Wenr, 153 (1992) 331-350 331

Effect of roughness and sliding speed on the wear friction of ultra-high molecular weight polyethylene

and

T. S. Barrett*, G. W. Stachowiak and A. W. Batchelor The Department of ~echan~al Engineering, The U~ive~~ of Western A~~a~~ Nedlands, Western Austdiu 6009 (Australia)

(Received April 22, 1991; revised and accepted July 15, 1991)

Abstract

The friction and wear of ultra-high molecular weight polyethylene (UHMWPE) pins sliding against a stainless steel disc were measured for sliding speeds ranging from 1.25 to 10 m s-l and disc surface roughnesses R, from 0.07 to 0.53 pm ms-. Frictional heating was controlled by air jets and surface temperature measured with an IR pyrometer. It was found that the wear of UHMWPE is critically dependent on surface temperature and that, when the temperature exceeds a critical value, wear proceeds in a series of discrete steps caused by the sudden loss of a molten or softened layer of polymer. Wear was also influenced by surface roughness. An optimum surface roughness, ie. a minimum of wear was found at low and medium sliding speeds. At the highest speed tested, however, the influence of roughness on wear rate was much less distinct. Scanning electron photomi- crographs of worn pins and disc surfaces revealed evidence of melting by IJHMWPE at high sliding speeds and abrasion at high surface roughnesses. Transfer fifms on disc surfaces were limited to isolated deposits of polymer wear particles.

1. Introduction

UHMWIE is a polymer which allows sliding against steel or other surfaces with very low wear coefficients and a moderate coefficient of friction. Studies of the friction and wear characteristics of this material have usually been limited to low sliding speeds as is the case with other polymers. A detailed model of transfer films to explain polymer wear and friction in terms of an interfacial film acting as a preferential sliding surface has been developed [l-3]. Studies of wear between UHMWFE and a smooth steel counterface at low sliding speeds revealed that thin transfer films formed on the counterface, Wear of the ALE proceeded by two mechanisms: an adhesive process which occurred immediately after sliding began and a fatigue process which did not appear until after a long period of sliding had elapsed [4, 51. However, wear and friction processes in polymers sliding against rough surfaces at elevated sliding speeds have hardly been studied despite its considerable practical importance. UHMWPE is a wear-resistant polymer with a moderate coefficient of friction against a steel counterface. The design of bearings and sliding contacts made from UHMWFE or other polymers is usually based on a PV limit where P is the apparent contact pressure and V is the sliding speed. For design purposes it is assumed that, when loads and

*Present address: Interseal Ltd., Alfred Cove, Western Australia 6154, Australia.

0 1992 - Elsevier Sequoia. AU rights reserved

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sliding speeds exceed a critical PVvalue, wear rises precipitately to cause rapid failure of the bearing. This design concept is often very inaccurate and has to be modified in practice by setting multiple values of PV with each value of PV corresponding to a narrow range of sliding speed. More information on wear processes at and beyond the PY limit could enable improvements in the properties of UHMWPE bearing materials and allow the development of a more accurate design criterion.

Wear of materials at high sliding speeds is largely controlled by melting or softening due to the high interface temperatures at the sliding contact 161. This form of wear in which the sliding materials are separated by films of molten material has been demonstrated in metals [7], ice [8] and polymers [9]. It has even been speculated that the molten layers separate the wearing surfaces by hydrodynamic lubrication [lo, 111 since the coefficient of friction under melting wear can be relatively low. It is generally observed that the friction coefficient rises with increasing sliding speed until the interface temperature is sufficient for one of the sliding surfaces to melt. Beyond this temperatures friction is inversely proportional to sliding speed. This characteristic has been modelled by the concept of thermal control of friction where it is speculated that at high sliding speeds the friction coefficient varies to maintain a constant interface temperature [12]. Polymers which do not melt or soften but instead degrade at high temperatures such as polytetrafluoroethylene do not show a declining friction characteristic at high sliding speeds [13]. Thermoplastics such as nylon and high density polyethylene reveal a friction characteristic similar to metals and ice, i.e. a maximum in friction with increasing sliding speed at a contact temperature close to a melting point [9]. ALE is possibly an extreme case since, even compared with other polymers, the molten UHMWPE is so viscous that it is effectively solid. Friction might therefore not show a significant change when the contact temperature exceeded the softening or melting point. The wear rates in all these studies of polymers showed a sharp rise at the sliding speed or surface temperature where friction decreased.

Studies of the relationship between surface roughness of a metal counterface and polymer wear have shown that increased roughness causes abrasion of the polymers [14]. Separate investigations revealed that an optimum surface roughness exists where a minimum in wear may occur 1157. Reasons or modeis to explain the effect of roughness on wear that are available in the literature do not provide any detailed explanation of this minimum. It is speculated that the effect of counterface roughness is to modify friction by allowing the asperities from the solid metal to penetrate more deeply into the molten layers of polymer. A progressive increase in viscosity of the molten polymer with depth of asperity penetration would lead to greater frictional resistance when the molten polymer has a relatively low viscosity. However, molten UHMWPE is known to have a very large viscosity so that only a relatively small decrease in friction with melting is expected and the effect of roughness should also be limited. The influence of roughness on the friction coefficient of other polymers with smaller melt viscosities should be more pronounced.

In this work, the results of a study of the wear of UHMWPE pins on a stainless steel counterface for various sliding speeds and counter-face roughnesses but constant load are presented. A model of wear at high sliding speeds is introduced.

2. Theory

Polymer wear is known to be dependent on interfacial contact temperature. It was not possible to measure the contact temperature directly so a formula developed

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by Challen and Dowson [16] was applied for the contact temperature. The following equations give an estimate of the temperature rise for UHhWPE pins sliding on stainless steel:

Tf= F,,W+ FtbT,, (1)

F = 1.65FsF,,,p@.5 P F-i-F U-. s m

F tb

= F,- Q.65FJJ-Q~s

F +F s m U-.

(2)

where Tf C is the interfacial contact temperature or flash temperature, Tb C is the disc surface temperature, W (N) is the load, U (m s-) is the sliding velocity, p is the coefficient of friction, Fs (K s N-l m-) is a constant and F, (K sin N- m-In), is a constant. The effect of forced cooling was not specifically included in the derivation of these equations and it was assumed that negligible inaccuracy would result provided that the disc temperature was accurately measured.

3. Experimental details

Unidirectional constant-speed sliding was selected for experimentation to facilitate measurement of the effect of sliding speed and contact temperature on wear and friction.

3.1. Experimental apparatus A pin-on-disc test rig was used to investigate the wear and friction of UHMWPE

in unidirectional sliding. UHMWPE pins were loaded against a stainless steel disc with friction measured by strain gauges and wear monitored by a linear displacement transducer contacting the pin. A schematic diagram of the test rig is shown in Fig. 1.

At high sliding speeds a polymer pin is prone to soften and wear catastrophically and, to obtain valid data, careful temperature control at the sliding interface is essential. The temperature was controlled by cooling the disc and pin with air jets directed at the disc surface at the pin. A heating element was fitted underneath the disc to provide uniform heat to the disc independent of frictional effects. An IR pyrometer was used to measure the disc surface temperature in the wear track adjacent to the pin. Figure 2 illustrates the cooling and temperature measurement systems.

0.5 HP DC motor load

Fig. 1. Schematic diagram of pin-on-disc test apparatus.

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~-~ LED transmitter/receiver

Fig. 2. Schematic diagram of temperature control and measurement systems for pin-on-disc test apparatus: LED, light-emitting diode.

The disc was driven by a 770 W d.c. electric motor with a maximum speed of 1150 rev min- with a variable-speed controller. The fluctuation in any set speed was less than + - 1% and was monitored by a digital tachometer. The disc was directly coupled to the motor and with a motor maximum speed of 1150 rev min-, a disc diameter of 100 mm was chosen for these tests. At a 45 mm wear track radius, the maximum motor speed corresponds to a linear sliding speed of approximately 16 m

-1 s . The linear variable-displacement transducer (LVDT) used to measure wear was

rigidly mounted on a frame above the loading arm of the pin. With a high value of amplification of the output signal, movements as small as 1 pm could be accurately measured. The LVDT was calibrated by placing a metric dial gauge above the pin

*holder which was then moved by a screw. The strain gauges for friction measurement were calibrated using weights connected by a pulley and cord to the pin holder. The IR pyrometer was calibrated by heating the disc and measuring the temperature with a mercury thermometer. A stainless steel disc of medium roughness, R,,=0.14 pm was selected for these tests. Several temperatures were tested and, at an emissivity setting of 0.23, the readings of the pyrometer and mercury thermometer agreed to within 1 K. During wear tests, some discs acquired a dark transfer film in the wear track. The calibration was repeated and a correction made to the emissivity factor.

3.2. Experimental materials The discs were made of stainless steel with a composition of 18 wt.% Cr, 10

wt.% Ni, 3 wt.% MO, 0.1 wt.% C, 1 wt.% Si and 2 wt.% Mn. Stainless steel was

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Pin Design as used in preliminary tests

Pin Design as used in 2nd round tests

i- 3mm

Fig. 3. Dimensions of polymer pins.

chosen to prevent any influence on wear rates by small abrasive particles of rusted iron. The wear of UHMWPE has previously been shown to be extremely sensitive to small levels of contamination [17]. The discs were prepared to various levels of surface roughness by machining to a fine turned finish followed by hand finishing with emery paper. The roughness R, of the discs were set at the following levels: 0.07 pm, 0.14 pm, 0.24 pm, 0.29 pm, 0.44 pm and 0.53 pm (centre-line average). The randomness of the surface roughness was checked by measuring the surface roughness in radial and circumferential directions for each disc using a Taylor-Hobson Talysurf Model 5. Ten values of surface roughness were obtained for both the circumferential and the radial directions and a t test at 2.5% level of significance applied to the data. No significant difference between the two roughnesses was found.

The polymer pins were made of GUR 415 UHMWFE. The average molecular weight of this material is 5.5 x lo6 and the measured density was 925 kg rne3. The Vicat softening point is 136 C. Rockwell Hardness 52 HR and tensile strength at 20 C is 40 MPa. The pins were machined into two forms shown in Fig. 3: one of these is the conical profile with an end-flat diameter of 3 mm which has been used elsewhere [16], and the other consists of a rod-shaped projection of 3 mm diameter from the main body of the pin. The latter design was intended to provide better cooling of the wearing surface and was found to have slightly different wear characteristics from the conical pin.

3.3. Experimental procedure Before testing, the discs and pins were carefully cleaned in ethanol and dried in

hot air to remove any dust or greasy contaminants. The pins were then weighed by an analytical balance accurate to 0.1 mg and their dimensions measured in an optical profilometer to provide wear data additional to the data recorded from the LVDT. Wear debris accumulated on the pin and obscured the wear scar profile and caused erroneously low readings of mass loss. Once material is expelled from the contact, then it becomes wear debris even though it remains adhered to the wearing specimen. The data recorded by these latter methods of measurement were therefore not included. The pins and discs were then fitted into the apparatus and a load of 34.33 N (3.5

336

kgf) applied to the pin (with spring force from the LVDT compensated for). The corresponding contact pressure for this load is 4.85 MPa. The pins sustained rapid creep immediately after loading and wear tests were not begun until the LVDT indicated that the initial creep had ceased. The same problem was found by other workers measuring the wear of UHMWPE [18]. The length of the pin which controls LVDT measurements is also affected by changes in temperature. The disc was therefore heated to the test temperature before loading the pin onto the disc. Tests were then performed over a 1 h period which results in a sliding distance of 18 km at a sliding speed of 5 m s-r. The sliding speeds selected were 1.25 m s-r, 2.5 m s-, 5 m s-l and 10 m s-r while the disc temperatures were 44 C, 50 C, 66 C and 82 C respectively. The pins and discs were removed after testing and examined later by scanning electron microscopy (SEM) for evidence of wear mechanisms and transfer films.

4. Results

Friction and wear coefficients were found for UHMWPE sliding on stainless steel at various roughnesses, sliding speeds and disc temperatures for constant load. All three parameters were found to have a strong effect on friction and wear. Micrographs of the worn pins and discs showed that changes in wear and transfer film mechanisms concurred with variations in wear and friction.

4.1. Wear and friction data Figure 4 shows a graph of wear coefficient W. surface roughness at various sliding

speeds. The disc surface temperature in these tests is 50 C which provides severe but not extreme conditions for the UHMWPE material. An optimum counterface roughness R, of between 0.14 and 0.24 pm is apparent for all sliding speeds apart from the maximum speed of 10 m s-l. The effect of optimum roughness is very marked as the wear values at 0.001 mm3 N- m-l represent the limits of measurement and the real wear rates could be much lower. For the highest velocity tested of 10 m s-l, the minimum wear recorded was between R,= 0.14 and 0.29 pm but there appears to be no large decrease in wear rate close to an optimum roughness. The change occurring between a sliding velocity of 5 and 10 m s-l is very marked and worth further investigation.

, E z ?i

1 .E 1:

ii ;,OOl, .,v. ,- , .I . I.

00 0.1 02 0.3 0.4 0.5 0.6

Surface Roughness, Ra &tn

Fig. 4. Effect of counterface surface roughness on wear coefficient at various sliding speeds.

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The friction coefficients show much less variation with surface roughness or even sliding velocity than the wear coefficients. Figure 5 shows the friction coefficients for the same tests as in Fig. 4. The friction coefficients appear to maintain a nearly constant value for all values of R, apart from the lowest and highest R, values where a small rise in friction was recorded. At the highest value of surface roughness tested, R,= 0.53 pm, a small increase in friction with increasing sliding speed was found.

The interfacial temperature which is the sum of disc temperature and the flash temperature was found to exert a very strong effect on wear rates. Figure 6 shows a graph of the friction and wear coefficients of UHMWPE as a function of interfacial temperature for a counterface roughness of 0.13-0.14 pm.

A very strong effect of contact temperature on wear rates is evident; at around 136 C, which is the softening point of the UHMWPE, wear coefficients rise to extremely high levels of more than lo- mm3 N- m-. The friction coefficient declines by a small amount with increasing temperature but the effect is minor compared with the variation in wear rate. The effect of sliding speed at this level of roughness on

s 0.4 -

z

z 0.3 -

5

E 0.2 - 0

E r s 0.1 -

- 1.25mk - 2.5mk

- 5.oml.s

_ lO.Om/s

0.0 11 0.0 01 0.2 0.3 04 0.5 0.6

Surface Roughness, Ra pm

Fig. 5. Effect of counterface roughness on friction coefficient at various sliding speeds.

0.5 3o E z ;;i

0.4 $ s F 20 y

E 0.3 & - o Coefficient of Friction

z _ Wear Coefflclent E 0.2

-, 0

0 10 z

E B

t 0.1 0

s

;

0.0 O 60 60 100 120 140 160

i

Interfacial Contact Temperature, C

Fig. 6. Friction and wear coefficients of UHMWPE at 0.14413 pm counterface roughness vs. interfacial contact temperature.

338

wear and friction is dependent on its relation to contact temperature. The PV value at temperatures just short of the rapid rise in wear ranges from 5 to 40 MPa m s-l which is far in excess of accepted PI/ limits of 0.05 MPa m s-l for UHMWPE.

When the contact temperature exceeds the critical value, the nature of wear changes as well as its magnitude. Figures 7-9 illustrate recordings of wear and friction ZIS. time. Data of tests for a counterface roughness R, of 0.14 pm and sliding speed of 5 m s-r for varying disc temperature is shown. Figure 7 exhibits data for a disc temperature of 38 C. It can be seen that the wear rate varies in a continuous manner even if there is considerable variation in differential wear rates. The friction coefficient is steady but with small variations occurring. A similar pattern occurs at a disc temperature of 50 C which is shown in Fig. 8.

A Friction Coefficient 0.38 _

0.35.

Friction Coefficient

Time, minutes Time, minutes

-40 20 1

E 1-5 lo

50

f g -10 0 2

z-

Fig. 7. Recordings of wear and friction for 0.14 pm disc roughness, 5 m 5-l sliding speed and 38 C disc temperature.

Fig. 8. Recordings of wear and friction for 0.14 j.cm disc roughness, 5 m s- sliding speed and disc temperature 50 C.

0.35 . Friction Coefficient

0.32 -

Fig. 9. Friction and wear recordings at 0.14 pm disc roughness, 5 m s- sliding speed and 66 C disc temperature.

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0.38

0.35 Friction Coefficient

Fig. 10. Friction and wear recordings at 0.53 pm disc roughness, 5 m s-* sliding speed and 50 C disc temperature.

There is a very strong similarity in wear and friction characteristic between 38 and 50 C, indicating that temperatures in this range have very little effect on wear and friction. This condition persists until contact temperatures very close to the limit are reached. Figure 9 shows wear and friction recording at a disc temperature of 66 C.

At a disc temperature of 66 C the wear has changed from continuous loss of material by small increments to a stepped characteristic. Chunks of polymer 2-4 pm thick are lost in a precipitate manner while, during the remainder of the test, very little wear occurs. The frictional force remains almost constant during these fluctuations in wear.

When there is a combination of an elevated disc roughness and a high contact temperature, a combined form of continuous and stepwise wear occurs. Figure 10 shows friction and wear recordings at 0.53 pm disc roughness, 5 m s-r sliding speed and 50 C disc temperature. The increased roughness coincides with a higher coefficient of friction and contact temperature. Wear progresses as a continuous form of wear which is assumed to be the result of abrasion and the step-form wear associated with softening of the polymer at the sliding interface. Even when polymer is softened, it is still subject to abrasion. The most rapid wear measured occurred when a rough and hot disc was rotated against a polymer pin.

4.2. Micrographs of wear sugaces Micrographs were collected of both the worn end of the pin and the wear track

of the disc. The predominant feature at high disc roughness was abrasion of the pin as can clearly be seen in Fig. 11 which shows a pin surface after sliding against an R,= 0.44 m disc at 1.25 m s-l sliding speed and 50 C disc temperature.

Most of the worn surface is covered by a series of straight sharp-edged grooves aligned to the direction of sliding which indicates abrasion by asperities or sharp- edged scratches on the metal counterface. When the sliding speed is raised to 10 m S -l, then the higher frictional heating inhibits abrasion and a more irregular surface

Fig. 11. Micrograph of worn pin surface after sliding against and R,=0.44 pxn disc at 1.25 m s- and at 50 C disc surface temperature.

Fig. 12. Micrograph of worn pin surface after sliding against an R,=0.44 pm disc at 10 m s- and 50 "C disc temperature.

morphology results. Figure 12 shows a micrograph of an R, = 0.44 pm disc sliding at 10 m s- against a pin at 50 C surface temperature.

There are some traces of abrasion revealed by scratches aligned to the direction of sliding but the dominant feature is the smearing of tongues of polymer and irregular wave features. Both these tongues and waves are also aligned to the direction of sliding. It appears that softening by frictional heat has caused the surface layers of polymer to deform of flow continuously in the direction of sliding. When roughness is reduced too close to the optimum value, e.g. 0.14 pm, a different pattern of wear to~~aphy is found. Figure 13 shows a micrograph of the worn end of a pin after

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Fig. 13. Micrograph of worn pin surface after sliding against an R,=0.14 pm disc at 2.5 m s-l and at a disc temperature of 50 C.

Fig. 14. Worn pin end after sliding at 1.2.5 m s-l against an R,=0.07 pm disc at 50 C disc temperature.

sliding against an R, 0.14 pm disc at 2.5 m s - sliding speed and 50 C disc surface temperature.

Direct evidence of abrasion is not present; instead a layer of sheared material covers the surface. When the surface roughness is lowered still further to the minimum tested, the process of shearing becomes more extreme. Figure 14 shows a high magnification view of a worn pin after sliding at 1.25 m s-l against an R.= 0.07 pm disc at 50 C disc temperature.

When the sliding speed is raised to the highest level tested while maintaining a low surface roughness, then the surface topography simplifies to a smooth surface

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with only a few grooves present. These grooves may be caused by ploughing from either an abrasive contaminant or a lump of solidified polymer bonded to the counterface. Figure 15 shows a micrograph of a worn pin end after sliding against an R,= 0.07 ,um disc at 10 m s- * and 50 C disc surface temperature.

When the interfacial contact temperature was in the vicinity of 75 CT, irregular striations or waves covered the worn pin surface. These striations occurred even when abrasion was present. Figure 16 shows a micrograph of a worn pin after sliding at 5 m s- against a disc with R,=0.53 pm and 50 C surface temperature.

Fig. 15. Micrograph of worn pin end after sliding at 10 m s- against an R,=0.07 pm disc at 50 C disc temperature.

Fig. 16. Micrograph of worn pin after sliding against an R,=O.53 pm disc at 50 C disc surface temperature and 5 m s-* sliding speed.

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The striations are oriented almost normally to the direction of sliding and have a wavelength of approximately 10 pm. The striations are similar to those observed in other studies of UHMWPE at high sliding speeds [9] and were explained as caused by elastic recovery of molecules after frictional contact.

When a temperature sufficient to cause rapid wear was reached, gross softening of the polymer pin became evident. On the trailing edge of the pin a lip of softened material formed which extended up to 1 mm from the wear face. Figure 17 shows the worn surface of a pin after sliding against an R,=0.14 pm disc at 5 m s-l and 123 C disc temperature. The interfacial contact temperature is calculated as 143 C. The wear coefficient in this instance is the highest measured at 2.5X1O-5 mm3 N- m-.

Micrographs of the disc wear track revealed that the occurrence of transfer films depended on the disc roughness and sliding speed. In most tests, very little polymer was transferred to the steel disc and only in the form of lumps. Figure 18 shows a wear track on a disc surface after sliding against a pin at 10 m s-l with an R,= 0.24 pm disc surface roughness.

Wear debris accumulates in the grooves on the disc and the widespread globular shape of debris is a sign of frictional melting or softening. When a rougher disc is used, abrasion of polymer by sharp-edged grooves in the disc becomes pronounced. Figure 19 shows a micrograph of an R,= 0.44 pm disc after sliding at 5 m s-.

When smoother discs were tested, transfer material was either absent or extremely rare and it is concluded that the fine powdery debris which tends to form transfer films rather than be expelled from the contact is the result of abrasion.

Three types of wear debris were found: fine powdery debris as shown in Fig. 18, filament debris as shown in Fig. 20 and ribbon-form debris as shown in Fig. 21. Filament debris was observed attached to the trailing edges of pins after sliding at high speeds. Figure 20 shows a micrograph of debris from the trailing edge of a pin after sliding against an R,=0.24 pm disc at 10 m s-l.

At lower sliding speeds the debris changed form to ribbons of highly compressed material. The width of the ribbon could be as large as 1 mm but the thickness was

Fig. 17. Worn surface of pin after sliding at 5 m s-l against an R,=0.14 pm disc with disc surface temperature of 123 T.

Fig. 18. Micrograph of an R,=0.24 pm disc after sliding at 10 m SC.

Fig. 19. Micrograph of an R, =0.44 pm disc after sliding at 5 m s-1.

less than 10 pm. Figure 21 shows a typical ribbon collected from a pin after sliding at 5 m s-l against an R,=0.14 pm disc.

Longitudinal striations possibly due to disc abrasion cover the ribbon surface while the extent of corrugation or folding of the wear debris is notable. It is evident that the debris has been compressed and extruded from the pin surface by frictional forces. At higher speeds, frictional contact temperatures are higher so that material is expelled as thin streamers of resolidified material.

5. Discussion

The experimental results confirmed the significance of two basic factors controlling the wear of polymers against a hard counterface: contact temperature and counter-face

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Fig. 20. Micrograph of debris at trailing edge of a pin after sliding against an R,=U.24 pm disc at 10 m s-l.

Fig. 21. Ribbon-form debris from a pin after sliding at 5 m s-l against an R,=0.14 pm disc.

roughness. For these tests, the effect of variation in sliding velocity was largely dependent on its relation to the contact temperature.

The primary effect of counterface roughness was to accelerate wear by abrasion and at high levels of roughness R, such as 0.53 pm wear was at least 1000 times more rapid than for the smoother surfaces tested. Some counterface roughness was found to be essential for low wear as was found by other workers [15]. It was observed by SEM that polymer wear particles tending to adhere to the disc, i.e. those forming a rudimentary transfer film, were preferentially located at grooves and scratches in a rough surface. The edges of grooves and prows of displaced material present on rough surfaces have been previously identified as the main cause of rapid abrasion of

346

UHMWPE [17]. When a very smooth counterface slides against the polymer, there is no abrasion to form polymer wear debris which is a precursor of the transfer film. Wear and friction are consequently much higher for a very smooth surface. Friction reduction by interposed debris may be affected by the same general principle as occurs when a small amount of sand is present between a metal object and a stone floor. If a limited quantity of sand is present, the friction coefficient is smaller than for a floor perfectly devoid of sand. This model is a further application of the principle that in dry sliding there should be a third body present between the sliding surfaces to minimize friction and wear [19]. Only a sparse transfer film adhering to the disc was observed in most tests, but loosely attached powdery debris might be more effective in reducing friction and wear and yet be easily lost after testing. Friction and wear coefficients display approximate agreement in that the minimum in friction occurs at the same minimum for wear for most of the sliding speeds tested. The minimum in wear coefficient was found to occur at a surface roughness R, of between 0.14 and 0.24 pm which is similar to that found elsewhere at approximately 0.1 pm [15].

When the sliding speed is very high, i.e. 10 m s-l, polymer wear and friction are only marginally influenced by roughness and the optimum roughness observed at lower sliding speeds disappears almost completely. This change is perhaps a result of the dominance of polymer softening mechanisms at higher sliding speeds. Polymer softening is controlled by temperature, and roughness would only influence wear rates to the extent that high levels of roughness accelerate abrasion of softened polymer. This effect can be seen as a rise in wear rates between R,=0.53 pm and R,=0.44 pm. Wear of a frictionally softened polymer remains high; so there is no minimum in wear when an optimum amount of transfer film is abraded as is the case at lower sliding speeds.

The dominant effect of temperature is demonstrated by the dramatic rise in wear at a temperature close to the Vicat softening point of UHMWPE. Wear ZIS. temperature was measured at a counterface roughness close to the observed minimum for wear rates (0.14 pm) withvarying disc temperature and sliding speed. The observed precipitate rise in wear at the softening point of the polymer is very similar to previous studies [16]. However, it was also confirmed here that, irrespective of whether interfacial temperature is raised by heating the counterface or by the selection of high sliding speeds, this has very little effect on wear. For this level of roughness, the effect of sliding speed on wear is determined by frictional temperature. It is only at high levels of roughness that an additional effect of sliding speed in the form of a proportional increase in abrasion with sliding speed becomes important. The trend to increasing friction with roughness at high sliding speeds is consistent with a model in which counterface asperities protrude deeper into the molten or softened surface polymer layer to find stronger resistance to sliding from cooler and more viscous polymer further from the source of frictional heat. The small change in friction coefficient arises because wear tests are confined to temperatures only slightly above the softening point of UHMWPE. If higher temperatures were tested, then friction coefficients might be lower as predicted by the thermal control of friction model [20] but wear rates would probably be extraordinarily high and prevent testing.

The kinetic form of wear changes from the typical linear and continuous characteristic to a non-linear discontinuous progression of wear by discrete steps. The step-form wear is explained in terms of the formation and sudden expulsion of a semiliquid or softened layer of polymer from the sliding contact when the remaining solid polymer can no longer support the load. The concept of a partially softened wear face is illustrated in Fig. 22.

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4 Pin Velocity

1.65T,

(T, = melting or softening temperature)

Fig. 22. Model of step-form wear in terms of a partially softened wear face.

The trailing edge of the pin is the hottest part of the sliding contact; here melting or softening is initiated and it spreads over the rest of the contact area. Load (or a major portion of it) becomes concentrated on the remaining solid area until this collapses or is sheared to cause the measured step increment of wear. The depth 04 wear is in the range 2-4 wrn and this is close to the depth of the softened layer observed by other workers in tests at high sliding speeds [9]. When the counterface is rough as well as moving at a high sliding speed, abrasion is superimposed 09 this chunk wear and a wear kinetic of steps plus continuous wear or a slope results. The stepped progress of wear observed here for polymers resembles that observed for metals in dry sliding [21] although the reasons for this are different. In the cited example, step changes in both positive and negative measured wear depth are due to the formation and release of transfer films. Once the contact temperature is raised to still higher values, then chunk wear is replaced by continuous rapid wear. In this process, melting occurs simultaneously over the whole of the pin face so that, as soon as the polymer contacts the hot disc, then it softens or melts and is expelled from the wearing contact.

The coefficient of friction of UHMWPE tends to decrease as the sliding speed is raised even though the change is relatively small. The friction coefficient is also unaffected by the rise in wear at temperatures close to the softening point. It would be of practical benefit to suppress the wear rise at the softening point or else to postpone it to a higher temperature. A polymer composite could be manufactured with improved heat transfer characteristics, eg. fibres of high conductivity material could be added to the polymer. The orientation of the fibres should be normal to the sliding contact to maximize heat transfer from the wear face. A fibre with compatibility

348

to steel is required because of the known propensity of normally oriented fibres to cause seizure [22].

The ribbon-form wear debris is similar to debris from tests where UHMWPE was worn against a steel disc under dry conditions [4, 51 and while lubricated by water [20]. This form of wear is associated with low or medium temperatures against smooth counterfaces where polymer is sheared or extruded from the contact. A reduction in the area of true contact was observed when a PTFE pin was slid against a glass disc [23]. It is possible that a similar process occurs here, causing a concentration of shear forces at small areas of the pin to produce ribbons approximately 1 mm wide. The change from ribbon-form debris to streamers would be caused by a change in contact geometry with increased surface temperature from a few contact areas of approximately 1 mm width to more numerous bands of softened polymer aligned to the direction of sliding [9]. Molten or softened material is expelled from the sliding contact and before cooling, surface tension forces the strand of worn material to a near-cylindrical shape.

The results suggest that a transition from slow to rapid wear occurs at a critical temperature. This aspect of the data accurately confirms the temperature dependence of UHMWPE wear found elsewhere [16]. Consequently a failure criterion based on a critical temperature which is close to the polymer softening temperature, ambient temperature, contract stress, sliding speed and an average value of friction coefficient would appear to be suitable for practical applications. The failure criterion could be based more on the equation for interfacial temperature given in eqns. (1) and (2). The range of loads and speeds tested so far, however, are not comprehensive and it cannot be assumed that the failure temperature at high sliding speeds and low loads will be the same as at low sliding speeds and high loads. In studies of the wear of nylon at moderate speeds and high loads, a critical temperature was found at ap- proximately 150 C which is far below the melting temperature of crystalline nylon at 220 C [24]. An extensive series of tests covering practical limits of load and sliding speed are required before a general criterion for the wear transition of a polymer can be made. It is hypothesized that the critical temperature would decline with increasing contact stress because, at high loads, rapid shear of the polymer would occur at a lower temperature.

6. Conclusions

An experimental study of the wear and friction of UHMWPE at various sliding speeds, counter-face temperatures and surface roughnesses revealed the following characteristics.

An optimum counterface roughness for minimum wear and friction was found with very low wear rates occurring in a narrow range of roughnesses. This effect appears to be, however, confined to low and moderate sliding speeds. At high sliding speeds, wear was influenced by surface roughness but a distinct optimum in surface roughness was not found.

The effect of sliding speed on wear and friction is mostly influenced by its contribution to interfacial temperature. When a critical interfacial temperature close to the softening point of UHMWPE is exceeded, wear becomes very rapid while friction shows a small decline.

During rapid wear at high sliding speeds and inter-facial temperatures, wear occurs in a series of steps where a sudden release of molten or softened surface polymer

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occurs. The effect of increased surface roughness at high sliding speeds is to raise the friction coefficient and consequently the interfacial temperature. Rapid step-form wear with a superimposed abrasive wear component may then result. The increase in friction with roughness is consistent with a hydrodynamic model of molten polymer separating the counterface and pin.

Three types of wear debris were found; powdery or larger lumped debris from abrasion, ribbon-shaped debris at intermediate contact temperatures resulting from shear of the polymer, and strands of polymer that had been molten or softened in the contact. Reinforcement of the polymer to prevent shear should improve the wear resistance of UHMWPE at moderately severe sliding speeds and loads against smooth counterfaces. A composite material to facilitate heat transfer from the contact might inhibit softening and allow use of the polymer at significantly higher speeds and loads.

The conventional design criterion for polymer bearings, i.e. the limiting PVvalue, predicts failure at loads and sliding speeds far less than actually occurs. A more accurate criterion based on the interfacial contact temperature would allow exploitation of polymer bearings at much higher levels of speed and load. More tests are required, however, to confirm the validity of a constant limiting temperature, particularly at high contact loads.

Acknowledgment

The authors would like to thank the Department of Mechanical Engineering, University of Western Australia for supporting this work.

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