Effect of duty cycle and treatment time on electrolytic plasma oxidation of commercially pure Al samples

Plasma electrolytic oxidation (PEO) is an environmentally friendly anodizing technique that produces ceramic coatings using high voltage discharges. To the present work this technique was applied to treat aluminum surfaces using a pulsed current supply and sodium silicate electrolyte. The effect of the duty cycle and treatment time on the morphology, phase composition and tribo-mechanical properties of the formed layer was investigated. The current density was maintained at 12 A/dm2, with two different duty cycles, 30% and 50%, for total period of 100 μs. For each duty cycle two different treatment times (2 min and 12 min) was used. During each treatment, the voltage was monitored to determine the start of the rupture of the passive oxide layer. It was demonstrated that the increase in the duty cycle from 30% to 50% influenced the thickness and porosity of the film, verified by scanning electron microscopy (SEM) images. Phase γ-Al2O3 was predominant on the surface of the samples treated by PEO, although occurrence of mullite and α-Al2O3 were also observed. There was an inversion in the intensities of the diffraction peaks Al (200) and Al (111) after the formation of the oxide phase, suggesting a preferential oxidation in the Al (111) direction. The PEO process allows tuning of the hardness, yield strength and wear resistance by controlling film thickness, porosity and phases present.