Glass for smartphones: engineering meets a “traditional” material

Since the release of the new iPhone 12 there has been a lot of talk about the innovative glass with nano-ceramic crystals, renamed Ceramic Shield by Apple, with which the Cupertino company wanted to decree its supremacy in the field of glasses for smartphones. However, this was only the latest in a long series of technological advancements that have occurred in recent years in the field of glass for consumer electronics. One of the leaders in the production of high-tech glass is the American company Corning®, well known for its Gorilla Glass® reinforced glasses and for Pyrex®, the iconic borosilicate glass with high resistance to thermal shock also used in the oven dishes.

A survey carried out by Corning showed that consumers believe that durability is the main buying factor for a smartphone after the brand . After all, who has never experienced the unfortunate inconvenience of damaging the screen of their mobile phone? Maybe even recently bought. For this reason, the efforts focused on enhancing the resistance of the screens to shocks and scratches have increased exponentially and with them the complexity of the materials used to achieve this purpose.

Smartphone glass: is it really a fragile material?

When one thinks of glass, one imagines, mistakenly, an intrinsically “fragile” material that is not very capable of resisting mechanical stress of any kind. You will be surprised to know that the theoretical resistance of glass, i.e. the one calculated on the basis of the chemical bonds within the material, is around 40 GPa. To give you an idea of ​​what 40 GPa is, know that a high-strength steel has a mechanical strength of “only” 1-3 GPa! However, in reality glass does not have these mechanical characteristics: its real resistance is well below that of a steel. This is due to the low fracture toughness of the glass, which is around 1 Mpa m 1/2 against 20-120 Mpa m 1/2 of a steel.

Mechanical properties of glass. Credits: Archivetro

You can interpret this property of the material as its sensitivity to the presence of defects: the lower the fracture toughness, the more the mechanical strength of the material drops compared to the theoretical one in the presence of even microscopic defects. For example, considering a modern glass for smart devices having a fracture toughness of 0.75 Mpa m 1/2 with a scratch on the surface of a tenth of a millimeter, its mechanical tensile strength drops to 84 MPa, only 0.21 % of the theoretical one! This is the reason why avoiding the formation of defects during the glass production processes, as well as increasing the scratch resistance of the same, is crucial in extending the life of our electronic devices .

Resist scratches

To increase the chances of survival of the screens in our busy days it is necessary to limit the formation of defects that could compromise their resistance. Fortunately, the hardness of the glass comes into play here, which has nothing to envy to steels since it is around 6 GPa (Vickers hardness). Hardness is a mechanical property that expresses the resistance to the penetration of a material, or equivalent, to localized plastic deformation. To put it simply, think that the harder material affects the less hard one. Therefore, in principle, any object that comes into contact with your phone must have a harder hardness than glass in order to damage it by engraving it.

The Mohs scale for hardness. Credits: National Park Service
The Mohs scale for hardness. Credits: National Park Service

A hardness scale rarely used in the technical field, but very useful for understanding the concept, is the Mohs scale. This hardness scale is based on a simple “what affects what” test to classify the materials tested. Many common objects have a Mohs hardness lower than 5-6 of tempered glass, so it is difficult for keys to scratch your shiny latest generation smartphone.

The chemically strengthened smartphone glass

Even in the best of cases, glass is never "scratchproof" and therefore there will always be a certain probability of having a defect that could undermine the resistance of the smartphone screen. So how to remedy the problem? The answer is the temper of the glass. As already explained in another article , glass can be strengthened in two ways: with thermal tempering and with chemical tempering. Both result in the formation of a layer of compressive stresses on the surface of the material, which acts against the propagation of cracks in the presence of mechanical stress. The final result of the treatment is therefore an improved fracture toughness with a consequent lower sensitivity to defects and greater mechanical resistance.

In glass intended for screens, the thicknesses are very small and consequently the only practical choice is chemical tempering which allows to obtain even very thin compression layers. Chemical hardening takes place through the ion exchange process in which the glass is immersed in a solution containing potassium nitrate (KNO 3 ) in order to force the exchange of the sodium Na + ions contained in the glass with the potassium K + ions supplied by the solution .

Chemically toughened smartphone glass: before and after ion exchange. Credits: neg-co.jp
Chemically toughened smartphone glass: before and after ion exchange. Credits: neg-co.jp

Since the size of potassium ions is 37% greater than that of sodium ions, a compressive stress is generated in the glass due to the deformation of its structure. You can imagine the process as if you were trying to replace some people (sodium ions) with rugby players (potassium ions) in a bus at rush hour, surely they would all be tighter.

Article by Axel Baruscotti

The Glass article for smartphones: engineering meets a “traditional” material comes from Tech CuE .