Temperature dependent atomic-scale modeling of interfaces in cemented carbides

Doctoral thesis, 2020

Material properties can now be calculated directly from first principles using density functional theory (DFT) which has a great predictive power and can, in cases that are difficult to approach experimentally, provide crucial insights on the atomic and electronic level. Such cases include the thermodynamics of interfaces and surfaces which are crucial factors for the structure and macroscopic properties of many materials, where cemented carbides are one example.

Cemented carbides, or hardmetals, are composite materials manufactured by means of powder metallurgy, where carbide and binder metal powders are mixed, pressed, and sintered into a dense material. In this way the material gets a unique combination of hardness from the carbide and toughness from the binder. Cemented carbide is, therefore, an excellent choice of material in application where high hardness, wear-resistance, and toughness are crucial.

In this thesis bulk, interface, and surface thermodynamics in cemented carbides are studied using DFT, but also using other atomistic descriptions derived from DFT including analytical bond order potential (ABOP), cluster expansions (CE) and force constant (FC) models. Further, free energies are calculated using methods such as thermodynamic and temperature integration from both molecular dynamics (MD) and Monte Carlo (MC) simulations, quasi harmonic approximation (QHA), effective harmonic models (EHM) from ab-initio molecular dynamics (AIMD), surface stress for liquid surface free energy and calculation of work of adhesion from separation and joining simulations.

Wetting of WC surfaces and WC/WC grain boundaries is investigated in WC-Co and WC-Ni cemented carbides at elevated temperatures and it is concluded that, at liquid sintering temperatures, wetting of WC surfaces is only partial in C-rich materials while perfect in W-rich materials. Further, WC/WC grain boundaries are predicted to be stable also at liquid phase sintering temperatures. WC/WC grain boundary sliding is shown to be facilitated by infiltration of binder phase of only a few atomic layers proportion. Moreover, the hexagonal and cubic WC phases are investigated at high temperatures and a phase diagram is generated. Finally, the formation of thin cubic carbide films (complexions) in WC/Co phase boundaries is studied in both undoped and Ti-doped cemented carbides. These films are predicted at liquid phase sintering temperatures in both cases and also at solid state sintering temperatures in the Ti-doped case. In Ti-doped cemented carbides, the Ti atoms are found to mostly segregate to the second layer of the thin film and leave an essentially pure W layer towards Co.