Advancement of plasma electrolytic oxidation towards non-valve metals

Discharge phenomena in electrolytes were discovered many decades ago. However, the research and development of plasma oxidation in electrolytes for surface engineering have only truly been actively performed in recent years, particularly stimulated by a review paper entitled Plasma Electrolysis for Surface Engineering published in 1999. Among plasma electrolysis processes, plasma electrolytic oxidation (PEO) caught the most attention from academic and industrial communities. However, PEO has been largely restricted to valve metals, such as Al, Mg and Ti and their alloys. This research intends to explore the current boundaries of the PEO process and extend it into non-valve metals. The paper therefore first describes the coating growth mechanisms in non-valve metals, which are different from the conventional PEO (conversion) process on valve metals. Particular emphasis is given to revealing details of the coating deposition behavior through direct observations and analysis of non-valve metallic substrates and their coatings formed at different timescales of the PEO process. Copper and ferrous materials are chosen as appropriate non-valve metal substrates and their coating properties in terms of mechanical, corrosion, thermal and electrical aspects are summarized. The results show that high hardness and adhesion strength, high corrosion resistance, good thermal and electrical insulation performances are achievable for PEO-derived coatings on non-valve metals. These properties also open doors for emerging applications, for example in electrical vehicles. Such applications are presented by reporting the coating application on an e-motor bearing and brake disc as two examples. Future research perspectives are discussed with a hope to stir a new wave of PEO research for non-valve metals as realized by the first review paper for the previous wave of plasma electrolysis research.