Effects of cathodic voltages on the structure and properties of ceramic coatings formed on NiTi alloy by micro-arc oxidation

Micro-arc oxidation (MAO) ceramic coatings were prepared on biomedical NiTi alloy in constant voltage mode using a pulsed bipolar power supply. It was found that cathodic voltage has an influence on the structure and properties of ceramic coating. The coatings formed at relatively high cathodic voltages (>10 V) have deteriorative surface morphologies, with some cracks on the surface. The thickness and surface roughness of the MAO coatings increase with increasing cathodic voltage. The MAO coatings are composed of γ-Al2O3 as the only crystalline phase at different cathodic voltages, and the crystallinity can be enhanced through increasing the cathodic voltages. The Ni concentration on the surface layer increases with increasing cathodic voltage. The bond strength of the coatings to the substrate is more than 40.0 MPa while the cathodic voltage applied for MAO is 0 V. The corrosion resistance of coated NiTi decreases with increasing cathodic voltage. The corrosion current density increases from 2.88 × 10−9 A cm−2 for the MAO0 to 6.30 × 10−8 A cm−2 for the MAO30 and the corrosion potential decreases from −0.225 V for the MAO0 to −0.303 V for the MAO30.