Mechanisms of plastic deformation in pristine and pressure-densified sodium silicate and aluminosilicate glasses: A molecular dynamics investigation

Understanding how glasses deform under sharp contact is essential for predicting and improving their resistance to surface damage and crack initiation. Here we use molecular dynamics simulations to examine nanoindentation in sodium silicate (NS) and sodium aluminosilicate (AL) glasses, both in pristine and pressure-densified states, with the aim of disentangling the relative contributions of densification and shear flow to plastic deformation. By resolving density and atomic free volume changes, stress and strain fields, atomic rearrangements and coordination defects around the indenter, we show that plasticity is strongly composition-dependent. In pristine NS, deformation is dominated by irreversible densification and persistent Si over-coordination localized beneath the indent. In contrast, AL glasses accommodate loading primarily through shear-mediated rearrangements of Al and Na, with Al[5], TBOs and Na coordination increasing during loading but largely recovering upon unloading. Pre-densification suppresses further volumetric compaction in both compositions and shifts the dominant mechanism toward shear flow, yielding broader non-affine displacement zones and a characteristic pile-up at the indent boundary. These results demonstrate that the balance between densification and shear is governed by the structural roles of Na and Al and can be tuned by pressure history. The mechanistic insights obtained here provide guidelines for designing silicate glasses with tailored resistance to surface damage and improved indentation response.