COMPARISON OF EFFECTS OF CVD Al_2O_3 AND PRE- OXIDIZED Al_2O_3 LAYERS ON THE THERMALLY GROWN OXIDE FORMED ON Ni-BASED SUPERALLOY

摘要

Al_2O_3 thin-films were prepared on the surface of a single-crystal Ni-based superalloy by chemical vapor deposition (CVD) and pre-oxidation. In comparison to uncoated superalloy, the superalloy samples coated with CVD and pre-oxidized Al_2O_3 layers exhibited beneficial effects on the morphology and growth rate of the thermally grown oxide (TGO) formed on the alloy surface upon high-temperature oxidation. Furthermore, we observed that the as-deposited CVD layer consisted of only α-Al_2O_3 whereas the as-prepared pre-oxidized layer contained α-Al_2O_3 as the major phase and small amounts of θ-Al_2O_3 and mixed oxides. These observations suggested that the phase purity of the initial Al_2O_3 layer, whether prepared by CVD and pre-oxidation, could be a critical factor in dictating the TGO growth rate.