Revealing ‘invisible’ subsurface structural change/damage in silicate glass made by ‘nearly-elastic contact’ with a spherical smooth surface

This research delves into the hypothesis that elastic contact with an external object can generate `invisible' surface defects, consequently reducing the apparent load to failure, or strength, of glass. This theory, while longstanding, has left the intricate details associated with such defects largely unknown. Utilizing vibrational spectroscopic techniques—with a depth sensitivity on the order of approximately 100 nm—and chemical etching, the study investigates subsurface structural alterations in and around the area subjected to nominally-elastic contact with a Hertzian indenter. Direct contact with the spherical indenter led to the recovery of over 99.96 % of the surface topography, suggesting a near-perfect post-indentation surface. However, surface-sensitive vibrational spectroscopy unveiled structural changes in the subsurface silicate network. Additionally, a shift in the vibrational spectrum was observed in areas that had experienced elastic deformation along with the contact region and fully recovered upon unloading. This shift indicates residual structural changes, even in these non-contacted regions. When the indenter tip was slid across the surface under loads much lower than the nominal yield strength, interfacial friction made the top-most region more susceptible to base-catalyzed hydrolysis. The depth affected by friction was significantly shallower than the length scale of the principal stress fields at maximum load during Hertzian indentation. Despite the glass surface maintaining its microscopic defect-free appearance, these subsurface structural changes—induced by indentation and friction—are proposed to be key factors responsile for the decrease in the apparent load to failure of glass.