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This study investigates the influence of thermally grown oxide (TGO) on the lifetime of thermal barrier coatings (TBCs). In the experiments, cold sprayed NiCoCrAlY powder was deposited as the bond coat and plasma sprayed YSZ was deposited as the topcoat. The TBCs were subjected to various treatments to induce different types of TGOs at the bond coat-topcoat interface. Three types of oxide were detected, including spinel, chromium oxide, and alumina. Thermal cycling tests show how the oxides contribute to cracking and spalling in the topcoat layer and how the morphologies and constituents of the oxides can change with time and temperature.

Plasma-sprayed ceramic coatings are deposited such that flattened splats together with some nonmelted particles are present in the coatings. In this study, the nonmelted particles in plasma-sprayed alumina coatings were examined by scanning electron microscopy, confocal Raman analysis, and X-ray diffraction analysis with the aim of quantitative evaluation of the coating microstructure. Results showed that the nonmelted particles can be clearly identified from the cross-sectional microstructure due to the morphology that results from the high hardness of the nonmelted particles. The obvious gap at the interface between nonmelted particles and the surrounding splats suggests weak interface bonding. Raman analysis revealed that there was little α-Al2O3 phase in the flattened splats region, which confirms that this phase in the coating appears only from nonmelted particles. Attention should be paid to the weak bonding of the nonmelted particles relative to the flattened splats during the preparation of samples for quantitative characterization of coating microstructure.

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