Examining the effect of temperature on steam oxidized silicon bond coats

The SiO₂ thermally grown oxide (TGO) is a life-limiting factor for environmental barrier coatings (EBCs). EBCs in aeroengines face a variety of operating temperatures so the resulting effect on SiO₂’s crystallinity and growth are necessary to better understand EBC failure. Here, the temperature effect of steam oxidation on SiO₂ crystallinity, SiO₂ growth and silicon bond coat residual stress are investigated by coupling electron microscopy image analysis and Raman spectroscopy stress mapping. This novel characterization approach offers a new perspective for microstructure and stress evolution in EBCs. From this study, image analysis reveals the internal oxidation of SiO₂ on air plasma spray splat boundaries and the faster growth of an amorphous SiO₂ compared to β-cristobalite SiO₂. Consequently, stress mapping characterizes the greater inhomogeneity in the residual stress distribution in the silicon bond coat with an amorphous SiO₂ and the effect of cracking on stress relief. As a result, silicon bond coat cracks near amorphous SiO₂ TGO are expected to result in an EBC with a reduced lifetime.