Interaction of Nitrogen With WC and Nitrogen-Assisted Sintering of Hardmetals

Nitrogen-containing WC powders were prepared upon WC synthesis as well as by nitridation of commercial WC powder in order to study the impact on final grain size in hardmetals. The first show uniform and ? depending on the nitrogen pressure ? relatively high nitrogen content throughout the particles. The a and c lattice parameters of the hexagonal unit cell of such a hexagonal W(C,N) decrease as compared to pure WC. Upon hardmetal sintering nitrogen is evolved from this powder up to 1400�C due to the low nitrogen diffusivity in WC. This leads to substantial final nitrogen porosity which makes these powders inappropriate for usual hardmetal sintering. In contrast, nitrided WC powders show only slightly lower lattice parameters because of a thin nitrogen-enriched rim, with unchanged interior. This nitrogen-rich rim is dissolved upon sintering evolving all the contained nitrogen. Pore-free hardmetals can thus be obtained from this powder. The nitrogen-rich rim prevents WC from low-temperature grain growth upon sintering due to the stabilising effect of nitrogen, leading to a substantially finer final hardmetal microstructure.