Mixed-alkali effect on hardness and indentation-loading behavior of a borate glass system

In this work, we investigate the mixed alkali effect on both hardness and indendation-loading behavior of the 75B₂O₃•xLi₂O•(25-x)Na₂O (mol%) glass series. It is found that the mixed alkali effect manifests itself as a minimum in the extent of the indentation size effect (ISE). This minimum could be connected to that of the plastic deformation induced by pressure. We also study the structural responses to indentation using the Raman spectroscopy. The results show that indentation leads to both incorporation of three-coordinated boron (B^III) into medium-range order units and transformation of B^III into four-coordinated boron (B^IV). This implies that the mixed-alkali effect on both hardness and indendation-loading behavior could be associated with the contributions of both short-range and medium-range order units to the structural changes.