Thin single-phase yttrium-based environmental barrier coating systems for SiC/SiC CMCs

Abstract

Two thin yttrium silicate-based EBC systems for SiC/SiC CMCs that comprised of three layers were produced via (reactive) magnetron sputtering with pure yttrium disilicate (YDS) and yttrium mono silicate (YMS) layers. The systems survived 1000 1-hour furnace cycle tests at 1250 °C in air interrupted by active cooling for 10 min. One system showed partial delamination of the YMS. The YDS intermediate layer stopped deleterious reactions among YMS and silicon and the silica formation did not cause spallation of the silicates. Some vertical cracks in YMS left possible attack paths for water vapor. Rapid water vapor testing was performed at 1250 °C in 100 vol% water vapor at 10 m/s for 2 h. This test proved that a thin but hermetic and phase pure YMS top layer can protect the system from volatilization: single phase X2-YMS was not attacked by steam whereas a mixed beta-YDS/X2-YMS coating showed volatilization of SiO2.