10 September 2014

Gorilla My Dreams

Lots of whining about no sapphire display in the 6s continues. This is especially true of the plungers who expected to get that cottage in Bimini off of the GT Advanced Tech stock they bought cheap, cheap a year ago. Sniff.

The likelihood that any company is foolish enough to make a pure sapphire screen for 5+ inch phone is just about 0.0 (well, may be not; no information on how the screen is built and not coming to a Best Buy near you any time soon). Why? The modulus of elasticity for sapphire is 345 GPa, while for Gorilla Glass (don't know whether all versions) is 71.5 GPa. It's called Young's modulus E (not, alas, in memory of yours truly), and it matters. It means that sapphire lacks ductility; twist it or bend it, and it shatters. This is well known. The reason it can be used in conventional watch faces is because a) they're generally 1 to 1&12; inches in diameter and b) the cases are themselves far more rigid than a 5+ inch phone. While a regular watch face, sapphire or mineral glass, can be punctured, torque stress is effectively zero in normal use. Can't say quite that for phones which spend most of their time in the ass-end pockets of pimply adolescents.

There would appear to be two ways to use sapphire for a 5+ inch phone screen. Either fabricators of sapphire figure out how to shave it off boules wafah tin, thus without wasting most of the boule in the process, and, figure out a way to laminate it to (Gorilla?) glass. Or, the engineers figure out a way to vapor deposit sapphire to glass. A quick innterTubes search yielded lots of links to deposition on sapphire, but none to deposition of sapphire. Which isn't surprising. Although... the innterTubes does show lots of papers (most for $$$) describing vapor deposition of aluminum oxide, which is the base molecule of sapphire; not crystal, though.

As to the "slicing" bit, turns out, they have; basically, boil sheets off the boule. Very cool engineering!! Hot off the presses, too.

Here is a piece on Apple patenting a laminate. A year ago.
The use of a sapphire outer surface with a glass inner surface for the display may be used in future iDevices where the two sapphire surfaces are laminated together with the glass providing support for the display and the sapphire providing scratch resistance and durability advantages.

I swear; I wrote the first paragraph before I stumbled on the Apple patent. I swear to the Great Jehovah. In any case, Apple/GT couldn't git er done for the 6s.

Anyway. The reason sapphire becomes practical in laminate, is that for such thin sheets, bending stiffness improves ductility. Somewhere in materials' science I expect there's a bunch of algebra which demonstrates that a sufficiently thin sheet of sapphire is more elastic. Durned if I could find it though, but it sure looks like Apple thinks so.

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