SCHOTT solutions no. 1/2013 > Research and Technology

Photo: SCHOTT/C. Costard

On the Way to Super Strong Glass

The industry is demanding ever thinner, yet sturdy, transparent materials. Applied Research at SCHOTT therefore has one clear objective: developing glass that is considerably stronger.

Thilo Horvatitsch

Legend has it that the Roman Emperor Tiberius was presented with an indestructible flexible glass around the year 30 A.D. However, because he was afraid that gold, silver and bronze would lose value, he destroyed the glassmaker’s workshop.

So did that mark the end of the dream of coming up with bendable, indestructible glass? Certainly not for glass researchers at SCHOTT. Today’s glass products hardly use even one percent of this potential. Our goal is to use more of it,” says Dr. Markus Kuhr, an expert on rigidity. But this isn’t an easy task because metals and plastics are flexible and can be bent very easily. However, if they are subjected to too much stress, this will deform them plastically. Based on our experience, this sounds impossible with glass. After all, glass can hardly be bent or stretched at all before it suddenly breaks. This also explains why glass, ceramics and crystals are referred to as so-called brittle fracture materials.
SCHOTT researchers perform sophisticated scratch tests to obtain differentiated results on the scratch resistance of various types of glass. Photo: SCHOTT/C. Costard
Microscopically small cracks can be observed on glass surfaces. And when mechanical pressure is applied, their ”peaks” don’t become rounded like metals do because of plastic flow. These types of peaks can cause a crack to spread into the middle of the glass and break the glass if enough tensile stress is applied.

A technique that is capable of increasing the strength of glass to roughly five times its normal value, despite the inherent mechanisms that weaken it, is now being used to increase the strength of glass so it can be used as cover glass in smartphones or in aircraft and locomotive glazing. A so-called ion exchange process is now being used to induce a roughly 30 to 120 micrometer deep chemical change in the composition of the glass, depending on the glass type and application, that creates a compressive stress zone. So, if a crack spreads out from the surface to penetrate into the glass and thus cause destruction, it first needs to overcome this enormous compressive stress of up to 1000 MPa like getting past a dam. As a result, the respective product, a cover glass for a smartphone, for example, will be far less likely to break when mechanical stress is applied. Furthermore, this technique can also be used to increase scratch resistance and impact and bending strength.
SCHOTT Research and Development has access to state-of-the-art devices for chemically tempering glass, characterizing the bulk material, performing microstructural analyses of glass surfaces and various types of fatigue tests like this two-point bending test (Photo below left), for instance. Photo: SCHOTT/C. Costard
Besides the absolute strength of the compressive stress, the question of how deeply these ion exchanges take place in glass is also quite important. After all, the product will only lose its strength if cracks like scratches manage to penetrate through the zone. This chemical ion exchange processes has long been used in aircraft and high-speed train glazing to improve the strength of the windshields. In the meantime, SCHOTT has introduced an incredibly broad spectrum of treated glasses under its umbrella brand Xensation® for use in various touch technologies.

So what are SCHOTT researchers working on at the moment? ”Due to the fact that a product’s useful properties are defined by its microscopic characteristics, we have set ourselves the goal of coming up with a glass that is as flexible as possible at the atomic level,” explains Material Developer Dr. Ulf Dahlmann. ”We would like to give this glass some of the plastic properties of metals and polymers without sacrificing its outstanding properties like strength and optical quality. The reason for the brittle fracture behavior of glass can be found deep inside the nature of atomic bonds and chemical structures in glass. Understanding these relationships is the key to finding new solutions.”

Photo: SCHOTT/C. Costard

In other words, the goal is to increase its ability to plastically deform under pressure. SCHOTT researchers are already on the trail of certain glass forming elements that enhance these properties. Nevertheless, they also need to be bonded in an appropriate manner inside the right glass network. A difficult challenge that requires models of the glass structure and tests on structural mechanics, but also glass chemists and technologies that display the relationships in an experimental manner and push back the boundaries of what is technically and commercially possible.

So what visions are behind these types of developments? Perhaps developing a glass that is capable of reacting to the effects of scratches by using the right mixture of hardness and softness to ensure that the edges of possible scratches no longer break off in a brittle way, but rather flow away plastically and are thus less visible. Or ultra-thin glasses that are flexibly bendable, much like fiber optics, and thus enable completely new production processes and products. SCHOTT is already developing a glass product with the highest possible edge strength for this very reason. Here, too, the way in which cutting techniques interact and how the material reacts are both important factors.

SCHOTT is therefore well prepared to develop the even stronger glasses we will be seeing in the future. ”Over the next ten years, flexible, ultra-thin and light glasses could well contribute to a real technology and product revolution in the areas of electronics, lighting and household technology, and in mobility and industrial applications,” says Dr. Rüdiger Sprengard, head of Product Development with SCHOTT Research. Maybe someday it will actually become reality: ”Vitrum flexile,” the flexible, indestructible glass from the ancient world. <

Increased strengt through ion exchange