The influence of particle size distribution for homogeneity of ytterbium silicate coating prepared by atmospheric plasma spraying

Environmental barrier coatings (EBCs) are applying for higher temperature operated turbine engine using with SiC-based ceramic matrix composites (CMCs) to prevent reducing the thickness by oxidation-volatilization cycle in high-temperature and water vapor environments. Although Yb₂Si₂O₇ has attracted attention in terms of the similar coefficient of thermal expansion (CTE) to the SiC and excellent high-temperature steam resistance, and the Yb₂Si₂O₇ coatings which has vertical cracks running through the thickness are reported. These cracks are caused by the high thermal stresses attributed to formation of Yb₂SiO₅ which has higher CTE than that of the Yb₂Si₂O₇ and SiC. Since Yb₂SiO₅ is formed due to the volatilization of SiO₂ in Yb₂Si₂O₇ during the melting, it can be suppressed by reducing the heat energy into the feedstock adjusting the spray conditions, but which may entrap the unmelted particles into the coating.

In this study, we focused on the particle size distribution (PSD) of feedstock, and investigated the relevance of the PSD of Yb₂Si₂O₇ feedstock, the volatilization amount of SiO₂, and the Yb₂SiO₅ formed in the coating. It is confirmed that volatilization of SiO₂ tends to occur in specific particle size. Subsequently, we examined and discussed the optimal PSD of feedstock to obtain homogeneity Yb₂Si₂O₇ coating.