Standardization of Laboratory Methods to Evaluate Friction Behavior

22

Standardization

of

Laboratory

Methods

to

Evaluate

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Friction Behavior of Polymer-Based Materials

A. I. Sviridyonok and Yu. E. Kirpichenko Institute of Mechanics of Metal-Polymer Systems, Byelorussian S.S.R. Academy of Sciences, Gomel, U.S.S.R.

T h e paper d e a l s with the problem of s t a n d a r dization of friction a n d wear test methods for p o l y m e r — b a s e d materials. A review i s p r e s e n t e d of the test methods a v a i l a b l e i n the d e v e l o p e d countries. T h e i r c l a s s i f i c a t i o n i s g i v e n with the c o n s i d e r a t i o n of friction p r o c e s s modelling. T h e r e a s o n s of data d i s c r e p a n c i e s a r e d i s c u s s e d between different s o u r c e s . It i s c o n c l u d e d that p r o c e d u r e s for comparative tests should include tests for g e n e r a l i z e d regimes. A p r o c e d u r e i s d e s c r i b e d for evaluation of the tribological p r o perties of p o l y m e r — b a s e d materials a s s u g g e s t e d in I M M S AS B S S R which i s r e v i e w e d i n ISO as a draft of International Standard. F r i c t i o n test data a r e a r r a n g e d a s a map o n the material a n d u s e d to compile a bank of s t a n d a r d i z e d r e f e r e n c e information o n the tribological properties of polymeric materials.

In r e c e n t y e a r s a v a r i e t y o f n o v e l a n t i f r i c t i o n polymer-based m a t e r i a l s h a v e b e e n d e v e l o p e d a n dt h e k n o w n i m p r o v e d at different r e s e a r c h c e n t e r s a n dl a b o r a t o r i e s of t h e w o r l d . Howe v e r , e v e n a v e r y l e a r n e d e x p e r t i n t h e field of friction i s i n a difficulty to e v a l u a t e t r u l y t h e p u b l i c i t y a n dl i t e r a r y d a t a w h e n making comparative analysis o r selecting proper materials for particular friction applications. T h e reasons lie i n the u n a v a i l a bility of c o m m o n methods f o r friction testing of materials a s well a s evaluating criteria o n the friction characteristics of materials. E x p l a n a t i o n a n ds e l e c t i o n of e v a l u a t i n g c r i t e r i a l a r g e l y d e p e n d u p o n t h e factors that influence t h e friction c h a r a c t e r i s t i c s of polymers, a n dcapabilities of t h e testing equipment.

0097-6156/85/0287-0333$06.00/0 © 1985 American Chemical Society

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

POLYMER WEAR AND ITS CONTROL

334 F a c t o r s Influencing the Metal-Polymer Pairs

Friction

in

T h e factors that influence the friction a n d w e a r p r o c e s s e s can b e c o m b i n e d into three g r o u p s : i) s t r u c t u r a l ( g e o m e t r i c a l d i m e n s i o n s , s h a p e o f t h e p a r t s , g e o m e t r y o f c o n t a c t , e t c . ); ii) p h y s i c a l ( p r e s e n c e of v i b r a t i o n s , t h e r m a l c o n d i t i o n s , d u r a t i o n of c o n t i n u o u s o p e r a t i o n , p r e s e n c e of r a d i a t i o n , e l e c t r i c o r magnetic fields, etc.); and iii) c h e m i c a l o n e s ( w o r k i n g e n v i r o n m e n t s a n d t h e i r a c t i v i t y t o w a r d s t h e m a t e r i a l s r u b b e d ). Each factor in turn h a s its o w n features. F o r instance, p h y s i c a l factors m a y be constant or v a r i e d in the direction or values; g r e a t i s t h e v a r i e t y of t h e c o n t a c t g e o m e t r y , i . e . it m a y be a sphere, a plane or a cylinder, or any combination thereof, to g i v e p o i n t , l i n e o r p l a n e c o n t a c t s ( 1 ). In v i e w of t h e a b o v e , of i n t e r e s t a r e t h e d a t a r e p o r t e d by the O r g a n i z a t i o n for E c o n o m i c C o o p e r a t i o n a n d Development ( O E C D ) with the h e a d q u a r t e r s i n P a r i s resulted from a P r o g r a m of C o m p a r a t i v e W e a r T e s t s c o n d u c t e d o n t h e same materials (copper, brass, bronze and steel) in different labor a t o r i e s of d i f f e r e n t c o u n t r i e s o n d i f f e r e n t t e s t i n g e q u i p m e n t (). T h e a n a l y s i s of t h e t e s t r e s u l t s s h o w e d t h a t t h e v a l u e s of w e a r r e s i s t a n c e obtained from different laboratories differed almost 20—fold. Similar pictures are now observed with polym e r i c m a t e r i a l s . T h i s l a c k of c o r r e l a t i o n b e t w e e n t h e t e s t r e s u l t s u n d e r l i n e s the l o s s e s o w i n g to t h e u n a v a i l a b i l i t y of c o m m o n t e s t i n g p r o c e d u r e s a n d e x p l a n a t i o n of t h e m a i n f a c t o r s that i n f l u e n c e g r e a t l y t h e f r i c t i o n c h a r a c t e r i s t i c s of m a t e r i a l s . S p e c i f i c p h y s i c a l a n d m e c h a n i c a l b e h a v i o r of polymeric m a t e r i a l s a l l o w s to i d e n t i f y t h e m a i n f a c t o r s t h a t i n f l u e n c e the friction process: * temperature in the contact zone * v i s c o e l a s t i c b e h a v i o r of p o l y m e r s u n d e r l o a d (creep) * dimensions * c h e m i c a l a c t i v i t y of t h e e n v i r o n m e n t s i n t h e contact zone * o v e r l a p p i n g ratio for t h r u s t b e a r i n g s or encompassing angle b e t w e e n the testing shaft a n d polymeric bearing or b u s h i n g tested * t h e p r o c e s s e s of p o l y m e r t r a n s f e r o n t o r u b b e d m e t a l l i c surfaces. It i s c l e a r t h a t t h e i n f l u e n c e o f t h e s e f a c t o r s u p o n f r i c t i o n and wear s h o u l d be studied in combination with friction p a r a meters s u c h as load, sliding velocity, and surface roughness. T h e c o n v e n t i o n a l m e t h o d s of f r i c t i o n t e s t i n g provide m e a s u r e m e n t of t h e m a i n t r i b o m e t r i c a l c h a r a c t e r i s t i c s t h a t s p e c i f y the m a t e r i a l a p p l i c a b i l i t y for d r y o p e r a t i o n at g i v e n loads and sliding velocities. T h e main tribometrical characterist i c s for p o l y m e r s are a s follows: * load—carrying capacity with minimum deformation * wear rate * c o e f f i c i e n t of f r i c t i o n * s e r v i c e life of b e a r i n g s i n t e r m s of P, V, a n d T. t


22.

SVIRIDYONOK AND KIRPICHENKO

Friction Behavior Evaluation

335

A l l t h e s e f a c t o r s a r e i n t e r r e l a t e d to a c e r t a i n d e g r e e , a n d to o b t a i n t r u e a n d u s e f u l d a t a for d e s i g n i n g f r i c t i o n u n i t s , testing methods are r e q u i r e d that are c a p a b l e of e s t a b l i s h i n g t h e influence of the main friction factors and conditions o n the i n t e r r e l a t i o n of t h e c h a r a c t e r i s t i c s mentioned.

Methods

of

Friction

Investigation

T h e m e t h o d s a v a i l a b l e for e v a l u a t i n g the tribometric character i s t i c s c a n b e c l a s s i f i e d b y t h e m o d e of m o d e l l i n g t h e f r i c t i o n processes. T h e s e are tribometric schematics modelling the following: * s i m p l e a c t s of f r i c t i o n a l i n t e r a c t i o n ** p e r f o r m a n c e of p a r t i c u l a r f r i c t i o n u n i t s a n d p a r t s * service conditions. T h e m e t h o d s of t h e f i r s t g r o u p a r e m o s t l y i n t e n d e d for research laboratories and developing most d y n a m i c a l l y . Of t h e s e are the methods d e s i g n e d for s t u d y i n g the s t r u c t u r e — a n d f r i c t i o n p r o p e r t i e s of m a t e r i a l s (j.), (3_—6.), a n d t h o s e modelling s i m p l e a c t s of f r i c t i o n interaction on a certain structure level (7—j9). Several procedures a n d i n s t r u m e n t s of t h i s group are unique, however, one meets officially approved and universal o n e s f o r t a c k l i n g u r g e n t p r o b l e m s s u c h a s s e l e c t i o n of c o n s t i tuents for s e l f - l u b r i c a t e d c o m p o s i t i o n s , s t u d y i n g adhesion and deformation p r o c e s s e s at r u b b i n g ( l f j ) , (11). T h e s e c o n d g r o u p is for l a b o r a t o r y estimation of the performance of a c t u a l f r i c t i o n u n i t s s u c h a s plain bearings, gear or c h a i n drives, or other standard friction units on s p e c i a l s t a n d s . T h e p u r p o s e of t h e s e t e s t s i s to o b t a i n d a t a for p r e d i c t i n g p e r f o r m a n c e of a c t u a l f r i c t i o n assemblies. T h e t h i r d g r o u p of m e t h o d s f o r f r i c t i o n t e s t i n g i s at p r e s e n t m o s t i m p o r t a n t i n s t u d y i n g t h e e f f e c t of f r i c t i o n parameters a n d m a i n f a c t o r s u p o n t h e v a l u e s of t r i b o m e t r i c a l c h a r a c t e r i s t i c s . T h e v a r i e t y of t e c h n i q u e s f o r f r i c t i o n t e s t i n g a n d d e v i c e s for t h e i r r e a l i z a t i o n ( . 1 2 ) , (.13) indicate a wide search for "universal" p r o c e d u r e s for testing materials. H o w e v e r , the standards available (14-16) describe in most instances t h e w a y of e s t i m a t i o n of certain tribometrical characteristics ( c o e f f i c i e n t of f r i c t i o n , w e a r r a t e ) f o r p a r t i c u l a r c o n d i t i o n s of material applications. T h e data obtained d u r i n g these tests find l i m i t e d u s a g e , f a i l to g i v e c o m p l e t e u n d e r s t a n d i n g of t h e effect of f r i c t i o n p a r a m e t e r s and main factors u p o n the characteristics m e n t i o n e d , a n d d o n o t a l l o w c o m p a r i s o n of t h e m a t e r i a l s . T h e s e a r c h for complete u n d e r s t a n d i n g of f r i c t i o n propert i e s l e d to the m e t h o d s ( . 1 7 ) , (.18) a c c o u n t i n g for the combined e f f e c t s of t h e m a i n f a c t o r s . From Ref. (jL7) relations are found for the friction coefficient, temperature, wear rate versus sliding velocities a n d loads. T h e n b y the d a t a obtained, a s e t of c u r v e s i s d r a w n i n P - V coordinates, having the s a m e v a l u e s of t h e f r i c t i o n c o e f f i c i e n t , temperature, and wear r a t e . It i s c l e a r t h a t g r e a t d i f f i c u l t i e s a r i s e i n obtaining and u s i n g t h i s v o l u m e of i n f o r m a t i o n . C r e a s e (jL8) finds only


336


relations for the friction coefficient v e r s u s loads a n d t e m p e r a tures ( K = f(P), K = f(T) ) , and maximum tolerable pressure v e r s u s temperature ( P max = f(T)). This provides a p r e d i c t i o n of t h e w e a r r e s i s t a n c e for f r i c t i o n u n i t s w i t h t h e relation K = f(T) using equation T

r

fr

= T

env

+

c • f • P • V

where Tf i s the temperature i n the friction zone; T n v is the t e m p e r a t u r e of t h e e n v i r o n m e n t ; c is a coefficient dependent o n the assembly design; f is the c o e f f i c i e n t of friction; P i s the unit load o n the bearing, and V is the sliding velocity. r

N o v e l M e t h o d s for A c c e l e r a t e d of F r i c t i o n C h a r a c t e r i s t i c s

e

Evaluation

In t h e I n s t i t u t e of M e c h a n i c s of M e t a l - P o l y m e r Systems, B y e l o r u s s i a n S S R A c a d e m y of S c i e n c e s , U S S R a method M R 7 4 - 8 2 (_20) f o r e v a l u a t i n g t h e t r i b o t e c h n i c a l p r o p e r t i e s o f polymer-based materials h a d been developed based on the g e n e r a l i z a t i o n of t e c h n i q u e s a n d t a k i n g into c o n s i d e r a t i o n t h e f r i c t i o n b e h a v i o r of p o l y m e r s a n d p o l y m e r - b a s e d composites (12). With polymeric materials in friction pairs the working c a p a c i t y of t h e a s s e m b l y i s l o s t f o r t h r e e r e a s o n s : i) temperature rise i n the friction z o n e a b o v e the limiting value, ii) w e a r e x c e e d i n g t h e t o l e r a b l e limit, and iii) loads over the y i e l d p o i n t of t h e p o l y m e r i c m a t e r i a l . In v i e w of t h i s a n d o t h e r w o r k s ( 1 9 , 2 1 - 2 3 ), the f o l l o w i n g c r i t e r i a of a c c e l e r a t e d p r e d i c t i o n of f r i c t i o n p r o p e r t i e s for p o l y m e r i c m a t e r i a l s w e r e s e l e c t e d w h e n d e v e l o p i n g the method mentioned: * A c o m p l e x of l o a d s a n d v e l o c i t i e s a t w h i c h t h e w e a r rate of t h e p o l y m e r i c m a t e r i a l i s b e l o w 0 . 2 5 yum/h [ P V 2 5 J • "* A c o m p l e x o f l o a d s a n d v e l o c i t i e s a t w h i c h t h e t e m p e rature i n the friction zone i n c r e a s e s significantly along with t h e f r i c t i o n m o m e n t , i . e. fPv] regimes are reached c o r r e s p o n d i n g to t h e l o s s of w o r k i n g c a p a c i t y b y t h e p o l y m e r i c material. ^ ^ * C o e f f i c i e n t of f r i c t i o n f and wear K. = , S P where A h i s l i n e a r w e a r of t h e polymeric specimen; S i s the rubbed distance, and P is the load o n the specimen. T h e s e l e c t i o n of f r i c t i o n p a r a m e t e r s i n o r d e r to d e t e r m i n e the a b o v e c r i t e r i a for e v a l u a t i n g the friction p r o p e r t i e s was b a s e d o n t h e a n a l y s i s a n d s t a t i s t i c a l treatment of t h e d a t a o n t h e r e g i m e s o f f r i c t i o n u n i t s o p e r a t i o n w h e r e i t i s b e n e f i c i a l to use self-lubricated polymeric materials. T h e characteristic o p e r a t i o n r e g i m e s w e r e f o u n d to b e i n t h e r a n g e of s l i d i n g velocities from 0.01 u p to 5 . 0 m/s and loads from


22.

SVIRIDYONOK AND

KIRPICHENKO

Friction Behavior Evaluation

337

0.1 u p to 20 M P a . 9 0 % of t h e u n i t s e x a m i n e d s h o w e d a s u r f a c e - t o - s u r f a c e contact; 80% of t h e s e were cylinders with i n s i d e c o n t a c t s and 20% were bearings with butt-end c o n t a c t s . T h u s , the s c h e m a t i c "shaft-partial bearing i s most s u i t a b l e f o r a g e n e r a l i z e d m o d e l of t h e f r i c t i o n c o n t a c t . The procedure suggested is r e a l i z e d as follows: the c o m p l e x of l o a d s a n d v e l o c i t i e s c o r r e s p o n d i n g to t h e l o s s of working c a p a c i t y b y the p o l y m e r i c m a t e r i a l i s f o u n d automati cally with friction machines UMT, SMT-1 and computer system SM-1, F i g u r e 1. A minimum l o a d from Table I is a p p l i e d to a c l e a n e d r u n - i n s p e c i m e n w i t h a b l o c k of l o a d s e l e c t i o n . T h e b l o c k of v e l o c i t y s e l e c t i o n p r o v i d e s a minimum s l i d i n g v e l o c i t y from Table I as well. S i g n a l s from the d e v i c e s that m e a s u r e the t e m p e r a t u r e , f r i c t i o n moment a n d w e a r go to t h e r e g i s t e r w h e r e t h e y a r e r e c o r d e d ( F i g u r e 2 ) and to t h e b l o c k s h o w i n g t h e e x t r e m e v a l u e s . A f t e r t h e t e m p e r a t u r e and the f r i c t i o n moment b e c o m e s t e a d y , the b l o c k s h o w i n g the extreme values, forms a s i g n a l being recorded by a bridge de vice w h i c h f o r c e s t h e v e l o c i t y - s e l e c t i o n - b l o c k to g i v e another v a l u e from T a b l e I. A f t e r t h e w h o l e s e t of v e l o c i t i e s were taken or a limiting l o a d r e a c h e d for a g i v e n material, the s i g n a l f r o m t h e p u l s e c o u n t e r g o e s to the c o i n c i d e n c e c i r c u i t w h i c h s w i t c h e s on the l o a d i n g d e v i c e that p r o v i d e s a next l o a d l e v e l . F o r this l o a d l e v e l the s a m e algorithm g i v e s a limiting s l i d i n g velocity. A f t e r t h e r e s u l t s of a g r o u p of t e s t s h a v e b e e n t r e a t e d m a t h e m a t i c a l l y , a c u r v e i s c o n s t r u c t e d i n t h e c o o r d i n a t e s of load (P) and velocity (v) c o v e r i n g t h e s e t of l i m i t i n g loads and velocities [PV] . T h e s e t of l o a d s a n d v e l o c i t i e s at w h i c h t h e w e a r r a t e reaches 0.25 yum/h f o r t h e p o l y m e r i c m a t e r i a l i s d e t e r m i n e d from the c u r v e of t h e [PV] l i m i t i n g values. From Table I a minimum v e l o c i t y i s s e l e c t e d and a l o a d a p p l i e d that p r o v i d e s p r e s s u r e o n t h e f r i c t i o n u n i t of o.5 [PV] . A f t e r 1 0 0 h o p e r a t i o n the w e a r i s m e a s u r e d . S h o u l d its v a l u e be o v e r 25 yum, a n o t h e r t e s t i n g i s c o n d u c t e d at a l o w e r l o a d i n g l e v e l . S h o u l d the w e a r be b e l o w 25 yum, t h e l o a d i s i n c r e a s e d up to a n e x t l e v e l . S i m i l a r t e s t s a r e c o n d u c t e d f o r a l l t h e r e s t r a n g e s of sliding velocities. The test r e s u l t s are presented as c u r v e s f r o m w h i c h a c o m p l e x of l o a d s a n d v e l o c i t i e s i s s e l e c t e d t h a t c o r r e s p o n d s to w e a r r a t e of 25 yum after 100 h o p e r a tion (Figure 3). T r i b o t e c h n i c a l p r o p e r t i e s of 30 p o l y m e r - b a s e d m a t e r i a l s w i d e l y u s e d i n e n g i n e e r i n g h a v e b e e n s p e c i f i e d a c c o r d i n g to the U S S R T e s t i n g M e t h o d M R - 7 4 - 8 2 . T h e h i g h d e g r e e of r e p r o d u c i b i l i t y a l l o w s a s u c c e s s f u l a p p l i c a t i o n of t h e d a t a obtained in designing practice. T h e w e a r r a t i o of t h e m a t e r i a l s t e s t e d t h a t c o r r e s p o n d s to t h e w e a r r a t e of 0.25 yUm/h i s characterized by a wide r a n g e of v a l u e s f r o m 0.2 u p to 2.1, w h i c h d e p e n d to a l a r g e extent on the t e m p e r a t u r e i n the f r i c t i o n z o n e . T h u s b y u s i n g t h e d e p e n d e n c e of t h e w e a r r a t i o o n t h e t e m p e r a t u r e i t i s p o s s i b l e to f o r e c a s t t h e p e r f o r m a n c e of a t r u e f r i c t i o n u n i t as it had b e e n s i m i l a r l y d o n e elsewhere (18, 2 l ) . 1 1


338


F i g u r e 1. C e n t e r w h e r e t r i b o t e c h n i c a l p r o p e r t i e s of materials a r e tested. T e s t s a r e c a r r i e d out on standard machines SMT-1, and computer system SM-1.

UMT

T, K

450

- 0.3 -

400

-

350

-

300

F i g u r e 2. D e p e n d e n c e of f r i c t i o n z o n e t e m p e r a t u r e ( T ) ( c u r v e 2 ) a n d t h e c o e f f i c i e n t of f r i c t i o n ( f ) ( c u r v e l ) upon PV. F o r l o a d v a l u e s from Pmin u p to P m a x with stepwise v e l o c i t y i n c r e a s e , a limiting v a l u e f o r s l i d i n g v e l o c i t y (Vmax) i s found at w h i c h the m a t e r i a l suffers f r i c t i o n a l f a i l u r e . I n t h e c o u r s e of t e s t i n g t h e c o e f f i c i e n t of f r i c t i o n ( c u r v e l ) a n d t e m p e r a t u r e i n t h e s u b s u r f a c e l a y e r of the p o l y m e r i c s p e c i m e n ( c u r v e 2) are recorded.



Ρ,

V,

·

I.

s

M P a

m

Table

-1

V a l u e s of

0.25

1.25

0.5

0.25

0.1

Velocities

0.1

and

0.05

(P)

0.01

Loads

(v)

50.0 20.0 10.0

5.0

2 5

2.5

V

5.0

P

3.5

and

2.5

[PV]

1.0

Finding

0.5

for

3

δ*

§r•

Si δ*

I

5

Ο

η χ m

α

>

Ο

§

c/a

Standardization of Laboratory Methods to Evaluate Friction Behavior

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