Tungsten borides

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^[1][2] and that of WB₄ is ~30 GPa for loads exceeding 3 N.^[3]

Structure of hexagonal WB₂

Structure of orthorhombic β-WB

Synthesis

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,^[1] and WB₄ crystals can be grown by arc-melting a mixture of elemental tungsten and boron.^[3]

Structure

WB₂ has the same hexagonal structure as most diborides (AlB₂, MgB₂, etc.).^[4] WB has several forms, α (tetragonal), β (orthorhombic) and δ (tetragonal).^[2]

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.^[2]

Properties

Material	Vickers hardness (GPa)	Bulk Modulus (GPa)	Melting point (°C)
W2B			2670
WB	~20		2655
WB2	~20		2365
WB4	~30