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Fiber coatings have been used to modify fiber-matrix interfacial forces, and thus control mechanical properties of continuous fiber ceramic composites. It has been shown that the properties and thickness of the interlayer influence composite properties such as matrix cracking and ultimate strength, toughness and interlaminar shear. The effects of fiber coating properties and thickness on fiber-reinforced SiC matrix composites fabricated employing CVI techniques have been examined. Correlations between interface condition, mechanical properties and failure mechanisms have been made.
Woven fabric ceramic composites fabricated by the chemical vapor infiltration method are susceptible to high void content and inhomogeneity. The condition of such materials may be characterized nondestructively with ultrasonic methods. In this work, longitudinal and shear waves were used in the quantitative determination of elastic constants of NicalonTM/SiC composites as a function of volume percent of porosity. Elastic stiffness constants were obtained for both the inplane and out-of-plane directions with respect to fiber fabric. The effect of porosity on the modulus of woven fabric composites was also modelled and compared to the measured results. Scan images based on the amplitude and time-of-flight of radio frequency (RF) ultrasonic pulses were used for evaluating the material homogeneity for the purpose of optimizing the manufacturing process and for correlation with the mechanical testing results.
A forced-flow thermal-gradient chemical vapor infiltration process has been developed to fabricate composites of thick-walled tubular geometry common to many components. Fibrous preforms of different fiber architectures (3-dimensionally braided and filament wound) have been investigated to accommodate components with different mechanical property requirements. This paper will discuss the fabrication of tubular, fiber-reinforced SiC matrix composites and their mechanical properties.
Chemical vapor deposition has been utilized to produce ternary, multiphase coatings of various compositions of silicon carbide (SiC) with Ti, Cr, and Mo. Thermodynamic calculations have been performed for a variety of experimental conditions in each system. Scanning, transmission and analytical electron microscopy, and X-ray diffraction techniques have been used to characterize the microstructures and to determine compositions.
A process has been developed for the fabrication of a ceramic composite of SiC fibers in a chemical vapor infiltrated (CVI) SiC matrix. Early specimens produced by this technique exhibited nonuniform fracture toughness behavior; regions of brittle fracture with no fiber pullout were interspersed with regions of good toughness where fiber pullout predominated. Microscopic and spectroscopic analyses of fiber surfaces revealed that this behavior may have been related to a thin, discontinuous layer of predominantly silica which, when present, prevented tight bonding of fibers and CVI matrix. Consequently, efforts to control interfacial bond strengths and enhance fracture toughness via fiber pretreatment with CVI overcoatings have had mixed success depending upon the overcoating specie. Chemical and microstructural characterizations by analytical electron microscopy of these composites are presented and correlated with composite mechanical property data.
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