Alkali salt vapour deposition and in-line ion exchange on flat glass surfaces

Glassy materials provide a unique set of properties, such as transparency, high hardness and good chemical durability, forming ability, low cost production and the possibility of recycling. They are therefore used in a wide range of applications, e.g. windows, containers, displays, thermal insulation, optical lenses, data transport, or, e.g. as bioactive materials. The fact that glass is a relatively hard material originates from the nature and the alignment of bonds of the vitreous network. The mechanical strength, on the other hand, mainly depends on the presence of defects in the glass surface, see Figure 1, and, hence, the defect resistance of this surface. As a consequence, the practical use of glass is often limited by its brittleness, which is associated with statistical failure. There exist a variety of ways to increase the strength of glass; most of them involve modifications of the glass surface. A method, which has received much attention, is chemical strengthening. It is based on the exchange of smaller ions in the glassy matrix by larger ions from a molten salt, e.g. Na is replaced by K, see Figure 2. The larger ions are literally squeezed into the sites of the smaller ions, generating compressive stresses in the glass surface, which counteract external tensile stresses. The most commonly described route of chemical strengthening is K–(Li,Na) ion exchange, which was discovered independently by Kistler and Acloque. Chemical strengthening of glass has recently been Alkali salt vapour deposition and in-line ion exchange on flat glass surfaces