Simulation of glass network evolution during chemical strengthening: Resolution of the subsurface compression maximum anomaly

Chemical strengthening of glass by ion exchange below its glass transition temperature has become a well-established technological process in the glass industry since its introduction in the 1960s. However, some questions related to the basic science of the ion exchange process still remain. In this paper we focus on the observation of a maximum in compression that is often observed below the surface, rather than at the surface as predicted by the Cooper theory. In some glasses, this anomaly appears in a dramatic form, where the surface compression reduces to zero and can even become tensile upon continued ion exchange. We provide an explanation of this anomaly based on a mechanism of volume shrinkage (densification) triggered by the large hydrostatic pressure inherent to ion exchange in some glasses. We propose a mechanical spring-and-dashpot model for the pseudo-viscoelastic behavior of glass to identify the mechanisms and associated time scales that can reproduce the experimental observations. Next, we modify Sane and Cooper's analysis by introducing a new “V-factor” to account for the identified relaxation effects (fast β-relaxation, slow α-relaxation, and stress release due to volume and shear viscosities). The approach accurately reproduces the subsurface compression maximum anomaly. Our model therefore enables a clearer perspective on the evolution of the glass network during the chemical strengthening process.