Effect of current step-down on the growth and hardness of PEO coatings on Al6061 alloy

In this study, plasma electrolytic oxidation (PEO) treatments were conducted on Al6061 plates using silicate containing alkaline electrolyte. The hardness variation with the growth of the PEO coating was investigated under two different regimes of applied AC current. The first was a continuous supply with a constant current density of 500 mA/cm(2) for 60 or 90min. The other was a current supply of 500mA/cm(2) for 60 min followed by a current step-down to 200 mA/cm2 and continuous treatment for 30 min. In the case of a constant current for 60 min, the coating thickness and Vickers hardness increased to 150 m and 1300 Hv, respectively. With further processing in the same manner, the coating grew thicker while its hardness gradually decreased to 900 Hv. In the case of a current step-down, no significant thickness increase was observed, but there was a hardness increase up to 1800 Hv. Analysis of the PEO coatings by SEM/EDS and XRD revealed that all their function layers were comprised mainly of alpha-Al2O3 with hardness dependent on density. The current step-down method contributes to the densification of the function layer in a PEO coating, and thus increases the hardness. (C) 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11