For oxide ceramic reinforcement fibers, the well-known material systems Al2O3, Al2O3-SiO2, Al2O3-Y2O3 and ZrO2 are used at HTL. Dopants e.g. with Ce, Fe, Mg or Y are used to control the grain growth. At the HTL, fibers with radial gradients of grain size are also developed to meet specific strength and creep requirements.
Non-oxide reinforcing fibers are made from SiC. Here, there is close cooperation with the company BJS Ceramics. In addition, a high-entropy fiber made of SiBNC was developed at the HTL, which has particularly good creep resistance at high temperatures due to its amorphous structure.
Fiber cross sections are usually round and lie in a range between 10 and 100 µm. Thicker fibers can be interesting for use in MMC, for example, because they have high kink resistance, while thinner fibers offer advantages in textile processing. Hollow fibers, e.g. for filtration purposes, or fibers with other cross-sections, e.g. kidney shape, can also be produced to enhance interfacial effects. In addition, other functional ceramic fibers are being developed besides hollow fibers, e.g. piezoelectric fibers. The number of fibers in the roving can be varied within wide limits. For example, rovings with only about 200 filaments can be produced for particularly thin-walled structures. For the production of woven fabrics, different roving types can be used in warp and weft direction in order to minimize the stresses caused by the ondulation by using different numbers of filaments or fiber diameters. Fiber design is supported at the HTL by simulation calculations.