Molecular dynamics simulation of deformation in SiO2 and Na2O-SiO2 glasses

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The atomic scale deformation behavior of SiO2 and Na2O-SiO2 glasses is studied by Molecular Dynamics (MD) simulation methods. First, the pressure exerted on the simulation box is varied and the effects of isotropic compression and expansion imposed on both glasses are investigated. The SiO2 glass shows the well-known densification under a compression pressure of 7 GPa or above. In contrast, no densification is observed for the Na2O-SiO2 glass. Next, the shape of the simulation box is varied and the effects of shear deformation exerted on both glasses are investigated. In the case of the SiO2 glass, the shear deformation brings about an increase in shear stress up to over 4 GPa at first; however, further shearing of the simulation cell results in a decrease in shear stress, accompanied with densification. On the other hand, in the case of Na2O-SiO2 glass, shear stress rises up to about 1.5 GPa at first and the further tilting keeps almost the same value of shear stress, without densification. These calculated results suggest that two different behaviors of atomic scale deformation exist. The exchange of Si-O bonds is the main event occurring in SiO2 glass during deformation. This phenomenon leads to the densification. In contrast, in the case of Na2O-SiO2 glass, mobile sodium ions can accommodate their positions to relieve stress so that the breaking of Si-O bonds is considerably reduced. This phenomenon displays a behavior similar to plastic flow before fracture appears.

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