VC-doped WC-NbC-Co hardmetals

The influence of the addition of 0.9, 1.8, 3.6 and 5.4 wt.% VC on the microstructure and mechanical properties of WC-NbC-12 wt.% Co with 10, 20, 30 or 40 wt.% NbC hardmetals prepared by conventional liquid phase sintering in vacuum for 1 h at 1420 °C was investigated. Microstructural analysis revealed that the WC grain size as well as the amount of residual WC-Co in the sintered hardmetals was strongly influenced by the VC and NbC content. The WC grain growth could be strongly limited and even completely inhibited by the combined addition of NbC and VC, whereas the NbC phase converted into a (Nb,W,V)C solid solution during sintering. Microstructural investigation was supported by thermodynamic equilibrium calculations to predict the composition of the NbC phase as a function of the NbC and VC content in the starting powder. The fracture toughness and hardness of the hardmetals were evaluated and explained in terms of the final WC grain size and the residual WC-Co fraction in the hardmetal.