Size-dependent toughness and strength in defective 3C-SiC nanowires

Abstract

This paper presents an atomistic understanding of effective toughness and strength in defective 3C-SiC nanowires of different diameters. We consider a set of high-symmetry vacancy defect clusters and employ a combination of density functional theory and molecular dynamics simulations to calculate stress in the nanowires, using an energy-based approach that does not require use of any macroscopic geometric information of the nanowire. Our results suggest that for defect-free nanowires, cracks nucleate from one of the corners of the hexagonal cross section, whereas for defective nanowires—regardless of the size of the defect core—cracks nucleate from the edge of the defect core. With increasing diameter, both strength and toughness increase in defective or defect-free nanowires. Furthermore, defects alter the size-dependent effective toughness and strength of the nanowire: the larger the size of the defect, the stronger the size-dependence of effective toughness and strength. A single vacancy in a 8.0 nm diameter nanowire reduces effective toughness and strength by around 16.5% and 3.4%, respectively. As diameter approaches ∞, effective stiffness approaches the bulk behavior—whereas neither strength nor toughness approaches the behavior of the bulk. This is primarily because of the presence of the surface and associated sustained stress-localization in the nanowire. Effective toughness and strength are, therefore, controlled by the local critical events and not by the macroscopic features of the nanowire. Additionally, both toughness and strength decrease nonlinearly with increasing temperature due to thermal softening of the material—and this thermal softening is, however, weakly dependent on the size of the defective regime.