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). Both fiber rovings and textile 2D and 2.5D preforms or prepregs can be processed. Using the production facilities, both material samples and prototypes can be manufactured.
Ceramic Fiber and Matrix Development
The most important characteristics of CMC materials are their high fracture toughness and damage tolerance under thermal and mechanical stress compared to monolithic ceramics. The CMC material group includes non-oxide ceramics (NO-CMC), carbon-fiber reinforced SiC (C/SiC), silicon-carbide-fiber-reinforced SiC (SiC/SiC) and oxide-fiber-reinforced oxide ceramics (O-CMCs). For the material design, Fraunhofer-Center HTL selects suitable starting materials - fibers and matrices - to obtain the desired material properties. Typical fibers for processing are carbon, aluminum oxide, mullite and silicon- carbide fibers. Ceramic fibers are developed inhouse from polymer precursors and manufactured up to pilot scale. The fibers are further processed as fiber 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 Liquid Silicon Infiltration Process, shortened LSI process (see Hofbauer, P.; Rädlein, E.; Raether, F.: Fundamental Mechanisms With Reactive Infiltration of Silicon Melt Into Carbon Capillaries), by infiltrating a silicon melt into porous carbon preforms. With O-CMCs, typically matrices based on aqueous powder dispersions in the system Al2O3-SiO2-ZrO2 are used. They are reinforced using Al203-, Mullit- or Basalt fibers.
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 statistically secures the measured material properties.
Manufacturing of CMC Components
Fiber composite components are manufactured at the Fraunhofer-Center HTL as prototypes or as small series in closed-loop processes. The basis for this is successful material development with selected matrices and textile fiber preforms, where the material properties are adjustable. Our development service includes component manufacture in-line with fiber requirements as well as manufacturing. For the heat and high-temperature treatment of fiber-reinforced composite components, multiple heat treatment facilities as well as a specific high temperature furnace system up to 2400 °C are available. Components from fiber-reinforced ceramics are designed on the basis of finite element analyses.
Technical Equipment and Process Technology
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 up scaling of newly developed ceramic fibers, a fiber pilot plant is under construction, which allows a production in ton scale. For the manufacturing of green bodies hot presses, a CNC-controlled winding system and a continuously operating prepreg system for the coating of fiber preforms are at disposal. Other plants for textile fiber processing are situated at the application center for Textile Fiber Ceramics TFK at the location Münchberg.
The composites are consolidated by heat treatment up to about 200 °C and by high temperature treatment up to 2400 °C conversed into CMC. By means of mechanical processing of individual components and high temperature joining techniques, Fraunhofer-Center HTL can create very complex component geometries. Ceramic coatings enable use under corrosive environmental conditions and thus an increase in service life. 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 after successful prototype manufacturing. The manufacturing can be completed customer-specifically by quality assurance measures. Hence a transfer of results and technology into the production scale is possible.
Characterization of CMC
At the Fraunhofer-Center HTL, a series of non-destructive, mechanical and thermo physical testing methods is available. The structure and homogeneity of CMC-materials can be analyzed with non-destructive testing (NDT) 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 approx. 1500 °C. A large number of standardized procedures (DIN, EN, ISO, ASTM) can be used. This can be realized with both quasi-static and dynamic-cycled stress profiles, so-called fatigue tests on CMCs. Fiber preforms, samples and components can be analyzed before and after the examination with computed tomography and other analysis methods to detect damage or material changes.