Effect of Silicon Carbide and graphite additives on the pressureless Sintering mechanism and microstructural characteristics of Ultra-High Temperature ZrB۲ Ceramics Composites

The effect of SiC content, additives, and process parameters on densification and microstructural properties of pressureless sintered ZrB۲– (۱–۱۰ wt %) SiC particulate composites have been studied. The ZrB۲–SiC composite powders mixed by Spex mixer with ۱-۲wt% C (added as graphite powder) and CMC have been cold-compacted and sintered in argon environment in the temperature range of ۱۸۰۰–۲۱۰۰ºC for ۲hs. The amount of densification is found to increase with sintering duration and by prior holding at ۱۲۰۰-۱۶۵۰ºC for reduction of oxide impurities (ZrO۲, B۲O۳ and SiO۲) on powder particle surfaces via the formation of new phases such as ZrSi۲ and ZrC in the system. Presence of SiC with average size smaller than that of ZrB۲ appears to aid in densification by enhancing green density, increasing C content by erosion of milling media, and inhibiting matrix grain growth. Both of SiC and C appear to aid in reduction of oxide impurities. The shrinkage of samples was measured, and the microstructure of samples was examined using X-Ray Diffraction and scanning electron microscopy (SEM), equipped with EDS spectroscopy. Room temperature mechanical properties were examined. Sintering temperature has a great effect on relative density, porosity, water absorption, hardness, fracture toughness, oxidation resistance, Strength and microstructure of these composites. The highest relative density, (۹۹.۶۵%), was obtained in ZrB۲–۱۰wt. %SiC–۲ wt. %C composites sintered at ۲۰۰۰ºC for ۲hs.