Loading...
Thumbnail Image
Publication

Atomic-scale deformation mechanisms in MAX phases and MXenes

Plummer, Gabriel
Research Projects
Organizational Units
Journal Issue
Embargo Expires
2023-09-30
Abstract
MAX phase ceramics and their two-dimensional derivatives, MXenes, are both large, emerging families of materials. MAX phases, ternary metal carbides and nitrides, have a layered crystalline structure in which strongly-bonded ceramic layers are interleaved with more weakly-bonded metallic layers, and MXenes can be produced via selective etching of these layers. Both systems have received significant research attention due to a number of unique, highly tunable properties. Despite this, there remain a number of fundamental knowledge gaps, especially regarding their deformation, which currently keep them in the early development stages for many applications. Therefore, the work in this thesis is focused on providing insights into the atomic-scale, structure-property relationships associated with their mechanical behavior through novel atomistic simulations. To enable these simulations, new, highly-accurate interatomic potentials were developed and validated against relevant properties. Several different key deformation mechanisms were investigated in MAX phases, including basal slip and dislocations, kinking, and delamination cracking. Calculations of basal dislocation core structures revealed that rearrangement of the weakly-bonded metallic layer can result in less-mobile dislocations, but dislocation pairs can also form, which become much more mobile due to greater core spreading. Kinking was determined to result from a coupling of atomic layer buckling and subsequent nucleation of dislocations. Importantly, kinked regions remain highly elastically strained, which can contribute to the reversibility of the mechanism. Delamination cracking is closely linked to kinking, as when enough dislocations accumulate during the process, they can spontaneously nucleate a cleavage crack at the metal-ceramic interface. In MXenes, simulation results demonstrated that superior in-plane mechanical properties can be achieved through increased monolayer thickness and surface termination coverage, albeit at the cost of out-of-plane flexibility. The presence of vacancies can significantly degrade MXene mechanical properties, but their effect can be mitigated through appropriate surface termination engineering. Overall, the fundamental insights gained from these atomic-scale deformation studies of both MAX phases and MXenes should serve as a solid foundation for better understanding the origin of their unique mechanical properties and provide a guide to help better engineer them for specific applications.
Associated Publications
Rights
Copyright of the original work is retained by the author.
Embedded videos