Wear resistance of micro-arc oxidation coatings on biomedical NiTi alloy

Protective coatings were successfully formed on biomedical NiTi alloy by micro-arc oxidation (MAO) using pulsed bipolar power supply. The coating surface exhibits a typical MAO porous structure, and the coating mainly consists of O, Al, Ti and Ni, with the atomic concentration of 65.11%, 27.77%, 2.20% and 2.8%, respectively. The thickness of MAO coating is about 24.0 μm when the duration time of the MAO treatment was 90 min at 400 V constant voltage treatment. XRD analysis showed that micro-arc oxidation coating is composed of γ-Al2O3 and α-Al2O3. The wear resistance of the coatings was investigated by ball-on-disk friction test. The microhardness of the NiTi alloy is greatly enhanced due to the formation of Al2O3 coating after micro-arc oxidation treatment. The friction coefficient of the coated NiTi is stable at 0.85 and the wear resistance is increased by 9 times compared with uncoated NiTi. The wear mechanism transforms from abrasive-dominant for the uncoated sample to adhesive-dominant for the coated sample.