MB. Ruggles-Wrenn, P D. Laffey Composites Science and Technology 68(2008)2260-2266 2265 b Fig. 11. Fracture surface of the dNS specimen tested in creep at 6.5 MPa at 1200C in steam. 10m um Fig. 12. Fracture surface of the DNS specimen tested in compression to failure following 100 h at 4.0 MPa at 1200" in steam. steam. However, while prior creep in air resulted in strengthening creep run-out was achieved only at 4 MPa. The run-out specimen of the matrix, retained properties in Table 2 suggest that prior retained only 75% of its ILSS For applied stress >5 MPa, the pres- creep in steam causes weakening of the matrix. The retained ilSs ence of steam drastically reduced creep lifetimes. The hydrother of the specimen pre-crept in steam is noticeably lower than the al weakening of the alumina matrix due to incorporation of ILSS of the as-processed composite. ydrogen defects during testing at 1200C in steam may be behind Recent studies [35,36 show that water attacks grain bound the degraded creep performance of the N720/A in steam. aries and degrades the strength of the polycrystalline alumina. In tests of short duration conducted in air, the failure occurs pri- Kronenberg et al. [36 found two types of hydrogen defects incor- marily through delamination of the woven 0/90 fiber layers from porated in alumina specimens subjected to heat treatment at tem- the matrix-rich regions, with minimal fiber fracture. generally om 850c to 1025C under 1500 MPa only one fiber layer is associated with delamination For test dura- hydrostatic pressure in the presence of water, followed by com- tions >100 h, the failure mechanism includes considerable fiber pression tests at temperatures in the 630-850C range The infra- fracture. It is possible that the matrix undergoes additional sinter red measurements revealed interstitial hydrogens in the bulk and ing during the long-term tests conducted in air. In tests conduct molecular water clusters near surfaces, grain boundaries and in steam, the failure mechanism is dominated by fiber fracture. cracks of the hydrothermally treated alumina specimens. Further more, the presence of hydrogen defects reduced the yield stress of Acknowledgements fine-grained alumina by a factor of six. It is possible that hydrogen defects that are introduced into the alumina matrix of the n720/A The financial support of Dr R. Sikorski and Dr J. Zelina, Propul- specimens during tests conducted at 1200C in steam are behind sion Directorate, Air Force Research Laboratory is highly the degradation of creep performance in steam. 4. Concluding remarks References The creep behavior of the N720/A ceramic composite in inter- I1 Ohnabe H, Masaki SOnozuka M, Miyahara K,Sasa TPotential application of laminar shear was assessed and the interlaminar shear properties were measured at 1200C in laboratory air and in steam using [21 Zawada LP Staehler J Steel S Consequence of intermittent exposure to strength(ILSS)was 8.25 MPa. Prior aging for 24 h at 1200C in (31 Parlier M, Ritti MH. State of the art and perspectives for oxide/oxide steam had no effect on ILSS. The N720/A composite exhibits pri- mary and secondary creep regimes in air. In steam, primary, sec. [4 Mattoni MA, Yang JY, Levi CG, Zok Fw-2: 211-21. Zawada LP. Effects of combustor rig ondary and tertiary creep regimes are observed. Creep strains us-matrix oxide composite. Int J Appl Ceram Technol 2(2):133-40 accumulated in steam are at least an order of magnitude larger [5] Parthasarathy TA, Zawada LP, John R, Cinibulk MK, Zelina J. Evaluation of than those produced in air. Creep strain rates were approximately 21×10-7s- I in ai, and ranged from34×10-6to7.×10-5s-1 Technol 2005:2(2):122-32 novel combustor wall application. Int JApp Ceram in steam. At 6.5 MPa, creep rate in steam is at least two orders of posite member and m No.5601674,1997 magnitude higher than the creep rate produced in air. [7 Sim SM, Kerans R]. Slurry infiltration and 3-D woven composites Ceram Eng Creep run-out was achieved in all tests conducted in air. The 18) Moore EH, Mah T, Keller KA 3D composite fabrication through matrix slurry un-out specimens exhibited substantial increase in ILSS In steam, pressure infiltration. Ceram Eng Sci Proc 1994: 15(4): 1 13-20steam. However, while prior creep in air resulted in strengthening of the matrix, retained properties in Table 2 suggest that prior creep in steam causes weakening of the matrix. The retained ILSS of the specimen pre-crept in steam is noticeably lower than the ILSS of the as-processed composite. Recent studies [35,36] show that water attacks grain bound￾aries and degrades the strength of the polycrystalline alumina. Kronenberg et al. [36] found two types of hydrogen defects incor￾porated in alumina specimens subjected to heat treatment at tem￾peratures ranging from 850 C to 1025 C under 1500 MPa hydrostatic pressure in the presence of water, followed by com￾pression tests at temperatures in the 630–850 C range. The infra￾red measurements revealed interstitial hydrogens in the bulk and molecular water clusters near surfaces, grain boundaries and cracks of the hydrothermally treated alumina specimens. Further￾more, the presence of hydrogen defects reduced the yield stress of fine-grained alumina by a factor of six. It is possible that hydrogen defects that are introduced into the alumina matrix of the N720/A specimens during tests conducted at 1200 C in steam are behind the degradation of creep performance in steam. 4. Concluding remarks The creep behavior of the N720/A ceramic composite in inter￾laminar shear was assessed and the interlaminar shear properties were measured at 1200 C in laboratory air and in steam using double-notch shear test specimens. The interlaminar shear strength (ILSS) was 8.25 MPa. Prior aging for 24 h at 1200 C in steam had no effect on ILSS. The N720/A composite exhibits pri￾mary and secondary creep regimes in air. In steam, primary, sec￾ondary and tertiary creep regimes are observed. Creep strains accumulated in steam are at least an order of magnitude larger than those produced in air. Creep strain rates were approximately 2.1 107 s1 in air, and ranged from 3.4 106 to 7.0 105 s1 in steam. At 6.5 MPa, creep rate in steam is at least two orders of magnitude higher than the creep rate produced in air. Creep run-out was achieved in all tests conducted in air. The run-out specimens exhibited substantial increase in ILSS. In steam, creep run-out was achieved only at 4 MPa. The run-out specimen retained only 75% of its ILSS. For applied stress P5 MPa, the pres￾ence of steam drastically reduced creep lifetimes. The hydrother￾mal weakening of the alumina matrix due to incorporation of hydrogen defects during testing at 1200 C in steam may be behind the degraded creep performance of the N720/A in steam. In tests of short duration conducted in air, the failure occurs pri￾marily through delamination of the woven 0/90 fiber layers from the matrix-rich regions, with minimal fiber fracture. Generally, only one fiber layer is associated with delamination. For test dura￾tions >100 h, the failure mechanism includes considerable fiber fracture. It is possible that the matrix undergoes additional sinter￾ing during the long-term tests conducted in air. In tests conducted in steam, the failure mechanism is dominated by fiber fracture. Acknowledgements The financial support of Dr. R. Sikorski and Dr. J. Zelina, Propul￾sion Directorate, Air Force Research Laboratory is highly appreciated. References [1] Ohnabe H, Masaki S, Onozuka M, Miyahara K, Sasa T. Potential application of ceramic matrix composites to aero-engine components. Composites: Part A 1999;30:489–96. [2] Zawada LP, Staehler J, Steel S. Consequence of intermittent exposure to moisture and salt fog on the high-temperature fatigue durability of several ceramic–matrix composites. J Am Ceram Soc 2003;86(8):1282–91. [3] Parlier M, Ritti MH. State of the art and perspectives for oxide/oxide composites. Aerospace Sci Technol 2003;7:211–21. [4] Mattoni MA, Yang JY, Levi CG, Zok FW, Zawada LP. Effects of combustor rig exposure on a porous-matrix oxide composite. Int J Appl Ceram Technol 2005;2(2):133–40. [5] Parthasarathy TA, Zawada LP, John R, Cinibulk MK, Zelina J. Evaluation of oxide–oxide composites in a novel combustor wall application. Int J App Ceram Technol 2005;2(2):122–32. [6] Szweda A, Millard ML, Harrison MG. Fiber-reinforced ceramic–matrix composite member and method for making. US Patent No. 5 601 674, 1997. [7] Sim SM, Kerans RJ. Slurry infiltration and 3-D woven composites. Ceram Eng Sci Proc 1992;13(9–10):632–41. [8] Moore EH, Mah T, Keller KA. 3D composite fabrication through matrix slurry pressure infiltration. Ceram Eng Sci Proc 1994;15(4):113–20. Fig. 11. Fracture surface of the DNS specimen tested in creep at 6.5 MPa at 1200 C in steam. Fig. 12. Fracture surface of the DNS specimen tested in compression to failure following 100 h at 4.0 MPa at 1200 in steam. M.B. Ruggles-Wrenn, P.D. Laffey / Composites Science and Technology 68 (2008) 2260–2266 2265