A computational study of interface structures and energetics in cemented carbides and steels

Doktorsavhandling, 2010

Cemented carbides are hard composite materials of great industrial importance. Because of their combination of hardness and toughness, they are used e.~g. for cutting, drilling, turning and milling. The material is produced by means of powder metallurgy, where powders of carbide and metal are sintered together into a hard and dense material. To retain a fine WC-Co microstructure, additions of VC, Cr3C2, TiC etc. are often made to inhibit WC grain growth. In this thesis, a computational study of interface structures and energetics in WC-Co cemented carbides is presented. The investigation is performed in the density functional theory framework with a plane-wave pseudopotential method. To make predictions of interface energies, coherent atomic interface configurations are used. We have calculated the stability of thin layers of cubic TiC, VC, CrC, NbC, MoC, HfC and TaC in the interface between WC and Co. Due to a lowering of the WC/Co interface energy, we predict that thin VC films of two atomic layers are stable at the basal WC/Co interfaces at high temperature sintering conditions under which grain growth occurs. The predicted atomic structure is in agreement with available high-resolution electron microscopy images. A comparison between the stability of various carbide films in both basal and prismatic WC/Co interfaces provides a consistent explanation for the experimentally known effectiveness of VC as grain growth inhibitor. Furthermore, we have extended our modeling of interfaces to include the effect of misfit in semicoherent interfaces. Information of interface energetics from atomistic simulation is coupled to a continuum model of elasticity in the Peierls-Nabarro scheme to account for misfit dislocations. The method is applied to the low misfit Fe/VN interface in steels, and we find that its interface energy is dominated by elastic energy.