The iPhone 4 glass – so exactly what is it?

Jul 19, 2010

I have been hearing alot about the new iPhone 4′s glass front and back panels. Apple has deviated quite far from its last few generation iPhones in that it has gone from the glass front, with aluminised bezel and plastic backing – to a fully glass front and back. Apple claim that the new iPhone’s glass is “Chemically strengthened to be 20 times stiffer and 30 times harder than plastic, the glass is ultradurable and more scratch resistant than ever.”. So I got thinking – from the basic information on the Apple website, and some educated guesses – can I figure out exactly what is in Apple’s new “special” glass.
According to Apple’s website the glass is an Aluminosilicate which has been “chemically strengthened to be 20 to 30 time harder than plastic.”. An Aluminosilicate glass in its most basic form is a glass containing aluminium oxide (otherwise known as alumina – Al2O3), and silica. The problem is, that both alumina and silica are both network formers, and as such require very high temperatures to form a homogeneous glass. In order to reduce the melt temperature of the glass, glass manufacturers will add a network modifier such as lithium, sodium or potassium. The addition of nucleation agents such as TiO2 or TiO+ZrO2 and the selection of the optimum heat treatment schedule controls the distribution and morphology of the final crystal structures. Many other components can be added in order to optimise the crystalline phases and hence the glass–ceramic properties. Sodium aluminosilicates and barium–sodium aluminosilicates have high expansion and can be strengthened by surface compression techniques such as the application of a low expansion glaze.

So if I were to be a betting man, I would stick a few dollars down on the “special” Apple glass being a glass-ceramic similar to a barium-sodium aluminosilicate, which has had a long annealing to remove internal stresses, and then been surface treated. This would give the properties described by Apple.

The common mis-conception (or possible mis-leading by Apple) is the durability of the glass panels. The problem with glass is that, although it is hard in terms of scratch resistance, it is rather brittle when subjected to tensile loading (e.g a small drop onto a hard surface, such that the impact is on a side or corner). The lovely Apple image showing the iPhone 4 glass panel under going a flexion test it great publicity, but rather exaggerates the glasses durability. Slow loads are ok – impacts are not so good.

Either way I like the idea of using glass. The design of having the glass raised slightly above the surrounding bezel is not such a good idea. It would not surprise me if Apple change their design to keep the glass but sink it slightly lower than the bezel edge and have a thin rubber gasket between the glass and the metal bezel.