Oxidation and failure mechanisms of ytterbium disilicate-based coatings on SiC/SiC composites in wet oxygen at 1350 °C

The oxidation and failure behavior of SiC/SiC-Si/Yb₂Si₂O₇ and SiC/SiC-Si/mullite/Yb₂Si₂O₇ in wet oxygen at 1350 °C was investigated. Cracking and spalling occurred in the Si/mullite/Yb₂Si₂O₇ due to greater thermal stress. Al elements in mullite diffused into Yb₂Si₂O₇ and thermal growth oxide (TGO) in unpeeled Si/mullite/Yb₂Si₂O₇, which enhanced the bonding at the mullite/Yb₂Si₂O₇ interface and suppressed TGO cracking, but also compromised its structural stability and corrosion resistance. The Si/Yb₂Si₂O₇ coating could remain intact and adhere tightly to the SiC/SiC substrate. But TGO formed in Si/Yb₂Si₂O₇ induced delamination due to exacerbated thermal mismatch, and it cracked due to phase transformation. Wet oxygen diffused into SiC/SiC substrate through cracks in Yb₂Si₂O₇-based environmental barrier coatings (EBCs). A layered SiO₂ formed on the SiC matrix in SiC/SiC-Si/mullite/Yb₂Si₂O₇ due to passive oxidation, while a porous SiO₂ layer formed in SiC/SiC-Si/Yb₂Si₂O₇ due to active-passive oxidative cycles. Both Si/Yb₂Si₂O₇ and Si/mullite/Yb₂Si₂O₇ have limitations in long-term protection for SiC/SiC, thereby new designs and material screening are needed.