boron carbide

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.