Oxidation behavior of supersonic air plasma sprayed Yb₂SiO₅/Si and Yb₂Si₂O₇/Si coating for CVD-SiC coated C/C composites in wet oxygen at 1773 K: Experimental and first-principle calculation

The protective effect of Yb₂SiO₅/Si and Yb₂Si₂O₇/Si coatings on CVD-SiC coated C/C composites in high temperature wet oxygen environments was investigated by first-principle calculation and experiment, respectively. The calculation results reveal that the Yttrium silicate (Yb₂SiO₅, YbMS) phase exhibits stronger bonding energy compared to the yttrium disilicate (Yb₂Si₂O₇, YbDS) phase. Moreover, the Si-O bond within the YbMS structure is more stable under similar water vapor corrosion conditions. Consequently, the YbMS structure contributes to the formation of a thinner thermally grown oxide (TGO) and diffusion reaction layer (DRL) at the interface between the Si bond coat and YbMS topcoat under wet oxygen conditions at 1773 K. However, the higher coefficient of thermal expansion (CTE) of the YbMS coating resulted in the formation of larger penetration cracks in YbMS-coated C/C samples during cooling. After 20 h of oxidation in a water vapor environment at 1773 K, the weight change of the YbMS-coated sample was −4.374 %, while the YbDS coated sample was −0.610 %.