- Email: [email protected]
Wear mechanism of a composite plastic bearing
111
Wear, 36 (1976) 111 - 117 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
WEAR MECHANISM OF A COMPOSITE PLASTIC BEARING
I. D. GRAHAM and G. H. WEST University of Manchester, Institute of Science and Technology, Manchester M60 1 QD (Gt. Britain)
Sackville Street,
(Received July 18, 1975)
Summary The wear behaviour of a high temperature, composite plastic bearing material (polyphenylene sulphide, PTFE, lead oxide and graphite in the ratio 55 : 25 : 10 : 10) has been studied in journal bearing configuration against steel. The composition of the rubbing surfaces was studied using electron probe microanalysis (EPMA) and the physical properties of the composition at operating temperature were determined from shear modulus measurements. The results reveal a high lead content of the contact areas, suggesting that rubbing involves the lamellar shearing of a lead oxide and possibly also a graphite layer which is aided by a relatively soft matrix containing PTFE.
Introduction The usefulness of plastics for the fabrication of low duty gears and lightly loaded sleeve bearings has been recognized for some time but difficulties arise in the use of many plastics in a relatively pure form for medium and heavy duty applications. This is due in part to their physical properties in the form of relatively low softening points, high expansion coefficients and creep susceptibility and the comparatively poor wear under these conditions. Attempts to overcome these disadvantages resulted in the development of a range of thermosetting resin impregnated materials and recently in multicomponent formulations. The latter comprise mixtures of polymeric and non-polymeric components often in the form of a relatively thin layer on a metal backing and have found fairly wide application in the engineering industry [l] . Examples include mixtures of polytetrafluoroethylene (PTFE) and bronze with or without certain other components, and PTFE, graphite and lead oxide in a matrix of a thermosetting polymer such as an epoxy or polyphenylene sulphide (PPS) [2]. It is with materials of the last composition that this investigation is particularly concerned. The optimum composition was determined by standard wear tests and was found to be 25% PTFE, 10% lead oxide and 10% graphite with the balance being the matrix polymer PPS. Relatively small composition changes have a marked
112
effect on wear and the objective of the work described is to investigate of the factors involved in determining the wear of these materials.
some
Wear behaviour The fillers incorporated in the PPS all possess lubricating qualities. PTFE has been recognized as exceptional in that its friction coefficient is appreciably lower than that of most other materials (0.04 - 0.1) and this is accompanied by a low wear rate at light loadings. As the loading increases, however, so does the wear until at high pressures it becomes poor [ 31. In wear tests involving plastic-metal contact in pin-and-plate configurations a film of highly oriented PTFE has been observed on the metal counterface and on this basis it has been suggested that wear proceeds by the removal and renewal of this film, which occurs readily at high loads f4]. It has also been suggested that metallic fillers incorporated in the polymer assist the adhesion of this film by either physical or chemical means, thereby reducing wear. The importance of graphite as a dry lubricant has long been realized and this is thought to be related to its lamellar structure, which allows the formation of a layer of low shear strength and low wear on rubbing surfaces, possibly with the aid of any metallic filler present 121. The situation is complicated by the possible presence of varying wear mechanisms in differing experimental configurations since it is known that transfer film formation may depend on whether or not the wear debris can escape from the contact area, and comparison of results obtained using differing wear configurations can be difficult for this reason. The influence of the physical and in particular the mechanical properties of a bearing on its wear has been the subject of a number of previous investigations [ 5,6] . For a plastic-plastic contact it has been suggested that both the shear modulus and the extension to break might be significant [7] and the influence of hardness (largely controlled by the matrix polymer) for a composite plastic bearing has also been investigated [ 8] . Procedure The approach adopted in the investigation of possible wear mechanisms involved physical property determination at the bearing operating temperature and analysis of the rubbing surfaces in journal bearing tests in which a plastic bush was in contact with a hardened steel shaft. Physical characterization The components of the optimised composition (PPS (Ryton VI, Phillips Petroleum Ltd.), PTFE (Fluon Ll69B, ICI Ltd.), graphite (Foliac No. 1371, Rocol Ltd.) and lead monoxide (Technical Grade, BDH)) were mixed in a ball mill and pre-cured at 266 “C for 16 h and 371 “C for 1 h. The mixture was then ground and compression moulded into bars 6.35 mm X 10 mm X 74 mm at a temperature of 385 “C and pressure of 23 MN rne2. The structure
113
A
Fig. 1. Optical micrographs of mouldings of polyphenylene sulphide composition and the pure polymer (inset): A, lead monoxide particle; B, graphite particle; dark areas, PTFE; light areas, PPS.
of the moulding was revealed by optical microscopy after it had been mounted in bakelite and polished with gamma-alumina (Fig. 1). The overall dispersion is seen to be reasonably homogeneous, the identity of the components being determined by comparison with micrographs of single-component mixtures. Of the various mechanical characteristics of a material one of those most frequently related to friction and wear behaviour is shear strength. This is associated with the adhesive theory of wear which tends to be favoured for many polymeric materials [ 71. Also associated with wear is the temperature rise due to frictional heating and this necessitates the determination of the shear modulus over a range of temperature. The method adopted involved the calculation of the shear modulus G from the slope of the torque T uersus angular deflexion @curves for specimens 63.5 mm X 6.35 mm X 1.27 mm thick cut from the original moulding using a microtome diamond saw. The formula employed is due to Nielson [9] : G=-
16L
dT CD311 0 d@ @=c
where L, C and D are the specimen length between clamps, its width and thickness and p is a shape factor. The apparatus employed was a modified Clash-and-Berg torsion tester. Tests were conducted over the range 20” 200 “C, the results being shown in Fig. 2. The deflexion curves revealed that above 100 “C the behaviour was largely non-linear. The rapid drop in modulus over the region 90 ’ - 105 “C is probably related to the glass transition temperature Tg of the uncured PPS, which has been found to be 85 “C by
114
SHEAR
MODULUS
VS
TEMPERATURE
IO9
"I I z 0 10S
I 0’
t
I
I
I
0
50
100
150
TEMP
I 200
“C
Fig. 2. Shear modulus us. temperature.
DTA [lo] . The reduced magnitude of the modulus reduction compared with that usually observed in amorphous polymers is likely to be related reduced chain mobility above Tg due to cross-linking during moulding.
to
Bearing manufacture and wear testing Bearings were prepared by centrifugally casting a suspension of the components in toluene on the inside of a brass bush heated to 360 “C and machining to an internal diameter of 25.482% mm. The bush was then cut into two using a slitting saw (Fig. 3). An optical micrograph of a cross section of the coating and bush prepared in a similar manner to the moulded
115
CENTRE
LINE
I BRASS BUSH I C-
+ ,025 _ .ooo
A 125.48 B = 258
mm
mm
Fig. 3. Dimensions of bearing.
Fig. 4. Optical micropaph (section) of a polymer film; A, lead monoxide particle; B, graphite particle; dark areas, PTFE; light areas, PPS.
specimen is shown in Fig. 4. It can be seen that the film is of a reasonably uniform composition but displays a slightly different structure of the PTFE filler, probably attributable to the reduced pressure applied during processing. Wear testing consisted of operating the split bearings against a hardened tool steel shaft 25 mm in diameter under a range of conditions until a satisfactory transfer film and uniform wear rate resulted [ 111. A load of 178 N and a speed of 1.6 m s-l was found to be suitable.
116 TABLE 1 Concentration ratios of PPS, PTFE, lead oxide and iron in the bearing transfer film and the bush surfaces (wt. %)
Transfer film Bush surface Bulk average value (from mixture composition)
PTFE/PPS
PbO/PTFE
Fe/PPS
16.3
1220
172
5.5
830
45.5
40
6.4
Surface analysis
Chemical microan~ysis was carried out using an electron probe microanalyser (Cambridge Microscan 5). A focussed beam of electrons 2 - 3 mm in diameter with a penetration of 1 - 2 pm produces X-rays characteristic of the elements present and the weight fractions can be calculated from the Xray intensity in conjunction with a calibrated standard. Errors result from varying degrees of sub-surface absorption and mutual excitation effects and the method cannot be used for low atomic number elements. Microan~ysis was applied to specimens cut from both shaft and bush, the average concentration of the elements sulphur, fluorine, lead and iron being determined along a line 270 mm long. From the elemental analysis the concentrations of PPS, PTFE and lead oxide could be calculated. The results are shown in Table 1. Discussion A comparison of the values given in Table 1 reveals a significant absence of PTFE in both the worn bush and the transfer film surfaces, the latter in particular being le~~om~ated. The concentration of graphite cannot be determined by this procedure owing to inaccuracy in the evaluation of carbon and the difficulties of resolution into the various forms present. The presence of iron in large proportion in the film implies a penetration of the steel shaft by the electron beam, and hence a film thickness of less than 1 - 2 rum. This is in agreement with surfaee electron micrographs, which revealed a film sufficiency thin for the ma~h~ing marks of the shaft to be distinguishable beneath it, and is in contrast to the relatively thick films observed with some other plastics [4]. The iron in the bush is significant as it reveals a mechanism for back transfer from the shaft. These results suggest that in the case of composite bearings of this general type the low wear is likely to be related to the presence on the rubbing surfaces of a relatively thin film of lead oxide or lead with the possible inclusion of graphite. Both these materials exhibit laminar shear, which may be facilitated by the PTFE acting as a low energy sub-surface. Experimental and theoretical evaluation of the surface running temperatures indicate that a temperature of 150” - 160 “C is attained [ll],which
117
implies that the matrix is in the lower shear modulus, softer condition. This could be important in assisting the observed transfer of lead to the shaft and might also reduce adhesive wear. These results suggest some sim~a~ty between soft metal and plastic bearings as far as possible weax behaviour is concerned. References 1 G. C. Pratt, Recent developments in PTFE-based dry bearing materials and treatments, Proc. Inst. Mech. Eng. London, 181 (Part 30) (1966 - 7) 58. 2 G. H. West and J. M. Senior, Tribology, 6 (1973) 269. 3 K. Tanaka, Y. Uchiyama and S. Toyooka, Wear, 23 (1973) 153. 4 B. J. Briscoe, A. K. Pogasian and D. Tabor, Wear, 27 (1974) 19. 5 J. K. Lancaster, Wear of carbon and graphitic materials sliding against rough metal surfaces, Proc. of the Lubrication and Wear Convention, Inst. Mech. Eng., London, (1963) p. 190. 6 T. L. Oberle, J. Met., 3 (1951) 438. 7 G. H. West and J. M. Senior, Wear, 19 (1972) 37. 8 A. Ghafoor, J. M. Senior, R. H. Still and G. H. West, Polymer, 15 (1974) 577. 9 0. W. Trayer and H. W. March, Nat1 Advis. Comm. Aeronaut., Rep. 334. 10 J. N. Short and H. Wayne, Chem. Technol., 2 (1972) 481. 11 C. A. Jones, M. SC Dissertation, UMIST, 1974.
Wear, 36 (1976) 111 - 117 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
WEAR MECHANISM OF A COMPOSITE PLASTIC BEARING
I. D. GRAHAM and G. H. WEST University of Manchester, Institute of Science and Technology, Manchester M60 1 QD (Gt. Britain)
Sackville Street,
(Received July 18, 1975)
Summary The wear behaviour of a high temperature, composite plastic bearing material (polyphenylene sulphide, PTFE, lead oxide and graphite in the ratio 55 : 25 : 10 : 10) has been studied in journal bearing configuration against steel. The composition of the rubbing surfaces was studied using electron probe microanalysis (EPMA) and the physical properties of the composition at operating temperature were determined from shear modulus measurements. The results reveal a high lead content of the contact areas, suggesting that rubbing involves the lamellar shearing of a lead oxide and possibly also a graphite layer which is aided by a relatively soft matrix containing PTFE.
Introduction The usefulness of plastics for the fabrication of low duty gears and lightly loaded sleeve bearings has been recognized for some time but difficulties arise in the use of many plastics in a relatively pure form for medium and heavy duty applications. This is due in part to their physical properties in the form of relatively low softening points, high expansion coefficients and creep susceptibility and the comparatively poor wear under these conditions. Attempts to overcome these disadvantages resulted in the development of a range of thermosetting resin impregnated materials and recently in multicomponent formulations. The latter comprise mixtures of polymeric and non-polymeric components often in the form of a relatively thin layer on a metal backing and have found fairly wide application in the engineering industry [l] . Examples include mixtures of polytetrafluoroethylene (PTFE) and bronze with or without certain other components, and PTFE, graphite and lead oxide in a matrix of a thermosetting polymer such as an epoxy or polyphenylene sulphide (PPS) [2]. It is with materials of the last composition that this investigation is particularly concerned. The optimum composition was determined by standard wear tests and was found to be 25% PTFE, 10% lead oxide and 10% graphite with the balance being the matrix polymer PPS. Relatively small composition changes have a marked
112
effect on wear and the objective of the work described is to investigate of the factors involved in determining the wear of these materials.
some
Wear behaviour The fillers incorporated in the PPS all possess lubricating qualities. PTFE has been recognized as exceptional in that its friction coefficient is appreciably lower than that of most other materials (0.04 - 0.1) and this is accompanied by a low wear rate at light loadings. As the loading increases, however, so does the wear until at high pressures it becomes poor [ 31. In wear tests involving plastic-metal contact in pin-and-plate configurations a film of highly oriented PTFE has been observed on the metal counterface and on this basis it has been suggested that wear proceeds by the removal and renewal of this film, which occurs readily at high loads f4]. It has also been suggested that metallic fillers incorporated in the polymer assist the adhesion of this film by either physical or chemical means, thereby reducing wear. The importance of graphite as a dry lubricant has long been realized and this is thought to be related to its lamellar structure, which allows the formation of a layer of low shear strength and low wear on rubbing surfaces, possibly with the aid of any metallic filler present 121. The situation is complicated by the possible presence of varying wear mechanisms in differing experimental configurations since it is known that transfer film formation may depend on whether or not the wear debris can escape from the contact area, and comparison of results obtained using differing wear configurations can be difficult for this reason. The influence of the physical and in particular the mechanical properties of a bearing on its wear has been the subject of a number of previous investigations [ 5,6] . For a plastic-plastic contact it has been suggested that both the shear modulus and the extension to break might be significant [7] and the influence of hardness (largely controlled by the matrix polymer) for a composite plastic bearing has also been investigated [ 8] . Procedure The approach adopted in the investigation of possible wear mechanisms involved physical property determination at the bearing operating temperature and analysis of the rubbing surfaces in journal bearing tests in which a plastic bush was in contact with a hardened steel shaft. Physical characterization The components of the optimised composition (PPS (Ryton VI, Phillips Petroleum Ltd.), PTFE (Fluon Ll69B, ICI Ltd.), graphite (Foliac No. 1371, Rocol Ltd.) and lead monoxide (Technical Grade, BDH)) were mixed in a ball mill and pre-cured at 266 “C for 16 h and 371 “C for 1 h. The mixture was then ground and compression moulded into bars 6.35 mm X 10 mm X 74 mm at a temperature of 385 “C and pressure of 23 MN rne2. The structure
113
A
Fig. 1. Optical micrographs of mouldings of polyphenylene sulphide composition and the pure polymer (inset): A, lead monoxide particle; B, graphite particle; dark areas, PTFE; light areas, PPS.
of the moulding was revealed by optical microscopy after it had been mounted in bakelite and polished with gamma-alumina (Fig. 1). The overall dispersion is seen to be reasonably homogeneous, the identity of the components being determined by comparison with micrographs of single-component mixtures. Of the various mechanical characteristics of a material one of those most frequently related to friction and wear behaviour is shear strength. This is associated with the adhesive theory of wear which tends to be favoured for many polymeric materials [ 71. Also associated with wear is the temperature rise due to frictional heating and this necessitates the determination of the shear modulus over a range of temperature. The method adopted involved the calculation of the shear modulus G from the slope of the torque T uersus angular deflexion @curves for specimens 63.5 mm X 6.35 mm X 1.27 mm thick cut from the original moulding using a microtome diamond saw. The formula employed is due to Nielson [9] : G=-
16L
dT CD311 0 d@ @=c
where L, C and D are the specimen length between clamps, its width and thickness and p is a shape factor. The apparatus employed was a modified Clash-and-Berg torsion tester. Tests were conducted over the range 20” 200 “C, the results being shown in Fig. 2. The deflexion curves revealed that above 100 “C the behaviour was largely non-linear. The rapid drop in modulus over the region 90 ’ - 105 “C is probably related to the glass transition temperature Tg of the uncured PPS, which has been found to be 85 “C by
114
SHEAR
MODULUS
VS
TEMPERATURE
IO9
"I I z 0 10S
I 0’
t
I
I
I
0
50
100
150
TEMP
I 200
“C
Fig. 2. Shear modulus us. temperature.
DTA [lo] . The reduced magnitude of the modulus reduction compared with that usually observed in amorphous polymers is likely to be related reduced chain mobility above Tg due to cross-linking during moulding.
to
Bearing manufacture and wear testing Bearings were prepared by centrifugally casting a suspension of the components in toluene on the inside of a brass bush heated to 360 “C and machining to an internal diameter of 25.482% mm. The bush was then cut into two using a slitting saw (Fig. 3). An optical micrograph of a cross section of the coating and bush prepared in a similar manner to the moulded
115
CENTRE
LINE
I BRASS BUSH I C-
+ ,025 _ .ooo
A 125.48 B = 258
mm
mm
Fig. 3. Dimensions of bearing.
Fig. 4. Optical micropaph (section) of a polymer film; A, lead monoxide particle; B, graphite particle; dark areas, PTFE; light areas, PPS.
specimen is shown in Fig. 4. It can be seen that the film is of a reasonably uniform composition but displays a slightly different structure of the PTFE filler, probably attributable to the reduced pressure applied during processing. Wear testing consisted of operating the split bearings against a hardened tool steel shaft 25 mm in diameter under a range of conditions until a satisfactory transfer film and uniform wear rate resulted [ 111. A load of 178 N and a speed of 1.6 m s-l was found to be suitable.
116 TABLE 1 Concentration ratios of PPS, PTFE, lead oxide and iron in the bearing transfer film and the bush surfaces (wt. %)
Transfer film Bush surface Bulk average value (from mixture composition)
PTFE/PPS
PbO/PTFE
Fe/PPS
16.3
1220
172
5.5
830
45.5
40
6.4
Surface analysis
Chemical microan~ysis was carried out using an electron probe microanalyser (Cambridge Microscan 5). A focussed beam of electrons 2 - 3 mm in diameter with a penetration of 1 - 2 pm produces X-rays characteristic of the elements present and the weight fractions can be calculated from the Xray intensity in conjunction with a calibrated standard. Errors result from varying degrees of sub-surface absorption and mutual excitation effects and the method cannot be used for low atomic number elements. Microan~ysis was applied to specimens cut from both shaft and bush, the average concentration of the elements sulphur, fluorine, lead and iron being determined along a line 270 mm long. From the elemental analysis the concentrations of PPS, PTFE and lead oxide could be calculated. The results are shown in Table 1. Discussion A comparison of the values given in Table 1 reveals a significant absence of PTFE in both the worn bush and the transfer film surfaces, the latter in particular being le~~om~ated. The concentration of graphite cannot be determined by this procedure owing to inaccuracy in the evaluation of carbon and the difficulties of resolution into the various forms present. The presence of iron in large proportion in the film implies a penetration of the steel shaft by the electron beam, and hence a film thickness of less than 1 - 2 rum. This is in agreement with surfaee electron micrographs, which revealed a film sufficiency thin for the ma~h~ing marks of the shaft to be distinguishable beneath it, and is in contrast to the relatively thick films observed with some other plastics [4]. The iron in the bush is significant as it reveals a mechanism for back transfer from the shaft. These results suggest that in the case of composite bearings of this general type the low wear is likely to be related to the presence on the rubbing surfaces of a relatively thin film of lead oxide or lead with the possible inclusion of graphite. Both these materials exhibit laminar shear, which may be facilitated by the PTFE acting as a low energy sub-surface. Experimental and theoretical evaluation of the surface running temperatures indicate that a temperature of 150” - 160 “C is attained [ll],which
117
implies that the matrix is in the lower shear modulus, softer condition. This could be important in assisting the observed transfer of lead to the shaft and might also reduce adhesive wear. These results suggest some sim~a~ty between soft metal and plastic bearings as far as possible weax behaviour is concerned. References 1 G. C. Pratt, Recent developments in PTFE-based dry bearing materials and treatments, Proc. Inst. Mech. Eng. London, 181 (Part 30) (1966 - 7) 58. 2 G. H. West and J. M. Senior, Tribology, 6 (1973) 269. 3 K. Tanaka, Y. Uchiyama and S. Toyooka, Wear, 23 (1973) 153. 4 B. J. Briscoe, A. K. Pogasian and D. Tabor, Wear, 27 (1974) 19. 5 J. K. Lancaster, Wear of carbon and graphitic materials sliding against rough metal surfaces, Proc. of the Lubrication and Wear Convention, Inst. Mech. Eng., London, (1963) p. 190. 6 T. L. Oberle, J. Met., 3 (1951) 438. 7 G. H. West and J. M. Senior, Wear, 19 (1972) 37. 8 A. Ghafoor, J. M. Senior, R. H. Still and G. H. West, Polymer, 15 (1974) 577. 9 0. W. Trayer and H. W. March, Nat1 Advis. Comm. Aeronaut., Rep. 334. 10 J. N. Short and H. Wayne, Chem. Technol., 2 (1972) 481. 11 C. A. Jones, M. SC Dissertation, UMIST, 1974.