Oxidation behavior of ceramic coatings on Ti-6Al-4V by micro-plasma oxidation

Compound ceramic coatings prepared on Ti-6Al-4V alloy by pulsed bi-polar micro-plasma oxidation (MPO) in NaAlO₂ solution were oxidized under different temperature in air. The phase composition and surface morphology of the coatings before and after oxidation were investigated by X-ray diffractometry and scanning electron microscopy, respectively. Meantime, the weight gains and the high temperature oxidation characteristics of the coated samples were investigated. The results show that the coatings prepared by MPO were composed of a large amount of Al₂TiO₅ and a little α-Al₂O₃ and rutile TiO₂. And the oxidation process of the coated samples included the decomposition of the Al₂TiO₅ in the coating, the oxidation of the substrate and the changes of the coating structure. After high temperature oxidation, the increase of α-Al₂O₃ in the coating was due to the decomposition of Al₂TiO₅, whereas the increase of rutile TiO₂ in the coating was attributable to both the decomposition of Al₂TiO₅ and the oxidation of the Ti substrate. The main crystalline of the coatings became rutile TiO₂ after the oxidation of 1000 °C for 1 h. The decomposition of Al₂TiO₅ in the coating occurred at 900 and 1000 °C, and its half decomposition time was less than 1 h at 1000 °C. Increasing oxidation temperature or extending oxidation time, the weight gains of coated samples was increased to different extent. However, the weigh gains of the coated samples was much lower than that of the substrate, so the ceramic coatings improved the oxidation resistance of Ti alloy greatly under the experimental conditions.