Ion exchange strengthening and thermal expansion of glasses: Common origin and critical role of network connectivity

Various commercial glasses are strengthened via ion exchange at their surface, wherein small alkali ions from the glass are replaced by larger ones from a surrounding molten salt bath, thereby inducing the formation of a compressive stress layer. However, the level of stress induced by ion exchange is typically significantly lower than its theoretical limit (so-called network dilation anomaly). Here, based on molecular dynamics simulations of sodium aluminosilicate glasses, we assess the influence of network connectivity on stress development upon ion exchange. We show that, similarly to thermal expansion, the propensity for ion exchange-induced expansion is controlled by the connectivity of the network. This suggests that tuning the network connectivity may lead to the development of higher compressive stress upon ion exchange and, thereby, to stronger glasses.