In order to evaluate the high-temperature durability of the plasma sprayed thermal barrier coating (TBC) systems in connection with their coating characteristics such as the coating microstructures and interfacial natures depending on the coating processing, high-temperature oxidation test was conducted at 1000 and 1100°C under both the isothermal and thermal cycle conditions for several kinds of TBC systems. Specimens with different coating features were prepared systematically by controlling different coating parameters such as the sorts of the ceramic top-coat powders and the post-spraying heat-treatment conditions.
High-temperature oxidation behavior was found to depend strongly on both the sorts of top-coat powder and the heat-treatment conditions after spraying such as a combination of treatment temperature and atmosphere. Then, the top-coat structure with large numbers of micropores and microcracks introduced by hollow spherical powder was proved to have the superior top-coat spalling resistance even in the thermal cycle conditions as compared to those with only a few microdefect by angular powder, because of the thermal stress relaxation effect associated with the preexisting microdefects. It was also clarified that the segmented top-coat made by short spray distance using the angular powder was much effective in suppressing the top-coat spalling. Furthermore, the appropriate reheat-treatment in argon atmosphere delayed the development of the thermally grown oxides (TGO) layer which is formed at top-coat/bond-coat interface. On the contrary, the combination of heat-treatment at higher temperature and in air atmosphere was revealed to promote the development of the heterogeneous TGO layer with many voids and to cause the premature top-coat spalling for TBC system with the dense top-coat by angular powder.
From the microstructural analyses of TGO layer using TEM-EDS, it was confirmed that Y-Al-mixed oxide particles existed selectively at Al₂O₃ grain boundary in TGO layer and Y content in these oxide particles tended to increase as the testing temperature was raised.
Affecting factors for high-temperature oxidation property of TBC systems was discussed in connection with the coating parameters for developing the high performance TBC system.