Densification of fused silica

As compared to other optical glasses, when fused silica (SiO₂) is ground or polished, it has some unique features, such as low surface microroughness and subsurface damage, low grinding-induced residual stresses, and low removal rates. When subjected to sufficiently large compressive stresses, fused silica glass will densify permanently. It also has higher hardness than other optical glasses. On the other hand, fused silica's Young's modulus and fracture toughness are in the middle range of the optical glasses. So, compared to other optical glasses, fused silica behaves differently under compressive loads. It is also known that shear stress will facilitate densification. Onset of densification is governed by a linear combination of mean hydrostatic stress and effective shear stress. Plastic strains include both densification and shear plastic flow. We modeled this behavior by a simple constitutive law including both shear flow and densification. Based on this material model, cavity expansion in an infinite medium was studied analytically and numerically. We also used the constitutive model to interpret nano-indentation tests on fused silica via finite element simulations of indentations by spherical and Berkovich indenters. Based on the same material model, scratching by spherical and conical tips also was studied. The effects of densification on surface quality during grinding and polishing were suggested based on these simulations.