Plasma electrolytic oxidation of hafnium

This paper presents the results of the investigation of plasma electrolytic oxidation (PEO) of hafnium. Atoms ionized during the PEO micro-discharging were identified using optical emission spectroscopy. The spectral line shape analysis of the hydrogen Balmer line Hβ indicated the presence of two types of micro-discharges characterized by electron number densities of around 2.5·1021 m− 3 and 1.3·1022 m− 3. Scanning electron microscopy and X-ray diffraction were employed to investigate surface morphology and phase composition of the PEO coatings obtained. The coatings were crystalline and composed of monoclinic HfO2. Diffuse reflectance spectroscopy has shown that HfO2 coatings have a broad absorption band in the range from 200 nm to 400 nm. Optical band gap of HfO2 coatings was around 5.4 eV, as estimated from absorption spectra. Photoluminescence measurements show that HfO2 coatings have broad emission band in the visible region, with a maximum at around 480 nm. The highest photoluminescence was obtained for the excitation wavelength of 270 nm. Intensity of photoluminescence increased with PEO time and is related to an increase of oxygen vacancy defects in HfO2 coatings formed during the process.