Nanostructured tungsten carbides by thermochemical processing

Influence of the precursor structure on nanostructured W-C phase evolution during reduction and carburization of WO3 under hydrogen environment was investigated. Nanostructured WC was obtained from a chemically synthesized precursor consisting of hexagonal, monoclinic and partially amorphous WO3 and amorphous C by heat treatment at 1000 degrees C. The phase analysis of the heat treated product also revealed a small fraction of amorphous WC. No trace of W2C, W or WC1-x could be detected. Under identical heat treatment condition, a precursor comprising of commercial grade crystalline (monoclinic) WO3 and graphite (crystalline C) led to pure W2C while a precursor containing commercial grade crystalline (monoclinic) WO3 and partially amorphous carbon (chemically synthesized) resulted to a blend of WC and W2C phases. The W2C phase, as obtained from the commercial grade WO3, was orthorhombic (Pbcn). In contrast, the W2C derived from the chemically synthesized precursor showed hexagonal (P6(3)/mmc) structure. The mean crystallite size of pure WC, as estimated by Warren-Averbach's Fourier analysis of the XRD peak profile, was about 38(+/- 5) nm. The pure W2C, on the other hand, was quite coarse (0.5-0.8 mu m). The difference in the type of product phase(s) was attributed to the improved interface between the reacting species in case of chemically synthesized WO3 and C. (C) 2010 Elsevier B.V. All rights reserved.