Effect of CNTs addition on microstructure, ablation property and mechanism of ZrC-SiC coating for C/C-ZrC-SiC composites

ZrC-SiC coating with gradient inner layer was fabricated on C/C-ZrC-SiC composites by pack cementation. The effects of CNTs addition on the microstructure, ablation property, and mechanism of ZrC-SiC coating were investigated. The results show that the CNTs affected the structure and phase composition of coatings. With CNTs addition, the residual ZrSi2 is low and ZrCx phase possesses higher C/Zr molar ratio, and the SiC phase was generated in the superficial layer. CNTs addition also resulted smaller grain size, reduced thickness, and increased open porosity of the coating. Ablation tests demonstrate that the CNTs decreased the ablation resistance of the ZrC-SiC coating. After ablation, the ZrC-SiC coating was intact and can effectively protect C/C-ZrC-SiC composites from ablation, whereas carbon fibers were exposed in the ZrC-SiC-CNT coating center, indicating the failure of the coating. The degraded ablation property of ZrC-SiC-CNT coating was caused by the inferior structure of the coating. The residual CNTs, thinner coating thickness, reduced grain size (more grain boundaries) and increased SiC phase on the superficial layer weakened the structural integrity of ZrC-SiC-CNT coating, resulting in degraded ablation property.