Ultra-fine Tungsten Carbide Powder Process
Transition metal carbides, particularly tungsten carbide, are rather attractive due to their physical and mechanical properties. They are characterized by the high melting point, unusual hardness, low friction coefficient, chemical inertness, oxidation resistance, and excellent electric conductivity. Nowadays, highly dispersed tungsten carbide powders appear to be very important for production of wear-resistant parts, cutters, non-iron alloys, etc.
It is well known, that fine-grained alloys demonstrate better mechanical properties in comparison with coarser alloys of the same composition under the same terms. Use of ultrafine tungsten carbide powder or nanosized tungsten carbide powders is one of the most efficient ways to produce new materials with required properties.
There are several phases of tungsten carbide; the most important ones are WC and W2C. Though W2C is unstable at T=1300°C, in most cases the mixture of WC and W2C is observed in the synthesis products. Precipitation of the single phase of WC is only possible in the narrow area of the technological parameters.
The production of pure highly dispersed powder of metal tungsten within the first stage. The initial material is very pure WO3, tungsten acid or ammonium tungstate.
The second stage includes carbonization of tungsten by carbon in the graphite furnace with hydrogen atmosphere. Depending on the type of the furnace, atmosphere, and carbon content the reaction occurs according to the scheme:
2W + C → W2C or W + C → WC.
The obtained tungsten carbide powder has particles of the indefinite melted form, minimum 3 – 5 μm in size and contains 5 % of W2C minimum. The reduction terms greatly influence the characteristics of the metal powder and forming carbide.
Thermochemical synthesis of nano-phased tungsten carbide powders was also studied. It consisted of two stages. At first, nano-phased powders of metal tungsten were synthesized by reduction of various tungsten salts and chemical decomposition of vapor of volatile tungsten compounds. Then nano-phased tungsten carbide with the particle size of ~30 nm was obtained by carbonization at low temperature in the medium of controlled active carbon-containing gas phase.
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