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Oxide Ceramic Fiber Development at the ITCF Denkendorf

Oxide ceramic fibers are key components of ceramic matrix composites (CMCs), which form a new class of light-weight high temperature resistant materials with exceptional properties. As CMCs combine the advantages of a monolithic ceramic material (corrosion resistance, high strength and high temperature stability) with a non-brittle fracture behavior, a high damage tolerance and an extreme thermo-shock resistance, there is an increasing interest in their industrial applications, particularly by replacing highly legated steel. Important technical fields with growing requirements are: power generation with stationary gas turbines as well as combustion chambers and engines of aircrafts, rockets and space vehicles.
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Oxide ceramic fibers determine the properties of CMCs and therefore have to meet special requirements such as high strength, long-term high temperature stability as well as excellent resistance against oxidation, corrosion and creep. Generally, the creep rate of the polycrystalline ceramic fibers increases with decreasing grain size and the creep resistance of the commercially available ultrafine grained fibers is comparatively low. Particularly under mechanical stress and at high temperatures exceeding 1100 °C, they tend to creep and brittleness increases due to grain growth, which can ultimately lead to failure of the entire device. The optimization of the ceramic fiber properties with regard to high creep resistance while maintaining strength and associated long-term high temperature resistance still represents a major topic in ongoing research in the field of ceramic fibers.
Research at the DITF Denkendorf focuses on the development of continuous oxide ceramic fibers of various compositions and started as early as in 1989. The complete production process has been studied intensively comprising the design of spinning dopes, the development of the dry spinning process as well as the thermal treatment including pyrolysis, calcination and sintering processes. Corundum and mullite fibers have achieved a high level of development in the past years. Recent investigations of the fiber properties by others have shown the high potential of these two fiber types. Currently, the transfer of the technology into industrial scale is under progress.

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Current research projects focus on the improvement of creep resistance, the reduction of grain growth in long time applications and the improvement of the textile processability of oxide ceramic fibers. For this purpose, the chemical compositions of the oxide ceramic fibers are further varied in order to optimize the structures and the mechanical properties.
Yttrium aluminum garnet (YAG) fibers are high performance fibers with high temperature stability, high modulus and strength, high oxidation resistance and excellent creep resistance. As YAG is characterized by a very high melting point of 1940 °C it is very attractive for high temperature applications. Furthermore, it is chemically inert in reducing and oxidizing atmosphere and it is the oxide with the highest creep resistance. Therefore, YAG fibers have the potential to outperform the commercial oxide ceramic fibers in terms of creep resistance.
The microstructural optimization of corundum fibers by the incorporation of zirconia results in zirconia toughened alumina (ZTA) fibers with a substantially inhibited grain growth at high temperatures. ZTA exhibits enhanced fracture toughness and almost doubled flexural strength in comparison to alumina. ZTA fibers with these properties not only enable more complex structures due to improved textile processability but also exhibit a higher corrosion resistance compared to other fiber types.

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References:

[1] D. Schawaller, B. Clauß, M. R. Buchmeiser, Macromol. Mater. Eng., 297, 2012, 502-522.
[2] S. Pfeifer, M. Bischoff, R. Niewa, B. Clauß, M. R. Buchmeiser, J. Eur. Ceram. Soc., 34, 2014, 1321-1328.
[3] S. Pfeifer, P. Demirci, R. Duran, H. Stolpmann, A. Renfftlen, S. Nemrava, R. Niewa, B. Clauß, M. R. Buchmeiser, J. Eur. Ceram. Soc., 36, 2016, 725-731.