Influence of heating rate on formation of nanostructured tungsten carbides during thermo-chemical processing

The structure, morphology and growth mechanism of nanostructured W-C phases during its synthesis by thermo-chemical processing at different heating rates was investigated. Higher heating rates of 8 and 4 K/min led to the formation of core–shell type nanoparticles via sequential formation of the trigonal ε-W2C (Space group:P3¯m1) followed by the hexagonal α-WC layer (Space group: P6¯m2). It appears that the formation of the core–shell possibly initiates by the nucleation of metastable W2C to form octahedral core, which subsequently act as the nucleation site for the growth of the lower order α-WC from the surface layer. The surface layer encapsulating the metastable phase possibly restricts the coarsening of the particles to a size less than 100 nm. Slow heating rate of 1 K/min, on the other hand, leads to the formation of single phase ε-W2C with abnormal particle size growth and epitaxial morphology along (0 0 1) crystallographic plane. This observation confirms that the anisotropic lattice volume expansion in the crystalline ε-W2C induced diffusion of carbon leading to abnormal particle coarsening.