Reduction of carbide grain growth in WC-VC-Co by sintering in a nitrogen atmosphere

In order to produce a harder and more wear resistant material than WC-Co, a WC-VC-Co hardmetal was sintered in nitrogen at a pressure of 1 bar to reduce the growth of (W,V)C grains created during sintering. The hardmetal was analysed using scanning and transmission electron microscopy (SEM and TEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffractometry (XRD). The cubic carbonitride (W,V)(C,N) grains were found to be of two types, those with higher vanadium content and the others with lower vanadium content. The presence of nitrogen restricted the growth of the cubic carbide grains giving a much narrower grain size distribution than in a material produced from an identical powder but sintered in vacuum. The nitrogen potential was higher in the bulk than in the sintering atmosphere resulting in outward diffusion of nitrogen and inward diffusion of V. This created a gradient zone of approximately 50 μm depleted of cubic carbonitride and enriched with binder phase. Remarkably, the WC grain size in the gradient zone was not larger than in the bulk.