Ablation properties of ZrC-SiC-HfB2 ceramic with different amount of carbon fiber under an oxyacetylene flame

In the present work, a carbon fiber reinforced ZrC-20 vol% SiC-15 vol% HfB2 composite was produced by pressureless sintering method. Carbon fiber with various weight percentages (0, 10, 20, 30) was added to the milled ZrC-SiC-HfB2. The mixed powders were pressed and sintered at 2200 degrees 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. ZrSiO4 and HfSiO4 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 SiO2 and the decrease in the pinning effect of ZrSiO4 and HfSiO4 phases as well as the phase transformation of the remained ZrO2 and HfO2. As a result, as the carbon fiber was increased, the ablation resistance of the composite was decreased.