Tribochemistry of Lubricating Oils, Volume 45 (Tribology and Interface Engineering)

TRIBOCHEMISTRY OF LUBRICATING OILS

TRIBOLOGY AND INTERFACE ENGINEERING SERIES Editor B.J. Briscoe (U.K.) Advisory Board M.J. Adams (U.K.) J.H. Beynon (U.K.) D.V. Boger (Australia) P. Cann (U.K.) K. Friedrich (Germany) I.M. Hutchings(U.K.)

Vol. Vol. Vol. Vol. Vol.

24 25 26 27 28

Vol. 29 Vol. 30 Vol. 31 Vol. 32 Vol. Vol. Vol. Vol.

33 34 35 36

Vol. 37 Vol. 38 Vol. 39 Vol. Vol. Vol. Vol. Vol.

40 41 42 43 44

J. Israelachvili (U.S.A.) S. Jahanmir (U.S.A.) A.A. Lubrecht (France) I.L. Singer (U.S.A.) G.W. Stachowiak (Australia)

Engineering Tribology (Stachowiak and Batchelor) Thin Films in Tribology (Dowson et al., Editors) Engine Tribology (Taylor, Editor) Dissipative Processes in Tribology (Dowson et al., Editors) Coatings Tribology - Properties, Techniques and Applications in Surface Engineering (Holmberg and Matthews) Friction Surface Phenomena (Shpenkov) Lubricants and Lubrication (Dowson et al.. Editors) The Third Body Concept: Interpretation of Tribological Phenomena (Dowson et al.. Editors) Elastohydrodynamics - '96: Fundamentals and Applications in Lubrication and Traction (Dowson et al., Editors) Hydrodynamic Lubrication - Bearings and Thrust Bearings (Frene et al.) Tribology for Energy Conservation (Dowson et al.. Editors) Molybdenum Disulphide Lubrication (Lansdown) Lubrication at the Frontier - The Role of the Interface and Surface Layers in the Thin Film and Boundary Regime (Dowson et al., Editors) Multilevel Methods in Lubrication (Venner and Lubrecht) Thinning Films and Tribological Interfaces (Dowson et al., Editors) Tribological Research: From Model Experiment to Industrial Problem (Dalmazet al.. Editors) Boundary and Mixed Lubrication : Science and Applications (Dowson et al., Editors) Tribological Research and Design for Engineering Systems (Dowson et al., Editors) Lubricated Wear - Science and Technology (Sethuramiah) Transient Processes in Tribology (Lubrecht, Editor) Experimental Methods in Tribology (Stachowiak and Batchelor)

Aims & Scope The Tribology Book Series is well established as a major and seminal archival source for definitive books on the subject of classical tribology. The scope of the Series has been widened to include other facets of the now-recognised and expanding topic of Interface Engineering. The expanded content will now include: • colloid and multiphase systems; • rheology; • colloids; • tribology and erosion; • processing systems; • machining; • interfaces and adhesion; as well as the classical tribology content which will continue to include • friction; • contact damage; • lubrication; and • wear at all length scales.

TRIBOLOGY AND INTERFACE ENGINEERING SERIES, 45 EDITOR: B.J. BRISCOE

TRIBOCHEMISTRY OF LUBRICATING OILS ZENON PAWLAK University of Technology and Agriculture Bydgoszcz, Poland

Er^SHVIFR

2003 ELSEVIER Amsterdam - Boston - London - New York - Oxford - Paris San Diego - San Francisco - Singapore - Sydney - Tokyo

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PREFACE Recognition of tribology as a truly interdisciplinary field is growing and this book provides a review of the rapidly developing field of tribochemistry. The interaction of chemistry under boundary lubrication is known as tribochemistry. Chemical reactions between some additives from lubricants and activated surfaces produce antiwear tribofilm. Friction and wear expose fresh surfaces which are active catalysts to the reaction of lubricants, especially for the action of the phosphorus, sulfur and chlorine, the antiwear additives. The detailed physical and chemical processes occurring in the contact zone are still not well understood. The word "Tribochemistry" is defined as lubrication under engine conditions that is a compromise between numerous limitations forced by the nature of the physical and chemical properties of the lubricant, the material surfaces, the oil formulation, and the life required. In boundary lubrication, additives react with surfaces under tribological conditions, resulting in the formation of tribofilm. These tribofilms have generally been poorly characterized and thus the mechanism of film growth and removal (wear) is largely unknown. This is particularly true for synthetic oils and for oils with metal-free additives, which are increasingly being used as an environmentally friendlier alternative to the ones containing heavy metals. Considerable progress has been made in studying tribofilms in the last decade. A number of important surface and thin film techniques have been developed in recent years, which are contributing to a better understanding of at least some tribochemical processes of boundary lubrication. In recent years, X-ray absorption near edge structure XANES spectroscopy, a powerful technique for tribofilm characterization, was used to identify a layered structure (surface and bulk) of tribofilms. The chemistry of tribofilms generated by the combination of zinc dialkyldithiophosphate (ZDDP) and molybdenum dialkyldithiocarbamate (MoDTC) has been examined. "Tribochemistry of Lubricating Oils" provides a broad overview of the many tribochemical problems which may arise under harsh engine oil conditions. In the conditions of lubrication, the degradation of engine oil is a function of time as well as temperature. Tribochemical processes have been observed and utilized technologically for a long time, and the action of many lubricant additives is based on tribochemical processes. The conscious recognition of tribochemistry as a science is recent. The term "tribochemistry" introduced by Thiesen in 1967 has attracted less attention than the term "tribology" introduced by P. lost in 1966. We

vi

Preface

can expect the term "tribochemistry" to have an expotentional effect on the development of this technologically important field. This volume is a comprehensive text that attempts to deal with the tribochemical reactions in hydrocarbon formulations affecting the tribofilm formation on metal surfaces. The most important factor governing the tribochemical processes under boundary lubrication is connected with the action of soft-core and hard-core reverse micelles, RMs. The book covers a very broad spectrum of topics, e.g., additives interactions, acid-base processes in lubricating formulations and the importance of solubilization. Emphasis is on chemical interpretations of the phenomena of tribochemistry of reverse micelles, surface tribochemistry, and current analytical techniques of metal surfaces. The book starts off with an introduction to tribochemistry, the science concerned with chemical reactions affecting the tribofilm formation on activated metal surfaces. The "Tribochemical Tree" summarizes our knowledge of some of the most important processes proceeding in the bulk lubricant and on mechanically activated surfaces. The next chapter is a review of current practice in lubrication of internal combustion engines and lubricant design. The role of individual lubricant components and their use in mineral and synthetic formulations is covered. This is followed by a discussion of the tribochemical effects of additive interactions. The heart of the manuscript is chapters, "Tribochemical nature of antiwear film", "Surface tribochemistry and activated processes", and "Analytical techniques in lubricating practices". Topics covered include tribofilm formation, organomolybdenum compounds in surface protection, catalytic activity of rubbing surfaces, introduction of some techniques for evaluation of tribofilms composition and analytical techniques for evaluation of lubricant degradation. Examples of the appHcation of basic concepts are introduced, eg., acidity and basicity in the process of lubricant deterioration. The final chapter, "Environmental issues", addresses lubricant performance and suggests ways to improve fuel efficiency of engines by the use of lower viscosity grade oils and the use of modifiers to reduce metal-metal friction. The book can be used by practicing scientists or engineers seeking a review of the state of knowledge of the tribochemistry field; however, it can also be used in the education of graduate students' one semester courses in chemical and mechanical engineering. For tribology courses offered in mechanical engineering departments, the chemistry concepts and application examples presented add specific knowledge and greater appreciation of the need to consider all aspects of tribological processes. Engineers looking for guidance on problems involving acidbase reactions in oil formulations, role of inverse micelle in lubrication and concerns with the effect of organomolybdenum compounds in engine surface protection will find the book very useful. Essentially, the content meets its objective of helping to make reverse micelles formation and solubilization processes

vii

Preface

more understandable to lubricant chemists, and explains the principles of the chemistry of tribofilm formation. It was a very rewarding experience to write a book which fills an important gap in the reference material on tribochemistry. May 2003, Salt Lake City

Zenon Pawlak

Acknowledgments The University of Gdansk, Poland, has played a crucial role in my teaching and research on the thermodynamics of hydrogen bonding in nonaqueous solvents and engine oils physicochemistry, where I was Professor of Chemistry until 1990. Over the last 20 years I have benefitted from the help of many people, and need to mention a few. Professor Lucjan Sobczyk, University of Wroclaw (Poland), deserves a special word of thanks because he inspired me in my hydrogen bonding research and he made it possible for me to start my own research program. During my sabbatical years, 1972-1975 in Florida University, Gainesville, I worked with Professor Roger G. Bates ("pH man"), who taught me that in solution chemistry there are other species besides solvated proton (H'^soiv)' but the proton is the most important yet still very unknown. Therefore, I began the study of proton transfer reactions in nonaqueous solvents and we pubhshed ten papers. In 1979 I was a recipient of a Deutscher Akademischer Austauschdient Fundation. I used this scholarship to work with Professor George Zundel in the Physical Chemistry Institute, University of Munich, where we studied proton transfer reactions in aprotic solvents. During my sabbatical year (1981-82) at Leicester Polytechnic (Great Britain), I had the pleasure of working with Professor Malcolm F. Fox and Professor Jim D. Picken. It was a memorable time of the Solidarity movement in Poland. During that time we studied acid-base interactions and micellar processes in engine oils when engines were run on bio-gas. The 1982-1983 academic year was spent at the University of Texas, Austin, with Professor Joseph J. Lagowski's research group. We studied the dissolution of gold by solvated electrons in liquid ammonia. I enjoyed my research in Utah, from 1986 to 1990, at Brigham Young University, Provo, collaborating with Professor Reed Izatt and Professor Noel L. Owen, where calorimetric and electrometric studies were applied for used and fresh engine oils and FTER spectrometric studies of solvated electrons in liquid ammonia with wood surface and wood components were analyzed. My home in Utah is a great place to write this book while I have continued to work in environmental chemistry in Salt Lake City. I have enjoyed skiing many winters in Utah and this provided me with inspiration to write about tribochemistry.

viii

Preface

First of all, I would like to thank Professor Duncan Dowson for his personal input and support to publish this book and Professor Brian Briscoe for constructive comments and remarks. It is my pleasure to express thanks to Professor Bamey E. Klamecki, University of Minnesota, Peter Cartwright, PE, MinneapoUs; Dr. Pierre A. Willermet, Ford Motor Co; Professor Lucjan Sobczyk, University of Wroclaw (Poland); Professor Czeslaw Kajdas, Warsaw University of Technology (Poland), and Professor Stanislaw Plaza, University of Lodz (Poland) for reviewing part or all chapters of my book and friendly comments. A special word of thanks is due to my friend. Professor Ryszard Pifkos, Medical School in Gdansk (Poland), for his permanent support and valuable suggestions during the preparation of the book. A word of thanks is due to my friend, Professor Beniamin Lenarcik, University of Technology and Agriculture, Bydgoszcz (Poland) for his support, where I am currently a Professor of Chemistry and the Head of Physicochemistry of Surfaces Department. I would like to give many thanks to my family for all their support during preparation of this manuscript. I also would like to thank the library staff of Brigham Young University, Provo and University of Utah, Salt Lake City, Utah for their help in finding many references and for permission of using technical resources during the preparation of the manuscript. I would like to thank all those undergraduate and graduate students from Gdansk University whose participation in the engine oils physicochemistry research program over many years has been a permanent inspiration. The author will accept with gratitude all suggestions aimed at the improvement of this work. Finally, I would like to thank the following publishers for granting us permission to reproduce the figures listed below: Figure 2.3: Reprinted from Lubrication, Vol. 44, 1958, pp. 173-186, with permission from Society of Tribologists and Lubrication Engineers. Figures 2.6 and 2.7: Reprinted from D. Klamann, Lubricants and Related Products. Synthesis, Properties, Applications, International Standards: Verlag Chemie, Weinheim. Copyright 1984, with permission from John Wiley and Sons. Figure 2.8: Reprinted from Tribol. Int., Vol. 24, M.F. Fox, Z. Pawlak and D.J. Picken, Inverse micelles and solubihzation in hydrocarbon formulations, pp. 341349. Copyright 1991, with permission from Elsevier.

Preface

ix

Figure 2.12: Reprinted from Petroleum Review, Vol., 47, 1993, pp. 84-87, with permission from The Institute of Petroleum. Figure 3.2: Reprinted with permission from Critical Rev. Anal. Chem., Vol. 21, 1990, pp. 257-278. Copyright CRC Press, Boca Raton, Florida. Figure 3.3: Reprinted from Y. Moroi, Micelles Theoretical and Applied Aspects, Plenum Press, New York. Copyright 1992, with permission from Plenum Press. Figure 4.1: Reprinted from the Proceedings of European Academy of Surface Technology, M. Kasrai, G.M. Bancroft, K.F. Laycock, H.A. Tan, and X.H. Feng, November 1993, Schwabischgmund, Germany E.G. Leuze, SaulgagAVurtt, 1994, pp. 79-85, with permission of the authors. Figure 4.2: Reprinted from Phys. Rev. B, Vol. 51, Z. Yin, M. Kasrai and M. Bancroft, X-ray-absorption spectroscopic studies of sodium polyphosphate glasses, pp. 742-750. Copyright 1995, with permission of the authors. Figure 4.3: Reprinted from Wear, Vol. 202, Z. Yin, M. Kasrai, M. Fuller, G.M. Bancroft, K. Fyfe and K.H. Tan, Application of soft X-ray absorption spectroscopy in chemical characterization of antiwear films generated by ZDDP: the effect of physical parameters, Part I, pp. 172-191. Copyright 1997, with permission from Elsevier. Figure 4.4: Reprinted from Tribol. Int., Vol. 31, M.L.S. Fuller, M. Kasrai, G.M. Bancroft, K. Fyfe and K.H. Tan, Solution decomposition of zinc dialkyl dithiophosphate and its effect on antiwear and thermal film formation studied by Xray absorption spectroscopy, pp. 627-644. Copyright 1998, with permission from Elsevier. Figure 4.6: Reprinted from D.C. Koningsberger and R. Prins, Eds., X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES, Wiley, New York. Copyright 1988 with permission from John Wiley and Sons. Figure 4.7: Reprinted from D. Dowson et al., Eds., Lubricants and Lubrication, Elsevier Science B.V. M. Kasrai, M. Fuller, M. Scaini, Z. Yin, G.M. Bancroft, M.E. Fleet, K. Fyfe and K.H. Tan, Study of tribochemical film formation using Xray absorption and photoelectron spectroscopies, pp. 659-669. Copyright 1995, with permission of the authors.

X

Preface

Figure 4.8: Reprinted from Spectrochim. Acta , Vol. 49A, N.E. Lindsay, R.O. Carter, ffl, P.J. Schmitz, L.P. Haack. R.E. Chase, J.E. DeVries and P.A. Willermet, Characterization of films formed at a lubricated cam/tappet contact, pp. 2057-2070. Copyright 1993, with permission from Elsevier. Figure 4.9: Reprinted from Wear, Vol. 66, H.J. Mathieu and D. Landolt, The application of Auger electron spectroscopy to the study of wear surfaces, pp. 87100. Copyright 1981, with permission from Elsevier. Figure 4.10: Reprinted from Tribol. Int., Vol. 25, P.A. Willermet, R.O. Carter, III and N.E. Boulos, Lubricant-derived tribochemical films-An infra-red spectroscopic study, pp. 371-380. Copyright 1992, with permission from Elsevier. Figure 5.1: Reprinted from Lubrication Engineering, Vol. 51, 1995, pp.182-194, with permission from Society of Tribologists and Lubrication Engineers. Figure 5.2: Reprinted from Tribol. Int., Vol. 32, P.D. Fleischauer and J.R. Lince, A comparison of oxidation and oxygen substitution in M0S2 solid film lubricant, pp. 627-636. Copyright 1999, with permission from Elsevier. Figures 5.3, 5.4, 5.5, 5.6: Reprinted from Tribology Transactions, Vol. 41, 1998, pp. 69-77, with permission from Society of Tribologists and Lubrication Engineers. Figure 5.7: Reprinted from Tribol. Int., Vol. 31, C. Grossiord, J.M. Martin, Th. Le Mogne, C. Esnouf and K. Inoue, M0S2 single sheet lubrication by molybdenum dithiocarbamate, pp 737-743. Copyright 1998, with permission from Elsevier. Figure 6.1: Reprinted from JSAE Review, Vol. 16, M. Tomita, H. Komo, Y. Nomura, M. Nozawa, S. Yamaguti and Y. Toda, Study on deterioration of engine oil and its sensing, pp. 283-285. Copyright 1995, with permission from Elsevier. Figures 6.2 and 6.3: Reprinted from Chemistry and Industry, 20 July, 1987, pp. 470-473, with permission from Society of Chemical Industry, Figure 6.4: Reprinted from Lubrication Engineering, Vol. 56, 2000, pp. 23-29, with permission from Society of Tribologists and Lubrication Engineers. Figure 6.5: Reprinted from Tribol. Int., Vol. 24, M.F. Fox, Z. Pawlak and D.J. Picken, Acid-basedeterminationof lubricating oils, pp. 335-340. Copyright 1991, with permission from Elsevier.

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xi

Figures 6.8 and 6.9: Reprinted from Tribol. Int., Vol. 23, M.F. Fox, J.D. Picken and Z. Pawlak, The effect of water on the acid-base properties of new and used IC engine lubricating oils, pp. 183-187. Copyright 1990, with permission from Elsevier. Figure 6.10: Reprinted from Wear, Vol. 139, W.S. Moon and Y. Kimura, Wearpreventing property of used gasoline oils, pp. 351-365. Copyright 1990, with permission from Elsevier. Figure 7.1: Reprinted from Synthetic Lubricants and High Performance Functional Fluids, L.R. Rudnick and R.L. Shubkin, Eds., 2nd Edition, Marcel Dekker, New York. Copyright 1999, with permission from Marcel Dekker.

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Xlll

CONTENTS Chapter 1. INTRODUCTION TO THE TRIBOCHEMISTRY CONCEPT Chapter!.

LUBRICATION CHEMISTRY

2.1. 2.2. 2.3. 2.4. 2.5.

The Role of Lubricants in Engine Lubricant Additives Tribochemical Interactions of Additives Synthetic Engine Oils Lubricant Requirements and Specifications Problems

Chapter 3. MICELLAR STRUCTURE OF LUBRICATING FORMULATIONS 3.1. 3.2. 3.3. 3.4.

Reverse Micelles in Tribochemical Processes Micellar Solubilization in Lubrication Tribochemistry of Hard-Core Reverse Micelles Tribochemical Interactions of Acid-Base Chemistries Problems

Chapter 4. TRIBOCHEMICAL NATURE OF ANTIWEAR FILMS

1 11 11 17 36 48 56 64 67 67 77 91 112 118 121

4.1. Tribochemical Characterization of Antiwear Films 4.2. The Chemistry of Tribofilm Formation 4.3. Techniques for Evaluation of Metal Surfaces Problems

121 133 143 159

Chapter 5. SURFACE TRIBOCHEMISTRY OF ACTIVATED PROCESSES

161

5.1. Chemical Nature of Metal Surfaces 5.2. Catalytic Activity of Rubbing Surfaces 5.3. Tribochemical Reactions on Surfaces

161 170 180

xiv

Contents

5.4. Organomolybdenum Compounds in Surface Engines Protection Problems Chapter 6. 6.1. 6.2. 6.3. 6.4. Chapter 7.

ANALYTICAL TECHNIQUES IN LUBRICATING PRACTICES

217

Evaluation of the Degradation of Lubricants.. Engine Oil Condition Monitoring Oil Acidity and Basicity Engine Oil Evaluation Tests Problems

217 223 237 258 263

ENVIRONMENTAL ISSUES

267

7.1 Recyclability, Biodegradability and Toxicity 7.2 Clean Air and Energy Efficient Cars Problems Chapter 8.

188 214

267 277 299

LUBRICATING OILS - RELATED ACRONYMS AND TERMS

301

References

317

Appendix

351

Subject Index

357

Chapter 1

INTRODUCTION TO THE TRIBOCHEMISTRY CONCEPT It was known that lubricants could be used to influence bothfi'iction and wear, yet the physical and chemical processes by which this is achieved have been explored recently and are still being revealed to us. Duncan Dowson, 1979 The term "tribochemistry"originated from two words "tribo" (Greek: tribos, tribein - a rubbing, to rub) and "chemistry" of reactions occuring between the lubricant and the surfaces. It relates directly to the combinations of all sciences and technologies associated with the chemical reactions in mineral and synthetic oil formulations and with tribofilm formation on metal surfaces under boundary lubrication. Tribochemistry is of practical importance because lubricants are an essential component of engine and other machine operations. Mechanical energy is lost at the power train due to changes occurring in lubrication, friction, and wearing processes. Because of this interplay of friction and chemistry, these reactions are called tribochemical. Tribochemistry not only exploits the fundamentals of chemistry and physics, but it also encompasses other sciences, especially mechanics and material sciences. The experimentally observed influence of friction on the chemical reaction is the acceleration of tribochemical reactions. Technological importance of tribochemistry extends from antifriction action of lubricants to material surface quality (Briscoe et al., 1992; Dowson, 1998; Fischer, 1988a and 1988b; Fuller et al., 1997 and 1998; Grossiord et al., 1998 and 1999; Heinicke, 1984; Hsu et al., 1988 and 2002a; Kajdas , 2001; Lindsay et al., 1993; Luisi et al., 1988; Martin, 1999; Martin et al., 1986a, 2000a and 2001; Muratov et al., 1998; Sakurai, 1981; Spikes, 2001; Stolarski, 1999; Willermet, 1998; Willermet et al., 1991, 1995a and 1997; Varlot et al., 1999, 2000 and 2001; Yin et al., 1997a and 1997b). The basic processes of dissolution, acid-base interaction, micellization, solubilization, oxidation and reduction take place in oil formulation. During engine operation, additives of the lubricant interact continuously with engine surfaces and themselves. Thus, there is a progressive change in the surface due to the lubrication, friction, and wearing processes, tribofilm formation, and oxidation. All these processes are presented and discussed throughout this book. Surfactant additives are fundamental to reverse micelles (RMs) formation in oil

2

Chapter 1

formulation. The formation of RMs, due to dipole-dipole interactions between the polar heads of the surfactant molecules, may play an important role in oil formulation. The structure of individual surfactant-stabilized reverse micelles, together with their mutual interactions, have been well characterized over recent years (Bascom et al., 1959; Das et al., 1992; Gon and Kumar, 1996; Inoue, 1993; Inoue and Watanabe, 1983; Kertes and Gutman, 1976; Kertes and Markowits, 1968; Kertes et al., 1970; Luisi et al., 1988; Pawlak, 2001; Pileni, 1989a; Reemik, 1965; Singleterry, 1995; Ward and du Reau, 1993; Willermet, 1998). Due to the continuous input of thermal energy necessary to maintain mechanical work, tribological systems are in progressive equilibrium. In the tribosystem, the flow of energy is accompanied by an increase in entropy of the total system and is reflected by the tribochemical reactions and deterioration of lubricant quality. Our understanding of tribosystems has been seriously limited by a lack of kinetic information on critical reactions in hydrocarbon formulation and critical reactions at interfaces.

/

^

^

\

/

I" ^Z" Low or N on polar ^ sol V ents ^' ^'Ij^^* Intermol E cular <»m...M.% inte R actions Ashless di S persant + ZDDP \ ^ ^^f.MM^* Metallic det E rgent + ZDDP *>)\\\n((<» / Hard & soft core M icelles formation \ •im..MiMut Antiox I dation (ZDDP) t^.MMum?* ^ Molecular de C omposition J" Acid-base r E actions
Fig. \A. Tribochemical "TREE"

Introduction to the Tribochemistry Concept Lubricant resilience and buffering against change after all mechanical operations become so powerful that they influence global chemical processes. Lubricant quality is degraded by the creation of hazardous chemicals from burning and oxidation processes. Extension of the life of lubricants will require the creative integration of chemical, mechanical, and tribological understanding (Dowson, 1998; Pirro and Wessol, 2001; Spikes, 2001; Stachowiak and Batchelor, 2001). A "Tribochemical Tree" shown in Fig. 1.1 summarizes our knowledge of some of the most important processes occurring in the bulk oil and the effects of those processes on the mechanically activated surfaces under the boundary lubrication conditions (Dowson, 1998; Kajdas, 2001; Martin, 1999; Pawlak, 2001; Sakurai, 1981; Spikes, 2001; Willermet, 1998; .Yin et al., 1997a and 1997b).

mic

a r b t r a r y

u n i t s

low-polar

intermediate

high-polar

Dielectric constant (e) Fig. L2. Dependence offree-energychanges (AG° ^jj of micellization of surfactants in high, intermediate, and low-polar solvents. Normal micelle (M) to reverse micelle (RM) transition asfrinctionof polarity (e) of the medium. Surfactants in a medium of optimum dielectric constants 38 to 41 do not aggregate but remain in the monomeric state (n)

3

4

Chapter 1

The additive mixtures interact in a variety of ways, both in the bulk oil and on surfaces. Tribochemical interactions of additives in the oil formulation are discussed in Chapter 2. Surfactant molecules, when dissolved in base oil, are capable of self-organization to form aggregates such as soft-core reverse micelles (RMs). The polar or charged head groups of these molecules with the counter ions form the interior of the micelle (core), and the hydrocarbon chains made up its external shell. The most important factor governing the tribochemical reactions under boundary lubrication is connected with the action of soft-core and hard-core reverse micelles discussed in Chapter 3. The role of solvent structure and dielectric effects on the micelle formation have been extensively studied over the past fifty years (Das et al., 1992; Gon and Kumar, 1996; Kertes and Gutman, 1976; Kertes and Markovits, 1968; Kertes et al., 1970; Luisi et al., 1988; Pileni, 1989a; Reemik, 1965; Ward and du Reau, 1993). The nature of micelles, whether normal or reverse, depends on the polarity of the medium. Several interesting features have been obtained with respect to the aggregation and micelle structure with changing polarity of the medium. If the medium polarity decreases progressively as, for instance, in the series: water (e = 80.1), monoethylene glycol (e = 38.7), and chloroform (e = 4.8), the surfactant initially existing as a normal micelle (M) with a non-polar core, gradually changes to an intermediate monomeric state (n) to form a reverse micelle (RM) with a polar core, see Fig. 1.2 (Das et al., 1992; Gon and Kumar, 1996). The free-energy changes of micellization (AG^j^^J of the surfactants are also schematically shown in Fig. 1.2. The micelle formation process and structure can be described by thermodynamic functions (AG^j^i^? ^H°j„i^, AS^'j^jJ, physical parameters (surface tension, conductivity, refractive index) or by using techniques such NMR spectroscopy, fluorescence spectroscopy, small-angle neutron scattering and positron annihilation. Experimental data show that the dependence of the aggregate nature, whether normal or reverse micelle is formed, depends on the dielectric constant of the medium (Das et al., 1992; Gon and Kumar, 1996; Kertes and Gutman, 1976; Ward and du Reau, 1993). The thermodynamic functions for micellization of some surfactants are presented in Table 1.1. Micellization occurs in order to reach minimum free energy (AG^j^^^,) in the system, once the process of adsorption at the interface has reached saturation. Thermodynamic measurements in dilute low-polar solvents surfactant solutions give values for the enthalpy change on micellization (Aff^^J in the range of (-50.2 to -119.9 kJ • mol"^) and the associated entropy change (AS^^jJ in the range (-63.8 to -143.1 J • mol"^ K"^), see Table 1.1. The free energy change (AG°^jJ comprise enthalpy and entropy contributions according to the equation 1.1: AG^„ = AH°,„ - TAS",,,

(1.1)

and are negative in the range (-15,9 to -87.0 kJ • mol'^) which can indicate self-

Introduction to the Tribochemistry Concept association processes. Table l.L Thermodynamic functions for micellization of surfactants at 25 °C Surfactant^ Thermodynamic functions

A

B C Reverse micelle (RM)

D

E Normal micelle (M)

Free energy, AG°^, kJ • mol-^

-60.7

-63.8

-15.9

-87.0

-335

to

-502

Enthalpy, A//°^j, kJ • mol'

-79.5

-105.4

-50.2

-116

-167

to

+586

Entropy, A^°,,, J • mol-^ • K-^

-63.8

-143.1

-115.1

-96.7

+837

to +2,510

^The following surfactants have been tested : (A) Potassium benzenesulfonate in heptane (Kertes and Gutman, 1976), (B) Tri-«-dodecylammonium bromide in cyclohexane (Reemik, 1965), (3) Tri-^-dodecylammonium bromide in benzene (Reemik, 1965), (D) Triw-dodecylammonium tetrachloroferrate in benzene (Kertes et al., 1970), (E) Most surfactants in aqueous solutions (Ward and du Reau, 1993) The enthalpy and entropy changes of micellization have been calculated for benzenesulfonate and alkylammonium salts in low-polar solvents suggesting that micellization is essentially an enthalpy-driven effect. The aggregation can take place at low concentrations of surfactant and can have different aggregation numbers. The absence of well-defined critical micelle concentrations (CMC) for some systems in the low-polar solvents was observed (Kertes and Gutman, 1976). In aqueous solutions, the resulting free energy changes (AC^iJ ^^e negative (in the range -502 to -335 kJ • mol'^) indicating spontaneous processes. The entropy changes are dominant, contributing most to the overall free energy changes as a result of changes in the water structure. In water, formation of micelles is an entropy-driven process which favors the shielding of the amphiphilic hydrocarbon moiety from the solvent (Kertes and Gutman, 1976; Ward and du Reau, 1993). Several mechanisms of action of reverse micelles are generally recognized. These are as follows: (1) Prevention of agglomeration of insoluble oxidation products and sludge particles generated by the combustion of diesel fuels as discussed in Chapter 2, (see also Fig. 1.3).

6

Chapter 1

(2) Solubilization of oxidation products, e.g., organic acids (HA) and some additives (ZDDP) by soft-core reverse micelles and inorganic acids (HS) by hardcore reverse micelles in oil formulations as shown in Fig. 1.3.

HA

m ZDDP ZDDP H A (H20)n< ' HA ZDDP ZDDP

ZDDP

Soft-core reverse micelle '^^—O metal alkyl phenate ^ sludge particle HA organic acid ZDDP zinc dialkyldithiophosphate

(^^i^::^^^^—

Hard-core reverse micelle metal alkyl benzenesulfonate ^cacoa) colloidal core HS sulfiir acid, nitrogen acid

Fig, 1.3. Solubilization of oxidation products, e.g., organic acids (HA) and some additives (ZDDP) by soft-core and inorganic acids (HS) by hard-core reverse micelles in oil formulations

The kinetics and thermodynamics of the solubilization and localization of guest molecules into reverse micelles in lubricants are little known. Questions such as: What are the driving forces responsible for the uptake of molecules into reverse micelles? What are the kinetic steps in solubilization? Where is the location of the guest molecules in a water pool or interface? - all still await a definitive answer (Luisi et al, 1988; Pawlak, 2001; Willermet, 1998). The presence of chemical guest species in the water pool of soft-core RMs can modify the organization of the micellar components. The chemical guest species may compete with the surfactant for water molecules to build its own hydration shell. Ions may be specifically bound to the charged groups of the micellar wall resulting in dramatically changed properties of micelles. The rate of release of some solubilizates from a soft-core reverse micellar system to an adjacent oil phase across a planar interface has been recently investigated (Albery et. al., 1987; Luisi et al., 1988). A related process would be the transfer of solubilizates (e.g., ZDDP) from reverse micelles into a co-existing

7

Introduction to the Tribochemistry Concept activated metal surface in the tribochemical system as discussed in Chapter 3 (Lindsay et al., 1993; Yin et al, 1997). The mechanically activated surface in the presence of ZDDP is involved in the formation of polyphosphate tribofilm (Martin, 1981; Martin et al., 2000a and 2001; Varlot et al., 2000; Willermet et al., 1991; Yin et al. 1997). In the absence of ZDDP, hard-core RMs, e.g., (R-C6H4-SO3)2Ca-10CaCO3 form compacted, amorphous metal carbonate films. Borated succinimide can also provide antiwear protection by formation of mixed iron-boron oxides film (Shirahama and Hirata, 1989; Varlot et al., 1999 and 2001). Reported effects of detergents on tribofilm formation in the presence of ZDDP range from synergistic to antagonistic (Willermet, 1998). Succinimide dispersants adversely affect the tribofilm formation of ZDDP, and can be influenced by the strength of complex formation in the bulk phase. Chapters 4 and 5. Zinc poly(thio)phosphate "thermal films" have been recently recognized to be precursor reaction products in the formation of polyphosphate glasses tribofilm (Bascom et al., 1959; Bovington and Dacre, 1984; Fuller et al., 1997; Martin, 1999; Varlot et al. 1999; Willermet et al. 1991). The rearrangement of zinc dialkyldithiophosphate initiated by a double alkyl group migration from oxygen atoms to sulfur atoms (Fuller et al., 1998; Jones and Coy, 1981; Varlot et al., 2000), is believed to be the result of the following reaction mechanism (equation 1.2): RO. (s=P-S-)^Zn

RS. — ^

RO Zinc dialkyldithiophosphate or Zn[(RO)2PS2]2

(0=P-0-)^Zn

^^^^

RS A linkage isomer (LI-DDP) or Zn[02P(SR)2]2

As a result of tribochemical reactions and the presence of oxygen or hydroperoxide in oil, adsorbed ZDDP and LI-ZDDP on the surface are thermooxidatively decomposed to give long-chain polyphosphates Zn(P03)2. Zn[02P(SR)2]2 (adsorbed) + O2 (or ROOH) -* Zn(P03)2 + sulfur species

(1.3)

As the rubbing continues, the polyphosphate layer comes into closer contact with water in oil and is hydrolyzed to give a short-chain polyphosphate, e.g., Zn2P207 (Fuller et al., 1998). During the mechanically activated processes (friction coefficient, /i > 0.4), a nascent surface is generated, and in the presence of oxygen, an iron oxide is formed. The acid - base reaction between polyphosphate glasses (hard Lewis bases) and the oxides (hard Lewis acids) is

Chapter 1

8

thermodynamically favorable according to the hard and soft acids and bases (HSAB) principle (Pearson, 1997). HSAB has been described in the literature (Gellman and Spencer, 2002; Martin, 1999; Martin et al., 2001), 5Zn(P03)2 + Fe,03 or

*

Zn(P03)2 + 2FeO Long-chain phosphate

-

FezZnaPioQ,

+ 2 ZnO

Fe2Zn(P04)2 Short-chain phosphate

(1.4) (1.5)

(VS) Very severe wear conditions, jLi > 0.8 - nascent surfaces and adhesive wear - disruption of the phosphate tribofllm - formation of Fe/Zn sulfides

(MS) More severe wear conditions, |Li>0.4 - eHmination of Fe oxide particles by acid-base reactions - formation of short-chain Zn/Fe phosphate glasses

(MS) More severe wear conditions, |i>0.4 - bi-layered tribofilm formed of: - Zn polyphosphates and - short-chain phosphate glasses containing FeS precipitates

(MW) Mild wear conditions, |i < 0.4 - thermal film formed by long-chain Zn polyphosphates

Fig. 1.4. The cycle of tribochemical fibn formations durmg the tribological mild, more severe, and very severe wear conditions initiated by thermooxidative decomposition of the ZDDP additive m the steel-on-steel combination (not to scale) The role of organic sulftir species (other than those in thiophosphate form) in the tribochemical process is to react immediately with the nascent iron surfaces and ZnO to form metal sulfides (FeS, ZnS) embedded in the short chain phosphate matrix. ZnO

+

S"'

-

ZnS

+

O"'

(1.6)

Also, the presence of sulfur in the phosphate chain Zn6(Pio029S2) can promote the

Introduction to the Tribochemistry Concept formation of ZnS as precipitates in the short-chain polyphosphate material. The antiwear mechanism of ZDDP has been recognized as the ability of phosphate glasses to digest the oxides in the acid-base tribochemical reactions (Martin et al., 1986a). A layered structure of the tribofilm is composed of short-chain polyphosphates covered on the top by a zinc polymer-like phosphate. Fig. 1.4 is a diagram of the antiwear mechanism of the phosphates under the tribological wear conditions. The distribution of the friction coefficient (|LI) is illustrated over rotating surface conditions in a full tribological cycle. From the phase of minimum friction MW (mild wear) to the phase of MS (more severe wear), the friction increases showing a maximum friction VS (very severe wear), and from a maximum friction it decreases to the more severe (MS) conditions (Fischer, 1988a and 1988b; Hsu et al, 1997; Martin, 1999; Muratov et al., 1998; Plaza, 1997; Shpenkov, 1995a). The tribochemical reactions based on the hard and soft acids and bases (HSAB) principle linked to the friction coefficient (|i) are summarized above. The effect of tribological processes consists of the formation of mixed short-chain phosphate glasses containing sulfide precipitates. The thermal film made of long-chain zinc polyphosphates is formed on the surface. When friction increases, the process of transformation of phosphorus compounds into short-chain phosphate glasses is observed and iron sulfide abrasive particles are eliminated by tribochemical acid-base reactions. Under very severe wear conditions (nascent metal surface creation), an iron sulfide is formed, which will be mixed with the phosphate glasses tribofilm.

Problems

1.1 Tribochemistry Tribochemistry is the science concerned with the chemical reactions in mineral and synthetic formulations affecting the tribofilm formation on metal surfaces during the boundary lubrication processes. What are the differences in the concept of electron sharing in the liquid processes and on metal surfaces? 1.2

Additives Name two oil additives and explain their activities.

1.3 Interactions Give the relative order of strength of interactions (weak, medium, strong) in the following series of pairs in low-polar solvent, dielectric constant 8 < 10: (a) solvent + additive, (b) Calcium benzenesulfonates ^ reverse micelles formation, (c) strong acid (HS) + N-base, (d) weak acid (HA) + N-base, (e) soot particle + reverse micelle, and (f) mechanically activated surface processes and molecular decomposition.

9

10

Chapter 1

1.4 Tribochemical Tree Considering the tribochemical interactions in Figure 1.1, which of the following pairs illustrate hydrogen bonding or solubilization or acid-base process: (a) metallic detergent + HCl, (b) hard-core carbonate-sulfonate RMs + HCl, (c) metallic detergent + ZDDP, and (d) soft-core RMs + CH3COOH? 1.5 Oil formulation (a) By using specific examples, what is meant by the "tribological system in progressive equilibrium"?, (b) What information about the molecular structure of additives is indicated by the oil formulation requirements? 1.6 Oil formulation What is a distinctive feature about the oil formulation of each of the following: (a) base oils and (b) selected additives? 1.7 Reverse micelles Where in the reverse micelles would you find each of the following: (a) water, (b) organic acids, (c) molecules of detergents? 1.8 Reverse micelles Where in the soft-core or hard-core reverse micelles will you find each of the following: CaC03, H2O, calcium benzenesulfonates?

11

Chapter 2 LUBRICATION CHEMISTRY Perhaps the least evident, yet in the long run the most important features of lubricant development, were the involvement of physicists, chemists, material scientists and engineers in separate and combined studies which play no small part in the progress towards a recognition of the multi-disciplinary nature of the problems at hand, and the growing recognition that a knowledge of lubricant properties should form an integral part of the overall approach to machine design. Duncan Dowson, 1979 2el. The Role of Lubricants in Engine The development of modem engine and transmission technologies would have been impossible without advanced lubricant additives chemistry and lubricant formulation. Modern engine oils contain a wide range of additives which are blended with base oils to form a complete package capable of meeting demanding performance requirements (Havet et al., 2001; Klamann, 1984; Mortier and Orszulik, 1997; Pirro and Wessol, 2001). The primary purpose of high quality lubricant design is to maintain engines and transmissions in design condition as long as possible while enhancing vehicle operational characteristics. Vehicle design advances are starting points for the development of new performance requirements which, in turn, stimulate the development of new lubricant and additive technologies. If the performance of such additives mixtures could be readily predicted by combining the known performance characteristics of components, then formulating oils to achieve performance objectives by incorporating new additive chemistries would be a relatively straight forward task. However, it is frequently the case that additives interact in a variety of ways, both in bulk oil and on surfaces, resulting in synergism or antagonism which greatly complicate the task of oil formulation (Shirahama and Hirata, 1989; Spikes, 1989; Stachowiak and Batchelor, 2001; Willermet, 1998). In addressing natural resources conservation and environmental concerns, vehicle systems are being driven towards higher durability, longer term emission control and enhanced fuel efficiency. Environmental concerns have initiated significant changes in fuel technology, such as unleaded gasoline for catalyst systems, as well as a variety of alternative "cleaner" fuels.

12

Chapter 2

The development of chemical classes of lubricant additives is thus directly linked with advances in transportation technologies, and places greater challenges on lubricants operating over wider temperature extremes (Copan and Richardson, 1992; Dickens, 1991). The primary objectives of lubrication of reciprocating engines are the prevention of wear and the maintenance of power-producing ability and efficiency. These objectives require that lubricants function effectively to lubricate, seal, control frictional properties, prevent excessive wear and seizure of moving parts, protect against corrosion, keep surfaces and oil ways clear, cool and permit operation at extremes of temperatures. Engine oil formulations consist of (a) base oil and viscosity improver (7296%), and (b) additive package (4-28%). Fully formulated gasoline engine oils typically contain one, perhaps two dispersants, one or two detergents, an antiwear agent, oxidation inhibitors, corrosion inhibitors, foam inhibitors, viscosity modifiers, and pour point depressants. Most of these materials are surface active and may interfere with antiwear tribofilm formation (Rounds, 1989; Yamaguchi et al., 2002a). The products used to lubricate modem engines are complex mixtures of additives in a base oil. The composition of a typical engine oil (Salino and Volpi, 1987) is shown in Table 2.1. Table 2.1. Concentration range of main additives used in the formulation of engine oils Material

Weight (%)

Material

Weight (%)

SAE 30 or 40 base oil Metallic detergent Ashless dispersant Zinc dithiophosphate

71.5 2 1 0.5

Antioxidant/Wear Friction modifier Anti-foam agent Pour point depressant

0.1 0.1 2 0.1

to to to to

96.2 10 9 3.0

to to to to

2.0 3.0 15 ppm 1.5

The optimization of antioxidant systems is required to enhance the performance of the base oil by decreasing lubricant oxidation. Oxidation of the lubricant undergoes three stages. In the inhibited stage, properties of the oil are relatively stable and the oxidation extent is very small and is predominantly affected by temperature and by the concentration of antioxidants. The second oxidation stage is the break-down stage (increase in oxidation products), and the final stage is characterized by slow oxidation rates due to oil viscosity increases. Selection of antioxidants must provide an appropriate balance of performance for various applications to control oil thickening and its associated effects: corrosion, sludge and deposit formation (Barnes et al., 2001; Cemy et al., 2001; Mortier and Orszulik, 1997).

Lubrication Chemistry

13

Zinc dialkyldithiophosphates (ZDDPs) function mainly as antioxidants and antiwear additives. Molecules of ZDDPs adsorb on metal surface to participate in surface tribofilm formation under conditions of boundary lubrication. The solid tribofilms are formed at the metal surface to protect even under conditions of coarse contact under load (Bom et al., 1992). Incorporation of detergent and dispersant additives into a lubricant as surface-active agents (or surfactants) is used to provide the solubilizing action of engine sludge deposits at low temperature by dispersants, and at high temperature by detergents. Surfactants are basic compounds with a very important fiinction in acid deactivation (or solubilization). Detergents and dispersants have long chain hydrocarbons: R- is an alkyl group preferably with 12 or more carbon atoms, and a polar part. Examples of such surfactants are detergents which include calcium and magnesium sulfonates (RS00")2M^'^, phenates (RC6H40")2M^^, carboxylates (RCOO")2M^^, phosphonates RPOg^lVf^ and carbonate-sulfonate hard-core reverse micelles (RMs). Ashless dispersants are the most widely used types, such as the substituted polyisobutylene amine succinimides (mono-substituted, m-PIBS and bis-substituted, b-PIBS), succinate esters, Mannich bases, and phosphorus types, see Chapter 2.2 for formulas (Inoue and Watanabe, 1983; Papke and Rubin, 1992; Vipper and Watanabe, 1981). Detergents with additional basicity number (TBN). The extra base built into detergents accomplishes the very important job of neutralizing acid combustion products. The alkaline metal carbonates or borates in colloidal form dispersed in a base oil formulation must be predominantly amorphous as hard-core carbonatesulfonate or carbonate-phenate reverse-micelles (RMs). These RMs have been previously known as overbased detergents. The detergent additives with additional basicity used in the formulation of engine oils contain up to 30 times more metals than the corresponding neutral sulfonates detergent. The detergents with a total base number TEN in the range of 50 to 300 are usually employed in the formulation of modem engine oils for diesel engines. The alkyl groups Rmay have from 12 to 30 carbon atoms. Typical parameter values (molecular weight, metal : acid ratio, TEN, detergent (%) , and sulfur (%)) for hard-core micellar sulfonate and for alkylphenate sulfide detergents are presented in Table 2.2 (Salino and Volpi, 1987). Phosphonates have been employed less frequently as detergent additives in engine oils because it is difficult to obtain micellar calcium carbonate in oil formulations. Typical phosphonate formulas (phosphonate, thio-pyrophosphonate, and thio-phosphonate) are shown in Chapter 2.2. (Ramakumar et al., 1992; Willermet et al., 1991 and 1995b).

Chapter 2

14

Table 2.2. Typical parameter values for calcium sulfonates, alkylphenyl sulfides and alkylsalicylates detergents with additional total base number (TBN, mgKOH/g oil) from calcium carbonate. Formation of hard-core reverse micelles (RMs) of calcium carbonatesulfonate or alkylphenate and soft-core revere micelles (RMs) of calcium alkylsalicylate in oil formulations Parameter

Hard-core RMs

Hard-core RMs

(R-QH4-S03-)2Ca2^

S(C6H3RO

• (CaC03),

Molecular weight of acid Metal type [Metal]/[Acid] TBN Detergent (%) Sulftir (%)

Sulfonic 375 to 700 Ca, Mg, Ba 1 to 30 0 to 500 10 to 45 0.5 to 4

%c^''

• (CaC03),

S-alkylphenol 160 t o 600 Ca,Ba 0.8 to 10 50 to 300 30 to 50 0 to 4

Soft-core RMs (HO-CgHjRCOOO^Ca^^

Carboxyli c 250 to Ca 1 to 50 to 10 to None

1000 10 200 45

The mechanism of action of surfactant additives. The operation of internal combustion (IC) engines results in the formation of by-products: gases, soot particles, acids, water and free radical sources.

^ ^^^^^^^

soot particle surfactant molecule

Fig. 2.1. Steric stabilization mechanism of soot particles by formation of reverse micelles and solubilization A portion of these by-products enters the crankcase via blow-by gases and adsorption in the thin lubricant film. It seems that these mechanisms are partly

Lubrication Chemistry

15

dependent on the degree of aggregation of additives in the base oil. It has been demonstrated that in a fresh oil, detergent additives take the form of reverse micelles in equilibrium with the saturated monomer solution. Depending on their chemistry, they are involved in solubilization, neutralization and adsorption (Pawlak, 2001; Sakurai, 1981). Solubilization of insoluble oxidation products and soot particles. Reverse micelles (RMs) formations manage the prevention of agglomeration and the contamination process of insoluble oxidation particles and soot particles by both steric stabilization (Fig.2.1) and electrostatic stabilization mechanisms (Fig.2.2). The steric stabilization mechanism provides a physical barrier to agglomeration of particles by adsorption on particle surfaces. Adsorbed dispersant acts as a physical barrier to attraction between particles. The electrostatic stabilization mechanism develops a surface charge on the particles by proton transfer reaction between soot particle surface and detergent or hydrogen bonding formation complex. The base oil polarity performs an important role. Solvated molecules in solution attach themselves to insoluble particles via their polar fragment. This leads to creation of an electrostatic field that prevents these particles from clumping. Dispersants form hydrogen bonds with hydroxylic compounds found in used engine lubricating oils. This physical effect contributes to the internal cleanliness of engine by minimizing the agglomeration and subsequent deposition of sludge particles in a similar way as detergents act (Arkin and Singleterry, 1949; Baker et al., 1954; Ford, 1968; Honig and Singleterry, 1954; Honig and Singleterry, 1956; Inoue and Nose, 1987; Inoue and Watanabe, 1965; Inoue et al. 1983; Mathews and Hirschhom, 1953). Neutralization (or deactivation) of acidic products. During combustion in gasoline or diesel engines, certain materials in the fuel, such as sulfur and antiknock scavengers, can bum to form strong acids. It has been found that the inclusion of highly alkaline materials in the oil will help to neutralize these strong acids as they are formed. The sulfonates and phenates may be neutral or highly alkaline as carbonate-sulfonate, carbonate-phenate (hard-core RMs); that is, they may contain more of the basic material to neutralize mineral acids, i.e., strong inorganic acids. This neutralization reduces corrosion and corrosive wear and minimizes the tendency of these acids to cause oil degradation. Organic acids may be formed as a result of progressive oxidation of the oil, and these acids may be deactivated (solubilization) by hydrogen bonding complex formation with detergents and dispersants (soft-core RMs). See the detailed discussion in Chapter 3.0 (Denison, 1994; Kreuz, 1970; Salino and Volpi, 1987). Adsorption on metal surfaces (especially the piston) provides protection against the deposition of pollutants. Surface protection is the result of physical

16

Chapter 2

adsorption. Here too, the polar part of the additive faces the metal surface. In this case, the hydrocarbon chain must be small enough to ensure adhesion of the additive since the additive's solvation by hydrocarbons is minimal. The size of a hydrocarbon chain to which its polar group is bound must thus be adjusted to ensure the optimum performance of the detergent. It has been found that the detergent's optimum molecular weight ranges between 375 and 700. Since detergents are physically adsorbed on metal surfaces, they compete with other additives by the same mechanism of action. This negative synergy is particularly evident with regard to the so-called "friction modifier" additives sometimes used to reduce friction and hence fuel consumption. Laboratory tests have shown that the effectiveness of the modification of the coefficient of friction is considerably reduced when detergent additives are also present (Barcroft and Park, 1986; Barcroft et al., 1982).

V n O ^ -

^ SH

cr

SHV SH-VHAXSH

HA J^^HA^ 'X / HA ^ SH

^= ^

OH-s- .

P-H£V..HA-V^^ I n J^-"'^'^ P-H^ ^ /^ " " ^ ^^\ /0---HA >0 U

Fig. 2.2. Electrostatic stabilization mechanism by proton transfer (AAAOH^S) or by hydrogen bonding complex formation (AAAAO..HA), where SH is a strong acid, e.g., sulfuric acid, and HA represents an organic acid Addition of detergent additives has always been of great importance in the formulation of oils for diesel engines; in fact they are "dirtier" than gasoline engines. The temperature of the oil in a diesel engine is higher and this enhances oxidation and the formation of insoluble substances. The combustion of diesel fuel produces soot and SO3. Added to this is the fact that diesel engines are much less sensitive to the accumulation of oil-derived ash in their combustion chambers, which means that a much higher amount of metal-based detergents can be added. In a gasoline engine, in fact, excessive formation of ash deposits in the combustion chamber alters the compression ratio. This produces a risk of preignition that may even result in perforation of the crown of the piston. The question of ash formation has been promoted by a series of studies to determine the extent to which better results are obtained by adding one metal rather than another to a detergent additive. A comparison of the total base number TBN versus the same

Lubrication Chemistry

17

ash level (about 1%) has shown that metal additives offer increasing basicity TBN in the following order: barium (4.8), sodium (7.9), calcium (8.2) and magnesium (9.4), (Pirro and Wessol, 2001; Salino and Volpi, 1987; Stachowiak and Batchelor, 2001).

2,2. Lubricant Additives The development of modem engine and transmission technologies would be impossible without lubricant additives. From its conception in the early 1900s, the lubricant additives industry has worked in partnership with the oil and the automotive industries to enhance durability and performance of engine and drive line systems through lubricant design. Vehicle design advances are the starting point for the development of the new performance requirements, which in turn stimulate the development of the new lubricant and additives technologies (Copan and Richardson, 1992). There is a growing need for lubricant additives to play a greater role as design components as the engine and transmission design becomes more complex. Engines and transmissions are having to meet increasingly severe operating stress and ever widening range of operating conditions. This is driving Original Equipment Manufacturers (OEMs) towards new designs, in some cases using new material technology. A total formulation approach with the additive as a part of the design process will be required to meet the demanding needs of future equipment designs operating in diverse markets and providing protection to the environment. Additives for the future lubricants will be required to maintain oil consumption control over the life of the equipment while providing a high level of thermal stability and enhancing low temperature fluidity. For dispersants, the need to control sludge, soot and general engine deposits will become more pronounced. Likewise, detergents will play a key role in minimizing deposits and will have to provide the correct sulfated ash and total base number TBN balance to meet the needs of new engines and emerging fuel types. Anti-wear additives will be critical in minimizing wear and hence controlling oil consumption at the elevated temperatures experienced in new equipment (Copan and Richardson, 1992; Dowson et al., 1998). Table 2.3 shows additive chemistry development. Starting from the 1930s, the discovery of antioxidant-antiwear chemistry such as zinc dithiophosphates revolutionized the transportation industry. Successive generations of additive chemistry are linked in time with higher performance levels of lubricated systems (Bovington and Castle, 2001; Copan and Richardson, 1992; Dowson et al, 1998; Rizvi, 1999). What does the future hold for additive technologies to engines as well as driving applications?

18

Chapter 2

Table 2.3. Additive chemistry development from 1930 to 2000 1930s - Pour point depressants - Zinc dithiophosphates - Lead soap gear oil additives - Active sulfrir gear oil additives (EP) additives - Sulfur - chlorine extreme pressure 1950s - Superbased motor oil detergents - Phenate detergents - Viscosity modifiers - Mono-functional ashless dispersants 1970s - Inhibitor technology - Boron, sulfrir, zinc-based drive line additives - Super high performance diesel oil additive - Additives for universal oils in farm tractors - Wet brakes and hydraulic systems - Fuel-saving multigrade gear oil additives 1990s - Synthetic lubricants - Organometallic molybdenum oil additives - Crown ethers as antiwear and antioxidation additives - Macrocyclic (bicyclic and polycyclic) polyamine dispersants and viscosity index (VI) improvers

1940s - Multipurpose antiwear additives - Phenolic antioxidants - Sulfonate and salicylate detergents

1960s - Multi-frinctional ashless dispersants - Sulfiir-phosphorus gear oil additives - Friction modifiers 1980s - Sulfur-phosphorus EP gear oil additive - Multi-frinctional power transmission fluid additive - Diversification of additives chemistries in all functional classes

2000s - The impact of engine design changes - Ash-free organic N-base-calixarene detergents (nitrogen-containing N-bases and alkane glycol substituted calixarenes)

New performance requirements of dispersants to manage the contamination processes are as follows: (a) in the high temperature zones of the engine, these insoluble products participate in varnish formation; (b) in cooler regions of the engine, sludge deposits can accumulate when the dispersancy of the system is overcome.

Lubrication Chemistry

FOAM INHIBITION

>v

19

^

HCB »*A^ AB»^

\t'<

[ RING SCUFFING V ^ PROTECTION

^^

A^Afy. V

/^EARING ( CORROSION I PROTECTION

^

y

/TEARING

\

) / I

^r/^^'^^ Fig. 2.3. The "Additive Tree" illustrates some of the functions which lubricating oil additives perform (Anon, 1958) The impact of engine design changes. There is a variety of possible engine design changes which are intended to enable the attainment of emissions targets. Engine design trends influence the following: direct fuel injection (gasoline and diesel), particulate trap, NO^ traps, exhaust gas re-circulation, low sulfur fuel, modified valve actuation, reduced bearing areas, low friction coatings, modified piston ring design, and cylinder bore coatings. Some of the variable valve lift design and valve timing devices may prove to be sensitive to lubricant viscosity and formulation in a way which is unexpected. Some of the cylinder bore coatings, which will be deployed will show different

20

Chapter 2

responses (to lubricant additives) to those which we see for ferrous metallurgy. A final complication is whether improved vehicle aerodynamics and reduced sump volumes will impose even greater thermal stress upon the lubricant. Finding costeffective solutions to some of these challenges will be difficult to meet. All of them will present a significant challenge to future lubricant formulators (Bovington and Castle, 2001; Taylor, 1998; Warra et al., 2001). The cost of development The explosion of additive chemistries available, as well as the complexity of developments to meet the European and international performance requirements, have escalated the costs required for lubricant design. Development and optimization of a new high performance engine lubricant meeting key international specification and OEM requirements, is in the range of several million dollars to develop and engine test. By contrast, bench testing is a cost and time effective method to evaluate new additives and formulations (Yamaguchi et al., 2002a). The additive industry must continue to have deep commitment to research and development. This investment will support the development of future technology to meet the requirements for transportation of the future. The partnership between oil company, additive supplier, and OEM will continue to meet new challenges and add value to future products. There is considerable activity in the area of both engine and transmission design with various forces of change initiating these new developments. The environmental benefits gained from utilization of advanced lubricant additive technology will permit the internal combustion engine to remain a viable transportation energy source into the twenty first century (Copan and Richardson, 1992; Crawford, 1991; Dowson et al., 1998; Mani and Shitole, 1997). For most applications of petroleum oil lubricants, some chemical materials (additives) must be added to enhance a desirable property or to overcome natural deficiency. Modem engine oils contain a wide range of additives which are blended with base oils to form a complete package capable of meeting demanding performance requirements. The performance of additive mixtures results in detergency, wear protection, oxidation inhibition and corrosion inhibition in the bulk and on surfaces, see Fig. 2.3. Additives are used to reduce thermal and oxidative degradation, reduce deposits, change viscosity characteristics, minimize rust and corrosion, control frictional behavior, reduce wear, prevent destructive metalmetal contact, and control foaming. Additives may modify physical properties (viscosity, pour point, foaming) and chemical behavior (detergency, oxidation, corrosion, wear, extreme pressure resistance) of lubricating formulations. Detergency normally utilizes adsorption on metal surfaces to minimize high temperature engine varnish and lacquer deposits. Detergent additives take the form of reverse micelles in equilibrium; reverse micelles prevent particles from clumping and participate in solubilization of oxidation products. Wear due to scuffing, welding, and surface disintegration is exemplified by valve train wear

Lubrication Chemistry

21

(Bell, 1998; Michalski et al., 2000; Shirahama et Hirata, 1989; Soeijma et al., 1999; Williamson and Perkins, 1992; Zhu and Taylor, 2001). Oxidation stability refers to the ability of the oil to resist degradation from reaction with oxygen at elevated temperatures. The adverse effects of poor oxidation stability are similar to those of poor thermal stability. Thermal stability refers to the ability of the oil components, both base oil and additive, to resist degradation or "cracking" due to the effects of heat. The products of thermal decomposition may be corrosive and form a deposit. Corrosion of engine parts (of non-ferrous metals) occurs during the winter months for the most part under low temperature driving conditions. Water vapor may condense in the crankcase, and rusting (of ferrous metals) may occur if the motor oil does not have adequate rust inhibiting properties (Anon, 1958). The relationship between additive types and their function, in general, can be applied to lubricants for engines, transmissions and rear axles in automobiles, trucks, off-highway equipment and a wide variety of industrial equipment. The lubricating formulation for a specific application is a difficult task which requires identification of performance needs, knowledge, and selection of the appropriate base oils and additives. Many types of additives perform more than one function. For example, zinc dithiophosphates protect metal surfaces from wear and corrosion and prevent oxidation processes, thus acting as multifunctional compounds (Schilling and Bright, 1977). Table 2.4 lists some of the commonly used additives types, typical compounds, and their purpose and functions. Lubricant additives can be grouped into three main functional areas (Anon, 1969; Mortier and Orszulik, 1997; Pirro and Wessol, 2001; SAE, 1983): (A) surface protective additives; (B) performance additives; (C) lubricant protective additives. (A) Surface protective additives: (i) antiwear (AW)/extreme-pressure (or temperature) improver (EP), (ii) anticorrosion and rust inhibitor, (iii) detergent and dispersant, and (iv) friction modifier. Examples of protective additives are listed in Table 2.4. (/) Antiwear (A W)/extreme-pressure (or temperature) improver (EP). The distinction between antiwear and extreme-pressure is related to two types of chemistry being involved. Antiwear is based on the prevention of damage caused by moderate or sporadic loadings. Extreme-pressure chemistry is based on the protection from shock loading and continually applied heavy or sliding loads. One of the primary functions of lubricants is to reduce wear.

22

Chapter 2

Table 2.4. Commonly used additives Additive Type

Typical Compounds (Purpose and Function)

(A) Surface protective additives i) Antiwear (AW)/ extreme pressure (or temperature) improver (EP)

Zinc dithiophosphates, organic phosphates, chlorine and sulfur compounds, sulfides and disulfides, crown ethers, calcium carbonate-benzoate hard-core RMs (Reduce friction wear and prevent scoring and seizure; chemical reaction with metal surface to form strong tribofilm)

(ii) Anticorrosion and rust inhibitor

Zinc dithiophosphates, sulfonic acids compounds, phosphoric acid derivatives, nitrogen compounds, fatty acid amides, carboxylic acid derivatives, Ca carbonate-benzenesulfonate hard-core RMs (Prevent corrosion and rusting of metal parts; preferential adsorption of polar compounds on metal surface to provide a protective tribofilm)

(iii) Detergent and dispersant

Detergent: Sulfonates, phosphonates, phenates and salicylates, ash-free bases-calixarenes. (Soft-core reverse micelles formation, solubilize organic acids and neutralize strong acids induced by blow-by gases, reduce laquer). Dispersant: Succinimides, succinate esters, Mannich bases and phosphorus compounds types, macrocyclic bicyclic and polycyclic polyammes (Keep surface free of deposits and contaminants dispersed in the lubricant, disperse sludge, carbon, and other precursors, prevent agglomeration by dispersing particles in fluid)

(iv) Friction modifier

Fatty alcohols, fatty amines, amides, phosphoric acid esters, organometallic modifier, molybdenum compounds (Preferential adsorption of surface-active material)

(B) Performance additives (v) Pour point depressant

Polymethacrylates, long-chain alkyl phenols, polyacrylamides (Enable lubricant to flow at low temperature; modify wax crystal formation to reduce interlocking)

(vi) Viscosity index improver

Polyisobutenes, polymethacrylates, olefin co-polymers, polyalkylstyrenes (Reduce the rate of viscosity change with temperature; polymers expand with increasing temperature to counteract oil thinning)

Lubrication Chemistry

23

Table 2.4. Commonly used additives (continued) Additive type

Typical compounds (purpose and function)

(C) Lubricant protective additives (vii) Antioxidant

Aromatic amines, hindered phenols, sulfurized phenols, zinc dithiophosphates, crown ethers (Retard oxidative decomposition; decompose peroxides and terminate free radicals; prevent or slow down oxidation of the base oil at high temperatures)

(viii) Antifoamant

Silicone polymers, organic copolymers (Prevent lubricant from a persistent fr)am; reduce surface tension to speed collapse of foam)

(ix) Other additives

Preservatives, adhesives, odorants

Wear can result from a number of different processes, such as corrosion, metalto-metal contact, or abrasion by solid particles. Corrosion wear can start from acidic products of combustion (Kreuz, 1969); mechanical wear from metal-tometal contact or abrasion is normally prevented by hydrodynamic lubrication with an oil film thick enough to keep moving parts separated. Under extreme pressure conditions such as high load, low speed, and low lubricant viscosity, the thickness of the lubricant film can change to a step referred to as boundary lubrication, where the lubricant film can actually rupture and allow metal-to-metal contact (Fein and Kreuz, 1965; Fein and Villforth, 1973). This condition may exist between the piston ring and cylinder wall, the rocker and valve tip interfaces in the engine,,and between gears in transmissions and rear axles of heavy equipment. Heat from friction between mating surfaces provides energy for the chemical reaction between the phosphorus, sulfur and chlorine atoms of extreme-pressure additives and metal surfaces that results in a protective coating. The coating may be an iron sulfide (the friction coefficient of steel is 0.78, the coefficient of FeS is 0.39), iron phosphate, or some other organometallic compound. The action of polar substances is based purely on adsorption and chemisorption as preliminary conditions. Multifunctional zinc dialkyldithiophosphates, (R-0)2PS2Zn acts not only as extreme-pressure additives but also as antioxidants and corrosion inhibitors (Aktary et al, 2001; Asseff, 1941; Barnes et al., 2001; Giammaria and Woodbury, 1946; Papay, 1998; Rutherford and Miller, 1941). The use of carbonate-benzenesulfonate hard-core RM additive drastically changes the build-up mechanisms and the resulting structure of the antiwear surface film. Considering these results, the main difference between the antiwear action of the ZDDP molecules and the hard-core RMs is clear. In the case of

24

Chapter 2

ZDDP, the antiwear film formation requires that a chemical reaction occurs between the additive and the metallic surface. In the case of hard-core RMs, the mineral material CaC03 is directly introduced in the sliding contact and undergoes small physicochemical changes during the film build-up. Consequently no chemical reaction with the substrate surfaces is required. The crown-ether compounds as boundary lubricants and antioxidation additives. On the sliding surface, bromobenzo-15-crown-5 coordinates with ferrous ions and forms a strong reaction layer which protects the underlying metal surface. In the base stock solution, the crown ring can capture the metal ions which catalyze the oxidation of oil formulation (Brois and Gutierrez, 1987, 1989, 1992 and 1994; Le Suer and Norman, 1965 and 1966; Moreton, 1998). Bromobenzo-15-crown has excellent antiwear, antifriction and antioxidation properties, better than the ZDDP tested. Boron nitride (BN) as a lubricant additive. The structure of boron nitride (BN) is similar to that of graphite and molybdenum disulfide M0S2. Boron nitride is very effective in reducing wear and less effective in friction reduction if used as a lubricant additive. The binding between layers is almost entirely by means of weak van der Waals forces (Nagashima, 1996). As a solid lubricant, BN was inferior to graphite and M0S2 except for its high thermal stability and its white color. In sliding of bearing steel vs. iron or steel vs. steel, addition of 1 wt % BN results in the reduction of wear by more than an order of magnitude; however, the coefficient of friction is slightly increased with steel/steel and decreased with steel/iron (< 0.08) surfaces. The chemical state of boron on the wear scars was examined by X-ray photoelectron spectroscopy. In the case of steel/steel surface, a peak binding energy of 191.3 eV indicates the dominant existence of some boron oxide B2O3. In contrast, in the case of the steel/iron spectrum, the majority of boron remains as BN at a binding energy of 190.3 eV. This distinction of components would be responsible for the different friction and wear characteristics of steel/steel and steel/iron combination (Kimura et al., 1999). (ii) Anticorrosion and rust inhibitor. Rust inhibitors form a coherent adsorbed film on metal surfaces. Surface active, organic surfactants like friction modifiers additives are often antagonistic towards extreme-pressure agents. A neutral calcium sulfonate rust inhibitor is very effective in preventing dibenzyldisulfide reaction on steel surfaces at elevated temperatures. The mechanism of extremepressure antagonism, as friction modifiers, involves masking the rubbing surfaces (Damrath and Papay, 1998; Spikes and Cameron, 1974), Corrosion is a chemical attack on metal surfaces, and rust is a specific type of corrosion involving ferrous metals (Fe, Co, Ni). The process of combustion introduces corrosive materials in automotive lubricants, which are composed of

Lubrication Chemistry

25

organic and mineral acids into engine crankcase oils (Kreuz, 1969, 1970). Moisture contamination in transmission and gear oil can also react with some extreme-pressure additives, hydrolyzing them to form acids and making them corrosive. Polar functional groups of the additives are adsorbed on metal surfaces and this provides a barrier to prevent corrosive materials from contacting the metallic surfaces. Zinc dithiophosphates, dithiocarbamates and benzotriazoles are commonly used to protect copper-lead bearings from the type of corrosion present at concentrations of 50 to 300 mg/kg of oil. Rust inhibitors derived from sulfonates and amines also form absorbed films on ferrous metal, which prevents corrosive material from reaching the metal surface. (Hi) Detergent and dispersant terms are often used interchangeably when discussing engine oils. Both are long chain hydrocarbons with polar head groups. Detergents are metal salts of organic acids having a polar head group containing alkaline metal ions; dispersants utilize nitrogen and oxygen for polarity. The hydrocarbon chain of these additives helps to solubilize or suspend the debris in the oil, and to neutralize acids which form from fuel combustion products. If a distinction is made, detergents are normally considered for deposit control at high temperatures and dispersants for controlling low-temperature sludge. Inclusion of detergent and dispersant additives into a lubricant is important to prevent harmful sludge deposit and carbon. Sludge deposits in some parts of the engine lead to engine shut-down and repair. Detergents and dispersants have a direct effect on minimizing harmful engine exhaust emissions, increasing engine life, and controlling oil consumption by maintaining clean 'in-tune' engine operation (Mortier and Orszulik, 1997; Willermet, 1998). Their major applications is in engine oils; other applications include gear and transmission oils and tractor hydraulic oils. Detergents in hydrocarbon formulation. Interactions between the polar groups of metallic detergents promote their aggregation into soft-core reverse micelles in hydrocarbon formulations. A multifunctional additive, carbonatesalicylate, carbonate-phenate, and carbonate-sulfonate form hard-core RMs (Fox et al., 1991b; Fu et al., 1996; Sun et al., 1996). When engines operate at high temperatures, the engine oil is an important heat transfer fluid that prevents overheating of areas such as piston rings, under-crowns, and skirts. Exposure to these localized high temperatures and blow-by gas accelerates the rate of oil decomposition to produce deposits. These deposits can prevent free movement of piston rings and keep them from forming the desired seal between the piston and cylinder wall. Varnish and lacquer deposits that can form on the piston undercrown and skirts reduce the rate at which the piston transfers heat to the engine oil. The products of high temperature lubricant oxidation are highly-acidic polar

26

Chapter 2

oxygenates which have low solubility in mineral oil and a strong affinity for other polar compounds and metal surfaces. They readily polymerize, and when trapped in ring grooves, they further decompose to hard carbonaceous deposits (Kreuz, 1969 and 1970). Typical detergent additives are normal or basic barium, calcium or magnesium salts of substituted long-chain alkyl compounds (Moreton, 1998). They react with highly-acidic deposit precursors to neutralize them and keep them in suspension as very small particles. Detergents have varying capacity to provide engine rust protection. The main functions performed by detergents are: oxidation inhibition, high temperature detergency, acid neutralization, and rust protection. There are four major classes of detergents: sulfonates, phenates, salicylates, and phosphonates. Sulfonates are the most widely used detergent additives; the use of salicylate is limited while phosphonate use is minimal. Sulfonates and phenates provide excellent engine antirust properties but phenates provide lower ash content at equal alkalinity when compared to sulfonates (Bartz, 1993; Bhushan, 2001a; Pirro and Wessol, 2001; Stachowiak and Batchelor, 2001). The formulae of major classes of detergents are shown in Table 2.5. Table 2.5.

Type of detergent: sulfonate, phenate, salicylate and phosphonate

Type of detergent

Chemical formula'

Neutral Sulfonate Carbonate-sulfonate micellar compound Synthetic Sulfonate Normal Phenate Salicylate

(RS03-),M2^ (RS03-)2M^^-(M^"C03^a (R'H2CRHC-C6H4-S03-)2M'^

(R-C^H^-OO^M^^

(R-C6H4-ROO O^M""^ ^M'^ = Ca, Mg

I R-P-0 II

I I R-P-S-P-R I I

S I R-P-0 11

(1)

(2)

(3)

0 Phosphonates: (1) Phosphonate (2) Thiopyrophosphonate (3) Thiophosphonate

0-M

0

0

0-M-O

0-M

Increasing environmental awareness is prompting a search for metal-free or ashfree detergents. The compounds of organic nitrogen-containing bases and alkaline glycol-substituted calixarenes function to disperse sludge and neutralize acidic material in lubricating oils (Brois and Gutierrez, 1987, 1989, 1992 and 1994; Le Suer and Norman, 1965 and 1966; Moreton, 1998).

Lubrication Chemistry

27

Detergents at surfaces. Neutral and basic detergents provide less wear protection than ZDDP solutions and much less than a base oil alone. A multifunctional additive carbonate-detergent can provide wear protection comparable to that of ZDDP. The choice of metal, anion and basicity can affect wear and friction with formulated lubricants. With rubbing, a coherent carbonate film is formed which separates metal surfaces, thus preventing wear. In the absence of ZDDP, carbonate-detergent RMs form compacted, amorphous metal carbonate films which are robust up to contact pressures on the order of 100 MPa. Expulsion of the organic detergent from the carbonate-detergent RMs is possible because the detergent portion is adsorbed (rather than strongly bound chemically) on the carbonate particles (Fu et al., 1996; Glasson et al., 1993; Griffiths and Hayes, 1989; Habeeb and Stover, 1987; Jahanmir, 1987; McGeehan et al., 1985; Matsuoka et al., 1983; Shirahama and Hirata, 1989; Willermet, 1998; Yamada et al., 1992). Dispersants in hydrocarbon formulation. Typical dispersants have a polar functional group attached to a long bulky hydrocarbon group. Dispersants utilize oxygen and/or nitrogen polarity and do not contain metal ions. This enables them to adsorb on contaminant particles as soot and keep them in suspension so they cannot agglomerate to form sludge (Kreuz, 1969, and 1970). Dispersants that do not contain metallic components are called "ashless". Substituted long chain alkenyl succinimides are one of the many classes of additives that have gained acceptance as ashless dispersants. Self-association has been reported for polyisobutylene amine succinimide (mono-substituted, m-PIBS)-type dispersants. The driving force for association of ashless dispersants is H-bonding between amino groups. Aggregation numbers for m-PIBS are more than twice those for bis-PIBS due to steric effects of the alkenyl groups (Fox et al., 1991b; Inoue and Watanabe, 1983). Engines operated under light-duty, short-trip, stop-and-go conditions rarely reach normal operating temperatures. This type of service leads to formation of sludge which blocks oil passages. Sludge is a complex mixture of products from fuel combustion, water, carbon, and oxidized oil, that forms agglomerates and is no longer soluble in the engine oil. Automatic transmission fluids are not required to contend with the quantities of contaminants typical for engine oils, but they still must prevent sludge formation. Wear debris from clutch place and oil decomposition products is kept from forming sludge by the same types of ashless dispersants used in engine oils (Bovington and Dacre, 1984; Lacoste, 1968). The detergents developed for diesel engines do not prevent formation of sludge in gasoline engines. In gasoline engines, which tend to run colder and less efficiently, higher levels of water are produced. In response to this problem, the first ashless dispersants were developed (Du Pont, 1954), became the dominant dispersant types by 1970, and continue to be used today.

Chapter 2

28 OH

OH I

C6H3-CH2-NHRN-CH2-C6H3 R

Mannich base

R

H-,C,

CH3 I ' HQC "" C I CHo C

CH2 II

\

H3C

I

CH;^

^

-CH2—C-CH2-CH —C-OH CH2-C-OCH2C(CH20H)3 X

X = 11-52

O

Succinates

H,C,

CH3 I H3C~C"rCH2—C—rCH2

\

H3C

o

CH,

CHo

^ C CH2~CH 0>. I ^N-fcH2CH2NH^H CH2-C'^ ^ ^y

Succinimides

X = 11-52

H.C,

CHo I H3C - C+CH2~C —hCH2-"C-CH2-CH-P-(OCH2-CH-OH)2 CH3

\

H3C

I

X = 11-52

CHgJx

S

Phosphorus dispersant

Fig. 2.4. Type of dispersants: Mannich base, polyisobutylene succinate ester (succinates), polyisobutylene amine succinimide (succinimides, PIBS) and phosphorus dispersant The above ashless dispersants solubilize combustion generated water, which leads to formation of sludge. The types of ashless dispersants such as succinates, succinimides, Mannich bases and phosphorus compounds are shown in Fig. 2.4. There are two types of Mannich bases, one derived from high molecular weight alkyl-substituted phenols (Cahill and Piasek, 1984; Gutierrez et al. 1990, Lundberg and Emert, 1989; Smyser and Cengel, 1983; Tramontini and Angiolini, 1994; Worrel, 1975) and one from partially oxidized polyenes (West, 1978). Dispersants at surfaces. As in the simple systems, bis-substituted polyisobutyleneamine succinimides (bis-PIBS) give less wear than mono-

Lubrication Chemistry

29

substituted polyisobutyleneamine succinimide (m-PIBS) or a succinic acid ester; high molecular weight dispersants give less wear than low molecular weight dispersants. A borated mono-substituted polyisobutyleneamine succinimide, mPIBS form mixed iron-boron oxides. Although PIBS dispersants are not antiwear agents, borate PIBS can provide some antiwear protection in the absence of ZDDP, probably by formation of mixed iron-boron oxides. The m-PIBS is found to play an important role in the formation of the boron-iron oxide film, it (m-PIBS) prevents the precipitation of boric acid and assists with the adsorption and fusion of boric acid on the sliding surface (Matsuoka et al., 1983; Shiomi et al., 1992; Shirahama and Hirata, 1989). New family of dispersants and viscosity index (VI) improver. Macrocyclic polyamine and polycyclic polyamine compounds are useful as dispersants and viscosity index (VI) improvers in both gasoline and diesel engine lubricating oil formulations (Brois and Gutierrez, 1987). Dispersants and VI improver additives are formed by reactions of macrocyclic polyamines and polycyclic polyamines with substituted succinic anhydride or carboxylic acids. Also suitable are mixed donor macrocyclic amines containing nitrogen-oxygen, nitrogen-sulfur, and nitrogen-sulfur donor groups, as depicted in Fig. 2.5, which can be acylated to give useful lubricant additives. Polycyclic polyamine compounds contain 2 to 3 rings and 5 to 6 atoms in each ring; these atoms include 3 to 4 nitrogen atoms, at least one being an NH group. Preferably, 2 to 3 NH groups per molecule are present in a compound. These compounds can be represented by the formulae in Fig. 2.5. The dispersants and viscosity improvers were evaluated for sludge (SIB test), varnish potency (Vemlnh test), and comparative tests with prior art products were also conducted. A series of the Sludge Inhibition Bench test (SIB) and Vemish Inhibition Potency test (Vemlnh) evaluations were carried out comparing bicyclic, macrobicyclic and polycyclic polyamine dispersants and viscosity index (VI) improvers, and compared for activity against a compound representative of current commercially suitable products of mono- and bis-substituted polyisobutylene amine succinimides. The sludge results (SIB test) are reported as milligrams of sludge per 10 grams of oil. The less new sludge formed, the more effective the additive as a dispersant and viscosity index (VI) improver. The amount of varnish potency is rated from one to seven with higher number being the greater amount of varnish. In the (SIBA/^emlnh) tests, the sludge and varnish potency results showed that macro- and polycyclic additives are much higher quality than monoand bis-succinimide products. The dispersants prepared by the reaction of hydrocarbon substituted succinic acid anhydrides with polyamines to give linear mono- and bis-succinimides, are well known lubricating oil additives commercially available (Gutierrez and Brois, 1980; Le Suer and Norman, 1965 and 1966; Song et al., 1993, 1994 and 1995).

30

Chapter 2

CH3

H-N"^ ^N'

Aza polycyclic ring assemblies (aza polycycles)

H-N

\J'

j^__^ I

N-H

l,10-diaaza-4,7-dithia18-crown-6

^^Y{ I

^' ^ O-diaaza-4,7,13,16-tetrathia18-crown-6

N-H

l,10-diaaza-18-crown-6


Fig. 2.5. Polycyclic polyamines and macrocyclic polyamines (crown ethers) as multifunctional oil additives It was discovered that certain macrocyclic polyamine and polycyclic polyamine compounds possess significant properties as multifunctional viscosity improvers (VI), especially the capability to be a highly effective dispersant in both gasoline and diesel engine lubricating oil formulations. Lubricating oils containing these additives exceed the highest qualification standards for such oils (Brois and Gutierrez, 1987). The case study. Three formulated oils were each evaluated in the Caterpillar lH-2 test which is an industry and government accepted test for the dispersancy and overall effectiveness of diesel oil lubricants. The engine test uses a single cylinder Caterpillar lH-2 diesel 5%" x 6^/2", 240 hour test WTD (Weighted Total Demerits), and the target specification is a value in the 90-100 range. The test uses a bicyclic polyamine additive as a novel dispersant viscosity index improver (VI). The purpose of the test is to determine the effect of oil on ring sticking, wear, and accumulation of deposits.

31

Lubrication Chemistry

The WTD is a cumulative rating based on observation of deposits in the groove and land areas of the piston and lacquer on piston skirts which have been weighted and calculated in accordance with the test procedure. The evalated lubricating oils also contained, where required, conventional viscosity improver, a metal detergent and ZDDP in mineral base oil. The WTD ratings were as follows: (a) base formulation (contained a borated polyisobutylene succinic anhydridealkaline polyamine dispersant) with value of WTD = 189; (b) comparison formulation (a multifunctional commercial dispersant-viscosity improver, "Acryloid 1155" which is a C-vinyl pyridine ethylene copolymer graft) with value of WDT =195; (c) new formulation (contained viscosity index improver bicyclic polyamine additive) with value of WDT =119 (Brois and Gutierrez, 1987). (iv) Friction modifier. These compounds function by reducing boundary friction coefficients under high pressure and sliding conditions, as are encountered in the valve train (Bell, 1998; Bovington and Castle, 2001; Michalski et al., 2000; Shirahama and Hirata, 1989; Soejima et al., 1999; Williamson and Perkins, 1992; Zhu and Taylor, 2001). Automatic transmissions, limited-slip differentials, power take-off units and wet-brake systems require lubricants with specific frictional properties for proper clutch engagement. Some units require a quick clutch lockup, while others require a small amount of slippage prior to lock-up for a smooth engagement. A majority of fractional requirements (viscosity and choice of the additives) are determined by equipment design and the materials used in clutch plates (Lacoste, 1968; Nann and Pinchbeck, 1966). These additives, known as "friction modifiers", act generally by forming thin layers on the friction surfaces due to physical adsorption. They represent polar oil-soluble materials such as fatty alcohols, amides or salts whose effect increases with increasing molecular mass and in the order: alcohol

<

ester

<

unsaturated acid

<

saturated acid

Since the effect drops suddenly when the temperature reaches the melting point of the given fatty acid or salt, the outstanding effect of the fatty acids has been related to an interaction with the metal surface. A second class of friction modifiers is made up of organomolybdenum compounds: MoDTC - molybdenum dithiocarbonate and MoDDP - molybdenum dithiophosphate. These function by the formation of platelets of M0S2 on the surface (Grossiord et al., 1998). It is understood that these platelets are dispersed in the polyphosphate tribofilm. The combined use of organomolybdenum compounds and ZDDP showed synergistic effect by reducing both friction and wear and promoting fuel economy in motor vehicles (Akijama et al., 1993; Braithwaite and Greene, 1978; Greene and Risdon, 1981), seeChapt. 5,

32

Chapter 2

(B) Performance additives: (v) pour point depressants, and (vi) viscosity index (VI) improvers. Table 2.4 lists examples of performance additives. (v) Pour point depressants. Mineral oils thin out at high temperatures and thicken at low temperatures. These are natural properties of oil that are determined by the crude oil and the refinery processing it receives. Mineral oils selected for a particular lubricant should allow that the lubricant remains fluid over the entire temperature range it may encounter in service. Paraffin wax present in most refined oils comes out of solution at low temperatures in the form of wax crystals. At these temperatures, the oil itself is still capable of flowing, but an interlocking network of wax crystals will not permit flow. Polymeric additives such as polymethacrylates at concentration of 0.1 to 0.5% (Neher and Hollander, 1938) or condensation products of chlorinated wax (Davis and Blackwood, 1931) and long-chain alkyl phenols (Sanin, 1961) can be used to interfere with wax crystal growth and alter the formation of the interlocking crystal network. {vi) Viscosity index (VI) improvers. Once a viscosity index had been defined in 1929 there was a desire to improve the viscosity index of base-oils (Dean and Davis, 1940). Modification by the addition of polymers allowed year round operation of lubricants in a wide range of climates. The viscosity index of automotive engine oil has received considerable attention. Starting requires low viscosity at low temperature but normal operation requires maintaining an adequate fluid film near 149^C. Mineral oils have been able to meet these wide temperature-range viscosity requirements through the use of high-molecularweight polymeric additives known as VI improvers. At low temperatures, these polymers are sparingly soluble in oil and exist as closely coiled chains with little influence on the viscosity of the oil. As the temperature increases, the polymer becomes more soluble and expands into loose, random coils. These expanded polymers restrict movement of the oil molecules and serve to reduce the rate at which oil thins out with increasing temperature. Various polymethacrylates, olefin co-polymers, hydrogenated polyisoprene and styrene-butadiene co-polymers have been used as VI improvers. The amount of thickening they provide depends on the size of the polymer molecule. A larger or higher molecular weight polymer of the same type will generally contribute to more thickening. Shear stability is another important property of VI improvers. Larger long-chain polymers are more likely to be broken apart by shear forces between moving surfaces. Once this type of permanent shearing takes place, the polymer contributes less to high-temperature thickening. Therefore, formulation of high VI oils using polymeric VI improvers require selection of a polymer that will continue to provide adequate high-temperature thickening in service. The addition of VI improvers to "multigrade oils" makes it possible to cover several SAE viscosity grades (according to ASTM D2270) with only one

33

Lubrication Chemistry

base oil (Fig.2.6). Engine oil viscosity classification is presented in Table 1, Appendix.

2 3 Vf Improver. mass %

^

Fig. 2.6. Viscosity index as a function of VI improvers for base oils of the same origin (Klamann, 1984) (C) Lubricant protective additives: (vii) antioxidant, (viii) antifoamant, and (ix) other additives. Examples of protective additives are listed in Table 2.4. (vii) Antioxidant Additives that reduce oil oxidation are called antioxidants, e.g., aromatic amines, hindered phenols, sulfurized phenols, and zinc dithiophosphates. Antioxidants act to retard oxidation of the lubricating oils, thus preventing the formation of corrosive products. Much research has been done on the nature of oxidation, but it is not fully understood in all of its aspects (Barnes et al., 2001; Cemy and al., 2001; Gilks, 1964; Mahoney, 1968; Stadtmiller and Smith, 1986; Walling, 1957). The process is extremely complex. Antioxidants act in two different ways, by inhibition of peroxides or by radical scavenging. ZDDPs exhibit both types of antioxidant behavior. In oxidation, it is generally postulated that organic peroxy radicals (ROO •) are formed by the removal of a hydrogen atom from the hydrocarbon chain (RH) with the resultant formation of a "free radical" (R •) which reacts with oxygen to form a peroxy radical:

34

Chapter 2 RH + Acceptor (e.g., NO2) - R • + H-Acceptor

Initiation

R-

Propagation

+ O2

-

ROO-

These peroxy radicals act as the precursors for further oxidation. They may remove a hydrogen atom from another hydrocarbon molecule to form a hydroperoxide (ROOH ) and another free radical in a chain reaction: ROO • + RH

-

ROOH

+

R-

The process is further complicated by decomposition reactions: RCOOH

-

RO-

+

OH-

which result in a variety of organic compounds such as alcohols, ketones, aldehydes, and acids which may further oxidize and react with one another to form high molecular mass polymers. Some of these polymers may be oil soluble, resulting in a viscosity increase of the lubricant; others may be oil insoluble and drop out as sludge, carbon or varnish. Both the initiation and continuation of the oxidation are materially affected by temperature (oxidation rates are doubled for each 10°C rise in temperature), but may also be catalyzed by the presence of various metals or by light. The termination of the oxidation reaction may result from the exhaustion of the oxygen supply in lubrication systems or from the formation of stable products R • + R • -^ R-R) in the oxidation chain reaction. Antioxidant or oxidation inhibitors may function as chain terminating agents by reacting with free radicals to form stable products, by acting as peroxide decomposers, or they may act as metal passivators to prevent catalytic effects. The chain terminating additives are usually aromatic amines, phenols, or sulfides. Those that inhibit the catalytic effect of metallic ions such as Cu, Fe, Pb, Mn, and Co are generally organic sulfides, phosphites, or thiophosphates. Although oxidation or corrosion inhibitors are frequently referred to separately, many of the phosphorus and sulfur containing compounds are effective in both applications. The entire matter of oxidation is affected by many things, including the temperature of the lubricant and the material of construction of the equipment in which the lubricant is used. It is also materially affected by inherent resistance of the base oil to oxidation. This resistance is affected by the crude source and refining process (which in turn affects the relative amounts of paraffinic, aromatic and naphthenic hydrocarbons). Because of these fundamental differences, lubricants respond differently to different additives. The final choice of the additive must be based, therefore, on actual laboratory tests according to ASTM D-2272 and practical tests

Lubrication Chemistry

35

in the lubricant to be inhibited. Metal compounds can have an oxidizing or reducing effect, see Fig. 2.7 (Klamann, 1984). 1000

800

600 h

UOQ

200 \

Fig. 2.7. Oxidation tendency of mineral oils at 175°C as a function of base oil, degree of refming, and the addition of metals and inhibitors. Graph shows: (a) highly refined oil (Oil 1); (b) Oil 1 with Fe wire; (c) Oil 1 with Cu wire; (d) Oil 1 with sulfur-phosphorus inhibitor; (e) Oil 1 with Fe wire and sulfur-phosphorus inhibitor; (f) nor-mally refined oil (Oil 2); (g) Oil 2 with sulfur-phosphorus inhibitor; (h) Oil 1 with Ca detergent; (i) Oil 1 with Ca detergent and sulfur-phosphorus inhibitor (Klamann, 1984) Metal compounds reach the lubricating oil by surface abrasion or the corrosive action of acidic oxidation products. The combustion fuel products with metal ions can be combined in a complex form and thus "masked" by so-called deactivators. Materials previously referred to as corrosion and rust inhibitors also function as metal ion deactivators, due to their ability to form a coating on the metal surface. The degradation of ZDDP is very complex and much work has been done to understand the mechanism of its degradation and its interactions with other additives. Some workers have tried to follow the reactions by oxidizing ZDDP in the laboratory (Bum et al., 1971a, 1990a and 1990b; Harrison and Kikabhai, 1987; Paddy et al., 1989, 1990) and others follow the reaction of ZDDP in an engine environment (Korcek et al., 1981; Peng et al., 1994). In the decomposition of cumene hydroperoxide (CHP) in the presence of ZDDP, the disulfide was identified as an initial degradation product, and zinc sulfate was identified as a

36

Chapter 2

final product. In organic solvents, the basic form of ZDDP dissociates to form zinc oxide which inhibits hydrolysis of ZDDP. The degradation process of ZDDP in a fiilly formulated engine oil was seen to be temperature dependent, formed a complex with succinimide dispersant, and the process was complicated. As a rule, however, the final formulation of products must be subjected to timeconsuming practical tests. This is particularly true for motor oils, where only the practical test in the engine can assess, for instance, high temperature efficiency of dithiophosphates and the frequently antagonistic effects of dispersants and oxidation inhibitors in a given additive combination (Klamann, 1984; Mortier and Orszulik, 1997). (viii) Antifoamant All automotive lubricants which are subjected to sufficient agitation will entrain air and produce foam. This is undesirable because it increases exposure of the lubricants to oxygen and thereby increases the rate of oxidative decomposition. Entrained air and foam also reduce lubricant efficiency as a coolant and hydraulic fluid. Polymers of silicone and organic copolymers are commonly used to control foam. These additives have lower surface tension than the lubricant and low solubility in the lubricant. This permits them to spread over the surface of the foam bubbles at a concentration high enough to weaken the oil film, causing it to collapse (Ross, 1967). Liquid silicones (in particular, polydimethylsiloxanes) are the most efficient antifoam agents at concentration of 0,0001% to maximum 0.001% (Beerbower and Bamum, 1961). The foam forming tendency is assessed according to ASTM D 982 from the foam volume and foam stability. (ix) Other additives. Preservatives. Suitable preservatives are imidazolines, amidoacetals and hexahydrotriazines; however, their usage must be controlled in order to prevent skin damage and waste water problems such as toxicity to fish. They are used in metal working or hydraulic applications to prevent bacteria, yeasts and fungi from growing. Odorants. Natural or synthetic base oils and additives can possess a specific odor which can be due to aging or oxidation products. This odor can be masked by odorants. The addition of ca. 0.01%) combinations of natural or synthetic oils such as pine or citronella oil has given particularly good results.

2.3. Tribochemical Interactions of Additives Additive interactions take place in lubricating oil formulation and at surfaces (Kajdas, 2001; Spikes, 1989). At surfaces, additive interactions should protect metallic engine surfaces from corrosion, prevent rusting, build-up of varnishes, agglomeration of particles, and form low friction and protective films, hi the base

Lubrication Chemistry

37

oil, additives enhance the performance of the mixture through prevention or slowing of oxidation of fluids at high temperatures, inhibit sludge and deposit formation, control soot thickening, and reduce the change in viscosity with changes in oil temperatures. The chemical interactions (synergistic or antagonistic) involve direct reactions in lubricating oil formulation and on surfaces. To clarify this, the following two topics are considered: (I) Intermolecular interactions between additives; (II) Zinc dialkyldithiophosphates as multifunctional additives in lubrication formulations. (I) Intermolecular interactions between additives. The term "additive interaction" is used to describe the performance of interactions between additives of the same or different class, e.g., combinations of additives give different antiwear performance than an individual additive. The phenomenon is called "synergism", when the interactions are increased, or "antagonism" when they are reduced. Additives used in lubricating oil formulations are not neutral compounds and interact with one another to change their properties in ways which are only partially understood. Differences between single and mixed system additives have been studied by dye solubilization (Vipper et al., 1985), osmometry (Vipper and Watanabe, 1981), spectroscopy (Gallopoulos and Murphy, 1971), fluorescence depolarization (Kaufman and Singleterry, 1957), X-ray diffraction and electron spectroscopy (Giddings and Barett, 1971; Luisi and Straub, 1984). Fig. 2.8 summarizes the most commonly observed interactions between additives of different classes. These interactions in oil formulations are mainly acid-base, e.g., a strong acid will convert a carboxylate to the molecular acid and will increase decomposition of ZDDPs (Inoue and Watanabe, 1983; Rounds, 1978). Mixtures of metallic detergents, such as phenates, sulfonates, phosphonates, and salicylates with ashless dispersants such as succinimides and benzylamine, together with zinc dialkyldithiophosphate (ZDDP), can lead to new effects. The possible interactions between these main additives used in lubricating formulations when dissolved/dispersed in hydrocarbon media are shown in Fig. 2.8 together with an indication of the intensity of those respective interactions. In this section of the chapter we consider intermolecular interactions between some additives specified in Fig. 2.8. (A) strong intermolecular interactions: dispersant-ZDDP; (B) medium intermolecular interactions: detergent-dispersant; (C) weak intermolecular interactions: detergent-ZDDP. (A) Strong intermolecular interactions: dispersant-ZDDP. In hydrocarbon formulations, polyisobutyleneamine succinimide (PIBS) as a class has been found to form complexes with ZDDP (Ganc and Nigarajan , 1991; Harrison et al., 1992; Inoue and H. Watanabe, 1981 and 1983; Kulp et al., 1992; Ramakumar et al..

38

Chapter 2

1992; Shiomi et al., 1992). The association is between the zinc cation (or phosphorus atom) and amino group on the PIBS chain (>Zn...NH< ; >P...NH< ), hence ZDDP plus mono-substituted m-PIBS interactions are much stronger than ZDDP plus bis-substituted, bis-PIBS. Another approach to reducing the ZDDPPIBS interaction is to use PIBS borates, PIBSffH2BO-3 (Brois and Gutierrez, 1994; Shiomi etal., 1992). Metallic detergents

p ZDDP

Ashless dispersants

/ \

)_

< -^

_>

:>•

Nonionic surfactants

Strong interaction Medium interaction Weak interaction Very weak interaction

Fig. 2.8. Intermolecular interactions between additives (Fox et al., 1991a) In the hydrocarbon formulations, succinimides interact strongly with ZDDP due to acid-base interaction between its own polar groups and the alkyl groups of ZDDP (Inoue and Watanabe, 1981) as shown in Fig. 2.9. The interaction of ZDDP-succcinimides pair has conflicting interpretations in the existing literature. Some scientists (Furey, 1973; Gallopoulos and Murphy, 1971) agree in their observations that ZDDP forms association complexes with amino group of PIBStype dispersants and this complexation has so far been proved to be antagonistic to antiwear action (Fig. 2.9). Other scientists state that because of the association tendency of the two additives, PIBS coated contacts can effectively adsorb the ZDDP molecules (or its decomposition products), (Forbes et al., 1970). The antioxidant nature of ZDDP has often been considered to be antagonistic to its antiwear performance. The most probable reason that can be given for this is the depletion of active ZDDP in the antioxidant action. In the case of diesel engines, special types of interaction are reported. Incomplete combustion of diesel fuel coupled with oil particulates in diesel engines lead to the formation of thick

39

Lubrication Chemistry

black carbonaceous soot with tremendous surface activity. This surface-active soot entraps the effective ZDDP decomposition products, resulting in dramatic wear rates. Further, it absorbs the active additives present in the bulk of the lubricant, leading to component wear.

s

s

RO. II

RO. II

RO 1 S—Zn-

RO 1

":P-NH2CH2CH2-

NH2CH2CH2-

s- - Z n -

Fig» 2.9, Intermolecular interaction between dispersants (succinimides) and zinc dialkyldithiophosphates (ZDDPs) Dispersant-ZDDP interactions at surfaces. The dispersant reduces the amount of ZDDP available for tribofilm formation by forming complexes to increase wear in 4-ball and valve train tests. The borated PIBS dispersants may participate in the formation of a borate component in the antiwear film. PIBS dispersants adversely affect the antiwear activity of ZDDP. The stronger the complexation, the greater the adverse effect on wear. It may well be that this effect is due largely to keeping ZDDP in suspension and away from the surface (Rounds, 1978; Shiomi et al., 1992; Shirahama and Hirata, 1989; Willermet, 1998). In XANES spectra of tribofilms, both the phosphorus and sulfur signals are very weak compared to ZDDP used alone. For all the concentrations (1%, 2%, 5%) of dispersant PIBS used, antiwear polyphosphate films were formed and unreacted ZDDP was not present in the film. These results imply that antiwear films are much thinner than either tribofilm generated by ZDDP alone or ZDDP with the detergent. This confirms that the PIBS dispersants compete for ZDDP adsorption on the surface (Yin, 1997b). The role of soot as a promoter of the engine wear was studied as boundary lubrication in the presence of two additives, a ZDDP antiwear and a polybutylene succinimide dispersant (Diatto et al., 1999). Increasing wear rates have long been reported with soot laden oils for engine components such as cylinder liners, piston rings, valve train systems and bearings (Gautam et al., 1998). The two additives tested, a ZDDP antiwear and a polybutylene succinimide dispersant, were mixed with the soot-like contaminant and showed a significant increase in wear. Results gave evidence of antagonistic interactions between the two additives under contaminated (2% soot) oil conditions, and no correlations between induced wear and the colloidal aggregation have been found. ZDDPs have been reported to form complexes with dispersants, and this complexation has so far been proved to be antagonistic to antiwear action (Forbes et al., 1970; Furey, 1973; Gallopoulos and Murphy, 1971; Rounds, 1989; Yin et al., 1997b). It was found that dispersants

40

Chapter 2

may cause depletion of antiwear species at the interfaces, competing with ZDDP derived compounds for adsorption on the metal surface. (B) Medium intermolecular interactions: detergent-dispersant In hydrocarbon formulations, the principal interaction between ashless dispersants (e.g., succinimide) and metallic detergents (e.g., phenate, salicylate and sulfonate) may be ascribed to the acid-base interactions between the anion of metallic detergents and the amino group of the succinimide as shown in Fig. 2.10. —N—CH2-CH2— COO"...H


>

sulfonate

>

phenate

The formation of the strong complex ( >NH..."OOC-C6H4-OH...NH< ) has been reported (Hsu and Lin, 1983; Inoue and Watanabe, 1983; Vipper and Watanabe, 1981). Detergent-dispersant interactions at surfaces. In 4-ball wear tests, an ashless dispersant was found to have an adverse effect on ZDDP-sulfonate-carbonate hardcore RM additives. A high molecular weight Schiff base had the worst effect, followed by a bis-PDBS; m-PIBS had the least adverse effect. Interactions among additives affects valve train wear. One of the effects is that a succinimide together with other additives increases the decomposition temperature of ZDDP (Ramakamur, 1994; Shirahama and Hirata, 1989). (C) Weak intermolecular interactions: detergent-ZDDP. In hydrocarbon formulations, the alkylbenzenesulfonates and phenates are antagonistic to

Lubrication Chemistry

41

antiwear performance of ZDDPs (Okrent, 1961; Quality and Martin, 1969). Sulfonates do not react with ZDDP (Gallopoulos and Murphy, 1971) but take part in the mechanism of oxidation inhibition by ZDDP in base oils. The presence of ZDDP with sulfonates lowers the rate of wear of engine components. A wide range of types of interactions have been suggested to explain this. The interaction of ZDDP with carbonate-sulfonate RMs was considered to have significant influence on wear-reducing properties of ZDDPs (Inoue and Watanabe, 1983; Shirahama and Hirata, 1986). The carbonate-sulfonate RMs solubilize ZDDP and the solubilization subsequently retards the decomposition of ZDDP. ZDDP's decomposition is presumably the rate-determining step of antiwear action of ZDDP. Other work (Inoue and Watanabe, 1983) indicates that carbonate-sulfonate RMs give extremely good synergistic enhancement to the antiwear action of ZDDP. Detergents, such as barium and calcium sulfonates, are powerful block adsorbents of ZDDP on metal surfaces (Inoue and Watanabe, 1983; Plaza, 1987a). It has been suggested (Plaza, 1987a, Kapsa, 1981) that neutral sulfonates act as solubilizing agents, preventing the agglomeration of a protective antiwear film. Carbonate-sulfonate RMs themselves can form protective antiwear films, and the effectiveness of this appears to be promoted by the presence of ZDDP. The presence of ZDDP with sulfonates lowers the rate of wear of engine components. Also, the interaction between carbonate-sulfonate RMs and ZDDP was reported to be an irreversible reaction (Papke and Rubin, 1992;Willermetetal. 1995b). A multifunctional additive of carbonate-detergents RMs retarded the decomposition of ZDDP in the ISOT test (Yamada et al., 1992). Mixtures of ZDDP plus carbonate-detergents RMs additive have been reported to have synergistic effects on detergency, see Chapter 3.3 on tribochemical interactions of hard-core RMs and ZDDP (Inoue, 1993; Ramakumar et al., 1994; Willermet, 1995a and 1995b; Yin et al., 1997) Detergent-ZDDP interactions at surfaces. A neutral calcium salicylate or phenate plus ZDDP solution had no effect on wear in a motored valve train test, had a graded detrimental effect in 4-ball test (Yamada et al., 1992) and an antagonistic response between ZDDP and neutral sulfonate (Ramakumar et al., 1992). Calcium sulfonate was reported to shift the ZDDP (neutral-basic) equilibrium toward basic ZDDP, which has been reported to give higher wear rates than neutral ZDDP. A basic detergent might displace ZDDP from metal surfaces more effectively than neutral detergents. A multifunctional additive carbonatedetergent would shift the ZDDP (neutral-basic) equilibrium toward basic ZDDP more effectively than calcium sulfonate alone, causing a higher wear rate. Multifunctional carbonate-sulfonate (-salicylate or -phenate) hard-core RM additives had an antagonistic effect on wear in a motored valve train test and a graded negative effect in a 4-ball test (Yamada et al., 1992). Other laboratories

42

Chapter 2

have reported that synergistic effects with carbonate-detergents hard-core RMs affect the structure of antiwear films (Ramakumar et al., 1992; Rounds, 1978; Willermet et al., 1991, 1992, 1995a and 1995b). The growing polyphosphate chains undergo acid-base reaction with metal carbonate, forming shorter chains and incorporating the metal cation into the phosphate structure (Willermet, 1998). Adding detergents to ZDDP affects film structure both by chemically modifying the phosphate film and, at sufficiently high levels of detergent, by forming a second metal carbonate phase. After replacement of part of the zinc by calcium or magnesium from phosphate tribofilm, reduction in phosphate chain length was observed (Willermet, 1998). The XANES spectra of the tribofilms were recorded in the bulk FY mode, and in the surface TEY mode from the ZDDP and calcium phenate. The (S) L-edge and (P) L-edge XANES spectra indicate that sulfur in sulfide form was only present on the topmost part of the film; the phosphorus signaled that the topmost surface contained relatively long polyphosphates, and the bulk chemistry of the film contained mostly shorter-chain polyphosphates. Using a range of concentrations of two detergents, it was concluded that calcium phenate influences the film formation much more than calcium sulfonate, and thus calcium phenate solubilize excess ZDDP from film surfaces more effectively (Yin et al., 1997).

(II) Zinc dialkyldithiophosphates as multifunctional additives in lubrication formulations The ZDDP interactions in hydrocarbon formulation. Zinc dithiophosphates, especially ZDDP (zinc dialkyldithiophosphate), are the most widely used (for over a half a century) multifunctional antioxidant/antiwear additives in engine oil formulations (Dowson, 1998; Mortier and Orszulik, 1997). The structure of ZDDP is similar in outline to that of the sulfonates, but ZDDP itself is not a dispersant and does not form its own micelles in hydrocarbon media. The ZDDP additive is used rarely as the sole additive in oil formulations. ZDDP forms selfassociates, and at higher temperatures, basic ZDDP is transformed to neutral ZDDP (Ganc and Nigarajan, 1991; Habeeb and Stover, 1988). The better performance of the s-ZDDP with secondary alkyl groups over p-ZDDP with primary alkyl groups (Fuller et al., 1997; Matsuoka et al., 1983; Shiomi et al., 1992), alkyl ZDDP over aryl ZDDP (Smolenski and Kabel, 1983), and short alkyl chains over long ones was observed (McGeehan et al., 1985). The major degradation pathway of ZDDP in rubbing contact is oxidation which produces the disulfide [(RO)2PS]2 and a basic salt [(RO)2PS]5Zn20. The adsorption studies of neutral and basic ZDDPs from n-hexadecane solution on black carbon have demonstrated that these compounds are strongly oxidized to bis(0,0dialkyloxyphosphino-thiolyl)disulfide (DS) at 80''C. Analysis of the soot using X-

Lubrication Chemistry

43

ray fluorescence techniques showed an increase in the ratios of concentration of zinc to phosphorus (3:1) onto the soot in comparison to the original compound (0.5:1); this indicated that ZDDPs are oxidized onto the soot surface. Investigations of DS adsorption on black carbon at this temperature has revealed that DS is oxidized to elemental sulfur (ES) which may by due to corrosive wear of the rubbing surfaces (Plaza and Margielewski, 1993). Oxidation of ZDDP salts to the bis(0,0-dialkyloxyphosphino-thiolyl)disulfide (DS) onto soot is the major reason for the antagonistic effect of this form of carbon on the performance of zinc dialkyldithiophosphate. Decomposition of ZDDP takes place in the presence of oxygen, either coming from oxygen dissolved in engine oil or from peroxy radicals and hydroperoxides. Solution studies of the oxidation of zinc dialkyl dithiophosphates by peroxy radicals have shown that disulfides are major reaction products (Paddy et al., 1990; Rossi and Imparato, 1971; Willermet, 1998; Willermet et al., 1983; Willermet and Kandah, 1984). The ZDDP interactions at surfaces. ZDDPs provide wear protection by decomposing and reacting with rubbed metal surfaces to provide a protective coating. The relative effectiveness of a given ZDDP thus depends on its stability (or conversely, reactivity) at the temperatures encountered in an operating engine. For instance, aryl ZDDPs are relatively stable, decomposing only at high temperatures (230°C in oil). Such temperatures may be found in the piston ring area of heavy-duty diesel engines. Consequently, aryl ZDDPs provide good piston and ring scuffing protection in diesel engines. Temperatures in the camshaft and valve lifter area of gasoline and light-duty diesel engines are, however, much lower. Consequently, aryl ZDDPs do not provide good wear protection in these applications. Alkyl ZDDPs are less thermally stable than aryl ZDDPs and are generally more effective in preventing camshaft and valve lifter wear in these engines. ZDDPs also function as antioxidants, primarily by acting as peroxide decomposers (Bum et al., 1971b; Howard et al., 1973). ZDDP is known to interact with most other additives employed in these formulations; e.g., ZDDP is solubilized by soft-core and hard-core micelles and has been considered for reduced antiwear performance (Inoue, 1993; Kapsa et al., 1981; Rounds, 1981; Shiomi et al., 1986; Willermet 1995a and 1995b; Yin et al., 1997). Based on the tribofilm formation (polyphosphate) and the presence (or absence) of unchanged ZDDP in the film, we can conclude that the additives compete with the adsorption of ZDDP on the surface (Varlot et al., 2000; Yin et al, 1997a and 1997b). The antiwear and antioxidant additive, zinc dialkyldithiophosphate, is a key ingredient in the great majority of engine oil formulations, and other lubricant applications such as hydraulic fluids and gear oils. The ZDDP-derived tribochemical films have been studied by a number of laboratories, but their mode

44

Chapter 2

of formation and composition requires structural studies. The prediction of antiwear benefits and optimization of the ZDDP dosage has become a very complex task. The chemistry of the mode of action of ZDDP itself is complicated and so the nature of its interactions with other additives needs investigating all the more. Interaction between ZDDP and fatty acids, again in lubricating oil formulations, shows a considerable amount of mechanical test wear. The antiwear property of ZDDP is reduced by fatty acid additive due to the adsorbed layer of fatty acid, and the solubilization process, which disturbs the function of ZDDP (Otsubo, 1975), see Fig. 2.11. Acidic conditions accelerate ZDDP decomposition and basic barium sulfonate detergents increase the rate of the ZDDP thermal decomposition reaction pathway. ZDDPs have been reported to form complexes with amines and succinimide dispersants (Gallopoulos, 1964; Heilweil, 1969; Rounds, 1976).

"^^^^^^

ZDDP

ZDDPO ZDDP ZDDP ZDDP

"V

ZDDP

Y ZDDP ZDDP

\

surfactant molecule

Fig. 2.11. Solubilization mechanism of zinc dialkyldithiophosphate molecules (ZDDP) by soft-core reverse micelles (RMs) The mechanism of ZDDP action is rather complex. It reacts chemically with metallic surfaces to form new condensed products, possibly FeDDP (Belin et al., 1989; Bell et al, 1992; Martin, 1999; Willermet et. al., 1991), Zn(P03)2, Zn^{?0,\ and Zn2P207 (Yin et al., 1997a and 1997b). In commercial lubricants, ZDDP is always associated with other additives and some interferences have been observed as a well-known antagonism exists between ZDDP and calcium sulfonate detergents in automotive lubricant formulations (Kapsa, 1975). It has been shown that X-ray absorption near-edge spectroscopy (XANES) provides the most sensitive technique so far for the chemical characterization of antiwear films where detergents could interfere with the ZDDPs by coadsorbing on the surface (Yin et al. 1997b). Using a range of concentrations, it has been found that calcium sulfonate interacts with ZDDP only at high concentrations (>2%), whereas calcium phenate affects the film formation even at low concentrations. Using XANES spectroscopy, investigators have observed for the first time

Lubrication Chemistry

45

direct processes of the detergent and dispersant interactions with ZDDP on the surface. These interactions manifest themselves in three ways as far as the ZDDP adsorption and phosphate formation is concerned: (a) tribofilms formed in the presence of detergents and dispersants even at low concentrations, are free from unreacted ZDDP; (b) tribofilms generated in the presence of detergents and dispersants are thinner than tribofilms produced in the absence of the additives; and (c) polyphosphates formed in the presence of additives are of shorter chain length. Additives, especially dispersants, compete with the adsorption of ZDDP on the surface and thus much thinner layers of ZDDP remain on the sliding surfaces. Obviously, the relation between the adsorption and concentration of ZDDP on the surface will affect the film thickness and chain length. The classes of additives which reportedly interact with ZDDP are sulfonate-type detergents and polyisobutylene succinimide (PIBS)-type of dispersants. ZDDP decomposes by a number of routes involving free radical and redox processes. Film composition varies from the iron-rich bonding layer, through the zinc phosphate layer to the outer surface, which contains organic material incompletely converted to precursor species. The polyphosphate chain length may vary as a function of depth into the film and the conditions under which the film is formed. Formation of polyphosphate tribofilms from simple ZDDP solutions is promoted by self-association of ZDDP molecules, which increases the local concentration of ZDDP, The phosphorus (P) L-edge XANES spectra of tribofilm that were generated during 6 hrs from ZDDP only, have shown it to be a mixture of zinc polyphosphate plus 25% to 35% of unreacted ZDDP (Bell et al., 1992; Fuller et al., 1997; Willermet et al., 1991, 1992, 1995a; Yin et al., 1997a and 1997b). It has been found that ZDDP solutions form surface tribofilms typically 120 to 200 nm thick on rubbed steel surfaces at lOO'^C. The development of the film is more strongly dependent on the extent of rubbing than on temperature. ZDDP films form at temperatures as low as 40^C, but the equilibrium thickness of the film increases markedly with increase of temperature. The films formed by ZDDP are quite rough, this suggests that ZDDPs increase friction by an effective surface roughening which promotes a boundary lubrication condition at the expense of elastohydrodynamic (EHD) lubrication (Taylor et al., 2000). Combustion generated soot in the crankcase of diesel engines in the presence of ZDDP can significantly increase engine wear surfaces. Zinc salts have significantly better antiwear behavior than DS. Wear volumes in the case of DS are about 5 fold larger. The antiwear properties of neutral and basic salts of ZDDP in the presence of soot, in contrast to the antiwear activity of bis(0,0dialkyl-oxyphosphino-thiolyl) disulfide (DS) under the same condition, are strongly dependent on time of friction. Wear volume after a 15 minute wear test is 3.5 times smaller for DS in comparison to both ZDDPs and falls to 1.1 after 90 minutes (Margielewski and Plaza, 1997).

46

Chapter 2

Phosphorus, originating from engine oil, poisons emission devices (Caracciolo and Spearot, 1976; Spearot and Caracciolo, 1977). To reduce engine oil phosphorus concentrations without compromising engine durability, and thereby reducing phosphorus poisoning of emission control devices, an optimum ZDDP (or mixture with ZDDP) should be used. The three approaches reducing the poisoning of emission devices are (Willermet and Kandah, 1984): to develop more phosphorus-resistant catalysts, reduce the oil consumption of engines, and reduce the concentration of phosphorus in the engine oil. Research is being conducted by others using the first approach, but no significant break-through has been achieved. Engine designers have successfully employed the second approach, and oil consumption is currently even lower than O.lL/1,000 km reported in 1992. Further reduction in oil consumption will come with greater difficulty, and may not be desirable; wear can increase to an undesirable level as consumption is reduced, because of inadequate lubricating film. Some consumption is also desirable from the additive replenishment stand point. Thus, further reductions in phosphorus poisoning of current emission control devices will most likely be achieved by reducing engine oil phosphorus content. ZDDPs are formed by the reaction of phosphorus pentasulfide (P2S5) dimer with alcohol (ROH) or phenol (dithiophosphoric acid ester formation), followed by reaction with zinc powder or zinc oxide or with zinc chloride and sodium hydroxide solutions (see below). The various types of ZDDPs differing in Rgroup structures are shown in Table 2.6 (SAE, 1983; Yamaguchi et al., 1966). Scheme of the formation of zinc dialkyldithiophosphate, ZDDP: OH / 2(S=P-SH) \ OH

+

ZnO

Dithiophosphoric acid ester

=

OH / (S=P-S-)2Zn + H2O \ OH Zinc dialkyldithiophosphate (monomeric)

Three different structural forms of ZDDP have been proposed (Armstrong et al., 1998): the monomeric form Zn[PS2(OR)2]2 the dimeric or neutral form Zn2[PS2(OR)2]4 the basic form Zn4[PS2(RO)2]60 The reactivity of the lubricating oil ZDDP additives was investigated by molecular orbital techniques (Armstrong et al., 1998). Semi-empirical quantum chemistry methods were used to model the structures of some of the complexes

Lubrication Chemistry

Al

expected to be formed during the reaction of the additive with an oxygen-rich steel surface. Table 2.6. Types of ZDDPs (primary, secondary) with R-group chain structure (long, medium, short) and examples of alcohols ZDDP type

R-group chain structure example

Chemical name

Abbreviation

Short-chain primary

-C-C-C1 C

isobutyl or isoamyl alcohol

C4-C5p

Short-chain secondary

c 1 c-cc 1 c-c-c11 cc c

isopropyl alcohol

C3-C6S

3,3-dimethyl-2butanol

C3-C6S

isooctyl alcohol

C8p

polypropyl phenol

Aryl

Medium -chain secondary

Long-chain primary

1 C-C-C-C-C1 1

c c

Aryl

R-C^H^-

C (carbon) = CH3, CH2,, CH or C; p = primary., s = secondary

In order to mimic the attack of ZDDP onto the oxide surface (FeO), the structure of the possible complexes formed between an O^" ion and ZDDP was examined. The oxide anion was allowed to interact with the positively charged atoms (zinc and phosphorus), and partially negatively charged sulfur atoms of the additive molecule. The heats of complex formation (Oxide ion + ZDDP ^ ZDDPiOxide) and total energies determined for each complex were reported (Armstrong et al., 1998). The monomeric form of ZDDP, which corresponds to the formula Zn[PS2(OR)2]2, has the zinc atom surrounded by four sulfur atoms arranged in a distorted tetrahedron. The attack of the oxygen anion on any of the atoms in the monomer molecule causes the breakdown of both four membered rings. The most stable of the three complexes was found to be the structure in which O^' is bound to the zinc atom. It is interesting to note that this is the only complex in which a

48

Chapter 2

ring structure is formed. This suggests that the presence of a five membered ring in the complex contributes to its stabiHty. The other two complexes have an open structure. This implies that the presence of four membered rings in the monomeric form of ZDDP induces strain in the molecule. For dimeric or neutral forms of ZDDP, Zn2[PS2(OR)2]4, the lowest energy corresponds to the complex in which the oxygen anion was allowed to approach one of the sulfur atoms belonging to the eight membered ring. The breakdown of two bonds results in the formation of two independent anions. Each of these structures maintains one four membered ring present in the dimer ZDDP molecule. An open structure is instead obtained when the O^' anion is attached to one of the phosphorus atoms in the four-membered ring. The basic form of ZDDP, Zn4[PS2(RO)2]60, has a structure in which the central oxygen atom is surrounded by four zinc atoms in tetrahedral geometry, and the six 0,0-dialkyl-dithionate groups are attached to the six edges of the tetrahedron (Armstrong et al., 1998). For the basic ZDDP, it was found that the lowest energy corresponds to the attack of the oxide ion on one of the sulfur atoms contained in the additive molecule. The attack induces the cleavage of the three bonds, namely two P-S bonds and one Zn-S bond. Overall, the relative stability of the three forms was found to increase in the order: monomeric, dimeric, and basic. Analysis and identification ofZDDPs. The thermal decomposition of ZDDPs on a stainless metal surface was identified by the use of external reflection Fourier transform infrared spectroscopy (FTIR), ^^P-MAS NMR, and scanning electron microscopy (SEM), (Harrison and Brown, 1991). The authors (Willermet et al., 1992) studied tribochemical films formed from fully-formulated engine oils which contained ZDDPs using reflection-absorption infrared spectroscopy (IR), X-ray photon spectroscopy (XPS), and Auger spectroscopy. XPS was used to study the effects of ZDDPs on hydrocarbon films adsorbed on iron substrates of various base stocks and three different ZDDPs (Rhodes and Stair, 1993). The molecular composition of the initial film was studied by inelastic electron tunneling spectroscopy (lETS) (Yamaguchi and Ryason, 1993). The kinetics of hydrolysis of ZDDPs were analyzed by ^^P-NMR (Bum et al., 1995). ZDDPs have been analyzed in the laboratory by thin layer chromatography (TLC) (Coates, 1971), and the lubricating additives were analyzed by non-aqueous capillary electrophoresis (Thibon et al., 1999) and by normal phase high performance liquid chromatography (Lambropoulos et al., 1996).

2.4. Synthetic Engine Oils The 1990s brought a new urgency to the concept of environmental responsibility. This has led to an increased need for functional fluids that are both

Lubrication Chemistry

49

biodegradable and low in toxicity. Synthetic engine oils offer the freedom to formulate oils that are better for the engine, better for the environment, and better for the driver. The API rating of SG gasoline engine oil became more stringent in 1993 as API tightened their formulations involving different base stock oils in the manufacturing process. Many companies came out with "new" formulations and synthetic engine oils during 1992. Synthetic and semisynthetic engine oils are now receiving more attention. Semisynthetic oils are a combination of synthetic fluids and mineral oils. In these oils there is some reduction in friction, increase in film strength, and they oxidize at the same rate as a mineral oil product. These marginal benefits are realized at a higher cost than that of mineral engine oils. If you are going to synthetics, go all the way to fiill synthetics and gain all the benefits (Miller, 1993). The synthetic engine oils are becoming more common today and have properties which can provide superior performance. Compared to mineral-based engine oils, the oxidation resistance is high, the friction is low, with much better low-temperature characteristics. The film strength is higher and detergents with dispersants are very effective. A 5 to 10 percent improvement in fuel economy in gasoline and diesel engines is gained. Oil drain and filter change can be extended. The synthetic oil provides easier cold starting and excellent protection in both warmer and colder climates. The disadvantages of synthetic engine oils are higher cost (roughly 3 to 5 times that of mineral oil) and poor cost effectiveness in applications where dirt is a problem. In a fully synthetic oil, there is almost certainly some mineral oil present. The chemical components used to manufacture the additive package and the viscosity index improver (VI) contain mineral oil. When all these aspects are considered, it is possible for a "fully synthetic" engine oil to surpass mineral oil (Shubkin, 1993). Synthetic oils fall into general ASTM classification: (a) synthetic hydrocarbons (poly-a-olefins, alkylated aromatics, cycloaliphatics); (b) organic esters (dibasic acid esters, polyol esters, polyesters); (c) other fluids (polyalkylene glycols, phosphate esters, silicates, silicones, polyphenyl esters, fluorocarbons). Some applications of the synthetic fluids are motor oil, trucks, marine diesel, transmissions and industrial lubricants, aviation and aerospace lubricants, fireresistant fluids, and greases. Specifications for several military lubricants can be met only by a synthetic product. All commercial and military jet aircraft engines use synthetic lubricants, in addition to the space shuttle, NASA, and nuclear submarines. The U.S. finished lubricant market, which includes synthetic and petroleumbased products, was 11 billion liters in 1997. Lubricants for automotive applications consumed more than half (52%) of the volume, split between consumer and commercial applications, at 31% and 21%), respectively. The remaining quantity was used for various industrial applications, including hydraulic fluids, equipment oils, and process oils. Penetration into the lubricant

Chapter 2

50

market has been slow for synthetic oils, which today represent less than 5% of the total market. Price has been a major factor, with synthetic products costing three to five times more than mineral oils. Lubricants are formulated products composed of a base stock, which is either a mineral or synthetic oil, and various specialty additives designed for specific performance needs. Additive levels in lubricants range from 1 to 25% depending on the application. Synthetic base stocks are oligomers of small molecules, synthesized to a defined molecular weight. Important performance indicators include viscosity index which measures the viscosity index behavior over a temperature range, oxidative stability, and pour point. The performance of synthetic and mineral oils (Morse, 1998; Shubkin, 1993) is summarized in Table 2.7.

Table 2.7. Performance of synthetic and mineral lubricant oils. Relative evaluation of viscosity, stability (thermal, oxidation, hydrolytic, volatility), lubricity (wear protection and fatigue) and environmental impact Parameters

Mineral oils

Poly-aolefms

Di-ester glycols

Polyol esters

Polyalkylenes

Phosphate

fair/good good good

excellent good very good

very good good very good

very good good very good

very good good very good

fair/good good fair/good

good fair excellent poor/fair

very good very good excellent very good

good very good fair good

good excellent fair good

good good good good

fair good fair fair/good

good excellent fair/good

good excellent good

fair good fair

fair good fair/good

good good fair

excellent excellent fair

good excellent

good excellent

good excellent

poor/fair very good

3x

5x

5x

5x

esters

Viscosity Low temp. High temp. Viscosity index

Stability Thermal Oxidative Hydrolytic Volatility

Lubricity No additives With additive Fatigue life

Environmental performance Low toxicity Biodegradability

good fair

excellent poor

Price compared to mineral oil Ix

3x

51

Lubrication Chemistry

These synthetic products are free of the heavy metals, sulfiir, and nitrogen compounds which are often present in mineral oil and can cause serious equipment wear. Petroleum-based oils have a wide range of molecular weights within a distillation fraction. By contrast with these refined base stocks, synthetic base stocks are more highly controlled for structure and composition. This results in tighter molecular weight and fewer extraneous functional groups present, which may have impact on higher flash point, fewer volatility, higher viscosity index, and lower pour point. Table 2.7 compares viscosity, stability, lubricity (wear protection properties), environmental performance and prices of synthetic fluids and mineral oils. Poly((x-olefms) or PAOs, polyol esters and diesters are now used in automotive and marine engine oils. To understand how an ester lubricates, it is important to consider its behavior in the different lubrication regimes, especially boundary lubrication when the properties of the bulk lubricant (e.g. viscosity) are of minor importance. The chemical properties of the lubricant responses under extreme conditions will become increasingly important. The polar ester will preferentially stick to the surface of metal when a small amount of ester is added to a low viscosity nonpolar fluid (PAO), (Randies, 1999; Spikes, 1999). When the two metal surfaces come closer together, the polar ester molecules stay in the contact zone. The use of fully synthetic engine oil formulations has produced some improvement in viscosity control and engine cleanliness in the piston and valve train areas over petroleum-based oils (Boehringer, 1975; Frame et al., 1989; Kennedy, 1995; Lohuis and Harlow, 1985). Viscosityy volatility andpour point Poly(a-olefms), diesters and polyol esters usually show improved performance of viscosity, higher viscosity index, significantly lower pour point, and lower volatility (%) loss, compared to petroleum base stock (Table 2.8). Table 2.8. Comparison of viscosity, volatility and pour point for synthetic and mineral base stock Parameter/Test

Viscosity ,cSt* IOO°C -40 °C

Mineral oil PAO-4 Diester Polyol ester

4.0 3.9 4.3 4.1

Solid 2500 3500 3750

Viscosity Index 127 123 161 130

Volatility Loss (%) 16 13 7.7 3.8

Pour Point °C -22 -69 <-65 -69

' Centistoke (mmVs) Higher viscosity index base stocks, whether petroleum or synthetic, will exhibit

52

Chapter 2

lower viscosity loss upon temperature increase. This property increases film strength for hydrodynamic and elastohydrodynamic lubrication in the engine, and will improve protection for bearings. PAO and ester base stocks show significantly lower pour point than petroleum oil. This advantage is due to molecular structure and the lack of crystalline wax particles, present in some refined petroleum oils. Fully synthetic engine oil lubricants offer excellent low temperature flow and viscosity properties.(Demmin etal., 1992; Lakes, 1999). Thermal, oxidative and hydrolytic stability. Organic esters and PAO inhibited lubricant base stocks resist oxidative and thermal degradation better than petroleum-based oil: petroleum (121 °C), PAO (121-177°C), diesters (149-177°C), and polyol esters (177-218°C). Oxidative stability -^ petroleum base oil < PAO < diesters

< polyol esters

These full synthetics have been all PAO or ester based, and a mixture of ester plus PAO. The addition of PAO or ester to petroleum based engine oils for improved oxidative stability has shown average quality results. The addition of lower levels of PAO or ester base stock <15 wt % to petroleum based formulations show little or no improvement in the thermo-oxidative engine test. The predominant automotive synthetic base stocks (PAO, diesters, polyol esters) do not show any hydrolytic instability in engine oil applications. Lubricity - wear protection and fatigue. The coefficient of friction is a measure of lubricity of a lubricant. The esters, being more polar, are attracted to the metal surfaces and form monolayers. These thin layers reduce the coefficient of friction at the surface. The surface phenomena that determine the behavior of boundary lubricants can be described in the following terms: physically and chemically adsorbed layers and tribochemically formed films. Esters stick to the surface better than mineral oils. Since ester groups are polar, they form physical bonds with metal surfaces. At high loads, esters will tend to form chemisorbed films. Under extreme boundary conditions, esters tend to break down to form acids which leads to wear protection and friction reduction (Randies, 1999). These acids readily react with freshly exposed metal surfaces to form metal carboxylates tribofilms. Since ester groups are polar, they can compete with antiwear or EP agents for the metal surface. They can cover the metal surfaces instead of the antiwear additives, resulting in higher wear characteristics. The lubricity of an ester in a fully formulated fluid is not always easy to predict. Esters can be blended with mineral oil and with other synthetics such as PAOs. Esters have much lower

Lubrication Chemistry

53

coefficients of friction than mineral oils. In modem engine oil formulations, antiwear capability is predominantly provided by the additive package. Esters are more polar than both petroleum and PAOs and may compete at the metal surface with any polar additive (corrosion inhibitors or antiwear agents). Applications. Table 2.9 summarizes the possibilities of synthetic lubricants, by listing information on the main applications of some of the most important fluids (Bartz, 1999; Miller, 1993; Rudnick and Shubkin, 1999). Table 2.9. More important synthetic oils and their applications Oil type

Main application

Synthetic hydrocarbons Poly(a-olefms) Engine oils, industrial (hydraulic, compressor, bearings) Polyisobutenes Two-stroke engine, electrical insulation, metal forming oils Alkylated aromatics Low temperature oils (engines, gears, hydraulics) Organic esters Diesters Polyol esters Others Phosphate esters Silicone oils Halogenated fluids Polvphenyl ethers

Turbine oils, mixing components with PAO Turbine oils, gear, compressor, hydraulic oils ^^^^ resistant hydraulic fluids, gas turbine oils ^*Sh temperature hydraulic fluids, brake fluids, compressor oils Extremely fire-resistant hydraulic oils Radiation-resistant, heat transfer fluids

In 1996 market, the most widely used additives were as follows: poly(aolefins) (PAO) which are oligomers of decene (48% - in engine, industrial lubricants-compressor, hydraulic, and gear oils), polyalkylene glycols (23% - in brake fluids, metalworking and gear oils), polyol esters (16% - in aviation turbine, engine, compressor, hydraulic, and gear oils), organic diesters (8% - in aviation turbine and engine oils, and mixing components for PAO), and phosphate esters (5% - in fire-resistant hydraulic fluids and gas turbine oils). Synthetic lubricants offer great reliability, especially in the high temperature and more demanding applications, allowing users' equipment to operate for longer periods without requiring an oil change. Also, stability of the synthetics means less wear and dirt buildup on machinery caused by degradation of the lubricant. Synthetic motor oil, introduced in 1976, will maintain exhaust emissions within EPA standards for at least 320,000 km and improve engine performance at high speeds and during cold weather. Reduced equipment wear and maintenance

54

Chapter 2

requirements can translate into significant cost savings for users of synthetic lubricants. The high performance segment seems to offer the best opportunity. It is certain that as engine requirements continue to become more and more severe, premium quality demand will increase. Synthetic lubricants will continue to play a major role in meeting that demand. Fully synthetic motor oils are proven daily by motorists, many commercial airliners in their jet engines, in our nuclear submarine fleet, in military jets, tanks, and other vehicles, even in the NASA space shuttle (Bartz, 1999). The benefits of using synthetic engine oils. The benefits that can be engineered into a synthetic engine oil fall into several main aspects. These are: (A) engine wear protection; (B) improved fuel and oil economy; (C) environmental protection. The following summarizes how these aspects can be improved by the use of synthetic oils. Every synthetic engine oil has its own balance of benefits (Miller, 1993; Mills, 1993; Patter et al., 1981). Synthetic engine oils are capable of superior performance for the following reasons: (A) Engine wear protection, (a) A full lubricant film is provided by the synthetic engine oil and wear is reduced to a minimum. There is great film strength of the synthetic as compared with mineral oils; (b) High-temperature wear protection: a breakdown of the lubricant viscosity film is protected by using wider viscosity range oils; synthetic oil is more shear stable than mineral oil. Synthetic oil showed a 10% increase in viscosity and mineral oil showed a 135% increase after 64 hrs of operation at an oil-sump temperature of 150^C. Both oils were SAE lOW-50; (c) Start-up wear protection: possesses superior volatility, provides almost instant lubrication, better resistance to oxidation, used oil has the same low temperature flow rates as it had when it was fresh. One cold start engine with mineral oil at minus 20°C is equivalent to some hundred miles of driving wear of an engine; (d) Warm-up wear protection: base stocks have a high affinity for metal surfaces and increased film-strength performance; the high viscosity of an oil at very low temperatures provides a thick film of lubricant that minimizes surface-to-surface contact. This protects against wear at low temperature; (e) Improved piston deposit control: deposit in the piston ring grooves is lower by a factor of 3 to 5 over traditional mineral engine oils. Excessive deposit buildup leads to less compression, increased combustion gas blow-by, and increased oil consumption; (f) Improved inlet valve deposit formation: engine oil consumed by passing through the piston rings into the combustion chamber or from mist generated in the crankcase and vented back to the engine air intake, can form deposits on the engine intake valves. Synthetic engine oils can be formulated for low-volatility, high viscoelasticity to promote cleaner burning and contribute to

Lubrication Chemistry

55

reduced inlet valve deposit formation; (g) Reduced Turbo charger and combustion chamber deposit; deposits can block the oil flow to the Turbo charger bearings and lead to Turbo charger seizure. Synthetic engine oils will maintain the Turbo charger considerably cleaner than mineral engine oil. Lower oil consumption in the combustion chamber can considerably reduce deposits (causing pre-ignition) and minimize engine damage; (h) Minimized oil system deposit: the benefit is significant by low levels of sludge; the whole engine is in a clean condition; keeps engine efficiency at maximum; keeps emissions to a minimum. (B) Improved fuel and oil economy, (a) Synthetic engine oil can lower fuel consumption by more than 5% over mineral engine oil. There is lower internal friction due to even molecular structure; (b) The CAFE figures should be based on a more realistic, long-distance fuel economy testing; (c) Oil and filter changes were performed every 40,000 km for poly-oc-olefin. (PAO) based synthetic SAE 5W-20 SE-CC oil, and every 8,000 km in identical vehicles operated on SAE lOW SE mineral oil. (C) Reduced environmental damage, (a) Extends the useful life of an engine and maintains the engine in peak performance conditions so as to minimize emission and maximize fuel conservation; (b) Reduced oil consumption will reduce pollutants from lubricants; (c) Extends the drain interval of the engine oil to reduce the quantity of used oil for disposal; (d) Collection and recycling (or controlled burning at high temperatures) are environmentally preferred methods of used engine oil disposal. The results of tests relating to the use of synthetic oil poly-a-olefin (PAO) and mineral oil in automotive crankcase application (Mills, 1993; Patter et al., 1981) are described below. The case study. The following section examines in detail the results of testing for all the major areas of automotive applications. PAO and mineral oils were compared employing identical additive packages at identical concentrations. (1) Conditions: temperature = 165°C, time = 120 h. The test was designed to measure the thermal and oxidative stability of the fluid as it is splashed on the hot metal surfaces inside an engine. The measured increase in viscosity was 54% for the mineral oil and only 3% for the PAO oil. (2) The Peter Wl engine test after 108 h. The test measured the increase in viscosity (108% for mineral and 20%) for PAO oil) and measured also the amount of wear, as determined by bearings weight loss: 14.1 mg for mineral and 14.5 mg for PAO oil. (3) Low temperature performance. The cold crank simulation test is of vital interest to any car owner living in a cold climate. The advantage of a PAO-based formulation in the crankcase is immediate and obvious on a cold winter morning;

56

Chapter 2

it is the difference of being able to start the car or not. The test was conducted at minus 25 °C, the viscosity of the PAO fluid was 600 Centipoise, but the mineral oil with high viscosity index (HVI)) became solidified. (4) Hot oil oxidation test (manual transmission and rear axle oils). Temperature of the test was lOO^'C. After 16 h, the viscosity of the PAO fluid increased by only 21% but the mineral oil product became too viscous to measure. (5) Thermal stability test is the panel cooker thermal stability test. Test conditions: Panel temp. SIO'^C, sump temp. 12rC, operations: 6 min splash and baked for 1.5 min. At the end of the test, the panels are rated for cleanliness. A completely clean panel has a rating of 10. The mineral oil panel was covered with deposits, indicating a lack of thermal stability and rating for cleanliness was 0; PAO rating was 8. (6) Hydrolytic stability. The test method required treating the fluids with 0.1% water and maintaining the fluid at ITC for up to 200 h. Samples were withdrawn at 20-h intervals, and the flash points were measured by the closed cup method. A decrease in flash point was interpreted as being indicative of hydrolytic breakdown to form low molecular weight products. The PAO showed no decrease in flash point under any of the test conditions. (7) Improved fuels and oil economy. A test program involving a total of 182 vehicles with PAO showed a weighted average fuel saving of 4.2 %>. In 10 different tests on oil consumption, the average improvement was 55.9%. (8) Performance in extended oil drain field service. This conclusion was based on a 161,000 km test using parkway police cruisers. Oil and filters changes were performed every 40,000 km with PAO; 8,000 km with mineral oil. 2o5o Lubricant Requirements and Specifications Engine oil viscosity classification. Probably the most important single property of a lubricating oil is its viscosity. It is a factor in the formation of lubricating films under both thick and thin film conditions which : (a) affects heat generation in bearings, cylinders, and gears; (b) governs the sealing effect of the oil and the rate of consumption or loss, and (c) determines the ease with which machines may be started under cold conditions. SAE (Society of Automotive Engineers) recommended Practice J300d classified oils for use in automotive engines by viscosities determined at either 100°Cor-18°C. The ranges for grades (viscosities) in this classification are shown in Table 1, Appendix, Viscosity grades denoted as W (Winter): OW, 5W, low, 15W, 20W, and 25W, are measured at low temperature viscosities cranking (-30, -25, -20, -15,-10, and -5°C , respectively), and measured with a minimum viscosity required at 100°C. Viscosity grades without the W, such as 20, 30, 40, 50 and 60, have minimum and maximum viscosities, which are measured only at

Lubrication Chemistry

57

100°C. If an oil meets the SAE lOW specification at a lower temperature and SAE 30 at 100°C, the oil has an SAE grade of lOW-30. The same is true with 5W-30, lOW-40, 15W-40, and the other multigrades. In the US 5W-20 is recommended in winter for easier starting, and 5W-30 in summer for greater heat protection. What the numbers mean: 5W part means that the oil functions like a straight 5-weight (very thin) at (W)inter temperatures, and when hot, like a 20- or 30- weight 'straight' oil (thicker, more heat resistant). In these classifications, grades with the suffix W (Winter) are intended primarily for use where low ambient temperatures will be encountered, while grades without the suffix (non-W) are intended for use where low ambient temperatures will not be encountered. Oils can be formulated that will meet the -20°C limits of one of the W grades and the lOO^'C limits of one of the non-W grades. For example, an oil can be formulated to meet the -20''C limits for the l o w grade and the lOO^'C limits for the 40 grade. It can then be designated an SAE lOW-40 grade or referred to as a multigrade or multiviscosity oil. Oils of this type generally require the use of VI improvers (see Additives) in conjunction with a petroleum or synthetic lubricating oil base. Lubricant requirements and specifications. The development of North American classification systems for the SAE began in 1911 on the basis of viscosity alone. In 1947, the API (American Petroleum Institute) developed a performance classification system for engine oils in terms of regular, premium and heavy duty lubricants. By 1952, the API and ASTM (American Society for Testing and Materials) further advanced the system to generally classify engine oils. In 1970, the TRIPARTITE of API, ASTM, and SAE was established to manage a completely new API classification system, see Fig. 2.12 (Haycock, 1993).

Fig. 2.12» The US "Tripartite" of API, ASTM and SAE (Haycock, 1993)

58

Chapter 2

Since the development of performance specifications from the 1930s and beyond, the API, Military, OEM (Original Equipment Manufacturer) and European ACEA requirements have reflected the introduction of the nevs^ engine technologies as well as the performance of lubricants in the field. The implementation of the new performance requirements will be driven by field experience for realistic correlation with laboratory test tools. The increase in the number of performance classifications for passenger car and commercial diesel grades has accelerated dramatically from the 1930s (when 3 classifications were in place) through the 1980s (with over 25 active classifications in US and Europe). This increased complexity reflects the diverse market of vehicles and the specific performance requirements demanded by OEMs. This new API classification system was finalized during 1970 and consists of two categories: 'S' (covering gasoline engine oils sold in Service Stations for use in passenger cars and light tracks, gasoline, or spark ignition), and a 'C category (for oils for use in commercial, farm, construction and off-highway vehicles, mainly diesel engines, or compression ignition). An oil can meet more than one classification, for example, API SG/CD or CE/SG. The API SG category was formally adopted in March 1988 and it meets the requirements of 1989 model year passenger cars. Similarly, the API CE was adopted in 1988 and recommended by all American heavy duty manufacturers. In 1990 it was replaced by API CF-4 for four stroke engines. Plans to issue an API "S" classification next year are well advanced. There are seventeen engine service classifications as follows (Booser, 1997; Pirro and Wessol, 2001; Wills, 1980): - Nine in category S (Service): SA, SB, SC, SD, SE, SF, SG, SH, and SJ - Eight in category C (Commercial): CA, CB, CC, CD, CE, CF, CG, and CH The chronology of engine, gear and automatic transmission lubricants development since 1930 (Copan and Richardson, 1999) is illustrated in Table 2, Appendix. Automotive engines and engine oil requirements are changing at an increasing rate. Gasoline engine categories that formerly remained in effect for 7-8 years (SF, SG) are now being upgraded every 2-3 years. Each recent upgrade has introduced more stringent requirements and testing standards, along with a higher cost of development. A new generation of diesel engine will require longer engine life, extended drain intervals and increasingly strict emission regulations. More engine builders are considering synthetic oils or components as a technical solution for the upgraded oil requirements (Lakes, 1999). The API classification system is open ended, so that additional classifications can be added when needed. There is a relationship between API engine service classifications and MIL, OEM and ACEA classifications of the oil quality standards previously outlined. These relationships are shown in Table 2,

Lubrication Chemistry

59

Appendix. There are also minimum performance requirements for each classification that are defined in terms of performance in prescribed engine tests. The minimum performance levels in those tests are at least equal to those required to meet the standard. This approach will provide the user with better assurance that oil marketed as being suitable for a particular service classification is suitable. Oil recommendations by field use. The following comments provide a guide to the types and viscosities of oils usually recommended for internal combustion engines used in the various major fields of applications (Pirro and Wessol, 2001; Wills, 1980). Passenger car. Most passengers cars have four stroke cycle gasoline engines. Current recommendations for these cars are oils which meet requirements for API Service SJ and ILSAC GF-2. These oils provide good protection against low temperature deposits and corrosion, protect against wear, and provide excellent protection against oxidation, thickening, and high temperature deposits under the most severe conditions of high speed operation or towing, even at high ambient temperatures. The viscosities usually recommended are SAE 20W-20 or 30 for temperatures down to about 18°C, and SAE 20W-20 or lOW for lower temperatures. For extreme low temperatures, SAE 5W, 5W-20, or 5W-30 oils may be recommended. Multiviscosity SAE lOW-30 or 5W-30 oils are often recommended for year-round use, and are now the primary recommendation of several U.S. engine manufacturers. European engine manufacturers often recommend higher viscosities, particularly multiviscosity oils where SAE 20W-40 and 20W-50 oils are frequently preferred. The recommendations for passenger car diesel engines are generally similar to those for four cycle gasoline engines; that is, oils for API Service CD or SJ/CD. Some manufacturers permit the use of multiviscosity oils, while others prefer single viscosity types. The rotary engines used in passenger cars generally require SAE lOW-30 oils for API Service SD or SE quality, and for two cycle gasoline engines, the oil used is usually of either SAE 30 or 40 viscosity and is formulated specifically for this service. Truck and bus. The recommendations for gasoline engines in trucks and buses are similar to those of passenger cars; that is, for API Service SJ. The viscosities used are somewhat lower, typically SAE 30 or 40 for summer use and SAE 20W20 or SAE lOW for winter use. SAE 30 oils are used year-round in many cases where operation is more or less continuous, vehicles are stored inside, or starting aids can be used. Multiviscosity SAE lOW-30, 15W-40, 20W-40, and 20W-50 oils are being used to an increasing extent to take advantage of the improved starting and fuel economy they provide.

60

Chapter 2

There are considerable variations in the oils recommended for diesel truck and bus engines. In general, oils for API Service CC are recommended for naturally aspirated engines, and oils for API Service CF-2 for supercharged engines. However, the two cycle engines are somewhat sensitive to ash content in oil; thus, the usual recommendation for supercharged or naturally aspirated engines is an oil for API Service CC with certain restrictions on the ash content. Some of the four cycle engines also have shown sensitivity to the additive system; therefore, various manufacturers have special requirements over and above the basic API service recommendations. Viscosities used are usually SAE 30 for summer use and SAE 20W-20 or l o w for winter use. SAE 30 oils are specified for year round use by some manufacturers. Where liquefied petroleum gas (LPG) engines are used in trucks or buses, oils for API Service SJ are often used for convenience, although somewhat lower quality oils may be satisfactory. In some cases, special oils containing no organometallic detergents (ashless additives) are recommended. Table 2.10. Summary of oil use over time Passangercar

1949

1972

1992

2002

Power/rmp (PS/min"^) Oil consumption (L/1000 km) Oil change interval (km)

34/4,200 0.5 1,500

100/5,000 0.25 10,000

113/5,600 <0.1 15,000

124/5,900 <0.1 18,000

Over the period of approximately 40 years as illustrated in Table 2.10, higher performance output was attained in engines by virtue of design criteria and the availability of enhanced lubricant additives and base fluid technologies (Copan and Richardson, 1992). These achievements have been accomplished with reduced quantities of lubricant required for maintenance and operation. Although the lubricant is a minor overall factor in vehicle operating costs, the investment in technology development has resulted in reduced oil waste and oil consumption, with substantially enhanced vehicle durability. New lubricant requirements and specifications ofpassenger car motor oils. The new requirements for passenger car motor oils will likely be designated API "SK". The API "SK" requires no new engine tests compared to those used to document API SJ performance, but to meet this new quality level, the test will have to be run to the Chemical Manufacturers Association (CMA) Code of Practice and be subject to the ASTM Multiple Test Acceptance Criteria. Oils will also need to meet the ILSAC GF2, MIL-L-2104G physical and chemical requirements. After 1994, new tests were introduced including a new Sequence

Lubrication Chemistry

61

VF, Sequence IID replacement, new Sequence VI and a Caterpillar 1G2 replacement. More stringent requirements on volatility and seal compatibility may also be included. The API SJ motor oil test (adapted in 1996) and ILSAC GF-2 are comprised of a set of engine tests for defining minimum oil performance requirements. These tests are as follows: bearing wear and corrosion (Sequence L-38), valve corrosion (Sequence II-D), sludge formation (Sequence III-E), degradation wear products (Sequence V-E), and fuel economy (Sequence VI-A), (Lakes, 1999). For abbreviations see Chapter 8. North America diesel engine oils are predominantly for heavy-duty commercial use. The API CH-4 and CG-4 commercial categories and the MIL-L-2104G military category use a set of engine tests for defining minimum oil performance requirements. These tests are as follows: valve train wear (GMPT 6.2L), viscosity control (CaterpillarIN 3116/DD, 6V92TA/Bosch Injector), filter plugging/piston deposits (CAT 3116), Wear/filter plugging (Cummins 444XT), viscosity increase (soot) (Mack T-8), bearing wear, corrosion (CRC L-38), and sludge formation (Sequence III-E), (Lakes, 1999). MIL (U.S. military specifications). Since the Army specifications were becoming outdated, new specifications were prepared and issued in 1970 and 1980, and previous specifications became obsolete. A relationship between API and MIL classifications is shown in Table 2, Appendix. Many of the old MIL specifications are obsolete, but the most current "obsolete" ones are MIL-2104B, C, and D, replaced by MIL02104E, and MIL-46152C replaced by MIL-46152D. MIL-46152D is for both gasoline and diesel engines in commercial vehicles used in federal and military vehicles. Gasoline performance would be at the API SG level while the diesel engine performance might only be at MIL-2104B or API CC level. Some of these oils will be rated SG/CD, and CE. In Europe. Engine oil classifications represent a blend of quadripartite agreement (ACEA, CEC, ATC, ATIEL) and individual OEM requirements. New specifications for gasoline and diesel oils were introduced by the CCMC (Comitte des Constructeurs d'Automobiles du Marche Commun) in 1983. This organization consisted of twelve European car makers who all had individual veto power of any proposals presented to the group. Eleven of the twelve members resigned in protest of one member vetoing a proposal all the other members considered important. This group did not include Ford of Europe or General Motors of Europe. Their purpose was to present a common front against Japanese imports and establish standards in their industry. Work started immediately to establish the CLCA (Comitte de Liaison de la Construction de I'Automobile) with few differences from the CCMC except for the elimination of the veto and adaption of an 80 percent majority rule. Based on the North American model, sequences

62

Chapter 2

included European engine and bench tests. CLCA was short-lived and was replaced by the ACEA (Association des Constructeures Europeans d'Automobiles) with a different structure and membership which included Ford and General Motors. The ACEA is headquartered in Brussels. ACEA is reviewing the overall European classification system in view of combined input from the oil industry and lubricant additive manufacturers. The classifications were extensively revised in 1984 and 1991. The latest sequences include G-4, G-5 for gasoline (G) engines and PD-2 for passenger diesel (D) and D-4 and D-5 for industrial diesel engines. They are as follows: G-4 minimum quality for passenger car gasoline engines, G-5 low viscosity fuel economy oil for passenger car gasoline engines, PD-2 for passenger car diesel engine, D-4 minimum quality for commercial vehicle diesel engines, and D-5 similar to D-4, but for more severe service or extended oil drain for industrial engines. The specifications issued by the new ACEA will become an important factor in engine oils in the United States because Ford and General Motors are now included as members of that group. In Europe, ACEA, which represents the interest of European engine manufacturers, has set up a committee to examine engine oil quality requirements for the engines they produce. In contrast to API Engine Service Classification, which defines several oil quantities, the ACEA system defines only one oil quality which is roughly equivalent to API Service SE. Engine tests for minimal oil performance for ACEA include the following (Lakes, 1999): sludge formation (sequence III-E), high temperature test (Peugeot TU3M/KDX, 4 cylinder, 1.3L), wear test (Peugeot TU3M/KDX, degradation wear products, sequence V-E), black sludge test, and fuel economy test (Mercedes-Benz Ml 11E20, 4 cylinder, 2L). In Europe in the near term, the ACEA, representing the original equipment manufacturers, will continue to use the existing CCMC specifications, namely G4, G-5 for gasoline engine lubricants, and D-4, D-5, PD-2 for diesel engine lubricants. Future requirements for Europe will likely be based on the need for improved fuel economy driven by a proposed European carbon dioxide (CO2) tax. New engine bench tests are being developed to measure the fuel economy benefits of lubricants. Environmental issues including oil recycling and potential chemical restrictions will also play a part in defining lubricant requirements for Europe. In Europe, a new requirement is the continuing demand to improve lubricant quality in the heavy duty diesel market. A notable example is the defined need of Mercedes-Benz to have lubricants of higher quality than the current sheet 228.3 specification. Such a lubricant will require improved piston cleanliness, cylinder wear and bore polishing characteristics together with improved soot handling. Using the OM 364A tests limits, it is evident how the wear, cleanliness and bore polishing requirements have become significantly more severe with the higher level of specifications as they have emerged. Mercedes-Benz also requires well documented field tests in their new range of engines.

Lubrication Chemistry

63

Japanese vehicle manufacturers. In general, they have relied on the API classification system to recommend engine oils for service-fill applications. There is a parallel with Europe (CEC) in the Japanese Automobile Standards Organization JASO; they have developed four engine test procedures without giving pass/fail limits. The parallel is not complete because neither within nor outside Japan do the vehicle manufacturers require any of these tests to be run to meet their own specifications. Transmission lubricants. A relationship between GM and Ford classifications is presented in Table 2, Appendix. An automatic transmission requires smooth shifting from gear to gear. Both detergents and dispersants play significant roles in automatic transmission fluid ATF in additive packages. The major role of sulfonates in ATFs is to stabilize friction characteristics. Dispersants are used in ATFs to suspend thermal decomposition and oxidation products. The following automatic transmission fluid classification is obsolete: GM Type A, GM Type ASuffix A, and GM Dexron. Automatic transmission fluids such as Dexron IV, Mercon IV, and DemlerChrysler MS 9602 are currently used for service. In Europe, improved thermal stability suitable to meet the requirements of the new transmission designs is a key feature, together with improved low temperature fluidity and gear shift ability. The OEMs continue to seek fluids with extended drain or fill-for-life capability. Gear lubricants. The 1926 saw the development of the first commercial automotive hypoid gears; however, it was until 1942 that the first federal specification was issued to define multi-purpose gear oils based on performance testing. Since that time, performance categories API GL-4 and GL-5 have been defined by US military specifications. The 1957 API GL-4 classification designates service in hypoid and other automotive gears operated under highspeed, low-torque or low-speed, high-torque conditions. In 1962, API GL-5 classification was issued for automotive equipment operated under high-speed shock load and more severe service conditions. API GL-6 is now an obsolete classification designed for specific high off-set hypoid gears in high speed performance conditions. API has established automotive gear oil service or performance designations for automotive manual transmission and differentials. Key ingredients in gear lubricants are extreme pressure EP agents which protect gear teeth surfaces and provide extended gear life. Dispersant are used to provide cleanliness. They must be compatible with the EP agents. Detergents have minimal use in gear lubricants because they are liable to be incompatible with EP agents. The chronology of development of gear oils is illustrated in Table 2, Appendix. The general description of this classification is as follows: GL-1 is specified for warm gear axles and some manual transmissions under mild

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service. It usually contains rust and oxidation inhibitors, defoamants and pour point depressants; GL-2 is specified for warm gear service more severe than GL-1; GL-3 is specified for moderate to severe service; GL-4 is specified for hypoid gears under severe service; This classification is now obsolete. The package contained a zinc additive combination; GF-5 is specified for hypoid gears under shock load and severe service; GF-6 is obsolete, and was never adopted by API. In the automotive gear oil market, two new North American classifications are being developed by ASTM, namely PG-1 andPG-2; PG-1 covers the development of a category for manual transmissions used in trucks and buses. The key performance needs are: thermal stability, seal compatibility, copper corrosion, anti-wear, high temperature lubricant stability and foaming. PG-1 is known as GL-7; PG-2 is the development of a category intended for final drive axles with performance beyond GL-5. In this case, the performance needs are: thermal stability, seal compatibility, surface fatigue, copper corrosion, and maintaining GL-5 tests. PG-2 is now known as GL-8. After 1994, most automotive gear oils have been rated as GL-5, GL-7, and GL-8, all in one product.

Problems 2.1 Benefit of good lubrication In 1966, the first comprehensive study of how friction, lubrication and wear affected a country was funded by the British Ministry of State for Education and Sciences. A remarkable set of figures was reported to the British Government by the committee headed by H. Peter Jost (the Jost Report). The Jost Report estimates of the effect of improved tribology are as follows: (a) reduction in energy consumption through lower friction by 7.5%, (b) savings in lubricant cost of 20%, (c) savings in maintenance repair and replacement costs of 20%) and (d) savings in investment through increased life of 5%. Based on potential savings estimates for improved tribological processes, what are your potential savings? 2. 2 Functions of lubricant What is the distinctive role of engine oil in each of the following: (a) friction, (b) durability, ( c) wear, (d) corrosion, (e) surface, (f) heat? 2. 3 Oil viscosity index classification Using Table 1 (Appendix) and Chapter 2.5 as references, write viscosity index values for winter and summer engine oils.

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What are the possible values of viscosity index for: (a) w^inter (W) (centipoise, cP) engine oils: SAE 0, SAE 5W, SAE lOW, SAE 15W, SAE 20W, SAE 25W, (b) viscosity grades (centistoke, cSt) engine oils without W such as: SAE 20, SAE 30, SAE 40, SAE 50, SAE 60, (c) multigrade oil SAE lOW-20? 2.4 Engine oil temperature Determine whether cold starts or high temperature operations will wear engine surface most. Compare and contrast vegetable, mineral and synthetic lubricants. 2.5 Viscosity classification In the description of SAE grade oils, such as SAE 5 W, does the W designate a viscosity that applies to warm or winter temperatures? Which multigrade engine oil designation is incorrect: SAE 5W-20, SAE lOW-50, orSAE5W-10W-30? 2.6 Diesel and gasoline engine oil What is the first letter of API services classification for a diesel and gasoline engine oil: A, B, C, D or S? Which of the following is the most current and updated gasoline and diesel classification: SF, SG or SH and CD II, CE or CF-4? Why are gasoline oil ratings such as SA, SB, SC, SD and SE obsolete? 2.7 Sulfur content The diesel fuel sulfur content might be very high ( > 1.5 %) and the ability of the engine oil to handle the acidity conditions might be exceeded. Thus, the bearing material will appear to be etched. In the setting of high sulfur fuel content, high TBN engine oil must be used and replaced more frequently. What is the current trend in petroleum industry? 2.8 Viscosity index (VI) improver (a) Demonstrate that the viscosity index as plotted in Figure 2.6 illustrates the viscosity index improver requirement, (b) Show that multiplication of polymeric viscosity index improver makes it possible to utilize it in several SAE viscosity grades, e.g., SAE 10, SAE 20, SAE 30, SAE 40, SAE 50. 2.9 Synthetic oils By using Table 2.7, "Performance of synthetic and mineral lubricant oils", consider the benefits of using synthetic engine oils in several main aspects of (a) engine wear protection, (b) improved fuel and oil economy and (c) environmental protection. Which of the synthetic oils: phosphate esters or alkylated aromatics (PAO) are the most common synthetic fluids used in automotive motor oils today? 2.10 Oxidation of mineral oils By using data in Figure 2.7, discuss the oxidation tendency of mineral oils as a function of base oil, degree of refining and the addition of metals and additives.

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2.11 Additives In the Table 2.4 "Commonly used additives", lubricant additives have been separated into three main functional groups: (a) surface protective additives, (b) performance additives and (c) lubricant protective additives. List those lubricant additives that exhibit more than one of the listed functions.

67

Chapter 3

MICELLAR STRUCTURE OF LUBRICATING FORMULATIONS Many, if not most additives are present in oils not as individual molecules in solution, but as single or multi-component aggregates (inverse micelles), ordered structures or chemical complexes. Pierre A. Willermet, 1998

Soft acids prefer to associate with soft bases, and hard acids prefer soft bases, and hard acids prefer to associate with hard bases. Ralph G. Pearson, 1966

3.1. Reverse Micelles in Tribochemical Processes Surface-active agents, or surfactants, all share interesting physicochemical characteristics at surfaces and interfaces. Surfactants (detergents and dispersants) are long chain hydrocarbons with polar headgroups which are called dipoles. Surfactants are molecules which consist of two well defined parts: one which is oil-soluble hydrophobic and another which is water-soluble hydrophilic. The hydrophobic part is non-polar and usually consists of aliphatic or aromatic hydrocarbons. The hydrophilic part is polar and interacts strongly with water.

Normal micelle (M) in water and polar solvents

Soft-core reverse (RM) micelle in hydrocarbon formulation

Hard-core reverse (RM) micelle in hydrocarbon formulation

Hard-core reverse (RM) micelle in hydrocarbon formulation

Fig. 3.1. Schematic representation of normal micelle (M) in water, a soft-core reverse micelle (RM) and hard-core reverse micelles (RM) in hydrocarbon formulation, (\AAAO ) detergent molecule Surfactants fulfil many functions, such as detergency, micelle stabilization, interfacial tension reduction, wetting, and so on. In hydrocarbons, however, surfactants are not capable of lowering the surface tension, because these solvents

68

Chapter 3

have a low surface tension themselves. Instead, in hydrocarbon solutions, surfactans form soft-core reverse micelles (RMs). Fundamental studies of surfactants in hydrocarbons have been carried out, mainly from the point of view of solubility, micelle formation and solubilization (Ekwall, 1969; Kertes and Gutman, 1976; Kon-no, 1993; Pileni, 1989a). Surfactants are classified on the basis of the charge carried by the polar headgroup as anionic, cationic, nonionic, and amphoteric. Surfactant headgroups are dipoles, especially ionic ones that exist as ion pairs in hydrocarbon solvents. Electrostatic dipole-dipole attraction between headgroups in hydrocarbon solvents is the driving force for the formation of reverse micelles, or micellar aggregates, see Fig. 3.1 and Fig. 3.2. RMs with a dense hydrated polar soft-core have been named "soft-core reverse (or inverse) micelles". When a colloidal core (CaC03)j, or (CuO)„ is surrounded by surfactant molecules, such aggregates are called "hard-core reverse (or inverse) micelles". The polar head-groups are joined together to form the hydrophilic softcore of these aggregates, and the hydrophobic tails are extended into the hydrocarbon solvent. Micelle formation in hydrocarbon solvents appears to be due principally to electrostatic dipole-dipole interaction of the polar head-groups, especially in the presence of water molecules, and probably due to either ionic attraction or specific coordination bonding between atoms (Chevalier and Zemb, 1990; Eicke, 1980; Ekwall et al., 1972; Kertes and Gutman, 1976; Kon-no, 1993; Luisi and Straub, 1984; Pileni, 1989a). The surfactant concentration range above which micelles are formed is called the critical micelle concentration (CMC). Studies have clearly established that most surfactants aggregate above the critical micelle concentration (CMC) to form normal micelles in polar solvents such as water, while they aggregate to form reverse micelles in hydrocarbon solvents. The critical micelle concentration for detergents, similar in type to those used in lubricating oil formulations, are extremely low, ranging between 10"^ to 10"^ M. ' H,Ov • t o ® H2O ®< Above the critical micelle ^ H,o ^ ^ Hp; concentration, the additional detergent appears almost exclusively in a micellar form, and the concentration of molecularly dissolved detergent increases only very slightly. The saturation of the Fig. 3.2. A soft-core reverse micelle solution provides maximum with cationic surfactant head groups and adsorption potential and the a branched chain (Mclntire, 1990) preponderance of micelles ensures

Micellar Structure of Lubricating Formulations

69

the immediate availability of the adsorbate. A number of experimental techniques by measurements of physical properties (interfacial tension, surface tension, osmotic pressure, conductivity, density change) applicable in aqueous systems suffer frequently from insufficient sensitivity at low CMC values in hydrocarbon solvents. Some surfactants in hydrocarbon solvents do not give an identifiable CMC; the conventional properties of the hydrocarbon solvent solutions of surfactant compounds can be interpreted as a continuous aggregation from which the apparent aggregation number can be calculated. Other, quite successful, techniques (light scattering, solubilization, fluorescence indicator) were applied to a number of CMCs, e.g., alkylammonium salts, carboxylates, sulfonates and sodium bis(2-ethylhexyl)succinate (AOT) in hydrocarbon solvents, see Table 3.1 (Eicke, 1980; Kertes, 1977; Kertes and Gutman, 1976; Luisi and Straub, 1984; Preston, 1948). The thermodynamic equilibria of surfactant molecules in hydrocarbon solutions involve four fundamental processes: dissolution, micellization, solubilization and interfacial processes, see Fig. 3.3 (Kertes and Gutman, 1976; Kon-no, 1993 ; Moroi, 1992). gaseous phase or liquid phase

i j^y-^ \ \

solid phase rZ/~0

^"^^"-^O a a' , ,.

micellar solution phase

Fig. 3.3. Four fundamental processes for thermodynamic equilibria of surfactant molecules in hydrocarbons: (1) dissolution of molecules into solution, (2) micellization (or aggregation) of dissolved molecules, (3) adsorption (solubilization) of molecules at an interface, and (4) interfacial processes of surfactant molecules (oW\A = surfactant molecule), (Moroi, 1992)

70

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The dissolution process. In hydrocarbon solvents, ionic surfactants form undissociated ion-pairs in solution, which have been confirmed by conductivity and NMR measurements. The dissolution seems to proceed in two stages as the system is warmed: swelling of the surfactant lattice by solvent penetration and then, at a critical solution temperature, as the forces holding the surfactant lattice together are overcome, a swelling without limit or a disaggregation. The importance of the solubility parameter for the swelling, dissolution and size of the formed aggregates has been demonstrated for many surfactants (Fowkes, 1967; Kertes and Gutman, 1976; Little, 1966). The micellization equlibria. Three main factors play a critical role in the formation of reverse micelles: (a) the interaction between the polar groups; (b) the interaction of the hydrophobic (nonpolar) parts, and (c) environmental factors. Water that is bound to the surfactant aggregates plays a role in interactions with the polar groups. It was shown that micelles can bind considerably more water than is accounted for by hydration of the alkali metal ions. This was taken as evidence that the carboxylate or sulfonate groups bind water through ion-dipole interaction and/or hydrogen bonds. Fatty carboxylic acid and carboxylate groups are linked by hydrogen bonds (AHA", A(HA)2"), A(HA)j^). These effects have been demonstrated in many other surfactant systems (Ekwall, 1969; Ekwall and Mandell, 1967; Ekwall and Solyom, 1967; Ekwall et al., 1972).

Table 3.1. Reverse micelle formation in hydrocarbon solvents Surfactant

Solvent

Sodium bis(2-ethylhexyl)sulfosuccinate Sodium bis(2-ethylhexyl)carboxysuccinate Sodium bis(2-ethylhexyl)sulfosuccinate Sodium bis-butylsulfosuccinate Sodium bis-dodecylsulfosuccinate Hexanolamine oleate Dodecylamine propionate Calcium stearate Alkylammonium benzoate

/?-Heptane ^-Octane Cyclohexane Benzene Benzene Benzene Cyclohexane Benzene Cyclohexane

Number of molecules in the micelle 13 6 12 9-25 4-17 3 10 22 9

The aggregation number of metallic detergents varies with length and structure of the hydrocarbon groups, the type of the cation, and the polarity of the solvent. The aggregation number of sodium, magnesium, calcium and barium dinonylnaphthalene sulfonates in hydrocarbon solvents lies between 10 and 15 and is twice that of nonylphenates (Fowkes, 1962; Heiweil, 1964; Inoue and Watanabe,

Micellar Structure of Lubricating Formulations

71

1965 and 1983; Kaufman and Singleterry, 1955 and 1957; Peri, 1958; Reemik, 1965). The stronger intermolecular interactions between the polar groups of the salicylate promote their aggregation in low polar solvents and show that RMs of salicylate have a very characteristic structure with aggregation numbers of 15 to 30 (Vipper and Watanabe, 1981). The micelles formed in the non-polar solvents would seem in general to be relatively small, the aggregation number being of the order of 4 to 30. When the micelles are nearly spherical, as they seem to be at low concentrations, the low aggregation numbers are due to steric factors. The space in a spherical soft-core micelle permits only the accommodation of a limited number of polar groups; this also applies to the number of bulky hydrocarbon groups in the outer parts of the micelle (Eicke, 1980; Ekwall, 1972; Kertes and Gutman, 1976; Rounds, 1976). Ashless dispersants such as sulfosuccinates do not form rigid micelles like the metallic detergents. In the associated state, hydrogen bonding occurs between the amino groups and is affected by the size of the respective alkenyl groups. The aggregation number of mono-succinimides (m-PIBS) is more than twice that of the bis-succinimides (b-PEBS) due to the steric effects of the weak alkenyl groups of the surfactant in the micelle structures thus formed (Fontana, 1968; Forbes and Neustadter, 1972; Inoue and Watanabe, 1983; Vipper et al., 1968; Watanabe, 1971). The reason for the use of the bis-derivatives of the succinimides or benzylamines is to increase the number of the alkyl groups per molecule in the dispersant, giving better dispersancy per number of nitrogen atoms present. In addition, the hydrogens of the amino groups are more masked for the bisderivative and thus not so involved in undesirable reactions (Watson, 1975). The ashless dispersants are moderately associated, forming small micelles with n < 10, in hydrocarbon solvents. The relationship between the aggregation and molecular structure of surfactants has been examined. A factor preventing the growth of the aggregate in low polarity solvents is steric interaction between the hydrophobic groups. The aggregation number in benzene solution of straight-chain dialkyl sodium sulfosuccinates (from dibutyl- to didodecyl-compounds) decreases rapidly (from 15 to 7) with an increasing total number of carbon atoms of the alkyl chains, until this exceeds 16, but slowly thereafter. A homologue with straight chains has a higher aggregation number than one with branched chains. Similar results were obtained with alkylammonium halides in various non-polar solvents (Kertes and Gutman, 1976; Kon-no, 1993). In the case of ionic surfactants, a reduction in aggregation number will result when the counter-ion radius is increased (Kon-no and Kitahara, 1971a). The change in the size of aggregates as a function of the structure of ionic surfactants was studied (Eicke, 1980; Kon-no and Kitahara, 1971a; Kon-no et al., 1983; Matijevic, 1993; Ward and C. du Reau, 1993). The aggregation numbers of anionic surfactants (sodium bis(2-ethylhexyl)sulfosuccinate and sodium 1,3-(2-

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Chapter 3

ethylhexyl)-2-propyl-sulfonate, (-carboxylate and -sulfate) for different polar groups follow the order: sulfonates >

carboxylates > sulfates

in various apolar solvents (benzene, cyclohexane, n-heptane, n-octane, and tetrachloromethane, see Table 3.1 (Inoue and Watanabe, 1983; Ko-no and Kitahara, 1971a; Kertes and Gutman, 1976). The polar group of surfactants is one of the essential factors determining the size of soft-core reverse micelles. In general, reverse micelles have moderate aggregation numbers: carboxylates in toluene (5 to 20), calcium cetylphosphate plus calcium alkylphenolate in lube oil (20 to 30), tetra-n-alkyl-ammonium salts in benzene (3 to 25), and sodium bis(2-ethylhexyl)sulfosuccinate in dodecane (32), (Bascom et al., 1959). Let us consider a simple association equilibrium between surfactant monomers S and the micelles M of aggregation number n nS

^ M^

The micelle formation constant K^^. is therefore written as K„,c = [ M „ ] / [ S r The determination of the various n and K^^ values can be done either graphically or numerically ((Eicke, 1980; Moroi, 1992; Kertes and Gutmann, 1976). At equilibrium, the total free energy must reach minimum. From the definition of a minimum, it follows that AG ^^^ = 0. The total free energy change AG ^^^ is written AG^i,= A G ° _ + RTlnK^,, and since at equilibrium, AG ^^j^ "^ 0, AG°„,, = -RTlnK^, Thermodynamic measurements in dilute hydrocarbon surfactant solutions give negative values for the enthalpy change on micellization (Aff^^J, and the entropy change (AS^'^J in the negative range (see Chapter 1, Table 1.1 for comparison). The free-energy change (AG°^J comprises enthalpy and entropy contribution according to the equation AG"^, = AH°^, - TAS°^, and are in negative range which can indicate self-association process. The

Micellar Structure of Lubricating Formulations

73

thermodynamic functions for micellization of some surfactants are presented in Chapter 1, Table 1.1. The solubilization. Let us assume that complex MP forms by the interaction of reverse micelles M with a polar substance P; (M + P ^ MP) in which the association constant K is expressed by K = [MP] / [M] [P]. The determination of the association constant K values can be done either graphically or numerically (Kertes and Gutmann, 1976). The K values can be used to estimate thermodynamic quantities such as AG°, Aff and AS°. The interactions of polar substances, such as methanol, ethanol and acetone, with Aerosol OT in toluene systems were examined (Kon-no, 1993). The negative AG° value was found in the decreased order methanol > ethanol > acetone and the contribution to AG° was |AH°| > |TAS°|. This result indicates that the solubilization of three solutes with Aerosol OT micelle is an enthalpy driven process. The solubilization phenomenon in hydrocarbon surfactant solutions is defined as a lowering of the activity of any solubilizates. The location of solubilizates in micelles can be investigated using probe molecules which indicate the surrounding conditions. A solute (additive) can be located in reverse micelles in different solubilization sites in the water core, in the interfacial region or in the bulk solvent. Solubilization into the water cores increases the inner volume at constant interfacial area, resulting in radial growth. If the micelle is too small to receive a solute molecule without deformation, e.g., at low water content, a segregation occurs between small free molecules and the large objects which are covered with surfactant (Chatenay et al., 1987; Encinas and Lissi, 1986; Pileni et al., 1985). The accepted rules for solubilizate position and the factors influencing solubilization are derived from many observations (Mukerjee, 1979; Sepulveda, 1974): (a) compounds such as acids, amines and alcohols with polar groups are located in the micellar soft-core, and nonpolar hydrocarbon groups at the micellar surface; (b) nonpolar aliphatic hydrocarbons and aromatic hydrocarbons are located in the outer micellar surface. The total solubilizate concentration is a function of the stepwise association constant, K, between a solubilizate and a monomer concentration solubilizate. It is very useful to consider the factors influencing solubilization. These values determine the general behavior of solubilization as follows: (a) as long as the value of association constant, K, and monomer concentration in solubilizate remain constant, the total solubilizate concentration increases linearly above the CMC (Imae et al., 1986; Prapaitrakul and King, 1985); (b) the total solubilizate concentration increases with alkyl chain length for a series of homologous surfactants. This effect can be attributed to the decrease in CMC, increase in K, and the closer packing of surfactant molecules at the micellar surface (Abu-Hamdiyyah and Rahman, 1987; Moroi and Matuura, 1988,

74

Chapter 3

Prapaitrakul and King, 1985); (c) the association constant K usually decreases with temperature but the total concentration of solubilizates increases with temperature owing to an increase in monomeric solubilizate concentration (Birdi et al., 1979; Moroi and Matuura, 1983). Interfacial processes ofsurfactants. The area of mixing between water and hydrocarbon or between surfactant headgroups and solvent is called the interface. The interface is therefore defined as a region where neither water nor hydrocarbon may be considered free of the other component. According to the definition of the interface, each type of hydrophilic group has a well defined hydration number. These numbers are usually obtained by comparing the micelle hydrodynamic volume with the dry volume. It should be noted that interfacial area depends strongly on its localization in a system with high "surfactant parameter" (with vl^X ^^^ different from one). Surfactant parameter (v/a^lj is calculated on purely geometrical grounds since v is the surfactant molecular volume, a^ the surface area per polar head, and 1^ the length of a linear alkyl chain of n carbon atoms (Chevalier and Zemb, 1990; Tanford, 1972). V = 27.4 + 0.0269 n (nm'); and

1,= 1.5 + 0.1265 n (nm)

The surface area per polar head is mainly determined by head interaction and is itself the sum of van der Waals attraction and electrostatic repulsion and can be estimated as follows: a, = (e'D/2eT)^'' With D = 0.5 nm, e ~ 40, i = 50mN/m, a^ ~ 0.6 nm^, where e is the charge per surfactant molecule, T the oil-water surface tension, D the thickness of the double layer, and e the local dielectric constant. Surfactant parameter for AOT: if v ~ 0.6 nm^, 1^, ~ 0.8 nm, a^ ~ 0.6 nm^, this gives v/aj^, > 1. This indicates that reverse micelles will be formed and that the water swelling ability is as large as observed (Langevin, 1989). It is known that when a short-chain oil penetrates easily into surfactant layers, v increases. When the surfactant chains are loosely packed, the chain length 1^ is shorter because of chain folding. The area per polar head a^ is only slightly affected by the presence oil or alcohol penetration. The ratio of the surface area per headgroup (a^,, nm^) to the average section of the hydrophobic part (v/lj defines the spontaneous curvature R^p of the interface. The surfactant parameter (v/a^l^) value is the fimdamental geometric quantity for structural descriptions. In case of sodium dodecylsulfate (SDS), the surface area per headgroup at a micellar interface is 0.6 nm^/molecule A value (v/a^l^,) higher than one indicates the spontaneous bending of the interface towards oil, while a lower value means spontaneous bending towards water (direct micelles). The

Micellar Structure of Lubricating Formulations

75

"surfactant packing parameter" gives a good idea of the shape of the aggregates which will form spontaneously as presented below (Langevin, 1989). In water (normal micelles)

Shape of the micelle

In hydrocarbons RMs

v/aol^ < 1/3 1/3
spherical micelles rod-like micelles lamellar phases

v/a^lc > 3 < v/aJe < 3 2 < v/aj. < 2 1/2

When both oil (o) and water (w) are present, o/w microemulsions will be formed when v/a^lc < 1; w/o microemulsions when v/a^lc > 1, and lamellar phases when v/aJ,--1 (IsraelachviHetal., 1980; Mitchell and Ninham, 1981). The v/a^l, ratio depends on the surfactant chemical structure (1^ and v) and on surface repulsions between headgroups (a^), (Mitchell and Ninham, 1981). When repulsions increase, the surfactant parameter {yl?iX) increases and micelles get smaller. As a consequence, size and CMC are related: surfactants with low CMC aggregate into large molecules, while the higher the CMC, the smaller the micelles. Physicochemical microenvironment of soft-core RMs. The dipole moment and polarizability interactions between the polar headgroups of the surfactant molecules affect the size of the aggregate in solution. A low dipole moment of the higher aggregate will limit the growth of the aggregate in solution. The aggregation can be attributed to the ion pairs or other polar groups, hydrogen bonds between them, and, in some cases, also to coordination of end groups around the central ion (Ekwall et al., 1972; Kertes, 1977; Kertes and Gutman, 1976). The cohesive forces in soft-core micelles are covalent or of a purely electrostatic nature. The presence of a polar head (either charged or uncharged) in the surfactant molecule is subject to the different forces of water molecules which hydrate the polar head and molecules of less structured bulk water. As has been shown, an increase in temperature seems to produce exceptionally high water content in the hydrocarbon solution of several surfactants. The temperature at which increased water solubilization is affected by the presence of electrolytes, and it is the cations that are active in the case of anionic surfactants, and the anions in the case of cationic surfactants (Kon-no and Kitahara, 1970 and 1971b). Soft-core reverse micelles are spherical or ellipsoidal aggregates consisting of a water core separated from a continuous apolar phase by a surfactant shell. It is well known that in the absence of water, some surfactants such as sodium bis(2ethylhexyl) sulfosuccinate (AOT) can form dry aggregates, while others such as sodium dodecyl sulfate (SDS) or hexadecyl-trimethylamonium bromide (CTAB) need a cosurfactant, e.g., a short chain alcohol, to form micelles.

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Much interest has been focused on solubilizing various amounts of water in reverse micelles. The micellar solutions can solubilize considerable amounts of water; this is bound to the polar groups of the surfactant molecules by ion-dipole or dipole-dipole attraction. The properties of water solubilized by RMs are different from those of bulk water and are sensitive to the water pool parameter, Wo = [H20]/[Surfactant]. Assuming the water molecules in the oil droplets are spherical, the radius of the sphere is expressed as (Luisi et al, 1988): r^(water pool radius) = Oymo^^smf)-^^ where the volume of the water molecule (v) is usually taken to be approximately 0.03 nm^ and (asu^f) has a value approximately 0.6 nm^ at high water content. There is an empirical relationship between the parameter, Wo, and the water pool radius, r^ (ranging up to 20 nm ) which can be expressed as (Flecher et al., 1987;Garcia-Carmana, 1992; Luisi etal., 1988): r^(water pool radius)/nm = 0.175 Wo The maximum amount of the structured water in the micelles corresponds to Wo of about 10 (Stenoius, 1984). Above this amount, part of the water is "free"or unstructured. When water in the system exceeds the hydration requirements, softcore reverse micelles will consist of three different microenvironments: surfactant apolar tails, bound water with polar head of surfactant, and free water. Free water properties and structure become closer to those of bulk water as Wo increases. At low values Wo < 10, the properties of solubilized water such as density, viscosity, mobility and hydrogen bonding deviate greatly from those of bulk water. The size and shape of the RMs are critically dependent on the number of water molecules available per polar head of the surfactant. Water is highly immobilized in the micellar interior at low Wo < 10 and the mobility increases with increasing Wo, gradually approaching that of bulk water. Properties of water in the water pool in soft-core RMs shows change of the activity of water (aH+) in AOT hydrocarbons as a function of the water pool parameter Wo in hexadecane, dodecane and octane (Higuchi and Misra, 1962). Wo equals 25, thus categorizing the pools as medium sized; Wo = 2 to 10 for small pools, and 30 to 50 for large pools. In small micelles, the hydration of the polar heads and counter ions is not complete so that the entire water present is highly bounded and oriented in solvation shells (sulfonate and sodium ions). Measurements of water activity vs. micelle parameter Wo display a hyperbolic shape reaching a value higher than 0.98 at Wo = 10 (Higuchi and Misra, 1962; Zulauf and Eicke, 1979)»

Micellar Structure of Lubricating Formulations

11

3.2. Micellar Solubilization in Lubrication Solubilization of water, Detergency is defined as the ability of surfactant molecules to solubilize water molecules or polar substances in soft-core and hardcore RMs. Thus, micellization and solubilization are competitive processes. Any solubilized probe molecule causes a decrease in the CMC. Solubilization describes the dissolution of a solid, liquid or gas by an interaction with surfactant molecules. Addition of water has a dramatic effect on surfactant aggregation in hydrocarbons because hydrogen bonding has an appreciable stabilizing effect on reverse micelles. Solubilization for reverse micelles is phenomenologically similar to the adsorption processes (Eicke and Christen, 1978; Kitahara, 1980; Kitahara et al., 1976; Singleterry, 1955). In engine oils, detergent-dispersant additives solubilize or deactivate water, organic acids and sludge precursors produced by combustion of the fuel or oxidation of the base oil. Solubilization of water by micelles of calcium dinonylnaphthalenesulfonate CaDNNS in benzene solution did not change with temperature in the range 10°C to 90''C. Water is solubilized by the strong iondipole interactions with the cation in DNNS micelles at the initial stage of solubilization. Further solubilization proceeds by hydrogen bonding with the water near the cation and has a property similar to bulk water at this stage. The quantity of water relate directly to the aggregation number of the micelle, and calcium DNNS solubilized 16 moles water per mol calcium DNNS (Inoue and Nose, 1987; Little and Singleterry, 1964). Investigation by IR and NMR spectra show that all of the water molecules in inverse micelles are hydrogen bonded by sulfonate additives. The amount of water necessary to form a minimum number of hydrogen bridges which would be sufficient to support aggregate growth and also to solubilize the final particles is far below the detectable amount of water in organic solvents. The cooperative hydrogen-bond formation mechanism leads simultaneously to closed micellar structures (Eicke, 1980; Fowkes, 1967). All experimental observations clearly show that the structure of water within reverse micelles is significantly different than that of bulk water. This is not surprising if one considers the high charge density in the water caused by the ionized surfactant heads. The structure of water in AOT reverse micelles demonstrated by IR, NMR and ESR spectra indicates that local polarity and viscosity of the micellar water pools were different from that of bulk water. Moreover, the local polarity and viscosity depends on the water content of the reverse micelles (Smith and Luisi, 1980). The size and characteristics of soft-core reverse micelles are critically dependent upon the water content of the solution. The water present tends to accumulate with the soft-core to form an isolated pool which may exhibit unique properties. At a low ratio of water to surfactant (Wo < 10) the activity of the water is greatly diminished as compared with that of bulk water. The quantity of water

78

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solubilized by polyisobutyleneamine succinimides is different than that for sulfonates (or their mixtures) in hydrocarbons. Water molecules interact strongly with the cation of the sulfonate through strong ion-dipole forces at the initial stage of the solubilization, followed by further solubilization through hydrogen bonding with the initial water. The process of water solubilization in a reverse micellar system shows that water is solubilized by hydrogen bond formation with succinimides and the amount of water solubilized is less than that needed for sulfonates. It is argued that the micellar cavity formed by succinimides in hydrocarbon solvents is smaller than that formed by sulfonates. When mixed micelles of sulfonate and succinimide are formed, the amount of water solubilized is less than the sum of the water that would have been solubilized by individual additives acting separately. Overall, the amount of water solubilized by the mixture decreases with an increase in the concentration of succinimide. The interaction between the polar groups of both additives reduces their overall ability to interact with water (Inoue and Watanabe, 1981; Watanabe, 1971). Micellar solubilization of water by surfactants having different polar groups follows the order: Succinimide < mixed (sulfonate + succinimide) < sulfonate For small amounts of solubilized water, as a polar additive, the stability of the micelle is markedly increased, as shown by a decrease in the CMC, On the other hand, large amounts of water as a polar additive decrease the stability of the micelle. It is known that a solution of AOT in iso-octane solubilized up to 50 moles of water per mole of surfactant. As the concentration of water increases, the isotropic reverse micellar solution changes to a water-in-oil microemulsion. A clear understanding of the complex analyte-micelle-water pool interactions, especially analyte concentration and pH at the head group interfacial region, is under intensive study (Cline Love and al., 1984; Little and Singleterry, 1964; Luisi and Straub, 1984; Mclntire, 1990). The characteristics of the water pool of reverse micelles has been explored by ^H, ^^Na, ^^C, ^^P-NMR spectroscopy. Since the initial association process in RMs is not totally understood, and because of the low CMC, aggregation studies from NMR are rather scarce. Direct determination of a CMC in the diethyl hexyl phosphate /water/benzene system (at Wo = 3.5) was possible because the chemical shift of ^^P in phosphate groups is very sensitive to hydration effects. The structure and state of water in RMs and particularly at low water content has received considerable attention. The proton chemical shifts have been explored in AOT/water/heptane, methanol, chloroform, isooctane and cyclohexanone. The water behavior in small reverse micelles is close to that of the corresponding bulk ionic solution. Until now, the effect of a solute on micellar structure was not well

Micellar Structure of Lubricating Formulations

79

understood, and driving forces were unknown. There is a lack of theory to predict solubilization of a given compound in RMs, despite the growing number of observations from NMR spectroscopy (James, 1975; Llor and Zemb, 1989). It is generally accepted that the soft-core RMs contain amounts of water equal to or less than hydration of water of the polar part of the surfactant molecules, whereas in microemulsions the water properties are close to those of the bulk water (Fendler, 1984). At relatively small water to surfactant ratios (Wo < 5), all water molecules are tightly bound to the surfactant headgroups at the soft-core reverse micelles. These water molecules have high viscosities, low mobilities, polarities which are similar to hydrocarbons, and altered pHs. The solubilization properties of these two systems should clearly be different (El Seoud, 1984). The advantage of the RMs is their thermodynamic stability and the very small scale of the microstructure: 1 to 20 nm. The radii of the emulsion droplets are typically 100 nm (Fendler, 1984; El Seoud, 1984). The water in the RMs is considered to be a composite of two different types: the "bound water" region, and the remaining "free water" region. On the basis of the IR data up to a Wo = 4, the water solvates the AOT ion-pair, further increasing in the water concentration up to a Wo =10, probably giving rise to a hydration shell around the new-separated ions of AOT. Further increasing water concentration gives rise to the so called "free water". It has been shown by various physico-chemical techniques that the water of the reverse micelle behaves differently from normal water, especially at low concentrations (Wo < 10). Solubilization of water by such micelles promotes dissociation of ion pairs in the micelle to form micellar free ions. Water solubilization also allows solubilization of inorganic salts not otherwise taken up by the micelle. The water solubilized by the soft-core reverse micelles of the oil soluble surfactant can subsequently solubilize inorganic salts which were originally oil-insoluble. This phenomenon is known as "secondary solubilization". This is defined as the solubilization of a material which the micelle can take up only when another solubilizate such as water is already present (Aebi and Wiebush, 1959; Arkin and Singleterry, 1949; Baker et al., 1954; Fulton et al., 1953; Inoue and Nose, 1987; Kon-no and Kitahara, 1972; Mathews and Hirschhom, 1953). The first water entering the soft-core is coordinated as water of hydration, but as more is introduced, the hydrated cations begin to dissociate giving in effect a highly concentrated solution. The first portion of water is solubilized strongly by sodium dioctylsulfosuccinate in a hydrocarbon medium. The water is held so strongly by magnesium dinonylnaphthalene sulfate that it reduces the amount of acetic acid in the micelles from benzene solution. The concept of the hydrated micelle core as a concentrated solution is most helpful when considering the phenomenon of solubilization. The effect of high concentrations of water on RMs in oil is thus seen to depend on the properties of both the anion and cation. The

Chapter 3

80

solubilization of water is encountered with benzene solution of barium phenylstearate, barium (2-ethylhexyl) sebacate, and many of the dinonylnaphthalene sulfonates. These detergents take up 3 to 8 moles of water and become saturated, so that they can exist in equilibrium with a drop of pure water. It is apparent that the role of water in RMs is complex and relatively unexplored. In lubricating oil detergency, the solubilization of water may be significant in water transport mechanisms by acting as a reservoir or cushion against temporary surges of water condensation in an engine crankcase, and because it modifies the ability of the micelles to solubilize other polar substances (Honig and Singleterry, 1954 and 1956; Kaufman and Singleterry, 1957 and 1958; Mathews and Hirschhom, 1953).

Table 3.2. The physical properties of water in the water pool (WP) in reverse micelles of sodium bis-2-ethylhexyl sulfosuccinate (Aerosol OT or AOT) in isooctane. The spectroscopic properties of the hydrated electrons (e\q) in the micellar water pool (Wo) Wo=[H20]/[AOT]

Band maximum (nm)

Spectral changes

WP changes and H2O status

Without AOT additive Water

720

Spectrum of Q\^

WP is absent

5M NaC104

650

Blue-shift spectra

WP is absent

60

710

Spectrum of e'aq

Bulk water status

20

710

Spectrum of e'aq

Bulk water status

15

710

Spectrum shifted, broader

Free water present

10

690

Spectrum shifted, broader

Free water present

6

650

Spectrum shifted, broader, less intense; Lifetime of e" ^ decreased

Bounded water only, slower motion

<5

Absent

The water in the pool is notable to hydrate electrons

Tetrahedral structure not observed, water is bound, low mobilities, high viscosities

In the presence of AOT

aq

Hydrated electron probe inverse micelles. Hydrated electrons (e'^q) are expected to be a very good probe to test the water pool of reverse micelles. The physical properties of hydrated electrons obtained by pulse radiolysis in AOT reverse micelles were experimentally determined (Calvo-Perezet al., 1981; Pileni,

81

Micellar Structure of Lubricating Formulations

1989a; Pileni et al., 1982 ; Wong et al., 1975). By decreasing the water content, changes in the absorption spectrum of hydrated electrons are observed. At a Wo value above 15, the {Q^ concentration is independent of the water content in the water pool, whereas at lower values, it falls with decreasing Wo. At Wo above 20, a transient optical spectrum obtained in AOT reverse micelles is characterized by an absorption band of hydrated electrons centered at 710 nm, see Table 3.2 and Table 3.3 (Llor and Rigny, 1986; Pileni, 1989b; Pileni et al., 1982). In the presence of soft-core micelles at high water content (Wo = 60), the absorption spectrum consists of a broad band with a maximum around 710 nm. To explain the blue-shift and the broadening of the hydrated electron absorption spectra, the effect of sodium perchlorate in aqueous solution was studied. Without any additive, the absorption spectrum of the hydrated electrons was centered at 720 nm, and on adding 5M sodium perchlorate to the water solution, a blue-shift of the absorption spectrum was observed with a maximum centered at 650 nm. The blue-shift observed in the reverse micelles at low Wo is due to the high concentration of sodium ions (and ion-pairs) in the micellar core, at Wo = 5 and [Na] = 10 M of the sodium sulfonate (Pileni et al., 1982).

Table 3.3. Some geometrical parameters of reverse micelles of AOT in cyclohexane Wo^= [H20]/[A0T]

Aggregation No.

WP' radius (nm)

Aggregate radius (nm)

15

160

2.58

3.48

10

127

2.08

3.07

6

86

1.55

2.57

3

47

1.0

2.03

^WP = micellar water pool By decreasing the water content in the core (Wo < 15), a decrease in the hydrated electron concentration occurs. At low values (Wo < 6) all molecules of water interact very strongly with the micelle core and electrons are less attracted in the process of hydration. At Wo values lower than 5, no solvation of the electrons has been observed in reverse micelles. When Wo increases, the water in the center of the pool is partially attracted by the hydration process of electrons. The absorption spectra of hydrated electrons in the core of the micelle are shifted toward short wavelengths compared to bulk water, thus showing that the water in the pool is different from the bulk water. This could be due to the fact that the sodium cations are very active in interaction with electrons (Llor and Rigny, 1986; Pileni, 1989b; Pileni et al., 1982; Wong et al., 1976).

82

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To interpret experimental data with hydrated electrons in reverse micelles, investigators propose the following model (Bakale et al., 1981; Calvo-Perez, 1981; Wongetal., 1975): (a) at low water content in the core Wo values, the water in the pool is highly immobilized and is not able to solvate electrons; (b) when Wo increases, the water in the center of the pool remains free from the interface and the probability of electron solvation also increases. The fact that the absorption spectrum of hydrated electrons inside the micelle is shifted towards short wavelengths compared to bulk water shows that the water in the pool is different from the bulk water. This could be due to the fact that the local concentration of Na^ is high; (c) when Wo increases further, the probability of electron capture and solvation by the water pool increases and the hydrated electrons spectra become closer to that observed in bulk water; (d) at Wo above 20, the spectral properties of hydrated electrons are almost identical to those in bulk water. This fact suggests strongly that at Wo > 20 the water in the center of the micelles has the same properties as bulk water. These results are in agreement with those obtained from fluorescence measurements (Zinsli, 1979). The small enthalpy of hydration of the electron (-160 kJ/mol) compared with that of the sodium ion (-406 kJ/mol) and of the proton (-1,140 kJ/mol) leads to the conclusion that the electron is a less energetically favorable hydrated species in the water pool of the micelle (Burgess, 1970; Hart and Anbar, 1970; Robinson et al., 1986). NMR spectroscopy studies on the ^H and ^^Na, and Raman scattering investigations of the state of water in AOT reverse micelles at low (Wo = 5) values suggest the following: (a) strong hydration of the sodium ion and formation of [Na(H20)„^] solvate in the inner aqueous core, (b) the bonded water near the polar surface suffers a loss of structure due to interaction with sodium ions, (c) the regular tetrahedral structure of water is not observed, and (d) the free water is not present when water molecules are highly immobilized by interaction with sodium ions and is not able to solvate electrons (Mathews and Hirschhom, 1953; Thomson and Gierasch, 1984; Wong et al., 1977; Zulauf and Eicke, 1979) Effective pH values in soft-core RMs. Characterization of the acidity in the aqueous soft-core is important as ionizable compounds are solubilized in the water pool. The micellar core has a very high degree of organization of water. The water pool within reverse micelles is a different solvent than bulk water. The most interesting range of water content corresponds to rather small water pools (waterto-surfactant ratio of 3 to 10) in which peculiar properties of water cause the largest changes in behavior as compared to their behavior in bulk water. A water to surfactant ratio of 1:1 represents a very small, almost undetectable, quantity of

Micellar Structure of Lubricating Formulations

83

water. But, as has been pointed out, the water concentration can be far smaller and still support the growth of the aggregates. In a reduced water pool, proton transfer leads to the formation of contact ion pairs, which cannot be efficiently stabilized in a medium of low polarity, as observed in the vicinity of the interface of larger micelles. NMR experiments and fluorescence lifetime studies on xanthene dyes have shown that the water contains less hydrogen bonding than normal bulk water. Highly bound water is thus unable to hydrate a proton, because hindered rotation reduces the acceptor character of water molecules and affects thermodynamic and kinetic properties (Llor and Rigny, 1986; Robinson et al., 1986; Rouviere et al., 1979). It is tempting to use the classical concept of pH and pK^, but several difficulties arise when applying these concepts to confined water in reverse micelles. Since the water in reverse micelles is a new solvent, the conventional determination of pH of the water pool is difficult. The micellar solubilization of oil-insoluble dyes is a well known phenomenon (Arkin and Singleterry, 1948; Fendler, 1984; Klevens, 1950; Rodgers, 1981; Ross, 1951). Water solubilized by RMs in hydrocarbon solvents by different surfactants (anionic, cationic and zwitterionic) exhibits two absorption bands in the near infrared spectral region (5200-4700 cm'^) due to water populations (Sunamoto et al., 1980), one assigned to the surfactant polar heads and the other to water dispersed in the bulk phase. Three methods have been proposed to evaluate the acidity in the water pool of soft-core of reverse micelles: (i) the glass electrode; (ii) ^^P-NMR spectrometry; and (iii) pH indicators and buffers. (i) The glass electrode. It is not possible to insert a pH electrode into a water pool of reverse micelles and read the pH directly. One can only determine an operational pH, namely that of the water prior to mixing with the surfactant/low polar solvent. The pH at the active site is unknown and the external pH must be used. The pH in a solution of AOT in heptane (Wo > 25) in the presence of solubilized phosphate and borate buffers has been measured by means of a glass electrode (Menger and Yamada, 1979). The authors used a glass electrode to measure the electropotential of various micellar solutions. At medium water content (Wo > 20), the amount of water is sufficient to respond to the glass electrode properly. At low water content with low polarity solvents, the response of the electrode is nonlinear and the results cannot be trusted. It was found that in most cases there was small difference, about 0.4 pH units, between the pH water pool (pH^), and the pH of the starting buffer (pH J in bulk water. Thus, pH^j refers to the commonly used pH scale in bulk water (as measured directly by a glass electrode), whereas pH^ refers to an empirically defined acidity scale (Smith and Luisi, 1980). The difference (pH^ - pH^^) is measured under a variety of conditions, and this permits the determination of an operational acidity in the

84

Chapter 3

micelle water pool as a function of the pH^^ at which the micelles are prepared. (ii) ^^P'NMR spectroscopy. An empirical acidity scale for water pools in reverse micelles has been defined by measuring the ^^P-chemical shift of phosphate buffers. The chemical shift of the phosphorus atom ^^P in some compounds depends on pH, a property which can be applied to anionic reverse micelles (sodium octanoate, AOT), (Smith and Luisi, 1980; Valeur and Bardez, 1989). A solubilized phosphate buffer provides a convenient probe of the pH in the core of the micelles by observation of the ^^P chemical shift in sodium octanoate/hexanol/water, and in AOT/water/octanoate. In the sodium octanoate system, the ^^P shift has been calibrated first in phosphate buffers with the internal pH of the RMs of about 8. hi AOT, the pH deduced from the ^^P shift of the water core of the RMs slightly decreases as the water content changes: pH = 7.5 (at Wo = 7), pH = 12 (at Wo - 50). This method is based on the fact that the ^^Pchemical shift of phosphate depends on the pH of phosphate buffer solutions before and after injection into reverse micelles, under the assumption that the pK^ of the phosphate ions (H2P04' ^ HP04^" + H ^); does not change from water to the micellar solution. The phosphate standard meets certain requirements: (a) it is of relatively small size; (b) it is confined in the water pool; and (c) has no tendency to adhere to the hydrophobic parts of the surfactants, nor to their negatively charged heads (Smith and Luisi, 1980). (Hi) Use ofpH indicators and buffers. The most common method is to inject into reverse micelles various organic dyes (indicators) whose absorption spectra depend on pH, and observe the shift of those spectra with respect to the injected water solution. Dye solubilization shows that the micellar cores in hydrocarbon solutions can be either strongly acidic or basic. The determination of the pK^ of an indicator in the presence of detergent aggregates in non-aqueous solvents is intrinsically more complex than in aqueous system. The classical concept of pH in the aqueous core reverse micelles can be used when the center of water pools is large enough to accommodate a sufficient amount of free water, (Wo > 10). In small water pools (Wo < 10) or at the interface, the problem becomes extremely difficult. Attempts to get direct or indirect information on hydrogen ion activity are futile, because hydrogen ion activity distribution is not uniform throughout the aqueous core. A large difference exists between the interface and the center of the soft-core micelles. Acidity in small water pools or at the interface must be viewed rather in terms of the tendency of water molecules to donate or accept protons. j9-Nitrophenol in bulk water has a pK^ of 7.14 (95% of/^-nitrophenol and is ionized as/?-nitrophenolate at pH 8.4). The pK^ of/^-nitrophenol adsorbed inside AOT reverse micelles in heptane in water pool (Wo = 25.8), shows the pK^ = 11.6, a 4.5 units increase. This is attributed to adsorption at the pool interface where

85

Micellar Structure of Lubricating Formulations

phenolic -OH groups can form hydrogen bonds with the sulfonates at the soft-core RMs of the AOT. Unfavorable desorption equilibria preclude ionization. The acidity of pool-incorporated/?-nitrophenol is not sensitive to pool size (Wo = 2 to 10 for small pools and 30 to 50 for large pools). Spectroscopic pK^ determination ofj!?-nitrophenol in the water pool of AOT containing imidazole buffer in heptane is intriguing. The data obtained suggest that imidazole (Im) displaces pnitrophenol (HOR) from the interface according to the reaction (Menger and Saito, 1978): RM^HOR), +(Im), = RM,,(HOR),.,Im,., + (HOR), where a < z and z is a 28-fold excess of n; RM^q = reverse micelle with stabilized water pool, and Wo = 10 to 36. Table 3. 4. The ionization of/?-nitrophenol (c = 1.22x10"'* M) for different water pool (Wo) parameters of AOT reverse micelles in heptane containing an excess of imidazole Wo = [H20]/[A0T] 10.3 25.8 25.8 25.8 25.8 36.1

[AOT], M 0.11 0.011 0.022 0.043 0.11 0.11

[Imidazole], M 0.201 0.193 0.193 0.193 0.201 0.201

(%) Ionized p-nitrophenol 16 42 55 65 70 85

/?-Nitrophenol moves into the water phase, away from the AOT sulfonate groups, where the pK, assumes a more normal value (0.4 to 0.8 unit larger than that in bulk water). Raising the imidazole concentration increases the percent of displaced phenol, from a competitive adsorption process. Only 50% of interfacial /^-nitrophenol is removed by a 28-fold excess of imidazole, indicating that the interface binds/^-nitrophenol preferentially by phenolic hydroxy -OH group to the sulfonate anion. The imidazole molecule possess fewer acidic protons in position H-2 and H-5 and has shown formation of complexes with AOT in CCI4. The next step deals with the dependency of/?-nitrophenol ionization on pool size at constant concentration of AOT and imidazole. The results are shown in Table 3.4 for three cases in which Wo = 10.3 (16%/7-nitrophenolate), 25.8 (70%) and 36.1 (85%) at 0.11 M AOT and 0.20 M imidazole (El Seoud and Fendler, 1975; Menger and Saito, 1978). A high imidazole concentration (0.20 M) should displace a decreasing percentage of phenol as the pool size diminishes (there is less free water to

86

Chapter 3

solubilize the phenol). This was found to be the case when [AOT] = 0.11 and Wo = 10.3, 25.8 and 36.1. The pK^ of solubilized phenol changes only from 7.93 to 7.74 as Wo increases from 10.3 to 36.1. Since only four or five water molecules are needed to solvate each AOT ion pair, all pools examined have sufficient unbound water to stabilize phenolic species. Experiments with small pool (Wo < 5) reverse micelles, when [AOT] = 0.11 M, [Im] = 0.20 M and a water pool containing 1.22 x 10""^ M /7-nitrophenol, seem unable to replace j!?-nitrophenol at the interface (Frank et al., 1973; Higuchi and Misra, 1962; Menger and Saito, 1978). When an inorganic phosphate buffer or sodium hydroxide is present, without imidazole, only weakly acidic phenol of pK^ -11.6 can be detected. Since the phenolic group is adjacent to anionic sulfonates of the AOT, ionization is weakened. /7-Nitrophenol in the water pool is influenced primarily by electrostatic interaction between molecular imodazole (Im) and the anionic surfactant group. When excess imidazole (but not methanol or n-butanol) is added to the water pool, a fraction of the interfacial/?-nitrophenol is displaced into the water pool. Solubilization of acids and polar compounds in engine oils. The principal actions of detergent-dispersant additives in engine oil include surface chemical aspects such as solubilization of polar compounds produced by combustion of a fuel and stabilization of engine sludge. Hard-core carbonate-sulfonate (or -phenate and - salicylate) RMs, as well as common oil-soluble surfactants and alkylammonium salts can solubilize various polar compounds. The solubilized compounds go into the micelle interior cavity consisting of ionic or polar groups. The micelles solubilize or deactivate polar compounds, such as water, organic acids and sludge. Also, mineral acids, such as sulfuric acid or nitric acid, are neutralized by the alkaline components of hard-core RMs. Carbonate or borate micellar systems are conventionally employed as the alkaline component in hardcore RMs. The actual solubilization limit depends on the temperature, the nature of surfactant, the concentration of water, and on the nature of the acid. Irrespective of size or the specific properties of the solubilized molecules, very little is known about the thermodynamics or the kinetics of the solubilization process. The association of the solute with the interface can be checked using techniques capable of yielding detailed microscopic information at the molecular level (e.g. NMR, ESR, fluorescence, hydrated electrons). Some acids associate strongly with the reverse micellar interface, and those do not control the solubilization. This depends on the strength of the interactions with the interface and the hydrophobic effect on the acid side chain (Albery et al., 1987; Leodidis and Hatton, 1989; Luisi et al., 1988; Steinmann et al., 1986). There are some possible residence sites for hydrophilic solutes in reverse micelles.

Micellar Structure of Lubricating Formulations

87

Acidic molecules can be adsorbed internally (a) and (b) or externally (c), see Fig. 3.4.

Fig. 3.4. The anionic type soft-core reverse micelles with p-nitrophenol (ROH) located: (a) on the internal wall of the anionic group, there is no "free" water, Wo < 10; (b) internally hydrated by "free water" in the water pool unless Wo > 10; (c) the acidic molecules can be adsorbed externally by soft-core RMs at the interface of an detergent-stabilized water pool, Wo > 30 The piCa of acidic substances in soft-core reverse micelles differ considerably from that of the dilute aqueous solution. The pK^ changes are about 0.5 to 2.5 units indicating a lower acidity in the micelle core than in water solution. pKa(ROH, in soft-core micelle) > pK^ (ROH, in bulk water) Solubilization of carboxylic acids, such as acetic acid and oleic acid, by dinonylnaphtalenesulfonates (DNNS) in hexane was studied by infrared spectroscopy. Since DNNS salts form reverse micelles and have an aggregation number of approximately 7 in low polar solvents, carboxylic acids would be solubilized in the polar core of the RMs (Inoue and Nose, 1987; Inoue et al. 1965). Most studies of engine lubricating oil have been concerned with dispersant additive effects on the efficiency of solubilization. The interaction mechanism between additives is based either on micelle formation or synergistic effects between the molecules. For example, succinimides form micelles in hydrocarbon media in the presence of a third compound which interacts strongly with succinimide. The micellar solubilization of a wide range of polar compounds such as water, high molecular weight organic acids, and phenols, is widely known and exploited in technical applications (Arkin and Singleterry, 1949; Ford, 1968; Honig and Singleterry, 1954 and 1956; Inoue and Watanabe, 1983; Mathews and Hirschhom, 1953;Papke, 1998). The acid deactivation mechanism in hydrocarbon media is supplementary to the neutralizing action of carbonate-surfactant hard-core RMs. Traces of strong sulfur, nitrogen or halogen acids are scavenged by neutral detergents, with the

88

Chapter 3

strong acid reacting with carboxylate or sulfonate micelles to form mineral salts and long chain acids which will be retained in the micelle. If the hard-core RMs contain hydroxyl or bicarbonate ions, the acid will be permanently neutralized and form less corrosive reaction products. On the other hand, if the micelle contains a carboxylate detergent, then strong acids will convert it to its metal salt and form high molecular weight organic acids with rust-inhibiting properties. Petroleum sulfonates form soft-core RMs in hydrocarbons and are capable of solubilizing acids (Baker et al., 1954; Luisi and Straub, 1984; McBain and Hutchinson, 1955). Ashless succinimide-type additives are highly effective for deactivating pyruvic acid, and acetic or sulfuric acid . The solubilized acids exist either as a micellar solution, micro-emulsion, hydrogen bonded complex, or an ion-pair species. Solubilized acids are much less active than their unsolubilized form. Ashless dispersants such as polyaminoalkenyl-succinimides are widely used for low temperature sludge protection in engine lubricants. The precursors of the sludge are reactive intermediates, such as aldehydes, acids or hydroxy-acids. These may further react to give insoluble products which can be deposited as a sludge on engine surfaces, depending on the presence of lead salts and water. Succinimides show a high tolerance for water together with the ability to neutralize acids by combination through hydrogen bonding formed when the acid concentration exceeds the total base number of the additives (Bell and Groszek, 1965; Hall, 1969; Fontana, 1968; Gallopoulos, 1967; Ranney, 1968; Watanabe, 1971). Solubilization of ZDDP by soft-core RMs in the bulk oil has been considered as a reason for reduced antiwear performance. The tribofilm formation is strongly influenced by detergents: a phenate RMs additive affects the film formation at low concentrations, and sulfonate RMs affects the tribofilm formation only at high concentrations (over 2%). The polyisobutylene succinic anhydride polyamide (PIBS-PAM) dispersant in formulated engine oil interacts strongly with ZDDP. The polyphosphate tribofilms formed in the presence of the these additives have shorter chain length compared to ZDDP alone and contain less decomposed ZDDP in the tribofilm. The antiwear films were produced on hardened A2 steel using the oil with additives and a wear machine. X-ray absorption near edge structure spectroscopy was used to analyze the chemical nature of the tribofilm. The surface (TEY) and bulk (FY) spectra were recorded. A layered structure of polyphosphate films was identified. Based on the presence (or absence) of unchanged ZDDP in the tribofilm, we can conclude that the additives compete with the adsorption of ZDDP on the surface. The following conclusions were formulated (Inoue and Watanabe, 1983; Kapsa et al., 1981; Rounds, 1981; Shiomi et al., 1986; Yin et al., 1997a and 1997b): - Calcium sulfonate RMs (c < 2%) + ZDDP - polyphosphate tribofilm and unreacted ZDDP; - Calcium sulfonate RMs (c > 2%) + ZDDP - polyphosphate tribofilm only;

Micellar Structure of Lubricating Formulations

89

- Calcium phenate RMs (c ~ 0.5%) + ZDDP -* the chain length of polyphosphate on the surface is longer than in the bulk; - PIBS-PAM surfactant RMs (c ~ 1%) + ZDDP -> short chain polyphosphate tribofilm only. If less ZDDP is adsorbed on the surface, a greater percent is transformed as a short chain phosphate tribofilm. Micellar solutions of detergents such a sulfonates, phenolates and salicylates enhance the solubilizing properties of succinimide additives. For small concentrations of about 0.25% w/w sulfonate and 2.8%) succinimide, a synergistic effect is observed, but for high sulfonate concentrations, the quantity of acid solubilized is less than that observed for succinimide alone (Bradley and Jaycock, 1972). A different mechanism exists for the solubilization of those weak acids and for strong acids. The different interaction between the amine groups in the polar head of a succinimide micelle and a weak acid (WH), and for a strong acid (SH), indicates that the weak acid is much better solubilized as: Weak acid (WH): R3N + mWH = R3N...(HW)^, where m is 2-6 Strong acid (SH): R3N + SH - R3NH^Swith the resulting association complex R3N...(HW)^ being much less polar than the ion pair R3NH'^S". For strong acid (sulfuric acid), solubilization (with proton transfer) can make the succinimide salt more polar and thus less soluble. If the polyamine chain is in excess, the association complex becomes more insoluble. The molar ratio of solubilized weak acid to additive, M(acid)/M(additive), is 2 to 6 times higher than that for a strong mineral acid. The ability of succinimide additives to deactivate acidic contaminants by solubilizing them is important in reducing corrosion and sludge deposition. The amount of solubilized acid in an engine oil depends upon the properties of all of the additives and the interactions among them (Bradley and Jaycock, 1972). The oil analyses have shown that the TBN values of lubricating oils deplete completely while at the same time, the corrosion rate can be considerably reduced. The relationship between the solubilization of large quantities of acid, total base number (TBN), and total acid number (TAN) values with the rate of corrosion is still unresolved. TAN values are not a good prediction of corrosion, and the source of extra TBN is much more important in the neutralization of corrosive acids than the simple numerical value of TBN. The effect of hard-core RMs shows poor correlation between used oil sample TAN values and the potential for bearing corrosion (Denison, 1944; Kreuz, 1970). Where corrosion rates are reduced by treatment with hard-core reverse micelle detergent, and no significant reduction in TAN has occurred, corrosion protection must have occurred by a

90

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mechanism other than acid neutralization. Acids were probably deactivated but TAN measurements did not reflect this. Additives of similar TBN value gave large differences in corrosion protection, some providing corrosion protection by a mechanism other than acid neutralization; e.g., solubilization of the corrosive acids. As an example, neutral detergents were blended with corrosive, used-diesel lubricant oil at a treatment rate calculated to give a neutral soap concentration equivalent to an analogous hard-core RMs detergent. The corrosion protection performance often separate neutral detergents ranged from excellent to a complete lack of protection. The associated TAN values ranged from 5.4 to 9.6, compared with 8.3 for the used diesel oil alone. The detergents which were effective in reducing corrosion, but caused little change in TAN, probably solubilized the acids. These detergents do not contain any hard-core reverse micellar component. Differences in corrosion inhibition between the detergents probably reflect their different surface activity (Cartwright and Carey, 1980). Corrosivity of used oils. The classical determination of TBN and TAN involves a titrimetric procedure, whereby the oil sample is dissolved in a particular solvent system and neutralized by strong acid or strong base (ASTM D664 or 2896), equivalent to (IP 171 or 276). TBN and TAN values do not correlate with corrosivity and the titrimetric analysis has a very limited ability to differentiate between acids of varying strengths. A quantitative differential infrared spectroscopy technique used to monitor the neutralization reaction is more meaningful, since the technique applies to reactions in hydrocarbon solvents. The classical reaction between corrosive acids and hard-core RMs results in formation of the metal salt of the acid and carbonic acid: 2R(CH2)„COOH + MCO3

C=0, IRabsorbance at 1720 cm"^

^

[R(CH2)3COO-]2M'" + H2CO3

COO", IRabsorbance at 1580 cm'

with the weaker corrosive organic acid remaining unchanged. For the weaker, but corrosive, organic acids the above reaction does not go to completion even when the amount of hard-core RMs is in excess. The corrosive activity on copper/lead bearings for typical carboxylic acids, such as decanoic, lauric, palmitic, stearic, and oleic acids, as 1% w/w solutions in a lubricating oil base stock with excess of hard-core RMs, measured by infrared spectroscopy, supports the observation for the corrosive activity of used lubricating oils. An increase in total acidic number (TAN) is generally either an indication of contamination with acidic combustion products or the result of oil oxidation. Corrosion of bearing metals by used lubricating oils requires the presence of both acids and peroxides and probably takes place by a two-step mechanism. In the first step, the peroxide reacts with the metal to form a metal

Micellar Structure of Lubricating Formulations

91

oxide and, in the second step, the oxide reacts with the acid to form a metal salt (Asseff, 1968; Cartwright and Carey, 1980; Denison, 1944). Used oil corrosivity, measured in the laboratory and engine test, was found to be relatively independent of used oil TAN values. For high contamination levels (TAN 12 to 18), where the new oil TAN level was 5, the levels of residual basicity were about TBN 3. The type of acid formed depends upon the service undertaken by the engine /oil system and some are corrosive to lead while others are not. Generally, the tests show that the basicity present in the system does not neutralize the acids formed. There is a need for a better understanding of the significance of TBN and TAN since their contribution to used oil corrosivity and engine wear is often not evident and at best is poorly understood. (Carey et al., 1978). For example, when certain used oil samples reach TBN values of zero, then TAN increases with service use and appears to be independent of both initial TBN values and base number retention (Holmes and Overton, 1978). The corrosivity of high-acidity used oils can be controlled by blending with new oil, see Table 3.5 (Carey et al., 1978; Holmes and Overton, 1978).

Table 3.5. Corrosivity of new and used oil blends Ratio of new/used oils Parameters

TAN(mgKOH/g„„) TBN(mgKOH/gJ Insolubles (%) Corrosion test (mg)

0/100 used oil

25/75 new/used

50/50 new/used

100/0 new oil

8.3 0.8 3 2828

6.4 2.5 2.3 1282

4.8 4.8 1.5 106

2.2 8.2 25

Blending only changed TBN and TAN by the ratio of new/used oil, while the base reserve of the new oil had no neutralizing effect on the used oil TAN value. Corrosion tests in a CLR-L-38 engine for the new/used blends of 0/100 and 25/75 showed these to be very corrosive, whereas the 50/50 blend was much more effective at reducing corrosion. The rates of corrosion do not show a linear correlation with either TAN or TBN.

3.3. Tribochemistry of Hard-Core Reverse Micelles The hard-core reverse micelles considered in this chapter are composed of an amorphous Ca (Mg) carbonate (or borate) colloidal core surrounded by benzensulfonate, phenate or salicylate molecules strongly bonded to the colloidal

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core. Carbonate (or borate)-sulfonate RMs have been recognized as an efficient multifunction class of antiwear and anticorrosive additives. Hard-core reverse micelles are indispensable for internal combustion engine oils because of the need to neutralize strong acids. They are not always indispensable in other applications, e.g., gear oils, automatic transmission fluids, and other industrial oils. The hardcore borate-sulfonate RMs have a good acid neutralization ability, probably because calcium borate has a hydration water by nature (Inoue and Nose, 1987; Liston, 1992; Mansot et al., 1993a and 1994). The shape and size of micellar particles can be determined using transmission electron microscopy (TEM) and photo-correlation spectroscopy (PCS). The size distribution of the hard-core RMs measured by PCS was confirmed by TEM technique: the hard-core RMs micelles of carbonate-sulfonate had mean particle sizes between 10 and 20 nm while the neutral sulfonate formed micelles with a diameter of 2 nm (Chinas-Castilio and Spikes, 2000). This constitutes a reverse micelle association whose active parts consist of a core carbonate plus a shell of surfactant. Micellar carbonate additives are prepared under conditions such that carbonate is formed by chemical reaction in the presence of surfactant. The commercial concentrates are dispersed in hydrocarbon oil, stabilized by an adsorbed layer of surfactant. They contain about 20 to 35 wt% (or 8 to 15 vol%) of inorganic carbonate, and 18 to 30 wt% (or 30 to 35 vol%) of a surfactant. The carbonate particles have a radius of about 3 nm and the surfactant layer thickness is about 2 nm (Marsh, 1987). The results of preparing additives with various surfactant concentrations from 20 to 35 wt% indicated incomplete surface coverage at low surfactant concentrations. High surfactant concentrations (35 vol%) result in smaller hard-core sizes and hence higher surface area for surfactant adsorption. The size of the water core does not primarily determine the size of the particle but nonetheless provides a stabilizing medium. It is also found in this case that the size of the particles increases as the Wo value increases. It was suggested (Kandori, 1988) that the formation of particles occurs via fusion of reverse micelles containing precursor particles. To obtain a stable system containing the carbonate additives (particles of < 10 nm), special types of manufacturing processes are required: solvent-free / alkoxide process or oxide / hydroxide process (Marsh, 1987). In industrial oil formulations, a variety of surfactants are used to coat calcium, barium and magnesium carbonate colloidal particles. Colloidal dispersions of inorganic carbonates can fulfil most requirements as effective acid-neutralizing additives with the following features: strong base, stable after reaction with acid, neutralization product not harmful, clear and stable oil solution. Calcium may be preferred for specialized diesel engine applications, and magnesium for rust control in gasoline car engines. The sizes of the reverse micelles and the carbonate hard-core and surfactant layer thickness were investigated using small angle neutron scattering methods (SAXS), (Giasson et al., 1992; Markovic et al..

93

Micellar Structure of Lubricating Formulations

1984). Typical hard-core RMs sizes (nm): (a) (CaCOgX Ca-sulfonate values are: surfactant layer 2.2 nm , hard-core diameter 3.9 nm, total diameter 8.2 nm. (b) (CaC03X Ca-phenate values are: 1.5 nm, 2.9 nm and 5.9 nm, respectively (see Table 3.6).

Table 3.6. Typical hard-core RMs of sulfonate and phenate, sizes and variation in detergent content (Marsh, 1987) Carbonate and (detergent) CaCOj (Ca sulfonate) MgCOj (Mg sulfonate) CaCOj (Ca sulfonate) CaCOj (Ca phenate)

Diameter (nm)^

Detergent layer (nm)^ thickness (R2-R1)

Core size 2Ri

Overall size 2R2

3.9 7.8 21.3 2.9

8.2 11.8 25.4 5.9

2.15 2.0 2.05 1.5

11.4 11.2 10.9 10.4

1.45 1.65 1.7 1.7

Wt. % detergent 20 25 30 35

8.5 7.9 7.5 7.0

^The sizes were determined by small angle neutron scattering method (SANS), (Markovic et al., 1984). In order to interpret the SANS results, a concentric sphere model was used. A spherical homogeneous core particle of radius R^, and the total spherical units were taken as R2, giving the thickness of the adsorbed layer as (R2 - Ri) Infrared analysis showed that the hard-core reverse micelles were initially adsorbed, and then subsequently destroyed, with the amorphous carbonate core deposited in the contact. The initially-formed film gradually develops over time of rubbing to reach a thickness corresponding to four particle diameters in some cases (Georges et al., 1995; Palermo et al., 1996). Negligible reduction of friction was observed in reciprocating motion but the hard-core RMs micelles showed a reduction in wear scar diameter of 30% compared to the base oil. The wear reducing properties of hard-core RMs result from the formation of a very strong deposit by an adhering and possibly chemically transformed (crystallization to calcite) surface layer of metal carbonate. With the soft-core reverse micelles tested, however, phosphorus and boron-based antiwear additives result from the formation of a thick, surface tribofilm (Adams and Godfrey, 1981; Chinas-Castilio and Spikes, 2000; Taylor et al., 2000).

94

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Following are described the physical properties of hard-core reverse micelles, the effect of changing the surfactant, the exchange solubilizates, and some specific aspects, e.g., technological relevance. To clarify this, the following five topics are considered: (i) Micellar carbonate as anticorrosive/antiwear additive; (ii) Tribochemical performance of micellar carbonate (borate) + ZDDP; (iii) Modified hard-core RMs with a sulfiirized carboxylic acid; (iv) Effect of hard-core RMs on the stability of ZDDP; (v) Micellar CuO and Cu as multifunctional additives. (i) Micellar carbonate as anticorosive/antiwear additive. Structure of hardcore reverse micelles with colloidal dispersion of a mineral salt stabilized by a detergent is shown in Fig. 3.5.

Hard-core reverse micelle alkyl benzenesulfonate colloidal core

Fig. 3.5. Structure of hard-core carbonate-benzenesulfonate RMs. The mineral core is mainly made up of amorphous carbonate. The carbonate colloidal core has a radius of about 3 nm and the surfactant layer thickness is about 2 nm The hard-core RMs of Ca, Mg, Na carbonates (Belle et al., 1990; Brooks and Shlimkan, 1979; Hunt, 1989; Ottewill et al., 1992) or calcium borate (Inoue, 1993; Inoue and Nose, 1987) stabilized in colloidal form with a detergent such as alkyl aryl sulfonate, alkyl phenate, alkyl naphthenate or alkyl salicylate, have good antiwear properties which are associated with the build-up of an antiwear surface film. This film consists mainly of calcium carbonate micro-crystallities aggregated through an amorphous inter-granular phase. The antiwear action of such an adherent film is due to its micro-granular structure which ensures that during sliding, shearing occurs essentially in the interfacial film (Mansot et al., 1993a).

Micellar Structure of Lubricating Formulations

95

The term "hard-core RMs" is used to describe dispersions containing an excess of colloidal carbonate over that required to neutralize the sulfonic acid. The excess is found in calcium carbonate trapped in a micellar structure. The total base number (TBN) of the additive so obtained is 368 mg KOH/g of oil. The crude additive consisted of 33 wt% CaCOg (Delfort et al., 1995; Giasson et al., 1992). Some typical results of the core particle with the overall diameter and the detergent layer thickness are shown in Table 3.6 (Marsh, 1987). The case study. The sulfonate samples were all made of the same detergent, and good agreement was obtained among the measured sulfonate surfactant layer thicknesses. The phenate was made with a shorter chain detergent. The effect of additives with various surfactant concentrations, from 20 wt% to 35 wt% calculated by computer modeling, indicated that a packed detergent layer should have a thickness of about 1.7 nm. The results indicate incomplete surface coverage at low detergent concentrations. High detergent concentrations result in smaller core size and hence higher surface area for detergent adsorption (Marsh, 1987). Friction tests were carried out on a plane-on-plane tribometer. The specimens were immersed in a 2 wt% of additive in n-dodecane. The results for the three different colloidal additives A, B, and C with TNB values of 305, 447, and 470 mg KOH/g oil, respectively) show that the size of grains (mean diameters of 6.8, 8.6, and 9.3 nm, respectively) present in the antiwear film, is slightly larger than the micellar core sizes (mean diameters of 4.2, 6.0 and 7.2 nm, respectively) of the antiwear additives used during the friction test (Mansot et al., 1993a). Wear results show that the carbonate-alkylaryl sulfonate RMs have very good antiwear properties. Evaluation of wear (loss of volume V, mm^) of the ring as a function of the sliding distance (L, mm) for different lubricants, namely pure n-dodecane, a solution of traditional antiwear additive ZDDP (2 wt%) in n-dodecane, and carbonate-di-dodecylbenzenesulfonate RMs 2 wt% in n-dodecane was reported as the wear rate (dV/dL, mm^, mm"^) ratio 6 : 1.4 : 1, respectively. Losses of volume V of the ring as a function of its sliding distance L for different lubricants are reported by (Mansot et al., 1993a). Chemical and structural changes of the hard-core RMs. The surfaces are then coated with a blue antiwear film and examined in an optical microscope. It has been demonstrated that the wear particles, generated in the lubricant during antiwear regime, have the same form and structure as the antiwear film. Quantitive elemental analyses and molecular structure studies of the antiwear film were performed by means of electron energy loss spectroscopy (EELS). A comparison between the carbon in the micelles and wear debris shows significant changes in the near-edge structure. The carbon that is contained in the micelles (especially the organic chains) undergoes severe chemical changes during the

96

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build-up of the antiwear film, such as oxidation (C=C ~* C=0). The quantitive analysis showed that the carbon concentration was lower (about 40%), and concentrations of Ca and O were higher (20 to 30%) in the antiwear film than in the micelles. The iron concentration coming from rubbing surfaces in the wear particles is very low (about 3%) and confirms the superior antiwear properties of the hard-core RMs (Mansot et al., 1993a). The analytical results obtained on the wear particles allows the authors to conclude that during friction, hard-core RMs undergo chemical and structural changes. Fragmentation and oxidation of the organic chains occur, which, under the physical conditions existing in the sliding contact, lead to the loss of the organic shell. Simultaneously, the metastable amorphous calcium carbonate undergoes crystallization into the calcite structure. The sizes of the crystallites are strongly correlated with the original size of the mineral core of the micelles. The following tribochemical mechanism is proposed for the film build-up to confirm the antiwear properties of the hard-core RMs (Mansot et al., 1993a): (a) the feeding of the rubbing surface is achieved by adsorption of the hard-core RMs onto the friction surfaces; (b) during the rubbing process, the micelles lose their organic shell and their mineral cores crystalize and aggregate to form a polycrystalline film adherent to the metallic surfaces; (c) tribological film delamination occurs leading to protection of the rubbing surfaces. Considering these results, the main difference between the antiwear action of the ZDDP soft-core RMs and the hard-core RMs is clear. In the case of ZDDP soft-core RMs, the antiwear film formation requires that chemical reactions occur between the additive and the metallic surfaces. In the case of hard-core RMs, the mineral material (CaC03) is directly introduced to the sliding contact and undergoes small physicochemical changes during the film build-up. Consequently no chemical reaction with the substrate surfaces is required. Hard-core RMs of carbonate-alkylaryl sulfonates (OCABS) are prepared by reaction of carbon dioxide with calcium or magnesium oxide (or hydroxide) in the presence of a surfactant (Delfort et al., 1995; Mansot et al., 1993a). Because of their alkaline reservoir they are able to neutralize the acidic by-products resulting from oxidation of oil and from fuel combustion products (blow-by). The extended X-ray absorption fine structure (EXAFS) results show that the hard-core RMs are made of amorphous calcium carbonate, the calcium atoms being surrounded by nearly six oxygen neighbors at distance 0.24 nm (local order close to that of calcium in calcite), (Mansot et al., 1993b). Consequently, an OCABS micelle can be described as a metastable amorphous calcium carbonate core surrounded by an organic shell made of the alkylaryl sulfonate groups as shown in the spectroscopic images. The fact that the alkylaryl sulfonate groups are chemically bonded to the mineral core explains the fact that the micelles remain stable even beyond their original medium. In this case, amphophilic groups cannot leave the surface of the particles and exchange with the molecules

Micellar Structure of Lubricating Formulations

97

present in the solvent, as happens in the case of classical soft-core reverse micellar system. It also explains why such dispersions are retained when diluted with pure solvent. The hard-core RMs studied by others (Markowic and Ottewill, 1986; Markovic et al., 1984) by means of small angle neutron scattering (SANS) are described as hard spheres composed of a calcium carbonate core surrounded by an organic adsorbed layer. The additive concentrates are blended into finished engine lubricants at levels ranging from approximately 1.5 (wt%) in gasoline passenger cars to 30 mass per cent in marine diesel lubricants. The choice between calcium and magnesium carbonate depends on the required performance of finished oil. Magnesium may be preferred for specialized diesel engine applications, and calcium for rust control in gasoline car engines. Sodium carbonate is sometimes used in addition to calcium and magnesium, and can provide additional benefits in rust and oxidation control (Bray et al., 1975; Marsh, 1987). The EXAFS spectroscopy results strongly confirm the existence of local order in the mineral part of lead isooctane reverse micelles in dodecane and reveal quantitative information concerning the first and second coordination shells. The radial distribution functions (RDFs) exhibit peaks at around 0.19 nm, corresponding to the first shell of oxygen atoms and at around 0.35 nm corresponding to the shell of lead. Analytical transmission electron microscopy (ATEM) indicates the size of the mineral core of the micelles (1-1.5 nm) and the discoid shape of the particles when the micelles aggregate (Mansot et al. 1994). The degradation of carbonate-sulfonate caused by water, and the choking of a suction filter, have been reported (Inoue and Nose, 1987; Yano et al., 1999). The degradation process of carbonate-sulfonate RMs in the presence of water: nCaC03-(MS03R)m + z H p - nCaCOg + z(H20)-(MS03R)m (Sulfonic acid micelles) nCaCOa + nCaC03 -> mCaC03 (Ca carbonate co-agglomerate) Regarding the degradation mechanism of calcium carbonate, it is postulated that sulfonic acid is consumed to form micelles in oil emulsions as shown by the reactions above, causing particles of calcium carbonate to expose themselves, which in turn, cause those particles to co-agglomerate and degrade. Degraded particles of calcium carbonate are formed in oil and deposited on the inside of the flow restrictor, eventually leading to its choking. As indicated by the results of oil A 6100 ppm Ca (0 wt% water), oil B 9 ppm Ca (0 wt% water) do not produce the deposition. Oil C 6100 ppm Ca, which contains 30 wt% water, did produce deposition, while oil D 9 ppm Ca and 30 wt% water did not produce deposition. These results show that both calcium detergent and a high content of water are necessary for the deposition to occur.

98

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(ii) Tribochemical interactions of micellar (carbonate) and ZDDP. The borate-detergent RMs with sulfonate, salicylate, and phenate were evaluated in comparison with micellar carbonate-detergent RMs. Since alkaline earth metal salts of boric acid have been known to be rust preventive additives, boratedetergent RMs probably have good rust preventive performance. Furthermore, since boric acid itself acts as both an oxidation inhibitor and an antiwear additive, it is expected that borate-detergent RMs would show good oxidation stability and antiwear performance. The case study. The hydrolytic stability, oxidation stability, thermal stability, friction and wear characteristics of both borate and carbonate RMs of sulfonate, salicylate and phenate are shown in Table 3.7 (Inoue, 1993). Table 3.7. A comparison of the physico-chemical parameters of micellar borate-detergent RMs with micellar carbonate-detergent RMs in mineral oil

Detergent

Sulfonate Salicylate Phenate

Hydrolytic stability

TBN

(mg/g)

W

(2)^ (mgKOH/g)

0.7 (2.5)' 1.5(4) 2.0(5)

180(320) 200(170) 185(250)

Oxidation stability

(37

(min) 180(160) 150(130) 175 (140)

Thermal stability (4)(1 to 10) 6(5) 9(2) 9(6)

Friction

Wear scar

(5)^ (N)

(6)^ (mm)

3.6 (5.3) (4.4)

0.4 (0.5) 0.4 (0.5) 0.4 (0.6)

-

^Methods: (1) ASTMD2619; (2) ASTM D2896; (3) ASTM D4742 by the thin fihn oxygen uptake test (TFOUT) at 160°C; (4) hot tube test at 290°C (Okhawa and Seto, 1984); (5) ASTM D2714, N - Newton; (6) ASTM D4172 ^Numbers: borate (carbonate) (1) Hydrolytic stability (ASTM D2619 method). The borate-detergent RMs showed superior hydrolytic stability as compared to those of carbonate-detergent RMs regardless of the type of detergents, e.g., the results of insolubles [mg/g]: of borate-sulfonate micelle system is 0.7 and carbonate-sulfonate is 2.5 (Table 3.7). The hydrolytic stability is due to the properties of calcium borates whose crystal form was not changed (based on IR spectra) during the test. (2) TBN (ASTM D2896 method). The amount of an alkaline reservoir contained in the hard-core RMs of metallic detergents is expressed as a total base number (TBN) and defined in terms of mgKOH per gram of the product (mgKOH/g). The TBNs of borate-detergent RMs of metallic detergents were from 180 to 200 mgKOH/g oil and carbonate-detergent RMs of metallic detergents were from 170 to 320 mgKOH/g oil.

99

Micellar Structure of Lubricating Formulations

{3) Oxidation stability (ASTMD4742 method at 160 ^Q. Oxidation stability was evaluated by the thin film oxygen uptake test TFOUT (ASTM D4742) at 160°C. As shown in Table 3.7, the borate micelles showed better oxidation stability compared to that of carbonate micelles in the same detergent type. For example, oxidation stability of a micellar borate-sulfonate system from 180 min decreased to 160 min for micellar carbonate-sulfonate. The good oxidation stability obtained by micellar borate-detergents might be explained by the action of boric acid liberated by the reaction of borates and oxidative products. The boric acid formed during oxidative degradation then acts as an hydroperoxide deactivator (Yamada et al., 1992). Oxidative degradation test ISOT. The oxidative stability of RMs of borates and carbonates with sulfonates and salicylates were evaluated in the presence of ZDDP. The concentration of secondary type zinc dialkyldithiophosphate was 1 wt% and that of the detergents was 2.2 wt% in SAE 10 mineral oil. Table 3.8 shows the results (Inoue, 1993).

Table 3.8. Oxidation stability of mutifunctional additives (carbonate-sulfonate and boratesulfonate RMs) in the presence of zinc dialkyldithiophosphate in SAE 10 mineral oil. Initial oxidation (In) time (min) and the remaining oxidation (Re) time (min) after 100 hrs oxidative degradation (ISOT) test^ Formation of hard-core RMs

ZDDP alone ZDDP + Ca borate ZDDP +Ca carbonate

Without additives In Re (min)

70

-

FO^

Sulfonate additive In Re (min)

110 45

75 15

Salicylate additive In Re (min)

70 75

45 15

^Initial (In) and remaining (Re) oxidation time were determined by TFOUT test. ^ZDDP wasftillyoxidated (FO) after 35 hrs duration of oxidative degradation (ISOT) test. Oxidative degradation test ISOT (adapted as JIS K2514, duration time 100 hrs at 150°C): 1% of the ZDDP with the 2.2 wt% micellar borate (or carbonate)detergents were dissolved in SAE 10 mineral oil and oxidized at 150°C with the Indiana Stirring Oxidation Test (ISOT). The oil was periodically withdrawn during the (ISOT) test and the remaining oxidation lifetime was determined by the Thin Film Oxygen Uptake Test (TFOUT) the ASTM D4742 method at 160°C. In this test 2 wt % of the additive(s) was dissolved with API SG grade hydrocracked base oil (Inoue, 1993). As shown in Table 3.8, the induction time for oxidation (In) lifetime of the oil

100

Chapters

containing 1 wt% of the ZDDP alone was about 70 min. The oxidative degradation lifetime of the oil containing ZDDP alone was obtained after a 35 min test. The addition of ZDDP to micellar carbonate-sulfonate (or salicylate) decreased the induction time (In) lifetime to about 45 minutes, but the oxidative degradation during 100 hrs ISOT test was much less than that of the oil containing ZDDP alone. The micellar carbonate-detergent system significantly prevented the oxidation of the oils containing ZDDP. The micellar carbonate-sulfonate system deactivates the active acidic material formed from ZDDP itself (Yamada et al., 1992), which acts as a strong antioxidant. On the other hand, the oil containing borate-detergent RMs and ZDDP increased the initial oxidation time. The calcium borate micelles did not decrease the induction time of the oil containing ZDDP as shown in Table 3.8. The oxidative degradation of micellar borate + ZDDP was much smaller than that of the oil containing carbonate RMs and ZDDP. Such performance suggests that the use of the micellar calcium borate-sulfonate system makes it possible to formulate oils having good oxidation stability. (4) Thermal stability. Thermal stability was evaluated with the hot tube test at 290°C, Table 3.7. The apparatus and method were described elsewhere (Okhawa and Seto, 1984) and results were presented as the lacquer rating, which rated on a scale of 0 (black) to 10 (colorless and transparent). 2 wt% of micellar carbonate and borate with detergents were dissolved in SAE 50 base oil. As presented in Table 3.7, the borate micelles showed much better thermal stability on a scale of 0 to 10 (sulfonate 6, salicylate 9, and phenate 9 units) than those of the micellar carbonate (sulfonate 5, salicylate 2, and phenate 6 units). In particular, the thermal stability of borate-salicylate and borate-phenate RMs was found to be excellent. (5) Friction force (ASTM method D2714'the Falex block-on-ring test). The friction characteristics were evaluated for the new oils and oils after oxidative degradation. Oxidative degradation was carried out with ISOT at 165 °C for 24 hours, 2 wt% of the micellar borate and carbonate with detergents were dissolved in hydrocracked base oils. The friction force for micellar carbonate-detergent RMs was increased appreciably by the oxidative degradation, whereas the friction force for micellar borate was nearly unchanged. The borate micelles have good oxidation stability, see Table 3.7. (6) Wear scar (ASTM D4172 method four ball wear test). The antiwear performance was evaluated by the diameters of the wear scars in four-ball wear tests. The borate-detergent RMs showed better antiwear performance as compared with those of the carbonate-detergent RMs. Interestingly, the antiwear performance was dependent on the detergent type in the case of carbonate-

Micellar Structure of Lubricating Formulations

101

detergent RMs; however, almost the same wear scar was obtained for the boratedetergent RMs. Li all tests, the borate-detergent RMs showed better performance in comparison with carbonate micelles. Both hydrolytic stability and superior antiwear performance were ascribed to the borate-detergent RMs. Good oxidation stability might be explained by the action of boric acid, which acts as an oxidation inhibitor, liberated by the reaction with oxidative products. Borate-detergent RMs have recently been recognized as an efficient multifunction class of anti-corrosiveantiwear additives. The additive acts as an antiwear agent by the formation of a calcium borate glass tribofilm material. The use of borate-salicylate RMs in the friction test leads to the formation of a thin amorphous film, which can be described as a composite material. This material is composed of a calcium and iron borate matrix, with some embedded crystallites of calcite which are from impurities in the calcium borate micelles. The boron behaves as a glass former like phosphorus does with the ZDDP additive. The combination of ZDDP and boratesalicylate RMs is expected to be synergistic, due to the formation of a mixed phosphate-borate glass tribofilm (Inoue, 1993; Mansot et al., 1993a; Normand et al., 1998;Varlotetal., 1999), Tribochemical reactions between borate-salicylate RMs + ZDDP in boundary lubrication were investigated in a reciprocating friction and wear tester by analyzing collected wear debris by Transmission electron microscopy (TEM) and Electron energy-loss spectroscopy (EELS). Micellar borate (CB) RMs and ZDDP produce long chain oxide glasses as antiwear tribofilm. The friction coefficient in the steady-state conditions is 0.12 for ZDDP, 0.09 for CB and 0.11 for ZDDP/CB. Considering the evaluation of friction in the different cases, it seems that ZDDP is first acting in the presence of mixtures, then CB becomes active after a few minutes and finally the steady-state behavior is obtained in the presence of borophosphate. It is anticipated that under shear, the glasses behave as liquid-like species because the glass transition temperature (Tg) of borophosphates is known to be very low (around 200 °C). The combination of borate-detergent RMs with ZDDP produces calcium and zinc borophosphate glass tribofilms which could correspond to the empirical formula Ca4Zn4P4B402i. There is evidence for P-O-B bonding in this glass, indicating an atomic scale mixing of the two additives in the tribo contact. No iron is seen in the tribofilm composition (Varlot et al., 1999; Warren etal., 1998). With the selection of proper water content and reaction temperature, a new type of hard-core RMs sulfonates which contained an ultrafine calcium borate as an alkaline component was prepared. Since both reactants, calcium hydroxide and boric acid, are oil insoluble, water is necessary as a reaction medium in the presence of neutral calcium sulfonates. When water content was less than 5% of the sulfonates used, the rate of the reaction was too low and unreacted materials were present in significant quantity as coarse particles. With the water content

Chapter 3

102

between 10% and 20%, only a trace amount of coarse particles were produced in borate-sulfonate hard-core RMs synthesized. As mentioned above, both reactants are solid and react with each other in the water phase more than 10 % and less than 20%) of the sulfonates (Inoue and Nose, 1987). (Hi) Modified hard-core RMs with a sulfurized carboxylic acid To improve antiwear and extreme-pressure performance of the micellar carbonate, a chemical modification of these colloidal species was performed. This modification consists in partial esterification of the calcium carbonate core with a sulfur-containing carboxylic acid, leading to a comicellized calcium carbonate and calcium carboxylate. This sulfur functionalization of micellar calcium carbonate leads to compounds which exhibit additional extreme-pressure performances. Two sulfurized carboxylic acids i.e., dithiodiglycolic acid and 4,4'-(l,3,4-thiadiazole2,5-diyl)bis(thiabutanoic) acid (DMTD/AA) are capable of producing colloidal RMs. It is expected that the calcium carboxylate resulting from the reaction between the acid and CaCOg is located in or near the polar core and does not diffiise in the hydrocarbon fraction. The three carbonate-sulfonate additives which differ in their core/shell ratio were used as starting materials, see Fig. 3.6 and Table 3.9 (Papke and Rubin, 1992).

Ca2+

dialkyi benzenesulfonate S2(CH2COO)2Ca = calcium dithiodiglycolic carboxylate Fig. 3.6. Partial (10%) neutralization of colloidal CaCOjWith dithiodiglycolic acid leading to comicellized calcium carbonate and calcium dithiodiglycolic carboxylate The core/shell ratio is defined as the ratio of the weight of the carbonate core divided by the weight of the surfactant. All modified and non-modified micellar compounds were evaluated as antiwear and extreme-pressure additives at different

Micellar Structure of Lubricating Formulations

103

concentrations in neutral solvent mineral oil in four-ball tests (ASTM D2783 standard method). From this test, antiwear and extreme-pressure data were determined, such as welding load, load wear index and wear scar diameter under a load of 40 decanewtons (daN), 60 daN and 80 daN, respectively. First, evaluations were performed on the three original nonmodified hard-core RMs, then evaluations were performed on the sulfixr-functionalized ones. Four-ball test data (results only for one concentration 10 wt% additive) are summarized in Table 3.9 (Delfort et al., 1995 and 1999) for: (a) Non-modified hard-core carbonate-sulfonate RMs; (b) Carboxylic acid-modified hard-core carbonate-sulfonate Rms. Table 3.9. The antiwear characteristics and extreme-pressure properties of non-modified and modified hard-core carbonate-sulfonate RMs in mineral oil measured by a four-ball test under 60 decanewtons (daN) load. RMs Core/shell ratio^ (TBN)*'

Non-modified RMs

Mineral oil only 0.98 (300) 1.96(410) 2.58 (500)

2.16 0.85 0.40 0.38

Modified RMs with carboxylic acids Acid^ Acid'^

Weai• scar diameter (mm) 0.70 0.49 0.41

1.55 0.47 0.71

Welding load (daN) Mineral oil only 0.98 (300) 1.96(410) 2.58 (500)

120 180 220 240

200 250 290

210 300 370

Load wear index (daN) Mineral oil only 0.98 (300) 1.96(410) 2.58 (500)

18 29 38 38

30 33 49

28 41 47

^Core shell ratio = weight of carbonate core / weight of the detergent; ""Total base number (mg KOH/gJ; '^Dithiodiglycolic acid, S2(CH2COOH)2; ^4,4'-(l,3,4thiadiazole-2,5-diyl)bis-(thiabutonic) acid. Both acids are added to convert 10% of the calcium carbonate alkalinity reserve.

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(a) Non-modified hard-core carbonate-sulfonate RMs. For all carbonatesulfonate micelles considered, the extreme-pressure performances such as welding load, and the load wear index as well, increased quite regularly with the concentration of additives in the focus area. The three additives differ in their core/shell ratio. It appears that both the welding load and load wear index seem to be almost linearly related to the weight of CaC03. The wear scar diameter (mm) behavior of fiinctionalized sulfonates vs. the calcium concentration does not differ from that of the non-modified sulfonates. It seems that the antiwear performances are less dependent on the available calcium carbonate in the oil and more so on the micellar characteristics of the additives. (b) Carboxylic acid-modified hard-core carbonate-sulfonate RMs. Better extreme-pressure properties were obtained with DMTD/AA acid-modified sulfonate rather than with dithiodiglycolic acid ones. The welding load of the products (daN) vs. the total sulfur concentration in oil is almost linearly dependent on the sulfur concentration for each of the three hard-core RMs-type precursors. Chemical incorporation of sulfurized species into the hard-core RMs leads to products with improved extreme-pressure characteristics, such as welding load, and the load wear index. The functionalization of the micellar calcium carbonate by neutralization with dithioglycolic acid is illustrated in Figure 3.6. Experiments have shown that detergent molecules of calcium alkylarylsulfonate alone do not provide any antiwear properties in four-ball tests performed in the range of concentrations considered. The antiwear properties can be attributed to the CaC03 colloidal particles. Four-ball test data are listed in Table 3.9 (Delfort et al., 1995 and 1999). Modified hard-core RMs by phosphosulfurized compound. Improved extreme-pressure and antiwear properties have also been obtained with the introduction of some chemical species, such as sulfur, phosphorus or boron derivatives, into the colloidal core (Delfort et al., 1998; Inoue, 1993; Inoue and Nose, 1987). Welding loads, load wear index and wear scar diameter at 5 vv1;% of a CaC03 core surrounded by a calcium alkylaryl-sulfonate surfactant shell, and modified by phosphosulfurized calcium carbonate core were evaluated for calcium dialkyl dithiophosphate (CaDTP) and calcium trithiophosphate (CaTTP) with the four-ball extreme-pressure test (ASTM D2783 standard method). Both modified products exhibit improved extreme-pressure performances (welding load and load wear index), while their antiwear properties (wear scar diameter) compared to those of the original micellar substrate remain at least at the same level. Welding loads and load wear index at 5 wt% concentration (as the extreme pressure performances) are improved compared to those of the original nonphosphosulflirized substrate (Delfort et al., 1998). The welding load (daN), wear load index (daN) and wear scar diameters (mm) under a load 60 daN behavior are

105

Micellar Structure of Lubricating Formulations

illustrated in Table 3.10. The w^ear scar diameters after one hour under a load of 60 daN for fiinctionalized products remain slightly larger than those of the original one, but they still remain below^ 0.5 mm. The introduction of phosphorized species into the mineral micellar core of the additive leads to products with improved fourball extreme-pressure performance, with a small advantage for the calcium dialkyldithiophosphate structure. The initial intrinsic antiwear character of the original substrate is not significantly affected by the functionalization and can even be improved at a low concentration of additive (Delfort et al., 1998).

Table 3.10. The antiwear performance of hard-core RMs^ modified by phosphposulfiirized compound under 60 decanewtons (daN) load. Medium Oil only RMs only RMs + CaTTP additive RMs + CaDTP additive

Welding load (daN) 120 240 290 310

Load wear index (daN)

Wear scar diameter (mm)

18 38 50 53

2.16 0.38 0.42 0.42

CaTTP = calcium trithiophosphate; CaDTP = calcium dialkyl dithiophosphate; daN = decaNewtons ^RMs are formed of calcium carbonate surrounded by calcium alkylaryl-sulfonate surfactant shell and modified by phosphosulfurized calcium carbonate core. Synthesis of oil soluble micellar calcium thiophosphate was performed in a one-step process involving the reaction of calcium oxide, tetraphosphorus decasulfide and water in the presence of an alkylaryl sulfonic acid. This product could be defined as a calcium thiophosphate hard-core surrounded by a calcium alkylarylsulphonate shell in accordance with a reverse micelle type association in oil. Three micellar products with the same chemical nature core were prepared, each with different core/shell ratio of: 0.44, 0.92 and 1.54. Better performances are expected with products of higher core/shell ratios. The antiwear performance of micellar calcium carbonates is directly linked to the size of the mineral CaC03 colloidal particles. At a concentration of 2 % micellar cores, no antiwear effect is observed whatever the micellar size. At an intermediate concentration of 4 % of micellar cores, the wear scar diameter is clearly dependent on the micellar size, slipping from 1.70 mm to 1.10 mm, then to 0.79 mm when the core diameter moves from 4.37 nm to 6.07 nm, then to 6.78 nm. Size dependence is increased at a concentration of 5 % in colloidal cores. This clearly confirms the size dependence of the micellar cores on their antiwear performance (Delfort et al..

106

Chapters

1996 and 1999). (iv) Effect of hard-core RMs on the stability of ZDDP, Since ZDDP in gasoline engine oils is consumed in approximately 300 hours of use (10,000 to 15,000 km) it is important to reduce the rate of decomposition of ZDDP. It is reported that the degradation of ZDDP is dependent on: strong acids, thermal factor, and retardation effect (Yamada et al., 1992): (a) strong acids: the presence of strong acids, e.g., sulfonic type, promotes decomposition significantly, even at 120''C. The catalytic decomposition of ZDDP itself generates the sulfur acids. A carboxylic acid which is a much weaker acid than sulfonic acid has a very small effect on the degradation of ZDDP; (b) thermal factor: high temperature, over 140°C, is a factor in acid-catalyzed decomposition; (c) the retardation effect: the presence of hard-core RMs with the sulfonates, salicylates, and phenates seems promising for high performance engine oils. The retardation of the decomposition of ZDDP by hard-core RMs can be properly explained in terms of neutralization of the sulfur acids into inactive species. Micellar sulfonate and salicylate, having borate as the alkaline micellar hard-core, shows better retardation effect on the decomposition of ZDDP than carbonate-sulfonate (salicylate) hard-core RMs. The neutral (normal) phenates, but not sulfonates and salicylates, also retards the decomposition of ZDDP, as does the hard-core micellar phenate. Phenates are much stronger bases than salicylates and sulfonates: phenates pK,(Hp) ~ 4

>

salicylates pK,(Hp) ~ 11

> sulfonates pK,(H,0) - 13.3

A combination of ZDDP and hard-core RMs leads to a synergistic effect of metallic detergents on the degradation of ZDDP. These phenomena are observed in many tests and can be explained in terms of: (a) the acid neutralization property of hard-core RMs that leads to the prevention of decomposition of ZDDP (in the valve train wear test and the thin film oxygen uptake test), (b) the competitive adsorption of detergents that reduce the effective concentration of ZDDP on the metal surface (in the four-ball test), (c) the formation of mixed films on the metal surface, formed through the decomposition of ZDDP in the presence of hard-core RM's (the coefficient of friction in the Falex wear test). X-ray absorption study of tribofilms generated from a combination of ZDDP and borate-sulfonate RMs was used to determine the chemistry of tribochemical films at the surface and the bulk The calcium phosphate content in the tribofilm generated from either ZDDP + borate-sulfonate RMs or from ZDDP + calcium sulfonate soft-core RMs is similar (Varlot et al., 2001), Calcium sulfonate S(+5) undergoes disproportion reaction to form sulfate S(+6) and sulfite S(+4), and the presence of ZDDP affects the disproportion process. Close to the steel surface, the

Micellar Structure ofLubricating Formulations

107

tribofilm is mainly composed of the species originating from the borate-sulfonate micelles and not from ZDDP, This suggests that under rubbing conditions, the borate-sulfonate micelles interact more effectively than ZDDP on the surface. ZDDP and the borate-sulfonate micelles interact with each other under rubbing conditions and form calcium phosphate. The results for surface processes generated from ZDDP and borate-sulfonate RMs are summarized in Table 3.11 and Table 3.12. The antiwear mechanism of ZDDP alone can be decomposed depending on the severity of the wear test (Martin, 1999; Martin et al., 2000a): (a) in the mild conditions the long-chain zinc polyphosphate film is generated on the surface; (b) in the more severe conditions, any oxide abrasive particles are immediately eliminated by the short-chain Fe/Zn polyphosphate glass. The acidbase reaction governs the tribofilm composition; (c) in severe conditions (extreme pressure), nascent metal surfaces can be produced by the removal of the tribofilm itself by the wear process. The sulfide species present in the tribofilm can quickly react with the iron (in acid-base rection, FeS ).

Table 3.11. The chemistry of tribofilm generated by multifunctional additives composed of soft-core and hard-core reverse micelles (RMs) in oil formulation. Evaluation of tribofilm by XANES spectroscopy (Varlot et al, 2001) Surface processes under rubbing conditions

Soft-core RMs

Hard-core RMs

ZDDP + calcium sulfonate

ZDDP + calcium boratesulfonate

Tribofilm composition

Calcium phosphate present

Calcium phosphate present

Sulfonate (S^O status

Disproportion to sulfate (S'O and sulfite (S'^)

Disproportion to sulfate (S'O and sulfite (S^^ in high concentration

Micelles in tribofilm

RMs not identified

RMs and sulfonate present

Antiwear function of ZDDP Wear scar width {jjm)

Retarded by RMs

Retarded by RMs

90 (ZDDP in base oil 125)

205^ (235 in oil base only)

* The presence of the unreacted borate-sulfonate RMs at the tribofilm compared to softcore RMs. Most effective antiwear additive gives the smallest wear scar width. It was proven that the carbonate-sulfonate hard-core RMs, but not the calcium carbonate powder, has an effect on acid neutralization and performance. The effective diameter of the micelles is 5 to 10 nm, and it is known that the smaller the size of the micelle, the greater its ability to neutralize acids. In order to develop high performance calcium carbonate in RMs, it is important to understand

108

Chapter 3

the formation mechanism of micellar solutions. Any experiment with micellar calcium carbonate detergents prepared from powdered calcium carbonate generates faulty results. Micellar calcium carbonate additives must be prepared by chemical reaction in the presence of detergent. The colloidal carbonate particles have a diameter of about 5 to 10 nm and the detergent layer thickness is about 2 nm. Some laboratories simulating this micellar process used powdered calcium carbonate (ground in a ball mill until the particles are reduced to less than 1 /^m) added at a 2 % level to a blend of ZDDP, with the 5 TBN for calcium sulfonate. No antiwear benefit was observed for the calcium carbonate powder addition. Very different results were obtained in wear performance with powdered micellar calcium carbonates. As shown, the oxidative degradation test of ZDDP in the presence of the calcium carbonate powder had no effect. The strong effect of acids on the decomposition of ZDDP was confirmed. The addition of ptoluenosulfonic acid promoted the decomposition of ZDDP significantly, even at 120°C5 whereas no decomposition of ZDDP occurred after 120 hours without acids (Mansot et a l , 1993a and 1993b; Marsh, 1997; Rounds, 1989; Yamada et al., 1992).

Table 3.12. The chemistry of tribofilm generated by multifunctional additives composed of soft-core and hard-core reverse micelles (RMs) in oil formulation. Evaluation of the surface and the wear of particles by X-ray Photoelectron Spectroscopy (XPS) and Highresolution Electron Energy Loss Spectroscopy (EELS), (Martin et al., 2000a; Varlot et al., 1999) Surface processes under rubbing conditions

Soft-core RMs

Hard-core RMs

ZDDP + succinimide (PIBSI)

ZDDP + calcium boratesalicylate

Tribofilm composition

Calcium and zinc borophosphates with oxides, sulfonates and nitrates

Zinc polyphosphates

Additive status in tribofilm

ZDDP => zinc polyphosphate PIBSI =* iron oxide

ZDDP ^ zinc polyphosphate Calcium borate-salicylate calcium borate glass

Micelle status in tribofilm

PIBSI identified, oxidized species, residual succinimide

Salicylate not identified

Antiwear function of ZDDP

Hindered by presence of succinimide

ZDDP acting first in the mixture

109

Micellar Structure of Lubricating Formulations

Comparative friction tests under boundary conditions suggest three types of interactions between ZDDP and carbonate-sulfonate RMs in the oil phase (Kapsa et ai.5 1981): (a) chemical interactions between ZDDP and hard-core RMs of carbonate-sulfonate lead to an effective ZDDP concentration decrease; (b) the detergent effect due to the presence of calcium sulfonate molecules prevents materials from agglomeration during running; (c) the specific role of the hard-core reverse micelles. Wear rates in ring wear tests (thickness worn, expressed in meters per disk revolution) have been calculated at the beginning and at the end of each test. Wear surface analysis was done for n-dodecane with additives present in four tests. Based on values of ring wear rates (thickness worn) ratios R were calculated. The ratio R = (Thickness worn before and during films formation) / (thickness worn after films formation); e.g., R value of the calcium sulfonate = 3.9 nm / 1.5 nm = 2.6. Calcium sulfonate, TBN = 0.06

R = 2.6

Micellar carbonate-sulfonate, TBN = 47.3

R= 5

ZDDP(wt%=l)

R = 20

ZDDP + micellar carbonate-sulfonate

R = 20

As shown above, ZDDP showed comparable ratio value R to that of (ZDDP + micellar carbonate-sulfonate RMs). Electrical contact resistance (ECR) studies have shown tribofilm formation in the case of carbonate-sulfonate RMs, but here it appears that the tribofilms have only "poor" antiwear properties. The presence of ZDDP in the solution make the wear rates decrease. Auger analyses of surface coating mainly indicated the presence of P, S, Zn, typically associated with ZDDP activity; and also C, O and Fe. Concerning the wear rate, it is obvious comparing cases of n-dodecane and carbonate-sulfonate RMs, that an oxide film makes the wear decrease and one could think that in the case of n-dodecane + ZDDP, an effect of ZDDP is to permit the formation of an oxide film. For the case of ZDDP + hard-core micellar carbonate-sulfonate, the film covering the worn surface is mainly composed of calcium and oxygen. Notracesof elements present in ZDDP were found. Under these conditions, there is strong evidence for the existence of a large interfering mechanism between ZDDP and the carbonate-sulfonate RMs. The case study. The composition of the surface tribofilms formed by ZDDP and of carbonate-phenate RMs in a cam and tappet friction apparatus were examined using a combination of surface analysis techniques. Adding carbonatephenate RMs to ZDDP resulted in partial replacement of zinc by the detergent metal and loss of the higher molecular weight phosphates in favor of ortho- and

110

Chapters

pyro-phosphates. Highly refined paraffin oil was blended with conventional additives, but without polymeric VI improver, see Table 3.13. - Case (A) was a simple solution of ZDDP with the mixture of secondary propyl and hexyl groups. - Case (B) was ZDDP + polyisobutylene succinimide dispersant, and a hard-core micellar carbonate-phenate RMs, [Ca] = 890 ppm. - Case (C) was similar to case (B), but contained five times higher concentration of carbonate-phenate RMs, [Ca] = 4220 ppm. The effect of ZDDP, dispersant and carbonate-phenate RMs on the tribofilm composition is given in Table 3.13. Table 3.13. The effect of ZDDP, dispersant and carbonate-phenate hard-core RMs on the tribofilm formation in paraffmic oil (Willermet et al., 1995a) Case

Additives

Tribofihn composition^

A

ZDDP only

Composed of long or high molecular weight chains such as me/a-phosphates (P03^") with zinc as a cation

B

ZDDP + polyisobutylene succinimide dispersant + calcium carbonate-phenate; [Ca] = 890 ppm

Composed of inorganic, low molecular weight amorphous short chain ortho(P04^") and pyro- (P2O7'*") phosphates (±20% of zinc was replaced by calcium in phosphate tribofihn)

C

ZDDP + polyisobutylene succinimide dispersant + calcium carbonate-phenate; [Ca] = 4220 ppm

Low-molecular weight phosphates, short chain ortho- and pyro-phosphates (±50% of zinc replaced by calcium in phosphatefilm;some carbonates detected m tribofilm)

""Average tribofilm thickness (nm): (A) 53, (B) 60, and (C) 40;frictioncoefficient ~0.L The tribofilms were generated from base stock solutions of a secondary (sZDDP) and a mixture of s-ZDDP and detergent and dispersant, in a cam and tappet friction apparatus, using reflectance-absorbance infrared, Auger, and XPS spectroscopies. The data support the conclusions that in the absence of hard-core RMs the films were essentially zinc phosphates and that adding hard-core RMs resulted in partial replacement of zinc by the metal of the detergent. In the presence of hard-core RMs, only short chain phosphates were observed. Films were low in sulfur, with sulfur being present mostly as a sulfide. The carbonate increased at high levels of hard-core RMs and was then easily identified by IR spectroscopy (Willermet et al., 1995a),

Micellar Structure of Lubricating Formulations

111

(v) Micellar CuO and Cu as multifunctional additives. The hard-core reverse micelles are composed of colloidal CuO (or colloidal metal Cu) core surrounded by oleic acid strongly bonded to the core, see Fig. 3.1. The shape and size of micellar particles can be determined using transmission electron microscopy and a roentgenography method. The colloidal particles of a copper oxide containing additives which look like needles, but pure metal copper particles resemble spheres. The dispersion particles have a crystalline structure of copper oxide with a thickness of about 10 nm and a length up to 40 nm. The metal copper colloidal particles diameters are about 25 nm. The colloidal particles are easily dispersable in oils, fuel, and water, forming a stable colloidal solution (Shpenkov, 1995a). Micellar copper particles (and copper oxide) can give maximum benefit when used as multifunctional additives in liquid lubricants or greases, internalcombustion (IC) engines, fuels, cutting fluids, and hydraulic fluids. Micellar copper oxide is prepared under conditions such that copper oxide is formed by chemical reaction (CUSO4 + NaOH) in the presence of oleic acid as the surfactant and dispersed in hydrocarbon oil (Shpenkov, 1995a; Shpenkov and Sagatowski, 1992). Fundamental data with micellar copper additives in motor oil were obtained by Shpenkov from an internal-combustion engine test (Shpenkov, 1995b) using the micellar CuO additive. The micellar CuO additive is characterized by a high improvement in lubricant and fuel properties. A concentration of the additive at about 0.006% is needed in the motor oil and fuel products to note provide improvement of its maintenance properties. Effectiveness of mineral motor oil Superol CD 15W/40 containing micellar CuO additives after 100 hrs in a test engine at maximum power and torsional moment are shown below (Shpenkov, 1995a and 1995b): - motor oil consumption is decreased to 22%; - fuel consumption is decreased by 2 - 5.8%); - the fuel combustion temperature is decreased by 10 - 15 °C; - compression in a combustion chamber is increased by 4 - 5.7%; - the temperature in the oil film insignificantly decreased (test with AlmenFalex tribometer); - the wear of the engine is essentially decreased (about 10%, in a four-ball tester). Transfer and adhesion of the micellar particles accelerates the surface modification, self-reducing and forming a fine copper film as a selective transfer phenomenon. The term "selective transfer" (ST) is an adopted abbreviation for decreased wear and the friction coefficient, which is a result of the formation of a thin copper film induced by boundary lubrication (Garkunov and Kragelsky, 1968). The formation of the tribofilm of pure copper was reported in such different tribosystems as airplane landing gear (cylindrical coaxial repricating bushes) lubricated by glycerin and refrigerator compressor bearings lubricated by

112

Chapters

freon (Garkunov, 1989; Kragelsky and Alisin, 2001). Selective transfer is the most pronounced manifestation of tribochemistry of the boundary friction processes. Many physical and chemical theories were applied including nonequilibrium thermodynamics and self-organization. Physico-chemical aspects of friction under ST conditions were addressed in the work (Kuzharov et al., 1981). A natural way to use the selective transfer phenomenon in real tribosystems was followed in motor oil with the use of the concept of micellar lubricants in 1992 1995, when a micellar copper additive was patented and experiments were performed (Shpenkov, 1995a and 1995b; Shpenkov and Sagatowski, 1992). The use of colloidal copper oxide additives is especially promising for traditional friction pairs made from metals and their alloys. Formation of the tribofilm layer on friction surfaces occurs under the effect of the field in the electrochemical metali-lubricant-metal2 system, owing to formation of electro-potential (emf), forming free copper tribofilm (Shpenkov, 1995a). Since the process of tribofilm formation takes place during the friction process, disintegration of the reverse micelles takes place in a tribochemical reaction, where a redox reaction occurs, and copper oxide reduces to free copper. Other processes responsible for formation of a fine copper tribofilm are mentioned in (Shpenkov, 1995a). In particular, copper ion deposition on steel is explained by contact substitution of iron for copper in solution. If sufficient metallic iron is in contact with the copper micelle, iron dissolution (friction will saturate the surface quicker) and copper deposition will continue until the activity ratio of their ions satisfies the equation: ^cu(2+) ' ^Fe(2+) ~

1"

Thus, with ferric ion activity equal to unity, copper activity is only 10"^^, i.e., the solution is almost completely free of copper ions. Fuel combustion with 0.005% copper micellar additives lowers the concentration of harmful components in the exhaust gas. Concentration of CO, NO^, and CH are decreased by up to 10%. Also soot and smoke decrease by 15% at low revolutions. The CuO particles in the flame are in an ionized state (Cu"^^, O"^) converting carbon oxide into carbon dioxide and hydrogen into water (Shpenkov, 1995b).

3.4. Tribochemical Interactions of Acid-Base Chemistries Tribochemical interactions of acid-base in tribosystem have been observed at two levels: (i) acid-base reaction in oil formulation; and (ii) acid-base reaction on metal surfaces.

Micellar Structure of Lubricating Formulations

113

(i) Acid-base reactions in oil formulation. Many physical and chemical processes occur in internal combustion engine lubricating oil systems, the majority being acid-base interactions in formulated hydrocarbons. The stoichiometry of acid-base combinations in non-aqueous solutions is more complex than in the case of aqueous solutions. N-base can combine with several associated molecules of carboxylic acids B...(HA)n, and inorganic acids, SH can form the ion pair BffS" with N-bases in engine oil formulations (Fox et al., 1991b). In mineral and synthetic engine oil formulations, hydrogen bond formation between anionic or molecular bases (A", B) and their conjugate acids HA or Bff, have a heavy impact on the activity of acids and their salts. For example, in mixtures of carboxylic acid with excess of sodium carboxylate in hydrocarbon and aprotic solvents, the activity of the acid is greatly decreased because of homoconjugation: A+ AHA- +

HA nHA

^ ^

AHAA-(HA)„^i

Kf(AHA) Kf(A-(HA)„^i

where Kf(AHA-) and Kf(A"(HA)n+i is the formation constant for the acid (HA) and base (A) (Van Looy and Hammett, 1959). The complex AHA- or A-(HA)n+i is called a homoconjugate, while hydrogen bonding with a non-conjugate acid, HR, is called a heteroconjugate AHR- (Davies, 1968; Kolthoff et al., 1968; Pawlak, 1972a and 1972b; Van Looy and Hammett, 1959). The formation constants K/BHB^) for the hydrogen bonding of cationic acids, Bff with free N-bases (B). Bff BH^

+ +

B nB

^ -

BHB^ Bff(B)„,,

Kf(BHBO KXBHXB)„,i

have much lower values than for association of AHA, but very stable (BO...H...OB)^ conjugates are formed with amine N-oxides (Chmurzyriski, 1994; Chmurzyiiski and Pawlak, 1998; Fritch and Zundel, 1981; Gilkerson and Rolph, 1965; Pawlak, 1986; Pawlak and Wawrzynow, 1983). Strong hydrogen bond interactions appear to play a very important role in engine oil formulations stabilizing conjugate-forming acids such as R(CH2)nCOOH, ArOH, and R(CH2),NH3^ see Table 3.14. Carboxylic acids and phenols R(CH2)nC00H, ArOH, and cationic R(CH2)nNH3^ conjugated acids exist in engine oil formulations together with their own or different bases, and will interact to form hydrogen-bonded complexes (Fox e t a l , 1991b). [R(CH2),COO.H.OOC(CH2)„R]-; [ArO.H.OAr]"; [R(CH2)„NH2.H.NH2(CH2)„R]^

114

Chapters

Table 3.14. The acid-base reactions in oil formulation Additives

Sulfuric acid, SH or organic acid, WH

R(CH2)„C00-Me" ArOMe* R(CH,)„NH2 R3N

+ + + +

SH SH SH nWH

=

Products Conjugate acid

Conjugate base

R(CH2)„C00H ArOH R(CH2)„NH3^SR3N...(HW)„

+ +

Me^SMe-'S-

where n = 2 to 6

When this conjugation occurs, the level of active (corrosive) acid is substantially decreased. No simple quantitive correlation has been shown between the acidity (pKJ of acids in hydrocarbon formulation and low polar solvents (Coetzee, 1967). Acid-base interaction with and without proton transfer (PT) (BH^A", B...(HA)^) has been related to acid and base enthalpies of reaction (Pawlak and Bates, 1982), the infrared carbonyl stretching band and gradual appearance of the asymmetric COO" band (Lindeman and Zundel, 1972; Magoiiski and Pawlak, 1982), changes in pH (Kuna et al., 1982; Pawlak et al., 1982), NMR proton chemical shifts (Magoiiski and Pawlak, 1982), and dipole moments (Sobczyk and Pawelka, 1979). These parameters depend upon the acid-base strength of the partners, ApK/PT) the difference between the pKa(acceptor) and pKa(donor) on the water scale (Sobczyk, 2001). ApK,(PT) = [pK,(H20),acceptor - pK,(Hp),donor)] The values of ApK/PT), which indicate proton transfer between acid and base, resulting in the formation of the ion pair (B + HA = BH'^A") in less polar solvents like benzene, cyclohexane, toluene, and trichloroethylene with dielectric permittivity < 10, are between 4 to 8 units (Dega-Szafran and Dulewicz, 1981; Jadzyn and Malecki, 1972; Sobczyk and Pawelka, 1973; Zeeger-Huyskens and Huysken, 1981). The ApKa(PT) values show that for less polar solvents, aliphatic acids are about 10^ to 10^ times weaker than protonated aliphatic N-baseH^, whereas aromatic carboxylic acids and phenols are about 10"^ to 10^ times weaker. There is no absolute acidity scale for substances in low polar solvents. The majority of detergents are rather weak bases and can only participate in acid-base reactions with strong inorganic acids. Proton transfer processes will occur in low dielectric permittivity solvents when ApKa(HA), which is the difference between the acidity of proton acceptor and proton donor, is more than five units on the water scale. Acid-base reactions in hydrocarbon oil formulations and low polar media can be formulated as acid-base association constants between an acid and base:

Micellar Structure of Lubricating Formulations

HA(acid) + B(base)= BHA(salt);

KBHA =

115

[BHA]/[HA] [B]

Simplificaion of the acidity or basicity scale in low polar media consists of a series of equilibrium constants corresponding to the reaction above, with either B or HA being the reference compound. The acid-base association constants for the equilibria of substances interacting within these solvents, will be seen to fall mainly in the range logKgH^ = 2 to 7 (Davies, 1968). In engine oil acid-base reactions with proton transfer, logKeH+A. > 10^, occurs in systems, e.g., sulfuric acid + carbonate or sulfuric acid + phenate; however, the majority of reactions belong to the group without proton transfer, logKg ^^ e.g., carboxylic acid + succinimide or phenol + succinimide. In engine oil, the decrease in total base number (TBN) can only be partially attributed to an increased concentration of acidic products in the oil. Many studies have shown that the major decrease in formulated lubricating oil takes place in the first few hours of service life, when the oil oxidation process has not yet developed (Pawlak, 1980; Pawlak et al., 1985). This is supported by total acid number (TAN) values found during the same period of oil use. The decrease in TBN does not result solely from the interaction of the alkaline additives with acidic oxidation products, but also reflects the possible interaction of the additives themselves. In addition, at this time, the high molecular weight compounds will also be undergoing cleavage and decomposition. Acid-base reactions in non-aqueous solvents presented here are well characterized for small molecules (Bruckenstein and Saito, 1965). The larger, long chain molecules in solvents of low dielectric constant form aggregates above certain concentrations. (ii) Acid-base reactions on metal surfaces. The strong adsorption of basic molecules on a metal surface is usually considered an electron donation process from the base to the metal, e.g., an acid-base reaction. A + :B -

A:B

AH"

where A (metal atom) is a Lewis acid, or electron acceptor, and B is a base, or electron donor. The acid-base complex, A : B, can be an organic molecule, an inorganic molecule, a complex ion, or anything that can be held together by even weak chemical bonds (the cohesive energies, -AH°, kJ/mol). Both metals and non-metals can be either (a) or (b) type of acids depending on their charge and size. Since the features which promote class (a) behavior are those which lead to low polarizability, and those which create type (b) behavior lead to high polarizability, it is convenient to call class (a) acids "hard" acids and class (b) acids "soft" acids. We then have the useful generalization that "hard acids prefer to associate with hard bases, and soft acids prefer soft bases" (Pearson, 1997). This is the Principle of Hard and Soft Acids and Bases, or the

116

Chapters

HSAB Principle. Complexes with neutral metal atoms typically contain the soft base characteristic of class (b). From the positive oxidation state of metals, it can be predicted that metals at the zero oxidation state will always be class (b) acids. Typical examples of acids (or bases) being called class (a) hard and class (b) soft are listed below (Pearson, 1997). Classification of Lewis Acids: - Class (a) hard acids: ff, Li^ Na", K^ Mg'\ Ca'", AP", Cr'^ Co'^ Fe'^ As'^ Mo'^ B(OR)3, RP02^ SO3, CO2 - Class (b) soft acids: Cu^ Ag^ Cs", Cd'", Pt'", Hg'", RS\ T, Br", HO", RO", I2, Br2, trinitrobenzene, O, CI, Br, I, M^ (metal atoms), bulk metals. - Borderline: Fe'", Co'", Ni'", Cu'", Zn'", Pb'", SO2, NO" Classification of Lewis bases: - Class (a) hard bases: H2O, OH", F , CH3COO-, PO/", ^0,^\ CI", CO3'-, CIO/, N03-,R0H, RO-, R2O, NH3, RNH2, O'- Class (b) soft bases: R2S, RSH, RS", T, SCN", 8203^ R3P, R3AS, (RO)3P, CN", RNC, CO, C2H4, C^H^, H-, R-, S'" - Borderline: C6H5NH2, C5H5N, N3-, Br", N02-, SO3'-, N2 The idea that a metal atom in the zero oxidation state is both a soft acid and soft base can be used to explain surface reactions of metals. Soft bases such as carbon monoxide and olefins are strongly adsorbed on surfaces of the transition metals. Bases containing P, As, Sb, Se, and Te in low oxidation states are strongly adsorbed, blocking the active sites (Pearson, 1966). The clean surfaces are incomplete solids, in that the surface atoms have no nearest neighbors in one of the three-dimensional coordinate system. This means that there are atomic orbitals, both filled and empty, which are not being used to form surface orbitals. Some remarks can be made about properties of metal surfaces without knowing the detailed structures. Because surface atoms are higher energy than bulk atoms, this leads to surface tension. Metal surface tension can be described as the excess energy needed to form a unit area of surface. Statistically, the electronic chemical potential for surface metals must be the same as for the bulk atoms; however, the work function (WF) for surface atoms will be different from that of bulk atoms There is experimental evidence for changes of the metal surface energy charge. The higher surface energy will develop a positive charge when the valence band is less than half full. The lower surface energy will become more negative as the valence band fills, having a smaller work function (Saillard and Hoffmann, 1984; Shustorovich and Baetzold, 1980). In an effort to reduce the surface energy (surface tension), a reaction between

Micellar Structure of Lubricating Formulations

117

the surface atoms and the adsorbate molecules acting as the second reactant leads to chemisorption. The atoms of the surface will adsorb the adsorbate molecule, especially small molecules, such as oxygen or water. The study of surfaces is a difficult one, and a number of very special experimental methods have been developed (Hamers, 1996; Somorjai, 1981). In the chemisorption process, a transfer of electron density between the two reactants takes place with bondbreaking in the adsorbate. Due to the acid-base character of the surface a high value of the softness also leads to better interaction with the substrate (Baetzold, 1983; Stair, 1982; Sung and Hoffmann, 1985). From the chemist's point of view, the cohesive energy of solids is their most important property. The cohesive energy (AE^^) is defined as the energy required to dissociate one mole of solid M(s) into its constituent gas atoms M(g): M(s) = M(g)

AE,exp

The mechanism by which a tribofilm actually forms on the surface has been given by several investigators (Martin, 1999; Martin et al., 2001; Willermet et al., 1992 and 1995b; Yin et al., 1997a and 1997b) and is proposed on the basis of the hard and soft acids and bases (HSAB) principle. During the wear processes the acid-base reactions between the nascent surface, oxygen dissolved in the lubricant, and phosphates take place. The principle of soft and hard acid and bases (HSAB) has been shown to be useful in rationalizing of surface processes (Pearson, 1973 and 1997). The HSAB principle states that hard acids (Fe surface) prefer to coordinate with hard bases (oxygen, phosphates). Hard interactions are normally ionic. Iron oxides can be readily formed and the antiwear mechanism starts to interact with the polymeric zinc metaphosphate, Zn(P03)2. 3Fe

+

2O2

Zn(RO)4P2S4 + Oa (or ROOH)

-*

FcjOa (and FeO)

-

Zn(P03)2 + Sulfur species

5Zn(P03)2

+

FePs

-

Fe2Zn3Pio03| + 2ZnO

Zn(P03)2

+

FeO

-

FeZnP207

Zn(P03)2

+

2FeO

-

Fe2Zn(P04)2

Also, other intermediate reactions starting between Fe203 and zinc thiophosphate Zn6(Pio029S2) can promote the formation of ZnS as precipitates in the short chain polyphosphate material VQZX\(?2^1)2' The results of tribochemical processes should be composed of Fe/Zn short-chain polyphosphates covered by

118

Chapters

a zinc polymer-like.phosphate at the top (Martin, 1999; Martin et al., 2001). Summary: Characterization of the acidity in the hydrated soft-core RMs is important as soon as ionizable compounds are to be solubilized in the water pool. The micellar soft-core RMs has a very high degree of organization of water. The water pool within reverse micelles is a different solvent than bulk water. The most interesting range of water contents corresponds to rather small water pools (waterto-surfactant ratio 3 to 10) in which peculiar properties of water cause the largest changes in the behavior as compared to their behavior in bulk water. A water to surfactant ratio of 1:1 represents a very small, hardly detectable amount of water. But, as has been pointed out, the necessary water concentration can be far smaller and still support the growth of the aggregates. Borate-salicylate hard-core RMs have recently been recognized as an efficient multifunction class of anticorrosive-antiwear additives. They are composed of nanometer size particles of calcium borate micellized by an organic shell of salicylate molecules. The additive acts as an antiwear agent by the formation of a calcium borate glass tribofilm material. The hard-core RMs contained in oil undergo important structural and chemical changes. First, the organic shell detaches from the micelles and becomes oxidized under extreme conditions of temperature, pressure and mechanical shear. The liberated calcium carbonate reservoirs can crystallize, leading to the formation of a polycrystalline tribofilm which poorly adheres to metallic surfaces. This has been observed in the case of carbonate-salicylate hard-core RMs. The use of borate-salicylate hard-core RMs in the friction test leads to the formation of a thin amorphous film, which can be described as a composite material. This material is composed of a calcium and iron borate matrix with some embedded crystallites of calcite from impurities in the borate-sulfonate micelles. The boron behaves as a glass-former like phosphorus does with the ZDDP additive. The combination of ZDDP and boratesalicylate RMs is expected to be synergistic, due to the formation of a mixed phosphate-borate glass tribofilm.

Problems 3.1 Solubilization By reference to Figure 3.1, "The soft-core RMs", explain the solubilization process of carboxylic acids, such as acetic acid and oleic acid by calcium phenolate RMs in engine oil. 3.2 Equilibria How do the following processes change in equilibria of surfactant molecules in mineral or synthetic oil: (a) dissolution, (b) micellization, (c) solubilization, (d) interfacial process? (see interpretation of Figure 3.3).

Micellar Structure of Lubricating Formulations

119

3.3 Aggregation number Three main factors play a critical role in aggregation of soft-core reverse micelles: the interaction between polar groups, the interaction of the hydrophobic (non-polar) part, and environmental factors. Compare aggregation numbers of some surfactants in low polar solvents specified in Table 3.1. 3.4 Acids and bases No simple quantitative correlation has been shown between the acidity, pK^, of acids (or conjugated bases) in mineral, synthetic formulation and low polar solvents. N-base (B) can combine with several associated molecules of carboxylic acids B...(HOOCR)n, and inorganic acid (SH) can form the ion pair BH^S". Write net equations for each of the following interactions: (a) strong acid SH with R(CH2)nNH2, (b) strong acid SH + ArOMe^, and (c) carboxylic acid with N-base, R3N. 3.5 Micellar interactions Write net equations for the following: (a) carbonatebenzenesulfonate RMs solution with strong acid SH, (b) soft-core phenolate RMs and strong acid SH, and (c) soft-core carboxylate reverse micelles solution with carboxylic acid. 3.6 Water pool (Wo) in reverse micelle What information about soft-core micellar structure is indicated by the experiment with hydrated electrons as listed in Table 3.2? Explain, why at Wo = [water] / [detergent] < 5, the water in the water pool is highly immobilized and is not able to hydrate electrons. 3.7 Calcium borate-detergent and calcium carbonate-detergent hard-core RMs Calcium borate is stabilized in hard-core RMs with detergents such as sulfonate, salicylate and phenate and evaluated in comparison with calcium carbonatedetergent RMs. The hydro lytic stability, oxidation stability, thermal stability, friction and wear characteristics of both systems are shown in Table 3.7, Why have borate-detergent RMs been recently recognized as an efficient multifunctional class of anticorrosive-antiwear additive? 3.8 Soft-core and hard-core reverse micelles, RMs The retardation of the decomposition of ZDDP by soft-core and hard-core RMs appears promising for high performance engine oils as shown in Table 3.11. Compare the chemistry of tribochemical films generated by multifunctional soft-core and hard-core RMs systems. 3.9 Neutralization and solubilization What are the differences and similarities (if any) in the concepts of neutralization and solubilization of oleic acid and sulfuric acid by soft-core RMs and hard-core RMs in lubrication formulations?

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121

Chapter 4

TRIBOCHEMICAL NATURE OF ANTIWEAR FILMS

Tribochemical reactions may be assessed to have the most diverse effects on friction, lubrication, and wear processes in the formation of the tribofilm in surface protection. Gerhard Heinicke, 1984

ZDDP tribofilms are composed of inorganic polymer material: on the top surface the long chain polyphosphate is a zinc phosphate and in the bulk the short chain polyphosphate is a mixed Fe/Zn phosphate with a gradient concentration. Jean Michel Martin, 2001

4.1. Tribochemical Characterization of Antiwear Films The current understanding of the structure of ZDDP-based tribofilms derives mainly from XANES spectroscopy (Armstrong et al., 1997; Bancroft et al., 1997; Ferrari et al., 1999a; Fuller et al., 1995, 1998 and 2002; Martin et al., 2001; Yin et al., 1993 and 1997a), infrared spectroscopy (Harrison and Brown, 1991; Lindsay et al., 1993; Willermet et al., 1992 and 1995b), and a comprehensive multi-technique (AES/XPS/XANES) approached on the basis of the hard and soft acids and bases (HSAB) principle (Martin, 1999, Martin et al., 2001;). Most prominent models involve a two-layer structure. The layer in contact with the metal surface is thought to be composed of short-chain polyphosphates, and a top layer, consists of long-chain zinc polyphosphate. In order to better understand the mechanism of antiwear functions it is essential to identify the compounds, as well as the elements, constituting the tribofilm. With the exception of XPS and X-ray diffraction techniques, all other physical techniques to date have focused on elemental analysis. Extended X-ray absorption, fine structure EXAFS and, in particular. X-ray absorption near-edge structure XANES techniques have, been shown to be very sensitive for structural and chemical speciation. The K-edge EXAFS spectroscopy has been used to study antiwear additives and tribofilms of some metals (Koningsberger and Frins, 1988; Martin et al, 1986a; Belin et al., 1989). Our objective in this chapter is to derive a chemical mechanism for the formation, composition and structure of surface tribofilms. These tribofilms result from interactions among additives, the base stock, the surface and the ambient

122

Chapter 4

environment under the contact conditions. Accordingly, an adequate understanding of the process involved in friction and wear reduction through tribofilm formation in lubricated systems requires knowledge and the consideration of: (A) The chemical characterization of tribofilm on sliding surfaces; (B) X- ray spectroscopy for chemical speciation of antiwear tribofilms; (C) The effect of physical parameters on tribofilm chemistry. (A) The chemical characterization of tribofilm on sliding surfaces. A number of studies have been performed on the chemical nature of antiwear tribofilms generated on steel surfaces using ZDDPs, but there is a disagreement as to chemical composition. The conventional analytical surface techniques are not sensitive enough to allow chemical speciation. The chemical nature of the tribofilm formed by degradation of ZDDP (zinc dialkyldithiophosphate) has been characterized by a number of different surface techniques: X-ray fluorescence XRP spectroscopy, electron probe micro analysis EPMA in conjunction with scanning electron microscopy SEM, energy dispersive X-ray EDX, two widely used surface techniques (X-ray photoelectron spectroscopy XPS, and Auger electron spectroscopy AES), and recently X-ray absorption near-edge spectroscopy XANES, see Table 4.1 (Ferrari et al., 1999a; Coy and Jones, 1981; Martin, 1999; Martin et al., 2001; Valrot et al., 2000; Watts, 1990; Yin et al., 1997a and 1997b). Using XRF and EPMA in conjunction with SEM and EDX the antiwear films were found to consist of P, S, O, and Zn (Brown et al., 1992; Rounds, 1993). The application of XPS and AES surface techniques promoted deeper understanding of the antiwear mechanism: elemental composition of the chemical species, valence of the elements, and depth distribution. Chemical speciation, e.g., phosphate and S (sulfide or sulfate) can be obtained from binding energies. Many researchers have studied the tribochemical nature of the film using XPS and AES surface techniques. The authors reported that antiwear films consisting of Zn, P, O, Fe, and P were present as a phosphate, and S present as a sulfide. Also, the formation of zinc phosphate and iron sulfide, and the presence of iron oxide is necessary as an intermediate step in forming a protective tribofilm (Bird and Galvin, 1976; Glaeser et al., 1993; Jahanmir, 1987; Martin, 1999; Martin et al., 2001; Spedding and Watkins, 1982; Watkins, 1982). The tribofilms formed from a ZDDP were composed of long chain phosphates with zinc as a cation but with a mixture of a ZDDP + detergent and ZDDP + dispersant, only short-chain phosphates with zinc and calcium as cations are formed in the presence of a sulfide (Barcroft and Park, 1986; Willermet et al., 1995a). One approach to obtain structural information on surface tribofilms is provided by reflectance-absorbance infra-red spectroscopy RAIR. ZDDP is known to decompose along either thermal or thermo-oxidative pathways. The studies of

Tribochemical Nature ofAntiwear Films

123

degradation products of ZDDP for samples generated under these circumstances showed similarities of product formation. The similarity between the spectrum for the precipitate formed in the thermo-oxidative way and that for the tribochemical film is striking, particularly if the bands at 1300 to 1500 cm'^ and 1000 cm"^ are assigned to residues of the ZDDP. These spectra suggest that the tribochemical film was formed by thermo-oxidative degradation, rather than by thermal degradation (Willermet et al., 1992; Yin et al., 1997a). Studies of carbonate-sulfonate RMs in engine oil on the antiwear performance of ZDDP, using reflectance-absorbance infrared spectroscopy RAIR, X-ray photoelectron spectroscopy XPS and Auger electron spectroscopy AES, showed that the tribofilms were inorganic amorphous phosphates, mainly orthophosphate (P04^") and pyrophosphate (?2^i^) associated with zinc and magnesium (from the hard-core RMs) (Willermet et al., 1991 and 1992). The tribofilms consist of amorphous, short-chain phosphates (ortho- and metaphosphates), with some evidence of sulfur incorporated into the phosphate chain structure. Tribofilm structure depends not only on the lubricant composition of the substrate material, but also on the severity of the rubbing contact. The ZDDP typically forms phosphates under antiwear conditions, whereas sulfides and sulfates are typically formed under severe or very severe conditions, />^ > 0.8 (Willermet et al., 1997). Some fundamental aspects of tribochemistry of ZDDP have been investigated using an ultrahigh vacuum UHV analytical tribometer on chemisorbed films previously formed on a steel surface (Martin et al., 1996). The steel surface was immersed in a solution of ZDDP at a concentration of 2 wt.% in PAO synthetic lubricant base at 130°C for 24 hours. The chemistry of the treated steel surfaces was investigated by XPS and AES, and friction tests were carried out in UHV just after the analysis. At the end of the friction tests, AES microanalysis was performed both inside and outside the wear scar. The AES analysis showed that sulfide was present inside the wear scar and phosphorus was eliminated from the surface, presumably as wear debris. Summary data of the tribofilm composition after ZDDP degradation are presented in Table 4.1, The study of the neutral and basic diisobutyl (primary) ZDDPs form were shown to be in equilibrium. [(RO)2PS2]6Zn40 basic

^

3 [(RO)2P(S)S]2Zn +ZnO neutral

This transformation depends on the temperature and polarity of the solvent. Lower temperatures and lower polarity favor the basic ZDDP species (Yamaguchi et a l , 1996). From initial solution of 89% basic + 11% neutral at 26°C, the equlibrium changes at 150°C to 67 % and 33%, respectively (Varlot et al., 2000).

124

Chapter 4

Table 4.1. The tribofilm composition after ZDDP degradation based on results of surface techniques Surface techniques^ and the tribofihn composition XRF The four-ball test wear surface after thermal decomposition was composed mainly of Zn, S and P (Rounds, 1975). XPS, AES The antiwear film consisted of Zn, P, O, S and Fe; P was present as a phosphate and S present as a sulfide (Bird and Galvin, 1976; Spedding and Watkins, 1982; Watkins, 1982). - The formation of zinc phosphate on topmost surface and chemisorbed iron sulfide layer close to the metal substrate was observed (Glaeser et al., 1993; Jahanmir, 1987). - The engine oil containing ZDDP and carbonate-detergents RMs formed orthophosphate and pyrophosphate tribofilms. In the presence of detergent and dispersant only short-chain phosphates were observed (Willermet et al., 1991, 1992 and 1997; Yin et al., 1997b). - XPS depth profiling of tribofilms formed in 30 min, 60 min and 12 hrs had been performed, the thickness of the first tribofilm was ~5 nm and the second film was ~30 nm. For both fibns, Zn, S, P, C, O and a small amount of Fe were seen on the surface. (Bell et al., 1992; Cao et al., 1990, Chao et al., 1994; Georges et al., 1979; Glaeser et al., 1993; Jahanmir, 1987; Lindsay et al., 1993; Martin, 1999; Martin et al., 2001, Rhodes and Stair, 1993; Willermet etal., 1993). EXAFS Tribofilms were amorphous with long-chain polyphosphates with Fe as the cation (Belin et al., 1989 and 1995; Martin et al., 1986a). XANES The chain length ofpolyphosphate is related to the alkyl groups in ZDDP. When zinc was present m the additives, sulfiir in the film remained in the reduced form S^', but when zinc was absent, sulfur was oxidized to the sulfate. The degree of phosphate polymerization can be quantified for sodium/zinc phosphate glasses and antiwear tribofilms. The spectra intensity is proportional to poly-phosphate chain length (Ferrari et al., 1999b; Kasrai et al, 1993 and 1996; Yin et al., 1993, 1995, 1997a and 1997b). - The tribochemical and thermally generated fibns contained mostly a mixture of short-and long-chain phosphates. The sulftir in tribofilms is always present as sulfide (Canning et al., 1999; Fuller et al., 1997; Yin et al., 1997a). - A Cameron-Plint friction machine generated tribofilms with two-layer structure: a zinc polyphosphate "thermal film" overlying a mixed short-chain phosphate glass, containing iron sulfide precipitates. A tribochemical reaction between the zinc polyphosphate and the iron oxides species is proposed on the basis of the hard and soft acid and base HSAB principle (Martin, 1999; Martm et al, 2001). XPS,AES,RAIR The tribofilms were amorphous phosphates which are mainly orthophosphate (PO/) and pyrophosphate (P207^") associated with Zn and Ca derived from carbonate-detergent RMs (Willermet e t a l , 1991, 1992, 1995a).

Tribochemical Nature ofAntiwear Films

125

Table 4.1. (Continued) Surface techniques^ and the tribofilm composition - ^H, ^^P products were identified: trialkyltetrathiophosphate as oil soluble products and a white precipitate, which was rich in Zn and O and low in P and S, compared with the original ZDDP (Coy and Jones, 1981). ECR Both sulfonates and ZDDPs form electrically resistive film. Electrical contact resistance measurements may help to guide the work of synthetic and mineral oil formulation chemists (Komvopoulos, et al., 2002; Yamaguchi et al., 1997). - ZDDP alone quickly forms a nonconducting durable antiwear film; high TBN phenate forms a nonconducting, reasonably durable antiwear film; the combination of ZDDP and high TBN phenate form nonconducting, non-durable fihns; dispersant alone forms a poor, non-durable fihn; dispersant interferes with the ability of either ZDDP or phenate detergent to form a durable film; reducing the levels of ZDDP, phenate and dispersant simultaneously results in a non-durable film that takes longer to form; future low phosphorus engine oils will most likely require supplemental antiwear additives to maintain durable antiwear films (Yamaguchi et al., 2002a). XANES, IPS The tris-[p-(perfluoroalkylether)-phenyl]phosphine dissolved in perfluoropolyalkyl-ether oil decomposed on the surface, within the wear track, forming a tribofilm composed of a polyphosphate glassy material (Cutler et al., 1999). Optical Interferometry The tribofilm was unevenly-distributed with its roughness, was more strongly dependent on the extent on rubbing than on temperature (Gunsel et al., 1993; Taylor et al., 2000). ^XRF = X-ray fluorescence spectroscopy, XPS = X-ray photoelectron spectroscopy, AES = Auger electron spectroscopy, XANES ^ X-ray absorption near edge spectroscopy, RAIR = Reflectance-absorbance infrared spectroscopy, EXAFS = X-ray absorption finestructure spectroscopy, ECR = Electric contact resistance, NMR = Nuclear magnetic resonance spectroscopy, IPS = Imaging photoelectron spectromicroscopy. In a recent study four independent groups of investigators compared the wear performance of neutral and basic ZDDP: neutral [(RO)2P(S)S]2Zn and basic [(RO)2PS2]6Zn40 forms of zinc dialkyl (or diaryl) dithiophosphate, primary diisobutyl ZDDP, secondary di-isopropyl ZDDP, and aryl di(para-tert-octyl)phenyl ZDDP. One group, investigated the adsorption of these additives on AI2O3 and relative wear performance using inelastic electron tunneling spectroscopy. Both the adsorption and wear performance of the aryl ZDDP is markedly different from the alkyl ZDDPs. Tribochemical films were generated from engine oils containing a variety of additives and investigated by reflectance-absorbance infrared RAIR,

126

Chapter 4

XPS and AES spectroscopies. The other group reported that the tribofilms were inorganic amorphous phosphates, mainly orthophosphate (P04^"), and pyrophosphate (P207^) associated with zinc and magnesium from hard-core RMs. Tribofilms were low in sulfur, being present mostly as a sulfide. A recent XANES experiment showed that the ZDDP tribofilm generally has a two-layer structure: a short-chain phosphate layer covered by a thin, long-chain polyphosphate layer. XANES spectra analysis of tribofilms did not show a substantial difference between neutral and basic ZDDPs (Fuller et al., 1997; Martin, 1999; Martin at al, 2001; Varlot et al., 2000; Willermet et al., 1991 and 1992; Yamaguchi et al., 1993 and 1996). The uncertainty in the mechanism of antiwear tribofilm formation derives in part from observations that exposing metal surfaces to heated ZDDP/oil solution forms films similar to those generated in a tribochemical way. From the utility standpoint, both thermal and tribochemical films seem to provide protection from wear. Thus, the current model involves both a tribochemical and thermooxidative component for the decomposition of ZDDP and tribofilm formation (Aktary et al., 2001; Bancroft et al., 1997; Fuller et al., 1997 and 1998; Martin, 1999; Willermet etal., 1995b; Yin etal., 1997a). The case study. The XANES spectroscopy at the (P) L-edge and (S) L-edge spectra has been used to characterize the chemical nature of tribochemical and thermally generated films from several ZDDP antiwear agents in neutral and basic (primary and secondary) alkyl as well as aryl form (Fuller et al., 1997). Thermal decomposition (oxidation) samples of the ZDDPs in air were prepared by heating the pure solid powders in a small vial in an oven at 100 to 200 °C (see Table 4.2). The thermally generated films were prepared by suspending steel coupons in an Erlenmeyer flask with the preheated oil solutions containing the additives at concentrations similar to those used for the tribofilms, and in air. The tribochemical films were generated on A2 steel using a Cameron-Plint wear machine (100°C, 225N and 25 Hz) with a rubbing time of 0.5 to 12 hrs. The characterization of tribofilm and thermal film is presented in Table 4.2. At lower temperature, thermally generated film has more similarity to tribofilm. The thermal films, as in the case of the tribofilms, contain mostly a mixture of short-and long-chain polyphosphates. The chemical states of phosphorus and sulfur in each neutral and basic pair were very comparable. The aryl phosphate films contain long-chain polyphosphate throughout the film, whereas the alkyl phosphate films are composed of long-chain polyphosphates in the bulk. Also, the aryl polyphosphate films contain more unchanged ZDDP. The (P) L-edge XANES spectra of tribofilms and thermal films generated from the neutral di-isopropyl ZDDP along with the model compounds (zinc metaphosphate and zinc pyrophosphate) are very similar and compare well with model compounds. The surface may also play an important role in catalysis of the thermal decomposition and provide oxygen for phosphate formation. There is also

Tribochemical Nature ofAntiwear Films

111

experimental indication that hydroperoxide (ROOH) plays an important role in providing oxygen for replacing sulfiir by oxygen in ZDDP oxidation occurring in the transformation of ZDDP to polyphosphate (Fuller et al., 1995 and 1997; Willermet et al., 1995b).

Table 4.2. Chemical characterization of tribochemical and thermal fihns generated from zinc dialkyldithiophosphates (ZDDPs) using (P) L-edge and (S) L-edge XANES spectroscopy (Fuller et al., 1997) Zinc di-isopropyldithiophosphate (primary)

Zinc di-isobutyldithiophosphate (secondary)

Zinc 6\'{p-tert' octyl)-phenyldithiophosphate

Thermal decomposition/oxidation of solid ZDDP

Decomposed at 200°C (as polyphosphates and sulfides).

Decomposed at 200°C (as polyphosphates and sulfides).

Stable at 200°C.

Thermal film in oil solution

Mixture of short and long chain polyphosphates. Sulfur is absent at 200°C; present as sulfate at low temp.

Mixture of polyphosphates. Above 200°C, sulfur is absent in the film; present as sulfate at low temp.

Mixture of polyphosphates. Above 200°C, sulfiir is absent in the film; present as sulfate at low temp.

Tribochemical film in oil solution

Long chain polyphosphate on the topmost surface and short chain polyphosphates in the bulk. S is present as sulfide.

Long chain polyphosphate on the topmost surface and short chain poly-phosphates in the bulk. S is present as sulfide.

Long chain polyphosphate throughout the tribofilm. Contain more unchanged ZDDP.

Tribofilms

The polyphosphate chain length on the surface of the tribofilms, generated from the neutral salts, contains long-chain polyphosphates. The tribofilm generated from the neutral salts not only contains long-chain polyphosphates, but also contains more unchanged ZDDP. The XANES spectra generated from diisopropyl and di-isobutyl ZDDP are very similar (Fuller et al., 1997). On the other hand, the tribofilms of (para-tert-octyl) phenyl ZDDP both in the neutral and basic form apparently contain more long-chain polyphosphate and unchanged ZDDP. The neutral form of zinc dithiophosphate provides superior valve train wear performance relative to the basic form of ZDDP in Sequence VE engine tests (Yamaguchi, 1999).

128

Chapter 4

(B) X-ray spectroscopy for chemical speciation of antiwear tribofilms. More recently, it has been shown that, with the use of synchrotron radiation, X-ray absorption near edge spectroscopy (XANES) is a powerful surface analytical technique. One of the most attractive advantages of the XANES technique is its ability to provide essential information concerning the bulk FY and surface TEY microstructure under ambient conditions. Using the (P) L-edge with two detection modes showed that the ZDDP tribofilm generally has a two-layer structure: a short chain phosphate layer covered by a thin, long-chain polyphosphate layer. XANES of the (?) L- edge spectrum shows well defined peaks whose intensities increase with chain length so that the number of phosphorus atoms in polyphosphate can be determined with good accuracy. XANES spectroscopy also has some limitations in terms of quantitative analysis, multi-elemental analysis and thickness probe determination (Martin et al. 2001). The X-ray absorption spectroscopy is capable of probing the local structural environment around selected atoms (Ferrari etal., 1999a). XANES spectroscopy has been used to study the composition and mechanism of antiwear tribofilm formation. The absorption XANES spectra were recorded in total electron yield (TEY) versus fluorescence yield (FY) detection to investigate the chemical nature of P, S, Ca, O and Fe on the surface and in the bulk, respectively. The application of XANES surface TEY mode which analyzes the top ~5 nm layer, and the FY technique which analyzes the -50 nm layer of the bulk, taken together, give a marvelous opportunity to study nondestructively the antiwear tribofilms. Both techniques can be used under a wide variety of conditions; e.g., the formation of tribofilms at different rubbing times, load, concentrations, temperatures and surface roughness (Kasrai et al., 1993 and 1996; Koningsberger and Prins, 1988; Martin et al., 2001; Yin et al., 1997a). First, the XANES is much more bulk FY sensitive; and second, the resolution of the FY spectra is usually considerably better. The (P) L-edge and (P) K-edge XANES spectra of several model compounds are shown in Fig. 4.1 (Kasrai et al., 1994). In Figure 4.1 the (P) K-edge XANES spectra of the antiwear agent ZDDP (zinc isopropyldithiophosphate) is plotted along with the spectra of three phosphates. Li the (P) K-edge spectra, the strong peak a is due to the transition of electrons from the phosphorus Is orbital to the p-like empty antibonding orbitals below or within the conduction band. The same peak is observed for the three phosphates (spectra b-d). The only major difference is that the phosphate spectra are shifted to higher energies compared with ZDDP. In ZDDP, the central phosphorus is coordinated to two oxygen and two sulfur atoms, whereas in the phosphates, the phosphorus is bonded to four oxygen atoms. The main difference between XPS and XANES, as Fig. 4.1. indicates, is the fact that the features of the peaks a-d (relative intensities and positions) change from one compound to another. For example, the chemical state and the local environment of phosphorus in orthophosphate (Zn3(P04)2) pyrophosphate

Tribochemical Nature ofAntiwear Films

129

Q^2i^2^i) ^^d metaphosphate (Na3P3 0^) are the same. In all cases, phosphorus is coordinated to four oxygen atoms, but the local symmetry is different. As seen in Fig. 4.1 this difference is reflected very strongly in these spectra. In contrast, these compounds all give essentially identical XPS spectra. Peaks e and d are missing in the spectrum of ZDDP. Peak e appears whenever the central atom is coordinated to 3 or more strong electronegative atoms such as oxygen (Sutherland et al., 1993). The presence of the phosphate and the fine structures on the left shoulder of strong peak d will indicate the local symmetry and the structure of the phosphate.

2140

2160 2180 Photon Eaergy (eV)

130

140 150 Plioton Energy (^Y)

150

Fig. 4.1. (P) K-edge XANES spectra of model compounds (on the left) and (P) Ledge XANES spectra of the same models (on the right): (a), (A) ZDDP; (b), (B) Na3P309; (c), (C) Na2P207; and (d), (D) Zn3(P04)2, (Kasrai et al, 1994) In order to compare peak areas, the peak intensities of peaks a and b have been normalized to peak c. The relative area of peak a versus the number of phosphorus atoms in polyphosphate (chain length), increases sharply from P04^" (n = 1) to PigOss^^ (n = 18), then falls off relatively sharply for higher polyphosphates as shown in Figure 4.2 (Yin et al., 1995).

Chapter 4

130

Data from Fig. 4.2 can be used to directly determine the number of phosphorus atoms in sodium polyphosphate glasses. Up to n = 18, the change in the number of phosphorus atoms versus the peak area is large and as a result, the chain length can be determined with good accuracy. The results of this investigation have been applied to characterize the nature of polyphosphate in tribochemical films (Yin et al., 1993). It has been shown recently that XANES spectroscopy is much more sensitive to the chemical environments of elements such as phosphorus, sulfur and others than any other technique to date. XANES was used to analyze films generated on steel surfaces (Yin et al., 1993, 1997a and 1997b), and to identify the chemical nature of phosphorus and sulfur in complex matrices such as antiwear tribofilms (Brown et al., 1992; Fuller et al., 2002; Kasrai et al., 1990). 0.20

0.15 h (0

< >

0.10 h

0)

a::

0.05

0.00

Fig. 4.2. Number of phosphorus atoms in polyphosphates (chain length) as a function of relative area of peak a (Yin et al, 1995) In order to identify the chemical nature of phosphorus and sulfur in complex matrices such as antiwear tribofilms, it is essential to compare the spectra of films with different model compounds in which the local chemical environments of phosphorus and sulfur are known. The high resolution of the technique allows characterization of the chemical nature of phosphorus and sulfur in the tribofilm. Investigators have shown that the chain length of polyphosphate is related to the length of alkyl groups in ZDDP. By comparison of the L-edge XANES spectra of the tribofilms with the spectra of model compounds with known structures, it has been possible to speciate the chemical nature of phosphorus and sulfur in the antiwear tribofilms. When zinc was present in the additives, sulfur in the film remained in the

Tribochem ical Nature ofA ntiwear Films

131

reduced form as sulfide. But when zinc was absent, sulfur was oxidized to sulfate (film generated from iso-propyl DDP-without Zn), (Kasrai et al., 1994). It seems that zinc acts as an antioxidant for sulfur and also catalyzes the polymerization of phosphates. The primary role of sulfur species in the tribochemical processes is passivation by sulfur species on nascent surfaces caused by a severe wear process (extreme pressure conditions): Fe^^ + S^" -^ FeS, the heat of formation AH^ (FeS) = -1.04 eV. The sulfide can prevent adhesion and also the attack of oxygen species, AHf(FeO) = - 2.82 eV (Mori, 1995). There is enough sulfur in engine oil to initiate acid-base reactions in the formation of zinc sulfide (Martin, 1999). Organic sulfur species other than in thiophosphate form can react with the ZnO produced by the phosphate reactions according to the following: ZnO + S^' ^ ZnS + O^'. The zinc sulfide can also be directly produced if the polyphosphate contains sulfur atoms in the polymer chain (thiophosphate), for example: FeZn-thiophosphate + Fe203 -^ FeZn-pyrophosphate +ZnS Both zinc and iron sulfides have to be present in a mixed Fe/Zn polyphosphate matrix and iron sulfide is present in the more severe tribological conditions. The XANES analytical technique is considerably more chemically sensitive than XPS, and the degree of phosphate polymerization was quantified for sodium phosphate glasses (Fuller et al., 2002; Yin et al., 1995), zinc phosphate glasses (Kasrai et al., 1995), and the antiwear tribofilms. (C) The effect of physical parameters on tribofilm chemistry. The effect of physical parameters (adsorption, rubbing, temperature, concentration, load, surface roughness, tribofilm thickness) and on the chemistry of tribofilms was studied by many investigators, see Table 4.3. Table 4.3. The effect of physical parameters such as adsorption, rubbing, temperature, concentration, load, surface roughness on the antiwear performance of ZDDPs Physical parameters and the chemical nature of antiwear tribofilms Adsorption. Sulfur and phosphorus ratios change with rubbing time. Tribofilms accumulated on the surface become thicker with time. All co-additives cause reduction in ZDDP surface coverage under all conditions studied. A surface force apparatus (SFA), and atomic force microscopy were used to determine tribofilm thickness, molecular structure and mechanical properties. For the neutral ZDDP, monomolecular layer thickness is 1 nm and for basic ZDDP it is 1.6 nm (Sutherland et al., 1993; Barnes et al., 2001; Bee et al, 1999; Dacre and Bovington, 1983; Georges et al, 1998; Paddy et al., 1990; Wu and Dacre, 1997).

132

Chapter 4

Table 4.3. (Continued) Physical parameters and the chemical nature of antiwear tribofilms Rubbing. Tribofilm thickens and the friction coefficient increases with rubbing time. Rubbing time affects the chemical composition of the antiwear tribofilm. At short rubbing times, adsorbed ZDDP on the surface remains unreacted. As the rubbing time increases, long-chain polyphosphates are formed on the topmost surface and ZDDP is consumed, but short-chain polyphosphate is present in the bulk. Sulfur species are in the reduced form of sulfide. The long-chain polyphosphate layer is generally very thin (~5 nm) compared to the short-chain polyphosphate layer (-20 nm). ZDDP is not recommended for lubricating aluminum alloy surfaces. The adsorption of ZDDP on iron surfaces and oxide powders increases with ZDDP concentration. The higher the concentration of ZDDP in solution, the more ZDDP is detected on the surface. If the concentration is too low, only short chain polyphosphate is found in the film (Bovington and Dacre, 1984; Coy and Jones, 1981; Dacre and Bovington, 1981, 1983; Dickert and Rowe, 1967; Habeeb and Stover, 1987; Jones and Coy, 1981; Martin etal., 1984, 1986b and 2001; Palacios, 1986; Plaza, 1987b; Sutherland et al., 1993; Taylor et al, 2000; Tonck et al., 1999; Wan et al., 1997; Willermet and Kandah, 1993; Willermet et al., 1983, Yin et al., 1997a). Temperature. Tribofilm thickens and antiwear performance is affected by temperature. High temperature gives a thicker tribofilm and worse performance. The Zn:S:P ratio varied with temperature. At 150°C, no unchanged ZDDP is present on the surface. At 200°C, the tribofilm contains mostly short-chain polyphosphates. Only at 200°C, sulfur is detected as sulfate (Bird and Galvin, 1976; Chao et al., 1994; Harrison and Brown, 1991; Lindsay et al., 1993; Palacios, 1987; Rhodes and Stair, 1993; Rounds, 1993; So and Lin, 1994; Spedding and Watkins, 1982; Taylor et al., 2000; Wu and Dacre, 1997; Yin et al., 1997a). Load, Tribofilm composition changes with increasing load; when load increases, the concentration of sulfur increases and phosphorus decreases. Under a high load such as 400 N, no unreacted ZDDP is present in the tribofilm. When the load is lowered to 40 N, unreacted ZDDP can be detected on the tribofilm surface. The tribofilms were found to grow thicker as the load temperature was increased until a critical temperature was reached (Glaeser et al, 1993; Jahanmir, 1987; Palacios, 1987; Plaza et al. 2001, Sheasby et al., 1990; Yin etal., 1997a). Surface roughness. The rougher surface requires a longer time to form a tribofihn. The wear scar width increased with increasing rubbing speed. Two A2 steel coupons with surface roughness of R^ = 10 nm (polished) and 293 nm (unpolished) were used to prepare the tribofilms. Smoother surfaces favor decomposition of ZDDP and formation of longchain polyphosphate. No unchanged ZDDP was found in the tribofilm when a polished surface was used (Sheasby et al., 1990; So et al., 1993; Yin et al., 1997a). Tribofilm thickness. Depth profiling results show that the 12 hour test tribofilm is much thicker than the 0.5 hour test film. The tribofilm thickness results from the balance of the rate of tribofihn growth, and the rate of removal by wear, and is not correlated to the wear scar width to any great extent (Bell, 1995; Fuller et al., 1998 and 2000; Georges et al., 1998; Jahanmir, 1987; Lindsay et al., 1993; Palacios, 1987; Yin et al., 1997a; Tonck et al., 1999). Several groups found fi-om adsorption tests that tribofilm became thicker w^ith time; sulfur and phosphorus composition on the surface changed with rubbing

Tribochemical Nature ofAntiwear Films

133

time; the wear rate increased in the first 20 hours then leveled off, and film thickness and friction coefficient increased with rubbing time and then leveled off. The rate of adsorption and decomposition of a given ZDDP increased with increasing concentration. Thermal decomposition has been accepted as the major mechanism of antiwear tribofilm formation. High temperature gives a thicker film and poor antiwear performance. The effects of rougher surface and lower sliding speed on the tribofilm formation required a longer rubbing time. 4.2. The Chemistry of Tribofilm Formation Surface analytical techniques such as XANES are a powerfiil technique for antiwear tribofilm characterization. Using the (P) L-edge and (S) L-edge XANES spectroscopy, researchers have been able to extract from spectra far more information than previously known; e.g., short-, and long-chain polyphosphates, and partially decomposed (or partially unreacted) ZDDP on the surface (Armstrong et al., 1997 and 1998; Barnes et al., 2001; Ferrari et al., 1999a and 1999b ; Kasrai et al., 1994 and 1998; Martin, 1999; Martin et al., 2001; Yin et al., 1993, 1997a and 1997b). This part of the chapter includes descriptions of: (A) The mechanism of antiwear tribofilm formation; (B) The effect of detergents and dispersants on tribofilm formation. (A) The mechanism of antiwear tribofilm formation. A new mechanism and chemistry for antiwear tribofilm formation is proposed from XANES spectra after considering many effects of physical parameters is summarized in Table 4.3, and discussed below: (a) The antiwear tribofilm in its initial stage of rubbing contains unreacted ZDDP and long chain polyphosphates. The ZDDP and long-chain polyphosphates are predominantly present on the topmost layer. Over a long rubbing time (12 hrs), ZDDP was consumed to form long-chain polyphosphates on the surface and shortchain polyphosphates in the bulk. For the short rubbing times, the sulfur spectra contained signals from ZDDP; but at the later stages, sulfur spectra resembled the ZnS spectra plus a small amount of alkyl sulfides; (b) ZDDP adsorption on the surface is increased when load or temperature is increased. The antiwear tribofilms generated in oil at 150°C were similar to the tribofilm generated at a 400 N load at 100°C. In the case when ZDDP was consumed and rubbed deeper in the system, the polyphosphate tribofilm broke down to form short chain polyphosphates or orthophosphates; (c) It has been shown by many investigators that during tribofilm formation of iron phosphates (Belin et al, 1989; Martin et al., 1986a) and zinc phosphates (Willermet et al., 1995a), the zinc polyphosphates were predominantly present on

134

Chapter 4

the surface while iron phosphates were predominantly in the bulk. This has also been confirmed by studies based on XANES spectra; d) The chemical nature and role of sulfur in film formation is not clear. The nature of cations associated with sulfides is not well established, ZnS, FeS, alkyl sulfides have been suggested. XANES spectra indicate the presence of ZnS. In order to distinguish between ZnS and FeS, their (S) L-edge XANES spectra were examined. It was found that FeS (powder) was very sensitive to surface oxidation and the surface was covered with a sulfate layer, but FeS can still exist in the bulk (Bell and Delargy, 1993, Bell et al.,1992; Yin et al., 1993). The case study. Tribofilms were generated from commercial ZDDP (iso-butyl (85%) + n-octyl (15%) after rubbing in the wear machine for 5 and 30 min, 6 and 12hrs. Also, a tribofilm was made in two stages: the tribofilm that was generated in 30 min was further rubbed for 5.5 hrs without ZDDP in the base oil. The (?) L-edge spectra of these tribofilms are shown in Fig. 4.3. Comparing spectrum D with spectra A, B, and C in Fig. 4.3 shows that when the rubbing time was increased from 5 min to 12 hrs, peak a2 and the shoulder aj decreased in intensity. The relative intensity of peak a2 is similar in the 5-min tribofilm (spectrum A) and 30-min tribofilm (spectrum B), but is reduced and shifted to lower energy in the 6-hour film (spectrum C). The relative intensities and positions of peaks c and d in all the spectra remain the same and similar to the spectra of model polyphosphates (Yin et al., 1995). The amount of unreacted ZDDP in the 6-hour tribofilm (spectrum C) was found to be 25%). Films A and B had about 35%) unreacted ZDDP. Spectrum E in Fig. 4.3 was taken in two stages. The total rubbing time of this sample (0.5 h with ZDDP + 5.5 h without ZDDP in oil = 6 h) was the same as that of C, but peaks a^ and a2 are not observed for this tribofilm. Because the FY spectrum of this sample is still intense, an excellent thick tribofilm is present after 5.5 h rubbing without ZDDP. Based on the intensity calibration curve (Yin et al., 1997a), investigators evaluated the chain length corresponding to this tribofilm which was found to be about 3 from TEY spectrum and close to a pyrophosphate or orthophosphate from the FY spectrum. Comparing spectra E and D, it is noticed that the intensities of peaks a and b are much stronger in D and thus the chain length must be much longer. Using the calibration curve, investigators found that the ratio of peak (a/c) is greater than that expected for any linear polyphosphate studied. Thus, it was concluded that the higher intensity of peak a in spectrum D is indeed due to higher polyphosphate chain length (>20 or cyclic metaphosphate) and not to a ZDDP impurity. In contrast to TEY detection which probes a ~5 nm depth of the film, FY mode probes >50 nm (Kasrai et al., 1996). In Fig. 4.3 it was shown that the spectra measured for the model polyphosphates in TEY and FY modes were similar indicating that surface and bulk species are the same. Examining spectra F-H, it is immediately clear that the intensity of peaks a and b in (FY) spectra is much weaker than that in the corresponding TEY spectra A-C,

Tribochemical Nature ofAntiwear Films ""1

ZDDP-1 Fllmi

'-•^'

c

r-

A

,

135 p-

i4

5 min

30 mJn

4

A

\

d

1

W]

4v

\

(F)

(0)

(H)

12 hr

0.5 hr-f5.5| ihrinBO 130

140 150 Photon Energy (eV)

130

J L140 150 Photon Energy (eV)

Fig. 4.3. (P) L-edge XANES spectra of ZDDP tribofilms generated at different rubbing times measured by TEY (surface) mode (left) and FY (bulk) mode (right). (A, F) 5 min, (B, G) 50 min, (C, H) 6 hr, (D, I) 12 hr, and (E, J) 0.5 hr with ZDDP+ 5.5 hr rubbing without ZDDP or the bridging oxygen (BO). ZDDP consists of the secondary iso-butyl (85%) and n-octyl (15%) groups (Yin et al., 1997a) This suggests that the polyphosphate chain length is much shorter in the bulk than on the surface. But the broad shoulder indicates that the spectra still contain small components from ZDDP, which can be trapped during rubbing when the wear rate is high. Spectra I and J in all aspects are very similar and show that these bulk films are ortho- or pyrophosphate (Yin et al., 1997a). The tribofilm chemistry changed with ZDDP oil solution heating time. A linkage isomer (LI-ZDDP) of ZDDP is proposed as an important precursor for film formation Fuller et al., 1998; Varlot et al., 2000). The rearrangement of ZDDP (reaction eq, 4.1) is initiated by a double alkyl group migration from oxygen atoms to sulfur atoms (Jones and Coy, 1981) and the shift to higher energy of LI-ZDDP (2154.5 eV) compared to ZDDP (21533 eV), (Fuller et al., 1998) is believed to be the result of a reaction mechanism.

Chapter 4

136

RO \ (S=P-S-)2Zn / RO

RS \ (0=P-0-)2Zn / RS

Zinc dialkyldithiophosphate ZDDP

(4.1)

A linkage isomer (LI-ZDDP) of ZDDP

The XANES data suggest (for a thermal film generated at 100°C, 48-108 h) that it is more likely that the surface adsorbed species in Fig. 4.4, spectrum [B], is a rearranged ZDDP, known as LI-ZDDP (linkage isomer of ZDDP). T

1

]

pi

P K-e
j\ \

ZDDP

b \ \

p]

1

Thermal Film ioo*c (48~108ti)

^ Diaulphide

p3 140

y 2 ISO

\

Zn^CPO^),

2 ISO

1

21 7 0

P t i o t o n E n e r g y <eV)

Fig. 4.4. Phosphorus (P) K-edge XANES spectra of a thermooxidative tribofilm generated at 100°C (48-108 h heating time) for model compounds: ZDDP, disulfide and Zn orthophosphate. The surface adsorbed species of the thermal film [B] is a rearranged ZDDP (linkage isomer, LI-ZDDP). A shoulder in the thermooxidative tribofilm indicates that there is some orthophosphate present (Fuller et al, 1998) Peak a of the thermal film (spectrum B) is shifted by 1.2 eV to higher energy from peak a of the ZDDP (spectrum A) and peak a of the disulfide (spectrum C), and yet peak a of the thermal peak (spectrum B) does not correspond to peak b of the zinc orthophosphate (spectrum D), revealing that this species is a phosphate or the original ZDDP.

Tribochem ical Nature ofAntiwear Films

137

A shoulder in the thermal film (spectrum B) does not correspond to peak b of the zinc orthophosphate (spectrum C), indicating that there is some orthophosphate present in the thermal film. As the spectra suggest, the species in (spectrum B) obviously cannot be a disulfide. The LI-ZDDP has been observed on a metal surface for the first time after being immersed in a thermally rearranged ZDDP oil solution. A linkage isomer as LI-ZDDP, in which alkyl groups have migrated from oxygen atoms to sulfiir atoms, is proposed as the intermediate surface species. In this rearrangement, all the sulfiir originally bonded to Zn, is partially or totally replaced by oxygen. The shift to higher energy of LI-ZDDP compared to ZDDP, is believed to be the result of migration of an alkyl group from the oxygen atom to a sulfiir atom. Alkyl groups are electron donating thus decreasing the effective electronegativity of the oxygen atom bonded to the alkyl group. The oxygen atom, now bonded to the electron deficient zinc atom can pull more electron density away from the phosphorus atom, thereby shifting the absorption edge to higher energy. A rearranged ZDDP or LI-ZDDP has been proposed previously as an intermediate in the thermal decomposition of ZDDP in solution studied by IR, and by 31-P N M R spectroscopy. This is the first time this proposed LI-ZDDP has been observed on a metal surface in air after being immersed in a thermally decomposing ZDDP oil solution (Coy and Jones, 1981; Dickert and Rowe, 1967; Fuller et al., 1998; Jones and Coy, 1981). These results were summarized and can be outlined as follows in reaction equations from 4.1 to 4.7: (a) the preheated ZDDP in oil solution over 100°C leads to formation of LI-ZDDP (see reaction eq. 4.1, page 136) as an intermediate in antiwear film formation, (b) the LI-ZDDP is present in thermal films, as can be determined by (P) K-edge XANES spectroscopy, but 31-P NMR cannot differentiate LI-ZDDP from ZDDP conclusively, (c) the LI-ZDDP is reduced in the tribofilms (LI-ZDDP reacts with the rubbing surface) to form a polyphosphate, (d) the tribofilms generated from totally decomposed ZDDP oil solutions (150°C for 24 h or 200''C for over Ih preheated), indicate that a different film formation mechanism accounts for the short-chain polyphosphate, (e) the polyphosphate tribofilm formed from rapidly decomposed ZDDP oil solutions is not as effective in reducing wear as a film derived from unheated ZDDP oil solution, (f) the ZDDP oil solution heated at ISC'C for various lengths of time, showed a decrease in polyphosphate chain length as ZDDP thermal solution decomposition progressed. Considering present understanding of the reactions of ZDDP in solution and on surfaces (Hasfie et al., 1993; Martin 1986a; Martin et al., 2001; Willermet et al., 1995a and 1995b; Yin et al., 1997a), a possible mechanism for the tribofilm formation is presented: Zn[(RO)2PS2]2 (solution) Zn[(RO)2PS2]2 (solution)

-- Zn[(RO)2PS2]2 (adsorbed) - Zn[02P(SR)2]2 (LI-ZDDP (in solution)

(4.2) (4.3)

138

Chapter 4

Zn[02P(SR)2]2 (solution) -> Zn[(02P(SR)2]2 (LI-ZDDP (adsorbed) Zn(RO)4P2S4 + O2 (or ROOH) - Zn(P03)2 + sulfur species 2Zn(P03)2 + 3H2O -> Zn2P207 + 2H3PO4 Zn(P03)2 + FeO - FeZnP207 5Zn(P03)2 + Fe203 - Fe2Zn3Pio03i + 2ZnO

(4.4) (4.5) (4.6) (4.7a) (4.7b)

The first step (eq. 4.2) is an adsorption process. ZDDP in solution is adsorbed on the rubbing surfaces. As time goes by, ZDDP is converted into LI-ZDDP (eq. 4.3) which in turn, will be adsorbed on the surface along with ZDDP (eq. 4.4). Using XANES spectroscopy, no adsorbed ZDDP was detected on steel surfaces at temperatures lower than lOC'C but adsorbed ZDDP is clearly present at 150°C and above, especially with no nitrogen protection. There are clear ZDDP signals detected on the tribofilm surface with shorter rubbing times (Yin et al., 1997a) . If the concentration is too low, there will not be enough ZDDP adsorbed on the surface, which will lead to the formation of short-chain polyphosphates or thin tribofilms (Yin et al., 1997a). As a result of tribochemical reactions and the presence of oxygen or peroxide in oil, adsorbed ZDDP and LI-ZDDP on the surface is thermo-oxidatively decomposed to give a long-chain polyphosphate. The second step (eq. 4.5) is a thermal oxidative process. This initiates the reaction of ZDDP with oxygen, and enhances the decomposition. Since oxygen and/or hydroperoxide is present in the oil, decomposition is not a pure thermal degradation. The main products on the surface are zinc polyphosphates with minor amounts of zinc sulfides. As the rubbing continues, the polyphosphate layer comes into closer contact with water in oil and is hydrolyzed to give short-chain polyphosphates (eq. 4.6). The third step (eq. 4.7a and eq. 4.7b) is the formation of pyrophosphate or orthophosphate respectively, and requires iron cations. After the tribofilm containing long-chain phosphates is formed, it is converted to short- chain phosphates after long rubbing times, see Fig. 4.3 (Yin et al., 1997a). These reactions are proceeding under very favorable conditions of contact temperatures and loads. The temperature in the interface can rise up to -700°C, and the surface plays an important role. Basically, a tribochemical reaction between zinc polyphosphate and iron oxides species is energetically favorable from the point of view of the hard and soft acid and bases (HSAB) principle (Bovington and Dacre, 1984; Dacre and Bovington, 1983; Katnack and Hummel, 1958; Kotvis et al., 1993; Martin et al., 2001; Pearson, 1997; Yin et al., 1997a). This mechanism is in agreement with the mechanism proposed by others (Belin et al., 1989 and 1995; Martin et al., 1986a, Willermet et al., 1992) using extended X-ray absorption fine structure spectroscopy (EXAFS) and infrared spectroscopy. When ZDDP is present in the lubricant formulation, the radial distribution function (RDF) indicates that crystalline iron oxide diffuses into the polyphosphate network material.

Tribochemical Nature ofAntiwear Films

139

Disulfide f(RO)2PS2j2 Studies of decomposition of ZDDP in solution have led researchers to believe that the disulfide [(RO)2PS2]2 may be an intermediate in the formation of the polyphosphate antiwear tribofilms derived from ZDDP. Studies of the oxidation of ZDDPs by peroxyl radicals (Paddy et al., 1990; Rossi and Imparato, 1971; Willermet et al., 1983; Willermet and Kandah, 1984) have shown that [(RO)2PS2]2 is a major reaction product. The (P) L-edge XANES spectra of the surface TEY and bulk FY of disulfide are very different (Fuller et al., 1998). The general features of the bulk FY disulfide spectrum correspond to those of the ZDDP spectrum with similar peak energy position, see Fig. 4.4. The surface spectrum TEY of the disulfide appears to be a mixture of bulk disulfide and long chain zinc polyphosphate. The surface disulfide may have partially decomposed to phosphate. The (P) L-edge and K-edge XANES spectroscopy is unable to differentiate between ZDDP and disulfide. The presence of disulfide on the metal surface has been proposed (Plaza, 1987b) in that an iron oxide surface not only adsorbs but also assists the oxidation of ZDDP to disulfide. There was disulfide in the unheated ZDDP in oil solution (about 13%) but in the preheated ZDDP oil solution, it was much less than 10%. The 31-P NMR spectra of residual ZDDP and the presence of disulfide were quantified. After 6 h of heating at 150°C, about 25% of the original amount of ZDDP, and 3.4% of disulfide were detected in solution. The absence of signals in 31-P NMR spectrum of ZDDP, disulfide or intermediate products in solution after 1 h of heating at 200''C was very evident. What is interesting here is that a polyphosphate antiwear film is formed even after 6 h of heating the ZDDP in oil solution at 200°C. Perhaps fine colloidal particles of ZDDP decomposition products are responsible for the formation of these polyphosphate films (Fuller et al., 1998). The ZDDP oil solution produced a long-chain polyphosphate tribofilm whereas the corresponding disulfide (no zinc present) yielded a simple phosphate antiwear film (Kasrai et al., 1994). The effect of zinc in the antiwear agent is very significant. The chemical nature of sulfur and phosphorus, in the films of isopropyl DDP (without zinc), is quite different from those with zinc. When zinc is present, phosphate in the film tends to polymerize more, and zinc acts as an antioxidant for sulfur in protecting the sliding surfaces. (B) The effect of detergents and dispersants on tribofilm formation. The tribofilm composition and antiwear performance of ZDDP in lubricant oil can be affected by many physical parameters; e.g., adsorption, rubbing, temperature, load, concentration and chemical; e.g., detergents and dispersants, and has been studied extensively (Bird and Galvin, 1976; Glaeser et al., 1993; Jahanmir, 1987; Martin et al., 1986a and 2000a; Rounds, 1975; Spedding and Watkins, 1982; Watkins, 1982; Willermet et al, 1991, 1992 and 1995b; Yin et al., 1993 and 1997b). The ZDDP additive is used rarely as the sole additive in oil formulations. The finished engine oil contains other additives to improve dispersancy, detergency

140

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and anti-rust lubricant performance. The interactions of ZDDP with detergents and dispersants have been studied widely and are well documented in review articles, see Chapter 2. It is generally believed that ZDDP is transformed and that its transformation product adsorbs on and/or reacts with the rubbing surfaces to form the protective antiwear surface layer. The rate of ZDDP transformation can be retarded by soft-core RMs. Solubilization between soft-core reverse micelles and ZDDP has also been considered to be a reason for reduced antiwear performance. A related process would be the transfer of solubilizates (e.g., ZDDP) reverse micelles into a co-existing activated metal surface in the tribochemical system during friction and formation of polyphosphate tribofilms (Inoue and Watanabe, 1983; Kapsa et al., 1981; Lindsay et al., 1993; Ramakumar et al., 1992; Rounds, 1981 and 1986; Shiomi et al., 1986; Silver, 1978; Yin et al., 1997b). When detergents and dispersants were present along with ZDDP in the system, the polyphosphate chain length was reduced (Inoue ans Watanabe, 1983; Willermet et al., 1995b)„ It can be concluded that there was a strong micellar interaction between soft-core RMs and ZDDP. Using a range of concentrations (wt%): 0.5, 1 and 2 , it was found that calcium sulfonate lowers ZDDP activity at high concentrations (>2 wt%) (Yin et al., 1997b). For a detergent concentration of 2%, there is almost no unreacted ZDDP on the surface. These results clearly indicate that soft-core RMs at over 2% detergent concentration interact with ZDDP, and as a result, less of the ZDDP is on the surface. The surface coating composition is slightly different from that of using ZDDP alone. This is probably due to the decrease of effective concentration of ZDDP caused by the solubilization of ZDDP by soft-core RMs formed by the detergent (Kapsa et al., 1981). The phosphorus and sulftir L-edge XANES spectra of the tribofilms phenateZDDP were recorded in the bulk FY mode, and in the surface TEY mode for three concentrations of calcium phenate. The surface TEY spectrum assigned for the lowest concentration (0.5 wt%) of the calcium phenate has been assigned to the absence of (unreacted) ZDDP. The chain lengths (number of P) in the phosphates in the tribofilms were estimated; they decreased from -44 in 0.5 wt% calcium phenate to -^8 in 1 wt% calcium phenate and even further to ~5 in 2 wt% calcium phenate. The same trend is followed in the bulk FY mode. This indicates that the polyphosphate chain lengths are shorter in the bulk without detergents (Yin et al., 1997a). The chain lengths in FY spectra are estimated at 12, 7 and 1 in for 0.5 wt%, 1 wt.% and 2 wt% calcium phenate, respectively. Thus the topmost surface measured by TEY contains relatively long chain polyphosphates, and the bulk chemistry of the film measured by FY contains mostly shorter chain polyphosphates. The bulk FY signal is relatively strong, indicating that the films formed with calcium phenate are relatively thick (several hundred angstroms). Summary of the tribofilm compositions is displayed in Table 4.4. The (S) L-edge

Tribochemical Nature ofAntiwear Films

141

XANES spectra measured the surface chemistry and the TEY probe indicated that sulfur for all concentrations was in the sulfide form. Table 4.4. The tribochemical effect of micellar additives (calcium sulfonate, calcium phenate) + ZDDP (1.2 wt%) on the polyphosphate chain lengthfromXANES spectrum measured in the bulk FY technique and in surface TEY technique (Yin et al., 1997b) Calcium sulfonate or Calcium phenate (wt%)

Tribofilm composition (%) unreacted ZDDP

0.5

35

1.0 2.0

Chain length, number of (P). Surface (TEY)~5 nm Calcium sulfonate 44

Chain length, number of (P) Bulk (FY) -50 nm Calcium phenate 12 7

all reacted

1

Effects of rubbing time in the presence of dispersants with ZDDP on tribofilm formation, A range of concentrations of a dispersant (1,2, and 5 wt%) were used to form tribofilms with ZDDP (1.2 wt%), (Yin et al., 1997b). For all the concentrations used, antiwear polyphosphates tribofilms were formed and unreacted ZDDP was not present in the tribofilms. In the XANES spectra of tribofilm, both P and S signals are very weak compared with the case when ZDDP was used alone. This result implies that an antiwear tribofilm is much thinner than either tribofilm generated using ZDDP alone or ZDDP and a detergent (Yin et al., 1997b). This confirms that the soft-core RMs compete for ZDDP solubilization on the surface. The rubbing time affects the composition of the antiwear tribofilm. At short rubbing times, polyphosphates and unreacted ZDDP are present in the film. As the rubbing time increases, unreacted ZDDP is consumed to form polyphosphates. The tribofilms formed in the presence of dispersants are very thin. This could be due to interaction between ZDDP and soft-core RMs, modifying the concentration of the ZDDP, or merely a reflection of the RMs competing effectively for adsorption sites on the metal surface (Yin et al., 1997a and 1997b). The antiwear mechanism of ZDDP in the presence of dispersants has been studied and it has been concluded that ZDDP forms an association complex with an amino group of a succinic type dispersant, and this complexation has been proved to be antagonistic to antiwear action (Gallopolous and Murphy, 1991: Rounds, 1986; Shiomi et al., 1986; Willermet et al 1995b). The solubilization of ZDDP helps the adsorption of ZDDP on the surface, thus improving the antiwear performance of the additives (Forbes et al., 1970b). RMs would decrease the

142

Chapter 4

adsorption rate of ZDDP on steel surfaces, making the engine oil last longer (Harrison et al., 1992). A combination of dispersants results in good antiwear performance regardless of the poor antiwear characteristics of the dispersant itself (Shirahama and Hirata, 1986). The tribofilms formed in the presence of dispersants are thinner than films produced using ZDDP alone. Polyphosphates formed in the presence of dispersants are of shorter chain length. The dispersant competes with the adsorption of the ZDDP on the surface, and thus much thinner layers of ZDDP remain on the sliding surfaces. Summary of the tribofilm compositions is displayed in Table 4.5. Table 4. 5. The chemical nature of the tribofilm generated on steel surfaces using ZDDPs in the presence of detergents and dispersants (30 min wear machine). Results based on interpretation of phosphorus and sulfur L-edge tribofilm XANES spectra (Yin et al., 1997b) Additives^

Chemical nature of antiwear tribofilm and composition (unreacted %)

ZDDP only

Zinc polyphosphate + unreacted ZDDP (35%)

ZDDP + sulfonate andphenate RMs ZDDP + 0.5% sulfonate ZDDP + 1% sulfonate ZDDP + 2% sulfonate ZDDP + 0.5%) phenate ZDDP + 1%) phenate ZDDP + 2%) phenate ZDDP + dispersant

Zinc polyphosphate + unreacted ZDDP (35%) Zinc polyphosphate + unreacted ZDDP (5%) Zinc polyphosphate, total reacted ZDDP Zinc polyphosphate + unreacted ZDDP (5%)) Zinc polyphosphate + zinc sulfide Zinc polyphosphate, total reacted ZDDP

ZDDP + 1 % PIBS-PAM ZDDP + 2% PIBS-PMA ZDDP + 5%) PIBS-PAM

Zinc polyphosphate, total reacted ZDDP Zinc polyphosphate, total reacted ZDDP Zinc polyphosphate, total reacted ZDDP

^ZDDP: secondary alkyl ZDDP [(iso-butyl (85%) + ^-octyl (15%)]; Dispersant: polyisobutylene succinic anhydride polyamide (PIBS-PAM), Additives interact in a variety of ways, both in the bulk oil and on surfaces. Knowing the nature of additive interactions and their impacts on performance can help in oil formulations and improve lubricated systems. Additive interactions in the bulk phase have been particularly useful in understanding synergism and antagonism between different classes of additives. Many of these interactions affect processes on surfaces.

Tribochemical Nature ofAntiwear Films

143

4.3. Techniques for Evaluation of Metal Surfaces Analytical techniques for the identification of the structure and chemistry of solid surfaces are based on detection of particles: photons, electrons, and ions. The techniques are classified according to the particles or radiation used to excite the sample and detected particles (emission) to obtain information about the sample. Fig. 4.5 schematically shows the interaction of the excitation sources with the solid surfaces of materials; various forms of energy are emitted from the solid surfaces and detected by energy analyzers. Excitation Photons, electrons, ions i l l /

Emission Photons, electrons, ions t t t /

ii/iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii/iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiin Solid sample Fig. 4.5. Excitation of a metal surface and detection of emitted particles In Table 4.6 some analytical techniques are summarized in terms of their input and measured radiations (Briggs and Seah, 1990 and 1992; Bubert and Janett, 2002; Miyoshi and Chung, 1993; Watts, 1990). Table 4.6. Techniques used to analyze various forms of energy emitted on solid surfaces of metals Emission particles

Techniques

Electrons

Auger Electron Spectroscopy, Extended X-Ray Absorption Fine Structure, Low-Energy Electron Diffraction, Scanning Electron Microscopy, Surface Extended X-Ray Absorption Fine Structure, Ultraviolet Photoelectron Spectroscopy, X-Ray Absorption Near Edge Fine Structure, and X-Ray Photon Spectroscopy.

Ions

Ion Scattering Mass Spectroscopy, Laser Microprobe Mass Analyzer, Rutherford Backscattering Spectroscopy, Secondary Ion Mass Spectroscopy.

Photons/X-rays

Electron-Probe Micro-Analysis, Electron-Spin Resonance, Laser MicroProbe, Photo-Desorption, Photoelectron Spectroscopy, y-R^Y Absorption Spectroscopy, X-Ray Fluorescence, and Other (IR, NMR, UV, Visible, y-R^y, and X-Ray Absorption Spectroscopy).

The information needed about surfaces and interfaces is threefold: (i) composition (all atomic species involved and their concentrations);

144

Chapter 4

(ii) electronic structure (nature of bonding); (iii) crystallographic structure (structural arrangement of atomic species). (i) The composition can be obtained most easily using such techniques as Auger electron spectroscopy (AES), photo emission spectroscopy (PES), highresolution electron energy loss (EEL) spectroscopy, thermal desorption spectroscopy (TDS), secondary ion mass spectroscopy (SIMS), or Rutherford ion scattering spectroscopy (RISS). The crystallographic and electronic structures are linked since one is the consequence of the other. (ii) The electronic structure is largely revealed by photo emission spectroscopy (PES), which provides detailed information on the bonding orbitals or to high-resolution electron energy loss (EEL) spectroscopy, which is particularly valuable for identification of chemisorbed molecules. (iii) crystallographic structure. Low-energy electron diffraction (LEED) was the first technique developed for the determination of crystallographic structures. Spectroscopic structural tools measure a signal in response to a local electronic excitation (electrons, photons, or ions). Electrons are detected in angle-resolved photo emission extended fine structure (AREFS), extended appearance potential fine structure (EAPFS), and extended electron loss fine structure (EXELFS) measurements, while ions are detected in electron stimulated ion desorption (ESD), photon stimulated ion desorption studies (PSD), and secondary ion mass spectroscopy (SIMS). The surface extended X-ray absorption fine structure (SEXAFS) method can use either an electron or an ion detection signal (Koningsberger and Prins, 1988). The classification of analytical techniques may be considered in terms of incident and emitted radiation, resolution, and sensitivity, according to Table 4.7, which lists eight of the many possible techniques (Briggs and Seah, 1990; Buckley, 1981; Watts, 1990). Many of the surface analysis techniques were introduced into many laboratories over the years of 1968 to 1970. This resulted from the maturing of clean vacuum systems which could achieve pressures, down to 10"^ Pa. At these low pressures, it is possible to obtain and maintain atomically clean surfaces. Some of the techniques described in this chapter used most widely today are Auger electron spectroscopy. X-ray photoelectron spectroscopy, electron-probe micro-analysis, low energy electron diffraction, scanning electron microscope, ion scattering spectroscopy, and secondary ion mass spectroscopy. The solid surface, after liberation of electrons, can be analyzed directly by AES, XPS, ISS, and EPMA (nondestructive techniques), or by liberation of ions from surfaces using SIMS (involving the destruction of the surface). Apart from the surface techniques, reflectance-absorbance infrared (RAIR) spectroscopy has also been employed for film characterization (Lindsay et al., 1993; Yin et al., 1993). Some

Tribochemical Nature ofAntiwear Films

145

of these techniques can provide chemical information on how elements exist on a solid surface, the structure of a particular compound, or a simple elemental analysis. EPMA, AES and XPS can provide information on chemical bonding states and identify the particular compound. XANES contains information of the stereochemical details (coordination geometry and bond angles) and EXAFS gives information about local structures in terms of atomic radial distribution (distances) around the central atom. Some techniques are very deep probes while others are strictly surface oriented; e.g., with the EPMA and SIMS, the probe depth is 1 nm; with XPS, 2 to 7 nm; with EXAFS, 0.5 nm, with XANES, 5 to 50 nm, with AES, 1 to 3 nm, and with ISS, it is only 0.3 nm. Table 4.7. Features of various analytical techniques for surface analysis Technique^

Analysis type^

Best resolution

Sensitivity

Depth of analysis

Effective year

AES EXAFS ISS RBS SIMS UPS XANES XPS

CandE CandE E E E E CandE CandE

5 nm 6eV Imm 1mm 50//m

0.3% 1% 1% 1% <1 ppm 1% 0.5% 0.3%

1 to 3 nm 0.5 nm outer atom 1 mm 1 nm 3 nm 0.5 to 50 nm 2 to 7nm

1968 1970 1967 1967 1967 1969 1970 1967

5 fjm.

0.2 eV 5 yum

Technique abbreviations: AES = Auger Electron Spectroscopy; EXAFS = Extended XRay Absorption Fine Structure; ISS = Ion Scattering Spectroscopy; SIMS = Secondary Ion Mass Spectroscopy; UPS = Ultraviolet Photoelectron Spectroscopy; XANES = X-Ray Absorption Near Edge Structure; XPS (or ESCA) = X-Ray Photoelectron Spectroscopy; ^Analysis type: C = chemical, E = elemental Several physical techniques were used in the past to analyze the composition of tribochemical films. Electron-probe micro-analysis (EPMA), (Sheasby et al., 1990; Singh et al., 1990) and X-ray fluorescence spectroscopy (XRF), (Rounds, 1975; Watkins, 1982) have shown the presence and spatial distribution of elements such as O, P, S, and Zn. Auger electron spectroscopy (AES) has been used for both depth profiling and chemical analysis of the films (Cao et al., 1990; Jahanmir, 1987; Mathieu et al., 1981; Schumacher et al., 1980; Sieber et al., 1983). Photoelectron spectroscopy (XPS) has also been widely used by several invesfigators in the past (Bird and Galvin, 1976; Cao et al., 1990; Watkins, 1982). Time-of-flight secondary ion mass spectrometry (TOE SIMS), (Bell and Delargy, 1993; Bell et al., 1992) and extended X-ray absorption fine structure (EXAFS),

Chapter 4

146

(Martin et al., 1986a) have been used for the chemical speciation of tribochemical films. It has been shown recently that L-edge X-ray absorption near-edge structure (XANES) spectroscopy is much more sensitive to the chemical environments of elements such as P, S and Si than any other techniques (Brown et al., 1992; Kasrai et al, 1990; Kasrai et al., 1995; Martin et al., 2001; Yin et al., 1993, 1997a and 1997b). Reactions of thermal decomposition of ZDDP in the solid and oil phase have been studied using infra-red spectroscopy (IR ) (Lindsay et al., 1993; Spedding and Watkins, 1982; Willermet et al., 1992) and ^H and ^^P nuclear magnetic resonance spectroscopy (NMR) (Bovington and Dacre, 1984; Coy and Jones, 1981; Dacre and Bovington, 1983; Molina, 1987; Spedding and Watkins, 1982; Willermet et al, 1995b). Among other things, investigators identified compounds as oil-soluble products and a precipitate which was rich in zinc and oxygen and low in phosphorus and sulfur, compared with the original ZDDP. They have also identified the product as a mixture of zinc phosphate and an unknown sulfur compound and various zinc phosphates or thiophosphates. In addition, ZDDP absorbed in the monolayer range becomes depleted of zinc. These investigators proposed a mechanism for film formation based on the decomposition products and the nature of the alkyl group. Table 4.8. X-ray and spectroscopic notation Quantum numbers

X-ray suffix

n

1

j

1 2 2

0 0 1

V2 V2 V2

2

1

3 3

0 1

3 3 3

1 2 2 etc.

3/2 V2 V2

3/2 3/2 5/2

1 1 2 3 1 2 3 4 5 etc.

X-ray level

K L,

U L3

M, M, M3 M4 M5

etc.

Spectroscopic level lsl/2 2s 1/2 2pl/2 2p3/2 3sl/2 3pl/2 3p3/2 3d3/2 3d5/2 etc.

Nomenclature. The spectroscopic nomenclature is directly equivalent to that used for spectroscopy X-ray, and is related to the various quantum numbers such as the principal quantum number n, the electronic quantum number 1, the total angular momentum quantum number j , and the spin quantum number s, which can take

Tribochemical Nature ofAntiwear Films

147

either of the values iVi. Where: n = 1, 2, 3, 4,..., are designated K, L, M, N,... respectively; 1 = 0, 1, 2, 3,... and j = 1 + s (can take the values of V2, 3/2, 5/2, 7/2),... are given conventional suffixes, 1, 2, 3, 4,... according to the listing in Table 4.8 (Briggs and Seah, 1990). This description of the summation is known as j-j coupling. X-ray absorption spectroscopy include XANES, EXAFS and SEXAFS techniques. X-ray absorption spectroscopy was first used in the 1920s for structural investigations of matter. The observed fine structure near the absorption edges was referred to as "Kossel structure", but the structure extending for hundreds of eV past the edge was called "Kronig structure". The latter is what is now called extended X-ray absorption fine structure (EXAFS) (Koningsberger and Prins, 1988; Stohr, 1992). During the 1970s, when EXAFS was developed into a powerfiil structural analysis tool, most of the recorded EXAFS spectra were too complicated for interpretation. One exception was K-shell excitation spectra of low-Z molecules (composition of elements of low atomic number, i.e. Z < ~ 20) adsorbed on surfaces with binding energies in the 250-750 eV range: carbon (285 eV), nitrogen (400 eV), oxygen (535 eV), and fluorine (685 eV). Since then, significant progress in the understanding of the near edge structure in molecules, inorganic complexes, biological systems, crystalline and disordered solids, and chemisorbed atoms and molecules has been made (Koningsberger and Prins, 1988; Stohr, 1992). hi recent years, the near edge structure has been mostly referred to as X-ray absorption near edge structure (XANES), (Bianconi, 1980) or the near edge X-ray absorption fine structure (NEXAFS), (Stohr and Jaeger, 1982). The term XANES is most commonly used for solids and inorganic complexes while NEXAFS is used more in conjunction with surfaces. In fact, the name NEXAFS was created in part because it rhymes with SEXAFS, the surface version of EXAFS (Citrin, 1986; Koningsberger and Prins, 1988). The term NEXAFS is specifically used for K-shell spectra of low-Z molecules. Low Z-molecules are defined as those consisting of hydrogen, carbon, nitrogen, oxygen, and/or fluorine atoms, which are particularly important in surface chemistry. Interest in X-ray absorption spectroscopy techniques and particularly in EXAFS, XANES and SEXAFS, has increased enormously since synchrotron radiation facilities were developed at Stanford University 1974. X-ray absorption spectroscopy is a powerful tool in determining the local structure near a particular atom. X-ray absorption spectra of a surface atom can be divided in two parts: (i) the X-ray absorption near edge structures (XANES), and (ii) the extended X-ray absorption fine structure (EXAFS) (Bianconi, 1980). From the analysis of XANES and the EXAFS, results of structural information like symmetry, chemical bonding, interatomic distance and the coordination number, can be obtained. It is important to note that the requirements for good surface XANES and EXAFS spectra are different. The resolution in the X-ray

148

Chapter 4

spectra, determined by the optical setup (monochromators or mirrors) should be high for the XANES (A < 0.2 eV) and can be low for the EXAFS (A ^ 6 eV) (Bianconi, 1980). The spectral features of XANES have been interpreted as the result of multiple-scattering resonances of the low kinetic energy photoelectrons. Examples of the strong and sharp XANES peaks above the continuum threshold and below the beginning of the weak EXAFS oscillations in the absorption spectra of condensed molecular complexes, are shown in Fig. 4.6.

A/\

05

B

0.0

1//

/v^

V/

--

'

-,.^^ - ^ — - - |

-0,3

1 -^—XANES

-0.6

EXAFS—-

K4Fe"(CN)5-3Hj0 —-K,F/{CN)8

-0.9

^1

7100

h

I

}

7140

7180

1 7220

E(eV)

1 7260

Fig. 4.6. Iron K-edge X-ray absorption spectra of KjFeCCN)^ and KiFeCCN)^ showing XANES resonances and EXAES oscillations (Koningsberger and Prins, 1988) In Fig. 4.6, the relative absorption is plotted against energy, showing a relative variation of the absorption coefficient of about 30% in the XANES region, compared to the EXAFS modulation of less than 4%. Three regions can be identified in the X-ray absorption spectrum: (1) The low-energy XANES region of about 8 eV, called' "edge or threshold region". The absorption edge is the absorption with a few eV determined by the core excitations; (2) The region of multiple scattering in the continuum, called the "XANES region"„ The XANES region is determined by the local site symmetry and chemical bonding, and extends over an energy range of 30 to 60 eV above the absorption edge, where strong peaks with fine structure appear before the threshold of the EXAFS oscillations;

Tribochemical Nature ofAntiwear Films

149

(3) The region of single scattering at higher energies, called the "EXAFS region". EXAFS is used as a standard technique to measure interatomic distances and coordination numbers. XANES spectroscopy has been used to study the composition and mechanism of antiwear tribofilm formation. The photo-absorption XANES spectra were recorded in total electron yield (TEY) versus fluorescence yield (FY) detection to investigate the chemical nature of P, S, Ca, O and Fe on the surface and in the bulk, respectively. When the spectra from the TEY mode and the FY mode were compared, the two techniques provided very useful information (Kasrai et al., 1993 and 1996; Yin et al., 1997a): (a) the photon resolution is < 0.2 eV at the phosphorus and sulfur L-edge regions, (b) when the tribofilm is too thin (<50 nm), the FY signal is too weak to be used. This is due to the fact that fluorescence yield (FY) is orders of magnitude lower than the total electron yield (TEY); (c) the line width broadening in the TEY method can be attributed to surface effects. The TEY method probes both the surface and near surface of the sample. It is well known that surface atoms, due to fewer neighbors and lower symmetry, have slightly different binding energy than the bulk atoms; (d) the low fluorescence yield FY is not a deterrent factor in the measurement, since the background noise is much less than that in the total electron yield TEY method; (e) the spectral patterns for homogeneous samples are very similar, but peaks are slightly better resolved in the FY mode (see Table 4.9); (f) peaks a, b and c in FY spectra are shifted slightly to lower energy while peak d is shifted to higher energy by 0.2 to 0.5 eV (shifts are due to surface effects); (g) when bulk and surface compositions are the same, TEY and FY measurements provide equivalent results; (h) when the composition of the surface and the bulk is different and the spectrafi-omTEY and FY and the modes compared, a layered structure was found in most of the films; there was a longer-chain polyphosphate on the topmost surface and a shorter-chain polyphosphate in the bulk; (i) the bulk FY spectra obtained show two significant advantages over the surface spectra TEY. The assignments of the peaks position of XANES spectra are summarized in Table 4.9. EXAFS spectrum extends from --40 eV to -800 eV above the absorption threshold. EXAFS spectroscopy gives information only on the local structure around the absorbing atom within a distance of about 0.5 nm. The iron K-XANES spectra in Fig. 4.6 can only be explained if all carbon and nitrogen neighbors are included. The position of the strong XANES peaks A and B moves toward higher energy with contraction of the Fe-C distance and their splitting depends on the CN distance. The relative intensity and line shape of XANES features depend on bond angles. The difference between the two spectra shown in Fig. 4.6 has been associated with a larger distortion of the octahedral symmetry in the [Fe(CN)6]'^' rather than the [Fe(CN)6]^' cluster, in agreement with neutron diffraction data (Bianconi et al, 1982). The surface XANES spectra, where the excited electrons strongly interact with many atoms, contain information on relative orientations and

150

Chapter 4

bond angles of atoms surrounding the absorbing atom. The symmetry of a molecular cluster can be easily determined by comparing the XANES of an unknown system with that of known systems ((Koningsberger and Prins, 1988).

Table 4.9. The assignments of the peaks of phosphate and polyphosphate compounds in the (P) L-edge XANES spectra Peak position and assignment of (P) L-edge spectra Peak position a andb. Peak a is separated by ~ 0.9 eV from b, which indicates that peaks b and a are the spin-orbit spHtting of the 2p level (Yin et al. 1995). The relative intensities of peaks a and b are directly related to the number of phosphorus atoms in the polyphosphate structure (or polyphosphate chain length). The intensity increases from orthophosphate to metaphosphate (with bridging oxygen, P-O-P), (Kasrai et al., 1995) and polyphosphate compounds (Yin et al. 1995). In an undisturbed tetrahedral structure, the intensities of peaks a and b are very small (Sutherland et al., 1993). Fig. 4.1 shows that the peak a position shifts to high energy when chain length increases from P04^' (n = 1), to PsOie^" (ri "^ 5). There is no obvious increase in energy above n = 5. The plot of peak a intensity versus the number of phosphorus atoms in polyphosphates has a very sharp increase up to n ~ 20 and then falls off (see Fig. 4.2). Peak position c. The intensities of peak c are related to four coordinated polyphosphates (Kasrai et al, 1995). Thus, the appearance of this peak in the spectra of the fihns indicates the presence of the a phosphate and the fine structure on the left shoulder of strong peak d indicates the local symmetry and the structure of the phosphate (Kasrai et al., 1994). Peak position d. This peak is observed whenever the absorbing atom is bonded to three or more electronegative atoms such as oxygen. The intensity of the peak is qualitatively related to the kind and number of bonded electronegative atoms (Yin et al., 1993). Peak d is always present at the same energy position for all phosphates, whether crystalline or glassy^; however, when oxygen is replaced for other elements with lower electronegativity, the peak is shifted to lower energy (Yin et al., 1993). Peak d is usually assigned to d-like shape resonances (Yin et al., 1995). Peak position e. Peak e, sometimes referred to as a shape resonance peak, appears whenever the central atom is coordinated to three or more strongly electronegative atoms such as oxygen (Sutherland et al, 1993). Another important difference between the ZDDP spectrum and phosphates is the fact that peak e is missing in the spectrum of ZDDP (see Fig. 4.1). ^Except for sodium orthophosphate (Na3P04), sodium pyrophosphate (H2i^2^T)^ pentasodium triphosphate (NajFjOio) and sodium metaphosphate (N^P3Q) which are in crystalline forms, the rest of the phosphates are glasses.

Tribochemical Nature ofAntiwear Films

151

Following are some examples how XANES spectroscopy can be used to determine the local geometrical stucture: (a) in the XANES photoionization process, the excited photoelectron generates "multiple-scattering resonances"; (b) it is not a direct method for the determination of the binding energy of core levels (XPS and ESCA are direct probes); (c) XANES is very sensitive to small atomic displacements; (d) transition from chemisorption to oxidation can be observed; (e) it contains information on the stereochemical details (bond angles and coordination geometry), (f) spectral features are much stronger than EXAFS features; (g) high-resolution spectra are required for XANES data analysis (AE > 0.2 eV energy bandwidth); (h) XANES deals with the structure of a few electron volts to several tens of electron volts above the edge (Kasrai et al., 1995). The experimental setup for surface XANES studies is similar to SEXAFS studies, where X-ray absorption is measured by recording the intensity of emitted electrons or ions. The elastic scattering is essentially determined by the position of adjacent atoms and weakly affected by valence electron distribution. As a result, the XANES technique gives much better chemical differentiation of the spectra than XPS. In XPS spectra, one broad peak is assigned to Ipyi ^^d 2py2 spin orbit components. The 2py2 P^^^ position of 134.5 eV is very close to that expected for a metaphosphate (Briggs and Seah, 1990). In the XANES spectrum, the envelope peaks a, b and c can be compared with the XPS spectrum. This envelope not only shifts in energy, it also has several fine structures related to the local environment of P in metaphosphate. In the XANES spectrum, the relative intensities of peak a and b are directly related to the polyphosphate chain length (Yin et al., 1995). Another interesting feature of the XANES spectrum is the presence of peak d, which is sometimes referred to as a shape resonance peak. This peak is observed whenever the absorbing atom is bonded to three or more electronegative atoms such as oxygen. The intensity of this peak is qualitatively related to the kind and numbers of bonded electronegative atoms (Sutherland et al., 1993; Yin et al., 1993 and 1995). Another important feature of the XANES spectra is the narrow line widths of the peaks. Peaks a and b in the XANES spectrum have bandwidth of-0.8 eV, whereas the individual peaks (not resolved) in the XPS spectrum have a bandwidth of > 1.6 eV. As a result, XANES spectra give much better chemical differentiation than the XPS spectra (Kasrai et al., 1995; Martin., 1999). In order to understand the mechanism of tribochemical film formation, utilization of different surface analytical techniques to investigate the chemical nature of the film is essential. The principles of XPS and XANES techniques are well known (Briggs and Seah, 1990 and 1992; Kasrai et al., 1995; Stohr, 1992; Yin, et al., 1993). In XPS, the bonding energies of electrons are measured in atoms or molecules using a monoenergetic photon source. The resolution of the photoelectron spectra, among other things, is strongly related to the bandwidth of the monoenergetic photons. The technique, in general, yields information about

Chapter 4

152

the elemental composition and oxidation state of the sample. For example, phosphorus (P) 2p binding energy in a phosphate (P04^") is higher than that of phosphine (PH3). In XANES spectroscopy, a variable monoenergetic photon source is used to probe the unoccupied energy level of an atom in a sample. In a similar way to XPS, the absorption edge position is related to the oxidation state of the atom in question; however, the fine structure observed, along with the absorption edge jump, is related to the local structure (symmetry) of the absorbing atoms. To illustrate these points, phosphorus (P) 2p XPS and (P) L-edge XANES spectra of sodium metaphosphate (NaP03) are compared in Fig. 4.7 (Kasrai et al., 1995). In the (P) L-edge XANES spectra, the shift of the peaks to higher energy is evident as the phosphorus (P) structure changes from a formal oxidation state of phosphorus of -3, to the phosphates with phosphorus in the +5 oxidation ,, —ri »— 1 state. Phosphorus in the -3 oxidation state is bonded to three carbon atoms, whereas phosphorus in the +5 oxidation state (eq. ZDDP) is bonded to two oxygen and two sulfur atoms. Similar chemical shifts are generally observed in the case of XPS spectra, indicating that the binding energy of the (P) 2p levels has shifted to higher energies. In contrast to XPS, which 13a 13B 134gives a broad 2p doublet spectrum BLadln£ Enarffy (aV) with no fine features, the XANES spectra are rich in structural details. The L-edge XANES method shows particular value in differentiating phosphorus compounds in the same oxidation state. The monobasic phosphate (CaHP04) pyrophosphate (]^?i^2^i) ^^d methaphosphate (NaP03) all contain phosphorus in the +5 oxidation state, bonded to oxygen, but their local symmetries are 135 140 145 ISO different. These differences are Pbiotozi Ezxargy (aV) reflected very strongly in their spectra. XPS is sensitive to the Fig. 4.7. X-Ray photoelectron (XPS) oxidation state of phosphorus and thus phosphorus (P) 2p spectrum and (P) Lis capable of distinguishing between edge XANES spectrum of sodium orthophosphate and, for example. metaphosphate (Kasrai et al., 1995)

1

f 1 \

M

1

\

1

'

Tribochem ical Nature ofA ntiwear Films

153

phosphide, but is unable to differentiate between a phosphate and polyphosphate. It is also very interesting to note that changing the alkyl group from isopropyl to n-butyl alters the chemical nature of the film (Yin et al., 1993). Extended X-ray absorption fine structure (EXAFS) can be used to obtain information about the arrangement of atoms in the vicinity of the absorbing atom. Because synchrotron radiation sources typically have X-ray intensities three or more orders of magnitude greater in the continuum energies than standard X-ray tube sources, the time of measuring a spectrum for concentrated samples decreases from about a week to minutes (Lytic et al., 1975). Most importantly, the measurements of dilute samples that could not even be contemplated before, are now feasible. The introduction of the Fourier transform changed EXAFS from a confusing scientific curiosity to a quantitive tool for structure determination. The surface extended X-ray absorption fine structure (SEXAFS) method can use either electron or ion detection signal. In practice, there are many differences in techniques used in experimentation. SEXAFS experiments have to be carried out in ultrahigh vacuum, which is not the case for EXAFS and XANES. Surface techniques SEXAFS require ultra-high vacuum of 10'^ Pa. The two basic questions in surface tribochemistry are what atoms are there and how are they arranged? The SEXAFS technique is able to differentiate between different atoms (Z to AZ < 1), in bond lengths (R to AR < 1%), in bond angles (a to Aa < T), and in coordination number N (AN < 10 %). At present, no surface structural technique can fulfil all these demands but SEXAFS satisfies most of them. Bulk EXAFS measurements can be performed at atmospheric pressure, but SEXAFS measurements require an ultrahigh-vacuum (UHV) environment. SEXAFS studies also require a different detection technique than bulk EXAFS measurements. One area of surface science where SEXAFS and XANES is able to show its full strength is the study of chemisorbed molecules (Koningsberger and Prins, 1988). X-Ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES) and other techniques. The surface analytical techniques used most successfully are X-Ray photoelectron spectroscopy (XPS), Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) or (ASCA), and Electron probe microanalysis (EPMA). Diffraction techniques, although applicable to very thin films, can only give positive results with crystalline compounds (Bird and Galvin, 1976). For a complete understanding of extreme pressure additives in engine oil, it is necessary to know the compounds, rather than the elements, that are present on the surface. The difficulties in identification are due to the fact that the layers are extremely thin, are frequently amorphous and that worn surfaces present difficulties in the application of many techniques of analysis. The investigators used the study of the film formation on immersed and rubbed

154

Chapter 4

surfaces to learn about the differences in composition. The various conditions of static immersed tests and rubbed test differences can be expressed as follow: The static immersed test: specimens are prepared by heating them in oil formulations with additives, film is formed on oxide contaminated surfaces; load is not involved in specimen preparation, only temperature can be a comparable parameter; very different compounds are formed from the same additives solution. The rubbed test: Test performed on the wear machine or in engine oil; tribochemical film formed on oxide-free fresh surfaces; specimen is produced under high-load and low-load conditions; only temperature can be a comparable parameter; very different compounds are formed from the same additives solution. The many studies demonstrate that static immersion tests can be a very misleading procedure as frequently very different compounds are formed from the same additive solution. XPS has been used to analyze surface layers. In XPS, a surface is irradiated with X-rays of known energy. Electrons are liberated from the atoms present in the surface and these electrons are collected and separated according to the energy they possess. From the energies of the incident radiation and the emitted electrons, the bonding energies of the electrons are deduced. This reveals the elements from which they come. The advantages of XPS are: (a) the depth of the analyzed layer is on the order of 1 to 2 nm; (b) light elements are analyzed; and (c) the binding energies of electrons vary slightly with their state of chemical combination, e.g., sulfur can be shown to be present as sulfide or sulfate (Bird and Galvin, 1976). XPS is also known by the acronym ESCA (electron spectroscopy for chemical analysis), XPS as an analytical technique has the ability to identify not only the elements present in the samples but also their chemical state, i.e. the spectra Fe°, Fe^^, and Fe^^. XPS spectra can be quantified in a very straightforward manner. Quantification of Auger electron spectroscopy (AES) data is rather more complex and the accuracy obtained is generally not as good. The two methods (XPS and AES) have come to be regarded as the most important ones of surface analysis in the context of material science. All commercial spectrometers are now based on vacuum systems designed to operate in the ultra high vacuum range of 10'^ to 10'^^ mbar, and now it is generally accepted that XPS and AES experiments must be carried out in this pressure range. Important parameters for characterization of phosphate glasses are the P/O atomic ratio which is equal to n/(3n + 1) and the ratio of bridging oxygens (P-O-P) to non-briding oxygens (-P=0 and P-O-Zn). This ratio is equal to (n - l)/2(n +1), where n = 1 is for zinc orthophosphate, n = 2 for pyrophosphate and the metaphosphate corresponding to a very long polymeric chain (linear or cyclic). XPS, for example, can give both the P/O atomic ratio and the briding oxygens/non-bridging ratio. XPS core level data on the tribofilm clearly show that

Tribochemical Nature ofAntiwear Films

155

some sulfur is in sulfide form, but XPS cannot easily separate the metal sulfide form from the thiophosphate due to the weak chemical shift ((Martin et al., 1999; Martin, 2001). Auger electron spectroscopy cannot tell us anything about phosphate chain length, but has the advantage of giving good differentation between the oxide and phosphate forms by looking at the chemical shift on the OKLL line (Grossiord et. al., 1999). In principle, the same energy analyzer may be used for both XPS and AES and, consequently, the two techniques are often found in the same analytical instrument. The sources of the primary radiation for the two methods are a soft X-ray tube and an electron gun. The nomenclature employed to describe photoelectrons and Auger electrons is different: XPS electrons are described by the appropriate quantum numbers, i.e. Is, 2p3/2, 3d5/2, 4f7/2, while Auger electrons are identified by the X-ray notation (shell name), i.e., K, L, M, N, and so on (Watts, 1990). XPS, like LEED and AES, requires the use of a vacuum environment. In XPS a wear surface is exposed to a beam of monochromatic X-rays causing electrons with kinetic energies characteristic of the parent atom to be ejected from the specimen. A spectrum is obtained by plotting the total number of electrons ejected from the surface as a fimction of kinetic energy. AES provides a good technique for elemental analysis while XPS provides chemical binding information. Thus, with XPS, it is possible to identify from binding energies the nature of the compounds in which the elements exist on the surface. The binding energies of functional groups of organic compounds, particularly those containing carbon: CC, C-N, C-O, C-Br, C-F, and C-S bonds can be very useful in the determination of lubricating structures on wear surfaces. Some researchers have preferred to use XPS since it gives better quantification of the elements of interest (Zn, P, S) than Auger spectroscopy and also some indication of their valency state. Most investigators, however, look at surfaces under dynamic engine conditions and start with XPS analysis of cam noses and lifters from a Volvo B20 engine test on typical fully formulated engine oil (Buckley, 1978; Watkins, 1982). This shows a very different picture than the static tests in that the surface contained Zn, P, S, Ca, and Mg. XPS analysis shows valency information on the surface molecules. The elements of binding energy show clearly the presence of sulfide, pentavalent phosphorus (phosphate) and zinc cations. Routes from ZDDP to zinc and phosphorus as phosphates and sulfur as iron sulfide at the surface have been demonstrated: zinc phosphate by physical adsorption and FeS formation via the oxide layer and elemental sulfur generated from alkyl sulfides plus Fe203. Characterization of the bearing surface film, and film formed in a lubricated cam/tappet friction apparatus have been analyzed by reflectance-absorption infrared. X-ray photoelectron (XPS) and Auger electron (AES) spectroscopies (Lindsay et al., 1993). The two lubricants used were similar to fiilly formulated engine oils.

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156

One oil contained a soluble molybdenum friction modifier additive while the other did not. The IR bands in the 1200 to 1100 cm'^ and 650 to 600 cm"^ regions have been assigned to the symmetric PO'2 stretching and symmetric bending modes, respectively, observed in divalent cations in metaphosphate glasses. Both XPS and Auger analyses were performed for tribofilm. The Auger results revealed higher concentration of Fe, Zn, Mg and P, but lower levels of carbon relative to XPS. Phosphorus is present in phosphate form and sulfur as sulfide in unmodified oil. The oil containing a soluble molybdenum friction modifier additive formed films which were thinner and less continuous than films formed from the unmodified oil. The film was dominated by magnesium phosphate which was identified by AES, and by the combination of XPS and JR. Sulfur was present as sulfate and sulfide while molybdenum was present as Mo^"^ and Mo^^, as shown in Fig. 4.8 (Lindsay et al., 1993).

239-6

B i n d i n g Energy

(eV)

Fig. 4.8. XPS Mo3d spectra from the fihn produced from engine oil containing a soluble molybdenum friction modifier additivefromthe outer ring region (A), the middle ring region (B), and the center ring region (C), as well as the spectrum of the agent, M0S2 (D), (Lindsay et al., 1993) The Molybdenum Mo3d5/2 spectra from the different ring areas are displayed in Fig. 4.8, The binding energy of 232.8 eV of the Mo^^ species is present in the outer ring (62% Mo^^ and 38% Mo^O, middle ring (100% Mo^O and center ring (100%) Mo^^). The Mo"^^ species in the outer ring (A) was found to have a binding energy of 229,7 eV, which is the same as the binding energy of a M0S2 reagent, shown in Fig, 4.8, spectrum (D). The peak occurring in the spectra of Fig. 4.8 at

Tribochemical Nature ofAntiwear Films

157

a binding energy of 226.8 eV, is the S2s sulfide line (D) of M0S2 reagent, which was run on the same instrument, and the outer ring (A). Auger electron spectroscopy (AES) is particularly suited for surface analysis (depth 0 . 5 - 1 nm). AES depth profile analysis was employed to determine the thickness and composition of surface reaction layers formed under test conditions in the Reichert wear apparatus in the presence of four different ZDDPs additives at different applied loads (Schumacher et al., 1980). Using elemental sensitivity factors the concentration of the four elements (S, P, O, C) was determined at three locations corresponding to a depth of 1.8, 4.3, and 17 nm. No significant correlation between wear behavior and carbon or oxygen content of the reaction layer was observed. A steady state sulfur concentration is reached after a very short friction path. Contrary to the behavior of sulfur, phosphorus concentration in the presence of ZDDPs increases steadily with friction path, and no plateau value is reached.

Ekclfon tnefijy (cV)

Fig. 4.9. Auger electron spectroscopy (AES) spectrum of a specimen subjected to a Reichert wear test in the presence of ZDDP additive (Mathieu et al., 1981) Fig. 4.9 presents a typical Auger spectrum for a specimen subjected to a Reichert wear test, which was analyzed without using ion sputtering (Mathieu et al., 1981). Iron, oxygen, carbon, sulfur and phosphorus Auger peaks can be identified. The amplitude of the peaks is a measure of the concentration, but quantitative elemental analysis depends to some extent on the matrix effects. The resultant signal is displayed as a differential spectrum (dN/dE). The Auger spectra are overlapped by other emitted electrons. To eliminate background, differential spectra are recorded rather than the direct (pulse counted) energy spectrum.

Chapter 4

158

Electron-probe micro-analysis (EPMA) can show the presence and spatial distribution of elements present in thin films. The technique does not, however, give direct information on the compounds present and it is less easy to apply to the analysis of light elements such as oxygen and carbon (Bird and Galvin, 1976). The surface topography of the bearing ball wear scars was studied using scanning electron microscopy (SEM) and electron probe micro-analysis (EPMA) techniques. In view of the interest in the development of new extreme pressure additives, nitrogen and sulfur heterocycles (arylamino-oxadiazoles and arylaminothiadiazoles) have been evaluated in liquid paraffin. The results were compared with the commercial sulfur-phosphorus gear oil package additives. The concentration of sulfur, oxygen, carbon, and iron were measured with a line analysis and were represented by intensity (counts per second). The composition of the chemically reacted surface films appears to be similar at lower and upper applied loads. The different intensities of sulfur and oxygen were clearly detected at the wear scar (Singh et al., 1990).

Fig. 4.10. Infra-red absorption spectra for model compounds (a, b, c) and a typical tribochemical film spectrum (d). (a) magnesium phosphate, (b) thermal decomposition product (ZDDP under 120°C); (c) thermooxidative decomposition product (ZDDP) and thefreeradical initiator azo bisisobutyronitrile under air at 120°C); (d) tribochemical film spectrum (Willermet et al., 1992)

I600

«400

I200 lOOO 800 Wave-numbers

600

400

A reciprocating wear tester was used to investigate the nature of antiwear boundary lubrication films formed by several ZDDP additives in mineral oil. The surface films were analyzed by SEM-EDX, EPMA and scanning Auger. The antiwear film contained S, O, Zn, and P elements with trace amounts of Fe and C. Once antiwear films are formed they can be removed by enhanced roughness and the presence of hydroperoxides (Sheasby et al., 1990). One approach to obtaining structural information on surface films is reflectance-absorbance infra-red (RAIR) spectroscopy and Fourier Transform IR (Foster, 1999). Model materials were prepared by thermal and thermooxidative decomposition of zinc diisopropyl-dithiophosphate. Lubricant-derived

Tribochemical Nature ofAntiwear Films

159

tribochemical films were prepared in a cam and cam follower combination (cam follower is sometimes referred to as a tappet) friction apparatus using fully formulated engine oils containing zinc diisopropyl-dithiophosphate. Spectra for the various model compounds are compared with a typical spectrum for the tribochemical film in Fig. 4.10 (Willermet et al., 1992) and suggest that: (a) the tribochemical film was formed by thermooxidative degradation; (b) the phosphates were not present as phosphate glasses; (c) the film is predominantly amorphous. The tribochemical film produces two major bands, one centered between 1130 cm'^ and 1185 cm"^ and one centered at -620 cm'^ Amorphous ortho- and pyrophosphate were prepared and their spectra seem to be consistent with the spectra of the tribochemical films. The films had two broad major bands consistent with the phosphorus-oxygen bonds. This indicates that the tribochemical film was not composed of phosphate glasses as expected, but rather predominantly amorphous orthophosphate or pyrophosphate formed by thermooxidative decomposition of zinc dialkyldithiophosphate. These groups may be connected by the metal cations.

Problems 4.1 The tribofilm composition On the basis of Table 4.1, what analytical surface techniques were used to evaluate the tribofilm composition after ZDDP degradation? The tribofilm conditions include the following: (a) the tribofilm consists of Zn, P, S, O and Fe and (b) the tribofilm contains mostly of a mixture of short-and long-chain phosphates and sulfur is present as zinc sulfide. What analytical technique has the ability to identify elements and their chemical state, e.g., Fe° or Fe^^, in case (a), and compounds in case (b)? More about analytical techniques for evaluation of the metal surfaces can be found in chapter 4.3. 4.2 Physical parameters By using Table 4.3. "The effect of physical parameters such as adsorption, rubbing, temperature, concentration, load, surface roughness on antiwear performance of ZDDPs", which term is described by the physical parameter: (a) the tribofilm accumulated on the surface becomes thicker with adsorption time; (b) long-chain phosphates are formed on the topmost surface, but short-chain phosphates were present in the bulk; (c) sulfur and phosphorus ratio changes with rubbing time; and (d) when load increased the concentration of (S) and decreased that of (P)?

160

Chapter 4

4.3 Equilibrium ofZDDP The rearrangement of ZDDP is initiated by a double alkyl group migration from oxygen atoms to sulfur atoms (Jones and Coy, 1981), and the shift to higher energy of LI-ZDDP (2154.5 eV) compared to ZDDP (2153.3 eV), (Fuller et al., 1998). How does the chemistry of the film change with preheating? (see interpretation of Fig. 4,4).

RO \ (S=P-S-)2Zn / RO

->

Zinc dialkyldithiophosphate (ZDDP)

RS \ (0=P-0-)2Zn / RS A linkage isomer (LI-ZDDP) of ZDDP

4.4 Chemical parameters What is meant by chemical parameters in oil formulation? Which of the following is a chemical parameter: adsorption, detergent, dispersant, concentration, ZDDP, or surface roughness? 4.5 X-ray spectroscopy On the basis of interpretation of phosphorus and sulfur L-edge tribofilm XANES spectra in Table 4.4 and 4.5, what is the chemical nature of antiwear tribofilm and composition of the following pairs: (a) ZDDP + detergent, (b) ZDDP + dispersant? 4.6 Tribofilm composition Explain the polyphosphate chain length in the bulk and on the surface when ZDDP is in the presence of calcium sulfonate and calcium phenolate soft-core RMs (for solution see Table 4.4). 4.7 Polar additives in oil formulation What type of interactions can you predict in the engine oil two-component system ZDDP + dispersant: (a) in the hydrocarbon formulation, and (b) on the surface? Consider the stronger the complexation, the greater adverse effect on wear.

161

Chapter 5

SURFACE TRIBOCHEMISTRY AND ACTIVATED PROCESSES

'^God made solids, but surfaces were made by the Devil" Wolfgang Pauli, 1926

"Changing the chemistry of the lubricant can also change the chemistry of the metal surface" Donald K Buckley, 1981

5.1. Chemical Nature of Metal Surfaces In examining any mechanical lubrication system (the tribological system, or tribosystem), three tribometric components must be considered: (I) the metal surface to be lubricated; (II) the effectiveness of the lubricant; and (III) the environment. (I) The metal surface to be lubricated constitutes the foundation of the tribological system. There are a number of tribometric parameters for a solid surface, which include friction, wear, and adhesion. Tribological processes can occur in the contact area between two friction surfaces, which include physical, chemical and tribochemical events (Buckley, 1997) such as: (a) physical adsorption; (b) chemisorption; (c) boundary lubrication; and (d) wear. (a) Physical adsorption is a physical phenomenon by which molecules are attracted to a surface by van der Waals electrostatic forces. If the interaction involves less than 50 kJ/mol energy, the process is one of physical adsorption; over 50 kJ/mol, the process is similar to chemisorption. Physical adsorption is a temperature-dependent phenomenon as well as being pressure dependent; it is very much reduced at high temperatures, and even adsorption of base oil molecules is likely to be ineffective at temperatures much above 200°C (Studt, 1989). The adsorption of additives is determined by the polar groups. Comparing the adsorption of acids, alcohols and esters, acids are the most strongly adsorbed and esters are the least strongly adsorbed.

Chapter 5

162 esters

<

alcohols

<

acids

(5.1)

Thus, by the introduction of a carboxylic acid group into the additive molecule, its concentration on metallic surfaces can be greatly increased. A single monolayer of adsorbed molecules, e.g., of aliphatic acid, causes a dramatic reduction in the friction coefficient. The surface forces technique (SFT) has made it possible to control friction at a single monolayer of adsorbed species, and the friction force microscope technique (FFT) makes it possible (monomolecular film) to study unreactive surface lubricant molecules on the atomic level (Bhushan et al, 1995a and 1995b; Granick et al., 1995 ; Israelachvili and McGuiggan, 1990; Levins and Vanderlick, 1994; Marti, 1993; Stolarski, 1990). (b) Chemisorption is an interaction that involves strong chemical bonds between the liquid components and the solid surface. It has been shown in experiments that anti-wear additives, mild extreme-pressure additives, friction modifiers, and other classes of additives operate by adsorption (Mori and Imazumi, 1988; Smith and McGill, 1957). The adsorption of any lubricant component is affected by the surface activity of the reacting metal: (a) on a real metal surface under static conditions; the surfaces are covered with metal oxides and organic contaminants; (b) on the nascent metal surfaces; when the surface layers are mechanically removed and nascent areas of high chemical activity form on the fresh surfaces.

Table 5.1. Adsorption of organic compounds on metal surfaces under static conditions and on the nascent steel surfaces Compound (dielectric constant)

Static surface

Nascent surface adsorption activity (sec"^)

^-Hexane (1.89) Cyclohexane (2.02) 1-Hexene(2.05) Benzene (2.28) Diethyl disulfide (15.9) Methyl propionate (5.4) Propionic acid (3.3) Propyl amine (5.3)

No adsorption No adsorption No adsorption No adsorption Weak adsorption Weak adsorption Strong adsorption Strong adsorption

No adsorption No adsorption Strong adsorption (0.28) Strong adsorption (0.39) Strong adsorption (0.35) Medium adsorption (0.18) V. weak adsorption (0.05) V. weak adsorption (0.01)

The compounds studied were divided into two groups, based on their adsorption activity, as highly reactive and relatively unreactive compounds. Organic compounds adsorb through electron donation to the metal surface (Fischer et al., 1977; Stair, 1982). The two saturated hydrocarbons, n-hexane and

Surface Trihochemistry and Activated Processes

163

cyclohexane, did not adsorb on the nascent steel surface. Benzene and 1-hexene, which have TT-electrons, showed high activity. As it can be seen in Table 5.1 the less reactive compounds have polar functional groups. Functional groups in the adsorbate molecules affect their adsorption activity (Forbes et al., 1970b; Hironaka et al., 1978 Mori and Imazumi, 1988). The effect of the particular functional group on adsorption, however, is not always the same as seen on oxide surfaces. For example, carboxylic acids adsorb strongly on metal oxide surfaces, and the heat of adsorption of stearic acid is higher than that of the corresponding ester, methyl stearate (Hironaka et al., 1978). For adsorption on the nascent surface, however, propionic acid is a poor adsorbate, and the adsorption activity of propionic acid is lower than that of methyl propionate. Although propyl amine adsorbs easily on metal oxide surfaces, the adsorption activity of propyl amine is low on the fresh steel surfaces. Also, the heat of adsorption of organic sulfides on iron oxide is less than that of esters (Forbes et al., 1970b), but the results of adsorption activity on the nascent surface were the opposite. It is thus noteworthy that the chemical nature of the nascent surface of steel is often opposite to that of oxide-covered metal surfaces, with respect to adsorption. The adsorption of organic compounds on nascent surfaces can be considered as an acid-base reaction. According to the hard-soft acid and bases HSAB principle (Ho, 1977), polar compounds such as carboxylic acid and amine (with lone pair electrons on oxygen or nitrogen) are classified as "hard bases". A hard base reacts more easily with a hard acid than with a soft acid. Metals are classified as soft acids which react much more easily with soft bases than hard bases. The results in Table 5.1 can be explained with this concept. The soft bases (benzene, 1-hexene, diethyl disulfide) react easily with the nascent surface as a soft acid. On the other hand, the hard bases such as propionic acid, stearic acid, propyl amine and trimethyl phosphate exhibit a very low activity (Fischer et al., 1997a and 1997b; Mori and Imazumi, 1988). Chemisorption is a monolayer process, where exchange of orbital electrons occurs between adsorbed molecules and atoms of the surface, causing the formation of new chemical compounds. Chemisorbed molecules are much more strongly retained than adsorbed molecules, giving greater resistance to high temperatures, and mechanical shear. Organic chemisorbed films are not likely to be usefiil above 300 °C. Aliphatic amine and aliphatic acids have also proved to be specifically suitable for the build-up of anti-wear protective layers. If the heat of binding is used as a measure of the strength, then in comparing very similar additives, increasing adsorption heat decreases wear. For instance, in an aluminum-steel pairing, 12-carbon amines are more strongly bound to the fractional surfaces than 12-carbon alcohols and these in turn are more strongly bound than 12-carbon carboxylic acids (Teipathi and Groszek, 1972). As Table 5.2 shows, amines also reduce most effectively the metal-metal contact. This is

164

Chapters

a function of the portion of aluminum and iron contained in the lubricant. The reaction layer developed tribochemically and bound to the base by strong adhesion leads to a reduction of the friction coefficient, /i, from 0.16 to 0.038 and thus offers considerably improved lubrication (Teipathi, 1971). Table 5.2, Content of aluminum and iron in paraffinic oil under severe rubbing conditions, T = 217°C (Tomaru et al, 1977) Twelve-carbon {C^^ chain length compounds Metals Aluminum (ppm) Iron (ppm)

Acids

Alcohols

Amine

40 18

21 8

14 4

Adsorption of carboxylic acids, alcohols and amine on steel surfaces takes place by hydrogen-bonding of monomeric molecules to the oxide and hydroxide layers on the surface. Aliphatic amines, alcohols, and acids have also proved to be especially suitable for build-up of anti-wear protective layers, and greatly reduce the friction coefficient. Adsorbed and chemisorbed films are very effective in reducing friction and mild wear under moderate rubbing; however, they fail fairly rapidly under severe rubbing conditions and therefore are not very effective in preventing wear and seizure. (c) Boundary lubrication is a type of lubrication in which the volume of the lubricating substance is insufficient to separate the rubbing surfaces. Contact between surface asperities is thus frequent, and occurs with a significant dissipation of energy. Boundary lubrication usually occurs under high load and low speed conditions in bearings, gears, cam and tappet interfaces, piston rings and liner interfaces, pumps, transmissions, etc. Under boundary lubrication conditions, interactions between the two surfaces take place in the form of asperities colliding with each other. Mechanical interactions at these contacts produce elastic and plastic deformation, friction, heat, and sometimes wear (Buckley, 1981; Shpenkov, 1995; Stolarski, 1987 and 1991). Under slight variations of the friction mode and conditions, the properties of surfaces are changed. Molecules of substances in the friction mode are in a particularly excited state. That the excitation in friction is unique is demonstrated by the fact that some materials can be synthesized only in friction mode and conditions. This state cannot be attained without the boundary friction by simply heating the materials in pairs to the temperature reached in friction. When the boundary friction occurs the excitation becomes so high that photo and electronic emissions and ionization of the ambient atmosphere occur around the interacting surface (Hsu et al., 2002;

165

Surface Tribochemistry and Activated Processes

Kuzharov and Suchkov, 1980; Shpenkov, 1995a). The wear of surfaces is much greater and the coefficient of friction is much higher than in elastohydrodynamic lubrication (see Stribeck-Hersey curve coefficient of friction). The friction is extremely sensitive to the presence of small amount of various substances in the lubricant. Detergents tend to act as friction modifiers by forming chemisorbed films. When the melting point in the range of 120 to 200°C is reached, they desorb and the boundary lubricating properties are lost. Chemical reactions between lubricant molecules and surfaces usually accompany formation of tribofilm (Bernard, 1983, Fischer etal., 1997a and 1997b; Hsu and Gates, 2001; Yin et al., 1997a). The viscosity of the bulk lubricant is of minor importance in boundary lubrication. Synthetic fluids, such as poly-oc-olefin and PAO/ester blends offer a number of inherent performance advantages over conventional petroleum-based engine oils. When the surfaces come closer together, the PAO is squeezed out of the contact zone. At high loads, the esters preferentially stick to the metal surface, tend to form chemisorbed layers and then a tribofilm is formed by chemical reaction. Since ester groups are polar, they can compete with antiwear or extreme pressure agents for the metal surface. Under extreme boundary conditions, esters tend to break down to form a metal carboxylate film capable of reducing friction between rubbing surfaces (Randels, 1999). Incorporation of reactive substances containing sulfiir, chlorine, and nitrogen or free oxygen in the lubricant stimulate reaction with the nascent metal surfaces to provide boundary lubrication. (d) Wear is defined as the progressive loss of material from the surface of a contacting body as a result of mechanical processes, hi practice, the two important forms of mechanical wear are adhesive and abrasive wear. Archard's wear equation is commonly used to correlate data from wear experiments (Archard, 1953; Hsu et al., 1997; Plaza, 1997; Stolarski, 1979). Wear volume = (wear coefficient • load • distance of sliding) / hardness

(5.2)

Equation (5.2) indicates that, given a sliding device in which a specified load must be moved a specific distance, there are just two ways of minimizing adhesive wear, namely to use hard materials or to achieve a low wear coefficient. An important fact to bear in mind when planning to minimize adhesive wear is that changes in bearing materials which reduce the wear coefficient that covers a range from 10"^ to 10"^, are much more likely to reduce the wear than are changes in hardness alone. This covers a range of 2.5 orders of magnitude. Wear occurs when the shear strain reaches a certain critical value of the coefficient of friction, resulting in surface disruption and wear (Hsu et al., 1997). There are several criteria which can be used to classify the different types of

166

Chapter 5

wear, e.g., according to the types of friction, according to wear mechanisms, or the shape of the wear particle (Mang and Dresel, 2001). Based on the wear mechanism, there are four principal types of wear: adhesive, abrasive, corrosive, and surface fracture wear; adhesive wear is the only one which can never be eliminated (Rabinowicz, 1984). Adhesion takes place when the boundary lubricant layer in the lubricant and the phosphate or metal oxide film is disrupted. In general, depending on the severity of the situation, there are three types of adhesive wear: mild wear, more severe wear, and very severe wear (extreme pressure). The tribochemical processes and mechanism of ZDDP was studied as a function of the severity of the wear conditions, see Table 5.3 (Hsu et al., 1997; Martin, 1999; Plaza, 1997; Podsiadlo and Stachowiak, 2002). Table 5.3. The adhesive wear (mild wear, more severe wear, very severe wear) properties and tribofilm formation on metal surfaces (Hsu et al. 1997) Mild wear

More severe wear

Very severe wear

ix^ < 0.4, w.c.'^-lO-^ particle size < 0.2 /^m, plastic deformation, lubricating film, oxidation film, long-chain zinc polyphosphate tribofilm.

/i > 0.4, w.c. -10-^ particle size (20 - 2 /im), shear strain layer, adhesive wear, local abrasive wear, shortchain Fe/Zn polyphosphate tribofilm.

/^ >0.8, w.c.~10-\ particle size (200 - 20 /^m). temp, induce wear, severe galling wear, nascent surfaces, iron sulfide tribofilm.

^the coefficient of Mction. When the coefficient of friction is less than 0.4, shear strain accumulates under repeated sliding until it reaches a critical value, and then surface wear occurs; %e coefficient of wear (w.c). Under mild wear conditions, tribochemistry and local plastic deformation dominate, and the friction coefficient is less than 0.4. The metal's reactivity with the environment, such as lubricant molecules, additives and oxygen, is important in adequately describing the wear process. In this case, no sizeable wear particles are observed, while wear coefficients are typically in the range of 10"^ to 10"^. In mild wear conditions, the long-chain zinc polyphosphate "thermal film" is generated on the surface. The transition temperature of the glass phosphate is about 200-300''C, Some kind of viscous flow of the magma state glass is formed to separate the two metal counter-faces. Under more severe conditions, the shear strain concentration in the subsurface layers dominates the wear process. When the coefficient of friction is less than 0.4, shear strain accumulates under repeated sliding until it reaches a critical value.

Surface Tribochemistry and Activated Processes

167

and then surface wear occurs. When the coefficient of friction is larger than 0.4, surface wear by deformation occurs immediately. Typical wear coefficients are around of 10"^ to 10"^, while wear particle sizes are in the range 2 to 20 yum. In more severe conditions, any iron oxide abrasive particles are immediately eliminated by the phosphate glass. The formation of short-chain mixed Fe/Zn polyphosphate can control the contact zone. The tribochemical reactions between the phosphates and the oxides are governed by the hard and soft acids and bases (HSAB) principle (Martin, 1999; Pearson, 1966, 1973 and 1997; Stair, 1982). In very severe conditions (extreme pressure), the surface lubrication films become largely ineffective. In this region, both the nominal contact stresses and local asperity contact stresses are important to the wear process. Such sliding systems generally produce wear coefficients in the range 10'^ to 10"* and wear particle sizes in the range 200 to 20 /^m. In very severe conditions, nascent metal surfaces can be produced by the disruption of the zinc phosphate glass tribofilm itself by the wear process. In this case, the residual organic sulfur species present in the lubricant can react with the metal surface to form iron sulfide, and adhesive wear can be avoided. Surface energy (surface tension) or interfacial tension of solids is related to the adhesive behavior of solids in solid-state contact. It is simply the work required to pull apart a solid and thereby generate a new surface. Surface energies for solids have been measured by various techniques and calculated for a number of crystalline solids . The surface energy decreases drastically with impurities, e.g., sulfur content (Buckley, 1981 and 1985; Loomis, 1985; Mathieu et al., 1981; Somorjai, 1981). There are some examples of how the chemistry of the solid surface can be affected: (1) the presence of 0.5% sulfur reduces the wear of the solid surfaces, (2) the presence of carbon in a graphite form in cast iron is responsible for good wear characteristics, (3) the fresh metal surface interaction with the environment forms oxides, nitrides, or hydroxides tribofilms, (4) interactions of lubricant additives form surface compounds, (5) increasing the load or the sliding speed brings more energy to the interface. (II) The effectiveness of the lubricant is measured by its ability to prevent wear and reduce friction. The only gauge for the effectiveness of a lubricant is its ability to provide a protective film or coating on the surface to minimize friction and wear. The more effective it is in minimizing wear and reducing friction, the better the lubricant. Even with effective surface lubrication, however, metal can be transferred from one surface to another. Thermal stability is valuable because it gives an indication of the highest temperature at which the lubricant can be used. The higher oxidation limit and thermal stability of some synthetic fluids result in better high temperature behavior than is possible with mineral oils. The extraordinary high temperature performance of polyphenyl ether becomes obvious. Maximum service temperature for permanent and temporary service periods are

168

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given in Table 5.4, and it becomes obvious that some synthetic fluids are superior to mineral oils (Buckley, 1981; Lansdown, 1990; Rudnick and Shubkin, 1999). Table 5.4. Maximum service temperature for mineral and synthetic lubricating oils Oil Mineral oils Polyphenyl ether Perfluorohydrocarbons Carboxylic acid esters

Permanent temp. (°C)

Temporary temp. (°C)

90-120 310-370 280-340 170-180

130-150 420-480 400-450 220-230

Liquid lubrication mechanism. There are four defined regimes of liquid lubrication: hydrodynamic (thickness of lubricant film (h), h > 0.25 /^m), elastohydrodynamic (h ~ 0.025 to 2.5 //m), boundary (h ~ 0.0025 /^m), and mixed. These regimes are dependent on oil viscosity (Z) and relative velocity (V); and are inversely proportional to the load (L), (ZV/L). Fig. 5.1, known as the StribeckHersey curve, depicts these regimes in terms of friction coefficient versus viscosity, velocity, and load (ZV/L) (Fusaro, 1995). The first regime is known as hydrodynamic lubrication. This regime is characterized by complete separation of the surfaces by a fluid film that is developed by the flow of the fluid through the contact region. The second lubrication regime known as elastohydrodynamic lubrication comes into effect where loads are high enough to cause elastic deformation of the surfaces, but speed and viscosity are not high enough to produce film thicknesses greater than about 0.25 //m. Usually, during hydrodynamic and elastohydrodynamic lubrication, no wear takes place because there is no contact between the sliding surfaces. As the thickness of the oil film decreases to values below 0.0025 /im, the boundary lubrication regime comes into play. In this regime, asperity contact between the sliding surfaces takes place, and the lubrication process becomes the shear of the chemical compounds on the surface. This is dependent on lubrication additives within the oil that produces compounds on the surface, which have the ability to shear and provide lubrication. Boundary lubrication is highly complex, involving surface topography, physical and chemical adsorption, corrosion, catalysis and reaction kinetics. The transition between the elastohydrodynamic and boundary regimes is not sharp, and there exists a region, called the mixed lubrication regime, which consists of some elastohydrodynamic and some boundary lubrication (Bovington, 1999; Dowson and Ehret, 1999; Fusaro, 1995; Spikes, 1999). This is characterized by a fluid and by surface changes: hydrodynamic (continuous fluid film, negligible deformation); elastohydrodynamic (continuous fluid film, elastic deformation), and boundary (contact, elastic and plastic deformation).

Surface Tribochemistry and Activated Processes

169

(IQ) The environment can alter lubricant properties, and affect the lubricantsolid surface interaction (friction, adhesion and wear). For example, many oils contain high concentrations of dissolved oxygen. Some oils dissolve gas (e.g., oxygen) as much as 50 times their volume. The friction coefficient measured with various coverages of tungsten surface by oxygen dropped from 3.0 to about 1.3. By the time the surface had been covered by a full mono-layer of oxygen no further changes were observed in friction.

h " 0.025 to 2.5 iim (10-* to 10*^in.)

^:^

/

^*^

f

/)-0.0025 urn (10-^n.) '

^ .150

i

o

8 .001

(Vi»co$ity)(Velocity) 2V (Load) ' L

Fig. 5.1. Stribeck-Hersey curve coefficient of friction (f or JLI) as function of viscosity (Z) • velocity (V) / load parameter (L); h = thickness of lubricant fihn (Fusaro, 1995) Entrapped oxygen and water vapor in lubricants can act as anti-wear additives to form protective surface films. Metals are known to catalyze decomposition of certain lubricants, and decomposition temperatures may be reduced by 60°C or more. This is particularly true of bearing surfaces, on which the surface energy may be increased by stress-induced dislocations and by freshly exposed metal surfaces. An example of the way a tribochemical surface is affected by a polymerization process is vinyl chloride. If the load is increased from 0.1 to 0.5 kg in the presence of a vinyl chloride atmosphere, the Auger spectra of iron oxide surface shows a marked increase in the concentration of vinyl chloride on the surface (Buckley, 1981).

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The lubrication system is extremely complex. The mechanism of lubrication is partly dictated by the nature of interactions between the lubricant and the solid surface. Additives blended into lubricating oil formulations either adsorb onto the sliding surfaces, eg., fatty alcohols, fatty amines, amides, phosphoric acid esters (friction modifiers), or react with the surface, eg., ZDDP, MoDTC, MoDDP organic phosphates (extreme pressure). Some interactions affecting the surfaces of metals include: adsorption, chemisorption, and tribochemical reactions-these form new compounds on the surface and lubrication by reaction products (Bhushan and Gupta, 1991; Briscoe et al., 1973; Briscoe and Evens, 1982; Heinicke, 1984; Hsu and Klaus, 1978 and 1979; Klaus and Tewksbury, 1987; Lansdown, 1990; Liston, 1993; McFadden et al., 1998; Studt, 1989).

5.2. Catalytic Activity of Rubbing Surfaces Tribochemistry is concerned with chemical reactions in fluid formulations affecting tribofilm formation on metal surfaces. The surfaces possess enhanced activity due to stored energy accumulated by mechanical actions (Rowe and Murphy, 1974). Tribochemistry deals with relations between mechanical work and mass transformation. The term tribochemistry called by the others mechanochemistry (Kubo, 1971) is based on the following definition: "Tribochemistry is a branch of chemistry dealing with the chemical and physicochemical changes of solids due to the influence of mechanical energy" (Heinicke, 1984; Thiessen et al., 1967). Tribochemistry is the stimulation (usually acceleration) of chemical reactions by friction (Muratov, 1998). Several mechanism have been proposed to describe tribochemical interactions between materials (Fischer, 1988a, 1998b and 1997; Fischer and Mullins, 1992). Among these are frictional heat generation, transformation of the material (plastic deformation) and defect generation, exposure of fresh surfaces and charge-induced effects. The reactivity of mechanically-activated (rubbing) surfaces is the subject of extensive study in the field of catalysis. Reactions at clean metal surfaces serve as a reference point for metal-lubricant reactions at rubbing surfaces. Frictional energy can result in extremely high local temperatures which, in turn, induce chemical reactions between the metal substrate and lubricant. New mechanically created fresh surfaces can emit exoelectrons from the metal surfaces in relation to chemisorption-induced electrons and photon emission (Kramer, 1950; Nakayama et al., 1995; Sujak et al., 1974; Wei and Lytic, 1976; Young and Williams, 1968). There are various surface techniques, eg., optical interferometry high fi-equency capacitance and resistance method, which demonstrate the build up of the physically and chemically reacted film during boundary lubrication in scuffing reactions (Anghel et al., 1997; Dowson et al., 1996; Smalley and Cameron, 1996).

Surface Tribochemistry and Activated Processes

171

Emission of electrons (EE) from wearing metal surfaces (fresh metal surfaces) may result in catalytic actions, e.g., causing so-called tribochemical reactions under boundary lubrication conditions, which govern tribological behavior. To determine the exoelectron mechanism for metals, systematic work on the behavior of EE for a combination of engine metal surfaces, surrounding additives and oxygen gas species is necessary (Hsu et al., 2002a; Nakayama et al., 1995; Schey, 1983; Wei and Lytic, 1976). Exoelectrons are known to be emitted from nascent or fresh solid surfaces by mechanical action such as abrasion, cutting, surface fatigue, wear, forming and so on. There are two types of EE associated with dark emission termed "triboemission and after emission" (Ksi}dsiS, 1985a, 1989 and 1994; Nakayama and Hashimoto, 1991 and 1992 ; Nakayama et al., 1992 and 1995; Thiessen, 1965). Triboemission refers to the emission of particles during tribological damage, while after-emission refers to that from fresh surfaces after surface damage (Nakamyama et al., 1995). Triboemission is caused by a high energy nascent surface with an extremely short life time, while after-emission intensity decreases with time. The after-emission intensity depends on the rate of the reaction between the metal surface and the surrounding additives or oxygen species. Negative particles observed during after-emission are considered to be electrons (Uebbing and James, 1970; Wei and Lytic, 1976). The electron emission from fresh metal surface is thought to be caused by the interaction between the fresh metal surface and the surrounding active chemical or oxygen species. Exoemission = (emitted particles)

Triboemission + (negative ions, radicals, positive ions, electrons and photons)

After-emission (electrons)

Triboemission time is extremely short while after-emission time is much longer. The enhanced surface activity caused by rubbing processes produces exoelectrons emission, catalytic and structural factors, increased surface temperature, and pressure (Rowe and Murphy, 1974). When surfaces of tribological systems are involved in the mechanical activity of rubbing, direct reactions of surface adsorbed films with solid surfaces take place. The mechanically activated clean surface (nascent surface) of the metals and alloys is extremely reactive. Tribofilm formation is caused by the interaction between the metal (M, substrate) nascent surface under high energy and chemisorbed molecules of additive (adsorbate) (Buckley, 1981). The processes on nascent metal surfaces are:

172

Chapter 5

(a) formation of radicals by breaking the 0 = 0 , S-S, C-C or C-Cl bonds; (b) tribofilm formation radicals condensing on the metal surface. Adsorption on nonactivated metal (M) surfaces: substrate (M) + adsorbate M + Oxygen (O2) M + Dibenzyl disulfide (DBDS) M + RCl

=

product M.. .O2 (adsorption) M...DBDS (adsorption) M...C1R (adsorption)

Chemisorption on metal (M) nascent surfaces ^^^^(nascent) (nascent) (nascent)

+ + +

2M0 + MCI + MPO3 +

O2

RCl ZDDP

=

(-AHf) (-AHf) (-AHf)

+ + +

electrons emission electrons emission electrons emission

To examine the tribochemical processes of various fresh metal surfaces in the presence of oxygen, nitrogen and water vapor electron emission was measured. It is known that the energy of excited electrons comes from the reaction of O2 with the freshly exposed metal surface (see Table 5.5) (Ferrante, 1976; Gesell et al., 1970; Moucharafieh and Olmsted, 1971; Nakayama et al., 1995; Wei and Lytic, 1976). Emission intensity was compared with literature values of the electron work function (FW), (Hodgeman et al., 1962) and heat of formation (AHf) of hydroxides, oxides and nitrides (Dean et al., 1973). Table 5.5. The metal electron work function (WF, eV), emission electron intensity (I, cps) and heat of formation (AHf, eV) of hydroxides M(OH)n, oxides Mj„0„ and nitrides Mj^N^ after cutting (Nakayama et al., 1995) WF

In water vapor

(eV) M

M(OH)„

AHf (eV)

I (cps)

MA

AHf (eV)

I (cps)

M^N„

AHf (eV)

I (cps)

A1(0H)3 Ti(0H)3 Mo(OH)3 Cd(0H)2 Fe(0H)2 Cu(0H)2

-13.2 -4.82 -1.80 -5.78 -5.89 -4.06

AI2O3

-

-17.3 -9.45 -5.64 -2.64 -2.76 -1.61 -0.32

2200 300 4 2.5 1 «1 «1

AIN TiN

-

40 50 <1 <1 1 «1 «1

-2.5 -3.17 -0.36 +1.37 -0.11 +0.77 +2.90

160 240 «1 «1 1 «1 «1

Al Ti Mo Cd Fe Cu Ag

3.8 4.2 4.3 3.9 4.5 4.3 4.3

In oxygen gas

TiO^ M0O2

CdO FeO CuO Ag20

In nitrogen gas

M02N

CdN Fe4N CU3N AgN3

Units: eV = electron volt, cps = counts per second, at oxygen and nitrogen gas pressure of 3 X 10'^ Pa, and at water vapor pressure of 4 x lO"^ Pa Table 5.5 shows that the after-emission electrons intensity (I) clearly increases for titanium and aluminum having the highest heat of formation (AHf) values. The

Surface Tribochemistry and Activated Processes

173

lack of exoelectron emission during exposure of strain-free aluminum to oxygen (Ferrante, 1976), while exoelectrons emission from aluminum during abrasion of aluminum by a steel blade, or steel brush, was observed by many workers (Allen and Tucker, 1981; Anderson and Klemplerr, 1960; Nakayamaet al., 1984). Some metals react with nitrogen to form nitrides. Emission of electrons from various metals cut in N2 gas is also driven by the heat of formation (AHf) of the nitrides. For titanium, having the highest negative AHf value of nitrides, the afteremission electron intensity was highest, while aluminum having the second highest negative AH^ value showed the second most emission electron intensity. These negative AHf values are high enough for electron emission to exceed the threshold value of the electron work function (WF). In Mo and Fe, having negative AHf values smaller than the WF values, the electron emission was negligibly small. For metals with endothermic nitride formation, electron emission was also negligible. The emission intensity in N2 gas also increased with the increase in the heat of formation of nitride. This demonstrates that not only the oxidation reaction but also other surface reactions contribute to the emission of exoelectrons when the negative AHf value exceeds the WF value of the surface (Nakayama et al., 1995). Electron emission was also observed in water vapor pressures of 4 x 10""^ Pa as seen in Table 5.5. Ti and Al emitted electrons intensely, while other metals emitted weakly but distinctly. The metal hydroxides other than Mo(OH)3 and Cu(OH)2 have sufficiently high negative heat for electron emission. The good correlation between the exoelectrons emission intensity and the standard heat of formation (AHf) of the compounds strongly suggests that the electrons are excited by the exothermic reactions (Nakayama et al., 1995; Wei and Lytle, 1976). Exoemission intensity is related to the maximum possible kinetic energy of the electrons, which can be expressed as tribochemical energy (TribEn) Tribochemical energy

=

AHf

-

WF

(5.3)

where AHf is the heat of formation of the compound, e.g., oxide, and WF is the electron work function, since the number of electrons escaping from the metal surface will be increased with the kinetic energy of the electrons. The excited electron may have a chance to escape into the medium if it is properly directed and is energetic enough to overcome the electron work function (WF) of the element„ The work function (WF) of the cut fresh metal surface, with all metals following abrasion and probably also oxidation, hydration and contamination, would exhibit a WF lower by 1 to 2 eV than the reported strain-free metal surface (Grunberg and Wright, 1955; Huber and Kirk, 1966). Under boundary friction conditions, the exposed metal surface is extremely reactive due to mechanical activation. Exoemission occurs when a material surface is disturbed by plastic deformation, abrasion, fatigue cracking, and phase

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deformation. Heat generation, fresh surfaces, wear debris and the emission of exoelectrons were observed from transformation processes of the surface (Kajdas, 1994). It was reported that the electron work function (WF) of a freshly evaporated magnesium film changed between 3.3 to 1.8 eV (Gesell and Arakawa, 1972). Emission of electrons from a solid surface is the result of a chemical reaction, e.g., in the Auger spectra, a large peak is due to the adsorption of oxygen on the surface of magnesium. The chemical reaction to form the surface film of magnesium oxide liberates energy in the form of electron emission. Electron emission of a deformed metal should characterize the respective local changes in the exit of electrons from the site of dislocation. The intensity of exoelectron emission from an abraded magnesium surface was very strong, which is simply AHf - WF ^ - 6.24 + 3.46 = - 2.78 (eV), due to formation of stable magnesium oxide (Ferrante, 1976; Wei and Lytic, 1976). Emission parameters should correlate, for instance, with corrosion resistance of a stressed metal (Hoar and Scully, 1964; Swann, 1963; Zamiryakin et al., 1969). The study of surface behavior is not of recent origin. In the 1920s, Joffe (1928) observed an increase in the ductility in the presence of water on solid potassium chloride. In 1930s, Roscoe (1926) published that the presence of oxides on certain metals, such as cadmium, produced a surface hardening effect. In the 1920s and 1930s, Rehbinder (Rehbinder and Likhtman, 1957) recognized that the presence of certain organic acids on the surfaces of solids resulted in a surface softening or a reduction in the mechanical properties of solids. The Rehbinder effect produced increases in plasticity in the presence of surface active materials. The darkening of a photographic plate by a freshly abraded metal surface in its vicinity has been known since 1896. The phenomenon was ascribed to the action of "metallic vapor" at room temperature, and reported the emanation of a hydrogen peroxide product (Colson, 1986;Russel, 1897). In 1950 (Kramer, 1950) first used the name exoelectrons (EE) to describe the low-energy electron emission from a metal solid surface during surface deformation following exothermic processes. The emission of electrons from the surface is not, as has been believed over the years by Kramer and many other investigators, a result of the deformation or mechanical working of the solid surface and the generation of clean surface. Instead, it is the result of a chemical interaction, as in the case of magnesium, and in chemical interaction with oxygen, there is emission or liberation of energy from the solid surface. The subject has been actively re-examined (Churchill, 1939; Grunberg and Wright, 1952) and it was reported that H2O2 can be formed when free electrons, water, and oxygen were available simultaneously. It thus appears that an electron, rather than H2O2 is the elementary product of the oxidation reaction.

175

Surface Tribochem is try and Activated Processes

The energy transfer processes during the chemisorption on metal surfaces have been generally explained in terms of electron transfer from metal to incoming species (Cox et al., 1983; Gesell et al., 1970; Prince et al., 1981). F

=

F

•'-'energy released

4-

-'-'chemisorptive emission

F

C^ A^

^-'photon emission

V*^*^/

Electron and proton involvement in energy release has been proposed by Gesell (Gesell et al., 1970). hi the Auger emission model, chemi-emission or "potential" ejection mechanisms have been responsible for a large number of reported phenomena (Delchar, 1967; Kasemo, 1974; McCarrol, 1969; Prince et al., 1981). The electron emission only occurs if E \ > 2WF, and photons are to be expected if E \ > WF, where WF is the electron work function and E \ is the effective electron affinity of the adsorbate at the position where electron transfer occurs. The E \ values for oxygen, bromine and chlorine atoms are 5.7, 7.8 and 8.0 eV, respectively (Cox et al., 1983), while the reported values of electron work functions (WF) of the metals are in range of 2.98 to 4.77 (Nakayama et al., 1995). Applying a value of WF below 2.27 eV (expected to be reduced for the fresh metal surfaces) to the Auger model satisfies the condition of the Auger emission model of E \ > 2WF. Accordingly, electron emission is possible in an O2 atmosphere according to the Auger model. It has been recognized for many years that films on solid surfaces can influence the mechanical behavior of a solid. Mechanical activation gives rise to the input of a considerable quantity of energy at the surface. This energy can produce a number of changes in the nature of the solid surfaces and the lubricant additives that interact with the solid surface to provide a very stable surface tribofilm (see Table 5.6) Table 5.6. Typical wear processes for metal-on-metal sliding systems Boundary lubrication PROCESSES The two surfaces may contact each other. Elastic and plastic deformation. Frictional heat. Wear.

=>

Nascent surfaces

=•

EXOEMISSION Triboemission (radicals, electrons, photons, positive ions, X-ray emission). After-emission electrons.

Tribofilm formation STRUCTURE Upper layer long chain polyphosphates. Lower layer short chain polyphosphates,

For many years, it has been well known by researchers that examining the reaction characteristics or interaction characteristics by means of static immersion reaction studies of additives and oils with solid surfaces, can produce misleading results. With static immersion experiments, the steel surfaces are covered with

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oxides and adsorbates and the active element of the additive e.g., sulfide, chloride or phosphorous, in the oil does not have an opportunity to interact directly with the nascent metal surface. For tribochemistry to be accomplished, clean (nascent) metal must be exposed. The best way to expose a nascent metal surface is mechanical activity such as rubbing the solid surface (by varying extreme loads, rollers in contact). The other way that a clean surface can be produced is to use a clean vacuum system which can achieve pressures of 1x10'^ Pa. At these pressures, it is possible to obtain and maintain atomically clean surfaces. The most common technique used today for generating clean surfaces (i.e., to remove adsorbates and oxides from material surfaces) is ion bombardment of the surface with argon or some other inert gas to sputter away surface contaminants (Buckley, 1981), Analytical surface techniques such as Auger Emission Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) analysis are extremely useful in identifying the chemistry of the solid surfaces (Buckley, 1981; Briggs and Seah, 1990). Table 5.7 is a summary of the XPS spectra data for rollering surfaces in oil containing dibenzyl disulfide under various conditions: - immersed in the oil containing dibenzyl disulfide; - in a cutting experiment in the oil; and - for the roller in contact at a load of 75 kg in the oil. Table 5.7. Spectral differences of sulfur binding energy on steel surfaces in the absence and presence of mechanical activity by X-ray photoelectron spectroscopy (XPS) (Baldwin, 1976; Bird and Galvin, 1976) Spectra intensity and binding energy Test conditions

Oxide 168.0 [eV]

Sulfide 162.0 [eV]

Immersed, static Cutting experiment Mechanical activity

Very intensive Medium Weak

Absent Very intensive Very intensive

In the absence of mechanical activation, the XPS spectrum is different from that obtained in the presence of mechanical activation. Thus, sulfur compounds that are formed on the surface (162 eV) appear in the spectra that are not present in the spectra for static immersion. Cutting produces a change in the surface chemistry and the reaction product formed appears to be similar to that obtained by mechanical activation. Thus, mechanical activity plus increased surface temperatures on a solid surface tend to promote surface chemical reactions. A lubricant interacting with metal oxide can produce entirely different reaction products from that of a lubricant interacting with rubbing metal surfaces.

Surface Tribochemistry and Activated Processes

177

Mechanical activity at the surface such as load, speed, and variations in surface energetics, play a role in surface chemistry. For example, if a clean metal surface is exposed to materials such as oxygen, chlorine, and sulfur, an interaction goes on. There is no activation energy necessary to achieve the reaction of the species with the metal surface to form surface compounds. Tribological nascent surfaces and, more generally, tribological surfaces are believed to be more reactive than, for instance, single crystals, since their reactivity may be enhanced by the presence of many surface defects. Chemical interactions occuring in a boundary lubricated contact under industrial conditions are complex and their study requires a simplified model of boundary lubrication. The in-situ analytical ultra-high vacuum tribometer (UHV) enables analysis of sample surfaces before the friction process and the understanding of additive tribochemistry. The techniques of UHV surface tribometer have been applied with some success to determine the reaction products and to understand the action of boundary additives (Boehm et al., 1999; Buckley, 1968; Gellman, 1992; Le Mogne et al., 1999; Martin et al., 1996; McFadden and Gellman, 1995). The UHV environment allows the study of atomically clean metal surfaces, free of oxides or other contaminants. The sample surface can be studied by in-situ analyses Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). In addition to these analyses, there are several possibilities for imaging the sample surface: scanning electron microscopy (SEM), scanning auger microscopy (SAM), and X-ray photoelectron spectroscopy. The SEM image reveals the presence of debris but neither wear nor film are visible inside the wear scar. As SAM and XPS imaging are sensitive, they reveal a contrast which cannot be seen by optical or SEM imaging. The main issue is to introduce the lubricant components inside the UHV chamber during friction. Molecules may be adsorbed on surfaces prior to friction or introduced in a vapor phase. As the molecules have to be in the gaseous state, only low molecular weight compounds can be considered. These molecules are used to simulate the heavy lubricant components by their functional groups. Experimental modeling of boundary lubrication is based on the introduction of molecules which simulate lubricant components in the analytical UHV tribometer. The carbon-carbon double bond present in the olefinic compounds has been shown to enhance their reactivity towards nascent surfaces. The formation of strongly bonded compounds is far more likely to occur for unsaturated compounds, e.g., 1hexene and benzene. On steel, they exhibit good friction properties, contrary to saturated ones, e.g., «-hexane, which do not seem to significantly affect friction properties (Boehm et al., 1999; Igari et al., 1996). The atomic force microscope (AFM), (Aktary et al., 2001; Binnig et al. 1986; Warren et al., 1998) and the lateral force microscope (LFM), (Mate et al., 1987) are valuable tools for characterizing the forces involved between surfaces in contact, both lubricated and unlubricated. A particular strength of AFM/LFM is

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the ability to operate under liquids. In this way, the effects of a liquid medium on friction can be observed - a very promising approach for studying lubrication on a nanometer scale. The lateral forces of surfaces in fractional contact under electrolytes are dependent both on pH and on the isoelectric points (lEP) of the tip and sample materials (Marti et al., 1995; Hahner et al., 1997). The effects of a liquid medium can be observed on friction on an atomic scale and forces on single asperities. As can be seen in Table 5.8, the normal load used in pin-on-disk and LFM differs typically by at least seven orders of magnitude (Feldman et al., 1998). The AFM gave high resolution images together with an estimate of the distribution of the film thicknesses (Pidduck and Smith, 1997). Table 5.8. Comparison of some tribological parameters in nano- (lateral force microscope, (LFM)) and macro-scale Parameters

Nano (LFM)

Macro (pin-on-disk)

Contact area Load Pressure Velocity

10-^^ m^ 0-200 X 10-'N 0-2 GPa 0.05-12/^ms-*

10-'m' 1-lON 0.1 - 1 MPa 10yums-'-260mms-^

Plastic deformation (strain). When two surfaces of ductile materials are placed in contact and the load exceeds the elastic limit of one of the two materials, plastic deformation or strain occurs. The plastic deformation of one surface when two surfaces are in solid-state contact can occur in the presence or absence of lubricants. In fact, in some instances, the presence of lubricants can increase the deformability of the solid surfaces by a mechanism such as the Rehbinder effect. Plastic deformation of the solid surface is, therefore, observed in the presence of lubricants. Plastic deformation is accommodated by the generation of slip lines for dislocation flow in the solid surface. Dislocations are line defects in the solid and they are site of higher energy state on the surface. Thus, they interact or react more rapidly with certain chemical agents than do the bulk surfaces (Buckley, 1981; Lunarska and Samatowicz, 2000). These highly energetic surfaces can lead to rapid surface oxidation in the presence of oxygen (oxide formation), or other chemical reactions with environmental species. The results of enhanced surface activity are known as stress corrosion and corrosion fatigue. The enhanced surface activity causes plasticization and additional deformation of the dissolving solid. Tribochemical systems are thermodynamically in non-equilibrium state with irreversible processes proceeding. The process of dislocation generation and motion under plastic flow of solids is essentially irreversible. In polycrystalline solids, different orientation of separate grains causes different resistance to the applied stress

Surface Tribochemistry and Activated Processes

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(Gutman, 1994; Hsu and Klaus, 1978 and 1979). To solve the problem of corrosion protection of the rubbed metal, inhibitors are introduced to lubricating systems. Three inhibiting additives (Inl, In2 and In3) have been chosen in the investigation of tribochemical behavior of steel under continuous deformation at a strain rate of 34 %/min (Gutman, 1967) in an electrochemical experiment. The observed different efficiency of n-decyl-3hydroxy-pyridinium chloride Inl, alkyl hexamethyleniminium bromide In2 and sodium bromide In3 inhibitors is, to a large degree, due to different mechanisms of their action. The action of inhibitor Inl is steel surface passivation, which may be due to the presence of OH" groups in the inhibitor molecule. Inhibitor Inl showed a lower efficiency of protection under the condition of elastic-plastic strain, promoting the destruction of the coating layer. Inhibitor In2 completely eliminated the tribochemical effect. The protective properties of inhibitor In2 are caused by the combined actions of organic cation and anion (synergistic effect). The stability of the protective action during the entire deformation process is ensured by the organic cation. The mechanism of its action is characteristic of compounds of this type. The principal property of this mechanism is a high cation chemisorption due to a donor-acceptor type interaction of the molecule's pielectrons with d-electrons of the metal surface. The protective effect does not involve film forming requiring a certain time. Inhibitory chemisorption proceeds rapidly on a newly formed steel surface. In the case of plastic deformation of steel, some corrosion protection effects by the inhibitor In3 sodium bromide were observed. Consequently, the tribochemical effect of inhibition is achieved at the expense of substances which do not form brittle surface films. They are chemisorbed at a rate exceeding that of surface renewal in the process of plastic deformation. Thus, the three characteristics, namely, inhibiting effect, surface activity and adsorptional plasticization action of organic cations, are correlated according to the Rehbinder effect (Likhtman et al., 1964). The higher the inhibitive capacity of additives, the higher the plasticization extent. Apparently, a significant inhibition of tribochemical dissolution of steel by additives on the basis of 1,3-dioxanes may be attributed to their ability to form a strong elastic adsorbed film on the metal. Two oxygen atoms in meta-position of a six-member ring impart electron-donor properties to the system. Strong chemisorption is achieved at the expense of pi-electrons. At elevated temperatures, polymerization of the chemisorbed inhibitor is possible, with increasing inhibitor-metal bonding (Gutman, 1994). A change in the surface tension does not exert essential influence on the mechanical properties of metals and vice versa; however, a significant decrease in surface tension caused by adsorption should promote corrosion (Kolotyrkin, 1967). Plastic deformation can arise at the site of the stress concentrations, i.e., notches, asperities, scratches, inclusions and surface irregularities (Chiu, 1999).

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5.3. Tribochemical Reactions on Surfaces The chemical reactions which occur in boundary lubrication are referred to as tribochemical reactions, which possess increased activity due to stored energy accumulated by mechanical processes. The tribochemical reactions may be enhanced by the following factors: exoeiectrons, surface structure, elevated temperature and high pressure. In the electron transfer process involving the shifting of a near surface electron from the metal to an adsorbed surface layer, the liberated electrons may be removed from the free electron cloud in the metal. The objective of designing reactive boundary additives is to ensure that little or no reaction takes place under steady state contact, but that adequate reaction occurs at highly loaded asperity contacts to provide protective films on the surfaces. It follows that conventional extreme-pressure additives will react continuously at high rates with metal surfaces at temperatures above 200 to 250''C. Such a continuous reaction will in fact be a form of corrosive attack. Conventional additives will, in any case, be unstable and decompose at temperatures above 250''C. The extreme-pressure additives consist mostly of chlorine, sulfur and phosphorus compounds reacting tribochemically with the exposed metal surface during mechanical treatment and developing a well adhering and easy to shear protective layer of sulfides, phosphides, or chlorides (Allum and Forbes, 1968; Kajdas, 2001). The additive molecules or specific reactive intermediate generated by exoeiectrons may react together directly at metal surfaces, catalyzed by thermal or electronic effects of rubbing parts, e.g., condensation tribopolymers can form between diacids and diols on rubbed surfaces (Furey, 1973). Also, one additive may chemically react with a rubbed surface to form a tribofilm, and the tribofilm interacts with an other additive, e.g., friction modifying additives adsorb strongly on metal sulfides formed by extreme pressure additives (Hironaka et al., 1975). Similarly, succinimides adsorb very strongly on surfaces treated with ZDDPs (Forbes et al, 1970a). It is well known that organic molybdenum compounds are effective friction modifiers (FMs) for automotive engine lubricants (Korcek at al., 1996; Nagakari et al., 1997; Parenago et al., 2001; Yagishita and Igarashi, 1991). To understand the wear mechanism in valve train wear tests, samples of the worn tappet surface were analyzed for surface elements by electron probe microanalysis (EPMA) and X-ray photo electron spectroscopy (XPS). Results of EPMA analysis of the worn surface in terms of concentration of phosphorus and sulfur atoms for oil with primary ZnDDP without MoDTC, showed an increase of zinc and sulfur intensity after 100 hrs of test time, in spite of decreasing phosphorus intensity. Examination of the worn surface by XPS with primary and secondary ZDDP with addition of MoDTC showed the presence of M0S2 in the tribofilm. Using mixtures of ZDDP and MoDTC, the friction coefficient is reduced, and wear is comparable to that of using ZDDP alone (Kasrai et al., 1997).

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181

In the presence of friction reducing additives in engine oil, a friction coefficient of 0.04 was observed which increased up to 0.12 after the additives had been depleted (Johnson et al., 1997a). In an interna! combustion engine, the lubricant is oxidized in the combustion chamber and peroxides are formed. These peroxides are responsible for corrosive wear, which is itself a tribochemical reaction. The antioxidant action of the ZDDP is to scavenge the peroxides and reduce corrosive wear. The extreme pressure additives (organic chlorides and sulfides) react with the surface and cause controlled local corrosion of the welding contact. The other form of tribochemistry is the polymerization of the hydrocarbons (Tonck et al., 1979) and the formation of a hard layer of friction polymers on surface (Kajdas et al., 2001; Molina et al., 2001) in the friction contact. Polymers dissolved in lubricants are influenced by the high shear rates; this leads to cracking of the molecules, with a concomitant lowering of the molecular weight of the polymer and permanent loss of viscosity (Fischer, 1988b; Lauer et al., 1982). Auger emission spectroscopy (AES) surface analysis was used to confirm the presence or absence of oxygen on the surface. The formation of sulfide films may be displaced by other surface active elements such as oxygen, where, for example, the oxide of the metal AHf (FeO) = - 2.82 eV is thermodynamically more stable than the sulfide AHf (FeS) = 1.04 eV. The sulfide may form initially simply because there is a sufficient concentration of sulfur available at the surface for interaction with the solid surface. With exposure to oxygen, however, the sulfur concentration begins to decrease. The friction coefficient associated with displacement of sulfur from iron surfaces by oxygen was not markedly altered by the presence of the oxide film. In fact, iron sulfide yields a friction coefficient of approximately 0.5 and iron oxide gives about an equivalent friction coefficient when the surfaces are completely saturated, but an examination of the surface with Auger emission spectroscopy revealed a complete substitution (an oxide film for a sulfide film) of surface films (Buckley, 1981; Gellman and Spencer, 2002). X-ray Photoelectron Spectroscopy (XPS) tests were conducted on surfaces lubricated with a sulfur-containing extreme pressure additive, dibenzyl sulfide (Baldwin, 1976; Bird and Galvin, 1976). The films can arise from the use of additives that contain sulfur, phosphorus, chlorine, bromine, or boron and the differences in reactivity are affected by the formation of protective layers. Triboinduced electrons are said to activate the formation of iron halides, iron phosphates and iron sulfides (Dorison and Ludema, 1985; Grunberg, 1966; Kajdas, 2001; McFadden et al., 1998 ). When a chemical reaction takes place, e.g., oxygen interacts with aluminum to form aluminum oxide, a large oxygen peak is seen at approximately 500 eV in the Auger electron spectra (Benndorf et al., 1977; Nakayama et al., 1995).

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Mixed^lement compounds. The phosphorus-sulfur compounds have been found to be effective extreme-pressure antiwear additives and are widely used. Lubricants in which sulfur and phosphorus are introduced as separate compounds or are mixed in additives, have shown similar results, i.e., a phosphorus-rich layer is found under moderate loads and a sulfur-rich one under more severe loading (Masuko et al., 1994). The mechanism of film formation from mixed-element additives and even their chemical compositions are very poorly understood (McFadden et al., 1998). Boron-containing additives are emerging as replacements for the phosphorus compounds discussed above. It is expected that boron-iron compounds (Fe2B or FeB) play an important role in preventing seizure, and that oxoborates may providefriction-reducingproperties (Adams and Godfrey, 1981; Junbin and Janxiu, 1981). Phosphorus containing additives include esters of phosphoric acids, derivatives of thiophosphoric acids, phosphites (Davey, 1950; Sakuri and Sato, 1970), and the metal salts of dithiophosphoric acid diesters, the best known of these compounds being the ZDDPs. The structure of ZDDP films, the composition of the film formed, and the mechanism of action on the molecular level are summarized in Chapter 4. Lubricants containing chlorinated hydrocarbons are typically used as antiseizure additives in the metalworking industry. Some chlorine-containing hydrocarbons function by producing iron chloride on the metal surface (Kotvis et al., 1991). The correlation between chemical reactivity and load carrying capacity of oil containing extreme pressure additives can be assumed to be as follows:

Table 5.9. The correlation between load capacity and chemical reactivity for extreme pressure EP additives EP additives containing S, P, CI

Sulfur

Correlation factor^ (load/chemical reactivity)

14

>

Phosphorus 4

>

Chlorine 1

^ For calculation method see Sakurai and Sato, 1966 As shown by the correlation factor (Table 5.9), sulfur compounds exhibit higher load-carrying capacity than the phosphorus and chlorine compounds of the same chemical reactivity (Sakurai and Sato, 1966 and 1970). Organic sulfides are the most widely used sulfiir-containing extreme-pressure additives (Forbes, 1970; Forbes and Reid, 1973; McFadden et al., 1998; Kajdas, 1994 and 2001; Plaza and Gruziriski, 1996; Plaza et al., 1994; Sakurai and Sato,

Surface Tribochemistry and Activated Processes

183

1966)c Sulfur-containing additives include organic mono- and polysulfides, elemental sulfur as well as a wide variety of sulfurized fats and hydrocarbons. Phosphorus containing additives include esters of phosphoric acids, derivatives of thiophosphoric acids and phosphites (Davey, 1950; Sakurai and Sato, 1970). It is generally accepted that the extreme performance of disulfides (R-S-S-R) is better than that of monosulfides (R-S-R), (Allum and Ford, 1965, Forbes, 1970; Mayer etal., 1979). dibenzyl sulfide >

diphenyl sulfide >

diphenyl disulfide > (DPDS)

dibenzyl disulfide (DBDS)

The chemical structure of mono- and disulfides has a distinct effect on performance. Tribochemical effectiveness of diphenyl DPDS and dibenzyl DBDS disulfides strongly depends on the decomposition products. The main products of thermal decomposition in a hydrocarbon solution are as follows (Plaza and Gruzinski, 1996; Plaza et al., 1994): DBDS DPDS

-

elemental sulfur

+

benzyl + thiol

hydrogen + sulfide

phenyl ring + thioethers

phenyl + thiol

diphenyl sulfide

sulfurized hydrocarbons

Some thermally degraded byproducts react tribochemically with the friction iron surface to produce sulfide FeS. This is seen, for example, when elemental sulfur reacts tribochemically with the rubbing surface; however, much greater quantities of elemental sulfur release are seen from dibenzyl DBDS than diphenyl DPDS disulfide, thus qualifying DPDS as a better lubrication performer (Plaza, 1987c and 1989; Plaza at al., 1997, 1999 and 2000). The negative-ion-radical action mechanism (NIRAM) approach was proposed by Kajdas (Bhushan and Kajdas, 2001; Kajdas, 1985a, 1985b, 1987 and 1994). The most important factor governing the tribochemical reactions under boundary friction is associated with the interaction of exoelectrons with lubricating oil additives. The NIRAM approach considers several processes: (a) emission process and creation of positively-charged sites, generally on tops of asperities; (b) action of the emitted electrons with some additives causing formation of reactive negative ions and free radicals; (c) reaction of negative ions with activated metal surfaces, and other reactions, e.g., free radical reactions, forming tribofilm. It has to be considered that the formation of reactive intermediate negative ions and free radicals on the rubbing surfaces follows tribofilm formation from most

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important additives, such as, phosphorus, sulfur and chlorine compounds. Action of low-energy electrons with the most important lubricant additives responsible for tribofilm formation, e.g., zinc dithiophosphate, molybdenum dithiocarbonate or molybdenum dithiophosphate, are less documented in terms of the NIRAM approach. The differences in antiwear properties of disulfides are related to their ability to be physisorbed about 100 to 1000 times faster than monosulfide on metal surfaces. The differences can be explained in terms of the lower energy needed for the formation of the same number of RS' ions from disulfides (Kajdas,1994). The exposed metal surface is extremely reactive to lubricant components, especially antiwear and extreme-pressure additives resulting the formation of a film on the contact surface. The reaction of emitted electrons of low energy (1 to 4 eV) with molecules of oil additives adsorbed on the friction surface may lead to formation of negative ions and negative ion radicals. The investigator (Kajdas, 1994 and 1985) pointed out the indispensability of the metal oxide film on the rubbing surface from the viewpoint of the theory of sulfide film formation. It is known that under rubbing conditions, the interaction and degradation of some additives should be considered in terms of reduction-oxidation processes (Kajdas, 1995). Exoelectrons, with energy average of about 3 eV, are attached to the molecules (AB) and negative ions are formed (AB). The activated surface spots form a new protective triboflm on solid contacts during the friction. Friction process —> Activated surface + negative ions —^ Tribofilm formation iI chemisorption 11 tribochemical reactions i1 Organic compounds —^ Organometallic -4 Inorganic (high temp, 3 eV compounds compounds exoelectrons) formation formation Following are some examples of specific reactive intermediates generated by low-energy electrons NIRAM approach (Kajdas, 1994 and 2001): (a) esters under boundary lubrication conditions form the carboxylate tribofilm. The carboxylate formation is due to the ester C-0 bond cleavage leading to the formation of RCOO" ions which react with activated surfaces; (b) the aromatic hydrocarbons can yield two types of negative ions; (c) alcohols, the basic decomposition process of fatty alcohol, is the splitting of three hydrogen atoms from, for example, n-Cg H17 OH alcohol and four anions are formed, which are chemisorbed on the positively charged areas of rubbing surfaces; (d) organosulfur compounds; (e) tribopolymer additives. The formation of a radical-anionic reactive intermediate during boundary friction was assumed for lauryl methacrylate, diallyl phthalate, vinyl acetate and lactam. Most recently, lactam compounds were found to be very effective antiwear additives for ceramic materials as well as metals (Furey and Kajdas, 1998 and 1999; Kajdas et al., 2001;

Surface Tribochem istry and Activated Processes

185

Molina etal., 2001). The basis process of sulfides decomposition is the cleavage of the C-S bond R-S-R

+ e

^

RS-

+

R°

and, in the case of disulfides, decomposition -S-S- bond cleavage R-S-S-R

+ e

-V

RS"

+

RS°

where: RS° + e'(electron of lower energy) —^ RS" Thus, it has been concluded that the disulfide will exhibit more efficient loadcarrying properties due to the fact that lower energy is needed for the formation of the same number of RS" ions. Tribochemical reactions of dibenzyl disulfide in the presence of a second additive (e.g., ZDDP, chlorinated paraffin, amines, phenol, barium alkylbenzylsulfonate, and polyisobutenylsuccinimide) were investigated (Plaza, 1989). All the systems tested reduced the concentration of elemental sulfur in the oil to low loads. Formation of FeS was reduced in the presence of chlorinated paraffin, barium alkylbenzylsulfonate, ZDDP, and polyisobutenylsuccinimide. The amines and hindered phenol had little or no effect on iron sulfide formation at higher loads. Organic disulfides and chloroorganic compounds in boundary lubrication conditions form iron sulfide (AHf = -1.04 eV), iron chloride (AHf = -3.54 eV), or iron bromide (AHf = -2.59 eV), which decrease wear and protect against seizure. Some chlorinated and brominated hydrocarbons in the presence of DBDS produce strong synergism in severe conditions involving lubrication in metal cutting (McCarroll et al., 1978; Mould at al., 1973; Plaza, 1989; Plaza et al., 1993; Sakuri etal., 1967). It has been recognized in the boundary lubrication regime that the oxygen dissolved in lubricating oils without additives plays an important role in promoting the formation of protective oxide films on rubbing surfaces, which can enhance their lubricating performance or provide seizure resistance. The oxygen dissolved in oils containing extreme pressure (EP) additives appears to have significant effects on lubricating performance. The optimum composition of oxides and sulfides of the tribofilm seems to bring about superior lubricating performance as a result of synergism on the rubbing surfaces (Bjerk, 1973; Murakami et al., 1958; Murakami and Sakamoto, 1999; Sakai et al., 199; Tomaru et al., 1977). Changing the dissolved oxygen concentration in oil containing diphenyl disulfide showed that the highest load carrying capacity and the lowest friction and wear were found at the optimum concentration of dissolved oxygen (Sakai, 1992). In the evaluation of extreme pressure (EP) organic disulfide, the difference in the reactivity of the additive, the concentration of dissolved oxygen, and severity of operating conditions should be considered. Reported (Forbes, 1970) EP

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performance of organic sulfides could be related to the ease of cleavage of the carbon-sulfur (C-S) bond. In dibenzyl sulfur compounds such as DBDS, the C-S bond is most easily broken due to the stability of the benzyl radical or ion. In contrast, in DPDS as a diphenyl sulfur compound, the breakage of the C-S bond is not easy because of the stability of the phenyl mercaptyl radical or ion. The detailed mechanism of interaction between dissolved oxygen and organic disulfides in oils has not yet been clarified. To evaluate the role of dissolved oxygen in the synergistic lubrication mechanism of oils containing organic disulfides, four-ball tests were conducted under increasing temperature (see Table 5.10) (Murakami and Sakamoto, 1999).

Table 5.10. The concentration of oxygen in FeO and sulfur in FeS on the rubbing metal surface estimated by the electron probe micro-analysis (EPMA) technique (Murakami and Sakamoto, 1999)

Additives^

FeO oxyg en increase, AO (cps)

FeS sulfur increase. AS (cps)

Relative concentration. AS/AO

100°C

200°C

100°C

200°C

lOOX

200°C

0.4 1.3 0.6 0.7 0.6

0.8 5.5 3.0 2.1 3.1

_ 0.2 0.1 0.1 0.1

_ 1.4 0.5 0.5 0.4

_ 0.15 0.17 0.14 0.17

_ 0.26 0.17 0.24 0.13

DBPC + B DBPC + B + DBDS DBPC + B + DBDS+O2 DPDS + B + DPDS DPDS + B + DPDS+O2

"Antioxidant additives: DBPC == 2,6-di-/er/-butyl-p-cresol, B = alkyl-diphenyl amine; Extreme-pressure additives: DBDS = dibenzyl disulfide (C6H5-CH2-S-S-CH2-C6H5 ), DPDS = diphenyl disulfide (C^Hs-S-S-CgH^); AO and AS = the increase in the oxygen and sulfur concentration on the rubbing surfaces from the virgin surfaces; cps = counts per second; O2 = oil samples with blow-in oxygen As shown in Table 5.10, not only the amount of sulfur, but also that of oxygen, significantly increased on the rotating balls at 200°C. It should be noted that the increase in sulfur for DBDS is higher than for DPDS. The previous studies (Murakami et al., 1956; Sakai et al., 1991 and 1992) indicated that the effectiveness of a protective film depends on the ratio of sulfide to oxide films. As shown in Table 5.10, a higher value of AS/AO appears to induce deterioration of the surface film. Antioxidant (DBPC)

+

amine +

O2 (without organic sulfide, DPDS)

Antioxidant (DBPC) +

amine +

O2 + DPDS (S = 0.013 wt%)

Surface Tribochemistry and Activated Processes

187

Protection by surface film formation is likely to control the occurrence of scuffing. The addition of DBDS to the antioxidant DBPC enhances the loadcarrying capacity, and the concentration of dissolved oxygen also controls the load-carrying capacity. With addition of DBPC alone; however, the blowing-in of oxygen was not effective. The coexistence of two kinds of anti-oxidant additives, DBPC and alkyl-diphenyl amine, reduced the scuffing load, hi the twostep constant temperature test (lOO^'C -^ 200°C), both showed the highest scuffing load. Dissolved oxygen enhanced the scuffing load by almost two fold. Table 5.11. The correlation of tribochemical energy (AHf - WF) between the heat of formation (AHf) and the work function (WF) for the stable compoundsfromabraded metal surfaces at25°C Metal

Work function, WF^ (eV)

Al

3.8

Fe

4.5

Zn

4.6

Compound

AHf^ (eV)

(AHf-WF) (eV)

AICI3 AIPO4 FeS FeO FeP04 FeCl2 FeBr2 Zn(P03)2

-7.30 -17.5 -1.04 -2.82 -13.5 -3.54 -2.59 -21.6 -26.0 -30.0 -3.61 -2.13 -3.59 -9,75 -2.47 -2.44 -3.77 -6.10 -7.70

-3.50 -13.7 +3.46 +1.68 -9.0 +0.96 +1.91 -17.0 -21.4 -25.4 +0.99 +2.47 +0.61 -5.55 +1.73 +1.86 +0.53 -1.80 -3.40

ZnjPjOy

Ti

4.2

Zn3(P04)2 ZnO ZnS TiO TiOj

Mo

4.3

TiS M0S2 M02S3 M0O2 M0O3

^Mean value for a crude comparisonfromref (Weast et al., 1990). 4 eV/molecule = 96.5 kJ/mole (Hodgeman et al., 1962). The correlation between the heat of formation AHf and the work function WF. Table 5.11 is showing the heat of formation of AHf of the oxides, sulfides, chlorides and phosphorus compounds and the average work function (WF) of the elements. The work function is a complicated physical property related to the crystal orientation, surface composition, and chemisorption (Shpenkov, 1995). Taking an aluminum surface as an example (Huber and Kirk, 1966), the adsorption of dry oxygen to one monolayer coverage will lower the work function by 0.05

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eV, whereas the presence of H2O will lower the work function by 1 eV. Thus, the listed work function values are only meant for a rough comparison. In spite of all uncertainties, the tribochemical energy can be correlated with AHf for stable compounds, which is simply (AHf - WF). All investigated above metals (Al, Fe, Zn, Ti, and Mo) following oxidation, abrasion, and contamination would exhibit changed work function values comparing with those shown in Table 5.11. As seen in Table 5.11, oxidation and other surface reactions contribute to the emission of tribochemical energy when the negative AHf value exceeds the WF value of the surface. The tribochemical energy (AHf - WF), due to the chemisorption of adsorbates, e.g., oxygen or sulfur on zinc surface, is not of the same order of magnitude as the yield of chemisorption of ZDDP on zinc surface. The tribochemical energy comes from the energy released by chemisorption of the ZDDP on the fresh zinc surface.

5o 4a Organomolybdenum Compounds in Surface Engines Protection This part of the chapter includes descriptions of: (I) Sputter - deposited M0S2 films; (II) The combination of MoDTC/ZDDP in surface protection; (III) Tribochemically and thermally generated antiwear films using MoDTC/ZDDP; (IV) Friction and wear capabilities of MoDTC/ZDDP; (V) M0S2 tribofilm formation directly from MoDTC in the boundary contact. (I) Sputter-deposited M0S2 film. The M0S2 layered formation is a well known lamellar solid lubricant film with a hexagonal crystal structure (Braithwaite and Greene, 1978; Grossiord et al., 1998; Korcek et al., 1999; Lansdown, 1999; Mitchell, 1984; Yamamoto and Gondo, 1989). High purity films (where the S/Mo ratio is > 1.97) prepared under ultra high vacuum conditions (Donnet et al., 1993) and tested without exposure to atmosphere have coefficient of fi'iction (/^) < 0.002. The films with S:Mo ratios ranging from 1.5 to 1.9, prepared in standard sputterdeposition chambers (i.e., background pressures of-IxlO"* Pa), had coefficients of friction (//) from 0.007 to 0.1 in inert environment (Roberts, 1992). Molybdenum disulfide films tested under inert conditions have relatively long lifetimes in both sliding and rolling contact conditions, as well as low friction. Films tested in moderately humid air (> 30% RH) have short lifetimes and higher coefficients of friction. Poor wear performance has been attributed to oxidation of M0S2.X and subsequent disruption of the facile sliding of crystallites within or between films (Gardos, 1995). Oxidation of MoS2/MoS2.xO^ films to form M0O3, either during storage or operation, generally has a negative effect on lubricant performance. M0O3 can

Surface Tribochemistry and Activated Processes

189

display reasonably low coefficients of friction among oxides, but not nearly as low as that of M0S2. Oxygen substitution is ubiquitous among sputter-deposited films except for those prepared in a ultra-high vacuum (UHV) analytical tribometer, and there is evidence that increasing quantities of substitutional oxygen decrease both friction and wear during sliding and rolling contacts. Typical M0S2 films contain large quantities (up to 20 %) of oxygen substituted for sulfur in individual crystal lattices. Oxygen substitution has the effect of oxidation which involves a change in oxidation number of the central molybdenum atoms within the crystals; however, pure M0S2 films have the lowest friction of all films. Fig. 5.2 is a schematic representation of the apparent friction properties of M0S2 films as a function of the bulk substitutional oxygen concentration.

M0S2 Dominates

MoS2.xOx Dominates

i^

^

n^

n «2 10 — • ' 10"^—^

t

t

10% 1% Total % of Oxygen in Film Fig. 5. 2. Schematic dependence offrictioncoefficient for M0S2 films as a function of oxygen concentration in the film. Coefficients of friction (//) for pure M0S2 are significantly lower than for MoS2.xOx. The peak in the ij. curve occurs at some relatively low concentration of oxygen in thefilms.Additional substitution of oxygen in the M0S2 has the combined effect of atomically smoothing the surface and reducing the coefficient offrictionyU (Fleischauer and Lince, 1999) The minimum coefficient of friction for pure M0S2 is significantly lower than that for MoS2.xOx5 and the peak in the coefficient of friction curve occurs for relatively low concentrations of oxygen in the film. The friction rises rapidly for

190

Chapters

small quantities of substituted oxygen (of the order of 1 atomic %) in that discontinuities in the otherwise smooth surface of sulfur atoms are created and that these discontinuities cause energy barriers to the sliding of two such basal surfaces (Fleischauer and Lince, 1999). Additional substitution of oxygen in the basal surfaces has the combined effect of atomically smoothing the surface somewhat and of modifying the electron density and lattice spacing, both factors tending to lower the attraction of adjacent nanocrystal basal surfaces and reduce the coefficient of friction. The combination of the X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and of extended X-ray absorption fine structure (EXAFS) data leads to the inescapable conclusion that conventional, sputter-deposited M0S2 films contain oxygen substituted into the trigonal prism of the M0S2 hexagonal lattice forming a MoS2.xOx phase. The significant shift of the Mo XPS peak to higher binding energies indicated oxidation of the Mo(IV) center in M0S2 to Mo(VI), typical of M0O3. Films reacted with air if they were not desiccated or stored in vacuum. Some films appeared to be more reactive than others (Fleischauer and Bauer, 1986). M0O3 accumulated around the edges of the M0S2 crystals, H2S is liberated, and eventually a white powder formed out of previously silvery-black films. The significant point to be made is that oxidation and oxygen substitution are very different processes that influence film performance in dramatically different ways. Substitution is ubiquitous in conventionally sputterdeposited film and does not degrade performance substantially, while oxidation occurs after the deposition and always has a negative effect on performance (Fleischauer and Lince, 1999). (II) The combination of MoDTC/ZDDP in surface protection. Oil soluble Mo-S compounds are well established as antiwear and extreme pressure additives in oils, which reduce friction and wear. The combined use of organomolybdenum compounds, e.g., molybdenum dialkyldithiocarbamate [MoDTC or Mo(dtc)2] and molybdenum dialkyldithiophosphate [MoDDP or Mo(ddp)2]), and zinc dialkyldithiophosphates (organozinc compounds) simply called in this book ZDDPs, e.g., zinc dialkyldithiophosphate [ZDDP or ZnDDP or Zn(ddp)2], showed a synergistic effect by reducing both friction and wear and promoting fuel economy in motor vehicles. It has been reported that addition of MoDTC brought about a few percent fiiel savings (Akiyama et al., 1993; Braithwaite and Greene, 1978; Greene and Risdon, 1981). Compounds such as MoDTC and MoDDP were used as additives (see structures below), where R was 2-ethylhexyl in both molybdenum compounds (Kasrai et al. 1998; Mitchell, 1984; Yamamoto and Gondo, 1989). The effect of MoDTC compounds as a friction modifier in automobile engines has been studied since 1980 and more intensively after 1990 (Braithwaite and Greene, 1978; Graham and Spikes, 1999; Grossiord et al., 1998; Johnson et al., 1997a; Kasrai et al., 1998; Korcek et al., 1997 and 1999; Le Mogne

Surface Tribochemistry and Activated Processes

191

et al., 1999; Martin et al., 2000b; Mitchell, 1984; Muraki and W a d a , 1995 a n d 2002; Stipanovic et a l , 1996; Tohyama et a l , 1996; Tripaldi et al., 1999; Parenago et al., 2 0 0 1 ; Wei et al., 1993; Y a m a m o t o and Gondo, 1986, 1989 and 1994). The effects o f combustion products in internal combustion engines on t h e performance of engine oil containing M o D T C and Z D D P were determined in laboratory and engine experiments (Johnson et al., 1995 and 1998). Based on these studies, it can be concluded that: (a) M o D T C and Z D D P act as both peroxyl radical (R02-) trapping and hydroperoxide ( R C O O H ) decomposing antioxidants; (b) both M o D T C and Z D D P are consumed during engine operation; (c) the friction reducing capabilities of the M o D T C and Z D D P additive systems can be extended by addition of a peroxyl radical-trapping antioxidant; (d) t h e friction-reducing capabilities of the M o D T C and Z D D P additive systems are very much affected b y oxidative conversion occurring during inhibited oxidation. •^X^

y^v

N-C / R

{j^ y^ l?/\ / ^ Mo ^Mo C-N \ / \ / \ / \ S S S R

Molybdenumdialkyldithiocarbamate [Mo(dtc)2orMoDTC]

RO S O S O S OR ^, X UA A- u 1 \ / v | | y ' v | | > ' v / Molybdenum dialkylP Mo Mo P dithiophosphate / \ / \ / \ / \ [Mo(ddp) 2 or MoDDP] RO S S S OR When present together, MoDTC and ZDDP formed very strong antioxidant systems (Korcek et al., 1996). There must be very strong synergism between these two compounds and intermediates derived from them. Addition of MoDTC (0.04 wt% of molybdenum) in combination with ZDDP (0.1 wt% of phosphorus) in engine oil indicates a strong interaction in terms of generating a scuffing protection tribofilm (Nagakari et al., 1997). It has been shown that when both additives are present together, they undergo ligand exchange in two-step reactions (Jensen et al., 1997; Korcek et al., 1999; Yagishita and Igarashi, 1991). Mo(dtc)2 + Zn(ddp)2 - Mo(dtc)(ddp) + Zn(ddp)(dtc) - Mo(ddp)2 + Zn(dtc)2 mixed ligand exchange full ligand exchange Both original additives are gradually consumed due to oxidation since they act as antioxidants (Korcek at al., 1996). This exchange leads first to the formation of the mixed products and, in the second step, full ligand exchange takes place to produce molybdenum dialkyldithiophosphate (Mo(ddp)2) and zinc dialkyldithiocarbamate (Zn(dtc)2). The equilibrium in this reaction favors Mo(dtc)2 and Zn(ddp)2 formation, and the

192

Chapters

equilibrium constant (K^q) for the reaction of Mo(ddp)2 and Zn(dtc)2 is 520 at 20°C and 393 at 30°C. The large change observed suggests that at a higher temperature the equilibrium constant would be much lower. Conversely, this would mean that the equilibrium constant for the reaction of Mo(dtc)2 with Zn(ddp)2 would be smaller and the concentration of exchange products would be significantly higher at elevated temperatures (Yagishita and Igarashi, 1991). Studies of ligand exchange during oxidation indicate that the equilibria in these reactions are affected by preferential consumption of Zn(ddp)(dtc) and/or Zn(dtc)2 for antioxidant reactions. The ligand exchange reactions in different base oils may proceed to a different extent, eg., a polyalphaolefm oil PAO does not contain any antioxidant, so the extent of the exchange in this oil is much greater than in mineral oil, which may contain natural oxidation inhibitors. The ligand exchange and friction reduction characteristics may also be affected by the presence of other engine oil additives, e.g., carbonate-sulfonate hard-core RMs and antioxidants (Jensen et al., 1998a and 1998b; Korceketal., 1999). The interaction of MoDTC with ZDDP in tribofilm formation and thermal film formation of MoDTC and ZDDP on steel surfaces was studied by using XANES spectroscopy (Fuller et al., 1998; Kasrai et al., 1994 and 1998; Yin et al., 1993, 1997a and 1997b) with X-ray photoelectron spectroscopy (Kasrai et al., 1998) or the X-ray photoelectron spectroscopy technique only (Grossiord et al., 1998; Muraki and Wada, 1995; Stipanovic et al., 1996; Tohyama et al., 1996; Wei et al., 1993; Yamamoto and Gondo, 1986 and 1989). The main factor controlling the durability of Mo additive performance is the consumption of this additive, and the consumption can be postponed by the presence of other antioxidants. Since friction reduction by MoDTC appears to be lost below a concentration of 180 ppm Mo, the presence the antioxidants, eg., dibenzylsulfide, ZDDP, dialkyldiphenylamine, hindered phenol, or hindered phenol disulfide, serves to reduce MoDTC consumption, thereby extending the time for which the solutions are able to reduce friction (Graham et al., 2001b). In the XANES technique, it is essential to compare spectra of films with those of different model compounds in which the local chemical environments of the phosphorus and the sulfur are known (Brown et al., 1992; Fuller et al., 1998). Optimizing friction reduction and other important performance properties of engine oils requires a fundamental understanding of tribochemical reactions at the molecular level. The MoDTC/ZDDP system has already found practical applications but still needs further improvement. The reviews and published data on one such additive system, MoDTC/ZDDP, suggest that there is a strong relation among processes of friction reduction, antiwear protection and oxidation. Strong interactions between additives in the bulk oil during engine operation and the resulting products of these processes are believed to be precursors that lead to formation of antiwear and low-firiction tribofilms in boundary contacts (Kawai et al., 1998; Korcek et al., 1999). Electron spectroscopy (XPS), which is often used

Surface Tribochemistry and Activated Processes

193

for surface study, does not prove that hexagonal M0S2 has been generated, because the presence of both M0O2 and FeS give very similar XPS Mo3d/S2s photopeaks (Bird and Galvin, 1976; Grossiord et al., 1998). The XANES studies of tribofilms, generated from MoDTC and mixtures of MoDTC and ZDDP, indicate that the films are composed of an MoS2-like film in both cases, and a sulfate only from MoDTC alone. The presence of low concentrations of ZDDP is sufficient to prevent the oxidation of sulfur to sulfate and of Mo (IV) to Mo (VI). The tribofilms generated from the mixture of ZDDP/MoDTC are composed of MoS2-like layers, ZnS-like films and unchanged MoDTC. The (P) XANES spectra the tribofilms generate from mixtures of ZDDP/MoDTC are composed of medium/long-chain polyphosphates on the surface and short-chain polyphosphates in the bulk. Thermal antiwear films were produced on steel from oil containing mixed MoDTC and ZDDP in various concentrations at 150°C. In these thermal antiwear films, sulfate dominates the films; the Mo in the thermal film is mostly composed of M0O3, and the phosphorus is mostly short chain polyphosphate. As thermal antiwear films are generated in a much longer time (6 hr) compared with the tribofilms (0.5-1 hr), the presence of oxygen in oil should play an important role (Kasrai et al., 1998; Stipanovic et al., 1996).

(Ill) Tribochemically and thermally generated antiwear film using MoDTC/ZDDP Tribochemically generated tribofilm - sulfur characterization. The effect of the concentration of a MoDTC on tribofilm formation using the Plint reciprocating wear ring machine on a 52100 steel coupon was investigated. The case study. The tribofilm generated on the coupon was rubbed 5 to 60 min in oil containing the additives for XANES analysis and wear scars measurement. The sulfur L-edge XANES spectra of tribofilms generated from combinations of MoDTC/ZDDP are presented in Fig. 5.3 (Kasrai et al., 1998). Spectrum (A) is for a film generated from neat ZDDP. Peaks b and c are aligned closely with those of ZnS (E), but peaks e a n d / d o not match well (Yin et al., 1993 and 1997a). Spectrum (B) (100 ppm Zn and 1000 ppm Mo) shows close similarity with spectrum (G) of M0S2. As the concentration of ZDDP is increased, the spectral features change. At an equal concentration of ZDDP and MoDTC (Spectrum C), the spectrum is dominated by ZnS peaks. At a high concentration of MoDTC (Spectrum D), the spectrum still closely resembles that of ZnS; however, as seen in spectrum C in Fig. 5.3 peaks a, b, and c are also shifted by -0.5 eV to higher energies relative to M0S2. A new peak, peak d, has appeared in spectrum D generated from a mixture of [Zn] = 1000 and [Mo] = 2000 ppm, and also from a mixture of [Zn]= 1000 and [Mo] = 1500 ppm. Some of these anomalies are due to the fact that in the M0S2, spectrum (G) peaks 1, 2, and 3 are much weaker than

Chapter 5

194

those of ZnS in spectrum E. Peak d in the (S) L-edge spectrum, originating from unchanged MoDTC further complicates the spectrum. The main difference between tribofilms generated from MoDTC alone and mixtures of ZDDP/MoDTC is the lack of sulfate signals in the mixed films. The presence of as low as 100 ppm ZDDP is sufficient to prevent oxidation of sulfiir-to-sulfate and Mo(IV) to Mo(VI) (Kasrai et al, 1998).

[Zn]2000 [Mo]0 tZn] 100 {Mol 1000 [Zn] 1000 {Mo] 1000

{Zn} 1000 (Mo] 2000

160

165

170

176

180

185

190

Photon Energy (eV)

Fig. 5.3. Sulfur (S) L-edge XANES spectra recorded in TEY (surface) mode of tribofilms generated from combinations of ZDDP and MoDTC, as compared with model compounds (E,F, G). Peaks b and c are aligned closely with those of ZnS, and are shifted by -0.5 eV to higher energy relative to M0S2, and peaks e and/do not match well (Kasrai et al., 1998) The spectra of the tribofilms generated from mixtures of ZDDP/MoDTC have also been recorded at the (S) K-edge using the surface TEY technique in the energy range 2460 to 2490 eV (Kasrai et al, 1998). The main peaks of the starting materials, ZDDP/MoDTC, lie betw^een first peak M0S2 (-2741 eV) and second peak ZnS (-2473 eV), see Table 5.12.

Surface Tribochem is try and Activated Processes

195

Table 5.12. The tribofilms generatedfrommixture of ZDDP/MoDTC and recorded at the sulfur (S) K-edge using XANES surface TEY technique Mixture of ZnDDP/MoDTC

Tribofihn composition

[Zn]' neat [Zn] 1000 [Zn] 1000 [Zn] 1000 [Zn] 1000 [Zn] 1500

ZnS only ZnS^ and M0S2, ZnS^ and M0S2, M0S2 only M0S2 only ZnS and M0S2

+ + + + +

[Mo] 500 [Mo] 1000 [Mo] 1500 [Mo] 2000 [Mo] 1000

^[Zn] and [Mo] in ppm; 'Speaks in spectrum have ahnost equal intensities For a tribofilm generated from neat ZDDP, the main component in the spectrum is ZnS. For tribofilms generated from both ZDDP/MoDTC ([Zn] = 1000 + [Mo] = 500 ppm), and ([Zn] = 1000 + [Mo] = 1000 ppm), in both cases ZnS and M0S2 were present. These spectra clearly show that, for equal concentration of MDTC and ZnDDP, or for higher concentration ZDDP than MoDTC, the surface concentrations of the ZnS and M0S2 appear to be similar on the steel surface. Spectra of tribofilms generated from 1500 and 2000 ppm Mo and 1000 ppm Zn concentrations in both cases, are dominated by M0S2. On the other hand, if the concentration of ZDDP is higher than MoDTC ([Zn] - 1500 and [Mo] = 1000 ppm), ZnS and M0S2 are present in almost equal concentrations. Thermally generated antiwear film - sulfur characterization. Thermal antiwear films were produced on steel from oil containing mixed MoDTC and ZDDP in various concentrations at 150°C. The (S) L-edge spectra of thermal films along with the model compounds are illustrated in Fig. 5.4 (Kasrai et al., 1998). Spectrum (B) is for the antiwear film generated from equal concentrations of ZnDDP and MoDTC. Comparing the spectra of the antiwear films with those of Na2S04 (A) and ZnS (E), indicates that reduced as well as oxidized forms of sulfiir are present in the antiwear film. Peaks a and h originate from the reduced forms, and c and d from the oxidized forms. It seems that the film is mostly composed of an oxidized sulfur form and very little or no reduced form. The XPS spectra indicate that Mo in thermal films is mostly composed of M0O3, and there is no indication of any trace of a MoS2-like antiwear film. Spectrum C shows that, as was observed in tribochemical films, peaks a and h align closely with those of ZnS. The main difference between the thermal antiwear film and tribofilm is the fact that tribochemical films were free of any oxidized form of sulfiir. The thermal antiwear films are dominated by the oxidized sulfiir forms. Spectrum D for neat MoDTC shows peak Z? (165.6 eV) which does not overlap with that of M0S2. It is shifted from the peak position of unchanged

196

Chapter 5

MoDTC (164.0 eV). "T

.

1

r

^

,---.,1

,

p

S L-edge

Na^SO^

[Mo] 1000 [ZnllOOO

[Mojo [Znl 1000 ^ [Mo] 1000 |Zn]0

ZnS

MoS^

160

166

170 175 180 Photon Energy (eV)

186

190

Fig. 5.4. Sulfur (S) L-edge XANES spectra of thermal antiwear films recorded in TEY (surface) mode generatedfromcombination of ZDDP and MoDTC, as compared with model compounds. Peaks a and b originatefromthe reduced forms of sulfiir, and c and dfromthe oxidized forms of sulfur (Kasrai et al., 1998) The wear scar measurements were performed for tribofilms generated from keeping MoDTC constant ([Mo] = 1000 ppm) and varying the ZDDP concentrations from [Zn] = 100-2000 ppm. For the ZDDP concentration < 1000 ppm, the wear scars are about twice as large as those of neat ZDDP (75 yum) (antisynergistic effect). Then, at [Zn] concentration >1000 ppm, the values coincide. Also, the wear scar measurements of tribofilms, generated at constant ZDDP concentration ([Zn] = 1000 ppm) and varying MoDTC concentrations of [Mo] = (100-2000 ppm), were performed. At the concentration of [Mo] < 1000 ppm, the wear scars are very small (--70) and similar to neat ZDDP. Above

197

Surface Tribochem is try and A ctivated Processes

concentration of 1000 ppm of Mo, the wear scar width increases. These differences were supported by XANES phosphate films characterization over the concentration range (Kasrai et al., 1998). The main difference between tribofilms generated from MoDTC alone and the combination of ZDDP and MoDTC is the lack of sulfate signal in the mixed films, see Table 5.13. It is suggested that the presence of sulfate is the cause of the increase in friction.

Table 5.13. The composition of tribofilms and thermally generated films from the combination of ZDDP and MoDTC. Interpretation of sulfrir (S) L-edge XANES spectra recorded in TEY surface technique (Kasrai et al., 1998) ZDDP and MoDTC, ppm^

Tribofilms

Thermal antiwear fihns

[Zn] neat [Mo] neat

The ZnS formation A trace of sulfide, mostly sulfate The M0S2 and ZnS formation

Mostly ZnS04 formation The sulfate dominates in the film A trace of ZnS, mostly sulfate

[Zn] = [Mo] - 1000

^Zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) were used as additives. The metal [Zn] and [Mo] concentrations are in ppm. Thermally deposited antiwear films were made in an oil solution containing the additives at 150°C for six hours. The tribofilms were generated at 100°C and rubbing time of 5 to 60 minutes. Wear scar and coefficient of friction. Study of friction coefficient and wear scar were performed for tribofilms generated from a range of MoDTC and ZDDP concentrations, see Table 5.14 (Kasrai et al., 1998). The wear scar for base oil is - 2 2 5 yum wide. The wear scar for neat ZDDP shows a - 7 5 /^m value, over the whole range of ZDDP concentrations (100-2000 ppm).

Table 5.14. The wear scar and coefficient of friction for tribofilms generated from the combination of ZDDP + MoDTC Base oil + ZDDP + MoDTC, ppm Base oil only [Zn] 100 to 2000 [Mo] 100 to 2000 [Mo] 1000+ [Zn] 100 to 1000 [Zn] 1000 +[Mo] 100 to 1000

Wear scar, fim. 225 75 125 130 70

Coefficient of friction >0.09 0.09 0.09 0.04 0.04

198

Chapters

It was shown by XANES characterization that, even with 100 ppm ZDDP, a good polyphosphate film was formed on the surface and it was thick enough to protect the surface. The wear scars for neat MoDTC for the whole range of concentrations (100 - 2000 ppm) are much larger than for ZDDP and are almost constant around 125 yum, but still lower than those for base oil alone. The friction coefficients for tribofilms generated from neat ZDDP [Zn] = 100-2000 ppm and neat MoDTC [Mo] = 100-2000 ppm are very similar (-^ 0.09). The friction reduction must be related to the formation of a pure MoS2-like film and wear is comparable to using ZDDP alone. Tribofilms generated from neat MoDTC were mixed with a sulfate. The presence of sulfate was particularly clear in the sulfur L-edge spectra (see Fig. 5.4). The proportion of sulfate was reduced when the concentration of MoDTC was increased; however, a small amount of sulfate apparently can affect the friction reducing properties of the tribofilm. Tribofilms generated from a constant concentration of 100 ppm Mo and various concentrations of ZDDP [Zn] = 100-2000 ppm show a similar low friction coefficient (-0.04) over the whole range. As was observed from XANES spectra, only 100 ppm Zn is enough to protect a MoS2-like film from oxidation (Kasrai et al., 1998). The friction coefficients for tribofilms generated from a constant concentration of ZDDP ([Zn] = 1000 ppm) and varying the MoDTC concentrations of [Mo] = 100-2000 ppm are -0.04 for a Mo concentration > 500 ppm. Below 500 ppm, the coefficient is high but still lower than both with neat ZDDP and MoDTC. The sulfur (S) L-edge and K-edge XANES spectra showed signals for M0S2 that were very weak for a molybdenum concentration of <500 ppm, which is in good agreement with these measurements. Synergistic effects for the combination of MoDTC/ZDDP and the friction and wear results are in good agreement between the authors (Kasrai et al., 1998; Muraki and Wada, 1995). Tribochemically generated tribofilm-phosphorus characterization. Combined results of the phosphorus L-edge and K-edge XANES tribofilm spectra generated from a mixture of MoDTC and ZDDP correspond to medium-chain length polyphosphates. Short-chain polyphosphates were found for thermal antiwear films generated from oil containing MoDTC and ZDDP in various concentrations at 150°C. The XANES spectra clearly show that the counter ion in phosphates is zinc rather than iron. The XANES results do not support the formation of iron phosphate (Kasrai et al., 1998). Phosphorus is present in zinc dialkyldithiophosphate and deactivate platinum catalysts in engine exhaust systems. Using zinc, antimony and oxothiomolybdate dialkyl-dithiocarbamate (MoDTC) complexes alone or in combination with other lubricating oil additives appears to solve this problem. Antimony complexes are used also as extreme pressure agents while molybdenum ones as friction modifier additives (Hill et al., 1994). Analyses of Zn(dtc)2 decomposition using the thermogravimetric method, flash vacuum pyrolysis, and four-ball machine

Surface Tribochem is try and A ctivated Processes

199

tribolological tests with GC-MS analysis were performed (Celichowski et al., 1977). The most promising antiwear and extreme pressure additive for lubricants is bismuth dialkyldithiocarbamate, also oil soluble (Chen and Dong, 1997; Otto, 1994; Tuli et al., 1995). Because of environmental and toxicological considerations, it is desirable to provide lubricating oil compositions which are zinc-less and do not contain phosphorus. Benzotriazole derivatives containing the R2NC(=S)S- group possess excellent load-carrying capacity, are antioxidative, anticorrosive, and have friction-reduction and antiwear properties better than ZDDP (Ren etal., 1994). (IV) Friction and wear capabilities of MoDTC/ZDDP. The presence of effective friction reducing additives in engine oil serves to reduce overall engine friction and diminish the potential for wear. These additives not only reduce friction but also provide good wear protection (Arai et al., 1997; Kawai et al.,1998; Yamada, 1997a). Issues of designing such oils and their performance characteristics were recently reviewed (hioue and Yamada, 1997). MoDTC/ZDDP containing oil formulations that possess better retention of friction-reducing capabilities, have recently been developed and introduced (oil RO-97) in Japan (Kawai et al, 1998) and Europe. The evaluation of engine oil RO-97 showed that the friction reducing capabilities are gradually depleted with mileage accumulation. The friction coefficient increased from 0.057 to 0.14 after 7,200 km. In recent evaluation of two commercially available MoDTC/ZDDP oils in the prototype Sequence IIIF test, which evaluates high temperature antiwear capabilities of engine oils at ISS^'C, it was shown that one of these oils, 5W-20, provides good protection in this test (cam and lifter wear was less than the proposed GO-3 oil limit), while the 5W-30 oil failed to do so (Korcek et al., 1999). It is generally accepted that the friction-reducing capabilities of MoDTC are attributed to in-situ formation of M0S2 from MoDTC directly (without ZDDP present) in the boundary contact as a result of mechano-chemical and thermooxidative reactions (Grossiord et al., 1998; Yamamoto and Gondo, 1994). The formation of M0S2 is accompanied by formation of an antiwear tribofilm, a solid phosphate matrix, from ZnDDP-derived precursors. Formation of a low friction/antiwear film structure when both MoDTC and ZDDP are present is outlined in the literature. The presence of solid boundary tribofilms is the result of a dynamic process involving the film formation and removal. These two processes may not necessarily occur at the same time, but depending on the conditions, one or the other may prevail. Solid film formation results from chemical reactions occurring in the boundary contact involving oil additives, especially MoDTC/ZDDP transformation products. Factors influencing the tribochemical processes of additive systems containing MoDTC/ZDDP depend on concentrations of bulk lubricant reacting species, the rate of adsorption of these species on the surface and temperature (Korcek et 1999; Willermet 1995a; Yagishita and Igarashi, 1991; Yin et al., 1997a).

200

Chapter 5

Table 5.15. Protective tribofilm formation from degradation of molybdenum dithiocarbamate (MoDTC), molybdenum dithiophosphate (MoDDP) and molybdenum amine-ester complex (MoAC) directly and in combination with ZDDPs or sulfur compounds Additive System and Observed effect M0S2 tribofilm formation directly from MoDTC or MoDDP MoDTC, Tribochemically generated film is composed of M0S2 and sulfate on the surface and thermally formed fihns are composed of M0O3 and sulfate. The XANES spectra are very sensitive to small changes in the chemical environment (Kasrai et al., 1998). - M0S2 was formed on the rubbing surfaces in air and oxygen environment and was not formed in nitrogen and argon atmosphere. (Yamamoto et al., 1994). - The tribofilm formation of M0S2, M ^ 0.04, was explained by a two-step tribochemical reaction of MoDTC (Grossiord et al., 1998). - Friction reduction is strongly dependent upon temperature and MoDTC concentration at 100°C and 0.05 wt.%, // = 0.15 and a 0.18 wt.%, M = 0.08 (Graham et al., 1999). - It appears that M0S2 forms, and friction is reduced, only when direct solid-solid contact occurs, i.e., in true boundary lubrication conditions (Graham et al., 2001a). MoDTC, MoDDP, M0S2 and FeP04 were identified on the rubbing surfaces from MoDTC and MoDDP, respectively. The unrubbed surfaces contained mainly M0O3 (Yamamoto and Gondo, 1987). - All the molybdenum-containing additives show a sensible improvement in fiiel efficiency, 3% (Braithwaite and Greene, 1978). - MoDTC gives higher friction than MoDDP (Tripaldi et al., 1999). MoDT Carbonamide. An increase in the load carrying capacity and a decrease in the values of wear and // = 0.05 are observed (Tripathi et al., 2000). Antiwear capabilities of MoDTC or MoDDP with ZDDP MoDTC+ ZDDP. A synergistic effect in reducing friction and wear has been attributed to anti-wear films containing primarily M0S2 and polyphosphates. The counter ion to the phosphates is zinc, rather than iron. The XANES spectra indicate that sulfur and phosphorus form M0S2 and polyphosphate chains (Kasrai et al., 1998). - The degradation of both MoDTC and ZDDP gives improved sustainable fuel economy (Kubo et al., 1995). The synergistic reduction in friction is by ZDDP in conjunction with MoDTC (Muraki and Wada, 1994 and 2002). - The ZDDP layer acts as the source of sulfur, reducing oxidation of M0S2. The fuel economy was improved (Stipanovic et al, 1996). - The friction reduction is from MoDTC deterioration. Other antioxidants are essential to extend friction reducing capability (Korcek et al., 1996). - Ligand exchange reactions are affected by oxidation, leading to formation of full ligand exchange products (MoDDP, ZDDP).

Surface Tribochemistry and Activated Processes

201

Table 5.15. (Continued). Additive System and Observed effect - Improved fuel efficiency and extended service intervals are observed (Korcek et al, 1997; Johnson et al., 1997a). - The friction reducing ability is completely depleted when ZDDP is consumed, but while MoDTC is still present (Johnson et al., 1995). - An active intermediate, MoDDP, provides more efficient radical trapping (antioxidant activity) than either ZDDP or MoDTC (Johnson et al., 1997b). - The friction reducing capabilities are not only affected by the additives and ligand exchange products, but also base oil can have a dramatic effect (Johnson et al., 1997c). - At high temperature, active precursors to friction reduction are Mo(dtc)(ddp) and Mo(ddp)2; at low temperature such precursors are Mo(dtc)2 and Mo(dtc)(ddp) (Jensen et al., 1998a). - Scuffing protection for oil with p-ZDDP is good; for oil with s-ZDDP is poor. The presence of M0S2 in the tribofilm and the formation of M0O3 in oil (pri-ZDDP) were identified (Nagakari et al, 1997). - The friction is reduced. Low content of ZDDP is sufficient to prevent the oxidation of sulfur to sulfate and Mo(IV) to Mo(VI). ZDDP facilitates the formation of a better M0S2 tribofilm (Kasrai eat al., 1997). - ZDDP had a role in increasing the wear resistance and promoting the formation of M0S2 (Muraki et al., 1997). - The M0S2 forms from MoDTC, but MoDDP is more prone to form polymeric surface films (Mitchell, 1984). MoDTP + ZDDP. The surface film is effective in reducing friction. M0S2 and FeP04 were formed on rubbing surfaces while M0O3 and FeP04 were formed on unrubbed surfaces (Yamamoto and Gondo, 1986). MoDTC^ MoDDP or Mo AC + S-containig compounds. A strong synergism generates a drastic friction coefficient reduction below 0.065 (Tripaldi et al., 1999). MoDDP + ZDDP + dispersant. Ternary mixtures show enhanced antifriction and antiwear properties of Mo compounds (Lin, 1995). MoDTC + ZDDP or dibenzosulfide or dithiuranu The MoDTC can be protected against thermooxidative degradation by the addition of ZDDP or dibenzosulfide or dithiuram. The rate of formation of M0S2 in the rubbing contact appears to be lost below -180 ppm Mo (Graham etal., 2001b). Table 5.15 summarizes protective tribofilm formation from degradation of molybdenum dithiocarbamate (MoDTC), molybdenum dithiophosphate (MoDDP) and molybdenum amine-ester complex (MoAC) directly and in combination with ZDDPs or sulfur compounds. Temperature effect on reduction of friction. Thermal stabilities of molybdenum dialkyldithiophosphates (MoDDP) are much lower (below 180 ""C) than those of the corresponding MoDTC, ~300°C. The type of ZDDP must also

202

Chapter 5

be considered when selecting appropriate additive systems. MoDDP is formed from MoDTC ligand exchange with ZDDP. It is interesting to note that primary and secondary MoDDPs formed by Hgand exchange of MoDTC with prim-ZDDP and sec-ZDDP decompose at around 175 and 120°C, respectively. This must be considered in applications of these additives at elevated temperatures. The friction coefficient testing indicates that the MoDTC/ZDDP oil tested (R097) exhibits very low friction coefficients (-0.05) at 105°C up to about 4000 miles of operation while friction coefficients at 45''C are twice as high during the same period (Korcek et al, 1999). Effects of aging on friction properties of fuel-efficient engine oils. The effect of laboratory and vehicle aging on the ability of an oil containing a molybdenum dialkyldithiocarbamate (MoDTC) additive to provide reduced friction has been investigated (Johnson et al., 1995, 1997a and 1997b; Korcek et al., 1996 and 1999). Results of these studies show that during engine operation, reducing additives are consumed during oil use and the friction reducing benefits are lost causing a loss of some full efficiency benefits before the end of the service interval. It was concluded that future engine tests designed to evaluate the fuel efficiency of engine oils must involve oil use characteristics prior to the fuel economy determination, in order to reflect the effects of customer use. This aging, to be realistic, must be more extensive than the one used in the current fuel economy engine test, ASTM Sequence VIA, that ages the oil only for the purpose of stabilizing its viscosimetric properties. The SAE 5W-20 oil (with ZDDP present in the oil formulation) was used in the laboratory aging test, ASTM Sequence VIA and Vehicle aging tests. The oil was formulated with MoDTC friction modifier, the molybdenum content [Mo] = 700 ppm (Johnson et al., 1995). The Case Study, Comparison of laboratory test, sequence VIA and vehicle aging tests of the SAE 5W-20 engine oil modified by MoDTC additive, [Mo] = 700 ppm. Used oil samples were analyzed for the following properties: MoDTC (%) left, antioxidant (%) left, ZDDP (%) left, and friction- reduction effectiveness. The case study includes a description of: (a) Laboratory aging test and evaluation, 15 hour test; (b) Vehicle aging test and evaluation, 6,672 km test; (c) ASTM sequence VIA fuel economy test and evaluation, 16 hour test; (d) A modified ASTM sequence VIA test and evaluation, 96 hour test. (a) Laboratory aging test and evaluation. Tests were conducted in a batch reactor with oil at 160''C using blow-by components to accelerate oil degradation (Johnson and Korcek, 1991). The oil (60cm^) was exposed to a gas stream containing 20% oxygen, 680 ppm NO2, and the balance was nitrogen. The total gas flow was 200 cm^/min. Samples were withdrawn periodically for analysis.

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203

The results of this test are presented in Table 5.16. The fresh oil provides a low friction coefficient o f - 0.04. After 14 hours of accelerated oil aging, friction reduction is apparent only during the mid portion of the test temperature above 104°C„ Finally, after 15 hours of laboratory aging, the friction coefficient reached a value typical of that of non-modified oils (0.12). Analysis of the 15 hour sample revealed that, despite the loss of nearly 1/3 of the original friction reduction, MoDTC remained in the oil. Also, approximately 25% of the radical-trapping antioxidant capacity remained and the extent of oxidation and nitration was low. Analysis by ^^P NMR, however, revealed that the ZDDP was completely depleted in this sample. Experimentally, fresh ZDDP was added to the sample and the 15hour friction test was repeated. Adding ZDDP to laboratory aged oil showed that the friction capability was recovered (// ~ 0.05). The friction-reducing capability of the oil, as measured in a laboratory test, is completely depleted when ZDDP is consumed but while MoDTC is still present. Table 5.16. Comparison of laboratory aging ASTM, sequence VIA and vehicle aging tests of SAE 5W-20 engine oil modified by an MoDTC additive, [Mo] = 700 ppm Parameter

Laboratory aging 15 h test

Vehicle 6672 km test

Sequence VIA 16 h test

Modified sequence VIA 96 h test

MoDTC^ % active Antioxdant^, % active Nitration'', Abs/cm Oxidation'', Abs/cm ZDDP'^ Friction reduction^ Plint test TE77, /i

32 25 10 12 Depleted None 0,12

30 48 13 16 Depleted None 0.12

75 79 6 7 Present Good 0.04

10 26 N/A 17 Depleted None 0.12

Analyzed by: Reversed phase HPLC using a Cg column (Jensen et al., 1998a); ^ peroxyl radical titration procedure; ''IR using a Nicolet Oil Analyzer; "^monitored with P NMR ; Tlint reciprocating wear machine (b) Vehicle aging test and evaluation. Tests were conducted in a 1993 Lincoln Town Car with a 4.6L SOHC V8 engine. The vehicle was operated on a Labeco Road Simulator, using an accelerated vehicle mileage accumulation cycle for 6,437 kilometers. The aging cycle consisted of approximately one stop and one stop retard per 1.6 km with an average speed of approximately 74 km/h. Oil and coolant temperatures were between 88 and 93"C during the aging tests conducted at ambient temperatures of 4 to 16°C. Samples were collected at 1,609 km intervals, and were analyzed to determine changes in additives content and possible effects onfi-iction-reducingability. One quart of fresh oil was added after

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the 3,454 km sample was taken. The results of this test are presented in Table 5.16. Changes in antioxidant capacity and MoDTC concentrations in the oil during vehicle operation were recorded (Johnson et al., 1995): Antioxidant % remained (distance portion)

76 (0.25); 64 (0.5); 56 (0.75); 48 (LO)

MoDTC % remained (distance portion)

66 (0.25); 48 (0.5); 42 (0.75); 30 (1.0)

These findings reveal that MoDTC is depleted slightly faster than total antioxidant capacity and that substantial fractions of both radical-trapping antioxidants (48%) and MoDTC (30%) remain in the oil at 6,672 km. ZDDP was completely depleted in the oil with the friction coefficient being // ~ 0,12, which falls in the range of non-friction modified oils. Comparisons of data for laboratory and vehicle aged tests of 5W-20 modified oil, presented in Table 5.16, indicate that 15 hours of accelerated aging tests simulate the 6672 km vehicle aging fairly well. The concentrations of MoDTC remaining are similar and ZDDP is depleted in both cases. (c) ASTM sequence VIA fuel economy test and evaluation. The oil was evaluated in a 4.6L SOHC V8 engine operated according to the dynamometer test procedure developed for the Sequence VIA fuel economy test. This test includes a 16 hour "aging" process to "stabilize" the oil. The aging takes place at an oil temperature of 125°C and is followed by the testing at various speeds and loads andoiltemperaturesthatare varied in steps from 105°C to 70''C to 45''C. Total test time, including "aging", is 25 hours. The results of this test are presented in Table 5.16, This dynamometer engine test is milder than the in-vehicle-use testing and modified Sequence VIA testing. This is indicated by the fact that 75% of the MoDTC and 79 % of the antioxidant capacity remained at the end of the sequence VIA compared to 30%, 48% and 10%, 26%), respectively in the vehicle and modified sequence VIA tests. Also, ZDDP was not completely depleted as it was in other tests and the friction test showed a low friction coefficient (yU ~ 0.04). The Sequence VIA aging cycle to stabilize the tested oil is milder than running 6,672 km of vehicle aging. (d) A modified ASTM sequence VIA test and evaluation. The more severe engine-aging procedure used in this testing consisted of operating the current ASTM Sequence VIA engine at 3000 rpm, 135''C oil temperature and 30 kW load. Oil samples were withdrawn periodically for analysis. Results from a 96 hour aging test for the contents of antioxidants, MoDTC and effect of aging on boundary friction coefficient are shown in Table 5.16. Changes in antioxidant capacity (26%) and MoDTC concentration (10%)) in the oil during the vehicle testing are quite large. The ZDDP was completely depleted

Surface Tribochemistry and Activated Processes

205

and the oil lost friction-reduction characteristics, with the friction coefficient becoming /^ ~ 0.12. Testing after 96 hours of aging, using the fuel economy portion of the current Sequence VIA, showed the fuel economy improvement of 1.4% in the standard test had been reduced to 0.4% after aging. A fraction of the decrease is attributable to a viscosity increase of 12%. The maximum benefits of fuel-efficient engine oils will be realized only if changes in friction-reducing capabilities and viscometric properties during service are minimized. Frictionreducing capability had been lost before the MoDTC was depleted. Recovery of some of thatfi-iction-reducingability upon addition of fresh ZDDP indicates that some, as yet undefined, process involving ZDDP is needed in order to activate the friction-reducing benefits of MoDTC in this formulation. Because of concerns related to adverse effects of emission system components, engine oil specifications set limits on the maximum amount of phosphorus that may be in an oil formulation; therefore, increasing the initial concentration of ZDDP is not allowable. For the same reasons, adding additional ZDDP during service is not acceptable. Thus, other approaches to formulating oils for long lasting effectiveness of this friction modifier must be considered. Evaluation of oil derived fuel efficiency benefits in future engine tests, intended for engine oil standardization, should include an aging cycle representative of aging in customer service. Otherwise, the fiiel efficiency benefit indicated by those tests could be exaggerated (Johnson et al., 1995 and 1997a). Prolonging friction-reduction capabilities ofZDDP/MoDTC additives. One alternative for prolonging friction-reducing effectiveness may be to minimize the consumption of ZDDP and MoDTC due to antioxidant reactions. Both of these additives are consumed, not only by providing antiwear or antifriction performance, but also due to antioxidant reactions. The peroxyl radical-trapping antioxidant, 2,6-di-rer/-butyl-4-methylphenol (MPH), reduces the rate of consumption of MoDTC (Korcek et al., 1996; Mahoney et al., 1978). Analysis of samples from similar experiments with MPH and ZDDP by IR indicates that MPH also reduces ZDDP consumption. The efficiency in inhibiting the oxidation process for some of the antioxidants was characterized using a peroxyl radical titration technique (Mahoney et al., 1978). hi this test,fi-eeradicals are generated from azo-bis-isobutyronitrile, and the rate of oxygen consumption is monitored. When an antioxidant is present, there is an inhibition period during which peroxyl radicals are trapped and the rate of oxidation is reduced. The antioxidant 2,6-di^er^butyl-4-methylphenol (MPH) is regarded here as 100% efficient in inhibition of oxidation (Johnson et al, 1997b). Comparison of MPH with other antioxidants gives the following sequence: MoDTC (44%) < ZDDP(51%) < MoDDP(64%) < MPH (100%) Rates of depletion of the two components (MoDTC and ZDDP) in a ternary

206

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system that also contains the peroxyl radical antioxidant, 2,6-di-^^r/-butyl-4methylphenol, reduced the rate of consumption of both components by about three times (Johnson et. al., 1997b). In the laboratory aging procedure (Johnson and Korcek, 1991), high temperature antioxidant capabilities were assessed at 160''C in a batch reactor using blow by components to accelerate oil degradation. The sample analyzed after five hours of accelerated aging of (MoDTC + ZDDP) without MPH, had lost its ability to reduce friction coefficient to below that which would be observed for a ZDDP - only system. The five hour sample from the mixture (MoDTC + ZDDP) containing added MPH still provided substantial friction reduction and the results are very similar to those observed for the fresh mixtures, yU ~ 0.05. The behavior of MoDTC + ZDDP containing additive systems under oxidation conditions have shown that (Johnson et al., 1997b): (a) MoDTC does not fiinction initially as a radical-trapping antioxidant but is converted to an intermediate during oxidation that is an effective radical-trapping antioxidant; (b) combinations of MoDTC and ZDDP do not trap peroxyl radicals initially, but are definitely converted to an intermediate that is a very effective radical-trapping antioxidant; (c) efficient radical-trapping intermediates are formed from interactions between MoDTC and ZDDP during high-temperature oxidation; (d) MoDDP provides more efficient radical-trapping antioxidant activity than either ZDDP or MoDTC; (e) addition of a hindered phenol extends the inhibition period and reduces the rates of MoDTC and ZDDP consumption. The results of this investigation indicate that it should be possible to extend the friction reducing capabilities of MoDTC containing engine oil through carefiil selection of antioxidants and formulation of the overall additive and base oil systems. The effectiveness of MoDTC and MoDDP is strongly influenced by synergism and antagonism of the additives, e.g., sulfur containing additives (Sarin et al., 1994b; Lin, 1995). In terms of extreme pressure properties, the combinations of organosulfur compounds with MoDTP and molybdenum amine-ester complex MoAC are antagonistic in general. The combination of MoDTC with dibenzyl disulfide (DBDS) was found to be antagonistic. However, interaction between MoDTC and sulfiirized isobutylene (SIB), and sulfurized fat (SF) were in general, synergistic in nature. The antiwear performances of various sulfur (S) additives in combination with MoDTC and MoDDP were found to be synergistic; however, in the case of MoAC, the trend was antagonistic in general. The best synergistic combination of three organomolybdenum and four organosulfur additives was found to be between MoDTC and sulfurized isobutylene (SIB) (Sarin et al., 1994b). The synergism of MoDTC and SIB was sustained, even in the presence of other competitive additives, such as detergents, dispersants, antioxidants, rust and corrosion inhibitors, antifoamants, etc. The wear scar (WSD) decreased from 1.3 mm to 0.8 (39%), the weld load improved from 400 to 450 kgf (12.5%) and the coefficient of friction improved from 0.085 to 0.065 (23.5%)). The mechanism of

Surface Tribochemistry and Activated Processes

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friction reduction and wear prevention of organomolybdenum compounds involves chemical reactions between the Mo compounds and the rubbing metal surfaces to form a Mo-containing protective tribofilm (Bird and Galvin, 1976). (V) M0S2 tribofilm formation directly from MoDTC in the boundary contact. Over the last thirty years, considerable research has been carried out to investigate the mechanism by which soluble molybdenum additives function. The molybdenum additives reduce friction by forming a reactive tribofilm containing M0S2. The tribofilm can be analyzed by X-ray photoelectron spectroscopy (XPS) (Yamamoto and Gondo, 1987), Auger electron spectroscopy (AES), electron diffraction (Isoyama and Sahurai, 1974; Ming et al., 1963), Raman spectroscopy and atomic force microscopy (AFM), (Graham et al., 2001a) and XANES on immersed and rubbed surfaces (Kasrai et al., 1998). It has been variously suggested that (Graham and Spikes, 1999): (a) sulfur-containing additives are needed to promote tribofilm M0S2 formation (Mitchell, 1984); (b) antioxidants are required to prevent molybdenum additives from behaving as peroxidedecomposers and thereby being consumed (Graham and Spikes, 1999; Johnson et al., 1997b and 1998); (c) ligand exchange between the dialkyldithiocarbamate moiety and the dialkylphosphate in zinc dialkylphosphate is an important stage in the friction-modifying reaction of the molybdenum dialkyldithiocarbamate (Johnson et al., 1997b); (d) antiwear additives promote the formation of M0S2 films by reducing the rate of their removal by wear (Muraki et al., 1997). It is deduced that competitive adsorption between ZDDP and MoDTC onto the surface occurred during the run, because Mo concentration on the surface with ZDDP + MoDTC oil was lower than that obtained with the oil containing MoDTC alone. The adsorption of the additive on a surface results in formation of a film composed of the decomposition compounds or reaction compounds of the additives with the metal surface. The decline in the Zn/Mo atomic concentration ratio in the film composition with test time indicates that the decomposition products of ZDDP, which had a high coefficient of friction, were preferentially formed and subsequently. Mo compounds were formed. Similarly, an increase in the concentration of M0S2 with time was also observed. A necessary condition for the production of an effective surface film for reducing friction is the previous formation of the compounds derived from ZDDP. The molybdenum dialkyldithiophosphates (MoDDP) were able to reduce friction effectively in mineral oil in the absence of other additives. The MoDTC only reduces friction to boundary values of about 0.095; however, mixtures of MoDTC and ZDDP can give much lower boundary friction coefficient values, below 0.04 (Graham and Spikes, 1999; Tripaldi et al., 1999). It has been reported that low friction films of molybdenum disulfide (M0S2) are formed from MoDTC or MoDDP if ZDDP is not present. MoDDP seems more prone to form polymeric surface films directly in the boundary contact as a result

Chapter 5

208

of mechanical-chemical and thermo-oxidative reactions. Also, molybdenum complexes containing no sulfur were capable of forming M0S2 when they were added to oils containing sulfur compounds (Braithwaite and Greene, 1978; Grossiord et al, 1998; Kasrai et al., 1998; Mitchell, 1984; Yamamoto and Gondo 1989, 1994).

[Mol 100

3

[Mo] 1000

^

[Mo] 2000 MODTC MoS^

160

166

170 175 ISO Photon Energy (eV)

185

190

Fig. 5.5. The (S) L-edge XANES spectra recorded in TEY (surface) mode of tribofilm generated from net MoDTC, as compared with model compounds MoDTC and M0S2. Peaks a, b, and c are related to the reduced form of sulfur, whereas peaks d and e are the sulfate peaks. Peaks 1, 2, and 3 are MoSj. The photon resolution at the Ledge is -0.2 eV (Kasrai et al., 1998) The effect of the concentration of MoDTC on tribofilm formation using the Flint reciprocating wear ring machine on a 52100 steel coupon was investigated (Kasrai et al., 1998). The film generated on the coupon was rubbed 5 to 60 min in oil containing the additives for XANES analyses and wear scar measurements. The sulfur (S) L-edge XANES spectra of tribofilms, along with those of neat MoDTC and M0S2, were recorded in the TEY (surface) mode, and are shown in Fig. 5.5.

Surface Tribochemistry and Activated Processes

209

Spectra A, B, and C indicate that the Mo concentration in oil increases from 100, 1000 to 2000 ppm. Spectrum A shows only a trace of a reduced form of sulfur and contains mostly sulfate. Spectra B and C contain both reduced and oxidized species in different proportions. The higher the concentration of Mo in oil, the higher the reduced form of sulfur. Spectrum D of neat MoDTC shows both sets of peaks. The sulfate peaks most likely originate from surface oxidation. The nature of the reduced form is not entirely clear. For example, peaks a, b, and c in spectrum C do not align with peaks 1, 2, and 3 of M0S2. All the peaks are shifted to higher energy by 0.5 to 1 eV. The film contains a relatively higher proportion of sulfate on the surface than in the bulk. The (S) L-edge XANES bulk (FY mode) spectra recorded were very weak, indicating that the film is thin (<10 nm), (Kasrai et al., 1998). The (S) K-edge XANES spectra of tribofilms along with neat MoDTC, M0S2 and Na2S04, recorded in the TEY (surface ) mode, are shown in Fig. 5.6.

S KHBdge

[MoJ 100 CMo]500

(A) (B)

(C)

t

(D)

(E)

(F)

2460

2470 2480 Photon Energy (eV)

2490

Fig. 5,6. The (S) K-edge XANES spectra recorded in the TEY mode (surface) of tribofilms generated from neat MoDTC, as compared with model compounds MoDTC, M0S2 and Na2S04. Peaks 1 and 2 are related to those of M0S2 and the sulfate, respectively. The photon resolution at the K-edge is ~1 eV (Kasrai etal, 1998)

210

Chapters

Observations ofpeaks. Fig. 5.5 and Fig. 5.6. In (S) K-edge spectra (Fig. 5.6), peak 1 is observed in spectra (A-D) and aligns well with the main peak of M0S2 spectrum E, whereas peak 2 aligns with the sulfate spectrum F. In (S) L-edge spectra (Fig, 5.5), at lower concentrations of MoDTC, both reduced and oxidized forms of sulfur are present (see spectra A and B). The proportion of the oxidized form was greater at the (S) L-edge due to the shallow probing of TEY detection. The TEY detection method at the L-edge and K-edge penetrates to depths of ~5 and -25 nm, respectively (Kasrai et al., 1996). Also, there are differences in the photon resolution of the spectra in the two regions of energies, -0.2 eV (at the Ledge) versus -1 eV (at the K-edge). From the L-edge and K-edge XANES spectra, it is possible to conclude that the film contains a relatively higher proportion of sulfate on the surface than in the bulk. From the combined sulfur (S) L-edge and K-edge results, it can be concluded that tribofilms generated from neat MoDTC are composed of at least two layers. At a low concentration of Mo (100 ppm), the surface (down to ~5 nm) is composed mostly of sulfate. At this concentration, sulfate is also present in the bulk, but the bulk contains a good proportion of a MoS2-like film. At a high concentration of Mo (>1000 ppm), both the surface and the bulk contain mostly MoS2-like species (Kasrai et al., 1998). Wear of aluminum alloys. The effect of molybdenum dithiocarbonate (MoDTC) on friction and wear properties between an aluminum alloy and steel was investigated (Arai and Yamamoto, 2000). The steel penetrates into the aluminum, resulting in strong adhesion and large adhesive wear of the aluminum alloy. Reducing the weight of automobiles is one way to achieve higher fuel economy, and for this purpose aluminum alloys are commonly used for housing of drive train units, cylinder blocks, bodies, and so on. Aluminum alloys including higher silicon content are used where high resistance to wear is required (Barber etal., 1991). If aluminum alloy parts are applied in internal combustion engines, they are lubricated by engine oils that include ZDDP, which is well known as a good antiwear agent for steel. ZDDP is not effective in preventing the wear of aluminum alloys (Wan et al., 1997). MoDTC can prevent the adhesive wear of aluminum alloys by generating a surface film over higher sliding speed ranges. The surface film reduces the specific wear of the aluminum alloy to less than approximately 10"^ mm^.N'^ and is comparable to the steel value. The surface film consists of a mixture of molybdenum trioxide and molybdenum sulfide. Electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy were conducted to characterize the rubbing surfaces. In order to better understand the fundamentals of friction reduction by molybdenum dithiocarbamate and to relate it to chemical changes and transfer phenomena of the tribofilm, an ultrahigh vacuum (UHV, 10"^ Pa) tribometer was used (Grossiord et al, 1998; Martin et al., 1999). Specifically, this tribometer allows in situ Auger electron spectroscopy (AES), X-ray photoelectron

Surface Tribochemistry and Activated Processes

211

spectroscopy (XPS), surface analysis and imaging inside and outside wear scars of both jfriction counterfaces. This represents an indirect way to analyze, in situ, the tribochemistry of the two counterfaces with Hmited tribological perturbations. The boundary lubrication is essentially due to the action of polar molecules which are chemisorbed on the surface and which then penetrate the contact zone. In a sense, boundary lubricated rubbing surfaces are so close to each other that no free (non adsorbed) molecules coming from the lubricant phase can enter between these surfaces. Thus the two friction surfaces can be analyzed by spectroscopic tools with confidence, because there will be neither change due to contamination nor to ionic etching. In the reciprocating pin-on-flat tribometer, friction reducing properties of MoDTC additives were shown to be associated with a transfer mechanism of nanometer thick M0S2 layers between the two counter faces. Once transferred (one or two single sheets originating from the tribofilm), the layer is found to be chemically bound to metallic iron by sulfur atoms. AES and microspot XPS were successfully used here. Elemental imaging allows the localization of the transfer film and the wear debris to be clearly seen. As AES and XPS imaging are sensitive at nanometer scale depth, they can reveal contrasts which cannot be seen by optical or even SEM imaging. The association of friction experiments in UHV and powerful analytical tools appears to be a very efficient way to highlight the mechanism of the friction-reducing effect of organic molybdenum compounds (Martin et al., 1999). UHV friction tests on MoDTC tribofilm have been performed and give evidence for the mechanism of single sheet lubrication by highly-dispersed M0S2 phases. The tribological test using MoDTC as a lubricant additive gave a very low steady-state friction coefficient, about 0.04, with good reproducibility (Grossiord et al., 1998). The mechanism of fi'iction-reduction of MoDTC is attributed to the effect of sliding between single layers of M0S2 only, rather than to intra-sliding in a M0S2 3-D crystal. Highly dispersed M0S2 sheets are present in a carbon matrix in the tribofilm material. The 2-D M0S2 single sheets are formed by degradation of the MoDTC molecule by electron transfer mechanisms activated by the friction process. X-ray photoelectron spectroscopy (XPS) indicates that the tribofilm has a complex structure and the composition seems to be homogeneous in thickness. The top surface of the film is oxidized, as evidenced by the presence of sulfates and oxides. M0S2 might be present in the whole film in a relatively small proportion (a few atomic percent). Another species present in the film is attributed to M0O3 (a few percent also). Practically, in these conditions, only a few per cent of dispersed M0S2 is sufficient to lubricate at the same level as pure M0S2. From the chemical point of view, the formation of M0S2 from MoDTC can be explained by a two-step process. Fig. 5.7 (Grossiord et al., 1998).

212

Chapter 5 electron transfer V ^Ho.

R"

.rO. Mo

'S^^"^^Vg

N—C R

/ V

S

-

.C—N

S^^

R

^R

'

>'\

;

;s

0=Mo! Mo=0 • ^'^Z

MoS. ^ Moa thjtiram disuindc

>S ^ / / Mo.' Mo Mo. Mo,

>s \s ^s

MoS? sheet

MoOj

molybdenum oxide

\

Fig. 5.7. Chemical process of the formation of M0S2fromMoDTC. The first phase corresponds to electron transfer on Mo-S chemical bonding, leading to the formation of free radicals. The second phase represents the recombination of two radicals with the formation of thiuram disulfide, where the other radical decomposes into M0S2 and M0O2 (Grossiord et al., 1998) Electron transfer occurs on Mo-S chemical bonding in MoDTC, which leads to the formation of three radicals: one corresponding to the core of the MoDTC molecule and the others to the chain ends. Chain-end radicals then recombine to form thiuram disulfide, whereas the core radical decomposes into M0S2, which crystallizes into sheets, and M0O2, which can by oxidized in the presence of O2. It is very remarkable that a few per cent of highly-dispersed M0S2, in a carbonbased material, is as efficient as a pure crystallized M0S2 thin film. Experiments proved that even two-dimensional M0S2 arrangements can lubricate efficiently. The friction reduction due to the presence of MoDTC is attributed to M0S2 formation by the tribochemical reaction (Grossiord et al., 1998; Chermette et al., 2001). The surface film of M0S2 was formed with molybdenum dithiocarbamate (MoDTC) on the rubbing surfaces in an environment containing oxygen. In contrast, in nitrogen and argon, M0S2 was not formed, and the molybdenum compounds formed had poor friction and wear characteristics (Yamamoto and Gondo 1989 and 1994). The surface film composed of M0S2 was effective in

Surface Tribochemistry and Activated Processes

213

reducing wear and friction,// = 0.05, while the molybdenum compound formed in the nitrogen or argon environment had no ability to prevent direct contact between the rubbing surfaces and to reduce the friction, yU = 0.14. It was a necessary condition for forming films composed of M0S2 that the environment of the rubbing process contained oxygen at a concentration above a certain level (6.5 vol. %). Both MoDTC and MoDDP reduced friction and wear when tested in a reciprocating sliding test in a pure hydrocarbon. MoDTC formed a surface film composed mainly of M0S2. MoDDP formed fewer molybdenum compounds in the rubbing surfaces, which were composed of M0S2 and FeP04 at 120°C, and M0S2, M0O3 and FeP04 at 200°C. It is well known that iron phosphate has a high wear resistance; therefore, MoDTC is superior in reducing friction to MoDDP. The oil containing MoDDP also offers a low coefficient of friction (yU = 0.06), in a low temperature stage up to about 120°C; however, the coefficient of friction began to increase with increasing oil temperature, from // -0.12 at 200°C. The coefficient of friction of MoDTC decreased to below 0.05 at 200°C. Therefore, MoDDP seems to decompose on the rubbing surfaces at about ISO^'C, 30 degrees lower than the static decomposition (the rubbing surface activation effect). The static decomposition of MoDTC occurs at about 300°C. The decomposition products of MoDDP appear to increase the coefficient of friction (Yamamoto and Gondo, 1989). MoDDP is known to possess good friction-reducing properties which are due to M0S2 formation, a well known lamellar solid lubricant. To find the relation between the friction-reducing properties of MoDDP and surface film chemistry, several friction tests were performed with MoDDP at a concentration of 1 wt% in PAO synthetic lubricant base. After an induction period of a few cycles, the coefficient of friction reaches a steady-state value near 0.04. The presence of a transfer film is evidenced by Auger electron spectroscopy AES and by scanning Auger microscopy (SAM) and is composed of Mo, S, and Fe, whereas wear debris contains mainly phosphorus and oxygen. This AES observation is in favor of the presence of pure M0S2 through which iron is detected (Le Mogne et al., 1999; Sarin et al., 1994a; Sun et al., 1990). The steel surface of the flat was immersed in the solution of oxymolybdyl dithiophosphate (DDP)2Mo02 at a concentration of 2 wt% in a PAO synthetic lubricant base. The immersion time was 5 hr and the temperature was 100°C (Martin et al., 1996). The X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy analysis of the reaction film indicates that before friction, a relatively thick chemisorbed film composed of phosphorus (P) and sulfur (S) is present on the steel surface. The friction test was carried out in ultra high (UHV) analytical tribometer just after the analysis and in the same conditions as the untreated surface.

214

Chapters

The friction coefficient drops to a value below 0.05, characteristic of ultralow friction. This is a well known phenomenon encountered when using MoDDP additives in lubricating tests (Tung et al., 1988; Zheng et al., 1986). This demonstrates that the ultralow friction is mainly due to a tribochemical transformation of the solid reaction film. In the ASE spectrum analysis a molybdenum sulfide tribofihn formed on the wear scar showed a S : Mo peak ratio near 9, which is in agreement with a pure M0S2 spectrum. Surface Raman analysis was used to characterize the reaction products formed by MoDTC additives on the rubbed metal surfaces. M0S2 is evident as shown by the two characteristic Raman peaks, at 376 cm'^ and 412 cm"^ It is noteworthy that in mixed rolling/sliding, higher loads were needed to form a friction-reducing film than in pure sliding. The precise way that solid-solid contact and rubbing promotes MoDTC reaction is not yet known. It could be related to removal of the oxide film, to plastic deformation, to the freshly exposed metal surface or simply to asperity flash temperature excursions. The abruptness and rapidity of the appearance of M0S2 and consequent friction reduction, suggests that the reaction is autocatalytic (Graham et al., 2001a; Willermet et al., 1997).

Problems

5.1 Adsorption Using data from Table 5.1 "Adsorption of organic compounds on the metal surface under static conditions and on the nascent steel surfaces", discuss the adsorption activity for each of the following: (a) saturated hydrocarbons (n-hexane, cyclohexane), (b) the compounds which have 7i-electrons (benzene, 1-hexene), (c) compounds with functional polar groups (propylamine, propionic acid). 5.2 Tribochemical reaction The formation of an iron sulfide tribofilm may be displaced by other surface active elements such as oxygen, where the oxide of the iron heat of formation AHf = -2.82 eV is thermodynamically more stable than the sulfide AHf = -1.04 eV. Using the data from Table 5.11, compare the heat of formation of molybdenum oxide and molybdenum sulfide. 5.3 Thermodynamic parameters The tribochemical energy for the stable compounds of tribofilm comes from the heat of formation AHf and work function WF: (AHf - WF). Using the data from Table 5.11, compare the tribochemical energy for iron compounds and explain formation of the tribofilms of FeS and FeO in spite of positive tribochemical energy.

Surface Tribochemistry and Activated Processes

215

5.4 Lubrication mechanism How do you expect each of the following regimes to change in going from thickness of lubricating film h to a coefficient of friction, |i in the Stribeck-Hersey curve Fig.5.1? There are three defined regimes of liquid lubrication: (a) hydrodynamic, h -25 |im, (b) elastohydrodynamic, h = 0.025 to 2.5 (xm, (c) boundary, h = 0.0025 |j.m. What information about lubricant structure is indicated by the Stribeck-Hersey curve? 5.5 Catalytic activity of rubbing surfaces (a) By reference to Table 5,5, find the metal hydroxides, oxides and nitrides that illustrate their highest exoelectrons emission intensity (I, cps), (b) Calculate the tribochemical energy (TribEn); TribEn = (AHf - WF) for the listed compounds and correlate them with measured exoelectrons emission intensity (I, cps). Explain differences. 5.6 Nascent surface Explain the difference in the concept of liquid lubrication mechanism in: (a) hydrodynamic, (b) elastohydrodynamic and (c) boundary lubrication. Which of the following characterize (a), (b), and (c) lubrication regime: continuous fluid film, negligible deformation, complete separation of the surfaces, elastic and plastic deformation, no wear takes place, no contact between the sliding surfaces, involving surface topography, physical and chemical adsorption, catalysis and reaction kinetics, and tribochemical film formation? 5.7 Deposited M0S2 With reference to Fig. 5.2, what is the role of M0S2 in the tribological friction system: (a) pure lubricant M0S2 film, (b) M0S2/M0O3 mixture film? 5.8 Tribochemically formed M0S2 film The tribofilms generated from combination of ZDDP/MoDTC have been recorded in Fig. 5.3 at the (S) L-edge using XANES surface technique. What concentration of ZDDP is sufficient to prevent oxidation of sulfur-to-sulfate and Mo (FV^) to Mo (VI)? The main difference between tribofilms generated from MoDTC alone and mixtures of ZDDP/MoDTC is the lack of sulfate signals in the mixed film. 5.9 Molybdenum additives By using data from Table 5.15, determine a protective tribofilm formation from the degradation of molybdenum dialkyldithiocarbamate (MoDTC), molybdenum dialkyldithiophosphate (MoDDP) directly and in combination with ZDDPs or sulfur compounds.

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217

Chapter 6 ANALYTICAL TECHNIQUES IN LUBRICATING PRACTICES Infrared spectroscopy is one of the most fundamental tools for the study of lubricant degradation andfor condition monitoring of oils during service. John P. Coates, 1984 6.1. Evaluation of the Degradation of Lubricants A liquid lubricant has many functions: (a) protecting engines from wear by minimizing friction with a film formation between moving parts, (b) maintaining cleanliness and acting as a coolant by heat exchange, (c) being lifeblood of the engine to combat internal and external contamination as well as doing its original job of sealing, cooling, and lubricating the engine. The components of the lubricant are predominantly a base fluid containing specifically designed additives. It is these additives which provide the lubricant's functions and also protect the base fluid from degradation, as the fluid transports the additives around the engine. The majority of the base fluids used in modem diesel and gasoline engines are hydrocarbon based and are prone to oxidative degradation (Booser, 1997; Mortier and Orszulik, 1992; Stachowiak and Batchelor, 2001). A mineral engine oil in service will chemically and physically change during the period of service by its exposure to heat and oxygen, the formation of acids, and the catalytic metals it comes into contact with in the application. Oxidation, resulting from high temperature, will change the chemical structure of the base oil and the additives are depleted depending on the length of service and the conditions imposed on them by the applications. Oil in service will darken in color, increase in viscosity, insoluble resins will form deposits, organic acids will be formed. Additives will retard these chemical changes and produce an odor. Dirt can build up in a system, causing accelerated wear. Wear metals can act as a catalyst in the oxidation process as can the acids formed by the oxidation of the oil. Products of oxidation will act as a catalyst for further oxidation. The right principle of lubrication is to put the right lubricant in the right place at the right time. The right oil is determined by many factors (e.g., viscosity, correct additives, operational conditions). The following are examples of a

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detergent composition evaluation of oil A and B performed on gasoline and diesel engines (Salino and Volpi, 1987). Gasoline engine oils A andB are subjected to a 120 h test on a 1500 cc engine designated to assess their resistance to high temperatures. Comparison of the pistons clearly shows that oil A passed the test without incident, while oil B had considerable wear and tear owing to the seizure of pistons. Oil A contained a detergent with calcium and magnesium, 2 : 1 , whereas oil B contained calcium only. The ash content 0.9% and other components were the same. The results clearly show how performance of the engine is influenced by the type of detergent employed. It may be supposed that the seizure observed with oil B resulted from the following: (a) an excessive build-up of deposits on the piston crown causing its overheating; (b) formation of thick lacquer and carbon deposits due to overheating in the piston grooves and the area. In a gasoline engine the contamination comes from many sources (Denis et al., 1997; Schwartz, 1991, 1992 and 1997) such as: (a) fuel dilution from cold starts, short trips, and low engine temperature; (b) by-products of poor combustion or incorrect fuel-to-air ratios and blow-by past the rings in a worn engine; (c) wear metals contribute to the physical contamination as well as act as oxidation catalysts; (d) leaking gasket can allow a combination of water and antifreeze into the lubricating system; the water leaches out some additives, e.g., ZDDP, and vaporizes them when the engine reaches operating temperature, glycol oxidizes rapidly with heat and form deposits in the oil. Diesel engine oils A andB were subjected to a 100 h test on a 1300 cc engine with oil A containing a balanced sulfonate-phenate system and oil B containing sulfonates only (Salino and Volpi, 1987). Both formulations contained combined calcium-magnesium additives and had the same ash level (about 1.5%). This engine was chosen because it is a particularly heavy soot former and thus provides a good source of a reference standard for the cleanliness of the engine and the control of wear. It was evident that oil A resulted in a cleaner piston and less formation of carbon deposits on the piston after test with oil B. Moreover, there were no signs of wear on the cams and tappets after the test with oil A, whereas considerable evidence of wear could be seen on three of the eight tappets of the engine lubricated with oil B. It can be speculated that the antioxidant property of the phenate included in formulation A resulted in better control of the formation of insoluble substances and thus ensured better dispersion of the soot inevitably formed during the combustion of diesel fiiels. Today's diesel car engines are of the prechamber type, as opposed to the direct injection engine used on commercial vehicles. This means that the combustion of diesel fiiel leads to the formation of a large amount of soot, which is the main source of insoluble substances in engine

Analytical Techniques in Lubricating Practices

219

oils. It has also been shown that the effect of antiwear additives is diminished by the presence of soot, since the additives are partly adsorbed by the soot itself and extracted from the oil. In a diesel engine the contamination comes from the above and additional sources such as: (a) soot as a normal by-product of diesel engine will increase the viscosity of the oil and become abrasive as its level increases in the oil; (b) the fuel dilution will reduce the film strength of the oil to a point where accelerated wear occurs; (c) the fuel dilution will reduce the flash-point of the oil; (d) a diesel engine oil will also be exposed to the formation of acid from the sulfurized fuel max.5%; a diesel engine will have a high TBN to assist it in combating the acid formation. In large marine diesel engines, the highest temperatures and pressures occur at a place furthest from the oil feed-point. When sliding partners come in to contact, either due to dynamic effects or inadequate lubrication, the chemical reactions between the oil, its additives, environmental oxygen and the substrate are dominant. The ability to form boundary films that have a lower wear rate and friction than the unlubricated system is thus important. In the diesel engine, the oil, aside from lubricating, must also fulfil other important tasks, such as hindering the deposition and furthering the removal of soot, and neutralizing the acid condensates. The influence of pressure and temperature on the wear rate was examined for an experimental oil containing zinc dialkyldithiophosphate ZDDP as a wear reducing additive and calcium carbonate-sulfonate or carbonate-phenate as a detergent and acid neutralizer. ZDDP generally lowers the wear rate. The calcium sulfonate additions to the motor oil used blocked the wear inhibiting properties of the ZDDP under the existing conditions (Wuthrich and Desponds, 1990). Monitor oil recovery. Looking at changes in the oil deterioration indicators in relation to the distance driven, we see that the TBN and TAN indicate a particular trend of deterioration up to about 5,000 km, but between 5,000 and 8,000 km, the deterioration phenomenon decreases. ASTM D893 describes the centrifuge metod to determine insolubles in used oil at a speed 650 G. The insolubles separate out at a speed of 20,000 G increase in a large uniform proportion, but cannot be separated out at a speed of 650 G determined by ASTM D893. The reason is that insolubles in gasoline engine oil are mainly composed of combustion products and oxidation products, and these are thought to be dispersed in the oil by the action of dispersing agents. So, at a low centrifugal separation speed, only relatively large particles, mainly consisting of abrasion particles, are separated out, and finer particles are missing at low speed. Monitoring these insolubles seperated out at 20,000 G in the oil represents the oil deterioration indicators.

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220

Using the new light-reflecting type of deterioration sensor to accurately detect the concentration of finer insolubles in the oil enables more precise evaluation of oil deterioration. The basic elements of this construction are the light emitting element, the prism, and the light receiving element. There is a correlation between the increase in concentration of insolubles separated out at a speed of 20,000 G and sensor output (Tomita et al., 1995). The oil deterioration mechanism, various deterioration indicators (viscosity change, total base number (TBN), total acid number (TAN), insolubles concentration, etc.) have been proposed and their respective detecting methods were reported. The oil deterioration component is divided into two general categories: deterioration of the base oil and ingression of foreign matter. Ultimately, however, particles are considered to exist in engine oil in the form of insoluble elements. Fig. 6.1 shows the mechanism of the oil deterioration (Tomita etal., 1995). Oil Deterioration Deterioration of Base Oil Exhaustion of (TBN\)

}

Oxidation Product y (TAN / )

Ingression of Foreign Matter Complete Combustion (NOx) Unburnt Fuel

}

Combustion Products

( TBN-^^ Varnish, Resin, Sludge

Wear Particles Molding Sand, etc.

Formation of Insolubles Fig. 6.1. Mechanism ofengine oil deterioration (Tomita etal., 1995)

The remaining useful life evaluation routine (RULER) is a useful monitoring program for used engine oils. The RULER system is based on a voltammetric method (Jefferies and Ameye, 1997; Kauffman, 1989 and 1994). The data allows the user to monitor the depletation of two additives ZDDP and the phenol/amineff antioxidant. The RULER results were compared to other standard analytical techniques, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), total base number (TBN), total acid number (TAN), and viscosity to determine any correlation between the techniques (Jefferies and Ameye, 1997 and 1998). The test concluded that the RULER instrument can

Analytical Techniques in Lubricating Practices

221

rapidly determine the effective antioxidant concentration in used oils. When compared to the original oil, a plot of additive depletion versus oil age can be made, allowing an estimate of the oil's condition to be made. The RULER instrument is a useful monitoring system for engine oils, owing to its rapid measurement, accuracy, low sample volume, and its freedom from interferences such as soot. The engine oil is required to function in a very hostile environment. It is subjected to high temperatures, high shear stress and chemical attack by fuel combustion products 'blow-by'. These products are corrosive and also cause degradation of the lubricating oil, resulting in viscosity increase, the formation of sludge in the bulk oil and carbonaceous deposits on the piston and rings. Effective lubrication of the internal combustion (IC) engine will minimize friction and wear. This lubrication is accomplished by circulation of oil from the crankcase shown in Fig. 6.2 (Marsh, 1977).

Wear

Stud^e

Fig. 6.2. Schematic of the intemal combustion engine (Marsh, 1977) Oil additives must be used to minimize the degradation of the lubricant. Initially, the degradation rate is slow due to the protective action of the additives; however, the rate dramatically increases as soon as the additive package is depleted. Without such additives, modem engines would quickly cease to function. The engine lubricating oil contains a balanced mixture of additives to minimize the effect of deterioration. Inorganic acids lead directly to corrosive wear if not neutralized by basic additives. A particularly critical example is the formation of sulfuric acid from high sulfur content of diesel fuels, such as

Chapter 6

222

commonly used in marine engines. Acidity in the lubricant can also lead to rusting of critical moving parts such as valve lifters, and to degradation of the oil producing insoluble products which are precursors of carbonaceous piston deposits. Acids resulting from fiiel combustion are neutralized by basic additives. Organic acids resulting from oil oxidation processes are deactivated by inverse micelles which are formed in the lubricating formulations (low acidity products are not neutralized). Oxidation is catalyzed by free radicals in the blow-by and by soluble u-on salts formed by reaction of wear debris with oil-soluble acids. Wear taking place in regions of boundary lubrication such as the valve train is controlled by zinc dialkyldithiophosphate antioxidant. Insoluble sludge can build up in the crankcase. This is especially true at low temperatures, when appreciable quantities of dispersed water are present. The surfactant additives are very effective at keeping the sludge materials and the carbonaceous precursors dispersed in oil. Fig. 6.3 (Marsh, 1977) is a summary of the overall processes taking place. Water Fuel combustion-^ Inorganic acids Corrosion rWear Organic Acids Soluble and Lube r Oxidation-^O, S, N, species-•dispersed •Viscosity \ Degradation /'' \ products increase \ Wear

/ • Iron

\ Insolubles

• Varnish sludge

Fig. 6.3. Degradation pathways of lubricating oil in the internal combustion engine (Marsh, 1977) Recent trends in automobile engine design have imposed more severe conditions on engine oils. These include higher temperatures, higher pressures and the presence of increasing amounts of blow-by gases caused by higher compression ratios. Under such conditions, oils deteriorate and acidic components increase, owing to oxidation, thermal degradation, contamination and additive depletion. Engine design, operating conditions and fiiel characteristics are generally considered to be the factors affecting engine oil deterioration (Kimura et al., 1996; Moon and Kimura, 1990). Deterioration of diesel engine oils has been known to promote considerable wear of engine parts through the process of preferential adsorption of ZDDP by soot, thus retarding the formation of wear-

Analytical Techniques in Lubricating Practices

223

preventing films and wear rate increase. It is also possible that some large aggregates of soot intrude into gaps between sliding surfaces and cause abrasion (Hirose et al., 1985; Kawamura et al., 1988; Rounds, 1977 and 1986). In contrast, with gasoline engine oils, it is believed that this deterioration does not have significant effect on their wear-preventing property (Fujita et al., 1983; Kawamura et al., 1982 and 1985); however, used gasoline oils containing hydroperoxides can cause significant cam lobe wear (Habeeb and Stover, 1987; Habeeb et al., 1987; Hsu et al., 1988; West et al., 1986). The oil is periodically replaced to perform its intended functions effectively and to sustain reliability of the components and system. This activity is carried out as a part of preventive maintenance as per the manufacturer's recommendation. The oil change frequency is a function of various factors including operating and environmental conditions, and maintenance practices. The cost of lubricating oil represents an important component of the operating cost of a vehicle and is about 60 to 70% of the fuel cost. The analysis of engine oils is widely used to monitor overall preventive maintenance programs. According to the engine tests by some European manufacturers, engine life is represented as 1,000 hours or 150,000 km for the overhaul period at maximum load and revolution conditions. During 1,000 hours of testing, oil drain is every 150 hours or 15,000 km (Kaleli and Khorramian, 1998). Test of the degree of deterioration of engine oil fall into two broad categories (Fox et al., 1990 and 1991a; Masters, 1995; Pawlak et al., 1985 and 1989; Smolenski and Schwartz, 1994 and 1997).

6,2. Engine Oil Condition Monitoring (I) Physical characteristics of the oil condition monitoring: wear metals analysis, fiiel contamination, viscosity, chromatography, flash point, water content, insoluble test, blotter test, direct reading ferrography, differential scanning calorimetry and colorimetry. (II) Chemical characteristics of the oil condition monitoring.* IR and FT-IR spectrometry by specified chemical groups (oxidation, soot, nitration, sulfation, additives). (I) Physical characteristics of the oil condition monitoring Wear metal analysis. ASTM D4951 and ASTM D5185 (ICP-AES). A wide variety of different metals occur in engine oil. The metal accumulation is a consequence of wear, corrosion, and from the air induction system. The wear metal values change with the maker and model of the equipment and with the type of service, including working environment, drain interval and filter change

224

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interval. Among the techniques that can be used are Graphite atomic absorption spectrometry GFAA, Inductively coupled plasma atomic emission spectrometry, ICP-AES, Inductively coupled plasma mass spectroscopy, ICP-MS and X-ray fluorescence, XRF (Banasal and McElroy, 1993; Hodges, 1975; Jones, 1987; Nadkami, 1991). Metal concentrations are normally low and increase slowly with longer operating periods. Maximum concentration limits for different elements of wear in engine oils were measured (Kaleli and Khorramian, 1998; Smolenski and Schwartz, 1994 and 1997). The content (in ppm) is as follows: Pb

Sn

5-40

5-15

Cu

5-40

Si

Fe

Cr

Al

10-20

40 - 200

10-30

15-40

Ag

Zn and P

Ba, Ca, Mg

Na

B

5-10

> 0.08%

lower than higher than higher than fresh oil fresh oil fresh oil

The "classic combinations" of elements arise due to specific problem identification. Some of the elements such as silicon, iron, chromium and aluminum are from the wear of liners and rings pistons or from the air induction system and contamination. Lead, tin and aluminum are from the wear of bearings and pistons, from lack of lubrication and coolant contamination. A sudden upward change above the maximum concentration limit mentioned above of any metallic element suggests an increased wear rate, and possibly abnormal operating conditions (Erickson and Taylor, 1984). The interpretation of wear analyses is often greatly enhanced by additional tests which detect contamination of fuel, water and antifreeze. Fuel contamination "fuel dilution", ASTM D3525. Excessive unbumed fuel in the oil can reduce oil viscosity and reduce oil-film thickness. A drop in viscosity can, if serious, cause bearing damage. Higher levels of unbumt fuel greater than 2% usually indicate a fuel line leak or plugged injector. Fuel is often accompanied by water and acids which degrade oil. If caused by short-trip driving, the oil-change intervals should be reduced. There are three principal methods (viscosity, chromatography and flash-point) for measuring fuel contamination. Viscosity is one of the most important properties of an oil and is defined as its resistance to flow (Erickson and Taylor, 1984). High-temperature, high-shear viscosity measured at shear conditions typical of operating engines, is determined with a Cold-Cranking Simulator CCS, using ASTM D5293. The pumping

225

Analytical Techniques in Lubricating Practices

viscosity of CCS indicates whether or not an engine oil will flow to the oil pump inlet in order to provide adequate oil pressure during initial stages of engine operation (Smolenski and Schwartz, 1994 and 1997). For a crankcase oil, a high viscosity index is desirable. A high viscosity index will provide more uniformity in viscosity at various temperatures, which in turn will provide consistent lubrication over a wide range of temperatures. At low temperatures, a relatively low-viscosity oil is desired to permit adequate cranking speed during starting and adequate flow to the oil pump and engine during operation. At high temperatures in a running engine, the oil viscosity must be high enough to maintain adequate film thickness between rotating or rubbing parts to minimize wear. Using a higher-viscosity oil generally reduces oil consumption and blow-by, but increases friction associated with oil film shearing. In a used oil, a deviation of less than 25% from the ISO viscosity range is usually considered normal. Greater deviation from this standard value may cause unnecessary heat generation and an increase in energy consumption and wear in internal combustion engines (ASTM D445) (SAE, 1984). As fuel dilution increases, viscosity decreases. Because viscosity is affected by other factors such as oxidation (viscosity increase), long chain molecules suffer from mechanical shear (drop in viscosity); the combination of these factors in used oils makes viscosity measurements rather insensitive and often misleading. Lubricant samples of 15W50 synthetic oil were examined every 2,000 km and continued up to 21,500 km (Kaleli and Khorramian, 1998). Below is an example of oil performance by measuring the Kaleli viscosity changes during (I.C.) engine operation. Trip length, km

0

2000

8000

14000

21500

Viscosity at 40°C, cSt

121.5

115.7

124.8

137.2

155.3

Viscosity at 100°C,cSt

19.0

17.9

18.8

19.5

21.3

The acceptable upper limit at 100°C is 23.8 cSt (SAE, 1984). Gas chromatography (GC): high performance liquid chromatography (HPLC), and a variety of detection devices (infrared, ultraviolet, fluorescence and mass spectrometry) are used for identifying the specific compounds eluting from the HPLC column, or the specific chemical compound represented by a given GC peak (Adlard and Matthews, 1975; Hodges, 1975; Norris, 1979). Following are examples of the application of gas chromatography to problems in the analysis of petroleum or petroleum products: (a) the determination of water and glycol in used engine oil - a known amount of acetone is added to the sample as an internal standard, and the areas of the water and glycol peaks are compared to the area of the acetone peak to determine percent of water and glycol, (b) the determination

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of gasoline dilution in lube oil, (c) the determination of diesel fiiel dilution in lube oil, (d) the ability of GC to distinguish between viscosity grades of SAE lOW, 20W, 40, and lOW-40 motor oil. The GC-mass spectrometric analysis can detect and identify specific solvents or diluents in lube oils and thus precisely pinpoint the source of contamination. Thin-layer chromatography (TLC) has been used for estimating the remaining useful life of engine oil including depletion of the multifunctional additive ZDDP. It was found that the soot from the oil did not interfere with the spot intensity since these particles did not travel with the mobile phase. The TLC technique has the potential to be a good supporting technique for estimation of chemical additives depletion (Brook et al., 1975; Coates, 1971). This technique is comparatively easy, very cheap, does not need sophisticated and expensive instruments and takes less than an hour. Flash-point (FP) is the lowest temperature at which the substance gives off sufficient vapor to form an inflammable mixture with air. For used engine oils, FP indicates contamination of the oil by a volatile product such as gasoline or diesel fuel (SAE, 1984). Fuel contamination in engine oil such as fuel dilution will reduce oil viscosity and oil-film thickness. Excessive fuel dilution is normally taken as an indication of fuel system problems and the need for an oil drain. When an oil is diluted, no adequate hydrodynamic lubricating film can be maintained between the moving parts, resulting in increased wear and possible bearing failure. The flash-point method, ASTM D93, is quick and sensitive and a small fuel concentration 1%, will reduce the lube oil's flash-point. In used oil, a drop by more than 30% in flash-point shows an excessive dilution. Lubricant samples of a 15W50 fully synthetic oil were examined every 2,000 km and continued up to 21,500 km (Kaleli and Khorramian, 1998). Following is an example of oil performance by measuring the flash-point changes during an IC engine operation. Trip length, km

0

2000

8000

14000

21500

Flash-point at (°C)

210

168

140

138

128

The acceptable lower limit is 150T (SAE, 1984). There is 30% decrease in flashpoint, which is still at the acceptable level. The optimum oil drain interval for a fully synthetic 15W50 was found to be 11,000 km. Water content higher than 0.2% levels needs to be recorded as it can indicate a severe coolant leak. The crackle test is a useful screening test for the presence of water in oil. The test can be conducted by placing a few drops of oil in a small

227

Analytical Techniques in Lubricating Practices

cup made from aluminum foil and heating rapidly over a small flame or on a hot plate. When a positive "crackle test" is observed, a quantitive test for water by distillation ASTM D95 or by a Karl Fischer coulometric titration ASTM D4928 should be made. The presence of water in a lubricant system is indicative of contamination through leaking seals, blow-by of combustion gases, coolant seepage or improper storage or application of the oil. Free water is a prime cause of rusting, sludging and impaired lubrication. Excessive water contamination in engine oil can cause increase wear in an engine. Water also causes corrosion and "additives drop out," that is precipitation of additives from the oil (Warning limits: > 0.1% to > 0.5% long-trip service; > 4 %o water observed in oil in short-trip service without engine failure (Smolenski and Schwartz, 1994 and 1997). Test on insolubles, ASTM DS93. Solids insoluble in toluene are generally considered to be soot while those insoluble in pentane include soot and resins, mostly result of oxidation processes. Under long-trip service conditions, high content of pentane insolubles generally indicates that the dispersant in the oil is no longer performing its intended function. Under extreme short-trip winter driving conditions, water can condense in the engine oil and ASTM D893 registers the water as if it were insoluble material (warning limits: > 1% to > SYo; values > 8% found in short-trip service without engine failure), (Smolenski and Schwartz, 1994). The test for oils with a high content of detergent is not always reliable. Any wear metals, debris and dirt will also count as insolubles, leading to misleading results. Blotter test. This involves putting a spot of oil onto a piece of absorbent paper then heating it to allow the oil to spread. The darkness of the developed spot gives an indication of the amount of soot while the extent to which it travels indicates the state of dispersion. The paper test is qualitative, but can be quantified photometrically. Direct reading ferrography studies were carried out to investigate particle number distribution in large (5yum) and small (1 to 2yum) wear debris (Hunt, 1993), Trip length, km

2000

4000

7000

9000

11000

13000

Optical density of large particles (5 ijva)

44

55

68

73

105

130

Optical density of small particles (1 to 2//m)

35

40

50

57

75

100

Initially, particle optical density values increased slowly, but in the vicinity where the additives get depleted (-10,000 km), particle density values increased

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dramatically indicating excessive wear of lubricated components (Bijwe et al., 2000). The abrasivity of surfaces is related to the shape of surface profiles and can be described numerically. The morphology of wear particles, in terms of their shape, size, and surface texture, reflects the complex nature of the wear process involved in particle formation (Stachowiak, 1998 and 2000; Stachowiak and Podsiadlo, 1999 and 2001). The cumulative evaluation of turbine lubricating oil system, using techniques such as automatic particle counters, ferrography, ICP-AES spectroscopy, and scanning electron microscopy indicated the involvement of very small iron particles in the size range of 1-10 microns or less, and abrasive wear silica particles in the size range of 10-40 microns (Korycki and Wislicki, 1991). Differential scanning calorimetry DSC and colorimetry has been used to estimate stability of lubricants and the antioxidant package (Fox et al., 2001); Kauffman and Rhine, 1988a). However, using pressurized conditions to suppress evaporation of oil in a DSC cell has proved to be a more effective tool than ordinary DSC. The sealed pan differential scanning calorimetry (SP-DSC) technique which uses an oxygen purged sealed pan, has been recently developed and has been exploited for analyzing three types of oxidized oils under induced stress in the laboratory (Bowman and Stachowiak, 1996). Lower instrumental cost, easy operation and high repeatability compared to high pressure DSC (HPDSC) are reported to be its major advantages. Table 6.1. The induction temperatures T^ (temperature at which oxidation starts) are given below for SAE l o w 30 engine oil. Trip length, km

0

4000

8000

10200

11000

12800

Induction temp. Ti(°C)

286

233

224

218

207

196

The data indicate that the higher the induction temperature (Tj) the higher the oxidation stability and the residual useful life, RUL. With continued use, antioxidants get depleted and oil becomes more and more oxidized. The criteria, i.e., 30 % reduction in T^, indicate the end of useful life for an oil system. In contrast to the DSC method, the colorimetric method determines the hydroperoxide decomposing capabilities of the antioxidant species in the oil sample (Enmanji, 1979). The SP-DSC has a lower instrumental cost and is simpler and safer to operate, while the HP-DSC technique has a lower operational cost and does not require sample preparation (shorter total analysis time). The colorimetric method is much less expensive and much easier to operate in comparison to DSC methods. It is capable of accurately predicting the operating time at which oil samples possess 0 hours of the residual useful life (RUL).

Analytical Techniques in Lubricating Practices

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Table 6.1. Comparison of the high pressure (HP) and sealed pan differential scanning calorimetric techniques (SP-DSC/ HP-DSC) and the colorimetric technique for oil change requirements (Kauffinan and Rhine, 1988a) Parameter

High-pressure DSC

Sealed pan-DSC

Colorimetric

Sample size Sample preparation Procedure Safety Measured parameter

2.5^1 No Complex High O2 pressure Oxidation time

0.2/^1 Yes Simple No Oxidation time

Remaining useful life Instrument cost

Linear $20000 $0.30

Nonlinear $15000 $0.80

15/^1 Yes Simple Toxic chemicals Decoloration induction time Nonlinear $2000 $0.08

Operational cost per test

High frequency oscillator procedures have been used to determine TBN and TAN. These give sharp breaks in the end point and indicate TBN/TAN values comparable with those obtained potentiometrically (Caughly and Joblin, 1969; Fernandez et al., 1978 and 1979). Conductometric analysis using a 2-propanol/toluene/water solvent mixture in the titration of fresh and used oil samples (Armitage et al., 1987, Pawlak, 1980) and benzene/alcohol solvent mixture for the titration of phenolates and sulfonates (Labre and Briant, 1960) have been found to give satisfactory results; however, benzene is a class A carcinogen and is no longer used in most laboratories. Conventional thermometric titrimetry was used to determine base and acid content in light and heavy petroleum derivatives, including lubricating oils (Quilty, 1967), crude oils (BorruU et al., 1986a) and petroleum distillates (Borrull et al., 1986b; Greenhow and Nadjafi, 1979). Studies show that the acids in the oils not determined by the thermometric method are weaker than pKa(H20) =10. Voltammetry. The voltammetric techniques are based on the current-voltagetime relationship at microelectrodes. To perform voltammetry, the oil/antioxidant sample is dissolved in a solvent containing an electrolyte and a three-electrode system (glassy carbon working electrode, a platinum wire reference electrode, and platinum wire auxiliary electrode) is inserted into an oil/solvent solution. A fresh oil typical of the application (100% standard) and the solvent system (0 % standard) is used to calibrate the voltammetric instrument for % remaining antioxidant determination (Kauffman, 1989 and 1991). The oil sample (0.1 to 0.5g) is diluted with 5 mL of acetone or ethanol containing a dissolved electrolyte and + 325 mesh sand (Ig). For aromatic amine-

230

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type antioxidants (aircraft engine oils), the solvent system was acetone and the electrolyte was lithium perchlorate. For hindered phenol-type antioxidant (diesel engine oil and hydraulic fluid) and phenol degradation products (phosphate esters), the solvent system was water/ethanol and electrolyte was potassium hydroxide. For ZDDP-type additives (diesel engine oil and hydraulic fluid), the solvent system was water/acetone solution and the electrolyte was lithium perchlorate. When the oil/solvent/sand mixture was shaken (-20 seconds) to get complete reaction, the insoluble oil coated the sand and, upon standing, the agglomerated particulates quickly settled out to produce a clear solution for analysis. The voltage of the auxiliary electrode was scanned from 0.0 to 1.0 V at a rate of 0.5 V/second. The height (or area when integration available) of the peak(s) produced by the voltammetric method for the fresh (100%) and used oil or fluids was then used to calculate the percent of remaining useful life of the tested fluid parameter TAN and TBN (Kauffman, 1998): TAN = (BLNK rdng - Smp rdng)/g of smp x (0.28 mg KOH /rdng Diff ) TBN = (Smp rdng - Baseline rdng) x (TBN fresh oil /rdng Diff) Where: BLNK = blank; Smp = sample; rdng= reading; Diff = difference The voltage of the auxiliary electrode is increased linearly with time and the current produced at the surface of the working electrode is recorded as a function of voltage. When the electrode voltage equals the oxidation potential of an antioxidant in the diluted oil, the antioxidant undergoes electrochemical oxidation increasing the current flow at the working electrode and producing a peak in the voltammogram. The voltage of the peak depends upon the concentration of the antioxidants. The sensitivity (peak height) of voltammetry to antioxidants is also dependent upon the pH of the solution: e.q., TAN and TBN analysis is performed in ethanol/acetone solution in the presence of HCl acid. The TBN and TAN voltammetric techniques require less than 1 g of oil, 5 mL of acetone or ethanol solution, and less than two minutes for each analysis, and are performed in vials using a portable instrument. The voltammetric techniques do not involve titrations and, consequently, do not rely on endpoints which are operator dependent. On-board type oil deterioration pH sensor. Considering practical use as an on-board type pH sensor, it is desirable to use rigid materials for the electrodes which are thought not to be damaged due to mechanical vibrations of the engine. Therefore, oxidized stainless steel and lead electrode were selected as the pHresponse electrode and the reference electrode, respectively. The signal of the newly developed sensor gradually increases as the deterioration of the oil

Analytical Techniques in Lubricating Practices

231

proceeds. When new SD-grade oil was used, the sensor signal was 0.5 V; the sensor signal increases to about 0.95 V after traveling 4,000 km. In case of using new SG-grade oil (0.5 V) the sensor signal was about 0.85 V after traveling 6,000 km. The relationship between the sensor signal and the pH value of the used-oil (8 brands, SD and SG grades) diluted with a solvent was investigated by mixing toluene, 2-propanol, and water (method ASTM D2896). As the pH value of the oil diluted with the solvent-mixture decreased, the sensor signal increased and a good linear relationship was observed. TBN used-oils obtained by the titration analysis of the test oils are also calibrated with sensor signals (V). The sensor signal is about 0.5 V for new oils and increases to about 0.95 V to the oils condition where TBN equals 0. The proper oil change can be accurately recommended with no relation to brands and grades (Morishita et al., 1993).

(II) Chemical characteristics of the oil condition monitoring Oil condition monitoring is aimed at measuring the changes of oil deterioration. Monitoring can help identify engine problems, but oil condition monitoring can also be used to determine optimum oil change periods. During use, engine oil will become degraded due to the effects of heat and contamination by combustion gases. This is the result of the oil being in various high temperature zones of the engine in the presence of oxidizing atmosphere (air and often nitrogen oxides). Oxidation is a complex process and this is compounded by the large number of chemical species that are present in the oil. The outcome of these effects can be described as: oxidation, soot, nitration and sulfation (Booser, 1997). Infrared spectroscopy IR. Infrared spectrometry is one of the most fundamental tools for the study of lubricant degradation and for condition monitoring of oils from a used lubricant (Coates, 1986; Coates and Setti, 1983, 1984 and 1985; Coates et al., 1984). It is well established as a quantitative technique for the analysis of additives in lubricants. Infrared spectroscopy involves passing a beam of infrared light through an oil sample and measuring the fraction of radiation absorbed at various wavelengths, to provide a spectrum which yields much qualitative information on the oil condition. When a sample of fresh oil is available for comparison, differential IR can be used to obtain semiquantitive information on the extent of oil contamination (fiiel, water, glycol, soot), oxidation, nitration, and the concentration of various oil components (Smolenski and Schwartz, 1994 and 1997). All three contributions to oil deterioration: oxidation, nitration and sulfation, produce measurable effects in the IR spectrum. Oxidation causes an increase in the carbonyl absorption peak at 1730 cm"\ nitration increases the nitro absorption at 1630 cm\ and sulfation increases sulfate

Chapter 6

232

absorption at 1150 cm'^ A summary and assignments of bands and regions of formulated and used lubricants is given in Table 6.2 (Bellamy, 1980; Coates, 1971 and 1986; Coates and Setti, 1984; Coates et al., 1984).

Table 6,2. A summary of main infrared bands and regions of formulated and used lubricants Absorption, band, cm'^

Functional group

Product identification

3500 3600-3000

0-H 0-H

Phenol Glycol, alcohols

2800-2200

OH...H

H-bond aldehydes

1850-1600 1790-1770 1745-1735 1720-1715 1705-1680

C=0 C=0 C=0 0=0 C=0

Carbonyl comp. Lactones Esters Carboxylic acids Aldehydes, ketones

1610-1550 1630-850

coo-

Carboxylates Organic nitrates

0-N=0

Origin

Interferences

Antifreeze

Phenolic acid (broad)

Oxidation Oxidation Oxidation Oxidation

Imide ester Polymetacrylate

Succinimides Succinimides

Nitrooxidation

Alcohols, lactones

1270-1000 1150

ethers, sulfates

1070-1030 810-780 860-855

C-0 C-H

1010-950 675-625

P-O-C P=S

CO32-

Ethylene glycol Aromatic Carbonates

Antifreeze

Phosphorus additive

Antiwear additive Antiwear additive

ZDDP

Antioxidant Aromatic

Basic detergents

Fourier transform infrared spectroscopy FT-IR. The measurement of individual degradation products with FT-IR is very simple, quick and precise. A reference sample spectrum of new oil is required to subtract electronically from the oil sample spectrum. The spectra of the fresh oil and the used oil sample are obtained individually in the same cell. The results - both spectra and the "differential" spectrum are stored in the computer in absorbance format, a form that varies linearly with concentration. The spectral data is evaluated in terms of degradation/contamination and additive depletion by observing either positive or negative absorbance values.

Analytical Techniques in Lubricating Practices

233

After a short period of use in the average engine, changes start to occur. Initially, a loss of the zinc based antiwear/antioxidant additive ZDDP is observed by negative absorptions at 1000 cm"^ and 715 cm^ Oxidative degradation of oil follows soon after and this is observed by positive absorptions, represented by carbonyl, hydroxy, nitro and C-O- species. The IR spectroscopy of lubricants can reflect additive depletion and the formation of oxidation products (Coates and Setti, 1984; Coates et al., 1984). IR monitoring of oxidation process. Monitoring lubricants by infrared spectroscopy is a well established technique. The infrared spectra of oxidized (used) engine oil samples can be split into three parts: (a) above 1900 cm"^ (b) 1900 to 1500 cm"^ and (c) below 1500 cm"\ The spectral changes in the region between 1900 to 1500 cm'^ in commercial automotive oils (SAE lOW/40, API service SE) operated in a Toyota 20R engine over a 8000 km period were evaluated (Coates and Setti, 1984). Major absorptions bands (1900 to 1500 cm'^) of the spectra are [cm"^]: 1732 (oxidation, carbonyl esters), 1710 (oxidation, carbonyl ketones/acids), 1629 (nitrate esters), 1605 (carboxylates) of used oils. Substantial increase in the carbonyl absorbency at 1720 cm"^ is a measure of carboxylic acid formation. Recent studies of the infrared spectra of oil oxidation products have shown that this type of evaluation is not representative (Coates and Setti, 1983 and 1984). For most inhibited oils the process is very complex and a wide variety of carbonyl containing oxidation products is typically formed covering the spectral region from 1800 cm'^ to 1550 cm'^ with the formation of esters, ketones, lactones, carboxylates, etc. It is more meaningful to use an integrated area over the carbonyl region. In this way, all carbonyl containing materials may be monitored together. Under the conditions of high temperatures and exposure to air, oil undergoes partial oxidation to form a range of components which eventually separate out as varnishes and sludge. Not only do these cause deposits and an increase in viscosity, but they are also weak acids. Oil antioxidant/antiwear additives may be depleted. The FTIR spectroscopic method was developed to rapidly and quantitatively determine the contribution of carboxylic acids to the total acid number (TAN) of lubricating oils (Dong et al, 2000 and 2001). In the analysis of oil containing nonCOOH acidic components (e.g., nitrogen or sulfiir-based additives), the FTIR values obtained will not match the TAN values provided by the standard ASTM method, as the FTIR method devised only measures COOH groups. As such, it may be necessary in the future to qualify the analysis by using a term such "Carboxyl Value" (CV) rather than TAN. In the case of monitoring the relative changes in TAN with time in used oils, the FTIR results should largely parallel the chemical TAN results. The changes in the carboxylic acid contribution values were found to be directly related to the soot levels and inversely related to the amount of ZDDP antiwear additive present. The analytical procedure consists of

234

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splitting the sample into two 5 g portions, adding 10% (w/w) KOH/hexanol stock solution to one, in order to convert the carboxylic acid to carboxylate salt, and 0.38 g of pure hexanol to the other. The two samples are mixed for 30 seconds in vortex mixer, aspirated in the IR cell, and scanned for one minute. When the two scans are completed, spectral data are automatically processed to present the final TAN value, the total analysis time being about 5 min/sample. The FTIR method, when programmed and automated using a continuous oil analysis and treatment (COAT) system, provides a simple and accurate means of determining carboxylic acid contributions to TAN values (Dong et al., 1997, 2000 and 2001). Soot is generally the main cause of oil deterioration in diesels. This is the result of partially burnt fuel (smoke) which becomes dispersed in oil as sub-micron sized particles of carbon. Soot interferes with antiwear additives, can increase cam and lifter wear, oil viscosity, and low temperature starting problems. The detergent and dispersant additives in oil coat the particles and prevent coagulation into larger particles. As the soot level increases, these additives become exhausted resulting in an increase in viscosity and tendency to form deposits on the pistons. Engine failures have occurred with as little as 3% soot. The level of the soot content was quantified by FTIR spectroscopy (Coates et al., 1984; McGeehan and Fontana, 1980). Soot was measured by determining the difference in the base line values at 2200 cm'^ and 1817 cm"^ this base line shift correlates with relative changes in soot content. The amount of the soot content is obtained by subtracting the absorbance at 1817 cm'^ from that at 2200 cm'^ (Dong et al., 2000 and 2001). Nitration. Substantial increase in the nitro-absorption at 1630 cm"^ is observed, but it is known to interfere with the measurement of carboxylate carbonyl compounds. This is visually distinguished as a narrow band around 1630 cm"^ that is overlapped by the broader carboxylate band that occurs about 1600 cm" ^ (Coates et al., 1984). One way to overcome this problem is to take the second derivative of the difference spectrum. Because of high combustion temperatures particularly in natural gas engines - nitrogen in the air becomes oxidized, forming nitrogen oxides (NOJ. Upon reaction with oil, these also form varnishes, acids and an increase in viscosity. The increase of blow-by gas flow due to excessive nitro-oxidation wear of engine components causes severe nitro-oxidation in heavyduty diesel engine load conditions, very similar to that observed in gasoline engines (Iwakata et al., 1993). The blow-by gas contains NO^ from the combustion gas and its concentration is 1/7 to 1/3 of that in the exhaust gas. A strong absorption of nitrate ester (1630 cm"^), that is generally not found in the oil from new engines, is in the infrared spectrum of the oil deteriorated under excessive wear and high load.

Analytical Techniques in Lubricating Practices

235

Sulfation. The formation of sulfate is easily explained in diesel systems when the fuel has a high sulfur content (>0.5%). Substantial increase in the sulfate absorption at 1150 cm"^ tends to be overlapped with absorption from oxidation products (Coates et al, 1984). It is sometimes difficuh to rationalize that the sulfate formed is solely fuel related. Results from gasoline engine samples also indicate a high degree of sulfate formation. The fuel used, however, typically contains an order of magnitude less sulfur than the average diesel fiiel. One possible explanation is that sulfur compounds in the formulated lubricant have been oxidized to sulfates (Coates et al., 1984). When the fiiel containing sulfur is burnt, the resulting sulfur oxides produce strong acids which can cause corrosion problems if not neutralized. This is principally a problem with high sulfur (>0.5%) diesel fiiels and sewage gas methane (high in hydrogen sulfide), (Fox et al., 1990; Pawlak et al., 1988). IR and XANES monitoring of additive performance. A hindered phenol antioxidant and a zinc dialkyldithiophosphate are considered. Changes in hydroxy species (O-H) at 3650 cm'^ during the oxidation of antioxidant (2,6-di-tert-butyl-4methylphenol) showed decay with time (Coates and Setti, 1984, Spitz, 1980).

0««

»ySf«-m.It

FR£SM OK

Fig. 6.4. Plot of IR absorption peak at 979 cm"' for various distances indicating residual for oil systems I and II. P, = recommended engine oil periodicity, P^ = newly recommended periodicity level (Bijwe et al, 2000)

236

Chapter 6

The oxidation of ZDDP inhibited oil was studied for depletion of two absorption bands characteristic of the additive at 975 cm"^ (P-O-C) and 646 cm "^ (P-S), (Willermet and Wright, 1979) and by XANES techniques (Yamaguchi et al., 2002b). The infrared (C=0) band has been associated with the formation of oxidation products. Differential IR spectra (difference between peaks of base stock and used oil) for all the oils indicating ZDDP absorbance peak at 979 cm'^ are shown in Fig. 6.4 (Bijwe et al., 2000). By calculating the peak areas and assuming 100% area for ZDDP in the fresh oil, the percentages of ZDDP left were calculated and are shown below. Peak areas at 979 cm'^ showed fairly linear behavior with the amount of ZDDP. System L Oil was taken from a Mazda vehicle with Mazda diesel engine with specifications 4 cylinders in line, overhead valves (OVH) with brake horse power (BHP) 86.5 at 3000 rpm and torque 22.9 Kg.m at 2000 rpm. Mazda, km run

0

4000

8000

11000

12200

13200

(%) ZDDP active

100

23.6

15.3

7.2

5.1

2.0

System 11. Oil was taken from a Toyota (model 13 B) with Toyota engine, with specifications of 4 cylinders, direct injection (DI) diesel engine with output 90HP at 3400 rpm and max. torque 22.5 Kg.m at 2200 rpm. The average length of a trip in each case was approximately 80 km. SAE 10W30 grade oil was used in both the diesel engine vehicles and was identical in all respects including additives. In the long term, it mainly showed that the engines were operating correctly even though the oils acted differently. Toyota, km run

0

4000

9000

11000

12200

12600

Active ZDDP (%)

100

58

24

5.0

0.72

0.72

Absorption bonds due to water, fuel and ethylene glycol can also be used to indicate their presence. Two absorption bands that occur nominally at 1070 cm"^ and 1040 cm'^ are characteristic of ethylene glycol and a large absorption is also present around 3400 cm"' for water/glycol presence. Any contamination of unburned fuel residues is present as non-volatile aromatic hydrocarbons. Three absorption bands were selected that were characteristic of the gasoline fuel: 3016 cm-^, 803 cm"' and 469 cm"' (Coates and Setti, 1983). Another quantitative spectroscopic technique for the analysis of additives in lubricants and hydrocarbons is Raman spectroscopy (Coates, 1975). Theoretically, Raman spectroscopy should be as good as infrared for quantitative analysis. The

Analytical Techniques in Lubricating Practices

237

intensity of a Raman band varies as a linear function of concentration, which is in contrast to the logarithmic relationship experienced with infrared. This difference in spectral output is important when considering the concentration ranges that may be determined for each technique. Raman is most suited to the determination of the compounds at concentrations in excess of 5%. Two additional aspects should be considered when analyzing additives in oils: (a) Many functional group vibrations are inherently weak (e.g., C=0 and C-O-C vibrations). This can reduce the sensitivity and selectivity of the technique for the analysis of many additives; (b) Matrix interferences such as fluorescence, sample color, and suspended particles restrict direct application of quantitative Raman to lubricant applications. In many cases, an indirect approach which involves a sample clean-up procedure, such as chromatography, has been adopted. Recently, supercritical fluid chromatography was found to be very useful for analyzing ZDDP in oils (Barnes et al., 2000). The first use of XANES spectroscopy was applied for examining zinc dithiophosphate depletion in used diesel engine oils (Yamaguchi et al., 2002b), since it had previously been used for surface studies only. The XANES data for phosphorus and sulfur K-edge have been obtained for Cummins engine oil samples at 50 hours, 100, 150 and 200 hours. The fresh oil spectrum (P) K-edge has a single peak a at 2151.5 eV which corresponds to ZDDP, however, moving to the 100 hour, 150, and 200 hour spectra, a new peak Z? at 2153.5 eV begins to grow. This new peak corresponds closely to the spectrum of zinc phosphate. The (S) K-edge XANES spectra of the fresh oils indicates two major peaks, a and b. Peak a is closely related to the ZDDP (reduced form S"^) and peak b corresponds to sulfonate. After 200 h engine test oil samples, peak b grew in intensity and also gradually moved to higher energy. The energy position of peak b in spectra of 200 h is very close to that of calcium sulfate. This suggests that sulfur from ZDDP and calcium sulfonate are oxidized to sulfate as oil works in the engine. The XANES technique is very sensitive nondestructive technique for monitoring the ZDDP consumption in engines. Comparison of this XANES technique with a more traditional technique, ^^P nuclear magnetic resonance (NMR) spectroscopy, showed that XANES spectroscopy has data consistent with NMR.

6.3- Oil Acidity and Basicity The acid-base determination of petroleum products in media of low polarity with micellar properties has not been an easy analytical problem to solve (Fox et al., 1987 and, 1991a; Pawlak et al, 1989). Many lubricants for internal combustion engines contain high content calcium carbonate, calcium borate

238

Chapter 6

additives, ashless dispersants and nitrogenous polymeric compounds. Metal carbonates remain in these media in colloidal form (Abbott and Farley, 1968; Giddings and Barrett, 1971; Pawlak et al., 1989; Salino and Volpi, 1987). Analysis of phosphate, sulfonate or phenolate using the ASTM D-664 HCl method (Annual Book ASTM, 1985) has shown approximately 10-40% less base content than is obtained using the ASTM D-2986 HCIO4 method (Abbott and Farley, 1968; Giddings and Barrett, 1971; Toida and Uchinuma, 1970; Uchinuma et al., 1969a, b). Determination of colloidal carbonates in mixtures of chlorobenzene and glacial acetic acid with perchloric acid as a titrant has been successful (Beckett and Tinley, 1961; Fox et al., 1991a; Rimmer, 1965). The TBN and TAN are used as criteria for the remaining service life in used oils. While a modem additive-type oil may have both acid and base numbers, it is recognized that there is no valid correlation between these numbers and the corrosiveness of the oil in service (Asseff, 1977; Thorton, 1945). In petroleum oils, no ionization of acids, bases or salts takes place. Thus, the pH concept cannot be used directly (Frewing, 1962); however, the properties of base and acid mixtures in low-polar media have been studied extensively (Pawlak et al.,1989). Information from these studies has been used to overcome the difficulties inherent in analyzing petroleum products. Many laboratories have modified the procedure by dissolving the petroleum product in alcohol (Evens and Davenport. 1931) or in mixtures such as dimethyl sulfoxide-chlorobenzene (Jantzen, 1971; Kahsnitz and Mohlmann, 1967) and isopropyl alcohol-toluene-water (Lykken, 1946; Lykken et al., 1944; Pawlak et al., 1989). Potentiometric (Evens and Davenport, 1931; Ferguson, 1950; Gibb and Gibbson, 1959; Jantzen, 1971; Kahsnitz and Mohlmann, 1967; Lykken et al., 1944; Lykken, 1946; Rescorla et al., 1937), conductometric (Labre and Briant, 1960; Pawlak, 1980), high frequency (Fernandez et al., 1978 and 1979), calorimetric (Borrull et al., 1986a, b; Quilty, 1967; Wasilewski et al., 1964), voltametric (Kauffman, 1998) or chemical indicator (Annual Book ASTM, 1985; Ferguson, 1950; Gibb and Gibbson, 1959) methods are used to detect the end point. Convenient ways of expressing the base additives or acid additive/degradation product concentrations in petroleum products are the concepts of total base number, TBN, and total acid number, TAN (IP Standards, 1969; Lykken et al., 1944). The oil oxidation, nitration and sulfation products all produce acids which can cause corrosion. Engine oils therefore contain basic (alkaline) additives which can react with and neutralize these acids. Because not all the bases measured are of equal strength, and some are functioning rather as dispersants, the condemning limit is not zero TBN but rather a pragmatic 50% of the original value. This value can vary from one oil to another depending on the particular blend of additives used, A single TBN measurement is of practically no value as it gives no information about how corrosive the oil is and how long will it last. The 50% rule

Analytical Techniques in Lubricating Practices

239

is used because not all the bases have the same strength. Approximately 50% of the bases measured will actually be the salts of weak acids and so of little NEUTRalizing ABility (NEUTRAB). NEUTRAB

=

TBN

-

[weak bases]

Relating TBN to sulfur also presents problems as sulfur is not the only cause of TBN loss, as oxidation, nitration and even halogens can have a very important effect on gasoline engine oils. Diesel engine lubricants require a minimum level of TBN; the accepted critical value is 1 mg KOH g"^ oil for each 1% of sulfur in the fiiel. Variations of the basicity concentration with time during a diesel engine run have been predicted (Dyson et al., 1957). The quantities of concern are fuel, oil consumption rates, fiiel sulfur content and oil charge. TBN

=

S (1 + 0.35 f y Q )

where TBN = base concentration of the oil, in mg KOH g"^ oil; S = percentage of fuel sulfur content with arbitrary addition of 0.1% by weight; f = ratio of fuel consumption to oil consumption; y = percentage conversion ratio, the ratio between the rate of neutralization of additive in the crankcase and the chemical equivalent of the fiiel sulfur throughput; Q = safety factor depending on the makeup procedure. High values of TAN and low values of TBN, compared to those of the fresh oil, indicate that the oil has lost some of its ability to neutralize acids. When the TBN is less than 2 (ASTM D2896), the oil reserve alkalinity is depleted; when the TAN is greater than 7 (ASTM D664), the oil is acidic and may not adequately protect the engine from corrosion (Smolenski and Schwartz, 1994). The total base number (TBN) provides a measure of the remaining activity of alkaline agents such as detergents and other basic components that neutralize acid (such as SO2) generated in an IC engine. TBN of a used oil indicates the usefulness of the oil to neutralize acids from blow-by. Oils designated for extended operation under severe conditions have a high alkalinity. In order to avoid corrosion, engine oils should be replaced when their TBN falls to 50% of the original level (ASTM D445), (Erickson and Taylor, 1984). As the basic additives neutralize acids, the resulting products (salts) are not completely neutral but slightly acidic. As the most effective (strongest) bases are used up, the remaining (weaker) ones produce progressively more acidic products. The TBN method using 2-propanol/toluene/water as a solvent and hydrochloric acid as titrant served for a number of years. Initially presented as a tentative method in 1942, after several revisions it was published as an accepted IP method: IP 177 (ASTM D-664). In 1964, however, serious discrepancies appeared when

240

Chapter 6

new additives were used. For chlorobenzene/acetic acid solvent media, all bases weaker than acetate give no inflection in the titration curves. Many users modified the IP 177 version by using a back titration method (Abbot and Farley, 1981) or, alternatively, by using an acetic acid-chlorobenzene solvent and perchloric acid as a titrant (Giddings and Barrett, 1971). The latter was proposed in IP Standards (1969) and as a tentative standard method IP 276 (or ASTM D-2896) in IP 171, and subsequently accepted as a full standard. IP 177 has been found to give low TBN results with poor repeatability for many additives (Giddings and Barrett, 1971). In particular, low results have been obtained for highly over-based products and those containing nitrogenous polymeric dispersants. Hydrochloric acid, HCl, as a relatively weak acid in 2propanol, interacts incompletely with some basic compounds. Inflections in the titration plots are often obscured or missing but this problem does not occur with the stronger perchloric acid in the IP 276 method because the reaction is rapid and complete. TBN values obtained for the same oil sample using the IP 276 method are higher than those obtained with the IP 177 method. These differences in TBN values obtained for the different titration methods mean that the neutralization goes to completion in the former. Differentiation between TBN values is observed when the sample contains additives with different basicities, for example, calcium sulfonate, carbonate and hydroxide (Luneva and Pavlova, 1977; Lyashenko et al., 1973; Monin and Pavlova, 1978). TBN determination of diesel engine oil using three different acids, HCIO4, H2SO4 and HCl in chlorobenzene/acetic acid and toluene/2-propanol/H20 solvent mixtures respectively, exhibits three curves which give TBN values in the order HCIO4 > H2SO4 > HCl (Hooks, 1975). The total acid number (TAN) is a measure of the concentration of acids in oils. TAN arises from the weak acidic components of the fresh oil and acids formed during combustion and oil oxidation. High values of TAN and low values of TBN, compared to values for the fresh oil, indicate that the oil has lost some of its ability to neutralize acids, and corrosion of engine components is more likely to occur (Smolenski and Schwartz, 1994). The TAN is a measure of the amount of all these acidic products but cannot be thought of as complimentary to TBN (as TBN decreases, TAN increases). As the name implies, TAN measures acids of all strengths. Provided some measurable bases remain, all these acids will be weak and cause little problem; however, if the TBN decreases to the point where free acids such as sulfuric and nitric acid are no longer neutralized and as strong acids they will subsequently cause corrosion. No oil should be allowed to degrade to the point where strong acid number (SAN) exists. A number of labs find the TAN test more useful than TBN simply because detecting the limiting value is easier and less ambiguous. A positive TBN value indicates the absence of free strong acids. If free acids of the strength of sulfuric acid are present, then a TBN of zero is obtained.

Analytical Techniques in Lubricating Practices

241

Several methods for measuring TBN and TAN have been reported; the most widely used are summarized in Table 6.3, some as standard methods, some as proposed methods (Armitage et al., 1987; Pawlak, 1980).

Table 6.3. Techniques and methods for the determination of total base number (TBN, mg KOH g-^ oil) and total acid number (TAN , mg KOH g' oil), (Fox et al., 1991) Technique

Methods, medium, titrant

Comments

Emf

TBN , TAN: IP 177/ASTM D 664. Mixture of toluene /2-propanol/H20.TBN (titrant):HCl in propanol; TAN:KOH in 2-propanol(IP, 1969).

No inflection point for heavily used oils. Lengthy titration time. Incomplete neutralization caused by water content.

Emf

TBN:IP 276; ASTM D 2896. Mixture of chlorobenzene/ glacial acetic acid (1:2). Titrant: HCIO4 in glacial acetic acid.

Rapid equilibrium. Sharp brake at the end point. Clear inflection obtained.

High frequency (Caughly and Joblin, 1969: Fernandez, et al., 1979b)

TBN: mixture of toluene/2-propanol/H20. HCl in 2-propanol. TAN: mixture of toluene and ethanol. (CH3)4NOH in ethanol.

Reduces the analysis time. Sharp break at the end point. Eliminates contamination of electrode.

Conductivity (Armitage et al, 1987; Pawlak, 1980;Larbre and Briant, 1960)

TBN Mixture of toluene/2propanol/H20/. HCl in 2-propanol, or mixture of benzene/ alcohol.

Sharp break at the endpoint. Eliminates contamination of electrode. Less time consuming. Buffer solution is eliminated.

Calorimetry (Borrull et al.,1986a,b; Quality, 1967).

TBN, TAN. Mixture of toluene/2-propanol. KOH in 2propanol. HCl in 2-propanol.

Eliminates contamination of electrodes. Less time consuming Simpler in procedure.

Voltammetry (Kauffman, 1998).

TBN, TAN. Solvent: acetone, ethanol. A three-electrode system, requires lithium perchlorate

Uses small samples (5 ml). No titration required. Less time consuming. Eluninates contamination of electrodes.

242

Chapter 6

Total acid number (TAN), is determined for petroleum products normally using alcoholic potassium hydroxide as the titrant and toluene/2-propanol/H20 as the solvent. Only acids with pY^j^2^) values < 10 will be effectively titrated in this system (Ferguson, 1950; Gibb and Gibbson, 1959; Lykken et al., 1944; Pawlak et al., 1985). Differentiation between phenols and carboxylic acids titrated in suitable basic solvents such as pyridine, n-butylamine and ethylenediamine have been reported (Deal and Wyld, 1955; Moss et al., 1948). For mixtures of phenols and carboxylic acids, two separate endpoints generally occur, corresponding to the two substances of different acidic strength. Thus, for the titration mixtures of phenol, p¥^J^2^) "^ 10.0, and benzoic acid (4.2) in ethylenediamine, separate end points are obtained for each of the acids present. Potentiometric determination of the acid number in aircraft turbine oil shows that optimum results are obtained in chlorobenzene/dimethyl sulfoxide and toluene/dimethyl sulfoxide (3:1 ratio), using tetra-n-butylammonium hydroxide as the titrant (Jantzen, 1971). Studies showed that a chlorobenzene / dimethylsulfoxide solvent ratio (2:1) with tetramethylammonium hydroxide as the titrant was the most suitable combination for the determination of acidic compounds in petroleum products (Kahsnitz and Mohlmann, 1967). The case study. Comparision of the total base numbers obtained by the potentiometric and conductometric methods. More than 100 samples of CA SAE-30 (-40) and CB SAE-30 (-40) oils have been analyzed by both potentiometric IP 177 and conductometric methods in toluene/2-propanol/H20 mixture (Fox et al., 1991a). The oil samples were obtained from several sources, such as heavy duty diesel engines, one running on biogas containing 0.1% and 1.24 % hydrogen sulfide from a water reclamation works, and another from a large marine diesel in an ocean-going vessel. For the potentiometric titration of samples of a 'Marinol' heavy duty marine lubricating oil, the curves fall off with the service life of the oil. The sigmoidal curve for the fresh oil titration (curve 4), the shape of the plot is affected down for the used oil titration (curves 2 and 3). No inflection point was observed when the potentiometric method was used for more than 40% of the used oil samples. For these oil samples, the end-point E^^^j^ (2,4,6-trimethylpyridine + HCl) method has to be used, as described in IP 177, and shown in Fig. 6.5. In contrast, the conductometric titration plots for the same samples as analyzed by the above potentiometric titrations have a common form, that of two intersecting straight lines (plots 1' - 4' in Fig. 6.5). The shape of the plot is not affected by the service life of the oil sample. TBN values obtained from the conductometric titration method are higher than those obtained using the potentiometric method on the same samples. The discrepancies between the potentiometric and conductometric TBN results for the same oil samples may be partially eliminated when the empirical potential, Eemp5 obtained from samples for which a sharp inflection occurs (see curve 4 in

Analytical Techniques in Lubricating Practices

243

Fig. 6o5), is substituted for the E^^ff potential to determine the end point.

1

ZJv300

"

. 2 1 3

^

4

— y

^err^p 200 ^boff

,

7

/

j

^ 3' ^

.„

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._

^

2'

~ /

4'

-|

100 1'

O i

~|

^--^ J

i

,,.

{

3 HCt.

4

! . ..

mt

Fig. 6.5. Typical potentiometric (1,2,3,4) and conductometric (l',2',3',4') titration curves forCB SAE-30 Marinol engine oils. Plots: fresh oil (4,4'), used oils (1,1'; 2,2'; 3,3'). Medium: toluene (50%), 2propanol (47.5%), water (2.5%), v/v mixture. The end-point = E^, -'bufFer (2,4,6-trimethyl-pyridine + HCl). The empirical potential, E^^p, obtained from samples for which a sharp inflection occurs (see curve 4), and is substituted for the E^^^ potential to determine the end point (Fox et al., 1991b) While the IP 177 method specifies a solvent system containing 0.5% water, some workers investigated the effect of increasing the water content on the titration plot of conductometric system. A gradual increase in the water content of the toluene/2-propanol/water system from 0.5% to 3%) (by volume) did not affect the end-point of the titration. The slopes of the two straight intersecting lines on the plot increased, but the appearance of a white suspension from a previously clear solution for the same samples becomes more pronounced as the water content increases. The conductometric titration method has several advantages over the potentiometric titration method. It is applicable in a straight forward manner, without back titration or other modifications, to the determination of TBN for a wide range of petroleum products including fresh and heavily used oils. The conductometric method is quick and easy to perform, with two intersecting lines at the equivalent point, also contamination of electrodes is eliminated. The

244

Chapter 6

method is suitable for fast automatic procedures, being quicker and simpler than either of the IP potentiometric methods (Armitage et al., 1987; Pawlak, 1980). Once the addition of an aliquot of titrant to the solution is thoroughly mixed by the internal magnetic stirrer, as shown by disappearance of the refractive index gradients, the conductivity reading is constant. A complete automatic analysis by the conductometric method, from sampling, dilution, titration and cleaning/drying of the cell ready for the next sample, takes about four minutes (Armitage et al., 1987). In contrast, the potentiometer reading takes considerably longer to equilibrate for the same addition of titrant. It must be concluded that the reason for the IP determinations of TBN by the potentiometric titration method taking relatively long equilibration times following each aliquot addition in non-aqueous solutions is due to the low diffusion within the glass/calomel combination electrode. The conductometric electrode consists of two platinum plates, as with a standard laboratory conductivity cell. Despite the appearance of a black deposit on the surface of the electrodes, no deterioration in performance was found. An occasional washing in trichloroethane was used. In contrast, glass/calomel electrodes often require unexpected exchange and a spare electrode must always be available. The standard deviation of the TBN determinations by the conductometric titration method for used oils is consistently smaller than for the IP potentiometric methods. For the additive package component, TBN determinations, the conductometric titration gives a standard deviation close to that of the IP 177 or ASTM D2896 potentiometric method. Table 6.4. The determination of TBN by the IP 177 and IP 276 potentiometric titration methods is not always directly comparable. A previous conductometric study (Armitage et al., 1987; Fox et al., 1991a; Pawlak, 1980) showed that the mixture of toluene/2-propanol/water was a suitable solvent for the TBN titration of engine oils and gave a clear end-point at the intersection of two straight lines, as in Fig. 6.5 (curves 1' through 4'). TBN results obtained from IP 276 (potentiometric), IP 177 (potentiometric) and conductometric titration methods, compared for oil samples from diesel engines using the Ratella oil, are initially high but drop more rapidly due to the higher H2S content in biogas than for the HDX-30 oil running on normal diesel fiiel. Overall, it is evident that the TBN results are very similar using the three methods for fresh oils, but this is not the case for used oils. Samples of lubricating oil from a spark ignition engine running at constant load using a natural gas as a fuel. Table 6.4, show much greater disparity between IP 276, conductometric and IP 177 TBN values, the latter giving results for some samples which are less than 50% of the other methods. The conductometric TBN results lie closer to those for IP 276 for fresh and used oils. The increase in TBN found for used oils can be explained by the volatility of the lighter hydrocarbon fractions present in the lubricating oil formulation.

Analytical Techniques in Lubricating Practices

245

The TBN values obtained for the fresh, unused, lubricating oil additive package components show results from conductometric and IP 177 (potentiometric) methods being 90% to 98 % and 85% to 90%), respectively, of the corresponding IP 276 (potentiometric) values. The TBN values for some selective products such as zinc dialkyl-dithiophosphate (ZDDP) was observed as an inflection point using the IP 276 (potentiometric) back titration method and also the conductometric method. Table 6.4. Comparative experimental TBN [mg KOH g"^] values obtained by the potentiometric and conductometric method for petroleum products (Fox et al, 1991)

Sample type

Potentiometric ASTM D2896 chlorobenzene/ acetic acid

Potentiometric ASTM D664 2-propanol/ toluene/HjO

Conductometric Proposed 2-propanol/ toluene/HjO

110±2' 146±2

93±3 103±3

97±2 140±3

89±2

80±2

88±1

225±2 ST^iO.S"

210±6 Nil Nil

220±4 Nil 16.0±0.6

(A) Additive pack components Ca/Ba CO3 - sulfonate, Ca sulfonate-succinimide Ca/BaCOj-sulfonatenonylphenate CaCOj sulfonatesuccinimide Calcium carbonate Zinc dialkyldithiophosphate

is.eio.s-^

(B) Blended automotive lubricants, after Spark ignition engine, natural gas

Diesel engine, biogas with 1.24%ofH2S

Time,hrs 0 100 300 400 S.D.

12.5 23.4 23.7 23.1 ±0.5

4.7 11.5 13.0 11.1 ±0.4

11.9 18.8 19.4 18.1 ±0.2

0 70 100 150 300

19.1 17.5 15.3 14.4 13.6

18.2 16.4 10.9 9.1 7.1

18.6 14.2 14.2 12.3 11

^S.D, = standard deviation; Vhite suspension remains throughout titration, no visible titration curve, TBN obtainedfromreference carbonate curve; ""Back titration method

246

Chapter 6

This shows that the ZDDP antioxidant may react with HCIO4, with HjS liberation, and that this reaction contributes to the observed TBN value, Table 6.4. Finely ground calcium carbonate titrated as a base in a chlorobenzene/acetic acid solvent mixture, using the potentiometric IP 276 method, does not appear to dissolve. No visible inflection is observed for the plot during the course of the potentiometric titration. A colloid suspension of an alkaline earth carbonate, usually calcium carbonate, is the active species in engine oil. In the cases where the particles of calcium carbonate are present as a white suspended mass, titrimetric methods give titration curves that are difficult to interpret (Beckett and Tinsley, 1961). The carbonate is held in a colloidal state by the surface active alkylphenolate. High carbonate content does not give any substantial improvement in the detergency of the additive. The mechanism of carbonate action evidently consists in neutralization of acidic products formed by oil oxidation (Moss et al, 1948). One method liberates carbon dioxide by acid treatment which is absorbed in a mixture of organic solvents and then determines TBN by titration with standard sodium methoxide solution (Rimmer, 1965). A method for determining carbonate in overbase oil additives and blends is based on the method published in 1961 (Patchomik and Shalatin, 1961). The conductometric method is superior to the current standard potentiometric methods, with the advantages of: (a) the sharp break, or intersection, of the end point; (b) the continued quality of the end point for the fresh, used and heavily used lubricating diesel and petrol lubricating oils with no need for back titration; (c) an enhanced reproducibility, compared to potentiometric methods, particularly for used and heavily used oils; (d) reduced analytical time for each analysis, due to very rapid equilibrium for each aliquot, compared to the potentiometric method; (e) elimination of electrode contamination and replacement. One major problem of potentiometric methods, for whatever solvent system used, is the slow, diffusion-controlled response of the electrodes which are used. The conductometric method is generally more convenient and reproducible than the potentiometric method. The general order of magnitude for TBN is: IP 276 (potentiometric) > conductometric > IP 177 (potentiometric) The basicity of lubricating oils can be determined with much greater accuracy by means of conductometric analysis than by potentiometry. The case study. A comparison of the TBN and TAN values obtained by potentiometric, conductometric^ and calorimetric methods. The potentiometric ASTM D-2986 method was used as a standard technique and calorimetry and conductometry were used as supporting methods (Pawlak et al., 1989). The results

247

Analytical Techniques in Lubricating Practices

obtained are given in Table 6.5. Total base number (TBN) values, obtained using various titration methods, show that the calorimetric back titration and conductometric methods give results which agree fairly well with potentiometric measurements (ASTM D2986) in chlorobenzene-glacial acetic acid mixtures and HCIO4 as a titrant. Other studies (Fox et al., 1991a) have shown that the conductometric method gives TBN values 20 to 40% higher for most used oils than values obtained using the ASTM D-664 potentiometric method, but corresponds closely to results obtained with the ASTM D2986 technique.

Table 6. 5. Total base number (TBN) and total acid number (TAN) [mg KOH g'^ oil] and heat of neutralization -AH [kJ mol'^] of lubricating oils (Pawlak et al., 1989)

Oil

Emf

sample number

Method A*"

Cond Method B''

Back Cal Method C"

TBN

TBN

TBN

1 2 3 4 5 6 7 8 9 10 11

5.63^ 5.97 6.05 9.13 8.84 8.92 10.10 37.26 28.40 2053 7.63

5.52 6.21 6.60 9.15 9.94 8.93 10.42 37.56 29.14 19.13 8.24

5.75 6.73 6.82 9.54"= 9.43 9.12 10.07'= 38.05'=

27.8r 20.73^ 9.12

Direct Cal Method D'' TAN

AH

1.20 0.83 0.94 2.52 1.15 2.46 2.82 4.63 4.44 1.76 1.91

-6.2 -3.7 -7.7 -5.8 -10.5 -11.3 -15.9 -10.5 -15.9 -20.4 -19.2

"*Average standard deviation of each TBN was ±0.18; TAN was ±0.05, and heat of neutralization ±1.25. ^Method A: The potentiometric technique ASTM D-2986 (Annual Book, 1985), 0.1 HCIO4 in glacial acetic acid was used as the titrant and chlorobenzene-glacial acetic acid (2:l,v/v) was used as the titration medium. ^Curve of two zones. Method B: The conductometric technique (Pawlak, 1980), 0.1 HCl in isopropanol was used as the titrant and toluene-isopropanol-water (50+49.5+0.5,v/v) was used as the titration medium. Method C: The calorimetric back titration method, the sample of engine oil was dissolved in a mixture of toluene-isopropanol-water (50:49.5:0.5,v/v) and 0.25 M HCIO4 in isopropanol-toluene (1:1) was used as the titrant; the excess acid was titrated by 1 M (CH3)4NOH in isopropanol. Method D: The determination of the TAN and AH was performed by direct calorimetric titration. (CH3)4NOH in isopropanol was used as the titrant and a toluene-isopropanol-water (50:49.5:0.5,v/v) was used as the solvent

The ASTM D664 potentiometric method is not very satisfactory for carbonated high base calcium (barium) phenolates or sulfonates (Abbott and Farley, 1968; Annual Book ASTM, 1985; Giddings and Barrett, 1971; Pawlak et al., 1989). It

248

Chapter 6

has been shown (Labre and Briant, 1960) that phenolates or sulfonates can be determined with much greater accuracy in a (benzene + alcohol) medium by conductometric rather than by potentiometric titration. It has been also shown that results in a (chlorobenzene + glacial acetic acid) medium are essentially identical with calculated values and correlate better with engine wear than those determined by the ASTM D-664 method. Titration with perchloric acid (ASTM D2986 method) in a levelling mixture of chlorobenzene and glacial acetic acid ( 2 : 1 , v/v) may determine not only phenolates and nitrogen bases but also carbonates. In the determination of TBN by calorimetric back titration, the basic titrant (CH3)4NOH) neutralizes the unconsumed titration acid HCIO4 and any nonvolatilized acid present already or liberated from the basic additives. These acids are neutralized in order of decreasing strength. If the acidity strengths of these acids differ sufficiently from each other, they give separate and distinct inflection zones on thermometric titration curves, except for samples 4 and 7-11, which give two zones (for HCIO4 and weak acid), see Table 6.5. An estimation of the amounts of weak acid was made from direct titration on the samples (method D). Most oil samples have TAN of 1 to 2 units (mg KOH g'^ oil); however, samples 8 and 9 have an acidity index of around 4.5 units. Heat of neutralization of acidic products ranges from -3.7 to -20.4 kJ mol'^ and indicates the presence of weak acids in engine oil samples. The precision of calorimetric titration is comparable with conductometric and potentiometric methods. The advantages of calorimetric and conductometric techniques are rapidity, good reproducibility and avoidance of poisoning of detectors by organic substances. The case study. Performance of heavy duty diesel engine oil-TBN considerations. The TBN measurements are informative, easy, and quick; it can be misleading to base the judgment of an oil's performance solely on one criterion. Some detergents do not effectively neutralize all acidic species present in the lubricant, thereby keeping their own base while in fact oil may no longer provide sufficient protection against corrosion of bearings. It is recommended that, at a minimum, total acid number TAN measurements be included in any analysis. Recently, many governments have mandated the use of low sulfur fuel for diesel engines. Normally, such a mandate would be expected to lower TBN requirements; at the same time, however, there is a drive towards extended service intervals (Schiemann et al., 1995) which tends to push TBN requirement higher. Several engine builders (OEMs) have set TBN limits for condemning used oil. One OEM suggests that a reduction in TBN (ASTM D4739) to one-third of the initial value provides a guideline for the drain interval. Another OEM requires fresh lubricants with a minimum TBN ASTM D2896 of at least 20 times the fuel sulftir level for pre-chamber engines, and 10 times for direct injection engines (Van Dam et al., 1997). The difference between the ASTM D4739 and D2896

Analytical Techniques in Lubricating Practices

249

methods, comes in the choice of acid used, and the solvent in which he oil is dissolved to run the test. The ASTM D2896 uses a stronger acid (HC O4) than D4739 and a more polar solvent system. The combination of a stronger acid and a more polar solvent results in a more repeatable method. For some lubricant additives types, D4739 does not measure all the base that is present. Table 6.6 show^s some typical values for the ratio of two TBNs ASTM D4739/D2896 methods.

Table 6. 6. The TBN difference between ASTM D4739 and ASTM D2896 methods Additive type

TBNs ratio D4739/D2896^

Additive type

TBNs ratio D4739/D2896

Phenate detergent Sulfonate detergent

0.96 0.96

Ashless dispersant Amine antioxidant

0.48 0.0

'D4739 method: titrant HCl; solvent: mixture of toluene/2-propanol/H20; D2896 method: titrant HCIO4; solvent: mixture of chlorobenzene/glacial acetic acid In general, D4739 gives lower results, but the difference between the two methods is not significant. The D4739 results are lower for all ashless additives, especially for some amine oxidation inhibitors. Regardless of these differences, we have chosen to show only ASTM D4739, as it has become the accepted procedure for used oils. Characterization of used engine lubricants requires a complement of analyses consisting of TBN, TAN, wear metal content, oxidation, soot content, and viscosity. Some characteristic correlation parameters of the deteriorated engine oils are summarized in Table 6.7.

Table 6. 7. Results of the total base number TBN, total acid number TAN, lead content from the field and bench tests of diesel engine oil (Van Dam et al, 1997)

Detergent in oil formulation Ca phenate Ca sulfonate Mg sulfonate Pure base oil

Field'' testB

Field'' teste

Bench testD

FieW testE

D2896 TBN^

D664 TAN [mgKOH/goil]

Lead [mg/kg]

TAN TBN

[mgKOH/goil]

FTIR [Abs unit]

2.7 2.8 4.0 -

2.8 3.2 4.7 4.7

20 25 30 -

Field*' test A

[mgKOH/goil]

1.6 2.2 2.8 -

1.4 0.7 2.6

Tresh TBN oil with phenate, or sulfonate (5.7 mg^oH /gou)? ^32,200 km run

7.8 2.0 14 -

250

Chapter 6

Test A Field test used oil TBN (D2896 method) during the test, 32,200 km in diesel engine. TBN depletion rates for calcium sulfonate and calcium phenate were found to be similar. A magnesium sulfonate, however, behaved differently; its TBN depletion rate was slower. The wear metal analyses show a different picture, the magnesium sulfonate containing oil is most corrosive. The difference in TBN is that the magnesium sulfonate, possibly being a weaker base in the lubricating oil, may not neutralize all acids-thus reserving base which shows up as higher TBN. Also a higher TAN level for magnesium sulfonate oil would be expected. TAN levels for the magnesium containing detergents increase more rapidly throughout the drain period. Tests Acid neutralization test. The same three field test formulations, identical in every way except for the detergent, were treated with 5 TAN of oleic acid (a weak organic acid) and titrated for TAN by method D664. In addition, the same amount of acid was added to the base oil as a reference. As shown in Table 6.7, the magnesium sulfonate did not reduce the TAN at all over the base oil case. The greatest reduction in TAN was observed with calcium phenate, but calcium sulfonate also provided a meaningful reduction in TAN. This is the same ranking as the TAN increase observed in the field test. Test C The used oil lead content analyzed after 32,186 km run from a diesel engine, shown in Table 6.7, indicates that the lead corrosion rate is highest for the magnesium sulfonate containing oil. Test D High temperature 200°C oxidation bench test. The impact of high temperature oxidation on base depletion in the hotter regions of the engine was examined. A bench test was run for 4 hours at 200^C in the presence of air but no metal catalyst. Table 6.7 shows the TBN by the D4739 method and TAN by the D664 method of three oils at the end-of-test (EOT). Severe TBN depletion occurred with all three oils, but, like the field test, magnesium sulfonate showed the highest TBN (2.6), and also the highest TAN (2.8); the TAN/TBN ratio =1.1. This demonstrates that the magnesium sulfonate does not neutralize acidic oxidation products as effectively as calcium phenate. Test E Oxidation field test. The results were obtained by Fourier transform infra-red (FTIR) spectroscopy. In the sump oil field test temperature was from 90 to 1 lO^'C, and in the oil bench test temperature was \l(fC. In both field and bench tests data indicates that the magnesium sulfonate-containing oil gave higher oxidation level, and the calcium sulfonate-containing oil gave lower oxidation levels. Each of the evaluated detergent types has its strengths and weaknesses. Some detergents which have good TBN retention are less effective in neutralizing acids, and may cause lubricants to be less oxidatively stable. Acid-base study of glacial acetic acid and water extracts of lubricants. pH tests include: (1) Glacial acetic acid extracts of lubricants; (2) Water extracts of lubricants.

Analytical Techniques in Lubricating Practices

251

TBN and TAN parameters were originally developed for quality control of fresh oils. Their adoption for used oils is of limited value because neither gives a direct measure of oil corrosivity and their values depend on knowledge about the original oil. Because acid corrosion is caused mainly by hydronium ions (HgO^), the measurements of their concentration (pH) give a good indication of how corrosive an oil is becoming. The test requires the oil to be diluted with the suitable solvents and measured with the glass electrode of a pH meter. A fresh oil has a pH between 7 and 8 and this decreases steadily with use. At a certain point, the pH begins to decrease more rapidly and this is when the oil needs to be changed. For 4% oil solution, the condemning inital pH (IpH) limit is 4.5 (measured by ASTM D 664 or IP 177 methods); this IpH value is presented as the first point on a TBN titration curve. The pH method (Masters, 1995) differs from IpH in that it uses a more concentrated oil in the solvent mixture (20%), which is less affected by the solvent and results in a greater signal from the oil. Correlating pH with TBN and TAN gives the condemning value of 3.5 for 20% oil in toluene/2-propanol/water solvent mixture. The solution is largely non-aqueous, a certain amount of drift occurs during the first five minutes, but generally after this initial settling period, repeatable readings can be obtained from one sample to another within a minute or so.

550

500

L'^\

Lm\

350

"5 C8SAE30

Fig. 6.6. Relationship between pKb(water) of N-bases and half- neutralization potential E1/2 for potentiometric base titration in glacial acetic acid. 1) 4aminopyridine (4.83), 2) morpholine (5.30), 3) 2,4,6-trimethylpyridine (6.68), 4) pyridine (8.85), 5) p-toluidine (8.88), 6) o-toluidine (9.61), 7) 3acetylpyridine (10.82), 8) chloroaniline (11.36), 9) pyrazole (11.47), 10) diphenylamine (13.10) (Pawlak et al, 1985)

252

Chapter 6

(1) Glacial acetic acid extracts of lubricants. The basic compounds from fresh and used engine oils of the Marinol type were extracted with glacial acetic acid and the extracts were analyzed by conductometric and potentiometric methods (Pawlak et al., 1985). Half-neutralization potentials, E1/2, (Deal and Wyld, 1955) of oil samples were determined and compared with the standard pKb (N-bases), Fig. 6.6. The extraction of organic bases with glacial acetic from the engine oils and their conductometric titration indicates the presence of two kinds of bases with different dissociation constants, Fig. 6.7. Potentiometric titration shows only one kind of base.

Fig. 6.7. Conductometric titration curves of basic components extractedfromengine oil CB SAE 30 into glacial acetic acid. Curve 1 fresh oil, 2 - after 45 h of use, 3 - after 625 h of use (Pawlak et al., 1985) The strength of the basic compounds pK^ in engine oil estimated in glacial acetic acid is from 11 to 12.5 units in water scale (see Fig. 6.6). Also, it has been proved that 2,4,6-trimethylpyridine, pKb(H20) = 6.68, applied as a reference in the ASTM D664 emf technique to obtain the inflection point on the badly developed titration curves, is too strong of a base (Pawlak, 1980) and should be replaced by the base having pKb(H20) about 11, e.g., 2-chloroaniline, pyrazole, or 3-acetylpyridine. (2) Water extracts of lubricants. In general, water should not be present in internal combustion (IC) engine lubricating oils. The presence of more than a small amount (> 0.2%) of water indicates either a cooling system leak or condensation due to cold operation (Haith, 1970). Water can also accumulate in

Analytical Techniques in Lubricating Practices

253

formulated lubricating oils during prolonged storage or service and will produce significant changes in their subsequent service properties. The presence of water will change not only the TBN value but also the dispersant properties of the oils. The dispersion of water-contaminated oil decreases with both increasing temperature and increasing solids contaminant content. These changes are more pronounced for some oils and only slight for others (Kozhekin et al., 1978; Shishigin and Belganovich, 1976; Somov et al., 1978). The presence of water accelerates oxidation, as indicated by the increased intensity of the carbonyl infrared band for the oil. The anti-corrosion properties of the test lubricating oils are decreased after water contamination and subsequent separation. The use of water-contaminated oil causes an increase in piston ring wear by a factor of two. The deposition rate of carbonaceous matter on the cylinder-piston group parts is increased by a factor of 2-3 when water contaminated oil is used as compared to uncontaminated oils (Kozhekin et al., 1978; Shishigin and Belganovich, 1976; Somov et al., 1978). After 100 hrs of shipboard engine operation at 80% rated power with water contaminated oil, its TBN value dropped by 58%, whereas that value only dropped 22% for the dry oil. Similarly, the benzene insoluble contaminant content was 2.8%) and 0.7%), respectively. Micellar solubilization is said to remove water and polar contaminants from the engine oil to prevent particulate coagulation. The reverse micelles cores in nonaqueous solvents frequently exhibit marked basic or acidic character, in marked contrast to that prevailing in aqueous solvents (Forbes and Neustadter, 1972). At relatively small water/surfactant ratios, of the order of 5, all water molecules are tightly bound to the surfactant head groups in the polar cores of the reverse micelles. These water molecules have high viscosities, low mobilities, polarities which are similar to hydrocarbons and also altered pHs. An increase in the size of the water pool leads to the formation of larger aggregates and at some point there is a transition from a reversed micelle structure to water/oil microemulsions (Fendler, 1982; Luisi and Straub, 1984), Oil (reverse micelles) + H2O (excess) = H2O /oil microemulsions The effect of the presence of water in oil in terms of alkaline reserve of oil, pH (extract) and TBN value was investigated (Fox et al., 1990; Pawlak et al., 1985). The soluble acid/base content has been determined by extraction into either DI water, 7%) synthetic sea water solution or aqueous ethanol mixture (1:1, v/v). Extraction of oil samples by aqueous ethanol mixture (1:1, v/v) was applied to new and used samples of an SAE 30 oil. The relationship between pH extracted in aqueous ethanol and TBN value is shown in curve I, Fig. 6.8. We cannot distinguish between the fresh (TBN = 6.91) or used oils on the basis of extracted pH for 2 < TBN < 7. Below TBN values of 2, it is possible to differentiate between the oils because the pH extract is now close to a linear

Chapter 6

254

function of TBN. Total base number and total acid number values have been determined for a set of oil samples (Pawlak et al., 1985), curves 11 and III of Fig. 6.8. The two values cross over at a value of TBN = 2, after which the TAN values increase sharply and become meaningful. TBN values decrease sharply after the crossing point. The oil is now acidic and corrosive. The implications of the curves in Fig. 6.8 taken together are that the presence of water in oil could be potentially more damaging for "well-used" oils in service than for a fresh oil.

10 8 X

a

6

rsCs (J

.rr - ^

^

o

r

en E

^

6

1 2

_._

\>.TBN

T

J.

III

'5

6 4

TAN

2 1

03

1 1

r\ J

^

1

1 3

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1

1 I^

I

3ao

680

,Time,h

^

5

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J

7

Fig. 6.8. pH changes after extraction form SAE-30 oil by aqueous ethanol (1:1, v/v) mkture against TBN, curve I. (A)freshoil sample, (o) used oil samples. Extraction, 2 g oil/75 cm^ aqueous ethanol. hiset: the relation of TBN (curve II), and TAN (curve III) as a ftmction of engine running time (Fox et al., 1990) The real solubility in water of the basic compounds from the lubricating oils is more adequately described by Table 6.8. It shows some trends between the type of engine oil and its water extract expressed as TBN3q.ext values. For the used oils, the relative change was between 14% and 35%, indicating that water has a more severe effect on oils that have been in service than on fresh oils. The change in pH of the extract does not correlate with the loss of additives, as measured by the change in TBN. Only some, but not all, basic species extracted into water from the lubricating oil samples are hydrolyzed so as to contribute to pH change. The pH of water extracts for fresh oils is usually higher than that of used samples of the same oils. The pH of a fresh oil (SAE-30) extract was 9.4, decreasing to 7.9 after 15 hrs of service, whereas the TBN values for the same samples over the

Analytical Techniques in Lubricating Practices

255

same time increased from 6.5 to 7.6, and corresponding TBN values dropped from 6.5 to 5.1. These changes are for the same TBN of oil, SAE-30 monograde and HDX-30, but used in different engines using different fuels. Extraction into 7% synthetic sea water used the same range of new and used oil samples as was used for extraction into aqueous ethanol, Fig. 6.9. Varying amounts of oil, between zero and 25 g/150 cm^ for each sample, were extracted into 150 cm^ aliquots of water. Two types of behavior may be seen, the first where TBN < 2 has a lowest alkalinity change (AALK) value, samples I - IV, and where alkalinity content remains below the level of alkalinity of the 7% sea water.

Table 6,8. Relative total base number TBN change following aqueous extraction from engine lubricating oils (Fox et al., 1990)* Type of oil (fuel)

Oil sample

Test time, hrs

TBN [mgKOH/goil]

(%) TBN extracted^

HDH -30 (biogas with 0.2% H2S)

fresh used

0 10 to 100

6.5 5.1 to 4.9

14 20 to 30

SAE - 30 (diesel, 1% sulfur)

fresh used

0 10 to 450

6.0 6.1 to 5.3

25 27 to 35

SAE - 30 (high lead gasoline)

fresh used

0 15 to 96

6.5 7.6 to 10.8

22 14 to 19

Multigrade 20/50 (biogas)

fresh used

0 10 to 350

6.6 7.3 to 350

9 20 to 24

* Extraction conditions: 1 g oil/75 cm^ DI water at 298K, followed by shaking for 1.5 hr; \%) TBN extracted = (TBN - TBN.^.^^ )/TBN x 100 The second is where TBN > 2 for the oil samples VI - X and where the AALK value increases with the amount of oil to a maximum of about 16 g oil/150cm^ water. The value of (AALK) gradually increases with the service life of the oil, as TBN decreases, showing that the resistance to water contamination decreases as TBN decreases. The oil most resistant to water contamination is the fresh sample, V, on the basis of its low solubility for the basic compounds in water. The percentage of TBN that can be extracted into water significantly increases with the service life of lubricating oil. Well-used oils are therefore more susceptible to degradation by water contamination than fresh oils. The presence of significant amounts of water in lubricating oil is serious and should form a part of any lubricant condition monitoring system. It could be argued that there is a case for dehydration cartridges in the oil circulation system, in an analogous

256

Chapter 6

manner to the accepted particulate oil filters.

2.0 TBN

-

X IX

2.62 4.08

1.0

VIII

U.39

0.5

VII

4.39

1.5

0

_

.^—_.

0.5

o

-O—"

K

-

r\ -—

1 n

1

1 10

I 15

I 20

Q

VI V

5.43 6.91

IV III II I

1 .43 0.90 0.51 0.43

1 25

g ^ . , / 1 5 0 cm'"H.,0 ^011 2

Fig. 6. 9. Extraction of basic additivesfromSAE-30 engine oil samples in 7% sea water. Curve V fresh oil, curve I - IV, VI - X used oil samples. Relationship between A alkalinity = f (m^ii); where A alkalinity = AlK^^t ALK^eawater (Fox etal, 1990) The typical detergent-dispersant additives used in modem lubricating oils are metallic detergents/sulfonates, phenolates, phosphonates, salicylates, ashless dispersants/succinimides and benzylamines. Water is solubilized by strong iondipole interactions. The solubilization of water (Watanabe, 1970) by hydrogen bond formation with succinimides and the amount solubilized is smaller than that solubilized by sulfonates. The difference is ascribed to the smaller micelle cavity of succinimides relative to sulfonates. Mixed micelles of naphthalene-sulfonate-succinimide show weaker solubilization capacity than that of individual additives. The solubilization of water in a micellar system is closely related to the micelle core (Fontana, 1968). Addition of water to this non-polar solution, as engine lubricating oil is, produces a new set of phenomena. For small amounts of water, the micellar aggregates show swelling by uptake of water. The highly bounded water in reversed micelles makes surfactants less effective.

Analytical Techniques in Lubricating Practices

"t

y

i

T ' »

257

.•.,,.,.^y|,,.

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t I t

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-i

fc

300 600 UMdtlme.h

1

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900

300 600 UMdtfme.h

Fig. 6.10. Changes in physical and chemical characteristics of oils SI (SF grade) and Nl (SG grade) with use time: (a) TAN (ASTM 664); (b) TBN (by the HCl ASTM D664 and by the HCIO4ASTM D2896); (c) viscosity at 40°C (ASTM D445); (d) insoluble; (e) IR bands of P-O-C (970 cm'O and S=P (660 cm'O; IR bands of differential C=0 (1700 cm'O, COOH (1700 c m ^ and NO (1630 cm'O, (Moon and Kimura., 1990)

258

Chapter 6

6.4„ Engine Oil Evaluation Tests The case study L Deterioration of engine oil Changes in some physical and chemical characteristics (TAN, TBN, viscosity, insolubles, IR spectra, and wear) of SI (SF grade) and Nl (SG grade) oil with use are summarized in Fig. 6.10. Deteriorated samples of SI oil were obtained during an endurance test on a 550 cm^ two-cylinder gasoline engine. Li the course of the run of about 40,000 km in equivalent running distance over 879 h, six samples of about 500 cm^ each were taken out, followed by filling with fresh oil to the specified amount each time. Nl oil was deteriorated by a stop-and-go-type test in a 1400 cm^ four-cylinder gasoline engine, and 12 samples were taken out during the run for 96 cycles in 384 h. The sample weight was 430-600 g and the consumed amount was replenished each time. In Fig, 6.10 (a), TAN values of the SI oil show a steady increase, while that of Nl, after a small initial drop, quickly increases until its final value is reached. The initial drop in TAN is possibly caused by the depletion of some additives which contributed to the high value of the fresh oil. Some differences of TAN values exist among the fresh oils, depending on their formulation: that is, 1.35 mgKOH g'^ for SI and 2.61 mgKOH g"^ for Nl. The amount of wear is not significantly different when lubricated with either of the fresh oils. The wear-preventing property of engine oils changes significantly with the degree of deterioration in gasoline engines. An increase in TAN above the initial value has a definite correlation with the amount of wear. The change is only slight until the increase in TAN becomes about 1 mgKOH g"^ a further increase in TAN causes wear to increase sharply, showing a nearly parabolic relation with TAN (Moon and Kimura, 1990). Wear experiments have been carried out on a three-roller-on-ring machine. Some characteristic correlations (ATAN) of the deteriorated engine oils used in the wear experiment are summarized in Table 6.9. An effect of deterioration of petrol engine oil on wear was that an increase in the total acid number of the deteriorated oil above an initial value had a definite correlation with wear of steel. Similarly, the decrease of TBN is more dramatic for Nl, Fig. 6.10 (b). This can be observed with data determined by the HCl (ASTM D664) method and the HCIO4 (ASTM D2896) method, though they are quantitatively different. Table 6.9. The deterioration of engine oil as a function of an increase in ATAN ATAN [mgKOH /gdi]

0.1

0.5

1

1.5

2

3

4

Wear amount [lO'mm']

10

10

10

18

30

60

140

Analytical Techniques in Lubricating Practices

259

The increase in TBN (HCIO4 method) for the last sample of Nl may be attributed to the fact that some basic particles in used engine oils are aggregated at a much higher rate under low temperature conditions, as expected from the increase in insolubles in Fig, 6.10 (d). In Fig. 6.10 (c), viscosity values by the ASTM D445 method of both SI and Nl show a sharp initial drop owing to the shear of viscosity index improvers, and the value tends to increase thereafter at a much higher rate with Nl. The insolubles, Fig. 6.10 (d), increase markedly with Nl but with SI they remain at a very low level. TBN is indirectly related to the wear-preventing property of oils and TAN in fresh oils varies among them according to their formulations, TBN mgKOH g'^ that is: 5.20 for SI and 7.23 for Nl by the HCl method, and 6.58 SI and 9.03 Nl by the HCIO4 method. With used oils, it represents remaining basicity, which should neutralize acidic materials attack. Then, irrespective of its initial value, the acidic attack seems to be activated when TBN becomes smaller than some critical value. For all the oils tested, including heavily deteriorated oils (N2, N3, S2, Dl), wear is minimized when TBN is larger than 2 mgKOH g"^ as determined by the HCl method (6 units by the HCIO4 method). Once TBN becomes smaller than this value, the amount of wear generally increases, but considerable scatter is found among the data in this region. The increase in wear is most drastic for heavily deteriorated oils, and moderate for others. In Fig. 6.10 (e), the IR bands P-O-C (970 cm"^) and S=P (660 cm'^) for the used oil samples are plotted against use time. The Figure shows a decrease in concentration of specific groups initially contained in additives, e.g. ZDDP. The changes are more rapid with Nl, and the S=P peak with Nl tends to increase after passing through the minimum. The increase in S=P absorbance for Nl is considered to be connected with consumption rate and sampling rate of the oil as well as low temperature conditions. Fig. 6.10 (f) shows changes in some absorbance of differential IR spectra, which are obtained by subtracting those for the fresh oil samples, implying an increase in the amount of selected chemical groups as the deterioration proceeds. Similarly to the changes given above, the absorbance of C=0 (1700 cm'^), COOH (1720 cm"^) and NO (1630 cm"^) increases with time for both oils, but more rapidly for N l ; in particular, the NO peak is minimized for S1. The case study 2. Evaluation of ZDDP (primary, secondary) with R-group chain structure (long, medium, short) in oil formulation was done during the field tests. The various aspects of valve train wear performance of lubricating oils through field testing in a passenger car vehicle were compared with laboratory data (Smolenski and Kabel, 1983). The experiment was conducted to: (a) determine which of the major zinc dithiophosphate and mixtures provide the best antiwear/antioxidant performance at a set phosphorus concentration;

Chapter 6

260

(b) evaluate a number of oils including low phosphorus formulations; (c) test performance of individual additives, additive interactions, and engine variability. Table 6.10. Physical and chemical values for oil tests Oil code

A

B

C

D

E

F

G

H

I

ZDDP chain code": short (S), medium (M), long (L), primary (p), secondary (s) ZDDP code""

Sp

Lp

-

Sp/ Lp

Ms/ Lp

Ss/ Lp

Sp/ Lp

Sp

Ms/ Lp

TAN^ TBN'^

3.8 7.2

3.6 7.1

3.3 7.1

3.5 7.2

3.7 7.2

3.7 7.1

3.6 7.1

3.6 7.6

3.5 6.9

75

63

Oxidation Induction time by differential scannig calorimetry [min] 86 78 120+ 84 97 94 107

Metals mass (%) Ca Mg Zn/P -Cabs#

-Medium value 0.22— -Medium value < 0.01-Medium value 0,12— 8

6

^C4-5p = short-chain primary, C8p = long-chain primary, C6s = medium-chain secondary, C3-6s = short-chain secondary (see Chapter 2, Table 2.8 for chain structure). ''ZDDP code : (A) C4-5p; (B) C8p; (C) Aryl; (D) C4-5p/C8p; (E) C6s/C8p; (F) C3-6s/C8p; (G) C4-5p/C8p; (H) C4-5p; (I) C6s/C8p. ""Total acid number and total base number (fresh oil) Evaluation of the field tests in 56 passenger cars and laboratory analyses of lubricating data included: viscosity, TBN, TAN, ZDDP (active), dispersants, oil consumption rate, engine deposits, camshaft and valve lifter wear. Passenger car vehicles (56 taxi cabs) were equipped with 3.8 L, V-6 engines with D-500 lifters and C-4 emission control systems and tested. Seven of the nine oils were SAE 20W-30 oils; the remaining two were SAE lOW-30 oils. Eight taxi cabs operated on C oil and six taxi cabs on each of the other oils: A, B, D, E, F, G, H, I. All taxi cabs were operated in low speed, stop-and-go service in the metropolitan San Diego areao The test length was planned to be 160,000 km. The vehicles were ftieled with unleaded gasoline. Oils and filters were changed every 12,000 ± 1,200 km. The recommended oil-change interval for this type of service is 4,800 km. Camshaft and lifter valve wear protection performance was expected to vary with

Analytical Techniques in Lubricating Practices

261

the ZDDP type. At the final step of the test, engines from nine taxi cabs operating each on a different oil type, and camshafts and valve filters from the remaining 47 taxi cabs have been removed and inspected. Test results for nine taxi cabs using different ZDDP and ZDDP-type mixtures are presented in Table 6.10 (Smolenski and Kabel, 1983). Other results and observations in a number of performances in the taxi cab test are as follows: Used oil analysis. Used oil samples were analyzed for the following properties (Annual Book ASTM, 1985): viscosity ASTM D445, total base number ASTM D2896, total acid number ASTM D664, pentane insolubles ASTM D896, mass % zinc ASTM D811, mass % iron ASTM D811), "Active" zinc (differenfial infrared), carbonyl absorbance (differential infra-red). Over 250 samples were analyzed. Viscosity. Average used oil viscosity increased from 30% to 80%, and the ranges of viscosity increases for individual drain samples to maximum values were 80% to 210% at 40^C. Since the oil-change interval in this test is two-and-one half times more frequent than the recommended interval for this type of service, it is not expected that observed thickening (less than 220%) would cause any engine performance problems within the recommended oil-change interval. Oil thickening could result from either volatilization of light components in the oil or from oil oxidation processes. The good correlation between the viscosity at 40°C and the carbonyl absorption at -1710 cm"^ in the IR spectrum of engine oil indicates that thickening increase is primarily due to oil oxidation. The decreasing total base number is proportional to the concentration of oxidized species in the engine oil. The correlation between carbonyl absorbance and TBN was excellent for the lubricants, r = 0.82. Oxidized contaminants should be related to acidity of oils. However, only TBNs of used oils were determined during the test. The TBNs of used oil samples were between 1 to 2 units, which is substantially less than the nominal 7 mg KOH/g oil in the new oils. '^Active'' ZDDP. Differential Infrared Spectroscopy (DIR) was used to determine the concentration of ZDDP in the used oil samples by measuring the absorbance of the P-O-C band at 1,000 cm"^ The ZDDP concentrations of the used oil samples were generally less than 0.05 mass percent (as zinc), which is substantially less than the nominal 0.12 mass percent in the fresh oils. There was no correlation between camshaft and lifter valve wear and amount of ZDDP remaining in the used oil. This result supports other observations that the decomposition of ZDDP results in other compounds which may also exhibit some antiwear properties.

262

Chapter 6

Oil consumption rate. The oil consumption rate for each engine was computed by dividing the total volume of oil additions, in increments of 0.95 L, by the total test range. The average oil consumption rate for the oils ranged from 0.16 to 0.30 L/1,000 km. Oil consumption rates for individual engines ranged from 0.086 to 0.54 L/1,000 km. Engine deposit. Only one engine on each oil was tested for deposits. Based on engineering judgment, the oils were similar with respect to deposit control Oil ring deposit weights varied considerably among the oils (0.86 to 3.40 g), but were not considered to be excessive considering the extended oil-change interval. Sludge, varnish, and intake valve deposits were rated using Coordinating Research Council (CRC) merit rating scales, in which 10 units denotes a clean part and 0 denotes a part completely covered with heavy deposit. CRC sludge ratings varied from 9.3 to 9.6. Only one oil did not meet the Sequence V-D sludge requirement (9.4 min) for SF quality. Camshaft and lifter wear. Wear was computed by comparing measurements of compression ring gap, camshaft lobe lift, and lifter crown height to nominal, new part values. Variability in the new camshaft lobe lift and lifter crown height was 25 /^m and 12 //m, respectively. Camshaft lobes were considered excessively worn when wear was more than 500 /um. Camshaft and lifter wear was generally lowest with the A, F, H, and I oils (average 110 and maximum 450 /^m/161,000 km), which contained predominantly short-chain primary and secondary ZDDPs. Wear was greater with the C, B and G oils (average 220 and maximum 1,200 //m/161,000 km), which contained primarily long-chain primary and aryl ZDDPs A. Thus, it doesn't appear that any synergism occurs when ZDDPs are mixed. The taxi camshaft and lifter wear results correlated fairly well (r = 0.67) with Sequence HID wear results on similar test oil. There are several engine tests designed to evaluate engine lubricant valve train wear performance, hi Europe, the Daimler Benz 2400cc diesel engine and Volvo B 1986cc gasoline engine are commonly used for running standardized CEC (Coordinating European Council) for the development of performance tests for lubricants and engine fiiel tests. In the USA, the Oldsmobile 5700cc and the Ford Pinto 2300cc gasoline engines are used for the ASTM Seq. HID and VD Tests, respectively. In Japan, the Toyota 20R 2200cc and the Nissan L-16 1600cc have been approved as JASO (Japanese Automobile Standards Organization) Standard test method M328. The effects of additives on valve train wear in 100 hour Toyota 20R test. (a) Two sec-ZDDPs with different carbon numbers and one pri-ZDDP were selected. Between the two sec-ZDDPs, the shorter chain one had better anti-wear performance; pri-ZDDP had significantly poorer performance. This clearly indicates that the type of alkyl group of ZDDPs is the most important factor. The

Analytical Techniques in Lubricating Practices

263

differences in anti-wear performance of the three ZDDPs could be explained by the difference in their decomposition temperatures: 208, 213 and 233 °C. The lower the decomposition temperature, the more chemically active is the ZDDP. The phosphorus level was fixed at 0.05 wt %, the minimum level used in Japanese cars. Phosphorus is known to be the main cause of catalyst poisoning, but the presence of alkaline metals in oil reduces the catalyst poisoning effect of ZDDP; (b) The three dispersant (succinic ester, mono-succinimide and bis-succinimide) contents were fixed at 5 wt%. Bis-succinimide showed the best anti-wear performance. The difference between mono- and bis- types in performance could be explained by the tendency to form complexes with ZDDP (the degree of steric hindrance and higher nitrogen content in mono- should be considered); (c) The three metal detergents (Ca-sulfonate 1.08 wt%, Mg-sulfonate 0.84 wt%, and Ca-salicylate 2,15 wt%) were tested with the mole ratio Me/P = 2. Wear test indicated that Mg-sulfonate exhibites less wear than Ca-sulfonate and Casalicylate; (d) Three viscosity index (VI) improvers with different thickening power were selected. Two were of olefin copolymer OCP type with molecular weights MW = 126,000 (concentration of 18 wt%), and MW = 350,000 (concentration of 8 wt%); also one polymetacrylate type PMA with MW = 410,000 (concentration of 5.5 wt%). ZDDP had the most significant effect on wear in the Toyota 20R test followed by the dispersant, detergent and VI improver. A field test was conducted to evaluate the valve train wear in a 2.3 L OHC (over-head cam) engine with new technology crankcase lubricants; these oils also passed the V-D test (Haris and Zakalka, 1983). Oils formulated with secondary alkyl zinc dithiophosphate (ZDDP) wear inhibitor provided significantly better wear protection than two different primary alkyl ZDDPs. Secondary alkyl ZDDP demonstrated good wear protection at a phosphorus content as low as 0.07 (wt%).

Problems 6.1 Lubricant deterioration See Fig. 6.1 for the mechanism of the oil deterioration. Explain the difference in concept of some indicators: viscosity, TBN, TAN, insolubles. 6.2 Degradation of engine oil By using the schematic of the (IC) engine in Fig. 6.2 and the degradation pathways of the lubricating oil in an internal combustion diesel engine of Fig, 6.1 and Fig. 6.3, where in the engine would you find each of the following processes: (a) inorganic acids formation, (b) organic acids formation, (c) viscosity increase, (d) varnish sludge formation?

264

Chapter 6

6.3 Physical characteristics of the oil condition monitoring In terms of the description in Chapter 6.2, p. 223, specify the analytical techniques that can be used during engine operation satisfying the following conditions: (a) wear metal analysis, (b) level of fuel contamination, (c) viscosity changes, (d) flash-point changes, (e) water content, (f) morphology of wear distribution, (g) induction temperature. 6.4 Chemical characteristics of the oil condition monitoring By using the description and data given in Chapter 6.2, p. 231, specify the analytical techniques measuring the changes of oil deterioration: soot, oxidation, nitration and sulfation. 6.5 Fourier Transform Infrared Spectroscopy FTIR Where in the lubricant formulation would you find each of the following: (a) oil contamination (water/glycol at 3400 cm"\ fuel at 3016 cm"^ 803 cm"^ and 469 cm-^ glycol at 1070 cm"^ and 1040 cm\ soot at 2200 cm"^ and 1817 cm"^); (b) oil deterioration: oxidation at 1730 cm"^ nitration at 1630 cm"\ sulfation at 1150 Qm\ ZDDP at 975 cm'^ and 646 cm"^, hindered phenols at 3650 cm'^ ? By using the data given in Table 6.2, correlate the functional groups with mentioned compounds and consider interferences when you have mixture of compounds in the oil formulation. 6.6 Basicity and acidity of oilformulation On the basis of the total base number (TBN) and total acid number (TAN), and by using techniques and methods for the determination of TBN and TAN from Table 6.4, answer the following questions: (a) How is it that the measured TBN may be greater for used oil than fresh one; (b) How could you test experimentally your hypothesis in a particular case? 6.7 Conductivity and emf techniques By using the values of Figure 6.5, explain the differences in the concept of TBN determination in the conductivity and potentiometric techniques. Give a good example of each of the following: (a) show that, for solution containing very used oil, the potentiometric titration curve falls off and empirical potential, E^^^ is used to determine the end point, (b) explain differences between the empirical potential, E^j^p and the buffer potential, ^buff-

6.8 Used oil acidity Explain the concept of basicity of oil samples with the TBN values high and low after extraction in sea water as plotted in Figure 6,9; illustrate (a) acidic and (b) basic character of aqueous solution. 6.9 Deterioration of engine oil By using data in Figure 6.10, "Changes in physical and chemical characteristics of oils SI (SF grade) and Nl (SG grade)...", analyze the following: (a) TAN, (b) TBN, (c) viscosity, (d) insolubility, (e) ZDDP, (f) oxidation.

Analytical Techniques in Lubricating Practices

265

6.10 The ZDDP deterioration By reference to Table 6.10 and the case study 2 "Evaluation of ZDDPs", in field tests of 56 passenger car vehicles (taxi cabs) and laboratory analysis of lubricating data were included: viscosity, TBN, TAN, ZDDP (active), dispersants, oil consumption rate, engine deposits, camshaft and valve lifter wear. Which of the major ZDDPs and ZDDPs mixtures provide the best antiwear and antioxidant performance?

This Page Intentionally Left Blank

267

Chapter 7 ENVIRONMENTAL ISSUES Everywhere the production, application, and disposal of lubricants has to cover the requirements of the best possible protection of our nature and environment in general and of the living beings in special. WilfriedJ. Bartz, 1998 lA. Recyclability, Biodegradability and Toxicity There is a growing awareness of the need to protect the environment in which we hve. This significantly impacts on the oil and additives industries and offers many opportunities for new initiatives in product development and marketing. Taking the European Community in 1985 as an example, of the 4.5 million tons of the total lubricants marketed, some 0.75 million tons were burnt; 0.6 millions tons were totally unaccounted for, and 0.1 million ton were poured down drain deliberately (CONCAWE, 1985; Randies et al., 1991). Statistics classify lubricating oils for automotive use into four categories: gasoline engine oils, diesel engine oils, gear oils and others (including automatic transmission fluids, twostroke engine oils and others). Used engine oil is recognized as posing a carcinogenic risk to man. The main carcinogenic substances in used oil are polycyclic aromatic hydrocarbons (PAHs) with 3-7 rings such as benzopyrene, benzanthracene and chrysene (Cosmacini et al., 1988). There are three basic methods of disposal of used oil: (i) burning as fuel; (ii) disposal as toxic/hazard waste; (iii) distillation/hydrotreatment re-refining to produce base oil. (i) Burning as fuel: Energy gain is about 90%. Some treatment of the oil will be required to ensure that emission standards for this material are not exceeded when oil is used as fuel. Principal environmental concerns are polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), dioxin, hazardous metals (Ba, Pb, Cd, Hg, Cr, As, Se, Ag) and the content of S, P, and CI. (ii) Disposal as toxic/hazard waste: Energy input is required to achieve high temperature combustion to ensure complete oxidation of PAHs, PCBs, and dioxin.

268

Chapter 7

If the content of some contaminants in the used oil exceeds predetermined concentrations, incineration is mandatory. Incineration and pyrolysis are the common techniques, with thermal requirements of 1200°C. No energy gain results from this process. (Hi) Distillation/hydrotreatment re-refining to produce base oil: It has 70% recovery rate of lube oil With regard to re-refining, the process is able to generate a product of sufficiently high quality, but the residue can be highly toxic and/or potentially carcinogenic. The cost of disposal of these wastes is usually high. Most of the used oil re-refming technology installed within recent years has included a catalytic hydrogenation finishing step. The hydrogenation reaction chemistry ensures that the final products from PCBs dechlorination are hydrocarbons. In the United States alone, over 100 million galons of used oil are recovered each year using this type of process (Brinkman et Dickson, 1995; Brinkman et al., 1995). The development of environmental regulations or legislation to regulate the use of lubricants exists in Austria, Canada, Hungary, Japan, Poland, Scandinavia, Switzerland, the USA and the European Union, (Bartz, 1998). In the USA, currently there are no laws requiring the use of environmentally compatible lubricants, but two regulations may have a significant impact on the use and disposal of conventional lubricants: Executive Order 12873 (EO 12873), and Great Lakes Water Quality Initiative (GLWQI). The EO 12873 encourages the use of recovered materials - use of recycled oils, and environmentally preferable products - use of environmentally compatible oils where possible to meet requirements. The GLWQI is intended to maintain, protect and restore the unique Great Lakes resource - water quality. Austria is the only country with a law that bans the use of mineral oil based lubricants in particular applications, e.g. chain saw oils. The preferred course of action is to reduce toxicity and increase biodegradalibity. Often, there is an agreement to define lubricants as environmentally acceptable if they meet the following requirements: rapidly biodegradable, nontoxic against human beings, nontoxic against fish and bacteria. Following are fields of applications for environmentally acceptable lubricants', outboard two-stroke engine oils, chain saw and saw frame oils, railway wheel tread lubricants, mold parting compounds, wire rope lubricants, truck centralized system lubricants, hydraulic oils for machinery in building and bridge construction, deep workings and underground workings, hydraulic oils for forest and agricultural equipment, lubricants for sewage-treatment plants, water dam plants and lock gate mechanism, lubricants for food machinery, lubricants for snow mobiles and ski run maintenance equipment, metal working and metal forming processes, internal combustion engines and hydraulic systems in general (Bartz, 1998),

Environmental Issues

269

Environmentally accepted lubricants. The development of criteria appropriate for engine oils is not a simple procedure. A scheme has been proposed that recognizes the environmental benefits. This offers oil companies the opportunity to market new products with environmental benefits such as biodegradability or employing recycled products. Recently, the environmental behavior of lubricants such as emissions, safe handling, toxicity, and biodegradability has come under consideration. Environmentally considerate lubricants for the automotive and engineering industries are becoming increasingly important features of the growing public awareness. For a lubricant to be assessed as environmentally considerate, several aspects of its performance should be considered (Bates, 1995; Bartz, 1998; Coy et al., 1995; Droy, 1993; Mani and Shitole, 1997; Randies et at., 1991). These are: (i) good performance of a lubricant; (ii) low bio-accumulation; (iii) low toxicity; (iv) reduced or clean exhaust emissions; (v) biodegradability. (i) Performance of lubricants. Synthetic lubricants can provide superior performance to traditional mineral based oils. The advantage of their low volatility and increased thermal stability is that they often have longer service life, which benefits the environment. As we become more concerned with the environment, there is no doubt the use of synthetics will increase. New engine oil will need to be developed with different characteristics which will require novel solutions. Environmental demands and are as follows: (a) improved fuel economy (reduced viscosities, reduced friction, special viscosity improvers), (b) reduced oil consumption (unconventional base oils, improved seal compatibility), (c) extended oil life (improved thermo-oxidative stability), (d) extended engine life (improved detergents and antiwear additives), (e) beneficial effects on emission/after treatment hardware (new additives), (f) technological and environmental sensitivity (no halogens, limited metal types/concentrations, new organic compounds), (g) recyclability (limitations the polycyclic aromatic hydrocarbons content and high chlorine levels (Havet et al., 2001; Waara et al., 2001). (ii) Low bioaccumulation. Oil has a tendency to accumulate in the environment. The reduction in additives levels, used to give the oil better performance, is coming under attack, because they cause bio-accumulation problems. It can be seen quite clearly that this type of hydrocarbons is accumulating in a dip in the road, near river mouths, and lakes, and finally in sediments. It was found that polycyclic aromatic hydrocarbons (PAHs) which represent < 1% of the volume of a used oil (Cosmacini et al., 1988) are highly

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270

mutagenic, especially the benzopyrene compounds. Bioaccumulation is the concentration (i.e., accumulation) of chemicals from water or food in living organisms, Bioaccumulation is determined by the degree of uptake, distribution, metabolism, and elimination of the chemical in an organism. A typical example are polychlorinated biphenyls (PCBs), which are soluble in lipids, persistent and accumulate in fish. The ratio expressed by the concentration in an organism to the concentration in water is called the bioconcentration factor (BCF). As with the lethal concentration 50 (LC50, the bioconcentration factor is generated and used for environmental hazard assessment purposes (Droy and Randies, 1999).

4 r

LL O 3 CQ O

5

/ JL

I

)r 1 h Q.

6

10

12

LogP Fig. 7.1. Schematic representation of the relationship between octanol-water partition coefficient log P and bioconcentration factor log BCF in aquatic species (Droy and Randels, 1999) The bioconcentration factor (BCF) is an expensive and difficult test and can be replaced by an estimation of relative lipophilicity of the material, the partition coefficient (log P). This physico-chemical parameter (log P) is determined by measuring the distribution ratio of the material between octanol and water (Droy, 1993; McKim et al, 1985). The logarithm of this ratio is the partition coefficient (log P); however, materials with a log P of <1 or > 6 or 7 are not expected to bioconcentrate. Graphic representation of the relationship between the octanolwater partition coefficient, log P, and predicted bioconcentration, log BCF, has the

Environmental Issues

271

linear and inverse relationships (Lambda shape). Fig. 7.1 is a schematic representation of the linear and inverse relationship between log P and predicted log BCF, where BCF = [concentration of chemical in organism]/[concentration of chemical in water]. The inverse relationship between two parameters occurs, with a log P about 6, and log BCF above 3. Regulatory bodies have generally categorized all materials with log P > 3 as bioaccumulators, and materials with log P > 7 are not expected to bioconcentrate (Shubkin, 1993). Based on a review of 1992 regulations, a decision tree for environmental acceptance in regard to aquatic toxicity and environmental fate was developed (Droy, 1993). The LC50 values below 10 ppm are unacceptable in all cases. The LC50 > 100 ppm is readily biodegradable would be environmentally acceptable. If a material is not readily biodegradable and a question exists as to its potential persistence, one must evaluate bioconcentration possibilities; hence, the introduction of the log P and fish bioconcentration test criteria. If a chemical has a log P between 3 and 7, the material would be considered a suspect bioconcentrator, and then a definite fish bioconcentration test should be run. A "passing grade" for this test is a bioconcentration factor, BCF < 100. If a material that is not readily biodegradable (or is slightly toxic with lethal concentration of LC50 > 10 or < 100 ppm) passes the log P and/or fish bioconcentration test, it must subsequently pass a test for "relative" (i.e., CEC L-33-T-82) or "inherent" biodegradability in order to be rated environmentally acceptable. Biodegradation for a material can be described as being readily, inherently, or relatively biodegradable under either aerobic or anaerobic conditions. Tests for ready biodegradability are the most stringent. A material that biodegraded in a ready biodegradability test would be considered to likely degrade in a rapid and complete manner in the environment. Tests for inherent biodegradability are less stringent - a material that degrades in this test may not be considered to be biodegradable in the environment. Recently, a test for relative biodegradability has been developed. The CEC L-33-T-82 test was designed for water-insoluble materials and compares the biodegradation of a component to the degradation of reference mineral oils (Droy, 1993). (Hi) Low toxicity. The possible hazard of chemicals in the environment has become widely accepted by many countries. As a result, legislation has now been passed in many parts of the world (Japan-1973; USA- 1979), and will soon have such legislation in Canada, Australia, New Zeland and European countries (EU), which requires that, before new substances can be marketed (or manufactured), ecotoxicological data must be generated (Randies et al., 1991). In 1989, a substance list (KBWS-classification) concerning classifications into waterendangering classes was published (Substances List, 189). The toxic potential of a particular chemical substance is evaluated by addressing the intrinsic toxicity of the material itself and understanding the magnitude of the exposure to an

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individual, population or ecosystem. Toxicity evaluation end points include lethality and effect on reproduction and growth. The most commonly determined endpoint is LC50, which is defined as the concentration of material (in air or water) needed to kill 50% of a population within a specific time period. With direct oral or dermal doses, this parameter is referred to as the LD50 or lethal dose 50. This particular parameter is the most important number used by regulatory bodies worldwide in assessing the toxicity of a particular material (Droy, 1993). Regarding ecotoxicity, a general rule of thumb exists according to which materials with an LD50 value > 1000 mg/kg are low or non-toxic. In fact, ecotoxicity represents the toxic effect of a lubricant on plants and animals (not on human health). Regarding the toxicological potential of lubricants, base oils and additives have to be examined. The toxicological behavior of the base oils can be found in the CONCAWE report "Health Aspects of Lubricants" (CONCAWE, 1987). Conventional mineral-based stocks have low toxicity by inhalation; the acute oral LD50 for rats was higher than 10,000 mg/kg, and by skin absorption, the acute dermic LD50 for rats was higher than 2000 mg/kg. For human ingestion, low viscosity products will cause serious effects, and for short-time dermal contact no problem was found. The toxicity of the majority of lubricant additives is not classified as dangerous. The Additive Technical Committee (ATC) has developed a classification system, naming 35 classes of additives divided into six broad groups of classification (Caines and Haycock, 1996). Of the most important additive types, only ZDDP and some long chain calcium alkylaryl sulfonates are classified as irritants, e.g., the acute oral (rat) and dermal (rabbit) typical LD50 [mg/kg] (Bartz, 1998). LD50 values are as follows: (a) for zinc alkyl dithiophosphate > 2000 (oral) and > 2000 (dermal); (b) for zinc alkaryl dithiophosphate > 5000 (oral) and > 2000 (dermal); (c) for calcium long chain alkarlyl sulfate > 5000 (oral) and > 3000 (dermal); (d) for calcium long chain alkyl phenate > 10 000 (oral) and > 2000 (dermal); (e) for calcium long chain alkyl phenate sulfide > 5000 (oral) and > 2000 (rat) (dermal); (f) for polyolefin amide alkeneamine >10 000 (oral) and > 2000 (dermal). Due to the different fate of lubricants during their use, e.g. contamination by fuel and combustion products of engine oils, the toxicity of used lubricants may be significantly different than that of fresh oils. Fluids which are considered environmentally friendly must not only be biodegradable, but also relatively nontoxic, in both their initial form and degradation products. Their effects on flora and fauna must be minimal. There are two common tests to evaluate toxicity: the Microtox and the Rainbow Trout bioassay. Considerable environmental toxicity testing has been carried out on esters fluids. Esters cause minimal acute toxicity by ingestion and skin absorption.

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273

(iv) Reduced or clean exhaust emissions. In smoke emitted in diesel exhaust gases, particulates have been found to include a significant contribution from lubricating oil. The poly cyclic aromatic hydrocarbons (PAHs) over 50% are present in exhaust particulates emission from the PAH accumulated in the engine oil. A decrease of 25% in the total PAH content has been found by using a carboxylic ester over mineral oil (Randies et al., 1991). Successively more stringent regulations in the US and the rest of the world are being proposed. These regulations are requiring significant alterations to engine design and exhaust system. The consequences of these engine design changes and the implications for the lubricant formulations are: use of exhaust gas recirculation, sensitivity of exhaust after-treatment devices, total removal of phosphorus and sulfur, higher and sustained fuel economy (Bovington and Castle, 2001). Exhaust gas recirculation will be deployed increasingly to reduce emissions of nitrous oxides. The consequence for the lubricant is that the higher soot loadings, which result, cause unacceptable increases in viscosity and increased wear (Bovington and Castle, 2001). The viscosity increase results from the agglomeration of soot particles in the lubricant and can be minimized by the use of dispersants. If increased levels of dispersant are deployed this can pose problems with wear since solubilization of ZDDP by dispersants (Korcek et al., 2000) influences adversely the formation of protective films. Inadequately solubilized soot will cause serious erosion of tribofilm and cause enhanced wear. Sensitivity of exhaust - after treatment devices. It is clear that excessive deposition of phosphorus and sulfur on the catalyst can cause the reduction in system efficiency. Oil phosphorus contaminant comes from the oil additive ZDDP. The reduction in its use adversely affects both antiwear and antioxidation performance. Sulfur comes from the base oil, antiwear additives, detergents, organomolybdenum friction modifiers, and from the fuel. There is strong pressure from OEMs to reduce the sulfur level of the fuel, and to reduce the sulfur contamination of the catalyst, which results from presence of sulfur in oil. Total removal ofphosphorus and sulfur would require the use of synthetic base-oils and new additive systems to provide antiwear antioxidation protection. Synthetic base-oil PAOs or esters have high values of viscosity improver VI and low temperature operating properties. The lubricants in diesel engines require a reduction in Ca carbonate-sulfonate concentrations. This may be less of a problem when ultra low sulfur diesel fiiel is widely deployed, since a significant part of the requirement for these additives arises from the need to neutralize sulfur oxides from combustion processes. Corporate Average Fuel Economy CAFE. Requirements necessitated the use of lower viscosity oils, which give improvement in friction (traction) hydrodynamic (HD) and elasto-hydrodynamic lubrication (EHD) full film conditions.

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(v) Biodegradability. In biodegradation processes, a chemical compound is transformed or eliminated by the biological action of living organisms. In general terms, biodegradability is the tendency of a lubricant to be ingested and metabolized by microorganisms. Complete biodegradability indicates that the lubricant has essentially returned to nature. Partial biodegradability usually indicates that one or more components of the lubricant is not degradable. There are several processes associated with biodegradation: oxygen consumption, carbon dioxide formation, and substrate loss. The test of biodegradation can be evaluated using conventional analytical methods like infrared spectroscopy (IR), total organic carbon (TOC), dissolved organic carbon (DOC), and chemical oxygen demand (COD), (Mortier and Orszulik, 1997; Van Donkelaar, 1990; Shubkin, 1993). The presence of additives in the lubricants is known to have a significant effect on the rate of biodegradation. The additive components which contain transition metals have a very negative effect on biodegradation, while on the positive side, are nitrogen and phosphorus additives. The most common mineral oil substitutes consist of vegetable oils (natural esters) and synthetic esters. Repassed and canola oils are the most common base stock for vegetable base lubricants. Additives for biodegradable lubricants must also be biodegradable and non-toxic, or at least not interfere with the biodegradation of the base fluid. Additives which are considered appropriate for biodegradable lubricants must contain no chlorine or heavy metals and should not be controlled under any occupational health and safety regulation. Simple, biodegradable lubricants were in widespread use until the discovery of petroleum oil in the late 1800s. With the development of the petrochemical industry, large volumes of base stocks become readily available, displacing natural products for reasons of lubricity, stability and economics. Along with advancement of analysis and awareness of the environment have come increasing concerns over the effects of petroleum products being released into the environment, leading to a reinvestigation of vegetable oils and readily biodegradable products. Degradation can occur by abiotic and biotic means including hydrolysis, photolysis, and biodegradation. Therefore, before conducting tests for biodegradation, one should assess the hydro- and photolytic capabilities of a substance. From ecolabeling perspective, it seems that most of the interest in degradation is in the area of biodegradation. Determining the biodegradation capabilities of a material is very difficult. This is because most of the methods for biodegradation tests have been developed for use with water-soluble materials and many of the petroleum materials, including synthetic lubricants, are hydrophobic. The biodegradation test method for water-insoluble substances, the CEC L-33-A93, is dealing with this problem (Droy, 1993; Randies et al., 1991). Eight test methods are currently available for determining biodegradability. These tests

275

Environmental Issues

methods are the OECD 301 series of standard tests and are recognized by EU, the EPA and ISO, Table 7.1 lists the seven test methods for outboard two-stroke oils (Mortier and Orszulik, 1997; Randels et al, 1991; Rudnick and Shubkin, 1999). Table 7.1. Biodegradation tests for oils - worldwide Test name^ Closed Bottle OECD CEC Sturm AFNOR MITI EPA

Substance (mg/L)

Bacteria (cells/mL)

Parameter

2 to 10 5 to 40 2,500 10 to 20 40 100 5 to 40

250 50 to 250 1,000,000 1,000,000 1,000,000 1,000,000 50 to 250

0, DOC" C-H (IR) COj DOC 0,

co^

(%) pass level 60 70 70 60 70 60 70

Test days 28 28 21 28 28 28 28

'Closed Bottle test (method: OECD 30ID); OECD, Organization for Economic Cooperation and Developments (method: OECD 301E); CEC, Coordinating European Council (method: CEC-L-33-A-93); Sturm (method: OECD 301B); ANFOR (method: OECD 301A); MITI (OECD 301C); EPA, U.S. Environmental Protection Agency (method: modified shake flask test measure carbon dioxide evaluation, method 560/6-82003); ISO, International Organization for Standardization (method: ISO 9439:1990); ^DOC = Dissolved organic carbon Test method summaries: The CEC-L-33-A-93 test - 150 mL water plus nutrient salts, 0.05 mL oil/CCl4 solution and 1 mL of inoculum (the mixture of bacteria and other microorganisms) are poured into an Erlenmeyer flask with cotton wool stopper; conditions: stirred at 25'^C in a darkened room for 21 days. The CEC-L-33-A-93 test for relative biodegradability for water-insoluble materials, compares the biodegradation of a component versus the degradation of reference mineral oils. This test, though not as yet accepted for regulatory or complete in its development, is gaining acceptance in Europe (Droy, 1993). The microbes (bacteria, fungi) must utilize the test compound if they are to grow and it is this utilization that is monitored by measuring CO2 evolution, O2 depletion or dissolved organic carbon (DOC) reduction. Chemical structure influences the biodegradability which decreases with increasing chain length and molecular weight. The different classes of synthetic lubricants vary greatly in biodegradability. Branched compounds have lower biodegrabilities than linear compounds (Droy and Randies, 1999; Basu et al., 1998). The biodegradability of various lubricants measured using a Coordinating European Council (CEC-L-33-A-93) test showed a range of biodegradation from very high (100%) to very low (5%), see Table 7.2 (Randies et al., 1991; Mortier

276

Chapter 7

and Orszulik, 1997). Low viscosity poly((X-olefins), e.g., 2 and 4 cSt, can be classified as relatively biodegradable in aqueous systems as defined by the CEC L-33-A-93 test; however, this does not mean that all poly(a-olefms) are biodegradable. The concept of biodegrability is very difficult to define. The extremely low water solubility of poly(a-olefins) decreases the available surface area for bacterial degradation. If high biodegrabilities are required, linear polyol esters tend to be used.

Table 7. 2. Biodegradability of some lubricants. Method: CEC-L-33-A-93 test Lubricant type Hydrocracked White oils Mineral oils Vegetable oils Poly(a-olefins) Alkylbenzenes

Biodegradability (%) 25-80 25-45 10-45 75 - 100 20-80 5-25

Lubricant type Diesters Polyol esters Aromatic esters Polypropylene glycols Polyethylene glycols Polyethers

Biodegradability (%) 50-100 55 - 100 0-95 10-30 10-70 0-20

In North America biodegradable fluids have found a widespread use in hydraulic and forestry applications. Other applications include saw guide oils, valve oils, turbine oils, and rail curve grease. The forestry industry currently uses environmentally friendly chain bar oils and has expanded to using biodegradable lubricants for the blade and saw guides in the mills. A biodegradable, nontoxic natural and synthetic ester based fluid was developed specifically for saw guides. This product has not only decreased operational costs, but has solved the mineral oil related health problems. After 6000 hrs of testing in Alberta's power generation plants, a synthetic ester based fluid seems to be comparable to mineral oil, with the added benefit of being biodegradable in +21 days (Goyan et al., 1997). With technical, economical and environmental aspects of cutting processes, the major concerns are cutting tools and the cutting fluid applied. In cutting steel, the elevated temperature and pressure may reach the values of over 1,000''C and 200 MPa respectively. Almost 100% of the energy input in metal cutting is converted into heat and dissipated into the environment. The cutting fluids address reduction of friction and removal of the heat generated during the cutting process. At the same time, cutting fluids not only represent additional environmental problems, but generate separation, cleaning, treatment and health hazard problems (Anon, 1996). To operate under such circumstances, cutting oils have required effective boundary lubricants such as extreme-pressure (EP) additives and oiliness agents.

Environmental Issues

277

In particular, organochlorine compounds, such as chlorinated paraffins, have been preferred as EP additives because of their superior cutting performance to other boundary lubricants (Kubo et al., 1999). However, recent concerns about environmental issues have raised the problems of using lubricants containing chlorinated chemical compounds. The amount of chlorine in waste oils has been regulated because chlorine causes serious air pollution when those oils are disposed by incineration. Also, specific chlorinated paraffins have been reported to be carcinogenic (National Toxicology Program, 1986). To eliminate cutting fluids, dry cutting may result in substantial cost savings. Additionally, the principles of minimal cutting fluid applications have emerged (Heisel and Lutz, 1993; Wakabayashi et al., 1998). The application of biodegradable multi-layer type cutting fluids has been tested with excellent results (Kaldos, 1997). In Europe, biodegradable oils are required by law to be used in lakes, and they account for about 10% of the sales of the two-cycle engine oil. In Japan, biodegradable oils are used in engines for motorboat racing, but otherwise, these oils are not widely used because of the lack of environmental regulations (Kubo etal., 1999). Environmental ecolabelling. In order to characterize and label environmentally acceptable products, authorities and institutions in different countries have developed environmental labelling schemes and special signs and labels. It is the aim of these signs to draw the attention of users to environmentally acceptable lubricants. Some environmental labels have been developed in the USA (green cross/green seal), Canada (maple leaf), Japan (eco mark-arms embracing the globe), Scandinavian countries (white swan), Austria (symbol developed by Hundertwasser), Germany (blue angel), Poland (beetle on leaf), European Union (blossom symbol), and Hungary (green tree), (Bartz, 1998).

7.2. Clean Air and Energy Efficient Cars The environmental aspects of lubricants (Kubo et al., 1999; Waara et al., 2001) often encounter problems such as human health and safety, waste oil disposal, greenhouse effect, and oil resource consumption. The fuel efficiency improvement contributed by engine oil is thus becoming increasingly important in the US and European markets (Hoshino and Nakada, 1995). Following are some ways to improve the fuel efficiency performance of an engine lubricant: Use a lower viscosity grade oil to reduce fluid friction under high speed driving conditions at low temperatures. Low-viscosity and low-volatility base oils, such as poly-(x-olefin PAO or hydrorefined base oils, are necessary for the next generation of fuel-saving engine oils, such as those required to meet specification

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of GF-3 oil. Hydrorefined base oils have some advantages over conventional refined base oils such as low-volatility and durability. Use friction modifiers to reduce metal-to-metal contact friction under lowspeed driving conditions at high temperatures. This is particularly effective when lower-viscosity-grade oils are used (Kubo et al., 1993). There have been many studies of the effects of friction modifiers on engine friction reduction, with oilsoluble molybdenum compounds being the most widely evaluated. Molybdenum dithiocarbamate (MoDTC) gives superior friction reduction characteristics in combination with several other additives, and the formation of M0S2 on the worn surface where zinc dialkyldithiophosphate (ZnDDP) is the sulfur supplier additive under lubricating conditions (Grossiord et al., 1998 ; Johnson et al., 1995 ; Kasrai et al.,1998; Muraki and Wada, 1993; Nagakari et al., 1997). Sulfiir compounds contribute to M0S2 formation on the worn surface and also fiinction as antioxidants (Kubo et al., 1996; Yamada et al., 1997). Also, salicylate-based detergents improve the engine friction reduction and durability characteristics of MoDTC (Yamaguchi and Inoue, 1995). It is known that salicylate has both antioxidant and friction reduction capabilities owing to its molecular structure. Basic mechanisms of pollution are the processes of "combustion" and "friction" and their interplay. Friction causes contamination manifested in intensive energy dissipation in the form of heat, noise and vibrations. Cars emit about 60% of the consumed energy through combustion and friction. About 30% of the CO2 in the atmosphere has been emitted by automobiles, which contributes to the warming of the climate through the greenhouse effect (Kandeva, 1997). The legislation on emissions in the USA, European Community and Japan has a wide influence on the design of equipment and potentially on the lubricant requirements for new engines. Examples of emission requirements are shown in Tables 7.3 and 7.4, for both passenger and diesel car engines. The parameters for which the limits are quoted include particulates, oxides of nitrogen, unbumed hydrocarbons and carbon moide. Table 7.3 demonstrates the increasingly stringent requirements for these parameters (Calvert et al., 1963; Copan and Richardson, 1992). Auto emissions control has already a 30 year history. Exhaust emission federal standards for new cars were first set in 1968 (1966 in California), and became more stringent every couple of years until the early 1980s. The federal and California standards for formaldehyde are 0.01 g/km for 1993. California has set in place a program of future emissions standards more stringent than those in the 1990 Federal Clean Air Act Amendments that are designed to go by stages to low emission levels, including a requirement that 10% of the new cars in the year 2003 have "zero emissions". The most important urban air pollution problem is photochemical smog. Hydrocarbons HCs, their oxidation products, and oxides of nitrogen NO^, which denote the sum of NO and NO2, in a few hundred meters of air above our major cities, react in the presence of sunlight to produce strong

279

Environmental Issues

oxidizing compounds of which ozone is the most prevalent. The strategy adopted to minimize smog was a major reduction in unbumed HC emissions with lesser reductions in NO^. Unbumed carbon-containing compounds in the exhaust are fuel HCs and partial oxidation products that escape burning in the automobile spark-ignition engine. Carbon monoxide emissions are significant when the engine is operated under fuel-rich conditions; that is, when air in the fuel-air mixture that enters the engine cylinder is insufficient to convert all fuel carbon to CO2. Rich mixtures are used as the engine approaches wide open throttle because they produce the highest possible power from the engine. Also, they help with combustion stability during engine warm-up and, in older cars, at idle. Oxides of nitrogen are formed from nitrogen and oxygen in engine cylinder at the high temperatures created during the fuel combustion process, and are fixed from the reformation of oxygen and nitrogen by the rapid cooling of the exhaust gases. For over 20 years, catalytic converters in the engine exhaust system have reduced the emission of each of the three pollutants HCs, NO^, and CO that leave the engine's cylinders by a factor of 5 to 10 lower before the exhaust enters the atmosphere.

Table 7. 3. US and European emission exhaust standards (cars)[g/mile; g/km] Legislation ion emission Pre-control 1966 1968 1970 1973 1975 1980 1981 1983 1993 1994-7 1997 1997 2001 2004-9

California, CA US Federal Standard US Federal Standard US Federal Standard US Federal Standard US Federal Standard US Federal Standard California, CA California, CA US Fed. Stand. "Tier 1" Low emission vehicle, CA Ultra low emission, CA US Federal Standard US Fed. Stand. "Tier 2"

European emission regulation 2000 2005

HC^

CO

NO.

10.6; 6.6 6.3; 3.9 6.3; 3.9 4.1; 2.6 3.0; 1.9 1.5; 0.9 0.41; 0.26 0.41; 0.26 0.39; 0.24 0.25; 0.16 0.41; 0.26 0.08; 0.05 0.04; 0.03 0.41; 0.26 0.07; 0.04

84.0; 52.2 51.0; 31.7 51.0; 31.7 51.0; 31.7 28.0; 17.4 15.0; 9.3 7.0; 4.4 3.4; 2.1 7.0; 4.4 3.4; 2.1 3.4; 2.1 3.4; 2.1 1.7; 1.1 3.4; 2.1 3.4; 2.1

4.1; 2.6 6.0; 3.7 6.0; 3.7 6.0; 3.7 3.0; 1.9 3.1; 1.9 2.0; 1.2 1.0; 0.6 0.4; 0.25 0.4; 0.25 0.4; 0.25 0.2; 012 0.2; 0.12 0.2; 0.12 0.1; 0.06

0.20; 0.12 0.10; 0.06

2.3; 1.4 1.0; 0.6

0.15; 0.09 0.08; 0.05

^ NMOG, Non-methane organic gases (total of nonoxygenated and oxygenated HC).

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Cold start The concept and the performance of a device to remove hydrocarbons from engine exhaust during cold start were investigated (Yang and Kung, 1994). The device consisted of a hydrocarbon adsorbent and a metal oxide that could react with the hydrocarbon by oxidation to form carbon dioxide and water. The chemical reactions involved are shown below: C,Hy + nMO^

--

M o _ +Z/20,

^

XCO2 + y/2H20 + nMO^.,

(7.1)

MO,

(7.2)

The metal could then be regenerated later in the drive cycle by reaction with oxygen. Using a mixed oxide containing Cr, Co, Fe, and Al; ZSM-5 zeolite as the adsorbent; and propene, propane, or toluene as the hydrocarbon, the efficiency of hydrocarbon removal was measured in the laboratory during a 2 min period when the unit was heated from room temperature at a rate of 1 SO'^C/min. The efficiency was found to be suppressed by the presence of water vapor, but not affected by presence of CO and CO2 With the mixture of 10% H2O, 2% CO, 0.6% O2, 2000 ppm hydrocarbon, and the balance helium, the efficiencies were 92, 78, and 57% for toluene, propene, and propane, respectively. The evaporative HC emissions from vehicles fall into three categories: (a) diurnal, so called "diurnal breathing" of the fuel tank can produce evaporative emissions of as much as 50 g of HCs per day on hot days; (b) hot soak, emissions occur just after the engine is shut down and the residual heat from the engine heats up the fiiel system; (c) running losses, emissions occur as gasoline vapors are expelled from the tank while the car is driven and the fuel in the tank becomes hot, and when vehicle is filled at the service station (Calvert et al., 1963). The results of a 1987 field test of vehicle emissions in Los Angeles, in which measurements of pollutant concentrations in the air were used to compute actual on-the-road vehicle emissions, surprised the technical community (Pierson et al., 1990). Concentrations of CO and HCs averaged 2.7 and 3.8 times higher, respectively, than predicted by the emission inventory models that are the basis for predictions of mobile source emissions. More recent studies show that the measured values of HCs and CO are about 2 times the predicted concentrations. The remote-sensing and road side data show that about 50% of the CO and HC emissions come from 10% of the vehicles. What was not expected were the high emission rates detected in the worst 20% of more recent model cars (Calvert et al., 1963). Approaches to reduced motor vehicle emissions from cars fall into four categories: (i) technological improvements and modifications to the motor vehicle; (ii) improvements in fuels that reduce mass emissions of pollutants; (iii) inspection and maintenance programs (IM); (iv) transportation control measures.

Environmental Issues

281

(i) Technological improvements and modifications to the motor vehicle. Basic engine improvements include improved air and fliel distribution, m'xing, and combustion; reduced oil consumption, tighter tolerances on engine design and manufacture; and design of the total exhaust system for low heat loss and fast warm up. Control system changes include improvements in the ignition system, in airflow measurement, in the fuel metering and injection system, better integration of the engine-transmission-catalyst systems, and more precise control of the systems. Improved exhaust treatment includes increased catalyst size and precious metal loading, more effective catalyst formulations, mounting of the catalyst closer to the engine, and special cold start techniques. Improvements in these areas over the next decade or so offer continued reduction in exhaust emissions by the factor of 2 to 3, but with generally higher initial complexity and cost (Calvert et al., 1963). There are other opportunities, such as electrically heated catalyst, that might satisfy the regulations but are unlikely to be nearly as effective in use. With several kilowatts of electrical power, the catalyst can be heated before vehicle start-up to temperatures which effectively remove CO and HCs. A longer term opportunity also being explored aggressively is the lean NO^ catalyst. If an effective catalyst technology that removes NO^ when the engine is operated lean (that is, with excess air) can be developed, then this technology will allow some efficiency gain to be made in gasoline-fueled spark-ignition engines, and will also help the diesel emission problem substantially. New evaporative HCs emissions from gasoline-fiieled vehicle test procedures have been developed and are being implemented. More effective control systems for evaporative emissions are under development. (ii) Improvements in fuels that reduce mass emissions of pollutants or result in emissions that are less reactive in the atmosphere. Two important fuel modifications during the past 25 years have been enacted. In the early 1970s, the removal of lead from gasoline was begun, with the required sale of unleaded gasoline nationally to meet the needs of catalyst-equipped cars. Limitation of fiiel vapor pressure has recently helped to control evaporative emissions. The next phase of fuel changes to reduce emissions follows the three separate approaches of alternative fuels and reformulated gasoline such as: (a) expanded use of alternative or clean fuels such as natural gas, liquid petroleum gas, and alcohols have been proposed as part of a strategy to reduce automotive emissions; (b) reformulated gasoline: a term that refers to gasoline with modified chemical and physical properties to reduce the mass and air pollution impact of emissions; (c) changes in gasoline properties. Natural gas and electricity provide the least air quality problems, but neither is without emission problems. Compressed natural gas or liquid natural gas seem particularly promising for use in buses and in large-fleet applications. The alcohol fuels, especially when blended with gasoline and used in flexible fuel vehicles.

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provide little or no air quality advantages beyond the reduction of CO. The addition of ethanol to gasoline is generally counterproductive with respect to ozone formation. The two changes in gasoline properties to have a widespread beneficial effect are reductions in sulfur and vapor pressure. The reduction in fuel sulfur increases catalyst efficiency for HCs, CO, and NO^. The reduction in fuel vapor pressure has a direct effect on the generation of vapor in fuel systems. Evaporative emissions may make up 50% or more of HC emissions from modem automobiles, especially on hot days. Methanol and MTBE, added to HC fuels, increase formaldehyde emissions, and ethanol and MTBE increase acetaldehyde emissions, as would be expected from their chemical structures and reaction mechanisms. A recent study on changing fuel composition (aromatics reduction by 45 to 20%, olefins reduction by 20 to 5%), reformulated (oxygenate) fuels, and distillation characteristics T90) offers no clear advantage in exhaust emissions (see Tables 7.5 and 7.6). (Hi) Inspection and maintenance programs (IM). IM programs are intended to reduce in-use vehicle emissions through identification and repair of vehicles that do not meet exhaust emission standards. Today, IM is typically a measurements of tail pipe emissions at two different engine speeds with no load on the engine. Inspection and maintenance programs have generally not met their goals for the following reasons: (a) evaporative emissions have not been tested; (b) the repair of too many high-emitting vehicles has been waived because of cost limitations; (c) the tail pipe test on the operation of unloaded engines is not representative of on-road emissions; (d) testing has often been performed incompetently; (e) cheating has taken place through collusion between the tester and vehicle owners modifying their vehicles before and after scheduled testing. The EPA has proposed an enhanced IM program that would require a dynamometer test under varying engine loads, would test evaporative emissions, and would raise the repair cost limit substantially. The enhanced IM program still has some serious defects. Most notably, it lacks the necessary components of significant on-road, in situ, remote sensing to validate the emission reductions and to catch cars that have been tampered with between inspections. An important aid to the maintenance of good emission control over the useful life of each vehicle is the incorporation of on-board engine and emission control system diagnostics (OBD). These devices are combinations of sensors, computer diagnostics, and warning lights that alert the driver and maintenance personel to problems that affect emission control systems. Both the 1990 Clean Air Act and the California Air Resources Board (CARB) require that during the next few years, extensive OBD capability be built into new vehicles, as requirements for the performance of emission control systems.

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(iv) Transportation control measures. Real world emissions from the mobile fleet are undoubtably much higher than those produced from the stationary test on individual cars. An IM program should include the measurement of emissions of the mobile fleet as they occur in actual operation. The remote monitoring of tail pipe emissions of passing cars is a cost-effective tool that can allow the identification of high emitters, which can then be targeted for repair. The combination of remote-sensing programs with EM programs on the higher emitting vehicles is an especially attractive strategy. Prospect for the future. The reduction in emissions is predicted to come from new car standards, cleaner fuels, and from more effective inspection/maintenance programs. Reformulated gasoline is useful in reducing tail pipe and evaporative emissions. Some of the proposed gasoline volatility and composition changes in reformulated gasoline will produce improvements such as reductions in vapor pressure, light olefins, sulfur, and T-90. The others such as oxygenate addition and aromatics reduction do not seem worth the cost. The use of methanol and ethanol blended with gasoline in flexible fuel vehicles offers no significant improvement in emissions over the use of reformulated gasoline. Finally, because motor vehicles are now the source of less than half of the total urban emissions into the atmosphere, major reductions in stationary source emissions will likely be needed to achieve significant improvement in urban air quality (Brekken and Durbin, 1998). Reassessment of auto emission standards was due March 1998. The EPA must determine whether, for air quality purposes, there is a need to tighten controls on hydrocarbons, carbon monoxide, nitrogen oxides, and particulates. The study may also call for further regulating the sulfur content of gasoline. The current standards, known as "Tier 1", were implemented in the 1994 model year and regulate the same combustion byproducts. The upcoming "Tier 2" study would affect passenger cars and light-duty trucks such as sport utility vehicles. This reassessment was originally slated for release in June 1997, a delay that prompted a lawsuit by the Sierra Club, The recommendations would take effect no earlier than 2004. The California Low-Emission Vehicle program 1997 is an example of available technology which we need to study and implement as federal standards. That program contains five levels of progressively more stringent vehicle emission standards. The EPA will close the loophole for light trucks and sport utility vehicles. That loophole allows US-made sport utility vehicles used for routine transportation to benefit from older, more lenient emission standards originally intended for the smaller trucks used in farming and construction (Cooney, 1998a). Researchers agree that conventional high-sulfiir gasoline can poison a catalyst, but is the effect reversible? Establishing a program to put clean cars on the road five years ahead of a Clean Air Act mandate, the "Big Three" automakers agreed voluntarily to begin producing low-emission automobiles by the end of 1998 for

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sale in the northeastern states. These low-emission vehicles (LEVs) would achieve emission reduction of 50% for NO^ and 70% for hydrocarbons. Low emission vehicles planned for sale would not cut HCs and NO^ emissions if the cars were fueled with high sulfur gasoline commonly sold in the United States. Most gasoline sold in the U.S. has fairly high sulfur content. The national average is 350 ppm; as much as one-fourth of gasoline has a sulfur level of 500 ppm or higher. California, the nation's smoggiest state, now has a statewide sulfur standard of 30 to 40 ppm in gasoline. Most northeastern states, also suffering from high ozone levels, sell reformulated gasoline with sulfur levels of 150 ppm. EPA 1992 studies demonstrated that sulfur affects emission controls by poisoning the catalyst in the LEVs advanced catalytic converter. The studies indicated that high sulfur levels can cause a vehicle's on-board diagnostic equipment to malfunction. Sulfur atoms can bond with and block reactive sites on the catalyst surface, preventing the catalyzed reactions that break down NO^ and hydrocarbons. This effect has been known for several years. The catalyst manufacturers are doing research on this, and they have said that the state of art is not here. There are questions that concern us whether the sulfur on the catalyst is permanent or reversible. Studies already show that under certain conditions, the sulfur can be driven off the catalyst; a vehicle must be running at full throttle for at least 30 seconds, through 10 cycles, with accelerations from 50 to 110 km before sulfur at 600 ppm can be purged from the catalyst. The catalyst must be extremely hot, but such conditions don't exist in the real world. In March 1998, the petroleum producers and refiners joined forces and asked EPA to set regional low-sulfur gas requirements by 2004. Under this proposal, only dealers in 22 eastern states comprising the Ozone Transport Assessment Group (OTAG), would sell conventional gasoline with a sulfur content of 150 ppm in summer month, while the western states (except California (30 ppm/80 ppm cap) would sell gas with a 300-ppm level. High-sulfur gasoline contributes to particulate matter, so will not help states meet the new PM25 standard. If the country does choose to require low-sulfur fuel nationwide, it can break the barriers for many new technologies (Cooney, 1998c). The case study. All 103 diesel engine vehicles included in this study were recruited on the basis of their having high HC and /or CO emissions as determined by the remote sensing measurements. Seventy one of these vehicles were repaired as a part of the program and were retested. Seventeen vehicles in the fleet initially emitted visible smoke from the tailpipe and were classified as "smokers"„ The fleet ranged in age of 6 to 22 years, with a median age of 12.3 years. The average fleet particulate emission (PM-10) rate was 0.086 g/km, while the average emission rate for smokers was 0.246 g/km. The average emission rates for HC, CO,andNO^were3.9,46.3 and 0.75 g/km for the pre-repair fleet and 0.93, 17.8 and 0„93 for post-repair fleet. Ninety-nine percent of the vehicles had lover HC emission, 93%) had lower CO, and 55% had lower NO^ after repair (Cadle et al..

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1997). Currently, the industry is laying the groundwork for the next generation of diesel engine categories (API PC-7 and PC-8). Driving both the North American and European changes are the more stringent emission requirements imposed by national legislature. U.S. Federal Regulations mandate the profile shown in Table 7.4 (Rudnick and Shubkin, 1999). Table 7.4. Heavy duty-diesel engines-worldwide emission limits [mg/mile or mg/km] Model year

HC

CO"

NO"

Particulate

Fuel sulfur

1985 1987 1988 1991 1994 1998 2002

1.9/1.2 1.3/0.8 1.3/0.8 1.3/0.8 1.3/0.8 1.3/0.8 1.3/0.8

37.1/23.1 15.5/9.6 15.5/9.6 15.5/9.6 15.5/9.6 15.5/9.6 15.5/9.6

10.6/6.6 10.6/6.6 6.0/3.7 5.0/3.1 5.0/3.1 4.0/2.5 2.0/1.2

0.60/0.37 0.60/0.37 0.60/0.37 0.25/0.16 0.10/0.06 0.10/0.06 0.10/0.06

0.28/0.17 0.28/0.17 0.28/0.17 0.28/0.17 0.05/0.03 0.05/0.03 0.05/0.03

^HC = Hydrocarbons, CO = Carbon monoxide, NO^ = Oxides of nitrogen Diesel engines, which are used in the larger vehicles, are important sources of particles and NG^, but emit relatively low amounts of CO and HCs. Diesel particulate emissions can, over time, be controlled. The control of NO^ is problematic, and an appropriate technology is not available. Lean NO^ catalysts are being pursued but conversion efficiencies remain low. Diesel exhaust is a complex mixture of gases and fine particles, and contains over 40 substances that EPA considers hazardous, including benzene. The researchers found that exposure to diesel exhaust at a concentration of about 2 jj^^vc^ will cause cancer. The environmental groups are recommending a switch to liquefied natural gas (Cooney, 1998b) and oxygenated fuels (McCormick et al., 1997). Fatty acid mono-ester diesel fiiel (or biodiesel) has advantage over petroleumbased fuel in being a renewable source of energy, virtually free of sulfur and aromatic compounds (Bagley et al., 1998). Biodiesel fuel reduces total particle volume concentration and does not increase any of the potentially toxic, health related emissions. Changing the fuel composition can either raise or lower engine emissions (see Table 7.5). A consortium of 14 oil companies and the three major auto manufacturers has released its first data from a major testing program analyzing how gasolines can be altered to reduce emissions. The research program called the Auto/Oil Air Quality Improvement Research Program, is an unprecedented

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cooperative effort by industry. These data show emissions are a complicated problem and that gasoline composition changes will not be equally effective in all vehicle technologies. The first phase of the program entails measuring emissions for a large variety of vehicle and fiiel options and modeling the data to predict the ozone impact from these emissions. An analysis of relative costs of the ozone control alternatives will also be conducted. The initial comprehensive testing program will involve 29 different fuels and 53 vehicles, eventually resulting in more than 2200 emission tests. The intent is to determine for the first time, just how the different fuels and vehicles interact to produce exhaust emissions (Hansen, 1977). Table 7.5. Fuel composition change and engine emission. Test data for 1989 models vehicles (Hansen, 1991) Fuel composition change(%) Aromatics 45-20

MTBE 0-15

Olefins 20-5

T90 182- 138°C

Change in mass emission (t) increase (%) and (i)d.ecrease (%) tNO(+2) ICO (-13) iHC(-6)

INO(4-2) iCO(-12) lHC(-5)

iNO(-6)

rco(+2) tHC(+6)

tN0(+5) TCO(+l) lHC(-22)

HC = Hydrocarbons; CO == Carbon monoxide; NO = Nitrogen oxide; MTBE = Methyl tert-butyl ether; T90 = the temperature at which 90% of the fuel's mass has been evaporated by distillation. The decrease in the heavy gasoline components, Cg to Cg alkanes and C^ to Cg aromatics and alkenes Three groups of reformulated fuels were used in new, old and prototype vehicles. The first was reformulated gasoline, some of which contained methyl tert-butyl ether (MTBE). The second consisted of methanol and gasoline combinations, including what is called M85 with 85% methanol and 15%o gasoline. The third group was reformulated gasoline with ethanol or ethyl tertbutyl ether. The percentage of olefins and aromatics in the blends affect ozone formation, so aromatics were varied from 45 to 20%o of the gasoline and olefins from 20 to 5%. The 90%) evaporative temperature (T90) - the boiling range measure - was varied from 182 to 138°C. The quantity of MTBE ranged from 0 to 15%, the maximum allowed by current law. Using 1989 vehicles, the tests showed that reducing aromatics from 45%o to 20%) reduced exhaust emission by 6%). Adding 15%) MTBE reduced exhaust hydrocarbons by another 5%o, and reducing olefin concentration from 20%) to 5%o

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increased exhaust hydrocarbons by 6%. The single largest effect was a 22% reduction in hydrocarbons in exhausts found by reducing the distillation temperature from 182 to 138°C. For other pollutant emissions, the test showed that carbon monoxide could be cut by reducing aromatics and adding MTBE, but no changes came from reducing olefins or changing the T90. Nitrogen oxide emissions were lower with reduced olefins but increased by reducing the T90. Other changes had little effect on nitrogen oxide levels. For the 1983 to 1985 models, with up to 130,000 km on them, the results were somewhat different. Reducing aromatics cut hydrocarbons by 10% and adding MTBE reduced them by another 7%. Cutting olefines reduced hydrocarbons by 6%), but reducing T90 caused only a 2Vo reduction in hydrocarbons. Other pollutant changes were similar in new cars, except that reducing aromatics decreased nitrogen oxides. Much of the difference was explained by the use of fuel injectors in new cars and carburetors in older vehicles. These results did not include evaporative emissions, which may be a significant source of vehicle emissions. They also did not include all the gasoline variables being explored or the results of tests with methanol-blended fiiels. These results will be released in near future. The second phase of the research program will look at more advanced autos, including all-methanol and compressed natural gas cars, and will consider fuels mandated by legislation for their effectiveness. To reduce motor vehicle emissions, the Clean Air Act amendments of 1990 mandate the use of reformulated and oxygenated gasolines. Starting in 1995, reformulated gasoline has been required in nine areas with serious ozone air quality problems; oxygenated gasoline has been required since 1992 during the wintertime in ~ 40 CO non-attainment areas (Krichstetter et al., 1996). A minimum fuel oxygen content of 2.7 wt%) is required for all gasoline sold in CO non-attainment areas during the high-CO portion of the year. Most large urban areas in California are required to sell oxygenated gasoline. However, in California, the required oxygen content has been set at a lower level of 1.8 to 2.2 wt%) because the concerns of increased NO^ emissions. Light-duty vehicle emissions were measured in the San Francisco Bay area in August, 1994 (lowoxygenate, oxygen content 0.3 wt%; MTBE was the only oxygenate), and during October 1994 (high-oxygenate, oxygen content 2.0 wt%; 80%) of oxygenate was MTBE and 20%) was ethanol), and the data are presented in Table 7.6 (Krichstetter e t a l , 1996). As shown in Table 7.6 the measured pollutant emission concentration for high oxygenated gasoline is lower for carbon oxide and VOC by 21 and 18%, respectively. Similarly, formaldehyde emissions increased by 13%o, acetaldehyde did not change significantly, and benzene emission decreased by 25%). A similar reduction in CO emissions (-16%)) was measured during the Colorado oxy-fuels program (Bishop and Stedman, 1990). Acetaldehyde is formed as a partial oxidation product of ethanol, analogous to the formation of formaldehyde from

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MTBE. During phase 1 of the Oil/Auto program, an increase in fuel oxygen content from 0 to 2.7% by weight reduced stabilized CO emissions by 23% for 1989 model year vehicles and by 16% for 1983-1985 model year vehicles (Hochhauser et al., 1991). The reduction in VOC emissions (-18%)) and the intensivity of NO^ emissions to fuel oxygen content also is consistent with results reported elsewhere (Gething, 1991; Hochhauser et al., 1991; Hoekman, 1992).

Table 7.6. Impact of low and high oxygenated gasoline use on California light-duty vehicle emission (Krichstetter et al, 1996)

Contaminant

Low oxygenate^ 0.3 (wt%) MTBE''

High oxygenate' 1.6 (wf;/o) MTBE and 0.4 (wt%) ethanol

Change (%)

Pollutant emission [mg/L] CO VOC NO, Fonnaldehyde Benzene Acetaldehyde

78.20 4.17 7.56 58.9 235 13.7

61.6 3.41 7.53 66.3 176 14.3

-21 -18 0 +13 -25 +4

^Low-oxygenate = 0.3 (wt%) oxygen contentfromMTBE, High-oxygenate = 2 (wt%) oxygen contentfromMTBE 80% and ethanol 20% 'MTBE = methyl tert-h\xty\ ether Phase 2 of the California program took effect in the second half of 1996 and required more extensive changes in gasoline properties (Krichstetter et al.,1999). These changes included further reduction of summertime Reid vapor pressure RVP to 47 kPa maximum (in phase 1 RVP was reduced from 61 to 53 kPa); reduction of benzene, total aromatic, olefin, and sulfur contents in gasoline; addition of oxygenates; and reductions in distillation temperatures, T50 and T90. Estimates of the effect of phase 2 reformulated gasoline RFG, on vehicle emissions, including cold-start, running exhaust, and evaporative emissions, are the reduction of VOC by 17%), NO^ and CO by 11%), and toxic air contaminants by 30%o during the first year of use. Phase 2 RFG was introduced in the San Francisco Bay Area and pollutant emissions were measured at the 1100 m long Caldecott tunnel during the summers of 1994 and 1997. Between the 1994 to 1997, emissions of carbon monoxide decreased by 3 l%o, non-methane volatile organic compounds (VOC) decreased by 43%o, nitrogen oxides decreased by 18%), and vehicle emission of benzene was estimated to be a 30 to 40%) reduction. The use of RFG increased formaldehyde

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emissions by about 10%, while acetaldehyde emissions did not change significantly. The combined effect of phases 1 and 2 of California's RFG program included a 20% reduction in gasoline vapor pressure (Krichstetter et al.,1999). Fuel economy has been a key issue in North America for many years, regulated by the Corporate Average Fuel Economy (CAFE) requirements, which will continue to become more stringent through to the year 2005. The proposed EPA requirement in miles per US gallon/or km per L is: 27/11.5 (1990 year), 30/12.8 (1993), 37/15.7 (1995), 46/19.6 (2000). The probable emergence of a carbon dioxide tax in Europe will focus attention on the issue of fuel economy. A performance investigation group has been initiated within CEC to develop suitable tests for both gasoline and diesel engines to tailor the lubricant contribution to fiiel economy. The various studies into ahemate fuels may also provide lubricant opportunities for power plants using new fuel types. Alternate fuels. There is worldwide activity in investigating the commercial and technical feasibility of using alternate fuels to replace, at least in part, the traditional gasoline and diesel fuels. The fiiels under consideration are: methanol, ethanol, natural gas (compressed and liquified), repassed methyl ester, propane (liquified), reformulated gasoline, hydrogen, vegetable oils, electricity, and ultra clean coal. The most viable near-term choices are methanol, reformulated gasoline, compressed natural gas, and propane. Large scale electric vehicle technology application was scheduled for the mid 1990s while hydrogen and ultra clean coal are the new century technologies. Hydrogen has been studied as a possible replacement for fossil fuels, it bums cleaner and uses renewable resources. A new method is able to extract hydrogen from certain forms of sugars, such as cellulose, lactose and starch. The first step is to use enzymes to form glucose from complex sugars. Other enzymes from microorganisms are used to convert the glucose into hydrogen. The complex sugars are abundant. Cellulose is found in wood products such as newspapers. Lactose is a milk sugar that is prevalent in cheese production waste whey. Starch can be derived from many sources. Jonathan Woodward and his group at the Oak Ridge National Laboratory in 1996 were able to increase the rate of hydrogen production, and almost doubled the efficiency of the process. They hope that genetic engineering will provide a more abundant and cheap supply of catalysts and that the catalyst can be reused, thus bringing the cost down (Anon, 1997). The Clean Air Act in North America is driving the move towards methanol powered vehicles. It is anticipated that 100%) of buses are capable of using methanol fuel. Further predictions indicate 1 million methanol powered vehicles in North America by the end of the 20*^ century. Environmental factors are strongly influencing the design characteristics of new equipment. Future lubricants incorporating performance enhancing additives will have to meet the

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challenge of lubricating the new equipment while being fully compatible with such diverse components as seal materials and exhaust gas treatment devices. One of the primary forces of change in engine and transmission design is the rapidly growing need to protect the environment, driven by a growing need to protect public awareness of environmental issues. Passenger car engines. In North America, development of two-cycle engines has a high priority at the three largest manufacturers, namely General Motors, Ford and Chrysler. There is also interest in Japan and in Europe, and test programs and developments are now moving forward. The benefits of two cycle engines are as follows: (a) lower costs of production and lower fuel consumption of 25% to 30% reduction, (b) approximately half of the size/weight of conventional engines, (c) greater simplicity and smoother than conventional engines. Diesel engines. The need to reduce emission, both in North America and in Europe, present a very difficult technical challenge to engine manufacturers regarding the use of specific lubricants. Design changes include redesigned combustion chambers, reduced piston crown clearance, high piston rings, improved exhaust gas recirculation and emission control via particulate traps or catalysts. The OEM's are also designing engines with reduced oil consumption as high oil consumption significantly contributes to particulate emissions. Future lubricants will play a key role in maintaining low levels of oil consumption through excellent wear and deposit control.

7.2. Clean Air and Energy Efficiency Cars U.S. does not have an energy problem but an oil problem. The nation is essentially self-sufficient in all energy sources except for oil, and, to make up for it, the U.S. imports approximately 2/3 of the oil it uses. Most of this oil powers the 200 million automobiles and trucks used by the nation's 250 million people. This enormous fleet generates most of our four main pollutants: carbon monoxide, nitrogen oxides, hydrocarbons, and particulates. There are many proposed alternative solutions and alternative fuels that claim to reduce dependence on foreign oil and clean the air. These proposed solutions can be compared by looking at the efficiencies of the proposed alternatives to the current gasoline vehicle (Taylor, 1998). Besides the conventional gasoline-powered internal combustion engine, five other systems have gained publicity recently: battery electric, hybrid electric, natural gas, hydrogen fuel cell, and gasoline fuel cell vehicles. All are active pursued by major manufacturers such as General Motors, Honda and Toyota (Brekken and Durbin, 1997).

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Battery electric vehicles (BEV). Electric motors are very efficient, taking their energy from batteries which are recharged at home or recharging stations. The very heavy batteries can store energy for a short range only. But we can not ignore the energy required to haul the massive hydrogen storage tanks in the vehicle. For the same stored energy, the high pressure tank volume for hydrogen is 16 times greater than for gasoline. The manufacturing of the ultrapure hydrogen required by the use of electric energy to electrolyze water into hydrogen and oxygen or by reforming methane, requires extra energy consumption. Hybrid combustion electric vehicles (HEV). The hybrid electric vehicle (HEV) uses an electric motor powered by electricity from a generator, which is powered by a small gasoline internal combustion engine (ICE). The ICE provides a small fraction (e.g. 10 to 20%) of the maximum power, and since the vehicle uses only 10% of its maximum power on average, power is drawn off from onboard batteries. When the vehicle requires less power, excess energy from the engine is stored in the batteries. Since the gasoline engine is always running at one speed it is optimized for efficiency, which eliminates throttling and other losses. Some lost energy is associated with charging and discharging inefficiencies. Engine on/off cycling produces the highest emissions. Advanced nickel metal hydride batteries store three times more energy per weight than lead batteries (approx. 100 W • hr per kg). This can serve for a 600-km radius vehicle; still, this energy density is small compared with gasoline at 10,000 W • hr per kg. Moreover, these batteries are approx. three times as expensive as lead batteries. The battery life is approx. 33,220 km and replacement cost is approx. $5,000. Natural gas vehicle (NOV). NGV vehicles use combustion engines and pressurized natural gas fuel instead of gasoline. The main constituent of natural gas is methane with an octane value of 130; NGV engine compression ratios can reach as high as 15:1. Higher compression ratios increase engine efficiency. Emissions below California's strict ULEV standards are obtainable. The disadvantage of natural gas vehicles is the lack of filling stations and the increased volume requirement for the pressurized natural gas fuel storage cylinders. Fuel cell vehicle - hydrogen fuel (FCVH2). FCVH2 are electric vehicles that use a cell to combine oxygen in the air with hydrogen to produce water and electricity for the electric motor. The main remaining problem includes distributing the fuel from production plants and storing it at high pressures on the vehicles. The broad flammability limits of hydrogen (3 to 75%) by volume in air) make this system potentially very hazardous. Fuel cell vehicle - gasoline fuel (FCVR), A second family of hydrogen cell vehicles is being developed to use gasoline or methanol fuel. This fuel cell with

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fuel reformer (FCVR) extracts hydrogen from the hydrocarbon fuel. The system must produce hydrogen cleanly enough to avoid poisoning the fuel cell with carbon monoxide and fuel components. The efficiency calculation of each stage in the fuel processing sequence was multiplied to arrive at the overall efficiency. At 100% efficiency, the chemical energy contained in 10.2 L of gasoline would be sufficient to move the fivepassenger car 600 km on which calculations were based; however, much more energy is needed at the source because of the inefficiency of energy conversion into useful work and other inefficiencies and losses. For instance, in the case of natural gas, the efficiency is approx. 90% for fuel transport from well to gas station, 95% for station compression of the gas into high-pressure vehicle tanks, 88% for weight penalty in carrying those tanks, and 26% for engine and transmission efficiency. If these are multiplied, it is shown that 51.1 gasoline L equivalent of energy are used to do 10.1 L of work. The hybrid electric system being explored by some Japanese and European manufacturers uses a combustion engine to charge the battery, thus eliminating the need for access to electric recharging stations. This concept seems like a fine idea at first glance. The battery would serve as an energy reservoir constantly refreshed by combustion engine recharging. In normal driving, the average engine power required is only 10% of the peak power with the battery providing the remaining 90%) peaking power when necessary. Electric batteries typically provide approx. lOOW per kg. A 75-kW total power plant (five passenger, 2.5 L, four-cylinderengine compact car) would require a 675-kg battery to provide this peaking power. The gasoline version of this car weighs only approximatelly 1,000 kg in total. There is a significant efficiency penalty in carrying this heavy battery. Fuel economy in urban driving is inversely proportional to weight. The overall efficiency of the 600-km hybrid car is only approximatelly two-third that of the same car powered by natural gas. There are some ways to increase vehicle efficiency (Brekken and Durbin, 1998): (a) Decrease base weight by using aluminum and lighter materials. Improve streamlining to reduce aerodynamic drag and the hauling energy intensity (HEI), The HEI is the ratio of the total fiiel energy stored on-board divided by the product of the vehicle weight and the vehicle range, as shown in equation: HEI = Energy/Weight • Range [Watt-hours/kg • km]

(73)

The HEI describes the amount of energy required to haul 1 kg of the vehicle over a distance of 1 km„ The HEI is a measure of efficiency - it is the inverse of a vehicle L divided by the weight of the vehicle; (b) Increase the efficiency of energy conversion devices such as the engine, fiiel cell, electric motor and gasoline

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Environmental Issues

reformer; (c) Increase vehicle power-to-weight (PWR) ratios of power providing devices such as engines, fuel cells, and peaking power batteries; (d) Increase the storage density for batteries and gaseous fuels. Gasoline and liquid fuels are so energy dense that no improvements are necessary; (e) Increase performance of the car by decreasing power-to-weight (PWR) ratio. Following is a summary and comparison of existing alternative vehicles. Battery electric (EVl), Hybrid, and natural gas vehicles have recently been introduced to the public. Only the electric car EVl is currently available to the U.S. public (Oldham, 1997). The NECAR III is a concept car using steam reforming of methanol to provide hydrogen for a cell (Anon, 1997). The Prius is a parallel hybrid electric vehicle, where both the motor and the engine can turn the wheels (Yamaguchi, 1998). The P2000 is a concept hybrid electric vehicle (Bucholtz, 1997).

Table 7.7, Performance of existing altemative vehicles and future concept vehicles (SAE 1998) Vehicle efficiency

EVl Electric

NECAR III Methanol

Prions Gasoline

P2000 Diesel

HEI PWR MPG/kmPL Well to wheel MPG/kmPL

0.094 76 169/68 61/26

0.293 n/a 49/21 32/14

0.216 48 66/28 57/24

0.397 62 60/26 57/24

HEI = hauling energy intensity (W • hr/kg • km) PWR = power to weight ratio (W/^g) MPG = miles per gallon or kmPL = kilometers per liter U.Sgalon = 3.785 L Table 7.7 shows vehicle efficiency corrected to miles per gallon (km per L) of equivalent gasoline energy. Vehicle HEI, PWR, and MPG/kmPL above are calculated from information given in the sources. The well to wheel MPG is calculated by multiplying the 'Vehicle MPG/kmPL' in Table 7.7 by calculated stage 1 efficiency. The P2000 uses diesel (the diesel stage 1 efficiency is assumed to be 0.86 or equal to the stage 1 efficiency for gasoline). The Prius uses gasoline. The NECAR III reforms on-board methanol. In Table 7.7 methanol is assumed to be produced from natural gas at an efficiency of 65% (Boroni-Bird, 1995). The Prius has an incredibly low HEI of 0.216 Whr/(kg • km). This corresponds to a fuel conversion efficiency of 46% if the HEI 100 value of 0.10 Whr/(kg • km) is assumedo Overall, these vehicles are about as efficient as the models predict. A

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summary of the comparative efficiency of the various alternatives to the standard passenger car is shown in Table 7.8 (Brekken and Durbin, 1997). Table 7.8 shows miles (km) per gallon of fossil fuel at the well, and not the gallons installed in the vehicle. This provides the true measure of fuel economy. For the five-passenger, 600 km range car the Fuel Cell Car is slightly more efficient than the Natural Gas Car; however, it is not a viable alternative. The technology is not available and is unlikely to ever be useful for routine operation. It is frightening to consider millions of vehicles carrying hydrogen fuel, which is not only flammable, but, when leaked into air, can explode with trivial ignition energy over an extremely broad range of concentration. Fuel cells are inherently costly. In 1991, the estimated cost for a 75-kW fuel cell was $300,000. Optimistic estimates claim that the cost of a 75-kW fuel system can be lowered to $5,000. This is to be compared with the cost of a conventional gasoline engine of about $1,000. Table 7. 8. The comparative efficiency of the various altemativefoelsystems for the fivepassenger, 372 mile (600 km) range car, and the two-passenger, 93 mile (150 km) range car Miles per equivalent gallon or km/L of gasoline energy Fuel system car Fuel cell vehicle-hydrogen fuel (FCVH2) Fuel cell vehicle-gasolineftiel(FCVR) Natural gas vehicle (NGV) Series hybrid electric vehicle (SHEV) Gasoline vehicle Battery electric vehicle-cadmium (BEV)

372 mile (600 km) range car

93 mile (150 km) range car

14.5 (34) 13.2(31) 12.8 (30) 11.1(26) 8.5 (20) 4.7(11)

27.2 (64) 20.4 (48) 21.7(51) 17.4(41) 13.6(32) 25.9(61)

Table 7.8 examines altemative fuel systems for the two-passenger, 150 km range car. Note that as the range and payload are reduced, the penalty of heavy fuel storage weights for the electric car is diminished and the efficiency is increased considerably from 11 to 61 MPG (4.7 to 25.9 km/L), while gasoline and hybrid electric cars go from about 20 to 32 MPG (8.5 to 13.6 km/L). Similarly, the natural gas car, which carries heavier fiiel storage tanks, goes from 30 to 51 MPG (12.8 to 21.7 km/L). In the case of ethyl alcohol as a vehicle fuel, called a "renewable" substitute for gasoline, it actually wastes petroleum. It is well known that the production of alcohol from crops such as com requires more than onethird petroleum from crop production and processing than the alcohol can replace. The price of the ethyl alcohol at the pump will need to be subsidized at (54 cents

Environmental Issues

295

per gallon) by the U.S. government (Brekken and Durbin, 1997). Almost as efficient as the Fuel Cell Car is the existing natural gas car. Approximately 1.4 million such cars are being driven in the world, with only approx. 40,000 in the U.S. The overwhelming advantage of the natural gas car is quite clear. Such an engine produces approximately one-third less carbon dioxide and is exceptionally clean burning. Honda recently revealed a prototype naturalgas-fuel led vehicle with an exhaust that was cleaner than the air in some cities. Natural gas is especially gentle on engines. Most of the engine wear in gasoline engines occurs during starting and cold running, when excess fuel is supplied to overcome the poor vaporization of cold gasoline. This excess fuel washes the lubricant off cylinder walls, increasing engine wear. A gaseous fuel requires no excess fuel under cold conditions. Natural gas cars have been run 500,000 miles (805.00 km) without perceptible engine wear. Most importantly, a widespread switch from gasoline to natural gas would solve the oil problem and free the U.S. from the shackles binding it to unstable oil-producing countries. Domestic oil would suffice for chemical feedstock. Domestic natural gas reserves are huge, cheap, and adequate for all transportation needs over the next century. Our air pollution and oil importation problems continue in spite of the decreased emission and increased vehicle efficiency since 1970 (Brekken and Durbin, 1998). Federal emission standards have reduced average automobile emissions by over 90 percent on a per mile basis since 1970. The average fuel economy of the U.S. vehicle fleet increased from 13.5 MPG (5.7 km/L) in 1970 to 21.5 MPG (9.7 km/L) in 1994. From 1970 to 1994, U.S. automobile registration increased at an average annual rate of 2.1 percent. The average annual miles traveled per vehicle increased by 0.7 percent per year since 1970 to an average of 13,186 miles (21,200 km) in 1994. In 1994, American vehicles averaged 13 trips per week and 347 miles (558 km) traveled per week - both highest in the world. Cheap gasoline is one of the most important reasons why we drive our vehicles so much. Increased taxation in Europe and Japan accounts for the factor of three or more higher prices relative to gasoline sold in America (Davis and McFarlin, 1996). Inexpensive fuel also influences vehicle design and consumer preferences. Vehicles keep getting bigger and vehicle power-to-weight ratios PWR keep increasing. An EPA data base of vehicle statistics shows that the average PWR of new car models in the U.S. increased by 30 percent from 1986 to 1997. The rapid grow of the Sport Utility Vehicle is a vivid example of the current popularity of large, powerful vehicles (US EPA, Office of Mobile Sources). In the model year 1997, U.S. vehicles averaged 24.3 MPG (10.3 km/L). These same vehicles would average 28.3 MPG (12 km/L), 4.3 MPG (1.8 km/L) increase or 18% improvement if they weighed and had PWR equal to the average of the 1986 model year vehicle. The same data also shows that decreasing the weight by 100 kg on a 1997 model year vehicle increases efficiency by 1.7 MPG (0.7 km/L)

296

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l.Wo improvement in fiiel economy for a constant vehicle PWR. Decreasing 1997 vehicle PWRs by 10% increases efficiency on average of 0.5 MPG (0.2 km/L) 2.2% improvement in fuel economy, (US EPA, Office of Mobile Sources). Consumers demand and buy larger and more powerful vehicles, and this trend is unlikely to change any time in the near future as long as gasoline is cheap. Added vehicle weight and power will continue to prevent improved engine efficiency from translating into increased vehicle MPG. Emissions from vehicles keeps decreasing, but increased vehicle mileage and the small proportion of vehicles in use with malfunctioning pollution control equipment continues to create air pollution (Calvert et al., 1963). Therefore, the push for even cleaner and more efficient alternative vehicles continues. This push will need to be stronger in the future since alternative vehicles currently occupy only 0.2% of the U.S. market (US DOE, 1995). Air quality standards. The Environmental Protection Agency EPA has established health-based National Ambient Air Quality Standards (NAAQS) for six pollutants: carbon monoxide, ozone, nitrogen oxide, sulfur dioxide, particulate matter, and lead. The current NAAQS are presented in Table 7.9. (US EPA, 1993). Table 7.9. National ambient air quality standards Pollutant 1. Particulate matter (PM,o) 1 a.Particulate Matter (PM2 5) 2. Sulfur Dioxide (SO2) 3. Ozone (O3) 4. Carbon Monoxide (CO) 5. Nitrogen Dioxide (NO2) 6. Lead (Pb)

Concentration (exposure time) 24 hrs Yearly 150 //g/m^ 65 //g/m^ 0.14 ppm 0.08 ppm 35 ppm (1 hr) 1.5 /ig/m^ (quarter)

50 //g/m^ 15 /ig/m^ 0.03 ppm 0.08 ppm 9 ppm (8 hrs) 0.053 ppm

Pollutant concentrations higher than the standards are considered harmfiil to health and below the standards are considered acceptable. Following is a short description of sources, effects on health and effects on the environment for the six pollutants. Particulate matter (PM-10, PM-l.S) is made up of dust, smoke and soot. The source is wood burning, diesel and other fuels, industrial plants, agriculture (plowing, burning of fields), unpaved roads. Health effects include nose and throat irritation, lung damage, bronchitis, and early death. Environmental effect

Environmental Issues

297

is that particulate matter is the main source of haze that reduces visibility. Sulfur dioxide. The sources are burning coal and oil, especially high sulfur coal from Eastern US, and industrial processes (paper and metal industry). Health effects include breathing problems, which may cause permanent damage to lungs. Environmental effects are as follows: SO2 is an ingredient of acid rain (acid aerosols), which can damage trees and life in lakes. Acid aerosols can also reduce visibility. Ozone (ground-level ozone is the principal component of smog). The source are chemical reactions of pollutants, nitrogen oxides (NO J , and volatile organic contaminants (VOcs) smog-formers. Cars are important sources of VOCs. Health effects include breathing problems, reduced lung function, asthma, irritated eyes, stuffy nose, reduced resistance to colds and other infections, and ozone may speed up aging of lung tissue. Environmental effects are that ozone can damage plants and trees, and smog can cause reduced visibility. Carbon monoxide (a clear colorless, odorless gas). The source is the burning of gasoline, wood, natural gas, coal, and oil. Carbon monoxide (CO) can have very serious effects on health because it is absorbed easily by hemoglobin which subsequently reduces ability of blood to bring oxygen to body cells and tissues. Acute CO poisoning can cause death within 24 hrs. Carbon monoxide is produced primarily by motor vehicles. Because motor vehicle emissions are the major source of CO, the highest levels occur during the morning and evening rush hours. The worst problems are found where large numbers of slow-moving cars congregate, such as in parking lots or during traffic jams. Carbon monoxide problems are greater in winter because cold weather makes motor vehicles run less efficiently, and strong inversion layers may develop near the ground, trapping pollutants. The EPA has established two national standards for carbon monoxide: 35 ppm over a one-hour period and 9 ppm averaged over an eight-hour period. These values may be exceeded only once during a given year at any given location. Once a location measures a second exceeding of either standard, it is considered to be in violation of that standard. Exceeds of the eight-hour CO standard are more widespread than one-hour exceed, and are the primary target of pollution control efforts. Nitrogen dioxide. The major sources of nitrogen dioxide (NO2) are motor vehicles, power plants, aircraft, as well as high temperature combustion processes used in industrial work. During high temperature combustion, the nitrogen in the air reacts with oxygen to produce nitrogen oxide (NO), a reddish-orange-brown gas. Photochemical reactions convert NO into NO2, Nitrogen dioxide (NO2) is considered a criteria pollutant due to adverse health effects. The current standard

298

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for NO2 is that an annual average value cannot exceed 0.053 ppm. Effects of nitrogen oxides on the environment, personal comfort, and well-being include impacts on vegetation, materials, visibility, rates of acidic deposition and symptomatic effects on humans. Nitrate aerosols are the most significant contributors to the urban visibility problems (Utah Division of Air Quality, 1996). Lead. The sources are leaded gasoline, paint, smelters (metal refineries), and manufacture of lead storage batteries. Health effects are as follows: causes brain and other nervous systems damage (children are at special risk), causes cancer in animals, causes digestive and other health problems. Effects on the environment include harming wildlife. A powerful human carcinogen S-nitrobenzantrone found in diesel-exhaust and airborne particulates. The compound discovered in the exhaust fumes of diesel engines may be the strongest carcinogen ever analyzed, according to Japanese researchers at Kyoto University (Enya et al., 1997; Pearce, 1997). They warn that a major source of this chemical is a heavily loaded diesel engine, and that it could be partly responsible for a large number of lung cancer cases in cities. The compound, 3-nitrobenzathrone (nitro-PAHs) produced the highest score ever reported in an Ames test, a standard measure of the cancer-causing potential of toxic chemicals. The Ames test measures the number of mutations the compound causes in the DNA of standard bacteria strains (Ames et al., 1975). In a test with a strain of Salmonella typhimurium, 3-nitrobenzanthrone recorded more than 6 million mutations per nanomole. This compared to the score of 4.8 million for its nearest rival, 1,8-dinitropyrene, which is also found in diesel exhaust and until now, had been the most powerful known mutagen. 3-Nitrobenzathrone is a nitrated polycyclic aromatic hydrocarbon. It is produced during reactions between ketone by-products of burning fuel and airborne nitrogen oxides that take place on the surface of hydrocarbon particles in diesel exhaust. It reveals a "remarkable increase" in emissions when engines are working under heavy load. The concentration of 3-nitrobenzatrone in diesel exhaust particle extracts was found to be 0.6 and 6.6 /ig/g for engine loads of 6% and 80%, respectively. In the atmospheric environment, during the night time when concentration of ozone becomes high, reaction of benzanthrone with N02-ozone occurs preferentially on the surface of airborne particulates (not in the gaseous phase) to give mutagenic 3-nitrobenzanthrone, because more than 96% of the total amount of benzanthrone in the atmospheric environment is known to exist in the adsorbed state (Ligocki and Pankow, 1989), This may show the potential danger of engine overloading. In a further experiment, the researchers found that the compound causes "considerable chromosomal aberrations" in the blood cells of mice, suggesting that it is likely to have similar effects on other mammals, including humans (Enya et al., 1997). Tiny combustion particles, many of them from diesel exhaust, have

Environmental Issues

299

been estimated to cause 10,000 deaths in Britain and 60,000 in the USA each year (Bown, 1994). The US EPA has found that thousands of modem heavy-duty diesel truck engines run cleanly during mandatory performance testing, but give off substantially more pollutants in normal highway use. EPA, the Justice Department, and state pollution control authorities are checking to see if the engines were designed to skirt the Clean Air Act control of smog. Manufacturers are arguing that they have done nothing wrong, claiming that the engines are designed to minimize emissions during the tests and stop and go urban driving (Anon, 1998).

Problems 7.1 Recyclability There are three basic methods of disposal of used oils. Which of the following is the most economical: (a) use as fuel, (b) disposal as toxic/hazard waste, (c) re-refining. 7.2 Environmentally acceptable lubricant For such a lubricant, several aspects of its performance should be considered: good performance as lubricant, low bioaccumulation, low toxicity, reduced or clean emission, biodegradability. Which of the above conditions can not be satisfied for a lubricant used in the car, 7.3 Biodegradation test methods By using the test method summaries of Table 7.1, explain the differences in concept of CEC and Close bottle test. Why the presence of additives (nitrogen and phosphorus excluded) in the lubricants is known to have a significant effect on the rate of biodegradation? 7.4 Biodegradation of lubricants With reference to Table 7.2, compare (a) vegetable, (b) mineral lubricants, and (c) synthetic lubricants, 7.5 Emission standards With reference to Table 7.3 and Table 7.4 "US emission (cars, diesel) exhaust standards" calculate percentage of progress (a) for present requirements for 1992 cars and diesel, and (b) compare requirements for cars and diesel for HC, CO and NO^ . 7.6 Reformulated fuels On the basis of Tables 7.5 and 7.6, explain what fuel composition can lower emission: (a) aromatics 45 -* 20%, (b) methyl tert-butyl ether, MTBE 0 - 15%, (c) olefines 20 - 5%, (d) T90 182 - 138°C.

300

Chapter 7

1.1 Energy efficiency car By using data from Table 7.8, discuss the performance of the following: (a) Hydrogen fuel cell electric (PCVH2), (b) Fuel cell vehicle-gasoline fuel (FCVR), (c) Natural gas vehicle (NGV), (d) Series hybrid electric vehicle (SHEV), and (d) Battery electric vehicle-cadmium (BEV).+

301

Chapter 8

LUBRICATING OILS - RELATED ACRONYMS AND TERMS

AAMA Abrasion ACEA ACS Additive

Adhesion

Adsorption AES ANFOR Anionic surfactant

American Automobile Manufactures Association. Surface loss of material due to fractional forces. Association des Constructeurs Europeens dAutomobiles-(European Motor Vehicle Builders Association). American Chemical Society. Chemical compound added to a base oil to improve oil quality; as any substance that aids in the lubrication process at the rubbing interface by functionality other than viscosity. Intermolecular energy of attraction between the separate molecules making up a homogeneous liquid or solid. The energy consists of contribution from Lifshitz-van der Waals components in all cases. Process in which molecules are concentrated on a surface by physical forces. Auger Electron Spectroscopy. The effect was discovered by Pierre Auger in 1925, Association Francaise de Normalisation. An ionic surface-active agent (surfactant) in which surface active moiety is the anion, e.g., sodium stearate, Ci7H35COONa^

Antifoam

AFV AGMA AHEM AIAM Alison-C4 AMA ANSI

Substance used to destabilize, or inhibit the formation of foam. Commercial antifoams are usually some form of poly(dimethylsiloxane) polymer, but short chain alcohols are frequently effective. Alternative Fuel Vehicle. American Gear Manufacturers Association. An organization for the promotion of industrial gear lubricant standards. Association of Hydraulic Equipment Manufacturers. Association of International Automobile Manufacturers. Test measures diesel engine oil friction retention and wear. American Manufacturers Association. American National Standards Institute.

302 Antioxidant

Antiwear

APE API

Ashless dispersant ASLE ASM ASME ASTM ATA ATC Autoignition AW Biodegradation BOD

Boundary lubrication

BRT

C CA (API)

CAA CAAA

Chapters Chemical compound or substance that inhibits oxidation, decomposes peroxides and terminates free radicals, eg., ZDDP, hindered phenols, aromatic amines. Additive (AW), normally zinc dithiophosphate (ZDDP), which is added to lubrication formulation to prevent scuffing of the moving parts. Antiwear agent (forming tribofilm with a metal surface during friction process). Association of Petroleum Engineers. American Petroleum Institute. Consensus group within the oil industry responsible, among other duties, for defining lubricant quality standards, and for enforcing compliance with the standards where their symbol is used. Additive containing no metallic elements, e.g., ashless dispersant: succinimides, succinate esters. American Society of Lubrication Engineers (now STLE). American Society of Materials. American Society of Mechanical Engineers. American Society for Testing and Materials. American Trucking Association. Additive Technical Council (European Petroleum Additive Industry Association, European CMA). The temperature where an oil will ignite without a source of ignition. see Antiwear. Biodegradation is the breakdown of the chemical by organisms. Biochemical Oxygen Demand. This refers to the amount of oxygen consumed by microbes when metabolizing or degrading a compound. The lubricant film between the two surfaces is no longer a liquid layer, instead the lubrication two surfaces are separated by film of only molecular dimensions and may contact each other. The properties of lubricant other than viscosity. Ball rust test: the new bench test to replace the Sequence II D engine test to measure rust and corrosion at low temperatures„ Commercial (Fleets, Contractors, Farmers, Diesel). This is diesel classification operated in mild to moderate duty with high quality fuel and occasionally has included gasoline engines in mild service. This classification is obsolete. Clean Air Act. Clean Air Act Amendment,

Lubricating Oils - Related Acronyms and Terms

CAFE CARB Caterpillar IN Caterpillar IP Caterpillar IK, IM-PC CB (API) Cationic surfactant

303

Corporate Average Fuel Economy. The average fuel economy of the fleet of cars sold by manufacturer. California Air Resources Board. Uses a single-cylinder Caterpillar test engine (148.8 CID). Single cylinder engine test designed to measure piston deposit control of engine oil. Test designed to measure diesel oil piston deposits oil consumption. Diesel engines oil designed for this service were introduced in 1949, This classification is obsolete. An ionic surface-active agent in which the surface active moiety is the cation, e.g., cetyl trimethylammonium bromide (CTAB) Ci6H33N(CH3)3^Br-.

CC (API) CCMC CCPA CCS CD (API) CE (API) CEC

CEE CEFIC CEN CEPA CERLA CF (API) CF-2 (API)

Diesel classification adapted in 1961. This classification this is obsolete. Comite des Constructeurs d'Automobiles du Marche Commun. European motor builders association (precursor of ACEA). Canadian Chemical Producers Association. Cold cranking simulator (ASTM D5293). This is the diesel classification adapted in 1955. This classification is obsolete. This is the diesel classification adapted in 1980. This classification is obsolete Conseil Europeen de Co-ordination pour les Developments des Essais de Performance des Lubrifiants et desCombustibles pour Moteurs (European Coordinating Council of Motor and Petroleum Industries: test standardization like ASTM). Conseil Europeen Economique. Conseil Europeen des Federations de I'lndustrie. Chimique (European Chemical Industry Council). Conceil Europeen de Normalization. Canadian Environmental Protection Agency. Comprehensive Environmental Response, Compensation and Liability Act, The diesel engines oils designated for this service were introduced in 1994 and may also be used when API service category CD is recommended. This is the diesel classification adopted in 1994. These oils do not necessarily meet the requirements of CF or CF-4.

304

CF-4 (API)

Chapter 8

This is the diesel classification adopted in December 1990 and describes oils for use in high speed, four-stroke diesel engines. The newest diesel classification, at this time, is CF-4, which exceeds the service requirements of CE category. CG-4 (API) Service category CG-4 designated to meet 1995 exhaust emission standards for use in high speed four-stroke-cycle diesel engines CH-4 New classification for the generation of heavy-duty engine oils. Chemisorption The process of adsorption characterized by a chemical reaction between the adsorbate and adsorbent, where exchange of orbital electrons occurs. Chemical Industries Association (part of CEFIC). CIA Cleveland Open Cup apparatus used to determine the flash and Cleveland fire point of all. CMA Chemical Manufacture's Association (standard body composed of additive manufactures (U.S.). CMAQ Congestion Mitigation and Air Quality Program. Critical micelle concentration. The concentration in solution at CMC which a surface-active agent forms multimolecular aggregates which are in kinetic equilibrium with monomer. Cetane number. The performance rating of a diesel fuel CN expressed as the percentage of cetane that must mixed with liquid methylnaphthalene to produce the same ignition performance as the diesel fuel being rating. Compressed Natural Gas. CNG CNPC China National Petroleum Corporation. COD Chemical Oxygen Demand. COD refers to the amount of oxygen consumed during chemical oxidation with hot dichromate. CONCAWE Conservation of Clean Air and Water (Europe). Concentration Ppm-parts per million (/ug/mL, 10'^ g/mL, mg/L, lO'^g/L); ppbunits parts per billion (ng/mL, 10"^ g/mL, /ugfL, 10'^ g/L); pptparts per trillion (pg/L, 10"^^ g/mL, ng/L, 10'^ g/L). Centipoise = mPa.s (SI unit). cP Coordinating Research Council (an American standardizing body CRC for performance testing). Chromatography Any analytical procedure in which the components of a mixture are separated by flow through a packed column, e.g., gas-liquid chromatography. Corrosive wear Wear process in which chemical reactions predominate. The sump at the bottom of the engine which covers the engine Crankcase crankshaft and in which the oil is retained when it is not circulating through the engine.

Lubricating Oils - Related Acronyms and Terms

305

cSt Cummnis M i l Cummins NTC400 CUNA

Centistoke = mmVs. Heavy duty engine test to measure crosshead wear. Diesel engine oil is a measure of piston deposit, oil composition, and wear. Commission Technica Di Unificazione Nel TAutoveicolo (CEC).

DD Defomant

Detroit Diesel. Very small amount is added to many different products which allows air in the oil to escape more rapidly. All oils will "foam", but an oil that does not release air is said to "foam". Oil additive that prevents deposit from forming on engine surfaces and may remove previously formed deposits. Detergents are metal salts of organic acids and have a polar end containing a metal ion. e.g., neutral sulfonate (Cj7H35S03)2Ca. Typical compounds: neutral and basic (contain base reserve, usually as metal carbonate) sodium, calcium and magnesium sulfonates, phenates, carboxylates, and salicylates. These additives act by two mechanisms. (1) They neutralize inorganic acids that result from lubricant oxidation. (2) They associate with sludge and varnish precursors and keep them dissolved in oil. Both of these mechanisms prevent deposit formation, thereby keeping metal surfaces clean. Dexron 11 (General Motors trademark specification for automatic transmission fluid, ATF), Dexron-II issued 1993; Dexron-III issued 1998. Dexron-IV issued 2000. Detergent inhibitor formulation, or direct injection (normally diesel), or drive ability index. Deutsches Institut fiir Normung (German Standards Institute). Public shorthand for the class of chlorinated dibenzodioxins. Often refers only to the number of the class 2,3,7,8tetrachlorodibenzodioxin, considered to be most toxic. Oil additive that keeps the engine clean by holding in suspension the insoluble particles for oil oxidation and fuel combustion formed during engine operation. Dispersants have a polar fiinctional group appended to a large hydrocarbon group. Dispersants utilize oxygen and /or nitrogen for polarity and do not contain metal ions. Typical compounds alkenylsuccinimides and succinate esters, Mannich products, and alkenylphosphonic derivatives. Dissolved Organic Carbon. This is the amount of carbon present in the a test compound that is in aqueous solution. Department of Defense (U.S.).

Detergent

DEXRON

DI DIN Dioxin Dispersant

DOC DOD

306 DOE DOT DPMA DSC DVM

Chapter 8 Department of Energy (U.S). Department of Transportation (U.S.). Dispersant polymethacrylate viscosity modifier. Differential Scanning Calorimetry - used to measure onset oxidation temperatures of oils. Dispersant viscosity modifier.

European Community; European Commission; Environmental. Council (Japan); Environment Canada; energy conserving. Engine Fuels Technical Committee (CEC). EFTC Elastohydrodynamic (lubrication). EHD Emulsifier Substance used to promote or aid the emulsification of two liquids and to enhance the stability of the emulsion. Mixture of two liquids which are not to miscible with each other, Emulsion e.g., oil-in-water cutting fluid. Water-in-oil is classified as an inverted emulsion. The emulsions droplets are typically 1000 nm size. Soft-core reverse micelles size 10 to 20 nm.. Engler viscosity Another method of measuring the viscosity of an oil, primarily in Europe. Extreme Pressure (EP), an additive, normally a sulfur-phosEP phorus combination, effective in reducing wear, and preventing scuffing, scoring, and seizure under higher pressure contact. Environmental Protection Agency (U.S.). EPA Energy Policy Act. EPACT European Program to Investigate Emission from Fuels and EPEFE Engines (and advisory group consisting of 14 motor companies (ACEA) and 32 oil companies (EUROPIA)). Electron probe microanalysis (method to examine the EPMA composition of surface films). Engine Test of Lubricants Panel (LP). ETLP European Union. EU European Union of Independent Lubricant Manufacturers. EULIM European Petroleum Industry Association„ EUROPIA Electric vehicle. EV Extreme-pressure Compound typically containing sulfur, chlorine, or phosphorus lubricant which reacts with surface of the metal or tool to form a sulfide, chloride, or phosphide compound which has a low shear strength. EC

FBP FCEV FERC

Final boiling point. Fuel cell electric vehicle. Federal Energy Regulatory Commission (U.S.).

Lubricating Oils - Related Acronyms and Terms

307

Popular method of analysis which detects ferrous, non-ferrous, and non-metallic particles, both large and small, that could be suspended in the lubricant. Fire point see Flash point. The lowest temperature at which a material will sustain burning for five minutes (ASTM D 92). The temperature at which an oil, when subjected to a source of ignition or a flame, will ignite and continue to bum. Typically the fire point is about 10°C above the flash point. Federation Internationale des Societies d'Ingenieurs des FISITA Techniques de I'Automobile. The lowest temperature at which a material to form a vapor with Flash-point air will ignite (to flash or "poof) (ASTM D 92). The vapors will ignite and then go out. Four ball test Test which is used to determine the relative wear-ball preventing properties of lubricants under boundary test lubrication conditions. Two values are reported - load wear index and weld point fi'om two procedures: EP test ASTM D 2596 and wear test ASTM D2266. There are four steel y2-inch balls. Three of the balls are held together in a cup filled with lubricant while the fourth ball is rotated against them. Resistance to motion. Friction Friction modifier Agents such as PTFE and molybdenum compounds which are very effective at reducing friction at surface contacts. FZG test German gear test for evaluating EP properties. Use in the United States is becoming more common. Results will tend to demonstrate the real performance of a gear oil. Ferrography

G-1 GC GDI GEO GF-1 GF-2

GF-3

European classification for gasoline engine oils. Gas chromatography. Gasoline direct injection. Gas engine oil; lubricant used for natural gas engine. ILSAC PCMO oil classification standard. ILSAC PCMO oil classification standard effective 1997. The fuel economy requirement differ with multigrades and ranges from 0.5 to 1.4% and was replaced by ILSAC GF-3 by the year 2000. ILSAC PCMO oil classification after GF-2, in effect 2000.

Hard-core RMs Hard-core reverse micelles are composed of detergent molecules, e.g., calcium sulfonate strongly bonded to calcium carbonate core; detergent layer of 2.2 nm, hard-core diameter of 3.9 nm, total diameter of 8.2 nm.

Chapter 8

308 HD HDD Hydrophile

Hydrophobe

Heavy duty. Heavy duty diesel. An abbreviation for diesel enginesthemselves and also for the oil used in those engines. Group in a molecule which is preferentially water-soluble, usually through hydrogen bonding; also a surface which is wetted by water. Group in a molecule (or surface) which is not soluble in or wetted by water.

ISOT

Initial boiling point. Institution of Chemical Engineers (U.K.). indirect injection (diesel). International Energy Agency. Institute Francais du Petrole (France). Investigation Group Lubricants (CEC). International Lubricant Standards Approval Committee. Association of AMA and JAMA to develop unified lubricant standards. Specification GF-1 was issued in 1991 for use in North America. CCMC has declined to participate at this time. Vehicle Inspection and Maintenance Program (U.S.). Institution of Mechanical Engineers U.S.). Institute of Petroleum (U.K.). International Standards Organization, which has set viscosity standards for industrial oils. Viscosity is measured in centistoke at Celsius temperatures, eg., cSt at 40''C. This method is known as "kinematic viscosity". Indiana stirred oxidation test (adapted as JIS K 2514).

JAST JPI JSAE

Japan Society of Tribologists. Japan Petroleum Institute. Japan Society of Automotive Engineers.

Kinematic

see ISO.

LA

Lubricant additive {see additive). Lethal concentration to kill 50% of test population. Amount of the material when dosed orally or dermally which results is death of 50% of the test animals, usually rats or mice. It is generally expressed in terms of mg/kg of body weight. Low emission vehicle. Liquified natural gas.

IBP IChemE IDI lEA IFP IGL ILSAC

I/M IMechE IP ISO

1^^50

LD 50 LEV LNG

Lubricating Oils - Related Acronyms and Terms Load Load-carrying

Low pour LPG LRI

Lubrication

309

Force measured during machine testing expressed in units such as pound-force, newton, or kilogram-force. Maximum load or pressure which can be sustained by the capacity of lubricant (when used in a given lubricant system under specific lubricant conditions) without failure of moving bearings or sliding contact surfaces as evidenced by seizure or welding. Oil which will pour at a low temperature point Liquified petroleum gas. Lubricant Review Institute; a body associated with SAE which qualifies heavy-duty engine oil and gear lubricants for the industry and the U.S. military. Application of an oily or greasy substance in order to reduce friction. If a layer of lubricant is thick enough, it reduces the coefficient of friction (yu). For a very thin layer of lubricant, boundary layer lubrication takes place.

Engine test used by Mack Track Company in specifying T-6, T7 heavy-duty diesel lubricants. T-6 (piston deposit, oil consumption, and oil thickening); T-7 (oil thickening). Metal deactivator Additive which prevents the oxidation-increasing catalytic effect of certain metals on the oil in the lubrication system, copper, lead, and iron are the most common. Cummins heavy-duty engine test (CH-4) to measure crosshead Mil wear. Mill Mercedes-Benz gasoline engine oil sludge test (CEC-L-53-T95) and fuel economy test (CEC-L54-T-96). MERCON Ford Motor Company trademark ATF specification. Automatic transmission fluid key test requirements. Micelle (normal Oriented aggregate of surface-active molecules formed in or reverse) solution when the solubility limit for single molecules (monomers) has been reached. Such aggregates may contain from approximately 5 to 100 molecules. Thus, in water, the hydrophilic portion of the surfactant of molecules is on the outside and this aggregation is named a normal micelle. In organic low-polar solvents, the lipophilic moieties are on the outside and the aggregation is termed an inverse or reverse micelle. Military (U.S.). MIL Motor Industry Research Association (U.K.). MIRA Ministry of International Trade and Industry (Japan). MITI Ministry of Defense (U.K.). MOD Mack- T-6

310 MoDDP MoDTC MOFT MSDS MTAC Multiviscosity or multigrade oil

MVEG MVMA

NAAQS NACE NDVM NEB NGEO NIST NLEV NLGI NMHC NMOG NOACK NORA NPA NRC OCP OEM

Chapter 8 Molybdenum dialkyIdithiophosphate. Molybdenum dialkyldithiocarbamate. Minimum Oil Film Thickness. Material Safety Data Sheets (U.S.). Multiple Test Acceptance Criteria. Mineral oil to which a viscosity index improver has been added in order to reduce the thinning effect of heat. An or SAE 10 W30 or OW-40 is an oil which would meet the SAE lOW specification at -18°C (that is what the W means) and meets the SAE 30 or 40 specification at 100°C. A multiviscosity oil cannot be a SAE 30-40 because that would mean it had two different viscosities at 100°C. Fluids with very high natural viscosity indexes, such as some synthetic fluids, are also classified as multiviscosity. Motor Vehicle Emission Group (Europe). Motor Vehicle Manufacturers Association (U.S. passengers cars). A North American OEM association. As part of ILSAC, it participates in promoting the ILSAC engine oil standards. National Ambient Air Quality Standards (U.S.) National Association of Corrosion Engineers (U.S.) Non-dispersant viscosity modifier. National Energy Board (U.S.). Natural Gas Engine Oil. National Institute of Standards and Technology (U.S.). National low emission vehicle. National Lubricating Grease Institute (U.S.). Non-methane hydrocarbons. Non-methane organic gases (includes alcohols). NOACK volatility, DIN 51851 (ASTM 5800). National Oils Recyclers Association (U.S.). National Petroleum Association (U.S.). National Resources Canada.

Olefin copolymer viscosity modifier or viscosity index improver. Original equipment manufacturer. The company that makes the vehicle or unit, e.g.. Ford, GM. ON Octane number. A numerical measure of the antiknock properties of motor fuel, based on percentage by volume of isooctane in a standard reference fuel. Organic sulfide Extreme pressure (EP) additives, eg., dibenzyl disulfide, diphenyl sulfide.

Lubricating Oils - Related Acronyms and Terms

311

OSHA OTA OTC

Occupational Safety and Health Administration (U.S.). Office of Technology Assessment (U.S.). Ozone Transport Commission (U.S.).

PAH PAJ PAO PCBs

Poly aromatic hydrocarbons. Petroleum Association of Japan. Poly-a-olefin base stock of various viscosity classifications. Any of the chlorinated derivatives of biphenyl (also called diphenyl). European classification for passenger diesel engine oils. High temperature test is to evaluate an engine oil ability, under high temperature. Antioxidant based on 2,6-di-^gr^butyl chemistry or other alkylated phenolics. Polyisobutylene. Polyisobutenyleneamine succinamide; b-PIBS bis-substituted PIBS; m-PIBS monosubstituted PIBS. Polyisobutenyl succinic anhydride; precursor for ashless dispersants. Particulate matter less than 2.5 /^m and 10 yUm in diameter. Polymetacrylate viscosity index improver or viscosity modifier. Pensky-Martin closed cup-flash point test. Polynuclear aromatic. Measure of lubricant low-temperature flow which is 3 °C above the temperature at which a normally liquid petroleum product maintains fluidity. Oil forms a honeycomb or crystals at low PPD (pour point depressant). An additive used to lower the pour point of an oil by depressant modifying the structure of wax crystals.

PD-1 Peugeot TU3M/KDX Phenolic antioxidant PIB PIES PIBSA PM2 5;PMio PMA PMC PNA Pour point

PPD

RBPT test

RFG RME Rust inhibitor

Ready biodegradability (or readily biodegradable) test is an arbitrary definition whereby compounds achieve a 'pass' level in one of five named tests (OECD, Sturm, AFNOR, MITI or closed bottle, and rotating bomb oxidation test or RBOT. Reformulated gasoline. Repassed methyl ester. Lubricant additive; minimizes rust by preferential adsorption of polar compounds on metal surface to provide a protective film and/or neutralization of corrosive acids, e.g., fatty acids, nitrogen compounds, carbonate-alkylbenzenosulfonate.RMs. Water is solubilized by soft-core Rms.

312 SA (API)

Saybolt

SAE

SB SB (API)

SC (API)

SD (API)

SE(API)

Sequence test

Sequence IID

Sequence Ill E, III F

Chapter 8 Formally for utility gasoline and diesel engine service: simple mineral oil, no performance required and is not currently recommended by any engine manufacturer. This classification is obsolete. see SSU and SUS. A method of measuring viscosity in the U.S. The time (in seconds) for 60 ml of fluid to flow through a Saybolt Universal Viscosimeter at 40°C to 100°C. This method, although very common, is being replaced by kinematic method. Society of Automotive Engineers. Consensus body responsible, among other duties, for establishing the need for and the details of automotive and aerospace standards. Lubricant viscosity standards are their responsibility. An industry group which has set the viscosity brackets for motor oil grades and automotive gear oil grades (i.e., SAE 30, 15W-40, 80W-90). When a number such as 10 is followed by a W, it means that specification is measured at -18°C. A Table 1 in the Appendix section illustrates those SAE grades. Styrene-butadiene viscosity modifier or viscosity index improver. Minimum duty gasoline engine service: inhibited oil (no detergents) since the 1930's and provide only antiscuff capability, and resistance to oil oxidation and bearing corrosion. This classification is obsolete. This service is typical of gasoline engines in 1964 through 1967 models of passengers cars and some trucks. This classification is obsolete. 1968 Gasoline Engine Maintenance Service: this service is typical of gasoline engines in 1968 through 1970 models of passengers cars and small trucks. This classification is obsolete. 1972 Gasoline Engine Warranty Maintenance Service: designated for gasoline engines in passengers cars and some trucks. This classification is obsolete. A series of (ASTM) industry standardized tests used to determine the quality of test crankcase lubricants (i.g., Sequence II D, III E, V E, VI A). Uses a 1978 General Motors eight-cylinder commercial engine (350 CID). This test evaluates an engine oil ability to resist rust and corrosion. Uses a 1986 GM V-block six-cylinder commercial engine. Evaluation criteria: sludge, varnish, and viscosity changes.

Lubricating Oils - Related Acronyms and Terms

313

Uses a Ford four-cylinder commercial engine (2.3L). This test is to simulate stop-and-go driving to measure sludge, varnish, cam wear and oil screen plugging. Sequence Uses a 1993 Ford 8-cylinder commercial engine (4.5L). This VI A, VI B test is designed to measure fuel efficiency properties of an engine oil (VIA for ILSAC GF-2. VI B for ILSAC GF-3). Sequence Uses a single-cylinder test engine (42.3 CID). This engine test CRC-L-38 is similar to L-38 except that fuel used is unleaded; test measures bearing corrosion, sludge, oil oxidation, varnish, and viscosity change. SF (API) Oil which greatly increases engine protection for 1980 and later warranty. This oil maybe used where SC, SD and SE oils are recommended. 1989 warranty for current gasoline engines. SG oils also meet SG (API) the properties of the diesel classification CC and replace SE and SF oils for gasoline engines. Ratings such as SA, SB, SC, SD, and SE are obsolete. SH (API) Service requirement for gasoline engines and dated in 1993 for use in current and earlier all passenger cars, vans and trucks. Super High Performance Diesel Oil. SHPDO Spark ignition. SI Sludge inhibition bench test. SIB Styrene-isoprene viscosity modifier or viscosity index improver. SIV Adapted for gasoline engines in 1996 for use in current and SJ (API) earlier passenger-car, sport utility vehicles, vans and light trucks. Soft-core RMs Polar head groups e.g., sulfonate R-S03'M^ are drawn together to form hydrophilic and the hydrophobic tails are extended into the bulk hydrocarbon solvent to form reverse micelles (RMs). RMs have a dense polar soft-core, generally bounded together with a certain amount of water, which promotes micelle formation, SSU or SUS Saybolt Second Universal or Saybolt Universal Second viscosity measure. The time (in seconds) for 60 ml of fluid to flow through a Saybolt Universal Viscosimeter at 40°C to 100°C. Sump Crankcase or oil reservoir of an internal combustion engine. Boundary between two phases. The term surface and interface Surface are often used interchangeably. Surface tension Mechanical properties of the interfacial layer between two fluid phases can be tension expressed in terms of those of a geometrical surface of uniform tension called the surface tension. Society of Tribologists and Lubrication Engineers. STLE

Sequence VE,VF

314

Chapter 8

Total acid number; a measure of a lubricant's acidity. Total base number; a measure of the acid-neutralizing property of a lubricating oil. TFOUT Thin film oxidation test. TOST test Turbine oxidation oil stability test, or TOST. This a measure of an oil resistance to test oxidation in hours. Tribology The science of the mechanisms of friction, lubrication, and wear of interacting surfaces that are in relative motion. Tribochemistry The science concerned with all the all chemical reactions in mineral and synthetic formulation affecting the tribofilm formation on metal surfaces. It is a branch of chemistry different types of energy and catalysis. Tribochemistry deals with the relations between tribosystem and chemical changes of the surface layer. Tribochemical Reaction induced by rubbing, usually referring to reactions that reaction are not reactions observed by purely thermal means. Tribolological Also called tribosystem and tribomechanical system, whose functional behavior is system connected with interacting system surfaces in relative motion. Toxic substances control act (U.S.). TSCA TAN TBN

ULEV

Ultra low emission vehicle.

Viscosity

The measurements of the resistance of a fluid to flow. The higher the viscosity, the more resistance to flow. A "heavy" oil has more resistance to flow than a "light" oil. Normally measured at 40°C and 100°C for Saybolt Universal and for other classifications; dynamic viscosity (units: poise or centipoise); kinematic viscosity (stoke or centistoke). A measure of the rate of change in viscosity with temperature higher means less viscosity change (ASTM D2270). The rate at which an oil resists thinning with temperature. Oil with a VI of 150 resists thinning better than an oil with a VI of 100. A multigrade motor oil resists thinning better than mono-grade motor oil as temperature increases. Commonly called "VI". Viscosity index improver; additive that increases the viscosity index beyond that which viscosity can be obtained using ordinary refining methods. A VI improver increases an oil index resistance to thinning as it is heated. It is commonly used in multiviscosity or improver multigrade motor oils. Since a VI improver increases the viscosity as well as the viscosity index, it must be taken into consideration when formulating an oil.

Viscosity Index (VI)

VI Improver

Lubricating Oils - Related Acronyms and lerms

WOCNOV Waste oil

WF

ZEVs ZDDP

31b

(Example: Taking an oil in the SAE 10 range, adding a VI improver, would give us some oil like SAE 20W-40. Adding the VI improver to an SAE 30 w^ould yield an SAE 40). See multiviscosity the definition of SAE 10 and SAE 40 oils. Volatile organic compound/or volatile organic fraction. Used oil which contains in excess of 1000 ppm chlorine, 5 ppm arsenic, 2 ppm disposal cadmium, 10 ppm chromium, 100 ppm lead, or having a flash point of less than 40°C is considered to be hazardous waste and cannot be burned for energy recovery or used in any way to produce fuel. Waste oil containing more than 1000 ppm chlorine is considered to be contaminated with chlorinated solvents or PCB and is designated as hazardous waste. Work Function is an electron work function of the elements; a quantity (eV) that determines the extent to which emission will occur. The experimental method: thermoionic, field emission, photoelectric, and contact potential difference at the experimental conditions (e.g., vacuum of 10"^ Pa, clean surfaces, and identifi-cation of crystal-face distribution). Zero emission vehicle. Zinc dialkyldithiophosphate, a widely used antiwear and antioxidant additive for motor oils and industrial fluids; also referred to as ZDTP, ZDP, and "zinc".

Information source: ASTM, 1986; Becher, 1990; Bowden and Tabor, 1950 and 1964; Dorinson and Ludema, 1985; Kajdas, 1990; Kroschwitz and Howe-Grant, 1995; Miller, 1993; Rowe, 1969; Rudnick and Shubkin, 1999; lUPC, 1971; Weast, 1990.

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351

APPENDIX Table 1. Engine Oil Viscosity Classification: SAE J300 (April 1997)"

SAE

Low

Low

viscosity grade

temperature

temperature

OW 5W

low

15W 20W 25W 20 30 40 40 50 60

CO

CC)

cranking viscosity (cP, max)''

pumping viscosity (cP max, with no yield stress)""

3250 3500 3500 3500 4500 6000

60000 60000 60000 60000 60000 60000

at-30 at - 2 5 at - 2 0 at - 2 5 at - 2 0 at - 5

-

at - 4 0 at-35 at-30 at - 2 5 at - 2 0 at-15

-

Kinematic viscosity (cSt) at 100°C min"^

3.8 3,8 4.1 5.6 5.6 9.3 5.6 9.3 12.5 12.5 16.3 21.9

Kinematic viscosity (cSt) at lOO'C min"^

_ < 9.3 <12.5 <16.3 <16.3 <21.9

<16.1

High shear viscosity (cP), at 150°Cand lOV^min'

2.6 2.9 2.9' 3.7^ 3.7 3.7

^ All values are critical specifications as defined by ASTM D 3244 ^ ASTM D 5293 "" ASTM D 4684. Note that the presence of any yield stress detectable by this method constitutes a failure regardless of viscosity "^ ASTM 445 ' ASTM D 4683 or ASTM D 4741 'OW-40, 5W-40, and lOW-40 grades ^ 15W-40,20W-40,25W-40 and 40 grades

Appendix

352

Table 2. Development of engine, gear and transmission lubricants. Relationship between API (U.S.) MIL (U.S. Army), and ACEA (Europe) classifications Part A of Table 2 Engine ubricants Year

API

MIL

1930

SA SB CA CB CD

2104

1940 1950

1960 1970

CC SC SD SE

1980 1987

SF CE SG

1990 1992 1993 1994

CF-4

1995 1996 1998

45199A 45199B 2105B 2104C 46152 46152A 46152B 2104D 46152C 46152D 2104E 46152E

2104G

Gear lubricants API

MIL

2-105B

2104 A 45199

2104F SH CF CF-2 CG-4 SJ CH-4

ACEA

1

G-1/G-2/G-3 D-1/D-2/PD-1 G-4/G-5/D-4/D-5/PD-2

GL-1 GL-2 GL-3 GL-4 GL-5

2105 2105 A

GL-6

21058/Al 2105C 2105C/A1

2105B

353

Appendix P a r t s of Table 2. Transmssion lubricants Year

GM

1940 1950

Type A Type A SuffixA Dexron

1960 1970 1980 1990 2000

Dexron II (F/F) Dexron II (S/F) Dexron II (U) Dexron III Dexron IV

Ford M2C 33-A-B M2C 33-C-D M2C 33-F M2C33-G M2C 138-CJ M2C 166-M Mercon III Mercon IV

Table 3. Most important US (ASTM), Coordinating European Council CEC (L), and SAE (J) Standards Test Methods for Testing Lubrication Formulated ASTM No. D 93-00 D 94-00 D 97-96 D 445-97 D D D D D D D

664-95 892 -98 893-97 974-97 975-98 1401 -98 1744-92

D 2622-98 D 2670 D 2896 D 2983 D 3120

Standard Test Method for: Flash Point by Pensky-Martens Closed Cup Tester Saponification Number of Petroleum Products Pour Point of Petroleum Products Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosities) Acid Number of Petroleum Products by Potentiometric Titration Foaming Characteristics of Lubricating Oils Insolubles in Used Lubricating Oils Acid and Base Number by Color-Indicator Titration Specification for of Diesel Fuel Oils Water Separability of Petroleum Oils and Synthetic Fluids Determination of Water in Liquid Petroleum Products by Carl Fischer Reagent (discontinued in 2000) Sulfur in Petroleum Products by Wavelenght Dispersive X-Ray Fluorescent Spectrometry Standard Method for Measuring Wear Properties of Fluid Lubricants (Falex Pin and Vee Block Methods). Total Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration Low-Temperature Viscosity of Automotive Fluid Lubricants Measured by Brookfield Viscometer Standard Test Method for Trace Quantities of Sulfur in Light liquid Petroleum Hydrocarbons by Oxidative Microculometry.

354

Appendix

Table 3. (Continued) ASTM No. D 3525 D4172 D 4294 D 4628 D 4739 D 4742 D 4927 D 4951 D 5119 D 5185 D 5293 D 5302 D 5480 D 5533 D 5480 D 5800

Standard Test Method for: Test Method for Gasoline Diluent in Used Gasoline Engine Oils by Gas Chromatography Wear Preventive Characteristics of Lubricating Fluid (Four Ball Method) Sulfur in Petroleum Products by Energy-Dispersive X-Ray Fluorescence Spectroscopy Analysis of Barium, Calcium, Magnesium, and Zinc in Unused Lubricating Oils by Atomic Absorption Spectroscopy Base Number Determination by Potentiometric Titration. Oxidation Stability of Gasoline Automotive Engine Oils by Thin Film Oxygen Uptake (TFOUT) Elemental Analysis of Lubricant and Additive Components, Barium, Calcium, Phosphorus, Sulfur, and Zinc, by Wavelength Dispersive Xray Fluorescence Spectroscopy Determination of Additive Elements in Lubricating Oils by hiductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) Evaluation of Automotive Engine Oils in CRCL-38 Spark Ignition Engine Determination of Additive Elements, Wear Metals, and Contaminants in Used Lubricating Oils and Determination of Selected Elements in Base Oils by ICP-AES Apparent Viscosity of Engine Oils Between -5 and -30 ^C Using the Cold-Cracking Simulator Evaluation for Automotive Engine Oils in the Sequence VE Spark Ignition Engine Engine Oil Volatility by Gas Chromatography Evaluation of Automotive Engine Oils in the Sequence IIIE Spark Ignition Engine Motor Oil Volatility by Gas Chromatography Evaporation Loss of Lubricating Oils by NOACK Method

Appendix

355

Tables. (Continued) CEC No. L-Ol-A-79 L-02-A-78 L-24-A-78 L-49-A-93 L-41-T-88

SAE No. Jl 83 J300 J357 J1423

Standard Test Method for: Engine Oils Crankcase Oils Using the Petter AVI Single Cylinder Diesel Engine Oil Oxidation and Bearing Corrosion Test Using Petter Wl Single Cylinder Gasoline Engine Engine Cleanliness Under Severe Condition Using the Petter AVB Supercharged Diesel Engine Lubricating Oil Evaporative Losses (Using NOACK Evaporative Tester) Evaluation of Sludge-hihibition Qualities of Motors Oils in a Gasoline Engine (Using a Mercedes Benz M102E Engine) Standard Test Method for: Engine Oil Performance and Engine Service Classification (Other than "Energy Conserving") Engine Oil Viscosity Classification Standard Physical and Chemical Properties of Engine Oils Classification of Energy-Conserving Engine Oils for Passengers Cars, Vans, and Light

This Page Intentionally Left Blank

357

S U B J E C T INDEX

Abrasion, 165, 166 Acidity in water pool of RMs, 82 chemical shift of ^^P, 84 glass electrode, 85 pH indicator and buffer, 84 Acids in oil formulation, 113 inorganic, 15, 16,89, 113, 114 organic, 15, 16,89, 113, 114 Acid-base reactions on metal surfaces, 115-117 Acid-base reactions in oil formulation, 113-115 acid-base formation constant, 113, 115 glacial-acetic acid extracts, 250, 251 proton transfer process, 89, 90, 114 water extract of lubricants, 250-256 without proton transfer, 114 Acid neutralization, 15, 88 Acid deactivation, 15, 88 see also Total acid number Activated surface, 184 Additive tree, 19 Additives chemistry development, 18, 20 Additives functions, 19, 22-25 Additive package, 12 Additives anticorrosion and rust inhibitors, 22, 24 antifoamants, 23, 36 antioxidants, 23, 33 antiwear improver, 21, 22 boron nitride, 24 chemistry development, 18

crown-ether, 24, 30 detergents formulas, 26 developments, 20 dispersants formulas, 28 extreme-pressure improver, 21 friction modifiers, 22, 31 intermolecular interactions, 37, 38 microcyclic polyamines, 29, odorants, 36 performance additives, 22, 32, 33 polycyclic polyamines, 30 pour point depresant, 22, 32 preservatives, 36 protective additives, 23, 33-36 synergism and antagonism, 31, 37, 41 surface protective, 21-33 viscosity index improver, 22, 32 Adhesion, 165, 166 see also Wear Adsorption, 15, 16, 161-164, 172 physical, 161, 172 chemical, 162-164, 172, 179 Aerosol OT, 69-72, 78-82 Auger electron spectroscopy, 156-157 Aggregation, 70-75, 81 aggregation number, 70-75, 81 aggregation radius, 81 polar group factor, 72 shape of aggregates, 75 solvent factor, 3, 70-72 Air quality standards, 296 carbon monoxide, 297 ozone, 297 lead, 297 nitrogen oxides, 297

358

Subject Index

particulate matter, 296 sulfur dioxide, 297 Alternate fuels, 289 Analytical surface techniques, 124, 127,143-159,217 Analytical techniques in lubrication, 217 Anticorrosion inhibitor, 22, 24 Antifoamants, 23, 36 Antioxidants, 23, 33-36, 203 see also Oxidative stability ashless, 23, 33-36 interaction with additives, 27-31, 36-40 optimization, 12 Anti-wear (AW) additives, 21-23 action of ZDDPs, 137, 138 borate-sulfonate RMs, 98, 106 interaction with additives, 38-41 MoDTC + ZDDP, 200 thermal stability of ZDDPs, 127 Antiwear film, 121-133, 193-199 see also Tribofilm AOT, see Aerosol OT, 69-72, 78-82 API classification system, 57, 58 Archard's wear equation, 165 Ash, 17 Ashless dispersants, 27, 71 Asperities, 132 see also Surface roughness Association complex, 89 Association constants, 72, 73, 113-115 Atomic force microscope AFM, 177, 178 Auger electron spectroscopy AES, 154-158, 181 Autoxidation of hydrocarbons, 33, 34 Bioaccumulation, 269 Biodegradable fluids, 276 Biodegradability, 267, 274, 276 Biodegradation mineral oils, 276

synthetic oils, 276 vegetable oils, 276 Biodegradation tests, 275 Borated m-PIBS, 27 Boron nitride, 24 Boundary friction, 64, 173 Boundary lubrication, 164-169, 174175 see also Friction modifiers Chain length, 74 CAFE legislation, 273 Carcinogenic risk, 298 Catalytic converter, 279 Cationic surfactant, 68 CEC test, 275 Chemical oxygen demand (COD), 274 Chemical characteristics of engine oil condition monitoring, 231 - 237 infrared spectroscopy IR, 231 IR monitoring of additives, 235, 261 Fourier transform IR, 232 oxidation process IR monitoring, 233 soot, 234 nitration, 234 sulfation, 235 XANES monitored additives, 235 Chemical nature of metals surfaces, 161 Chemisorption, 162-164, 172, 184 Clean Air Act (U.S.), 289 Chlorinated additives, 182 Chemical reactions in oil formulation, 36-42 Chemical nature of metals surfaces, 161-165 Classification acid and bases HSAB, 116 Clean surface, 174 Clean exhaust emission, 272 Closed bottle test, 275

Subject Index CMC see Critical micelle concentration COD see Chemical oxygen demand Coefficient of friction, 166 Coefficient of friction curve, 169 Coefficient of wear, 166, 167 Cold start, 279 Colloidal structure, 94 Consumption of lubricants, 60 Corrosion, 24 corrosivity, 90-91 see also Wear Corrosion inhibitors, 22, 24 Critical micelle concentration CMC, 68 Crown ethers, 30 Degradation of lubricants, 221-223 Deterioration of lubricants, 220, 230, 257 Deposit, 25, 218-223 engine, 262 piston, 218, 219 see also Sludge; Varnish Detergency, 19, 20 Detergents, 22, 25-27, 40, 41 additional basicity, 13 classification, 26 interaction with dispersant, 40 interaction with ZDDPs, 40 see also Phenates; Phosphonates; Salicylates; Sulfonates; Total base number Dielectric constants, 3 Diesel engines, 16, 218, 219 Diesters, 50-53 Differential scanning calorimetry, 228 Dipole-dipole attraction, 1, 68, 76 Dispersants, 13, 22, 25, 27-31, 37-40 at surfaces, 28 interaction with detergents, 40 classification, 28 interactions with ZDDPs, 37-39

359

macrocyclic polyamines, 30 Mannich, 28 polycyclic polyamines, 30 Dissolution of surfactants, 69 Distillation, 268 Disulfide, 139, 183-185 Ecolabelling, 277 Ecotoxicity see also Environmental Issues Effectiveness of lubricants, 167 Elastohydrodynamic lubrication, 168 Electrical contact resistance, 125 Electrostatic stabilization, 16 Emission of electrons, 170 Emitted particles, 171 Emission, 171,284-289 phosphorus poisoning, 45, 46 Emulsions, 75 Energy efficiency car, 290-296 battery-electric, 290 comparative efficiency cars, 294 fuel cell-hydrogen, 291 fuel cell-gasoline, 291 future concept vehicle, 293 hybrid combustion-electric, 291 vehicle efficiency, 292-296 Engine design-changes, 19, 20 internal combustion IC, 221 schematic IC engine, 221 wear protection, 54, 55 Engine oils acidity and basicity, 238-251 classification, 56-58 classification in Europe, 62 condition monitoring, 223 consumption, 60 contamination, 218, 219 corrosivity, 90, 91 development and optimization, 20 deterioration mechanism, 220, 258 deterioration pH sensor, 230

360

Subject Index

degradation, 217, 222, 257-263 efficient engine oils, 202 environmental performance, 50 economy, 55, 204 European classification, 62, 376 evaluation test, 258 film thickness, 169 formulation, 12 IR monitoring, 235 Japanese classification, 63 military (US) specification, 61, 376 modified by MoDTC, 203 monitor recovery, 219 monitoring physical tests, 223 new sequence test, 61 oil change intervals, 60 oxidation process, 12 , 233 passenger car recommendations, 59 pH sensor, 230 performance, 60 preservatives, 36 requirements and specification, 5664 service classification, 58, 59, 376 thermal stability, 21 track and bus recommendations, 59, 60 viscosity classification, 56, 57, 375 Enthalpy, 4, 5 Enthalpy of hydration, 82 Entropy, 4, 5 Environmental issues, 267 acceptable lubricants, 268, 269 alternate fuels, 289 biodegradability, 267, 273, 276 biodegradation tests, 275 clean exhaust emission, 272, 273 ecolabelling, 277 exhaust gas recirculation, 273 low bioaccumulation, 269 low toxicity, 50, 271 oxygenated fuel, 287, 288 pollutants emission, 286-288

prospect the future, 283 reduction of environmental damage, 55 reformulated fuel, 286-289 EP additives, see also Extreme pressure additives Equilibrium constants, 72-73, 113 formation Kf(BHB^ AHA), 113 micellization formation, 72, 73 solubilization association, 73 Esters see also Synthetic esters; Vegetable oils European emission regulations, 279 Exhaust emission, 272, 273 Exoelectrons, 171-174, 184 Exoemission, 171 Extreme-pressure (EP) additives, 2122,182 Fatigue, see also Wear Ferrography, 227 Flash point, 226 Fluorocarbons, 69 Foam stability, see also Antifoamants Free energy change, 5, 72 Free radicals, 34 Friction organomolybdenum compounds, 188 temperature effect, 201, 202 Friction coefficient, 79, 166 Friction modifier, 22, 31 see also Boundary lubrication Friction forces, 100 Friction reduction, 205 Fuel economy test, 204 Fourier transform IR spectroscopy, 232 Fuel composition change 285, 286 Fuel consumption, 204, 289, 293 Future concept vehicles, 283, 293

Subject Index

Gas chromatography, 225 Gear lubricants, 63, 376 Hard-core RMs, 6, 67, 68, 71, 91-112 see also Reverse micelles antiwear characteristics, 93, 102-110 borate-carbonate RMs, 98, 106 borate-phenate RMs, 98 borate-sulfonate RMs, 98-101, 106 carbonate-benzenosulfonate RMs, 93-98 carbonate-salicylate RMs, 98-101 chemical and structural changes, 95 cupper oxide-oleic acid RMs, 111, 112 degradation RMs, 97 disintegration RMs, 112 friction characteristics RMs, 96, 100 hydrolytic stability RMs, 98 modification hard-core RMs,, 102-105 oxidative degradation test, 99 oxidation stability, 98, 99 XANES monitoring, 235 physico-chemical parameters, 98-102 sizes, 92, 93, 105 structure, 94 thermal stability, 100 tribochemistry hard-core RMs, 91 tribofllm composition, 107, 108 tribofilm XANES evaluation, 107 wear characteristic RMs, 100 ZDDP + hard core RMs, 99, 100, 106-110 Hard-soft acid-bases (HSAB) in tribofllm 7, 8, 116, 117 Heat of formation, 172, 187 compounds, 187 hydroxides, 172 nitrides, 172 oxides, 172, 187

361

phosphates, 187 sulfides, 187 Hydrated electrons, 80, 81 in bound water RMs, 80 in bulk water core RMs, 80 in water pool (WP) RMs, 81 probe (WP) in RMs, 81 spectral properties in RMs, 80, 81 Hydrodynamic lubrication, 167, 168 Hydrogen bonds, 39, 40, 113-115 anionic homocomplexes (AHA), 113 cationic homocomplexes (BHB^), 113 hydrogen bonding complexes, 113, 114 in reverse micelles core, 71, 85 proton transfer process, 89, 90, 114 Hydrolytic stability of synthetic oils, 50,52 Hydrophobic, 67 Hydrophilic, 67 Hydroperoxide decomposers, 34 Intermolecular interactions, 37-42 detergent + dispersant, 40 dispersant at surfaces, 40, 44 detergent + ZDDP in hydrocarbons, 37-40 detergent + ZDDP at surfaces, 41 dispersant + ZDDP in hydrocarbons, 38 dispersant + ZDDP at surfaces, 39 intensity (strong, medium, weak), 37,38 synergism and antagonism, 37 ZDDP in hydrocarbons, 42 ZDDP at surfaces, 43 Interfacial processes, 74-75 Infrared spectroscopy, 158, 159, 231-237 lubricant degradation, 231-233 oxidation monitoring, 233

362

Subject Index

surface tribofilm, 158, 159 ZDDP monitoring, 235, 236, 258261 Ionic surfactants, 68 lon-dipole attraction, 76 Ionization in (WP) RMs, 84-86 Ion-pair effects, 69, 89 Japan motor oil classification, 63 Kramer electron emission, 174 Lacquer, 19 Liquid lubrication mechanism, 168 Load wear, 21, 103, 105 see also Extreme pressure additives Low ash oils additives, 27 Low temperature viscosity, 51 Lubricants additives, 17, 18 developments, 18 film thickness, 169 formulation, 12 gear, 63, 377 low toxicity, 271 performance, 60 passenger car, 59 role in engine, 11 requirements, 56, 57, 60-64 service classification, 58, 376 specification, 56, 57, 61 track and bus, 60 transmission, 63, 377 water extract, 254, 257 Lubrication mechanism, 168 boundary, 168, 169 elastohydrodynamic, 168, 169 hydrodynamic, 168, 169 Lubricity, 50, 52 Microcyclic polyamines, 30 Micelles, association constant, 72

see also Reverse micelles; Hardcore RMs; and Soft-core RMs critical micelle concentration CMC, 68 formation constant, 12,92 , 113 free energy change, 3, 4, 72 interfacial tension, 69 micellar properties, 98 micelle core, 4 micelle shape, 75 micellization equilibria, 70 normal micelle, 67 surface tension, 69 water pool radius, 76 Metal surfaces, 161 see also Surface; Surface properties; Wear analytical techniques, 143, 144 chemical nature 162-164 chemisorption, 162 nascent surface, 162, 163, 175-177 physical adsorption, 161, 162 static condition surface, 162, 163 Micellar structure and aggregation, 71 Mineral oil biodegradability, 267, 273, 276 recyclability, 267 toxicity, 50, 267, 271,272 Mixed lubrication, 168, 169 Molecular weights, 14 MoDDP, see Molybdenum dialkyldithiophosphate MoDTC, see Molybdenum dialkyldithocarbamate Molybdenum disulfide, 189, 190 formation mechanism, 213, 21 oxygen in tribofilm, 189, 190 XANES spectra, 208-210 tribofilm formation, 200-207 ultra-high vacuum tribometer, 211 Molybdenum dialkyldithiocarbamate, 191

Subject Index

antiwear capabilities, 193, 194,200, 201 coefficient of friction, 197 evaluation test MoDTC, 202-205 friction and wear, 199-206 friction reduction, 203-205 friction test of MoDTC, 211-214 fuel economy, 204 formation of M0S2, 207-210, 213 MoDTC + ZDDP in surface, 190, 192-198,206 thermal film, 195, 196 XANES tribofilm characteristcs, 198 Molybdenum dithiodiphosphate, 191, 213 Multifunctional lubricants, 32, 56-61 Multigrade oils, 32, 33, 56-61 see also Viscosity Nascent surfaces, 7, 8, 78, 162, 163, 175-177 Natural gas, 281, 294, 295 Nitration, 203, 204 Negative-ion-radical-action (NIRAM), 183-185 Neutralization, 15 Nitrogen acids solubilization, 6 Non-aqueous systems, 113-115 Nuclear magnetic resonance NMR ^^P-chemical shift, 84 solubilization measurements, 77-80 water pool in RMs, 78 Odorants, 36 Organomolybdenum compounds, 188 see also MoDTC, MoDDP, M0S2 combination with ZDDP, 190-198, 202-207 friction and wear, 199-207 engine surface protection, 188-214 Organosulfiir compounds, 182-186

363

Oxidation of mineral oils, 12, 34, 35, 203 Oxidative stability, 50, 52 Oxygen in lubricants, 168, 169 Oxygenated gasoline , 287, 289 PAH, see Polyaromatic hydrocarbons PAOs, see Polyalphaolefins Paraffinic and synthetic oils hydrolytic stability, 50, 52 oxidation stability, 50, 52 thermal stability, 50, 52 see also Mineral oils; Synthetic oils Phenates, 13, 18,22,26 see also Detergents pH in soft-core RMs, 83-86 by glass electrode, 84 by P-NMR spectroscopy, 84 by indicators and buffers, 84 effective pH values, 82-86 water pool RMs changes , 80 pH water lubricant extract, 252-256 Phosphate esters, 49, 50, 53 Phosphonate detergents, 13, 22, 26 see also Detergents Phosphorus dispersants, 28 Phosphorus removal, 273 Photoelectron spectroscopy (XPS), 143, 145,153 Physical adsorption, 15, 16, 162, 172 Physical characteristic engine oil, 223 calorimetry, 241 colorimetry, 229 conductometric analysis, 229, 241 differential scanning calorimetry, 228 electrometric, 241 ferrography, 227 flash point, 226 fuel contamination, 224 gas chromatography, 225 insoluble test, 227 thermometric analysis, 229

364

Subject Index

thin-layer chromatography, 226 voltammetry, 229, 241 viscosity, 224, 261 water content, 228 wear metal analysis, 223 Plastic deformation, 178, 179 P-NMR spectroscopy, 84 pH in water pool of RMs, 84 Polymetacrylates, 22, 32 Polyalphaolefms (PAOs), 49-53 Polyaromatic hydrocarbons (PAH), 267, 298 Polichlorinated biphenyls (PCBs), 268 Polybutenes, 22 Polyisobutylene (PIB), 28, 29 Polyisobutylene amine succinimide (PIES), 28 b-PIBS-bis-substituted, 28, 29 m-PIBS-mono-substituted, 28, 29 Pollutants emission, 284-289 Polymers, 22, 32 Poly cyclic polyamines, 29, 30 Pour point depressant, 22, 32 Preservatives, 36 Proton transfer reactions, 16, 89, 90 Reactions in oil formulation, 112-118 Recyclability, 267 burning as a fuel, 267 disposal as toxic/hazard waste, 267 distillation/hydrotreatment, 268 Raman spectroscopy, 236, 237 Remaining useful life RULER, 220 Reverse micelles RMs, 2-7, 67 aggregation, 70, 71, 81 antiwear hard-core RMs, 93, 94 bounded water, 80 borate-phenate RMs, 98, 108 buffers, 84, 85 bulk water, 80 carbonate-phenate RMs, 13, 93, 95 carbonate-sulfonate RMs, 93, 95

dissolution, 70 electron in water pool (WP) Rms, 80 formation, 1- 4, 67-69 formation constant, 72 free water pool WP, 87 geometrical parameters, 81, 92, 93 hard-core RMs, 6, 67, 68, 71, 92, 103 IR study, 109, 110 micellization, 70-72 modified Ca sulfonate RMs, 102-104 p-nitrophenol in RMs, 84-87 preparation hard-core RMs, 92 shape of RMs, 75 sizeofRMs, 81,92, 93,95 soft-core RMs, 6, 67-69, 75, 76 solubilization, 77 structural studies 3-2, 3-27 water pool parameter Wo, 76, 80, 81 water pool radius WP, 76, 81 water in RMs formation, 78-82 ZDDP in hard-core RMs, 89, 98101, 105-110 XANES study, 106-109 RMs see Reverse micelles Rust inhibitor, 22, 24 Salicylates, 26, 40 see also Detergents Selective transfer. 111, 112 Sequence oil tests, 61 Silicone oils, 53 Sludge,6, 13, 15, 17,22,25-29, 218-223 inhibition bench test (SIB), 29 sludge formation, 6, 220, 221 Soft-core RMs see Reverse micelles Solubilization, 6, 14, 15, 73, 77 acids, 86-89 association constant, 73

Subject Index efficiency soft-core RMs, 78-80 polar group factor, 78 polar compounds, 86-89 secondary, 79 sludge particles, 6, 14, 15 strong acids, 6, 8 thermodynamics, 73 water, 77-80 weak acids, 86-90 ZDDP, 6, 44, 88, 89 Solvation forces, 16, 68 Solvents and micelles formation, 3, 70 Soot, 45 Sputter deposited M0S2 film, 188 Stribeck-Hersey curve, 169 Sturm test, 275 Succinimide, 22, 28 Sulfides, 182-186 Succinates, 22, 28 Sulfation, 235 Sulfonates, 22, 26 see also Detergents and dispersants Sulfur see also Organosulfur compounds in mineral oils, 182-186 removal, 273 sulfur acids, 6 sulfur dioxide, 296 Surface activated, 184 adsorption of ZDDP, 137 analytical techniques, 124, 125, 143159 energy, 167 nascent surface, 7, 8, 163 protective additives, 22 tribofilm formation, 8,131, 137 roughness, 132 Surface analytical techniques Auger electron spectroscopy (AES), 124, 145, 153-157, 176 Electric contact resistance ECR, 125

365

Electron-probe micro-analysis EPMA, 157, 158 Extended X-ray absorption fine structure EXAFS, 12, 145, 153 Imaging photoelectron spectromicro-scopy, 125 Ion scattering mass spectroscopy ISS, 145 Nuclear magnetic resonance spectroscopy, 124 Optical interferometry, 125 Reflectance-absorbance infrared spectroscopy RAIR, 124, 158, 159 Rutherford backscattering spectroscopy RBS, 145 Scanning electron microscopy, 158 Secondary ion mass spectroscopy SIMS, 145 Surface extended X-ray absorption fine structure SEXAFS, 153 Ultraviolet photoelectron spectroscopy UPS, 145 X-ray photoelectron spectroscopy, XPS, 124, 145, 153-156,176, 177 X-ray absorption near edge structure, XANES, 124, 125, 145, 147-152 X-ray fluorescence XRF, 124 Surface properties catalytic activity of surfaces, 170-179 roughness on tribofilm formation, 132 see also Asperities roughnes, 132 techniques description, 145 tribofilm composition, 124, 125 Surface tension, 167, 179 Surfactants, 67, 68 adsorption, 15, 16 anionic, cationic, amphoteric, 68 concentration, 12, 68 dissolution, 69

366

Subject Index

functions, 67 interfacial processes, 74 mechanism of action, 14, 69 micellar parameters, 68, 69, 81 micellization and the medium, 3, 68, 70 parameter v/aolc^, 74 polar group factor, 72, 78 succinimide, 28, 39, 40 succinate ester, 28 thermodynamic equilibrium 3-5, 72, 73 Synergism between additives, 11,37, 179, 198 Synthetic lubricants, 48-54 applications, 53 benefits using synthetic lubricants, 54-56 classification, 49 comparison with mineral oils, 51 engine wear protection, 54 environmental performance, 50, 55 esters and poly(a-olefins), 51 fully synthetic, 49 hydrolytic stability, 50, 52, 98 improved fuel and oil economy, 55 lubricity, 50, 52 oxidative stability, 50, 52, 99 oxidative degradation, 99 pour point, 51 reduced environmental damage, 55 test performance, 56 thermal stability, 50, 52, 100 viscosity, 50, 51 volatility (% loss), 51 wear protection, 54, 55 TAN see Total acid number TBN see Total base number Test synthetic and mineral oils, 58 Test on insolubles in engine oil, 227 Thermal stability, 21, 50 Thermodynamic equilibria, 69

Thermodynamic functions, 3, 4, 72, 73 enthalpy of micellization AH^j^^^, 4, 5, 72 entropy of micellization AS^^j^? 5,72 free energy change AG^^^^^, 3, 4, 72 Thin-layer chromatography, 226 Total acid number TAN, 240-250 Total base number TBN, 239-250 additional basicity number, 13 back titration, 245 conductometric titration curves,, 242-246 comparison TBN methods, 247-249 corosion rate, 89 potentiometric titration curves, 242244 techniques and methods, 241 Toxicity, 50,267, 271,272 Transmission lubricants, 63, 377 Tribochemistry, 1, 170 Tribochemical tree, 2 Tribochemical energy, 173, 187, 188 Tribochemical interactions additives, 36 hard-coreRMs, 6, 92-112 in oil formulation, 112-115 modified hard-core RMs, 93, 102-105 on metal surfaces, 7-9, 115-118 Triboemission, 171 Tribofilm antiwear film, 44, 45 chemical characterization, 121-127 chemical speciation, 127-129 composition, 124 detergent-dispersant effect, 139-142 effect of micelles, 141, 142 formation mechanism, 133-136 load and temperature, 132 physical parameters, 131, 133 chemical parameters, 133-143

Subject Index

polyphosphates chain length, 130 reactions of tribofilm formation, 137-138 rubbing time, 131, 141 surface roughness, 132 thermally generated film, 127 thickness, 132 tribochemical film, 8, 127, 195-199 XANES spectra, 127-130, 197 zinc presence in tribofilm, 130, 139 US emission standards, 279, 285 Used lubricants burning as a fuel, 267 collection, 267 disposal, 267 environmental impact, 268 Vacuum tribometer (UHV) 177, 211 Van der Waals forces, 74 Varnish, 220 see also Deposits Vehicle efficiency, 292, 293 Vehicle performance aging test evaluation, 203 diesel engine emission limits, 285 emission reduction, 280-289 fuel improvement, 281, 282 fuel composition, 286 inspection and maintenance, 282 control measures, 282 modification motor vehicle, 280, 281 passenger cars emission limit, 279 prospect for the future, 283, 284 transportation control measures, 282, 283 Varnish inhibition potency test, 29 Viscosity classification, 56, 57, 375 increase during oxidation, 224, 225, 260,261 viscosity changes, 262 see also Multigrade oils; Viscosity

367

index; Viscosity index improver, 22, 32 use of polybutylenes, 22, 32 use of polymetacrylates, 22, 32 use of styrene-butadiene co-polymers, 32 Volatility, 50, 51 Voltammetry, 229, 241 Water content in lubricants, 226 content in RMs, 75 hydrogen bonds, 39, 40 surface and interfacial tension, 74 Water pool (WP) of RMs, 78-81, 84 activity, 76, 77 hydration of electrons, 80 parameter number Wo, 76-82 pH, 84 properties, 76 radius, 76, 81 spectroscopic changes, 80 Wo, see Water pool parameter RMs WP, see Water pool RMs Wear, 8, 165-167 abrasive, 165, 166 adhesive, 165, 166 corrosive, 165 scar, 100 see also Corrosion Weighted total demerits (WTD) test, 30,31 Work function WF, 173-175, 187, 188 correlation with heat of formation, 187 X-ray absorption spectroscopy, 143-153 EXAFS, 143-147, 152 SAXAFS, 153 XANES, 143-152

368

Subject Index

peak position XANES spectra, 150-153 spectra and XPS comparison, 151, 152 X-ray photoelectron spectroscopy, 152-154,157, 176, 177 X-ray and spectroscopic notation, 146, 147 ZDDPs, 13,42-48 active, 261 analysis and identification, 48 adsorption, 131 antioxidation activity, 33-36 antiwear mechanism, 137-138 basic-neutral equilibrium, 123 chain structure, 47 friction of MoDTC/ZDDP, 199-201 interaction in hydrocarbons, 42, 43 interactions with additives, 37-39 interaction at surfaces, 43 effect of detergents on tribofilm 139-142 effect of dispersant on tribofilm, 139-142

linkage isomer LI-ZDDP, 136, 137 monomeric, dimeric and basic, 46-48 primary and secondary type, 47 reactivity, 46-48 retardation of decomposition, 106 solubilization by RMs, 44 stability in hard-core RMs, 105 surface-ZDDP reaction, 138 synthesis of ZDDPs, 46 thermal decomposition, 126, 127 tribofilm formation mechanism, 122-132 tribofilm XANES specta, 44, 45 ZDDP types, 47 Zinc dialkyldithiocarbomate, see ZnDTC Zinc dialkyldithiophosphate, see ZDDP Zinc diaryldithiophosphate, see ZDDP see also Anti-wear agents; Extreme pressure additives Zinc sulfide, 130, 131, 139