The protectiveness of thermally grown oxides on cold sprayed CoNiCrAlY bond coat in thermal barrier coating

This paper presents the results of an oxidation behavior study for a thermal barrier coating (TBC) with air plasma sprayed yttria-stabilized zirconia top coat and CoNiCrAlY bond coat deposited using low pressure plasma spray (LPPS) and cold spray (CS). The TBC is subjected to isothermal oxidation and creep tests at 900°C and evaluated using scanning electron microscopy, energy dispersive x-ray spectrometry transmission electron microscopy and electron backscatter diffraction. The thermally grown oxide (TGO) developed in the TBC with the LPPS bond coat was composed of only a-Al ₂O ₃ and the TGO developed in the TBC with a CS bond coat is composed of a-Al ₂O ₃ and c-Al ₂O ₃. Despite the presence of this metastable c phase, the TGO in the CS specimens exhibits a dense microstructure and lower amounts of mixed oxides. The correlation between c-Al ₂O ₃ and the formation of mixed oxides was investigated through the measurement of c-Al ₂O ₃ thickness ratio and mixed oxides coverage ratio. It was found that the mixed oxides coverage ratio is inversely proportional to the c-Al ₂O ₃ thickness ratio.