The Structure and Properties of the Promising Ultra-High-Temperature HfB₂–HfC–SiC Ceramics Obtained from Heterophase SHS Powders

This work continues the earlier studies focusing on fabrication of heterophase micropowders and consolidated ceramics based on HfB₂–HfC–SiC ultra-high-temperature boride/carbide compositions via self-propagating high-temperature synthesis (SHS) and hot pressing (HP). The effect of NH₄Cl addition on the morphology and microstructure of the SHS powders was studied. Composite micropowders characterized by particle size of 0.2–10 μm and 40–50% content of the submicron-sized fraction were fabricated. The structure, mechanical and thermophysical properties, kinetics and mechanism of high-temperature oxidation of hot-pressed ceramic materials composed of 57–72 wt % HfB₂, 14–20 wt % HfC_x, 10–14 wt % SiC, and 8–15 wt % HfO₂ were studied. They are found to have hardness up to 18.9 GPa, crack resistance up to 9.7 MPa m^0.5, bending strength up to 400 MPa, temperature diffusivity up to 22.6 mm²/s, and thermal conductivity up to 59 W/(m K). The power law describes their oxidation kinetics. The protection mechanism against oxidation involves the formation of a multilayered heterogenous oxide film consisting of HfO₂, HfSiO₄, and borosilicate glass.