Structural relaxation in soda lime silicate glass below the glass transition temperature during ion exchange

Chemical strengthening based on ion exchange, IE, is a crucial process for improving the mechanical properties of glass through the formation of a surface residual compression layer. Herein, we investigate the IE of float soda lime silicate glass at temperatures from 380°C to 490°C, with a focus on how the medium-range structure evolves and its relation to stress-relaxation phenomena. Using positron annihilation lifetime and Raman spectroscopy, it is shown that the free volume is substantially reduced by potassium incorporation starting at the lowest IE temperature (380°C), while no changes are observed in the silicate ring structure or bonding angles. When the maximum compressive stress is achieved (430°C), the free volume and the positron lifetime reach a minimum. Moreover, the Raman bands shift, indicating a narrowing of the Si-O-Si bond angle in the vicinity of the alkali cations. The transition between the squeezed and relaxed configuration is sharp, suggesting the existence of two well-defined configurations of the silicate backbone. As the IE temperature further increases, the residual stresses relax. This is coupled with an increase in the free volume and a relaxation of the Si-O-Si bonding angles toward the original configuration. These results clarify that (i) the free volume shrinkage can partially account for the residual stress relaxation through a mechanism not involving a re-construction of the network; (ii) potassium can fill different types of sites accessible at different temperatures, with higher temperatures activating sites straining the silicate framework; (iii) stress relaxation at “high temperature” mirrors a relaxation of the medium-range structure.