Examining Glass Substrates:An Insider’s Guide to Stronger & Scratch Resistant Glass Gary L. Barrett, Chief Technology Officer, Touch International
IntroductionOne of the biggest benefits to integrating projected capacitive touch technology intointeractive displays is its durability, potentially lasting beyond the use of most devices.This is because the most common touch surface is now glass, as opposed to flexibleplastic in legacy products. However, while the touch panel may never wear out, the glasscan break, which often ends the touch function. So, making the touch panel lesssusceptible to breakage makes the device a better one.In the pursuit of making better touch panels, touch manufacturers have experimentedwith many kinds of glass. Currently, the most common types of glass used in touchpanels include, basic soda-lime, chemically strengthened, and the now very popularalkali-alumina-silicate glass, often referred to as Gorilla Glass. Tempered glass andsapphire glass are also occasionally used.BackgroundWindow glass, used for almost all glass products, is commonly called soda-lime glass orfloat glass. It is made of 75% silica (SiO2), soda or sodium carbonate (Na2CO3), and limeor calcium oxide (CaO), along with some other trace elements. These trace elementsactually affect the color, which you can see from the edge, and vary by the manufacturers- green in the case of most U.S. manufacturers and blue from the rest (some customerscare about the color of the glass). In a continuous process, these raw ingredients aremixed and melted. Once liquid, the elements flow over the edge of the melting chamberand become a long river of hot liquid-glass floating on molten tin where it spreads to theright thickness and is slowly cooled into flat glass. This process is the reason commonwindow glass is sometimes called float glass, though the other-chemistry glass we willdiscuss can also be termed “float glass”. Because of the difficulty in starting up thesemachines, they run continuously for at least 5 years before they are turned off, rebuilt,and restarted. The image below illustrates the float glass manufacturing process. Source: Tangram Technology, LTD Fig. 1
Testing GlassCommon tests used to classify glass (or plastic) include measuring breakage resistance,with a drop ball test, pencil hardness, through a scratch test, and light transmission, usinga spectrophotometer. For reference, regular soda-lime glass has a pencil hardness of 7Hand light transmission of 92%.Breaking GlassTo test break resistance via a drop ball test, a 2” diameter steel ball weighing 0.5Kg isdropped systematically at varying heights up to 1.3 meters onto the glass below. Theimage below illustrates the standard setup for a drop ball test.Drop Ball Test Steel Locking PVCTouch Sensor bonded on LCD Stand Source: Touch International Fig. 2
Chemical StrengtheningMany years ago, it was discovered that glass could be strengthened by putting thesurfaces in compression. One way of doing this is to replace the sodium ions (from thesodium carbonate) with potassium ions (usually from potassium nitrate [KnO3]); thepotassium ions will bond with the carbonate (remember valence from chemistry class)better than sodium, and they are bigger than the sodium ions. In a molten bath ofpotassium nitrate, the glass is soaked, allowing the big potassium ions to trade placeswith the smaller sodium ions, and when the glass cools, the surface where the exchangetook place, is in compression, and therefore stronger.The chart below compares the strength of chemically strengthened (CS) float glass toordinary float glass. Source: http://www.xinology.com/en/eg_cpml_temper-chemicalI.HTM Fig. 3It is important to note that this transition of ions only occurs at the surface of the glasswhere the glass and potassium salts connect. This hot-soak process should take 16 hoursat 380°C to reach 90% of the theoretical maximum strength. It is believed that it takes 30hours to get 100% of the maximum strength. Some companies cheat and only soak for 8hours, which interrupts the ion exchange, with a corresponding reduction in strength.
Tempered GlassTempered glass is made by heating glass to around 620°C, usually in a conveyor oven,and then quickly cooling, often called quenching, the surface, with jets of cold air. Thisputs the surface into compression, and the hotter glass underneath into tension. If regularglass will break at 10,000 PSI, tempered glass will break around 25,000 PSI.Far more tempered glass is produced each year than chemically strengthened glass and isfound most commonly in glass doors and appliances. It is cheaper than CS glass, andwhen it breaks, is usually less hazardous; however, the process often leaves opticalimperfections and will often not work with the thinner glass needed for touch screenapplications. A touch screen mounted onto the backside of a shop window is the mostcommon place to see a tempered glass touch screen, and occasionally in outdoor securityaccess or transit applications.Sapphire GlassSapphire Glass, which is nearly pure Aluminum Oxide (Al2O3), provides the ultimate inscratch resistance. Historically, it was used as watch crystals, and today is often used ingrocery store UPC scanners that have heavy cans dragged over them all day. Sapphireglass is made by heating the aluminum oxide to melting point and then drawing into aboule, and when cooled, cut and polished into a window. Due to its high cost and limitedsize, it is only occasionally used for special touch applications. Source: Corning Museum of Glass Fig. 4
Alkali-Alumina-Silicate (AAS) GlassThe hot, new “super-glass” is currently Alkali-Alumina-Silicate (AAS), also known asaluminosilicate or high-ion-exchange glass, which is similar to soda-lime glass, exceptthat it has more aluminum oxide. In its raw state, it has slightly better light transmissionand slightly better break resistance than regular soda-lime (note the word slightly).Corning has brought much interest and attention to this glass in the past few years,spending millions of dollars promoting its break-resistant glass product, Gorilla 2. Fromthe iPhone 4 to emerging touch screen keyboards, Gorilla Glass is becoming associatedwith a host of cutting edge products. In response, the big guns of the glass business,Schott (Germany) and Asahi (Japan) have released Xensation and Dragon Trail,respectively. Due to Corning’s slick marketing campaign, however, Gorilla Glasscurrently has a strong edge in this market.Products likely to be scratched, including the smart phone in your pocket, will benefitfrom this glass, but at six times the cost of regular glass, expect to pay a premium.Alkali-Alumina-Silicate glass is made either as fusion glass (Corning) (See Figure 3) orby the float process (Schott and Asahi) described above. For fusion glass, the moltenalumina-silicate glass falls over the edge of the melting box on two sides, and meetstogether below the box and “fuses” into one sheet. Corning uses the fusion process tomake Pyrex (boro-silicate glass), and it has some advantages because there is no residualtin, which can be a problem for other chemicals, such as the ones used in LCDs. Also ofnote, some believe the tin can impede the potassium/sodium ion exchange so the glass issometimes polished before chemically strengthening; Asahi does not believe polishing isnecessary and does not recommend it.The chemical strengthening process is the same for alkali-alumia-silicate glass as it is forordinary soda-lime glass. The big difference is that soda-lime and AAS glass surfaces arenot the same. In the case of alumina-silicate, there are bigger holes in the surface, whichallows the potassium ions to go deeper into the glass and thus create a deepercompression layer. After chemically strengthening, the alumina-silicate becomes Gorilla,Xensation and Dragon Trail glass.Here is the kicker: In terms of scratch resistance, and break resistance, there is very littledifference between ordinary chemically strengthened soda-lime glass and Gorilla,Xensation and Dragon Trail.So why pay big money for glass with high-ion-exchange? It is a term the glass guys liketo call depth-of-layer (DOL) or thickness, which refers to the ion exchange which ismuch deeper in AAS glass. This deep ion exchange allows AAS glass to take a lot ofscratching and still retain its break resistance.The original iPhone, through the iPhone 3GS, did not use Gorilla Glass. The use ofGorilla Glass was touted as a big advance in the iPhone 4; however, there were morereports of breakage on the iPhone 4. The breakage spike was originally attributed to thehigh tension level in the ultra-thin new glass, but was later was found to be a designproblem with the phone. It should be noted that all chemically strengthened glass,
whether float or AAS, is very vulnerable to cracking if the edge is hit sharply in any way.The iPhone ushered in the era of flush mount surface (no protective bezel), so greatercare must be given to the exposure of the glass edge.SummaryIs High-Ion-Exchange Glass worth the higher price tag? Unfortunately, we cannot showyou a comparison chart on the benefits of these high-ion-exchange products, because themanufacturers consider them proprietary and are subject to non-disclosure agreements.By observation, and through Touch International’s experience with these products, hereare our findings: 1. High-Ion-Exchange Glass demonstrates much better break resistance after scratching. 2. Ordinary float glass, which has been chemically strengthened, performs almost as well in break tests if there is no scratching of the surface. 3. Among the three big brands of AAS glass, the performance is generally about the same. Differences include depth-of-layer (over-all scratch resistance resulting in breakage) and performance in the ball drop test (mostly due to time in the chem- strengthening bath).