Fracture- and Indentation-Induced Structural Changes of Sodium Borosilicate Glasses

It is known that indentation using a diamond indenter induces permanent densification to result in plastic or inelastic deformation of glass. However, it is still unclear whether a high tensile stress causes such a structural change or not. In this study, fracture- and indentation-induced structural changes of glasses with the compositions of 20 Na2O − 40x B2O3 – (80−40x) SiO2 (in mol%, x = 0, 0.5, 1, 1.5) are investigated. Two-point bending tests of the glass fibers are performed to apply a high tensile stress to the glasses, and Vickers indentation tests of the glasses are also carried out for comparison. The structural change of the glass is evaluated by using Raman spectroscopy. It is elucidated that a tensile side of the fractured fiber for every composition shows a permanent structural change, which is characterized by a lower wave number shift of Raman peak assigned to the Si-O-Si bending vibration mode. It is also found that the behaviors of Raman peaks of the fractured fiber are opposite to those of the indented glass under a high compressive stress.