August 1998 Processing and Performance of an All-Oxide Ceramic Composite (b) 1 mm (c) (d) 溶答容20m 200m Fig. 5. An approach to minimize matrix cracking in slurry-infiltrated composies: (a) depicts the topography of the fiber cloth in the as-received condition and(b)after coating with a paste of short alumina fiber. (c) and(d) show in-plane and cross-sectional views of the resulting composites where only minimal microcracking can be detected surements have been used to assess the incidence of damage also of the above order, but these strains are much closer to the evolution(from modulus changes), as well as the occurrence of values measured in tows extracted from the woven fabric. internal friction(from hysteresis strains ). 34, 35 The results are (Section Ill(I). These observations are in agreement with the reminiscent of those found for fiber-dominated carbon-matrix superior stability of the N720 fiber. They also suggest that the composites, such as carbon/carbon and SiC/carbon. 5.,33, 36-39 composite manufacturing process do not cause significant In the 0 /90 orientation, the tensile response is essentially degradation of the reinforcement beyond that expected from linear with only small inelastic strains, Fig. 6(a), similar to that hermal of the pristine fibers. Notwithstanding the observed in SiC/carbon systems, ,39 Fig. 6(b). The behavior is higher stability of the N720 fiber, these composites have sig- quite reproducible, with stress-strain curves for a given volume nificantly lower strengths at ambient temperature than those tively narrow band. The initial moduli (-100 and-60 GPa for reflecting the initially lower strength of the N720 fiber and the N610 and N720 composites, respectively) are consistently indicating that strength retention in the N610 fiber is still ad- larger than those expected from the axial fibers alone (fEr /2 of equate after processing. Further work is clearly needed to un- -75 and-50 GPa, respectively ), reflecting a contribution from derstand and quantify the effects of processing on fiber degra- the matrix and the transverse reinforcements. The average fail ure strain for 14 specimens from five different panels of N610 In the 0o/90 orientation, the fracture plane is ill-defined composite(33%39% fiber) sintered at 1200.C is 0.27% with the fiber tows breaking over a wide range of axial loca- These failure strains are smaller than those expected for pris tions, spanning a distance of-I cm(Fig. 7(a)). The locations of ine tows of comparable gauge length, even after weaving, but the fiber breaks within an individual tow also exhibit a broad are consistent with the values reported for N610 fibers sub- distribution, typically -l mm in length(Fig. 7(b). These ob- jected to thermal cycles similar to those used in composite servations validate the efficacy of the porous matrix as a crack processing. 24 The failure strains of the N720 composites are deflection medium both within and between the fiber tows Thesurements have been used to assess the incidence of damage evolution (from modulus changes), as well as the occurrence of internal friction (from hysteresis strains).34,35 The results are reminiscent of those found for fiber-dominated carbon-matrix composites, such as carbon/carbon and SiC/carbon.5,33,36–39 In the 0°/90° orientation, the tensile response is essentially linear with only small inelastic strains, Fig. 6(a), similar to that observed in SiC/carbon systems,5,39 Fig. 6(b). The behavior is quite reproducible, with stress–strain curves for a given volume fraction and set of processing conditions falling within a rela￾tively narrow band. The initial moduli (∼100 and ∼60 GPa for the N610 and N720 composites, respectively) are consistently larger than those expected from the axial fibers alone ( f Ef/2 of ∼75 and ∼50 GPa, respectively), reflecting a contribution from the matrix and the transverse reinforcements. The average fail￾ure strain for 14 specimens from five different panels of N610 composite (33%–39% fiber) sintered at 1200°C is 0.27%. These failure strains are smaller than those expected for pris￾tine tows of comparable gauge length, even after weaving, but are consistent with the values reported for N610 fibers sub￾jected to thermal cycles similar to those used in composite processing.24 The failure strains of the N720 composites are also of the above order, but these strains are much closer to the values measured in tows extracted from the woven fabric29 (Section III(1)). These observations are in agreement with the superior stability of the N720 fiber. They also suggest that the composite manufacturing process does not cause significant degradation of the reinforcement beyond that expected from thermal exposure of the pristine fibers. Notwithstanding the higher stability of the N720 fiber, these composites have sig￾nificantly lower strengths at ambient temperature than those made with N610 fibers (∼140 MPa vs >200 MPa, respectively), reflecting the initially lower strength of the N720 fiber and indicating that strength retention in the N610 fiber is still ad￾equate after processing. Further work is clearly needed to un￾derstand and quantify the effects of processing on fiber degra￾dation and its effects on composite performance. In the 0°/90° orientation, the fracture plane is ill-defined, with the fiber tows breaking over a wide range of axial loca￾tions, spanning a distance of ∼1 cm (Fig. 7(a)). The locations of the fiber breaks within an individual tow also exhibit a broad distribution, typically ∼1 mm in length (Fig. 7(b)). These ob￾servations validate the efficacy of the porous matrix as a crack deflection medium both within and between the fiber tows. The Fig. 5. An approach to minimize matrix cracking in slurry-infiltrated composies: (a) depicts the topography of the fiber cloth in the as-received condition and (b) after coating with a paste of short alumina fiber. (c) and (d) show in-plane and cross-sectional views of the resulting composites where only minimal microcracking can be detected. August 1998 Processing and Performance of an All-Oxide Ceramic Composite 2081