Cobalt capping on WC–Co hardmetals. Part I: A mechanism explaining the presence or absence of cobalt layers on hardmetal articles during sintering

Abstract A reason for the presence or absence of Co thin layers on a surface of WC–Co hardmetals during sintering has been a riddle for a long time. As a result of sintering, such thin shiny Co layers are sometimes present on the surface of WC–Co articles and their formation is referred to as “Co capping” in the literature. Although the cobalt capping phenomenon is discussed in the literature, its mechanism is not well understood and there is no information on its technological implementations. Here we propose a new mechanism explaining the presence or absence of cobalt layers on hardmetal articles during sintering. The proposed mechanism is based on considering wetting phenomena of WC by liquid Co on the surface of hardmetal articles and capillarity phenomena acting on the liquid Co in narrow channels between WC grains in the hardmetal near-surface layer. The mechanism explains all the phenomena of the Co layer formation during sintering of various hardmetal grades followed by either fast or slow cooling.