Bonding strength, thermal shock and oxidation resistance of interlocking (Zr,Hf)C-SiC/SiC double-layer coating for C/C composites

In order to alleviate the mismatch of CTEs (coefficient of thermal expansions) and increase the bonding strength between the outer UHTCs (ultra-high temperature ceramics) coating and the inner SiC coating on C/C composites, a double-layer (Zr,Hf)C-SiC/SiC coating with interlocking structure was designed and prepared on C/C composites by pack cementation. The microstructure, bonding strength, thermal shock, and oxidation resistance of the coating were studied. The results showed that the construction of a coarse inner Si-based coating with microholes was beneficial for the outer UHTCs coating materials infiltrating into it, and thus realized a gradient distribution of UHTCs in the whole coating. The bonding strength and the thermal shock tests revealed that the adhesion strength between the inner layer and the outer layer was significantly increased owing to the zigzagged interface. After exposure to 1500 °C for 220 h in air, the coated specimens only had a mass loss of 2 mg·cm⁻², which was attributed to the alleviation of the mismatch of CTEs and the better interface bonding strength.