Tungsten boride

Tungsten borides are compounds of tungsten and boron. Their most remarkable property is high hardness. The Vickers hardness of WB or WB₂ crystals is ~20 GPa and that of WB₄ is ~30 GPa for loads exceeding 3 N.

Single crystals of WB_2−x, x=0.07–0.17 (about 1 cm diameter, 6 cm length) were produced by the floating zone method, and WB₄ crystals can be grown by arc-melting a mixture of elemental tungsten and boron.

WB₂ has the same hexagonal structure as most diborides (AlB₂, MgB₂, etc.). WB has several forms, α(tetragonal), β (orthorhombic) and δ (tetragonal).

δ-WB and WB₂ crystals have metallic resistivities of 0.1 and 0.3 mΩ·cm, respectively. The oxidation of W₂B, WB and WB₂ is significant at temperatures above 600 °C. The final oxidation products contain WO₃ and probably amorphous B₂O₃ or H₃BO₃. The melting temnperatures of W₂B, WB and WB₂ are 2670, 2655 and 2365 °C, respectively.

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