On Gorilla Glass

Stress helps glass resist damage. By incorporating it into the manufacturing process, Engineers at Corning, Inc., in N.Y., can give a normally fragile material super-strength. Their Gorilla Glass product now forms the screens of more than 1,000 different devices, from smartphones to tablets to televisions.

To avoid building flaws into the material, Corning creates large, flat panes of Gorilla Glass mechanically. During the process, the molten glass is suspended by its top edge, leaving it untouched by human hands—or anything else. Despite their stability, these sheets cannot prevent future damage...yet. The next step is to apply stress to the glass, compressing its molecules to strengthen the material and enable it to resist flaws.

Cut to appropriate sizes, the unfinished Gorilla Glass then takes a bath in a molten solution of potassium salts. This process leaches small sodium ions out of the glass and replaces them with larger potassium ions. The large particles squeeze the sheet from the outside in, compressing the material.

This creates two outer layers squeezing inwards, towards a central layer that balances out the internal forces by pushing back.

"You have an equilibrium of stress and tension," explained Marcus Haynes, a senior applications engineer at Corning. "There's a layer of compressive stress, then a layer of central tension, where the glass wants to press out, then another layer of compressive stress."
Compressing the surface of the glass makes it stronger, able to resist blows and scratches rather than breaking immediately.

"Even if you damage the glass, the flaw is contained within that compressive stress layer," Haynes elaborated. "It doesn't allow the flaw to expand." In order to break a Gorilla Glass screen, a flaw would have to penetrate through the compressive layer and into the tension layer.

Although the ingredients that go into Gorilla Glass also help the material withstand damage, stress is the real key to its abilities.

Source: Physics Buzz