Unbreakable

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The minute hand of this watch prototype is made of glass and filled with a fluor

The minute hand of this watch prototype is made of glass and filled with a fluorescent liquid. Image: Ulysse Nardin

Can glass flow at room temperature and thus withstand hard impacts’ A theory from the 1970s predicted exactly this. Empa researchers have now provided the proof. The results could form the basis for robust 3D printed glass microarchitectures.

No one in the world has ever seen what we have measured," says Rajaprakash Ramachandramoorthy. "We’ve tracked the breaking properties of glass further than any previous research team." Raj - his nickname among colleagues - works in the Empa lab "Mechanics of Materials and Nanostructures" led by Johann Michler. The team explores material properties on a very small scale: Machines they have developed themselves use tiny stamps to press columns just a few micrometres thin, so-called micropillars. In the electron microscope, and with the help of the most precise force measurements, they can observe how the micropillar breaks or deforms. These results allow the researchers to draw conclusions about the internal structure of materials.

Ramachandramoorthy and his colleague Jakob Schwiedrzik have now used this method to examine fused silica glass - and discovered properties that have little to do with the macroscopic world of glass: Glass is as tough as modelling clay when pressed very slowly, it can tear and burst when pressed a little faster - and at very short, fast pressure impulses it behaves again tough and yielding. "Our slowest compression test took about 20 minutes," says the Empa researcher. "Our most rapid impulse on the micropillar, on the other hand, took only 100 microseconds. It is comparable to the blow of a hammer."

For the first time, the Empa researchers now provide empirical values and measurement data for material properties that could thus far only be postulated in theory. The deformation properties of amorphous materials such as glass were predicted at the end of the 1970s independently by two physicists, Frans Spaepen at Harvard University and Ali S. Argon at the Massachusetts Institute of Technology (MIT). Both theories, we will call them "glass flow theories" direction and say: Whoever wants to build a break-resistant micromechanical system, should rather build it from glass than from crystalline silicon. Because glass withstands high forces better.

But this could never be proven experimentally. Only very slow compression tests were possible at the microscale. On the other hand, at the macroscale, dynamic experiments with high speeds trigger a shock wave in the test specimen that superimposes the deformation mechanisms. Then something happens we all know from experience: The glass breaks as soon as the shock wave encounters a material defect that almost always occurs in large test specimens.

Ramachandramoorthy and Schwiedrzik found a suitable solution at Empa: They use micro-columns of etched glass that are so small that, statistically speaking, they no longer contain any material defects that could distort the results. At the same time, the shock wave is no longer a problem: It now runs so quickly through the microscale test specimen that the effect does not interfere with the hammer blow that shifts the glass atoms. Now both effects can be documented separately. For the first time, the 40-year-old glass flow theory of Spaepen and Argon can be verified through experiments.

Research at Empa has already attracted a great deal of attention in the Swiss watchmaking industry and among manufacturers of microelectromechanical systems (MEMS). They could provide the basis for 3D-printed glass microcomponents that could be built into shock-resistant watches or robust measuring instruments. In March 2019, the Swiss luxury watch brand Ulysse Nardin presented the prototype of a watch called FREAK neXt, which was equipped with a luminous minute hand made of glass. The glass component, a very fine capillary filled with fluorescent liquid, was produced by the Swiss specialist company Femtoprint using 3D printing. But from a prototype for the watch case to a shock-resistant series product, there is still a lot of development to be done. The Empa researchers now want to lay the foundations for this.

In the meantime, Rajaprakash Ramachandramoorthy is not only in demand as a supplier of new ideas for the watch industry. After they published the results with pure silicate glass, manufacturers of float glass and borate glass asked for an expert opinion. They all want to know more about the micro-properties of their products. The next research projects are already underway. "With our method, we can basically investigate any material system including amorphous materials, metals, polymers, biomaterials and ceramics," he says.

He also wants to use the machinery developed in the Empa lab in Thun for an extensive series of experiments: "In the future, we want to measure the mechanical properties of materials between minus 150 degrees and 1000 degrees Celsius at a wide range of testing speeds." Looking at the properties of tiny microcolumns could soon pave the way for some major material innovations.