Densification energy during nanoindentation of silica glass

Load/unioad displacement curves at room temperature (humidity 49%) for silica glass have been measured in the penetration range of 0.5-1.2 mum using a Vickers nanoindentation technique (load/unload speed 50 mN/s). Deformation energies have been estimated for the first time. The universal (dynamic) hardness, H-u, and elastic recovery, E-R, at the penetration depth, h(t), of 1.0 mum are H-u = 4.1 GPa and E-R = 0.7. The following energies for total deformation, U-t, elastic deformation, U-e and plastic deformation (i.e., densification during loading), U-p, are obtained: U-t = 190, U-e = 135 and U-p = 55 kJ/mol at h(t) = 0.5 mum and U-t = 139, U-e = 96 and U-p = 43 kJ/mol at h(t) = 1.0 mum. All these deformation energies increase with decreasing penetration depth. It is found that plastic deformation energies of 38-55 kJ/mol for 0.5 < h(t) < 1.2 mum 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.