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Effect of the degree of crystallinity on the friction and wear of poly(ethylene terephthalate) under water lubrication
Wear, 111 (1986)
63 - 72
63
EFFECT OF THE DEGREE OF CRYSTALLINI’IY ON THE FRICTION AND WEAR OF POLY(ETHYLENE TEREPHTHALATE) UNDER WATER LUBRICATION YOSHINORI Faculty
YAMADA and KYUICHIRO
of Engineering,
Kanazawa
TANAKA
University,
(Received September 10,1985;accepted
Kanazawa
920 (Japan)
January 2,1986)
summary By means of a pin-on-disk type of wear-testing apparatus, coefficients of friction and wear depths of various poly(ethylene terephthalate) (PET) specimens with different degrees of crystallinity were measured at sliding speeds of 0.01 and 0.1 m s-l under a constant load of 10 N with water lubrication. The coefficients of friction with lubrication were lower than those for unlubricated conditions and there was little dependence on the degree of crystallinity. However, the wear rates with lubrication were higher than those obtained under dry conditions and decreased with increasing degree of crystallinity, in contrast with the unlubricated wear rates. These results are discussed on the basis of the plasticization of worn polymer surfaces by water. The worn surfaces and frictional tracks were observed by optical and electron microscopy. The frictional tracks for low crystallinity PET were smooth with many fine scratches and characteristic of transfer wear. For high crystallinity, frictional tracks were somewhat different from those for low crystallinity PET.
1. Introduction It is well known that the friction and wear of polymeric materials under boundary lubrication are complicated by the presence of lubricant fluid. Possible explanations have been proposed by several investigators to interpret the observed wear behaviour, and especially the fact that the wear rate is higher in the lubricated condition than in the dry condition. Evans [l] showed that for amorphous polymers, such as poly(2,6dimethyl 1,4-phenylene oxide) and poly(methy1 methacrylate), a higher wear rate was observed with lubrication using the fluids whose solubility parameters were close to those of polymeric materials. Moreover, he reported that for semicrystalline polymers, such as polytetrafluoroethylene (PTFE) and polyacetal, lubricant fluids primarily influenced wear via their 0043-1646/66/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
64
effect on the transfer film formation; where transfer plays little part in the dry wear process, fluids do not greatly change the wear rate but, where transfer is responsible for a very low wear rate, lubricant fluids appreciably increase the wear rate. He also showed, however, that the wear rate was exceptionally high in all polymers studied when water was used as the lubricant fluid. Lancaster [2] reported that water and aqueous solutions inhibit the formation of transfer films on a metal counterface, and the wear rates of carbon-fibre-reinforced polymers are generally greater than those occurring during dry sliding. In contrast, the effect of the plasticization of polymers, which may be caused by absorption of lubricant fluids, must also be taken into account to understand the friction and wear behaviour of polymeric material under lubrication. Rubenstein [3] studied the friction of polymers with boundary lubrication by various fluids and suggested that the *mechanical strength of polymers may be reduced by the plasticization of polymer surfaces because of the permeation of lubricant fluids into the polymer surfaces. Tanaka [4] suggested that the wear rates of semicrystalline polymers such as PTFE, poly(aceta1) and nylon 6 with water lubrication became higher than those under unlubricated conditions because of the modification of the polymer surface by water. As shown by Lasoski and Cobbs [ 51, the permeation of water into semicrystalline polymers may occur preferentially in their amorphous phases. In order to obtain evidence for plasticization with water lubrication, the effect of crystallinity on the friction and wear of polymers should be examined both under dry conditions and with water lubrication. One of the best polymers for such examination is poly(ethylene terephthalate) (PET), because its crystallinity can be varied over a wide range by heat treatment. We have reported elsewhere the results of a study of the effect of crystallinity on the friction and wear of PET in the dry condition
[61. The purpose of the present work is to examine the effect of water lubrication on the friction and wear of PET and to clarify the wear mechanism in the water-lubricated condition.
2. Experimental
details
2.1. Poly(e th ylene tereph thalate) specimens Four sets of PET specimens with different crystaUinities were used, similar to the specimens used in the previous work [6]. The conditions of heat treatment and the degrees of crystallinity are listed in Table 1. One original PET plate, 2 mm thick and almost amorphous, was supplied from Teijin Ltd. Two sets of specimens, 1-L and l-H, were prepared by annealing the original amorphous plates, 1-O. Another original PET film, 0.5 mm thick with a high crystallinity, produced by the Toray Industries Inc. was also used for specimens after it had been annealed. The crystallinity of the specimens was determined from their densities by calculation using values of 1.328 g
65 TABLE 1 Sample preparation and physical characteristics of samples Sample
Density (g cmm3)
1-o
1.337
1-L
1.313
1-H
1.392
2-H
1.417
Vickers’ hardness
Heat treatment
8
26
Unannealed
39
38
As 1-O but annealed at 120 “C! for 20 min
55
42
As 1-O but annealed at 210 “C for 60 min
75
46
This film (original density, 1.399 g cme3) was annealed at 200 “C for 90 min
Degree of crystallinity (%I
condition
cme3 and 1.450 g cm -3 for the densities of amorphous and crystalline phases respectively. The Vickers hardness of specimens was measured under a load of 1.0 N and a loading time of 20 s. 2.2. Experimental procedure The pin-on-disk type of wear-testing machine used in the previous work [6] was again employed in this work. The diameter of the polymer pins was 3 mm. Stainless steel (SUS 304) was used for the material of the counterface disk. The surface of the disk was finished with 1500 grade waterproof abrasive cloth and the surface roughness R, of the disk was about 0.02 pm. The frictional force and the wear depth of polymer pins were measured under a load of 10 N at sliding speeds of 0.01 and 0.1 m s-‘. Small drops of deionized water were continuously supplied in front of the pin specimen surface during the sliding experiments. The measurements were started after pre-rubbing against a 1000 grade emery paper placed on the disk to obtain close contact between the pin surface and the counter-face.
3. Experimental results and discussion 3.1. Dependence of friction on degree of crystallinity The coefficients of friction for various specimens with water lubrication are illustrated in Fig. 1 as a function of sliding speed. The bars attached to the circles indicate the range of measured friction coefficients. As shown in Fig. 1, the coefficient of friction depends very little on the sliding speed at speeds below 0.1 m s-l. At sliding speeds over 0.1 m s-l, the coefficient of friction decreased rapidly with increasing sliding speed and reached a very small value of 0.04. A similar dependence of the friction coefficient on the sliding speed is observed for all PET specimens studied. In the present work the sliding speed at which the friction coefficient begins to decrease rapidly
66
,I :ri;i-i_iI-, (a)
0
(d)
‘ii.:
0.01 sliding
0.1 velocity
1 V , m/s
Fig. 1. Dependence of coefficient of friction on sliding speed for various PET specimens: (a) 1-O; (b) l-L;(c) 1-H; (d) 2-H.
is almost independent of the hardness of specimens. As described in the previous papers [ 4,7], it is expected that the lubricated condition at a speed of 0.1 m s-l and with a load of 10 N ensures approximate boundary lubrication and that a sliding speed of 0.01 m s-l ensures perfect boundary lubrication. Figure 2 and Fig. 3 show friction coefficients p and wear depths h measured at sliding speeds of 0.1 m s-l and 0.01 m s-l respectively. The coefficients of friction are almost constant during sliding, although small variations were observed in the initial sliding stage. The coefficients of friction at sliding speeds of 0.01 and 0.1 m s-l are shown as a function of degree of crystallinity in Fig. 4. In this figure, the results from previous work [6] are also shown by a broken line for comparison with the results with lubrication. It is observed that the coefficients of friction with lubrication are lower than those with no lubrication. Slight decreases and increases in friction coefficient with increasing crystallinity are seen at sliding speeds of 0.01 m s-l and 0.1 m s-l respectively. However, the results indicate that the friction coefficient of PET is very weakly dependent on the degree of crystallinity with boundary lubrication by water. 3.2. Dependence of the wear rate on degree of crystallinity with water lubrication The wear depths for various specimens are shown in Figs. 2 and 3 as a function of sliding distance. It is noted that wear, except for specimen 2-H, proceeds approximately in a steady state throughout the wear process. Such behaviour was observed also in the dry condition [ 61. The wear rate tends to
67
(a)
sliding distance L. km
2 4 6 6 10 sliding distance L. km
sliding distance L,km
?lidir$ dish&e
(d)
’
L?km
1 IO
Fig. 2. Coefficients of friction and wear depths for various PET specimens with a sliding speed of 0.1 m s-l as a function of sliding distance: (a) 1-O; (b) 1-L; (c) 1-H; (d) 2-H.
(4
sliding distance L, km
(c)
sliding
--
distance
L. km
(b)
(d)
sliding distance
L, km
sliding distance
L, km
Fig. 3. Coefficients of friction and wear depths for various PET specimens with a sliding speed of 0.01 m s-l as a function of sliding distance: (a) 1-O; (b) 1-L; (c) 1-H; (d) 2-H.
68
---__ --________----I~-~ 05
=0.
s 0.3
‘;
.v-
;
02
$
0.1
5 OK) ”
crystallinity
W, %
100
20 40 crystollinity
60
W ,
80
7.
Fig. 4. Dependence of coefficient of friction on the degree of crystallinity: 0, lubricated condition, 0.1 m s-l; 0, lubricated condition, 0.01 m s-l; ---, dry condition, 0.1 m s-1. Fig. 5. Dependence of wear rate on the degree of crystallinity: 0, lubricated condition, 0.1 m s-l; 0, lubricated condition, 0.01 m s-l; - - -, dry condition, 0.1 m s-l.
be lower with increasing degree of crystallinity. With the specimen of the highest crystallinity, specimen 2-H, an initial wear stage showing a higher wear rate than the steady wear was observed. The wear rates in the steady wear stage are shown in Fig. 5 as a function of degree of crystallinity. For comparison, the results at a sliding speed of 0.1 m s-l with no lubrication [6] are also shown, by a broken curve, in the figure. It is remarkable that the wear rate with lubrication is much greater than that without lubrication and decreases with increase in crystallinity, in contrast with the wear rate without lubrication, As shown in Fig. 5, the ratio of the wear rate with lubrication to that without increases markedly as the crystalhnity decreases. In other words, the wear rate of PET with water lubrication is reduced as the crystallinity is increased. This fact suggests that the increase in wear with water lubrication is essentially due to permeation of water into the amorphous phase of PET. The wear rates at 0.1 m s-i are considerably lower than those at 0.01 m s-l. This may be due to a contribution of the wear-reducing action of hydrodynamic lubrication which may partially exist even at a sliding speed of 0.1 m s-l, although its action was not apparent from the results presented in Fig. 4. It is well known that the wear rate of polymeric materials rubbing against metal counter-faces is higher with lubrication than under dry conditions [ 1,2,4,7]. As described earlier, explanation for such wear behaviour proposed by several researchers can be classified into two categories: modification of counter-faces by polymer transfer and modification or plasticization of worn polymer surfaces by water. Figure 6 shows optical micrographs of counterfaces rubbed against PET pins under lubrication. It is clearly
Fig. 6. Optical micrographs of frictional tracks on the counterfaces: m s-l; (b) sample l-H, 0.1 m s-l.
(a) sample l-L, 0.1
observed that polymer transfer films are formed on the counterfaces not only under dry conditions but also with water lubrication. Moreover, it should be noted that the wear of PET proceeds steadily from the initial wear stage with lubrication as well as without lubrication. These observed facts suggest that the wear behaviour of PET may not be influenced markedly by the polymer transfer films formed on the counterface during sliding. Consequently, it seems that higher wear with water lubrication is not attributed to the prevention of polymer transfer film formation with water. Tanaka [4] observed that the wear rates of semicrystalline polymers, such as PTFE, polyethylene, nylon 6 and polyacetal, with water lubrication become higher than those without and suggested that the modification of the polymer wear surface by water may be responsible for the higher wear rate with water lubrication. The modification or plasticization of polymers by water may preferentially occur in the amorphous phase of polymers. Lasoski and Cobbs [ 51 reported that the permeation of PET films by water increased with increasing volume of amorphous phase in the PET. As shown in Fig. 5, the wear rate with lubrication decreases and the ratio of wear rate with lubrication to that under dry conditions decreases markedly with increasing crystahinity, in other words, with decreasing volume fraction of the amorphous phase. Therefore the observed facts can be explained by plasticization of the amorphous phase by water permeation. In order to obtain further information about the wear process, worn surfaces of polymer pins and frictional tracks on the counterfaces were observed by transmission electron microscopy (TEM). Figures 7 and 8 are TEM micrographs of replicas of the polymer wear surfaces and the frictional tracks respectively. As shown in Fig. 7, the worn surfaces are very smooth with many fine scratches parallel to the sliding direction. This feature on worn surfaces is observed for many semicrystalline polymers rubbed against smooth metal counterfaces and is characteristic of their transfer wear [4]. In the dry condition, much rougher worn surfaces were observed for high crystallinity PET [6]. This suggests that the wear mechanism for high crys-
Fig. 7. Electron micrographs (b) sample l-H, 0.01 m s-l.
of worn
surfac
tallinity PET with lubrication differs somewhat from that without lubrication. Figure 8 shows TEM micrographs of friction tracks. Coherent transfer films, with many fine scratches parallel to the sliding direction, are observed on the counterfaces for low crystallinity PET (Fig. 8(a)). Such a feature is also characteristic of the transfer wear of polymeric materials. In contrast, for high crystallinity PET, some characteristic features are observed as shown in Figs. 8(b), 8(c), 8(d) and 8(e). It should be noted that fine ridges, which may be lame&r crystahites, and films coexist on the track as shown in the micrographs in Figs. 8(b) and 8(c), and debris consisting of crystal&s is also observed in the micrographs in Figs. 8(d) and 8(e). The features of transfer for high crystallinity PET specimens show rougher surfaces than that for low crystallinity PET. This suggests that the wear mechanism of PET with lubrication, as well as under dry conditions, is influenced by the presence of the crystalline phase. In the previous paper [6], with no lubrication, it was shown that the predominant wear mode for low crystallinity PET was film transfer, but surface fatigue seemed to play an important role in the wear process for high crystallinity PET. Also, with lubrication, surface fatigue may be partly responsible for the wear process of high crystallinity PET. However, the fact that wear of high crystallinity PET was rather lower than that of low crystallinity PET may suggest that surface fatigue was suppressed when plasticization of the amorphous phase by water occurred.
4. Conclusions (1) The friction of PET with boundary lubrication by water was slightly dependent on the degree of crystallinity. (2) The wear rate of PET with water lubrication was higher than that under dry conditions and slightly decreased with increase in the degree of crystalhnity.
(a)
(d)
(e) Fig. 8. Electron micrographs of frictional tracks on the counterfaces: (a) sample l-0, 0.01 m s-l; (b) sample l-H, 0.01 m s-l; (c) higher magnification of (b); (d) sample 2-H, 0.01 m s-l; (e) higher magnification of (d).
(3) The higher wear rate of PET with water lubrication seemed to be due to surface plasticization which occurred as a result of permeation of water molecules into the amorphous phase of PET.
72
Acknowledgments The authors wish to thank Teijin Ltd. and Toray Industries Inc. for their kind supply of PET samples.
References 1 D. C. Evans, Polymer-fluid interactions in relation to wear. In D. Dowson, M. Godet and C. M. Taylor (eds.), Froc. 3rd Leeds-Lyon Symp. on Tribology, Leeds, 1976, Mechanical Engineering Publications, London, 1978, p. 47. 2 J. K. Lancaster, Lubrication of carbon fibre-reinforced polymers, Part I, Water and aqueous solutions, Wear, 20 (1972) 315. 3 C. Rubenstein, Lubrication of polymers, J. Appl. Phys., 32 (1961) 1445. 4 K. Tanaka, Friction and wear of semicrystalline polymers sliding against steel under water lubrication, J. Lubr. Technol., 102 (1980) 526. 5 S. W. Lasoski, Jr., and W. H. Cobbs, Moisture permeability of polymers, I, Role of crystallinity and orientation, J. Polym. Sci., 36 (1959) 21. 6 Y. Yamada and K. Tanaka, Effect of the degree of crystallinity on friction and wear of poly(ethylene terephthalate), In L. H. Lee (ed.), Polymer Wear and Its Control, in Am. Chem. Sot. Symp. Ser., 287 (1985) Chapter 24. 7 Y. Yamada and K. Tanaka, Friction and wear behaviour of PTFE-based composites under water lubrication, Zyunkatsu, 29 (1984) 209 (in Japanese).
63 - 72
63
EFFECT OF THE DEGREE OF CRYSTALLINI’IY ON THE FRICTION AND WEAR OF POLY(ETHYLENE TEREPHTHALATE) UNDER WATER LUBRICATION YOSHINORI Faculty
YAMADA and KYUICHIRO
of Engineering,
Kanazawa
TANAKA
University,
(Received September 10,1985;accepted
Kanazawa
920 (Japan)
January 2,1986)
summary By means of a pin-on-disk type of wear-testing apparatus, coefficients of friction and wear depths of various poly(ethylene terephthalate) (PET) specimens with different degrees of crystallinity were measured at sliding speeds of 0.01 and 0.1 m s-l under a constant load of 10 N with water lubrication. The coefficients of friction with lubrication were lower than those for unlubricated conditions and there was little dependence on the degree of crystallinity. However, the wear rates with lubrication were higher than those obtained under dry conditions and decreased with increasing degree of crystallinity, in contrast with the unlubricated wear rates. These results are discussed on the basis of the plasticization of worn polymer surfaces by water. The worn surfaces and frictional tracks were observed by optical and electron microscopy. The frictional tracks for low crystallinity PET were smooth with many fine scratches and characteristic of transfer wear. For high crystallinity, frictional tracks were somewhat different from those for low crystallinity PET.
1. Introduction It is well known that the friction and wear of polymeric materials under boundary lubrication are complicated by the presence of lubricant fluid. Possible explanations have been proposed by several investigators to interpret the observed wear behaviour, and especially the fact that the wear rate is higher in the lubricated condition than in the dry condition. Evans [l] showed that for amorphous polymers, such as poly(2,6dimethyl 1,4-phenylene oxide) and poly(methy1 methacrylate), a higher wear rate was observed with lubrication using the fluids whose solubility parameters were close to those of polymeric materials. Moreover, he reported that for semicrystalline polymers, such as polytetrafluoroethylene (PTFE) and polyacetal, lubricant fluids primarily influenced wear via their 0043-1646/66/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
64
effect on the transfer film formation; where transfer plays little part in the dry wear process, fluids do not greatly change the wear rate but, where transfer is responsible for a very low wear rate, lubricant fluids appreciably increase the wear rate. He also showed, however, that the wear rate was exceptionally high in all polymers studied when water was used as the lubricant fluid. Lancaster [2] reported that water and aqueous solutions inhibit the formation of transfer films on a metal counterface, and the wear rates of carbon-fibre-reinforced polymers are generally greater than those occurring during dry sliding. In contrast, the effect of the plasticization of polymers, which may be caused by absorption of lubricant fluids, must also be taken into account to understand the friction and wear behaviour of polymeric material under lubrication. Rubenstein [3] studied the friction of polymers with boundary lubrication by various fluids and suggested that the *mechanical strength of polymers may be reduced by the plasticization of polymer surfaces because of the permeation of lubricant fluids into the polymer surfaces. Tanaka [4] suggested that the wear rates of semicrystalline polymers such as PTFE, poly(aceta1) and nylon 6 with water lubrication became higher than those under unlubricated conditions because of the modification of the polymer surface by water. As shown by Lasoski and Cobbs [ 51, the permeation of water into semicrystalline polymers may occur preferentially in their amorphous phases. In order to obtain evidence for plasticization with water lubrication, the effect of crystallinity on the friction and wear of polymers should be examined both under dry conditions and with water lubrication. One of the best polymers for such examination is poly(ethylene terephthalate) (PET), because its crystallinity can be varied over a wide range by heat treatment. We have reported elsewhere the results of a study of the effect of crystallinity on the friction and wear of PET in the dry condition
[61. The purpose of the present work is to examine the effect of water lubrication on the friction and wear of PET and to clarify the wear mechanism in the water-lubricated condition.
2. Experimental
details
2.1. Poly(e th ylene tereph thalate) specimens Four sets of PET specimens with different crystaUinities were used, similar to the specimens used in the previous work [6]. The conditions of heat treatment and the degrees of crystallinity are listed in Table 1. One original PET plate, 2 mm thick and almost amorphous, was supplied from Teijin Ltd. Two sets of specimens, 1-L and l-H, were prepared by annealing the original amorphous plates, 1-O. Another original PET film, 0.5 mm thick with a high crystallinity, produced by the Toray Industries Inc. was also used for specimens after it had been annealed. The crystallinity of the specimens was determined from their densities by calculation using values of 1.328 g
65 TABLE 1 Sample preparation and physical characteristics of samples Sample
Density (g cmm3)
1-o
1.337
1-L
1.313
1-H
1.392
2-H
1.417
Vickers’ hardness
Heat treatment
8
26
Unannealed
39
38
As 1-O but annealed at 120 “C! for 20 min
55
42
As 1-O but annealed at 210 “C for 60 min
75
46
This film (original density, 1.399 g cme3) was annealed at 200 “C for 90 min
Degree of crystallinity (%I
condition
cme3 and 1.450 g cm -3 for the densities of amorphous and crystalline phases respectively. The Vickers hardness of specimens was measured under a load of 1.0 N and a loading time of 20 s. 2.2. Experimental procedure The pin-on-disk type of wear-testing machine used in the previous work [6] was again employed in this work. The diameter of the polymer pins was 3 mm. Stainless steel (SUS 304) was used for the material of the counterface disk. The surface of the disk was finished with 1500 grade waterproof abrasive cloth and the surface roughness R, of the disk was about 0.02 pm. The frictional force and the wear depth of polymer pins were measured under a load of 10 N at sliding speeds of 0.01 and 0.1 m s-‘. Small drops of deionized water were continuously supplied in front of the pin specimen surface during the sliding experiments. The measurements were started after pre-rubbing against a 1000 grade emery paper placed on the disk to obtain close contact between the pin surface and the counter-face.
3. Experimental results and discussion 3.1. Dependence of friction on degree of crystallinity The coefficients of friction for various specimens with water lubrication are illustrated in Fig. 1 as a function of sliding speed. The bars attached to the circles indicate the range of measured friction coefficients. As shown in Fig. 1, the coefficient of friction depends very little on the sliding speed at speeds below 0.1 m s-l. At sliding speeds over 0.1 m s-l, the coefficient of friction decreased rapidly with increasing sliding speed and reached a very small value of 0.04. A similar dependence of the friction coefficient on the sliding speed is observed for all PET specimens studied. In the present work the sliding speed at which the friction coefficient begins to decrease rapidly
66
,I :ri;i-i_iI-, (a)
0
(d)
‘ii.:
0.01 sliding
0.1 velocity
1 V , m/s
Fig. 1. Dependence of coefficient of friction on sliding speed for various PET specimens: (a) 1-O; (b) l-L;(c) 1-H; (d) 2-H.
is almost independent of the hardness of specimens. As described in the previous papers [ 4,7], it is expected that the lubricated condition at a speed of 0.1 m s-l and with a load of 10 N ensures approximate boundary lubrication and that a sliding speed of 0.01 m s-l ensures perfect boundary lubrication. Figure 2 and Fig. 3 show friction coefficients p and wear depths h measured at sliding speeds of 0.1 m s-l and 0.01 m s-l respectively. The coefficients of friction are almost constant during sliding, although small variations were observed in the initial sliding stage. The coefficients of friction at sliding speeds of 0.01 and 0.1 m s-l are shown as a function of degree of crystallinity in Fig. 4. In this figure, the results from previous work [6] are also shown by a broken line for comparison with the results with lubrication. It is observed that the coefficients of friction with lubrication are lower than those with no lubrication. Slight decreases and increases in friction coefficient with increasing crystallinity are seen at sliding speeds of 0.01 m s-l and 0.1 m s-l respectively. However, the results indicate that the friction coefficient of PET is very weakly dependent on the degree of crystallinity with boundary lubrication by water. 3.2. Dependence of the wear rate on degree of crystallinity with water lubrication The wear depths for various specimens are shown in Figs. 2 and 3 as a function of sliding distance. It is noted that wear, except for specimen 2-H, proceeds approximately in a steady state throughout the wear process. Such behaviour was observed also in the dry condition [ 61. The wear rate tends to
67
(a)
sliding distance L. km
2 4 6 6 10 sliding distance L. km
sliding distance L,km
?lidir$ dish&e
(d)
’
L?km
1 IO
Fig. 2. Coefficients of friction and wear depths for various PET specimens with a sliding speed of 0.1 m s-l as a function of sliding distance: (a) 1-O; (b) 1-L; (c) 1-H; (d) 2-H.
(4
sliding distance L, km
(c)
sliding
--
distance
L. km
(b)
(d)
sliding distance
L, km
sliding distance
L, km
Fig. 3. Coefficients of friction and wear depths for various PET specimens with a sliding speed of 0.01 m s-l as a function of sliding distance: (a) 1-O; (b) 1-L; (c) 1-H; (d) 2-H.
68
---__ --________----I~-~ 05
=0.
s 0.3
‘;
.v-
;
02
$
0.1
5 OK) ”
crystallinity
W, %
100
20 40 crystollinity
60
W ,
80
7.
Fig. 4. Dependence of coefficient of friction on the degree of crystallinity: 0, lubricated condition, 0.1 m s-l; 0, lubricated condition, 0.01 m s-l; ---, dry condition, 0.1 m s-1. Fig. 5. Dependence of wear rate on the degree of crystallinity: 0, lubricated condition, 0.1 m s-l; 0, lubricated condition, 0.01 m s-l; - - -, dry condition, 0.1 m s-l.
be lower with increasing degree of crystallinity. With the specimen of the highest crystallinity, specimen 2-H, an initial wear stage showing a higher wear rate than the steady wear was observed. The wear rates in the steady wear stage are shown in Fig. 5 as a function of degree of crystallinity. For comparison, the results at a sliding speed of 0.1 m s-l with no lubrication [6] are also shown, by a broken curve, in the figure. It is remarkable that the wear rate with lubrication is much greater than that without lubrication and decreases with increase in crystallinity, in contrast with the wear rate without lubrication, As shown in Fig. 5, the ratio of the wear rate with lubrication to that without increases markedly as the crystalhnity decreases. In other words, the wear rate of PET with water lubrication is reduced as the crystallinity is increased. This fact suggests that the increase in wear with water lubrication is essentially due to permeation of water into the amorphous phase of PET. The wear rates at 0.1 m s-i are considerably lower than those at 0.01 m s-l. This may be due to a contribution of the wear-reducing action of hydrodynamic lubrication which may partially exist even at a sliding speed of 0.1 m s-l, although its action was not apparent from the results presented in Fig. 4. It is well known that the wear rate of polymeric materials rubbing against metal counter-faces is higher with lubrication than under dry conditions [ 1,2,4,7]. As described earlier, explanation for such wear behaviour proposed by several researchers can be classified into two categories: modification of counter-faces by polymer transfer and modification or plasticization of worn polymer surfaces by water. Figure 6 shows optical micrographs of counterfaces rubbed against PET pins under lubrication. It is clearly
Fig. 6. Optical micrographs of frictional tracks on the counterfaces: m s-l; (b) sample l-H, 0.1 m s-l.
(a) sample l-L, 0.1
observed that polymer transfer films are formed on the counterfaces not only under dry conditions but also with water lubrication. Moreover, it should be noted that the wear of PET proceeds steadily from the initial wear stage with lubrication as well as without lubrication. These observed facts suggest that the wear behaviour of PET may not be influenced markedly by the polymer transfer films formed on the counterface during sliding. Consequently, it seems that higher wear with water lubrication is not attributed to the prevention of polymer transfer film formation with water. Tanaka [4] observed that the wear rates of semicrystalline polymers, such as PTFE, polyethylene, nylon 6 and polyacetal, with water lubrication become higher than those without and suggested that the modification of the polymer wear surface by water may be responsible for the higher wear rate with water lubrication. The modification or plasticization of polymers by water may preferentially occur in the amorphous phase of polymers. Lasoski and Cobbs [ 51 reported that the permeation of PET films by water increased with increasing volume of amorphous phase in the PET. As shown in Fig. 5, the wear rate with lubrication decreases and the ratio of wear rate with lubrication to that under dry conditions decreases markedly with increasing crystahinity, in other words, with decreasing volume fraction of the amorphous phase. Therefore the observed facts can be explained by plasticization of the amorphous phase by water permeation. In order to obtain further information about the wear process, worn surfaces of polymer pins and frictional tracks on the counterfaces were observed by transmission electron microscopy (TEM). Figures 7 and 8 are TEM micrographs of replicas of the polymer wear surfaces and the frictional tracks respectively. As shown in Fig. 7, the worn surfaces are very smooth with many fine scratches parallel to the sliding direction. This feature on worn surfaces is observed for many semicrystalline polymers rubbed against smooth metal counterfaces and is characteristic of their transfer wear [4]. In the dry condition, much rougher worn surfaces were observed for high crystallinity PET [6]. This suggests that the wear mechanism for high crys-
Fig. 7. Electron micrographs (b) sample l-H, 0.01 m s-l.
of worn
surfac
tallinity PET with lubrication differs somewhat from that without lubrication. Figure 8 shows TEM micrographs of friction tracks. Coherent transfer films, with many fine scratches parallel to the sliding direction, are observed on the counterfaces for low crystallinity PET (Fig. 8(a)). Such a feature is also characteristic of the transfer wear of polymeric materials. In contrast, for high crystallinity PET, some characteristic features are observed as shown in Figs. 8(b), 8(c), 8(d) and 8(e). It should be noted that fine ridges, which may be lame&r crystahites, and films coexist on the track as shown in the micrographs in Figs. 8(b) and 8(c), and debris consisting of crystal&s is also observed in the micrographs in Figs. 8(d) and 8(e). The features of transfer for high crystallinity PET specimens show rougher surfaces than that for low crystallinity PET. This suggests that the wear mechanism of PET with lubrication, as well as under dry conditions, is influenced by the presence of the crystalline phase. In the previous paper [6], with no lubrication, it was shown that the predominant wear mode for low crystallinity PET was film transfer, but surface fatigue seemed to play an important role in the wear process for high crystallinity PET. Also, with lubrication, surface fatigue may be partly responsible for the wear process of high crystallinity PET. However, the fact that wear of high crystallinity PET was rather lower than that of low crystallinity PET may suggest that surface fatigue was suppressed when plasticization of the amorphous phase by water occurred.
4. Conclusions (1) The friction of PET with boundary lubrication by water was slightly dependent on the degree of crystallinity. (2) The wear rate of PET with water lubrication was higher than that under dry conditions and slightly decreased with increase in the degree of crystalhnity.
(a)
(d)
(e) Fig. 8. Electron micrographs of frictional tracks on the counterfaces: (a) sample l-0, 0.01 m s-l; (b) sample l-H, 0.01 m s-l; (c) higher magnification of (b); (d) sample 2-H, 0.01 m s-l; (e) higher magnification of (d).
(3) The higher wear rate of PET with water lubrication seemed to be due to surface plasticization which occurred as a result of permeation of water molecules into the amorphous phase of PET.
72
Acknowledgments The authors wish to thank Teijin Ltd. and Toray Industries Inc. for their kind supply of PET samples.
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