Sink-in/pile-up formation and crack nucleation mechanisms of high purity fused silica and soda-lime silica glass during nanoindentation experiments

Abstract To improve the fabrication qualities and service performance of optical glass, the mechanical response mechanisms of two typical optical glass materials, high purity fused silica (HPFS) and soda-lime silica glass (SLSG), are investigated by nanoindentation experiments under loads below 500 mN. The mechanical properties of glass are reflections of its molecular structure. The molecular structure of HPFS is an amorphous SiO2 3D crosslink network structure with free volume and nonbridging oxygens (NBOs). For SLSG, due to the metallic ions packed into the free volume of the SiO2 network and the directionless property of the ionic bonds, its elastic modulus, Poisson's ratio and densification deformation resistance are higher than those of HPFS. The hardness and shear flow resistance of SLSG are lower than those of HPFS. An analytical model of the strain field after the nanoindentation unloading process was developed considering the permanent densification deformation. The mechanisms of the sink-in and pile-up formations and crack nucleation were explored using the analytical model of the strain field. The borderline cracks nucleate at the sinking region of the HPFS during the unloading process due to the free volume stacked at the surface. The mechanisms of radial crack nucleation in SLSG are the tensile stress concentration at the elastic-plastic interface near the surface induced by residual stress.