Decoupling of indentation modulus and hardness in silicate glasses: Evidence of a shear- to densification-dominated transition

Prediction of the composition dependence of Calcium aluminosilicate (CAS) glasses mechanical properties is essential to aid in the development of new glasses with tuned properties for specific applications. In this investigation, we use microindentation to measure the hardness (H) and modulus (M) and the inelastic volume that dissipates the inelastic energy is elucidated using annealing and AFM to decompose it into contributions from densification and shear flow. We chose a family of CAS glasses with a constant CaO/Al₂O₃ ratio but varying SiO₂ content, as well as a commercial >99% SiO₂ glass. We observe a decoupling between M & H, M systematically decreases upon increasing SiO₂ content, however, H exhibits a non-monotonic behavior with a minimum around 79% SiO₂. The inelastically deformed volumes show a high contribution from shear flow for SiO₂-poor glasses and a high contribution of densification for SiO₂-rich glasses, demonstrating a shear to densification dominated transition between these two extremes.