Universal hardness and elastic recovery in Vickers nanoindentation of copper phosphate and silicate glasses

Load/unload displacement (penetration) curves for yCuO_x·(100-y)P₂O₅ glasses (y=45, 50) with different copper valence states, silica and soda-lime silicate glasses at room temperature (humidity ∼64%) have been measured using a Vickers nanoindentation technique, and universal hardness H_u and elastic recovery E_R have been estimated in the load range of 0.1-1 N. Copper phosphate glasses have the values of H_u=1.7-2.5 GPa and E_R=0.55=0.65, depending on the Cu⁺/Cu²⁺ ratio. The glasses with high Cu⁺ contents show large densifications (plastic deformation) during loading and small elastic recoveries during unloading compared with the glasses with no Cu⁺ ions. These deformation behaviors are well explained qualitatively by considering lower coordination number and more covalent bonding of Cu⁺ compared with Cu²⁺, which have been confirmed from X-ray photoelecton spectra. Silica glass has E_R≒0.77, which is large compared with E_R≒0.63 in a soda-lime silicate glass. This large difference leads to the smaller value in H_u of silica glass than that of a soda-lime silica glass. It is confirmed that load displacement curves in nanoindentation tests are well fitted with quadratic equations.