Water vapor corrosion behavior of SiC/Si/mixed Yb₂Si₂O₇-Yb₂SiO₅ environmental barrier coating for C/C composites

Tri-layer environmental barrier coatings (EBCs) were developed to provide complete encapsulation for carbon/carbon (C/C) composites, consisting of SiC inner layer, Si bond layer, and mixed Yb₂Si₂O₇-Yb₂SiO₅ topcoats with varying mass ratios. The protective properties of the EBCs were assessed by exposing the coated samples to a corrosive gas mixture of 50 % O₂ and 50 % H₂O at 1500 °C, the mass change, phase composition, and microstructure evolution were investigated. The results showed that a small amount of Yb₂SiO₅ doping could improve water vapor corrosion resistance, but the excessive content of Yb₂SiO₅ led to a mismatch of the coefficient of thermal expansion (CTE) with inner coating, resulting in coating cracking. The mixed topcoat with 60 wt% Yb₂Si₂O₇ and 40 wt% Yb₂SiO₅ showed superior water vapor corrosion resistance. After corrosion for 20 h at 1500 °C, no penetrating cracks of large size appeared, the thickness of thermal growth oxide (TGO, 4.6 ± 0.4 μm) was relatively lower, and the C/C substrate was effectively protected from damage.