Permanent Densification of Calcium Aluminophosphate Glasses

High-temperature densification of oxide glasses influences their interatomic distances and bonding patterns, resulting in changes in the mechanical and chemical properties. Most high-pressure investigations have focused on aluminosilicate and aluminoborosilicate based glasses, due to their relevance for the glass industry as well as the geological sciences. Relatively few studies have explored the pressure-induced changes in the structure and properties of phosphate-based glasses, although P ₂O ₅ is an important component in various multicomponent oxide glasses of industrial interest. In this work, we investigate the influence of permanent densification on the structure, mechanical properties (Vicker's hardness), and chemical durability (weight loss in water) of binary CaO-P ₂O ₅ and ternary CaO-Al ₂O ₃-P ₂O ₅ glasses. The densification of bulk glasses is obtained through isostatic compression (1–2 GPa) at the glass transition temperature. The binary CaO-P ₂O ₅ series is prepared with varying [CaO]/[P ₂O ₅] ratios to obtain glasses with different O/P ratios, while the ternary series CaO-Al ₂O ₃-P ₂O ₅ is prepared with a constant O/P ratio of 3 (metaphosphate) but with varying [CaO]/([CaO]+[Al ₂O ₃]) ratio. Using Raman and ³¹P NMR spectroscopy, we observe minor, yet systematic and composition-dependent changes in the phosphate network connectivity upon compression. On the other hand, ²⁷Al NMR analysis of the compressed CaO-Al ₂O ₃-P ₂O ₅ glasses highlights an increase in the Al coordination number. We discuss these structural changes in relation to the pressure-induced increase in density, Vicker's hardness, and chemical durability.