Chemical structure and mechanical properties of soda lime silica glass surfaces treated by thermal poling in inert and reactive ambient gases

This study employed thermal poling at 200 degrees C as a means to modify the surface mechanical properties of soda lime silica (SLS) glass. SLS float glass panels were allowed to react with molecules constituting ambient air (H2O, O-2, N-2) while sodium ions were depleted from the surface region through diffusion into the bulk under an anodic potential. A sample poled in inert gas (Ar) was used for comparison. Systematic analyses of the chemical composition, thickness, silicate network, trapped molecular species, and hydrous species in the sodium-depleted layers revealed correlations between subsurface structural changes and mechanical properties such as hardness, elastic modulus, and fracture toughness. A silica-like structure was created in the inert gas environment through restructuring of Si-O-Si bonds at 200 degrees C in the Na-depleted zone; this occurred far below T-g. This silica-like surface also showed enhancement of hardness comparable to that of pure silica glass. The anodic thermal poling condition was found so reactive that O-2 and N-2 species can be incorporated into the glass, which also alters the glass structure and mechanical properties. In the case of the anodic surfaces prepared in a humid environment, the glass showed an improved resistance against crack formation, which implies that abundant hydrous species incorporated during thermal poling could be beneficial to improve the toughness.