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Hafnium Carbonitride for Hypersonic Vehicles

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hafnium carbonitride (HfC) is a highly refractory mixed anion ceramic material composed of hafnium, carbon and nitrogen. It has many desirable properties including a high melting point and hardness. However, synthesis of HfC is challenging as it requires a high temperature and long processing time. Currently, HfC is manufactured using powder metallurgy from reduction of hafnium(IV) oxide or by carbothermal reduction from its oxide precursors. These methods require a substantial amount of energy and produce large and heterogeneous samples.

Recently, atomistic simulations have predicted that a Hf-C-N material could exhibit a higher melting point than hafnium carbide and Ta4Hf1C5 (TaTaC). To validate this prediction, a series of niobium-hafnium carbide (HfTa) compounds were synthesized by self-propagating high-temperature synthesis. The stoichiometric and metastable compositions of these samples were determined by thermodynamic analysis, while the oxidation behavior was investigated experimentally. The results demonstrated that the addition of nitrogen enhances the stability and oxidation resistance of (Hf,Ta)C compounds.

Hot structures for hypersonic vehicles need excellent thermal shock resistance and strength at elevated temperatures. Carbon/carbon (C/C) composites are considered the best candidate for these applications. Unfortunately, C/C composites rapidly oxidize at elevated temperatures limiting their use to air-cooled applications. Current protective coatings of C/C composites fail at elevated temperatures or experience poor bonding with the substrate due to a lack of thermal conductivity. This effort developed a hafnium carbo nitride (HfCN) coating for C/C composites that was processed using a new method called reactive solution infiltration.