Formation Mechanism of Ceramic Films Fabricated by Micro-arc Oxidation on Aluminum Alloy

The growth dynamics of micro-arc oxidation (MAO) films on 2A12 aluminum alloy was investigated. The microstructure, morphology, composition and phase constituents under different oxidation time conclitions were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that the total film thickness increased linearly and the growth speed towards outer space was faster than that towards substrate. While the ration of dense layer in the film increased firstly then and decreased. The results of SEM show that many crater-mouth like traces formed by the plasma discharge were observed on the coating surface, and the thickness on the cross section became the same and the film/substrate interface became more even with the increase of MAO time. The results of XRD reveal that with increasing the oxidation time the content of γ-Al2O3 in the ceramic coatings inereased first and then decreased, while the content of α-Al2O3 increased. The process can be divided into six steps, including block layer formation, dielectric breakdown of the block layer, broadening of discharge channel, Al2O3 formation through the reaction in the channnel, melted oxide spraying out-cooling-solidification-phase transference, and formation of compact and loose layers.