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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 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.

MaterialVickers hardness (GPa)Bulk Modulus (GPa)Melting point (°C)
W2B2670
WB~202655
WB2~202365
WB4~30

References

References

  1. (1995). "Preparations and Some Properties of W2B, δ-WB and WB2 Crystals from High-Temperature Metal Solutions". Japanese Journal of Applied Physics.
  2. (1995). "Preparation of WB2−x single crystals by the floating zone method". Journal of Crystal Growth.
  3. (2011). "Tungsten tetraboride, an inexpensive superhard material". Proceedings of the National Academy of Sciences.
  4. (1966). "Tungsten Diboride: Preparation and Structure". Science.
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