Materials for Tribology

MATERIALS FOR TRIBOLOGY

TRIBOLOGY SERIES Advisory Board W.J. Bartz (Germany, F.R.G.) R. Bassani (Italy) B. Briscoe (Gt. Britain) H. Czichos (Germany, F.R.G.) D. Dowson (Gt. Britain) K. Friedrich (Germany, F.R.G.) N. Gane (Australia)

Vol. 1 Vol. Vol. Vol. VOl. Vol. VOl. Vol.

2 3 4 5 6 7 8

VOl. 9 VOl. 10 Vol. 11 VOl. 12 Vol. 13 Vol. 14 Vol. 15 Vol. 16 Vol. 17 Vol. 18 VOl. 19 VOl. 20

W.A. Glaeser (U.S.A.) M. Godet (France) H.E. Hintermann (Switzerland) K.C. Ludema (U.S.A.) T. Sakurai (Japan) W.O. Winer (U.S.A.)

Tribology - A Systems Approach to the Science and Technology of Friction, Lubrication and Wear (Czichos) Impact Wear of Materials (Engel) Tribology of Natural and Artificial Joints (Dumbleton) Tribology of Thin Layers (Iliuc) Surface Effects in Adhesion, Friction, Wear, and Lubrication (Buckley) Friction and Wear of Polymers (Bartenev and Lavrentev) Microscopic Aspects of Adhesion and Lubrication (Georges, Editor) Industrial Tribology - The Practical Aspects of Friction, Lubrication and Wear (Jones and Scott, Editors) Mechanics and Chemistry in Lubrication (Dorinson and Ludema) Microstructure and Wear of Materials (Zum Gahr) Fluid Film Lubrication - Osborne Reynolds Centenary (Dowson et al., Editors) Interface Dynamics (Dowson et al., Editors) Tribology of Miniature Systems (Rymuza) Tribological Design of Machine Elements (Dowson et al., Editors) Encyclopedia of Tribology (Kajdas et al.) Tribology of Plastic Materials (Yamaguchi) Mechanics of Coatings (Dowson et al., Editors) Vehicle Tribology (Dowson et al., Editors) Rheology and Elastohydrodynamic Lubrication (Jacobson) Materials for Tribology (Glaeser)

TRIBOLOGY SERIES, 20

MATERIALS FOR TRIBOLOGY William A. Glaeser Engineering Mechanics Division, Battelle, Columbus, Ohio, USA

ELSEVIER Amsterdam London New York Tokyo

1992

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands

L i b r a r y o f Congress C a t a l o g i n g - i n - P u b l i c a t i o n Data Giaeser. W i l l i a m A. M a t e r i a l s for t r i b o l o g y / W i l l i a m A. Glaeser. p. cm. -- I T r i b o l o g y s e r l e s ; 20)

Includes b i b l i o g r a p h i c a l references and i n d e x . ISBN 0-444-88495-5 (U.S.) 1. T r l b o l o g y . I . T i t l e . 11. Series. TJ1075.G53 1992 621.8'9--dc20

9 1-4835 1

CIP ISBN 0 444 88495 5

0 1992 ELSEVIER SCIENCE PUBLISHERS B.V. All rights reserved. No part of this publication 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 prior written permission of the publisher, Elsevier Science Publishers B.V., Copyright & Permissions Department, P.O. Box 521, 1000 A M Amsterdam, The Netherlands. Special regulations for readers in the U.S.A. - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the publisher. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Printed in The Netherlands

DEDICATION

T h i s book i s d e d i c a t e d t o my w i f e , B e t t y , who has k e p t me going

on t h i s long p r o j e c t . W.A. Glaeser

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vii

CONTENTS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

............................ INTRODUCTION ........................... METALLURGY OF STEELS ....................... C a r b o n Steels . . . . . . . . . . . . . . . . . . . . . . . . . SELECTION OF STEEIS . . . . . . . . . . . . . . . . . . . . . . . . PEARLITIC STEELS ......................... MARTENSITIC STEELS ........................

8

22

.........................

24

INTRODUCTION

.

CHAPTER 1 STEELS

STAINLESS STEELS

9

10 13

18

18

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 . . . . . . . . . . . . . . . . . . . . . . . . . . 37 BEARING STEELS . . . . . . . . . . . . . . . . . . . . . . . . . 41 MANGANESE STEELS SELECTING STEELS FOR MINING AND CONSTRUCTION INDUSTRIES . . . . . . 4 1 CHAPTER 2 . COPPER BASE BEARING MATERIALS . . . . . . . . . . . . . . . .4 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 INTRODUCTION 48 T I N BRONZES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 LEADED T I N BRONZES 51 COPPER LEAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 ALUMINUM BRONZE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 MANGANESE BRONZE . . . . . . . . . . . . . . . . . . . . . . . . . 56 BERYLLIUM COPPER . . . . . . . . . . . . . . . . . . . . . . . . . . 56 POROUSBRONZES BEARING PROPERTIES OF COPPER BASE MATERIALS . . . . . . . . . . . . 5 9 TOOL STEELS

viii

. . . . . . . . . . . . . . . . . . . . . . . . . . . 63 ELECTRICAL CONTACTS . . . . . . . . . . . . . . . . . . . . . . . . 64 CHAPTER 3 . SOFT METAL BEARING MATERIALS . . . . . . . . . . . . . . . . . 69 SOFT METALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 72 BABBITTS OR WHITEMEAL . . . . . . . . . . . . . . . . . . . . . . . GEAR BRONZES

. . . . . . . . . . . . . . . . . . . . . . . . 78 Bearing Materials . . . . . . . . . . . . . . . 79

Babbitt Fatigue Aluminum Based

. . . . . . . . . . . . . . . . . .81 ALLOY SELECTION AND DESIGN . . . . . . . . . . . . . . . . . . . . 84 87 ZINC BEARING ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . 88 GOLD. SILVER AND INDIUM . . . . . . . . . . . . . . . . . . . . . . B i m e t a l . T r i m e t a l M a t e r i a1 s

CHAPTER4

. CAST IRON

INTRODUCTION

.......................... ...........................

89

90

PROPERTIES OF CAST IRONS

.....................

91

METALLURGY OF CAST IRONS

.....................

94

. . . . . . . . . . . . . . . . . . 103 . SURFACE HARDENING . . . . . . . . . . . . . . . . . . . . . . . . . 106 WEAR PROPERTIES OF CAST IRONS . . . . . . . . . . . . . . . . . . 1. 0 7 112 HIGH ALLOY CAST IRONS . . . . . . . . . . . . . . . . . . . . . . . 114 CHAPTER 5 . CARBON GRAPHITE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . 115 USAGE I N TRIBOLOGY 119 WEAR OF CARBON GRAPHITE . . . . . . . . . . . . . . . . . . . . . . MECHANICAL SEALS . . . . . . . . . . . . . . . . . . . . . . . . . 122 124 SLEEVE BEARINGS . . . . . . . . . . . . . . . . . . . . . . . . . . 126 THRUST BEARINGS AND VANES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 CARBON BRUSHES HEAT TREATMENT OF CAST IRONS

ix

CHAPTER 6

. CERAMICS AND S P E C I A L ALLOYS

INTRODUCTION CERAMICS

. . . . . . . . . . . . . . . . 130 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

.............................

132

. . . . . . . . . . . . . . 139 . Heat T r e a t a b l e Ceramics . . . . . . . . . . . . . . . . . . . . 145 CERAMIC TOOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4 8 STRUCTURE AND PROPERTIES OF CERAMICS

. . . . . . . . . . . . . . . . 1. 5 0 151 CERMETS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 GLASSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REFRACTORY METALS AND ALLOYS . . . . . . . . . . . . . . . . . . .1 5 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 SUPER ALLOYS CERAMIC ROLLING CONTACT BEARINGS

. . . . . . . . . . . . . . . . . 1. 6 6 H i g h T e m p e r a t u r e P r o p e r t i e s O f S u p e r A l l o y s . . . . . . . . . .1 6 8 CHAPTER 7 . POLYMERIC MATERIALS . . . . . . . . . . . . . . . . . . . . . 177 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 . . . . . . . . . . . . . . . . . . . . . . 180 PROPERTIES OF POLYMERS MATERIALS FOR NUCLEAR REACTORS

DESIGN OF PLASTIC BEARINGS

....................

182

. . . . . . . . . . . . . . . . . . . . 187 H i g h T e m p e r a t u r e P l a s t i c s . . . . . . . . . . . . . . . . . . .1 9 4 ELASTOMERS-RUBBER . . . . . . . . . . . . . . . . . . . . . . . . . 200 PLASTICS USED I N TRIBOLOGY

. . . . . . . . . . . . . . . . . . . . . 201 BEARINGS . . . . . . . . . . . . . . . . . . 2.0 5

PROPERTIES OF ELASTOMERS DESIGN OF ELASTOMER

. . . . . . . . . . . . . . . . . . . . . . . . 207 . . . . . . . . . . . . . . . . . . . . . . . . . . 209 WEAROFRUBBER REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 1 APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 .2 1 INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 TYPES OF ELASTOMERS

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XI

LIST

OF

TABLES

OF

MATERIAL

PROPERTIES

Carbon steels Mechanical properties ...............................................

14

Physical properties .................................................

15

Stainless steels Mechanical properties ...............................................

24

Physical properties .................................................

25

Tool steels Mechanical properties ...............................................

33

Physical properties .................................................

34

Roll ing contact bearing steels Mechanical properties ...............................................

36

Physical properties .................................................

37

Relative hardnesses o f Steels. Minerals and Carbides .....................

45

Porous bearing materials PV limits .......................................

58

Wear properties o f bearing bronzes .......................................

59

Copper base materials Mechanical properties ...............................................

66

Physical properties .................................................

67

Soft metal elements mechanical & physical properties .....................

71

xii

Soft metal bearing alloys mechanical & physical properties ...............73 Mulilayer bearing material load capacities...............................

83

Cast iron mechanical and physical properties .............................

92

Cast irons abrasion resistance ..........................................

110

Carbon graphite Mechanical properties ..............................................

117

Physical properties ................................................

118

Seal wear properties ...............................................

120

PV values ..........................................................

125

Carbon electric brush properties ...................................

128

Ceramics Mechanical and physical properties ...........................

133

.

138

Thermal shock resistance...........................................

141

Critical velocities ................................................

144

Modified zirconias. mechanical & physical properties ............... 147 Cermets. mechanical and physical properties .............................

154

Glasses, mechanical and physical properties .............................

156

Wear of superalloys .....................................................

159

High temperature alloys Mechanical properties ..............................................

162

Physical properties ................................................

163

Cobalt base alloys, abrasive wear properties ............................

165

xiii

High temperature m a t e r i a l s E f f e c t o f temperature on thermal c o n d u c t i v i t y ......................

170

E f f e c t o f temperature on s h o r t t i m e t e n s i l e s t r e n g t h ............... 171 E f f e c t o f temperature on Young's Modulus ...........................

172

E f f e c t o f temperature on thermal expansion .........................

173

E f f e c t o f temperature on thermal d i f f u s i v i t y .......................

174

E f f e c t o f temperature on thermal s t r e s s r e s i s t a n c e f a c t o r .......... 175 E f f e c t o f temperature on s p e c i f i c heat c a p a c i t y ....................

176

Plastics Mechanical p r o p e r t i e s ..............................................

180

Physical p r o p e r t i e s ................................................

181

PV values ..........................................................

184

D e f l e c t i o n temperatures ............................................

198

Wear r a t e s .........................................................

199

Elastomers Hardness comparisons ...............................................

202

Mechanical p r o p e r t i e s ..............................................

203

Physical p r o p e r t i e s ................................................

204

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1

INTRODUCTION

M a t e r i a l s used i n t r i b o l o g i c a l a p p l i c a t i o n s are, o r t h e most p a r t , common m a t e r i a l s used f o r general e n g i n e e r i n g a p p l i c a t i o n s . There a r e some m a t e r i a l s designed s p e c i f i c a l l y f o r b e a r i n g s as, f o r i n s t a n c e b a b b i t t s , leaded bronze, a1 uminum-t i n , woven PTFE-g1 ass, s i n t e r e d bronze- 1ead-PTFE and T r i b a l oy. Many conventional e n g i n e e r i n g m a t e r i a l s have been adapted t o t r i b o l o g i c a l uses. For instance, A I S I 52100 s t e e l and M50 t o o l s t e e l a r e b o t h used i n r o l l i n g c o n t a c t bearings. Special h e a t t r e a t m e n t s have been developed f o r t h e s e two a l l o y s t o enhance t h e i r endurance under r o l l i n g c o n t a c t c o n d i t i o n s . There a r e many o t h e r examples o f a d a p t i o n o f m a t e r i a l s t o s p e c i f i c t r i b o l o g i c a l a p p l i c a t i o n s . These w i l l be found i n t h e ensuing c h a p t e r s . U n f o r t u n a t e l y , f o r t h e t r i b o l o g i s t , i n f o r m a t i o n on m a t e r i a l s he uses i s s c a t t e r e d t h r o u g h t h e l i t e r a t u r e , causing c o n s i d e r a b l e waste o f t i m e i n searching f o r d e s i r e d m a t e r i a l p r o p e r t i e s . Often, t h e p a r t i c u l a r m a t e r i a l o f i n t e r e s t i s found b u r i e d i n l i s t i n g s o f o t h e r m a t e r i a l s n o t used f o r b e a r i n g s , gears, brakes, o r machine t o o l s . The i n t e n t o f t h i s handbook i s t o p r o v i d e a comprehensive r e f e r e n c e f o r m a t e r i a l s used i n t r i b o l o g i c a l a p p l i c a t i o n s . The a u t h o r has used h i s l o n g experience i n s e l e c t i n g m a t e r i a l s f o r a wide v a r i e t y o f f r i c t i o n and wear a p p l i c a t i o n s t o develop a d a t a base o f m a t e r i a l s f o r t r i b o l o g y . I n a d d i t i o n i n f o r m a t i o n has been s e l e c t e d f r o m t h e l i t e r a t u r e on t h e b e h a v i o r o f these m a t e r i a l s i n bearings, seals, gears, brakes, c l u t c h e s , w i r e rope, v a l v e s cams and wear s u r f a c e s and i s i n c l u d e d i n t h e d e s c r i p t i v e t e x t . The m a t e r i a l s have been grouped i n f a m i l i e s , r e l a t i n g t o t h e i r compos t i o n . The f o l l o w i n g c l a s s i f i c a t i o n s have been s e l e c t e d :

CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER

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

STEELS COPPER BASE BEARING MATERIALS SOFT METAL BEARING MATERIALS CAST IRON CARBON GRAPHITE CERAMICS AND SPECIAL ALLOYS POLYMERIC MATERIALS

2

Each c a t e g o r y i n t h e l i s t i n g on t h e p r e v i o u s page i s t h e s u b j e c t o f a c h a p t e r i n t h e handbook. A s h o r t t a b l e i s p r o v i d e d a t t h e b e g i n n i n g o f each chapter l i s t i n g t h e ranges o f s e l e c t e d p r o p e r t i e s f o r t h e m a t e r i a l s i n t h a t c h a p t e r . I n a d d i t i o n t h e r e a r e s h o r t summaries o f t h e t r i b o l o g i c a l a p p l i c a t i o n s t h i s c l a s s o f m a t e r i a l s i s used f o r . Therefore, on t h e f i r s t page o f each c h a p t e r one can f i n d a g u i d e f o r s e l e c t i o n o f m a t e r i a l s - a t l e a s t f o r t h e f i r s t c u t . Tables o f m a t e r i a l p r o p e r t i e s a r e i n c l u d e d i n t h e body o f each c h a p t e r . The p r o p e r t i e s chosen f o r a g i v e n c l a s s o f m a t e r i a l s r e p r e s e n t those p r o p e r t i e s r e l e v a n t t o t h e t r i b o l o g i c a l use o f t h e m a t e r i a l s i n t h e c h a p t e r . I n o r d e r t o keep t a b l e s i z e t o a reasonable l e v e l , several p r o p e r t i e s t a b l e s may be used i n one c h a p t e r . For instance, i n Chapter 8 ( S o f t M e t a l s ) , a separate t a b l e i s p r o v i d e d f o r pure metals such as g o l d , s i l v e r , t i n , e t c . A second t a b l e i s p r o v i d e d f o r b a b b i t t s , b o t h l e a d based and t i n based. The p r o p e r t i e s d a t a f o r t h e s e l e c t e d m a t e r i a l s have been o b t a i n e d f r o m a number o f sources. The data, t h e r e f o r e , a r e o n l y as good as t h e source from which t h e y have been taken. Some data, i n p a r t i c u l a r , b e a r i n g l o a d c a p a c i t y , wear c o e f f i c i e n t s and PV values have been developed i n t h e B a t t e l l e T r i b o l o g y Laboratory. Other sources i n c l u d e t h e B a t t e l l e Defense M a t e r i a l s I n f o r m a t i o n Center, A l l o y s D i g e s t , ASM M e t a l s Handbook, M e t a l s Reference Book, Wear, Proceedings o f ASME Wear Conferences and ASME Wear C o n t r o l Handbook. C e r t a i n m a t e r i a l s p r o p e r t i e s a r e q u i t e s p e c i f i c f o r t r i b o l o g y - as, f o r instance, PV, wear c o e f f i c i e n t , maximum b e a r i n g s t r e n g t h , hardness and c o e f f i c i e n t o f f r i c t i o n . Other p r o p e r t i e s such as t e n s i l e s t r e n g t h and s p e c i f i c h e a t a r e i n more g e n e r a l use b u t have a p p l i c a t i o n i n c e r t a i n t r i b o l o g i c a l areas. C o e f f i c i e n t o f f r i c t i o n i s v e r y s p e c i f i c b u t p r e s e n t s a v e r y d i f f i c u l t problem i n i t s meaning, accuracy and r e l e v a n c e . C o e f f i c i e n t o f f r i c t i o n i s used i n several mathematical r e l a t i o n s h i p s as f o r i n s t a n c e , t h e f o r m u l a f o r power l o s s i n a s l e e v e b e a r i n g : HP

=

7.93 f L RPM D x

[l]

Where: coefficient o f f r i c t i o n Normal l o a d on b e a r i n g , l b RPM = S h a f t speed, RPM D = S h a f t diameter, i n . f L

= =

3

If, in the above relation, the friction coefficient is measured while the bearing is in operation under the load and lubrication condition in question, the power calculation should be accurate. I f , however, the friction coefficient must be assumed, errors amounting to several orders of magnitude are possible. Unfortunately, friction coefficient is not a material property like tensile strength which can be assigned to each material. Instead, it is a proportionality constant which indicates that tangential resistive force varies 1 inearly with normal bearing load: F = f L Generally, under well controlled surface conditions, this proportionality holds. However, under very heavy loads, frictional heating or changing surface environment, the proportionality no longer applies. If one measures friction forces in a bearing, considerable fluctuation can be detected in the friction level as the bearing runs. Depending on the sensitivity and response characteristics o f the friction measuring device, friction force variations can appear very complex. This presents a problem in selection of a friction value. Often, an average value is used. However, spectrum analysis may be more appropriate - especially when friction peaks are of concern. Because of the above mentioned barriers to the definition of reliable friction values, the author of this handbook has refrained from including friction coeffient values for many of the materials. Some exceptions have been made, however, for self lubricating materials like carbon graphite and ptfe to indicate the level of friction reduction. Estimates of friction coefficient ranges for material classes have been included to show the effect of self lubricating materials on friction levels for comparison purposes. The material properties chosen for inclusion in this handbook were selected on the basis of frequency of use in tribological applications. Some properties, like fracture toughness, are appropriate to a class of materials -namely ceramics. Fracture toughness influences wear or erosion rates of brittle materials.

4

A r e l a t i o n s h i p has been developed b y Evans and M a r s h a l l [ 2 ] i n which a b r a s i v e wear r e s i s t a n c e i s r e l a t e d t o f r a c t u r e toughness and modulus:

0.5 0.625 Abrasive Wear Resistance

=

[KIc

H

0.8 ]/ (E/H)

Where: H KIc E

= = =

Hardness F r a c t u r e Toughness Young's Modulus

Thermal c o n d u c t i v i t y i s s i g n i f i c a n t f o r e s t i m a t i o n o f temperature developed b y f r i c t i o n a l h e a t i n g . Functions r e l a t i n g t o " f l a s h temperature" a r e used i n t h e p r e d i c t i o n o f l u b r i c a n t f i l m breakdown. F l a s h temperature, o r l o c a l temperature s p i k e s a t s l i d i n g c o n t a c t s generated b y f r i c t i o n a l h e a t i n g i s used i n t h e a n a l y s i s o f t h i n f i l m o r boundary l u b r i c a t e d surfaces t o estimate threshold s l i d i n g v e l o c i t i e s f o r contact f a i l u r e . Czichos [ 3 ] has summarized t h e work o f B l o k , Jaeger, Holm and Archard [ 4 , 5, 6, 71 i n t h e f o l l o w i n g r e l a t i o n s h i p :

dT = 0 . 2 5 CNL Where :

f = friction coefficient

g p

g r a v i t y constant p e n e t r a t i o n hardness = density c = s p e c i f i c heat k = thermal d i f f u s i v i t y v = velocity C = c o n s t a n t ; 1 - 0.5, depending on t h e v a l u e o f L dT = f l a s h temperature r i s e above ambient =

=

5

F l a s h temperature i s i n f l u e n c e d b y l o a d and speed, d e f o r m a t i o n mode and t h e thermal p r o p e r t i e s o f t h e c o n t a c t i n g m a t e r i a l s . Not o n l y i s t h e dT v a l u e i n f l u e n c e d , b u t t h e f o r m u l a must be a l t e r e d f o r s p e c i f i c c o n d i t i o n s . As can be seen, t h e above analyses r e q u i r e a number o f p h y s i c a l and mechanical properties f o r the materials i n s l i d i n g contact. I n d r y s l i d i n g systems where m a t e r i a l s w i t h l o w thermal c o n d u c t i v i t y a r e used such as i n h i g h speed s h a f t f a c e s e a l s and c l u t c h e s and brakes, thermal shock can be a s i g n i f i c a n t f a c t o r i n t h e wear process. I f f r i c t i o n a l l y developed s u r f a c e h o t spots develop r a p i d l y and a r e quenched, f r a c t u r e o f b r i t t l e m a t e r i a l s from t h e thermal shock can r e s u l t . Thermal shock can cause a c a t a s t r o p h i c i n c r e a s e i n wear. Thermal shock r e s i s t a n c e i s a d i f f i c u l t parameter t o d e f i n e o r measure i n a m a t e r i a l . Mehrotra [8] has developed a r a n k i n g system f o r comparing t h e r e l a t i v e thermal shock r e s i s t a n c e o f v a r i o u s ceramic m a t e r i a l s . He uses two r e l a t i o n s : KIc/Ea KIck/Ea Where: KIc E a k

= = = =

F r a c t u r e toughness E l a s t i c Modulus C o e f f i c i e n t o f l i n e a r thermal expansion Thermal c o n d u c t i v i t y

R a t i o (1) does n o t c o n t a i n thermal c o n d u c t i v i t y and can be used t o check o u t m a t e r i a l s w i t h l o w thermal expansion and E as p o t e n t i a l l y good i n thermal shock r e s i s t a n c e . S i a l o n i s one o f those m a t e r i a l s and i t performs w e l l as a h i g h speed c u t t i n g t o o l . R a t i o ( 2 ) c o n t a i n s thermal c o n d u c t i v i t y and w i l l s o r t o u t t h e e f f e c t o f thermal c o n d u c t i v i t y on thermal shock r e s i s t a n c e f o r ceramic m a t e r i a l s w i t h s i m i l a r expansion and e l a s t i c p r o p e r t i e s . S i l i c o n c a r b i d e , w i t h i t s h i g h thermal c o n d u c t i v i t y , a l s o has t h e h i g h e s t thermal shock r e s i s t a n c e among t h e ceramics.

6

Dow and Burton [9] have developed a relationship for determining velocity at which hot spots developed on a sliding surface begin move about the surface, causing an increase in the wear rate. This called thermal mechanical instability (TEM). The relationship is as Vcr =

a critical to rapidly process is follows:

16 K2 [ ( f a E )2 ~ k z ]

Where: Vcr

=

K

=

f

=

a

=

E k

= =

p

=

z

= =

c

critical sliding speed thermal conductivity coefficient of friction coefficient of linear thermal expansion Young's modulus thermal diffusivity; (Klpc) density width of slider specific heat

The thermoelastic instability criterion has been used in the estimation of excessive wear for given ceramic materials for cylinder liner and piston ring materials for advanced low heat loss diesels [ l o ] . In this work, materials like partially stabilized zirconia were predicted to produce excessive wear by thermal shock because of their low thermal diffusivity. Silicon carbide or silicon nitride, on the other hand, were predicted to not develop thermal instability under diesel operating conditions. These predictions were verified by experiment Note that the coefficient of friction is one parameter in the equation. t was derived experimentally in the above work. The preceding examples show that there are a number of material properties needed to solve equations used in tribologica applications. These properties include hardness, tensile strength, density, Young's modulus, specific heat, fracture toughness, thermal conductivity and thermal expansion coefficient. In add it ion, some tribological properties are useful for selection of materials. These include PV, maximum bearing strength, and maximum operating temperature. Other properties of use to tribologists include electrical resistivity and melting point. In the study of high strain deformation effects in the near surface region in the wear zone, stacking fault energy is of use as well as the work hardening coefficient at very high strain levels.

7

These two p r o p e r t i e s a r e n o t r e a d i l y a v a i l a b l e f o r a l l m a t e r i a l s . Some s i m p l e b i n a r y a l l o y s and pure m e t a l s have these values recorded. T h e r e f o r e a l i m i t e d number o f m a t e r i a l s w i l l have t h i s i n f o r m a t i o n p r o v i d e d i n t h i s handbook. F i n a l l y , something must be s a i d about t h e wear r e s i s t a n c e o f e n g i n e e r i n g m a t e r i a l s . There a r e a number o f equations which have been developed f r o m wear t e s t d a t a and used i n s p e c i f i c a p p l i c a t i o n s such as brakes and c l u t c h e s , f a c e seals, automobile t i r e s , m i n i n g machinery and machine t o o l s f o r e s t i m a t i n g component l i f e f o r g i v e n c o n d i t i o n s . For these examples, some f i g u r e o f m e r i t o r measure o f wear r e s i s t a n c e i s o f t e n used. S p e c i f i c wear, a b r a s i v e wear r e s i s t a n c e f a c t o r o r wear c o e f f i c i e n t a r e some examples. Wear c o e f f i c i e n t i s p r o b a b l y used more t h a n any o t h e r parameter. I t i s used c o n s i s t e n t l y i n t h e ASME Wear C o n t r o l Handbook [ll]. Wear d a t a i s b e i n g r e p o r t e d i n t h e l i t e r a t u r e u s i n g t h e wear c o e f f i c i e n t . Wear c o e f f i c i e n t s have been i n c l u d e d i n some c h a p t e r s , u s i n g s e l e c t e d d a t a o b t a i n e d i n w e l l documented and accepted t e s t s .

The ASTM has developed an a b r a s i o n t e s t f o r l o w s t r e s s a b r a s i o n c o n d i t i o n s . Low s t r e s s a b r a s i o n i s a form o f a b r a s i o n i n which a b r a s i v e p a r t i c l e s r u b over a s u r f a c e under a c o n t a c t s t r e s s which does n o t r e s u l t i n f r a c t u r e o f t h e p a r t i c l e s . I n t h e ASTM t e s t , sand passes between a r o t a t i n g r u b b e r wheel and t h e t e s t specimen. The ASTM Dry Sand/Rubber Wheel Abrasion Tests a r e s p e c i f i e d under G 65-81. The wear values f r o m t h i s t e s t a r e r e p o r t e d i n volume l o s s i n c u b i c m i l l i m e t e r s p e r ASTM procedure. The volume l o s s i s determined f o r a s p e c i f i e d number o f wheel r e v o l u t i o n s under a g i v e n wheel f o r c e . Data from these t e s t a r e q u i t e r e p r o d u c i b l e . Data a r e p r o v i d e d f o r some m a t e r i a l s i n t h i s handbook. Coatings f o r c o n t r o l o f wear a r e f i n d i n g an i n c r e a s i n g use i n machinery. The economical advantage o f u s i n g a l o w c o s t and ‘ e a s i l y formed m a t e r i a l as t h e b u l k o f a p a r t w i t h a small amount o f h i g h c o s t wear r e s i s t a n t m a t e r i a l l i k e a c a r b i d e a p p l i e d t o t h e s u r f a c e s expected t o see wear i s b e i n g c o n s i d e r e d more and more i n new designs. The importance o f t h e s e m a t e r i a l s i n t r i b o l o g y cannot be ignored. However, t h e t o p i c o f wear r e s i s t a n t c o a t i n g s i s l a r g e enough t o t a k e up an e n t i r e volume. I n a d d i t i o n , new developments i n c o a t i n g technology such as diamond and diamond-1 i k e c o a t i n g s a r e coming a l o n g r a p i d l y . Therefore, c o a t i n g s a r e n o t covered i n d e t a i l i n t h i s t e x t . Some u s e f u l t a b l e s , e x t r a c t e d from t h e l i t e r a t u r e , a r e i n c l u d e d i n t h e appendix. Readers a r e d i r e c t e d t o o t h e r t e x t s on wear r e s i s t a n t c o a t i n g s i n c l u d i n g B u d i n s k i ’ s [12].

8

CHAPTER

1 -

STEELS

MELTING POINTS 1400 - 1500'C HARDNESS

180 - 790 VICKERS

TENSILE STRENGTH 100 - 2700 MPa YOUNGS MODULUS

6.9Et4

- 2.OEt5 MPa

THERM COND 25 - 50 WATT/m k THERM EXPANS 1.1

- 1.5 Et05/'C

DENSITY 6.4 - 8 . 0 Et03 Kg/m3

TYPES OF STEEL ALLOYS USED I N TRIBOLOGY Carbon S t e e l - Forged o r hardened b y h e a t t r e a t m e n t A l l o y S t e e l - C o n t a i n i n g a l l o y i n g elements t o improve h a r d e n a b i l i t y A u s t e n i t i c S t a i n l e s s S t e e l - Not hardenable b y heat t r e a t m e n t M a r t e n s i t i c S t a i n l e s s S t e e l - Heat t r e a t a b l e t o harden Tool S t e e l - Hot hardness, r e s i s t a n c e t o a b r a s i o n APPLICATIONS FOR STEELS Shafting o r bearing journals Gears B a l l and r o l l e r b e a r i n g s Tools and d i e s Wire rope Wheels and r a i l s Fasteners Pumps and compressors Knives Thrust bearings Drill bits

9

INTRODUCTION

Steels have a wide and diversified use for Tribologica applications. Utilization, rather than specific formulation has been the rule for this class of materials. A few steels have been modified in compo ition and heat treatment methods for use in bearings. A I S I 52100 steel is used for ball and roller bearings and is subjected to special vacuum melting practices for inclusion control and is given a special heat treatment to minimize residual austenite content and to ensure dimensional stability. These practices are discussed later in the chapter. A I S I 440C stainless steel is also used for rolling contact bearings - principally for elevated temperature and some corrosive environments. This alloy has been modified to refine the carbide structure for roll ing contact applications. Some tool steels with the required grinding properties have also been utilized for elevated temperature rolling contact bearings. More recently, hardened steels made by powder metallurgy techniques have been developed to provide better control over microstructure and inclusion content to improve contact fatigue life of high speed aircraft bearings. Manganese steel or Hadfield steel is used in the mining, earth moving and railroad industries where high toughness, impact resistance and abrasive wear resistance are needed. This alloy contains 1% carbon and 11 to 14% manganese. It is an austenitic steel which transforms to martensite when abraded heavily. This results in a tough core with a hard skin that continually renews itself as it is worn off. This alloy is capable of extensive work hardening under abrasive impact conditions. Steel is used extensively in machinery for load bearing components like shafting, gearing, housings, cable, thrust surfaces, etc. For efficiency, steel surfaces are being coated with wear resistant materials. In this way, the structural strength of the steel can be used for large components while small amounts of expensive wear resistant material is applied to specific areas where it is needed. Stainless steel is used extensively in reactor components where resistance to heat and corrosion is essential. It also finds use in the petrochemical and food processing industries for much the same reasons. Stainless steel is also used in cutlery, flat ware and cooking utensils.

10

METALLURGY OF STEELS Two abundant elements, i r o n and carbon a r e t h e b a s i s f o r t h e s t e e l i n d u s t r y . Carbon i s s o l u b l e i n i r o n i n small amounts and s t e e l s cover a range o f carbon - i r o n a l l o y s f r o m 0.005 w t % carbon t o 2.00 w t % carbon. The i r o n - c a r b o n e q u i l i b r i u m diagram, w e l l e s t a b l i s h e d from y e a r s o f research, p r o v i d e s a map o f p o s s i b l e phases based on temperature and carbon c o n t e n t . An i r o n - c a r b o n e q u i l i b r i u m diagram f o r carbon w t % up t o 6.67% i s shown i n f i g u r e 1.1. The p o r t i o n o f t h e diagram f r o m 0% t o 2.0% carbon covers s t e e l s . H i g h e r carbon c o n t e n t s a r e t y p i c a l f o r c a s t i r o n s . I r o n - c a r b o n - s i l i c o n diagrams a r e used i n c a s t i r o n m e t a l l u r g y and a r e discussed i n c h a p t e r 4 ( c a s t i r o n ) . R e f e r r i n g t o f i g u r e 1.1, s e v e r a l phases o f i r o n - c a r b o n a r e shown: a u s t e n i t e , f e r r i t e , gamma i r o n and c e m e n t i t e . These a r e a l l e q u i l i b r i u m phases. Gamma i r o n i s n o t o f consequence i n s t e e l m e t a l l u r g y and w i l l n o t be discussed. A u s t e n i t e e x i s t s a t e l e v a t e d temperature (above 727'C). Carbon i s an a u s t e n i t e s t a b i l i z e r and i s more s o l u b l e i n a u s t e n i t e (2.11 wt%) t h a n i n f e r r i t e (0.022wt%). As temperature i s reduced from 1148'C, t h e s o l u b i l i t y o f carbon i n a u s t e n i t e decreases and carbon i s r e j e c t e d as i r o n c a r b i d e (Fe3C) o r cementite. Two phases, c e m e n t i t e and a u s t e n i t e e x i s t t o g e t h e r as shown i n t h e diagram. A t 727'C, a u s t e n i t e c o n v e r t s t o f e r r i t e and c o n s i d e r a b l e carbon i s r e j e c t e d from s o l u t i o n as c a r b i d e and t h e two phases, f e r r i t e and c e m e n t i t e e x i s t together. F o r t u n a t e l y f o r engineers, carbon r e j e c t i o n i s a v e r y s l u g g i s h process and w i t h r a p i d enough c o o l i n g f r o m t h e a u s t e n i t e r e g i o n , a s u p e r s a t u r a t e d s o l u t i o n o f carbon i n i r o n r e s u l t s . The l a t t i c e s t r a i n caused b y t h e excess carbon produces a phase change t o body centered t e t r a g o n a l ( a d i s t o r t e d c u b i c s t r u c t u r e ) w i t h v e r y h i g h hardness. T h i s s t r u c t u r e i s n o t shown on t h e e q u i l i b r i u m diagram and i s known as m a r t e n s i t e . M a r t e n s i t e i s d e s i r a b l e as a h i g h l y wear r e s i s t a n t form o f s t e e l . M a r t e n s i t e r e q u i r e s r a p i d c o o l i n g as achieved b y quenching. Slower c o o l i n g r a t e s w i l l produce o t h e r m i c r o s t r u c t u r e s having e q u a l l y u s e f u l e n g i n e e r i n g p r o p e r t i e s . These m i c r o s t r u c t u r e s i n c l u d e b a i n i t e ( a m a r t e n s i t e - l i k e s t r u c t u r e ) and p e a r l i t e ( a l a m e l l a r s t r u c t u r e o f f e r r i t e sandwiched between c e m e n t i t e p l a t e s ) . The hardness o f t h e s e m i c r o s t r u c t u r e s - e s p e c i a l l y m a r t e n s i t e and b a i n i t e a r e s i g n i f i c a n t l y i n f l u e n c e d b y carbon c o n t e n t . F i g 1.2 shows hardness as a f u n c t i o n o f carbon content f o r martensite. The above m e t a l l u r g i c a l f a c t o r s make f o r a wide v a r i e t y o f s t e e l s produced b y v a r y i n g amounts o f carbon and d i f f e r e n t heat t r e a t m e n t s . T h i s makes s t e e l a very v e r s a t i l e engineering material.

11

3

Figure 1.1 Iron-carbon equilibrium diagram

12

Not only are there a large variety of steels to choose from, but within those grades of steels, various heat treatments are p o s s i b l e resulting in a variety of microstructures. The microstructure of steel can have an effect on wear resistance.

1100 1000 900

800

> 700

I 6 ln 600 (I)

bainilic microslruclures

f 500

2

400

300 200 100

0 0

0.20 0.40 0.60 0.80 1.00 1.20

Carbon, %

Figure 1.2 Hardness as a function of carbon content for martensite

13

S t e e l s can e x i s t i n t h r e e b a s i c s t a t e s : a u s t e n i t i c , m a r t e n s i t i c and p e a r l i t i c . There are many v a r i a t i o n s on these s t a t e s i n c l u d i n g m i x t u r e s o f each, m a r t e n s i t i c o r a u s t e n i t i c m a t r i x c o n t a i n i n g c a r b i d e s , mixed f e r r i t e and p e a r l i t e , b a i n i t e e t c . Each one o f t h e s e s t a t e s has c h a r a c t e r i s t i c wear p r o p e r t i e s . Other p r o p e r t i e s such as f r a c t u r e toughness, c o r r o s i o n r e s i s t a n c e , c o s t , m a c h i n a b i l i t y , f a t i g u e s t r e n g t h , creep r e s i s t a n c e and t e n s i l e s t r e n g t h come i n t o c o n s i d e r a t i o n when choosing a grade o f s t e e l f o r a p a r t i c u l a r a p p l i c a t i o n . We w i l l d i s c u s s these b a s i c s t e e l m i c r o s t r u c t u r e s i n d i v i d u a l l y . Wear r e s i s t a n c e can be c o n t r o l l e d i n s t e e l s b y heat t r e a t m e n t , b y c a r b u r i z i n g o r n i t r i d i n g , b y work hardening o r b y t h e a p p l i c a t i o n o f hard c o a t i n g s . These f a c t o r s w i l l be discussed s e p a r a t e l y .

Carbon Steels Carbon s t e e l s can e x i s t i n a number o f d i f f e r e n t s t a t e s : p e a r l i t i c , b a i n i t i c , m a r t e n s i t i c and a u s t e n i t i c . Carbon s t e e l s r e v e r t t o t h e a u s t e n i t i c s t a t e when heated t o t h e temperature range 750'C t o 100O'C. A u s t e n i t e decomposes t o c a r b i d e s and f e r r i t e ( i r o n ) a t room temperature. The s t e e l a l s o changes d e n s i t y d u r i n g t h e change f r o m a u s t e n i t e t o another phase so t h a t t h e development o f m a r t e n s i t e f r o m a u s t e n i t e r e s u l t s i n an expansion o f t h e p a r t . T h i s e f f e c t can be c r u c i a l i n o p e r a t i o n o f b e a r i n g s and w i l l be discussed 1a t e r . Mechanical and p h y s i c a l p r o p e r t i e s and composition o f s e l e c t e d carbon s t e e l s a r e found i n t a b l e s 1.1 a,b,& c . P l a i n carbon s t e e l s having more t h a n 0.4'' C c o n t e n t can be hardened b y heat t r e a t m e n t . With a l l o y a d d i t i o n s o f N i o r Cr e t c t h e c r i t i c a l carbon c o n t e n t can be reduced below 0.4%. P e a r l i t e i s an e q u i l i b r i u m phase i n s t e e l c o n s i s t i n g o f f e r r i t e sandwiched between c e m e n t i t e ( i r o n c a r b i d e ) p l a t e s . I t forms d u r i n g slow c o o l i n g below t h e e u t e c t o i d temperature (around 600'C). P e a r l i t e forms from t h e decomposition o f t h e h i g h temperature a u s t e n i t e phase produced b y h e a t i n g t h e s t e e l above 160OaC and h o l d i n g l o n g enough t o d i s s o l v e a l l c a r b i d e s . P e a r l i t e e x h i b i t s s u p e r i o r wear r e s i s t a n c e t o a s - c a s t s p h e r i o d i z e d s t e e l . The p e a r l i t e phase w i l l r e s i s t deep p e n e t r a t i o n o f h a r d a b r a s i v e p a r t i c l e s and m i n i m i z e t h e depth o f plowing.

14

Table l . l a Mechanical P r o p e r t i e s of Carbon S t e e l s

MATERIAL

1040 1040 1095 1095 1118 1118 4320 4340 4340 4620 4620 4820 4820 81845 81845 8620 8620 9310 9310 Austenitic-Mn C 1080 C1080 Conversion f a c t o r s : MPa x 0.145 = k s i

FORM

CARBURIZED

CARBURIZED CARBURIZED CAST CARBURIZED CARBURIZED CAST

TEMPER HEAT TREAT Annealed HT,300F t e HT 500F t e ANNEALED HT,300F t e Annealed Annealed Annealed HT, 500F t e Annealed HT, 300F t e HT, 300F t e Annealed Annealed HT,400F t e HT,300F t e Annealed HT,3OOF t e Annealed HT,1850F t e HT,400F t e Annealed -

HARDNESS VICKERS DPH 150 540 710 200 360 165 225 260 550 230 740 690 230 195 617 789 180 694 200 200 404 224

TENSILE YOUNGS STRENGTH MODULUS MPa MPa 552 896 1480 690 7 58 448 752 979 1724 607 827 1379 7 58 64 1 2041 1296 607 1241 689 965 1303 820

2.07E+05 2.07E+05 2.07E+05 2.07E+05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.07E+05 2.07Et05 2.07E+05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.00Et05 2.07Et05 2.07Et05

15

Tab1 e 1. l b

P h y s i c a l P r o p e r t i e s Carbon Steel s

MATERIAL

1040 1118 4320 4340 4820 81845 8620 9310 Aust.-Mn C 1080

MAX OP TEtjP C

KIc MPa m4 55.00 167.00 77.00

260 538

77.00 22.00

204

DENSITY THERM CON kg/ WATT/m CU METER K 7,75Et03 7.86Et03 7.85Et03 7.75Et03 7.86Et03 7.75Et03 7.75Et03 7.75Et03 8.03Et03 7.75Et03

THERM EXP R E S I S m/m micro

/'C

50.69 51.55 38.06 37.54 45.00

1.12E-05 1.22E-05 1.13E-05 1.46E-05 1.55E-05 1.26E-05 1.48E-05 1.46E-05 1.84E-05 1.47E-05

37.50 46.71 12.63 46.71

HEAT CAPACITY ohm-cm J/kg/'K

19.00 14.00 30.00 30.00 26.00 30.00 20.00 68.30 18.00

5.OEt2 4.6Et2 4.6Et2 4.6Et2 4.6Et2 4.6Et2 4.6Et2 5.OEt2 5.OEt2 4.2Et2

Conversion f a c t o r s : kg/m

3

x 3.613E-5

=

l b / c u i n : kg/m

W/m/K x 0.578 = BTU/ft/hr/'F J/kg/K x 2.388E-4 =BTU/lb/'F

3

x 0.001

=

gm/cc ( S G )

or c a l / g / C

Table l . l c Composition o f Carbon S t e e l A l l o y s MATERIAL 1040 C 1095 C 1118 C 4320 C 4340 C 4620 C 4820 C 81845 C 8620 C 9310 C Austenitic-Mn C1080

COMPOSITION .4,Mn 1.0,Mn .16,Mn .2,Mn .4,Mn .2,Mn .2,Mn .45,Mn .2,Mn .1,Mn

.7,P .04,S .05,Si .2,Fe 98.6 .4,P .04,S .05,Fe 98.6 1.4,Fe 98.44 .55,Si .25,Ni 1.7,Cr .5,Mo .25 .7,P .025,S .025,Si .3,Ni 1.7,Cr .8,Mo .25,Fe 96 .55,Si .3,Ni 1.7,Mo .25 .6,P .035,S .04,Si .3,Ni 3.5,Mo .25,Fe 95 .8,Si .3,Ni .3,Cr .4,Mo .1,B .0005,Fe 98 .8,Si .2,Cr .5,Ni .6,Mo .2,Fe 97.5 .5,Si .2,Cr 1.2,Ni 3.2,Mo 1,Fe 94.2 C 1.0,Mn 12,Si .2,Fe 87 C .8,Mn . 7 , S i .2,Fe 98

16

Rapid quenching of austenite produces martensite. This i s a single phase containing dissolved carbon with a highly stressed lattice. The result is a very hard, abrasion resistant material. Martensite is the most wear resistant phase which can be developed in plain carbon steel. However it is relatively brittle until tempered by reheating after quenching. Selection of the correct quench rate is governed by the temperature-timetransformation (TTT) characteristics of the alloy. The chart in figure 1.3 shows TTT characteristics for a plain carbon steel. The C curve at the upper temperatures reflects the rate of phase change at a given temperature. For instance, if one slowly cools from austenite to below the eutectoid temperature and holds at about 700'C until the C curve is passed through (cooling curve 1) a coarse pearlite is formed after quenching when well within the knee of the curve. Holding at a lower temperature still above the knee of the curve (cooling curve 2) produces finer pearlite. If one cools rapidly to below the knee of the C curve (say about 500'C) and holds until the transformation is complete, one forms bainite (cooling curve 3 ) . Bainite is a feathery or acicular phase somewhat more wear resistant than pearlite. On quenching rapidly below the knee of the C curve until one passes through the Ms line , martensite i s formed (cooling curve 4 ) . Note that the martensite transformation i s not completed even at temperatures close to the boiling point of water (M90 line). This means that at room temperature there can still be some metastable austenite in the martensite. The retained austenite can transform when heavily strained and cause small, but important dimensional changes in a part. Note that the complete transformation does not occur until below freezing temperature. This is why heat treatment of ball bearing steel involves subzero cooling. This will be discussed under bearing steels.

17

800

700

600

h

," 500

coarse pearl ite

I

v 0)

L

3 c,

2

400

aJ cz

fij + 300

200

100

0

lo-'

Irn

2 4 s

tf

I

!

Id

Ih I

I

I

I

I

I

I

I

I

10

10'

I 03

I 04

I

10'

Time (seconds) Figure 1.3 Temperature-Time-Transformation Diagram for Plain Carbon Steel

18

SELECTION OF STEELS

Pearlitic Steels: Carbon Steels are inexpensive and ideal for large components. Steels are probably the most used materials in conditions requiring wear resistance. A great deal of information on the wear properties and methods for selection for steels can be found in the ASM metals handbook 1131.

Pearlitic steels are hardened by heat treatment. By heating the part to the austenitizing temperature and quenching, a microstructure is developed containing lamellar pearlite composed o f plates o f iron carbides (cementite) sandwiching iron or ferrite. Free carbides may also be present. The resulting microstructure is about five times harder than the hardness of the original austenite. Wear resistance increases with increasing amount of pearlite. Increased carbon content raises the percentage of carbides and pearlite in the structure as well as the hardness and wear resistance. Increased cooling rate reduces the carbide lamellar spacing and increases the resistance to abrasion. Reducing the area o f ferrite in the structure reduces the chances of adhesion and galling. In addition, the increased hardness provides better support for oxide films which also inhibit wear. The wear process produces extreme plastic deformation in the near surface region of steel. The extent of the plastic flow will, to some extent, govern the amount of wear. The pearlitic structure will tend to minimize the depth of heavy deformation. As the structure is subjected to contact stress and shear, the average spacing in the lamellar structure is reduced by deformation processes and essentially increases the resistance to penetration. Toughness - Although wear resistance increases monotonically with hardness, there is a hardness level above which the metal toughness decreases to a point where microfracture in the surface becomes part of the wear process and wear increases. A chart showing the effect of toughness on wear resistance i s shown in figure 1.4.

19

A

Glass

0 Tool Steel 0

7-

Gray Cast Iron

Nodular Cast Iron

6-

-

0

5-

I

-

I

9

-3 4-

I

u a 8

-e

atm

c

0

ul

i

01

a

:2- I P f g I.

0

I

3-

0

a 0

I

1

F i g u r e 1.4

E f f e c t o f M a t e r i a l Toughness on Wear Resistance [14]

20

H a r d e n a b i l i t y - There i s some c o n f u s i o n between t h e terms hardness and h a r d e n a b i l i t y . Hardness i s t h e r e s i s t a n c e t o p e n e t r a t i o n o r s c r a t c h i n g on a g i v e n l o c a t i o n on a p a r t . H a r d e n a b i l i t y i s t h e d e p t h t o which a s t e e l p a r t can be hardened t o a s e l e c t e d hardness l e v e l f o r a g i v e n c o o l i n g r a t e d u r i n g h e a t t r e a t i n g . Thus, i f a one i n c h diameter s t e e l s h a f t must have a hardness o f Rc 50 h a l f way i n t o t h e c e n t e r , an a l l o y must be chosen which w i l l have t h e h a r d e n a b i l i t y r e q u i r e d t o produce t h a t hardness p a t t e r n w i t h a reasonable quench r a t e . Hardness i s c o n t r o l l e d b y quench r a t e . Quench r a t e depends on t h e r a t e o f d i f f u s i o n o f heat i n t o a b l o c k o f m e t a l . That i s , t h e h i g h e s t quench r a t e i s a t t h e s u r f a c e o f t h e p a r t . The f u r t h e r one goes i n t o t h e p a r t , t h e l o w e r t h e quench r a t e . A t some depth, one reaches a p o i n t where no hardening occurs. Thus, i n s e l e c t i n g a s t e e l and i t s heat t r e a t m e n t , one must c o n s i d e r t h e t h i c k n e s s o f t h e s e c t i o n and t h e depth t o which a c e r t a i n hardness l e v e l must be maintained. For instance, i n a s t e e l gear, a h i g h hardness i s r e q u i r e d on t h e s u r f a c e t o r e s i s t s c u f f i n g . High hardness i s a l s o r e q u i r e d below t h e s u r f a c e t o t h e d e p t h a t which t h e maximum shear s t r e s s o c c u r s t o r e s i s t contact fatigue. The t o o t h must a l s o have a g i v e n c o r e s t r e n g t h and f r a c t u r e toughness l e v e l t o p r e v e n t bending f a i l u r e . The s e l e c t i o n o f t h e p r o p e r a l l o y must n o t o n l y c o n s i d e r maximum hardness and toughness a c h i e v a b l e b u t a l s o t h e h a r d e n a b i l i t y and i t s e f f e c t on t h e hardness g r a d i e n t t h r o u g h t h e t o o t h s e c t i o n . A g r i n d i n g b a l l f o r b a l l m i l l i n g can l o s e c o n s i d e r a b l e p a r t o f i t s volume t o wear and s t i l l be a usable p a r t . Therefore, t h r o u g h hardness o r h i g h hardness t o t h e c e n t e r o f t h e b a l l i s r e q u i r e d f o r an economic r e s u l t . The h a r d e n a b i l i t y o f t h e a l l o y s e l e c t e d f o r t h e b a l l , then, w i l l be an e s s e n t i a l c o n s i d e r a t i o n and w i l l be d i f f e r e n t f r o m t h a t f o r t h e gear.

21

Required: 45 HRC at 3/4 radius for a 44 mm diameter shaft, oil quenched 120

E

100

E

L -

c1I

80

n L 0

60 4 0-

20 0 4

0

12

8

16

20

Distance from quenched end, 1/16 in. 70 I

6 5 ....................

i ...................~....................

..... .............;.....................

....................

................... j................... j ....................

...................

30 0

4

8

12

16

Results: Minimum hardenability o f 4140H produces 49 HRC

Figure 1.5 Example o f Use o f Hardenability Charts for 4140H Steel

20

22

H a r d e n a b i l i t y d a t a f o r s t e e l s can be found i n t h e f o r m o f end quench and e q u i v a l e n t c o o l i n g r a t e c h a r t s . An example i s shown i n f i g u r e 1.5 [15]. ( F o r more d e t a i l s on h a r d e n a b i l i t y and h a r d e n a b i l i t y curves see ASM M e t a l s Handbook, n i n t h e d i t i o n , 1978, pgs 471-525). I n f i g u r e 1.5, an A I S I 4140H s t e e l w i t h a carbon c o n t e n t o f 0.38% i s r e q u i r e d t o have a hardness o f a t l e a s t 48RC a t t h e t h r e e - q u a r t e r - r a d i u s o f a 1.75 i n c h b a r when quenched i n o i l . E n t e r i n g t h e e q u i v a l e n t c o o l i n g r a t e c h a r t a t 1.75 in.(44.5mm) diameter and f i n d i n g t h e e q u i v a l e n t d i s t a n c e from t h e quenched end o f a t e s t standard b a r (0.4 i n ) , one e n t e r s t h e h a r d e n a b i l i t y c h a r t a t 0.4 inches f r o m t h e quenched end and f i n d s a hardness v a l u e o f 49 Rc. Thus t h e a l l o y s e l e c t e d i s satisfactory.

Martensitic Steels: The m a r t e n s i t i c s t r u c t u r e i n s t e e l s , produced b y heat t r e a t i n g ( u s u a l l y b y a r a p i d quench) i s t h e most wear r e s i s t a n t s t r u c t u r e i n carbon s t e e l s . For t h e same hardness, p e a r l i t i c s t e e l s w i l l n o t r e s i s t a b r a s i v e wear as w e l l as m a r t e n s i t i c s t e e l s . B a i n i t e , a s t r u c t u r e s i m i l a r t o m a r t e n s i t e and achieved w i t h slower quench r a t e s has about t h e same wear r e s i s t a n c e as m a r t e n s i t e , b u t a g r e a t e r toughness. High carbon m a r t e n s i t e i s u s u a l l y tempered a t about 230 C f o r s t r e s s r e l i e f and an i n c r e a s e i n f r a c t u r e toughness and has t h e b e s t wear r e s i s t a n c e o f any o f t h e s t e e l s t r u c t u r e s . I f m a r t e n s i t i c s t e e l must be tempered below 50 HRC (as, f o r i n s t a n c e t o p r e v e n t c h i p p i n g under impact l o a d s ) , i t i s i n f e r i o r t o p e a r l i t i c s t e e l a t t h e same hardness. M a r t e n s i t e i s a metastable s u p e r s a t u r a t e d s o l u t i o n o f carbon i n i r o n . T h i s produces a v e r y f i n e homogeneous needle-1 i k e c r y s t a l s t r u c t u r e which w i l l show no p r e f e r e n t i a l wear o f a second phase. However, m a r t e n s i t e w i l l t e n d t o decompose and produce c a r b i d e s when heated. Therefore, i t i s n o t recommended f o r temperatures much above 200°C. T h i s i n c l u d e s s u r f a c e h e a t i n g f r o m f r i c t i o n . High carbon m a r t e n s i t i c s t e e l s w i l l o f t e n c o n t a i n some r e t a i n e d a u s t e n i t e f r o m t h e heat t r e a t m e n t . As l o n g as t h e percentage o f r e t a i n e d a u s t e n i t e i s low, i t can be b e n e f i c i a l i n p r o v i d i n g some toughening and under heavy a b r a s i o n w i l l t r a n s f o r m t o untempered m a r t e n s i t e and w i l l be more r e s i s t a n t t o wear. I n r o l l i n g c o n t a c t bearings, and i n c l o s e f i t t i n g p a r t s , r e t a i n e d a u s t e n i t e can be d e t r i m e n t a l because i t w i l l cause a s w e l l i n g o f t h e p a r t when i t transforms, r e s u l t i n g i n b i n d i n g o f small c l e a r a n c e b e a r i n g s .

23

Austenite and Ferrite: The addition of sufficient manganese to steels will stabilize austenite at room temperature. If the austenite is stabilized sufficiently so that it does not transform even under plastic deformation, the steel will show better wear resistance than ferritic steels with the same carbon content. Manganese i s used in tough impact resistant steels for mining and dirt moving machinery to increase abrasion resistance. Martensite is formed during impacting and will add resistance to gouging with little reduction in toughness. To summarize: the relative wear resistance of various steel microstructures, an increase in wear resistance is noted as one goes from pearlite through austenite t o martensite. This trend is shown graphically in figure 1.6 - a plot of gouging wear ratio vs carbon content. Note that the highest carbon iron-base alloys are the high chromium white irons. The chart shows little increase in gouging wear resistance for these alloys - however - they have an important role in certain abrasive wear conditions as is described in chapter 4. (Cast Irons)

0

1.o

2.0

3.0

4.0

Carbon Content, 96

Figure 1.6 Gouging Wear Ratio as a Function of Carbon Content and Microstructure [16]

24

Stainless Steels There a r e t h r e e b a s i c t y p e s o f s t a i n l e s s s t e e l used i n wear c o n t r o l : A u s t e n i t i c o r 18-8 s t a i n l e s s s t e e l Martensitic stainless steel PH s t a i n l e s s s t e e l

The mechanical and p h y s i c a l p r o p e r t i e s and chemical c o m p o s i t i o n o f a s e l e c t e d group o f s t a i n l e s s s t e e l s a r e found i n Table 1.2 a,b, & c .

Table 1.2a

Mechanical P r o p e r t i e s o f S t a i n l e s s S t e e l s

MATERIAL

SS SS SS SS SS SS SS

ss SS SS SS SS SS SS SS SS SS

15-5PH 16-25-6 17-4PH 304 304HN 316 316 347 410 410 440C 440C 440CM ALLEGHENY A-286 NITRONIC 60 UHB AEB-L UNILOY 19-9DL

Conversion f a c t o r s : MPa x 0.145 = k s i

CONDITION

HEAT TREAT

WROUGHT H900 HOT ROLLED Hardened WROUGHT H900 ANNEALED

HOT WORK

Bar

Annealed Cold R o l l ANNEALED HT, 1000 t e Annealed Annealed HT, 600F t e F u l l y aged Annealed Hot R o l l ed

HARDNESS DPH

TS MPa

1379 1103 1379

MODULUS MPa

420 320 400 150 200 150 200 150 257 135 257 650

1.96Et05 1.97E+05 1.96Et05 1.97Et05 2.00Et05 1.96Et05 1.96E+05 1.93E+05 1.96E+05 1.96Et05 2.00Et05 2.00Et05 2.00E+05

689 586 758 620 758 517 862 1379

330 200 310 217

1000 717 2.00Et05 689 2.07Et05 751

Table 1.2b P h y s i c a l P r o p e r t i e s o f S t a i n l e s s S t e e l s STAINLESS GRADE SS SS SS SS SS SS

ss

SS SS SS SS SS SS SS

MAX TEMP DENSITY OC Kg/cu METER

15-5PH 16-25-6 17-4PH 304 304HN 316 347 410 440C 440CM ALLEGHENY A-286 NITRONIC 60 UHB AEB-L UNILOY 19-9DL

500 649 500 900 700 649 815 649 316 760 760

7.75€+03 8.03€+03 7.75E+03 8.03E+03 8.03E+03 8.03E+03 7.89E+03 7.68E+03 7.68E+03 7.68E+03 7.90E+03 7.66E+03 7.75E+03 7.92E+03

CONDUCT. WATT/m/K

EXPANS. m/m/oC

16.20 17.0 24.90 29.41

1.12E-05 1.60E-05 1.12E-05 1.74E-05 1.91E-05 1.12E-05 1.87E-05 1.17E-05 1.02E-05

1427 1427 1427 1482 1538

20.0

1.78E-05

1427

28.35

1.10E-05

649

17.82 17.8 16.20

Conversion f a c t o r s : kg/m

3

W/m/K

x 3.613E-5 x 0.578

=

=

l b / c u i n : kg/m

x 0.001

BTU/sqft/ft/hr/'F

J/kg/K

x 2.388E-4 =BTU/lb/'F

m/m/"C

x 0.55

=

3

in/in/'F

o r cal/g/-C

M.P. C

=

gm/cc (SG)

RESIST. ohm-cm

HEAT CAP. J/kg/K

77.00

4.2E+02

77.00

4.2€+02

74.00 72.00 57.00 60.00

4.2E+02 4.2E+02 4.2E+02 4.2E+02 4.2E+02

98.0 98.2 58

4.2E+02 4.2E+02

26

A u s t e n i t i c s t a i n l e s s s t e e l : This class of steels is used to a great extent for

its superior corrosion resistance. Having the most effective balance of carbon, chromium and nickel for corrosion resistance, these steels resist many chemical reagents, sea water, 1 iquid metals, rusting and reactor grade 316'C (600OF) water. These are the 300 series steels ( AISI 304, 316, etc) and contain about 18% chromium, 8% nickel and 0.1 carbon. They are notorious for their tendency for adhesion and galling. Even though they quickly develop a tough passivating oxide film when exposed to air, their tendency for low work hardening rates causes easy rupture of the oxide and virgin metal contact. Austenitic stainless steels cannot be hardened by heat treatment and therefore have no easy means for improving wear resistance. Two series of austenitic stainless steels are available: 200 series and 300 series. The 200 series have a portion of the nickel replaced by manganese or nitrogen. The 200 series stainless steels respond well to work hardening - achieving maximum values in the stainless alloys. However, 200 series stainless steels are not used as much as other grades in tribological applications. Austenitic stainless steels are used for high temperature, aggressively corrosive conditions and nuclear reactor appl ications for the most part. These steels resist corrosion in 316'C reactor water and in chemical processing plants operating at temperatures up to 900 C. The chromium - nickel combination produces an oxide in oxidizing conditions that is very slow growing and tough. As long as this oxide is not disrupted, corrosion is insignificant over long periods of exposure. However, this oxide provides little protection from wear. Since the 300 series stainless steels have relatively low carbon content for optimum static corrosion resistance, they do not have the carbides present in wear resistant steels. In addition, 300 stainless steels are highly susceptible to adhesion and galling. There is little wear data in the literature for austenitic stainless steels. Most of the data is found in reactor handbooks. ASM metals handbook shows the gouging wear resistance of AISI 316 stainless and it has the lowest resistance of all other steels tested. The tendency for austenitic stainless steels to gall and seize - especially when self-mated presents a serious problem when they are needed in an aggressive environment and sl iding contact is involved. Schumacker has attributed high nickel content in 300 series stainless alloys to sensitivity to galling [17,18]. Bhansali [19] has suggested that the high nickel content of 300 series stainless steels increases the stacking fault energy of the alloy. Stacking fault energy has an influence on the deformation mode of metals

27

High s t a c k i n g f a u l t energy a l l o w s h e a v i e r d e f o r m a t i o n i n a metal t h a n does low s t a c k i n g f a u l t energy - f o r a g i v e n shear s t r e s s l e v e l . B h a n s a l i concludes t h a t s t a c k i n g f a u l t energy can i n f l u e n c e g a l l i n g o r adhesion t e n d e n c i e s b u t n o t general wear p r o p e r t i e s o f an a l l o y . T h i s i s t r u e because t h e s u r f a c e condition ( n a t i v e oxide, l u b r i c a n t , contamination) i s a strong m o d i f i e r o f t h e wear p r o p e r t i e s o f a m a t e r i a l . However, i f t h e c o n t a c t s t r e s s i s such t h a t t h e d e p t h o f p l a s t i c d e f o r m a t i o n i s l a r g e , near s u r f a c e p l a s t i c f l o w can cause f o l d i n g o f t h e s u r f a c e l a y e r s i n t o t h e subsurface r e g i o n , exposing f r e s h metal t o 1 i k e l y adhesion and g a l l i n g . However,the near s u r f a c e d e f o r m a t i o n o c c u r r i n g w i t h wear i n v o l v e s such h i g h l e v e l s o f s t r a i n , t h a t t h e d e f o r m a t i o n modes a s s o c i a t e d w i t h s t a c k i n g f a u l t energy l e v e l s a r e o f t e n l o s t i n h i g h s t r a i n modes. I t i s t r u e , however, t h a t n i c k e l c o n t e n t does seem t o i n f l u e n c e t h e tendency f o r g a l l i n g i n stainless steels. Carbon c o n t e n t and n i c k e l c o n t e n t have s i g n i f i c a n t i n f l u e n c e on t h e work hardening c o e f f i c i e n t o f s t a i n l e s s s t e e l s . F o r 18Cr-8Ni s t e e l s , carbon c o n t e n t below 0.06% r e s u l t s i n a drop i n work hardening c o e f f i c i e n t [20]. T h i s i s shown i n f i g u r e 1.7. The e f f e c t o f n i c k e l c o n t e n t on work hardening exponent i s shown i n f i g u r e 1.8. Note t h a t g o i n g from 8% t o 12% r e s u l t s i n a s i g n i f i c a n t drop i n t h e work hardening exponent. T h i s i s f u r t h e r evidence t h a t 316 s t a i n l e s s should have p o o r g a l l i n g r e s i s t a n c e .

F i g u r e 1.7 E f f e c t o f Carbon Content on Work Hardening F a c t o r i n S t a i n l e s s S t e e l [21]

28

Table 1 . 2 ~Chemical Composition o f S t a i n l e s s S t e e l s

MATERIAL

CHEMICAL

COMPOSITION

SS 15-5PH C .07,Mn 1.0,P .04,S .03,Si l , C r 14.5,Ni 4,Cu 3,Cb .3,Fe 76 SS 16-25-6 C .12,Mn 2,Si l , C r 16,Ni 25,Mo 6,Fe 49.9 SS 17-4PH C .05,Cr 16.5,Ni 4.0,Cu 4.0,Fe 75.5 SS 304 C .08,Mn 2,Si l , C r 18,Ni 8,Fe 71 SS 304HN C .08,Mn 2 , P .045,S .03,Si l , C r 18,Ni 8,N .2,Fe 70.6 SS 316 C . l , C r 18,Ni 14,Mo 3,Fe 65 ss 347 C .08,Cr 18,Ni l l , T i . 8 Fe 70 SS 410 C .15,Mn 1.0,Si .5,CR 1 2 , Fe 86.5 SS 440C C 1,Mn 1.25,Si l , P .04,S .04,Cr 18,Mo .75,Fe 78 SS 440CM C .08,Mn 1 , S i .6,Cr 15,Ni 26,Mo l , T i 2,Va .3 Fe 54 SS ALLEGHENY A-286 C l,Mn 1 , S i 1,Cr 15,Mo 4,Va .15,Fe 87.25 SS NITRONIC 60 C .1,Mn 8,Si 4,Cr 17,Ni l , N 1,Fe 63 SS UHB AEB-L C .68,Mn .6,Si .38,Cr 13.2,Fe 85.2 SS UNILOY 19-9DL C .4,Mn 1 , S i .5,Cr 19,Ni 9,Mo 1.5,W 1.5, CbtTa .4,Fe 67

I55

145

e

2

155

v <

-

C-.05-.07

.SI-.bl .51-.bS

19.60-10.11.021-.01b

I15

z

2

115

I L

g

I05

0

0”

95

U

I5

75

b

8

10

I1

I4

16

In

10

N I C K E L ,*I.

F i g u r e 1.8 E f f e c t o f N i c k e l Content on Work Hardening F a c t o r f o r S t a i n l e s s S t e e l [22]

29

Resistance to galling of austenitic stainless steels has been improved by the substitution of nitrogen for nickel in the alloys - without detrimental effect on corrosion properties. Among the nitronic series of nitrogen containing steels, the best performance has been attained by Nitronic 60, containing 0.13 % nitrogen and relatively high silicon content ( 4 % ) . This alloy is listed in Table 1.2, the stainless steel properties table. Recent advances in nitrogen alloying in austenitic stainless steels has produced new alloys containing considerably more nitrogen with increased tensile and yield properties [23]. By melting alloys under nitrogen pressure in a hot isostatic pressure furnace and holding for an hour followed by reduction to room temperature, still maintaining the pressure, successful nitrogen alloying was achieved. Using gas pressures of 200 MPa (2000 atm) nitrogen contents as high as 4.0 % were acheived. A CrN phase forms in the high nitrogen containing stainless steels Martensitic stainless steel: Martensitic stainless steels are those containing more than 11.5% chromium, low nickel content (see table 1 . 2 ~ )and can be hardened by heat treatment. The compositions of typical martensitic stainless steels are shown in Table 1.2, as 400 series. Because of their martensitic structure when heat treated, these stainless steels are much more resistant to galling than the 300 series. However, the 400 series are not as corrosion resistant. Figure 1.9 shows the effect of chromium content in iron on corrosion rate in dilute nitric acid. Note that the difference between 15% and 18% chromium corresponds t o a change of more than one order of magnitude in corrosion rate. Thus, the low chromium martensitic steels like 410 are also low in corrosion resistance - although they are better than carbon steel. Ordinary atmospheric conditions without heavy industrial contamination, even at high humidity will not cause rusting of 400 series stainless steels. In marine and industrial environments, however, the 400 series stainless steels are not as resistant as the 300 series. The martensitic stainless steels are also susceptible to stress corrosion cracking when used in the heat treated condition [24].The hardenable grade, 440C is used in many wear applications, including rolling contact bearings - discussed below under "bearing Steels". Nickel, together with chromium, enhances the corrosion resistance of stainless steels. The 18 Cr-8 Ni content of austenitic stainless steel is an optimum for corrosion resistance. Austenitic stainless steels are preferred over martensitic steels for heat exchanger tubing in water cooled nuclear reactors and for ordinary boiler heat exchangers. Austenitic stainless steels are used in liquid sodium and NaK heat exchangers also.

30

1.000

I00

10

I

Chromium Content of Alloy, Wt %

Figure 1.9 Corrosion Resistance of Stainless Steels in Dilute Sulfuric and Nitric Acids as a Function of Chromium Content [25]

Addition of chromium to carbon steel increases its hardenablity. It tends to retard the formation of ferrite and pearlite so that the martensite structure can be achieved with a much slower cooling rate than would be required for carbon steels and many alloy steels. Thus, most martensitic stainless steels can be hardened by air quenching and distortion of parts can be considerably reduced.

31

Optimum mechanical properties are achieved in martensitic steels by heating to a temperature range which will result in a maximum amount of austenite before quenching. The temperature range is 980 - 1090'C (1800 - 2000°F). Proper tempering must follow quenching. Figure 1.10 shows the effect of tempering temperature on hardness of 12% Cr stainless steel. Note that the steels do not soften significantly until a temperature of 480'C (900'F) i s reached. The rise in the hardness after the softening range is due to secondary hardening - the result of carbide formation. If secondary hardening is accomplished, the corrosion resistance of the alloy is reduced, because the chromium in solution has been reduced by combining with carbon to form carbides.

Figure 1.10 Effect of Tempering Temperature on Hardness of 12% Cr Stainless Steel Tempered for 2 Hr [26]

32

Precipitation Hardening Stainless Steels: This class of stainless steels is used in high temperature and corrosive environments requiring wear resistance. Applications include pump casings and impellers to combat erosion-corrosion and shafting for graphite journal bearings in hot water pumps used in nuclear reactors. Precipitation hardened (PH) grades are selected for optimum corrosion resistance together with high hardness and tensile properties. These grades are superior to the 400 series for these combined properties. There are a range of grades of PH stainless steels available including 18Cr-8Ni compositions. Typical grades are shown in Table 1.2. Grade 17-4 PH is martensitic at room temperature and attains full strength by aging by precipitation of a copper rich phase. The sister alloy, 17-7 PH is austenitic in the annealed condition and therefore, is readily cold formed. It is hardened by transforming to martensite which is then aged by precipitating an aluminum rich phase. Precipitation hardening alloys require special heat treating procedures beyond the scope of this text. Heat treating texts or specialists should be consulted if the exact procedures are of concern. There are a series of 18-8stainless steels containing molybdenum which can be precipitation hardened. These are the PH-55A series. These alloys have been developed to provide combined corrosion, abrasion and erosion resistance. They also will resist excessive oxidation at elevated temperature (538 - 760'C). Precipitation produces a finely dispersed sigma phase in a matrix of austenite and dispersed ferrite. Welding produces carbide precipitation in the dispersed ferrite and thus avoids development of a detrimental carbide network. The PH-55A series show high resistance to stress corrosion. General corrosion resistance is similar to 18-8stainless materials because very little of the corrosion inhibiting elements are used in the hardening process. One important use for this series of alloys i s for erosion-corrosion resistance in mixed acid slurries used in the manufacture of phosphoric acid 1271.

33

Tool Steels Tool s t e e l s were developed t o r e s i s t wear a t f o r m i n g and c u t t i n g temperatures. Therefore, t o o l s t e e l s have been used i n a number o f a p p l i c a t i o n s r e q u i r i n g wear r e s i s t a n c e and h o t hardness. There a r e several t y p e s o f t o o l s t e e l used f o r t r i b o l o g i c a l a p p l i c a t i o n s : [28]

O i l hardening c o l d work t o o l s t e e l s ........... 01 - 07 High carbon High chromium c o l d work ...........D2 - 07 H11 - H16 Chromium h o t work ............................ T1 - T15 Tungsten h i g h speed ........................... M1 - M50 Molybdenum h i g h speed .........................

Table 1.3a

Mechanical P r o p e r t i e s o f Tool S t e e l s

MATERIAL

TOOL TOOL TOOL TOOL TOOL TOOL TOOL TOOL TOOL

STEEL STEEL STEEL STEEL STEEL STEEL STEEL STEEL STEEL

FORM

CPM 1OV D2 H11 M2 M50 M50 01 01 T15

Conversion f a c t o r s : MPa x 0.145 = k s i

CPM

TEMPER HEAT TREAT

HARDNESS VICKERS DPH

HT, 975 t e HT,600F t e HT,500F t e HT,300F t e HT,600F t e Annealed HT,600F t e Annealed HT,1100 t e

763 590 598 765 760 160 577 185 83 2

TENSILE YOUNGS STRENGTH MODULUS (MPa) MPa

1930 2068 2758 2758 683 1930 586

2.20Et05 2.07Et05 2.07Et05 2.07Et05 2.03Et05 2.00Et05 2.13Et05 2.13Et05 2.07Et05

34

P h y s i c a l P r o p e r t i e s o f Tool S t e e l s

Table 1.3b

MAX OP TEMP

MATERIAL

e

TOOL TOOL TOOL TOOL TOOL TOOL TOOL

STEEL STEEL STEEL STEEL STEEL STEEL STEEL

CPM 1OV D2 H11 M2 M50 01 T15

DENSITY THERM CON kg/ WATT/m CU METER K

538 538 427 150 590

7.47Et03 7.75Et03 7.75Et03 8.03Et03 7.75Et03 7.85Et03 8.19Et03

THERM EXP

m/m

/e

HEAT CAPACITY J/kg/K

1.19E-05 29.41 38.06

1.10E-05 1.01E-05 1.08E-05 1.08E-05 1.10E-05

4.2Et2 5.OEt2

Conversion f a c t o r s : kg/m

3

x 3.613E-5

W/m/K x 0.578 J/kg/K

=

=

l b / c u i n : kg/m

3

x 0.001

=

gm/cc ( S G )

BTU/ft/hr/f

x 2.388E-4 =BTU/lb/p

o r cal/g/C

Table 1 . 3 ~ Chemical Composition o f Tool S t e e l s

MATER IA L

TOOL TOOL TOOL TOOL TOOL TOOL TOOL

STEEL STEEL STEEL STEEL STEEL STEEL STEEL

CHEMICAL COMPOSITION

CPM 1OV C 2.45,Mn .5,S .07,Si .90,Cr 5.25,Va 9.75,Mo 1.3,Fe 79.8 C 1.5,Si .4,Mn .3,Cr 12,Va .4,Mo 1,Fe 96.5 D2 H11 C .35,Cr 5,MO 1.5,Va .4,Si .9,Mn .3,Fe 91.6 M2 C .8,W 6,Mo 5,Cr 4,Va 2,Fe 82.2 C .8,Mn .25,Si .25,P .015,S .015,Cr 4,Ni .1,Va 1,Mo 4.5Fe89 M50 01 C .9,Mn 1,Cr .5,W .5,Fe 97 C 1.5,Cr 4,W 12,Va 5,CO 5,Fe 72.5 T15

35

Cold Work Steels: These steels can be used at room temperature and provide

high wear resistance for low cost. They are high carbon steels with tungsten, manganese and chromium. The 0 series can be oil quenched while the D series must be air quenched. The D series have high chromium and carbon and are deep hardening with hardenabil ity accentuated by small amounts of tungsten and molybdenum. Maximum operating temperature for these steels is 370 - 430'F (188 - 223'C). Chromium Hot Work Steels: These steels contain medium chromium, tungsten and

vanadium. The relatively low carbon and total alloy content provides a tough material with hot hardness capability. H11 steel is found in supersonic airframes where combined wear resistance and toughness is required for highly stressed parts. This material resists softening for prolonged periods of temperatures in the vicinity of 540'C. High Speed Tool Steels: These tool steels were designed for machining at high removal rates o f work material. Steel with high hot hardness are necessary to

combat the effects of frictional heating at high cutting speeds. The T series and the M series fit this requirement. Their composition and properties can be found in Table 1.3. The T series or tungsten high speed steels have tungsten, molybdenum, chromium, vanadium and cobalt with the highest level of tungsten. Carbon content ranges between 0.70 and 1.5. These steels can be hardened to over Rockwell C 65 and T I 5 is considered the most wear resistant of this series but poor in toughness. The M series or molybdenum high speed steels have as principal elements molybdenum, tungsten, chromium, vanadium and cobalt with molybdenum the main element, ranging from 4 to 9 % Mo. Their carbon content ranges from 0.80 to 1.5. These grades are less expensive than the T series and have somewhat higher toughness. The wear properties of some tools steels are compared in figure 1.11 [29]. Note that the highest carbon content tool steel possesses the lowest wear rate.

36

0.025

16

m

.. Y

0.020

14

a 2 0.015

12

N ,

t

m L

-

M2 M2S

0

M7 M2 M1O - M 4 1 M2 M 2s

. -

10 N c ._

8

-6 -4 T15 - 2

CI1

0 7

0

F i g u r e 1.11 Wear Rates f o r Various Grades o f Tool S t e e l [30]

Table 1.4a Mechanical P r o p e r t i e s o f R o l l i n g Contact Bearing S t e e l s MATERIAL

4320 4620 4820 50100 51100 52100 52100 52100 MOD 8620 9310 SS 440C SS 440C SS 440CM TOOL STEEL M2 TOOL STEEL M50 TOOL STEEL M50 Conversion f a c t o r s : MPa x 0.145 = k s i

FORM

TEMPER

HARDNESS VICKERS DPH

Annealed CARBURIZED HT. 300F t e CARBURIZED HT, 300F t e Annealed Annealed HT, 9OOF t e Annealed Annealed CARBURIZED HT,300F t e CARBURIZED HT,300F t e HT,600F t e Annealed HT,300F t e HT,600F t e Annealed

TENSILE STRENGTH MPa

225 740 690 225 200 413 200 200 789 694 650 257

752 827 1379 689 689 1379 689 689 1296 1241 1379 862

765 760 160

2758 2758 683

YOUNGS MODULUS MPa 2.07E+05 2.07E+05 2.07E+05 2.07E+05 2.07E+05 2.00E+05 2.00E+05 2.07E+05 2.07E+05 2.07E+05 2.00Et05 2.00E+05 2.00Et05 2.07E+05 2.03E+05 2.00Et05

37

Tab1 e 1.4b

P h y s i c a l P r o p e r t i e s o f R o l l i n g Contact B e a r i n g S t e e l s

MATERIAL

MAX OP TEMP O C

4320 4820 50100 51100 52100 52100 MOD 8620 9310 SS 440C SS 440CM TOOL STEEL M2 TOOL STEEL M50

260 260 260 260

316 538 427

THERM EXP m/m/ 'C

RESIS micro ohm-cm

HEAT CAPACITY J/kg/K

38.06 45.00 43.25 43.25 43.25 43.25 37.50 46.71 29.41

1.13E-05 1.55E-05 1.24E-05 1.24E-05 1.24E-05 1.24E-05 1.48E-05 1.46E-05 1.08E-05

30.00 31.00

4.6E+2

30.00 20.00 60.00

4.6E+2 4.4E+2 4.2E+2

38.06

1.01E-05 1.08E-05

DENSITY

THERM CON WATT/m k9/ CU METER K

7.85Et03 7.86Et03 6.37E+03 6.37E+03 6.37E+03 6.37E+03 7.75E+03 7.75E+03 7.47E+03 7.47E+03 8.03Et03 7.75E+03

Conversion f a c t o r s : kg/m 3 x 3.613E-5 = l b / c u i n : kg/m 3 x 0.001 W/m/K x 0.578 = BTU/sqft/ft/hr/'F J/kg/K x 2.388E-4 = BTU/lb/'F o r cal/g/'C m/m/"C x 0.55 = in/in/'F

=

4.2E+2

gm/cc (SG)

Bearing Steels R o l l i n g c o n t a c t b e a r i n g s ( b a l l and r o l l e r b e a r i n g s ) a r e made o f a l i m i t e d number o f s t e e l grades. The p r o p e r t i e s o f these s t e e l s a r e shown i n Table 1.4 a,b. Chemical compositions a r e shown i n Table 1 . 4 ~R o l l i n g c o n t a c t b e a r i n g s have some s t r i n g e n t requirements f o r m a t e r i a l p r o p e r t i e s i n c l u d i n g h i g h dimensional s t a b i l i t y , h i g h hardness (Rc 58 - 64), h i g h f i n i s h c a p a b i l i t y and h i g h clean1 iness. The l a s t requirement i s i m p o r t a n t t o promote l o n g f a t i g u e l i f e s i n c e r o l l i n g c o n t a c t b e a r i n g s can f a i l f r o m c o n t a c t f a t i g u e . Hard i n c l u s i o n s a c t as s t r e s s r a i s e r s and s i t e s f o r t h e i n i t i a t i o n o f f a t i g u e s p a l l s . Low i n c l u s i o n c o n t e n t d e r i v e s f r o m s p e c i a l m e l t i n g processes i n p r o d u c i n g an i n g o t . C u r r e n t l y , t h e p r a c t i c e i s t o m e l t b y e l e c t r i c f u r n a c e u s i n g carbon d e o x i d i z i n g and vacuum degassing. Other p r a c t i c e s i n c l u d e consumable e l e c t r o d e vacuum m e l t i n g [31]. Heat t r e a t i n g i s designed t o e l i m i n a t e as much r e t a i n e d a u s t e n i t e as p o s s i b l e t o ensure dimensional s t a b i l i t y . Quenching i s f o l l o w e d immediately b y c o o l i n g t o subzero temperatures p r i o r t o tempering.

38

Table 1.4 c Chemical Composition o f Bearing S t e e l s COMPOSITION

MATERIAL

4320 4620 4820 50100 51100 52 100 52100 MOD 8620 9310 9310 SS 440C SS 440CM TOOL STEEL M2 TOOL STEEL M50

C .2,Mn .55,Si .25,Ni 1.7,Cr .5,Mo .25 C .2,Mn .55,Si .3,Ni 1.7,Mo .25 C .2,Mn .6,P .035,S .04,Si .3,Ni 3.5,Mo .25,Fe 95 C l,Mn .3,Si .2,Cr 1,Fe 97.5 C l,Mn .3,Si .2,Cr .5,Fe 98.1 C l,Mn .5,P .025,S .025,Si .3,Cr 1.5,Fe 96.5 C l,Mn l , S i .6,Cr 1,Fe 96.4 C .2,Mn .8,Si .2,Cr .5,Ni .6,Mo .2,Fe 97.5 C .1,Mn .5,Si .2,Cr 1.2,Ni 3.2,Mo 1,Fe 94.2 C .1,Mn .5,Si .2,Cr 1.2,Ni 3.2,Mo 1,Fe 94.2 C l,Mn 1.25,Si l , P .04,S .04,Cr 18,Mo .75,Fe 78 C .08,Mn l , S i .6,Cr 15,Ni 26,Mo l , T i 2,Va .3 Fe 54 C .8,W 6,Mo 5,Cr 4,Va 2,Fe 82.2 C .8,Mn .25,Si .25,P .015,S .015,Cr 4,Ni .1,Va 1,Mo 4.5Fe89

A I S I 52100 s t e e l i s a t h r o u g h hardening s t e e l . I t can be t h r o u g h hardened i n s e c t i o n s up t o 13mm (1/2 i n c h ) t h i c k . For l a r g e r s e c t i o n s , a m o d i f i e d 52100 s t e e l i s used. Several grades a r e g i v e n i n ASTM standard A 485-63. H a r d e n a b i l i t y o f t h e 52100 a l l o y s t e e l s can be increased b y i n c r e a s i n g Mn c o n t e n t . Two m o d i f i e d a l l o y s a r e shown i n t a b l e 1.5 [32]. I f adequate protection against oxidation p r o v i i e d , A I S I 52100 s t e e l b e a r i n g s can be used a t temperatures up t o 200 C (400 F ) .

is

39

Table 1.5. AISI 52100 M o d i f i c a t i o n s f o r Thick S e c t i o n s

S e c t i o n Thickness 13 mm - 19mm (1/2" - 3/4")

C Mn Si Cr Mo

0.9 - 1.05 0.95 - 1.25 0.45 - 0.75 0.9 - 1.2 0.06

Over 19 mm

0.85 - 1.0 1.4 - 1.7 0.5 - 0.8 1.4 - 1 . 8 0.06

For r e s i s t a n c e t o normal atmospheric c o r r o s i o n , s t a i n l e s s 440C s t e e l i s used f o r b a l l bearings. I t w i l l n o t r e s i s t s a l t w a t e r c o r r o s i o n , however. 440C i s t h e usual c h o i c e f o r p r e c i s i o n i n s t r u m e n t and g y r o b e a r i n g s , where even small amounts o f c o r r o s i o n i n t h e b a l l paths can degrade t h e i r performance. 440C s t a i n l e s s s t e e l can be used a t temperatures up t o 200'C (400'F) i n o x i d i z i n g c o n d i t i o n s . T h i s m a r t e n s i t i c s t a i n l e s s can be hardened t o about Rc 58 and does n o t have t h e c o n t a c t f a t i g u e s t r e n g t h t h a t AISI 52100 has. 440C s t a i n l e s s s t e e l a l s o has a l e s s d e s i r a b l e c a r b i d e d i s t r i b u t i o n a f t e r heat t r e a t m e n t t h a n other bearing steels.

R o l l e r b e a r i n g s a r e g e n e r a l l y made f r o m c a r b u r i z e d s t e e l . The tough c o r e developed i n c a r b u r i z e d and heat t r e a t e d s t e e l i s needed t o a l l o w r o l l e r bending w i t h o u t f r a c t u r e . A number o f c a r b u r i z i n g grades o f s t e e l a r e used f o r r o l l e r bearings. Choice i s p r e d i c a t e d on t h e b a s i s o f h a r d e n a b i l i t y and c o r e s t r e n g t h . For instance, AISI 8620 i s used f o r b e a r i n g s w i t h r e l a t i v e l y t h i n s e c t i o n s . Table 1.6 shows t h e p r e f e r r e d grades f o r l i g h t , heavy and v e r y heavy s e c t i o n r o l l e r b e a r i n g s .

40

Table 1.6 C a r b u r i z i n g Grades o f S t e e l Used f o r R o l l e r Bearings

MATERIAL

BEARING WEIGHT

A I S I 8620

L i g h t Section

A I S I 4620

Heavy S e c t i o n

A I S I 4820

Very Heavy S e c t i o n

The case d e p t h f o r c a r b u r i z e d r o l l e r b e a r i n g components must be s u f f i c i e n t t o p r o v i d e maximum hardness a t t h e maximum shear s t r e s s l e v e l . When t h e case boundary c o i n c i d e s w i t h t h e maximum shear s t r e s s depth, e x t e n s i v e subsurface f r a c t u r e a t t h e zone boundary can occur. C a r b u r i z e d s t e e l s have an advantage i n t h a t compressive r e s i d u a l s t r e s s e s i n t h e hardened case increases f a t i g u e l i f e . Conventional c a r b u r i z e d b e a r i n g s cannot be used i n a i r c r a f t gas t u r b i n e s because o f temperature 1 i m i t a t i o n s . A r e l a t i v e new c a r b u r i z i n g a l l o y , Timken CBS 600, has been developed f o r gas t u r b i n e s capable o f o p e r a t i n g as r o l l i n g c o n t a c t b e a r i n g elements a t temperatures approaching 315' C (600)' F.

H i g h Temperature Bearings: For r o l l i n g c o n t a c t b e a r i n g s t h a t have t o o p e r a t e a t temperatures above 315'C (6OO0F) t o o l s t e e l s a r e used. Tool s t e e l b e a r i n g s a r e a l s o used i n h i g h performance gas t u r b i n e engines f o r t h e i r s u p e r i o r wear r e s i s t a n c e and f a t i g u e r e s i s t a n c e . M2, M10 and M50 t o o l s t e e l s p r o v i d e good h o t hardness p r o p e r t i e s and l e n d themselves t o manufacture i n t o p r e c i s i o n r o l l i n g elements. The a p p l i c a t i o n o f secondary hardening o r p r e c i p i t a t i o n o f complex a l l o y c a r b i d e s d u r i n g tempering i s t h e b a s i s f o r i n c r e a s i n g t h e o p e r a t i n g temperature o f b e a r i n g s t e e l s t o 538' C (1000' F ) [33]. T h i s has been acheived i n Timken's CBS 1000, Vasco's X-2 and General E l e c t r i c ' s m o d i f i e d M 50. Vasco's a l l o y i s i n c l u d e d i n Table 1.4. Ceramic b a l l b e a r i n g s a r e a l s o used i n h i g h temperature a p p l i c a t i o n s and a r e d e s c r i b e d i n c h a p t e r 6, "Ceramics and Special A l l o y s " .

41

Manganese Steels

Manganese or austenitic steels are used where high toughness as well as wear resistance is required. Applications include mining machinery, ore processing machinery and earth moving construction machinery. By increasing the Mn alloy content in high carbon steels, it is possible to produce a stable austenite with high carbon content. Addition of other alloying constituents like molybdenum and silicon, it is possible to make a heat treatable or hardenable steel with carbides included in an austenitic matrix. Manganese steels also have considerable capacity for work hardening. The change in hardness during gouging abrasion for manganese steels is compared with martensitic steels in figure 1.12. Manganese steel also tends to transform to martensite during heavy working, thus forming a renewable hard surface as it wears. The manganese steels come in an array of compositions and heat treatments [ 3 4 ] . One of the well known commercial manganese steels is Hadfield steel. Hadfield steel contains from 1.1 to 1.25 percent carbon and 11 to 14 percent manganese. Another class of manganese austenitic steels are the lean manganese high carbon steels. These steels are not as tough as Hadfield steel but have higher abrasion resistance and are useful for large section castings where residual cooling stresses must be minimized. Hadfield steel is best suited for heavy impact and gouging abrasion. Parts requiring moderate toughness but better abrasion resistance such as earth moving equipment should use leaner alloys containing perhaps 6 percent manganese and 1.4 carbon. Austenitic steel has low resistance to low stress or scratching abrasion. Often a martensitic steel will do better in this circumstance.

SELECTING STEELS FOR MINING AND CONSTRUCTION INDUSTRIES

A relatively large tonnage of steel construction industries. Components conditions. There are three general of machinery: high stress abrasion,

alloys are used in the mining and of these machines see heavy abrasive wear types of abrasion encountered in this type low stress abrasion and gouging abrasion.

High stress abrasion occurs when a part slides over hard, sharp asperities and the asperity contact stress is large enough to fracture abrasive particles, as in running a snow plow blade over concrete pavement.

42

Low stress abrasion occurs when loose abrasive particles are slid over - as when a drag line bucket slides over loose gravel. Abrasive particles are not fractured and the metal surface develops parallel abrasion scratches. Gouging abrasion occurs when large , hard chunks of material move over a machine component surface under such 1 arge surface forces that pieces of the metal are gouged out - as in rock crushing machinery. These are important distinctions because they influence the choice of material to control wear. In high stress abrasion, the hard sharp asperities tend to dig into soft ferrite and machine it out. Therefore, a martensitic structure or structure with large carbides is require for wear resistance. Low stress abrasion can be controlled with a pearlitic or bainitic structure- thus allowing less costly steels to be used. Gouging abrasion will remove material with a fine carbide structure. Gouging abrasion is also often accompanied by impact and therefore toughness is necessary. High carbon austenitic manganese steel is used in rock crushers with success. Figure 1.13 shows the relation between toughness and gouging wear ratio (the lower the ratio, the lower the wear) [35]. Note that the materials in band A lose gouging abrasion resistance as their toughness is increased. These are materials like AISI 4340 steel which, in the quenched and high temperature tempered condition consists of a mixture of fine carbides in a ferrite matrix. This material has high fracture toughness but very poor gouging abrasion resistance. If the alloy is quenched and tempered at low temperature, its toughness is much lower but its gouging abrasion resistance is good. Contrast this with band B, representing austenitic manganese steels where a great range of toughness levels is available without influencing good abrasion resistance.

In general, abrasion resistance is improved as carbon content is increased. Of course, austenitic manganese steels are limited to a maximum carbon content of about 1.5% before they become brittle. For carbon steels and alloy steels, martensitic structure is the best for abrasion resistance. Carbon content can be adjusted for the desired level of toughness. But as with austenitic manganese steels, the higher the carbon, the greater the abrasion resistance. It must be pointed out that there are other abrasion resistant mater a1 s that are used very successfully in the mining industry. They include high chromi um white irons, stellites and some super alloys. These materials are de cribed in chapter 6.

43

Figure 1.13 Relation o f Gouging Wear Resistance to Toughness.

44

The s e v e r i t y o f abrasion i s i n f l u e n c e d b y t h e r e l a t i v e hardness o f t h e abrading medium and t h e s t e e l i t i s wearing and b y t h e s i z e d i s t r i b u t i o n o f the abrasive p a r t i c l e s . I t i s i m p o r t a n t t o c o n s i d e r t h e b u l k hardness and t h e hardness o f h a r d phases ( c a r b i d e s ) i n s t e e l m i c r o s t r u c t u r e i n r e l a t i o n t o t h e expected a b r a d i n g media. For instance, i f q u a r t z i s t h e a b r a d i n g m a t e r i a l , a u s t e n i t i c manganese s t e e l would n o t be a p p r o p r i a t e because i t i s n o t h a r d enough. Table 1.7 shows t h e r e l a t i o n between hardness o f most m i n e r a l s and s t e e l phases and c a r b i d e s . I t shows t h a t f o r g a r n e t abrasion, a t l e a s t chromium c a r b i d e and b e t t e r s t i l l , molybdenum c a r b i d e should be i n t h e s t e e l m i c r o s t r u c t u r e t o r e s i s t abrasion.

F i g u r e 1.14 shows t h e a b r a s i o n r a t e f o r p a r t i c l e s o f m i n e r a l s o f g i v e n hardness as a f u n c t i o n o f t h e hardness o f t h e m a t e r i a l b e i n g abraded. I t i s i m p o r t a n t t o n o t e t h a t t h e r e i s a narrow r e g i o n i n which a s t e e l becomes s e n s i t i v e t o a b r a s i o n as hardness o f t h e a b r a s i v e increases. Thus, t h e i n a d v e r t e n t i n t r o d u c t i o n o f a s l i g h t l y h a r d e r a b r a s i v e i n t o t h e environment o f a g i v e n s t e e l p a r t c o u l d c o n c e i v a b l y cause a sudden l a r g e unexpected i n c r e a s e i n a b r a s i v e wear.

0 0

u (r

r

I Machinery steel -HRC 30

I

1

1

J

m

I I

1 I I

Limestone

5 I

I I

Gloss

I I Flint

I

I

I

I Gronite

I

I Corundum

I I I

I

Shcon torbide

F i g u r e 1.14 A b r a s i v e Wear Rate o f S t e e l s and Other M a t e r i a l s as a F u n c t i o n o f M i n e r a l Hardness.

45

Table 1.7 Relative Hardness of Steels, Minerals and Carbides [ 3 6 ] Mineral Talc Bituminous coal Gypsum Calcite F1 uor i te Apatite Fel spar Magnetite Orthoclase F1 int Quartz

Garnet

Corundum

Diamond

Hardness, Knoop 20 35* 40 130 175 250 305 435 550 575 620 710 820 840 1025

Steel

Carbide

Ferrite Austenitic manganese 0.3% C martensite

Aust. fully worked 0.4% C Martensite 0.6% C Martensite

1360 1750 1750 1800 2020 2080 2660 2700 2955

FeC CrC TaC MoC

wc vc BC

Tic

7570

* some coals contain considerable silica, making them

VERY abrasive to steel

46

CHAPTER

2.COPPER

BASE

BEARING

MATERIALS

HARDNESS 28 - 390 VICKERS

TENSILE STRENGTH

58

- 1378 MPa

YOUNGS MODULUS 5.24Et4 THERM COND

28 - 121 WATT/m k

THERM EXPANS DENSITY

- 1.31Et5 MPa

2.16E-6 - 1.67E-5

7.75Et3 - 8.86Et3 KG/cum

HEAT CAPACITY - 0.4

Kj/kg k

TYPES OF COPPER BASE BEARING MATERIALS

Tin Bronzes - c a s t Phosphor Bronzes - wrought Leaded Bronzes - c a s t Copper l e a d - c a s t B e r y l l i u m Copper - c a s t Aluminum Bronzes - c a s t APPLICATIONS FOR COPPER BASE BEARING MATERIALS H e a v i l y loaded, slow moving j o u r n a l b e a r i n g s f o r heavy machinery Gears Airframe bearings Seals Springs Pump b e a r i n g s Non s p a r k i n g c o n t a c t s Marine hardware Nonmagnetic b e a r i n g s

K

47

INTRODUCTION

Copper-base alloys have been used for bearings and sliding surfaces in machinery since the introduction of the first steam engines. The copper-tin alloys, known as bearing bronzes, come in a large array of alloy compositions with combinations of aluminum, lead, tin, iron, manganese and nickel. These alloys make excellent bushings and journal bearings for heavily loaded, slow speed heavy machinery. Off road construction and agricultural machinery uses large numbers of these bearings. Bronze bearings are also used in oscillating motion, boundary lubricated bearings such as aircraft landing gear assembl ies and control surface hinges and linkages. Bronze bearings have been used at bearing stress levels as high as 242 MPa (35,000 psi) [37]. Copper base alloys have been popular because of their unique compatibility with steel journal materials together with good bearing strength. The bronzes in particular, perform well under boundary lubrication conditions. Petroleum oil based greases work very well with bronzes. It is believed that the tin content is an important element in the surface chemistry of boundary lubrication [38]. Beryllium copper can be produced with a strength comparable to steel. (But with a Young’s modulus about 0.6 of steel). With its high corrosion resistance and nonmagnetic properties, it provides a substitute for steel in systems where ferromagnetic materials are excluded. It can also be used in hazardous conditions where explosive gases exclude materials which produce sparks when rubbed. ALthough copper and tin are relatively expensive metals, over 68 million kg (75,000 tons) of copper are used in bearings each year [39]. Bearing bronzes come in two forms - cast and wrought. Certain compositions, such as aluminum bronzes with more than 8% aluminum cannot be cold worked and are often used in the as cast condition. This produces a microstructure with characteristic features which will be discussed later. Cold rolled or forged bronzes can achieve high strength levels by virtue of work hardening. However, where stress corrosion cracking may threaten, the amount of cold work may have to be drastically restricted or special alloys used.

48

TIN BRONZES T i n bronzes c o n t a i n 5 - 15 w t % t i n and a small amount o f z i n c . Phosphorous i s added f o r d e o x i d i z i n g . Phosphorous a l s o combines w i t h copper t o f o r m Cu3P, which s t r e n g t h e n s t h e a l l o y . Too much phosphorous, however, w i l l r e s u l t i n b r i t t l e n e s s . G r e a t e r t h a t 0.01% P can r e s u l t i n h o t shortness. Phosphorous has a l s o been found t o i n h i b i t good boundary l u b r i c a t i o n [40]. Although t h e c o p p e r - t i n phase diagram shows v e r y l o w s o l u b i l i t y o f t i n i n copper, c a s t i n g experience has shown t h a t below 8% t i n , c a s t i n g s a r e f u l l y s i n g l e phase a. F i g u r e 1 shows an approximate c o n s t i t u t i o n diagram f o r as c a s t c o p p e r - t i n a l l o y s . N o t i c e t h a t when c h i l l c a s t , 8% t i n a l l o y s w i l l c o n t a i n some 6 phase. The c a s t s t r u c t u r e w i l l be cored, meaning t h a t t h e t i n d i s t r i b u t i o n i n d e n d r i t e s w i l l v a r y from t h e c e n t e r o f t h e d e n d r i t e t o t h e o u t e r boundary. A t y p i c a l m i c r o s t r u c t u r e o f b e a r i n g a l l o y C90900 (88%cu, 10%Sn, 2%Zn) i s shown i n f i g u r e 2 . The l a r g e d a r k d e n d r i t i c phase i s t h e l o w t i n c o n t e n t cored zone. ( T h i s i s t h e r e s u l t o f t h e t i n f r e e z i n g l a s t d u r i n g c o o l i n g f r o m t h e m e l t ) . The 6 phase can be seen as a s m a l l e r , w e l l d i s p e r s e d 1 i g h t g r a y m a t e r i a l . T h i s i s an i n t e r m e t a l l i c compound, CuglSng, and i t i s harder t h a n t h e m a t r i x . T h i s hard phase tends t o s t r e n g t h e n t h e a l l o y . I n c r e a s i n g t h e t i n c o n t e n t , t h e r e f o r e , increases t h e average hardness o f t h e t i n bronze a l l o y . Too much t i n w i l l r e s u l t i n a l o s s i n f r a c t u r e toughness, however. Z i n c i s added t o t h e a l l o y t o d i s p l a c e t i n and a l l o w a h i g h e r percentage o f 6 . I f t h e a l l o y i s h e l d f o r a l o n g t i m e a t t h e a n n e a l i n g temperature, t h e 6 phase w i l l t e n d t o p r e c i p i t a t e along g r a i n boundaries causing e m b r i t t l e m e n t . The h a r d 6 phase i s u s e f u l t o a i d i n w e a r - i n o r p o l i s h i n g o f t h e s t e e l o r c a s t i r o n s h a f t i n g s u r f a c e i t i s r u n a g a i n s t . Thus, a s h a f t w i t h a t u r n e d f i n i s h may s t a r t o u t i n a j o u r n a l b e a r i n g w i t h s t e e l a s p e r i t y c o n t a c t , b u t as t h e h i g h spots a r e l e v e l e d , t h e b e a r i n g w i l l t e n d toward hydrodynamic l u b r i c a t i o n . T i n bronzes, w i t h o u t l e a d a d d i t i o n s a r e c o n s i d e r e d moderately h i g h s t r e n g t h a l l o y s , good f o r gears and h e a v i l y loaded bushings. Aluminum bronze i s considered t o be a h i g h s t r e n g t h b e a r i n g bronze.

49

F i g u r e 2 . 1 C o n s t i t u t i o n Diagram f o r a s - c a s t C o p p e r - t i n Alloys. (Approx)

Y

+

s

/’ ,-’

8’

o + s

s+

‘- ~.

I

1

I

10

M

30

t

Weiqhf Percent TNn

F i g u r e 2.2 M i c r o s t r u c t u r e o f Cast

Alloy C90500 (SAE 62)

50

LEADED TIN BRONZES

Lead is virtually insoluble in copper. The two metals are immiscible when molten. Therefore, when a mixture of lead and copper freeze from the melt, the copper freezes first and the lead freezes out as free globules in the copper matrix. Leaded bronzes are made from the addition of lead to copper tin a1 1 oys. The copper lead equilibrium diagram is shown in figure 3. Note that there is a small solubility of lead in copper - it is about 0.003%. The eutectic freezes at about 953' C while the lead phase,0 , freezes at a much lower temperature resulting in free lead distributed in the copper matrix. The microstructure of a leaded bronze is shown in figure 4. In the micrograph, the hard 6 phase can be seen as the light gray phase while the lead can be seen as dark globules. There are a number of grades of leaded bronzes, covering a range of lead percentages. Up to about 20% lead is added to tin bronzes. The general usage alloys cover the range 1 - 10% lead. As lead displaces tin in the alloy, the amount of hard 6 phase is reduced as is the strength of the alloy. Highly leaded tin bronze (10%Pb and higher) is not used where impact or heavy cyclic loads are encountered. Lead also produces hot shortness in tin bronzes and therefore the operating temperature is of concern. Free lead in bearing alloys provides intrinsic sol id lubrication. Transfer of lead to the mating shaft will provide a soft, low shear strength coating which will help to maintain a reduced friction and lessen overheating during loss of oil or grease lubrication. Experiments have demonstrated that lead can transfer from leaded bronze to stainless steel and produce a very thin, smooth lead film [41]. Leaded bronze bearings will also conform to shaft misalignment or waviness so that the contact area is maximized. This helps to distribute the load over as large a surface as possible. Thus the probability of full film hydrodynamic lubrication is increased. In a steel bearing, on the other hand, shaft misalignment can lead to damaging edge loading with break down of the hydrodynamic film in the concentrated contact zone.

51

COPPER LEAD

A binary mixture of copper and lead has been used for a simple, self lubricating bearing material. Copper-lead mixtures contain from 20 to 40% lead, the lead existing in pockets just as it does in leaded bronzes. Copperlead is not used by itself as a bearing material. It is usually overlayed on steel strip by powder metallurgy techniques. The steel backing is used to provide strength. Copper-lead is used as an under layer in tri-metal babbitt bearing construction. The copper-lead is similar in hardness to the babbitt overlay at room temperature but harder at elevated temperatures.

"C

Atomic Percentage Lead

"f

Weight Percentage Lead

Figure 2.3 Constitution Diagram for Copper-lead Alloys

52

F i g u r e 2 . 4 M i c r o s t r u c t u r e o f Leaded Bronze (7% Pb). Alloy C93200 (SAE 660)

53

The c o p p e r - l e a d on s t e e l backing p r o v i d e s good f a t i g u e s t r e n g t h , h i g h l o a d c a p a c i t y and a b i l i t y t o p e r f o r m a t moderately e l e v a t e d temperatures. The m a t e r i a l has been used f o r heavy d u t y main and c o n n e c t i n g r o d b e a r i n g s i n t r u c k s , d i e s e l engines and i n t e r n a l combustion a i r c r a f t engines.

ALUMINUM BRONZE Aluminum bronzes, o r copper base a l l o y s c o n t a i n i n g between 5% and 11% aluminum a r e s t r o n g e r and tougher t h a n t h e t i n bronzes. Aluminum s t r e n g t h e n s t h e a l l o y and increases c o r r o s i o n r e s i s t a n c e . Aluminum bronzes can a l s o o p e r a t e a t e l e v a t e d temperatures. H e a v i l y loaded bushings have been operated a t temperatures up t o 260' C (500' F ) . The phase diagram f o r copper - aluminum i s shown i n f i g u r e 2.5. T h i s diagram shows t h a t w i t h a d d i t i o n s up t o 8% aluminum, t h e a l l o y i s a s o l i d s o l u t i o n a phase. Above 8% aluminum, R phase i s i n e q u i l i b r i u m w i t h a above 560a C . The R phase i s m a r t e n s i t i c i n s t r u c t u r e and can be r e t a i n e d b y quenching. A l l o y s c o n t a i n i n g more t h a n 8% aluminum can be hardened by heat t r e a t m e n t b y quenching from about 9000 'C and tempering a t 400 t o 600' C f o r an hour. T h i s can produce a hardness o f up t o 230 HV. Bronze b e a r i n g s are r a r e l y , i f ever, heat t r e a t e d . They a r e used i n t h e annealed c o n d i t i o n . However, heat t r e a t m e n t can be used f o r g e a r i n g a p p l i c a t i o n s . A number o f combinations o f h o t working and heat t r e a t m e n t have been developed [42]. As t h e aluminum c o n t e n t o f t h e a l l o y i s increased, t h e toughness reduces. A t 14% aluminum, t h e a l l o y i s v e r y b r i t t l e b u t can o n l y be used f o r s p e c i a l a p p l i c a t i o n s 1 i k e deep drawing d i e s . N i c k e l and i r o n a r e added t o aluminum bronzes t o improve d u c t i l i t y . T h e a f i e l d can be extended b y a d d i t i o n o f t h e s e elements and t h e r e f o r e R i s n o t r e t a i n e d below 600' C. A second phase i s formed i n these a l l o y s which increases t h e mechanical s t r e n g t h w i t h o u t decreasing t h e d u c t i l i t y a p p r e c i a b l y . The m i c r o s t r u c t u r e o f a t y p i c a l b e a r i n g grade aluminum bronze c o n t a i n i n g 3% i r o n i s shown i n f i g u r e 6. The m a t r i x i s a and t h e a c i c u l a r phase i s 0 . The pepper-1 i k e phase i s i r o n p r e c i p i t a t e .

MANGANESE BRONZE Manganese bronze i s s i m i l a r t o aluminum bronze i n t h a t i t i s a h i g h s t r e n g t h bronze used f o r heavy l o a d c o n d i t i o n s . I t i s o f t e n used f o r a worm gear m a t e r i a l . Manganese added t o t h e a l l o y has t h e e f f e c t o f s t a b i l i z i n g t h e R phase. Manganese bronze i s e a s i e r t o c a s t and f a b r i c a t e t h a n t h e aluminum n i c k e l bronzes. Where c o r r o s i o n r e s i s t a n c e i s i m p o r t a n t , t h e manganese c o n t e n t must be k e p t t o 1% maximum.

54

L A W

Weight Percentage Aluminum

Figure 2.5 Copper-aluminum Equil ibrium Diagram

55

Figure 2.6 Photomicrograph o f Cast A1 uminum Bronze A1 l o y C95400 Microstructure

56

BERYLLIUM COPPER

The strength of age hardened beryllium copper can approach that of steel.(Its elastic modulus is a little over half that of steel, however). The material has the high thermal conductivity of copper and can be used at temperatures as high as 315' C (600' F ) . It has excellent corrosion resistance to both marine and industrial environments. It has been used for heavily loaded grease lubricated bushings for bearing loads as high as 50,000 psi. The phase diagram for beryllium copper is shown in figure 2.7. Note that beryllium is soluble in copper up to 16% at 866' C and the solubility drops off to about 1% at 200' C. Therefore, a 2% Be Cu alloy can be precipitation hardened. Alloy 25 is a commonly used grade of beryllium copper containing 1.85% Be. Its microstructure after heat treating is shown in figure 8. The photomicrograph shows a dark matrix with tetrahedral strain lines, the result o f precipitation hardening. The white islands are Be -rich I3 phase. The matrix hardness is Rc 39 and the D phase is Rc 55. Hardened beryl1 ium copper bearings require hardened steel shafting preferably tool steel or AISI 4340. Lubrication is essential and must be maintained without interruption. Precautions must be taken when fabricating or processing beryllium copper alloys to prevent release of metal dust. Inhalation of dust from processing beryllium copper is a serious health hazard. POROUS BRONZES

Porous sintered bronzes which hold liquid lubricant are used in many "lubricated for 1 ife" appl ications such as fractional horsepower electric motors, machine tools, business machines and aircraft accessories. A popular composition contains 90% copper and 10% tin. The bearings are made by blending powdered copper and tin in the correct proportions and die pressing into bearing shapes under high pressure. The green forms are then sintered to strengthen the material. The bearing surface is sized by ball burnishing. The parts are then impregnated with a liquid lubricant. The process makes possible manufacture of very inexpensive bushings.

57 57

Porous bronze bearings come in a large range of sizes, porosity, lubricant and composition. Some grades contain from 1 t o 3.5% graphite to improve self lubricating properties under lubrication starvation. High porosity bearings are used for large lubricant reservoir, high speed, 1 ightly loaded conditions. More dense composites containing less oil and more graphite are used for oscillating motion or reciprocating load conditions. At high speeds, hydrodynamic lubrication conditions are possible in porous bronze bearings [ 4 3 ] . Porous bronze bearings are often rated on the basis of PV (bearing pressure in psi x velocity in fpm). Table 2.1 shows some PV limits for a group of porous bearings. 2

I 0 1400 ,

l

6

4

,

I

I

'

8 '

I0

i ' i

14

'

I

20

25 30

< ' I

60

40

80

1 ' 1 ' 1

1

I

1300

I200

ll0C

IOOC

WC

u

c 3

5

80C

Ql

a

E

c

70C

6CK

50C

40C

30C

zoc

I

10

I 20

I

30

I I \ 1 , 40 50 60 Atomic Percent Beryllium

)

70

(

(

80

Figure 2.7 2.7 Equilibrium Equilibrium Diagram Diagram for for Copper-Beryl1 Copper-Beryl1ium ium Figure

1

90

,

0

58

Figure 2.8 Photomicrograph of BeCu microstructure Table 2.1. PV Limits For Porous Bearing Materials Mater i a1

PV psi-fpm

P, Static psi

Vmax f Pm

Bronze

50,000

8,000

1,200

Lead-Bronze

80,000

3,500

1,500

Copper-Iron

35,000

20,000

225

Iron

30,000

10,000

400

A1 uminum

50,000

4,000

1,200

(From Machine Design Magazine [ 4 4 ] ) psi-fpm x 35.03 = MPa-m/sec MPa x 0.145 = ksi fpm x 0.005 = m/sec

59

BEARING PROPERTIES OF COPPER BASE MATERIALS

The various copper base bearing formulations can be compared for their potential use as bearings. Wear factor is one measure of performance and is shown in Table 2.2 for several bearing alloys. The wear factor was measured for heavily loaded, slow moving, grease lubricated bearings and is shown for two load conditions for each material - a high load and a moderate load. It shows the effect of load on increased wear. The wear factor is computed by dividing the volumetric wear by the total distance slid during a wear test. Mechanical and physical properties are summarized in tables 2.3 and 2.4 at the end of this chapter.

Table 2.2 Wear Properties of Bearing Bronzes

Material

Hardness BHN

Bearing Pressure Mpa (ksi)

Wear Factor cu mm/m (cu in/ft x 10-9)

Leaded Bronze

65

0-14 (0-2) 14-40 (2-6)

0.006 (120) 0.033 (620)

Tin Bronze

75

0-14 (0-2) 14-40 (2-6)

0.003 (50) 0.013 (250)

Aluminum Bronze

170

0-100 (0-15) 100-200 (15-30)

0.001 (25) 0.007 (125)

Beryl 1 ium Copper

380

0-550 (0-80)

0.001 (20)

Conversion factors: MPa x 0.145 = ksi The table above provides a comparison of the wear behavior of four classes of copper base materials. In heavily loaded, boundary lubricated operating conditions, frictional heating is often the limiting factor.

60

When the bearing temperature reaches the 1 ubricant decomposition temperature, the bearing will either fail or the wear rate will increase several orders o f magnitude. As bearing pressure is increased, friction will increase, causing an increase in heating and a decrease in the maximum velocity at which moderate wear conditions will exist. A wear map has been developed for C93200 Bronze based on bearing test data [ 4 5 , 461 and is shown in figure 2.9.

-

4000

24 -

3500

21 -

3000

28

N

2$ ul

e

17-

2

High terrperature

2500

8

14-

5

2000

01

10-

D)

m

z

m

1500

7-

1000

35 -

500

0-

Moderate wear

0

0

10

20

30

40

50

60

..

Velocity. M/sec

Figure 2.9 Wear Design Map for Heavily Loaded, Slow Moving Bronze Bearings

Cycler (

* 10 to * 45O. 8 - 12 fym) (.04 - .06 d r e c )

Figure 2.10 Design Chart f o r Aluminum Bronze Bushings Operating Under Heavy Loads and Oscillating motion 61

62

In the wear map in figure 2.9, using bearing stress as the ordinate and sliding velocity as the abscissa, various operating zones are established for boundary operating conditions. An approximate hydrodynamic zone is a1 so included to indicate how far or near the designer might be from that more desirable condition. The high temperature zone to the upper right of the chart is the forbidden zone for bearing operation. High wear, moderate wear and high friction zones are indicated. Thus when choosing a bearing, knowing the load and sliding velocity, one can estimate the potential life of the bearing and its feasibility for a given application. If one lands in the high friction zone, the bearing stress can be decreased by increasing the bore size. A high wear zone might indicate the need for using a harder bearing material such as aluminum bronze. It is important to keep in mind that changing bearing materials also changes other critical operating properties. For instance, the harder bearing materials are less tolerant to lubricant starvation and will not accommodate shaft misal ignment or other geometric defects. For heavily loaded bearings subjected to oscillating motion and pulsating load, a design chart has been developed for grease lubricated aluminum bronze The chart shows two conditions: bearings [47] as shown in figure 2.10. 0.1% increase in I.D. and 0.3% increase in I.D. from wear. One chooses the maximum wear that can be tolerated and the desired life in cycles and determines the maximum bearing stress allowable from the chart. The bearing stress i s based on projected area of the bearing I.D. From the allowable bearing stress, the bearing size can be calculated.

A bronze bearing design program on floppy disk has been developed by Copper Development Corporation and provides design methods for both hydrodynamic and boundary lubrication conditions [48].

63

G E A R BRONZES

Bronzes have been used for gearing for a long time. They offer some distinct advantages. Bronzes can stand high sliding velocities in worm gearing and therefore are used with steel worm drivers. Bronze is corrosion resistant and is used in gearing in many marine applications. Fabrication is relatively inexpensive - bronze gears can be stamped from plate. Bronzes have high thermal conductivity and are excellent under boundary lubrication and therefore are optimum for gearing which involves considerable sliding contact (worm gears, hypoid gears ) . One of the more popular grades of bronze for gears is 89 Other gear bronzes are listed in table 2 . 3

- 11

bronze (11% tin)

Table 2 . 3 Gear Bronzes (from D.Dudley [ 4 9 ] )

Mater i a1

Features

Phosphor bronze

good fatigue resistance

Manganese bronze

tough

A1 uminum bronze

tough, 1 ighter than manganese bronze

S i 1 icon bronze

non magnetic material applications

64

ELECTRICAL CONTACTS

Copper and its alloys are most frequently used for electrical contacts because of the need for low and stable contact resistance and reasonable cost. The noble metals like gold and platinum are used as coatings or platings for cost savings so that sol id metal contacts 1 ike commutators are usually copper base. For contacts involving sliding contact such as commutators and brushes, the copper must be alloyed to provide reasonable wear resistance and structural strength. Alloying copper decreases its electrical conductivity. The metal i s quite sensitive to small amounts of solute. For instance, 0 . 1 % phosphorous in solution reduces the conductivity by 50% while 1.0% of tin in solution reduces the conductivity by about 40%. The relative conductivities of several copper base materials and electric contact alloys are shown in figure 2.11. The conductivity increases to the right on the chart. I5

Y)

Leaded Yellow B r a s .

IITI

Leaded Tln B r o n s e

I

I

I

. .

Sillcon Bronze

I Nickel Silver

I Brami

8e;ylllum'Copper

I

35

ZD 15

Cu-Zn)

1

10

1

Sa,.-n

Alumlnum B r o n m

-

1

I

Goi

[ili,

TI

nium

Zinc

1 I Figure 2.11

MagneBlum

65

Heat treatment can be used to increase the strength of a copper alloy and improve its conductivity. Precipitation hardening of beryllium copper takes the beryllium out o f solution and precipitates it as a separate phase. The remaining copper matrix is purer and, since it is the major volume in the alloy, conductivity is much closer to that of pure copper. By adding chromium and silicon in the right proportions and precipitation hardening, the chromium and silicon are taken out of solution and precipitated as chromium silicide and a high strength, high conductivity alloy is the result.Recent advances in mechanical alloying processes makes possible mixtures o f copper and chromium resulting in a fine dispersion of chromium in copper, strengthening the mater i a1 . Some wear resistant copper alloys that are used as electrical conductors are listed in table 2.4.

Table 2.4 Strengthened Copper Alloys for Electrical Contacts [50] Hardness,BHN

Mater i a1

Electrical Conductivity % IACS *

0.8 Cr, 0.08 Si, Bal Cu

100

80

0.6 Si, 2.4 Co, Bal Cu

120

65

0.6 Si, 3.0 Ni, Bal Cu

140

45

2.6 Co, 0.4 Be, Bal Cu

220

45

* Percent conductivity compared with annealed pure copper with a resistance o f 1.7421

pohm-cm

66

Mechanical and P h y s i c a l p r o p e r t i e s and c o m p o s i t i o n o f s e l e c t e d copper base b e a r i n g a l l o y s a r e summarized i n t a b l e s 2.5, 2.6 and 2.7.

Table 2.5 Mechanical P r o p e r t i e s o f Copper Base Bearing M a t e r i a l s

MATERIAL

A1 -Ni-Fe Bronze BERYLLIUM COPPER BERYLLIUM COPPER C51100 Phos Brnz C60800 A1 Brnz C61000 A1 Brnz C61000 A1 Brnz C86300 Mn Brnz C87200 S i Brnz C93200 SAE 660 C93700 SAE 64 C94300 H i Pb Brnz C98600 CU-Pb Conversion f a c t o r s : MPa x 0.145 = k s i

FORM WORKED CAST

Cast Cast Cast Cold-work Cast Cast Cast Cast Cast Cast Cast Cast Cast

TEMPER HEAT TREAT

Age harden Annealed Hard Annealed Annealed Hard Annealed Annealed Annealed Annealed Annealed Annealed

HARDNESS TENSILE YOUNGS VICKERS STRENGTH MODULUS DPH MPa MPa

170 393 80 145 70 100 165 225 85 80 60 48 28

655 1378 483 552 414 358 448 793 310 276 207 166 59

l.lOEt05 1.31Et05 1.31Et05 l.lOEt05 1.21Et05 7.93Et04 7.93Et04 9.79Et04 1.03Et05 1.03Et05 7.58Et04 7.24Et04 5.24Et04

67

Table 2.6 P h y s i c a l P r o p e r t i e s o f Copper Base B e a r i n g M a t e r i a l s

DENSITY THERM CON THERM EXP MELT POINT KG/ m/m WATT/m CU METER K

MATER IAL

/ O C

A1 -Ni-Fe Bronze BERYLLIUM COPPER C51100 Phos Brnz C60800 A1 Brnz C61000 A1 Brnz C86300 Mn Brnz C87200 S i Brnz C93200 SAE 660 C93700 SAE 64 C94300 H i Pb Brnz C98600 Cu-Pb

7.85Et03 8.30Et03 8.86 Et o 3 8.30Et03 7.78Et03 7.83Et03 8.36Et03 8.86Et03 8.86Et03 9.30Et03 9.13Et03

35.50 121.10 83.73 79.58 69.20 35.47 28.37 58.13 46.71 62.63 294.10

1.62E-05 2.16E-06 1.78E-05 1.80E-05 1.78E-05 2.17E-05 1.67E-05 9.72E-06 1.83E-05 1.85E-05 1.97E-05

O C

1060 982 1060 1054 1041 923 971 1038 1038

Conversion f a c t o r s :

kg/m 3 x 3.613E-5 W/m/K J/kg/K

x 0.176

=

= l b / u i n : kg/m 3 x 0.001 BTU/ft /hr/'F

5

x 0.238E-4 =BTU/1 b/'

F

=

gm/cc (SG)

RESIS HEAT micro CAPACITY ohm-cm J/gK

0.04 8.00 8.70 10.00 11.50 7.20 28.57 14.28 16.84 18.86

0.38 0.38 0.38 0.38 0.38 0.38 0.38 0.38 0.38

68

Table 2.7 Composition o f Copper Base Bearing M a t e r i a l s

MATERI AL

A1 -Ni-Fe Bronze B e r y l 1 ium copper C51100 Phos Brnz C60800 A1 Brnz C61000 A1 Brnz C86300 Mn Brnz C87200 S i Brnz C93200 SAE 660 C93700 SAE 64 C94300 H i Pb Brnz C98600 Cu-Pb

COMPOSITION

A1 Be cu cu cu A1 Sn cu cu cu cu

9,Fe 4.5,Ni 5,Mn 1.5, Cu 8 1.8,Co,Ni .2,Cu 99.5 95.6,Sn 4.2,P .2 95, A1 5 92, A1 8 5,Cu 63,Fe 3,Mn 3,Zn 25 1, Pb .5,Zn 5 , F 3 2 . 5 , A l 1.5, S i 1-5, Cu 84 83,Sn 7,Pb 7,Zn 3 80,Sn 10,Pb 10 70,Pb 25,Sn 5 65,Sn .5,Pb 34.5

69

CHAPTER

3 - SOFT

METAL

BEARING

MATERIALS

M.P. RANGE: 157C - 1063C HARDNESS: 5 BHN - 100 BHN DENSITY: 2.7 g/CC

-

19 g/CC

TYPES OF SOFT METAL BEARINGS: Lead and Tin based babbitts Aluminum-tin bearing metals Mu1 t i -1 ayer steel backed ha1 f round bearings Gold, Silver and Indium plating Zinc-aluminum bearing metals APPLICATIONS FOR SOFT METAL BEARING MATERIALS: Crank and main bearings for internal combustion engines Crank and main bearings for reciprocating compressors Engine camshaft bearings Electric motor bearings Gearbox and transmission bearings Pi1 low blocks Railroad car axle bearings Gearbox thrust washer Wear plates in gear and piston pumps Thrust bearing tilting pads

70

SOFT METALS

Many metals in their pure state are very soft. For example, iron, used as a basis for tools, structural parts, rails and gears is softer than zinc when refined to a high purity. Metals with a hardness below 70 DPH (-70 Brinell) have use in bearing applications wherein the bearing material is much softer than the material in rubbing contact with it. The reason for use of soft metals in bearings is their ability to imbed abrasive debris and neutralize it, to conform to the harder surface in contact so as to provide maximum bearing area for load support and to act as sacrificial components in critical or expensive machinery for fail -safe requirements. For instance, it is better that the soft metal bushing wipes out than to have the hardened shaft, expensive and difficult to replace, fail by heavy surface damage. Experience has shown that b-subgroup metals like silver, cadmium, indium, tin, gold, lead and bismuth are compatible with cast iron and steel [51]. That is, they have a low susceptibility to adhesion and galling with iron. Soft metals are also used as solid lubricants, coated on strong metal backings. Very thin soft metal coatings on high strength substrates provide a low shear surface while the substrate supports the normal load. Bowden and Tabor [52], in early tribological research demonstrated that there was an optimum soft metal coating thickness for minimum coefficient of friction. This effect is illustrated in figure 3.1.They showed that a minimum friction was obtained for a film about one micron thick deposited on tool steel.

Film thickness, cm Fig. 3 . 1 Friction of a soft metal film as a function of film thickness (From Bowden & Tabor)

Table 3 . 1 P r o p e r t i e s of s o f t metal elements

MATERIAL

HARDNESS

VICKERS DPH

TENSILE STRENGTH MPa

YOUNGS

MODULUS MPa

DENSITY KGI CU METER

THERM CON WATTIm K

ALUM1NUM ANT IMONY CADM IUM COPPER GOLD INDIUM

37 59

53 10

6.89E+04 7.79E+04

221 16

42 51 55 25

68 193 138 3

5.52E+04 1.04E+05 7.45E+04 1.10E+04

IRON LEAD

52 36

275 41

2.07E+06 1.38E+04

92 393 294 69 71 35

PLATINUM SILVER

37 54 31 65

124 207 15 103

1.45E+05 7 . 1 OE+04 4.14E+04 6.89E+04

TIN

ZINC

71 415 64 112

THERM EXP

M/M / oc

MELT

RESIS

POINT C

micro C A P A C I T Y ohm-c

HEAT J/Kg

2.35E-05 9.00E-06 3 . 1 OE-05 1.70E-05 1.40E-05 2.48E-05

649 631 321 1083 1064 179

5.00 70.00 6.83 1.50 2.20 9.00

0.84 0.27 0.21 0.38 0.13 0.23

1.21E-05 2.90E-05 9.00E-06

1537 327

10.00 24.30

0.46 0.13

1769

10.60

1.91E-05 2.35E-05 3.10E-05

961 231 419

1.60 11.50 5.90

0.11 0.19 0.17 0.42

72

Those single phase soft metals having a hardness of less than 70 DPH possess a variety of other physical and mechanical properties and therefore provide a number of alternatives in selection. Table 3.1 shows a listing of 11 such metals with their properties. Commercial bearing alloys based on soft metals - mostly tin, lead, copper and aluminum are available as full plain bearings or thrust bearings. Large bearings are usually cast from an alloy while smaller bearings 1 ike automotive engine bearings are made from soft metal alloys coated on steel backings. Perhaps the most familiar soft bearing alloys are the babbitts. These have been in use since the early 19th century when printer’s type metal was utilized for machinery by casting it into holes drilled in iron housings. BABBlllS OR WHITEMEAL

Babbitt or whitemetal bearing materials represent a class of alloys developed specifically for bearing applications. Their use has been largely for hydrodynamic journal bearings - especially in automotive engines. The constitutuents in these alloys have been chosen to provide a bearing surface that will conform to misalignment, embed dirt and, in case of interruption of lubricant, will melt or flow to limit shaft damage. Materials chosen for this application include lead, tin, cadmium, aluminum and silver. Zinc alloys, not as common, are used for inexpensive bearings. The physical and mechanical properties of some commercial soft bearing materials are summarized in table 3 . 2 . Lead Based Babbitts The lead based bearing alloys contain a high percentage of lead with various combinations of tin and antimony. The resulting alloys consist of soft lead-tin-antimony sol id solution with a hard antimony-tin intermetall ic phase. The SbSn phase is cubic in structure and is distributed as fine cubes throughout the structure. Lead based babbitts have been used in the United States in large volume in automotive engines. Lead babbitts are inexpensive and generally perform as well as the more expensive tin based babbitts. The lead based babbitts possess relatively low fracture toughness and low fatigue strength as compared with the other soft metal bearing materials. For large bearings, the tin antimony phase tends to segregate during solidification. This is especially true for the centrifugal casting method. Copper additions to the alloy will control segregation. Up to 2 percent copper can be used.

Table 3.2 P r o p e r t i e s of Commercial S o f t Metal A l l o y Bearing M a t e r i a l s

MATER I AL

A1 1100 A1 750 LEAD BABBITT 13 LEAD BABBITT 15 LEAD BABBITT 7 LEAD BABBITT 8 T I N BABBITT 1 T I N BABBITT 2 T I N BABBITT 3 Z I N C 12 Z I N C 27

HARDNESS VICKERS DPH

30 45 20 20 22 11 17 24 26 93 107

TENSILE STRENGTH MPa 90 159 69 69 69 69 81 81 81 296 414

YOUNGS

MODULUS MPa 6.83E+04 7.10E+04 2.90E+04 2.90E+04 2.90E+04 2.90E+04 5.03E+04 5.24E+04 5.10E+04 8.27E+04 7.58E+04

DENSITY THERM CON KG/ WATT/m CU METER K

2.71E+03 8.30E+03 1.08E+04 1.02E+04 9.69E+03 1.05E+04 7.47E+03 7.47E+03 7.47E+03 6.09E+03 4.98E+03

THERM EXP M/M

/oc

221.44 183. 24.22

2.20E-05 2.30E-05

24.22 24.22 53.63 51.90

1.24E-05 2.40E-05

115.91 124.56

1.17E-05

MELT PO INT C 649 649 240 28 1 240 240 223 240 240 404 427

74

Developments in automotive engines in the late 60s and in the 70s resulted in higher rpm and hotter running engines. This resulted in the production of acids in the lubricants which tend to attack lead. In addition, the change to unleaded gasoline exacerbated the problem and has forced the development of more corrosion resistant automotive engine bearings. The use of thin layer construction with corrosion resistant top coatings has helped increase the load capacity and corrosion resistance o f the conventional babbitts. (The multilayer bearing is discussed below) The standard lead based bearing compositions are listed in table 3 . 2 The 15% sb alloy (alloy 15 ) has been popular for automotive use. In this application, the alloy is strip lined to a steel backing. The thin coating on a steel backing improves the babbitt resistance to elevated temperatures and fatigue. Tin coatings over the lead babbitt help to protect the alloy from corrosion. In large diesel and natural gas engines the lead babbitt is applied to steel shells but the babbitt layer is relatively thick and fatigue strength is 1 imited. Tin Based Babbitts Tin has long been recognized as an exceptional bearing material. High tin content bearing materials generally contain 7 to 8" antimony and 3 to 4% copper. Two hard intermetallic phases are possible in these alloys: SbSn, a cuboid shaped phase and Cu6Sn5, a needle shaped phase. (These intermetallics are also found in the lead base babbitts). The composition diagram shown in figure 3 . 2 [53] shows the four possible microstructures which might be found in these alloys. The areas labeled A and B represent the compositions used commercially. In zone A, the soft matrix contains an intermetallic, Cu6Sn5 in the form of needles. In zone B, with antimony content above 7.5%, the two phases, SbSn and Cu6Sn5 are present. The microstructure should look like that shown in figure 3 . 3 . Since the matrix is a supersaturated solution of antimony in tin, SbSn will tend to precipitate out when bearing stress and sliding contact causes heating of the surface. The presence of either or both of these intermetallic phases has little influence on bearing properties. However, fatigue strength is maximum when the intermetallic phases are present as very fine dispersals in the matrix. Fatigue strength falls off rapidly with increasing temperature. The fatigue strength temperature properties are shown in figure 3 . 4 . Above 130 C these alloys are unsuitable for service involving fatigue loading.

75

1

2

3

4

5

6

7

8

*/* Copper

Figure 3.2 Tin-antimony-copper phase diagram portion showing divisions

76

Figure 3.3 Micrograph o f tin based babbitt showing intermetallics

Cadmium is added to the alloy up to about 1% to improve strength. Both hardness and fatigue strength are increased by the addition o f cadmium. Lead content is restricted to about 0.35% in the high quality alloys to minimize lead-tin eutectic which melts at about 183 C. Lead also reduces the corrosion resistance o f the alloy. Zinc is restricted also because in small quantities it tends to create a yellow tinge in the alloy.

77

60

50

40

.- 7 h s b

3cu

30

I

20

I

10

Temperature,

OC

From G . C . P r a t t

Figure 3.4 Effect o f temperature on tin babbitt fatigue properties

78

The tin-7.5% Sb-3.4% Cu alloy is a popular one because of its excellent antiseizure properties [ 54 3 Even under oil starvation conditions , the alloy shows little tendency for adhesion. Wiping or local melting is more likely to occur. Since this alloy is soft, hardened shafts are not required. The alloy has good corrosion resistance except in water contamination of the lubricant. For marine environment where salt water may be encountered in bearings, an alloy resistant to corrosion in salt water is available. The composition is 30% Zn, 68.5% Sn, 1.5% Cu. The alloy is anodic to steel in sea water. It has been used in stern tube propeller bearings. Cadmium addition to tin babbitt can increase the mechanical properties of the alloy. One percent cadmium addition can increase the fatigue rating by approximately 40%. Babbitt Fatigue

Cracking and spalling of lead or tin based bearing materials is generally caused by fatigue. Poor bonding of the babbitt layer to the steel backing can also cause spalling of the babbitt shell. Cadmium containing alloys can reduce the bond strength o f the babbitt to the metal backing when rapid cooling occurs. It is easy to recognize poor bonding by the nature of the surface left after a piece falls out of the bearing. The steel backing can be seen where babbitt has left. An example of this can be seen in figure 3.5.

10 x Figure 3.5 Fatigue of babbitt with bonding failure

79

Aluminum based Bearing Materials Aluminum - tin alloys are used in place of babbitt where increased fatigue strength is required. It is reported that aluminum-tin alloys have a fatigue strength three times greater than tin or lead based babbitts [55]. The fatigue strength of these alloys is much superior at elevated temperatures. Therefore, they are very useful in high performance high speed engines. These alloys are similar to copper-lead alloys in that the two constituents are immiscible. Tin is soluble in aluminum at temperatures above 600 C. When the alloy solidifies the tin precipitates out as a separate phase. Copper is added to the alloy to strengthen the aluminum matrix. The aluminum base alloys, in addition to having high fatigue strength compared with the lead or tin babbitts, are also corrosion resistant, have high thermal conductivity and are less expensive than tin based alloys. On the other hand, aluminum based alloys have a high thermal expansion rate relative to steel and cast iron and therefore require larger room temperature clearances. The embedability and galling resistance of the aluminum based alloys are not as good as lead based or tin based alloys. Therefore, the 6% tin aluminum alloy requires a thin overlay of lead-tin to prevent galling. Hardened steel or chromium plated steel journals are also recommended for use with aluminum based bearing alloys. High tin content aluminum alloys provide better compatibility with steel. A 20% tin alloy is used in high speed engines, operating on a very thin hydrodynamic oil film. Conventionally cast 20% tin alloy produces a continuous network of tin surrounding the aluminum constituent. This structure weakens the alloy - especially at elevated temperature. The network structure can be modified by 20% cold reduction followed by annealing at 500C for one hour. This produces a microstructure of small unconnected pools of free tin embedded in an aluminum matrix.The high tin-aluminum alloys are used as linings bonded to steel backings. The bearing alloy is cast, rolled into strip and heat treated, The strip of tin-aluminum is then roll bonded to a precoated steel shell. The steel is precoated with pure aluminum or nickel. These bearings, strengthened with 1% copper, can be used in high speed small engines without a soft lead-tin overlay. Hardened steel shafting is not essential. A less expensive aluminum based alloy can be made with lead instead of

tin [56]. Although lead is immiscible in aluminum, it will go into solution in aluminum at high temperature (1050C for 10% Pb or 1250C for 20% Pb). Cooling from these high temperatures develops serious segregation problems especially in thick castings. Lead and aluminum can be combined by sintering the two constituents in powder form on a steel strip and rolling the composite into a cylinder.

80

Commercial a l l o y s now c o n t a i n a l e a d b a b b i t t . SAE 718 aluminum and SAE 19 b a b b i t t have been used f o r t h e a l l o y . The aluminum-lead a l l o y e x h i b i t s f a t i g u e s t r e n g t h equal t o t h e aluminum-20% t i n a l l o y and wear r e s i s t a n c e s u p e r i o r t o i t [57]. The Aluminum-babbitt a l l o y a l s o demonstrated v e r y good a n t i s c o r i n g properties.

A s i l i c o n strengthened aluminum has a l s o found use as a b e a r i n g m a t e r i a l . C o n t a i n i n g 11% s i l i c o n , 1% copper t h i s a l l o y has been used i n t u r b o charged d i e s e l s i n t h e U.K. The a l l o y i s r e l a t i v e l y h a r d and i s used w i t h a t h i n o v e r l a y o f l e a d - t i n . The a l l o y has performed s u r p r i s i n g l y w e l l as an engine b e a r i n g , h a v i n g e x c e l l e n t h i g h temperature f a t i g u e s t r e n g t h and corrosion resistance

The composition o f commercial b e a r i n g a l l o y s d e s c r i b e d above a r e summarized i n t a b l e 3.3

Table 3.3 Composition o f s o f t metal a l l o y s

MATER I AL

A1 1100 A1 750 LEAD BABBITT 13 LEAD BABBITT 15 LEAD BABBITT 7 LEAD BABBITT 8 TIN BABBITT 1 TIN BABBITT 2 TIN BABBITT 3 Z I N C 12 Z I N C 27

COMPOSITION

Si Sn Pb Pb Pb Pb Sn Sn Sn Zn Zn

& Fe 1,Cu .1,Mn .05,Zn . l , A l 6.5,Cu l , N i l , A l 91.5 83,Sb 10,Sn 6,As .25,Cu .5 82.5,Sb 15,Sn 1,As 1,Cu .6 75,Sb 15,Sn 10,As .5,Cu . 5 80,Sb 15,Sn 5,As .5,Cu .5 91,Sb 4.5,Cu 4.5 89,Sb 7.5,Cu 3.5 84,Sb 8,Cu 8 88.2,Al 11,Cu .75,Mg .02 70.8,Al 27,Cu 2.2,Mg .015

99

81

Bimetal, Trimetal Materials Multilayered bearing materials are composed of thin layers of soft metals on a steel backing or "shell". The structure of this class of bearings is diagramed in figure 3.6. The intention is to increase the load carrying capacity of a soft material while retaining the imbedability and low shear properties. Fatigue strength is also improved with the use of trimetal design. This type of bearing is used mostly in automotive engine applications. Multilayer bearing materials include copper-lead, aluminum,tin babbitt, silver, indium, tin and lead. The materials are put down on a steel backing in various combinations, depending on the end use. In other words these bearings can be custom designed for specific operating conditions involving fatigue, corrosion, lubricant and bearing loading. The coatings are applied to flat steel strip by sintering, electrodeposition, roll bonding and vapor deposition and the strip roll formed into cylinders or half shells. The wall thickness of the soft bearing material coatings is less than 5 mm. Using a thin layer of a soft metal on a strong steel substrate allows much higher load capacity while maintaining the conformability and compatibility with the journal material. Just as the soft metal layer thickness effects friction as shown in figure 3.1, fatigue strength is a function of babbitt layer thickness. The effect of babbitt liner thickness on relative fatigue life of a multilayer bearing is shown in figure 3.7 [58].

Ti n- 1 ead Copper-lead Steel

Figure 3.6 Diagram of the multilayer soft metal bearing

82

Low carbon steel is usually used as a backing material for multilayer bearings. In bimetal systems, tin babbitt, lead babbitt, aluminum-tin alloy and copper-lead are used as surface layers. The surface layer thickness ranges between 0.50 mm to .025 mm. In some applications, where expensive machinery is involved, a bearing bronze backing material is used to provide added insurance against journal damage during overload or loss of lubricant.

Trimetal systems use an intermediate layer of high fatigue strength material (leaded bronze, copper-lead, aluminum-tin, silver) and a tin or lead babbitt surface layer. The surface layer is usually electrodeposited to a thickness of about 0.025 mm.

Layer thickness Figure 3 . 7 Fatigue life as a function of babbitt layer thickness

83

Another v e r s i o n o f t h e m u l t i l a y e r b e a r i n g i s t h e l e a d indium b e a r i n g [59]. T h i s one has been designed f o r h i g h power, s m a l l e r c a p a c i t y automobile engines o f t e n u s i n g t u r b o c h a r g i n g . T h i s b e a r i n g has a c o n t i n u o u s l y c a s t bronze on s t e e l w i t h a l e a d indium o v e r l a y . The o v e r l a y t h i c k n e s s range i s

0.0130

-

0.015mm.

The l o a d c a r r y i n g c a p a c i t i e s o f t h e v a r i o u s m u l t i l a y e r b e a r i n g m a t e r i a l s i s shown i n t a b l e 3.4. These b e a r i n g s a r e manufactured i n l a r g e numbers w i t h h i g h p r e c i s i o n and r e l a t i v e l y low c o s t .

Table 3.4 M u l t i l a y e r Bearing M a t e r i a l Load C a p a c i t i e s

Backing Layer

I n t e r m e d i a t e Layer

Leaded Bronze T i n Bronze A1 uminum- t i n Steel Steel Steel Steel Steel Leaded Bronze Aluminum-t i n Steel Steel Steel Steel Steel Steel

* 0.075

mm c o a t i n g

none none none none none none none none none none Copper-1 ead Copper-1 ead Leaded bronze Aluminum-t i n Silver Bronze

Surface Layer

Load C a p a c i t y MPa

none none none Tin Babbitt Lead B a b b i t t Aluminum-t i n Copper-lead Leaded Bronze Lead B a b b i t t Lead B a b b i t t Lead B a b b i t t * Lead B a b b i t t Lead B a b b i t t Lead B a b b i t t Lead B a b b i t t Lead indium

21 34 28 15 15 31 38 55 48 41 21 55 69 55 83 62

84

Note t h e s u b s t a n t i a l i n c r e a s e i n l o a d c a p a c i t y w i t h t h e use o f t h i n m u l t i l a y e r s t r u c t u r e i n comparison t o t h i c k w a l l b e a r i n g s t r u c t u r e . Thick t r i m e t a l l a y e r s a r e u s u a l l y used i n l a r g e b e a r i n g s . The e f f e c t on l o a d c a p a c i t y can be seen i n t a b l e 3.4.

Lead, and t o some e x t e n t , copper a r e s u b j e c t t o c o r r o s i o n from a c i d s produced i n engines from combustion and l u b r i c a n t thermal decomposition. Therefore i n some engine a p p l i c a t i o n s , a p r o t e c t i v e f l a s h c o a t i n g o f indium o r t i n i s used over a copper-lead o r a l e a d b a b b i t t s u r f a c e l a y e r Since t h e r e a r e a l a r g e number o f grades o f m u l t i l a y e r b e a r i n g m a t e r i a l s , s e l e c t i o n f o r s p e c i f i c a p p l i c a t i o n s can be a complicated process. Cost may be a d e c i d i n g f a c t o r i n choosing between s e v e r a l grades. R e l a t i v e c o s t s can change i n t i m e owing t o t h e f l u c t u a t i o n o f market p r i c e s f o r i n d i v i d u a l me a1 s and changes i n m a n u f a c t u r i n g processes. Research c o n t i n u e s t o f i n d ways t o reduce t h e c o s t o f m a n u f a c t u r i n g s o f t metal p r e c i s i o n b e a r i n g s . S e l e c t i o n o f s p e c i f i c b e a r i n g a l l o y s cannot be based on t h e i n f o r m a t i o n p r o v i d e d here. T h i s i n f o r m a t i o n i s intended f o r guidance i n t h e range o f m a t e r i a l s a v a i l a b l e . Bearing manufacturers have s p e c i a l i s t s who can be c a l l e d on f o r s p e c i f i c s e l e c t i o n s .

ALLOY SELECTION AND DESIGN

Although t h e o r i g i n a l b a b b i t t a l l o y s f i r s t used i n t h e e a r l y 1 9 t h c e n t u r y were n o t designed s p e c i f i c a l l y f o r j o u r n a l b e a r i n g s b u t were e x i s t i n g a l l o y s used i n p r i n t i n g , t h e y proved s u c c e s s f u l as b e a r i n g a l l o y s . I n t h e 2 0 t h c e n t u r y , r e f i n e m e n t and m o d i f i c a t i o n o f b e a r i n g a l l o y s has been a c o n t i n u i n g process because o f changing o p e r a t i n g c o n d i t i o n s and t h e search f o r l e s s expensive p r o d u c t s . Modern b e a r i n g a l l o y s have d e r i v e d from a number of p r i n c i p l e s and compromises.

85

An example of the complexity of bearing alloy development is found in the account o f the development of a leaded aluminum bearing for automotive use [60]. Using a rating called "relative bearing Performance", representing a weighted combination o f a bearing's resistance to fatigue, wear, scoring and corrosion, several types of bearings were compared. The results are shown in figure 3.8. This showed that inexpensive lead base bearings are suitable for low performance engines and the next choice for high performance engines were the expensive aluminum based alloys or more reasonable copper-lead alloys. Copper-lead, however, was inadequate because of low corrosion resistance to acids developed in high performance engines. Therefore, there existed a gap in the available bearing materials, as shown in figure 3.8 .

W

u

c m

Ei0

rc L W

CL

ul c

.r

r---1

L

m aJ m W

>

.r

c,

.

I

I

I I

I

I

5 7

W CT

Lead babbitts

Alumium-tin

A1 -Cu-Sn

Figure 3.8 Relative bearing performance for various bearing materials

86

Aluminum-lead was an inexpensive alternative but had inherent problems in manufacture. Note, from the aluminum-lead constitution diagram in figure 3.9 that lead is virtually insoluble in aluminum at room temperature. A 5% lead content can exist in solution at 815 C, allowing mixing and casting of the two constituents. The cast alloy would be a mixture of free lead globules in an aluminum matrix. This would be similar to a copper-lead bearing material. Two problems had to be overcome: (1) the lead would be susceptible to corrosion and ( 2 ) over 2% lead produces a weak bond when bonding to a steel backing. The first problem was solved by using a lead-tin babbitt with sufficient tin t o provide corrosion resistance. The second problem was solved by developing a casting method that produced a graded lead content throughout a solid section. By machining the cast plate so that one side contained minimum free lead and the other side contained 5% lead, the material could rolled t o a sheet, cut into bearing inlays and bonded to the steel backing with the low lead side.

Figure 3.9 Aluminum-lead constitution diagram

87

ZINC BEARING ALLOYS

Zinc is an inexpensive substitute for babbitt or bronze materials. A1 hough not found in general use, large bearings such as hot mill runout table bearings are an appropriate application, saving approximately 20% over bronze. Zinc alloy bear ngs were used in Germany during World War I1 because of critical copper shortages. Zinc-aluminum a loys are used for plain bearings. Two commercial alloys are listed in table 3.2. Note that the alloys contain 12 to 27% aluminum. Sometimes zinc die casting alloys are used so that a cast housing can serve as a bearing, eliminating the necessity of casting in or pressing in a bronze bearing. The zinc bearing alloys can be used in the cast condition, much the same as bronze bearings. Zinc-aluminum alloys have a number of advantages besides economy. For instance, their low elastic modulus allows zinc bearings to conform to elastic bearing loads. Zinc bearing materials can support large loads for slow moving, grease lubricated conditions. In fact, they will support loads which leaded bronze bearings are used for [61]. Zinc bearing alloys have limitations that must be considered if their use is anticipated. Thermal expansion rates for zinc alloys are higher than bronze or cast iron and therefore, journal bearings made of zinc a loy require larger clearances than used in equivalent bronze bearings. Zinc alloys also loose strength quickly when heated. These alloys are also sens tive to lubrication starvation. Loss of lubricant can result in adhesion and galling of zinc alloy bearings .

88

GOLD, SILVER AND INDIUM

The precious metals gold, silver and platinum are used as coatings on harder metals to provide both corrosion protection and solid lubrication. These metals provide soft, easily sheared coatings as long as the coating is thin enough. The metals are used in thin coatings electrodeposited or sputter coated on metal substrates. There is an optimum coating thickness for minimum friction conditions as was discussed early in the chapter. Wear will eventually remove the coating and therefore a compromise between minimum friction and maximum life may be necessary when using this approach. Gold and platinum are used in electrical contacts not only because of their desired electrical conductivity but also for their corrosion resistance especially atmospheric in industrial atmospheres. Their ability to act like solid lubricants also is desirable for moving electrical contacts. Very thin gold coatings are used on connections for removable printed circuit boards used in high quality intstruments and computers. These coatings will last for several removals and insertions before they wear off. Gold coatings have been used on precision instruments operating in vacuum or inert environments. This includes coatings on precision ball bearings. Unalloyed or low alloy gold is susceptible to adhesive type wear and resulting noisy low energy electrical contacts. In addition, precious metal electrical contacts will develop "friction polymers" during sl iding contact in atmospheres containing organics. This can provide effective lubrication - but if polymer develops in sufficient quantity, it can separate the contacting metal surfaces. For these reasons, it is often desirable to use a lubricant with a sliding electric contact. Silver has been used in heavy duty engine journal bearings for aircraft and diesels. Silver is plated over a steel backing and an overlay of lead and a flash coating of indium is used t o provide breakin and corrosion protection. Silver has also been used as an antiscuffing coating in engine cylinders.

89

CHAPTER

4 - CAST

IRON

MELTING POINTS

HARDNESS

150

-

THERM EXPANS DENSITY

'C

895 VICKERS

TENSILE STRENGTH YOUNGS MODULUS

1160 - 1450

110 - 689 MPa 1.03Et05

1.24E-05

0.27 kg/mJ

TYPES OF CAST IRONS Gray Cast I r o n White I r o n Nodular Cast I r o n Ma1 1eabl e Cast I r o n High A l l o y I r o n s

APPLICATIONS FOR CAST IRONS Automotive c r a n k s h a f t s and connecting r o d s Piston r i n g s Brakes and c l u t c h e s Gears D i e blocks Grinding b a l l s Machine ways and s l i d e s Crushers Cams and t a p p e t s Valves Pumps Wheels and r o l l e r s f o r heat t r e a t furnaces Crane wheels

-

-

2.21Et05 MPa

2.5E-06 K

90

INTRODUCTION

Cast iron is an iron-carbon-silicon alloy with carbon content between 2.5 and 4.0 percent. The alloy can also contain silicon, manganese, sulphur and phosphorous. The alloying additions are used to modify the structure and properties of cast iron. Cast iron has been used in machinery since the beginning of the industrial revolution. It is a low cost material with structural strength similar to steel and can be used for the fabrication of large structures such as machine bases, engine blocks and large brackets. It is ideal for casting because of its ease of flow when molten. Cast irons are known for their good tribological properties. They are resistant to wear in boundary lubrication conditions and can be heat treated to be resistant to aggressive abrasive conditions. Cast irons can be used at elevated temperatures (gray iron can be used at 400' C ) . There are many grades of cast irons and often the terms of reference can be confusing. Gray cast iron, the most used of the irons, is an iron-carbonsilicon alloy in which much of the carbon exists as flake graphite. The matrix in which the graphite is imbedded can be ferrite (pure iron), pearlite, bainite or martensite. The particular matrix structure can be developed by heat treatment.Gray cast iron is so named because when it is fractured, the fracture face looks gray because of the free graphite in it. White iron is cast iron in which the carbon exists as iron carbides. When white iron is fractured the fracture face looks white owing to the presence of a large fraction of carbides. White iron can be achieved by rapid quenching a normally gray iron composition. Malleable iron is an iron-carbon alloy in which the carbon exists as nodular graphite. Malleable iron is formed from white iron by heat treating. The result is an alloy with much improved fracture toughness and increased elastic modulus (compared to gray cast iron). The hardness, of course, is reduced. This material resembles steel in mechanical properties and yet can be fabricated by casting - a low cost advantage over steel. Heat treatment required to achieve the nodular structure however, is long and expensive. Nodular or Ductile or Spherulitic Graphite (SG) iron is an iron-carbon alloy in which the carbon again exists as graphite nodules or spheres. In this alloy, the nodular graphite is formed by addition of small amounts of spheroidizing agents such as magnesium or cerium during casting. Nodular iron is less expensive than malleable because it does not require extensive heat treating to achieve its nodular structure.

91

High alloy cast irons are conventional cast irons with extra amounts of chromium, silicon, molybdenum or nickel added to improve abrasion resistance, corrosion resistance or for high hot hardness requirements. These alloys generally have hard chromium carbides or have an austenitic matrix. Some of the familiar alloys in this category are the Ni-hards and the Ni-resists. High chromium white irons are found in the mining and ore processing industries. PROPERTIES OF CAST IRONS

The general characteristics of each of the classes of cast irons are summarized in table 4 . 1

Table 4 . 1 Characteristics o f the Five General Types o f Cast Iron Type of Iron

Toughness

Hardness

Elastic Modulus

Wear Resistance

White Iron

Low

High

High

High

Gray Iron

Moderate

Moderate

Low *

Moderate

Ductile Iron

High

Moderate

Higher

Moderate

Malleable Iron

High

Moderate

15% greater Moderate than ductile

High Alloy Iron

Low

High

Same as steel

Best abras erosion resistance

&

* Some gray irons have elastic moduli almost half that of steel, The mechanical and physical properties o f cast irons are summarized in table 4 . 2

92

Table 4.2 Mechanical and Physical P r o p e r t i e s o f Cast I r o n s

MATERIAL

A536 NODULAR ABEX PACE DURIRON HC-250 MEEHANITE AQ N I-HARD NI-HARD 4 NI-RESIST 1 Pearl i t i c M a l l e a b l e

HARDNESS VICKERS DPH

350 380 530 600 196 655 560 150 248

HARDNESS BRINELL BHN

TENSILE STRENGTH MPa

974 606 110 689 340 379 620 207 723

331 360

500 550 196 587 522 150 248

YOUNGS MAX OP MODULUS TEMP 'C GPa

168.90 197.17 124.09 217.16 152.00 172.35 172.35 103.41 172.35

D E N S I T Y CONDUCT Kg/ WATT/m CU METER K

.

x 3.613E-5 = l b / c u i n : kg/m

3

44.70 17.30

816 538

7.47E+03 7.20E+03

39.79

1.27E-05 1.94E-05

7,30E+03

40.00

1.00E-05

x 0.001 = gm/cc ( S G )

W/m/K x 0.578 = B T U / s q f t / f t / h r / ' F J/kg/K x 2.388E-4 = BTU/lb/'F or cal/g/.C m/m/'C

x 0.55 = i n / i n / ' F

CHEMICAL COMPOSITION

MATERIAL

A536 NODULAR ABEX PACE DURIRDN MEEHANITE AQ N 1-HARD NI-HARD 4 NI-RESIST 1 P e a r l i t i c Malleable

C C C C C C C C

J/kg/'K

816

MPa x ,145 = k s i 3

/'C

HEAT CAPACITY

8.47E-06 4.19E+02 1.24E-05 5.44E+02 8.90E-06 1.02E-05 7.44E+00 9.36E-06

Conversion f a c t o r s :

kg/m

m/m

7,64E+03 7,20E+03 7,47E+03 7.34E+03 7.60E+03

399

3.5.Si 2,Mn 1,P ,065.Mg .04,Fe 93 (approx comp. ) 1.6.Mn 1 . 5 . S i 2.0,Cr 28,Ni 2,Mo 2,Fe 63 .85,Si 14.5,Mn .5,P .07.S .08, Fe 84 3,Mn 2,Si 1.8,Cu .5,Ni 2,Mo .5,Fe 90.2 3.5,Si .5.Ni 4,Cr 2,Fe 89.5 3 . 5 . S i 1.5.Mn.5.Ni 6 , c r 8,Fe 80.5 3,Si 1.5.Ni 15,Cu 6,Cr 2,Fe 71.5 2.5.P 1 . 0 , s . 6 . S i 1 . 0 , Fe 95

14.50

EXPAN

93

Elastic modulus. The graphite structure in gray cast irons influences the modulus of elasticity. Elastic modulus drops with increasing graphite content. The effect of flake graphite is much greater than nodular graphite. Angus [ 6 2 ] reports that the ratio of (elastic modulus CI/elastic modulus steel) ranges between 0.4 and 0.7 in as-cast pearlitic gray cast iron.In most cast irons there is no proportional limit in the tensile stress-strain diagram. Elastic moduli are determined by a tangent to the stress strain curve at 0.10 strain. Poisson’s ratio. Poisson’s ratio in grey iron tends to fall as tensile stress increases owing to the opening up of graphite cavities. It generally remains constant under compressive stresses. Angus [ 6 3 ] shows a range of values from 0.25 to 0.08 depending on the load and graphite structure. Impact resistance. Gray cast iron exhibits moderate impact resistance especially in low phosphorous containing alloys. When the phosphorous content exceeds 0.7%, impact resistance drops. Nodular iron shows much higher impact resistance than gray iron. White iron and high alloy cast irons exhibit low impact resistance. Thermal conductivity. The thermal conductivity of gray cast iron increases with increasing carbon content - as long as the carbon exists as graphite. Pearlite will tend to reduce conductivity. Nodular iron exhibits a 20 to 30 percent lower thermal conductivity than gray iron. White iron and high alloy cast irons also exhibit lower thermal conductivities because of the carbide content. Gray iron is similar to steel in thermal conductivity levels. Electrical resistivity. The electrical resistivity of cast iron ranges between 25 and 80 microhms/cm. Carbon and silicon content are the most effective

moderators of resistivity. Flake graphite produces high resistivity and the finer the graphite flakes, the lower the resistivity. For instance, gray cast iron with coarse graphite flakes has a resistivity of 104 microhms/cc while fine flake iron is 77.4 microhms/cc. Ferritic nodular iron has the lowest resistivity of the cast irons (-60 microhms-cm). Magnetic properties. Although cast irons are not as good permanent magnets as magnet steels, the temperature coefficient of magnetic loss and impact loss are much lower in cast iron. Nonmagnetic cast irons are produced by adding austenite stabilizers to iron. Nickel and manganese alloys are used. Damping capacity. Damping properties of gray cast irons are an important advantage in their use in bearings. Gray cast iron has exceptionally high damping capacity. Compared with mild steel, ferritic gray iron damping capacity is almost an order of magnitude greater. Thus, when fretting, stick slip or noise is a problem, gray cast iron is a good candidate to consider.

94

Hardness. Gray cast iron is often graded according to hardness. Care must be exercized, however, that the way in which the hardness reading is taken, the rate of cooling in the area where the hardness is determined and the microstructure of the iron are considered. A casting can experience various cooling rates depending on section thicknesses and therefore have a range of hardness levels over a component, depending on where the hardness readings are taken. Gray iron, with its heterogeneous structure of soft graphite in pearl ite and some carbides represents a range of hardnesses. Hardness measurements should provide an average of this range. Thus the Brine11 hardness system is used because the indentor covers a large enough area to include all phases in the structure. The hardness values for gray irons shown in this chapter have been converted to Vickers from BHN to maintain consistency in terms for the text. Since hardness values for gray cast iron are an average of soft and hard phases, it is dangerous to compare the hardness of cast iron with the hardness of steels when trying to choose a wear resistant material. A high carbon gray iron might exist as fully hardened martensite matrix with soft graphite imbedded and have a Rockwell C hardness of 40 - 45 but could be very wear resistant and virtually unmachinable because of its hard constituents. A steel of the same hardness might not compare at all in wear resistance and machinabil ity. METALLURGY OF CAST IRONS

The iron-carbon-silicon phase diagram, shown in figure 4 . 1 , shows that for the cast irons, the possible microstructures include iron carbide, pearlite and transformed austenite. I f the cooling rate is slow, the austenite transforms to pearlite and graphite. However, if the cooling rate is sufficiently rapid, carbon forms carbides rather than graphitizing. When the iron-carbon-silicon diagram is compared with the iron-carbon diagram shown in the upper right corner, it can be seen that addition of silicon changes the eutectic compositions (lowers the carbon content for the eutectic) and complicates the diagram with ranges o f transformation rather than sharp phase change boundaries. Gray iron can be pearlitic or ferritic depending on the speed of cooling during casting. Its structure consists of flake graphite in a ferrite or pearlite matrix. The structure is shown in figure 4 . 2 .

95

Nodular iron can be formed when a nodularizing agent is added to the alloy. The structure is different from gray iron only in morphology of the graphite phase. A an example of nodular structure is shown in figure 4.3. Note the ferrite surrounding the nodules of graphite. The ferrite shown in figure 4.3 is excessive in amount. The ferrite can all but be eliminated with controlled casting processes.

,

I

Carbide

I

I

I

I

2.0

10

I

i i

1 3.0

Carbon Content-Percent

4

I

Iec

0

By Weight

a. Iron-carbon diagram 900

+

,

800 IFerrite Austenite Carbide

+

700

600

"C Carbon Content-Percent

B y Weight

Figure 4.1 Iron-carbon-silicon equilibrium diagram (2% silicon) (from Walton [63]

96

Figure 4.2 Microstructure o f Gray Iron

Figure 4 . 3 Nodular iron

2M642

Figure 4 . 4

3Mo 7 V White iron

97

As s t a t e d above, w h i t e i r o n r e s u l t s from r a p i d c o o l i n g , p r e v e n t i n g e q u i l i b r i u m s t r u c t u r e s shown i n f i g u r e 4 . 1 f r o m f o r m i n g a t room temperature. An example o f t h e m i c r o s t r u c t u r e f o r w h i t e i r o n i s shown i n f i g u r e 4.4. The room temperature composition o f w h i t e i r o n c o n s i s t s o f d e n d r i t e s o f p e a r l i t e surrounded by i r o n c a r b i d e s . I t i s c a l l e d w h i t e i r o n because when f r a c t u r e d , i t s f r a c t u r e f a c e i s s i l v e r y w h i t e . Gray c a s t i r o n f r a c t u r e f a c e s l o o k g r a y because o f t h e g r a p h i t e f l a k e s i n t h e s t r u c t u r e . The h i g h carbon c o n t e n t and s i l i c o n i n c a s t i r o n s make them e x c e l l e n t c a s t i n g a l l o y s . They a r e v e r y f l u i d i n t h e l i q u i d s t a t e and do n o t f o r m d i f f i c u l t s u r f a c e f i l m s d u r i n g c a s t i n g . The i r o n - c a r b o n diagram ( f i g 4 . l a ) shows a e u t e c t i c o r l o w m e l t i n g p o i n t a t about 4.3% carbon. A t t h a t composition, t h e i r o n m e l t s a t about 1150 C. C a s t i n g i s f a c i l i t a t e d b y t h e l o w e s t m e l t i n g temperature. (Most commercial c a s t i r o n s c o n t a i n between 2.5 and 4 p e r c e n t carbon). A d d i t i o n o f s i l i c o n o r phosphorous w i l l l o w e r t h e carbon l e v e l a t which t h e e u t e c t i c occurs. S i l i c o n a l s o a l t e r s t h e c o m p o s i t i o n o f p e a r l i t e phase and t h e s o l u b i l i t y o f carbon i n a u s t e n i t e . Note t h a t t h e a d d i t i o n o f two p e r c e n t s i l i c o n t o c a s t i r o n reduces t h e carbon l e v e l a t which t h e e u t e c t i c occurs t o about 3.6 p e r c e n t as shown i n f i g u r e 4.1. The c a s t i r o n s , t h e r e f o r e d i f f e r s i g n i f i c a n t l y f r o m s t e e l s i n t h a t t h e y c o n t a i n a p p r e c i a b l y more carbon and g e n e r a l l y a l s o c o n t a i n s i l i c o n .

The i r o n - c a r b o n - s i l i c o n composition o f c a s t i r o n s makes p o s s i b l e a v a r i e t y o f m i c r o s t r u c t u r e s depending on t h e r a t e o f c o o l i n g and t h e e u t e c t i c p o i n t . S i l i c o n extends t h e t r a n s f o r m a t i o n range as w e l l . One can produce gray, w h i t e o r n o d u l a r i r o n depending on c o m p o s i t i o n and c o o l i n g r a t e . O f t e n t h e t y p e o f i r o n can be r e l a t e d t o t h e carbon e q u i v a l e n t (CE). The f o l l o w i n g f o r m u l a i s used t o determine t h e CE: CE

=

T o t a l Carbon% t ( S i % t P%)/3

T h i s v a l u e can be used i n t h e i r o n - c a r b o n diagram i n f i g u r e 4 . l a as t h e carbon c o n t e n t . For instance, an i r o n w i t h carbon c o n t e n t o f 3.2%, s i l i c o n 2% and phosphorous 1.3% would have a CE o f 4.3 and would be a t t h e e u t e c t i c p o i n t . The diagram i n f i g u r e 4.5 ( f r o m Walton) [ 6 4 ] shows t h e e f f e c t o f carbon s i l i c o n c o n t e n t on t h e t y p e o f i r o n s which f o r m d u r i n g s o l i d i f i c a t i o n . The carbon and s i l i c o n l e v e l s a t which t h e s t r u c t u r e changes form s t e e l t o c a s t i r o n can be seen i n t h e diagram. The d o t t e d l i n e s d e f i n e t h e zone i n which most commercial c a s t i r o n s f a l l . Note t h a t w h i t e i r o n s t e n d t o appear a t t h e lower CE l e v e l s .

98

As an iron composition cools from the melt, it will reach a temperature at which austenite will begin to form. This is shown in figure 4.1 by the vertical line representing a CE of 3.5. At the eutectic temperature, the remaining melt rejects carbon in iron carbide form and a eutectic mixture o f iron carbide and austenite occurs. As the CE is increased, the amount o f eutectic mixture increases and the amount of austenite decreases. Although iron carbide forms more easily than graphite because it does not require complete separation of carbon from iron, graphite is the most prevalent constituent o f commercial cast irons. Silicon is a graphitizing agent and therefore serves to promote graphite in the final structure after solidification resulting in gray iron. In addition, slow cooling and high carbon content promote graphitization. Rapid cooling rate favors iron carbide and white iron structure.

+

%c 3%

Si =4.3

. ~ u c t i l ed n s

4.0

I

c

e

2

3.c

I

I

c

” c V

5 2.0

1.c

C

I

I

I

1.o

2.0

3.0

Silicon Content-Percent

Figure 4.5 Silicon and carbon content ranges for various cast irons

99

A s t h e c o o l i n g proceeds i n t h e diagram i n f i g u r e 4.1,

t h e s o l u b i l i t y o f carbon i n a u s t e n i t e decreases and f u r t h e r carbon i s r e j e c t e d f r o m t h e s o l u t i o n and i t d e p o s i t s on t h e s u r f a c e o f t h e e x i s t i n g g r a p h i t e f l a k e s . When t h e f i n a l phase change temperature range i s reached, t h e a u s t e n i t e c o n v e r t s t o f e r r i t e and i r o n c a r b i d e - o r p e a r l i t e . The a l l o y composition and t h e c o o l i n g r a t e through t h i s i n t e r v a l w i l l have an i m p o r t a n t e f f e c t on t h e amount o f f r e e f e r r i t e ( s o f t i r o n w i t h v e r y low carbon c o n t e n t ) i n t h e p e a r l i t e m a t r i x . I n i r o n c a s t i n g , t h e f i n e n e s s o f p e a r l i t e and i t s carbon c o n t e n t w i l l depend on t h e a n a l y s i s o f t h e a l l o y and i t s c o o l i n g r a t e b u t a p o r t i o n o f t h e m a t r i x t h a t i s p e a r l i t e i s determined b y t h e amount o f carbon t h a t remains i n s o l u t i o n . Wear r e s i s t a n c e r e q u i r e s l o w f e r r i t e c o n t e n t and f i n e p e a r l i t e s t r u c t u r e

White iron White i r o n s f o r m f r o m a l l o y s l o w i n s i l i c o n c o n t e n t , w i t h CE s u f f i c i e n t l y below t h e e u t e c t i c p o i n t o r w i t h s u f f i c i e n t r a p i d c o o l i n g r a t e . The s t r u c t u r e o f w h i t e i r o n c o n s i s t s o f d e n d r i t e s o f p e a r l i t e surrounded by i r o n c a r b i d e .

Gray iron Gray i r o n , s u i t a b l e f o r t r i b o l o g i c a l use, cons s t s o f t y p e A f l a k e g r a p h i t e imbedded i n a f i n e p e a r l i t e m a t r i x w i t h l i t t l e f r e e f e r r i t e . When s p e c i f y i n g g r a y c a s t i r o n hardness, p e a r l t e s t r u c t u r e and g r a p h i t e s t r u c t u r e should be included.

Ma1 1 eabl e and nodul ar irons I n f i g u r e 4.5, one f i n d s a zone i n t h e 1.5 s i l i c o n - 2 . 5 carbon c o o r d i n a t e s marked m a l l e a b l e i r o n . T h i s d e f i n e s t h e compositions o f w h i t e i r o n s which can be transformed t o m a l l e a b l e i r o n b y h e a t i n g i n t o t h e a u s t e n i t i c zone and h o l d i n g a t temperature f o r a l o n g p e r i o d o f t i m e . T h i s decomposes t h e metastable c a r b i d e s t o produce i r r e g u l a r aggregates o f g r a p h i t e . Nodular i r o n , on t h e o t h e r hand i s produced b y a d d i t i o n o f magnesium r e s u l t i n g i n t h e f o r m a t i o n o f s p h e r i c a l nodules o f g r a p h i t e d u r i n g c a s t i n g .

N i Resist and N i Hard irons There i s o f t e n c o n f u s i o n about t h e d i f f e r e n c e s between these two c l a s s e s o f c a s t i r o n s . N i Resist. i s an a u s t e n i t i c g r a y o r n o d u l a r i r o n developed c h i e f l y f o r c o r r o s i o n and o x i d a t i o n r e s i s t a n c e . N i Hard i s a l o w a l l o y chrome-nickel w h i t e i r o n developed f o r abrasion r e s i s t a n c e .

100

N i c k e l i n i r o n promotes g r a p h i t e f o r m a t i o n , suppresses p e a r l i t e and s t a b i l i z e s a u s t e n i t e . Up t o 4.5% n i c k e l i s used t o promote m a r t e n s i t i c s t r u c t u r e and i s t h e b a s i s f o r t h e N i Hard a l l o y s . Chromium i s added t o a c t as a c a r b i d e s t a b i l i z e r and t o o f f s e t t h e g r a p h i t i z i n g i n f l u e n c e o f n i c k e l . S i l i c o n i s k e p t t o a l o w l e v e l i n t h e m a r t e n s i t i c N i Hards. Above 6.5% n i c k e l , a u s t e n i t e i s r e t a i n e d i n t h e c a s t i r o n s t r u c t u r e . N i R e s i s t a l l o y s c o n t a i n n i c k e l , copper & manganese i n v a r i o u s combinations. These a d d i t i o n s r e s u l t i n an a u s t e n i t i c s t r u c t u r e s t a b l e down t o room temperature. T h i s i s what t h e N R e s i s t a l l o y s a r e based on. N i c k e l c o n t e n t s above 18% produce an a l l austen t i c i r o n which i s nonmagnetic.

High chromium w h i t e irons High chromium i r o n s are used i n t h e m i n i n g i n d u s t r y f o r t h e i r g r e a t a b r a s i o n r e s i s t a n c e . As has been mentioned, chromium suppresses g r a p h i t e f o r m a t i o n and s t a b i l i z e s c a r b i d e s . With h i g h carbon l e v e l s , chromium a t between 12 and 27 p e r c e n t and some molybdenum, a v e r y hard w h i t e i r o n i s produced. A u s t e n i t i c i r o n s heat t r e a t e d t o m a r t e n s i t e have much b e t t e r a b r a s i o n r e s i s t a n c e than p e a r l i t i c i r o n s . The r e l a t i v e a b r a s i o n r e s i s t a n c e s o f these h i g h a l l o y i r o n s a r e shown i n f i g u r e 4.6 ( f r o m Diesburg and B o r i k ) [ 6 5 ] . S i l i c o n i s k e p t a t a l o w l e v e l t o m i n i m i z e t h e tendency t o form p e a r l i t e . Molybdenum h e l p s t o suppress p e a r l i t e and s p h e r o i d a l c a r b i d e s a f e r r i t e matrix. Hardenability i s a l s o increased. High chromium w h i t e i r o n s can be q u i t e b r i e . However, w i t h t h e r i g h t composition and m i c r o s t r u c t u r e , abrasion r e s s t a n t h i g h chromium i r o n s w i t h good toughness can be produced.

High Silicon or Duriron irons I r o n s w i t h 14 t o 24 p e r c e n t s i l i c o n e x h i b i t h i g h c o r r o s i o n r e s i s t a n c e t o a c i d s , e s p e c i a l l y h y d r o c h l o r i c a c i d . The s i l i c o n a d d i t i o n produces a tough, c o r r o s i o n r e s i s t a n t s i l i c o n o x i d e f i l m . Carbon i s k e p t t o n o t l e s s t h a n 0.35 p e r c e n t and n o t more t h a n 1 p e r c e n t . The c o n s t i t u t i o n diagram f o r these a l l o y s i s shown i n f i g u r e 4.7. Note t h a t a t 14 p e r c e n t s i l i c o n one has a s o l i d s o l u t i o n o f s i l i c o n i n gamma i r o n . The carbon e x i s t s as g r a p h i t e . A t s i l i c o n c o n t e n t above 15.2%, a new phase, n, appears. The n phase which appears a t between 825 and 1030' C i s so s l u g g i s h i n changing phase t h a t i t w i l l e x i s t i n t h e room temperature a l l o y .

101

High s i l i c o n i r o n i s a l o w s t r e n g t h b r i t t l e m a t e r i a l . Residual t e n s i l e s t r e s s e s can be annealed o u t b y h e a t i n g i n an a n n e a l i n g furnace. High s i l i c o n i r o n has e x c e l l e n t wear r e s i s t a n c e t o wet a b r a s i v e .

High phosphorous i r o n s Phosphorous increases t h e f l u i d i t y o f i r o n and has been used f o r c a s t i n g o f t h i n s e c t i o n s where f l o w p r o p e r t i e s o f t h e m e l t a r e i m p o r t a n t . A phosphorous c o n t e n t o f 0.2% produces an i r o n phosphate o r s t e a d i t e phase o f small d i s p e r s e d phosphate p a r t i c l e s which e t c h w h i t e i n t h e m i c r o s t r u c t u r e . A phosphorous c o n t e n t o f more t h a n 0.5% w i l l r e s u l t i n a continuous s t e a d i t e phase surrounding t h e p e a r l i t e c o l o n i e s . T h i s tends t o e m b r i t t l e t h e a l l o y . S t e a d i t e i s v e r y hard and i t s presence i n i r o n improves d r y wear r e s i s t a n c e .

\ 0.10

\

\

\ \

'

Spheroidite Matrix ( O I Mo)

0.09

\

\

\

\ \

0.08

\

W

Pearlite Matrix

Y

W

k

5

\

0.07

\

0.

5

15Cr 2Mo 1Cu (3 3C) 17Cr 1 5Mo 1 c u (3 oc) 0.06 0 0 0 0 l 8 C r O t o 3Mo 1Cu (2 9C) 2 o* 18Cr 2Mo 1Cu ( 3 OC) M vv 18Cr 2Mo 1Cu (3 3C)

01 AA

Y)

3

\

'

\

\

770+\

\ \

'

\

\

\

620'\

0.05

0.04 Matrix 0.03

Matrix I

I

I

10

20

30

I

40

1

I

50

60

HRC

f i g u r e 4 . 6 Abrasion Resistance o f High Chromium I r o n s ( P i n Test)

102

SILICON,

Wt

"/o

Figure 4.7 Iron-Sil icon Equil ibrium Diagram (from Angus

[66] )

103

HEAT TREATMENT OF CAST IRONS

Heat treatment is used to alter the microstructure of the as-cast product. Structural elements which can be influenced include graphite, ferrite, cementite (iron carbide), pearl ite, austenite and martensite. Hardening and softening the alloys are reversible processes. That is, irons hardened by quenching - like steel - can be annealed and irons that have been annealed to soft ferrite can be hardened by increasing the combined carbon content from the graphite phase. Carbides can be dissociated to graphite - but the graphite cannot be altered - it is a stable phase. A substantial improvement in wear resistance can be obtained by a variety of

hardening treatments. These include quenching and tempering, martempering, austempering, induction hardening flame hardening, nitriding and carbon i trid ing .

Stress Relief... In many tribolog cal applications, the dimensional stabi i ty of the bearing or rubbing surface is important to the precision of operat on of the device. Internal casting stresses are difficult to prevent in cast iron foundry practices. The residual stresses can cause gradual distortion as parts are machined. Internal stresses can also cause creep in a part if it is subject to elevated temperatures. Some plastic yielding can occur in castings as they age at room temperature. Stress relief by heat treatment can reduce residual stresses up to 80%. Heating a part for a few hours at 600' C will suffice.

... Cast iron can be heat treated to a martensitic structure just like steel. However, the resulting structure will contain flake or nodular graphite as it occurred in the original casting. To heat treat successfully, the cast iron must start out pearlitic in structure. In this condition, a part can be machined after casting and then the appropriate heat treatment carried out followed by a finish grind. This applies to both gray iron and nodular iron.

Hardening

The time-temperature-transformation diagram is used in designing heat treatments just as it is in steel heat treatment. A typical diagram for 2% silicon iron is shown in figure 4.8.

104

BHN 1200

Pearlite 202

Fine Pearlite 285

Bainite 401

MF

I

I 1

\ I 102

Schematic transformation diagram for a low silicon gray iron illustrates isothermal transformation at three different temperatures. Martensite formstion is by interrupted quenching.

Martensite 555 I 104

I 106

Time, Seconds

Figure 4.8 Time-Temperature-Transition Diagram for Cast Iron (from Walton)

105

Heat treatment begins with austenitizing the part by heating to above the critical temperature - the temperature at which pearlite decomposes to austenite (containing carbon in solution). The part must be held at temperature long enough to complete the decomposition of pearl ite and solution of carbon in the iron. Ferritic cast iron would require a very long furnace time to produce enough dissolved carbon because the only source of carbon is from the graphite - a more stable form than iron carbide. The austenitizing temperature ranges between 850 - 880' C (1560 - 1600' F) and is influenced by the silicon content. Increasing silicon content increases the critical temperature. The best response to hardening is obtained with cast irons containing total carbon between 2.8 and 3.2 percent and silicon between 1.3 and 2.1 percent. Quenching is done in oil. Water quenching may cause cracking in some parts. The TTT diagram indicates that a full quench in oil i s rapid enough to produce a martensitic structure. The diagram in figure 4.8 shows several heat treatment methods to achieve a variety of structures. Slow cooling keeps the process above the knee of the transformation curve and produces a pearlitic structure. If the part is quenched in a constant temperature bath and held while transformation takes place, a variety of compositions are possible depending on the holding temperature - as shown in figure 4.8. Thus a fine pearlite or a bainite can be produced with the difference in hardness as shown. This process of isothermal transformation is called austempering. The process not only is useful to control ultimate microstructure, but also allows large sections to cool slowly, preventing large temperature gradients with ensuing residual stresses or cracking. The shock of rapid cooling, then,is eliminated. If the quench is interrupted at a temperature well below the knee of the TTT curve but above the martensite transition temperature, held for a time long enough for the temperature gradients to level out and then quenched to martensite, a hardened part is produced with improved mechanical properties over one that is fully quenched. The interrupted quench is known as martempering and is often recommended for wear resistant parts.

** Quenching and tempering of gray and nodular irons can reduce corrosion resistance to dilute HCL.

106

Surface Hardening. .. Pearl itic and nodular cast iron can be surface hardened to white iron hardness levels for maximum wear resistance combined with toughness of the bulk material. Flame hardening, laser hardening and induction hardening will produce about the same results as in steel. Case hardness levels as high as 600 Vickers (55Rc) can be achieved by these means. Case depths are generally about 1.8 mm ( 70 mils). Noses on cam shafts can be hardened to over 600 Vickers with chromium contents from 0.2 to 0.7 percent. Flame hardening can produce pitting in cast iron with coarse graphite structure. Close control of the flame hardening conditions is required if pitting is to be avoided in even moderately coarse grained graphite gray irons 1631. Nitriding ... Gaseous nitriding can be used as a surface hardening method in special alloy cast irons containing aluminum and chromium. This will produce a surface hardness of 900 Vickers ( 67Rc) and also provides residual compressive stresses - further inhibiting surface fatigue type wear. Gray and nodular iron can be nitrided by cyaniding or the salt bath method. The resulting surface hardness is 790 Vickers (64Rc) and the case depth is about 10 pm (0.4 mils). Cyaniding not only improves wear resistance, reduces gall ing tendencies, improves fatigue strength but a1 so improves corrosion resistance. Hardenability . . . The same principles used in determining hardenabil ity of steels discussed in chapter 1 apply to cast irons. Alloying is used to improve the hardenability of cast irons - principally Cr, Ni and Mo are the alloying elements used.

107

WEAR PROPERTIES OF CAST IRONS

Pearlitic gray and nodular cast irons are the least expensive and easiest to use of wear resistant materials. The role o f graphite in the structure is complex but does appear to provide some solid lubrication when lubricant starvation occurs or under dry sliding conditions. The graphite phase also will inhibit tendency for adhesive contacts to grow and develop galling. Graphite will hold liquid hydrocarbon lubricants and provide a surface lubricant reservoir for dry starts or temporary loss of lubricant. Graphite can be detrimental to some rubbing applications. Under high contact stress, flake graphite can contribute the breakout of metal flakes during wear. The process is illustrated in figure 4.9. The SEM micrograph in (a) shows the flaking type of wear associated with high friction and heavy contact stress that can occur in pearlitic gray cast iron. The worn surface has been sectioned in (b) to show the subsurface process leading to the accelerated wear. As surface deformation occurs under heavy contact stress and high friction,a shallow layer smears out and forces metal to move around the graphite, opening long subsurface fissures. The fissures extend to the surface producing loose laps which break off as flakes. If this process occurs, hardening by treatment is in order. Ferrite caps ... These features develop in nodular iron as a result of the graphite nodules. High contact stress and friction will cause the ferrite phase surrounding the graphite nodules to flow over the nodules. The flowed material will not adhere to the graphite and will break off as small, work hardened "caps"[67]. The process is shown in figure 4.10. Ferrite caps can be formed during the machining process and should be removed before the part is put in service. The caps shown in figure 4.10 were in an as machined crankshaft for a gas compressor. The broken out caps embedded in the babbitt bearings, eventually accumulating sufficient concentration to cause abrasion of the crankshaft. Effect of microstructure ... Much has been written on the effect of gray iron microstructure on its wear properties. Walton summarizes information up to 1970 and the references are numerous. As has been noted the graphite phase is influential in wear resistance and both the average size and distribution can be important. Graphite size must be large enough to be effective. At the same time the number of flakes must be limited and the distribution should be even. The effect of concentration of graphite flakes on wear is shown in figure 4.11 [68]. Fine pearlite and minimum amounts of ferrite are also desirable for optimizing wear resistance.

108

a. Surface wear (SEM micro)

b. Microstructure o f wear

Figure 4.9 Effect o f flake graphite on wear of gray iron under high contact stress

Figure 4.10 Ferrite caps on ground nodular iron shaft

109

5

.12

a

a

.lo

0

- .08 v)

C

22 .06 CJ .04 .02

0

Figure 4.11 Effect of graphite flake size on wear of gray iron (from Walton)

... Cavitation is deleterious to graphitic gray cast iron. Gray irons with large graphite flakes and soft matrix are particularly susceptible. Ductile irons with martensitic matrix are the most resistant to cavitation.

C a v i t a t i o n erosion

110

Running-in of Gray Cast Iron ... Engine cylinders made o f gray cast iron will wear less provided they are run-in properly. The running-in process not only smooths the surface so that hydrodynamic lubrication of piston rings becomes more likely, but also a thin layer of iron oxide and graphite has been detected [69]. Presumably, this coating inhibits asperity welding and adhesive wear. Abrasion Resistance.. . Gray cast iron in the pearl itic condition does not perform well under severe abrasion conditions. By severe conditions is meant high stress abrasion, where the contact stresses are large enough to crush the abrasive medium and the abrasive is harder than the pearlite matrix. Heat treating the cast iron to achieve a martensitic structure improves the abrasion resistance although the flake graphite phase limits the resistance to high contact stress. Heat treated nodular iron behaves better in high stress abrasion. Figure 4.12 shows the increasing difference in abrasion resistance between the two types of cast iron as the surface stress is increased [70]. The relative abrasion resistance of cast irons is summarized in table 4.3

Table 4.3 Abrasion Resistance of Cast Irons * Material

Hardness V i ckers

Abrasion Factor**

565

0.4

Martensitic AISI 4150 Steel 750

0.6

Alloyed White Cast Iron

550

0.8

Unalloyed White Cast Iron

425

1.0

Gray Cast Iron

200

1.5

Ni -Hard (martensi t ic)

* From ASM Metals Handbook, 9th Edition [71] ** Ratio of weight loss o f sample to weight loss of AISI 1020 Steel

111

25

-+Tool

Steel

P

20

4 -Gray Cast Iron

/

- * -Nodular Iron

/

/

In

z

6

15

ai

e

m

/

a

8 3 .-% VI

E n a

/

lo

P 5

0

0

1

2

3

4

Surface Pressure, MPa

Figure 4.12 Abrasion resistance o f gray and nodular irons

5

112

HIGH ALLOY CAST IRONS

Low alloy white irons have been the cheapest abrasion resistant cast irons in use for many decades. The white irons have austenitic, pearlitic or martensitic matrixes with iron carbide inclusions. The martensitic grades exhibit the best resistance to abrasion. However, in order to achieve the martensitic structure, a rapid quench i s required and this results in a very brittle product. Therefore, either selected surface zones are made white iron by casting against "chill surfaces" or the irons are alloyed with appropriate amounts of nickel and chromium to encourage martensite production with slower cool ing rates. These are the Nihards, containing 3-5% Ni, 1-2.5% Cr & 2.5-3.5% C. The need for greater resistance to abrasive wear in the mining and ore processing industries has resulted in a series of high alloy cast irons containing 6-28% Cr, 1-4% C and other alloying constituents including Mo, Ni, Cu and Va. These alloys can be heat treated to produce ferritic, austenitic or martensitic matrix containing eutectic carbides - mostly chromium carbides. These carbides are much harder than iron carbides and harder than quartz -the most prevalent hard abrasive found in mining. The martensitic and austenitic high alloy cast irons are mostly used for abrasion resistance. The ferritic grades are used for high temperature oxidat ion resistance. The relative abrasion resistance of some high alloy cast irons are summarized in figure 4.6 [72]. Note that the austenitic white irons perform slightly better than the martensitic. The work hardening properties and tendency to transform to martensite under high shear stresses may be the explanation. Of course, high carbon content austenite is known to be very wear resistant. The low carbon variety is soft and prone to galling. Brittleness of high alloy irons are of some concern - especially in applications involving impact such as ore crushers, ball mills and pneumatic tools. Fracture toughness in these alloys can be controlled by microstructure and alloy content. Fracture toughness for high chromium-molybdenum irons as a function of carbide content is shown in figure 4.13 [73]. Note that the austenitic white irons demonstrate higher fracture toughness at lower carbide content. Thus, in view of the good abrasion resistance of austenitic high alloy irons, they might be considered in applications requiring toughness as well.

113

There are a large number o f high alloy cast irons. Each one has been developed for a given abrasion and corrosion problem. Choice o f the appropriate alloy may require the aid of an alloy producer.

0

10

20

30

Carbide Volume, %

Figure 4.13 Fracture toughness o f white irons

40

50

114

CHAPTER

5 .

CARBON

GRAPHITE

MELTING POINT..

. ...4375'C

HARDNESS RANGE.. .15 - 2 8 KNOOP DENSITY RANGE ...1.7 - 1.9 g/CC ELASTIC MODULUS ...7Et3 - 1.5Et5 MPa ELEC RESISTIVITY .....1.5 - 6.2 Mill iohm-cm

GRADES OF CARBON-GRAPHITE: Resin bonded Carbon (Phenolic filled with ground flake graphite) Hard Carbon (calcined coke t powdered graphite self bonded) High Thermal Conductivity Carbon Graphite (Graphitized carbon heated to 2760,248>'C) Metal Impregnated Carbon Graphite Resin Impregnated Carbon Graphite Oxidation Inhibited High Temperature Graphite Soft Machinable Carbon Graphite APPLICATIONS FOR CARBON GRAPHITES: Water 1 ubricated bearings Seal rings Cryogenic bearings Gas Bearings Electric brushes Aircraft brakes Pump vanes

115

INTRODUCTION Carbon g r a p h i t e i s a v e r y i n e r t s o l i d s t r u c t u r a l m a t e r i a l manufactured f r o m m i x t u r e s o f carbon and ground up n a t u r a l and manufactured g r a p h i t e . The carbon can s t a r t i n t h e f o r m o f lamp b l a c k , p e t r o l e u m coke and p i t c h coke. C a l c i n e d p i t c h coke, used i n h a r d carbon g r a p h i t e grades, i s h a r d enough t o s c r a t c h most metals. Various mixes o f carbon and g r a p h i t e , o f t e n bonded w i t h p i t c h , a r e heated and c o m p l e t e l y carbonized o r heated t o a " g r a p h i t i z i n g temperature" (above 2760'C). The c a r b o n i z i n g o f t h e b i n d e r produces a s t r u c t u r a l s o l i d . The e l a s t i c modulus can be l o w o r q u i t e h i g h depending on t h e c o n s t i t u e n t s . G r a p h i t i z i n g a l t e r s t h e s t r u c t u r e o f t h e b i n d e r and carbonaceous f i l l e r s somewhat. X-ray d i f f r a c t i o n i n d i c a t e s t h a t t h e carbon-carbon bond d i s t a n c e changes. However, t h e g r a p h i t i z a t i o n t r e a t m e n t can produce 2 t o 5 t i m e s increase i n thermal and e l e c t r i c a l c o n d u c t i v i t y . P o r o s i t y can be c o n t r o l l e d so t h a t a range o f f r o m 20% t o l e s s t h a n .01% p o r o s i t y can be o b t a i n e d i n carbon g r a p h i t e s . D e t a i l s o f t h e manufacture o f c a r b o n - g r a p h i t e can be found i n two e x c e l l e n t t e x t s b y Paxton & Shobert on manufactured carbon [74,75].

USAGE IN TRIBOLOGY Carbon g r a p h i t e i s a s e l f l u b r i c a t i n g m a t e r i a l w i t h h i g h r e s i s t a n c e t o c o r r o s i o n b y most aggressive media except s t r o n g l y o x i d i z i n g a c i d s . It w i l l r e t a i n i t s s t r e n g t h a t v e r y h i g h temperatures. The chemicals t h a t a t t a c k carbon g r a p h i t e a r e l i s t e d i n Table 5.1. The temperature l i m i t a t i o n s o f carbon g r a p h i t e a r e r e l a t e d t o o x i d a t i o n . I n an o x i d i z i n g atmosphere, carbon g r a p h i t e i s s a t i s f a c t o r y up t o about 500'C. With o x i d a t i o n i n h i b i t o r a d d i t i v e s , t h e upper l i m i t can be pushed t o 650'C. A carbon e l e c t r o d e i n a carbon a r c lamp operates i n open a i r a t about 4000'K and o x i d i z e s a t a r a t e o f about 3 inches p e r hour. The bending s t r e n g t h and t e n s i l e modulus o f carbon g r a p h i t e a c t u a l l y increases somewhat w i t h i n c r e a s i n g temperature. Carbon g r a p h i t e i s a m a t e r i a l w i t h l o w thermal expansion (about 1/4 t h a t o f s t e e l ) . Thermal c o n d u c t i v i t y o f t h e h i g h e r c o n d u c t i v i t y grades i s comparable t o aluminum. The m a t e r i a l i s v e r y d i m e n s i o n a l l y s t a b l e d u r i n g thermal e x c u r s i o n s and i s n o t s u s c e p t i b l e t o thermal shock. Carbon g r a p h i t e i s a l o w modulus m a t e r i a l - about 10 t o 15 t i m e s l o w e r t h a n s t e e l . I t i s a l s o b r i t t l e and r e q u i r e s c a r e i n h a n d l i n g i n c l u d i n g m i n i m i z a t i o n o f impact l o a d s and t e n s i l e s t r e s s s t a t e s when used i n d e s i g n o f machine elements.

116

Table 5 . 1 Chemicals corrosive to carbon graphite Bromine Chlorine, hot 1 iquid Chromic acid Chrome pl at ing solution Fluorine Hydrofluoric acid Iodine Nitric acid Sulfuric acid, over 75% concentration

The lubricating ability of carbon graphites depends on adsorbed water or organic materials. When used in vacuum or very low humidity conditions, carbon graphite will not lubricate and will tend to wear rapidly, releasing a powder wear debris which is deliquescent. Specially treated carbon graphites are available for use in vapor free environments. A special high altitude brush grade was developed during World War I 1 for use in electric motors in high flying planes. The properties of some commercial carbon graphites used in tribological applications are summarized in Tables 5.2 and 5.3 The composition of the various grades in tables 5 . 2 and 5.3 are shown in Table 5.4.

117

Table 5.2 Mechanical P r o p e r t i e s o f Carbon Graphites

MATER IAL

P-29 P - 658RC 486 G- 1 PM- 103 Pyrol y t i c Graphite P-03 P-15 P-2w P-5 P - 5Ag P-9 s-95 Conversion f a c t o r s : MPa x 0.145 = k s i

FORM

HARDNESS TENSILE YOUNGS MAXIMUM VICKERS STRENGTH MODULUS TEMP DPH MPa MPa ' c

Molded Molded

540 730

Molded Molded CVD Baked Bab Imprg Baked Baked Ag Imprg Baked

180 660 600 750 320 680 820 480 650

48.32 48.32 6.89 20.68 68.90 689.40 37.92 62.05 27.58 41.36 68.94 27.58 96.52

1.80Et04 2.4Et04 2.07Et04 1.38Et05 2.76Et04 1.24Et04 2.62Et04 6.89Et03 1.38Et04 2.62Et04 1.45Et04 1.38Et04

230 260 399 204 400 538 204 288 316 260 288 371

118

Table 5.3

Physical p r o p e r t i e s o f c a r b o n - g r a p h i t e

MATER IAL

DENSITY kg/ CU METER

Graphite C r y s t a l P-29 P - 658RC 486 G- 1 PM- 103 Pyrol y t i c Graphite P-03 P-15 P-2w P-5 P - 5Ag P-9 s-95

*

2.26Et03 2.26Et03 1.80Et03 1.83Et03 1.66Et03 1.94Et03 5.90Et03 1.94Et03 1.94Et03 2.41Et03 1.66Et03 1.66Et03 2.35Et03 1.66Et03 8.30Et03

THERMAL EXPANS CONDUCTIVITY m/m Watt/m k /'C

400 6 12 8.6 1.73 43.00 69.20 10.38 17.30 8.65 25.95 12.11 138.40

0.36E - 06 3.00E-05 1.60E-06 1.50E-06 9.18E-06 8.10E-06 9.36E-06 6.66E-06 7.74E-06 4.60E-06 9.90E-06 8.91E-06 4.60E-06 3.96E-06 3.96E-06

P a r a l l e l t o basal planes

t Normal t o basal planes

Con e r s i o n f a c t o r s : 3 kg/m x 3.613E-5 = l b / s u i n : kg/m x 0.001 W/m/K x 0.176 = BTU/ft /hr/'F

Y

J/kg/K x 0.238E-4 =BTU/lb/'

F

=

gm/cc (SG)

RESIT micro ohm-cm

HEAT CAPACITY J/g K

4.00 4.00 3.8

8.5* 8.5t

2.50

0.60 4.10 1.80 4.50 2.0 3.80

119

Table 5.4 Composition of Carbon-Graphite Grades MATERIAL P-29 P-658RC 486 G-1 PM-103 Pyrolytic Graphite P-03 P-15 P-2w P-5 P- 5Ag P-9 s-95 T-0054

COMPOSITION Polyester impregnated Resin impregnated Mechanical carbon Molybdenum disulf ide, metal bonded Carbon graphite C, Babbitt Carbon graphite Carbon graphite C, Silver Carbon graphite Graphite, inorganic impreg

WEAR OF CARBON GRAPHITE

Carbon graphite wears by abrasion. Usually the surface against which it slides is harder and its asperities will tend to grind material off the carbon surface. Therefore, surface roughness of the mating part in a carbon graphite sliding system is very important to wear control. When carbon graphite is lubricating effectively, a transfer film develops on the counter surface and graphite vs graphite results. Under these conditions, wear rate is minimum and transfer continues as the transfer film wears away. The wear of the transfer film is slow. As with other tribological materials, the wear rate of carbon graphite is rapid during a short "wear-in period and then levels off to a very low linear process. The wear-in period represents the initiation and development of a transfer film. Most bearing appl ications for carbon graphite involve relatively low bearing stress. Wear does not follow a linear relationship with load. After a critical load, wear rate increases rapidly. Bearing stress should be kept below 50 psi for reasonable life. 'I

120

Mating materials are an important consideration when using carbon graphite in sliding contacts. Those carbon graphites containing hard carbons, calcined petroleum, charcoal, glassy carbon require stell ite, chromium plate, ceramics (Tungsten Carbide, Alumina) as mating surfaces. Chromium plate, ceramics, hardened steel, cast iron, and carbon graphite make good mating materials for "soft" carbon graphites containing high percentages of natural graphite and soft metal or polymer impregnants. Aluminum and bronze or brass are inferior as mating materials. Massaro [76] has demonstrated the importance of selecting the correct mating materials for seal ring applications. Using the materials shown in Table 5.5, tested seals made up of various combinations. Face type seals were run for 100 hours sealing 46'C water. The shafts turned at 1750 rpm resulting in a rubbing speed of 9.1 m/sec. The results for three seal nose contact pressures are shown in Table 5.6. Note the significant difference in carbon ring wear when reaction bonded silicon carbide was used as compared with tungsten carbide.

Table 5.5 Materials used in Massaro's face seal wear tests Carbon Material Density Hardness Material Identification gm/cc Scler C* Seal nose Grade A Hard Carbon-graphite Resin impreg.

1.83

95

Grade B Medium Hard Carbon-graphite Resin impreg.

1.80

85

Grade C Medium hard Carbon-graphite Resin impreg

1.80

84

*Schlerescope C

121

Table 5.6 Wear data

-

nose materials v s ring materials sealing warm water

Ring material Fused Alumina WC Contact press. kPa 500 770

Si Graphite* 1500

Nose Grade A Frict Coeff. Nose wear** Ring wear

.08 .051

.12 .0025

.07 .09

.0002

.0003

.oooo

Nose grade B Frict coeff. Nose wear Ring wear

.10 .134

.09 .017

.0002

.oooo

.07 .162 .0002

Nose grade C Frict coeff. Nose wear Ring wear

.ll .220 .0000

*Siliconized graphite ** Wear rate, mm/100 hr

.14

.06

.009

.oooo .oooo

.oooo

122

MECHANICAL SEALS

One of the most important uses for carbon graphite is in mechanical seals. Some typical seal designs in which carbon graphite is used are shown in Fig. 5.1. Nose pieces in face seals, rings in turbine seals, vanes in positive displacement pumps and rings for dry gas piston compressors use carbon graphite as a basic material. Carbon graphite is an ideal material for these purposes because of several properties. First of all, it is self lubricating and, therefore less apt to damage the smooth, flat surfaces required for effective sealing. Secondly, it has a low elastic modulus which helps in providing conformity to a parallel surface to minimize th seal gap over the entire contact region. Thirdly, it can be manufactured to a high degree of flatness and precision. Fourthly, it has a structure that lends itself to the generation of hydrodynamic lubricating films between para lel flat surfaces. And finally, it is inert.

'sup

1. Anti-Rotation Device 2. cup 3. Extended Length 4. Primary-Seal Carrier 5. Primary-Seal Ring, Carbon 6. %a1 Bore 7. Sealing Face 8. Sealing-Face Inner Diameter 9. Sealing-Face Outer Diameter

10. SealingFace Width 11. Seal Nose 12. Seal Outer Diameter 13. Secondary Seal (ORing) 14. Secondary -Seal Land 15. Secondary-Seal-Land Diameter 16. Washer 17. Wave Spring

a. Face Seal

1. Back Ring, Carbon 2. Coil Spring 3. Cover Ring, Carbon 4. Garter Springs 5. Primary-Seal Ring, Carbon

6. Runner 7. Seal Housing 0. Shaft 9. Spring Adapter

b. Circumferential Seal

Figure 5.1 Typical seal designs using carbon seal elements

123

When sealing gases, the self lubricating property and ability to finish to a flat very smooth surface makes carbon graphite work. The sealing elements are brought together by spring pressure or a combination of spring pressure and gas pressure. Sliding contact occurs at the sealing surface unless gas film support is designed into the system. Owing to its low modulus, a seal ring, reacting to the pressure drop across it will elastically deform. The result is a lifting of the carbon graphite surface away from its contact on the high pressure side and contact on the low pressure side. Under proper conditions, the surfaces can be separated a very small amount by a process known as leakage hydrodynamics [77]. If the seal is to be exposed to changing pressure differentials, it is best to try to select a carbon with a high elastic modulus to minimize distortion. If there are hydrocarbons present in the gas being sealed, heating o f the seal interface becomes important. At high sliding speeds, the seal interface can generate sufficient heat to cause polymerization of hydrocarbons and a buildup of a soft solid on the carbon and the mating surface. This material will cause leakage. This effect can be minimized by using a grade of carbon graphite with some mild abrasive characteristics. The carbon graphite will tend to clear the hydrocarbon from the mating surface as it deposits. Carbon graphite grades with distinct hard phase in a soft matrix will tend to wear to a surface topography that encourages hydrodynamic lubrication in a liquid environment. The microstructure shown in Fig. 2.2 consists of fine crystallites of graphite distributed in an amorphous matrix. The wear rates of the two phases are different and the graphite grains tend to project from the surface after wear in. These small plateaus can act like pad bearings because of the fluid dynamics of the liquid flowing past them. In this way two parallel flat surfaces can be supported on a very thin fluid film, thus reducing wear and friction [78]. The gap developed between the sealing surfaces is small enough so that leakage of a viscous fluid is negligible. The process i s complicated by the tendency for the carbon graphite to elastically deform under the pressure gradient across the seal. Care must be taken in design of the seal components that advantage is taken of the self generating hydrodynamic film.

124

There are a number of grades of carbon graphite used in seals. Each grade has a somewhat different microstructure. Generally they contain a graphite phase , some porosity and, possibly a resin impregnant. Figure 5.2 shows the microstructure of ATJ-S. The black areas are pores - but since they are not interconnected, the permeablility is low. The white accicular phase is graphite.

Figure 5.2

Microstructure of ATJ-S carbon graphite

SLEEVE BEARINGS

Carbon graphite is best known for its use in water lubricated bearings. Grades which are not influenced by radiation flux were used as pump bearings in the first successful nuclear reactors. Carbon is virtually immune to gamma, beta and alpha radiation. There is little effect from neutron irradiation at dosages below 10 neutrons per cc. In these applications, the bearings had to perform in reactor grade water at 145'C. Pressurized water is highly corrosive to many metal bearing alloys but does not effect carbon graphite at all. In addition, the carbon graphite provided insurance for re1 iable operation of the pumps even in case of loss of fluid. The bearings were capable of operating without lubricant for a period of time. The immunity to galling or seizure of carbon graphite provided additional impetus for their use. Precipitation hardened stainless steel was used as shafting for these bearings [79].

125

Carbon graphite expansion rate is about 1/3 of that for steel and therefore, the effect on operating clearance must be considered when sizing bearings. For water lubricated bearings, a very close clearance is required to induce film lubrication owing to the very low viscosity of the lubricant. The shaft will expand more than the bearing when heated and cause loss of clearance. Clearance can be kept constant at operating temperatures by shrink fitting the carbon graphite bearing in as steel sleeve and pressing the sleeved bearing into the steel housing. The carbon graphite will be under compression in the sleeve and as the steel housing expands during heating, the carbon graphite will press out against the confining wall and follow it as it expands. In this way, a nearly constant clearance can be maintained in the bearing. Because o f the very thin fluid film generated in water lubricated bearings, the surfaces of the bearing and shaft must be finished to a polished condition. It is especially important that the steel shaft surface be smooth since it will wear the carbon graphite when it comes into contact with it. For successful bearing operation, the shaft surface should be in the neighborhood of 5 to 10 microinches cla. Shrink fitting carbon graphite bearings in metal housings is also useful to reduce fracture tendency under impact loading and to prevent rotation of the bearing in the housing. For unlubricated bearings, wear and heat generation are two important criteria to consider when selecting materials. The use of PV criterion for estimating carbon graphite performance can be helpful. Shobert recommends the PV levels shown in Table 5.7. Table 5.7. PV Values for Carbon Graphite - Steel Combinations Lubrication

Oil Moist Air Dry air Hi Temp ( > 500C) *P= psi : V= fpm

PV*

150,000 15,000 15,000 15,000

Remarks

Adjuvants required Ox idat ion inh i b i t ion required

126

It must be remembered that the PV level is a measure of heat generation and that the friction coefficient will influence this value as much as the load and sliding velocity. In conditions where the upper limit of PV is approached, frictional heating can el iminate resin impregnated carbon graphites and materials with minimum friction (containing significant graphite) should be considered. Carbon graphite bearings can be used over a wide range of temperatures from cryogenic to 500'C - with adequate attention to either adjuvants for supplementing water vapor or protection from oxidation. As long as they are not exposed to excessive abrasive or impact conditions, performance is reliable. Since the material is inert, it can be used in special applications such as food processing equipment,corrosive fluid pumping, solvents hand1 ing, or where dimensional stability, low friction or resistance to thermal shock is required. Other unique properties include low thermal expansion, low density, high electrical and thermal conductivity. Properties for various grades of carbon graphite are listed in table 5.1. THRUST BEARINGS AND VANES

Carbon graphite performs very well as self lubricating thrust bearings and as sliding vanes in vane pumps. Under these conditions, the material operates under boundary lubrication if a process fluid is present and therefore, the self lubricating capability of the material is important to endurance and re1 iabil ity. Flanges are not recommended for sleeve bearing thrust surfaces. Instead, allowing the thrust to be carried on the end of the bearing is usually satisfactory. A small shoulder on the end of the bearing for thrust is appropriate. If larger thrust surfaces are needed, a separate thrust ring is recomended.These can be metal jacketed to reduce chances of edge chipping and cracking.

127

Vane pumps f o r b o t h gases and l i q u i d s o f t e n use carbon g r a p h i t e vanes. Common examples where t h e y a r e used a r e a i r blowers, and pumps f o r g a s o l i n e , r e f r i g e r a n t s , s o l v e n t s and w a t e r . For gases, hard a b r a s i o n r e s i s t a n t grades o f carbon g r a p h i t e a r e used f o r vane m a t e r i a l . The pump housing i s made o f a hardened s t a i n l e s s s t e e l coated w i t h hard chromium p l a t e o r f l a m e sprayed m a t e r i a l which has been p o l i s h e d t o a 4 t o 6 m i c r o i n c h f i n i s h . For l i q u i d pumps, c o n s i d e r a t i o n must be g i v e n t o t h e tendency o f t h e carbon g r a p h i t e t o s w e l l i n water o r o t h e r p o l a r l i q u i d s , The i n c r e a s e i n dimension can be as much as .05%. To p r e v e n t t h i s , t h e carbon g r a p h i t e must be f i r e d t o above 1100'C t o reduce t h e micropore s t r u c t u r e . One added advantage o f carbon g r a p h i t e f o r vane pump use i s i t s h i g h f a t i g u e s t r e n g t h i n bending. Paxton mentions one grade o f carbon g r a p h i t e having an endurance l i m i t s i m i l a r t o s t e e l . A r e d u c t i o n i n s t r e n g t h o f about 30% was found a t t h e endurance l i m i t .

C A R B O N BRUSHES Carbon and carbon g r a p h i t e m a t e r i a l s have l o n g been used as b r u s h m a t e r i a l s i n r o t a t i n g e l e c t r i c a l machinery because o f t h e i r good e l e c t r i c a l c o n d u c t i v i t y , l o w wear, l o w c o n t a c t r e s i s t a n c e and l o w f r i c t i o n . The s e l e c t i o n and d e s i g n o f carbon brushes i s a h i g h l y s p e c i a l i z e d f i e l d . A number o f t e x t s e x i s t on t h i s s u b j e c t alone [80,81,82]. There a r e a number o f grades used f o r e l e c t r i c brush a p p l i c a t i o n s . These m a t e r i a l s i n c l u d e carbon g r a p h i t e , e l e c t r o g r a p h i t e (carbon p r o d u c t heat t r e a t e d a t a temperature above 2525'C), metal impregnated g r a p h i t e and a b r a s i v e impregnated carbon g r a p h i t e f o r keeping t h e commutator c l e a n . Table 5.8 shows t y p i c a l brush a p p l i c a t i o n s and t h e carbon grades used.

128

Table 5.8 Electric brush applications for carbon.*

Appl ication

Type

Pressure Velocity Hardness 2 KPa m/sec MN/m

Wear Coeff 7 x 10

Subfract HP Metal-Graphite Motors

23

1.9

80

Fract HP Motors

Carbon Graphite

63

17.5

100

Automotive

Metal-Graphite

48

4.2

40

7.3

98

11.8

30

0.64

162

7.1

180

A1 ternators Elec-Graphite Starters

Metal -Graphite

7.2

111

134

Diesel-Elec Elec-Graphite

55

45

90

0.85

Aircraft

68

23.5

90

2.4

Elec-G

t

BaF 2

* Brushes running against copper. Moisture and oxygen are important components in a high endurance electrical brush system. When the dew point drops below -1O0C,untreated brush wear will increase from about 100 microns per hour to 10 cm/hr with a release of a large cloud of dust. High altitude, arctic climate and space environment can raise this problem. For this purpose, carbon graphite impregnated with barium fluoride, molybdenum disulfide and other additives are used. These additives do the job that water vapor does and prevent rapid dusting wear. Some gaseous environments can be destructive to carbon brush operation owing to chemical reactions. Those contaminants to be wary of when using carbon brushes include chlorine, hydrogen sulfide, HC1, sulfur dioxide and ammonia.

129

OTHER

USES

Carbon graphite has found uses in other Tribological applications. Some successful ones include meter bearings, balls seats, jet engine exhaust nozzle roller bearings, rupture disks, flow meter rotors, chemical metering pumps and self a1 igning spherical seat bearings.

130

CHAPTER

6 - C E R A M I C S AND

SPECIAL

MELTING POINTS HARDNESS TENSILE STRENGTH YOUNG'S MODULUS DENSITY THERMAL EXPANSION THERMAL CONDUCTIVITY FRACTURE TOUGHNESS, K I c

ALLOYS

1275'C - 3600'C 157 DPH - 3500 DPH 517 MPa - 2400 MPa 150 - 550 GPa 2.2Et3 - 1.7E+4 Kg/m3 2.3E-6 - 1.78E-5 m/m 1.6 - 176 W/m 0.7 - 12 MPa m4

TYPES C ERAMI CS CERMETS GLASSES REFRACTORY METALS SUPER ALLOYS HARD FACING ALLOYS

APPLICATIONS Bearings & Seals i n S l u r r y Pumps Bearing & Seals i n L i q u i d Metal Pumps Rocket Bearings & Seals C o n t r o l Rod Bearings i n Reactors Gas T u r b i n e P a r t s Low Heat Loss D i e s e l Engine C y l i n d e r L i n e r s & Valves S t e r l i n g Engine P i s t o n s Exhaust Valves i n I n t e r n a l Combust i o n Engines M i n i n g Equipment Rock D r i l l i n g B i t s H i g h Speed Tools High Speed Angular Contact B a l l Bearings

131

INTRODUCTION

The materials discussed in this chapter are used mainly in tribological applications involving high temperatures and/or highly corrosive environments. Although these materials are very wear resistant, economics usually precludes their use solely for wear reduction. There are three classes of materials to be discussed: Ceramics and Cermets Cobalt and nickel base superalloys Refractory metals (molybdenum,tungsten,zirconium,titanium) High temperature bearings, seals and gearing have seen two periods of development in the U . S . during the last 50 years. The first period was in the 50s and early 60s when 315'C (600'F) pressurized water and liquid metal heat transfer systems were being developed for nuclear reactors and development of rocket engines was under way. Considerable testing o f the wear properties of cobalt and nickel based alloys and ceramics 1 ike silicon carbide, aluminum dioxide and silicon nitride was carried out at high temperatures and in unusual environments. Much good information was generated and is still available in publications from that period [83]. The second period started in 1978 with the large scale commitment to the development of the "adiabatic" diesel engine as part of an energy conservation program [84]. This development period has extended through the decade of the eighties and been concerned mostly with the use of ceramics in internal combustion engines with very high combustion chamber temperatures. In addition to the diesel engine applications, there has been a continual striving t o use ceramic materials in high performance gas turbines and in other heat engines such as the Sterling engine. Nickel base and iron base superalloys received considerable attention during the development of nuclear power generating stations. Wear and corrosion testing was done in 316'C pressurized water, liquid sodium, liquid NaK and liquid salts. The results of those tests are available to the engineering community. Data has been extracted from these sources and included in this chapter.

132

Other uses for ceramics and special alloys include high speed cutting tools, rock drill s, coal gasification components, high temperature water pumps, valves and heat exchanger parts, pump parts for liquid slurry pipe lines, high speed rolling contact bearings and high speed tape recording systems. Ceramics are costly not only in their manufacture but also in fabricating parts from them. Quality control is difficult and very important in the consistency of results in application. Ceramic parts have to be taken from their molded or sintered shape to the final part by time consuming and expensive grinding operations. Special techniques are required to eliminate imbedded debris from grinding which can have disastrous results when it becomes loose in the operating system. Because of the above problems associated with fabrication of ceramic parts, ceramic coatings are finding greater usage as they are developed. In this way, the bulk of a part can be made of a relatively inexpensive metal alloy and the wear surface then coated with the appropriate ceramic material. The same procedure is used for special alloys, too. Ceramics have been used by man for many thousands of years. They are found in the ruins of ancient cities in the forms of pottery, bricks and decorative tiles. It has been only relatively recently that ceramics have been used in machinery. The ceramics used for wear resistance constitute a small percentage of the uses of ceramics today. The materials selected for review in this chapter represent the grades which have been appl ied to tribological problems and found to be effective. These materials are very pure compared to bricks and t i 1 es .

CERAMICS

Engineering ceramics can be divided into the following groups: Borides Carbides Nitrides Single oxides Mixed oxides Sil icides G1 asses

133

The range of properties t h a t these groups cover i s shown i n f i g u r e s 6.1 t o 6.5 (found i n t h e excellent handbook by Lynch, Ruderer and Duckworth [ 8 5 ] ) . Note t h a t t h e r e a r e some exceptions t o t h e general t r e n d s a s , f o r instance, t h e very low e l a s t i c modulus f o r boron n i t r i d e ( B N ) and t h e very high e l a s t i c modulus f o r tungsten carbide ( W C ) . The very high thermal c o n d u c t i v i t i e s of s i l i c o n carbide (SIC) and b e r y l l i a (BeO) are notable a l s o . Note, a l s o , t h a t t h e thermal conductivity o f Be0 drops more than an order of magnitude a t elevated temperature. I t can be seen t h a t t h e r e i s a l a r g e range of possible combinations of p r o p e r t i e s afforded by t h e d i f f e r e n t c l a s s e s of engineering ceramics. The mechanical and physical p r o p e r t i e s of s e l e c t e d grades o f ceramics a r e presented i n t a b l e s 6.1.

Table 6 . 1 P r o p e r t i e s o f E n g i n e e r i n g Ceramics

MATERIAL

HARDNESS TENSILE VICKERS STRENGTH OPH MPa

ALUMINA BERYLLIA BORON CARBIOE CHROMIUM CARBIDE GLASS, QUARTZ GLASS, SODA HP TIC K 1626 PSZ 1027 PSZ 2016 PSZ MS P S Z TZ3Y PSZ 2191 SiAlON SILICA SILICON CARBIDE SILICON NITRIDE SPK SN80 TITANIA TITANIUM TITANIUM T I TAN I UM TUNGSTEN ZYS2Y20A

1500 3200 2600

3000 1400

1600 1158 286 1780 853 2700 1300

Zr02 CARBIDE OIBORIOE N I T R I OE CARBIDE Zr02

900 3000 3500

262 103 172 262 110

YOUNGS MODULUS MPa

KIc MPa

mf

3.00

OENSITY

1760

3.88E+03 2 . 2 1E+03 6,64E+03 6.64Ec03 2.49E+03 2.49E+03 5.54E+03 6.09E+03

6.00 0.68 0.70

1649

345 276 172

10.00

689

9.00

1172 1020 450 103 103 524 545

6 50

io.oa 8.50 7.70 0.60 4.60 4.00 5.30 2.70

896 7.00

zoo0 1500 1400

MAX OP TEMP 'C

896 2399

12.00 10.00

THERM CON THERM EXP HEAT m/m CAPACITY

Kg/ WATT/m Cu Meter K

34.60 1.73 19.03 19 03 1.56 17.30 19.03

1371 1371

1482 1400 2399 1649 1482

5.54E+03 6,09E+03 5.81E+03 3.24~+03 3.04E+03 3.04~+03 3.04E+03 4.15E+03 4.15E+03 4,43E+03 4.43E+03 5.54E+03 3,04E+03 5.54Ec03

1.73 2.94 2.94 21.30 1E4.70 147.05 30.00

25.09 5.02 25.95 25.95 65.74 1.56

5.02

I'C

7. i a ~ - o 6 3.60E-05 2.50E-06 9.80E-06

1. O ~ E - O ~ 8.60E-06 9.50E-06 i.98~-05 i.00~-05 1. O ~ E - O ~ 1.00E-05 3.04E-06 1.30E-05 4.00E-06 2.30E-06 8.10E-06 9.4UE-06 8.00E-06 8.10E-06 8.00E-06 6.00E-06 1 .o w 0 5

JIgK

0.13 2.09 1 67 0.84 1.26 1 26

1.05 0.67

0.50 0.84 0.67 1.05 0.85 0.84 0.84 1.05 0.84

0.25

134

Figure 6.1 Theoretical Density and Melting or Decomposition Temperatures for Classes o f Ceramics

Aeltina Point or Decomposition Temperature, F

Theore tical Density. g/cm3

Material Class

P

1 BORIDES

I

I

r

n

I

-

N

-

(

N

n

0

I

--a s

CARBIDES

O”0

NITRIDES

-02

SINGLE OXIDES

0

M I X E D OXIDES

SULFIDES

SlLlClDES

I

1

METALLOID ELEM E N T S (B.Ge,Si) MISC. METALLOID

n lca N

W

NTERMETALLICS

-

135

Mater'al Class

10

0

20

30

40

50

60

70

80

BORIDES

CARBIDES

Legend NITRIDES

OC

+

70 F 20OC

IN SINGLE OXIDES

MIXED OXIDES

T - -.-

--OMg0*A1203

SULFIDES

SlLlClDES

lo6 psi

0

I0

20

30

40

50

60

70

80

YOUNG'S MODULUS

F i g u r e 6.2 Range of Young's Modulus Values f o r Various Classes o f Ceramics ( f r o m Lynch, Ruderer & Duckworth)

136

Class

110

0.2

0.4

0.6

0.8

1.0

1.2

BORIDES

1.4

1.6

Legend

CARBIDES

NITRIDES

SINGLE OXIDES

M I X E D OXIDES

SULFIDES

SlLlClDES

Percent

0

0.2

0.4

0.6

0.8

1.0

12

1.4

1.6

LINEAR THERMAL EXPANSION

Figure 6.3 Range of Linear Thermal Expansion for Various Classes of Ceramics (from Lynch, Ruderer & Lynch)

137

'm I Legend

Sic

--a7 0 F 2000F

I

16 11 I

NITRIDES

I

I

............ ............ ........... .......................

J-

-

1

1

1Btu/(hr)(ft)(F)lO

10

20

30

40

50 FIdlOO

I20

140

- J T H E R M A L CONDUCTIVITY

F i g u r e 6.4 Range o f Thermal C o n d u c t i v i t y Values f o r V a r i o u s C l a s s e s o f Ceramics ( f r o m Lynch, R u d e r e r & D u c k w o r t h )

138

Class

Microhardness, lo3 Kg/mm2

BORIDES

CARBIDES

N l T R IDES SINGLE OXIDES

M I X E D OXIDES

SULFIDES

I

SlLlClDES

M IC RO H AR DNESS F i g u r e 6.5 Range o f Microhardness Values f o r c l a s s e s o f Ceramics (from Lynch, Ruderer & Duckworth)

139

STRUCTURE AND PROPERTIES OF CERAMICS

Ceramics differ from metal alloys in that they are ionic or covalent bonded crystals while the metallic bond is by free electrons. Metals can be melted and allowed to crystalize in a poured shape as they cool. Ceramics require the bonding o f solid crystalline powders to produce bulk shapes. Ceramics are held together by relatively low melting point glassy binders or by self bonding of crystals compressed and sintered at high temperature. The bonds between atoms in covalently bonded ceramic materials are very strong and highly directional. Thus bonds are highly stable and the result is high melting points. Hafnium carbide has perhaps the highest melting point, 4150’C (7500’F). These materials also have very low Poisson’s ratios ( as low as 0.10). Silicon is a good example of a covalent ceramic. Metallic bonds are highly nondirectional and therefore dislocations can move under much lower applied stress. Ionic bonded crystals are held together by the attraction of arrays of positive and negative ions. In single crystals, the resistance to slip is relatively low. Thus, single crystal MgO and NaCl have mechanical properties similar to metals and have been used to model metals in dislocation studies. However, the slip systems in ionic materials are more limited than in metals and when ionic crystals are combined to make polycrystall ine materials, deformation will be inhibited by the inability of individual crystals to accommodate to deformation changes in neighboring crystals. Thus cracks will develop along grain boundaries. Some ceramics like alumina have complex sl ip systems because positive and negative ions are located on parallel slip planes and dislocations must extend normal to the slip plane in order to move through the crystal without changing the force fields in the lattice. This makes slip in polycrystalline alumina very difficult and hence increases the brittleness of this material. Because of the way in which ceramics are made, it is almost impossible to achieve theoretical density in a fabricated part. That is, ceramics are porous. Because of their porosity and the presence of microcracks, ceramics are brittle. Because it is difficult to control the porosity of ceramic bodies, Weibull statistics are sometimes used in reporting strength values from multiple tests. This practice is not used very much for very dense ceramics.

140

... Ceramics are generally used at high temperatures or in situations where frictional heating generates high surface temperatures. Therefore, resistance to thermal shock is essential in these applications. Rapid cooling is more detrimental than rapid heating because it puts the surface in tension. Therefore the design must minimize quenching conditions. High thermal gradients are built up in ceramics as a result of their low thermal conductivities and small existing cracks grow into fractures owing to the low fracture toughness of ceramics.

Thermal shock

It has been suggested that thermal shock resistance is a function of thermal expansion rate, elastic modulus and thermal conductivity. The relation is: TSR

- ok/Ea

Where

a is the fracture stress k is the thermal conductivity E is the elastic modulus a is the thermal expansion coefficient

The TSR values of some ceramic materials and hard metals are compared in table 6.2

141

Table 6.2 Thermal Shock Resistance of Ceramics

Material

Beryl 1 ia Tungsten Carbide PSZ MS Chromium Carbide HP Tic PSZ 2191 Boron Carbide Alumina Sil ica Titanium Carbide Silicon Carbide SiALON Silicon Nitride Tool Steel Carbon Graphite S95

Thermal Therma1 Expansion Conductivity

36 6 10 9.8 8.6 10 2.5 7.1 12.96 8

4 3.04 2.3 10

4

1.7 1.56 1.7 19 17.3 2.9 19 34.6 164.7 26 147 21.3 30 38 138.4

TSR

0.01 0.42 0.6 1.4 1.7

1.5 3 3.4 3.5 6 10.3 10.95 22 50 244

Conversion factors: x 0.176 = BTU/ft3/hr/"F

W/m/K

Note the very low thermal stress resistance value for beryllia. This material has a high elastic modulus, high thermal expansion coefficient and low fracture tensile strength. Note, also, the larger thermal stress resistance for silicon nitride as compared with silicon carbide. Silicon nitride has a larger tensile strength than silicon carbide. The high conductivity carbon graphite grade shown in the table shows an exceptionally high thermal shock resistance. The very low elastic modulus for carbon graphite is responsible for this unusual value. Even though PSZ exhibits high fracture toughness for a ceramic material, its thermal shock resistance is low owing to its poor thermal conductivity and high thermal expansion coefficient. This is a significant factor in its performance under high velocity sliding conditions along with its poor thermal mechanical stress properties, explained below. The above thermal shock relationship is only an indicator - it should not be used as a precise measure for fracture.

142

Other approaches t o e s t i m a t i n g m a t e r i a l s p r o p e n s i t y t o crack under thermal shock have been suggested, i n c l u d i n g use o f t h e G r i f f i t h c r i t e r i o n f o r b r i t t l e f r a c t u r e [86].

.

D u r i n g h i g h speed s l i d i n g on d r y surfaces, Thermal Mechanical I n s t a b i l i t y . . m a t e r i a l s can experience t r a n s i e n t h o t spots. The process r e s p o n s i b l e f o r t h i s phenomenon i s known as thermal mechanical i n s t a b i l i t y ( T M I ) . It has a l s o been c a l l e d thermal e l a s t i c i n s t a b i l i t y . Ceramics a r e s u s c e p t i b l e t o TMI because o f t h e i r poor thermal d i f f u s i v i t y p r o p e r t i e s . When TMI occurs, r e d h o t t o w h i t e h o t s t r e a k s can be seen on t h e s l i d i n g s u r f a c e exposed t o view. These s t r e a k s o f t e n t e n d t o move back and f o r t h across t h e wear p a t h i n a p e r i o d i c f a s h i o n . An example o f c l a s s i c TMI i s shown i n f i g u r e 6.6. T h i s was developed i n a b u t t o n vs d i s k c o n f i g u r a t i o n . The d i s k was r o t a t e d a t h i g h speed and t h e s t r e a k s were photographed i s s u i n g f r o m t h e t r a i l i n g edges o f t h e b u t t o n s .

F i g u r e 6.6 H i g h temperature s t r e a k s on a h i g h speed b u t t o n - d i s k ceramic m a t e r i a l t e s t e r . Streak temperatures e s t i m a t e d a t 880'C [87]

143

Thermal distortion of the sliding surfaces is responsible for the development of hot spots. The process is self energizing, in that a few asperity contacts produce localized rapidly increasing temperatures. Owing to the inability of the material to dissipate heat the asperity expands and increases in height above the surface. This point, then takes most of the contact load and the heat input increases. Temperature and expans ion increase at an exponential rate until the asperity softens or vaporizes and another asperity takes up the contact support and begins the same process. Rapidly changing high temperature locations on the surface result. A diagram o f this process i s shown in figure 6.7. For a material with poor thermal shock properties, TMI can be devastating. Hot spots jumping all over the surface will produce spalling and accelerated wear.

TMI has been developed [88]. The onset of TMI can be predicted as a critical velocity for a given material and sliding velocity. The critical velocity, Vcr can be derived from the following relationship:

A mathematical model of

Vcr

=

4kz/(pE)znr]z

(89)

Where: Vcr k P a E rl

d C Z

critical sliding velocity thermal conductivity = coefficient of friction = coefficient of thermal expansion = Young's modulus = thermal diffusivity (k/dc) = density = specific heat = width of slider = =

Critical velocity is the approximate sliding velocity above which thermal instability sets in. Note that contact load does not influence the onset of thermal instability. Load will influence the amount of frictional heat generated.

144

r T h e r r n a l stress crack

Figure 6.7 Diagram of TMI process

There is a significant difference between critical velocity values for different ceramics - depending on their thermal and mechanical properties and the coefficient of friction at the sliding interface. Some materials are compared in table 6.3.

Table 6.3 Critical Velocities for TMI for Several Ceramics (From Dufrane [ g o ] ) Material

Coefficient of Friction (estimated)

PSZ (Yittria stabilized)

.12 (Lubricated)

ATTZ

.12 (Lubricated)

Si3N4

Sic

"c r cm/sec (fpm) 8.9

(17.6)

15

(29.7)

.08

538

(1065)

.07

1100

(2178)

145

The l a r g e d i f f e r e n c e s i n c r i t i c a l v e l o c i t y f o r t h e m a t e r i a l s shown i n t a b l e 6.3 can be seen and were proven b y Dufrane i n s i m u l a t e d engine experiments [go].

Heat treatable ceramics .... T r a n s f o r m a t i o n toughened ceramics have been a r e c e n t development i n ceramic technology. By m o d i f y i n g t h e s t r u c t u r e w i t h small amounts o f a d d i t i v e s , toughening can be achieved b y t h e development o f b e n e f i c i a l r e s i d u a l s t r e s s e s . Z i r c o n i a toughened alumina (ZTA) i s a f i n e g r a i n e d alumina c o n t a i n i n g 10 t o 20 % z i r c o n i a . T h i s m o d i f i c a t i o n o f alumina produces an i n c r e a s e i n s t r e n g t h and e l a s t i c modulus p l u s a two t o t h r e e f o l d i n c r e a s e i n f r a c t u r e toughness w i t h o u t an increase i n d e n s i t y . ZTA cannot be used a t temperatures above 900'C because o f much i n c r e a s e d creep. Other ceramics have been toughened i n a l i k e manner - s i l i c o n n i t r i d e - z i r c o n i a , f o r instance. Z i r c o n i a i t s e l f can be toughened b y a d d i t i v e s and h e a t t r e a t i n g . Oxides o f calcium, magnesium and y t t r i u m have been s u c c e s s f u l l y used i n t h i s process. O r i g i n a l l y , these a d d i t i v e s were used t o s t a b i l i z e t h e c u b i c s t r u c t u r e z i r c o n i a achieves a t s i n t e r i n g temperatures. When p u r e s i n t e r e d z i r c o n i a i s c o o l e d t o room temperature, t h e c u b i c phase t r a n s f o r m s t o a m o n o c l i n i c phase w i t h an accompanying l a r g e volume change. The r e s u l t i n g t e n s i l e s t r e s s e s w i l l cause e x t e n s i v e c r a c k i n g i n t h e s o l i d body. F u l l y s t a b i l i z e d z i r c o n i a remains c u b i c b u t has a h i g h thermal expansion c o e f f i c i e n t and t o g e t h e r w i t h l o w thermal expansion r a t e , r e s u l t s i n v e r y poor thermal shock r e s i s t a n c e .

I f t h e amount o f s t a b i l i z e r i s reduced, a p a r t i a l l y s t a b i l i z e d z i r c o n i a (PSZ) i s produced. R e f e r r i n g t o t h e phase diagram f o r z i r c o n i a - m a g n e s i a shown i n f i g u r e 6.8, i t can be seen t h a t t h e a d d i t i o n o f magnesia reduces t h e temperature o f t h e c u b i c t o t e t r a g o n a l phase change. S t a r t i n g w i t h t h e 8 t o 11 % Mgo c o n t e n t used i n PSZ, one can s i n t e r a t a reasonable temperature.Since phase changes i n t h i s m a t e r i a l a r e s l u g g i s h , one ends up w i t h a c u b i c m a t e r i a l c o n t a i n i n g a c e r t a i n amount o f t e t r a g o n a l phase. T h i s m a t e r i a l has a much l o w e r thermal expansion c o e f f i c i e n t t h a n f u l l y s t a b i l i z e d z i r c o n i a because o f t h e volume expansion on c o o l i n g . The r e s u l t i n g m a t e r i a l , however, i s l o w i n s t r e n g t h . I f , however, t h e m a t e r i a l i s s i n t e r e d i n t h e c u b i c r e g i o n and aged i n t h e c u b i c t t e t r a g o n a l r e g i o n a f i n e p r e c i p i t a t e o f t e t r a g o n a l phase r e s u l t s . T h i s phase tends t o t r a n s f o r m t o m o n o c l i n i c when s t r e s s e d causing l o c a l areas t o go i n t o compression and t h u s i n h i b i t i n g crack p r o p a g a t i o n [91]. The r e s u l t i n g f r a c t u r e toughness i s almost t w i c e t h a t o f s i l i c o n c a r b i d e and y e t , t h e hardness and a b r a s i o n r e s i s t a n c e are retained.

146

Figure 6.8 Phase Diagram f o r zirconia - MgO system [ 9 2 ]

147

An a l l t e t r a g o n a l z i r c o n i a (TZP) i s a v a i l a b l e t h r o u g h t h e s i n t e r i n g o f submicron z i r c o n i a powder, t h u s suppressing t h e t r a n s i t i o n t o m o n o c l i n i c . TZP has e x c e p t i o n a l l y h i g h h o t hardness and toughness. The d i f f e r e n c e between PSZ and TZP i s shown i n t a b l e 7.4.

Table 6.4 Mechanical and P h y s i c a l P r o p e r t i e s o f M o d i f i e d Z i r c o n i a s

Density Flex Strength Hardness Youngs Mod F r a c t . Tough. Max Temp.

PSZ

Z TA

5.2 - 5.9

3.9

600

800

900

330

1300

2000

1300

1900

200

450

200

380

7-12

2 - 5

6 - 8 900

Therm Cond

1.3 - 1.5

Expans Coef

8 - 12

Conversion f a c t o r s : MPa 0.145 = k s i 3 kg/m x 3.613E-5 = l b / s u i n : kg/m x 0.001 W/m/K x 0.176 = BTU/ft /hr/'F

6 -10

TZP

5.6

900

900

3.8

1300

1.7 - 3.5

26

7 - 15

8.0

8.0

4

=

Alumina

gm/cc (SG)

148

These advanced ceramics are being used increasingly in difficult wear problems. These include tool inserts, cam lobes, high temperature seals, high temperature water pump bearings etc. The PSZ materials have been considered for use in low heat loss diesel engines for cyl inder 1 iners and valve seats. Experimental work has shown, however, even though the material has good fracture toughness and strength at elevated temperature, the critical velocity for thermal mechanical instability is below the normal piston velocities expected in these advanced engines. The result is thermal shock induced spalling caused by thermal spikes [ 9 3 ] . CERAMIC TOOLS

Ceramics are finding application as tool materials for high speed finishing, high removal rates and machining difficult to machine materials. The advantage ceramics provide is hot hardness, high wear resistance and resistance to corrosion. Two classes of ceramics are generally used for metal cutting: alumina and silicon nitride. Because of brittleness, these materials are modified to provide a tool which will stand up under machining conditions. Alumina is modified with titanium carbide, zirconia and silicon carbide whiskers. One of the first modifications used was "black ceramic" or alumina with 25 to 40% Tic. This increases the hardness and the thermal conductivity of the alumina. Zirconia transformation toughened alumina (ZTA) is obtained by the addition of 10 -20% zirconia. The differences in properties of ZTA and pure alumina are shown in table 6.4. Alumina with higher zirconia content and traces of tungsten carbide are also in use for tool materials as well as silicon carbide whisker reinforced alumina [94]. Silicon nitride has the desired combination of high temperature mechanical properties, resistance to scaling and good thermal shock resistance. The thermal conductivity is almost twice that of alumina-Tic and the thermal expansion of about half. Thus, as has been pointed out earlier, thermal shock is proportional to thermal conductivity and inversely proportional to thermal expansion. Note in table 6.2 that thermal shock resistance of silicon nitride is 22 while that of alumina is 3 . 4 . Silicon nitride comes in many forms because of the different additives that are required to promote sintering. Alumina and silica are the most common additives used.

149

The SiALONs are a series of compositions combining silicon nitride and alumina. One benefit derived from the use o f alumina is its inertness. The presence of alumina inhibits cratering wear associated with chemical reaction between the tool and the workpiece. Although SiALON is not as hard as titanium carbide modified alumina, its wear resistance and hot hardness is impressive. The hot hardness of a number of tool materials is shown in the chart in figure 6.9. Note that Sialon maintains its hardness better than the other materials from 6OO0C to 1000°C. It should be pointed out that silicon nitride will react with steel at elevated temperatures so that Sialons are not usually used for cutting steels. Alumina-Tic is more appropriate. For cutting superalloys, sialons and Sic whisker reinforced alumina have been used. Temperature.

O F

750 1100 1500 1800 2200 2550

32

400

0

200 400 600 800 1000 1200 1400 Temperature. a C

Figure 6.9 Hot Hardness o f Some Ceramic Tool Materials [95]

150

CERAMIC ROLLING CONTACT BEARINGS

Silicon nitride has been evaluated extensively for use as a rolling element in ball bearings. The interest stems from requirements for advanced gas turbines for aircraft. Its advantages for that application include low mass (only 40% that of M50 tool steel) which reduces the centrifugal force loading of balls against the races at high speeds, high hot hardness, hence lower contact area and less heat generation, and better corrosion resistance. Low mass balls in angular contact bearings will not be forced up the race at high speed, thus requiring less thrust load to insure traction. Very smooth finish and close to theoretical density for the silicon nitride balls is required for effective performance. Rolling contact experiments with silicon nitride running against M-50 have demonstrated contact fatigue lives comparable to AISI 52100 and M-50 steels [96, 971. Silicon nitride rolling elements perform well in turbine bearing 1ubricants.The relative rolling contact fatigue lives of silicon nitride, M-50 tool steel and AISI 52100 bearing grade steel are shown in the stress -life plots in figure 6.10.0ther ceramics such as alumina and silicon carbide have been evaluated as possible low mass rolling elements but have not been as effective as silicon nitride.

6.5

............

\ El:

N

E

In-

6 :

In w

g

5.5

:

r

% I

5 :

.A

-.-.

3

4.5

: I

4

\..

-.

3

I

.........

."".I

--*-HS-l

I 0 SIN

- 6 N C -

I 3 2 SIN

\..

: : 'L : -.-A : . . . . . . . . . . . . ..-

-.

x...

\..

from R.J. Parker 8 E.V. Zaretsky Trans ASK (JOLT) July 1915, 350

I

Figure 6.10 Rolling Contact Fatigue Life of Hot-Pressed Silicon Nitride Balls and Steel Balls from 5 Ball Fatigue Tester .

151

C E RMETS Cermets are aggregates of small ceramic particles in a matrix of metal. The ceramic constituent provides the high hardness and resistance to wear while the metal provides the fracture toughness and tensile strength and acts as a binder for the ceramic particles. Probably tungsten carbide, cobalt bonded, is the most familiar engineering cermet. Other engineering cermets include: Tic-Ni A1 umina-Cr Tungsten Carbide Cobalt bonded tungsten carbide has long been used for machine tools and in rock drilling bits. The cermet is produced by liquid phase sintering of cobalt and WC. The milled WC particles and cobalt powder are thoroughly mixed and formed into the part by high pressure. The green part i s then heated to about 1400 'C to melt the cobalt phase.

152

I600

I I

LlOUlD

-

I too

!

I

I

I

1000

0

CO

I I

10

2 0 30 4 0 SO 60 70 80 TUNGSTEN C A R B I D E (WC), Wt%

90

1

10

wc

Figure 6.11 Tungsten carbide - Cobalt Phase Diagram [98]

153

Referring to the phase diagram in figure 6.11 and assuming a common grade of tungsten carbide with 94% WC and 6% Co by weight, as the temperature of the mixture is raised slowly, WC dissolves into the cobalt by solid state diffusion. (The solubility of WC in cobalt at 1000°C is 2%). When the temperature reaches 1400'C and is held, a liquid having the composition as shown by C will form. The liquid then wets the WC crystals and is drawn into the interstices by capillary action. By this means, a high density body is produced. Depending on the percentage of cobalt present, the cermet will either be a three dimensional network of WC with cobalt filling the voids in the network, or a matrix of cobalt with WC imbedded in it. In some cases, WC particles may be separated by a very thin film of cobalt.Since the solubility of WC in cobalt decreases as the temperature is lowered. Fine WC crystals precipitate from the cobalt matrix as the part is cooled after sintering. This precipitation process tends to strengthen the cobalt binder and make the cermet stronger. Additions of Tic and TaC to tungsten carbide cermets are used to reduce cratering of tools in machining of steel.

Tic cermets with cobalt, nickel and mixtures of Co, Ni and Cr are used for high temperature wear appl ications since they provide superior oxidation resistance, thermal shock resistance and high strength at elevated temperature as compared with WC cermets. Chromium-alumina cermets provide the extreme inertness of alumina and the increased toughness of the metal binder. These materials are extremely resistant to high temperature oxidation. Impact resistance is not as good as the WC and Tic cermets. The alumina cermets resist attack by liquid metals and have been used for bearing and seal parts in liquid metal pumping systems in liquid sodium heat transfer systems. Properties of some of the above cermets are listed in table 6.5

154

Table 6.5 Properties of Some Cermets Cermet

Hardness HV

Tensile Strength MPa

Young‘s Modulus GPa

3%CO WC 6%Co WC 1O%Co wc 25%Co WC 20%Ni Tic K162B*

1400 1300 1100 860 1300 513

4240 4233 4137 3068 524 712

0.72 0.72 0.60 0.48

LT-lB**

0.4 0.26

Poisson‘s Thermal Thermal Ratio Conduct. Expans. W/m/K OC x .24 .28 .25 .25

121 100 71 19

5.5 5.8 7.0 11.1 9.5 8.5

* 25% Ni, 5% Mo, 70% Tic **19% A1203, 2% Ti02, 59% Cr, 20% Mo GLASSES

Glasses are non crystalline sol id metal oxides. Glass is not a supercooled liquid because it has short range order. When glass is cooled from the liquid state,its viscosity increases and there is no crystal1 ization and no sudden change in density and mechanical properties as it solidifies to the glassy state. A glass transition temperature does exist, however, for the thermal expansion coefficient. That is, the expansion coefficient is much less in the glassy state as compared with the liquid. The temperature-viscosity properties for several different glasses are shown in figure 6.12.

155

I5 13

II h

"E

93;Z v

-

.-x

7 1 ;

.-5(>

5 "

2

s

3 I

200

400

600

XOO

loo0

1200

1400

1600

Temperature ( " C )

Figure 6.12 Viscosity-temperature curves for various glasses. [99]

156

The principal glass forming oxides include Si02, B203 & P 0 . Other glass 2 5 formers include oxides of arsenic and germanium, lead fluoride and selenium and sulphur. All other oxides are found as additives to these glass formers. Thus, during high temperature sliding, materials which will oxidize to the glass forming oxides can form glassy surface films. Some glasses used in engineering applications are listed in table 6.6. Table 6.6 Properties o f some gl asses Glass

Densiiy Mg/m

Silica

2.2

700

100-120

72

0.17

5.5

1.45

Soda Lime 2.5

550

80-100

69

0.21

87

1 .o

52

0.23

91

62

0.20

33

87

0.25

42

120

0.24

57

Hi Lead

Hardness Tensile Young's DPN Strength Mod, MPa GPa

4.3

290-340

Borosilic. 2 . 2

550-600

Alum.si1 ic.2.5

580-630

Pyroceram 2.6

620-640

80-100

185

Conversion factors: MPa 0.145 = ksi 3 kg/m x 3.613E-5 = lb/$uin : kg/m x 0.001 W/m/K x 0.176 = BTU/ft /hr/'F

Poisson's Ratio

4

=

gm/cc or Mg/m

Expansion Thermal Coeff. Comd. 'C x W/m/'c

3

(SG)

1.2

3.6

157

Glasses find a limited use in tribological applications owing to their brittleness. Any appl ication that involves concentrated contact can lead to local spalling and chipping. Glass has been used for thread guides in yarn spinning and fabric making machines. Glass fibers are used as reinforcement for plastic bearing materials. Glass reinforced PTFE makes a good self lubricating sleeve bearing. (See chapter 7 ) Because of the fine surface finish possible in glass, it has been used for self acting air bearings in lightly loaded fractional horsepower electric motors [loo]. Glass has been used in transparent bearings to study lubricant cavitation and EHD films [ l o l l . Note the wide range of thermal expansion properties and elastic moduli in table 6.6. Pyroceram, shown in the table is a crystallized glass. It is made by precipitating small amounts of metal crystals in the glass. Heat treating will cause glass to crystallize around the metal nuclei and produce a fine grained very hard and abrasion r sistant glass-ceramic. Pyroceram is translucent owing to its crystal ized state. It has high strength and high thermal shock resistance, making it useful in bearing and seal applications. It can be bonded to metals readi y.

REFRACTORY METALS AND ALLOYS

For very high temperature sl iding and rolling contact conditions such as reactor cool ing systems, rocket seals, reentry vehicle control surfaces, hot working machines etc, metals with structural strength and fracture toughness are available. Some of the properties required for such systems include high me1 ting temperature, high hot hardness, dimensional stability, viable oxide layers, corrosion resistance and resistance to galling and seizure. Dimensional stability involves the ability of a material to resist permanent changes in size at stresses below the elastic limit. Dimensional instability at elevated temperature is usually the result of solid state reactions such as transformation or decomposition of unstable phases and relaxation of internal stresses. For instance, in cobalt base alloys, the cobalt will change from c.p.h. structure to f.c.c above 430'C and the change in volume resulting from this phase change can distort the part. High melting point is an indicator of the maximum temperature limit at which an alloy can be used. Those materials with high melting points are usually those with high temperature strength.

158

Oxidation characteristics will strongly influence the maximum service temperature of an alloy. If the alloy forms a strong, tough oxide scale, it generally will resist galling and excessive wear. Phase changes in the alloy at elevated temperatures can change the oxidation characteristics to undesirable products. Some metal oxides change their oxidation rate drastically at a given critical temperature. For instance, molybdenum produces a tight protective oxide to 760'C. Above this temperature, so called catastrophic oxidation takes place in which the Moo3 vaporizes and no longer protects the metal surface. Alloys containing glass forming constituents such as silicon will often develop a glassy oxide when subjected to high temperature sliding. This oxide tends to be protective and reduces the friction.

Refractory Metals.. . Refractory metals are those metals having melting points at least as high as chromium (1875'C). Metals which fall into this category and have been used in tribological appl ications include chromium, molybdenum, tantalum and tungsten. For high temperature use, these metals are usually used in alloys with improved oxidation resistance. Tantalum is used as a metal matrix binder in an MoS2 base compact. Molybdenum has been used in two alloy forms, TZM and %Ti for high temperature sliding contacts where nonoxidizing conditions exist. Examples include inert gas lubricated bearings, liquid sodium or NaK bearings and high vacuum conditions. TZM alloy can be used at temperatures as high as 870'C in non oxidizing environment. Molybdenum has a high elastic modulus and low thermal expansion coefficient. Therefore it is attractive for high temperature shafting. Molybdenum shows excellent compatibility with molybdenum disulfide sol id 1 ubricant. When molybdenum is used in 1 iquid sodium, boundary lubrication conditions can be produced when small amounts of oxygen are present. The surface oxide of molybdenum will react with sodium to form sodium molybdate, a renewable solid 1 ubr icant .

159

SUPER ALLOYS

Super alloys include iron, cobalt and nickel base alloys capable of service above 650'C. Of these three, probably the iron base alloys are the least oxidation resistant and least wear resistant. However, the iron base chromiummolybdenum-nickel alloy 16-25-6in the nitrided condition will form a tight spinel type oxide scale when lubricated with a solid lubricant at elevated temperature. The Inconels and Hastelloys have excellent oxidation resistance at temperatures up to 870'C. However, they require lubrication to prevent adhesion and galling. Inconel X has performed well with a calcium fluoride base lubricant over the temperature range 100' - 815'C [102]. Super alloys have been evaluated in dry sliding at 870'C and their relative wear damage observed. The results are 1 isted in table 6.7, below.

Table 6.7 Wear of Superalloys Sliding Against Inconel X at 815'C (1600'F) (From Amateau & Glaeser [lo21 Load 1.9 kg; Velocity 0.02 m/s Material

Renee 41 Hastelloy C M-252 Incoloy 901 J 1570 Inconel 700 Inconel X A-286*

Friction Coeff. Initial Final

0.47 0.34 0.31 0.44 0.42 0.66 0.85 0.88

0.25 0.29 0.27 0.28 0.30 0.35 0.35 0.32

Order of Increasing Damage 1

2 3 4

5 6 7 8

* Age hardenable iron base superalloy used in gas turbines

160

Nickel Base Alloys ... High nickel content alloys (40 to 50 % Ni) provide resistance to oxidat ion at elevated temperatures and good high temperature strength. Since nickel alone is relatively soft and adhesive, nickel alloys containing chromium, aluminum and titanium for hardening are used for wear resistance. Lubrication, however, is almost a must in most applications. Oxidation of nickel alloys aids in the reduction of adhesion during sliding contact. Generally, a significant reduction in friction is seen at temperatures above 540'C. This is often a reversible process - as the temperature drops, the friction rises after reaching a critical temperature. Cobalt Base Alloys ... Many hard facing alloys are cobalt base materials. The Stellites, Colmonoys, etc. are the more familiar cobalt alloys. These alloys have excellent high temperature oxidation resistance. Their high temperature strength is not as good as nickel base alloys. Cobalt reacts differently to high temperature than nickel. A plot of friction versus temperature is shown in figure 6.13. The plot shows that cobalt exhibits its lowest friction level at room temperature. As the temperature increases, friction gradually rises and in the range between 480'and 59O0C,the friction rises rapidly to a high level. In this range, surface damage increases also. A decrease in friction is seen at higher temperatures. These effects are attributed to complex oxidation processes influenced by temperature 1 eve1 . Cobalt also transforms from c.p.h. to f.c.c at about 430'C change has been associated with frictional changes.

and this phase

Tribaloys ... A relatively recent development in high temperature alloys for wear resistance has been the Tribaloy series. These alloys are either cobalt base or nickel base and contain molybdenum , chromium and silicon. The alloy combination results in a hard intermetallic phase, known as Laves phase. This produces a hardened superalloy with high hardness and wear resistance at elevated temperature. These materials have excellent abrasion resistance. The abrasion resistance of Tribaloys is compared with other cobalt base materials in figure 6.14. Properties and composition o f super alloys are listed in tables 6.8 - 6.10

161

TEMPERATURE O F

F i g u r e 6.13 E f f e c t o f Temperature on C o b a l t F r i c t i o n [lo31

162

Table 6.8 Mechanical P r o p e r t i e s o f High Temperature A l l o y s

MATER IAL

FORM WORKED CAST

TEMPER HEAT TREAT

HARDNESS VICKERS DPH

TENSILE YOUNGS MAX OP STRENGTH MODULUS TFMP MPa MPa C ~

PLATED Heat 400C Electroless N i PLATED As Depos. Electroless N i HASTELLOY C CAST ANNEALED INCONEL 718 CAST Aged MOLYBDENUM, 0.5% T i ARC CAST MOLYBDENUM TZM ARC CAST MONEL K-500 CAST Rene 41 CAST STELLITE 1 CAST STELLITE 3 C h i l l Cast STELLITE 6 CAST STELLITE F CAST STELLITE STAR J CAST STOODY 6 CAST T i t a n i u m 6A1-4V Boronized HT T i t a n i u m 6A1-4V CAST Annealed TRIBALOY T-400 TRIBALOY T-700 TRIBALOY T-800 TUNGSTEN PM SINTER DRAWN VANASIL 77 CAST Age Hdnd WASPALOY Annealed WASPALOY HT, PH WAUKESHA WAUK. 54 ZIRCALLOY 2 CAST ANNEALED

689 55 1 1379 689 790 689 1419

2.00Et05 2.00Et05 1.97Et05 2.00Et05 3.24Et05 3.17Et05 1.79E+05 2.13Et05 2.41Et05

393

834

2.10Et05

675 526 1700 301 655 485 739 257 140 157 373 165 220 185

517 689

2.55Et05 2.07Et05 1.10Et05 1.10Et05 2.69Et05 2.14Et05 2.41Et05 2.76Et05 9.30Et04 2.11Et05 2.11Et05 1.52Et05 1.59Et05 9.66Et04

1050 500 216 410 264 325 320 562

896 689 689 276 827 275 517 586 517

320 320 649 538 649 538

649 400 400 704 649 704 260 87 1 87 1 649 400

163

Tab1e 6.9 P h y s i c a l P r o p e r t i e s o f H i g h Temperature A1 1oys

MATERIAL

DENSITY THERM CON KG/cu m WATT/m K 4.18 135.70 13.84 131.50 128.02 13.84 20.20

Electroless N i 8.03Et03 HASTELLOY C 8.86Et03 INCONEL 718 8.03Et03 MOLYBDENUM TZM 1.02Et04 MOLYBDENUM, 0.5% T i 1.02Et04 MONEL K-500 8.30Et03 Ren6 41 8.30Et03 STELLITE 1 9.13Et03 STELLITE 3 STELLITE 6 8.30Et03 STELLITE F STELLITE STAR J 8.86Et03 STOODY 6 8.30Et03 T i t a n i u m 6A1-4V 4.43Et03 T i t a n i u m 6A1-4V 4.43Et03 TRIBALOY T-400 9.13Et03 TRIBALOY T-700 8.86Et03 TRIBALOY T-800 8.66Et03 TUNGSTEN 1.70Et04 VANASIL 77 2.77Et03 WAS PALOY 8.14Et03 WASPALOY 8.14Et03 WAUKESHA 8.86Et03 WAUK. 54 8.86Et03 ZIRCALLOY 2 6.70Et03

THERM EXP

m/v

/ c

1.20E-05 1.25E-05 1.31E-05 5.40E-06 5.40E -06 1.40E-05 1.57-05

1304 1370 2610 2610 1316

1275

176.46

7.27 7.27 176.46 136.67 171.27 167.00 126.29 12.11 12.11 27.68 25.95 14.50

MELT PQINT C

1.44E-05 1.44E-05 9.44E-06 9.44E-06 1.78E-05 1.85E-05 7.00E-06 1.62E-05 1.39E-05 1.39E-05 3.24E-06 4.50E-06 6.50E-06

3

x 3.613E-5

W/m/K x 0.578 J/kg/K

=

=

3

x 0.001

BTU/sqft/ft/hr/*F

x 2.388E-4

m/m/"C x 0.55

l b / c u i n : kg/m

=

=

BTU/lb/'F

in/in/'F

o r cal/g/*C

=

90.00 140.00 13.00 50.00

1288 1243 1288 3410 538

gm/cc (SG)

HEAT CAPACITY J/kg/'K

3.85Et02 4.60Et02 2.51Et02 2.93Et02 2.93E+02 4.50Et02

104.00 104.00 91.00

171.00

Conversion f a c t o r s : kg/m

RESIS micro ohm-cm

5.86Et02 5.86Et02

5.65 8.37Et02 5.23Et02 74.00

164

Table 6.10 Chemical Composition of High Temperature A l l o y s MATER IAL

Electroless N i HASTELLOY C INCONEL 718 MOLYBDENUM, 0.5% T i MOLYBDENUM TZM MONEL K-500 Rene 41 STELLITE 1 STELLITE 3 STELLITE 6 STELLITE F STELLITE STAR J STOODY 6 T i t a n i u m 6A1-4V TRIBALOY T-400 TRIBALOY T-700 TRIBALOY T-800 TUNGSTEN VANASIL 77 WASPALOY WAUKESHA WAUK. 54 ZIRCALLOY 2

COMPOSITION

P C C C

1-12,Ni 88-99 .15,W 4,Fe 6,Cr 16,Mo 17,Va .3,Mn 1, S i 1, N i 54.6 . 1 , S i .75,Mn .5,Cu .75,Ni 50,Cr 18,Co 5,Mo3,A1.8,Til,Fe .02,Ti .5,Mo 99.4 T i 0.5,Zr 0.1,C 0.02,Mo 99.25 N i 65,Al 3,Fe 2,Mn 1.5,C .25,Si 1.0,Ti .5,Cu 27 C r 19,Co 11,Mo 10,Fe 5,C 0.09,Ti 3,Al 1.5,Ni 50 C r 30,C 2.5,Si 1,Mo 1,Fe 3,Ni 3,W 12,Co 47.5 N i 3,Si l,Mn l , C r 30,W 12,C 2.5,Co 50 C 1.1,Cr 28,W 4,Co 67 C 2, S i 1, Cr 26, N i 23, W 12, Fe 1, Co 35 C 2.5, Mn 1, S I 1, Fe 3,Ni 2.5,Cr 32,W 17,Co 41 C 1.2,Si 1.2,Fe l , N i l , C r 30,W 5,CO 60.5 C . l , A l 6,V 4, T i 90 Co 62,Mo 28,Cr 8,Si 2,C .08 N i 50,Mo 32,Cr 15,Si 3,C .08 Co 52,Mo 28,Cr 17,Si 3,C .08 N i 7,Cu 3,W 90 S i 22,Zn .l,Cu 1.5,Fe .75,Ti .15,Mn .1,Ni 2.2,Mgl,Va.l,Al C .1,Mn .5,Si .75,Cr 20,Ni 57,Mo 4,Co 13,Ti 3,Al l , Z r .1 C .05,Si .3,Mn 8,Sn 4,Bi 4,Mo 3,Cr 12,Ni 68 N i 80,Pb 4,Sn 8,Zn 7,Mn 1 Sn 1.5,Fe . l , C r .1,Ni .05,Zr 98.4

165

I

I

/ ASTM Rubber Wheel Abrasion Test / / 2000 revolutions, 13.6 kg load I

; dry sand

I

I

II

Figure 6.14 Abrasive wear properties o f cobalt base alloys

166

MATERIALS FOR NUCLEAR REACTORS

Since the early 1950s there has been considerable work done in the U . S . , Great Britain and France on the study of corrosion and wear of high temperature materials in liquid sodium and high temperature water.The applications include coolant pump bearings, control rod guides, fuel rod separators and heat exchange tube and sheet assemblies. Liquid sodium is used as a heat exchange fluid operating at temperatures up to 659'C. It is used because it remains liquid over a large temperature range at relatively low pressure. Reactor grade water is used at temperatures to 343'C and is pressurized to high pressure to keep it in the liquid phase. Boiling water cooled reactors use water at lower temperatures and much lower pressure. Both of these heat exchange media are extremely corrosive and the variety of materials which are resistant to them and not susceptible to radiation damage is limited. Liquid Sodium: Material combinations from tests made in 659'C high purity sodium at the Liquid Metal Engineering Center [lo51 which showed promise in slow speed sl iding contact include: Udimet 630 LT-2 Haynes 273 Stell ite 1 K-1626 K- 1628

vs vs vs vs vs vs

K-95 LT-2 Stel 1 ite 1016 K-95 K-95 Hastelloy C

Some material combinations which proved to be unsatisfactory included: M-1 Tool Steel vs TZM vs Molybdenum vs

M-1 Tool Steel Hastelloy C Inconel 718

167

Compositions of the above listed materials are included in the table below:

Mater i a1 Udimet 630 Hastelloy C Haynes 273 Stel 1 ite 1016 Stell ite 1 K- 1628 K-95 LT-2

Composition

Ni 53, Cr 17, Fe 18, Mo 3, Nb 6, A1 .5, C .04, Ti 1 54, Co 2.5, Cr 15, Mo 16, W 4, Fe 6, C .08, V .35 57, Cr 16, Mo 17, W 4.5, Fe 5.3, C 0.1 48.5, Cr 32, W 17, C 2.5 55.5, Cr 30, W 12, C 2.5 Tic 63, Ni 25, Mo 5, Nb 5.3 wc 91, co 9 W 60, Cr 25, Alumina 15 Ni Ni Co Co

Sodium-metal -oxide 1 ubrication As has been pointed out, it is possible to achieve boundary lubrication conditions in liquid sodium with certain alloys. Early work at Battelle [lo51 revealed that the friction coefficients of alloys containing molybdenum, tungsten or chromium are lower in sodium containing minute amounts of oxygen than in inert gaseous environment at the same temperature. Roberts [lo61 then showed that molybdenum, tungsten and chromium oxides are stable below 427'C and hence alloys containing them should form complex double metal oxides which behave like solid lubricants. (Sodium molybdate was known to be a solid lubricant). Further, there was a combined effect o f temperature and oxygen level in the sodium which encouraged formation of boundary films. The Liquid Metal Engineering Center developed a sodium boundary lubrication map from plots of free energy curves [107]. This map i s shown in figure 6.15.

168

HIGH TEMPERATURE PROPERTIES OF SUPER ALLOYS

High temperature materials are generally used in bearings, seals and gears because they have hot hardness and resist rapid oxidation. Elevated temperature changes other room temperature physical and mechanical properties which may be ignored during the design process. For instance, thermal shock resistance factor, discussed on page 5, is a function o f thermal conductivity, elastic modulus and thermal expansion coefficient. This factor will differ depending on the temperature the component is required to operate at. The thermal expansion coefficient generally increases by an order of magnitude with an increase in temperature of 100O'C. Some materials, like graphite, for instance, show relatively small increases in expansion coefficient with large temperature increases.Therma1 conductivity increases with temperature for some materials and decreases with temperature with others. These effects must be considered when trying to predict performance involving multi-factor relationships at elevated temperatures. Charts showing the change in properties of a number of high temperature materials with increasing temperature are shown in figures 6.16 - 6.23. These charts were produced by Sibley, Mace, Greiser and Allen in 1960 [88].

169

5 PPm

1 Oxygen, P P ~ Figure 6.15 Temperature-oxygen map for boundary lubrication o f austenitic stainless steel in liquid sodium.

170

Temprroture. F

0 .L

3.0

0 .f 2.0 0.4

I .o

02

0.8 0.6

0.s 0.

0.4

0.08

-

0.3

g

0.2

E

-

z "

a 0.w -Y

-

-E a04

-E

0

-P

\ Y)

\

O.'

aoe

9

0.02

am 0.05 0.04

0.03

0.01 0.008

0.006 0.02 0.004

0.01

0.002

Figure 6.16 Effect of temperature on the thermal conductivity of high temperature materials.

171

30

Temperature, C

Figure 6.17 Effect of Temperature on Average Short Time Tensile Strength

172

Temperature, F 5 00

I

\I

I

or AI,03 - C r ( 7 0 : 3 0 )

~ A I , O , porcelain Y

-- - - -- -\-

Sic-Si, N

,

A

m--I

I

I

I

I

I

I

2 00

400

600

S ~ O , gloss

Graphite

,

aoo

loo0

1200

Temperature, C

Figure 6.18 Elastic Modulus as a Function o f Temperature

1400

173

2.:

2.c

steel

/

iC-Ni-Mo

I .I

01 C

k a C

-

1 .:

.-0

porcelain

e 0

a x

W

b

.-c

-I

-

0

0.f

$

7Grophite

O.d

.-

I

Si 0, gloss

C

200

400

600

000

1000

1200

1400

Temperature, C

Figure 6.19 Thermal Expansion o f High Temperature M a t e r i a l s

1600

174

I .a

ae 0.6

0.4

0.3

0.2

0) 0

2

0.1

0

0.08

E

0.02

0.01

0.008

0.004 0

1

1

200

400

1

1

600

800

1000

I 200

140

Temperature, C

Figure 6.20 Thermal Diffusivity as a Function o f Temperature

175

Temperature, F I001

I

T Srophite

/ 10

I

f

I 200

400

600

800

loo0

I200

Temperature, C

Figure 6.21 Transient Thermal Stress Resistance Factor as a Function o f Temperature

1400

176

Temperoture,

F

5 00 0.7

0.6

0.5

-L ! Y

F

-

LL

c

n

04c

\

a

1

0.4 L

uu0

--. 0

\

0

0.3

0

I

.-U c .U

a v)

0.2

//.-l----

alloy

<-Mo

0 .I

I

0

200

400

600

000

I000

1200

Temperature, C

Figure 6.22 Specific Heat Capacity as a Function of Temperature

I( 00

177

CHAPTER

7 -

POLYMERIC

MATERIALS

MELTING POINTS 135 - 365'C HARDNESS 5 - 115 VICKERS TENSILE STRENGTH 6 - 200 MPa YOUNGS MODULUS 0.7 - 24 GPa THERMAL EXPANSION 3.9E-06 - 1.8E-04 m/m DENSITY 9.6Et02 - 2.5Et03 Kg/m

Elastomers HARDNESS 10 - 100 SHORE A TENSILE STRENGTH 7 - 34 MPa MAX OPERATING TEMP 70 - 316'C THERMAL EXPANSION 9.9E-06 - 1.4E-05 m/m 3 DENSITY 8.3Et02 - 1.4Et03 Kg/m

Appl i c a t i o n s Low n o i s e g e a r i n g Water l u b r i c a t e d p l a i n b e a r i n g s S e l f - 1u b r i c a t i n g bearings R e t a i n e r s f o r r o l l i n g elements i n r o l l i n g c o n t a c t b e a r i n g s Automotive Suspension B a l l sockets Corrosion r e s i s t a n t bearings Low maintenance a i r f r a m e b e a r i n g s Human j o i n t prostheses P i s t o n r i n g s f o r u n l u b r i c a t e d pumps Valves Tires Seals and O - r i n g s Brakes and c l u t c h e s Footwear D r i v e b e l t s and conveyor b e l t s

178

INTRODUCTION

Plastics have found increasing use in tribological applications during the past 40 years. These materials are different from metals and ceramics in that they are generally amorphous rather than crystal1 ine. Plastics are polymers or long chains of molecules usually synthesized from simpler molecules. These organic materials are light weight, corrosion resistant (don’t rust), low modulus, often low friction, and easily shaped. Most plastics have tensile strengths less than 35 Mpa (5000 psi). Their strength can be increased by an order of magnitude by reinforcing with high strength fibers. The elastic modulus of steel is 210 GPa (30 Mpsi) while the elastic moduli of unfilled polymers are less than 3.5 GPa (500,000 psi). It is obvious, in view of these differences, that the use of plastics or polymers in bearings, seals and gears requires a different approach to design. This chapter deals with plastics and rubber and rubber-like materials all classified as polymers. There are two general classes of polymers: Thermoplastics: Acetal Nylons Polycarbonate Polyphenylene sulfide (PPS) Polytetrafl uoroethylene (PTFE) Pol yetheretherketone (PEEK) Pol yethersulfone (PES) Polyethylene Thermosets: A1 kyd polyester EPOXY Phenol ics Polyurethanes Polyimides (Thermoplastic polyimides are available also)

179

Thermoplastics start as melted fluids and are solidified by cooling. These materials can be melted again and again. Thermosets can start out as fluids but harden by chemical reaction, forming cross linking or a three dimensional network. Once hardened, these materials cannot be melted, but will decompose at high temperature. Heating will soften the structure somewhat, but not enough for conventional forming processes. Polymers can be produced in a large variety of compositions. They can be mixed or alloyed, modified with various fillers and made into composite materials by lamination, weaving and filament winding. It is beyond the scope of this manual to include all grades of polymeric materials commercially available even those most often chosen for tribological appl ications. Those presented in this chapter have been selected as representative of the wide variety of polymers in use.

180

PROPERTIES OF POLYMERS The mechanical and p h y s i c a l p r o p e r t i e s o f a s e l e c t e d group of p l a s t i c s a r e summarized i n t a b l e s 7.1 and 7.2

Table 7.1 Mechanical P r o p e r t i e s o f Some P l a s t i c s

Material

ACETAL t PTFE ARAMI D DELRIN DUROID 5600 FEP I P C 1832 NYLON 6 NYLON 66 NYLON TF PEEK t GRAPHITE PHENOLIC RT DUROID RYTON R4 TORLON T-0454 UHMWPE VESPEL SP1 VESPEL SP21

Form

MOLDED FIBER CAST MOULDED CAST SINTERED CAST CAST CAST MOULDED LAM1NATED

Hardness T e n s i l e Vickers Strength DPH MPa

Youngs Modulus GPa

Densjty kg/m

57 120 76 18 21 69 159 62 55 207 62 34 131 6 57 41 83 62

1.93 7.58 6.89 13.79 0.69 11.03 2.07 2.76 2.07 24.10 6.89 1.38 11.72 24.60 10.34 0.69 3.45 4.14

1.40Et03 1.30Et03 1.46Et03 1.90Et03 2.21Et03 1.66Et03 1.38Et03 l.llEt03 l.llEt03 1.43Et03 1.38Et03 2.49Et03 1.66Et03 1.38Et03 1.52Et03 9.60Et02 1.38Et03 1.38Et03

40 35 5 5 15 15 20

115

40

SINTERED SINTERED

Conversion f a c t o r s : MPa x 0.145 = k s i kg/m3 x 3 . 6 1 3 ~ 1 0 - =~ l b / i n 3

50 50 35

181

Table 7.2 Physical P r o p e r t i e s o f Some P l a s t i c s

MATERIAL

Thermal Conductivity W/m/K

ACETAL t PTFE ARAMI D DELRIN DU DUROID 5600 FABROID FEP I P C 1832 NYLON 6 NYLON 66 NYLON TF PEEK t GRAPHITE PHENOLIC RT DUROID RULON LD RYTON R4 TORLON T-0454 UHMWPE VESPEL SP1 VESPEL SP21

Me1t Point 'C

Heat Capacity J/kg/K

170 0.21 0.67 41.52 0.35 0.35 0.17 0.35 0.35 0.35 0.21 0.17

4.00E-05 3.90E-06 1.93E-05 1.84E-05 2.34E-05 1.80E-04 2.16E-05 9.00E-05 6.30E-05 2.00E-05 2.87E-05 2.34E-05

3.98 0.35 0.35 0.35 5.71 5.71

*

Decomposition temperature conversion factors: W/m/K x 0.176 = BTU/ft2/h/"F

m/m/"C

x 0.56

J/kg/K

x 0 . 2 3 8 ~ 1 0 - =~ BTU/lb/'

=

Thermal Expansion m/m/ 'C

in/in/'F F

6.00E-05 7.02E-05 6.30E-05 2.34E-05

1.17 175 0.96 275 290 220 260

1.17 1.67 2.08

334 1.25 327* 290 275* 365* 135 365* 365*

1.84 1.00

182

Table 7.3 Composition of Some Plastics Material

DELRIN DU

DUROID 5600 FABROID RT DUROID RULON LD

FEP IPC 1832 NYLON 6 NYLON 66 NYLON TF TORLON PEEK t GRAPHITE PHENOLIC RYTON R4 UHMWPE T-0454 VESPEL SP1 VESPEL SP21

Composition Acetal PTFE/Lead,bronze sinter on steel backing PTFE - Ceramic filled PTFE - Glass woven fabric Fibre reinforced PTFE Mineral filled PTFE F1 uor inated Ethylene Propyl ene Polyphenylene sulfide resin, fibre filled Pol yam i de Polyamide Polyamide Polyamide/imide t graphite Pol yetheretherketone t graphite fiber Cotton fabric laminated, phenolic binder Polyphenylene sulfide resin Ultra high molecular weight polyethylene Polyimide, carbon & graphite filled Polyimide t 15% graphite Polyimide t 40% graphite

DESIGN OF PLASTIC BEARINGS Plastics require special design considerations because of some properties much different from metal alloys. Plastics have low thermal conductivity and therefore tend to local ize temperature spikes from frictional heating rather than conduct the heat away. For thermoplastic materials, excessive sliding velocity can produce enough heat input to melt the surface. Thermoset plastics will char and decompose with excessive temperature. Plastics also have high thermal expansion coefficients and therefore frictional heating will cause distortion and loss of bearing clearance. Because of the above factors, an essential criterion in the design of plastic bearings is frictional heat generat ion. Operating temperatures must be predicted and generous clearances provided to accommodate expansion. One criterion often used in the design of plastic bearings is PV level. That is - bearing pressure x sliding velocity. PV is a rough indicator of heat generation. Friction coefficient is also an important component.

183

The rate of heat generation in a bearing

=

Force of friction x sliding velocity (V) Load x coefficient of friction ( p ) XV

=

Rate of heat dissipationocbearing area ( A ) Temperature risea Load/A (pV) or, pPV Thus, for a given plastic material, its temperature rise during sliding contact is proportional to friction coefficient x bearing pressure x sliding velocity. If one could assume a friction coefficient that does not vary appreciably among plastic materials then, PV affords a rough measure of the operating limits for a plastic material. This is especially important for thermal plastics because we do not want to operate them beyond their softening point. One cannot totally ignore friction coefficient, however, and some knowledge of this factor under given operating conditions helps to more accurately assess the operating potential of a given plastic bearing material. In addition to the importance of not ignoring friction coefficient in estimating friction heating consequences, PV cannot be used as a linear function. This is because experience shows that sliding velocity is more potent in the development of frictional heating than is bearing pressure. This may be because the replenishment to beneficial surface films on the sliding surfaces is more difficult at high sliding velocities. PV values for several plastic materials have been summarized in figure 7.1. This bar graph contains information from several sources [109,110,111].The PV values found tended to vary with each source - so that the values in figure 7.1 are averages of those values.

2. E v)

z

$

Figure 7.1 PV values for a variety o f polymers

PTFE glass filled

PTFE bronze filled

Polyeurethane PTFE

PES carb + PTFE

Polyimide

Polycarbmate glass

Polycarbonate

Phenol fabric lamina

PEEK PTFE C filled

Nylon carbon filled

Nylon

A c e d carbon filled

Aced

0

0

2

Iu 0

0 0

PV Value, PSI x FPM (thousands) P 0

a 0

UI 0

184

185

Plastic bearings must be sufficiently retained in their housing to prevent relative motion between the housing and the bearing. An interference fit is often used. There is always a danger that the relatively soft material will eventually loosen and slip. Adhesives can be used and are most used where the plastic bearing material is made in a thin sheet and attached to a metal backing by adhesive. The plastic-metal backing unit is then pressed into the housing. Examples of plastic-metal backing design are shown in figure 7.2.

Photograph courtesy of The Barden Corporation Figure 7 . 2 Metal Backed Plastic Sleeve Bearings

186 hhhhh

Plastics have low young's modulus compared to metals ( two orders of magnitude less than steel, for instance). Therefore, plastics are not suitable for use in high contact stress applications - especially as ball bearings. Some plastics (nylon and fabric reinforced phenolics) have been used as spur gears in relatively low load conditions. Plastic materials are relatively soft and will imbed harder materials such as metals and minerals. The loaded soft material can then act like a grinding wheel and cause serious abrasive wear on the mating part. If a steel or nodular iron shaft is used with a plastic bearing, the surface finish of the steel or iron is extremely important to the wear life of the bearing. Ridges in the steel surface will act like a file and tend to cut into the softer plastic. Plastics tend to creep under load much more than metals do. Thus, the initial breakin "wear" of a heavily loaded plastic bearing can be mostly permanent deformation as the shaft sinks into the bearing surface. This "bedding in" must be taken into account in the design of the bearing system. Some plastics like PTFE will cold flow under very small loads. This material must be reinforced and confined by lamination, weaving, impregnation into rigid matrices etc. Many plastics will absorb water and swell. The humidity of the environment will affect the amount of water absorption. Water absorbtion will not only affect dimensional stability of the part but will also change mechanical properties. If the results of water take u p are intolerable, the polymer can be modified by alloying with material to inhibit water take up or a polymer with low water absorbtion properties substituted.

187

PLASTICS USED IN TRIBOLOGY Nylon

Nylons are polyamides, thermoplastic. They are i-intified by numbers which indicate the number of carbon atoms in the monomers. Some grades are made from the combination of diamines and organic acids. Thus nylon 6/6 is made of a 6 carbon diamines plus a 6 carbon adipic acid. The more carbon atoms, the lower the melting point. Thus many grades of nylon are available - not including the filled nylons. Nylon might be called the queen of the tribological polymers. After its invention during World War 11, it rapidly took over many of the applications where laminated phenolics were used. In addition, because of its good mechanical properties it found its way into parts previously made of metal. Gears, latches, bushings, linkages, many moving parts in business machines and appl iances are made of nylon. Nylon is easily molded and can be b ended with many lubricating and reinforcing materials like PTFE, silicone oi , aramid, carbon and glass fibers and MoS2 solid lubricant. It is resistant to greases and oils. Most attractive of all, is the low cost in production. Nylon parts can be used in demanding performance mechanisms such as precision timers, data processing equipment and telephone equipment. One might expect that since polymers have low elastic modulus - are more compliant in elastic contacts - that plastic gears would not have to be as accurate in geometry as metal gears. However, excessive tooth meshing error in plastic gears can cause excessive noise and contribute to premature tooth failure from fatigue. By correct design of injection molds and the use of fillers such as glass beads, nylon gears can be molded to acceptable accuracy [112]. This is possible even though nylon is a high shrinkage material. With suitable allowance for predictable shrinkage after cool ing, required accuracy can still be achieved. Nylon has a relatively low softening point - around 120'C and, therefore, both ambient temperature and friction induced temperature are important considerations in sl iding contact applications. Increasing load or sl iding velocity will result in frictional heating increase and at a critical PV, wear rate will increase precipitously. This problem is aggravated by the low thermal conductivity (400 to 600 times less than babbitt and bronze bearing materials) and high thermal expansion of nylon. These drawbacks can be alleviated somewhat by the use of metal and high modulus fiber fillers.

188

Nylon will absorb water and swell when exposed to water vapor or immersed in water. This factor must be taken into consideration when selecting a clearance value for a water lubricated bearing, for instance. Nylon bearings can be pretreated by immersion in heated water. Lower absorbtion grades of nylon have been developed. Nylon is capable of a large number of modifications and mixtures to produce unique characteristics. Recent developments include IPNs (interpenetrating networks) involving nylon and silicone for improved self lubricating properties. Reinforcement with aramid fibers produces an improvement in wear resistance in addition to virtually no wear of mating metal parts. Acetal (Delrin) Acetals are polyoxymethylenes - thermoplastics having good load bearing properties and low coefficient of friction. They are similar in tribological behavior to nylons. Wear properties and friction are similar to nylon. Acetals can be modified with lubricant additives such as PTFE and molybdenum disulfide. They can be reinforced with glass, carbon or aramid fibers and mineral fillers. These additives improve dimensional stability, stiffness and compressive strength. Acetals provide an inexpensive way to manufacture mechanical parts 1 ike gears, ball sockets, linkages, cranks, plain bearings. etc. Injection molding i s the most widely used process. Acetals are used to a great extent in automobiles and appliances. PTFE PTFE was invented as a high temperature polymer. It was soon noted that the polymer had a slippery feel and its self lubricating properties were exploited.PTFE ( p o l y t e t r a f l u o r o e t h y l e n e ) commonly known as Teflon, is a fully fluorinated polymer with a melting point o f 327'C. However, its viscosity at melting point is very high, making conventional extrusion or injection molding of parts impractical. Powder metallurgy molding or extrusion techniques are used for making PTFE parts. Heating above the melting point can produce thermal decomposition and the evolution of poisonous gas.

189

PTFE has a low dielectric constant ( 2 . 1 ) . The polymer can be used as a bearing material over a wide temperature ranging from cryogenic temperatures to 260'C. In its pure state, its wear resistance is low and its cold flow properties make it impractical for most load bearing applications. Thus it is used filled with various stiffeners like glass, graphite, metal fibers and metal and ceramic powders. PTFE is also applied to metal surfaces as a non stick film. It can be drawn into a filament and woven with glass or polymer fibers to make a fabric. This fabric can be cut and adhered to metal backing to make a low friction bearing capable of supporting heavy loads [113]. Woven PTFE fabric bearings have been successfully used in heavily loaded plain bearing applications where lubrication is not practical. These self lubricating bearings are used in aircraft airframe applications and require little or no maintenance. Generally the fabric bearings are subject to slow speed, low amplitude oscillatory motion. Loads can cycle also. The wear characteristics for typical fabric bearings have been determined. Figure 7.3 shows the wear of two types of fabric bearings at several load levels and as a function of number of journal oscillations. (Speed of oscillation was 10 cpm and amplitude was t/- 45 degrees). Note that at the high load there is a rapid increase in diametral clearance over the first 500 cycles. A large percentage of that apparent wear is permanent deformation or compaction of the fabric. Note also that the glass fibre fabric shows less wear than the cotton fabric.

190

0.5

0.4

E

E

0.3

.-E

0.2

0.1

0 0

0.5

1

1.5

2

2.5

3

3.5

Cycles of journal oscillation (thousands)

Figure 7.3 Wear of teflon fabric plain bearings operating at high bearing pressures , oscillating motion (10 cpm) simulating airframe operation.

191

Another self lubricating plain bearing design using PTFE is the DU or Permaglide system. These bearings are made by sintering PTFE, bronze and lead powder on a steel backing. A section through such a bearing is shown in figure 7.4. The material consists of roughly 3 layers: about 0.01 to 0.03 mm of lead and PTFE, a PTFE-bronze layer about 0.2 to 0.35 mm thick and a steel or bronze strip about 0.75 mm thick. (Bronze backing is used if corrosion resistance is important). With ;he steel backing, the bearing can support a bearing stress of up to 320 N/mm . The sintered lead-bronze-PTFE design provides an excellent anchor for the PTFE and also makes for a higher thermal conductivity body. Thus, these bearings are capable of higher sliding velocities than other PTFE base bearing materials. Its rated PV value is 1.8 to 2.0 N/mmz. The PTFE sintered strip i s stamped into flat circular parts for thrust bearings and rolled on mandrels to form plain bearings.

STEEL 1OpmFigure 7.4 Metallographic section of a PTFE-lead-bronze sintered bearing

192

FEP (fluorinated ethylene propylene) and PFA (perfluoroalkoxy resin) are similar to PTFE in that they are crystalline and predominantly linear chains of CF, - CF and are inherently low friction materials,. PFA has somewhat better high temperature mechanical properties and can be used to 260'C. Both of these resins have the advantage of being melt processed by conventional injection molding and extrusion processes. Other fluorinated resins with similar low friction and high temperature resistance are also available [114]. Melt processable fluorocarbons can be reinforced with glass and carbon fibers and such resins as ETFE, ETCFE and PVDF-TFE, having hydrogen bonding capability adhere very well to fiber reinforcing materials. Thus tensile and flexural strength can be significantly increased. FEP and PFA are not improved by fiber reinforcement owing to poor fiber adhesion to the matrix. FEP thin film is used as glazing in solar energy collectors [115]. Polyethylene Polyethylene in the ultra high molecular weight form (UHMWPE) is used in a number of tribological applications. One important use is in artificial human joint implants. This form of PE has an average molecular weight of from 3 million to 6 million. It is fabricated by compression molding or ram extrusion. UHMWPE has a high crystallinity and thus high tear resistance and resistance to penetration. This results in a plastic with high wear resistance (claimed 10 times greater abrasion resistance than mild steel), toughness and low coefficient of friction. It also resists stress cracking. Its low friction properties can be enhanced by addition of silicones or solid lubricants like graphite or molybdenum disulfide. In its pure form, it complies with FDA and USDA requirements for use in processing foods, water and pharmaceuticals. Its toughness can be enhanced by the addition of graphite fibers. Wear testing of UHMWPE for use in human joint implants have demonstrated its exceptional wear resistance. In tri-pin-on-disk experiments involving UHMWPE pins sliding against austenitic stain1 ss steel while immersed in distilled water wear was measured as low as 1 mm5 per 1800 hours [ 1161. The load was 150 N. Some creep of the plastic was observed.

193

Phenol ics Thermosetting phenol ic-laminates are used in a number of tribological applications. These composites are made by laying up fabric, matt or paper impregnated with phenolic resin into layers and applying heat and pressure. The result is a hard, dense material with directional properties. The "end grain" exposes porous fibers and when oil impregnated, will perform like an oilite bearing. If the material i s to be subjected to bending stress, the load must be appl ied normal to the laminations. Any loading which tends to separate or buckle laminations should be avoided. Phenolic laminates outperform most plastics in impact resistance, superior mechanical properties and dimensional stability. Phenolics will retain up to 80 % of their Elastic Modulus at 240aC. Nylon, on the other hand will have lost 60 % of its modulus at 160'C. A number of materials are used for the laminations. They include cotton fabric, hard paper, woven glass fabric, glass mat. These fabrics may also include graphite, molybdenum disulfide and silicone fluid impregnants. Phenolic laminates are used as rolling contact bearing retainers (Figure 7.5) gears, cams, rotor vanes in vane pumps, spools, valve plates for gas compressors, piston rings, valve disks and plane bearings.

Photograph courtesy of Garlock Bearings Inc Figure 7.5 Ball Bearing with Phenolic Retainer

194

HIGH TEMPERATURE PLASTICS A number of grades of plastics have been developed to provide structural

strength at elevated temperatures (260 - 300' C). Most of these materials also operate at cryogenic temperatures. These plastics have taken the place of metal alloys like Monel, bronze, cast iron and steels in specific problem applications where corrosion resistance, low friction and resistance to adhesion were required at elevated temperatures Polyimides Polyimides are derived from aromatic diamines and aromatic dianhydrides. High temperatures are required to process polyimides. Powders can be compression molded at temperatures between 260' and 450'C and sintered using powder metallurgy techniques. Therefore production of parts can be expensive. Polyimides are generally used with fillers. Two fillers used most often are graphite and PTFE. Polyimides moulded with controlled porosity are impregnated with liquid lubricants similar to oilite bearings. The polyimide body for oilite bearings provides a fail-safe system for dry-up o f lubricant. They are superior to lubricant impregnated nylon because of their higher heatdeflection temperature. (temperature at which permanent deformation occurs under a given 1 oad) . Polyimides can be used at temperatures up to 260' C and will withstand short excursions to 480'C. They have no melt or glass transition temperature even though they are classed as thermoplastics. Therefore the upper temperature limit of polyimide is limited by thermal degradation. Sudden failure can occur after sufficient degradation. Thermal expansion is low compared to other polymers like acetal and nylon. Polyimides have good fatigue properties Fatigue endurance 1 imit in bending is typically of the order of 50 MPa for 107 cycles at room temperature. Polyimides are used as piston rings, valves, rolling contact bearing retainers, non lubricated seals, bearings in copiers, business machines and space vehicle components.

195

Polyamide-imide Polyamide-imide are high temperature thermoplastics for extrusion, injection molding and compression molding. They exhibit good mechanical properties at temperatures up to 200'C. These materials are noncrystalline. Of the commercially available thermoplastics, polyamide-imide has the highest glass transition temperature (270 - 285'C). Heat deflection temperatures are high while thermal expansion is low. PAIs have found extensive use in the aerospace industry, replacing metal parts because of their high strength/weight ratio. They are used as seals and bushings in hydraulic equipment and compressor and jet engine parts. Aram i ds Aramid resins are aromatic polyimide fibers. These are high strength linear rodlike polymer chains. They have low density, high tensile strength and metal-like compressive stress - strain characteristics. Aramid fibers are often used as reinforcing fillers in composite materials. The fibers are not abrasive and are light weight. (About 40% lower than glass) The fiber has outstanding heat resistance. They have a useful temperature range from -190 to 200'C. Aramid fibers have been used as substitutes for asbestos. Aramid yarns can be woven into fabrics and combined with resins and processed in the same way that glass or other yarn fabrics are. Because of the low density - high strength properties of aramids, they are used in many aircraft space vehicle and missile applications. Fibers are used in elastomer reinforcement for tires, drive belts and conveyors. Frictional properties and heat resistance makes aramids useful for clutch and brake applications. PEEK Polyetheretherketone is a thermoplastic with exceptional high temperature properties. It has a melting point of 335'C and a glass transition at 143'C. The structure of this polymer produces mechanical properties similar to bronzes plus radiation resistance and low flammability. Filled with glass or carbon fibers, the mechanical properties are further enhanced. The 30% carbon fiber filled grade provides maximum rigidity and load bearing capability. Addition of solid lubricants to the mix reduces friction and makes for an attractive tribological material which is easier to process than polyimides. PEEK can be molded into many complex parts.

196

PEEK is finding numerous applications in the aerospace industry, replacing aluminum because of its superior performance at high temperature. Low friction plain bearing grades are available with PTFE additives. PEK Polyetherketone has a somewhat higher glass transition temperature than PEEK. It can be used where the thermal properties of PEEK are marginal. Polysul fone Polysulfone (PSO) is a transparent, high performance, heat resistant thermoplastic. It has a glass transition at 190'C. Its structure makes it retain its rigidity at high temperatures. PSO has high resistance to acids, alkalies, salt solutions and oils. It can be used continuously in steam up to 150'C. It is sanctioned by the FDA for food contact. It is finding use in the food processing industry and for many medical instrumentation appl ications. PSO has a tensile impact strength of 200 ft-lb/ sq in. When filled with 15%

PTFE and 30% glass fiber, it has a PV rating of 35,000 psi-fpm at 100 fpm [117].

PPS Polyphenylene Sulfide is a semicrystalline polymer with good thermal resistance and dimensional stability. It is a thermoplastic material and is usually injection molded. In order to produce a part with high dimensional stability, precise mold temperature control is necessary. If not controlled, too low a mold temperature will result in a material not fully crystallized. If the part is then put into service at a temperature higher than the original mold temperature, further crystallization will take place with ensuing shrinkage. Because of its low melt viscosity, PPS can be loaded with and reinforcing agents. Its melt temperature is 285'C and withstand short term temperatures up to 260'C. It is also No solvents below 200'C are known to dissolve PPS. Unlike PPS does not absorb water.

up to 70% of fibers the polymer can flame resistant many of the polymers,

197

PPS can be filled with PTFE, carbon fiber, glass fiber and silicone to make it useful as a tribological material. With 15% PTFE and 30% glass fiber, it has a PV rating of 35,000 psi-fpm at 100 fpm [118]. Polymer Selection As has been pointed out, frictional heating of polymers is a primary concern in selecting material for a tribological application. Ambient temperature is important, also, if it exceeds the softening point of a given polymer. Remember that the surface temperature will be the frictional temperature rise plus the ambient temperature. The deflection temperature is used to estimate the point at which a loaded polymer part will fail. The deflection temperatures at 264 psi pressure are shown for a number of polymers in figure 7.6. This chart can be used to rank polymers for their thermal endurance. The chart shows deflection temperatures for both unfilled and filled polymers. Note that some polymers show a significant improvement with fillers added. Polyimide ranks high in thermal endurance. Polyamide-imide shows 1 ittle improvement with added fillers. Re1 at ive Wear

-m Q

Figure 7.7 sh s t e ranking of a number o f polymers in terms of wear. Wear rates are 10 in min/ft-lb hr (wear factors). That is, the higher the bar, the higher the wear rate. The chart shows that nylon has the best wear resistance while PEEK shows relatively low wear resistance. The polymers in figure 7.7are all filled - some of them have solid lubricants added.

Polyimide

Polyamide-imide

Polysulfone

PEEK

Acetal

PPS

Polycarbonate

Nylon

FEP

UHMWPE

Phenolics

0

0

OI 0 0

A

0

0

0

N

cn

A

0

cn

N

Deflection temperature, "C

0 0

w 0

cn

0 0 0

P

198

Figure 7.6 Deflection Temperatures for Polymers, Neat and Reinforced [118]

A.

7-i

Figure 7.7 Comparison o f wear rates of filled polymers

v)

-5

3

Y

2

-0

P

(D

1

d

2

0 -h

v)

.2 . CD

7 7

2Bl

0 -h

3

0

v)

7.

w

-0

3

0 0

u u

(D

-5

S

cn

z fn

0,

Y

a

-

I -!

0

PEEK

Polysulfone

PPS

Phenolics

Polyimide

Polycarbonate

Acetal

UHMWPE

FEP

Nylon

0

w 0

h3 0

-L

0

P 0

Relative wear

rate 0)

0

VI 0

0

199

200

E LAST0MERS-RU BBER Natural rubber (latex) was the first elastomer used in engineering. It was and has continued to be a remarkable material. It has excellent strength and resilience and high resistance to tearing and abrasion. Natural rubber bonds well to metals, making it ideal for vibration isolation devices. The material has low hysteresis, making it ideal as a tire material (little heat build up). Natural rubber is degraded by ozone and swells and becomes sticky in mineral oil and solvents. Rubber in its raw form is gummy and useless for load bearing applications. By the process o f vulcanization the long chain polymer is cross 1 inked through sulfur bonds to develop a three dimensional structure. Additives such as carbon black are used to modify its properties. Vulcanized rubber is a thermoset polymer and, therefore, cannot be melted or injection molded in its vulcanized form. Rubber is molded by compression molding or injection molding o f uncured material and curing in the mold. This requires a heated mold [119]. High injection pressures and control of the uncured rubber temperature in the injectors is required in this process. The resulting product, scraps, flash and discards cannot be melted down for recycling as can thermoplastics. During World War 11, interruption of supplies o f natural rubber spurred the development of synthetic rubber. Synthetic rubber is made from petroleum based products such as butadiene, styrene and acrylonitrile. The result was a whole new array of elastomers with a wide range of properties including resistance to oils and solvents, resistance to weathering, and high temperature tolerance. These products, 1 ike vulcanized rubber were thermosets and required similar molding processes. During the 1960s, thermoplastic elastomers (TPEs) were developed [119]. These materials revolutionized the elastomer processing industry, opening it up to utilizing the techniques used for all thermoplastic or meltable polymers. Thus, thermoplastic elastomers can be repeatedly softened by heating and scrap parts can be reprocessed. Techniques such as vacuum forming, blow molding, and high velocity injection molding can be used. Significant reduction in production costs over conventional vulcanized materials has been achieved with TPEs. Thermoplastic polyurethane is one of the best examples of the exciting vistas opened for tribological applications with the development of TPEs.

20 1

PROPERTIES OF ELASTOMERS

Elastomers exhibit very large elastic strain and low elastic modulus. Most polymers extend elastically to about 1% . Elastomers can achieve extensions up to 1000%. Elastomers are non-Hookean and the stress-strain curve is S shaped. When the strain reaches about 400 to 500% elongation, the modulus rises rapidly to values close to those of plastic materials. Rubber can take a permanent set under high compressive strains. This occurs over an extended time period as the result o f creep. The Poisson’s ratio of elastomers is close to 0.5 - in other words, they are virtually incompressible. The coefficient of thermal expansion of unstretched rubber is positive while that of the stretched material is negative [120]. This is an important effect for consideration in 0 ring design. Elastomer Hardness.. . Hardness is probably more meaningful for rubbers than for plastics like nylon. Consistent readings can be obtained using a hardened steel ball indentor. There is little creep involved in rubber hardness readings. Standard hardness tests for elastomers require the specification and use of a given specimen thickness. Hardness values are quoted in International Rubber Hardness Degrees (IRHD) or Shore A or D Durometer readings. The range of hardnesses for elastomers is illustrated in figure 7.8. It can be seen that the concept of hardness is different from metals where permanent deformation or yield point is a significant factor. Note that PTFE, which cold flows easily, still has an apparent hardness greater than rubber which does not yield pl ast ical ly.

202

IRHD 0 10 20 30 40 50 60 70 80 90 95 l l l l Shore A II ~ I ~1 I - I I- I - II - I - II - I - lI ~ I l 0

Shore D

10 I

20

I

30 40 50 60 70 80 90 100

I I I l-1-1-1-1-1-1_1~1

Rubber inner car band tube tire 0 ring

100

I

go1 f ball

I

I

I

I

bone

Figure 7.8 Comparison of Elastomer Hardness Values (from Crawford)

I

203

The mechanical and p h y s i c a l p r o p e r t i e s o f some s e l e c t e d elastomers used i n t r i b o l o g i c a l a p p l i c a t i o n s a r e summarized i n t a b l e s 7.4 and 7.5 The values g i v e n a r e t y p i c a l o f t h e m a t e r i a l l i s t e d . Most o f t h e elastomers come i n a number o f grades w i t h a range o f p r o p e r t i e s . F o r instance, urethane comes i n a range o f hardnesses f r o m 50 Shore A t o 75 Shore D durometer.

Table 7.4

Mechanical P r o p e r t i e s o f Some Elastomers

MATERIAL

BUTYL NEOPRENE NITRILE SILICONE URETHANE NATURAL RUBBER ETHYLENE-PROPYLENE POLY BUTADIEN E VITON

FORM

MOULDED MOULDED MOULDED MOULDED MOULDED MOULDED MOULDED MOULDED MOULDED

Conversion F a c t o r s MPa x 0.145 = k s i Kg/m3 x 3 . 6 1 3 ~ 1 0 - =~ l b / i n 3

HARDNESS TENSILE Shore A STRENGTH MPa

30-100 40-95 20-90 10-85 10-100 30-100 30-90 35-90 60-90

21 21 21 7 34 25 10 10 13.6

MAX OP GLASS DENSITY TEMP TRANSIT Kg/cu m OC TEMP 'C

149 116 149 316 116 70 125 70 116

-75 -49 -22 -120 -70 -58 -85

8.30Et02 1.25Et03 l.llEt03 1.38Et03 1.11Et03 9.30Et02 8.60Et02 9.20Et02 l.llEt03

204

Table 7.5 P h y s i c a l P r o p e r t i e s o f Some Elastomers

MATERIAL

THERM CON Watt/m

' k

BUTYL NEOPRENE NITRILE SILICONE URETHANE V I TON

0.17 0.17 0.17 0.17 0.09 0.09

THERM EXP

m/m / O C

9.90E-06 9.95E-05 1.39E-05 2.52E-05 4.32E-05

ELEC R E S I S micro ohm-cm

3.50Et10 1.00Et15 2.00Et12

conversion f a c t o r s : W/m/K x 0.176 = BTU/ft*/h/"F m/m/"C x 0.56 = i n / i n / ' F

POLYMER COMPOSITION

Butuyl Neoprene Nitrile Silicone Urethane Viton

Isobutylene-isoprene Chl oroprene Butad i e n e - A c r y l on itrile P o l y s i l oxane Diisocyanate polyester V i n y l idene f l u o r i d e - h e x a f l uoropropylene

205

DESIGN OF ELASTOMER BEARINGS Natural Rubber Natural rubber has the unique abil ity to withstand considerable deformation and to snap back to its original size. Natural rubber is used for spring mounts for structures and machinery. It provides the needed resilience and shock absorbtion. Bridge bearings have been made o f rubber. Natural rubber provides high fatigue resistance and low heat build up during flexing. In designing a rubber compression mount, it is important to determine the stiffness of the spring. For a simple compression block as shown in figure 7.9 the stiffness relations are as follows: Compressive Stiffness, Kc Shear Stiffness, Ks

=

=

E A/h

GA/h

Where: Ec= compressive modulus A = cross sectional area G = shear modulus h = thickness

For a bushing, torsional stiffness, kt = nGL/(d-' - D - 2 ) and Axial stiffness ka = 2.73/1og (d/d). More detailed design methods are included in Gobel [121\0 Moduli for rubbers are a function of their Shore hardness readings. The stiffness and compressive moduli are a function of shape factor, s . The shape factor is the ratio of the bearing area to the thickness of the rubber mount. For a compressioa block, s = BD/2h (BtD). The compression modulus, Ec = E / ( 1 t 2ks) . Table 7.6 gives Young's Modulus, Shear Modulus and Shape factor for rubbers of various hardness.

206

Y

L

Figure 7.9 Natural Rubber Compression Block Table 7.6 Hardness and Elastic Moduli f o r Rubbers of Various Hardness (from Crawford [118]) Hardness Shore A

30 40 50 60 70

E MN/W

0.92 1.50 2.20 4.45 7.35

G

k

MN/m2

0.30 0.45 0.64 1.06 1.73

0.93 0.85 0.73 0.57 0.53

207

TYPES OF ELASTOMERS Butyl Rubber

Butyl rubber has been used in inner tubes and automotive window weatherstripping. It is resistant to ozone, acids, alkalis and oils. It is also recommended for use with phosphate ester type hydraulic fluids, silicone fluids and greases. It has an excellent resistance to gas permeation. The rubber has high hysteresis and will build up heat when subjected to flexing. It tends to stiffen at temperatures below -5O'C. Butyl is also incompatible with other synthetic rubbers - thus it cannot be mixed with them. N i t r i l e Rubber

Nitrile rubber is noted for its resistance to petroleum products. It has therefore found considerable use in oil seals. The rubber requires fillers (carbon black) and plasticizers to achieve the desired mechanical properties. The high oil resistance means some sacrifice o f low temperature flexibility. It is also subject to deterioration from ozone, sunlight or weathering. Most U . S . Military rubber specifications for fuel and oil resistant o rings require nitrile base compounds. Nitriles are superior to most elastomers in resistance to cold flow and permanent set. In addition this material has good tear resistance and abrasion resistance. Nitriles are recommended for use with refrigerants, si icate ester lubricants and high aniline petroleum oils. Ethyl ene-Propyl ene

This elastomer is a relatively new addition to the rubber industry. This material combines excel lent resistance to phosphate ester type hydraul ic fluids but also has good resistance to high temperatures, chemicals, abrasion and weathering. It is not resistant to petroleum oils, however. EP is used for conveyor belts and seals. It is recommended for use with steam, phosphate esters, water, silicone oils and greases, dilute acids and alkalies, alcohols and automotive brake fluids.

208

F1 uorocarbons Fluorocarbons are used extensively in the seal industry for lip type seals and O-rings. With a wide temperature range (up to 316'C) and resistance to chemicals and fluids many mil itary and aerospace seal appl ications have fluorocarbons in them. Later developments in formulating fluorocarbons have produced greatly improved compression set properties. Neoprene Neoprene is unusual in having limited resistance to both petroleum oils and n ozone. I also has a broad service temperature range (-54' to t149'C). addition it is moderate in cost. Neoprene is recommended for use with refrigerants, petroleum oils, mild acids, and silicate ester lubricants.

Polybutad iene This is a relatively inexpensive rubber having better abrasion resistance than most natural rubber compounds. This elastomer, when blended with natural rubber or styrene-butadiene rubber, is found in most automobile tires. This mix shows good resistance to flex cracking. The cost can be kept down by taking advantage of its ability to absorb large amounts of carbon black and petroleum oil. Silicone Rubber Sil icone rubber has a wide temperature range for appl ications (-100 to 300'C). Actually, there is a group of elastomers made from silicon, oxygen, hydrogen and carbon classed as silicone rubbers. As a group, they have poor tensile strength and therefore tear easily and have poor abrasion resistance. The material is ideal for o ring applications owing to its high compression set resistance. In seal applications, a maximum operating temperature of 232'C in dry air is recommended [ 1 2 2 ] . Silicone is unaffected by ozone and is resistant to hot oil. Its inertness makes it useful for food machinery and surgical applications.

209

Urethane Polyurethanes come in a broad range of products including foams, elastomeric moldings and extrusions. Thermoset polyurethanes are made by crossl inking. Tightly crosslinked PURs are very rigid and strong while a lower degree of crossl inking results in a more rubber-1ike material. PUR elastomers can be used at temperatures up to 178'C. This material has excellent abrasion resistance and resistance to chemicals. High toughness and tensile strength has made this material very useful for drive couplings. The material is also used in industrial wheels, small gears and heel lifts for women's shoes.

WEAR OF RUBBER

Because of its extreme elasticity, rubber wears in a different mode from metals. When rubber surfaces soften owing to frictional heating, adhesion occurs and rubber tends to transfer to the mating surface. During abrasive wear, the severity of wear is associated with the formation of "abrasion patterns" or sets of parallel ridges at right angles to the direction of sliding. An example is shown in figure 7.10 [123]. These ridges are very much like what one finds on the end of a rubber eraser after using it. This pattern can be inhibited by changing the direction of sliding periodically. When this is done, the abrasive wear rate decreases significantly. As might be expected, the softer compounds tend to form abrasion patterns more readily. Tear resistance is also an important factor in abrasion pattern tendency. Rubber is an effective material to combat erosive wear. Because of the elasticity of rubber, impingement of particles on a rubber surface produces elastic deformation only and, therefore, the eroding particles bounce off harmlessly. Rubber is readily cut, however, and if the particles are sharp and impinge at a glancing angle, cutting wear will result. Thus, the closer the impingement angle is to the normal to the surface, the less likely will rubber wear by erosion.

210

40X

Figure 7.10 Abrasion Pattern on a Rubber Surface (From W.A. Glaeser unpublished 1991)

bg’s s

21 1

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Structures, Properties and Applications, Nonferrous Founder's Society, 1963 (51) Metals Handbook Ninth Edition,Volume 3, ASM pg 805 (52)

F.P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Oxford Press, 1954, pg 115.

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(68) Charles F. Walton, Gray & Ductile Iron Castings Handbook, Iron Founder's Society, 1971, pg 345 (69) R.S. Montgomery, Run-in and Glaze Formation on Gray Cast Iron Surfaces, Wear, 14, 1969, pp 99-105 (70) Karl-Heinz Zum Ghar, Microstructure and Wear of Materials, (70) 22, Elsevier Tribology Series, 10, p 285 (71) ASM Metals Handbook Vol 3, 9th Edition, pp 582-583 (72) D.E. Diesburg & F. Borik, Optimizing Abrasion Resistance and

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(77) S . S . Bupara, A.J. Walowit and C.M. Allen, Gas Lubrication and Distribution of High Pressure Mainshaft Seals for Compressors, Third International Conference on Fluid Sealing, April 10, 1963 B3 (78) D.B. Hamilton, J.A. Walowit & C.M. Allen, A theory of

Lubrication by Microirregularities, Trans ASME, JBE, Paper no 65-Lub 11, 1965 (79) D.J. DePaul, Corrosion and Wear Handbook for Water Cooled Reactors, TID 7006 U.S. AEC, March, 1957 (80) E.I. Shobert, Carbon Brushes, Chemical Publishing Co., New York, 1965

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Reactors, U.S. Atomic Energy Commission, TID 7006/March 57, and N.J. Hoffman, D.E. Goggin and J.J. Droher, Friction and Wear Screening Tests of Materials in Sodium, Liquid Metal Engineering Center, LMEC-70-10 (84) Martin J. Devine, Proceedings of a Workshop on Wear Control to Achieve Product Durability, Analytical Rework/Service Life Project Office, NAval Air Development Center, Warminster, Pa, U.S.A. (85) J.F. Lynch, C.G. Ruderer and W.H. Duckworth, Engineering

Properties of Selected Ceramic Materials, American Ceramic Society, 1966 (86) D.P.H. Hasselman, Unified Theory of Thermal Shock Fracture

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1982 (89) T.A. Dow and R.A. Burton, The Role of Wear in the Initiation

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21 7

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Heat Engines, Ceramic Engineering and Science Proceedings, Vol 7, No 7-8, The American Ceramic Society, 1986, 1052-1059 (91) A.H. Heuer, Transformation Toughening in Zr02 Containing Ceramics, Jnl American Ceramic SOC, 70(10) 1987, 689-698 (92) Ernest M. Levin & Howard F. McMurdie, Phase Diagrams for Ceramicists 1975 Supplement, The American Ceramic Society, 117

Dufrane, Sliding Performance of Ceramics for Advanced Heat Engines, Ceramic Engineering and Science Proceedings,

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(7-8) , 1986, 1052-1059

(94) Ranga Komanduri & Shyam Samanta, Ceramics, Metals Handbook, Vol 16, ninth edition, ASM 1989, 98-103. (95) Metals Handbook, 9th Edition, Volume 3, ASM, 1980, 462 -464 (96) Richard J. Parker & Erwin V. Zaretsky, Fatigue Life of High-

Speed Ball Bearings with Silicon Nitride Balls, Trans ASME JOLT, July 1975, 350-357 (97) H.R. Baumgartner, Evaluation of Roller Bearings Containing

Hot Pressed Silicon Nitride Rolling Elements, Second Army Technology Conference, Ceramics for High Performance Applications, Hyannis, Mass, Nov 1973 (98) Robert F. Stoops, Cermet Technology, Bulletin 8 2 , North Carolina State-University,Raleigh, N C., 1965, 13 (99) E.B. Shand, Engineering Glass, Modern Materials, Vol 6, Academic Press, 1968, 247

Unpublished work performed at Battel e Institute, Columbus, Ohio D.B. Hamilton, J.A. Walowit and C.M. Allen, A theory of Lubrication by Microirregularities, ASLE-ASME Lubrication Conference 1965, Paper 65-Lub-11 H.E. Sliney, Lubricating properties of some bonded fluoride and oxide coatings for temperatures to 1500°F,NASA Tech Note D-478, 1960. M.F. Amateau & W.A. Glaeser, Survey of Materials for HighTemperature Bearings And Sliding Applications, Wear, 1964, 1-34

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(104) R.T. Foky, M.B. Peterson and C. Zapf, Frictional Characteristics of Cobalt, Nickel and Iron as Influenced by Their Surface Oxide Films, ASLE Trans., 6, (1963) 29-39 (105) N.J. Hoffman, D.E. Goggin and J.J. Droher, Friction and

Wear Screening Tests of Materials in Sodium, LMEC-70-10, 1970 (106) J.W. Kissel, W.A. Glaeser & C.M. Allen, Frictional Behavior

of Sodium Lubricated Bearing Materials in Controlled High Temperature Environment, Wear, 5, 446-457 (1966) (107) W.H. Roberts, Friction and Wear Behavior of Sliding Bearing Materials in Sodium Environment at Temperatures to 6OO0C, U.K. Atomic Energy Authority TRG-Report-1269, (1966) (108) N.V. Hoffman, D.E. Goggin and J.J. Droher, Friction and

Wear Screening Tests of Materials in Sodium, LMEC-71-7 pp 49-56 (1971) (109) Machine Design, Sliding Bearing Materials, June 20, 1974, Pg42

(110) 1989 Guide to Selecting Engineering Materials, Advanced Materials & Processes, ASM Iternational, pg 116

(111) Allen J. Klein, Polymers Wage War Against Wear, Advanced

Materials

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Processes, March 1986, 31-36

(112) John E. Thegerge, Peter J. Cloud & Barry Arkles,How to Get High-Accuracy Plastic Gears, Machine Design, Sept 6, 1973,

140-145 (113) W.A. Glaeser & K.F. Dufrane, Woven teflon Fabric Bearings, Mechanical Engineering, Feb 1975, 16 -19 (114) J.M. Crosby, C.A. Carreno & K.L. Talley, New Reinforcing Technology Provides Stronger Fluorocarbons, Machine Design, Jan 21, 1982, 79 - 84. (115) Modern Plastics Encyclopedia, Fluorplastics, 1990, pg 25 (116) D.Dowson, John Atkinson & K . Brown, The Wear of High

Molecular Weight Polyethylene with Particular Reference to its use in Artificial Human Joints, Advances in Polymer Friction and Wear, Plenum Press, 1974, 533-551 (117) Guide to Selected Engineered Materials, 1989, ASM publication, 116-119

219

(118) R. Crawford, Plastics and Rubber, Engineering Design and

Applications, Mechanical Engineering Publications, London, 1985 (119) Benjamin M. Walker, Handbook of Thermoplastic Elastomers, Van Nostrand Reinhold Co, 1979 (120) Oliver H. Wyatt and David Dew Hughes, Metals, Ceramics and Polymers, Cambridge University Press, 1974, pg 308 (121) E.F. Gobel, Rubber Springs Design, Newnes-Butterworth, London, 1974 (122) Parker 0-Ring Reference Guide, Parker Seal Group,

Lexington, Ky. (123) A. Schallamach, Friction and Abrasion of Rubber, Wear, Vol 1, 348-417, 1958

This Page Intentionally Left Blank

22 1

APPENDIX

Contents of the appendix include:

1 . Tables of special material properties sorted in decending order a. b. c. d. e. f. g. h. i.

Critical velocity ................................... Stacking fault energy ............................... Fracture toughness .................................. Hardness ............................................ Specific hardness ................................... Thermal diffusivity ................................. Thermal stress resistance factor .................... Plasticity index .................................... Maximum operating temperature ....................... j. Maximum Bearing Stress ..............................

220 222 224 226 229 232 234 236 239 242

The above tables are derived from a data base containing all of the materials described in this book . The special properties were calculated using the appropriate equations and the resulting values sorted in decending order . 2 . A hardness conversion nomogram and a table of crystal structures and lattice constants for pure metals is also included: 243 k . Hardness conversion nomogram ........................ 1 . Crystal structures and lattice constants ............244

222

CRITICAL VELOCITY Dow and Burton (1) have recognized a thermal instability which can develop between two sliding surfaces - usually dry surfaces - in which contacting asperities heat up to near incandescence . These "hot spots" tend to move rapidly over the surfaces as they develop, reach maximum temperature and wear away, transferring their contact support to other nontouching asperities. The process, known as Thermoelastic instabil ity tends to spread back and forth across surfaces in a regular, cyclic manner. The process has been modeled, assuming that initial asperity contact causes rapid local thermal expansion of the contacting asperities, transferring most of the load support to a few contacts resulting in a runaway increase in temperature at the contacts. The hot spots soften, melt or vaporize and are destroyed, allowing other asperities to come into contact. This thermal instablility can produce surface damage in brittle materials as a result of thermal shock or thermal fatigure. Wear of more ductile materials increases significantly with the onset of thermoelastic instability. (This is different from gross surface frictional heating which is also detrimental to sliding systems when hot hardness levels are exceeded) Thermoelastic instability is influenced by the sliding material's thermal diffusivity, thermal expansion characteristics and elastic moduli. A critical velocity above which a given material will exhibit TEM can be estimated by the fol 1 owing re1 at ion :

V*

=

2

40K / ( p ~ z E ) ~kz

where :

V* K /L

a

E k z

critical sliding velocity, mm/sec thermal conductivity = coefficient of friction = coefficient of thermal expansion = Young's modulus = thermal diffusivity = width of slider = =

On the following page the critical velocities of some materials are listed for conditions of unity friction coefficient and per cm of width. To determine a velocity for a given friction coeffifient and sl der width, divide the velocity value in the table by 1/(p) and l/z. As an example, the critical velocity of Sic runn ng unlubricated with a coefficient of friction of 0.2 would be about 40 cm/sec per cm of slider width. If the system were lubricated with a solid lubricant and the friction coefficient were lowered to 0.06, the critical velocity would be about 1540 cm/sec per cm of slider width. 1)

T.A. Dow

&

R.W. Burton, Jnl Lub Tech, ASME, F, 9 5 ( 1 ) , 71-75, 1973

223

MATERIAL

C93700 SAE 64 BERYLLIUM COPPER TITANIUM D I BORIDE INDIUM GOLD SILVER COPPER LEAD LEAD BABBITT 7 UHMWPE A1 1100 C93200 SAE 660 ALUMINUM SILICON NITRIDE BORON CARBIDE VESPEL SP1 SILICON CARBIDE VANASIL 77 MOLYBDENUM, 0.5% T i C94300 H i Pb Brnz MOLYBDENUM TZM C61000 A1 Brnz TIN TITANIUM NITRIDE HASTELLOY C ZINC C51100 Phos Brnz ANTIMONY CADMIUM C60800 A1 Brnz 1040 I RON C87200 S i Brnz 1118 FEP NYLON 66 4320 C86300 Mn Brnz NYLON 6 9310 SS 440C T i t a n i u m 6A1-4V SILICA 1095 C1080 ABEX PACE

CRITICAL VELOCITY mm/s ec/ 10 104 67 19 19 8 5 5 3 3 3 3 2 2 2 2 2 1.6 1.6 1.5 1.5 1.4 1.3 1.o 0.9 0.9 0.9 0.9 0.8 0.7 0.6 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

4340 8620 SS 316 SPK SN80 Zr02 DUROID 5600 CHROMIUM CARBIDE HP TIC PHENOLIC INCONEL 718 TITANIUM CARBIDE Rene 41 WASPALOY SS Gal 1 -Tough MONEL K-500 Austenitic-Mn TITANIA ARAMID KS-105 ALUMINA PSZ 2191 TUNGSTEN CARBIDE BERYLLIA

1.86E-01 1.81E-01 1.44E-01 1.33E-01 1.25E-01 1.01E-01 1.01E-01 9.71E-02 9.20E-02 8.20E-02 8.09E-02 8.05E-02 7.14E-02 6.72E-02 4.76E-02 4.56E-02 4.41E-02 3.06E-02 2.59E-02 1.38E-03 5.47E-04

224

STACKING FAULT ENERGY The stacking fault energy of a metal is a measure of its work hardening properties and capability of plastic deformation. Stacking fault energy is the energy required to form a defect in the regularity of the stacking of atomic planes in a crystal. The higher the stacking fault energy, the more difficult it is to form a stacking fault or to split a dislocation into partials. With high stacking fault energy, plastic slip is easier because partial dislocations can be pinched together, allowing cross slip and preventing pile up of moving dislocations. One should expect a larger work hardening rate for metals with low stacking fault energy. Stacking fault energy can be reduced in some metals by alloying. For instance, adding aluminum to copper results in significant decreases in stacking fault energy. There is much controversey over the effect of stacking fault energy on the wear properties of metals. In some cases, where heavy deformation occurs in the near surface region of, for instance, copper-aluminum with varying amounts of aluminum, very little difference can be detected. Zum Ghar [ l ] has found that copper-nickel alloys show substantial increases in abrasion resistance when the nickel content is greater than 60% and there is a resulting substantial increase in stacking fault energy. He attributes the increased wear resistance to increasesd capabil ity for plastic strain and reduced tendency for microcutting. The importance of stacking fault energy in wear mechanisms may, in some cases be exaggerated and care should be exercised in using it as the only criterion in the selection of materials for wear resistance. The table on the next page gives stacking fault energies for several metals and a few alloys. The values given are subject to considerable error ( as much as 50%). Therefore, the table should be used only to indicate the ranking of the materials in it.

1 Karl-Heinz Zum Gahr, Microstructure and Wear of Metals, Tribology Series, 10, Elsevier, 1987

225

STACKING FAULT ENERGY MATERIAL

RHENIUM NICKEL FE-SI AlUMINUM ZINC ALUMINUM PLATINUM COPPER GOLD SILICON INDIUM SILVER LEAD COBALT SS 304HN a BRASS C60800 A1 Brnz C61000 A1 Brnz WASPALOY GRAPHITE

SFE erg/cm 750 450

400 200

140 135 95 70

40 40 30

25 25 20 20

14 10 5 4

1

226

FRACTURE TOUGHNESS Fracture toughness is a measure of the ability o f a material to resist the propagation of a brittle crack. Thus, materials exposed to mechanical impact must have fracture toughness to prevent cracking and fracture. Fracture toughness can play a role in the wear of materials. The more brittle the material is - as with very hard metal alloys and brittle ceramics the more likely fracture toughness will be an important parameter to consider in the selection of wear resistant materials. Hornbogen [2] has developed an equation relating wear with elastic modulus and fracture toughness in which wear is proportional to E/KIc2.

2 E. Hornbogen, The Role of Fracture Toughness in the Wear of Metals, Wear, 33, 1975, 251 - 259

227

MATERIAL

KIc MPa

m4 4340 SS ALLEGHENY A-286 ABEX PACE INCONEL 718 SS 15-5PH 8620 4820 21-4N T i t a n i u m 6A1-4V 21-55N 1040 SS 17-4PH 4820 Austenitic-Mn ZINC 27 ZINC 12 TOOL STEEL VASCO X-2 STELLITE F TUNGSTEN CARBIDE ZYS2Y20A Zr02 PSZ 2016 PSZ 1027 PSZ MS PSZ 2191 S iA1 ON TITANIUM DIBORIDE PSZ TZ3Y BORON CARBIDE SPK SN80 Zr02 SILICON CARBIDE SILICON NITRIDE ALUMINA TITAN IA GLASS, SODA GLASS, QUARTZ SILICA

167.00 125.00 98.00 96.00 8 1 .OO 77.00 77.00 76.00 71 .OO 60.50 55.00 48.00 38.00 22.00 21.40 21.40 14.00 12.00 12.00 10.00 10.00 10.00 9.00 8.50 7.70 7.00 6.50 6.00 5.30 4.60 4.00 3.00 2.70 0.70 0.68 0.60

228

MATERIAL HARDNESS Wear resistance is often 1 inked directly to hardness. The harder the material, the better the wear resistance. Of course this breaks down when a metal is hardened to the point of brittleness and microfracture begins to play a role in the wear process. Hot hardness is also important when a material is to be used in a heated system. The following table has been developed to show the ranking of various materials in hardness value. Most of the steels shown in the table are in their hardened condition. DPH hardness was used so that all materials could be compared. For some materials like plastics, this meant converting to DPH from another hardness scale - entailing some error.

229

HARDNESS VALUES OF MATERIALS MATERIAL

TITANIUM DIBORIDE BORON CARBIDE TITANIUM CARBIDE HP TIC SILICON CARBIDE CHROMIUM CARBIDE TITANIUM NITRIDE SiAlON T i t a n i u m 6A1-4V PSZ MS TUNGSTEN CARBIDE ALUMINA ZYS2Y20A Z r 0 2 K 1628 SILICON NITRIDE PSZ TZ3Y Eiectroless Ni T I TAN I A 15Cr- 3Mo SILICA TOOL STEEL T15 P-5Ag 8620 TOOL STEEL M2 TOOL STEEL CPM 1OV TOOL STEEL M50 Super N i t r a l l o y TRIBALOY T-800 P- 658RC FERRO T I C 9310 P-5

SS 440C 81845 P-15 HC-250 P-03 TOOL STEEL H11 TOOL STEEL D2 TOOL STEEL 01

HARDNESS DPH 3500 3200 3000 3000 2700 2600 2000 1780 1700 1600 1500 1500 1400 1400 1300 1158 1050 900 895 853 832 820 789 765 763 760 740 739 730 695 694 680 675 675 675 655 655 650 650 650 617 617 600

600 598 590 577

MATERIAL

STELLITE 1 NI-HARD 4 1095 P-29 1040 DUR IRON STOODY 6 TOOL STEEL VASCO X-2 TRIBALOY T-700 P-9 4820 SS 15-5PH 52 100 INCONEL 7 1 8 C1080 SS 17-4PH STELLITE 6 BERYLLIUM COPPER ABEX PACE WASPALOY 4340 A536 NODULAR SS ALLEGHENY A-286 H-46 MOLYBDENUM TZM P-2w SS 1 6 - 2 5 - 6 MONEL K-500 SS UHB AEB-L PSZ 2 1 9 1 MOLYBDENUM, 0.5% T i 21-55N SS 410 TUNGSTEN SS 440C 4620 P e a r l i t i c Ma1 1e a b l e C86300 Mn B r n z 50100 4320 C 1080 WAUK. 5 4 SS UNILOY 19-9DL HASTELLOY C 1118 SS NITRONIC 60 51100

HARDNESS DPH 562 560 550 540 540 530 526 520 485 480 424 420 413 410 404 400 393 393 380 373 350 350 330 330 325 320 320 320 310 286 264 264 257 257 257 255 248 225 225 225 224 220 217 216 210 200 200

230

MATERIAL

52100 MOD A536 NODULAR SS 3 1 6 SS Gal 1-Tough MEEHANITE 81845 ZIRCALLOY 2 G- 1 C61000 A1 B r n z WAUKESHA A1 -N i- Fe B r o n z e WASPALOY ss 3 4 7 NI-RESIST 1 SS 3 0 4 SS 3 1 6 C51100 Phos B r n z A1 3 9 0 VANASIL 7 7 PHENOLIC ZINC 27 C61000 A1 B r n z ZINC 1 2 C87200 S i B r n z C93200 SAE 6 6 0 C60800 A1 B r n z C93700 SAE 6 4 VESPEL SP1 UHMWPE C94300 H i Pb B r n z ZINC A1 1918 IRON A1 750 COPPER ARAMID KS-105 ANTIMONY TORLON DELRIN

GOLD VESPEL s p 2 1 SILVER A1 1100 C98600 Cu-Pb T I N BABBITT 3 T I N BABBITT 2

HARDNESS DPH 200 200 200 200 196 195 185 170 165 165 157 157 150 150 150 150 145 145 140 115 107 100 93 85 80 70 60 50 50 48 47 45 45 45 42 40 40 40 35 35 35 34 30 28 26 24

MATERIAL

NYLON 6 6 LEAD BABBITT 13 T I N BABBITT 1 NYLON 6 IPC 1832 ALUMINUM LEAD LEAD BABBITT 8 TIN URETHANE FEP DUROID 5600 VITON SILICONE NEOPRENE NITRILE BUTYL INDIUM

HARDNESS DPH 20 20 17 15 15 15 14 11 8 6 5 5 3 3 2 2 2 1

23 1

SPECIFIC HARDNESS The following pages contain a listing of the specific hardness of materials (DPH hardness/density). They are listed in decending order - that is, the best materials for minimum weight and maximum hardness are at he top. For instance, titanium carbide has about the same hardness as partially stabilized zirconia (PSZ TZ3Y). However, a part made o f titanium carbide, with a specific hardness of 0.68 would be would be only about 30% o f the weight of one made of partially stabilized zirconia (SH = 0.19). If fracture toughness were and important consideration, the PSZ would have better fracture toughness and design might have to be modified to use less volume of material.

232

MATERIAL

SILICON CARBIDE TITANIUM DIBORIDE TITANIUM CARBIDE S i A1 ON HP TIC TUNGSTEN CARBIDE BORON CARBIDE SILICON NITRIDE P-5 P-658RC CHROMIUM CARBIDE ALUMINA T i t a n i u m 6A1-4V TITANIUM NITRIDE T-0054 P-5Ag P-03 P-29 PSZ MS P-9 SILICA P-15 ZYS2Y20A Zr02 K 1628 T I TAN1A P-2w PSZ TZ3Y Electroless N i PM- 103 FERRO TIC TOOL STEEL CPM 1OV 8620 TOOL STEEL T15 TOOL STEEL M50 Super N i t r a l l o y TOOL STEEL M2 9310 G- 1 N I -HARD SS 440C TRIBALOY T-800 PHENOLIC HC-250 81845 s-95 TOOL STEEL H11 STELLITE STAR J

SPECIFIC HARDNESS DPH/DENS

0.887 0.790 0.677 0.549 0.542 0.493 0.482 0.427 0.409 0.399 0.391 0.387 0.384 0.361 0.351 0.349 0.310 0.300 0.289 0.289 0.280 0.256 0.253 0.230 0.217 0.193 0.190 0.131 0.114 0.109 0.102 0.102 0.102 0.098 0.095 0.095 0.090 0.088 0.088 0.087 0.085 0.083 0.080 0.080 0.078 0.077 0.076

MATERIAL

NI-HARD 4 DURIRON TRIBALOY T-400 1040 T i t a n i u m 6A1-4V TOOL STEEL VASCO X-2 52100 STOODY 6 Electroless N i STELLITE 1 A1 390 Super N i t r a l l o y TRIBALOY T-700 SS 15-5PH 4820 C 1080 UHMWPE SS 17-4PH INCONEL 718 ABEX PACE VANASIL 77 PSZ 2191 STELLITE 6 WASPALOY 4340 H-46 SS ALLEGHENY A-286 SS UHB AEB-L SS 16-25-6 MONEL K-500 VESPEL S P I SS 440C 21-55N P e a r l i t i c Ma11eabl e 4340 SS 410 MOLYBDENUM TZM 52100 MOD ARAMID KS-105 TORLON C1080 C86300 Mn Brnz 4320 ZIRCALLOY 2 SS UNILOY 19-9DL MEEHANITE 1118

SPECIFIC HARDNESS DPH/DENS 0.075 0.074 0.072 0.070 0.068 0.066 0.065 0.063 0.062 0.062 0.058 0.056 0.055 0.054 0.054 0.052 0.052 0.052 0.051 0.051 0.051 0.049 0.047 0.046 0.045 0.043 0.040 0.040 0.040 0.039 0.036 0.034 0.034 0.034 0.034 0.033 0.032 0.031 0.031 0.029 0.029 0.029 0.029 0.028 0.027 0.027 0.027

233

SS Gal 1 -Tough 1095 SS NITRONIC 60 MOLYBDENUM, 0.5% T i VESPEL SP21 81845 Austenitic-Mn Steel SS 316 SS 304HN WAUK. 54 HASTELLOY C DELRIN ZINC 27 C61000 A1 Brnz NI-RESIST 1 A l - N i - F e Bronze WASPALOY ss 347 SS 304 NYLON 66 C51100 Phos Brnz Z I N C 12 TUNGSTEN C61000 A1 Brnz A1 1100 NYLON 6 C87200 S i Brnz BERYLLIUM COPPER I P C 1832 C93200 SAE 660 C60800 A1 Brnz C93700 SAE 64 ZINC ANTIMONY IRON ALUMINUM URETHANE A1 750 A1 1918 C94300 H i Pb Brnz COPPER T I N BABBITT 3 SILVER C98600 Cu-Pb DUROID 5600 CADMIUM BUTYL LEAD BABBITT 7 FEP SILICONE NITRILE NEOPRENE GOLD Lead

0.026 0.026 0.026 0.026 0.025 0.025 0.025 0.025 0.025 0.025 0.024 0.024 0.021 0.021 0.021 0.020 0.019 0.019 0.019 0.018 0.016 0.015 0.015 0.013 0.011 0.011 0.010 0.010 0.009 0.009 0.008 0.007 0.007 0.006 0.006 0.006 0.005 0.005 0.005 0.005 0.005 0.003 0.003 0.003 0.003 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.001

234

THERMAL DIFFUSIVITY Thermal diffusivity is a measure of a material’s ability to dissipate heat from a highly localized spot on the surface. During sliding contact, asperity contacts can develop high localized temperatures owing to frictional heating . Thus, the asperity contact can potentially heat to the softening or melting point. A material with a high thermal diffusivity can operate at relatively high sliding velocity and high friction without severe wear because hot spots dissipate rapidly. In a ceramic sliding condition, if both contact surfaces are ceramic, the material with the lowest thermal diffusivity will wear the most. If a metal surface slides against a ceramic surface, wear is minimized by choosing a metal with a high thermal diffusivity. For instance, molybdenum would make an excellent surface to slide against silicon carbide. The relation for thermal diffusivity is as follows:

K

=

k/pc

Where:

K

=

k p c

=

Thermal diffusivity, cmz/sec Thermal conductjvity, cal/sec/cmz = Density, gm/cm = Heat capacity, cal/gm C

235

MATERIAL

SILVER GOLD COPPER ALUMINUM A1 1100 SILICON CARBIDE ALUM1NA VANASIL 77 TIN MOLYBDENUM TZM CADMIUM INDIUM HASTELLOY C ZINC BERYLLIUM COPPER C51100 Phos Brnz LEAD C61000 A1 Brnz C94300 H i Pb Brnz C93200 SAE 660 1040 IRON 1095 C 1080 1118 TITANIUM NITRIDE C93700 SAE 64 LEAD BABBITT 8 C86300 Mn Brnz 9310 4320 4340 8620 A1 -Ni-Fe Bronze ANT IMONY SILICON NITRIDE SS 440C C87200 S i Brnz SS 410 TITANIUM CARBIDE ZIRCALLOY 2 P-2w MONEL K-500 T I TAN IUM D I BOR I D E

THERMAL DIFFUSIVITY cm2/sec

1.724 0.994 0.939 0.810 0.729 0.633 0.585 0.448 0.430 0.422 0.416 0.344 0.325 0.309 0.287 0.206 0.200 0.195 0.147 0.143 0.127 0.123 0.118 0.118 0.117 0.117 0.115 0.113 0.099 0.099 0.087 0.087 0.087 0.083 0.080 0.077 0.077 0.074 0.063 0.058 0.053 0.051 0.047 0.046

MATERIAL

Rene 41 SS 316

ABEX PACE SS 304 INCONEL 718 VESPEL S P l CHROMIUM CARBIDE SS Gal 1-Tough HP T I C WASPALOY Titanium 6A1-4V Austenitic-Mn TUNGSTEN CARBIDE BORON CARBIDE TITANIA GLASS, QUARTZ BERYLL IA DUROID 5600 UHMWPE NYLON 66 NYLON 6 ARAMID KS-105 PHENOLIC FEP

THERMAL D I FFUSI V ITY cm2/sec 0.044 0.040 0.038 0.033 0.031 0.030 0.028 0.027 0.025 0.023 0.023 0.021 0.017 0.014 0.012 0.004 0.003 0.002 0.001 0.001 0.001 0.001 0.001 0.001

236

THERMAL STRESS REISISTANCE FACTOR When ceramic and cermet materials are used in high speed sliding conditions (such as rocket seals) they become susceptible to thermally induced heat checking or cracking. Localized hot spots flow plastically and then cool into the elastic region after the hot spot passes. Tensile stresses develop in the surface when the stretched material cools and shrinkage cracks develop. It has been found that wear rates of high speed sliding ceramics vary inversely about the 1-1/4 power of the thermal stress resistance factor [3]. Therefore ceramics or cermets for these operating conditions should have high thermal stress resistance factor and high thermal diffusivity. A material with high thermal stress resistance has high elastic limit of strain or ratio of tensile strength to modulus o f elasticity, and low thermal expansion. The thermal stress resistance factor is determined from the fol 1 owing express ion :

R

=

s(1-u)/Ea

Where:

R

Thermal stress resistance factor tensile strength u = poisson’s ratio a = coefficient of thermal expansion s

= =

A listing of ceramic and cermet materials are presented on the following page with their thermal stress resistance factors in decreasing order. A few metals have been included for comparison.

Note that partially stabil ized zirconia exhibits high thermal stress resistance. However, because of its poor thermal conductivity, it is still subject to thermal shock cracking.

3 Sibley, Mace, Grieser and Allen, WADD TR 60-54,Wright Air Development Center, Ohio, 1959

237

MATERIAL

M-50 Tool Steel SILICON NITRIDE PSZ TZ3Y PSZ 2191 1040 Steel, HT PSZ MS TUNGSTEN CARBIDE Ni-Hard TITANIUM CARBIDE SPK SN80 Zr02 BORON CARBIDE Tribaloy T-400

ALUMINA HP TIC CHROMIUM CARBIDE PSZ 1027 SILICON CARBIDE SILICA BERYL L IA

Therm Stress Resistance Factor 1255.00 734.30 586.21 498.32 401.00 344.83 270.83 235.00 232.14 187.56 156.25 143.00 99.11 96.90 71.81 69.66 68.18 21.04 7.58

238

PLASTICITY INDEX Plasticity index is a measure of the amount of plastic deformation expected between contacting asperities during solid contact. It is possible to have contact conditions where the real contacts are predominently elastic. This requires relatively smooth surfaces and a low elastic modulus/hardness ratio. Greenwood and Williamson [4] in a study of contact mechanics developed a relation which they called the Plasticity Index shown below: =

(E*/H) (as ks)'

Where: Plasticity Index E* = E/(l-V 2 ) E = Young's modulus v = Poisson's ratio as = rms roughness ks = peak radius =

Greenwood and Williamson stipulated that when the plasticity index fell below 1.0, the probability that most of the asperity contacts would be elastic was high. Above a value of 1.0 the contact would be predominently plastic, even at very 1 ight loads. This non-dimensional surface texture parameter is useful for estimating the probability of plastic deformation and roughening of two surfaces in sliding contact. - As long as the bearing stress does not exceed 3Y, where Y is the tensile yield strength of the material. It also can be useful in determining the reauired surface finish to provide mostly elastic asperity contact in contacting parts. On the following pages a listing of metals and alloys with the r E*/H va ues. 4 J.A. Greenwood and J.B.P. Williamson, Contact of Nomina ly Flat Surfaces, Proc Roy SOC, A295, 1966, 300-319

239

MATER IAL

CONDITION

MOLYBDENUM TZM ANNEALED C1080 ANNEALED 50100 ANNEALED WAUKESHA 4320 ANNEALED HASTELLOY C ANNEALED 4820 ANNEALED 4620 ANNEALED ZINC 12 SS Gal 1 -Tough ANNEALED A536 NODULAR ANNEALED 4620 HT. 300 t e 4340 ANNEALED C61000 A1 Brnz ANNEALED SS 440C ANNEALED SS 410 HT,1000 t e C51100 Phos Brnz HARD 21-55N ANNEALED WAUK. 54 ZINC 27 A1 -Ni-Fe Bronze P e a r l it i c Ma11e a b l e NI-RESIST 1 SS UHB AEB-L VANASIL 77 Age Hdnd MEEHANITE HARDENED SS 16-25-6 HEAT TREAT 4340 A1 390 PRECIP HARD WASPALOY MONEL K-500 STELLITE 6 ANNEALED ZIRCALLOY 2 ABEX PACE HT,400F t e C1080 PRECIP HARD SS 17-4PH HEAT TREAT 4820 AGED INCONEL 718 52100 HEAT TREAT Martens. A536 NODULAR HARD C61000 A1 Brnz N I TR I DED Super N i t r a l l o y PRECIP HARD SS 15-5PH TRIBALOY T-700 ANNEALED C86300 Mn Brnz STELLITE 1 FERRO TIC TOOL STEEL VASCO X-2HEAT TREAT TRIBALOY T-400

E*/H 109.34 103.46 103.00 103.00 103.00 101.93 100.76 100.76 99.68 95.79 94.63 90.88 89.13 88.84 87.17 85.67 85.24 81.93 80.76 79.42 78.73 77.87 77.25 . . 74.76 74.49 70.94 67.59 66.21 63.40 63.37 62.76 59.76 58.46 58.14 57.36 55.04 54.66 54.64 54.24 54.07 53.84 53.28 52.42 49.38 48.75 48.11 46-68 46.05 46.00 ~

~~

240

MATERIAL Electroless Ni STOODY 6 1040 S t e e l STELLITE STAR J TOOL STEEL 01 T i t a n i u m 6A1-4V HC-250 TOOL STEEL D2 STELLITE 3 TOOL STEEL H11 81B45 BERYLLIUM COPPER TRIBALOY T-800 NI-HARD 4 SS 440C 9310 TOOL STEEL CPM 1OV Super N i t r a l 1oy TOOL STEEL M2 TOOL STEEL M50 NI-HARD 8620 TOOL STEEL T15 DURIRON Electroless Ni T i t a n i u m 6A1-4V

CONDITION As D e p o s i t HEAT TREAT HEAT TREAT ANNEALED HARDENED HEAT TREAT HEAT TREAT HEAT TREAT AGE HARDEN HEAT TREAT HEAT TREAT HEAT TREAT NI TR IDED HEAT TREAT HEAT TREAT HEAT TREAT HEAT TREAT ANNEALED Heat 400C HEAT TREAT

E*/H 44.80 44.06 42.92 42.34 41.50 41.06 40.56 39.28 38.91 38.75 37.56 37.35 36.59 34.49 34.47 33.39 32.40 31.32 30.29 29.98 29.48 29.37 27.85 26.24 21.34 7.27

24 1

Maximum o p e r a t i n g temperature (Temp a t which s o f t e n i n g , m e l t i n g o r decomposition occurs) MATERIAL

HEAT TREAT

SILICA ALUMINA GLASS, QUARTZ SILICON CARBIDE PSZ 2191 SILICON NITRIDE SiAlON PSZ 2016 PSZ 1027 SS Gal 1 -Tough Annealed SS 304 Annealed WASPALOY HT, PH WASPALOY Annealed NI-HARD NI-HARD 4 ss 347 Annealed SS ALLEGHENY A-286 F u l l y aged SS UNILOY 19-9DL Hot R o l l e d TRIBALOY T-400 TRIBALOY T-800 T-0054 SS 410 Annealed SS 16-25-6 Hardened MOLYBDENUM TZM INCONEL 718 Aged STELLITE STAR J H-46 SS 316 Cold R o l l e d 2690 WAUK. 54 TRI BALOY T- 700 SS 410 HT, 1000 t e SS 316 Annealed 4820 HT 300F t e 4820 Annealed TOOL STEEL VASCO X-2 HT,1000 t e 21-55N Annealed P-03 NI-RESIST 1 TOOL STEEL M2 HT, 300F t e MOLYBDENUM, 0.5% T i MONEL K-500 HT,500F t e TOOL STEEL H I 1

MAX OP TEYP C 2399 1760 1649 1649 1482 1482 1400 1371 1371 982 900 871 87 1 816 816 815 760 760 704 704 650 649 649 649 649 649 649 649 649 649 649 649 649 620 538 538 538 538 538 538 538 538 538

242

MATER I A L

TOOL STEEL M50 BERYLLIUM COPPER 1095 BERYLLIUM COPPER TOOL STEEL M50 A536 NODULAR T i t a n i u m 6A1-4V PM-103 ZIRCALLOY 2 T i t a n i u m 6A1-4V 486 MEEHANITE VESPEL SP21 103 CGAG s-95 VESPEL S P l Electroless N i Electroless Ni SS 440C P-5 SILICONE SS 440C P-2w P-9 P-658RC 4340 DUROID 5600 4340 52 100 P - 5Ag 52100 MOD VANASIL 77 50 100 C61000 A1 Brnz RT DUROID RYTON R4 52100 A1 1918 51100 TORLON C61000 A1 Brnz C60800 A1 Brnz TIN BABBITT 2 LEAD BABBITT 7 LEAD BABBITT 13 LEAD BABBITT 8 VITON

HEAT TREAT

Annealed Age harden Annealed Annealed HT,600F t e Annealed Annealed Annealed

HT 400C As Depos. Annealed Impreg HT,600F t e

Temp 260C Annealed Annealed Annealed Age Hdnd Annealed Hard HT,900F t e Annealed Annealed Annealed

MAX OP TEP ! C 427 427 427 427 427 425 400 400 400 400 399 399 371 371 371 371 320 320 316 316 316 316 288 288 269 260 260 260 260 260 260 260 260 260 260 260 260 260 260 260 260 260 238 232 232 232 232

243

MATERIAL

Carbon P-29 T-0454 T I N BABBITT 1 PEEK t GRAPHITE T I N BABBITT 3 G- 1 A1 750 FEP IPC 1832 FABROID DU Austenitic-Mn P-15 LEAD BABBITT 15 C98600 Cu-Pb TOOL STEEL 01 A1 1100 BUTYL NITRILE C93200 SAE 6 6 0 C93700 SAE 64 ZINC 12 PHENOLIC URETHANE NEOPRENE DELRIN NYLON 6 NYLON 66 UHMWPE NYLON TF

HEAT TREAT

HT 1850

MAX OP TEYP C

F

Annealed HT,600F t e P u r e A1 Annealed Annealed

230 228 221 220 220 204 204 204 204 204 204 204 204 204 176 150 149 149 149 149 121 121 121 116 116 100 85 85 77 60

244

BEARING STRENGTH FOR SEVERAL JOURNAL BEARING MATERIALS

MATERIAL

IPC 1832 52100 FABROID BERYLLIUM COPPER TRIBALOY T-800 P-5 C61000 A1 B r n z TRIBALOY T - 4 0 0 TRIBALOY T - 7 0 0 P-2w A1 1918 A1 750 VESPEL S P l VESPEL SP21 PEEK t GRAPHITE RYTON R4 NYLON 6 NYLON 6 6 DU PHENOLIC T I N BABBITT 2 LEAD BABBITT 7 LEAD BABBITT 8 T I N BABBITT 1 LEAD BABBITT 13 LEAD BABBITT 15 NYLON TF C93200 SAE 6 6 0 C98600 Cu-Pb C93700 SAE 64 UHMWPE RULON LD ZINC 12 ZINC 27

FORM

SINTERED WOVEN CAST BAKED CAST BAKED CAST CAST SINTERED SINTERED MOULDED CAST CAST SI NTERED LAMINATED CAST CAST CAST CAST CAST CAST CAST CAST CAST CAST SINTERED CAST CAST

BEARING STRENGTH MPa 827.28 689.40 413.64 344.00 248.18 206.82 206.00 124.09 124.09 82.73 62.05 62.05 55.15 55.15 50.00 41.36 41.36 41.36 41.36 34.47 33.09 27.58 26.89 26.20 25.51 24.10 20.68 17.24 13.79 10.34 6.89 6.89 0.69 0.69

245

HARDNESS CONVERSIONS The charts below provide an approximate comparison o f the various hardness scales found used for measuring material hardness properties. The comparisons are not to be considered accurate but are intended to show the relative positions or hardness values.

1107

300

5

200

SCHLERESCOPE

ROCKWELL C

100

DPH 20 50

ROCKWELL B ROCKWELL 30 T

g03 1 120

10 60

80

ROCKWELL M

BARCOL 5

100 loo 90 50 30

100 140 160

B S SOFTNESSS BHN

6 0 4 40-

60 20

-

ROCKWELL R

30

SHORE D SHORE A (DUROMETER)

246

CRYSTAL STRUCTURES AND LATTICE CONSTANTS FOR METALLIC ELEMENTS

Element

Structure

L a t t i c e constants, A a

Aluminum Antimony B e r y l 1 ium Cadmium Carbon, Diamond Graphite, a Graphite, R C hrom ium Cobalt, a

R Copper Go1d Hafnium Indium Ir i d ium Iron, a T

> 1404'C Lanthanum, a

U Lead Lithium ( c o l d worked) Magnesium Manganese ,ao

0 727-1095 5 1095-1133 C

T

Mercury Molybdenum Nickel Niobium Pal 1ad ium P1 a t inum Rhen iurn Rhodium Rub i d ium Ruthinium Silver Sod ium T a n t a l urn

FCC

Rhombohedral CPH CPH Diamond c u b i c Hexagonal Rhombohedral BCC CPH FCC FCC FCC CPH FC Tetrangonal FCC BCC FCC FCC CPH FCC FCC BCC FCC C PH CPH Cubic Cubic FC Tetragonal Rhombohedral BCC FCC BCC FCC FCC C PH FCC BCC CPH FCC BCC BCC

4.049 4.506 2.285 2.979 3.568 2.461 2.461 2.885 2.507 3.552 3.615 4.078 3.206 4.594 3.839 2.866 3.571 2.94 3.762 5.307 4.949 3.509 4.40 3.08 3.209 8.912 6.313 3.782 2.006 3.147 3.524 3.301 3.890 3.924 2.761 3.803 5.63 2.704 4.086 4.291 3.303

b

6.075

247

Element

Structure

T e l l u r ium Tin, a g r a y R, w h i t e Titanium, a I3 Tungsten Uranium, a Vanad ium Zinc Zirconium, a R

Hexagonal Diamond c u b i c Te t r a g o n a l C PH BCC BCC Orthorhombic BCC CPH C PH BCC

L a t t i c e Constant, A a b

4.456 6.47 5.831 2.950 3.33 3.165 2.858 3.039 2.664 3.230 3.62

5.877

This Page Intentionally Left Blank

249

INDEX

Abrasion resistance 110, 1 1 1 Chromium 101 Abrasion resistance vs particle hardness 44 Abrasive wear 44 mining industry 112 Abrasive Wear Resistance 4 cobalt base alloys 165 Acetals 187 Adhesion 27 Adiabatic diesel 131 Air bearings 157 Alumina 148 Aluminum based bearing alloys aluminum-lead diagram 86 lead addition 79 Aluminum based bearing materials 79 babbitt containing 80 copper adddition 79 silicon strengthened 80 A1 uminum bronze bearing design chart 61 heat treatment 53 microstructure 55 nickel & iron additions 53 Aluminum-lead 86 Antimony 71 Aramid 179 Aramids 194 fibers 194 Austenite 94, 100 cast iron 99, 103 Austenitic stainless steels 26 Automotive bearings 74 Babbitt bearing materials fatigue life vs layer thickness 82 selection and design 84 Babbitt bearings fatigue 78 fatigue failure example 78 marine environment 78 Babbitts 72 antiseizure alloy 78 copper in lead babbitts 72 effect of temperature on fatigue 77 fatigue 74 lead base babbitts 72 tin base babbitt microstructure 76 tin base babbitts 74 tin-antimony-copper diagram 75 unleaded gasol ine 74

250

Ball bearing retainers phenol ics 192 Bearing bronzes 47 hot shortness 48 leaded tin bronzes 50 metallurgy 48 microstructure 49 porous bronzes 56 tin bronzes 48 wear design map 60 wear properties o f porous bronzes 59 Bearing load capacities 83 Bearings cryogenic 126 water lubricated 124, 125 Beryl 1 ium copper 47, 56 copper-beryl1 ium diagram 57 health hazard 56 heat treatment 65 microstructure 58 precipitation hardening 56 Boundary lubrication 48, 63 liquid sodium 167 Bronze bearing a1 1 oys aluminum bronze 53 leaded bronze microstructure 52 manganese bronze 53 SAE 660 52 Bronze bearing design program 62 Bronze bearings grease lubrication 62 Bushings for oscill itory motion 61 Butyl rubber 206 Cadmium 71, 76, 78 Carbon equivalent 9 7 Carbon equivalent (CE) 98 Carbon graphite bearings 124, 125 brush operating conditions 128 brushes 127 compositions 119 corrosion 116 counter materials 120 dusting 128 grades 114 lubricating properties 116 manufacture 115 mechanical properties 117 microstructures 124 physical properties 118 property ranges 114

25 1

Carbon graphite (cont’d) seal ring wear 121 seal rings 120 seals 122, 123 thrust bearings 126 uses 114 vane pumps 126, 127 water vapor 128 wear 119 Carbon steels austenite 13 bainite 13, 16 composition 15 Gouging wear resistance vs C 23 hardenabil ity 20, 22 hardenability charts 21 heat treatment 16, 18 martensite 13, 16 mechanical properties 14 pearlite 13, 16 physical properties 15 TTT diagram 16, 17 wear vs toughness 19 Carburized roller bearings 40 Cast iron 107 Cast iron abrasion resistance 110, 111 abrasion resistance of high Cr irons 101 alloying elements for abrasion resistance 108 chromium in cast iron 100 damping capacity 93 ductile or spherulite 72 duriron 100 effect of graphite on wear 108 ferrite caps 107 fracture toughness of white irons 113 graphite phase 98, 107 gray cast iron 72 gray iron 99 gray iron microstructure 96 hardness 94 hardenabil ity 106 heat treatment 103, 105 high alloy cast irons 73, 112 high chromium irons 100 high phosphorous irons 101 high silicon irons 100, 101 impact resistance 93 magnetic properties 93 malleable 72 malleable iron 99 mechanical & physical properties 92

252

Cast iron (cont’d) metallurgy 94 modulus 93 ni hard 99, 112 ni resist 99 nickel in cast iron 100 nitriding 106 nodular 72 nodular iron 95, 99 nodular iron microstructure 96 poisson’s ratio 93 property ranges 89 si & C 98 Si & C ranges in cast irons 98 silicon & phos addition 97 surface hardening 106 thermal conductivity 93 TTT diagram 104 types 89 uses 89 wear properties 107 wear resistance 91 white iron 72, 99 white iron microstructure 96 thermal conductivity 93 Cavitation erosion 109 Cementite cast iron 103 Ceramic ball bearings 40 Ceramic tools 148 Ceramics 139 critical velocities 144 density 134 heat treatment 145 hot hardness of ceramic tool materials 149 mechanical & physical properties of Zirconia 147 microhardness 138 plastic deformation 139 poisson‘s ratio 139 property ranges 130 rolling contact bearings 150 structure 139 tempertature 1 imits 134 thermal conductivity 137 thermal expansion 136 thermal shock 140 thermal shock resistance 141 TMI 144 tools 148 types 132 uses 130 young’s modulus 135

253

Cermets properties 154 wear 151 Coatings wear resistant 209 Cobalt base alloys abrasive wear properties 165 hard facing 160 Cobalt friction vs temperature 161 Cobalt transformation temperature 160 Copper base alloys 46, 51 alloy C95400 microstructure 55 bearing properties 59 composition 68 copper lead 51 copper-aluminum diagram 54 copper-lead diagram 51 copper-tin diagram 49 mechanical properties 66 physical properties 67 properties range 46 types 46 Copper base aloys appl ications 46 Critical velocity 114, 222 DU 190 PV rating 190 Elastic modulus 93 Elastomers property ranges 176 Electric motor brushes carbon graphite 127 Electrical contacts 64, 88 Copper alloy properties 65 copper base 64 electrical conductivity of copper alloys 64 Engine bearing materials 81 Engine bearings 80, 84, 85, 88, 110 Engines PSZ materials 148 Ethylene 206 Ethylene-propylene 206 Fatigue 78 FEP 191 Ferrite 72 cast iron 99 Ferrite caps 107, 109 Flake graphite 72 Flame hardening 106 Flash temperature 4 F1 uorocarbons 207

254

Fracture toughness high alloy cast irons 112 white irons 113 listing 226 Friction 70 Friction plastics 182 Friction coefficient carbon graphite 121 Frictional heating 140 Galling 79 Gear bronzes operating conditions 63 properties 63 Glass transition rubber 202 G1 asses composition 154 properties 156 thread guides 157 viscosity-temperature characteristics 155 wear resistance 157 Gold 88 Gouging abrasion 42 Gouging wear 23 Graphite phase 94 Hadf ield steel s austenite stabilization 23 Hard facing alloys high temperature application 160 Hardness conversion nomogram 245 Hardness of steels & minerals 45 Heat treatable ceramics 145 High speed engine bearings 79 High stress abrasiom 41 High temperature alloys composition 164 mechanical properties 162 physical properties 163 High temperature bearings 158 High temperature materials elastic modulus vs temperature 172 short time tensile strength vs temperature 171 specific heat vs temperature 176 thermal conductivity vs temperature 170 thermal diffusivity vs temperature. 174 thermal expansion vs temperature 173 thermal stress resistance vs temperature 175 High temperature plastics 193 Hot hardness 40 Human joint implants 191

255

Hydrodynamic lubrication 123 Inconel 164 oxidation resistance 159 Indium 71, 83, 88 Induction hardening 106 IPNs 187 Iron-carbon-si?icon diagram 94, 95 Iron-silicon diagram 102 K-162B

166

Laser hardening 106 Lattice constants 246 Lead 72 Lead-indium overlay 83 Leakage hydrodynamics 123 Liquid sodium sliding contact materials 166 Low stress abrasion 42 Lubrication 115 carbon graphite 122 Martensite cast iron 103 Martensitic steels 22 Maximum allowable bearing stress 244 Maximum operating temperature 241 Mechanical seals 122 carbon graphite 123 Mol vbdenum high temperature bearings 158 liquid sodium bearings 158 TZM 158 Mu1 t il ayer-bear ing lead-indium 83 Multilayer bearing materials 82 load capacities 83 Natural rubber 204 Neoprene 207 Ni hard 91, 99 Ni resist 91, 99 Nickel base a1 loys oxidation resistance 160 Nitrile rubber 206 Nylon 179, 186 water absorbt ion 187 P-29

microstructure 124

P-658- RC

microstructure 124 PAIs 194 pearlite 72, 94, 97 cast iron 99, 103 Pearlitic steels 18 toughness 18

256

wear resistance 18 PEEK 179, 194 PEK 195 Phenolic 179 Phenol ics 192 laminations 192 Plasticity index 238 Plastics composition 180 mechanical properties 179 physical properties 180

PV values

183

sleeve bearing 184 water absorbtion 185 Platinum 88 Poisson’s ratio 93 Polyamide- imide 194 Polyamides 186 Polybutadine 207 Polyethylene 191 Polyimides 193 Polymeric materials property ranges 176 uses 176 Polymers classes 177 deflection temperature 196 deflection temperatures 197 selection 196 Polysulfone 195 Polyurethanes 208 PPS 195 Precipitation hardening 65 Precision ball bearings 88 PTFE 187 PTFE-lead-bronze microstructure 190 PV plastics 182 PV rating carbon graphite 125, 126 limits for porous bronzes 58 porous bronze 57 Pyroceram 157 Reactors sl iding contact materials 166 Refractory metals and alloys 157 oxide and scale 158 Retained austenite 37 Untempered martensite 22 Roller bearing steels 39

257

Roll ing contact bearing steels 40 carburizing grades 40 composition 38 contact fatigue 37 high temperature bearings 40 mechanical properties 36 physical properties 37 Rolling contact bearings ceramic fatigue 150 ceramics 150 Rubber compositions 203 compression 200 hardness 200 hardness and moduli 205 hardness comparison chart 201 mechanical properties 202 natural rubber 199 physical properties 203 Rubber bushing torsional stiffness 204 Ryton 179 SG iron 72 SiALONS 149 Silicon nitride 150 tools 148 Silicone rubber 207 Silver 72, 88 Sodium-metal-oxide lubrication 167 Soft metal bearing alloys composition 80 multi layer - trimetal 81 properties 73 Soft metal bearing materials friction as a function of film thickness 70 properties 71 property ranges 69 types 69 Uses 69 Solid lubricants 70 Specific hardness 209, 231 Stacking fault energy 26, A3 Stainless steels 24 17-4 PH 32 200 series 26 300 series 26 400 series 29 composition 28 corrosion resistance 29, 30 galling 26 galling resistance 29

258

Stainless steels, (cont’d) martensitic 29 mechanical properties 24 nitrogen alloying 29 PH steels 32 PH-55A 32 physical properties 25 tempering 31 work hardening factor v s Ni 28 work hardening v s C 27 Steadite 101 Steel martensite hardness v s carbon 12 Steels abrasion resistance 41 AISI 440C 39 AISI 52100 39 AISI 52110 38 chromium hot work steels 35 cold work steels 35 ferrite, austenite, cementite 10 gouging wear v s toughness 43 hadfield steel 9 Hadfield steels 41 hardenab i 1 i ty 30 iron-carbon diagram 1 1 manganese steels 41 martensite 10 metal 1 urgy 10 properties 8 reactor components 9, 26 types 8 uses 8 work hardening coefficient 27 Stellite 164 Sterling engine 131 Stress re1 ief cast iron 103 Super alloys high temperature wear 159 property ranges 130 wear resistance 159 Teflon 179, 187 Teflon fabric 188 Thermal diffusivity 174, 234 Thermal mechanical instabil ity 143 ceramics 142 Thermal shock resistance 5 Thermal stress resistance factors 236 Thermoelastic instability 6 Thermoplastic elastomers 199

259

Thermoplastics 177, 178, 186 Thermosets 177 definition 178 Tin 72 Titanium carbide 153 TMI 143 Tool steels cold work steels 35 composition 34 hot work steels 35 mechanical properties 33 physical properties 34 types 33 wear rates 35 Tools cermets 151 Tools steels high speed tool steels 35 Torlon 179 Toughness 41 Transfer 119 Tribaloys 160, 164 Trimetal bearings 81, 82 TSR ceramics 141 Tungsten carbide 151, 153 Tungsten carbide-cobalt diagram 152 TZM 166 Ud imet 166 UHMWPE 179, 191 Vane pumps carbon graphite vanes 127 Vespel 179 Wear aluminum bronze 61 carbon graphite 116, 119 carbon graphite seal rings 121 high temperature materials 159 nylon 186 oscillating aluminum bronze bearings 62 Plastic bearings 185 polymers 198 reactor coolant environment 166 rubber 208 teflon fabric 189 wear properties of cast irons 107 wear properties of porous bronzes 59 Wear coefficient carbon brushes 128 Wear factor bronze bearing alloys 59

260

Wear maps bearing bronze 60 Wear reistant coatings 209 Wear resistance cast irons 91 high silicon iron 101 Whitemetal 72 Zinc 72 Zinc bearing alloys 87 Zinc-aluminum 87 Zircalloy 164 ZIrconia PSZ 145 Zirconia TZM 145 Zirconia-MgO diagram 146