Strengthening of Soda-Borosilicate Glasses by Ion Exchange Processes

Flexible electronics and displays rely on strong thin borosilicate glasses. Furthermore, borosilicate glass play a vital role in pharmaceutical packaging, particularly for the container of liquid medicine in the auto-injectors. Because of the risk of failure due to the fracture of the glass ampule the costumers need to purchase several units of auto-injector; also, the injector price increases dramatically. This renewed the interest in the strengthening of soda-borosilicate glass. Moreover, the applications of strong thin borosilicate glasses in flexible electronics attracted attention. Chemical strengthening is a practical means of improving the mechanical performance of soda borosilicate glasses. The chemical strengthening process involves the immersion of an alkali-silicate glass in a molten nitrate salt containing potassium ions at temperatures below the glass-transition temperature where the replacement of small alkali ions in the glass with larger potassium ions from the molten salt occurs. The glass composition, salts impurities, temperature and time, are crucial factors of the treatment. Furthermore, applying an electric field can speed up the process and improves the efficiency. This study investigates the impact of potassium for sodium ion exchange and its parameters on the final strength of alkali borosilicate glasses. Alkali borosilicate tubes, used in pharmaceutical packaging, were subjected to ion exchange in potassium nitrate salts containing different impurities. The initial surface flaws have a significant impact on the final strength due to the limited case depth of surface compression in alklai borosilicate glass subjected to ion exchange. Nonetheless, the results revealed that the sodium poisoning of salt has a limited influence on strengthening; conversely, even a small amount of calcium spoils the strengthening. The replacement of sodium ions with calcium is thermodynamically favoured with respect to Na/K ion-exchange. Calcium can penetrate into the glass surface and prevent the replacement of sodium with potassium and, consequently, the generation of compressive stress. Interestingly, performing electric field assisted ion exchange, EF-IE, for 10 min produces an ion-exchanged layer as deep as conventional strengthening for 4 hours in soda borosilicate glass. Electric field assisted ion exchange also augments the glass strength and makes the glass more damage resistant; however, the initial defects on the glass surface have an adverse influence on the efficiency, as expected. Applying an electric field changes the governing mechanism of ion exchange and accelerates the penetration of potassium ions into the glass; furthermore, the glass structure of the layer undergone electric field assisted ion exchange is modified during the process. Although EF-IE generates a strong surface compression in glass, the inhomogeneous distribution of residual stress is a drawback. Performing EF-IE using AC E-fields produces homogenous ion-exchanged layers in glass and is, probably, a practical approach to balancing the residual stress in glass. Chemical strengthening of a thin alkali-borosilicate glass ,D 263 Teco®, is also investigated in the present work. Na/K ion exchange improves the glass strength three times. Although the surface compression generated by ion-exchange in alkali borosilicate glasses is not as strong as typically used glasses for chemical strengthening ( alklai aluminosilicate), it can be used to improve the mechanical properties of borosilicate glass. Annealing prior to the ion exchange increases the compressive stress generated on the surface; however, its effect on strengthening is trivial. The compressive stress produced by Na/K ion exchange in thin alkali borosilicate glass improves the damage resistance and the bending strength of glass. Due to the limited thickness of samples, heat treatments with high heating and cooling rates can be conducted. Such heat treatments can be used to carry out surface relaxation and improve the strength of samples by “surface relaxation”. A fast heat treatment after ion exchange improves the finals strength samples about 40%.