Assessment of microstructure and tribological performance of ZrO2-reinforced MAO coatings on 7075 aluminum alloy using grey relational analysis

In this study, micro-arc oxidation (MAO) coatings containing different concentrations of ZrO2 were fabricated on 7075 Al alloy in a silicate-based electrolyte to evaluate their microstructural evolution, surface characteristics, and tribological performance. The coatings were produced at 390 V for 30 min in an electrolyte containing 10 g/L Na2SiO3 with ZrO2 additions of 0, 3, 6, and 9 g/L. Surface and cross-sectional morphologies were examined by SEM/EDS, while surface topography and wear track profiles were characterized by profilometry. Tribological behavior was evaluated under dry sliding conditions using a reciprocating tribometer. The results revealed that ZrO2 incorporation markedly influenced the coating growth behavior and surface morphology. The Zr0-MAO coating exhibited a rough and porous structure with large discharge channels, whereas the addition of ZrO2 led to a more compact and homogeneous morphology with reduced porosity. Among the coated samples, Zr6-MAO exhibited the most refined microstructure and the lowest surface roughness (Ra = 4.24 µm). This sample also showed the best tribological performance, with the lowest coefficient of friction (0.27), wear volume (0.0065 mm3), and wear rate (1.63 × 10−4 mm3/N·m). To provide a comprehensive multi-criteria assessment, grey relational analysis (GRA) was employed by simultaneously considering coating thickness, surface roughness, friction coefficient, wear volume, and wear rate. The GRA results were consistent with the experimental findings and identified Zr6-MAO as the optimum coating condition. These findings demonstrate that a moderate ZrO2 addition is highly effective in improving the compactness and wear resistance of MAO-coated 7075 Al alloy.