The science behind smartwatch scratch resistance

The science behind smartwatch scratch resistance

Is the sapphire crystal on the Apple Watch worth the extra money?

Published: April 24, 2015 08:00 PM

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The Apple Watch with a sapphire crystal survived a level-9 Mohs pick in our scratch test.

Durability is a greater concern for smartwatches than for any other personal electronics device. Sure, cellular phones can take a beating, but at least you can protect them in a case. Smartwatches, by design, are exposed to the elements and vulnerable to any hazards that your arms encouter. With millions of consumers now taking delivery of the new Apple Watch, that’s going to leave a lot of people wondering just how gingerly they need to treat these expensive electronic timepieces.

For the Apple Watch, the question of durability translates pretty directly into cost, since the base-model Apple Watch Sport is made from different materials than the more expensive Apple Watch version. The face on the $350 to $400 aluminum-bodied Sport model is constructed from what Apple describes as “Ion-X” glass, which is essentially a hardened glass similar to the Corning Gorilla Glass found on many smartphones. (In fact, Apple’s description of how Ion-X glass is made is almost identical to Corning’s description of how Gorilla Glass is made.) The stainless-steel Apple Watch ($550 to $1,100) and gold Apple Watch Edition ($10,000 to $17,000) models have a sapphire-crystal face, a super-hard material used on high-end watches from manufacturers such as Rolex and Breitling.

Since a base-model Apple Watch is a $200 premium over a base-model Apple Watch Sport, we wondered just how much tougher the sapphire crystal is than the Ion-X glass and whether it's worth the extra money.

Looking for a new digital timepiece? Our smartwatch buying guide can help get you educated in a hurry. And see our earlier coverage of the Apple Watch, below.

A chart of the Mohs scale. (Click on the scale to see it full size.)

There are several ways to test material hardness. One of the most accessible is a 203-year-old scale created by the 19th-century German geologist Friedrich Mohs, which compares the scratch resistance of minerals relative to one another. The Mohs scale, as it is known, uses 10 minerals of increasing hardness as reference points. It rates talc as 1 and goes all the way up to diamond at a 10 rating. (You can see a chart of the scale, below.) The idea is that each mineral on the scale can scratch every mineral ranked below it. Consumer Reports has a Mohs hardness kit that we use in our test labs. It contains a series of picks to represent each level of the Mohs scale.

"The Mohs scale uses common materials, so it's easy to understand," says James A. Harrington, a professor of materials science and engineering at Rutgers University. We consulted with Harrington to ask about the science of glass and sapphire. "When it comes to their structure, glasses and crystals are as different as apples and oranges," he says. Harrington describes glass as like "a frozen liquid" whereas sapphire is an incredibly hard solid with a high melting point.

Sapphire is a form of corundum, a mineral that occurs naturally, but can also be manufactured synthetically. (Ruby gemstones are just corundum crystals with red-colored impurities.) Corundum rates a hardness of 9 on the Mohs scale, just under diamond.

We attached the picks from our Mohs kit to a rig that we use to test all the smartwatches. That rig is designed to apply the same amount of force for each trial. We swap the picks out for each step up the Mohs scale.

So how did Apple's watches fare? The sapphire crystal performed as expected, which is to say very well. It survived a 9-rated pick from our kit. The Apple Watch Sport made it up to a 7-rated pick without damage, but was scratched by an 8-rated pick.

So the face of the Apple Watch is definitely harder than that of the Apple Watch Sport. But the performance of the hardened glass of the Sport model is pretty impressive as well. An 8 on the Mohs scale is equivalent to topaz, just one step below sapphire, and it means that it takes quite an abrasive material to scratch Apple's glass. (We also tried a completely unscientific attempt on the Sport model with a steel key, and it didn't scratch the glass.)

One last note. We've seen an early video of a blogger taking sandpaper and a knife blade to a glass face component purportedly from an Apple Watch Sport. In that video, his knife didn't scratch the Ion-X glass, but sandpaper did. It turns out that corundum is commonly used in sandpaper. ("You can find it in Home Depot," says Harrington.) So it's not that suprising that sandpaper would scratch the glass of the Apple Watch Sport. The lesson: Keep your belt sander away from your Apple Watch Sport, and keep your diamond rings away from your Apple Watch.

—Glenn Derene

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