Effect of KOH to Na₂SiO₃ Ratio on Microstructure and Hardness of Plasma Electrolytic Oxidation Coatings on AA 6061 Alloy

In this study, plasma electrolytic oxidation (PEO) process has been employed to fabricate alumina coatings on AA 6061 aluminum alloy from an electrolyte containing water glass (Na₂SiO₃) and alkali (KOH). The effect of deposition time and the alkali to water glass (KOH: Na₂SiO₃) composition ratio on the coating morphology and properties are studied. The different phases of the oxide layer and microstructure are investigated by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. The results indicate that initially γ-Al₂O₃ forms in the coating, and as the processing time is increased from 5 to 60 minutes, α-Al₂O₃ phase becomes prominent. Further, higher the content of Na₂SiO₃, higher is the hardness and coating growth rate due to the formation of stable α-Al₂O₃ and Al-Si-O phase. It has been reported that the optimum properties of the PEO coatings can be obtained at a ratio of KOH: Na₂SiO₃ ≈ 15:10 followed by 10:10.