Chemical Tempering (Chemically Strengthened Glass)

Chemical tempering was the first advanced materials science experiment that I ever performed. I attended a free ASM materials science camp, and by random selection I ended up with a group that performed chemical tempering by ion exchange (I will even include those results later in the article).

It took me a few tries to understand chemical tempering, but with the help of my illustrations, I hope you can understand it on the first try!

Chemical tempering is a way to strengthen glass by changing the chemical composition of the glass’s surface. The most common method of chemical tempering is by ion exchange, which expands the volume of glass at the surface. Like thermal tempering, chemical tempering creates a compressive layer on the surface of the glass which prevents cracks from opening, increasing the glass’s strength.

Chemically tempered glass may also be called chemically strengthened glass, chemically reinforced glass, or chemically toughened glass.

Basics of Glass Tempering

Chemical tempering is one type of glass tempering (the other kind is thermal tempering). Tempering is a way to make glass stronger by creating a compressive stress at the surface. (Don’t confuse this with steel tempering, which actually makes steel softer).

If you don’t know why a compressed surface is good for glass, click here to expand.

Click here to find out more.

Glass is a ceramic. Ceramics are very hard, but also brittle. Ceramics have directional bonding, which means that if you break a bond between two atoms, the bond will not easily reform with the next atom. 

That’s why ceramics–including glass–are so brittle. When atoms slide past each other, they form an atomically sharp crack. If forces open the crack past its critical size, the crack will travel at the speed of sound, connect with all the other micro-cracks and shatter the ceramic into thousands of shards.

However, if the ceramic experiences compressive forces, cracks will be forced shut. They won’t grow. This is why ceramics are much stronger in compression than tension.  Architecture involves arches and domes because these structures are made of ceramics, and arches use the force of gravity to keep the part in compression.

To create this beneficial compressive stress, engineers change the volume of the outside of the glass. For example, suppose you joined two pieces of glass together. Imagine that both sides started out as the same size, but then you caused the right side to increase in volume (I’ll explain how this is possible later). 

Since both sides need to stay joined, the right side wants to force the leftt side to expand (tension), and the left side wants to force the right side to contract  (compression). This causes the left side to experience compressive stress, and the right side to experience tensile stress.

For cracks to travel from left to right, they will need to overcome the compressive force.

    $$F_{critical} = F_{crack growth} - F_{compressive}$$

Where F_{critical} is min force to shatter everything. F_{crack growth} is force needed to open the crack.

For cracks to travel from right to left, they will be assisted by the tensile force.

    $$F_{critical} = F_{crack growth} + F_{tensile}$$

Obviously when you temper glass, you don’t want one side to be weaker than the other. To make both sides strong, you need to hide the compressive layer inside the glass. As long as the tensile forces do not initiate a critical crack by themselves, a crack will need to start from the outside, where the glass is strong! (However, once the crack does reach the inside, everything will explode.)

So instead of a side-to-side volume change, engineers temper glass with an inside-outside volume change.

There are two ways to temper glass: thermal tempering and chemical tempering. This article is about chemical tempering, but if you want to learn more about thermal tempering, I have written an article about Prince Rupert’s drops, which work because of thermal tempering.

How does Chemical Tempering work?

Chemical tempering creates a compressive outer layer on glass by expanding the volume of the outside surface of the glass. One common way to do this is by ion exchange.

Regular soda-lime glass (window glass) is made of SiO2 with some sodium atoms interspersed.

In ion exchange, engineers “trade” the sodium ions for a larger ion, such as potassium. Since potassium and sodium have similar chemical properties, a certain amount of ion exchange can happen just because of diffusion. With proper manipulation of thermodynamics and chemistry, it’s even possible to make the potassium prefer to replace sodium, even though the potassium is larger and strains the lattice.

Since the silicon and oxygen atoms don’t move, but now a larger atom is in the structure, the atomic arrangement is stressed. The glass experiences compressive force where the ion exchange has occurred. Since the ion exchange happens on the surface of the glass, the outside is strong.

And there you have it–chemical tempering by ion exchange! The outside has expanded in a layer of compressive strain, so the center of the glass must have a compensating tensile strain.

When I did this in high school, we placed glass slides in a bath of potassium nitrate at 450°C for 24 hours and 72 hours, and compared the strength (3 point bend test) with untreated glass slides.

As you can see, tempering the glass for a single day nearly tripled the glass’s strength. Longer times for ion exchange resulted in a slight decrease of strength. My theory for this is that–since the ion exchange happens at high temperatures–the thermal energy at long time will give the lattice enough time and energy to expand slightly, reducing the strain imposed by the oversized potassium ion. It’s also possible that potassium ions have started to replace sodium ions at the very center of the glass–this would decrease the difference in volume between the surface and core of the glass, reducing the compression layer.

Chemical Tempering vs Thermal Tempering

Thermal tempering is a physical process, chemical tempering is a chemical process.

In general, thermal tempering is less precise, but much cheaper to perform. Chemical tempering has many advantages over thermal tempering, but is more expensive. If thermal tempering will work for a specific application, it is probably the better option. However, many applications require chemical tempering.

Chemical tempering can be done on thin pieces of glass. The thickness of the compression layer is a function of time in ion exchange, not a function of overall glass thickness. 

Likewise, glass with non-uniform shapes or curves may have problems with thermal tempering. Certain portions of the glass can hold heat better than other (resulting in different cooling rates and tempered strength), and the sudden thermal stress may distort curves. Curve distortion is especially critical for applications requiring good optical properties.

Since thermal tempering may distort the final dimensions of the glass, chemical tempering is ideal for glasses where transparency and optical properties are especially important.

Furthermore, since chemically tempered glass does not have such a huge layer of tension, it can be cut, drilled, or otherwise processed after the tempering. Such processes often decrease the strength of the glass, but they don’t immediately shatter chemically tempered glass, like they would with thermally tempered glass.

Perhaps most importantly, chemically tempered glass can be 2-3 times as strong as thermally tempered glass.

Thermal tempering has a few advantages over chemical tempering, as well. Thermally tempered glass tends to have a much more powerful layer of tension, which will shatter broken glass into very small fragments. These fragments could cut someone, but they are not as dangerous as large shards that come from broken chemically tempered glass..

Additionally, the price of chemically tempering glass increases a lot with bigger pieces of glass. Sometimes, it’s not possible to find a huge vat to hold a dangerous chemical, so chemical tempering is not an option.

Applications of Chemical Tempering

Chemical tempering can be used in any application where it is worth spending money to make the glass stronger than thermal tempering.

Chemical tempering is the only option for tempering if the glass is thin, has complex shapes, or requires precise optical characteristics.

Chemically tempered glass is especially used for phone screens and screen protectors because it’s stronger than thermally tempered glass, and you’d rather have a few long cracks instead of the entire screen shattering. Additionally, the phone screen is very thin, so thermal tempering is impossible anyway.

Likewise, many glasses lenses are made of chemically tempered glass. This can make them scratch resistant without any optical distortion.

High-end fighter jets may have cockpit glass that is thermally tempered–the curved glass would not be ideal for thermal tempering.

I would also expect that many observatories have telescopes made from chemically tempered glass (although I checked for outer space telescopes, and the Hubble telescope has a different glass, Zerodur, which is used because of its low thermal expansion).

Gorilla glass, by Corning, is the most famous brand of chemically tempered glass.

Final Thoughts

Chemical tempering is a beautiful application of chemistry to materials science. 

Chemical tempering strengthens glass by ion exchange which creates a compressive outer layer of the glass. While this method is more expensive and time-consuming than thermal tempering, chemical tempering can achieve stronger glass. It is also a more-controlled process, allowing it to be used to temper fine glass parts such as phone screens.

References and Further Reading

Check out Corning’s gorilla glass procedure.

Click here to read a free review paper on chemical tempering.

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