Correlation between plasma electrolytic oxidation treatment stages and coating microstructure on aluminum under unipolar pulsed DC mode

Alumina coatings were deposited on 6061 aluminum alloy substrates using plasma electrolytic oxidation (PEO) in an alkaline silicate electrolyte. Different electrical parameters, including frequency, duty cycle, and current density were applied to obtain PEO coatings. Scanning electron microscopy (SEM) and microhardness tests were used to investigate the coating microstructure and properties. For each sample, the voltage-time response of the PEO process was obtained and compared with surface morphology, and coating cross sections and thicknesses. Special consideration was given to the connection between the electrical parameters, different stages of the PEO process, and the morphology and microstructure of the coatings. Applying different electrical parameters changed the duration and ratio of the PEO stages in the voltage-time response curve and affected the growth rate, surface morphology, microstructure and microhardness of the coatings. We report the new finding of a correlation between the stage of the PEO process and the microstructure and morphology of the coating.