Characterization of CVD Bonded Tyranno~® Fibers Oxidized at High Temperatures

摘要

The oxidation of highly porous ceramic matrix composites (PCMCs) based on different Tyranno~® fibers has been analyzed by means of thermogravimetry and electron microscopy. Both uncoated fibers and PCMC materials exhibit parabolic kinetics between 900℃ and 1250℃, these being faster for Ti-doped than for Zr-doped Tyranno fibers. Oxide layers in Ti-doped fibers are porous and partially crystalline, whereas in Zr-doped materials a significant fraction of relatively coarse β-SiC grains is still found embedded in the amorphous silica matrix. On the other hand, the CVD-SiC coatings exhibit higher oxidation rates from the outer surface than from the inner one, a phenomenon that has been associated not only with the more difficult access of oxygen to the inner face but also with the highly <111> textured structure of these coatings, for which very different oxidation rates have been published for the inward and outward directions. Cracking phenomena observed above 1100℃ for long dwelling times do not lead to an acceleration of the oxidation process, which could be due to the simultaneous crystallization of the amorphous silica layers.