Effect of the sintering parameters on the liquid Co migration in WC-Co

The formation of a Co gradient from the surface to the bulk of a sintered WC-Co occurs during sintering, under certain condition, due to the liquid Co migration. This phenomenon leads to the formation of the functionally graded WC-Co (FG-WC-Co). Depending on the Co gradient, it is possible to modify the local mechanical properties of the material. The FG-WC-Co with a lower Co content on the surface and higher Co amount in the bulk, offers a trade-off between the wear resistance and the fracture toughness. This is important in case of tool applications. Differently, the formation of a Co layer on the surface due to a higher surface Co content is preferred in case of joining process of the WC-Co with other materials. This is the so called “Co capping”. Even if the liquid Co migration is extensively discussed in literature, the mechanism for its formation is not well understood. In this paper, to further investigate the mechanism, the effects of binder content and the sintering condition, using different near-industrial sintering cycles, were studied. Has result, it was observed that the Co migration depends on the temperature and carbon gradient into the material. In case of the absence of the temperature gradient, the Co migration occurs from a region with high C content to a region with lower C amount. If the Co migration occurs towards the surface, the necessary to minimize the surface energy, leads to the formation of a Co layer on the surface.