Materials research in space with Swiss participation
Metallic glass is similar in color to white gold, but has the hardness of quartz glass. At the same time, it is elastic and resistant to salts or acids. It can be processed in 3D printing, for example for medical implants, or also in injection molding. However, a great deal of materials research still needs to be done before this is possible. At Empa, Antonia Neels, who [...]
Editorial - May 5, 2022
Materials research in weightlessness: Scientists from the University of Ulm during a melting test in Novespace's Zero-G Airbus. (Photo: Airbus Defence and Space) Metallic glass is similar in color to white gold, but has the hardness of quartz glass. At the same time, it is elastic and resistant to salts or acids. It can be processed in 3D printing, for example for medical implants, or also in injection molding. However, a great deal of materials research still needs to be done before this is possible. At Empa, Antonia Neels, head of the Empa X-ray Center, is working on this mysterious material. Her team is investigating the internal structure of metallic glass using various X-ray methods, thereby discovering correlations with properties such as deformability or fracture behavior. Even for professionals in materials science, metallic glasses are a tough nut to crack: "The closer we look at the samples, the more questions arise," says Antonia Neels. This spurs the researchers' ambition all the more.
Into space for materials research
Nun soll in einigen Monaten eine Probe von metallischem Glas ins All fliegen. Auf der internationalen Raumstation ISS werden die Materialeigenschaften in der Schwerelosigkeit untersucht. Eine Forschergruppe unter Beteiligung der Empa hat die Proben vorbereitet und bei der europäischen Raumfahrtagentur ESA zum Weltraumflug angemeldet. Die Speziallegierung liefert die Firma PX Group aus La Chaux-de-Fonds, die Materialien für die Uhrenindustrie und die Zahnmedizintechnik herstellt. Mit im Team sind auch die Forscher Markus Mohr und Hans-Jörg Fecht vom «Institute of Functional Nanosystems» der Universität Ulm sowie Roland Logé vom «Laboratory of Thermomechanical Metallurgy» der EPFL in Neuchâtel. Die Herstellung von metallischem Glas ist nicht ganz einfach: Im Vergleich zu Fensterglas müssen die speziell ausgewählten Metall-Legierungen bis zu hundertfach schneller abgekühlt werden, damit sich die Metallatome nicht zu Kristallgittern zusammenlagern. Nur wenn die Schmelze schockartig erstarrt, bildet sie ein Glas. In der Industrie werden dünne Folien metallischer Gläser erzeugt, indem die Schmelze zwischen schnell rotierende Kupferwalzen gepresst wird. Forscher giessen bisweilen ihre Proben in Gussformen aus massivem Kupfer, das die Wärme besonders gut abführt. Doch grössere, massive Werkstücke aus metallischem Glas sind mit diesen Methoden nicht machbar.
The 3D printing helps further
One possible way out of the dilemma is 3D printing using the so-called powder bed process. A fine powder of the desired alloy is heated for a few milliseconds with a laser. The metal grains fuse with their neighbors to form a kind of foil. A thin layer of powder is then placed on top, the laser fuses the freshly applied powder with the underlying foil, and a three-dimensional workpiece is gradually created from many briefly heated powder grains.Empa researcher Antonia Neels heads the Center for X-ray Analysis. She is an expert in metallic glasses and will analyze the samples from the ISS. (Image: Empa) This method requires a fine dosage of the laser pulse. If the laser burns too weakly on the powder, the particles do not fuse together and the workpiece remains porous. If the laser burns too strongly, it also melts the lower layers again. The multiple melting allows the atoms to rearrange themselves, forming crystals - and that's the end of metallic glass.
Tracking down the secrets with X-ray methods
At Empa's X-ray center, Antonia Neels' team has already analyzed several such samples from 3D printing experiments. The results, meanwhile, always raise new questions. "Some evidence suggests that the mechanical properties of the glasses do not deteriorate, but on the contrary actually improve, if the sample contains small crystalline fractions," says Neels. "Now we are looking into the question of how large this crystal fraction in the glass has to be, and what kind of crystals have to form to increase, for example, the pliability or impact strength of the glass at room temperature."
To track down crystal growth in an otherwise amorphous environment, Empa's experts use various X-ray methods. "With radiation of different wavelengths, we can learn something about the structure of the crystalline parts, but also determine close-order phenomena of the atoms in the sample - in other words, determine the properties of the chemical bonds," explains Neels. In addition, X-ray imaging analysis, known as micro-CT, reveals something about density fluctuations in the sample. This indicates phase segregation and crystal formation. However, the density differences between the glassy and crystalline regions are minute. Therefore, detailed image processing is needed to visualize the three-dimensional distribution of the crystalline portions.
Why material research in weightlessness?
Materials research to date using 3D methods has its limitations. Above all, the question of the temperatures at which the aforementioned crystals form and how they grow still needs to be clarified. Thermo-physical parameters such as viscosity or surface tension play a role here. Experiments on the ISS offer ideal conditions for their analysis. In order to test the behavior in weightlessness, the first droplets of metallic glass were brought into suspension on an experimental basis as early as 2019, on board an Airbus A310, which carried out a so-called parabolic flight. In the experiment called TEMPUS (crucible-free electromagnetic processing under zero gravity), the glass droplet consisting of palladium, copper, nickel and phosphorus was held in suspension by means of a magnetic field and heated up to 1500 degrees Celsius by induction. During the cooling phase, two short pulses of induction current caused the glowing droplet to oscillate. A camera recorded the experiment. After landing, the material sample was analyzed at Empa's X-ray center. However, since more in-depth analyses require a longer duration of weightlessness than that possible on a parabolic flight, a material sample has now been registered for a flight in the European COLUMBUS module of the ISS. The ISS-EML electromagnetic levitation furnace has been installed there since 2014. In each case, 18 material samples fly along, are automatically exchanged and can be observed by researchers on Earth via video stream. The metallic glass from Switzerland will go into space with the next batch of samples.
Computer simulation for new casting processes
From the far more detailed data of the space flight, the researchers want to generate a computer simulation of the melt. This will bring all the answers together in a unified model through a combination of experiments on Earth and in space: At what temperature is there what viscosity and surface tension? When do crystals of what composition, size and orientation form? How does this internal material structure influence the properties of the metallic glass? From all these parameters, the researchers want to develop a manufacturing method together with the industrial partner PX Group, in order to be able to produce the coveted material in a defined form. So in the next few years, there is still a lot for the materials researchers in all the teams involved to do. Source and further information: Empa
