Optimization of Heat Treatment Parameters for Yb-Disilicate Environmental Barrier Coatings

In aircraft engines, environmental barrier coatings (EBCs) are responsible for protecting ceramic matrix composites (CMCs) from the corrosive effects of water vapor at elevated temperatures. In this work, ytterbium disilicate (YbDS) was plasma-sprayed using optimized spray parameters that avoid Si evaporation, without any substrate pre-heating, resulting in a partially amorphous EBC. A series of heat treatments were used to crystallize the YbDS-based EBC and bring its CTE closer to the CTE of the SiC CMC substrate. This project aimed to optimize the heat treatment for the as-sprayed EBCs by in-depth analysis of the effect of time and temperature on the phase formation and crystallinity of Yb-disilicate coatings. For this purpose, heat treatment was carried out at 1100, 1200 and 1300 °C for 2 to 10 hours. Metastable phases are formed during the crystallization process at 1100 °C. To achieve around 94-97% YbDS as the major crystalline phase, it is necessary to increase the heat treatment time at 1100 °C or increase the temperature to 1200 or 1300 °C. The microstructure and porosity measurements were performed on the cross section of the as-sprayed, heat treated with stable phases, and heat treated with both stable and metastable phases EBCs.