Formation of Al2O3–ZrO2 composite coating on zirconium by micro-arc oxidation

Al2O3–ZrO2 composite coating was prepared on zirconium by micro-arc oxidation in an aqueous NaAlO2-containing electrolyte. The coating is composed of α-Al2O3, tetragonal ZrO2 (t-ZrO2) and monoclinic ZrO2 (m-ZrO2), and firmly adherent to the substrate. The maximum hardness of the coating reaches 1630 Hv0.2 N, and the friction coefficient of the coating against steel is as low as 0.115 at 200–1000 N loads under oil-lubricated condition. The structure and the formation mechanism of the Al2O3–ZrO2 composite coating have been investigated.