Incorporation of zirconia nanoparticles into coatings formed on aluminium by AC plasma electrolytic oxidation

Composite ceramic coatings were formed on aluminium by AC plasma electrolytic oxidation (PEO) using Na(6)P(6)O(18) or Na(2)SiO(3) center dot 5H(2)O/KOH electrolytes with monoclinic zirconia nanoparticles in suspension. The coatings grown in Na(2)SiO(3) center dot 5H(2)O/KOH electrolyte revealed gamma-Al(2)O(3) and amorphous phase; alpha-Al(2)O(3) and AlPO(4) were additionally produced with the Na(6)P(6)O(18) electrolyte. Higher temperature zirconia phases, possibly tetragonal and orthorhombic, in addition to the monoclinic phase, were indicative of elevated temperatures at sites of microdischarges. Further, local melting resulted in zirconium-rich dendrites in the coating formed in silicate electrolyte. Zirconium was mainly located in the relatively compact, outer layer of the coating, constituting similar to 70-90% of the coating thickness. Nanoparticles appeared to be incorporated at the coating surface and following transport to the interface regions between the inner and outer layers along short-circuit paths through the outer coating.