Joumal of the American Ceramic Sociery--Bertrand et al. Vol 84. No 4 properties, including an improved lifetime in static fatigue in air at C. Droillard and J. Lamon, "Fracture of 2D Woven siC/iC cvi 700C. The effect of strong interfaces operates through the opposites with Multilayered Interphases, deviation of the crack within the PyC sublayers and limited Nonlinear Stress-Strain Behavior in Micro Influence of Interfacial pa- debonding and crack-opening displacement, which increase the s,"lm.J. fract.,82,297-316(1996) oxygen propagation path and decrease the quantity of oxygen Lissart and J, Lamon,"Damage and Failure in Ceramic Matrix Minicompos- migrating toward the fibers. ites: Experimental Study and Model, Acta Mater, 45 [3]1025-44(1997). Lamon, R. Naslain, E. La The mechanical behavior of the minicomposites under tension Besmann, "Properties of Multilayered Interphases in SiC/SiC CVI Comp exhibited the features previously observed on SiC/SiC composites Weak And"Strong Interfaces, " J. Am Ceram Soc., 81[9]2315-26(199 haracterized by a wide curved domain and large strains at Composites with Multilayered (Byc-sSicn, lntesolh sta ic Fatigue of 2D SicI/sic saturation of matrix cracking that were close to ultimate failure. As pointed out in previous papers, these features were not affected by trand, F. Germain, R Paillet, and J. Lamon,Thermomechanical Behavior of 2D-SiC/SiC Composites with Nanoscale Multilayered(PyC-SiC)m Interphases, the presence of multilayered fiber coatings, including stiff Sic Ad Compos. Lett, 8(61315-21(1999) sublayers. Deflection of the matrix cracks was observed in the Pyc er, and J. Lamon, "Interfacial Behavior the microstructure of the fiber surface that consisted of a thin layer for fficicont mdlilavered n terd a. o mite s ren sinte pre in Gs a g ng5 of free carbon. Such a microstructure was previously detected in SiC/SiC composites reinforced with treated Nicalon fibers (NL 202)and was shown to lead to strong fiber/coating bonds The t values estimated using various methods were comparable with(Pyrocarbon-Sic) Nanoscale Multilayered Interphases,"JAmCeramSoc Nicalon NL 202 fibers Bertrand, K. Pastor, F. Ronchail, and J. Lamon, "Determination of mechanical Statistical Properties of Hi-Nicalon Fibers and Tow acques,A. Guette, F. Langlais, and X. Bourrat,"Characterization of Predictions of the mechanical behavior of minicomposites from SiC/C(B)SiC Microcomposites by Transmission Electron Microscopy,J. Mater. constituent properties were in good agreement with experimental J. Lamon, F. Rebillat, and A. G. Evans, ""Microcomposite Test Procedure for results and allowed the extraction of consistent T values Evaluating the Soc,78[2]401-405(1995) Marshall, B. N. Cox, and A G. Evans, "The Mechanics of Matrix Cracking Acknowledgments eston, G. A. Cooper, and A, Kelly, ""Single and Multiple Fracture The authors wish to thank B Humez for help with mechanical testing, P. Forio for Properties of Fiber composites, Conference Proceedings of the National ph help with computations, x. Bourrat for help with TEM observations, S. Goujard for valuable discussion, and J. Forget and C. Dupouy for preparation of the manuscript. N. Morscher,"Tensile Stress Rupture of SiC,SiCm Minicomposites with Carbon and Boron Nitride Interphases at Elevated Temperatures in Air,J.Am Ceram. Soc.,80[S]2029-42(1997) ces S. Bertrand, O. Boisron, R. Pailler, J. Lamon, and R. Naslain, "(PyC/SiC), and (BN/SiC), Nanoscale-Multilayered Interphases by Pressure-Pulsed CVI, Pp. 321-24 R. Naslain, "Fiber-Matrix Interphases and Interfaces in Ceramic Matrix Compos- in Key Engineering Materials, Vols. 164-165. Trans Tech Publications, Aedermanns- ites Processed by CVI, Compos. Interfaces, 1 [3] 253-86(1993). Droillard,"Elaboration et Caracterisation de Composites a Matrice SiC et 2 H. M. Yun and J. A. DiCarlo, "High Temperature Contraction Behavior of Interphase Sequence C/SiC Ph D. Thesis No. 913. University of Bordeaux, Polymer-Derived SiC Fibers, "Ceram. Eng. Sci. Proc., 18 3]126(1997) Bordeaux, France, 1993 2S. Bertrand and R. Pailler, unpublished work.properties, including an improved lifetime in static fatigue in air at 700°C. The effect of strong interfaces operates through the deviation of the crack within the PyC sublayers and limited debonding and crack-opening displacement, which increase the oxygen propagation path and decrease the quantity of oxygen migrating toward the fibers. The mechanical behavior of the minicomposites under tension exhibited the features previously observed on SiC/SiC composites with strong fiber/coating bonds. The force–deformation curve was characterized by a wide curved domain and large strains at saturation of matrix cracking that were close to ultimate failure. As pointed out in previous papers, these features were not affected by the presence of multilayered fiber coatings, including stiff SiC sublayers. Deflection of the matrix cracks was observed in the PyC sublayers within the coating. This deflection pattern was related to the microstructure of the fiber surface that consisted of a thin layer of free carbon. Such a microstructure was previously detected in SiC/SiC composites reinforced with treated Nicalon fibers (NL 202) and was shown to lead to strong fiber/coating bonds. The t values estimated using various methods were comparable to those measured on SiC/SiC composites reinforced with treated Nicalon NL 202 fibers. Initial tow twisting exerted a certain influence that was reflected by the presence of a higher density of cracks in the internal matrix. Predictions of the mechanical behavior of minicomposites from constituent properties were in good agreement with experimental results and allowed the extraction of consistent t values. Acknowledgments The authors wish to thank B. Humez for help with mechanical testing, P. Forio for help with computations, X. Bourrat for help with TEM observations, S. Goujard for valuable discussion, and J. Forget and C. Dupouy for preparation of the manuscript. References 1 R. Naslain, “Fiber-Matrix Interphases and Interfaces in Ceramic Matrix Compos￾ites Processed by CVI,” Compos. Interfaces, 1 [3] 253–86 (1993). 2 C. Droillard, “Elaboration et Caracte´risation de Composites a` Matrice SiC et Interphase Se´quence´e C/SiC”; Ph.D. Thesis No. 913. University of Bordeaux, Bordeaux, France, 1993. 3 C. Droillard and J. Lamon, “Fracture Toughness of 2D Woven SiC/SiC CVI Composites with Multilayered Interphases,” J. Am. Ceram. Soc., 79, 849–58 (1996). 4 L. Guillaumat and J. Lamon, “Fracture Statistics Applied to Modelling the Nonlinear Stress–Strain Behavior in Microcomposites: Influence of Interfacial Pa￾rameters,” Int. J. Fract., 82, 297–316 (1996). 5 N. Lissart and J. Lamon, “Damage and Failure in Ceramic Matrix Minicompos￾ites: Experimental Study and Model,” Acta Mater., 45 [3] 1025–44 (1997). 6 F. Rebillat, J. Lamon, R. Naslain, E. Lara-Curzio, M. K. Ferber, and T. M. Besmann, “Properties of Multilayered Interphases in SiC/SiC CVI Composites With ‘Weak’ And ‘Strong’ Interfaces,” J. Am. Ceram. Soc., 81 [9] 2315–26 (1998). 7 S. Pasquier, J. Lamon, and R. Naslain, “Tensile Static Fatigue of 2D SiC/SiC Composites with Multilayered (PyC–SiC)N Interphases at High Temperatures in Oxidizing Atmosphere,” Composites—Part A, 29A, 1157–64 (1998). 8 S. Bertrand, F. Germain, R. Pailler, and J. Lamon, “Thermomechanical Behavior of 2D-SiC/SiC Composites with Nanoscale Multilayered (PyC–SiC)n Interphases,” Adv. Compos. Lett., 8 [6] 315–21 (1999). 9 K. L. Rugg, R. E. Tressler, and J. Lamon, “Interfacial Behavior During Creep of Microcomposites at Elevated Temperature,” J. Eur. Ceram. Soc., 9, 2297–303 (1999). 10C. Droillard, J. Lamon, and X. Bourrat, “Strong Interface in CMCs, a Condition for Efficient Multilayered Interphases,” Mater. Res. Soc. Proc., 365, 371–76 (1995). 11F. Heurtevent, “Nanoscale (PyC–SiC)n Multilayered Interphases–Application as Interphase in Thermostructural Composites” (in Fr.); Ph.D. Thesis No. 1476. University of Bordeaux, Bordeaux, France, 1996. 12S. Bertrand, P. Forio, R. Pailler, and J. Lamon, “Hi-Nicalon/SiC Minicomposites with (Pyrocarbon–SiC)n Nanoscale Multilayered Interphases,” J. Am. Ceram., Soc., 82 [9] 2465–73 (1999). 13S. Bertrand, K. Pastor, F. Ronchail, and J. Lamon, “Determination of Mechanical and Statistical Properties of Hi-Nicalon Fibers and Tows”; to be published. 14S. Jacques, A. Guette, F. Langlais, and X. Bourrat, “Characterization of SiC/C(B)/SiC Microcomposites by Transmission Electron Microscopy,” J. Mater. Sci, 32, 2969 (1997). 15J. Lamon, F. Rebillat, and A. G. Evans, “Microcomposite Test Procedure for Evaluating the Interface Properties of Ceramic Matrix Composites,” J. Am. Ceram. Soc., 78 [2] 401–405 (1995). 16D. B. Marshall, B. N. Cox, and A. G. Evans, “The Mechanics of Matrix Cracking in Brittle-Matrix Fiber Composites,” Acta Metall., 33 [11] 2013 (1985). 17J. Aveston, G. A. Cooper, and A. Kelly, “Single and Multiple Fracture”; in Properties of Fiber Composites, Conference Proceedings of the National Physical Laboratory, Vol. 15. IPC Science and Technology Press Ltd., Surrey, U.K., 1971. 18G. N. Morscher, “Tensile Stress Rupture of SiCf/SiCm Minicomposites with Carbon and Boron Nitride Interphases at Elevated Temperatures in Air,” J. Am. Ceram. Soc., 80 [8] 2029–42 (1997). 19S. Bertrand, O. Boisron, R. Pailler, J. Lamon, and R. Naslain, “(PyC/SiC)n and (BN/SiC)n Nanoscale-Multilayered Interphases by Pressure-Pulsed CVI”; pp. 321–24 in Key Engineering Materials, Vols. 164–165. TransTech Publications, Aedermanns￾dorf, Switzerland, 1999. 20H. M. Yun and J. A. DiCarlo, “High Temperature Contraction Behavior of Polymer-Derived SiC Fibers,” Ceram. Eng. Sci. Proc., 18 [3] 126 (1997). 21S. Bertrand and R. Pailler; unpublished work. M 794 Journal of the American Ceramic Society—Bertrand et al. Vol. 84, No. 4