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Friction and wear characteristics of lead and its compounds filled polytetrafluoroethylene composites under oil lubricated conditions
Tribology International Vol. 31, No. 7, pp. 361–368, 1998 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0301–679X/98/$19.00 ⫹ 0.00
PII: S0301–679X(98)00045–0
Friction and wear characteristics of lead and its compounds filled polytetrafluoroethylene composites under oil lubricated conditions Zhao-Zhu Zhang*, Qun-Ji Xue, Wei-Min Liu and Wei-Chang Shen
The friction and wear properties of Pb, PbO, Pb3O4, or PbS filled polytetrafluoroethylene (PTFE) composites sliding against GCr15 bearing steel under both dry and liquid paraffin lubricated conditions were studied by using an MHK-500 ring-block wear tester. The worn surfaces and the transfer films of these PTFE composites formed on the surface of GCr15 bearing steel were then investigated by using a scanning electron microscope (SEM) and an optical microscope, respectively. Experimental results show that filling Pb, PbO, Pb3O4 or PbS to PTFE can greatly reduce the wear of the PTFE composites, but the wear reducing action of Pb3O4 is the most effective. Meanwhile, PbS increases the friction coefficient of the PTFE composite, but Pb and Pb3O4 reduce the friction coefficients of the PTFE composites. However, the friction and wear properties of lead or its compounds filled PTFE composites can be greatly improved by lubrication with liquid paraffin, and the friction coefficients of the PTFE composites can be decreased by one order of magnitude. Optical microscope investigation of transfer films shows that Pb, PbO, Pb3O4 and PbS enhance the adhesion of the transfer films to the surface of GCr15 bearing steel, so they greatly reduce the wear of the PTFE composites. However, the transfer of the PTFE composites onto the surface of GCr15 bearing steel can be greatly reduced by lubrication with liquid paraffin, but the transfer still takes place. SEM examination of worn surfaces shows that the interaction between liquid paraffin and the PTFE composites creates some cracks on the worn surfaces of the PTFE composites; the creation and development of the cracks reduces the load-carrying capacity of the PTFE composites, and this leads to deterioration of the friction and wear properties of the PTFE composites filled with lead or its compounds under higher loads in liquid paraffin lubrication. 1998 Elsevier Science Ltd. All rights reserved. Keywords: PTFE composites, lead, compounds, oil lubrication, friction and wear, frictional surfaces
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Friction and wear characteristics of lead: Zhao-Zhu Zhang et al.
Introduction Lead (Pb) and its compounds PbO, Pb3O4, and PbS etc. are important inorganic fillers for polymers. Their effects on the friction and wear behaviours of polymers and the interactions between polymers and lead compounds have been studied by some scholars1–6. It is known that the lubricity of PbO and Pb3O4 etc. is usually very poor at room temperature; they have been used as high-temperature lubricants in practice. Briscoe et al.1 found that filling Pb3O4 to high density polyethylene (HDPE) can greatly reduce the wear of the HDPE composite, but Pb3O4 increases the friction coefficient of the HDPE composite. Pocock et al.2 studied the interactions between PTFE and lead oxides such as PbO and Pb3O4 etc. by using differential scanning calorimetry (DSC). They found that no reaction occurred between PTFE and lead oxides from room temperature to 450°C. But Gao et al.3 found, by using DSC and X-ray photoelectron spectroscopy (XPS), that chemical reaction between PTFE and lead oxides occurred under static and frictional conditions. Gong et al.4–6 studied the effects of Pb, PbO and PbS etc. on the friction and wear of PTFE composites. They found that Pb, PbO and PbS etc. can greatly reduce the wear of the PTFE composites, but some of them increase the friction coefficients of the PTFE composites. Until now, almost all of the above studies have been carried out under dry friction (unlubricated) conditions. With enlargement of application fields of PTFE-based composites in practice, it is essential to study the friction and wear behaviours of lead or its compounds filled PTFE composites under lubricated conditions. It is known that many polymers wear much more in water than in air7–9. Watanabe et al.10–12 found that the wear of the PTFE composites filled with only glass fibres is much greater than that of other PTFE composites in water. However, until now, much less information has been available on friction and wear behaviours of the PTFE composites filled with lead or its compounds under oil lubricated conditions. The purpose of this work is to study the friction and wear behaviours of lead or its compounds filled PTFE composites under oil lubricated conditions, and give some insights into the friction and wear mechanisms of the PTFE composites in oil lubrication. It is expected that this study may be useful to the application of the lead or its compounds filled PTFE composites under oil lubricated conditions in practice. Experimental details The friction and wear tests were carried out on an MHK-500 ring-block wear tester (Timken wear tester) with a steel ring, which is 49.2 mm in diameter and 13.0 mm in length, rotating on a PTFE composite
Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 97, Lanzhou, 730000, People’s Republic of China *Corresponding author Received 13 January 1998; revised 25 March 1998; accepted 7 July 1998
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block, which is 12.3 ⫻ 12.3 ⫻ 18.9 mm in size. The steel ring, made of GCr15 bearing steel (its chemical composition is listed in Table 1), was polished with number 900 grade SiC abrasive paper to a surface roughness of Ra ⫽ 0.15 m. Meanwhile, the surfaces of the PTFE composite blocks were polished with number 800 grade SiC abrasive paper to a surface roughness of Ra ⫽ 0.2–0.4 m. Materials used for preparing the PTFE composites include PTFE powder with a grit size of about 30 m, Pb powder about 45 m, PbO, Pb3O4, and PbS powders about 76 m. The proportion of Pb, PbO, Pb3O4, and PbS powders as fillers in PTFE in each case was 30% by volume. Firstly, Pb, PbO, Pb3O4, and PbS powders were mixed completely with the PTFE powder, respectively. Secondly, these mixtures were moulded into the blocks by compression moulding under the pressure of 50 MPa. Finally, these PTFE composite blocks were sintered at 380°C for 3 h in air, and then cooled freely to room temperature. Five kinds of PTFE-based composites, such as PTFE, PTFE ⫹ 30(v)% Pb, PTFE ⫹ 30(v)% PbO, PTFE ⫹ 30(v)% Pb3O4, and PTFE ⫹ 30(v)% PbS composite, were prepared. The lubricating oil used in the experiments was liquid paraffin, which was added to the rubbing surfaces at a rate of 30 drops per minute during the tests. The friction and wear tests were performed at room temperature in ambient atmosphere with a sliding speed from 1.0 to 2.5 m/s and loads from 100 to 400 N for the dry friction conditions or from 100 to 1200 N for the oil lubricated conditions. Each friction and wear test was performed for 30 min. Before each test started, the surfaces of the PTFE composite block and the GCr15 bearing steel ring were cleaned by rubbing with a soft cloth dipped in acetone and then dried in air. The friction coefficient was determined by measuring the friction torque, while the friction torque was detected by a torque measuring system, so the friction coefficient could be calculated by the formula of friction coefficient for the Timken wear tester. The friction coefficient was the value in the steady stage of friction (the last 10 min) for each test. In this work, three to five samples were tested at each condition, the friction coefficient and wear were the average values of these tests for each condition. The wear was detected by the weight loss of the PTFE composite blocks after each test to an accuracy of 0.1 mg. Finally, the worn surfaces of the lead or its compounds filled PTFE composites were examined by using a JEM-1200EX/S scanning electron microscope (SEM), while the transfer films of the PTFE composites formed on the surface of GCr15 bearing steel ring were investigated by using an optical microscope.
Results and discussion Friction and wear properties under dry friction conditions
The friction and wear results of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under the load of 100 N and the sliding
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Friction and wear characteristics of lead: Zhao-Zhu Zhang et al. Table 1 Chemical composition of GCr15 bearing steel (wt.%) C 0.950–1.050
Mn
Si
Cr
P
S
Fe
0.200–0.400
0.150–0.350
1.300–1.650
⬍ 0.027
⬍ 0.020
Remainder
Table 2 Friction and wear results of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under dry friction condition (speed, 1.5 m/s; load, 100 N; time, 30 min) Material PTFE PTFE PTFE PTFE PTFE
⫹ ⫹ ⫹ ⫹
Friction coefficient 30(v)% 30(v)% 30(v)% 30(v)%
Pb PbO Pb3O4 PbS
0.257 0.248 0.261 0.232 0.395
speed of 1.5 m/s in the dry friction condition are shown in Table 2. It can be seen from Table 2 that, under the given conditions in this experiment, PbO has little effect on the friction coefficient of the PTFE composite, while PbS increases the friction coefficient of the PTFE composite. However, Pb and Pb3O4 reduce the friction coefficients of the PTFE composites, and the friction reducing action of Pb3O4 is better than that of Pb. The results in Table 2 also show that the wear of the PTFE composites can be greatly reduced by filling Pb, PbO, Pb3O4, or PbS to PTFE. The wear of Pb3O4 or PbS filled PTFE composites can be decreased by two orders of magnitude compared to that of pure PTFE, while the wear of Pb filled PTFE composite can be decreased by one order of magnitude. This indicates that the wear reducing actions of Pb3O4 and PbS are much better than those of Pb and PbO. The wear reducing action of Pb3O4 is the most effective, but that of PbO is the worst. Therefore, the friction and wear reducing actions of Pb3O4 are the most effective of the lead and its compounds. The variations of friction coefficients and wear with load for lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under dry friction conditions are shown in Figs 1 and 2, respectively. It
Fig. 1 Variations of friction coefficients with load for lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under dry friction conditions (sliding speed, 1.5 m/s)
Wear (mg) 385.4 20.5 125.6 1.1 3.3
Fig. 2 Variation of wear with load for lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under dry friction conditions (sliding speed, 1.5 m/s) can be seen from Fig 1 that the friction coefficients of lead or its compounds filled PTFE composites decrease with the increase of load under dry friction conditions. Under loads from 100 to 300 N in dry friction conditions, the friction property of the PTFE ⫹ 30(v)% Pb3O4 composite is the best, and that of the PTFE ⫹ 30(v)% PbS composite the worst. The results in Fig 2 show that the wear of these PTFE composites increases with the increase of load, and the wear of Pb3O4 or PbS filled PTFE composites is lower than that of Pb or PbO filled PTFE composites by a factor of 1–2 orders of magnitude under different loads in dry friction conditions. When the load is higher than 200 N, the wear of the PTFE ⫹ 30(v)% Pb composite increases sharply, but the wear of the PTFE ⫹ 30(v)% Pb3O4 composite is still the lowest. Therefore, the PTFE ⫹ 30(v)% Pb3O4 composite can be used as a kind of PTFE composite which has excellent friction and wear reducing properties under dry friction conditions. It is well known that PTFE-based composites are viscoelastic materials, their deformation under load is viscoelastic. Therefore, the variations of friction coefficients with load for lead or its compounds filled PTFE com-
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property of the PTFE ⫹ 30(v)% Pb composite is better than those of the other PTFE composites under lubrication of liquid paraffin. The results in Fig 4 show that the wear of the PTFE composites increases with the increase of load under lubrication of liquid paraffin. Under higher loads (> 600 N) in liquid paraffin lubricated conditions, the wear reducing property of the PTFE ⫹ 30(v)% Pb composite is better than those of the other PTFE composites. Therefore, the PTFE ⫹ 30(v)% Pb composite exhibits excellent friction and wear reducing properties under lubrication of liquid paraffin. Fig. 3 Variations of friction coefficients with load for the lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under lubrication of liquid paraffin (sliding speed, 2.5 m/s) posites can be described by the equation ⫽ KN (n ⫺ 1) , where is the friction coefficient, K is a constant, N is the load applied, and n is also a constant with value between 2/3 and 1 (2/3 ⬍ n ⬍ 1). According to this equation, the friction coefficients of the lead or its compounds filled PTFE composites decrease with the increase of load. Friction and wear properties under oil lubricated conditions
The variations of friction coefficients and wear with load for lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under lubrication of liquid paraffin are shown in Figs 3 and 4, respectively. Comparison of the friction and wear results in Figs 3 and 4 to those in the dry friction conditions shows that the friction and wear properties of lead or its compounds filled PTFE composites can be greatly improved by lubrication with liquid paraffin, and the friction coefficients of the PTFE composites can be decreased by one order of magnitude. Meanwhile, the results in Fig 3 show that, under lubrication of liquid paraffin, the friction coefficients of these PTFE composites first decrease with the increase of load, and then increase with the increase of load when the load is higher than 600 N. However, the friction reducing
Fig. 4 Variation of wear with load for lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under lubrication of liquid paraffin (sliding speed, 2.5 m/s) 364
The variations of friction coefficient and wear rate with sliding speed for the PTFE ⫹ 30(v)% Pb composite sliding against GCr15 bearing steel under lubrication of liquid paraffin are shown in Figs 5 and 6, respectively. The results in Figs 5 and 6 show that the friction coefficient and the wear rate of the PTFE ⫹ 30(v)% Pb composite first decrease with the increase of sliding speed, and then increase as the sliding speed increases. At the sliding speed of 2.0 m/s, the friction coefficient and the wear rate of the PTFE ⫹ 30(v)% Pb composite are the lowest. It is believed that, with the increase of sliding speed under liquid paraffin lubrication, a layer of lubricating oil film can be more easily formed on the frictional surfaces, then the lubrication condition at the rubbing surfaces can be greatly improved, this would lead to the decrease of the friction and wear of the PTFE composite. However, with the further increase of sliding speed, the temperature increase at rubbing surfaces results in the reduction of mechanical strength and load carrying capacity of the PTFE composite, and so, in turn, leads to the increase of the friction and wear of the PTFE composite. When sliding speed is a constant, the variations of friction coefficients with load for the PTFE composites filled with lead or its compounds under lower loads (⬍ 600 N) in liquid paraffin lubrication can be explained by the Stribeck’s curves of friction coef-
Fig. 5 Variation of friction coefficient with sliding speed for the PTFE ⫹ 30(v)% Pb composite sliding against GCr15 bearing steel under lubrication of liquid paraffin (load, 1000 N)
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Friction and wear characteristics of lead: Zhao-Zhu Zhang et al.
sliding against GCr15 bearing steel under lubrication of liquid paraffin. The results in Fig 7 show that, at a constant sliding speed under lubrication of liquid paraffin, the friction coefficient of the PTFE decreases with the increase of load. Therefore, the variations of friction coefficients with load for the lead or its compounds filled PTFE composites under lower loads (⬍ 600 N) in liquid paraffin lubrication can be explained properly by the Stribeck’s curve as given in Fig 7.
Fig. 6 Variation of wear rate with sliding speed for the PTFE ⫹ 30(v)% Pb composite sliding against GCr15 bearing steel under lubrication of liquid paraffin (load, 1000 N) ficients against the Sommerfeld variable N/P, where is the viscosity of liquid paraffin, N is the rotation speed of GCr15 bearing steel ring and, P is the load applied13,14. It is known that, at a constant sliding speed, the temperature at frictional surfaces increases with the increase of load, while the viscosity of liquid paraffin decreases with the increase of temperature but increases with the increase of load. Therefore, under the conditions in this experiment, the effect of viscosity on the Sommerfeld variable N/P is so small as compared to the effect of load on it that the N/P can be simplified to N/P. Fig 7 gives variation of friction coefficient as a function of velocity/load for pure PTFE
Fig. 7 Variation of friction coefficient as a function of velocity/load for pure PTFE sliding against GCr15 bearing steel under lubrication of liquid paraffin
Under higher loads (> 600 N) in liquid paraffin lubrication, the temperature increase at frictional surface results in the reduction of mechanical strength and load carrying capacity of the PTFE composites, so the friction and wear of the PTFE composites increase. Meanwhile, it was found in the experiments that there were some serious deformation or obvious cracks on the worn surfaces of lead or its compounds filled PTFE composites under certain loads in liquid paraffin lubrication. These loads under which serious deformation or obvious cracks occurred on the worn surfaces of the PTFE composites are the load limits of the PTFE composites. Fig 8 gives the load limits of lead or its compounds filled PTFE composites sliding against GCr15 bearing steel at the sliding speed of 2.5 m/s in liquid paraffin lubrication. It can be seen from Fig 8 that the load limits of the PTFE, PTFE ⫹ 30(v)% PbO and PTFE ⫹ 30(v)% PbS composites are 1000 N, while the load limits of the PTFE ⫹ 30(v)% Pb3O4 and PTFE ⫹ 30(v)% Pb composites are 600 N and 1200 N, respectively. It is believed that the interactions between liquid paraffin and the PTFE composites, especially the absorption of liquid paraffin into the surface layers of the PTFE composites, reduce the mechanical strength and load carrying capacity of the PTFE composites15,16, this leads to the deterioration of the friction and wear properties of the PTFE composites under higher loads (> 600 N) in liquid paraffin lubrication. Therefore, the friction and wear of the PTFE composites increase with the increase of load under higher loads (> 600 N) in liquid paraffin lubrication. When the load increases to the load limits of the PTFE composites, the wear of the PTFE composites increase sharply. Since the load carrying capacity of the PTFE ⫹ 30(v)% Pb composite is higher than that of the other PTFE composites, so the friction and wear
Fig. 8 Load limits of lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under lubrication of liquid paraffin (sliding speed, 2.5 m/s)
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properties of the PTFE ⫹ 30(v)% Pb composite are better than those of the other PTFE composites under higher loads (> 600 N) in liquid paraffin lubrication. Optical microscope investigation of transfer films
The optical micrographs of the transfer films formed on the surface of GCr15 bearing steel for the PTFE composites filled with lead or its compounds under both dry and liquid paraffin lubricated conditions are shown in Figs 9 and 10, respectively. It can be seen from Fig 9 that there are some obvious transfer films formed on the surface of GCr15 bearing steel for the PTFE composites filled with Pb, PbO, Pb3O4 or PbS under dry friction conditions, but no obvious transfer films formed on the surface of GCr15 bearing steel for pure PTFE. Correlating the above investigations with the results of friction and wear tests in dry friction conditions, it is believed that Pb, PbO, Pb3O4 and PbS enhance the adhesion of the transfer films to the surface of GCr15 bearing steel, and thus promote the transfer of the PTFE composites onto the surface of GCr15 bearing steel, so they greatly reduce the wear of the PTFE composites1,17. Meanwhile, the results in Fig 9 show that the transfer films formed on the surface of GCr15 bearing steel for Pb, PbO or Pb3O4 filled PTFE composites are much thicker than those for PbS filled PTFE composite. This indicates that the adhesion
Fig. 10 Optical micrographs of transfer films formed on the surface of GCr15 bearing steel for the PTFE composites filled with lead or its compounds under lubrication of liquid paraffin (128 ⫻) (sliding speed, 2.5 m/s): (a) PTFE ⫹ 30(v)% Pb, 1000 N; (b) PTFE ⫹ 30(v)% PbO, 300 N; (c) PTFE ⫹ 30(v)% Pb3O4, 300 N; and (d) PTFE ⫹ 30(v)% PbS, 1000 N between the surface of GCr15 bearing steel and the transfer film of PbS filled PTFE composite is weak, so it is difficult to form uniform transfer films on the surface of GCr15 bearing steel for PbS filled PTFE composite. However, Pb, PbO or Pb3O4 filled PTFE composites can easily form transfer films on the surface of GCr15 bearing steel, then the friction between the PTFE composites and GCr15 bearing steel can be transformed to the friction between the PTFE composites and its transfer films formed on the surface of GCr15 bearing steel. Therefore, the friction properties of Pb, PbO or Pb3O4 filled PTFE composites are better than that of PbS filled PTFE composite under dry friction conditions. Comparison of the results in Fig 10 with those in Fig 9 shows that the transfer of the PTFE composites filled with lead or its compounds onto the surface of GCr15 bearing steel can be greatly reduced by lubrication with liquid paraffin, but the transfer still takes place18,19. It is believed that, under lubrication of liquid paraffin, the formation of lubricating oil films on the rubbing surfaces changes the contact form of the friction pair, and then greatly improves the lubrication condition of the frictional surfaces, so the friction and wear properties of the PTFE composites can be greatly improved. The above analysis results are consistent with the results of the friction and wear tests. SEM investigation of worn surfaces
Fig. 9 Optical micrographs of transfer films formed on the surface of GCr15 bearing steel for the PTFE composites filled with lead or its compounds under the dry friction condition (128 ⫻ ) (sliding speed, 1.5 m/s; load, 100 N): (a) PTFE, (b) PTFE ⫹ 30(v)% Pb, (c) PTFE ⫹ 30(v)% PbO, (d) PTFE ⫹ 30(v)% Pb3O4, and (e) PTFE ⫹ 30(v)% PbS 366
It was found in the experiments that the width of the wear scar on the worn surface of pure PTFE was about 12 mm under the dry friction condition, while that of the PTFE ⫹ 30(v)% PbO composite was about 7 mm, but the width and the depth of the wear scars on the worn surfaces of Pb, PbS or Pb3O4 filled PTFE composites were much smaller than those of pure
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PTFE and the PTFE ⫹ 30(v)% PbO composite. Fig 11 gives electron micrographs of the worn surfaces of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under dry friction conditions. It can be seen from Fig 11 that the worn surface of the PTFE ⫹ 30(v)% PbO composite is very smooth, but there are still some smaller wear scars in the large wear scar of pure PTFE. Meanwhile, the wear scars on the worn surfaces of PbS or Pb3O4 filled PTFE composites are smaller than those of Pb filled PTFE composite. Therefore, it can be deduced that filling Pb, PbO, PbS or Pb3O4 to PTFE can greatly reduce the wear of the PTFE composites, but the wear reducing actions of PbS and Pb3O4 are better than those of Pb and PbO. Electron micrographs of the worn surfaces of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under lubrication of liquid paraffin are shown in Fig 12. It can be seen from Fig 12 that there are still some obvious wear scars on the worn surface of pure PTFE under lubrication of liquid paraffin, but no obvious wear scars on the worn surface of lead or its compounds filled PTFE composites. However, there are some obvious cracks on the worn surfaces of Pb, PbS or Pb3O4 filled PTFE composites. Therefore, it can be deduced that the interaction Fig. 12 Electron micrographs of worn surfaces of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under lubrication of liquid paraffin (sliding speed, 2.5 m/s): (a) PTFE, 1000 N; (b) PTFE ⫹ 30(v)% Pb, 1000 N; (c) PTFE ⫹ 30(v)% PbO, 1000 N; (d) PTFE ⫹ 30(v)% Pb3O4, 600 N; and (e) PTFE ⫹ 30(v)% PbS, 1000 N
between liquid paraffin and the PTFE composites, especially the absorption of liquid paraffin into surface layers of the PTFE composites, creates some cracks on the worn surfaces of the PTFE composites. The creation and development of these cracks reduces the mechanical strength and load carrying capacity of the PTFE composites; this leads to the deterioration of the friction and wear properties of the PTFE composites under higher loads in liquid paraffin lubrication. The above investigation and analysis results are consistent with the results of the friction and wear tests.
Fig. 11 Electron micrographs of worn surfaces of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under the dry friction condition (sliding speed, 1.5 m/s; load, 100 N): (a) PTFE, (b) PTFE ⫹ 30(v)% Pb, (c) PTFE ⫹ 30(v)% PbO, (d) PTFE ⫹ 30(v)% Pb3O4, and (e) PTFE ⫹ 30(v)% PbS
Conclusions Under load of 100 N and sliding speed of 1.5 m/s in the dry friction condition, PbO has little effect on the friction coefficient of the PTFE composite, while PbS increases the friction coefficient of the PTFE composite. However, Pb and Pb3O4 reduce the friction coefficients of the PTFE composites, and the friction reducing action of Pb3O4 is better than that of Pb. Under dry friction conditions, filling Pb, PbO, PbS, or Pb3O4 to PTFE can greatly reduce the wear of the PTFE composites. However, the wear reducing actions of PbS and Pb3O4 are much more effective than those of Pb and PbO, and the wear reducing action of Pb3O4 is the most effective.
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Under dry friction conditions, the friction coefficients of Pb, PbO, PbS, or Pb3O4 filled PTFE composites decrease with the increase of load, but the wear of these PTFE composites increases with the increase of load. Under different loads in the dry friction conditions, the wear of Pb3O4 or PbS filled PTFE composites is lower than that of Pb or PbO filled PTFE composites by a factor of 1–2 orders of magnitude. Meanwhile, the friction and wear properties of the PTFE ⫹ 30(v)% Pb3O4 composite are better than those of Pb, PbO or PbS filled PTFE composites under the loads from 100 to 300 N in the dry friction conditions. The friction and wear properties of Pb, PbO, PbS, or Pb3O4 filled PTFE composites can be greatly improved by lubrication with liquid paraffin, and the friction coefficients of these PTFE composites can be decreased by 1 order of magnitude compared to those in the dry friction conditions. Under lubrication of liquid paraffin, the wear of Pb, PbO, PbS, or Pb3O4 filled PTFE composites increases with the increase of load, but the friction coefficients of the PTFE composites first decrease with the increase of load, and then increase as the load increases. The friction and wear properties of the PTFE ⫹ 30(v)% Pb composite are better than those of the other PTFE composites under higher loads (> 600 N) in liquid paraffin lubrication.
posites. The creation and development of these cracks reduces the mechanical strength and load carrying capacity of the PTFE composites. This would lead to the deterioration of the friction and wear properties of the PTFE composites under higher loads in liquid paraffin lubrication.
References 1. Briscoe, B. J., Pogosian, A. K. and Tabor, D., Wear, 1974, 27, 19. 2. Pocock, G. and Cadman, P., Wear, 1976, 37, 129. 3. Gao, J., Zhao, J. and Dang, H., In ASLE Proc. 3rd Int. Conf. on Solid Lubrication, ed. E. E. Bisson. ASLE, Park Ridge, NJ, 1984, p. 308. 4. Gong, D., Xue, Q. and Wang, H., Wear, 1989, 134, 283. 5. Gong, D., Zhang, B., Xue, Q. and Wang, H., Wear, 1990, 137, 25. 6. Bahadur, S. and Gong, D., Wear, 1992, 158, 41. 7. Zhongqian, Y., Manquing, L. and Hailing, K. In Wear of Materials, ed. S. K. Rhee, A. W. Ruff and K. C. Ludeman. ASME, New York, 1981, p. 153. 8. Stolarski, T. A., Wear, 1980, 58, 103. 9. Clarke, C. G. and Allen, C., Tribol. Int., 1991, 24, 109. 10. Watanabe, M., Wear, 1992, 158, 79. 11. Tanaka, K. In Friction and Wear of Polymer Composites, ed. K. Friedrich. Elsevier, Amsterdam, 1986, p. 137. 12. Lancaster, J. K., Wear, 1972, 20, 315.
Pb, PbO, Pb3O4 and PbS enhance the adhesion of the transfer films to the surface of GCr15 bearing steel, and thus promote the transfer of the PTFE composites onto the surface of GCr15 bearing steel, so they greatly reduce the wear of the PTFE composites. However, the transfer of the PTFE composites onto the surface of GCr15 bearing steel can be greatly reduced by lubrication with liquid paraffin, but the transfer still takes place.
13. Dickens, P. M., Sullivan, J. L. and Lancaster, J. K., Wear, 1986, 112, 273.
The interaction between liquid paraffin and the PTFE composites, especially the absorption of liquid paraffin into the surface layers of the PTFE composites, creates some cracks on the worn surfaces of the PTFE com-
18. Zhang, Z. Z., Shen, W. C. and Liu, W. M., Wear, 1996, 193, 163.
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14. Yamaguchi, Y. Tribology of Plastic Materials. Elsevier, Amsterdam, 1990, p. 203. 15. Briscoe, B. J., Stolarski, T. A. and Davies, G. J., Tribol. Int., 1984, 17, 129. 16. Evans, D. C. Proc. 3rd Leeds-Lyon Symp. on Wear of Nonmetallic Materials. Mechanical Engineering Publications, London, 1978, p. 47. 17. Bahadur, S. and Tabor, D., Wear, 1984, 98, 1.
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Friction and wear characteristics of lead and its compounds filled polytetrafluoroethylene composites under oil lubricated conditions Zhao-Zhu Zhang*, Qun-Ji Xue, Wei-Min Liu and Wei-Chang Shen
The friction and wear properties of Pb, PbO, Pb3O4, or PbS filled polytetrafluoroethylene (PTFE) composites sliding against GCr15 bearing steel under both dry and liquid paraffin lubricated conditions were studied by using an MHK-500 ring-block wear tester. The worn surfaces and the transfer films of these PTFE composites formed on the surface of GCr15 bearing steel were then investigated by using a scanning electron microscope (SEM) and an optical microscope, respectively. Experimental results show that filling Pb, PbO, Pb3O4 or PbS to PTFE can greatly reduce the wear of the PTFE composites, but the wear reducing action of Pb3O4 is the most effective. Meanwhile, PbS increases the friction coefficient of the PTFE composite, but Pb and Pb3O4 reduce the friction coefficients of the PTFE composites. However, the friction and wear properties of lead or its compounds filled PTFE composites can be greatly improved by lubrication with liquid paraffin, and the friction coefficients of the PTFE composites can be decreased by one order of magnitude. Optical microscope investigation of transfer films shows that Pb, PbO, Pb3O4 and PbS enhance the adhesion of the transfer films to the surface of GCr15 bearing steel, so they greatly reduce the wear of the PTFE composites. However, the transfer of the PTFE composites onto the surface of GCr15 bearing steel can be greatly reduced by lubrication with liquid paraffin, but the transfer still takes place. SEM examination of worn surfaces shows that the interaction between liquid paraffin and the PTFE composites creates some cracks on the worn surfaces of the PTFE composites; the creation and development of the cracks reduces the load-carrying capacity of the PTFE composites, and this leads to deterioration of the friction and wear properties of the PTFE composites filled with lead or its compounds under higher loads in liquid paraffin lubrication. 1998 Elsevier Science Ltd. All rights reserved. Keywords: PTFE composites, lead, compounds, oil lubrication, friction and wear, frictional surfaces
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Introduction Lead (Pb) and its compounds PbO, Pb3O4, and PbS etc. are important inorganic fillers for polymers. Their effects on the friction and wear behaviours of polymers and the interactions between polymers and lead compounds have been studied by some scholars1–6. It is known that the lubricity of PbO and Pb3O4 etc. is usually very poor at room temperature; they have been used as high-temperature lubricants in practice. Briscoe et al.1 found that filling Pb3O4 to high density polyethylene (HDPE) can greatly reduce the wear of the HDPE composite, but Pb3O4 increases the friction coefficient of the HDPE composite. Pocock et al.2 studied the interactions between PTFE and lead oxides such as PbO and Pb3O4 etc. by using differential scanning calorimetry (DSC). They found that no reaction occurred between PTFE and lead oxides from room temperature to 450°C. But Gao et al.3 found, by using DSC and X-ray photoelectron spectroscopy (XPS), that chemical reaction between PTFE and lead oxides occurred under static and frictional conditions. Gong et al.4–6 studied the effects of Pb, PbO and PbS etc. on the friction and wear of PTFE composites. They found that Pb, PbO and PbS etc. can greatly reduce the wear of the PTFE composites, but some of them increase the friction coefficients of the PTFE composites. Until now, almost all of the above studies have been carried out under dry friction (unlubricated) conditions. With enlargement of application fields of PTFE-based composites in practice, it is essential to study the friction and wear behaviours of lead or its compounds filled PTFE composites under lubricated conditions. It is known that many polymers wear much more in water than in air7–9. Watanabe et al.10–12 found that the wear of the PTFE composites filled with only glass fibres is much greater than that of other PTFE composites in water. However, until now, much less information has been available on friction and wear behaviours of the PTFE composites filled with lead or its compounds under oil lubricated conditions. The purpose of this work is to study the friction and wear behaviours of lead or its compounds filled PTFE composites under oil lubricated conditions, and give some insights into the friction and wear mechanisms of the PTFE composites in oil lubrication. It is expected that this study may be useful to the application of the lead or its compounds filled PTFE composites under oil lubricated conditions in practice. Experimental details The friction and wear tests were carried out on an MHK-500 ring-block wear tester (Timken wear tester) with a steel ring, which is 49.2 mm in diameter and 13.0 mm in length, rotating on a PTFE composite
Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 97, Lanzhou, 730000, People’s Republic of China *Corresponding author Received 13 January 1998; revised 25 March 1998; accepted 7 July 1998
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block, which is 12.3 ⫻ 12.3 ⫻ 18.9 mm in size. The steel ring, made of GCr15 bearing steel (its chemical composition is listed in Table 1), was polished with number 900 grade SiC abrasive paper to a surface roughness of Ra ⫽ 0.15 m. Meanwhile, the surfaces of the PTFE composite blocks were polished with number 800 grade SiC abrasive paper to a surface roughness of Ra ⫽ 0.2–0.4 m. Materials used for preparing the PTFE composites include PTFE powder with a grit size of about 30 m, Pb powder about 45 m, PbO, Pb3O4, and PbS powders about 76 m. The proportion of Pb, PbO, Pb3O4, and PbS powders as fillers in PTFE in each case was 30% by volume. Firstly, Pb, PbO, Pb3O4, and PbS powders were mixed completely with the PTFE powder, respectively. Secondly, these mixtures were moulded into the blocks by compression moulding under the pressure of 50 MPa. Finally, these PTFE composite blocks were sintered at 380°C for 3 h in air, and then cooled freely to room temperature. Five kinds of PTFE-based composites, such as PTFE, PTFE ⫹ 30(v)% Pb, PTFE ⫹ 30(v)% PbO, PTFE ⫹ 30(v)% Pb3O4, and PTFE ⫹ 30(v)% PbS composite, were prepared. The lubricating oil used in the experiments was liquid paraffin, which was added to the rubbing surfaces at a rate of 30 drops per minute during the tests. The friction and wear tests were performed at room temperature in ambient atmosphere with a sliding speed from 1.0 to 2.5 m/s and loads from 100 to 400 N for the dry friction conditions or from 100 to 1200 N for the oil lubricated conditions. Each friction and wear test was performed for 30 min. Before each test started, the surfaces of the PTFE composite block and the GCr15 bearing steel ring were cleaned by rubbing with a soft cloth dipped in acetone and then dried in air. The friction coefficient was determined by measuring the friction torque, while the friction torque was detected by a torque measuring system, so the friction coefficient could be calculated by the formula of friction coefficient for the Timken wear tester. The friction coefficient was the value in the steady stage of friction (the last 10 min) for each test. In this work, three to five samples were tested at each condition, the friction coefficient and wear were the average values of these tests for each condition. The wear was detected by the weight loss of the PTFE composite blocks after each test to an accuracy of 0.1 mg. Finally, the worn surfaces of the lead or its compounds filled PTFE composites were examined by using a JEM-1200EX/S scanning electron microscope (SEM), while the transfer films of the PTFE composites formed on the surface of GCr15 bearing steel ring were investigated by using an optical microscope.
Results and discussion Friction and wear properties under dry friction conditions
The friction and wear results of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under the load of 100 N and the sliding
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Friction and wear characteristics of lead: Zhao-Zhu Zhang et al. Table 1 Chemical composition of GCr15 bearing steel (wt.%) C 0.950–1.050
Mn
Si
Cr
P
S
Fe
0.200–0.400
0.150–0.350
1.300–1.650
⬍ 0.027
⬍ 0.020
Remainder
Table 2 Friction and wear results of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under dry friction condition (speed, 1.5 m/s; load, 100 N; time, 30 min) Material PTFE PTFE PTFE PTFE PTFE
⫹ ⫹ ⫹ ⫹
Friction coefficient 30(v)% 30(v)% 30(v)% 30(v)%
Pb PbO Pb3O4 PbS
0.257 0.248 0.261 0.232 0.395
speed of 1.5 m/s in the dry friction condition are shown in Table 2. It can be seen from Table 2 that, under the given conditions in this experiment, PbO has little effect on the friction coefficient of the PTFE composite, while PbS increases the friction coefficient of the PTFE composite. However, Pb and Pb3O4 reduce the friction coefficients of the PTFE composites, and the friction reducing action of Pb3O4 is better than that of Pb. The results in Table 2 also show that the wear of the PTFE composites can be greatly reduced by filling Pb, PbO, Pb3O4, or PbS to PTFE. The wear of Pb3O4 or PbS filled PTFE composites can be decreased by two orders of magnitude compared to that of pure PTFE, while the wear of Pb filled PTFE composite can be decreased by one order of magnitude. This indicates that the wear reducing actions of Pb3O4 and PbS are much better than those of Pb and PbO. The wear reducing action of Pb3O4 is the most effective, but that of PbO is the worst. Therefore, the friction and wear reducing actions of Pb3O4 are the most effective of the lead and its compounds. The variations of friction coefficients and wear with load for lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under dry friction conditions are shown in Figs 1 and 2, respectively. It
Fig. 1 Variations of friction coefficients with load for lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under dry friction conditions (sliding speed, 1.5 m/s)
Wear (mg) 385.4 20.5 125.6 1.1 3.3
Fig. 2 Variation of wear with load for lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under dry friction conditions (sliding speed, 1.5 m/s) can be seen from Fig 1 that the friction coefficients of lead or its compounds filled PTFE composites decrease with the increase of load under dry friction conditions. Under loads from 100 to 300 N in dry friction conditions, the friction property of the PTFE ⫹ 30(v)% Pb3O4 composite is the best, and that of the PTFE ⫹ 30(v)% PbS composite the worst. The results in Fig 2 show that the wear of these PTFE composites increases with the increase of load, and the wear of Pb3O4 or PbS filled PTFE composites is lower than that of Pb or PbO filled PTFE composites by a factor of 1–2 orders of magnitude under different loads in dry friction conditions. When the load is higher than 200 N, the wear of the PTFE ⫹ 30(v)% Pb composite increases sharply, but the wear of the PTFE ⫹ 30(v)% Pb3O4 composite is still the lowest. Therefore, the PTFE ⫹ 30(v)% Pb3O4 composite can be used as a kind of PTFE composite which has excellent friction and wear reducing properties under dry friction conditions. It is well known that PTFE-based composites are viscoelastic materials, their deformation under load is viscoelastic. Therefore, the variations of friction coefficients with load for lead or its compounds filled PTFE com-
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property of the PTFE ⫹ 30(v)% Pb composite is better than those of the other PTFE composites under lubrication of liquid paraffin. The results in Fig 4 show that the wear of the PTFE composites increases with the increase of load under lubrication of liquid paraffin. Under higher loads (> 600 N) in liquid paraffin lubricated conditions, the wear reducing property of the PTFE ⫹ 30(v)% Pb composite is better than those of the other PTFE composites. Therefore, the PTFE ⫹ 30(v)% Pb composite exhibits excellent friction and wear reducing properties under lubrication of liquid paraffin. Fig. 3 Variations of friction coefficients with load for the lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under lubrication of liquid paraffin (sliding speed, 2.5 m/s) posites can be described by the equation ⫽ KN (n ⫺ 1) , where is the friction coefficient, K is a constant, N is the load applied, and n is also a constant with value between 2/3 and 1 (2/3 ⬍ n ⬍ 1). According to this equation, the friction coefficients of the lead or its compounds filled PTFE composites decrease with the increase of load. Friction and wear properties under oil lubricated conditions
The variations of friction coefficients and wear with load for lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under lubrication of liquid paraffin are shown in Figs 3 and 4, respectively. Comparison of the friction and wear results in Figs 3 and 4 to those in the dry friction conditions shows that the friction and wear properties of lead or its compounds filled PTFE composites can be greatly improved by lubrication with liquid paraffin, and the friction coefficients of the PTFE composites can be decreased by one order of magnitude. Meanwhile, the results in Fig 3 show that, under lubrication of liquid paraffin, the friction coefficients of these PTFE composites first decrease with the increase of load, and then increase with the increase of load when the load is higher than 600 N. However, the friction reducing
Fig. 4 Variation of wear with load for lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under lubrication of liquid paraffin (sliding speed, 2.5 m/s) 364
The variations of friction coefficient and wear rate with sliding speed for the PTFE ⫹ 30(v)% Pb composite sliding against GCr15 bearing steel under lubrication of liquid paraffin are shown in Figs 5 and 6, respectively. The results in Figs 5 and 6 show that the friction coefficient and the wear rate of the PTFE ⫹ 30(v)% Pb composite first decrease with the increase of sliding speed, and then increase as the sliding speed increases. At the sliding speed of 2.0 m/s, the friction coefficient and the wear rate of the PTFE ⫹ 30(v)% Pb composite are the lowest. It is believed that, with the increase of sliding speed under liquid paraffin lubrication, a layer of lubricating oil film can be more easily formed on the frictional surfaces, then the lubrication condition at the rubbing surfaces can be greatly improved, this would lead to the decrease of the friction and wear of the PTFE composite. However, with the further increase of sliding speed, the temperature increase at rubbing surfaces results in the reduction of mechanical strength and load carrying capacity of the PTFE composite, and so, in turn, leads to the increase of the friction and wear of the PTFE composite. When sliding speed is a constant, the variations of friction coefficients with load for the PTFE composites filled with lead or its compounds under lower loads (⬍ 600 N) in liquid paraffin lubrication can be explained by the Stribeck’s curves of friction coef-
Fig. 5 Variation of friction coefficient with sliding speed for the PTFE ⫹ 30(v)% Pb composite sliding against GCr15 bearing steel under lubrication of liquid paraffin (load, 1000 N)
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Friction and wear characteristics of lead: Zhao-Zhu Zhang et al.
sliding against GCr15 bearing steel under lubrication of liquid paraffin. The results in Fig 7 show that, at a constant sliding speed under lubrication of liquid paraffin, the friction coefficient of the PTFE decreases with the increase of load. Therefore, the variations of friction coefficients with load for the lead or its compounds filled PTFE composites under lower loads (⬍ 600 N) in liquid paraffin lubrication can be explained properly by the Stribeck’s curve as given in Fig 7.
Fig. 6 Variation of wear rate with sliding speed for the PTFE ⫹ 30(v)% Pb composite sliding against GCr15 bearing steel under lubrication of liquid paraffin (load, 1000 N) ficients against the Sommerfeld variable N/P, where is the viscosity of liquid paraffin, N is the rotation speed of GCr15 bearing steel ring and, P is the load applied13,14. It is known that, at a constant sliding speed, the temperature at frictional surfaces increases with the increase of load, while the viscosity of liquid paraffin decreases with the increase of temperature but increases with the increase of load. Therefore, under the conditions in this experiment, the effect of viscosity on the Sommerfeld variable N/P is so small as compared to the effect of load on it that the N/P can be simplified to N/P. Fig 7 gives variation of friction coefficient as a function of velocity/load for pure PTFE
Fig. 7 Variation of friction coefficient as a function of velocity/load for pure PTFE sliding against GCr15 bearing steel under lubrication of liquid paraffin
Under higher loads (> 600 N) in liquid paraffin lubrication, the temperature increase at frictional surface results in the reduction of mechanical strength and load carrying capacity of the PTFE composites, so the friction and wear of the PTFE composites increase. Meanwhile, it was found in the experiments that there were some serious deformation or obvious cracks on the worn surfaces of lead or its compounds filled PTFE composites under certain loads in liquid paraffin lubrication. These loads under which serious deformation or obvious cracks occurred on the worn surfaces of the PTFE composites are the load limits of the PTFE composites. Fig 8 gives the load limits of lead or its compounds filled PTFE composites sliding against GCr15 bearing steel at the sliding speed of 2.5 m/s in liquid paraffin lubrication. It can be seen from Fig 8 that the load limits of the PTFE, PTFE ⫹ 30(v)% PbO and PTFE ⫹ 30(v)% PbS composites are 1000 N, while the load limits of the PTFE ⫹ 30(v)% Pb3O4 and PTFE ⫹ 30(v)% Pb composites are 600 N and 1200 N, respectively. It is believed that the interactions between liquid paraffin and the PTFE composites, especially the absorption of liquid paraffin into the surface layers of the PTFE composites, reduce the mechanical strength and load carrying capacity of the PTFE composites15,16, this leads to the deterioration of the friction and wear properties of the PTFE composites under higher loads (> 600 N) in liquid paraffin lubrication. Therefore, the friction and wear of the PTFE composites increase with the increase of load under higher loads (> 600 N) in liquid paraffin lubrication. When the load increases to the load limits of the PTFE composites, the wear of the PTFE composites increase sharply. Since the load carrying capacity of the PTFE ⫹ 30(v)% Pb composite is higher than that of the other PTFE composites, so the friction and wear
Fig. 8 Load limits of lead or its compounds filled PTFE composites sliding against GCr15 bearing steel under lubrication of liquid paraffin (sliding speed, 2.5 m/s)
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properties of the PTFE ⫹ 30(v)% Pb composite are better than those of the other PTFE composites under higher loads (> 600 N) in liquid paraffin lubrication. Optical microscope investigation of transfer films
The optical micrographs of the transfer films formed on the surface of GCr15 bearing steel for the PTFE composites filled with lead or its compounds under both dry and liquid paraffin lubricated conditions are shown in Figs 9 and 10, respectively. It can be seen from Fig 9 that there are some obvious transfer films formed on the surface of GCr15 bearing steel for the PTFE composites filled with Pb, PbO, Pb3O4 or PbS under dry friction conditions, but no obvious transfer films formed on the surface of GCr15 bearing steel for pure PTFE. Correlating the above investigations with the results of friction and wear tests in dry friction conditions, it is believed that Pb, PbO, Pb3O4 and PbS enhance the adhesion of the transfer films to the surface of GCr15 bearing steel, and thus promote the transfer of the PTFE composites onto the surface of GCr15 bearing steel, so they greatly reduce the wear of the PTFE composites1,17. Meanwhile, the results in Fig 9 show that the transfer films formed on the surface of GCr15 bearing steel for Pb, PbO or Pb3O4 filled PTFE composites are much thicker than those for PbS filled PTFE composite. This indicates that the adhesion
Fig. 10 Optical micrographs of transfer films formed on the surface of GCr15 bearing steel for the PTFE composites filled with lead or its compounds under lubrication of liquid paraffin (128 ⫻) (sliding speed, 2.5 m/s): (a) PTFE ⫹ 30(v)% Pb, 1000 N; (b) PTFE ⫹ 30(v)% PbO, 300 N; (c) PTFE ⫹ 30(v)% Pb3O4, 300 N; and (d) PTFE ⫹ 30(v)% PbS, 1000 N between the surface of GCr15 bearing steel and the transfer film of PbS filled PTFE composite is weak, so it is difficult to form uniform transfer films on the surface of GCr15 bearing steel for PbS filled PTFE composite. However, Pb, PbO or Pb3O4 filled PTFE composites can easily form transfer films on the surface of GCr15 bearing steel, then the friction between the PTFE composites and GCr15 bearing steel can be transformed to the friction between the PTFE composites and its transfer films formed on the surface of GCr15 bearing steel. Therefore, the friction properties of Pb, PbO or Pb3O4 filled PTFE composites are better than that of PbS filled PTFE composite under dry friction conditions. Comparison of the results in Fig 10 with those in Fig 9 shows that the transfer of the PTFE composites filled with lead or its compounds onto the surface of GCr15 bearing steel can be greatly reduced by lubrication with liquid paraffin, but the transfer still takes place18,19. It is believed that, under lubrication of liquid paraffin, the formation of lubricating oil films on the rubbing surfaces changes the contact form of the friction pair, and then greatly improves the lubrication condition of the frictional surfaces, so the friction and wear properties of the PTFE composites can be greatly improved. The above analysis results are consistent with the results of the friction and wear tests. SEM investigation of worn surfaces
Fig. 9 Optical micrographs of transfer films formed on the surface of GCr15 bearing steel for the PTFE composites filled with lead or its compounds under the dry friction condition (128 ⫻ ) (sliding speed, 1.5 m/s; load, 100 N): (a) PTFE, (b) PTFE ⫹ 30(v)% Pb, (c) PTFE ⫹ 30(v)% PbO, (d) PTFE ⫹ 30(v)% Pb3O4, and (e) PTFE ⫹ 30(v)% PbS 366
It was found in the experiments that the width of the wear scar on the worn surface of pure PTFE was about 12 mm under the dry friction condition, while that of the PTFE ⫹ 30(v)% PbO composite was about 7 mm, but the width and the depth of the wear scars on the worn surfaces of Pb, PbS or Pb3O4 filled PTFE composites were much smaller than those of pure
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Friction and wear characteristics of lead: Zhao-Zhu Zhang et al.
PTFE and the PTFE ⫹ 30(v)% PbO composite. Fig 11 gives electron micrographs of the worn surfaces of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under dry friction conditions. It can be seen from Fig 11 that the worn surface of the PTFE ⫹ 30(v)% PbO composite is very smooth, but there are still some smaller wear scars in the large wear scar of pure PTFE. Meanwhile, the wear scars on the worn surfaces of PbS or Pb3O4 filled PTFE composites are smaller than those of Pb filled PTFE composite. Therefore, it can be deduced that filling Pb, PbO, PbS or Pb3O4 to PTFE can greatly reduce the wear of the PTFE composites, but the wear reducing actions of PbS and Pb3O4 are better than those of Pb and PbO. Electron micrographs of the worn surfaces of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under lubrication of liquid paraffin are shown in Fig 12. It can be seen from Fig 12 that there are still some obvious wear scars on the worn surface of pure PTFE under lubrication of liquid paraffin, but no obvious wear scars on the worn surface of lead or its compounds filled PTFE composites. However, there are some obvious cracks on the worn surfaces of Pb, PbS or Pb3O4 filled PTFE composites. Therefore, it can be deduced that the interaction Fig. 12 Electron micrographs of worn surfaces of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under lubrication of liquid paraffin (sliding speed, 2.5 m/s): (a) PTFE, 1000 N; (b) PTFE ⫹ 30(v)% Pb, 1000 N; (c) PTFE ⫹ 30(v)% PbO, 1000 N; (d) PTFE ⫹ 30(v)% Pb3O4, 600 N; and (e) PTFE ⫹ 30(v)% PbS, 1000 N
between liquid paraffin and the PTFE composites, especially the absorption of liquid paraffin into surface layers of the PTFE composites, creates some cracks on the worn surfaces of the PTFE composites. The creation and development of these cracks reduces the mechanical strength and load carrying capacity of the PTFE composites; this leads to the deterioration of the friction and wear properties of the PTFE composites under higher loads in liquid paraffin lubrication. The above investigation and analysis results are consistent with the results of the friction and wear tests.
Fig. 11 Electron micrographs of worn surfaces of the PTFE composites filled with lead or its compounds sliding against GCr15 bearing steel under the dry friction condition (sliding speed, 1.5 m/s; load, 100 N): (a) PTFE, (b) PTFE ⫹ 30(v)% Pb, (c) PTFE ⫹ 30(v)% PbO, (d) PTFE ⫹ 30(v)% Pb3O4, and (e) PTFE ⫹ 30(v)% PbS
Conclusions Under load of 100 N and sliding speed of 1.5 m/s in the dry friction condition, PbO has little effect on the friction coefficient of the PTFE composite, while PbS increases the friction coefficient of the PTFE composite. However, Pb and Pb3O4 reduce the friction coefficients of the PTFE composites, and the friction reducing action of Pb3O4 is better than that of Pb. Under dry friction conditions, filling Pb, PbO, PbS, or Pb3O4 to PTFE can greatly reduce the wear of the PTFE composites. However, the wear reducing actions of PbS and Pb3O4 are much more effective than those of Pb and PbO, and the wear reducing action of Pb3O4 is the most effective.
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Under dry friction conditions, the friction coefficients of Pb, PbO, PbS, or Pb3O4 filled PTFE composites decrease with the increase of load, but the wear of these PTFE composites increases with the increase of load. Under different loads in the dry friction conditions, the wear of Pb3O4 or PbS filled PTFE composites is lower than that of Pb or PbO filled PTFE composites by a factor of 1–2 orders of magnitude. Meanwhile, the friction and wear properties of the PTFE ⫹ 30(v)% Pb3O4 composite are better than those of Pb, PbO or PbS filled PTFE composites under the loads from 100 to 300 N in the dry friction conditions. The friction and wear properties of Pb, PbO, PbS, or Pb3O4 filled PTFE composites can be greatly improved by lubrication with liquid paraffin, and the friction coefficients of these PTFE composites can be decreased by 1 order of magnitude compared to those in the dry friction conditions. Under lubrication of liquid paraffin, the wear of Pb, PbO, PbS, or Pb3O4 filled PTFE composites increases with the increase of load, but the friction coefficients of the PTFE composites first decrease with the increase of load, and then increase as the load increases. The friction and wear properties of the PTFE ⫹ 30(v)% Pb composite are better than those of the other PTFE composites under higher loads (> 600 N) in liquid paraffin lubrication.
posites. The creation and development of these cracks reduces the mechanical strength and load carrying capacity of the PTFE composites. This would lead to the deterioration of the friction and wear properties of the PTFE composites under higher loads in liquid paraffin lubrication.
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Pb, PbO, Pb3O4 and PbS enhance the adhesion of the transfer films to the surface of GCr15 bearing steel, and thus promote the transfer of the PTFE composites onto the surface of GCr15 bearing steel, so they greatly reduce the wear of the PTFE composites. However, the transfer of the PTFE composites onto the surface of GCr15 bearing steel can be greatly reduced by lubrication with liquid paraffin, but the transfer still takes place.
13. Dickens, P. M., Sullivan, J. L. and Lancaster, J. K., Wear, 1986, 112, 273.
The interaction between liquid paraffin and the PTFE composites, especially the absorption of liquid paraffin into the surface layers of the PTFE composites, creates some cracks on the worn surfaces of the PTFE com-
18. Zhang, Z. Z., Shen, W. C. and Liu, W. M., Wear, 1996, 193, 163.
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14. Yamaguchi, Y. Tribology of Plastic Materials. Elsevier, Amsterdam, 1990, p. 203. 15. Briscoe, B. J., Stolarski, T. A. and Davies, G. J., Tribol. Int., 1984, 17, 129. 16. Evans, D. C. Proc. 3rd Leeds-Lyon Symp. on Wear of Nonmetallic Materials. Mechanical Engineering Publications, London, 1978, p. 47. 17. Bahadur, S. and Tabor, D., Wear, 1984, 98, 1.
19. Zhang, Z. Z., Shen, W. C. and Liu, W. M., Wear, 1996, 196, 164.
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