Preliminary Study on Preparation of Black Ceramic Coating Formed on Magnesium Alloy by Micro-arc Oxidation in Carbon Black Pigment-contained Electrolyte

Abstract

Based on orthogonal design, black ceramic coating was obtained on AZ91D magnesium alloy with addition of carbon black pigment in electrolyte by micro-arc oxidation (MAO). The blackness, thickness, phase, morphology and composition of the coating were analyzed by visual detection, eddy current thickness meter, XRD, digital cameras and EDS. The reasonable preparation parameters were obtained, and the influences of current density, carbon ink concentration, Na2WO4 concentration, Na2SiO3 concentration and sodium citrate concentration on the blackness and thickness of the coating were studied. The results showed that adopting constant current mode, the ceramic coating with good color uniformity, blackness of 0.9 and thickness over 30μm could be prepared on AZ91D magnesium alloy surface by MAO under the conditions of current density 3A/dm2, carbon ink concentration 25mL/L, Na2WO4 concentration 1.5g/L, Na2SiO3 concentration 6g/L and sodium citrate concentration 0.3g/L. Carbon ink concentration and current density were the main effect factors of the coating blackness and thickness, respectively. Coating blackness rose significantly with the increasing of carbon ink concentration and coating thickness increased with the increasing of current density at first and then decreased. Coating blackness and thickness all increased with the increasing of Na2WO4 concentration. As Na2SiO3 concentration increased, coating blackness gradually decreased, and there was little change of coating thickness. Sodium citrate concentration has little influence on coating blackness, while coating thickness decreased as it increased.