EFFECT OF PARTICULATE VOLUME FRACTION ON MECHANICAL PROPERTIES OF PRESSURELESS SINTERED ZrB2-SiC ULTRA-HIGH TEMPERATURE CERAMIC COMPOSITES

A comparative study has been carried out on microstructure and properties of ZrB2-10, 20 or 30 volume percent SiC composites, processed by pressureless sintering at 2000 degrees C for 2 h, using B4C (3 wt.%) and C (2 wt.%) in the form of phenolic resin as sintering additives to promote densification by removal of surface oxides. The microstructures of the sintered composites have been characterized using X-ray diffraction, scanning and transmission electron microscopy. The effect of SiC volume fraction on densification, mechanical properties, thermal shock and ablation resistance of ZrB2-SiC composites has been investigated. It has been found that the relative density increases from 93 to 97%, and average matrix (ZrB2) grain size is reduced with increase in the volume fraction of SiC, which acts as grain-growth inhibitor. Young's modulus, hardness and indentation fracture toughness have been found to increase with SiC content. The thermal shock resistance studied by water quenching from temperatures of 800-1200 degrees C and ablation resistance determined by exposure to neutral oxy-acetylene flame, have been found to be maximum for the ZrB2-20 volume percent SiC composite.