Structure of Tungsten Carbide

WC structure, carbon atoms are gray

There are two forms of WC, a hexagonal form, α-WC, and a cubic high temperature form, β-WC, which has the rock salt structure. The hexagonal form can be visualized as made up of hexagonally close packed layers of metal atoms with layers lying directly over one another, with carbon atoms filling half the interstices giving both tungsten and carbon a regular trigonal prismatic, 6 coordination. From the unit cell dimensions the following bond lengths can be determined; the distance between the tungsten atoms in a hexagonally packed layer is 291 pm, the shortest distance between tungsten atoms in adjoining layers is 284 pm, and the tungsten carbon bond length is 220 pm. The tungsten-carbon bond length is therefore comparable to the single bond in W(CH₃)₆ (218 pm) in which there is strongly distorted trigonal prismatic coordination of tungsten.

Molecular WC has been investigated and this gas phase species has a bond length of 171 pm for ¹⁸⁴W¹²C.

Physical properties of Tungsten Carbide

Tungsten carbide is high melting, 2,870 °C (5,200 °F), extremely hard 8.5–9.0 Mohs scale at 22 GPa Vickers hardness (Vickers Hardness number = 2242) with low electrical resistivity (1.7–2.2×10⁻⁷ Ohm·m), comparable with metals (e.g. vanadium1.99×10⁻⁷ Ohm·m).

WC is readily wetted by both molten nickel and cobalt nikel and cobalt. Investigation of the phase diagram of the W-C-Co system shows that WC and Co form a pseudo binary eutectic. The phase diagram also shows that there are so-called η-carbides with composition (W,Co)₆C that can be formed and the fact that these phases are brittle is the reason why control of the carbon content in WC-Co hard metals is important.

Chemical Properties of Tungsten Carbide

There are two well characterized compounds of tungsten and carbon, WC and tungsten semicarbide, W₂C. Both compounds may be present in coatings and the proportions can depend on the coating method.

WC can be prepared by reaction of tungsten metal and carbon at 1400–2000 °C. Other methods include a patented fluid bed process that reacts either tungsten metal or blue WO3 with CO/CO₂ mixture and H₂ between 900 and 1200 °C. Chemical vapor deposition methods that have been investigated include:

• Tungsten hexachloride with hydrogen, as reducing agent and methane as the source of carbon at 670 °C (1,238 °F)

WCl₆ + H₂ + CH₄ → WC + 6 HCl

Tungsten hexachloride

• reacting tungsten hexafluoride with hydrogen as reducing agent and methanol as source of carbon at 350 °C (662 °F)

WF₆ + H₂ + CH₃OH → WC + 6 HF + H₂O

• At high temperatures WC decomposes to tungsten and carbon and this can occur during high temperature thermal spray, e.g. high velocity oxygen fuel (HVOF) and high energy plasma (HEP) methods.

• Oxidation of WC starts at 500–600 °C. It is resistant to acids and is only attacked by hydrofluoric acid/nitric acid (HF/HNO₃) mixtures above room temperature. It reacts with fluorine gas at room temperature and chlorine above 400 °C (752 °F) and is unreactive to dry H₂ up to its melting point.

WC has been investigated for its potential use as a catalyst and it has been found to resemble platinum in its catalysis of the production of water from hydrogen and oxygen at room temperature, the reduction of tungsten trioxide by hydrogen in the presence of water, and the Isomerisation of 2,2-dimethylpropane to 2-methylbutane. It has been proposed as a replacement for the iridium catalyst in hydrazine powered satellite thrusters.