Fiber-Matrix interactions in brittle matrix composites
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INTRODUCTION
B R I T T L E matrix composites offer a new structural high-temperature material. Monolithic ceramics suffer from low fracture toughness and fracture energy values. Toughening methods, including phase transformation toughening, whisker reinforcement, and fiber reinforcement, are used to improve fracture properties of ceramic materials. Brittle fiber-brittle matrix composites offer a unique way to obtain "ductile ceramics." Ceramic fibers, including graphite and silicon carbide, exhibit very high strength values. The most challenging problem is to transform the outstanding mechanical properties of these fibers into mechanical properties of ceramic matrix composites. Fiber-matrix interactions play a crucial role in determining the mechanical behavior of brittle matrix composites, i1-5[Inherent limitations of graphite or silicon carbide matrix composites result from the matrix cracking and microcracking during composite fabrication processes. The presence of these cracks was found to be detrimental to the mechanical properties of carbon-carbon (C-C) composites. Carbon-carbon composites are, at present, manufactured from three different matrix precursors: (1) thermosetting resins with high carbon yield, (2) pitches, and (3) chemical vapor infiltration (CVI) pyrolytic carbon.J6,7,81 All these precursors have their own inherent limitations. Resins including phenolics have low carbon yield (about 50 pct) and a high linear shrinkage (about 20 pct). These two factors yield a severe matrix cracking and prestresses during initial pyrolysis process called carbonization. In order to improve the fracture properties of carbon-carbon composites, a hightemperature treatment process called graphitization has W. KOWBEL, Assistant Professor, and H.T. TSOU, Graduate Student, are with the Department of Mechanical Engineering, Auburn University, Auburn, AL 36849. H.L. LIU, Graduate Student, is with the Depamnent of Materials Science, University of Florida, Gainesville, FL 32601. This invited paper is based on a presentation made in the symposium "Structure and Properties of Fine and Ultrafine Particles, Surfaces and Interfaces" presented as part of the 1989 Fall Meeting of TMS, October 1-5, 1989, in Indianapolis, IN, under the auspices of the Structures Committee of A S M / M S D . METALLURGICAL TRANSACTIONS A
to be involved. However, during this process, which takes place between 2500 ~ and 3000 ~ additional thermal stresses are introduced which can become detrimental to the strength values. Processing composites from pitch has the advantage of a much smaller linear shrinkage and a higher carbon yield. Chemical vapor infiltration with carbon is used'for the densification of the C-C skeleton obtained by either a resin or liquid pitch impregnation. Silicon carbide matrix composites can be obtained by the silicon carbide CVI infiltration or a pyrolysis of an organosilazane polymer precursor. [9,1~ Matrix cracking may result from the thermal or internal stresses generated during the composite fabrication. The presence of
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