The development of ceramic fibers is one of the main areas at Fraunhofer-Center HTL. This covers the entire process chain in the manufacture of ceramic fibers. The fiber development begins on the laboratory scale on highly flexible devicessystems and can be up-scaled to pilot plant scale. Usually, the transfer to the customer is carried out afterwards. Fraunhofer-Center HTL can provide support for the design of the systems required by the customer and assist in commissioning of the processes. Oxidic reinforcing fibers are made of aluminum oxide or mullite. Custom zirconium oxide or yttrium aluminum garnet fibers can also be developed. Non-oxide reinforcing fibers are made from silicon carbide (SiC) or SiBN3C. There is also experience with the development of functional fibers (lead zirconate titanate). The fiber diameter is between 10 and 100 µm.
As precursors, metalorganic polymers or sol-gel systems are used, which allow a large diversity of oxide and non-oxide ceramic fibers. The spinning masses are specifically adapted to the spinning process used. The rheological properties in particular are of great importance here.
The ceramic fibers are mainly spun using the melt spinning process (solid sol-gel precursors and preceramic polymers) and solution spinning processes / dry spinning processes (preceramic polymer solutions and liquid sol-gel precursors) (Publication: Fabrication and upscaling of spinning processes). All spinning processes are carried out as continuous spinning processes of rovings with 1 – 1000 filaments. Monofilament spinning systems for hollow fibers and two-component fibers as well as for thick continuous fibers with diameters > 100 µm are also available (Publication: Fabrication of Large Diameter SiC Monofilaments). All spinning processes can be carried out in air or under inert gas. The conditions in the spinning shaft, such as the atmosphere, temperature and gas flow, can be adapted to the specific requirements of the spinning mass.
After the spinning process, the pyrolysis and sintering of the spun fibers is undertaken, whereby the transformation to ceramic fibers occurs. Pyrolysis and sintering can be carried out continuously up to temperatures of 1800 °C in air and 2000 °C under inert conditions.
The fiber manufacturing is accompanied by extensive possibilities for fiber testing, whereby the testing of fibers is also offered as a service, certified according to diverse DIN EN standards (see Testing of ceramic fibers).
In addition to the ceramic fibers, fiber coatings are also being developed. The purpose of the coating is to adjust a fiber-matrix interface, enabling damage-tolerant behavior of ceramic composites. Another objective is the protection of the ceramic fiber from corrosive attack. For the application of the fiber coating, a wet-chemical route – comparatively inexpensive – is predominantly pursued. Here, process speeds of up to 1000 m/h are being achieved. The HTL has experience in the application of non-oxide and oxide material systems. Multi-layer systems made of different ceramics can also be manufactured using wet chemistry. Typical fiber coatings consist of silicon carbide, silicon nitride, boron nitride, zirconium oxide or spinel. We develop new coating precursors for our customers and optimize existing systems. During the application of the layers, process parameters are identified, allowing a homogeneous fiber coating with the desired layer thickness. In addition, there are various possibilities for the characterization of the fiber coatings. The further processing of the fibers is usually carried out with textile processes.