Initial microstructure and composition evolution of Al2O3 coatings fabricated by cathode plasma electrolytic deposition under constant voltage

At present, the research on cathode plasma electrolytic deposition (CPED) mainly focuses on improving the coating properties, while the initial structure and formation mechanism of CPED coating are less studied. This work aims to study the initial deposition behavior of Al2O3 coating by CPED at different time scales systematically. The results show that the coating consists of lamellar Al(OH)3 in the pre-plasma discharge period. After reaching the breakdown potential, the preparation process of Al2O3 can be considered as a cyclic process: plasma discharge, injection of molten oxide, sintering and solidification, and stacking. Such an iterative process results in a gradual thickening of the coating. Later in the deposition process, the raised structure of the coating surface flaps off and coating thickness decreases. In addition, the high temperature during plasma discharge results in local melting of the substrate surface, which is co-deposited with Al2O3.