FORMATION OF UHMWPE POLYMERIC TRANSFER FILMS ON SLIDING GLASS COUNTERFACES - EARLY AND STEADY-STATE WEAR STUDIED BY TRANSMISSION ELECTRON-MICROSCOPY

At a temperature of 30-degrees-C polymeric transfer films were generated on glass counterfaces during small-amplitude oscillatory sliding contact with an ultra-high molecular weight polyethylene (UHMWPE) pin under a constant load of 6.5 MPa. Early discontinuous and continuous (steady-state) transfer films were studied with vertical platinum-carbon replication and transmission electron microscopy (TEM). Nanometre-scale UHMWPE deposits in these transfer films have been visualized for the first time. Nanometre-scale particles averaging 20.6 +/- 6.3 nm (5-40 nm) were deposited on glass in the early stages of UHMWPE film transfer. After formation of a continuous transfer film, UHMWPE particles of slightly smaller dimensions, 13.5 +/- 6.1 nm (2-41 nm), were deposited on the transfer film surface. In addition, micrometre-scale particles (0.1-6.4 mum) were found at the ends of the wear track. At high magnification, a fraction of the UHMWPE polymer chains observed in particle surfaces and in the transfer film surface nearby were not oriented in the sliding direction. Some crystal- or plate-like particles of UHMWPE were seen in the transfer film. Plate-like and micrometre-scale spherulitic inclusions were also identified on the mostly amorphous UHMWPE pin surface fractured at liquid nitrogen temperatures. The high frequency of nanometre-scale UHMWPE particles in contrast to the less numerous micrometre-scale deposits near the ends of the wear track suggests that the nanometre-scale deposits contribute significantly to transfer film formation and to the wear characteristics of UHMWPE.