A computational study of interfaces in WC–Co cemented carbides

Abstract

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.

Interfaces between Model Co-W-C Alloys with Various Carbon Contents and Tungsten Carbide

“…Therefore, incomplete wetting of WC by the model binder alloy with the high carbon content is presumably related to the special features of the Co-based alloys oversaturated with carbon, which corresponds well to the theoretical prediction that is obtained in ref. [ 3 ].…”

“…In ref. [ 3 ], a dependence of the wetting efficiency on the carbon chemical potential in the binder phase is predicted, which can explain the better wetting of WC that was observed experimentally at low carbon contents.…”

Fracture Energy Measurement of Prismatic Plane and Σ2 Boundary in Cemented Carbide

“…The cleavage energies (defined as twice the surface energy) in these two scenarios have been determined to be 13.85-14.09 Jm À2 and 7.25-7.33 Jm À2 , respectively, in the literature. [14][15][16] Four W-C bonds are broken per unit surface in the former compared to two in the latter. This explains the difference in energy by a factor of 2.…”

Interface characteristics in cemented carbides with alternative binders

“…Indeed the performance of the alloys is closely related to the existence of a carbide skeleton across the material and grain boundaries are constituents of the carbide skeleton. For cobalt binder, calculations using the density functional theory (DFT) predict that half a monolayer (0.5 ML) of cobalt segregates at grain boundaries, which decreases their energy and prevents the complete infiltration during sintering [23]. Experimental investigations by energy dispersive X-ray spectroscopy (EDS) in transmission electron microscopy (TEM) confirmed the presence of cobalt in grain boundaries [24].…”

“…Carbide/binder interface structure and energy are also important parameters to consider as they influence the wettability of the carbide phase by the binder, the sintering process as well as the strength of interfaces [23,32]. Lattice coherency at interfaces also seems an important feature that would improve the strength of the alloy [33].…”