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GORILLA GLASS: unbelievably unbelievably tough and flexible Most people know that smart phones and many notebook computers use Gorilla Glass but few would know just how tough or flexible this glass is. Unlike ordinary window glass it can be flexed by extreme amounts and it is very hard. In this article we describe how it is manufactured. By Dr David Maddison T hese days, most people have smart phones or tablet computers but may not be aware of the special material that constitutes the front face of most of these devices. These devices often receive rough treatment as they are accidentally dropped, sat upon or otherwise abused and so have to be extremely rugged overall – but most of all the cover glass. It is designed to be thin, tough, optically clear and extremely smooth and is mostly a glass known by the Corning 16  Silicon Chip trade name of ‘Gorilla Glass’. While glass materials are generally brittle there are a variety of special treatments available to make them extremely hard or tough. Examples include lamination of two or more sheets to make automotive windshields or bullet-resistant glass; thermally or chemically-toughened glass as usually used on the side windows of cars and glass tables or doors in homes; and modern glass bottles, some of which will not even always break if dropped on a concrete surface. In portable electronic devices such as smart phones and tablet computers the marketing emphasis is on how thin a device can be made (even though many would prefer a slightly thicker device with a decent battery life!). Every single component is made as thin as possible, including the cover glass. Since normal glass is not strong enough in its usual form when it is made to the desired thinness, special compositions and treatment of glass is siliconchip.com.au needed that makes it both strong and tough as well as thin. Plastic, an obvious candidate material, generally cannot be produced with the look or feel that is found desirable on the working surfaces of smart devices. Glass composition Glass has been manufactured by mankind since around 3500BC where it was produced in Mesopotamia. Before that, natural glass such as volcanic obsidian was used and extensively traded by stone age societies. Glass, in its most basic form is made from silicon dioxide (silica, SiO2) or (mostly) beach sand. Unfortunately this is not practical to use for large scale production (except for specialised applications requiring strength and heat resistance) as it melts at a very high temperature (between 1600 and 1725°C). So other constituents are added which help lower the melting point and also add certain desirable properties. In most common commercial glasses such as window glass, both limestone (calcium carbonate which is turned into lime, CaO) and sodium carbonate (Na2CO3) are added to the silica base along with some other ingredients. This type of common glass is known as soda-lime-silica glass. It is cheap Samples of Gorilla Glass in various thicknesses. and useful but not especially strong. Other common glasses based on silica include sodium borosilicate glass (Pyrex), lead oxide (crystal) glass and aluminosilicate composition (used in fibreglass, halogen light globes and other applications requiring resistance to high temperatures and thermal shock). Corning uses an aluminosilicate glass for its Gorilla Glass. It is roughly intermediate in properties and cost between a common basic soda-lime-sili- Exterior of Glass Interior of Glass 120 80 40 0 40 80 120 Stress (MPa) Cross section of piece of glass showing both the stress profile at the surface and also at the interior. Overall the forces balance but the surface is in compression, meaning that cracks have difficulty starting. The interior is in tension where cracks could easily propagate should they reach that area but they normally don’t, due to the surface of the glass being in compression. siliconchip.com.au ca glass and the exotic pure silica glass and offers a good mix of properties such as good mechanical properties and workability of the melted glass. A typical composition for aluminosilicate glass (but not the secret one used in Gorilla Glass) is silica (SiO2) 57%, alumina (Al2O3) 16%, lime (CaO) 10%, magnesia (MgCO3) 7%, barium oxide (BaO) 6% and boric oxide (B2O3) 4%. How to strengthen the glass Phone glass has to be tough. One way to turn ordinary brittle glass into a much stronger form is by a special chemical or thermal treatment that puts the surface of the glass in compression compared to the interior of the glass. In other words, the surface is made to expand and would tend to occupy a greater volume than it formerly did but it is constrained from doing so as it is held back by the bulk of the glass comprising the interior. The atoms of the surface glass are thus squeezed closer together. This has the effect that it makes it much harder for cracks to initiate or continue to grow should a crack become initiated as the crack edges are being pushed together. If there is no cracking, there can be no failure of the glass. In addition to making the initiation or propagation of cracks harder by putting the surface in compression, crack initiation can also be made much more difficult by ensuring the glass surface is extremely smooth, with a minimal November 2014  17 The atomic structure of the same material, silicon dioxide, shown in both a highly ordered crystalline form (quartz) and amorphous form (silica glass). The red dots represent silicon atoms while the blue represent oxygen atoms. Elements of the basic crystalline structure can be seen in the glassy form at a short range but there is also great randomness to the structure at a longer range. amorphous atomic structure. After the discovery of glass-ceramic, the Corning company started looking at other ways to strengthen glass. It began work with chemically treated toughened glass in the 1960s and developed a product called “Chemcor” as part of its “Project Muscle” initiative to make a glass product that was strong, light and close to unbreakable. This glass was used in small quantities in some early US “muscle cars” in order to lighten their weight as well as some space craft windows and tableware. Ultimately this glass was considered too strong for most applications and its use waned. A further concern was the way in which, when it did fail, the glass would explode which was number of microscopic defects that CorningWare are a hybrid between a reason that some safety eyeglasses can act as crack initiation centres. traditional glass and ceramic materials that used this product were recalled. The number of surface defects in and possess both types of structure. The glass fell out of favour as glass can be minimised by acid treat- Structurally, glass is said to be amor- a lightweight material in muscle ments, polishing or by the cars because, despite the way the glass is cast. One weight saving, human ADVANTAGES OF GORLLA GLASS example is by forming the head impact forces were • Highly scratch resistant. glass onto a smooth molten found, in experiments, to • Very thin and compatible with touch screen technology. be much greater on the metal surface such as tin, as in the case of float glass. Chemcor glass compared • Very lightweight. Gorilla Glass is made with with laminated glass • Allows thin and lightweight devices without fragility. an extremely smooth surface which had been in use • Available thicknesses 0.4 - 2.0 mm. which not only makes it in cars since the 1930s. • Low distortion of underlying image. extremely transparent but The latter glass had some • Optically clear. makes the image of the un“give” thus reducing derlying display distortion• Extremely smooth. head injuries. free. It also makes the glass Project Muscle was • Recyclable. pleasant to touch and resistshelved in 1971. The ant to staining. phous (a pseudo-random structure) Chemcor class of chemically toughGlass technology has continued while ceramic materials are crystalline ened glass products then became a to improve throughout the ages but (a regular structure). Gorilla Glass is solution in search of a more suitable particularly from the 20th century a pure glass with a pseudo-random, problem to solve. until the present as material behaviour came to be understood at the atomic and molecular level. Glass strength in particular has undergone constant improvement. Thermally toughened glass was first patented in 1900. Laminated glass, which was invented in 1903, was an accidental discovery. Similarly by accident, in 1953 a Corning scientist discovered a remarkable form of material called a glassceramic which they called Pyroceram which found its way into laboratory ware, missile nose cones and microwave oven ware. In 1958, a consumer product was released into the market The 1965 Dodge Coronet A990 muscle car which was specially lightened by place which is familiar to most people a number of modifications, including the removal of some trim and the use today – hard-to-break CorningWare of Corning Chemcor glass windows instead of regular glass. It was an early, dinnerware. “hi-tech” consumer-end use of chemically strengthened glass. There is an Glass-ceramic materials such as unexpected connection between this car and the modern smart phone.... 18  Silicon Chip siliconchip.com.au The fusion draw process. Two streams of molten glass flow down both sides of a v-shaped trough, rejoin at the base of the v and then solidify. It is a continuous and highly automated process. Along comes Apple Then along came Steve Jobs from Apple Inc. In the various published accounts of how Apple requested a new glass product for the iPhone, certain details seem to vary but the following account is supported in the biography of Steve Jobs by Walter Isaacson and also quotes of Jobs and the Corning CEO in 'Wired' magazine. Steve Jobs unveiled the original iPhone to the public on 9th Jan 2007 and it was released on 29th June 2007. For some period before the original unveiling it is said that Steve Jobs had been carrying around an iPhone prototype in his pocket and was upset that the plastic face had become scratched from keys in his pocket. Presumably the prototype screen face was made of plastic. He ordered his staff to find a solution and time was running out before the release date. He wanted a scratch-resistant glass face on the phone. In February 2007 Steve Jobs decided to visit Corning in New York where he met with the CEO Wendell Weeks. He told him that he wanted to have tens of thousands of square metres of very thin, ultra-strong and ultra-scratchresistant glass, a product that did not yet exist and within a seemingly impossible time frame of six months (or less). According to Walter Isaacson, Steve Jobs said to Wendell Weeks “This is what I want, a glass that can do this”. Wendell Weeks said “We once created siliconchip.com.au a type of process that created something called Gorilla Glass”. Steve Jobs said “No, no, no. Here’s how you make really strong glass”. Wendell then said, “Wait a minute, I know how to make glass. Shut up and listen to me”. Wendell Weeks then described the process to make Gorilla Glass. Steve Jobs then said “Fine. In six months I want enough of it to make – whatever it is – a million iPhones”. Wendell then said “We don't have the capacity – none of our plantes make the glass now.” Steve Jobs looked at him and said what he said to Steve Wozniak 20 or 30 years earlier: “Don’t be afraid, you can do it”. Wendell Weeks later told the biographer “I just sat there and looked at the guy. He kept saying, ‘Don’t be afraid. You can do this.’”. While Chemcor glass would be a good starting point this product was not suitable as it was, because it was made much thicker at around 4mm and Apple’s requirement was for glass that was around 1.3mm thin. It was not known if the Chemcor process could be scaled to make much thinner glass and whether the chemical toughening would work with that thinness and the glass still remain ultra-strong. In fact, before Steve Jobs came on the scene with Apple’s requirements, in 2005 Corning had already resurrected internal interest in the Chemcor glass, known in-house as 0317. Motorola had introduced the Razr V3 flip phone which used a glass front screen instead of plastic and Corning wondered if there could be a speciality market for this kind of glass for phones and other small devices like watches. Such a glass would need to be strong like Chemcor but very thin and smooth. Marketers concluded there KNO3 BATH GLASS SURFACE GLASS (CS) COMPRESSIVE STRESS LAYER Composition, depth of layer (DOL) and compressive (CS) are key characteristrics. DOL refers to the depth of the compressive stress layer. (TS) TENSILE STRESS AREA O2 Si Al K:1.33Å Na: 0.97Å Model is for illustration purposes only, illustration is not to scale The aluminosilicate glass is placed in a bath of molten potassium nitrate, KNO³ (common name, saltpetre) and some of the sodium atoms near the surface (grey, 0.97Å in diameter) leave the glass and are replaced by much larger potassium ones (yellow, 1.33Å in diameter). This results in the surface layer being placed in compressive stress and the interior layer in tensile stress. Other elements present in the composition of the glass are oxygen, silicon and aluminium. November 2014  19 Computer model of atomic structure near surface of Gorilla Glass during the ionexchange process, looking from the inside out. Note the larger yellow potassium atoms entering the surface, which are replacing the smaller grey sodium atoms. was a demand for glass of this nature but researchers had not got far by the time in February 2007 when Apple started demanding massive quantities of it. Nevertheless, the project to make glass for this speciality market had the codename “Gorilla Glass”. Fortunately, Corning, while it is a big company, thinks like a small company with a can-do attitude. After the meeting with Steve Jobs, Corning started experimenting with different glass compositions and was coming close to something suitable by the end of March 2007. Inventing a new manufacturing process and equipment for the glass in the short time frame provided was totally out of the question and Corning had to find ways to adapt existing manufacturing processes and equipment to make the glass. And it had to be amenable to producing very large amounts of materials for the millions of iPhones expected to be made. There is only one way to make nearperfect, thin sheets of glass in very large quantities. This is the fusion draw process and it is a proprietary technology that Corning is highly experienced with and a reason for their technological leadership. It is capable of producing exceptionally flat and smooth uniform-thickness sheets of glass with surfaces that are free of contamination (unlike the float 20  Silicon Chip glass process where one surface comes in contact with molten tin). The fusion draw process was first developed in the 1960s for the production of the aforementioned muscle car windshields, shelved and then reinvigorated for Gorilla Glass. The process is also used by Asahi Glass Co, Nippon Electric Glass, and Samsung Corning Precision Glass for making thin, large area glass for flat screen display panels. Fusion draw is a continuous flow process (also known as the overflow down-draw method). Molten glass is continuously loaded into a V-shaped trough and overflows on both sides, runs down the sides and then both flows join together at the bottom of the V to make a continuously moving sheet of glass flowing toward the ground. As the glass progresses it cools and solidifies. Auto- mated machinery cuts off pieces of the glass and stacks them. As with all well-thought-out processes, it sounds simple but just imagine the massive amount of fine tuning that would have been required to make it work perfectly. It was critically important to get the composition of the glass just right as it had to melt at a usable temperature, had to have the right viscosity and it had to produce a glass with reasonable optical and mechanical properties as well as being amenable to the chemical toughening process. This involved juggling the proportions of the six standard ingredients in aluminosilicate glass (see above) as well as a seventh secret ingredient. A Corning YouTube video of the process can be seen at http://youtu. be/q4ZU7zUxdM8 (or search for “The Fusion Process: At the Core of Corning’s Glass Innovations” on YouTube). Once the glass has been cut into sheets and stacked, it is still not Gorilla Glass. It has to be cut and shaped to final size and then surface toughened by an ion-exchange-process. How is Gorilla Glass toughened? After the fusion draw process described above the glass is sent to another factory for cutting to final shape and then it is subjected to an ion-exchange process to place the surface of the glass in compressive stress which is the key to the great strength of this glass. The ion-exchange process involves dipping the glass in a molten bath of potassium nitrate for a period of A 38cm diameter sapphire “boule” from GT Advanced Technologies. To make cover glass for smart devices pieces have to be sliced off with a diamond saw or using a laser process. Sapphire is a potential competitor to Gorilla Glass and is more scratch-resistant but much more expensive and not quite as transparent. siliconchip.com.au time. This causes the small sodium atoms to leave the surface regions of the glass while larger potassium ions enter the glass. Gorilla Glass itself is made in the USA but it is sent to a factory in China for final cutting and toughening. The iPhone 6 – Gorilla Glass or Sapphire Crystal? Apple has never stated what it uses for the cover glass in its various smart products but it is certain that Gorilla Glass was used for the iPhone 1 and it is widely believed that Gorilla Glass is what it uses for all other past and current models of iPhones and iPads. There was recently much speculation as to whether the iPhone 6 would use Gorilla Glass or sapphire crystal as its front glass but it has now been established that the main display glass is not sapphire. Sapphire crystal (aluminium oxide, -Al2O3) is used as the “glass” on high end watches now because of its high hardness and therefore scratch resistance but in that application it is relatively thick and heavy. Sapphire crystal is many times more expensive, 1.6 times heavier and requires 100 times more energy to produce than glass and it is not quite as transparent as Gorilla Glass. Gorilla Glass is strong (but not indestructible). Here a 0.7mm thick sample of Gorilla Glass 2 is subject to a 70mm deflection in a three point bending test. Due to its higher refractive index, sapphire has been claimed to be more reflective than Gorilla Glass. Also, because it is not quite as transparent as Gorilla Glass, the underlying display would need to be brighter and thus more energy-consuming. While it is more scratch-resistant than glass, sapphire does still break. In terms of scratch resistance, sapphire has a hardness of 9 on Mohs scale compared with 10 for diamond and around 7 for Gorilla Glass 3. Nevertheless, Gorilla Glass is not likely to scratch in normal use. The iPhone 5 already uses sapphire for the camera lens cover and the 5S also uses it for the fingerprint scanner. Speculation that Apple may use sapphire for the display in the iPhone 6 arises from its deal with GT Advanced Technologies to supply sapphire materials but these may currently be just for the camera lens and fingerprint scanner (should one be installed) or other products such as the Apple Watch. GT Advanced Technologies certainly has the capability to make very large crystals or “boules” (single crystal synthetic ingots) of sapphire for special applications as shown in the illustration of a piece that is 38cm in diameter. Ultimately, it is thought that Apple did not proceed with sapphire for the main screen on the iPhone 6 because of both supply and cost. GT Technologies was thought not to be capable of ramping up production of flawless large pieces of crystal for an estimated 80 million iPhone 6s. Sapphire will be used on the Apple Watch which will have a much smaller display (38mm or 42mm depending on model) than the phone. Gorilla Glass is not unbreakable Testing Gorilla Glass in the laboratory to measure mechanical properties. siliconchip.com.au Some people have interpreted the great strength of Gorilla Glass to mean that the product is unbreakable. Nothing is unbreakable and it is certainly true that sometimes the Gorilla Glass of smart phones and tablets does break if the devices are dropped onto concrete or other hard surface. There are numerous “tests” people November 2014  21 have done that can be viewed on YouTube in which the outcome is that the glass either does not break under the challenge or it does. What is impressive is the significant amount of abuse that the glass will usually take before it finally breaks. It is interesting that people are prepared to sacrifice their perfectly good phones for these demonstrations. Like any advanced electronic devices phones and tablets have to be treated with respect. As part of their ongoing quality procedures, Corning obtains whatever phones it can with failed Gorilla Glass and examines the failure mode and tries to see how such failure modes can be avoided. Future development Apart from smart phones, tablet and slate computers Gorilla Glass is being increasingly used in notebook computers, as a large cover glass on large scale interactive digital displays, digital signage and marker boards. In interior architecture, designers are exploring the possibilities of making entire walls with Gorilla Glass. Other emerging applications will rely upon glass properties such as it being sleek, cool to the touch and readily cleanable, possessing exceptional damage resistance and being compatible with touch screen technologies. It can also be printed on if necessary. Other anticipated developments include different surface treatments to make the surface antimicrobial (already unveiled), less reflective and less susceptible to fingerprints. MILESTONES IN THE DEVELOPMENT OF GORILLA GLASS • Billions of devices have now been produced that utilise Gorilla Glass. Since the release of Gorilla Glass 1 in 2007 it has been used on 2,400 different models of smart devices, over 33 brands. • Gorilla Glass 2 was released in 2012 and achieved a 20 percent reduction in thickness compared with the previous formulation while keeping its damage resistance, toughness and scratch resistance. The reduction of thickness allowed thinner devices with greater touch sensitivity and brighter images. • Gorilla Glass 3 was released in January 2013 with a feature known as Native Damage Resistance. It is up to 40% more scratch resistant and three times more damage resistant than Gorilla Glass 2. To achieve this the glass was reformulated and extensive atomicscale computer modelling was undertaken. • In July 2013 Corning introduced Gorilla Glass NBT for touch screens on notebook computers. The glass provides improved scratch resistance, reduced scratch visibility and better retained strength when a scratch does develop. According to Corning's marketing materials this glass avoids a common form of damage that happens when a notebook computer screen is closed and there is something on the keyboard such as a pen or USB drive, which causes the screen to break. • Antimicrobial Gorilla Glass has now been developed and unveiled and incorporates ionic silver, which renders the surface of the device anti-microbial for the life of the device. Conclusion At no time in history have materials scientists understood more about the behaviour of materials in terms of their atomic and molecular structures. In addition, never has the ability to do computer modelling of materials been greater than now. This enables scientists and engi- neers to develop materials with properties that were not conceivable even decades ago. Gorilla Glass is but one example of a highly engineered material that improves our technological progress and makes our daily lives so much easier. There will be many more materials SC like it. Companion products Corning has also developed companion materials for Gorilla Glass: Willow glass is a flexible borosilicate glass for display substrates. It will allow the use of flexible, printed displays. Delivered on a roll like newsprint, it is anticipated that the display elements will be printed onto the glass akin to a newspaper printing operation. In roll form the glass can be up to 1.3m wide by 300m long and in sheet form it can be up to 1.1 by 1.2m and 0.1 to 0.2mm thick. Lotus glass is designed as a substrate for OLED and next-generation LCD displays. It is to be used as a display backplane in conjunction with Gorilla Glass for cover material. 22  Silicon Chip Another of the many stringent tests Gorilla Glass is subjected to in the laboratory. siliconchip.com.au