Klaus Jopp, Hamburg
Collaborations on Composites
After the glass industry, the aluminum industry has now discovered “Fortadur,” a fiber-reinforced glass produced by SCHOTT Medica. New research findings give reason to believe that this family of composites has a promising future.
Some years ago, SCHOTT researchers developed a new material that SCHOTT Medica now produces and markets with growing success under the brand name “Fortadur.” This unusual material is a fiber-reinforced glass in which the ceramic fibers are made from either carbon (C) or silicon carbide (SiC). Thanks to these built-in “reinforcers,” a glass is formed with tailored properties that are anything but typical of this normally brittle material. “Composites like this one are characterized by their unique breaking and flexural strength,” says Hans-Hermann Leiß, Director of Production at SCHOTT Medica GmbH in Wertheim. “Fortadur” also has a very high resistance to temperature changes and, depending on the type of glass and fibers, can withstand air temperatures of up to 450° C (“Duran”/C) or even 1,200° C (glass ceramic/SiC).
Interesting fields of application
The glass industry was the first to utilize the enormous potential of this new class of engineering materials. When handling hot components, for example in bottle manufacturing, there are a number of tools and machine parts that are exposed to high temperatures. They must be sturdy, offer a good service life and ensure that no traces are left behind on the glass products.
“Fortadur” has a very good breaking and flextural strength.
It tolerates temperatures of up to 1,200° C and can be used to produce components in complex geometries.
The demands on the production of bottles and glass are enormous, especially considering the normal production targets of between three and five million pieces per day. For this reason, fiber-reinforced glass is an excellent choice as an engineering material for the fabrication of grippers, sliders, racks and transport devices. Metals cannot be used for this equipment because they would withdraw too much warmth from the bottles, which can result in cracks. Graphite is one alternative, but it is breakable. “Fortadur” is used both in glassworks and also in factories in which the glass is processed into a particular shape.
The characteristics that have been so successful with glass are also applicable to other materials, such as ceramics and metals. In the meantime, the aluminum industry has begun to use composites as well. The advantage of “Fortadur” in aluminum applications is that it is not wetted by the metal, which, as a particularly lightweight engineering material, is growing in importance in automobile construction, in the aircraft and aerospace industries and in rail vehicles.
With composites, processes can be accelerated because conversion times are shortened and the service life is extended. The foothold in the aluminum industry succeeded through the production of piston rings. Other applications include hooks, stirrers and scoops. “We are actually supplying the aluminum industry more and more. ‘Fortadur’ is definitely a promising product,” confirms Armin Reiche, Managing Director of SCHOTT Medica, which has one of the biggest hot presses in the world. And this device is responsible for the most important process step.
Continuous fibers are first coated with glass powder in a kind of whirlpool bath, then wound on reels in wide bands and cut into defined segments called tapes. These tapes are pre-dried in stacks (prepregs) and then fused in pairs at 1,000° C and 100 atmospheric pressure into the nearly poreless, finished composite. This occurs with an upper punch and lower punch; the compression ratio is about 1:4.
Searching for better production methods
Despite all the enthusiasm for this special kind of glass, one drawback has still not been overcome. The silicon carbide fibers produced by companis like UBE and Nippon Carbon are very expensive. So SCHOTT is pursuing several strategies to come up with better solutions. For example, a new manufacturing process has been developed in which short, randomly distributed or matted fibers about one centimeter long are used instead of the continuous fiber. This is a technological improvement because it reduces production costs, while ensuring isotropic, i.e. identical in all directions, features. The mechanical properties, for example, remain virtually the same. Thanks to this isotropy, other geometries are possible, which increases the potential for new applications. “We are now working up to 80 or 85 percent with matted fibers,” says Leiß.
Continuous fibers are coated with glass powder in a whirlpool bath ...
... and wound on a reel to wide bands.
The Fraunhofer Institute for Silicate Research (ISC) in Würzburg under the supervision of Professor Gerd Müller, with whom SCHOTT is also collaborating, has taken a different approach. Conventional reinforced fibers are often too expensive for the desired applications. For this reason, ISC is mainly searching for fiber types that will be more economical. Thus researchers are investigating SiBN3C fibers, which contain boron, nitrogen and carbon in addition to silicon. They differ from conventional silicon carbide types because of their improved oxidation and temperature stability.
A good starting position
„Fortadur“ weist durch den hohen Faseranteil von 40 %, ganz gleich ob mit Siliziumcarbid- oder Kohlenstoffverstärkung, eine nahezu schwarze Farbe auf. Komposite, die transluzent oder sogar transparent wären, hätten ein deutlich größeres Einsatzpotenzial, zum Beispiel für durchwurf- oder durchschusshemmende Scheiben. „Dazu ist es notwendig, dass Glas und Faser identische Brechungsindices haben“, erläutert SCHOTT-Materialexperte Prof. Dr. Wolfram Beier. Entsprechende Möglichkeiten lotet die Technische Universität Ilmenau im Verbund mit der Universität Chemnitz und der Bergakademie Freiberg aus; die Arbeiten werden von der Deutschen Forschungsgemeinschaft gefördert.
No matter if it is reinforced with silicon carbide or with carbon, “Fortadur” is virtually black due to its high fiber content of 40 percent. If such composites were translucent or even transparent, they would have the potential for a much wider range of application, for example panes that can withstand shots and flying objects. “For this application, the glass and fibers must have an identical refractive index,” explains SCHOTT’s materials expert Professor Wolfram Beier. In collaboration with the University of Chemnitz and the Technical University and Mining Academy in Freiberg, the Technical University of Ilmenau is researching various possibilities. The German Research Community (DFG) is supporting this project.
The use of matted fibers is a technological advance. Compared with manufacturing methods based on continuous fibers, composites can not only be made more economically, but also permit the production of components with other geometries because of their isotropic features.
There are many options in this field. In fact, SCHOTT itself is investigating the use of aluminosilicate glasses and “Ceran” type glass ceramics for fiber reinforcement. There are also other potential areas of application. For example, clutch disks and facings could be made from fiber-reinforced glass. The same is true of brake disks and linings. Such components could be used not only in street vehicles, but also in high-speed trains like the German ICE, the French TGV and the Japanese Shinkansen. The advantages of components made from composites are manifold: temperature stability, a high coefficient of friction, better wear resistance, good comfort behavior and lower prices compared with ceramic materials. With this in mind, it is clear that the future of fiber-reinforced glass has just begun. And SCHOTT has a very good starting position. “We are the only company that already has marketable products,” says Armin Reiche.