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There has been a lot of news recently regarding the joint venture between GT Advanced Technologies (NASDAQ:GTAT) and Apple (NASDAQ:AAPL) to build a factory to manufacture crystalline sapphire for use as display cover glass for upcoming Apple products. I am long GTAT so I've enjoyed the resulting run up in the stock, but I am also long Corning (NYSE:GLW) who manufactures Gorilla Glass, the current cover glass of choice for most high-end mobile touch-screen devices. If crystalline sapphire is superior to Gorilla Glass, should Corning investors be concerned that Gorilla Glass will be supplanted by sapphire? Maybe.

I think there are many reasons that Gorilla Glass will continue to be a popular choice for cover glass including cost, ease of manufacturing, thinness, weight, and environmental impact. However, here I want to focus on a physical reason, namely the degradation of flexural strength caused by microscopic abrasions.

The Material Science Matters

In almost every measurable way, it appears that crystalline sapphire is mechanically superior to Gorilla Glass. Take a look at the summary table below pulled from some GTAT marketing material. I confirmed the Gorilla Glass specifications (Gorilla Glass 2) with Corning's own marketing material.

sapphire vs gorilla glass

Gorilla Glass is significantly less dense (lighter at same volume) and has a slightly smaller dielectric constant, which I suspect makes it perform slightly better as a touchscreen. My reasoning is as follows. A dielectric is a material that increases the capacitance of a capacitor at a given electric field strength (voltage). A capacitive touchscreen works by sensing the dielectric effect of a finger when it touches it. If the screen itself has a larger dielectric, the change induced by a finger is proportionally reduced and so is the sensitivity of the touch interface. A finger probably has a dielectric about ten-times the screen's so the difference may be negligible. I have to do more research to confirm this.

In regard to the other mechanical properties listed, sapphire seems to have a significant advantage. For example, sapphire has approximately three-fold greater hardness when compared to Gorilla Glass meaning it is less likely to be scratched. To be more specific, hard materials like sapphire, quartz, and diamond, have greater resistance to permanent deformation when subjected to friction.

The elastic modulus is a ratio of stress (applied pressure usually) over strain (stretch). It is a measure of stiffness. An elastic modulus of 1 GPa (gigapascal) means it would take over 140,000 pounds per square inch of pressure to stretch the material in question by a factor of two. The problem with elastic modulus as a descriptor of a material is that it says little about a material's toughness, that is, the ability of a material to deform without breaking. Most materials fail (break) way before they stretch any meaningful amount.

Some measures of toughness are given in the table, namely fracture toughness and flexure strength. Again it seems that sapphire has the upper hand (although no flexure strength is given for Gorilla Glass). On the other hand, tests like the one conducted in this Corning-produced video seem to suggest Gorilla Glass is much tougher then Sapphire. The following screen shot is taken from the end of the video. The test being conducted most closely measures flexure strength (the value missing for Gorilla Glass in the table above).

(click to enlarge)

The sapphire sample shatters under 161 pounds of force while the Gorilla Glass remains intact even under about two and a half times as much force. The message Corning is trying to convey is that sapphire is not tough, it is in fact brittle (has a smaller fracture toughness). Which marketing material do you trust? There is truth in both.

Bend But Don't Break

The apparent discrepancy between the toughness numbers presented in the table above (showing sapphire to be tougher) and the video (showing Gorilla Glass to be tougher) can be explained as follows. The Corning test was conducted after subjecting the samples to the types of abrasions a smartphone would experience under normal use. These abrasions etch microscopic fissures in the surface of the samples. The strength of sapphire is a product of its crystalline structure. In fact, sapphire cover glass isn't glass at all. Glass by definition is an amorphous solid, not crystalline. The imperfections created in the sapphire's crystalline structure by the abrasions act like fulcrums upon which deformation forces are focused. Moreover, I suspect that the orderly structure of crystalline sapphire allows cracks to propagate larger distances more easily once they get started. (The fracture strength reported in the above table seems to counter this idea. To know for sure, I would need to know the test conditions under which this number was determined. I suspect that fracture toughness may not be an accurate predictor of long-range crack propagation.) On the other hand, the amorphous structure of glass probably helps prevent the long-range propagation of cracks. The message Corning is trying to get across is that glass may not be as strong under perfect conditions compared to perfectly crystalline sapphire but in the real world, it will stand up better to normal wear and tear.

One More Thing

The figure below is a schematic representation of the flexure strength test conducted in the Corning video above.

During such a test, the cover glass acts like a beam loaded at its center. Under such a load, the top of the beam (location B) will be subjected to compression (the molecules are pressed together) while the bottom of the beam (location A) is subjected to tension (the molecules are pulled apart). In general, solid-state matter resists compression much better than tension. Moreover, micro-fissures will affect the tensile strength of the cover glass much greater than its strength under compression. For this reason. I find it likely that scratches on the side opposite the applied load (location A in the diagram) will reduce the flexure strength much more than scratches on the load side (location B in the diagram). With a smartphone, loads are usually applied to the top of the cover glass (side B) and side A is under the cover glass and not subjected to scratches. For this reason, abrasions may not affect the strength of the cover as much as one might otherwise think. I would like to know if both sides of the cover-glass samples in the Corning video were subjected to abrasions instead of just one surface like the cover glass of a real smartphone. If only one side was subjected to abrasions, I would like to know if this side was facing upward or downward during the test as I feel this would most definitely affect the results.


When a load is applied to cover glass, the glass (or crystalline sapphire) itself can act like a lever magnifying the force. For this reason, a larger display (bigger lever) would be more susceptible to breaking than a small display when both are subjected to the same load. At the same time, a larger display has more surface area on which abrasion-caused micro-fissures can accumulate. Under ideal conditions, the flexure strength of a homogeneous defect-free piece of cover glass will be equal to its tensile strength and crystalline sapphire will be much tougher. But in the real world, where micro-fissure causing abrasions occur, glass is more likely to be tougher, especially for larger screens. For all of these reasons (combined with cost and ease of manufacture), I suspect that Gorilla Glass will continue to be the cover glass of choice for larger displays like those on tablets, phablets, and televisions, regardless of whether crystalline sapphire gains traction. Since large displays have more surface area than small displays, this will translate to good sales and a bright future for Corning. On the other hand, crystalline sapphire is incredibly resistant to large scratches and is very tough so long as micro-fissures do not accumulate on the surface and the display is sufficiently small to prevent the magnification of too much force at these imperfections. So, there is a bright future for crystalline sapphire too, especially in small form factors like smartwatches and smartphones although I wouldn't be surprised if adoption with smartphones (even the iPhone) is limited due to the costs outweighing the benefits.

Source: GT Advanced Technologies Vs. Corning: Crystalline Sapphire Is Better (And Worse) Than Gorilla Glass