Effect of water vapour on preferential orientation of thermally grown oxide at 1050 °C

The crystal structure and hardness of thermally grown oxide (TGO) layer formed on a NiCoCrAlY bond coat in thermal barrier coatings (TBCs) prepared by electron-beam physical vapor deposition were studied. The hardness and the relationship between the crystal structure and defects depended on the oxidising atmosphere. The presence of excess water vapour during high-temperature oxidation promoted the transformation of delta- to alpha-Al2O3. The TGO layer formed in water vapour environments exhibited higher hardness and fewer defects than that formed in air. These results offer an opportunity to design the long-lifetime TBCs. Furthermore, the grain orientation of the TGO layer was affected by the presence of water vapour; the preferential orientation of alumina was < 0 0 1 > in air + water vapour environment.