**Author details**

Figure 6 Interdiffusion coefficient for potassium in the different baths.

Figure 7 Depth of penetration for potassium during the ion-exchange process in the different baths. The interdiffusion coefficients are in very good agreement with data reported in previous works [9, 10]. It is also confirmed that the ܦഥ is not really affected by the presence of limited amounts of Na (up to 5%) in the KNO3 bath [11]. Conversely, the surface concentration and, consequently, the concentration of potassium in the sub-surface layers are lower when the glass is treated in the Na-containing bath. On this basis, the K surface concentration in Equation (7) scales with the concentration in the used salt. Accordingly, the residual stress on the surface (shown in Figure 1) is higher when very pure KNO3 bath is used, while the case depth does not change to an appreciable extent. Nevertheless, the effect of Nacontaining salts on the final strength is substantially negligible, as shown in Figures 3 and 4. Clearly, this is mainly associated to the experimental scatter of the strength measurement, related to the typical dispersion on the surface defect sizes. In addition, due to the limited exchanging time used in the present work, some of the flaws are not "fully" reinforced; as a matter of fact, starting from the strength of the as-cut glass (ranging from ≈ 100 MPa to 400 MPa) and assuming, for simplicity, semicircular surface cracks, one can calculate that flaw sizes vary from ≈5 to ≈80 µm. Therefore, according to the case depth (Figure 2) and K penetration (Figure 7) results, it is evident that only a portion of the surface defects are completely "immersed" in the residual compressive stress field.. Deeper defects,, in a simplified model that considers flaws as invariant and perfectly closed during the ionexchange process, are conversely subjected to a residual stress that changes from highly

The interdiffusion coefficients are in very good agreement with data reported in previous works [9, 10]. It is also confirmed that the *D*ˉ is not really affected by the presence of limited amounts of Na (up to 5%) in the KNO3 bath [11]. Conversely, the surface concentration and, consequently, the concentration of potassium in the sub-surface layers are lower when the glass is treated in the Na-containing bath. On this basis, the K surface concentration in Equation (7) scales with the concentration in the used salt. Accordingly, the residual stress on the surface (shown in Figure 1) is higher when very pure KNO3 bath is used, while the case depth does not change to an appreciable extent. Nevertheless, the effect of Na-containing salts on the final strength is substantially negligible, as shown in Figures 3 and 4. Clearly, this is mainly associated to the experimental scatter of the strength measurement, related to the typical dispersion on the surface defect sizes. In addition, due to the limited exchanging time used in the present work, some of the flaws are not "fully" reinforced; as a matter of fact, starting from the strength of the as-cut glass (ranging from ≈ 100 MPa to 400 MPa) and assuming, for simplicity, semicircular surface cracks, one can calculate that flaw sizes vary from ≈5 to ≈80 μm. Therefore, according to the case depth (Figure 2) and K penetration (Figure 7) results, it is evident that only a portion of the surface defects are completely "immersed" in the residual compressive stress field.Deeper defects, in a simplified model that considers flaws as invariant and perfectly closed during the ion-exchange process, are conversely subjected to a residual stress that changes from highly compressive on the surface to slightly tensile at a certain depth (below ≈12 μm). The effect of the initial flaw sizes, i.e. of the surface quality of the bare glass, appears to be more important in the ion-exchange process than the presence of limited amount

The presence of a small amount (up to 5 wt%) of NaNO3 in potassium nitride bath partially influences the strengthening process of soda-lime-silicate glass when the treatment is carried

**Figure 7.** Depth of penetration for potassium during the ion-exchange process in the different baths.

ABCDEFG

**Salt**

0

of Na in the salt bath.

**4. Conclusions**

5

10

15

**Depth of Penetraion (μm)**

20

25

30

160 Ion Exchange - Studies and Applications

Hamid Hassani\* and Vincenzo M. Sglavo

\*Address all correspondence to: hamid.hassani@studenti.unitn.it

Department of Industrial Engineering, University of Trento, Trento, Italy
