Aluminosilicate glass
5-15 usd/kg
Adding alumina (Al2O3) to silica glass – in combination with modifying cations (positively charged ion), such as alkaline earth or rare earth oxide – leads to a glass with improved mechanical, optical and thermal properties. There are several different types, which are categorised according to their physio-chemical properties, and many of which are not in large-scale production. Well-known trade name examples include Gorilla, Xensation, Dragontrial and Dinorex. While the price for plain aluminosilicate is not so much – alumina is not an expensive ingredient – the chemical strengthening process adds considerable time and cost to the finished part.
Unlike float glass, which is limited to 0.4-25 mm thick, aluminosilicate cover glass is produced by the vertical fusion process in-air, allowing thicknesses down to 50 microns or so, with a perfect finish. As with glass fibres, the higher aspect ratio of thinner glass allows it to be very flexible.
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Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Aluminosilicate glass, produced in volume by companies such as Corning and Schott, exhibits very good chemical strengthening properties. It is also referred to as high ion exchange (HIE) glass. Specific examples include lithium aluminosilicate (LAS) glass and lithium alumino borosilicate (LABS) glass. LAS glass also forms the backbone of some high performance glass ceramics, such as Schott Ceran, which is used to make astronomical mirrors and cooktops.
Aluminosilicate glass has been exploited in many demanding applications, that require a hard, transparent barrier. Examples include aircraft cockpit windows, cover glass for phones and tablets, automotive interiors, and technical products such as sensor cover glass (fingerprint sensors and camera imaging for example), medical drug dispensing cartridges, scanners and lighting. As a composite, laminated with float glass, it holds potential for use in lighter weight automotive glazing, including roofs, windows and lighting (offering 30% weight reduction).
Even though it has been known for sometime, it wasn’t until full-screen phones and tablets emerged as a huge market opportunity that its benefits were fully utilised. The advantage of chemical strengthening is that it doesn’t distort the glass like heat strengthening, or affect light transmission. In practice, the glass is placed into a molten salt bath (such as potassium nitrate, KNO3) at high temperature. Through a process of ion exchange, smaller ions (sodium, Na) in the substrate are replaced with large ions (potassium, K) from the salt bath. This stuffing creates high levels of compression in the surface. Surface compression values can reach up to 700 MPa, and depth of layer (DOL) from 50 microns to 1 mm. The hard surface provides exceptional resistance to flexural breakage, scratching and thermal shocks. Real-life performance is highly dependent on flaws in the glass. In the case of chemically strengthened glass, damage has to penetrate through the pre-compressed layer to cause failure.
