Fracture Mechanisms\nand Crack Propagation in Monolayer\nTi<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> under Nanoindentation:\nThe Influence of Surface Terminations and Vacancy Defects

Monolayer MXenes are a novel class of two-dimensional\ntransition\nmetal carbides/nitrides with fascinating physicochemical properties.\nDespite recent advances in the study of MXenes’ mechanical\nproperties, a comprehensive understanding of the fundamental physical\nmechanisms that affect fracture due to surface terminations and vacancy\ndefects in MXenes under nanoindentation remains largely unexplored.\nHere, we address this gap using molecular dynamics simulations and\nnanoindentation theory to investigate the effects of surface terminations\nand vacancy defects on the fracture behavior of Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> MXenes. By inducing the rupture\nof monolayer MXenes through nanoindentation, we find that bare Ti<sub>3</sub>C<sub>2</sub> exhibits brittle fracture behavior. The presence\nof surface terminations and vacancy defects reduces the load-carrying\ncapacity and flexibility of MXenes. Interestingly, surface terminations\nincrease the stiffness of the structure, while vacancy defects have\nthe opposite effect. We also find that high concentrations of surface\noxidation impart ductile fracture characteristics to MXenes and increase\nthe maximum crack length at failure. Additionally, defects exceeding\nthe critical concentration can effectively prevent brittle crack propagation\nby causing frequent crack deflection and blunting crack tips. Combining\nthese findings, we propose a new strategy to synergistically enhance\nthe fracture toughness of MXenes through high concentrations of surface\noxidation and vacancy defects exceeding the critical concentration\nwithout significantly affecting strength and stiffness, thereby avoiding\ncatastrophic failure in MXene monolayers due to brittle fracture.\nThis work provides fundamental insights into the mechanical properties\nand fracture mechanisms of monolayer MXenes.