Comparative study of microstructure and ablation behaviour of C/C-HfC-SiC composites prepared under two different conditions

C/C-SiC-HfC composites were prepared using reactive melt infiltration (RMI) under a vacuum (noted as HS-V) and normal pressure (noted as HS-NP), separately. The distribution of the introduced SiC and HfC ceramics can be regulated by adjusting the RMI pressure. Different pressures of the environment at the high temperature stage led to the difference in volatilization of Si in the melt and the ratios of C and HfSi₂ in unit volume on the surface of the sample, resulting in the different composition of the sample surface. A HfSi₂-HfC-SiC mixed layer was formed on the surface of HS-NP, while HfC coating was formed on the surface of HS-V. HS-NP possessed better ablation performance than HS-V. After ablation for 120 s, its mass ablation rate decreased by 87.81%. The presence of HfSi₂ improved the densification of HfO₂ film, which made the ablation resistance of HS-NP better than HS-V. This work provides a guide for choosing an idea pressure condition during RMI to obtain C/C-SiC-HfC composites with high ablation resistance.