Effects of NaAlO₂ concentrations on structure and characterization of micro-arc oxidation coatingsformed on biomedical NiTi alloy

The MAO coating was prepared on biomedical alloy by micro-arc oxidation (MAO) in sodium aluminate (NaAlO₂) solution using pulsed bipolar power supply. It was found that the coating consists of O, Al, Ti and Ni, and Ni concentration in the coating surface is much lower than that of Ni in the NiTi substrate. The concentration of sodium aluminate (NaAlO₂) solution has an influence on the surface morphologies, surface roughness, corrosion resistance and bonding strength. XRD analysis reviewed that MAO coating formed in solution of 0.05 M sodium aluminate (NaAlO₂) is amorphous, the coating was composed of crystallize γ-Al₂O₃ when the sodium aluminate (NaAlO₂) concentration is from 0.075 M to 0.15 M. The intensity of γ-Al₂O₃peaks increased with increasing the concentrations of NaAlO₂. The corrosion resistance and bonding strength of the MAO coatings increased with the increasing NaAlO ₂ concentration. Especially, the bonding strength of MAO coating to NiTi substrate was up to 28.0 MPa when NaAlO₂ concentration is up to 0.15 M.