Structure and tribological properties of micro-arc oxidation coatings for reduction of Ni2+ ion release on biomedical NiTi alloy

The ceramic coating was prepared on the NiTi alloy by Micro-arc oxidation (MAO) in a sodium aluminate (NaAlO2) solution at constant current density. The surface of the coating displays a rough, porous structure. The elements of the coatings mainly contain Al, Ti, O, and Ni. The concentration of Ni in the surface is 1.63 at%, which is greatly lower than that of Ni in the NiTi substrate. XPS analysis revealed that Al exist as Al–O bond of Al2O3, Ti existed as Ti4+(TiO2), Ti3+(Ti2O3), Ti2+(TiO), and Ti0(metallic Ti), and Ni exist as Ni2O3, NiO and metallic Ni in the coating surface. Thin-film X-ray diffraction shows that the only Al2O3 phase can be confirmed in the coating. Potentiodynamic polarization curves indicate that the corrosion resistance of the Al2O3 coating is better than that of uncoated NiTi. The amount of Ni leached from coated NiTi sample is 20.97% of that from the uncoated NiTi sample after immersion 40 d. The Tribological behavior of uncoated and coated NiTi alloy was evaluated using a computer-controlled ball-on-disk WTM-1E tribometer.