Plasma electrolytic oxidation of titanium in heteropolytungstate acids

This article is a review of our recent research of plasma electrolytic oxidation (PEO) process of titanium in heteropolytungstate acids (12-tungstosilicic acid and 12-tungstophosphoric acid). It has been detected that spatial density of microdischarges is the highest in the early stage of the PEO process, while the percentage of oxide coating area covered by active discharge sites decreases with PEO time. The elements and their ionization stages present in PEO microdischarges are identified using optical emission spectroscopy technique. The spectral line shape analysis of hydrogen Balmer line Hβ (486.13 nm) indicates presence of two types of microdischarges during PEO. The discharges are characterized by relatively low electron number densities of Ne ≈ 0.8 × 1015 cm− 3 and Ne ≈ 2.1 × 1016 cm− 3. Oxide coatings formed by PEO process were characterized by AFM, SEM–EDX and XRD. The elemental components of PEO coatings are Ti, W and O. The oxide coatings are partly crystallized and mainly composed of WO3 and anatase.