Authors:
Matt McDowell, Austin Leach and Ken Gall
Summary:
As the physical dimensions of a material are reduced to the nanometer size scale, experimental determination of mechanical becomes increasingly difficult. At the bulk scale, mechanical testing is performed with relative ease, by loading a specimen of defined geometry under uniaxial tension and recording values of force and displacement, then extracting relevant properties such as Young’s Modulus. For one-dimensional nanostructures (e.g., nanowires), mechanical testing under uniaxial tension is not yet feasible for many reasons. Recently, new techniques have been developed for the nanomechanical characterization of nanowires, based on the application of a transverse force on a nanowire by an Atomic Force Microscope (AFM). The applied force from the AFM and the tip displacement are monitored and used to extract the elastic properties of the nanowire using principles of continuum beam-bending theory. These tests can provide a quantitative understanding of nanomechanical behavior; however, they give little insight into the qualitative nature of elastic deformation, and specifically why the extracted Young’s modulus values may exhibit variations unseen in bulk materials. In addition, continuum beam-bending theory does not take into account surface energy and surface stress, both of which have been shown to strongly influence the mechanical behavior of nanometer scale materials. Unlike uniaxial tension, specimens subjected to a bending load will exhibit a distribution of axial stress through the cross-section from a maximum (tensile) value at the free surface, to a maximum (compressive) value at the opposite surface. Since in nanowires subjected to bending loads, the stresses are concentrated at the free surfaces, it is paramount to understand the impact of surface energy and surface stress on the resulting mechanical response. Using atomistic simulations we have investigated the bending behavior of metallic nanowires of varying axial and surface orientation to systematically determine the impact of nanowire structure and geometry on the resulting elastic response. Our results show that continuum, strain-energy methods are sufficient to determine the elastic properties of metal nanowires. The calculated elastic modulus values are shown to be independent of free-surface orientation and are consistent with the modulus extracted from simulated tensile deformation of identical nanowire geometries. Furthermore, as the nanowire diameter is increased, the elastic modulus was observed to approach that of a bulk specimen (of corresponding orientation). This research indicates that AFM bending test are an acceptable technique for nanomechanical characterization, and should be considered a sufficient replacement for tensile tests to determine the mechanical properties of nanometer-scale structures.
Source:
Materials Research Society Fall 2007 Meeting (Abstracts)