Toughness and strength anisotropy among high-symmetry directions in 3C-SiC

Abstract

This paper presents a quantitative understanding of toughness and strength anisotropy in 3C-SiC under uniaxial deformation. We consider four high-symmetry crystallographic directions including [100], [110], [111], and [112¯] for loading, and find that both toughness and strength are the maximum along the [100] direction and the minimum along the [111] direction. The maximum anisotropy in crack nucleation-toughness is 145% and in fracture toughness 126%, relative to the [111] direction. The corresponding anisotropies in fracture strain and fracture strength are found to be 62% and 36%, respectively. An atomistic analysis shows that bonds deform uniformly for loading along the [100] direction, whereas for loading along the [110], [111], or [112¯] directions, bonds deform nonuniformly and it breaks the symmetry of the local atomic structure. The nonuniform bond deformation creates different sets of bond lengths and forms the atomistic basis for the direction-dependent mechanical behavior. The simulations are conducted with four different interatomic potentials including the Stillinger-Weber, Tersoff, Vashishta, and Environment Dependent Interatomic Potentials. It is found that only the Stillinger-Weber potential exhibits first-principles accurate strength and toughness as well as brittlelike fracture. Also, there is a sizeable difference among the potentials in terms of the crack nucleation toughness and strength. We find the difference to originate from the dissimilarity in the forcing function and its derivative in the nonlinear regime of mechanical deformation. A mathematical analysis suggests that it is essential for the forcing function to accurately represent the first-principles accurate forcing function, at least up to the maximum bond force, to produce accurate fracture properties and patterns.