Characterization of self-sealing MAO ceramic coatings with green or black color on an Al alloy

Abstract

Micro arc oxidation (MAO) coatings with green or black color were obtained on a 6061 Al alloy in a new Cr-containing electrolyte system. Compared to the white coatings obtained with traditional electrolytes, this black coating has the characteristics of self-sealing pores and different chemical compositions. The microstructure and composition of the different coatings were comparatively studied by field-emission scanning electron microscopy (FESEM), energy dispersive X-ray spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS), respectively. The mechanical property, tribological behavior and corrosion resistance of the MAO coatings were systematically investigated using nano-indentation, tribological tester and electrochemical corrosion tests. The results indicated that the green MAO coating with a porous structure had Cr₂O₃ distributed into the Al₂O₃ ceramic coating with a little of K₂Cr₂O₇ addition into the base solution, and the black MAO coating with a self-sealing microstructure contained Cr₂O₃ and CrO₃ with a large amount of K₂Cr₂O₇ addition into the base solution. The hardness values of the MAO coatings with an increase of Cr at% increased first and then decreased due to the compact structure and different compositions of the MAO coatings. Compared with the other MAO coatings, the self-sealing black MAO coating with a high Cr content showed greatly improved wear resistance and corrosion resistance for its special microstructure. Furthermore, the formation mechanism of these MAO coatings with Cr addition has been revealed and the relationship between microstructure and properties has been discussed.