Microstructure evolution and properties of Hf/Zr-based UHTCs modified C/C composites prepared by reactive melt infiltration method

Different Hf/Zr-based UHTCs modified C/C composites were fabricated via the reactive melt infiltration (RMI) adopting HfSi₂, ZrSi₂, and HfSi₂-ZrSi₂ hybrid alloys with different molar ratios (3:1, 1:1, 1:3). The microstructural evolution of the composites, variation of mechanical properties, and ablation performance of the composites in the absence of in-situ coating were studied. The C/HfC-ZrC-SiC composites were all divided into the exterior region with more UHTC phases and the interior region with more SiC and silicides, and the UHTC-rich region enlarged with the HfSi₂ content increased in the RMI alloy. The flexural strengths of C/HfC-ZrC-SiC were enhanced to 238.37 ± 20.42 Mpa and 237.26 ± 17.57 Mpa when adopting HfSi₂-ZrSi₂ alloys with molar ratios of 1:1 and 1:3. As the HfSi₂ content in the RMI alloy increased, the oxide layer became compact with sintering degree increased, enhancing the ablation resistance of the composites and reducing the mass ablation rate.