The Fusion Draw
While you might think of glass in its manufactured form, the truth is it’s a material we find in nature. Certain rocks and minerals become glass after coming into contact with high temperatures. This occurs naturally along lava flows and places where lightning has hit the ground.
Humans have been creating glass for millennia. Furnaces capable of generating incredible heat melt the right type of rocks down into what we call a glass melt. At this stage, you can shape the glass in many ways, including using a tube to push air into the mass. We call this technique glass blowing.
Commercial glass tends to come from three main sources. The first is sand, which we refer to chemically as silicon dioxide. That’s the type of material Corning uses in its manufacturing process. The other two types of materials in commercial glass include limestone and sodium carbonate.
Corning takes the silicon dioxide (SiO2) and combines it with other chemicals before melting it down into a glass melt. The resulting glass is aluminosilicate — that means the glass contains aluminum, silicon and oxygen. The glass also contains sodium (Na) ions, which become important in the next phase of manufacturing.
Corning pours the molten glass into a V-shaped trough but doesn’t stop at filling the trough to the top. The company continues to add molten glass until the glass begins to overflow the sides of the trough. Automated robotic arms draw the sheets of glass from the edge of the trough. Each sheet is just over half a millimeter thick.
If you were to use this glass for a screen on your electronic devices, you’d end up with a very clear covering. But it’s not damage-resistant like Gorilla Glass — it’s just aluminosilicate glass. To give Gorilla Glass its ability to withstand scratches and cracks, Corning gives these sheets of glass a little bath.
The Ion Exchange
The real secret behind Gorilla Glass involves a chemical process called an ion exchange. An ion is an atom that has either gained or lost an electron and so carries a net charge. Electrons are negatively-charged sub-atomic particles. An ion’s net charge is negative if it has an extra electron or positive if it lost an electron. Elements in their atomic form have a neutral charge because the number of electrons matches the number of protons, which are positively charged.
So what do ions have to do with glass? The aluminosilicate glass from the first phase of the manufacturing process contains sodium ions. Corning dips these sheets of glass into a bath of potassium ions. If you look at the periodic table of elements, you’ll see that sodium is just above potassium. Dmitri Mendeleev, the man responsible for creating the periodic table, arranged elements by atomic weight and grouped elements possessing similar qualities together. Both sodium and potassium belong to a group known as active metals. These are metals that react strongly with other substances.
Sodium is higher on the periodic table than potassium, which means an atom of sodium is smaller than an atom of potassium. You might think that at the atomic scale size doesn’t matter but it turns out that’s not the case! If you could take the sodium ions out of the aluminosilicate glass and replace them with larger potassium ions, the sheet of glass would experience compression.
Imagine you have a net. The line in the net is flexible but taut — there’s not a lot of give. In each hole of the net there’s a golf ball held into place. Now imagine that you replace all the golf balls with baseballs. That’s similar to what’s happening on an atomic level with an ion exchange.
So how does it work? To replace sodium with potassium, you first must break the ionic bond sodium has with the glass. That’s why the potassium salt bath is so hot — Corning says the bath reaches a temperature of 400 degrees Celsius (752 degrees Fahrenheit). At this temperature, the energy (heat) breaks down sodium’s ionic bond to the aluminosilicate. But one of the qualities lower active metals have is that they can maintain an ionic bond at higher temperatures than the lighter active metals. Potassium weighs more than sodium — that 400 degrees Celsius (752 degrees Fahrenheit) isn’t enough to keep potassium ions and the aluminosilicate apart.
After a nice hot dip in the potassium bath, the aluminosilicate emerges compressed by potassium ions. The compression creates a protective layer on the glass and gives it strength that normal glass doesn’t have. And the environmentally conscious can rest easy — Gorilla Glass is recyclable.
For more Detail: How Gorilla Glass Works