LAUSR

Abstract The macrokinetic features of combustion in the Ta-Zr-B system were studied. Combustion is characterized by spin mode, suggesting the limiting role of gas-phase mass transfer of reagents. The mechanism of chemical reactions and phase formation in combustion wave was discussed. Primary layers of tantalum and zirconium borides were detected in the preheating zone at temperatures below the melting point of the reagents. After zirconium and boron melt, the temperature in the combustion zone reaches its maximum and zirconium diboride precipitates out of the oversaturated solution. Powders with a grain size of 1–3 μm were fabricated and hot-pressed into dense ultra-high-temperature ceramics (UHTCs). Boride ceramics with the record-setting hardness of 70 GPa, Young's modulus of 594 GPa, and elastic recovery of 96% were obtained. The measured heat conductivity of the solid solution (Zr,Ta)B2 was equal to 35–42 W/m K. Plasma torch tests demonstrated high oxidation resistance of the obtained ceramics at 2900–3000 °C.