Phase evolution in synthesis of nanocrystalline WC-η composite powder by solid-state in situ reactions

Replacement of Co with η phase (e.g. Co₆W₆C and Co₃W₃C) is proposed as an effective approach to resist against corrosion of WC-Co coatings in environment of molten zinc. In the present work, the nanocrystalline WC-η powder was firstly synthesized as the coating material by a novel method of in situ reactions using metal oxides and carbon black as raw materials. The phase evolution in the process of in situ synthesis was investigated by thermodynamic calculations and experiments. It was found that the initial WO₃ Co₃O₄ and C transformed firstly to CoWO₄, then to W and W₂C, and eventually to η and WC. The formation of η phase decreases the energy barrier for WC formation and hence the synthesis temperature for the composite powder. The duplex phase constitution of WC and η was obtained with optimized conditions, and the composite powder had a mean particle size of about 190 nm with inner grain size of about 50 nm. The amount of Co in the composite is controllable by adjusting the carbon addition to the starting materials.