Growth mechanism of black ceramic layers formed by microarc oxidation

Black ceramic layer containing V2O3 was obtained on aluminum alloy surface by microarc oxidation. When ammonium metavanadate (NH4VO3) with a 6g/L concentration was added into the commonly used (NaPO3)6 and Na2SiO3 solution, obtained ceramic layers change from white appearance to black. Surface and cross-section morphology of the ceramic layer were revealed by SEM. In-layer concentration and chemical state of vanadium were investigated by EDS and XPS, respectively. Layer thickness was measured by eddy current thickness meter. Compared with inner sublayer, an outer sublayer with a thickness of approximate 4μm formed on the surface which shows higher vanadium concentration. It is proposed that the V2O3 formed in this sublayer causes the obtained black appearance. During the microarc oxidation process, VO3− contained in the solution adsorbed on the surface of metastable oxide phases and transit to vanadium oxide by the instantaneous high temperature in the heat-affected zone of microarc. It is found that the growth of the black ceramic layer has two different stages. At the first stage the micro-zones containing vanadium are formed and expanded. After the microarc oxidation processing was started for 3min, the adsorption of VO3− on the metastable oxides plays a primary role in the growth of the black ceramic layers.