Control of surface morphology of carbide coating on Co-Cr-Mo implant alloy.

Wear of materials used in artificial joints is a common failure mode of artificial joints. A low wear rate for implants is believed to be critical for extending implant service time. We developed a carbide-coated Co-Cr-Mo implant alloy created in plasma of methane and hydrogen mixed gas by a microwave plasma-assisted surface reaction. The carbide-coated Co-Cr-Mo has a unique "brain coral-like" surface morphology and is much harder than uncoated Co-Cr-Mo. The effect of plasma processing time and temperature on the surface morphology of the top carbide layer was studied toward optimizing the surface coating. The ratios of average roughness, Ra, core roughness, Rk, and summation of core roughness, reduced peak height (Rpk) and reduced valley depth (Rvk), Rk+Rpk+Rvk, for the 6-h/985 degrees C coating to those for the 0.5-h/985 degrees C coating were 1.9, 1.7, and 1.9, respectively. The ratios of Ra, Rk, and Rk+Rpk+Rvk for the 4-h/1000 degrees C coating to those for the 4-h/939 degrees C coating were 2.3, 2.3, and 2.0, respectively. With the proper combination of plasma processing time and temperature, it may be possible to change the thickness of the peak-valley top cluster by fourfold from approximately 0.6 microm to approximately 2.5 microm. Finally, the growth mechanism of the carbide layers on Co-Cr-Mo was discussed in the context of atomic composition analysis.

Wear behavior of carbide coated Co-Cr-Mo implant alloy.

The wear behavior of a new type of metal carbide surface coating on Co-Cr-Mo implant alloy was studied. The coating was created using a microwave plasma-assisted reaction. Codeposition of impurity diamond film, diamond particles, and soot was prevented by controlling process conditions. Wear tests were carried out using a sapphire ball-on-Co-Cr-Mo disc unidirectional sliding configuration with harsh conditions of high contact stress and slow sliding speed in both no-lubrication, and deionized water lubrication environments. In the case of uncoated Co-Cr-Mo discs, the effect of deionized water lubrication was remarkable and reduced the wear factor by one order of magnitude compared to the no-lubrication tests. The wear factor of carbide coated Co-Cr-Mo discs was slightly smaller than that of uncoated Co-Cr-Mo discs with deionized water lubrication (2.7 x 10(-6) mm3 N(-1) m(-1) vs. 4.2 x 10(-6) mm3 N(-1) m(-1)). The addition of deionized water lubrication did not greatly affect the wear factor of carbide coated Co-Cr-Mo discs. The influence of surface geometry resulting from the "brain coral-like" surface morphology of carbide layers on wear behavior was analyzed considering stress concentrations and effective contact area.