The characterization of the oxide based coating synthesized on pure zirconium by plasma electrolytic oxidation

• Monoclinic ZrO2 and tetragonal ZrO2 phases were the main constituents of the coating.
• Monoclinic ZrO2/tetragonal ZrO2 ratio doesn't depend on PEO coating thickness.
• Plasma channels were revealed by cross-sectional fracture surfaces.
• The equiaxed clusters unique to Zr were formed on the surface of the coating.

The pure zirconium metal was coated by plasma electrolytic oxidation in silicate containing basic electrolyte for the cumulative periods of 5, 10, 20, 30, 45, 60, 90, and 120 min successively for the total coating duration of 120 min. The phase content, surface morphology and chemical composition of the PEO coating were characterized by XRD, SEM and profilometry after each coating time successively. The equiaxed clusters unique to Zr were formed on the surface of the coating. The number of plasma channels decreased but the sizes and intensities of the sparks increased by increasing the process duration and consequently the amount of materials ejaculated through these channels increased. The thickness and the surface roughness of the coating were increased by the process duration. The presence of cracks and pores resulted in a lower value of microhardness of the coating compared to the theoretical value of ZrO2. The plasma channels containing Si rich and radially grown Zr rich crystals were clearly observed from the cross sectional examination of intentionally created fracture surface. The characterizations carried out upon successive coating and successive removal of this coating confirmed that monoclinic ZrO2/tetragonal ZrO2 ratio was not changing with coating thickness.