Mixed cation effect in sodium aluminosilicate glasses

While the composition dependence of Vickers hardness of network glasses has received significant attention in the glass literature, the underlying deformation mechanisms and their respective resistances are not yet fully understood. Here, we investigate the relationship among Vickers hardness, network structure, and the resistances associated with the deformation processes in mixed cation glasses by partially substituting magnesium for calcium and calcium for lithium in sodium aluminosilicate glasses. We use Raman and 27Al NMR spectroscopies to obtain insights into the structural and topological features of these glasses and we use AFM to quantify the resistances associated with each deformation process under Vickers indentation. We demonstrate that the mixed cation effect manifests itself as a maximum in the amount of bonded tetrahedral units and as a minimum in liquid fragility index, glass transition temperature, Vickers hardness, and plastic flow. The observed minima in fragility, glass transition temperature, and plastic flow are ascribed to bond weakening in the local structural environment around the network modifiers. The elastic properties of the investigated system are found to be compositionally independent. Therefore, based on the AFM measurements, we suggest that the minimum in Vickers hardness is closely correlated to the minimum in plastic flow. Both of these properties are related to the translational motion of structural units, and hence, both may be related to the same underlying topological constraints.