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Carbon fiber reinforced carbon-silicon carbide (C/C-SiC) sandwich structures have been developed using the Liquid Silicon Infiltration process and the in situ joining method. They offer high mass-specific stiffness, low thermal expansion, and high environmental stability. Potential application areas are highly precise satellite structures, like optical benches. In this study, sandwich samples were manufactured using prepregs based on 2D carbon fibre fabrics and a phenolic resin precursor. Carbon fibre reinforced polymer preforms for folded and grid-cores, as well as for the skin panels were manufactured using autoclave technique. In the second step, the sandwich components were pyrolyzed, leading to C/C preforms. For the build-up of the sandwich samples, two skin panels were joined to a core structure and subsequently, the resulting C/C sandwich preform was siliconized. C/C-SiC sandwich samples were tested under shear load. Shear strength, modulus, and fracture strain were determined and compared to the results obtained by analytical calculation. The shear properties were dependent on the fiber orientation in the core structure as well as on the core type and orientation. The sandwich shear stiffness obtained in the tests was close to the expected theoretical values, calculated on the basis of the material properties and the core geometry.

In a new design approach, C/C–SiC sandwich structures have been realized via liquid silicon infiltration and in situ joining methods. In the first process step, the carbon fiber reinforced polymer preforms for the planar skin plates as well as for the core structures were manufactured via warm pressing and autoclave technique, using C fiber fabrics preimpregnated with phenolic resin. Thereby, two types of C/C–SiC core structures were used: foldcore and grid core. In the second process step, the sandwich components were pyrolyzed separately, leading to porous C/C preforms and subsequently joined to a C/C sandwich structure, using an adhesive. In the last process step, the C/C structure was infiltrated with molten Si, and the SiC matrix was built up by a chemical reaction of Si and C, leading to a permanently joined C/C–SiC sandwich structure. For mechanical characterization, sandwich specimens were manufactured and tested in 4 point bending.

Yttrium monosilicate (Y 2 SiO 5 ) is considered a promising material for environmental barrier coatings (EBCs) for ceramic matrix composites (CMCs). Y 2 SiO 5 coatings were deposited on all-oxide and non-oxide CMCs by magnetron sputtering, respectively. As deposited Y 2 SiO 5 is X-ray amorphous, homogenous, and virtually free of cracks and macropores. Annealing performed between 1273 and 1473 K induces crystallization of metastable X 1 –Y 2 SiO 5 , apatite-type Y 4.67 (SiO 4 ) 3 O and finally stable X 2 –Y 2 SiO 5 . Annealing also produces significant pore coalescence and re-crystallization. Experiments performed with Al 2 O 3 -based CMCs prove the fundamental instability of the Y 2 SiO 5 /Al 2 O 3 interface at high temperatures, however no detrimental melting is observed and a well-bound Y 2 Si 2 O 7 /YAG double layer is developing. Experiments with CVD-SiC coated C/SiC CMCs in different atmospheres revealed that SiC oxidation to SiO 2 favors adhesion of Y 2 SiO 5 coatings. The SiO 2 layer on top of SiC reacts with Y 2 SiO 5 to highly porous Y 2 Si 2 O 7 which is able to bridge gaps at the Y 2 SiO 5 /SiC interface. However, the thin SiO 2 interphase may be critical with respect to thermal cycling. As a preliminary conclusion, sputtered Y 2 SiO 5 coatings are considered more advantageous for all-oxide CMCs.

In the first part of this thesis the influence of silicon (Si) alloying on the thermal stability of alumina thin films was investigated by means of transmission electron microscopy (TEM). These alumina thin films were deposited by filtered cathodic arc (FCA) and by magnetron sputter ion plating (MSIP). Amorphous and crystalline FCA alumina thin films were deposited on Si wafers, and crystalline MSIP alumina thin films were deposited on Si wafers and on cemented carbide cutting tools. The thin films were annealed at elevated temperatures before TEM investigations to determine their thermal stability. In the second part of this thesis MSIP alumina thin films, deposited on cemented carbide cutting tools, were investigated in the TEM after using them in several cutting tests. Furthermore, so called WHIPOX-material was investigated. WHIPOX is an alumina fibre matrix composite with a porous matrix. TEM investigations of Al2O3 thin films showed that Si-alloying stabilizes the amorphous and the gamma-Al2O3 phases. Si-alloying of the amorphous FCA Al2O3 thin film led to at least a 290 °C higher crystallization temperature. In addition investigations of annealed thin films showed that Si-alloying increases the thermal stability of gamma-Al2O3 by at least 100 °C. The morphology of the porous, nanocrystalline FCA gamma-Al2O3 layer was stabilized to a 300 °C higher temperature. By Si-alloying of crystalline MSIP Al2O3 thin films deposited on Si wafers, the thermal stability range of gamma-Al2O3 was increased by 100 °C and the formation temperature of the alpha-Al2O3-phase was increased by 250 °C. Moreover Si influenced the phase-transfomation sequence: The unalloyed crystalline MSIP Al2O3 thin film showed a direct transition of the gamma- into the alpha-phase, while the Si alloyed MSIP Al2O3 thin film first transformed into theta- before the formation of alpha-Al2O3 was observed. By using another substrate, such as cemented carbide cutting tools coated with (Ti,Al)N for improved adhesion, the coating system could be used in cutting tests. However, investigations of the annealed coating system proved, that a (Ti,Al)N-layer decreases the thermal stability range of gamma-Al2O3: The formation of alpha-Al2O3 is accompanied by the decomposition of the metastable, cubic (Ti,Al)N-layer into the cubic TiN- and the hexagonal AlN-phase, the oxidation of the (Ti,Al)N-layer and the Ti-diffusion into the Al2O3-layer. The thermal stability of gamma-Al2O3 could be enhanced by annealing in vacuum instead of annealing in atmosphere as well as by Si-alloying of the Al2O3- and (Ti,Al)N-layers. The combined results lead to the assumption that Si was not incorporated into the Al2O3-lattice, but segregated preferentially as amorphous Si or SiO2 at grain boundaries of the nanocrystalline gamma-Al2O3-grains and thereby reduced diffusion and hence crystal growth. Thus, gamma-Al2O3 was presumably stabilized due to Si-alloying by stabilizing the size of the nanocrystals. In case of the Si-alloyed Al2O3/(Ti,Al)N-layers deposited on cemented carbide, the Si alloying of the (Ti,Al)N-layer can also explain the stabilization of gamma-Al2O3. Less (Ti,Al)N-layer-oxidation can also be explained by the amorphization effect of Si-alloying on the Al2O3-layer, which was observed for all crystalline FCA and MSIP Al2O3 thin films. The higher the amorphous content, the less grain boundaries and therefore possible diffusion pathways for O, Al and Ti exist. Investigations of the MSIP Al2O3/(Ti,Al)N-coating system after use in cutting tests of a case hardening steel, showed that it could withstand the thermomechanical stress at selected cutting parameters. Neither delamination of the coating nor phase transformation was observed. Investigation of WHIPOX-material sintered at 1200 °C showed the existence of small, below 100 nm in size, Y-stabilized ZrO2-nanoparticles homogeneously dispersed within the Al2O3-matrix. Moreover it showed that the ZrO2-particles got into the matrix during preparation and that they could be found between Al2O3-matrix grains and between fibre-matrix grains and therefore could influence the sintering activity. This presumption was confirmed by the result that ZrO2-free prepared WHIPOX-samples showed higher sintering activities than ZrO2-containing samples. Furthermore the sintering activity was enhanced by increasing the sintering temperature to 1300 °C.

Carbon fibre reinforced SiC, the most widely used CMC materials, can be divided up into two main types, C/SiC and C/C-SiC, mainly differing in their microstructural design. C/SiC materials are characterized by C fibres, embedded almost separately in SiC matrix. Therefore, fibre preforms are used and the SiC matrix is built up by Chemical Vapour Infiltration (CVI) or Polymer Infiltration and Pyrolysis (PIP). In contrast, C/C-SiC materials are based on porous Carbon/Carbon (C/C) preforms. Thereby, not the single fibres, but dense C/C bundles, consisting of several hundreds of C fibres, embedded in a C matrix, are again embedded in a SiC matrix, build up by Liquid Silicon Infiltration (LSI). In this presentation, the influences of the manufacturing methods on the morphology and properties of the resulting CMC materials, as well as on the feasibility of components are shown, and an overview of current applications is given.

Abgeleitet von den charakteristischen Eigenschaften faserkeramischer Werkstoffe werden zehn Richtlinien fur die Bauteilkonzeption mit Faserkeramik erlautert und anhand ausgewahlter Bauweisen beispielhaft dargestellt. Besonderes Augenmerk wird dabei auf Fugestellen und Krafteinleitungsbereiche gerichtet. An das Entwicklungspotential faserverstarkter Keramiken werden hohe Erwartungen geknupft. Die Keramik-Werkstoffwissenschaftler erwarten eine Steigerung der Risszahigkeit und der Schadenstoleranz im Vergleich zur unverstarkten Keramik. Die Leichtbaukonstrukteure hoffen, mit Hilfe eines hochtemperaturbestandigen, schadenstoleranten Werkstoffs mit einer Dichte < 3 g/cm 3 neue Anwendungsgebiete erschliesen und auf dem Gebiet heiser, tragender Strukturen zum Teil die Superlegierungen ersetzen zu konnen. Der Gedanke, im Hochtemperaturbereich faserverstarkte keramische Werkstoffe einzusetzen, ist nicht neu. Schliest man die kohlenstofffaserverstarkten Kohlenstoffe (C/C) und die verstarkten Glaser mit ein, kann man auf eine uber 20-jahrige Erfahrungszeit zuruckblicken. Die Firma Aerospatiale fertigt heute dreidimensional verstarkte C/C-Dusen fur Raketentriebwerke bis 1,5 m Durchmesser und 1,5 m Lange in automatisch gesteuerten Fertigungseinrichtungen. Die Firma SGL ist in der Lage, 2 D-C/C-Formen mit 1,3 m Durchmesser und 2 m bis 5 m Lange fur das superplastische Umformen von Titan-Triebwerkseinlaufen herzustellen. Die Nase und Flugelvorderkanten des Space Shuttle sind aus C/C mit Oxidationsschutz und unter Extrembedingungen mehrfach erfolgreich flugerprobt. Thermomechanisch hochbelastete Strukturen aus Faserkeramikwerkstoffen, wie C/C, C/C-SiC, C/SiC und SiC/SiC stellen insbesondere fur zukunftige, wiederverwendbare (geflugelte) Raumtransportsysteme (RTS) und Hyperschallflugzeuge eine Schlusseltechnologie dar, da es fur die hierbei auftretenden Temperaturbereiche keine Werkstoffalternative gibt. Faserkeramiken finden ihre Anwendungen im Bereich "Heiser Strukturen" des Thermalschutzsystems, in thermisch hochbelasteten Klappen, Stosstangen sowie im Triebwerksbereich (Einlauf, Brennkammer, Duse).