Properties of Aluminium Alloy Ceramic Coating Prepared by Microarc Oxidation

Under the condition of keeping impulse amplitude of alternating voltage constant, microarc oxidation of aluminium alloy in aqueous solution containing sodium silicate and sodium hydroxide was carried by means of alternating current power supply. It is found that the current passing through the sample is divided obviously into five different stages with the oxidation time. The ceramic coatings prepared in the different stages were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The result shows that the ceramic coatings mainly contain 7-Al_2O_3, α-Al_2O_3 phases; the γ-Al_2O_3 and α-Al_2O_3 contents are close relative to oxidation time. The difference of γ-Al_2O_3 and α-Al_2O_3 contents between external layer and internal layer of the coating is mainly caused by the change of cooling rate of molten alumina in microarc zone.