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 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.