A kinetic model for cobalt gradient formation during liquid phase sintering of functionally graded WC–Co

Functionally graded WC–Co composites offer solutions to the trade-off between wear resistance and fracture toughness in WC–Co composites. Although liquid phase sintering is the most economical and viable method for producing WC–Co parts, the main challenge in fabricating a functionally graded WC–Co composite using liquid phase sintering is that the cobalt content homogenizes across the layers resulting in a sintered part with a uniform cobalt content. However, a cobalt gradient can be established after sintering by introducing an initial gradient in carbon within a WC–Co bi-layer prior to sintering. In this paper, a kinetic model is used to describe the graded microstructure as a function of sintering time and temperature as well as other factors including the volume fraction of Co3W3C (η) phase, liquid migration pressure and carbon content. The model predicts that the rate of the increase of the thickness of the WC–Co product layer with respect to the sintering time reduces with the volume fraction of η phase. This is attributed to the effect of the phase reactions between the carbon and η phase. A good agreement is observed between the model and experimental results.