Ion exchange: a diffusion process resulting in residual stress influencing diffusion.

What is it about?

The chemical strengthening of glass results from an ion exchange process in which smaller alkali ions in glass are replaced with larger alkali ions from a molten salt bath. This interdiffusion process leads to a buildup of chemical stress in the glass. However, traditional modeling of the ion exchange process has not fully accounted for interaction effects between mass diffusion and the chemical stress developed during the process. In this study, we develop the general theory of coupling between diffusion and stress, resulting in a single flux equation with a concentration- and stress-dependent interdiffusion coefficient. We apply the theory to the specific cases of chemically strengthened soda lime silicate and aluminosilicate glasses, demonstrating the impact of interaction terms on concentration profiles and interdiffusion coefficient. Following a phenomenological approach, this study demonstrates the effect of the interdiffusion on stress generation and vice versa to account for deviations from the simple expressions published hitherto in the literature.

Featured Image

Why is it important?

Our findings show that interdiffusion and residual stress can be coupled in a single flux equation resembling a Fickian diffusion equation with a variable diffusion coefficient. Additionally, it allows comparing the effects on diffusion coefficient coming from the coupling term and the stress dependence of the self-diffusion coefficients of the single ions. The resulting diffusion equation is approached by the numerical finite differences method. It also put in evidence how chemical stress may result complicated by the interdiffusion character of ion exchange.

Perspectives