Molecular dynamics (MD) simulations of uniaxial tension and compression tests on hexagonal prism [11] -SiC nanowires

In the present work, we report results from molecular dynamics (MD) simulations of uniaxial tension and compression tests on hexagonal prism [111] -SiC nanowires. The simulation results show that after relaxation for 300,000 time-steps without any constraints, the nanowires were observed to elongate along the length direction without visible bending or buckling. At the two ends of nanowires, deformation was more severe and nonuniform, showing the end effect. We excluded a few lattices at each of the two ends and studied the deformation without the end effect. At the atomic level, the initial deformation induced by free surfaces was inhomogeneous, which changed from the surface atomic layer to the interior atomic layer. The layer-averaged potential energy per carbon (or silicon) atom had the highest value at the surface and dropped gradually from the surface to the interior. Clearly, the energy distribution was inhomogenous at the atomic level in the nanowire. Following Gibbs approach to surface excess energy, we analyzed surface energy, surface stress, and surface elastic constants of a nanowire by treating a nanowire as a composite of a hypothetical bulk phase, a true two-dimensional geometric surface phase, and a true one-dimensional geometric edge phase. Furthermore, the hypothetical nanowire, two-dimensional surfaces and one-dimensional edges were all assumed to be thermodynamically homogeneous, which was necessary for the study of the continuum concepts of surface energy, surface stress, surface elastic constants and the Young s modulus of nanowires. The stress-free status of the bulk body was taken as the reference state. Then, we found that the initial strain of SiC nanowires without any applied loads was at the magnitude of 1 % and changed with the nanowire cross-sectional perimeter, exhibiting that the smaller the cross-sectional perimeter, the larger the initial strain. The uniaxial tension and compression tests were simulated on the initially equilibrated nanowires. This means the initial equilibrium state is the reference state to calculate the work done by an applied load and then changes in the surface energy and in the internal energy of the nanowire. For a finite piece of nanomaterial without any applied loads, its surface induced an initial deformation on it and generated an internal stress field in the hypothetical bulk, surface and edge phases. At equilibrium, the total force among the three phases self-balanced. With this theoretical approach, we successfully analyzed the MD simulation results of the size-dependent specific surface energy and Young s modulus.