On the structure and properties of hydrothermally toughened soda–lime silicate float glass

Hydrothermal treatments of soda–lime silicate glass cause a remarkable improvement in the resistance to flaw formation with an increase of the critical load to ≈1 kgf. This remarkable effect is achieved even if the reaction layer between the glass and water solution is well below 1 µm. Positron Doppler broadening spectroscopy (DBS) reveals that the hydrothermal treatment causes a drop in the free volume of the glass network near the surface connected with the diffusion of molecular water whose presence was further confirmed by Fourier transformed infrared (FTIR) spectroscopy and secondary ion mass spectrometry (SIMS). Based on FTIR and SIMS, we also argue that the hydrothermal ion exchange is a double-step process: first H+ substitutes Na+ in the network, and following molecular water permeates the system. Moreover, we show that the presence of water in the network is fundamental in stabilizing the modified glass surface leading to toughening. Once molecular water is released, the network is quickly polymerized and becomes more brittle. Hydrothermal toughening is only a partially reversible process, once water is released it is not possible to reobtain the same properties with a second treatment. Finally, it is shown that air and tin side of the float glass perform differently as a result of different water diffusion kinetics related to dissimilarities in the network density.