Rare-earth silicates (i.e., R 2 SiO 5 and R 2 Si 2 O 7 ; R = rare-eath elements) have increasingly attracted attention for their application to environmental barrier coatings (EBCs). EBCs are necessary to protect ceramic matrix composites against high-temperature corroding environments in hot-section components of advanced gas-turbine engines since the mid-2000s . To clarify an optimal layered system and suitable processes, degradation behaviors of EBC systems, including rare-earth silicates, under water vapor and oxygen atmospheres at high temperatures have been extensively investigated, especially within the past decade , along with countermeasures against Ca-Mg-Fe-Al-silicate (CMAS) attacks . The degradation processes are determined by the selected compositions of EBC layers and by multiple closely related microstructural factors with temporal changes (e.g., crystallinity, grain size, hetero-interface structure, porosity, cracks, layer thicknesses, and related thermal stresses). Therefore, through simplified and modeled experiments, fruitful insights into the degradation mechanisms and improvements of EBCs could be achieved.