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

Abstract

For a series of conventional soda-lime-silicate glasses with increasing Al2O3 content, we investigated the thermal, mechanical, and structural properties before and after K+-for-Na+ ion-exchange strengthening by exposure to molten KNO3. The Al-for-Si replacement resulted in increased glass network polymerization and lowered compactness. The glass transition temperature (Tg), hardness (H) and reduced elastic modulus (Er), of the pristine glasses enhanced monotonically for increasing Al2O3 content. H and Er increased linearly up to a glass composition with roughly equal stoichiometric amounts of Na2O and Al2O3 where a nonlinear dependence on Al2O3 was observed, whereas H and Er of the chemically strengthened (CS) glasses revealed a strictly linear dependence. Tg, 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 27Al nuclear magnetic resonance (NMR) revealed the sole presence of AlO4 groups in both the pristine and CS glasses. 23Na 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 (Tg) and an exothermic step below Tg; the latter stems from the relaxation of residual compressive stresses. The nanoindentation-derived hardness at low loads and <5 mol% Al2O3 showed evidence of stress relaxation for prolonged ion exchange treatment. The crack resistance is maximized for molar ratios n(M(2)O)/n(Al2O3)≈1 for the CS glasses, which is attributed to an increased elastic energy recovery that is linked to the glass compactness.

Hardness and scratch resistance of chemically strengthened alkali‐borosilicate thin glass

“…Overall, the magnitude of CS is consistent with the results from an earlier study using the same glass 27 . For comparison, notable larger surface compression on the order of ∼500 MPa is commonly realized in standard soda–lime silicate glasses, 4 whereas Na + /K + ‐ion exchange enables permanent surface compressive stress up to ∼1 GPa in sodium‐aluminosilicate glasses 15–17 . This large variability among different glasses can be explained at least partially by variations in the alkali oxide concentrations, in particular the amount of K + ions present in each glass before ion exchange.…”

“…To effectively prevent the generation of defects during glass processing or handling, surface compressive layers extending to depths greater than 50 µm are required 2,10,11 . The implementation of Al 2 O 3 was found to significantly improve the interdiffusion of alkali cations inside the glass, 4,12–14 leading to layer depths on the order of 10 2 µm along with the creation of very high levels of surface compressive stress up to ∼1 GPa 15–17 . This points out the central role of an optimized glass composition and adapted network topology for the efficiency of the ion exchange process 18 .…”

“…2,10,11 The implementation of Al 2 O 3 was found to significantly improve the interdiffusion of alkali cations inside the glass, 4,[12][13][14] leading to layer depths on the order of 10 2 µm along with the creation of very high levels of surface compressive stress up to ∼1 GPa. [15][16][17] This points out the central role of an optimized glass composition and adapted network topology for the efficiency of the ion exchange process. 18 Besides Al 2 O 3 , also other network forming oxides, such as P 2 O 5 , 19,20 ZrO 2 , 12 TiO 2 , 21,22 or B 2 O 3 , 20,23,24 and their specific effects on the buildup of a residual surface compressive layer through chemical strengthening have been studied in the past.…”

“…27 For comparison, notable larger surface compression on the order of ∼500 MPa is commonly realized in standard soda-lime silicate glasses, 4 whereas Na + /K + -ion exchange enables permanent surface compressive stress up to ∼1 GPa in sodium-aluminosilicate glasses. [15][16][17] This large variability among different glasses can be explained at least partially by variations in the alkali oxide concentrations, in particular the amount of K + ions present in each glass before ion exchange. During Na + /K +ion exchange, a majority of the smaller Na + ions inside the glass surface are replaced by larger K + ions from the molten salt bath (see Figure 1A).…”

Effect of complex nucleating agents on crystallization behavior of CaO–Al2O3–SiO2–ZnO–Na2O–K2O glass‐ceramics

“…[6][7][8][9] Many studies have been reported on enhancing the mechanical strength of CAS glass frit through chemical strengthening or crystallization. [10][11][12] Chemical strengthening involves an ion exchange process that takes place within a molten salt bath. During this process, previously contained alkali ions in glass composition, such as Na + , are exchanged with larger ions like K + , leading to the formation of compressive stress on the surface.…”