Incorporation of zirconia into coatings formed by DC plasma electrolytic oxidation of aluminium in nanoparticle suspensions

A study has been carried out to investigate the incorporation of zirconia into alumina-based coatings formed on aluminium using DC plasma electrolytic oxidation in phosphate and silicate electrolytes containing zirconia nanoparticles in suspension. The nanoparticles were incorporated mainly near the coating surface and within cavities inside the coatings. Additionally, a silicon-rich, surface deposit appeared to entrain the nanoparticles of the silicate electrolyte. Zirconia was also present in cellular microstructures of the main alumina-rich coating layers, suggesting melting at >1710 °C and subsequent solidification. The coatings frequently contained tetragonal zirconia in addition to monoclinic zirconia, with the presence of zirconia also affecting the proportions of γ- and α-alumina in favour of the former phase.