Understanding the effects of compositional changes in ion exchange

The primary methods for consistently improving the strength of silicate glasses is the use of thermally or chemically introduced residual stresses, with the latter technique having become increasingly important in recent years. Chemical strengthening is based on ion exchange which is mainly a diffusional process, takes place in a molten salt by exchanging larger alkali for smaller ones to generate surface compression below the glass transition temperature. Commonly the focus is on the level of stresses that can be introduced by ion exchange, with relatively little attention being paid to other structural and property changes that arise from the compositional changes inherent in ion exchange. This project involves investigation of the effects of exchanging pairs of ions, ionic species, process time and temperature on the enhancement of mechanical properties of ion-exchanged strengthened alkali – alkaline earth silicate glasses as well as the effects of compositional changes arising from ion exchange by making bulk glasses with equivalent compositions to glasses produced by ion exchange. The structures of these bulk glasses have been assessed using Raman and IR spectroscopies and the results compared to those obtained on the ion exchanged glasses. In addition, the mechanical properties, including fracture toughness, of the bulk glasses have also been assessed. In this research the primary focus has been Na/K ion exchange but ion exchange of different alkali ions such as Na+ for Li+, K+ for Na+, Cs+ for Na+ or K+ have been examined using a salt paste method. Modifications in the Na+ - K+ ion–exchanged glasses are observed to be associated with structural band changes in the silica network structure; notably Qn distribution changes based on ion-exchange temperature and time. However, the structural changes cannot all be a direct consequence of ion-exchange; some of the changes shown should be due to the stress. The Raman Spectroscopy and FTIR reflectance spectra suggest observation of similar features in both ion exchange glasses and the as-melted glasses. The ion – exchange process aids to improve mechanical properties; however, as the amount of potassium content increases in the glass composition, toughness, hardness, elastic moduli all decrease. Consequently, it can be seen that although the structural changes are similar in both Na+-K+ ion-exchanged glasses and equivalent potassium-containing as-melted glasses, examination of the mechanical properties gave better results for the ion-exchange strengthened glasses.