Ultra-hard ceramic coatings fabricated through microarc oxidation on aluminium alloy

Ultra-hard ceramic coatings with microhardness of 2535 Hv have been synthesized on the Al alloy substrate by microarc oxidation (MAO) technique. The effects of anodic current density ( ja) and the ratio of cathodic to anodic current density ( jc/ja) on the mechanical and corrosion resistance properties of MAO coatings have been studied by microhardness and pitting corrosion tests, respectively. In addition, the phase composition and microstructure of the coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results show that the coatings prepared at high anodic current density consist mainly of a-Al2O3, while those fabricated at low anodic current density are almost composed of g-Al2O3. Microhardness test shows that the coatings have high microhardness, and the highest one is found in the coating formed at ja = 15 A/dm2 and jc/ja = 0.7. Pitting corrosion test shows that the structure of coatings is strongly influenced by the varying jc/ja.