Wear-resistant coatings on aluminium produced by plasma anodising-A correlation of wear properties, microstructure, phase composition and distribution

In the recent decades, various process windows have been found for plasma anodising of aluminium surfaces to produce wear-resistant alumina coatings. The coatings offer a high hardness and provide an excellent bonding to the substrate material, thus preventing spallation under mechanical or tribological load. In the present study, coatings with a high abrasive wear resistance and a hardness of up to 12 GPa were produced in an electrolyte of 5 g/l sodium metasilicate and 5 g/l potassium hydroxide at a current density of 30 A/dm2. To understand the reasons for the high wear resistance, the morphology as well as the phase composition and distribution within the coating were examined globally and locally using X-ray diffraction with conventional and grazing incidence and electron backscatter diffraction. The analyses show that the coating globally is comprised of approximately one third of α-alumina, one third of γ-alumina and one third of amorphous alumina with locally varying phase content.