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General characteristics of SiC-based woven fabrics The typical XRD patterns of the as-received and heat-treated SiC-based ceramic woven fabrics are shown in Fig. 2. The peaks show some broadening due to the small grain size of the fibers. The existence of three peaks in the 2¦Èvalues of 35.9, 60.2, and 71.8¡ãshowed the presence of ¦Â-SiC, detected in both the as-received and heat-treated samples. However, after the heat treatment an additional phase, SiO2(¦Á-cristobalite), was identified at around the 2¦Èvalue of 22.0¡ã, indicating oxidation of the sample. This passive oxidation leads to the formation of a protective silica film, protecting the fibers from further oxidation [22]. Under equilibrium conditions, quartz is the stable phase at room temperature; however, in this case cooling from 1573 K (heat-treatment temperature) was not slow enough for reconstructive phase transitions from cristobalite to quartz, which resulted in a displacive transition, causing the distortion of cristobalite and the existence of ¦Á-cristobalite at room temperature [23]. Figure 3a shows the SEM micrographs of the as-received PN-type fibers. A continuous oxide layer can be observed on the surface of the fibers on the SEM micrograph of the heat-treated PN (PN-H) fabric (Fig. 3b). In some areas the continuity of the oxide layer was lost,most probably during handling of the bundles in the course of the observation of the fracture surface. At a higher magnification, the thickness of the oxide layer around the fiber was determined to be ~1 ¦Ìm (Fig. 3c). The morphology and structure of the S8-type fibers showed similar characteristics to the PN-type in both as-received and heat-treated states. Micrographs of the as-received and heat-treated ZE8(ZE8-H)-type fibers are shown in Fig. 4a and b, respectively. In the case of the ZE8-H, detailed observations revealed that there was no clear continuous oxide layer around the fibers, but rather only localized oxidation of the fiber, indicated by arrows in Fig. 4b. The ZE8 fabric is described as heat resistant by the manufacturer, a claim that is verified by the results of our experiments. |
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RXMCDM: ½ð±Ò+1 2014-06-12 20:47:34
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RXMCDM: ½ð±Ò+1 2014-06-12 20:47:34
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