Mechanical, thermal, and structural investigations of chemically strengthened Na2O–CaO–Al2O3–SiO2 glasses

For a series of conventional soda-lime-silicate glasses with increasing Al 2 O 3 content, we investigated the thermal, mechanical, and structural properties before and after K + -for-Na + ion-exchange strengthening by exposure to molten KNO 3 . The Al-for-Si replacement resulted in increased glass network polymerization and lowered compactness. The glass transition temperature ( T g ), hardness ( H ) and reduced elastic modulus ( E r ), of the pristine glasses enhanced monotonically for increasing Al 2 O 3 content. H and E r increased linearly up to a glass composition with roughly equal stoichiometric amounts of Na 2 O and Al 2 O 3 where a nonlinear dependence on Al 2 O 3 was observed, whereas H and E r of the chemically strengthened (CS) glasses revealed a strictly linear dependence. T g , on the other hand, showed linear increase with Al-for-Si for pristine glasses while for the CS glasses a linear to nonlinear trend was observed. Solid-state 27 Al nuclear magnetic resonance (NMR) revealed the sole presence of AlO 4 groups in both the pristine and CS glasses. 23 Na NMR and wet-chemical analysis manifested that all Al-bearing glasses had a lower and near-constant K + -for-Na + ion exchange ratio than the soda-lime-silicate glass. Differential thermal analysis of CS glasses revealed a “blurred” glass transition temperature ( T g ) and an exothermic step below T g ; the latter stems from the relaxation of residual compressive stresses. The nanoindentation-derived hardness at low loads and <5 mol% Al 2 O 3 showed evidence of stress relaxation for prolonged ion exchange treatment. The crack resistance is maximized for molar ratios n ( M (2) O)/ n (Al 2 O 3 ) ≈1 for the CS glasses, which is attributed to an increased elastic energy recovery that is linked to the glass compactness.