Wear of Poly(tetrafluoroethylene) - ACS Publications - American

quely low surface free energy of PTFE in its wear behaviour. This point is discussed briefly in the conclusion to this paper where the significance of...
0 downloads 0 Views 2MB Size
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.

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

152

POLYMER WEAR AND

ITS

CONTROL

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.

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

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

11.

BRISCOE

153

Wear ofPoly(tetrafluoroethylene)

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

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

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

154

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

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 .

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

11.

BRISCOE

155

Wear of Poly (tetrafluoroethylene)

(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):

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

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

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

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

156

P O L Y M E R W E A R A N D ITS

CONTROL

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.

11.

BRISCOE

157

Wear of Poly(tetrafluoroethylene)

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;

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

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

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

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

158

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

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

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

11.

BRISCOE

159

Wear of Poly(tetrafluoroethylene)

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.

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

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

5

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

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

160

POLYMER WEAR AND

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 ) .

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

11.

BRISCOE

Wear of Poly(tetrafluoroethylene)

161

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

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

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

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

162

POLYMER WEAR AND

ITS

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

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

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

11.

BRISCOE

Wear of Poly(tetrafluoroethylene)

163

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

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

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

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 .

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

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

11.

BRISCOE

165

Wear of Poly(tetrafluoroethylene)

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.

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

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 .

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

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

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

11.

BRISCOE

167

Wear of Poly (tetrafluoroethylene)

100 484 90

80

.1007.R.H.

Air (40%R.H)

•0%RH.

Ν^Οΐ&Η)

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

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 ) .

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

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

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.

Literature Cited 1. 2.

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

3. 4. 5. 6. 7. 8.

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

Steijn, R.P., American Soc. for Metals, 9-12 May 1967. Evans, D.G.; Lancaster, J.K., "The Wear of Polymers" in "Materials Science and Technology, Wear", Ed. Scott, D., Vol.13, Academic Press, New York, 36-140, 1979. Barteney, G.M.; Laurentev, V.V., "Polymer Friction and Wear", trans. Payne, D.B., ad. Lee, L.H., Chemistry Publisher, Lenin­ grad, 1972, Eng.Ed., Elsevier, The Netherlands, 1981. Bely, V.A. et a l . , "Friction and Wear in Polymer-based Materials", trans. Granville-; Jackson, P., Pergamon Press, 1982. Lee, L.H. ed., "Advances in Polymer Friction and Wear", Vol.5A, 5B. Plenum Press, New York, 1974. Dowson, D., Godet, M.; Taylor, C.M., ed. "The Wear of NonMetallic Materials", Mechanical Engineering Publications Ltd., 1978. Ludema, K.C., Glaeser, W.A., Rhee, S.K., et al. ed., "Wear of Materials", various editions 1977, 1979, 1981, Am.Soc.Mech. Engnrs. Briscoe, B.J.; Tabor, D., "Friction and Wear of Polymers", Ch. I, "Polymer Surfaces" ed. Clark, D.T. and Feast, W.J., Wiley, New York, 1978. "The Sliding Wear of Polymers" in "Fundamentals of Tribology" ed. Suh, N.P. and Saka, Ν., MIT Press, Cambridge, Mass., 1980. Briscoe, B.J., Tribology International, 231-242, August 1981 and "Tribology of Polymers" in "Physicochemical Aspects of Polymer Surfaces" ed. Mittal, K.L., Plenum Press, New York,1981. Schallamach, Α., J.Polymer Sci., 9 (5) 1952, 385. Schallamach, A., Wear, 1, 1958, 384. Tabor, D., "The Hardness of Metals", Oxford University Press, 1951. Tabor, D., "Indentation Hardness and its Measurement; Some Cautionary Comments". Private Communication. Archard, J . F . , "Wear" in Interdisciplinary Approach to Friction and Wear, ed. Ku, P.M., NASA. SP-181 Washington DC, 1968. Briscoe, B.J., Evan, P.D.; Lancaster, J.K., J.Mat.Sci., in press. Johnson, K.L., "Aspects of Friction", in "Friction and Trac­ tion", ed. Dowson, D. et a l . , Westburg House, London 1981. Lamy, B., Tribology International, 17 (1), 1984, 35. Tabor, D., "Wear - A Critical Synoptic View" in "Wear of Materials 1977", ed. W.A. Glaeser et a l . , American Society of Mechanical Engineers, 1977. Bethune, B. J.Mat.Sci., 11, (1976), 199. Hutchings, I.M., Winter, R.E.; Field, J . E . , Proc.Roy.Soc., A348, 1976, 379.

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

11. BRISCOE

21. 22. 23. 24. 25. 26. Downloaded by MONASH UNIV on November 14, 2015 | http://pubs.acs.org Publication Date: September 12, 1985 | doi: 10.1021/bk-1985-0287.ch011

27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45.

Wear of Poly(tetrafluoroethylene)

169

Ahman, L. ; Oberg, Α., "Wear of Materials", 1983, Am.Soc. Mech.Engrs. Tilly, G.P., Wear, 23, 1973, 87. Kragehlskii, I.V., "Friction and Wear", Butterworth, London, 1965. Lancaster, J.K., "Abrasion and Wear of Polymers" in "Encyclo­ pedia of Polymer Science and Engineering", Vol. I, John Wiley & Sons, 1985. Lancaster, J.K., Proc.Inst.Mech.Engrs.Lub. and Wear Group, 190, 1964. Ratner, S.B.; Farberova, I.I. in "Abrasion of Rubbers", ed. James, D.I., McLaren & Sons, London, 1967, 23, 297. Warren, J.H.; Eiss, N.S. in "Wear of Materials 1977", ed. Glaeser et al, American Society of Mechanical Engineers, 1977, 494. Ratner, S.N., Farberova, I.I., Radyukeuich, O.V.; Lure, E.G., Soviet Plastics, 7, 1964, 37. Lancaster, J.K., Plastics and Polymers, 41, 1973, 297. Briscoe, B.J.; Ni, Ζ., "Wear of Materials 1983", ed. Ludema, K.C., American Society of Mechanical Engineers, 1983, 643. Champ, D.H., Southern, E . , Thomas, A.G. in "Advances in Poly­ mer Friction and Wear", Polymer Science and Technology 5A, ed. Lee, L.H., Plenum Press, New York, 1974, 133. Hertzberg, R.W. and Manson, J.A., "Fatigue of Engineering Plastics", Academic Press, New York, 1980. Briscoe, B . J . , Phil.Mag., 43A, (3), 1981, 511. Pooley, C.M.; Tabor, D., Proc.Roy.Soc. A329, 1972, 251. Sviridyonok, A.I., Belyi, V.A., Smurugov, V.A.; Saukin, V.G., Wear, 25, 1973, 301. Bowers, R.C.; Zisman, W.A., Eng.Chem.Prod.Res.Develop., 13, 1974, 115. Jain, V.K.; Bahadur, S. in "Wear of Non-Metallic Materials", ed. Dowson, D. et a l . , Mechanical Engineering Publications Ltd, London 1978. Bonfield, W., Edwards, B.C. ;Markham, A . J . , Wear, 37, 1973, 153. Tanaka, K.; Uchiyama, Y. in "Wear of Materials 1977", ed. Glaeser, W. et a l . , American Society of Mechanical Engineers, New York, 1977, 499. Kar, M.K.; Bahadur, S. in "Wear of Materials 1977", ed. Glaeser et al, American Society of Mechanical Engineers, New York, 1977, 501. Arkles, B.C.; Schireson, M.J., Wear, 39, 1976, 177. Czichos, H. in "Wear of Non-Metallic Materials", Mechanical Engineering Publication Ltd., ed. Dowson, D. et al, London, 1978, 285. Tanaka, K. in Wear of Materials 1981", ed. Rhee, S.K. et a l . , American Society of Mechanical Engineering, New York, 1981,98. Eiss, N.; Bayraktaroglu, H.M., Trans.Amer.Soc. of Lubri.Eng., 23, (3), 1980, 269. Dowson, D., Challen, J.M., Holmes, K.; Atkinson, J.R. in "Wear of Non-Metallic Materials", Mechanical Engineering Pub­ lications Ltd., London, 1978, 99.

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

POLYMER WEAR AND ITS CONTROL

170 46. 47. 48. 49. 50.

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

51. 52. 53. 54. 55. 56. 57. 58.

Briscoe, B.J.; Steward, M.D., in Materials Performance and Conversation", Inst.Mech.Engrs. Publication, London 1978. Lancaster, J.K.; Hollander, D.E., Wear, 25, 1973, 1955. Eiss, N.S.; Milloy, S.C. in "Wear of Materials 1983", ed. K.C. Ludema, American Society of Mechanical Engineers, New York, 1983, 650. Tanaka, K.; Uchiyama, Y. in "Advances in Polymer Friction and Wear", 5B, ed. Lee, L.H., Plenum Press, New York, 1974, 499. Hu, T.-Y. Eiss, N.S. in "Wear of Materials 1983", ed. K.C. Ludema, American Society of Mechanical Engineers, New York, 1983, 636. Briscoe, B.J. in "Adhesion 5", ed. Allen, Κ., Applied Science Publishers, London, 1981. Czichos, H., "Systems Approach to Tribology", Elsevier, The Netherlands, 1979. Pocock, G.; Cadman, P., Wear, 37, 1976, 129. Pepper, S.V., J . Applied Phys. 45(7), 1976, 2949. Lancaster, J.K. Private Communication. Briscoe, B . J . , Steward, M.D.; Groszeck, Α., Wear, 42, 1977,99. Lewis, R.B., American Society of Automotive Engineers, Inc., Nation. Combined Fuels and Lubrication and Transportation Meetings, Houston, Texas, 1969. Briscoe, B.J., Stolarski, T.A. ; Davis, G., Tribology International, 17, 1984, 121. ;

RECEIVED January 23, 1985

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