Abstract: 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 γ-Al₂O₃, α-Al₂O₃ phases; the γ-Al₂O₃ and α-Al₂O₃ contents are close relative to oxidation time. The difference of γ-Al₂O₃ and α-Al₂O₃
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.

Key words: microarc oxidation, ceramic coatings, surface morphology, phase component