You Can Drop The iPhone 4 Three Times

A couple of folks from took a brand new shiny iPhone 4, did some unscientific drop tests, and captured it on video. Warning: for reasons not completely known to me the video was a bit difficult to watch. Maybe it had something to do with my thinking that the abused iPhone 4 would have been so well taken care of if it was mine, despite the color. I’m still waiting on the white one but it seems the wait will be quite a bit (read Statement by Apple on White iPhone 4). Back to the drop test. The iPhone 4’s hardened Gorilla cover glass, claimed to be comparable in strength to sapphire crystal and 30 times harder than plastic did eventually break after the fourth drop but that also means for those of you with butterfingers you’ve got three chances with your iPhone 4. But, how strong is sapphire crystal anyway?

Before we dive into answering that question, let’s go back to what Jonathan Ive said in the iPhone 4 video (at around 5:20 into the video) about the cover glass:

We also developed a custom glass comparable in strength to sapphire crystal but about thirty times harder than plastic. This glass is not only used on the front of the phone but also on the back.

I googled around found that sapphire is the non-technical term for non-red, non-white corundum. Red corundum is called ruby and considered a different gemstone; white corundum is pure corundum and everything else including clear corundum is sapphire. More info about sapphire can be found at Wikipedia.

The Mohs scale of mineral hardness is generally used to measure the scratch resistance of minerals and it is so called because Friedrich Mohs, a German mineralogist developed the scale in 1812 (source: Wikipedia). There is also an absolute hardness scale for minerals. For instance, corundum has a Mohs hardness scale of 9 and an absolute hardness scale of 400. Diamond, which is the only natural substance that is harder than corundum, has a Mohs scale of 10 and an absolute hardness scale of 1600. Diamonds are four times harder than corundums.

The only natural material that would scratch the front and back of the iPhone 4 is a diamond if we are to believe Ive. That’s simple enough, but I don’t know how to go about assessing what it means that this Apple-developed custom glass is thirty times harder than plastic. That was a useless statement since there are so many different types of plastic, some that are almost indestructible. But plastic is not normally hard, or highly scratch-resistant.

What is confusing is that plastic is generally not associated with scratch-resistance but more with ruggedness or flexural strength. And that ruggedness is in turn strongly associated with plastic’s characteristic of being bendable, as shown in photo above from Apple’s iPhone 4 video. This custom glass, according to Ive, is highly scratch resistant due to its sapphire-equivalent strength but I think he meant to say that it is also more durable or has more flexural strength, not harder, than plastic, by thirty fold. But is it?

If the iPhone 4 had cover plastic instead of cover glass I don’t think it would have shattered after being dropped four times. That would be because plastic is generally more durable than glass, which is generally more brittle. It would certainly have had terrible scratches though because cement, I would think, is harder than plastic. In other words, cement has a higher Mohs scale than plastic. On the other hand Apple’s custom cover glass would not endure any scratches if scratched on concrete for the opposite reasons: sapphire-level hardness having a higher Mohs scale than concrete.

The iPhone 4 should be highly scratch resistant back and front with almost no known natural substances, with the exception of diamonds, being able to scratch it. If only this was true. Joshua Topolsky’s iPhone 4 got scratched. Have a look at the photos here. Did he have a diamond on him that scratched it? Maybe. A number of other materials, synthetic materials, could have been the culprit: cubic boron nitride (Mohs scale: 9.9), boron carbide (9.5), silicon carbide (9.4), tungsten carbide (9.0). But I doubt it. iFixit confirmed the front panel was made of Gorilla glass from Corning and since Jonathan Ive stated the same custom glass was used for the front and rear scratch resistance should be the same on both sides. So what gives? How is the iPhone 4 being scratched?

I understand glass will eventually break but this is custom glass that is as strong as sapphire and thirty times harder than plastic.

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