Hardness is not toughness: Why your phone's screen may not scratch, but will shatter

Photo by MSVG/Flickr

During a vacation earlier this month, my wife and I put our smartphones in a big, loose beach bag, the kind with bamboo handles. The idea was to forget about them while we baked inside an oily SPF 50 shell.

Instead, we took them out two hours later–her iPhone 5, my HTC One (the M7/2013 model)–and found them each cracked, or at least deeply scratched, across their screens. My screen line runs from top to bottom, almost right across the middle. My wife's iPhone screen line runs in a quarter-circle around the bottom-left corner. You can feel either break if you run a fingernail across them, but both phones still work. It's just annoying, in the same part of one's mind that notifies you of typos and mispronunciations and slightly un-level paintings.

Both phones went through the same experience: placed in a bag, walked down a boardwalk to a beach, then occasionally jostled as sunscreen or a book was obtained. What cracked or deeply scratched them? Almost certainly, the answer is the same as with every phone: gravity, or sand-type particles.

When we find a scratch on our phone screen, our mind naturally turns to the usual suspects: keys, coins, pens, carabiners, random objects in one's purse or pocketbook. But believe me when I say that Corning's Gorilla Glass, and the variants and offshoots used in most modern phones, is made to specifically resist those scratches.

The proof comes in both video form (fun!) and something you might remember from chemistry class (eh).

The video is from XDA Developers, a go-to site for anyone hacking around on phones. Erica Griffin uses glass screen protectors from Spigen, which are almost exactly as thick and scratch-resistant as Gorilla Glass. Watch as she runs through a dozen or more protectors, scratching them with different sandpapers, smacking them with keys, and shaking them in a big with coins. For the most part, no scratches for the common stuff.

The reason is somewhat simple: objects only scratch objects that are softer than their own hardness. Corning has not publicly released a Mohs scale of hardness for any of the three versions of Gorilla Glass, but it is pretty tough. From a Guardian report on Gorilla Glass and its competitors:

The hardness of a material is rated in Mohs, where talc is rated as 1 Mohs and diamond 10 Mohs. Glass ranks around 5.5 to 7 Mohs, but sapphire crystal has a hardness of 9 Mohs, making it only slightly less hard than diamond.

The Mohs rating of Corning’s Gorilla Glass, which is the chemically hardened glass used to make many smartphone screens including those found on the Google Nexus 5 and Samsung Galaxy S4, has not been released, but it is expected to be around 7 Mohs.

Your keys, your coins, your pens and mint tins and door access badges: they are not as hard as your phone glass, so they do not scratch it.

Photo by Flickr/bittbox

What is as hard as your phone's screen is sand, grit, and other particles that have bits quartz, topaz, and other such minerals in them. Your brain doesn't equate tiny particles with big scratches, but it's science, my good sirs and madams. And fine white sand, the kind on Florida beaches, may very well be what caused the smaller, circular scratch on my wife's phone.

Apple is, according to 9to5 Mac and those watching the materials and manufacturing markets, potentially using sapphire crystal, in whole or in part, in a future iPhone screen, possibly the next iPhone 6. Other phone makers might be soon to follow.

Sapphire is already covering the camera on the iPhone 5, and synthetic sapphire costs are heading in the direction of full-screen covers.

Sapphire hovers around a 9 on the Mohs hardness scale. You can literally rub a concrete block around on Sapphire and laugh as you wipe away harmless dust.

Corning, as you might imagine, begs to differ. Chief among the differences touted for Gorilla Glass 3 over sapphire crystal are its impact resistance: based on its own tests, Corning states that "sapphire would break more easily than Gorilla Glass." There is also cost, weight, and lower environmental impact, and other phone-specific matters, like brightness capability.

But "hardness" is not "toughness." A diamond is the hardest natural substance known on Earth, but lightly tap it the wrong way with a hammer, and now you've got diamond dust. It's the same way for sapphire: hit the wrong angle while getting out of your car, or walking down the street, or having loose pockets while standing over concrete, and now your phone looks like mine.

I have yet to find an interesting solution to the hardness/toughness compromise. One comment at MIT's Technology Review points to lamination. My own experience points to a solution that some really find distasteful: a full-body phone case, one which raises slightly above the face of the phone, so as to prevent the majority of face-down smashes. There is also an insurance policy, learning to live with non-critical cracks, and buying a cheaper phone if you know you're a klutz.

It seems like our smartphones might be finally winning the war against sand. But gravity? Gravity always wins.

This story, "Hardness is not toughness: Why your phone's screen may not scratch, but will shatter" was originally published by ITworld.