Properties of aluminium oxide coating on aluminium alloy produced by micro-arc oxidation

Aluminium oxide coating was directly synthesized on aluminium alloy by micro-arc oxidation (MAO) in NaAlO2 solution. XRD results show that the coating consists of α-Al2O3 and γ-Al2O3 in its whole thickness; the content of α-Al2O3 in the coating is 64±4%. The coating consists of a relatively compact inner layer and a porous outer layer, and its porosity is 7-10%. The density of the coating is 3.47g/cm3, its highest hardness 22 GPa and elastic modulus 279 GPa appear at around 12 μm from interface. The thermal expansion coefficient of the coating is 7.38×10−6 K−1. The coating can suffer the thermal impact of 873 K; it indicates that the aluminium oxide coating possesses a good thermal shock-resistant and adhesion strength to the substrate.