Boron Carbide

Boron carbide (chemical formula approximately B₄C) is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications. With a Mohs hardness of about 9.497, it is one of the hardest materials known, behind cubic boron nitride and diamond.

Boron carbide was discovered in the 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 B₄C. 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 B₁₂ icosahedra. These features argued against a very simple exact B₄C empirical formula. Because of the B₁₂ structural unit, the chemical formula of "ideal" boron carbide is often written not as B₄C, but as B₁₂C₃, and the carbon deficiency of boron carbide described in terms of a combination of the B₁₂C₃ and B₁₂C₂ 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. 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.