Radial variation of elastic properties of SCS-6 silicon carbide fiber
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John H. Cantrell and William T. Yost NASA Langley Research Center, Mail Stop 231, Hampton, Virginia 23681-0001 (Received 21 December 1993; accepted 26 April 1994)
The upper and lower bounds of the bulk, shear, and Young's moduli are calculated pointby-point along the radius of SCS-6 silicon carbide fibers using the Hashin-Shtrikman equations from considerations of the Auger spectra along the fiber radius. The calculated values are in agreement with measurements of the average Young modulus obtained over relatively large radial regions of the fiber using scanning acoustic microscopy (SAM). The validity of the Hashin-Shtrikman bounds calculations is enhanced by the agreement found between experimental SAM V(z) curves for fixed z along the fiber radius and the theoretical bounds of such curves obtained from the bounds of the calculated bulk and shear moduli.
I. INTRODUCTION Ceramic fibers are being used as reinforcement materials in both metal and ceramic matrix composites developed for high temperature applications. The high tensile strength, low mass density, low thermal expansivity, and high resistance to degradation in hostile environments have made these fibers attractive for structural applications. One of the most widely used fibers is SCS-6 silicon carbide fiber.1 The SCS-6 fibers are produced by high temperature (1100-1300 °C) chemical vapor deposition of a 50 /nm thick silicon carbide layer on a 33 fim diameter carbon filament. The SiC layer in turn is coated with two layers of carbon for shielding. The outer layer of carbon improves the strength of the fiber and tailors the matrix-fiber interfaces for the fabrication of composites. The microstructure of SCS-6 fibers has been studied extensively by Ning and Pirouz2 and Ning et al? using transmission electron microscopy (TEM), high resolution transmission electron microscopy (HREM), and electron diffraction. According to their observations the central or core portion of the SCS-6 fiber is a filament 33 fxm in diameter, consisting of turbostratic carbon (TC) blocks having dimensions in the range 1-50 nm. The c-axes of these blocks are randomly oriented throughout the core region. Surrounding the core region is a 1.5 /bum thick coating of pyrolitic carbon, consisting of TC blocks with dimensions in the range 30-50 nm that are arranged with the c-axes of the blocks preferentially (but not exclusively) aligned along the direction radial to the carbon filament axis. This inner coating of pyrolitic carbon thus has microtexture. Immediately outside the pyrolitic carbon 2298 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 9, Sep 1994 Downloaded: 14 Mar 2015
layer are four layers of silicon carbide. The layer SiC-1 adjacent to the inner carbon coating consists of rod-like grains of /3-SiC. Nearest the inner carbon coating the yS-SiC grains are roughly 5-15 nm in length and are randomly oriented. At larger distances from the inner carbon coating the grains become larger (of the order 50-150 nm in length) and become increasingly aligned along the fiber rad
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