Quantifying the densification and shear flow under indentation deformation in borosilicate glasses

Borosilicate glasses are used ubiquitously for a wide range of applications, where their mechanical properties play a critical role. However, the deformation mechanisms governing the sharp contact response of these glasses remain poorly understood. Herein, we analyze the role of elastoplastic response in determining the indentation deformation mechanisms for a range of borosilicate glass compositions. The series of glasses were made by varying the SiO₂-to-B₂O₃ molar ratio while maintaining a constant content of network modifying alkali and alkaline earth oxides. We employed nanoindentation followed by annealing below the glass transition temperature to quantify the contribution of densification and shear flow as a function of glass composition. Interestingly, we observe that the volume recovery upon annealing is inversely proportional to the hardness of the glasses. This suggests that the resistance to permanent deformation is closely related to the network connectivity of the glasses, which in turn governs the mechanism of deformation under sharp contact loading. Overall, we show the important role of alkali and alkaline earth modifiers in governing the composition-dependent indentation behavior of borosilicate glass series.