Tungsten borides are compounds of tungsten and boron. Their most remarkable property is high hardness. The Vickers hardness of WB or WB<sub>2</sub> crystals is ~20 GPa and that of WB<sub>4</sub> is ~30 GPa for loads exceeding 3 N.
Single crystals of WB<sub>2âÂÂx</sub>, x = 0.07âÂÂ0.17 (about 1 cm diameter, 6 cm length) were produced by the floating zone method, and WB<sub>4</sub> crystals can be grown by arc-melting a mixture of elemental tungsten and boron.
WB<sub>2</sub> has the same hexagonal structure as most diborides (AlB<sub>2</sub>, MgB<sub>2</sub>, etc.). WB has several forms, ñ (tetragonal), ò (orthorhombic) and ô (tetragonal).
ô-WB and WB<sub>2</sub> crystals have metallic resistivities of 0.1 and 0.3 mé÷cm, respectively. The oxidation of W<sub>2</sub>B, WB and WB<sub>2</sub> is significant at temperatures above 600 ðC. The final oxidation products contain WO<sub>3</sub> and probably amorphous B<sub>2</sub>O<sub>3</sub> or H<sub>3</sub>BO<sub>3</sub>. The melting temperatures of W<sub>2</sub>B, WB and WB<sub>2</sub> are 2670, 2655 and 2365 ðC, respectively.