Are all “tempered glass” screen protectors scam? - Page 1
Forum section: electronics / general

author: Logan
author_rank: Frequent Contributor
timestamp: December 22, 2020, 02:39:33 pm
content: Hello.As I know, real tempered glass would completely shattered if anywhere on the glass is damaged. But I see those screen protector films which claim to be “tempered glass” just crack partially when dropped. So they are all scams?Thanks.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: December 22, 2020, 03:42:31 pm
content: Well what do you define as a scam?They do work as intended, and takes a lot of the impact, I've managed to shatter the protector completely with my phone screen intact.Also, "real" tempered glass involves a tensioned layer in the middle of the glass, so I suspect there is a minimum thickness needed to properly temper glass.it might just be slightly pre-stressed.

author: TMM
author_rank: Frequent Contributor
timestamp: December 22, 2020, 03:54:39 pm
content: They are most likely chemically strengthened glass - exactly the same as the OEM digitizer glass. It is stronger than untreated glass but still turns into shards instead of pebbles when it fails.

author: coppice
author_rank: Super Contributor
timestamp: December 22, 2020, 04:44:16 pm
content: Quote from: Logan on December 22, 2020, 02:39:33 pmUnsupported tempered glass, like that used for most of the windows in most cars, generally shatters into small pieces when broken. However, when you laminate 2 thin sheets of tempered glass with a layer of plastic, like most car windscreens, the glass splits and rarely shatters. Those screen protectors are like the latter. They have a very thin tempered glass sheet over a thin plastic sheet. The plastic sticks the sheet to the phone and provides some resilience, so a strike to the glass is buffered from the phone's own glass, and most times only the protector's glass breaks. Its quite remarkable how rarely the phone's own glass breaks with these protectors on. It usually takes a massive impact.

author: tom66
author_rank: Super Contributor
timestamp: December 22, 2020, 09:17:46 pm
content: I suspect they aren't really tempered glass as you've noticed they tend to chip and crack rather than catastrophically fail. I've always interpreted them to be a sacrificial shield, with the intention that they absorb damage to reduce the need to replace the whole screen.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.

author: tooki
author_rank: Super Contributor
timestamp: December 22, 2020, 09:29:25 pm
content: Quote from: tom66 on December 22, 2020, 09:17:46 pmThat'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?

author: tom66
author_rank: Super Contributor
timestamp: December 22, 2020, 10:59:40 pm
content: 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.

author: m98
author_rank: Frequent Contributor
timestamp: December 23, 2020, 12:58:17 pm
content: 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.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: December 23, 2020, 09:49:50 pm
content: 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.

author: tooki
author_rank: Super Contributor
timestamp: January 04, 2021, 10:40:49 pm
content: Quote from: m98 on December 23, 2020, 12:58:17 pmThanks for the explanation! (Just a little correction, "natrium" is actually called "sodium" in English.)