Hafnium(IV) carbide
Hafnium carbide is a chemical compound of hafnium and carbon. With a melting point of about 3900 °C it is one of the most refractory binary compounds known. However, it has a low oxidation resistance, with the oxidation starting at temperatures as low as 430 °C. This compound will be soon be seen on future spacecrafts functioning as heat-resistant shields.
Hafnium carbide is usually carbon deficient and therefore its composition is often expressed as HfCx. It has a cubic crystal structure at any value of x.
Hafnium carbide powder is obtained by the reduction of hafnium oxide with carbon at 1800 to 2000 °C. A long processing time is required to remove all oxygen. Alternatively, high-purity HfC coatings can be obtained by chemical vapor deposition from a gas mixture of methane, hydrogen, and vaporized hafnium chloride. Because of the technical complexity and high cost of the synthesis, HfC has a very limited use, despite its favorable properties such as high hardness and melting point.
The magnetic properties of HfCx change from paramagnetic for x ≤ 0.8 to diamagnetic at larger x. An inverse behavior is observed for TaCx, despite its having the same crystal structure as HfCx.
