At Fraunhofer-Center HTL manufacturing and testing methods for fiber-reinforced composite components are developed and implemented. The emphasis is on the development of Ceramic Matrix Composites (CMCs) using carbon or ceramic fibers made of silicon carbide and aluminum oxide respectively mullite. Both fiber rovings and textile 2-2.5D preforms or prepregs can be processed.
The fiber composite components are produced as prototypes or small series in a closed process chain. For that, a successful development of materials is essential adjusting the material properties. Our development service includes load-conforming fiber alignment, material and component design (see Component Design), and the production of CMC components. Numerous thermal treatment facilities and special furnaces up to 2400 °C are at disposal for the thermal and high temperature treatment of fiber-reinforced composites. .
The most important characteristic of CMC materials is their high fracture toughness under thermal and mechanical stress compared to monolithic ceramics. The CMC material group includes carbon-fiber reinforced SiC (C/SiC), silicon-carbide-fiber-reinforced SiC (SiC/SiC) and oxide-fiber-reinforced oxide ceramics (O-CMCs). The Fraunhofer-Center HTL selects suitable starting materials - fibers and matrices - to obtain the desired material properties. Typical fibers for processing are carbon, aluminium oxide, mullite and silicon- carbide fibers. These are used as rovings or as textile fiber preforms. For non-oxide CMCs the matrices preferably include carbon precursors and silicon polymers, which are converted to ceramics by high-temperature processes. In addition, carbide matrices are produced with the so-called LSI-process, by infiltrating a silicon melt into porous carbon preforms (see Melt infiltration). With O-CMCs, typically matrices based on aqueous powder dispersions in the substance system Al2O3-SiO2-ZrO2 are used. Fraunhofer-Center HTL utilizes different material concepts (Weak interface-, Weak matrix- composites) to adjust the quasi ductile properties of fiber composites. A closed process chain in laboratory scale is available for the production of samples for new fiber composites, often carried out as small-format plates or tubes. Fraunhofer-Center HTL develops reproducible manufacturing methods and secure the measured material properties statistically.
For the production of fiber composite components pilot plants are available at Fraunhofer-Center HTL that allow upscaling to large component dimensions up to 1000 mm. For the manufacturing of green bodies a hot press, a CNC-controlled winding system and a prepreg system are at disposal. Other plants for fiber processing are situated at the application center for Textile Fiber Ceramics TFK (see Textile Engineering). The composites are consolidated by heat treatment up to about 200 °C. A conversion to high-temperature resistant CMCs is accomplished by high temperature treatment up to 2400 °C. By means of mechanical processing of individual components and high temperature joining techniques Fraunhofer-Center HTL can create very complex component geometries. An advanced 5-axis machining center is at disposal for the intermediate and finishing processes. When needed, Fraunhofer-Center HTL also offers a concept to small-series production of components. The manufacturing can be completed customer-specifically by quality assurance measures. Hence a transfer of results and technology into the production scale is possible.
The structure and homogeneity of materials can be analyzed with non-destructive testing methods. Using NDT, material defects, which occur during manufacturing or whilst in use, can be detected. Mechanical tests are possible at room temperature and at temperatures up to 1400 °C. This can be realized with both quasi-static and dynamic test cycles (fatigue tests). The samples and components can be analyzed before and after the examination with computed tomography and other non-destructive methods to detect damage (see Material Testing).