Al–AlN composite coatings on AZ31 magnesium alloy for surface hardening and corrosion resistance

Al–AlN composite coatings were prepared using filtered cathodic vacuum arc deposition on AZ31 magnesium alloy substrates. With increasing substrate bias, AlN phase content increased and Al phase content decreased, resulting in increased hardness of the coatings. Transmission electron microscopy morphology of the composite coatings at 100 V bias showed that the metallic Al phase was embedded in the AlN matrix. The coatings significantly enhanced the corrosion resistance of the AZ31 magnesium alloy. The maximum hardness was 512 HV obtained at a bias of 300 V and a minimum corrosion current density was 1.913 × 10⁻⁶ A/cm² attained at a bias of 100 V. The effect of substrate bias on microstructure, coating thickness, and residual stress in the coatings was studied in details.