Structure evolution and mechanical properties of ion-exchanged silicate glass

Soda-lime silicate glass (SLS glass) is one of the most commercialized glasses with a huge extent of applications from homewares and kitchenware appliance to cover for touchscreen gadgets. The latter has gained notable attentions recently since the majority of available touchscreen gadgets have covers from other glass families such as aluminosilicate. Technically, these types of glasses are difficult to produce and very costly, therefore, the prices of articles which are made by these glasses are high. In comparison, soda-lime silicate glass has lower price because of globally growing production and promising future. However, due to the intrinsic weaker mechanical properties, the application of SLS glass as cover of touchscreen gadgets is restricted. Several techniques have been applied to improve the mechanical properties of SLS glass. Among them chemical tempering is one the most promising technique. Typically, the chemical tempering is done by an ion-exchange process where sodium atoms contained in the glass are substituted by potassium ions diffusing from the molten salt. The effect of variables such as glass composition, molten bath composition, temperature, and time is crucial in the ion-exchange process. Particularly, selecting an unsuitable time and temperature of the process can affect mechanical properties of glass through a stress relaxation phenomenon. Therefore, optimization of the time and temperature can guarantee efficient reinforcement of glass. In this PhD research, three different temperatures (430°C, 450°C, and 470°C) and five different times (4 h, 8 h, 24 h, 48 h and 168 h) selected for chemical tempering of glass samples in pure molten KNO3 and molten KNO3 systematically poisoned by NaNO3. The compressive residual stress and case depth were determined by optical methods, the flexural strength was measured by a ring-on-ring test method and the surface chemical composition of the glass was analysed by Energy Dispersion X-ray Spectroscopy (EDXS). The resistivity of treated glass against forming surface cracks was studied by Vickers hardness and scratch test. To study the structural evolution, micro-Raman (μ-Raman) spectroscopy was used. The results pointed out that below addition of 0.5 wt% NaNO3, the ion-exchange process is always effective. Indeed, compressive residual stress, flexural strength, surface concentration and potassium penetration in Na-containing baths are substantially identical to values recorded on glasses treated in “pure” KNO3. Actually, case depth and interdiffusion coefficient are invariant with respect to the sodium content at least up to 1 wt%. No significative difference between “tin” and “air” side are revealed. Influence of time of tempering on Na-K exchange process showed that the concentration of K+ on the surface of glass was increased by increasing the duration of of the process. Compressive residual stress, on the other hand, was decreased by time due to the surface structural relaxation. A surface crack tendency under a Vickers indenter and scratch test gave the evident that K/Na ion-exchange process for more than 24 h is responsible for an indent mechanical reinforcement. A structural reorganization of the glass network occurred and a higher number of Q2 and Q3 species were present in the tempered glasses with respect to the pristine one. Such re-polymerization could account for a more plastic behavior under Vickers indentation and scratch test, making the material less susceptible to surface cracking. The case depth of tempered samples was increased by temperature at the expense of the compressive residual stress due to the stress relaxation. Relative concentration of K+ on the surface, as well, increased by the temperature. The increase in the temperature of ion-exchange process led to increase in the tendency of glass against formation of crack under Vickers indenter and scratch test. The limit of this propensity is tempering at 470 °C for 48 h as the formation of radial cracks and shorter plastic deformation regions under scratch test observed. From structural point of view, Na-K exchange caused reorganization of the glass network which is responsible for a more plastic behavior under Vickers indentation and scratch test.