Tribological behaviour of cast and wrought Co–Cr–Mo implant alloys

Abstract

Cobalt alloys are currently employed in the development of hip implants due to their relatively good strength and wear properties. However, in some instances the exhibited wear rates are excessive and alloy modification to improve this is possible. In the present work, the wear properties of a cast and a wrought Co–Cr–Mo alloy sliding against ultrahigh molecular weight polyethylene (UHMWPE) were investigated as functions of the implemented heat treatment. For this purpose a pin-on-disc wear testing machine was used. It was found that athermal epsilon martensite was developed by solution heat treating (HT) at 1225°C for 1 h. The steady state friction coefficients found were 0·245 (for a low carbon wrought alloy) and 0·220 (for a high carbon cast alloy) under dry air with wear factors of 2·78 × 10^-7 and 1·80 × 10^-7 mm³ N^-1 m^-1, respectively. The as cast Co–Cr–Mo alloy exhibited a higher friction coefficient initially, but this was reduced under steady state. Annealing these two alloys reduced the friction coefficient on the as cast, but not on the wrought alloy. In addition, annealing heat treatments did not improve the UHMWPE wear rates or volume per cent debris loss with either alloy. It was suggested that athermal epsilon martensite might increase wear by promoting the formation of wear debris at wide stacking faults in the wrought Co–Cr–Mo counterface. No changes were found in the exhibited volume fractions of athermal martensite after wear testing, suggesting that plastic deformation is mainly confined to the UHMPWE counter surfaces.