Tungsten carbide powders are obtained by direct carbonization of tungsten powder.
The synthesis of bulk tungsten carbides by carburization of W metal or of WO3with mixtures of CH4in hydrogen at various pressures has been studied in temperature programmed experiments. The resulting solids have been characterized by elemental analysis, X-ray diffraction, XPS analysis, and specific surface area measurements. The carburization occurs in two distinct steps: W2C is formed in the first step taking place at about 650°C at atmospheric pressure with a 20% CH4–H2mixture, while the formation of WC occurs only at higher temperatures. During carburization some free carbon is deposited, the importance of which is very much dependent on CH4partial pressure and on the temperature of carburization. It has also been shown that direct carburization of WO3by CH4–H2does not take place, but that the carburization occurs via the reduction of WO3to W metal. The rate of reduction of WO3and that of carburization of W metal are very much dependent on, respectively, hydrogen partial pressure and CH4partial pressure. The extent of reduction of WO3into W metal required for carburization which takes place also depends on CH4partial pressure, indicating a competition between carburization of W metal at the surface and diffusion of W metal into the bulk of the solid.
Submicron tungsten carbide (WC) powder is obtained with ammonium tungsten bronze (ATB). The samples are characterized by laser particle size analyzer and scanning electron microscope (SEM). The analysis confirms that submicron tungsten powder is obtained with ATB through the deoxidation process in the hydrogen gas (Rate of purity: 99.99 %, dew point: -40 °C). And then submicron tungsten powder is carbonized in the hydrogen gas. The volume percent content of submicron WC powder between 0 μm and 1 μm is 83.7 %. Therefore, ATB is advantageous to prepare submicron WC powder via the conventional deoxidation and carbonization processes.
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