Improvement of wear resistance and safety of cobalt-chromium alloys for metal-on-metal type artificial joint

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Friction-wear properties of a Co-29mass%Cr-6mass%Mo alloy (ASTM F799-95) but reducing the amount of nickel were evaluated with friction-wear test using a pin-on-flat type reciprocating friction tester in air and phosphate buffered saline, PBS(-), as a quasi-biological environment under applied stress of 1.0, 3.5 and 5.0MPa to understand the performance of this alloy for metal-on-metal type artificial hip joints. In addition, metal ions dissolved in PBS(-) after the test was quantified. As a result, wear loss in PBS(-) is much smaller than that in air and increases with the increase of applied stress in both air and PBS(-), while the coefficient of friction in steady state during the test is larger in PBS(-) than in air. Wear damage in PBS(-) is smaller than that in air, however more small scale of irregularity is observed in PBS(-) than that in air.Nitrogen ions were implanted in the alloy with reducing the amount of nickel in order to improve the friction-wear properties. The friction … More -wear properties of unimplanted and nitrogen-ion-implanted Co-Cr-Mo alloys were evaluated. As a result, the wear loss in PBS(-) was much smaller than that in air and increased with the increase of applied stress in both air and PBS(-), while the friction coefficient in a steady state during the test was larger in PBS(-) than in air, according to the lubrication behaviors of wear debris and PBS(-). Nitrogen-ion implantation is effective to decrease the friction coefficient in PBS(-) at the sliding interface between mutual Co-Cr-Mo alloys as well as to decrease the wear debris in PBS(-).Changes in the composition of the surface layer of the alloy with ion implantation, autoclaving, and immersion in Hanks' solution as a simulated body fluid were characterized using X-ray photoelectron spectroscopy to evaluate the stability of the material and predict the safety and tissue compatibility of the material. After N-implantation, nitrogen atoms existed as cobalt nitride in the substrate just under the surface oxide and NH_3 or NH_4 in the surface oxide, and these amounts increased with the increase of the dose. Chromium was concentrated in the surface oxide but depleted in the substrate with ion implantation. Cobalt was preferentially oxidized by autoclaving and depleted in the substrate after autoclaving. Less