TRIBOLOGY OF MINIATURE SYSTEMS
TRIBOLOGY SERIES Advisory Board W.J. Bartz (Germany, F.R.G.) R. Bassani (Italy) B. Bri...
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TRIBOLOGY OF MINIATURE SYSTEMS
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. VOI.
2 3 4
5 6
7
a
Vot. 9 Vol. 10 VOl. 11 Vol. 12 Vol. 13
W.A. Glaeser (U.S.A.) M. Godet (France) H.E. Hintermann(Switzerland) K.C. Ludema (U.S.A.) G.W. Rowe (Gt. Britain) T. Sakurai (Japan) W.O. Winer (U.S.A.)
Tribology -A Systems Approach to the Science and Technology of Friction, Lubricationand Wear (Czichos) ImpactWear of Materials (Engel) Tribology of Naturaland ArtificialJoints (Dumbleton) Tribology of Thin Layers (Iliuc) Surface Effectsin Adhesion, Friction,Wear, and Lubrication(Buckley) Frictionand Wear of Polymers (Bartenevand Lavrentev) MicroscopicAspects of Adhesion and Lubrication(Georges, Editor) IndustrialTribology -The PracticalAspects of Friction, Lubrication and Wear (Jones and Scott, Editors) Mechanicsand Chemistry in Lubrication (Dorinsonand Ludema) Microstructureand Wear of Materials (Zum Gahr) Fluid Film Lubrication- Osborne Reynolds Centenary (Dowson et al., Editors) InterfaceDynamics(Dowson et al., Editors) Tribology of Miniature Systems (Rymuza)
TRIBOLOGY SERIES, 13
TRIBOLOGY OF MINIATURE SYSTEMS Zyg munt Rymuza Institute of Design of Precise and Optical Instruments, Warsaw University of Technology, Warsaw,Poland
ELSEVIER Amsterdam- Oxford- New York -Tokyo
1989
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat25 P.O. Box 211,lOOOAE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655 Avenue of the Americas New York, NY 10010
ISBN 0-444-87401-7 WOI. 13) ISBN 0-444-41677-3 (Series) Elsevier Science Publishers B.V., 1989 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./Physical Sciences & Engineering Division, P.O. Box 1991, 1000 BZ Amsterdam, The Netherlands. Special regulations for readers in the USA - 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 USA. All other copyright questions, including photocopying outside of the USA, should be referred to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified.
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 materials herein. Printed in The Netherlands
The t r i b o l o g y of m i n i a t u r e s y s t e m s i s q u i t e d i f f e r e n t from t h e t r i b o l o n o f l a r g e machinery. T h i s book i s i n t e n d e d t o c o v e r b o t h t h e b a s i c c o n c e p t s of t h e t r i b o l o g y o f m i n i a t u r e s y s t e m s and some areas of i t s p r a c t i c a l a p p l i c a t i o n . The a u t h o r ' s a i m i s t o q i v e a compact s u r v e y o f t h e s p e c i f i c problems e n c o u n t e r e d i n t h i s d i s c i p l i n e and p r e s e n t a volume which w i l l a l s o b e u s e f u l
in
s o l v i n g p r o f e s s i o n a l e n g i n e e r i n g problems i n t h e f a s t - g r o w i n q f i e l d of p r e c i s i o n e n g i n e e r i n g and m i c r o t e c h n o l o g y ( m e c h a t r o n i c s )
.
The s u i t a b i l i t y of v a r i o u s m a t e r i a l s and l u b r i c a n t s f o r t h e t r i b o l o g i c a l s y s t e m s i n m i n i a t u r e mechanisms i s d i s c u s s e d . The t r i b o l o g i c a l p r o p e r t i e s and t h e f r i c t i o n and wear p r o p e r t i e s which o c c u r i n s u c h s y s t e m s a r e a n a l y s e d . S p e c i f i c l u b r i c a t i o n problems
are d i s c u s s e d i n d e t a i l ; i n p a r t i c u l a r , t h e u s e of s p e c i a l t r i b o l o g i c a l c o a t i n g s t o s o l v e many d i f f i c u l t l u b r i c a t i o n problems and t o o b t a i n h i g h w e a r r e s i s t a n c e of t h e r u b b i n g e l e m e n t s i s considere d . The s p e c i a l i n v e s t i g a t i o n t e c h n i q u e s u s e d t o c h a r a c t e r i z e m i n i a t u r e t r i b o l o g i c a l s y s t e m s and t h e i r e l e m e n t s ( e . q . l u b r i c a n t s ) a r e p r e s e n t e d . The t r i b o l o q i c a l a s p e c t s of many of t h e most common a s s e m b l i e s found i n m i n i a t u r e mechanism and e l e c t r o m e c h a n i s m design a r e a n a l y s e d and some p r a c t i c a l s u g g e s t i o n s are p u t f o r w a r d f o r t h e r a t i o n a l d e s i g n of such s y s t e m s . A l s o s p e c i a l t r i b o l o g i c a l problems such a s t h o s e m e t i n computer t e c h n o l o q y , b i o e n g i n e e r i n g
etc., a r e discussed. The book i s i n t e n d e d f o r t r i b o l o g i s t s ( b o t h s e a s o n e d r e s e x c h ers and newcomers) s t u d y i n q t h e problems of t h i s s p e c i f i c b r a n c h o f t r i b o l o g y and also f o r p r a c t i c i n q e n q i n e e r s a c t i v e i n t h e d e s i g n , m a n u f a c t u r e and e x p l o i t a t i o n of v a r i o u s m i n i a t u r e s y s t e m s . T h i s volume s h o u l d a l s o be u s e f u l i n u n i v e r s i t i e s , b o t h € o r l e c t u r e r s and s t u d e n t s . The a u t h o r would b e cjlad t o r e c e i v e any comments o r remarks r e a d e r s may have a b o u t t h e book. T h i s monograph, a s t h e f i r s t p r e s e n t a t i o n on a n academic l e v e l of o u r p r e s e n t knowledqe o f t h e t r i b o l o g i c a l b e h a v i o u r of m i n i a t u r e s y s t e m s , i s b a s e d on t h e r e s u l t s o f much t r i b o l o g i c a l and o t h e r m u l t i d i s c i p l i n a r y r e s e a r c h which would have b e e n i m p o s s i b l e t o u n d e r t a k e w i t h o u t t h e h e l p o f many c o l l e a g u e s a c t i v e i n t h e same f i e l d .
VI
The a u t h o r i s g r a t e f u l t o P r o f . H .
C z i c h o s and P r o f . G. G l a s e r
f o r t h e i r k i n d r e v i e w s o f t h e o u t l i n e of t h e book and f o r t h e i r h e l p f u l s u g g e s t i o n s . Thanks a r e a l s o d u e t o t h e reviewer, Dip1.-Phys. H.
T i s c h e r . The a u t h o r would l i k e t o t h a n k P r o f . W. T r y l i f i s k i , h i s
eminent u n i v e r s i t y t e a c h e r , f o r t h e i n t e r e s t h e showed i n t h e work and f o r h i s v a l u a b l e comments. Many c o l l e a g u e s were k i n d enough t o d i s c u s s w i t h t h e a u t h o r t h e problems c o n s i d e r e d i n t h i s volume a n d / o r to make a v a i l a b l e v a r i o u s i t e m s c i t e d i n t h e t e x t . These a r e : Doz. M. A j a o t s , I n g . F . A u b e r t , Dip1.-Ing.
G . Bankmann, D r . M.K. B e r n e t t , I n g . A. Braun, E r r , DOZ. F. F r a n e k , Dr.-Ing. J . G e h r i g , D r . H . E . Hintermann, I n g . A . Huber, Dip1.-Phys. M. Huck, D r . A. Kropiewnick i , D r . A. Maciszewski, D r . M. Massin, I n q . M. M a i l l a t , D r . K.L. M i t t a l , Dip1.-Ing. E . N a j a s e k , A. R a s t a w i c k i M Eng., Dip1.-Chem. A. R e n f e r , Dr.-Ing. G . RBgnault, P r o f . A. RUSS, M r . W . Stehr, D r . M. T i l l w i c h , M r . M. Treboux. Thank you v e r y much f o r t h i s help. The a u t h o r i s g r a t e f u l t o t h e f o l l o w i n g c o p y r i g h t owners f o r p e r m i s s i o n t o r e p r o d u c e some o f t h e f i g u r e s and t a b l e s i n t h i s book: t h e American S o c i e t y of L u b r i c a t i o n E n g i n e e r s , p u b l i s h e r of "ASLE T r a n s a c t i o n s " , B u t t e r w o r t h S c i e n t i f i c L t d . , p u b l i s h e r of " T r i b o l o g y I n t e r n a t i o n a l " , C a r l Hanser V e r l a g , p u b l i s h e r o f " T r i b o l o g i e und S c h m i e r u n g s t e c h n i k " , E l s e v i e r S e q u o i a , p u b l i s h e r of "Wear" , t h e I n s t i t u t e of E l e c t r i c a l and E l e c t r o n i c s E n g i n e e r s Inc. , p u b l i s h e r o f " I E E E T r a n s a c t i o n s on Magnetics" , P e n t o n P u b l i s h i n g I n c . , p u b l i s h e r o f "Machine Design", S p r i n g e r - V e r l a g , p u b l i s h e r o f t h e volumes " T r i b o l o g i e : Reibung-Verschleiss-Schmierung" , and Vogel-Verlag XG Wirrzburg, p u b l i s h e r o f "Maschinenmarkt". Many t h a n k s a r e a l s o due t o t h e s t a f f of E l s e v i e r S c i e n c e Publishers f o r t h e i r c a r e i n bringing t h i s p r o j e c t t o completion.
Dip1.-Ing.
F.
Warsaw, January 1989
Zygmunt Rymuza
VI I
CONTENTS List of abbreviations used for materials 1.
2.
3.
4.
5.
6.
Introduction 1 Materials 6 6 2.1. Metals 9 2.2. Sintered metals 2.3. Minerals, ceramics, composites 15 2.4. Polymeric materials 20 Unfilled polymers 20 2.4.1. 2.4.2. Filled polymers 28 Lubricants 33 33 3 . 1 . Introduction 3.2. Oils 33 3.3. Greases 54 68 3.4. Solid lubricants Unlubricated systems 73 73 4.1. Metallic systems 4.2. Polymeric systems 83 4.2.1. Metal-polymer systems 83 4.2.2. Polymer-polymer systems 109 4.3. Other systems 131 Lubricated systems 149 5 . 1 . Metallic systems 149 5.1.1. Solid metals 149 Sintered metals 164 5.1.2. 5 . 2 . Polymeric systems 173 5.2.1. Metal-polymer systems 173 5.2.2. Polymer-polymer systems 197 203 5 . 3 . Other systems Lubrication problems 211 211 6 . 1 . Introduction 6.2. Preventing oil from spreading or creeping 6.2.1. Introduction 2 1 2 Fundamentals 212 6.2.2. 6.2.3. Methods 214 6.2.4. Coatings (epilames) 2 1 6
212
VIII 6.2.5.
Self-coating (autoepilamizing) 2 2 7 Coating (epilame) technology 230 6.3. Estimation of optimum volume of oil deposit 237 6.4. Lubricant durability 243 253 6.5. Lubrication under extreme conditions 6.6. Lubrication of polymeric systems 261 7. Special tribological coatings 269 269 7.1. Introduction 269 7.2. Anti-friction coatings 7 . 3 . Anti-wear coatings 285 302 8. Experimental techniques 8.1. Introduction 302 8.2. Friction 302 Introduction 302 8.2.1. 8.2.2. Oscillating motion of the rubbinq element 303 8.2.3. Unidirectional motion of the rubbing element 312 325 8.3. Wear 8.4. Thermal effects 331 8.5. Quality of lubricants and coatings (epilames) 333 8.5.1. Introduction 333 Lubricity of lubricants 334 8.5.2. 8.5.3. Physicochemical properties of lubricants 336 8.5.4. Effects of interactions in lubricant-rubbing elements-ambient systems 369 8.5.5. Properties of coatings and epilames 379 8.6. Cleaning 381 8.7, Special investigations 392 9. Tribological aspects of Eine mechanism assemblies 404 9 . 1 . Introduction 404 9 . 2 . Typical plain bearings 404 435 9 . 3 . Special bearings 9.4. Rolling bearings Q41 9 . 5 . Guides 448 9.6. Gears and transmissions 453 9.7. Couplings: clutches and brakes 463 467 9.8. Contacts, brushes 1 0 . Special tribological problems 481 11. Closing comments 508 6.2.6.
References 509 Subject Index 558
IX
LIST OF ABBREVIATIONS USED FOR MATERIALS ABS
Acrylonitrile /butadiene/
CF
Carbon f i b r e
DMPS
Dimethy l p o l y s i l o x a n e Ethylene/tetrafluoroethylene copolymer F l u o r i n a t e d e t h y l e n e / p r o p y l e n e copolymer
ETFE FEP GF
Glass f i b r e
HDPE
High d e n s i t y p o l y e t h y l e n e
LDPE
Low d e n s i t y p o l y e t h y l e n e
PA
Polyamide
PA1
P o l y amide- i m i d e
s t y r e n e copolymer
PAM
Polyacrylamide
PBTP
Poly ( b u t y l e n e t e r e p h t h a l a t e )
PC
Polycarbonate
PCA
Polycaproamide
PCTFE
PEI
Polychlorotrifluoroethylene Polyethylene Polyetheretherketone Polyetherimide
PESU
P o l y e t h e r s u l f one
PETP
Poly (ethylene terephthalate)
PFA
Perfluoroalkoxyethylene
PE PEEK
PI
Polyimide
PMMA
Poly (methyl m e t h a c r y l a t e )
PMP
Polymethy l p e n t e n e
P NP
P e n t a p l a s t (Penton)
P OM
Polyoxymethy l e n e
POM c
P o l y o x y m e t h y l e n e ( p o l y a c e t a l ) copolymer
POM h
P o l y o x y m e t h y l e n e homopolymer
PP
Polypropylene
PPO
Poly (phenylene o x i d e )
PPS
Poly (phenylene s u l p h i d e )
PPSU
P o l y s u If o n e
PS PSZ
Polystyrene Partially stabilized zirconia
PTFE
Polytetraf luoroethylene
PUR
Polyurethane
X
PVC
Po l y ( v i ny 1 bu t y r a 1) Poly ( v i n y l c h l o r i d e ]
PVDC
Poly ( v i n y l i d e n e c h l o r i d e )
PVDF
Poly ( v i n y l i d e n e f l u o r i d e )
SAN
S t y r e n e / a c r y l o n i t r i l e copolymer
SR
S i l i c o n e rubber
UHMWPE
Ultrahigh-molecular-weight polyethylene
PVB
O t h e r d e s i g n a t i o n s of m a t e r i a l s ( i n c l u d i n g l u b r i c a n t s , s o l v e n t s , e t c . ) , are r e g i s t e r e d t r a d e m a r k s .
1
1, INTRODUCTION The t r i b o l o g y of m i n i a t u r e s y s t e m s i s a s p e c i a l a r e a o f t r i bology s c i e n c e b e c a u s e t h e r u b b i n g e l e m e n t s a r e u s u a l l y of v e r y s m a l l dimensions. The e n e r g y d i s p o s i t i o n i n t h e c o n t a c t i n g a r e a i s q u i t e d i f f e r e n t from t h a t i n l a r g e machines. The t r i b o l o g i c a l p r o p e r t i e s o f m i n i a t u r e s y s t e m s t h e r e f o r e d i f f e r from t h o s e composed of e l e m e n t s of l a r g e dimensions ( r e f . 1 ) . The t r i b o l o g y o f m i n i a t u r e systems i s r e l a t i v e l y p o o r l y d e v e l oped. According t o t h e B u n d e s a n s t a l t f t i r M a t e r i a l p r U f u n y (BAM) i n B e r l i n , p u b l i c a t i o n s i n t h i s f i e l d are o n l y a b o u t 1 . 3 % o f t h e t o t a l t r i b o l o g i c a l w o r l d p u b l i c a t i o n s p e r y e a r ( r e f . 2 ) . The r a p i d growth o f p r e c i s i o n e n g i n e e r i n g i n r e c e n t y e a r s (and t h e dynamic p r o g r e s s which can be f o r e s e e n f o r t h e coming y e a r s ) i n t h e p r o d u c t i o n o f m e c h a n i c a l and e l e c t r o m e c h a n i c a l d e v i c e s f o r v a r i o u s a p p l i c a t i o n s which embody m i n i a t u r e t r i b o l o g i c a l s y s t e m s h a s n o t been matched i n t h e development o f t h e t r i b o l o g y s c i e n c e of s u c h s y s t e m s . T r i b o l o g i c a l s y s t e m a n a l y s i s b a s e d on t h e g e n e r a l d e s c r i p t i o n o f s t r u c t u r e , i n p u t and o u t p u t can be a p p l i e d t o m i n i a t u r e s y s t e m s
i s d e f i n e d by t h e s e t of i t s e l e m e n t s , t h e i r p r o p e r t i e s and t h e i n t e r a c t i o n s between t h e e l e ments. The i n p u t o f t h e t r i b o l o g i c a l s y s t e m i s d e s c r i b e d when l o a d , ( r e f . 2 ) . The s t r u c t u r e of a s y s t e m
s l i d i n g s p e e d , motion form ( s l i d i n g , r o l l i n g , c o n t i n u o u s , s t a r b s t o p ( i n t e r m i t t e n t ) , e t c . ) , s l i d i n g t i m e , a m b i e n t c o n d i t i o n s and d i s t u r b a n c e s i f any a r e g i v e n . The o u t p u t i s t h e u s e f u l e n e r g y , i n f o r m a t i o n o r m a s s s i g n a l t r a n s f e r r e d t o t h e o t h e r s y s t e m f r o m t h e anal y s e d t r i b o l o g i c a l system. The t r i b o l o g i c a l s y s t e m i s a l s o c o n n e c t e d w i t h t h e environment by t h e t r i b o l o g i c a l l o s s e s i . e . e n e r g y , f r i c t i o n and m a s s l o s s e s (wear) which i n f l u e n c e i t s s t r u c t u r e a n d f u n c t i o n . The f u n c t i o n i s d e s c r i b e d by t h e r e l a t i o n s h i p s between t h e o u t p u t and i n p u t q u a n t i t i e s . The m i n i a t u r e t r i b o l o g i c a l s y s t e m i s a c o n w i n a t i o n of rubbinc; e l e m e n t s of s m a l l or v e r y s m a l l d i m e n s i o n s , u s u a l l y less t h a n 5 mm, used i n s m a l l mechanisms f o r t h e t r a n s m i s s i o n of i n f o r m a t i o n o r
s m a l l e n e r g y q u a n t i t i e s ( d o s e s ) . The d i s p o s a b l e e n e r g y i n s u c h mechanisms is u s u a l l y s t r i c t l y l i m i t e d . The t e c h n i c a l f u n c t i o n o f t h e t r i b o l o g i c a l system i s d e s c r i b e d by d e f i n i n g t h e r e l a t i o n s h i p s between t h e e n e r g y f l o w s ( s t r e a m s ) , which are t h e c a r r i e r s of i n -
f o r m a t i o n , i n t h e o u t p u t and i n p u t o f t h e s y s t e m . The r e q u i r e m e n t t h a t t h e energy l o s s e s i n t h e m i n i a t u r e t r i b o l o g i c a l system b e a s s m a l l a s p o s s i b l e i s u s u a l l y t h e m o s t i m p o r t a n t a n d d i f f i c u l t cond i t i o n f o r a d e s i g n e r t o f u l f i l l . A l s o m a s s losses ( w e a r ) must b e s m a l l s i n c e t h e y o f t e n e x e r t a c o n s i d e r a b l e e f f e c t on t h e f u n c t i o n i n g of t h e s y s t e m and on i t s d u r a b i l i t y . The s e t of t h e s y s t e m ' s e l e m e n t s c o n s i s t s o f two r u b b i n g p a r t s ( e l e m e n t s ) , wear d e b r i s and any l u b r i c a n t u s e d . The p r o p e r t i e s of e l e m e n t s a r e d e t e r m i n e d by t h e i r g e o m e t r i c a l and m a t e r i a l f e a t u r e s . The c o n s t r u c t i o n a l s h a p e s of t h e r u b b i n g e l e m e n t s o f m i n i a t u r e t r i b o l o g i c a l systems a r e u s u a l l y
cylinders or p l a t e s
( r e f s . 3 - 6 ) . The
e x t e r n a l m a c r o s t r u c t u r e d e s c r i p t i o n c o n s i s t s of nominal dimensions and t h e i r t o l e r a n c e s . The m i c r o s t r u c t u r e i s g i v e n by t h e s u r f a c e r o u g h n e s s o f t h e e l e m e n t s . The most c h a r a c t e r i s t i c f e a t u r e of t h e dimensions systems is t h e i r s m a l l nominal dimensions ( a l s o less t h a n 1 mm) and v e r y h i g h r a t i o s of t h e t o l e r a n c e s t o t h e n o m i n a l d i m e n s i o n . F o r d i m e n s i o n s i n t h e r a n g e 1-5 mm, t h e r e l a t i v e t o l e rance u n i t according t o the IS0 standard is about 0.16-0.40,
de-
c r e a s i n g r a p i d l y as t h e n o m i n a l d i m e n s i o n i n c r e a s e s . F o r d i m e n s i o n s < 1 mm t h e r e l a t i v e t o l e r a n c e u n i t i n c r e a s e s r a p i d l y a s t h e n o m i n a l
d i m e n s i o n d e c r e a s e s and when t h e n o m i n a l d i m e n s i o n i s 0 . 1 mm it i s a b o u t 4.5
( r e f . 3 ) . The r e l a t i v e c l e a r a n c e s i n m i n i a t u r e j o u r n a l
b e a r i n g s when t h e n o m i n a l d i a m e t e r s a r e < 5 mm may e x c e e d 1 0 % . I n t h e polymeric b e a r i n g bushes o f t e n used i n such b e a r i n g s , t h e r a t i o of t h e w a l l t h i c k n e s s t o t h e nominal b e a r i n g d i a m e t e r c a n r e a c h 1 0 0 % . S i m i l a r p r o p o r t i o n s a r e found i n b e a r i n g s w i t h a s i n t e r e d
porous bush. The r u b b i n g s u r f a c e s o f t h e e l e m e n t s o f m i n i a t u r e t r i b o l o g i c a l s y s t e m s are u s u a l l y v e r y smooth. T h i s i s e s p e c i a l l y t h e case w i t h s u r f a c e s made o f s t e e l (e.9. j o u r n a l s u r f a c e s ) which are o f t e n r o l l e r b u r n i s h e d t o Ra < 0 . 1 6 ,um. T y p i c a l materials used i n rubbing e l e m e n t s are: m e t a l s b r a s s , bronze; m i n e r a l s a g a t e ; and polymers
-
-
-
steel,
corundum ( r u b y , s a p p h i r e ) , c h a l c e d o n y ,
polyamides (PA)
,
polyacetals
(POM). S p e c i a l
o i l s and g r e a s e s ( a l s o s o l i d l u b r i c a n t s ) a r e a p p l i e d a s l u b r i c a n t s . These m a t e r i a l s must n o t m i g r a t e from t h e l u b r i c a t e d s y s t e m s i n c e t h e l u b r i c a t i o n o f t e n o c c u r s o n l y once d u r i n g t h e assembly p r o c e s s ("for-life'' lubrication) with, e.g.
one d r o p o f o i l . The a g e i n g re-
s i s t a n c e and c h e m i c a l i n t e r t n e s s o f s u c h l u b r i c a n t s m u s t b e h i g h . The p r e s e n c e o f w e a r d e b r i s i n t h e f r i c t i o n area o f a m i n i a t u r e t r i b o l o g i c a l s y s t e m h a s an i m p o r t a n t e f f e c t on i t s f u n c t i o n
3
and on t h e t r i b o l o g i c a l p r o c e s s e s d u r i n g i t s o p e r a t i o n . The wear d e b r i s can r e s u l t i n l a r g e displacements of t h e rubbing elements ( r e l a t i v e t o t h e nominal d i m e n s i o n s o f e l e m e n t s o r c l e a r a n c e s ) which can l e a d t o s e r i o u s f a i l u r e s . The i n t e r a c t i o n between t h e a f o r e m e n t i o n e d e l e m e n t s o f t r i b o l o g i c a l s y s t e m s i s v e r y complex and h a s been r e l a t i v e l y l i t t l e i n v e s t i g a t e d . The i n t e r a c t i o n s between t h e e l e m e n t s can b e a n a l y s e d
on t h r e e c o n c e p t u a l p l a n e s : t h e m e c h a n i c a l work p l a n e , t h e t h e r m a l p l a n e and t h e m a t e r i a l p l a n e ( r e f . 2 ) . Although t h e f l o w o f mechani c a l energy i n t h e m i n i a t u r e t r i b o l o g i c a l system i s r e l a t i v e l y s m a l l , b e c a u s e o f t h e v e r y s m a l l d i m e n s i o n s o f t h e e l e m e n t s and t h e s m a l l a r e a s of c o n t a c t , t h e i n t e n s i t y of t h e e n e r g y stream ( f l u x ) c a n b e v e r y h i g h . Energy d i s s i p a t i o n o c c u r s m a i n l y a t t h e t r a n s f o r mation i n t o thermal energy, s i n c e t h e s t o r e d s t r a i n e n e r g y a s s o c i a t e d w i t h t h e deformation of elements i s r e l a t i v e l y small. Because of t h e e n e r g y ( f r i c t i o n a l ) l o s s e s , t h e t h e r m a l e n e r g y i s m a i n l y s t o r e d by t h e r u b b i n g e l e m e n t s and wear p a r t i c l e s , s i n c e t h e s m a l l s i z e of t h e r u b b i n g e l e m e n t s h i n d e r s t h e h e a t t r a n s f e r from t h e f r i c t i o n a r e a . T h i s i s e s p e c i a l l y t h e c a s e f o r m i n i a t u r e s y s t e m s w i t h e l e m e n t s m a n u f a c t u r e d from polymers o r m i n e r a l s . The mass t r a n s f e r between t h e e l e m e n t s i s c o n d i t i o n e d by t h e thermal energy flow p r o c e s s s i n c e t h i s energy flow a c t i v a t e s t h e t r i b o p h y s i c a l p r o c e s s e s . Such p r o c e s s e s p l a y an i m p o r t a n t r o l e p a r t i c u l a r l y i n t h e m i n i a t u r e t r i b o l o g i c a l systems w i t h polymeric elements ( r e f . 6 ) . Even a v e r y s m a l l m a s s t r a n s f e r c a n have an import a n t e f f e c t on t h e s y s t e m ' s f u n c t i o n , s i n c e b e c a u s e o f t h e s m a l l a r e a o f c o n t a c t t h e v a r i a t i o n s i n t h e l i n e a r d i m e n s i o n s of t h e rubb i n g e l e m e n t s c a n be v e r y l a r g e . One o f t h e m o s t i m p o r t a n t p r o p e r t i e s of l u b r i c a t e d m i n i a t u r e s y s t e m s i s t h e s t r o n g e f f e c t of t h e a g e i n g o f t h e l u b r i c a n t on t h e t r i b o l o g i c a l p r o c e s s e s . The e f f e c t of t h e a n t i - m i g r a t i o n c o a t i n g s ( e p i l a m e s ) on t h e l u b r i c a n t s h o u l d a l s o be t a k e n i n t o c o n s i d e r a t i o n . The a f o r e m e n t i o n e d s e t of i n p u t q u a n t i t i e s s i g n i f i c a n t l y a f f e c t s t h e o p e r a t i o n of m i n i a t u r e t r i b o l o g i c a l s y s t e m s . The c o n t a c t p r e s s u r e s c a n b e v e r y h i g h . Although t h e a b s o l u t e l o a d s are n o t o f t e n h i g h , t h e s p e c i f i c loads may be v e r y h i g h i n d e e d b e c a u s e of t h e h i g h c u r v a t u r e o f t h e c o n t a c t i n g s u r f a c e s and t h e v e r y s m a l l d i m e n s i o n s and a r e a s of c o n t a c t . T h i s means t h a t i n a c t u a l p r a c t i c e t h e r e a l i s t i c c o n t a c t p r e s s u r e s c a n sometimes be more t h a n s e v e r a l t h o u s a n d MPa. The s l i d i n g s p e e d i s u s u a l l y n o t h i g h e r t h a n 0 . 2 m / s e v e n though t h e a n g u l a r s p e e d s o f j o u r n a l s are h i g h b e c a u s e j o u r n a l
4
d i a m e t e r s are s m a l l . The motion c h a r a c t e r i s t i c s d i f f e r w i d e l y f r o m one s y s t e m t o a n o t h e r . The e l e m e n t s o f t e n r o t a t e , a l t h o u g h l i n e a r r e c i p r o c a t i n g s l i d i n g i s a l s o o f t e n u s e d , and i n some mechanisms t h e r e may be start-stop
( i n t e r m i t t e n t ) and o s c i l l a t i n g m o t i o n w i t h impact l o a d s .
The motion d e s c r i p t i o n c o n s i s t s o f t h e t r a j e c t o r y o f t h e moving e l e m e n t , s p e e d v a r i a t i o n s and f r e q u e n c y o f movements. T h i s i s a very important element i n t h e s e t d e s c r i b i n g t h e i n p u t of a miniat u r e t r i b o l o g i c a l system. The o p e r a t i n g t e m p e r a t u r e r a n g e i s o f t e n v e r y w i d e . F o r example, t h e m i n i a t u r e t r i b o l o g i c a l systems used i n a i r c r a f t i n s t r u m e n t a t i o n
are exposed t o t e m p e r a t u r e v a r i a t i o n s from - 6 0 to +12OoC. The atbient h u m i d i t y c a n a l s o v a r y and t h e i n s t r u m e n t s o p e r a t i n g i n t h e chemical o r m e t a l l u r g i c a l i n d u s t r y f o r example are e x p o s e d t o a c o r r o s i v e atmosphere. I n s t r u m e n t s w i t h m i n i a t u r e t r i b o l o g i c a l s y s t e m s a r e o f t e n u s e d i n vacuum c o n d i t i o n s i n s p a c e . The s l i d i n g d i s t a n c e o r s l i d i n g time of m i n i a t u r e r u b b i n g e l e ments o f t e n h a s t o b e v e r y l o n g e.g. 1 0 years o f e x p l o i t a t i o n . The v i b r a t i o n s t o which t h e mechanisms of some i n s t r u m e n t s are exposed ( e . g . some of t h o s e u s e d i n v e h i c l e s ) a f f e c t t h e t r i b o l o g i c a l p r o cesses i n t h e s e s y s t e m s . The d u s t i n e s s o f t h e atmosphere and t h e
p r e s e n c e of r a d i a t i o n have a s i g n i f i c a n t i n f l u e n c e on t r i b o l o g i c a l p r o p e r t i e s , p a r t i c u l a r l y i n t h e case of l u b r i c a t e d m i n i a t u r e s y s tems. The q u a n t i t i e s d e s c r i b i n g t h e losses o f t h e t r i b o l o g i c a l s y s t e m a r e t h e e n e r g y l o s s e s due t o f r i c t i o n ( u s u a l l y d e s c r i b e d by t h e f r i c t i o n c o e f f i c i e n t ) and t h e mass l o s s ( w e a r r a t e ) . An i m p o r t a n t r e q u i r e m e n t f o r m i n i a t u r e t r i b o l o g i c a l s y s t e m s i s t h e s t a b i l i t y of t h e f r i c t i o n c o e f f i c i e n t d u r i n g t h e p e r i o d o f e x p l o i t a t i o n and a s h o r t running-in
time ( s t a b i l i z i n g t h e f r i c t i o n c o e f f i c i e n t and
wear i n t e n s i t y ) . The v a r i a t i o n s i n t h e f r i c t i o n c o e f f i c i e n t s h o u l d
be v e r y s m a l l s i n c e it is o f t e n v e r y i m p o r t a n t t o e l i m i n a t e t h e s t i c k - s l i p e f f e c t s during s l i d i n g . The i n s t r u m e n t a t i o n u s e d i n t r i b o l o g i c a l s t u d i e s o f m i n i a t u r e s y s t e m s p r e s e n t s s p e c i a l r e q u i r e m e n t s . The s m a l l d i m e n s i o n s o f t h e rubbing e l e m e n t s , v e r y s m a l l a b s o l u t e t r i b o l o g i c a l l o s s e s observed under o f t e n extreme c o n d i t i o n s make t h e t r i b o l o g i c a l s t u d i e s of such s y s t e m s v e r y d i f f i c u l t and time a b s o r b i n g . The d e v i c e s which u s e m i n i a t u r e t r i b o l o g i c a l s y s t e m s a r e v e r y v a r i e d ; r a n g i n g from h o u s e h o l d d e v i c e s , computer p e r i p h e r a l s , m e d i c a l i n s t r u m e n t s , and image and sound r e c o r d e r s , t o t h e a p p a r a t u s
5
i n s p a c e c r a f t o r s a t e l l i t e s . T h i s book c o n c e r n s t h e t r i b o l o g y o f m i n i a t u r e systems b u t d o e s n o t a t t e m p t d e t a i l e d d e s c r i p t i o n s of part i c u l a r t r i b o l o g i c a l systems. It i s a s y n t h e s i s of t h e r e s u l t s of i n v e s t i g a t i o n s of c e r t a i n s y s t e m models and a t t e m p t s t o g e n e r a l i z e , on t h e academic l e v e l , t h e problems of t h i s s p e c i f i c area o f t r i b o l o g y . The d i s c u s s i o n t h e r e f o r e d o e s n o t c o n c e n t r a t e on p a r t i c u l a r s y s t e m s b u t p r e s e n t s a s y n t h e s i s of t h e r e s e a r c h c a r r i e d o u t so f a r , and as s u c h s h o u l d be h e l p f u l i n s o l v i n g p r a c t i c a l problems i n any m i n i a t u r e mechanism used i n modern p r o f e s s i o n a l o r h o u s e h o l d instruments. T h i s book is not a m o r p h o l o g i c a l e n g i n e e r i n g e n c y c l o p e d i a ; r a t h e r it p r e s e n t s t h e s t a t e o f t h e a r t o f m i n i a t u r e t r i b o l o g i c a l s y s t e m s .
The m a t e r i a l s and c o m b i n a t i o n s of materials u s e d i n s y s t e m s o p e r a t i n g under u n l u b r i c a t e d c o n d i t i o n s , s y s t e m s which u s e a s p e c i a l i n s t r u m e n t l u b r i c a n t ( d e s c r i b e d i n d e t a i l i n t h e book) or i n which t h e rubbing elements a r e c o a t e d ( a n t i - f r i c t i o n , anti-wear c o a t i n g s ) a r e d i s c u s s e d . The l u b r i c a t i o n problems of m i n i a t u r e t r i b o l o g i c a l s y s t e m s and t h e u s e of a n t i - m i g r a t i o n c o a t i n g s
( e p i l a m e s ) , are a l s o
a n a l y s e d . The methods and i n s t r u m e n t a t i o n u s e d i n t h e i n v e s t i g a t i o n s a r e a l s o p r e s e n t e d . Moreover, t h e a c t u a l p a r t s ( v a r i o u s t y p e s o f b e a r i n g s , g e a r s , c o u p l i n g s , e l e c t r i c a l c o n t a c t s e t c . ) used i n m i n i a t u r e mechanisms o r e l e c t r o m e c h a n i s m s a r e a n a l y s e d from a t r i b o l o g i c a l p o i n t o f view: a l s o some s p e c i a l c a s e s o f t h e problems of t r i b o l o g i c a l s y s t e m s i n v a r i o u s areas ( e . g . c o m p u t e r s , m e d i c a l d e v i c e s ) a r e a l s o d i s c u s s e d . I t i s i n t h e a u t h o r ’ s aim t h a t t h i s book s h o u l d be a b a s i c r e f e r e n c e book t o a s s i s t i n s o l v i n g t h e t r i b o l o g i c a l problems o f v a r i o u s s m a l l and micro-mechanisms. T h i s i s a c h i e v e d by p r e s e n t i n g t h e l a t e s t d e v e l o p m e n t s i n t h e s c i e n c e o f m i n i a t u r e t r i b o l o g i c a l systems through t h e s t r u c t u r a l , g e n e r a l i z e d d i s c u s s i o n o f r e s e a r c h r e s u l t s i n t h i s v e r y i m p o r t a n t and growing a r e a of t h e t r i b o l o g y s c i e n c e .
6
2 , MATERIALS
The metals u s e d i n t h e m a n u f a c t u r e o f t h e s l i d i n g e l e m e n t s o f m i n i a t u r e mechanisms a r e s t e e l s , b r a s s e s , b r o n z e s , b a b b i t t s , cadm i u m , aluminium and s i l v e r - b a s e d a l l o y s , e t c . F r e e c u t t i n g s t e e l
( a low c a r b o n s t e e l , a v e r a g e c o n t e n t 0 . 1 % C , r a i s e d p h o s p h o r u s c o n t e n t ) is o f t e n u s e d f o r s h a f t s , p i v o t p i n s e t c . , and i s f u r n i s h e d i n t h e form o f s t r a i g h t b a r s made t o v e r y c l o s e t o l e r a n c e s . I t
i s r a t h e r b r i t t l e and u n s u i t a b l e f o r cold-working p r o c e s s e s . High for pivot pins, leafed p i n i o n s h a f t s wherever l o n g e r s e r v i c e l i f e i s r e q u i r e d . E l e m e n t s c a r b o n (1%C ) b r i g h t s t e e l i s used e . g .
made o f h i g h c a r b o n s t e e l are, as a r u l e , s u b j e c t e d t o h e a t t r e a t ment. Here s p e c i a l o i l - q u e n c h e d s t e e l ( t h e r e f o r e o n l y s l i g h t l y deformed by h e a t t r e a t m e n t ) i s w i d e l y u s e d ( r e f . 3 ) . S t a i n l e s s s t e e l w i t h a chrome c o n t e n t o f between 1 2 and 1 4 % i s used f o r e l e m e n t s whose r u b b i n g s u r f a c e s a r e r e q u i r e d t o be c o r rosion-resistant
e.g.
i n a damp, t r o p i c a l c l i m a t e . Because s u c h
s t e e l i s n o t q u i t e r u s t l e s s and i t s r u s t - p r o o f i n g improves w i t h s u r f a c e q u a l i t y , t h e e l e m e n t s have t o b e c a r e f u l l y p o l i s h e d . F o r a c i d ambient c o n d i t i o n s , a u s t e n i t i c s t a i n l e s s s t e e l h a s t o be employed. Mild s t e e l ( c a r b o n c o n t e n t below 0 . 2 % ) and f r e e c u t t i n g s t e e l s can be u s e d i n s t e a d o f s t a i n l e s s ( e . g . chrome and chrome-nickel
s t e e l s ( r e f . 3 ) ) when modern h a r d non-porous c o a t i n g s a r e a p p l i e d (see C h a p t e r 7 . 3 ) . Simple c a r b u r i z a t i o n or c y a n i d i n g o f m i l d s t e e l , o r n i t r i d i n g o f n i t r i d i n g s t e e l s improves t h e i r h a r d n e s s and w e a r resistance. Leaded wrought h i g h - t e n s i l e b r a s s e s c o n t a i n i n g 5 8 % c o p p e r and 2 % l e a d , o r 6 3 % c o p p e r and 1 . 5 % l e a d , a r e e a s i l y m a c h i n a b l e and are
good b e a r i n g m a t e r i a l s ( r e f s . 3 , 7 , 8 ) . They a r e b r i t t l e i n c o l d c o n d i t i o n s . Basic 6 3 Cu
-
b r a s s i s d u c t i l e and p e r f e c t l y s u i t a b l e
f o r c o l d working c o n d i t i o n s . For c a s t i n g e l e m e n t s , b r a s s e s composed of 6 0 % c o p p e r , 1.5% l e a d and t h e r e s t z i n c , a r e u s e d . C a s t bronzes
( a v e r a g e c o n t e n t : 85% c o p p e r , 5% t i n , 5 % z i n c , 5 %
l e a d ) a r e good b e a r i n g m a t e r i a l s . The c o m p o s i t i o n o f some t y p e s o f cast bronzes i s given i n T a b l e 2 . 1
(ref. 9)
.
7 TABLE 2.1 COMPOSITION OF CAST BRONZES
ALLOY No.
(ref. 9)
AOOITIONAL COMPONENTS i n % cu
89-91 87-89 85-87 84-87 78-81
75-79 69-77
Sn
Zn
Pb
9-11
0.5 0.5 0.5 1.o 3.0 3.0 3.0
1.0 1.0 1.0
11-13 13-15 9-11 9-11 7-9 3.5-5.5
4-6 8-1 1 13-17 18-23
- -P S Fe 0.2 0.2 0.2 0.25 0.25 0.25 0.25
0.4 0.4 0.2 0.1 0.05
0.05 0.05 0.05
0.05 0.05
- --
A l l o y s 1 and 2 a r e n o t v e r y i n t e r e s t i n g a s b e a r i n g m a t e r i a l s i n comparison w i t h a l l o y 3 . These b r o n z e s a r e n o t i m p a c t - r e s i s t a n t b u t they are corrosion-resistant
and t h i s i s t h e i r main a d v a n t a g e . The
a l l o y s w i t h a r e l a t i v e l y h i g h l e a d c o n t e n t (numbers 4 t o 7 ) , demons t r a t e v e r y good t r i b o l o g i c a l p r o p e r t i e s and can be used a t temper a t u r e s f o r b a b b i t t s (see below) and a t h i g h s p e c i f i c p r e s s u r e s (10-20 MPa) i n r u b b i n g a g a i n s t s t e e l p i v o t s (hardened t o 50-60 HRC)
.
T h e i r d i s a d v a n t a g e i s low c o r r o s i o n r e s i s t a n c e . Alloy 4 i s t h e most c o r r o s i o n - r e s i s t a n t of t h e s e a l l o y s . Lead- o r t i n - b a s e d a l l o y s ( b a b b i t t s ) a r e v e r y good b e a r i n g mat e r i a l s . They can be d i v i d e d i n t o t h r e e groups: h i g h - t i n a l l o y s ( t i n 2 808,
l i t t l e o r no l e a d ) : h i g h - l e a d a l l o y s ( a b o u t 80% l e a d
and 1-128 t i n ) : and a l l o y s w i t h i n t e r m e d i a t e p e r c e n t a g e s of t i n and l e a d . Apart from l e a d and t i n , t h e s e a l l o y s c o n t a i n antimony and copper. The a p p l i c a t i o n of such a l l o y s i s l i m i t e d t o low o r medium l o a d s . These a l l o y s a r e n o t s u s c e p t i b l e t o c o r r o s i o n . Cadmium-based a l l o y s ( 9 8 % cadmium, 2% n i c k e l o r 98% cadmium, 1 % s i l v e r , 1% c o p p e r ) have a s t r u c t u r e c o n s i s t i n g of a s o f t m a t r i x c o n t a i n i n g h a r d e r c r y s t a l s o f i n t e r m e t a l l i c compounds. The cadmiumbased a l l o y s a r e c h a r a c t e r i z e d by a low c o e f f i c i e n t of f r i c t i o n , low wear and a h i g h l o a d - c a r r y i n g c a p a c i t y . They a r e n o t a s wearr e s i s t a n t a s l e a d bronze f o r example b u t t h e y c a n be u s e d w i t h an unhardened s h a f t . The b e a r i n g c l e a r a n c e may be g r e a t e r t h a n f o r a b a b b i t t b u t less t h a n f o r a l e a d bronze b e a r i n g . Aluminium-based a l l o y s (aluminium w i t h 6-7% t i n and a s m a l l amount o f copper and n i c k e l , aluminium w i t h 6-7% t i n and 1.5-2.5% s i l i c o n ) a r e s i m i l a r t o b a b b i t t s a s regards t h e i r wear-resistant p r o p e r t i e s but t h e y a r e a l s o e x t r e m e l y c o r r o s i o n - r e s i s t a n t . The disadvantage o f s u c h m a t e r i a l s i s t h e i r h i g h c o e f f i c i e n t of e x p a n s i o n .
8
They may b e u s e d a s l i n i n g s on a s t e e l b a s e . S i l v e r - b a s e d a l l o y s are used f o r e l e c t r o - d e p o s i t i o n of a t h i n
mm) i s d e p o s i t -
l a y e r on a s t e e l s u p p o r t . The s i l v e r l a y e r ( 0 . 3 - 0 . 5
e d w i t h a n i n t e r m e d i a t e l a y e r of c o p p e r or n i c k e l . A 0.02-0.03
mm
f i l m of l e a d and indium i s t h e n d e p o s i t e d o n t o p o f t h e s i l v e r , a n d t h e indium d i f f u s e d i n t o t h e l e a d by h e a t t r e a t m e n t a t 1 8 O O C . N i c k e l i n t h e t e c h n i c a l l y p u r e s t a t e i s s u i t a b l y h a r d and c o r I t c a n b e u s e d on s h a f t s , p i v o t p i n s , g e a r s , and
rosion-resistant.
f o r working i n h i g h l y c o r r o s i v e media s u c h a s h o t w a t e r . N i c k e l s i l v e r (10 t o 3 0 % n i c k e l , 55 t o 63% c o p p e r and t h e r e m a i n i n g p e r c e n t a g e o f z i n c ) i s c o r r o s i o n - and w e a r - r e s i s t a n t . M a g n e t i c a l l y s o f t m a t e r i a l s w i t h h i g h wear r e s i s t a n c e are b a s e d on Fe
-
-
A 1 o r Fe
Si
l i s t e d i n Table 2.2.
Pe
A 1 a l l o y s ( r e f . 10). Some s u c h a l l o y s are
-
Ni,
Fe
-
Co
-
N i o r Fe
-
C r a l l o y s are
c o r r o s i o n - r e s i s t a n t magnetic materials (see Table 2 . 2 )
.
T A B L E 2.2 COMPOSITION OF WEAR-RESISTANT ALLOYS BASED ON IRON
( a ) AND CORROSION-RESISTANT
(b) MAGNETICALLY SOFT
COMPONENTS % ALLOY
No.
a)
1
2
3 4
b)
Al
Cr
Re
Si
15.8-16.4 15.7-16.1 5.2-5.6
1.7-2.5 1.7-2.1
2.1-2.5
-
Ce,huric acid (99.8%) and K 1 (99%)
17.65
Corundum i s h i g h l y r e s i s t a n t t o c h e m i c a l a c t i o n e v e n a t h i g h t e m p e r a t u r e s and i n t h e p r e s e n c e of r a d i o a c t i v e water vapcur ( r e f . 7 ) .
16
The f l u o r i n e - c o n t a i n i n g a c i d s a t t a c k it a t t e m p e r a t u r e s above 3OO0C, however. The h a r d n e s s of t h e m a t e r i a l i n f l u e n c e s t h e s m o o t h n e s s o f t h e s u r f a c e and t h e c o e f f i c i e n t of f r i c t i o n . The v e r y h i g h e l a s t i c i t y modulus i s , b e s i d e s h a r d n e s s , t h e c h a r a c t e r i s t i c f e a t u r e of corundum. Agate ( T a b l e 2 . 7 )
is a
c a l and c h e m i c a l p r o p e r t i e s .
OW c o s t m i n e r a l w i t h i n f e r i o r mechani-
S i t a l l is a crystalline glass material
w i t h s i m i l a r p r o p e r t i e s t o a g a t e . I t can b e used as a s u b s t i t u t e f o r a g a t e i n t h e m a n u f a c t u r e o f low l o a d e d m i n i a t u r e s l i d i n g e l e ments. S p i n e l c o n s i s t s o f a m i x t u r e of magnesium and aluminium o x i d e s (MgO, A 1 2 0 3 ) a n d i s n o t a s h a r d as corundum ( 9 o n t h e Mohs' s c a l e ) . The c h e m i c a l , o p t i c a l , e l e c t r i c a l and m e c h a n i c a l p r o p e r t i e s of s p i n e l are c o m p a r a b l e t o t h o s e of corundum, b u t t h e p r i c e is s o m e what l o w e r . S p i n e l i s u s e d p a r t i c u l a r l y when l a r g e p a r t s h a v e t o b e made, a s it p o s s i b l e t o o b t a i n l a r g e r c r y s t a l s of rough s y n t h e t i c s p i n e l t h a n of corundum. 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) ( Z r O Z
-
-
9 % MgO) seems t o b e a n a t t r a c t i v e t r i b o l o g i c a l m a t e r i a l .
The h a r d e s t m a t e r i a l s , s u c h a s diamond ( u s e f i i l when s t i c k - s l i p e f f e c t s must b e e l i m i n a t e d ( r e f . 1 7 ) ) , b o r o n n i t r i d e o r b o r o n c a r b i d e , c a n be a p p l i e d when h i g h wear r e s i s t a n c e i s needed i n e x t r e m e c o n d i t i o n s ( r e f s . 1 8 , 1 9 ) . J a s p e r , g l a s s and g a b b r o h a v e b e e n found ( r e f . 2 0 ) t o be v e r y u s e f u l a s m a t e r i a l s f o r t h e r u b b i n g e l e m e n t s o f b e a r i n g s o p e r a t i n g i n a vacuum when t h e c o u n t e r f a c e i s manufact u r e d from a p o l y m e r i c c o m p o s i t e . Ceramic m a t e r i a l s s u c h a s s i n t e r e d A 1 2 0 3 ,
Zr02,
Si3N4, S i S i C ,
SiALON a r e v e r y i n t e r e s t i n g a s s p e c i a l b e a r i n g m a t e r i a l s b e c a u s e o f t h e i r r e s i s t a n c e t o wear and h e a t ( r e f s . 2 1 - 2 d ) .
Some p r o p e r t i e s o f
t h e aforementioned ceramic materials a r e l i s t e d i n T a b l e 2.d. c h a r a c t e r i s t i c p r o p e r t y o f SiALON i s i t s u l t r a - h i g h
The
resistance to
oxidation a t elevated temperatures. I t i s a l s o corrosion-resistant i n l i q u i d metals: A l l C u , F e , Zn and s t e e l , i n H2S04 and H C 1 a c i d s , i n borax, a l k a l i s , e t c . T h i s m a t e r i a l i s a l s o h i g h l y wear-resistant. The r e a c t i o n - s i n t e r e d ,
fine-grain,
s o l i d s i l i c o n carbide Purbide
REFEL ( P u r e Carbon C o ; ) c a n work up t o
140OOC.
High d e n s i t y f e r r i t e s w i t h a r e s i d u a l p o r o s i t y of 0 . 5 % o r l e s s have a h i g h r e s i s t a n c e t o wear and e l e m e n t s made o f s u c h m a t e r i a l s
a r e c h a r a c t e r i z e d by t h e p r e c i s i o n w i t h which t h e y c a n be cut, ground o r p o l i s h e d ( r e f . 7 ) . Such m a t e r i a l s a r e v e r y u s e f u l i n t h e manuf a c t u r e o f t a p e r e c o r d e r h e a d s . An example of t h e h i g h l y wear-res i s t a n t m a t e r i a l u s e d i n t a p e r e c o r d e r h e a d s i n HAVAR, made by
17
Hamilton, which has a good permeability and retains virtually no detectable magnetism after high magnetic field exposure (ref. 29). TABLE 2.8 SOME PROPERTIES OF CERAMIC MATERIALS I
S i 3N4
MATERIALS
REACT I ON S I NTERED
PROPERTIES Dens i t y , 3 mg/mm
S i 3N4 HOT PRESSE
2.6
SiSiC REACT I ON S I NTEREO
3.2
3.0
S i ALON
REACT I ON S I NTEREO
3.2
3'2 HOT PRESSE
3.9
Tensi l e s t r e n g t h , MPa
260
690
300
430
480
Modulus o f e l a s t i c i ty,GPa
220
310
350
300
365
Coe f f i c i en t o f thermal expansion,
10-6/K Thermal cond u c t iv i t y , W/m.K Maximum app l i c a t i o n tern p e r a t u r e , OC
3.2
3.2
16
(20°C)
14.2 (1200°C)
25 (20OC)
14
(1200°C
4.5 (20OC) 40 (lOOO°C
200
3.2 25 (20°C)
7 (10OO0C
9.0
26 (20°C) 5 (12OOOC
1500
1500
1400
1400
1750
Wei b u l 1 ' s rnodul us
10
15
10
15
10
Opened porosity, %
20
0
0
0.01
0
Composite materials of various structures are very interesting for use in bearings, especially for making unlubricated bearing elements. Composites are usually formed by a combination of metal, ceramic materials and polymers with various geometrical features and properties. The metallic glasses such as Fes7Co18B14Si1, Fe40Ni40P14B6 Fe76Cr4C12Pd demoqstrate very good tribological properties (refs. 30, 31). The composites of borosilicate glass reinforced withgraphite fibres are also of interest (ref. 32). The metallic glasses, graphite-fibre-reinforced ceramic or glass materials can be applied where there are high temperatures and a corrosive environment, and in these conditions they show better tribological properties than metal or polymer matrix materials. Composite materials based on crystalline glass sitall are sintered using, for example, Sitall-3 (manufactured on the base of Si02,
18 A1203,
1000
-
Ba203, MgO and f l u o r i d e s and w i t h a c o m p r e s s i v e s t r e n g t h o f 1 2 0 0 MPa
( r e f . 3 3 ) ) c o n t a i n i n g 30% m e t a l ( n i c k e l , c o p p e r ,
t i n , o r b r o n z e ) . The b e s t wear and a n t i - f r i c t i o n p r o p e r t i e s when r u b b i n g a g a i n s t s t e e l i n a vacuum, w e r e e x h i b i t e d by a c o m p o s i t e c o n t a i n i n g b r o n z e powder ( r e f . 3 3 ) . A n o t h e r p o s s i b i l i t y i s t o s i n t e r sitall-based
c o m p o s i t e s w i t h a p o r o s i t y o f 2-39,
impregnated w i t h
PTFE o r c o n t a i n i n g 30% c o p p e r and i m p r e g n a t e d w i t h PTFE. Such m a t e r i a l s are c o r o s s i o n - r e s i s t a n t
and c a n work i n a g a s , l i q u i d o r
vacuum e n v i r o n m e n t . The c o e f f i c i e n t o f f r i c t i o n i n a i r i s 0.25-0.31, i n a vacuum 0.2-0.25
and t h e maximum s p e c i f i c l o a d s are between 8 a n d
li) MPa ( r e f . 12). Ceramic c o m p o s i t e s s u c h a s cermets ( m e t a l bounded ceramics e . g . chromium bonded t u n g s t e n c a r b i d e ) , c a n be u s e d a s d r y b e a r i n g m a t e r i a l s working a t t e m p e r a t u r e s o v e r SOOOC ( r e f . 3 4 )
. Generally,
b o t h t h e c o e f f i c i e n t o f f r i c t i o n and t h e wear r a t e a r e v e r y h i g h , and so i s t h e c o s t : t h e r a n g e o f a p p l i c a t i o n s i s v e r y l i m i t e d . Hard carbon ( g r a p h i t e ) o r carbon ( g r a p h i t e ) - m e t a l m a t e r i a l s are very u s e f u l a s b e a r i n g m a t e r i a l s a t t e m p e r a t u r e s o f up t o 350OC ( t y p i c a l c o m p o s i t e s ) . However, t h e s e m a t e r i a l s a r e r e l a t i v e l y weak a n d b r i t -
t l e which makes b e a r i n g s d i f f i c u l t t o f i t i n t o t h e i r h o u s i n g s and l i m i t s t h e l o a d c a p a c i t y t o a b o u t 1.5 MPa, a l t h o u g h i m p r e g n a t i o n by m e t a l s can double t h i s f i g u r e ( r e f . 3 4 ) . C a r b o n - g r a p h i t e s a r e s e l f - l u b r i c a t i n g , h e a t - r e s i s t a n t and i n e r t i n t h e p r e s e n c e o f a l l b u t t h e s t r o n g e s t o x i d i z i n g a g e n t s . The maximum working t e m p e r a t u r e s f o r c a r b o n g r a p h i t e s a r e a b o u t 4 0 0 ,
65U
and 750OC i n a i r , w a t e r v a p o u r and c a r b o n d i o x i d e r e s p e c t i v e l y . They c a n b e u s e d i n e l e m e n t s o p e r a t i n g i n t h e vacuum and c o l d o f o u t e r s p a c e and c a n be s a f e l y b r o u g h t i n t o c o n t a c t w i t h f o o d s , beve r a g e s , d r u g s o r m e d i c a l a p p l i a n c e s and c a n b e s t e r i l i z e d w i t h o u t damage. C a r b o n - g r a p h i t e s c a n a l s o b e i m p r e g n a t e d w i t h m o l t e n metals, c e r a m i c s , r e s i n s , l u b r i c a n t s e t c . The s i l i c o n i z e d g r a p h i t e s a r e g r a p h i t e m a t e r i a l s w i t h s u r f a c e which c a n b e c h e m i c a l l y c o n v e r t e d t o s i l i c o n c a r b i d e . Such m a t e r i a l s have t h e wear r e s i s t a n c e o f s i l i c o n c a r b i d e w h i l e r e t a i n i n g some o f t h e l u b r i c i t y o f g r a p h i t e . C a r b o n - g r a p h i t e m a t e r i a l s c a n b e e a s i l y machined o r molded t o s i z e ; t h e y have b e e n s u c c e s s f u l l y u s e d t o make b e a r i n g b u s h e s , d e m n s t r a t i n g v e r y good t r i b o l o g i c a l p r o p e r t i e s ,
including i n conditions of
w a t e r and g a s l u b r i c a t i o n ( r e f s . 3 5 , 3 6 ) . A n i n i a t u r e b e a r i n g made o f Purebon P-9 m a t e r i a l
( P u r e Carbon Co.) w i t h a b o r e d i a m e t e r o f mm h a s b e e n a p p l i e d t o a w a t t - h o u r meter f o r w h i c h t h e l i f e expectancy i s 30 y e a r s (without l u b r i c a t i o n ) , s l i d i n g speed
0.523f0.007
19
below 0 . 0 1 m / s
and t e m p e r a t u r e r a n g e - 5 5 O t o 5OoC
( r e f . 3 ' 7 ) . Some
properties of carbon-graphites a r e l i s t e d i n Table 2.9. TABLE 2.9 PROPERTIES OF SOME CARBON-GRAPHITES
MATERIAL
MANUFACTURER PROPERTIES Dens i t y , mg/mm 3
PUREBON
PUHEBON
p-9
P-692
PURE CARBON C O .
1.75
1.85
Compressive strength, MPa
150
220
Modulus o f e l a s t i c i t y , GPa
14
21
I EK-200
WITH
RINGSDORF-WERKE GmbH BAD GOOESBERG (F.R.G.
1.70 100
9.5
1.90 250
14
C o e f f i c i e n t o f thermal expansion,
10-6/K Thermal c o n d u c t i v i t y , W m - l Temperature 1 i m i t ,
K-l
4.3
6.5
3.7-4.6
5.7
12.3
12.3
25.2
25.2
OC
Neut r a 1 a tniosphe r e O x i d i z i n g atmosphere
800
300
300 300
40 0
200
Metal-based c o m p o s i t e s a r e c h a r a c t e r i z e d by t h e i r h a r d s t r u c t u r e f i l l e d w i t h various a n t i - f r i c t i o n materials. S i n t e r e d i r o n , c o p p e r , aluminium, b r a s s o r b r o n z e , C r - C o a l l o y - b a s e d m a t e r i a l s w i t h 1 0 - 1 5 % g r a p h i t e a r e a p p l i c a b l e o v e r a wide t e m p e r a t u r e r a n g e ( - 2 0 0 t o 6OO0C f o r i r o n - b a s e d m a t e r i a l ) and d e m o n s t r a t e a s u f f i c i e n t l y low c o e f f i c i e n t of f r i c t i o n ( a s low a s 0 . 0 5 ) . M a t e r i a l s
w i t h porous b r o n z e , Cu
-
Nil
Cu
-
Pb l a y e r s a r e m a n u f a c t u r e d u s i n g
s t e e l b a c k i n g . The s i n t e r e d p o r o u s l a y e r i s i m p r e g n a t e d w i t h a low
m e l t i n g p o i n t a l l o y , PTFE or l u b r i c a n t . A c t u a l l y , PTFE and l e a d i m p r e g n a t e d i n t o s i n t e r e d p o r o u s b r o n z e ( G l a c i e r DU) h a s p r o b a b l y found a w i d e r r a n g e of a p p l i c a t i o n s t h a n a n y o t h e r b e a r i n g m a t e r i a l
(see r e f . 3 4 ) . Porous s t e e l m a t e r i a l s impregnated w i t h e . g . A g - C u - Zn - Cd a l l o y ( r e f . 1 2 ) a r e l o a d - r e s i s t a n t when o p e r a t i n g i n a vacuum a g a i n s t s t a i n l e s s o r n i t r i d e d s t e e l . The optimum p o r o s i t y of such c o m p o s i t e s i s 3 0 - 4 0 % . The c o e f f i c i e n t of f r i c t i o n i s 0.04-0.05 when l u b r i c a t e d e l e m e n t s rub i n a i r a t 5 0 - 7 0 MPa.
20
2,4,
POLYMERIC MATERIALS
2.4.1.
UNFILLED POLYMERS
U n f i l l e d bearing (anti-friction) polymers are principally thermplastic
materials s u c h a s polyamides, p o l y a c e t a l s , p o l y t e r e p h t h a l a t e s e t c . The most i n t e r e s t i n g o f them, a s f a r as t h e molding of b e a r i n g e l e ments i s c o n c e r n e d , w i l l be d i s c u s s e d h e r e . Polyamides ( P A ) a r e c r y s t a l l i n e polymers w i t h a h i g h d e g r e e o f c r y s t a l l i n i t y , which i s i m p o r t a n t f o r good t r i b o l o g i c a l p r o p e r t i e s . Polyamide 6 h a s v e r y good t r i b o l o g i c a l p r o p e r t i e s b u t it i s s e n s i t i v e t o m o i s t u r e . A t room t e m p e r a t u r e (2OoC) and 6 5 % r e l a t i v e hum i d i t y , t h e m a t e r i a l c o n t a i n s a b o u t 3 % water ( r e f . 3 8 ) . The o t h e r polyamides ( P A 6 6 , PA 6 1 0 , PA 11, PA 1 2 ) a r e more r e s i s t a n t t o t h e p r e s e n c e of m o i s t u r e and e l e m e n t s d e m o n s t r a t e h i g h e r d i m e n s i o n a l s t a b i l i t y . The p r i n c i p a l p r o p e r t i e s and some t r a d e m a r k s of t h e main t y p e s o f polyamides a r e g i v e n i n T a b l e 2.10. TABLE 2.10 MAIN TYPES OF POLYAMIDES
MATERIAL
PROPERTIES
PA 6
PA 66
4KULON M,K ULTRAMID B ORGAM I DE 3EETLE GR I LON
ZYTEL 101 ULTRAMI 0 A TECHNYL A MARANYL A D URETHAN
1.13-1.15 80
1.13-1.15
PA 6 1 0 ZYTEL 3 1 TECHNYL OP ULTRAMID S
PA 1 1
R I LSAN
DO
80
60
85
1500
2000
1400
60
34 ( a t 1% def o r mat i on)
Coefficient of thermal expansion, 1 0 - 5 1 ~ Thermal c o n d u c t i v i t y , W/m K Speci f i c h e a t , J/g.K Maximum o p e r a t i n g temperature o f beari n g , OC Surface f r e e 2 mJ/m energy, S o l u b i l i t y parameter 103J0.5,J .5
1.07-1.09 60
8-10 0.28
1.9
8-10 0.24 1.7
75
70
50
46
30
26
20 ( a t 1 % def o rmat i on) 9 0.22 1.6
60
VESTAM I D G R I LAM I D
--
_ I _ -
Dens; t y , mg/mm 3 Tensi l e strength,MPa Tensi l e e l o n g a t i o n , % Modulus o f e l a s t i c i t y , MPa Compress i ve s t r e n g t h , MPa
PA 12
1.04-1.05 300-330
1.01-1.02 60 250-300
1150
1100
50
50
13
11
50
0.29 1 .8
50
0.33 1.7 50
35 27
25
25.5
21
P o l y a c e t a l s (POM) a r e h i g h l y c r y s t a l l i n e , t h e y a r e s t r o n g and r i g i d , and have good m o i s t u r e , h e a t , and s o l v e n t r e s i s t a n c e . The homopolymers are h a r d e r , have h i g h e r r e s i s t a n c e t o f a t i g u e , a r e more r i g i d , and d e m o s t r a t e h i g h e r t e n s i l e and f l e x u r a l s t r e n g t h w i t h g e n e r a l l y lower e l o n g a t i o n . The copolymers remain s t a b l e i n long-term, h i g h - t e m p e r a t u r e s e r v i c e and o f f e r e x c e p t i o n a l resistance t o t h e e f f e c t s o f immersion i n water a t h i g h t e m p e r a t u r e s . N e i t h e r type resists s t r o n g a c i d s , b u t
t h e copolymer is v i r t u a l l y unaffect-
e d by s t r o n g b a s e s ( r e f s . 3 9 , 4 0 ) . The t y p i c a l p r o p e r t i e s o f p o l y a c e t a l s are l i s t e d i n T a b l e 2 . 1 1 . TABLE 2.11 TYPICAL PROPERTIES OF POLYACETALS, POLYCARBONATES AND POLYTEREPHTHALATES
MATERIAL
POM h
POM c
OELRl N
HOSTAFORM C ULTRA FORM CELCON (KEMATAL) TARNOFORM
PROPERTIES D e n s i t y , mg/mm 3 T e n s i l e s t r e n g t h , MPa Tensile elongation, % Modulus o f e l a s t i c i t y , MPa Compressive s t r e n g t h , MPa C o e f f i c i e n t o f thermal expansion, 10-51~ Thermal c o n d u c t i v i t y , W/m.K S p e c i f i c heat, J/g.K Maxi mum opera t i ng tempe. r a t u r e o f b e a r i n g , OC S u r f a c e f r e e energy, 2 mJ/m S o l u b i l i t y parameter, lo3J0.5;1 -5
PC -LEXAN MA KROL0 N MERLON S I NVET
PETP
PBTP
ARNITE A HOSTADUR K FR-PET RYNITE
UALOX CELANEX POCAN 0 E R OT ON ARNITE T HOSTAOUR B
1.37 74
1.31-1.32
-1.4; 70 25-75
1.41 60 60-75
I .20
65 90
50 1 goo
3500
2800
2300
125
110
85
10.0
0.2:
0.30
1.5
1.5
57 150-300 1900
85
7
7
4-4
0.20 1.2
0.24 1.05
0.23 1.1
90
80
100
55
50
43
40
33
47
44
21
23
20
21
21
P o l y c a r b o n a t e ( P C ) (see Table 2 . 1 0 )
,
low-crystalline material,
h a s h i g h i m p a c t s t r e n g t h o v e r a wide t e m p e r a t u r e r a n g e . The materi a l i s c h a r a c t e r i z e d by good d i m e n s i o n a l s t a b i l i t y even w i t h humid-
i t y changes o r d u r i n g s h o r t p e r i o d s i n b o i l i n g w a t e r . i s u n a f f e c t e d by g r e a s e s , o i l s and a c i d s , b u t s o l u b l e h y d r o c a r b o n s , e s t e r s and k e t o n e s . I t i s w e a t h e r p r o o f , composition is observed a f t e r a long p e r i o d ( 2 y e a r s )
Polycarbnate i n chlorinated b u t some de(refs. 12, 41).
22
P o l y e t h y l e n e t e r e p h t h a l a t e (PET?) and po l y b u t y l e n e tere&thalate (PBTP)
( T a b l e 2.10) a r e c o m p e t i t o r s o f p o l y a s e t a l s . They c r y s t a l -
l i z e r a p i d l y , flow r e a d i l y , and have h i g h c r e e p r e s i s t a n c e and low m o i s t u r e a b s o r p t i o n . T h e i r main advantage i s t h e i r t o u g h n e s s . They a r e c h e m i c a l l y r e s i s t a n t t o a broad r a n g e of media such as aliphatic hydrocarbons, g a s o l i n e , carbon t e t r a c h l o r i d e , p e r c h l o r o e t h y l e n e , o i l s , f a t s , a l c o h o l s , g l y c o l s , e s t e r s , e t h e r s and d i l u t e a c i d s and b a s e s . PETP and PBTP a r e a t t a c k e d , however, by s t r o n g a c i d s and bases. The p o l y o l e f i n s s u c h as p o l y e t h y l e n e and p o l y p r o p y l e n e have
s i m i l a r chemical s t r u c t u r e s b u t t h e y d i f f e r i n t h e i r c r y s t a l l i n e s t r u c t u r e , and c o n s e q u e n t l y i n t h e i r t r i b o l o g i c a l p r o p e r t i e s . Polye t h y l e n e s (PE) a r e c l a s s i f i e d i n t o t h r e e c a t e g o r i e s a c c o r d i n g t o d e n s i t y : l o w , medium and h i g h . A f o u r t h t y p e is ultrahigh-molecularweight p o l y e t h y l e n e (UHMWPE) w i t h a m o l e c u l a r w e i g h t of o v e r 3 1 0 0 0 0 0 . Low-density PE (LDPE) i s v e r y tough and f l e x i b l e b u t d e
m o n s t r a t e s r e l a t i v e l y l o w h e a t r e s i s t a n c e ( T a b l e 2 . 1 2 ) . For b e a r i n g a p p l i c a t i o n s , it can be added t o o t h e r polymers t o improve t h e i r
--
t r i b o l o g i c a l p r o p e r t i e s . High-density PE ( H D P E ) i s more r i g i d and d e m o n s t r a t e s a d e q u a t e h i g h impact s t r e n g t h , e s p e c i a l l y a t l o w t e m p e r a t u r e s . I t is u s e f u l f o r b e a r i n g s f i l l e d w i t h v a r i o u s substances
(see Chapter 2 . 4 . 2 )
. Both
m a t e r i a l s demonstrate e x c e l l e n t i n e r t -
n e s s , n e a r - z e r o m o i s t u r e a b s o r p t i o n , good e l e c t r i c a l p r o p e r t i e s , a low c o e f f i c i e n t o f f r i c t i o n and e a s e of p r o c e s s i n g . TABLE 2.12
T Y P I C A L PROPERTIES OF POLYETHYLENES. POLYPROPYLENE AND POLYPHENYLENE OXIDE I
HDPE
I
UHMWPE
PP
H 0s TALE N , LUPOL E N , VESTOL EN A, ALATHON IOSTALEN PP 'ESTOLEN P, POLYETHYLENE. ENJAY, MARLEX, DYLAEI, T E N I T E , ALKATENE, BAYLON,HOSTALEN GUR, IOVOLEN 'ROPATHENE, SUPROLEN, 1000 RCH , M I L L 1 ON H I -2EX IARLEX PP, E N I T E PP, ARLONA
-2
0.910-0.925
4- 16
t i c i t y , MPa
3
4
0.941-0.965
0.928-0.941
20-40
27-40
90-800
20-1000
200-500
100-300
400-1200
140-750
NORY L
5 0.905
1.06-1.10
35
50-70
10-20
50-60
1100
2400-2600
I
23 TABLE 2.12
I
(continued) 2
1 Compressive s t r e n g t h , MPa Coefficient o f thermal ex an sion, 10- / K Thermal conducti v i t y , W/m.K S p e c i f i c heat, J/g.K Maximum o p e r a t i n g temperature of b e a r i n g , OC Surface f r e e energy, mJ/m Solubi 1 i t meter, l o ! J ~ ~ ! ~ i .!
y -
38-48
20-25
1
8
40
I
50
35
I
6-7
6-12
3.3-3
.a
0.35 2.4
30 33
Ultrahigh-molecular-weight PE has entirely different properties from those of conventional polyethylenes (ref. 4 2 ) . It is also available with a crosslinked structure - which is obtained by a process of irradiation - giving the material outstanding thermoplastic heat resistance and strength; UHMWPE is wear-resistant at a low coefficient of friction and chemical-resistant. The material can be satisfactorily used for the rubbing elements of artificial joints in the human body (ref. 11) Polypropylene (PP) is unsuitable for rubbing elements but its chemical and electrical properties can be put to good use in the mechanical parts of electrical devices. Specially stabilized PP can also be used for elements exposed to oxidative conditions and UV radiation. Polyphenylene oxide (PPO) is characterized by having the lowest water absorption of the engineering thermoplastics, outstanding dimensional stability and excellent mechanical and thermal properties (see Table 2 . 1 2 ) . PPO is resistant to acids, bases and detergents but the material is attacked by many halogenatedor aromatic hydrocarbons. PPO rubbing against other thermoplastics demonstrates good friction properties but its wear resistance is not high (refs. 4 3 ,
.
44).
The styrene group of thermoplastics (Table 2.13) is characterized by medium price, hardness, rigidity and thoughness and dimensional stability even at low temperatures. Materials such as ABS (Acrylonitryle-butadiene-styrene copolymer) and SAN (Styrene-acrylonitrile copolymer) have enough strength, impact resistance and w a r
24
resistance to be used for certain elements in bearings (mechanism frames). Styrene copolymer parts are practically unaffected by water, salts, most inorganic acids, food acids and alkalis, especially under stress. Polystyrene (PS) is rigid and brittle and has low heat resistance. It is soluble in most aromatic and chlorinated solvents but insoluble in alcohols such as methanol, ethanol, normal heptane and acetone. Only polystyrene which has been modified by, for example, silicone is interesting as a bearing material(see Chapter 2 . 4 . 2 ) . TABLE 2.13 TYPICAL PROPERTIES OF ABS,
SAN AN0 POLYSTYRENE
AB S
MATERIAL
IMPACT NOVOOUR, BAKELITE TYBRENO, AFCORYL.
PROPERTIES
3 1.01-1.05 mg/mm 40 s t r e n g t h , MPa elongation, % 5-20 of elasticity, MPa 2300 Compressive s t r e n g t h , MPa 50-60 C o e f f i c i e n t o f thermal 10-51~ expans i on, 5.3 Thermal c o n d u c t i v i t y , W/m.K 0.17 1.4 S p e c i f i c heat, J/g*K Maximum o p e r a t i n g temperature o f bearin 50 Surface f r e e energy, 2 mJ/m S o l u b i l i t y parameter lo3J0.5;1.S
Density, Tensile Tensi l e Modulus
8;
PS
SAN
TERLURAN, ABSON, A B S , CYCOLAC ABS, LUSTRAN, VESTOOUR, URTAL, RAVIKRAL
LURAN TYR'L SAN
HOSTYREN POLYSTYROL VESTYRON LUSTREX FOSTARENE 0 I STRENE
1.04
1.04-1.10
5-70
1.04-1.06 40-50 5-25
1.5-3.7
35-70 1 .O-2.3
I 40 0 -2400
2500-260 0
2800-3900
2800-4200
55-85
95-200
I .02-1.05
35-45
5.6
4.6
0.17 1.4
0.17
50
60-80
3.5-4.0
80-110 3-5
1.4 50
50
50
43
45
19
18
Fluoropolymers (Table 2 . 1 4 ) such as polytetrafluoroethylene [PTFE), fluorinated ethylene propylene (FEP) perfluoroalkoxyethylene (PFA), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), ethylene tetrafluoroethylene (ETFE) demonstrate high chemical inertness, high- and low-temperature stability, low friction (especially PTFE) and excellent electrical properties. Their resistance to wear and creep is low, but these characteristics can be
TABLE 2.14 TYPICAL PROPERTIES OF FLUOROPOLYMERS
MATERIAL
TEFLON HALON FLUON POLY FLUON THIOKOL TFE HOSTAFLON
PROPERTIES Density, mg/mm 3 T e n s i l e strength, MPa T e n s i l e elongation, % Modulus o f e l a s t i c i t y , MPa Compressive strength, MPa C o e f f i c i e n t o f thermal expansion,
PT FE
10-5/K
Thermal c o n d u c t i v i t y , W/m.K S p e c i f i c heat, J/g.K Maximum o p e r a t i n g temperature o f bearing, Surface free energy, mJ/m2 Solubi 1 i t y parameter, 103~~.5ni’*5
OC
FEP
2.13-2.30 23 300 350-700 8-12
2.12-2.17 20 300
5.5-8.4
4.6-5.8
0.26 1.05
0.21 1.1
50 19 13
-I
TEFLON FEF rEFLON PFA
2.12-2.17 28 300
3 50
45
6.7
0.27
KYNAR
1.75-1.78 35-50 100-300 1100 70 8.0-8.5
0.1-0.14
PCTFE
ETFE
KEL-F HALON HOSTAFLON C2 PLASKON PCTFE
TEFZEL
2.13 37 125 1300 30-50 4.8-15 (below and above 6OoC) 0.27 0.9 60 31 15.5
1 .70
45
275 830 5.2
0 -25 60
N Ln
26
improved by compounding t h e r e s i n s w i t h i n o r g a n i c f i b r e s , o r p a r t i c u l a t e materials (see C h a p t e r 2 . 4 . 2 )
.
H e a t - r e s i s t a n t polymers ( T a b l e 2.15) a r e a p p l i c a b l e f o r b e a r i n g elements o p e r a t i n g a t e l e v a t e d temperatures. Polyimide ( P I ) , polyamide-imide
( P A I ) and p o l y e t h e r i m i d e
( P E I ) a r e t h e most h e a t - and
f i r e - r e s i s t a n t polymers known. They a r e f o r m u l a t e d a s t h e r m o s e t s and t h e r m o p l a s t i c s . P o l y i m i d e s a r e a v a i l a b l e m o s t l y as l a m i n a t e s and s h a p e s , molded p a r t s , and s t o c k s h a p e . P o l y i m i d e p a r t s c a n o p e r a t e c o n t i n o u s l y i n a i r a t a b o u t 30U°C
but s e r v i c e temperature
f o r i n t e r m i t t e n t e x p o s u r e c a n r a n g e from c r y o g e n i c t o a s h i g h a s 50OoC. The m a t e r i a l i s w e a r - r e s i s t a n t ,
has a l o w c o e f f i c i e n t o f fric-
t i o n and i s i n a f f e c t e d by e x p o s u r e t o d i l u t e a c i d s , aromatic and a l i p h a t i c hydrocarbons, ester, e t h e r s , a l c o h o l s , Freons, h y d r a u l i c f l u i d s , f u e l s and k e r o s e n e : however, it i s a t t a c k e d by d i l u t e alkal i s and c o n c e n t r a t e d i n o r g a n i c a c i d s . Polyamide-imide r e s i n s c a n be molded i n t o complex, p r e c i s i o n p a r t s i n c o n v e n t i o n a l i n j e c t i o n molding machines. P A 1 h a s h i g h c r e e p - r e s i s t a n c e and d i m e n s i o n a l s t a b i l i t y even a t e l e v a t e d temper a t u r e s and l o a d e d . I t h a s good r a d i a t i o n and c h e m i c a l . r e s i s t a n c e ( e . g . a g a i n s t a l i p h a t i c and a r o m a t i c h y d r o c a r b o n s , h a l o g e n a t e d solv e n t s , a c i d s and b a s e s o l u t i o n s ) . I t i s a t t a c k e d , however, by h i g k - t e m p e r a t u r e c a u s t i c m a t e r i a l s , steam and some a c i d s . PA1 a b s o r b s m o i s t u r e : f o r example, a t 50% r e l a t i v e h u m i d i t y and 2U°C i t absorbs a b o u t 1%m o i s t u r e (by w e i g h t ) i n
1 0 0 0 h; molded PA1 p a r t s m u s t b e
d r i e d t o a c h i e v e maximum perfomance a t high t e m p e r a t u r e s ( r e f . 4 0 )
.
P A 1 h a s v e r y good t r i b o l o g i c a l p r o p e r t i e s .
( i n t r o d u c e d i n 1 9 8 2 ) i s plyetherimide ( P E I ) . P E I i s a n amorphous e n g i n e e r i n g t h e r m o p l a s t i c w i t h o u t s t a n b A r e l a t i v e l y new m a t e r i a l
i n g p r o c e s s a b i l i t y and e x c e l l e n t , s t a b l e e l e c t r i c a l p r o p e r t i e s over
a wide r a n g e o f t e m p e r a t u r e s and f r e q u e n c i e s . High h e a t and c r e e p r e s i s t a n c e , h i g h s t r e n g t h and a h i g h modulus o f e l a s t i c i t y a r e c h a r a c t e r i s t i c s o f t h i s m a t e r i a l . Molded o r e x t r u d e d p a r t s made o f PEI c a n b e machined u s i n g c o n v e n t i o n a l t e c h n i q u e s and c a n be bonded t o g e t h e r o r t o d i s s i m i l a r materials u s i n g u l t r a s o n i c , adhesive o r s o l v e n t methods. P E I i s r e s i s t a n t t o m i n s r a l a c i d s , a l i p h a t i c hydroc a r b o n s and a l c o h o l s , m i n e r a l - s a l t s o l u t i o n s , d i l u t e d bases a n d f u l l y h a l o g e n a t e d h y d r o c a r b o n s b u t i t i s a t t a c k e d by p a r t i a l l y hal o g e n a t e d s o l v e n t s s u c h a s methylene c h l o r i d e and t r i c h l o r o e t h a n e and by s t r o n g b a s e s . P o l y s u l f o n e s ( p o l y p h e n y l e n e s u l f o n e (PPSU) and p o l y e t h e r s u l f o n e (PESU)) o f f e r t h e h i g h e s t performance p r o f i l e s of a n y o f t h e t h m -
TABLE 2.15 TYPICAL PROPERTIES
O F HEAT-RESISTANT POLYMERS PPS TPX
RYTON
(R-4;
PPS + 40% GLASS FIBRES)
PROPERT I ES Dens i t y , m g / m 3 Tensile strength,
MPa
Tensile elongation,
0.83 30
20°C 25OoC
%
20% 25OoC Modulus o f e l a s t i c i t y ( f l e x u r a l ) , MPa
Compressive s t r e n g t h , MPa C o e f f i c i e n t o f thermal expansion, Thermal c o n d u c t i v i t y , W/m.K S p e c i f i c h e a t , J/g.K
15
1.67 120 1.25
PI
PA I
TORLON VESPEL (PI + 15% ;RAPHITE)
PE I -
PPS u
PESU
ULTEM
UDEL
V I CTREX
1.51 65 40
4.5
1.4 190 65
15
1.2
105 7-8
1.24 70
1.37 85
50-100
40-80
22 2ooc 25OoC
lo-'/,
Maximum a d m i s s i b l e b e a r i n q t e m p e r a t u r e , S o l u b i l i t y parameter, 103J0.5~7~1.5
1500
6.5
0.17
11700
145 2.2 0.3
11000 6200
4.7
5000 3500
2
0.9
500
3.1 0.2
2.18 OC
120
250 20.5
320
19.5
300
200
-
2500 (tensile modulus) 95 ' 3.1 0.26 1.5 ( a t 15OoC) 180
2700
0.13-0.1E
200
28 p l a s t i c s p r o c e s s a b l e on c o n v e n t i o n a l i n j e c t i o n and e x t r u s i o n mchine r y . The p o l y s u l f o n e p a r t s remain t r a n s p a r e n t a t s e r v i c e t e m p e r a t u -
res a s h i g h a s 230°C.
C o n t i n u o u s u s e i n a i r o r i n steam a t r a t e d
t e m p e r a t u r e s d o e s n o t c l o u d , c r a z e , or o t h e r w i s e d e s t r o y t h e i r t r a n s p a r e n c y . Thermal s t a b i l i t y and o x i d a t i o n r e s i s t a n c e a r e e x c e l l e n t a t s e r v i c e t e m p e r a t u r e s o f a b o u t 20OoC.
Creep of p o l y s u l f o n e s
i s e x c e p t i o n a l l y low a t e l e v a t e d t e m p e r a t u r e s and under c o n t i n o u s
-
load
c r e e p a t 1 2 O o C i s l e s s t h a n t h a t of p o l y a c e t a l o r h e a t - r e -
s i s t a n t ABS a t room t e m p e r a t u r e ( r e f . 4 0 ) . P o l y s u l f o n e s d e m o n s t r a t e h y d r o l i t i c s t a b i l i t y which, combined w i t h h e a t r e s i s t a n c e , r e s u l t s i n e x c e p t i o n a l r e s i s t a n c e t o b o i l i n g water and steam. An i m p o r t a n t d i s a d v a n t a g e o f p o l y s u l f o n e s , however, i s t h e i r a b s o r p t i o n o f ultrav i o l e t r a y s , which g i v e s them poor w e a t h e r r e s i s t a n c e . P o l y p h e n y l e n e s u l p h i d e (PPS) i s a v a i l a b l e i n v a r i o u s g l a s s f i b r e and m i n e r a l / g l a s s - f i b r e - r e i n f o r c e d g r a d e s f o r i n j e c t i o n o r compression molding and i n powder form f o r s l u r r y c o a t i n g , f l o c k i n g and e l e c t r o s t a t i c s p r a y i n g . O u t s t a n d i n g c h e m i c a l r e s i s t a n c e and high-temperature
and e x c e l l e n t d i m e n s i o n a l s t a b i l i t y w i t h a low a n d
p r e d i c t a b l e s h r i n k a g e r a t e make t h i s m a t e r i a l v e r y u s e f u l i n p r e c i s i o n e n g i n e e r i n g a p p l i c a t i o n s . The w e a r - r e s i s t a n c e and r e l a t i v e l y
low c o e f f i c i e n t of f r i c t i o n a r e s a t i s f a c t o r y ( r e f . 4 5 ) . Polymethyl p e n t e n e (PMP) d e m o n s t r a t e s good c h e m i c a l and h e a t r e s i s t a n c e ( u p t o 170°C and f o r s h o r t p e r i o d s up t o 2 2 O O C ) . I t i s t r a n s p a r e n t i n t h i c k s e c t i o n s b e c a u s e i t s c r y s t a l l i n e a n d amorphous p h a s e s have t h e same r e f r a c t i o n i n d e x . E l e m e n t s m a n u f a c t u r e d from PMP a r e h a r d and s h i n y . They s h o u l d n o t b e exposed t o l o n g t e r m U V radiation. 2.4.2.
FILLED POLYMERS
F i l l e d polymers a r e i n t e r e s t i n g b e c a u s e f i l l i n g improves t h e i r m e c h a n i c a l and t r i b o l o g i c a l p r o p e r t i e s . T h e r m o p l a s t i c and t h e r m o s e t polymers a r e f i l l e d w i t h r e i n f o r c i n g s u b s t a n c e s o r w i t h i n t e r n a l l u b r i c a n t s ( o r w i t h b o t h kinds of f i l l e r ) .
or b e a d s , m i n e r a l s o r carbon i s a v e r y e f f e c t i v e way t o s i g n i f i c a n t l y improve t h e i r m e c h a n i c a l and t h e r m a l p r o p e r t i e s . The t y p i c a l c o n t e n t o f r e i n f o r c i n g s u b s t a n c e i s 2 0 t o 4 0 % by w e i g h t . G l a s s f i b r e r e i n f o r c e m e n t generall y improves t h e m e c h a n i c a l p r o p e r t i e s o f p l a s t i c s by a f a c t o r o f two o r more ( r e f s . 4 0 , 4 6 ) . The g l a s s f i b r e improves t h e c r e e p - and w e a r - r e s i s t a n c e of p o l y m e r i c m a t e r i a l b u t g l a s s f i l l e r s i n c r e a s e R e i n f o r c i n g polymers w i t h g l a s s f i b r e
29
t h e wear o f t h e m a t i n g s u r f a c e and t h e c o e f f i c i e n t of f r i c t i o n . The l u b r i c a t i o n o f g l a s s - r e i n f o r c e d p l a s t i c s i s a v e r y e f f e c t i v e way t o o f f s e t t h e n e g a t i v e wear e f f e c t s a n d t o improve t h e i r t r i b o l o g i c a l p r o p e r t i e s (see Chapter 5 . 2 )
.
Carbon f i b r e p r o v i d e s t h e h i g h e s t s t r e n g t h modulusl heat-deflect i o n t e m p e r a t u r e , and c r e e p and f a t i g u e - e n d u r a n c e v a l u e s . The t h e r mal c o n d u c t i v i t y i n c r e a s e s and t r i b o l o g i c a l p r o p e r t i e s a r e s i g n i f i c a n t l y improved (low c o e f f i c i e n t o f f r i c t i o n ; s o f t e r with g l a s s
-
-
a s compared
c a r b o n f i b r e s have less e f f e c t on t h e wear o f t h e m a t -
i n g s u r f a c e ) . Another u s e f u l p r o p e r t y o f c a r b o n - f i b r e - r e i n f o r c e m e n t o f polymers i s t h e i r low volume r e s i s t i v i t y and s u r f a c e r e s i s t i v i t y . Carbon f i b r e s a l s o d i s s i p a t e s t a t i c c h a r g e e f f e c t i v e l y . The d i s a d vantage of t h e i r a p p l i c a t i o n i s t h e h i g h e r c o s t . T y p i c a l p r o p e r t i e s o f some g l a s s - r e i n f o r c e d t h e r m o p l a s t i c p o l y m e r s a r e l i s t e d i n Table 2.16.
The i m p o r t a n t e f f e c t s o f g l a s s f i b r e
a d d i t i o n a r e less water a b s o r p t i o n ( e s p e c i a l l y less mold s h r i n k a g e and t h e r m a l e x p a n s i o n .
h i g h f o r PA 6 ) ,
and
TABLE 2.16 TYPICAL PROPERTIES OF 30% GLASS FIBRE
MATE R I A L
-
REINFORCED THERMOPLASTICS
PA 6
POM h
1.37 150
1.63 140 2
PC
PBTP
AB S med i um impact
1.52 135 2-4
1.28 100 3-4
PROPERT 1 ES D e n s i t y , mg/mm’ Tensi le s t r e n g t h , MPa Tensile elongation, % F l e x u r a l modulus o f e l a s t i c i t y , MPa Compress i ve s t r e n g t h ,MPa C o e f f i c i e n t o f thermal expansion, 10-5/~ Thermal c o n d u c t i v i t y , W/m. K S p e c i f i c heat, J/g-K Maximum o p e r a t i n g ternper a t u r e o f b e a r i n g , OC
3-4 8000 150
1.43 130 4-6 8200 150
9500 120
9500 165
1.7
2.4
1.3
1.2
0.34 1.4
0.37 0.9
0.25
0.29
110
100
To improve t r i b o l o g i c a l p r o p e r t i e s
1 .6
1.45 125
3 8200
165 1.4
1.2
1.3 115
7500 100
PPSU
100
60
200
w e a r - r e s i s t a n t thermoplastics
:an a l s o b e i n t e r n a l l y l u b r i c a t e d i n a v a r i e t y of ways. The t y p i c a l and optimum l e v e l of l u b r i c a t i n g
f i l l e r v a r i e s depending o n t h e
t y p e o f f i l l e r and polymer, as f o l l o w s ( r e f . 4 0 ) : PTFE 1 5 - 2 U % ; s i l i c o n e 1 - 5 % ; PTFE/silicone 1 5 - 2 0 % ; g r a p h i t e 10%; MoS2 2-5%. L u b r i c a n t s c a n b e used w i t h o r w i t h o u t g l a s s - o r c a r b o n f i b r e r e i n f o r c e ments. PTFE ( o r sometimes P E ) p a r t i c l e s d i s p e r s e d i n t o a thermo-
30 p l a s t i c r e s i n g r e a t l y improve wear c h a r a c t e r i s t i c s and l e a d t o a d e c r e a s e i n t h e f r i c t i o n c o e f f i c i e n t and t e m p e r a t u r e o f t h e r u b b i n g s u r f a c e s . S i l i c o n e o i l d r o p s (2-10 w) d i s p e r s e d i n a b a s e polymer m i g r a t e t o t h e r u b b i n g s u r f a c e by d i f f u s i o n and b e c a u s e o f exclusion from t h e m a t r i x . The r e s u l t o f t h i s m i g r a t o r y a c t i o n i s t h e c o n t i nuous g e n e r a t i o n o f s i l i c o n e f i l m a s s u r i n g boundary o r mixed l u b r i c a t i o n . G r a p h i t e powder and MoS2 improve t r i b o l o g i c a l p r o p e r t i e s s i g n i f i c a n t l y l e s s t h a n P T F E / s i l i c o n e l u b r i c a n t s . An i m p o r t a n t a r e a of a p p l i c a t i o n f o r g r a p h i t e - l u b r i c a t e d
t h e r m o p l a s t i c s i s i n compo-
n e n t s o p e r a t i n g i n aqueous e n v i r o n m e n t s . S p e c i a l a t t e n t i o n must b e p a i d t o s u c h p o l y m e r i c m a t e r i a l s a s PTFE ( a n d o t h e r f l u o r o p o l y m e r s ) , PE and p o l y u r e t h a n e s ( P U R )
. The
use of t h e s e m a t e r i a l s i s i n e f f e c t i v e w i t h o u t f i l l i n g ( r e i n f o r c e ment) b e c a u s e o f h i g h wear r a t e s and p o o r m e c h a n i c a l p r o p e r t i e s . The e f f e c t o f v a r i o u s f i l l e r s on t h e wear r a t e o f PTFE i s p r e s e n t e d ( r e f . 4 7 ) . PTFE f i l l e d w i t h b r o n z e a n d g r a p h i t e (com-
i n Table 2.17
p o s i t e G l a c i e r DQ1) h a s a p a r t i c u l a r l y l o w wear r a t e . G r a p h i t e a s a f i l l e r ( G l a c i e r DQ2) i s u s e f u l f o r b e a r i n g e l e m e n t s o p e r a t i n g i n l i q u i d s which a r e c o r r o s i v e t o w a r d s b r o n z e . A r e c e n t i n n o v a t i o n i s t h e u s e o f polymer f i l l e r s (e.9. Rulon J c o m p o s i t e ) . Such compsites c a u s e less damage t o s o f t m e t a l s h a f t s ( e . g . b r a s s , aluminium) t h a n PTFE c o n t a i n i n g h a r d f i l l e r s ( e . g . g l a s s f i b r e , a s b e s t o s f i b r e ) AL c o m p o s i t e s
(refs. 4d-50).
(Pampus F l u o r o p l a s t ) a r e f i l l e d w i t h
g l a s s , b r o n z e o r c a r b o n . The d r y c o e f f i c i e n t o f f r i c t i o n o f f i l l e d PTFE r a n g e s from 0 . 1 t o 0.25 depending on t h e s l i d i n g c o n d i t i o n s ( r e f s . 5 0 - 5 2 ) . R e l a t i v e l y l a r g e r u n n i n g c l e a r a n c e s of t h e b e a r i n g s a r e r e q u i r e d because of t h e high thermal expansion c o e f f i c i e n t . TABLE 2.17 EFFECT OF VARIOUS FILLERS ON WEAR RATE OF PTFE
FILLER
CONTENTS
COMMENTS
% 1
2
3 7000-14000
Glass f i b r e
22.2
5.5
G l a s s f i b r e + MoS2 Carzon o r g r a p h i t e
12.2+2.3 26.3
5.0
23.8+2.3
4.0
Bronze
+
MoS2
....................
4.0
4 p=0.233 MPa ( f o r f i 1 l e d PTFE) , P= 4.7 N ( f o r unf i 1 l e d PTFE) ; v=0.75 m / s , Hardness o f m a t i n g s u r f a c e HRC 18-22
..............................................
31 T A B L E 2.17
(continued)
Dry wear o f most PTFE c o m p o s i t e s i s a s s i s t e d by t h e t r a n s f e r of w e a r d e b r i s t o t h e m a t i n g s u r f a c e . I n t h e p r e s e n c e o f w a t e r and most o t h e r l i q u i d s t h i s t r a n s f e r d o e s n o t t a k e p l a c e a n d wear r a t e s may i n c r e a s e ( r e f . 5 3 ) . C o n s e q u e n t l y s u c h materials are s u s c e p t i b l e t o f l u i d c o n t a m i n a t i o n . PTFE c o m p o s i t e s may a l s o e x h i b i t h i g h wear r a t e s under c o n d i t i o n s o f a b r a s i v e w e a r , when a b r a s i v e p a r t i c l e s a r e p r e s e n t and p r e v e n t t h e t r a n s f e r o f wear d e b r i s t o t h e m a t i n g surface. F i l l i n g HDPE w i t h r e l a t i v e l y h a r d m a t e r i a l s r e s u l t s i n less wear. The wear r a t e c o m p o s i t i o n depends on t h e k i n d of f i l l e r a n d properties
( s u c h a s d i s p e r s i t y ) , p e r c e n t a g e c o n t e n t and s u r f a c e
t r e a t m e n t ( r e f s . 5 1 , 5 4 ) . Some wear r a t e d a t a o f f i l l e d HDPE s l i d i n g a g a i n s t s t e e l s u r f a c e ( R = 0 . 6 3 pm) are l i s t e d i n T a b l e 2.18 a ( r e f . 5 1 ) . The optimum p a r t i c l e s i z e f o r i r o n f i l l e r i s 3 0 0 - 6 0 0 Qm; f o r A 1 2 0 3 it i s 2 0 p m . The maximum c o n t e n t o f f i l l e r s h o u l d b e no more t h a n 25% ( b y v o l u m e ) . T A B L E 2.18 WEAR I N T E N S I T Y
OF
HDPE C O M P O S I T I O N S RUBBING A G A I N S T S T E E L ( R a = 0.63 ,urn)
The u s e o f Pb30rl and CuO a s f i l l e r s f o r LDPE or HDPE i s v e r y e f f e c t i v e ( r e f . 4 7 ) . The e f f e c t o f t h e a d d i t i o n o f b o t h f i l l e r s
32
(Pb304 (CuO) is more effective than the addition of a single filler. Filling LDPE gives better results than filling HDPE. The main role of filler in this case is the formation of a transferred film of polymer on the counterface. Polyurethanes (PUR) can be filled with typical fillers (e.g. glass fibre: Urafil, Urasar by Fiberfil Inc.) and can also be modified by the introduction of active epoxide oligomers (ref. 55). The best addition was found to be the oligomer based on allilphenolformaldehyde resin. The modified polyurethane composites can best be used as friction materials, because of their high coefficient of friction (d.9-1.0). Fibre reinforced thermosetting resins offer improved properties as well as cost/performance benefits; such composite materials are strong, demonstrate dimensional stability awl are lighter than most metals. More than 9 5 % of reinforced thermoset parts are based on polyester and epoxy resins; others are phenolics and silicones (ref. 40). Phenolic composites have good tribological properties, particularly those incorporating solid lubricants (e.g. Railko PX) or impregnated with oil. The materials can be used in bearings or gears operating at low loads and sliding speeds, although they are more lolerant of abrasive and fluid contamination (refs. 4 8 , 5 0 ) . High temperature stable polymeric composites such as polyimides, e.g. Vespel or Devaplas, have good tribological properties at elevated temperatures. Materials reinforced with glass or carbon-fibre and containing PTFE as a lubricant, such as polyphenylene sulphide, polyvinylidene fluoride, polyetheretherketone (PEEK), polytetrafluorethylene, polyethersulfone and perfluoroalkoxy-modified tetrafluoroethylene are wear-resistant at high temperatures ( 200-3OO0C) (ref. 5 6 ) . Other polymeric materials with good tribological properties are those which contain grease or are impregnated with oil. Some of the commercially available materials of this kind are oil-filled polyamides (Nylasint, Oilon, Sovietic SAM 3 , SAM 4 andSAM 5 (ref. 57) containing mineral oil and other lubricants) and oil-filled polyacetals (Railko PV 80, SAY 6, SAM 7 ) . When rubbing against steel, these materials demonstrate a low coefficient of friction (ca. 3 . 2 ) and wear intensity (under 0.2 pm/km) when the temperature of the rubbing surfaces ranges from 0 to 12OoC (ref. 57).
33
3 , LUBR I CANTS 3,1, INTRODUCTION It is important to choose the right lubrication for miniature rubbing elements and this is not always straightforward. The requirements f o r instrument lubricants, particularly oils, are as follows : 1) Long-term durability, since fine mechanisms are usually for life lubricated, 2 ) High lubricity, resulting in low coefficient of friction and wear - usually at high specific loads up to 2 5 0 0 MPa and frequently l o w or very lob7 sliding speeds, 3 ) Ageing-resistance should be assisted by low-evaporation, chemical inertness (e.g. against epilame coatings and rubbing materials) and non-migration (high surface tension) from the operating or stored miniature couple during several years of use, since small or very small doses of oils (as little as g ) are introduced during the assembly process of rubbing elements, 4) Low variations of viscosity during ambient temperature changes over a wide range (e.g. from - 5 0 to +2OO0C), 5 ) Resistance to the effect of special environments: climate, gases, vacuum, microorganisms, radiation, industrial or polymer gases or vapours, dust, etc., 6 ) Fulfilling occasional special requirements such as high or very low electrical conductivity when applied to electrical instruments. Oils, greases and solid lubricants are used to lubricate the rubbing elements of instruments. Oils are the most satisfactory lubricants because of their ability to improve the tribologicalproperties of lubricated systems, but their use is sometimes limited by migration and also temperature or other special ambient conditions.
3,2,
OILS
Natural and synthetic oils are used to lubricate miniature elements. Natural oils are mineral-fatty (e.g. neat's-foot oil) oil blends or mineral oils containing many special additives (sometimes
34
as many as 3 0 0 (refs. 5a, 59)) to improve their properties. The traditional clock oils are usually based on mixtures of neat's-foot oil and mineral oil (Fig. 3.1, ref. 60).
NePt'S
0
20
40
- foot
Oil 60
80
100%
Mineral o d
F i g . 3 . l . M i x i n g m i n e r a l and f a t t y o i l s t o produce c l a s s i c c l o c k o i l s . 1 - nonspreading o i l s w i t h good l u b r i c a t i n g p r o p e r t i e s , 2 - o i l s d e m o n s t r a t i n g r e l a t i v e l y good l u b r i c i t y and ageing r e s i s t a n c e used f o r l u b r i c a t i o n o f s t e e l - m i n e r a l systems, 3 - o i l s w i t h good chemical s t a b i l i t y b u t r e l a t i v e l y low l u b r i c i t y , 4 o i Is d e m o n s t r a t i n g r e l a t i v e l y h i g h a g e i n g r e s i s t a n c e and medium l u b r i c i t y b u t h i g h spreading.
-
Oils 1 (Fig. 3.1) have relatively good chemical stability and lubricity but they spread easily. Oils 2 are sufficiently chemically stable, have better lubricity and less tendency to spread; they can be used for the lubrication of, e.g. steel-brass or steel-bronze pivot bearings. Oils 3 demonstrate a lower chemical stability but good lubricity and they are nonspreading. They are useful for lubricating steel-mineral systems. Oils 4 have high lubricity and are nonspreading but are chemically unstable and can be used to lubricate elements made of materials that do not havb a catalytic effect on o i l (e.9. minerals). Such oils contain many stabilizing additives, the most important being, antioxidants. Some commercially available, traditional clock oils are listed in Table 3.1. In general, the disadvantages of traditional clock oils are poor ageing-resistance, great viscosity variations as a function of temperature, narrow temperature range of application and rela-
TABLE 3.1 EXAMPLES OF TRADITIONAL CLOCK OILS
MANUFACTURER
'ROPE RT I ES
01 L
v i scos i t y , (20'~) 2 mm / s , (50OC) ~ pI p i c a t i on temperature range,
a t 100°C,
SORTE 1
100 30
- 18
OC
Pour p o i n t , OC T o t a l a c i d number (TAN), mg KOH/g S a p o n i f i c a t i o n number, mg KOH/g Surface t e n s i o n , mN/m o r s p r e a d i n g (brass, 24 h) ( r e l a t i v e l y ) Evaporation rate, % Age i ng r e s i s t a n c e Appl i c a t i o n ( m a t e r i a 1 s of 1 u b r i ca t e d elements)
- 5 days
VEB TECHNISCHE WACHSE, JENA ( G . D. R)
D r . TILLWICH GmbH, HORB-AHLDORF (F.R.G.)
3
SORTE
SORTE 5
U.S.S.R. MBP-12
-
~~
a
MOEBIUS ET FILS ALLSCHW I L (SWITZERLAND)
b - 16 h a t 105OC,
0.1 160 34.5
l.Sa
-32 0.1
I
1.4a
-
GOST 7934.1-74
1 2.3b 27d Steel, Jewels, Brass, A
2
1.5
80 -31 0.15
b
d
-20 0.70
- 15
50 -20
0.20
74
14d grass, A l , netal s
(4 h a t 5OoC),
61-70 19-22 -10
33 - 18
33.7
Yetals, minerals ( jewe 1 s ) c
80 -13 0.25 180
142
127 31 -15 80 -27 0.22 96
-
31 33 1 .Sb 2.1 9d S i n t e r e d M e t a l s , m i n e r a 1s
0.2c l e t a 1s , ni nerals
I V i s c o s i t y r i s e (%) a t Baader's t e s t , 95OC, Cu. 02, 840 h.
W
ln
36 t i v e l y h i g h e v a p o r a t i o n r a t e s . M o d i f i c a t i o n s such as d e c r e a s i n g t h e p o u r p o i n t o r t h e a d d i t i o n o f MoS2 o r g r a p h i t e t o l u b r i c a t e o f h i g h l y loaded r u b b i n g c o u p l e s a r e p o s s i b l e (Moebius o i l s 8032,8036 (8043) and 8034 (8042) r e s p e c t i v e l y ) . S p e c i a l l y r e f i n e d m i n e r a l o i l s c o n t a i n i n g many a d d i t i v e s a r e a l s o u s e d t o l u b r i c a t e m i n i a t u r e s y s t e m s . Such p a r a f f i n i c o i l s a r e s u f f i c i e n t l y chemically s t a b l e , b u t t h e i r l u b r i c i t y is r e l a t i v e l y poor and t h e y have a s t r o n g t e n d e n c y t o s p r e a d . The i n s t r u m e n t mine r a l o i l s c a n b e a p p l i e d t o t h e l u b r i c a t i o n of i n s t r u m e n t s o p e r a t i n g a t low t e m p e r a t u r e s b u t t h e o p e r a t i o n t e m p e r a t u r e s h o u l d n o t v a r y much b e c a u s e of g r e a t v i s c o s i t y c h a n g e s . The p r o p e r t i e s o f s e v e r a l i n s t r u m e n t m i n e r a l o i l s are p r e s e n t e d i n T a b l e 3 . 2 .
Theoils
D-2 and D-5 a r e from t h e M i c r o g l i s s g r o u p ( b y Moebius e t F i l s ) developed e s p e c i a l l y f o r h i g h p r e s s u r e c o n t a c t s . They c o n t a i n e x t r e m e
p r e s s u r e (E.P.)
and w e a r - r e d u c i n g a d d i t i v e s b a s e d on a molybdenum
o r g a n i c compound c o m p l e t e l y sol.uhle i n m i n e r a l o i l s . They h a v e i m proved o x i d a t i o n s t a b i l i t y as compared w i t h t h e t r a d i t i o n a l c l o c k o i l s . The o i l s o f g r o u p D a r e a p p l i e d w i t h a n o i l e r , w h i l e t h e o i l s g r o u p L a r e s u i t a b l e f o r immersion p r o c e d u r e s . The o i l s of g r o u p L contain an a d d i t i o n a l anticorrosive agent. Pressure r e s i s t a n c e ( t e s t e d using a f i v e - b a l l apparatus) f o r t h e Microgliss group of oils D
+
L i s 230-350 N as compared w i t h 80-250 N f o r t r a d i t i o n a l
clock o i l s ( r e f . 6 1 ) . S y n t h e t i c o i l s a r e v e r y i n t e r e s t i n g l u b r i c a n t s w i t h many a p p l i c a t i o n s i n p r e c i s i o n e n g i n e e r i n g . The main a d v a n t a g e of them i s t h e i r h i g h c h e m i c a l s t a b i l i t y , b u t t h e i r l u b r i c i t y is poor a n d t h e y g e n e r a l l y have a s t r o n g t e n d e n c y t o s p r e a d ( e s p e c i a l l y t h e o i l s b a s e d on p o l y s i l o x a n e s ) . The main g r o u p s of s y n t h e t i c i n s t r u m e n t o i l s a r e t h o s e b a s e d on e s t e r s , p o l y e t h e r s ( e v e n t u a l l y w i t h f r e e a l c o h o l g r o u p s ) , p o l y g l y c o l s and p o l y s i l o x a n e s . Ester-based instrument o i l s generally demonstrate r e l a t i v e l y low c h e m i c a l s t a b i l i t y ( a s compared w i t h o t h e r s y n t h e t i c o i l s ) and t h e y c o n t a i n s t a b i l i z e r s . The main a d v a n t a g e of such o i l s i s t h a t t h e y have good l u b r i c i t y and a r e n o n s p r e a d i n g . The f i r s t s y n t h e t i c i n s t r u m e n t o i l i n t r o d u c e d i n 1928/1929 w a s b a s e d on o r g a n i c phosphate (tricresylphosphate).
T h i s o i l , w i t h good l u b r i c i t y and h i g h
s u r f a c e t e n s i o n , decomposes i n t h e p r e s e n c e o f m o i s t u r e . F r e e phosphate a c i d is a very corrosive agent, attacking steel ( i r o n ) surf a c e s i n p a r t i c u l a r . T h i s kind o f o i l is s u b j e c t t o v i s c o s i t y changes d u e t o t e m p e r a t u r e v a r i a t i o n s ( i n t h e same way a s t r a d i t i o n a l c l o c k o i l s ) . O t h e r e s t e r - b a s e d o i l s a r e more c h e m i c a l l y
TABLE 3.2 INSTRUMENT MINERAL OILS
B MANUFACTURER
OIL
PROPERTIES
v i scos i t y , 2 mm / s ,
(20'~)
(5OoC) ~ p p l i c a t i o nt e m p e r a t u r e range,
I
EXXON
ESSO AVIATION INSTRUMENTAL OIL
MOEB I US ET FILS, ALLSCHW I L
D-2
D-5
(SWITZERLAND)
11
TEY $ ::E
J ENA (G.D.R.)
POLAND, U.S.S.R.
MWP
10.8
6.3-8.5
(3OoC) OC
-50
-50 100
80
Pour p o i n t , O C T o t a l a c i d number (TAN), mg KOH/g Saponi f i c a t i on numbe r, mg KOH/g S u r f a c e t e n s i o n , mN/m Evaporation rate, %
- 57
-32
-18
-60
0.05 20.7 32.1 5 days
19.6 34.5
23.8
a t 100°C
0.83
-- 42
--
s t a b l e and have good t r i b o l o g i c a l p r o p e r t i e s . Some of t h e ester-based i n s t r u m e n t o i l s a r e l i s t e d i n T a b l e 3.3. The o i l S i l b e r C is a p p l i c a b l e t o t h e l u b r i c a t i o n of m i n e r a l e l e m e n t s , b u t S i l v e r B can b e u s e d f o r l u b r i c a t i n g metals ( s t e e l , b r a s s , A l - a l l o y s ,
sintered
m e t a l s ) and m i n e r a l s b u t n o t p o l y m e r i c e l e m e n t s . The o i l s b a s e d upon U . S .
M i l i t a r y S p e c i f i c a t i o n MIL-L-GO85
A a r e e s p e c i a l l y useful
f o r l u b r i c a t i n g miniature b a l l bearings. E t h e r o i l s a r e a g e i n g - r e s i s t a n t b u t t h e i r p r o p e r t i e s change a f t e r long-term e x p o s u r e t o s u n l i g h t . Rubbing e l e m e n t s l u b r i c a t e d w i t h e t h e r o i l s must n o t be h i g h l y l o a d e d . The o i l s have a v e r y low e v a p o r a t i o n r a t e and a r e n o n s p r e a d i n g . Examples o f s u c h o i l s a r e E l g i n M56 a and Iy56 b , w i t h viscosities a t 2OoC of 9 0 and 120 mm 2 / s 0
r e s p e c t i v e l y and a -30 C pour p o i n t . The maximum o p e r a t i n g temperat u r e however i s 80-100°C. -resistant.
P o l y e t h e r o i l s are o x i d a t i o n - and h e a t -
T h e i r v i s c o s i t y c h a r a c t e r i s t i c s as a f u n c t i o n of t e m -
p e r a t u r e are s i m i l a r t o t h o s e o f m i n e r a l o i l s . T h e i r e v a p o r a t i o n
r a t e s a r e l o w b u t s u c h o i l s have h i g h p o u r p o i n t s , i n t h e r e g i o n of -3OOC. Polyphenylether o i l s demonstrate e s p e c i a l l y high thermal s t a b i l i t y , h i g h o x i d a t i o n and r a d i a t i o n r e s i s t a n c e , c h e m i c a l i n t e r t n e s s and r e l a t i v e l y good l u b r i c i t y ( r e f s . 62-64). F l u o r i n a t e d 2 p o l y a l k y l e t h e r w i t h a v i s c o s i t y of 15 mm / s a t 4OoC c a n r e s i s t gamma-ray r a d i a t i o n 0 . 8 MeV a t a d o s e o f 1 0 0 J ( r e f . 64). Fluorinated p o l y e t h e r o i l s have been s a t i s E a c t o r i l y a p p l i e d t o t h e l u b r i c a t i o n of s m a l l motor b e a r i n g s o p e r a t i n g a t a t e m p e r a t u r e o f a b o u t 15U°C f o r s e v e r a l thousand h o u r s ( r e f . 65). The f l u o r i n a t e d p o l y e t h e r
o i l s Krytox 143 AB and K r y t o x 143 AC (Du P o n t p r o d u c t s ) have a 2 v i s c o s i t y of 85 and 270 mm / s ( a t 33OC) r e s p e c t i v e l y . They c a n b e u s e d i n t h e t e m p e r a t u r e r a n g e s -40 - +232OC and -30
-
+288OC re-
s p e c t i v e l y and a r e non-f lammable. P o l y a l k y l g l y c o l e s ( r e f . 66) w i t h e t h e r and a l c o h o l g r o u p s (alkylaryloxydibutyleneglycoles) a r e u n s a p o n i f i a b l e and d e m o n s t r a t e e x c e l l e n t c h e m i c a l s t a b i l i t y . They a r e used a s b a s e m a t e r i a l i n t h e Moebius Synta-A-Lube
s e r i e s of i n s t r u m e n t o i l s . They c o n t a i n
1 . 0 8 - 1 . 8 % of a h i g h e r m o l e c u l a r
synthetic carboxylic acid with
e t h e r t o improve t h e l u b r i c i t y . O t h e r a d d i t i v e s s u c h as a n t i o x i d a n t s , a c o p p e r d e a c t i v a t o r and p r e s s u r e r e s i s t a n c e EP a d d i t i v e g u a r a n t e e t h e i r u s e f u l n e s s . The o t h e r i m p o r t a n t a d v a n t a g e s o f s u c h o i l s a r e r e m a r k a b l e a d h e r e n c e ( n o n s p r e a d i n g ) and e x t r e m e l y low e v a p o r a t i o n r a t e s . The m o s t r e c e n t f o r m u l a o f t h e Synta-?--Lube series of o i l s a l s o a s s u r e s good t r i n o l o g i c a l p r o p e r t i e s i n m i n i a -
TABLE 3.3 ESTER
-
BASED INSTRUMENT OILS
MANUFACTURER
OIL
O r . T I LLW I CH GmbH, HORB-AHLDORF (F.R.G.)
SILBER
PROPERTIES
v i scos i t y , (20'~) 2 mm / s , (50°C) A p p l i c a t i o n temperature range,
c
I
-5 80
-15 0.27
198 0.3 30
I
ESSO
AEROSHELL FLUID 12?
UNlVlS P l p
NUODEX I N C . PISCATAWAY, NEW JERSEY
ANDEROL L401
26
27
24
-50 120
-60 120
-54
90
-44
-57
75
33 (40°C)
29
Pour p o i n t , OC T o t a l a c i d number (TAN), mg KOH/g S a p o n i f i c a t i o n number, mg KOH/g E v a p o r a t i o n r a t e , % (16 h a t 105OC) Ageing r e s i s t a n c e ( v i s c o s i t y r i s e , %) (Baader's t e s t 95OC, Cu, 02, 840 h )
SILBER B
110
99 OC
VEL*
SHELL
19
-35
-20 100 -35 0.30 173 1.8
149 -63
0.25 225 1.9
3
2.2
I
* VEL
o i l demonstrates e s p e c i a l l y h i g h s u r f a c e t e n s i o n (33.3 mN/rn)
,
a - MIL-L-6085A
w W
40 ture systems operating in humid atmospheres (see Chapter 6.5. , Fig. 6.ld). The typical properties of the Moebius synthetic oils are listed in Table 3.4. TABLE 3.4 MOEBIUS ET FILS SERIES OF SYNTHETIC OILS WITH ETHER AN0 ALCOHOL GROUPS
OIL
GF-pxSYNTA-A-LUBE
PROPERT I E S
Dens i t y , mg/mrn 3 v i scos i t y , rnm2/s
0.895
0.888
90 20 SYNTA v I sco LUBE 0.920
9030 S Y NTA FR I GO LUBE
0.908
:
11 - 5 31 - 5 150 625 3800
8OoC 50OC 20oc ooc - 20%
8.9 24
-
-3OOC -4OOC
Maximum o p e r a t i n g , t e m p e r a t u r e , Pour p o i n t , O C S a p o n i f i c a t i o n number, nig KOH/g S u r f a c e t e n s i o n a t 25OC, mN/m Contact angle, 0 : o n ruby on steel Evaporation rate, % (5 days a t 100OC)
OC
266 1450 13500
98 390 2000 5800
-
70
70 -40
14.5
45
6.5 16
58
-
80 -40 0.8 34.8
180 380 2250 9000
60
0.8
-43 0.8
33.8
33.0
22-25 15-19
22-25 15-18
24-29 20-24
14-17
0.5
0.5
0.4
0.5
-50 0.9 32.7 19-23
Synthetic oil based on polysiloxanes are very useful as lubricants because of their high chemical stability, heat resistance, wide range of operating temperature and low evaporation rate. The disadvantages of such oils are low surface tension (spreading) and poor lubricity. The basic chemical formula of polysiloxanes is as f01lows :
CH3
R
CH3
CH 3 I
I
- Cf13 - Si I
CH3
R
CH3
By taking various n numbers, the length of the macromolecule chain can be controlled and as a result the viscosity can be changed without changing the chemical structure of the substance. When R groups are CH3 or phenyl C6H5 radicals, dimethylpolysiloxane and
41 methylphenylpolysiloxane are obtained respectively. In methylalkylpolysiloxane the group R is replaced with the group CHJ
.
I
I
- 0 - S i -
O - S i -
I
I
R
(CH2)
I CH3
The fluorinated CH3 I
O - S i -
CH3 I
or chlorinated
- Si I
I
C2H4
CH 2C1
I
CF3
polysiloxanes demonstrate better tribological and anti-migration properties than dimethyl-, methyl-, phenyl- or methylalkylpolysiloxane (refs. 6 7 - 6 9 ) . Polymeric materials in particular can be very effectively lubricated with polysiloxanes (ref. 70). Several instrument oils based on polysiloxanes are listed in Table 3.5. Mineral base oils containing dispersed polymer particles are also of interest. The clock oil XU 4 3 0 (see Table 3 . 1 ) with PTFE particles ( < 5 um, specific surface 2000 m 2 /kg) and fluorinated alkenylalkylether as dispersal agent, demonstrated very good tribological properties when used to lubricate a steel-diamond tribological system operating in a vacuum 4 Pa (ambient temperature 10°C) (ref. 1 7 ) . We must include among the special lubricating liquids those natural substances which demonstrate extremely high lubricity, reduce effectively friction and wear; they are useful when the lifetime of a tribological system is short and loads are very high. Prostheses are lubricated with synovial fluid, which also demonstrates excellent lubricity in typical metal, mineral and polymeric tribological systems (refs. 11, 71). Another natural lubricant is formed by the microbes of Bacillus mucilaginosus; it is a high-polymeric substance containing 95% polysaccharides and 5 % protein, used e.g. to prepare a surface for calibration of glass vessels. This sticky, dark coloured liquid density 0.976-0.985 mg/mm3 in-
-
TABLE 3.5 INSTRUMENT OILS BASE0 ON POLYSILOXANES
MANUFACTUREd
I
O r . TlLLWlCH GrnbH, HORB-AHLOORF (F.R.G.)
SILICONUL n PROPERT I ES
I
SILICONDL V500
I
I
I
OOW CORNING
I
GENERAL
I KLUBER
I
MUNCHEN (F. R . G)
U.S.S.R.
(UN I S I L )
510
2 V i s c o s i t y , mm / s :
5OoC
82
20oc ooc -6OOC Pour p o i n t , OC A p p l i c a t i o n t e m p e r a t u r e range, OC
Evaporation rate, %
368
52 (38OC) 50 (25OC
46 5
I
1026
2
-6 7 -49 180 180 a t 105OC, 16h
0.1
0.2
-73 -68 204
300
I
1
-70 upon GOST 7934.1 a t maximum operating temperature d u r i n g
’’
I
4.5
I
3.5
1
2.2
43 creased the maximum admissible load of a metallic steel-steel tribological system by 3-4 times as compared with the best of the normal lubricants (ref. 72). The properties of the base oil are improved by the use of special additives. They are soluble o r uniformly dispersed throughout the carrier medium. The most important are antioxidants, which minimize the formation of resins, varnishlacidsf sludge and polymers. These are substances such as phenols, amines, organic sulphides, zinc dithiophosphate. The friction modifiers (fatty acids, fatty amines, fats) and anti-wear additives (zinc dialkyldithiophosphate, tricresyl phosphate) are very important for improving the lubricity of an oil. Oil-soluble fluorinated telomers,especially fatty acids and their amine salts, are very effective as anti-wear additives, forming a reaction film containing iron fluoride (ref. 73). The viscosity index improvers such as polyisobutylenes, polymethacrylate and polyacrylates, and extreme pressure additives such as sulphurized fats, olefins, chlorinated hydrocarbons, lead salts of organic acids, and amine phosphates also improve the oil. The friction reducers improve boundary lubrication and oil-film retention (even after draining); they also reduce frictional losses (ref. 7 4 ) . The complex soluble molybdenum compounds are very useful for improving ageing- and wear-resistance and can also act as extreme pressure additives (ref. 75). These additives, used instead of MoS2 in the Moebius instrument oils of Microgliss group D + L based on stabilized mineral oils, ensure very good load carrying qualities in the oil. For the best effect, other additives such as corrosion inhibitors (zinc dithiophosphates, sulphurized terpenes, phosphosulphurized terpenes, sulphurized olefins), rust inhibitors (amine phosphates, fatty acids etc.), and dispersants to prevent and retard sludge formation, must be added very carefully, bearing in mind which additives have already been put in (refs. 6 2 , 76). Instrument oils are usually very complex substances and to produce them requires a great deal of experience; there are only a few specialized producers, and these are mostly small firms. Instrument oils are normally used for the lubrication of metal and mineral rubbing elements, but special oils are manufactured for the lubrication of polymeric elements. They demonstrate high oxidation stability and are chemically inert to almost all the thermoplastic polymers commonly used as materials for miniature rubbing elements (ref. 7 ’ 7 ) . These synthetic oils are based on e.g. alkylaryloxybuthyleneglycols (Moebius oils), partially fluorinated poly-
44
e t h e r s o r p o l y s i l o x a n e a l c o h o l ( D r . T i l l w i c h GmbH o i l s ) . The p r o p e r t i e s o f some o f t h e s e o i l s a r e l i s t e d i n T a b l e 3.6.
The a p p l i c a t i o n
o f a n t i - m i g r a t i o n c o a t i n g s ( e p i l a m e s ) i s a d v i s a b l e when t h e s e o i l s a r e t o b e used (see C h a p t e r 6 . 2 ) . F i x o d r o p KlLI
Examples o f s u c h e p i l a m e s a r e
(Moebius) a n d A n t i s p r e a d M2/20U
(Dr.
T i l l w i c h GmbH).
The o i l f o r t h e l u b r i c a t i o n o f a m i n i a t u r e s y s t e m o p e r a t i n g u n d e r e x t r e m e c o n d i t i o n s must b e c h o s e n c a r e f u l l y ( s e e a l s o C h a p t e r 6 . 5 ) . When t h e t e m p e r a t u r e i s v e r y low, s a y below -2OoC,
t h e most i m p r t a n t
factor which s h o u l d b e t a k e n i n t o c o n s i d e r a t i o n i s t h e rise i n v i s c o s i t y o f t h e o i l w i t h d e c r e a s i n g t e m p e r a t u r e . The o i l s b a s e d on p o l y s i l o x a n e s a r e a p p l i c a b l e a t low t e m p e r a t u r e s (see T a b l e 3 . 5 ) , and s o a r e some e s t e r - b a s e d and m i n e r a l o i l s ( T a b l e s 3.3 a n d 3 . 2 ) . Compounds s u c h a s RCOOCaHl1
and RCH2OCOC8Hl1
i n t h e molecule, demonstrate little m
having r a d i c a l s R as
e i n v i s c o s i t y as a f u n c t i o n
o f t e m p e r a t u r e , a n d t h e y h a v e a v e r y low p o u r p o i n t ( f r o m - 7 3
to
-5OOC) and r e l a t i v e l y low v i s c o s i t y ( r e f . 7 6 ) . The compound w i t h t h e summarized f o r m u l a C 1 5 H 2 6 0 2
h a s a p o u r p o i n t o f -66OC
r a t i o o f t h e v i s c o s i t i e s a t -40°C
and 100°C
and a
n o t h i g h e r t h a n 55.
T h e s e compounds seem v e r y p r o m i s i n g as b a s e l i q u i d s f o r low-temper a t u r e i n s t r u m e n t o i l s . O i l s o b t a i n e d by t h e r e a c t i o n o f a - o l e f i n s C6 - C l0
o r t h e i r mixture with c y c l i c aromatic
h y d r o c a r b o n s C6-Cl0
o r t h e i r m i x t u r e w i t h h y d r o c a r b o n s C6-Cl0
having a l k y l g r o u p s , have 2 a pour p o i n t of a b o u t -5lOC and a v i s c o s i t y h i g h e r t h a n 5 mm / s a t 9 9 O C and l o w e r t h a n
2 8 0 0 0 mm / s a t -4OOC
(ref. 79).
High t e m p e r a t u r e i n s t r u m e n t o i l s m u s t b e e f f e c t i v e f o r l u b r i c a t i n g m i n i a t u r e s y s t e m s when t h e o p e r a t i n g t e m p e r a t u r e i s h i g h e r t h a n 150OC. T h i s r e q u i r e m e n t c a n b e f u l f i l l e d b y o i l s b a s e d o n p l y s i l o x a n e s and p o l y e t h e r s . Some o i l s b a s e d o n p o l y s i l o x a n e s c a n be
used when t h e t e m p e r a t u r e r e a c h e s 230-250°C
( s e e T a b l e 3 . 5 ) . The
p r i n c i p a l problem w i t h s u c h o i l s i s f i n d i n g a d d i t i v e s t o s t a b i l i z e them a t h i g h t e m p e r a t u r e s , w h e r e c l a s s i c a l a n t i o x i d a n t s s u c h a s a l k y l p h e n o l s do n o t work. The t h e r m a l l y s t a b l e a d d i t i v e s (e.g. mtal-silicone-organic
compounds) w e r e found t o improve t h e o x i d a t i o n 0
r e s i s t a n c e of p o l y s i l o x a n e s a t t e m p e r a t u r e s o v e r 2 0 0 C o v e r lotimes ( r e f . 8 0 ) . Such a d d i t i v e s a r e u s e d i n t h e S o v i e t o i l s l i s t e d i n T a b l e 3.5.
F l u o r i n a t e d o i l s s u c h a s V e r s i l u b e F 5 0 , FS 1265 ( T a b l e 3 . 5 ) ,
K 7132 (Table 3.6) are useful lubricants because of their g o d l u b r i c i t y even a t
TABLE 3.6 SPECIAL INSTRUMENT OILS FOR LUBRICATION
OF POLYMERIC MINIATURE SYSTEMS
OIL
MANUFACTURER
PROPERT I E S Viscosity,
2 mm / s ,
I
OOC
20oc
I
4OoC Pour p o i n t , OC A p p l i c a t i o n t e m p e r a t u r e range,
OC
S u r f a c e t e n s i o n , mN/m Evaporation rate, % Ageing r e s i s t a n c e (Baader's t e s t D I N 51554), %
K 7132 mv
K 7132
K 2363
10000
K 2363 21000
O r . TlLLWlCH GmbH, HORB-AHLDORF
1250 434 123 (50°C)
49776 10328 4890
820 510
350
K 4563 2400
1
(F.R.G.)
24000 20700 14100
9015
4200 2400 1500
9024
MOEBIUS ET FILS, ALLSCHWIL (SWITZERLAND)
1450 266
625 150
31 (at
-44
- 26
-40
-20
200
200 28
-41 -30 27 120 120 20.6 20.9 a t 1050C. 16h -33
-
-62 -40 120 21
9027
-40 - 29 70
45 5OoC)
7600 1040 130
-40
33.8
-20
-18
-7
90
80
34.8
35.5
a t 1OOOC, 5 days
0.31
0.5
1
2
1 1.'" I
( a f t e r 12 days)
0.4 2
46 high temperatures. Polyether o i l s a r e t h e m o s t s t a b l e a t higher t e m p e r a t u r e s b u t t h e aforementioned c o n s t r a i n t s on t h e i r u s e s h o u l d b e t a k e n i n t o c o n s i d e r a t i o n . The e l e m e n t s o f m i n i a t u r e s y s t e m s l u bricated with these high temperature o i l s should be epilamized with f l u r o p o l y m e r - b a s e d e p i l a m e s (see C h a p t e r 6 . 2 ) . I t i s p r a c t i c a l l y i m p o s s i b l e t o u s e t h e above-mentioned i n s t r u -
ment o i l s when t h e o p e r a t i n g t e m p e r a t u r e i s o v e r 25OoC. I t i s poss i b l e t h a t t h e problem o f t h e o i l l u b r i c a t i o n o f m i n i a t u r e s y s t e m s working a t h i g h e r t e m p e r a t u r e s may b e s o l v e d by u s i n g s y n t h e t i c some f l u o r i n a t e d esters
s i l a h y d r o c a r b o n s t h e r m a l l y s t a b l e t o 37OoC,
( s t a b l e t o 30OoC) , p a r t i c u l a r l y esters p r o d u c e d u s i n g f l u o r i n a t e d a l c o h o l s and t r i e t h y l a c e t i c a c i d , f l u o r i n a t e d p o l y a l k y l e t h e r s , o r f l u o r t r i a z i n e compounds t h e r m a l l y s t a b l e t o 34OoC ( r e f . 8 1 ) . A l k y l n a p h t h a l e n e s a r e p r o m i s i n g components f o r h i g h t e m p e r a t u r e o i l s b e c a u s e t h e y h a v e h i g h c h e m i c a l s t a b i l i t y and g e n e r a l l y improve o i l s i n u s e under extreme c o n d i t i o n s ( r e f . 8 2 ) . O i l s o p e r a t i n g i n a vacuum s h o u l d h a v e a v e r y low e v a p o r a t i o n
r a t e . The p o l y e t h e r o i l s a r e o f most i n t e r e s t i n t h i s r e s p e c t , b u t o i l s based on p o l y s i l o x a n e s o r esters a r e a l s o u s e f u l ( r e f s . 9 , 83, 84, 85; see a l s o C h a p t e r 6 . 5 ) . The o i l t4n-605 l i s t e d i n T a b l e 3 . 5 c a n b e u s e d t o l u b r i c a t e m i n i a t u r e s y s t e m s o p e r a t i n g i n a vacuum P a ) a n d low o r h i g h t e m p e r a t u r e c o n d i t i o n s .
( t o 1.33
A e o r s h e l l F l u i d 1 2 and A n d e r o l 4 0 1 D ( T a b l e 3 . 3 ) , m e e t i n g t h e MIL-L-6085
A s t a n d a r d , are s u i t a b l e f o r t h e l u b r i c a t i o n o f s p a c e
i n s t r u m e n t a t i o n . The i n s t r u m e n t o i l "Gold" ( D r . T i l l w i c h GmbH Horb-Ahldorf
,
F .R.G.)
has s i m i l a r properties
t e m p e r a t u r e r a n g e of - 7 0 ,
, with
,
an application
+8OoC, and i s u s e f u l f o r t h e l u b r i c a t i o n
of m e t a l , m i n e r a l and p o l y m e r i c
( e x c e p t ABS) m i n i a t u r e r u b b i n g el-
m e n t s . The a f o r e m e n t i o n e d XU 430 o i l f i l l e d w i t h PTFE p a r t i c l e s has been used e f f e c t i v e l y i n s p a c e a s a l u b r i c a n t i n a steel-diamond t r i b o l o g i c a l system i n a F o u r i e r ' s i n t e r f e r o m e t e r
(refs. 17, 86).
O i l which w i l l b e e x p o s e d t o r a d i o a c t i v i t y m u s t b e c a r e f u l l y
c h o s e n . The p o l y e t h e r - b a s e d o i l s a r e most s u i t a b l e when gamma-radia t i o n a n d h i g h t e m p e r a t u r e s o c c u r , f o r example i n t h e i n s t r u m e n t a t i o n of n u c l e a r i n s t a l l a t i o n s
( r e f s . 9 , 63, 6 4 ) . Such o i l s a r e a l s o
s a f e when e x p l o s i v e s u b s t a n c e s a r e p r e s e n t
( f o r example i n oxygen
h a n d l i n g pumps). I n t r o p i c a l climates w h e r e m i c r o o r g a n i s m s a r e p r e s e n t a n d t h e h u m i d i t y is h i g h , t h e u s e of h u m i d i t y - i n e r t o i l s s u c h a s t h e new Moebius Synta-A-Lube
9 0 1 0 , 9 0 2 0 , 9030 ( s e e T a b l e 3 . 4 )
T i l l w i c h GmbH K 4563 o i l ( s e e T a b l e 3 . 6 )
,
or Dr.
i s recommended. The MCT
47 and MPT s e r i e s o f o i l s from t h e U.S.S.R. 6.5)
,
( r e f . 8 7 , see a l s o C h a p t e r
c o n t a i n a c t i v e f u n g i c i d e s making them r e s i s t a n t a g a i n s t m i c r o -
o r g a n i s m s . F o r s y n t h e t i c o i l s u s e d i n c o n d i t i o n s of h i g h h u m i d i t y o r i n t h e p r e s e n c e of w a t e r , t h e h y d r o l y s i s o f complex esters a n d a d d i t t i v e s i s a s e r i o u s t h r e a t ( r e f . 8 8 ) , a s it d e c r e a s e s t h e a n t i - f r i c t i o n and a n t i - w e a r a c t i o n of t h e o i l s . The o i l s u s e d f o r t h e l u b r i c a t i o n o f e l e c t r i c a l c o n t a c t s , s u c h
as c o n n e c t o r s , s w i t c h e s or p r i n t e d c i r c u i t b o a r d s (see C h a p t e r 9 . 8 ) must d e m o n s t r a t e e x c e p t i o n a l l y good l u b r i c i t y , h i g h o x i d a t i o n - r e s i s t a n c e , non-migration
from t h e c o n t a c t s and a l s o e l e c t r i c a l re-
s i s t i v i t y higher than 1 5 M Q . m .
O i l c a n n o t improve t h e r e s i s t a n c e
of c o n t a c t s ( r e f s . 8 9 - 9 1 ) . The o i l s h o u l d n o t p o l y m e r i z e b e c a u s e o f t h e c a t a l y t i c e f f e c t of m e t a l s ,
i n p a r t i c u l a r p l a t i n u m . When p o l y -
m e r i c materials a r e used, t h e o i l c a n n o t p l a s t i c i z e t h e material. The p o l y e t h e r o i l s s u c h as 0s-124
(by Monsanto) w e r e f o u n d t o a t -
t e n u a t e t h e r e s i s t a n c e of p a l l a d i u m c o n t a c t s and t h e y c a n o p e r a t e i n a c o r r o s i v e atmosphere ( r e f s . 6 4 , 92).The s p e c i a l s y n t h e t i c o i l s f o r e l e c t i c a l c o n t a c t s m a n u f a c t u r e d by DODUCO K G d e m o n s t r a t e v e r y good p r o p e r t i e s ( r e f s . 9 0 , 9 3 ) . The p r o p e r t i e s o f t h e s e o i l s a r e l i s t e d i n Table 3.7.
The p r o b l e m o f t h e p r o p e r l u b r i c a t i o n of elec-
t r i c a l c o n t a c t s i s , however, f a r from s o l v e d ( r e f . 9 4 ) . TABLE
3.7
SPECIAL DODUCONTA - OILS FOR LUBRICATION OF ELECTRICAL CONTACTS MANUFACTURED BY DODUCO KG, D r . F. OURRWACHTER, PFORZHEIM (F.R.G.)
E=-2
610
PROPERT I ES
V i s c o s i t y , mPa.s 2ooc
8OoC Pour p o i n t , O C Flash p o i n t , OC R e s i s t i v i t y a t 20°C, GRVmm Temperature c o e f f i c i e n t o f r e s i s t i v i t y (0-80 'C), 10-2K-1 App 1 i c a t i o n
405
235
32 -35 230
21
-40
-55
-60
238
270
5
60
200
220 100
2 ;ene r a 1, i i g h wear-decreasing iroperties
L
47
21
1 Contact 1oad 0.1-5N. metallic and p o l y m e r i c contact
Contact 1oad 0.1-2N. noble metals
Contact load l e s s t h a n 0.2N, ga 1v a n i zed metals
An i m p o r t a n t p r o b l e m i s t h e damping o f o s c i l l a t i o n (or i m p a c t s p r o d u c i n g n o i s e ) i n m e a s u r i n g i n s t r u m e n t s , q u a r t z - c l o c k mechanisms
48
e t c . The p r o p e r t i e s of a h i g h v i s c o s i t y o i l s u i t a b l e f o r t h i s p u r p o s e a r e l i s t e d i n T a b l e 3 . 8 . T h i s o i l c a n a l s o b e u s e d t o lubricate p o l y m e r i c m i n i a t u r e s y s t e m s . The u s e of K 7132/10000 a n d K 2363/2100 ( T s b l e 3 . 6 ) o i l s i s a l s o recommended. The e s t e r - b a s e d o i l A n d e r o l 4 0 1 D ( T a b l e 3.3) c a n be used as a shock a b s o r b e r f l u i d . TABLE 3.8 PROPERTIES OF SPECIAL DAMPING INSTRUMENT OIL D 861 ( D r . -AHLDORF, F.R.G)
TlLLWlCH GmbH, HORB-
PROPERTIES
2 V i s c o s i t y , mrn / s ,
4565 1375 5 23 -3 1
OOC
20oc 40OC
Pour p o i n t , OC A p p l i c a t i o n temperature range,
- 20
OC
90 S u r f a c e t e n s i o n , mN/m Evaporation rate, 105OC/16h, % Ageing r e s i s t a n c e (Baader's test, Base f l u i d
34.6
1 .14
DIN 51554),%
1.2 Polyoxyalkylene
O p e r a t i n g c o n d i t i o n s must b e t a k e n i n t o c o n s i d e r a t i o n when sel e c t i n g a n o i l , s i n c e most m i n i a t u r e s y s t e m s o p e r a t e u n d e r boundary o r mixed l u b r i c a t i o n . When hydrodynamic l u b r i c a t i o n i s r e q u i r e d , t h e v i s c o s i t y o f t h e o i l needed c a n b e c a l c u l a t e d ( s e e C h a p t e r 9.2). When t h e s l i d i n g s p e e d i s h i g h e r and t h e c o n t a c t p r e s s u r e i s l o w , a lower o i l v i s c o s i t y c a n b e c h o s e n . When t h e s y s t e m u n d e r c o n s i d e r a t i o n w i l l b e o p e r a t i n g u n d e r boundary o r mixed l u b r i c a t i o n , t h e f o l l o w i n g f o u r groups o f f a c t o r s s h o u l d b e t a k e n i n t o consideration: t h e s t r u c t u r e o f t h e t r i b o l o g i c a l s y s t e m and e x t e r n a l l o a d ( s l i d i n g speed, c o n t a c t p r e s s u r e , n a t u r e o f t h e motion
-
see C h a p t e r 1 ) ;
ambient c o n d i t i o n s ( t e m p e r a t u r e , humidity, e t c . ) ; " t i m e f a c t o r s " ( p e r i o d o f o p e r a t i o n o f t h e system i n r e l a t i o n t o p e r i o d s o f idlen e s s ) ; and s p e c i a l r e q u i r e m e n t s f o r t h e o i l ( r e f . 9 5 ) . G e n e r a l recommendations f o r t h e s e l e c t i o n o f a n i n s t r u m e n t o i l f o r t h e l u b r i c a t i o n of a m i n i a t u r e t r i b o l o g i c a l s y s t e m a r e l i s t e d i n T a b l e 3.9 ( b a s e d o n r e f . 9 5 ) . The m o s t i m p o r t a n t c r i t e r i a i n t h e s e l e c t i o n of a n i n s t r u m e n t o i l are a l w a y s t h e l u b r i c i t y , a g e i n g r e s i s t a n c e a n d
l i t t l e change i n v i s c o s i t y as a f u n c t i o n o f t e m p e r a t u r e v a r i a t i o n s . The a b i l i t y o f t h e o i l t o d e c r e a s e f r i c t i o n and wear canbe estimated from t h e a n a l y s e s o f l u b r i c a t e d s y s t e m s g i v e n i n C h a p t e r 5 , by cons u l t i n g t h e m a n u f a c t u r e r o r , i d e a l l y by c a r r y i n g o u t t r i b o l o g i c a l i n v e s t i g a t i o n s on t h e a c t u a l t r i b o l o g i c a l s y s t e m t o b e l u b r i c a t e d .
T A B L E 3.9 R E L A T I O N S H I P S BETWEEN T H E P R O P E R T I E S OF INSTRUMENT O I L S AND O P E R A T I N G C O N D I T I O N S I N M I N I A T U R E SYSTEM ( r e f . 9 5 ) P R O P E R T I E S OF 011
VISCOS I T Y r) mm2/ S
3t OPERATING CONDITIONS
Contact pressure, MPa t o 100 over 100 S l i d i n g speed, m/s t o 0.02 over 0.02 Shearing f o r c e , mN t o 10 a v e r 10 ournal diameter ( i n j o u r n a l b e a r i n g s ) , nnn r a d i a l clearance
ro 20 + + + + +
consumption 2 (see Chapter 6 . 3 ) verage o p e r a t i n g temperature,
OC
+ +
!
,
AGE I NG EVAPORAR E S I S T A N C E T I O N RAT1
- -OVER TO OVER OOC)
20 -
).3
iIGH
0.3 -
LOW
TO 1
6 -
;PREAD I NG
-
DVER .O 1 -
3VER
+ +
+
+
+
+ +
+
+
+
+ +
+
+
+
+
+
+
2 - -
1.5 3.5 +
+
+
+
+
+
+
+
+ +
+
+ +
+
L I F E T l ME years
- ro LOW
I_
+
+
LUBR I C I T Y
%
OVER 2
-
+ + +
+
+
+
+ +
+
4-
+ +
loads, number o f g ( a c c e l e r a t i o n o f
I””‘
+
+
+
R a t i o o f period of actual o p e r a t i o n (use) t o period o f e x p l o i t a t i o n ( t o t a l l i f e t i m e )
---
+
--
+
+
+
+
+ +
I
+
+
-- -
+
Oils with very low surface tension (say below 30 mN/m) have a tendency to migrate from the miniature system. The use of anti-migration coatings (epilamizing) is then called for (see Chapter 6.2). The autoepilamizing oils can also be considered (Chapter 6.2.5). Another possibility is a special lubrication procedure involving the addition of a special oil to the last rinse during the cleaning of the microelements. This oil can adhere to the lubricated surfaces as a partly plastic lubricant layer and in this way a lasting lubricating effect is obtained (ref. 96). The oil, Miracle-Plastic, consists of two liquids which are added to the last two rinsing baths. The first liquid is a mineral oil dissolved in solvent (higher boiling point benzine). The second liquid is silicone oil (polydimethylsiloxane). Silicone oil is non-soluble in the retained lubricant, and consequently, remains on the lubricated surfaces after the two films dry one on top of the other. One actually produces the lubricating properties and the other one the protective layer, which reduces wear and tear of the functional film. Soviet o i l s of the OKB-122 series, (see Chapter 5.4) are suitable for this kind of two-layer lubrication (refs. 97, 9 8 ) and give a relatively good nonspreading effect: they are a mixture of mineral and silicone oils. In comparison with conventional lubrication methods, this method saves manufacturing time but the whole element becomes covered with a thin oil film. Oils such as polyphenyl ethers can be applied to the cleaned surfaces (of e.g. electrical contacts, magnetic media - see Chapters 9 . 8 and 10 respectively) by immersion and withdrawal at room temperature from a e.g. 0.5% solution in l,l,l-trichloroethane. The solvent quickly evaporates, leaving a thin residual film of lubricant. Non-spreading oil can be obtained by the addition of small particles of magnetic material such as magnetite Fe304 to a typical oil. Magnetite powder with granules measuring 15-30 nm is often used in magnetic fluids such as those used in seals and hardly affects the anti-wear properties of the oil (liquid). A drop of oil containing 10% (by weight) of Fe304 and 1-5% (by weight) of a surface active agent for stabilizing the mixture in a magnetic field, does not spread (refs. 99, 100). The contact angles of instrument oils treated in this way and placed on a steel surface (Ra=i).32 um) were ca. 22, 24, 23 and 33O for the dimethyl-, methylphenyl-, methylchlorophenyl- polysiloxanes and fluorinated polyether respectively and were constant for a long period (about 72 h). The drop of oil was kept in a magnetic field with a strength of 300 mT with the
51 v e c t o r of f i e l d i n t e n s i t y p e r p e n d i c u l a r t o t h e s u r f a c e o f t h e s t e e l p l a t e on which t h e d r o p was p l a c e d . T r i b o l o g i c a l t e s t s c a r r i e d o u t w i t h t h e 5 - b a l l a p p a r a t u s i n a magnetic f i e l d w i t h a s t r e n g t h o f 500 mT a t a t e m p e r a t u r e o f 20OoC and w i t h l u b r i c a t i o n by p o l y e t h e r o i l c o n t a i n i n g magnetic p a r t i c l e s , have shown
t h a t the operating
p e r i o d b e f o r e s e i z u r e was t w i c e a s long a s f o r t h e same o i l where t h e r e was no magnetic f i e l d . The t h e r m a l decomposition o f t h e polyether o i l
+
magnetic p a r t i c l e s
b e g i n s a t a b o u t 20OoC.
+
s u r f a c e a c t i v e a g e n t composition
Drops of s y n t h e t i c o i l s based on plysiloxanes
o r p o l y e t h e r s which a r e f i l l e d w i t h magnetic p a r t i c l e s and p l a c e d i n magnetic f i e l d do n o t s p r e a d when t h e s u b s t r a t e ( e . g . r u b b i n g element) i s made o f s t e e l o r g l a s s ( r e f . 9 9 ) . The i n c o m p a t i b i l i t y o f some o i l s and t h e m a t e r i a l s used f o r t h e elements i n m i n i a t u r e system i s a complex problem. The c a t a l y t i c e f f e c t and r a p i d o x i d a t i o n of o i l s i n t h e p r e s e n c e of c o p p e r , e s p e c i a l l y when wear d e b r i s i s p r e s e n t , is t h e main problem and t h i s can be s t u d i e d by u s e o f B a a d e r ' s t e s t f o r example ( s e e Chapters 6.4 and 8 . 5 . 4 ) .
The e f f e c t o f copper on o i l d e g r a d a t i o n t h r o u g h c a t a -
l y t i c o x i d a t i o n i s due t o t h e a b i l i t y o f i t s i o n s t o d i s s o l v e i n o i l more r e a d i l y t h a n i o n s o f o t h e r m e t a l s ( r e f . 1 0 1 ) . The e f f e c t s of i r o n , N i
-
C r on t h e o x i d a t i o n p r o c e s s a r e a l s o c o n s i d e r a b l e ,
although 1Ji
-
C r and aluminium have l e s s e f f e c t on o i l . d e g r a d a t i o n
t h a n copper o r i r o n because N i forms A 1 2 0 3 ,
-
C r a p p e a r s a s an a l l o y a n d a l m i n i u m
which has an a m p h o t e r i c c h a r a c t e r and r e a c t s w i t h
s t r o n g a c i d s a s a b a s e ( r e f . 1 0 1 ) . I n g e n e r a l , t h e harmful e f f e c t s caused by s i n g l e m e t a l s d e c r e a s e when o t h e r m e t a l s a r e p r e s e n t . The c a t a l y t i c d e t e r i o r a t i o n o f i n s t r u m e n t o i l s i s v e r y danger when v i r g i n s u r f a c e s of m e t a l s a r e produced b e c a u s e of t h e i n t e n s i v e wear ( r e f . 1 0 2 ) . When f r e t t i n g i s e x p e c t e d t h e t r i b o c o r r o s i v e e f f e c t s and any complex t r i b o c h e m i c a l r e a c t i o n s ( r e f . 103) r e s u l t i n g i n t h e o i l d e t e r i o r a t i n g s h o u l d be a n a l y s e d . S t u d i e s of f r e t t i n g i n s t e e l - s t e e l systems when m i n e r a l and s y n t h e t i c o i l s were used show t h a t t h e r e i s 1 e s s . t r i b o c o r r o s i o n when m i n e r a l o i l s ( i n p a r t i c u l a r p a r a f f i n i c ) a r e used
(ref. 104).
The h i g h e r t r i b o c o r r o s i o n a t f r e t t i n g when p o l y s i l o x a n e o r p l y s i l o x a n e o r p o l y e t h e r o i l s a r e used is a t t r i b u t e d t o t h e r i g i d molecules w i t h h i g h mesomerism and t h e poor l u b r i c i t y o f such o i l s . The t r i b o c o r r o s i o n a t f r e t t i n g i s h i g h e r when t h e boundary l a y e r of t h e l u b r i c a n t i s n o t formed o r t h e a d s o r p t i o n o f m o l e c u l e s i s poor. E s t e r o i l s can b e p l a c e d between m i n e r a l and p o l y s i l o x a n e o r poly-
e t h e r o i l s . Increasing t h e v i s c o s i t y of t h e o i l decreases t h e t r i -
52
bocorrosion. This effect is particularly characteristic of mineral oils. The use of polyether or polysiloxane oils for lubrication of zinc phosphated steel elements increases the tribocorrosion as compared with the tribocorrosion of unlubricated systems or those lubricated with mineral oils. Oils containing MoS2 are no use since an increase in the quantity of MoS2 in the oil increases the tribocorrosion. When mineral and ester based oils operate under boundary lubrication conditions in the presence of oxygen, products of oxidation are produced which react with an iron surface at temperatures considerably lower than boundary (or mixed) lubrication temperatures (ref. 1 0 5 ) . These processes can lead to the formation of polymrized oxidation sludge, cause corrosive wear and influence reactions of other surface-active species. These effects, although generallydeleterious, could conceivably be beneficial in some circumstances, for example under extreme pressures (refs. 105, 106). When polymeric systems are lubricated, the complex tribophysical and tribochemical effects at the interface during rubbing cause the oil to deteriorate (refs. 77, 107, 103, see also Chapter 6.6). The manufacturer will usually indicate which instrument oils are compatible with the various polymers. The special oils used for lubricating polymeric miniature systems are compatible with most polymers used in practice. However, the ester oils are incompatible with nitriles; for example, the use of ABS material is not recommended when such oils are used. The solubility parameters of the oil and polymsr material used should be different. The solubility parameter of oil can be roughly determined using eqn. ( 8 . 4 4 ) or by multiplying the dielectric constant of the oil by 6.73 (ref. 109) where the solubility parameter is expressed in 1 0 ~ ~ ~ 1 ~ m - 3The 12. dielectric constant of an oil is very important when selecting a lubricant for a particular metal-polymer or polymer-polymer system (see Chapter 5.2). The method of determining the dielectric constant of instrument oils is described in Chapter 8.5.3 and in ref. 110. Values of the dielectric constant as a function of temperature for several instrument oils are presented in Figs. 3.2 and 3.3. Variations in the dielectric constant can be used as a control parameter for estimating the ageing of instrument oils (ref. 110). Minerals oils and other combustible oils are incompatible with high-pressure oxygen: special fluorinated lubricants must be used in breathing apparatus (ref. 76). Sometimes, for example during the manufacture of f o o d s t u f f s , p h a r m a c e u t i c a l s and chemicals, even the
53
smallest l e a k o f o i l may be u n a c c e p t a b l e . I n s u c h c a s e s t h e u s e o f a p r o c e s s f l u i d as a l u b r i c a n t may b e c o n s i d e r e d ; t h i s c a n eliminate t h e need f o r s e a l s and g l a n d s . The a n t i s t a t i c a g e n t s i n c o r p o r a t e d i n t o p o l y e t h y l e n e o r polyamide f i l m s u s e d i n packing have been shcwn t o have a d v e r s e e f f e c t s on c o n t a c t w i t h p r e c i s i o n m i n i a t u r e b e a r i n g s and t h e i r l u b r i c a n t s ( r e f . 111).
w
Temperature,
OC
F i g . 3.2. D i e l e c t r i c c o n s t a n t (& o f i n s t r u m e n t m i n e r a l (4,6) and c l a s s i c c l o c k Moebius 8000, 2 - Moebius 8030, 3 - XU 120, Is (1,2,3,5) v s . temperature. 4 - Moebius 0-5, 5 - XU 430, 6 - MWP.
oi
-
M i n e r a l s o i l s and c l a s s i c c l o c k o i l s based on m i n e r a l and f a t t y o i l m i x t u r e s d e m o n s t r a t e s l i g h t t o x i c i t y and a r e n o n - c o r r o s i v e when p u r e . D i e s t e r - b a s e d o i l s a r e s l i g h t l y t o x i c and a r e s l i g h t l y c o r r o s i v e t o n o n - f e r r o u s metals w h i l e s l i g h t l y t o x i c complex ester a r e c o r r o s i v e t o some n o n - f e r r o u s metals when h o t . T y p i c a l plysiloxanes
a r e n o n - t o x i c and n o n - c o r r o s i v e w h i l e c h l o r i n a t e d p o l y s i l o x a n e s a r e n o n - t o x i c b u t a r e c o r r o s i v e t o f e r r o u s metals i n t h e p r e s e n c e of w a t e r ( r e f . 7 6 ) . P o l y g l y c o l s and polyphenyl e t h e r s e x h i b i t low t o x i c i t y and a r e n o t c o r r o s i v e t o metals.
54
\
'0
40
60
80
I00
Temperature, O C F i g . 3.3. D i e l e c t r i c c o n s t a n t (E) o f i n s t r u m e n t s y n t h e t i c o i l s v s . temperature. 1 - S i l b e r K 7132 mv, 2 - D i e s t e r o i l Nycolube 1 1 B (Nyco S.A., P a r i s , France), 3 - Synta-A-Lube 9010, 4 - K u n s t s t o f f i j l K 2363 b l a u , 5 - OKEi 122-16 ( s i l i c o n e mineral o i 1, U.S.S.R.).
-
F o r h i g h s l i d i n g s p e e d s and r e l a t i v e l y low c o n t a c t p r e s s u r e s , g a s e s a r e sometimes u s e d i n s t e a d o f o i l s f o r hydrodynamic l u b r i c a t i o n . The low v i s c o s i t y o f g a s e s makes a v e r y low f r i c t i o n c o e f f i c i e n t p o s s i b l e (see C h a p t e r 9 . 3 ) . Gas l u b r i c a t i o n c a n b e u s e d a t h i g h e r o r l o w e r t e m p e r a t u r e s t h a n o i l o r g r e a s e . I t s main advant a g e s a r e t h e r i g i d i t y it b r i n g s t o h i g h s p e e d p r e c i s i o n b e a r i n g s s u c h as t h o s e i n d e n t a l d r i l l s and p r e c i s i o n g r i n d i n g s p i n d l e s , and t h e f a c t t h a t i t a v o i d s s e a l i n g and c o n t a m i n a t i o n p r o b l e m s when a s u i t a b l e p r o c e s s gas i s a v a i l a b l e t o a c t a s t h e gas l u b r i c a n t .
G r e a s e s a r e d i s p e r s i o n s of o r g a n i c o r i n o r g a n i c t h i c k e n e r ( s ) i n o i l . Greases are i n t e r e s t i n g l u b r i c a n t s f o r m i n i a t u r e s y s t e m s when h i g h e r e n e r g y l o s s e s due t o f r i c t i o n c a n b e a c c e p t e d t h a n i s t h e c a s e w i t h o i l l u b r i c a t i o n , and t h e main p u r p o s e o f l u b r i c a t i o n i s a
55 d e c r e a s e i n t h e r a t e of wear. The problem o f t h e m i g r a t i o n o f t h e l u b r i c a n t from t h e s y s t e m i s n o t a s s e r i o u s a s i n t h e case o f o i l l u b r i c a t i o n . The g r e a s e c a n a l s o a c t a s a n
a n t i c o r r o s i v e , damper,
shock a b s o r b e r o r s e a l . The g r e a s e u s u a l l y c o n t a i n s 5-35% ( b y w e i g h t ) o f t h i c k e n e r . The b a s e o i l is m i n e r a l o r s y n t h e t i c : u s u a l l y ester o r p o l y s i l o x a n e f l u i d . Many o f t h e a d d i t i v e s u s e d i n t h e l u b r i c a t i n g o i l s a r e equ a l l y e f f e c t i v e i n g r e a s e s . I n o r g a n i c and o r g a n i c compounds a r e u s e d a s t h i c k e n e r s . Soaps u s e d a s t h i c k e n e r s a r e p r o d u c e d from c a r b o x y l i c a c i d s o r t h e i r g l y c e r i d e s ( f a t s and o i l s ) and a l k a l i o r a l k a l i n e - e a r t h h y d r o x i d e s and a l c o h o l s . They a r e p r e s e n t i n t h e g r e a s e i n t h e form o f c h a r a c t e r i s t i c f i b e r s t r u c t u r e s . S o a p s o f uns a t u r a t e d f a t t y a c i d s a r e more s o l u b l e i n m i n e r a l o i l s b u t t h e l o w o x i d a t i o n s t a b i l i t y of f a t t y a c i d s l i m i t s t h e i r a p p l i c a t i o n . The most i m p o r t a n t g r e a s e s w i t h s o a p t h i c k e n e r i n them a r e t h o s e b a s e d on c a l c i u m and l i t h i u m s o a p s . The o t h e r t h i c k e n e r s a r e t h e inorganic ones ( s i l i c a , o r g a n o p h i l i c b e n t o n i t e s )
,
p i g m e n t s ( a l i z a r i n , anthraq-
u i n o n e , i n d i g o , a z o i n d a t h r e n e , p h t a l o c y a n i n e d y e s t u f f s , Ultrarnarine B l u e ) , and p o l y m e r s ( P E , PP, PTFE, PA,
P I , c o n d e n s a t i o n p r o d u c t s of
a l k y l p h e n o l f a t t y a c i d and formaldehyde, p o l y u r e a s )
( r e f s . 59, 6 2 ) .
O r g a n o p h i l i c b e n t o n i t e s and p o l y u r e a s a r e u s e d i n g r e a s e s f o r u s e
a t m o d e r a t e t e m p e r a t u r e s where t h e y a r e more s u i t a b l e t h a n normal g r e a s e s . O r g a n i c t h i c k e n e r s a r e u s e d i n p o l y s i l o x a n e o i l s t o produce h i g h - t e m p e r a t u r e g r e a s e s . Some s p e c i a l g r e a s e s a l s o c o n t a i n s o l i d l u b r i c a n t s s u c h as MoS2 o r g r a p h i t e . The i n s t r u m e n t g r e a s e s m a n u f a c t u r e d w i t h m i n e r a l o i l a s a b a s e l i q u i d d e m o n s t r a t e r e l a t i v e l y low o x i d a t i o n r e s i s t a n c e , narrow range o f o p e r a t i n g t e m p e r a t u r e and a r e l a t i v e l y h i g h e v a p o r a t i o n r a t e ( r e f . 1 1 2 ) . The p r o p e r t i e s o f some i n s t r u m e n t g r e a s e s b a s e d o n min-
e r a l o i l are l i s t e d i n Table 3.10. The g r e a s e "BOX" c a n b e u s e d as a n a n t i - c o r r o s i v e w h i l e "BOZ" g r e a s e c o n t a i n i n g a b o u t 1%(by weight) MoS2 c a n be u s e d f o r l u b r i c a t i o n of h i g h l y l o a d e d m i n i a t u r e s y s t e m . The g r e a s e s b a s e d on s y n t h e t i c o i l a r e m o s t l y b a s e d o n e s t e r o r p o l y s i l o x a n e o i l s . Such g r e a s e s h a v e a wide r a n g e o f o p e r a t i n g t e m p e r a t u r e , l o w e v a p o r a t i o n r a t e and h i g h a g e i n g r e s i s t a n c e . The p r o p
e r t i e s o f some e s t e r - b a s e d i n s t r u m e n t g r e a s e s a r e l i s t e d i n T a b l e 3.11.
A s w e l l as b e i n g s u i t a b l e f o r u s e a t low t e m p e r a t u r e s , some
o f t h e s e g r e a s e s c a n be u s e d when t h e o p e r a t i n g t e m p e r a t u r e rises a b o v e 153OC. G r e a s e s 7 6 1 , 7 9 4 and 7 9 9 c a n b e u s e d f o r t h e l u b r i c a t i o n of m i n i a t u r e p o l y m e r i c s y s t e m s when t h e r u b b i n g e l e m e n t o r e l e m e n t s a r e made of PA, POM, PTFE, PBTP.
Ln
TABLE 3.10
cn
PROPERTIES OF SOME INSTRUMENT GREASES BASED ON MINERAL O I L
I
GREASE
FETT 852
MANUFACTURER
I PROPERTIES
1
I
ALVANIA RS (V3872)
D r .T I LLW I CH GmbH, HORB -AH LOORF (R.F.G.)
[
AEROSHELL (GREASE 5 )
INSTRUMENTENFETT BR2
BOX DOW CORN I NG
SHELL
I
BOZ
TECHNISCHE WACHSE, JENA (G.O.R.) I
Thickener Drop p o i n t , OC P e n e t r a t i o n a t 25OC A p p l i c a t i o n t e m p e r a t u r e range,
I A1 s t e a r a t e
Li stearate
107 41 2 OC
265-29 5
-25
- 25
80
120
-40
TABLE 3.11 PROPERTIES OF SOME .ESTER-BASED INSTRUMENT GREASES
ANDE ROL
I SOFLEX
GREASE PDP 38 C X 1000 MANUFACTURER PROPERTIES
SHELL
DOW CORNING
M i croge 1 26 0 26 0 296 280 a t t e m p e r a t u r e 39OC (22h) Li stearate
Thickener Drop p o i n t , OC P e n e t r a t i o n a t 25OC Evaporation rate, % A p l i c a t i o n t e m p e r a t u r e range,
NUOOEX I N C . , PISCATAWAY , NEW JERSEY
OC
1
'
1
-62 150
-54
20 5
KLUBER, MUNCHEN (F.R.G.)
I L i stearate
120
SUPER TEL
Li stearate
57
The g r e a s e s b a s e d on p o l y s i l o x a n e s d e m o n s t r a t e h i g h c h e m i c a l s t a b i l i t y , a g e i n g r e s i s t a n c e , a wide t e m p e r a t u r e r a n g e and a h i g h o p e r a t i n g t e m p e r a t u r e . The p r o p e r t i e s o f t h e s e g r e a s e s a r e l i s t e d i n T a b l e 3.12. Grease NP 51 c o n t a i n s a n a d d i t i o n o f 6 % (by w e i g h t ) MoS2, which makes i t a p p l i c a b l e i n h i g h l y l o a d e d t r i b o l o g i c a l s y s t e m s . Very s o f t g r e a s e K 7132 c a n a l s o be u s e d t o l u b r i c a t e p o l y -
meric systems ( e x c l u d i n g t h o s e w i t h e l e m e n t s made o f ABS). P r o p e r t i e s of t h e Xrytox 240 AC g r e a s e s (manufactured by Du Pont) based on p o l y e t h e r o i l s , a r e a s f o l l o w s : p e n e t r a t i o n ( a t 25OC) 283 and 282 r e s p e c t i v e l y : o p e r a t i n g t e m p e r a t u r e r a n g e - -4U to232OC and -34 t o 287OC r e s p e c t i v e l y . They a l s o h a v e a r e l a t i v e l y h i g h resistance t o radiation. The Bel-Ray Company o f F a r m i n g d a l e , N e w J e r s e y , h a s d e v e l o p e d a n a n t i - s e i z e and a n t i - s t i c k / s l i p h e l d i n a non-weeping,
p a s t e o f powdered PTFE s e c u r e l y
o r non-bleeding,inorganic g e l system t h a t is
h e a t s t a b l e (Bel-Ray 6 3 8 6 0 ) . I t w i l l n o t m e l t or r u n when exposed
t o h i g h t e m p e r a t u r e s , making i t s u i t a b l e f o r l o n g t e r m u s e a t up t o 1 2 O o C (from -7OC)
and i n t e r m i t t e n t e x p o s u r e t o 26OoC.
This p a s t e
c o n t a i n s 348 ( b y w e i g h t ) PTFE, h a s no d r o p p o i n t , and h a s a penetrat i o n v a l u e o f 284. S p e c i a l g r e a s e s which c a n b e u s e d a s l u b r i c a n t s u n d e r h i g h v a c uum c o n d i t i o n s a r e a p p l i e d f o r s e a l i n g s t a t i o n a r y and moving elements i n vacuum a p p a r a t u s and f o r l u b r i c a t i n g s p a c e i n s t r u m e n t a t i o n l o c a t e d o u t s i d e t h e h e r m e t i c a l l y s e a l e d s p a c e c r a f t . Such g r e a s e s must d e m o n s t r a t e good l u b r i c i t y , a v e r y l o w e v a p o r a t i o n r a t e ( m i n i mum p r e s s u r e of s a t u r a t e d v a p o u r s ) and s h o u l d resist l o w and h i g h t e m p e r a t u r e s . The s e a l i n g g r e a s e s a r e u s u a l l y b a s e d on m i n e r a l o i l s t h i c k e n e d w i t h s o l i d h y d r o c a r b o n s and r u b b e r . The p r o p e r t i e s of sane g r e a s e s o f t h i s k i n d are l i s t e d i n T a b l e 3.13.
The c h a r a c t e r i s t i c s
o f some g r e a s e s a p p l i e d t o t h e l u b r i c a t i o n o f s p a c e i n s t r u m e n t a t i o n
are g i v e n i n T a b l e 3.14. T h i c k e n e r s used i n g r e a s e s a p p l i c a b l e f o r t h e l u b r i c a t i o n o f vacuum o p e r a t i n g s y s t e m s a r e u s u a l l y n o t c h e a p s i n c e t h e y c o n s i s t of m a t e r i a l s l i k e f l u o r o p o l y m e r s , polyisobutylene, c e r e s i n and n a t u r a l r u b b e r . Very c h e a p material s u c h as a t a c t i c p o l y p r o p y l e n e (PP) which i s sometimes used as a t h i c k e n e r , i s n o t s u i t a b l e f o r g r e a s e s which must work i n h i g h vacuum i n s t a l l a t i o n s . An i n t e r e s t i n g t e s t o f s y n t h e t i c vacuum s e a l i n g g r e a s e s b a s e d on m i n e r a l o i l s and u s i n g a t a c t i c PP a s a t h i c k e n e r i s d e s c r i b e d i n r e f . 113. PP (20-30% by w e i g h t ) was mixed i n o i l heated to 160 k 10°C. A f t e r c o o l i n g , a l i g h t brown g r e a s e w i t h t h e c o n s i s t e n c y of v a s e l i n e w a s o b t a i n e d . Two s u c h g r e a s e s are compare w i t h t h e g r e a s e
TABLE 3.12 PROPERTIES
OF
SOME INSTRUMENT GREASES BASED ON POLYSl LOXANES 1
I
GREASE
MANUFACTURER PRDPERT I ES Thickener Drop p o i n t , OC P e n e t r a t i o n a t 25OC A p p l i c a t i o n t e m p e r a t u r e range,
PTFE FETT
K 7132
D r . TlLLWlCH GmbH, HO RB -AH L DO R F (F. R.G.)
DOW CORNING
GENERAL ELECTRI C
(F. R. G.) Na compound > 220
Powdered PTFE
230
355 OC
-30 200
KLUBER, MUNCHEN
- 73
-73
232
180
265-295 -60 230
CHEM IEWERK. NUNCHRITZ (G.D.R.) L i stearate 200
270-31 0
-30 150
TABLE 3.13 GREASES USED FOR SEALING I N VACUUM APPARATUS
GREASE
AP I EZON AP 100
MANU FACT URER
1
AP 101
I
SHELL
PROPE RT I ES Compos i t i on
Drop p o i n t , OC A p p l i c a t i o n temperature range,
M i n e r a l o i 1, sol i d h y d r o c a r bons. a d h e s i v e additive, polyi s o p r e n e rubber
10
5
. 10-11
BHMM HT[-300
KLUBE R, MUNCHEN (F. R . G. )
U.S.S.R.
Fluorinated polyalkylether, PT FE
48, 44, 44, 125
47 OC
-40
30
Vacuum t o , Pa Remarks
L,M,N,T
BARRIERTA L55/3 HV L55/5 HV
180 10-11
I
Glass v a l v e s o f P r e s s u r e o f s a t l a b o r a t o r y apu r a t e d vapours p a r a t u s , can 10-7, o p e r a t e i n cont a c t w i t h acids, Pa bases, o r g a n i c (Apiezons L,M,N so 1v e n t s respectively)
Mineral o i l , distillation residue of p e t r o l , rubber
62 -30
250
10-7 Stable i n water, vapour, aggress i v e media. Used i n b a l l anc s a f e t y valves
1'.3 * loM2 V i scosi t y 5000 Pa s ( a t OOC). Used i n moving e l e ments s e a l i n g
cn
TABLE 3.14
0
VACUUM GREASES USED IN SPACE
GREASE
SPACE 2110 (1)
ROPERT I E S
8135
FBA7
FM8,
FMlO
FPlO
F55M,F55MS
UNlSlLlCON HOCHVACUUMFETT TK HV
BRITISH PETROLEUM M i nera 1 o i l , oleophi 1 i c graphite w i t h lead, a n t i ox id a n t and a n t i -corr o s i o n addi tives
ompos i t i on
,ppIication
STRUCTOSCORAL
t e m p e r a t u r e range,
Synthetic oi I , oleoohi l i c graphite nrith lead
OC
F l u o r i natec M i n e r a l o i polyether, s o l i d hyPTFE d r o c a rbons
lineral >i 1, PTFE
-65
-65
200
2 50
I 0-9
10-IO
( FM8 ) ,o-lil
Low s l i d i n g speed (100 rpm) b e a r i ngs
Higher 5 1 i d i ng speed bear i ngs
(FMlO) S e r i e s o f space and vacuum s e a l i n g greases
'olyloxane, 'TFE ji
-65 175 10-5
P o l y s i loxanes
61 BHMl4 HIT-300
(see T a b l e 3.13)
i n T a b l e 3.15.
N e w c h e a p g r e a s e s con-
t a i n i n g a t a c t i c ?P a s a t h i c k e n e r h a v e h i g h e r d r o p p o i n t s and l o w e r e v a p o r a t i o n r a t e s t h a n BHMM
HIT-300 g r e a s e .
S p e c i a l g r e a s e which c a n b e u s e d a t h i g h p r e s s u r e s and t e m p e r a t u r e s may b e o b t a i n e d by a d d i n g PTFE ( 0 . 0 5 - 6 % by w e i g h t ) , c o p p e r p a l m i t a t e (0.1-5)
,
MoS2 ( 3 - l d ) , and g l y c e r o l ( 0 . 5 - d . 6 )
t o a typical
s o a p - c o n t a i n i n g g r e a s e ( r e f . 1 1 4 ) . T h i s g r e a s e c a n be u s e d f o r t h e l u b r i c a t i o n of e l e c t r i c a l c o n t a c t s , a s c a n m i c r o c r y s t a l l i n e wax, a v e r y u s e f u l c o n t a c t l u b r i c a n t when f r e t t i n g d o e s n o t o c c u r (ref.92), and s u c h g r e a s e s a s K o n t a s y n t h o r S y n t h e s i n ( K l i i b e r , Miinchen F.R.G.)
or S w i t c h C o n t a c t Compound ( C a s t r o l , G B ) . I n t h e case of l i t h i u m g r e a s e s based on m i n e r a l o i l s , t h e c r i t i c a l t e m p e r a t u r e o f g r e a s e d i s o r i e n t a t i o n c a n b e i n c r e a s e d by making them u n d e r optimum c o n d i t i o n s o f i s o t h e r m i c c r y s t a l l i z a t i o n (18OoC, 1 h )
( r e f . 1 1 5 ) . The
boundary l a y e r on s t e e l c a n b e h e a t - r e s i s t a n t up t o 2OO0C
(instead
o f 13OoC i n a t y p i c a l l i t h i u m g r e a s e ) when i n t e r m i t t e n t h e a t i n g i s applied. The thhOtrOpiC g r e a s e " T h i x o - g r e a s e
9 4 1 5 " , m a n u f a c t u r e d by
Moebius e t F i l s ( A l l s c h w i l , S w i t z e r l a n d ) , i s b a s e d o n d i e s t e r s w i t h e t h e r g r o u p s , h a s p u r e s y n t h e t i c aluminium t r i s t e a r a t e a s a thickener and c o n t a i n s a d d i t i v e s t o p r o v i d e e x c e l l e n t r e s i s t a n c e t o h i g h cont a c t p r e s s u r e s and a s i z a b l e r e d u c t i o n o f w e a r and f r i c t i o n i n l u b r i c a t e d m i n i a t u r e s y s t e m s . I t c a n b e u s e d i n s t e a d o f o i l when i t
i s i m p o r t a n t t h a t t h e l u b r i c a n t d o e s n o t m i g r a t e . The p e n e t r a t i o n of t h e g r e a s e i s ca.350,
e v a p o r a t i o n r a t e a t 100°C
less t h a n 0 . 6 %
( a f t e r 2 2 h o u r s ) , and t h e a p p l i c a t i o n t e m p e r a t u r e r a n g e -25 to 8OoC. The f r i c t i o n c o e f f i c i e n t ( n e a r l y t h e same i n d r y o r humid a i r ) of a journal steel-ruby
($3 0 . 1 1 mm) m i c r o b e a r i n g l u b r i c a t e d w i t h t h i s
g r e a s e i s p r a c t i c a l l y t h e same a s f o r a b a s e o i l l u b r i c a n t ( 0 . 1 1 ) . Greases t o be used mainly i n o p t i c a l i n s t r u m e n t s a r e u s u a l l y t h i c k , demonstrating high adherence t o t h e l u b r i c a t i n g s u r f a c e s . T h e i r u s e i n screw and c o r e l o c k i n g d e v i c e s a s s u r e s a s m o o t h e r mot i o n t o g i v e p r e c i s e a d j u s t m e n t c o n t r o l . They a r e u s e d on s l i d i n g and s p i n d l e mechanisms i n a wide a r r a y o f i n s t r u m e n t s s u c h as t e l e s c o p e s , b i n o c u l a r s , cameras, r a n g e f i n d e r s , v a r i a b l e c o n d e n s e r s , p o t e n t i o m e t e r s a n d c o i l s . The p r o p e r t i e s of some o f t h e s e g r e a s e s a r e l i s t e d i n T a b l e 3.16. Jena (G.D.R.)
The s p e c i a l g r e a s e s u s e d by C a r l Zeiss
( m a n f a c t u r e r o f o p t i c a l i n s t r u m e n t s ) , b a s e d on mineral
o i l s and t h i c k e n e d w i t h A 1 o r L i s t e a r a t e s , a r e l i s t e d i n Table 3.17 and 3.18. G r e a s e s of t h e FZ-A1 s e r i e s h a v e e i t h e r a s h o r t - f i b r o u s s t r u c t u r e (kz series) o r a long-fibrous s t r u c t u r e ( l g series).
m
TABLE 3 . 1 5
N
COMPARISON OF THE PROPERTIES O F VACUUM SEALING GREASES BASED ON MINERAL OtLS AND HAVING ATACTIC PP AS THICKENER WITH GREASE B H M M Hll - 300 ( s e e T a b l e 3.13) ( r e f . 113) I
GREASE PROPERT I ES D r o p p o i n t , OC E v a p o r a t i o n r a t e , % by w e i g h t a t 200 OC d u r i n g h 1
3 6 a t 100 1
OC
BM-1 VACUUM OIL (BY WEIGHT) PP
+ 25%
EM-4 VACUUM O I L
25% PP
BHMB
~n-
62
70
70
+
0.30 0.80 1.50
0.41
0.05
0.08
0.08 0.14
0.13 0.20 0.18
0.97 1.80
3..0
8.2 14.0
during h
3 6 Humidity absorption, Vacuum to, Pa
%
0.13 2.6
.
4
.
1.3
.
300
TABLE 3.16
GREASES USED FOR LUBRICATION OF OPTICAL INSTRUMENTS
GREASE
BEL-RAY VISCOLUBE GREASE I
MANUFACTURER
3EL-RAY COMPANY I N C . :ARMINGDALE, NEW JERSEY
1 Friz- I
SERXTELL2A
MV2
i e l s o f l a r g e molecular geight s y n t h e t i c f l u i d s
CARL Z E l S S JENA CHEMISCHE LABOR (G.D.R.)
KLUBER, MUNCHEN (F.R.G.)
I
I
Syn the t i c thickent r based on hydrocarbon o i 1
GEmrEFETT V 232
B
ROPERT 1 ES mpos i ti on
FETT SOVl sco
L i stear a t e as thickener
Organic thickener
qineral o i 1 , 41 stearate
Me1t i ng poi n t rop p o i n t , O C e n e t r a t i o n a t 25OC p p i~c a t i o n temperature range
216 328
-7 120
emarks, a d d i t i o n a l a p p l i c a t i o n !
204 99 312 334 -2 -18 93 105 Viscous, Gel o f lower sticky v i scos i t y fluid for eas i er movement o f component core-locking and s l i d i n g mechan i sms
250
Screws on binoculars
-60 150 Preci s i oi screws, gears, 9Y r o scopes
-60 180 White. Bearings o f spindles o f spi nni ng frames
90 220-250 - 20 90 Sticky
160 180-215 -20 140
m w
64
CT
0
a
v
s W 7 ln ln
-
W
N -I
CT
d >m 0 W ln
3 W
ln -
CT W ln
I
-I
a a
U - U h
* o
M W
0
0
M ?
I 0
N u )
I 0
b
0
0
M
0
7
-
I
-
m L n o
0
N I
m
0
7
0 4 -
0
n
L
q n
N
I n
4.
Ln
m LN n
L
N
-
\
D
o
00 I
Ln
f
\D I L n o
N F
m N
I
M
0
L n o
N c o 7
CT
0
a
a
v
W z
7 ln
-ln W
N -I
CT
a
V
m
> 0 W ln
3 lW n
-
N 7
-I
.-
0 7
-I
.-
N
-I
.-
0
co -I
.-
0
-f
-1
.N 0
.-
-1
W
CT
W ln
a a I
ln
W -
k
W R
R
CT 0
-
3 A
.-
W U
M l n
21 W
2:
w l n
I-u
a E
-14
m w a =
t a
65 They c a n b e u s e d between 15 and 50OC. Greases o f t h e FG-Li series c a n b e a p p l i e d when t h e a m b i e n t t e m p e r a t u r e is b e t w e e n -35 a n d 55OC ( o r -50 t o 1 3 0 O C when mechanisms o t h e r t h a n o p t i c a l i n s t r u m e n t s a r e l u b r i c a t e d ) . The d r o p p o i n t o f t h e s e g r e a s e s i s b e t w e e n 190 and
25OOC. The p e n e t r a t i o n i s g i v e n i n T a b l e 3.18.
The s t r u c t u r e o f t h e
L i 13 and L i 12 g r e a s e s i s l o n g - f i b r o u s a n d t h a t o f t h e other greases
short-fibrous. cussed i n r e f . 3.4,
The t r i b o l o g i c a l p r o p e r t i e s of t h e s e g r e a s e s aredis116; some of t h e s s p r o p e r t i e s c a n b e found i n F i g s .
3 . 5 and 3 . 6 .
1
-0.4
-0.2
tag v
I
0 0.2 ( v i n fD-*m/s)
0.4
.*
0.6
F i g . 3 . 4 . F r i c t i o n f o r c e F ( c i r c u m f e r e n t i a l f o r c e ) v s . c i r c u m f e r e n t i a l speed ( s l i d i n g speed) between r o t a t i n g e x t e r n a l c y l i n d e r ( r i n g ) h a v i n g f r i c t i o n s u r f a c e 1 000 mm2 and d i a m e t e r 55 mm, and i n t e r n a l c o a x i a l c y l i n d e r when grease of t h e FG-A1 s e r i e s i s i n t r o d u c e d between t h e f r i c t i o n s u r f a c e s ( T a b l e 3 . 1 7 ) . Thickness o f grease l a y e r was 3-20 gm. Ambient t e m p e r a t u r e 2OoC ( r e f . 1 1 6 ) .
66
c 0 00
FG 12 - ~ i
g 2.151 c
.a
LL
v
0
- Li 08
1 /FG
x h
LL 0 a
1.551
d
u
,
/
/
0.951
-0.4
-0.2
1
I
0
0.2
0.4
0.6 *
Fig. 3.5. F r i c t i o n f o r c e F ( c i r c u m f e r e n t i a l f o r c e ) v s . c i r c u m f e r e n t i a l speed v ( s l i d i n g speed) f o r the FG-Li s e r i e s of greases (see Table 3 . 1 8 ) . For d e t a i l s o f f r i c t i o n system used see c a p t i o n t o F i g . 3 . 4 ( r e f . 1 1 6 ) .
67
3.351.
F F6-A16 GA -1 *\9 \'\ 10 A
2.751N
'9 h
LL
2.151-
1.551.
0.951
K
I/T1
-20
,
-10
1
1
I
0
20
40
Temperature
I
I
50
*
OC
F i g . 3.6. F r i c t i o n a l f o r c e F ( c i r c u m f e r e n t i a l f o r c e ) v s . ambient temperature T f o r FG-Li s e r i e s o f greases. S l i d i n g speed 0 . 0 1 m / s . For d e t a i l s o f f r i c t i o n system used see c a p t i o n t o F i g . 3.4 ( r e f . 116).
68
3,4, SOLID
LUBRICANTS
S o l i d l u b r i c a n t s a r e more and more f r e q u e n t l y u s e d s i n c e t h e y
are v e r y good l u b r i c a n t s f o r t r i b o l o g i c a l s y s t e m s o p e r a t i n g u n d e r e x t r e m e c o n d i t i o n s : low and h i g h t e m p e r a t u r e s , vacuum, r a d i a t i o n , c o r r o s i v e environment, h i g h c o n t a c t p r e s s u r e s , h i g h frequencyoscill a t i n g motion, o r s t a r t - s t o p
( i n t e r m i t t e n t ) motion (e.g.
i n gas
b e a r i n g s ) . The most w i d e l y u s e d s o l i d l u b r i c a n t s a r e i n o r g a n i c s u b s t a n c e s : g r a p h i t e and molybdenum d i s u l p h i d e . The good l u b r i c a t i n g p r o p e r t i e s o f g r a p h i t e are due t o t h e l a y e r e d l a t t i c e s t r u c t u r e a n d a l s o depend o n a d s o r b e d f i l m s , i n p a r t i c u l a r of water v a p o u r , which p r o v i d e s u r f a c e s w i t h low a d h e s i o n . The u s e o f g r a p h i t e i s t h e r e f o r e e f f e c t i v e i n a humid a t m o s p h e r e ; i n a vacuum i t loses i t s anti- f r i c t i o n p r o p e r t i e s . A t t e m p e r a t u r e s o v e r 45OoC it f o r m s c a r b o n d i o x i d e . When s u i t a b l e o x i d e l a y e r s a r e p r e s e n t g r a p h i t e k e e p s i t s l u b r i c a t i n g p r o p e r t i e s u p t o 60OoC. M i x t u r e s o f g r a p h i t e w i t h m e t a l o x i d e s ( s u c h as PbO) o r m e t a l s a l t s a d h e r e t o m e t a l s u r f a c e s and r e d u c e f r i c t i o n c o n s i d e r a b l y ( r e f . 6 2 ) . The f r i c t i o n c o e f f i c i e n t o f g r a p h i t e i n a normal a t m o s p h e r e i s a b o u t 0 . 2
( r e f . 117). G r a p h i t e
i s a good l u b r i c a n t a t room t e m p e r a t u r e and a t 5OO0C b u t n o t a t i n t e r m e d i a t e t e m p e r a t u r e s ; a t room t e m p e r a t u r e , e n v i r o n m e n t a l cont a m i n a n t s s e p a r a t e g r a p h i t e l a m e l l a e and t h e y d e s o r b a t h i g h e r t e m p e r a t u r e s , and a t t e m p e r a t u r e s above 500°C,
as m e n t i o n e d b e f o r e ,
oxides a i d i n t h e l u b r i c a t i o n process ( r e f . 1 1 7 ) . Molybdenum d i s u l p h i d e (MoS2) h a s a lamellar s t r u c t u r e a n d c r y s t a l l i z e s i n t h e h e x a g o n a l s y s t e m w i t h t r i g o n a l symmetry. The s u l p h u r l a y e r s f o r m i n g t h e s u r f a c e o f t h e MoSZ c r y s t a l g i v e s t r o n g a d h e s i o n t o m e t a l s u r f a c e s . The f r i c t i o n c o e f f i c i e n t o f MoS2 c a n b e v e r y low ( l e s s t h a n 0 . 2 )
,
e s p e c i a l l y when v e r y h i g h c o n t a c t p r e s -
s u r e i s a p p l i e d , a l t h o u g h i n t h e p r e s e n c e o f water t h e f r i c t i o n c o e f f i c i e n t and
wear i n c r e a s e . U n l i k e g r a p h i t e , MoS2 i s a good
l u b r i c a n t i n a h i g h vacuum, d e m o n s t r a t i n g v e r y low f r i c t i o n c o e f f i c i e n t ( b e l o w 0 . 0 5 ) . A t e m p e r a t u r e i n c r e a s e from 2 0 t o 100°C reduces t h e f r i c t i o n c o e f f i c i e n t s i n c e environmental contaminants are d e t r i m e n t a l t o l u b r i c a t i n g e f f e c t i v e n e s s . Above 100°C, c o e f f i c i e n t remains r e l a t i v e l y s t a b l e MoS2 o c c u r s a b o v e 37OoC.
the friction
u n t i l s e v e r e o x i d a t i o n of
The s m a l l e r t h e p a r t i c l e s o f MoS2 a r e , t h e
higher t h e oxidation rate w i l l be. G r a p h i t e and MoS2 o r i e n t r a p i d l y when r u b b e d b e c a u s e o f t h e c r y s t a l s t r u c t u r e . The b a s a l p l a n e s become p a r a l l e l t o t h e f r i c t i o n s u r f a c e which f a c i l i t a t e s e a s y s h e a r . Powder b l e n d s o f h i g h - g r a d e
69 g r a p h i t e and MoS2 l e a d t o s y n e r g i s n or e q u i v a l e n c e , d e p e n d i n g o n o p e r a t i n g c o n d i t i o n s , w i t h r e g a r d t o f r i c t i o n a n d wear ( r e f s . 1 1 8 , 1 1 9 ) . T h i s phenomenon c a n b e e x p l a i n e d i n terms o f the physicochenical
p r o p e r t i e s o f t h e s o l i d l u b r i c a n t s . The f o r m a t i o n and t r a n s f e r o f l u b r i c a n t f i l m i s i n f l u e n c e d b y p a r t i c l e s i z e a n d by t h e r e l a t i v e volumes of t h e MoS2 and g r a p h i t e i n t h e b l e n d . The d i s t i n c t s y n e q i s n
w a s o b s e r v e d i n b l e n d s c o n t a i n i n g 1 5 t o 35% b y w e i g h t g r a p h i t e and i n b l e n d s w i t h c o n s i d e r a b l y h i g h e r amounts o f g r a p h i t e
-
from 6 0 t o
80%. The p r o c e s s of i n t e r c a l a t i o n , i . e . ,
t h e formation of chemical
compounds b y t h e i n s e r t i o n o f a t o m i c o r m o l e c u l a r s p e c i e s i n t h e v a n d e r Waals g a p b e t w e e n p l a n e s of l a m e l l a r s o l i d s c a n g r e a t l y improve t h e i n t r i n s i c l u b r i c a t i n g p r o p e r t i e s of such s o l i d s
( r e f . 120).
The a d d i t i o n o f 1 9 . 8 % ( b y w e i g h t ) of CoC12 t o g r a p h i t e w a s f o u n d t o i n c r e a s e l i f e of t h e l u b r i c a n t by more t h a n f i v e t i m e s , w h i l e g r a p h -
i t e + 1 9 . 3 % N i C 1 2 c o m p o s i t i o n more t h a n d o u b l e d t h e l o a d - c a r r y i n g c a p a c i t y compared w i t h g r a p h i t e and was e q u i v a l e n t t o EloS2. The amount by which t h e l i f e t i m e i s p r o l o n g e d d e p e n d s o n t h e c o n c e n t r a t i o n o f i n t e r c a l a n t i n g r a p h i t e and t h e r e s u l t i n g i n c r e a s e i n i n t e r l a y e r c a r b o n s p a c i n g due t o i n t e r c a l a t i o n . Another i n o r g a n i c l a m i n a r s o l i d l u b r i c a n t i s g r a p h i t e f l u o r i d e (CFxIn, where x c a n v a r y from a b o u t 0.3 t o 1.1. T h i s l u b r i c a n t c a n be roughly d e s c r i b e d a s a l a y e r l a t t i c e i n t e r c a l a t i o n
compound of
g r a p h i t e ( r e f . 1 2 1 ) . The g r a p h i t e f l u o r i d e d o e s n o t o x i d i z e i n a i r , b u t i t decomposes t h e r m a l l y above 54OoC t o f o r m c a r b o n t e t r a f l u o r o m e t h a n e , o t h e r l o w m o l e c u l a r w e i g h t f l u o r o c a r b o n s and c a r b o n . The f a i l u r e t e m p e r a t u r e f o r ( C F x ) n i s 49OoC, (4OOOC).
h i g h e r t h a n t h a t of MoS2
(CFxIn e x h i b i t s h i g h p l a s t i c i t y w i t h i n a l u b r i c a t e d con-
t a c t . The o r d e r o f p l a s t i c i t y i s ( C F x ) n > MoS2 > g r a p h i t e . (CFx)n i s p a r t i c u l a r l y u s e f u l i n polymide-bonded (CFx) c o a t i n g s (refs. 121, 1 2 2 ) . Graphite f l u o r i d e demonstrates b e t t e r l u b r i c a t i n g p r o p e r t i e s
t h a n g r a p h i t e i t s e l f b u t n o t q u i t e a s good a s XoS2. T e m p e r a t u r e , h u m i d i t y and t h e C:F r a t i o a f f e c t its p e r f o r m a n c e . D i c h a l c o n i d e s s u c h as WS2, s y n t h e t i c Nb1.158S2
have v e r y good
l u b r i c a t i n g p r o p e r t i e s ( r e f s . 1 2 1 , 1 2 3 ) . The p r o p e r t i e s o f W S 2 a r e s i m i l a r t o MoS2 b u t t h e o x i d a t i o n r e s i s t a n c e o f W S 2 a t t e m p e r a t u r e s h i g h e r t h a n 34OoC i s b e t t e r t h a n MoSZ. The loss of l u b r i c a t i q qual-
i t i e s of WS2 a n d MoS2 i n a i r c o i n c i d e s w i t h t h e t e m p e r a t u r e s a t which r a p i d c o n v e r s i o n t o t h e o x i d e o c c u r s ( r e f . 1 2 1 ) . I n a n i n e r t g a s o r vacuum t h e maximum o p e r a t i n g t e m p e r a t u r e i s a f u n c t i o n o f t h e t h e r m a l d i s s o c i a t i o n rates r a t h e r t h a n t h e o x i d a t i o n r a t e s o f
70
t h e l u b r i c a n t s . WS2 d e m o n s t r a t e s b e t t e r e l e c t r i c a l c o n d u c t i v i t y t h a n many
o t h e r d i c h a l c o n i d e s and i s u s e d i n e l e c t r i c a l c o n t a c t s .
The s y n t h e t i c Nb1.158S2
( s y n t h e s i z e d by niobium p e n t o x i d e r e a c t i n g
w i t h hydrogen s u l p h i d e a t 70OoC) when added t o l i t h i u m s o a p g r e a s e s d e m o n s t r a t e s a lower f r i c t i o n c o e f f i c i e n t a n d b e t t e r e x t r e m e p r e s s u r e p r o p e r t i e s t h a n MoS2 ( r e f . 1 2 3 ) . I n o r g a n i c , nonlaminar s o l i d s s u c h as PbO, CaF2 d e m o n s t r a t e good l u b r i c a t i n g p r o p e r t i e s t h a n k s t o t h e i r low s h e a r s t r e n g t h . S o f t oxi d e s ( e . g . PbO, B 0 ) h a v e r e l a t i v e l y low f r i c t i o n c o e f f i c i e n t s , 2 3 e s p e c i a l l y a t h i g h t e m p e r a t u r e s where t h e i r s h e a r s t r e n g t h i s reduced t o s u c h a d e g r e e t h a t d e f o r m a t i o n o c c u r s by p l a s t i c f l o w rat h e r t h a n by b r i t t l e f r a c t u r e . S n c r e a s i n g s u r f a c e t e m p e r a t u r e red u c e s b o t h c r y s t a l l i n e s h e a r s t r e n g t h and g l a s s v i s c o s i t y w i t h i n t h e s l i d i n g c o n t a c t . PbO l u b r i c a t e s e f f e c t i v e l y o v e r a narrow t e m p e r a t u r e r a n g e (500OC t o 650°C) and a t low s l i d i n g s p e e d s b u t i t i s even more e f f e c t i v e a t h i g h s l i d i n g s p e e d s and h i g h t e m p e r a t u r e s ( r e f . 1 2 1 ) . B o r i c o x i d e (B203) e x h i b i t s a h i g h f r i c t i o n c o e f f i c i e n t when t h e t e m p e r a t u r e i s lower t h a n 50OoC ( r e f . 1 1 7 ) . Near t h e m e l t i n g p o i n t , t h e f r i c t i o n c o e f f i c i e n t o f b o r i c o x i d e d e c r e a s e s to less t h a n 0 . 1 0 . T h i s i s b e c a u s e of a marked r e d u c t i o n i n t h e f o r c e n e c e s sary f o r viscous shear. Chemically s t a b l e f l u o r i d e s such a s CeF3, CaF2, L i F o r BaF2 c a n b e u s e d a s s o l i d l u b r i c a n t s . Cerium f l u o r i d e (CeF3) h a s a t y s o n i t e o r m o d i f i e d l a t t i c e l a y e r s t r u c t u r e . I t i s water i n s o l u b l e and res i s t a n t t o t h e r m a l d i s s o c i a t i o n and c h e m i c a l r e d u c t i o n . I t s m e l t i n g p o i n t i s 1438OC and i t s t h e r m a l e x p a n s i o n c o e f f i c i e n t m a t c h e s t h o s e of n i c k e l b a s e s u p e r a l l o y s and s t a i n l e s s s t e e l . CaF3 s i g n i f i c a n t l y reduces w e a r i n I n c o n e l a l l o y s , b o t h i n a i r and a r g o n atmospheres up t o 1 OOO°C; i t s powder i s a good f i l m - f o r m e r and t h e fusion-boded CeF3 c o a t i n g s have p o s s i b i l i t i e s a s s o l i d l u b r i c a n t s ( r e f . 1 2 4 ) . When u s e d a s a n a d d i t i v e i n g r e a s e s , CeF3 s u b s t a n t i a l l y i n c r e a s e s t h e l o a d - c a r r y i n g c a p a c i t y of a good g r e a s e , and r e d u c e s w e a r w i t h o u t i n f l u e n c i n g t h e f r i c t i o n c o e f f i c i e n t . The wear r e d u c t i o n i s s l i g h t l y b e t t e r t h a n MoS2 ( b o t h a t room t e m p e r a t u r e and a t 100°C)
( r e f . 1 2 4 ) . Compared w i t h MoSZ and CeF3
,
g r a p h i t e appears t o be
l e s s e f f e c t i v e as a g r e a s e a d d i t i v e when u s e d e i t h e r a l o n e o r a s a
m i x t u r e w i t h CeF3. CeF3 h a s s e v e r a l a d v a n t a g e s o v e r MoS2; i t is curr e n t l y 30% cheaper, has higher temperature s t a b i l i t y . Its off-white colour can be helpful i n c e r t a i n circumstances. CaF2, LiF and BaF2 l u b r i c a t e a t h i g h t e m p e r a t u r e s and o v e r a w i d e r r a n g e of t e m p e r a t u r e s t h a n PbO. C o a t i n g s w i t h c o m p o s i t i o n s
71
from t h e CaF2/BaF2 b i n a r y e u t e c t i c s y s t e m c a n o p e r a t e from a b o u t 500 t o 95OoC, d e m o n s t r a t i n g a f r i c t i o n c o e f f i c i e n t o f a b o u t 0 . 1 (ref. 121). O t h e r i n o r g a n i c s o l i d l u b r i c a n t s s u c h a s metal h a l i d e s (cadmium i o d i d e , cadmium c h l o r i d e , cadmium bromide, c o b a l t c h l o r i d e , l e a d i o d i d e and m e r c u r i c i o d i d e ) a r e h i g h l y c o r r o s i v e . Ammg t h e s o l i d l u b r i c a n t s w e c a n i n c l u d e t h i n f i l m s o f s o f t metals s u c h as l e a d , cadmium, t i n , indium, s i l v e r and g o l d u s e d t o c o a t h a r d s u b s t r a t e s . They a r e e s p e c i a l l y u s e f u l when b o t h l u b r i c a t i o n and c o r r o s i o n p r o t e c t i o n are r e q u i r e d , b u t t h e y g e n e r a l l y e x h i b i t higher f r i c t i o n c h a r a c t e r i s t i c s than l a y e r laminar s o l i d s such as MoS2 (see C h a p t e r 7 . 2 ) . O r g a n i c compounds
-
f a t s , s o a p s , waxes, polymers and t h e r m a l l y
s t a b l e s u b s t a n c e s such as p h t a l o c y a n i n e s - a r e a l s o u s e d a s s o l i d l u b r i c a n t s . They g i v e a low f r i c t i o n c o e f f i c i e n t b u t c a n n o t b e used a t t e m p e r a t u r e s h i g h e r t h a n t h e i r m e l t i n g p o i n t s . Metallic s o a p s
a r e u s e d , f o r example, a s t h i c k e n e r s i n g r e a s e s ( s e e C h a p t e r 3 . 3 ) . Polymers s u c h a s PTFE o r P I a r e w i d e l y u s e d a s s o l i d b o d i e s and a l s o o f t e n a s f i l m s . Using PTFE, a r e m a r k a b l y low f r i c t i o n c o e f f i c i e n t c a n b e e x p e c t e d - from 0 . 0 1 t o 0.04 from a t e m p e r a t u r e a s low
a s t h a t o f l i q u i d hydrogen t o t h e d e c o m p o s i t i o n t e m p e r a t u r e of t h e polymer ( r e f . 1 2 1 ) . PTFE u s e d i n powder form a s a n a d d i t i o n t o o i l s o r g r e a s e s ( t h i c k e n e r ) c a n r e d u c e s t i c k - s l i p e f f e c t s and demnstrates good a n t i - s e i z u r e p r o p e r t i e s ( s e e C h a p t e r s 3.2 and 3 . 3 ) . P h t a l o c y a n i n e s (metal f r e e o r c o p p e r ) d e m o n s t r a t e l u b r i c a t i n g p r o p e r t i e s i n f e r i o r t o MoS2 b u t under c e r t a i n c o n d i t i o n s a r e s u p e r i o r tograph-
i t e . The f o r m a t i o n of c h e l a t e s bonds them s t r o n g l y t o m e t a l s u r f a c e s . T h e i r most i m p o r t a n t u s e i s a s a t h i c k e n e r i n h i g h temperat u r e greases. S o l i d l u b r i c a n t s b a s e d on c a p r o l a c t a m , p r o d u c t s o f r e a c t i o n s of
metal ( s u c h as F e , Co, Cu, T i ) o x i d e s h y d r a t e s w i t h a c r y s t a l l i n e c a p r o l a c t a m , d e m o n s t r a t e good l u b r i c a t i n g p r o p e r t i e s ( r e f . 1 2 5 ) . Such s o l i d l u b r i c a n t s are p e r i o d i c c o l l o i d s y s t e m s w i t h t h e p r o p e r t i e s of t h i x o t r o p i c g e l s y s t e m s t a c t o i d s c h a r a c t e r i z e d by t h e i r
-
v e r y low s h e a r s t r e n g t h . The m e c h a n i c a l l u b r i c a t i n g a c t i v i t y o f t h e l u b r i c a n t i s b a s e d on t h e e n l a r g e m e n t s o f t h e c1earanc.e between t h e r u b b i n g s u r f a c e s when t h e r u b b i n g r e g i o n i s f i l l e d w i t h t h e v i s c o u s f l o w of c o l l o i d p a r t i c l e s , which makes a s i g n i f i c a n t r e d u c t i o n i n
wear and f r i c t i o n p o s s i b l e . The l u b r i c a t i n g a b i l i t y o f t h e s e l u b r i c a n t s i s a l s o d e p e n d e n t on t h e i r p h y s i c o c h e m i c a l and c h e m i c a l i n t e r a c t i o n s w i t h t h e r u b b i n g s u r f a c e s ( r e f s . 125, 1 2 6 ) .
12
S o l i d l u b r i c a n t s a r e used p r i n c i p a l l y a s d i s p e r s i o n s i n o i l s and g r e a s e s . They a r e a l s o a p p l i e d sometimes a s powders, b u t t h i s k i n d o f l u b r i c a t i o n i s i n e f f e c t i v e b e c a u s e t h e l o o s e powders c a n b e e a s i l y pushed o u t of t h e c o n t a c t r e g i o n . The s o l i d l u b r i c a n t s a r e t h e r e f o r e o f t e n mechanically burnished. Another p o s s i b i l i t y is t o u s e r e s i n s t o make compacts o f s o l i d l u b r i c a n t m a t e r i a l s and i f t h e compact i s b r o u g h t i n t o c o n t a c t w i t h t h e s u r f a c e o f a s o l i d r u b b i n g e l e m e n t , l u b r i c a n t f i l m w i l l b e c o n s t a n t l y r e g e n e r a t e d ( f o r example i n b a l l b e a r i n g s , see C h a p t e r 7 . 2 and 9 . 4 ) .
Chemical and e l e c t r o -
chemical methods f o r f o r m i n g m e t a l s u r f a c e s a r e a l s o u s e f u l f o r saw s o l i d l u b r i c a n t s . Dry s u r f a c e a n t i - f r i c t i o n c o a t i n g s formed by v a r i o u s s o l i d l u b r i c a n t s a r e o f most i n t e r e s t i n m i n i a t u r e s y s t e m s
(see C h a p t e r ' 7 . 2 ) . They c a n be a p p l i e d by b o n d i n g , s p u t t e r i n g , o r i o n p l a t i n g ( s o f t metals) ( r e f s . 1 1 7 , 1 2 1 , 1 2 7 , 1 2 8 ) . A n t i - f r i c t i o n dry coatings a r e discussed i n d e t a i l i n Chapter 7.2. Magnetic f i e l d s e n e r g y c a n a l s o b e used t o i n t r o d u c e t h e s o l i d lubricant particles into
t h e r u b b i n g r e g i o n and k e e p them t h e r e .
Such l u b r i c a n t s c o n t a i n a n a d d i t i o n of 1 0 - 3 0 % by w e i g h t of f e r r o m a g n e t i c m a t e r i a l s and t h e r u b b i n g e l e m e n t s s h o u l d b e made o f magn e t i c m a t e r i a l s ( r e f s . 1 2 9 , 1 3 0 ) . T h i s method c a n e x t e n d t h e l i f e -
t i m e of a t r i b o l o g i c a l s y s t e m c o n s i d e r a b l y , making it one o f t h e most e f f e c t i v e ways t o a p p l y s o l i d l u b r i c a n t s .
73
4 , UNLUBRICATED SYSTEMS
41 I
I
METALLIC SYSTEMS V a r i o u s m e t a l l i c t r i b o l o g i c a l s y s t e m s a r e u s e d i n s m a l l mecha-
n i s m s . A t y p i c a l example i s j o u r n a l b e a r i n g s where t h e j o u r n a l s
a r e made o f f r e e - c u t t i n g o r s t a i n l e s s s t e e l and t h e b e a r i n g p l a t e s which make u p t h e f r a m e o f t h e mechanism a r e made o f l e a d e d h i g h -
s t e e l journal
t e n s i l e b r a s s . The r o u g h n e s s o f t h e r o l l e r - b u r n i s h e d
i s u s u a l l y Ra
MPa ( t h e a m p l i t u d e A)
,
the fre-
quency f o f t h e l o a d v a r i a t i o n s from 0 . 1 7 to 0 . 5 1 Hz, and when t h e c o n s t a n t a v e r a g e c o n t a c t p r e s s u r e was k e p t p = 8 MPa and s l i d i n g s p e e d v = 0.00157 m / s i n v e s t i g a t i o n s ( r e f . 1 3 4 ) h a v e shown t h a t t h e bearing l i f e of a dynamically loaded b ear in g i s double t h a t
t o a comparable s t a t i c a l l y l o a d e d b e a r i n g . When t h e b e a r i n g i s o n l y r e q u i r e d t o o p e r a t e f o r a s h o r t p e r i o d , t h e l o a d c a n be 3 t i m e s h i g h e r t h a n t h e maximum a d m i s s i b l e l o a d f o r t h e o p e r a t i o n o f t h e same b e a r i n g o v e r a l o n g p e r i o d . The r e l i a b i l i t y f u n c t i o n s f o r b e a r i n g s a n a l y s e d u n d e r s t a t i c and dynamic l o a d i n g , o b t a i n e d by t h e u s e o f t h e Gumbells t h e o r y , c a n be e x p r e s s e d a s f o l l o w s :
76
t + 150 7182 p-1.62 -1.42 V
+
45
- 3.811
(4.la)
where t i s i n s . The l i f e o f a b e a r i n g o p e r a t e d u n d e r s t a t i c l o a d i n g i s g i v e n by (refs. 134, 1 3 5 ) :
-t
= 23700 p
where
t
-1.62
V
-1.42
(4.2a)
is a v e r a g e life ( i n s ) , p i s i n MPa a n d v i s i n m / s .
T h i s f u n c t i o n i s shown g r a p h i c a l l y i n F i g . 4 . 2 .
\ W i n g speed
Fig. 4.2.
L i f e (:)
I
mm/s
o f s t a t i c a l l y loaded s t e e l - b r a s s m i n i a t u r e j o u r n a l b e a r i n g .
77
The l i f e of a d y n a m i c a l l y l o a d e d b e a r i n g i s
t
= 834
+
133 f
-
130 A
(4 .2b)
where t i s i n s , f i n Hz and A i n MPa. The c o r r e s p o n d i n g g r a p h i s g i v e n i n F i g . 4.3.
Amplitude , MPa
F i g . 4 . 3 . L i f e ( f ) o f d y n a r a i c a l l y loaded s t e e l - b r a s s m i n i a t u r e journal b e a r i n g .
N o c o r r e l a t i o n between t h e d i a m e t r a l c l e a r a n c e and t h e l i f e of
t h e b e a r i n g was found ( r e f . 134). The r e l a t i o n s h i p between b e a r i n g l i f e and b e a r i n g c l e a r a n c e i s p r e s e n t e d i n F i g . 4.4. When i n v e s t i g a t e d u s i n g a s t a n d a r d ASTM pendulum ( s e e C h a p t e r 8.2.2)
,
t h e f r i c t i o n c o e f f i c i e n t i n a s t e e l - b r a s s system w a s d i s -
t i n c t l y lower t h a n t h a t i n a s t e e l - s t e e l ( s p h e r e - p l a t e ) t r i b o l o g i c a l system ( r e f . 4 3 ) . S t e e l - s t e e l
s y s t e m s a r e less u s e f u l t h e r e -
f o r e , s i n c e t h e a d h e s i o n between t h e u n l u b r i c a t e d r u b b i n g e l e m e n t s
78
i s very high ( r e f . 1 3 6 ) .
A 50
0
4-
0
0
3-
o
o
0
0
0
0
0
0
o o
2-
0
0
0
0
0 0
0
1-
0
0
I
I
1
25
35
45
Diametrol clearonce
I
55
*
65
, pm
F i g . 4.4. R e l a t i o n s h i p between b e a r i n g d i a m e t r a l c l e a r a n c e and 1 i f e o f m i n i a t u r e (0 3 m m ) s t e e l - b r a s s j o u r n a l b e a r i n g . C o n t a c t p r e s s u r e 6 MPa, s l i d i n g speed 0.63 mm/s
(ref.
134)
.
Wear s t u d i e s o f medium c a r b o n s t e e l r u b b i n g o n i t s e l f
( r e f . 137)
show t h a t t h e h a r d n e s s a n d t h e s t a t e o f o x i d a t i o n of t h e r u b b i n g s u r f a c e s a r e t h e p r i n c i p l e f a c t o r s i n d e t e r m i n i n g wear r a t e s . When t h e h a r d n e s s i s i n c r e a s e d from 2 0 0 HV t o 6 0 0 HV, t h e l i n e a r wear
r a t e d e c r e a s e s 6 f o l d . When a t r i b o l o g i c a l s y s t e m ( p i n - o n - d i s k ) w a s o p e r a t e d u n d e r vacuum c o n d i t i o n s , t h e h i g h e s t wear r a t e was s t e e l s a m p l e s was 4 5 0 HV, w h i l e t h e l o w e s t was found when t h e i r h a r d n e s s w a s 600 HV. The w e a r r a t e found when t h e h a r d n e s s o f t h e
o f s a m p l e s w i t h a h a r d n e s s of 4 5 0 HV was 6-10
times h i g h e r t h a n
s a m p l e s w i t h a h a r d n e s s of 6 0 0 HV; t h e wear r a t e o f s a m p l e s w i t h a h a r d n e s s o f 2 0 0 HV w a s a p p r o x i m a t e l y t h e a r i t h m e t i c mean o f t h e
79
wear rates of the samples with 450 and 600 HV hardness. For a tribological system operating in a vacuum, higher wear rates were observed than in normal air conditions and severe material transfer occured. The dominating tribo-oxidative process in normal air conditions and the adhesive wear mechanism in a vacuum were observed. For systems operating in air, the friction coefficient was 0.25 at the begining of the test and had increased to about 1.0 by the end of the investigation Austenitic stainless steel rubbing on itself in air at room temperature (ref. 138) involves equal rates of wear in both elements (pin-on-flat plate system). The wear is the result of prow formation. Prows are formed by the adhesive transfer of material from one surface to the other and have a layered structure composed of thin platelets formed by asperity interaction. The prows formed under low loads demonstrate reduced ductility which is probably caused by oxygen pick-up and their high martensite content. Prows formed under high loads are more ductile and are flattened by repeated interaction. Studies of austenitic stainless steels sliding on tool steel (in argon) (ref. 139) show that their tribological behaviour depends on the ease of formation of strain induced martensite, the nature of the transfer process and the relative hardness of the stainless steel sample. When the hardness of the tool steel sample is higher the transfer of the stainless steel to the tool steel surface occurs: when the hardness of the stainless steel is higher the transfer of the tool steel to the stainless steel occurs. The oxidational wear of steel-steel systems operating at elevated temperatures is the main cause of damage to the rubbing elements (refs. 140, 144). Stainless steels with their generally good oxidation resistance might be expected to be more wear-resistant at elevated temperatures than other steels. Oxidational wear can also occur in other than steel-steel metallic systems; for example, it can occur in steel-copper (ref. 142), or copper-copper systems (ref. 1431, even when the sliding speeds and loads are small and the ambient temperature is not elevated. Oxidation processes play a major role in producing wear damage in the fretting of steels at room temperature (ref. 144) Adhesive wear in metallic systems is typically accompanied by the transfer of material. Transfer occurs when the shear strength of the adhesive bond between two asperities is greater than that of transferring material but the nature of the geometry is also
.
80
sometimes i m p o r t a n t ( e . 9 . f o r a s l i d e r on a f l a t s u r f a c e o f t h e same m a t e r i a l , t r a n s f e r g e n e r a l l y o c c u r s from t h e f l a t p a r t t o t h e s l i d e r ( r e f . 145) )
. The
o c c u r r e n c e o f t h e t r a n s f e r i s o b v i o u s from
t h e a p p e a r a n c e o f a c o a t i n g o f c o p p e r - c o l o u r e d m a t e r i a l on t h e
s t e e l . A c l e a r c o n n e c t i o n e x i s t s between t h e t r a n s f e r l a y e r and t h e g e n e r a t i o n o f l o o s e wear d e b r i s ( r e f . 1 4 6 , 1 4 7 , 1 4 8 ) . S t u d i e s o f copper-based a l l o y s s l i d i n g a g a i n s t h a r d s t e e l s show t h a t t r a n s f e r l a y e r s o r p a t c h e s d e v e l o p on t h e s l i d i n g s u r f a c e s d u r i n g p r o l o n g e d s l i d i n g . However, when t h e s t a i n l e s s s t e e l s l i d e s a g a i n s t samples made o f Cu-Nil Cu-A1,
Cu-A1203, o r Cu-Be
a l l o y s , t h e t r a n s f e r l a y e r s begin t o develop e a r l i e r , even b e f o r e l o o s e wear d e b r i s c a n b e d e t e c t e d ( r e f . 1 4 6 ) . S i m i l a r t o o l s t e e l s l i d i n g a g a i n s t Cu
-
t e s t s on
1.8 w t % B e and c o p p e r d i s p e r s i o n
hardened w i t h A 1 2 0 j o r C u z O p a r t i c l e s , a l s o i n d i c a t e d t h a t m a t e r i a l t r a n s f e r p l a y s a s i g n i f i c a n t r o l e i n b o t h f r i c t i o n and wear i n t h e s e s y s t e m s . The w e a r i s h i g h when t h e s m a l l t r a n s f e r p a r t i cles a r e not dispersed o r separated b e f o r e they can accumulate t o
form t h e t r a n s f e r l a y e r ; t h e u s e o f a f l u i d l u b r i c a n t i s o n e way t o p r e v e n t t h i s a c c u m u l a t i o n ( r e f . 1 4 6 ) . Wear e q u a t i o n s ( s u c h a s A r c h a r d ' s laws o f wear) a r e i n a d e q u a t e f o r r e a l s l i d i n g s y s t e m s b e c a u s e t h e y do n o t i n c o r p o r a t e t h e e f f e c t s o f material t r a n s f e r , a l t h o u g h t h e s t u d i e s d e s c r i b e d i n r e f . 1 4 9 , u s i n g pin-on-disk s y s t e m s i n which p i n s made o f b r a s s , c o p p e r and aluminium w e r e rubbed a g a i n s t d i s k s o f s t e e l , b r a s s , c o p p e r and aluminium, showed t h a t f o r d r y a d h e s i v e w e a r t h e f i r s t and t h i r d l a w s o f A r c h a r d ' s t h e o r y a r e v a l i d , i . e . t h a t t h e volume of worn m a t e r i a l i s proport i o n a l t o t h e s l i d i n g d i s t a n c e and i n v e r s e l y p r o p o r t i o n a l t o t h e y i e l d stress, o r t h e h a r d n e s s of t h e s o f t e r m a t e r i a l . R e a l i s t i c
wear e q u a t i o n s s h o u l d t a k e i n t o a c c o u n t t h e p r o p e r t i e s o f b o t h s l i d i n g p a r t n e r s (and t h e t r a n s f e r material which forms between them) , a s w e l l a s g e o m e t r i c a l and e n v i r o n m e n t a l e f f e c t s ( r e f s . 146, 1 4 7 ) . Geometrical f a c t o r s and a d h e s i o n a r e b o t h i m p o r t a n t b e c a u s e r e v e r s i n g m a t e r i a l s which h a v e a g i v e n g e o m e t r y , o r changing t h e i r geometry, c a n have profound e f f e c t s which a r e n o t y e t w e l l c h a r a c t e r i z e d o r u n d e r s t o o d ( r e f s . 1 4 7 , 1 4 8 ) . I n pin-on-disk s t u d i e s ( r e f . 1481, t h e p r e f e r r e d t r a n s f e r d i r e c t i o n w a s from t h e d i s k t o t h e p i n ( t h e w e a r l o s s from t h e d i s k i s l a r g e r ) ard a d h e s i o n studies have shown t h a t t h e p r e f e r r e d t r a n s f e r d i r e c t i o n is from t h e cohes i v e l y weaker material t o t h e c o h e s i v e l y s t r o n g e r o n e . When t h e l o a d s d u r i n g rubbing are lower t h a n t h e y i e l d s t r e n g t h o f t h e w e a k e r m a t e r i a l , t h e w e a r depends o n t h e p r o c e s s e s o f f o r m a t i o n
81 and r u p t u r e o f o x i d e s o f t h e r u b b i n g m a t e r i a l s ( r e f . 1 5 0 ) . The wear o f a l u m i n i u m - s i l i c o n a l l o y s d e p e n d s o n t h e s i l i c o n c o n t e n t ( r e f . 151)
.A
h i g h e r s i l i c o n c o n t e n t l e a d s t o improved
wear c h a r a c t e r i s t i c s . T h i s h a s b e e n shown f o r h y p e r e u t e c t i c a l u minium-silicon a l l o y s c o n t a i n i n g 17-26% (by weight) S i r rubbing ( p i n ) a g a i n s t a t o o l s t e e l d i s k . I n c r e a s i n g t h e volume f r a c t i o n o f p r i m a r y s i l i c o n r e s u l t s i n t h e t r a n s i t i o n from m i l d t o s e v e r e wear o c c u r r i n g a t a h i g h e r l o a d . The c o m p o s i t i o n a l v a r i a t i o n s which a f f e c t m a t r i x h a r d n e s s h a v e a much smaller i n f l u e n c e o n w e a r r a t e s t h a n v a r i a t i o n s i n s i l i c o n c o n t e n t . The wear b e h a v i o u r of p u r e aluminium (when aluminium p i n s r u b a g a i n s t a s o f t s t e e l d i s k ) exh i b i t s two d i s t i n c t r e g i o n s o f wear i n t h e low l o a d r a n g e s marked by a c l e a r t r a n s i t i o n p o i n t ( r e f . 1 5 2 ) . A t v e r y low l o a d s , wear i s a p p a r e n t l y c o n t r o l l e d by a p u r e l y o x i d a t i v e mechanism, w h i l e a t m o d e r a t e l y h i g h l o a d s it c h a n g e s t o a combined o x i d a t i v e - c u m m e t a l l i c t y p e o f wear. The t r a n s i t i o n p o i n t i s a f u n c t i o n o f t h e s l i d i n g s p e e d , t h e r e l a t i v e h a r d n e s s a n d r o u g h n e s s o f t h e d i s k surface, and a l s o t h e a m b i e n t c o n d i t i o n s o f t e m p e r a t u r e and h u m i d i t y . S e v e r e , p u r e l y m e t a l l i c wear o c c u r s o n l y a t v e r y h i g h l o a d s . The a d d i t i o n o f g r a p h i t e (maximum 2 % by volume) t o aluminium improves
i t s wear r e s i s t a n c e r e m a r k a b l y and d e c r e a s e s t h e f r i c t i o n c o e f f i cient (ref. 153). S i n t e r e d m e t a l s , s u c h a s a l l o y s b a s e d o n i r o n and c o n t a i n i n g phosphorus, are v e r y i n t e r e s t i n g as a n t i - f r i c t i o n materials s i n c e t h e y can o p e r a t e without t h e a d d i t i o n of l u b r i c a n t ( r e f s . 154, 1 5 5 , 1 5 6 ) . Such a l l o y s c o n t a i n 0 . 2
-
2 % P. I n Fe-P-S
alloys,
phosphorus makes t h e m a t e r i a l more h e t e r o g e n e o u s , f a c i l i t a t i n g t h e r u n n i n g - i n p r o c e s s , and s u l p h u r lowers t h e f r i c t i o n c o e f f i c i e n t .
- ( 0 . 5 - 11%P - ( 0 . 9 - 1 ) %S mater i a l o b t a i n e d by e x t r u s i o n i s 0.18-0.33 ( r e f . 1 5 5 ) . The a d d i t i o n o f 2.5 - 5 % M o o r 0 . 6 - 1%C improves t h e m a t e r i a l ' s p r o p e r t i e s . I n t h e nitriding-carburizing process of phosphatizing s i n t e r e d
The f r i c t i o n c o e f f i c i e n t o f Fe
i r o n m a t e r i a l , a w e a r - r e s i s t a n t s u r f a c e l a y e r c o n t a i n i n g Fe (CNP) m n alloy
t y p e compounds i s c r e a t e d ( r e f . 1 5 7 ) . For s i n t e r e d Fe-P-X
c o n t a i n i n g 1 - 4 % Cu '(where X i s 2 - 4 % N i l M o o r c a r b i d e a l l o y cont a i n i n g Mn, C r , M o and F e )
,
a t s l i d i n g a g a i n s t s t e e l , when t h e
c o n t e n t s of P and Cu a r e i n c r e a s e d t h e f r i c t i o n c o e f f i c i e n t and wear d e c r e a s e ( r e f . 1 5 6 ) . Such a l l o y s c a n b e u s e d a s a n t i - f r i c t i o n and w e a r - r e s i s t a n t materials. Phosphorus is a l s o i n t r o d u c e d i n t o s i n t e r e d c o p p e r o r b r o n z e m a t e r i a l s t o improve t h e i r m e c h a n i c a l and a n t i - f r i c t i o n p r o p e r t i e s
82 ( r e f s . 154, 1 5 5 ) . I n c o p p e r - b a s e d a l l o y w i t h 8-11% Sn, t h e a d d i t i o n of 0.2-1.7%
P l o w e r s t h e s i n t e r i n g t e m p e r a t u r e a t which m a t e r i a l
w i t h good t r i b o l o g i c a l p r o p e r t i e s i s o b t a i n e d . Amorphous m e t a l s and a l l o y s s u c h a s i r o n , c o b a l t a n d n i c k e l a l l o y s h a v e a f r i c t i o n c o e f f i c i e n t o f 0.2-0.5
and a r e l a t i v e l y low
wear r a t e ( r e f . 1 5 8 ) . The a d h e s i v e wear p r o c e s s i n s u c h m a t e r i a l s g e n e r a l l y i n v o l v e s t h r e e s t a g e s : t h e f i r s t s t a g e shows a smooth s u r f a c e c r e a t e d by t h e removal o f a s p e r i t i e s , t h e s e c o n d i n v o l v e s s e v e r e p l a s t i c deformation as w e l l a s c r a c k n u c l e a t i o n and propagat i o n , and t h e t h i r d s t a g e p a r t i a l l y c h a n g e s t h e wear mechanism from a d h e s i v e t o a b r a s i v e a s a c o n s e q u e n c e o f t h e embedment o f f r a c t u r e particles a t the interface. The f r i c t i o n c o e f f i c i e n t f o r p u r e metals w a s f o u n d t o b e r e l a t ed t o t h e t h e o r e t i c a l t e n s i l e s t r e n g t h , t h e o r e t i c a l s h e a r s t r e n g t h and a c t u a l s h e a r s t r e n g t h o f m e t a l s ( r e f . 1 5 9 ) . By r u b b i n g t h e m e t a l s a g a i n s t t h e m s e l v e s a t a v e r y low s l i d i n g s p e e d ( 0 . 0 1 2 m/s) , -8 a t l o w l o a d s a n d i n a h i g h vacuum o f 3 1 0 P a , it w a s f o u n d t h a t t h e h i g h e r t h e s t r e n g t h of t h e m e t a l , t h e lower t h e c o e f f i c i e n t of f r i c t i o n . The wear r e s i s t a n c e o f p u r e metals i n c r e a s e d w i t h increasi n g c o v a l e n t b i n d i n g e n e r g y ( r e f . 1 6 0 ) . Metals w i t h a p u r e m e t a l l i c bond, s u c h a s Ag, Al, Cu and Mg, a l l h a v e s i m i l a r , r e l a t i v e l y low wear r a t e s , b u t p u r e metals w i t h a m e t a l l i c , p a r t i a l l y c o v a l e n t bond, s u c h a s N i l Mo or W , a r e c h a r a c t e r i z e d by h i g h wear r a t e s which i n c r e a s e a s t h e c o n t r i b u t i o n o f t h e c o v a l e n t bond becomes more i m p o r t a n t . The r e l a t i o n s h i p between t h e
wear r a t e a n d b i n d i n g
e n e r g y c a n b e d e s c r i b e d w i t h p a r a b o l a which a p p r o a c h e s t h e b r i z o n t a l axis. T y p i c a l wear c o e f f i c i e n t v a l u e s f o r a d h e s i v e wear i n m e t a l - o n -metal sliding systems are10-3 while f o r abrasive wear they
-
-
are l o m 2 ( r e f . 1 6 1 ) . S e v e r e g a l l i n g wear o c c u r s when two c l e a n metals w i t h a h i g h d e g r e e o f m e t a l l u r g i c a l c o m p a t i b i l i t y s l i d e over each o t h e r ; t h e w e a r c o e f f i c i e n t s are u s u a l l y i n t h e a n d t h e w e a r p a r t i c l e s i z e s i n t h e r a n g e of range of 200-20 ,um ( r e f .161) With l e s s c o m p a t i b l e metals, m o d e r a t e wear occ u r s ; when t h e c o n t a c t p r e s s u r e i s low t h e t y p i c a l wear c o e f f i c i e n t s are around w h i l e wear p a r t i c l e s i z e s a r e i n t h e r a n g e of 20-2 ,urn. The w e a r c o e f f i c i e n t a p p e a r s i n t h e Holm-Archard r e l a t i o n s h i p f o r a d h e s i v e a n d a b r a s i v e wear:
.
wear c o e f f i c i e n t
*
load
*
sliding distance
(4.3)
wear volume = hardness
83
For metals i n c o n t a c t w i t h metals, t h e p r e s e n c e o f oxygen red u c e s t h e a d h e s i o n and c o r r e s p o n d i n g l y t h e f r i c t i o n c o e f f i c i e n t , a s compared w i t h metals i n t h e c l e a n s t a t e . The f r i c t i o n a t various loads i n t h e p r e s e n c e of oxygen i s i n t e r m e d i a t e between t h e c l e a n s t a t e and t h e l u b r i c a t e d s t a t e ( r e f . 1 6 2 ) . The w e a r r a t e o f metals
i s a l s o r e d u c e d by t h e p r e s e n c e of oxygen s i n c e t h e o x i d e s which form a c t e i t h e r a s a l u b r i c a n t o r a n a b r a s i v e m a t e r i a l , depending o n t h e i r h a r d n e s s ( r e f s . 1 1 7 , 1 6 3 ) . For a comparison o f t h e hardn e s s of some metals and o x i d e s , see T a b l e 4 . 2 ( b a s e d o n r e f . 1 6 3 )
.
TABLE 4.2
MOHS HARDNESS OF METALS AND METAL O X I D E S
METAL
HARDNESS
METAL 0x1 DE
Sn
1 .8
Al
2.2
cu20
Zn
2.6
Z nO
cu
3
M"304
3.5 ca. 5.5
Fe CuN iMn Hardened s t e e l
6-6.5
Nitrided steel
a
Sintered hard s t e e l
9
Mg (OH)
Fe203 Mg0 Mn203 Sn02
N i t r ides 2'3
HARDNESS 2.6
3.5-4 4-4.5 5-5.5 5.5-6
6 6-6.5
6.5-7 7.5-8.5 9
The p r e s e n c e o f h a r d o x i d e s ( s u c h a s A 1 2 0 3 ) between r e l a t i v e l y s o f t s u r f a c e s ( s u c h a s A l ) r e s u l t s i n a r e l a t i v e l y h i g h wear r a t e .
42 I
POLYMER I C SYSTEMS
I
4.2.1
. METAL-POLYMER
SYSTEMS
Metal-polymer t r i b o l o g i c a l s y s t e m s a r e u s e d more and more f r e q u e n t l y i n m i n i a t u r e mechanisms. A r e l a t i v e l y low f r i c t i o n c o e f f ic i e n t and s u f f i c i e n t l y h i g h w e a r - r e s i s t a n c e c a n b e a c h i e v e d i n t h e s e s y s t e m s by t h e p r o p e r s e l e c t i o n o f t h e polymer a n d t h e m e t a l t o be u s e d . S t e e l i s t h e most s u i t a b l e metal f o r u s e a s a c o u n t e r f a c e f o r polymeric elements ( r e f s . 1 6 4 , 1 6 5 ) . T y p i c a l s l i d i n g systems i n m i n i a t u r e mechanisms - j o u r n a l b e a r i n g s have a j o u r n a l made o f f r e e c u t t i n g s t e e l and a b e a r i n g bush i n i n j e c t i o n -moulde d , t h e r m o p l a s t i c polymer; t h e j o u r n a l s u r f a c e i s u s u a l l y r o l l e r
84 b u r n i s h e d t o Ra < 0 . 2 p m . The wear r a t e o f t h e j o u r n a l i s n e g l i g i b l e compared t o t h e w e a r r a t e o f t h e p o l y m e r i c b e a r i n g b u s h (when t h e p o l y m e r i c m a t e r i a l i s n o t r e i n f o r c e d ) . S t u d i e s have shown t h a t t h e b e s t t r i b o l o g i c a l p r o p e r t i e s o f s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g s a r e o b t a i n e d when t h e p o l y m e r i c b e a r i n g b u s h e s a r e made from POM or PA m a t e r i a l s ( r e f s . 166-180). B e a r i n g s w i t h POM b e a r i n g b u s h e s d e m o n s t r a t e b e t t e r t r i b o l o g i c a l p r o p e r t i e s t h a n t h o s e w i t h PA b u s h e s . The f r i c t i o n c o e f f i c i e n t of s t e e l
-
POM b e a r i n g s i s less t h a n 0 . 3 w h e r e a s f o r s t e e l
PA b e a r i n g s it can b e a s much a s 0 . 4 5 .
The d e p e n d e n c e o f t h e f r i c -
t i o n c o e f f i c i e n t o n t h e s l i d i n g s p e e d f o r t h e s e b e a r i n g s i s shown i n F i n s . 4 . 5 a n d 4.6.
-1
-2
-3 -4
~
41
0.2
0.3
t
i-
Sliding speed, m/s
F i g . 4 . 5 . F r i c t i o n c o e f f i c i e n t v s . s l i d i n g speed f o r steel-POM h m i n i a t u r e j o u r n a l b e a r i n g s . Nominal diameter 2.15 mm, e x t e r n a l diameter o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2.1 mm, d i a m e t r a l c l e a r a n c e 1 .5-2%. 1 - c o n t a c t pressure p = 0 . 5 MPa, 2 - 1 MPa, 3 - 2 MPa, 4 3 MPa.
-
85
/ 2.0 3.0
0.2
t I
0 . T
0.3
Sliding speed, rn/s
F i g . 4 . 6 . F r i c t i o n c o e f f i c i e n t vs. s l i d i n g speed f o r steel-PA 6 m i n i a t u r e j o u r n a l b e a r i n g s . Nominal diameter 2 . 1 5 mm, e x t e r n a l d i a m e t e r o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2.1 rnm, d i a m e t r a l c l e a r a n c e 1 .5-2%. 1 - c o n t a c t p r e s s u r e p = 0 . 5 MPa, 2 - 1 MPa, 3 - 2 MPa, 4 - 3 MPa.
The r e l a t i o n s h i p b e t w e e n t h e f r i c t i o n c o e f f i c i e n t , t h e s l i d i n g speed and t h e c o n t a c t p r e s s u r e can be expressed with t h e following formula : f = (alv
+
-(a3v + a4) a2)p
(4.4)
w h e r e f o r b e a r i n g s u s i n g PA 6 , PA 6 6 , POM h o r PO14 c , t h e v a l u e s o f a l , a2, a3 a n d a4 r e s p e c t i v e l y , 0.12;
PA 66
-
0.3,
0.125,
0.5,
a r e : PA 6
0.12;
POM h
-
-
0.45,
0.2,
0.16,
0.1,
0.3,
0.5, 0.13;
POI4 c - 0 . 1 1 , 0 . 1 3 5 , 0 . 4 2 5 , 0 . 2 5 . T h e v a l u e s of p a n d v i n e q n . ( 4 . 4 ) s h o u l d be i n t r o d u c e d i n MPa a n d i n m / s r e s p e c t i v e l y .
86
The b e a r i n g s w i t h POM b e a r i n g b u s h e s d e m o n s t r a t e b e t t e r wear r e s i s t a n c e t h a n PA 6
wear r a t e s f o r POM
-
-
b a s e d b e a r i n g s . A c o m p a r i s o n of t h e r a d i a l
and PA
-
based bear in g s (based on t h e s t u d i e s
d e s c r i b e d i n r e f . 1 7 4 ) is presented i n F i g . 4 . 7 .
1
01
I
I
I
2
3
*
F i g . 4 . 7 . R a d i a l wear r a t e v s . c o n t a c t p r e s s u r e f o r s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g s . Nominal d i a m e t e r 2.15 nun , e x t e r n a l d i a m e t e r o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2.1 mm, d i a m e t r a l c l e a r a n c e 1 .5-2%, s l i d i n g speed 0.0167 m / s , s l i d i n g d i s t a n c e 3 0 km. 1 - P A 6, 2 - P A 66, 3 - POM c , 4 - POM h .
I n c r e a s i n g t h e c o n t a c t p r e s s u r e r e s u l t s i n a n i n c r e a s e i n t h e radial wear r a t e , p a r t i c u l a r l y f o r PA
-
b a s e d b e a r i n g s , b u t when t h e con-
t a c t p r e s s u r e i s h i g h enough ( a b o v e 2 MPa) t h e i n c r e a s e i n t h e r a d i a l wear r a t e of POM
-
based bearin g s i s a l s o s i g n i f i c a n t .
I n c r e a s i n g t h e c o n t a c t p r e s s u r e above 3 MPa r e s u l t s i n a dramatically h i g h e r wear r a t e ( r e f s . 1 6 8 , 1 7 0 ) . The e f f e c t o f t h e s l i d i n g s p e e d
87 o n wear i s smaller t h a n t h e e f f e c t of t h e c o n t a c t p r e s s u r e ( r e f s . 1 6 8 , 1 7 0 , 1 7 4 ) . The w e a r r a t e i n t e n s i t y c a n g e n e r a l l y be assumed
t o be a s t r a i g h t l i n e c o r r e l a t e d w i t h t h e pv ( p v
-
-
c o n t a c t pressurer
s l i d i n g speed) parameter of miniature steel-polymer j o u r n a l
bearings. O t h e r m a t e r i a l s a l s o a p p l i c a b l e f o r t h e b u s h e s i n t h i s k i n d of b e a r i n g a r e t h e p o l y t e r e p h t h a l a t e s PETP a n d PBTP ( r e f s . 1 6 8 , 1 7 0 , 1 8 1 ) . The f r i c t i o n c o e f f i c i e n t of t h e s e b e a r i n g s i s simil a r t o t h o s e w i t h PA b e a r i n g b u s h e s b u t t h e wear r a t e i s s l i g h t l y
175-177,
h i g h e r . O t h e r s u i t a b l e b e a r i n g m a t e r i a l s a r e PPO, PC a n d P I ( r e f s . 175-177). O p e r a t i n g c o n d i t i o n s s u c h as ambient t e m p e r a t u r e and h u m i d i t y play on important role i n t h e o p e r a t i n g behaviour of steel-polymer m i n i a t u r e j o u r n a l b e a r i n g s ( r e f s . 11, 1 8 2 - 1 8 4 ) . A h i g h e r t e m p e r a t u r e lowers t h e f r i c t i o n c o e f f i c i e n t f o r steel-POM m i c r o b e a r i n g s ( F i g . 4.8,
based on r e f . 1 8 4 ) .
t
-101
0
1
1
20
40
I
60
1
80
t
Temperature , OC F i g . 4.8. F r i c t i o n c o e f f i c i e n t v s . t e m p e r a t u r e i n steel-POM h m i c r o b e a r i n g s (nominal d i a m e t e r 0.14 mm). Load 16 mN, s l i d i n g speed 3.5 m / s ( r e f . 1 8 4 ) .
aa The e f f e c t of h u m i d i t y on t h e f r i c t i o n c o e f f i c i e n t o f s t e e l - P O M b e a r i n g s i s n e g l i g i b l e ( r e f . 1 8 4 ) b u t f o r PA-based b e a r i n g s a n i n -
crease i n h u m i d i t y c a n lower t h e f r i c t i o n c o e f f i c i e n t and wear rate ( r e f . 1 8 5 ) .
During t h e o p e r a t i o n o f s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g s , t h e r e i s a marked t r i b o e l e c t r i f i c a t i o n e f f e c t ( r e f . 186). The p o t e n t i a l produced i n a b e a r i n g w i t h PA 6 b u s h e s r i s e s i n a r e l a t i v e l y s h o r t time t o s e v e r a l v o l t s ( + ) and s t a b i l i z e s a t t h i s l e v e l . The e f f e c t of t h e s l i d i n g s p e e d and t h e c o n t a c t p r e s s u r e o n t h e po’zential produced i s p r e s e n t e d i n F i g s . 4 . 9 and 4 . 1 0 .
j )
2 0.3 Sliding speed m/s
,
F i g . 4.9. P o t e n t i a l produced vs. s l i d i n g speed f o r s t e e l - P A 6 m i n i a t u r e j o u r n a l b e a r i n g s . Nominal d i a m e t e r 0 2 . 1 5 mm, e x t e r n a l d i a m e t e r o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2.1 mm, d i a m e t r a l c l e a r a n c e 1 . 5 - 2 % . C o n t a c t p r e s s u r e p : 1 - 3 MPa, 0 . 5 MPa. 2 - 2 MPa, 3 - 1 MPa, 4
-
89
Contact pressure, MPa
F i g . 4 . 1 0 . P o t e n t i a l produced v s . c o n t a c t p r e s s u r e f o r steel-PA 6 m i n i a t u r e j o u r n a l b e a r i n g s . Nominal d i a m e t e r 0 2 . 1 5 mm, e x t e r n a l d i a m e t e r o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2 . 1 mm, d i a m e t r a l c l e a r a n c e 1 . 5 - 2 % . S l i d i n g speed v : 1 - 0 . 0 6 7 m / s , 2 - 0 . 0 1 6 7 m / s .
The c o r r e l a t i o n between t h e p o t e n t i a l produced and t h e t r i b o l o g i c a l p r o p e r t i e s of t h e b e a r i n g s h a s been a n a l y s e d ( r e f . 186). Other m i n i a t u r e t r i b o l o g i c a l systems such a s t h e s t e e l s p h e r e - p l a t e i n o s c i l l a t i o n motion i n t h e ASTM pendulum o r t h e s p h e r e -
see Chapter 8 . 2 ) , denonstrate s i m i l a r t r i b o ' l o g i c a l p r o p e r t i e s t o t h e aforementioned m i n i a t u r e j o u r n a l b e a r i n g s ( r e f s . 43, 44, 106, 166, 169, 170, 171).
-oscillating t a b l e ( p l a t e )(UTI set-up,
F i l l e d polymers used a s m a t e r i a l s f o r t h e b e a r i n g bushes o r o t h e r s l i d i n g e l e m e n t s i n m i n i a t u r e mechanisms have b e t t e r t r i b o l o g i c a l p r o p e r t i e s t h a n u n f i l l e d polymers used f o r t h e same purpose. T h i s i s e s p e c i a l l y t r u e of polymers f i l l e d w i t h s o l i d l u b r i c a n t s such a s PTFE ( o r P E ) , g r a p h i t e o r MoS2. The f r i c t i o n c o e f f i c i e n t and t h e wear r a t e g e n e r a l l y d e c r e a s e ( r e f s . 164, 1 6 6 , 1 6 8 , 169, 170, 174, 178, 179). When t h e polymer i s r e i n f o r c e d , e . g . , w i t h g l a s s f i b r e , t h e e f f e c t of t h e r e i n f o r c e m e n t on t h e t r i b o -
90
l o g i c a l p r o p e r t i e s d e p e n d s o n t h e o p e r a t i n g c o n d i t i o n s ( r e f s . 175187-1891. The n a t u r e of t h e r e i n f o r c i n g m a t e r i a l a l s o a f f e c t s t h e t r i b o l o g i c a l p r o p e r t i e s of t h e s y s t e m s . When r e i n f o r c e d p l y m e x
-177,
i s u s e d i n metal-polymer s y s t e m s , e . g . i n m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s , t h e r e i s w e a r of t h e m e t a l ( s t e e l ) e l e m e n t ( r e f s . 1 7 0 , 1 7 1 , 1 7 4 , 188, 1 8 9 ) . S t e e l - p o l y m e r j o u r n a l b e a r i n g s which h a v e a b e a r i n g b u s h made
from polymer r e i n f o r c e d w i t h g l a s s f i b r e (25-35% by w e i g h t ) demons t r a t e a r e l a t i v e l y h i g h f r i c t i o n c o e f f i c i e n t ( u p t o 0 . 8 ) . The f r i c t i o n c o e f f i c i e n t , a n d a l s o t h e wear r a t e o f t h e s t e e l j o u r n a l , c a n b e r e d u c e d by t h e s i m u l t a n e o u s a d d i t i o n o f a s o l i d l u b r i c a n t s u c h a s PTFE, g r a p h i t e o r MoSZ. F i l l i n g t h e polymer w i t h g l a s s f i b r e i s g e n e r a l l y more e f f e c t i v e t h a n f i l l i n g it w i t h glass b e a d s , a l t h o u g h t h e o p p o s i t e i s t r u e i n t h e c a s e o f PBTP w i t h a r e l a t i v e l y low s h e a r modulus o f , s a y , 750 MPa ( r e f s . 1 8 8 , 1 8 9 ) . G l a s s b e a d s r e l e a s e d by w e a r may a c t a s a b r a s i v e s . The wear r a t e o f r e i n f o r c e d polymer m a t e r i a l s d e p e n d s t o a g r e a t e x t e n t o n t h e o p e r a t i n g c o n d i t i o n s . The wear r a t e s o f some r e i n f o r c e d polymers u s e d a s m a t e r i a l s f o r b e a r i n g b u s h e s i n m i n i a t u r e steel-polymer j o u r n a l b e a r i n g s are p r e s e n t e d i n F i g s . 4 .ll and 4 . 1 2
( p l o t s a r e based on t h e d a t a found i n r e f s . 1 7 4 and 1 7 0 ,
1 7 1 , 1 7 2 r e s p e c t i v e l y ) . An i n c r e a s e i n c o n t a c t p r e s s u r e a n d / o r
s l i d i n g s p e e d r e s u l t s i n a h i g h e r wear r a t e . A t h i g h l o a d s , thermal e f f e c t s p l a y a n i m p o r t a n t r o l e i n t h e wear p r o c e s s . S i n c e g l a s s - f i l l e d polymers g i v e a h i g h e r c o e f f i c i e n t o f f r i c t i o n t h e h e a t i s g r e a t e r t h a n i n b e a r i n g s w i t h b e a r i n g b u s h e s made from unreinforced polymers. A t h i g h l o a d s ( h i g h p v
-
contact pressure x sliding
s p e e d ) , t h e t h e r m a l d e c o m p o s i t i o n o f t h e polymer may o c c u r i n t h e neighbourhood of t h e f i b r e s ( r e f . 1 9 0 ) , s i n c e f i b r e s exposed on t h e surface tend t o act as hot spots during s l i d i n g . This effect
i s p r o b a b l y t h e main r e a s o n why t h e w e a r r a t e o f POM b e a r i n g bushes i s lower t h a n t h e wear r a t e o f g l a s s f i b r e f i l l e d POM b e a r i n g b u s h e s ( r e f s . 175-177,
see T a b l e 4 . 3 ) .
The f r i c t i o n c o e f f i c i e n t s o f s e v e r a l f i l l e d p o l y m e r s when t e s t e d i n a p l a t e - s p h e r e s y s t e m ( U T I a p p a r a t u s , see C h a p t e r 8 . 2 ) a r e shown i n F i g . 4 . 1 3
( b a s e d o n r e f s . 1 7 1 , 1 7 2 ) . The f r i c t i o n
c o e f f i c i e n t s a r e g e n e r a l l y similar t o t h o s e o b s e r v e d i n m i n i a t u r e j o u r n a l b e a r i n g s ( r e f s . 1 6 7 , 1 7 0 , 1 7 1 , 1 7 2 , a n d p e r s o n a l communic a t i o n from t h e a u t h o r o f r e f s . 1 7 5 - 1 7 7 ) . The w e a r r a t e of t h e rubbing p l a t e s under t h e c o n d i t i o n s d e s c r i b e d i n t h e c a p t i o n t o F i g . 4.13 w a s n e g l i g i b l e .
91
A
1
100 .
L
Sliding distance , k m
F i g . 4 . 1 1 . R a d i a l wear r a t e s o f p o l y m e r i c b e a r i ng bush o f s t e e l -polymer m i n i a t u r e j o u r n a l b e a r i n g s w i t h bushes made o f PA 6 + 25% g l a s s f i b r e + 4% MoSg ( p l o t s 1 and 2) o r PA 6 + 25% g l a s s f i b r e + 4% g r a p h i t e ( p l o t s 3 and 4 ) v s . s l i d i n g d i s t a n c e . Nominal diameter 2.15 mm, e x t e r n a l diameter o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2.1 mm, r e l a t i v e b e a r i n g c l e a r a n c e 1 .5-2%, sl i d i n g speed 0 . 0 6 7 m / s . 1,3 - cont a c t pressure p = 0 . 5 MPa, 2,4 - 0.2 MPa.
92
I
i
I
10
20
30
*
sliding distonce , km
F i g . 4.12. R a d i a l wear r a t e o f p o l y m e r i c b e a r i n g bush o f s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g s w i t h bushes made f r o m u n r e i n f o r c e d a n d r e i n f o r c e d p o l y m e r s v s . s l i d i n g d i s t a n c e . Nominal b e a r i n g d i a m e t e r 1 mm, e x t e r n a l d i a m e t e r 2.7 mm, b e a r i n g l e n g t h ( l e n g t h o f c o n t a c t w i t h t h e j o u r n a l ) 1 mm, r e l a t i v e c l e a r a n c e 3-6%. 1 - PC ( s l i d i n g speed 314 mm/s, c o n t a c t p r e s s u r e 0 . 2 2 5 MPa), 2 - PA 6 6 (13.1 mm/s, 2.5 MPa), 3 - PBTP ( 1 3 . 1 m m / s , 1 .25 MPa), 4 - PBTP + 28% g l a s s beads 5 - 50 ,urn (13.1 mm/s, 1 . 2 5 MPa), 5 - PBTP + g l a s s f i b r e (13.1 m m / s , 1 , 2 5 MPa), 6 - PA 6 6 + 50% g l a s s f i b r e ( 1 3 . 1 m m / s , 1 . 2 5 MPa), 7 - PA 6 6 + g l a s s beads ( 1 3 . 1 mm/s, 1,25 MPa), 8 - PC + 3 0 % g l a s s f i b r e (314 mm/s, 0.225 MPa).
93 TABLE 4.3 SPECIFIC WEAR RATE OF POLYMERIC BEARING BUSH I N STEEL-POLYMER JOURNAL BEARINGS. JOURNAL MADE OF STAINLESS STEEL, GROUND SURFACE TO R = 0 . 1 - 0 . 2 g m , NOMINAL BEARING DIAMETER 5 mm, EXTERNAL DIAMETER 1 0 mm, B E A R ~ N GLENGTH 2.8 mm ( r e f s . 175-1 77)
.
SPECIFIC WEAR RATE,
km MPa
POLYMERIC MATERIAL v = 0.26 m/s p = 0.075MPa PA 66 PA 6 6 + 25% g l a s s f i b r e PA 12 PA 12 + 30% g l a s s f i b r e POM c POM c + 30% g l a s s f i b r e POM c + PTFE g l a s s f i b r e PC PC + 30% g l a s s f i b r e PPO ABS PETP PI P I + inorganic f i l l e r PTFE + i n o r g a n i c f i l l e r
v = 0.026m/s p = 0 . 7 5 MPa
v=0.0026 m / s p=7.5 MPa
8.2
9.1
7.0
6.6
10.6 2.5 1 .o
9.9 2.3 0.3
14.3 11.5 6.5 16 0.81 68 1 .8 2.9
44 1.3 13 1.5
13 200 0.7
3.3 0.20
0.13
51 0.41 7.3 2.4
15 28 1 .1 2.0 0.07 0.30
4.8 8.2 20
3.6 2.1 0.57 0.91
f
F i g . 4 . 1 3 . F r i c t i o n c o e f f i c i e n t f o r p l a t e - s p h e r e s y s t e m (UTI a p p a r a t u s , s e e C h a p t e r 8 . 2 ) . O s c i l l a t i n g p o l y m e r p l a t e 1 0 x 6 ~ 2mm, s t e e l s p h e r e ( b e a r i n g s t e e l ) 1/2". Load 6 . 1 N, s l i d i n g speed lOmm/s. 1 - PA 66 + 40% c a r b o n f i b r e , 2 PETP + 30% g l a s s f i b r e , 3 - PBTP + 30% g l a s s f i b r e , 4 - PBTP, 5 - PC, 6 - PC + 30% g l a s s f i b r e , 7 - PC + 30% g l a s s f i b r e + 15% PTFE, 8 - PA1 + 3% T i 0 2 + 0.5% PTFE, 9 - P A 1 + 3% T i 0 2 + 0.5% PTFE, 10 - POM h ( D e l r i n 500 NC 1 0 ) . 11 - POM h c h e m i c a l l y l u b r i c a t e d ( D e l r i n 500 CL) ( r e f . 1 7 1 ) .
-
94
The wear p r o c e s s i n m i n i a t u r e s t e e l - p o l y m e r s y s t e m s ( e . g . journ a l b e a r i n g s ) is of a n adhesive-cohesive n a t u r e s i n c e t h e s u r f a c e of t h e s t e e l e l e m e n t i s v e r y smooth, s l i d i n g s p e e d s a r e v e r y low and t h e c o n t a c t p r e s s u r e may b e h i g h . The t r a n s f e r o f t h e p o l y -
m e r i c m a t e r i a l t o t h e s t e e l s u r f a c e a n d s u c c e s s f u l s h e a r i n g o f new l a y e r s a r e t y p i c a l o f u n f i l l e d m a t e r i a l s s u c h a s PA
a n d POM (refs.
1 9 1 - 1 9 3 ) . The t h i c k n e s s o f t h e f i l m o f t r a n s f e r r e d m a t e r i a l s t a b i l i z e s b e f o r e s t e a d y - s t a t e wear o c c u r s d u r i n g c o n t i n u o u s s l i d i n g . The wear of t h e polymer o c c u r s by t h e p r o c e s s e s o f t r a n s f e r t o t h e c o u n t e r f a c e and t h e s u b s e q u e n t d e t a c h m e n t of t h e f r a g m e n t s f r o m t h e t r a c k ( r e f . 1 9 4 ) . The i n c r e a s e i n f i l m t h i c k n e s s is l i m i t e d b y t h e a d h e s i v e bond r e s i s t a n c e o f t h e polymer f i l m t o t h e m e t a l s u r f a c e . When t h e maximum t h i c k n e s s o f t h e f i l m i s r e a c h e d , t h e f i l m c a n l o s e i t s l o a d c a p a c i t y , i s d e s t r o y e d and e s c a p e s as w e a r debris when t h e e l a s t i c s t r a i n e n e r g y s t o r e d i n it becomes g r e a t e r t h a n o r e q u a l t o t h e a d h e s i o n a l energy a c t i n g o v e r t h e i n t e r f a c e ( r e f . 1 9 5 ) . The r e l a t i o n s h i p between t h e r a d i a l w e a r i n t e n s i t y o f t h e polymeric b e a r i n g bush i n m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s and t h e maximum t h i c k n e s s o f t h e polymer t r a n s f e r r e d f i l m i s p r e sented i n Fig. 4.14
( r e f . 1 9 6 ) . The i n c r e a s e i n f i l m t h i c k n e s s i s
accompanied by a n i n c r e a s e i n wear i n t e n s i t y ; t h i s e f f e c t may b e due t o t h e l o w e r h e a t t r a n s f e r from t h e f r i c t i o n r e g i o n by t h e
s t e e l s h a f t v i a t h e t h i c k e r polymer t r a n s f e r r e d f i l m . Nevertheless, t h e t r a n s f e r o f t h e polymer m a t e r i a l i n m e t a l - p o l y m e r
systems i s
g e n e r a l l y b e n e f i c i a l b e c a u s e o f i t s s e l f - l u b r i c a t i n g a c t i o n . The amorphous polymers ( e . g . P C , PPO) do n o t t r a n s f e r c o h e r e n t f i l m s t o t h e c o u n t e r f a c e d u r i n g s l i d i n g a n d t h e i r wear r a t e i s r e l a t i v e l y high (see Table 4 . 3 )
.
The wear d e b r i s formed d u r i n g s l i d i n g between a polymer and
s t e e l s u r f a c e a r e u s u a l l y e l l i p s o i d i n s h a p e , t h e t h i c k n e s s of t h e p a r t i c l e depending on t h e f o r m a t i o n p r o c e s s ( r e f s . 1 9 4 , 1 9 7 , 1 9 8 ) . I f t h e p a r t i c l e s e s c a p e from t h e i n t e r f a c e as s o o n a s t h e y are formed, t h e y a r e l e s s t h i c k t h a n i f t h e y a r e t r a p p e d between t h e s l i d i n g s u r f a c e s a n d bond a g a i n w i t h t h e polymer s u r f a c e . A h i g h e r
wear r a t e o f t h e polymer i s u s u a l l y accompanied by l a r g e r wear p a r t i c l e s ( r e f s . 1 9 4 , 1 9 7 , 1 9 8 ) . The r e l a t i o n s h i p between t h e r a d i a l wear i n t e n s i t y of t h e p o l y m e r i c b e a r i n g b u s h i n m i n i a t u r e steel-polymer j o u r n a l b e a r i n g s i s presented i n F i g . 4.15 196).
(ref.
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*
, pm
F i g . 4 . 1 4 . Radial wear i n t e n s i t y o f p o l y m e r i c b e a r i n g bush i n a s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g v s . t h i c k n e s s o f t r a n s f e r r e d polymer f i Im. C i r c l e s : beari n g h o l e 1 mm, e x t e r n a l b e a r i n g bush d i a m e t e r 2.7 mm, b e a r i n g l e n g t h 1 mm. r e l a t i v e c l e a r a n c e 4-6%, s l i d i n g speed 1 3 . 1 mm/s, c o n t a c t p r e s s u r e 1.25 MPa. Squares: b e a r i n g h o l e 5 mm, e x t e r n a l b e a r i n g bush d i a m e t e r 10 mm, b e a r i n g l e n g t h 2.8 mm, r e l a t i v e c l e a r a n c e 1%, s l i d i n g speed 2.6 mm/s, c o n t a c t p r e s s u r e 0.75 MPa. 1 - POM h, 2 - POM C , 3 - PA 11, 4 - PA 66, 5 PBTP, 6 PA 12. 7 - PETP.
-
-
The a d h e s i v e wear o c c u r r i n g i n t h e m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g c o r r e l a t e s w i t h t h e a d h e s i v e bonding f o r c e between t h e r u b b i n g s u r f a c e s which may be d e t e r m i n e d o n t h e b a s i s of t h e t h e o r y of f l u c t u a t i o n s of Van d e r Waals d i s p e r s i o n f o r c e s ( r e f s . 1 9 9 , 2 0 0 ) . Using I s r a e l a s h v i l y ' s
formula (see refs. 2 0 1 , 2 0 2 ) ,
t h e a d h e s i v e f o r c e p e r u n i t c o n t a c t area Za f o r t h e f l a t c o n t a c t i n d i e l e c t r i c - d i e l e c t r i c s e p a r a t e d by t h e l a y e r o f t h e d i e l e c t r i c
96
l i q u i d can be calculated as follows:
where h r e p r e s e n t s h/2‘ii(h
-
Planck’s c o n s t a n t ) , H i s t h e distance
between s u r f a c e s ( t h i c k n e s s o f t h e l i q u i d l a y e r ) , u ,
is the fre-
quency of h i g h e s t a b s o r p t i o n , which may b e t a k e n as b e i n g t h e same f o r t h e m a j o r i t y o f d i e l e c t r i c s , a d e q u a t e t o t h e u l t r a v i o l e t wavel e n g t h ( A o ) o f 1 0 0 nm
,
a n d El0,
E20
and &30 a r e t h e d i e l e c t r i c
c o n s t a n t s of t h e two d i e l e c t r i c s a n d t h e s e p a r a t i n g l i q u i d r e s p e c tively.
Wear particle thickness p m
F i g . 4 . 1 5 . R a d i a l wear i n t e n s i t y o f p o l y m e r i c b e a r i n g bush i n a s t e e l -polymer m i n i a t u r e j o u r n a l b e a r i n g v s . wear p a r t i c l e t h i c k n e s s . S l i d i n g speed 26 mm/s, c o n t a c t p r e s s u r e p = 0.75 MPa. 1 - P I , 2 POM c , 3 - PC, 4 - P E T P , 5 - PA 6 6 , 6 - PA 1 2 , 7 PPO, 8 ABS.
-
-
-
97
F o r metal-polymer c o n t a c t w i t h o u t l i q u i d E 3 0 = 1, E 2 0 -m and w e o b t a i n
h oo (El0 -
I
1) (4.6)
The a d h e s i v e f o r c e p e r u n i t a r e a of c o n t a c t i n c r e a s e s parabol i c a l l y with t h e i n c r e a s e i n d i e l e c t r i c c o n s t a n t of t h e polymeric
m a t e r i a l . The c o r r e l a t i o n between t h e r a d i a l wear i n t e n s i t y o f t h e p o l y m e r i c b e a r i n g bush i n m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s and t h e r a t i o o f
rmp/ec( e c -
c o h e s i v e e n e r g y d e n s i t y ) i s discussed
elsewhere ( r e f . 1 9 6 ) . For steel-polymer m i n i a t u r e j o u r n a l b e a r i n g s w i t h a b e a r i n g bush made from u n f i l l e d polymer, t h e a d h e s i v e - c o h e s i v e p r o p e r t i e s of t h e polymers seem t o p l a y a dominant r o l e i n t h e wear mechanism
when t h e c o n t a c t p r e s s u r e i s r e l a t i v e l y h i g h a n d t h e s l i d i n g s p e e d v e r y s m a l l ( s a y below 0 . 0 2 m / s ) .
The f r i c t i o n a l h e a t i n g i n t h e
f r i c t i o n region i s then negligible (say a temperature rise i n t h e f r i c t i o n r e g i o n a T < 3 K)
. In
s u c h s i t u a t i o n s t h e wear o f t h e p o l y -
m e r m a t e r i a l o f t h e b e a r i n g bush depends o n b o t h t h e a d h e s i v e b n i i s a t t h e i n t e r f a c e between t h e r u b b i n g e l e m e n t s and o n t h e c o h e s i v e
p r o p e r t i e s of t h e m a t e r i a l b e i n g worn. The g e n e r a l f o r m u l a r e l a t i n g t o t h e volume of worn polymer m a t e r i a l ( V )
,
t a k i n g i n t o con-
s i d e r a t i o n t h e a d h e s i o n a l e n e r g y o n t h e i n t e r f a c e (Ea) a n d t h e c o h e s i o n a l e n e r g y of t h e polymer ( E c ) , c a n be w r i t t e n a s f o l l o w s : V = f
Ea
(4.7)
( 7 ) C
The a d h e s i o n a l e n e r g y on t h e i n t e r f a c e c a n be d e t e r m i n e d u s i n g t h e formula: I
Ea =
wps d 2
'Po
(4.8)
where W
is t h e s p e c i f i c energy of adhesion a t t h e polymer-steel PS i n t e r f a c e , d2 t h e bearing h o l e diameter, 1 t h e bearing l e n g t h , and
YO
t h e h a l f a n g l e o f c o n t a c t between j o u r n a l and b e a r i n g b u s h when
t h e r a d i a l wear w > 0 . The h a l f a n g l e o f c o n t a c t , 19, following formula:
I
,
can be c a l c u l a t e d using t h e
98
cos
, y o --
+
-d12
(s
+
2a
(s
2 dl
where dl
+
+
2wI2
+
(dl
+
s)
2 (4.9)
2a + 2w)
is t h e journal diameter, s t h e diametral clearance, a t h e
d e f o r m a t i o n o f t h e p o l y m e r i c b e a r i n g b u s h u n d e r l o a d , and w t h e r a d i a l w e a r o f t h e polymeric b e a r i n g bush. The d e f o r m a t i o n o f t h e p o l y m e r i c b e a r i n g bush u n d e r a p a r t i c u l a r load can be estimated using t h e formula: Pm a = g -
(4 .lo)
E
where g i s t h e w a l l t h i c k n e s s o f t h e b e a r i n g b u s h , pm t h e maximum v a l u e of t h e r e a l i s t i c c o n t a c t p r e s s u r e , a n d E t h e e l a s t i c i t y modulus o f t h e polymer u s e d , The maximum v a l u e o f t h e r e a l i s t i c c o n t a c t p r e s s u r e pm was d e t e r m i n e d ( r e f . 203) t o be pm -
3.97 Tsin
yo
p
,
0.4
Ceramic F i g . 4.34. Wear o f s t e e l and ceramic i n f r e t t i n g t e s t s . O s c i l l a t i o n o f ceramic d i s k r u b b i n g a g a i n s t A l S l 52100 s t e e l b a l l , 0 10 mm 800 HV. frequency 20 Hz, a m p l i t u d e 0 . 2 mm, l o a d 20 N , number o f o s c i l l a t i o n s
4.8 106.
137 Amorphous metals and a l l o y s u s e d i n s l i d i n g c o n t a c t w i t h ceramic demonstrate i n t e r e s t i n g t r i b o l o g i c a l p r o p e r t i e s
( r e f s . 30,
3 1 , 158, 2 6 2 ) . When t h e amorphous i r o n a l l o y s Fe40N38M04B18 Fe67CO18B14Sil and FeglB13.5Si3.5C2
,
( f o i l t h i c k n e s s 0.030-0.033mn)
a r e rubbed a g a i n s t a s a p p h i r e h e m i s p h e r e i n a vacuum a t r e l a t i v e l y low s p e e d s ( 0 . 0 5 - 1 . 5 mm/s)
a n d low l o a d s ( 0 . 2 - 2 . 5
p h i r e s p h e r e d i a m e t e r i s 3.2 mm)
,
N , when t h e s a p -
t h e i r f r i c t i o n c o e f f i c i e n t in-
creases from 1 . 0 - 1 . 5 a t room t e m p e r a t u r e t o 1.8-2.4 a t 35OoC ( r e f . 3 0 ) . This i s due t o a n i n c r e a s e in a d h e s i o n r e s u l t i n g f r o m s u r f a c e s e g r e g a t i o n of b o r i c o x i d e a n d / o r s i l i c o n o x i d e . The wear resistance o f t h e amorphous a l l o y Fe67Co18B4Sil w a s s u p e r i o r t o t h a t o f 304 s t a i n l e s s s t e e l . During t h e s l i d i n g p r o c e s s , c r y s t a l l i t e s 10-150
nm i n s i z e a p p e a r on t h e wear s u r f a c e . Between 500 and 75OoC t h e f r i c t i o n c o e f f i c i e n t i s low ( 0 . 2 - 0 . 3 ) and r e m a i n s c o n s t a n t because o f b o r o n n i t r i d e s e g r e g a t i o n on t h e f o i l s u r f a c e . The s e g r e g a t i o n o f c o n t a m i n a n t s coming from t h e b u l k o f t h e m a t e r i a l t o t h e s u r f a c e upon h e a t i n g i s r e s p o n s i b l e f o r t h e f r i c t i o n b e h a v i o u r o f t h e s e s y s t e m s i n a vacuum. I n a r g o n and a i r c o n d i t i o n s , t h e f r i c t i o n c o e f f i c i e n t o f Fe67C016B14Sil amorphous a l l o y r u b b i n g a g a i n s t
a 6 . 4 nun s a p p h i r e h e m i s p h e r e r i d e r i s 0 . 2 , t h e same as f o r 304 s t a i n l e s s s t e e l , a n d d o e s n o t depend on l o a d o r s l i d i n g s p e e d w i t h i n t h e a f o r e m e n t i o n e d r a n g e s . T h e r e i s no v i s i b l e wear o n t h e amorphous a l l o y . I f a 3.2 mm s a p p h i r e r i d e r i s u s e d t h e r e are marked d i f f e r e n c e s n o t o n l y i n f r i c t i o n , b u t a l s o i n w e a r . The f r i c t i o n c o e f f i c i e n t i n i t i a l l y low i n c r e a s e s w i t h s l i d i n g d i s t a n c e u n t i l eq'uilibrium c o n d i t i o n s are reached, 0.2
f o r t h e amorphous
a l l o y and a b o u t 0 . 5 f o r t h e s t e e l . Oxide w e a r d e b r i s p a r t i c l e s a r e g e n e r a t e d on t h e amorphous a l l o y s u r f a c e , w h i l e m e t a l wear d e b r i s p a r t i c l e s are p r i m a r i l y g e n e r a t e d o n t h e s t e e l s u r f a c e . I n t h e c a s e o f t h e amorphous a l l o y m i c r o s c o p i c b r i t t l e f r a c t u r e s a p p e a r a n d t h e o x i d e l a y e r s b r e a k u p , p r o d u c i n g f i n e o x i d e wear d e b r i s p a r t i c l e s . However, t h e s a p p h i r e a d h e r e s t o t h e s t e e l a c r o s s t h e i n t e r f a c e and w i t h t a n g e n t i a l m o t i o n , s e p a r a t i o n t a k e s p l a c e i n t h e
s t e e l and t h e bonds of t h e c o h e s i v e l y weaker s t e e l r u p t u r e ; c a v i t i e s t h e n form i n t h e m a t e r i a l . When t h e amorphous a l l o y Fe81B13.5Si3.5C2
(Ra = 2 ,um)
i s rub-
b e d a g a i n s t a s t e e l r o l l c o a t e d w i t h a 50 ,um t h i c k l a y e r (R, = 3 p n ) o f 80%WC-14%Ni-3.5%Cr-0.8%B-0.8%Fe-O.8%Si-O.l%C
it wears by plough-
i n g ; as t h e r o u g h n e s s i n c r e a s e s , t h e wear mode c h a n g e s t o microc u t t i n g and t h e n t o c r a c k n u c l e a t i o n and p r o p a g a t i o n ( r e f . 1 5 8 ) . A f t e r a n i n c u b a t i o n p e r i o d , t h e mass loss i n c r e a s e s l i n e a r l y w i t h
138
duration or sliding distance; the wear of this particular mrphous alloy increases with sliding speed, which is quite contrary to most crystalline metals. For wear resistance, the alloy is preferred in the crystallized state for sliding against smooth surfaces at low loads, but jn the amorphous state when the roughness and load are higher. The friction coefficient and wear intensity of boron nitride sliding on steel depend mainly on the sliding speed (ref. 18). When a certain sliding speed is reached (say over 10 m/s at the contact of two rolls @ 5 and 100 mm respectively, at load 20 N), the friction coefficient decreases to 0.5 (at low total wear in the tribological system) initially because of the formation of B2O3 oxides and finally by phase changes from sphalerite and wurtzite structures to graphitization of the ceramic surface. Similar graphitization of a silicon carbide surface (when heated to 15OO0C) with the graphite functioning as an abrasion and friction reducer was observed by Buckley and Miyoshi (ref. 257). The sliding of ceramic under load is accompanied by a plastic flow in ceramics such as magnesium oxide, aluminium oxide and silicon carbide under relatively modest conditions of rubbing contact (refs. 24, 257). The presence of surface films such as adsorbates, has a marked influence on the adhesion friction andwear, altering the amount of plastic deformation that will occur during rubbing. The wear encountered with ceramics is generally of an adhesive and abrasive nature. Fracture pits and multiangular wear debris, having crystallographically-oriented sharp edges,'have been observed with ferrite-ferrite and silicon carbide-siliconcarbide contacts (ref. 257). Relatively small tangential stresses can easily produce fracture at the surface of diamond-silicon carbide systems, for instance. Abrasive wear also occurs when a third particle harder than one or both of the rubbing surfaces becomes trapped at the interface. It can remove material from one or both surfaces. Such third-body abrasion can result in catastrophic wear in ceramic or other systems (ref. 263). Sic, Al2O3, SiAlON and PSZ demonstrate very good tribological properties when rubbing on themselves (ref. 24). The tests were carried out using a barrel roll-plate system where the diameter of the roll was 34 nun (width 10 nun, curvature radius 100 m m ) , the length of the plate was 10 mm and its thickness 5 nun. The system was operated at sliding speeds in the range of 0.1-4 m/s and applied load 2-40 N. The surface roughness, Ra, of the roll was 1 ,urn
139 and of t h e p l a t e 0.02-0.05 ,um. The f r i c t i o n c o e f f i c i e n t f o r Al2O3, PSZ, S i A l O N and S i c s y s t e m s was a b o u t 1 . 0 5 , 0 . 9 5 ,
0.95 a n d 0.75
r e s p e c t i v e l y a t a s l i d i n g s p e e d o f 0.25 m / s and a f t e r a s l i d i n g d i s t a n c e o f 2 km, and s l i g h t l y d e c r e a s e d w i t h i n c r e a s i n g l o a d . The f r i c t i o n c o e f f i c i e n t as a f u n c t i o n of t h e s l i d i n g s p e e d i n i t i a l l y i n c r e a s e d , r e a c h i n g a maximum o f 1.1 a n d 0.9 f o r A1203 a n d SiAlON s y s t e m s r e s p e c t i v e l y ( a t s l i d i n g s p e e d ca. 0 . 8 m / s ,
load 5 N , after
s l i d i n g d i s t a n c e 2 kn) w h i l e f o r PSZ a n d S i c s y s t e m s i t r e a c h e d a maximum v a l u e o f 0 . 9 and 0 . 8 r e s p e c t i v e l y ( s l i d i n g s p e e d c a . 0 . 3
m/s);
when t h e s l i d i n g s p e e d i n c r e a s e d t o 4 m/s, t h e f r i c t i o n co-
e f f i c i e n t decreased t o 0.65,
0.60,
0.55 a n d 0 . 5 5 f o r Al203, S W N ,
PSZ and S i c s y s t e m s r e s p e c t i v e l y . The w e a r o f t h e p l a t e i n c r e a s e s
p a r a b o l i c a l l y as t h e a p p l i e d l o a d i n c r e a s e s , w h i l e i n c r e a s i n g t h e s l i d i n g s p e e d i n i t i a l l y c a u s e s i t t o d e c r e a s e r a p i d l y ( h a v i n g min-
i m u m a t ca. 0 . 5 m / s ) from 55 ,um ( d e p t h o f t h e wear c r a t e r ) , 2 8 , 20 ,um ( a t 0 . 1 m / s , l o a d 5 N , a f t e r s l i d i n g d i s t a n c e 2 km) f o r SiAlON, A1203 and Sic s y s t e m s r e s p e c t i v e l y t o minimum v a l u e s 1 8 , 1 4 a n d 1 2 ,um, which t h e n i n c r e a s e t o 6 0 ,
55 and 1 6 ,um r e s p e c t i v e l y .
The wear
o f PSZ measured i n t h e r a n g e o f 0.3-1 m / s a l s o d e c r e a s e d from a b o u t 36 t o 28
m. A1203 wears by m i c r o c u t t i n g w h i l e w e a r by abra-
s i o n i s c h a r a c t e r i s t i c o f SiAlON m a t e r i a l i n p a r t i c u l a r . The r e l a t i o n s h i p between t h e s h e a r modulus K, o f t h e m a t e r i a l u s e d a n d t h e
w e a r c r a t e r d e p t h hc c a n b e e x p r e s s e d by t h e l i n e a r r e l a t i o n s h i p hc = -0.23 where hc i s i n +mi and K,
Ks + 53
(4.39)
i n GPa.
A t a low s l i d i n g s p e e d ( 1 m/s) t h e f r i c t i o n c o e f f i c i e n t f o r
Si3N4 s l i d i n g on i t s e l f i s 0 . 8 5 i n d r y a r g o n and n i t r o g e n a n d 0 . 8 i n l a b o r a t o r y a i r and oxygen ( r e f . 264) s y s t e m s : h e m i s p h e r e p i n @ 6 mm,
.Pin
wear ( i n p i n - o n - p l a t e i n d r y g a s e s ) as
l o a d s 1-30 N ,
compared t o wear of t h e p l a t e was n e g l i g i b l e . I n 98% humid a r g o n and a i r , t h e w e a r o f t h e p i n a n d t h e p l a t e w a s t h e same, v e r y s n a l l i n b o t h cases. I n d r y g a s e s , wear o c c u r s by f r a c t u r e and no e v i d e n c e of p l a s t i c d e f o r m a t i o n w a s o b t a i n e d . I n w e t g a s e s c o n t a i n i n g water v a p o u r , a t r i b o c h e m i c a l r e a c t i o n p r o d u c e s an amorphous s u b s t a n c e , p r o b a b l y a h y d r a t e d s i l i c o n o x i d e . T h i s i n c r e a s e s t h e adh e s i o n between wear p a r t i c l e s a n d p r o v i d e s a l a y e r on t h e s u r f a c e which p r o t e c t s t h e m a t e r i a l from f u r t h e r wear. The wear asymmetry o b s e r v e d between p i n and p l a t e c a n be e x p l a i n e d by t h e f o r m a t i o n o f a p r o t e c t i v e s h o e o f wear p a r t i c l e s a d h e r i n g t o t h e p i n b e c a u s e o f i t s s p h e r i c a l s h a p e . Aluminium s i l i c a t e s l i d i n g o n i t s e l f c a n
140
have a friction coefficient as low as 0.1 (ref. 2 6 5 ) and low-mass alumina ceramic ( 9 9 . 8 % A 1 2 0 3 1 sliding on itself G . 2 (ref. 2 6 6 ) Hot-pressed boron carbide B q C rubbing on itself at high temperatures in air or in a vacuum demonstrates very'good tribological properties (Fig. 4 . 3 5 , ref. 19). During sliding in air at very high temperatures, the dimensions of the samples increase because of the oxidation of the rubbing surfaces. At 5 0 - 6 0 0 ° C the B 2 O 3 grains and B O ( 0 H ) and B ( O H ) 3 fibres are present, at higher temperatures B(OH)3 with B 2 O 3 grains and r A l 2 O 3 , and at above 1000°C the surface layer consists of B ( O H l 3 only.
.
F i g . 4 . 3 5 . Wear and f r i c t i o n c o e f f i c i e n t o f h o t - p r e s s e d B 4 C s l i d i n g on i t s e l f as a f u n c t i o n o f t e m p e r a t u r e . C y l i n d r i c a l bushes r u b b i n g f r o n t a l l y ; i n t e r n a l d i a m e t e r 8 mrn, e x t e r n a l d i a m e t e r 16 mm, l e n g t h 1 5 mm, s l i d i n g speed 0.01 m/s, c o n t a c t p r e s s u r e 1 MPa. 1 1 Pa vacuum, 2 - a i r .
-
141
The ceramic-polymer s y s t e m s t e s t e d i n s l i d i n g and r o l l i n g i n a vacuum d e m o n s t r a t e i n t e r e s t i n g t r i b o l o g i c a l p r o p e r t i e s ( r e f . 2 0 ) . PC ( o r f i l l e d PTFE)
( d i s k ) s l i d i n g i n a pin-on-disk
system a g a i n s t
g l a s s o r d i a s p o r e g i v e s a f r i c t i o n c o e f f i c i e n t o f 0.03-0.04
(glass)
and 0.05 ( d i a s p o r e ) w i t h no wear o f t h e p i n and v e r y l i t t l e wear o f t h e d i s k (vacuum 1 . 3 3
Pa, s l i d i n g speed 0.5 m / s ,
l o a d 25 N ,
s l i d i n g d i s t a n c e 1 km). I n a s y s t e m s i m u l a t i n g a r o l l i n g b e a r i n g , t h e b e s t f r i c t i o n and w e a r b e h a v i o u r was f o u n d f o r d i a s p o r e - P C ( o r f i l l e d PTFE)
( t h e two r i n g s and s i x b a l l s r o l l i n g b e t w e e n them
were made o f d i a s p o r e a n d t h e c a g e f o r b a l l s was made o f PC ( o r f i l l e d pTFE).Of t h e o t h e r m i n e r a l s u s e d ( g l a s s , g a b r o a n d m a r b l e ) , t h e b e s t r e s u l t s w e r e o b t a i n e d when g l a s s was u s e d i n s t e a d o f t h e diaspore. Alumina c e r a m i c s c a n r e p l a c e t h e t r a d i t i o n a l metal i m p l a n t s i n a r t i f i c i a l h i p s ( r e f . 2 6 7 ) . The s i n g l e - c r y s t a l
alumina (sap-
p h i r e ) d e m o n s t r a t e s b e t t e r wear b e h a v i o u r t h a n a p o l y c r y s t a l a l u -
mina and i s b e t t e r t h a n s t e e l m a t e r i a l s i n r u b b i n g a g a i n s t UHMWPE. Glass i s a m a t e r i a l w i t h some d i s t i n c t i v e t r i b o l o g i c a l features ( r e f . 1 6 2 ) . When i t i s i n c o n t a c t w i t h metal i n m o i s t a i r , m e t a l i s t r a n s f e r r e d t o t h e g l a s s and t h e f r i c t i o n c o e f f i c i e n t i s t y p i c a l l y from 0 . 5 t o 0 . 7 ,
depending on t h e s h e a r s t r e n g t h of t h e m e t -
a l i n v o l v e d . I n a vacuum, g l a s s t r a n s f e r s t o t h e m e t a l , a n d t h e f r i c t i o n c o e f f i c i e n t is about 0.5.
I n o t h e r words, t h e f r i c t i o n
c o e f f i c i e n t s a r e s i m i l a r b u t t h e mechanisms of f r i c t i o n a r e comp l e t e l y d i f f e r e n t . This i s because t h e f r a c t u r e properties of g l a s s are s t r o n g l y a f f e c t e d by water; water v a p o u r a d s o r p t i o n d e -
creases t h e s t r e n g t h o f g l a s s . I n a vacuum, t h e f r i c t i o n c o e f f i c i e n t f o r m e t a l s s u c h as aluminium o r i r o n s l i d i n g on g l a s s i s a b o u t t h e same as t h e f r i c t i o n c o e f f i c i e n t o f g l a s s s l i d i n g on g l a s s . When s l i d i n g on metal i n a vacuum, g l a s s t r a n s f e r s o n t o t h e
metal s u r f a c e and t h e m e t a l s u r f a c e becomes c h a r g e d w i t h g l a s s ; a s a r e s u l t , t h e g l a s s i s e f f e c t i v e l y s l i d i n g o n g l a s s . The adh e s i v e wear p r o c e s s i s dominant when g l a s s i s r u b b i n g a g a i n s t glass. B o r o s i l i c a t e g l a s s 7 7 4 0 P y r e x of d e n s i t y 2 . 2 3 mg/mm3 , reinforced w i t h g r a p h i t e f i b r e s (modulus from 230 t o 6 9 0 G P a , s t r e n g t h 2.2-3.1 GPa, d i a m e t e r 6.5-11 ,um, volume c o n t e n t s i n t h e m a t r i x m a t e r i a l 3 1 . 5 - 7 1 . 8 % ) , d e m o n s t r a t e s good t r i b o l o g i c a l p r o p e r t i e s when s l i d i n g on p e a r l i t i c g r a y c a s t i r o n ( r e f . 3 2 ) . A g l a s s p i n ( @ 4 . 7 6 mm) was p r e s s e d a g a i n s t a d i s k u n d e r a l o a d o f 89 N and a t s l i d i n g speeds of 0 . 2 4 ,
0 . 6 0 and 0.96 m / s .
I t w a s found t h a t u n i d i r e c t i o n -
142
a1 fibre composites have lower wear rates and a lower coefficient of friction than chopped random fibre composites. High modulus fibre composites demonstrate a lower wear rate and coefficient of friction than high strength fibre composites. The friction coefficient and wear for both composite increase with increase in the sliding speed. At low sliding speeds there is only mild wear; a thin, smooth film appears on the composite surface and there is minimal plastic deformation of the counterface. At high speeds, wear is severe: extensive pitting of the thick surface film on the composite, plastic deformation and pitting of the counterface can be observed. The friction coefficients were from 0.09 ( 6 0 % high modulus fibres) to 0 . 3 9 ( 3 1 . 5 % discontinuous high strength fibres). The lowest wear (at friction coefficient 0.17) was observed when ultrahigh modulus fibres ( 6 7 % ) were used, and the highest when the glass was reinforced with high strength discontinuous fibres. Tests have been carried out to compare the wear resistance of brass, various steels, Fe-Cr-A1 alloy and titanium when sliding on glass fibre; they show that the wear resistance of porcelain, nitrided steel ( 0 . 4 5 % C ) , borated mild steels, nitrided Fe-Cr-A1 alloy and carburized titanium is respectively 5 5 , 7 8 , 68, 7 8 and 2 9 0 times more than the wear resistance of brass (ref. 2 6 8 ) . The industrial use of borated mild steel elements to guide glass fibres in textile machines supports the results of these laboratory studies. Glass-polymer systems can exhibit interesting tribological properties (refs. 1 9 0 , 194, 2 6 9 ) . The friction and wear in polymer (pin, 9 3 nun)-glass (disk, @ 60 nun) systems was found to depend on the sliding speed (ref. 2 6 9 ) . Polymer wear depends to some extent on the time taken to reach the critical speed and is around lpm/km or less in the lower speed range (for applied load 1 0 N, and sliding speeds 0.1-3 m/s). At the critical speed, the wear of the polymer rapidly increases. The values of the critical speed for LDPE, HDPE, PP, PA 6 and POM h are 0.5, 1.5, 1.1, 1.2 and 1 . 6 m/s respectively. The irrespective friction coefficients are 0.9, 0.4-0.8 (lower at lower sliding speed), 1 . 4 - 0 . 8 (higher at lower sliding speed, rapidly increasing at the critical speed) 2-1.3 (high at low speeds), and 0.5-0.6. The wear of the polymers on glass in the lower speed range is mainly due to the small local transfer of molten polymer to the frictional track; the wear in the higher speed range is due to the outflow of molten polymer through the rear edge of the polymer pin. LDPE wears by shearing
143
(at a distance from the frictional surface) and melting, and a substantial film is laid on the frictional track in the area of contact. The addition of carbon or glass fibre to POM c ( 2 0 % and 2 5 % by weight respectively) reduces the friction coefficient by 3 and 1 . 5 times respectively (for unfilled POM c it is 0.6-0.5) and clearly reduces the wear when the sliding speed (in the aforementioned pip -on-disk system, load 50 N) is higher than 0.6 m/s (ref. 1 9 0 ) . At lower speeds, the wear of glass-fibre-reinforced POM c is significantly higher than that of unfilled POM c when rubbing against qlass. The friction coefficient for PTFE similarly reinforced is 0.25 (practically the same for glass or carbon fibre reinforcement) but the wear of carbon-fibre-reinforced PIE5 is higher than glass-fibre-reinforced PTFE, especially at lower speeds. The wear of fibre-filled POM c is much greater than that of fibre-filled PTFE. Fibre-filled PQM c exhibits in general a much higher friction in sliding on steel than on glass. The high wear of glass-fibre-reinforced c is probably due to the marked thermal decomposition of polyacetal around the glass fibres on the frictional surface. The wear of PTFE (pin) rubbing against glass (disk)is connected with the transfer and return of the PTFE particles to and from the glass surface (ref. 1 9 4 ) . Graphite and carbon-graphite materials have good tribological qualities, their frictional properties being particularly good (refs. 3 4 - 3 7 , 1 5 4 , 162, 2 7 0 - 2 7 6 ) . The counterface for such materials are usually stainless steel, tool steels, hard chromeplated steels, hard coated aluminium, sintered metals, bronze, cast iron, ceramics, or the same graphite or carbon-graphite material. The surface roughness should be small. The friction coefficient against metals is about 0 . 2 The friction in air is controlled mainly by the presence of water molecules adsorbed on the frictional surface. During sliding, the carbon (graphite) material is transferred to the counterface. Radial journal bearings with a steel journal and a bearing bush made from carbon (graphite) materials demon’strate very good friction and wear behaviour (ref. 2 5 ) . The wear of such bearings (those with bushes made of electrographitized carbon) depends mainly on the load applied; for bearings which have bushes with a bearing hole diameter of 2 0 nun, external diameter 30 nun and length 2 0 nun, the maximum pv (p - contact pressure, v - sliding speed) is 0 . 3 MPa m/s at a sliding speed of 1 m / s , and 5 times higher at a sliding speed of 0.1 m/s (ref. 3 5 ) . When electrographitized carbon is impregnated with thermosetting polymer, the bearings can be used
.
144
at sliding speeds below 1 m/s and contact pressures below 0.1 MPa when ambient temperature is below 14OoC. Miniature bearing bushes manufactured from such materials and which have a bearing hole diameter of 0.5 mm, external diameter 3 mm and length 1 mm, operating at a sliding speed of 0.01 m/s and at very low contact pressure, have been successfully appled in flow-meters. Purebon hard carbon is used on guide bushing (bearing hole diameter 0.523 5 0.007, external bushing diameter 3.15 mm, bushing length 1.02 mm) for the rotor shaft in watt-hour meters designed to operate in the temperature range - 5 0 to 55Oc, at very low contact pressure, at a sliding speed below 0.01 m/s, and with a required life of 30 years at a constant friction coefficient (in a filtered air environment) (refs. 37, 273). The wear of hard carbon, hard carbon with resin impregnant and graphite with anti-oxidant impregnant varies with the ambient temperature. For example, the wear of hard carbon rubbing against hard chrome plate on steel increases rapidly with increase in temperature, while the wear of hard carbon with resin impregnant actually decreases in the range of 20-300°C. The wear rate of graphite with anti-oxidant impregnant decreases and reaches its minimum at about 12OoC and then rapidly increases as a function of temperature (ref. 37). The friction coefficients for bearing steel sliding on graphite materials or hard carbons are similar to the friction coefficients of such carbon-graphite materials rubbing against themselves (i.e. 0.2-0.3)(ref. 361, although the chrome stainless steel in steel-carbon-graphite systems has a slightly lower friction coefficient. Hard carbon material (super-hard coal sinter, microhardness 'up to 20 MPa) sliding against corundum (surface roughness Ra = 0 . 0 4 ,urn) has the lowest friction coefficient (ca. 0.16) and wear of any of these systems. In this sinter, the interphase material particles with low microhardness (ca. 1.2 MPa) act as a solid lubricant. Such materials can be used in gas bearings operating in air. Graphitized carbon materials impregnated with anti-friction metals (Ag, Cu, Cd, bronze, babbits, 95% Pb - 5% Sn or 70% Cu 30 % Pb alloys) have good tribological properties (refs. 154,272). Carbon-ceramic materials can be used in journal bearings or seals when the sliding speed is below 150 m/s, contact pressure 0.3-0.5 MPa, and temperature 600-650°C. Carbon-polymeric materials can be used in compressors or pumps at sliding speeds up to 15 m/s, contact pressure up to 20 MPa and at temperatures between-200 and 15OoC
145
(ref. 274). The carbon material manufactured by Nippon Carbon, containing 13-50% A1 or its alloy, 0.5-10% A1203 (concentrated mainly on the surface), demonstrates good frictional properties and extremely good wear behaviour (ref. 275). The Supragraf lamellae graphite materials manufactured by James Walker and Co. Ltd. have high chemical inertness, high thermal resistance and low density ( 1 . 4 mg/mm3), and can operate at temperatures of between -200 and 5OO0C in an oxidative atmosphere or at up to 250OoC in a reduced or inert atmosphere whilst having a friction coefficient of 0.05 (ref. 276). Tribological systems with an element or elements manufactured from graphite or carbon-graphite materials demonstrate better tribological properties in a vacuum than in air. The carbon transfer films are the key to effective operation (ref. 162). The lowest friction coefficients for vacuum-operated, 100% electrographitized carbon were obtained at sliding against electrolytic iron, copper or silver (0.15, 0.15, 0 . 2 0 ) and relatively low friction (0.2) and wear at sliding against aluminium oxide and stainless steel (friction coefficient ca. 0 . 4 ) (ref. 162). Pyrolitic graphite prepared by high temperature decomposition of hydrocarbons demonstrates the effect of orientation; during sliding on gold in a vacuum the friction coefficient was higher (up to 0.6) at prismatic orientation than at basal orientation (up to 0.4)(ref. 162). No gold was observed during sliding in basal orientation while during sliding in prismatic orientation small spheres of transferred gold could be seen. The prismatic orientation of graphite is from 500 to 1000 times more chemically active than the basal orientation. A single crystal diamond, a form of carbon, in sliding contact with metals demonstrates a decreasing friction coefficient with an increase in d-bond character of the transition metals (ref. 162). Titanium and zirconium, which are chemically very active, exhibit very strong interfercial bonding to diamond and a friction coefficient of about 0.7 (in a vacuum) while, by contrast, rhodium and rhenium have relatively low coefficients of friction ( 0 . 4 ) . The fact that oxygen increases the friction coefficient is related to the relative chemical thermodynamic properties and bonding of carbon to oxygen. All the aforementioned metals transfer to the surface of diamond during sliding. The advantage of using diamond is that no stick-slip effects occur in metal-diamond sliding systems (ref. 17). Some small mechanisms have tribological systems consisting of
146 elements made of materials such as paper (computer peripherals) or materials in a special form (perforated material, magnetic tapes). In the case of magnetic tapes they consist of finely dispersed r-Fe2O3, Cr02 or BaO.GFe203 particles bonded by used of polymeric binders onto a flexible substrate such as PETP. The magnetic tape is moved against a stationary (audio or computer) or rotating (video) read-write magnetic head which is generally made of Ni-Zn or Mn-Zn ferrite or Permalloy. The direct physical contact between the tape and the head takes place during the start-stop operations of the tape drive. Generally the friction in such systems is high at high humidity (above 60% relative humidity)(refs. 277, 278). Frictional damage to the tape increases the friction between the tape and the head. The damage to the tape is mainly due to a plastic flow of the binder in the magnetic layer (ref. 278). Ferrite heads causes less frictional damage to the tape than Permalloy heads and they have a much lower friction coefficient than Permalloy heads (0.20 . 3 at 35% relative humidity compared to 0.8-1.0 for Permalloy heads). The specific wear rate of Permalloy and hard Permalloy (containing niobium) heads is of the order of mm3/N.m and of Mn-Zn ferrite heads loq7 mm3/N.m (ref. 278). The wear of magnetic materials increases with relative humidity, the increase beinq more noticeable for ferrite heads than for Permalloy heads. The wear of the materials proceeds mainly by the abrasive action of hard magnetic powders contained in the magnetic layer of the tape, but the variation in the mechanical properties of the binder in the magnetic layer with relative humidity and possible tribochemical reactions in such conditions also influence the wear (refs. 277-279). Wear by paper occurs because of the abrasive action of small, hard particles contained in the paper (usually Si02, different minerals , Ti02) (refs. 280, 281). The abrasive wear by paper is proportional to the load and sliding distance and for Knoop hardness of the abraded material less than 7500 MPa, the wear is inversely proportional to hardness. Generally there are three categories of wear dependence on hardness (this was recognized by testing with paper a variety of materials, ranging from elastomers to diamond (ref. 281)) : the aforementioned inverse linear dependence, a transitional category of abrasive hardness and a third category where the hardness H, of the abraded materials is greater than the hardness of the abrasive. For all three categories the dependence of wear on hardness can be described in the form Hin; in the category
147 i n which t h e a b r a s i v e s a r e h a r d e r t h a n t h e a b r a d e d m a t e r i a l , n m l ; i n t h e c a t e g o r y i n which t h e h a r d n e s s e s are c o m p a r a b l e , n = l O ;
and
i n t h e c a t e g o r y i n which t h e a b r a s i v e s are s o f t e r , n x 5 . The app r o x i m a t e h a r d n e s s o f a b r a s i v e s i n p a p e r i s a b o u t 1 0 0 0 0 MPa. The m a t e r i a l s which are most r e s i s t a n t t o w e a r by p a p e r a r e diamond, s a p p h i r e , T i c and WC. I n mechanisms i n which m e t a l l i c e l e m e n t s are i n c o n t a c t w i t h
r a w c o t t o n , h i g h wear and s p a r k i n g c a n o c c u r b e c a u s e o f a t r i b o e l e c t r i c a l c h a r g e ( r e f s . 282, 2 8 3 ) . The a p p l i c a t i o n o f p o l y m e r s or p o l y m e r i c c o a t i n g s on t h e m e t a l l i c e l e m e n t s i s a d v a n t a g e o u s i n s u c h mechanisms. When r a w c o t t o n was s l i d i n g o n LDPE, PVB, PCA, PNP, epoxy (ED 1 6 ) and f u r a n - e p o x y o l i g o m e r s , a t s l i d i n g s p e e d s
0-8 m / s a n d c o n t a c t p r e s s u r e s 0.001-0.05
MPa, t h e h i g h e s t f r i c t i o n
c o e f f i c i e n t s were found f o r PVB a n d LDPE ( c a . 0 . 4 ) f o r PNP a n d ED 1 6 ( c a . 0 . 2 5 ) .
and t h e l o w e s t
The v a l u e s o f t h e t r i b o e l e c t r i c a l
c h a r g e d e n s i t y were h i g h e s t f o r LDPE and ED 1 6 a n d l o w e s t f o r PVB and PNP. The t r i b o e l e c t r i c a l f r i c t i o n component i s a s much as 50-60% of t h e t o t a l f r i c t i o n f o r c e i n s u c h s y s t e m s . The f r i c t i o n between a copper-MoS2 compound(32% by volume, s i n t e r e d a t 650°C i n a vacuum) and a s t e e l or s a p p h i r e s p h e r e (@ 1
mm) i n a vacuum 2 l o q 7 P a and a t s l i d i n g s p e e d 4 ,um/s and N i s accompanied by t h e t r a n s f e r o f t h e MoS2 f i l m
l o a d 0.2-0.5
( 1 . 2 n m ) ( r e f . 2 8 4 ) . The f r i c t i o n c o e f f i c i e n t i s 0 . 0 0 8 .
The p r o c e s s o f r o u g h n e s s f o r m a t i o n and w e a r o f human t e e t h under t h e f r i c t i o n of a t o o t h b r u s h
used w i t h a d e n t i f r i c e o c c u r s
according t o the s t r a i g h t l i n e l a w a s a function of t h e s l i d i n g
t i m e ( r e f . 2 8 5 ) . The t i m e n e e d e d f o r r e a c h i n g t h e u l t i m a t e s t e a d y s t a t e s u r f a c e r o u g h n e s s , ( R a = 0.3-0.5 ,urn), i s a b o u t 4 . 5 h . A f t e r t h e s u r f a c e r o u g h n e s s h a s s t a b i l i z e d , f u r t h e r wear o c c u r s w i t h o u t any c h a n g e i n t h e s u r f a c e r o u g h n e s s . The e f f e c t o f a c i d i t y i n t h e mouth i s q r e a t e r t h a n t h e s i m p l e a b r a s i v e e f f e c t o f t h e t o o t h b r u s h and d e n t i f r i c e . Summarizing t h e above c o n s i d e r a t i o n s , it c a n b e s t a t e d t h a t i n t h e case o f ceramics t h e f r i c t i o n a n d wear a r e a n i s o t r o p i c ; adhes i v e wear i s accompanied by a b r a s i o n , s u r f a c e c h e m i s t r y i s v e r y i m p o r t a n t t o f r i c t i o n and w e a r b e h a v i o u r , a n d c o n t a m i n a n t s o n t h e i r s u r f a c e a f f e c t t h e i r t r i b o l o g i c a l p r o p e r t i e s . These remarks a r e a l s o t r u e o f c a r b o n - g r a p h i t e m a t e r i a l s . A t h i g h t e m p e r a t u r e s graphi t i z a t i o n of t h e ceramic s u r f a c e may o c c u r and a r e d u c t i o n i n w e a r and f r i c t i o n c a n b e e x p e c t e d . Humidity h a s a r e m a r k a b l e e f f e c t on t h e b e h a v i o u r o f t r i b o l o -
148 gical systems such as the magnetic tape-head systems used in audio, computer or video applications. The wear of materials by paper depends on the hardness of the material used.
149
5 , LUBRICATED SYSTEMS 5,1, METALLIC SYSTEMS 5.1.1.
S O L I D METALS
Typical examples of boundary or mixed lubricated minuature systems are clock-type bearings with a steel journal (usually made of free cutting or stainless steel and roller-burnished to %MPa, l u b r i c a t i o n w i t h MWP m i n e r a l o i 1 .
Polymers can only be effectively lubricated when the liquids (oils) used have different solubility parameters from the polymers. In the event that the solubility parameters are equal, the polymer is dissolved in the oil and the wear rate is high (refs. 5 3 , 1 0 7 ) . The solubility parameter & (in l o 3 q m)of instrument oils used to lubricate miniature systems can be estimated using the formula 6 ~ 6 . 7 &3 (where & is the dielectric constant of the oil) (ref. 2 0 5 ) . The solubility parameters of polymers can be found in Chapter 2.4. The polysiloxanes have a different solubility parameter ( 1 1 . 2 l o 3 from most polymers and are characterized by a relatively low surface tension that assures good wettability
49)
189 o f polymers. The problems o f t h e l u b r i c a t i o n o f p o l y m e r i c s y s t e m s w i l l b e d i s c u s s e d i n C h a p t e r 6 . 6 . S p e c i a l i n s t r u m e n t o i l s which c a n be used f o r t h e l u b r i c a t i o n o f m i n i a t u r e p o l y m e r i c s y s t e m s are described i n Chapter 3 . 2 . The wear o f l u b r i c a t e d m i n i a t u r e steel-polym.er j o u r n a l b e a r i n g s ( u s u a l l y l u b r i c a t e d w i t h one d r o p of o i l ) i s t h e e f f e c t of a d h e s i v e - c o h e s i v e i n t e r a c t i o n s on t h e i n t e r f a c e ( i f t h e t h e r m a l e f f e c t s can be n e g l e c t e d , i . e . t h e temperature rise i n t h e f r i c t i o n a r e a A T < 3 K ; see F i g . 5 . 2 2 ) . t e r i a l worn, V l ,
The volume o f t h e p o l y m e r i c ma-
c a n t h e r e f o r e be found u s i n g t h e f o l l o w i n g f o r (4.7) (ref. 196):
mula, s i m i l a r t o eqn.
(5.19) The l u b r i c a t e d wear c a n be d e t e r m i n e d f o r b e a r i n g s o p e r a t i n g under boundary o r mixed l u b r i c a t i o n . S i n c e , a s h a s a l r e a d y been mentioned, t h e h i g h e l a s t i c i t y o f t h e p o l y m e r i c b e a r i n g bush i s f a v o u r a b l e t o t h e hydrodynamic l u b r i c a t i o n e f f e c t a t r e l a t i v e l y low s l i d i n g s p e e d s , t h e s l i d i n g speed a t which t h e f r i c t i o n c o e f f i c i e n t i s l o w e s t , i . e . when t h e hydrodynamic e f f e c t b e g i n s t o occ u r , s h o u l d b e d e t e r m i n e d . The a u t h o r ' s s t u d i e s d e s c r i b e d elsewhere ( r e f . 360) have shown t h a t i n t h e c a s e o f m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s , t h e a n g u l a r s p e e d o f t h e j o u r n a l a t which t h e hydrodynamic l u b r i c a t i o n b e g i n s ( w ) c a n b e e x p r e s s e d by t h e f o r mula :
w
= CK
P
Y
hmin
(5.20)
fid where C and K a r e p a r a m e t e r s , p i s t h e c o n t a c t p r e s s u r e , y t h e rel a t i v e c l e a r a n c e , hmin t h e minimum t h i c k n e s s o f t h e o i l f i l m ,
,u t h e v i s c o s i t y ( d y n a m i c a l ) o f t h e o i l , and d t h e b e a r i n g h o l e d i ameter. Taking i n t o a c c o u n t t h e e l a s t i c i t y o f t h e s l i d i n g s u r f a c e , t h e p a r a m e t e r C c a n be d e t e r m i n e d by f i r s t o f a l l e s t i m a t i n g t h e c o e f f i c i e n t D i n t h e f o l l o w i n g way: D =
pd Erhmin
(5.21)
190
moduli and Poisson's ratios of contacting materials respectively). Knowing D , the parameter C can then be found:
c =
1
l + D
(5.22)
- m1
where ml equals 0 . 1 5 when 0.01GDDg 0 . 2 and 0 . 1 for D > 0 . 2 . Taking into consideration the outflow of oil from the bearing, the coefficient K was experimentally determined (for miniature steel-polymer journal bearings lubricated with instrument oil) as (5.23) K = b3 ~ p + 3 c where parameter b3 equals - 2 . 2 5 l o 5 and c2 can be expressed by the formula :
+
c3 = 2 7 p
7540
(5.24)
where p is contact pressure in MPa. The pressure drop a p (expressed in MPa in eqn. ( 5 . 2 3 ) ) between the gap where the thickness of the oil film is minimum and the gap where the distance between the journal and the bearing bush is maximum can be calculated using the following formula:
AP
=
rl(cos
el +
cos
e,)
-(
1
hmin
- s +l a )
(5.25)
where y1 is the surface tension of the oil, 0 1 and e2 are the contact angles of the oil on the materials used, s is the diametral clearance, and a the deformation of the polymeric bush under load; for hmin see eqn. (5.20). The minimum thicknes of the oil film hmin can be assumed to be (5.26)
where R,1 and R,2 are roughness height parameters (according to IS0 standard) of the steel and polymeric bearing bush surfaces respectively, The polymeric bearing bush deformation a can be calculated using eqn. (4.10). To determine the adhesional energy acting over the interface (EaI1 in eqn. ( 5 . 1 9 ) ) I the parameter CL in eqn. (5.19) needs to be known. oi can be determined by the analysis of the experimentally found Stribeck's curve and by use of eqn. ( 5 . 1 6 ) . Investigations into miniature steel-polymer journal bearings lubricated with typical instrument oils ( M W P mineral oil, OKB 1 2 2 - 1 6 silicone mineral oil and XU 4 3 0 traditional clock oil) have
191 shown that the friction coefficient f of the bearings in the range of non-hydrodynamic lubrication can be approximated using the following formula: f = fa(l
-
0.12 p
e
0*34
(5.27)
)
For p, p and d , see eqn. (5.20); p, ,u and d should be introduced in MPa, mPa.s and mm respectively; v is the sliding speed in m/s; for fd see eqns. (4.4) and (5.161. The fd should be calculated for every p and v value using eqn. (4.4). Since eqn. (4.4) was determined at vc0.5 m/s, f cannot be calculated from eqn. (5.27) when the sliding speed is higher than 0.5 m/s, because the fd value is unknown. The sliding speed is usually relatively low and eqn. (5.19) is valid at relatively low sliding speeds and contact pressures when thermal effects are negligible, so the parameter in eqn. (5.16) can be determined by use of eqn. (5.27). From eqn. (5.16) we have
- ff
-
1
fd 7 “( 1) + 1
(5.28)
1
The ratio of fd/fl was estimated experimentally and can be taken to be approximately al v + a2 -(a3 v + a,) + a14 P f, (1 - 0.12 p o . 3 P 4 Jd 5 a13 -U = (5.29) =1
where p , p and d are as in eqn. (5.20) and are in MPa, mPa-s and nun respectively, Vh is the sliding speed at which hydrodynamic lubrication begins (friction coefficient is minimum), v is the sliding speed, (vh and v are in m/s), and Xn is the outflow parameter for the oil, (other than XU 430); for K, see eqns. (5.20) and (5.23); for a l l a2, a3 and a4, see eqn. (4.4); parameters a13 and a14 for PA 6, PA 66, POM h and POM c are 0.07, 0.05, 0.03, 0.035 and 0.2, 0.35, 0 . 3 0 , 0.26 respectively. The ratio f/fl can be determined after estimating fd/fl because a s a result of eqn. (5.27) , - f-
- - fd
‘d
fl
(1
- 0.12 p
(5.30)
192 After determining fd/fl, the parameter &can be calculated from eqn. (5.28). The avalues determined were intoduced into eqn. (5.19) and the adhesional energy acting on the interface, Eall, I was determined (yo in eqn. (5.19) was calculated using eqn. (4.9)). After determining Ea,l the respective values of Ec,l (see eqns. (5.19) and (4.15)) were found on the basis of the experimentally determined radial wear rates and the calculated volumes of the worn polymeric material (estimated by use of eqn. (4.18)). The relationship between V1 and EcI1 is presented in Fig. 5.23 (ref.196). This relationship can be approximated by the following formula:
(5.31) for V1, EaI1 and EcI1 see eqn. (5.19); a15 and alG are parameters. The value of the parametera16= -1.25 in eqn. (5.31) is the same for all polymer+oil combinations, while the values of parameter a15 are as follows: POM c + XU 430 oil, 0.45 POM h + XU 430, 1.87 PA 6 + XU 430 , 2.34 POM c + MWP (or OKB 122-161, 1.11 POM h + MWP (or OKB 122-16), 3.0 and PA 6 + MWP (or OKB 122-16) , 3.95 The differences between the characteristic curves are probably due to the way in which the lubricant influences the wear process of each polymer. The nonpolar mineral MWP and silicone-mineral OKB instrument oils are fairly similar, while the traditional clock oil XU 430 contains polar additive (fatty oil); when the polymer is more polar and has higher surface free energy, the difference between lubrication with nonpolar oil is more pronounced. The resistance of the adsorbed oil film and also the interactions in the oil-polymer system (ref. 107, see also Chapter 6.5) also probably affect the differences in thecharacteristic curves V1 vs. EaI1/ECI1. It is clear from Fig. 5.22 that when the temperature rise A T in the friction area of lubricated steel-polymer bearings is high enough (nT>3 K), the thermal frictional energy produced should be taken into consideration in the wear analysis. Consideration of the relationship between thermal effects and wear in lubricated bearings gives similar to the tests on unlubricated steel-polymer miniature journal bearings (see Chapter 4.2.1); the relationship is plotted in Fig. 5.24 (ref. 196).
193
0
-0.08
0
0
a
0
0.06 b
0
0
0
A
0
D
8
0
B
m
0.04
0
L
'
>*
8 8
0
0
00
@Q
0.02
n A
¶
O
A rn
A 0
0 0
QO
.
D A
00 0
..
0
UP
1
0POMh +xu430 BPOMh 8POPlC DPOMC
0
I
1
1
1
20
40
60
80
1
100
1
I20
F i g . 5.23. Volume o f p o l y m e r i c m a t e r i a l worn (Va,]) vs. r a t i o of ad he si onal energy a c t i n g o ver t he i n t e r f a c e (E a , l ) t o cohesional energy ( E c , l ) f o r l u b r i c a t e d m i n i a t u r e steel-polymer j o u r n a l b e a r i n g . B e a r i n g h o l e diameter 2.15 mm, e x t e r n a l bush di a met er 6 m m , l e n g t h 2.1 mm, r e l a t i v e c l e a r a n c e 1.5%, s l i d i n g speed 0.067 m/s, c o n t a c t p r e s s u r e p = 3 MPa a t l u b r i c a t i o n w i t h MWP m i n e r a l o i l and OKB 122-16 s i l i c o n e -mi n era l oil and p E MPa a t l u b r i c a t i o n w i t h XU 430 o i l .
1
140
*
194
(h (3.36,15) 3.0 0 PA6 A POMh A PDMc
7
D PA6
7
}tMWP
2.5
m POMh 2.0 E
x
A A DA 0
1.7
a
-
POMc PA6+25%GFJ
o PA6 7 PDMh / t X U 430 POMC a PA6+25% GFJ
A
A
m
A'%
d
3
-
m m
A A
1.0
0.5
0
1
I
10
20
1
30
I
40
F i g . 5.24. R a d i a l wear i n t e n s i t y I w 1 vs. a p p a r e n t s t o r e d f r i c t i o n a l energy d6nsi t y (ASFED, see Chapter 4.2.1) f o r l u b r i c a t e d m i n i a t u r e bearing. Bearing hole diameter 2 . 1 5 mm, e x t e r n a l bush d i a m e t e r 6 mm, l e n g t h 2.1 mm, r e l a t i v e c l e a r a n c e 1.5%, s l i d i n g speed v E < 0 . 0 6 7 , 0 . 1 8 2 > m/s , c o n t a c t p r e s s u r e p E < 3 , 7 > MPa.
I
50
c
195
This relationship can be approximated with the following formula: I = a17(et,l)a18 il w,l
(5.32)
*
where I W I 1 is the radial wear intensity in ,um/km, and etIl the apparent stored frictional energy density (ASFED, see Chapter 4.2.1) for lubricated bearings, measured in MJ/mm3, parameters a17 and a18 are 106.6 and - 1.4866 respectively. The relationship (5.32) can be used to predict wear in bearings similar to the lubricated miniature steel-polymer journal bearings analysed since eqn. (5.32) means that a20 a19 IW,l = et,l
(5.33)
where I w I l is the same as in eqn. (5.32), etll is the maximum density of the thermal energy stored in the polymeric material in mJ/mm3 (see eqn. (4.19)), and parameters a19 and a20 are 23.12 and 0.3273 respectively. The temperature rise in the friction area, which needs to be known in order to determine etll, can be estimated using a formula similar to that for unlubricated bearincjs (eqn. (4.21)). Since the friction coefficient for lubricated bearings can be determined from eqn. (5.27), by taking into consideration eqn. (4.4) the following formula can be used for estimating the temperature rise hT1 for lubricated bearings:
- (a3v+a4)
AT^ = 1150 601pv(alv+a2)p
(1-0.12 p
(5.34)
For p I vI p , d, A , k and 6olsee eqns. (4.4), (4.21) and (5.27); the values of the parameters a l l a2' a3 and a4 are given after eqn. (4.4). The coefficient J01 for lubricated bearings, as found in experiments on miniature steel-polymer journal bearings ((3 2.15 nun), can be taken for PA 6, PA 66, POM h and POM c bearings as 6,1 =0.06p'0'6 and for the bearings PA 6 + 25% glass fibre Sol = 0.08 p-0.5 (contact pressure p in MPa). By combining eqns. (4.19), (5.32) and (5.34), the radial wear intensity of lubricated miniature steel-polymer journal bearings similar to those investigated can be estimated. The radial wear rate of lubricated bearings as a function of the sliding distance can be predicted in a similar way as for unlubricated bearings (see Chapter 4.2.1). The value of Cr (eqns.
196 (4.22) and (4.23)) needed in eqn. (4.241, as estimated from experimental results, is 1.5 kJ for a bearing lubricated with MWP mineral oil or OKB 122-16 silicone-mineral oil and 2 kJ for bearings lubricated with X U 430 or XU 120 traditional clock oils. The tl parameter in eqn. (4.24) can be calculated on the basis that for lubricated bearings operating under contact pressure pEMPa and sliding speed v < 0.2 m/s, tl = m2 v0.05 p-Om3 (v is in m/s , p in MPa) where m2 for bearings based on P A , POM and PA 6 + glass fibre and lubricated with MWP or OKB oils and XU 430 or XU 120 oils can be taken as 2.5, 2.4, 2.3 and 2.3, 2.2, 2.1 respectively. The values of t and u (eqn. (4.25)) for some lubricated steel-polymer miniature journal bearings are listed in Table 5.2 (ref. 196). TABLE 5.2 PARAMETERS t AND u (see eqn. (4.25) - w = t L U , where L AS DETERMINED E X P E R I M E N T I A L L Y FOR L U B R I C A T E D M I N I A T U R E B E A R I N G S . B E A R I N G HOLE D I A M E T E R 2 . 1 5 mm, EXTERNAL BUSH 2 . 1 mm, R E L A T I V E CLEARANCE 1.5-2%, S L I D I N G SPEED 0 . 0 6 7 3 MPa.
i s i n km and w i n ,urn) STEEL-POLYMER JOURNAL DIAMETER 6 mm, LENGTH m/s, CONTACT PRESSURE
The "adhesive-cohesive'' formula (5.31) can be applied to predicting the wear in lubricated steel-polymer miniature journal bearings (when bearing hole diameter is ca. 2 m m ) , when the thermal effects are small (temperature rise in the friction area bT1
wa
'z k' = MAX (W,)
(6.4)
214 P r a c t i c a l e x p e r i e n c e w i t h i n s t r u m e n t o i l s shows t h a t the optimum r e l a t i o n s h i p between t h e a d h e s i o n o f t h e o i l t o a s o l i d s u r f a c e and t h e t e n d e n c y t o s p r e a d o c c u r s a t t h e c o n t a c t a n g l e 0 = 25-40°. 6.2.3.
METHODS
The p r e v e n t i o n o f o i l m i g r a t i o n c a n b e a c h i e v e d by the g m t r i c a l o r c h e m i c a l m o d i f i c a t i o n o f t h e s o l i d s u r f a c e . An example o f geo-
metrical m o d i f i c a t i o n o f t h e s u r f a c e o f t h e m i n e r a l c o v e r b e a r i n g o f a b a l a n c e i s shown i n F i g . 6 . 1 , where t h e o i l c a n o n l y r e a c h and f i l l the barrier circle.
F i g . 6.1. Ge o m e t ri ca l m o d i f i c a t i o n o f t h e s u r f a c e o f a cover balance b e a r i n g t o prevent o i 1 m i g r a t i o n .
The o t h e r g e o m e t r i c a l a p p r o a c h t o n o n s p r e a d i n g i s b a s e d o n the e d g e e f f e c t ( F i g . 6 . 2 ) . For t h e d i s p l a c e m e n t o f t h e d r o p o n t h e v e r t i c a l p l a n e , t h e s u r f a c e o f t h e d r o p must t u r n o n t h e a n g l e 90°.
For t h i s
t h e d r o p h e i g h t must b e enough t o change t h e c o n t a c t a n g l e of 90'. A d d i t i o n a l e n e r g y i s needed f o r t h i s e f f e c t and t h e o i l c a n n o t c r o s s t h e edge b a r r i e r . The e f f i c i e n c y of t h i s method h a s b e e n shown ( r e f . 9 5 ) i n t h e c a s e o f t h e d r o p s of c l a s s i c c l o c k o i l s l a i d o n ruby
p l a t e s . The c o n t a c t a n g l e s were a b o u t 20° a n d t h e o v e r l o a d s required t o d i s p l a c e t h e d r o p w e r e a b o u t 30 g i n t h e c e n t r e of t h e p l a t e but
215
a t t h e e d g e were 1 0 0 - 1 2 0 q . I t is a l s o known ( r e f s . 3 9 4 - 3 9 8 )
that
t h e t e n d e n c y t o s p r e a d i n c r e a s e s w i t h t h e i n c r e a s e i n surface r o u g h n e s s from Ra = 0 . 6 3 p m t o Ra = 0 . 0 2 p m r e s u l t s i n a n i n c r e a s e i n t h e c o n t a c t a n g l e from 13O t o 2 4 O and i n t h e i r r e m i s s i b l e o v e r l o a d s from 17.5 q t o 31.1 g ( r e f . 9 5 ) .
F i g . 6.2.
The edge e f f e c t d u r i n g o i l d r o p s p r e a d i n g .
The f o l l o w i n g c h e m i c a l methods o f s o l i d s u r f a c e m o d i f i c a t i o n
are u s e d : 1) T h e s u r f a c e i s c o m p l e t e l y c o a t e d b y m o n o l a y e r s o f low s u r f a c e
e n e r g y compounds on which o i l d r o p s w i t h h i g h e r s u r f a c e t e n s i o n
w i l l not spread, 2 ) The o i l d r o p i s s u r r o u n d e d w i t h a narrow r i n g c o a t i n g ( e p i l a m e )
o f a compound w i t h low s u r f a c e e n e r g y , or t h e e n t i r e s u r f a c e o f t h e e l e m e n t is c o a t e d and t h e c o a t i n g i s t h e n removed f r o m t h e
area where t h e o i l d r o p w i l l b e l a i d ( F i g . 6 . 3 ) . T h i s i s t h e s 0- c a 11ed
I'
s t o p - o i 1" method ,
3 ) The s o l i d s u r f a c e i s c h e m i c a l l y t r e a t e d t o l o w e r t h e s u r f a c e
f r e e energy.
Barrier film I
F i g . 6.3.
S t o p - O i l method f o r p r e v e n t i n g o i l m i g r a t i o n .
216 The f i r s t method w a s i n v e n t e d b y Woog i n F r a n c e i n t h e n i n e t e e n t w e n t i e s , t h e s e c o n d method w a s d e v e l o p e d i n U S Navy l a b o r a t o r i e s a f t e r t h e l a s t w a r , and t h e c h e m i c a l t r e a t m e n t method w a s i n v e n t e d by Osowiecki a n d a p p l i e d i n i n d u s t r y i n S w i t z e r l a n d i n 1 9 6 4 . The c o a t i n g ( e p i l a m e ) i s l a i d o n t h e s o l i d s u r f a c e from l i q u i d o r g a s s o l u t i o n ( o r d i s p e r s i o n ) . The t h i c k n e s s o f t h e l a y e r u s u a l l y d o e s n o t e x c e e d 0 . 5 ,urn. The a c t i v e p o l a r g r o u p s o f t h e e p i l a m e macromolecules a r e p h y s i c a l l y a d s o r b e d o r c h e m i s o r b e d o n t h e s o l i d s u r f a c e . O s o w i e c k i ' s method is v e r y e f f e c t i v e i n t h e case o f m i n e r a l s o l i d s u r f a c e s . The e f f e c t o f t h e c h e m i c a l t r e a t m e n t of t h e s u r f a c e
i s v e r y s t a b l e and f o r example, u l t r a s o n i c c l e a n i n g o r c h e m i c a l i n t e r a c t i o n s b e t w e e n t h e o i l a n d t h e m a t e r i a l o f t h e s u r f a c e l a y e r do n o t c h a n g e t h e e n e r g e t i c a l p r o p e r t i e s o f t h e s u r f a c e . Only s t r o n g a l k a l i s o r a c i d s can s p o i l t h e t r e a t e d s u r f a c e l a y e r o f ruby o r s a p p h i r e j e w e l s . O s o w i e c k i ' s method h a s b e e n a p p l i e d by Reno S.A. and by KIF P a r e c h o c i n S w i t z e r l a n d t o p r e v e n t o i l m i g r a t i n g from t h e r u b b i n g r e g i o n of t h e c o v e r b e a r i n g i n t h e watch b a l a n c e , e s p e c i a l l y i n t h e I n c a b l o c system. 6.2.4.
COATINGS (EPILAMES)
The r e q u i r e m e n t s f o r t h e s p e c i a l c o a t i n g s ( e p i l a m e s ) t o p r e v e n t o i l from s p r e a d i n g or c r e e p i n g are a s follows ( r e f . 3 9 0 ) : 1) Low s u r f a c e f r e e e n e r g y , 2 ) S t r o n g adhesion t o t h e s o l i d s u r f a c e , 3 ) High c h e m i c a l s t a b i l i t y , 4 ) The c o a t i n g compound must b e i n e r t t o t h e o i l a n d t h e c o a t e d ma-
terials ' 5 ) Broad t e m p e r a t u r e r a n g e o f u s e , 6 ) S u i t a b l e h a r d n e s s and s h e a r s t r e n g t h ,
7) Homogeneity o f t h e c o a t i n g l a y e r on t h e whole s o l i d s u r f a c e ( t h i c k n e s s , m e c h a n i c a l and p h y s i c o c h e m i c a l p r o p e r t i e s )
,
8 ) The s m a l l e s t p o s s i b l e t h i c k n e s s , 9 ) The compound f o r t h e e p i l a m e s h o u l d be s o l u b l e i n common s o l -
vents,
10) The t e c h n i q u e s f o r d e p o s i t i n g c o a t i n g s must n o t b e t o o s o p h i s t i cated. The s u r f a c e f r e e e n e r g y o f t h e e p i l a m e d e p e n d s o n t h e c h e m i c a l s t r u c t u r e of t h e m a c r o m o l e c u l e s . The s u r f a c e free e n e r g y o f t h e epilame d e c r e a s e s f o r t h e macromolecules w i t h s i d e c h a i n s o f t h e f o l l o w i n g s t r u c t u r e s ( r e f . 387) :
217 -CH3
>
-CF2-
>
-CF2H
) -CF3
The e p i l a m e s w i t h
-CF3 g r o u p s a r e c h a r a c t e r i z e d by t h e i r low s u r face f r e e energy ( c a 6 m J / m 2 , Table 6 . 1 )
.
TABLE 6.1. CRITICAL SURFACE TENSION OF WETTING, TURES OF SURFACE GROUPS ( r e f . 387)
rc
OF LAYERS WITH DIFFERENT CHEMICAL STRUC-
SURFACE CONSTITUTION
A. Fluorocarbon s u r f a c e s -C F3 -CF2H -CF
8. Hydroc rbon
3
6 15
and -CF2-
17
-CF2
18
-CH2- CF
20
3
-CF2-CFH-
22
-C F2-CH2-
25
- CFH- CH 2-
28
u r f aces -CH3 ( c r y s t a l ) -CH
3
(monolayer)
20-22 22-24
-CH2-
31
=CH- (phenyl r i n g edge)
35
C . Chlorocarbon s u r f a c e s -CCIH-CH2-
39
- C C I 2-CH2-
40
=cc 1
43
0. N i t r a t e d hydrocarbon s u r f a c e s -CH20N02 ( c r y s t a l )
40
(monolayer) -C(NO ) 2 3 -CH2NHN02 ( c r y s t a l )
42
44
The compound f o r t h e c o a t i n g i s u s u a l l y d e p o s i t e d o n t h e s o l i d s u r f a c e from a l i q u i d of g a s s o l u t i o n ( d i s p e r s i o n ) . The PTFE c o a t i n g s a r e normally a p p l i e d from d i s p e r s i o n i n w a t e r b u t it i s d i f f i c u l t t o m e e t t h e r e q u i r e m e n t of a b r i e f h e a t i n g p e r i o d a t h i g h t e m p e r a t u r e s , which i s n e c e s s a r y t o s i n t e r t h e PTFE p a r t i c l e s , obt a i n good f i l m a d h e s i o n and e l i m i n a t e t h e water and a d d i t i v e s . This t r e a t m e n t is o f t e n t o o i m p r a c t i c a l f o r t h e m a n u f a c t u r e o f i n s t r u ments. A more g e n e r a l l y a p p l i c a b l e and s i m p l e r method, which d o e s
218
i s t h e u s e of s o l u b l e compounds. The
n o t r e q u i r e any p r e - t r e a t m e n t ,
low s u r f a c e - e n e r g y f i l m s c a n b e r e a d i l y l a i d down from a s o l u t i o n i n a n a p p r o p r i a t e s o l v e n t s u c h as x y l e n e h e x a f l u o r i d e o r b e n z o t r i f l u o r i d e w h i c h , a f t e r slow e v a p o r a t i o n , l e a v e a s u i t a b l e c o a t i n g . The compounds a c t u a l l y u s e d on t h e s o l i d s u r f a c e h a v e t h e i m p o r t a n t a d v a n t a g e o f n o t b e i n g r e a d i l y d i s s o l v e d by any o f t h e c l e a n i n g s o l v e n t s commonly u s e d i n t h e i n s t r u m e n t f i e l d . T h r e e g r o u p s of compounds a r e commonly used f o r t h e e p i l a m e s : o r g a n i c materials such as o l e i c o r s t e a r i c a c i d s ; p o l y s i l o x a n e s ; and f l u o r i n a t e d p o l ymers. Woog i n F r a n c e f i r s t d e v e l o p e d and p a t e n t e d a p r o c e s s i n which a d i l u t e s o l u t i o n o f t h e f a t t y a c i d s i n a v o l a t i l e s o l v e n t
w a s used t o c o a t t h e b e a r i n g s u r f a c e s w i t h a t h i n f i l m (epilame) o f f a t t y a c i d . An example o f t h i s t y p e o f epilarne is A r e t o l , manufact u r e d by Moebius e t F i l s i n A l l s c h w i l ( S w i t z e r l a n d ) . The n e x t s t e p was t h e d e p o s i t i n g e p i l a m e s from t h e g a s p h a s e . An i n e r t g a s , e . g . a r g o n , f l o w i n g t h r o u g h t h e chamber w i t h f a t t y a c i d , t a k e s i t s evapo r a t e d a t ( 1 2 0 O C ) m o l e c u l e s and b r i n g s them t o t h e c o n t a i n e r w i t h t h e e l e m e n t s t o be c o a t e d . The e s s e n t i a l d i s a d v a n t a g e s o f t h e f a t t y -acid epilames are: 1) C r i t i c a l s u r f a c e t e n s i o n o f w e t t i n g
T~
i s v e r y h i g h (ca. 30 nN/m),
2 ) Chemical r e a c t i o n o f t h e e p i l a m e w i t h t h e c l a s s i c o r g a n i c o r min-
e r a l instrument o i l s a c c e l e r a t e s t h e i r ageing, 3) Great c h a n g e s i n t h e e p i l a m e p r o p e r t i e s o c c u r w i t h t i m e and tern perature, 4 ) T y p i c a l s o l v e n t s u s e d i n t h e i n s t r u m e n t f i e l d c a n remove t h e epilame
.
E p i l a m e s b a s e d on p o l y s i l o x a n e s show some i n t e r e s t i n g properties. A series o f t h e s e e p i l a m e s h a s b e e n e l a b o r a t e d by Cetehor i n Besanpn ( F r a n c e ) ( r e f s . 64,68) -401)
,
. As
c o n c l u d e d by Tabor and W i l l i s
(refs. 399-
t h e a d s o r p t i o n o f d i m e t h y l p o l y s i l o x a n e (DMPS) molecules o n t o
a c o p p e r s u r f a c e is p o s s i b l e when DMPS i s d e p o s i t e d from t h e s o l u t i o n ( e . g . i n Freon 113) and t h e n h e a t e d t o 100°C. The t h i c k n e s s ( s ) o f t h e a d s o r b e d DMPS l a y e r i s a p a r a b o l i c f u n c t i o n v s . t i m e ( t )( s = fi).A f t e r a b o u t 2 0 0 h o u r s o f h e a t i n g t h e c o a t i n g t h i c k n e s s i n c r e a s e s t o a b o u t 0 . 1 1 ,UIII. According t o Tabor and W i l l i s , t h e good a d s o r p t i o n of DMPS o n t o a m e t a l l i c s u r f a c e i s due t o t h e p l a r groups -COOH
o b t a i n e d as a r e s u l t of t h e o x i d a t i o n of m e t h y l g r o u p s -CH3
i n t h e macromolecules o f DMPS
.
The f i r s t e x p e r i m e n t s c a r r i e d o u t by C e t e h o r showed t h a t t h i s method c a n b e a p p l i e d t o t h e p r o d u c t i o n o f e p i l a m e s o n m e t a l l i c and m i n e r a l s u r f a c e s . The e p i l a m e s f i l m w a s o b t a i n e d u s i n g DMPS ( w i t h
219
v i s c o s i t y 30000 mm2/s a t 20OC) d i s s o l v e d i n F r i g e n 1 1 3 . The e l e m e n t w a s immersed i n t h e s o l u t i o n a n d t h e n h e a t e d a t 100°C f o r 2 h o u r s . The r e m a i n s o f t h e n o n a d s o r b e d compound were c l e a n e d by u l t r a s o n i c s i n c h l o r i n a t e d s o l v e n t . The c r i t i c a l s u r f a c e t e n s i o n o f t h i s s o r t o f e p i l a m e i s 2 4 mN/m. On a s u r f a c e l i k e t h i s , o i l s w i t h a s u r f a c e t e n s i o n o f 25-30 mN/m c a n b e l a i d down. The c o n t a c t a n g l e s are 20-30'. The maximum v a l u e o f t h e work of a d h e s i o n , W a r f a c e t e n s i o n o f t h e o i l i s 27 mN/m.
o c c u r s when t h e s u r -
The DMPS e p i l a m e s o b t a i n e d by
t h i s i n s i t u method may o n l y b e removed m e c h a n i c a l l y . The d e v e l o p m e n t of t h e p o l y s i l o x a n e e p i l a m e s h a s b e e n d i r e c t e d t o t h e o x i d i z e d p o l y s i l o x a n e compounds d i s s o l v e d i n s o l v e n t s . F o r t h i s , the DMPS w i t h p o l a r g r o u p s -COOH h a s b e e n p r e v i o u s l y formed by t h e o x i d a t i o n o f DMPS i n a n o z o n e a t m o s p h e r e a t a t e m p e r a t u r e o f
100°C. About 0 . 5 % o f DMPS i s t h e n d i s s o l v e d i n F r i g e n 113.The microelements are inmersed i n t h i s solution f o r 2-4 minutes. A f t e r d r y i n g , t h e
elements are washed w i t h c h l o r i n a t e d s o l v e n t t o remove t h e unadsorbed m o l e c u l e s o f t h e compound. The p r o p e r t i e s o f t h i s e p i l a m e a r e s i m i -
l a r t o t h e o b t a i n e d by t h e i n s i t u method. T h i s t y p e of e p i l a m e h a s b e e n p a t e n t e d , a n d was i n d u s t r i a l l y m a n u f a c t u r e d by Rhone-Poulenc (Rhodorsil) i n France. The c r i t i c a l s u r f a c e t e n s i o n of w e t t i n g i s r e l a t i v e l y h i g h . To l o w e r t h e
tc,
rc
f o r these epilames
f l u o r i n a t e d polysiloxanes a r e
a p p l i e d . The o x i d a t i o n i n t h e o z o n e a t m o s p h e r e o f f l u o r i n a t e d p o l y s i l o x a n e FS 1265 (Dow C o r n i n g ) c o n t a i n i n g t h e g r o u p s CF2-CF2 made
it p o s s i b l e t o manufacture a n epilaine w i t h
rC
= 17-19 mN/m.
i c a l s u r f a c e t e n s i o n o f w e t t i n g , a b o u t 1 7 mN/m,
The c r i t
w a s achieved i n
e p i l a m e s b a s e d o n t h e f l u o r o p o l y s i l o x a n e s w i t h (CF2)n-CF3
groups
o x i d i z e d i n t h e o z o n e a t m o s p h e r e . The c o n t a c t a n g l e f o r o i l s w i t h a s u r f a c e t e n s i o n o f 26-27 mN/m on s u c h e p i l a m e s i s 30-35O. E f f e c t i v e e p i l a r n e s h a v e b e e n d e v e l o p e d i n t h e Naval R e s e a r c h L a b o r a t o r y i n Washington D . C .
( r e f s . 4 0 2 , 4 0 3 ) . They a r e b a s e d o n
c o m m e r c i a l l y a v a i l a b l e f l u o r i n a t e d polymers whose low
;yc
values
r e n d e r them e s p e c i a l l y a t t r a c t i v e f o r s u c h a p p l i c a t i o n . The b e s t compounds a r e f l u o r i n a t e d esters of p o l y ( a c r y l i c a c i d ) or poly(mtha c r y l i c acid) with the values of
lowest v a l u e o f
;yc
( 6 mN/m)
rc
11.1 and 1 0 . 6 r e s p e c t i v e l y .
The
is found o n a s u r f a c e w i t h a n a d s o r b e d
f l u o r i n a t e d f a t t y a c i d monolayer. The f i r s t f l u o r i n a t e d c o a t i n g s b a s e d o n PTFE o r c o p o l y m e r s o f t e t r a f l u o r o e t h y l e n e ( T P E ) a n d h e x a f l u o r o p r o p y l e n e (HFP) w i t h a criti c a l surface tension 404)
,
rc
1 8 . 5 a n d 1 6 . 2 mN/m r e s p e c t i v e l y ( r e f s . 4 0 2 , were v e r y l i m i t e d i n t h e i r a p p l i c a t i o n b e c a u s e of t h e rigorous
220
s u r f a c e t r e a t m e n t r e q u i r e d t o d e p o s i t t h i n , c o n t i n o u s , adherent p l y m e r c o a t i n g s , s i n c e i n o r d e r t o a s s u r e good f i l m a d h e s i o n , t h e objects t o b e c o a t e d need a b r i e f b a k i n g i n a n over o r t o t a l immersion i n a f l u i d i z e d b e d o f t h e h o t polymer powder a t h i g h t e m p e r a t u r e s . The f l u o r o p o l y e s t e r s , however, c a n b e d e p o s i t e d a s a n a d h e r e n t f i l m from a s o l u t i o n i n a v o l a t i l e f l u o r o c h e m i c a l s o l v e n t a t room temperature. The two f l u o r o p o l y e s t e r s a r e o n l y u s e f u l w i t h t h e S t o p - O i l method b e c a u s e t h e a d h e s i o n o f t h e o i l t o t h e f l u o r o p o l y e s t e r c o a t i n g i s v e r y poor due t o t h e i r low s u r f a c e e n e r g y . A s l i g h t j a r o f t h e i n s t r u m e n t might d e t a c h t h e o i l d r o p . The o i l d r o p i s t h e r e f o r e p l a c e d on t h e h i g h e n e r g y s u r f a c e and s u r r o u n d e d by t h e low e n e r g y anti-migration coating. R e s e a r c h ( r e f s . 402-404) h a s b e e n c a r r i e d o u t on f l u o r i n a t e d polymers ( p r o d u c t s o f t h e Minnesota ?lining and K a n u f a c t u r i n g Co.)
- pentadecafluoro-octyl m e t h a c r y l a t e ( PFOMA) and C a F 7SO 2MC 3 H 7C 2H 4OCOCH=CH 2 , 2-N- propy l p e r f l u o r o o c t a n e s u l f o n a m i d o / e t h y l a c r y l a t e (PFOSEA) and iyc 10.6 and 11.1 mN/m r e s p e c t i v e l y . The polymers w e r e s u p p l i e d i n t h e form o f a 20% s o l u t i o n i n p u r e x y l e n e h e x a f l u o r i d e . A measured q u a n t i t y o f
w i t h t h e f o r m u l a s C7F15CH2OCOC(CH3)=CH2, 1 H
t h e s o l u t i o n w a s poured o n t o t h e smooth s u r f a c e s o f t h e t e s t p a n e l . The s o l v e n t w a s t h e n e v a p o r a t e d s l o w l y u n d e r c o n t r o l l e d c o n d i t i o n s a t room t e m p e r a t u r e i n a d u s t - f r e e environment f o l l o w e d by e x p o s u r e f o r 24 h o u r s i n a c l e a n vacuum oven a t 5OoC and 1 0 kPa. The r e s u l t i n g smooth and s p e c u l a r f i l m s were between 0.15 and 0.23 mm t h i c k , f i r m and t r a n s p a r e n t , and were s o a d h e r e n t t h a t o n l y s m a l l fragments c o u l d b e removed from t h e p a n e l by c h o p p i n g t h e c o a t i n g . The p a n e l s used w e r e s t a i n l e s s s t e e l , P y r e x , g l a s s epoxy, p a p e r b a s e epoxy, p a p e r b a s e p h e n o l i c , f i b e r - r e i n f o r c e d and p a p e r - r e i n f o r c e d Bakelite. The p a n e l s w e r e immersed a t room t e m p e r a t u r e f o r p e r i o d s o f one and f o u r weeks i n o n e of t h e f o l l o w i n g o r g a n i c l i q u i d s : hexadecane,para f f i n o i l , tricresylphosphate, dicyclohexyl , propylene carbonate, and b i s ( 2 - e t h y l h e x y l ) s e b a c a t e . C o a t i n g s on m e t a l s u r f a c e s ( s t a i n less s t e e l , b r a s s and aluminium) w e r e a l s o s t u d i e d . E q u i l i b r i u m
c o n t a c t a n g l e s o f hexadecane and w a t e r on t h e d r i e d s u r f a c e s w e r e measured b e f o r e and a f t e r e x p o s u r e i n t h e o r g a n i c l i q u i d s . These i n v e s t i g a t i o n s ( r e f s . 402,403) r e v e a l e d t h a t t h e r e was no change i n t h e p h y s i c a l a p p e a r a n c e o f t h e c o a t i n g s a f t e r immersion i n n o n p o l a r l i q b i d s such a s hexadecane o r p a r a f f i n o i l , and o n l y o c c a s i o n a l p i t t i n g o r o p a q u e n e s s were n o t e d a f t e r p r o l o n g e d exp o s u r e i n some p o l a r l i q u i d s . W e t t a b i l i t y w a s o n l y s l i g h t l y a f f e c t -
221 e d . The polymer c o a t i n g s on m e t a l s u b s t r a t e s were u n a f f e c t e d by e v e n t h o s e p o l a r o r g a n i c o i l s which had some a d v e r s e e f f e c t s o n t h e
same c o a t i n g s on r e s i n o u s s u r f a c e s , e m p h a s i z i n g t h e i n f l u e n c e o f t h e u n d e r l y i n g s u r f a c e . The a d h e s i v e q u a l i t i e s o f t h e f i l m s f o r any o f t h e p a n e l s o r m e t a l l i c s u r f a c e s t e s t e d were n o t a f f e c t e d by exp o s u r e t o a n e n v i r o n m e n t o f 1 0 0 % R.H.
and 4 9 O C f o r one week.
These i n v e s t i g a t i o n s ( r e f s . 402,403) i n t o t h e p r e v e n t i o n o f o i l s p r e a d i n g ( S t o p - O i l method) showed t h a t f o r m e t a l and m i n e r a l s u r f a c e s t h e two p o l y m e r s i n v e s t i g a t e d were v e r y good. I f d u r i n g n o r m a l u s e t h e i n s t r u m e n t w i l l b e s u b j e c t e d t o a b r u p t t e m p e r a t u r e c h a n g e s from subzero t o high temperature i n c o n d i t i o n s of high humidity condit i o n s , t h e PFOSEA c o a t i n g s h o u l d b e p r e f e r r e d b e c a u s e t h i s p l y e s t e r w i t h s t a n d s s u c h c o n d i t i o n s ; o t h e r w i s e PFOMA s h o u l d b e u s e d . The PFOMA c o a t i n g ( 2 % PFOMA s o l u t i o n i n h e x a f l u o r o x y l e n e ) h a s b e e n t e s t e d and u s e d f o r t h e p r e v e n t i o n o f l u b r i c a n t m i g r a t i o n from t h e b a l l b e a r i n g s of servo-motors
( r e f . 4 0 5 ) . A v a r i e t y of l u b r i -
cant types w e r e considered: s i l i c o n e s , d i e s t e r s , s i l i c o n e - d i e s t e r b l e n d s , g r e a s e s , soap-thickened d i e s t e r s , soap-thickened
silicone-
- d i e s t e r blends ( a l l d i e s t e r l u b r i c a n t s contained 0.5% antioxidant and 2 . 0 % r u s t i n h i b i t o r ) . The l i q u i d l u b r i c a n t s r a n g e d i n v i s c o s i t y from 1 0 0 0 t o 4 2 0 0 0 nun2/,
a t -53.9OC.
B a r r i e r f i l m s o f PFOMA l e s s
t h a n 1,um t h i c k were d e p o s t e d on t h e f o r e h e a d s u r f a c e s o f t h e b a l l - b e a r i n g r i n g s . T h e s e i n v e s t i g a t i o n s showed t h a t PFOMA c o a t i n g s a r e v e r y e f f e c t i v e i n p r e v e n t i n g t h e l u b r i c a n t from s p r e a d i n g . B e a r i n g s t r e a t e d w i t h such a b a r r i e r have i n c r e a s e d t h e l i v e s o f serw-motors from 300 t o n e a r l y 4000 h o u r s . A PFOMA c o a t i n g c a n b e a c c i d e n t a l l y formed o n t h e r u b b i n g s u r -
f a c e s o f t h e b a l l b e a r i n g s . When t h e y become worn, t h e t r a c e s of PFOMA may r e s u l t i n d r y r u b b i n g o f m e t a l - t o - m e t a l
surfaces. I f t h e
l u b r i c a n t d o e s n o t q u i c k l y come between t h e r u b b i n g s u r f a c e s , s e i z u r e is p o s s i b l e . I n h i s i n v e s t i g a t i o n s i n t o gyroscope b e a r i n g s , Ahlborn ( r e f . 4 0 6 ) showed t h a t when t h e s l i d i n g s p e e d o f t h e b a l l i s less t h a n 1 . 0 7 m / s
and t h e r e a l i s t i c s p e c i f i c l o a d s
( 8 0 0 MPa,
p r a c t i c a l l y no s e i z u r e s o c c u r . S e i z u r e i s p o s s i b l e o n l y when t h e v i s c o s i t y o f t h e l u b r i c a n t i s t o o h i g h and t h e o i l c a n n o t q u i c k l y come between t h e m e t a l l i c s u r f a c e s a f t e r t h e d e s t r u c t i o n o f t h e PFOMA l a y e r and when t h e h e a t i n g i n t h e r u b b i n g r e g i o n i s very high. S e l e c t i n g t h e optimum s o l v e n t f o r t h e p r e p a r a t i o n o f b a r r i e r e p i l a m e f i l m s i s v e r y i m p o r t a n t . B e r n e t t ( r e f . 407) h a s i n v e s t i g a t e d t h e e f f e c t o n PFObIA f i l m o f a l t e r n a t i v e s o l v e n t s f o r t y p i c a l h e x a f l u o r o x y l e n e (HFX) ( w i t h b o i l i n g p o i n t 116OC)
. These w e r e
:
222 1 ) a more v o l a t i l e
and
more
easily
available
CC12F-CC1F2
( F r e o n TF, b o i l i n g p o i n t 48OC) and 2 ) a s l i g h t l y more v o l a t i l e s o l v e n t a n a l y z e d a s i s o m e r s o f perfluorobutyltetrahydrofuran (PBTF) ( b p = 92-102OC) . I n t h e s e i n v e s t i g a t i o n s f l u o r o p o l y m e r f i l m s p r e p a r e d from o n e F r e o n TF and two d i f f e r e n t PBTF s o l u t i o n s were compared w i t h f i l m s p r e p a r e d from HFX s o l u t i o n f o r w e t t a b i l i t y and s t a b i l i t y a f t e r exposure t o e l e v a t e d t e m p e r a t u r e s i n a i r o r i n c o n t a c t w i t h o r g a n i c l i q u i d s : d i m e t h y l s i l o x a n e DC 200 ( 5 0 m2/s a t 2OoC1 ( 2-ethylhexyl)
s e b a c a t e , V e r s i l u b e F-50
from G e n e r a l E l e c t r i c Co.) t o NRL MB-20B) additives)
and NRL-20B
, Nye
, bis
(chlorinated polysiloxane
s y n t h e t i c instrument o i l
(formulated
( m i x t u r e of H e r c o l u b e A , d i e s t e r , and
( r e f . 4 0 8 ) . The t e s t s c a r r i e d o u t showed t h a t a t a m b i e n t
t e m p e r a t u r e s , F r e o n TF i s a s u i t a b l e s u b s t i t u t e f o r HFX i n many
cases. F o r c e r t a i n r e q u i r e m e n t s , however, s u c h a s p r e v e n t i o n o r g a n i c l i q u i d s p r e a d i n g a t e l e v a t e d t e m p e r a t u r e s , polymer s o l v e d i n HFX o r polymers s o l v e d i n PBTF ( t w o d i f f e r e n t ) a r e s u p e r i o r t o polymer d i s s o l v e d i n F r e o n TF, p r o v i d e d t h e r e were no t r a c e i m p u r i t i e s p r e s e n t i n e i t h e r s o l v e n t . The r e s u l t s i n d i c a t e t h a t whenever F r e o n TF
i s u s e d as a s o l v e n t f o r t h e s e f l u o r o p o l y m e r s , t h e p e r f o r m a n c e o f t h e f i l m s is a d v e r s e l y a f f e c t e d . W e t t a b i l i t y and a d h e s i o n o f t h e f i l m s p r e p a r e d from PFOMA d i s s o l v e d ( 2 0 % ) i n HFX, 2 0 % s o l u t i o n o f PFOMA i n BFX d i l u t e d t o 2 % w i t h HFX, s o l u t i o n o f 2 % PFOMA i n F r e o n TF, s o l u t i o n o f 2 % PFOMA i n PBTF-a a n d s o l u t i o n of 2 % PFOMA i n PBTF-b, w e r e a l m o s t u n a l t e r e d by e x p o s u r e s t o 21OoC i n a i r f o r 11 h o u r s and t o 100°C i n o i l s f o r 18 h o u r s . F i l m s of polymer d i s s o l v e d i n F r e o n TF and i n PBTP-a had t h e same
rc a s
f i l m s of polymer d i s s o l v e d i n HFX ( 2 0 % s o l u t i o n ) ; how-
e v e r , e x p o s u r e t o e l e v a t e d t e m p e r a t u r e s i n a i r , and e s p e c i a l l y i n c o n t a c t w i t h o i l s , p r o d u c e d g e n e r a l l y b a d and o c c a s i o n a l l y d i s a s t r o u s e f f e c t s o n w e t t a b i l i t y i n polymer d i s s o l v e d i n F r e o n TF films. F u r t h e r i n v e s t i g a t i o n s h a v e b e e n c a r r i e d o u t t o d e m o n s t r a t e hav t h e polymer c o n c e n t r a t i o n ( w h i c h g o v e r n s t h e f i l m t h i c k n e s s ) ard the s o l v e n t t y p e a f f e c t t h e q u a l i t y o f c u r e d b a r r i e r f i l m s o f PFOMA. T h e s e i n v e s t i g a t i o n s ( r e f . 4 0 9 ) r e p o r t t h e r e s u l t s of a n experimen-
t a l s t u d y o f t h e e f f e c t s o f t h e s e v a r i a b l e s on b a r r i e r f i l m w e t t a b i l i t y , s u r f a c e , and polymer p r o p e r t i e s , u t i l i z i n g c o n t a c t a n g l e and m i c r o s c o p y s t u d i e s , and r e l a t e them t o t h e i r o p t i m i z a t i o n i n b a r r i e r f i l m o i l r e p e l l e n c y a p p l i c a t i o n s . The r e s u l t s show t h a t o f t h e s e v e n s o l v e n t s u s e d : I-IFX, 1,1,2-trifluoro-1,2,2-trichloroethane (CClF) , p e r f l u o r i n a t e d c y c l i c e t h e r C8F160 (PCE) and t h e i r m i x t u r e s : HFC/CClF 7 5 / 2 5 , PCE/CClF 7 5 / 2 5 , PCE/CClF 90/10 and PCE/CClF/HFX
223
80/10/10, t h e b e s t is t h e P C E / C C l F 9 0 / 1 0 m i x t u r e . The m o s t e f f e c t i v e b a r r i e r f i l m f o r m u l a t i o n of t h e s e s o l v e n t s was i n a 0 . 2 % w/w s o l u t i o n o f PFOMA. T h i s f o r m u l a t i o n c o n s i s t e n t l y g a v e smooth (0.5-0.15 ,um t h i c k ) , uniform, h i g h l y o i l - r e p e l l e n t
f i l m s . Barrier f i l m s on
p o l i s h e d metal s u b s t r a t e s w e r e more r e s i s t a n t t o l u b r i c a n t s t h a n t h o s e on g l a s s s u b s t r a t e s . The c h o i c e o f s o l v e n t s i n t h e c a s t i n g s o l u t i o n a p p e a r e d t o b e t h e most i n f l u e n t i a l f a c t o r . I t is p s t u l a t e d t h a t c h a n g e s i n s u r f a c e t e n s i o n g r a d i e n t s and
s o l u b i l i t i e s during
s e l e c t i v e e v a p o r a t i o n from mixed s o l v e n t s a f f e c t t h e s u r f a c e p r o p e r t i e s of t h e d r i e d f i l m s . S c a n n i n g e l e c t r o n m i c r o s c o p y showed d i f f e r e n c e s i n u n i f o r m i t y and also i n d i c a t e d s u r f a c e c h a n g e s induced by e x p o s u r e o f t h e f i l m s t o l u b r i c a n t s a t e l e v a t e d (lOO°C) temperatures. The f o l l o w i n g t h r e e o i l s , r e p r e s e n t a t i v e of l u b r i c a n t s u s e d f o r m i n i a t u r e b e a r i n g s , were u s e d f o r c o m p a t a b i l i t y s t u d i e s ; M i l i t a r y
a formulated e s t e r - d i e s t e r instrument
S p e c i f i c a t i o n MIL-L-81846, o i l with
rl
= 25.5 mN/m,
u s e d i n b e a r i n g s i n normal o p e r a t i n g con-
d i t i o n s ; a n unformulated chlorophenyl p o l y s i l o x a n e ,
;yl
= 2 1 mN/m
,
u s e d f o r h i g h t e m p e r a t u r e b e a r i n g o p e r a t i o n ; and b i s ( 2 - e t h y l h e x y l ) sebacate, a d i e s t e r base stock, unformulated, with
rl
= 31.1 mN/m
( t h e s e b a c a t e was p e r c o l a t e d by a l u m i n a b e f o r e u s e ) . T r i p l y d i s t i l l e d water hexadecane
(rl (rl
= 72.0 = 27.6
mN/m), mN/m)
m e t h y l e n e i o d i d e (fl = 50.8 mN/m)
and
w e r e s t a n d a r d r e f e r e n c e l i q u i d s used
f o r c o n t a c t a n g l e measurement. M e t h y l e n e i o d i d e and h e x a d e c a n e had b e e n p u r i f i e d t h r o u g h v a r i o u s a d s o r b e n t columns p r i o r t o u s e . The w e t t a b i l i t y t e s t s show t h a t a f t e r immersion i n t h e o i l s t h e s u r f a c e o f t h e f i l m s became p i t t e d a n d rough and t h e c o n t a c t angles
were l o w e r t h a n b e f o r e immersion. These c h a n g e s , however, w e r e s m a l l , e s p e c i a l l y f o r f i l m s d e p o s i t e d o n m e t a l s ( p a s s i v a t e d a n d unp a s s i v a t e d b e a r i n g s t e e l s ) , The p o s s i b i l i t y o f o i l e x p o s u r e hcreasi n g t h e s u r f a c e p o l a r i t y w a s examined and t h e d e t e r m i n a t i o n ( u s i n g t h e g r a p h i c a l method o f Fowkes ( r e f . 4 1 0 ) ) of t h e d i s p e r s i o n comp o n e n t o f t h e s u r f a c e f r e e e n e r g y o f t h e PFOMA f i l m showed t h a t t h e d had d e c r e a s e d s l i g h t l y a f t e r o i l immersion i n d i c a t i n g a pssible JJs c h a n g e i n s u r f a c e p o l a r i t y ( f o r t h e c o n t r o l PFOMA f i l m s , d w a s very close to the
rc
rs
v a l u e , i n d i c a t i n g no p o l a r c o n t r i b u t i o n ) . P h y s i c a l
a l t e r a t i o n s by t h e l u b r i c a n t s u c h as s w e l l i n g , c r a z i n g a n d t h e l i k e
w e r e n o t s e e n , b u t should b e c o n s i d e r e d i n any d i s c u s s i o n of polym e r - o i l i n t e r a c t i o n s . I t would a p p e a r t h a t s u r f a c e r o u g h n e s s i s n o t a m a j o r f a c t o r , and t h a t a s l i g h t change i n t h e s u r f a c e p o l a r i t y o f PFOMA may o c c u r on o i l e x p o s u r e .
224
F l u o r i n a t e d m a t e r i a l s ( s u c h a s PFOMA) u s e d f o r b a r r i e r f i l m s have t h e i m p o r t a n t a d v a n t a g e o f n o t b e i n g r e a d i l y d i s s o l v e d by a n y of t h e c l e a n i n g s o l v e n t s commonly u s e d i n t h e i n s t r u m e n t f i e l d ( r e f . 4 0 2 ) . O r g a n i c s o l v e n t s s u c h a s x y l e n e o r p e t r o l e u m e t h e r had no
a d v e r s e e f f e c t o n PFOMA c o a t i n g s o n s t a i n l e s s s t e e l , b r a s s and aluminium s u r f a c e when t o t a l l y immersed f o r 3 weeks a t a m b i e n t t e m p e r a t u r e . Ammonia-containing aqueous w a t c h c l e a n i n g s o l u t i o n s a r e c a p a b l e o f d e t a c h i n g t h e c o a t i n g a f t e r p r o l o n g e d immersion w i t h o u t d i s s o l v i n g i t . F l u o r i n a t e d s o l v e n t s s u c h a s F r e o n TF u n d e r s t a n d a b l y a c t a s good s o l v e n t s o n t h e polymer. T h r e e i n d u s t r i a l l y m a n u f a c t u r e d a n t i - m i g r a t i o n coatings (epilams) a r e l i s t e d i n Table 6 . 2 .
These t h r e e c o a t i n g s are manufactured on
t h e b a s i s o f f l u o r o p o l y m e r m a t e r i a l and t h e c r i t i c a l s u r f a c e tension of w e t t i n g of t h e s e epilarnes i s t h e r e f o r e v e r y low. The a d m i s s i b l e t e m p e r a t u r e r a n g e f o r t h e d r i e d c o a t i n g s i s v e r y broad. The techniques f o r d e p o s i t i n g c o a t i n g s a r e v e r y s i m p l e a n d c h e a p . The d i s a d v a n t a g e of t h e s e anti-migration c o a t i n g s l i e s i n t h e handling o f them;there m u s t b e adequate v e n t i l a t i o n t o avoid vapour build-up and t h e s u r f a c e s need t o b e c a r e f u l l y c l e a n e d b e f o r e d e p o s i t i n g c o a t i n g s (see Chapter 6 . 2 . 6 )
.
I n v e s t i g a t i o n s c a r r i e d o u t i n r e c e n t y e a r s o n metal-on-polymer and polymer-on-polymer m i n i a t u r e s y s t e m s h a v e shown t h a t l u b r i c a t i n g such c o m b i n a t i o n s of m a t e r i a l s i s a v e r y e f f e c t i v e way t o s i g n i f i c a n t l y r e d u c e t h e i r w e a r r a t e s (see C h a p t e r 5 . 2 ) . T h e r e i s s t i l l a p r o b l e m , however, w i t h t h e o i l d r o p m i g r a t i o n from t h e rub-
b i n g r e g i o n . The c a u s e o f o i l m i g r a t i o n i n p o l y m e r i c s y s t e m s i s n o t q u i t e c l e a r ; p u r e p o l y m e r s have r e l a t i v e l y v e r y low (
< 60
rc
mN/m) ( r e f . 1 8 5 ) . Because o f t h e low work of a d h e s i o n W a be-
tween polymer and o i l , t h e d r o p c a n b e e a s i l y d i s p l a c e d o n t h e p o l ymeric s u r f a c e . Fox and Zisman ( r e f . 4 1 1 ) h a v e shown t h a t t h e s p r e a d i n g o f l i q u i d s on low-energy s o l i d s u r f a c e s d o e s n o t depend on t h e p o l a r and n o n p o l a r c o m b i n a t i o n s o f p o l y m e r s and l i q u i d s . The a d h e s i o n o f h y d r o c a r b o n s u r f a c e s t o l i q u i d s i n a l i p h a t i c h y d r o c a r b o n s wasbetter t h a n i n l i q u i d s c o n t a i n i n g o x y g e n ' o r f l u o r i n e b u t n o t as good a s i n a r o m a t i c h y d r o c a r b o n s . The a d s o r p t i o n e n e r g y of h y d r o c a r b o n l i q u i d s on low-energy s u r f a c e s w a s l a r g e l y d u e t o d i s p e r s i o n i n t e r a c t i o n s ; t h e o b s e r v e d h e a t s o f a d s o r p t i o n s e t a maximum v a l u e for t h e dispersion p o t e n t i a l i n t h e s e systems ( r e f . 4 1 2 ) . The i r r e g u l a r i t i e s and r o u g h n e s s o f r e a l i s t i c polymer e l e m e n t s a f f e c t t h e s p r e a d i n g o f o i l s . The f o r m a t i o n o f a n a d h e s i v e bond be-
TABLE 6 . 2 INDUSTRIALLY MANUFACTURED ANT I - M I GRAT ION COATINGS (EPI LAMES) *
DESCRIPTION AND PRODUCER PROPERT I ES
FC-721 3 M COMPANY MINNEAPOLIS, MINNESOTA (U.S.A.)
F1 uoropo 1 ymer Freon TF (2% s o l u t i o n )
Ma t e r ia 1 Solvent C r i t i c a l surface tension o f w e t t i n g ;Yc, o f d r i e d f i l m
11-12
mN/m
Density, mg mn-3 B o i i i n g p o i n t , OC F i l m t h i c k n e s s , ,urn
1.5 48 1.3 ( d i p c o a t i n g ) and 0.05 (FC-721 d i l u t e d t w e n t y f o l d )
ANTI SPREAD M2/200/50 D r . TlLLWlCH GmbH. HOKB-AHLDORF ( F .R .G .) F1 uoropo 1 yme r F r i g e n 113 (0.5 or 2% s o l u t i on) T e s t s i l i c o n e (50 mm2 s - ' a t 20°C) o i 1 forms a d r o p w i t h c o n t a c t a n g l e 5-45O 1.54
FIXDDROP BS, K MOEBIUS e t Fils,ALLSCHWIL, BASEL (SWITZERLAND) F I u o r o p o l ymer F r i gen 113 TR-T (Hoechs t ) Freon (Du Pont) S i l i c o n e o i I forms a d r o p w i t h c o n t a c t a n g l e a b o u t 45
1.54
47.6 0.5 (0.5% s o l u t i o n ) 2 (2% s o l u t i o n )
47.6
0.01 -0.05
Thermal s t a b i 1 i t y of d r i e d
film, Flash p o i n t Toxi c i t y Appearance Chemical s t a b i Ii t y
Product iv i t y Storage
1
OC
S t a b l e u p t o 205 Nonf lammable C l e a r and c o l o r l e s s Non-solubi l i z i n g s o l v e n t s such as heptane, t o l u e n e , acetone and w a t e r D i p p i ng, b r u s h i n g , s p r a y i ng . A i r d r y i n g : 15-20 s When i n c o n t a c t w i t h d i e s t e r o i l s , s h o u l d be baked f o r 15 min a t 100°C Should be s t o r e d a t o r near room t e m p e r a t u r e
-75 t o +200 Nonf 1ammab 1e A t 250°C f l u o r i n e i s s e c r e t e c C l e a r and c o l o r l e s s Non-sol u b i 1 i z i ng s o l v e n t s . Can be used f o r c o a t i ngs dep o s i t e d on polymers Dipping, b r u s h i n g , s p r a y i n g . D i p p i n g : 5-10 s a t 20°C. D r i e s a t between room temper a t u r e and 8OoC i n 1-15 m i n ca. T O O g p e r m2 12 months i n c l o s e d c o n t a i n e r
' - 7 5 t o +200
, Nonf lamnable A t 250OC f l u o r i n e i s s e c r e t e C l e a r and c o l o r l e s s Non-solubilizing solvents. Can be used for c o a t i n g s dep o s i t e d o n polymers (K) Dipping, b r u s h i n g , s p r a y i n g . D r i e s a t 5O-8O0C i n 2-5 m i n
12 months i n c l o s e d c o n t a i n e
N
N Ln
226
tween a l i q u i d and a s o l i d s u r f a c e depends on t h e development of a maximum a r e a o f m o l e c u l a r c o n t a c t and t h e d i s p l a c e m e n t o f a i r from the micro-irregularities
on t h e s u r f a c e . The r a t e of p e n e t r a t i o n o f
l i q u i d s i n t o c a p i l l a r i e s and s l i t s on t h e s u r f a c e a f f e c t s t h e dyna-
mics of f l o w . I n g e n e r a l t h e dynamics of s p r e a d i n g a r e c o n t r o l l e d by t h e c o e f f i c i e n t c o s E l m / ( g q ) ; where g i s a f a c t o r which depends on t h e geometry of t h e system ( 7 i s t h e v i s o s i t y of a l i q u i d )
r1
( r e f . 4 1 3 ) . The dynamics of DMPS s p r e a d i n g on PTFE depend on t h e volume of d r o p s and t h e r o u g h n e s s o f t h e s u b s t r a t e t o g e t h e r w i t h t e m p e r a t u r e and v i s c o s i t y ( r e f . 4 1 4 ) . The s p r e a d i n g v e l o c i t y w a s shown t o grow w i t h d e c r e a s i n g d r o p volume, t h e e f f e c t b e i n g more pronounced a t h i g h v i s c o s i t i e s of DMPS. The s p r e a d i n g on t h e rough s u r f a c e w a s s l o w e r t h a n on t h e smooth one owing t o t h e e n e r g y bar-
r i e r c r e a t e d by s u r f a c e i n h o m o g e n e i t i e s . An i n c r e a s e i n t e m p e r a t u r e increases t h e spreading velocity. The m i g r a t i o n of t h e o i l from p o l y m e r i c m i n i a t u r e b e a r i n g s i s p r o b a b l y a l s o a f f e c t e d by t h e a p p l i c a t i o n o f f i l l e r s , p l a s t i c i z e r s , s o l i d l u b r i c a n t s e t c . F i l l e r s such as g l a s s or c a r b o n r i s e t h e s u r f a c e f r e e e n e r g y of polymers ( r e f . 2 0 6 ) . The inhomogeneity of t h e s u r f a c e s c a n b e v e r y h i g h . The c o a t i n g ( e p i l a m e ) c a n b e u s e d t o make t h e polymer s u r f a c e uniform o r a s a b a r r i e r f i l m . The d e p o s i t i n g o f c o a t i n g s on polymers i s a q u i t e n o v e l and d i f f i c u l t problem. The f o l l o w i n g two ways a r e p o s s i b l e : 1) m o d i f i c a t i o n o f t h e s u r f a c e p r o p e r t i e s o f p o l y m e r i c s o l i d s by t h e a d s o r p t i o n of a p p r o p r i a t e e . g .
p a r t i a l l y f l u o r i n a t e d compounds a t p o l y -
mer-air i n t e r f a c e s d u r i n g t h e f o r m a t i o n o f t h e polymer s u r f a c e s , and 2 ) d e p o s i t i n g a t y p i c a l c o a t i n g ( e . g . f l u o r o p o l y m e r ) as on m e t a l l i c and m i n e r a l s u r f a c e . The f i r s t s o l u t i o n h a s been s t u d i e d i n t h e l a b o r a t o r y ( r e f . 4 1 5 ) . I t w a s found t h a t f o r t h e polymers b e i n g looked a t ( P S I PMMA, PAM, PVDC copolymer c o n t a i n i n g 2 0 % p o l y a c r y n i t r i l e ) t h e p a r t i a l l y f l u o r i n a t e d compounds u s e d ( a t concentrat i o n & 1%by w e i g h t ) w e r e a b l e t o d e c r e a s e t h e c r i t i c a l s u r f a c e
rcl
tension o f PMMA, PAM and PVDC from 39, 35-40 and 38-44 mN/m t o a b o u t 1 9 , 11 and 20 mN/m r e s p e c t i v e l y . The f l u o r i n a t e d compounds used w e r e so i n s o l u b l e i n t h e PS t h a t t h e y e i t h e r s e p a r a t e d o u t a s a n o t h e r p h a s e w h i l e t h e s o l v e n t e v a p o r a t e d o r else t h e y formed c l o u d y , opaque f i l m s . T h e s e l a r g e d e c r e a s e s i n c r i t i c a l s u r f a c e t e n s i o n r e f l e c t e d t h e change i n s u r f a c e c o m p o s i t i o n which had t a k e n p l a c e , t h e polymer m o l e c u l e s b e i n g r e p l a c e d i n t h e i n t e r f a c e by c l o s e l y packed CF2 and CF3 g r o u p s . The f l u o r i n e - c o n t a i n i n g s u r f a c e a c t i v e a g e n t s were e q u a l l y e f f e c t i v e when added t o t h e monomer prior
227 t o p o l y m e r i z a t i o n o r t o s o l u t i o n s o f t h e polymer i n a v o l a t i l e solv e n t . The less v i s c o u s t h e polymer i s , t h e more r a p i d l y a d s o r p t i o n e q u i l i b r i u m w i l l b e a t t a i n e d . One f u r t h e r p r o p e r t y of a f i l m formed by t h i s t e c h n i q u e i s t h a t it s h o u l d b e s e l f - h e a t i n g
-
t h a t i s , any
s u r f a c e a c t i v e m o l e c u l e s l o s t from t h e f i l m w i l l b e r e p l a c e d by t h e d i f f u s i o n o f a d d i t i o n a l m a t e r i a l i n t o t h e i n t e r f a c e . The r a t e o f s e l f - h e a t i n g w i l l be d e p e n d e n t upon t h e r a t e of d i f f u s i o n o f t h e f l u o r o c a r b o n d e r i v a t i v e s i n t h e b u l k polymer, and may b e accelerated by h e a t i n g t h e s o l i d polymer or o t h e r w i s e l o w e r i n g i t s v i s c o s i t y .
a normal polymer c o a t i n g s ( e p i 1 a m e s ) from t h e s o l u t i o n of base material (e.g. fluoropolymer) r e q u i r e s consideraThe d e p o s i t i o n
t i o n of t h e i n t e r a c t i o n s i n b a s e m a t e r i a l - p o l y m e r and ( e s p e c i a l l y ) s o l v e n t - p o l y m e r s y s t e m s . The r e s u l t s o f T i l l w i c h and S t e h r ' s studies ( r e f s . 4 1 6 , 4 1 7 ) showed t h a t f l u o r i n a t e d p o l y m e r s , p a r t i c u l a r l y fluorinated polyacrylates
,
c a n be used t o p r e p a r e c o a t i n g s (epilams)
f o r p o l y m e r i c s u r f a c e s . Of t h e c h l o r i n a t e d , a l c o h o l , e s t e r , and k e t o n e s o l v e n t s , t h e b e s t i s 1,1,2-trifluoro-1,2,2-trichloroethane
(CClF). I t i s i n e r t t o t h e polymers commonly u s e d i n p r e c i s i o n eng i n e e r i n g , e v a p o r a t e s r a p i d l y and i s r e s i s t a n t t o a g e i n g ( t h e r e
w e r e no p o l y m e r i z a t i o n e f f e c t s i n t h e a n t i - m i g r a t i o n c o a t i n g which w a s i n a c l o s e d c o n t a i n e r f o r 24 months and i n c o n t a c t w i t h a i r f o r 3 months).
The problem o f f i n d i n g a n optimum c o a t i n g f o r t h e s o l i d s u r f a c e o f materials u s e d i n p r e c i s i o n e n g i n e e r i n g n e e d s f u r t h e r s t u d y . E s p e c i a l l y important are t h e t r i b o l o g i c a l p r o p e r t i e s of epilames i f t h e y are a c c i d e n t a l l y p l a c e d i n t h e rubbing r e g i o n o r i f o i l g e t s o n t o t h e e p i l a m e s u r f a c e . I n t h e case o f hydrodynamic l u b r i c a t i o n o f a m i n i a t u r e b e a r i n g , t h e e p i l a m e b a r r i e r f i l m p l a y s an i m p o r t a n t r o l e i n p r e v e n t i n g , t h e o i l from s p r e a d i n g from t h e h i g h o i l p r e s s u r e r e g i o n . The v a l u a b l e e f f e c t o f l u b r i c a t i o n on t h e t r i b o l o g i c a l p r o p e r t i e s o f p o l y m e r i c m i c r o c o u p l e s and t h e problems w i t h o i l m i g r a t i o n from t h e r u b b i n g r e g i o n i n s u c h b e a r i n g s g i v e added importance t o t h e technique
o f e p i l a m i z i n g polymers. The e f f e c t s o f t h e
t r o b o e l e c t r i f i c a t i o n o f polymers must be c o n s i d e r e d i n p a r t i c u l a r . 6.2.5.
SELF-COATING (AUTOEPILAMIZING)
The i d e a o f s e l f - c o a t i n g i s v e r y s i m p l e . The o i l ( s p e c i a l l y p r e p a r e d o r w i t h s p e c i a l a d d i t i o n s ) does n o t s p r e a d a f t e r b e i n g l a i d on t h e s o l i d s u r f a c e b u t a l a y e r w i t h low s u r f a c e e n e r g y i s formed. T h i s l a y e r i s formed a s t h e r e s u l t o f c h e m i c a l a c t i o n o r
the selective adsorption of molecules e.g. the very active molecules of the special additives in the lubricant. Attempts have been made to synthesize a self-coating oil at the Naval Research Laboratory in Washington D.C. (refs. 367, 402, 410). Three classes of non-spreading liquids have been distinguished: 1) Autophobic liquids examplified by molten stearic acid, octyl alcohol, tricresyl phosphate and trichlorodiphenyl, 2 ) Numerous esters able to spread completely on metal surfaces b u t unable to spread on glass, silica or sapphire. The ester hydrolyzes immediately upon adsorbing onto these hydrated solid surfaces; of the two products of the hydrolytic reaction, the one with the greater average lifetime of adsorption remains to coat the surface with a close-packed monolayer thereby blocking further progress of the hydrolysis reaction. If this protective monolayer has a critical surface tension of wetting which is less than the surface tension of the liquid ester, nonspreading results, 3 ) Liquids whose surface tension is so high and adhesional energy so low that the energy of adhesion is lower than that of cohesion and spreading is thus thermodynamically impossible. If such liquids existed they would differ from auto-phobic liquids in not leaving a film behind them when rolled over a horizontal polished solid surface. Pure non-spreading instrument oils with good lubrication properties and chemical stability have unfortunately never been found. The best results have been obtained with the esters. The oils can be prevented from spreading by the addition of selected solutes which act in one of three ways. The first way is based on the ability of the solute to adsorb onto a high-energy surface and form a monolayer with a critical surface tension of wetting less than the surface tension of the original oil. The second way is based on the addition of a more volatile solute which creates a surface tension gradient at the edge of an oil drop that opposes the spontaneous spreading of the oil. The third way is based on the ability of the solute to react chemically with the solid surface, forming a low energy surface. The autoepilamizing effect of the DMPS oils is obtained (refs. 6 1 , 68, 288) by adding < l % of oxidized fluorinated polysiloxane (with groups CF2-CF2) to the original DMPS. The DMPS oil with the addition of 1% of polar fluorinated polysiloxane formed, on metal surfaces, drops with a contact angle of 12-15'. However the contact
-
229
a n g l e f o r a 0.5% s o l u t i o n ( o f t h e o x i d i z e d f l u o r o p o l y s i l o x a n e i n DMPS) on metal s u r f a c e s w a s a b o u t 35'.
I t h a s been i m p o s s i b l e t o
achieve t h i s autoepolamizing e f f e c t f o r t h e o t h e r polysiloxanes w i t h t h i s method b e c a u s e o f problems o f s o l u b i l i t y . The r e s u l t s of some i n t e r e s t i n g i n v e s t i g a t i o n s a r e r e p o r t e d i n r e f . 4 1 9 . 90 of t h e v a r i o u s f l u o r i n a t e d compounds ( w i t h g r o u p s -CF3 and -CF2) w e r e t e s t e d . The b a s e o i l s were: met hyl al kyl pol ys i l oxanes , chlorinated methylpolysiloxanes, f l u o r i n a t e d polysiloxanes, mineral o i l s , s y n t h e t i c esters and some multicomponent i n s t r u m e n t o i l s (MBC-30-9, NS-6n, MBL-12, Synt-A-Lube). T h e s e i n v e s t i g a t i o n s proved t h a t t h e a d d i t i o n o f f l u o r i n a t e d c a r b o n a c i d s C6F13COOH with 0.2-0.3% by w e i g h t i s e f f e c t i v e i n p r e v e n t i n g a l l o f t h e t e s t e d o i l s from s p r e a d i n g . The c o n t a c t a n g l e s of t h e a u t o e p i l a m i z i n g o i l s r a n g e d from 15'
t o 60°.
The d i f f e r e n c e s i n t h e c o n t a c t a n g l e f o r s t e e l ,
b r a s s and r u b y s u r f a c e s were v e r y small. The c o n t a c t a n g l e s o f MBL-12
and Synt-A-Lube o i l s c o n t a i n i n g 0 . 3 % C6F13COOH were 2 0 and
12O respectively on steel surfaces. A t t h e drop diameters 1 . 0 0
MBL-12)
and 0.95 mm (Synt-A-Lube)
the e f f e c t i v e drop "shearing"
c e n t r i g u g a l a c c e l e r a t i o n was o v e r 4 0 0 g ( f o r p u r e o i l s a b o u t 15 g and 4 g f o r MBL-12 and Synt-A-Lube
oils respectively).
The a u t o e p i l a m i z i n g e f f e c t on t h e s t e e l and s a p p h i r e s u r f a c e s o f t h e b i s ( 2 - e t h y l e x y l ) s e b a c a t e (EHS) , a l i p h a t i c esters, squalane, n-hexadecane, m i n e r a l o i l and p o l y s i l o x a n e s (DC 200, DC 510, DC 550, DC710) was a c h i e v e d ( r e f . 4 2 0 ) by i n c l u d i n g a f l u o r i n a t e d h y d r o c a r -
bon o r p o l y s i l o x a n e compound i n t h e t e s t l i q u i d . The n o n s p r e a d i n g e f f e c t w a s a c h i e v e d by l o w e r i n g ( w i t h t h e a d d i t i v e ) t h e s u r f a c e t e n s i o n o f o r i g i n a l l i q u i d by l e s s t h a n 5-6 mN/m.
The a d d i t i o n o f
f l u o r i n a t e d hydrocarbon r e s u l t e d i n t h e f o r m a t i o n o f w e l l - a d s o r b e d -CF3 o r -CF2 monolayers. The p o l y s i l o x a n e formed v e r y t h i c k adsorbed f i l m s on s t e e l , b r a s s , s a p p h i r e , q u a r t z and g l a s s ; t h e s e f i l m s had low
yc
and o i l d i d n o t s p r e a d on them. The c o n c e n t r a t i o n of p o l y -
siloxane i n t h e l i q u i d s t e s t e d (except i n squalane) w a s
< 1 % (by
w e i g h t ) and maximum 5% i n t h e case o f f l u o r o e s t e r s . The l o n g t e r m n o n s p r e a d i n g o f t h e t e s t e d l i q u i d s w a s a c h i e v e d when t h e a d d i t i o n o n l y s l i g h t l y d e c r e a s e d t h e s u r f a c e t e n s i o n o f t h e l i q u i d and had good s e l e c t i v e a d s o r p t i o n p r o p e r t i e s ( e . 9 . a d d i t i o n o f t h e p o l y s i l o x a n e t o a l i p h a t i c e s t e r s o r f l u o r i n a t e d e t h e r s t o EHS) forming l a y e r s with
rc 0 . 5 mg, i.e., it is comparable with oil volumes introduced into miniature bearings. The oil ageing grade can be described with an ageing number ( A Z )
AZ =
E ( ~ 1 7 7 0cm-l)
+
(E-1720
cm-l)
+
E (-1610
cm-’)
(6.19
E ( 1 4 6 0 cm-’)
where E (....I in the numerator is the absorptivities at the wave numbers of intensive formation of ageing chemical products in mineral instrument oils (Fig. 6.10) i.e. 1780 1 7 6 0 cm-l C = 0 in anhydrides, lactones; 1740 1 7 0 0 cm-l C = 0 in esters, aldehydes, ketones, fatty acids; 1620 1 5 9 5 cm-l C = 0 in metal salts.
... ... ...
Wave length , pm --c
-
Nave number ,cm”
F i g . 6.10.
I R spectrogram o f a n m i n e r a l i n s t r u m e n t o i l ( r e f . 4 4 4 )
248
The wave number 1460 cm-l has been chosen as the standard because after ageing the absorptivity in this band does not change (and it is also at its highest). The absorptivity E is defined as JO
E = log -
(6.16)
J
where Jo and J are light intensities before and after the test respectively. The results of Huber's investigations into the relationships between A 2 and the increase in the viscosity and acidity number of the special instrument oil MS2 (paraffin oil with antioxidants) are presented in Fig. 6.11 and Fig, 6.12 respectively.
A 200
-
q-150 L
-
I\r
Q)
n E 2
c 0
100 -
c
.-1
Iu
cn -x
50
-
I 01
I
I
200
400
1
I
600
800
c
viscosity increase, % Fig. 6.11.
Ageing number
AZ
and i n c r e a s e i n v i s c o s i t y o f a p a r a f f i n o i l ( r e f .
4441.
The oil had been aged using a modified Baader's test (DIN 51554). The ageing characteristics of the oil MS2 and pure paraffin oil are presented in Fig, 6.13. It is possible to see that the A 2 values for MS2 and pure paraffin oil differ significantly. Good correlation exists between AZ and the increase in viscosity and acidity
249
number,
250
20C ru Q
n E
15C
=3
c
m 100
c . A
QI 0
q
5c
0
I
M
I
80 Increase of acidity number, % 40
60
c
F i g . 6 . 1 2 . Ageing number (AZ) and i n c r e a s e i n a c i d i t y number o f a p a r a f f i n o i l ( r e f . 444).
Huber’s IR method can also be applied to the analysis of the ageing resistance of instrument oils based on polyalkylglycol with ether and alcohol groups which are often applied for the lubrication of miniature systems. Mineral oils or greases with a lot of additives can also be studied using this method. In this case, neglecting the soap bands in the spectrogram is necessary, or other spectroscopic techniques, e.g. differential IR spectroscopy, should be used. The IR spectroscopy method for the estimation of the ageing of a small amount of grease used as lubricant in a rolling bearing has been applied by Bilobrov et al. (ref. 4 4 5 ) . The grease Ciatim-203, applied for the lubrication of aircraft instrumentation, is based on paraffin (transformer) oil condensed with a Li-soap of stearic
250 a c i d and c o n t a i n i n g a n t i o x i d a n t s . A c o r r e l a t i o n w a s found between
E1720/E1450 (where E - i s IR a b s o r p t i v i t y a t 1720 c m - l d 1450 cm-' r e s p e c t i v e l y ) and t h e o x i d a t i o n g r a d e o f t h e g r e a s e . The E1720/E1450 e q u a l t o 0.9 h a s b e e n t a k e n a s t h e c r i t e r i o n v a l u e of t h e maximum a d m i s s i b l e o x i d a t i o n g r a d e of t h e g r e a s e ( t h e 8.5% ( i n w e i g h t ) o f t h e g r e a s e o x i d i z e d a t 0 . 9 E1720/E1450).
a, t
Instrument oil MS2 0
I
1
I
20
40
60
c.
F i g . 6 . 1 3 . Ageing number (AZ) as a f u n c t i o n o f ageing t i m e ( r e f .
444).
The o i l a g e i n g dynamics i n a c t u a l w o r k i n g s y s t e m s have b e e n s t u d i e d by o i l p r o d u c e r s over l o n g p e r i o d s of o p e r a t i o n . A s a n example, t h e i n c r e a s e i n v i s c o s i t y o f v a r i o u s i n s t r u m e n t o i l s a f t e r n a t u r a l a g e i n g i s p r e s e n t e d i n F i g . 6.14 ( r e f . 446). The e f f e c t of a d d i n g a n t i o x i d a n t t o c l a s s i c c l o c k o i l and t h e a p p l i c a t i o n of s y n t h e t i c o i l ( p o l y a l k y l g l y c o l w i t h e t h e r and a l c o h o l g r o u p s ) c a n be c l e a r l y o b s e r v e d . The comparison o f t h e e f f e c t of a n a t u r a l a g e i n g o f c l a s s i c c l o c k o i l and s i l i c o n e o i l o n t h e a m p l i t u d e of a b a l a n c e i n a watch i s shown i n F i g . 6 . 1 5
( r e f . 64). A f t e r 18 months
of o p e r a t i o n , t h e a m p l i t u d e of t h e b a l a n c e l u b r i c a t e d w i t h s i l i c o n e
o i l d e c r e a s e d by 3.5% w h e r e a s t h e a m p l i t u d e of t h e b a l a n c e lubricate d w i t h c l a s s i c c l o c k o i l d e c r e a s e d by a s much as 17.2% a f t e r o n l y 12 months o f o p e r a t i o n ,
251
I
I
I
c
I
3 Ageing (operat h g ) time, years 2
1
01
F i g . 6.14. I n c r e a s e i n v i s c o s i t y o f i n st rument o i l s d u r i n g n a t u r a l ageing. 1 - pure n e a t ’ s - f o o t o i l ; 2 - p u r e c l a s s i c c l o c k o i l ; 3 classic clock o i l ( w i t h a n t i o x i d a n t s ) ; 4 - s y n t h e t i c o i l - p o l y a l k y l g l y c o l (Synt-A-Lube) (ref.450)
-
.. --
Ho ri za ntal
- \
250
I
O -
-\ \
f
A
I
I
Vertical
I
‘0
4
I
I
12 16 Ageing(operat1ng) time, months 8
I
*
20
r i g . 6.15. A m p litud e o f a watch b al a nce as a f u n c t i o n o f n a t u r a l ageing ( o p e r a t i ng ) t i m e a t h o r i z o n t a l and v e r t i c a l watch p o s i t i o n s and l u b r i c a t e d w i t h s y n t k t i c and n a t u r a l o i l s ( r e f . 6 4 ) .
252
The problems involved in the estimation of natural lubricant ageing need further investigation. IR spectroscopy methods can only be used to estimate ageing in a few kinds of natural lubricants. The determination of the ageing number (AZ) does not give direct information about the tribological effect of ageing. The relationships between viscosity, acidity number etc., and AZ should be determined for the practical significance of the estimated AZ values. It is necessary to develop a standard method for the objective evaluation of the ageing resistance of instrument lubricants (refs. 440,
447).
The system should be made up of the following elements: 1 ) A device for mechanical-dynamic simulation of lubricant egeing, 2 ) Microviscometer
with a wide measuring range suitable for small 3 0.5 mm ) amounts of lubricant, 3 ) A device for a quick estimation of the acidity number of small (
20'
(9.7)
'PO
where A1, A2 and c are parameters; y o is as in eqn. ( 9 . 5 ) (in radians) ; and p is the bearing contact pressure (p = N/(dl)), where N is the applied normal load, d the bearing hole diameter, and 1 the length of contact between the journal and the bearing bush). can be determined using the folThe half angle of contact, lowing formula:
yo,
where
do
= Es
.N
mm
,
N being the applied normal load, E the elas-
ticity modulus of the bearing bush material, and s the diametral clearance; kl, m, c, A l l A 2 , a and b for various miniature journal bearings are listed in Table 9 . 2 (based on ref. 2 0 3 ) . Having these values and using eqns. ( 9 . 6 ) or ( 9 . 7 ) the value of pmcan be found and compared with the admissible value of the yield stress for softer material. When pm is known, the radial deformation of the element under load can be determined by use of eqn. (4.10) (based on Hookers law). For polymeric bearings, the important thing to determine is the so-called seizure number (ref. 646), which should not be exceeded for safe operation. The seizure number, L,, is defined in
TABLE 9.2 COMPUTED VALUES OF PARAMETERS kl,
* * 0
m, c , A l , A * , a and b PARAMETERS
kl
BEAR I N G
rn
C
Al
A2
a
b
Unl ubr i cated
0.77
0.32
3.97
0.49
1.12
1.60
0.40
Lubricated
0.70
0.27
3.98
0.31
1.17
1.83
0.44
Unlubricated
1.40
0.44
3.99
0.49
1.12
1.56
0.40
Lubricated
1.24
0.40
3.98
0.28
1.19
1.85
0.45
Uniubricated
0.77
0.35
3.97
0.32
1.16
1.87
0.44
Lubricated
0.71
0.34
3.97
0.28
1.17
1.89
0.44
Unl ubr i cated
0.78
0.39
3.97
0.37
1.15
1.87
0.44
Lubricated
0.72
0.38
3.97
0.33
1.16
1.89
0.44
Unl ubr i c a t e d
0.64
0.21
3.98
0.31
1.17
1.83
0.44
Lubricated
0.67
0.24
3.98
0.27
1.18
1.83
0.44
Un’ubricated
2.18
0.49
3.98
0.25
1.20
1.88
0.45
Lubricated
2.17
0.49
3.98
0.24
1.20
1.89
0.45
Steel-brass
S t e e 1 -ruby
PA 6 S t e e 1 -po 1 ymer POM h
Polymer (POM h)-polymer (PA 6)
polymer(PA 66+20% PTFE)-polymer(PA 66+30% glass f i b r e s )
1
411
the following way: Ls -
PV -
(9.9)
Y
where p and v are respectively the bearing contact pressure and sliding speed at which the temperature rise in the friction area is so high that necking (tightening) of the bearing bush on the journal occurs and thermal ratchetting occurs in the bearing; L+J is the relative radial clearance. Taking into consideration eqn. ( 9 . 2 ) , putting kt = s ( s being the diametral clearance) and introducing A T from eqn. (4.21), the following formula can be used for the estimation of Ls for unlubricated miniature steel-polymer journal bearings: Ls =
[(1-25) 1150
J fdd In TF1 CC
+ k2]h 1
(1+J) (1-k
1
(9.10)
where the nomenclature can be found after eqns. ( 4 . 2 1 ) and ( 9 . 2 ) . The value of the friction coefficient fd can be predictedusing eqn. (4.5) or determined experimentally. The tribological properties of the bearings are frictional losses (defined by the friction coefficient or torque) and mass losses (defined by the wear rate). The life of the bearing can be defined by the length of time up to seizure (the rapid and high increase of the friction coefficient or torque) or by the time needed for the assumed wear rate limit to be exceeded. The steady-state friction torque in miniature bearings during prolonged use is often a very important factor in their design. The estimation of the aforementioned losses in a tribological system is especially meaningful for miniature journal bearingswhen radial wear can reach significant values, as in the case of polymeric bearing. The tribological behaviour of the commonly used steel-polymer bearings can be defined by determining their so-called wear and frictional torque numbers (refs. 6 4 7 , 6 4 8 ) . The frictional torque Mf is expressed by the formula
(9.11)
J ,
- Ip,
412 where d is the nominal diameter of the bearing hole, 1 is the length of contact of the journal and bearing bush, p(q) is the contact pressure distribution, and qA the half angle of contact when radial wear is taken into consideration (see eqn. (4.9) Since from Archard's formula it results that (ref. 649)
.
(9.12) (where w(g) is the radial wear of the bearing bush measured perpendicularly to friction surface (Fig. 9.3); dw = dw(V)cos(P , L is the sliding distance, p(9) is as in eqn. (9.11) , and Wo is the wear modulus), the frictional torque, Mf, after the introduction of p((p) from eqn. (9.12) to eqn. (9.11) and integration, can be expressed as follows: I
=
2
d 2 If Wo
sinqi
(9.13)
After taking into consideration the static balance equation of the journal (see Fig. 9.31, p cos 4 dl = dl
f
p(y) (cos
+ f sinq )ydq
(9.14)
0
and remembering that tanfi = f, from eqn. (9.12) we obtain w
=
2P
(9.15)
If we take into consideration the suggestion from ref. 647, the wear number Lw can be expressed as follows: (9.16)
when f = 0 we obtain Lw the same as in ref. 647. The frictional torque Mf can therefore be determined from the formula (9.17)
413
Fig.
9.3. Contact
o f journal ( 1 ) w i t h b e a r i n g bush ( 2 ) . I
For determining To eqn. (4.9) can be applied. If we accept the suggestion from ref. 648 the frictional torque number LM will be given by
The variation in the frictional torque Mf depends on the variation in the value of the frictional torque number during the wearing out process: 1 2 Mf = 7 d lfpLM
(9.19)
when the coefficient of friction is assumed to be constant. The frictional torque increases to a maximum of 1.273 times the value of frictional torque at zero wear. Wear prediction in unlubricated and lubricated miniature steel-polymer journal bearings is discussed in detail in Chapters 4.2.1 and 5.2.1 respectively.
414
Steel-brass miniature journal bearings operating under a very high load (bearing contact pressure over 7 MPa and sliding speed below 2 m/s) will not seize up when the operating time is very short. The reliability functions of such unlubricated bearings are unlubricated bearings are given in Chapter 4.1. The reliability model of the seizure of such bearings, based on the adhesion-decohesion hypothesis of seizure, results in the following general reliability formulae (ref. 6 5 0 ) : - the distribution function of time period to the bearing's seizure F(t) = 1
-
-
exp
[A1q-
+ ,u 0 )t - - e *1
1
(9.20)
the frequency function -*lt)]
expLq(l-e A, -slt)-(A,+ ,uO)t]
(9.21)
- the failure (seizure) rate (9.22)
where p 0 and,u1 are parameters of distribution and frequency functions respectively, and is constant. In miniature metal or mineral bearings (generally when the bearing bush is made of relatively hard material), which usually operate under boundary lubrication conditions, the size and shape of surface irregularities are major factors in bearing performance (ref. 6 5 2 ) . The misalignment of the journal and two bearing surfaces and the ordinary surface roughness and waviness normally found on 'miniature journal bearings can prevent line contact. The contact is then made at two points on the bearing (bush). The journal, being misaligned and having surface irregularities of its own, starts by rolling over a series of surface irregularities on the bearing, resulting finally in its rolling up the bearing wall to a height at which neither rolling nor interlocking can be sustained, and in two-point contact and relatively steady-state slip (Fig. 9 . 4 ) . depends on the fricThe efficiency of the bearing system, tion and surface irregularities, and can be expressed by the formula (ref. 6 5 2 )
A,
Tb,
415
1-12b =
U
-
U -
( l + - ) -U l2
(9.23)
P
where l1 and l 2 are input and output thrust and load moment arms, P is the input thrust, and G the shaft assembly weight (see also Fig. 9 . 4 ) .
Input thrust P
F i g . 9.4. C o n t a c t o f J o u r n a l ( 1 ) w i t h b e a r i n g s u r f a c e (2) h a v i n g i r r e g u l a r i t i e s ( 3 )
.
When the journal slips on one surface irregularity (Fig. 9 . 4 ) factor u is calculated from the formula u = R - sine cos 0
(9.24)
where R is the journal radius, and @the angle of friction: f = tan@(f is the friction coefficient) and contact angle 6 can be determined as follows:
416
S
( l + - 2R -
r -12R
(9.25)
where s is the diametral clearance, and r the height of the surface irregularity. If, however, surface irregularities interlock, a condition that applies at breakaway, then u
= R
sin ( 4 + 2 e )
(9.26)
If the journal slips on two surface irregularities, then u = R
sin 4 EiEZi-3
(9.27)
and if the surface irregularities are removed (the condition corresponding to the condition where u is minimum) eqns. ( 9 . 2 4 ) , ( 9 . 2 6 ) and ( 9 . 2 7 ) all reduce to u
=
R sin#
(9.28)
The maximum efficiency qbmax is calculated as follows:
bmax
--
1 - 1 + -
U
I1 U
(9.29)
12
The efficiency is zero when
-GP
30 60 Half angle of contact , O
90
F i g . 9 . 1 1 . F r i c t i o n a l t o r q u e number f o r c y l i n d r i c a l j o u r n a l b e a r i n g ( 1 ) and s p h e r i c a l ( b a l l cup) b e a r i n g (2), b o t h w i t h a r e l a t i v e l y s o f t b e a r i n g bush such a s polymer.
428
In centre bearings, a conical journal with a spherical end is placed in a spherical recess or in a conical recess with a concave spherical centre portion. In both cases the recess radius is greater than the radius of the spherical end of the journal. Two cases of a journal supported on centre bearings should be noted: the one where the force (load) acts along the shaft and the one where the force acts transversely upon the shaft. In the first case the frictional torque Mfcl may be e s t d t d using the formula (refs. 3, 383)
(9.51)
-1 - - 1
where P is the axial load, f is the friction coefficient, q 1 , q 2 and El, E2 are Poisson's ratios and elasticity moduli for journal and bearing materials respectively, and R and Rb are the radii j of the spherical end of the journal and the spherical recess of the bearing respectively. When the force (load) acts normally to the journal's axis, one component of the frictional torque, the effect of the pivoting friction, can be neglected and therefore the frictional torque Mfc2 can be predicted using the following formula (ref. 3): Mfc2 = NR.f coS@ 3
(9.52)
where N is the normal load, R . and f are as in eqn. ( 9 . 5 1 ) , and Q 3 is the so-called "rolling upwards" angle (the friction angle at which the journal begins sliding on the block). The rolling upwards angle in the case of contact of sphere-endd journal with a spherical bearing may be predicted as follows: tan4