a transition-metal carbide

Start with a close-packed lattice of heavy metal atoms — tantalum, hafnium, titanium, tungsten — and slip small carbon atoms into the roomy gaps between them, the interstitial holes. The result is a transition-metal carbide, an 'interstitial compound' where the little carbon guests brace the metal cage from inside. That small change buys astonishing properties: some of the highest melting points of any solid. Tantalum carbide (TaC) and hafnium carbide (HfC) melt near 3900 degrees C, and a mixed Ta-Hf carbide holds the long-standing record near 4000 degrees C — hotter than almost anything else will tolerate as a solid.

The important carbides are TiC, ZrC, HfC, TaC and WC. For the early transition metals (groups 4 and 5) the monocarbide usually takes the rock-salt structure — carbon in the octahedral holes of a face-centred-cubic metal array — and it tolerates wide nonstoichiometry, sitting happily as MC_x with x below 1 (many carbon sites simply left empty). Like the borides they conduct heat and electricity, and they are extremely hard, roughly 20 to 30 GPa. Tungsten carbide is the odd one out structurally (hexagonal) but is by far the most used: bonded with cobalt metal it becomes cemented carbide, the everyday material of drill bits and cutting inserts.

Carbides matter as ultra-high-temperature ceramics for the very sharpest, hottest leading edges, as the hard phase in cutting tools and wear parts, as hard coatings, and even as nuclear fuel and cladding (uranium and silicon carbide). The honest caveats echo the diborides: for all their sky-high melting points, they oxidise readily in hot air (the melting point advertises the wrong strength — it is oxidation that limits service), they are brittle, and they are so refractory that sintering them dense needs very high temperatures and usually applied pressure.