Surface characterization and anodic polarization of nitrogen-ion-implanted nickel-free Co-Cr-Mo alloy

Nitrogen ions were implanted in a nickel-free Co-Cr-Mo alloy in amounts of 10¹⁹, 10²⁰, and 10²¹ ions m^-2 with an acceleration energy of 150 keV and ion-beam current-density of 10 ^-15 mA m^-2 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 ^-, that is, the oxide was hydrated or oxyhydroxidized. After N ²⁺ implantation, nitrogen atoms existed as cobalt nitride in the substrate just under the surface oxide and NH₃ or NH₄ 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 ²⁺-implanted Co-Cr-Mo alloy with an amount of 10²⁰ ions m^-2 showed the highest corrosion resistance.