Chemical tempering process

In chemical tempering, the glass is strengthened through two main ion-exchange processes, each of which plays a crucial role in altering the surface properties of the glass to improve its strength and durability.

High temperature ion exchange process

In the temperature range between the softening point and the transition point of glass, a thin layer with a smaller expansion coefficient than the glass matrix is formed on the glass surface through ion exchange between glass and molten salt. When cooled, a compressive stress layer is formed on the glass surface due to the inconsistent contraction of the surface layer and the matrix. The magnitude of the stress can be calculated by formula (4-6).

σs=E(1-v)-¹(a1—a2)△T (4-6)
Where o₈——surface stress, MPa;
E——glass elastic modulus, GPa; v——Poisson’s ratio;
a₁,a₂——expansion coefficients of inner and outer layers of glass, 1/℃; △T——temperature difference, ℃.

The high-temperature ion exchange method is to replace the alkali metal ions with a large radius in the glass with alkali metal ions with a small radius. The specific method is: to place the Na₇ (-Al₂O₃SiO₂ system glass in a high-temperature molten salt containing lithium ions, so that the Na+ on the surface of the glass is exchanged with Li with a smaller radius than them, and then cool to room temperature. Since the expansion coefficient of the surface layer containing Li+ is different from that of the inner layer containing Na+ or Li+, residual compressive stress is generated on the surface and strengthened. At the same time, if the glass contains Al₂O₃, TiO₂, and other components, through ion exchange, a crystalline surface layer of β-eucryptite (LiO·Al₂O₃·2SiO₂) or β-spodumene (Li0·Al₂O·4SiO₂) with a very small linear expansion coefficient is formed on the surface layer. Since the expansion coefficient of the surface layer is different from that of the glass matrix, the matrix cools and shrinks, and the surface layer prevents it from shrinking. The surface layer is subjected to compressive stress, so that the glass is obtained, and the strength can be as high as 700MPa.
For example, a glass containing 57% to 66% (mass fraction, the same below) SiO₂, 13.5% to 23% Al₂O₃,
38% to 11% Na₂O, and 10% to 1% Li₂O is immersed in a molten salt of Li+, Na+ Ag+ at 600 to 750°C. The Na+ in the glass is replaced by Ag+ or Li+, producing a double exchange layer, that is, the outer side is β-eucryptite and the inner side is a lithium metasilicate crystallized glass layer, which can greatly increase the strength.

Except for special products, general products are not produced by high-temperature methods because the processing temperature is high and the energy consumption is large. In addition, the required materials are the most expensive among alkali metals, which increases the production cost.

Low temperature ion exchange process

The low-temperature ion exchange process is to immerse the glass in a molten salt containing metal ions with a larger radius than the alkali metal ions in a temperature range not higher than the glass transition point, and ion exchange occurs between the glass and the molten salt. Large ions exchange small ions (such as K + replaces Na + in glass, K + radius is 0.133 nm, Na + radius is ), and the volume difference between the exchanged ions causes a “crowding” effect on the surface layer of the glass, forming a surface stress layer, which improves the strength of the glass. Figure 4-3 shows the exchange of K + and Na +. Although the exchange speed is slower than that of high-temperature ion exchange, it has practical value because the manufacturing cost of low-temperature ion exchange is low and the glass does not deform during processing.

The specific method is to put Na₂O-Al₂O₃-SiO₂ glass into a molten potassium nitrate (KNO₃) tank and release K+ in potassium nitrate salt to replace Na+ in the glass on the surface of the glass. Since the ion exchange is carried out at a temperature lower than the strain point, the glass does not show viscous flow. Since the radii of K+ and Na+ are different (the radius of K+ is 0.133nm, and the radius of Na+ is 0.099nm), the K+ with a large ion radius occupies the vacant position vacated by the Na+ with a small ion radius, resulting in a surface “crowding” phenomenon, which leads to a large compressive stress in the surface layer, thereby strengthening the glass.

The compressive stress of the surface layer of chemically tempered glass can be calculated by formula (4-7).

σs=(1−ν)⋅VE⋅ΔV
(4-7)
where os——surface stress, MPa;
E——glass elastic modulus, GPa; v——Poisson’s ratio;
V——the volume of glass before ion exchange, m³; △V——the volume difference caused by ion exchange, m³.
According to theoretical calculations, industrial Na₂O-CaO-SiO₂ glass generally contains about 15% (molar fraction) of Na₂O and a density of about 2.5g/cm³. It is calculated that 1cm³ of glass contains about 7×1021 Na+. If all these Na+ are replaced by K+, a volume change of about 4.5% will be generated, so the compressive stress on the glass surface caused by ion exchange is calculated to be about 900MPa, but this calculated value is difficult to achieve in actual production. Burggreaf has proved that the density of glass obtained by ion exchange is much higher than that of glass of the same composition obtained by conventional melting steps, while the volume change is less than the expected value, so the observed stress is also lower than the calculated value. In addition, there is also stress relaxation during the ion exchange process. In addition, the thickness of the ion exchange infiltration layer with commercial value is generally between 20 and 30 μm. If there are microcracks that penetrate the infiltration layer deeply, the compressive stress generated by ion exchange and the tensile stress balanced with it will act on the microcracks at the same time. Moreover, the tip of the microcrack is just in the tensile stress zone, which also affects the improvement of strength.

(1) Low-temperature ion exchange process flow As shown in Figure 4-4
(2) Production process formula and parameters
① Molten salt material
Main material: KNO₃ (chemically pure grade) 85%~98% (mass ratio).
Auxiliary additives: Al₂O₃ powder, potassium silicate, diatomaceous earth, others 2%~15% (mass ratio).
② Molten salt temperature of salt bath The general temperature is 380~500℃.
③ Exchange time Determined according to product enhancement needs and processing temperature, the exchange time will generally not be extended due to the glass thickness increase.
④ Design furnace temperature

Low-temperature preheating furnace: 200～300℃. High-temperature preheating furnace: 350～400℃. In molten salt tank: 410～500℃. High-temperature cooling furnace: 350～450℃. Medium-temperature cooling furnace: 200～300℃. Low-temperature cooling furnace: 150～200℃.
⑤ Selection of salt bath material The composition of molten salt determines its strong corrosiveness. To maintain the long-term activity of molten salt and the safety of production, the high-temperature corrosion resistance of salt bath materials should be good. Generally, most molten salts can be contained in stainless steel or high-silicon glass salt baths. Molten salts containing chloride ions are corrosive to stainless steel, so it is best to be contained in high-silicon glass salt baths. In actual production, to prevent accidents, the above-mentioned salt bath must be placed in a larger heat-resistant metal pool filled with fine sand (the temperature can be controlled at ±1℃).

Contact Us

Your feedback fuels our growth, and your questions drive our solutions.

We value your feedback, inquiries, and suggestions. Please feel free to get in touch with us

General inquiries

Please contact us via sales@bo-glass.com, and we will reply to you as soon as possible.

Interested to work with us

Drop your resume at info@bo-glass.com
and we will get back to you shortly.

We uses the contact information you provide to us to contact you about our relevent content, products, and services.