Entwicklung neuartiger Pufferschichten für exzellente Performanz von UHTCs; Untersuchung der Diffusionsmechanismen
Herstellung und Eigenschaften von Funktionsmaterialien
Thermodynamik und Kinetik sowie Eigenschaften der Phasen und Gefüge von Werkstoffen
Zusammenfassung der Projektergebnisse
A ZrB2-based ceramic, containing short Hi-Nicalon SiC fibers, was fabricated with a Mo-impermeable buffer layer sandwiched between bulk and the outermost oxidation resistant ZrB2–MoSi2 layer, in order to prevent inward Mo diffusion and associated fiber degradation reactions. This additional layer consisted of ZrB2 doped with either Si3N4 or with the polymer-derived ceramics (PDCs) SiCN and SiHfBCN. By means of scanning-electron microscopy (SEM) imaging and complementary EDS elemental mapping and electron diffraction it was demonstrated that this tailored sample geometry provides an effective diffusion barrier to prevent the SiC fibers from deterioration due to reactions with Mo or Mo-compounds. In contrast, the structure of the SiC fibers in a reference sample without buffer layer is strongly degraded by MoSi2 diffusion into the fiber core. The comparison of the three buffer-layer systems showed a moderate alteration of the fiber structure in the case of Si3N4 addition, whereas in the PDC-doped samples hardly any structural change within the fibers was observed. Microstructural investigations via transmission electron microscopy (TEM) confirmed the formation of the high-temperature stable phases BN, SiC, Zr(Hf)O2 within the buffer layers of all compositions. The chemistry of the residual glasses is basically characterized as Si-O-Zr-B-N, which varies from point to point, depending on species and amount of the phases crystallized. Mo is rarely detected as constituent of the glassy phases or as small, isolated precipitates of Mo5Si3 within the buffer layers, but never inside the SiC-fiber enriched bulk or within the fiber structure. At the bulk/ buffer layer interface, the formation of a reaction zone in each system was observed. This reaction zone consists of ZrB2 and a ZrBxCyN1-x-y solid solution in case of Si3N4 as buffer layer dopant or ZrB2 and ZrO2 or Zr(Hf)O2 respectively in the PDC systems. The formation of reaction zones proves the chemical interaction between bulk and buffer layer. A stepwise reaction mechanism is deduced, based on the continuous progression of a reaction zone that propagates towards the ZrB2–MoSi2 top layer. The progression of such a reaction zone as a consequence of the different eutectic melts forming in the different layers, that is, first in the SiC fiber-containing bulk, then in the buffer layer itself, and finally in the top layer at high temperature, allows for an effective separation of the ZrB2–MoSi2 top layer from the SiC fibers. Subsequent oxidation at 1500°C and 1650°C for 15 min did not affect the efficiency of all three buffer layers, since no structural changes regarding buffer layer and fibers were observed, as compared to the non-oxidized samples. This electron microscopy study marks a significant advancement in the research of ZrB2-based UHTCs by incorporating buffer layers, thus enabling previously unfeasible combinations of SiC and MoSi2 within a single ZrB2-based composite. This sets the stage for further research and refinements aimed to fully realize the potential of these materials for aerospace applications. Such FG UHTCs are potential components in nose tips and wing leading edges of vehicles during their short time of atmospheric re-entry.
Projektbezogene Publikationen (Auswahl)
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Understanding the oxidation behavior of a ZrB2–MoSi2 composite at ultra-high temperatures. Acta Materialia, 151, 216-228.
Silvestroni, Laura; Stricker, Kerstin; Sciti, Diletta & Kleebe, Hans-Joachim
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Prevention of SiC‐fiber decomposition via integration of a buffer layer in ZrB 2 ‐based ultra‐high temperature ceramics. Journal of the American Ceramic Society, 105(7), 4960-4973.
Stricker, Kerstin; Silvestroni, Laura & Kleebe, Hans‐Joachim
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An Electron Microscopy Study on the Prevention of SiC-Fiber Decomposition via the Integration of a Buffer Layer into ZrB2-based Ceramics
Kerstin Stricker
