The development of ceramic fibers is one of the key issues of the Fraunhofer-Center HTL. The entire process chain is covered in ceramic fiber production. The fiber development begins on a laboratory scale on highly flexible systems and can be scaled up to pilot plant scale. Afterwards the transfer to the customer is typically carried out. Thereby the Fraunhofer-Center HTL can also assist in designing the plants required by the customer and assist during the running of the processes.
Organometallic polymers or sol-gel systems are used as precursors, which allow a wide variety of material oxide and non-oxide ceramic fibers. Spinning masses are specifically adapted for the relevant spinning method. The rheological properties are of great importance, in particular.
The fiber spinning techniques are primarily melt-spinning (solid sol-gel precursors and preceramic polymers) and solution spinning/dry-spinning (preceramic polymer solutions and liquid sol-gel precursors) (see Nöth, A.; Hietel, D.; Rothmann, M.: Fabrication and upscaling of spinning processes of ceramic high-tech fiber production). All spinning techniques are carried out with endless fiber techniques of rovings that are containing 1 – 1000 filaments. The Fraunhofer-Center HTL also has monofilament spinning machines for hollow fibers and bi-component fibers as well as for thick filaments with diameters greater than 100 µm available (see Nöth, A.: Fabrication of large diameter SiC monofilaments by polymer route). All spinning techniques can be performed in air or inert atmosphere. The conditions in the spinning tower – atmosphere, temperature, gas flow – can be adapted to the specific requirements of the spinning material.
Pyrolysis and sintering of spun fibers are performed after the spinning process, by which the transformation into ceramic fiber takes place. Pyrolysis and sintering can be continuously carried out up to temperatures of 1800 °C in air and 2000 °C under inert conditions.
The fiber production is accompanied by extensive options for fiber testing, whereas the testing of fibers is also offered as a service by various DIN EN standards (see Fiber Testing).
Fiber coatings are developed in addition to the ceramic fiber at the Fraunhofer-Center HTL. The purpose of the coating is to set a fiber-matrix interface, which provides a damage tolerant behavior of ceramic composites. Another purpose is to protect the ceramic fiber against corrosive attacks. A comparatively inexpensive wet chemical route is mainly used during the application of coatings. Velocities up to 1000 m/h are currently achieved during this process. Fraunhofer-Center HTL has experiences in applying non-oxide and oxide material systems and non-oxide-oxide hybrid systems. Fraunhofer-Center HTL develops novel coating precursors and optimizes existing systems for its customers. Process parameters are identified during the application of the layers, which allow an homogenous coating of the fiber with the desired thickness. Furthermore, there are various possibilities for the characterization of fiber coatings. Further processing of fibers usually is done using textile techniques.