# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 2 Glass Strengthening: The Ion-Exchange Contribution
### Chemical Strengthening: Applications  
Although the chemical toughening process of glass presents expensive production costs due to its high time consuming if compared with a simple thermal tempering process, the possibility of obtaining greater mechanical strengthening in large glass surfaces with small thicknesses makes this process particularly suitable for many technological applications, ranging from the architectural (e.g., windows for buildings and palaces), automotive and transport sectors (e.g., windshield for airplanes and vehicles) up to the electronics and military ones (e.g., panels for displays in many electronic devices; air-to-ground missile launch tube protective covers). Figure 2 shows some outstanding applications related to chemical strengthened glasses. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Thermal Strengthening** & **Chemical Strengthening** \\ \hline Strength (max. value range) & \(\sim\)(200–400) MPa & \(\sim\)(800–1000) MPa \\ Surface compression layer & Thick & Thin \\ Stress distribution profile & Parabolic & More flat \& square \\ Minimum sample thickness & 2–3 mm & \(<\)1 mm \\ Process Time & Short (mins) & Long (hours) \\ Process Cost & Cheap & Expensive \\ Glass Composition & No relevant & Alkaline glass (i.e., soda-line \\ Product Shape & Simple geometries & Unusual shape \\ \hline \hline \end{tabular}
\end{table} 
Table 1: A comparison between thermal and chemical glass strengthening methods. In daily life, storing and sharing of data, from those personal to public ones, is almost delegated to electronic devices, such as smartphones, tablets, and lap-tops. The safety of these data does not only concern their protection at software level from any external attacks or a sudden failure of the operating system, but also requires the physical protection of the same devices, with particular reference to some of their most vulnerable elements such as monitor and/or display. For this reason, CORNING(r) Inc., Asahi Glass Corporation (AGC), and SCHOTT AG, three world leaders in glass manufacturing, have developed in the recent years different high ion-exchange (HIE) tempered aluminosilicate glasses (for instance, the Gorilla(r) series from CORNING(r), the Dragontrail(tm) from AGC, and the Xensation series from SCHOTT AG, respectively) with outstanding resistance to breakage and scratches in order to increase the protection for cover and touch screens, greatly reducing the risk of damage due to potential impact or fall and, consequently, avoiding any repair costs associated with this unpleasant event [130; 131; 132; 133]. In all these cases, the characteristics of high surface resistance (>900 MPa in the best cases), flexibility (>700 MPa as a bending stress) and lightness (glass thickness < 1 mm, generally) are conferred to the pristine aluminosilicate glasses by the special chemical tempering treatment based on Na\({}^{+}\leftrightarrow\) K\({}^{+}\) ion-exchange, where the large K\({}^{+}\) ions, substituting the host Na\({}^{+}\) ions, force the generation of surface compression stress, supporting the closure of the cracks, thereby increasing the strength of the material and preventing the formation of others. In this context, Apple, although employing Corning Gorilla glass technology to protect the displays of its smart devices, in recent years made a strong effort in the search of alternative solutions to further increase the conventional method of glass strengthening. Different strategies have been investigated, such as the implementation of a patterned asymmetric method based on double ion-exchange process in order to increase the compression depth over at least one localized region, with particular reference to the bend zones of the glass cover, as highlighted in some Apple patents recently published by the US Patent &  
Figure 2: Some relevant application fields of glass chemical strengthening. Trademark Office [134; 135; 136].