In situ growth of ZrO2–Al2O3 nano-crystalline ceramic coatings via micro arc oxidation of aluminum substrates

Micro arc oxidation technique was employed to grow zirconia–alumina porous layers. Considering XPS, XRD, and EDX results, the layers mainly consisted of α-Al2O3, γ-Al2O3, monoclinic ZrO2, tetragonal ZrO2. Fractions of these phases were observed to change based on the fabrication conditions. Zirconia phases formed not only on the surface, but also in the layers depth. Increasing the voltage as well as utilizing thicker electrolytes resulted in higher zirconium concentration. The average crystalline size of the ZrO2 and the Al2O3 phases was determined as about 57 and 75 nm. AFM studies revealed that the surface roughness increased with voltage and electrolyte concentration. Morphological evaluations, performed by SEM, showed that the microstructure of the layers strongly depended on the synthesis conditions. The layers revealed a porous structure with a pores size of 40–300 nm. Microcracks were observed to appear when the electrolyte concentration and the applied voltage increased. Finally, a formation mechanism was put forward with emphasis on the chemical and the electrochemical foundations.