Sol-Gel AL 2 O 3 Coatings on SiC Fiber
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SOL-GEL AL20 3 COATINGS ON SiC FIBER 3 Xiao, 1 Beng Jit Tan, 1, 2 Steven L. Suib, 1,2* and Francis S. Galasso Youming 1 Department of Chemistry, University of Connecticut, Storrs, CT 06269-3060 2 Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3060 3 United Technologies Research Center, East Hartford, CT 06108
"Towhom correspondence should be addressed. ABSTRACT Coating of SiC (BP-SIGMA) fibers with alumina by a sol-gel process did
not cause degradation even after heating to 1000°C in air for 24 h. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Auger electron spectroscopy (AES) and scanning electron microscopy (SEM ) methods were used to study the coating fiber interface. INTRODUCTION Silicon carbide monofilament has been used to reinforce metal and ceramic matrix composites. Unfortunately, during the composite formation process, handling, including touching, cutting and rewinding of SiCIW (BP-SIGMA) fibers results in a decrease intheir tensile strength. To solve this problem, carbon has been used as a coating to protect the SCS-6 SiC fiber [1], but the carbon is oxidized at elevated temperatures. In our laboratories, attempts have been made to coat a (BP-SIGMA) SiC/W fibers with TiB 2, B, W, and TiN to improve their handleability [2]. The former two coatings degraded the fiber during the coating process while the latter two did not. However, W and TiN will also eventually be oxidized at elevated temperatures. Therefore, this study was undertaken to determine how to produce a more oxidation resistant coating without degrading the SiC fiber. Sol-gel processing is a widely accepted method for making materials and it has been of growing interest in recent years [3-7]. One of the main reasons for the popularity of sol gel methods is that thin films or coatings can readily be obtained by controlling the dipping and drying process of the substrate [8,9]. In addition, a very homogeneous film can be obtained because all the starting materials are mixed at the molecular level in the solution [10,11]. Therefore, a sol-gel process was selected which involved coating the fiber with aluminum isopropoxide followed by heat treatment to produce an A1 2 0 3 coating. EXPERIMENTAL PROCEDURE A. Sn~hpsis The alumina sol solution used for coating monofilament SiC fibers was made from commercial aluminum isopropoxide (Alfa Corporation, 98% purity Ward Hill, MA). The preparation of the sol solution followed Yolda's procedure [12]. The hydrolysis was carried out with distilled, deionized water, at about 75 0C with constant stirring for 30 minutes. The molar ratio of water to aluminum was kept at 100 to 1 (about 0.56 M AI-isopropoxide). The peptization was done by adding 2.0 M nitric acid dropwise until the pH of the sol reached 3.5. Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society
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The SiC fibers, which were obtained from British Petroleum (BP-SIGMA), were dip-coated with the above freshly prepared sol solution, then dried in air. Calcinations were then carried out
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