Sven Johansson

Interface structure in a WC–Co alloy co-doped with VC and Cr 3 C 2

The effect of carbide additions is investigated at the atom scale by studying the interface between the cubic carbide phase and the WC prismatic facet in a VC Cr 3 C 2 co-doped WC Co alloy using high-resolution transmission electron microscopy. The alloy was prepared from WC powder about 0.5 lm in size and sintered at 1400°C for 2 h. The orientation relationship between WC and the MC carbide at the prismatic facet of WC is the same orientation as the one found at a WC/(Cr, W)C interface in Cr 3 C 2 -doped alloys. the image of WC shows triangular patterns indicating the position of the C atomic columns in this crystal. In MC crystal, some elongation of the bright areas parallel to the interface is observed on the simulation. The presence of dislocations at interfaces can be related to the parametric mismatch between the two neighboring crystals, to the presence of different stable structural units, or to an angular deviation between the crystals.

Theory of ultrathin films at metal-ceramic interfaces

A theoretical model for understanding the formation of interfacial thin films is presented, which combines density functional theory calculations for interface energies with thermodynamic modeling techniques for multicomponent bulk systems. The theory is applied to thin film formation in VC-doped WC–Co cemented carbides. It is predicted that ultrathin VC films may exist in WC/Co interfaces at the high temperature sintering conditions where most of the WC grain growth occurs, which provides an explanation of the grain growth inhibiting effect of VC additions in the WC–Co system.

First-principles derived complexion diagrams for phase boundaries in doped cemented carbides

This article reviews a method for calculating an equilibrium interfacial phase diagram depicting regions of stability for different interface structures as function of temperature and chemical potentials. Density functional theory (DFT) is used for interfacial energies, Monte Carlo simulations together with cluster expansions based on DFT results for obtaining configurational free energies, and CALPHAD-type modeling for describing the thermodynamic properties of the adjoining bulk phases. An explicit case, vanadium doped cemented carbides, is chosen to illustrate the progress in the research field and the interfacial diagram, a complexion diagram, for the phase boundary WC(0001)/Co is constructed as function of temperature and chemical potentials.

A computational study of interfaces in WC-Co cemented carbides

Interfaces in WC-Co cemented carbides have been investigated using the density functional theory (DFT). Six different model WC/WC grain boundaries are considered, together with the corresponding WC surfaces and WC/Co phase boundaries. The contribution to the grain boundary energies arising from misfit is estimated using an analytical bond order potential (ABOP) and the effect of magnetism is investigated using spinpolarized and non-spinpolarized calculations. A systematic study of adsorption of Co to WC surfaces, Co segregation to WC/WC grain boundaries and Co substitution at WC/Co phase boundaries has been carried out. Adsorption of Co to most WC surfaces is predicted, and result in a monolayer coverage of Co and sometimes a mixed Co/W or Co/W monolayer. The WC surfaces will become prewetted with Co as soon as the atoms become mobile at finite temperatures. Co substitutional segregation is predicted to all model WC/WC grain boundaries in 0.5 monolayer proportion. The segregation of Co to grain boundaries stabilizes the continuous skeleton network of hard WC grains in cemented carbides. Using the obtained interfacial energies, the wetting and the driving force for cobalt grain boundary infiltration are discussed. A dependence on the wetting efficiency on the carbon chemical potential is predicted, which could be an explanation for the better wetting observed experimentally under W-rich conditions.