silicon carbide

Silicon carbide is a compound of silicon and carbon, SiC, and it is one of the hardest and most heat-tough materials humans make, second only to diamond and a few others in hardness. First made in a furnace in 1891 and sold as carborundum, it has been grinding, sanding, and polishing the world ever since, and today it also lines rocket nozzles, brake discs on supercars, and power electronics that run scorching hot.

Its toughness of bonding comes from being almost purely covalent, essentially diamond with every other carbon swapped for silicon: each atom bonds strongly and directionally to four neighbours in a rigid tetrahedral network. Those strong short bonds give a hardness around 9.5 on the Mohs scale, a very high stiffness (Young's modulus near 400 GPa), stability to well above 1600 degrees C, and excellent resistance to wear and chemical attack. Unusually for a ceramic it also conducts heat very well and, when doped, conducts electricity, which is why it is prized for high-temperature, high-power semiconductor devices. It is hard to sinter, so it is often made by reaction bonding or by growing it directly from vapour.

As a structural ceramic SiC is the go-to when you need hardness plus real heat resistance together, for abrasives, cutting tools, kiln furniture, pump seals, armour plate, and heating elements. The same honest caveat applies as to all ceramics: for all its hardness and heat resistance it is brittle, with modest fracture toughness, so it excels under compression and wear but shatters under impact or sudden thermal shock. Its low, controlled thermal expansion helps it survive temperature swings better than most, but it is still a ceramic, not a metal.