November 2002 Interface Design for Oxidation-Resistant Ceramic Composites 2605 1.3 0.9 10(c) t/R<6% 0.8 20(BN) s.12/Q。=12575100C 0.7 0.75 0.6 0 0.5 t<0.5μm:E≤E 04 00.2040.60.8 -0.1-0.0500.050.10.150.20.25 R}{(Ect4}1+0.5h}tR}{(axa)-1}{1+0.5(E,/E} Fig. 8. Universal plots can be used to obtain the properties of an"effective" transverse fiber that can be substituted for a fiber pl odels that use transversely isotropic moduli and CTE: plots of (a)effective modulus and CTE in the transverse direction for fibers lots are a good approximation for up to 0.5 um thick(n)coating on an 8 um fiber radiL bols e and a refer to the modulus and Cte, subscripts t, c, and f stand for the transverse, coating, and fiber, respectively. Asterisk(* ffective properties.(Plot(b) was corrected for an er in the oniginal reference, where the matrix on the right-hand side in Eq. (8)should be gauge length of bridging fibers resulting from short debond lengths oating-it can be as high as 0.7 for an elastic anisotropy that are, in turn, a consequence of high T. As discussed earlier. om the He and Hutchinson%,60 analysis is not satisfied matrix cracks in high-strength material deflect into multiple the coating. A similar discrepancy has been noted for interfacial cracks, rather than a single debond. Therefore, crack on criteria using a laminate geometry. Although this deflection for this CMC is decided primarily by fracture anisotropy result is not well understood, it is encouraging with regard to the within the coating, rather than at the coating/fiber or coating/ development of alternative coatings in that the fracture energies matrix interface. Unusual fiber pushout load-deflection curves and the sliding friction may not be required to be as low as uggest substantial effects of rough interfaces, and subsequent previously thought. In any event, many of the coating approaches analysis implies that the critical strain energy to propagate cracks discussed later are likely to exhibit sufficiently high fracture in this interfacial region may be as high as 25 J/m". This is more energy and friction to greatly restrict debond lengths. It is helpful than half the fracture energy across the strongest graphite planes to know that, although the composites discussed above exhibit The criterion of fracture energy anisotropy of 1/4 or less( for an matrix crack spacings of from one to three fiber diameters, J d prc fiber 0.6 LONGITUDINAL TENSILE STRAIN(%) Fig 9. Tensile stress-strain behaviors in tension measured on the two-dimensional SiC/SiC composites fabricated from ()untreated or()treated Nicalon (ceramic grade)fibers. Complex crack deflection within the coating on treated fibers(schematic upper left)leads to higher friction than smooth interfacial ailure with untreated fibers (lower right).gauge length of bridging fibers resulting from short debond lengths that are, in turn, a consequence of high . As discussed earlier, matrix cracks in high-strength material deflect into multiple interfacial cracks, rather than a single debond.67 Therefore, crack deflection for this CMC is decided primarily by fracture anisotropy within the coating, rather than at the coating/fiber or coating/ matrix interface. Unusual fiber pushout load–deflection curves suggest substantial effects of rough interfaces, and subsequent analysis implies that the critical strain energy to propagate cracks in this interfacial region may be as high as 25 J/m2 . 66 This is more than half the fracture energy across the strongest graphite planes. The criterion of fracture energy anisotropy of 1/4 or less (for an isotropic coating—it can be as high as 0.7 for an elastic anisotropy of 6) from the He and Hutchinson59,60 analysis is not satisfied, even in the coating. A similar discrepancy has been noted for deflection criteria using a laminate geometry.64 Although this result is not well understood, it is encouraging with regard to the development of alternative coatings in that the fracture energies and the sliding friction may not be required to be as low as previously thought. In any event, many of the coating approaches discussed later are likely to exhibit sufficiently high fracture energy and friction to greatly restrict debond lengths. It is helpful to know that, although the composites discussed above exhibit matrix crack spacings of from one to three fiber diameters, Fig. 8. Universal plots can be used to obtain the properties of an “effective” transversely isotropic fiber that can be substituted for a fiber plus coating in models that use transversely isotropic moduli and CTE: plots of (a) effective modulus and (b) effective CTE in the transverse direction for fibers with coatings. Plots are a good approximation for up to 0.5 m thick (t) coating on an 8 m fiber radius (R). Symbols E and refer to the modulus and CTE, respectively; subscripts t, c, and f stand for the transverse, coating, and fiber, respectively. Asterisk (
) denotes effective properties.88 (Plot (b) was corrected for an error in the original reference, where the matrix on the right-hand side in Eq. (8) should be inverted.) Fig. 9. Tensile stress–strain behaviors in tension measured on the two-dimensional SiC/SiC composites fabricated from (I) untreated or (J) treated Nicalon (ceramic grade) fibers. Complex crack deflection within the coating on treated fibers (schematic upper left) leads to higher friction than smooth interfacial failure with untreated fibers (lower right).350 November 2002 Interface Design for Oxidation-Resistant Ceramic Composites 2605