Studies of composition and properties of mullite coatings on the aluminium alloy LY12

The surface modification of aluminum and its alloys using micro-plasma oxidation (MPO) technology are investigated. MPO is a cost-effective plasma electrolytic process, which can form ceramic coatings on aluminum and its alloys thereby increasing their wear resistance and corrosion resistance. In this work, the ceramic coatings are deposited from a Na₂SiO₃ solution using an alternating current power source. Phase composition as function of depth into the coating, microstructure and elemental distribution were investigated by XRD and EPMA(electronic Probe Microanalysis). It is seen from the back scattered electron micrograph that the ceramic coatings are compact with good cohesion/adhesion between the substrate and the ceramic coatings. The XRD results show that outer layer of the ceramic coating consists of mullite and γ-Al₂O₃. In the ceramic coating to the substrate direction, the mullite content of the coating decreases, while the α-Al₂O₃ and γ-Al₂O₃ content increases. On the other hand, the colour of ceramic coating transforms from black to white. In the middle of the coating the Si content is lowest, and the microhardness is highest.