Coefficient of friction under dry and lubricated conditions of a fracture and wear resistant P/M titanium-graphite composite for biomedical applications

Abstract

Recent studies have shown that aseptic loosening of orthopaedic implants is a biomechanical phenomenon initiated mechanically and propelled by biological responses to the presence of wear debris released from the biomaterial. A triphasic composite, fabricated by the heterogeneous sintering of titanium and graphite powders was developed to address the fracture and wear performance of titanium. The composite is designed to smear graphite on both articulating surfaces and hence reduced wear and maintained a low friction tribosystem with fracture properties better than conventional bioceramics. The composite is made up of ductile titanium and a colony of hard, wear resistant titanium carbide produced by the controlled sintering of titanium and graphite particles. Free graphite is present in varying quantities depending upon compaction pressure and initial graphite composition. Initial graphite content composites of 4% and 8% were made with different compaction pressure to give a range of porosities from 10% to 45%. The coefficient of frictional was measured on a pin-on-disc (hardened steel) configuration. Under dry state, the coefficient of friction was observed to reduce with increasing porosity. Graphite smearing and the entrapment of debris by the pores contributed to the reduction of the wear components which in turn reduced the coefficient of friction. Under lubricated conditions, the sintered titanium and its composites were observed to be independent of porosity and pore size. The frictional behaviour of the titanium-8% graphite composites showed first, a titanium carbide dominated-wear stage and the second, a free graphite smearing stage. The results proved the concept that the coefficient of friction of titanium-graphite composites approaches that between graphite-graphite surfaces after the running-in period. This proof of concept is the first realisation of a biomaterial that could reduce the coefficient of friction on both articulating surfaces, a step closer to the development of fracture and wear resistant biomaterials that also protects the other counter surface. © 1998 Elsevier Science S.A.