At Fraunhofer-Center HTL, manufacturing and testing methods for fibre-reinforced composite components are developed and implemented. The emphasis is on the development of Ceramic Matrix Composites (CMCs). Both fibre 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.
Since CMCs have so far been very expensive and can only be used in special applications, new types of low-cost CMCs are also being developed at the HTL.
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-fibre-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 - fibres and matrices - to obtain the desired material properties. Typical fibres for processing are carbon, aluminium oxide, mullite and silicon- carbide fibres. Ceramic fibres are developed inhouse from polymer precursors and manufactured up to pilot scale. The fibres are further processed as fibre rovings or as textile fibre preforms. For non-oxide CMCs, the matrices preferably include carbon precursors and silicone 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 (Publication: Fundamental Mechanisms With Reactive Infiltration), 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 fibres.
Fraunhofer-Center HTL utilizes different material concepts (Weak interface-, Weak matrix- composites) to adjust the quasi ductile properties of fibre composites. A closed process chain in laboratory scale is available for the production of samples for new fibre 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
Fibre composite components are manufactured at 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 fibre preforms, where the material properties are adjustable. Our development service includes component manufacture in-line with fibre requirements as well as manufacturing. For the heat and high-temperature treatment of fibre-reinforced composite components, multiple heat treatment facilities as well as a specific high-temperature furnace system up to 2400 °C are available. Components from fibre-reinforced ceramics are designed on the basis of finite element analyses.
Technical Equipment and Process Technology
For the production of fibre composite components, pilot plants are available at Fraunhofer-Center HTL that allow upscaling to large component dimensions up to 1000 mm. For the upscaling of newly developed ceramic fibres, a fibre pilot plant is under construction, which allows 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 fibre preforms are at disposal. Other plants for textile fibre 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 centre 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 Fraunhofer-Center HTL, a series of non-destructive, mechanical and thermophysical testing methods are available. The structure and homogeneity of CMC-materials can be analyzed with non-destructive testing (NDT) methods. Using NDT, material defects, which occur during the 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. Fibre 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.