EFFECT THE IN

OF SURFACE

SLIDING P.

LIQUID LAYERS

MEDIA OF

ON A

THE

DEFORMATION

POLYMER-METAL

OF PAIR

FRICTION V.

Nazarenko

and

F.

I. Skripnik

UDC 678.539.621+620.193.5

The r e l a t i o n between the M g h - e l a s t i c component of p o l y u r e t h a n e d e f o r m a t i o n and the d e f o r m a tion of Lhe s u r f a c e l a y e r s of m e t a l s h a s been investigated for sliding friction in v a r i o u s media. The smfface l a y e r s of p o l y m e r and m e t a l a r e p l a s t i c a l l y deformed. T h e r e is a c e r t a i n c o r r e lation between the coefficient of friction and the amount of deformation.

The study of the d e f o r m a t i o n p r o c e s s e s in the s u r f a c e l a y e r s of a p o l y m e r - m e t a l friction p a i r in the p r e s e n c e of l u b r i c a t i n g agents is of i n t e r e s t in connection with the engineering applications of polyurethanes. P o l y u r e t h a n e s have high w e a r r e s i s t a n c e , excellent damping p r o p e r t i e s , h a r d n e s s , flexibility, and elasticity and, m o r e o v e r , r e s i s t s t h e a c t i o n of v a r i o u s r e a g e n t s . Ths m a k e s t h e m suitable f o r u s e as a sealing m a t e r i a l in a r t i c u l a t e d a s s e m b l i e s with applications in the a i r c r a f t industry [1]. We have investigated the c o r r e l a t i o n between the h i g h - e l a s t i c component of the polyurethane d e f o r m a tion and the d e f o r m a t i o n of the s u r f a c e l a y e r s of m e t a l in sliding friction in the p r e s e n c e of l u b r i c a t i n g m e d i a . As l u b r i c a n t s we used MS-20 and MK-8 + 25% MS-20 aviation oils, the aviation fuels T - 1 k e r o s e n e and B-95/130 gasoline, AMG-10 aviation oil for hydraulic s y s t e m s , and AM-70/10 a l c o h o I - g l y c e r o l shocka b s o r b e r fluid, since these a r e the m e d i a in which m o s t articulations function. The e x p e r i m e n t s w e r e p e r f o r m e d on a specially designed friction machine that m a d e p o s s i b l e the combined m e a s u r e m e n t of the d e f o r m a t i o n of the p o l y u r e t h a n e in e x t e r n a l friction and t h e f o r c e of friction [2]. The p h o t o e l a s t i c method was u s e d to i n v e s t i g a t e the h i g h - e l a s t i c s t r a i n s . The d e f o r m a t i o n was d e t e r mined f r o m tt~e intensity of illumination of the i n t e r f e r e n c e f r i n g e s in p o l a r i z e d light [3 ]. The d e h ) r m a t i o n of the s u r f a c e l a y e r s of the o t h e r m e m b e r of the friction p a i r was e s t i m a t e d f r o m the change in dislocation density. L i F s a l t and s i n g l e - c r y s t a l zinc, in which the d i s l o c a t i o n s a r e c l e a r l y r e v e a l e d by etching, w e r e e m p l o y e d in the f o r m of plates m e a s u r i n g 5 × 10 × 20 m m s e c u r e d in the jaws of an a t t a c h m e n t on the f r i c t i o n machine. The moving s p e c i m e n was a ring of p o l y u r e t h a n e with an outside d i a m e t e r of 26 m m . The Shore A h a r d n e s s of the p o l y u r e t h a n e was 56. In Figs. 1 and 2 the dislocation density, h i g h - e l a s t i c strain, and coefficient of friction a r e plotted against the frfLction path for a p o l y u r e t h a n e - l i t h i u m fluoride p a i r in the p r e s e n c e of a lubricant. AS the f r i c t i o n path i n c r e a s e s , the h i g h - e l a s t i c s t r a i n a t f i r s t i n c r e a s e s , then attains a constant value. The a b s o lute magnitude and the n a t u r e of the i n c r e a s e in h i g h - e l a s t i c s t r a i n depend on the lubricant. In the p r e s e n c e of MS-20, MK-8 + 25% MS-20, and AMG-10 oils the absolute h i g h - e l a s t i c s t r a i n does not exceed 0.3% at a n o r m a l load N = 0.3 kgf (in all c a s e s the velocity was constant), w h e r e a s under the s a m e f r i c t i o n conditions the p r e s e n c e ef aviation fuels and a l c o h o l - g l y c e r o l m i x t u r e caused i n c r e a s e s in the h i g h - e l a s t i c d e f o r m a tion of the p o l y u r e t h a n e up to 0.53-0.83%. The coefficient of f r i c t i o n also depends on the n a t u r e of the lubricant. It i n c r e a s e s with i n c r e a s e in the h i g h - e l a s t i c s t r a i n s , but the dependence is not direct: w h e r e a s the m ~ x f m u m coefficient of f r i c t i o n (0.45) is o b s e r v e d in the p r e s e n c e of B-95/130 gasoline, which is evidently a s s o c i a t e d with an i n c r e a s e in the adhesion Kiev Institute of Civil Aviation Engineers. T r a n s l a t e d f r o m Mekhanika P o l i m e r o v , No. 1, pp. 147-149, J a n u a r y - F e b r u a r y , 1970. Original a r t i c l e s u b m i t t e d April 1t, 1969. © 1972 Consultants Bureau, a division of Plenum Publishing Corporation, 227 g%st 17th Street~ New York, N. Y. 10011. All rights reserved. This article cannot be reproduced [or any purpose whatsoever withouL permission of the publisher. A copy of this article is available from the publisher for $15.00.

131

0.7

o,s~ 6,0"~

u

[

'

o/ ~0~'

0

O,J

0.3

tO00

~000 3000 0000

F r i c t i o n p a t h , ram

Fig. 1

JO00~ 0 £ ~

3

Frictiorz path, mm

Fig. 2

Fig. 1. Variation of dislocation density (1-3), coefficient of friction (4-6), and high-elastic strain (7-9) with increase in the friction path for a lithium fluoride-polyurethane friction pair: 1, 4, 7) dry friction; 2, 5, 8) in MS-20 oil; 3, 6, 9) in MK-8 + 25% MS-20 oil. Fig. 2. Effect of liquid media on the variation of the dislocation density (1, 4, 7, 10), coefficient of friction (2, 5, 8, 11), and high-elastic deformation (3, 6, 9, 12) with increase in the friction path for a lithium fluoride-polyurethane friction pair: 1-3) in B-95/130 gasoline; 4-6) in AMG-10 oil for hydraulic systems; 7-9) in T-1 gasoline; 10-12) in AM-70/10 alcohol-glycerol mixture (glycerol 70%, alcohol 20%, water 10%). between the rubbing parts, under the same conditions the greatest Mgh-elastic strain (0.83%) is observed inthe presence of T-1 kerosene. This is obviously associated with changes in the physicomechanical properties of polyurethane exposed to friction in various media. Different liquids plasticize polyurethane (weaken the intermolecular forces by penetrating into the amorphous regions) differently: T-1 kerosene, AM-70/10 mixture, and B-95/130 gasoline have stronger plasticizing properties than lubricating oils. The reduced plasticizing effect of the oils as compared with the fuels is evidently attributable to the presence in the oils of various polymers-polyisobutylenes, polymethaerylates, e t c . - a s special viscosity additives. The large molecules of these polymers, like long thread or fibers, reduce the cross section and retard the flow of the low-viscosity component of the oil preventing it from penetrating into the amorphous regions of the polyurethane [4]. A comparison of the graphs in Figs. 1, 2, and 3 shows that both the absolute magnitude and the nature of the increase in the high-elastic strain are less dependent on the nature of the other member of the friction pair: there is little difference in the absolute values of the high-elastic strains for friction against lithium fluoride crystals and single-crystal zinc. Despite the fact that the hardness of the polyurethane is much less than that of metal, in particular, that of zinc and lithium fluoride crystals, the surface layers of these materials deform plastically in friction against polyurethane. The dislocation density depends on the friction conditions and the medium. The plastic deformation of the surface layer of lithium fluoride in sliding friction against polyurethane develops on the f i r s t two to four meters, after which it is stabilized and remains almost constant. The dislocation density is directly dependent on the coefficient of friction: the greater the coefficient of friction, the higher the dislocation density. Thus, the greatest coefficient of friction (0.44) and the most rapid increase in dislocation density with friction path are observed for sliding friction in a B-95/130 gasoline medium. Under the same friction conditions the dislocation density for zinc single crystals is somewhat less with respect to both absolute magnitude (the maximum value reaches 1.5 • 107 - c u r v e 1in Fig. 3) and rate of increase as a function of the friction path (the maximum value is reached after 3.5-4 m).

132

g)

~8

=t

o,6 't~/0'[

o.5

o

°l

3

o,8 °~

io

o

20O0

4O00

F r i c t i o n p a t h , mm

Fig. 3. Variation of the dislocation density (1, 2), high-elastic deformation (3, 4), and coefficient of friction (5, 6) with increase in the friction path for zinc-polyurethane friction pair: 1, 3, 6)dry friction; 2, 4, 5) friction in MS-20 oil. Subsequent polishing and etching of the zinc surface showed that at a normal load of 0.3 kgf the depth to which the dislocations extend is 10-20 ~. LITERATURE I.

2. 3. 4,

CITED

Structural Properties of Plastics [in Russian], Moscow (1967). P. V. Nazarenko, O. V. Zaitsev, I. 5. Ag~ov, and B. 5. Kostetskii, Mekhan. P o l i m , No. 3, 539 (1967). P. V. Nazarenko, O. V. Zaitsev, and B. 5. Kostetskii, Mashinovedenie, No. 5 (1967). A. F. /M~senov, Aviation Fuels, Lubricants, and Special Fluids [in Russian], Moscow (1965).

133