Al2O3-ZrO2 nanostructured coatings using DC plasma electrolytic oxidation to improve tribological properties of Al substrates

Alumina-zirconia nanostructured coatings were formed on 7075 Al alloy through Plasma Electrolytic Oxidation (PEO) operated in aDC potentiostatic mode. The composite coatings were produced in the range of 425-500 V in an alkaline electrolyte containing 4 g/LK2ZrF6. Tribological properties of coatings were investigated using dry sliding wear test against WC balls with a pin-on-disc tribometer. Wear rates were evaluated using optical profilometer. It was shown that the nanostructured alumina-zirconia composite coatings can be formed at voltages >= 450 V. The coating thickness and roughness were in the range of 15.2-24.2 mu m and 0.68-2.35 mu m, respectively. The distribution of Al, Zr and O in the coatings was uniform. Increasing the PEO voltage led to porosity increment and formation of the high temperature tetragonal zirconia phase (t-ZrO2). Significant enhancement in tribological properties for coated samples was achieved: under optimum conditions, corresponding to the PEO treatment at 500 V for 200 s, the coating wear rate of 2.62 x 10(-6) mm(3) N-1 m(-1) and friction coefficient of 0.22 were recorded that are about 120 and 3 times lower than those for the substrate. (C) 2015 Elsevier B.V. All rights reserved.