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Are all “tempered glass” screen protectors scam?
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tooki:

--- Quote from: tom66 on December 22, 2020, 09:17:46 pm ---I did have one smartphone that definitely had tempered glass, I had it in my pocket and bumped into the side of my car while unloading shopping and the entire screen shattered.  It was a budget Android phone that I'd purchased to replace my sadly deceased Moto G4 that was damaged by water.

--- End quote ---
That's kinda how phone screens tend to behave anyway. Do we know that chemical strengthening doesn't end up creating a similar effect (the way internal stresses are distributed) as thermal tempering?
tom66:
I think there's a difference.  A tempered screen is under compressive/tensile forces so any crack will be catastrophic.  Whereas most smartphone screens seem to crack in one place, there's limited spread of the failure.

My guess is the cheapest devices use tempered glass, but the more premium devices use things like Gorilla Glass which as you mention are chemically treated non-tempered glasses.  The chemical treatment doesn't put the glass under forces similar to tempering. 
m98:
First of all, you need to notice that the glass used for screen protectors and as a screen cover glass is far more advanced than what you find at your local bus-stop. Gorilla Glass and the likes are Alkali-Aluminosilicate glasses that get chemically tensioned by exchanging the smaller natrium for potassium ions in a molten salt bath.
Since the compressed surface layer created by chemical tensioning is very thin, the glass still shatters in big shards, which is actually advantageous for the application. If you want safety-glass that shatters into thousands of dull pieces, which is probably the kind of tempered glass you had in mind, you put regular soda-lime-glass though a classical thermal tempering process.
SVFeingold:
Chemical strengthening does indeed create similar tension/compression stresses. It is how the glass is made more resilient to bending stress. The more the surface is in compression, the more you will have to bend it to get it into tension and initiate a fracture. The untreated core of the glass and the compression/tension interface is very close to the neutral bending plane and so doesn't experience as much stress. Generally if you damage the glass to a depth less than the thickness of the compression layer it will chip or scratch. Dig into the tension/compression interface and it will crack.

There is all kinds of neat stuff you can learn from a cracked screen. Where exactly the crack was initiated, based on the crack pattern, what type of damage, and even how much stress the glass was under when it broke. Glass failure analysis is pretty fascinating.

But preferably, you want to avoid the need to do failure analysis in the first place. And for that the screen protectors generally do what they're supposed to. :)
tooki:

--- Quote from: m98 on December 23, 2020, 12:58:17 pm ---First of all, you need to notice that the glass used for screen protectors and as a screen cover glass is far more advanced than what you find at your local bus-stop. Gorilla Glass and the likes are Alkali-Aluminosilicate glasses that get chemically tensioned by exchanging the smaller natrium for potassium ions in a molten salt bath.
Since the compressed surface layer created by chemical tensioning is very thin, the glass still shatters in big shards, which is actually advantageous for the application. If you want safety-glass that shatters into thousands of dull pieces, which is probably the kind of tempered glass you had in mind, you put regular soda-lime-glass though a classical thermal tempering process.

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Thanks for the explanation! (Just a little correction, "natrium" is actually called "sodium" in English.)
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