CVD of Silicon Carbide on Structural Fibers: Microstructure and Composition
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CVD OF SILICON CARBIDE ON STRUCTURAL FIBERS: MICROSTRUCTURE AND COMPOSITION Lisa C. Veitch, Francis M.Terepka, Suleyman A.Gokoglu, NASA Lewis Research Center, Cleveland, OH
ABSTRACT Structural fibers are currently being considered as reinforcements for intermetallic and ceramic materials. Some of these fibers, however, are easily degraded in a high temperature oxidative environment. Therefore, coatings are needed to protect the fibers from environmental attack. Silicon carbide (SiC) was chemically vapor deposited (CVD) on Textron's SCS6 fibers. Fiber temperatures ranging from 1350 to 1500 °C were studied. Silane (SiH 4) and propane (C3H.) were used for the source gases and different concentrations of these source gases were studied. Deposition rates were determined for each group of fibers at different temperatures. Less variation in deposition rates were observed for the dilute source gas experiments than the concentrated source gas experiments. A careful analysis was performed on the stoichiometry of the CVD SiC-coating using electron microprobe. Microstructures for the different conditions were compared. At 13500C, the microstructures were similar;however, at higher temperatures, the microstructure for the more concentrated source gas group were porous and columnar in comparison to the cross sections taken from the same area for the dilute source gas group. INTRODUCTION Ceramic fibers are of interest as reinforcing materials for advanced ceramic and intermetallic matrix composites for aerospace applications. For many of these commercially existing fibers, additional coatings will be needed to protect the fibers from oxidation. These coatings will also serve to enhance the fiber/matrix interfacial properties. Silicon carbide coating has been considered to protect the outer carbon-rich coatings of Textron Specialty Materials' SCS6 silicon carbide fiber and can be synthesized by chemical vapor deposition (CVD) techniques. A number of different precursors, conditions and reactors have been discussed in the literature [1-3]. In this work, the deposition rates, stoichiometry and microstructure of the CVD SiC on SCS6 was examined. EXPERIMENTAL For the deposition experiment, a vertical batch fiber reactor was used to coat 30 cm long fibers [4]. Deposition temperatures ranged from 1350 to 1500 0C, and SiH, and C3 H, were used as the source gases with H2 as the carrier gas. A 1:9 Si:C atom ratio of source gases was held constant for all deposition experiments. Two distinct sets of experiments were conducted, based on different source gas concentrations. Group A experiments were performed using 3 mole% SiH 4 and 9 mole% CH 8 . Group B experiments were conducted using 0.3 mole% SiH, and 0.9 mole% C3H8. The temperature of the fibers during the CVD process was monitored at the center (15 cm) of the fiber with a two-colored optical pyrometer. Mat. Res. Soc. Symp. Proc. Vol. 250. ©1992 Materials Research Society
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RESULTS AND DISCUSSION The CVD SiC coating thickness on the fibers was measured using a split image microscop
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