Medical Cobalt-based Alloy Structure And Wear Performance

In recent years metal-on-metal artificial hip implants made of CoCrMoC alloy are gaining renewed interests due to the excellent wear resistance of the alloys. In the present paper, we investigated the casting CoCrMo and CoCrMoC alloys whose chemical composition is based on ASTM F75. By means of optical microscopy, scanning electron microscopy (SEM), energy spectrum, X-ray diffraction (XRD), hardness test and wear resistance measurement, the effects of various heat-treatments on microstructure and wear resistance property were studied.The results showed that the as-cast CoCrMo and CoCrMoC alloys exhibited a typical coarse dendritic microstructure composed of FCC matrix and a small amount of HCP martensitic phase. In CoCrMoC alloy there are many irregular eutectic carbides which existed in interdendritic arms. The homogenization of CoCrMoC alloy was not completed when the solution treatment temperature was under 1100℃. When the temperature rose to 1200℃ or above, the homogenization of the matrix was fully occurred and the dendritic structure disappeared. The alloy consisted of a signal FCC phase. It was found that apparent dissolution and spheroidization in CoCrMoC alloy was observed at 1300℃. Isothermal aging at 800℃ in the two alloys promoted the FCC-->HCP martensitic phase transformation. The amount of HCP phase increased with increasing aging time and the FCC matrix totally transformed HCP phase after 24 hours. For the CoCrMoC alloy, the ageing also promoted the precipitation of dispersed small carbides on the (111) habit plane of the FCC matrix.The results of wear resistance tests showed that the wear resistance of CoCrMo alloy was hardly affected by the heat treatments. In contrast, the wear resistance property of CoCrMoC alloy was strongly influenced by the heat treatments. The wear resistance of the CoCrMoC alloy was effectively improved by the solution treatment above 1100℃, with the 1200℃ being the optimum temperature. Subsequent annealing treatment deteriorated the wear resistance of the CoCrMoC alloy.The relationship between wear resistance and microstructure was also discussedin this paper. It was believed that the wear resistance of the two alloys was hardly affected by the HCP phase. The solution of the carbon into the FCC matrix, which strengthened the FCC matrix, is the major reason for the improvement of wear resistance of the solution treated CoCrMoC alloy. The decrease of the wear resistance after aging maybe caused by the dispersed small carbides which formed in aging process, because those carbides promoted the abrasive wear.