Densification Energy during Nanoindentation of Silica Glass

Abstract

Load/unload displacement curves at room temperature (humidity 49%) for silica glass have been measured in the penetration range of 0.5–1.2 μm using a Vickers nanoindentation technique (load/unload speed 50 mN/s). Deformation energies have been estimated for the first time. The universal (dynamic) hardness, Hu, and elastic recovery, ER, at the penetration depth, ht, of 1.0 μm are Hu= 4.1 GPa and ER= 0.7. The following energies for total deformation, Ut, elastic deformation, Ue, and plastic deformation (i.e., densification during loading), Up, are obtained: Ut=190, Ue=135 and Up= 55 kJ/mol at ht= 0.5 μm and Ut= 139, Ue= 96 and Up= 43 kJ/mol at ht=1.0 μm. All these deformation energies increase with decreasing penetration depth. It is found that plastic deformation energies of 38–55 kJ/mol for 0.5 < ht < 1.2 μm are very close to the activation energy (46–54 kJ/mol) for the recovery of densification in silica glass, but are very small compared with the single bond strength (443 kJ/mol for Si—O bond) of SiO2.