Anticorrosive yet conductive Hf/Si3N4 multilayer coatings on AZ91D magnesium alloy by magnetron sputtering

Hafnium monolayer and Hf/Si3N4 multilayer coatings were deposited on AZ91D magnesium alloys via magnetron sputtering. The microstructure, conductivity and corrosion behaviors of these coatings were investigated. It was found that the columnar growth of the Hf sublayer was suppressed in the Hf/Si3N4 multilayer coating. This multilayered structure resulted in the relief of residual stress and the decrease in porosity. These changes in microstructure inhibited the permeation of corrosion media into the substrate and decreased the diffusion rates of corrosion products. The Hf/Si3N4 multilayer coatings with 10 bilayers showed the best anticorrosion performance revealed by the lowest corrosion current density of 2.798 μA/cm2 in electrochemical system and the smallest corrosion area of 0.22% after 48 h salt spray test, respectively. Additionally, the introduction of Si3N4 sublayer has little impact on the conductivity of the multilayer coating due to the shadowing effect during the deposition.