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Why sapphire iPhone screens could be Apple's next big thing

Marco Tabini | July 30, 2014
Because of its higher strength, less sapphire is needed to make future mobile devices, thus making them lighter and thinner.

This problem is traditionally solved by making glass thicker, or--ironically--interleaving it with panes of plastic, as is the case, for example, with most modern car windshields. This approach, however, wouldn't help much in the case of a smartphone, since nobody would presumably want to carry around (or buy) a device protected by a one-inch pane of heavy glass.

As told in Walter Isaacson's biography, Jobs's quest for an iPhone-worthy glass took him all the way to Corning, a small city of 12,000 in south-central New York state that was named after the railroad tycoon who funded most of its original development in the 1800s--and is also home to a Fortune 500 glass manufacturer that shares its name.

Corning (the company) knew a thing a thing or two about making glass tough. It had built the windows for the Space Shuttle, and spent decades researching and developing special glass that would retain high degrees of strength even when laid out in thin sheets for applications like car racing and aerospace manufacturing, where weight comes at a premium.

Gorilla on a diet
At Corning, Jobs found--and eventually convinced CEO Wendell Weeks to mass produce--a material that the company had dubbed "Gorilla glass." As clear as plate glass, Gorilla sported a higher Mohs value and much better strength, making it perfect for Apple's upcoming family of smartphones.

Gorilla glass begins its life with a composition that is similar to regular glass: A brew that includes aluminum, silicon, oxygen, and alkali like sodium is melted together into a substance that lacks a specific overall structure--what chemists call an amorphous solid. This is then cooled and floated into sheets of arbitrary thickness.

Unlike regular glass, however, Gorilla undergoes a further chemical process, during which it is submerged in potassium nitrate--a salt that you may better know as one of the compounds that give bacon its pink color and keep it from spoiling into a cesspool of harmful bacteria intent on destroying humanity.

This is no ordinary brine, however: The nitrate is heated to more than 300 degrees Celsius, at which point it melts. When the plain glass comes into contact with the molten salt, a process known as ion exchange causes the sodium ions inside the glass to be replaced by the potassium in the nitrate.

Because potassium atoms are much bigger than their sodium counterparts, when the glass cools down again, they push harder against each other, creating a layer of compression stress in the finished material that helps prevent cracks from forming, thus making Gorilla glass more shock resistant while retaining the imperviousness to scratches that makes glass screens more desirable than those made of plastic.


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