A tribological study of cobalt chromium molybdenum alloys used in metal-on-metal resurfacing hip arthroplasty

Abstract

Some modern metal-on-metal bearings have undergone early failure and have exhibited a different microstructure from early hip replacement systems that produced very long lifetimes. This has led to investigations as to why different microstructural conditions, from ostensibly the same alloy chemistry, have had differing levels of survival. This paper is concerned with the effect of thermal treatment and hence the resulting microstructure on the wear properties of coupons using a micro-abrasion test. A series of cobalt–chromium–molybdenum (CoCrMo) alloy (ASTM F-75 98, ISO 5832-4: 1996; BS 7252: Part 4: 1997) sample coupons with different combinations of thermal treatments (solution heat treatment, hot isostatic pressing or sintering) were studied. Metallographic studies have been used to determine the volume fraction of carbide present. The carbide level and morphology varied markedly between samples with respect to thermal history. This work has established that microstructure and in particular the volume fraction, size and distribution of carbides is critical to the development of a low wear rate system. As-cast materials were determined to have greater abrasive wear resistance when compared to single or multiple heat treated materials.