Mechanical properties of nanoscale and atomic scale materials: Nanocrystalline copper and graphene

The first part of this study is about the mechanical properties of free-standing nanocrystalline Cu films. A versatile fabrication process using micromachining techniques is established to prepare submicron thick free-standing Cu films with an average grain size of 40 nm. An automatically controlled plane-strain bulge test system is implemented to characterize the mechanical behavior of the nanocrystalline samples. The specimens are fabricated by thermal evaporation and their mechanical properties are characterized by a plane-strain bulge test setup. The results show that the nanocrystalline Cu films have much higher strength than their micron-size-grain counterparts. Micrographs also show some characteristic deformation mechanisms of nanocrystalline metals, such as grain boundary sliding accompanied with dislocation activities, and grain rotation. In addition, one interesting phenomenon of those specimens, residual strain recovery, is studied. Strain recovery rate is obtained by recording the profile of the deformed film with a laser scanning confocal microscope profilometer. A numerical model employing grain boundary diffusion mechanism is presented to explain this phenomenon.;The second part of my research is to study the mechanical properties of graphene, a single atomic layer of carbon. Free-standing circular single layer graphene films are fabricated by exfoliation of graphene from bulk graphite onto Si wafer with arrays of micron-diameter wells. Mechanical properties of those films are characterized by nano-indentation technique with atomic force microscope (AFM). A numerical model is applied to analyzing the experimental data to determine the Young's modulus and film pre-stress. Moreover, a finite element simulation is performed to study the nonlinear elastic behavior of graphene. Results show that nonlinear elasticity of graphene is not negligible when it undergoes large deformation. This result motivated first principle calculations to study the nonlinear elasticity of graphene more precisely. Higher order elastic constants (up to fifth order elastic constants) for graphene are calculated by determining the electron density distributions and the elastic strain energy density as a function of deformation state with density functional theory. The derived nonlinear elastic constitutive model has been implemented into a user material subroutine with a small viscous terms for the commercial finite element code ABAQUS. Finite Element Simulations are performed to simulate the nano-indentation tests on free-standing circular graphene membranes. The consistency between the results from the experiments and the numerical calculations is strong evidence for the validation of the nonlinear elastic constitutive model.