Journal of the American Ceramic Society-Bertrand et al. Vol. 82. No 9 introduced in Eq.(1). This conclusion is supported by the The mechanical behavior of the minicomposites under ten- predictions of the mechanical behavior that are reported in the sion was unaffected by the presence of nanoscale multilayered following nterphases. The minicomposites exhibited the well-known (4 Assessment of the Minicomposite Characterization classical features of ceramic-matrix composites. Furthermore, the interfacial shear stress was not influenced by multilayering The force-deformation curves of the minicomposites were the interphase. A double deflection of the matrix cracks in the tistical parameters that are given in Table ll for various T val- Interphase/matrix and fiber/interphase interfaces was detected ues, using the previously mentioned model, which has been was observed after matrix cracking saturation detailed and validated elsewhere. 3,31 Figure 10 illustrates the The measured interfacial shear stresses (T 50 MPa)are d correlation that is generally observed between the pre ions and the experiments, which indicates that the materials comparable to those measured on Nicalon/Py C/SiC minicom- posites that have been made via I-CVI, in which debonding data given in Table I. For instance, for the minicomposites with Sio, and free carbon at the surface of the fibers. which con- single PyC layer, the agreement is excellent when T= 100 titutes the weakest link in the region that is located between MPa. This result supports the above-mentioned conclusion that he fiber and the matrix the T values given in Table I are underestimations The measured T values. as well as the mic ion, characterize rather weak interfacial bonding, in compari- V. Conclusion son to the features of the strong interfacial bonds in the SiC/Sic composites that are reinforced with treated Nicalon fiber. 4 The investigation of Hi-Nicalon/SiC minicomposites has The influence of various parameters was examined. The use multilayered interphases that are deposited viandres. of Hi-Nicalon fiber led to minicomposites由hchd sure-pulsed chemical vapor infiltration(P-CVI)can replace the the Nicalon-fiber-reinforced minicomposites. This difference in the mechanical behavior was attributed preponderantly to the ith(PyC/SiC)m microscale multilayered interphases that have stiffness of Hi-Nicalon fiber, as well as negligible residual en deposited via isothermal-isobaric chemical vapor infiltra- stresses as a result of comparable coefficients of thermal ex tion(I-CVI) pansion for the fiber and the matrix Initial tow twisting caused a higher density of cracks in the internal matrix of minicomposites. The twisting also caused ongitudinal matrix crac (a) Finally, predictions of the mechanical behavior of minicom- posites from constituent properties are consistent with the mea sured properties and the analysis 2150E7-0 Acknowledgments: The authors wish to thank B Humez for help with the mechanical tests. x. Bourrat for the tem observations. and R. Naslain for References R. Naslain,"Fiber-Matrix Interphases and Interfaces in Ceramic Matrix T=50 MPa H C. Cao E. Bischoff o. Sbaizero M. Ruhle. A. Evans D. B. Marshall and J.J. Brennan, " Effect of Interfaces on the Properties of Fiber-Reinforce Ceramics, "J. Am. Ceram Soc., 73[6] 1691-99 0.4 in Ceramic Matrix Min Deform盘tion(%) ess of 2-d Woven sic/s osites with Multilayered Interphases, "J. Am. Ceram. Soc., 79 14 R. P. Boisver, R. K. Hutter, and R J. Diefendorf, "Interface Manipulation in ed Mechanical Performa experiment bS. Goujard, P. Dupel, R. Pailler, and F. "Method of Manufac- bers and Matrix, and Material Obtained International Pat. No. wO 95/09136 SEP,1995 lain, F. Langlais, and R. Fedou, "The CVI-Processing of Ceramic Matrix Composites, "J. Plnys. C5, Sa prediction w.I. Lackey,"Review, Status and Future of the Chemical vapor Infiltration Process for Fabrication of Fiber-Reinforced Ceramic Composites, "Ceram. Eng Se.Pro,10[7-8577-84(1989) "T. M. Besmann, R. A. Lowden, D. P. Stinton, and T. L. Starr, "A Method for Rapid Chemical Vapor Infiltration of Ceramic Composites, "J. Phys. C5, Supp/ 1oH. C, Chang, T F. Morse, and B. W. Sheldon, "Minimizing Infiltration Mater. Process. Manyf. Sci., 2, 437(1994) w.A. Bryant, "" Producing Extended Area Deposits of Uniform Thickness 0,2 by a New Chemical Vapor Deposition Technique, J. Cryst, Growth, 35, 257 (1976) Deformation (90) Droillard, "Elaboration and Characterization of Sic-Matrix Composites Bordeaux, france, Jur curves of SiC/SiC minicomposites and x. bourret, "Strong Interface in CMCs, A 100/0)))and(b)a nanoscale mult 364109② (in Fr ) Ph D. Thesis N ultilayered Interphases, " Mater Res. Soc. Symp. Proc. Heurtevent, "Nanoscale(PyC/SiC). Multilayered Interphases-introduced in Eq. (1). This conclusion is supported by the predictions of the mechanical behavior that are reported in the following. (4) Assessment of the Minicomposite Characterization The force–deformation curves of the minicomposites were predicted from the constituent properties and flaw-strength sta￾tistical parameters that are given in Table II for various t val￾ues, using the previously mentioned model, which has been detailed and validated elsewhere.3,31 Figure 10 illustrates the good correlation that is generally observed between the pre￾dictions and the experiments, which indicates that the materials data and the analysis are pertinent. However, fitting was im￾proved with t values that were larger than the experimental data given in Table I. For instance, for the minicomposites with a single PyC layer, the agreement is excellent when t 4 100 MPa. This result supports the above-mentioned conclusion that the t values given in Table I are underestimations. V. Conclusion The investigation of Hi-Nicalon/SiC minicomposites has demonstrated that pyrocarbon/silicon carbide ((PyC/SiC)n) na￾noscale multilayered interphases that are deposited via pres￾sure-pulsed chemical vapor infiltration (P-CVI) can replace the single PyC interphase. A similar conclusion has been attained with (PyC/SiC)n microscale multilayered interphases that have been deposited via isothermal–isobaric chemical vapor infiltra￾tion (I-CVI).4 The mechanical behavior of the minicomposites under ten￾sion was unaffected by the presence of nanoscale multilayered interphases. The minicomposites exhibited the well-known classical features of ceramic-matrix composites. Furthermore, the interfacial shear stress was not influenced by multilayering the interphase. A double deflection of the matrix cracks in the interphase/matrix and fiber/interphase interfaces was detected via scanning electron microscopy, and complete debonding was observed after matrix cracking saturation. The measured interfacial shear stresses (t ≈ 50 MPa) are comparable to those measured on Nicalon/PyC/SiC minicom￾posites that have been made via I-CVI,3 in which debonding also occurred at the fiber/interphase interfaces. This typical location of the debond is attributed to the presence of a layer of SiO2 and free carbon at the surface of the fibers, which con￾stitutes the weakest link in the region that is located between the fiber and the matrix. The measured t values, as well as the microscope observa￾tion, characterize rather weak interfacial bonding, in compari￾son to the features of the strong interfacial bonds in the SiC/SiC composites that are reinforced with treated Nicalon fiber.4 The influence of various parameters was examined. The use of Hi-Nicalon fiber led to minicomposites that exhibited higher stresses, but a lower strain to failure, in comparison to that of the Nicalon-fiber-reinforced minicomposites. This difference in the mechanical behavior was attributed preponderantly to the stiffness of Hi-Nicalon fiber, as well as negligible residual stresses as a result of comparable coefficients of thermal ex￾pansion for the fiber and the matrix. Initial tow twisting caused a higher density of cracks in the internal matrix of minicomposites. The twisting also caused longitudinal matrix cracks. Finally, predictions of the mechanical behavior of minicom￾posites from constituent properties are consistent with the mea￾sured properties and the analysis. Acknowledgments: The authors wish to thank B. Humez for help with the mechanical tests, X. Bourrat for the TEM observations, and R. Naslain for valuable discussion. References 1 R. Naslain, “Fiber–Matrix Interphases and Interfaces in Ceramic Matrix Composites Processed by CVI,” Compos. Interfaces, 1 [3] 253–86 (1993). 2 H. C. Cao, E. Bischoff, O. Sbaizero, M. Ru¨hle, A. G. Evans, D. B. Marshall, and J . J. Brennan, “Effect of Interfaces on the Properties of Fiber-Reinforced Ceramics,” J. Am. Ceram. Soc., 73 [6] 1691–99 (1990). 3 N. Lissart and J. Lamon, “Damage and Failure in Ceramic Matrix Mini￾composites: Experimental Study and Model,” Acta Mater., 45 [3] 1025 (1997). 4 C. Droillard and J. Lamon, “Fracture Toughness of 2-D Woven SiC/SiC CVI-Composites with Multilayered Interphases,” J. Am. Ceram. Soc., 79 [4] 849–58 (1996). 5 R. P. Boisver, R. K. Hutter, and R. J. Diefendorf, “Interface Manipulation in Ceramic Matrix Composites for Improved Mechanical Performance,” Proc. Jpn.—U.S. Conf. Compos. Mater., 4, 789 (1989). 6 S. Goujard, P. Dupel, R. Pailler, and F. Heurtevent, “Method of Manufac￾turing a Composite Material with Lamellar Interphase between Reinforced Fi￾bers and Matrix, and Material Obtained,” International Pat. No. WO 95/09136, S.E.P., 1995. 7 R. Naslain, F. Langlais, and R. Fedou, “The CVI-Processing of Ceramic Matrix Composites,” J. Phys. C5, Suppl., 50, 191 (1989). 8 W. J. Lackey, “Review, Status and Future of the Chemical Vapor Infiltration Process for Fabrication of Fiber-Reinforced Ceramic Composites,” Ceram. Eng. Sci. Proc., 10 [7–8] 577–84 (1989). 9 T. M. Besmann, R. A. Lowden, D. P. Stinton, and T. L. Starr, “A Method for Rapid Chemical Vapor Infiltration of Ceramic Composites,” J. Phys. C5, Suppl., 50, 229 (1989). 10H. C. Chang, T. F. Morse, and B. W. Sheldon, “Minimizing Infiltration Times during the Initial Stage of Isothermal Chemical Vapor Infiltration,” J. Mater. Process. Manuf. Sci., 2, 437 (1994). 11W. A. Bryant, “Producing Extended Area Deposits of Uniform Thickness by a New Chemical Vapor Deposition Technique,” J. Cryst. Growth, 35, 257 (1976). 12C. Droillard, “Elaboration and Characterization of SiC-Matrix Composites with Multilayered C/SiC Interphase” (in Fr.); Ph.D. Thesis No. 913. University of Bordeaux, France, June 19, 1993. 13C. Droillard, J. Lamon, and X. Bourrat, “Strong Interface in CMCs; A Condition for Efficient Multilayered Interphases,” Mater. Res. Soc. Symp. Proc., 365, 371–76 (1995). 14F. Heurtevent, “Nanoscale (PyC/SiC)n Multilayered Interphases— Fig. 10. Comparison of the predicted and experimental force– deformation curves of SiC/SiC minicomposites with (a) a single PyC sublayer ((100/0)1) and (b) a nanoscale multilayered interphase ((20/50)10). 2472 Journal of the American Ceramic Society—Bertrand et al. Vol. 82, No. 9