high temperature stability. At high temperatures WC decomposes to tungsten and carbon and this can occur during high-temperature thermal spray, e.g., in high velocity oxygen fuel (HVOF) and high energy plasma (HEP) methods. Oxidation of WC starts at . It is resistant to acids and is only attacked by hydrofluoric acid/nitric acid (HF/) mixtures above room temperature. It reacts with fluorine gas at room temperature and chlorine above and is unreactive to dry up to its melting point. Finely powdered WC oxidizes readily in hydrogen peroxide aqueous solutions. At high temperatures and pressures it reacts with aqueous sodium carbonate forming sodium tungstate, a procedure used for recovery of scrap cemented carbide due to its selectivity. Physical properties Tungsten carbide has a high melting point at , a boiling point of when under a pressure equivalent to , a thermal conductivity of 110 W·m−1·K−1, and a coefficient of thermal expansion of 5.5 µm·m−1·K−1. Tungsten carbide is extremely hard, ranking about 9 to 9.5 on the Mohs scale, and with a Vickers number of around 2600. It has a Young's modulus of approximately 530–700 GPa, a bulk modulus of 630–655 GPa, and a shear modulus of 274 GPa. It has an ultimate tensile strength of 344 MPa, an ultimate compression strength of about 2.7 GPa and a Poisson's ratio of 0.31. The speed of a longitudinal wave (the speed of sound) through a thin rod of tungsten carbide is 6220 m/s. Tungsten carbide's low electrical resistivity of about 0.2 µΩ·m is comparable with that of some metals (e.g. vanadium 0.2 µΩ·m). WC is readily wetted by both molten nickel 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 that can be formed and the brittleness of these phases makes control of the carbon content in WC-Co cemented carbides important. In the presence of a molten phase such as cobalt, abnormal grain growth is known to occur in the sintering of tungsten carbide, with this having significant effects on the performance of the product material. Structure There are two forms of WC, a hexagonal form, α-WC (hP2, space group Pm2, No. 187), and a cubic high-temperature form, β-WC, which has the rock salt structure. The hexagonal form can be visualized as made up of a simple hexagonal lattice of