- Email: [email protected]
Friction and wear properties of metal powder filled PTFE composites under oil lubricated conditions
•
...-a.
WEAR ELSEVIER
Wear 210 (1997) 151-156
Friction and wear properties of metal powder filled PTFE composites under oil lubricated conditions Zhao-Zhu Zhang *, Qun-Ji Xue, Wei-Min Liu, Wei-Chang Shen Laborato~" of Solid Lubrication. Lanzhou Institute of Chemical Physics, ChineseAcademy of Sciences. Lan~ou. 730000. People's Republicof China Received I I September |996; accepted 11 March 1997
Almract Four kinds of polytetrafluoroethylene (PTFE)-based composite, pure PTFE, PTl~+3Ovoi.%Cu, PTFE+30vol.%Pb and + 30vol.%Ni composite, were prepared. The friction and wear properties of these metal powder filled FIFE composites si:ding against GCri5 bearing steel under both dq:, and lubricated condLions were studied using an MHK-500 ring-block wear tester. The worn surfaces of the ~ composites and the transfer films formed on the surface of GCr 15 bearing steel were examitted using scanning elcctrcnt microscopy (SEM) and optical micrc:scopy respectively. Experimental results show that the friction and wear properties of the PTFE composites can be greatly improved by liquid paraffin lubrication. The wear of these PTFE composites can be decreased by at least I to 2 orders of rnagnittule compared with that under dry friction conditions, while the friction coefficients can be decreased by I older of magnitude. SEM and optical microscopy investigations of the rubbing surfaces show that metal fillers of Cu, Pb and Ni not only raise the load carrying capacity of the PTFE :omposites, but also promote transfer of the PTFE composites onto the counterfaces, so they greatly reduce the wear of the PIFE composites. However, the transfer of these PTFE composites onto the counterfaces can be greatly reduced by liquid paraffin lubrication, but transfer still takes place. © ! 997 Elsevier Science S.A. ICeywords: PTFEcomposites: Metal fillers; Friction and wear; Oil lubrication: Frictional surfaces
1. Introduction PTFE-based self-lubricating composites have been successfully used in many fields, so the friction and wear mechanisms of trI'FE-based composites have been studied by many coworkers [ I-5]. However, almost all of the studies concerned dry friction conditions (unlubricated conditions). Briscoe et al. [ 6-8 ] studied the lubricated friction and wear of some polymers. They found that the absorption of fluid into the surface layers of polymers can change the mechanical p~operties of the polymers, and so, in turn, influence the friction and wear of the polymers. Watanabe et al. [9-1 ! ] investigated the friction and wear behavior of polymer composites in aqueous environments (water). They pointed out that many polymers wear much more in water than in air, and the wear of F r F E composites filled with only glass fibers is much greater thar, that of other composites in water. However, with enlargement of the application fields of FTFEbased composites in practice, it is essential to study the friction and wear behavior of PTFE-based composites under oil lubricated conditions. Until now, much less information * Corresponding author. 0043-1648/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved Pil S 0 0 4 3 - 1 6 4 8 ( 97 ) 0 0 0 5 2 - 5
has been available on the friction and wear behavior of PTFE composites under oil lubricated conditions. The purpose of this study is to investigate the friction and wear behavior of PTFE composites filled with metal powders under oil lubrication and give some insight into the friction and wear mechanisms of PTFE composites under oil lubrication. It is expected that this study may be useful to the application of ~ composites in practice.
2. Experimental details In this experiment, the friction and wear tests were carried out on an MHK-500 ring-block wear tester (Timken tester) with a steel ring 49.2 mm in diameter and 13.0 mm in length rotating on a PTFE composite block 12.3 mmX 12.3 mm x 18.9 mm in size. The steel ring, made of GCrI5 bearing steel (C 0.950%-1.050%, Mn 0.200%-0.400%, Si 0.150%0.350%; Cr 1.300%-1.650%; P <0.027%, S <0.020%, Fe the remainder), was polished with number 900 grade SiC abrasive paper to a surface roughness of R.--- 0.15 Itm. The blocks of PTFE composites were polished with number 800
152
Z.-Z Zhong et at. I Wear210 (1997) 151-156
grade SiC abrasive paper to a surface roughness of R a : 0.2 ~ 0.4 ttm, washed in acetone and dried in air. Materials used for preparing PTFE composites include PTFE powder with a grit size of about 30 ixm, Pb powder about 45 tLm, Cu and Ni powders about 76 tim. First, the metal fillers Cu, Pb and Ni were mixed completely with PTFE powder respectively, then the mixtures were molded and sintered into the blocks. Four kinds of FIFE-based composite, pure FIFE, PTFE + 30vol.%Cu, FIFE + 30vol.%Pb and + 30vol.%Ni composites, were prepared. The friction and wear tests were performed at room temperature in atmosphere at the sliding speed of 1.5 m s - J under a load from 100 N to 400 N for dry friction conditions, and at the sliding speed of 2.5 m s - t under a load from 400 N to 1200 N for oil lubricated conditions. The lubricating oil used in this experiment was liquid paraffin, the oil was added to the rubbing surfaces at a rate of 30 drops per minute during the testing. Before each test started, the surfaces of the GCrl 5 bearing steel ring and PTFE composite block were cleaned by rubbing with a soft cloth dipped in pure acetone and then dried in air. Each friction and wear test was performed for 30 rain. The wear was detected by the weight loss of the PTFE composite blocks after each test by an analytical scale (precision 0. I mg). The friction coefficient was determined by measuring the friction torque, while the friction torque was detected by a torque recorder, so the frictio~.-'oefficient could be calculated by the formula of friction coefficient for the Timken tester. The friction coefficient was the average value of that in the last 10 min. The worn surfaces of the PTFE composites were examined by a JEM-1200 EX/S analytical electron microscope (made in Japan), while the transfer films of the PTFE composites formed on the surface ofGCrl 5 hearing steel ring were investigated by optical microscopy,
3. Results and discussion 3. i. Friction and wear properties in dry friction condition The friction coefficients and wear of PTFE composites sliding against GCrl5 bearing steel under dry friction conditions are shown in Figs. I and 2 respectively. It can be seen from Fig. 1 that the friction coefficients of F I F E + 30vol.%Cu and PTFE+ 30vol.%Pb composites are lower than that of pure ZrFE, but the friction coefficient of the PTFE + 30vol.%Ni composite is higher than that of pure PTFE. This indicates that Cu and Pb fillers decreased the friction coefficients of the PTFE composites, but the Ni filler increased the friction coefficient. It can be seen from Fig. 2 that the wear of PTFE composites filled with Cu, Pb and Ni can be reduced by I to 2 orders of magnitude compared with that of pure FTFE. This indicates that Cu, Pb and Ni fillers greatly reduced the wear of the ~ composites, but the wear reducing action of Cu was the most effective, that of Ni was second, and the wear reducing action of Pb was the worst.
P11=E÷:m~A~)Cu
PTFE+3~(~b
PI1FE+30~4~ F ~ ' t i ~ cm~clmt Fig. I. Friction coefficientsof PTFEcomposites under dry friction conditions
(l.Sms -j. 100N).
PTFE+30%(v)Cu
PTFE+30%(v)Ni PTFE+30%(v)Pb PTFE
0
100 2 0 0 3 0 0
Wear(me)
o0=
0o
Fig. 2. Wearof P'ITEcompositesunderdry frictioncondilions ( 1.5 m s- *. lOON).
Variations of friction coefficients with load for PTFE composites filled by metal powders sliding against GCr 15 bearing steel under dry friction conditions are shown in Fig. 3. it can be seen from Fig. 3 that the friction coefficient of the PTFE + 30vol.%Pb composite decreases with the increase in load from ! 00 N to 400 N. However, for PTFE + 30vol.%Cu and PTFE + 30vol.%Ni composites, the friction coefficients decree, s' with the increase in load firstly, and then increase with the increase in load. Fig. 4 gives the variations in wear with load for PTFE composites filled with metal powders sliding against GCrI5 bearing steel under dry friction conditions. It can be seen from Fig. 4 that the wear of PTFE composites filled by Cu, Pb and Ni powders increases with the increase in load. When the load is higher than 3CO N, the wear of PTFE + 30vol.%Ni and PTFE + 30vol.%Pb composites increases sharply, but the wear of the PTFE + 30vol.%Cu composite is still very low even under higher loads, and the wear of the PTFE + 30vol.%Cu composite is 1 to 2 orders of magnitude lower than that of Pb and Ni filled PTFE composites under different loads.
Fdctloncoeflk:ient 0.45 0.4~ 0.36
PTFE÷30~,(v)Cu :PTFE÷30%lv)Pb
0.3
PTFE*30%(v)NI
0.25 0.2 i
|
i
L o ~ (N) Fig. 3. Variations of friction coefficients with load for PTFE composites filled with metal under dry friction conditions ( 1.5 m s- t ).
153
z.-z. Zhang et al. / Wear210 (!~,~7115i-156 (nW) 400
3OO
200
~Xrl~+3~v)l~
lOO ---
0 t~o
]
2~o
aoo
40o
t.e~(N)
Fig. 4. Variationsof wear with load for IrfFE composites filled with metal under dry frictionconditions ( 1.5 m s - t ). Since PTFE-based composites are viscoelastic materials, their deformation under toad is viscoelastic. Therefore, the variation of friction coefficient with load follows the equation I~=KN <'-I~, where p, is the friction coefficient, K is a constant, N is the applied load, and n is also a constant, with value between 2/3 and I ( 2 / 3 < n < l). According to this equation, the friction coefficients of PTFE composites decrease with the increase in load. However, with further increase in load, the temperature at the frictional surface rises. leading to the increase in viscoelastic deformation and reduction in the load carrying capacity of the PTFE composites. Therefore, the friction and wear inereame. When the load increases to the load limit of the PTFE composites, the friction and wear increase sharply. It is known that, of the metal fillers Cu, Pb and Ni, the hardness of Ni is the highest, that of Cu is second, and the hardness of Pb is the lowest. Comparison of the friction and wear results from Figs. !-4 shows that there are no direct one-to-one correspondence relations between the hardness of metal fillers and the anti-friction and anti-wear properties of the F I F E composites under dry friction conditions. 3.2. Friction and wear properties under oil lubricated conditions
The variations of friction coefficients and wear with load for PTFE composites sliding against GCrl5 bearing steel under lubrication with liquid paraffin are shown in Figs. 5 and 6 respectively. It can be seen from Fig. 5 that, comparing the friction results with those under dry friction conditions, the friction coefficients of the F I F E composites can be reduced by one order of magnitucle. For PTFE + 30vol.%Cu Frk:tlon coefficient 0.05
P'rFE
and PTFE + 30vol.%Ni composites, the friction coefficients decrease with the increase in load from 400 N to 1200 N. However, for pure PTFE and the PTFE + 30vol.%Ph composite, the friction coefficients decrease with the increase it: load firstly, and then increase as the load increases. The results in Fig. 6 show that, comparing the wear results with those under dry friction conditions (see Fig. 4), the wear of the PTFE composites can be decreased by 1 to 2 orders of magr~itude under liquid paraffin lubrication. Meanwhile. the wear of PTFE composites increases with the increase in load. However, the wear of pure VrFE and the P T I ~ + 30vol.%Fb composite increases sharply under higher loads. It can be seen from the results in Figs. 5 and 6 that, when the load is higher than 800 N, the order of the. fiiction coefficient and wear of PTFE composi,,es from low to high is as follows: PTFE + 30vol.%Ni, PTFE + 30vol.%Cu, PTFE + 30vol.%Pb and pure ZPFE. Meanwhile, it was found in the experiments that there were cracks vertical to the sliding direction in the subsurface of pure PTFE under the load of 1000 N, while some serious deformation occurred to the PTFE+ 30vol.%Pb composite under the load of 1200 N. However, there were no cracks or serious deformation on the surfaces of PTFE + 30vol.%Cu and PTFE + 30vol.%Ni composites even under the load of 1200 N. Therefore, 1000 N and 1200 N were the load limits of pure PTFE and PTFE + 30vol.%Pb composite under the conditions in this experiment respectively, but the load limits of PTFE + 30vol.%Cu and VrFE + 30vol.%Ni composites were higher than 1200 N. Therefore, it can be deduced that the load carrying capacities of the PTFE composites were greatly improved by metal fillers of Cu, Pb and Hi. Under higher loads ( above 800 N) and oil lubrication, the greater the hardness of the metal filler, the higher the load carrying capacity of the composite, and the better the friction and wearreducing properties of the PTFE composite. Since the friction and wear of the PTFE + 30vol.%Ni composite are the lowest of all u,der higher loads and liquid paraffin lubrication, the variations of friction coefficient and wear rate with sliding speed for the PTFE + 30voi.%Ni composite are given in Figs. 7 and 8 respectively. It is obvious that the variation in friction coefficient with sliding speed for the PTFE + 30vol.%Ni compom,~ is very small, but the wear rate of the PTFE+ 30vol.%Ni composite decreases with increasing sliding speed, it is believed that, with the increase Wear (rag) 25
PTFE
0.045 0,04
PTFE+30%(v)Cu
O.035 O.O3 0.025
PTFE+30%lv)NI -~.-
0.02
Load IXtOON) Fig. 5. Variations of friction coefficients with load for PTFE composites under lubricationwith liquid paraffin (2.5 m s-t).
20
PTFE+30~(v)Cu
15
"-*PTFE÷30~(v)Pb
10 PTFE+30~(v)NI
5 0
: 4
6
.
"7
,
8
10
12
Load ( X t m )
Fig. 6. Variationsof wear with load for ~ with liquidparaffin (2.5 m s- ').
composites under lubrication
154
Z -Z Zhan8 et ~1. / Wear 210 (1997) 151-156
FiaaelfioJ~
(ZO~l
posites; this would lead to the increase in the friction coefficient and wear. When me loads increased to the load limits of the PTFE composites, the friction and wear of the PTFE composites increased sharply.
3.3. Opticalmicroscopy investigationsof transferfilms 1.0 t.5 2.0 25 rain ~Jln~ Fig. 7. Friction cocf~k:n( of ~ - I - ~ o l . % N i ~ i ~ at different sliding speedsund~ l u ~ x ~ with liquid lxu~n ( 1200N). W ~ r nm(X~e~4 q ~ 12 10 9
8
7
....... L:-I Fig. 8. Vadation of wear rate with sliding speed for PTl~+3Ovol.%Hi emnpo~te under lebdcation with liquid paraffin ( 1200N). in sliding speed, a layer o f luMicating oil film can be more easily formed on the frictional surface, thus the lubrication condition of the frictional surface can be greatly improved, leading to the decrease in the wear rate. Therefore, comparing
the friction and wear results from Figs. 5-8, it can be coneluded that the PTFE + 30vol.%Ni composite can be used in practice as a kind of PTFE composite which has excellent friction and wear reducing properties under oil lubricated conditions. It is known that PTFE-based composites are viscoelastic materials. When a rigid ring of GCrI5 bearing steel slides against a viscoelastic block of PTFE composite, vi~oelasiic deformation may occu;" ~othe PTFE composite under the load applied; the contact between the rigid ring and the viscoelastic block becomes an arc plane contact in the rubbing area. Therefore, when the sliding speed is constant, the variations of friction coefficier~ts with load for FIFE composites under oil lubrication can be described by the formula pox VlNIP, where p. is the friction coefficient, v/is the viscosity of the oil, N is the rotation speed of the steel ring and, P is the load applied [ 12-16]. At a constant sliding speed, the temperature at the frictional surface increases with the increase in load, while the viscosity of the lubricant oil decreases with the increase in temperature but increases with the increase in load. The variations of viscosity with tempera0Jre and load show that the effect of viscosity on friction coefficient is so small that the formula/~(x vlNIP can be approximated by itcxN/P. Therefore, the friction coefficient decreases with the increase in load. However, with a further increase in load, the increase in temperature at the frictional surface would result in the increase in def~'ma,,ion and the reduction in the mechanical strength and load carrying capacity of PTI~ com-
Fig. 9 shows optical micrographs of the transfer films of PTFE composites formed on the surface of C,Crl5 bearing steel under dry friction conditions. It can be seen from Fig. 9 that there are obvious transfer films formed on the counterfaces ofCu, Pb and Ni filled PTFE composites, but no obvious transfer film on the countert'ace of pure FIFE. Correlating the above investigations with the friction and wear results from Figs. ! and 2, it can be deduced that the metal fillers Cu, Pb and Ni enhanced the adhesion of transfer films to the counterfaces, promoted the transfer of PTFE composites, so they greatly reduced the wear of ~ composites [3,4]. Meanwhile, it is clear that the thickness and the uniformity of the transfer films formed on the counterfaces of PTFE + 30vol.%Cu and PTFE + 30vol.%Pb composites are greater than those of the PTFE + 30vol.%Ni composite. This indicates that PTFE + 30vol.%Cu and PTFE + 30vol.%Pb composites could easily form uniform transfer films on the counterfaces. Then th,~friction between the PTFE composites and the surface of GCrl5 bearing steel was transformed to friction between FIFE composites and the transfer films on the counted'aces, so the friction coefficients of the PTFE composites were greatly reduced. Since the adhesion between the transfer film of the PTFE + 30vol.%Ni composite and the counterface was weak, it was difficult to form a uniform transfer film on the surface ofGCr 15 bearing steel. Therefore, the friction coefficient of the PTFE + 30vol.%Ni composite is higher than those of ~ + 3 0 v o l . % C u and F I F E + 30vol.%Pb composites under dry friction conditions. Fig. 10 shows optical micrographs of the transfer films of FIFE composites formed on the surface of C,Crl 5 bearing
Fig. 9. Oplicalmkrographsofthe transfer61msofPTFEcompositesformed on the surface of C-CrI5bearing steel under dry friction conditions ( 1.5 ms-'. 100 N) (128×): (a) pure PTFE; (b) PTFE+30voI.%Cu; (c) PTFE+ 30vol.%Pb:(d) PTFE+ 30vol.%Ni.
Z - Z 7_.hang et al. I Wear 2 ! 0 ( 1997~ ! 51 - ! 56
| 55
!.
Fig. I O.Oplical microgmphs oft he wansfer films of FI'FE composiles formed on the surface of GCrl5 bearing steel under liquid paraffin lubrication (2.5 m s-~, 1000 N) ( 1 2 8 × ) : (a) pure PTI~: (b) PTFE+30vol.CkCu: (c) FIFE + 30vol.%Pb: (d) PTFE + 30vol.%Ni.
Fig. ! !. Electron m i e m g n ~ of the worn mrfiw.es of ~ composites under do' friction conditions (1.5 m s - * ) : (a) pure PTFE, 100 N; (b) PTFE+3Ovol.e~Cu, 300 N; (c) PTl~+3Ovol.%Pb, 300 N; (d) PTFE + 30vol.%Ni, 300 N. .,.
steel under liquid paraffin lubrication, it can be seen from Fig. lO that there are no obvious transfer films of PTFE composites formed on the counterfaces under liquid paraffin lubrication except for the PTFE + 30vo!.%Pb composite, but the transfer film of the F I F E + 30vol.%Pb composite is very thin. Comparison of the transfer films with those under dry friction conditions indicates that the transfer of PTFE composites onto the counterfaces was greatly reduced by oil lubrication, but the transfer still took place. Correlating the results in Fig. I0 with the wear results in Fig. 6, it can be concluded Ihat the sharp increase in wear of the PTFE + 30vol.%Pb composite under higher loads and liquid paraffin lubrication resulted in the transfer of PTFE + 30vol.%Pb composite onto the counterface.
3.4. Scmming electron microscopy (SEM) im'esligations of worn surfaces It was found in the experiments that, under dry friction conditions, the width of the wear scar on the worn surface of pure PTFE was about 12 ram, but the wear scars on the worn surfaces of PTFE composites filled with Cu, Pb and Ni were much smaller than those of pure PTFE. This indicates that C,s, Pb and Ni greatly r::duced the wear of the PTFE composites. Fig. 11 shows electron micrographs of the worn surfaces of PTFE composites under dry friction conditions. It can be seen from Fig. I I that, for Ca, Pb and Ni filled PTFE composites, the wear scars on the worn surface of the PTFE + 30vol.%Pb composite are the largest, while those of the PTFE + 30vol.%Cu composite are the smallest. This indicates that the wear of PTFE + 30vol.%Cu composite is the lowest, that of PTFE + 30vol.%Ni composite is second, and that of PTFE + 30vol.%Pb composite is the highest. All of the above analytical results are consistent with the wear data. Fig. ! 2 shows electron micrographs of the worn surfaces of PTFE composites under liquid paraffin lubrication. It can
~,.
•
... ~..
~ ..~..~; ~.
Fig. 12. Electron microgral~S of ll~ worn surfaces of ~ composites under liquid paraffin lubrication (2.5 m s-'. 11300N): (a) pure P T ~ ; (bl, PTFE + 30vol.~Cu; (¢) F I F E + 30vol.%rPb; (d) PTFE +30VOI.%Ni.
be seen from Fig. 12 that there are wide and deep wear scars on the worn surface of pure PTFE, but no obvious wear scars on the worn surfaces of PTFE composites filled with Cu, Pb and Ni under oil lubrication. This indicates that severe wear occurred of pure PTFE under these conditions. However, since the load carrying capacities of them ~ composites were greatly improved by metal fillers Cu, Pb and Ni, the wear of the composites was greatly reduced [ 1,2l. Comparison of the wear scars in Fig, 12 with those in Fig. I I shows that the wear reducing properties of the PTFE composites can be greatly improved by oil lubrication. The above investigation and analysis results are also consistent with the wear data under oil lubricated conditions. 4. Conclusions
!. Metal fillers of Cu and Pb reduce the friction coefficients of the PTFE composites, but Ni increases the friction coefficient.
156
7...7. Zl~ng ¢t al. / Wear 210 (1997) 151-156
2. The wear ofCu, Pb and Ni filled PTFE composites can be decreased by 1 to 2 orders of magnitude compared with that of pure zrF'E. Meanwhile, of the metal fillers Cu, Pb and Hi, the wear reducing action of Cu is the most effecfive, that of Ni is second, and that of Pb is the worst. 3. Metal fillers Cu, Pb and Ni not only raise the load carrying capacity of the PTFE composites, but also promote the transfer of these PTFE composites onto the counterfaces, 4. The friction and wear properties of the PTFE composites can be greatly improved with liquid paraffin lubrication. The wear of the PTFE composites can be decreased by 1 to 2 orders of magnitude compared with that under dry friction conditions, while the friction coefficients can be decreased by i order of magnitude. 5. Under lubrication of liquid paraffin, the wear of the composites increases with the increase in load, while the friction coefficients decrease with the increase in load. However, when the loads increase to the load limits of the FTFE composites, their friction and wear increase sharply. 6. The transfer of FIFE composites onto the surface of GCrI5 bearing steel can be greatly reduced by liquid paraffin lubrication, but rite transfer still takes place, 7. The PTFE + 30vol.%Ni composite has excellent friction and wear reducing properties under liquid paraffin lubrication, so this PTFE composite can be used in practice under oil lubrication. I[l~r~q,eQ¢l~ [ 11 LK, Lancaster, The effect of carbon fiber reinforcementon the friction and wear of polymers, $. Phys, D, 1 (1968)549. [2l J.K, Laacaster, Composites for ~ wear resistance: current aahieveruems and future prospects. In ILL. William (ed.), New Directions in IJdm'cation, Material, Wear and Surface Interactions, Noyes Publications, NJ, 1985, p. 320. [3] BJ. Briscoe, A.K. Pogosian and D. Tabor, The friction and wear of IIDEE: the action of lead oxide and copper oxide fillers, Wear, 27
(t974) tg. [41 BJ. Bris~oeand D. Tabor.The sliding wear of polymers: a brief review. In N.P. Suh and N. Saka (eds.),Fundamentals of Tribology,MIT Press.Cambddge, MA, 1980. p. 733. [51 K. Tanaka, Effectsof various fillerson tbe frictionand wear of PTFEbssed composites. In K. Frkdrich (ed.),Friclionand Wear ofPolymer CompGsites,Elsevier,Amsterdam, 1986, p. 137, [6] BJ. Briscoe, T.A. Stolarski and GJ. Davies, Boundary lubrication of polymers in model fluids. Tribal. Int., 17 (1984) 129. [?] C. Rubenstein, Lubrication of polymers, J. Appl. Phys.. 32 ( 1961 ) 1445. [8] D.C. Evans, Polymer-fluid interactions in relation to wear, Proc. 3rd leeds-Lyon $ymp. on Wear of Non.Metallic Materials, Mechanical Engineering Publications, London, 1978, p. 47. [9l M. Watanabe, Wear mechanism of tVrFE composites in aqueous environw,ents, Wear, 1~8 (1992) 79. [1o1 J.K. Lamcaster, Lubrication of carbon fiber-reinforced polymers, part I: water and aqueous solutions. Wear. 20 (1972) 315. Ill] Y. Zhongqian. L. Manquing and K. Hailing. Friction and wear cltamctedstics of water lubricated polymer composites, in S.K. Rite, A.W. Ruffand K.C. Ladema (eds.), Wear of Materials, ASME, Hew York, 1981, p. 153.
[12] V. Stepina and V. Vesely, Lubricants and Special Fluids, Elsevier, Amsterdam. 1992, p. 4. [13l P.M. Dickens, J.L. Sullivan and J.K. Lancaster, Speed effects on the dry and lubricated wear of po|ymers, Wear, 112 (1986) 273. [141 F.P. Bowden and D. Tabofo The Friction and Lubrication of Solids, Clarendon Press, Oxford, 1954, p. 250. [151 Y. Yamagechi, Tribolosy of Plastic Materials, Elsevier, Amsterdam, 1990, p. 203-255. [16] Z.-Z, Zlumg, W.-C, Shen, W.-M. Liu et al., Tribologicnl properties of FifE-based composite in different lubricant media. Wear, 196 (1996) 164.
Biographies Zhao-Zhu Zhang was born in 1965, graduated with a BS degree in Solid Physics from the Physics Department of Lanzhou University in 1988, and received his MS degree in Physical Chemistry from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences in 1991. Now he is a PhD student. His current research interests include studies of tribological properties and mechanisms as well as the tribochemistry and tribophysics of the frictional surfaces and interfaces of polymer-based self-lubricating composites under dry friction and oil lubrication conditions. Qun-Ji Sue graduated from the Department of Chemistry, Shandong University of China and received his MS degree at Lanzhon Institute of Chemical Physics in 1967. From 1980 to 1982, he worked at the University of Michigan. Since 1965, he has worked in the field of tribology with special emphasis on lubricated materials. Now he is a professor and head of the Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science; he is also a head of the Lubrication and Tribochemistry Branch of the China Society of Mechanical Engineers. Wei-Chang Shen was born in 1940, graduated from the Chemical Engineering Department of Zhejiang University in 1964, and since then he has worked in the I..anzhou Institute of Chemical Physics, Chinese Academy of Sciences. Now he is a professor, his current interests include studies of new polymeric self-lubricating composites and their tribological properties. Wei-Min Liu graduated with a BS degree in Chemistry from Shandong Normal University in 1984, and received his MS degree and Phi) degree in Tribology at Lanzhou Institute of Chemical Physics in 1987 and 1990 respectively. He joined the Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science in 1990. From 1993 to 1994, he spent one year at the Department of Chemical Engineering, Pennsylvania State University. Now he is a professor, his current rese2.,-ch interests include lubrication of ceramics, solid lubrication, and tribochemistry of lubricating oil additives.
...-a.
WEAR ELSEVIER
Wear 210 (1997) 151-156
Friction and wear properties of metal powder filled PTFE composites under oil lubricated conditions Zhao-Zhu Zhang *, Qun-Ji Xue, Wei-Min Liu, Wei-Chang Shen Laborato~" of Solid Lubrication. Lanzhou Institute of Chemical Physics, ChineseAcademy of Sciences. Lan~ou. 730000. People's Republicof China Received I I September |996; accepted 11 March 1997
Almract Four kinds of polytetrafluoroethylene (PTFE)-based composite, pure PTFE, PTl~+3Ovoi.%Cu, PTFE+30vol.%Pb and + 30vol.%Ni composite, were prepared. The friction and wear properties of these metal powder filled FIFE composites si:ding against GCri5 bearing steel under both dq:, and lubricated condLions were studied using an MHK-500 ring-block wear tester. The worn surfaces of the ~ composites and the transfer films formed on the surface of GCr 15 bearing steel were examitted using scanning elcctrcnt microscopy (SEM) and optical micrc:scopy respectively. Experimental results show that the friction and wear properties of the PTFE composites can be greatly improved by liquid paraffin lubrication. The wear of these PTFE composites can be decreased by at least I to 2 orders of rnagnittule compared with that under dry friction conditions, while the friction coefficients can be decreased by I older of magnitude. SEM and optical microscopy investigations of the rubbing surfaces show that metal fillers of Cu, Pb and Ni not only raise the load carrying capacity of the PTFE :omposites, but also promote transfer of the PTFE composites onto the counterfaces, so they greatly reduce the wear of the PIFE composites. However, the transfer of these PTFE composites onto the counterfaces can be greatly reduced by liquid paraffin lubrication, but transfer still takes place. © ! 997 Elsevier Science S.A. ICeywords: PTFEcomposites: Metal fillers; Friction and wear; Oil lubrication: Frictional surfaces
1. Introduction PTFE-based self-lubricating composites have been successfully used in many fields, so the friction and wear mechanisms of trI'FE-based composites have been studied by many coworkers [ I-5]. However, almost all of the studies concerned dry friction conditions (unlubricated conditions). Briscoe et al. [ 6-8 ] studied the lubricated friction and wear of some polymers. They found that the absorption of fluid into the surface layers of polymers can change the mechanical p~operties of the polymers, and so, in turn, influence the friction and wear of the polymers. Watanabe et al. [9-1 ! ] investigated the friction and wear behavior of polymer composites in aqueous environments (water). They pointed out that many polymers wear much more in water than in air, and the wear of F r F E composites filled with only glass fibers is much greater thar, that of other composites in water. However, with enlargement of the application fields of FTFEbased composites in practice, it is essential to study the friction and wear behavior of PTFE-based composites under oil lubricated conditions. Until now, much less information * Corresponding author. 0043-1648/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved Pil S 0 0 4 3 - 1 6 4 8 ( 97 ) 0 0 0 5 2 - 5
has been available on the friction and wear behavior of PTFE composites under oil lubricated conditions. The purpose of this study is to investigate the friction and wear behavior of PTFE composites filled with metal powders under oil lubrication and give some insight into the friction and wear mechanisms of PTFE composites under oil lubrication. It is expected that this study may be useful to the application of ~ composites in practice.
2. Experimental details In this experiment, the friction and wear tests were carried out on an MHK-500 ring-block wear tester (Timken tester) with a steel ring 49.2 mm in diameter and 13.0 mm in length rotating on a PTFE composite block 12.3 mmX 12.3 mm x 18.9 mm in size. The steel ring, made of GCrI5 bearing steel (C 0.950%-1.050%, Mn 0.200%-0.400%, Si 0.150%0.350%; Cr 1.300%-1.650%; P <0.027%, S <0.020%, Fe the remainder), was polished with number 900 grade SiC abrasive paper to a surface roughness of R.--- 0.15 Itm. The blocks of PTFE composites were polished with number 800
152
Z.-Z Zhong et at. I Wear210 (1997) 151-156
grade SiC abrasive paper to a surface roughness of R a : 0.2 ~ 0.4 ttm, washed in acetone and dried in air. Materials used for preparing PTFE composites include PTFE powder with a grit size of about 30 ixm, Pb powder about 45 tLm, Cu and Ni powders about 76 tim. First, the metal fillers Cu, Pb and Ni were mixed completely with PTFE powder respectively, then the mixtures were molded and sintered into the blocks. Four kinds of FIFE-based composite, pure FIFE, PTFE + 30vol.%Cu, FIFE + 30vol.%Pb and + 30vol.%Ni composites, were prepared. The friction and wear tests were performed at room temperature in atmosphere at the sliding speed of 1.5 m s - J under a load from 100 N to 400 N for dry friction conditions, and at the sliding speed of 2.5 m s - t under a load from 400 N to 1200 N for oil lubricated conditions. The lubricating oil used in this experiment was liquid paraffin, the oil was added to the rubbing surfaces at a rate of 30 drops per minute during the testing. Before each test started, the surfaces of the GCrl 5 bearing steel ring and PTFE composite block were cleaned by rubbing with a soft cloth dipped in pure acetone and then dried in air. Each friction and wear test was performed for 30 rain. The wear was detected by the weight loss of the PTFE composite blocks after each test by an analytical scale (precision 0. I mg). The friction coefficient was determined by measuring the friction torque, while the friction torque was detected by a torque recorder, so the frictio~.-'oefficient could be calculated by the formula of friction coefficient for the Timken tester. The friction coefficient was the average value of that in the last 10 min. The worn surfaces of the PTFE composites were examined by a JEM-1200 EX/S analytical electron microscope (made in Japan), while the transfer films of the PTFE composites formed on the surface ofGCrl 5 hearing steel ring were investigated by optical microscopy,
3. Results and discussion 3. i. Friction and wear properties in dry friction condition The friction coefficients and wear of PTFE composites sliding against GCrl5 bearing steel under dry friction conditions are shown in Figs. I and 2 respectively. It can be seen from Fig. 1 that the friction coefficients of F I F E + 30vol.%Cu and PTFE+ 30vol.%Pb composites are lower than that of pure ZrFE, but the friction coefficient of the PTFE + 30vol.%Ni composite is higher than that of pure PTFE. This indicates that Cu and Pb fillers decreased the friction coefficients of the PTFE composites, but the Ni filler increased the friction coefficient. It can be seen from Fig. 2 that the wear of PTFE composites filled with Cu, Pb and Ni can be reduced by I to 2 orders of magnitude compared with that of pure FTFE. This indicates that Cu, Pb and Ni fillers greatly reduced the wear of the ~ composites, but the wear reducing action of Cu was the most effective, that of Ni was second, and the wear reducing action of Pb was the worst.
P11=E÷:m~A~)Cu
PTFE+3~(~b
PI1FE+30~4~ F ~ ' t i ~ cm~clmt Fig. I. Friction coefficientsof PTFEcomposites under dry friction conditions
(l.Sms -j. 100N).
PTFE+30%(v)Cu
PTFE+30%(v)Ni PTFE+30%(v)Pb PTFE
0
100 2 0 0 3 0 0
Wear(me)
o0=
0o
Fig. 2. Wearof P'ITEcompositesunderdry frictioncondilions ( 1.5 m s- *. lOON).
Variations of friction coefficients with load for PTFE composites filled by metal powders sliding against GCr 15 bearing steel under dry friction conditions are shown in Fig. 3. it can be seen from Fig. 3 that the friction coefficient of the PTFE + 30vol.%Pb composite decreases with the increase in load from ! 00 N to 400 N. However, for PTFE + 30vol.%Cu and PTFE + 30vol.%Ni composites, the friction coefficients decree, s' with the increase in load firstly, and then increase with the increase in load. Fig. 4 gives the variations in wear with load for PTFE composites filled with metal powders sliding against GCrI5 bearing steel under dry friction conditions. It can be seen from Fig. 4 that the wear of PTFE composites filled by Cu, Pb and Ni powders increases with the increase in load. When the load is higher than 3CO N, the wear of PTFE + 30vol.%Ni and PTFE + 30vol.%Pb composites increases sharply, but the wear of the PTFE + 30vol.%Cu composite is still very low even under higher loads, and the wear of the PTFE + 30vol.%Cu composite is 1 to 2 orders of magnitude lower than that of Pb and Ni filled PTFE composites under different loads.
Fdctloncoeflk:ient 0.45 0.4~ 0.36
PTFE÷30~,(v)Cu :PTFE÷30%lv)Pb
0.3
PTFE*30%(v)NI
0.25 0.2 i
|
i
L o ~ (N) Fig. 3. Variations of friction coefficients with load for PTFE composites filled with metal under dry friction conditions ( 1.5 m s- t ).
153
z.-z. Zhang et al. / Wear210 (!~,~7115i-156 (nW) 400
3OO
200
~Xrl~+3~v)l~
lOO ---
0 t~o
]
2~o
aoo
40o
t.e~(N)
Fig. 4. Variationsof wear with load for IrfFE composites filled with metal under dry frictionconditions ( 1.5 m s - t ). Since PTFE-based composites are viscoelastic materials, their deformation under toad is viscoelastic. Therefore, the variation of friction coefficient with load follows the equation I~=KN <'-I~, where p, is the friction coefficient, K is a constant, N is the applied load, and n is also a constant, with value between 2/3 and I ( 2 / 3 < n < l). According to this equation, the friction coefficients of PTFE composites decrease with the increase in load. However, with further increase in load, the temperature at the frictional surface rises. leading to the increase in viscoelastic deformation and reduction in the load carrying capacity of the PTFE composites. Therefore, the friction and wear inereame. When the load increases to the load limit of the PTFE composites, the friction and wear increase sharply. It is known that, of the metal fillers Cu, Pb and Ni, the hardness of Ni is the highest, that of Cu is second, and the hardness of Pb is the lowest. Comparison of the friction and wear results from Figs. !-4 shows that there are no direct one-to-one correspondence relations between the hardness of metal fillers and the anti-friction and anti-wear properties of the F I F E composites under dry friction conditions. 3.2. Friction and wear properties under oil lubricated conditions
The variations of friction coefficients and wear with load for PTFE composites sliding against GCrl5 bearing steel under lubrication with liquid paraffin are shown in Figs. 5 and 6 respectively. It can be seen from Fig. 5 that, comparing the friction results with those under dry friction conditions, the friction coefficients of the F I F E composites can be reduced by one order of magnitucle. For PTFE + 30vol.%Cu Frk:tlon coefficient 0.05
P'rFE
and PTFE + 30vol.%Ni composites, the friction coefficients decrease with the increase in load from 400 N to 1200 N. However, for pure PTFE and the PTFE + 30vol.%Ph composite, the friction coefficients decrease with the increase it: load firstly, and then increase as the load increases. The results in Fig. 6 show that, comparing the wear results with those under dry friction conditions (see Fig. 4), the wear of the PTFE composites can be decreased by 1 to 2 orders of magr~itude under liquid paraffin lubrication. Meanwhile. the wear of PTFE composites increases with the increase in load. However, the wear of pure VrFE and the P T I ~ + 30vol.%Fb composite increases sharply under higher loads. It can be seen from the results in Figs. 5 and 6 that, when the load is higher than 800 N, the order of the. fiiction coefficient and wear of PTFE composi,,es from low to high is as follows: PTFE + 30vol.%Ni, PTFE + 30vol.%Cu, PTFE + 30vol.%Pb and pure ZPFE. Meanwhile, it was found in the experiments that there were cracks vertical to the sliding direction in the subsurface of pure PTFE under the load of 1000 N, while some serious deformation occurred to the PTFE+ 30vol.%Pb composite under the load of 1200 N. However, there were no cracks or serious deformation on the surfaces of PTFE + 30vol.%Cu and PTFE + 30vol.%Ni composites even under the load of 1200 N. Therefore, 1000 N and 1200 N were the load limits of pure PTFE and PTFE + 30vol.%Pb composite under the conditions in this experiment respectively, but the load limits of PTFE + 30vol.%Cu and VrFE + 30vol.%Ni composites were higher than 1200 N. Therefore, it can be deduced that the load carrying capacities of the PTFE composites were greatly improved by metal fillers of Cu, Pb and Hi. Under higher loads ( above 800 N) and oil lubrication, the greater the hardness of the metal filler, the higher the load carrying capacity of the composite, and the better the friction and wearreducing properties of the PTFE composite. Since the friction and wear of the PTFE + 30vol.%Ni composite are the lowest of all u,der higher loads and liquid paraffin lubrication, the variations of friction coefficient and wear rate with sliding speed for the PTFE + 30voi.%Ni composite are given in Figs. 7 and 8 respectively. It is obvious that the variation in friction coefficient with sliding speed for the PTFE + 30vol.%Ni compom,~ is very small, but the wear rate of the PTFE+ 30vol.%Ni composite decreases with increasing sliding speed, it is believed that, with the increase Wear (rag) 25
PTFE
0.045 0,04
PTFE+30%(v)Cu
O.035 O.O3 0.025
PTFE+30%lv)NI -~.-
0.02
Load IXtOON) Fig. 5. Variations of friction coefficients with load for PTFE composites under lubricationwith liquid paraffin (2.5 m s-t).
20
PTFE+30~(v)Cu
15
"-*PTFE÷30~(v)Pb
10 PTFE+30~(v)NI
5 0
: 4
6
.
"7
,
8
10
12
Load ( X t m )
Fig. 6. Variationsof wear with load for ~ with liquidparaffin (2.5 m s- ').
composites under lubrication
154
Z -Z Zhan8 et ~1. / Wear 210 (1997) 151-156
FiaaelfioJ~
(ZO~l
posites; this would lead to the increase in the friction coefficient and wear. When me loads increased to the load limits of the PTFE composites, the friction and wear of the PTFE composites increased sharply.
3.3. Opticalmicroscopy investigationsof transferfilms 1.0 t.5 2.0 25 rain ~Jln~ Fig. 7. Friction cocf~k:n( of ~ - I - ~ o l . % N i ~ i ~ at different sliding speedsund~ l u ~ x ~ with liquid lxu~n ( 1200N). W ~ r nm(X~e~4 q ~ 12 10 9
8
7
....... L:-I Fig. 8. Vadation of wear rate with sliding speed for PTl~+3Ovol.%Hi emnpo~te under lebdcation with liquid paraffin ( 1200N). in sliding speed, a layer o f luMicating oil film can be more easily formed on the frictional surface, thus the lubrication condition of the frictional surface can be greatly improved, leading to the decrease in the wear rate. Therefore, comparing
the friction and wear results from Figs. 5-8, it can be coneluded that the PTFE + 30vol.%Ni composite can be used in practice as a kind of PTFE composite which has excellent friction and wear reducing properties under oil lubricated conditions. It is known that PTFE-based composites are viscoelastic materials. When a rigid ring of GCrI5 bearing steel slides against a viscoelastic block of PTFE composite, vi~oelasiic deformation may occu;" ~othe PTFE composite under the load applied; the contact between the rigid ring and the viscoelastic block becomes an arc plane contact in the rubbing area. Therefore, when the sliding speed is constant, the variations of friction coefficier~ts with load for FIFE composites under oil lubrication can be described by the formula pox VlNIP, where p. is the friction coefficient, v/is the viscosity of the oil, N is the rotation speed of the steel ring and, P is the load applied [ 12-16]. At a constant sliding speed, the temperature at the frictional surface increases with the increase in load, while the viscosity of the lubricant oil decreases with the increase in temperature but increases with the increase in load. The variations of viscosity with tempera0Jre and load show that the effect of viscosity on friction coefficient is so small that the formula/~(x vlNIP can be approximated by itcxN/P. Therefore, the friction coefficient decreases with the increase in load. However, with a further increase in load, the increase in temperature at the frictional surface would result in the increase in def~'ma,,ion and the reduction in the mechanical strength and load carrying capacity of PTI~ com-
Fig. 9 shows optical micrographs of the transfer films of PTFE composites formed on the surface of C,Crl5 bearing steel under dry friction conditions. It can be seen from Fig. 9 that there are obvious transfer films formed on the counterfaces ofCu, Pb and Ni filled PTFE composites, but no obvious transfer film on the countert'ace of pure FIFE. Correlating the above investigations with the friction and wear results from Figs. ! and 2, it can be deduced that the metal fillers Cu, Pb and Ni enhanced the adhesion of transfer films to the counterfaces, promoted the transfer of PTFE composites, so they greatly reduced the wear of ~ composites [3,4]. Meanwhile, it is clear that the thickness and the uniformity of the transfer films formed on the counterfaces of PTFE + 30vol.%Cu and PTFE + 30vol.%Pb composites are greater than those of the PTFE + 30vol.%Ni composite. This indicates that PTFE + 30vol.%Cu and PTFE + 30vol.%Pb composites could easily form uniform transfer films on the counterfaces. Then th,~friction between the PTFE composites and the surface of GCrl5 bearing steel was transformed to friction between FIFE composites and the transfer films on the counted'aces, so the friction coefficients of the PTFE composites were greatly reduced. Since the adhesion between the transfer film of the PTFE + 30vol.%Ni composite and the counterface was weak, it was difficult to form a uniform transfer film on the surface ofGCr 15 bearing steel. Therefore, the friction coefficient of the PTFE + 30vol.%Ni composite is higher than those of ~ + 3 0 v o l . % C u and F I F E + 30vol.%Pb composites under dry friction conditions. Fig. 10 shows optical micrographs of the transfer films of FIFE composites formed on the surface of C,Crl 5 bearing
Fig. 9. Oplicalmkrographsofthe transfer61msofPTFEcompositesformed on the surface of C-CrI5bearing steel under dry friction conditions ( 1.5 ms-'. 100 N) (128×): (a) pure PTFE; (b) PTFE+30voI.%Cu; (c) PTFE+ 30vol.%Pb:(d) PTFE+ 30vol.%Ni.
Z - Z 7_.hang et al. I Wear 2 ! 0 ( 1997~ ! 51 - ! 56
| 55
!.
Fig. I O.Oplical microgmphs oft he wansfer films of FI'FE composiles formed on the surface of GCrl5 bearing steel under liquid paraffin lubrication (2.5 m s-~, 1000 N) ( 1 2 8 × ) : (a) pure PTI~: (b) PTFE+30vol.CkCu: (c) FIFE + 30vol.%Pb: (d) PTFE + 30vol.%Ni.
Fig. ! !. Electron m i e m g n ~ of the worn mrfiw.es of ~ composites under do' friction conditions (1.5 m s - * ) : (a) pure PTFE, 100 N; (b) PTFE+3Ovol.e~Cu, 300 N; (c) PTl~+3Ovol.%Pb, 300 N; (d) PTFE + 30vol.%Ni, 300 N. .,.
steel under liquid paraffin lubrication, it can be seen from Fig. lO that there are no obvious transfer films of PTFE composites formed on the counterfaces under liquid paraffin lubrication except for the PTFE + 30vo!.%Pb composite, but the transfer film of the F I F E + 30vol.%Pb composite is very thin. Comparison of the transfer films with those under dry friction conditions indicates that the transfer of PTFE composites onto the counterfaces was greatly reduced by oil lubrication, but the transfer still took place. Correlating the results in Fig. I0 with the wear results in Fig. 6, it can be concluded Ihat the sharp increase in wear of the PTFE + 30vol.%Pb composite under higher loads and liquid paraffin lubrication resulted in the transfer of PTFE + 30vol.%Pb composite onto the counterface.
3.4. Scmming electron microscopy (SEM) im'esligations of worn surfaces It was found in the experiments that, under dry friction conditions, the width of the wear scar on the worn surface of pure PTFE was about 12 ram, but the wear scars on the worn surfaces of PTFE composites filled with Cu, Pb and Ni were much smaller than those of pure PTFE. This indicates that C,s, Pb and Ni greatly r::duced the wear of the PTFE composites. Fig. 11 shows electron micrographs of the worn surfaces of PTFE composites under dry friction conditions. It can be seen from Fig. I I that, for Ca, Pb and Ni filled PTFE composites, the wear scars on the worn surface of the PTFE + 30vol.%Pb composite are the largest, while those of the PTFE + 30vol.%Cu composite are the smallest. This indicates that the wear of PTFE + 30vol.%Cu composite is the lowest, that of PTFE + 30vol.%Ni composite is second, and that of PTFE + 30vol.%Pb composite is the highest. All of the above analytical results are consistent with the wear data. Fig. ! 2 shows electron micrographs of the worn surfaces of PTFE composites under liquid paraffin lubrication. It can
~,.
•
... ~..
~ ..~..~; ~.
Fig. 12. Electron microgral~S of ll~ worn surfaces of ~ composites under liquid paraffin lubrication (2.5 m s-'. 11300N): (a) pure P T ~ ; (bl, PTFE + 30vol.~Cu; (¢) F I F E + 30vol.%rPb; (d) PTFE +30VOI.%Ni.
be seen from Fig. 12 that there are wide and deep wear scars on the worn surface of pure PTFE, but no obvious wear scars on the worn surfaces of PTFE composites filled with Cu, Pb and Ni under oil lubrication. This indicates that severe wear occurred of pure PTFE under these conditions. However, since the load carrying capacities of them ~ composites were greatly improved by metal fillers Cu, Pb and Ni, the wear of the composites was greatly reduced [ 1,2l. Comparison of the wear scars in Fig, 12 with those in Fig. I I shows that the wear reducing properties of the PTFE composites can be greatly improved by oil lubrication. The above investigation and analysis results are also consistent with the wear data under oil lubricated conditions. 4. Conclusions
!. Metal fillers of Cu and Pb reduce the friction coefficients of the PTFE composites, but Ni increases the friction coefficient.
156
7...7. Zl~ng ¢t al. / Wear 210 (1997) 151-156
2. The wear ofCu, Pb and Ni filled PTFE composites can be decreased by 1 to 2 orders of magnitude compared with that of pure zrF'E. Meanwhile, of the metal fillers Cu, Pb and Hi, the wear reducing action of Cu is the most effecfive, that of Ni is second, and that of Pb is the worst. 3. Metal fillers Cu, Pb and Ni not only raise the load carrying capacity of the PTFE composites, but also promote the transfer of these PTFE composites onto the counterfaces, 4. The friction and wear properties of the PTFE composites can be greatly improved with liquid paraffin lubrication. The wear of the PTFE composites can be decreased by 1 to 2 orders of magnitude compared with that under dry friction conditions, while the friction coefficients can be decreased by i order of magnitude. 5. Under lubrication of liquid paraffin, the wear of the composites increases with the increase in load, while the friction coefficients decrease with the increase in load. However, when the loads increase to the load limits of the FTFE composites, their friction and wear increase sharply. 6. The transfer of FIFE composites onto the surface of GCrI5 bearing steel can be greatly reduced by liquid paraffin lubrication, but rite transfer still takes place, 7. The PTFE + 30vol.%Ni composite has excellent friction and wear reducing properties under liquid paraffin lubrication, so this PTFE composite can be used in practice under oil lubrication. I[l~r~q,eQ¢l~ [ 11 LK, Lancaster, The effect of carbon fiber reinforcementon the friction and wear of polymers, $. Phys, D, 1 (1968)549. [2l J.K, Laacaster, Composites for ~ wear resistance: current aahieveruems and future prospects. In ILL. William (ed.), New Directions in IJdm'cation, Material, Wear and Surface Interactions, Noyes Publications, NJ, 1985, p. 320. [3] BJ. Briscoe, A.K. Pogosian and D. Tabor, The friction and wear of IIDEE: the action of lead oxide and copper oxide fillers, Wear, 27
(t974) tg. [41 BJ. Bris~oeand D. Tabor.The sliding wear of polymers: a brief review. In N.P. Suh and N. Saka (eds.),Fundamentals of Tribology,MIT Press.Cambddge, MA, 1980. p. 733. [51 K. Tanaka, Effectsof various fillerson tbe frictionand wear of PTFEbssed composites. In K. Frkdrich (ed.),Friclionand Wear ofPolymer CompGsites,Elsevier,Amsterdam, 1986, p. 137, [6] BJ. Briscoe, T.A. Stolarski and GJ. Davies, Boundary lubrication of polymers in model fluids. Tribal. Int., 17 (1984) 129. [?] C. Rubenstein, Lubrication of polymers, J. Appl. Phys.. 32 ( 1961 ) 1445. [8] D.C. Evans, Polymer-fluid interactions in relation to wear, Proc. 3rd leeds-Lyon $ymp. on Wear of Non.Metallic Materials, Mechanical Engineering Publications, London, 1978, p. 47. [9l M. Watanabe, Wear mechanism of tVrFE composites in aqueous environw,ents, Wear, 1~8 (1992) 79. [1o1 J.K. Lamcaster, Lubrication of carbon fiber-reinforced polymers, part I: water and aqueous solutions. Wear. 20 (1972) 315. Ill] Y. Zhongqian. L. Manquing and K. Hailing. Friction and wear cltamctedstics of water lubricated polymer composites, in S.K. Rite, A.W. Ruffand K.C. Ladema (eds.), Wear of Materials, ASME, Hew York, 1981, p. 153.
[12] V. Stepina and V. Vesely, Lubricants and Special Fluids, Elsevier, Amsterdam. 1992, p. 4. [13l P.M. Dickens, J.L. Sullivan and J.K. Lancaster, Speed effects on the dry and lubricated wear of po|ymers, Wear, 112 (1986) 273. [141 F.P. Bowden and D. Tabofo The Friction and Lubrication of Solids, Clarendon Press, Oxford, 1954, p. 250. [151 Y. Yamagechi, Tribolosy of Plastic Materials, Elsevier, Amsterdam, 1990, p. 203-255. [16] Z.-Z, Zlumg, W.-C, Shen, W.-M. Liu et al., Tribologicnl properties of FifE-based composite in different lubricant media. Wear, 196 (1996) 164.
Biographies Zhao-Zhu Zhang was born in 1965, graduated with a BS degree in Solid Physics from the Physics Department of Lanzhou University in 1988, and received his MS degree in Physical Chemistry from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences in 1991. Now he is a PhD student. His current research interests include studies of tribological properties and mechanisms as well as the tribochemistry and tribophysics of the frictional surfaces and interfaces of polymer-based self-lubricating composites under dry friction and oil lubrication conditions. Qun-Ji Sue graduated from the Department of Chemistry, Shandong University of China and received his MS degree at Lanzhon Institute of Chemical Physics in 1967. From 1980 to 1982, he worked at the University of Michigan. Since 1965, he has worked in the field of tribology with special emphasis on lubricated materials. Now he is a professor and head of the Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science; he is also a head of the Lubrication and Tribochemistry Branch of the China Society of Mechanical Engineers. Wei-Chang Shen was born in 1940, graduated from the Chemical Engineering Department of Zhejiang University in 1964, and since then he has worked in the I..anzhou Institute of Chemical Physics, Chinese Academy of Sciences. Now he is a professor, his current interests include studies of new polymeric self-lubricating composites and their tribological properties. Wei-Min Liu graduated with a BS degree in Chemistry from Shandong Normal University in 1984, and received his MS degree and Phi) degree in Tribology at Lanzhou Institute of Chemical Physics in 1987 and 1990 respectively. He joined the Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science in 1990. From 1993 to 1994, he spent one year at the Department of Chemical Engineering, Pennsylvania State University. Now he is a professor, his current rese2.,-ch interests include lubrication of ceramics, solid lubrication, and tribochemistry of lubricating oil additives.