Micro-second range pulsed DC plasma electrolytic oxidation on Ti and Nb

The presented research reports on the formation of oxide coatings on pure Ti and Ni substrates by micro-second pulsed plasma electrolytic oxidation in sodium phosphate electrolyte. The formation process is monitored by real-time imaging and optical emission spectroscopy. At the beginning of the process, micro-discharges appear randomly over the sample's surface and migrate towards the edges with prolonged processing. Emission lines originating from Ti substrate are well resolved, while appearance of high-resolution emission lines originating from Nb substrate in the visible range is reported for the first time. Subsequent characterization of formed oxide coatings on Nb shows that its roughness increases with PEO processing time, contrasting the case of Ti. It is observed that longer processing time of Ti substrate results in the anatase-to-rutile phase transformation, while pseudohexagonal TT-Nb2O5 phase is present throughout the processing of Nb substrate. Photoluminescent and photocatalytic properties of obtained coatings are dependent on their morphology and phase composition i.e., on processing time.