Atomic surface of cobalt-chromium-molybdenum alloy induced by novel green chemical mechanical polishing through controlling pH values and oxidation processes

Abstract

Cobalt-chromium-molybdenum (CoCrMo) alloy has superior wear resistance, high corrosion resistance and excellent biocompatibility. However, these properties make CoCrMo alloy become a difficult-to-machine material. Nevertheless, high-performance CoCrMo devices strictly require the surface roughness Ra less than 1 nm, arising a challenge for ultraprecision machining. To overcome the challenge, a novel green chemical mechanical polishing (CMP) is developed, including silicon oxide, hydrogen peroxide, tartaric acid and deionized water. After CMP, surface roughness Ra of 0.16 nm is achieved for CoCrMo alloy. Compared with those published on CoCrMo alloys previously, the surface roughness is the lowest. X-ray photoelectron and infrared spectroscopies reveal that hydrogen peroxide dominated the oxidation processes in CMP. Co oxides were softened and dissolved by hydrogen ions. Relatively, Cr and Mo oxides showed better stability and less dissolution in tartaric acid, avoiding excessive corrosion. The remaining oxides are removed by silicon oxide. The metal ions released are chelated by tartaric acid during the polishing process. The chelation formulas of metal ions and tartaric acid are further proposed. The developed novel green CMP provide new insights to manufacture atomic surface on an alloy with high corrosion and wear resistances, which is beneficial for the potential applications on medical devices.