In the search for materials that can withstand high temperatures the melting points of materials produced by a type of ionic substitution known as interstitial solid solution using the metallic elements tantalum (Ta) and hafnium (Hf)(where carbon fills the octahedral gaps between in the structure of the transition metal) were recently measured accurately. The resulting carbides are not only particularly hard but have a high measure of specific gravity.

Under extreme conditions such in high-power jet or rocket engines and leading edges of hypersonic aircraft and re-entry heat shields for space vehicles etc. the temperature resistance of the material is of critical importance. For this reason it was important for the researchers at London’s Imperial College to determine the precise melting point of tantalum carbide and hafnium carbide which was last measured around fifty years ago. Using laser heating technology they ascertained that HfC holds the crown with a melting point of 3,958 °C while TaC can ‘only’ manage 3,768 °C before it wilts. The kicker is the cost of these rare materials with Hafnium currently at 2,700 $/kg. A carbide alloy (TaC4HfC5) however because of the greater abundance of tantalum works out at ‘just’ 260 $/kg.
Recent reports indicate an alloy of the material melts at an even higher temperatures at around 3,990 °C but this has yet to be confirmed and production cost of the alloy is quoted at 9,500 $/kg.

Interestingly, these carbides which have a structure similar to ordinary cooking salt (NaCl), are both harder than steel and about twice as heavy. The density of HfC is 12.2 g/cm³ and TaC = 13.9 g/cm³.