# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 2 Glass Strengthening: The Ion-Exchange Contribution
### Chemical Strengthening
On the other hand, [PERSON], [PERSON], and others in their studies modified the [PERSON]'s analysis introducing a new term in the model, related to different relaxation contributions (i.e., the viscoelastic and structural one) together with the network hydrostatic yield strength in order to evaluate the subsurface maximum compressive stress when the process temperature is higher and/or close to the glass transition temperature \(\rm T_{g}\)[121, 122, 123, 124]. Also noteworthy is the method, recently proposed by [PERSON], which allows to control in real time the stresses generated in a glass as a result of the ion-exchange process from the knowledge of some parameters such as the elasto-optical coefficients, the dependence of the diffusion coefficients on the temperature and, finally, the function describing the time relaxation of stresses at the glass surface [118]. Differently from the  
Figure 1: Residual stress profile through the glass thickness for: (**a**) thermally tempered glass; (**b**) chemically toughened glass. d is the glass thickness; DoL is the depth of layer. Figure reprinted from [67] under the terms of the Creative Commons Attribution 4.0 International License. numerical and analytical approaches mentioned above, [PERSON] and co-workers have developed a very original method for evaluating the compressive stress in a chemically strengthened glass, connecting it directly to the glass structure on an atomic scale. The non-contact and non-destructive method is based on the \"stuffing\" effect and the knowledge of three structural parameters, represented by specific boson peaks in the micro-Raman spectra of the sample [125]. At the end of this brief overview on the main concepts underlying the chemical strengthening of a glass, it should be remembered that in addition to the two most important process parameters, time and temperature, two other factors influence the formation of stress in a chemically strengthened glass: the composition of the pristine glass and that of the molten salt solution in which it is immersed. In fact, the larger the difference between the atomic dimensions of the two ionic species involved in the exchange, the greater is the magnitude of the compressive stress near the surface. In this sense, sodium-potassium (Na\({}^{+}\leftrightarrow\) K\({}^{+}\)), lithium-sodium (Li\({}^{+}\leftrightarrow\) Na\({}^{+}\)), or lithium-potassium (Li\({}^{+}\leftrightarrow\) K\({}^{+}\)) exchanges are able to produce high compressive stresses with values that, practically, are close to 1 GPa in the best cases [11; 12; 13]. On the other hand, the composition of the pristine glass plays an important role on the diffusion rate of the ions and stress relaxation of the material during the exchange process. The common soda-line glass has excellent diffusivity values for the incoming ions but a rapid relaxation process which makes difficult the achievement of a good glass strengthening despite its high alkali content [126]. However, the most promising glass formulation is the one that refers to aluminosilicates which have the highest diffusivity values while ensuring the achievement of compressive stresses well above the possible relaxation effects of the glass matrix [127; 128]. In particular, the closer the alumina content is to that of the alkaline oxides, the higher the attainable strength of the glass following the exchange process [129]. For more details on the different characteristics of glass formulations, molten salt compositions, and ion-exchange typologies in glass strengthening, the reader is referred to the exhaustive reviews on the subject [11; 12; 13]. The main features of the two glass strengthening processes described above are compared in Table 1 below.