Chemically tempered glass

Chemically strengthened glass has several applications. Often it comes down to wanting to protect a device with a strong, thin glass – for example a touchscreen used in a demanding environment. It can also be when you need a strong, thin glass to protect a camera solution or a sensor. Most mobile phones today, for instance, are protected by chemically strengthened glass.

All glass can be scratched. But chemically strengthened glass has higher scratch resistance than ordinary glass, which is why it is often used in harsh environments. One of its advantages is that it is strong even though it is thin. If you need glass with the highest possible scratch resistance, sapphire glass is the solution

Chemically strengthened glass is made in a specially heated salt bath, where a chemical reaction starts an ion exchange. Here, smaller sodium ions are replaced by larger potassium ions, which in effect seals the surface. This makes the glass more resistant to scratches, temperature fluctuations, impact and pressure. A special glass type, aluminosilicate glass, is often used as well, as the salt bath works even more effectively on it and the glass becomes even stronger. Other types of glass can also be chemically strengthened, but the glass should contain at least 10% sodium.

These are two different ways of processing the glass, so it depends on the task. As a rule, thicker glass is best suited to thermal toughening and thinner glass to chemical strengthening – but more on that below.
Thermally toughened glass can be toughened down to 3 mm (2.8 mm). The glass becomes 4–6 times stronger and withstands temperature changes of up to 250°C (at 280°C the glass begins to anneal, which over time means it is no longer toughened). If thermally toughened glass breaks, it crumbles into small, blunt granules, which is why thermally toughened glass is also classified as safety glass.
Chemically strengthened glass is best when thin glass needs reinforcing, because chemical strengthening is a surface treatment. This also means the surface becomes stronger – that is, more scratch-resistant – than thermally toughened glass, while the strength in some cases increases by up to 10 times compared with untreated glass. Like thermally toughened glass, chemically strengthened glass withstands a temperature differential of 250°C, but it too will anneal over time if the temperature stays above 280°C. If chemically strengthened glass breaks, it also breaks into smaller pieces, but because these are not blunt, the glass cannot be approved as safety glass. This can be solved, however, by laminating the glass.

Plexiglass is a trade name – the correct product name is acrylic/PMMA.
In terms of impact strength, at the same thickness (e.g. 3 mm), polycarbonate is the strongest, followed by acrylic, then chemically strengthened Kemirit, then thermally toughened glass, and finally annealed soda-lime glass.
Glass is considerably more resistant to scratches. In relative order (hardest to softest): Kemirit (chemically strengthened aluminosilicate glass), chemically strengthened soda-lime glass, thermally toughened soda-lime glass, annealed soda-lime glass, polycarbonate, acrylic.
Temperature: chemically strengthened glass withstands up to 250°C, acrylic up to 70–80°C.

During chemical strengthening, the glass is placed in a special salt bath (usually with a potassium salt), where some of the glass’s small building blocks are replaced by slightly larger ones. Specifically, sodium ions in the glass surface are exchanged for larger potassium ions from the salt bath. The larger ions take up more space and compress the glass surface.
So when we chemically strengthen glass, we introduce a high compressive stress into its surface. This surface stress acts as a kind of “armour” that increases the glass’s resistance to damage.

Mirit Glas makes chemically strengthened glass in-house – the only manufacturer to do so in the Nordics. When the very large glass factories make chemically strengthened glass, it is usually in very large production runs, for example for mobile phone manufacturers. Mirit Glas can produce chemically strengthened glass in both smaller runs – for example for prototypes – and larger runs, and always in a shape tailored to the customer’s needs.

Aluminosilicate glass is the name for the glass type in which, in addition to ordinary sand, aluminium oxide has been added – normally between 20% and 40% of the glass. This allows the glass to take up the chemical strengthening better and faster than other types of glass. As a result, aluminosilicate glass also becomes stronger after chemical strengthening than other glass types that can be chemically strengthened.

Thermally toughened: no. Chemically strengthened: yes, but it is difficult to control.
When glass is thermally toughened, controlled stresses are introduced into it by heating it to +600°C and then rapidly quenching it. The surface cools much faster than the core. This process creates permanent compressive stresses in the surface and tensile stresses in the core. These stresses are released if you try to cut the glass, and the whole sheet breaks into small, blunt granules.
Chemical strengthening is a surface treatment in which smaller ions are exchanged for larger ones, and the process only reaches a short distance below the surface. It is therefore possible to cut the glass after chemical strengthening – although it can be difficult to control, as there are strong stresses in the surface.

Not quite. “Gorilla Glass” is a trademark (from Corning) for a specific aluminosilicate raw material that is chemically strengthened – that is, one particular product. Chemically strengthened glass is the process and product category itself. Mirit Glas uses similar aluminosilicate raw materials (Kemirit) that are equally well suited to chemical strengthening. What matters is the glass’s chemical composition, including a sodium content of at least 10%.

Chemical strengthening is particularly suited to glass thinner than 3 mm and can be carried out on glass as thin as 0.1 mm. Glass thicker than 3 mm can also be chemically strengthened, but because it is a surface treatment, the proportional impact and bending strength decreases the thicker the glass is. You do, however, retain the scratch and temperature resistance. For thicker glass, thermal toughening is often a better choice.

Yes. Mirit Glas makes chemically strengthened glass in-house – the only manufacturer to do so in the Nordics – in everything from prototypes and smaller runs to larger runs, tailored to the customer’s needs. Glass that is already curved can also be chemically strengthened. After strengthening, the glass can be ground, cut and drilled, but the exposed edges are not strengthened and reduce the strength at that particular point.

No, not adversely. Because chemical strengthening reinforces the glass through ion exchange (replacing ions in the surface) and does not affect the surface mechanically, the original optics are preserved. The optical quality is therefore high, which makes the glass well suited to screens, cameras, sensors and other solutions where clarity is essential.

Not on its own. Chemically strengthened glass shatters into small pieces – but not into the small, blunt pieces that thermally toughened glass produces – and therefore cannot be approved as safety glass on its own. If it needs to meet a safety requirement, it can be laminated. Alternatively, thermally toughened glass (at least 3 mm) is chosen where it is essential that the glass breaks into granules.

The ion exchange itself usually takes 16–18 hours. The glass is lowered into a tank of heated, molten potassium nitrate, where smaller sodium ions in the surface are exchanged for larger potassium ions. This creates a compression zone (compressive stress) in the surface and a tension zone in the core – and it is the compression zone that makes the glass stronger. Note that this is the process time; the total delivery time depends on the task.