Effect of pre-coating annealing on plasma electrolytic oxidation of aluminum alloy substrate

Abstract

Plasma electrolytic oxidation (PEO) was performed on aluminum alloy (AA6061) coupons to investigate coating initiation and the effect of substrate annealing on the PEO coating. The annealing was carried out in a rapid thermal annealing system using flashlight heating from the top, with temperatures ranging from T_ann = 500 to 650 °C. To avoid the batch-to-batch PEO variations, a single batch of multiple samples was employed, which exhibited a uniform coating thickness across six samples, with an average thickness of 36.2 ± 0.3 μm obtained after 40 min of PEO process. Morphological, compositional, and crystallographic characterizations revealed localized ingrowth coating during the initial PEO stage. As the process continued, SiO₂ from the Na₂SiO₃-based electrolyte was incorporated in the form of mullite into the predominantly γ-phase alumina coating. Annealing-induced surface roughening played a minor role in the coating roughness and thickness, but it resulted in increased sample-to-sample thickness variations. Annealing at 650 °C induced crystallographic texturing of the substrate, which had a minor effect on the structural and crystallographic properties of the PEO coatings. Hardness, corrosion resistance testing, and dielectric breakdown strength measurements of the PEO coatings demonstrate their potentials against corrosion under tribological and/or plasma conditions.