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In the present work, a carbon fiber reinforced ZrC-20 vol% SiC-15 vol% HfB 2 composite was produced by pressureless sintering method. Carbon fiber with various weight percentages (0, 10, 20, 30) was added to the milled ZrC-SiC-HfB 2 . The mixed powders were pressed and sintered at 2200°C for 2 h. The microstructure and mechanical properties of the specimens were investigated by Scanning Electron Microscope (SEM) equipped with EDS spectroscopy, XRD analysis and hardness and toughness tests. The ablation resistance of the composites was performed by oxyacetylene torch. The results showed that as the carbon fiber content was increased, the porosity and toughness were increased as well, while the hardness and density were decreased. During the ablation test, the outer surface of the composite underwent a sintering process and formed a dense layer of Zr-Hf-Si-O on the surface. ZrSiO 4 and HfSiO 4 were also created after cooling, so these phases played a pinning effect and prevented the crack propagation. The results also showed that linear and mass ablation rates were increased with increasing the weight percentage of the carbon fiber. SEM analysis with EDS further revealed that with increasing the carbon content, the defects in the formed oxide layer on the composite surface were increased due to the evaporation of SiO 2 and the decrease in the pinning effect of ZrSiO 4 and HfSiO 4 phases as well as the phase transformation of the remained ZrO 2 and HfO 2 . As a result, as the carbon fiber was increased, the ablation resistance of the composite was decreased.

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In this study, HfB2-SiC-graphite composites were fabricated using spark plasma sintering at 1900 ℃ for 10 min. The effect of graphite content on the microstructure and mechanical properties was studied; also, oxidation resistance of the prepared composites was investigated at 1400 ℃ for 32 h. Weight changes and thickness of the formed oxide layer were measured. The relative density, toughness and strength of the SPSed composites increased with raising graphite up 2.5 wt% and then decraesed. On the other hand, the hardness of the composites was decreased when graphite was added. Oxidation resistance of the composites was, thus, improved following graphite addition up 2.5 wt%, while the ecessive quantity of graphite caused poor oxidation resistance of the composites.

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