Deposition and properties of zirconia coatings on a zirconium alloy produced by pulsed DC plasma electrolytic oxidation

In this study, zirconium oxide coatings were produced on Zr–2.5 wt.% Nb alloy by a plasma electrolytic oxidation (PEO) process using a pulsed DC power source with various frequencies 1000, 2000 and 4000 Hz. When a relatively low termination voltage (i.e. 400 V) was set, the thickness of the coatings was in a range of 7–15 μm and the treatment time to reach the preset voltage was less than 5 min. The coatingsʹ phase structure, tribological and corrosion properties were characterized by X-ray diffraction (XRD), pin-on-plate sliding wear test and potentiodynamic polarization corrosion test, respectively. The results indicate that the thickest coating of 15 μm can be obtained by adopting the highest frequency pulsed DC, i.e., 4000 Hz and the coating has almost the same surface roughness as other two coatings. In spite of the difference in thickness, all the coatings showed a similar phase structure with the main phase of monoclinic ZrO2 and less tetragonal ZrO2 phase. Furthermore, all the PEO coated samples can improve wear and corrosion protection of the Zr alloy, among which the coating produced by the highest frequency (4000 Hz) achieved the highest corrosion polarization resistance, wear resistance and lowest friction coefficient under the testing conditions in this study.