**2. Experimental procedure**

Commercial soda-lime-silicate float glass sheets with nominal thickness of 4 mm were used. Table 1 summarizes the composition of the glass.


**Table 1.** Composition (wt%) of the float glass

The sheets, obtained from one single original plate, were cut into square samples of 50 mm × 50 mm. The edges of the specimens were rectified and polished with SiC abrasive paper. The samples were then rinsed and cleaned gently in water, avoiding any damage on the glass surface.

A semi-automatic chemical strengthening lab scale furnace was used for the ion-exchange treatment. In each run, 20 samples were placed in a stainless steel basket to be treated at 450 °C for 4 h, with 30 min preheating and 30 min post-cooling above the bath. At least 60 samples were treated in each bath with the same thermal and time conditions.

Pure KNO3 and NaNO3 salts from Sigma Aldrich were used. Pure potassium nitrate was systematically poisoned by adding a specific amount of NaNO3 as reported in Table 2.


**Table 2.** Amount of NaNO3 added to pure KNO3 in the considered salt baths

After each treatment, the samples were rinsed with water and carefully dried.

The surface residual stress and the case depth were optically measured by surface stress-meter (FSM-60LE, Luceo Co., Ltd., Japan). Bi-axial flexural test [7] was used to measure the strength. This was carried out with a ring-on-ring configuration with the upper loading ring and the lower support ring having a diameter of 8 mm and 40 mm, respectively. The actuator speed was 1 *mm min* . The strength was determined from the maximum load (F) as:

$$
\sigma\_F = K \frac{F}{h^2} \tag{2}
$$

where *h* is the thickness of sample and

knowledge. It was in the 20th century that scientist started to understand the surface of chemically tempered glasses and figured out an exchange between monovalent cations in glasses with silver and/or potassium cations in the molten salts [2, 3]. These investigations increased the industrial application of the ion-exchange process, especially with the aim of

Glass strength can be largely increased by the ion-exchange process, otherwise called chemical

glass, is responsible for the creation of bi-axial residual compressive stress in the surface layers of the material. Because glass products generally break due to excessive tension applied at a surface flaw, the introduction of surface compression strengthens the glass component.

During the ion-exchange treatment, the glass matrix is considered as a solid negatively charged

The replacement takes place through an interdiffusion process, according to the Nernst–Planck

*Na K Na Na K K D D*

coefficient. One important parameter in the interdiffusion phenomenon is clearly the concen‐ tration of ions in the molten salt. Some studies have shown that the presence of poisoning elements (already present in the salt or coming out from the glass as in the case of Na) can hinder the diffusion of K+ into the glass and slow down the exchange process, thus reducing the strengthening efficiency. An important issue in industrial practice is also the replacement/ renewal of the molten salt which is time and money consuming [5]. Some researchers [3, 6], on the other hand, believe that the ion-exchange process is not affected in the presence of

In the present work, a systematic analysis of the effect of small amount of sodium as poisoning element in the molten bath on the performances of the strengthened soda-lime-silicate float

Commercial soda-lime-silicate float glass sheets with nominal thickness of 4 mm were used.

**SiO2 Al2O3 Na2O K2O MgO CaO** 71 1 13 1 4 10

equations. The flux of the ion species scales with the interdiffusion coefficient [2]:

is the fractional concentration of alkali ion *i* (Na or K) and D*<sup>i</sup>*

*D*

or Na+

from a molten KNO3 bath at temperatures below the strain point of the

from molten KNO3) responsible for the generation of a compressive stress.

in soda-lime-silicate glass) can be replaced by larger

*D N DN* <sup>=</sup> <sup>+</sup> (1)

in an alkali-containing glass, with

the self-diffusion

enhancing the optical and mechanical properties of glass [4].

tempering. The exchange of small ions such as Li+

structure where some mobile ions (Na+

specific amount of poisoning elements.

**2. Experimental procedure**

**Table 1.** Composition (wt%) of the float glass

Table 1 summarizes the composition of the glass.

glass was carried out.

larger ions such as K+

154 Ion Exchange - Studies and Applications

monovalent ions (K+

where N*<sup>i</sup>*

$$K = \frac{3\left(1+\nu\right)}{2\pi} \left( \ln \frac{D\_{\text{S}}}{D\_{\text{L}}} + \frac{\left(1-\nu\right)\left(D\_{\text{S}}^2 - D\_{\text{L}}^2\right)}{0.72D^2\left(1+\nu\right)} \right) \tag{3}$$

where *DS* and *DL* are the radius of the upper and lower supporting ring, respectively, *D* the sample size (50 mm), and *ν* the glass Poisson's ratio. A certain number of as-cut samples were also tested in this way for comparison.

Some fragments were collected from the broken samples and used for determining the potassium penetration profile. The fragments were attached on an aluminum disk by conduc‐ tive adhesive tape and then coated by sputtering with Au-Pd alloy. Clean and flat portions of the fracture surface were analyzed in a Scanning Electron Microscope (SEM) (JSM5500, Jeol, Japan) and the potassium Kα signal was recorded on a path of ~30 μm long by using the Energy Dispersion X-ray Spectroscopy (EDXS) (EDS2000, IXRF System, USA) probe. The chemical composition of the external surface of the glasses after the ion-exchange process was analyzed in the same way in a region of about 0.5 mm2 .
