Reactive sputter deposition of alumina films on magnesium alloy by double cathode glow-discharge plasma technique

In order to overcome the problem of the corrosion resistance of AZ31 magnesium alloy, the nanocrystalline Al₂O₃ film was deposited on AZ31 magnesium alloy by double cathode glow-discharge plasma technique. The microstructure, chemical composition and elemental chemical state of the sputter-deposited nanocrystalline Al₂O₃ film were analyzed by means of scanning electron microscopy equipped with an energy dispersive spectroscope, X-ray diffraction), transmission electron microscope and X-ray photoelectron spectroscopy. The results indicated that the sputter-deposited nanocrystalline Al₂O₃ film consisted of single θ-Al₂O₃ phase with average grain size about 60 nm. The hardness and the elastic modulus of the as-deposited nanocrystalline Al₂O₃ film were about 17.21 GPa and 217 GPa measured by nanoindentation instrument, respectively. The corrosion behavior of the sputter-deposited nanocrystalline Al₂O₃ film in 3.5%NaCl solution was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy. The amount of porosity for the sputter-deposited nanocrystalline Al₂O₃ film calculated by two electrochemical methods was equal to 0.0086% and 0.168%, respectively. The sputter-deposited nanocrystalline Al₂O₃ film exhibited excellent corrosion resistance, which was attributed to its dense enough structure to prevent magnesium alloy from corrosion in aggressive solutions.