WC coarsening in cemented carbides during sintering. Part I: The influence of WC grain size and grain size distribution

Abstract

Kinetics of WC coarsening during sintering of WC-10 wt% Co cemented carbide grades was examined by use of submicron WC powders with a broad grain size distribution containing much nanograin fraction, medium-fine-grain powders with a narrow grain size distribution containing little nanograin fraction and medium-grain powders with a very narrow grain size distribution not containing nanograin fraction. Based on the kinetic curves re-constructed in the Arrhenius coordinates, values of the apparent activation energy for each carbide grade were obtained, which allowed the limiting stage of the WC coarsening process to be evaluated. The limiting stage of the WC coarsening process for the medium-grain grade is related to the diffusion of W and C atoms in the liquid binder during sintering. For the submicron grade the limiting stage of the WC coarsening process is related to interfacial reactions at WC/liquid interfaces. In this case, the diffusion of W and C atoms dissolved in the liquid binder is fast due to a very significant difference between sizes of the fine/nano WC grains and coarser WC grains. When increasing the sintering temperature and time above a certain level, the significant acceleration of WC coarsening takes place indicating the transition of the process to the stage of anomalous grain growth, at which the formation of abnormally large WC grains in the microstructure determines the whole WC coarsening process. The limiting stage of the WC coarsening process for the medium-fine-grain grade is mainly related to diffusion of W and C atoms dissolved in the liquid binder.