THE EFFECT OF GRINDING DIRECTION ON THE NATURE OF THE TRANSFER FILM FORMED DURING THE SLIDING WEAR OF ULTRAHIGH MOLECULAR-WEIGHT POLYETHYLENE AGAINST STAINLESS-STEEL

A study has been made concerning the nature of the transfer film formed during the reciprocating sliding wear of ultrahigh molecular weight polyethylene (UHMWPE) against a hardened stainless steel counterface under water lubrication. A range of UHMWPE has been subjected to water-lubricated, reciprocating sliding wear under a pressure of 10 MPa at a frequency of 2.5 cycles s-1 and an average speed of 0.25 m s-1. It has been found that the interfacial deformation mechanism involves shear of thin surface layers of polymer and that the transfer film on the steel counterface is essentially multilayered. The worn polymer surfaces and debris have been extensively characterized for structural changes using X-ray diffraction and differential scanning calorimetry. It has been found that the wear debris is significantly more crystalline than the bulk polymer. The crystallinity of the wear particles is believed to be similar to that of the transfer film. No coherent transfer film was found when sliding was conducted parallel to the grinding direction on the steel surface. Polymer transfer was patchy, the amount increasing as the sliding distance increased. The debris was also found to be less crystalline than debris formed when sliding was conducted perpendicular to the counterface. An increase in molecular weight did not significantly improve the wear resistance. Small variations in counterface roughness values (R(a)) 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.