Protective glass notes

Hello, Habr. We often write about the second displays in our smartphones, the advantages they bring and the unique experience. But any modern high-quality smartphone, especially having two displays, is unthinkable today without a reliable protective glass. In this sense, a peculiar standard was the use of products from Corning, which manufactures protective glasses under the Gorilla Glass brand. And if you don’t surprise anyone today with its presence in a good smartphone, tablet or laptop, then in the first YotaPhone, and in the upcoming launch for sale on YotaPhone 2, unique curved Gorilla Glass glasses, made on special order, are used.

The popularity and widespread prevalence of Gorilla Glass almost made the name of this brand a household name. Like "felt-tip pen", "thermos", "jeep" and many others . But if this honor was usually awarded to those companies that were the first to market products under their own brand, then the popularity of Gorilla Glass is associated with the excellent protective properties of this glass.

Corning's perennial research has enabled us to work out the technology for making glass that is highly scratch and impact resistant and has great flexibility.
')

Technical process

The manufacturing process proceeds as follows. The molten glass mass is poured into a chute, called an “isotube” (isopipe). The trough is filled until the molten mass begins to flow evenly on both sides. Flowing around the chute, both streams converge at its lowest point, from where they merge downward, cooling down and forming a thin sheet of glass, measured in microns thick. At this stage no physical contact with tools and accessories is allowed, so that no defects form on the surface.



After cooling, blanks are cut from glass sheets and subjected to chemical treatment. To do this, they are placed in baths filled with molten salt at a temperature of 400 degrees Celsius. An ion-exchange process begins to take place, during which sodium ions in the surface layer of glass are replaced by potassium ions. Since they are larger than sodium ions, when cooled, the surface layer is compacted, and a compressive stress occurs in it. Due to this, glass acquires a high-hard surface resistant to mechanical stress.

Surface curvature

YotaPhone was the first device, which used curved glass Gorilla Glass. In the first generation of the smartphone, Gorilla Glass 2 is 0.7 mm thick, and in the second generation, Gorilla Glass 3. In this case, specially designed curved glass is used to protect the second display. Above, we talked about the fact that the “standard” Corning process technology involves the smelting of extremely flat glass sheets.

Bending glass blanks attached to the production of the partner company Biel Crystal. From Corning, they get large sheets, and then carry out chemical etching to give anti-glare properties, cut out the workpiece, heat it and give it the desired shape. At the end of the applied oleophobic coating.

In turn, a touch film is applied to our production. This is a rather nontrivial task, given the curvature of the surface, but we managed to cope with the difficulties encountered.

Thoughts about the future

With all its advantages, Gorilla Glass, like any other protective glass, to some extent reflects the surrounding space, especially light sources, which can cause inconvenience when working with gadgets. This summer, the results of experiments to create glass with antireflection and superhydrophobic surface were published. Moreover, these studies were partially funded by the same Corning company.

The idea is as follows: first, a mask of polymer microparticles is applied on the surface of the glass. After that, it is treated with a mixture of hydrofluoric and sulfuric acids, which etches the surface through the holes formed after the application of the polymer mask. As a result, microroughnesses from 1 to 100 microns in length and from tens to hundreds of nanometers in height are formed.

Next, an ultrathin (4-8 nm) layer of copper is deposited on the surface of the glass, which repeats the microrelief formed by etching. Then the glass is heated for about 95 seconds at a temperature of 750 degrees Celsius, as a result of which copper is collected in nanodrops, randomly scattered over the surface of the asperities of the glass. The thicker the copper layer was applied, the larger the droplets formed and the lower their number per unit area.

Then the glass is subjected to reactive ion etching , due to which nano-columns on average 50–200 nm and 100 nm in diameter are formed on the surface. After that, the metal layer is removed from the glass.



As a result of all these steps, anti-reflective micro and nanorelief is created on the glass surface.



The resulting glass has much higher optical properties due to its low reflectivity. It can also be given a superhydrophobic property by applying a layer of silicon fluoride.

In combination with the mechanical properties of Gorilla Glass would be the perfect glass for use in gadgets. But it is too early to talk about the commercial application of the above process, since it requires a lot of research. For example, it is not entirely clear how much such a texture will be resistant to mechanical stress, how long it will be. Can she survive this?



How compatible is such a glass with a touch screen touch layer? Whether such texturing will not worsen the heat exchange properties of glass? There are still a lot of questions that require a large amount of research and testing to get answers to. In any case, the search for an effective and reliable antireflection technology for protective glass goes in different directions.