Evolution of Aluminium Layer Transformed into Ceramic Coating on Steel Substrate

The external aluminium layer was transformed into a ceramic coating by the treatment of plasma electrolytic oxidation (PEO) on aluminized steel. Characteristics of anodic voltage, thickness growth regularity, cross-sectional morphologies and compositions of the ceramic coating were investigated. The results show that the anodic voltages of aluminized steel and pure aluminum vary similarly at the initial PEO stage, but the voltage of aluminized steel decreases at later PEO stage. The aluminium layer is consumed, and the ceramic-coating thickness increases linearly. The FeAl layer begins to participate in PEO process as the aluminium layer is transformed completely into the ceramic coating. Nevertheless, the ceramic coating grows slowly and many micro-cracks are observed at the Al2O3/FeAl interface. The ceramic coating is mainly composed of Al, Si and O elements. It consists ofγ-Al2O3, mullite, andα-Al2O3 phase appearing only at the last PEO stage. The hardness distribution of the coating is regional and the maximum is about HV 1800.