Atomistic modeling of interfaces in WC-Co cemented carbides

Doktorsavhandling, 2015

Grain and phase boundaries are crucial in understanding the properties of real materials. In this thesis, interfaces in WC–Co cemented carbide are modeled atomistically using density functional theory (DFT) and analytic interatomic potentials. WC-Co is a strong and tough material of great technological importance, which is used for metal cutting, rock drilling, and various other applications. e material consists of the refractory carbide WC cemented in a ductile Co binder and is produced by liquid phase sintering of milled powders. Special and general WC/WC grain boundaries in cemented carbides are studied using DFT. The special Sigma = 2 twist grain boundary, the most frequently observed grain boundary, is studied in detail using a Peierls-Nabarro model combining DFT with elasticity theory. is grain boundary is predicted to have a low energy of about 0.7 J/m2 and be free from Co segregation, in agreement with experimental observations. For models of more general grain boundaries, DFT calculations predict larger grain boundary energies, on average 2.3 J/m2. e grain boundaries are also predicted to contain 0.5 monolayers of Co, in accord with atom probe measurements. WC and Co surfaces and WC/Co phase boundaries are also considered, and it is found that most WC surfaces should become pre-wetted with a full Co monolayer or a mixed Co/C or Co/W monolayer. e calculated interface energies are used to discuss the wetting of WC by Co, and a dependence on the carbon chemical potential is predicted, which is a potential explanation for the better wetting observed experimentally under W-rich conditions. Segregation of Mn, Fe, Co, Ni, Ti, V, Cr, Zr, Nb, and Ta, to WC/WC grain boundaries is further investigated. Fe, which is used as a binder phase in cemented carbides, is found to segregate in 0.5 monolayer proportion and have a strengthening effect on grain boundaries similar to Co. It is predicted that Ti and V may segregate to grain boundaries as monolayer thick cubic TiC and VC films of NaCl structure, and VC film segregation is found to have a potentially embrittling effect on grain boundaries. Lastly, WC/WC grain boundary sliding, which is believed to be an important high-temperature deformation mechanism in WC-Co, is investigated. An analytic interatomic potential describing WC-Co is developed and sliding simulations of two model grain boundaries show that the Co infiltration of grain boundaries lowers the required stresses by an order of magnitude in the solid state.