Densification energy during nanoindentation of silica glass

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, H_u, and elastic recovery, E_R, at the penetration depth, h_t, of 1.0 μm 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 μm and U_t = 139, U_e = 96 and U_p = 43 kJ/mol at h_t = 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 < h_t < 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.