Structural Changes upon Ambient Temperature Densification in Calcium Aluminoborosilicate Glasses: Relation to Indentation Crack Resistance

Improving crack initiation resistance is an important objective of current glass research. High-pressure and indentation experiments are considered important predictors for the compositional design of glassy materials with optimized performances. For developing insights into the stress dissipation mechanisms, it is important to understand their structural aspects at the atomic level. In the present contribution, the structural consequences of ambient-temperature densification via pressurization at 25 GPa were studied by solid-state NMR and Raman spectroscopy for a series of calcium aluminoborosilicate glasses with composition (70–x)SiO₂–xB₂O₃–15Al₂O₃–15CaO (x = 0, 5, 15, 25). Cold pressurization resulted in a significant increase in the average coordination numbers of both boron and aluminum, in line with previous results found in hot-compressed alkali and alkaline-earth aluminoborosilicate glasses. Advanced ¹¹B/²⁷Al double resonance experiments suggest that B–O–Al connectivities play an important role in the structural transformation processes, contributing to the experimentally observed increase of indentation crack resistance with increasing B₂O₃ content. Our results indicate that the extent of densification is a poor predictor of indentation crack resistance and highlight the significance of medium-range order effects.