Composition and thermal properties of the coating containing mullite and alumina

A ceramic coating was deposited on aluminium alloy using micro-arc oxidation (MAO). The composition and structure were analyzed by X-ray diffraction and electric probe microscope analysis. The density, porosity, thermal properties and adhesive strength of the coating were investigated by differential thermal analysis, thermal dilatometry and pull test. The results show that the coating consists of a mixture of α-Al2O3, γ-Al2O3, mullite and a little amorphous silicon oxide, and mullite and amorphous silicon oxide are mainly rich in the outer layer of the coating. The density of the coating is 2.88 g cm−3 and its porosity is 13–16%. The results of TG-DSC of the coating show that there is a weak exothermic peak at around 815 °C, and it corresponds to the crystallization of the amorphous phase SiO2 in the coating. The thermal expansion coefficient of the coating is 6.6 × 10−6 K−1. The coating can suffer the thermal impact of 600 °C, it indicates that the coating possesses of a good thermal shock resistant and adhesion strength to the substrate. The pull test shows adhesive strength of the coating to substrate is about 20 MPa.