Diamond powder incorporated oxide layers formed on 6061 Al alloy by plasma electrolytic oxidation

Highlights: • Diamond powder incorporated coating on Al alloy by plasma-electrolytic-oxidation (PEO). • The amount of diamond powder added affects hardness, corrosion and wear resistance. • Best performance of PEO coating is obtained at diamond powder added amount 6 g/L. • Performance of the PEO coating deteriorates at diamond powder added amount 9 g/L. In this study, diamond powder was incorporated into oxide layers formed on 6061 Al alloy by using plasma electrolytic oxidation (PEO) method. Diamond powder, 1.5 μm in average size, was added in an amount of 0, 3, 6 and 9 g/L, respectively, into the electrolyte containing 0.15 M Na₂SiO₃ and 0.16 M H₃BO₃. Phases, microstructure and composition of the oxide layers were investigated by XRD, SEM and EDX analyses. Micro-hardness of the coatings was measured using a micro-indenter. Corrosion performance of PEO coatings was evaluated using potentiodynamic polarization in a solution of 3.5% NaCl. Wear resistance was evaluated using a ball-on-disk tribometer. Both XRD and EDX spectra prove the incorporation of diamond into the coating. With increasing content of diamond powder in the electrolyte, analytical results indicate that the thickness, roughness, corrosion and wear resistance of the coatings increase. PEO coatings obtained with an optimum diamond powder content 6 g/L shows the best performance. The best corrosion resistance is 9.70 × 10⁸ Ω cm², the highest micro-hardness 855 HV, the lowest average friction coefficient 0.29, and showing the least weight and volume losses after wear tests. Performance of the coatings deteriorates when obtained at a higher content of added diamond powder, 9 g/L. This is due to the sudden increase of both porosity and roughness of the coatings. The variation in coating performance is explained based on the evolution of phase, microstructure, micro-hardness, porosity and roughness with the amount of diamond incorporation.