The heat treatment and wear behaviour of a Co-Cr-Mo alloy.

Studies have been conducted to determine the influence of heat treatment upon the microstructure and wear resistance of the CoCrMo Alloy used in artificial hip joints. By means of a detailed metallographic examination into the response of this alloy to solution treatment, a number of reactions have been identified. These include a carbide transformation from M23C6 to M6C and the occurrence of incipient melting. Discussion of these results, together with those of previous investigators, has enabled a series of quasi-ternary phase diagrams to be constructed, which demonstrate the importance of both temperature and alloy composition upon the solution treatment behaviour of CoCrMo Alloy. The influence of heat treatment upon the wear resistance of CoCrMo Alloy was investigated, using a pin-on-disc apparatus, for three different conditions of this alloy, namely as-cast, solution treated and solution treated and aged. The results from these tests showed that, due to a higher transition load from mild to severe wear form, superior wear resistance was exhibited by the as-cast material. This difference in wear behaviour has been discussed and found to be independent of either bulk or microhardness value. Rather it has been shown that the presence of coring in the as-cast alloy caused a concentration gradient of chromium across the microstructure, thereby allowing an adherent. oxide scale to be produced. For the two solution treated conditions, however, this concentration gradient no longer existed and thus a less adherent oxide scale containing Cr2O3 was formed. Confirmation of this hypothesis was obtained by the use of selected experiments, these involving binary Co-Cr and ternary Co-Cr-Mo alloys, together with short time solution treatment tests. It has been proposed that future work examine alloys of a lower carbon and chromium content, so as to facilitate solution treatment whilst maintaining wear resistance