On February 11th, 2020, Samsung introduced the worlds first folding glass phone: the Galaxy Z Flip. Samsung declared that it had broken the laws of physics by bending glass, specifically by making a leap from polymer screens to ultra-thin glass technology.
Samsung made it sound like it had invented a new proprietary form of glass, and the company even dubbed it Samsung Ultra Thin Glass (UTG). Weve done the impossible and created ultra thin glass that folds, the companys presentation claimed, adding that it protects your screen from scratches.
But as the world would see days later, physics isnt so easy to ignore.
Glass experts could have warned you about what happened next, when it turned out that Samsungs supposed leap does use polymer and is easy to scratch after all. Samsungs dust-resistant fiber shield immediately went on to fail an aggressive dust test, and the companys claims around glass have now retreated to the ridiculous tagline tough, yet tender.
But Samsung didnt lie about the primary innovation here: the Galaxy Z Flip is truly a folding glass phone. Its just that glass is actually made by German manufacturer Schott, its got a soft, scratchable plastic layer up top, and hopefully future folding glass phones wont require that extra protection.
To understand why, you should probably first understand how glass can fold at all.
Flexible glass is real
If Im being honest, Im still wrapping my head around it myself. Glass is fragile; even if youve somehow never broken glass yourself, you definitely know how it sounds when it shatters. Its tough to even imagine how such a brittle material could be turned into a bendable flip phone, which is why Im not surprised when Samsung found itself facing accusations that the Galaxy Z Flips ultra-thin glass might not be glass at all.
And yet the physics of folding glass is remarkably simple, according to two material scientists, as well as representatives from Gorilla Glass maker Corning and Schott itself. The short version: practically anything can be bent if you make it thin enough.
One of the most common misconceptions about glass is that it breaks because its inherently weak Corning
All the materials we know of that are very rigid can be bent to some extent, says Schotts Dr. Mathias Mydlak, a chemist who now leads business development for the companys ultra-thin glass. If you think of wood, a 2×4 cannot be bent, but if you chisel a very fine piece of it [] the same is true for glass, he told me late last year. While a normal windowpane might not seem to bend before breaking, a thin enough ribbon of glass actually can.
When you bend a material, glass or otherwise, what youre doing is naturally stretching the material on the outside of your bend and even glass has some flex. You could imagine a metallic spring between every two atoms and the spring elongating when you stretch the two atoms apart, says Erkka Frankberg, a research fellow at Tampere University who studies unconventional forms of glass.
The main gotcha: you can only stretch a materials chemical bonds so far before it breaks, a concept known as tensile strength. But if you use a thinner sheet of glass, youre stretching less of the material across the same space, bending fewer layers and stretching fewer chemical bonds, Mydlak says. That puts less tensile strain on the glass, letting you bend it tighter before it snaps. Thinning out the glass is one of the two main tricks to making it bend, says MIT associate professor of materials science Juejun Hu, because physics students learn that tensile strain scales linearly with thickness.
When you get glass down below a hundred microns thick around the thickness of a human hair thats when it can bend far enough for basic folding gadgets, experts say. If you want a phone that folds inward without a huge hinge gap, were talking tens of microns. Think aluminum foil.
That thinness is already possible with glass that exists today. I touched a 70-micron sheet of Schott glass four years ago, and bent it with my own hands. Samsung says its Z Flip uses a 30-micron-thick glass, and Schott says its gone down to 25 microns in the lab as well. Both Corning and Schott use somewhat similar techniques, each creating a floating glass sheet in midair by drawing thin ribbons of molten glass out of specially shaped vats.
But thin isnt enough by itself. The second main trick for bendable glass is to strengthen it against imperfections, particularly at the surface where they can cause catastrophic failures when the glass is stressed. A tiny air bubble, speck of dirt, or minuscule scratch during the manufacturing or handling process might be enough to destroy a piece of bendable glass, experts say. All the stresses you produce in the glass tend to concentrate on those small defects, and thats where it will produce a fracture, says Frankberg.
Thats where chemical baths and heat treatments can come in to strengthen and temper the glass, as weve written about before, but the trade-off is that they can make glass easier to scratch. And as your glass gets thinner thin enough to bend a scratch becomes even more worrying.
Scratches are deadlier
While Mydlak says that a piece of Schotts glass can go on forever in controlled conditions if it survives the first bend test, he admits that one scratch could change that. If you scratch it with something this is going to be an initial defect that can cause problems later on, he says. If a piece of glass with a large enough scratch is bent with enough stress concentrated on a pre-crack, says Frankberg, it will basically propagate with the speed of sound and catastrophically go through the whole material.
The thing to remember is that while glass is more scratch resistant than plastic, its not scratchproof it may scratch at a Mohs hardness of 5 or 6, instead of 2 or 3 for plastic, but it scratches anyhow. I bet you have some tiny scratches on your phones traditional glass screen right now. But Im also guessing you never need to bend your traditional screen like you would with a folding phone. Now its a way bigger deal.
Frankberg thinks thats probably why Samsungs Galaxy Z Flip still has a plastic screen protector up top. Not to prevent scratches or help it bend more easily, but as a sacrificial layer that gets scratched instead of the glass, so there are fewer potential catastrophic cracks down the road. Probably they receive fewer product returns due to the extra layer, he speculates.
Schotts Mydlak actually hinted as much to me late last year: Most probably you would not be touching the bare glass at all, but something on top of it, Mydlak said when I asked about whether scratches would be something to worry about with the first batch of folding glass phones.
I should add that we dont fully know what Samsung and its partners are doing with Schotts glass screen before adding it to the phone. Schott could only confirm that its delivering its ultra-thin glass to Samsung, saying that it cannot comment on any processing details of the raw glass material. And while Samsung has admitted that it comes with a pre-installed screen protector, the company announced on Wednesday that its also injecting the glass with a special material up to an undisclosed depth to achieve a consistent hardness, whatever that means. Were asking, but it sounds fairly hush-hush.
But Samsung does have another partner involved in that process. Its been working with Korean screen manufacturer Dowoo Insys on this glass since 2013, Yonhap News reports, and recently locked it down by becoming the largest shareholder in the company. Allegedly, Samsung has an exclusive on Dowoos process but not Schotts ultra-thin glass, and Samsung reportedly decided to use both for the Galaxy Z Flips screen.
What good is glass?
All of this still doesnt answer the question, though: what good is foldable glass if it scratches like plastic anyhow? One answer is that it may last longer; Schott says glass wont break down like plastics naturally do over time. Theres also optical clarity to consider. Plus, as Dieter Bohn noted, the Galaxy Z Flips glass polymer cover or no just plain feels nicer than the pure-plastic-covered original Galaxy Fold. The crease at the fold is less prominent and though it can be indented with a fingernail, it feels firmer than whats on the Galaxy Fold.
But Gorilla Glass maker Corning wants you to know the Galaxy Z Flips screen protector may not be necessary in a year or two. The company tells me samples of its new bendable glass are already in the hands of device manufacturers. It expects devices in the market in the next 12 to 18 months, and its shooting to deliver durability, scratch resistance, and optical clarity all in a single package for foldable devices.
While a rep couldnt yet say how Cornings doing that or explicitly confirm thered be no plastic involved, plastic doesnt appear to be the goal: Today, when you buy a phone with Gorilla Glass, youre touching glass thats what were working towards.
Mass-producing glass thin enough to reliably bend is a tough challenge, Corning and Schott agree. Mydlak told me last year that Schott was still figuring out a few production issues, like how to properly cut, pack, and ship reels of the very thinnest glasses it had developed without damaging them along the way. It sounds like the existence of the Galaxy Z Flip means its worked some of that out but low yields also may be one of the reasons folding phones are so incredibly expensive, and why Corning has yet to fully figure it out.
Its hard to blame Samsung for pushing so hard, even if both its foldables have stumbled out of the gate for one thing, it wants to sell folding screens to other companies, too, as it all-but-revealed in its latest press release. But that doesnt make it any less frustrating to see that a phone that was promised as scratch-resistant can easily be scratched, and it doesnt mean you need to buy in. Though if you do, Samsung tells The Verge that you can get a free screen protector and / or a one-time $119 screen replacement if your device does succumb to scratches.
Frankberg isnt yet convinced. My honest opinion as a materials scientist: I wouldnt buy these kinds of phones at this stage because the size of the scratch you need to break it is very very small, he says, pointing out how a single grain of sand could destroy a folding glass screen. Buying a very expensive phone, I would step back and see how the technology improves.
Mind you, Frankberg has an idea of how such screens might eventually evolve: hes in the very early stages of developing a form of glass that uses aluminum oxide instead of the traditional silica, one that can bend instead of breaking, and is exploring other alternative materials, too. (It wouldnt be the first time the worlds looked into alumina glass, and you may also be familiar with aluminum oxides condensed crystalline form, sapphire.)
I just wonder if wed actually call such a material glass if or when it ever arrives. Like JerryRigEverythings Zack Nelson is fond of saying when he scratch-tests a new glass screen, its hard to shake the stereotype. Glass breaks. If anyone ever comes up with something stronger than glass, theyll definitely be calling it something different.