Optimization of allotropic and hardness of aluminum oxide coating by PEO.

One of the most important methods of producing materials and oxide coating is plasma electrolytic oxidation (PEO). Coatings made on aluminum in PEO method have two allotropes of $\alpha$-Al$_2$O$_3$ and $\gamma$-Al$_2$O$_3$. Coatings containing $\alpha$-Al$_2$O$_3$ have more hardness and wear resistance; therefore, the main goal of the present study is to optimize aluminum oxide allotrope for increasing the ratio of $\alpha$-Al$_2$O$_3$ in coating made with PEO method. Results of the present study shows that the optimum electrolyte compound for making highest phase of $\alpha$-Al$_2$O$_3$ has 2.9 g/lit of KOH, 1.15 g/lit of Na4P2O7, and 0.34 g/lit of NaAlO2 and ratio of highest allotropic pick of $\alpha$-Al$_2$O$_3$ to highest allotropic pick of $\gamma$-Al$_2$O$_3$ (in XRD test) in the optimum condition is 0.622 and hardness is 1648 Vickers.