What is "boron carbide"

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

Boron carbide (chemical formula approximately BC) is an extremely hard boron– carbon ceramic, and covalent material used in tank armor, bulletproof vests, engine sabotage powders, as well as numerous industrial applications. With a Vickers Hardness of >30 MPa, it is one of the hardest known materials, behind cubic boron nitride and diamond.

Boron carbide was discovered in 19th century as a by-product of reactions involving metal borides, however, its chemical formula was unknown. It was not until the 1930s that the chemical composition was estimated as BC. There remained, however, controversy as to whether or not the material had this exact 4:1 stoichiometry, as in practice the material is always slightly carbon-deficient with regard to this formula, and X-ray crystallography shows that its structure is highly complex, with a mixture of C-B-C chains and B icosahedra. These features argued against a very simple exact BC empirical formula. Because of the B structural unit, the chemical formula of "ideal" boron carbide is often written not as BC, but as BC, and the carbon deficiency of boron carbide described in terms of a combination of the BC and BCBC units.

The ability of boron carbide to absorb neutrons without forming long-lived radionuclides makes it attractive as an absorbent for neutron radiation arising in nuclear power plants and from anti-personnel neutron bombs. Nuclear applications of boron carbide include shielding, control rod and shut down pellets. Within control rods, boron carbide is often powdered, to increase its surface area.