Date of Award
Doctor of Engineering (DEng)
Micro and Nanoscale Systems
Multi-scale atomistic calculations were carried out to understand the interfacial features that dictate the mechanical integrity of the metal/ceramic nanolaminates. As such, first principles density functional theory (DFT) calculations were performed to understand the electronic and atomistic factors governing adhesion and resistance to shear for simple metal/ceramic interfaces, whereas molecular dynamics (MD) simulations were performed to observe the impact of interfacial structures, such as misfit dislocation network geometries and orientation relationships, on interfacial mechanical properties.
For the DFT investigation, we choose metals with different crystal structures, namely - Cu (fcc), Cr (bcc) and Ti (hcp) along with a variety of NaCl-type ceramics (TiN, VN and CrN) to identify common descriptors for metal/ceramic interfacial behavior. Electron density was found to be a good descriptor of shear barrier heights between metal layers. DFT calculations were also used to explore doping-assisted strengthening of metal/ceramic interfaces. As such, Ti/TiN metal/ceramic interface was doped with Al, V and Cr, whereas Cu/TiN was doped with Ni, Zn and Sn. Only Al dopants had negative enthalpies of mixing at the Ti/TiN interface. For Cu/TiN, enthalpy of mixing dictated that Sn was not energetically suitable for doping at the interface, whereas both Ni and Zn were.
The addition of Al increased the barrier to shear displacement of pure Ti by ~18 % when they were in adjacent atomic layers. There was a general correlation between higher resistance to shear and the Al concentration at the Ti/TiN interface, which were attributed to two effects: destabilizing Al-Al interactions and Al drawing electron density from the ceramic into the Ti phase. In the case of Cu/TiN, Ni segregated at the interface forming sub-nanometer interlayers between Cu and TiN, whereas Zn formed a solid solution with Cu. Ni interlayers increased both the shear (by a factor of ~3.75) and tensile strength (~17%) to a significant degree coinciding with an increase in electron density between the layers. Using analysis form their partial density of states, Ni interlayers were found to accept more electrons from interfacial Ti into their more compact 3d-orbitals than Cu, which accepted more into available 4s-orbitals.
A new modified embedded atom method interatomic potential was developed to study semi-coherent metal/ceramic interfaces involving Cu, Ti and N, such as Cu/TiN and Ti/TiN. The MD simulations performed using our newly developed MEAM model suggested that interfacial energetics is not the dominant factor in selecting the orientation relationship of the Cu/TiN interface, and pointed to the role of kinetic pathways in selecting the actual orientation. In addition, the models were applied to study semi-coherent Ti(0001)/TiN(111) and Cu(111)/TiN(111) systems as well. Ti/TiN was stable with misfits accommodated away from the interface. Cu/TiN, in contrast, was more stable with misfits at the interface. A spiral pattern in the misfit dislocation networks was observed away from the Cu/TiN interface, similar to the metal/metal (111) semi-coherent interfaces. The theoretical shear strength calculated for Ti/TiN when the misfits were several layers away from the interface (ranging from 1200-1800 MPa) and for Cu/TiN with the misfit at the chemical interface (1.65 MPa), had reasonable agreement with experiment.
Miraz, Abu Shama Mohammad, "" (2021). Dissertation. 65.