How SCHOTT developers are producing ever stronger, yet thinner glasses -- The industry is demanding ever thinner, yet sturdy, transparent materials. Applied Research at SCHOTT is therefore pursuing a clear objective: to develop glass that is a lot stronger.
Hardly anyone is aware of the fact that glass has enormous intrinsic strength. Nevertheless, its surface inevitably contains invisible production-related microcracks. These extend on into the inside of the glass when the material is put to use and reduce the usable strength down to only a fraction of its theoretical value. Unlike metal, glass is unable to compensate for these types of cracks by way of so-called plastic flow. It breaks very easily when it is bent.
The industry constantly places higher demands on glass materials. Glass that is less than 1 mm thick, yet far less likely to break and scratch, like cover glasses for touch displays in mobile devices, for instance, is in great demand. The aluminosilicate glass Xensation® Cover is therefore considered to be one of the world’s strongest and most scratch resistant glasses and is thus a prime example of how the company’s developers can achieve these types of unique mechanical properties.
"We essentially focus on having a stable bulk material for the glass and producing an incredibly strong surface that contains as few defects as possible and stands up well to mechanical forces. Material and process development always go hand-in-hand," explains Dr. Rüdiger Sprengard, Head of Product Development at SCHOTT Research. On the material side, the goal is to achieve a synthesis of glass blocks with a high chemical bond. Gaining deeper insights into the relationship between the material composition and its atomic structure plays an important role here, as do ultramodern computer simulations.
Aluminosilicate glasses with high alkali content are considered to be the material class of choice. During the so-called ion exchange process that is used to chemically temper glass, small alkali ions are released from the glass at high temperatures and their position in the atomic network is taken by larger alkali ions. Enormous strength levels are generated inside the glass product when it is cooled down, due to the pressure the large ions exert on the glass network at the surface. The deeper the ions penetrate into the surface, the higher the compressive stress that puts a “clamp” on scratches and other defects and thus prevents the glass from breaking. The strength of the glass will not be impaired unless these defects exceed the depth of ion exchange.
SCHOTT has succeeded in producing various silicate glasses with previously unseen depths of oxygenated layers (DoL) of more than 120 micrometers and high compressive stress (CS) of up to 1000 megapascal (MPa) using optimized chemical tempering techniques. By comparison: the CS values of conventional soda-line window glass are only about half as high, the depth of exchange is limited to only 30 micrometers.
