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.
Synthesis Single crystals of WB2−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.
Structure WB2 has the same hexagonal structure as most diborides (AlB2, MgB2, etc.). WB has several forms, α (tetragonal), β (orthorhombic) and δ (tetragonal).
Properties 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 temperatures of W2B, WB and WB2 are 2670, 2655 and 2365 °C, respectively.
+ Properties Material Vickers hardness (GPa) Bulk Modulus (GPa) Melting point (°C) W2B 2670 WB ~20 2655 WB2 ~20 2365 WB4 ~30
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