| Summary: | A study has been made concerning the tribological behaviour of ultrahigh molecular weight polyethylene (UHMWPE) during water-lubricated reciprocating sliding wear. The experimental work has been extended to study also the effect of molecular weight, fillers, lubrication, counterface roughness and sliding configuration on the polymer's transfer characteristics. The wear behaviour of polytetrafluoroethylene (PTFE) has been included for comparative studies. The worn material was studied using stylus profilometry, optical microscopy, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), Transmission electron microscopy (TEM), X-ray fluorescence (XRF), X-ray diffraction (XRD), differential scanning calorimetry (DSC), infrared spectroscopy (IR) and mass spectrometry. The effect of two fillers, namely glass beads and a titanium-based inorganic filler on the friction and wear behaviour of UHMWPE has been investigated as a function of counterface roughness (Rₐ). It was found that the filled material exhibited lower wear rates than the unfilled material on the rougher counterface. The filled material was also found to be more sensitive to a change in Rₐ and showed higher wear rates than the unfilled polymer on the smooth counterface. A uniform and coherent transfer film is found on the rougher counterface but the transfer film for the titanium-based filler was patchy on the smooth counterface. No coherent transfer film was found when sliding was conducted parallel to the grinding direction on the steel counterface, resulting in relatively high wear rates. Polymer transfer was patchy, the amount increasing as the sliding distance increased. The observed phenomena are explained in terms of mechanical interlocking and chemical bonding of the polymeric material with the metal counterface. An increase in molecular weight did not significantly improve the wear resistance of the UHMWPE. Small variations in counterface roughness values (Rₐ) were found to have a much greater effect on the wear rates than changes in molecular weight. The steady-state wear rate of the polymer was furthermore found to be more dependent on an adherent transfer film rather than a change in bulk morphology. Although PTFE exhibited low friction coefficients, the high wear rates obtained by this polymer is explained by the polymer's inability to form a transfer film under water lubrication, while any film that forms under dry sliding wear is easily peeled off the surface. Significant improvements in wear are found when fillers are added to the polymer. The wear rates for PTFE under dry sliding are similar to those obtained for UHMWPE under water lubrication. Transfet of UHMWPE material to the metal counterface during sliding wear involves interlamellar shear of the polymer and results in the development of a highly oriented transfer film. Significant differences have been found in the degree of crystallinity, crystallite size and orientation in the deformed surface layers of the polymer and debris compared with those of the bulk polymer. The worn surface of the polymer shows slightly increased crystallinity but the crystallinity of the debris is much higher than that of the bulk whilst the crystallite size is much reduced.
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