Analyses of microarc oxidation coatings formed on Si-containing cast aluminum alloy in silicate solution

Abstract

A dense ceramic coating up to 130 μm thick was deposited on high silicon cast aluminum alloy by microarc discharge in silicate electrolyte. Its microstructure and composition were analyzed by scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and phase identification was performed by X-ray diffraction (XRD). In addition, the distributions of hardness, H, and elastic modulus, E, across the ceramic coating were determined by nanoindentation method. The coating has a three-layer structure. The profiles of H and E in the coating are similar. From the surface to the inner layer of the coating, H and E gradually increase. The inner layer is dense and hard, in which the H and E can reach about 15 GPa and 250 GPa, respectively. This coating consists of mullite, γ-Al2O3, γ-Al2O3 and amorphous SiO2 phases. The surface layer enriched in Si element has a high fraction of amorphous SiO2, where the Si element comes mainly from the electrolyte rather than the alloy substrate. However, the Si element from Al-Si substrate enhances the formation of mullite phase in the coating.