Wetting of surfaces and grain boundaries in cemented carbides and the effect from local chemistry

Artikel i vetenskaplig tidskrift, 2019

Wettability is an important factor in the sintering of cemented carbides. We study wetting of surfaces and grain boundaries in WC–Co och WC–Ni cemented carbides using density functional theory (DFT). Based on experimental observations of WC grain orientations in cemented carbides, relevant model interfaces are created. The local chemical composition at the interfaces is taken into account by substitution of interface atoms, and the effect of temperature is evaluated based on a companion study of temperature dependent interface energies in cemented carbides. The results indicate that the wettability of Ni and Co on WC surfaces are similar. Furthermore, the wettability of Co on the commonly occurring basal WC surface is better in W-rich materials compared to C-rich materials. At liquid phase sintering temperatures we get perfect wetting in W-rich materials, while only partial wetting in C-rich materials, which is in agreement with recent experiments on wetting in WC–Co cemented carbides. The segregation of binder phase atoms to WC/WC grain boundaries stabilize grain boundaries and make them more resistance against infiltration (wetting) by binder phase. We find that the amount of dissolved binder atoms in essentially all studied WC/WC grain boundaries are of half a monolayer proportion, which is in agreement with experimental studies. Further, in WC–Co there is a stronger resistance against grain boundary infiltration compared to WC–Ni. We find that the continuous skeleton of WC grains seen after sintering and which is crucial for the superior mechanical strength of the material exists already during liquid phase sintering.