Stability of nanostructured W-C phases during carburization of WO3

The formation and stability of nanostructured W-C phases during carburization of chemically synthesized WO3 were investigated by quantitative X-ray diffraction using Rietveld structural refinement. It was observed that despite a greater thermodynamic stability of WC (hexagonal; P-6m2) below the eutectoid temperature of 1523 K, WC1−x (cubic; Fm3m) was first to form followed by W2C (hexagonal; P63/mmc). However, WC1−x and W2C were metastable in the temperature range of 1123–1323 K and had a tendency to transform to W2C or WC, respectively. The observed phase formation sequence was attributed to the kinetic factors involved in the reduction of WO3 to nanostructured W (cubic; Im3m) followed by the diffusion of C in the as reduced nanostructured W leading to C-deficient tungsten carbides (WC1−x and W2C) in the surface layer. The C-deficient W-C phases gradually transformed to stoichiometric WC at elevated temperature by bulk diffusion of C.

► The sequence of formation of nanostructured W-C phases from WO3 was studied.
► WC1−x followed by W2C phase evolved ahead of a thermodynamically stable WC.
► Initial rapid C-diffusion in the as reduced nanostructured W led to WC1−x.
► Subsequent slow C-diffusion led to W2C, which transformed to WC on prolong annealing.