Indentation recovery of soda-lime silicate glasses containing titania, zirconia and hafnia at low temperatures

From thermal treatment of indentations made on glass, the densification contribution to the total indentation deformation can be estimated. In this work, we studied the physico-mechanical properties and low-temperature indentation recovery of soda-lime silicate glasses with the composition of (in wt%) 73SiO2–12CaO–15Na2O–xTiO2 (ZrO2 or HfO2), where x is 0, 1, 2, 4 and 8. The obtained results demonstrated that the addition of group IV B metal oxides (TiO2, ZrO2 and HfO2) into soda-lime silicate glass increased density, elastic moduli, Vickers hardness and brittleness due to the higher atomic weight of additives, increased network connectivity, higher strength of individual bonds formed and lower molar volume, respectively. Indentation recovery of glass samples containing 8 wt% TiO2 (G5), 8 wt% ZrO2 (G9) and 8 wt% HfO2 (G13) was comparatively estimated with that of a pure soda-lime silicate glass (G1), as a reference, on the basis of the depth change of indentation impressions at low temperatures, 25 °C and 100 °C, for different periods with a maximum of 48 h. It was found that the higher indentation recovery for all four samples was observed at both temperatures within the first 6 h due to stored strain energy in deformation zone but equilibrium was not reached even after 48 h because of low thermal treatment temperatures, which generally provide a thermodynamic driving force to recover. According to the glass composition, the indentation recovery rate follows the order: G13 < G9 < G5 < G1, which is mainly attributed to the decrease in densification with the increase the size of tetravalent metal cations introduced into soda-lime silicate glass. All the indentations made on the experimental glasses exhibited three common features: elasticity, viscoelasticity and plasticity.