Ultrahigh temperature ceramics (UHTCs) based on ZrB₂ and HfB₂ systems: powder synthesis, densification and mechanical properties

Phase pure ZrB2 and HfB2 powders were synthesized by boro/carbothermal reduction (BCTR) of ZrO2 and borothermal reduction of HfO2, respectively. Combining the thermodynamic calculations and XRD results, the reaction process of BCTR of synthesized ZrB2 powder was shown. The ZrB2 powder synthesized at 1650 degrees C showed a better sinterability, due to its smaller particle size and lower oxygen content. Most of the HfB2 particles obtained at 1600 degrees C presented nearly column-like morphology with 1-1.5 mu m size. Using the synthesized HfB2 powder and commercial alpha-SiC, hot-pressed HfB2-20 vol.% SiC composite has high flexural strength of 993 +/- 90 MPa and fracture toughness of 6.29 +/- 0.65 MPa.m(1/2), which were significantly higher than the reported values in literature.