Application of chemical vapor deposited yttria for the protection of silicon carbide fibers in a SiC/Ni 3 Al composite
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A. Boseb) Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 (Received 4 April 1990; accepted 1 August 1990)
A CVD process has been developed for coating Textron-Avco SCS-6 SiC fiber with yttria. Both Y(fod)3-H2O and Y(thd)3 (fod = l,l,l,2,2,3,3-heptafluoro-7,7-dimethyl4,6-octanedionato; thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) were examined as potential Y2O3 CVD precursors. Analysis of the deposits by Auger spectroscopy indicated significant F and C incorporation in the case of Y(fod)3 • H2O whereas, under appropriate conditions, Y(thd)3 gave a deposit which was essentially free of C and other impurities. GCFTIR analysis of the volatile products of the CVD process indicated isobutylene, tetrafluoroethylene, 1,1-difluoroethylene, fluoroform, and fluoroethylene for Y(fod)3 • H2O and mainly isobutylene and propylene for Y(thd)3. The precursor Y(thd)3 was chosen to deposit 1-2 /xm of yttria on short lengths of silicon carbide fibers. The coated fibers were then incorporated into a nickel aluminide (Ni3Al) matrix by reactive sintering, with yttria affording protection from the known SiC + 2Ni -»• Ni2Si + C degradation process. The SiC/Ni3Al composites, before and after annealing at 1000 °C for up to 100 h, were studied by using SEM and EMPA to determine the extent of reaction. With the exception of certain portions of the fibers that were inadequately coated with yttria, complete protection of the fibers was indicated.
I. INTRODUCTION
There has been considerable interest in ceramic fiber/intermetallic matrix composites.1 This is, in part, due to the lower density and superior oxidation resistance of intermetallics versus that of many superalloys. The present study involves the use of a reactively sintered nickel aluminide (Ni3Al) intermetallic matrix with Textron-Avco SCS-6 SiC fibers. The Textron-Avco SCS-6 fibers basically consist of chemically vapor deposited (CVD) silicon carbide on a pitch derived (ca. 40 iim diameter) carbon fiber substrate. Silicon carbide fibers were chosen due to their high strength, good oxidation resistance, and availability. One major drawback is the fact that silicon carbide has been shown to react with nickel-based alloys at temperatures above 700 °C.3'4 This reaction is dominated by the diffusion of nickel into the silicon carbide, and to a lesser extent diffusion of silicon into the matrix, leading to a reaction which forms various nickel silicides.2'3'5 A stable barrier coating would therefore be necessary if these materials are to be used, in contact, at temperatures greater than 700 °C. a)Presented
at the 14th Annual Conference on Composite Materials and Structures, American Ceramic Society Closed Sessions (January 1990), Cocoa Beach, Florida. b)Currently at the Southwest Research Institute. 2706
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 11, Nov 1990
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The work reported herein was carried out as an extension of the studies of R. L. Mehan and M.R. Jackson, using yttria as a reaction barrie
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