Preparation and formation mechanism of the zirconia coating on Al-Si alloys by plasma electrolytic oxidation

Zirconia coating was prepared on Al-Si alloys by plasma electrolytic oxidation (PEO) in the alkaline electrolyte containing Zr(OH)₄ powders. The composition and structure of the coating were investigated by SEM (scanning electron microscope) and XRD(X-ray diffraction). The results show that in the initial stages of oxidation, the growth of coating belongs to the stage of anodic oxidation controlled by electrochemical polarization. The growth of coating is characterized by outward growth with a faster growth rate. With increasing in time and thickness of the coating, the growth of coating is characterized by in-growth with the decrease of growth rate. The coating consists of t-ZrO₂, m-ZrO₂, α-Al₂O₃ and γ-Al₂O₃, in which t-ZrO₂ is dominant in phase with distributed in outer layer of the coating, while α-Al₂O₃ appears in inner layer of the coating. The coating surface exhibits particle structure with size of 1~2 μm. In the process of plasma electrolytic oxidation, Zr(OH)₄ powders move and deposit on the mouth of plasma discharge channel under electric field force to transform into ZrO₂ as a result of the high temperature of plasma discharge.