Yb2Si2O7 Environmental barrier coatings with reduced bond coat oxidation rates via chemical modifications for long life

Spallation of environmental barrier coating (EBC) induced by thermally grown oxide (TGO) resulting from steam oxidation is a key EBC failure mode. A logical approach to improve EBC life, therefore, is to reduce TGO growth rates. A study was undertaken to investigate whether TGO growth rates can be reduced by adding modifier oxides. It was based on a hypothesis that modifier oxides dissolve in SiO₂ TGO and modify the SiO₂ structure, making the TGO less permeable to oxidants. Using a current state-of-the-art EBC (Si/Yb₂Si₂O₇) as the baseline, the Yb₂Si₂O₇ layer was modified by adding Al₂O₃ or Al₂O₃-containing oxide compounds, such as mullite and YAG (Y₃Al₅O₁₂), and TiO₂. EBCs were processed using air plasma spraying. Steam oxidation tests and post-oxidation test oxidation kinetics, chemistry, microstructure, and phase analysis were used to test the hypothesis. The best modified EBC reduced the TGO thickness by ~87% compared with that of the baseline EBC in 90% H₂O + 10% O₂ at 1316°C under thermal cycling. Correlations between oxidation kinetics, chemistry, and microstructure of EBC and TGO were used to explain the effect of modifier oxides on reducing TGO growth rates.