Journal of the American Ceramic Society-Levi et al. Vol 8l. No 8 only occasional instances of large-scale voids that arise fro reduce the scale of the unreinforced matrix regions by filling air bubbles or pockets of unconsolidated slurry trapped by the he larger spaces in the preform with short fibers. This idea was filtration front. The large particles within the matrix are mul explored by coating the cloth with a paste of chopped alumina lite, and the finer particles are a mixture of alumina and mullite fiber with an average aspect ratio of -10. The paste fills the ( Fig. 4(a)). It is also evident upon analysis of the fracture cross-over regions, as depicted in Figs. 5(a)and (b), and at surfaces that the matrix is bonded to the fibers, presumably taches randomly aligned short fibers to the cloth surface. La through the same alumina bridges and/or precursor necks pres- ers of such cloth were assembled into a preform and processed ent within the mullite network(see Section IV) in th ne same manner. The results in Figs. 5(c) and(d)show that Examination at lower magnifications reveals the presence of the flaws have been largely suppressed. This approach, how- cracklike shrinkage flaws within the matrix, especially in re- ever, further decreases the packing efficiency and limits the gions devoid of fibers( Fig. 4(d)). The flaws are typically per- volume fraction of continuous reinforcement to -25% which is pendicular to the fibers and tend to form 2-D arrays on planes substantially below the levels typical of current CFCC com- parallel to the fiber cloth. The results in Fig 3 suggest that the posites(235%). Efforts to find suitable ways to minimize flaws evolve primarily upon matrix shrinkage during drying flaws without sacrificing the volume fraction of reinforcement The phenomenon is induced by the biaxial constraint imposed are continuing e reinforcements and enhanced by the presence of rather large unreinforced matrix regions, owing to the limitations to fiber packing when using woven cloth. Such cracking does not IV. Tensile Behavior at Room Temperature occur in unidirectional composites. )Because of the rela Stress-strain curves for both materials have been measured tively large openings of these drying cracks, they cannot be healed in tension in the 0/900 and +45 orientations, as needed for by the subsequent precursor and pyrolysis implementation in numerical design codes. The test procedures One way to alleviate the formation of shrinkage flaws is to have been described elsewhere 33 Periodic unload-reload mea- d m 100pm Fig. 4. Microstructural views of al on N610 woven fiber preforms: (a) matrix containing 20% AL2O3 and-37% porosity, (b )and(c) show good levels of inf ween the fiber layers, respectively, (d) matrix cracks produced during drying. Samples in(c)and(d)were given 10 impreonly occasional instances of large-scale voids that arise from air bubbles or pockets of unconsolidated slurry trapped by the filtration front. The large particles within the matrix are mul￾lite, and the finer particles are a mixture of alumina and mullite (Fig. 4(a)). It is also evident upon analysis of the fracture surfaces that the matrix is bonded to the fibers, presumably through the same alumina bridges and/or precursor necks pres￾ent within the mullite network (see Section IV). Examination at lower magnifications reveals the presence of cracklike shrinkage flaws within the matrix, especially in re￾gions devoid of fibers (Fig. 4(d)). The flaws are typically per￾pendicular to the fibers and tend to form 2-D arrays on planes parallel to the fiber cloth. The results in Fig. 3 suggest that the flaws evolve primarily upon matrix shrinkage during drying. The phenomenon is induced by the biaxial constraint imposed by the reinforcements and enhanced by the presence of rather large unreinforced matrix regions, owing to the limitations to fiber packing when using woven cloth. (Such cracking does not occur in unidirectional composites.14,18) Because of the rela￾tively large openings of these drying cracks, they cannot be healed by the subsequent precursor impregnation and pyrolysis. One way to alleviate the formation of shrinkage flaws is to reduce the scale of the unreinforced matrix regions by filling the larger spaces in the preform with short fibers. This idea was explored by coating the cloth with a paste of chopped alumina fiber with an average aspect ratio of ∼10. The paste fills the cross-over regions, as depicted in Figs. 5(a) and (b), and at￾taches randomly aligned short fibers to the cloth surface. Lay￾ers of such cloth were assembled into a preform and processed in the same manner. The results in Figs. 5(c) and (d) show that the flaws have been largely suppressed. This approach, how￾ever, further decreases the packing efficiency and limits the volume fraction of continuous reinforcement to ∼25%, which is substantially below the levels typical of current CFCC com￾posites ($35%). Efforts to find suitable ways to minimize flaws without sacrificing the volume fraction of reinforcement are continuing. IV. Tensile Behavior at Room Temperature Stress–strain curves for both materials have been measured in tension in the 0°/90° and ±45° orientations, as needed for implementation in numerical design codes. The test procedures have been described elsewhere.33 Periodic unload–reload mea￾Fig. 4. Microstructural views of all-oxide composites based on N610 woven fiber preforms: (a) matrix containing 20% Al2O3 and ∼37% porosity, (b) and (c) show good levels of infiltration within and between the fiber layers, respectively, (d) matrix cracks produced during drying. Samples in (c) and (d) were given 10 impregnation cycles to facilitate polishing. 2080 Journal of the American Ceramic Society—Levi et al. Vol. 81, No. 8