Molecular dynamics study of mechanical behavior in silica glass under uniaxial deformation

Molecular dynamics simulations of silica glass have been carried out to investigate the mechanical behavior upon uniaxial tension. Fractions of SiOx (x = 4, 5 and 6) units and OSiy (y = 2, 3 and 4) linkages change with the density. The densified samples of 3.59 and 4.29 g cm(-3) exhibit ductile behavior, at which transition of SiOx units occurs from higher- to lower-fold Si coordination and the Si-O bond lengths are stretched upon deformation. Geometries of SiOx units almost do not distort with increasing strain. The big voids are formed during tension process and the biggest voids belong to O-voids. Clusters of crystalline O atoms in the samples of 4.29 g cm(-3) are maintained and formed upon uniaxial tension, suggesting inhomogeneous elastic-plastic deformation. The sample of 2.35 g cm(-3), consisting of principal SiO4 units, exhibits brittle behavior, at which fraction of SiO4 units is almost maintained and the Si-O bond lengths are stretched with the strain. Si-4-voids and O-voids are mainly big voids (radii > 2.0 angstrom) but O-voids play pivotal roles in the evolution and coalescence of voids causing the crack propagation. The network structure of SiOx units rearranges close to normal state during the fractured free surfaces in the sample.