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 SiO 2 .