Steam Degradation of Ytterbium Disilicate Environmental Barrier Coatings: Effect of Composition, Microstructure and Temperature

Recession of environmental barrier coatings (EBC) in environments containing steam is a pressing concern that requires further research before their implementation in gas turbine engines can be realized. In this work, free-standing plasma-sprayed Yb₂Si₂O₇ coatings were exposed to flowing steam at 1350 and 1400 °C for 96 h. Three samples were investigated, one coating with a low porosity level (< 3%) and 1 wt.% Al₂O₃ representing traditional EBCs porosity levels; and two coatings with higher porosity levels (~ 20%) representing abradable EBCs. Phase composition and microstructural evolution were studied in order to reveal the underlying mechanism for the interaction between high temperature steam and ytterbium disilicate. The results show depletion of Yb₂SiO₅ near the surface and formation of ytterbium garnet (Yb₃Al₅O₁₂) on top of all three coatings due to the reaction with gaseous Al-containing impurities coming from the alumina furnace tubes. The 1 wt.% Al₂O₃ added to the EBC sample exacerbated the formation of garnet at 1400 °C compared to the abradable samples, which presented lower quantities of garnet. Additionally, inter-splat boundaries were visible after exposure, indicating preferential ingress of gaseous Al-containing impurities through the splat boundaries.