Atomic-scale modelling of interfaces in cemented carbides: wetting and strength

Licentiate thesis, 2017

Cemented carbides are composite materials of great technological and industrial importance. They have a unique combination of hardness and toughness and are therefore used widely as tool materials in applications where both high hardness and toughness are demanded. These applications include among others: mining, turning, cutting and milling. Cemented carbides are powder metallurgical products where powders are mixed, pressed, and sintered into a dense material. These materials consists of a hard carbide phase embedded in a more ductile metal binder phase. The interfaces in cemented carbides highly influence the behavior during sintering and consequently the microstructure of the final material. Further, the unique mechanical properties of cemented carbides are to a large degree dependent on its microstructure and the stability and strength of its interfaces. This thesis is a computational study of interfaces in WC--Co, WC--Fe, and WC--Ni cemented carbides. Density functional theory is used to calculate interface energies and the local chemistry of interfaces is varied by altering the termination of WC planes and by substitution of W and C atoms for binder phase atoms. In paper I we investigate wetting of WC surfaces and WC/WC grain boundaries by the binder phase, and the results indicate that the wettability of Ni on WC surfaces is potentially better compared to that of Co, which in turn is better than that of Fe. Additionally, the wetting of WC surfaces is better in W-rich alloys compared to C-rich alloys regardless of binder, which is in agreement with experiments on wetting in WC--Co cemented carbides. In paper II the segregation to WC/WC grain boundaries and its effect on the interface strength is investigated. We show that the segregation of binder phase atoms to WC/WC grain boundaries is in 0.5 monolayer proportion in essentially all studied grain boundaries in the three different cemented carbides. The segregation generally strengthens the grain boundaries regarding its resistance against fracture and infiltration by binder phase.