The Apple Watch will sport a crystal display when it launches early next year. Which may or may not be bad news for Corning, the makers of the current leader of the smartphone screen pack, Gorilla Glass. So what does sapphire offer that Gorilla glass does not, and what are the key points of difference between the two display materials?

What are Gorilla Glass and sapphire made of?

Gorilla Grass is Corning’s trademark for a chemically toughened glass that has been through an ion-exchange process. This is done by putting the sheet into 400C bath of molten potassium salt which causes an exchange between the larger potassium ions and the smaller sodium ions originally on the glass. As the glass cools these potassium ions provide higher protection against compressive stress, which means that surface scratches have to break the glass.

Sapphire is a crystalline form of aluminium oxide also known as Corundum. Corundum occurs naturally as a mineral, and can be transparent although impurities can lead to a color tint which when red is referred as ruby and and otherwise is known as a sapphire.

How hard is sapphire compared with Gorilla Glass?

On the Mohs scale of hardness, Gorilla Glass scores a 6.8, which puts it just below a mineral like Quartz. Sapphire is the second hardest naturally occurring material on the planet, behind diamond. and scores 9 on the Mohs scale of mineral hardness. This test matches one substance’s ability to scratch another – and so is a better indicator of scratch resistance than shatter resistance.

“Chemically strengthened glass can be excellent, but sapphire is better in terms of hardness, strength, and toughness,” said Matthew Hall, Director of the Center for Advanced Ceramic Technology at the Kazuo Inamori School of Engineering at Alfred University, quoted in Tech Crunch. “The fracture toughness of sapphire should be around 4 times greater than Gorilla Glass – about 3 MPa-m0.5 versus 0.7 MPa-m0.5, respectively.”

But does this Moh hardness test translate to real life situations?

Corning concedes that sapphire is more scratch resistant than Gorilla Glass but argues that in ‘normal mobile use’ sapphire would shatter more easily. It has put together a demo video which it claims indicates that Gorilla Glass is 2.5x less likely to shatter (see below).

But it doesn’t take an expert to see that a ‘ring on ring’ strength test is a long way from a normal use scenario. And the fact is that in real life, scratches on the display are the conduit for future fractures and shattering, so a screen with few or no scratches at all should be less likely to shatter when dropped.

What is the cost of sapphire vs Gorilla glass?

Corning has claimed that sapphire displays cost up to 10 times as much as Gorilla glass – but that claim was based on existing production output of sapphire. Apple has a history of tieing in suppliers to long-term contracts to ring-fence key components such as retina displays which leaves competitors struggling to get their hands on matching products.

How heavy is Sapphire crystal compared with Gorilla glass?

The density of Gorilla Glass is 2.54 g/cm3 while sapphire is 3.98 g/cm3. So sapphire is roughly 1.5x heavier than Gorilla glass. But recently published patents submitted by Apple indicate that it has devised a way to fuse sapphire and glass layers together that creates a laminate combining the durability of sapphire with the weight and flexibility advantages of glass.

Doesn’t sapphire crystal absorb more light than Gorilla glass?

The refractive index of sapphire crystal is 1.76 compared with 1.5 for Gorilla glass – which means there is a small trade-off between light transmission and durability. “It transmits less light which…means either dimmer devices or shorter battery life,” said Corning senior vice president Tony Tripeny.

But again that assumes that you would require the same depth of display, or that the display was solid sapphire, both assumptions that are open to question.

Certainly in Marques Brownlee’s YouTube video the reviewer made a point of noting the ‘high quality’ of the sapphire crystal display: “This thing is paper thin, literally the same thickness as a sheet of paper, and it’s perfectly see through. I would say zero per cent opacity.”

This thing is paper thin, literally the same thickness as a sheet of paper, and it’s perfectly see through. I would say zero per cent opacity.

Isn’t sapphire much more expensive than Gorilla Glass?

Currently sapphire is more expensive to produce than Gorilla glass, but how much more depends on who you ask. Corning claims that a sapphire display is about 10x the cost of an equivalent Gorilla Glass display, but GT Advanced Technologies says sapphire is currently about 3-4 times more expensive to produce.

But Apple has paid GT Advanced Technologies $578 million up front to build a plant in Arizona to produce sapphire exclusively for its products. Estimates put the production capacity at up to 200 million displays a year – a volume which would inevitable reduce the unit price of sapphire still further.

There are other cost benefits that going sapphire brings over and above the unit cost, for instance the reduced cost of replacing screens on iPhones broken under warranty or Apple Care protection. And, naturally, being the only phone (barring the $11,000 Vertue) with sapphire display technology is a genuine unique selling point with real competitive advantage.


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