Hafnium Carbonitride

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hafnium carbonitride is an advanced ceramic that has a high melting point and good thermo-mechanical properties. It is one of the most promising materials for future spacecraft components and reusable rocket modules because it can withstand extremely high temperatures.

Atomistic simulations have predicted that a material made with the right ratio of hafnium, nitrogen and carbon could have a melting point significantly higher than the previously highest-known melting point for any compound: more than 4,400 kelvins (the temperature of the surface of the Sun). Experimental testing in 2020 has confirmed that this material exists: a hafnium nitride-carbonitride mixture called (Hf,Ta)CN that melts at an even higher temperature than hafnium carbide.

This article reports the synthesis of this new material via solid-gas mechanochemical reaction in a planetary ball mill from elemental Ti, B and C under a nitrogen atmosphere. The mechanochemical reactions were accompanied by a chemical dissociation process in the sample to form a mixed phase exhibiting a face-centred cubic structure. The (Ti,B)CN phase was further densified by spark plasma sintering to form dense single-phase Hf(Ti,B)CN ceramics. The oxidation resistance of these materials was assessed by thermogravimetric analysis and static oxidation experiments under ambient conditions. The results show that the (Hf,Ta)CN ceramics exhibit a low oxygen diffusivity owing to the formation of a protective HfSiO 4 barrier layer.

The structural, electronic and elastic properties of the non-stoichiometric transition metal carbonitrides TiNxC1-x, ZrNxC1-x and HfNxC1-x (0x1) with rock-salt structures were analyzed using density functional theory. The lattice constants, densities and bulk moduli of these compounds were found to be linear functions of the nitrogen concentration x. The magnetic behavior of these compounds was also studied and a transition from paramagnetic to diamagnetic was observed with increasing x.