M.B. Ruggles-Wrenn et al /Composites Science and Technology 66(2006)2089-2099 1E02 neous linkage of creep-nucleated cracks occurred just prior 1E-03 to failure N310 Fiber at v,a0.15. 1100C Stress-rupture behavior is summarized in Fig. 7, where 1.E04 110o’c creep stress is plotted vs time to rupture at 900 and 1E-05 1100C for both composites. Creep-rupture life of the 1E-06 coated fiber composite increases considerably with decreas- 1E-07 ing temperature. At 1100C, the time to rupture was 14 h for the low creep stress of 40 MPa, while the 80 MPa test survived a mere 0. 4 h. At 900C, the run-out stress was Creep strain rate is< 10st 120 MPa. While the coated fiber CMc achieved a run- E10 out at stresses <120 MPa, the uncoated fiber composite 1000 failed after 97 h at 73 MPa, approaching but not achieving Creep Stress(MPa) a run-out, and survived only 5.5 h at 80 MPa In the case of Fig. 6. Minimum creep rate as a function of applied stress for N610/A the monazite containing composite, the 80 MPa creep test and N610/M/A ceramic composites at 900 and 1100C. Data for Nextel was interrupted after 145 h of creep, producing the creep 610 fibers(Wilson [40), and Nextel 610-reinforced composites(Zawada life at least 25 times that of the uncoated fiber CMc at 37]and Casas [41]are also shown. the same creep stress. The addition of the monazite coating significantly improved the creep life at 900C. obtained for N610/M/A agree well with the N610 fiber Retained strength and modulus of the two specimens data adjusted for V=0. 2. Furthermore, fitting the that reached run-out in creep tests are summarized in Table N610/M/A creep results with a temperature-independent 4. Both specimens retained over 90% of their tensile Norton-Bailey equation of the form strength; the modulus loss was limited to 10%. Considering that these specimens experienced only small creep strains (.4%), better than 90% retention of strength and modu- ields a stress exponent n x3.5, which is approximately lus is not very surprising. Prior creep had minimal effect on equal to that reported for the Nextel 610 fiber. (Here e is the failure strain. Failure strains for the specimens sub the minimum creep rate, A is a temperature-dependent jected to prior creep were only slightly lower than those coefficient that accounts for the activation energy and other for the as-processed material. The excellent strength reten- variables in the full form of the power law, and o is the ap- tion produced in the present study is consistent with the plied stress). Conversely, both N610/AS and N610/Mox results reported by Zawada et al. [37] who observed no composites exhibit lower creep rates compared to the decrease in tensile strength of the N610/AS composite N610/M/A cross-ply. Creep rates of the N610/AS [37] are approximately an order of magnitude lower than what ted from N610 fibers(see the fiber da adjusted for v=0. 15). The lower creep rates of the N610/AS(a composite reinforced with an eight-harness sa tin weave of Nextel 610 fibers) suggest that creep may be Alumina ffected by fiber weave architecture. At 900C, the creep strain rates of both the uncoated fiber composite and 150F N610/onazite/Alumina N610/M/A(with the exception of the creep strain rate at 150 MPa)are <10-8s-I. The creep rupture of N610/M/A at both test temperatures is likely dominated by rupture of the Nextel 610 fibers. Previous studies suggest that the mechanism controlling the stead creep of the Nextel 610 fibers is interface-reaction controlled diffusion creep with fine intergranular crack formation. Cracks continue to nucleate throughout the 1000100001000001000000 creep process until a critical crack density is reached, caus- ing spontaneous crack linka ge and Fig. 7. Creep stress vs time to rupture for N610/A and N610/M/A noticeable tertiary creep at 900C suggests that a sponta- ceramic composites at 900 and 1100C Table 4 Retained properties of the N610/M/A specimens subjected to prior creep at 900C Specimen Creep stress(MPa) Retained strength(MPa) Strength retention (% Retained modulus(GPa) Modulus retention(%) Strain at 80 173 l20 164obtained for N610/M/A agree well with the N610 fiber data adjusted for Vf = 0.2. Furthermore, fitting the N610/M/A creep results with a temperature-independent Norton–Bailey equation of the form e_ ¼ Arn yields a stress exponent n 3.5, which is approximately equal to that reported for the Nextel 610 fiber. (Here e_ is the minimum creep rate, A is a temperature-dependent coefficient that accounts for the activation energy and other variables in the full form of the power law, and r is the ap￾plied stress). Conversely, both N610/AS and N610/Umox composites exhibit lower creep rates compared to the N610/M/A cross-ply. Creep rates of the N610/AS [37] are approximately an order of magnitude lower than what would be expected from N610 fibers (see the fiber data adjusted for Vf = 0.15). The lower creep rates of the N610/AS (a composite reinforced with an eight-harness sa￾tin weave of Nextel 610 fibers) suggest that creep may be affected by fiber weave architecture. At 900 C, the creep strain rates of both the uncoated fiber composite and N610/M/A (with the exception of the creep strain rate at 150 MPa) are 6108 s 1 . The creep rupture of N610/M/A at both test temperatures is likely dominated by creep rupture of the Nextel 610 fibers. Previous studies [42–44] suggest that the mechanism controlling the steady-state creep of the Nextel 610 fibers is interface-reaction￾controlled diffusion creep with fine intergranular crack formation. Cracks continue to nucleate throughout the creep process until a critical crack density is reached, caus￾ing spontaneous crack linkage and failure. Lack of a noticeable tertiary creep at 900 C suggests that a sponta￾neous linkage of creep-nucleated cracks occurred just prior to failure. Stress-rupture behavior is summarized in Fig. 7, where creep stress is plotted vs time to rupture at 900 and 1100 C for both composites. Creep-rupture life of the coated fiber composite increases considerably with decreas￾ing temperature. At 1100 C, the time to rupture was 14 h for the low creep stress of 40 MPa, while the 80 MPa test survived a mere 0.4 h. At 900 C, the run-out stress was 120 MPa. While the coated fiber CMC achieved a run￾out at stresses 6120 MPa, the uncoated fiber composite failed after 97 h at 73 MPa, approaching but not achieving a run-out, and survived only 5.5 h at 80 MPa. In the case of the monazite containing composite, the 80 MPa creep test was interrupted after 145 h of creep, producing the creep life at least 25 times that of the uncoated fiber CMC at the same creep stress. The addition of the monazite coating significantly improved the creep life at 900 C. Retained strength and modulus of the two specimens that reached run-out in creep tests are summarized in Table 4. Both specimens retained over 90% of their tensile strength; the modulus loss was limited to 10%. Considering that these specimens experienced only small creep strains (0.4%), better than 90% retention of strength and modu￾lus is not very surprising. Prior creep had minimal effect on the failure strain. Failure strains for the specimens sub￾jected to prior creep were only slightly lower than those for the as-processed material. The excellent strength reten￾tion produced in the present study is consistent with the results reported by Zawada et al. [37], who observed no decrease in tensile strength of the N610/AS composite 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 10 100 1000 Creep Stress (MPa) Creep Strain Rate (s-1) N610 Fiber, 1100˚C Wilson, 2001 N610 Fiber at Vf = 0.2, 1100˚C N610/Monazite/Alumina, 1100˚C N610/Monazite/Alumina, 900˚C N610/Alumina, 900˚C Creep strain rate is < 10-8 s-1 N610/AS, 1100˚C Zawada, 2003 N610/Umox, 1100˚C Casas, 2004 N610 Fiber at Vf = 0.15, 1100˚C Fig. 6. Minimum creep rate as a function of applied stress for N610/A and N610/M/A ceramic composites at 900 and 1100 C. Data for Nextel 610 fibers (Wilson [40]), and Nextel 610-reinforced composites (Zawada [37] and Casas [41]) are also shown. 0 50 100 150 200 250 1 10 100 1000 10000 100000 1000000 Stress (MPa) N610/Monazite/Alumina UTS at 900˚ C N610/Monazite/Alumina UTS at 1100˚ C N610/Monazite/Alumina 1100˚ C N610/Monazite/Alumina 900˚ C N610/Alumina 900˚ C 0 50 100 150 200 250 1 10 100 1000 10000 100000 1000000 Time (s) Stress (MPa) N610/Monazite/Alumina UTS at 900˚ N610/Monazite/Alumina UTS at 1100˚ N610/Monazite/Alumina 1100˚ C N610/Monazite/Alumina 900˚ C N610/Alumina 900˚ C Fig. 7. Creep stress vs time to rupture for N610/A and N610/M/A ceramic composites at 900 and 1100 C. Table 4 Retained properties of the N610/M/A specimens subjected to prior creep at 900 C Specimen Creep stress (MPa) Retained strength (MPa) Strength retention (%) Retained modulus (GPa) Modulus retention (%) Strain at failure (%) B10-1 80 173 96 80 96 0.28 B10-2 120 164 91 75 90 0.29 2094 M.B. Ruggles-Wrenn et al. / Composites Science and Technology 66 (2006) 2089–2099