The Mechanism of PEO Process on Al–Si Alloys in Zirconate Solution

Zirconia coating was produced on Al-Si alloys by plasma electrolytic oxidation (PEO). The alkaline electrolyte containing Zr(OH)4 powders was used. The composition and structure of the coating were investigated by SEM and XRD. 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 mainly outward growth and the growth rate is faster. With elongated treated time and increased thickness of the coating, the growth of coating is mainly ingrowth. In contrast with the stage of anodization, the growth rate of plasma electrolytic oxidation is slower than anodization. The coating consists of t-ZrO2, m-ZrO2, α-Al2O3 and γ-Al2O3.T-ZrO2 is the main phase and distributes in outer layer of the coating, however, α-Al2O3 appears in inner layer of the coating. Many micro-particles appear on the coating surface with dimension of 1-2μm.. In the process of plasma electrolytic oxidation, Zr(OH)4 powders move and deposit on the mouth of plasma discharge channel under the effect of electric field force, then it is transformed to ZrO2 by the high temperature of plasma discharge.