Chemical strengthening of silicate glasses: Dangerous and beneficial impurities

Chemical strengthening is an industrial process used to mechanically reinforce alkali-containing glasses. In a typical procedure, sodium or lithium atoms contained in the glass are substituted by potassium ions diffusing from a molten potassium nitrate at temperatures below the strain point of the glass, thus creating a bi-axial residual compressive stress in the material surface which strengthens the component. Several variables like glass composition, molten bath composition, temperature and time can affect the efficiency of the ion-exchange process. An interesting aspect regards the presence of impurities in the bath, introduced with the raw salt or accumulated during the process, rarely considered in the past at least from a scientific or technological point of view as responsible for the resulting chemical, physical and mechanical performance. The problem addressed here is of direct industrial significance since the strengthening is strictly related to the efficiency of the ion-exchange process. In this work, commercial soda lime silicate float glass was considered and the effect of variable sodium, magnesium and calcium concentration in the molten bath on the efficiency of the ion exchange process was analyzed. Industrial potassium nitrate salts from different sources were also considered. Calcium and mostly magnesium were shown to represent dangerous contaminants of the potassium nitrate bath, making the ion-exchange process almost not efficient. The addition of limited quantities of silica was suggested as possible remedy for non-efficient salts.