Irradiation-induced hardening/softening in SiO2 studied with instrumented indentation

To understand the plastic deformation mechanism of SiO2 polytypes, we measured the mechanical parameters of He+-irradiated crystalline SiO2 (α-quartz, c-SiO2) and vitreous SiO2 (silica glass, v-SiO2) as functions of the irradiation dose, by using the instrumented indentation method combined with a finite-element analysis. We extracted the effects of local rotation and bending of the SiO4 framework (the degree of local structural freedom), which play key roles in the plastic deformation, and expressed the hardness change with a simple formula. For v-SiO2, the changes in the density and the number of broken bonds correlated well with the change in the degree of freedom. In contrast, for c-SiO2 the present formulation was insufficient to fully express the hardness change in the structural disordering regime. The structure change by irradiation peculiar to this material is discussed, based on the theoretical formulation.