Chemically strengthened glass finds a new application

The glass most people deal with every day—as cups, jars, windows, and lightbulbs—shatters easily. But not all glass is fragile. Some types, like the bulletproof stuff produced for windows used in banks, embassies, and head-of-state limousines, are downright tough.

Somewhere between the kind that cracks when two delicate wine glasses clink just a bit too hard and the variety that stands up to a speeding bullet lies chemically strengthened glass. Nowadays, the most common application of that kind of glass is on the screens of ­smartphones and other mobile devices. Chemically toughened glass has also been used for years in computer hard disks, photocopiers, and pharmaceutical devices.

But while the enormous popularity of smartphones and the nagging problem of people dropping their devices and cracking their screens keeps the spotlight focused on those applications, chemically toughened glass is finding a new one: automobile windshields. Last year, Ford began using hybrid materials containing strengthened glass in its GT sports car, the first production vehicle to do so.

Chemically strengthened glass is formed as large sheets or in other shapes, cut to size, and then subjected to a glass-hardening liquid chemical treatment, generally an ion-exchange process that debuted in the 1960s. But glass strengthening didn’t start then.

The idea of shatterproof safety glass first arose in 1903 when Edouard Benedictus, a French artist and chemist, made a fortuitous observation while working in his lab. As the story goes, when Benedictus knocked a glass beaker to the floor, he was surprised to see that the glassware broke but the pieces remained together. It turns out that a solution of nitrocellulose had dried inside the beaker, coating the glassware with a tough film that held the glass shards in place. After a period of development, Benedictus came up with a safety-glass laminate composed of two layers of plate glass that sandwiched a film of cellulose.

Modern bulletproof glass still relies on lamination. Also known as ballistic glass, the tough transparent armor, which fortifies vehicles used for transporting the pope, the U.S. president, and other dignitaries, often contains several layers of annealed glass bonded together with a film of poly(vinyl butyral), polyurethane, or other polymers. It can also be made from multiple bonded layers of glass and polycarbonate measuring up to several centimeters in thickness. Ballistic glass is strong, tough, and durable but much too heavy, thick, and expensive for mobile touch screens and ordinary automobile windshields. That’s where thin, chemically strengthened glass comes in.

“The science of chemical strengthening is relatively simple,” says Arun K. Varshneya, president of Saxon Glass Technologies. Immersing glass in a molten salt bath, typically potassium nitrate at around 450 °C, causes potassium ions to replace some of the sodium ions in a micrometer-thick layer on the glass surface. The difference in ion size (K⁺’s radius is roughly 0.38 Å larger than that of Na⁺) leads to compressive forces that toughen the surface by blocking the routes along which cracks could otherwise propagate.

In principle, the ion-exchange process works with any glass that contains sodium, and most glass does. Soda-lime glass, for example, which is made from silica, sodium carbonate (soda ash), and calcium carbonate (limestone), accounts for 90% of all manufactured glass.

In the 1990s, Saxon Glass customized the process to strengthen 1-mm-thick sodium borosilicate glass cartridges used in EpiPen injectors. The devices deliver epinephrine to treat life-threatening anaphylactic shock caused by severe allergic reactions. Varshneya notes that before the strengthened cartridges were developed, as many as one in 10 broke during injection. The failure rate for the strengthened glass version is less than one in a million, he says, adding that last year the company sold 30 million such cartridges.

Corning’s Gorilla Glass, an aluminosilicate material, and Dragontrail, an alkali aluminoborosilicate manufactured by Asahi Glass in Japan, also depend on potassium-sodium ion-exchange chemistry for glass strengthening. Because of these materials’ clarity, low weight, thinness, and high resistance to scratching and fracture, they have been used to make the protective cover glass for billions of smartphones, tablets, and flat-panel televisions. Now those properties are being put to use to make newly designed windshields.

According to Thomas Cleary, an automobile glass reliability manager with Corning, windshields on today’s cars typically consist of two layers of about 2.1-mm-thick annealed soda-lime glass held together by a roughly 0.8-mm-thick layer of poly(vinyl butyral), or PVB. Similar to the interlayer film in the safety glass designed by Benedictus a century ago, the PVB layer’s primary purpose is to reduce the likelihood that the glass will break—in this case, from flying rocks and road debris—and to retain broken glass in case it fractures. The plastic interlayer has seen numerous updates over the years, but the glass has remained relatively unchanged in almost a century, Cleary says.

Seeing an opportunity to reduce the weight of the windshield while maintaining its strength, Cleary and other Corning scientists teamed up with Ford researchers to study hybrids in which one of the sheets of soda-lime glass is replaced with a much thinner (0.5–1.0 mm) sheet of Gorilla Glass. That swap could reduce a windshield’s weight by about 30%, or nearly 7 kg, for some SUVs, corresponding to greater fuel efficiency and lower carbon dioxide emissions from the vehicles.

Tests of various combinations of layer thicknesses suggested that replacing the 2.1-mm-thick soda-lime-glass inner layer with a 0.7-mm-thick layer of Gorilla Glass, while leaving the PVB and outer glass layer unchanged, would most effectively reduce the windshield’s weight and maintain its requisite strength. As it turns out, the hybrid is not only lighter than conventional laminates but also much stronger.

The researchers conducted numerous tests that simulate the impact of blunt and sharp stones of various weights striking the windshield. Most windshield failures on the road are caused by strikes with sharp objects. In one set of tests simulating those conditions, researchers fired an industry standard diamond-tipped dart at conventional laminates and Gorilla Glass hybrids. It took more than twice as much impact energy to form star cracks in the hybrids as it did in the conventional laminates.

Cleary explains that the strength comes from a combination of the stout outer layer’s ability to impede damage from propagating far below the surface and the Gorilla Glass layer’s knack for flexing and thereby dissipating impact energy.

A related dart test showed that conditions that formed large star cracks in conventional laminates generated just a tiny chip in the outer layer of the hybrids. But Cleary knew from personal experience that tiny fractures can grow under the right conditions.

For example, he admits that once while watering plants near his driveway on a hot summer day, he turned the hose on his dusty car, which had a tiny windshield fracture. “As soon as the cold water hit the windshield, I heard the crack,” he says. The fracture split the windshield right down the middle. But Corning’s hybrid glass probably wouldn’t have suffered the same fate: Nearly 200 control tests with prechipped Gorilla Glass hybrids show that the hybrids withstand thermal shock while conventional materials fail.

In addition to passenger cars, strengthened glass’s reduced weight could also benefit trains, buses, and military vehicles, experts say. The damage-resistant glass may also make its way into wearable electronic devices, hurricane-resistant architectural windows, and tough pharmaceutical vials. More than 50 years after glass strengthening via ion-exchange chemistry made its debut, enthusiasts such as Varshneya see plenty of new opportunities for the lightweight, tough glass in the future.