Gorilla Glass 2 vs 3 January 25 2014, 2 Comments

We all know glass break, so the big question is why does glass manufacturer decide to use Gorilla Glass as phone screen cover? Nearly all flagship Phones of 2013/2014 like Apple iPhone 6/6+ Samsung S5, Note 4 used Gorilla Glass 3 by Corning, though some other manufacturer like Sony, OnePlus opted for Japanese branded toughened glass (i.e Asahi dragon tail) Modern toughened glass is the result of multiple thermal and chemical treatment processes, increasing the strength of the material compared to ordinary glass.

If you look at the surface of a sheet of glass under a microscope, you will find it is full of tiny flaws and micro cracks. These flaws make glass really susceptible to breakage. In material term, they are known as stress concentrator which can amplify stress by up to 10 times due to its shape. If enough stress is applied, these cracks will propagate, result in fracture AKA shatter. If you take 2 sheets of paper, one is perfect and one has a small tear in the centre. If you pulled on the sides of the sheets of paper, the sheet with the small tear will require considerably less force to rip. Now imagine if the small tear was at the edge of the sheet of paper, even less force is required for it to propagate and eventually tip the paper in half. Stress can build up very easily at edges and even more at sharp corners; this is why aircraft are required to have windows with rounded corners.

Gorilla Glass like any other glass breaks easily before chemical toughen. What makes Gorilla glass unique is the “alkali-aluminosilicate” composition which enable glass to be transform by chemical process. Similar to the tempered glass, which causes a compression force on the outer surface of the glass, and essentially close the crack or makes the crack rounder. This toughens the glass by closing up some of those micro cracks, but also make the glass safer – if the glass breaks it will shatter into small pieces instead of large dangerous shards. Aside from tempering, a chemical process known as “ion exchange” also toughens the material.

Since glass contains a lot of sodium ion from the manufacturing process. As it is dipped into a hot molten potassium bath, the potassium ions diffuse (move) into the glass and displace the sodium ions. Potassium is larger than sodium and this also causes compression force on the surface of the glass — like tempering — which toughens the glass.

Gorilla Glass 3 don't break? 

The difference in brittleness (correlated by Young’s Modulus) between Gorilla Glass 2 and 3 is 71.5 and 69.3 GPa respectively. This is a 3% difference, and barely any difference. Just as a comparison: glass is usually around 50-90 GPa, mother of pearl is on average 70 GPa. (lower=more elastic, higher=more brittle)The axial stress (aka Poisson’s Ratio) is almost exactly the same, 0.21 to 0.22, which can easily be down to systematic and rounding errors.The fracture toughness (how hard it is for a crack to elongate/propogate) actually decreased from 0.68 to 0.66 MPa m^0.5, so if you have a crack in your screen, it will spread 3% easier. in other words gorilla glass 3 is harder but more brittle. This basically means that Gorilla 3 glass is better scratch resistance than the gorilla 2, which means a lot less stress concentrate, see previous post. 
that you should definitely note that little scratches like those from your keys on your smartphone when both are in your pockets accumulate, and once this damage builds up under some tension the glass would crack. So it is good idea to get a screen protector :D (Don't forget to pick up your screen protector here)


 


For those of you who is curious about sapphire, also see: