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Factors affecting the quality of chemical tempering

Glass composition
Chemical tempering of glass is a treatment process that forms compressive stress on the glass surface based on the ion expansion mechanism. The compressive stress value is related to the volume change of the exchanged ions. From the perspective of ion exchange, it is most important to obtain an ion exchange thickness that meets the strength requirements in a short time. Generally, glass compositions with fast exchange speed and small stress relaxation are used.
Many scholars have studied the effect of AL₂O₂ on chemical tempering. The most studied is silicate glass, whose main component system and the role of each component in ion exchange are as follows:
① SiO₂-RO-R₂O;
② SiO-Al₂O₃-R₂O;

③ SiO₂-Al₂O₃-RO(Mg0 , CaO , SrO , Zn0 , BaO , PbO)-R₂O;

④ SiO₂-Al₂O₃-B₂O3-RO-R₂O;

⑤ SiO₂-Al₂O₃-RO-R₂O(ZrO₂ , TiO₂ , CeO₂)-R₂0.

In the components of systems ② to ⑤, when the SiO₂ content is below 50%, the chemical stability of the glass is poor, and when the content is above 65%, the raw materials are difficult to melt when producing glass. The SiO₂ content is suitable to be between 60% and 65%. Increasing the content of oxides such as Al₂O₃, ZrO₂, P₂O₅, and ZnO in silicate glass is beneficial to the chemical tempering enhancement effect.

Replacing SiO₂ with Al₂O₃ leads to an expansion of the structural network voids, which facilitates alkali ion diffusion. Additionally, the increase in volume is beneficial for the absorption of large-volume K ions, promoting ion exchange. The suitable amount of Al₂O₃ ranges from 1% to 17%. When the content is less than 1%, the chemical stability of the glass is poor, while content greater than 17% makes the raw materials difficult to melt during glass production. Increasing RO and decreasing SiO₂ have a negative effect on ion exchange, as the interaction of R+ with non-bridging oxygen is stronger than that with bridging oxygen. Using small amounts of RO to replace SiO₂ leads to a reduction in diffusion rate; smaller diameter R²+ ions polarize oxygen more strongly, forming a tighter bond, which weakens the R+ — O bond in R+ — OR²+. As a result, the diffusion coefficient of alkali ions in a glass containing small-radius R²+ is higher compared to a glass containing larger R* ions. Replacing SiO₂ with R+ also clogs alkali ion channels. Therefore, divalent metal oxides containing small ions have a limited impact on alkali ion diffusion, with ZnO and MgO being more favorable compared to CaO, SrO, BaO, and PbO. The addition of ZnO strengthens the glass, improves workability, and prevents glass devitrification.
The combined use of B₂O₃ and Al₂O₃ increases the depth of the reinforced layer and improves strength. For borosilicate glass, the reinforced layer after ion exchange reaches a thickness of 20–40 μm, with flexural strength increasing to 500–600 MPa, which is 10 to 20 times higher than before treatment. The combined use of ZrO₂ and Al₂O₃ also yields good reinforcement results, though the ZrO₂ content should remain below 10% due to melting difficulties and high forming temperatures above this level. Glass containing TiO₂ shows significant strength improvement after ion exchange. For instance, glass with 25.2% TiO₂ can achieve a flexural strength of up to 710 MPa after ion exchange.
Alkali metal oxides
In glass containing both Na₂O and K₂O, two types of sites accommodate different ion sizes. Many researchers have found that during ion exchange, the diffusion of alkali ions in the glass matrix is completed through ion jumps. In such glasses, there are four possible jump transitions for K+ (both K+ in the molten salt and K+ in the glass):
from a K+ position to an adjacent Na+ vacancy,
from a K+ position to an adjacent K+ vacancy,
from a Na+ position to an adjacent Na+ vacancy,
from a Na+ position to an adjacent K+ vacancy.
In case 1, compressive stress is generated due to the “crowding” effect since K+ has a larger radius than Na+. In case 2, the ion jump does not generate stress. Similarly, no stress occurs in case 3. In case 4, there is a stress relief process from the “crowding” state, opposite to case 1. Since the mobility of Na+ in the glass is much higher than that of K+, K+ from the molten salt tends to jump into Na+ vacancies in the glass and then to adjacent Na+ vacancies, as in case 1. The more ion exchanges occur the deeper the surface layer, leading to a greater compressive stress and a thicker compressive layer.
The diffusion of K+ from the molten salt medium to Na+ positions in the glass requires energy—specifically, the energy needed to push K+ into Na+ sites or expand Na+ vacancies to accommodate the larger K+ radius. This energy comes from the thermal energy gained by heating the molten salt. Assuming the static interactions between Na+ and K+ sites are equal, the activation energy for K+ gradually decreases after multiple diffusion events. Eventually, K+ reaches a certain depth within the surface layer of the glass, remains at Na+ positions, and stops jumping—signaling the end of the ion exchange process. Under suitable process conditions, K+ can be used to exchange with most alkali-containing (Na₂O, Li₂O) glasses to achieve certain strengthening effects, with Na₂O-CaO-SiO₂ and Na₂O-Al₂O₃-SiO₂ based chemically strengthened glass being the most widely used.
Molten salt is divided into
Molten salt is mainly composed of KNO₃ and other auxiliary additives, among which KNO plays a replacement role.

First, during the production process, the ions that diffuse from the inside of the glass accumulated in the molten salt will gradually increase, and the other ions that need to be replaced between the molten salt (liquid phase) and the glass surface will decrease. When the concentration of ions to be replaced on the glass surface decreases, the driving force of the exchange will weaken, and the exchange efficiency will decrease. When the exchange is carried out without changing the exchange time and exchange temperature, the ion concentration on the glass surface will decrease, which will reduce the strengthening effect of the glass. Therefore, it is necessary to continuously add new salt to reduce the concentration of Na+ in the molten salt.
Second, when there are other types of ions with ion radii smaller than the ion radius contained in the glass in the molten salt. The impact on the strength of the glass will be greater. Because this is a reverse exchange, there is a possibility that the ions with a small radius will replace the ions with a large radius on the glass surface, making the glass surface “loose”, which will greatly harm the strength of the glass. Table 4-3 lists the effects of trace impurities in molten salt on the bending strength of chemically tempered glass.

It can be seen from Table 4-3 that a small amount of Na+ mixed in the molten salt has little effect on the strength of the exchanged glass, while trace amounts of Ca²+ and Mg²+ significantly reduce the strength of the exchanged glass. In order to eliminate the influence of impurity ions, add about 2% K₂CO₃ to KNO₃ to cause the impurity ions to form carbonate precipitation, which can effectively enhance the tempering strength.
Effect of Trace Impurities in Molten Salts on Flexural Strength of Chemically Tempered Glass

4-3Effect of Trace Impurities in Molten Salts on Flexural Strength of Chemically Tempered Glass

Molten Salt Composition Processing Conditions Flexural Strength (MPa)
Pure KNO₃ Temperature: 450 ℃, Time: 2 h 236
Temperature: 450 ℃, Time: 4 h 269
KNO₃ + 0.137% Na⁺ Temperature: 450 ℃, Time: 2 h 214
Temperature: 450 ℃, Time: 4 h 261
KNO₃ - 0.137% Ca²⁺ Temperature: 450 ℃, Time: 2 h 118
Temperature: 450 ℃, Time: 4 h 119
KNO₃ + 0.137% Mg²⁺ Temperature: 450 ℃, Time: 2 h 195
Temperature: 450 ℃, Time: 4 h 253
Third, additives include accelerators and protective agents. Accelerators accelerate ion exchange and improve surface quality, such as KOH, K₂CO₃, KF, K₃PO₄, K₂SiO₃, K₂Cr₂O₇, K₂SO₄, KCl,
KBF₄, etc. After adding accelerators, ion exchange can be shortened to 25 minutes. The effect of adding accelerators to molten salt on the bending strength of flat glass is shown in Table 4-4.
Table 4-4: Effect of Accelerator on Chemical Tempering Performance

Table 4-4: Effect of Accelerator on Chemical Tempering Performance

Property KNO₃ Molten Salt KNO₃ + KOH (Trace) Molten Salt
Exchange Temperature (℃) 490 490
Exchange Time (min) 180 25
Stress Layer Thickness (μm) 40.6 50
Flexural Strength (MPa) 199.5 300.2

Ion exchange time

At a certain temperature, there is an optimal time to reach the maximum strength. When the glass begins to undergo ion exchange, the surface ion concentration and diffusion depth increase with time. At this time, because the concentration difference between the liquid phase (molten salt) and the solid phase (glass) at the interface between the glass surface and the molten salt is relatively large, the mass transfer driving force is large and diffusion is easy to carry out. As time goes by, this mass transfer rate will decrease with the accumulation of large-radius ions on the glass surface, but the rate of stress relaxation at constant temperature is constant. When the stress value generated by the diffused ions is less than the stress dissipated by stress relaxation, it will hurt the glass strengthening.

Table 4-5: Effect of Ion Exchange Time on Strength of 3mm Glass

Table 4-5: Effect of Ion Exchange Time on Strength of 3mm Glass

Ion Exchange Temperature (℃) Ion Exchange Time (h) Impact Strength (kg·m) Flexural Strength (MPa)
430 0.17 1.35 118.29
430 0.5 1.70 158.10
430 2 1.90 172.58
430 4 2.38 198.35
430 6 2.90 266.56
430 12 2.98 225.30
430 18 2.78 223.24
430 23 2.88 259.27
430 35 3.30 263.75

Ion exchange temperature

Without considering other factors, the distribution and diffusion depth of ion concentration on the surface of glass should increase with the increase in temperature, and the surface stress of glass should also increase with the increase of surface ion concentration and depth. But in fact, it is different. Because glass is an amorphous material, there is stress relaxation at high temperatures. Stress relaxation accelerates with the increase in temperature, which makes the speed of ion exchange decrease. Therefore, the surface stress obtained in the ion tempering process should be set by comprehensively considering the three factors of surface ion concentration, diffusion depth, and stress relaxation to set the ion exchange temperature.
When ion exchange is carried out at a temperature lower than the glass strain temperature, the speed of thermal diffusion is very slow. As the glass temperature increases, the ion exchange speed becomes faster, but the higher the temperature, the better.
It can be seen from Table 4-6 that when the treatment time is 6h, the chemical tempering strength reaches the optimal value in the temperature range of 410~495℃.

Table 4-6: Effect of Temperature on the Strength of 3mm Chemically Tempered Glass

Table 4-6: Effect of Temperature on the Strength of 3mm Chemically Tempered Glass

Ion Exchange Temperature (℃) Ion Exchange Time (h) Impact Strength (kg·m) Flexural Strength (MPa)
Original Glass - 0.4 60.30
370 6 2.54 181.6
410 6 2.90 266.56
430 6 2.60 280.35
450 6 2.40 268.56
450 6 2.32 233.25
470 6 2.08 208.00
480 6 2.02 170.46
490 6 1.80 138.29

Ion exchange capacity

The strength of chemically tempered glass is closely related to the radius of ion exchange. The greater the difference in ion radius, the greater the compressive stress on the glass surface. The thicker the ion exchange layer, the higher the bending strength of the glass surface. Selecting appropriate glass and molten salt components and increasing the amount of ion exchange can increase the compressive stress on the glass surface. For soda-lime-silicon architectural glass, KNO’s molten salt is selected. Theoretical calculations show that when the Na+ in the glass exchange layer is completely replaced by K+, the stress of the compressive stress layer on the glass surface can be as high as 3000MPa. However, when ion exchange is carried out, the concentration of K+ is only 50% to 70%, so the compressive stress on the glass surface is only a few hundred MPa.

Glass surface damage

As we all know, any damage on the glass surface will reduce the strength of the glass. For chemically tempered glass, the effect of surface damage on strength is more prominent. Because in chemically tempered glass, the stress distribution is not parabolic, and the depth of the surface compressive stress layer is very small. When the damage to the glass surface layer exceeds the thickness of the compressive stress layer, the strengthening effect no longer exists. Even if the damage does not exceed the compressive stress, the strength will be significantly reduced, because the surface pressure of chemically tempered glass changes sharply with the depth from the surface glass. Usually, the thickness of the compressive stress layer of chemically tempered glass is only tens of microns. Even if it is slightly damaged, the tempered strength of the glass will be severely attenuated.

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