Reactive Hot Pressing of HfB2-SiC-Ta4HfC5 Ultra-High Temperature Ceramics

Highly disperse and reactive (HfB2-SiC)@(Ta2O5-HfO2-C) composite powders were manufactured by sol-gel technology where Ta2O5-HfO2-C amorphous components were nanostructured and distributed in each other as uniformly as possible. The reactive sintering of the prepared composite powders at a relatively low temperature (1800 degrees C) with an exposure time of 30 min and the pressure 30 MPa yielded (HfB2-30 vol % SiC)-xTa(4)HfC(5) ultra-high temperature ceramics (UHTCs), where x = 5, 10, and 15 vol %, with a relative density of 75-78%. X-ray powder diffraction proved the complete conversion of tantalum and hafnium oxides to complex carbide Ta4HfC5. The average grain size as determined by scanning electron microscopy did not exceed 2-3 mu m in HfB2 and 30-60 nm for the Ta4HfC5 phase. Thermal analysis in flowing air showed that, in the temperature range 20-1400 degrees C, the increasing percentage of tantalum-hafnium carbide (the least oxidation resistant phase) leads to a systematic increase in oxidation-induced weight gain; however, a tendency to saturation is observed. The microstructural specifics of the oxidized surface are noticed depending on the composition of HfB2-SiC-Ta4HfC5 ceramics.