Mechanism of interface changes of Al₂O₃-Yb₂Si₂O₇/SiC coating for C/C composite under water-oxygen environment at 1500 °C

To solve the corrosion issues of C/C composites in water vapor and oxygen environment at ultra-high temperatures, Al₂O₃-Yb₂Si₂O₇/SiC coating (AYSC) was developed using a combination of the supersonic atmospheric plasma spraying and pack cementation methods. The pinning structure at the interfaces provides strong adhesion between SiC inner coating and Al₂O₃-Yb₂Si₂O₇ (AY) outer coating, as well as between the coating and C/C composite substrate. After the corrosion assessment in a 90 % H₂O-10 % O₂ environment at 1500 °C, Al₂O₃-Yb₂Si₂O₇/SiC coating demonstrated effective protection for the C/C composite for 120 h with a weight increase of 0.68 %. The formation of the new phase, Yb₃Al₅O₁₂, under water-oxygen condition was the key to the excellent anti-corrosion performance. A thermal growth oxide (TGO) layer, composed of SiO₂, formed at the interface between Al₂O₃-Yb₂Si₂O₇ coating and SiC coating, with its thickness increasing over time due to prolonged corrosion exposure. The accumulation of thermal stress, arising from the differing thermal expansion coefficients of the TGO and the coating, could ultimately lead to coating cracking, thereby diminishing the protective capability of the Al₂O₃-Yb₂Si₂O₇/SiC coating.