Effects of Ni content on microstructure and performance of MAO coatings on binary Al-Ni alloys

In this study, the pure Al and binary Al-Ni alloys with Ni addition of 1, 3, 6, and 10 wt % were coated via micro arc oxidation (MAO) in a tetraborate and phosphate based electrolyte for 60 min. With increased Ni content, the color of MAO coating became darker due to increased Ni-containing compounds on the surface. The Ni played a negative effect on the coating growth, decreasing both the coating thickness and surface roughness. The coatings on the Al-Ni alloys were mainly composed of gamma-Al2O3 and alpha-Al2O3, and the Ni facilitated the formation of alpha-Al2O3 in the coating. Nanoparticles of NiO were generated around the edge of discharge channels for Al-6Ni and Al-10Ni alloys after a certain oxidation time. All coatings exhibited a uniform and dense surface structure, with the outer layer being more compact than the inner layer. Many tiny pores were found in the inner layer. Despite the corrosion resistance was decreased, the anti-wear performance of MAO treated samples was improved with the increased Ni content in the matrix. While, the corrosion resistance of the coated samples decreased with the adding of Ni content in the matrix.