Micro- and nano-formations on the surface of plasma electrolytic oxide coatings on aluminum and titanium

In the majority of cases electric spark and arc discharges developed in condensed media result in the formation of micro- or nanosized particles. Micro- or nano-formations of different types are always present on the surface of oxide coatings formed under effect of electric spark and arc discharges (the approach is referred to as the method of plasma electrolytic oxidation (PEO) or microarc oxidation (MAO)). These formations (particles) can be extremely active in physical-chemical terms and determine the properties of a system at large. Nevertheless, the structure, composition, and mechanism of formation of the above micro- and nanostructures and their control on the surface of PEO-coatings of different chemical composition as well as their effect on the functional properties of the systems obtained using the PEO-method have not yet been studied in sufficient detail. The present paper is devoted to the examination of micro- and nano-formations on the surface of PEO-coatings: their shape, composition, and effect on the coatings properties. The objective of the paper was to generalize our earlier obtained data on micro- and nano-formations on the surface of PEO-coating on aluminum and titanium and emphasize the importance of the poorly developed field of knowledge related to PEO-coatings, their structure and formation mechanisms, and the possibilities of controlling their composition and functional properties.