WC coarsening in cemented carbides during sintering. Part II: Mechanisms of the influence of different grain growth inhibitors

Abstract

It was reported in the literature that the influence of grain growth inhibitors (GGIs) on the WC grain growth during sintering of cemented carbides is related to the formation of complexions at WC/Co interfaces at temperatures of liquid-phase sintering. However, this viewpoint was not confirmed experimentally, as such complexions were found upon cooling after sintering. The influence of different grain growth inhibitors on the kinetics of WC coarsening in WC-10 wt% Co cemented carbides was investigated. The presence of complexions having a thickness of nearly 1 to 3 nm at WC/Co interfaces was established by STEM, EDX and HRTEM as a result of adding VC, Cr₃C₂ and TaC to WC + Co. WC grains in WC-Co cemented carbides containing Mo₂C was characterized by the presence of near-surface layers of (W,Mo)C having a thickness of about 100 nm and absence of complexions at the WC/binder interface. The values of activation energies for all the GGIs except for Mo₂C lie in the range typical for the solid-state diffusion-controlled processes, therefore, the solid-state diffusion of W and C atoms through the nm-thick complexions is presumably a limiting stage of WC coarsening. Considering the activation energy and distribution of heavy elements in the binder for the samples doped with Mo₂C, one can assume that in the liquid binder containing dissolved W, C and Mo atoms, molybdenum suppresses the diffusion of tungsten atoms. Therefore, the rate of the tungsten atoms' diffusion in the liquid binder is likely to be a limiting stage of WC coarsening.