Tungsten carbide is part of the group of interstitial carbides that form from the group IV-VI transition d-metals. The compounds are formed of a combined covalent-metallic-ionic type of chemical bonding. This leads to properties that resemble a combination of metallic and covalent compounds.
For example, its metallic-like properties include high thermal and electrical conductivity (decreasing with increasing temperature). While its covalent-type properties include high harness and low plasticity 5–8. Tungsten carbide is more closely linked to the carbides formed by the body centred cubic (bcc) metals of subgroups VA and VIA 5. These compounds obey Hagg’s rule, meaning that the ratio of the atomic radius of the non-metal to the metal must be between 0.41 and 0.59 for the metal lattice to accept the non-metal (carbon) in its interstices. Tungsten carbide has a ratio of 0.553 5.
This means that carbon is significantly larger than the largest interstitial site in the tungsten lattice. The occupation of these sites by carbon leads to a change in symmetry (e.g. from body centred cubic to simple hexagonal) and a slight expansion of the lattice, resulting in stability of the structure 5. The change in crystal structure from that of the transition metal on formation of the carbide phase suggests that metal-carbon interactions dominate 8.
In the case of the bcc transition metals, carbides form with a cubic or hexagonal sublattice. This non-metallic sublattice consists of both interstitial carbon and interstitial voids (vacancies). The concentration of these vacancies and their distribution contribute largely to the carbides properties 5, 9.For tungsten monocarbide (WC), the phase of most interest, the equilibrium crystal structure is simple hexagonal with AAA metal atom packing (space group Pm2) 5, 10. This structure is not seen in pure tungsten metal and occurs due to the introduction of planes of carbon atoms. The tungsten atoms sit at the nodes of the lattice with carbon filling the 1/2, 2/3, 1/2 sites 10, 11.
Ditungsten carbide (W2C) and graphite are also of interest due to their formation either side of the line compound WC (see phase diagram in Figure 1) 5. The structure of W2C consists of a hexagonal type CdI2 structure with lattice periods a = 2.992, c = 4.721A and c/a = 1.578 10, 12, 13. Further information on the regions of stability regarding composition and temperature can be seen in Figure 1 5.ThermodynamicsTungsten carbide begins to form when the solubility limit of carbon is reached within tungsten metal.
This solubility limit is temperature dependant with a linear relationship 10, 14. Using this relationship the solubility limit of carbon in tungsten at the eutectic temperature (2710 ºC) is 0.7 at% 10, 14. This shows that even at very high temperatures, tungsten carbide will begin to form with relatively low levels of carbon. There are three main carbides of tungsten; lower W2C (which forms congruently from the melt at 3043 K), higher WC (which forms peritectically from graphite and the liquid at 3047 K) and a cubic carbide WC1-x, which also forms congruently from the melt at 3024 K, but is only stable above the eutectoid transformation at 2811 K. Many modifications of these phases have been observed to be stable within different temperature ranges (see Figure 1 5), however, many of them are not stable and are lost when annealed or mechanically worked 10