Subsurface structural change of silica upon nanoscale physical contact: Chemical plasticity beyond topographic elasticity

Surface defects or flaws on materials made by physical contacts with foreign objects can deteriorate their mechanical properties and limit technical applications. Thus, understanding the contact-induced subsurface damage is of great importance. Using nanoscale infrared spectroscopy and reactive molecular dynamics simulations, the subsurface structural changes of silica upon nanoindentation and nanoscratch are investigated. The results reveal an elongation of the SiO bond length distribution even after the topographically-elastic contact, indicating a "chemical plasticity" at the sub-Angstrom level. In the plastic region with subsurface densification, the SiO bond is found to be slightly longer than the pristine region, indicating the decrease in molar volume is accompanied with the elongation, not shortening, of the SiO bond. These results elucidate the structural damage of a material upon physical contact cannot be delineated based on the topographic deformation of the surface.