Investigation of wear behavior of graphite additive composite coatings deposited by micro arc oxidation-hydrothermal treatment on AZ91 Mg alloy

Composite coatings were deposited on AZ91 magnesium alloys by micro arc oxidation (MAO) and the MAO-hydrothermal treatment (HT). The MAO process of Mg alloys was performed in the silicate/phosphate-based electrolyte with graphite particles size of 5-10 and 75 µm and then the obtained samples were submitted to hydrothermal treatment at 150°C in the graphite-ethanol mixture. The structural and morphological properties of composite coatings were analized X-ray diffraction (XRD) and scanning electron microscopy (SEM). The wear resistance was examined with ball-on-disc tribometer. The addition of graphite to the MAO electrolyte solution increased the coating thicknesses and compactness and the surface morphologies acquired relatively a homogeneous appearance. The coating surfaces were enriched in graphite by HT and the wear test results showed that the wear resistance of the graphite-doped MAO-HT composite coatings was significantly improved.