High temperature capillarity in hardmetal surface layers

• A mechanism of the formation of Co layers on the hardmetal surface is proposed.
• The mechanism is based on considering capillarity phenomena of WC by liquid Co.
• The mechanism explains all the known Co capping phenomena of the WC–Co surface.

A reason for the presence or absence of thin Co layers on a surface of WC–Co hardmetal articles after sintering has been a riddle for a long time. Such shiny Co layers are sometimes present on the surface of sintered WC–Co articles and their formation is referred to as “Co capping” in literature. Here we propose a detailed mechanism explaining the presence or absence of such thin cobalt layers on hardmetal articles after sintering. The proposed mechanism is based on considering wetting phenomena of WC by liquid Co in hardmetal surface layers and capillarity pressures acting on the liquid Co in narrow channels between WC grains in the surface layers. The mechanism explains all the phenomena of the Co layer formation during sintering of various hardmetal grades followed by either fast or slow cooling.