Recent progress in surface modification of metals coated by plasma electrolytic oxidation: Principle, structure, and performance

In recent years, considerable research has long been devoted to the development of metallic materials with excellent surface properties through various surface modification techniques. A plasma electrolytic oxidation (PEO), one of the electrochemical coatings, has considered the eco-friendly wet coating in alkaline-based electrolytes where the surface characteristics of metal would be altered significantly by electrochemical reactions assisted by plasma discharges, resulting in the formation of hard, conformal, adhesive inorganic layer on the metal substrate. This review described a couple of the scientific principles including transient discharge behavior at breakdown, nucleation and growth of inorganic layer, and electrophoresis for incorporating inorganic particle. It outlined the essential microstructural features, which were related to defect structure, plasma-induced microstructural transformation, phase transition, and roles of inorganic agents. The protective nature of the present coating was highlighted by considering structural reliabilities, such as tribological and corrosion performances. In addition, the emerging applications arising from functional properties of the present coating, such as biomedical, catalysis, light, and energy performances, were reviewed. The benign approaches used to improve the structural and functional properties of coating layers are described utilizing pre- and post-treatments of PEO.