August 1998 Processing and Performance of an All-Oxide Ceramic Co 2085 principle, these 3-D fiber architectures should be readily adap (2) Notch Performance able to the present manufacturing route. They will be explored Ambient temp re tests on tensile specimens containing center holes and edge notches indicate moderate notch sensi vIl. Other Characteristics tivity, with notches causing somewhat greater strength degra- dation than holes(Fig. 14). Comparisons with matrix-domi- () Creep Strength nated CFCCs, such as woven Nicalon(Nippon Carbon Co Preliminary studies of the high-temperature creep character- magnesium aluminum silicate(MAS), indicate similar trends degradation phenomena occurring in these materials. The re in the relative strength reduction associated with center holes ults from constant-stress creep tests performed at 1200C on (Fig. 14(a)). Notably, the strength decreases by -25%over the the material with the Nextel 720 fibers are presented in Fig range of hole diameters of0mm≤2a≤10mm. Strong 13(a). These materials exhibit steady-state creep, unlike msg. similarities are also obtained in the strength characteristics of counterparts with SiC fibers Moreover, they have supe the oxide CFCCs and carbon/carbon in the presence of sharp rior, unexpectedly high, creep strengths, which render them as notches, as illustrated in Fig. 14(b). Comparisons of the latter serious candidates for 1200oC applications. The creep rates are data with predictions based on linear elastic fracture mechanics onsiderably lower than those expected from the fibers alone (LEFM)yield an inferred fraction toughness of-10 MPam at the same remote stress, as noted in Fig. 13(b). The implica However, the oxide CFCCs exhibit less notch sensitivity than tion is that the matrix within the fiber bundles is able to sustain the LEFM predictions (i.e, a), as manifested in the slope load by creeping at a rate comparable to that for the fibers, he strength versus notch length plot(Fig. 14(b) The moderate sensitivity of strength to the presence of holes without extensive cracking. This behavior also differs from that in the Sic-based systems has been attributed to two mecha- of other oxide matrix CFCCs. wherein the matrix contributes minimally to the composite creep strength. A potentially im nisms:45(1)the redistribution of stress around the holes, en- portant feature of these materials is the initial shrinkage of the dependent strength, which allows the material to sustain high shrinkage has been attributed to a change in the alumina cor stresses over the relatively small volumes that are subject to tent of the mullite phase within the fiber, which is originally hIs stress concentration In the carbon/carbon composites, the synthesized at 1350.C. The possible effects on composite creep stress redistribution is associated primarily with inelastic shear have yet to be understood 0°/90 Tensio r"3-1 15 3x103g Laminated Nicalon/MAs fa/w=0.2) Tensile Creep o Plain Weave Nicalon/Sic (a/w=0.2 N720 Com 2025303540 Hole Diameter, 2a(mm) Time (ks) N610 00c Composite N610 100°c 1200°c Fiber Data fr 982C wilson et al. [271) Simulations Tensile Stress (MPal (b) Notch Length, a (mm) Fig 13. Tensile creep response of all-oxide CFCCs based on N720 Effect of holes and notches on the strength of all-oxide fiber:(a)1200@C creep response at constant load and ites compared with results from the literature for Nicalon/SiC MAS, and carbon/carbon composites from Ref. 37. Tensile Ref 28. Data for no10 fibers from ref 28 are also strengths for the unnotched composites in(b) were -300 MPa for the the difference in creep properties between the two fibers carbon/carbon system and -230 MPa for the all-oxide materialprinciple, these 3-D fiber architectures should be readily adapt￾able to the present manufacturing route. They will be explored in future research. VII. Other Characteristics (1) Creep Strength Preliminary studies of the high-temperature creep character￾istics have been performed to assure that there are no serious degradation phenomena occurring in these materials. The re￾sults from constant-stress creep tests performed at 1200°C on the material with the Nextel 720 fibers are presented in Fig. 13(a). These materials exhibit steady-state creep, unlike their counterparts with SiC fibers.41–44 Moreover, they have supe￾rior, unexpectedly high, creep strengths, which render them as serious candidates for 1200°C applications. The creep rates are considerably lower than those expected from the fibers alone, at the same remote stress, as noted in Fig. 13(b). The implica￾tion is that the matrix within the fiber bundles is able to sustain load by creeping at a rate comparable to that for the fibers, without extensive cracking. This behavior also differs from that of other oxide matrix CFCCs, wherein the matrix contributes minimally to the composite creep strength.43 A potentially im￾portant feature of these materials is the initial shrinkage of the N720 fibers that occurs upon exposure to ∼1100°C.28 This shrinkage has been attributed to a change in the alumina con￾tent of the mullite phase within the fiber, which is originally synthesized at 1350°C. The possible effects on composite creep have yet to be understood. (2) Notch Performance Ambient temperature tests on tensile specimens containing center holes and edge notches indicate moderate notch sensi￾tivity, with notches causing somewhat greater strength degra￾dation than holes (Fig. 14). Comparisons with matrix-domi￾nated CFCCs, such as woven Nicalon (Nippon Carbon Co., Tokyo, Japan)/SiC and cross-ply laminates of Nicalon/ magnesium aluminum silicate (MAS), indicate similar trends in the relative strength reduction associated with center holes (Fig. 14(a)). Notably, the strength decreases by ∼25% over the range of hole diameters of 0 mm # 2a # 10 mm. Strong similarities are also obtained in the strength characteristics of the oxide CFCCs and carbon/carbon in the presence of sharp notches, as illustrated in Fig. 14(b). Comparisons of the latter data with predictions based on linear elastic fracture mechanics (LEFM) yield an inferred fraction toughness of ∼10 MPa?m1/2. However, the oxide CFCCs exhibit less notch sensitivity than the LEFM predictions (i.e., a−1/2), as manifested in the slope of the strength versus notch length plot (Fig. 14(b)). The moderate sensitivity of strength to the presence of holes in the SiC-based systems has been attributed to two mecha￾nisms:45 (1) the redistribution of stress around the holes, en￾abled by matrix cracking and fiber bridging, and (2) a volume￾dependent strength, which allows the material to sustain high stresses over the relatively small volumes that are subject to this stress concentration. In the carbon/carbon composites, the stress redistribution is associated primarily with inelastic shear Fig. 13. Tensile creep response of all-oxide CFCCs based on N720 fiber: (a) 1200°C creep response at constant load and (b) creep rates versus remote stress compared with data for pristine N720 fibers from Ref. 28. Data for N610 fibers from Ref. 28 are also shown to illustrate the difference in creep properties between the two fibers. Fig. 14. Effect of holes and notches on the strength of all-oxide composites compared with results from the literature for Nicalon/SiC, Nicalon/MAS, and carbon/carbon composites from Ref. 37. Tensile strengths for the unnotched composites in (b) were ∼300 MPa for the carbon/carbon system and ∼230 MPa for the all-oxide material. August 1998 Processing and Performance of an All-Oxide Ceramic Composite 2085