Wear of Poly(tetrafluoroethylene) - ACS Publications - American

11 Wear of Poly(tetrafluoroethylene) A Maverick or Not B. J. Briscoe

Downloaded by MONASH UNIV on November 14, 2015 | http://pubs.acs.org Publication Date: September 12, 1985 | doi: 10.1021/bk-1985-0287.ch011

Department of Chemical Engineering, Imperial College, London SW7 2BY, England

The wear characteristics of polytetrafluoroethylene (PTFE) have been widely studied; it is an important commercial polymer. This special attention has sometimes created a thesis that this polymer has very unusual or special wear characteristics when compared with the response of other polymers. This review compares the wear behaviour of PTFE with that of a range of polymers and examines the basis of this belief. The experimental evidence indicates that it is only in the area of transfer wear that a major contrast in characteristics is seen. Even in this restricted wear mode the differences are arguably ones of extent and not kind. The wear p r o c e s s e s observed i n p o l y t e t r a f l u o r o e t h y l e n e , PTFE, share many f e a t u r e s w h i c h a r e common t o o t h e r t h e r m o p l a s t i c s . When the polymer i s s l i d over r e l a t i v e l y smooth r i g i d c o u n t e r f a c e s i t forms t h i n t r a n s f e r r e d l a y e r s on i t s c o u n t e r f a c e under i s o t h e r m a l c o n t a c t c o n d i t i o n s even a t room temperature. The g e n e r i c c l a s s o f p o l y e t h y l e n e s behave i n a s i m i l a r , a l t h o u g h g e n e r a l l y l e s s e x t e n s i v e manner. The r a t e s o f t r a n s f e r wear, as the o v e r a l l p r o c e s s i s termed, a r e c o m p a r a t i v e l y h i g h but may be e f f e c t i v e l y reduced by the i n c l u s i o n o f hard secondary phases i n t o the polymer m a t r i x . Such an e x p e d i e n t i s w i d e l y adopted t o some e x t e n t i n almost a l l commercial p o l y m e r i c b e a r i n g s on some o c c a s i o n s . As the s c a l e o f the roughness o f the c o u n t e r f a c e i s i n c r e a s e d , t r a n s f e r wear i s r e p l a c e d b y a b r a s i v e wear. The hard a s p e r i t i e s on the c o u n t e r f a c e now cut i n t o the s o f t e r p o l y mer and m i c r o c h i p s o r f i n e f i l a m e n t s are removed from the polymer s u r f a c e . Hard f i l l e r phases a r e u s u a l l y i n e f f e c t i v e i n s u p p r e s s i n g t h i s mode o f wear a l t h o u g h f i l l e r p a r t i c l e s may m o d i f y the s u r f a c e topography o f the c o u n t e r f a c e . A g a i n t h i s i s commonly observed w i t h thermoplastics. In b o t h these regimes o f wear, h i g h r a t e s o f power d i s s i p a t i o n i n the c o n t a c t f i r s t l e a d t o s u r f a c e and then s u b s u r f a c e thermal s o f t e n i n g . These t h e r m a l e f f e c t s g e n e r a l l y l i m i t the maximum PV ( p r e s s u r e - v e l o c i t y ) product o f polymeric c o n t a c t s . Furthermore, 0097-6156/85/0287-0151$06.00/0 © 1985 American Chemical Society

In Polymer Wear and Its Control; Lee, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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f l u i d l u b r i c a t i o n o r boundary l u b r i c a t i o n o f PTFE i s n o r m a l l y i n e f f e c t i v e . The s p e c i f i c f r i c t i o n a l work o f the d r y c o n t a c t i s low and hence f l u i d or weak boundary l a y e r s o f f e r no g r e a t advantages and what i s more these l a y e r s w i l l o f t e n undermine the t e n a c i t y o f the adhesion o f t r a n s f e r l a y e r s to the c o u n t e r f a c e w i t h the r e s u l t t h a t the r a t e of wear i s i n c r e a s e d . There are s i m i l a r i t i e s i n the response of o t h e r " c o l d " o r i s o t h e r m a l t r a n s f e r r i n g of polymers. These t h i n g s a r e w e l l known and numerous s p e c i f i c papers and r e v i e w a r t i c l e s emphasise these p o i n t s from v a r i o u s v i e w p o i n t s ( 1 - 9 ) . I n summary, the t r i b o l o g y of PTFE, a l t h o u g h somewhat unusual i n some r e s p e c t s , i s not e x c e p t i o n a l l y d i f f e r e n t from t h a t o f o t h e r o r g a n i c polymers, p a r t i c u l a r l y low temperature ( g r o s s s o f t e n i n g below c a . 350°C) t h e r m o p l a s t i c s . The r e v i e w w i l l f o c u s on the way i n w h i c h PTFE d i f f e r s from o t h e r polymers b e a r i n g i n mind t h a t these d i f f e r e n c e s are not r e a l l y ones o f k i n d but of e x t e n t . Three t o p i c s w i l l be d i s c u s s e d : a b r a s i v e wear, t r a n s f e r wear and l u b r i c a t e d wear. A b r a s i v e Wear of PTFE There are two ways of a p p r o a c h i n g a b r a s i o n . The now c l a s s i c a l s i n g l e pass a b r a s i o n experiment w h i c h i n v o l v e s s l i d i n g the polymer over a rough s u r f a c e and measuring the mass l o s s p r o v i d e s a u s e f u l c o r r e l a t i o n w i t h m a t e r i a l and t o p o g r a p h i c a l p r o p e r t i e s . I n v a r i a b l y , t h i s experiment does not r e v e a l the m i c r o s c o p i c n a t u r e o f the wear p r o c e s s a l t h o u g h the s t u d y o f the form of the f r e e d e b r i s and d e b r i s adhered on the c o u n t e r f a c e o f f e r s a u s e f u l i n d i c a t i o n of the d e f o r m a t i o n p r o c e s s e s i n v o l v e d . An a l t e r n a t i v e approach i s t o study the e f f e c t s produced by " i s o l a t e d s t r e s s c o n c e n t r a t i o n s " ; a method used s u c c e s s f u l l y by Schallamach i n h i s e a r l y s t u d i e s w i t h e l a s t o m e r s (10, 1 1 ) . Schallamach used sharp n e e d l e s o r spheres to deform the rubber under the combined a c t i o n o f a normal l o a d and a f r i c t i o n a l f o r c e . A com p a r i s o n o f the response produced i n PTFE u s i n g b o t h experiments w i l l be b r i e f l y r e v i e w e d . I s o l a t e d s t r e s s c o n c e n t r a t i o n : s c r a t c h hardness V a r i o u s geometries may be used t o p r o v i d e model a s p e r i t y d e f o r m a t i o n s ; Tabor (12,13) has reviewed t h e s e i n s e v e r a l d e f i n i t i v e papers m a i n l y i n the c o n t e x t o f m e t a l s . C u r r e n t l y we f a v o u r the use o f cones as they are a r e l a t i v e l y s i m p l e geometry t o f a b r i c a t e . F i g u r e 1 i s a s k e t c h adopted from A r c h a r d ( 1 4 ) . I f we n e g l e c t the volume o f mate r i a l d i s p l a c e d out o f the p l a n e of the polymer s u r f a c e , t w o v e r y s i m p l e e x p r e s s i o n s f o r the f r i c t i o n , f , and "wear", W , may be o b t a i n e d ( 8 , 9 ) . The "wear" i s not the volume removed but the volume d i s p l a c e d ; the p r o d u c t o f the c r o s s s e c t i o n area o f the groove, A, and i t s l e n g t h , £, ( F i g u r e 1) v

(1)

and

A£ = W

ν

= — £tan θ πρ Ο

1

θ' i s the c u t t i n g angle formed by the cone and ρ

(2) i s a flow stress.



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A f t e r the cone has passed o v e r the polymer s u r f a c e the w i d t h , w, o f the groove remains l a r g e l y unchanged (say 10% r e d u c t i o n ) , but i n v a r i a b l y t h e r e i s a s i g n i f i c a n t r e l a x a t i o n o f the s t r a i n i n the d i r e c t i o n of the normal l o a d ( F i g u r e 1) (12,15). The experiment c a n t h e r e f o r e be c a r r i e d out c o n v e n i e n t l y by m e a s u r i n g , w, and hence A can be com p u t e d . F i g u r e 2 shows d a t a f o r such an experiment w i t h PTFE and polymethylmethacrylate, PMMA; the o r d i n a t e i s the A/W, where W i s the normal l o a d (IN) ( 1 5 ) . The s l i d i n g v e l o c i t y i s f i x e d a t 4.2 χ 10~5 m/s. The i n t e r e s t i n g f e a t u r e i s the c l o s e s i m i l a r i t y o f t h e two d a t a s e t s ,and the f a c t t h a t e q u a t i o n (2) i s a good a p p r o x i m a t i o n . The normal i n d e n t a t i o n hardness o f PMMA and PTFE are t y p i c a l l y i n the r a t i o o f t e n f o r cones i n the range o f , Θ, s e m i - e p i c a l a n g l e between 80° and 25°. A s i m i l a r r a t i o i s seen i n the s c r a t c h h a r d ness i n t h i s range. L u b r i c a t i n g b o t h c o n t a c t s produces a r e d u c t i o n i n A/W o f comparable amounts as would be e x p e c t e d from the d e c r e a s e i n the i n t e r f a c i a l t r a c t i o n (16). L u b r i c a t i n g s l i d i n g c o n t a c t s o f t h i s type g e n e r a l l y produces a r e d u c t i o n i n p e n e t r a t i o n i n c o n t r a s t t o the l u b r i c a t i o n e f f e c t s seen i n normal hardness s t u d i e s where l u b r i c a t i o n promotes p e n e t r a t i o n (12,16). F i n a l l y , i t i s a l s o noted t h a t the form o f l o a d and s l i d i n g v e l o c i t y v a r i a t i o n s seen i n t h e s e type o f d a t a f o l l o w , t o a f i r s t o r d e r , the r e l a t i o n s h i p g i v e n by e q u a t i o n (2) but where ρ i s a time-dependent v a r i a b l e . I n a s c r a t c h hardness experiment o f t h i s s o r t , i t i s p o s s i b l e t o make a s u b j e c t i v e assessment o f the n a t u r e o f the d e f o r m a t i o n i n terms of the r e l a t i v e c o n t r i b u t i o n o f p l o u g h i n g o r i r o n i n g and c u t t i n g o r chip formation. There seems t o be a c r i t i c a l v a l u e o f θ' above w h i c h p l o u g h i n g i s r e p l a c e d by c u t t i n g . The magnitude o f θ i s a f u n c t i o n o f many v a r i a b l e s i n c l u d i n g the depth o f p e n e t r a t i o n and the s t a t e o f l u b r i c a t i o n (16,18). T y p i c a l v a l u e s o f θ f o r PMMA and PTFE are r e s p e c t i v e l y 45° and 90° as judged from e l e c t r o n m i c r o graphs. A c t u a l l y , the r e a l p i c t u r e i s more complex as the geometry of the damaged s u r f a c e s are r a t h e r d i f f e r e n t . PTFE shows e x t e n s i v e p l a s t i c f l o w and f i b r i l a t i o n whereas i n PMMA t h e r e i s o f t e n e v i d e n c e of c r a c k f o r m a t i o n , F i g u r e 3 ( 1 5 ) . Bethune (19) and Lamy (17) have shown s i m i l a r r e s u l t s . As an e x p e d i e n c y we have compared PTFE and PMMA but a more com p r e h e n s i v e comparison w i t h o t h e r polymers l e a d s t o a s i m i l a r c o n c l u s i o n . The g e n e r a l b e h a v i o u r o f PTFE i s comparable i n k i n d t o t h a t seen w i t h o t h e r polymers. Perhaps the o n l y u n u s u a l f e a t u r e i s t h e marked e v i d e n c e o f s u r f a c e f i b r i l a t i o n . T h i s c o n c l u s i o n would l e a d us t o suppose t h a t the a b r a s i o n c h a r a c t e r i s t i c s o f PTFE would a l s o be r a t h e r s i m i l a r t o t h a t found w i t h o t h e r polymers. T h i s i s found t o be the c a s e . 1

1

A b r a s i o n by m u l t i p l e a s p e r i t y c o n t a c t s The s t u d y o f s i n g l e a s p e r i t y i n t e r a c t i o n s does not d i r e c t l y r e s o l v e a number o f the i m p o r t a n t q u e s t i o n s w h i c h a r i s e when we attempt t o u n d e r s t a n d a b r a s i v e wear. The c r i t i c a l c u t t i n g a n g l e f o r m a t e r i a l removal o f t e n exceeds the r e a l i s t i c s l o p e s o f a s p e r i t i e s ,and i n any event i t i s u s e f u l t o q u e s t i o n whether m a t e r i a l can be removed by m u l t i p l e i n t e r a c t i o n s which i n v o l v e grooving. There i s good c i r c u m s t a n t i a l e v i d e n c e t o suggest t h a t t h i s i s a n i m p o r t a n t f e a t u r e o f a b r a s i o n processes>and i t i s an important f a c t o r i n p a r t i c l e e r o s i o n



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recovery on unloading—'

F i g u r e 1. ( a ) A r i g i d cone o f s e m i - a p i c a l angle θ c u t t i n g through a s o f t p l a n e . (b) V a r i o u s s e c t i o n s o f F i g u r e 1(a) t o i n d i c a t e t h e i n i t i a l and f i n a l deformations. The t r a c k w i d t h i s w and the cone produces a c u t t i n g angle θ . 1

1 0

tan©

f o r t h e experiment F i g u r e 2. "Wear as a f u n c t i o n o f θ and t a n shown i n F i g u r e 1. Load I N ; s l i d i n g v e l o c i l no l u b r i c a t i o n ; temperature c a . 20°C. The wear i s c a l c u l a t e d from, w, t h e s c r a t c h w i d t h . Open symbols PMMA and c l o s e d symbols PTFE. These c u t t i n g angles a r e f u n c t i o n s o f l o a d , v e l o c i t y and the s t a t e o f l u b r i c a t i o n .


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(18,22). A p a r t from t h i s q u e s t i o n t h e r e must be c o n c e r n about t h e m o d i f i c a t i o n o f the s u r f a c e topography by l o o s e d e b r i s . I n a d d i t i o n , t h e r e are i m p o r t a n t d e t a i l s t o be c o n s i d e r e d r e g a r d i n g the s t r e s s d i s t r i b u t i o n s over the numerous a s p e r i t y c o n t a c t s . I f we take an u n c r i t i c a l v i e w and b u i l d on the s c r a t c h hardness model assuming t h a t the volume d i s p l a c e d , A, i n some way d e f i n e s t h e wear r a t e o f the whole assembly o f a s p e r i t i e s we produce a r e l a t i o n s h i p f o r the a b r a s i v e wear per u n i t d i s t a n c e (8,23):


Z=^tan0' Ρ, ο

(3)

K r a g e h l s k i i (24) and o t h e r s have a p p l i e d t h i s argument t o d u c t i l e m e t a l s . Θ* i s now some average s l o p e o r c u t t i n g angle and the Κ parameter e x p r e s s e s the p r o b a b i l i t y o f f o r m i n g a l o o s e wear p a r t i c l e i n a s i n g l e d e f o r m a t i o n . I n v a r i a b l y , Κ i s s m a l l and hence we sup pose t h a t m u l t i p l e encounters are r e q u i r e d t o l o o s e n d e b r i s ; a l t e r n a t i v e l y we have a f a t i g u e p r o c e s s o p e r a t i n g . L a n c a s t e r (24,25) and o t h e r s (26,27) have examined the v a r i a b l e s θ' and W i n s i n g l e pass a b r a s i o n ; the model i s a r e a s o n a b l e account o f t h e i r d a t a . Of more i n t e r e s t i s the s i g n i f i c a n c e o f Κ as proposed b y R a t n e r (28,29). The R u s s i a n s c h o o l c o n s i d e r e d t h a t Κ was r e l a t e d t o t h e i n v e r s e o f the toughness measured i n t e n s i o n i n a u n i t d e f o r m a t i o n . T h i s approach does n o t e x c l u d e the i d e a o f a f a t i g u e p r o c e s s b u t c l e a r l y the r o l e o f m u l t i p l e d e f o r m a t i o n s i s not s t r e s s e d . I t i s u s u a l t o w r i t e the R a t n e r r e l a t i o n s h i p (sometimes c a l l e d t h e R a t n e r - L a n c a s t e r r e l a t i o n s h i p ) as Ζ =K

(4)

1

ρ σε ο yy 1

by comparing (1) and ( 3 ) . Κ now c o n t a i n s the f a t i g u e parameter which s p e c i f i e s the p r o b a b i l i t y o f p a r t i c l e detachment, σ and ε a r e r e s p e c t i v e l y the s t r e s s and s t r a i n a t t e n s i l e r u p t u r e as m?asured^ i n a m a c r o s c o p i c sample. The p r o d u c t o f σ ε i s a p p r o x i m a t e l y e q u a l to ( Ύαάε. ο · rε The inadequacy o f r e f e r e n c i n g a b u l k r u p t u r e parameter, J o d e , to an i n t e r f a c i a l d e f o r m a t i o n i s o b v i o u s ; the s t r u c t u r e , c h e m i s t r y , q u a s i - h y d r o s t a t i c s t r e s s e s , temperatures and r a t e s o f s t r a i n a r e c e r t a i n l y not comparable i n the two c a s e s . A w e l l quoted example o f the q u a l i t y o f the R a t n e r - L a n c a s t e r r e l a t i o n s h i p i s shown i n F i g u r e 4. The wear o f PTFE f i t s i n t o the o v e r a l l t r e n d . The wear d e b r i s i n the case o f PTFE i s n o r m a l l y h i g h l y f i b r i l a r i n s t r u c t u r e however. The g l a s s y polymers, such as PMMA, form powders o f more r e g u l a r aspect r a t i o w i t h l e s s evidence o f e x t e n s i v e p l a s t i c flow. F i g u r e 5 shows the pronounced f i b r i l a r f o r m a t i o n s produced b y a PTFE p i n when i t i s s l i d over a n o p t i c a l d i f f r a c t i o n g r a t i n g . The v a l u e o f the c o r r e l a t i o n between toughness and a b r a s i v e wear may be t a k e n f u r t h e r w i t h PTFE. PTFE i s unusual i n the f a c t t h a t gamma i r r a d i a t i o n i n vacuum does n o t appear t o l e a d t o e x t e n s i v e c h a i n c r o s s - l i n k i n g but does produce a p p r e c i a b l e c h a i n s c i s s i o n ( 3 0 ) . The d e t a i l s a r e not c l e a r but s i g n i f i c a n t i n c r e a s e s i n d e n s i t y (and hence p o s s i b l e c r y s t a l l i n i t y ) are observed as the dose l e v e l i s i n c r e a s e d . Of more s i g n i f i c a n c e i n the p r e s e n t c o n t e x t i s the marked ε

y

y

v



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Figure 3. Surface damage produced by a truncated 15 cone under a load of IN. Evidence of f i b r i l a t i o n i s seen i n PTFE along with extensive p l a s t i c flow. The surfaces of PMMA and polycarbonate PC show that s i g n i f i c a n t b r i t t l e fracture occurs. 10'

1

.

1

Figure 4. A Ratner-Lancaster plot f o r eighteen nominally pure and commercially available polymers. Data obtained i n a " s i n g l e pass" over a rough s t e e l surface of σ 12. ym. The data f o r a PTFE i s shown as a closed symbol. In Polymer Wear and Its Control; Lee, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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r e d u c t i o n i n toughness. The d a t a shown i n F i g u r e 6 are p l o t t e d a c c o r d i n g t o a m o d i f i e d form o f e q u a t i o n (4) ( 3 0 ) : Ζ =

(5) σ 2ε y y where ρ i s assumed t o be p r o p o r t i o n a l t o σ . A c t u a l l y ρ w i l l exceed 8 because o f t h e q u a s i - h y d r o s t a t i c environment in°normal a p p r o a c h ^ The d a t a i n F i g u r e 6, n e g l e c t i n g the v i r g i n PTFE, f o l l o w Ζ α(σ ^ε )"" where η i s c a . 0.25. The wear i s a s l o w e r f u n c t i o n o f the iXcre'ase o f t h e toughness than e x p e c t e d . A l s o the v i r g i n p o l y mer wears more r a p i d l y t h a n the t r e n d i n d i c a t e d b y t h e r a d i a t i o n damaged specimens. There may be s e v e r a l reasons f o r t h i s r e s u l t b u t i n v i e w o f t h e crude n a t u r e o f t h i s model i t i s n o t prudent t o draw firm conclusions. I t i s however noted t h a t t h e s i z e o f t h e average wear d e b r i s p a r t i c l e s d e c r e a s e s v e r y s i g n i f i c a n t l y as t h e gamma dose i n c r e a s e s . T h i s e f f e c t does n o t e x p l a i n , i n i t s e l f , t h e o b s e r v a t i o n t h a t as the dose l e v e l i n c r e a s e s the wear i s l e s s t h a n e x p e c t e d on the b a s i s o f toughness measurements b u t i t may be a c o n t r i b u t o r y f a c t o r . The i m p o r t a n t p o i n t i s t h a t t h e toughness as measured i n a u n i t d e f o r m a t i o n may n o t be t h e i m p o r t a n t m a c r o s c o p i c v a r i a b l e . A parameter based on f a t i g u e s t r e n g t h may be more r e l e v a n t . There a r e i n d i c a t i o n s t h a t t h i s i s t h e case w i t h e l a s t o m e r s a l t h o u g h e a r l y work found r e a s o n a b l e c o r r e l a t i o n s w i t h m a c r o s c o p i c toughness ( 3 1 ) . S e v e r a l a u t h o r s have demonstrated a c l o s e c o r r e l a t i o n between t h e v a l u e o f t h e toughness measured i n a s i n g l e d e f o r m a t i o n and t h e f a t i gue c r a c k p r o p a g a t i o n (FCP) r a t e s measured i n c y c l i c l o a d i n g a t lower s t r e s s e s ; H e r t z b e r g and Manson have produced a u s e f u l summary o f t h i s a r e a ( 3 2 ) . The FCP r a t e s , R f , may be e x p r e s s e d b y the P a r i s e q u a t i o n w h i c h c o n t a i n s two m a t e r i a l p a r a m e t e r s , A and m;


n

R

f

= ΑΔΚ™

ΔΚ i s r e l a t e d t o t h e s t r e s s v a r i a t i o n g e n e r a t e d a t t h e c r a c k t i p d u r i n g t h e l o a d c y c l e . The parameter m c o v e r s a wide range and i n d e e d may be a f u n c t i o n o f ΔΚ. The v a l u e f o r PMMA i s about s i x ; f o r p o l y ( v i n y l i d e n e f l u o r i d e ) , PVDF, i t i s l e s s , about 0.3. Fora f i x e d v a l u e o f R^ t h e q u a n t i t y ΔΚ may be computed and compared w i t h t h e toughness, Κ . I t i s found t h a t f o r a wide range o f polymers Δ Κ 7 Κ : o . 5 where ΔΚ i s ΔΚ f o r t h e f i x e d v a l u e o f R \ I f we c o n s i S e r t h a t the r a t e o f wear i s p r o p o r t i o n a l t o R^ ( t h i s i s t h e b a s i s o f the Champ, Southon and Thomas (31) a n a l y s i s ) t h e n a number o f i n t e r e s t i n g a n a l y s e s may be e x p l o r e d . F o r example i f t h e f a t i g u e toughness o f PTFE i s comparable w i t h PVDF, i . e . m Ζ 0.3; t h e n we c o u l d t e n t a t i v e l y e x p l a i n the γ-irradiated PTFE d a t a g i v e n i n t h e above terms o f a c y c l i c f a t i g u e p r o c e s s ; R^ α (toughness)™. I n summary, a t p r e s e n t we may c o n c l u d e t h a t w h i l e f a t i g u e - b a s e d c o r r e l a t i o n s a r e c o n c e p t u a l l y more a c c e p t a b l e , c o r r e l a t i o n s based on toughness measured i n t e n s i l e t e s t s g e n e r a l l y f o l l o w the c o r r e c t t r e n d . As f a r as PTFE i s concerned t h e r e are no v e r y s p e c i a l f e a t u r e s , save f o r the u n u s u a l l y l a r g e a s p e c t r a t i o s found i n t h e a b r a s i v e wear d e b r i s . f

?

f



158


F i g u r e 5. An o p t i c a l image o f t h e t r a n s f e r o f f i l a m e n t s formed on an o p t i c a l g r a t i n g (312 l i n e s p e r i n c h ) a f t e r 28 t r a v e r a l s o v e r i t s s u r f a c e by a PTFE p i n under a normal l o a d o f 20N. The grating i s of s t e e l . OMrad

1.5

(O^Ey

3

8

15

25

40

) Cm^N- ] 1

2

F i g u r e 6. A b r a s i v e wear o f γ-irradiated PTFE p l o t t e d a g a i n s t γ- dosage and t h e q u a n t i t y (σ 2 ) " i . M e c h a n i c a l p r o p e r t i e s _^ measured a t ε ^ 10~2 . LoXd Ϊ 4 . 7 N; s l i d i n g v e l o c i t y 4 χ 10 m s ~ i ; " s i n g l e - p a s s " a b r a s i o n over a b r a s i v e papers o f σ v a l u e s ·, 6.2 ym; 0, 6.7 ym; Θ, 9.5 ym; 0 11.3 ym, 6, 15.8 ym. ε


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The t r a n s f e r wear modes i n PTFE I f PTFE has anomolous wear c h a r a c t e r i s t i c s , i t i s i n t h e a r e a o f what i s now d e s c r i b e d as t r a n s f e r o r a d h e s i v e wear. Much has been w r i t t e n on t h i s t o p i c and o p i n i o n s do not r e a l l y d i f f e r on t h e i m p o r t a n t f e a t u r e s o f t h i s p r o c e s s (34-43). B e f o r e d i s c u s s i n g PTFE i n p a r t i c u l a r i t i s u s e f u l t o b r i e f l y r e v i e w two g e n e r a l a s p e c t s o f t r a n s f e r wear p r o c e s s e s ; t h e t r a n s i t i o n between a b r a s i v e and t r a n s f e r modes and t h e c a t e g o r i e s o f t r a n s f e r wear.


The t r a n s i t i o n between a b r a s i v e and t r a n s f e r modes The i d e a o f a t r a n s i t i o n from a t r a n s f e r p r o c e s s t o an a b r a s i o n p r o cess i s r e a l l y a q u e s t i o n o f s e m a n t i c s . I n a t r a n s f e r process i t i s a d h e s i v e f o r c e s w h i c h t r a n s m i t t h e s t r e s s e s w h i c h u l t i m a t e l y remove s u r f a c e l a y e r s from t h e polymer m a t r i x . The adhesive f o r c e s are s u f f i c i e n t l y s t r o n g a t the s l i d i n g i n t e r f a c e t o cause c o h e s i v e f a i l u r e o f t h e polymer near t h e i n t e r f a c e and t h e polymer i s t r a n s f e r r e d onto the c o u n t e r f a c e . Simple models o f a b r a s i o n l i k e the ones reviewed above do not r e g a r d s u r f a c e f o r c e s as a c o n t r i b u t i n g f a c t o r a l t h o u g h we w i l l r e t u r n t o t h i s p o i n t l a t e r i n the c o n t e x t o f l u b r i c a t i o n . The d e f o r m a t i o n work i n a b r a s i o n i s t r a n s m i t t e d t o t h e s u r f a c e b y r i g i d g e o m e t r i c a s p e r i t y engagements. The s o f t m a t e r i a l may form f r e e c h i p s a l t h o u g h t h i s i s u n l i k e l y w i t h machined c o u n t e r f a c e s whose a s p e r i t y s l o p e s a r e n o r m a l l y l e s s than 20 . More l i k e l y i t i s a f a t i g u e p r o c e s s w h i c h i n v o l v e s t h e p l o u g h i n g and hence repeated d e f o r m a t i o n o f t h e polymer s u r f a c e . I n p r a c t i c e , o f c o u r s e , a d h e s i v e f o r c e s o r i n t e r f a c i a l f r i c t i o n w i l l be i n v o l v e d . A f u r t h e r c o m p l i c a t i o n a r i s e s when t h e wear d e b r i s b e g i n s t o m o d i f y t h e t o p o graphy o f the c o u n t e r f a c e ; t h i s i s t h e case even d u r i n g what i s termed " s i n g l e - p a s s " a b r a s i o n . There a r e c u r r e n t l y no c l e a r answers t o t h e r e l a t i v e importance o f a b r a s i o n and t r a n s f e r as t h e s c a l e o f t h e s u r f a c e roughness i s reduced. The problem does however have some p o t e n t i a l l y i m p o r t a n t p r a c t i c a l consequences; t h i s i n v o l v e s the o b s e r v a t i o n t h a t t h e r e i s o f t e n an optimum s u r f a c e roughness f o r minimum wear (41-43). Figure 7 i s an example f o r a p o l a r g r a p h i t e f i l l e d PTFE ( 4 3 ) . An optimum s u r f a c e roughness o f c a . 0.25 y m c . l . a. i s e v i d e n t . S i m i l a r r e s u l t s have been noted w i t h u l t r a h i g h m o l e c u l a r w e i g h t p o l y e t h y l e n e (UHMPE). I t i s tempting t o a s c r i b e t h i s p o i n t t o t h e t r a n s i t i o n r e g i o n between t r a n s f e r and a b r a s i o n ; a r e a s o n why t h e t r a n s f e r wear might decrease w i t h t h e s c a l e s u r f a c e topography up t o 0.25 ym c . l . a . i s d i s c u s s e d l a t e r . Above 0.25 ymc. 1. a a b r a s i o n predominates and the s e v e r i t y o f t h e a b r a s i o n i n c r e a s e s r a p i d l y as t h e c . l . a . r o u g h n e s s , σ, i s i n c r e a s e d . I n none o f these s t u d i e s was a c a r e f u l study of s u r f a c e topography c a r r i e d o u t . There a r e few s t u d i e s where t h i s has been done; a n o t a b l e e x c e p t i o n i s the work o f L a n c a s t e r and H o l l a n d e r ( 4 7 ) . They measured b o t h σ and IL the mean a s p e r i t y r a d i u s and hence computed a mean v a l u e o f Θ , t h e average s l o p e . They then found good c o r r e l a t i o n s between s i n g l e - p a s s a b r a s i v e wear,and the r a t i o σ/IL w h i c h i s r e l a t e d t o t a n θ . ( t a n θ = (σ/2ί^) .) C l e a r l y tRe t r e n d i n F i g u r e 7 may be due t o changes i n R^. E i s s has r e c e n t l y l o o k e d s p e c i f i c a l l y a t t h e i n f l u e n c e o f R^ i n the wear o f branched p o l v ^ t h y l e n e s and p o l y v i n y l c h l o r i d e ( 4 8 ) ; i t i s an important v a r i a b l e . 1

1

1

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I

1

I

I

I

I

1

1

0

Q2

OA

0.6

0.8

1.0

1.2

1.4

ITS C O N T R O L

U 1.6

Initial counterface roughness,c.l.a [ pm] F i g u r e 7. Wear r a t e as a f u n c t i o n o f i n i t i a l c o u n t e r f a c e r o u g h ness f o r a s e r i e s of carbon f i l l e d PTFE composites r u n n i n g i n a i r a g a i n s t a s t e e l c o u n t e r f a c e . The f i l l e r s are p r e s e n t at 10% b y weight and the carbons d i f f e r i n p a r t i c l e s i z e , a s p e c t r a t i o and s u r f a c e c h e m i s t r y . L i t t l e change i n c o u n t e r f a c e topography i s noted d u r i n g the course o f t h i s experiment; these carbons a r e not p a r t i c u l a r l y a b r a s i v e towards s t e e l . The e v i d e n c e o f a minimum i n wear i s not s t r o n g but i t i s c o n s i s t e n t w i t h o t h e r work. I n t h i s experiment t h e r e i s a d e t e c t a b l e i n v e r s e t r e n d i n the f r i c t i o n ; the lower the wear the h i g h e r i s the f r i c t i o n . The symbols r e f e r to carbons of d i f f e r e n t p a r t i c l e s i z e and a s p e c t r a t i o . Open symbol i s f o r a g r a p h i t e h i g h a s p e c t r a t i o s h e e t . The f u l l y c l o s e d symbol i s f o r a p a r t i c l e of lower aspect r a t i o . Both p a r t i c l e s were p r e p a r e d from a n u c l e a r grade g r a p h i t e , Ô ( 4 6 ) .


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Transfer modes; general c l a s s i f i c a t i o n Under a given set of contact conditions organic polymers divide into two types i n respect of t h e i r transfer behaviour; some form transferred films and others do not. In the case of PTFE we often observe a type of transfer which we may c a l l predominantly i s o t h e r mal or c o l d , "whole" t r a n s f e r . There i s some f r i c t i o n a l heating and chain s c i s s i o n but these e f f e c t s are not large when compared with some other polymers. PMMA, for example, shows no detectable trasnfer below i t s glass t r a n s i t i o n temperature although there may be transfer of low molecular weight species which have segregated on the free surface. At high s l i d i n g speeds surface melting occurs and transfer films are formed from the melt (49) with some chain s c i s s i o n (41,50). This i s hot, t o t a l t r a n s f e r . On the other hand, a highly cross-linked system, such as a rubber or r e s i n , only forms extensive transfer films v i a extensive chemical degradation at the interface during high speed s l i d i n g . This i s degraded t r a n s f e r . There i s no transfer when isothermal conditions are maintained although surface "bloom" may be transferred. PTFE i s therefore amongst a class of polymers which e x h i b i t s c o l d , t o t a l t r a n s f e r . I t i s the wear which arises from t h i s type of transfer which i s usually described as transfer wear. As we s h a l l see the behaviour of PTFE i s not unique but i t i s unusual i n i t s extent. Transfer wear The part that the transfer process plays i n the o v e r a l l wear process has been reviewed elsewhere (8,9). The basic elements can be ident i f i e d broadly as follows: (i) (ii) (iii) (iv)

the nature and magnitude of the adhesive forces which provide surface t r a n s i t i o n s ; the p o s i t i o n of the locus of junction f a i l u r e and the morphology and thickness of the t r a n s f e r l a y e r ; the magnitude of the adhesion of the transfer f i l m to the substrate and the means by which i t i s detached from the substrate ; and f i n a l l y the d e t a i l s of the processes whereby the f i l m i s displaced from the contact.

S i g n i f i c a n t rates of transfer are observed when the i n i t i a l i n t e r f a c i a l adhesion i s s u f f i c i e n t to develop cohesive f a i l u r e s i n the polymer under the action of the transmitted shear stresses. I f t h i s transferred layer i s weakly attached to the substrate, i t i s detached by the same tractions. Providing the geometry of the system allows, this material i s displaced from the contact. More f i l m i s transferred to the substrate and a high equilibrium wear rate r e s u l t s ; t y p i c a l l y i n a polymer pin-on-disc configuration about 10 nm of polymer i s removed from the pin during each cycle over the face of the d i s c . In confined or conforming contacts with



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CONTROL

u n f i l l e d polymer the wear i s l e s s s i m p l y because the d e b r i s cannot escape and back t r a n s f e r o c c u r s . A r a t i o n a l e , w h i c h e x p l a i n s much o f t r a n s f e r wear b e h a v i o u r , i s t h a t the polymer does not r e a d i l y t r a n s f e r on t o i t s own t r a n s f e r f i l m . I f the t r a n s f e r f i l m i s adhered s t r o n g l y t o the s u b s t r a t e o r cannot escape, the wear i s r e d u c e d , o f t e n by l a r g e f a c t o r s . These g e n e r a l comments a p p l y to some e x t e n t t o a l l c o l d , "whole" t r a n s f e r wear p r o c e s s e s . PTFE i s unusual i n t h a t i t t r a n s f e r v e r y r e a d i l y i n t h i s type of p r o c e s s ; more so t h a n any o t h e r known p o l y mer. F u r t h e r , the t r a n s f e r f i l m s have p a r t i c u l a r l y poor a d h e s i o n t o most s u b s t r a t e s and a l s o have e x t r e m e l y h i g h degrees o f o r i e n t a t i o n . Some c h a i n s c i s s i o n can be d e t e c t e d (38,42,50). They a r e a l s o uncommonly t h i n ; sometimes no more t h a n 10 nm i n t h i c k n e s s (some workers b e l i e v e they may be o f the o r d e r o f one m o l e c u l a r c h a i n c r o s s s e c t i o n d i a m e t e r i n t h i c k n e s s , c a . 0.8 nm). The f a c i l i t y shown by PTFE t o form h i g h l y o r i e n t e d and t h i n t r a n s f e r f i l m s i s not unique t o t h i s polymer. L i n e a r p o l y e t h y l e n e s and p o s s i b l y polyoxymethylene behave i n a s i m i l a r f a s h i o n a l t h o u g h not w i t h the same e f f i c i e n c y . The tendency to form h i g h l y o r i e n t e d f i l m s can be r e a d i l y d e t e c t e d i n the s p i n s e n s i t i v i t y o f the wear r a t e (see companion paper by Dr. T. S t o l a r s k i and the p r e s e n t a u t h o r ) . The wear r a t e s o f t h e s e smooth m o l e c u l a r p r o f i l e polymers (PTFE and l i n e a r p o l y t h e n e s ) d e c r e a s e s w i t h r e a l and apparent l o a d a x i s s p i n . A t the same time the f r i c t i o n i n c r e a s e s . The same t r e n d s are not seen w i t h branched p o l y e t h y l e n e s , w h i c h form r a t h e r p o o r l y o r i e n t e d t r a n s f e r r e d l a y e r s and PMMA w h i c h does not t r a n s f e r l a y e r s . T h i s experiment u n d e r l i n e s the i m p o r t a n t p r o c e s s o f i n t e r f a c e r e o r i e n t a t i o n and shear s o f t e n i n g w h i c h o c c u r s i n smooth m o l e c u l a r p r o f i l e p o l y m e r s . The i m p o s i t i o n o f s p i n about the l o a d a x i s d i s r u p t s t h i s p r o c e s s and as a r e s u l t the f r i c t i o n a l work i n c r e a s e s and the wear r a t e d e c r e a s e s . The change i n the f r i c t i o n a l work can be e x p l a i n e d q u i t e s i m p l y i n q u a l i t a t i v e terms. Why the t r a n s f e r wear s h o u l d decrease cannot be e a s i l y e x p l a i n e d . However, t h e r e i s a general observâtion w h i c h seems to be t r u e f o r many PTFE and o t h e r p o l y m e r i c c o n t a c t s w e a r i n g by the c o l d t r a n s f e r p r o c e s s ; the h i g h e r the f r i c t i o n the l o w e r i s the wear. F i g u r e 7 may be r e c a l l e d i n t h i s context. Other exponents i n d i c a t e t h a t when the f r i c t i o n i s h i g h the wear i s l e s s . The i n f e r e n c e t h a t has been drawn i s t h a t the h i g h e r f r i c t i o n promotes a s t r o n g e r a d h e s i v e f u n c t i o n a t the t r a n s f e r f i l m - s u b s t r a t e i n t e r f a c e ( 5 1 ) . We s h o u l d a l s o n o t e h e r e t h a t PTFE has a r a t h e r low f r i c t i o n when s l i d on smooth c o u n t e r f a c e s . I t i s now w o r t h summarising the c h a r a c t e r i s t i c s o f PTFE i n the t r a n s f e r wear mode. The b a s i c p r o c e s s resembles t h a t seen i n o t h e r t r a n s f e r r i n g p o l y m e r s ; many r a t n e r i l l - d e f i n e d and d i f f u s e s t e p s are i n v o l v e d . PTFE i s u n u s u a l i n s o f a r as i t t r a n s f e r s v e r y e f f i c i e n t l y to c l e a n s u r f a c e s and i t s t r a n s f e r f i l m s do not adhere s t r o n g l y . The f a c t t h a t the f r i c t i o n a l work i s r a t h e r low as a r e s u l t of i n t e r f a c i a l r e o r d e r i n g may be i m p o r t a n t h e r e . The low f r i c t i o n a l s o ensures a w i d e r o p e r a t i n g range o f q u a s i - i s o t h e r m a l o r c o l d t r a n s f e r . The s p e c i a l h i g h l y o r i e n t e d t r a n s f e r f i l m c h a r a c t e r i s a l s o shared w i t h l i n e a r polythenes although these m a t e r i a l s do not t r a n s f e r so r e a d i l y . For the p o l y t h e n e s i t appears t h a t the h i g h e r the m o l e c u l a r w e i g h t the l e s s pronounced i s



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the t r a n s f e r . The r e l a t i v e importance of m o l e c u l a r w e i g h t and morphology i n g o v e r n i n g the case o f PTFE t r a n s f e r has been examined by s e v e r a l a u t h o r s a s have t h e e x t e n t s of c h a i n s c i s s i o n and c r y s t a l l i n i t y change produced i n t h e wear d e b r i s . H i g h c r y s t a l l i n i t y and h i g h m o l e c u l a r weight seem t o i n h i b i t t r a n s f e r o r a t l e a s t t r a n s f e r wear ( 5 0 ) . F i g u r e 8 shows t h e i n f l u e n c e o f gamma r a d i a t i o n on the t r a n s f e r wear o f PTFE ( 3 0 ) . No c r o s s - l i n k i n g o c c u r s b u t t h e r e i s a s i g n i f i c a n t i n c r e a s e i n d e n s i t y and hence p r o b a b l y c r y s t a l l i n i t y . The r a t e of wear i s reduced t o l e v e l s comparable w i t h those o b t a i n e d w i t h good f i l l e r packages f o r doses i n excess of 20 M Rad. At this p o i n t the c r y s t a l l i n i t y i s c a l c u l a t e d as c a . 80% from d e n s i t y measurements; the v i r g i n polymer has a c r y s t a l l i n i t y o f ca.60%. Eiss and Hu found s i m i l a r t r e n d s a l t h o u g h they a l s o d e t e c t the expected t r e n d w i t h i n c r e a s i n g m o l e c u l a r weight ( 5 0 ) . In l o o s e terms, the presence of o r d e r e d u n i t s o r l o n g range a f f i n e c o n n e c t i o n s suppresses the n e c e s s a r y c h a i n m o b i l i t y r e q u i r e d to g e n e r a t e o r i e n t e d polymer s u r f a c e s and hence t h e f o r m a t i o n o f t h i n o r i e n t e d t r a n s f e r f i l m s b y r u p t u r e a t the o r i e n t e d s u r f a c e b u l k i s o t r o p i c polymer i n t e r f a c e . The s o l i d p a r t i c l e s i n c l u d e d i n PTFE t o reduce t r a n s f e r wear may a c t i n the same way ( 4 1 ) . A d i s c u s s i o n o f the wear o f PTFE would not be complete w i t h o u t some r e f e r e n c e t o PTFE c o m p o s i t e s . T h i s has been a p o p u l a r f i e l d of s t u d y s i m p l y because w i t h o u t f i l l e r s the wear o f PTFE i s n o r m a l l y u n a c c e p t a b l e . A good f i l l e r w i l l reduce t r a n s f e r wear r a t e s by up to t h r e e o r d e r s o f magnitude. V a r i o u s mechanisms have been proposed and the s u b j e c t has been r e v i e w e d b y t h e p r e s e n t a u t h o r (8,9) and o t h e r s (2,52). The s i m p l e s t i d e a i s t h a t f i l l e r s wear l e s s t h a n the polymer when exposed a t the i n t e r f a c e . They may a l s o suppress t r a n s f e r and improve t r a n s f e r f i l m a d h e s i o n . A good d e a l o f e f f o r t of h i g h q u a l i t y has been put i n t o the s e a r c h f o r c h e m i c a l l y induced a d h e s i o n p r o m o t i o n a t the t r a n s f e r r e d f i l m - s u b s t r a t e i n t e r f a c e but the e v i d e n c e i s e q u i v o c a l (53,54). Chemical changes a r e d e t e c t e d but t h e i r p r e c i s e c o n t r i b u t i o n t o t h e a d h e s i o n i s u n c e r t a i n i n comm e r c i a l a p p l i c a t i o n s . PTFE i s a remarkably s t a b l e polymer t o c h e m i c a l a t t a c k even a t s l i d i n g i n t e r f a c e s . To c o n c l u d e t h i s s e c t i o n we may s t a t e t h a t , w i t h r e s p e c t t o t r a n s f e r wear, PTFE i s not e n t i r e l y u n i q u e . Even i t s s p e c i a l t r a n s f e r b e h a v i o u r i s seen elsewhere. I t i s r e a l l y a m a t t e r of the e x t e n t not the k i n d of i t s c h a r a c t e r w h i c h i s r e m a r k a b l e . L u b r i c a t i o n o f PTFE There i s more i n t e r e s t i n the e f f e c t of m a r g i n a l l u b r i c a t i o n o r s p u r i o u s c o n t a m i n a t i o n t h a n i n f u l l o r i n t e n t i o n a l l u b r i c a t i o n . The v e r y i n d i f f e r e n t performance of PTFE i n the o r i g i n a l human h i p j o i n t replacements emphasises t h e probelm f a c e d w i t h PTFE and even some o f i t s composites. P r o v i d i n g that a f u l l f l u i d f i l m i s maintained t h e r e i s no s e r i o u s problem. However, i f d i r e c t c o n t a c t between the polymer and t h e c o u n t e r f a c e o c c u r s a t r a n s f e r f i l m i s d e p o s i t e d . The f l u i d n a t u r a l l y undermines the a d h e s i o n o f t h i s f i l m t o the subs t r a t e and a l s o f a c i l i t a t e s d e b r i s removal from the c o n t a c t ; a n a c c e l e r a t e d wear may then ensue. F i g u r e 9 i s the d a t a from L a n c a s t e r w h i c h c l e a r l y e x e m p l i f i e s t h i s p o i n t ( 5 5 ) . C e r t a i n f i l l e r s may improve m a t t e r s but i n g e n e r a l i t i s not a good p r a c t i c e to use



164

P O L Y M E R W E A R A N DITS C O N T R O L

Y dose [MradJ F i g u r e 8. Wear r a t e o f γ-irradiated PTFE as a f u n c t i o n o f γdose. C o u n t e r f a c e m i l d s t e e l , σ = 0 . 4 ym; c l o s e d symbols gamma damage i n vacuum; open symbols gamma damage i n a i r . I n b o t h cases t h e t r a n s f e r wear reaches a minimum a t about 20 M Rad. I n the case o f gamma t r e a t m e n t i n a i r s i g n i f i c a n t o x i d a t i o n o f t h e sample, p a r t i c u l a r l y t h e s u r f a c e , i s found t o o c c u r .



11.

BRISCOE

165


10 Κ solubility parameter, 0

5

1 5

20

F i g u r e 9. T r a n s f e r wear r a t e s i n " l u b r i c a n t s " o f two PTFE com p o s i t e s (·, P T F E - p o l y i m i d e and 0, PTFE-25% w.w. I c a r b o n f i b r e ) as a f u n c t i o n o f t h e s o l u b i l i t y parameter, δ, o f t h e l u b r i c a t i n g media. The wear of the d r y c o n t a c t s i s shown a t δ = 0 and t h e c a l c u l a t e d v a l u e o f 6 f o r PTFE i s c a . 6.0. The s o l u b i l i t y p a r a meter i s d e f i n e d as t h e square r o o t o f the c o h e s i v e energy d e n s i t y and i s t h e r e f o r e n e a r l y p r o p o r t i o n a l t o t h e square r o o t o f t h e s u r f a c e t e n s i o n , γ, o f t h e f l u i d . The t r e n d f o r the wear t o i n c r e a s e w i t h γ and 6 i s apparent. I n the d r y c o n t a c t secure t r a n s f e r f i l m s a r e formed but they a r e n o t e v i d e n t i n l u b r i c a t e d c o n t a c t s . I t i s r e a s o n a b l e t o suppose t h a t as γ i n c r e a s e s t h e w e t t i n g o f t h e s t e e l c o u n t e r f a c e improves and hence t h e t r a n s f e r f i l m s a r e more r e a d i l y d i s p l a c e d . Data adapted from L a n c a s t e r and Evans.


POLYMER WEAR AND

166

ITS

CONTROL

PTFE i n t h i s way. There are a l s o numerous commercial examples of where s m a l l amounts of even condensable vapours may a c c e l e r a t e wear by presumably d e s t a b i l i s i n g the f o r m a t i o n of good and s e c u r e l y a t t a c h e d t r a n s f e r r e d l a y e r s . F i g u r e 10 i s an i l l u s t r a t i o n w h i c h a l s o shows t h a t i n composites the e n v i r o n m e n t a l s e n s i t i v i t y o f the wear may be conveyed by the f i l l e r ; i n t h i s case c a r b o n ( 5 6 ) . As PTFE so r e a d i l y forms t r a n s f e r r e d l a y e r s , t h i s type of g r o s s "lubrication f a i l u r e i s an acute problem. I n many a p p l i c a t i o n s i t has t o be a c c e p t e d as a low d r y f r i c t i o n i s r e q u i r e d when f u l l f l u i d f i l m l u b r i c a t i o n i s l o s t . T h i s i s sometimes the case a t the ends o f the s t r o k e i n a r e c i p r o c a t i n g c o n t a c t . We have made no d i r e c t r e f e r e n c e to the r o l e p l a y e d by the u n i q u e l y low s u r f a c e f r e e energy of PTFE i n i t s wear b e h a v i o u r . This p o i n t i s d i s c u s s e d b r i e f l y i n the c o n c l u s i o n t o t h i s paper where the s i g n i f i c a n c e of s u r f a c e f r e e energy as a p r i m a r y v a r i a b l e i s argued t o be of no g r e a t consequence. A more u s e f u l n o t i o n i s the low c o h e s i v e energy d e n s i t y o f the polymer and the low s u r f a c e f r e e energy i s j u s t a m a n i f e s t a t i o n o f the low c o h e s i v e energy d e n s i t y . Having made t h i s p o i n t i t i s w o r t h s t a t i n g t h a t the low s u r f a c e f r e e energy of the polymer does i n h i b i t e x t e n s i v e w e t t i n g by f l u i d media and t h e r e are t e n t a t i v e examples where t h i s poor w e t t i n g phenomenon seems t o p l a y a p a r t i n promoting s t a r v e d f l u i d l u b r i c a t i o n (57, 5 8 ) . Poor w e t t i n g i n t e r f e r e s w i t h the g e n e r a t i o n o f e f f e c t i v e f l u i d entry i n the e n t r y r e g i o n .


1 1

Conclusions I f the PTFE's have s p e c i a l c h a r a c t e r i t i s the way i n w h i c h they e a s i l y r e o r i e n t e d t h e i r s u r f a c e s under the a c t i o n o f i n t e r f a c i a l shear s t r e s s e s . T h i s produces f i b r i l l a t i o n i n a b r a s i o n a g a i n s t rough s u r f a c e s and e x t e n s i v e t h i n o r i e n t e d f i l m t r a n s f e r on smoother c o u n t e r f a c e s . These t h i n g s a r e a s u r p r i s e i n such a h i g h m o l e c u l a r w e i g h t polymer w h i c h c o n t a i n s b o t h l a m e l l a and s p h e r u l i t e s . The amorphous r e g i o n s are w e l l above t h e i r g l a s s t r a n s i t i o n temperature but g e n e r a l l y the c o n t a c t o p e r a t e s a t a temperature a t l e a s t 200 C below i t s c r y s t a l l i n e temperature. There are two r e l a x a t i o n tem p e r a t u r e s near ambient but these are u n l i k e l y to be o f g r e a t s i g n i f i c a n c e i n the o v e r a l l p i c t u r e . Our b e s t v i e w i s t h a t because PTFE i s a v e r y weak van d e r Waals s o l i d of low c o h e s i v e energy d e n s i t y , as demonstrated by i t s u n i q u e l y low s u r f a c e f r e e energy, the c h a i n - c h a i n i n t e r a c t i o n s are not s t r o n g i n the m a t r i x . I t i s i t s h i g h m o l e c u l a r weight ( l a r g e e n t r o p y per c h a i n ) w h i c h keeps the m a t e r i a l i n the s o l i d s t a t e . A c t u a l l y PTFE s do not n o r m a l l y m e l t i n the g e n e r a l sense. The f a c t t h a t the c h a i n s are so smooth enables them t o pack e f f i c i e n t l y to form a c o h e r e n t s o l i d . The same smooth m o l e c u l a r topography a l s o f a c i l i t a t e s m o l e c u l a r s l i p and hence the r e l a t i v e d i s p l a c e m e n t o f m o l e c u l e s and m o l e c u l a r domains r e q u i r e d f o r s u r f a c e r e o r i e n t a t i o n even at low temperatures. The consequences on the type and r a t e o f wear and magnitude o f the s l i d i n g f r i c t i o n f o r PTFE need not be r e p e a t e d . I t i s a moot p o i n t as t o whether these t h i n g s make the PTFE s a m a v e r i c k polymer οτ n o t . There i s however a v a l u e i n r e g a r d i n g the wear c h a r a c t e r i s t i c s o f PTFE as b e i n g g e n r a l l y s i m i l a r t o t h a t o f o t h e r polymers and v e r y s i m i l a r t o c e r t a i n l i n e a r p o l y e t h y l e n e s . Here, the d i f f e r e n c e s are ones o f e x t e n t not k i n d . 1

1


11.

BRISCOE

167

Wear of Poly (tetrafluoroethylene)

100 484 90

80

.1007.R.H.

Air (40%R.H)

•0%RH.

Ν^Οΐ&Η)


70

60

50 Ε φ

ο i. 40 l_ ο φ

30

20

10

10 2 0 » 1

10 2030 10 2030 2 3

10 2030 10 2030 10 2030 1 3 2

filler [wt 7.]

F i g u r e 10. The i n f l u e n c e o f environment on t h e wear of t h r e e c a r b o n f i l l e d PTFE c o m p o s i t e s . (a) t h e e f f e c t o f r e l a t i v e humid i t y i n a i r , (b) t h e e f f e c t o f oxygen (as a i r ) compared w i t h n i t r o g e n a t a nominal z e r o h u m i d i t y . The t h r e e carbons a r e : 1. a n u c l e a r g r a p h i t e ; 2. t h e same n u c l e a r g r a p h i t e m i l l e d i n a p o l a r f l u i d ; and 3. an agglomerated f l a k e g r a p h i t e formed from n u c l e a r g r a p h i t e by m i l l i n g i n a h y d r o c a r b o n . The main mode o f wear i s by a t r a n s f e r p r o c e s s . The d e t a i l s o f t h e changes p r o duced by t h e environment a r e n o t r e s o l v e d b u t i t appears t h a t a s i g n i f i c a n t p a r t o f the e n v i r o n m e n t a l s e n s i t i v i t y i s due t o changes induced a t t h e f i l l e r - c o u n t e r f a c e i n t e r f a c e . F o r example, the f i l l e r p a r t i c l e s a r e w e a r i n g more r a p i d l y i n an oxygen environment, presumably by t h e p r o c e s s o f o x i d a t i o n ( 5 4 ) .



168 Acknowledgments

I am g r a t e f u l t o P r o f e s s o r D. Tabor, Dr. T. S t o l a r s k i , Dr. J . L a n c a s t e r and Mr. P a u l Evans f o r u s e f u l d i s c u s s i o n s w h i c h have h e l p e d i n the f o r m u l a t i o n o f the i d e a s e x p r e s s e d i n t h i s paper.

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