Self-lubrication mechanisms in polymer composites

Self-lubrication mechanisms in polymer composites A.I. Sviridyonok*

State of the art in the field of self-lubrication mechanism studies is considered. Experimental data, collected by tribologists in many countries are used for the development of quantitative approach to friction transfer and its part in self-lubrication. It is shown that understanding of selflubrication behaviour could be used in development of the efficient polymer composites designed for operation in friction assemblies without lubrication.

Keywords:

self-lubrication, friction transfer, polymer composites

Introduction The fields of application of self-lubricating materials in friction assemblies, where external supply of lubricants is impossible or not recommended, are growing in number. Today they include space and aeronautical technologies, motor transport, agricultural machines, vacuum and cryogenic instruments, medical and food processing equipment, metalworking centres, robots, electronic computers and electric drives, drying, textile and chemical equipment, domestic devices, etc. The modern triboengineering materials for such applications are mostly compositions of metallic, polymeric or ceramic matrices and functional fillers providing one or several mechanisms of self-lubrication. One of the most important elements of self-lubrication is the process of frictional transfer, ie the formation, in the friction zone, of a continuous or, more often, discrete interlayer-a 'third body'-separating the contacting surfaces and participating actively in the friction and wear processes. It is impossible to develop

Metal

adequate models, or fundamental friction and wear theories, for assemblies operating in the self-lubricating regime, without taking frictional transfer into account. All this became evident when polymeric materials were first used. Early work in this field appeared in the 1950s ~, but the most significant research into frictional transfer was undertaken some two decades ago, and this topic now attracts more and more attention world-wide. Considerable amounts of data have been accumulated and published in reviews, monographs and numerous original articles, eg Refs 2 to 13. A range of most important problems has been specified for which solutions are necessary to reveal the role of transfer in the friction and wear of polymers and polymer-based composites. Amongst those of greatest importance are the following: • Estimation of the kinetics of the process; morphology and quantitative characteristics of the transferred products depending on their structure and surface properties, external load-and-velocity and temperature parameters. • Analytical description of the process of formation and frictional behaviour of the 'third body', aimed at predicting the performance of friction assemblies.

~P ~rG~ucts of friction transfer

We ;c,es

• Search for methods of control for friction transfer. Let us consider in brief some results relating to the solution of these problems.

Experimental results

Fig 1 Fricton process pattern

Success in understanding the nature of friction transfer is wholly dependent on methodical experimental work. Only the application of precise physical and chemical methods of surface analysis, structural transformations, and geometric and physico-mechanical parameters is able to reveal relationships important for understanding the phenomenon in question.

*Metal-Polymer Research Institute of the USSR Academy of Sciences, Gomel, 32a Kirov Str., USSR.

The techniques of electron spectroscopy, IR-spectroscopy and ellipsometry have shown that the mor-

/ Composite

TRIBOLOGY INTERNATIONAL

0301-679X/91/020037-7 © 1991 Butterworth-Heinemann Ltd

37

A. I. Sviridyonok--Self-lubrication mechanisms in polymer composites

Fig 2 Photomicrographs of friction-transferred products." (a, b) HDPE," (c)--(e) PCA," (f) PTFE

38

February 91 Vol 24 No 1

A. I. Sviridyonok--Self-lubrication mechanisms in polymer composites

x103

-

10.0 7.5

_~

5.0

a_

2.5

6

z ~n •~c

4 2

e~ I ,8

3.6

5.4 t,

7.2

mS

Fig 4 Dependence on testing time of." (a) rate of A E counts; (b) total AE counts; (A) period of failure and of transfer film formation; (B) (TI, rz) life period of transferred layers; (1) PCA + HDPE," (2) AG4V + HDPE

PTFE

HDPE

PMMA

(160.5)

(267.5)

(376.2)

PA-6

Steel 45

(505.78)(12.54

x 103 )

ID

125.4

I

I

209

292.6

I

376.2

459.8

1

543.4

.....J

Ec

J cm -3

Fig 5 Direction of transfer at friction contact of materials having different cohesive energy densities E,

Fig3 Photomicrographs of (a) friction-transferred products" of amorphous PMMA; and (b) wear debris phology of the friction-transferred elements is rather diverse and the thickness of the transferred polymer layers can range from a few monomolecular layers to tens of micrometres. With the help of electron paramagnetic resonance, ESCA, X-ray structural analysis, mass spectrometry, etc the kinetics of structural rearrangements and structural transformations within the transferred layers have been studied down to deep thermal- and mechanodestruction of polymers. Acoustic emission techniques have confirmed the cyclic nature of the formation of the 'third body' and helped in the estimation of its 'life' cycle. It is difficult to find any universal dependences governing transfer, since for every friction pair there exists an optimum correlation between the load, speed and temperature which determines the optimum kinetics of the transfer process to give the most favourable antifriction characteristics. The main elements of this phenomenon can be estimated qualitatively. It has been shown that the direction of material transfer is associated with the TRIBOLOGY INTERNATIONAL

cohesive energy density, Ec, ie a material with a lower Ec is transferred onto a material with a higher Ec3"9"11"12. In a metal-polymer contact, polymerpolymer rubbing occurs at least partially, owing to transfer of the polymer onto the metal. Even the initial stages of friction lead to considerable structural transformations of the surface layers. There is a general correlation between the friction and the surface activity and molecular mobility of the polymeric chains; wear depends on the thickness of the transferred layers, their ability to remain on the mating surfaces, and the resistance to repeated deformation (fatigue). The study of molecular weight distribution has shown that for metal-polymer contacts the activity of the mating metallic surface is important for the dispersion of the transfer products. For instance, when high density polyethylene is rubbed against copper, the proportion of low molecular weight fraction is larger than when in contact with steel or aluminium. The surface roughness resulting from processing only slightly influences the frictional characteristics during transfer. For thermoplastics, temperature is the decisive factor; its optimum values lead to thin, wear-resistant, easily removed, surface layers. Above the limiting temperature, however, the wear of polymers increases sharply. 39

A. I. Sviridyonok--Self-lubrication mechanisms in polymer composites x l O -4 4

wo;-

2

Fig 7 Calculation scheme

2

4

6

8

x 10-q

M

Fig 6 Differential curves of molecular weight distribution of HDPE transfer products during rubbing: (1) Al, Cu; (2) steel 45 (in air); (3) Cu, steel 45; (4) Al (in vacuum) Analyses of results obtained on the friction transfer process has revealed the key elements of this mechanism of self-lubrication. Schematically, for a single contact spot, it can be represented as a sequence of events such as contact-adhesive interaction, shear of the surface layer, repeated plastic deformation, separation of a particle from the material of lower cohesive energy density, transfer onto the counterface, repeated deformation of the transferred layer, fatigue damage, dispersion and removal from the friction zone. This process (or its individual elements) is of a cyclic nature. The above is an idealized representation in many respects since, in reality, many of the elements of the process combine. Part of the material separated from the surface is usually found in a free-moving state, part of it may be in a molten and highly degraded state, the assembly design may prevent removal of the products from the friction zone, etc. However, the main elements of the process in question take place in most instances of frictional transfer. It should be emphasized that experimental data are available demonstrating the fact that frictional transfer is most active in vacuum as well as under the influences of an electric current. It has also been observed in boundary lubrication as well as during partial hydrodynamic lubrication. This is why, in the ideal situation, all the materials used in the production of friction parts should possess self-lubricating properties.

Theory Attempts have been made to describe analytically the process of frictional transfer from the viewpoint of the 40

adhesive model of friction interaction, and by using the theory of mass transfer 4""'~2.j4 ~7. Two important factors are worth mentioning which must be taken into consideration. First, the actual stresses developed during loading of a real contact; second, the molecular concept of polymer adhesion. When all the physical processes occurring in contacts are considered, the existing methods of estimating the influence of roughness do not give a complete picture, since the known calculations consider only so-called technological roughness, as specified by the appropriate standards. Experimental data are available showing that the technological roughness is covered with minor asperities, ie a sub-roughness. The Metal-Polymer Research Institute suggested, and has now verified, a two-level model for estimating the role of the sub-roughnesses that accounts for the joint contact deformations of the asperities of two levels (level I is the surface roughness resulting from the technological processing, which is measured by optical or probe techniques; level II is the sub-roughness characterizing asperities of molecular and atomic sizes ~s,2°. The asperities of each of the two levels are simulated by spheres with constant radii for each level. To make the model, the known procedure of calculating the actual contact area was employed except that the solutions of the problem of a rough elastic sphere on a smooth flat surface were applied instead of the Hertz dependences 2~. The dependence of the actual contact area on load is represented as a parameter function with respect to the dependence of the level I asperities d~:

A(d,) =A,D,

A , ( z - d , ) ~ , (z) dz Ii

P(d,) = A.D1

P , ( z - d , ) ~ , ( z ) dz

(1)

I1

where A, is the nominal contact area, A ~(x) and P(x) are solutions of the problem of rough sphere contact and D~ and qb(z) are the density and the distribution law of the roughness tips respectively. February 91 Vol 24 No 1

A. I. Sviridyonok--Self-lubrication mechanisms in polymer composites

w h e r e ~ i s Planck's constant, l is the gap between the surfaces, and ~ is a complex parameter describing the overlapping of the absorption spectra including interlayers (of the 'third body').

-3

The calculated results (from Eq (3)) and those from specific stimulation experiments23 indicated that an increased molecular interaction estimated from the absorption spectra leads to a corresponding increase in the friction force from one pair of polymeric materials to another.

3 -tt

~o

..J

-5 -6

-5

-Lt

I -3

Log P/E'A a

Fig 8 Real contact area versus load taking account of sub-roughness: ( 1 ~ 0.2; (2) 8; (3) 1; (4) deleted. I~ = (8/3)~2D2V2R1R2 is a complex non-dimensional topographic parameter, where ~2, D2 and R2 are height, density and sub-microasperity radius respectively and RI is microasperity radius The relationship between the deformation characteristics and the height of the different levels is described by: z-d1 =wo-d2

(2)

where z and w, are the height and deformation along the middle line of a microasperity, respectively, and d2 is the deformation extent of a sub-roughness (level II asperities) covering the microasperities in question. The calculated estimates of the main parameters of the asperities (ie height, density, radius of curvature) showed that the sub-roughness markedly affects the actual contact area. Its magnitude, as compared to the calculated area of contact where only level I asperities were considered, can drop by 10 to 100 times, depending on the parameters of the sub-roughness and the load in the contact. This fact is of importance in developing a theory and calculation techniques for estimation of the friction characteristics taking into account the atomic-molecular interactions of polymeric elements. The theory of molecular forces based on the idea of fluctuating electromagnetic fields found in condensed solid surfaces seems promising for the understanding and analysis of the molecular mechanism of surface phenomena during polymer friction with account being taken of the 'third body'. An important conclusion resulting from this theory is the fact that molecular bonding of surfaces, depending on their atomic-molecular structure and composition, is in fact determined by the spectroscopic characteristics of polymers. The strength of the molecular surface bonding is higher, the more the spectra overlap. For polymers the IR-region (from 1 to 25 t.Lm) is of great importance. The bonding force can be calculated 22 from: kf F = 8,n. l - 2- ~ rP TRIBOLOGY INTERNATIONAL

(3)

The fluctuation theory of molecular forces applied to friction and wear problems permits a common approach to the influence of various friction events upon the adhesive properties of polymers. The aim is to analyse the optical and dielectric properties of materials over a wide frequency range and the way in which these properties change in the course of frictional contact. Thus, an approximate understanding of the gaps (based on the developments of multi-level models) and the intensity of the molecular interaction in a contact is helpful not only in calculating the friction forces, load-velocity and temperature conditions of surface damage, but also in controlling the friction behaviour of the 'third body' (ie fragments of friction transfer) as well.

Mechanisms

of self-lubrication

Understanding the mechanisms of the frictional transfer process assists in finding efficient ways of controlling lubrication, ie the durability and energy parameters in tribotechnology, The main idea of developing selflubricated composites is to introduce into the surface layers of a material substances which, as a result of mechanical, physical or chemical effects during friction, can form on the mating surfaces 'long-lived' thin films of low surface activity and high fatigue resistance. In this case self-lubrication is produced by: • Interface sliding of structurally anisotropic components such as graphite, molybdenum disulphide or diselenides. •

Inter-chain sliding in linear thermoplastics such as polytetrafluoroethylene or polyolefins.

• Surface melting of fusible components such as lead, tin or polyethylene. • Surface thermal decomposition of metal-containing chemical compounds like formates or oxalates of ductile metals. • Formation of low-molecular weight fragments of polymeric molecules and their tribodestruction. PE

,

/,/-

Oil

Graphite

/

Fig 9 Functioning of a polymeric composite 41

A. I. Sviridyonok--Self-lubrication mechanisms in polymer composites Table I Antifriction self-lubricating composites based on polymers Parameters

Ultimate

SAM-3, based on polyamide

ADP-2, based on wood

SPAM-2

F4-VM

80

150

250

60

205

220

350

300

0.1-0.2

0.15-0.2

0.1

<0.10

compression stress, MPa Vicat softening point, °C Coefficient of friction Wear rate, tzm km

PVmax,

1.5 × 10 8 1.6 x 10 6 2 × 10 8 1.8 × 10 9 1

1.25

2

1.5

MPams

surface layer. Oil, g r a p h i t e and p o l y e t h y l e n e form lubricating layers of friction t r a n s f e r on the m a t e d surfaces d u r i n g frictional interactions, thus p r o v i d i n g low resistance to relative m o t i o n and high w e a r resistance. I n t e r f e r e n c e to the s t e a d y - s t a t e r e g i m e , o b s e r v e d w h e n increasing the load and velocity, causes heat l i b e r a t i o n . A t t e m p e r a t u r e s a b o v e 100°C local melting takes place in p o l y e t h y l e n e and it functions as a highly viscous lubricant; for e m e r g e n c i e s when t e m p e r a t u r e s reach 320-350°C lead melts, thus p r o t e c t i n g the surfaces against sticking and c a t a s t r o p h i c d a m a g e . W i t h the recent u n d e r s t a n d i n g of the m e c h a n i s m s of frictional t r a n s f e r and their role in ensuring sell'lubrication, r e s e a r c h e r s in the M P R I - M e t a I - P o l y m e r R e s e a r c h Institute have d e v e l o p e d a series of triboengin e e r i n g m a t e r i a l s w h o s e m a i n p r o p e r t i e s are listed in T a b l e 1. T h e s e m a t e r i a l s are now widely used in the m a n u f a c t u r e of b e a r i n g s , gears, s p r o c k e t s , seals and coatings.

References 1. Mittal D.C. and Pratt G,C. Friction. wear and physical properties of some filled PTFE bearing matcrials. Paper 101. P m c I Mech. E. Luh. and Wear ConiC, London, 19.58, 416-423 2. Makinson K.R. and Tabor D. The friction and transfer of PTFE. Proc. Roy'. Soc., 1964, Ser. A281, (1384). 49 3. Belyi V.A., Sviridyonok A.I., Petrokovets M.i. and Savkin V.G.

Friction and wear of polymer-based materials. Nauke i Technike, Minsk, 1976

4. Pogosyan A.K. and Oganesyan K.B. The friction transfer phenomenon: basic relationships and study methods. Soy. J. Frict. Wear (USSR), 1986, 7, (6), 31-38 5. Briscoe B.J. Tribology of polymers: state of an art. Physicochemical aspects of polymer surfaces, Proc. Int. Syrup. of ACS Meeting, 1981, New York, 387-412

6. Lancaster J.K., Play D., Godet et al Third body and wear of dry bearings based on PTFE fibres. Prob. Frict. WearJ. (USSR), 1980, (2), 114-125 7. Polymer wear and its control, ed L.H. Lee. ACS Symposium, Ser. 287, 1985, 422

8. Buckley D. Surface effects in adhesion, friction, wear and lubrication. Elsevier, 1981, 631 9. Kholodilov O.V. Role of operating regime in the mechanism of polymer wear and frictional transfer. Soy. J. Frict. Wear, 1984, 5, (4), 637-643 10. Korshak V.V., Gribova I.A. and Krasnov A.P. Tribochemical lubrication at polyheterarylines friction. High Mol. Wt Comp. J. ( USSR), 1985

11. Lancaster J.K. Transfer lubrication for high temperatures. A review. Trans. ASMB: J. Tribology, 1985, 107, (4), 437 Fig 10 T r i b o e n g i n e e r i n g c o m p o s i t e s based on p o l y m e r s : (a) m i x t u r e s o f polymers'; (b) w o o d

12. Jain V.K. and Rahadur S. Material transfer in polymer-polymer

sliding. Wear, 1978, 46, (1), 177-188 13. Konchits V.V., Meshkov V.V. and Myshkin N.K. Tribology of

Electrical Contacts. Nauka i Technika, Minsk, 1986, 256 • L i b e r a t i o n from m i c r o c a p s u l e s of liquid substances such as lubricating or p o l y m e r - f o r m i n g c o m p o u n d s . • T h e effect of high electric fields ( l u b r i c a t i o n by electric c u r r e n t s ) o r ionizing r a d i a t i o n ( e l e c t r o n o r g a m m a rays) d u e to the f o r m a t i o n of a m o l e c u l a r s m o o t h s t r u c t u r e , t r i b o d e s t r u c t i o n etc.

14. Belyi V.A., Kholodilov O.V. and Sviridyonok A.I. Acoustic

spectrometry as used for the evaluation of tribological systems. Wear, 1981, 69, (2), 309-319

15. Makushok E.M., Kalinovskaya T.V. and Belyi V.A. Mass Transfer in Friction Processes. Nauka i Technika, Minsk, 1978, 272

Z. The role of interfacial energy in the wear of polymeric journal bearings Wear, 1985, 104, (1), 6.5

16. Rymuzs

F o r a c o m p o s i t e m a t e r i a l it is w o r t h w h i l e to p r o v i d e several stages of l u b r i c a t i o n , for instance, by i n t r o d u c ing g r a p h i t e , oil-filled p o l y e t h y l e n e or l e a d into the 42

S. Estimation of wear particle thickness in polymer-metal sliding Wear, 1980, 63, 105-112

17. Kar M.K. and Bahadur

F e b r u a r y 91 V o l 24 N o 1

18. Belyi V.A., Petrokovets M.I. and Sviridyonok A.I. Some theoretical aspects of polymeric materials friction. In: Tribology and Antifriction Materials Science, Novocherkassk, 1980, 167-168

Soy. J. Frict. Wear, 1985, 6, (6), 982-989 21. Sviridyonok A.I., Korochkina T.V., Petrokovets M.I. and Chizhik S.A. On real contact area of rough spheres. Soy. J. Frier. Wear, 1985, 6, (1), 20-26

19. Demkin N.B. and Izmailov V.V. Machine parts surfaces and performance properties of contacts. Surface J. (USSR), 1982, II, 16-27

22. Belyi V.A., Smurugov V.A., Sviridyonok A.I. and Savkin V.G. Molecular interactions in polymers in friction contact zone. AS USSR Reports, 1978, 241, (3), 573-575

20. Sviridyonok A.I., Chizhik S.A. and Petrokovets M.I. Statistical model of elastic contacts with reference to the subroughness.

23. Belyi V.A., Sviridyonok A.I. and Smurugnv V.A. Surface events at friction. Surface J. (USSR), 1986, (3), 130-140

TRIBOLOGY INTERNATIONAL

43