boron carbide

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A compound of boron and carbon, especially B₄C, an extremely hard, black crystalline compound or solid solution. It is used as an abrasive, in control rods for nuclear reactors, and as a reinforcing filament in composite structural materials.

Wiktionary

1. n. inorganic chemistry A binary compound of boron and carbon, B4C, that is a very hard ceramic, and is used in tank armour, bulletproof vests etc.