Surface Engineering for Corrosion and Wear Resistance

SURFACE ENGINEERING FOR CORROSION AND WEAR RESISTANCE

Edited by J.R. Davis Davis & Associates

I O M

heatio M iaclse InfoTrm nate Sro i ty Materials Park, OH 44073-0002 www. asminternational. org

Communications

IOM Communications is a wholly owned subsidiary of the Institute of Materials IOM Book No. B751

Copyright © 2001 by ASM International® All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the written permission of the copyright owner. First printing, March 2001

Great care is taken in the compilation and production of this Volume, but it should be made clear that NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, ARE GIVEN IN CONNECTION WITH THIS PUBLICATION. Although this information is believed to be accurate by ASM, ASM cannot guarantee that favorable results will be obtained from the use of this publication alone. This publication is intended for use by persons having technical skill, at their sole discretion and risk. Since the conditions of product or material use are outside of ASM's control, ASM assumes no liability or obligation in connection with any use of this information. No claim of any kind, whether as to products or information in this publication, and whether or not based on negligence, shall be greater in amount than the purchase price of this product or publication in respect of which damages are claimed. THE REMEDY HEREBY PROVIDED SHALL BE THE EXCLUSIVE AND SOLE REMEDY OF BUYER, AND IN NO EVENT SHALL EITHER PARTY BE LIABLE FOR SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES WHETHER OR NOT CAUSED BY OR RESULTING FROM THE NEGLIGENCE OF SUCH PARTY. As with any material, evaluation of the material under end-use conditions prior to specification is essential. Therefore, specific testing under actual conditions is recommended. Nothing contained in this book shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered by letters patent, copyright, or trademark, and nothing contained in this book shall be construed as a defense against any alleged infringement of letters patent, copyright, or trademark, or as a defense against liability for such infringement. Comments, criticisms, and suggestions are invited, and should be forwarded to ASM International. ASM International staff who worked on this project include Scott Henry, Assistant Director of Reference Publications; Bonnie Sanders, Manager of Production; Nancy Hrivnak, Copy Editor; and Kathy Dragolich, Production Supervisor. Library of Congress Cataloging-in-Publication Data Surface engineering for corrosion and wear resistance / edited by J.R. Davis p. cm. Includes index. 1. Corrosion and anti-corrosives. 2. Mechanical wear. 3. Surfaces (Technology) I. Davis, J.R. (Joseph R.) TA462.S789 2001 620.1'1233—dc21 00-048537 ISBN: 0-87170-700-4 ASM International® Materials Park, OH 44073-0002 www. asminternational. org Printed in the United States of America

P r e f a c e

Corrosion, wear, or the combined effects of these destructive failure modes cost industrial economies hundreds of billions of dollars each year. One of the more effective means of mitigating damage due to corrosion and wear is to treat, or "engineer," the surface so that it can perform functions that are distinct from those functions required from the bulk of the material. For example, a gear must be tough and fatigue resistant yet have a surface that resists wear. For applications requiring only a moderate degree of impact strength, fatigue resistance, and wear resistance, a higher carbon through-hardening steel may be sufficient. For more severe conditions, however, a surface hardened steel may have to be used. What are the options? Should the gear be flame or induction hardened, carburized or nitrided, or would high-energy processes such as laser- or electron-beam hardening be more appropriate? As a second example, consider the use of steels for various outdoor structural applications. Steel is popular because it is inexpensive, strong, and easily fabricated. Unfortunately steel is highly susceptible to severe corrosion in many environments and must be coated to achieve a satisfactory service life. Once again there are a variety of options. Should the component be painted, hot dip galvanized or aluminized, electroplated, thermally sprayed, or clad with a more corrosion resistant material? For large steel components, such as bridge members, size, weight, and handling problems may limit the type of surface treatment considered. Finally, take into consideration parts that require wearresistant, thin-film coatings. Can more conventional chromium or hard nickel electroplating be used, or will harder coatings deposited by vapor deposition techniques or ion implantation be required? Will processing time or temperature be a factor in coating selection? From the above discussion, it is apparent that engineers are faced with a bewildering number of choices when selecting the appropriate surface engineering treatment for a specific corrosion and/or wear application. But where does one start? Where can a design engineer find practical guidelines to aid in the selection process? The answers to these questions

lie within Surface Engineering for Corrosion and Wear Resistance. In addition to devoting an entire chapter to process comparisons (see Chapter 7), this book contains dozens of useful tables and figures that compare surface treatment thickness and hardness ranges; abrasion and corrosion resistance; processing time, temperature, and pressure; costs; distortion tendencies; and other surface treatment characteristics that must be considered when choosing the right coating for the job. The starting point for this publication was an excellent overview published by the Institute of Materials (IOM) entitled "Surface Engineering to Combat Wear and Corrosion: A Design Guide," which was written by Keith Stevens (A.T. Poeton Ltd.). Assisting IOM in the project was AEA Technology pic. and their National Centre of Tribology located in Risley, United Kingdom. The IOM booklet presents a concise methodology for understanding corrosion and wear problems and the many factors that must be considered before selecting a surface treatment. Material from the IOM design guide can be found primarily in Chapter 7, "Process Comparisons," and Chapter 8, "Practical Design Guidelines for Surface Engineering." Special thanks are due to Stephen Harmer, the editor of the IOM "Design Guide" series, who also reviewed several key chapters, and Bill Jackson, Head of Publishing for IOM, who worked out the copublishing agreement with Scott Henry, Assistant Director of Reference Publications for ASM International. Other key contributions for this book originated from Volumes 4, Heat Treating, 5, Surface Engineering, 13, Corrosion, 18, Friction, Lubrication, and Wear Technology, and 20, Materials Selection and Design, of the ASM Handbook series and from the Metals Handbook Desk Edition, Second Edition. Of particular note are articles authored by Arnold R. Marder (Lehigh University) and Eric W. Brooman (Concurrent Technologies Corporation) originally published in Volume 20 of the ASM Handbook. These are acknowledged at the conclusions of Chapters 4, 5, 6, and 8. Tabular data comparing various surface engineering processes were also adapted from the ASM Materials Engineering Institute course "Surface Engineering Processes for Wear and Corrosion" developed by Ralph B. Alexander (R.B. Alexander & Associates). Joseph R. Davis Davis & Associates Chagrin Falls, Ohio

Contents

Preface .......................................................................................................

vii

1. Introduction to Surface Engineering for Corrosion and Wear Resistance ...........................................................................................

1

Surface Engineering to Combat Corrosion and Wear ........................................

3

2. Principles of Corrosion ......................................................................

11

Electrochemical Corrosion Basics .......................................................................

11

Corrosive Conditions ...........................................................................................

13

Forms of Corrosion ..............................................................................................

15

Uniform Corrosion ..................................................................................

15

Galvanic Corrosion .................................................................................

16

Pitting .....................................................................................................

19

Crevice Corrosion ..................................................................................

21

Erosion-corrosion ...................................................................................

22

Cavitation ...............................................................................................

23

Fretting Corrosion ..................................................................................

24

Intergranular Corrosion ..........................................................................

25

Exfoliation ..............................................................................................

26

Dealloying Corrosion ..............................................................................

26

Stress-corrosion Cracking ......................................................................

27

Corrosion Fatigue ...................................................................................

29

Hydrogen Damage .................................................................................

30

Coatings and Corrosion Prevention ....................................................................

31

Corrosion Testing .................................................................................................

35

Field Tests .............................................................................................

36

Simulated Service Tests .........................................................................

36

Salt Spray Tests .....................................................................................

38

Humidity Cabinet Tests ..........................................................................

39

Electrochemical Tests ............................................................................

39

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iii

iv

Contents

3. Principles of Friction and Wear .........................................................

43

Friction .................................................................................................................

43

Wear .....................................................................................................................

54

Classification of Wear .............................................................................

54

Abrasive Wear ........................................................................................

56

Solid Particle Erosion .............................................................................

61

Liquid Erosion ........................................................................................

68

Slurry Erosion .........................................................................................

69

Adhesive Wear .......................................................................................

72

Galling ....................................................................................................

75

Fretting ...................................................................................................

76

Rolling-contact Wear ..............................................................................

77

Lubrication ............................................................................................................

77

Modes of Lubrication ..............................................................................

78

Lubricants ..............................................................................................

78

Wear Testing ........................................................................................................

81

Test Methods .........................................................................................

81

4. Surface Engineering to Change the Surface Metallurgy .................

87

Selective Surface Hardening ...............................................................................

87

Flame Hardening ....................................................................................

87

Induction Hardening ...............................................................................

88

High-energy Beam Hardening ................................................................

90

Laser Melting .......................................................................................................

91

Shot Peening .......................................................................................................

93

5. Surface Engineering to Change the Surface Chemistry ..................

95

Phosphate Chemical Conversion Coatings ........................................................

95

Types of Phosphate Coatings ................................................................

96

Applications ............................................................................................

98

Chromate Chemical Conversion Coatings .......................................................... 100 Aluminum Anodizing ............................................................................................ 102 Chromic Anodizing ................................................................................. 102 Sulfuric Anodizing .................................................................................. 103 Hardcoat Anodizing ................................................................................ 104 This page has been reformatted by Knovel to provide easier navigation.

Contents

v

Sealing of Anodized Coatings ................................................................ 105 Corrosion Resistance of Anodized Aluminum ......................................... 106 Oxidation Treatments .......................................................................................... 108 Diffusion Heat Treatment Coatings ..................................................................... 110 Carburizing ............................................................................................. 112 Nitriding .................................................................................................. 113 Carbonitriding and Ferritic Nitrocarburizing ............................................ 115 Pack-cementation Diffusion Coatings ................................................................. 116 Ion Implantation ................................................................................................... 120 Laser Alloying ...................................................................................................... 122

6. Surface Engineering to Add a Surface Layer or Coating ................ 125 Organic Coatings ................................................................................................. 127 Paints ..................................................................................................... 128 Ceramic Coatings and Linings ............................................................................. 132 Glass Linings .......................................................................................... 132 Porcelain Enamels ................................................................................. 133 Concrete and Cementatious Coatings and Linings ................................. 134 High-performance Ceramic Coatings and Linings .................................. 136 Hot Dip Coatings .................................................................................................. 138 Batch and Continuous Processing .......................................................... 138 Coating Microstructure ........................................................................... 138 Galvanized Coatings .............................................................................. 139 Galvanneal Coatings .............................................................................. 142 Zinc-aluminum Coatings ......................................................................... 142 Aluminum Coatings ................................................................................ 143 Terne Coatings ....................................................................................... 144 Electrochemical Deposition ................................................................................. 145 Aqueous Solution Electroplating ............................................................. 145 Continuous Electrodeposition ................................................................. 147 Fused-salt Electroplating ........................................................................ 148 Precious Metal Plating ............................................................................ 149 Electroless Plating .................................................................................. 150 Composite Coatings ............................................................................... 151 Weld-overlay Coatings ......................................................................................... 153 This page has been reformatted by Knovel to provide easier navigation.

vi

Contents Thermal Spray Coatings ...................................................................................... 160 Cladding ............................................................................................................... 166 Corrosion Control through Cladding ....................................................... 166 Chemical Vapor Deposition ................................................................................. 168 Physical Vapor Deposition Processes ................................................................ 172 Thermoreactive Deposition/Diffusion Process .................................................... 176

7. Process Comparisons ........................................................................ 183 Process Availability .............................................................................................. 184 Corrosion Resistance .......................................................................................... 185 Wear Resistance .................................................................................................. 186 Cost of Surface Treatments ................................................................................. 190 Distortion or Size Change Tendencies ................................................................ 191 Coating Thickness Attainable .............................................................................. 192

8. Practical Design Guidelines for Surface Engineering ..................... 195 Surface-engineering Solutions for Specific Problems ......................................... 196 Structural Parts in Corrosive Environments ........................................................ 197 Base Material ......................................................................................... 197 Neutral Environments ............................................................................. 197 Specific Corrosive Environments ............................................................ 197 Parts in Static Contact with Vibration (Fretting) .................................................. 199 Base Material ......................................................................................... 199 Contact Conditions ................................................................................. 199 Fretting Fatigue ...................................................................................... 200 Oxidative Wear ....................................................................................... 200 Parts in Static Contact with a Product ................................................................. 200 Base Material ......................................................................................... 200 Specific Applications .............................................................................. 201 Parts in Sliding or Rolling Contact with Another Surface .................................... 201 Base Material ......................................................................................... 202 General Contact Conditions ................................................................... 202 Surface-engineering Options .................................................................. 203 Specific Contact Conditions .................................................................... 205

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vii

Parts in Low-load Sliding Contact with an Abrasive Product .............................. 206 Base Material ......................................................................................... 206 Specific Applications .............................................................................. 207 Parts in High-load Sliding or Erosion with an Abrasive Product ......................... 208 Base Material ......................................................................................... 208 Surface-engineering Options .................................................................. 208 Parts in Contact with Another Engineering Component in the Presence of an Abrasive and Corrosion Product or Environment .................................... 208 Base Material ......................................................................................... 209 Surface-engineering Options .................................................................. 209 Preprocessing and Postprocessing Heat Treatment .......................................... 209 Coating Thickness, Case Depth, and Component Distortion Considerations ............................................................................................... 210 Surface Roughness and Finishing ...................................................................... 213 General Design Principles Related to Surface Engineering ............................... 213 Design Guidelines for Surface Preparation Processes ....................................... 218 Design Guidelines for Organic Coating Processes ............................................. 219 Design Guidelines for Inorganic Coating Processes ........................................... 222 Other Important Considerations for the Design Engineer ................................... 226

Glossary .................................................................................................... 231 Index .......................................................................................................... 257

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CHAPTER

I

I n t r o d u c t i o n

t o

E n g i n e e r i n g C o r r o s i o n

a n d

S u r f a c e f o r W

e

a

r

R e s i s t a n c e

SURFACE ENGINEERING is a multidisciplinary activity intended to tailor the properties of the surfaces of engineering components so that their function and serviceability can be improved. The ASM Handbook defines surface engineering as "treatment of the surface and near-surface regions of a material to allow the surface to perform functions that are distinct from those functions demanded from the bulk of the material" (Ref 1). The desired properties or characteristics of surface-engineered components include: Improved corrosion resistance through barrier or sacrificial protection Improved oxidation and/or sulfidation resistance Improved wear resistance Reduced frictional energy losses Improved mechanical properties, for example, enhanced fatigue or toughness Improved electronic or electrical properties Improved thermal insulation Improved aesthetic appearance As indicated in Table 1, these properties can be enhanced metallurgically, mechanically, chemically, or by adding a coating. The bulk of the material or substrate cannot be considered totally independent of the surface treatment. Most surface processes are not limited to the immediate region of the surface, but can involve the substrate by

Table 1

Surface engineering options and property benefits

Surface treatment/coating type

Primary property benefits

Changing the surface metallurgy Localized surface hardening (flame, induction, laser, and electron-beam hardening) Laser melting Shot peening

Improved wear resistance through the development of a hard martensitic surface Improved wear resistance through grain refinement and the formation of fine dispersions of precipitates Improved fatigue strength due to compressive stresses induced on the exposed surface, also relieves tensile stresses that contribute to stress-corrosion cracking

Changing the surface chemistry Phosphate chemical conversion coatings Chromate chemical conversion coatings Black oxide chemical conversion coatings Anodizing (electrochemical conversion coating)

Steam treating Carburizing Nitriding Carbonitriding Ferritic nitrocarburizing Diffusion (pack cementation) chromizing Diffusion (pack cementation) aluminizing Diffusion (pack cementation) siliconizing Boronizing (bonding) Ion implantation Laser alloying

Used primarily on steels for enhanced corrosion resistance, increased plating or paint adhesion, and for lubricity (e.g., to increase the formability of sheet metals) Enhanced bare or painted corrosion resistance, improved adhesion of paint or other organic finishes, and provides the metallic surface with a decorative finish Used for decorative applications, e.g., the "bluing" on steel gun barrels Used primarily for aluminum for increased corrosion resistance, improved decorative appearance, increased abrasion resistance (hard anodizing), improved paint adhesion, and improved adhesive bonding (higher bond strength and durability) Used on ferrous powder metallurgy parts to increase wear resistance and transverse rupture strength Used primarily for steels for increased resistance to wear, bending fatigue, and rolling-contact fatigue Used primarily for steels for improved wear resistance, increased fatigue resistance, and improved corrosion resistance (except stainless steels) Used primarily for steels for improved wear resistance Improved antiscuffing characteristics of ferrous alloys Improved molten-salt hot corrosion Improved oxidation resistance, sulfidation resistance, and carburization resistance Improved oxidation resistance Improved wear resistance, oxidative wear, and surface fatigue Improved friction and wear resistance for a variety of substrates Improved wear resistance

Adding a surface layer or coating Organic coatings (paints and polymeric or elastomeric coatings and linings) Ceramic coatings (glass linings, cement linings, and porcelain enamels) Slip/sinter ceramic coatings Hot-dip galvanizing (zinc coatings) Hot-dip aluminizing Hot-dip lead-tin alloy-coatings (terne coatings) Tin plate (continuous electrodeposition) Zinc-nickel alloy plate (continuous electrodeposition) Electroplating

Electroless plating Mechanical plating Weld overlays Thermal spraying

Cladding (roll bonding, explosive bonding, hot isostatic pressing, etc.) Laser cladding Carbide (salt bath) diffusion Chemical vapor deposition (CVD) Physical vapor deposition (PVD)

Improved corrosion resistance, wear resistance, and aesthetic appearance Improved corrosion resistance Improved wear resistance and heat resistance Improved corrosion resistance via sacrificial protection of steel substrate Improved corrosion and oxidation resistance of steel substrate Improved corrosion resistance of steel substrate Improved corrosion resistance of steel substrate Improved corrosion resistance of steel substrate Depending on the metal or metals being electrodeposited, improved corrosion resistance (e.g., nickel-chromium multilayer coatings, and cadmium and zinc sacrificial coatings), wear resistance (e.g., hard chromium coatings), electrical properties (e.g., copper and silver), and aesthetic appearance (e.g., bright nickel or decorative chromium plating) Improved corrosion resistance (nickel-phosphorus) and wear resistance (nickel-phosphorus and nickel-boron) Improved corrosion resistance Improved wear resistance (hardfacing alloys) and corrosion resistance (stainless steel or nickel-base overlays) and dimensional restoration (buildup alloys) Primarily used for improved wear resistance (many coating systems including ceramics and cermets), but also used for improved corrosion resistance (aluminum, zinc, and their alloys) and oxidation resistance (e.g., MCrAlY), thermal barrier protection (partially stabilized zirconia), electrically conductive coatings (e.g., copper and silver), and dimensional restoration Improved corrosion resistance Improved wear resistance Used primarily for steels for improved wear resistance in tooling applications Improved wear (e.g., tools and dies), erosion, and corrosion resistance; also used for epitaxial growth of semiconductors Improved wear (e.g., tools and dies) and corrosion resistance, improved optical and electronic properties, and decorative applications

exposure to either a thermal cycle or a mechanical stress. For example, diffusion heat treatment coatings (e.g., carburizing/nitriding) often have high-temperature thermal cycles that may subject the substrate to temperatures that cause phase transformations and thus property changes, or shot-peening treatments that deliberately strain the substrate surface to induce improved fatigue properties. It is the purpose of this book, and in particular Chapters 4 to 6, to review information on surface treatments that improve service performance so that metallurgists, chemists, mechanical engineers, and design engineers may consider surface-engineered components as an alternative to more costly materials. Surface Engineering to C o m b a t Corrosion and W e a r The Economic Effects of Corrosion and Wear. The progressive deterioration, due to corrosion and wear, of metallic surfaces in use in major industrial plants ultimately leads to loss of plant efficiency and at worst a shutdown. Corrosion and wear damage to materials, both directly and indirectly, costs the United States hundreds of billions of dollars annually. For example, corrosion of metals costs the U.S. economy almost $300 billion per year at current prices. This amounts to about 4.2% of the gross national product. However, about 40% of the total cost could be avoided by proper corrosion prevention methods. Table 2 provides a breakdown of the cost of metallic corrosion in the United States. Similar studies on wear failures have shown that the wear of materials costs the U.S. economy about $20 billion per year (in 1978 dollars) compared to about $80 billion annually (see Table 2) for corrosion during the same period. Table 3 illustrates the extent of wear failures by various operations within specific industrial segments. Highway vehicles alone use annually 14,600 X 1012 Btu/ton of energy represented in lost weight of steel and 18.6% of this energy could be saved through effective wear-control measures. Table 2 Cost of metallic corrosion in the United States Billions of U.S. dollars Industry

All industries Total Avoidable Motor vehicles Total Avoidable Aircraft Total Avoidable Other industries Total Avoidable Source: Ref 2

1975

1995

82.0 33.0

296.0 104.0

31.4 23.1

94.0 65.0

3.0 0.6

13.0 3.0

47.6 9.3

189.0 36.0

Table 3 Industrial operations with significant annual wear economic consequences Industry

Utilities (28% total U.S. consumption)

Transportation (26% total U.S. consumption)

Mining

Agriculture

Primary metals

Operation

Loss mass (a), 10 12 Btu

Seate Accessories Bearings Reliability Total Brakes Valve trains Piston ring assemblies Transmission Bearings Gears Total Ore processing Surface mining Shaft mining Drilling Total Tillage Planting Total Hot rolling Cold rolling Total

185 120 55 145 505 (b) (b) (b) (b) (b) (b) (b) 22.80 13.26 10.70 5.58 52.34 16.85 2.47 19.32 14.30 0.14 14.44

(a) Assumes 19.2 X 106 Btu per ton of energy represented in lost weight of steel. (b) Lost mass not estimated. Source: Ref 3

Corrosive Wear. Complicating matters is the fact that the combined effects of wear and corrosion can result in total material losses that are much greater than the additive effects of each process taken alone, which indicates a synergism between the two processes. Although corrosion can often occur in the absence of mechanical wear, the opposite is rarely true. Corrosion accompanies the wear process to some extent in all environments, except in vacuum and inert atmospheres. Corrosion and wear often combine to cause aggressive damage in a number of industries, such as mining, mineral processing, chemical processing, pulp and paper production, and energy production. Corrosion and wear processes involve many mechanisms, the combined actions of which lead to the mutual reinforcement of their effectiveness. As listed in Table 4, 17 synergistic relationships among abrasion, impact, and corrosion that could significantly increase material degradation in wet and aqueous environments have been identified. The combined effects of corrosion and wear can also lead to galvanic corrosion in some applications, such as crushing and grinding (comminution) of mineral ores. Wear debris and corrosion products that are formed during comminution affect product quality and can adversely affect subsequent benefication by altering the chemical and electrochemical properties of the mineral system (Ref 5-8). Electrochemical interactions between minerals and grinding media can occur, causing galvanic coupling that leads to increased corrosion wear. More detailed information on galvanic corrosion can be found in Chapter 2.

Methods to Control Corrosion. Owing to its many favorable characteristics, steel is well suited and widely used for a broad range of engineering applications and is referenced here to demonstrate the various corrosion-control steps that can be considered. Steel has a variety of excellent mechanical properties, such as strength, toughness, ductility, and dent resistance. Steel also offers good manufacturability, including formability, weldability, and paintability. Other positive factors include its availability, ferromagnetic properties, recyclability, and cost. Because steel is susceptible to corrosion in the presence of moisture, and to oxidation at elevated temperatures, successful use of these favorable characteristics generally requires some form of protection. Methods of corrosion protection employed to protect steel include: Altering the metal by alloying, that is, using a more highly alloyed and expensive stainless steel rather than a plain carbon or low-alloy steel Changing the environment by desiccation or the use of inhibitors Controlling the electrochemical potential by the application of cathodic or anodic currents, that is, cathodic and anodic protection Applying organic, metallic, or inorganic (glasses and ceramics) coatings Application of corrosion-resistant coatings is one of the most widely used means of protecting steel. As shown in Table 1, there are a wide variety of coatings to choose from, and proper selection is based on the component size and accessibility, the corrosive environment, the anticipated

Table 4 Synergistic relationships between wear and corrosion mechanisms Abrasion Removes protective oxidized metal and polarized coatings to expose unoxidized metal, in addition to removing metal particles. Forms microscopic grooves and dents for concentration cell corrosion. Increases microscopic surface area exposed to corrosion. Removes strain-hardened surface layers. Cracks brittle metal constituents forming sites for impact hydraulic splitting. Plastic deformation by high-stress metal-mineral contact causes strain hardening and susceptibility to chemical attack. Corrosion Produces pits that induce microcracking. Microcracks at pits invite hydraulic splitting during impact. Roughens surface, reducing energy needed to abrade away metal. May produce hydrogen with subsequent absorption and cracking in steel. Selectively attacks grain boundaries and less noble phases of multiphase microstructures, weakening adjacent metal. Impact Plastic deformation makes some constituents more susceptible to corrosion. Cracks brittle constituents, tears apart ductile constituents to form sites for crevice corrosion, hydraulic splitting. Supplies kinetic energy to drive abrasion mechanism. Pressurizes mill water to cause splitting, cavitation, and jet erosion of metal and protective oxidized material. Pressurizes mill water and gases to produce unknown temperatures, phase changes, and decomposition or reaction products from ore and water constituents. Heats ball metal, ore, fluids to increase corrosive effects. Source: Ref 4

Weld overlay Friction surfacing Thermal spraying Carburizing Carbonitriding Nitrocarburizing Nitriding Mechanical working Electrochemical plate + diffusion Transformation hardening Surface alloying—lasers Hot dipping (galvanizing and aluminum) Mechanical plating Electroless plating Electrolytic plating Chemical vapor deposition Physical vapor deposition Resin or laquer"bonding Ion implantation Thickness, mm Fig. 1 Approximate thickness of various surface engineering treatments

temperatures, component distortion, the coating thickness attainable (Fig. 1), and costs. Many of these selection criteria are addressed in Chapters 6 to 8 in this book. Painting is probably the most widely used engineering coating used to protect steel from corrosion. There are a wide variety of coating formulations that have been developed for outdoor exposure, marine atmospheres, water immersion, chemical fumes, extreme sunlight, high humidity, and moderately high temperatures (less than about 200 0 C, or 400 0 F). The most widely used corrosion-resistant metallic coatings are hotdipped zinc, zinc-aluminum, and aluminum coatings. These coatings exhibit excellent resistance to atmospheric corrosion and are widely used in the construction, automobile, utility, and appliance industries. Other important coating processes for steels include electroplating, electroless plating, thermal spraying, pack cementation aluminizing (for high-temperature oxidation resistance), and cladding (including weld cladding and roll-bonded claddings). Applications and corrosion performance of these coatings are described in Chapter 6 in this book. Methods to Control Wear. As is described in Chapter 3 in this book, there are many types of wear, but there are only four main types of wear systems (tribosystems) that produce wear and six basic wear control steps (Ref 9). The four basic tribosystems are:

Relatively smooth solids sliding on other smooth solids Hard, sharp substances sliding on softer surfaces Fatigue of surfaces by repeated stressing (usually compressive) Fluids with or without suspended solids in motion with respect to a solid surface As shown in Fig. 2, the wear that occurs in these tribosystems can be addressed by coatings or by modifications to the substrate metallurgy or chemistry. The six traditional techniques applied to materials to deal with wear produced in the preceding tribosystems include: Separate conforming surfaces with a lubricating film (see Chapter 3 in this book). Make the wearing surface hard through the use of hardfacing, diffusion heat treatments, hard chromium plating, or more recently developed vapor deposition techniques or high-energy processes (e.g., ion implantation). Make the wearing surface resistant to fracture. Many wear processes involve fracture of material from a surface; thus toughness and fracture resistance play a significant role in wear-resistant surfaces. The use of very hard materials such as ceramics, cemented carbides, and hard chromium can lead to fracture problems that nullify the benefits of the hard surface. Make the eroding surface resistant to corrosion. Examples include the use of cobalt-base hardfacing alloys to resist liquid erosion, cavitation, and slurry erosion; aluminum bronze hardfacing alloys to prevent cavitation damage on marine propellers or to repair props that have Wear-causing effects Coatings to reduce wear Polymers/elastomers Electrochemical (plating, etc.) Chemical (CVD, electroless plating) Thermal spraying Fusion welding Thin films (PVD, sputtering, ion plating) Wear tiles Cladding (cast, explosion, hot rolling) Lubricants

Substrate treatments to reduce wear Through hardening Surface hardening (flame, induction, EB, laser) Diffusion of a hardening species (carburizing, nitriding, etc.) Laser/EB alloying Ion implantation Work hardening

Tribosystem

Surface wear

FlC. 2 Surface engineering processes used to prevent wear. CVD, chemical vapor deposition; PVD, " physical vapor deposition; EB, electron beam

suffered cavitation damage; nickel-base hardfacing alloys to resist chemical attack; and epoxy-filled rebuilding cements used to resist slurry erosion in pumps. Choose material couples that are resistant to interaction in sliding (metal-to-metal wear resistance). Hardfacing alloys such as cobaltbase and nickel-chromium-boron alloys have been used for many years for applications involving metal-to-metal wear. Other surfaceengineering options include through-hardened tool steels, diffusion (case)-hardened surfaces, selective surface-hardened alloy steels, and some platings. Make the wearing surface fatigue resistant. Rolling-element bearings, gears, cams, and similar power-transmission devices often wear by a mechanism of surface fatigue. Repeated point or line contact stresses can lead to subsurface cracks that eventually grow to produce surface pits and eventual failure of the device. Prevention is possible through the use of through-hardened steels, heavy casehardened steels, and flame-, induction-, electron beam-, or laserhardened steels. More details on these surface-engineering techniques can be found in Chapters 5 through 8 in this book. Material/Process Selection (Ref 10). Faced with the wide range of possibilities indicated in Table 1 and the discussions on "Methods to Control Corrosion" and "Methods to Control Wear," selection of surface engiPredict working environment from consideration of design Proceed with one-piece construction (see note below)

Yes

Identify material requirements for structure and surface Consider one-piece construction

Analyze service failures to assist selection of better materials No

Select substrate material to suit strength, heat, and corrosion needs

Note: One-piece construction is often least Select surfacing material expensive for small parts as some surfacing alloys to suit requirements are available as castings machined to finished size or as powder metallurgical parts. Select from surfacing processes suitable for chosen material and job, Reconsider (must satisfy needs for coating density, materials thickness, dilution, etc.) Decide if chosen process suits substrate material Yes None and design (adhesion, Reconsider process HAZ, access, distortion, and/or material etc.) No Decide manufacturing Yes details, procedures, Identify quality assurance Finalize choice of health and safety and control needs materials and process requirements, etc. Fig, 3 Checklist for surface engineering material/process selection. HAZ, heat-affected zone

neering material and process may seem difficult, but it is normally straightforward. Often there are constraints placed on the choice because of availability (e.g., laser melting and/or alloying are not widely used, and these processes can only be obtained by a special arrangement with laser job shops). In many cases there is a precedent, but when considering a new problem it helps to follow a checklist of the type shown in Fig. 3. The sequence of decisions to be made covers several fundamental points. The first is the need to be clear about service conditions, based on experience or plant data. This is the key to material selection. The second decision is the choice of application process for the material. This involves the question of compatibility with the coating material; that is, not all materials can be applied by all processes. A further question of compatibility arises between both material and process with the substrate, for example, whether distortion from high-temperature processes be tolerated. All these issues are covered in subsequent chapters in this book (see, in particular, Chapters 7 and 8).

References 1. CM. Cotell and J.A. Sprague, Preface, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p v 2. Economic Effects of Metallic Corrosion in the United States, Battelle Columbus Laboratories and the National Institute of Standards and Technology, 1978 and Battelle updates in 1995 3. "Tribological Sinks in Six Major Industries," Report Number PNL5535, Sept 1985, Pacific Northwest Laboratory, Richland, WA, operated for the U.S. Department of Energy by Battelle Memorial Institute (NTIS No. DE86000841) 4. DJ. Dunn. Metal Removal Mechanisms Comprising Wear in Mineral Processing, Wear of Materials, K.C. Ludema, Ed., American Society of Mechanical Engineers, 1985, p 501-508 5. R.L. Pozzo and I. Iwasaki, Pyrite-Pyrrhotite Grinding Media Interactions and Their Effects on Media Wear and Flotation, /. Electrochem. Soc, VoI 136 (No. 6), 1989, p 1734-1740 6. R.L. Pozzo and I. Iwasaki, Effect of Pyrite and Pyrrhotite on the Corrosive Wear of Grinding Media, Miner. Metall. Process., Aug 1987, p 166-171 7. K.A Natarajan, S.C. Riemer, and I. Iwasaki, Influence of Pyrrhotite on the Corrosive Wear of Grinding Balls in Magnetite Ore Grinding, Int. J. Miner. Process., VoI 13 1984, p 73-81 8. R.L. Pozzo and I. Iwasaki, An Electro-chemical Study of PyrrhotiteGrinding Media Interaction Under Abrasive Conditions, Corrosion, VoI 43 (No. 3), 1987, p 159-169

9. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, Inc., 1988, p 6-10 10. Engineering Coatings—Design and Application, 2nd ed., S. Grainger and J. Blunt, Ed., Woodhead Publishing Ltd., 1999, p 7

CHAPTER

Mm

P r i n c i p l e s

o f

C o r r o s i o n

CORROSION of metal is a chemical or electrochemical process in which surface atoms of a solid metal react with a substance in contact with the exposed surface. Usually the corroding medium is a liquid substance, but gases and even solids can also act as corroding media. In some instances, the corrodent is a bulk fluid; in others, it is a film, droplets, or a substance adsorbed on or absorbed in another substance. All structural metals corrode to some extent in natural environments (e.g., the atmosphere, soil, or waters). Bronze, brass, most stainless steels, zinc, and pure aluminum corrode so slowly in service conditions that long service life is expected without protective coatings. Corrosion of structural grades of cast iron and steel, the 400 series stainless steels, and some aluminum alloys, however, proceeds rapidly unless the metal is protected against corrosion. As described in Chapter 1, corrosion of metals is of particular concern because annual losses in the United States attributed to corrosion amount to hundreds of billions of dollars. Although emphasis in this Chapter has been placed on irons and steels, the electrochemical corrosion basics and the forms of corrosion described are applicable to all metallic materials. For more detailed information on the corrosion resistance of various metals and their alloys, the reader should consult the selected references listed at the conclusion of this Chapter, as well as Corrosion, VoI 13, of the ASM Handbook or Corrosion: Understanding the Basics, published by ASM International in 2000.

Electrochemical Corrosion Basics Electrochemical corrosion in metals in a natural environment, whether atmosphere, in water, or underground, is caused by a flow of electricity from one metal to another, or from one part of a metal surface to another part of the same surface where conditions permit the flow of electricity.

Current flow in conductor Metal anode

Metallic conductor between the anode and the cathode Metal cathode Oxygen or other depolarizer in electrolyte

Oxidation reaction occurs at anode

Electrolyte, water containing conductive salts Reduction reaction occurs at cathode

Current flow through the electrolyte Fig. 1 Simple electrochemical cell showing the components necessary for corrosion

For the flow of energy to take place, either a moist conductor or an electrolyte must be present. An electrolyte is an electricity-conducting solution containing ions, which are atomic particles or radicals bearing an electrical charge. Charged ions are present in solutions of acids, alkalis, and salts. The presence of an electrolyte is necessary for corrosion to occur. Water, especially salt water, is an excellent electrolyte. Electricity passes from a negative area to a positive area through the electrolyte. For corrosion to occur in metals, one must have (a) an electrolyte, (b) an area or region on a metallic surface with a negative charge, (c) a second area with a positive charge, and (d) an electrically conductive path between (b) and (c). These components are arranged to form a closed electrical circuit. In the simplest case, the anode would be one metal, such as iron, the cathode another, perhaps copper, and the electrolyte might or might not have the same composition at both anode and cathode. The anode and cathode could be of the same metal under conditions described later in this article. The cell shown in Fig. 1 illustrates the corrosion process in its simplest form. This cell includes the following essential components: (a) a metal anode, (b) a metal cathode, (c) a metallic conductor between the anode and the cathode, and (d) an electrolyte in contact with the anode and the cathode. If the cell were constructed and allowed to function, an electrical current would flow through the metallic conductor and the electrolyte, and if the conductor were replaced by a voltmeter, a potential difference between the anode and the cathode could be measured. The anode would corrode. Chemically, this is an oxidation reaction. The formation of hydrated red iron rust by electrochemical reactions may be expressed as follows:

(EqI)

(Eq 2) During metallic corrosion, the rate of oxidation equals the rate of reduction. Thus, a nondestructive chemical reaction, reduction, would proceed simultaneously at the cathode. In most cases, hydrogen gas is produced on the cathode. When the gas layer insulates the cathode from the electrolyte, current flow stops, and the cell is polarized. However, oxygen or some other depolarizing agent is usually present to react with the hydrogen, which reduces this effect and allows the cell to continue to function. Contact between dissimilar metallic conductors or differences in the concentration of the solution cause the difference in potential that results in electrical current. Any lack of homogeneity on the metal surface or its environment may initiate attack by causing a difference in potential, and this results in localized corrosion. The metal undergoing electrochemical corrosion need not be immersed in a liquid but may be in contact with moist soil or may have moist areas on the metal surface.

Corrosive Conditions If oxygen and water are both present, corrosion will normally occur on iron and steel. Rapid corrosion may take place in water, the rate of corrosion being accelerated by several factors such as: (a) the velocity or the acidity of the water, (b) the motion of the metal, (c) an increase in temperature or aeration, and (d) the presence of certain bacteria. Corrosion can be retarded by protective layers or films consisting of corrosion products or adsorbed oxygen. High alkalinity of the water also retards the rate of corrosion on steel surfaces. Water and oxygen remain the essential factors, however, and the amount of corrosion is generally controlled by one or the other. For example, corrosion of steel does not occur in dry air and is negligible when the relative humidity of the air is below 30% at normal or lower temperatures. This is the basis for prevention of corrosion by dehumidification. Water can readily dissolve a small amount of oxygen from the atmosphere, thus becoming highly corrosive. When the free oxygen dissolved in water is removed, the water becomes practically noncorrosive unless it becomes acidic or anaerobic bacteria incite corrosion. If oxygen-free water is maintained at a neutral pH or at slight alkalinity, it is practically

noncorrosive to structural steel. Steam boilers and water supply systems are effectively protected by deaerating the water. Additional information on corrosion in water can be found in Ref 1. Soils. Dispersed metallic particles or bacteria pockets can provide a natural electrical pathway for buried metal. If an electrolyte is present and the soil has a negative charge in relation to the metal, an electrical path from the metal to the soil will occur, resulting in corrosion. Differences in soil conditions, such as moisture content and resistivity, are commonly responsible for creating anodic and cathodic areas (Fig. 2). Where a difference exists in the concentration of oxygen in the water or in moist soils in contact with metal at different areas, cathodes develop at points of relatively high-oxygen concentrations and anodes at points of low concentration. Further information on corrosion in soils is available in Ref 2. Chemicals. In an acid environment, even without the presence of oxygen, the metal at the anode is attacked at a rapid rate. At the cathode, atomic hydrogen is released continuously, to become hydrogen gas. Corrosion by an acid can result in the formation of a salt, which slows the reaction because the salt formation on the surface is then attacked. Corrosion by direct chemical attack is the single most destructive force against steel surfaces. Substances having chlorine or other halogens in their composition are particularly aggressive. Galvanized roofing has been known to corrode completely within six months of construction, the building being downwind of an aluminum ingot plant where fluorides were always present in the atmosphere. Consequently, galvanized steel should not have been specified. Selection of materials and evaluation of service conditions are extremely important in combating corrosion. The response of various materials to chemical environments is addressed in Ref 3 and 4. Atmospheric corrosion differs from the corrosion action that occurs in water or underground, because sufficient oxygen is always present. In at-

Cathodic area (steel at top of pipe) Buried pipe

Oxygen diffusing into earth from ground surface Electrolyte 1 (soil with ground water high in oxygen content) •Current flow

Anodic area (steel at bottom of pipe)

• Electrolyte 2 (soil with ground water deficient in oxygen content)

Fe2+ (rust) pjo 2 A metal pipe buried in moist soil forming a corrosion cell. A difference ^* in oxygen content at different levels in the electrolyte will produce a difference of potential. Anodic and cathodic areas will develop, and a corrosion cell, called a concentration cell, will form.

mospheric corrosion, the formation of insoluble films and the presence of moisture and deposits from the atmosphere control the rate of corrosion. Contaminants such as sulfur compounds and salt particles can accelerate the corrosion rate. Nevertheless, atmospheric corrosion occurs primarily through electrochemical means and is not directly caused by chemical attack. The anodic and cathodic areas are usually quite small and close together so that corrosion appears uniform, rather than in the form of severe pitting, which can occur in water or soil. A more detailed discussion on atmospheric corrosion can be found in Ref 5. Forms of Corrosion The differing forms of corrosion can be divided into the following eight categories based on the appearance of the corrosion damage or the mechanism of attack: Uniform or general corrosion Galvanic corrosion Pitting corrosion Crevice corrosion, including corrosion under tubercles or deposits, filiform corrosion, and poultice corrosion Erosion-corrosion, including cavitation erosion and fretting corrosion Intergranular corrosion, including sensitization and exfoliation Dealloying Environmentally assisted cracking, including stress-corrosion cracking (SCC), corrosion fatigue, and hydrogen damage (including hydrogen embrittlement, hydrogen-induced blistering, high-temperature hydrogen attack, and hydride formation) Figure 3 illustrates schematically some of the most common forms of corrosion. More detailed information pertaining to recognition and prevention of these forms of corrosion can be found in Ref 6 and 7. Uniform Corrosion General Description. Uniform or general corrosion, as the name implies, results in a fairly uniform penetration (or thinning) over the entire exposed metal surface. The general attack results from local corrosion-cell action; that is, multiple anodes and cathodes are operating on the metal surface at any given time. The location of the anodic and cathodic areas continues to move about on the surface, resulting in uniform corrosion. Uniform corrosion often results from atmospheric exposure (especially polluted industrial environments); exposure in fresh, brackish, and salt waters; or exposure in soils and chemicals.

More noble metal

No corrosion

Pitting

Uniform

Exfoliation

Galvanic

Dealloying

Flowing corrodent

Erosion

lntergranular

Cyclic movement

Metal or nonmetal

Fretting

Crevice

Tensile stress

Cyclic stress

Stress-corrosion cracking

Corrosion fatigue

Flg. 3 Schematics of the common forms of corrosion

Metals Affected. All metals are affected by uniform corrosion, although materials that form passive films, such as stainless steels or nickelchromium alloys, are normally subjected to localized forms of attack. The rusting of steel, the green patina formation on copper, and the tarnishing of silver are typical examples of uniform corrosion. In some metals, such as steel, uniform corrosion produces a somewhat rough surface by removing a substantial amount of metal, which either dissolves in the environment or reacts with it to produce a loosely adherent, porous coating of corrosion products. In such reactions as in the tarnishing of silver in air, the oxidation of aluminum in air, or attack on lead in sulfate-containing environments, thin, tightly adherent protective films are produced, and the metal surface remains smooth. Prevention. Uniform corrosion can be prevented or reduced by proper materials selection, the use of coatings or inhibitors, or cathodic protection. These corrosion prevention methods can be used individually or in combination. Galvanic Corrosion General Description. The potential available to promote the electrochemical corrosion reaction between dissimilar metals is suggested by the galvanic series, which lists a number of common metals and alloys arranged according to their tendency to corrode when in galvanic contact (Table 1). Metals close to one another on the table generally do not have a strong effect on each other, but the farther apart any two metals are separated, the stronger the corroding effect on the one higher in the list. It is possible for certain metals to reverse their positions in some environments, but the order given in Table 1 is maintained in natural waters and the atmosphere. The galvanic series should not be confused with the sim-

Table 1

Galvanic series in seawater at 25 0C (77 0F)

Corroded end (anodic, or least noble) Magnesium Magnesium alloys Zinc Galvanized steel or galvanized wrought iron Aluminum alloys 5052, 3004, 3003, 1100, 6053, in this order Cadmium Aluminum alloys 2117, 2017, 2024, in this order Low-carbon steel Wrought iron Cast iron Ni-Resist (high-nickel cast iron) Type 410 stainless steel (active) 50-50 lead-tin solder Type 304 stainless steel (active) Type 316 stainless steel (active) Lead Tin Copper alloy C28000 (Muntz metal, 60% Cu) Copper alloy C67500 (manganese bronze A) Copper alloys C46400, C46500, C46600, C46700 (naval brass) Nickel 200 (active) Inconel alloy 600 (active) Hastelloy alloy B Chlorimet 2 Copper alloy C27000 (yellow brass, 65% Cu) Copper alloys C44300, C44400, C44500 (admiralty brass) Copper albys C60800, C61400 (aluminum bronze) Copper alloy C23000 (red brass, 85% Cu) Copper C! 1000 (ETP copper) Copper alloys C65100, C65500 (silicon bronze) Copper alloy C71500 (copper nickel, 30% Ni) Copper alloy C92300, cast (leaded tin bronze G) Copper alloy C92200, cast (leaded tin bronze M) Nickel 200 (passive) Inconel alloy 600 (passive) Monel alloy 400 Type 410 stainless steel (passive) Type 304 stainless steel (passive) Type 316 stainless steel (passive) Incoloy alloy 825 Inconel alloy 625 Hastelloy alloy C Chlorimet 3 Silver Titanium Graphite Gold Platinum Protected end (cathodic, or most noble)

ilar electromotive force series, which shows exact potentials based on highly standardized conditions that rarely exist in nature. The three-layer iron oxide scale formed on steel during rolling varies with the operation performed and the rolling temperature. The dissimilarity of the metal and the scale can cause corrosion to occur, with the steel acting as the anode in this instance. Unfortunately, mill scale is cathodic to steel, and an electric current can easily be produced between the steel and the mill scale. This electrochemical action will corrode the steel without affecting the mill scale (Fig. 4). A galvanic couple may be the cause of premature failure in metal components of water-related structures or may be advantageously exploited.

Electrolyte (water) (rust)

Current flow

Cathode (broken mil scale)

Anode (steel) Fig, 4 Mill scale forming a corrosion cell on steel

Galvanizing iron sheet is an example of useful application of galvanic action or cathodic protection. Iron is the cathode and is protected against corrosion at the expense of the sacrificial zinc anode. Alternatively, a zinc or magnesium anode may be located in the electrolyte close to the structure and may be connected electrically to the iron or steel. This method is referred to as cathodic protection of the structure. Iron or steel can become the anode when in contact with copper, brass, or bronze; however, iron or steel corrode rapidly while protecting the latter metals. Also, weld metal may be anodic to the basis metal, creating a corrosion cell when immersed (Fig. 5). While the galvanic series (Table 1) represents the potential available to promote a corrosive reaction, the actual corrosion is difficult to predict. Electrolytes may be poor conductors, or long distances may introduce large resistance into the corrosion cell circuit. More frequently, scale formation forms a partially insulating layer over the anode. A cathode having a layer of adsorbed gas bubbles, as a consequence of the corrosion cell reaction, is polarized. The effect of such conditions is to reduce the theoretical consumption of metal by corrosion. The area relationship between the anode and cathode may also strongly affect the corrosion rate; a high ratio of cathode area to anode area produces more rapid corrosion. In the reverse case, the cathode polarizes, and the corrosion rate soon drops to a negligible level. The passivity of stainless steels is attributed to either the presence of a corrosion-resistant oxide film or an oxygen-caused polarizing effect,

Electrolyte (water)

(rust) Current flow Cathode (steel) Anode (weld metal) \

FlC, 5 Weld metal forming a corrosion cell on steel. Weld metal may be an^* odic to steel, creating a corrosion cell when immersed.

durable only as long as there is sufficient oxygen to maintain the effect, over the surfaces. In most natural environments, stainless steels will remain in a passive state and thus tend to be cathodic to ordinary iron and steel. Change to an active state usually occurs only where chloride concentrations are high, as in seawater or reducing solutions. Oxygen starvation also produces a change to an active state. This occurs where the oxygen supply is limited, as in crevices and beneath contamination on partially fouled surfaces. Prevention. Galvanic corrosion can be prevented or reduced by proper materials selection (i.e., select combinations of metals as close together as possible in the galvanic series), insulating dissimilar metals, applying a barrier coating to both the anodic (less noble) and cathodic (noble) metal, applying a sacrificial coating (aluminum, zinc, or cadmium) to the cathodic part, applying nonmetallic films (e.g., anodizing aluminum alloys), and by providing cathodic protection.

Pitting General Description. Pitting is a type of localized cell corrosion. It is predominantly responsible for the functional failure of iron and steel water-related installations. Pitting may result in the perforation of water pipe, rendering it unserviceable, even though less than 5% of the total metal has been lost through rusting. Where confinement of water is not a factor, pitting causes structural failure from localized weakening while considerable sound metal still remains. Pitting develops when the anodic or corroding area is small in relation to the cathodic or protected area. For example, pitting can occur where large areas of the surface are covered by mill scale, applied coatings, or deposits of various kinds and where breaks exist in the continuity of the protective coating. Pitting may also develop on bare, clean metal surfaces because of irregularities in the physical or chemical structure of the metal. Localized, dissimilar soil conditions at the surface of steel can also create conditions that promote pitting. Electrical contact between dissimilar materials or concentration cells (areas of the same metal where oxygen or conductive salt concentrations in water differ) accelerates the rate of pitting. In closed-vessel structures, these couples cause a difference of potential that results in an electric current flowing through the water or across the moist steel from the metallic anode to a nearby cathode. The cathode may be copper, brass, mill scale, or any portion of a metal surface that is cathodic to the more active metal areas. In practice, mill scale is cathodic to steel and is found to be a common cause of pitting. The difference of potential generated between steel and mill scale often amounts to 0.2 to 0.3 V. This couple is nearly as powerful a generator of corrosion currents as is the copper-steel couple. However, when the anodic area is relatively large compared with the

cathodic area, the damage is spread out and usually negligible, but when the anode is relatively small, the metal loss is concentrated and may be very serious. On surfaces having some mill scale, the total metal loss is nearly constant as the anode is decreased, but the degree of penetration increases. Figure 4 shows how a pit forms where a break occurs in mill scale. When contact between dissimilar materials is unavoidable and the surface is painted, it is preferred to paint both materials. If only one surface is painted, it should be the cathode. If only the anode is coated, any weak points such as pinholes or holidays in the coating will probably result in intense pitting. As a pit, perhaps at a break in mill scale, becomes deeper, an oxygen concentration cell is started by depletion of oxygen in the pit. The rate of penetration by such pits is accelerated proportionately as the bottom of the pit becomes more anodic. Fabrication operations may crack mill scale and result in accelerated corrosion. Metals Affected. Pitting occurs in most commonly used metals and alloys. Iron buried in the soil corrodes with the formation of shallow pits, but carbon steels in contact with hydrochloric acid or stainless steels immersed in seawater characteristically corrode with the formation of deep pits. Aluminum tends to pit in waters containing chloride ions (for example, at stagnant areas), and aluminum brasses are subject to pitting in polluted waters. Despite their good resistance to general corrosion, stainless steels are more susceptible to pitting than many other metals. High-alloy stainless steels containing chromium, nickel, and molybdenum are also more resistant to pitting but are not immune under all service conditions. Pitting failures of corrosion-resistant alloys, such as Hastelloy C, Hastelloy G, and Incoloy 825, are relatively uncommon in solutions that do not contain halides, although any mechanism that permits the establishment of an electrolytic cell in which a small anode is in contact with a large cathodic area offers the opportunity for pitting attack. Prevention. Typical approaches to alleviating or minimizing pitting corrosion include the following: Use defect-free barrier coatings Reduce the aggressiveness of the environment, for example, chloride ion concentrations, temperature, acidity, and oxidizing agents Upgrade the materials of construction, for example, use molybdenumcontaining (4 to 6% Mo) stainless steels, molybdenum + tungsten nickel-base alloys, overalloy welds, and use corrosion-resistant alloy linings Modify the design of the system, for example, avoid crevices and the formation of deposits, circulate/stir to eliminate stagnant solutions, and ensure proper drainage

Crevice

Corrosion

General Description. Crevice corrosion is a form of localized attack that occurs at narrow openings or spaces (gaps) between metal-to-metal or nonmetal-to-metal components. This type of attack results from a concentration cell formed between the electrolyte within the crevice, which is oxygen starved, and the electrolyte outside the crevice, where oxygen is more plentiful. The material within the crevice acts as the anode, and the exterior material becomes the cathode. Crevices may be produced by design or accident. Crevices caused by design occur at gaskets, flanges, rubber O-rings, washers, bolt holes, rolled tube ends, threaded joints, riveted seams, overlapping screen wires, lap joints, beneath coatings (filiform corrosion) or insulation (poultice corrosion), and anywhere close-fitting surfaces are present. Figure 6 shows crevice corrosion in a riveted assembly caused by concentration cells. Occluded regions are also formed under tubercles (tuberculation), deposits (deposit corrosion), and below accumulations or biological materials (biologically influenced corrosion). Similarly, unintentional crevices such as cracks, seams, and other metallurgical defects could serve as sites for corrosion. Metals Affected. Resistance to crevice corrosion can vary from one alloy-environment system to another. Although crevice corrosion affects both active and passive metals, the attack is often more severe for passive alloys, particularly those in the stainless steel group. Breakdown of the passive film within a restricted geometry leads to rapid metal loss and penetration of the metal in that area.

Low metal ion concentration

Metal ion concentration cell

High metal ion concentration High oxygen concentration Oxygen concentration cell

Low oxygen concentration Fig. 6 Corrosion caused at crevices by concentration cells. Both types of concentration cells shown sometimes occur simultaneously as in a reentry angle in a riveted seam.

Prevention. Crevice corrosion can be prevented or reduced through improved design to avoid crevices, regular cleaning to remove deposits, by selecting a more corrosion-resistant material, and by coating carbon steel or cast iron components with epoxy or other field-applied or factoryapplied organic coatings. Erosion-Corrosion General Description. Erosion-corrosion is the acceleration or increase in the rate of deterioration or attack on a metal because of mechanical wear or abrasive contributions in combination with corrosion. The combination of wear or abrasion and corrosion results in more severe attack than would be realized with either mechanical or chemical corrosive action alone. Metal is removed from the surface as dissolved ions, as particles of solid corrosion products, or as elemental metal. The spectrum of erosioncorrosion ranges from primarily erosive attack, such as sandblasting, filing, or grinding of a metal surface, to primarily corrosion failures, where the contribution of mechanical action is quite small. All types of corrosive media generally can cause erosion-corrosion, including gases, aqueous solutions, organic systems, and liquid metals. For example, hot gases may oxidize a metal then at high velocity blow off an otherwise protective scale. Solids in suspension in liquids (slurries) are particularly destructive from the standpoint of erosion-corrosion. Erosion-corrosion is characterized in appearance by grooves, waves, rounded holes, and/or horseshoe-shaped grooves. Analysis of these marks can help determine the direction of flow. Affected areas are usually free of deposits and corrosion products, although corrosion products can sometimes be found if erosion-corrosion occurs intermittently and/or the liquid flow rate is relatively low. Metals Affected. Most metals are susceptible to erosion-corrosion under specific conditions. Metals that depend on a relatively thick protective coating of corrosion product for corrosion resistance are frequently subject to erosion-corrosion. This is due to the poor adhesion of these coatings relative to the thin films formed by the classical passive metals, such as stainless steels and titanium. Both stainless steels and titanium are relatively immune to erosion-corrosion in many environments. Metals that

Corrosion film

Water flow Impingement corrosion pits

Original metal surface

Metal tube wall Fig, 7 Schematic of erosion-corrosion of a condenser tube

are soft and readily damaged or worn mechanically, such as copper and lead, are quite susceptible to erosion-corrosion. Even the noble or precious metals, such silver, gold, and platinum, are subject to erosion-corrosion. Figure 7 shows a schematic of erosion-corrosion of a condenser tube wall. The direction of flow and the resulting attack where the protective film on the tube has broken down are indicated. Prevention. Erosion-corrosion can be prevented or reduced through improved design (e.g., increase pipe diameter and/or streamline bends to reduce impingement effects), by altering the environment (e.g., deaeration and the addition of inhibitors), and by applying hard, tough protective coatings. Cavitation General Description. Cavitation is a form of erosion-corrosion that is caused by the formation and collapse of vapor bubbles in a liquid against a metal surface. Cavitation occurs in hydraulic turbines, on pump impellers, on ship propellers, and on many surfaces in contact with high-velocity liquids subject to changes in pressure. The appearance of cavitation is similar to pitting except that surfaces in the pits are usually much rougher. The affected region is free of deposits and accumulated corrosion products if cavitation has been recent. Figure 8 is a simplified representation of the cavitation process. Figure 8(a) shows a vessel containing a liquid. The vessel is closed by an airtight plunger. When the plunger is withdrawn (Fig. 8b), a partial vacuum is created above the liquid, causing vapor bubbles to form and grow within Partial vacuum

Pressurized

Metal (a) Rest Quiescent liquid at standard temperature and pressure

(b) Expansion Liquid boiling at room temperature

(c) Compression Collapse of vapor bubbles

Metal oxide (d)

Approaching microjet torpedo

Destruction of metal oxide on impact

Repair of metal oxide at expense of metal

P J o - 8 Schematic representation of cavitation showing a cross section through a vessel and plunger enclosing a fluid. " (a) Plunger stationary, liquid at standard temperature and pressure, (b) Plunger withdrawn, liquid boils at room temperature, (c) Plunger advanced, bubbles collapse, (d) Disintegration of protective corrosion product by impacting microjet "torpedo." Source: Ref 8

the liquid. In essence, the liquid boils without a temperature increase. If the plunger is then driven toward the surface of the liquid (Fig. 8c), the pressure in the liquid increases, and the bubbles condense and collapse (implode). In a cavitating liquid, these three steps occur in a matter of milliseconds. As shown in Fig. 8(d), implosion of a vapor bubble creates a microscopic "torpedo" of water that is ejected from the collapsing bubble at velocities that may range from 100 to 500 m/s (330 to 1650 ft/s). When the torpedo impacts the metal surface, it dislodges protective surface films and/or locally deforms the metal itself. Thus, fresh surfaces are exposed to corrosion and the reformation of protective films, which is followed by more cavitation, and so on. Damage occurs when the cycle is allowed to repeat over and over again. Prevention. Cavitation can be controlled or minimized by improving design to minimize hydrodynamic pressure differences, employing stronger (harder) and more corrosion-resistant materials, specifying a smooth finish on all critical metal surfaces, and coating with resilient materials such as rubber and some plastics. Fretting Corrosion General Description. Fretting corrosion is a combined wear and corrosion process in which material is removed from contacting surfaces when motion between the surfaces is restricted to very small amplitude oscillations (often, the relative movement is barely discernible). Usually, the condition exists in machine components that are considered fixed and not expected to wear. Pressed-on wheels can often fret at the shaft/wheel hole interface. Oxidation is the most common element in the fretting process. In oxidizing systems, fine metal particles removed by adhesive wear are oxidized and trapped between the fretting surfaces (Fig. 9). The oxides act like an abrasive (such as lapping rouge) and increase the rate of material removal. This type of fretting in ferrous alloys is easily recognized by the red material oozing from between the contacting surfaces. Fretting corrosion takes the form of local surface dislocations and deep pits. These occur in regions where slight relative movements have occurred between mating, highly loaded surfaces.

Surface Oxide Bare Metal Metal and Oxide Debris Fig, 9 Schematic of the fretting process

Prevention. Fretting corrosion can be controlled by lubricating (e.g., low-viscosity oils) the faying surfaces, restricting the degree of movement, shot peening (rough surfaces are less prone to fretting damage), surface hardening (e.g., carburizing and nitriding), anodizing of aluminum alloys, phosphate conversion coating of steels, and by applying protective coatings by electrodeposition (e.g., gold or silver plating), plasma spraying, or vapor deposition (Ref 9). lntergranular Corrosion General Description. lntergranular corrosion is defined as the selective dissolution of grain boundaries, or closely adjacent regions, without appreciable attack of the grains themselves. This dissolution is caused by potential differences between the grain-boundary region and any precipitates, intermetallic phases, or impurities that form at the grain boundaries. The actual mechanism differs with each alloy system. Although a wide variety of alloy systems are susceptible to intergranular corrosion under very specific conditions, the majority of case histories reported in the literature have involved austenitic stainless steels and aluminum alloys and, to a lesser degree, some ferritic stainless steels and nickel-base alloys. Precipitates that form as a result of the exposure of metals at elevated temperatures (for example, during production, fabrication, and welding) often nucleate and grow preferentially at grain boundaries. If these precipitates are rich in alloying elements that are essential for corrosion resistance, the regions adjacent to the grain boundary are depleted of these elements. The metal is thus sensitized and is susceptible to intergranular attack in a corrosive environment. For example, in austenitic stainless steels such as AISI type 304, the cause of intergranular attack is the precipitation of chromium-rich carbides ((Cr5Fe)23C6) at grain boundaries. These chromium-rich precipitates are surrounded by metal that is depleted in chromium; therefore, they are more rapidly attacked at these zones than on undepleted metal surfaces. Impurities that segregate at grain boundaries may promote galvanic action in a corrosive environment by serving as anodic or cathodic sites. Therefore, this would affect the rate of the dissolution of the alloy matrix in the vicinity of the grain boundary. An example of this is found in aluminum alloys that contain intermetallic compounds, such as Mg5Al8 and CuAl2, at the grain boundaries. During exposures to chloride solutions, the galvanic couples formed between these precipitates and the alloy matrix can lead to severe intergranular attack. Susceptibility to intergranular attack depends on the corrosive solution and on the extent of intergranular precipitation, which is a function of alloy composition, fabrication, and heat treatment parameters. Prevention. Susceptibility to intergranular corrosion in austenitic stainless steels can be avoided by controlling their carbon contents or by

adding elements (titanium and niobium) whose carbides are more stable than those of chromium. For most austenitic stainless steels, restricting their carbon contents to 0.03% or less will prevent sensitization during welding and most heat treatment. Intergranular corrosion in aluminum alloys is controlled by material selection (e.g., the high-strength Ixxx and Ixxx alloys are the most susceptible) and by proper selection of thermal (tempering) treatments that can effect the amount, size, and distribution of second-phase intermetallic precipitates. Resistance to intergranular corrosion is obtained by the use of heat treatments that cause precipitation to be more general throughout the grain structure (Ref 10). Exfoliation General Description. Exfoliation is a form of macroscopic intergranular corrosion that primarily affects aluminum alloys in industrial or marine environments. Corrosion proceeds laterally from initiation sites on the surface and generally proceeds intergranularly along planes parallel to the surface. The corrosion products that form in the grain boundaries force metal away from the underlying base material, resulting in a layered or flakelike appearance (see, for example, the schematic shown in Fig. 3). Prevention. Resistance to exfoliation corrosion is attained through proper alloy and temper selection. The most susceptible alloys are the high-strength heat-treatable Ixxx and Ixxx alloys. Exfoliation corrosion in these alloys is usually confined to relatively thin sections of highly worked products. Guidelines for selecting proper heat treatment for these alloys can be found in Ref 10. Dealloying Corrosion General Description. Dealloying, also referred to as selective leaching or parting corrosion, is a corrosion process in which the more active metal is selectively removed from an alloy, leaving behind a porous weak deposit of the more noble metal. Specific categories of dealloying often carry the name of the dissolved element. For example, the preferential leaching of zinc from brass is called dezincification. If aluminum is removed, the process is called dealuminification, and so forth. In the case of gray iron, dealloying is called graphitic corrosion. In the dealloying process, typically one of two mechanisms occurs: alloy dissolution and replating of the cathodic element or selective dissolution of an anodic alloy constituent. In either case, the metal is left spongy and porous and loses much of its strength, hardness, and ductility. Table 2 lists some of the alloy-environment combinations for which dealloying has been reported. By far the two most common forms of dealloying are dezincification and graphitic corrosion. Copper-zinc alloys containing more than 15% zinc are susceptible to dezincification. In the dezincification of brass, selective removal of zinc

leaves a relatively porous and weak layer of copper and copper oxide. Corrosion of a similar nature continues beneath the primary corrosion layer, resulting in gradual replacement of sound brass by weak, porous copper. Graphitic corrosion is observed in gray cast irons in relatively mild environments in which selective leaching of iron leaves a graphite network. Selective leaching of the iron takes place because the graphite is cathodic to iron, and the gray iron structure establishes an excellent galvanic cell. Prevention. Dezincification can be prevented by alloy substitution. Brasses with copper contents of 85% or more resist dezincification. Some alloying elements also inhibit dezincification (e.g., brasses containing 1% tin). Where dezincification is a problem, red brass, commercial bronze, inhibited admiralty metal, and inhibited brass can be successfully used. Attack by graphitic corrosion is reduced by alloy substitution (e.g., use of a ductile or alloyed iron rather than gray iron), altering the environment (raise the water pH to neutral or slightly alkaline levels), the use of inhibitors, and avoiding stagnant water conditions. Stress-Corrosion Cracking General Description. Stress-corrosion cracking (SCC) is a cracking phenomenon that occurs in susceptible alloys and is caused by the conjoint action of a surface tensile stress and the presence of a specific corrosive environment. For SCC to occur on an engineering structure, three conditions must be met simultaneously, namely, a specific crack-promoting environment must be present, the metallurgy of the material must be susceptible to SCC, and the tensile stresses must be above some threshold value. Stresses required to cause SCC are small, usually below the macroscopic yield stress. The stresses can be externally applied, but residual stresses often cause SCC failures. This cracking phenomenon is of particular importance to users of potentially susceptible structural alloys because SCC occurs under service conditions that can result, often with no warning, in catastrophic failure. Failed specimens exhibit highly branched Table 2 Combinations of alloys and environments subject to dealloying and elements preferentially removed Alloy

Brasses Gray iron Aluminum bronzes Silicon bronzes Tin bronzes Copper-gold single crystals Monels Gold alloys with copper or silver Tungsten carbide-cobalt High-nickel alloys Medium- and high-carbon steels Iron-chromium alloys Nickel-molybdenum alloys

Environment

Element removed

Many waters, especially under stagnant conditions Soils, many waters Hydrofluoric acid, acids containing chloride ions High-temperature steam and acidic species Hot brine or steam Ferric chloride Hydrofluoric and other acids Sulfide solutions, human saliva Deionized water Molten salts Oxidizing atmospheres, hydrogen at high temperatures High-temperature oxidizing atmospheres Oxygen at high temperature

Zinc (dezincification) Iron (graphitic corrosion) Aluminum (dealuminification) Silicon (desiliconification) Tin (destannification) Copper Copper in some acids, and nickel in others Copper, silver Cobalt Chromium, iron, molybdenum, and tungsten Carbon (decarburization) Chromium, which forms a protective film Molybdenum

Table 3

Some environment-alloy combinations known to result in stress-corrosion cracking (SCC) Alloy system Aluminum alloys

Environment

Carbon steels

Copper alloys

Nickel alloys

Austenitic

Stainless Steels Duplex Martensitic

Titanium alloys

Zirconium alloys

Amines, aqueous Ammonia, anhydrous Ammonia, aqueous Bromine Carbonates, aqueous Carbon monoxide, carbon dioxide, water mixture Chlorides, aqueous Chlorides, concentrated, boiling Chlorides, dry, hot Chlorinated solvents Cyanides, aqueous, acidified Fluorides, aqueous Hydrochloric acid Hydrofluoric acid Hydroxides, aqueous Hydroxides, concentrated, hot Methanol plus halides Nitrates, aqueous Nitric acid, concentrated Nitric acid, fuming Nitrites, aqueous Nitrogen tetroxide Polythionic acids Steam Sulfides plus chlorides, aqueous Sulfurous acid Water, high-purity, hot X, known to result in SCC

Stress-corrosion cracking control

Mechanical

Metallurgical

Environmental

Change alloy composition

Modify environment

Relieve fabrication stresses

Change alloy structure

Apply anodic or cathodic protection

Introduce surface compressfve stresses

Use metallic or conversion coating

Add inhibrtor

Avoid stress concentrators

Reduce operating stresses

Use organic coating

Nondestructive testing implications for design

Modify temperature

Fig. 1 0

M e t hods used to control SCC. Source: Ket I I

cracks (see Fig. 3) that propagate intergranularly and/or transgranularly, depending on the metal-environment combination. Table 3 lists some of the alloy-environment combinations that result in SCC. This table, as well as others published in the literature, should be used only as a guide for screening candidate materials prior to further indepth investigation, testing, and evaluation. Prevention. Figure 10 summarizes the various approaches to controlling SCC. Surface engineering treatments like shot peening, metallic coatings, and organic coatings play a key role in controlling SCC.

Corrosion Fatigue General Description. Corrosion fatigue is a term that is used to describe the phenomenon of cracking, including both initiation and propagation, in materials under the combined actions of a fluctuating or cyclic stress and a corrosive environment. Corrosion fatigue depends strongly on the interactions among the mechanical (loading), metallurgical, and environmental variables listed in Table 4. Corrosion fatigue produces fine-to-broad cracks with little or no branching (see Fig. 3); thus, they differ from SCC, which often exhibits considerable branching. They are typically filled with dense corrosion product. The cracks may occur singly but commonly appear as families or parallel cracks. They are frequently associated with pits, grooves, or some other form of stress concentrator. Transgranular fracture paths are more common than intergranular fractures.

Table 4 Mechanical, metallurgical, and environmental variables that influence corrosion fatigue behavior Variable Mechanical

Metallurgical

Environmental

Type Maximum stress or stress-intensity factor, a max or Kmax Cyclic stress or stress-intensity range, ACT or AK Stress ratio, R Cyclic loading frequency Cyclic load waveform (constant-amplitude loading) Load interactions in variable-amplitude loading State of stress Residual stress Crack size and shape, and their relation to component size and geometry Alloy composition Distribution of alloying elements and impurities Microstructure and crystal structure Heat treatment Mechanical working Preferred orientation of grains and grain boundaries (texture) Mechanical properties (strength, fracture toughness, etc.) Temperature Types of environments: gaseous, liquid, liquid metal, etc. Partial pressure of damaging species in gaseous environments Concentration of damaging species in aqueous or other liquid environments Electrical potential pH Viscosity of the environment Coatings, inhibitors, etc.

Prevention. All metals and alloys are susceptible to corrosion fatigue. Even some alloys that are immune to SCC, for example, ferritic stainless steels, are subject to failure by corrosion fatigue. Both temporary and permanent solutions for corrosion involve reducing or eliminating cyclic stresses, selecting a material or heat treatment with higher corrosion fatigue strengths, reducing or eliminating corrosion, or a combination of these procedures. These objectives are accomplished by changes in material, design, or environment and by the application of surface treatments. Shot peening, nitriding of steels, and organic coatings can successfully impede corrosion fatigue. Noble metal coatings (e.g., nickel) can be effective, but only if they remain unbroken and are of sufficient density and thickness. The relatively low corrosion-fatigue strength of carbon steel is reduced still further when local breaks in a coating occur. Hydrogen Damage General Description. The term hydrogen damage has been used to designate a number of processes in metals by which the load-carrying capacity of the metal is reduced due to the presence of hydrogen, often in combination with residual or applied tensile stresses. Although it occurs most frequently in carbon and low-alloy steels, many metals and alloys are susceptible to hydrogen damage. Hydrogen damage in one form or another can severely restrict the use of certain materials. Because hydrogen is one of the most abundant elements and is readily available during the production, processing, and service of metals, hydrogen damage can develop in a wide variety of environments and circumstances. The interaction between hydrogen and metals can result in the formation of solid solutions of hydrogen in metals, molecular hydrogen, gaseous products that are formed by reactions between hydrogen and elements constituting the alloy, and hydrides. Depending on the type of hydrogen/metal interaction, hydrogen damage of metal manifests itself in one of several ways. Specific types of hydrogen damage, some of which occur only in specific alloys under specific conditions include: Hydrogen embrittlement: Occurs most often in high-strength steels, primarily quenched-and-tempered and precipitation-hardened steels, with tensile strengths greater than about 1034 MPa (150 ksi). Hydrogen sulfide is the chief embrittling environment. Hydrogen-induced blistering: Also commonly referred to as hydrogen-induced cracking (HIC), it occurs in lower-strength (unhardened) steels, typically with tensile strengths less than about 550 MPa (80 ksi). Line pipe steels used in sour gas environments are susceptible to HIC.

Cracking from precipitation of internal hydrogen: Examples include shatter cracks, flakes, and fish eyes found in steel forgings, weldments, and castings. During cooling from the melt, hydrogen diffuses and precipitates in voids and discontinuities. Hydrogen attack: A high-pressure, high-temperature form of hydrogen damage. Commonly experienced in steels used in petrochemical plant equipment that often handles hydrogen and hydrogen-hydrocarbon streams at pressures as high as 21 MPa (3 ksi) and temperatures up to 540 0C (1000 0F) Hydride formation: Occurs when excess hydrogen is picked up during melting or welding of titanium, tantalum, zirconium, uranium, and thorium. Hydride particles cause significant loss in strength and large losses in ductility and toughness. Prevention. The primary factors controlling hydrogen damage are material, stress, and environment. Hydrogen damage can often be prevented by using more resistant material, changing the manufacturing processes, modifying the design to lower stresses, or changing the environment. Inhibitors and post-processing bake-out treatments can also be used. Baking of electroplated high-strength steel parts reduces the possibility of hydrogen embrittlement (see Chapter 8 for additional information).

Coatings and Corrosion Prevention As described in the previous section, surface treatments, and in particular protective coatings, are widely used to control corrosion in its varying forms. The problems of corrosion should be approached in the design stage, and the selection of a protective coating is important. Paint systems and lining materials exist that slow the corrosion rate of carbon steel surfaces. High-performance organic coatings such as epoxy, polyesters, polyurethanes, vinyl, or chlorinated rubber help to satisfy the need for corrosion prevention. Special primers are used to provide passivation, galvanic protection, corrosion inhibition, or mechanical or electrical barriers to corrosive action. Corrosion Inhibitors. A water-soluble corrosion inhibitor reduces galvanic action by making the metal passive or by providing an insulating film on the anode, the cathode, or both. A very small amount of chromate, polyphosphate, or silicate added to water creates a water-soluble inhibitor. A slightly soluble inhibitor incorporated into the prime coat of paint may also have a considerable protective influence. Inhibitive pigments in paint primers are successful inhibitors except when they dissolve sufficiently to leave holes in the paint film. Most paint primers contain a partially soluble inhibitive pigment such as zinc chromate, which reacts with the steel

substrate to form the iron salt. The presence of these salts slows corrosion of steel. Chromates, phosphates, molybdates, borates, silicates, and plumbates are commonly used for this purpose. Some pigments add alkalinity, slowing chemical attack on steel. Alkaline pigments, such as metaborates, cement, lime, or red lead, are effective, provided that the environment is not too aggressive. In addition, many new pigments have been introduced to the paint industry such as zinc phosphosilicate and zinc flake. Barrier coatings are used to prevent the electrolyte from reaching the component surface. Examples of barrier coatings include painted steel structures, steels lined with thick acid-proof brick, steels lined with rubberlike materials, or steels electroplated with a noble (see Table 1) metal (e.g., chromium, copper, or nickel). Protection is effective until the coating is penetrated, either by a pit, pore, crack, or by damage or wear. The substrate will then corrode preferentially to the coating (since it is anodic to the coating material), and corrosion products will lift off the coating and allow further attack (Fig. 11). Generally, electroplated coatings that are completely free of pores and other discontinuities are not commercially feasible. Pits eventually form at coating flaws, and the coating is penetrated. The resulting corrosion cell is shown in Fig. 12. The substrate exposed at the bottom of the resulting pit corrodes rapidly. A crater forms in the substrate, and because of the

Rust Paint Steel (a)

(b) p jo "I \ Illustration of the mechanism of corrosion for painted steel, (a) A void " in the paint results in rusting of the steel, which undercuts the paint coating and results in further coating degradation, (b) Photograph showing blistering and/or peeling (undercutting) of paint where exposed steel is rusting.

Moist air

Noble metal coating (cathode)

Steel substrate (anode) f\a 1 2 Crater formation in a steel substrate beneath a void in a noble metal " coating, for example, passive chromium or copper. Corrosion proceeds under the noble metal, the edges of which collapse into the corrosion pit. Water drop

Substrate (M3)

Coating (M1)

Coating (M2)

FlC, 1 3 Corrosion pit formation in a substrate beneath a void in a duplex ^* noble metal coating. The top coating layer (M1) is cathodic to the coating underlayer (M2), which is in turn cathodic to the substrate (M3). As in Fig. 12, the coating tends to collapse into the pit.

large area ratio between the more noble coating and the anodic crater, the crater becomes anodic, and high corrosion current density results. Electrons flow from the substrate to the coating as the steel dissolves. Hydrogen ions (H + ) in the moisture accept the electron and, with dissolved oxygen, form water at the noble metal surface near the void. Use of an intermediate coating that is less noble than a surface coating but more noble than the base metal can result in the mode of corrosion shown in Fig. 13. This would be typical of a costume jewelry item with a brass substrate, an intermediate nickel coating, and a tarnish-resistant gold top coat. It is also exemplified by nickel-chromium coating systems. Sacrificial coatings, which corrode preferentially to the substrate, include zinc, aluminum, cadmium, and zinc-rich paints. Initially these sacrificial coatings will corrode, but their corrosion products are protective and the coating acts as a barrier layer. If the coating is damaged or defective, it remains protective as it is the coating that suffers attack and not the substrate. Figure 14 shows the sacrificial (galvanic) protection offered by a zinc coating to a steel substrate. Cathodic protection involves the reversal of electric current flow within the corrosion cell. Cathodic protection can reduce or eliminate corrosion by connecting a more active metal to a metal that must be

Water drop

Zinc coating (anode)

steel substrate (cathode) FlC. 14 Principles and mechanism of galvanic protection of a substrate by a ^* coating. Galvanic protection of a steel substrate at a void in a zinc coating. Corrosion of the substrate is light and occurs at some distance from the zinc.

protected. The use of cathodic protection to reduce or eliminate corrosion is a successful technique of long-standing use in marine structures, pipelines, bridge decks, sheet piling, and equipment and tankage of all types, particularly below water or underground. Typically, zinc or magnesium anodes are used to protect steel in marine environments, and the anodes are replaced after they are consumed. Cathodic protection uses an impressed direct current (dc) supplied by any low output voltage source and a relatively inert anode. As is the case in all forms of cathodic activity, an electrolyte is needed for current flow. Cathodic protection and the use of protective coatings are most often employed jointly, especially in marine applications and on board ships where impressed current inputs do not usually exceed 1 V. Beyond 1 V, many coating systems tend to disbond. Current source for cathodic protection in soils is usually 1.5 to 2 V. Choice of anodes for buried steel pipe depends on soil conditions. Magnesium is most commonly used for galvanic anodes; however, zinc can also be used. Galvanic anodes are seldom used when the resistivity of the soil is over 30 fl • m (3000 ft • cm); impressed current is normally used for these conditions. Graphite, high-silicon cast iron, scrap iron, aluminum, and platinum are used as anodes with impressed current. The availability of low-cost power is often the deciding factor in choosing between galvanic or impressed current cathodic protection. Figure 15 illustrates both types of galvanic protection systems. Protective coatings are normally used in conjunction with cathodic protection and should not be disregarded where cathodic protection is contemplated in new construction. Because the cathodic protection current must protect only the bare or poorly insulated areas of the surface, coatings that are highly insulating, very durable, and free of discontinuities lower the current requirements and system costs. A good coating also enables a single-impressed current installation to protect many miles of piping. Coal-tar enamel, epoxy powder coatings, and vinyl resin are exam-

ac line Insulated copper wire

Rectifier. Insulated copper wire Soil

Soil Active metal anode

Pipeline

Pipeline

Current

Current

Backfill (a)

Anode

Backfil

(b)

Fig, 1 5 Cathodic protection for underground pipe, (a) Sacrificial or galvanic anode, (b) Impressed-cur^* rent anode, ac, alternating current

pies of coatings that are most suitable for use with cathodic protection. Certain other coatings may be incompatible, such as phenolic coatings, which may deteriorate rapidly in the alkaline environment created by the cathodic protection currents. Although cement mortar initially conducts the electrical current freely, polarization, the formation of an insulating film on the surface as a result of the protective current, is believed to reduce the current requirement moderately. Cathodic protection is used increasingly to protect buried or submerged metal structures in the oil, gas, and waterworks industries and can be used in specialized applications, such as for the interiors of water storage tanks. Pipelines are routinely designed to ensure the electrical continuity necessary for effective functioning of the cathodic protection system. Thus, electrical connections or bonds are required between pipe sections in lines using mechanically coupled joints, and insulating couplings may be employed at intervals to isolate some parts of the line electrically from other parts. Leads may be attached during construction to facilitate the cathodic protection installation when needed.

Corrosion Testing Many tests exist for establishing the reliability of protective coatings on metal substrates. Existing tests and standards are under continuous development, and new tests are being designed. Organizations active in the development and standardization of corrosion tests for coatings include ASTM, NACE International, the Society of Automotive Engineers (SAE), the National Coil Coaters Association (NCCA), the International Standards Organization (ISO), international systems (e.g., DIN), and commercial (e.g., automotive, architectural, electronics), proprietary, and

military organizations. This section provides a brief review of the most widely used test methods including: Field tests Simulated service tests Laboratory (accelerated) tests (e.g., salt spray tests, humidity tests, and electrochemical tests) Table 5 lists selected tests used for determining the effectiveness of protective coatings in corrosive environments. More detailed information on testing of coated specimens can be found in several excellent sources. Gaynes (Ref 13) and Munger (Ref 14) give descriptions and the framework for effective use of tests and standards. Gaynes provides detailed descriptions including photographs, cross-listing ASTM to federal tests and a broader perspective encompassing the federal standard, miscellaneous tests, and some caveats of traditional testing. Munger offers practical material directed toward large structures and provides a listing based on ASTM standards. Altmayer (Ref 15) compiled a table of 13 applicable corrosion tests for 30 metallic, inorganic, and organic coating/substrate combinations. Other useful sources of information can be found in review articles by Simpson and Townsend (Ref 16) and Granata (Ref 12), which describe tests for metallic coatings and nonmetallic coatings, respectively.

Field Tests The most reliable performance data are obtained by field tests/surveys. One example would be to monitor and test the corrosion of autobody panels that sit in junkyards. Another example of in-service testing would be to monitor the behavior of the materials in a fleet of captive vehicles. This enables better control and recording of the exposure and driving conditions. The use of fleet vehicles also makes it possible to test coupons representing a larger database of materials.

Simulated Service Tests The most widely used simulated service test for static atmospheric testing is described in ASTM G 50, "Practice for Conducting Atmospheric Corrosion Tests on Metals." It is used to test coated sheet steels for a variety of outdoor applications. Test materials, which are in the form of flat test panels mounted in a test rack (Fig. 16), are subjected to the cyclic effects of the weather, geographical influences, and bacteriological factors that cannot be realistically duplicated in the laboratory. Test durations can last from several months up to many years. Some zinc-coated steel specimens have undergone testing for more than 30 years.

Table 5

Widely used tests for determining the corrosion resistance of protective coatings

Test Salt spray (ASTM B 117)

100% relative humidity (ASTM D 2247) Acetic acid-salt spray ASTM G 85, Al (formerly ASTM B 287) Sulfur dioxide-salt spray (ASTM G 85, A 4) Copper-accelerated salt spray, or CASS (ASTM B 368) FACT (formerly ASTM B 538) Accelerated weathering

Lactic acid

Acidified synthetic seawater testing or SWAAT (ASTM G 85, A3; formerly ASTM G 43)

Electrographic and chemical porosity tests

Adhesion (ASTM D 3359-90)

T-bend adhesion (ASTM D 4145)

Scab test Exterior exposure (ASTM D 1014) Service test data

Description and remarks

Most widely specified test. Atomized 5% sodium chloride (NaCl), neutral pH, 35°C (95 0F) (a), follow details of ASTM B 117, Appendix Xl. Emphasizes wet surfaces (nondrying), high oxygen availability, neutral pH, and warm conditions. Control of comparative specimens should be run simultaneously. Corrosivity consistency should be checked as described in ASTM B 117, Appendix X3. Notes: May be the most widely misused test. Requires correlation to service tests for useful results. Do not assume correlation exists. Widely used test. Condensing humidity, 100% RH, 38 0 C (100 0 F). Emphasizes sensitivity to water exposure Widely used test. Atomized 5% NaCl, pH 3.2 using acetic acid, 35 0C (95 0 F). More severe than ASTM B 117. The lower pH and the presence of acetate affect the solubility of corrosion products on and under the protective coatings. Atomized 5% NaCl, collected solution pH = 2.5-3.2, 35 0 C (95 0 F), SO2 metered (60 min • 35 cm3/min per m 3 cabinet volume) 4 times per day Atomized 5% NaCl, pH 3.2 with acetic acid, 0.025% cupric chloride-dihydrate, 35 0C (95°F). Galvanic coupling due to copper salt reduction to copper metal. More severe than ASTM B 117 Testing anodized aluminum specimens. Electrolyte as in salt spray or CASS test. Specimen is made the cathode to generate high pH at defects. Exposure of coated specimens to effects of ultraviolet radiation experienced in outdoor sunlight conditions, which may be combined with other exposures such as moisture and erosion. Exposure cabinets use carbon arc (ASTM D 822), xenon lamp (ASTM G 26), or fluorescent lamp (ASTM G 53). On substrates of brass and copper alloys, determines coatings porosity and resistance to handling (perspiration). Consists of immersion in 85% lactic acid solution, drying, and incubating above acetic acid vapors for 20 h to reveal discoloration spots at failure points or delaminations Atomized synthetic seawater (ASTM D 1141) with 10 mL glacial acetic acid per L of solution, pH 2.8 to 3.0, 35 0 C (95 0 F). More severe than ASTM B 117. The lower pH and the presence of acetate affect the solubility of corrosion products on and under the protective coatings. Pores and active defects in nonmetallic coatings can be revealed by color indication or deposit formation. On nickel substrates, dimethylglyoxime, or steel, potassium ferricyanide (ferroxyl test) indicator can be applied to surface on filter paper while substrate is made the anode. Alternatively, a substrate immersed in acidic copper sulfate can be made the cathode to form copper nodules at conductive coatings defects. Knife and fingernail test consists of cutting through the coating with knife or awl and dislodging coating with thumbnail or fingernail (pass/fail). The ASTM D 3359 test consists of "X" scribes or parallel cross-hatches followed by adhesive tape stripping of loosened coating. Combined flexibility and adhesion test consists of clamping end of coated flat metal panel in vise or similar tool bending (convex) through 90°, reclamping to bend through 180° to give "071" bend (where T is panel thickness and the numeral (0, 1, 2,...) is the number of panel thicknesses). Rebending over the 180° bend gives a IT bend. Adhesive tape is pressed down along edge of bend and any loose coating stripped off.

Cyclic testing consisting of short salt exposure, short drying period, and long period of high humidity. Undercutting from scribe is measured. Method for conducting exterior exposure tests of paints on steel. Well-defined exposure setup, not necessarily equivalent to service tests Performance data of coatings systems under use conditions. Slowest evaluation method; provides tangible results

FACT, Ford anodized aluminum corrosion test, (a) Note that dissolved CO 2 concentration at 0 0C (32 0F) is three times that of concentration at 35 0 C (95 0F) and can affect corrosion. Source: Ref 12

Flg. 1 6 Atmospheric corrosion test rack Salt Spray Tests

As indicated in Table 6, salt spray testing is the most popular form of testing for protective coatings. These tests have been used for more than 90 years as accelerated tests in order to determine the degree of protection afforded by both inorganic and organic coatings on a metallic base. Table 5 lists several widely used salt spray tests. The neutral salt-spray (fog) test (ASTM B 117—Method 811.1 of Federal Test Method 151b) is perhaps the most commonly used salt spray test in existence for testing inorganic and organic coatings, especially where such tests are used for material or product specifications. The duration of this test can range from 8 to 3000 h, depending on the product type of coating. A 5% sodium chloride (NaCl) solution that does not contain more than 200 ppm total solids and with a pH range of 6.5 to 7.2 when atomized is used. The temperature of the salt spray cabinet is controlled to maintain 35 + 1.1 or -1.7 0C (95 + 2 or - 3 0F) within the exposure zone of the closed cabinet. The acetic acid-salt spray (fog) test (ASTM G 85, Annex Al; Former Method B 287) is also used for testing inorganic and organic coatings but is particularly applicable to the study or testing of decorative chromium Table 6 Results of a survey to determine the most widely used tests for protective coatings Test Salt spray Immersion Outdoor Ultraviolet/condensation Accelerated/weathering Humidity/condensation Cathodic disbondment Adhesion Atlas cell test (NACE TMO174) Other physical tests Other chemical tests Flexibility

% respondents(a) 52 24 22 20 14 10 7 7 4 4 3 2

(a) Multiple tests used (total greater than 100%). Source: Ref 12

plate (nickel-chromium or copper-nickel-chromium) plating and cadmium plating on steel or zinc die castings and for the evaluation of the quality of a product. This test can be as brief as 16 h, although it normally ranges from 144 to 240 h or more. As in the neutral salt spray test, a 5% NaCl solution is used, but the solution is adjusted to a pH range of 3.1 to 3.3 by the addition of acetic acid, and again, the temperature of the salt spray cabinet is controlled to maintain 35 + 1.1 or -1.7 0C (95 + 2 or - 3 0F) within the exposure zone of the closed cabinet. The copper-accelerated acetic acid-salt spray (fog) test (CASS test), which is covered in ASTM B 368, is primarily used for the rapid testing of decorative copper-nickel-chromium or nickel-chromium plating on steel and zinc die castings. It is also useful in the testing of anodized, chromated, or phosphated aluminum. The duration of this test ranges from 6 to 720 h. A 5% NaCl solution is used, with 1 g of copper II chloride (CuCl2-2H2O) added to each 3.8 L of salt solution. The solution is then adjusted to a pH range of 3.1 to 3.3 by adding acetic acid. The temperature of the CASS cabinet is controlled to maintain 4 9 + 1 . 1 or —1.7 0C (120 + 2 or —3 0F) within the exposure zone of the closed cabinet. Humidity Cabinet Tests In a humidity cabinet the humidity is raised to a value chosen as appropriate to the material under test. The temperature is generally cycled, so that the specimen is exposed to alternating humid air and condensation. The apparatus is automated to ensure that conditions are controlled within narrow limits. Other corrodent materials, such as sulfur dioxide, may also be introduced. Examples of humidity cabinet tests include ASTM D 2247 and ASTM G 85 listed in Table 5. Electrochemical Tests Corrosion of metallic substances is an electrochemical process. An alternate approach to field or other accelerated tests in understanding and predicting metallic corrosion is the use of electrochemical parameters/ tests. Electrochemical tests often complement other test methods by providing kinetic and mechanistic data that would be otherwise difficult to obtain. Electrochemical tests are typically grouped as direct current (dc) or alternating current (ac) methods based on the type of perturbation signal that is applied in making the measurements. A number of investigators have used dc and ac electrochemical methods to study the performance and the quality of protective coatings, including passive films on metallic substrates, and to evaluate the effectiveness of various surface pretreatments. Several are discussed below.

Anodized Aluminum Corrosion Test. One such method is the Ford anodized aluminum corrosion test (FACT) listed in Table 5. This test involves the cathodic polarization of the anodized aluminum surface by using a small cylindrical glass clamp-on cell and a special 5% NaCl solution containing cupric chloride (CuCl2) acidified with acetic acid. A large voltage is applied across the cell by using a platinum auxiliary electrode. The alkaline conditions created by the cathodic polarization promote dissolution at small defects in the anodized aluminum. The coating resistance is decreased, more current begins to flow, and the voltage decreases. The cell voltage (auxiliary electrode to test specimen voltage) is monitored for 3 min, and the parameter cell voltage multiplied by time is recorded. A similar test, known as the cathodic breakdown test, involves cathodic polarization to -1.6 V (versus saturated calomel electrode, SCE) for a period of 3 min in acidified NaCl. Again, the test was designed for anodized aluminum alloys because the alkali created at the large applied currents will promote the formation of corroded spots at defects in the anodized film. The electrolytic corrosion test was designed for electrodeposits of principally nickel and chromium on less noble metals, such as zinc or steel. Special solutions are used, and the metal is polarized to +0.3 V versus the SCE. The metal is taken through cycles of 1 min anodically polarized and 2 min unpolarized. An indicator solution is then used to detect the presence of pits that penetrate to the substrate. Each exposure cycle simulates 1 year of exposure under atmospheric-corrosion conditions. The ASTM standard B 627 describes the method in greater detail. The paint adhesion on a scribed surface (PASS) test involves the cathodic polarization of a small portion of painted metal. The area exposed contains a scribed line that exposes a line of underlying bare metal. The sample is cathodically polarized for 15 min in 5% NaCl. At the end of this period, the amount of delaminated coating is determined from an adhesive tape pulling procedure. The impedance test for anodized aluminum (ASTM B 457) is used to study the seal performance of anodized aluminum. In this sense, the test is similar to the FACT test, except that this method uses a 1 V root mean square 1 kHz signal source from an impedance bridge to determine the sealed anodized aluminum impedance. The test area is again defined with a portable cell, and a platinum or stainless steel auxiliary electrode is typically used. The sample is immersed in 3.5% NaCl. The impedance is determined in ohms X 103. In contrast to the methods discussed previously, this test is essentially nondestructive and does not accelerate the corrosion process. Electrochemical impedance spectroscopy (EIS) offers an advanced method of evaluating the performance of metallic coatings (passive film forming or otherwise) and organic barrier coatings. The method does not accelerate the corrosion reaction and is nondestructive. The technique is

quite sensitive to changes in the resistive-capacitive nature of coatings. The technique has been used to evaluate phosphate coverage/stability on galvanneal, painted cold-rolled steel, electrogalvanized steel, and electrogal vannealed steel (Ref 16). It is also possible to monitor the corrosion rate with this technique. In this respect, the electrochemical impedance technique offers several advantages over dc electrochemical techniques in that the polarization resistance related to the corrosion rate can be separated from the high dc resistance of the dielectric coating. This is not possible with the dc methods.

References 1. Corrosion of Steels in Waters, ASM Specialty Handbook: Carbon and Alloys Steels, J.R. Davis, Ed., ASM International, 1996, p 408-429 2. Corrosion of Steels in Soils, ASM Specialty Handbook: Carbon and Alloys Steels, J.R. Davis, Ed., ASM International, 1996, p 430-438 3. Corrosion of Steels in Chemical Environments, ASM Specialty Handbook: Carbon and Alloys Steels, J.R. Davis, Ed., ASM International, 1996, p 439-151 4. Types of Corrosive Environments, Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000, p 193-236 5. Atmospheric Corrosion of Steels, ASM Specialty Handbook: Carbon and Alloys Steels, J.R. Davis, Ed., ASM International, 1996, p 393^07 6. Forms of Corrosion: Recognition and Prevention, Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000, p 99-192 7. Corrosion Control by Proper Design, Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000, p 301-362 8. H.M. Herro and R.D. Port, Cavitation Damage, The Nalco Guide to Cooling Water System Failure Analysis, McGraw-Hill, Inc., 1993, p 270-271 9. R.B. Waterhouse, Fretting Wear, Friction, Lubrication, and Wear Technology, VoI 18, ASM Handbook, ASM International, 1992, p 242-256 10. Intergranular and Exfoliation Corrosion, Corrosion of Aluminum and Aluminum Alloys, J.R. Davis, Ed., ASM International, 1999, p 63-74 11. R.N. Parkins, An Overview—Prevention and Control of StressCorrosion Cracking, Mater. Perform., VoI 24, 1995, p 9-20 12. R.D. Granata, Nonmetallic Coatings, Corrosion Tests and Standards: Application and Interpretation, R. Baboian, Ed., ASTM, 1995, p 525-530 13. N.I. Gaynes, Testing of Organic Coatings, Noyes Data Corp., 1977

14. CG. Munger, Corrosion Prevention by Protective Coatings, National Association of Corrosion Engineers, 1984, Chapter 12 15. F. Altmayer, "Choosing an Accelerated Corrosion Test," Met. Finish., 61st Guidebook and Directory Issue, VoI 91 (No. IA), Jan 1993, p 483 16. T.C. Simpson and H.E. Townsend, Metallic Coatings, Corrosion Tests and Standards: Application and Interpretation, R. Baboian, Ed., ASTM, 1995, p 513-524 Selected References Corrosion, VoI 13, ASM Handbook, ASM International, 1987 Corrosion Basics—An Introduction, L.S. Van Delinder, Ed., NACE International, 1984 Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000 M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill, 1986 H.H. Uhlig and R.W. Revie, Corrosion and Corrosion Control, 3rd ed., John Wiley & Sons, 1985

CHAPTER

4 J

P r i n c i p l e s a n d

o f W

F r i c t i o n e

a

r

FRICTION, WEAR, AND LUBRICATION are complex, interwoven subjects that may all affect the service life of a component or the efficient operation of a machine. While all three are important factors, the major emphasis in this Chapter will be on wear and the various methods used to reduce or prevent it, including the application of surface engineering treatments. More detailed information on the science and technology of friction, wear, and lubrication—known as tribology—can be found in Friction, Lubrication, and Wear Technology, Volume 18 of the ASM Handbook Friction Friction is the resistance to motion when two bodies in contact are forced to move relative to each other. It is closely associated with any wear mechanisms that may be operating and with any lubricant and/or surface films that may be present, as well as the surface topographies. The heat generated as a result of the dissipation of frictional interaction may affect the performance of lubricants, may change the properties of the contacting materials and/or their surface films, and, in some cases, may change the properties of the product being processed. Any of these results of frictional heating can cause severe safety problems because of the danger of mechanical failure of components due to structural weakening, severe wear (for example, seizure), or fire and explosion. In moving machinery, friction is responsible for dissipation and loss of much energy. It has been estimated, for example, that 10% of oil consumption in the United States is used simply to overcome friction. The energy lost to friction is an energy input that must continually be provided in order to maintain the sliding motion. This energy is dissipated in the

system, primarily as heat—which may have to be removed by cooling to avoid damage and may limit the conditions under which the machinery can be operated. Some of the energy is dissipated in various deformation processes, which result in wear of the sliding surfaces and their eventual degradation to the point where replacement of whole components becomes necessary. Wear of sliding surfaces adds another, very large component to the economic importance of friction, because without sliding friction, these surfaces would not wear. The need to control friction is the driving force behind its study. In many cases low friction is desired (bearings, gears, materials processing operations), and sometimes, high friction is the goal (brakes, clutches, screw threads, road surfaces). In all of these cases, constant, reproducible, and predictable friction values are necessary for the design of components and machines that will function efficiently and reliably. Important Terms and Concepts. It is useful to clearly separate the various terms and concepts associated with friction, such as "friction force," "friction coefficient," "frictional energy," and "frictional heating." These terms are defined subsequently in the context of solid friction, which can be defined as "the resistance to movement of one solid body over another." The movement may be by sliding or by rolling. The friction force is the tangential force that must be overcome in order for one solid contacting body to slide over another. It acts in the plane of the surfaces and is usually proportional to the force normal to the surfaces, N, or: (EqI) The proportionality constant is generally designated |JL or/and is termed the friction coefficient; which is the ratio between the friction force, F, and the load, N: (Eq 2) The friction coefficient typically ranges from 0.03 for a very well lubricated bearing, to 0.5 to 0.7 for dry sliding, and even >5 for clean metal surfaces in a vacuum. A JUL-value of 0.2 to 0.3 allows for comfortable walking; however, walking on ice is very difficult because the JUL-value for the ice/shoe pair may be <0.05, and a slippery floor may have a jx-value of 0.15. Nature has provided highly efficient lubrication to another component of walking, the knee joint which has a |ji-value of 0.02. In most cases, a greater force is needed to set a resting body in motion than to sustain the motion; in other words, the static coefficient of friction, juLs, is usually somewhat greater than the dynamic or kinetic coefficient of friction, juik.

A body of weight W on a flat surface will begin to move when the surface is tilted to a certain angle termed the friction angle, 6, as defined in Fig. 1. The static friction coefficient is given by: (Eq 3) This represents a simple way to measure |xs, but force measurements are more generally used to measure both the static and the dynamic, or kinetic, coefficients of friction. The results obtained from these measurements do, however, depend on the nature and cleanliness of the surfaces and also to some extent on the various characteristics of the measuring system. This dependence underscores the basic fact that the friction coefficient is not a unique, clearly defined materials property, as may become evident from the following discussion on "Basic Mechanisms of Friction." To overcome friction, the tangential force must be applied over the entire sliding distance; the product of the two is friction work. The resulting energy is lost to heat in the form of frictional heating and to other general increases in the entropy of the system, as represented, for example, in the permanent deformation of the surface material. Thus, friction is clearly a process of energy dissipation. Basic Mechanisms of Friction. Surfaces are not completely flat at the microscopic level. At high magnification, even the best polished surface will show ridges and valleys, asperities, and depressions. When two surfaces are brought together, they touch intimately only at the tips of a few asperities. At these points, the contact pressure may be close to the hardness of the softer material; plastic deformation takes place on a very local scale. Cold welding may form strongly bonded junctions between the two materials. When sliding begins, these junctions have to be broken by the friction force, and this provides the adhesive component of the friction. Some asperities may plow across the surface of the mating material, and

Body

(b) (a) FlC. 1 Incl|ned plane used to determine coefficient of static friction, (JLS. (a) Tilting flat surface ^* through smallest angle, 0, needed to initiate movement of the body down the plane. (b) Relation of the friction angle to the principal applied forces

the resulting plastic deformation or elastic hysteresis contributes to the friction force. Additional contributions may be due to wear by debris particles that become trapped between the sliding surfaces. Because so many mechanisms are involved in generating the friction force, it is clear that friction is not a unique materials property, but instead depends to some extent on the measuring conditions, on the surface roughness, on the presence or absence of oxides or adsorbed films, and so on. In spite of this complexity, the values of JUL obtained by different methods and by different laboratories tend to fall into ranges that are representative of the material pair in question under reasonably similar conditions; that is, values obtained by different laboratories tend to fall within —20 to 30% of each other if the testing conditions are generally similar. It is important, however, to understand that the values of JUL listed in this Chapter are intended only to provide rough guidelines and that more exact values, if needed, must be obtained from direct measurements on the system in question under its typical operating conditions. The deformation at asperities and junctions is extremely localized, and very high temperatures may therefore be generated over very short periods of time. At these local hot spots, rapid oxidation, plastic flow, or interdiffusion can take place, and these all affect the wear process. In some cases, sparks may even form. The temperatures obtained depend on how fast heat is generated (that is, on the operating conditions of load and velocity) and on how fast heat is removed (that is, on the thermal properties of the sliding surfaces). These temperatures can be calculated with some degree of certainty, as shown in Friction, Lubrication, and Wear Technology, Volume 18 of the ASM Handbook (see pages 39 to 44). Friction Coefficients for Selected Materials. The friction coefficient between solids sliding, or about to slide, over one another under the influence of a nonzero normal force is a function of several factors whose relative contributions vary on a case-by-case basis: Composition of the materials Surface finish of each solid Nature of the surrounding environment Force holding the solids in contact (load) Velocity of relative motion Nature of the relative motion (for example, unidirectional, back and forth, steady, variable, and so on) Nature of the contact (conforming versus nonconforming surfaces) Temperature of the interfacial region Prior sliding history of the surfaces Characteristics of the machine and fixtures in which the materials are affixed No single source has generated a comprehensive list of friction coefficients for materials under identical testing conditions; therefore, nearly all

existing handbooks rely on compilations of data produced under a variety of testing conditions. Readers should he aware of this shortcoming and use the values only as very approximate guides, unless their applications are exactly the same as those methods used in generating the data. The five tables of friction coefficient values in this Chapter contain both static and kinetic friction coefficients. They are arranged by material type as follows: Table 1: metals on metals Table 2: ceramics on various materials Table 3: polymers on various materials Table 4: coatings on various materials Table 5: miscellaneous materials Table 1 Friction coefficient data for metals sliding on metals and corresponding references and test conditions Metals tested in air at room temperature Material Fixed specimen Ag

Al Al, alloy 6061-T6

Au Brass, 60Cu-40Zn Cd Co Cr Cu

Cu, OFHC Fe

In Mg Mo

Friction coefficient

Moving specimen

Test geometry(a)

Ag Au Cu Fe Al Ti Al, alloy 6061-T6 Cu Steel, 1032 Ti-6A1-4V Ag Au Steel, tool Cd Fe Co Cr Co Cr Co Cr Cu Fe Ni Zn Steel, 4619 Co Cr Fe

IS IS IS IS IS IS FOF FOF FOF FOF IS IS POR IS IS IS IS IS IS IS IS IS IS IS IS BOR IS IS IS

0.50 0.53 0.48 0.49 0.57 0.54 0.42 0.28 0.35 0.34 0.53 0.49

Mg Mo Ti W Zn In Mg Fe Mo

IS IS IS IS IS IS IS IS IS

0.51 0.46 0.49 0.47 0.55 1.46 0.69 0.46 0.44

Static

0.79 0.52 0.56 0.41 0.41 0.46 0.44 0.46 0.55 0.50 0.49 0.56

Kinetic

0.34 0.23 0.25 0.29

0.24

0.41 0.48 0.51

0.82

Ref 1 1 1 1 1 1 2 2 2 2 1 1 3 1 1 1 1 1 1 1 1 1 1 1 1 4 1 1 1 1 1 1 1 1 1 1 1

(continued) (a) Test geometry codes: BOR, flat block pressed against the cylindrical surface of a rotating ring; FOF, flat surface sliding on another flat surface; IS, sliding down an inclined surface; POR, pin sliding against the cylindrical surface of a rotating ring; RSOF, reciprocating, spherically ended pin on a flat surface; SPOF, spherically ended pin on a flat coupon

Table 1

(continued) Material

Fixed specimen Nb Ni

Pb

Pt Sn Steel Steel, 1020 Steel, 1032

Steel, 52100

Steel, mild Steel, M50 tool Steel, stainless Steel, stainless 304 Stellite Ti

Ti-6A1-4V

W

Zn

Zr

Moving specimen

Test geometry(a)

Nb Cr Ni Pt Ag Au Co Cr Fe Pb Steel Ni Pt Fe Sn Cu Pb Steel, 4619 Al, alloy 6061-T6 Cu Steel, 1032 Ti-6A1-4V Ni3Al, alloy IC-396M Ni3Al, alloy IC-50 Steel, 1015 annealed Steel, dual-phase DP-80 Steel, O2 tool Steel, mild Ni3Al, alloy IC-50 Steel, tool Cu Steel, tool Al Steel, 17-4 stainless Ti Ti Ti-6A1-4V Al, alloy 6061-T6 Cu-Al (bronze) Nitronic 60 Steel, 17-4 stainless Steel, Type 440C stainless Stellite 12 Stellite 6 Ta Ti-6A1-4V Ti-6A1-4V Cu Fe W Cu Fe Zn Zr

IS IS IS IS IS IS IS IS IS IS SPOF IS IS IS IS SPOF SPOF BOR FOF FOF FOF FOF RSOF RSOF BOR BOR BOR BOR RSOF POR FOF POR IS POF POF FOF POF FOF POF POF POF POF POF POF POF FOF POF IS IS IS IS IS IS IS

Reference

Friction coefficient Static

Kinetic

Ref

0.46 0.59 0.50 0.64 0.73 0.61 0.55 0.53 0.54 0.90 0.80 0.64 0.55 0.55 0.74

0.47 0.32 0.31 0.36

0.23 0.54 0.48 0.47 0.55 0.43 0.41 0.36 0.38 0.36 0.44 0.35 0.45 0.53 0.36 0.36 0.41 0.47 0.51 0.56 0.55 0.75 0.63

0.80 1.40 0.54 0.38 0.25 0.23 0.32 1.08 0.70 0.74 0.55 0.49 0.62 0.68 0.53 0.21 0.60 0.48 0.40 0.36 0.38 0.27 0.31 0.31 0.37 0.29 0.36 0.53 0.30 0.31

1 1 5 5 4 2 2 2 2 6 6 7 7 7 3 6 3 2 3 1 8 8 1 8 2 8 8 8 8 8 8 8 2 8 1 1 1 1 1 1 1

Test condition

1. E. Rabinowicz, ASLE Trans., VoI 14, 1971, p 198 2. "Friction Data Guide," General Magnaplate Corporation, 1988

Plate sliding on plate at 50% relative humidity TMI Model 98-5 slip and friction tester, 1.96 N (0.200 kgf) load, ground specimens, 54% relative humidity, average of five tests 3. J.F. Archard, ASME Wear Control Handbook, M.B. Peterson and W.O. Pin-on-rotating ring, 3.9 N (0.40 kgf) load, 1.8 m/s (350 ft/min) velocity Winer, Ed., American Society of Mechanical Engineers, 1980, p 38 4. A. W. Ruff, L.K. Ives, and W. A. Glaeser, Fundamentals of Friction Flat block-on-rotating 35 mm (1 3/8 in.) diameter ring, ION (1.02 kgf) and Wear of Materials, ASM International, 1981, p 235 load, 0.2 m/s (40 ft/min) velocity 5. F.P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Sphere-on-flat, unspecified load and velocity Oxford Press, 1986, p 127 6. RJ. Blau and CE. DeVore, Tribol, Int., VoI 23 (No. 4), 1990, p 226 Reciprocating ball-on-flat, 10 Hz, 25 N (2.6 kgf) load, 10 mm stroke 7. RJ. Blau, J. Tribology, VoI 107, 1985, p 483 Flat block-on-rotating 35 mm (1 3/s in.) diameter ring, 133 N (13.6 kgf) load, 5.0 cm/s (2.0 in./s) velocity 8. K.G. Budinski, Proceedings of Wear of Materials, American Society Modified ASTM G 98 galling test procedure of Mechanical Engineers, 1991, p 289 (a) Test geometry codes: BOR, flat block pressed against the cylindrical surface of a rotating ring; FOF, flat surface sliding on another flat surface; IS, sliding down an inclined surface; POR, pin sliding against the cylindrical surface of a rotating ring; RSOF, reciprocating, spherically ended pin on a flat surface; SPOF, spherically ended pin on a flat coupon

It should be emphasized that the data in the tables are for unlubricated solids at room temperature and in ambient air. The references provided with each table list both the sources of the data for the table and a brief description of the testing conditions used to generate these data, if such information was available in the reference. If accurate friction information is required for a specific application, the use of carefully simulated Table 2 Friction coefficient data for ceramics sliding on various materials and corresponding references and test conditions Specm i ens tested in air at room temperature Material Fixed specimen

Ag Al Alumina

Boron carbide Cr Cu Fe Glass, tempered

Silicon carbide

Silicon nitride

Steel, M50 tool

Ti Tungsten carbide

Moving specimen

Test geometry(a)

Alumina Zirconia Alumina Zirconia Alumina Alumina Alumina WRA(b) WRZTA(c) ZTA(d) Boron carbide Alumina Zirconia Alumina Zirconia Alumina Zirconia Al, alloy 6061-T6 Steel, 1032 Teflon(e) Silicon carbide Silicon nitride Silicon nitride Silicon nitride Silicon carbide Silicon carbide Silicon carbide Silicon nitride Boron carbide Silicon carbide Silicon nitride Tungsten carbide Alumina Zirconia Tungsten carbide

RPOF RPOF RPOF RPOF SPOD SPOD SPOD SPOD SPOD SPOD POD RPOF RPOF RPOF RPOF RPOF RPOF FOF FOF FOF SPOD SPOD SPOD SPOD SPOD SPOD SPOD SPOD POD POD POD POD RPOF RPOF POD

Reference

1. K. Demizu, R. Wadabayashim, and H. Ishigaki, Tribol. Trans., VoI 33 (No. 4), 1990, p 505 2. PJ. Blau, Oak Ridge National Laboratory 3. RJ. Blau, Oak Ridge National Laboratory 4. RJ. Blau, Oak Ridge National Laboratory 5. C S . Yust, Tribology of Composite Materials, RK. Rohatgi, PJ. Blau, and CS. Yust, Ed., ASM International, 1990, p 27 6. B. Bhushan and B.K. Gupta, table in Handbook of Tribology, McGraw-Hill, 1991 7. "Friction Data Guide," General Magnaplate Corporation, 1988

Friction coefficient Static

0.17 0.13 0.10

Kinetic

Ref

0.37 0.39 0.75 0.63 0.50 0.52 0.33 0.53 0.50 0.56 0.53 0.50 0.61 0.43 0.40 0.45 0.35 0.14 0.12 0.10 0.52 0.53 0.71 0.63 0.54 0.67 0.84 0.17 0.29 0.29 0.15 0.19 0.42 0.27 0.34

1 1 1 1 2 3 4 5 5 5 6

7 7 7 6 4 2 3 4 2 3 6 6 6 6 6 1 1 6

Test condition 1.5 mm (0.06 in.) radius pin reciprocating on a flat, 4 N (0.4 kgf) load, 0.17 mm/s (0.0067 in./s) velocity, 50% relative humidity 1.0 N (0.10 kgf) load and 0.1 m/s (20 ft/min) velocity 10 N (1.0 kgf) load and 0.1 m/s (20 ft/min) velocity 9.5 mm (3/8 in.) diameter sphere-on-disk, 2 to 9 N (0.2 to 0.9 kgf) load, 0.3 m/s (60 ft/min) velocity 20 N (2.0 kgf), 3 mm/s (0.12 in./s) velocity TMI Model 98-5 slip and friction tester, 1.96 N (0.200 kgf) load, ground specimens, 54% relative humidity, average of five tests

(a) Test geometry codes: FOF, flat surface sliding on another flat surface; POD, pin-on-disk (pin-tip geometry not given); RPOF, reciprocating pin-on-flat; SPOD, spherically ended pin-on-flat disk; SPOF, spherically ended pin on a flat coupon, (b) WRA, silicon carbide whisker-reinforced alumina, (c) WRZTA, silicon carbide whisker-reinforced, zirconia-toughened alumina, (d) ZTA, zirconia-toughened alumina, (e) Teflon, polytetrafluoroethylene

Table 3 Friction coefficient data for polymers sliding on various materials and corresponding references and test conditions Specimens tested in air at room temperature Material(a) Fixed specimen

Friction coefficient Moving specimen

Test geometry(b)

Static

Kinetic

Ref

0.06 0.06 0.80 0.17 0.50 0.20 0.04 0.08

0.07 0.07

1 1 2 1 2 2 2 3

Polymers sliding on polymers Acetal Nylon 6/6 PMMA Polyester PBT Polystyrene Polyethylene Teflon

TW TW NSp TW NSp NSp NSp FOF

Acetal Nylon 6/6 PMMA Polyester PBT Polystyrene Polyethylene Teflon Teflon

0.07

Dissimilar pairs with the polymer as the fixed specimen Nylon 6 (cast) (extruded) Nylon 6/6 Nylon 6/6 (+ PTFE) PA 66 PA 66 (+ 15% PTFE) PA 66 (PTFE/glass) PEEK PEEK (+ 15% PTFE) PEEK (PTFE/glass) PEI PEI (+ 15% PTFE) PEI (PTFE/glass) PETP PETP (+ 15% PTFE) PETP (PTFE/glass) Polyurethane(c) Polyurethane(d) POM POM (+ 15% PTFE) POM (PTFE/glass) PPS PPS (+ 15% PTFE) PPS (PTFE/glass) Teflon

UHMWPE

0.24

Steel, mild Steel, mild Polycarbonate Steel, mild Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, mild Steel, mild Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Steel, 52100 Al, alloy 6061-T6 Cr plate Cu Ni (0.001 P) Steel, 1032 Ti-6A1-4V TiN (Magnagold) Steel, mild

TPOD TPOD TW TPOD BOR BOR BOR BOR BOR BOR BOR BOR BOR BOR BOR BOR TPOD TPOD BOR BOR BOR BOR BOR BOR FOF FOF FOF FOF FOF FOF FOF TOPD

0.25

0.24 0.09 0.13 0.15 0.27 0.17 0.15

0.35 0.37 0.04 0.35 0.57 0.13 0.31 0.49 0.18 0.20 0.43 0.21 0.21 0.68 0.14 0.18 0.51 0.35 0.45 0.21 0.23 0.70 0.30 0.39 0.19 0.08 0.11 0.12 0.27 0.14 0.12 0.14

4 4 1 4 5 5 5 5 5 5 5 5 5 5 5 5 4 4 5 5 5 5 5 5 3 3 3 3 3 3 3 4

0.27 0.35 0.16 0.21 0.31 0.25 0.23 0.28 0.60 0.32 0.37 0.26 0.38 0.28 0.38 0.31

6 1 1 1 7 7 6 6 7 1 6 1 6 1 6 1

Dissimilar pairs with the polymer as the moving specimen Steel, carbon Steel, mild

Steel, 52100 Steel, carbon Steel, 52100 Steel, mild Steel, carbon Steel, mild Steel, carbon Steel, mild Steel, carbon Steel, mild

POF TW TW TW POD POD POF POF POD TW POF TW POF TW POF TW

ABS resin ABS ABS + 15% PTFE Acetal Acetal HDPE HDPE LDPE Lexan 101 Nylon (amorphous) Nylon 6 Nylon 6 Nylon 6/6 Nylon 6/6 Nylon 6/10 Nylon 6/10

0.40 0.30 0.13 0.14

0.36 0.48 0.23 0.54 0.22 0.53 0.20 0.53 0.23

(continued) (a) ABS, acrylonitrile butadiene styrene; HDPE, high-density polyethylene; LPDE, low-density polyethylene; Lexan, trademark of the General Electric Co. (polycarbonate); nylon, one of a group of polyamide resins (see also PA); PA, polyamide; PBT, polybutylene terephthalate; PEEK, polyetheretherketone; PEI, polyetherimide; PETP, polyethylene terephthalate; PMMA, polymethylmethacrylate; POM, polyoxymethylene; PPS, polyphenylene sulphide; PTFE, polytetrafluoroethylene; PVC, polyvinyl chloride; UHMWPE, ultra high molecular weight polyethylene; Magnagold, product of General Magnaplate, Inc.; Teflon, trademark of E.I. Du Pont de Nemours & Co., Inc. (PTFE). (b) Test geometry codes: BOR, flat block-on-rotating ring; FOF, flat surface sliding on another flat surface; NSp, not specified; POD, pin-on-disk; POF, pin-on-flat; TPOD, triple pin-on-disk; TW, thrust washer test, (c) Green polyurethane. (d) Cream-colored polyurethane

Table 3

(continued) Friction coefficient

Material(a) Fixed specimen

Steel, Steel, Steel, Steel,

Moving specimen

Test geometry(b)

Nylon 6/12 PEEK (Victrex) Phenol formaldehyde PMMA PMMA Polycarbonate Polyesther PBT Polyethylene Polyimide Polyoxylmethylene Polypropylene Polypropylene Polystyrene Polystyrene Polysulfone PVC PTFE Teflon Teflon Teflon Teflon Teflon Teflon Teflon

carbon 52100 carbon mild

Steel, carbon Steel, mild Steel, carbon Steel, mild Steel, carbon Al, alloy 6061-T6 Cr plate Glass, tempered Ni (0.001 P) Steel, 1032 Ti-6A1-4V TiN (Magnagold)

TW TW POF POD POF TW TW TW POF POF POF TW POF TW TW POF POF FOF FOF FOF FOF FOF FOF FOF

Static

Kinetic

Ref

0.24 0.20 0.51

0.31 0.25 0.44 0.68 0.50 0.38 0.25 0.13 0.34 0.17 0.26 0.11 0.37 0.32 0.37 0.38 0.09 0.18 0.19 0.10 0.19 0.16 0.21 0.11

1 1 6 7 6

0.64 0.31 0.19 0.09 0.46 0.30 0.36 0.08 0.43 0.28 0.29 0.53 0.37 0.19 0.21 0.10 0.22 0.18 0.23 0.16

1 1 6 6 6 1 6 1 1 6 6 3 3 3 3 3 3 3

Test condition

Reference 1. "Lubricomp Internally-Lubricated Reinforced Thermoplastics and Fluoropolymer Composites," Bulletin 254-688, ICI Advanced Materials 2. RP. Bowden and D. Tabor, Appendix IV, The Friction and Lubrication of Solids, Oxford Press, 1986 3. "Friction Data Guide," General Magnaplate Corporation, 1988 4. J.M. Thorp, Tribol Int., VoI 15 (No. 2), 1982, p 69 5. J.W.M. Mens and A.W.J. de Gee, Wear, VoI 149, 1991, p 255 6. R.P. Steijn, Metall Eng. Quart, VoI 7, 1967, p 9 7. N.P. Suh, Tribophysics, Prentice-Hall, 1986, p 226

Thrust washer apparatus, 0.28 MPa (40 psi), 0.25 m/s (50 ft/min), after running-in for one full rotation Unspecified testing conditions TMI Model 98-5 slip and friction tester, 1.96 N (0.200 kgf) load, ground specimens, 54% relative humidty, average of five tests Three pin-on-rotating disk apparatus, 0.1 m/s (20 ft/min) Flat block-on-rotating ring, 1.5 MPa (0.22 ksi) pressure, 150 N (15 kgf) load, 0.1 m/s (20 ft/min) velocity 12.7 mm (0.500 in.) diameter ball-on-flat, 9.8 N (1.0 kgf) load, 0.01 mm/s (4 X 10"4 in./s) velocity Pin-on-disk, 4.4 N (0.45 kgf) load, 3.3 cm/s (1.3 in./s) velocity, 65% relative humidity

(a) ABS, acrylonitrile butadiene styrene; HDPE, high-density polyethylene; LPDE, low-density polyethylene; Lexan, trademark of the General Electric Co. (polycarbonate); nylon, one of a group of polyamide resins (see also PA); PA, polyamide; PBT, polybutylene terephthalate; PEEK, polyetheretherketone; PEI, polyetherimide; PETP, polyethylene terephthalate; PMMA, polymethylmethacrylate; POM, polyoxymethylene; PPS, polyphenylene sulphide; PTFE, polytetrafluoroethylene; PVC, polyvinyl chloride; UHMWPE, ultra high molecular weight polyethylene; Magnagold, product of General Magnaplate, Inc.; Teflon, trademark of E.I. Du Pont de Nemours & Co., Inc. (PTFE). (b) Test geometry codes: BOR, flat block-on-rotating ring; FOF, flat surface sliding on another flat surface; NSp, not specified; POD, pin-on-disk; POF, pin-on-flat; TPOD, triple pin-on-disk; TW, thrust washer test, (c) Green polyurethane. (d) Cream-colored polyurethane

Table 4

Friction coefficient data for coatings sliding on various materials

Specimens tested in air at room temperature Material

Fixed specimen Al, alloy 6061-T6

Au, electroplate

Friction coefficient Moving specimen

Test geometry(a)

Cr plate Ni (0.001 P) plate TiN (Magnagold)(c) 60Pd-40Ag, plate 60Pd-40Au, plate 70Au-30Ag, plate

FOF FOF FOF POF POF POF

Static

Kinetic

Ref

0.27 0.33 0.25

0.22 0.25 0.22 2.40 0.30 3.00

1 1 2 2 2

(continued) (a) Ams, Amsler circumferential, rotating disk-on-disk machine; FOF, flat surface sliding on another flat surface; POD, pin-on-disk; POF, pin-on-flat; SPOD, spherically ended pin-on-flat disk, (b) Teflon is a registered trademark of E.I. Du Pont de Nemours & Co., Inc. (polytetrafluoroethylene). (c) Magnagold is a product of General Magnaplate, Inc. (d) CVD, chemical vapor deposition

Table 4 (continued) Material Fixed specimen

Au, electroplate (continued)

Cr plate

Niobium carbide, coating Ni (0.001 P) plate

Steel

Steel, 1032

Steel, type 440C stainless Steel, bearing

Steel, stainless

Teflon(b)

TiC on type 44OC stainless steel

TiN on type 440C stainless steel

TiN (Magnagold)(c)

Friction coefficient Moving specimen

80Pd-20Au, plate 99Au-I Co, plate Au plate Au-0.6 Co, plate Pd plate Al, alloy 6061-T6 Ni (0.001 P) plate Steel, 1032 Teflon(b) Ti-6A1-4V Niobium carbide, coating Al, alloy 6061-T6 Cr plate Ni (0.001 P) plate Steel, 1032 Steel, D2 tool Teflon(b) TiN (Magnagold)(c) Cu film on steel In film on Ag In film on steel Pb film on Cu Cr plate Ni (0.001 P) plate TiN (MagnagoldXc) TiC on type 304 stainless TiN on type 304 stainless Chrome carbide SiC(CVD)(d) TiC(CVD)(d) TiN(CVD)(d) Al2O3, plasma-sprayed Cr plate Cr2O3, plasma-sprayed TiO2, plasma-sprayed WC-12 Co, plasma-sprayed Cr plate Ni (0.001 P) plate TiN (Magnagold)(c) Al Ti TiC on type 440C stainless steel TiN on type 440C stainless steel Al Steel, type 304 stainless Ti TiC on type 440C stainless steel TiN on type 440C stainless steel Al, alloy 6061-T6 Steel, 1032 Teflon(b) Ti-6A1-4V TiN (MagnagoldXc)

Reference 1. "Friction Data Guide," General Magnaplate Corporation, 1988 2. M. Antler and E.T. Ratcliff, Proceedings of the Holm Conference on Electrical Contacts, 1982, p 19 3. MJ. Manjoine, Bearing and Seal Design in Nuclear Power Machinery, American Society of Mechanical Engineers, 1967 4. FP. Bowden and D. Tabor, The Friction and Lubrication of Solids, Oxford Press, 1986, p 127 5. B. Bhushan and B.K. Gupta, Handbook ofTribology, McGrawHill, 1991, Table 14.16a 6. B. Bhushan and B.K. Gupta, Handbook ofTribology, McGrawHill, 1991, Table 14.65 7. B. Bhushan and B.K. Gupta, Handbook ofTribology, McGrawHill, 1991, Table 14.12

Test geometry(a)

Static

Kinetic

Ref

POF POF POF POF POF FOF FOF FOF FOF FOF FOF FOF FOF FOF FOF FOF FOF FOF SPOD SPOD SPOD SPOD FOF FOF FOF POD POD POD POD POD POD Ams Ams Ams Ams Ams FOF FOF FOF POD POD POD POD POD POD POD POD POD FOF FOF FOF FOF FOF

... ... ... ... ... 0.20 0.19 0.20 0.21 0.38 0.19 0.26 0.41 0.32 0.35 0.43 0.22 0.33 0.30 0.10 0.08 0.18 0.25 0.37 0.31 0.12 0.50 ... ... ... ... ... ... ... ... ... 0.09 0.15 0.15 0.50 0.65 0.22 0.25 0.27 0.29 0.50 0.05 0.65 0.30 0.38 0.16 0.26 0.25

1.80 2.40 2.80 0.40 0.60 0.19 0.17 0.17 0.19 0.33 0.13 0.23 0.36 0.28 0.31 0.33 0.19 0.26 ... ... ... ... 0.21 0.30 0.28 0.17 0.75 0.79 0.23 0.25 0.49 0.13-0.30 0.30-0.38 0.14-0.15 0.10-0.15 0.11-0.13 0.08 0.12 0.12 0.85 0.80 0.20 0.20 0.40 0.41 0.76 0.06 0.45 0.26 0.31 0.11 0.23 0.21

2 2 2 2 2 1 1 1 1 1 3 1 1 1 1 1 1 1 4 4 4 4 1 1 1 5 5 6 6 6 6 7 7 7 7 7 1 1 1 5 5 5 5 5 5 5 5 5 1 1 1 1 1

Test condition TMI Model 98-5 slip and friction tester, 1.96 N (0.200 kgf) load, ground specimens, 54% relative humidity, average of five tests Sphere-on-reciprocating flat, 0.49 N (0.050 kgf) load, 1.0 mm/s (0.039 in./s) velocity Flat plate-on-flat plate, 28 MPa (4.1 ksi) contact pressure, 0.25 mm/s (0.010 in./s) velocity Sphere-on-flat, low-speed sliding, 39.2 N (4 kgf) load Pin-on-disk, 12 N (1.2 kgf) load, 14 to 16 cm/s (0.55 to 0.63 in./s) velocity Pin-on-disk, 5 N (0.5 kgf) load, 1.0 cm/s (0.39 in./s) velocity, 50% relative humidity Amsler disk machine, 400 rev/min, 250 N (26 kgf) load

(a) Ams, Amsler circumferential, rotating disk-on-disk machine; FOF, flat surface sliding on another flat surface; POD, pin-on-disk; POF, pin-on-flat; SPOD, spherically ended pin-on-flat disk, (b) Teflon is a registered trademark of E.I. Du Pont de Nemours & Co., Inc. (polytetrafluoroethylene). (c) Magnagold is a product of General Magnaplate, Inc. (d) CVD, chemical vapor deposition

Table 5

Friction coefficient data for miscellaneous materials

Specimens tested in air at room temperature Friction coefficient

Material Fixed specimen

Brick Cotton thread Diamond Explosives(b) HMX(c) PETN(d) RDX(e) Lead azide [Pb(N3)2] Silver azide (AgN3) Glass, tempered

Glass, thin fiber

Glass, clean Graphite, molded

Graphite (clean) Graphite (outgassed) Hickory wood, waxed Ice

Leather Metal Mica (cleaved) Mica (contaminated) Nylon fibers Paper, copier Sapphire Silk fibers Steel (clean) Wood (clean)

Test geometry(a)

Static

Wood Cotton thread Diamond

UnSp UnSp UnSp

0.6 0.3 0.1

Glass Glass Glass Glass Glass Al, alloy 6061-T6 Steel, 1032 Teflon(f) Brass Graphite Porcelain Steel, stainless Teflon(f) Glass (clean) Al, alloy 2024 Al, alloy 2219 Graphite, extruded Graphite, molded Inconel X-750(g) Steel, type 304 stainless Steel, type 347 stainless Graphite (clean) Graphite (outgassed) Snow Bronze Ebonite Ice Ice Ice Metal (clean) Glass (clean) Mica (cleaved) Mica (contaminated) Nylon fibers Paper, copier Sapphire Silk fibers Graphite Metals Wood (clean)

RPOF RPOF RPOF RPOF RPOF FOF FOF FOF StOD StOD StOD StOD StOD UnSp FOF FOF FOF FOF FOF FOF FOF UnSp UnSp UnSp UnSp UnSp UnSp UnSp FOF UnSp UnSp UnSp UnSp UnSp FOF UnSp UnSp UnSp UnSp UnSp

Moving specimen

Reference

1. RP. Bowden and D. Tabor, Appendix IV, The Friction and Lubrication of Solids, Oxford Press, 1986 2. J.K.A. Amuzu, BJ. Briscoe, and M.M. Chaudhri, /. Phys. D, Appl. Phys., VoI 9, 1976, p 133 3. "Friction Data Guide," General Magnaplate Corporation, 1988 4. P.K. Gupta, /. Am. Ceram. Soc, VoI 74 (No. 7), 1991, p 1692 5. MJ. Manjoine, Bearing and Seal Design in Nuclear Power Machinery, American Society of Mechanical Engineers, 1967 6. F.P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Oxford Press, 1986

0.17 0.13 0.10

0.9-1.0 0.16 0.22 0.20 0.18 0.16 0.18 0.19 0.10 0.5-0.8

Kinetic

1 1 1 0.55 0.40 0.35 0.28 0.40 0.14 0.12 0.10 0.16-0.26 0.15 0.36 0.31 0.10

0.17 0.14

0.14 0.02 0.02 0.05-0.15 >0.01 0.6 0.5-0.7 1.0 0.2-0.4 0.15-0.25 0.28 0.2 0.2-0.3 0.1 0.2-0.6 0.25-0.5

Ref

0.02 >0.01

2 2 2 2 2 3 3 3 4 4 4 4 4 1 5 5 5 5 5 5 5 1 1 6 6 6 6 6 3

0.26

1 1 1

Test condition

Method unspecified Reciprocating, single-crystal flat sliding on smooth fired glass surfaces, range 5 to 20 gf (0.049 to 0.1962 N load), 0.20 mm/s (0.008 in./s) velocity TMI Model 98-5 slip and friction tester, 1.96 N (0.200 kgf) load, ground specimens, 54% relative humidity, average of five tests Strand lying on a rotating drum, 1.96 N (0.200 kgf) load, 8.5 mm/s (0.33 in./s) velocity Flat plate-on-flat plate, 28 MPa (4.1 ksi) contact pressure, 0.25 mm/s (0.010 in./s) velocity Unspecified method, 4.0 m/s (790 ft/min) at 0 0 C

(a) FOF, flat surface sliding on another flat surface; RPOF, reciprocating pin-on-flat; StOD, strand wrapped over a drum; UnSp, unspecified method, (b) Explosives reported here were tested as reciprocating, single-crystal, flat-ended pin-on-moving flat, (c) HMX, cyclotetramethylene tetranitramine. (d) PETN, pentaerithritol tetranitrate. (e) RDX, cyclotrimethylene trinitramine. (f) Teflon is a registered trademark of E.I. Du Pont de Nemours & Co., Inc. (g) Inconel is a product of INCO, Inc.

conditions or instrumentation of the actual machine should be conducted in lieu of using tabulated values, because even a small change in contact conditions (for example, sliding speed or relative humidity for some materials) can result in a marked change in the measured or apparent friction coefficient. More detailed information on friction measurement techniques can be found in the article "Laboratory Testing Methods for Solid Friction" in Friction, Lubrication, and Wear Technology, Volume 18 in the ASM Handbook.

Wear In general, wear may be defined as damage to a solid surface caused by the removal or displacement of material by the mechanical action of a contacting solid, liquid, or gas. Gradual deterioration is often implied, and the effects are for the most part surface-related phenomena; but these restrictions should not be rigorously applied when analyzing wear problems or failures. Neither should the assumption that wear is entirely mechanical be accepted, because chemical corrosion may combine with other wear factors. Classification of Wear Wear has been classified in various ways. One of the simplest classifications of wear is based on the presence or absence of effective lubricants— namely lubricated or nonlubricated wear. (The various types of lubrication/lubricants are described later in this Chapter.) Another possibility is to classify wear on the basis of the fundamental mechanism that is operating. Unfortunately, this approach is complicated by the fact that more than one mechanism may be operating at the same time and by the fact that those developing wear classification schemes have come from different backgrounds and experiences with wear. As a result, different classification schemes based on wear mechanisms have been developed and no one scheme is universally accepted, although most of them have reasonably similar features. Figures 2 and 3 show two wear classifications based on the work of Blau (Ref 1) and Budinski (Ref 2). Blau places wear processes into three categories based on the type of motion encountered, that is, sliding, impact, and rolling contact (Fig. 2). Budinski reduces wear processes into four categories, that is, abrasion, erosion, adhesion, and surface fatigue (Fig. 3). Although both of the wear classifications schemes shown in Fig. 2 and 3 have merit, they also point out the difficulties inherent in classifying wear processes based on the commonality of mechanism. A third approach to wear classification emphasizes the nature of the contacting materials and the experimental conditions, using descriptive terms that are widely understood and accepted. The following is an example of this type of classification:

Metal against nonmetallic abrasive High-stress gouging or grinding: Wet, as in ball and rod mills Dry, as in jaw-type or roll-type ore crushers Low-stress scratching or sliding: Wet, such as conveyor screws for wet sand Dry, as against plows or earthmoving devices operating in dry soil Impact of loose abrasive (erosion): Wet, as against impellers in slurry pumps Dry, as in sandblasting Metal against metal Sliding: Lubricated, such as engine crosshead or shaft in a bearing Nonlubricated, such as fasteners, nuts, and bolts Rolling: Lubricated, such as roller bearings and gears Nonlubricated, such as wheels on tracks Liquid or vapor impingement on metals Wet steam, such as turbines Combustion gases, such as gas turbines Water, such as pump impellers Cavitations, as in turbulent, high-velocity flowing liquids

Frequently, these conditions are combined in service so an application that was originally metal against metal may evolve into metal against nonmetal wear, such as the generation of oxide wear debris and the introduction of nonmetallic particles through imperfect seals. Other combinations include rolling with sliding and lubricated-nonlubricated situations.

Wear Sliding wear

Impact wear

Roling contact wear

Abrasive (cutting) wear 2-body Multibody (3-body)

2-body impact wear

Pure rolling contact

Adhesive wear Fatigue wear Delamination Fretting wear Polishing wear (chemo-mechanical abrasion)

Multibody impact wear Erosion Solids Liquids Gases Slurries Electric sparks Cavitation Bubbles Gets)

Rolling/sliding contact Sliding Impact

Roling

pjo 2 Major categories of wear classified by the type of relative motion encountered (sliding, impact, ^* and rolling contact). Using this classification system, galling, scuffing, and scoring are not strictly considered forms of wear because material is not necessarily removed (it may instead be displaced to one side). Rather, these latter phenomena are referred to as "surface damage/' Source: Ref 1

Wear

Abrasion

Erosion

Adhesion

Surface fatigue

Low sress

Solid impingement

Fretting

Pitting

High stress

Fluid impingement

Adhesive

Spalling

Gouging

Cavitation

Seizure

Impact

Polishing

Slurry erosion

Galling

Brinelling

Require hard, sharp surfaces imposed on softer surfaces

Require fluid action

Oxidative wear Require interaction between conforming surfaces

Require repetitive compressive stresses

FlC. 3 Major categories of wear based on abrasion, erosion, adhesion, and " surface fatigue. Source: Ref 2

Abrasive Wear

General Description. Abrasive wear is defined as wear due to hard particles or hard protuberances forced against and moving along a solid surface. This form of wear in metals is most frequently caused by nonmetallic materials, but metallic particles can also cause abrasion. Generally, a material is seriously abraded or scratched only by a particle harder than itself. Figure 4 shows the damage caused on the surface of a soft copper substrate abraded by a hard ceramic particle. The cost of abrasion is high and has been estimated as ranging from 1 to 4% of the gross national product of an industrialized nation. The effect of abrasion is particularly evident in the industrial areas of agriculture, mining, mineral processing, earth moving, and essentially wherever dirt, rock, and minerals are handled. Examples include plows, ore loading/ moving buckets, crushers, and dump truck beds. When two surfaces contact, wear occurs on both surfaces. Individuals and industry tend to focus on the wearing surface that has the greatest potential for their own economic loss, and consider the other surface to be the abrasive. For example, an individual walking up the stairs of a building would

Fig. 4 Scanning electron micrograph showing surface damage by chip for" mation, plastic deformation, and pickup of fragments of a ceramic particle abrading a copper surface

be more likely to think that his shoes, rather than the stairs, were experiencing abrasive wear, whereas the maintenance staff would have the opposite opinion. In actuality, both surfaces are being subjected to abrasive wear. The rate at which the surfaces abrade depends on the characteristics of each surface, the presence of abrasives between the first and second surfaces, the speed of contact, and other environmental conditions. In short, loss rates are not inherent to a material. With reference to the above example, changing the material of either the shoes or the steps could, and often would, change the wear on the opposite counterface. The addition of an abrasive, such as a layer of sand, on the steps would further change the situation, in that the sand would be the second surface that contacts both the shoes and the steps. Abrasive Wear Categories. Abrasion is typically categorized according to types of contact, as well as contact environment. Types of contact include two-body and three-body wear. The former occurs when an abrasive slides along a surface, and the latter, when an abrasive is caught between one surface and another. Two-body systems typically experience from 10 to 1000 times as much loss as three-body systems for a given load and path length of wear. Contact environments (Fig. 5) are classified as either open (free) or closed (constrained). Tests have shown that for a given load and path length of wear, the wear rate is about the same for both open and closed systems. However, measurements of the loss in closed systems will often appear higher than the loss in open systems. This probably occurs because most closed systems experience higher loads. As shown in Fig. 3, abrasion is often further categorized as being lowstress abrasion, high-stress abrasion, gouging abrasion, and polishing

Machining

Free-flow ore (b) (a)

Plow penetrating sandy soil

Jaw crusher

(O (d) Fig, 5 Types of contact during abrasive wear, (a) Open two-body, (b) Closed two-body. ^* (c) Open three-body, (d) Closed three-body

abrasion. As described in the following paragraphs, each of these forms of abrasion is characterized by varying amounts of surface or subsurface damage. Low-stress abrasion (scratching) is defined as wear that occurs clue to relatively light rubbing contact of abrasive particles with the metal. The criteria established for low-stress abrasion is that the forces must be low enough to prevent crushing of the abradant. Wear scars usually show scratches, and the amount of subsurface deformation is minimal. Consequently, the surface does not work harden appreciably. Parts such as screens, chute liners, blades, and belts that are exposed to sand slurries or abrasive atmospheres could experience low-stress abrasion. Many machine components such as bushings, seals, and chains that operate in dust will wear by low-stress abrasion. Figure 6(a) shows a surface that was subjected to low-stress abrasion. High-stress abrasion is wear under a level of stress that is high enough to crush the abrasive. Considerably more strain hardening of the metal surface occurs. The abrasion of ore grinding balls is an example of highstress abrasion in the mining industry. Other examples include abrasion experienced by rolling-contact bearings, gears, cams, and pivots. Figure 6(b) illustrates this form of wear.

(a)

(b)

(C)

(d)

Fig. 6 Schematics illustrating the four types of abrasive wear, (a) Low-stress abrasion where material " is removed by hard, sharp particles or other hard, sharp surfaces plowing material out in furrows, (b) High-stress abrasion characterized by scratching, plastic deformation of surfaces, and pitting from impressed particles. Damage is almost always more severe than low-stress abrasion, (c) Gouging abrasion where material removal is caused by the action of repetitive compressive loading of hard materials such as rocks against a softer surface, usually a metal, (d) Polishing wear where material is removed from the surface by the action of rubbing from other solids under such conditions that material is removed without visible scratching, fracture, or plastic deformation. The example shown is polishing metal removal with a buffing wheel. Source: Ref 2

Gouging Abrasion. The term gouging abrasion is used to describe the high-stress abrasion that results in sizable grooves or gouges on the worn surface (Fig. 6c). It occurs on parts such as crusher liners, impact hammers in pulverizers, and dipper teeth handling large rocks. Strain hardening and plastic deformation are the dominant factors. Polishing wear is an extremely mild form of wear for which the mechanism has not been clearly identified, but that may involve extremely finescale abrasion, plastic smearing of microasperities, and/or chemical corrosion (Fig. 6d). Surfaces that have been subjected to polishing wear are usually smoothed and brightened, but this smoothing or brightening requires material removal. It can cause loss of serviceability in some instances, for example, worn and slippery stair treads. Several mechanisms have been proposed to explain how material is removed from a surface during abrasion. These mechanisms include fracture, fatigue, and melting. Because of the complexity of abrasion, no one mechanism completely accounts for all the loss. Figure 7 depicts some of the processes that are possible when a single abrasive tip traverses a surface. They include plowing, wedge formation, cutting, microfatigue, and microcracking.

Plowing

Microfatigue

Wedge

Microcracking

Cutting Fig, 7 Five mechanisms of abrasive wear

Plowing is the process of displacing material from a groove to the sides. This occurs under light loads and does not result in any real material loss. Damage occurs to the near surface of the material in the form of a buildup of dislocations through cold work. If later scratches occur on this cold-worked surface, then the additional work could result in loss through microfatigue. When the ratio of shear strength of the contact interface relative to the shear strength of the bulk rises to a high enough level (from 0.5 to 1.0), it has been found that a wedge can develop on the front of an abrasive tip. In this case, the total amount of material displaced from the groove is greater than the material displaced to the sides. This wedge formation is still a fairly mild form of abrasive wear. The most severe form of wear for ductile material is cutting. During the cutting process, the abrasive tip removes a chip, much like a machine tool does. This results in removed material, but very little displaced material relative to the size of the groove. For a sharp abrasive particle, a critical angle exists for which there is a transition from plowing to cutting. This angle depends on the material being abraded. Examples of critical angles range from 45° for copper to 85° for aluminum (Ref 3,4). Brittle materials have an additional mode of abrasive wear, namely, microcracking or microfracture. This occurs when forces applied by the abrasive grain exceed the fracture toughness of the material. This is often the predominant mode of severe wear for ceramic materials, and is active in materials such as white cast irons.

Effects of Material Properties on Abrasive Wear. Although hardness is the most important factor in the resistance to abrasion, other properties such as elastic modulus, yield strength, fracture toughness, microstructure, and composition also play an important role. For example, the abrasion resistance of ferrous metals is highly dependent on three metallurgical variables: microstructure, hardness, and carbon content. The inherently hard martensitic structure is preferable to the softer ferritic and austenitic structures. This is especially significant in low-stress abrasion, where little subsurface deformation occurs. When high-stress abrasion is encountered, alloys with high work-hardened hardness values have improved wear resistance when compared with alloys with low work-hardened hardness values. Figure 8 compares the hardness of ferrous alloy constituents with that of various minerals. Prevention of abrasive wear is possible through proper material selection and the use of surface engineering treatments. A number of material families have demonstrated good resistance to abrasive wear. They are typically hard materials that resist scratching, and include ceramics, carbide materials, alloyed white cast irons containing hard chromium carbides (see Fig. 8), and hardened alloy steels. Applicable surface treatments include: Weld hardfacing coatings, for example, high-carbon iron-chromium alloys, tool steels, nickel-chromium-boron alloys, cobalt-base alloys, and austenitic manganese steels Ceramic or cermet thermal spray materials deposited by plasma spraying, detonation gun (D-gun), or high-velocity oxyfuel processes Hard chromium plating Case hardening treatments Selective hardening treatments, for example, flame hardening Wear plates, for example, white cast iron or manganese steels Hard coatings produced by vapor deposition, for example, TiN Solid Particle Erosion General Description. Solid particle erosion (SPE) is the loss of surface material that results from repeated impacts of small, solid particles. In some cases SPE is a useful phenomenon, as in sandblasting and highspeed abrasive waterjet cutting, but it is a serious problem in many engineering systems, including steam and jet turbines, pipelines and valves carrying particulate matter, and fluidized bed combustion systems. Solid particle erosion is to be expected whenever hard particles are entrained in a gas or liquid medium impinging on a solid at any significant velocity (greater than 1 m/s, or 3.3 ft/s). Manifestations of SPE in service usually include thinning of components, a macroscopic scooping appearance following the gas/particle flow field, surface roughening (ranging

from polishing to severe roughening, depending on particle size and velocity), lack of the directional grooving characteristic of abrasion, and, in some but not all cases, the formation of ripple patterns on metals. Solid particle erosion can occur in a gaseous or liquid medium containing solid particles. In both cases, particles can be accelerated or decelerated, and their directions of motion can be changed by the fluid. This is more significant in liquid media, and slurry erosion is generally treated as Microconstituent

Mineral

Hardness Knoop Mohs Diamond

Vanadium carbide (VC) Titanium carbide (TiC)

Silicon carbide (SiC) Tungsten carbide (WC) Chromium carbide [(M^r)7C3] Cementite (Fe3C) Martensite

Corundum Topaz Quartz Feldspar

Austen ite

Apatite Pearlite

Silica sand Olivene Taconite, Glass Leucite, lmenite Limonite

Flint, garnet Iron pyrite Magnetite Hematite

Ferrite

Fluorite Calcite

Gypsum

Siderite Dolomite Bauxite Biotite mica

Kaolin Anthracite Bituminous coal Plastics

Talc

Fifi. 8 ^*

Hardness of some carbides, minerals, and alloy microconstituents. In general, the harder the material or constituent, the higher the abrasion resistance

a different, though related, subject. In gaseous media, at least for particles larger than about 50 jxm, deflection of the particles by the gas stream can often be ignored in erosion tests. However, it should be borne in mind that in an engineering application, such effects can be quite important—as, for example, the spatial distribution and severity of erosion damage within turbines and the effects of particle size and the change of direction and speed when particles rebound from surfaces. Erosion versus Abrasion. The distinction between erosion and abrasion should be clarified, because the term erosion has often been used in connection with situations that might be better classed as abrasion. Solid particle erosion refers to a series of particles striking and rebounding from the surface, while abrasion results from the sliding of abrasive particles across a surface under the action of an externally applied force. The clearest distinction is that, in erosion, the force exerted by the particles on the material is due to their deceleration, while in abrasion it is externally applied and approximately constant. This serves as a good working definition of the difference between the two phenomena. A clear-cut distinction between erosion and abrasion is difficult in some cases, particularly for very dense particle distributions in liquid or gas media, in which a "pack" of particles can develop and slide across the surface, which would be classed as abrasion. Variables Influencing Erosion. In practice, erosion rarely takes place in inert atmosphere or vacuum, but at room temperature the effect of oxidation can generally be ignored. Erosion can be thought of as "pure," that is, there are no synergistic interactions between erosion and corrosion (erosion-corrosion phenomenon are described in Chapter 2). Variables affecting pure erosion can be broadly separated into three types: impingement variables describing the particle flow, particle variables, and material variables. The primary impingement variables are particle velocity (v), angle of incidence (a), and flux (particle concentration), a is defined here and throughout virtually all erosion literature as the angle between the incident particle direction and the particle surface. Particle rotational speed is an additional impingement variable; it is difficult to measure and has not been studied in much detail. Particle variables include particle shape, size, hardness, and friability (ease of fracture). Material variables include all the material properties, such as hardness, work hardening behavior, and microstructure. In the erosion literature, materials are broadly classified as ductile or brittle, based on the dependence of their erosion rate on a. Ductile materials, such as pure metals, have a maximum erosion rate, E, at low angles of incidence (typically 15 to 30°), while for brittle materials, such as ceramics, the maximum is at or near 90°. These two classical extremes are illustrated in Fig. 9. A variety of curves intermediate between these classical extremes exist and in some cases the same material exhibits behavior that shifts from one extreme to the other, depending on erosion conditions.

The erosion rate, E, is commonly given in terms of mass or volume of material removed per unit mass of erodent impacted, volume being preferred because it permits thickness loss comparisons between materials of different density. Implicit is the assumption that the dimensions of the eroded area and the particle concentration are unimportant, which is a good approximation for dilute flows. Metals and ceramics differ in the dependence of E on a, as mentioned above, and also in their response to velocity and particle size and shape. E generally shows a power-law velocity dependence: (Eq 4)

AI2O3 erosion rate, (g/g) x 10"3

Al M00-O erosion rate, (g/g) x 10"4

where k is a constant and n is a velocity exponent that generally depends on material and erosion conditions. The value of n usually falls in the range of 2 to 2.5 for metals and 2.5 to 3 for ceramics, although observations exist outside these ranges. Material removal in ductile materials involves considerable plastic flow, while in brittle materials, fracture is of primary importance, at least for higher angles of incidence. Theories predict that E should be inversely proportional to hardness for metals, while for ceramics there should be a much weaker dependence on hardness, but fracture toughness should be important. Most theories further predict no effect of particle size for metals, although it is often observed that E increases strongly with particle size, at least up to about 100 fxm. For ceramics, a particle size exponent is predicted and observed. Hansen (Ref 5) compared the erosion rates of a large number of alloys, ceramics, and cermets. Although limited to a particular set of conditions, Hansen's study provides the practicing engineer with a good comparison of erosion rates for a very broad range of materials under the same condi-

Angle of impingement, a Fig. 9 Erosion of 1100-O aluminum relative to Al2O3 when both are eroded *** by 127 mm SiC particles impinging at a velocity of 152 m/s (499 ft/s)

tions. Estimation of the effects of variables such as velocity on the erosion rates of different classes of materials can be made from the remainder of this article. It should be noted that the order of material rankings would change with any change of variables such as velocity, particle type or size, and angle of incidence. The erosion tests were performed using 27 |xm Al2O3 particles at normal incidence and 170 m/s (560 ft/s) at 20 and 700 0C (68 and 1300 0F) in nitrogen. As in most other studies, the tests were conducted with a gas-jet erosion apparatus in which particles are fed from a hopper into a nozzle, where they mix with and are accelerated by a flowing gas stream (ASTM G 76 as described later in this Chapter). Hansen (Ref 5) normalized the erosion rates by defining the relative erosion factor (REF) as specimen volume loss divided by that of a standard material, Stellite 6B (Table 6). Figures 10 and 11 show this data for

Table 6 Erosion test results for selected metals and ceramics evaluated at room temperature and at elevated temperature Test parameters: 90° impingement; 27 mm Al2O3 particles; 5 g/min (0.18 oz/min) particle flow; 170 m/s (560 ft/s) particle velocity; 3 test duration; N2 atmosphere Relative erosion factor (REF)(O Material

20 0 C (70 0F)

700 0 C (1290 0F)

0.54 1.00 1.16 1.06 1.61 1.00 0.73 0.56 0.83 0.85 0.62 0.78 0.61 0.79 0.54 0.57 0.57 0.80 0.62

(GE)

1.26 1.25 1.19 1.08 1.04 1.00 1.00 0.99 0.97 0.96 0.93 0.93 0.92 0.91 0.91 0.90 0.83 0.80 0.77 0.76 0.52 0.48

ZrB2-SiC-graphite (N) (UCAR) SiC-Si3N4 bond (Carbor) Recrystallized SiC (N) SiC-ceramic bond (Carbor) Recrystallized SiC (N) (N) B4C (N) 2MgO-25TiB2-3.5WC-bal Al 2 O 3 (OGC)

6.36 2.44 0.91 0.80 0.49 0.40 0.40 0.38 0.37

>5.00 3.43 1.15 0.32 1.38 0.38 0.12 0.21 0.36

Manufacturing method(a)

Composition (manufacturer)(b)

Metals Ti-6A1-4V Haynes 93 25Cr iron Stellite 6K Stellite 3 Stellite 6B Type 304 stainless steel Type 316 stainless steel Haynes 188 Haynes 25 Type 430 stainless steel HK-40 Inconel 600 RA 330 Incoloy 800H Beta III Ti Incoloy 800 RA 333 Inconel 671 Mild steel Molybdenum Tungsten

W C C W C W W W W W W C W W W W W W W W W W

17Cr-16Mo-6.3Co-3C-bal Fe (Stellite) 25Cr-2Ni-2Mn-0.5Si-3.5C-bal Fe (OGC) 30Cr-4.5W-1.5Mo-1.7C-bal Co (Stellite) 31Cr-12.5W-2.4C-balCo 30Cr-4.5W-1.5Mo-1.2C-bal Co (Stellite) 17Cr-9Ni-2Mn-lSi-balFe 17Cr-12Ni-2Mn-lSi-2.5Mo-bal Fe 22Cr-14.5W-22Ni-0.15C-bal Co (Stellite) 22Cr-15W-10Ni-1.5Mn-0.15C-bal Co (Stellite) 17Cr-lMn-lSi-0.1C-balFe 26Cr-20Ni-0.4C-bal Fe 76Ni-15.5Cr-8Fe (HA) 19Cr-35Ni-1.5Mn-1.3Si-bal Fe (RA) 32.5Ni-21Cr-0.07C-46Fe (HA) 11.5Mo-6Zr-4.5Sn-balTi 32.5Ni-46Fe-21Cr (HA) 25Cr-1.5Mn-1.3Si-3Co-3Mo-3W-18Fe-bal Ni (RA) 50Ni-48Cr-0.4Ti (HA) 0.15C-balFe

0.17

Ceramics ZRBSC-M Chromite Refrax20C HD 435 Carbofrax D HD 430 Si3N4 Norbide BT-9

HP PS PS PS HP HP PS

(continued) (a) W, wrought; C, cast; HP, hot pressed; PS, pressed and sintered, (b) Manufacturer: BW, Babcock and Wilcox; Carbor, Carborundum Co.; GE, General Electric Co.; HA, Huntington Alloy Products; N, Norton Co.; OGC, Oregon Graduate Center; RA, Rolled Alloys Corp.; Stellite, Stellite Div., Cabot Corp.; UCAR, Union Carbide Corp. (c) REF = Volume loss material/volume loss Stellite 6B

Table 6

(continued) Relative erosion factor (REF)(c)

Material BT-12 BT-Il ZRBSC-D BT-24 BT-IO Noroc 33 TiC-Al2O3 SiC CBN GE diamond

Manufacturing method(a) PS PS HP PS PS HP PS HP

Composition (manufacturer)(b) 1.5MgO-49TiB2-3.5WC-bal Al 2 O 3 (OGC) 1.7MgO-38TiB2-3.5WC-bal Al 2 O 3 (OGC) ZrB2-SiC (N) 2MgO-30TiB2-3.5WC-bal Al 2 O 3 (OGC) 2MgO-30TiB2-3.5WC-bal Al 2 O 3 (OGC) Si3N4-SiC (N) (BW) (N) (GE) (GE)

20 0 C (700F)

700 0 C (1290 0F)

0.35 0.33 0.32 0.32 0.30 0.20 0.19 0.12 0 0

0.16 0.26 0.07 0.20 0.25 0.42 0.30 0.02 0 0

(a) W, wrought; C, cast; HP, hot pressed; PS, pressed and sintered, (b) Manufacturer: BW, Babcock and Wilcox; Carbor, Carborundum Co.; GE, General Electric Co.; HA, Huntington Alloy Products; N, Norton Co.; OGC, Oregon Graduate Center; RA, Rolled Alloys Corp.; Stellite, Stellite Div., Cabot Corp.; UCAR, Union Carbide Corp. (c) REF = Volume loss material/volume loss Stellite 6B

metals and ceramics, respectively. Tungsten carbide-cobalt (WC-Co) cermets gave REFs from about 0.1 to 1.6, and REF was found to increase with binder content. The REFs of most metals were similar at 20 and 700 0 C (68 and 1300 0 F), typically within about 20% of unity (Fig. 10). The three lowest room-temperature REFs for metals were for tungsten (0.48), molybdenum (0.52), and 1015 steel (0.76), and the highest was for Ti6A1-4V (1.26). The 700 0C (1300 0F) erosion rate of the standard (Stellite 6B) was 20% higher than the room-temperature value, so that 7000 C (1300 0F) REF values greater than 0.8 represent increases of erosion rate with temperature for a given material. These results illustrate the unfortunate fact that alloy-strengthening mechanisms such as solution or precipitation hardening that increase hardness do not significantly improve erosion resistance. According to Hansen (Ref 5), if service experience reveals an erosion problem for a metallic component, substitution of another metallic alloy will generally provide little improvement. Most ceramics tested had REF values in the range 0.3 to 0.6, although a few were much higher, and a few were nearly zero. It is important to note here (as discussed later) that for erodent particles of lower hardness than Al 2 O 3 (used in Hansen's study), significant improvements of erosion resistance can be obtained when the ratio of particle to target hardness, HJHV is less than 1. Prevention. Various design solutions have been developed in which high erosion rates are avoided by reconfiguring the system—such as the blocked tee configuration, in which a tee joint with one end closed is used in place of a gradual bend in a pipeline to prevent low-angle impingement. A good example of the variety of engineering solutions to SPE is provided by the case of power-generating steam turbines, in which exfoliation of iron oxide scale formed on steel heater tubes generates large pieces of scale that are fragmented into approximately 100 |xm particles, causing erosion of turbine blades, shrouds, valves, rivets, and other components. Liquid droplet erosion is also present. Solid particle erosion solutions include minimizing of scale formation by using austenitic steels or chromiz-

Ti-6AI-4V Haynes 93 25Cr iron HaynesStellite6K Haynes Stellite 3 Haynes Stellite 6B Type 304 stainless steel Type 316 stainless steel Haynes 188 Haynes 25 Type 430 stainless steel HK-40

Fiff. 1 0 R e ' a t ' v e erosion factors for *** selected commercially available metals at an impingement angle of 90°. Stellite 6B cobalt-base alloy was used as the reference material. Source: Ref5

lnconel 600 RA-330 lncoloy 800H BetaHTi lncoloy 800 RA 333 lnconel 671 Mild steel Molybdenum Tungsten

ZRBSC-M

Specimen perforated

Chromite Refrax20C HD 435 Carbofrax D HD 430 Si3N4 Norbide BT-9

p j a -| I Relative erosion factors for ^* selected ceramics at an impingement angle of 90°. Ratings based on using Stellite 6B cobalt-base alloy as the reference material. Source: Ref 5

BT-12 BT-11 ZRBSC-D BT-24 BT-10 Noroc 33 TiC-AI2O3 SiC Cubic boron nitride Diamond Relative erosion factor (REF)

ing treatments, particle removal with cyclones or screens, application of plasma-sprayed or diffusion coatings to blades, and redesign of turbine configurations. Liquid Erosion General Description. Erosion of a solid surface can take place in a liquid medium even without the presence of solid abrasive particles in that medium. Cavitation, one mechanism of liquid erosion, involves the formation and subsequent collapse of bubbles within the liquid. The process by which material is removed from a surface is called cavitation erosion, and the resulting damage is termed cavitation damage. The collision at high speed of liquid droplets with a solid surface results in a form of liquid erosion called: liquid impingement erosion. Cavitation damage has been observed on ship propellers and hydrofoils; on dams, spillways, gates, tunnels, and other hydraulic structures; and in hydraulic pumps and turbines. High-speed flow of liquid in these devices causes local hydrodynamic pressures to vary widely and rapidly. In mechanical devices, severe restrictions in fluid passages have produced cavitation damage downstream of orifices and in valves, seals, bearings, heatexchanger tubes, and Venturis. Cavitation erosion has also damaged water-cooled diesel-engine cylinder liners. Liquid impingement erosion has been observed on many components exposed to high-velocity steam containing moisture droplets, such as blades in the low-pressure end of large steam turbines. Rain erosion, one form of liquid-impingement erosion, frequently damages the aerodynamic surfaces of aircraft and missiles when they fly through rainstorms at high subsonic or supersonic speeds. Liquid impingement and cavitation erosion are of concern in nuclear power systems, which operate at lower steam quality than conventional steam systems, and in systems using liquid metals as the working fluid, where the corrosiveness of the liquid metal can promote rapid erosion of components. Basic Mechanisms. Liquid erosion involves the progressive removal of material from a surface by repeated impulse loading at microscopically small areas. Liquid dynamics is of major importance in producing damage, although corrosion also plays a role in the damage process, at least with certain fluid-material combinations. The process of liquid erosion is not as well understood as most other wear processes. It is difficult to define the hydrodynamic conditions that produce erosion and the metallurgical processes by which particles are detached from the surface. Evidently, both cavitation and liquid impingement exert similar hydrodynamic forces on a solid surface. In any event, the appearance of damaged surfaces (Fig. 12) and the relative resistance of materials to damage are similar for both liquid impingement and cavitation erosion. Additional information on the mechanism of material removal during cavitation can be found in Chapter 2, "Principles of Corrosion."

pja 1 2 A cast steel feedwater-pump impeller severely damaged by cavitation. Note how ^" damage is confined to the outer edges of the impeller where vane speed was maximum.

Prevention. Damage from liquid erosion can be prevented or minimized by reducing the intensity of cavitation or liquid impingement through design, using erosion-resistant materials, for example, cobaltbase alloys and tool steels including weld overlays of these materials, or, under certain conditions, using elastomeric coatings. Slurry Erosion General Description. Slurry erosion is progressive loss of material from a solid surface by the action of a mixture of solid particles in a liquid (slurry) in motion with respect to the solid surface. If the solid surface is capable of corroding in the fluid portion of the slurry, the slurry erosion will contain a corrosion component. Figure 13 shows an example of slurry erosion. A slurry by definition is a physical mixture of solid particles and a liquid (usually water) of such a consistency that it can be pumped. The particles must be in suspension in the liquid, and most pumpable slurries contain at least 10% solids. Apparent Abrasivity. Typical pumpable slurries possess inherent "apparent abrasivity," which must be determined by testing to enable cost predictions for pump replacement parts or other equipment used for slurries. Apparent abrasivity, without inhibition, is the complex synergistic reaction of many factors (Fig. 14). This reaction, known as the MorrisonMiller effect (Ref 6), is such that the wear response of a given material in

Fig. 1 3 Schematic of slurry erosion.

Resistance of protective film of corrosion products to abrasivity of slurry

Corrosive liquid

Dissolved air (oxygen or environment)

Specimen

Liquid

Lap

Galvanic corrosion (if two metals involved)

Released corrosive connate water from ore particles

True abrasivity of solids (particle hardness, size, shape, and concentration)

Soluble elements in solids forming corrosive solution

Fig. 1 4 Synergistic effects of seven factors in slurry abrasivity

a certain slurry does not indicate how that material would respond to another slurry. Similarly, the effect of a certain slurry on one material does not indicate how it would affect another material. Other modes of wear are also encountered when handling slurries. As shown in Fig. 15, these include abrasion-corrosion (the most severe wear mode), scouring wear, abrasive metal-to-metal wear (crushing and grinding), high-velocity erosion, low-velocity erosion, saltation wear (rapid wear caused when particles are moved forward in a series of short intermittent bounces from a bottom surface), and cavitation.

Polymer with embedded abrasive particles

(b)

(a)

Velocity profile

(O

Pipe wall (e)

(d) Pipe wall

Collapsing vapor bubbles

Large tumbling rocks U)

(g)

Fig. 1 5 blurry erosion wear modes, (a) Abrasion-corrosion, (b) Scouring wear, with wear areas equal (left) and unequal (center and right), (c) Crushing and grinding, (d) High-velocity erosion. (e) Low-velocity erosion, (f) Saltation erosion, (g) Cavitation

Miller numbers are used to determine the abrasivity of slurries, based on the rate of metal loss from a standard 27% chrome-iron wear block that reciprocates through any slurry, on a rubber lap, with an imposed load of 22.2 N (5 lbf) placed on the wearing block. The higher the number, the greater the aggressive effect of the slurry on part life. The additional effect of corrosion (usually present in liquid slurries, even those mixed with distilled water) was slow to be recognized. This was because such chrome-iron is rather corrosion resistant and the original test actually fulfilled its objective to reveal the "true abrasivity" of the dry particles. The effects of both abrasion and corrosion must be recognized in the operation of any slurry-handling system. Table 7 lists typical Miller numbers for selected slurry materials. The wide variation in Miller numbers for some materials is due to the inclusion of varying amounts of "tramp" materials that usually occur with the basic mineral. Information about the factors that contribute to Miller number abrasivity can be found in the ASTM G 75 standard. Prevention of slurry erosion is accomplished through design changes, for example, lessening the severity of pipe bends or using replaceable

Table 7

Typical Miller numbers for selected slurry materials

Material

Alundum (400 mesh) Alundum (200 mesh) Aragonite Ash Ash, fly Bauxite Calcium carbonate Carbon Carborundum (220 mesh) Clay Coal Copper concentrate Detergent Dust, blast furnace Gilsonite Gypsum Iron ore (or concentrate) Kaolin Lignite Limestone Limonite Magnesium hydrate Magnetite Microsphorite Mud, drilling Nickel Phosphate Potash Pyrite Quartzite Rutile Salt brine Sand and sand fill Sea bottom Shale Serpentine Sewage, digested Sewage, raw Sodium sulfate Soda ash tailings Sulfur Tailings (all types) Tar sand Waste, nickel Waste, coal

Miller number(s)

241 1058 7 127 83, 14 9, 33, 50, 76, 134 14 14, 16 1284 34,36 6,10,21,28,47,57 19, 37, 58, 68, 111, 128 6,8 57 10 41 28, 37, 64, 79, 122, 157, 234 7,30 14 22, 30, 39, 43, 46 113 4 64, 71, 134 76 10 31 68, 74, 84, 134 1,2 194 99 10 11 51, 68, 85, 116, 138, 149, 246 11 53,59 134 15 25 4 27 1 24, 61, 91, 159, 217, 480, 644 70 53 22, 28

wear backs on 90° elbows in high-velocity slurry pipelines and protective coatings. These include hardfacing alloys (e.g., cobalt alloys), plasmasprayed ceramics and cermets, hard platings, ceramic and carbide wear tiles, ceramic-filled repair cements, chromized steels, cast cylinder liners, plastic-lined pipe, and basalt-lined pipe (Ref 2). Adhesive Wear General Description. Adhesive wear is defined as wear by transference of material from one surface to another during relative motion under load due to a process of solid-state welding (Fig. 16); particles that are removed from one surface are either permanently or temporarily attached to the other surface. Adhesive wear may be between metallic materials, ceramics, or polymers, or combinations of these. It is dependent on adhesion between the

material, and that, in turn, depends on surface films like oxides or lubricants, as well as the mutual affinity of one material for another. If loads are light and the natural spontaneous oxidation of a metal can keep up with the rate of its removal by wear, then that wear rate will be relatively low (the oxide acting as a lubricant). This is called mild wear. If loads are high and the protective oxide is continually disrupted to allow intimate metal to metal contact and adhesion, then the wear rate will be high. This is called severe wear. Theory. Solid surfaces are almost never perfectly smooth but rather consist of microscopic or macroscopic asperities of various shapes. When two such surfaces are brought into contact under a load normal to the general planes of the surfaces, the asperities come into contact and elastically or plastically deform until the real area of contact is sufficient to carry the load. A bond may then occur between the two surfaces that is stronger than the intrinsic strength of the weaker of the two materials in contact. When relative motion between the two surfaces occurs, the weaker of the two materials fails, and material is transferred to the contacting surface. In subsequent interactions, this transferred material may be retransferred to the original surface (probably at a different location) or may become totally separated as a wear debris particle of an irregular morphology (Ref 7). Formulas that have been proposed (Ref 8, 9), to describe this phenomenon are of the form: (Eq 5) where Vis the wear scar volume, S is the distance of sliding, L is the load, H is the indentation yield strength (hardness) of the softer surface, and k is a probability factor that a given area contact will fracture within the weaker material rather than at the original interface. Formulations similar to Eq 5 have been shown to describe adhesive wear over fairly wide ranges of sliding distances, under a variety of conditions, over limited ranges of load, and over limited ranges of hardness when the same classes of material were compared (Ref 10, 11). While initial theoretical considerations assumed bare metal-to-metal contact, later Load Bodyl

Motion

Flg. 1 6 Schematic of adhesive wear

work assumed that oxide films, adsorbed films, and/or lubricant effects could be accounted for by changing k or by using more complex formulations (Ref 12). It has also been proposed that true metal-to-metal adhesive wear occurs at some time after motion is initiated when surface films or contaminants are worn away. Presumably, therefore, more than one adhesive wear mechanism could be operating at any given time, depending upon the presence or absence of various surface films in local areas. Changes in the apparent value of k or klH as a function of load may be the result of penetration of such films at sufficiently high load or the generation of new films as a result of frictional heating. The wear coefficient, k, has been determined experimentally for a large number of materials couples under various test conditions and geometries. The values found range from about 10~3 to 10~8 (Ref 7). For example, representative values of k for the end of a cylinder sliding against the flat surface of a ring at 1.8 m/s (6 ft/s) under a 400 g load are given for various combinations of cylinder and ring materials in Table 8. In many laboratory experiments, a stationary specimen with a small surface area rubs against a moving specimen with a large area. This frequently leads to a much higher wear rate on the smaller specimen than on the larger because of the constant contact and associated heating of the smaller specimen. This relative area effect may influence the wear mechanisms operating and may not be representative of field use. For most practical applications, volume loss, as predicted by Eq 5, must be converted to a linear value representing penetration or decrease in length, for example, increase in diameter of a journal bearing bushing, reduction in shaft diameter, or reduction in the length of brush in an electric motor. Primary Material Parameters. Materials selection for adhesion resistance requires careful consideration of the operating environment of the workpiece in addition to the total functional performance required of the workpiece itself. Wear properties of the steels vary widely with processing and heat treatment. Polymers are selected for sliding contact applications because of inherent properties such as inertness to many chemicals, relatively low galling tendency, and self-lubricating properties. Ceramics Table 8 Wear coefficients for various combinations of materials under conditions of dry sliding Sliding combination Cylinder material

Ring material

Low-carbon steel 60-40 brass PTFE Bakelite Beryllium copper Tool steel Stellite Tungsten carbide Tungsten carbide

Low-carbon steel Hardened steel Hardened steel Hardened steel Hardened steel Hardened steel Hardened steel Low-carbon steel Tungsten carbide

Wear coefficient, k 7.0 6.0 2.5 7.5 3.7 1.3 5.5 4.0 1.0

X X X X X X X X X

10~ 3 10~ 4 10~ 5 10~ 6 10~ 5 10~ 4 10~ 5 10~ 6 10" 6

Hardness of softer member, 106 g/cm2

18.6 9.5 0.5 2.5 21.0 85.0 69.0 18.6 130.0

Wear coefficients given are for the end of a cylinder sliding against the flat surface of a ring at 1.8 m/s (6 ft/s) under a 400 g load.

are used where extreme resistance to high-temperature oxidation or resistance to highly corrosive materials or gases is required. Prevention. The following guidelines are recommendations to prevent adhesive wear in metals, polymers, and ceramics: Avoid sliding similar materials together, particularly metals. If fatigue due to repeated high-contact pressure is not likely to be a problem, then high hardness is a desired property. However, avoid sliding hard metals against hard metals in lubricated systems to avoid scuffing and to accommodate debris. Consider the effect of relative hardness of phases in materials. For example, a high-chromium cast iron may have a hardness of 400 HB, which is moderate. However, that cast iron may contain Cr7C3, which has a hardness of about four times that of 400 HB and will damage the countersurface considerably. The same applies to polymers, which seem rather soft relative to metals. However, wear-resisting polymers often contain glass or some other hard filler that wears metal counterfaces rather severely. Hard phases in one body may fragment and become embedded in the counterface, which causes abrasion if the fragments extend above the surface. Even if done inadequately, lubrication will reduce wear. Some lubrication can be applied by providing an atmosphere that is corrosive in order to form surface films, many of which produce lower friction than if that film were not present.

Galling General Description. Galling can be considered a severe form of adhesive wear. With high loads and poor lubrication, surface damage can occur on sliding metal components. The damage is characterized by localized macroscopic material transfer, that is, large fragments or surface protrusions that are easily visible on either or both surfaces. This gross damage is usually referred to as galling, and it can occur after just a few cycles of movement between the mating surfaces. Severe galling can result in seizure of the metal surfaces. The terms scuffing and scoring are also used to describe similar surface damage under lubricated conditions. Scuffing is the preferred term when the damage occurs at lubricated surfaces, such as the piston ring-cylinder wall contact. Scoring typically describes damage that takes the form of relatively long grooves. Primary Material Parameters. Materials that have limited ductility are less prone to galling, because under high loads surface asperities will tend to fracture when interlocked. Small fragments of material may be lost, but the resultant damage will be more similar to scoring than to galling. For highly ductile materials, asperities tend to plastically

deform, thereby increasing the contact area of mated surfaces; eventually, galling occurs. Another key material behavior during plastic deformation is the ease with which dislocations cross slip over more than one plane. In face centered cubic (fee) dislocations easily cross slip. The rate of cross slip for a given alloy or element is usually indicated by its stacking-fault energy. Dislocation cross slip is hindered by the presence of stacking faults, and a high stacking-fault energy indicates a low number of impeding stacking faults and an increased tendency to cross slip and, hence, gall. Table 9 lists the stacking-fault energies of four fee elements. Nickel and aluminum have poor galling resistance, whereas gold and copper have good galling resistance. Materials that have a hexagonal close-packed (hep) structure with a high cla ratio have a low dislocation cross slip rate and are less prone to galling. This explains why cobalt-base alloys and cadmium-plated alloys resist galling while titanium alloys tend to gall. Prevention of galling is accomplished through proper design, for example, parts should have sufficient clearance, because tightly fitted parts are more prone to galling. Adequate lubrication and various hard surface coatings also can help prevent galling. Control of surface roughness is another important factor. Highly polished surfaces (<0.25 (xm, or 10 ixin.) or very rough finishes (>1.5 |ixm, or 60 |min.) increase the tendency for wear and galling. It is theorized that very smooth surfaces lack the ability to store wear debris because of the absence of valleys between asperities, which means the asperities will have greater interaction. Also, lubricants will tend to wipe off the smoother surface. Too rough a finish results in interlocking asperities, which promote severe tearing and galling.

Fretting General Description. Fretting is a wear phenomenon that occurs between two mating surfaces; initially, it is adhesive in nature, and vibration or small-amplitude oscillation is an essential causative factor. Fretting is frequently accompanied by corrosion. In general, fretting occurs between two tight-fitting surfaces that are subjected to a cyclic, relative motion of extremely small amplitude. Fretting generally occurs at contacting surfaces that are intended to be fixed in relation to each other but that actually undergo minute alternating Table 9 Metals Gold Copper Nickel Aluminum Source: Ref 13

Stacking-fault energies of some common metals Stacking-fault energy, eVgs/cm2

30 40 80 200

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relative motion that is usually produced by vibration. The relative displacements between bodies are quite small (<50 to 150 jjim). Fretting further differs from ordinary wear in that the bulk of the debris produced is retained at the site of fretting. In ferrous materials, the fretting process creates a mass of reddish oxide particles. Fretting also occurs in nonoxidizing materials, such as gold, platinum, and cupric oxide. Common sites for fretting are in joints that are bolted, keyed, pinned, press fitted, or riveted; in oscillating bearings, splines, couplings, clutches, spindles, and seals; in press fits on shafts; and in universal joints, baseplates, shackles, and orthopedic implants. One additional problem with fretting is that it may initiate fatigue cracks, which, in highly stressed components, often result in fatigue fracture. Prevention. Fretting, because of the significant role of oxidation, is best combatted by oxidation-resistant coatings, for example, electroless nickel or softer self-lubricating coatings like silver or indium. Solid-film lubricants are also successfully employed. Additional information on prevention of fretting corrosion can be found in Chapter 2, "Principles of Corrosion."

Rolling-Contact Wear General Description. The rolling of one body over another, as in a rolling-element bearing, can result in repeated stressing of the subsurface material, the nucleation of microcracks, and the eventual production of pits and spalls. Because rolling-contact wear is generally produced by repetitive mechanical stressing, it is often associated with, or even referred to as, rolling-contact fatigue. Analysis of bearings and gears indicates that some degree of slip occurs in many rolling-contact situations, such as in the cam and roller assembly in an automobile valve train and in the engagement of gear teeth. Thus, it is common to observe sliding wear (e.g., scuffing or polished-looking areas) on components that are ordinarily considered to be in "rolling contact." Prevention of rolling-contact fatigue is accomplished through proper design and load ratings, lubrication, and the use of through-hardened or case-hardened premium quality (clean) alloy steels.

Lubrication One important means of reducing wear (as well as friction) is lubrication. Lubrication not only reduces the power consumption needed to overcome friction but also protects rolling and sliding contact surfaces from excessive wear. Even with lubrication, however, wear still occurs.

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relative motion that is usually produced by vibration. The relative displacements between bodies are quite small (<50 to 150 jjim). Fretting further differs from ordinary wear in that the bulk of the debris produced is retained at the site of fretting. In ferrous materials, the fretting process creates a mass of reddish oxide particles. Fretting also occurs in nonoxidizing materials, such as gold, platinum, and cupric oxide. Common sites for fretting are in joints that are bolted, keyed, pinned, press fitted, or riveted; in oscillating bearings, splines, couplings, clutches, spindles, and seals; in press fits on shafts; and in universal joints, baseplates, shackles, and orthopedic implants. One additional problem with fretting is that it may initiate fatigue cracks, which, in highly stressed components, often result in fatigue fracture. Prevention. Fretting, because of the significant role of oxidation, is best combatted by oxidation-resistant coatings, for example, electroless nickel or softer self-lubricating coatings like silver or indium. Solid-film lubricants are also successfully employed. Additional information on prevention of fretting corrosion can be found in Chapter 2, "Principles of Corrosion."

Rolling-Contact Wear General Description. The rolling of one body over another, as in a rolling-element bearing, can result in repeated stressing of the subsurface material, the nucleation of microcracks, and the eventual production of pits and spalls. Because rolling-contact wear is generally produced by repetitive mechanical stressing, it is often associated with, or even referred to as, rolling-contact fatigue. Analysis of bearings and gears indicates that some degree of slip occurs in many rolling-contact situations, such as in the cam and roller assembly in an automobile valve train and in the engagement of gear teeth. Thus, it is common to observe sliding wear (e.g., scuffing or polished-looking areas) on components that are ordinarily considered to be in "rolling contact." Prevention of rolling-contact fatigue is accomplished through proper design and load ratings, lubrication, and the use of through-hardened or case-hardened premium quality (clean) alloy steels.

Lubrication One important means of reducing wear (as well as friction) is lubrication. Lubrication not only reduces the power consumption needed to overcome friction but also protects rolling and sliding contact surfaces from excessive wear. Even with lubrication, however, wear still occurs.

On lubricated surfaces, the wear process is usually mild and generates fine debris of a particle size as small as 1 or 2 jxm. Abrasive wear or delamination wear predominates under lubricated conditions. Electron microscope examination of worn surfaces from lubricated assemblies frequently reveals a multitude of fine scratches oriented in the direction of relative motion. The fine debris generated by abrasion becomes suspended in the oil or grease. In devices using circulating-oil lubrication, advantage has been taken of the fact that wear debris can be analyzed by spectroscopy and that deterioration of the device by wear can be diagnosed from these results. This technique is used to monitor the condition of vital components in aircraft and locomotive engines. Modes of Lubrication There are several basic modes of lubrication. In all modes, contact surfaces are separated by a lubricating medium, which may be a solid, a semisolid, or a pressurized liquid or gaseous film. Hydrodynamic lubrication is a system in which the shape and relative motion of the sliding surfaces cause the formation of a fluid film having sufficient pressure to separate the surfaces. Hydrostatic lubrication is a system in which the lubricant is supplied under sufficient external pressure to separate the opposing surfaces by a fluid film. Elastohydrodynamic lubrication is a system in which the friction and film thickness between the two bodies in relative motion are determined by the elastic properties of the bodies in combination with the viscous properties of the lubricant at the prevailing pressure, temperature, and rate of shear. Dry-film (solid-film) lubrication is a system in which a coating of solid lubricant separates the opposing surfaces and the lubricant itself wears away. Boundary lubrication and thin-film lubrication are two modes in which friction and wear are affected by properties of the contacting surfaces as well as by the properties of the lubricant. In boundary lubrication, each surface is covered by a chemically bonded fluid or semisolid film, which may or may not separate opposing surfaces, and viscosity of the lubricant is not a factor affecting friction and wear. In thin-film lubrication, the lubricant usually is not bonded to the surfaces and it does not separate opposing surfaces. Lubricant viscosity affects friction and wear. Lubricants Almost any surface film can act as a lubricant, preventing cold welding of asperities on opposing surfaces or allowing opposing surfaces to slide across one another at a lower frictional force than would prevail if the film were not present. Lubricants may be either liquid or solid (in some cases, gas films may act as lubricants). One of the functions of a lubricant is to carry away heat generated by two surfaces sliding under contact pressure.

Liquid lubricants can dissipate heat better than solid or semifluid lubricants, but in all types, the shear properties of the lubricant are critical to its performance. Properties. Liquid lubricants maintain separation or opposing surfaces by pressure within the film, which opposes the contact force. This pressure may be generated within the film, usually as a result of the shape of the opposing surfaces, or the liquid may be forced between the opposing surfaces by pressure from an external source. Regardless of the means of creating pressure within the film, the opposing surfaces slide on a film of liquid. Friction and wear are directly influenced by the thickness and shear properties (viscosity) of the liquid. Where appropriate, the use of a high-viscosity lubricant usually results in a relatively thick film and a low wear rate. However, high sliding speeds cannot be accommodated by a viscous film, because excessive heat generated within the film causes it to become less viscous and to decompose chemically. Full-film (thick-film) lubrication, such as occurs under hydrostatic or hydrodynamic conditions, effectively separates asperities on opposing surfaces, whereas thin-film and boundary lubrication allow asperity contact. The differences among these three conditions of liquid lubrication are illustrated schematically in Fig. 17. Some special types of boundary lubricants, most notably the extreme pressure (EP) lubricants, react with a metallic surface, often at high temperatures, to produce a monomolecular film on the surface. This very thin film contaminates the mating surfaces and prevents metal-to-metal contact or adhesion. Extreme-pressure lubricants often contain extremely reactive constituents that re-form the film instantly if it is scraped off one of the surfaces. Film formation of this type is, in effect, corrosion; when it is uncontrolled or when the film is repeatedly scraped off and re-formed, deterioration of the surface can result. Solid-film lubricants must be adherent to be effective, or they allow metal-to-metal contact or introduce unwanted particles that roll and slide

Fluid film

Full-film lubrication

Fluid film

Thin-film lubrication

Boundary films

Boundary lubrication

PJa- \ 7 Schematic showing the relation of surface roughness to film thickness. Shown are conditions of full-film, thin-film, and boundary lubrication.

within the joint. When they can be kept within the joint, graphite and molybdenum disulfide make good lubricants because they shear easily in certain crystallographic directions. Hard, adherent oxide films, such as Fe3O4 on steel or anodized Al 2 O 3 on aluminum, withstand wear because they resist penetration and do not bond with most mating surfaces. Lubricating oils are relatively free-flowing organic substances that are used to lower the coefficient of friction in mechanical devices. They are available in a broad range of viscosities, and many are blended or contain additives to make them suitable for specific uses. In general, lubricating substances that are fluid at 20 0C (70 0F) are termed oils; lubricating substances that are solid or semifluid at 20 0C (70 0F) are termed greases or fats. Oils are derived from petroleum (mineral oils) or from plants or animals (fixed oils). Mineral oils are classified according to source (type of crude), refining process (distillate or residual), and commercial use. The commercial mineral oil base products consist mainly of saturated hydrocarbons (even though naphthene-base crudes are predominantly unsaturated) in the form of chain or ring molecules that are chemically inactive and do not have polar heads. These commercial products may or may not contain waxes, volatile compounds, fixed oils, and special-purpose additives. Fixed oils and fats differ from mineral oils in that they consist of an alcohol radical and a fatty-acid radical, can be reacted with an alkali (sodium hydroxide or potassium hydroxide, for example) to form glycerin or soap, cannot be distilled without decomposing, and contain 9 to 12.5% oxygen. All fixed oils are insoluble in water and, except for castor oil, are insoluble in alcohol at room temperature. Fixed oils are generally considered to have greater oiliness than mineral oils. Oiliness is a term that describes the relative ability of any lubricant to act as a boundary lubricant. Lubricating grease, as defined by ASTM, is a solid to semifluid product consisting of a dispersion of a thickening agent in a liquid lubricant. In more practical terms, most greases are stabilized mixtures of mineral oil and metallic soap. The soap is usually a calcium, sodium, or lithium compound and is present in the form of fibers whose size and configuration are characteristic of the metallip radical in the soap compound. Solid lubricants, which are solids with lubricating properties, can be maintained between two moving surfaces to reduce friction and wear. Numerous solid inorganic and organic compounds, as well as certain metals and composite materials, may be classified as solid lubricants. Molybdenum disulfide (MoS2), graphite, polytetrafluoroethylene (PTFE), and graphite fluoride (CFx) are the solid lubricants most commonly used. Several hundred different compounds and mixtures have been described as potential solid lubricants. Increasingly, solid lubricants are being vapor deposited for use in harsh environments in which liquids would evaporate or congeal.

Wear Testing (Ref 1) Because different types of wear occur in machinery, many different types of wear tests have been developed to evaluate effects of wear on materials and surface treatments. Consequently, the selection of the right type of wear test for each investigation is important in order to achieve useful and meaningful engineering data. More than one type of wear can attack the same part, such as sliding wear and impact wear in printing presses, or erosive wear and abrasive wear on extrusion machine screws for plastics. Sometimes wear can operate in the presence of corrosive or chemically active environments, and synergistic chemomechanical effects are possible. Selection of appropriate wear test methods begins with an assessment of the type of wear involved in each problem area. Wear testing is performed for one or more of the following reasons: to screen materials, surface treatments, or lubricants for a certain application; to help develop new, wear-resistant materials, surface treatments, or lubricants; to establish the relationship between the manufacturing, processing, or finishing methods applied to a certain machine part and its wear performance; or to better understand and model the fundamental nature of a certain type of wear. Surprising to some, the wear resistance of a given material is not a basic material property, like elastic modulus or yield strength. Rather, the wear behavior of a material depends on the conditions of its use. Therefore, the first step in wear testing is to recognize how the results of the work will be used. Only then can the appropriate test method(s), testing parameters, and a useful format for reporting the results be selected. Test Methods Standardized Wear Testing Methods. A list of ASTM standardized wear test methods, organized by type of wear or surface damage, is given in Table 10. This is by no means a complete list of all available test methods; many nonstandardized tests developed by individual companies or research organizations are also frequently used. Oftentimes if a suitable standard test method does not exist for a specific type of wear problem, an organization may decide to develop its own internal wear-testing standards best suited to its purposes. Variables to be Controlled in Wear Testing. Each wear mode is influenced by a different set of physical variables. Therefore, it is important to recognize what factors must be controlled, or at least monitored, in the design of wear testing procedures. Table 11 lists the major experimental variables that are controlled in conducting wear tests of various types. Environmental and other factors, which should he considered in interpreting the results of the wear tests, are also listed. Sometimes these secondary factors must be controlled in order to simulate a given application, but

Table 10

ASTM wear test methods grouped by wear type illustrated in Fig. 2

Form of wear

Abrasive wear, 2-body

Designation

G 56 G 132 G 119

Abrasive wear, 3-body

G 65 G 81 G 105

Erosive wear, cavitating fluid Erosive wear, liquid droplets Erosive wear, slurry

G 32

Erosive wear, solid particles Fretting wear

G 76 D 4170

Sliding wear

D 2266

G 73 G 75

D 2670 D 2882

D 2981 D 3702 D 3704

D 4172 D 5001

G 77 G 99 G 119 G 133 G 137

Surface damage, galling

G 98

Surface damage, scoring

D 2782

Title

Test Method for Abrasiveness of Ink-Impregnated Fabric Printer Ribbon Test Method for Pin Abrasion Testing Guide for Determining Synergism between Wear and Corrosion Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus Practice for Jaw Crusher Gouging Abrasion Test Test Method for Conducting Wet Sand/Rubber Wheel Abrasion Tests Test Method for Cavitation Erosion Using Vibratory Apparatus Practice for Liquid Impingement Erosion Testing Test Method for Determination of Slurry Abrasivity (Miller Number) and Slurry Abrasion Resistance Response of Materials (SAR Number) Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets Test Method for Fretting Wear Protection of Lubricating Greases Test Method for Wear Preventative Characteristics of Lubricating Grease (Four-Ball Method) Test Method for Measuring Wear Properties of Fluid Lubricants (Falex Pin and Vee Block Method) Test Method for Indicating Wear Characteristics of Petroleum and Non-Petroleum Hydraulic Fluids in a Constant Volume Vane Pump Test Method for Wear Life of Solid Lubricants in Oscillating Motion Test Method for Wear Rate of Materials in Self-Lubricated Rubbing Contact Using a Thrust Washer Testing Machine Test Method for Wear Preventative Properties of Lubricating Greases Using the (Falex) Block on Ring Test Machine in Oscillating Motion Test Method for Wear Preventative Characteristics of Lubricating Fluid (Four-Ball Method) Test Method for Measurement of Lubricity of Aviation Turbine Fuels by the Ball-on-Cylinder Lubricity Evaluator (BOCLE) Test Method for Ranking Resistance of Materials to Sliding Wear Using Block on Ring Wear Test Test Method for Wear Testing with a Pin-on-Disk Apparatus Guide for Determining Synergism between Wear and Corrosion Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear Test Method for Ranking Resistance of Plastic Materials to Sliding Wear Using a Block-on-Ring Configuration Test Method for Galling Resistance of Materials Test Method for Extreme-Pressure Properties of Lubricating Fluids

Means of wear measurement

Surface profiling or other method Mass loss Mass loss and corrosion-related measurements Mass loss Mass loss ratio Mass loss, normalized by wheel dimensions Mass loss Mass loss Mass loss

Mass loss Mass loss ratio Wear scar diameter "Teeth wear" apparatus-specific measurement of wear Mass loss

Number of revolutions to failure, as indicated by friction Thickness change

Wear scar width

Wear scar diameter Wear scar diameter

Wear scar width Ball: wear scar diameter, disk: profile Mass loss and corrosion-related measurements Ball: wear scar diameter, flat: profile Mass loss

Visual inspection, critical load for galling "OK" value of for load just below critical scoring condition

usually it is sufficient just to measure and document them as an aid to interpreting the data. Wear Testing Devices: Commercially Manufactured and Custom Made. Table 12 exemplifies the types of simple testing geometries common to evaluating the various forms of wear. While simple geometries, such as those in Table 12, represent one approach to testing, some wear

Table 11 types

Parameters that are commonly controlled and reported when conducting wear tests of various

Category

Subcategory

Sliding

Abrasive wear, 2-body

Abrasive wear, 3-body

"Adhesive"

Fretting wear

Polishing wear

Impact

2-body

Impingement, liquid and solid

Cavitation erosion

Rolling

Rolling contact fatigue Rolling with slip

Supplementary characterizations or variables(a)

Typical variables

Load (contact pressure), abrasive type, binder type, backing body, whether repeated contact or sliding against fresh abrasive lubricant or coolant, surface speed, temperature, duration of contact Load (contact pressure), abrasive type, concentration, hardness of counterbody(b), coolant or lubricant, whether repeated contact or continual motion against fresh abrasive surface, surface speed, temperature, duration of contact Load (contact pressure or stress), relative velocity, contact geometry, type of motion (unidirectional or oscillating), duration, sliding distance, or time of sliding, temperature Load (contact stress), contact geometry, amplitude of oscillation, frequency of oscillation, number of cycles or time, choice of lubricant Size of polishing medium, concentration of medium, liquid used for suspension, normal pressure, type of motion bodies (platten and specimen), time of exposure, temperature, substrate (pad type) Force of impact, speed of impact, geometry of contact, angle of impact, repetition rate, duration/number of impacts, temperature Average impact velocity, particle stream shape (by nozzle design), impingement angle of the stream to the surface, duration of exposure, temperature of the specimen and/or jet Test geometry, frequency of moving body oscillation, temperature of the fluid, fluid type, duration of exposure Load (elastic contact stress), rpm of roller(s), test duration, temperature Load (elastic contact stress), rpm of roller(s), % slip, test duration, temperature

Method of surface preparation, material characterization

Method of surface preparation, material characterization

Method of surface preparation, cleaning, surface finish of bodies, type of material/lubricant, method of supplying the lubricant, relative humidity Surface finish, relative humidity, debris characteristics Particle composition and geometric description, method of medium introduction, initial surface finish of specimen Material characterization, environment and relative humidity, surface finish of bodies Particle velocity or flux distribution, density of particles, particle shape description, particle size distribution, particle composition Material characterization

Lubricant/material characterization, surface finish of rollers Lubricant/material type, surface finish of rollers

(a) These quantities are often used to characterize the testing conditions or materials even though they may not be directly controlled in an experiment. In certain cases, they could be treated as variables themselves, (b) In certain types of 3-body abrasive wear tests, notably the dry sand-rubber wheel test, the hardness of the material that is pressing the loose abrasive particles against the test specimen can have a significant effect on the results.

machines are either one-of-a-kind or highly specialized for simulating a particular application. Commercially manufactured sliding and rollingcontact wear testing machines are available in a number of contact geometries. Abrasion and erosion testing machines are also commercially available. Some testers, called "universal" or "multimode" testers, are configured to permit the user to change the contact geometry from, for example, block-on-ring to pin-on-disk, using accessory fixtures and drive mechanisms. Several manufacturers or retail sellers of wear testing machines advertise on the Internet and can be found through key-word searches. As indicated earlier, wear and chemical attack can have synergistic effects. Special procedures have been developed to study these phenomena (ASTM G 119). Specialized commercial testing machines have also been developed to study such effects, like machines that simulate the movements of surgical knee and hip replacement components in bodylike fluids.

Table 12 Category

Sliding

Typical testing geometries for wear tests of various types Subcategory

Abrasive wear, 2-body

Abrasive wear, 3-body

"Adhesive"

Fretting wear Polishing wear Impact

2-body Impingement, liquid and solid

Cavitation erosion

Rolling

Rolling contact fatigue Rolling with slip

Testing geometry

Flat pin-on-rotating abrasive drum (spiral path), reciprocating pin-on-abrasive flat, flat pin-on-moving abrasive belt, traversing pin-on-abrasive disk (spiral path), twin rotating abrasive wheels Dry sand fed between a rotating rubber wheel and a flat coupon, reciprocating flat pin-on-a plate in a slurry bath, block-onrotating ring in a slurry bath, ball mill, or tumbling wear test Block-on-ring (flat or conformal face), pin-on-disk, double rubshoe on rotating disk, reciprocating pin-on-flat, flat-on-flat (thrust washer), pin clamped between V-blocks, ball spinning on three flats (120° apart) Oscillating pin-on-flat, pivoting ball-in-socket, clamped specimen on the sides of a tensile coupon Flat specimen-on-vibrating lap, flat specimens in an orbital polishing or lapping machine Repetitive "hammer"-on-flat Liquid jet aimed at the specimen, gas jet with entrained particles, spinning specimens through a gravity-fed stream of particles, centrifugal particle "slinger" apparatus Oscillatory "horn" suspended above the specimen in a fluid, flowing fluid through a submerged nozzle aimed at the specimen Disk-on-disk rolling contact (equal circumferential speed), rod spinning between three captive balls Disk-on-disk rolling contact (unequal circumferential speed)

As with mechanical testing in general, commercial wear testing machines are being computer automated. While automation has definite advantages, it also drives up the price of these machines. Testers with infrequent wear problems who do not want to make a significant capital investment in wear testing may be faced with the decision as to whether to construct their own machine, purchase a commercial machine, have a custom machine built, or obtain the services of a fee-testing laboratory. Published surveys, conducted years ago by organizations such as the American Society of Lubrication Engineers (now called the Society of Tribologists and Lubrication Engineers) and the European Space Agency, have revealed the existence of hundreds of different wear testing devices. Some of these devices have similar geometries and operational features; however, even relatively similar-looking machines can produce different wear results due to subtle differences in construction features (fixture stiffness, method of specimen mounting, mechanical damping capacity, natural frequencies, heat flow, etc.). The number of custom-designed wear testing devices probably exceeds the number of commercially produced machines. Unfortunately, people who decide to build their own wear testing machines may not be aware of certain subtleties in wear tester design and, thus, may ultimately generate questionable results or results that cannot be reproduced elsewhere. There is still a great deal of applied research needed to better understand differences in wear results arising from different machine designs and measurement techniques.

References

1. PJ. Blau, Wear Testing, Metals Handbook Desk Edition, 2nd ed., J.R. Davis, Ed., ASM International, 1998, p 1342-1347 2. K.G. Budinski, Wear Modes, Surface Engineering for Wear Resistance, Prentice Hall, 1988, p 15-43 3. AJ. Sedriks and T.O. Mulhearn, The Effect of Work-Hardening on the Mechanics of Cutting in Simulated Abrasive Processes, Wear, VoI 7, 1964, p 451 4. T. Sasaki and K. Okamura, The Cutting Mechanism of Abrasive Grain, Bull Jpn. Soc. Mech. Eng., VoI 12, 1960, p 547 5. J.S. Hansen, Relative Erosion Resistance of Several Metals, Erosion: Prevention and Useful Applications, STP 664, ASTM, 1979, p 148-162 6. R.A. Corbett, "A Modified G-75 Abrasion Test for Corrosive Environments," Corrosion Testing Laboratories, Inc., Newark, DE 7. L.E. Samuels et al., Sliding Wear Mechanisms, Fundamentals of Friction and Wear of Materials, D. A. Rigney, Ed., American Society for Metals, 1981, p 1 3 ^ 2 8. KP Bowden and D. Tabor, The Friction and Lubrication of Solids, VoI 1 and 2, Clarendon Press, 1950 and 1966 9. J.F. Archard, Contact and Rubbing of Flat Surfaces, /. Appl. Phys., VoI 24, 1953, p 981-988 10. M.M. Kruschov, Resistance of Metals to Wear by Abrasion; Related to Hardness, Institute of Mechanical Engineers Conference: Lubrication and Wear, 1975, p 655-659 11. R.C. Tucker, Jr., Plasma and Detonation Gun Deposition Techniques, Deposition Technologies for Films and Coatings, R.F. Bunshah et al., Ed., Noyes Publications, 1982, p 454^189 12. CN. Rowe, Some Aspects of the Heat of Absorption in the Function of a Boundary Lubricant, Trans. ASLE, VoI 9, 1966 13. L.K. Ives, M.B. Peterson, and E.P. Whitenton, "Galling: Mechanism and Measurement," National Bureau of Standards Report, p 33-40

Selected References Friction, Lubrication, and Wear Technology, VoI 18, ASM Handbook, ASM International, 1992 M.B. Peterson and W.O. Winer, Ed., Wear Control Handbook, American Society for Mechanical Engineers, 1980 D.A. Rigney, Ed., Fundamentals of Friction and Wear of Metals, American Society for Metals, 1981

J.A. Schey, Tribology in Metalworking: Friction, Lubrication, and Wean American Society for Metals, 1983 A.Z. Szeri, Tribology: Friction, Lubrication, and Wear, McGraw-Hill, 1980

CHAPTER

T f

S u r f a c e C

h

a

n

g

E n g i n e e r i n g e

t h e

t o

S u r f a c e

M e t a l l u r g y

THE SURFACE-ENGINEERING TREATMENTS described in this Chapter include selective surface hardening by flame, induction, or highenergy beam heating, laser melting and quenching, and shot peening. None of these processes change the surface chemistry, but rather they improve properties such as wear and fatigue by altering the surface metallurgy. Selective Surface H a r d e n i n g Surface hardening in a general sense involves many processes that improve the wear resistance of parts while utilizing the tough interior properties of the steel or cast iron component. In this section, surface hardening is limited to localized heat treating processes that produce a hard quenched surface without introducing additional alloying species. This approach consists of hardening the surface by flame, induction, laserbeam, or electron-beam heating. More detailed information on surface hardening of steels can be found in Heat Treating, Volume 4 of the ASM Handbook. Flame Hardening Flame hardening consists of austenitizing the surface of steel by heating with an oxyacetylene or oxyhydrogen torch and immediately quenching with water. After quenching, the microstructure of the surface layer consists of hard martensite over a lower-strength interior core of other steel morphologies such as ferrite and pearlite. A prerequisite for proper flame

hardening is that the steel must have adequate carbon and other alloy additions to produce the desired hardness, because there is no change in composition. Flame-hardening equipment utilizes direct impingement of a high-temperature flame or high-velocity combustion product gases to austenitize the component surface and quickly cool the surface faster than the critical cooling rate to produce martensite in the steel. This is necessary because the hardenability of the component is fixed by the original composition of the steel. Thus, equipment design is critical to success of the operation. Flame-heating equipment may be a single torch with a specially designed head or an elaborate apparatus that automatically indexes, heats, and quenches parts. With improvements in gas-mixing equipment, infrared temperature measurement and control, and burner rig design, flame hardening has been accepted as a reliable heat treating process that is adaptable to general or localized surface hardening for small or medium-to-high production requirements. The flame-hardening process is used for a wide variety of applications. These include (1) parts that are so large that conventional furnace treatments are impractical or uneconomical, (2) prevention of detrimental treatment of the entire component when only small segments of the part require heat treatment, and (3) use of less costly material to obtain the desired surface properties where alloyed steels would be normally applied. Flame hardening is limited to hardenable steels (wrought or cast) and cast iron. Typical hardnesses obtained for the flame-hardened grades depend on the quench media (Table 1). The practical level of minimum surface hardness attainable with water quenching for various carbon contents is shown in Fig. 1. Induction Hardening

Hardness, HRC

Induction hardening is a versatile heating method that involves placing a steel part in the magnetic field generated by high-frequency alternating

Carbon, % Fig. 1 Relationship of carbon content to minimum surface hardness attain^* able by flame or induction heating and water quenching. Practical minimum carbon content can be determined from this curve. Source: Ref 1

Table 1

Response of steels and cast irons to flame hardening Typical hardness, HRC, as affected by quenchant

Material

Water(b)

Air(a)

Oil(b)

50-60 55-62

52-58 58-62 58-62

45-55 50-55

52-57(c) 55-60

33-50 55-60 60-63 62-65 45-55 55-62 58-64

50-60 50-60

58-62 60-63

62-65 62-65

45-55 50-60 55-60 55-60

52-57(c) 55-60 58-62 61-63 50-55 52-56 58-62 53-57 56-60 52-56 55-60 52-60 52-57 55-63

55-62 60-64 63-65 63-65 55-60 55-60 62-65 60-63 62-65 60-63 62-64 55-60 58-62 62-64

58-62 62-65 58-62

63-65 64-66 62-65

Plain-carbon steels 1025-1035 1040-1050 1055-1075 1080-1095 1125-1137 1138-1144 1146-1151

Carburized grades of plain-carbon steels(d) 1010-1020 1108-1120 Alloy steels 1340-1345 3140-3145 3350 4063 4130-4135 4140-4145 4147^150 4337^340 4347 4640 52100 6150 8630-8640 8642-8660

52-56 58-62 53-57 56-60 52-56 55-60 48-53 55-63

Carburized grades of alloy steels(d) 3310 4615-4620 8615-8620

55-60 58-62

Martensitic stainless steels 410,416 414, 431 420 440 (typical)

A2-A1 49-56 55-59

41^4 42-47 49-56 55-59

52-56

49-48 48-52 35^3 52-56 56-59

Cast irons (ASTM classes) Class 30 Class 40 Class 45010 50007, 53004, 60003 Class 80002 Class 60-45-15 Class 80-60-03

52-56

43^8 48-52 35^5 55-60 56-61 35-45 55-60

(a) To obtain the hardness results indicated, those areas not directly heated must be kept relatively cool during the heating process, (b) Thin sections are susceptible to cracking when quenched with oil or water, (c) Hardness is slightly lower for material heated by spinning or combination progressive-spinning methods than it is for material heated by progressive or stationary methods, (d) Hardness values of carburized cases containing 0.90-1.10% C. Source: Ref 1

current passing through an inductor, usually a water-cooled copper coil. The depth of hardening increases as the frequency of the alternating current decreases. Other variables important to the process include the coil current, heating time, and the coil design (Ref 2). The specific ferrous alloys that are commonly used in induction surface hardening are the same as those used in flame hardening (Table 1). The minimum carbon contents to obtain specific surface hardness are also shown in Fig. 1. Electrical

Table 2

Comparison of flame- and induction-hardening processes

Characteristic Equipment Applicable material Speed of heating Depth of hardening Processing Part size Tempering Can be automated Operator skills Control of process Operator comfort Cost Equipment Per piece

Flame

Induction

Oxyfuel torch, special head quench system Ferrous alloys, carbon steels, alloy steels, cast irons Few seconds to few minutes 1.2-6.2 mm (0.050-0.250 in.)

Power supply, inductor, quench system Same

One part at a time No limit Required Yes Significant skill required Attention required Hot, eye protection required

1-10 s 0.4-1.5 mm (0.015-0.060 in.); 0.1 mm (0.004 in.) for impulse Same Must fit in coil Same Yes Little skill required after setup Very precise Can be done in suit

Low Best for large work

High Best for small work

Source: Ref 2

properties of the alloy are an important consideration when selecting induction treatment as a surface-hardening technique. In induction hardening, the electrical resistivity and magnetic properties of the alloy can produce significant differences in heating characteristics. Thus, different steels require differing induction-heating parameters. Table 2 compares the flame- and induction-hardening processes. High-Energy Beam Hardening Electron- and laser-beam methods use high-energy beams to heat treat the surface of hardenable steel. The electron-beam (EB) heat treating process uses a concentrated beam of high-velocity electrons as an energy source to heat selected parts of the steel component. In laser heat treatment, a laser beam is used to harden localized areas of ferrous parts. These processes are similar to flame and induction hardening, except that the need for quenchants is eliminated as long as a sufficient size workpiece is being used. Electron-Beam Hardening. In EB hardening, the surface of the hardenable steel is heated rapidly to the austenitizing temperature, usually with a defocused electron beam to prevent melting. The mass of the workpiece conducts the heat away from the treated surface at a rate that is rapid enough to produce hardening. Materials for application of EB hardening must contain sufficient carbon and alloy content to produce martensite. With the rapid heating associated with this process, the carbon and alloy content should be in a form that will quickly allow complete solid solution in the austenite at the temperatures produced by the electron beam. In addition, the mass of the workpiece should be sufficient to allow proper quenching; for example, the part thickness must be at least ten times the depth of hardening, and hardened areas must be properly spaced to prevent tempering of previously hardened areas. The most suitable materials for EB hardening are the same steels used in flame hardening (Ref 2):

1045 to 1080 carbon steels Medium- to high-carbon alloy steels (4140, 4340, 8645, 52100, etc.) Pearlitic matrix cast irons Wl, W2, Ol, O2, L2, L6, Sl, S2 tool steels There are two basic types of EB systems: stationary or movable. In the movable process, the workpiece is fixed and the gun is moved to produce heating for the hardening. Stationary guns require manipulation of the workpiece under the beam. However, in both cases the area to be hardened on the workpiece must be in a line of sight with the beam. Gun movement or workpiece manipulation is accomplished by computer control to produce any desired pattern, and the beam can be oscillated or pulsed by standard controls. To produce an electron beam, a high vacuum of 10~5 torr (10~3 Pa) is required in the region where the electrons are emitted and accelerated. This vacuum environment protects the emitter from oxidizing and avoids scattering of the electrons while they are still traveling at a relatively low velocity. Electron-beam hardening in hard vacuum units requires that the part be placed in a chamber that is sufficiently large to manipulate the gun or the workpiece. Out-of-vacuum units usually involve shrouding the workpiece; a partial vacuum (IO"2 torr, or 13 Pa) is obtained in the work area by mechanical pumps. Laser-Beam Hardening. Lasers can be used to perform selective hardening with hardening depths and material constraints similar to those of EB hardening. Laser transformation hardening produces thin surface zones, which are heated and cooled rapidly, resulting in very fine martensitic microstructures, even in steels with relatively low hardenability. This process produces typical case depths for steel ranging from 0.75 to 1.3 mm (0.030-0.050 in.) depending on the laser power range, and hardness values as high as 60 HRC. Laser processing has advantages over EB hardening in that laser hardening does not require a vacuum, wider hardening profiles are possible, and there can be greater accessibility to hard-to-get areas with the flexibility of optical manipulation of light energy. A major disadvantage of lasers is the need to use surface treatments to prevent reflectivity of the laser beam.

Laser Melting (Ref 3) Processing. Laser melting requires higher power densities than the levels used for laser transformation hardening. The workpiece is often made absorptive either by using coatings similar to those used for laser heating or by increasing surface roughness, for example, by sand blasting. Laser melting can harden alloys that cannot be hardened by laser transformation hardening. In ferritic malleable gray iron, melting enhances the diffusion of carbon, and the ensuing rapid quench produces a hardened region.

(a)

(b)

Fig. 2 Cross sections of laser-melted cast iron surfaces, (a) Gray iron, (b) Ductile iron. Source: Ref 3

Weight loss (mg/mm2)

Metallurgical changes with laser melting are in the forms of grain refinement, solid solutions, and fine dispersions of precipitates. All of these can contribute to the hardening and strengthening of the surface. Lasermelted surfaces of cast irons appear dendritic, as shown in Fig. 2(a) for gray iron and in Fig. 2(b) for ductile iron. Below the melt zone is the heataffected zone, which appears in lighter contrast in Fig. 2. In the solidified melt in cast irons, a ledeburite (mixture of austenite and cementite) structure generally forms. Hardening is caused by graphite dissolution for form cementite and austenite transformation to martensite. Wear Behavior of Laser-Melted Surfaces. Figure 3 shows that significant improvements in erosion behavior of gray and ductile iron are possible with laser melting. Improved wear resistance can also be obtained with laser-melted tool steels.

Ductile iron, as-received Gray iron, as-received Ductile iron, laser melted Gray iron, laser melted erosion

Test time (hours) PJa- 3 Erosive wear behavior of as-received and laser-melted gray and ductile "* irons. Source: Ref 3

Stress, % of tensile strength

Peened Stressed in bending

Peened

Not peened

Stressed in torsion Not peened Number of cycles to failure Fig. 4 Fatigue curves for peened and unpeened steel spring wires Shot Peening

Shot peening is a method of cold working in which compressive stresses are induced in the exposed surface layers of metallic parts by the impingement of a stream of shot, directed at the metal surface at high velocity under controlled conditions. It differs from blast cleaning in primary purpose and in the extent to which it is controlled to yield accurate and reproducible results. Although shot peening cleans the surface being peened, this function is incidental. The major purpose of shot peening is to increase fatigue strength (Fig. 4). The process has other useful applications, such as relieving tensile stresses that contribute to stress-corrosion cracking, forming and straightening of metal parts, and testing the adhesion of silver-plate on steel. More detailed information on shot peening can be found in Ref 4. References 1. T. Ruglic, Flame Hardening, Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 260-267 2. K.G. Budinski, Surface Hardening by Flame and Induction, Chapter 5, Surface Engineering for Wear Resistance, Prentice Hall, 1988,

p 120-137

3. K.P. Cooper, Laser Surface Processing, Friction, Lubrication, and Wear Technology, VoI 18, ASM Handbook, ASM International, 1992, p 861-872 4. T. Kostilnik, Shot Peening, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 126-135

CHAPTER

kJ

S u r f a c e C

h

a

n

g

E n g i n e e r i n g e

t h e

t o

S u r f a c e

C h e m i s t r y

SURFACE TREATMENTS that change the surface chemistry of a metal or alloy, but that do not involve intentional buildup or increase in part dimension, include: Chemical or electrochemical conversion treatments that produce complex phosphates, chromates, or oxides on the metal surface Thermochemical diffusion heat treatments that involve the introduction of interstitial elements, such as carbon, nitrogen, or boron, into a ferrous alloy surface at elevated temperatures Pack cementation diffusion treatments that involve the introduction of aluminum, chromium, or silicon into an alloy surface Surface modification by ion implantation, which involves the introduction of ionized species (virtually any element) into the substrate using a beam of high-velocity ions Surface modification by a combination of laser-beam melting and alloying

Phosphate Chemical Conversion Coatings

Phosphate coating is the treatment of iron, steel, galvanized steel, or aluminum with a dilute solution of phosphoric acid and other chemicals in which the surface of the metal, reacting chemically with the phosphoric acid media, is converted to an integral, mildly protective layer of

insoluble crystalline phosphate. The weight and crystalline structure of the coating and the extent of penetration of the coating into the base metal can be controlled by (Ref 1): Method of cleaning before treatment Use of activating rinses containing titanium and other metals of compounds Method of applying the solution Temperature, concentration, and duration of treatment Modification of the chemical composition of phosphating solution The method of applying phosphate coatings is usually determined by the size and shape of the article to be coated. Small items, such as nuts, bolts, screws, and stampings, are coated in tumbling barrels immersed in phosphating solution. Large fabricated articles, such as refrigerator cabinets, are spray coated with solution while on conveyors. Automobile bodies are sprayed with or immersed in phosphating solution. Steel sheet and strip can be passed continuously through the phosphating solution or can be sprayed. Phosphate coatings range in thickness from less than 3 to 50 |xm (0.12 mils). Coating weight (grams per square meter of coated area), rather than coating thickness, has been adopted as the basis for expressing the amount of coating deposited.

Types of Phosphate Coatings Three principal types of phosphate coatings are in general use: zinc, iron, and manganese. Zinc phosphate coatings encompass a wide range of weights and crystal characteristics, ranging from heavy films with coarse crystals to ultrathin microcrystalline deposits. Zinc phosphate coatings vary from light to dark gray in color. Coatings are darker as the carbon content of the underlying steel increases, as the ferrous content of the coating increases, as heavy metal ions are incorporated into the phosphating solution, or as the substrate metal is acid pickled prior to phosphating. Zinc phosphating solutions containing active oxidizers usually produce lighter-colored coatings than do solutions using milder accelerators. Zinc phosphate coatings can be applied by spray, immersion, or a combination of the two. Coatings can be used for any of the following applications of phosphating: base for paint or oil; aid to cold forming, tube drawing, and wire drawing; increasing wear resistance; or rustproofing. Spray coatings on steel surfaces range in weight from 1.08 to 10.8 g/m2 (3.5 X 10~3 to 3.5 X 10" 2 oz/ft2); immersion coatings, from 1.61 to 43.0 g/m2 (5.28 X 10~3 to 1.41 oz/ft2). Iron phosphate coatings were the first to be used commercially. Early iron phosphating solutions consisted of ferrous phosphate/phosphoric acid

used at temperatures near boiling and produced dark gray coatings with coarse crystals. The term iron phosphate coatings refers to coatings resulting from alkali-metal phosphate solutions operated at pH in the range of 4.0 to 5.0, which produce exceedingly fine crystals. The solutions produce an amorphous coating consisting primarily of iron oxides and having an interference color range of iridescent blue to reddish-blue color. A typical formulation for an iron phosphate bath is: Component Phosphate salts Phosphoric acid Molybdate accelerator Detergents (anionic/nonionic)

Composition, % 12-15 3-4 0.25-0.50 8-10

Basically, then, iron phosphate formulations consist of primary phosphate salts and accelerators dissolved in a phosphoric acid solution. It is the acid that initiates the formation of a coating on a metal surface. When acid attacks the metal and begins to be consumed, solution pH at the metal surface rises slightly. This is what causes the primary phosphate salts to drop out of solution and react with the metal surface, forming a crystalline coating. Although iron phosphate coatings are applied to steel to provide a receptive surface for the bonding of fabric, wood, and other materials, their chief application is as a base for subsequent films of paint. Processes that produce iron phosphate coatings are also available for treatment of galvanized and aluminum surfaces. Iron phosphate coatings have excellent adherence and provide good resistance to flaking from impact or flexing when painted. Corrosion resistance, either through film or scribe undercut, is usually less than that attained with zinc phosphate. However, a good iron phosphate coating often outperforms a poor zinc phosphate coating. Spray application of iron phosphate coatings is most frequently used, although immersion application also is practical. The accepted range of coating weights is 0.21 to 0.86 g/m2 (6.9 X 10" 4 to 0.26 oz/ft2). Little benefit is derived from exceeding this range, and coatings of less than 0.21 g/m2 (6.9 X 10~4 oz/ft2) are likely to be nonuniform or discontinuous. Quality iron phosphate coatings are routinely deposited at temperatures from 25 to 65 0C (80-150 0F) by either spray or immersion methods. Manganese phosphate coatings are applied to ferrous parts (bearings, gears, and internal combustion engine parts, for example) for break-in and to prevent galling. These coatings are usually dark gray. However, because almost all manganese phosphate coatings are used as an oil base and the oil intensifies the coloring, manganese phosphate coatings are usually black in appearance. In some instances, a calcium-modified zinc phosphate coating can be substituted for manganese phosphate to impart break-in and antigalling properties.

Manganese phosphate coatings are applied only by immersion, requiring times ranging from 5 to 30 min. Coating weights normally vary from 5.4 to 32.3 g/m2 (1.8 X 10~2 to 9.83 oz/ft2), but can be greater if required. The manganese phosphate coating usually preferred is tight and finegrain, rather than loose and coarse-grain. However, desired crystal size varies with service requirements. In many instances, the crystal is refined as the result of some pretreatment (certain types or cleaners and/or conditioning agents based on manganese phosphate) of the metal surface. Manganese-iron phosphate coatings are usually formed from high-temperature baths from 90 to 95 0C (190-200 0 F).

Applications On the basis of pounds of chemicals consumed or tons of steel treated, the greatest use of phosphate coatings is as a base for paint. Phosphate coatings are also used to provide: A base for oil or other rust-preventive material Lubricity and resistance to wear, galling, or scoring of parts moving in contact, with or without oil A surface that facilitates cold forming Temporary or short-time resistance to mild corrosion A base for adhesives in plastic-metal laminations or rubber-to-metal applications Corrosion Protection. Conversion of a metal surface to an insoluble phosphate coating provides a metal with a physical barrier against moisture. The degree of corrosion protection that phosphate coatings impart to surfaces of ferrous metals depends on uniformity of coating coverage, coating thickness, density, and crystal size, and the type of final seal employed. Coatings can be produced with a wide range of thicknesses, depending on the method of cleaning before treatment, composition of the phosphating solution, temperature, and duration of treatment. In phosphating, no electric current is used, and formation of the coating depends primarily on contact between the phosphating solution and the metal surface and on the temperature of the solution. Consequently, uniform coatings are produced on irregularly shaped articles, in recessed areas, and on threaded and flat surfaces, because of the chemical nature of the coating process. The affinity of heavy phosphate coatings for oil or wax is used to increase the corrosion resistance of these coatings. Frequently, phosphatecoated articles are finished by a dip in nondrying or drying oils that contain corrosion inhibitors. The articles are then drained or centrifuged to remove the excess oil. Medium to heavy zinc phosphate coatings, and occasionally, heavy manganese phosphate coatings are used for corrosion resistance when

supplemented by an oil or wax coating. Zinc phosphate plus oil or wax is usually used to treat cast, forged, and hot-rolled steel nuts, bolts, screws, cartridge clips, and many similar items. Manganese phosphate plus oil or wax is also used on cast iron and steel parts. Phosphate Coating as an Aid in Forming Steel. The contact pressure used in deep-drawing operations sets up a great amount of friction between the steel surface and the die. The phosphate coating of steel as a metalforming lubricant, before it is drawn: Reduces friction Increases speed of the drawing operations Reduces consumption of power Increases the life of tools and dies Wear Resistance. Phosphating is a widely used method of reducing wear on machine elements. The ability of phosphate coating to reduce wear depends on uniformity of the phosphate coating, penetration of the coating in to metal, and affinity of the coating for oil. A phosphate coating permits new parts to be broken in rapidly by permitting retention of an adequate film of oil on surfaces at that critical time. In addition, the phosphate coating itself functions as a lubricant during the high stress of break-in. Heavy manganese phosphate coatings (10.8 to 43.0 g/m2, or 3.5 X 10~2 to 0.14 oz/ft2), supplemented with proper lubrication, are used for wearresistance applications. Parts that are manganese phosphate coated for wear resistance are listed in Table 1. When two parts, manganese phosphated to reduce friction by providing lubricity, are put into service in contact with each other, the manganese coating is smeared between the parts. The coating acts as a buffer to prevent galling or, on heavily loaded gears, welding. The phosphate coating need not stand up for an extended length of time, because it is in initial movements that parts can be damaged and require lubricity. For example, scoring of the mating surfaces of gears usually takes place in the first few revolutions. During this time, the phosphate coating prevents close contact of the faces. As the coating is broken down in operation, some of it is packed into pits or small cavities formed in gear surfaces by the etching action of the acid during phosphating. Table 1 Parts immersion coated with manganese phosphate for wear resistance Part(a)

Components for small arms, threaded fasteners(b) Bearing races Valve tappets, camshafts Piston rings Gears(c)

Material

Coating time, min

Supplementary coatings

Cast iron or steel; forged steel

15-30

Oils, waxes

High-alloy steel forgings or bar stock Low-alloy steel forgings or bar stock Forged steel, cast iron Forged steel, cast iron

7-15 7-15 15-30 15-30

Oils, colloidal graphite Oils, colloidal graphite Oils Oils

(a) Coating weights range from 10.8 to 43.0 g/m2 (3.5 X 10~2 to 0.14 oz/ft2). (b) Coating may be applied by barrel tumbling, (c) Coating weights range from 5.4 to 43.0 g/m2 (1.8 X 10" 2 to 0.14 oz/ft2).

Long after break-in, the material packed into the pits or coating that was originally formed in the pits prevents direct contact of mating surfaces of gear teeth. In addition, it acts as a minute reservoir for oil, providing continuing lubrication. As work hardening of the gear surfaces takes place, the coating and the etched area may disappear completely, but by this time scoring is unlikely to occur.

Chromate Chemical Conversion Coatings Chromate conversion coatings are formed by a chemical or an electrochemical treatment of metals or metallic coatings in solutions containing hexavalent chromium (Cr 6+ ) and, usually, other components. The process results in the formation of an amorphous protective coating composed of the substrate, complex chromium compounds, and other components of the processing bath. Chromate conversion coatings are applied primarily to enhance bare or painted corrosion resistance, to improve the adhesion of paint or other organic finishes, and to provide the metallic surface with a decorative finish. Chromating processes are widely used to finish aluminum, zinc, steel, magnesium, cadmium, copper, tin, nickel, silver, and other substrates. Chromate conversion coatings are most frequently applied by immersion or spraying, but other methods of application, such as brushing, roll coating, dip and squeegee, electrostatic spraying, or anodic deposition, are used in special cases. Processing Steps. Chromate coatings are applied by contacting the processed surfaces with a sequence of processing solutions. The processing baths are arranged in a series of tanks, and the surfaces to processed are transferred through the sequence of stages by using manual, semiautomatic, or automatic control. The chromate coatings are usually applied to metal parts or to a continuous metal strip running at speeds to 5 m/s (lOOOft/min). The basic processing sequence consists of the following six steps: cleaning, rinsing, conversion coating rinsing, posttreatment rinsing or decorative color rinsing, and drying. In many applications, this sequence is expanded to accommodate pickling, deoxidizing, dyeing, brightening, and other rinsing stages, or the sequence can be shortened when cleaning or posttreatment rinsing is not necessary. More detailed information on key processing steps can be found in Ref 2. Corrosion Protection. Chromate conversion coatings provide excellent bare or painted corrosion protection to the metal. The level of protection depends on the substrate metal, the type of chromate coating used, and the chromium coating weight. In unpainted applications, corrosion protection for the different conversion coatings generally increases with

coating weight, and the upper limit of the coating weight is determined by the process limitations or by the color requirement. Table 2 lists typical corrosion data measured by the ASTM B 117 method. In painted applications, the conversion coating must improve corrosion resistance and provide for good paint adhesion. The upper limit of the coating weight for the painted surfaces is normally defined by the onset of weaker paint adhesion or of corrosion problems related to paint delamination. Opinions differ widely regarding the mechanism of corrosion protection provided by the chromate coatings. The most widely advanced concepts suggest that the chromate coatings provide a barrier insulation from the environment and inhibit the cathodic corrosion reactions. Hardness and Abrasion Resistance. The hardness of chromate coatings depends strongly on the temperature during chromating and drying. Freshly made wet films are very soft and can be easily damaged by abrasion. After drying, the films develop good hardness, which allows for safe handling. However, even the dry films are susceptible to severe scratching or abrasion. Health and Safety Considerations. The disposal of spent solutions and rinse waters requires waste treatment. Hexavalent chromium must be reduced to Cr 3+ before neutralizing and precipitation. Sodium pyrosulfite (Na2S2O5) is usually used as the reducing agent in smaller operations, while for larger plants, sulfur dioxide (SO2) is preferred for economic reasons. Wastewater treatment sludges from chromating operations are considered hazardous waste. As a result, the use and disposal of chromium and chromium compounds have received much regulatory attention because of the toxicity of chromium and indications that it is a cancer-causing agent. Due to worker health and safety concerns, alternatives to chromate conversion coatings are being sought. Unfortunately, there are currently no drop-in substitutes to chromate conversion coatings that adequately match their corrosion resistance, paint adhesion, and so forth. Possible elimination of chromate conversion coatings due to regulatory restrictions is particularly troublesome for the aircraft industry. In applying the coating to the entire aircraft aluminum structure, the subsequent Table 2 Typical salt-spray data for chromate coatings on zinc and aluminum Substrate Electroplated zinc

Hot-dip zinc Aluminum alloy 3003

Type of chromate coating

Time to corrosion stain, h

Untreated Clear Iridescent Olive drab Electrolytic Untreated Clear passivate Untreated Clear Yellow-brown

<4 24-48 100-200 100-400 1000 <4 24-100 <24 60-120 250-800

rinse process can generate large quantities of chromium-containing wastes. The challenges of adequately maintaining aging aircraft will help drive the search for effective chromate substitutes. A l u m i n u m Anodizing Aluminum anodizing is an electrochemical method of converting aluminum into aluminum oxide (Al2O3) at the surface of the item being coated. It is accomplished by making the workpiece the anode while suspended in a suitable electrolytic cell. Although several metals can be anodized, including aluminum, titanium, and magnesium, only aluminum anodizing has found widespread use in industry. A more detailed discussion on anodizing can be found in Ref 3. Because a wide variety of coating properties can be produced through variations in the process, anodizing is used in almost every industry in which aluminum can be used. The broadest classification of types of anodize is according to the acid electrolyte used. Various acids have been used to produce anodic coatings, but the most common ones in current use are sulfuric (H2SO4) and chromic (CrO3) acids. There are two types of H2SO4 anodizing. The first is a room-temperature H 2 SO 4 process termed conventional anodizing, and the second is a low-temperature H 2 SO 4 process termed hardcoat anodizing. In addition to CrO3, conventional, and hardcoat anodizing, a process known as sealing can be used to enhance certain characteristics. The three common types of anodize described above are usually controlled and described through the use of military specification MIL-A8625 (Table 3). It has become standard in the industry to describe anodic coatings with the type and class nomenclature outlined in this specification.

Chromic Anodizing The CrO3 anodizing process produces a coating that is nominally 2 |xm (0.08 mil) thick. It is relatively soft and susceptible to damage through abrasion or handling. The color of the class 1 coating ranges from clear to Table 3 Classification of anodizing processes according to MIL-A-8625 Thickness Type

I II III

Class

Description

Dye

Seal

fxm

mils

1 2 1 2 1 2

CrO3 anodize CrO3 anodize, dyed H 2 SO 4 anodize H 2 SO 4 anodize, dyed Hardcoat anodize Hardcoat anodize, dyed

No Yes No Yes No Yes

Yes Yes Yes Yes No Yes

1.3-2.5 1.3-2.5 7.5-15 7.5-15 46-56 46-56

0.05-0.1 0.05-0.1 0.3-0.6 0.3-0.6 1.8-2.2 1.8-2.2

gray, depending on whether the coating is sealed and on the alloy coated. The coating can be dyed to produce a class 2 coating; however, this is not generally done, because the coating is thin and does not retain the dye color well. About two-thirds of the coating thickness penetrates the base metal; one-third of the coating builds above the original base metal dimension. Thus, for a coating thickness of 2 juim (0.08 mil) per side, the dimensional change of the workpiece would be 0.7 \xm (0.028 mil) per side. Although the industry has adopted the penetration/buildup terminology, the terms are somewhat misleading. Actually, when the aluminum is converted to Al 2 O 3 it takes up more space—approximately 133% of the space previously occupied by the aluminum converted. The penetration/buildup terms are used only as a convenience in predicting dimensional change in a coated article. The corrosion resistance of this coating is very good. The coating will pass in excess of 336 h in 5% salt (NaCl) spray per ASTM B 117. Advantages and Uses. Although CrO3 anodizing is the least used of the three types of anodize, it has several advantages that make its use desirable. First, because CrO3 is much less aggressive toward aluminum than H2SO4, it should be used whenever part design is such that rinsing is difficult. Difficult rinsing designs would include welded assemblies, riveted assemblies, and porous castings. Second, a typical CrO3 anodize buildup is 0.7 |xm (0.028 mil) per side with good repeatability. Therefore, it is a very good coating to use when it is necessary to coat a precise dimension to size. Third, because CrO3 anodize produces the least reduction in fatigue strength of the three coatings, it should be used where fatigue strength is a critical factor. Fourth, the color of CrO3 anodize will change with different alloy compositions and the heat treat conditions; this makes it useful as a test of the homogeneity of structural components. Lastly, when properly applied, CrO3 anodize can be used as a mask for subsequent hardcoat anodize operations. Suitable Alloys. Most alloys can be successfully coated by the CrO3 process. Exceptions are high-silicon die-cast alloys and high-copper alloys. The rule for suitability is that any alloy containing more than 5% Cu, 7% Si, or total alloying elements of 7.5% should not be coated by this process. Relative Costs. Chromic anodize costs more than H 2 SO 4 but less than hardcoat anodize. SuIfuric Anodizing The H2SO4 process produces a coating that is normally 8 jmm (0.31 mil) in minimum thickness. Although harder than type 1 coatings, H2SO4 anodize may still be damaged by moderate handling or abrasion. The color of the class 1 coating is yellow-green because of the preferred sealing method of immersion in sodium dichromate (Na2Cr2O7). Clear coatings can also be produced by sealing in hot water. Clear coatings should be

specified by the notation "class 1, clear." This coating can also be dyed to produce a class 2 coating. This type of anodize produces the most pleasing colors of the three anodizing methods. Dyed H 2 SO 4 anodize coatings have deep colors with good repeatability. Like CrO3 anodize, H 2 SO 4 anodize coatings penetrate the base metal for two-thirds of their thickness and build above the original base metal dimension for one-third the total thickness. As with all types of anodize, the corrosion resistance of H 2 SO 4 anodize is very good; it has an ASTM B 117 salt-spray resistance of at least 336 h. Advantages and Uses. Sulfuric anodize is the most widely used type of anodize and has many desirable benefits. First, because it has a fairly hard surface, it can be used in situations that require light to moderate wear resistance. Applications include lubricated sliding assemblies and items subject to handling wear, such as front panels. Second, because it is the most aesthetically pleasing type of anodize, it should be used where final appearance is important. It can be dyed almost any color and produces deep, rich shades that make the item appear to be made of a material bearing a color throughout, rather than an applied coating. Lastly, because corrosion resistance is good, it should be used whenever corrosion resistance is needed and the specialized benefits of the other two anodize types are not required. Suitable Alloys. With the exception of high-silicon die-cast alloys, all alloys can be successfully coated with H 2 SO 4 anodize. Clarity and depth of color of the anodize increase with the purity of the alloy. Therefore, alloys should be chosen for maximum purity consistent with the physical requirements needed in the item. Relative Costs. Sulfuric anodize is the least costly and most widely available type of anodize. Hardcoat Anodizing The hardcoat anodize process produces a coating that is normally 50 |xm (2 mils) thick, although other thicknesses can be specified. The coating is extremely hard. It is described as file hard (equal to about 60-70 HRC). The color of the class 1 coating ranges from gray to bronze to almost black, depending on the alloy coated, the coating thickness, and the electrolyte temperature. The coating can be dyed to produce a class 2 coating. Because thick coatings are naturally very dark, only colors darker than natural are possible. Generally, this limits the dying of hardcoat to black in common processes. If a more extensive color choice is required, there are several proprietary hardcoat processes available to accomplish this. Hardcoat penetrates the base metal for one-half of its thickness and builds above the original base metal dimension for one-half of its thickness. Thus, for a thickness of 50 |xm (2 mils) per side, the dimensional change of the workpiece would be 25 |jim (1 mil) per side. Commercially available coating thickness tolerances are the greater of ±5 |mm or ± 10%

of the total targeted thickness. The corrosion resistance of the unsealed class 1 coating is very good and comparable to the other types of anodize. When the hardcoat anodize is sealed, as in a class 2 coating, it becomes the most corrosion-resistant type of anodize. Advantages and Uses. Hardcoat anodize, because of its variety of desirable properties, has found widespread use in manufactured products. First, because of it extreme hardness, it is used in situations in which wear resistance is required. Applications include valve/piston assemblies, drive belt pulleys, tool holders and fixtures, and many other items requiring wear resistance. Second, because of its excellent resistance to corrosion, hardcoat is used on aluminum components in harsh environments. These include outside exposure in salt air, marine components, automobile wash equipment, components for the aircraft and aerospace industries, and food preparation machines. Third, because hardcoat is an excellent electrical resistor, it can be used to insulate heat sinks for direct mounting of electrical or electronic equipment. Also, it is used in welding fixtures where some areas may need to be insulated from work. Fourth, because hardcoat is a naturally porous substance, it is used in many areas in which the bonding or impregnation of other materials to aluminum is needed. This coating bonds very well with paints and adhesives. Also, it can be impregnated with teflon (polytetrafluoroethylene, or PTFE) and many dry film lubricants to impart lubricating properties to the coating. Lastly, because of its desirable properties and also because it produces a buildup of coating, it is widely accepted as a salvage coating to restore worn or improperly machined parts to usable dimensions. Coating thicknesses in excess of 250 \xm (10 mils) per side are possible on some alloys with certain proprietary hardcoat processes. Suitable Alloys. Although almost all alloys can be coated, the 6000-series aluminum alloys produce the best hardcoat properties. As with the other anodize types, high-silicon die castings produce the lowest-quality coatings. Also, because the hardcoat process is sensitive to copper, alloys in the 2000 series should be avoided if possible. Alloys containing copper can be hardcoated, but only a relatively few commercial sources have the ability to coat these alloys with reliability. Relative Costs. Hardcoat anodize is the most expensive type of anodize. It is generally twice the cost of H2SO4 anodize and 50% more than CrO3 anodize. Sealing ofAnodized Coatings Because all of the anodic processes produce porous Al 2 O 3 coatings, it is often desirable to seal the coating to close these pores and to eliminate the path between the aluminum and the environment. Sealing involves immersing the coating in hot water; this hydrates the Al 2 O 3

and causes the coating to swell in order to close the pores. Conventional sealing is generally done at a minimum temperature of 95 0 C (200 0F) for not less than 15 min. There are also several proprietary nickel-base sealing agents available that area said to produce sealing at low temperature through catalytic action. Chromic and sulfuric anodizes are almost always sealed. However, because sealing softens the coating somewhat, hardcoat anodize is usually not sealed unless criteria other than hardness have the maximum importance in the finished coating. Corrosion Resistance of Anodized Aluminum In general, corrosion resistance of anodic coatings is greatest in approximately neutral solutions, but such coatings are usually serviceable and protective if the pH is between 4 and 8.5. More acidic and more alkaline solutions attack anodic coatings. Under atmospheric weathering, the number of pits developed in the base metal decreases exponentially with increasing coating thickness (Fig. 1). The pits may form at minute discontinuities or voids in the coating, some of which result from large second-phase particles in the microstructure. The pit density was determined by dissolving the anodic coating in a stripping solution that does not attack the metal substrate. After the 8V2year exposure, the pits were of pinpoint size and had penetrated less than 50 |xm (2.0 mils). Specimens with coatings at least 22 jxm (0.9 mil) thick were practically free of pitting. Weathering of anodic coatings involves relatively uniform erosion of the

Number of pits per square meter

Original anodic coating thickness, mils Panels exposed 8V2 years Industrial environments

Highly aggressive environments Less aggressive nonindustrial environments

Original anodic coating thickness, p.m Fifi. & 1 Number of corrosion pits in anodized aluminum 1100 as a function of * coating thickness. Source: Ref 4

Average erosion rate Alloy type

Exposure time, years

Remaining coating thickness, mils

Remaining coating thickness, jim

coating by windborne solid particles, rainfall, and some chemical reaction with pollutants. The available information indicates that such erosion occurs at a reasonably constant rate, which averaged 0.33 jxm/yr (0.013 mil/yr) for several alloys exposed to an industrial atmosphere for 18 years (Fig. 2). A three-year seacoast exposure of specimens of several alloys with 23 jmm (0.9 mil) thick sulfuric acid coatings caused no visible pitting except in several alloys of the Ixxx series and in a 2xxx alloy (Table 4). Alloys that exhibited pitting were not protected any more effectively by 50 |xm (2 mils) thick coatings. This confirms a general observation that optimal protection against atmospheric corrosion is achieved in the coating thickness range of 18 to 30 |xm (0.7-1.2 mils) and that thicker coatings provide little additional protection.

Fig. 2 Weathering data for anodically coated aluminum in an industrial at^* mosphere

Table 4 Results of three-year seacoast exposure testing of anodized aluminum alloys Alloy and temper

Results

Sheet 1100 2024-T3, alclad 5456-H343 5086-H34 6061-T6 7039-T6 7075-T6 7075-F, alclad 7079-T6

No visible pitting Edge pitting only No visible pitting No visible pitting No visible pitting No visible pitting Edge pitting only Edge pitting only Edge pitting only

Extrusions 6351-T6 6061-T6 6063-T5 6070-T6 7039-T6

No visible pitting No visible pitting No visible pitting No visible pitting Scattered small pits

H 2 SO 4 anodic coatings 23 ixm (0.9 mil) thick, sealed in boiling water on test panels 100 X 150 mm ( 4 X 6 in.) cut from sheet and extrusions

Oxidation Treatments Tool Steels. Oxidation is a well-established process used for high-speed steel cutting tools. Increases in tool life of up to 100% are achieved, mostly due to a decrease in friction, because of the hard oxide coating and the ability of the porous oxide to entrap lubricant and draw it to the tool/workpiece interface. Steam oxidation of a finished tool is accomplished either by exposing it to steam at a temperature of about 565 0C (1050 0F) or by treating it in liquid sodium hydroxide and sodium nitrate salts at approximately 140 0C (285 0F) for 5 to 20 min. These treatments result in a black oxidized layer that is less than 5 |xm (0.2 mil) thick and will not peel, chip, or crack, even when the tool is bent or cut. Tool life improvements due to steam oxidation are listed in Table 5. Steam Treating of Powder Metallurgy (P/M) Steels. Many P/M parts have traditionally been steam treated for improved wear resistance, corrosion resistance, and sealing capacity. In this process, P/M parts are heated in a specific manner under a steam atmosphere at temperatures between 510 and 570 0C (950 and 1060 0F) to form a layer of black iron oxide, identified as magnetite, in the surface porosity. Magnetite has a hardness equivalent to 50 HRC. Spalling or flaking of the surface oxide layer can occur if the process temperature exceeds 570 0 C (1060 0F) and process times exceed 4 h. The maximum thickness of the surface oxide layer should not exceed 7 juim (0.28 mil). Beyond this thickness, flaking can occur due to an increase in surface tensile stress. Sintered density is an important consideration when applying steam treating for improved strength and hardness. Its ability to increase the wear resistance of the substrate material depends on the available porosity for oxidation. As density is increased, the amount of oxide formed is Table 5 Machining tool life improvements due to steam oxidation Tool life Tool

M2 broaches M2 drills

M7 end mill tools A6 hobs M2 milling cutters M2 saw blades M2 taps Source: Ref 5

Application

Cutting AISI 1010 latch Drilling Bakelite plastic insulating blocks Phenolic terminal plates Drilling AISI4030 steel, 25 mm (1 in.) thick Cutting 8740 steel forgings Cutting teeth on AISI 3140 forged gear Two slots in 1020 steel Slotting 1020 steel bars Cutting 75 mm (3 in.) rods, austenitic steel Cutting SAE 52100 steel

Before steam treating

After steam treating

20 h per grind 10 holes

70 h per grind 25 holes

1700 holes per grind 17 holes

8500 holes per grind 81 holes

30 pieces 62.2% increased life

200 pieces 62.2% increased life

150 cuts per grind 2000 per grind 100% endurance at 0.52 m/s (102 sfm) 1800 pieces

306 cuts per grind 7000 per grind 120% endurance at 0.57 m/s(112sfm) 3000 pieces

Density, Ib/in.3

Apparent hardness, HRB

steam-treated

steam-treated as-sintered

as-sintered

Density, g/cm3 Fig, 3 Effect of steam treating on the hardness of sintered P/M carbon steels. Source: Ref 6

decreased, which minimizes the improvement in apparent hardness attributed to steam treating. This is shown in Fig. 3 for sintered steel. The increase in density and apparent hardness produced by steam treating is illustrated in the micrograph of a sintered steel (Fig. 4). By filling

Fig. 4 Micrograph of steam-treated structure of a P/M steel. Unetched. 200X

the porosity with a hard second phase, the P/M steel offers a better support to the indentation hardness tester. Figure 5 illustrates that the transverse rupture strength is increased significantly by steam treatment for low-carbon P/M steels, but only modestly for high-carbon (0.8% C) P/M steels. Diffusion H e a t Treatment Coatings The diffusion coatings described in this section involve heat treating processes that cause carbon, nitrogen, or a combination of the two to diffuse into the surface of a ferrous part to alter the surface chemistry/properties. As listed in Table 6, these processes include carburizing, nitriding, and carbonitriding. Each of these depends on the concentration gradient of the diffusing species, the diffusivity of the atomic species in the host material, and the time and temperature at which the process takes place. All carburizing and nitriding processes increase the surface carbon or nitrogen content of the alloy to allow the surface to respond to quench hardening. The heat treater usually relies on empirical data to determine how long to expose the part to achieve the desired carbon or nitrogen diffusion. The term used for the entire field of surface-hardening processes is case hardening, and the case indicates the depth of hardening below the surface. Although

Density, Ib/in.3

as-sintered steam-treated

as-sintered

Transverse rupture strength, ksi

Transverse rupture strength, MPa

steam-treated

Density, g/cm3 Fig. &# 5

E ect 0 steam

^ ^ treating on transverse rupture strength of sintered P/M carbon steels. Source: Ref 6

the depth of hardening decreases gradually because the diffused species does not stop abruptly, the effective case depth is considered to be the depth at which the hardness falls below 50 HRC. More detailed information on the case-hardening procedures described in this section can be found in Heat Treating, Volume 4 of ASM Handbook.

Table 6

Typical characteristics of carburizing, nitriding, and carbonitriding diffusion treatments

Process

Name of case

Process temperature oC

(op)

Case hardness, typical case depth

Hardness, HRC

Typical base metals

Low-carbon steels, low-carbon alloy steels Low-carbon steels, low-carbon alloy steels

Process characteristics

Carburizing Pack

Diffused carbon

815-1090 (1500-2000)

125 fim-1.5 mm (5-60 mils)

50-63(a)

Gas

Diffused carbon

815-980 (1500-1800)

75 jjim-1.5 mm (3-60 mils)

50-63(a)

Liquid

Diffused carbon and possibly nitrogen

815-980 (1500-1800)

50 jxm-1.5 mm (2-60 mils)

50-65(a)

Low-carbon steels, low-carbon alloy steels

Vacuum

Diffused carbon

815-1090 (1500-2000)

75 |xm-1.5 mm (3-60 mils)

50-63(a)

Low-carbon steels, low-carbon alloy steels

Gas

Diffused nitrogen, nitrogen compounds

480-590 (900-1100)

125 fjim-0.75 mm (5-30 mils)

50-70

Alloy steels, nitriding steels, stainless steels

Salt

Diffused nitrogen, nitrogen, compounds

510-565 (950-1050)

2.5 jjim-0.75 mm (0.1-30 mils)

50-70

Most ferrous metals including cast iron

Ion

Diffused nitrogen nitrogen compounds

340-565 (650-1050)

75 |xm-0.75 mm (3-30 mils)

50-70

Alloy steels, nitriding steels, stainless steels

Gas

Diffused carbon and nitrogen

760-870 (1400-1600)

75 (xm-0.75 mm (3-30 mils)

50-65(a)

Low-carbon steels, low-carbon alloy steels, stainless steels

Liquid (cyaniding)

Diffused carbon and nitrogen

760-870 (1400-1600)

2.5-125 |i,m (0.1-5 mils)

50-65(a)

Low-carbon steels

Ferritic nitrocarburizing

Diffused carbon and nitrogen

480-590 (900-1090)

2.5-25 |xm (0.1-1 mil)

40-60(a)

Low-carbon steels

Low equipment costs, difficult to control case depth accurately Good control of case depth, suitable for continuous operation, good gas controls required, can be dangerous Faster than pack and gas processes, can pose salt disposal problem, salt baths require frequent maintenance Excellent process control, bright parts, faster than gas carburizing, high equipment costs

Nitriding Hardest cases from nitriding steels, quenching not required, low distortion, process is low, is usually a batch process Usually used for thin hard cases <25 jxm (<1 mil), no white layer, most are proprietary processes Faster than gas nitriding, no white layer, high equipment costs, close case control

Carbonitriding

(a) Requires quench from austenitizing temperature. Source: Ref 7

Lower temperature than carburizing (less distortion), slightly harder case than carburizing, gas control critical Good for thin cases on noncritical parts, batch process, salt disposal problems Low-distortion process for thin case on lowcarbon steel, most processes are proprietary

Carburizing Carburizing is the addition of carbon to the surface of low-carbon steels at temperatures (generally between 850 and 950 0 C, or 1560 and 1740 0F) at which austenite, with its high solubility for carbon, is the stable crystal structure. Hardening of the component is accomplished by removing the part and quenching or allowing the part to slowly cool and then reheating to the austenitizing temperature to maintain the very hard surface property. On quenching, a good wear- and fatigue-resistant high-carbon martensitic case is superimposed on a tough, low-carbon steel core. Carburized steels used in case hardening usually have base carbon contents of about 0.2 wt%, with the carbon content of the carburized layer being fixed between 0.8 and 1.0 wt% (Ref 8). Carburizing methods include gas carburizing, vacuum carburizing, plasma (ion) carburizing, salt-bath carburizing, and pack carburizing. These methods introduce carbon by use of an atmosphere (atmospheric gas, plasma, and vacuum), liquids (salt bath), or solid compounds (pack). The vast majority of carburized parts are processed by gas carburizing, using natural gas, propane, or butane. Vacuum and plasma carburizing are useful because of the absence of oxygen in the furnace atmosphere. Salt-bath and pack carburizing have little commercial importance, but are still done occasionally. Gas carburizing can be run as a batch or a continuous process. Furnace atmospheres consist of a carrier gas and an enriching gas. The carrier gas is supplied at a high flow rate to ensure a positive furnace pressure, minimizing air entry into the furnace. The type of carrier gas affects the rate of carburization. Carburization by methane is slower than by the decomposition of CO. The enriching gas provides the source of carbon and is supplied at a rate necessary to satisfy the carbon demand of the work load. Most gas carburizing is done under conditions of controlled carbon potential by measurement of the CO and CO 2 content. The objective of the control is to maintain a constant carbon potential by matching the loss in carbon to the workpiece with the supply of enriching gas. The carburization process is complex, and a comprehensive model of carburization requires algorithms that describe the various steps in the process, including carbon diffusion, kinetics of the surface reaction, kinetics of the reaction between the endogas and enriching gas, purging (for batch processes), and the atmospheric control system. Possible models of each of these steps have been outlined (Ref 9). Vacuum carburizing is a nonequilibrium, boost-diffusion-type carburizing process in which austenitizing takes place in a rough vacuum, followed by carburization in a partial pressure of hydrocarbon gas, diffusion in a rough vacuum, and then quenching in either oil or gas (Ref 10). Vacuum carburizing offers the advantages of excellent uniformity and reproducibility because of the improved process control with vacuum furnaces, improved mechanical properties due to the lack of intergranular oxidation,

and reduced cycle time. The disadvantages of vacuum carburizing are predominantly related to equipment costs and throughput. Plasma (ion) carburizing is basically a vacuum process utilizing glowdischarge technology to introduce carbon-bearing ions to the steel surface for subsequent diffusion (Ref 11). This process is effective in increasing carburization rates because the process bypasses several dissociation steps that produce active soluble carbon. For example, because of the ionizing effect of the plasmas, active carbon for adsorption can be formed directly from methane (CH4) gas. High temperatures can be used in plasma carburizing because the process takes place in an oxygen-free vacuum, thus producing a greater carburized case depth than both atmospheric gas and vacuum carburizing (Fig. 6). Nitriding Nitriding is a process similar to carburizing, in which nitrogen is diffused into the surface of a ferrous product to produce a hard case. Unlike carburizing, nitrogen is introduced between 500 and 550 0C (930 and 1020 0F), which is below the austenite formation temperature (Ac1) for ferritic steels, and quenching is not required. As a result of not austenitizing and quenching to form martensite, nitriding results in minimum distortion and excellent control. The various nitriding processes (Table 6) include gas nitriding, liquid nitriding, and plasma (ion) nitriding. All hardenable steels must be quenched and tempered prior to nitriding. The nitriding process is used to obtain a high surface hardness, improve wear resistance, increase fatigue resistance, and improve corrosion resistance (except for stainless steel). The case structure of a nitrided steel, conDepth below surface, in.

Carbon, wt%

A Atmosphere carburized, 30 min B Vacuum carburized, 30 min C Plasma carburized, 30 min

Depth below surface, mm ar

F i f i , 6 C bon gradient profile of atmosphere, vacuum, and plasma carburiz& * ing of AISI 8620 steel at 980 0C (1800 0F) saturation conditions for 30 min and followed by direct oil quenching. Source: Ref 12

taining a diffusion zone with or without a compound zone (Fig. 7), depends on the type and concentration of alloying elements and the timetemperature exposure of a particular nitriding treatment (Ref 13). The diffusion zone is the original core microstructure with the addition of nitride precipitates and nitrogen solid solution. The compound zone is the region where y' (Fe4N) and 8 (Fe 23 N) intermetallics are formed. Commercial steels containing aluminum, chromium, vanadium, tungsten, and molybdenum are most suitable for nitriding because they readily form nitrides that are stable at the nitriding temperatures (Ref 14). The following steels can be nitrided for specific applications: Aluminum-containing low-alloy steels: Nitralloys Medium-carbon, chromium-containing low-alloy steels: 4100, 4300, 5100, 6100, 8600, 8700, and 9800 series Low-carbon, chromium-containing low-alloy steels: 3300, 8600, and 9300 series Hot-working die steels containing 5% Cr: H I l , H12, and Hl3 Air-hardenable tool steels: A2, A6, D2, D3, and S7 High-speed tool steels: M2 and M4 Nitronic stainless steels: 30, 40, 50, and 60 Ferritic and martensitic stainless steels: 400 series Austenitic stainless steels: 200 and 300 series Precipitation-hardened stainless steels: 13-8 PH, 15-5 PH, 17-4 PH, 17-7 PH, A-286, AM 350 (Ref 14), and AM 355

Microhardness (H)

Gas nitriding (Ref 14) is a case-hardening process that takes place in the presence of ammonia gas. Either a single-stage or a double-stage

Compound zone

Diffusion zone

Case depth (AY) PJa- 7 Factors affecting the microhardness profile of a nitrided steel. The hard" ness of the compound zone is unaffected by alloy content, while the hardness of the diffusion zone is determined by nitride-forming elements (Al, Cr, Mo, Ti, V, Mn). AX is influenced by the type and concentration of alloying elements; AY increases with temperature and decreases with alloy concentration. Source: Ref 13

process can be used when nitriding with anhydrous ammonia. The singlestage process, in which a temperature of 495 to 525 0C (925-975 0F) is used, produces the brittle nitrogen-rich compound zone known as the white nitride layer at the surface of the nitrided case. The double-stage process, or Floe process, has the advantage of reducing the white nitrided layer thickness. After the first stage, a second stage is added which either by continuing at the first-stage temperature or increasing the temperature to 550 to 565 0 C (1025-1050 0 F). The use of the higher-temperature second stage lowers the case hardness and increases the case depth. Liquid nitriding (nitriding in a molten salt bath) uses similar temperatures as in gas nitriding and a case-hardening medium of molten, nitrogenbearing, fused-salt bath containing either cyanides or cyanates (Ref 15). Similar to salt-bath carburizing, liquid nitriding has the advantage of processing finished parts because dimensional stability can be maintained due to the subcritical temperatures used in the process. Furthermore, at the lower nitriding temperatures, liquid nitriding adds more nitrogen and less carbon to ferrous materials than that obtained with high-temperature treatments because ferrite has a much greater solubility for nitrogen (0.4% max) than carbon (0.02% max). Plasma (ion) nitriding is a method of surface hardening using glow-discharge technology to introduce nascent (elemental) nitrogen to the surface of a metal part for subsequent diffusion into the material (Ref 13). The process is similar to plasma carburizing in that a plasma is formed in a vacuum using high-voltage electrical energy and the nitrogen ions are accelerated toward the workpiece. The ion bombardment heats the part, cleans the surface, and provides active nitrogen. The process provides better control of case chemistry, case uniformity, and lower part distortion than gas nitriding. Properties of plasma nitrided ferrous alloys are listed in Table 7. Carbonitriding and Ferritic Nitrocarburizing Carbonitriding introduces both carbon and nitrogen into the austenite of the steel. The process is similar to carburizing in that the austenite Table 7 Properties of ion nitrided ferrous metals Nitriding temperature

Type of metal Carbon steel Gray cast iron Alloy steel Nitriding steel Hot-work tool steel Cold-work tool steel High-speed tool steel Stainless steel

Designation

1045 G2500 4140 9310 Nitralloy 135 H13 D2 M2 303 17-4 PH

0

C

510-570 510-540 480-540 510-550 480-540 480-540 450-540 480-510 540-570 510-540

Source: R.B. Alexander and Associates, Inc., Huntington Woods, MI

Typical case depth

Compound zone thickness

°F

Surface hardness, HR15N

mm

mils

950-1050 950-1000 900-1000 950-1025 900-1000 900-1000 850-1000 900-950 100O-1050 950-1000

File hard 77-82 84-90 77-92 90-95 90-94 90-94 92-95 90-95 90-95

0.30-0.76 0.10-0.20 0.30-0.76 0.30-0.76 0.25-0.76 0.10-0.33 0.13-0.25 0.03-0.10 0.05-0.13 0.10-0.20

12-30 4-8 12-30 12-30 10-30 4-13 5-10 1-4 2-5 4-8

tun

3.8-15 5.1-10 3.8-15 3.8-10 1.3-10 2.5-5.1

mils

0.15-0.6 0.2-0.4 0.15-0.6 0.15-0.4 0.05-0.4 0.1-0.2

composition is enhanced and the high surface hardness is produced by quenching to form martensite. This process is a modified form of gas carburizing in which ammonia is introduced into the gas-carburizing atmosphere (Ref 16). As in gas nitriding, elemental nitrogen forms at the workpiece surface and diffuses along with carbon into the steel. Typically, carbonitriding takes place at a lower temperature and a shorter time than gas carburizing, producing a shallower case. Steels with carbon contents up to 0.2% are commonly carbonitrided; these include 1000, 1100, 1200, 1300, 1500, 4000, 4100, 4600, 5100, 6100, 8600, and 8700 series. Ferritic nitrocarburizing is a subcritical heat treatment process, carried out by either gaseous or plasma techniques, and involves the diffusion of carbon and nitrogen into the ferritic phase. The process results in the formation of a thin white layer or compound layer, with an underlying diffusion zone of dissolved nitrogen in iron, or alloy nitrides (Ref 17). The white layer improves surface resistance to wear, and the diffusion zone increases the fatigue endurance limit, especially in carbon and low-alloy steels. Alloy steels, cast irons, and some stainless steels can be treated. The process is used to produce a thin, hard skin, usually less than 25 |xm (1 mil) thick, on low-carbon steels in the form of sheet metal parts, powder metallurgy parts, small shaft sprockets, and so forth.

Pack-Cementation Diffusion Coatings The pack-cementation process originally involved pack carburizing, which is the process of diffusing carbon into the surface of iron or low-carbon steel by heating in a closed container filled with activated charcoal. The simplest and oldest carburizing process involves filling a welded sheet metal or plate box with granular charcoal that is activated with chemicals such as barium carbonate to assist the formation of carbon monoxide (CO). In the heated box, charcoal forms carbon dioxide (CO2), which converts to CO in an environment with an excess of carbon. The CO then forms atomic carbon at the component surface and diffuses into the part. The packcarburization process is of little commercial importance, although it is still done occasionally. It has given rise to other pack-diffusion processes including aluminizing, siliconizing, chromizing, and boronizing. Basic Principles. Pack cementation is a batch vapor-phase process that involves heating a closed/vented pack to an elevated temperature (e.g., 1050 0 C, or 1920 0F) for a given time (e.g., 16 h) during which a diffusional coating is produced (Ref 18). The traditional pack consists of four components: the substrate or part to be coated, the master alloy (i.e., a powder of the element or elements to be deposited on the surface of the part), a halide salt activator, and relatively inert filler powder. The master alloy, the filler, and halide activator are thoroughly mixed together, and the part to be coated is buried in this mixture in a retort (Ref 19). When the

mixture is heated, the activator reacts to produce an atmosphere of source element(s) halides that diffuse into the pack and transfer the source elements) to the substrate on which the coating is formed (Ref 20). Aluminizing. An aluminizing pack-cementation process is commercially practiced for a range of alloys, including nickel- and cobalt-base superalloys, and carbon, low-alloy, and stainless steels. Simple aluminide coatings resist high-temperature oxidation by the formation of an alumina protective layer and can be used up to about 1150 0C (2100 0 F), but the coating can degrade by spallation of the oxide during thermal cycling. For extended periods of time at temperatures in excess of 1000 0C (1830 0 F), interdiffusion of the coating will cause further degradation, and therefore practical coating life is limited to operating temperatures of 870 to 980 0C (1600 to 1800 0 F). Pack compositions, process temperatures, and process times depend on the type of base material to be aluminized and fall into the following classifications (Ref 21): Class

I II III IV

Alloy

Carbon and low-alloy steels Ferritic and martensitic stainless steels Austenitic stainless steels with 2 1 ^ 0 % Ni and iron-base superalloys Nickel- and cobalt-base superalloys

Temperature, 0F

Temperature, 0C

As a general rule, overall aluminum diffusion is slowed as the nickel, chromium, and cobalt contents increase. Thus, higher temperatures and longer processing times are required to produce greater aluminum diffusion thicknesses as the base material increases in alloy content. Stainless steels are oxidation resistant as a result of the formation of a thin chromium-rich oxide on the component surface. A similar reaction occurs in aluminized steels in which a thin, slower-growing aluminumrich oxide forms. Unlike chromium oxide, Al 2 O 3 does not exhibit volatility in the presence of oxygen above 927 0C (1700 0 F). Figure 8 compares

Steel type PJa 8 Oxidation of steel s in air at the temperature at which scaling is less &* than 10 mg/cm2. Source: Ref 22

an aluminized carbon steel with several alloys at a temperature in which scaling remains less than 10 mg/cm2 for oxidation in air. In sulfidizing environments, pack-aluminized coatings have excellent resistance to corrosive attack. In contrast to stainless steels, the aluminum-rich surface (50% Al) and diffusion zone (20% Al min) of the coating is far more resistant than chromium to sulfidation corrosion. Figure 9 compares the corrosion rates of bare and aluminized 9Cr-IMo steel in a hydrogen sulfide (H2S) environment. As mentioned in the preceding text, pack aluminizing is commonly carried out on nickel- and cobalt-base superalloys. Diffusion-coated superalloys develop an aluminide (NiAl or CoAl) outer layer with enhanced corrosion resistance. It is estimated that more than 90% of all coated gas turbine engine hot section blades and vanes made from superalloys are coated by pack cementation and related processes. Detailed information on protective diffusion coatings for superalloys can be found in Ref 24. Siliconizing, the diffusion of silicon into steel, occurs similarly to aluminizing. There are pack and retort processes in which parts are subjected to gas atmospheres that react with the heated part surface to produce nascent silicon that diffuses into the substrate to be coated. In a pure silicon pack that is activated with NH4Cl, SiCl4 and SiHCl3 gases form, which are reduced by hydrogen gas to deposit elemental silicon on the surface of the parts (Ref 20). Another process involves tumbling parts in a retort with SiC. When a temperature of 1010 0C (1850 0F) is reached, silicon tetrachloride gas is introduced, which reacts with the part and the SiC particles to produce a concentration gradient of silicon on the part surface as the silicon diffuses into the substrate. The process normally takes place on lowcarbon steels, and these steels develop case depths up to 1 mm (0.040 in.) with a silicon content of 13 wt% (Ref 25). Case depths developed on these siliconized steels have hardnesses of about 50 HRC and therefore can be used for wear resistance. The presence of silicon on the surface allows for

Corrosion rate, mils/yr

Corrosion rate mm/yr

Bare steel Aluminized steel

e at ve FlC 6# 9 R ' * corrosion rates of 9Cr-1 Mo alloy steel in 5 mol% H2S at 3550 kPa (515 psi) for 300 h. Source: Ref 23

the formation of a stable silicon dioxide (SiO2) phase in oxidizing environments and excellent corrosion resistance. Chromizing. Chromium can be applied in the same manner as aluminum and silicon to produce a chromium-rich coating, and many of the same principles of aluminizing packs apply to chromizing packs. Parts are packed in chromium powder with an inert filler such as aluminum oxide. A halide salt activator is added that changes to the vapor phase at the processing temperature and serves as a carrier gas to bring chromium to the surface of the part. Diffusion coatings can be formed on nickel-base superalloys by pack cementation using ammonium chloride as a chromiumalumina activator. These coatings usually contain 20 to 25 wt% Cr at the outer surface and involve approximately equal rates of interdiffusion of chromium and nickel. Significant depletion of aluminum and titanium from the alloy surface occurs, thus producing a coating that is a solid solution of the chromium in the remaining nickel-base superalloy. The deposited coating is usually overlaid with a thin layer of a-chromium, which must be removed chemically (Ref 20). In low-alloy steels, it has been shown that chromizing is much more complex, leading to microstructures that may behave detrimentally in some environments (Ref 26). In a chromized 2.25Cr-IMo alloy, the coating contains a thin outer layer (~5 |xm, or 0.2 mil) of mostly chromium (>80 wt%), which is essential for corrosion protection. Large columnar ferritic grains, containing between 30 and 15 wt% Cr, are found beneath the outer layer. The columnar grain boundaries, as well as the boundary between the outer chromium-rich layer and the columnar grains, are decorated with chromium carbides that were found to contribute to coating degradation. A layer of Kirkendall voids (also decorated by carbides), iron carbides at the coating/substrate interface, and a large decarburized zone in the substrate are also produced by the process. Evaluation of samples exposed to a fossil-fired boiler up to two years revealed two degradation mechanisms: cracking and sulfidation corrosion. Cracking of the outer coating layer allowed ingress of sulfur, resulting in intergranular sulfidation corrosion attack. Once the outer protective chromium layer has been breached, the columnar grain-boundary orientation promotes crack initiation and propagation along the carbides when the tube is subjected to axial thermal loading. However, it should be noted that chromized coatings have been used up to 10 years in some fossil-fired boilers. Therefore, stress and environmental conditions are critical to the successful use of these pack-cementation coatings, as long as the effect of the processing thermal cycle on the coating and substrate morphology is understood. Bonding, or boronizing, is a thermochemical surface-hardening process that can be applied to a wide variety of ferrous, nonferrous, and cermet materials. The boronizing pack process is similar to pack carburizing with the parts to be coated being packed with a boron-containing

compound such as boron powder or ferroboron. Activators such as chlorine and fluorine compounds are added to enhance the production of the boron-rich gas at the part surface. Processing of high-speed tool steels that were previously quenched hardened is accomplished at 540 0C (1000 0 F). Boronizing at higher temperature up to 1090 0C (2000 0F) causes diffusion rates to increase, thus reducing the process time. The boron case does not have to be quenched to obtain its high hardness, but tool steels processed in the austenitizing temperature range need to be quenched from the coating temperature to harden the substrate. Boronizing is most often applied to tool steels or other substrates that are already hardened by heat treatment. The thin (12-15 jjim, or 0.48-0.6 mil) boride compound surfaces provide even greater hardness, improving wear service life. Distortion from the high processing temperatures is a major problem for boronized coatings. Finished parts that are able to tolerate a few thousandths of an inch (75 fim) distortion are better suited for this process sequence because the thin coating cannot be finish ground (Ref 25). Although pack boriding is the most widely used boriding process, it is important to note that other thermochemical boriding techniques are also used. These include paste boriding, liquid (salt-bath) boriding, gas boriding, plasma boriding, and fluidized-bed boriding. These alternative techniques are described in Ref 27.

Ion Implantation Ion implantation involves the bombardment of a solid material with medium- to high-energy ionized atoms and offers the ability to alloy virtually any elemental species into the near-surface region of any substrate. The advantage of such a process is that it produces improved surface properties without the limitations of dimensional changes or delamination found in conventional coatings. During implantation, ions come to rest beneath the surface in less than 10 to 12 s, producing a very fast quench rate and allowing the development of nonequilibrium surface alloys or compounds. In almost all cases the modified region is within the outermost micrometer of the substrate, often only within the first few hundred angstroms (i.e., microinches) of the surface. Details of the process and associated equipment are documented in Ref 28. Ion implantation is commercially applied to various steels, tungsten carbide/cobalt materials, and alloys of titanium, nickel, cobalt, aluminum, and chromium, although applications are restricted to temperatures below 250 0C (480 0F) for steels and 450 0C (840 0F) for carbides. Advantages and limitations of the ion implantation process are outlined in Table 8. Applications. Table 9 lists some of the applications for the ion-implantation process. Ion-implantation surfaces produce exceptional results in

Table 8

Advantages and limitations of ion implantation

Advantages

Limitations

Produces surface alloys independent of thermodynamic criteria No delamination concerns No significant dimensional changes Ambient-temperature processing possible Enhance surface properties while retaining bulk properties High degree of control and reproducibility

Limited thickness of treated material High-vacuum process Line-of-sight process Alloy concentrations dependent on sputtering Relatively costly process; intensive training required compared to other surface treatment processes Limited commercial treatment facilities available

reducing wear, friction, and corrosion (Ref 28, 29). Commercial applications involve tooling, bearings, and biomedical components. Nitrogen implantation, especially in alloy surfaces containing elements forming stable nitrides, has found use in tools and dies such as cobalt-cemented tungsten carbide wire-drawing inserts. Nitrogen implantation has been especially successful in increasing the life (up to 20 times) of tools and parts used in the manufacture of injection-molded plastics. Ion implantation with nitrogen or titanium and carbon has provided increased tool life for stamping and other forming tools. For example, the life of punches and dies for the manufacturing of aluminum beverage cans has increased to 6 to 10 times that of untreated tooling. Table 10 lists examples of extending tool life with ion implantation. Titanium and cobalt-chromium alloy orthopedic prostheses for hip and knee joints are among the most successful commercial applications for ion-implantation components for wear resistance.

Table 9

Research and development applications for ion implantation

Surface properties modified

Substrates studied

Ions species used

Steels, WC, Ti, Co/Cr alloys, TiN coatings, electroplated Cr Steels

N, C

Fatigue

Ti alloys, steels

N, C

Fracture toughness

Ceramics: Al2O3, TiN

Ar

Aqueous corrosion catalysis

Steels, Ti alloys, Pt

Cr, Ta, Cr+P >10 1 7 ions/cm2

Oxidation

Superalloys

Y, Ce >10 1 5 ions/cm2

Electrical conductivity

Polymers

Ar, F 10 15 -10 17 ions/cm2

Optical: refractive index

Glasses, electrooptics

Li, Ar 10 15 -10 17 ions/cm2

Wear

Friction

Source: Ref 28

10-20 at.% >10 1 7 ions/cm2 Ti plus C implants >10 1 7 ions/cm2 >10 1 7 ions/cm2 10 15 -10 17 ions/cm2

Comments Ti, Co/Cr alloys largest use commercially in orthopedic devices Dual implants give amorphous surface layer Implantation effective for surface initiated fatigue Radiation damage critical; ion-induced compressive stress helpful Ion implant can mimic "normal" alloys; amorphous and unique surface alloys possible Low effective doses; implanted species stay at metal-oxide interface Permits chain scissoning, doping; conductivity approaches disordered metal levels Chemical doping and lattice disorder both important

Table 10

Examples of extending tool life via ion implantation Material

Application

Chasers

Tool steel

Draw die

D2

Flanging ring

D2

Cutting threads in 380 aluminum Drawing 3.5 mm (0.140 in.) hot-rolled steel 2 mm (0.080 in.) hot-rolled steel (flywheels) 1 mm (0.042 in.) cold-rolled steel Forming electrical connectors 13 mm (V2 in.) hot-rolled steel 1.3 mm (0.050 in.) Hastelloy (jet engine part) 0.08 mm (0.003 in.) 301 stainless steel Shaping vinyl siding Gaging powdered metal part Finish reaming gray cast iron Cutting rubber (belts) Injection-molding thermoset (20% glass) Tapping cold-rolled steel nuts Tapping 380 aluminum Welding aluminum to steel Compacting copper-base composite wire

Tool

Forming dies

Wl

Forming Pierce punches

D2 M2

Pilot pins

M2

Plastic forming Plug gage Reamers Slitters Sprue bushings

M2 Stainless steel O6 Carbide Carbide P20 (chromium plated)

Taps

M2 (chromium plated)

Thread form taps Ultrasonic electrodes Wire compacting dies

M2 D2 Inconel, M2

Results 5 X life 22 X life before polishing 80X life before polishing Pickup reduced, improved finish on part At least 60% longer life 12X life 30% longer life 5 X life Wear rate reduced for dies At least 2 X life 2 X life Friction reduced 30% At least 4.5 X life 8 X life 2X life 2.5 X life (mechanical wear) At least 16 X life

Source: Ref 30

Laser Alloying Processing. A technique of localized alloy formation is laser surface melting with the simultaneous, controlled addition of alloying elements. These alloying elements diffuse rapidly into the melt pool, and the desired depth of alloying can be obtained in a short period of time. By this means, a desired alloy chemistry and microstructure can be generated on the sample surface; the degree of microstructural refinement will depend on the solidification rate. The surface of a low-cost alloy, such as mild steel, can be selectively alloyed to enhance properties, such as resistance to wear, in such a way that only the locally modified surface possesses properties typical of tribological alloys. This results in substantial cost savings and reduces the dependence on strategic materials. One method of alloying is to apply appropriate mixtures of powders on the sample surface, either by spraying the powder mixture suspended in alcohol to form a loosely packed coating, or by coating a slurry suspended in organic binders (Ref 31). The use of metal powders in laser alloying is the least expensive, but, with appropriate process modifications, alloys in the form of rods, wires, ribbons, and sheets can also be added. Applications. Laser alloying has been primarily applied to improve corrosion resistance. A very common technique is to alloy steels with

chromium. An example of laser alloying to improve wear resistance is exhaust valves fabricated by Fiat Research Laboratory.

Acknowledgment Portions of this chapter were adapted from A.R. Marder, Effects of Surface Treatments on Materials Performance, Materials Selection and Design, Volume 20, ASM Handbook ASM International, 1997, p 470-490.

References 1. Phosphate Coatings, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 378-404 2. K.A. Korinek, Chromate Conversion Coatings, Corrosion, VoI 13, ASM Handbook, ASM International, 1987, p 389-395 3. M.F. Stevenson, Anodizing, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 482-493 4. W.C. Cochran and D.O. Sprowls, "Anodic Coatings for Aluminum," Paper presented at Conference on Corrosion Control by Coatings, Lehigh University, Nov 1978 5. T.D. Deming, Steam Treating Emerges as Important Cog in Metal Surface Engineering, Ind. Heat., Jan 1990, p 28-30 6. H.A. Ferguson, Heat Treating of Powder Metallurgy Steels, Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 229-236 7. K.G. Budinski, Diffusion Processes, Chapter 4, Surface Engineering for Wear Resistance, Prentice-Hall, 1988, p 78-119 8. G. Krauss, Steels: Heat Treatment and Processing Principles, ASM International, 1990, p 286 9. CA. Stickels and CM. Mack, Overview of Carburizing Processes and Modeling, Carburizing Processing and Performance, G. Krauss, Ed., ASM International, 1989, pi 10. J. St. Pierre, Vacuum Carburizing, Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 348 11. W.L. Grube and S. Verhoff, Plasma (Ion) Carburizing, Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 352-362 12. S.H. Verhoff, Ind. Heat., March 1986, p 22-24 13. J.M. O'Brien and D. Goodman, Plasma (Ion) Nitriding, Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 420-424 14. CH. Knerr, TC. Rose, and J.H. Filkowski, Gas Nitriding, Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 387-409

15. Q.D. Mehrkam, J.R. Easterday, B.R. Payne, R.W. Foreman, D. Vukovich, and A.D. Godding, Liquid Nitriding, Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 410-419 16. J. Dossett, Carbonitriding, Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 376-386 17. T. Bell, Gaseous and Plasma Nitrocarburizing, Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 425-^36 18. M.A. Harper and R.A. Rapp, Codeposition of Chromium and Silicon in Diffusion Coatings for Iron-Base Alloys Using Pack Cementation, Surface Modification Technologies IV, T.S. Sudarshan, D.G. Bhat, and M. Jeandin, Ed., TMS, 1991, p 415 19. R. Bianco, M.A. Harper, and R.A. Rapp, Codepositing Elements by Halide-Activated Pack Cementation, JOM, Nov 1991, p 68 20. G.W. Goward and L.L. Seigle, Diffusion Coatings for Gas Turbine Engine Hot Sections, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 611-617 21.L.K. Bennett and G.T. Bayer, Pack Cementation Aluminizing of Steels, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 617-620 22. W. Beck, "Comparison of Carbon Steel, Alonized Type 304 for Use as Dummy Slabs in Reheat Furnace Operation," Alon Processing, Inc., Tarentum, PA 23. T. Perng, "A Fundamental Study of the Noxso No/SO 2 Flue Gas Treatment," Noxso, 1984 24. High-Temperature Coatings for Superalloys, ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, J.R. Davis, Ed., ASM International, 2000, p 281-290 25. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988, p 111 26. BJ. Smith and A.R. Marder, Characterization of Chromium Diffusion (Chromize) Coatings in a High Temperature Coal Combustion Atmosphere, Surface Modification Technologies TV, T.S. Sudarshan, D.G. Bhat, and M. Jeandin, Ed., TMS, 1991, p 471 27. A.K. Sinha, Bonding (Boronizing), Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 437^147 28. J.K. Hirvoven and B.D. Sartwell, Ion Implantation, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 605-610 29. G.R. Fenske, Ion Implantation, Friction, Lubrication, and Wear Technology, VoI 18, ASM Handbook, ASM International, 1992, p 850-860 30. R.E. Hoisington, Extending Tool Life with Ion Implantation, G.E. TechnoL, Jan 1986, p 9-12 31. K.P. Cooper, Laser Surface Processing, Friction, Lubrication, and Wear Technology, VoI 18, ASM Handbook, ASM International, 1992, p 861-872

CHAPTER

%J

S u r f a c e t o

A

d

d

a o r

E n g i n e e r i n g S u r f a c e

L a y e r

C o a t i n g

THE SURFACE-ENGINEERING METHODS described in this Chapter are those that involve an intentional buildup or addition of a new layer on a metal substrate, that is, the application of a coating or lining. A wide range of processes are used to deposit metal, ceramic, and organic (paints or plastic and rubber linings) coatings or combinations of these materials (composite coatings). As shown in Table 1, each has their own distinct processing parameters (e.g., temperature, pressure, and time), advantages, and limitations. Coating materials or coating methods discussed below include: • • • • • • • • • • •

Organic coatings and linings Ceramic coatings Hot dip metallic coatings Electroplating (metal or composite coatings) Electroless plating (metal or composite coatings) Weld overlays (metal or cermet coatings) Thermal spraying (metal, plastic, ceramic, or composite coatings) Cladding (thick metal coatings) Chemical vapor deposition (metals, graphite, diamond, diamondlike carbon, and ceramics) Physical vapor deposition (metals, ceramics, or solid lubricants) Thermoreactive deposition/diffusion process (carbides, nitrides, or carbonitrides)

More detailed information on these coating processes can be found in Volumes 5 (Surface Engineering), 13 (Corrosion), and 18 (Friction,

Table 1

An overall comparison of various surface engineering processes Surface hardness(a), HRC

Corrosion resistance

0.01-0.5

68-70 (hard Cr) 70-72

Good (hard Cr) Good

2.5-250

0.1-10

49-70 (Ni-P)

1-1300 0.025-10 1-1000 0.025-250 51-3800

0.04-50 0.001-0.4 0.04-40 0.001-10 2-150

Thickness/depth

Process/ material

jtm

mils

Electroplating

2.5-640

0.1-25

Thin dense chromium Electroless plating CVD PVD Diamond DLC Thermal diffusion

0.25-13

°F

C

torr

kPa


Yes

No

<65

<150

Liquid

Atmosphere

0.01-0.3

Yes

Liquid

Atmosphere


Yes

10-60 1-10 <1-250
Reheat treatment No Reheat treatment No Reheat treatment

10-50

Yes Yes Yes Yes No, except for C No

10-20 1-8

No No

Reheat treatment No

I^

Yes

No

1-10, >10 large areas

Yes

Grinding (usually)

Yes

No

No

85-95

No Yes No Yes No

820-1200 95-540 <1090 >RT 480-1040

1500-2200 200-1000 <2000 >RT 900-1900

0.013-100 1O~8-O.O13 0.013-100 10-8-1.3 40->100

io- -o.i

No

400-570

750-1050

0.027-0.40

0.2-3

870-1040 Usually <150

1600-1900 Usually <300

Liquid Atmosphere 10~7-3 X 10~5 10~6-2 X 10"4

Cryogenic980 95-260

Cryogenic1800 200-500

TRD process Ion implantation

2.5-20 0.1-1

0.1-0.8 0.004-0.04

90-93 (VC) 80-90 (N)

IBAD

0.1-10

0.004-0.4

91-96 (BN)

Excellent

No Yes (beam) No (plasma source) Yes

Thermal spray Solid lubricants

100-2500

4-100

50-70 (WC)

Excellent

Yes

Good (MoS2)

Grinding/polishing, reheat treatment Polishing (sometimes) No

Atmosphere

185-205

No increase (MoS2)

Rework required

Visible change

Liquid

Very good (Ni-P) 90-92 (TiC) Very good 93-95 (B4C) Excellent 100-102 Good 85-95 Good 49-71 (C, N) Average to good 60-72

Processing time(b), h

<200

Average, poor on stainless steel Average Excellent

0.02-0.5

Processing pressure

<95

51-640

0.5-13

0

No

Ion nitriding

2-25

Substrate temperature

Line-ofsight

Yes

>RT

>RT

0.1-760 7

0.1-760 10"7-10 300->760

10"7-10-3 Usually atmosphere 8

io- -ioo

10"7-760

Grinding (sometimes)

CVD, chemical vapor deposition; PVD, physical vapor deposition; DLC, diamondlike carbon; TRD, thermoreactive deposition/diffusion process; IBAD, ion-beam-assisted deposition; RT, room temperature, (a) Actual HRC values, or equivalent values derived from microhardness measurements, (b) Includes heating and cooling time, but not precleaning and vacuum pumpdown time. Source: R.B. Alexander & Associates, Huntington Woods, MI

Lubrication, and Wear Technology) of the ASM Handbook as well as the references cited throughout this Chapter.

Organic Coatings Painting and the application of various organic (plastic and rubber) coatings and linings are among the most widely used surface-engineering processes. Paints or linings that act as protective film to isolate the substrate from the environment exist in a number of different forms. Sheet linings, commonly of the vinyl or vinylidene chloride family, are one such type of coating that can be either adhered to the surface to be protected or suspended as a bag within a tank, for example, to provide protection. Hotapplied organisols, or plastisols, again usually of the vinyl family, can also be applied to a surface, typically by dipping or flow coating, to provide a protective film. Powder coatings are being increasingly used to protect concrete-reinforcing rod, as pipeline coatings, and as coating materials in the original equipment manufacturing markets. Fine powders produced from high-molecular-weight resins of the thermoplastic vinyl and fluorinated hydrocarbon families or from thermoset resins of the epoxy and polyester families are applied to the surface to be protected by either electrostatic spray or fluidized-bed deposition. The metal being protected is usually preheated at the time of application, and after application it is reheated to an elevated temperature (generally from 150-315 0 C, or 300-600 0F). The specific time/temperature baking schedule depends on the metal temperature at the time of application and the type of powder being applied. Alternatively, some coating systems are characterized by the application method used. For example, for coil-coated metal sheet (commonly steel or galvanized steel), very specialized high-speed roller application equipment is used to coat the sheet steel as it is unwound from a coil. The paint used in the coil-coating process can be of virtually any generic type, although alkyds, polyesters, epoxies, and zinc-rich epoxy coatings are the most prevalent. Certain lining materials, such as hand lay-up fiberglass-reinforced plastics, are also used to protect steel from corrosion. Such coating systems usually consist of an epoxy primer applied to a blast-cleaned steel surface, followed by one or more polyester gel coats, with one or more layers of a fiberglass veil or woven roving mat laid within the gel coats as reinforcement. The system is then sealed with a layer of the polyester gel coat (a semiclear, 100% solids resin coat). Similarly, rubber linings are used to protect against corrosion. There are various types of rubbers, but they can generally be categorized as prevulcanized or postvulcanized (vulcanized after application). Similarly, rubbers (or elastomers) can be formulated with different hardnesses and chemical resistances. Commonly, a rubber

lining is a composite of two or three different types of rubbers adhered to each other and to the surface. The environmental resistance of some common rubber lining materials is summarized in Table 2. Paints Despite the importance of the coatings and linings discussed previously, the most commonly used organic materials are the liquid-applied (usually by brush, roller, or spray) coating and lining materials, that is, paints. Liquid-applied organic coatings have four basic components: a resin, a solvent, pigments, and other miscellaneous compounds. The resins, often called binders, are identified by their generic type, as indicated in Table 3. These are based on the organic compound structure that makes up the resin. The solvent can be either water or an organic solvent. Increasing environmental regulations are limiting the use of organic solvent-base systems. The pigments are added for such functions as rust inhibition, decreased permeability, to provide color, or to increase resistance to ultraviolet or weathering conditions. Typical pigments include zinc phosphate, zinc molybdate, zinc phosphorus silicate, zinc chromate, and strontium chromate. Miscellaneous compounds added include dryers, flowcontrol and gloss-control agents, and suspension agents. Although the resin or organic binder of the coating material is most influential in determining the resistance and properties of the paint, the type and amount of pigments, solvents, and additives will dramatically influence the application properties and protective capability of the coating system. Furthermore, hybridized systems can be formulated that are crosses between the categories. For example, an acrylic monomer or prepolymer can be incorporated with virtually any other generic type of resin to produce a product with properties that are a compromise between the acrylic and the original polymer. In many cases, this is advantageous, as in the mixing of Table 2 Common name

Environmental resistance of common rubber lining materials ASTM D 1418 designation

Butadiene rubber BR Natural rubber, NR, IR isoprene rubber Chloroprene CR rubber Styrene-butadience SBR (nitrile) rubber Acrylonitrile-butadiene NBR (nitrile) rubber Isobutylene-isoprene HR (butyl) rubber Ethylene-propylene EPM, EPDM (-diene) rubber Silicone rubber VMQ Fluoroelastomer FKM

Resistance to(a): Ozone

Oxidation

Water

Alkalies

Aliphatic

Aromatic

Halogenated

Alcohol

Acids

Permeability to gases

P P

G G

E E

F-G F-G

P P

P P

P P

G G

F-G F-G

Low Low

VG

VG

G

E

G

F

P

G

F-E

Low-medium

P

G

E

F-G

P

P

P

G

F-G

Low

P

F-G

E

F-G

E

G

P

VG

E

Very low

E

E

E

E

F

F-G

P

VG

G-E

Very low

O

E

E

G-E

P-G

P

P-F

P-G

F-E

Medium

E O

E O

E VG

P-F F-G

P-G E

P-G E

F G

F VG

G-VG F-E

(a) O, outstanding; E, excellent; VG, very good; G, good; F, fair; P, poor

High Low

vinyls and acrylics or the heat curing of alkyds and acrylics. In other cases, such as with an epoxy, acrylic modification can be a detriment. Table 3 lists the properties and applications of the principal coating resins. Additional information on coating resins can be found in Ref 1 to 3. Basic Function. Paint coatings may be applied for appearance, to meet functional requirements, or to meet combined function and appearance Table 3 Properties and applications of coating resins Resin

Acrylic

Alkyd

Forms available

Solvent, waterborne, powder

Drying method

Air dry, bake

Solvent, waterborne Air dry, bake

Favorable characteristics

Poor-fair adhesion, tendency to be brittle

Moderate, high

High gloss, flexibility, good durability, versatility

Poor alkali resistance, generally not hard, tendency to yellow, depending on resin Abrasion resistance, hardness, gloss, sensitivity to solvents

Low, moderate

Rapid chalking on exterior exposure, poor resistance to oxidizing acids, yellows in clears Adhesion, recoatability, high baking temperatures Low solids content, fair to good exterior durability, low flash point solvents Darkens, can only be used in darkcolored coatings

Moderate, high

Fair adhesion, may hydrolize under certain conditions Some types yellow and chalk readily on exterior exposure Tendency toward brittleness. Unmodified types require high baking temperatures Generally low solids, low flash points

High

Solvent

Epoxy

Solvent, waterborne, powder

Fluorocarbon

Solvent, powder

Bake

Highest exterior durability, chemical resistance

Nitrocellulose(a)

Solvent

Air dry, bake

Extremely fast drying, good hardness, abrasion resistance

Phenolic

Solvent, waterborne

Air dry, bake

Hardness, adhesion, resistance to chemicals, corrosion

Polyester

Solvent, waterborne, powder Solvent, waterborne, powder

Air dry, bake

High gloss, hardness, chemical resistance, high film build Chemical resistance, abrasion resistance, hardness, exterior durability High heat resistance, exterior durability, gloss and color retention

Polyurethane

Air dry, bake

Air dry, bake

Silicone

Solvent, waterborne

Air dry, bake

Vinyl

Solvent, powder

Air dry, bake

(a) Must be modified with other resins

Cost

Water white, outdoor durability, chemical, heat resistance

Chlorinated rubber

Air dry

Unfavorable characteristics

Water, alkali, acid resistance

Excellent adhesion, chemical resistance, flexibility, abrasion resistance, hardness

Chemical resistance, flexibility, fast air dry, formability, resistance to acid, alkali, abrasion

Moderate

Uses

Automotive topcoats, appliances, coil coatings, aluminum siding, general industrial use Trade sales enamels, trim paints, exterior enamels, general metal finishing

Maintenance coatings, ship bottom paints, swimming pool paints, chemical process equipment Maintenance paints, automotive primers, appliances, metal products

High

Coil coatings, siding

Low, moderate

Furniture finishes, touch-up lacquers, general-purpose product finishes, aerosol lacquers

High

Can linings, tank linings, maintenance paint on metals Wood finishes, coil coatings, specialty bake coats Aircraft finishes, maintenance paints, metal and plastic coatings Any finish for high heat resistance, exterior metal coatings

Moderate, high

High

Moderate

Can and tank linings, maintenance paints, metal decorating paints

needs. If the basic purpose is appearance, the gloss, color, and retention of these properties in service are emphasized. In some applications, functional requirements are of equal importance to appearance. On office furniture, for example, paint films must provide attractive appearance and resist marring and abrasion. On automobiles, paint films must be attractive in appearance, easily applied, and readily repaired, but be resistant to abrasion, marring, and impact as well as capable of protecting the underlying metal from corrosion. In other applications, such as corrosion protection of tanks or chemical equipment, the functional requirements of the paint film are of prime concern. Corrosion resistance is the most important functional requirement. Corrosion of steel and cast iron occurs in all common environments. The rate and extent of corrosion vary from mild attack in dry, clean environments to highly accelerated attack in marine or industrial areas where corrosive fumes are present in air. Table 4 lists paints selected for service in a wide range of corrosive conditions. The rate of the base metal corrosion where paints are used should not exceed approximately 1.3 mm/yr Table 4 Organic coatings selected for corrosion resistance in various environments Coatings

Applications

Outdoor exposure Oil paints Alkyds Amino resin-modified alkyds Nitrocellulose lacquers Acrylics

Buildings, vehicles, bridges; maintenance Trim paints, metal finishes, product finishes Automotive, metal awnings, aluminum siding Product finishes, aerosol lacquers Automotive finishes

Marine atmosphere Alkyds, chlorinated rubber, phenolics, epoxies, vinyls, vinyl-alkyds Urethanes

Superstructures and shore installations Clear marine varnishes

Water immersion Phenolics Vinyls Chlorinated rubber Urethanes Epoxies

Ship bottoms Ship bottoms, locks Ship bottoms, swimming pools Clear marine varnishes Ship hulls, marine structures

Chemical fumes Epoxies, chlorinated rubber, vinyls, urethanes

Chemical-processing equipment

Extreme sunlight Vinyls Acrylics Silicone alkyds

Metal awnings Automotive finishes Petroleum-industry processing equipment

High humidity Amino resin-modified alkyds Epoxies Catalyzed epoxies, chlorinated rubber, phenolics

Refrigerators, washing machines Air conditioners Maintenance; chemical and paper plants

High temperature Epoxies Modified silicones Silicones Inorganic zinc-rich

Motors, piping, 120 0 C (250 0F) max Stove parts, roasters, 205 0 C (400 0F) max Stove parts, roasters, 290 0C (550 0F) max; aluminumpigmented paints 650 0C (1200 0F) max Structural steel, chimneys to 370 0 C (700 0F)

(50 mils/yr). For corrosion rates above this, both in atmospheric and immersion service, or where catastrophic failure is of concern, paints should generally not be used. For more severe corrosion applications, alternative corrosion prevention measures such as the use of more corrosion-resistant alloys, sheet or rubber coatings and linings, fiberglass layups, and metallic coatings and claddings should be considered. In service, paint films are frequently required to resist exposure to highly deleterious materials. For example, decorative finishes, such as those on home laundry equipment, must resist detergents, and paint films on equipment powered by gasoline engines must withstand attack from gasoline. Paint films also may be required to resist acids and alkalis, solvents, staining, heat, impact, marring, and abrasion. Some coatings must be able to withstand flexing without cracking or flaking. Table 5 lists paints that have proven successful in withstanding mechanical and chemical action. Surface Preparation. The importance of proper surface preparation to the durability of any coating system cannot be overemphasized. Without proper surface preparation, the finest paint, applied with the greatest of skill, will fall short of its maximum performance or may even fail miserably. A coating can perform its function only so long as it remains intact and firmly bonded to the substrate. An adequately prepared surface not only provides a good anchor for the coating, but also ensures a surface free of corrosion products and contaminants that might shorten the life of the film by spreading along the coating/substrate interface and destroying adhesion or by actually breaking through the coating. Before being painted, metals usually are exposed to one or more fabricating processes, such as rolling, stamping, forming, forging, machining, and heat treating. In these processes, the metal surfaces pick up various contaminants that can either interfere with the adhesion of the paint film or allow corrosion to progress beneath the paint film and cause it to fail prematurely. The principle surface contaminants that adversely affect the performance of paint films include oils, greases, dirt, rust, mill scale, water, and Table 5 Paints selected for resistance to mechanical or chemical action Action

Abrasion Impact Marring Flexing Acids Solvents Detergents Staining Gasoline Alkalis Heat

Paint

Vinyls; plastisols; polyurethanes Epoxies; vinyls; polyurethanes Thermosetting acrylics; vinyls Epoxies; vinyls Chlorinated rubber; vinyls; epoxies Epoxies; phenolics Thermosetting acrylics; epoxies Thermosetting acrylics Alkyds; epoxies Phenolics Alkyd-amines; silicone resins

salts such as chlorides and sulfides. These contaminants must be removed from the surface before paint is applied. Selection of cleaning process is governed by the soil or contaminant to be removed, the degree of cleanness required, the type of paint to be applied, and the size, shape, material, and end use of the part. In addition, the speed with which the process runs will affect the cleaning characteristics. Methods of cleaning metal surface can be classified as: Mechanical cleaning, including power brushing, grinding, and abrasive blasting Chemical cleaning, including emulsion cleaning, solvent cleaning, vapor degreasing, alkaline cleaning, acid cleaning, pickling, and steam cleaning To meet rigid requirements for surface cleanness, mechanical and chemical cleaning methods can be used in conjunction. For example, before structural steel intended for an application involving exposure to corrosive chemical environments is painted, oil, grease, rust, mill scale, and any other surface contaminants must be completely removed. Chemical paints strippers or solvent cleaners are often used prior to mechanical cleaning to remove oil, grease, or old paint. More detailed information on various surface cleaning methods can be found in Surface Engineering, Volume 5 of the ASM Handbook.

Ceramic Coatings and Linings The ceramic coatings and linings discussed in this section include glass linings, porcelain enamels, cement linings, and high-performance ceramic coatings applied by various processing methods. Additional information on ceramic and cermet (ceramic-metal) coatings applied by vapor deposition processes or thermal spraying can be found later in this chapter. Glass Linings Glass, particularly low-expansion borosilicate glass (e.g., Pyrex), finds numerous application as a material of construction in the chemical, food, pharmaceutical, petrochemical, and electronics industries. The principal attributes of interest in these applications are chemical durability, thermal stability, resistance to thermal shock, ease of cleaning, transparency and economy—requirements identical to those in the laboratory but now scaled up to reflect process capacity. Modular glass components may be used to make systems such as distillation columns, boilers, scrubbers, and so on. Glass piping systems, glass-lined steel reactor vessels, and glass heat exchangers are some examples of applications in which corrosion or

product purity may dictate the use of glass. Increasingly, glass reaction vessels are also being used in the treatment of hazardous (and, again, corrosive) wastes. Glass-lined steel vessels are used as reaction vessels in applications in which the additional mechanical integrity of the outer steel shell is a safety imperative. An acid recovery or concentration unit is a good example. Thus, a sulfuric acid concentrator would consist of a glass-lined steel container with a horizontal tantalum boiler. Waste sulfuric acid streams can be concentrated to 90 wt% H2SO4. The recovered concentrate may still contain trace amounts of organics. Vapor that arises during the process is condensed in yet another glass heat exchanger and then goes in to a glass-phase separator where the organic and the water are separated for reuse. The glass linings or vessel walls are completely inert to sulfuric acid in all concentrations, so that the purity of the product is dependent only on the quality of the feed and makeup fluids. There are two additional points: • •

Abrasion resistance is not an issue with glass linings but can be a concern when alternate fluorocarbon linings are considered. Glass lining can be damaged through internal mechanical impact or abuse; in such cases, the repair is performed with tantalum patches or a similar material.

Modular glass components are employed in solvent recovery applications where the use of glass ensures product purity. Porcelain Enamels Porcelain enamels are glass coatings applied primarily to products or parts made of sheet steel, cast iron, or aluminum to improve appearance and to protect the metal surface. Porcelain enamels are distinguished from other ceramic coatings on metallic substrates by their predominantly vitreous nature and the types of applications for which they are used. These coatings are differentiated from paint by their inorganic composition and coating properties. They are fused to the metallic substrate at temperature above 425 0C (800 0F) during the firing process. Detailed information on the types of porcelain enamels and methods to apply them can be found in Ref 4. The most common applications for porcelain enamels are major appliances, water heater tanks, sanitary ware, and cookware. In addition, porcelain enamels are used in a wide variety of coating applications, including chemical processing vessels, agricultural storage tanks, piping, pump components, and barbecue grills. They also are used for coatings on architectural panels, signage, specially executed murals, and substrates for microcircuitry. Porcelain enamels are selected for products or components where there is a need for one or more special service requirements that

porcelain enamel can provide. These include chemical resistance, corrosion protection, weather resistance, abrasion resistance, specific mechanical or electrical properties, appearance or color needs, cleanability, heat resistance, or thermal shock capability. Corrosion Resistance. Porcelain enamel is widely used because of its resistance to household chemicals and foods. Mild alkaline or acid environments are generally involved in household applications. Table 6 presents examples of corrosive environments for which porcelain enamels are widely used for long periods of service. Special enamel compositions are available to resist most acids, except for hydrofluoric or concentrated phosphorics, to temperatures of 230 0C (450 0 F). These compositions also resist alkali concentration to pH 12 at 93 0C (200 0 F). Wear Resistance. The hardness of porcelain enamels ranges from 3.5 to 6.0 on the Mohs scale. Porcelain enamels show a high degree of abrasion resistance. Under abrasive test conditions where plate glass retains 50% specular gloss, porcelain enamel compositions retain from 35 to 85% specular gloss. Subsurface abrasion resistance varies with processing variables that affect the bubble structure of the enamel, that is, gas bubbles frozen in during cooling of the enamel. A decrease in abrasion resistance occurs with an increase in the number or size of gas bubbles. Enamel compositions are available that contain crystalline particles (from mill additions or devitrification heat treatment) that increase abrasion resistance as much as 50%. Concrete and Cementatious Coatings and Linings Cementatious linings have become one of the most widely used construction materials in designing protective linings for industrial installation in which high temperatures, aggressive corrosive media, and complicated substrate geometry exist, such as floors, trenches, sumps, ducts, chimneys, and other air pollution control equipment. They are used in various industries, including power, steel and metalworking, chemical, pulp and paper, refinery, waste treatment, and mining. Table 6 Applications in which porcelain enamels are used for resistance to corrosive environments Corrosive environment Temperature °C

op

PH

Corrosive medium

Bathtubs Chemical ware

<49 <100 <100 175-230

<120 <212 <212 350-450

5-9 12 1-2 1-2

Home laundry equipment Range exteriors Range oven lines, conventional Range burner grates Kitchen sinks Water heaters

<71 21-66 66-315 66-590 <71 <71

<160 70-150 70-600 70-1100 <160 <160

11 2-10 2-10 2-10 2-10 5-8

Water; cleansers Alkaline solutions All acids except hydrofluoric Concentrated sulfuric acid, nitric acid, and hydrochloric acid Water; detergents; bleach Food acids; cleansers Food acids; cleansers Food acids; cleansers Food acids; water; cleansers Water

Application

Inorganic monolithic linings have proved themselves in these industries because of their chemical resistance to both high and low concentrations of strong acids and solvents, thermal insulation that protects the substrates from extremely high temperatures, temperature resistance to 870 0C (1600 0 F), good compressive and flexural strength for environments in which stress and strain are factors, and abrasion resistance. Monolithic linings can be applied by cast or gunite (shotcreting) methods over old and new steel or concrete, as well as by brick and mortar masonry. Certain disadvantages were encountered during the development of the acid-resistant silicate cements. The silicate cements were not resistant to alkalies, hydrofluoric acid (HF), and fluoride salts. The sodium silicate (Na2SiO3) cements formed a growth salt when exposed to H2SO4 that put undue internal stresses on the structure of the material. Inorganic monolithic linings also have a certain amount of permeability compared to organic surfacing materials. Over time, acid can penetrate the lining and eventually reach the surface of the substrate. The problem is now being combated by using a dual-lining system, which includes a chemically resistant elastomeric membrane applied to the surface of the substrate. Dual Linings. In recent years, the trend toward using the superior technology of dual linings has emerged and is being recommended where corrosion problems occur. Figure 1 shows the design of a typical membrane/monolithic system in the chemical industry. Condensation occurs not only on the face of the acid-resistant lining, but it can also penetrate and condense on the substrate to be protected. Although acid-proof monolithic linings offer the proper chemical resistance, they are inherently inelastic, or brittle. In time, monolithic linings may tend to crack and absorb Chemical- and moisture-resistant monolithic lining

Permanently flexible elastomeric expansion joint

Flexible chemical- or moisture-resistant membrane

Steel or concrete substrate

Fig, 1 Schematic of a chemical-resistant dual-lining system that provides " double protection to the substrate in the form of a flexible membrane and a rigid surface layer. The flexible, corrosion-resistant membrane is applied in direct contact with steel or concrete substrates. It is then covered by the monolithic cement lining, which provides protection over a broad pH range as well as against high temperatures.

acids in acid gas condensate; therefore, it is advantageous to have a backup membrane. In many applications, the coefficient of thermal expansion of the monolithic lining may not match that of the substrate. Therefore, a flexible membrane will help accommodate stresses resulting from these differences in thermal expansion, as well as other mechanically induced stresses. High-Performance Ceramic Coatings and Linings High-performance ceramic coatings include high-temperature glasses; high-temperature coatings based on oxides, carbides, nitrides, and silicides; and cermets. Ceramic coatings are applied to metals to protect them from oxidation and corrosion at room temperature and at elevated temperature. Special coatings have been developed for specific uses, including wear resistance, chemical resistance, high reflectivity, electrical resistance, and prevention of hydrogen diffusion. Ceramic-coated metals are used for furnace components, heat treating equipment, chemical processing equipment, heat exchangers, rocket motor nozzles, exhaust manifolds, jet engine parts, and nuclear power plant components. These coating materials are deposited by firing (sintering) a slurry or slip that has been applied to a metal surface by spraying or dipping, by various thermal spray techniques discussed later in this chapter, by the pack cementation process described in Chapter 5, fluidized-bed processing, trowel coating, and electrophoresis. The method of applying the coating is restricted by the type of coating, the type of metal to be coated, and the size and configuration of the work. More detailed information on ceramic coating methods can be found in Ref 5. Silicate Glasses. Coatings prepared from glass powders, with or without additions of refractory compounds, have the greatest industrial usage of all ceramic coatings. Glass coatings are used for such applications as aircraft combustion chambers, turbines and exhaust manifolds, and heat exchangers. Variations in composition of the silicate-based glass are virtually unlimited. The spray-sinter process is the most commonly used method to deposit these coatings. Under certain conditions, electrostatic spraying also can be used. Oxides. Coatings based on oxide material provide underlying metal, except refractory metals, with protection against oxidation at elevated temperature and with a high degree of thermal insulation. Alumina (Al2O3), zirconia (ZrO2), and chromium oxide (Cr2O3) are the oxides most commonly used as coatings. Alumina coatings are hard and have excellent resistance to abrasion and good resistance to corrosion. Zirconia is widely used as a thermal barrier because of its low thermal conductivity. Chromium oxide coatings are used in a variety of wear-resistant applications (see Chapter 8 for details). Oxide coatings are usually applied by flame spraying or plasma spraying, although chromium oxide can be applied by spraying/dipping plus sintering.

Carbides as ceramic coatings are principally used for wear and seal applications, in which the high hardness of carbides is an advantage. These applications include jet engine seals, metalforming tools, tools and dies for ceramic and plastic processing, components used in the chemical and general processing industries, and machine elements. Commonly employed carbides include titanium carbide (TiC), silicon carbide (SiC), tungsten carbide (WC), chromium carbide (Cr3C2), and boron carbide (B4C). These ceramics are applied by thermal spraying or vapor deposition techniques. Nitrides are used for wear-resistant applications, most notably surface engineering of cemented carbide cutting tools and tool steels. Titanium nitride (TiN) is the most frequently employed coating, but titanium carbonitride (TiCN), hafnium carbide (HfC), titanium aluminum nitride (TiAlN), titanium zirconium nitride (TiZrN), and chromium nitride (CrN) have also been used commercially. These coatings are applied by vapor deposition techniques (Fig. 2). Silicides are the most important coating material for protecting refractory metals against oxidation. Silicide-based coatings protect by means of a thin coating of silica that forms on the coating surface when heated in an oxygen-containing atmosphere. Examples of silicide coatings include

(a)

(b)

(C) FlC 5 2 Nitride ceramic coatings deposited on cemented carbide substrates by physical vapor deposi* tion. (a) TiN. (b) TiCN. (c) TiAlN

molybdenum silicide (MoSi2), niobium silicide (NbSi2), and tantalum silicide (TaSi2). To improve the stability and adherence of the silica coating, other elements, such as chromium, boron, or aluminum are added to the coating formula. Silicide coatings are usually applied by some variation of the vapor-deposition process. H o t D i p Coatings Hot dip coatings are predominantly used to improve the aqueous corrosion of steel. Batch and Continuous Processing Processing of hot dip coatings involves either batch or continuous processing. The continuous process is more advantageous for sheet steels, whereas the batch process is normally used for individual parts. Details of the processing techniques are outlined in Ref 6. In the batch galvanizing process, the two types of conventional practices are the wet process and the dry process (Ref 7). The wet process involves a flux blanket on the top of the molten zinc bath to remove impurities from the surface of the steel and also to keep that portion of the surface of the zinc bath, through which the steel is immersed, free from oxides. In the dry process the steel is usually cleaned, treated with an aqueous solution, dried, and then dipped in the molten zinc bath. The molten zinc bath is maintained at temperatures between 445 and 455 0C (830 and 850 0F) and times in the range of 3 to 6 min. The time of immersion is used to control the thickness of the coating, which consists of iron-zinc alloy phases at the interface along with a top coat of pure zinc. Good cooling control is necessary because the zinc can continue to react with the substrate to produce further alloying and detrimentally affect the properties of the coating such as the spangle finish (or grain size). In continuous hot dip processing, welded coils of steel are coated at speeds of 200 m/min. The flux or Cook-Norteman line is similar to the batch process in that the sheet is cleaned and fluxed in line prior to immersion. The hot-processed continuous line is more complex in that the steel sheet is first cleaned at temperature in a reducing environment, annealed above the recrystallization temperature of about 700 0C (1290 0F) and then immersed in the molten bath. As the strip exits the bath, the thickness of the molten metal film is controlled by gas wiping dies that remove excess coating metal. After coating, the sheet is either cooled by forced air or subjected to an in-line heat treatment, called galvannealing, before being rewound into coil or sheared into cut lengths at the exit of the line. Coating Microstructure In general, the coating microstructure consists of the substrate, the interfacial alloy layer, and the overlay cast structure. Depending on the type

of coating, the microstructure and composition of these constituents changes. As expected, the substrate plays a major role in the type of coating obtained, and substrate composition can affect growth kinetics of the phases formed. For example, if the substrate contains silicon then the well-known Sandelin Effect can influence the iron-zinc phase reaction and consequently the thickness of the coating (Ref 8). Similarly, alloy additions to the steel to improve sheet formability, for example, interstitialfree (IF) steels with titanium, titanium/niobium, and phosphorus, can influence the microstructure of the iron-zinc phases in galvanized and galvannealed steel (Ref 9). Substrate grain size has also been shown to greatly affect the nucleation of the iron-zinc phases. In aluminum-containing baths, the structure formed first is an inhibition layer that is dependent on bath composition and prevents further alloying for a certain short time before the inhibition layer becomes unstable (Ref 10). When the zinc galvanizing bath contains only a trace of aluminum, zinc attack of the substrate is uniform and the phases that form are governed by the iron-zinc binary phase diagram. In zinc baths containing aluminum, the stability of the inhibition layer governs the amount of iron-zinc phases formed. Once the inhibition layer is no longer stable, outbursts or rapid growth of iron-zinc phases occur during hot dipping (Ref 11). During the thermal cycle of the galvannealed process, the inhibition layer dissolves and iron-zinc phase layer growth occurs in a controlled manner until the entire coating is made up of iron-zinc phases (Ref 12). Both galvanized and galvannealed alloy phase growth are determined by Fe-Al-Zn ternary diffusion, and the overlay cast microstructure greatly depends on aluminum content of the bath. The pure-zinc and low-aluminum coatings form an overlay of pure zinc (r|) phase. Zn-5 wt%Al (Galfan) solidifies as eutectic microstructure, and the Zn-55 wt%Al (Galvalume and Zincalume) solidifies as aluminum dendrites with zinc-rich interdendritic regions. The aluminum coatings (type I and type II) either form overlays of aluminum-silicon or aluminum alloy, respectively.

Galvanized Coatings Surface Finishes. Galvanized coatings are commonly characterized by surface spangles. In cross section, an Fe2Al5(Zn) inhibition layer develops first, preventing any iron-zinc intermetallic phase formation. The overlay layer is made up of dendrites of pure zinc (r\) phase and appears as a polycrystalline structure. The three surface finishes commonly produced are: • • •

Regular spangle, where the coating solidifies from the dipping temperature by air cooling Minimum spangle, where the coating is quenched using water, steam, chemical solutions, or by zinc powder spraying Extra-smooth temper roll finish carried out as an additional operation with regular and minimum spangle material

Alloying Effects. Aluminum is probably the most important alloying element added to the hot dip galvanizing bath, with different levels required to produce different properties in the bath (Ref 13). Aluminum levels of 0.005 to 0.02 wt% are added to brighten the initial coating surface. The effect is related to the formation of a continuous Al 2 O 3 layer on the coating surface that inhibits further oxidation by acting as a protective barrier layer. This effect is also responsible for the reduced atmospheric oxidation of the zinc bath. In addition, aluminum in the range of 0.1 to 0.3 wt% is added to the zinc bath to suppress the growth of brittle iron-zinc intermetallic phases at the steel coating interface by forming the Fe2Al5 (Zn) inhibition layer. The end of this incubation period is marked by the disruption of the initial layer, followed by rapid attack of the substrate steel. An increase in the incubation period depends on increased aluminum bath composition using a low bath temperature, having low bath iron content, agitation, and the presence of solute additions in the steel. Thus, during commercial production, the immersion time is kept below the incubation period in order to obtain a highly ductile product. Corrosion Protection. Zinc coatings add corrosion resistance to steel in several ways. As a barrier layer, a continuous zinc coating separates the steel from the corrosive environment. By galvanic protection, zinc acts as a sacrificial anode to protect the underlying steel at voids, scratches, and cut edges of the sheet. The sacrificial properties of zinc can be seen in a galvanic series where the potential of zinc is less noble than steel in most environments at ambient temperatures. In addition, after dissolution of the zinc metal, zinc hydroxide can precipitate at the cathodic areas of the exposed steel, forming a secondary barrier layer. Zinc corrodes at a slower rate than the steel substrate, although the corrosion rate of zinc varies depending on the atmosphere to which it is exposed (Ref 14), as shown in Fig. 3. Fabricability. During forming, especially stretch-forming operations, increased friction of the zinc can result in less total stretch before fracture. In severe forming operations, galling and coating pickoff can also occur. Furthermore, coating particulate buildup on die surfaces can lead to poor surface appearance of formed parts. Proper lubrication is essential in the design of any forming process, especially when forming zinc-coated parts. Weldability of zinc coatings is also an important property of the coating. Spot weldability properties are particularly important because most galvanized product is joined in this manner. Zinc coatings reduce the life of welding electrodes because the copper electrode alloys with zinc. This effect leads to higher resistance, localized heating, and increased pitting and erosion of the electrode tip. As a result, manufacturing costs increase because lower tip life reduces productivity due to frequent downtime in the welding operation to redress tips. Paintability. Although zinc coatings are often used in the as-coated state, some applications call for a painted surface, and therefore paintability is an important design property of the coating. It has been shown that

Service life, yr

Zinc coverage, g/m2 (oz/ft2)

Thickness of zinc, |jm (mils) Fig. 3 Service life (time to 5% rusting of steel surface) versus thickness of zinc ^* for selected atmospheres. Shaded area is thickness range based on minimum thicknesses for all grades, classes, and so forth, encompassed by ASTM A 123 and A 153. Source: Ref 15

large-spangle material is difficult to paint; therefore, most painted products are either minimum spangle or temper rolled. It is usually necessary to pretreat a hot dip galvanized coating with a zinc phosphate or complex oxide thin coating before prepainting. In the automobile industry, following the pretreatment most automobile bodies are primed with an electrophoretic paint (e-coat), and, as a result, resistance to e-coat cratering is an important property. At high e-coat voltages, sparking as a result of exceeding the dielectric properties of the deposited paint film causes localized heat generation, film disruption, and premature curing of the paint. After paint curing, these sparked areas form pinpoint craters that result in a paint surface with a detrimental appearance. Therefore, resistance to e-coat cratering, expressed in cratering threshold voltage, is an essential paintability property (Table 7). Table 7 Effects of hot dip coatings on threshold voltages for cratering of cathodic electrophoretic primer Type of surface

Uncoated bare steel Zinc Zinc-iron Zn-55A1 Aluminum Source: Ref 16

Cratering threshold, V

>400 275 225 375 >400

Galvanneal Coatings Galvanneal coatings are essentially diffusion coatings that expose the zinc galvanized steel to an annealing temperature around 500 0C (930 0F) to produce a fully alloyed coating containing iron-zinc intermetallic phases. This is accomplished by inserting heating and cooling capacity above the liquid zinc pot in order for the galvannealing process to be continuous. Good process control requires that the effects of heating rate, hold temperature and time, and cooling rate on the iron-zinc reaction kinetics be well understood. Galvanneal coatings have been classified as (Ref 12): • •

•

Type 0: Underalloyed coating containing predominantly £-phase Type 1: Optimal alloyed coating with less than a 1 |uim interfacial 7-layer and a top layer containing 8-phase interspersed with a small amount of £-phase Type 2: Overalloyed coating with a 7-phase more than 1 |jim and an overlay of 8-phase containing basal plane cracks

Fabricability. Formability is an important property in galvanneal coatings because iron-zinc intermetallic phases are considered brittle. As a result, powdering and flaking of the coating can occur during the forming operation, resulting in reduced corrosion resistance and impaired paintability. The type 1 coating was found to have the best formability properties (Ref 17), but as in most forming operations lubrication to improve metal flow is essential. Spot weldability of galvanneal coatings are improved over galvanized coatings because it is more difficult to these iron-zinc phases to alloy with the copper electrode. Paintability is also better than that of galvanized coatings because of the microscopically rough surface formed as a result of the iron-zinc alloy phases throughout the coating. However, galvanneal coatings are more prone to cratering during e-coating (Table 7). Conversely, corrosion resistance can be slightly reduced because of the increased iron in the coating from the iron-zinc phases; the galvanic potential is not as great as it is for pure zinc.

Zinc-Aluminum Coatings Zn-5AI alloy coating (Galfan) is near the eutectic point in the aluminum-zinc equilibrium phase diagram. Two compositions have been reported based on additions to the eutectic composition: small (up to about 0.5%) mischmetal additions containing lanthanum and cerium and additions of 0.5% Mg. These additions are made to improve the wettability and suppress bare spot formation as well as to produce a typical "minimized spangle" structure. The microstructure of Galfan is characterized by a two-phase structure, a zinc-rich proeutectoid r\ -phase surrounded by eutectic phase consisting of lamellae of a-aluminum and zinc-rich T]-phase. However, the microstructure can be varied depending on the

cooling rate. In the range of normal bath temperatures, 420 to 440 0C (790-825 0F) there is no visible intermetallic layer or at least an extremely thin layer (<0.5 juim) at the interface between the steel substrate and the overlay coating. Thus, Galfan coatings have excellent formability and cutedge corrosion protection. Zn-55AI alloy coating (marketed under the tradename Galvalume) contains about 1.5% Si added for the purpose of preventing an exothermic reaction at the coating overlay/substrate steel interface. As a result, the coating contains a-aluminum dendrites, zinc-rich interdendritic regions, and a fine dispersion of silicon particles, along with a prominent Fe-AlZn intermetallic alloy layer at the interface between the steel substrate and the overlay coating. The surface of the coating contains characteristic spangles that consist of aluminum dendrites with a clearly measurable dendrite arm spacing. Cooling rate after dipping can significantly refine the microstructure of the coating, increasing the number of silicon particles and constraining the growth of aluminum dendrites. Initially, the atmospheric corrosion of the Zn-55A1 coating takes place in the zinc-rich interdendritic regions, enabling the coating to exhibit galvanic protection. As the coating continues to corrode, the zinc corrosion products become trapped in the interdendritic regions and act as a further barrier to corrosion. Eventually, the aluminum dendrites, which also acted as a barrier layer, add to the corrosion protection, as does the Fe-Al-Zn intermetallic alloy layer. This results in a parabolic type of corrosion as evidenced in Fig. 4. Although its galvanic protection is less than that provided by galvanized coatings, Zn-55A1 is generally adequate to protect against rust staining at scratches and cut edges of the steel sheet. Aluminum Coatings

Average corrosion loss, |tim

Average corrosion loss, mils

Aluminum coatings are produced as type 1 coating, a thin (20-25 (xm) aluminum-silicon alloy coating, and type 2, a thicker (30-50 |xm) pure

Exposure time, yr Fig.&# 4 Corrosion losses of hot dip coatings in the industrial environment of Bethlehem, PA. Source: Ref 18

aluminum coating. Silicon is present in type 1 coatings in the range of 5 to 11 wt% to prevent formation of a thick iron-aluminum intermetallic layer at the coating/steel substrate interface. Instead, a thin Fe-Al-Si intermetallic layer is formed, allowing for good formability and coating adherence. These coatings are intended primarily for applications requiring improved appearance, good formability, and resistance to high temperatures, as in automobile exhaust components. The type 2 coating has a microstructure containing a pure aluminum overlay and a thick iron-aluminum intermetallic alloy layer. Thus, the formability and adhesion of this coating is limited by the poor ductility of the alloy layer. Nevertheless, the coating is used for outdoor construction applications (e.g., roofs, culverts, etc.) that require resistance to atmospheric corrosion (Table 8). The aluminum outerlayer offers excellent corrosion resistance because of the good barrier properties provided by the increased thickness of the coating (Fig. 4). Terne Coatings Lead-tin alloy hot dip coatings are widely known as terne coatings. Generally, 3 to 15% Sn is added to the bath in order to facilitate wetting of the steel substrate. Terne-coated steel has a long history of use in automotive fuel tanks and tubing because of its excellent weldability, solderability, and formability. Moreover, the low corrosion rate of terne allows it to function primarily as a barrier coating without the evolution of corrosion products that might otherwise clog fuel and hydraulic systems. Due to concerns about the effects of lead in the environment, work is now under way to find suitable replacement materials. Zinc alloy coatings (hot dip zinc-iron or electroplated zinc-nickel) with an organic topcoat are among the most likely candidates. Nickel-terne-coated steel includes an electrolytic flash coating of nickel (1-1.5 g/m2) underneath a conventional lead-tin coating for enhanced corrosion resistance. Applications are similar to the conventional lead-tin alloy coatings described previously.

Table 8 Coating thickness losses for galvanized steel and type 2 aluminized steel in atmospheric exposure Middletown, OH Years exposed

1 2 4 6 10 15

2.6 5.2 9.3 14.5 24.4

Kure Beach, NC Type 2

G90(a)

T>pe 2

G90

mils

Jim

mils

(xm

mils

jun

mils

0.1 0.2 0.37 0.57 0.96

0.5 0.7 1.2 3.1 2.9 5.3

0.02 0.028 0.047 0.12 0.11 0.21

7.0 8.6 12.7 16.2 23.5 26.0

0.28 0.34 0.5 0.64 0.93 1.02

1.4 2.4 3.8 4.5 6.0 6.7

0.04 0.09 0.15 0.18 0.24 0.26

(a) G90 galvanized steel has a coating weight of 0.90 oz/ft2 (270 g/m2). Source: Ref 19

Electrochemical Deposition

Electrochemical methods are well-established processes for applying metal coatings for improved surface properties of materials. Electrodeposition or electroplating is defined as the deposition of a coating by electrolysis, that is, depositing a substance on an electrode immersed in an electrolyte by passing electric current through the electrolyte. The process can take place in an aqueous electrolyte near ambient temperatures (called aqueous solution electroplating) or in a fused metal salt at high temperatures (called metalliding or fused-salt electroplating). Modifications of the electroplating process include occlusion or composite deposition plating. Excellent reviews of electroplating can be found in Ref 20 and 21. Wearand corrosion-resistance data for selected electrochemical coating methods are summarized in Table 9. Aqueous Solution Electroplating Aqueous solution electroplating provides decorative and protective finishes for use at ambient temperatures and in a variety of environments. Table 9 Characteristics of wear- and corrosion-resistant electrochemical finishes for engineering components Characteristic Maximum working temperature, 0 C Nontoxicity Covering complex shapes\ Thickness range, jim Wear Hardness, HV

Copper

Electroplated nickel

Electroless nickel

Electroless nickel + chromium

Chromium

Electroplated nickel + chromium

Comment

50

650

550

650

550

650

Excellent Medium

Very good Medium

Very good Excellent

Very good Very poor

Very good Very good

Very good Medium

12.5-500

12.5-500

12.5-500

12.5-500

60-150

200-300

Low friction, antistick

Poor

Poor

850-950 450-500 (900-1000 after heat treatment) Medium Excellent

12.5-500 + 25-50 12.5-500 + 25-50 Grinding needed over 200 |xm Indication of 850-950 850-950 abrasive-wear resistance Excellent

Excellent

Resistance to impact

Medium

Very good

Medium

Medium

Medium

Very good

Corrosion resistance

Very poor

Very good

Medium

Poor

Very good

Very good

Typical applications

Buildup; lubricant in forming; heat sink; selective case hardening

Buildup PVC under or molding instead of tools and chromium dies; in corrosive moving conditions; parts in printing process surfaces glass and rubber molds

Molds, tools, valves, rams, pistons shafts, gauges, dies, saw blades

High-temperature, antiseize bolting; ball valves; shafts

Marine crane rams and hydraulics, mine roof supports, print rolls

Source: Ref 22

Short times at higher temperatures possible

Indication of adhesive-wear resistance Thin coats and soft substrates prone to damage Nickel at least 50 jxm for corrosive environments

A main drawback in electroplating is the inability of achieving uniform deposition, which is related to the throwing power of the electrolyte. Throwing power is substrate-shape dependent and also depends on the anode/cathode configuration and the current density, as well as on the composition and conductivity of the electrolyte. A second difficulty is that not all metallic elements can be deposited. Another problem with electroplating is evolution of hydrogen at the electrodes when the cathode efficiency is less than 100%. If a ferrous substrate is to be plated, absorbed atomic hydrogen can cause embrittlement of the part. Unless the parts are heat treated to remove the absorbed hydrogen, they will be brittle and unusable for any application involving elastic strain. Substrate preparation for plating is critical to ensure good adhesion and surface quality. Maximum adhesion depends on both the elimination of surface contaminants in order to induce a metallurgical bond and the generation of a completely active surface to initiate plating on all areas. The cleaning steps for steel substrates usually involve precleaning, intermediate alkaline cleaning, electrocleaning, acid treatments, and anodic desmutting (Ref 23). Electrodeposited metals can have a very good bond to the substrate, but that bond will never be the same as a fusion bond, and poor bonds can go undetected unless techniques are used to test the actual bond strength. The electrodeposited coating usually ends up with a surface texture that is the same as the starting substrate surface texture, unless an intermediate leveling coating (such as copper) is used. Nevertheless, the electroplating process produces a coating with distinct advantages (Ref 24): The workpiece will not undergo distortion or metallurgical changes because the operating temperature of the bath does not exceed 100 0 C (212 0 F). Plating conditions can be adjusted to modify hardness, internal stress, and metallurgical characteristics of the coating. Coatings are dense and adherent to the substrate. The thickness of the coating is proportional to the current density and length of time of the deposition. Although deposition rate seldom exceeds 75 |xm/h (3 mils/h), it can be accelerated by forced circulation of the electrolyte and can be as high as 150 or 200 |xm/h (6 or 8 mils/h) for some metals in high-speed plating. There is no technical limit to the thickness of electrodeposits. Application of coatings is not confined to the line of sight. Although throwing power may be limited, the freedom of anode design and location is helpful. Areas not requiring deposition can be masked.

Only the tank size of the bath limits the dimensions of the part, although large parts such as gun barrels can be the tank itself; as another alternative, brush plating can be used. The process is suitable for automation and has economic advantages over other coating processes. In general, aqueous electroplating has minimal effect on substrate properties (apart from hydrogen embrittlement). Coated substrates can also be heat treated to promote interdiffusion, although this may result in concentration of elements at grain boundaries, causing embrittlement. Specific elemental electrodeposition processes and properties are reviewed in Ref 25; some examples are given here. Nickel plating is widely used for a corrosion- and wear-resistant finish. Typical applications, with a thin top coat of electrodeposited chromium, are decorative trim for automotive and consumer products and office furniture. Nickel deposits are also used for nondecorative purposes for improved wear resistance, for example, on pistons, cylinder walls, ball studs, and so forth. Chromium electroplating is also used as decorative and hard coatings. Colored and tarnish-resistant chromium decorative coatings are produced over a base deposit of copper and/or nickel for applications such as those noted above for nickel. Hard chromium coatings are used for hydraulic pistons and cylinders, piston rings, aircraft engine parts, and plastic molds, where resistance to wear, heat abrasion, and/or corrosion are required. Cadmium and zinc electroplating provides galvanic corrosion protection when coated on steel. Deposit thickness can vary between 5 and 25 |xm (0.2 and 1 mil), and typical applications for both coatings are found in Table 10. Cadmium is preferred for the protection of steel in marine environments, whereas zinc is preferred in industrial environments. Cadmium is also preferred for fastening hardware and connectors because its coefficient of friction is less than zinc. Cadmium is toxic and should not be used in parts that will have contact with food. Precautions for minimizing hydrogen embrittlement should be taken because cadmium plating is more susceptible to such embrittlement than any other plated metal.

Continuous Electrodeposition Electrogalvanizing. The development of continuous electrogalvanizing lines has produced a thin, formable coating that is ideal for deep drawing or painting. Automotive body panels are typically new applications for electrogalvanized zinc, zinc-nickel, and zinc-iron alloys. Processing details applicable to electrogalvanizing can be found in Ref 27. Tinplate is another continuous electrolytic plating process that has been used for the past 200 years to make containers for the long-term storage of food (Ref 28). The typical tinplate product consists of five layers: an

Table 10 Recommended minimum thicknesses and typical applications for zinc and cadmium coatings electrodeposited on iron and steel Coating thickness(a) Service conditions

mils

Chromate finish

Time to white corrosion in salt spray, h

Typical applications

Electrodeposited zinc Mild (indoor atmosphere; minimum wear and abrasion)

5

Moderate (mostly dry, indoor atmosphere; occasional condensation, wear, and abrasion)

8

Severe (exposure to condensation; infrequent wetting by rain and cleaners)

13

Very severe (exposure to bold atmospheric conditions; frequent exposure to moisture, cleaners, and saline solutions; likely damage by abrasion or wear)

25

1

5

0.2

0.2

0.3

0.5

None Clear Iridescent Olive drab None Clear Iridescent Olive drab None Clear Iridescent Olive drab None

12-24 24-72 72-100 12-24 24-72 72-100 12-24 24-72 72-100

Screws, nuts and bolts, wire goods, fasteners

Tools, zipper pulls, shelves, machine parts

Tubular furniture, window screens, window fittings, builders' hardware, military hardware, appliance parts, bicycle parts Plumbing fixtures, pole line hardware

Electrodeposited cadmium

Mild (see above) 8

0.3

Moderate (see above) 13

0.5

Severe (see above) 25 Very severe (see above)(b)

1

None Clear Iridescent Olive drab None Clear Iridescent Olive drab None Clear Iridescent Olive drab None Clear Iridescent Olive drab

12-24 24-72 72-100 12-24 24-72 72-100 12-24 24-72 72-100

Springs, lock washers, fasteners, tools, electronic and electrical components

Television and radio chassis, threaded parts, screws, bolts, radio parts, instruments

Appliance parts, military hardware, electronic parts for tropical service

24 24-72 72-100

(a) Thickness specified is after chromate coating, if used, (b) There are some applications for cadmium coatings in this environment; however, these are normally satisfied by hot dipped or sprayed coatings. Source: Ref 26

innermost layer of steel sheet, a tin-iron intermetallic compound layer, a free-tin layer, a thin passivation layer based on chromium oxide, and a top layer of oil film for lubrication. The corrosion characteristics of tinplate are documented in the literature (Ref 29). Fused-Salt Electroplating Fused-salt electroplating, which is commonly referred to as "metalliding," is a process for surface modification and surface hardening by electrodeposition from fused-salt electrolytes. Two unique aspects of this electrodeposition process are: (1) elements that cannot be plated by conventional processes may plate by fused-salt electrodeposition and (2) if the deposition rate is controlled to match the diffusion rate of the

deposition species in the substrate at the fused-salt temperatures 400 to 900 0C (750-1650 0 F), the substrate will develop a diffusion coating. In electroplating (Ref 30), the molten-salt medium in which the anode and cathode are immersed, consists of a soluble form of the metal to be plated dissolved in a molten-salt solvent, such as an alkali metal halide, that does not participate in the plating process. Coating thickness is determined by the electrical charge, and a sharp interface between the coating and substrate is maintained. At the higher-temperature plating range of the bath, some coating/substrate interdiffusion can occur. In metalliding (Ref 31), the element to be diffused is made the anode of the molten-salt electrochemical cell (usually fluoride) and the substrate is the cathode. A more electrochemically active anode diffuses into the cathode when the electrodes are connected. Because the process is diffusion controlled, a sharp coating/substrate interface does not exist; instead a diffusion gradient in the substrate occurs. Although fused-salt electroplating has only found limited application for refractory metals and ceramic coatings, some success has been obtained with the platinum-group metals. The process conditions for this technology are too stringent and economically unfeasible. Nevertheless, the process requirements for the electrodes, the melt, and cell operation have been outlined (Ref 30). On the other hand, metalliding is a unique electrodeposition process for applying elements that are difficult to electrodeposit on substrates that usually cannot be plated. It is an important process for improving the surface hardness and corrosion resistance of metals without producing significant dimensional changes. Small-scale, small-size, specialized or strategically important components can be considered for technological development for coating by metalliding if the cost justification can be made, in view of the fact that a one-step diffusion process can be achieved more easily.

Precious Metal Plating Silver, gold, and the platinum metal groups are electroplated by either aqueous solution electroplating or fused-salt electrodeposition. Both silver and gold are used for decorative purposes as well as industrial uses; the aqueous plating process is reviewed in Ref 32 and 33. Decorative applications of both elements still predominate, but silver has been successfully substituted for gold in some functional uses in electronics. Silver is used on metallic leadframes, the device that supports the majority of silicon chips. New silicon-to-silver bonding techniques have been used to replace the more expensive gold. However, in electrical contact applications, where long-term surface integrity is important, silver has not been able to replace gold because of its tendency to oxidize or sulfidize on the surface, increasing the contact resistance of the component. Both aqueous and fused-salt electrolytes have been used for plating the platinum group elements. Platinum has been used as a diffusion-barrier layer in aluminiding nickel-base alloys and MCrAlY coatings. Platinum

from the aqueous electrolyte is highly stressed unlike the fused-salt deposit. Substances particularly considered for platinum-group metal coating are the refractory group alloys of molybdenum, tungsten, tantalum, niobium, and vanadium, which tend to form volatile oxides at high temperatures thus reducing their usefulness as corrosion-resistant materials. However, cost is still a major factor, and these metals can be used for diffusion-barrier layers only if components are small and strategically important. Electroless Plating Electroless Nickel Plating. Electroless plating baths have been developed for copper, nickel, silver, gold, and a number of other metals, but the systems with the most importance for corrosion and wear applications are the nickel-phosphorus and nickel-boron systems. Electroless nickel plating is used to deposit nickel without the use of an electric current; thus it is sometimes called autocatalytic plating. In this process, the part is immersed in an aqueous solution containing metal salts, a reducing agent, and other chemicals that control the pH and reaction rates. The part acts as a catalyst for the reduction of the nickel ions by the reducing agent. The reducing agent causes the metal ion reduction and the nickel coating on the part continues to act as a catalyst as the plating process continues, unlike in electroplating where the ions pick up electrons from the cathode. When the process takes place using a hypophosphite-reducing agent, the finished nickel coating is not pure nickel, but contains phosphorus inclusions. Phosphorus content can be as high as 13% (Ref 34). When the process takes place using a borohydride compound reducing agent, the finished product is a nickel-boron alloy. The boron content can be as high as 5% (Ref 35). As applied, nickel-phosphorus coatings are uniform, hard, relatively brittle, lubricious, easily solderable, and highly corrosion resistant (Ref 35). Wear resistance equivalent to hard chromium coatings can be obtained when the coating is heat treated at low temperatures to produce a very hard precipitation-hardened structure. As applied, most of these coatings are amorphous metal glasses that when heated first form nickel phosphite (Ni3P) particles; at temperatures above 320 0C (610 0 F), the deposit crystallizes. Internal stresses are primarily a function of coating composition, and coating thickness uniformity can be easily controlled. Adhesion to most metals is excellent, and frictional properties are also excellent and similar to chromium. Nickel-boron coatings have excellent resistance to wear and abrasion, but because they are not completely amorphous they have reduced resistance to corrosive environments. Furthermore, they are much more costly than nickel-phosphorus coatings. As deposited, the microhardness of electroless nickel-phosphorus coating is about 500 to 600 HVN (48-50 HRC), equivalent to many hardened steels. After precipitation hardening, hardness values as high as 1100 HVN are reported, which is equivalent to commercial hard-chromium

coatings. Because of their high hardness, electroless nickel coatings have excellent wear and abrasion resistance in both the as-deposited and hardened condition (Table 11). Electroless nickel coatings can be easily soldered and are used in electronic applications to facilitate soldering of light metals such as aluminum. Electroless nickel is often used as a barrier coating; to be effective, the deposit must be free of pores and defects. In the as-deposited amorphous state, the coating corrosion resistance is excellent (Table 12), and in many environments is superior to that of pure nickel or chromium alloys. However, after heat treatment the corrosion resistance can deteriorate.

Composite Coatings Composite deposition plating is a further extension of aqueous solution electroplating or electroless coatings in that particles or fibers are suspended in the electrolyte, then occluded in the deposit. Oxides, carbides, silicides, refractory powder, metallic powder, and organic powder can be introduced into the electrolyte. The most widely used electrodeposited composites are cermet coatings, with Al2O3, ZrO2, titania (TiO2), and SiC added to increase strength, hardness, and wear resistance (Table 13). The amount of ceramic particles incorporated in the coating depends on the current density and the bath loading, that is, the amount of particulate in the suspension. It has been shown that coatings up to 40 vol% Al 2 O 3 were produced at 0.5 A/dm2 and 5.3 vol% bath loading. The amount OfAl2O3 incorporated into the coating was seen to decrease with increasing current density and decreasing bath loading (Ref 36). Hardness values ranged from approximately 250 to 580 HVN, depending on the amount OfAl2O3 incorporated. Metallic particles such as chromium can be introduced into a metal plating electrolyte (for example, nickel and cobalt), and the deposited composite can be subsequently heat treated to form high-temperature oxidation-resistant alloys. MCrAlY composites have been made by depositing 10 |xm CrAlY powder in a cobalt or nickel matrix. Heat treatment bonds Table 11 Comparison of the Taber abrasive wear resistance of electroless nickel coatings Heat treatment for 1 h Coating Watts nickel Electroless Ni-P(b)

Electroless Ni-B(c) Hard chromium

C

°F

Taber wear index, mg/1000 cycles(a)

None None 300 500 650 None 400 None

None None 570 930 1200 None 750 None

25 17 10 6 4 9 3 2

0

(a) CS-IO abraser wheels, 1000 g load, determined as average weight loss per 1000 cycles for total test of 6000 cycles, (b) Hypophosphite-reduced electroless nickel containing approximately 9% P. (c) Borohydride-reduced electroless nickel containing approximately 5% B

Table 12

Corrosion of electroless nickel coatings in various environments Corrosion rate Electroless nickelphosphorus(a)

Temperature

Electroless nickel-boron(b)

Environment

0

C

°F

jjim/yr

mil/yr

jun/yr

mil/yr

Acetic acid, glacial Acetone Aluminum sulfate, 27% Ammonia, 25% Ammonia nitrate, 20% Ammonium sulfate, saturated Benzene Brine, 3.5% salt, CO 2 saturated Brine, 3.5% salt, H2S saturated Calcium chloride, 42% Carbon tetrachloride Citric acid, saturated Cupric chloride, 5% Ethylene glycol Ferric chloride, 1% Formic acid, 88% Hydrochloric acid, 5% Hydrochloric acid, 2% Lactic acid, 85% Lead acetate, 36% Nitric acid, 1% Oxalic acid, 10% Phenol, 90% Phosphoric acid, 85% Potassium hydroxide, 50% Sodium carbonate, saturated Sodium hydroxide, 45% Sodium hydroxide, 50% Sodium sulfate, 10% Sulfuric acid, 65% Water, acid mine, 3.3 pH Water, distilled, N 2 deaerated Water, distilled, O 2 saturated Water, sea (3.5% salt)

20 20 20 20 20 20 20 95 95 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 95 20 20 20 100 95 95

68 68 68 68 68 68 68 205 205 68 68 68 68 68 68 68 68 68 68 68 68 68 68 68 68 68 68 205 68 68 68 212 205 205

0.8 0.08 5 16 15 3 Nil 5 Nil 0.2 Nil 7 25 0.6 200 13 24 27 1 0.2 25 3 0.2 3 Nil 1 Nil 0.2 0.8 9 7 Nil Nil Nil

0.03 0.003 0.2 0.6 0.6 0.1 Nil 0.2 Nil 0.008 Nil 0.3 1 0.02 8 0.5 0.9 1.1 0.04 0.008 2 0.1 0.008 0.1 Nil 0.04 Nil 0.008 0.03 0.4 0.3 Nil Nil Nil

84 Nil

3.3 Nil

40

1.6

(C)

(C)

3.5 Nil

0.14 Nil

Nil

Nil 1.7

42 0.2

0.008

90

3.5

Nil

Nil

(C)

(C)

Nil Nil Nil

Nil Nil Nil

11

0.4

Nil Nil

Nil Nil

(a) Hypophosphite-reduced electroless nickel containing approximately 10.5% P. (b) Borohydride-reduced electroless nickel containing approximately 5% B. ( c) Very rapid. Specimen dissolved during test. Source: Ref 35

Table 13

Mechanical properties of electrodeposited cermets Hardness, HV

Cermet

Ni Ni-2.02Al2O3 Ni-3.33TiO2 Ni-6.80Cr2O3 Ni-3.6(TiO2 + CrSi2) Ni-24Co Ni-23.4Co-3TiO2 Ni-22.5Co-6.07Cr2O3 Ni-23.7Co-3.8CrSi2 Ni-23.7Co-3.4(TiO2 + CrSi2)

As plated

Annealed

Yield strength(a), MPa

Elongation(a), %

187 275.4 354 409 283 280 383 462 302 359

118 247 254 295 209 150 219 285 207 211

93.0 67(a), 68.4(b) 222.4 284.0 198.5 120.6 206.6 264 196.4 196.5

6.26 3.75 1.50 2.60 2.3 2.0 1.2 1.6

(a) Annealed, (b) As plated. Source: Ref 22

the coating to the substrate and interdiffuses the cobalt and nickel with the CrAlY particles. The CoCrAlY coatings produced have been shown to be superior to some plasma sprayed CoCrAlY and pack aluminized coatings (Ref 22). Aluminum particles have been codeposited in a nickel matrix

Oxide thickness, ^m

Next Page

Time, h pja 5 Oxide depth versus time plotted on log-log scale for pure nickel and nickel-alloy & * coatings exposed in air at 800, 900, and 1000 0C. Source: Ref 37

and subsequently heat treated to produce a nickel-aluminum intermetallic coating with exceptional oxidation resistance (Ref 37) (Fig. 5). Polytetrafluoroethylene (PTFE), diamond, and SiC particles can also be incorporated into a nickel electroless plating for improved properties. Diamond and silicon carbide are used to enhance abrasion resistance; the surface hardness of these composites is reported to be 1300 HVN. Polytetrafluoroethylene is added to the electroless nickel bath to provide a composite coating with enhanced lubrication. Almost any particulate material can be deposited in a metallic matrix, provided the particles are sufficiently small to remain suspended in the bath and that the particles do not react chemically with the bath during electrodeposition.

Weld-Overlay Coatings Welding is a solidification method for applying coatings with corrosion, wear, and erosion resistance. Weld-overlay coatings, sometimes referred to as hardfacing, offer unique advantages over other coating systems in that the overlay/substrate weld provides a metallurgical bond that is not susceptible to spallation and can easily be applied free of porosity or other defects. Welded deposits of surface alloys can be applied in thicknesses greater than most other techniques, typically in the range of 3 to 10 mm.

Oxide thickness, ^m

Previous Page

Time, h pja 5 Oxide depth versus time plotted on log-log scale for pure nickel and nickel-alloy & * coatings exposed in air at 800, 900, and 1000 0C. Source: Ref 37

and subsequently heat treated to produce a nickel-aluminum intermetallic coating with exceptional oxidation resistance (Ref 37) (Fig. 5). Polytetrafluoroethylene (PTFE), diamond, and SiC particles can also be incorporated into a nickel electroless plating for improved properties. Diamond and silicon carbide are used to enhance abrasion resistance; the surface hardness of these composites is reported to be 1300 HVN. Polytetrafluoroethylene is added to the electroless nickel bath to provide a composite coating with enhanced lubrication. Almost any particulate material can be deposited in a metallic matrix, provided the particles are sufficiently small to remain suspended in the bath and that the particles do not react chemically with the bath during electrodeposition.

Weld-Overlay Coatings Welding is a solidification method for applying coatings with corrosion, wear, and erosion resistance. Weld-overlay coatings, sometimes referred to as hardfacing, offer unique advantages over other coating systems in that the overlay/substrate weld provides a metallurgical bond that is not susceptible to spallation and can easily be applied free of porosity or other defects. Welded deposits of surface alloys can be applied in thicknesses greater than most other techniques, typically in the range of 3 to 10 mm.

Most welding processes are used for application of surface coatings and on-site deposition can be more easily carried out, particularly for repair purposes. Weld overlays are very versatile because a large number of commercially available alloys can be selected to provide protection from a wide range of environmental degradation mechanisms. During weld-overlay surfacing, the coating material is raised to its melting point and then solidified on the surface of the substrate, which means that metals and alloys used for this purpose must have melting points similar to or less than the substrate material. The effectiveness of the weldoverlay coating depends mainly on the welding process and the overlay alloy composition. The welding process must be selected and optimized to apply protective overlays at high deposition rates and thermal efficiency, with good control over the overlay/substrate dilution and coating thickness. The overlay alloy composition must be selected to provide the required properties to prevent coating degradation, and the alloy composition must be readily weldable. Applicable Welding Processes. A number of welding processes are available for applying protective weld overlays, and many welding parameters must be considered when attempting to optimize a particular process for a given application. The process principles and their characteristics for some processes are summarized for comparison purposes in Table 14 and are described in Ref 38 and 39. The processes can be grouped as torch processes, arc welding processes, and high-energy-beam techniques. The torch process, oxyacetylene welding (OAW), is the oldest and simplest hardfacing process and involves simply eating the substrate with the flame and then melting the filler rod to get the hardfacing to melt. High-energybeam techniques use laser beam welding (LBW) or electron beam welding (EBW) to alloy the surface by adding alloy powders to the weld pool. In arc welding, the heat is generated by an arc between an electrode and the workpiece. Arc welding processes can be grouped into nonconsumable electrode processes and consumable electrode processes. Nonconsumable electrode processes, gas tungsten arc welding (GTAW) and plasma arc welding (PAW), both involve a tungsten electrode and the introduction of the filler metal (in the form of rod or wire in GTAW and powder in PAW). The arc melts the filler metal to form a molten pool that is protected from the atmosphere by an inert gas shield. In plasma arc welding, an additional inert gas flows through a constricted electric arc in the welding torch to form the plasma. In general, for consumable electrode processes, the arc is maintained between the consumable electrode and the workpiece. In shielded metal arc welding (SMAW), the electrode consists of a core wire surrounded by a flux covering, that upon melting forms a liquid slag and gas to protect the molten metal pool. In flux core arc welding (FCAW), the flux is contained in the core of the metallic tubular electrode, whereas in gas metal arc welding (GMAW) the consumable wire electrode and substrate metal is protected from the atmosphere by a gas fed axially with the

Table 14

Weld surfacing processes Approximate deposit thickness (min), mm

Process Oxyacetylene (OAW) Powder weld (PW) Shielded metal arc (SMAW) Gas tungsten arc (GTAW) Plasma transferred arc (PAW) Gas metal arc (GMAW)

Flux-cored arc (FCAW)

Submerged arc (SAW) Wire

Strip

Deposition rate, kg/h

Dilution single layer, %

1.5

<1

1-5

0.1

0.2-1

3

1-4

15-30

1.5

<2

5-10

2

<10

2-10

2

3-6

10-30

2

3-6

15-30

3

10-30

15-30

4

10-40

10-25

4

15-35

5-20

Bulk

Electroslag (ESW)

Typical uses Small area deposits on light sections Small area deposits on light sections Multilayers on heavier sections High-quality lowdilution work High-quality lowestdilution work Faster than SMAW, no stub-end loss; positional work possible Similar to GMAW. Mainly for ironbase alloys for high abrasion resistance

Heavy section work; higher-quality deposits than FCAW Corrosion-resistant cladding of large areas Similar to SAW wire but other alloys possible High-quality deposits at higher deposit rates than SAW. Limited alloy range

Source: Ref 39

wire through the welding gun nozzle. In submerged arc welding (SAW), the arc, which is submerged beneath a covering of flux dispenses from a hopper, melts the electrode, the surface of the workpiece, and some of the flux that protects the molten pool from oxidation. Electroslag welding (ESW) uses equipment similar to SAW for strip cladding. Processing Parameters. There are a large number of processing parameters that must be considered when attempting to optimize welding processes for surface application:

AU processes

Consumable processes

Voltage across the arc Current through the arc Current polarity Current pulsing parameters Travel speed of heat source Shielding gas type (except SAW)

Filler metal feed rate Electrode diameter Electrode extension ("stick-out" length) Nonconsumable processes GTAW electrode tip angle (vertex angle) PAW plasma gas flow rate

However, the important factors considered in terms of arc welding overlay parameter optimization and process performance include arc efficiency, melting efficiency, deposition rate, dilution, and coating thickness (Ref 40). Arc efficiency is only a function of the arc welding process; melting efficiency increases with increasing arc power and travel speed, and the maximum deposition rate is directly related to both the arc and melting efficiency. During the deposition of the weld-overlay coating, the base metal and the filler metal are melted and mixed in the liquid state to form a fusion bond. Depending on the weld-overlay coating thickness, if a large portion of the substrate is melted and allowed to mix appreciably with the weld overlay, dilution can cause the overall alloy content of the coating to be significantly reduced. The level of mixing is quantified as the dilution ratio and is one of the most important parameters in a surface application because the original filler metal mechanical and corrosion properties can be altered. The extent to which dilution occurs depends on the surfacing and substrate materials used, the welding process chosen, and the parameters employed. Table 14 indicates the range for dilution expected for the various processes employed. Figure 6 is a surfacing diagram that relates dilution for various arc welding processes according to filler metal feed rate and melting power (a function of arc and melting efficiency) and can be used to

Filler metal feed rate, mm3/s

Calculated dilutions Inoperable range

Operable range

Melting power, W Fig. 6 Effect of processing parameters on dilution with experimental data plotted for SAW ^* process. Source: Ref 41

facilitate process selection and parameter optimization in weld-overlay applications (Ref 41). During welding, the base metal is subjected to peak temperatures that are at least as high as the melting temperature of the substrate. The properties of the weld and the adjacent heat-affected zone (HAZ) strongly depend on the thermal history as dictated by the heat input. Preheating the part may be a necessary step in reducing the residual stress and distortion associated with welding. Preheat and maintenance of a specific minimum temperature during the welding cycle can also reduce the cooling rate to prevent the formation of a detrimental transformation region in the HAZ of ferrous alloys. Interpass temperature is another important factor that needs to be controlled in order to prevent increased dilution and HAZ grain growth at high temperatures. Postweld heat treatment can take many forms, depending on whether the weld-overlay coating needs to be stress-relief annealed or must be heat treated for specified properties. Wear and Corrosion Resistance. Excellent reviews of hardfacing metallurgy and the application of weld-overlay consumables are found in Ref 39 and 42. For overlay coatings, components are designed to provide resistance to various forms of wear, erosion, and corrosion over a large temperature range. Thus, properties such as hardness, microstructure, and corrosion resistance are more important for the coating than tensile strength and elongation, which are usually provided by the substrate material. Generally, coatings selected for wear resistance require high hardness as a characteristic, thus the term "hardfacing." It is believed that most hardfacing alloys develop their wear resistance by virtue of wear-resistance carbides (Ref 43). Almost all hardfacing alloys can be separated into two major groups based on chemical compositions of the primary solidified hard phases: Carbide hardening alloys, including cobalt-base/carbide (WC-Co) and some iron-base superalloys Intermetallic hardening alloys, for example, nickel-base superalloys, austenitic stainless steels, and iron-aluminides However, although increased hardness generally increases wear resistance, different microstructures containing the same carbide type can also have significant effect on wear resistance (Fig. 7). Erosion resistance of materials is very dependent on the erosion conditions, the effects of which are dominated by a number of variables including particle size, shape, composition, and velocity; angle of incidence; and temperature. Unlike wear properties, the erosion rate of weld-overlay coatings generally increases with increasing hardness (Fig. 8). However, the erosion resistance of weld-overlay alloys depends on whether the

Wear resistance, arbitrary units

Bainite

Martensite

Pearlite Ferrite: Low-carbon steels, annealed or normalized - Pearlite: Medium- to high-carbon steels, annealed or normalized Bainite: Austempered medium- to high-carbon steels Martensite: Quenched and tempered medium- to high-carbon steels and carburized steels

Ferrite

Hardness, HB Fig. 7 Effect of structure and hardness on abrasion resistance. Source: adapted from Ref 44

Volume erosion rate, 103 mm3/min

Plastically deformed Not deformed plastically

Armacor-M

Stellite-6

Hastelloy-22

B-60 High-chromium iron

Iron-aluminide TS-2 Type 316L

IN-625 Ultimet .

Type 420

Average Vickers microhardness at 400 0C (500 g load) 8 Volume steady-state erosion rates of weld-overlay coatings at 400 0C (750 0F) as a function of average microhardness at 400 0C (90° impact angle; alumina erodent). Source: Ref 45 FlC.

Weight change, mg/cm2

coating can be classified as a brittle or ductile material (Ref 45). Those materials that can be deformed plastically (ductile) produce a large plastic zone beneath the eroded surface, and the increased plastic zone size can be directly correlated to an improved steady-state erosion resistance. For those materials that cannot deform plastically (brittle), an increase in coating hardness sometimes may lead to a decrease in volumetric erosion rate. Thus, materials that can dissipate particle impact energy through plastic deformation (plastic zone) exhibit low erosion rates. However, for materials that do not deform plastically (no plastic zone) and do not undergo plastic deformation, the ability to resist brittle fracture (i.e., cracking) becomes a major factor that can control the erosion resistance. The corrosion resistance of weld-overlay coatings follows the corrosion-resistant properties of the bulk materials and is also dependent on the corrosive environment. Weld-overlay coatings provide resistance to oxidation and sulfidation. Dilution, as discussed previously, can be expected to modify the behavior of the coating alloy from the properties quoted for the undiluted bulk materials. In weld-overlay coatings such as austenitic steels, dilution can affect corrosion resistance because of a reduction in the effective chromium content or an increase in carbon content through carbon pickup from the substrate steel. Iron aluminides appear to be potentially important weld-overlay coatings for sulfidation environments. Figure 9 shows isothermal weight gain studies for a number of weld-overlay coatings exposed to H2S-H2-H2O-Ar gas mixtures at 800 0C (1470 0F) (Ref 46). This work showed that compositions containing at least 30% Al and >2% Cr had excellent sulfidation resistance, and, at increased chromium levels, corrosion rates increased but were still superior to other alloy classes such as stainless steels.

Time, h Fig.&# 9 Weight change versus time for specimens cut from iron-aluminide weld overlays and isothermal Iy exposed to H2S-H2-H2O at 800 0C (1470 0F). The elemental concentrations shown are in at.%; the balance is iron. Source: Ref 46

T h e r m a l Spray Coatings

Thermal spraying is a generic term for a group of processes that apply a consumable in the form of a spray of finely divided molten or semimolten droplets to produce a coating. A number of extensive reviews of the topic can be found in Ref 47 to 51. The characteristics that distinguish thermal spray processes from weld-overlay coatings are indicated as follows (Ref 51): Substrate adhesion, or bond strength, is dependent on the materials and their properties and generally is characterized as a mechanical bond between the coating and the substrate, unlike the metallurgical bond found in weld-overlay coatings. Spray deposits can be applied in thinner layers than welded coating, but thick deposits are also possible. Provided there is a stable phase, almost all material compositions can be deposited, including metals, cermets, ceramics, and plastics. Thermal spray processes are usually used on cold substrates, preventing distortion, dilution, or metallurgical degradation of the substrate. Thermal spray processes are line-of-sight limited, but the spray plume often can be manipulated for complete coverage of the substrate. Tables 15 and 16 compare thermal spraying, welding, and electroplating. Thermal spray processes can be classified into two categories, arc processes and gas combustion processes, depending on the means of achieving the heat for melting of the consumable material during the spraying operation. In the lower-energy electric arc (wire arc) spray process, heating and melting occur when two electrically opposed charged wires, comprising the spray material, are fed together to produce a controlled arc at the intersection. The molten material on the wire tips is atomized and propelled onto the substrate by a stream of gas (usually air) from a high-pressure gas jet. The highest spray rates are obtained with this process, allowing for cost-effective spraying of aluminum and zinc for the marine industry. In the higher-energy plasma arc spray process, injected gas is heated in an electric arc and converted into a high-temperature plasma that propels the coating powder onto the substrate at very high velocities. This process can take place in air with air plasma spraying (APS), or in a vacuum with vacuum plasma spray (VPS) or low-pressure plasma spraying (LPPS). For gas combustion processes, the lower-energy flame spray process uses oxyfuel combustible gas as a heat source to melt the coating material, which may be in the form of rod, wire, or powder. In the higher-energy, high-velocity oxyfuel combustion spray (HVOF) technique, internal combustion of oxygen and fuel gas occurs to produce a high-velocity plume

Table 15

Applications of thermal spraying, welding, and electroplating Electroplating

Application

Thermal spraying

Welding

Base metal

Identification of alloy by generic type is required. Almost any alloy can be sprayed. Must be known before coating material selection can be made Practical for buildup from 0.25 mm (0.010 in.) to 2.5 mm (0.100 in.) and sometimes greater Not useful Excellent results—no distortion

Precise identification of alloy is required. In steels, composition must be known. Some alloys are difficult or impossible to weld. Need not be known. It is usually sufficient to match the properties of the base metal. Practical, but comparatively costly, particularly on alloys requiring postweld heat treatment

Operating environment of finished component Restoring dimensions

Restoring strength Precision-dimensioned

Good to excellent results Distortion is a serious problem. It is frequently difficult to predict whether or not distortion will be within acceptable limits. Weld stresses may create problems.

Identification of alloy by generic type is required. Almost any alloy can be plated. Usually not required in machinery applications Practical up to 0.64 mm (0.025 in.) by tank plating and 0.13 mm (0.005 in.) by "brush" plating Not useful Excellent results—no distortion

Rotating or oscillating machinery

Usually excellent. No induced stresses in base metal

Limited fatigue

Excellent results if stress risers are not introduced by machining and if shot peening is done prior to spraying Usually excellent

Not recommended

Fair

Usually excellent if hydrogen embrittlement is prevented. No induced stresses in the base metal Excellent results if preceded by shot peening and followed by hydrogen removal when required Usually excellent

Excellent

Impractical

Good

Excellent

Excellent but expensive

Excellent

Antiwear or antifriction surface Galvanic anticorrosion coating Corrosion-resistant coating

Source: R,B. Alexander & Associates, Huntington Woods, MI

Table 16

Process requirements in thermal spraying, welding, and electroplating

Application

Thermal spraying

Preheating

Always used to remove moisture, otherwise temperature is held as low as possible. Usual preheat range is 95-150 0 C (200-300 0F) Work frequently done on a lathe with the spray gun mounted on tool post and the lathe used to machine to plan size None required, except for one process variation that fuses the coating after application

Auxiliary equipment operation

Postheating

Restoring to plan dimensions

Special machining and grinding techniques used. Finish machining is sometimes unnecessary after light plasma spray antiwear coatings.

Welding

Electroplating

Treatment varies from chilling to heating up to 425 0 C (800 0F) depending on composition of base metal.

Heat treatment sometimes required for stress relief after grinding on alloy steels

None

Work frequently done on a lathe with handheld brush plating wand. No auxiliary equipment used for a tank plating None required except for hydrogen bake-out heat treatment after chromium electroplating on high strength alloys

Frequently used for dimensional stability, stress relieving, and tempering depending on composition of base metal, geometry, and end use of the part Conventional machining and grinding

Source: R.B. Alexander & Associates, Huntington Woods, MI

Special grinding required on chrome plate

capable of accelerating powders at supersonic speeds and lower temperatures than the plasma processes. Continuous combustion occurs in most commercial processes, whereas the proprietary detonation gun (D-gun) process uses a spark discharge to propel powder in a repeated operating cycle to produce a continuous deposit. In the lower-energy processes, electric arc (wire arc) spray and flame spray processes, adhesion to the substrate is predominantly mechanical and is dependent on the workpiece being perfectly clean and suitable rough. Some porosity is always present in these coatings, which may present problems in both corrosion and erosion. The higher-energy processes—APS, VPS, LPPS, and HVOF processes—were developed to reduce porosity and improve adhesion to the substrate. In addition, these processes are capable of spraying materials with higher melting points, thus widening the range of applications to include high-temperature coatings and thermal and mechanical shock-resistant coatings. With these higher-energy processes, bond strengths are higher because of the possible breakup of any oxide films present on the particles or the workpiece surface, allowing for some diffusion bonding to take place (Ref 51). Typical design features of the various thermal spray processes are listed in Table 17. Properties of Thermal Spray Coatings. The variations in oxide content and porosity, as well as the chemical composition of the coating, greatly affect the properties of the deposit and, in the case of corrosion, the underlying substrate. The splat morphology and, more importantly, the splat/splat and splat/substrate interface are critical to properties such as bond strength, wear, erosion, and corrosion. The mechanical properties of thermal spray coatings are not well documented except for their hardness and bond strength. Table 18 contains typical mechanical property data for a large range of plasma sprayed materials; however, the sensitivity of the properties of the coatings to specific deposition parameters makes universal cataloging of properties by process and composition "virtually meaningless" (Ref 48).

Table 17

Typical design characteristics of thermal spray processes

Process

Flame Arc wire High-velocity oxyfuel (JetKote) Detonation gun Air plasma spray Vacuum plasma spray

Gas temperature, 0 C

3,000 NA

Particle, velocity, m/s

40 100

Relative Typical deposit Adhesion, Oxide Porosity, Spray rate, MPa content, % kg/h cost, low = 1 thickness, mm %

8 12

10-15 10-20

10-15 10

2-6 12

1 2

0.1-15 0.1 to >50

3,000 4,000 12,000

800 >70 800 >70 200-400 4 to >70

1-5 1-5 1-3

1-2 1-2 1-5

2-4 0.5 4-9

3 NA 4

0.1 to >2 0.05-0.3 0.1-1

12,000

400-600

ppm

<0.5

4-9

5

0.1-1

NA, not applicable. Source: Ref 51

>70

Table 18

Typical mechanical properties of plasma sprayed coatings Bond tensile strength(a)

Material

Rockwell macro/micro hardness

g/cm3

lb/ft3

2.48 7.20 9.90 8.96 7.95 7.48 7.06 14.15 4.17 16.90

155 449 618 559 496 467 441 883 260 1055

Density

MPa

ksi

8.3 21.4 57.2 55.2 23.4 33.1 54.5 46.9 41.4 40.0

1.2 3.1 8.3 8.0 3.4 4.8 7.9 6.8 6.0 5.8

45/58 HRH 65/142 HRB 70/1450 HR15N 65/1448 HRA 84/... HR15T 81/... HR15T 61/1344 HRC 65/1585 HRA 78/... HR15N 50/500 HRA

17.6 23.4 31.0 31.0 29.0 24.1 24.1 28.3 22.1 41.4 23.4 68.3 47.6 49.6 16.5 37.9 42.7 42.1 22.1 33.8 35.9 33.1 44.8 22.1

2.55 3.4 4.5 4.5 4.2 3.5 3.5 4.1 3.2 6.0 3.4 9.9 6.9 7.2 2.4 5.5 6.2 6.1 3.2 4.9 5.2 4.8 6.5 3.2

88/... HR15T 70/... HR30T 35/... HRC 90/... HR15 90/... HR15T 72/... HRB 83/... HR15T 88/... HR15T 81/... HR15T 79/... HR15T 79/... RH15T 80/490 HRB 80/510 HRB 90/250 HRB 78/60 HR15T 80/200 HRB 89/... HR15T 90/... HR15T 70/... HR15N 35/... HRC 28/... HRC 35/... HRC 35/... HR15N 95/... HRB

7.22 6.80 6.25 7.48 7.19 7.89 7.94 6.73 6.30 7.65 7.83 7.51 6.92 7.51 2.49 7.43 7.65 8.25 7.10 7.05 7.00 4.30 8.50 6.90

451 425 390 467 449 493 496 418 393 478 489 469 432 469 155 464 478 515 443 440 437 268 531 431

33.8 32.4 32.1 42.7 27.6 48.3

4.9 4.7 4.65 6.2 4.0 7.0

80/500 HR15T 86/500 HR15T 72/660 HR15N 92/250 HR15T 80/250 HRB 80/200 HRB

7.39 7.02 6.62 7.71 6.90 7.40

461 438 413 481 431 462

88/... HR15N 81/... HR15N 85/... HR15N 85/950 HR15N 84/950 HR15N 80/1850 HR15N .../1850HR15N 80/1850 HR15N

13.75 12.41 14.55 11.10 6.41 6.23

858 775 908 693 400 389

5.80

362

5.30 4.80 5.00 4.10

331 300 312 256

3^50 3.50 4.0 3.30 3.30 4.20

218 218 250 187 187 262

Pure metals Aluminum Copper Molybdenum (fine) Molybdenum (coarse) Nickel (fine) Nickel (coarse) Niobium Tantalum Titanium Tungsten Alloy metals 304 stainless 316 stainless 431 stainless 80Ni-20Cr (fine) 80Ni-20Cr (coarse) 40Ni-60Cu 35Ni-5In-60Cu 10Al-90Cu (fine) 10Al-90Cu (coarse) Hastelloy31(fine) Hastelloy 31 (coarse) 5Al-95Ni 20Al-80Ni 6Al-19Cr-75Ni 12Si-88Al 5Al-5Mo-90Ni Hastelloy X Hastelloy C 420 stainless 0.9C stainless Cast iron Ti-6A1-4V Monel 0.2C steel Metal composites 95Ni-5Al 80Ni-20Al 65Ni-35Ti 75Ni-19Cr-6Al 75Ni-9Cr-7Al-5Mo-5Fe 90Ni-5Al-5Mo Carbide powders and blends 88WC-12Co (cast fine) 88WC-12Co (cast coarse) 88WC-12Co (sintered) 83WC-17CO 75Cr3-C2-25NiCr (fine) 75Cr3-C2-25NiCr (coarse) 75Cr3-C2-25NiCr (composite) 85Cr3-C2-15NiCr

44.8 44.8 55.2 68.9 41.4 34.5

6.5 6.5 8.0 10.0 6.0 5.0

Ceramic oxides ZrO2 (calcinated) Chromium oxide 80ZrO2 -20yttria TiO2 Al 2 O 3 (white) 87 Al 2 O 3 -DTiO 2 60Al2O3-40TiO2 50Al2O3-50TiO2 Al2O3-gray (fine) Al2O3-gray (coarse) Magnesium zirconate

44.8 44.8 15.2

6.5 6.5 2.2

44.8 15.5 27.6

6.5 2.25 4.0

6.9

1.0

17.2

2.5

70/... 90/... 80/... 87/...

HR15N HR15N HR15N HR15N

90/... HR15N 90/850 HR15N 85/... HR15N 87/193 HR15N 85/... HR15N 75/... HR15N

(a) Over a grit-blasted surface roughened to 2.5- 4.1 n-m (100-160 (xin.) AA (arithmetic average). Source: Ref 50

Table 19 Abrasive wear data for selected thermal spray coatings Material Carballoy 883 WC-Co

Type

Wear rate, mm 3 /1000 rev

Sintered Detonation gun Plasma spray Super D-gun High-velocity oxyfuel

1.2 0.8 16.0 0.7 0.9

ASTM G 65 dry sand/rubber wheel test. 50/70 mesh Ottawa silica. 200 rpm. 13.6 kg (30 Ib) load. 3000-revolution test duration. Source: Ref 48

Erosion rate at 90°, mg/min

One of the most extensive uses for thermal spray coatings is in wear applications. Generally, the wear resistance of coatings increases with their density and cohesive strength, so that HVOF coatings provide the best wear resistance in contrast to plasma spray coatings (Table 19). Carbide cermets were found to be good for both wear and erosion environments, and the optimal amount of hard phase (oxide and carbide) has been determined for erosion resistance. A comparison between the presprayed powder and the actual coating microstructure showed that the retention of the FeCrAlY matrix is much better than the chromium carbide particles, which can form oxides during HVOF thermal spraying (Ref 52). Erosion tests show that both carbides and oxides increase the erosion rate of the coating (Fig. 10) and that low amounts of hard constituents are preferable for erosion resistance. Table 20 summarizes the various thermal spray coatings used to prevent various forms of wear. Thermal spray zinc, aluminum, and zinc-aluminum alloys are used for sacrificial galvanic protection for corrosion resistance on bridges, ships, and other large structures (Table 21). Other corrosion-resistant applications for thermal spray coatings include oxidation and sulfidation resistance in power boilers and other high-temperature uses. In a comparison of several different thermal spray techniques, it was found that hightemperature corrosion-resistant coatings must have compositions that promote the formation of protective oxides at splat boundaries, be dense enough so that protective oxides can form within and fill voids, and be

a Carbide Oxide

Measured constituent content, % Fig,

I 0 Steady-state erosion rates versus constituent composition. Source: Ref 52

thick enough to postpone the diffusion of corrosive species to the substrate material along the fast diffusion paths of the coating (Ref 53). It has been found that corrosion attack of the substrate generally decreases as the free path to the substrate (that is, the diffusion path the corrosive species takes to the substrate) increases. Thus, as in erosion and wear, the splat/splat and splat/substrate boundaries are critical to properties of the thermal spray coatings.

Table 20

Thermal spray coatings used for wear-resistant applications

Type of wear Adhesive wear

Abrasive wear

Surface fatigue wear Fretting: Intended motion applications

Fretting: Small amplitude oscillatory displacement applications: Low temperature (<540 0 C, or 1000 0F) High temperature (>540 0 C, or 1000 0F) Erosion

Cavitation

Coating material

Soft bearing coatings: Aluminum bronze

Coating process(a)

Applications

Tobin bronze Babbitt Tin Hard bearing coatings: Mo/Ni-Cr-B-Si blend Molybdenum High-carbon steel Alumina/titania Tungsten carbide Co-Mo-Cr-Si Fe-Mo-C Aluminum oxide Chromium oxide Tungsten carbide Chromium carbide Ni-Cr-B-SiCAVC (fused) Ni-Cr-B-SiC (fused) Ni-Cr-B-SiC (unfused)

OFW, EAW, OFP, PA, HVOF OFW, EAW OFW, EAW, OFP OFW, EAW, OFP

Babbitt bearings, hydraulic press sleeves, thrust-bearing shoes, piston guides, compressor crosshead slippers

PA OFW, EAW, PA OFW, EAW OFP, PA OFP, PA, HVOF PA, HVOF PA PA PA PA, HVOF PA, HVOF OFP, HVOF OFP, HVOF HVOF

Bumper crankshafts for punch press, sugar cane grinding roll journals, antigalling sleeves, rudder bearings, impeller shafts, pinion gear journals, piston ring (internal combustion); fuel pump rotors Slush-pump piston rods, polish rod liners, and sucker rod couplings (oil industry); concrete mixer screw conveyors; grinding hammers (tobacco industry); core mandrels (dry-cell batteries); buffing and polishing fixtures; fuel-rod mandrels

Molybdenum Mo/Ni-Cr-B-SiC Co-Mo-Cr-Si

OFW, PA PA PA, HVOF

Servomotor shafts, lathe and grinder dead centers, cam followers, rocker arms, piston rings (internal combustion), cylinder liners

Aluminum bronze Cu-Ni-In Cu-Ni Co-Cr-Ni-W Chromium carbide

OFW, EAW, PA, HVOF PA, HVOF PA, HVOF PA, HVOF PA, HVOF

Chromium carbide Tungsten carbide WC/Ni-Cr-B-Si-C(fused) WC/Ni-Cr-B-SiC (unfused) Chromium oxide Ni-Cr-B-SiC-Al-Mo Ni-Al/Ni-Cr-B-SiC Type 316 stainless steel Ni-Cr-B-SiC (fused) Ni-Cr-B-SiC (unfused) Aluminum bronze Cu-Ni

PA, HVOF PA, HVOF OFP, HVOF OFP, HVOF PA PA PA PA OFP, HVOF HVOF PA, HVOF PA, HVOF

Aircraft flap tracks (air-frame component); expansion joints and mid-span supports (jet engine components) Compressor air seals, compressor stators, fan duct segments and stiffeners (all jet engine components) Exhaust fans, hydroelectric valves, cyclone dust collectors, dump valve plugs and seats, exhaust valve seats Wear rings (hydraulic turbines), water turbine buckets, water turbine nozzles, diesel engine cylinder liners, pumps

(a) OFW, oxyfuel wire spray; EAW, electric arc wire spray; OFP, oxyfuel powder spray; PA, plasma arc spray; HVOF, high-velocity oxyfuel powder spray

Table 21 Typical corrosion-resistant applications for thermal spraying Application

Thermal spray materials

Pipe sections for saltwater pumps Oil platform components Bridges Bridge fabrication shops Chemical and water storage tanks Power transmission poles Piping in power plant cooling towers Cooling water pump difruser Concrete bridge structures Grandstands Ski lifts Decorative hand rails

Alloy 625 (Ni-22Cr-9Mo-5Fe-4Nb) or Al Al and Al alloys Zn, Zn-Al, or Al Zn, Zn-Al, or Al Alloy 625, Al, Zn, and a high-Cr/Ni alloy 85% Zn-15% Al Al Al Zn or Zn-Al Zn-Al Zn Zn

Source: Ref 3

Cladding Clad metals are bonded metal-to-metal laminar composite systems that can be fabricated by a number of processes. The principal cladding techniques include hot-roll bonding, cold-roll bonding, explosive bonding, and weld cladding (including laser cladding), although centrifugal casting, adhesive bonding, extrusion, and hot isostatic pressing have also been used to produce clad metals. Clad metals can be provided in plate, sheet, tube, rod, and wire forms. Most engineering metals and alloys can be clad. Cladding combinations that have been commercially produced on a large scale are shown in Fig. 11. The cladding of steel with stainless steel, copper, nickel alloys, titanium, and tantalum has become increasingly popular in the chemical processing industries. Applications include pressure vessels, reactors, heat exchangers, and storage tanks. Clad metals provide a means of designing into a composite material specific properties that cannot be obtained in a single material. The early use of clad metals in the jewelry industry combined the aesthetics of precious metals with the low-cost strength of base metals. These materials systems are currently being used for electrical and electronics applications, such as contacts and connectors with selectively clad (inlay) precious metals for low contact resistance and high reliability. Corrosion Control through Cladding Clad metal systems designed for corrosion control can be categorized as follows: Noble metal clad systems Corrosion barrier systems Sacrificial metal systems Transition metal systems

Magnesium Austenitic stainless steel Brass/bronze

Nickel

Copper

Carbon steel

Aluminum

Aluminum Nickel Copper Carbon steel Ferritic stainless steel Martensitic stainless steel Austenitic stainless steel Invar Titanium Commercial Requires development Flg. 11 High-volume commercially available clad metals

Proper design is essential for providing maximum corrosion resistance with clad metals. Noble metal clad systems are materials having a relatively inexpensive base metal covered with a corrosion-resistant metal. A typical example would be a carbon steel clad with a stainless steel or nickel-base alloy. Another group of commonly used noble metal clad metals uses aluminum as a substrate. For example, in stainless-steel-clad aluminum truck bumpers, the stainless steel provides corrosion resistance, and the aluminum provides a high strength-to-weight ratio. Corrosion-Barrier Systems. The combination of two or more metals to form a corrosion-barrier system is most widely used where perforation caused by corrosion must be avoided. This is shown schematically in Fig. 12. Low-carbon steel and stainless steel are susceptible to localized corrosion in chloride-containing environments and can perforate rapidly. When steel is clad with a stainless steel layer, the corrosion-barrier mechanism prevents perforation. Localized corrosion of the stainless steel is prevented; the stainless steel is protected galvanically by the sacrificial corrosion of the carbon steel in the metal laminate. Therefore, only a thin pore-free layer is required. Sacrificial metals, such as magnesium, zinc, and aluminum, are in the active region of the galvanic series and are extensively used for corrosion protection. The single largest application for cold-roll-bonded materials is stainless-steel-clad aluminum for automotive trim. The stainless steel

Low-carbon steel

(a) Low-carbon steel

Stainless steel (b)

Fig. 1 2 lustrations of the corrosion-barrier principle, (a) Solid carbon steel. (b) Carbon steel clad with stainless steel

exterior surface provides corrosion resistance, high luster, and abrasion and dent resistance, and the aluminum on the inside provides sacrificial protection for the painted auto body steel and for the stainless steel. The largest application for hot-roll-bonded materials—alclad aluminum—also falls into this category. In this case, a more active aluminum alloy is bonded to a more noble aluminum alloy. In service, the outer clad layer of aluminum corrodes sacrificially and protects the more noble aluminum substrate. Clad transition metal systems provide an interface between two incompatible metals. They not only reduce galvanic corrosion where dissimilar metals are joined, but they also allow welding techniques to be used when direct joining is not possible. Clad metals provide an ideal solution to the materials problem of dual environments. For example, in the application of small battery cans and caps, copper-clad, stainless steelclad nickel (Cu/SS/Ni) is used where the external nickel layer provides atmospheric corrosion resistance and low contact resistance. The copper layer on the inside provides the electrode contact surface as well as compatible cell chemistry. The stainless steel layer provides strength and resistance to perforation corrosion. Chemical Vapor Deposition Chemical vapor deposition (CVD) involves the formation of a coating by the reaction of the coating substance with the substrate. The coating species can come from a gas or gases or from contact with a solid as in the pack-cementation diffusion process described in Chapter 5. The process is more precisely defined as the deposition of a solid on a heated surface by a chemical reaction from the vapor or gas phase (Ref 54). In general, three processing steps are involved in any CVD reaction: (1) the production of a volatile carrier compound, (2) the transport of the gas to the deposition

site without decomposition, and (3) the chemical reaction necessary to produce the coating on the substrate. The numerous chemical reactions used in CVD include thermal decomposition (pyrolysis), reduction, hydrolysis, disproportionation, oxidation, carburization, and nitridation. These reactions take place singly or in combination and are controlled by thermodynamics, kinetics, mass transport, chemistry of the reaction, and processing parameters of temperature, pressure, and chemical activity. Chemical vapor deposition processes can be classified as either open reactor systems, including thermal CVD and plasma CVD, or as a closedreactor system, as in pack cementation. In thermal CVD, reactions usually take place above 900 0 C (1650 0 F), whereas plasma CD usually operates at temperatures between 300 and 700 0C (570 and 1290 0F) (Table 22). Using the lower-reaction-temperature plasma CVD enables coatings to be produced on substrates with low melting points or that otherwise would undergo solid-state transformations over the range of deposition temperatures. Furthermore, the low deposition temperature of plasma CVD coatings limits the stresses due to the large mismatches in thermal expansion that can lead to cracking and delamination of the coating. Materials that cannot ordinarily be deposited by electrodeposition-for example, the refractory metals tungsten, molybdenum, rhenium, niobium, tantalum, zirconium, hafnium, and so forth-are deposited using CVD processes. Typical products produced are crucibles, rocket nozzles, and other high-temperature components; linings for chemical vessels; and coatings for electronic components. These refractory metals are deposited at temperatures far below their melting points or sintering temperatures, and coatings can be produced with a preferred grain size and grain orientation. For example, tungsten that is deposited by the hydrogen reduction of the halide and deposition at a lower temperature (500 0 C, or 930 0F) gives a finer grain size with higher strength (83 MPa, or 12 ksi) than deposition at a higher temperature (700 0 C, or 1290 0F) (Ref 54). Wear-, erosion-, and corrosion-resistance applications extensively utilize CVD coatings, as do applications that require low friction characteristics. Table 23 lists the properties of typical CVD coating materials for these applications. Some materials, such as titanium diboride, titanium Table 22 Typical deposition temperatures for thermal and plasma CVD Deposition temperature

Thermal CVD Material

Silicon nitride Silicon dioxide Titanium carbide Titanium nitride Tungsten carbide Source: Ref 54

Plasma CVD

C

op

900 800-1100 900-1100 900-1100 1000

1650 1470-2010 1650-2010 1650-2010 1830

0

0

C

°F

300 300 500 500 325-525

570 570 930 930 615-975

Table 23

Selected wear and corrosion properties of CVD coating materials Coefficient of thermal expansion at 25 0 C (77 0F) 1(T6/K

Material

GPa

106 psi

Thermal conductivity, W/m-K

Titanium carbide

31.4

4.5

17

7.6

Titanium nitride

20.6

3.0

33

9.5

24.5-29.4 22.1

3.5-4.3 3.2

20-30 11

8 10

Silicon carbide

27.4

4.0

125

3.9

Titanium diboride

33.0

4.7

25

Alumina

18.8

2.7

34

4.2-7.1

200

14.2

180

Hardness

Titanium carbonitride Chromium carbide

Diamondlike carbon

Diamond

2SM9

98

6.6

8.3

2.9

Remarks

High wear and abrasion resistance, low friction High lubricity; stable and inert Stable lubricant Resists oxidation to 900 0 C (1650 0F) High conductivity, shock resistant High hardness, high wear resistance Oxidation resistant, very stable Very hard, high thermal conductivity Extreme hardness and high thermal conductivity

Source: Ref 54

carbide, and silicon carbide, provide extremely low wear rates. Table 24 lists specific production applications for the wear-, erosion-, and corrosion-protection provided by CVD coatings. The cutting-tool industry relies heavily on coatings. The technology associated with CVD has made some of its most important gains in this area. Major applications are represented by titanium carbide coatings on the majority of cemented (cobalt-bonded) tungsten carbide tools and both titanium nitride and carbonitride coatings on high-speed tool steel and cemented carbide tools. The materials identified in Table 23 can be used as multilayer structures that utilize the strongest characteristics of each layer of material. Nearly all coatings are multilayer systems that combine titanium nitride for lubricity and galling resistance; alumina for chemical inertness and thermal insulation; and titanium carbide, as well as titanium carbonitride, for abrasion resistance. Selecting the optimal combination of materials depends on the type of machining operation, the material to be machined, and other factors. Criteria for such a selection are summarized in Table 25. Titanium nitride, the most common coating material, is generally combined with a very thin undercoating of titanium carbide or titanium carbonitride to promote adhesion. Alumina coatings are preferred in highspeed machining applications in which oxidation resistance and high-temperature stability are the critical factors. Like titanium nitride, alumina is deposited on an intermediate titanium carbide layer.

Table 24 Wear-, erosion-, and corrosion-resistance applications of CVD Metalforming (noncutting) Tube and wire-drawing dies (TiN) Stamping, chamfering, and coining tools (TiC) Drawing punches and dies (TiN) Deep-drawing dies (TiC) Sequential drawing dies (Cr7C3) Coating on dressing sticks for grinding wheels (B4C) Ceramic and plastic processing Molding tools and dies for glass-filled plastics [Ti(CN)] Extrusion dies for ceramic molding (TiC) Kneading components for plastic mixing (TiC) Chemical- and general-processing industries Pump and valve parts for corrosive liquids (SiC) and abrasive liquids (TiB2) Valve liners (SiC) Positive-orifice chokes (SiC, TiB2) Packing sleeves, feed screws (TiC) Thermowells (SiC, Al2O3) Abrasive-slurry transport (WC) Sandblasting nozzles (TiC, B4C, TiB2) Textile-processing rolls and shafts (Al2O3, TiC, WC) Paper-processing rolls and shafts (TiC) Valves for coal-liquefaction components (TiB2) Cathode coating for aluminum production (TiB2) Oxidation-resistant coatings for carbon-carbon composites (SiC) Machine elements Gear components (TiN) Coating on stainless-steel spray-gun nozzles (TiC) Components for abrasive processing (TiC) Coating on ball bearings (TiC) Turbine blades (SiC, TiC) Nuclear Coating for neutron flux control in nuclear reactors (B4C) Coating for shielding against neutron radiation (B4C) Coatings for fusion reactor applications (SiC) Nuclear waste container coatings (SiC) Instruments Radiation sensor (SiC) Thermionic cathodes (W-Th) Target coatings for x-ray cathodes (W-Re) Note: TiN, titanium nitride; TiC, titanium carbide; B4C, boron carbide; Ti(CN), titanium carbonitride; SiC, silicon carbide; TiB2, titanium diboride; Al 2 O 3 , alumina; WC, tungsten carbide

Table 25 Criteria for selecting coating materials for cutting tools Property Oxidation and corrosion resistance; high-temperature stability Crater-wear resistance Hardness and edge retention Abrasion resistance and flank wear Low coefficient of friction and high lubricity Fine grain size (a) For each property, best material is identified first. Source: Ref 55

Best materials(a)

Diamond films grown by CVD exhibit outstanding properties approaching natural diamond, such as high electrical resistivity, high optical transparency, extreme hardness, high refractive index, and chemical inertness. Different film-deposition techniques and system configurations result in films with different characteristics. Diamond films can be grown using processing variables of different concentrations of methane in methane-hydrogen gas mixtures and flow rates (Ref 56). The CVD of diamond requires the presence of atomic hydrogen, which selectively removes graphite and activates and stabilizes the diamond structure. The basic reaction involves the decomposition of methane, which can be activated by microwave plasma, thermal means (hot filament), plasma arc, or laser.

Physical Vapor Deposition Processes Physical vapor deposition (PVD) processes involve the formation of a coating on a substrate by physical deposition of atoms, ions, or molecules of the coating species (Ref 57). There are three main techniques for applying PVD coatings: thermal evaporation, sputtering, and ion plating. Thermal evaporation involves heating of the material until it forms a vapor that condenses on a substrate to form a coating. Sputtering involves the electrical generation of a plasma between the coating species and the substrate. Ion plating is essentially a combination of these two processes. A comparison of the process characteristics of PVD, CVD, and ion implantation is provided in Table 26. Reviews of these processes can be found in the literature (for example, Ref 57, 59, and 60). Originally PVD was used to deposit single metal elements by transport of a vapor in a vacuum without involving a chemical reaction. Today, PVD technology has evolved so that a wide array of inorganic materials (including metals, alloys, compounds, or their mixtures) and organic compounds can be deposited. The PVD process occurs in a vacuum chamber and involves a vapor source and the substrate on which deposition occurs. Different techniques arise because of variations in atmospheres, vapor source heating method, and electrical voltage of the substrate, all of which contribute to the structure, properties, and deposition rate of the coating (Ref 60). The steps in deposition occur as follows: 1. Synthesis of the material deposited (transition from a condensed state, solid or liquid, to the vapor phase, or, for deposition of compounds, reaction between the components of the compound, some of which may be introduced into the chamber as a gas or vapor) 2. Vapor transport from the source to the substrate 3. Condensation of the vapors followed by film nucleation and growth

Table 26

Comparison of PVD, CVD, and ion implantation process characteristics

Process

Vacuum evaporation

Ion implantation

Processing temperature, 0 C

RT-700, usually <200

Line of sight

200-400, best <250 for N

Line of sight

Ion plating, ARE

RT-0.7 Tm of coating. Best at elevated temperatures

Sputtering

RT-0.7 Tm of metal coatings. Best >200 for nonmetals

CVD

Throwing power

300-2000, usually 600-1200

Moderate to good

Coating applications and special features

Coating materials

Chiefly metal, especially Al (a few simple alloys/a few simple compounds) Usually N (B, C)

Ion plating: Al, other metals (few alloys) ARE: TiN and other compounds

Line of sight

Metals, alloys, glasses, oxides. TiN and other compounds(a)

Very good

Metals, especially refractory TiN and other compounds(a), pyrolytic BN

Electronic, optical, decorative, simple masking Wear resistance for tools, dies, etc. Effect much deeper than original implantation depth. Precise area treatment, excellent process control Electronic, optical, decorative. Corrosion and wear resistance. Dry lubricants. Thicker engineering coatings Electronic, optical, wear resistance. Architectural (decorative). Generally thin coatings. Excellent process control Thin, wear-resistant films on metal and carbide dies, tools, etc. Free-standing bodies of refractory metals and pyrolytic C or BN

RT, room temperature; ARE, activated reactive evaporation; Tm, absolute melting temperature, (a) Compounds: oxides, nitrides, carbides, silicides, and borides of Al, B, Cr, Hf, Mo, Nb, Ni, Re, Si, Ta, Ti, V, W, Zr. Source: Ref 58

The PVD process produces coatings for a range of applications including electronics, optics, decoration, and corrosion and wear prevention. Only engineering uses, that is, corrosion- and wear-resistant coatings, are discussed below. The coatings used for wear applications are usually hard compounds, and, from the designers point of view, thin-film wear coatings can be used for the same type of applications as chromium electroplate. Physical vapor deposition coatings have hardnesses greater than any metal and are used in systems that cannot tolerate even microscopic wear losses. Most processes are operated on a batch basis, and the component size is limited by the size of the vacuum chamber. Provided that the substrate can be manipulated to face the coating source, the size and shape of objects are limited by the capital and operating expenditures involved rather than by the fundamental characteristics of the process. Furthermore, cleanliness of the substrate is critical and far exceeds surface preparation requirements for other coatings. Thermal evaporation is the oldest and probably the most widely used PVD technique. It accounts for the major proportion of both the equipment in use and the area coated. Thermal evaporation occurs in a hard vacuum of 0.1 to 10 mPa, at which pressures the mean free path of a gas atom, that is, the average distance the atom travels before colliding with another atom, is greater than the chamber dimensions. An atom evaporating from a source travels in a straight line; thus the process is line-of-sight limited, and coating around corners or reentrant angles is not possible without substrate manipulation. A description of the process and the equipment is found in Ref 61.

Insulative ceramic (yttriastabilized zirconia) Oxidationresistant bond coat Dense inner layer (ZrO2)

Gas turbine blade superalloy

Fig. 1 3 Cross section illustrating the strain-tolerant columnar ZrO2 microstructure of EB/PVD zirconia thermal barrier coatings. Source: Ref 62

Aluminum and chromium coatings for automotive trim are probably the largest application of this process. Hard coatings of chromium and Al 2 O 3 are sometimes deposited on steel or tungsten carbide tools, but this application has been replaced by sputter coatings. Aerospace applications use aluminum and nickel-chromium for corrosion protection and aluminum and silver as solid lubricants. Electron beam/physical vapor deposition (EB/PVD) is widely used in the gas turbine industry for applying MCrAlY metallic coatings on turbine blades and vanes for oxidation and corrosion protection (Ref 62). Zirconia thermal barrier coatings (TBCs) can also be deposited using this technique. Bond coats for EB/PVD TBCs are normally MCrAlY type coatings, similarly processed. The major advantage of EB/PVD TBCs is the columnar outer structure of the ZrO 2 (Fig. 13), which reduces the stress buildup within the body of the coating. These TBCs have superior degradation resistance that has been confirmed in gas turbine flight tests. For example, EB/PVD zirconia was found to be better than plasma sprayed zirconia or metallic MCrAlY after 4200 h tests of first-stage blades. Sputter coating is a vacuum process that involves the use of ions from a gas-generated plasma to dislodge coating atoms or molecules from a target made of the material that will become the coating. The plasma is established between the target and the substrate by the application of a direct-current potential or an alternating potential (radio frequency). An inert gas is introduced into the chamber to form the glow discharge plasma between the electrodes. The materials that can be sputter coated are pure metals, alloys, inorganic compounds, and some polymeric materials. A major restriction to be considered for the substrate material is the temperature of the process, which can range from 260 to 540 0C (500-1000 0 F). Details of the process and equipment have been reviewed (Ref 63). Sputtering is often used for depositing compounds and materials that are difficult to coat by thermal evaporation techniques. Engineering applications of sputter coatings include:

Corrosion and oxidation resistance, for example, nickel-chromium, MCrAlY, and polymers Lubrication, for example, silver, indium, MoS2, PTFE, selenides, silicides, and tellurides Wear resistance, for example, TiN, other nitrides, tungsten, molybdenum, carbides, borides, and diamondlike carbon Titanium nitride coatings are generally used for wear resistance, and Fig. 14 shows that TiN coatings increase the abrasion resistance of a hardened steel (Ref 57). Ion plating is a vacuum coating process in which a portion of the coating species impinges on the substrate in ionic form (Ref 57). The process is a hybrid of the thermal evaporation process and sputtering with the evaporation rate being maintained at a higher rate than the atoms that can be sputtered from the substrate. Some evaporant atoms pass through the plasma in atomic form, while some atoms collide with electrons from the substrate and become ions. They impinge on the substrate in ionic form, pick up electrons, and return to the atomic state, forming the coating. A detailed description of the process and equipment is provided in Ref 65. A variant process is reactive ion plating in which the metallic constituent (titanium) of the compound (TiN) is evaporated into the reactive gas mixture of argon and nitrogen that is enhanced by the glow discharge, depositing a golden-colored TiN coating on the substrate. Films of TiN are applied to a wide range of tools such as bits, punches, dies, taps, and so forth, to improve tool life by three to ten times (Ref 62). Figure 14 shows the improvement in abrasion resistance of reactive ion plating over other PVD sputtering processes. The results show that the higher-energy reactive ion plating process had more than an order of magnitude

52100 steel (60 HRC) RF-diode sputtered TiN dc-magnetron sputtered TiN Cathodic arc sputtered TiN Reactive ion plated TiN Relative low-stress abrasion resistance of TiN coatings (ASTM G 56 test apparatus) Fifi. 14 Effect of coating technique on the relative abrasion resistance of TiN on hardened steel applied by various processes. Source: Ref 64

improvement over the radio-frequency diode sputtered coatings (Ref 57). These results indicate that application technique should be made a coating selection factor.

Thermoreactive Deposition/Diffusion Process The thermoreactive deposition/diffusion process (TRD) is a method of coating steels with a hard, wear-resistant layer of carbides, nitrides, or carbonitrides. In the TRD process, the carbon and nitrogen in the steel substrate diffuse into a deposited layer with a carbide-forming or nitrideforming element such as vanadium, niobium, tantalum, chromium, molybdenum, or tungsten. The diffused carbon or nitrogen reacts with the carbide- and nitride-forming elements in the deposited coating so as to form a dense and metallurgically bonded carbide or nitride coating at the substrate surface. The TRD process is unlike conventional case-hardening methods, where the specific elements (carbon and nitrogen) in a treating agent diffuse into the substrate for hardening. Unlike conventional diffusion methods, the TRD method also results in an intentional buildup of a coating at the substrate surface. These TRD coatings, which have thicknesses of about 5 to 15 |xm (0.2-0.6 mil), have applications similar to those of coatings produced by CVD or PVD. In comparison, the thickness of typical CVD coatings (usually less than 25 |xm, or 1 mil) has about the same range as TRD coatings. Process Characteristics. The hard alloy carbide, nitride, and carbonitride coatings in the TRD method can be applied to steels by means of salt-bath processing (Ref 66-69) or fluidized beds (Ref 70). The carbide coating by salt-bath immersion was first developed in Japan and used industrially approximately 30 years ago under the name of the Toyota Diffusion (TD) coating process (Ref 66, 67). The TD method uses molten borax with additions of carbide-forming elements such as vanadium, niobium, titanium, or chromium, which combine with carbon from the substrate steel to produce alloy carbide layers. Because the growth of the layers is dependent on carbon diffusion, the process requires a relatively high temperature, from 800 to 1250 0C (1470-2280 0 F), to maintain adequate coating rates. Carbide coating thicknesses of 4 to 7 jxm are produced in 10 min to 8 h, depending on bath temperature and type of steel. The coated steels may be cooled and reheated for hardening, or the bath temperature may be selected to correspond to the steel austenitizing temperature, permitting the steel to be quenched directly after coating. Applications and Properties. The most common application area for TRD-processed steels is that of forming tools subjected to high wear and galling problems. Table 27 lists typical applications.

Table 27 Applications of TRD-processed tooling Application

Tool

Sheet metal working

Pipe and tube manufacturing Pipe and tube working Wire manufacturing Wire working Cold forging and warm forging Hot forging Casting (aluminum, zinc) Rubber forming Plastic forming Glass forming Powder compacting Cutting and grinding

Draw die, bending die, pierce punch, form roll, embossing punch, coining punch, shave punch, seam roll, shear blade, stripper guide pin and bushing, pilot pin, and so on Draw die, squeeze roll, breakdown roll, idler roll, guide roll, and so on Bending die, pressure die, mandrel, expand punch, swaging die, shear blade, feed guide, and so on Draw die, straightening roll, descaling roll, feed roll, guide roll, cutting blade Bending die, guide plate, guide roll, feed roll, shear blade Extrusion punch and die, draw die, upsetting punch and die, coining punch and die, rolling die, quill cutter, and so on Press-forging die, rolling die, upsetting die, rotary swaging die, closed-forging die, and so on Gravity-casting core pin, die-casting core pin, core, sleeve, and so on Form die, extrusion die, extrusion screw, torpedo, cylinder sleeve, piston, nozzle, and so on Form die, injection screw, sleeve, plunger, cylinder, nozzle, gate, and so on Form die, plunger, blast nozzle, machine parts, and so on Form die, core rod, extrusion die, screw, and so on Cutting tool, cutting knife, drill, tap, gage pin, tool holder, guide plate, and so on

TRD process

CVD method PVD method

Bonding

Plating Ferritic nitrocarburizing Sulfurizing Spark hardening

Hardened tool steel Cemented carbide

Microhardness, HV (50 g) FlC. 1 5

Surface hardness of carbide layers by TRD process in relation to other surface-hardening processes. Source: Ref 71

Punch

Mild steel

Die Lubricant: none No scuffing Slight scuffing Severe scuffing Spelling

Uncoated Nitrided Bonded N+ implanted Chromium plated Sulfurized Cemented carbide TiC + TiN (CVD) TiC (CVD) VC NbC

Wear loss, jam2

Wear

Coated

Fifi. 1 6 Comparative cross-sectional area of wear, scuffing, and spall ing on a & * die radius in a sheet steel-bending test. Source: Ref 71

The most important properties associated with TRD coatings are high hardness and wear resistance. Figures 15 and 16 compare the surface hardness and wear properties of TRD coatings with various other surfacehardening processes. Acknowledgment Portions of this chapter were adapted from A.R. Marder, Effects of Surface Treatments on Materials Performance, Materials Selection and Design, VoI 20, ASM Handbook, ASM International, 1997, p 470^90. References 1. J.P. Gossner and K.B. Tator, Painting, Surface Engineering, VoI 5, ASM Handbook ASM International, 1994, p 421-447 2. K.B. Tator, Organic Coatings and Linings, Corrosion, VoI 13, ASM Handbook, ASM International, 1987, p 399-418 3. Corrosion Control by Protective Coatings and Inhibitors, Corrosion: Understanding the Basics, J.R. Davis, Ed., ASM International, 2000, p 363-106

4. J.C. Oliver et al., Porcelain Enameling, Surface Engineering, VoI 5, ASM Handbook ASM International, 1994, p 456^68 5. W.W. Carpenter, Ceramic Coatings and Linings, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 469^81 6. Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994 7. D. Wetzel, Batch Hot Dip Galvanized Coatings, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 360-371 8. J. Foct, The Morphology of Zinc Coatings, The Physical Metallurgy of Zinc Coated Steel, A.R. Marder, Ed., TMS, 1994, p 1 9. CE. Jordan and A.R. Marder, Alloy Layer Growth During Hot-Dip Galvanizing at 450 0 C, Galvatech '95, Iron and Steel Society, 1995, p 319 10. A.R.P. Ghuman and J.I. Goldstein, Reaction Mechanisms for the Coatings Formed During Hot Dipping of Iron in 0 to 10 Pet Al-Zn Baths at 450-700 0 C, Metall Trans. A, VoI 2A, 1971, p 2903 11. Y. Hisamatsu, Science and Technology of Zinc and Zinc Alloyed Coated Steel Sheet, Galvatech y89, Iron and Steel Institute Japan, 1989, p 3 12. CE. Jordan and A.R. Marder, Morphology Development in Hot-Dip Galvanneal Coatings, Metall Mater. Trans. A, VoI 25A, 1994, p 937 13. A.R. Marder, Microstructural Characterization of Zinc Coatings, Zinc-Based Steel Coating System: Metallurgy and Performance, TMS, 1990, p 55 14. D.CH. Nevinson, Corrosion of Zinc, Corrosion, VoI 13, ASM Handbook, ASM International, 1987, p 755-769 15. Hot Dip Galvanizing for Corrosion Protection of Steel Products, American Galvanizers Association, 1989 16. H.E. Townsend, Continuous Hot Dip Coatings, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 339-348 17. CE. Jordan, K. Goggins, and A.R. Marder, Interfacial Layer Development in Hot-Dip Galvanneal Coatings on Interstitial Free (IF) Steel, Metall Mater. Trans. A, VoI 25A (No. 10), 1994, p 2101-2109 18. H.E. Townsend and J.C Zoccola, Mater. Perform. A, VoI 10, 1979, p 13-20 19. H.F. Graff, Aluminized Steel, Encyclopedia of Materials Science and Engineering, Pergamon Press, 1986, p 138-141 20. F. Lowenheim, Modern Electroplating, 3rd ed., John Wiley, 1974 21. J. W. Dini, Electrodeposition: The Materials Science of Coatings and Substrate, Noyes, 1993 22. M.G. Hocking, V. Vasatasree, and RS. Sidky, Metallic and Ceramic Coatings: Production, High Temperature Properties and Applications, John Wiley & Sons, 1989, p 206 23. J.R. Davis, Surface Engineering of Carbon and Alloy Steels, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994,

p 701-740

24. Engineering Coatings: Design and Application, S. Grainger, Ed., Abington Publishing, 1989, p 101 25. M.E. Browning, Section Ed., Plating and Electroplating, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 165-332 26. Quality Metal Finishing Guide, Metal Finishing Suppliers' Association 27. S.G. Fountoulakis, Continuous Electrodeposited Coatings for Steel Strip, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 349-359 28. AJ. Killmeyer, Tin Plating, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 239-241 29. DJ. Maykuth and W.B. Hampshire, Corrosion of Tin and Tin Alloys, Corrosion, VoI 13, ASM Handbook, ASM International, 1987, p 770-783 30. K.H. Stern, Electrodeposition of Refractory Metals from Molten Salts, Metallurgical and Ceramic Protective Coatings, K. Stern, Ed., Chapman and Hall, London, 1996, p 9 31. K.H. Stern, Metalliding, Metallurgical and Ceramic Protective Coatings, K. Stern, Ed., Chapman and Hall, London, 1996, p 38 32. A. Blair, Silver Plating, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 245-246 33. A.M. Weisberg, Gold Plating, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 247-250 34. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988, p 52 35. D.W. Baudrand, Electroless Nickel Plating, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 290-310 36. K. Barnak, S.W. Banovic, CM. Petronis, D.F. Susan, and A.R. Marder, Structure of Electrodeposited Graded Composite Coatings of Ni-Al-Al2O3, J. Microsc, VoI 185, part 2, Feb 1997, p 265 37. D.F. Susan, K. Barnak, and A.R. Marder, Diffusion and Oxidation Behavior of Electrodeposited Ni Al Particle Composite Coatings, Materials, Coatings and Processes for Improved Reliability of High Temperature Components, N.S. Cheruvu and K. Dannemann, Ed., to be published 1997 38. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988, p 209 39. Engineering Coatings, Design and Application, S. Grainger, Ed., Abington Publishing, 1989, p 33 40. J.N. DuPont and A.R. Marder, Thermal Efficiency of Arc Welding Processes, Weld. J., Dec 1995, p 406-s 41. J.N. DuPont and A.R. Marder, Dilution in Single Pass Arc Welds, Metall. Mater. Trans., VoI 27B, 1996, p 481

42. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988, p 242 43. M. Scholl, R. Devanathan, and R Clayton, Abrasive and Dry Sliding Wear Resistance of Iron-Molybdenum-Nickel-Silicon-Carbon Weld Hardfacing Alloys, Wear, VoI 135 (No. 2), 1990, p 355 44. K.H. Zurn Gahr, How Microstructure Affects Abrasive Wear Resistance, Met. Prog., Sept 1971, p 46-49 45. B.F. Levin, J.N. DuPont, and A.R. Marder, Weld Overlay Coatings for Erosion Control, Wear, VoI 181-183, 1995, p 810 46. RF. Tortorelli, LG. Wright, G.M. Goodwin, and M. Howell, HighTemperature Oxidation/Sulfidation Resistance of Iron-Aluminide Coatings, Elevated Temperature Coatings: Science and Technology II, N.B. Dahorte and LM. Hampikian, Ed., TMS, 1996, p 175 47. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988, p 219 48. R.C. Tucker, Jr., Thermal Spray Coatings, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 497-509 49. H. Herman and S. Sampath, Thermal Spray Coatings, Metallurgical and Ceramic Protective Coatings, K. Stern, Ed., Chapman and Hall, 1996, p 261 50. Thermal Spraying: Practice, Theory, and Application, American Welding Society, 1985 51. Engineering Coatings: Design and Application, S. Grainger, Ed., Abington Publishing, 1989, p 77 52. KJ. Stein, B.S. Schorr, and A.R. Marder, Erosion of Thermal Spray FeCrAlY-Cr3C2 Cermet Coatings, Elevated Temperature Coatings: Science and Technology II, N.B. Dahorte and LM. Hampikian, Ed., TMS, 1996, p 99 53. S.T. Bluni and A.R. Marder, Effects of Thermal Spray Coating Composition and Microstructure on Coating Response and Substrate Protection at High Temperatures, Corrosion, VoI 52, 1996, p 213 54. H.O. Pierson, Chemical Vapor Deposition of Semiconductor Materials, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 510-516 55. D.G. Bhat and RF. Woerner, Coatings for Cutting Tools, /. Met., Feb 1986, p 68 56. D.R. Chopra, A.R. Chourasia, M. Green, R.C. Hyer, K.K. Mishra, and S.C. Sharma, Diamond and Amorphous Films, Surface Modification Technologies IV, TS. Sudarshan, D.G. Bhat, and M. Jeandin, TMS, 1991, p 583 57. K.G. Budinski, Surface Engineering for Wear Resistance, PrenticeHall, 1988, p 138 58. Engineering Coatings, Design and Application, S. Grainger, Ed., Abington Publishing, 1989, p 119

59. D.M. Mattox, Section Ed., Vacuum and Controlled-Atmosphere Coating and Surface Modification Processes, Surface Engineering, VoI 5, ASM Handbook ASM International, 1994, p 495-626 60. M.G. Hocking, V. Vasatasree, and RS. Sidky, Metallic & Ceramic Coatings: Production, High Temperature Properties & Applications, J. Wiley & Sons, 1989, p 49 61. D.M. Mattox, Vacuum Deposition. Reactive Evaporation and Gas Evaporation, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 556-572 62. R.L. Jones, Thermal Barrier Coatings, Metallurgical and Ceramic Protective Coatings, K. Stern, Ed., Chapman and Hall, 1996, p 194 63. S.L. Rhode, Sputter Deposition, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 573-581 64. EJ. Lee and R.G. Bayer, Tribological Characteristics of Titanium Nitride Thin Coatings, Met. Finish., July 1985, p 39-^2 65. D.M. Mattox, Ion Plating, Surface Engineering, VoI 5, ASM Handbook, ASM International, 1994, p 582-592 66. T. Arai and N. Komatsu, Carbide Coating Process by Use of Salt Bath and its Application to Metal Forming Dies, Proc. 18th International Machine Tool Design and Research Conference, 14-16 Sept 1977, p 225-231 67. T. Arai, Carbide Coating Process by Use of Molten Borax Bath in Japan, /. Heat. Treat., VoI 18 (No. 2), 1979, p 15-22 68. T. Arai, H. Fujita, Y. Sugimoto, and Y. Ohta, Vanadium Carbonitride Coating by Immersing into Low Temperature Salt Bath, Heat Treatment and Surface Engineering, George Krauss, Ed., ASM International, 1988, p 49-53 69. I.E. Campbell, VD. Barth, R.F. Hoeckelman, and B.W. Gonser, Salt Bath Chromizing, J. Electrochem. Soc, VoI 96 (No. 4), 1949, p 262-273 70. T. Arai, J. Endo, and H. Takeda, Chromizing and Bonding by Use of a Fluidized Bed, Proc. International Congress Fifth Heat Treatment of Materials Conference, 20-24 Oct 1986, p 1335-1341 71. T. Arai and S. Harper, Thermoreactive Deposition/Diffusion Process, Heat Treating, VoI 4, ASM Handbook, ASM International, 1991, p 448-453

CHAPTER

M

P r o c e s s

C o m p a r i s o n s

PROCESS COMPARISONS discussed in this Chapter include: Process availability Corrosion resistance Wear resistance Cost Distortion or size change tendencies Thickness attainable In addition to the information presented below, tables and figures comparing surface-engineering process characteristics that appear in other Chapters should also be referred to. These are summarized in Table 1. Table 1 Additional process comparison data presented in other Chapters Source

Description

Chapter 1 Fig. 1 Table 1

Compares the thickness of various engineering coatings Categorizes the various surface-engineering options and lists their property benefits

Chapter 3 Table 4

Gives friction coefficient data for different coatings applied by various processes

Chapter 4 Table 2

Compares flame- and induction-hardening processes

Chapter 5 Table 6

Compares the typical characteristics of carburizing, nitriding, carbonitriding, and ferritic nitrocarburizing

Chapter 6 Fig. 14 Fig. 15

Fig. 16

Table 1

Compares the abrasion resistance of TiN coatings applied by various thin-film processes Compares the surface hardness of hardened tool steel and a cemented carbide with that of the following surface-hardening processes: TRD, CVD, PVD, boriding, chrome plating, electroless nickel-phosphorus plating, ferritic nitrocarburizing, sulfurizing, and spark hardening Compares the wear, scuffing, and spalling resistance of sheet-metal dies coated by the following surface-hardening processes: uncoated, nitrided, borided, nitrogen ion implanted, chrome plated, sulfurized, uncoated cemented carbide, TiC + TiN by CVD, TiC by CVD, VC by TRD, and NbC by TRD Compares the processing characteristics for electroplating, electroless plating, CVD, PVD, thermal diffusion, ion nitriding, TRD, ion implantation, ion-beam assisted deposition, and thermal spraying (continued)

TRD, thermoreactive deposition/diffusion process; CVD, chemical vapor deposition; PVD, physical vapor deposition

Table 1 (continued) Source

Description

Chapter 6 (continued) Table 9 Table Table Table Table Table

11 14 15 16 17

Table 19 Table 22 Table 26

Compares the wear and corrosion resistance of electroplated copper, electroplated nickel, electroless nickel, electroplated chromium, and electroless nickel + chromium Compares the Taber abrasion resistance of electroplated nickel, electroless nickel, and electroplated hard chromium Compares the characteristics of various weld overlay coatings Compares the applications of thermal spraying, welding, and electroplating Compares the process requirements in thermal spraying, welding, and electroplating Compares the design characteristics of flame, arc wire, high-velocity oxyfuel, detonation gun, air plasma, and vacuum plasma thermal spray processes Compares the abrasive wear resistance of tungsten carbide coatings applied by detonation gun, plasma, and high-velocity oxyfuel thermal spray processes Compares the deposition temperatures for thermal and plasma CVD Compares the processing characteristics of PVD, CVD, and ion implantation processes

Chapter 8 Table 1 Table 2 Table 3 Table 4 Table 5 Table 6

Compares thickness ranges and hardness levels of a wide range of surface-engineering processes Compares surface finish characteristics of various surface-engineering processes Compares size and weight limitations for different surface treatments Summarizes design limitations for surface preparation/cleaning processes Summarizes design limitations for organic coating processes Summarizes design limitations for inorganic (metal and ceramic) coating processes

TRD, thermoreactive deposition/diffusion process; CVD, chemical vapor deposition; PVD, physical vapor deposition

Process Availability One of the key considerations in the materials selection process is material availability and delivery time. This is especially true if a person/company has only a limited time for completing a part. Even without time constraints, materials engineers tend to use materials that are readily available. Similarly, the choice of a surface-engineering process is often based on process availability because poor logistics between the customer and surface treatment supplier can result in added shipping time and costs. In general, the long-established surface-engineering processes are available from numerous job shops in varied locations. These would include localized surface-hardening treatments, diffusion heat treatments such as carburizing and nitriding, weld surfacing, thermal spraying, electroplating, galvanizing, and painting. However, within these surface-treatment categories there may be a wide disparity in the availability of specific processes. For example, most heat treating job shops offer flame and induction localized hardening, but few have facilities for electron beam or laser localized surface hardening. The same can be said of diffusion heat treatments. In a survey of 800 commercial heat treating shops in the United States and Canada (Ref 1), 70% offered carburizing services, of which: 48% offered gas atmosphere carburizing 19% offered pack carburizing 12% offered salt-bath carburizing 5% offered carburizing in fluid beds

2% offered vacuum carburizing 1% offered plasma (ion) carburizing Thus, process availability might negate the selection of plasma carburizing over conventional methods, despite the reduced carburizing times and more uniform case depths associated with plasma methods. A similar situation exists for gas nitriding and plasma (ion) nitriding. The more specialized pack-cementation diffusion processes, such as aluminizing, chromizing, siliconizing, and boronizing, are usually carried out at companies that specialize in these processes. Some of these processes are also performed by aerospace companies, for example, aluminizing of jet engine turbine components. More recently developed coatings or surface modifications—such as chemical vapor deposition (CVD), physical vapor deposition (PVD), ion implantation, and laser melting, alloying, or cladding—are also performed by companies that specialize in these processes/coatings. For example, most cutting tool manufacturers offer CVD, PVD, or CVD + PVD processing. The availability of facilities offering various surface-engineering options can best be determined by contacting technical associations that offer information services for these surface treatments. Examples include: American Electroplaters and Surface Finishers Society American Galvanizers Association American Welding Society Association of Industrial Metallizers, Coaters, and Laminators Federation of Societies for Coating Technology National Paint and Coatings Association Powder Coating Institute Society of Vacuum Coaters Steel Structures Painting Council Thermal Spray Society or the International Thermal Spray Association The Society for Protective Coatings Descriptions of these organizations including their scope, addresses, telephone and fax numbers, web site or e-mail access, and so forth can be found in the Encyclopedia of Associations, published by Gale Group Publishing and available at most local public libraries.

Corrosion Resistance Corrosion-resistant protective coatings include various organic and inorganic coatings that provide barrier protection (e.g., a paint coating or multilayer electroplate) or sacrificial protection (e.g., zinc and aluminum

Table 2 Salt mist corrosion performance of various steels and coatings Surface

Coating life(a), h

Steels Low-alloy steel or low-carbon steel Induction-hardened carbon steel Stainless steel (316) Carburized mild steel Nitrided low-alloy steel Nitrocarburized/oxidized mild steel Nitrided stainless steel Phosphated mild steel Steam-tempered alloy steel

2 2 <2000 2 20 400 2 500-1000 300-500

Coatings (on mild steel) Hard chrome plate Crack-free chrome Electroless nickel (as-deposited) Hardened electroless nickel Electroless nickel + polymer Electroless nickel-PTFE Electroless nickel-SiC Nickel electroplate Nickel-ceramic electroplate Cadmium plate Zinc plate Zinc-9Ni plate Hot dip galvanized Hot dip aluminized PVD TiN Plasma sprayed ceramic High-velocity oxyfuel cermet Spray and fused nickel-chromium Slurry /sinter formed ceramic Aluimnum alloy 6082 Anodized Anodized + polymer in-fill

< 10 30-50 100-1000 50-500 1500 <20 <20 < 1000 500 <2000 1000 2000 1000 500 2 10 1000 2000 <2000 5 300 1800

PTFE, polytetrafluoroethylene. (a) Time at which five or more individual corrosion spots have formed on the upper facing of the test panel. Copyright AEA Technology pic; used with permission. Source: Ref 2

coatings). As described in Chapter 2, the corrosion resistance of these coatings is often determined by accelerated laboratory tests. Table 2 lists the results of the neutral salt-spray (fog) test described in ASTM B 117 on substrate and coating materials. These data should be used with caution because the corrosion response of a given coating changes from environment to environment. Coating suppliers should be consulted for final coating material selection. The type of coating process selected is dependent on the design factors described in Chapter 8.

Wear Resistance Hardness versus Wear Resistance. The wear processes that are usually mitigated by the use of hard surfaces are low-stress abrasion, wear in systems involving relative sliding of conforming solids, fretting wear, galling, and to some extent, solid-particle erosion (Ref 3). Unfortunately there are many caveats to this statement, and substrate/coating selection should be

Boron carbide CVD TiC coating P V D Ti N coating 1 Soft steel Silicon carbide Aluminum oxide ceramics I Cemented carbide cermets Boronizing of steels Plasma sprayed (AI2O3ZCr2O3ZWC-Co coatings) Chromium electroplate I Hardened tool steels Nitrided steel Fusion hardfacings LaserZEB harden CarbonitrideZcarburizeZcyanide Induction-hardened steels Flame-hardened steels Electroless nickel (before hardening)Znickel electroplate HSLA steels Mn steels (before work hardening) Hardness, kgZmm2 Fig, 1 Range of hardness levels for various materials and surface treatments. Source: Ref 3

carefully studied with proper tests carried out if necessary. Coating suppliers should also be consulted. Chapter 3 provides additional information on wear processes and the means to prevent specific types of wear. Figure 1 shows typical ranges in hardness for many of the surfaceengineering processes used to control wear. All of the treatments shown in this figure have hardness values greater than ordinary constructional steel or low-carbon steel. The surface-hardening processes that rely on martensitic transformations all have comparable hardness, and the diffusion treatments that produce harder surfaces are nitriding, boronizing (boriding), and chromizing. The hardest metal coating is chromium plate, although hardened electroless nickel plate can attain values just under that of chromium. The surfaces that exceed the hardness of chromium are the cermets or ceramics, or surfaces that are modified so that they are cermets or ceramics. These include nitrides, carbides, borides, and similar compounds. The popular solid ceramics used for wear applications—aluminum oxide, silicon carbide, and silicon nitride—generally have hardnesses in the range of 2000 to 3000 kg/mm2. As shown in Fig. 1, when materials such as aluminum oxide are applied by plasma spraying or other thermal spray process, they have hardnesses that are less than the same material in solid pressed-and-sintered form. This is because the sprayed

Table 3

Comparison of t h e r m a l spray methods

As described in the text, coating porosity affects coating hardness. Flame or exit plasma temperature

Method

Combustion powder Combustion wire Arc wire Plasma High-energy plasma Vacuum plasma D-gun HVOF

C

°F

lb/h

11

400

2,200

4,000

30

100

7

15

6-15

71

2,500

2,800

5,000

180

600

9

20

6-15

5,500 5,500 11,000

10,000 10,000 20,000

240 240 240-1,200

800 800 800-4,000

16 5 23

35 10 50

2-8 <2 <1

11,000

20,000

240-610

800-2,000

11

24

<0.5

3,100 3,100

5,600 5,600

3,000 2,000-5,000

1 14

2 30

<1 <0.5

m3/h

71 4.2 17-28 8.5 11 28-57

ft3/h

2,500 150 600-1,000 300 400 1,000-2,000

0

Atmosphere around particles

Maximum spray rate

Coating porosity, %

Gas flow

Particle impact velocity m/s

910 610-1,500

ft/s

kg/h

D-gun, detonation gun; HVOF, high-velocity oxyfuel. Source: Ref 4

materials contain porosity and oxides that are not contained in the sintered solid form. Table 3 shows the coating porosity that can be expected from variations in the thermal spray process. The other hard surface for tools, cemented carbide, has a hardness of about 2000 kg/mm2, about twice as hard as the hardest metal. Recently developed diamond and diamondlike carbon coatings deposited by CVD processing have hardness levels in excess of 5000 kg/mm2. Table 4 Low-stress abrasive wear rankings for various materials See text for details. Low wear rate 100 200 300 400 500 800 1,000

1,500 4,000 5,000 8,000 10,000 12,000 15,000

50,000

HVOF WC-Co CVD CrC (high-carbon low-alloy tool steel) CVD CrN (high-chromium tool steel) Carbide diffusion process PVD CrN, 30 jjim thick Hard chrome plate Sprayed and HIP chromium Plasma sprayed alumina-titania Electroless nickel-ceramic Boronized 316 stainless steel Plasma sprayed chromium oxide Spray and fused nickel-chromium-chromium carbide Carburized steel Induction-hardened 0.4% C steel Slurry/sinter formed ceramic Nitrided 316 stainless steel Hardened electroless nickel As-plated electroless nickel 0.4% C steel, normalized 316 stainless steel PVD CrN (2 |xm thick) Anodized aluminum alloy Aluminum alloy High wear rate

CVD, chemical vapor deposition; PVD, plasma vapor deposition; HIP, hot isostatically pressed. Copyright AEA Technology pic; used with permission. Source: Ref 2

Table 5 Erosive wear rankings for various materials Test conditions: 1000 ppm of silica sand in water with an impact velocity of 25 m/s (80 Ws) Low wear rate 100 200 300 700 800 1000 1500 2000

High-chromium iron weld overlay Spray and fused nickel-chromium-chromium carbide Boronized 316 stainless steel High-energy sprayed WC-Co Hard chrome plate Nitrided 316 stainless steel Electroless nickel Slurry/sinter formed ceramic PVD TiN 316 stainless steel High wear rate

PVD, plasma vapor deposition. Copyright AEA Technology pic; used with permission. Source: Ref 2

Test Results. Table 4 shows results of the ASTM G 65 dry-sand/rubberwheel test on various coatings. The low-stress abrasion resistance performance is indexed to that of the best quality tungsten carbide-cobalt (WC-Co) coating, denoted a value of 100, and is related to volume loss per revolution of the wheel under a fixed load, at constant speed and abrasive throughput. Table 5 shows the resistance of various coatings to erosive wear. The results are indexed to that of a high-chromium cast iron hardfacing alloy, again denoted by a value of 100. Table 6 shows typical adhesive dry rubbing wear values for surface treatments and coatings. These were determined from a pin-on-plate Table 6 Adhesive wear rates of various materials Wear rate, m3/N • m

Material Lubricated through-hardened steel HVOF WC-Co Plasma sprayed chrome oxide PVD TiN (not at high loads) CVD CrN or alumina Hard chrome plate Nitrided tool steel Nitrided stainless steel (not at higher loads) Slurry/sinter formed ceramic (not higher loads) Carburized steel Nitrided low-alloy steel Unlubricated through-hardened steel Glass-filled PTFE Anodized aluminum Hardened electroless nickel Electroless nickel, as plated Normalized, unlubricated steel Austenitic stainless steel Copper plate Electrolytic nickel plate Aluminum alloy Unfilled PTFE coating Cadmium and zinc plates Unfilled PFA or FEP polymer coatings Silver plate

HVOF, high-velocity oxyfuel; PVD, plasma vapor deposition; CVD, chemical vapor deposition; PTFE, polytetrafluoroethylene; PFA, perfluoro alkoxy alkaline; FEP, fluorinated ethylene propylene. Copyright AEA Technology pic; used with permission. Source: Ref 2

sliding test using a polished hardened steel pin rubbing against the treated surface at a load of 10 N/m2 (-100 gf/ft2). Cost of Surface Treatments Cost must be weighed against the performance required for the surfacetreatment system. A low-cost surface treatment that fails to perform its function is a wasted expense. Unfortunately, it is nearly impossible to give absolute comparative costs for different surface-engineering options. Often, a range of prices will be offered for a particular job from different, equally competent candidate suppliers. Probably the most important factor that relates to costs of producing a corrosion- or wear-resistant surface on a part is part quantity. Treating many parts usually allows economies in treatment and finishing. Another consideration when assessing surface treatment costs is part size. There are some critical sizes for each surface-treatment process above which the cost of obtaining the treatment may be high. A number of surface treatments require that the part fit into the work zone of a vacuum chamber. The cost of vacuum equipment goes up exponentially with chamber volume.

SAW FCAW GMAW SMAW OAW GTAW FLSP PSP EB and laser

Relative cost Fig, 2 Relative costs (based on pounds of alloy deposited) for various weld overlay and thermal spray processes. SAW, submerged arc welding; FCAVV, flux-cored arc welding; GMAW, gas metal arc welding; SMAW, shielded metal arc welding; OAW, oxyacetylene gas welding; FLSP, flame spraying; PSP, plasma spraying; EB, electron beam. Source: Ref 3

Thermochemical: Carburizing Nitriding Nitrocarburizing Electrochemical: Chromium Cobalt + Cr3C2 Electroless: Nickel Plasma sprayed: WC-Co AI2O3 Combustion gun sprayed: 13% Cr wire Ni-Cr-B and fuse Ni-Cr-B + WC and fuse Surface weld: Iron-base Cobalt-base Vapor deposited: CVD TiC PVD TiN Cost Fig. 3 Approximate relative costs of various surface treatments

Other factors to be considered are: The time required for a given surface treatment Fixturing, masking, and inspection costs Final finishing costs Material costs Energy costs Labor costs Environmentally related costs, for example, disposal of spent plating solutions Expected service life of the coating Because of these various factors, it is difficult to compare costs with a high degree of accuracy. Figures 2 and 3 provide some general guidelines for cost comparisons. Distortion or Size Change Tendencies Figure 4 shows the surface temperatures that are encountered in various surface-engineering processes. As indicated in the figure, the processes are categorized into two groups: one group produces negligible part dis-

Negligible part distortion on ferrous metal

Likely distortion Furnace fusing Metal tiding Ferritic nitrocarburizing Brazing of WC wear tiles

Wear plates Sleeving Repair cements

EB and laser melting

Ion implantation Sputter coating Thermal evaporation coatings CVD coatings Selective hardening Plating Carburizing and pack cementation Nitriding Fusion welding Quench hardening alloy and tool steels Thermal spray coatings Temperature, 0F Temperature, 0C FlC. 4 Maximum surface temperatures that can0 be anticipated for various surface-engineering processes. The dashed vertical line at 540 C (1000 0F) represents the temperature limit for 0 distortion for ferrous metals. Obviously, a temperature of 540 C (1000 0F) would melt a number of nonferrous metals, and it would cause distortion on metals such as aluminum or magnesium. However, this process temperature information can be used to compare the heating that will be required for a particular process. Source: Ref 3

tortion, and the other group contains processes that have varying potential for causing distortion. Obviously if a part could benefit from a surface treatment, but distortion cannot be tolerated, processes that require minimal heating should be considered.

Coating Thickness Attainable Figure 5 shows the typical thickness/penetration capabilities of various coating and surface treatments. As indicated in the figure, some surfaceengineering treatments penetrate into the surface and there is no intentional buildup on the surface. These are the surface-engineering processes described in Chapters 4 and 5. Other surface treatments coat or intentionally build up the surface. This is a selection factor. Can a part tolerate a buildup on the surface? If not, the selection process is narrowed to the

Carbonitriding Carburizing Flame hardening Laser hardening EB hardening Induction hardening Nitriding Pack cementation Cyaniding _ Ferritic nitrocarburizing Ion implant Depth of penetration of surface treatment, in.

Rebuilding cements Wear plates SAW hardfacing FCAW/GMAW hardfacing PAW hardfacing GTAW hardfacing Laser/EB hardfacing OAW hardfacing Electroless plating Electroplating FLSP coatings PSP coatings

CVD Ion plating PVD coatings (thermal) Sputter coating

Part surface

Normal thickness range

Surface coating thickness, in.

Fig. 5 Typical coating thickness/depth of penetration for various coating and surface-hardening processes. Source: Ref 2

treatments that penetrate into the surface. Other factors affecting the thickness of a given surface treatment include dimensional requirements, the service conditions, the anticipated/allowable corrosion or wear depth, and anticipated loads on the surface. Questions or concerns related to coating thickness should be discussed with the contractor. Available specifications should also be reviewed. Additional information regarding the thicknesses associated with various surface-engineering processes can be found in Chapters 4 to 6 and 8. References

1. W.L. Kovacs, Commercial and Economic Trends in Ion Nitriding/ Carburizing, in Ion Nitriding and Ion Carburizing, ASM International, 1990, p 5-12 2. K. Stevens, "Surface Engineering to Combat Wear and Corrosion: A Design Guide," The Institute of Materials, London, United Kingdom, 1997 3. K. Budinski, Selecting a Wear-Resistant Surface, Chapter 12, in Surface Engineering for Wear Resistance, Prentice-Hall, 1988, p 303-345 4. M.L. Thorpe, Thermal Spray: Industry in Transition, Adv. Mater. Process., VoI 143 (No. 5), 1993, p 50-56

O CHAPTER

\ J

P r a c t i c a l G u i d e l i n e s

D e s i g n f o r

S u r f a c e

E n g i n e e r i n g

THE DESIGN ENGINEER is faced with a wide range of options when selecting a surface treatment for a given problem or application. Some of the important factors described in this Chapter that must be considered before selecting a surface treatment include (Ref 1): The function of the component. Is it rolling, sliding, in static contact, and so forth? The base material. Is it a low-carbon steel, medium-carbon steel, lowalloy steel, a nonferrous alloy, and so forth? The fabrication method. Is it cast, welded, machined, and so forth? Temperature restrictions, that is, the temperature that must not be exceeded when carrying out a surface-engineering treatment. Will distortion of the component result? The interactions to which the component will be subjected, for example, sliding, rolling contact, static corrosion, including special requirements for strength or fatigue resistance The operating environment. Is it corrosive or abrasive in nature? Is it saline, oxidizing, caustic, and so forth? The temperature of the environment. What is the maximum temperature the component will likely see in service? The material from which any component or product in rubbing contact with the part is made, that is the counterface material and its hardness. Does the counterface material contain a hard abrasive filler? The predominant mode of degradation. Is it corrosion, wear, fatigue, and so forth?

The essential requirements for successful performance, for example, low-stress abrasion, high-stress abrasion, nonsticking, and so forth The contact load (maximum value) and likely contact area. Is it over a large area or concentrated? The contact conditions, for example, impact, cyclic loading, static loading, or sliding The required surface hardness of the component The requirements for surface roughness (or smoothness) Constraints on any final or finishing operations. Are there any critical dimensions or tolerances that must be met after processing? The required surface coverage and thickness of any treatment The geometry of the component. Are holes, sharp edges, enclosures, reentrants, and so forth, present? The overall size and weight of the component Are there special requirements that must be met, for example, Department of Defense (DOD), Food and Drug Administration (FDA)? Appearance, for example, color or texture

Surface-Engineering Solutions for Specific Problems (Ref 1) This section provides surface-engineering solutions for seven operating conditions: Structural parts, for example, pipes, pump and valve bodies, casings, housings, supports, rigs, tanks, and so forth, subjected to corrosive conditions in various environments A part in static contact with another engineering component with small relative motions or vibrations A part in static contact with a product that is being cast, molded, cured, and so forth A part in rolling contact with another part, for example, shafts, journals, pistons, rings, gears, seals, and tools for metal pressing, forming, drawing, and cutting A part under light mechanical load but which handles, rubs, or slides against an abrasive product, for example, paper, filled plastics, textile yarns, leather, friction materials for clutches and brakes, pharmaceuticals, and some foodstuffs like wheat and soy A part under high mechanical load, with or without impact, that handles or slides against abrasive or erosive materials, for example, coal chutes and conveyors, crushers, digging equipment, and so forth A part in rolling or sliding contact with another part in the presence of corrosive or abrasive materials, for example, pumps, valves, mechanical seals, and slurry handling

Emphasis is placed on the base material, operating conditions, and applicable surface treatments. As can be seen, alternative coating processes/ materials may be recommended for a given material/operating condition combination. Final selection may be based on some of the application and performance requirements listed above and further examined in subsequent sections of this Chapter.

Structural Parts in Corrosive Environments (Ref 1) If the part is structural with no sliding or rubbing contacts, then the main concern will be corrosion. Also, if there is cyclic loading—that is, fatigue—corrosion can considerably accelerate mechanical failures. Environmentally assisted cracking due to corrosion fatigue, stress-corrosion cracking (SCC), or hydrogen damage is discussed in Chapter 2. Base Material Base materials for structural parts are commonly an engineering steel, cast iron, stainless steel (most probably a ferritic or martensitic type), or an aluminum alloy. Neutral

Environments

If there is no concern about corrosion, but there is a requirement for improved strength or fatigue resistance, the following surface treatments should be considered: Shot peening Improving the surface quality and finish by grinding, lapping, or polishing Specific Corrosive

Environments

If there is concern about corrosion, then both the corrosive medium and the temperature are important. Also, if the part is in contact with another metallic component of a dissimilar material, then galvanically assisted corrosion, which accelerates failure, is very possible. For outdoor, normal atmospheric corrosion, consider: Hot dip galvanizing, which can provide prolonged protection even in polluted environments Thermally sprayed zinc or aluminum Electrolytic zinc Painting or powder coatings with appropriate surface preparation and priming Heavy electrolytic nickel provided there are no defects in the coating

Electroless nickel-phosphorus coating Aluminum ion plating Phosphating for moderate protection Anodizing, preferably sealed, for aluminum alloys For more hostile environments, including marine and aerospace where galvanic corrosion will be a major concern, consider: Hot dip galvanizing, which will provide moderate protection Thermally sprayed zinc or aluminum for moderate protection Electrolytic zinc or zinc-nickel alloy (10-14% Ni) coating followed by chromate passivation and an organic topcoat Painting, with appropriate preparation and priming, perhaps zinc or aluminum loaded Cadmium plate, preferably chromate passivated for maximum protection High-Temperature Oxidation and Corrosion. The substrates for high-temperature corrosion applications are often superalloys, stainless steels, or titanium alloys. Protective coatings to be considered include: Diffusion chromizing for oxidation resistance up to 750 to 800 0C (1380-1470 0F) Diffusion aluminizing for protection against oxidation, carburizing, and sulfur and vanadium corrosion in chemical plants and gas turbines; can be effective above 800 0C (1470 0F) Slurry/sinter formed ceramics (chromium oxide based) at temperatures up to 600 0C (1110 0F) Thermally sprayed coatings, for example, MCrAlY corrosion protection layers and ceramic-based thermal barriers For caustic environments, consider: Slurry/sinter-formed ceramics (chromium oxide based) Thermally sprayed ceramics, for example, chromium oxide, alumina, preferably sealed Cadmium plate, preferably chromate passivated for moderate protection Electroless nickel Heavy electrolytic nickel plating For acidic environments, consider: Slurry/sinter-formed ceramics (chromium oxide based) Thermally sprayed ceramics, for example, chromium oxide, alumina, preferably sealed

For stress-assisted corrosive conditions, consider shot peening followed by corrosion protection appropriate to environmental conditions, but only those processes applied at near-ambient temperature.

Parts in Static Contact w i t h Vibration (Fretting) (Ref 1) If the part is in contact with another engineering component, but with no relative movement, then the main concern will be with corrosion. If the mating part is a dissimilar metal, then galvanic corrosion will be a significant risk (see Chapter 2). If the contact also involves vibration or impact motion then fretting, fretting corrosion, or even fretting fatigue must be considered. This is the case with splines and couplings where motion is transmitted from one part to another via a loaded contact and in parts fastened or fitted together where there is a source of external vibration, for example, heat exchangers and bearing housings. Fretting-type failures are also found on chains, pulleys, and wire ropes. Base Material Fretting corrosion is most prevalent with steel parts where the oxidation process produces an obvious, distinctive, red oxide abrasive dust. Stainless steels are not immune, particularly ferritic types. Fretting of aluminum alloys produces a white oxide debris that is also very abrasive. Contact

Conditions

Fretting and Fretting Corrosion. With light loads or low-cycle fatigue, the effects of fatigue will usually be small, and the preferred solution is to reduce the tendency to oxidation by applying an inert coating. Consider the following treatments: Hot dip galvanizing Heavy electrolytic nickel or copper plating, which will provide a lowcorrosion surface but one with a tendency to gall and to wear quickly Electroless nickel, which will provide good oxidation protection, with extra wear resistance if hardened. Additional improvements can be obtained by adding a further solid-lubricant coating of molybdenum disulfide (MoS2), ideally in an epoxy binder Hard chrome plate for maximum wear protection Silver or indium plating, which provides a soft, ductile interface with good oxidation resistance Anodizing for protection of aluminum alloys, preferably sealed with a self-lubricating polymer such as polytetrafluoroethylene (PTFE)

Fretting Fatigue

With high loads or prolonged operation, fretting may lead to crack initiation followed by fretting fatigue. Since electroplating can impair fatigue resistance of the substrate, the best solutions are usually the intrinsically hard and tough thermally sprayed coatings. These include: Nickel-chromium for corrosion resistance and toughness in impact fretting Tungsten carbide-cobalt (WC-Co) for maximum wear resistance Nickel-chromium-chromium carbide for higher-temperature fretting Oxidative Wear If there is a small, slow-speed relative sliding between the parts, this may also lead to a fretting-type wear condition, with the oxidized wear debris trapped in the contact. It is common on chain links and wire ropes and sometimes occurs on pulleys. The only viable solution for wire ropes is regular oil or grease soaking. For parts under high loading, and that are traditionally made of highstrength engineering steels, there is usually no easy way to reduce the corrosive contribution to the wear process. The best approach is to increase the surface hardness so that it can resist the abrasion by the oxide debris. For example, consider: Local surface hardening, for example, flame, induction, or laser for medium-carbon steels Case hardening, for example, carburizing, carbonitriding for lowcarbon steels Nitriding or nitrocarburizing if the loads are not too high and the steel has some alloying elements such as chromium or molybdenum Parts in Static Contact w i t h a Product (Ref 1) This operating condition applies to molding, casting, and activities such as baking and curing when a product is held against the component surface for an extended time. The issue is not usually one of wear; rather the principal requirement is that the product and component will separate without adhesion or damage to either surface. However, in many cases, the product must first flow into the mold and, if pressure is applied during processing, the product may also creep across the surface as it cures or sets. All of these can cause wear. Base Material In many food applications the substrate will be stainless steel or an aluminum alloy. Dies and molds for plastic molding are most likely to be

made of alloy steel, aluminum, brass, or copper. Die casting of aluminum or zinc products will generally use hot-work (H-series) tool steels; glass molding uses tool steels, cast irons, and beryllium-copper alloys.

Specific Applications For food baking and molding, consider: Fluorinated polymer coatings. Fluorinated ethylene propylene (FEP) provides the best release. Perfluoro alkoxy alkane (PFA) gives release and wear resistance. PTFE provides best low friction. Ensure that the grade chosen is approved for food use. Electroless nickel plus PTFE, which will provide both low friction and good release properties, but not high wear resistance Anodizing plus PTFE seal for nonsticking aluminum alloy parts For plastic injection molding tools, consider: Nitriding of alloy steel parts when the plastic is filled and abrasive Hard chrome plate for steel, brass, or copper parts when abrasion is expected Ion implantation for improved wear resistance of alloy tool steels and chrome-plated parts Anodizing plus PTFE seal for nonstick and wear resistance with aluminum alloy parts For die casting, consider: Nitriding for H-series tool steels Physical vapor deposition (PVD) coatings, for example, TiN, TiAlN For glass molding, consider: Diffusion chromizing on cast iron molds for hot erosion-corrosion resistance Slurry/sinter formed ceramics (chromium oxide based) coatings Hard chrome plate (crack-free form is best)

Parts in Sliding or Rolling Contact with Another Surface (Ref 1) If the part is in sliding or rolling contact with another engineering component then, even if it is lubricated, there is the likelihood of adhesive wear. Adhesive wear can occur in many engineering situations, for example, shafts, journals, pistons and rings, cams, bearings, pads gears, seals, slide-

ways, and so forth, in metal cutting, drawing, and forming. In general, if both mating parts are metallic, it will be the softer part that suffers the greater wear and should be surface engineered. However, in cases where replacement of a particular part is difficult, then it is that part that should be protected, even at the expense of extra wear on the mating surface. If the mating part is nonmetallic, wear could still occur on the counterfacing component. If the counterface is plastic, determine whether it has any fillers that could cause abrasive wear. This would also be the case if the counterface was a ceramic. Base Material Common base metals include cast iron, low-carbon steel, mediumcarbon steel, alloy steel (including tool and bearing steels), stainless steel (austenitic, martensitic, or ferritic), aluminum alloys, titanium alloys or other nonferrous metals, for example, bronzes, copper, and brasses. General Contact Conditions Is the Part Lubricated? If it is, or could be, then wear, even without surface engineering, might be reduced by a factor of 1000 compared to running unlubricated. In lubricated systems under high load and at high speed (e.g., cams and tappets, piston bores and rings) there is still the possibility of scuffing. Is the Part Unlubricated yet There is a Need to Reduce Friction? For dry sliding it is important to specify the exact requirements. A low friction coefficient (see Chapter 3) can be defined as 0.1 or less and is generally achieved with polymers such as PTFE, but these have high wear rates. If, without surface engineering, the friction would be unacceptably high, for example, galling between two soft steel parts, then most surface treatments will reduce friction as well as reduce the wear. Is the Specific Loading High? Loads above 100 MPa (14.5 ksi) are considered high, in which case the hardness and thickness, or case depth, of the surface treatment is the critical factor. Both the substrate and the coating must be able to withstand that load. It is important to remember that rolling parts are often under high specific loading. What are the Requirements for Reducing Wear? In general, the higher the hardness of the surface layer, the lower will be the wear. It is vital to understand the consequences of the wear; for example, it may be that an increase in clearance between two parts must be avoided in service. If the wear is concentrated in a small area, then even a low wear rate will lead to a rapid increase in the clearance, and a high surface hardness is needed in that area. If the wear is spread out over a wider area the corresponding increase in clearance will be smaller, and a simpler, less expensive solution may be adopted. Is There an Element of Corrosion? For instance, if moisture or salt water is present there is a major risk of combined wear and corrosion that

can rapidly increase surface material loss. Select coatings with good corrosion resistance rather than high hardness. Corrosion is generally the more damaging feature.

Surface-Engineering Options For mild steel or cast iron parts, consider: Case hardening, that is, carburizing or carbonitriding for high hardness and load-carrying capacity. With case-hardening processes, however, distortion problems must be considered. Nitriding or nitrocarburizing. This gives a thin compounded layer. Electroless nickel and associated composites for corrosive wear. Heat treating at 400 0C (750 0F) will provide additional hardness and wear resistance. Ceramic-filled electroless nickel gives greater wear resistance. PTFE-filled electroless nickel will give low friction, but high wear if the load is high. Hard chrome plate for excellent wear protection, for example, on auto body dies. Select the thickness according to the load. Good for corrosive wear if protected with an underlayer of electrolytic nickel. Thermally sprayed metals or alloys. Use for wear and corrosion. Thermally sprayed ceramics or cermets, for example, WC-Co, alumina, chromium oxide, and so forth, for maximum wear resistance. If corrosion is likely, a corrosion barrier is required under the hard coating. Hot dip galvanizing on steel parts. Use if corrosive wear is likely. For medium-carbon steel parts, consider the surface treatments listed above for mild steels, plus local surface hardening, for example, flame, induction, or laser for maximum loading, and for large rolling components (e.g., large cylindrical roller bearings and tracks). For low-alloy steel parts (steels containing chromium, vanadium, and/or molybdenum), consider the surface treatments listed above for both mild and medium-carbon steels, plus nitriding or nitrocarburizing to give a diffused case. Follow with oxidation and oiling treatment for corrosive conditions. High-Alloy and Tool Steel Parts. Tool steels or high-speed steels (including AISI 440C, and the A, D, and M series steels) can be heat treated to high hardnesses and are wear resistant in their own right. For additional wear resistance and to reduce pickup, particularly in metalworking operations, consider: Nitriding or nitrocarburizing. Follow with oxidation and oiling treatment for reduced pickup. Used on warm forming tools. Hard chrome plate for excellent wear prevention, but postprocessing heat treatment may be necessary to prevent hydrogen embrittlement. Good for deep-drawing tools

PVD coatings, for example, TiN, CrN, and diamondlike carbon will give low friction and low wear under moderate loads. Used on cutting and cold-forming tools Chemical vapor deposition (CVD) coatings, for example, TiN, TiC/TiN, Al2O3, and TiC for higher loads than with PVD. Used on carbide inserts and other cutting tools, cold- and hot-forming tools Carbide diffusion, also called Toyota diffusion process (see Chapter 6), for high-carbon and precarburized steels. Uses a salt bath to produce a vanadium carbide (VC) layer For austenitic (300 series) stainless steel parts, consider: Electroless nickel and composites used as-deposited or heat treated to provide some wear resistance. Ceramic-filled electroless nickel will give greater wear resistance. PTFE-filled electroless nickel will give low friction but high wear if the load is high. Hard chrome plate for excellent wear resistance. Choose the thickness according to the load. Thermally sprayed metals or alloys for wear and corrosion Thermally sprayed ceramics or cermets, for example, WC-Co, chromium oxide, alumina, and so forth, for maximum wear resistance Nitriding or nitrocarburizing can produce a very high surface hardness. However, all corrosion resistance will be lost. For aluminum or titanium alloy parts, consider: Electroless nickel and composites for corrosive wear. Hardening at 400 °C (750 0F) will provide additional wear resistance. Ceramicfilled electroless nickel will give greater wear resistance. PTFE-filled electroless nickel will give low friction, but high wear if the load is high. Hard chrome plate for excellent wear resistance. Choose the thickness on the basis of the load. Thermally sprayed metals or alloys for wear and corrosion Thermally sprayed ceramics or cermets, for example, WC-Co, alumina, chromium oxide, and so forth, for maximum wear resistance Anodizing for wear protection of aluminum alloys; can be sealed with PTFE for reduced friction. Anodizing of titanium produces only a very thin decorative layer. Nitriding or nitrocarburizing for wear protection of titanium alloys (requires high-temperature processing) Bronze, Brass, and Copper Parts. Some copper and copper-base alloy substrates have relatively low load-carrying capacity. The principal options to reduce wear are:

Electroless nickel and composites for corrosive wear conditions. Heat treating at 400 0C (750 0F) will provide additional wear resistance. Ceramic-filled electroless nickel will give additional wear resistance. PTFE-filled electroless nickel will give low friction, but high wear if the load is high. Hard chrome plate for excellent wear resistance. Choose the thickness according to the load. Thermally sprayed metals or alloys for wear and corrosion Thermally sprayed ceramics or cermets, for example, WC-Co, chromium oxide, alumina, and so forth, coatings for maximum wear resistance Specific Contact Conditions Rolling and Rolling/Sliding Contact. In rolling-element bearings and similar rolling components, the base material is usually a temper-sensitive steel. Concern about the effect of surface engineering on tolerances and possible distortion leave few options. In some circumstances, the following surface-engineering options may be considered: PVD coatings, for example, TiN, CrN, MoS2, and so forth, but must be processed at below the tempering temperature of the steel Oxide treatments, for example, caustic treatment of needle rollers Hard chrome plate, using the thin, dense variety (restricted to approximately 5 |xm thick) For gears, where the motion is combined rolling and sliding, the main options are: Case hardening, that is, carburizing or carbonitriding, of low-carbon steels to give high hardness and wear resistance Local surface hardening of medium-carbon steels to give maximum load capacity Nitriding of alloy steels for lower loads Scuffing Conditions. Cams and tappets, cylinders and pistons, even when lubricated, can be prone to scuffing. Options include: Hard chrome plate for moderate-speed cylinder bores Electrolytic nickel/ceramic composite for cylinder bores in highrevving engines Nitrocarburizing for tappets and cams made from nitriding steels Diamondlike carbon for high-revving cams and tappets on a polished hard substrate Reducing Friction in Dry Sliding. For any base material, certain polymer systems can be considered:

PTFE in a binder, wet sprayed and cured PFA wet sprayed and melt flowed at 400 0C (750 0F) MoS 2 wet sprayed in a phenolic binder Electroless nickel plus PTFE heat treated to give improved wear performance Diamondlike carbon, but only on a hard, polished substrate Sliding against Nonmetallic, Abrasive Counterfaces. This operating condition might be the case for some plain or journal bearings or for mechanical seals. If the counterface is a polymer, it will probably contain an abrasive filler. If the counterface is a ceramic or cermet, then its surface roughness will greatly influence its abrasiveness. For all of these conditions, high hardness must be the basis on which a surface-engineering treatment is chosen. Examples include: Hard chrome plate, which is the best of the electroplates. Electroless nickel, even hardened is not recommended. Sprayed ceramic or cermet, for example, chromium oxide. A ceramic versus ceramic combination is possible.

Parts in Low-Load Sliding Contact w i t h an Abrasive Product (Ref 1) Many products are abrasive, either as a result of their basic structure and composition or through the action of added fillers or pigments. In lowload situations (as defined by the product areas discussed later in this section), the choice of surface treatment can be made primarily on the basis of surface hardness, since even very thin coatings are able to support the contact loads. The industrial areas covered in this section include textiles, printing, plastics, packaging, food, Pharmaceuticals, leather goods paints, inks, ceramic powders, and wood processing. It is assumed that the part in question is in direct contact with the product (e.g., a textile guide, a print roller, a wood-cutting tool, a food chute, etc.) and not with another engineering component. The applications also cover seals, where a nonmetallic, for example, a filled polymer or elastomer, part is in sliding contact with a shaft or a thrust pad. Base Material The substrate will usually be mild steel, low-alloy steel, austenitic stainless steel, or an aluminum alloy. Tool steels will be used for knives or other cutting or trimming tools.

Specific Applications Chipboard, Wood, or Composite Products and Ceramic Powder Handling. The content of wood products is always uncertain, with metal and mineral contaminants being common. The only safe solutions are thermally sprayed or welded ceramics or cermets, for example, WC-Co, alumina, chromium oxide, and so forth, for maximum wear and damage resistance. Synthetic Textiles (Nylon, Polyester), Glossy Newsprint, GlassFilled Plastics (Including Seals), and Pigmented Plastics Other than Black (i.e., Specifically White, Green, and Red). These are all abrasive as they contain inorganic pigments or fillers. A surface hardness of at least 1000 HV is needed to ensure acceptable part lives. Applicable coatings include: Thermally sprayed or welded ceramics or cermets, for example, WCCo, alumina, chromium oxide, and so forth, for maximum wear resistance Nitriding may be used on austenitic stainless steel substrates to achieve maximum hardness, but it destroys corrosion resistance. Hard chrome plate will provide good wear resistance in applications where the contact is not concentrated on one area of the part. It can be used on textile feed rollers but not on eye-guides. PVD coatings, for example, ceramics such as TiN or CrN Slurry/sinter-formed ceramics, that is, chromium-oxide-based composites loaded with ceramic particles Black and White Newsprint, Natural Textiles (Cotton, Wool), Cardboard and Packaging, Carbon-Fiber-Reinforced Plastics, BlackPigmented Plastics, Paints and Inks, Food Products, Leather, and Pharmaceutical Products. These are mildly abrasive and require a surface hardness more than 600 HV for effective protection. Effective coatings include: Thermally sprayed or welded ceramics or cermets, for example, WCCo, alumina, chromium oxide, for example, for maximum wear resistance Nitriding or nitrocarburizing on any alloy steel substrate Hard chrome plate will provide good wear resistance in all applications PVD coatings, for example, TiN or CrN Slurry/sinter-formed ceramics, that is, chromium-oxide-based composites loaded with ceramic particles Case hardening, that is, carburizing, carbonitriding for low-carbon steels to give high hardness and wear resistance Local hardening, that is, induction, laser, and so forth, for mediumcarbon steels

Anodizing of aluminum alloys (provides only limited protection and is best for dry food products under the lightest loads) Parts in High-Load Sliding or Erosion w i t h an Abrasive Product (Ref 1) When abrasion takes place under high loads, and where impact occurs and erosion is prevalent, then hardness alone is not a reliable parameter on which to select the appropriate surface treatment. Applications include coal chutes, mining conveyors, diggers, crushers, millers, extruders, cutters, and compactors. Erosive conditions also exist in turbines, impellers, and pipework. The surface must not only be hard, it must also be tough and resilient and able to withstand high specific loading without deforming into the substrate. Base Material The substrate is most likely to be constructional steel or low-alloy tool steels in plate form. Surface-Engineering

Options

Suitable surface treatments for high-stress abrasive conditions include: Welded or spray and fused coatings, including nickel or cobalt-base materials with high carbide content, deposited at least 2 mm (0.08 in.) thick Thermally sprayed and hot isostatically pressed (HIP) coatings, including nickel and iron-base materials, with a high content of the carbides of tungsten, chromium, and titanium, deposited 5 mm (0.2 in.) or more thick High-velocity oxyfuel (HVOF) thermally sprayed coatings including nickel or cobalt-base cermets with low porosity and high bond integrity, at least 1 mm (0.04 in.) thick Elastomer-based coatings for high-angle erosive situations where there is no abrasive, cutting element Parts in Contact w i t h Another Engineering C o m p o n e n t in the Presence of an Abrasive and Corrosion Product or Environment (Ref 1) When a component has surfaces that roll or slide against others with abrasive and/or corrosive product trapped between them, it creates the very extreme condition of three-body high-stress abrasive wear (see

Fig. 5 and 6 in Chapter 3). It is particularly common in pumps, valves, and mechanical seals that are working in abrasive slurries such as sand, water, and hydrocarbons found in oil and gas extraction. The condition is typified by a crushing and grinding action between the surfaces, perhaps the two sliding surfaces of a journal bearing, which breaks down the abrasive particles and continuously creates new cutting edges. When combined with corrosion this creates a very extreme wear situation. Base Material The substrate is most likely to be austenitic or ferritic stainless steel, high-alloy steels, nickel-base alloys (e.g., Inconels), or cast grades of Stellite (Co-Cr-W-C) materials. Surface-Engineering

Options

Surface treatments for three-body high-stress abrasive wear and corrosion applications are limited to those which provide a combination of hardness, toughness, load-carrying capacity, and corrosion resistance. They include: Welded or spray and fused coatings, including nickel or cobalt-base materials with high tungsten, chromium, or titanium carbide content Thermally sprayed and HIP coatings, including nickel and iron-base materials with high tungsten, chromium, or titanium carbide content HVOF coatings, including nickel or cobalt-base cermets with low porosity and high bond integrity Diffusion chromizing for combined corrosion and wear resistance, but only on substrates with a sufficient carbon content to produce a surface layer of chromium carbide Boronizing for high wear resistance of carbon and alloy steels, but without appreciable corrosion resistance. Stellites, sintered cemented carbides, and some sprayed coatings can be boronized to reduce wear of their binder phases. Hard chrome plate, provided the substrate is corrosion resistant and the situation is not too aggressive; crevice corrosion can undermine the plating Preprocessing and Postprocessing H e a t Treatment (Ref 1) Heat treatments performed before or after surface processing are carried out to: Relieve residual stresses Restore mechanical properties of the metal core Reduce the risk of hydrogen embrittlement

Stress Relieving. Residual stresses are left in components after manufacture, either from machining, casting, cold-forming, or forging operations. These residual stresses may cause distortion during subsequent coating or surface treatments, and it is advisable to carry out a stressrelieving step prior to the final surface treatment. The following times and temperatures are recommended for steels of varying strength levels: Tensile strength MPa

ksi

Stress-relief treatment

Up to 1100 1100-1650 1650-1800 Over 1800

Up to 160 160-240 240-260 Over 260

None required 1h at 190-280 0 C (375-535 0F) 18 h at 190 0 C (375 0F) 24 h at 190 0C (375 0F)

Restoring Core Strength. If the surface hardening or coating process involves high temperature, the core strength or hardness of a steel component can be compromised. In the case of carburizing, particularly if carried out in a sealed quench furnace, the part will be quenched and tempered as part of the process, and the core properties (where the carbon content will be lower than the case) will generally be restored. For hightemperature processes like boronizing or chromizing, there will need to be a subsequent heat treatment step to reharden the core for most applications. After a CVD or carbide diffusion process on high-alloy tool steel, there will usually need to be a vacuum heat treatment step to restore core properties. These coatings are thin and depend on adequate support from the substrate to perform properly. Heat Treatment to Avoid Hydrogen Embrittlement. With most of the electroplating processes, and in particular cadmium and hard chromium plating, and some of the chemical processes including electroless nickel, there is a risk of hydrogen embrittlement of high-strength steel components. It is essential to carry out a postprocessing heat treatment immediately after plating. The recommended treatments for steels of varying strength levels are: Tensile strength MPa

ksi

Heat treatment

Up to 1100 1100-1650 1650-1800 Over 1800

Up to 160 160-240 240-260 Over 260

None required 190-230 0 C (375-450 0F) for 2 h 0 190-230 C (375-450 0F) for 6 h 190-230 0C (375-450 0F) for 8 h

Coating Thickness, Case Depth, and Component Distortion Considerations (Ref 1) The thickness of a surface coating or case depth is governed by both the process characteristics and the cost. For example, in theory it would

be possible to build up a PVD coating thickness of 100 fim, but it would take so long as to be both impractical and too expensive. From the user's point of view, it is important to know the thickness of the surface layer so that its load-carrying capacity and potential service life can be assessed and any likely changes in dimensions of the component predicted. Assuming that the final dimensions of the part are critical, below are some guidelines to help the designer predict coating thickness and potential distortion problems. In addition, Table 1 lists the thickness ranges and hardness values for a wide range of coating/surface-hardening processes. Weld overlays will produce significant distortion of the part and a surface growth at least equal to the layer thickness. They will need to be surface ground. High-temperature diffusion processes such as carburizing can produce component distortion. The only option is to allow for postgrinding Table 1 Thickness ranges and hardness levels associated with various surfaceengineering processes Treatment Local surface hardened Carburized (case hardened) Nitrided or nitrocarburized

Boronized

Chromized Aluminized (diffusion) Phosphated Chromated Oxidized Ion implanted PVD TiN PVD CrN Diamondlike carbon CVD chromium nitride CVD chromium carbide CVD alumina Chromium plate Nickel plate Copper plate Cadmium plate Zinc plate Electroless nickel Electroless nickel/ceramic Hot dip galvanizing Electrogalvanized steel strip Hot dip aluminized steel strip Thermally sprayed chromium oxide Thermally sprayed alumina Thermally sprayed tungsten carbide/cobalt Thermally sprayed and spray and fused chromium carbide/nickel-chromium Slurry/sinter-formed ceramics

Substrate

Thickness or case depth

Hardness, HV

Medium-carbon steel Low-carbon steel Low-carbon steel Tool steel Stainless steel Mild steel Low-alloy steel Stainless steel (316) Stainless steel (316) Stainless steel (316) Low-carbon steel Various Steel Steel Various Various Various Stainless steel (316) High-carbon steel Steel Various Various Various Various Various Various Various Steel Low-carbon steel Low-carbon steel Various

1-10 mm 1-3 mm 5-10 |xm 50-200 |xm 20-50 jxm 10-20 |xm 20-30 |xm 30-40 |xm 20-50 fjim 20-50 fxm 4-7 |xm 1 or 2 |xm 3-5 [Am 0.1-1 |jim 1-5 |xm 2-20 |xm 1 or 2 [Am 10-15 [Jim 10-15 |xm 5-10 |xm 5-250 jxm 10 iim to 1 mm 10-250 [Am 5-10 [Am 5-10 |Am 5-50 [Am 5-50 |Am 20-250 [Am 5-10 [Am 5-10 [Am 20-100 |Am

700-900 700-900 400-600 800-1000 1000-1200 500-700 800-1000 1000-1200 300-400 400-500 -200 Not accurately known 250-350 Not accurately known 2000-3000 1800-2500 1500-2000 1100-1300 1500-2000 1500-2000 800-1000 250-650 70-90 -50 -50 500-1000 <1300 70-250 -70 -70 1200-1600

Various Various

20-100 jAm 20-100 [Am

1500-1800 1100-1600

Various

Up to 1 mm

1000-1100

Steel

20-100 |Am

1000-1200

PVD, physical vapor deposition; CVD, chemical vapor deposition. Source: Ref 1

to size. Case depths in the range 200 to 2000 |xm allow for such finishing. The same principles apply to local surface hardening such as induction or flame hardening, provided the depth of hardening is sufficient. CVD and carbide diffusion processes will produce some distortion and growth of 50 to 100% of the coating thickness. There is insufficient coating thickness (up to 20 |xm, usually less) to allow for postcuring grinding. Normally such processes are applied to tooling where trials and experience prove that dimensional accuracy can be consistently maintained at acceptable levels. Nitriding and nitrocarburizing produce only minimal distortion, with a small surface growth (a few microns). Such processes often produce a thin "compound" layer (10 juim) on the surface of the main case (200-500 |xm), and this is usually removed by a finish-grinding operation. Thermal spray processes impart little general heat to the part and, therefore little distortion. The surface growth will equal the coating thickness and, if the finish is important, they will need grinding. Spray and fused deposits, or coatings that are HIP after spraying, will grow and distort from the effects of high temperature. They will require grinding to improve surface finish. Slurry-based ceramic coatings are sintered at high temperature and experience surface growth equal to the coating thickness (typically 10-100 juum) that may cause some distortion. These coatings are normally left unfinished. Coatings for corrosion protection, for example, zinc or cadmium plating, phosphating, and chromating will produce surface growth in the range of 2 to 20 |jim. They cannot be finished after processing. Electroless nickel will produce growth equal to the coating thickness (typically 10-100 |xm) with no distortion unless the substrate is sensitive to the heat treatment temperature of 400 0C (750 0 F). Hot dipped galvanized coating thickness ranges from 10 to 250 jmm and is controlled by the steel chemistry, section thickness, and immersion time. Electrolytic Coatings. Hard chrome and heavy nickel and copper plating can vary in thickness from just a few microns to 250 jjim. Surface growth will equal the coating thickness and, except for the thinnest layers, they will need finish grinding. PVD coatings are usually less than 10 |xm thick and will produce minimal distortion. They are not finished after processing. Paints and polymer coatings are usually around 10 to 30 jxm thick. They are left as-coated. Anodizing produces no distortion. The surface growth is half that of the coating thickness, the coating growing 50% in and 50% out of the original aluminum alloy surface.

Table 2

Surface finish characteristics of various surface-engineering processes

Process

As-treated surface finish

Normal finishing operation

Overlays Thermal spray Local hardening Case hardening Nitriding and nitrocarburizing

Very rough Very rough May be rough and distorted May be rough and distorted Slightly roughened

Galvanizing Phosphating Oxidizing Electroplating

Slight roughening Slight roughening Slight roughening May roughen

Electroless nickel PVD CVD Ion implantation Shot peening Paints and polymers

Replicates surface finish Replicates surface finish Some roughening Replicates surface finish Deliberate alteration Some roughening possible

Grind Grind Grind Grind May be ground, but often used as-treated Not finished Not finished Not finished Chromium and copper usually ground, cadmium and zinc used as-plated Not finished Not finished Not usually finished Never finished Not finished Not finished

PVD, physical vapor deposition; CVD, chemical vapor deposition. Source: Ref 1

Surface

R o u g h n e s s

a n d

Finishing

(Ref

1)

The surface finish of the surface-engineered component will depend on the process itself and, in some instances, on the finish before it was processed. As described in the previous section, some parts will have to be ground after treatment because of distortion or growth, or to develop an acceptable finish. Others will be left untreated, regardless of their surface roughness. In some cases, the primary objective will be to preserve original surface texture without the need for postfinishing operations. Table 2 gives some general surface-finish guidelines relevant to the various surface-engineering treatments.

General Design Principles Related to Surface Engineering (Ref 2) There are a number of general design principles that apply to a variety of surface-engineering processes, while others are specific to individual treatments/techniques. These general principles are discussed in this section, and the following three sections discuss design aspects relating to: (1) surface preparation techniques, including cleaning, (2) organic coating processes, and (3) inorganic (metal and ceramic) coating processes. Fabrication Processes. Some methods of fabrication such as the forging, extrusion, molding, and casting of metals and ceramics can lead to surface defects that must be removed by subsequent surface-finishing techniques, such as grinding, lapping, and polishing or electropolishing, or hidden by techniques such as applying a leveling copper deposit before a decorative plated finish. Defects include laps, tears, cracks, pores, shrinkage cavities,

Quality surface finish

Part or component Functional requirements Design constraints Material(s) Size Weight Tolerances Pretreatment Posttreatment Ease of rework (stripping)

Fixture Part orientation Movement (agitation) Masking Shielding Manual operation Automatic operation Material(s) Size Weight Tolerances Ease of use Cleaning (stripping)

Equipment Process selected Design restrictions Modifications Ease of control Batch operation Continuous operation Single/mixed parts Accuracy (calibration) Reliability Reproducibility Maintainability

Fig. 1 Interrelation between the component, fixturing, and equipment limitations. Source: Ref 2

gating and venting residues, ejection marks, and, parting lines. Careful design of the casting or molding operation—including the dies, gates, vents, and overflows—will minimize finishing problems by ensuring such defects are avoided, occur on nonsignificant surfaces, or are hidden by specially incorporated design features, such as steps or ridges at parting lines. Whatever the type of material being cast or molded, dimensional and warpage allowances must be made in the design of the tooling (i.e., dies) to accommodate shrinkage and distortion during solidification and cooling. Otherwise, parts may be undersized or require excessive machining to obtain the specified dimensional tolerances. Control of fastening or joining processes also can influence surface finishing. For example, two flat surfaces riveted together produce cavities that can entrap processing solutions, impair coating, and lead to corrosion (Ref 3). Spot or tack welding is no better in this regard. However, a continuous weld—with a smooth bead and no weld spatter—will prevent this problem and make surface finishing easier. Also, the elimination of sharp edges and comers will prolong the life of grinding, polishing, and buffing belts and wheels. Component Size and Weight and Handling Problems. The size, dimensions, and weight of a part to be surface engineered have a direct influence on part handling and fixturing and the size and type of equipment that is used (Fig. 1). Put simply, there are two main issues in relation to size and weight of components: Are they too big for the process to accommodate, either in respect to the pretreatment surface preparation/cleaning facilities or plating tanks, vacuum chambers, and the like?

Are they so small, or too numerous, to make the holding, manipulating, or cleaning for the chosen process impractical or too expensive? Objects weighing in excess of 20 kg (about 50 Ib) will probably need hoists or overhead moving cranes to manipulate them through the cleaning lines and treatment chambers. Some heavy objects that are to be treated in front-loading furnaces can often be handled by a fork lift. Table 3 provides some likely limits on size and weight for various surfaceengineering processes. Aesthetics and Function. Another general consideration is that not all surfaces may require the same high standard of surface finish. While surfaces exposed to view must be aesthetically pleasing, and surfaces subjected to more aggressive conditions of exposure or use require durable coatings, hidden (internal) surfaces or less-exposed surfaces may not need such a high-quality finish. Specifications for surface finishes for a part depend not only on the design and end-use application, but also must take into account that the requirements may differ for different areas or surfaces on that part. A design should take this into consideration, as well as the fact that different types of equipment or equipment operation settings may be necessary for those areas and surfaces. Functional requirements of a part also influence the selection of surfacepreparation processes. For example, grinding processes can introduce stresses that could have a negative impact on fatigue properties. Choosing an alternative process, such as chemical milling, or mitigating the stresses by shot peening can alleviate the problem. Design Features. Shape and features such as recesses, holes, threads, keyways, slots, fins, and louvers can present problems to the finisher, and the severity of the problem can depend on the finishing technique. For example, when holes are included in thin sections that require a finishing Table 3

Size and weight limitations for various surface-engineering processes Largest dimension restraint

Weight restraint

Small parts

Overlays

None, assuming access

Not less than about 100 mm

Thermal spray Local hardening

Around 2 m None, assuming access

Shot peening Case hardening Nitriding and nitrocarburizing Galvanizing Phosphating Oxidizing Electroplating Electroless nickel PVD, CVD

None, assuming access Around 3 m Around 3 m

None, particularly if on-site work is possible Several tonnes None, particularly if done on-site Often done on-site About 1 tonne About 1 tonne

Around 30 m Around 5 m Around 1 m Around 3 m Around 1 m Around 1-3 m, usually smaller Around 1 m None, assuming access

10-15 tonnes Several tonnes About 1 tonne About 5 tonnes About 0.5 tonne About 0.5 tonne, usually lighter About 0.5 tonne None assuming access

Process

Ion implantation Paints and polymers

PVD, physical vapor deposition; CVD, chemical vapor deposition. Source: Ref 1

Down to 10 mm Not less than about 100 mm Not less than about 100 mm Not less than about 10 mm Not less than about 5 mm 20 mm or M8 fastener Not less than about 1 m Down to 1 mm Down to 10 mm Down to 5 mm Down to 10 mm Down to 10 mm Down to about 10 mm

operation such as grinding, if too much pressure is applied edges and comers might be chipped. If only a light pressure is used to avoid this possibility, then the desired finish might not be obtained. Another example is when paint is applied by conventional solvent spraying or when a part is electroplated, bowl-shaped recesses, blind holes, and similar features can trap the paint or plating solution, leading to areas that sag or do not cure properly (in the case of paint), or carry over trapped chemicals to subsequent processing steps (in electroplating). The latter can cause problems such as rinse-water contamination and increased waste-treatment costs. Also, solutions that are trapped can lead to blistering or delamination of the plated coating, especially if there is a posttreatment step that requires the part to be heated (such as for electroless nickel, cadmium, and hard chromium deposition). For parts that will be sprayed, especially with paint, another problem with deep recesses, closely spaced, large fins or partitions, and the like is the entrapment of air. The back pressure of entrapped air causes incomplete coverage at the bottom of the recesses. One way to avoid this problem, if a change of design is not possible, is to use an "airless" spraying technique (Ref 4). During electrostatic powder coating there is the problem associated with "Faraday cage" effect, in which the charged components of the powder- coating system are attracted by the high fields at the edges and comers of parts, causing excessive coverage there and incomplete coverage in other areas (Ref 5), as shown in Fig. 2. Rounding corners and edges, tapering the sides and decreasing the depth of recesses, minimizing the use of louvers or fins, or changing their dimensions are ways to avoid the Faraday cage effect. In electroplating, a similar phenomenon exists whereby the depositing metal or alloy ions are attracted to the high-current-density areas at edges and comers, and thicker coatings are obtained in those locations. Rounding such edges, changing dimensions to allow for the excessive buildup, or using shields and current "robbers" or "thieves" will help the finisher to obtain the desired coating thickness distribution. Reference 6 provides some examples of the use of such devices. Coating Part

Fig. 2 Faraday cage effect in powder coating. Adapted from Ref 5

Accessible outside diameter Beam or spray

Beam or spray Inaccessible outside diameter

Accessible outside diameter

Accessible inside diameter

Beam or spray

Beam or spray

Inaccessible inside diameter Thread Fig. 3 Some examples of line-of-sight limitations in spraying or ion-beam coating processes

In conventional paint spraying and many vacuum-deposition techniques, such as ion plating, ion implantation, PVD, and sputtering, attention has to be paid to the limitations imposed by the "line-of-sight" deposition process. Certain features, such as ridges, flanges, and fins, can shadow or mask areas behind them leading to incomplete or nonuniform coverage, as shown in Fig. 3 and 4. Similarly, if the aspect ratio of holes and recesses is too high (i.e., the depth is much greater than the diameter of the opening), it is not possible with line-of-sight limited techniques to penetrate to the bottom surfaces and coat them (Fig. 4). Decreasing the Beam or spray Shadowed areas Shadowed areas

Cross section of part Fig. 4 Design features that cause shadowing or poor coverage because of line-of-sight limitations

aspect ratio, providing rounded edges, and tapering the sides of ridges and fins or holes will help to facilitate finishing, as will lowering the height of features such as fins. Of course, rotating or translating a part in the spray plume also will help to obtain complete and more uniform coverage, but this approach usually requires longer times and more sophisticated finishing equipment and fixturing; hence, it often leads to higher costs. The same can be said for using multiple line-of-sight sources to obtain better coverage. Finally, as a general rule of thumb, parts of the same size, weight, design, and material should always be finished at the same time so that the finishing process(es) can be optimized for those parts. Batches of mixed parts should be avoided unless they share some common features, such as shape and substrate material.

Design Guidelines for Surface Preparation Processes Surface preparation, including cleaning, is the essential first step in all successful surface-engineering practice. To facilitate surface preparation prior to subsequent coating operations, there are a number of design features that must be considered. Abrupt changes in surface contours should be avoided, and features such as fine grooves, recesses, surface patterning, blind holes, and reentrant areas should be avoided because they will be inaccessible to polishing media or would trap polishing media, making subsequent cleaning more difficult. Such features also would entrap cleaning chemicals, making rinsing more difficult, or could possibly entrap air, preventing cleaning of these areas. Sharp comers and edges or protrusions can cause excessive wear of polishing wheels and belts and lead to uneven polishing because the high areas are polished at the expense of the surrounding lower areas. As mentioned earlier, rounding edges and corners is a good design precept, while minimizing the height of protuberances is beneficial, as is decreasing the aspect ratio of holes, grooves, and recesses. Large expanses of flat surfaces may be a problem if these are significant surfaces, especially if these surfaces must be polished to a reflective, mirrorlike finish. Imperfections are exaggerated. Minimizing the area of such surfaces and providing a slightly rounded contour will help to attain the desired finish and help with visual appearance. Simpler designs lend themselves to automatic finishing processes, while more complex designs may require manual surface-preparation techniques. If parts are to be mass finished (e.g., by tumbling or vibratory finishing) significant flat areas should be avoided. Otherwise, parts may stick together, and these occluded surfaces will not be finished. Designs that prevent access by the deburring or polishing media (such as small recesses

and holes) or that entrap the media (such as narrowly spaced ribs) should be avoided as mentioned above. When it is impractical or impossible to use mechanical polishing, chemical etching, chemical milling, or electropolishing can be used. The design principles for the latter are similar to those for electroplating, which is discussed later. In electropolishing, the workpiece is the anode, which is the opposite of electroplating. Current-density distribution is extremely important, as is the original surface of the pan being electropolished. In high-currentdensity areas on susceptible materials, the surface layers may be removed and etching of the substrate can occur. Polishing occurs on a microscopic scale, so macro features such as large grooves or scratch marks will not be removed, but will receive a luster and become more noticeable. Similarly, parting lines can be smoothed, but not removed; therefore, parting fines must be minimized by good die design and careful molding operations. Solvent cleaning is a fairly forgiving surface-finishing process, but part design can influence its efficacy, as already alluded to. If agitation or other cleaning aids are used, such as ultrasonic energy, care must be taken to prevent soft materials or thin and fragile features or cross sections from being damaged. The energy released during cavitation, for example, in ultrasonic cleaning is very large. If techniques such as plastic media blasting are used, the blasting parameters should be tailored to the part material and design, and the part should be designed to allow easy access by the media and easy removal of the media once the desired finish (cleanliness) is obtained. If a power spray washing technique is used, the part design should allow for proper drainage to conserve chemicals and minimize carryover to the next process step. Providing drainage holes may be necessary. These should be either a natural feature of the design or located on nonsignificant surfaces. As the design of a part becomes more complex, rinsing requirements become more stringent, and several rinsing stages may be necessary. If an air knife is used afterward to remove excess water, the part must be capable of withstanding the pressure or must be fixtured such that the air pressure does not distort any delicate design features while holding the part steady. Table 4 provides a summary of the design limitations of some surfacepreparation and cleaning processes and indicates which design features to avoid. Design Guidelines for O r g a n i c Coating Processes (Ref 2)

Organic coatings are applied by a variety of techniques, such as dipping, brushing, spraying, airless spraying, or electrostatic spraying. In addition, some primers are deposited using electrophoretic techniques, while electropolymerization is being looked at for certain types of organic coatings.

Table 4

Summary of design limitations for selected surface-preparation processes Design limitations

Process B lasting/deburring

Broaching/honing

Brushing/burnishing

Chemical milling

Conversion coating

Electrocleaning

Electropolishing

Etching

Grinding

Lapping/buffing

Pickling

Polishing

Solvent cleaning, immersion

Solvent cleaning, ultrasonic

Avoid recesses, holes, channels, and similar features (such as closely spaced ribs) that could trap blasting media Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by the blasting media Avoid intricate designs and surface features Typically used for inside diameters of tubes and other cylindrical parts, or for grooves, large holes, and other cavities Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness that could deflect Avoid sharp corners and edges Avoid intricate designs and surface features Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air or evolved gases and prevent chemical action from occurring or cause uneven attack Mask areas not to be attacked Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent surface chemical reactions from occurring or cause staining Mask areas not to be attacked Allow for electrical contact to be made on nonsignificant surfaces Avoid features that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air or evolved gases and would prevent cleaning from occurring or cause staining Allow for electrical contact to be made on nonsignificant surfaces Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air or evolved gases and prevent polishing action from occurring or cause staining Mask areas not to be attacked Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent etching action from occurring Avoid sharp corners and edges Avoid shallow intricate designs and surface features Mask areas not to be attacked Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness Avoid sharp corners, edges, and protuberances Avoid intricate designs and surface features Surfaces must be accessible to tools (preferably flat or simple, curved contours Avoid very thin cross sections/wall thickness that cannot withstand the local pressure and heat buildup Avoid sharp corners and edges Avoid intricate designs and surface features that would trap the lapping/buffing compounds Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent pickling action Avoid flat surfaces on small parts that could stick together, exclude the acid, and prevent the pickling action Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness Avoid sharp corners, edges, and protuberances Avoid intricate designs and surface features that could trap the polishing compound Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent cleaning from occurring Avoid flat or curved surfaces on small parts that could stick together during immersion and prevent cleaning of those surfaces Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent cleaning from occurring (continued)

Source: Ref 2

Table 4 (continued) Process Solvent cleaning, ultrasonic (continued) Stripping, chemical

Stripping, mechanical

Stripping, thermal

Design limitations Avoid thin cross sections that could be damaged by the energy released during cavitation Avoid features (e.g., small recesses, blind holes, cavities) that would trap smut and process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent coating removal from occurring Mask areas not to be attacked Avoid recesses, holes, channels, and similar features that could trap blasting media Avoid thin cross sections or intricate designs that could be damaged by the stripping media Mask areas not to be attacked Avoid thin cross sections or intricate designs that could be distorted by the thermal cycling Try to provide uniform cross-sectional mass throughout the part to help provide a uniform temperature distribution during heating

Source: Ref 2

Table 5 summarizes these techniques and the design limitations associated with each. Most of the techniques are line-of-sight limited, and the guidelines provided in the previous section, "Design Guidelines for Surface-Preparation Techniques," will apply. Allowance for drainage is important for processes that involve dripping or spraying. Avoiding sags and runs on large, flat, vertical surfaces can be accomplished by applying good coating practices and by minimizing such surfaces in the design of the part. Table 5 Summary of design limitations for selected organic coating processes Process Electrocoating

Electropolymerization

Painting, brushing or dipping

Painting, solvent spraying

Powder coating

Sol-gel coating

Solution coating

Source: Ref 2

Design limitations Allow for electrical contact to be made on nonsignificant surfaces Avoid features that could trap air and prevent wetting by process solutions Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle Allow for electrical contact to be made on nonsignificant surfaces Avoid features that could trap air and prevent wetting by process solutions Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle Surfaces must be accessible to application tools (preferably flat or simple, curved contours) Avoid features that would trap excess paint Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent coating from occurring Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle Surfaces must be accessible to application tools (preferably flat or simple, curved contours) Allow for fixturing/racking on nonsignificant surfaces Avoid features that would trap excess paint Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent coating from occurring Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle Allow for fixturing/racking on nonsignificant surfaces Allow for electrical contact to be made on nonsignificant surfaces Avoid deep recesses and blind holes that cause the "Faraday cage" effect Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle Allow for fixturing/racking on nonsignificant surfaces Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals Avoid thin cross sections or intricate designs that could be distorted by the thermal cycling Try to provide uniform cross-sectional mass throughout the part to help provide a uniform temperature distribution during heating cycle Allow for fixturing/racking on nonsignificant surfaces Avoid features (e.g. small recesses blind holes cavities) that would trap process chemicals Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent coating from occurring Avoid thin cross sections or intricate designs that could become distorted during drying/curing cycle

A few organic coating techniques use electric or electrostatic fields. Designing the fixtures and electrical grounding, such that points of contact are on nonsignificant surfaces, will improve the appearance of the coated part and give the impression of a better quality product. With spraying techniques, proper fixturing and racking of parts can improve the use of coating material because less empty space exists during a run. However, the parts should not be racked so closely together that they shield some surfaces and prevent some areas from being coated. Avoiding thin cross sections and good fixturing will help prevent distortion during the curing and baking steps used after paint or powder is applied. Optimizing a design for surface finishing, such as painting, becomes very important as coating thickness is reduced to 30 firn or less. Access to all surfaces must be possible, and any features that would prevent this should be avoided. This is because the dimensions of the solid components in the coating formulation (e.g., powder particle) are similar to the dimensions of the desired dry film thickness (Ref 7). For example, during the first part of curing, when the particles liquefy, the surface tension of the film formed will tend to pull it away from sharp corners or edges, resulting in poor coverage. If a design modification is not possible, the powder formulation should be changed to include higher-viscosity resins, and no, or only small amounts, of surfactants (Ref 7). Thin-film coatings are best applied to parts with simple geometries, with flat or curved surfaces, and few sharp edges. Earlier, the problem with the Faraday cage effect was mentioned. This phenomenon is further complicated by back-ionization with traditional corona-charging systems (Ref 5). Not only does the design of a recess, hole, or channel control the distribution of coating thickness, but the buildup of back-ionization at the areas of high field intensity lowers the effective charge of the powder particles, further reducing their ability to reach the bottom surfaces. Some possible design modifications were mentioned earlier, but if these are not possible, changing to a turbocharging system will help. Back-ionization is greatly reduced, and the absence of free ions between the gun and the part promotes better coverage of all surfaces (Ref 5). Design Guidelines for Inorganic Coating Processes (Ref 2) Inorganic finishes—including metal- and ceramic-based coatings—are applied by a variety of techniques, such as electroplating, electroless plating, thermal spraying, hot dipping, and various vapor-deposition techniques. Other techniques, such as ion implantation and laser melting/alloying, modify surface properties. Table 6 summarizes design limitations for these and other types of inorganic coating processes. Electroplating is widely used in industry to apply inorganic coatings, especially metals and alloys. Like some organic finishing processes, satisfactory coatings are only obtained when a uniform current density can be

Table 6

Summary of design limitations for selected inorganic coating processes

Process Anodizing

Cementation/diffusion

Cladding

Electroless plating

Electrophoretic plating

Electroplating (plating, electrodeposition)

Hot dipping, galvanizing

Inorganic painting, slurry coating

Ion implantation

Ion plating

Design limitations Allow for electrical contact to be made on nonsignificant surfaces Avoid, if possible, sharp edges and corners, ridges, blind holes, etc. that would prevent uniform density distribution Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent electrochemical reactions from occurring Avoid features that could trap evolved gases and cause staining Mask areas not to be anodized Surfaces must be thoroughly deburred and cleaned before cladding, so design principles for these processes also apply Avoid thin cross sections or intricate designs that could become distorted during thermal cycling Mask areas not to be coated Only for relatively simple shapes, especially with flat surfaces Surfaces must be thoroughly cleaned before cladding, so design principles for cleaning also apply Allow for fixturing/racking on nonsignificant surfaces Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent chemical reactions from occurring or cause staining Mask areas not to be coated Allow for electrical contact to be made on nonsignificant surfaces Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent surface chemical reactions from occurring or cause staining Mask areas not to be coated Allow for electrical contact to be made on nonsignificant surfaces Avoid, if possible, sharp edges and corners, ridges, blind holes, etc., that would prevent uniform current density distribution; or use current robbers and/or shields Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent deposition from occurring Avoid features that could trap evolved gases and cause staining Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by internal stress in the coating Mask areas not to be coated Allow for fixturing/racking on nonsignificant surfaces for discrete, small parts Best for relatively simple shapes (e.g., tubing) and flat surfaces Allow for excess coating material to drain quickly Allow for doctor blades or air knives to be used to obtain uniform coating thickness Avoid thin cross sections that could become distorted during thermal cycling Surfaces must be accessible (preferably flat or simple, curved contours) Allow for fixturing/racking on nonsignificant surfaces Avoid features that would trap excess paint Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Avoid features that could trap air and prevent coating from occurring Avoid thin cross sections or intricate designs that could become distorted during drying/fusing cycle Mask areas not to be coated Allow for electrical contact to be made on nonsignificant surfaces or use a conductive screen Avoid features that would shield the surface from the beam (line-of-sight limited) unless multiple beams are used or part is rotated/translated in beam Avoid high aspect ratio holes and recesses, grooves, etc., that would not allow the beam to reach the bottom surfaces Mask areas not to be coated Allow for electrical contact to be made on nonsignificant surfaces or use a conductive screen Avoid features that would shield the surface from the beam (line-of-sight limited) unless multiple beams are used or part is rotated/translated in beam

(continued) CVD, chemical vapor deposition; PVD, physical vapor d<sposition. Source: Ref 2

Table 6

(continued) Design limitations

Process

Ion plating (continued)

Laser melting/alloying

Mechanical (peen) plating

Passivation

Thermal spraying

Vapor deposition (CVD, PVD)

Avoid high aspect ratio holes and recesses, grooves, etc., that would not allow the beam to reach the bottom surfaces Mask areas not to be coated Allow for fixturing/racking on nonsignificant surfaces Avoid features that would shield the surface from the laser beam (line-of-sight limited) unless multiple beams are used or part is rotated/translated in beam Avoid high aspect ratio holes and recesses, grooves, etc., that would not allow the beam to reach the bottom surfaces Avoid thin cross sections or intricate designs that could be damaged by local heating during glazing Mask areas not to be treated Allow for fixturing/racking on nonsignificant surfaces on large parts Avoid features that could trap air and prevent activation by the process chemicals from occurring Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover of activating solutions Avoid recesses, holes, channels, and similar features that could trap peening media Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by the peening action Avoid sharp edges and corners that could be damaged by the peening media Avoid intricate designs and small surface features that cannot be reached by the peening media Provide good natural drainage or use drainage holes on nonsignificant surfaces to minimize carryover Mask areas not to be coated Allow for fixturing/racking on nonsignificant surfaces Avoid features (e.g., small recesses, blind holes, cavities) that would trap process chemicals or prevent satisfactory rinsing Avoid features that could trap air and prevent surface chemical reactions from occurring or cause staining Mask areas not to be attacked Allow for fixturing/racking on nonsignificant surfaces Design should allow for surface roughening to promote adhesion, so blasting design precepts also apply Avoid features that would shield the surface from the spray (line-of-sight limited) unless multiple sprays are used or part is rotated/translated in spray plume Avoid high aspect ratio holes and recesses, grooves, etc., that would not allow the spray to reach the bottom surfaces Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by the local heating and kinetic energy Mask areas not to be coated Allow for fixturing/racking on nonsignificant surfaces Avoid thin cross sections (such as fins, louvers, walls) that could be distorted by heating, if needed prior to coating deposition Vacuum processes are line-of-sight limited, so similar design precepts to those for ion plating will apply Mask areas not to be coated

CVD, chemical vapor deposition; PVD, physical vapor deposition. Source: Ref 2

established on all surfaces to be finished. Phenomena like the Faraday cage effect occur when design features prevent the establishment of a uniform current density distribution. As mentioned earlier, techniques relating to fixturing and racking can alleviate some of the problems. General design approaches are discussed in Ref 6 and 8 and summarized in Table 7. With the plating of fasteners, some special considerations apply, particularly in respect to threads (Ref 8). As might be expected, electroplated metals build up faster on apexes of the threads, and coverage can be minimal at the bottom of the grooves. ANSI Specification B 1.1 states that compared to flat surfaces, plating thickness builds up six times faster on the major diameter than the minor diameter and that this results in a fourfold buildup on the pitch diameter, as illustrated in Fig. 5. This is known as the "Rule of Four and Six."

Table 7 Influence of substrate design features on electroplateability Feature

Convex surfaces Flat surfaces Sharply angled edges

Flanges

Slots

Blind holes

Sharply angled indentations Flat-bottom

grooves V-shaped grooves

Fins

Ribs

Concave recesses

Poor design

Influence on electroplateability

Better design

Ideal shape. Easy to plate uniformly, especially where edges are rounded Use a 0.4 mm (0.015 in.) crown to minimize undulations caused by uneven buffing. Undesirable. Reduced coating thickness at center areas requires increased plating time to obtain a minimum thickness of durable electroplate. All edges should be rounded. Edges that will contact painted surfaces should have a minimum radius of 0.8 mm (0.03 in.). Large flanges with sharp inside angles should be avoided to minimize plating costs. Use a generous radius on inside angles and taper the abutment. Narrow, closely spaced slots and holes reduce electroplateability and cannot be properly plated unless corners are rounded. Must usually be exempted from minimum thickness requirements. Where necessary, limit depth to 50% of width. Avoid diameters of less than 6 mm (0.24 in.). Increase plating time and costs for a specified minimum thickness, and reduce the durability of the plated part. Inside and outside angles should be rounded generously. Deep V-shaped grooves cannot be satisfactorily plated and should be avoided. Shallow, rounded grooves are better. Increase plating time and costs for a specified minimum thickness and reduce the durability of the plated part. Narrow ribs with sharp angles usually reduce electroplateability; wide ribs with rounded edges pose no problem. Taper each rib from its center to both sides and round off edges. Increase spacing if possible. Electroplateability depends on dimensions. Increase plating time and costs for a specified minimum thickness.

Deep scoops

Spearlike juts

Rings

Buildup on jut will rob corners from their share of electroplate. Crown the base and round off all corners. Electroplateability depends on dimensions. Round off corners and crown from center line, sloping towards both sides.

Note: Distribution of electroplate on design shapes is intentionally exaggerated by solid black outline. Cross-hatched areas indicate part before plating.

Buildup on pitch diameter = 47"

Buildup on major diameter = 67

Plating thickness (7")

Fic 5 ^ u ' e °f ^our anc^ S'x as applied to coating external threads. Source: &# Ref 8

Similarly, plating inside holes can be difficult. The general rule of thumb is that if the hole diameter is x, the plating will occur to a depth of 2x. However, for blind holes, plating will only occur to a depth of x. Agitation, solution flow, maximizing the throwing power of the plating bath, and other aids can improve the situation somewhat, but the best approach is to eliminate or minimize holes with high aspect ratios during the product-design stage. In plasma-coating processes, the part design will have considerable influence over the operating parameters of the coating-deposition equipment. Complex shapes, blind holes, fins, slots, and similar features will dictate that a high vacuum pressure, low part temperature, and light plasma density be used (Ref 9). The converse will be true for simple geometries. In plasma processing, consideration also must be given to heating of the part by the plasma itself. Some design features with thin cross sections and low mass, such as fins, louvers, and bosses will heat up faster than the bulk material in the part. For parts that have been heat treated, or otherwise finished to provide desirable mechanical properties, overheating could destroy those properties or at least change the values detrimentally. Reference 9 provides some examples of process and equipment modifications to avoid such problems during plasma nitriding. Reference 10 discusses the effect of part geometry on the growth of the nitride layer during ion nitriding and how coating uniformity can be improved for grooved surfaces. As in electroplating, decreasing the aspect ratio (depth of groove to width of groove) has a positive effect. O t h e r Important Considerations for the Design Engineer Drawing up a Specification (Ref 1). The greatest source of dispute or rejection after surface engineering is lack of communication and understanding, not a failure of the process itself. It is critically important that the contractor is provided with all the needed information to ensure that the component can, and will be, treated exactly as the customer expects. By establishing the right partnership and communication channels with all members of the purchase/supply chain, satisfactory results can be anticipated.

When a customer is seeking a quotation or evaluating a possible surface-engineering process, it is vital that the contractor has the information listed below. This information should be viewed as the basis of a specification, to be agreed upon by both parties, with no margin for error or misunderstandings. Important information to be conveyed includes: A current issue engineering drawing of the part Any applicable standards, for example, ASTM, ANSI, internal company standards, or international standards Indication of part weight particularly if it is a large item Indication of part number and quantity to be treated, in total and in each batch Packaging and handling requirements. Indication if parts must be returned in original packing The base material, including composition or formal material specification. If there is more than one material in the part, make this point very clearly. If it is a high-strength material being used in a highly stressed situation (e.g., fatigue), then clearly state this. Its heat treatment history during fabrication What coating or treatment is required Which areas on the part are to be treated. Mark them on the drawing. Clearly show any areas that must not be treated and agree on masking principles Required thickness or case depth. Indication of a tolerance band allowance If the part must be coated or plated to a final dimension, provide an overall tolerance band. Remember that there will be two tolerances on the part dimensions, the manufacturing tolerances and those of the coating process. These will combine to a wider tolerance on the final dimensions. Manufacturing sizes may have to be altered to accommodate the coating thickness. Where, on its surface, can the part be supported or jigged (the area on which it is supported will be obscured and not treated); anywhere where it must not be jigged Specify any precoating and postcoating heat treatment (e.g., stress relieving or deembrittlement) that will be the responsibility of the contractor. Inform the contractor of any heat treatment done elsewhere prior to their receiving the work. Remember the more people in the chain the greater the opportunities for them to deny responsibility for problems. Required surface finish if it is controllable. Agree on any postfinishing procedures (e.g., a subcontracted grinding operation) if it is to be part of the treatment service. The inspection required, with a clear statement and understanding of what is acceptable and what must result in rejection of the finished part

Agree and establish responsibility for rework, if allowed, and scrapped work. Any approvals required, for example, DoD or FDA Date dispatched, date received by contractor, date required, and method of delivery and dispatch Environmental Regulation of Surface Engineering. Environmental protection regulations are often related directly or indirectly to surfaceengineering processes. This is particularly applicable for solvent cleaning procedures, cadmium and chromium electroplating, chromate conversion coatings, and organic coatings containing high amounts of volatile organic compounds (solvents). The chemicals used for such processes may pose serious health and environmental hazards. For information about specific regulatory requirements, permitting conditions, and enforcement issues, the design engineer is advised to seek assistance from federal, state, and local regulatory agencies; consulting engineering firms; and law offices. Another valuable source of information can be found in the Section "Environmental Protection Issues" in Surface Engineering, Volume 5 of the ASM Handbook. Articles contained in this Section describe various environmental statutes affecting selection of surface-engineering processes and review specific processes that can be used to replace cadmium coatings, chromium coatings, and chromate conversion coatings, as well as alternatives to vapor degreasing and wipe solvent cleaners.

Acknowledgment This chapter was compiled and adapted from two primary sources: K. Stevens, "Surface Engineering to Combat Wear and Corrosion: A Design Guide," The Institute of Materials, London, United Kingdom, 1997. With permission E.W. Brooman, Design for Surface Engineering, Materials Selection and Design, VoI 20, ASM Handbook, ASM International, 1997, p 820-827

References 1. K. Stevens, "Surface Engineering to Combat Wear and Corrosion: A Design Guide," The Institute of Materials, London, United Kingdom, 1997 2. E.W. Brooman, Design for Surface Engineering, Materials Selection and Design, VoI 20, ASM Handbook, ASM International, 1997, p 820-827

3. M. Henthorne, Corrosion Causes and Control, Chemical Engineering Series, McGraw-Hill, 1972, Part 7 4. D.L. Stauffer, Ed., Finishing Systems Design and Implementation, Society of Manufacturing Engineers, 1993, Chapter 1 5. S. Guskov, Faraday Cage, Finish Quality, and Recoating: New Technology for More Effective Powder Coating, Powder Coat, 1996, p 82-91 6. W.H. Safranek and E.W. Brooman, Finishing and Electroplating Die Cast and Wrought Zinc, Zinc Institute, 1973, Chapter 7 7. B. Fawer, Thin-Film Powder Coatings: Design and Application Issues, Powder Coat, VoI 7 (No. 7), 1996, p 56-63 8. L.W. Flott, Quality Control: Becoming a Better Customer, Met. Finish., VoI 94 (No. 2), 1996, p 79-82 9. R. Gunn, Industrial Advances for Plasma Nitriding, Ion Nitriding and Ion Carburizing, T. Spalvins and W.L. Kovacs, Ed., ASM International, 1990, p 157-163 10. MJ. Park et al., Effect of Geometry on Growth of Nitride Layer in Ion Nitriding, Ion Nitriding and Ion Carburizing, T. Spalvins and W.L. Kovacs, Ed., ASM International, 1990, p 203-209

Glossary of Terms A abrasion. (1) A process in which hard particles or protuberances are forced against and moved along a solid surface. (2) A roughening or scratching of a surface due to abrasive wear. (3) The process of grinding or wearing away through the use of abrasives. See also high-stress abrasion and low-stress abrasion. abrasive. (1) A hard substance used for grinding, honing, lapping, superfinishing, polishing, pressure blasting, or barrel finishing. Abrasives in common use are alumina, silicon carbide, boron carbide, diamond, cubic boron nitride, garnet, and quartz. (2) Hard particles, such as rocks, sand, or fragments of certain hard metals, that wear away a surface when they move across it under pressure. abrasive blasting. A process for cleaning or finishing by means of an abrasive directed at high velocity against the workpiece. abrasive erosion. Erosive wear caused by the relative motion of solid particles that are entrained in a fluid, moving nearly parallel to a solid surface. See also erosion. abrasive wear. The removal of material from a surface when hard particles slide or roll across the surface under pressure. The particles may be loose or may be part of another surface in contact with the surface being abraded. Compare with adhesive wear. accelerated corrosion test. Method designed to approximate, in a short time, the deteriorating effect under normal long-term service conditions. active metal. A metal ready to corrode or being corroded. adhesion. (1) In frictional contacts, the attractive force between adjacent surfaces. In physical chemistry, adhesion denotes the attraction between a solid surface and a second (liquid or solid) phase. This definition is based on the assumption of a reversible equilibrium. In mechanical technology, adhesion is generally irreversible. In railway

engineering, adhesion often means friction. (2) Force of attraction between the molecules (or atoms) of two different phases. Contrast with cohesion. (3) The state in which two surfaces are held together by interfacial forces, which may consist of valence forces, interlocking action, or both. adhesive wear. (1) Wear by transference of material from one surface to another during relative motion due to a process of solid-phase welding. Particles that are removed from one surface are either permanently or temporarily attached to the other surface. (2) Wear due to localized bonding between contacting solid surfaces leading to material transfer between the two surfaces or loss from either surface. Compare with abrasive wear. alclad. Composite wrought product composed of an aluminum alloy core having one or both surfaces a metallurgically bonded aluminum or aluminum alloy coating that is anodic to the core and thus electrochemically protects the core against corrosion. alkaline cleaner. A material blended from alkali hydroxides and such alkaline salts as borates, carbonates, phosphates, or silicates. The cleaning action may be enhanced by the addition of surface-active agents and special solvents. aluminizing. Forming of an aluminum or aluminum alloy coating on a metal by hot dipping, hot spraying, or diffusion. anodizing. Forming a conversion coating on a metal surface by anodic oxidation; most frequently applied to aluminum. arc spraying (ASP). A thermal spraying process using an arc between two consumable electrodes of surfacing materials as a heat source and a compressed gas to atomize and propel the surfacing material to the substrate. arc welding. A group of welding processes that produce coalescence of metals by heating them with an arc, with or without the application of pressure, and with or without the use of filler metal. atmospheric corrosion. The gradual degradation or alteration of a material by contact with substances present in the atmosphere, such as oxygen, carbon dioxide, water vapor, and sulfur and chlorine compounds. B barrel cleaning. Mechanical or electrolytic cleaning of metal in rotating equipment. barrel finishing. Improving the surface finish of workpieces by processing them in rotating equipment along with abrasive particles that may be suspended in a liquid. The barrel is normally loaded about 60% full with a mixture of parts, media, compound, and water. barrel plating. Plating articles in a rotating container, usually a perforated cylinder that operates at least partially submerged in a solution.

black oxide. A black finish on a metal produced by immersing it in hot oxidizing salts or salt solutions. blasting or blast cleaning. A process for cleaning or finishing metal objects with an air blast or centrifugal wheel that throws abrasive particles against the surface of the workpiece. Small, irregular particles of metal are used as the abrasive in gritblasting; sand, in sandblasting; and steel, in shotblasting. bluing. Subjecting the scale-free surface of a ferrous alloy to the action of air, steam, or other agents at a suitable temperature, thus forming a thin blue film of oxide and improving the appearance and resistance to corrosion. This term is ordinarily applied to sheet, strip, or finished parts. It is used also to denote the heating of springs after fabrication to improve their properties. bonding. Thermochemical treatment involving the enrichment of the surface layer of an object with borides. This surface-hardening process is performed below the Ac 1 temperature. Also referred to as boronizing. bright finish. A high-quality finish produced on ground and polished rolls. Suitable for electroplating. bright nitriding. Nitriding in a protective medium to prevent discoloration of the bright surface. bright plate. An electrodeposit that is lustrous in the as-plated condition. brinelling. (1) Indentation of the surface of a solid body by repeated local impact or impacts, or static overload. Brinelling may occur especially in a rolling-element bearing. (2) Damage to a solid bearing surface characterized by one or more plastically formed indentations brought about by overload. See also false brinelling. brush plating. Plating with a concentrated solution or gel held in or fed to an absorbing medium, pad, or brush carrying the anode (usually insoluble). The brush is moved back and forth over the area of the cathode to be plated. buffing. Developing a lustrous surface by contacting the work with a rotating buffing wheel. burnishing. Finish sizing and smooth finishing of surfaces (previously machined or ground) by displacement, rather than removal, of minute surface irregularities with smooth-point or line-contact fixed or rotating tools. butler finish. A semilustrous metal finish composed of fine, uniformly distributed parallel lines, usually produced with a soft abrasive buffing wheel; similar in appearance to the traditional hand-rubbed finish on silver. c calorizing. Imparting resistance to oxidation to an iron or steel surface by heating in aluminum powder at 800 to 1000 0C (1470-1830 0 F).

carbonitriding. A case-hardening process in which a suitable ferrous material is heated above the lower transformation temperature in a gaseous atmosphere of such composition as to cause simultaneous absorption of carbon and nitrogen by the surface and, by diffusion, create a concentration gradient. The heat treating process is completed by cooling at a rate that produces the desired properties in the workpiece. carburizing. Absorption and diffusion of carbon into solid ferrous alloys by heating, to a temperature usually above Ac3, in contact with a suitable carbonaceous material. A form of case hardening that produces a carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then reaustenitizing and quenching. case. In heat treating, that portion of a ferrous alloy, extending inward from the surface, whose composition has been altered during case hardening. Typically considered to be the portion of an alloy (a) whose composition has been measurably altered from the original composition, (b) that appears light when etched, or (c) that has a higher hardness value than the core. Contrast with core. case hardening. A generic term covering several processes applicable to steel that change the chemical composition of the surface layer by absorption of carbon, nitrogen, or a mixture of the two and, by diffusion, create a concentration gradient. The processes commonly used are carburizing and quench hardening; cyaniding; nitriding; and carbonitriding. The use of the applicable specific process name is preferred. CASS test. Abbreviation for copper-accelerated salt-spray test. cavitation. The formation and collapse, within a liquid, of cavities or bubbles that contain vapor or gas or both. In general, cavitation originates from a decrease in the static pressure in the liquid. It is distinguished in this way from boiling, which originates from an increase in the liquid temperature. There are certain situations where it may be difficult to make a clear distinction between cavitation and boiling, and the more general definition that is given here is therefore to be preferred. In order to erode a solid surface by cavitation, it is necessary for the cavitation bubbles to collapse on or close to that surface. cavitation corrosion. A process involving conjoint corrosion and cavitation. cavitation damage. The degradation of a solid body resulting from its exposure to cavitation. This may include loss of material, surface deformation, or changes in properties or appearance. cavitation erosion. Progressive loss of original material from a solid surface due to continuing exposure to cavitation. cementation. The introduction of one or more elements into the outer portion of a metal object by means of diffusion at high temperature.

checks. Numerous, very fine cracks in a coating or at the surface of a metal part. Checks may appear during processing or during service and are most often associated with thermal treatment or thermal cycling. chemical conversion coating. A protective or decorative nonmetallic coating produced in situ by chemical reaction of a metal with a chosen environment. It is often used to prepare the surface prior to the application of an organic coating. chemical deposition. The precipitation or plating-out of a metal from solutions of its salts through the introduction of another metal or reagent to the solution. chemical polishing. A process that produces a polished surface by the action of a chemical etching solution. The etching solution is compounded so that peaks in the topography of the surface are dissolved preferentially. chemical vapor deposition (CVD). A coating process, similar to gas carburizing and carbonitriding, whereby a reactant atmosphere gas is fed into a processing chamber where it decomposes at the surface of the workpiece, liberating one material for either absorption by, or accumulation on, the workpiece. A second material is liberated in gas form and is removed from the processing chamber, along with excess atmosphere gas. chromate treatment. A treatment of metal in a solution of a hexavalent chromium compound to produce a conversion coating consisting of trivalent and hexavalent chromium compounds. chromating. Performing a chromate treatment. chromizing. A surface treatment at elevated temperature, generally carried out in pack, vapor, or salt baths, in which an alloy is formed by the inward diffusion of chromium into the base metal. cladding. (1) A layer of material, usually metallic, that is mechanically or metallurgically bonded to a substrate. Cladding may be bonded to the substrate by any of several processes, such as roll cladding and explosive forming. (2) A relatively thick layer (1 mm, or 0.04 in.) of material applied by surfacing for the purpose of improved corrosion resistance or other properties. See also coating, surfacing, and hardfacing. clad metal. A composite metal containing two or more layers that have been bonded together. The bonding may have been accomplished by corolling, coextrusion, welding, diffusion bonding, casting, heavy chemical deposition, or heavy electroplating. coating. A relatively thin layer (<1 mm, or 0.04 in.) of material applied by surfacing for the purpose of corrosion prevention, resistance to hightemperature scaling, wear resistance, lubrication, or other purposes. coefficient of friction. The dimensionless ratio of the friction force (F) between two bodies to the normal force (AO pressing these bodies together: (|JL or/) = (F/N).

cohesion. (1) The state in which the particles of a single substance are held together by primary or secondary valence forces. As used in the adhesive field, the state in which the particles of the adhesive (or adherend) are held together. (2) Force of attraction between the molecules (or atoms) within a single phase. Contrast with adhesion. color buffing. Producing a final high luster by buffing. Sometimes called coloring. coloring. Producing desired colors on metal by a chemical or electrochemical reaction. See also color buffing. composite coating. A coating on a metal or nonmetal that consists of two or more components, one of which is often particulate in form. Example: a cermet composite coating on a cemented carbide cutting tool. Also known as multilayer coating. composite plate. An electrodeposit consisting of layers of at least two different compositions. contact corrosion. A term primarily used in Europe to describe galvanic corrosion between dissimilar metals. contact plating. A metal plating process wherein the plating current is provided by galvanic action between the work metal and a second metal, without the use of an external source of current. conversion coating. A coating consisting of a compound of the surface metal, produced by chemical or electrochemical treatments of the metal. Examples include chromate coatings on zinc, cadmium, magnesium, and aluminum, and oxide and phosphate coatings on steel. See also chromate treatment and phosphating. copper-accelerated salt-spray (CASS) test. An accelerated corrosion test for some electrodeposits and for anodic coatings on aluminum. core. In a ferrous alloy prepared for case hardening, that portion of the alloy that is not part of the case. Typically considered to be the portion that (a) appears dark (with certain etchants) on an etched cross section, (b) has an essentially unaltered chemical composition, or (c) has a hardness, after hardening, less than a specified value. corrodkote test. An accelerated corrosion test for electrodeposits. corrosion. The chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties. corrosion-erosion. See erosion-corrosion. corrosion fatigue. The process in which a metal fractures prematurely under conditions of simultaneous corrosion and repeated cyclic loading at lower stress levels or fewer cycles than would be required in the absence of the corrosive environment. corrosion inhibitor. See inhibitor. corrosion product. Substance formed as a result of corrosion. corrosion protection. Modification of a corrosion system so that corrosion damage is mitigated.

corrosion rate. Corrosion effect on a metal per unit of time. The type of corrosion rate used depends on the technical system and on the type of corrosion effect. Thus, corrosion rate may be expressed as an increase in corrosion depth per unit of time (penetration rate, for example, mils/yr) or the mass of metal turned into corrosion products per unit area of surface per unit of time (weight loss, for example, g/m2/yr). The corrosion effect may vary with time and may not be the same at all points of the corroding surface. Therefore, reports of corrosion rates should be accompanied by information on the type, time dependency, and location of the corrosion effect. corrosion resistance. The ability of a material to withstand contact with ambient natural factors or those of a particular, artificially created atmosphere, without degradation or change in properties. For metals, this could be pitting or rusting; for organic materials, it could be crazing. corrosion system. System consisting of one or more metals and all parts of the environment that influence corrosion. corrosive wear. Wear in which chemical or electrochemical reaction with the environment is significant. See also oxidative wear. covering power. (1) The ability of a solution to give satisfactory plating at very low current densities, a condition that exists in recesses and pits. This term suggests an ability to cover, but not necessarily to build up, a uniform coating, whereas throwing power suggests the ability to obtain a coating of uniform thickness on an irregularly shaped object. (2) The degree to which a porcelain enamel coating obscures the underlying surface. crevice corrosion. Localized corrosion of a metal surface at, or immediately adjacent to, an area that is shielded from full exposure to the environment because of close proximity between the metal and the surface of another material. cyaniding. A case-hardening process in which a ferrous material is heated above the lower transformation temperature range in a molten salt containing cyanide to cause simultaneous absorption of carbon and nitrogen at the surface and, by diffusion, create a concentration gradient. Quench hardening completes the process. D dealloying. The selective corrosion of one or more components of a solidsolution alloy. Also called parting or selective leaching. See also dezincification and graphitic corrosion. degreasing. The removal of grease and oils from a surface. Can be accomplished by immersion in liquid organic solvent, by solvent vapors condensing on the parts being cleaned (vapor degreasing), or by spraying the parts with solvent.

deposit corrosion. Corrosion occurring under or around a discontinuous deposit on a metallic surface. Also called poultice corrosion. detonation flame spraying, A thermal spraying process variation in which the controlled explosion of a mixture of fuel gas, oxygen, and powdered coating material is utilized to melt and propel the material to the workpiece. dezincification. Corrosion in which zinc is selectively leached from zinccontaining alloys leaving a relatively weak layer of copper and copper oxide. Most commonly found in copper-zinc alloys containing less than 85% Cu after extended service in water containing dissolved oxygen. See also dealloying and selective leaching. dichromate treatment. A chromate conversion coating produced on magnesium alloys in a boiling solution of sodium dichromate. differential coating. A coated product having a specified coating on one surface and a significantly lighter coating on the other surface (such as a hot dip galvanized product or electrolytic tin plate). diffusion coating. Any process whereby a base metal or alloy is either (1) coated with another metal or alloy and heated to a sufficient temperature in a suitable environment or (2) exposed to a gaseous or liquid medium containing the other metal or alloy, thus causing diffusion of the coating or of the other metal or alloy into the base metal with resultant changes in the composition and properties of its surface. diphase cleaning. Removing soil by an emulsion that produces two phases in the cleaning tank: a solvent phase and an aqueous phase. Cleaning is effected by both solvent action and emulsification. distortion. Any deviation from an original size, shape, or contour that occurs because of the application of stress or the release of residual stress. droplet erosion. Erosive wear caused by the impingement of liquid droplets on a solid surface. See also erosion. E electrochemical corrosion. Corrosion that is accompanied by a flow of electrons between cathodic and anodic areas on metallic surfaces. electrodeposition. (1) The deposition of a conductive material from a plating solution by the application of electrical current. (2) The deposition of a substance on an electrode by passing electric current through an electrolyte. Electroplating, electroforming, electrorefining, and electrotwinning result from electrodeposition. electroforming. Making parts by electrodeposition on a removable form. electrogalvanizing. The electroplating of zinc upon iron or steel. electroless plating. (1) A process in which metal ions in a dilute aqueous solution are plated out on a substrate by means of autocatalytic chemical reduction. (2) The deposition of conductive material from an autocatalytic plating solution without the application of electrical current.

electrolytic cleaning. A process of removing soil, scale, or corrosion products from a metal surface by subjecting it as an electrode to an electric current in an electrolytic bath. electrolytic deposition. Same as electrodeposition. electrolytic pickling. Pickling in which electric current is used, the work being one of the electrodes. electron beam heat treating. A selective surface hardening process that rapidly heats a surface by direct bombardment with an accelerated stream of electrons. electroplate. The application of a metallic coating on a surface by means of electrolytic action. electroplating. The electrodeposition of an adherent metallic coating on an object serving as a cathode for the purpose of securing a surface with properties or dimensions different from those of the substrate. electropolishing. A technique commonly used to prepare metallographic specimens, in which a high polish is produced making the specimen the anode in an electrolytic cell, where preferential dissolution at high points smooths the surface. electrotinning. Electroplating tin on an object. emulsion cleaner. A cleaner consisting of organic solvents dispersed in an aqueous medium with the aid of an emulsifying agent. environmental cracking. Brittle fracture of a normally ductile material in which the corrosive effect of the environment is a causative factor. erosion. (1) Loss of material from a solid surface due to relative motion in contact with a fluid that contains solid particles. Erosion in which the relative motion of particles is nearly parallel to the solid surface is called abrasive erosion. Erosion in which the relative motion of the solid particles is nearly normal to the solid surface is called impingement erosion or impact erosion. (2) Progressive loss of original material from a solid surface due to mechanical interaction between that surface and a fluid, a multicomponent fluid, and impinging liquid, or solid particles. (3) Loss of material from the surface of an electrical contact due to an electrical discharge (arcing). See also cavitation erosion and erosion-corrosion. erosion-corrosion. A conjoint action involving corrosion and erosion in the presence of a moving corrosive fluid, leading to the accelerated loss of material. erosivity. The characteristic of a collection of particles, liquid stream, or a slurry that expresses its tendency to cause erosive wear when forced against a solid surface under relative motion. exfoliation. Corrosion that proceeds laterally from the sites of initiation along planes parallel to the surface, generally at grain boundaries, forming corrosion products that force metal away from the body of the material, giving rise to a layered appearance. Most commonly associated with wrought aluminum alloys.

extreme-pressure lubricant. A lubricant that imparts increased loadcarrying capacity to rubbing surfaces under severe operating conditions. Extreme-pressure lubricants usually contain sulfur, halogens, or phosphorus. F false brinelling. (1) Damage to a solid bearing surface characterized by indentations not caused by plastic deformation resulting from overload, but thought to be due to other causes such as fretting corrosion. (2) Local spots appearing when the protective film on a metal is broken continually by repeated impacts, usually in the presence of corrosive agents. The appearance is generally similar to that produced by brinelling, but corrosion products are usually visible. It may result from fretting corrosion. This term should be avoided when a more precise description is possible. False brinelling (race fretting) can be distinguished from true brinelling because in false brinelling, surface material is removed so that original finishing marks are removed. The borders of a false brinell mark are sharply defined, whereas a dent caused by a rolling element does not have sharp edges and the finishing marks are visible in the bottom of the dent. fatigue. The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the ultimate tensile strength of the material. Fatigue failure generally occurs at loads that applied statically would produce little perceptible effect. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress. filiform corrosion. Corrosion that occurs under some coatings in the form of randomly distributed threadlike filaments. finish. Surface condition, quality, or appearance of a metal. finish grinding. The final grinding action on a workpiece, of which the objectives are surface finish and dimensional accuracy. fixture. A device designed to hold parts to be joined in proper relation to each other. flame hardening. A process for hardening the surfaces of hardenable ferrous alloys in which an intense flame is used to heat the surface layers above the upper transformation temperature, whereupon the workpiece is immediately quenched. flame spraying. A thermal spraying process in which an oxyfuel gas flame is the source of heat for melting the surfacing material. Compressed gas may or may not be used for atomizing and propelling the surfacing material to the substrate. fluidized bed. A contained mass of a finely divided solid that behaves like a fluid when brought into suspension in a moving gas or liquid. flux-cored arc welding (FCAW). An arc welding process that joins metal by heating them with an arc between a continuous tubular filler-metal

electrode and the work. Shielding is provided by a flux contained within the consumable tubular electrode. Additional shielding may or may not be obtained from an externally supplied gas or gas mixture. See also flux-cored elctrode. flux-cored electrode. A composite filler metal electrode consisting of a metal tube or other hollow configuration containing ingredients to provide such functions as shielding atmosphere, deoxidation, arc stabilization, and slag formation. Minor amounts of alloying materials may be included in the core. External shielding may or may not be used. fretting. A type of wear that occurs between tight-fitting surfaces subjected to cyclic relative motion of extremely small amplitude. Usually, fretting is accompanied by corrosion, especially of the very fine wear debris. Also referred to as fretting corrosion and false brinelling (in rolling-element bearings). fretting corrosion. (1) The accelerated deterioration at the interface between contacting surfaces as the result of corrosion and slight oscillatory movement between the two surfaces. (2) A form of fretting in which chemical reaction predominates. Fretting corrosion is often characterized by the removal of particles and subsequent formation of oxides, which are often abrasive and so increase the wear. Fretting corrosion can involve other chemical reaction products, which may not be abrasive. fretting fatigue. (1) Fatigue fracture that initiates at a surface area where fretting has occurred. The progressive damage to a solid surface that arises from fretting. Note: If particles of wear debris are produced, then the term fretting wear may be applied. fretting wear. Wear arising as a result of fretting. friction. The resisting force tangential to the common boundary between two bodies when, under the action of an external force, one body moves or tends to move relative to the surface of the other. friction coefficient. See coefficient of friction. fused-spray deposit. A self-fluxing spray deposit that is deposited by conventional thermal spraying and subsequently fused using either a heating torch or a furnace. G galling. (I)A condition whereby excessive friction between high spots results in localized welding with subsequent spoiling and a further roughening of the rubbing surfaces of one or both of two mating parts. (2) A severe form of scuffing associated with gross damage to the surfaces or failure. Galling has been used in many ways in tribology; therefore, each time it is encountered its meaning must be ascertained from the specific context of the usage. See also scoring and scuffing. galvanic corrosion. Corrosion associated with the current of a galvanic cell consisting of two dissimilar conductors in an electrolyte or two

similar conductors in dissimilar electrolytes. Where the two dissimilar metals are in contact, the resulting reaction is referred to as couple action. galvanic series. A list of metals and alloys arranged according to their relative corrosion potentials in a given environment. galvanize. To coat a metal surface with zinc using any of various processes. galvanneal. To produce a zinc-iron alloy coating on iron or steel by keeping the coating molten after hot dip galvanizing until the zinc alloys interdiffuse completely with the basis metal. gaseous corrosion. Corrosion with gas as the only corrosive agent and without any aqueous phase on the surface of the metal. Also called dry corrosion. See also hot corrosion, oxidation, and sulfidation. gas metal arc welding (GMAW). An arc welding process that produces coalescence of metals by heating them with an arc between a continuous filler metal electrode and the workpieces. Shielding is obtained entirely from an externally supplied gas. gas tungsten arc welding (GTAW). An arc welding process that produces coalescence of metals by heating them with an arc between a tungsten (nonconsumable) electrode and the work. Shielding is obtained from a gas or gas mixture. Pressure may or may not be used, and filler metal may or may not be used. general corrosion. (I)A form of deterioration that is distributed more or less uniformly over a surface. (2) Corrosion dominated by uniform thinning that proceeds without appreciable localized attack. See also uniform corrosion. gouging abrasion. A form of high-stress abrasion in which easily observable grooves or gouges are created on the surface. See also abrasion. graphitic corrosion. Corrosion of gray iron in which the iron matrix is selectively leached away, leaving a porous mass of graphite behind; it occurs in relatively mild aqueous solutions and on buried pipe and fittings. grinding. Removing material from a workpiece with a grinding wheel or abrasive belt. grit blasting. Abrasive blasting with small irregular pieces of steel, malleable cast iron, or hard nonmetallic materials. H hard chromium. Chromium electrodeposited for engineering purposes (such as to increase the wear resistance of sliding metal surfaces) rather than as a decorative coating. It is usually applied directly to substrate and is customarily thicker (> 1.2 |xm, or 0.05 mil) than a decorative deposit, but not necessarily harder. hardfacing. The application of a hard, wear-resistant material to the surface of a component by welding, spraying, or allied welding processes

to reduce wear or loss of material by abrasion, impact, erosion, galling, and cavitation. See also surfacing. hardfacing alloys. Wear-resistant materials available as bare welding rod, flux-coated rod, long-length solid wires, long-length tubular wires, or powders that are deposited by hardfacing. high-stress abrasion. A form of abrasion in which relatively large cutting forces are imposed on the particles or protuberances causing the abrasion and that produces significant cutting and deformation of the wearing surface. high-temperature hydrogen attack. A loss of strength and ductility of steel by high-temperature reaction of absorbed hydrogen with carbides in the steel resulting in decarburization and internal fissuring. holidays. Discontinuities in a coating (such as porosity, cracks, gaps, and similar flaws) that allow areas of substrate to be exposed to any corrosive environment that contacts the coated surface. honing. A low-speed finishing process used chiefly to produce uniform high dimensional accuracy and fine finish, most often on inside cylindrical surfaces. In honing, very thin layers of stock are removed by simultaneously rotating and reciprocating a bonded abrasive stone or stick that is pressed against the surface being honed with lighter force than is typical of grinding. hot corrosion. An accelerated corrosion of metal surfaces that results from the combined effect of oxidation and reactions with sulfur compounds and other contaminants, such as chlorides, to form a molten salt on a metal surface that fluxes, destroys, or disrupts the normal protective oxide. See also gaseous corrosion. hot dip. Covering a surface by dipping the surface to be coated into a molten bath of the coating material. See also hot dip coating. hot dip coating. A metallic coating obtained by dipping the substrate into a molten metal. hydrogen blistering. The formation of subsurface planar cavities, called hydrogen blisters, in a metal resulting from excessive internal hydrogen pressure. Growth of near-surface blisters in low-strength metals usually results in surface bulges. hydrogen damage. A general term for the embrittlement, cracking, blistering, and hydride formation that can occur when hydrogen is present in some metals. hydrogen embrittlement. A loss of ductility of a metal resulting from absorption of hydrogen. hydrogen-induced cracking. Step wise internal cracks that connect adjacent hydrogen blisters on different planes in the metal or to the metal surface. Also called stepwise cracking. hydrogen stress cracking. Cracking that results from the presence of hydrogen in a metal in combination with tensile stress. It occurs most frequently with high-strength alloys.

I

immersion cleaning. Cleaning in which the work is immersed in a liquid solution. immersion coating. A coating produced in a solution by chemical or electrochemical action without the use of external current. immersion plating. Depositing a metallic coating on a metal immersed in a liquid solution, without the aid of an external electric current. Also called dip plating. impact wear. Wear of a solid surface resulting from repeated collisions between that surface and another solid body. The term erosion is preferred in the case of multiple impacts and when the impacting body or bodies are very small relative to the surface being impacted. impingement. A process resulting in a continuing succession of impacts between liquid or solid particles and a solid surface. impingement attack. Corrosion associated with turbulent flow of liquid. May be accelerated by entrained gas bubbles. See also erosioncorrosion and impingement corrosion. impingement corrosion. A form of erosion-corrosion generally associated with the local impingement of a high-velocity, flowing fluid against a solid surface. impingement erosion. Loss of material from a solid surface due to liquid impingement. See also erosion. induction hardening. A surface-hardening process in which only the surface layer of a suitable ferrous workpiece is heated by electromagnetic induction to above the upper critical temperature and immediately quenched. inhibitor. A substance that retards some specific chemical reaction, for example, corrosion. Pickling inhibitors retard the dissolution of metal without hindering the removal of scale from steel. intergranular corrosion. Corrosion occurring preferentially at grain boundaries, usually with slight or negligible attack on the adjacent grains. intergranular stress-corrosion cracking (IGSCC). Stress-corrosion cracking in which the cracking occurs along grain boundaries. interrupted-current plating. Plating in which the flow of current is discontinued for periodic short intervals to decrease anode polarization and elevate the critical current density. It is most commonly used in cyanide copper plating. ion carburizing. A method of surface hardening in which carbon ions are diffused into a workpiece in a vacuum through the use of high-voltage electrical energy. Synonymous with plasma carburizing or glowdischarge carburizing. ion implantation. The process of modifying the physical or chemical properties of the near surface of a solid (target) by embedding appropriate atoms into it from a beam of ionized particles.

ion nitriding. A method of surface hardening in which nitrogen ions are diffused into a workpiece in a vacuum through the use of high-voltage electrical energy. Synonymous with plasma nitriding or glow-discharge nitriding. ion plating. A generic term applied to atomistic film-deposition processes in which the substrate surface and/or the depositing film is subjected to a flux of high-energy particles (usually gas ions) sufficient to cause changes in the interfacial region or film properties. K knife-line attack. Intergranular corrosion of an alloy, usually stabilized stainless steel, along a line adjoining or in contact with a weld after heating into the sensitization temperature range. L lapping. A finishing operation using fine abrasive grits loaded into a lapping material such as cast iron. Lapping provides major refinements in the workpiece including extreme accuracy of dimension, correction of minor imperfections of shape, refinement of surface finish, and close fit between mating surfaces. laser alloying. See laser surface processing. laser beam welding (LBW). A welding process that produces coalescence of materials with the heat obtained from the application of a concentrated coherent light beam impinging upon the joint. laser hardening. A surface-hardening process that uses a laser to quickly heat a surface. Heat conduction into the interior of the part will quickly cool the surface, leaving a shallow martensitic layer. laser melting. See laser surface processing. laser surface processing. The use of lasers to modify the metallurgical structure of a surface and to tailor the surface properties without adversely affecting the bulk properties. The surface modification can take the following three forms. The first is transformation hardening in which a surface is heated so that thermal diffusion and solid-state transformations can take place. The second is surface melting, which results in a refinement of the structure due to the rapid quenching from the melt. The third is surface (laser) alloying, in which alloying elements are added to the melt pool to change the composition of the surface. The novel structures produced by laser surface melting and alloying can exhibit improved electrochemical and tribological behavior. liquid carburizing. Surface hardening of steel by immersion into a molten bath consisting of cyanides and other salts. liquid honing. Producing a finely polished finish by directing an airejected chemical emulsion containing fine abrasives against the surface to be finished.

liquid nitriding. A method of surface hardening in which molten nitrogen-bearing, fused-salt baths containing both cyanides and cyanates are exposed to parts at subcritical temperatures. liquid nitrocarburizing. A nitrocarburizing process (where both carbon and nitrogen are absorbed into the surface) utilizing molten liquid salt baths below the lower critical temperature. localized corrosion. Corrosion at discrete sites, for example, crevice corrosion, pitting, and stress-corrosion cracking. low-stress abrasion. A form of abrasion in which relatively low contact pressures on the abrading particles or protuberances cause only fine scratches and microscopic cutting chips to be produced. lubricant. (1) Any substance interposed between two surfaces in relative motion for the purpose of reducing the friction or wear between them. (2) A material applied to dies, molds, plungers, or workpieces that promotes the flow of metal, reduces friction and wear, and aids in the release of the finished part. lubrication. The reduction of frictional resistance and wear, or other forms of surface deterioration, between two load-bearing surfaces by the application of a lubricant. luster finish. A bright, as-rolled finish, produced on ground metal rolls; it is suitable for decorative painting or plating, but usually must undergo additional surface preparation after forming. M matte finish. (I)A dull texture produced by rolling sheet or strip between rolls that have been roughened by blasting. (2) A dull finish characteristic of some electrodeposits, such as cadmium or tin. mechanical plating. Plating wherein fine metal powders are peened onto the work by tumbling or other means. The process is used primarily to provide ferrous parts with coatings of zinc, cadmium, tin, and alloys of these metals in various combinations. mechanical polishing. A process that yields a specularly reflecting surface entirely by the action of machining tools, which are usually the points of abrasive particles suspended in a liquid among the fibers of a polishing cloth. metallizing. Forming a metallic coating by atomized spraying with molten metal or by vacuum deposition. Also called spray metallizing. metal spraying. Coating metal objects by spraying molten metal against their surfaces. See also thermal spraying.

N nitriding. Introducing nitrogen into the surface layer of a solid ferrous alloy by holding at a suitable temperature (below Ac 1 for ferritic steels) in contact with a nitrogenous material, usually ammonia or molten

cyanide of appropriate composition. Quenching is not required to produce a hard case. See also bright nitriding and liquid nitriding. nitrocarburizing. Any of several processes in which both nitrogen and carbon are absorbed into the surface layers of a ferrous material at temperatures below the lower critical temperature and, by diffusion, create a concentration gradient. Nitrocarburizing is performed primarily to provide an antiscuffing surface layer and to improve fatigue resistance. Compare with carbonitriding. o oxidation. A corrosion reaction in which the corroded metal forms an oxide; usually applied to reaction with a gas containing elemental oxygen, such as air. Elevated temperatures increase the rate of oxidation. oxidative wear. (I)A corrosive wear process in which chemical reaction with oxygen or oxidizing environment predominates. (2) A type of wear resulting from the sliding action between two metallic components that generates oxide films on the metal surfaces. These oxide films prevent the formation of a metallic bond between the sliding surfaces, resulting in fine wear debris and low wear rates. oxyacetylene welding. An oxyfuel gas welding process in which the fuel gas is acetylene. oxyfuel gas welding (OFW). Any of a group of processes used to fuse metals together by heating them with gas flames resulting from combustion of a specific fuel gas such as acetylene, hydrogen, natural gas, or propane. The process may be used with or without the application of pressure to the joint, and with or without adding any filler metal. p pack carburizing. A method of surface hardening of steel in which parts are packed in a steel box with a carburizing compound and heated to elevated temperatures. This process has been largely supplanted by gas and liquid carburizing processes. pack nitriding. A method of surface hardening of steel in which parts are packed in a steel box with a nitriding compound and heated to elevated temperatures. passive. (1) A metal corroding under the control of a surface reaction product. (2) The state of the metal surface characterized by low corrosion rates in a potential region that is strongly oxidizing for the metal. phosphating. Forming an adherent phosphate coating on a metal by immersion in a suitable aqueous phosphate solution. Also called phosphatizing. See also conversion coating. physical vapor deposition (PVD). A coating process whereby the deposition species are transferred and deposited in the form of individual

atoms or molecules. The most common PVD methods are sputtering and evaporation. Sputtering, which is the principal PVD process, involves the transport of a material from a source (target) to a substrate by means of the bombardment of the target by gas ions that have been accelerated by a high voltage. Evaporation, which was the first PVD process used, involves the transfer of material to form a coating by physical means alone, essentially vaporization. Physical vapor deposition coatings are used to improve the wear, friction, and hardness properties of cutting tools and as corrosion-resistant coatings. pitting. (1) Forming small sharp cavities in a surface by corrosion, wear, or other mechanically assisted degradation. (2) Localized corrosion of a metal surface, confined to a point or small area, that takes the form of cavities. plasma arc welding (PAW). An arc welding process that produces coalescence of metals by heating them with a constricted arc between an electrode and the workpiece (transferred arc) or the electrode and the constricting nozzle (nontransferred arc). Shielding is obtained from hot, ionized gas issuing from an orifice surrounding the electrode and may be supplemented by an auxiliary source of shielding gas, which may be an inert gas or a mixture of gases. Pressure may or may not be used, and filler metal may or may not be supplied. plasma-assisted chemical vapor deposition. A chemical vapor deposition process that uses low-pressure glow-discharge plasmas to promote the chemical deposition reactions. Also called plasma-enhanced chemical vapor deposition. plasma carburizing. Same as ion carburizing. plasma nitriding. Same as ion nitriding. plasma spraying. A thermal spraying process in which a nontransferred arc of a plasma torch is utilized to create a gas plasma that acts as the source of heat for melting and propelling the surfacing material to the substrate. polishing. (1) Smoothing metal surfaces, often to a high luster, by rubbing the surface with a fine abrasive, usually contained in a cloth or other soft lap. Results in microscopic flow of some surface metal together with actual removal of a small amount of surface metal. (2) Removal of material by the action of abrasive grains carried to the work by a flexible support, generally either a wheel or a coated abrasive belt. (3) A mechanical, chemical, or electrolytic process or combination thereof used to prepare a smooth, reflective surface suitable for microstructural examination that is free of artifacts or damage introduced during prior sectioning or grinding. See also electropolishing. porcelain enamel. A substantially vitreous or glassy, inorganic coating (borosilicate glass) bonded to metal by fusion at a temperature above 425 0C (800 0 F). Porcelain enamels are applied primarily to compo-

nents made of sheet iron or steel, cast iron, aluminum, or aluminumcoated steels. poultice corrosion. A term used in the automotive industry to describe the corrosion of vehicle body parts due to the collection of road salts and debris on ledges and in pockets that are kept moist by weather and washing. Also called deposit corrosion or attack. powder flame spraying. A thermal spraying process variation in which the material to be sprayed is in powder form. precoated metal products. Mill products that have a metallic, organic, or conversion coating applied to their surfaces before they are fabricated into parts.

Q quench hardening. In ferrous alloys, hardening by austenitizing and then cooling at a rate such that a substantial amount of austenite transforms to martensite. quenching. Rapid cooling of metals (often steels) from a suitable elevated temperature. This generally is accomplished by immersion in water, oil, polymer solution, or salt, although forced air is sometimes used. R residual stress. (1) The stress existing in a body at rest, in equilibrium, at uniform temperature, and not subjected to external forces. Often caused by the forming or thermal processing curing process. (2) An internal stress not depending on external forces resulting from such factors as cold working, phase changes, or temperature gradients. (3) Stress present in a body that is free of external forces or thermal gradients. (4) Stress remaining in a structure or member as a result of thermal or mechanical treatment or both. Stress arises in fusion welding primarily because the weld metal contracts on cooling from the solidus to room temperature. robber. An extra cathode or cathode extension that reduces the current density on what would otherwise be a high-current-density area on work being electroplated. rolling-contact fatigue. Repeated stressing of a solid surface due to rolling contact between it and another solid surface or surfaces. Continued rolling-contact fatigue of bearing or gear surfaces may result in rolling-contact damage in the form of subsurface fatigue cracks and/or material pitting and spallation. rouge finish. A highly reflective finish produced with rouge (finely divided, hydrated iron oxide) or other very fine abrasive, similar in appearance to the bright polish or mirror finish on sterling silver utensils. rust. A visible corrosion product consisting of hydrated oxides of iron. Applied only to ferrous alloys.

S salt fog test. An accelerated corrosion test in which specimens are exposed to a fine mist of a solution usually containing sodium chloride, but sometimes modified with other chemicals. Also known as salt spray test. satin finish. A diffusely reflecting surface finish on metals, lustrous but not mirrorlike. One type is a butler finish. scaling. Forming a thick layer of oxidation products on metals at high temperature. Scaling should be distinguished from rusting, which involves the formation of hydrated oxides. See also rust. scoring. (1) The formation of severe scratches in the direction of sliding. (2) The act of producing a scratch or narrow groove in a surface by causing a sharp instrument to move along that surface. (3) The marring or scratching of any formed metal part by metal pickup on the punch or die. scouring. (I)A wet or dry cleaning process involving mechanical scrubbing. (2) A wet or dry mechanical finishing operation, using fine abrasive and low pressure, carried out by hand or with a cloth or wire wheel to produce satin or butler-type finishes. scuffing. (1) Localized damage caused by the occurrence of solid-phase welding between sliding surfaces, without local surface melting. (2) A mild degree of galling that results from the welding of asperities due to frictional heat. The welded asperities break, causing surface degradation. seal coat. Material applied to infiltrate the pores of a thermal spray deposit. sealing. Closing pores in anodic coatings to render them less absorbent. seizure. The stopping of relative motion as the result of interfacial friction. Seizure may be accompanied by gross surface welding. The term is sometimes used to denote scuffing. selective leaching. Corrosion in which one element is preferentially removed from an alloy, leaving a residue (often porous) of the elements that are more resistant to the particular environment. Also called dealloying or parting. See also dezincification and graphitic corrosion. sensitization. In austenitic stainless steels, the precipitation of chromium carbides, usually at grain boundaries, on exposure to temperatures of about 540 to 845 0 C (about 1000-1550 0 F), leaving the grain boundaries depleted of chromium and therefore susceptible to preferential attack by a corroding medium. Welding is the most common cause of sensitization. Weld decay (sensitization) caused by carbide precipitation in the weld heat-affected zone leads to intergranular corrosion. shielded metal arc welding (SMAW). An arc welding process that produces coalescence of metals by heating them with an arc between a

covered metal electrode and the workpieces. Shielding is obtained from decomposition of the electrode covering. Pressure is not used, and filler metal is obtained from the electrode. Also commonly referred to as stick welding. shotblasting. Blasting with metal shot; usually used to remove deposits or mill scale more rapidly or more effectively than can be done by sandblasting. shot peening. A method of cold working metals in which compressive stresses are induced in the exposed surface layers of parts by the impingement of a stream of shot, directed at the metal surface at high velocity under controlled conditions. siliconizing. Diffusing silicon into solid metal, usually low-carbon steels, at an elevated temperature in order to improve corrosion or wear resistance. solid lubricant. Any solid used as a powder or thin film on a surface to provide protection from damage during relative movement and to reduce friction and wear. sour gas. A gaseous environment containing hydrogen sulfide and carbon dioxide in hydrocarbon reservoirs. Prolonged exposure to sour gas can lead to hydrogen damage, sulfide-stress cracking, and/or stresscorrosion cracking in ferrous alloys. spalling. (1) The spontaneous chipping, fragmentation, or separation of a surface or surface coating. (2) A chipping or flaking of a surface due to any kind of improper heat treatment or material dissociation. sputtering. The bombardment of a solid surface with a flux of energetic particles (ions) that results in the ejection of atomic species. The ejected material may be used as a source for deposition. See also physical vapor deposition. steam treatment. The treatment of a sintered ferrous part in steam at temperatures between 510 and 595 0C (950 and 1100 0F) in order to produce a layer of black iron oxide (magnetite, or ferrous-ferric oxide, FeO-Fe2O3) on the exposed surface for the purpose of increasing hardness and wear resistance. stray-current corrosion. Corrosion resulting from direct-current flow through paths other than the intended circuit. For example, by an extraneous current in the earth. stress corrosion. Preferential attack of areas under stress in a corrosive environment, where such an environment alone would not have caused corrosion. stress-corrosion cracking (SCC). A cracking process that requires the simultaneous action of a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also excludes intercrystalline or transcrystalline corrosion, which can disintegrate an alloy without applied or residual stress. Stress-corrosion cracking may occur in combination with hydrogen embrittlement.

stress relieving. Heating to a suitable temperature, holding long enough to reduce residual stresses, and then cooling slowly enough to minimize the development of new residual stresses. strike. (I)A thin electrodeposited film of metal to be overlaid with other plated coatings. (2) A plating solution of high covering power and low efficiency designed to electroplate a thin, adherent film of metal. submerged arc welding (SAW). An arc welding process that produces coalescence of metals by heating them with an arc or arcs between a bare metal electrode or electrodes and the workpieces. The arc and molten metal are shielded by a blanket of granular, fusible material on the workpieces. Pressure is not used, and filler metal is obtained from the electrode and sometimes from a supplemental source (welding rod, flux, or metal granules). substrate. The material, workpiece, or substance on which the coating is deposited. sulfidation. The reaction of a metal or alloy with a sulfur-containing species to produce a sulfur compound that forms on or beneath the surface on the metal or alloy. sulfide stress cracking (SSC). Brittle fracture by cracking under the combined action of tensile stress and corrosion in the presence of water and hydrogen sulfide. superabrasives. Synthetically produced diamond and cubic boron nitride (CBN) used in a wide variety of cutting and grinding applications. superfinishing. A low-velocity abrading process very similar to honing; however, unlike honing, superfinishing processes focus primarily on the improvement of surface finish and much less on correction of geometric errors (dimensional accuracy). Also known as microhoning. surface damage. In tribology, damage to a solid surface resulting from mechanical contact with another substance, surface, or surfaces moving relatively to it and involving the displacement or removal of material. In certain contexts, wear is a form of surface damage in which material is progressively removed. In another context, surface damage involves a deterioration of function of a solid surface even though there is no material loss from that surface. Surface damage may therefore precede wear. surface finish. (1) The geometric irregularities in the surface of a solid material. Measurement of surface finish shall not include inherent structural irregularities unless these are the characteristics being measured. (2) Condition of a surface as a result of a final treatment. surface hardening. A generic term covering several processes applicable to a suitable ferrous alloy that produces, by quench hardening only, a surface layer that is harder or more wear resistant than the core. There is no significant alteration of the chemical composition of the surface layer. The processes commonly used are carbonitriding, carburizing, induction hardening, flame hardening, nitriding, and nitrocarburizing. Use of the applicable specific process name is preferred.

surface modification. The alteration of surface composition or structure by the use of energy or particle beams. Two types of surfacemodification methods commonly employed are ion implantation and laser surface processing. surface roughness. Fine irregularities in the surface texture of a material, usually including those resulting from the inherent action of the production process. Surface roughness is usually reported as the arithmetic roughness average, /?a, and is given in micrometers or microinches. surfacing. The deposition of filler metal (material) on a base metal (substrate) to obtain desired properties or dimensions, as opposed to making a joint. T tarnish. Surface discoloration of a metal caused by formation of a thin film of corrosion product. terne. An alloy of lead containing 3 to 15% Sn, used as a hot dip coating for steel sheet or plate. The term long terne is used to describe ternecoated sheet, whereas short terne is used for terne-coated plate. Terne coatings, which are smooth and dull in appearance (terne means dull or tarnished in French), give the steel better corrosion resistance and enhance its ability to be formed, soldered, or painted. thermal spraying. A group of coating or welding processes in which finely divided metallic or nonmetallic materials are deposited in a molten or semimolten condition to form a coating. The surfacing material may be in the form of powder, rod, or wire. See also arc spraying, flame spraying, plasma spraying, and powder flame spraying. thermal stresses. Stresses in a material resulting from nonuniform temperature distribution. thermal wear. Removal of material due to softening, melting, or evaporation during sliding or rolling. Thermal shock and high-temperature erosion may be included in the general description of thermal wear. Wear by diffusion of separate atoms from one body to the other, at high temperatures, is also sometimes denoted as thermal wear. thief. A racking device or nonfunctional pattern area used in the electroplating process to provide a more uniform current density on plated parts. Thieves absorb the unevenly distributed current on irregularly shaped parts, thereby ensuring that the parts will receive an electroplated coating of uniform thickness. See also robber. throwing power. The ability of a plating solution to produce a uniform metal distribution on an irregularly shaped cathode. Compare with covering power. tolerance. The specified permissible deviation from a specified nominal dimension, or the permissible variation in size or other quality characteristic of a part.

transformation temperature. The temperature at which a change in phase occurs. This term is sometimes used to denote the limiting temperature of a transformation range. The following symbols are used for irons and steels: In hypereutectoid steel The temperature at which austenite begins to form during heating The temperature at which transformation of ferrite to austenite is completed during heating The temperature at which austenite transforms to 8 ferrite during heating The temperatures of phase changes at equilibrium In hypereutectoid steel The temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling The temperature at which austenite begins to transform to ferrite during cooling The temperature at which 8 ferrite transforms to austenite during cooling The temperature at which transformation of austenite to pearlite starts during cooling The temperature at which transformation of austenite to martensite is completed during cooling The temperature at which transformation of austenite to martensite starts during cooling

Note: All these changes, except formation of martensite, occur at lower temperatures during cooling than during heating and depend on the rate of change of temperature. trees. Visible projections of electrodeposited metal formed at sites of high current density. tribology. (1) The science and technology of interacting surfaces in relative motion and of the practices related thereto. (2) The science concerned with the design, friction, lubrication, and wear of contacting surfaces that move relative to each other (as in bearings, cams, or gears, for example). tuberculation. The formation of localized corrosion products scattered over the surface in the form of knoblike mounds called tubercles. tumbling. Rotating workpieces, usually castings or forgings, in a barrel partly filled with metal slugs or abrasives, to remove sand, scale, or fins. It may be done dry, or with an aqueous solution added to the contents of the barrel. See also barrel finishing. U ultraprecision finishing. Machining processes used to alter surface characteristics such as finish, waviness, roundness, and so forth, with substantial removal of the work material. Examples include lapping and polishing of optical lenses, computer chips, or magnetic heads, and honing of cylinder liners. ultrasonic cleaning. Immersion cleaning aided by ultrasonic waves that cause microagitation. underfilm corrosion. Corrosion that occurs under organic films in the form of randomly distributed threadlike filaments or spots. In many cases, this is identical to filiform corrosion. uniform corrosion. (1) A type of corrosion attack (deterioration) uniformly distributed over a metal surface. (2) Corrosion that proceeds at

approximately the same rate over a metal surface. Also called general corrosion.

V vacuum carburizing. A high-temperature gas carburizing process using furnace pressures between 13 and 67 kPa (0.1 and 0.5 torr) during the carburizing portion of the cycle. Steels undergoing this treatment are austenitized in a rough vacuum, carburized in a partial pressure of hydrocarbon gas, diffused in a rough vacuum, and then quenched in either oil or gas. vacuum deposition. Deposition of a metal film onto a substrate in a vacuum by metal evaporation techniques. vacuum nitrocarburizing. A subatmospheric nitrocarburizing process using a basic atmosphere of 50% ammonia/50% methane, containing controlled oxygen additions of up to 2%. vapor degreasing. Degreasing of work in the vapor over a boiling liquid solvent, the vapor being considerably heavier than air. At least one constituent of the soil must be soluble in the solvent. Modifications of this cleaning process include vapor-spray-vapor, warm liquid-vapor, boiling liquid-warm liquid-vapor, and ultrasonic degreasing. vapor deposition. See chemical vapor deposition, physical vapor deposition, and sputtering. vapor plating. Deposition of a metal or compound on a heated surface by reduction or decomposition of a volatile compound at a temperature below the melting points of the deposit and the base material. The reduction is usually accomplished by a gaseous reducing agent such as hydrogen. The decomposition process may involve thermal dissociation or reaction with the base material. See also vacuum deposition. W wear. Damage to a solid surface, generally involving progressive loss of material, due to a relative motion between that surface and a contacting surface or substance. Compare with surface damage. wear debris. Particles that become detached in a wear process. weathering. Exposure of materials to the outdoor environment. welding. (1) Joining two or more pieces of material by applying heat or pressure, or both, with or without filler material, to produce a localized union through fusion or recrystallization across the interface. The thickness of the filler material is much greater than the capillary dimensions encountered in brazing. (2) May also be extended to include brazing and soldering. (3) In tribology, adhesion between solid surfaces in direct contact at any temperature.

white layer. (1) Compound layer that forms in steels as a result of the nitriding process. (2) In tribology, a white-etching layer, typically associated with ferrous alloys, that is visible in metallographic cross sections of bearing surfaces. white rust. Zinc oxide; the powder product of corrosion of zinc or zinccoated surfaces. wiped coat. A hot dipped galvanized coating from which virtually all free zinc is removed by wiping prior to solidification, leaving only a thin zinc-iron alloy layer. wiping effect. Activation of a metal surface by mechanical rubbing or wiping to enhance the formation of conversion coatings, such as phosphate coatings. wire flame spraying. A thermal spraying process variation in which the material to be sprayed is in wire or rod form. See also flame spraying. Z zincrometal. A steel coil-coated product consisting of a mixed-oxide underlayer containing zinc particles and a zinc-rich organic (epoxy) topcoat. It is weldable, formable, paintable, and compatible with commonly used adhesives. Zincrometal is used to protect outer body door panels in automobiles from corrosion.

Index

Index terms

Links

A Abrasion. See also Abrasive wear; High-stress abrasion; Low-stress abrasion. definition

231

Abrasion-corrosion

70

Abrasive, definition

231

Abrasive blasting, definition

231

71

Abrasive erosion. See also Erosion. definition

231

Abrasive metal-to-metal wear

70

Abrasive wear

56

categories by type of contact

57

definition

56

electroless nickel coatings gouging high-stress

231

151 57

hardness vs. wear resistance process comparisons

71

59

186 57

59

61 61

243 low-stress (scratching)

57

59

multibody

55

57

polishing

57

59

porcelain enamels prevention through surface treatments

134 61

62

75

202 rate of

57

rate rankings for various materials, low-stress

188

surface treatments for prevention

206

synergistic relationship with corrosion mechanisms test methods

164

5 82

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257

258

Index terms

Links

Abrasive wear (Continued) test variables to be controlled

81

of thermal spray coatings

164

thermal spray coatings applications

165

83

under lubricated conditions

78

versus erosion

63

wear testing devices

82

84

157

158

weld-overlay coatings Accelerated corrosion (weathering) test definition

37 231

Acetic acid-salt spray (fog) test (ASTM G 85), description

37

Acidified synthetic seawater testing or SWAAT (ASTM G 85, A3; formerly ASTM G 43), description

37

Acidity, of water, and corrosion

13

Acid treatments

38

146

Acrylics applications

130

characteristics, cost, and applications

129

in hybridized systems

128

modification of

128

thermosetting, resistant to mechanical and chemical action

131

Acrylonitrile-butadiene (nitrile) rubber, environmental resistance ratings

128

Active metal, definition

231

Additives, for paints

128

Adhesion. See also Adhesive wear. definition

231

Adhesion test (ASTM D 3359-90), description

37

Adhesive wear

56

coefficient

74

definition

72

description

72

fretting

76

galling

75

lubricants and

73

material combinations affected

72

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72 232

259

Index terms

Links

Adhesive wear (Continued) materials selection

74

rates of various materials, pin on plate sliding test

189

surface treatments for prevention

201

test variables to be controlled thermal spray coatings applications

81 165

wear testing devices

82

Aesthetic appearance

1

Agriculture, industrial operations and annual wear economic consequences

4

Aircraft, costs of metallic corrosion in U.S. (1975, 1995)

3

Air plasma spraying (APS)

84

160

abrasive wear rate

164

design characteristics

162

Alclad, definition

83

232

Alkaline cleaner definition

232

intermediate

146

Alkalinity, of water, and corrosion

13

Alkyd-amines, resistant to mechanical and chemical action

131

Alkyds

127

applications

130

characteristics, cost, and applications

129

resistant to mechanical and chemical action

131

Alloying, process availability

185

Alloy steels flame hardening

89

hardened, abrasive wear

61

Alpha-aluminum

142

Alpha-chromium, used in chromizing

119

Alumina

136

as chemical vapor deposition coating material

170

thermal sprayed, thickness ranges and hardness levels

211

Alumina/chromium oxide/tungsten carbide-cobalt coatings, plasma sprayed, hardness range

187

Alumina plasma sprayed coating, cost, relative

191

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143

171

211

260

Index terms

Links

Aluminizing applications

198

characteristics and requirements

117

definition

232

diffusion (pack cementation)

198

process availability

185

thickness ranges and hardness levels

211

201

Aluminum as anode material with impressed current

34

dealuminification

26

oxidation

16

pitting corrosion

20

pure, corrosion rate

11

Aluminum alloys corrosion rates

11

erosion

63

exfoliation

26

fretting corrosion

25

galvanic corrosion

17

intergranular corrosion

25

stress-corrosion cracking

28

Aluminum anodizing

102

applications

102

chromic anodizing

102

classification of types 2

102

corrosion pits per m as function of coating thickness

106

corrosion resistance of products

106

corrosion test

40

erosion

106

hardcoat anodizing

104

military specification (MIL-A-8625) for classification

102

sealing

102

sealing of coatings

105

sulfuric anodizing, types of

102

Aluminum bronze hardfacing alloys, to prevent cavitation damage

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7

64

209

261

Index terms Aluminum bronzes, dealuminification

Links 27

Aluminum coatings, applications

143

Aluminum ion plating, applications

198

Aluminum oxide ceramics, hardness range

187

Amino resin-modified alkyds, applications

130

Amsler circumferential rotating disk-on-disk machine (Ams) test geometry

52

Anaerobic bacteria

13

Anode

12 34

Anodic desmutting Anodic protection Anodized aluminum corrosion test, description

14

15

146 5 40

Anodizing applications benefits

198

199

201

204

208

212

2

definition

232

design limitations

223

to prevent fretting corrosion

25

Aqueous solution electroplating

145

Arc spraying (ASP), definition

232

Arc welding, definition

232

Arc wire spray process, characteristics

188

Atmospheric corrosion of anodized aluminum

106

definition

232

description

14

Atmospheric corrosion tests on metal, practice for conducting (ASTM G 50)

36

Austenitic manganese steels, as coatings

61

Austenitic stainless steels intergranular corrosion

25

stress-corrosion cracking

28

to prevent erosion

66

Autocatalytic plating

150

This page has been reformatted by Knovel to provide easier navigation.

152

262

Index terms

Links

B Back-ionization

222

Barrel cleaning, definition

232

Barrel finishing, definition

232

Barrel plating, definition

232

Batch galvanizing process, wet or dry

138

Batch processing, of hot dip coatings

138

Binders

128

Biologically influenced corrosion Black oxide, definition Black oxide chemical conversion coatings, benefits

21 233 2

Blast cleaning, definition

233

Blasting, definition

233

Blasting/deburring, design limitation

220

Block-on-ring (BOR) test geometry

47

Bluing

2

definition

233

Borates

32

Borides, as sputter coating material

175

Bonding. See also Boronizing. characteristics and requirements

119

definition

233

Borohydride compound reducing agents

150

Boron carbide as chemical vapor deposition coating material

171

as coating material

137

hardness range

187

Boronizing (boriding) applications

209

benefits

2

characteristics and requirements

119

process availability

185

thickness ranges and hardness levels

211

Boron powder

119 This page has been reformatted by Knovel to provide easier navigation.

210

263

Index terms Brasses

Links 11

Brazing, temperature range and distortion

192

Break-in

100

Bright finish, definition

233

Bright nitriding, definition

233

Bright plate, definition

233

26

Brinelling. See also False brinelling. definition

233

Broaching/honing, design limitations Bronzes

220 11

Brushing, design limitations

221

Brushing/burnishing, design limitations

220

Brush plating, definition

233

27

Buffing definition

233

design limitations

220

Burnishing definition

233

design limitations

220

Butadiene rubber, environmental resistance ratings

128

Butler finish, definition

233

C Cadmium, galvanic corrosion Cadmium electroplating, applications

17 147

148

applications

198

212

thickness ranges and hardness levels

211

Cadmium plating

Calorizing, definition

233

Carbide coatings, applications

137

Carbide diffusion process applications

204

benefits

2

Carbide hardening alloys This page has been reformatted by Knovel to provide easier navigation.

157

210

212

264

Index terms

Links

Carbide materials, abrasive wear

61

Carbides

25

Carbon, content effect on intergranular corrosion

25

Carbon alloy steels, electron-beam hardening

91

Carbonitride/carburize/cyanide coatings, hardness range

175

187

Carbonitriding applications

203

benefits

205

207

2

characteristics and requirements

110

coating thickness/penetration depth

192

definition

234

115

Carbon steels corrosion fatigue

30

crevice corrosion

22

electron-beam hardening

91

flame hardening

89

hydrogen damage

30

pitting corrosion

20

stress-corrosion cracking

28

Carburization resistance, surface engineering treatments for improvement

2

Carburizing

7

applications

203

205

210

211

benefits

2

characteristics and requirements

110

coating thickness/penetration depth

192

cost, relative

191

definition

234

phase transformations

3

process availability in U.S. and Canadian commercial heat treating shops

184

temperature range and distortion

192

thickness ranges and hardness levels

211

to prevent fretting corrosion Case, definition

25 234

This page has been reformatted by Knovel to provide easier navigation.

112

207

265

Index terms

Links

Case hardening

61

110

112

applications

200 207

203

205

definition

234

size and weight limitations

215

surface finish characteristics

213

thickness ranges and hardness levels

211

CASS test. See also Copper-accelerated salt-spray test. definition

234

Cast irons crevice corrosion

22

electron-beam hardening

91

flame hardening

88

galvanic corrosion

17

graphitic corrosion

26

laser melting

92

manganese phosphate coatings for parts

99

structural grades, corrosion rate

11

89 27

Cast steel, erosion

68

69

Cathode

12

14

15

Cathodic breakdown test, description

40

Cathodic protection

5 33

16

18

Cavitation

7

23

definition

234

thermal spray coatings applications

165

Cavitation corrosion, definition Cavitation damage definition

234 7

68

234

Cavitation erosion definition

15 70

16 71

68

234

test variables to be controlled

81

83

wear testing devices

82

84

32

135

Cement This page has been reformatted by Knovel to provide easier navigation.

192

266

Index terms

Links

Cementation, definition

234

Cementation/diffusion, design limitations

223

Cemented carbide cermets, hardness range

187

Cemented carbides, fracture of

7

Cement linings, benefits

2

Ceramic coatings applications

132 207

benefits

2

porcelain enamels Ceramic high-performance coatings and linings

133 136

applications

136

Ceramic linings

132

Ceramics abrasive wear

60

adhesive wear

74

erosion test results

65

fracture of

61

7

relative erosion factors

65

wear coefficients, adhesive wear

74

Ceramic thermal spray materials

61

67

Cermet coatings

151

applications

203 206

204 207

66

67

erosion test results Cermet thermal spray materials Checks, definition Chemical attack

61 235 8

Chemical conversion coating, definition

235

Chemical deposition, definition

235

Chemical etching

219

Chemical milling

219

design limitations Chemical polishing, definition Chemicals, as corrosion contributor

This page has been reformatted by Knovel to provide easier navigation.

220 235 14

205

267

Index terms Chemical vapor deposition (CVD)

Links 7

advantages, limitations, and processing parameters

126

applications

169

168 204

210

212 benefits

2

characteristics compared to PVD and ion implantation

172

chemical reactions

169

closed reactor systems

169

coating thickness/penetration depth

192

cost, relative

191

definition

235

design limitations

224

open reactor systems

169

plasma process

169

process availability

185

processing steps

168

size and weight limitations

215

surface finish characteristics

213

temperature range and distortion

192

thermal process

169

Chemistry, surface changes

2

Chlorimet 2, galvanic corrosion

17

Chlorimet 3, galvanic corrosion

17

Chlorinated rubbers Chlorine

173

31 131

129

130

212

235

14

Chloroprene rubber, environmental resistance ratings Chromate

128 31

Chromate chemical conversion coatings benefits

100 2

salt-spray test data on zinc and aluminum

101

Chromate treatment, definition

235

Chromating

211

Chromatizing, to prevent erosion Chromic anodizing This page has been reformatted by Knovel to provide easier navigation.

66 102

105

268

Index terms

Links

Chromium carbide abrasive wear

61

as chemical vapor deposition coating material

170

as coating material

137

171

211

Chromium carbide/nickel-chromium, thermally sprayed and spray and fused, thickness ranges and hardness levels

211

Chromium electroplating

147

187

Chromium nitride, as coating material

137

211

Chromium oxide, as coating material

136

211

201

203

204

205 209

206

207

201

209

Chromium plating applications

characteristics

145

cost, relative

191

thickness ranges and hardness levels

211

Chromium-rich carbides

25

Chromizing applications

198 210

characteristics and requirements

119

definition

235

process availability

185

thickness ranges and hardness levels

211

Cladding. See also Coating; Hardfacing; Surfacing benefits

7

166

2

corrosion control metal systems

166

definition

235

design limitations

223

high-volume, commercially produced metal combinations

166

process availability

185

techniques employed

166

Clad metal, definition

235

Clad transition metal systems

168

This page has been reformatted by Knovel to provide easier navigation.

167

269

Index terms

Links

Cleaning. See also Barrel cleaning; Blast cleaning; Electrolytic cleaning; Emulsion cleaner. alkaline

146

232

electro

146

220

precleaning

146

surface cleaning methods before painting

132

ultrasonic

219

Coal-tar enamel

34

Coatings. See also Chemical vapor deposition; Physical vapor deposition; Weld-overlay coatings aluminum

1

187

117

143

barrier

32

benefits

2

black oxide chemical conversion

2

borides

175

carbide

137

cementatious

134

ceramic

2

132

207

cermet

66

67

151

203 206

204 207

205

chlorinated rubber chromate chemical conversion

31 2

100

chromium carbide

137

170

chromium nitride

137

211

chromium oxide

136

211

cobalt-base alloys

61

composite

151

concrete

134

corrosion tests used definition

211

37 235

diamond-like carbon (DLC)

126

170

175

7

110

211 diffusion heat treatment distortion or size change tendencies elastomeric

3 191 2

This page has been reformatted by Knovel to provide easier navigation.

270

Index terms

Links

Coatings. (Continued) electroless nickel/nickel electroplate

187

electroless nickel-boron

151

electroless nickel-phosphorus

151

152

198

electrolytic nickel

197 205

199 212

203

electrolytic zinc

197

198

electroplated

32

221

epoxy

31

127

flame-sprayed

136

192

fluorinated ethylene propylene (FEP)

201

fluorinated polymer

201

fog test data

38

186

galvanized

140

141

of gold

33

hafnium carbides as high-carbon iron-chromium alloys as

137 61

of high-performance ceramic

136

high-velocity oxyfuel (HVOF) thermally sprayed

208

hot dip

138

indium

175

inorganic

222

130

96

97

iron phosphate laser/electron-beam hardened manganese phosphate

187 97

material availability and delivery time

184

MCrAlY metallic

174

of nickel

209

99

33

nickel-aluminum intermetallic

152

nickel-boron

150

of nickel-chromium

33

nickel-chromium-boron

61

nickel-phosphorus

150

nickel-terne

144

niobium silicide

137

nitride

137 This page has been reformatted by Knovel to provide easier navigation.

152

152

175

192

142

271

Index terms

Links

Coatings. (Continued) noble metal

32

organic

2

organic high-performance

31

organizations active in test development and standardization

35

127

oxide application

136

pack-cementation diffusion

116

perfluoro alkoxy alkane (PFA) applications

201

206

2

95

phosphate chemical conversion polyester

31

polyurethane

31

porcelain enamels applications

133

powder

127

process availability

184

process comparisons found in Chapters 1 to 6

183

resins as

128

sacrificial

33

salt mist corrosion performance of various steels and coatings

186

sealing of

105

silicate glass

136

silicide

137

silicon carbide

137

sol-gel

221

sputter

7

197

34

175

175

192

197

sulfuric anodized

104

tantalum silicide

137

terne

144

thermal spray

160

192

203

215

thickness available thicknesses of various surface engineering treatments

192 6

titanium carbide

137

titanium carbonitride

137

titanium nitride

61

to prevent cavitation

24

to prevent corrosion

31

This page has been reformatted by Knovel to provide easier navigation.

221

137

272

Index terms

Links

Coatings. (Continued) to prevent corrosion fatigue

30

to prevent crevice corrosion

22

to prevent erosion-corrosion

23

to prevent fretting corrosion

25

to prevent galling

76

to prevent galvanic corrosion

19

to prevent pitting corrosion

20

to prevent slurry erosion

71

to prevent stress-corrosion cracking

28

to prevent uniform corrosion

16

to reduce wear

77

29

7

trowel

136

tungsten carbide

137

tungsten carbide-cobalt

27

191

211 vinyl

31

Watts nickel

151

weld hardfacing

61

zinc

2

zinc-aluminum

142

zinc phosphate

96

zirconia thermal barrier Cobalt + chromium carbide, cost, relative

136 191

Cobalt-base alloys, as coatings

61

Cobalt-base hardfacing alloys

8

to resist erosion and cavitation

7

Coefficient of friction. See also Friction, coefficient of. definition

235

Coefficient of static friction. See Static coefficient of friction. Cohesion, definition

236

Color buffing, definition

236

Coloring. See also Color buffing. definition

236

Combustion gun spraying, cost, relative This page has been reformatted by Knovel to provide easier navigation.

191

174

200

273

Index terms

Links

Combustion powder spray process, characteristics

188

Combustion wire spray process, characteristics

188

Comminution

4

Composite coating

151

applications

203

definition

236

Composite deposition plating

145

Composite plate, definition

236

Compound white layer, ferritic nitrocarburizing

116

Compound zone

113

Concentration cell

14

Conductor

12

Contact corrosion, definition

236

Contact plating, definition

236

Contact stresses

8

Continuous electrodeposition

147

Continuous hot dip processing

138

Conversion coating. See also Chromate treatment; Phosphating. definition

236

design limitations

220

Cook-Norteman line

138

Copper erosion-corrosion

23

uniform corrosion

16

Copper-accelerated acetic acid-salt spray (fog) test (CASS test)(ASTM B 368). See also CASS test

39

definition

236

description

37

Copper alloys galvanic corrosion

17

pitting corrosion

20

stress-corrosion cracking

28

Copper-gold single crystals, dealloying corrosion

This page has been reformatted by Knovel to provide easier navigation.

27

204 151

115

205

274

Index terms

Links

Copper plating characteristics

145

thickness ranges and hardness levels

211

Copper-zinc alloys, dezinciflcation

26

Core, definition

236

Corrodkote test, definition

236

Corrosion. See also Specific forms of corrosion

13

abrasion

70

71

atmospheric

14

36

106

39

186

232 by salt spray on steels and coatings

37 236

conditions for

13

corrodent forms

11

dealloying

26

definition

11

economic effects forms of

236

3 15

galvanic

4

15

198

1

2

16

25 103

31

100

11

13

196

197

241 prevention

rates of materials compared structural parts, surface engineering solutions for synergistic relationships with wear mechanisms tests Corrosion-barrier systems Corrosion cell

5 35

231

167

168

18

Corrosion-erosion. See Erosion-corrosion. Corrosion fatigue

15

definition

236

description

29

mechanical, metallurgical, and environmental variables influencing behavior

29

prevention

30

This page has been reformatted by Knovel to provide easier navigation.

16

29

275

Index terms

Links

Corrosion inhibitor. See Inhibitor. Corrosion product, definition

236

Corrosion protection, definition

236

Corrosion rate

11

definition

237

Corrosion resistance definition

1

130

131

16

239

16

21

237

surface engineering treatments for improvement Corrosion system, definition Corrosion testing

2 237 35

field tests

36

organizations active in test development and standardization for coatings

35

simulated service tests

36

tests used for corrosion resistance of protective coatings

37

Corrosive wear. See also Oxidative wear definition

4 237

Costs factors affecting

190

of surface treatments

190

Covering power, definition

237

Cracking of chromized steels environmental

119 15

Cratering threshold voltage Crevice corrosion definition

141 15 237

Cross slip

76

Cutting

59

Cyaniding

192

definition

237

60

D Deaeration, of water, and corrosion

14

Dealloying. See also Dezincification; Graphitic corrosion

15

definition

237 This page has been reformatted by Knovel to provide easier navigation.

16

276

Index terms

Links

Dealloying corrosion

26

Dealuminification

26

Decarburization

27

Degreasing, definition

27

237

Dehumidification, to prevent corrosion

13

Delamination wear, under lubricated conditions

78

Depolarizing agents

13

Deposit corrosion

21

definition

238

Desiccation

5

Design of flame hardening equipment

88

to minimize cavitation

24

to prevent crevice corrosion

22

to prevent erosion-corrosion

23

to prevent hydrogen damage

31

to prevent slurry erosion

71

of wear testers

82

Design guidelines for surface engineering

195

Design limitations environmental regulations of surface engineering

228

inorganic coating processes

222

interrelation between the component, fixturing, and equipment limitations

214

organic coating processes

219

principles related to surface engineering

213

specifications

226

substrate features influence on electroplateability

225

surface preparation processes

218

Desiliconification

27

Destannification

27

Detonation flame spraying, definition

238

Detonation gun (D-gun) process abrasive wear rate

164

characteristics

188

design characteristics

162

This page has been reformatted by Knovel to provide easier navigation.

220

277

Index terms Dezinciflcation. See also Dealloying; Selective leaching definition

Links 26

27

238

Diamond, as particle additive for nickel electroless plating

153

Diamond coatings advantages, limitations, and processing parameters

126

chemical vapor deposition

170

172

Diamondlike carbon (DLC) coatings advantages, limitations, and processing parameters

126

as chemical vapor deposition coating material

170

as sputter coating material

175

thickness ranges and hardness levels

211

Dichromate treatment, definition

238

Differential coating, definition

238

Diffusion aluminizing

2

Diffusion (case)-hardened surfaces

8

Diffusion chromizing, applications

198

Diffusion coating, definition

238

Diffusion heat treatment coatings Diffusion (pack cementation) aluminizing applications

198 201

209

3

7

110

198

201

209

198

benefits

2

Diffusion (pack cementation) siliconizing benefits

2

Diffusion treatments

7

Diffusion zone

113

116

Dilution ratio

155

156

Diphase cleaning, definition

238

Dipping, design limitations

221

Dislocation cross slip Distortion

76 191

definition

238

design guidelines

210

Droplet erosion. See also Erosion. definition

238

This page has been reformatted by Knovel to provide easier navigation.

159

278

Index terms

Links

Dryers

128

Dry-sand/rubber-wheel test (ASTM G 65), data for various coatings

188

Duplex stainless steels, stress-corrosion cracking

28

Dynamic coefficient of friction

44

189

E e-coat cratering, resistance to

141

Economic costs industrial operations with significant annual wear consequences

4

metallic corrosion in the U.S

3

Elastomeric coatings and linings, benefits

2

Electrical properties

1

Electric arc (wire arc) spray process, design characteristics Electrochemical cell

162 12

Electrochemical coatings (plating)

7

Electrochemical conversion coatings, benefits

2

Electrochemical corrosion definition

11 238

Electrochemical deposition

145

Electrochemical impedance spectroscopy (EIS), description

40

Electrochemical tests

39

Electrocleaning

146

Electrocoating, design limitations

221

Electrodeposition definition

25

223

145

238

Electroforming, definition

238

Electrogalvanized steel strip, thickness ranges and hardness levels

211

Electrogalvanizing

147

definition

238

Electrographic and chemical porosity tests, description

37

Electroless nickel + chromium plating, characteristics

145

Electroless nickel (before hardening)/nickel electroplate coatings, hardness range

187

This page has been reformatted by Knovel to provide easier navigation.

220

279

Index terms

Links

Electroless nickel-boron coatings abrasive wear resistance

151

corrosion rate in various environments

152

Electroless nickel/ceramic plating, thickness ranges and hardness levels

211

Electroless nickel-phosphorus coatings abrasive wear resistance

151

applications

198

corrosion rate in various environments

152

Electroless nickel plating applications

150 212 198

199

201

204

205

150

152

145

cost, relative

191

size and weight limitations

215

surface finish characteristics

213

thickness ranges and hardness levels

211

advantages, limitations, and processing parameters benefits

210

203 206 characteristics

Electroless plating

152

7 126 2

coating thickness/penetration depth

192

definition

238

design limitations

223

Electrolyte

12

definition

12

Electrolytic cleaning, definition

239

Electrolytic copper plating, applications

199

Electrolytic corrosion test (ASTM B 627), description

40

Electrolytic deposition, definition

239

Electrolytic nickel coating, applications

197 205

Electrolytic pickling, definition

239

Electrolytic zinc coatings, applications

197

Electron beam (EB) This page has been reformatted by Knovel to provide easier navigation.

212

7

199 212 198

203

280

Index terms

Links

Electron beam-hardened steels

8

Electron-beam (EB) hardening

2

Electron beam heat treating, definition

239

Electron beam/physical vapor deposition (EB/PVD)

174

Electron beam welding (EBW)

154

Electronic properties

90

192

190

192

1

Electrophoresis, to apply ceramic coatings

136

Electrophoretic plating

219

Electroplate, definition

239

Electroplated nickel + chromium plating, characteristics

145

Electroplated nickel plating, characteristics

145

223

Electroplating advantages, limitations, and processing parameters

126

146

applications

6

210

benefits

2

coating thickness/penetration depth

192

comparison of applications of thermal spraying and welding

161

definition

145

design limitations

223

process requirements compared to those of thermal spraying and welding

161

size and weight limitations

215

surface finish characteristics

213

Electropolishing

219

definition

239

220

239

Electropolymerization

219

Electroslag welding (ESW), for weld-overlay coatings

155

Electrotinning, definition

239

Emulsion cleaner, definition

239

Encyclopedia of Associations

185

Environmental cracking, definition

239

Environmentally assisted cracking

15

16

Epoxies

31

127

130

131

Epoxy-filled rebuilding cements

This page has been reformatted by Knovel to provide easier navigation.

8

221

129

281

Index terms

Links

Epoxy powder

34

Erosion. See also Cavitation erosion; Erosion-corrosion

56

abrasive

231

of aluminum alloys

63

anodized, aluminum

106

cavitation

7

15

16

68 81

70

71

definition

239

description

61

high-velocity

70

of high-velocity oxyfuel spray coatings of laser-melted gray and ductile irons liquid

64

71

164 92 7

68

liquid impingement

68

low-erosion

70

manifestations in service

61

mechanisms of material removal

68

69

Miller numbers

71

72

particle to target hardness ratio and resistance

66

prevention

7

rain

68

rate

63

rate rankings for various materials

189

related to plastic deformation

159

71

66

69

64

164

saltation

70

71

slurry

62

69

solid particle erosion (SPE)

61

186

surface treatments for prevention

208

thermal spray coatings applications

165

variables influencing

63

versus abrasion

63

weld-overlay coatings Erosion-corrosion definition

157 15 239

Erosion tests (ASTM G 76) This page has been reformatted by Knovel to provide easier navigation.

64

65

16

22

282

Index terms Erosive wear, test methods

Links 82

Erosivity, definition

239

Etching, design limitations

220

Ethylene-propylene (-diene) rubber, environmental resistance ratings

128

European Space Agency

84

Exfoliation

15

definition

239

Explosive bonding, benefits

16

26

221

222

2 192

111

115

25

30

2

Exterior exposure test (ASTM D 1014)

37

Extreme-pressure lubricant, definition

240

F FACT test (formerly ASTM B 538), description

37

False brinelling, definition

240

Faraday cage effect

216 224

Fatigue

1

definition

240

Ferritic nitrocarburizing Ferritic stainless steels, corrosion of Ferroboron

119

Ferrous alloys, hardness range

61

Fiberglass, veil or woven roving mat reinforcing gel coats

127

Fiberglass layups, for corrosion resistance

131

Fiberglass-reinforced plastics, hand lay-ups

127

Filiform corrosion definition

15 240

Finish, definition

240

Finish grinding, definition

240

Fish eyes

31

Fixed oils

80

Fixture, definition Flakes

240 31

This page has been reformatted by Knovel to provide easier navigation.

62

283

Index terms Flaking, of P/M steels surface oxide layer Flame hardening applications benefits

Links 108 7 87

8

61

88

200

203

50

2

coating thickness/penetration depth

192

definition

240

Flame spraying cost for process

190

definition

240

design characteristics

162

to apply oxide coatings

136

Flame spraying (FLSP) coatings, coating thickness/penetration depth

192

Flat block-on-rotating ring (BOR) test geometry

50

Flat surface sliding on flat surface (FOF) test geometry

47

49

51

53

Floe process

115

Flow-control agents

128

Fluidized bed, definition

240

Fluidized-bed bonding

120

Fluidized-bed processing, to apply ceramic coatings

136

Fluorides

14

Fluorinated ethylene propylene (FEP) coatings, applications

201

Fluorinated hydrocarbon

127

Fluorinated polymer coatings, applications

201

Fluorocarbons, characteristics, cost, and applications

129

Fluoroelastomer, environmental resistance ratings

128

Flux

63

Flux-cored arc welding (FCAW). See also Flux-cored electrode. cost for weld-overlay coatings

190

definition

240

for weld-overlay coatings

154

Flux-cored arc welding/gas metal arc welding hardfacing, coating thickness/penetration depth

192

Flux-cored electrode, definition

241

This page has been reformatted by Knovel to provide easier navigation.

155

284

Index terms Flux line

Links 138

Fog test. See Neutral salt-spray test. Ford anodized aluminum corrosion test (FACT) test, description

37

Fretting

76

definition

241

description

76

prevention

77

sites

77

test methods

82

test variables to be controlled

81

thermal spray coatings applications wear testing devices Fretting corrosion definition

40

83

165 82

84

15 199

16

24

241

Fretting fatigue, definition

241

Fretting wear

186

definition

241

testing devices

82

84

Friction

43

angle

45

definition

43

241

friction coefficient

44

46

friction force

44

heating

43

mechanisms, basic

45

solid friction, definition

44

work

45

Frictional energy

1

Frictional heating

43

Friction angle

45

Friction coefficient. See also Coefficient of friction

44

ceramics sliding on metals and ceramics

49

coatings sliding on metals, ceramics, and polymers

51

definition

235 This page has been reformatted by Knovel to provide easier navigation.

45

45 46

74

285

Index terms

Links

Friction coefficient. (Continued) factors contributing to

46

metals sliding on metals

47

miscellaneous materials, sliding test geometry

53

polymers sliding on metals and polymers

50

Friction force

44

Friction work

45

Furnace fusing, temperature range and distortion

192

Fused-salt electroplating

145

Fused-spray deposit, definition

241

Fusion hardfacings, hardness range

187

Fusion welding

46

148

7

temperature range and distortion

192

G Galfan Galling. See also Scoring; Scuffing and adhesive wear

139

142

55

75

74

definition

241

galvanized coatings

140

hardness vs. wear resistance process comparisons

186

test methods

82

Galling test (ASTM G 98) Galvalume Galvanic corrosion

48 139

143

4

15

definition

241

description

16

galvanic series in seawater at 25°C (77°F)

17

prevention

17

19

surface treatments for prevention

198

199

Galvanic protection, by a coating

33

Galvanic protection systems

33

Galvanic series, definition

242

Galvanize, definition

242

This page has been reformatted by Knovel to provide easier navigation.

16

286

Index terms Galvanized coatings

Links 139

alloying effects

140

corrosion resistance

140

fabricability and weldability

140

paintability

140

Galvanized steel, corrosion of

14

Galvanized wrought iron, galvanic corrosion

17

141

17

Galvanizing design limitations hot dip

223 2

197

203

211 size and weight limitations

215

surface finish characteristics

213

Galvanneal, definition

242

Galvanneal coatings

141

142

Galvannealing

138

139

Gas bonding

120

Gas carbonitriding, characteristics and requirements

111

115

Gas carburizing, characteristics and requirements

111

112

113

113

114

Gaseous corrosion. See also Hot corrosion; Oxidation; Sulfidation. definition

242

Gas metal arc welding (GMAW) cost for weld-overlay coatings

190

definition

242

for weld-overlay coatings

154

Gas nitriding, characteristics and requirements

111

Gas tungsten arc welding (GTAW) cost for weld-overlay coatings

190

definition

242

for weld-overlay coatings

154

Gas tungsten arc welding hardfacing, coating thickness/penetration depth

192

General corrosion. See also Uniform corrosion. definition

242

Glass linings

132

benefits

2 This page has been reformatted by Knovel to provide easier navigation.

155

287

Index terms

Links

Glass linings (Continued) for steel vessels

133

Gloss-control agents

128

Gold corrosion of

17

for precious metal plating galvanic corrosion Gold alloys with copper or silver, dealloying corrosion

23

149 17 27

Gouging abrasion. See also Abrasion. definition

242

Graphite

80

as anode material with impressed current

34

galvanic corrosion

17

Graphite fluoride

80

Graphitic corrosion

26

definition

27

242

Gray irons dealloying corrosion

26

graphitic corrosion

27

laser melting

91

27

Grinding definition

242

design limitations

220

Grit blasting, definition

242

Gunite method

135

H Hafnium carbide, as coating material Halogens

137 14

Hard anodizing

2

Hard chromium

7

definition

242

Hard chromium coatings, abrasive wear resistance Hard chromium plating

This page has been reformatted by Knovel to provide easier navigation.

151 7

61

199

288

Index terms Hardcoat anodizing Hardfacing. See also Surfacing; Weld-overlay coatings definition

Links 104 7 242

Hardfacing alloys

157

Hardness and adhesive wear ranges

73

range of carbides, mineral, and alloy microconstituents

61

versus abrasion resistance

62

Hastelloy alloys, corrosion of

17

Health and safety precautions, cadmium electroplating Heat resistance, surface engineering treatments for improvement Heat treatments, preprocessing and postprocessing Heavy case-hardened steels Hexavalent chromium

62 20

147 2 209

226

8 100

High-carbon iron-chromium alloys, as coatings

61

High-carbon steels, decarburization

27

High-energy beam hardening

90

High-energy plasma spray process, characteristics

188

High-molecular-weight resins

127

High-nickel alloys, dealloying corrosion

27

High-nickel cast iron, galvanic corrosion

17

High-silicon cast iron, as anode material with impressed current

34

101

High-strength low-alloy steels, hardness range

187

High-strength steels, hydrogen embrittlement

30

31

High-stress abrasion

57

59

definition

243

High-temperature hydrogen attack definition

15 243

High-velocity oxyfuel combustion spray (HVOF) technique

160

164

High-velocity oxyfuel (HVOF) thermally sprayed coatings, applications

208

209

Holidays, definition

243

Honing

220

definition

243 This page has been reformatted by Knovel to provide easier navigation.

188

289

Index terms

Links

Hot corrosion. See also Gaseous corrosion. definition

243

Hot dip. See also Hot dip coating. definition

243

Hot dip aluminized steel strip, thickness ranges and hardness levels Hot-dip aluminizing, benefits Hot dip coatings

211 2 138

aluminum coatings

143

definition

243

galvanized coatings

139

galvanneal coatings

141

microstructure of coating

138

terne coatings

144

zinc-aluminum coatings

142

142

Hot dip galvanizing applications benefits

197

198

203

212

199

2

thickness ranges and hardness levels

211

Hot-dip lead-tin alloys coatings, benefits

2

Hot-dipped aluminum coatings, applications

6

Hot-dipped zinc-aluminum coatings, applications

6

Hot-dipped zinc coatings, applications

6

Hot dipping, design limitations

223

Hot isostatically pressed (HIP) coatings

2 212

Hot-processed continuous line

138

Humidity cabinet tests (ASTM D 2247, ASTM G 85), description

39

Humidity test, 100% relative (ASTM D 2247), description

37

Hydride formation

15

Hydrogen attack

31

Hydrogen blistering, definition Hydrogen damage definition

31

243 15 243

This page has been reformatted by Knovel to provide easier navigation.

208

30

209

290

Index terms Hydrogen embrittlement definition

Links 15

30

243

Hydrogen-induced blistering

15

Hydrogen-induced cracking (HIC)

30

definition

30

243

Hydrogen stress cracking, definition Hydrogen sulfide, causing hydrogen embrittlement Hypophosphite-reducing agents

243 30 150

I Immersion cleaning, definition

244

Immersion coating, definition

244

Immersion plating, definition

244

Impact, synergistic relationships with corrosion mechanisms

5

Impact wear

55

definition

244

test variables to be controlled Impedance test for anodized aluminum (ASTM B 457), description

81

83

40

Impingement. See also Erosion. definition

244

Impingement attack. See also Erosion-corrosion; Impingement corrosion. definition

244

Impingement corrosion, definition

244

Impingement erosion, definition

244

Impingement erosion wear, test variables to be controlled

81

83

Impingement impact wear, wear testing devices

82

84

Incidence, angle of

63

64

Inclined surface test geometry (IS)

47

Incoloy alloys, corrosion of

17

Inconel alloys, galvanic corrosion

17

Indium, as sputter coating material

175

Indium plating, applications

199

Induction-hardened steels

This page has been reformatted by Knovel to provide easier navigation.

8

20

291

Index terms Induction hardening applications benefits

Links 7

88

200

203

207

2

carbon content effect

88

coating thickness/penetration depth

192

definition

244

Information services, technical associations listing surface treatment providers Inhibitors

185 5

definition

31

244

in phosphate coatings

98

to prevent stress-corrosion cracking

28

to prevent uniform corrosion

16

Inorganic coatings, design limitations

222

Inorganic painting, design limitations

223

Inorganic zinc-rich coatings, applications

130

Intergranular corrosion definition

15

16

25

244

Intergranular stress-corrosion cracking (IGSCC), definition

244

Intergranular sulfidation corrosion attack, of chromized steels

119

Intermediate alkaline cleaning

146

Intel-metallic hardening alloys

157

Interpass temperature

157

Interrupted-current plating, definition

244

Interstitial-free (IF) steels

139

Ion-beam-assisted deposition, advantages, limitations, and processing parameters

126

Ion carburizing, definition

244

Ion implantation

7

120

advantages, limitations, and processing parameters

126

alloys suitable for

120

121

applications

120

201

benefits

2

characteristics compared to CVD and PVD

172

coating thickness/penetration depth

192

definition

244

design limitations

223

This page has been reformatted by Knovel to provide easier navigation.

173

122

292

Index terms

Links

Ion implantation (Continued) line-of-sight limitations

217

process availability

185

size and weight limitations

215

surface finish characteristics

213

temperature range and distortion

192

thickness ranges and hardness levels

211

Ion nitriding (plasma nit riding)

111 126

definition

245

Ion plating

7 175

coating thickness/penetration depth

192

definition

245

design limitations

223

line-of-sight limitations

217

Iron, corrosion of Iron aluminides, as weld-overlay coatings

13 159

Iron-chromium alloys, dealloying corrosion

27

Iron oxide scale

66

Iron phosphate bath

97

Iron phosphate coatings

96

Isobutylene-isoprene (butyl) rubber, environmental resistance ratings

128

Isoprene rubber, environmental resistance ratings

128

J Jet Kote spray process, design characteristics

162

K Kinetic coefficient of friction

44

Kirkendall voids, produced by chromizing

119

Knife-line attack, definition

245

Knoop hardness scale

This page has been reformatted by Knovel to provide easier navigation.

62

113

115

172

173

20

293

Index terms

Links

L Lactic acid test, description

37

Lapping, definition

245

Lapping/buffing, design limitations

220

Laser alloying. See also Laser surface processing Laser-beam hardening, description

2

122

91

Laser beam welding (LBW) cost for weld-overlay coatings

190

definition

245

for weld-overlay coatings

154

Laser cladding, benefits

2

Laser/electron beam alloying

7

Laser/electron-beam hardened coatings, hardness range

187

Laser/electron-beam hardfacing, coating thickness/penetration depth

192

Laser-hardened steels

8

Laser hardening

7

applications

200

benefits

203

207

91

185

17

23

2

coating thickness/penetration depth

192

definition

245

Laser melting. See also Laser surface processing

2 192

Laser melting/alloying, design limitations

224

Laser surface processing, definition

245

Lead, corrosion of Lead-tin alloy hot dip coatings, applications

16 144

Lead-tin solder (50-50), galvanic corrosion

17

Ledeburite

92

Lime

32

Line pipe steels, hydrogen-induced blistering

30

Linings

31

cementatious

134

ceramic

132

concrete

134

This page has been reformatted by Knovel to provide easier navigation.

294

Index terms

Links

Linings (Continued) dual

135

glass

132

of high-performance ceramics

136

inorganic monolithic

135

Liquid carbonitriding (cyaniding), characteristics and requirements

111

Liquid carburizing

111

definition

245

Liquid erosion

7

Liquid honing, definition

245

Liquid nitriding (salt nitriding)

111

definition

68 113

115

246

Liquid nitrocarburizing, definition

246

Liquid (salt-bath) bonding

120

Local hardening

213

Localized corrosion definition

215

13 246

Local surface hardening, thickness ranges and hardness levels

211

Low-alloy steels, hydrogen damage

30

Low-carbon steel, galvanic corrosion

17

Low-expansion borosilicate glass

132

Low-pressure plasma spraying (LPPS)

160

Low-stress abrasion, definition

246

Lubricants

7 126

definition

246

Lubricating films

7

Lubrication

77

boundary

78

circulating-oil

78

definition

246

diagnosis of wear by spectroscopy

78

dry-film (solid-film)

78

elastohydrodynamic

78

hydrodynamic

78 This page has been reformatted by Knovel to provide easier navigation.

162

164

73

78

295

Index terms

Links

Lubrication (Continued) hydrostatic

78

lubricants

78

modes

78

polytetrafluoroethylene added to electroless nickel baths

153

sputter coatings for

175

thin-film

78

to prevent adhesive wear

75

to prevent fretting corrosion

25

to prevent galling

76

Luster finish, definition

202

246

M Magnesium, galvanic corrosion

17

Magnesium alloys, galvanic corrosion

17

Magnesium anodes, for cathodic protection

34

Manganese phosphate coatings

97

parts immersed for wear resistance

99

Manganese steels before work hardening, hardness range as wear plates

187 61

Martensitic stainless steels Material/process selection checklist

28 8

Matte finish, definition

246

MCrAlY coatings

149 198

Mechanical plating

2

definition

246

peen plating, design limitations

224

Mechanical polishing, definition

246

Mechanical properties

1

Medium-carbon steels, decarburization

27

Metaborates

32

Metal ion concentration cell

21

This page has been reformatted by Knovel to provide easier navigation.

89

174

175

296

Index terms

Links

Metallic coatings and cladding, for corrosion resistance

131

Metalliding

145

Metallizing, definition

246

Metallurgy, surface changes

148

192

2

Metals adhesive wear prevention

75

erosion test results

65

fretting

76

galling

75

relative erosion factors

65

stacking-fault energies

76

wear coefficients, adhesive wear

74

67

Metal spraying. See also Thermal spraying. definition

246

Microcracking

5

59

Microfatigue

59

60

Miller numbers

71

72

Mill scale

17 20

18

Mineral oils

80

Mining, industrial operations and annual wear economic consequences

4

Mohs hardness scale

62

Molybdates

32

Molybdenum erosion rate

66

as sputter coating material Molybdenum disulfide as sputter coating material Molydenum silicide, as coating material

175 80 175 137

Monels, corrosion of

17

Morrison-Miller effect

69

Motor vehicles, costs of metallic corrosion in U.S. (1975, 1995) Multibody abrasive wear

This page has been reformatted by Knovel to provide easier navigation.

27

3 55

57

60

19

297

Index terms

Links

N Natural rubber, environmental resistance ratings

128

Neutral salt-spray (fog) test (ASTM B 117)

38

Nickel, galvanic corrosion

17

Nickel alloys, stress-corrosion cracking

28

Nickel-aluminum intermetallic coating

152

Nickel-base alloys, intergranular corrosion Nickel-base hardfacing alloys

25 8

Nickel-boron coatings

150

Nickel-chromium, as sputter coating material

175

Nickel-chromium boron alloys

8

Nickel-chromium-chromium carbide plating, applications

200

Nickel-chromium plating, applications

200

Nickel-molybdenum alloys, dealloying corrosion

186

152 61

27

Nickel-phosphorus coatings

150

152

Nickel plating

147

211

Nickel-terne coatings

144

Niobium, content effect on intergranular corrosion Niobium silicide, as coating material Ni-Resist, galvanic corrosion Nitride coatings Nitriding. See also Bright nitriding; Liquid nitriding applications

benefits

25 137 17 137

175

7

226

200 204 212

201 205

2

characteristics and requirements

110

coating thickness/penetration depth

192

cost, relative

191

definition

246

Floe process

115

phase transformations

3

process availability in commercial heat treating shops

185

size and weight limitations

215

This page has been reformatted by Knovel to provide easier navigation.

113

203 207

298

Index terms

Links

Nitriding. (Continued) steels treated in various applications

114

surface finish characteristics

213

temperature range and distortion

192

thickness ranges and hardness levels

211

to prevent corrosion fatigue

30

to prevent fretting corrosion

25

white nitride layer

115

Nitrocarburizing applications

200 204

cost, relative

191

definition

247

size and weight limitations

215

surface finish characteristics

213

thickness ranges and hardness levels

211

Nitrocellulose lacquers

129

Noble metal clad systems

167

Noble metals, erosion-corrosion

203 207

130

23

O Occlusion

145

Oil paints, applications

130

Organic coatings

2

design limitations

219

paints

128

Organisols, hot-applied

127

127

Oxidation definition

247

and fretting corrosion

24

rate of

13

reaction

12

resistance

1

treatments

108

weld-overlay coatings

159

This page has been reformatted by Knovel to provide easier navigation.

159

204 212

299

Index terms

Links

Oxidative wear definition

247

surface treatments for prevention

200

Oxide coatings, applications

136

Oxide treatments, applications

203

Oxide wear debris Oxidizing

205

55 211

213

215

155

190

Oxyacetylene welding (OAW) cost for weld-overlay coatings

190

definition

247

for weld-overlay coatings

154

Oxyfuel acetylene welding hardfacing, coating thickness/penetration depth

192

Oxyfuel gas welding (OFW), definition

247

Oxygen concentration cell

21

P Pack aluminizing, characteristics and requirements Pack carburizing definition

117 11

112

247

Pack-cementation diffusion coatings

116

aluminizing

117

bonding, or boronizing

119

chromizing

119

principles of process

116

siliconizing

118

Pack-cementation diffusion processes coating thickness/penetration depth

192

process availability in commercial heat treating shops

185

temperature range and distortion

192

to apply ceramic coatings

136

Pack nitriding, definition

247

Paint adhesion on a scribed surface (PASS) test, description Painting

40 6 221

This page has been reformatted by Knovel to provide easier navigation.

20

31

300

Index terms Paints applications benefits

Links 32

128

197

198

212

2

corrosion resistance as functional requirement

130

electrophoretic (e-coat)

141

function

129

resistant to mechanical and chemical action

131

size and weight limitations

215

surface cleaning methods used before application

132

surface contaminants

131

surface finish characteristics

213

surface preparation

131

Paint spraying, line-of-sight limitations

217

Particle rotational speed

63

Particle velocity

63

Parting corrosion. See Dealloying corrosion. Passivation, design limitations

224

Passive, definition

247

Paste bonding

120

Perfluoro alkoxy alkane (PFA) coatings, applications

201

206

Phenolics

35 131

129

Phosphate chemical conversion coatings. See also Phosphating. application method

96

applications

98

articles coated

96

benefits

2

for corrosion protection

98

iron phosphate coatings

96

manganese phosphate coatings

97

as metalforming lubricant in forming steel

99

thickness range and coating weight

96

types

96

wear resistance reduction on machine elements

99

weight and crystalline structure

95

This page has been reformatted by Knovel to provide easier navigation.

130

301

Index terms

Links

Phosphate chemical conversion coatings. (Continued) zinc phosphate coatings Phosphates

96 32

Phosphating. See also Conversion coating; Phosphate chemical conversion coatings. applications

198

definition

247

size and weight limitations

215

surface finish characteristics

213

thickness ranges and hardness levels

211

Physical vapor deposition (PVD)

7

advantages, limitations, and processing parameters

126

application methods

172

benefits

212

172

2

characteristics compared to CVD and ion implantation coatings, applications

172

173

201

204

205

207

210

212

52

coating thickness/penetration depth

192

cost, relative

191

definition

247

design limitations

224

line-of-sight limitations

217

process availability

185

processing steps

172

size and weight limitations

215

surface finish characteristics

213

thickness ranges and hardness levels

211

Pickling, design limitations

220

Pickoff of coatings, galvanized coatings

140

Pigments, added to resins

128

Pin-on-disk (POD) test geometry

49

50

Pin-on-flat (POF) test geometry

50

51

Pin-on-ring (POR) test geometry

47

Pitting

15

of anodized aluminum This page has been reformatted by Knovel to provide easier navigation.

106

16

19

302

Index terms

Links

Pitting (Continued) definition

248

Plasma arc welding (PAW), definition

248

Plasma arc welding (PAW) hardfacing, coating thickness/penetration depth

192

Plasma-assisted chemical vapor deposition, definition

248

Plasma bonding

120

Plasma (ion) carburizing. See also Ion carburizing. characteristics and requirements

112

113

115

characteristics and requirements

111

113

115

properties of treated metals

115

Plasma (ion) nitriding. See also Ion nitriding.

Plasma spraying characteristics

188

coating thickness/penetration depth (PSP)

192

cost for process

190

definition

248

to apply oxide coatings

136

to prevent fretting corrosion

25

Plasma transferred arc welding (PAW), for weld-overlay coatings

154

Plastic deformation, and erosion rate

159

Plastisols

127

Plating

155 131

8

design limitations

223

reactive ion

175

temperature range and distortion

192

Platinum as anode material with impressed current

34

corrosion of

17

for precious metal plating Plowing

149 59

definition

60

Plumbates

32

Polarization

13

Polishing. See also Electropolishing definition

219 248

This page has been reformatted by Knovel to provide easier navigation.

23 60

35

303

Index terms

Links

Polishing. (Continued) design limitations

220

test variables to be controlled

81

83

wear testing devices

82

84

31

127

129

199

201

202

203 206

204

205

31

129

131

2

133

Polyesters Polymer coatings and linings adhesive wear prevention applications

7 75 212

benefits

2

size and weight limitations

215

as sputter coating material

175

surface finish characteristics

213

Polyphosphate

31

Polytetrafluoroethylene (PTFE)

80

as lubricant

as particle additive for nickel electroless plating

153

as sputter coating material

175

Polyurethanes Porcelain enamels definition

248

Post-processing bake-out treatments, to prevent hydrogen damage

31

Poultice corrosion

15

definition

249

Powder coatings

127

Powder flame spraying, definition

249

Powder metallurgy (P/M) steels

108

Powder weld (PW), for weld-overlay coatings

155

Power-law velocity dependence

64

Power spray washing technique

219

Precious metal plating, applications

149

Precious metals, erosion-corrosion

23

Precipitation-hardened steels, hydrogen embrittlement

30

Precleaning

146 This page has been reformatted by Knovel to provide easier navigation.

197

221

304

Index terms Precoated metal products, definition Primary metals, industrial operations and annual wear economic consequences

Links 249 4

Pure zinc (η) phase

139

Pyrex, applications

132

Q Quenched-and-tempered steels, hydrogen embrittlement

30

Quench hardening, definition

249

Quench hardening alloy and tool steels, temperature range and distortion

192

Quenching, definition

249

R Reactive ion plating

175

Rebuilding cements, coating thickness/penetration depth

192

Reciprocating pin-on-flat test geometry (RPOF)

49

Reciprocating, spherically ended pin on a flat surface (RSOF) test geometry

47

Red lead

32

Reducing agents

150

Reduction, rate of

13

Relative erosion factor (REF)

65

Repair cements, temperature range and distortion

192

Residual stress, definition

249

Resins, as coatings

128

Robber

216

definition

53

67

249

Roll bonding, benefits

2

Rolling-contact fatigue

77

definition

249

test variables to be controlled

81

83

wear testing devices

82

84

Rolling-contact wear

55

77

surface treatments for prevention

201

Rolling with slip wear test variables to be controlled This page has been reformatted by Knovel to provide easier navigation.

81

83

305

Index terms

Links

Rolling with slip wear (Continued) wear testing devices

82

Rouge finish, definition

249

Rubbers

127

131

“Rule of Four and Six”

224

226

12

19

Rust definition

84

32

249

S Sacrificial metals for cladding

167

Salt-bath carburizing, characteristics and requirements

112

Salt fog test, definition

250

Salt nitriding (liquid nitriding), characteristics and requirements

111

Salt particles

15

Salt-spray test (ASTM B 117)

38

coating life data

113 113 101

186

description

37

sulfuric anodized coatings Salt water, as electrolyte

104 12

Sandelin Effect

139

Satin finish, definition

250

Scab test, description

37

Scaling. See also Rust. definition

250

Scoring

55

definition

75

250

of gears

99

test methods

82

Scouring, definition

250

Scouring wear

70

Scrap iron, as anode material with impressed current

34

Scuffing

55

definition

250

surface treatments for prevention

205

This page has been reformatted by Knovel to provide easier navigation.

71 75

115

306

Index terms

Links

Scuffing (Continued) of TRD processed sheet steel

178

Seal coat, definition

250

Sealing

102

definition

250

Seizure

75

definition

250

Selective hardening temperature range and distortion treatments

192 61

Selective leaching. See also Dealloying corrosion; Dezincification; Graphitic corrosion. definition

250

Selective surface-hardened alloy steels Selenides, as sputter coating material

8 175

Sensitization

15

definition

250

Service test data test, description

37

Shatter cracks

31

Sheet linings

26

127

Shielded metal arc welding (SMAW) cost for weld-overlay coatings

190

definition

250

for weld-overlay coatings

154

Shotblasting, definition

251

Shotcreting method

135

Shot peening

93

applications

199

benefits

2

definition

251

design limitations

224

fatigue curves for steel spring wires phase transformations

93 3

size and weight limitations

215

surface finish characteristics

213

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155

307

Index terms

Links

Shot peening (Continued) to prevent corrosion fatigue

30

to prevent fretting corrosion

25

to prevent stress-corrosion cracking

29

Silicate cements

135

Silicate glass coatings, applications

136

Silicates

31

32

Silicide coatings applications

137

as sputter material

175

Silicon bronzes, desiliconification

27

Silicon carbide as chemical vapor deposition coating material

170

as coating material

137

hardness range

187

as particle additive for nickel electroless plating

153

Silicon dioxide, as chemical vapor deposition coating material

169

Silicone alkyds, applications

130

Silicone resins, resistant to mechanical and chemical action

131

Silicone rubber, environmental resistance ratings

128

Silicones applications

130

characteristics, cost, and applications

129

modified, applications

130

Siliconizing

118

definition

251

process availability

185

Silicon nitride, as chemical vapor deposition coating material

169

Silicon-to-silver bonding techniques

149

Silver erosion-corrosion for precious metal plating galvanic corrosion as sputter coating material tarnishing

23 149 17 175 16

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171

308

Index terms

Links

Silver (Continued) uniform corrosion

16

Silver plating, applications

199

Sintered thermal spray process, abrasive wear rate

164

Sleeving, temperature range and distortion

192

Sliding wear

55

hardness vs. wear resistance process comparisons

186

surface treatments for prevention

201

test methods

208

82

Slip/sinter ceramic coatings, benefits

2

Slurry abrasivity

69

definition

69

Miller number values

71

Slurry coating, design limitations Slurry erosion

72

223 7

8

71

198

201

207

Slurry/sinter formed ceramics applications

212 thickness ranges and hardness levels Society of Tribologists and Lubrication Engineers

211 84

Sodium dichromate, for sealing of sulfuric anodized alloys

103

Sodium pyrosulfite, to reduce hexavalent chromium

101

Sodium silicate cements

135

Soil, as corrosion contributor

14

Sol-gel coating, design limitations

221

Solid lubricant, definition

251

Solid lubricants application, advantages, limitations, and processing parameters

126

Solution coating, design limitations

221

Solvent cleaning

219

immersion, design limitations

220

ultrasonic, design limitations

220

Solvents

128

Solvent spraying, design limitations

221

This page has been reformatted by Knovel to provide easier navigation.

309

Index terms Sour gas, definition

Links 251

Spalling aluminized coatings

117

definition

251

of P/M steels surface oxide layer

108

of TRD processed sheet steel

178

Spangle material

139

Sparking, of galvanized coatings

141

Specification, drawing up of one

226

Spectroscopy, to analyze wear debris in lubricants

78

Spherically-ended pin-on-a-flat coupon (SPOF) test geometry

47

Splat boundaries

164

Spraying/dipping plus sintering, to apply chromium oxide coatings

136

Spray-sinter process, to apply silicate glass coatings

136

141

49

Sputter coating coating thickness/penetration depth

192

temperature range and distortion

192

Sputtering. See also Physical vapor deposition

7

definition

251

line-of-sight limitations

217

Stacking-fault energy

76

Stacking faults

76

Stainless steels corrosion fatigue

30

corrosion rate

11

crevice corrosion

21

erosion-corrosion

22

galvanic corrosion

17

intergranular corrosion

25

nitriding

114

passivity

18

pitting corrosion

20

stress-corrosion cracking

28

Static coefficient of friction

44

Static friction coefficient. See Static coefficient of friction. This page has been reformatted by Knovel to provide easier navigation.

45

52

310

Index terms

Links

Steam treatment benefits

2

definition

251

Steels boronized, hardness range

187

carbonitriding treatments

116

corrosion inhibitors for corrosion protection methods

32 5

corrosion rate

13

erosion rate

66

flame-hardened, hardness range

187

flame hardening

88

fretting corrosion

25

induction-hardened, hardness range manganese phosphate coatings for parts

187 99

nitrided, hardness range

187

nitriding treatment

114

salt-spray test data, coating life

186

structural grades, corrosion rate

11

uniform corrosion

16

Strand lying on a rotating drum (StOD) test geometry

53

Stray-current corrosion, definition

251

Stress corrosion, definition

251

Stress-corrosion cracking (SCC) definition

89

15

16

251

Stress relieving, definition

252

Strike, definition

252

Stripping chemical, design limitations

221

mechanical, design limitations

221

thermal, design limitations

221

Strontium chromate

128

Structural parts in corrosive environments, surface engineering solutions

196

Styrene-butadiene (nitrile) rubber, environmental resistance ratings This page has been reformatted by Knovel to provide easier navigation.

128

197

27

311

Index terms

Links

Submerged arc welding (SAW) cost for weld overlays

190

definition

252

for weld-overlay coatings

155

Submerged arc welding hardfacing, coating thickness/penetration depth

192

Substrate, definition

252

Substrate treatments, to reduce wear

156

7

Sulfidation corrosion, of chromized steels

119

definition

252

resistance

1

weld-overlay coatings Sulfide stress cracking (SSC), definition Sulfur compounds Sulfur dioxide, to reduce hexavalent chromium Sulfur dioxide-salt spray test (ASTM G 85, A4), description

159 252 15 101 37

Sulfuric acid, in glass-lined steel vessels

133

Sulfuric anodizing

102

alloys suitable for

104

conventional anodizing

102

hardcoat anodizing

102

sealing of coatings

105

Superabrasives, definition

252

Super D-gun process, abrasive wear rate

164

Superfinishing, definition

252

Surface damage

2

103

55

definition

252

Surface engineering

2

definition

1

processes

213

properties or characteristics of components

215

1

Surface engineering material/process selection checklist

8

9

Surface fatigue

8

56

Surface fatigue wear, thermal spray coatings applications

This page has been reformatted by Knovel to provide easier navigation.

165

105

312

Index terms

Links

Surface finish aesthetics and functional requirements

215

characteristics of various surface-engineering processes

213

definition

252

design features

215

size and weight effect on surface-engineering processes

215

Surface hardening definition

7 252

to prevent fretting corrosion

25

Surface modification, definition

253

Surface roughness, definition

253

Surface spangles

139

Surfacing, definition

253

Suspension agents

128

T Tantalum, hydride formation Tantalum silicide, as coating material Tarnish

31 137 16

definition

253

T-bend adhesion test (ASTM D 4145), description

37

Technical associations, information services source for surface treatments

185

Tellurides, as sputter coating material

175

Terne, definition

253

Terne coatings

2

Thermal diffusion, advantages, limitations, and processing parameters

126

Thermal evaporation

172

Thermal insulation Thermal spray coatings

192

160 164

of alloy metals

163

aluminum, applications

197

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173

1

abrasive wear

applications

144

198

203

204

205

207 212

208

209

313

Index terms

Links

Thermal spray coatings (Continued) applications recommended for wear resistance

165

of carbide powders and blends

163

of ceramic oxides

163

ceramics, applications

198 205

corrosion-resistant applications and materials used

166

mechanical properties

163

of metal composites

163

process categories

160

properties

162

of pure metals

163

size and weight limitations

215

surface finish characteristics

213

temperature range and distortion

192

thickness ranges and hardness levels

211

versus weld-overlay coatings

160

zinc, applications

197

Thermal spraying. See also Arc spraying; Flame spraying; Plasma spraying; Powder flame spraying advantages, limitations, and processing parameters benefits

7 126 2

comparison of applications of welding and electroplating

161

definition

253

design limitations

224

methods compared

188

process requirements compared to those of welding and electroplating

161

to apply carbide coatings

137

to apply ceramic coatings

136

Thermal stresses, definition

253

Thermal wear, definition

253

Thermoreactive deposition/diffusion process (TRD)

176

advantages, limitations, and processing parameters

126

applications of tooling

176

This page has been reformatted by Knovel to provide easier navigation.

203 206

198

204

314

Index terms Thief. See also Robber definition

Links 216 253

Thin dense chromium coatings, advantages, limitations, and processing parameters

126

Thorium, hydride formation

31

Three-body abrasive wear (multibody wear)

55

57

7

8

Through hardening Throwing power definition

253

of electrolyte in electroplating

146

Thrust washer (TW) test geometry

50

Tin, galvanic corrosion

17

Tin bronzes, destannification

27

Tinplate (continuous electrodeposition)

2

147

Titanium content effect on intergranular corrosion

25

erosion-corrosion

22

galvanic corrosion

17

hydride formation

31

Titanium alloys erosion rate

66

stress-corrosion cracking

28

Titanium aluminum nitride, as coating material

137

Titanium carbide as chemical vapor deposition coating material

169

170

171

187 as coating material

137

Titanium carbonitride as chemical vapor deposition coating material

170

as coating material

137

Titanium diboride, as chemical vapor deposition coating material

171

170

171

169

170

61

137

187

211

Titanium nitride as chemical vapor deposition coating material as coating material as physical vapor deposition coating material This page has been reformatted by Knovel to provide easier navigation.

171

315

Index terms

Links

Titanium nitride (Continued) as reactive ion plating material

175

as sputter coating material

175

Titanium zirconium nitride, as coating material

137

Tolerance, definition

253

Tool steels as coatings

61

electron-beam hardening

91

hardened, hardness range

187

laser melting

92

nitriding

114

oxidation

108

Toughness

1

Toyota Diffusion (TD) coating process applications

176 204

Transformation temperature, definition

254

Transition metals, for cladding

168

Transportation, industrial operations and annual wear economic consequences

4

Trees, definition

254

Tribology, definition

254

Tribosystems

6

Triple pin-on-disk (TPOD) test geometry

50

Trowel coating, to apply ceramic coatings

136

Tuberculation

21

definition

254

Tumbling. See also Barrel finishing. definition

254

Tungsten erosion rate

66

as sputter coating material

175

Tungsten carbide as chemical vapor deposition coating material

169

as coating material

137

This page has been reformatted by Knovel to provide easier navigation.

171

316

Index terms

Links

Tungsten carbide-cobalt coatings applications

200

dealloying corrosion

27

plasma sprayed, cost

191

thermally sprayed, thickness ranges and hardness levels

211

Tungsten rhenium (W-Re), as chemical vapor deposition coating material

171

Tungsten thorium (W-Th), as chemical vapor deposition coating material

171

Two-body abrasive wear

55

57

U Ultraprecision finishing, definition

254

Ultrasonic cleaning

219

definition

254

Underfilm corrosion, definition Uniform corrosion definition

254 15 254

Uranium, hydride formation Urethanes, applications Utilities, industrial operations and annual wear economic consequences

31 130 4

V Vacuum carburizing characteristics and requirements

111

definition

255

Vacuum deposition, definition

255

Vacuum nitrocarburizing, definition

255

Vacuum plasma spraying (VPS)

160

112

162

164

25 138

61

137

31 130

127 131

129

188 Vapor degreasing, definition Vapor deposition. See also Chemical vapor deposition; Physical vapor deposition; Sputtering

255

Vapor plating. See Vacuum deposition. Vinyl

This page has been reformatted by Knovel to provide easier navigation.

317

Index terms

Links

Vinyl-alkyds, applications

130

Vinylidene chloride

127

Vinyl resin

34

W Water alkalinity effect on corrosion rate

13

as corrosion contributor

13

Watts nickel coatings, abrasive wear resistance Wear. See also Abrasive wear; Adhesive wear; Erosion; Rolling-contact wear

151 54

abrasive

56

adhesive

72

classification schemes

54

definition

54

economic effects

3

erosion

61

galling

75

parts in static contact with a product, surface treatments for rolling-contact

255

196

200

77

synergistic relationships with corrosion mechanisms Wear coefficient

5 74

Wear debris, definition

255

Wear plates coating thickness/penetration depth

192

temperature range and distortion

192

Wear resistance

1

process comparisons surface engineering treatments for improvement

186 2

Wear scar volume

73

Wear testing

81

computer automation

84

devices

82

purposes

81

standardized methods

81

82

variables to be controlled

81

83

This page has been reformatted by Knovel to provide easier navigation.

318

Index terms Wear tiles

Links 7

Weathering, definition Wedge formation Welding. See also Weld-overlay coatings definition

255 59

60

161 255

Weld-overlay coatings

153

abrasive wear resistance

157

158

applications

207

208

211 benefits

2

cost, relative

191

erosion

157

iron aluminides

159

oxidation resistance

159

processing parameters for optimization

155

size and weight limitations

215

sulfidation resistance

159

surface finish characteristics

213

thickness range of treatment

6

versus thermal spray coatings

160

welding processes available

154

White cast irons, wear of

60

White layer, definition

256

White rust, definition

256

Wiped coat, definition

256

Wiping effect, definition

256

61

Wire flame spraying. See also Flame spraying. definition

256

Work hardening

7

Wrought iron, galvanic corrosion

17

Z Zinc corrosion of

11

dezincification

26 This page has been reformatted by Knovel to provide easier navigation.

17

209

319

Index terms

Links

Zinc (Continued) as sacrificial coating material

33

Zincalume

139

Zinc-aluminum coatings

142

Zinc anodes, for cathodic protection

34

Zinc chromate

31

Zinc coatings, benefits Zinc electroplating, applications Zinc flake

34

128

2 147

148

32

Zinc molybdate

128

Zinc-nickel alloy plate (continuous electrodeposition), benefits Zinc phosphate

2 96

Zinc phosphorus silicate Zinc phosphosilicate

128 32

Zinc plating

211

Zinc-rich epoxy coatings

127

Zinc-rich η-phase

142

Zincrometal, definition

256

Zirconia thermal barrier coatings (TBCs)

136

Zirconium, hydride formation

31

Zirconium alloys, stress-corrosion cracking

28

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128

212

174