Micro-arc oxidation (MAO) is a plasma-assisted electrochemistry method to prepare protective ceramic coatings on aluminium alloys. Alloy elements of the Al-alloy substrate, such as Si, Cu, Mg and Li, have effects on the microstructure and composition of the MAO coatings. Usually, silicon distributes in the cast Al-Si alloy substrate as small laths and they cover approximately 10% of the substrate surface. Therefore, their effects on the growth process and microstructure of the MAO coatings are worthy of notice. In the present study, oxide coatings with a thickness of 15-18 µm were prepared on the ZL109 Al-Si alloy by MAO. The phase content, surface morphology and element distribution of the coatings were investigated by X-ray diffraction, grazing incidence X-ray diffraction, scanning electron microscope, and electron probe micro-analysis respectively. The average hardness of the coatings was 622.3 ± 10.2 HV0.05. The adhesive strength of the coatings is 40.55 ± 2.55 N, and the adhesion of the coatings could be rated as 5B by tape test according to ASTM D3359-17 standard test methods, which indicated a high adhesive strength between the MAO coating and substrate. The effects of silicon laths on surface morphology and composition of the coatings were discussed, and a model was put forward to describe the growth process of the MAO coatings on cast Al-Si alloys. The authors believe that the high silicon content of the substrate has no adverse influence on the structure and properties of the MAO coating on the ZL109 alloy.
Keywords: Al–Si alloys; coating; growth process; micro-arc oxidation.