Surface morphology, corrosion resistance and in vitro bioactivity of P containing ZrO2 films formed on Zr by plasma electrolytic oxidation

The present work was aimed at developing the corrosion resistant and bioactive oxide film on zirconium by plasma electrolytic oxidation in phosphate electrolyte. The effect of plasma electrolytic oxidation treatment time on surface morphology and corrosion resistance of the oxide films was further investigated. The phase composition, surface morphology, thickness and elemental composition of the oxide films were analyzed by X-ray diffraction and scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy. The corrosion behavior of substrate and oxide films in simulated body fluid environment was studied by open circuit potential and potentiodynamic polarization tests. The apatite forming ability of the oxide film was evaluated after immersing in simulated body fluid for 14 days. X-ray diffraction patterns show that the oxide films predominantly comprised of monoclinic zirconia with a small amount of tetragonal zirconia. With prolonging treatment time, phase transformation of tetragonal to monoclinic zirconia was observed. Scanning electron microscopy results show that for a treatment time of 2–8 min, uniform and highly dense oxide films, thickness varying from 3 to 14 μm with no obvious pores were formed and the phosphorous content in the films was found to be in the range of 2.8–6.8 at.%. Corrosion test results reveal that all oxide films improved their corrosion resistance especially in terms of pitting potential and showed superior passivity in simulated body fluid environment. Bioactivity test results confirm that plasma electrolytic oxidation treated zirconium was fully covered by apatite layer in simulated body fluid medium. The incorporation of phosphorous in oxide film during coating process significantly enhanced the apatite forming ability of zirconium. In conclusion, among all the plasma electrolytic oxidation coated samples, the 6 min coated zirconium with high corrosion resistance and bioactivity is a potential candidate as orthopedic implants.

► P incorporated ZrO2 films were formed on Zr by PEO at different treatment time.
► Na3PO4·12H2O electrolyte leads to the formation of dense oxide film on Zr.
► Treatment time of 6 min was found to be optimum to form corrosion resistance film.
► Phase transformation from t-ZrO2 to m-ZrO2 depends on PEO treatment time.
► P in oxide film enhances bioactivity of Zr in 7.4 pH simulated body fluid medium.