Gas-Solid Reaction of Ammonium Paratungstate(APT) to Produce Tungsten Carbide

The main application of tungsten carbide (WC) is the production of hardmetal, or cemented carbide Cemented carbides are composite materials consisting of a hard phase (WC) and a metal, Hardmetal is used for cutting, drilling and wear resistant pieces. Tungsten carbide used for producing hardmetal is conventionally synthesized by heating a mixture of metallic tungsten powder and carbon black in a graphite furnace under a flowing hydrogen atmosphere. The carburization reaction is described as a solid state reaction between W and C. This reaction is re- markably dependent on the diffusion of C in W. There- fore the reaction temperature is high and lies in the range 1400–1800°C. This technique is able to produce WC powders with a wide range of mean particle sizes.

The conventional synthesis technique is not able to produce powders under 150 nm in grain size. Very fine WC powders are also attractive for use in hardmetal because fine grained alloys exhibit higher hardness than coarser grained ones of the same composition, at the same toughness level.

The common solid reaction to produce WC requires a rather high temperature.  A new gas-solid synthesis procedure has been developed with lower temperatures and shorter production period.

Experimental procedure

  Ammonium paratungstate (APT) and tungsten blue oxide (TBO) are used as the tungsten sources. APT is used as received. TBO is prepared by heating APT, after milling it to reduce its particle size, at 600°C for 2 h under N2 flow.

CH₄ (99%) and H₂ (99%) are used as the carbon source and reducer. The carburization reaction is carried out in a horizontal alumina fixed bed reactor. A boat with 3 g of APT or TBO is placed into the reactor. The tempera- ture is raised at 5 °C min⁻¹ up to 850°C for APT and 820°C for TBO. The soaking time is 2 h in both cases. During heating and isotherm, a gaseous mixture CH4+H2 flows through the reactor at a rate of 20 lh−1. The composition of the mixture (95% H2–5% CH4) is controlled by two Brooks 5850TR flowmeters. APT and TBO were characterized by X-ray diffraction. TG/DTA of APT in argon was carried out to show how it decomposes. The carburized products of APT and TBO were characterized by XRD, SEM and laser scattering particle size measurement.

Conclusion

The WC particles have roughly the same size and shape as the APT and TBO particles. They consist of WC crystallites <1 mm.  The synthesis of WC through a gas–solid reaction between APT or TBO and a mixture of H2 and CH4 is possible at low temperature in short time due to the high reactivity of the tungsten precursors and the good contact between the reactants. The carburization of TBO should proceed through the reduction of different intermediate tungsten oxides up to tungsten metal by H2 and then the carburization of W by methane.

The high reactivity of APT and TBO are due to the decomposition and reduction steps, during which suc- cessive nucleation of different phases takes place, increasing the density. The small nuclei and the porosity created by the density change increase the reactivity. The reduction step is a very important part of the process since it can control the size and shape of the tungsten particles which will be carburized.

In short, this gas-solid synthesis method uses APT and TBO as the material, to produce very fine WC powder at a low temperature (850°C) and takes a shorter time. The size of the product is < 1 µm.

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