Effect of KOH Content on Structure and Bonding Strength of Red Mud Plasma Electrolytic Oxidation Composite Ceramic Coatings

Plasma electrolytic oxidation (PEO) of 5005 aluminum alloy was carried out in KOH-red mud electrolytes. Effects of KOH mass concentration (C_KOH=1.0, 2.0, 3.0, 4.0, 5.0 g·L^-1) on the critical arcing voltage of PEO and the thickness, phase composition, morphology and bonding strength of the ceramic coating were analyzed. The results show that the ceramic coatings prepared in electrolytes with different KOH content were mainly composed of γ-Al₂O₃ and α-Al₂O₃, but a small amout of CaCO₃, SiO₂ and Fe₂O₃ phases also appeared. When KOH content increased, the positive and negative critical arcing voltages of PEO decreased rapidly; the coating thickness showed a growth trend of "semi-parabolic" shape; the γ-Al₂O₃ content in the ceramic coating increased first, then decreased and then increased, the change trend of α-Al₂O₃ content was just the opposite, and the CaCO₃, SiO₂, Fe₂O₃ content increased first and then decreased; the surface porosity of the ceramic coating decreased first and then increased, the surface roughness kept increasing and the hardness and bonding strength increased first and then decreased.