Effects of Al₂O₃ nano-additive on performance of micro-arc oxidation coatings formed on AZ91D Mg alloy

Ceramic coatings were prepared on AZ91D Mg alloy by micro-arc oxidation (MAO) in aluminate electrolytes, with Al₂O₃ nano-additive suspending at different concentrations. Effects of nano-additive concentration on the structure, phase composition, hardness and anti-corrosion property of the MAO coatings were analyzed by scanning electron microscopy, X-ray diffraction, micro-hardness test and electrochemical method, respectively. The results revealed that Al₂O₃ nano-particles were mostly incorporated into ceramic coating chemically, transferred into MgAl₂O₄, rather than being trapped mechanically during MAO process. With the increase of Al₂O₃ concentration, the voltage-time response, content of MgAl₂O₄, hardness and anti-corrosion property increased. However, when the concentration varied from 10g/L to 15g/L, these behaviors and properties changed only a little. This result indicated that, after the concentration of Al₂O₃ nano-additive reaching 10g/L, the incorporation of Al₂O₃ nano-particles turned into a saturation state, due to the complex process during MAO treatment. Therefore, 10g/L might be a proper concentration for MAO coating to incorporate Al₂O₃ nano-particles.