Hi-Nicalon fiber-reinforced celsian matrix composites: Influence of interface modification
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Hi-Nicalon fiber-reinforced celsian matrix composites: Influence of interface modification Narottam P. Bansala) and Jeffrey I. Eldridge National Aeronautics and Space Administration, Lewis Research Center, Cleveland, Ohio 44135-3191 (Received 29 May 1997; accepted 10 October 1997)
Unidirectional celsian matrix composites having 42–45 vol % of uncoated or BN-SiC coated Hi-Nicalon fibers were tested in three-point bend at room temperature. The uncoated fiber-reinforced composites showed catastrophic failure with strength of 210 6 35 MPa and a flat fracture surface. In contrast, composites reinforced with coated fibers exhibited graceful failure with extensive fiber pullout. Values of first matrix cracking stress and strain were 435 6 35 MPa and 0.27 6 0.01%, respectively, with ultimate strength as high as 960 MPa. The elastic Young modulus of the uncoated and coated fiber-reinforced composites were 184 6 4 GPa and 165 6 5 GPa, respectively. Fiber push-through tests and microscopic examination indicated no chemical reaction at the uncoated or coated fiber-matrix interface. The low strength of composite with uncoated fibers is due to degradation of the fiber strength from mechanical damage during processing. Because both the coated- and uncoated-fiber-reinforced composites exhibited weak interfaces, the beneficial effect of the BN-SiC dual layer is primarily the protection of fibers from mechanical damage during processing.
I. INTRODUCTION
Fiber-reinforced ceramic matrix composites are being developed1,2 for high-temperature structural applications in aerospace, energy conservation, power generation, nuclear, petrochemical, and other industries. A number of glass and glass-ceramic matrices reinforced with continuous fibers having high strength and high modulus have been reported1,2 over the last two decades. Monoclinic celsian is a refractory material showing good oxidation resistance, phase stability up to ,1600 ±C, and low values of dielectric constant and loss tangent. It is, therefore, a promising matrix material3–5 for reinforcement with ceramic fibers for high temperature structural composites. Processing and properties of celsian glassceramic matrix composites reinforced with large diameter CVD SiC SCS-6 monofilaments,6 –8 multifilament small diameter Nicalon,9 and HPZ fibers10 have been described earlier. The details of fabrication of Hi-Nicalon fiber-reinforced celsian matrix composites have been reported elsewhere.11 In order to achieve stoichiometric composition, the celsian matrix was synthesized3,11 by solid-state reaction between the metal oxides. The objective of the present study was to investigate the effects of fiber-matrix interface modification on the properties of small diameter Hi-Nicalon fiberreinforced celsian matrix composites. Room temperature mechanical properties of uncoated and BN-SiC coated Hi-Nicalon fiber-reinforced celsian composites were measured in three-point flexure. The function of the a)
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