Journal of the American Ceramic Society-Zhu et al. Vol 81. No 9 ◆60MPa,A o 60 MPa, Alr 0.5 50 102 Time (s Fig. 16. Micrograph depicting the matrix crack in a specimen of the enhanced SiC/SiC composite crept at a load of 60 MPa in argon 1g modulus, normalized by the value of the first load- ne time to rupture of the enhanced SiC/SiC com nt load (60 MPa)in air and in argon at 1300oC the enhanced SiC/SiC argon is% of the original value, which is lower than that it is also higher thana o he at trd is several orders of magnitude SiC/SiC composite in air, and standard SiC/SiC composite in argon at1300°C lus between creep and fatigue has not yet been well understood (7) Creep ar ue cracks are always found at the large A possible interpretation is that the larger debonded length of pores among the fiber bundles ack propagation occurs the fibers(producing a larger strain in Fig. 3)under fatigue in the matrix of the enhanced composite. The crack an under creep leads to failure of the enhanced fibers at th propagation of the composite is controlled by both the bridged ame maximum stress. 2 Therefore. when the matrix cracks are fibers and the matrix similar in damage extent, fatigue failure occurs before creep Acknowledgment: The authors are grateful for the assistance of Mr S failure Ogawa in the mechanical tests. v. Conclusion References 1) The ultimate tensile strength (UTS)of the enhanced A.G. Evans, ""Perspective on the Development of High-Toughness Ceram- SiC/SiC composite is similar to that of the standard SiC/SiC ics,”J.Am. Cera.Soc,73{2]187-206(1990 composite; however, the strain at UTS of the enhanced SiC/SiC Goto and Y. Kagawa, ""Fracture Behavior and Toughness of a plain- composite is much higher than that of the standard SiC/SiC n SiC Fiber-Reinforced SiC Matrix Composite, Mater. Sci. Eng.A, A211,72-81 composite. The Youngs modulus that is calculated from the ndamental Research in Structural Ceramics for Service Near linear portion of the curve is -89 GPa, which is much lower 000°C,”J.Am. Ceram. soc,7692147-74(1993) than that of the standard SiC/SiC composite at 1300 C(200 A. G. Evans and Life Prediction Issues for High-Temperat ering Ceramics and Their Composites, ""Acta Mater., 45[1] 23-40 2) The stress exponent(n) for cyclic creep is 10, which is K and T. Dunyak, Fully-Reversed Cyclic higher than that for static creep(n= 8). Both stress exponents posites at Elevated Temperature, for creep are much higher than the stress exponent for the creep 6D. S. Fox, ""Oxidation Kinetics of Enhanced SiC/SiC, ""Ceram. Eng. Sci of SiC fibers(n=1-2.5).27-29 The creep-rate-controlling pro Proc,16|S87-84(1995 cess of the composite may be the creep of the fibers that are Behavior of a Sic-Fiber/SiC Composite at Elevated Temperatures,"Compos. by the matrix (3) For the enhanced SiC/SiC composite, the creep rates in Sc. Technol.,57,1629-37(1997) XM. Mizuno, S Zhu, Y. Nagano, Y Sakaida, Y Kagawa, and M. Watanabe argon are evidently higher than hoe that in argon at a given tures, J.Amz Ceram Soc. 79[12]3065-77(1996). stress. However, the creep resistance in air is in argon, because of the oxidation effects on creep for the standard SiC/SiC composite. The oxidation resistance of the Temperature, Mater. Sci. Eng, A, A220, 100-108(1996). enhanced SiC/SiC composite is much improved. and Fatique behavion o t sicaversi c com. sateant, ngh .mayer at resep (4) The creep rate of the enhanced SiC/SiC composite is F Lamouroux, M. Steen, and J. L. Valles,"Uniaxial Tensile and Creep the time to rupture of the enhanced SiC/SiC composite is much perimental evans,F W. Okrand Soc, 14, 529-37 1 Composite:LEX longer than that of the standard SiC/SiC composite in air, be cause of the improved oxidation resistance of the enhanced SiC/SiC composite 3. W. Holmes, Y. Park, and J Creep and Creep Recov- (5) Although the creep rate of the enhanced SiC/SiC avior of a SiC-Fiber Sia N, Matrix Composite, J. Am. Ceram Soc., 76 posite in argon is higher than that of the standard SiC/Si I4X. Wu and J. W. Holmes, "Tensile Creep and Creep-Strain Recovery Be- composite in argon, the time to rupture of the enhanced SiC/ havior of silicon Carbide fiber/calcium aly SiC composite is still longer than that of the standard SiC/SiC 可mmum可 the enhanced SiC/SiC composite posite, "J. Am. Ceram.个mcm olmes, ""Influence of Stress-Ratio on the Elevated Temperature Fatigue of a SiC Fiber-Reinforced Si3N4 Composite, J. Am. Ceram. Soc., 74 7]163945(1991) (6) At the same maximum stress, the cyclic-fatigue life of 7M. Elahi. K. Liao. J Lesko K. Reifsnider. and w. Stinchcomb. " Elevatedargon is ∼40% of the original value, which is lower than that under fatigue. The reason for the difference of the limit modu￾lus between creep and fatigue has not yet been well understood. A possible interpretation is that the larger debonded length of the fibers (producing a larger strain in Fig. 3) under fatigue than under creep leads to failure of the enhanced fibers at the same maximum stress.12 Therefore, when the matrix cracks are similar in damage extent, fatigue failure occurs before creep failure. IV. Conclusion (1) The ultimate tensile strength (UTS) of the enhanced SiC/SiC composite is similar to that of the standard SiC/SiC composite; however, the strain at UTS of the enhanced SiC/SiC composite is much higher than that of the standard SiC/SiC composite. The Young’s modulus that is calculated from the linear portion of the curve is ∼89 GPa, which is much lower than that of the standard SiC/SiC composite at 1300°C (200 GPa). (2) The stress exponent (n) for cyclic creep is 10, which is higher than that for static creep (n 4 8). Both stress exponents for creep are much higher than the stress exponent for the creep of SiC fibers (n 4 1–2.5).27–29 The creep-rate-controlling pro￾cess of the composite may be the creep of the fibers that are constrained by the matrix. (3) For the enhanced SiC/SiC composite, the creep rates in argon are evidently higher than those in air; consequently, the time to rupture in air is longer than that in argon at a given stress. However, the creep resistance in air is much lower than in argon, because of the oxidation effects on creep for the standard SiC/SiC composite. The oxidation resistance of the enhanced SiC/SiC composite is much improved. (4) The creep rate of the enhanced SiC/SiC composite is much lower than that of the standard SiC/SiC composite, and the time to rupture of the enhanced SiC/SiC composite is much longer than that of the standard SiC/SiC composite in air, be￾cause of the improved oxidation resistance of the enhanced SiC/SiC composite. (5) Although the creep rate of the enhanced SiC/SiC com￾posite in argon is higher than that of the standard SiC/SiC composite in argon, the time to rupture of the enhanced SiC/ SiC composite is still longer than that of the standard SiC/SiC composite. This observation implies that the addition of a glassy phase in the matrix of the enhanced SiC/SiC composite increases creep rates but much improves the total creep time to rupture. (6) At the same maximum stress, the cyclic-fatigue life of the enhanced SiC/SiC composite is several orders of magnitude higher than that of the standard SiC/SiC composite in air, and it is also higher than that of the standard SiC/SiC composite in argon at 1300°C. (7) Creep and fatigue cracks are always found at the large pores among the fiber bundles. Slow crack propagation occurs in the matrix of the enhanced SiC/SiC composite. The crack propagation of the composite is controlled by both the bridged fibers and the matrix. Acknowledgment: The authors are grateful for the assistance of Mr. S. Ogawa in the mechanical tests. References 1 A. G. Evans, ‘‘Perspective on the Development of High-Toughness Ceram￾ics,’’ J. Am. Ceram. Soc., 73 [2] 187–206 (1990). 2 K. Goto and Y. Kagawa, ‘‘Fracture Behavior and Toughness of a Plain￾Woven SiC Fiber-Reinforced SiC Matrix Composite,’’ Mater. Sci. Eng., A, A211, 72–81 (1996). 3 R. Raj, ‘‘Fundamental Research in Structural Ceramics for Service Near 2000°C,’’ J. Am. Ceram. Soc., 76 [9] 2147–74 (1993). 4 A. G. Evans, ‘‘Design and Life Prediction Issues for High-Temperature Engineering Ceramics and Their Composites,’’ Acta Mater., 45 [1] 23–40 (1997). 5 M. Elahi, K. Liao, K. Reifsnider, and T. Dunyak, ‘‘Fully-Reversed Cyclic Fatigue Response of Ceramic Matrix Composites at Elevated Temperature,’’ Ceram. Eng. Sci. Proc., 16 [4] 75–85 (1995). 6 D. S. Fox, ‘‘Oxidation Kinetics of Enhanced SiC/SiC,’’ Ceram. Eng. Sci. Proc., 16 [5] 877–84 (1995). 7 S. Zhu, M. Mizuno, Y. Nagano, Y. Kagawa, and H. Kaya, ‘‘Tensile Creep Behavior of a SiC-Fiber/SiC Composite at Elevated Temperatures,’’ Compos. Sci. Technol., 57, 1629–37 (1997). 8 M. Mizuno, S. Zhu, Y. Nagano, Y. Sakaida, Y. Kagawa, and M. Watanabe, ‘‘Cyclic Fatigue Behavior of SiC/SiC Composite at Room and High Tempera￾tures,’’ J. Am. Ceram. Soc., 79 [12] 3065–77 (1996). 9 S. Zhu, Y. Kagawa, M. Mizuno, S. Guo, Y. Nagano, and H. Kaya, ‘‘In Situ Observation of Fatigue Crack Propagation of SiC-Fiber/SiC Composite at Room Temperature,’’ Mater. Sci. Eng., A, A220, 100–108 (1996). 10S. Zhu, M. Mizuno, Y. Kagawa, J. Cao, Y. Nagano, and H. Kaya, ‘‘Creep and Fatigue Behavior of SiC-Fiber/SiC Composite at High Temperatures,’’ Mater. Sci. Eng., A, A225, 69–77 (1997). 11F. Lamouroux, M. Steen, and J. L. Valles, ‘‘Uniaxial Tensile and Creep Behaviour of an Alumina Fiber-Reinforced Ceramic Matrix Composite: I. Ex￾perimental Study,’’ J. Eur. Ceram. Soc., 14, 529–37 (1994). 12A. G. Evans, F. W. Zok, and R. M. McMeeking, ‘‘Fatigue of Ceramic Matrix Composites,’’ Acta Metall. Mater., 43 [3] 859–75 (1995). 13J. W. Holmes, Y. Park, and J. W. Jones, ‘‘Tensile Creep and Creep Recov￾ery Behavior of a SiC-Fiber Si3N4 Matrix Composite,’’ J. Am. Ceram. Soc., 76 [5] 1281–93 (1993). 14X. Wu and J. W. Holmes, ‘‘Tensile Creep and Creep-Strain Recovery Be￾havior of Silicon Carbide Fiber/Calcium Aluminosilicate Matrix Ceramic Com￾posites,’’ J. Am. Ceram. Soc., 76 [10] 2695–700 (1993). 15C. H. Weber, J. P. A. Lofvander, and A. G. Evans, ‘‘Creep Anisotropy of a Continuous-Fiber-Reinforced Silicon Carbide/Calcium Aluminosilicate Com￾posite,’’ J. Am. Ceram. Soc., 77 [7] 1745–52 (1994). 16J. W. Holmes, ‘‘Influence of Stress-Ratio on the Elevated Temperature Fatigue of a SiC Fiber-Reinforced Si3N4 Composite,’’ J. Am. Ceram. Soc., 74 [7] 1639–45 (1991). 17M. Elahi, K. Liao, J. Lesko, K. Reifsnider, and W. Stinchcomb, ‘‘Elevated Fig. 16. Micrograph depicting the matrix crack in a specimen of the enhanced SiC/SiC composite crept at a load of 60 MPa in argon. Fig. 15. Young’s modulus, normalized by the value of the first load￾ing (E/E0), versus the time to rupture of the enhanced SiC/SiC com￾posite under constant load (60 MPa) in air and in argon at 1300°C. 2276 Journal of the American Ceramic Society—Zhu et al. Vol. 81, No. 9