Nitrogen ions were implanted in a nickel-free Co-Cr-Mo alloy in amounts of 10 19 , 10 20 , and 10 21 ions m À2 with an acceleration energy of 150 keV and ion-beam current-density of 10 À15 mA m À2 to improve the friction-wear properties. 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. The surface oxide layer on the mechanically polished Co-Cr-Mo alloy consisted of oxidic species of cobalt, chromium, and molybdenum, and its thickness was about 2.5 nm. The surface film contained a large amount of OH À , that is, the oxide was hydrated or oxyhydroxidized. After N 2þ 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. Calcium phosphate was formed, and cobalt was preferentially dissolved during immersion in Hanks' solution. N 2þ -implanted Co-Cr-Mo alloy with an amount of 10 20 ions m À2showed the highest corrosion resistance.
Nitrogen ions were implanted in a Co-29 mass%Cr-6 mass%Mo alloy (ASTM F799-95) with reducing the amount of nickel in order to improve the friction-wear properties. The friction-wear properties of unimplanted and nitrogen-ion-implanted Co-Cr-Mo alloys were evaluated using a pin-on-flat-type reciprocating friction tester in air and phosphate-buffered saline, PBS(À), as a quasi-biological environment under applied stress of 3.54 MPa to understand the performance of this alloy for metal-on-metal-type artificial hip joints. As a result, the wear loss in PBS(À) was much smaller than that in air and increased with the increase of applied stress in both air and PBS(À), while the friction coefficient in a steady state during the test was larger in PBS(À) than in air, according to the lubrication behaviors of wear debris and PBS(À). Nitrogen-ion implantation is effective to decrease the friction coefficient in PBS(À) at the sliding interface between mutual Co-Cr-Mo alloys as well as to decrease the wear debris in PBS(À). Both in air and PBS(À), the surface of a N 2þ -implanted flat specimen was smoother than that of an unimplanted flat specimen. Cobalt phosphate was precipitated as a corrosion product on the pin during the friction-wear test.
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