Reacting CMAS with Yttrium Disilicate Environmental Barrier Coating
Motivated by unprecedented demands for more powerful jet engines, the development of SiC/SiC ceramic matrix composites (CMCs) pushes potential material temperature capabilities to upwards of 1300oC resulting in a 9-10% increase in cycle efficiency. The final step in fully adopting this technology lies in designing more capable barrier coatings to protect the SiC/SiC CMCs from water vapor induced oxidation. Yttrium Disilicate environmental barrier coatings (EBCs) are auspicious due to their favorable coefficients of thermal expansion, low volatility, and phase stability. However, a new problem arises when operating temperatures exceed 1300oC. Siliceous materials ingested by the engine melt and adhere to the surface. These siliceous materials vary from desert sand to volcanic ash and, in general, contain oxides of Calcium, Magnesium, and Aluminum, hence the abbreviation CMAS for CalciumMagnesium-Alumino-Silicate. When the engine is brought back to non-operating temperatures, say after landing, the CMAS deposits solidify forming a layer with a vastly different coefficient of thermal expansion, and cracks form in the multi-layered system. In order to prevent or mitigate the reaction between the EBC and CMAS, we must first be able to understand how these cracks form and use this information to predict how the system will behave. Designing more capable EBCs is key to fully adopting CMCs and revolutionizing the aerospace industry.