The concept of an oxidation resistant fiber-matrix interphase that deflects cracks by cleavage is explored. Sols with the nominal composition CaAl_(12)O_(19) were used to coat single-crystal alumina fiber and plates and yttrium-aluminum garnet fiber. Subsequent annealing, or consolidation in a matrix, results in strong texturing of the hibonite coating with basal planes parallel to the interface. Texture development within the hibonite interphase occurs by a complex series of phase transformations and reactions. Fiber pushout tests indicate high sliding resistance; however, strain-energy release rates of hibonite-bonded alumina laminates were found to be adequate to provide for crack deflection within the interphase via cleavage. Model composites containing hibonite-coated fibers were tested and showed evidence of crack deflection within the interphase.
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