R.R. Naslain et al/ Solid State ionics 141-142(2001)541-548 ites compared to that of similar minicomposites pre- multilayered interphases can actually be deposited pared with one single 100-nm pyrocarbon interphase. within a porous body by P-CVI. Further, the concept A TEM picture of such a multilayered interphase is of an interphase strongly bonded to the fiber surface shown in Fig. 6. The interphase was designed to and that of a multilayered interphase combining me- have an overall thickness of 700 nm (its thickness chanical fuses and glass formers, effectively increase being actually close to 600 nm), n=10(PyC-Sic) the oxidation resistance of SiC/SiC minicomposites sequences and elementary sublayer thicknesses of 20 nm for pyrocarbon and 50 nm for SiC(C). The fibers ere pretreated (proprietary treatment of Snecma) before depositing the interphase in order to achieve a 3.3. P-CV as a way to engineer self-healing matri- strong bonding between the fiber surface and the ces first pyrocarbon sublayer. Under such a condition cracks are again nicely deflected by each pyrocarbon The multilayering concept has been extended to sublayer [20]. The lifetimes of the minicomposites, the matrix itself through the infiltration of boron- as assessed through tensile static fatigue tests in air doped pyrocarbon mechanical fuses and various glass at 700@C under constant load (at a stress level higher formers, with a view to entrap oxygen 'far'from the than the proportional limit to microcrack the Sic fibers as it diffuses along the microcrack network matrix)were as follows: (i2 h for the minicompos- formed under load and, hence, to increase the oxida- ite with a 100-nm single Pyc interphase produced tion resistance from as-received fibers, (ii) 100 h for their counter- An example of such highly engineered composite part fabricated from pretreated fibers and (ii) 150 h is shown in Fig. 7. This minicomposite was fabri for the minicomposite prepared with the multilayered cated from a carbon fiber tow, from BX3-C H -H (PyC-SiC)o interphase from pretreated fibers. Simi- precursor for the boron-doped pyrocarbon(C(B)) lar results were obtained for minicomposites in which mechanical fuse and boron carbide (referred to as pyrocarbon was replaced by BN. Hence, complex Bu3 C2) glass former, on the one hand, and from MTS-H, precursor for the Sic glass former, on the other hand. The two first(C(B)/B13C2/C(B)/SiC) sequences(S, and S,)have a thickness of about 1 Matrix um, whereas the two others, S, and S4, display a ( Fiber surface thickness of 4 and 6 um, respectively(Fig. 7a, b) Each carbon fiber is actually coated by the S, and S, multilayers(single arrow) only(Fig. 7c). The third sequence coats a group of fibers( double arrow)and, finally, the last sequence(S4) acts as a multilayered seal-coating for the whole fiber tow(triple arrow) The lifetime of the minicomposite(tensile tests in air at 700%C under a constant 250-MPa stress). i.e. >70 h, was considerably improved with respect to that(1 h) of a conventional C/PyC/SiC minicomposite pyrocarbon interphase. Similar re- sults were obtained for real composites fabricated with a complex 2.5D fiber architecture [6]. These data show again how flexible and effective the p-cvi Fig. 6. Multiple matrix crack deflection in a Hi-Nicalon/(Pyc process is to engineer non-oxide ceramic matrices in SiC),o/Sic minicomposite prepared by P-CVI from propane and CMCs. In these matrices, oxygen diffusing along MIS-H ursors at T=950C under the following condi- extremely tortuous paths(tortuosi from and a-1/4 for the sic(C)), and P=3 kPa, Ig-2 s and a-6 the occurrence of multiple crack deflections)is en- for the SiC matrix(TEM picture from a longitudinal thin foil) trapped by forming liquid/ glass oxide phases with546 R.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) ites compared to that of similar minicomposites pre￾pared with one single 100-nm pyrocarbon interphase. A TEM picture of such a multilayered interphase is shown in Fig. 6. The interphase was designed to have an overall thickness of 700 nm its thickness Ž being actually close to 600 nm ,.Ž. ns10 PyC–SiC sequences and elementary sublayer thicknesses of 20 nm for pyrocarbon and 50 nm for SiC C . The fibers Ž . were pretreated proprietary treatment of Snecma Ž . before depositing the interphase in order to achieve a strong bonding between the fiber surface and the first pyrocarbon sublayer. Under such a condition, cracks are again nicely deflected by each pyrocarbon sublayer 20 . The lifetimes of the minicomposites, w x as assessed through tensile static fatigue tests in air at 7008C under constant load at a stress level higher Ž than the proportional limit to microcrack the SiC matrix were as follows: i 2 h for the minicompos- . Ž. ite with a 100-nm single PyC interphase produced from as-received fibers, ii 100 h for their counter- Ž . part fabricated from pretreated fibers and iii 150 h Ž . for the minicomposite prepared with the multilayered Ž . PyC–SiC interphase from pretreated fibers. Simi- 10 lar results were obtained for minicomposites in which pyrocarbon was replaced by BN. Hence, complex Fig. 6. Multiple matrix crack deflection in a Hi-NicalonrŽPyC– SiC. 10 rSiC minicomposite prepared by P-CVI from propane and MTS–H precursors at Ts9508C under the following condi- 2 tions: Ps3 kPa, t s5 s for pyrocarbon; Ps5 kPa, t s1 s R R and a s1r4 for the SiC C ; and Ž . Ps3 kPa, tRs2 s and a s6 for the SiC matrix TEM picture from a longitudinal thin foil . Ž . multilayered interphases can actually be deposited within a porous body by P-CVI. Further, the concept of an interphase strongly bonded to the fiber surface and that of a multilayered interphase combining me￾chanical fuses and glass formers, effectively increase the oxidation resistance of SiCrSiC minicomposites w x 7 . 3.3. P-CVI as a way to engineer self-healing matri￾ces The multilayering concept has been extended to the matrix itself through the infiltration of boron￾doped pyrocarbon mechanical fuses and various glass formers, with a view to entrap oxygen ‘far’ from the fibers as it diffuses along the microcrack network formed under load and, hence, to increase the oxida￾tion resistance. An example of such highly engineered composite is shown in Fig. 7. This minicomposite was fabri￾cated from a carbon fiber tow, from BX –C H –H 3 x y 2 precursor for the boron-doped pyrocarbon C B Ž Ž .. mechanical fuse and boron carbide referred to as Ž B C glass former, on the one hand, and from . 13 2 MTS–H precursor for the SiC glass former, on the 2 other hand. The two first C B ŽŽ. Ž. . rB C13 2rC B rSiC sequences S and S have a thickness of about 1 Ž . 1 2 mm, whereas the two others, S and S , display a 3 4 thickness of 4 and 6 mm, respectively Fig. 7a,b . Ž . Each carbon fiber is actually coated by the S and S 1 2 multilayers single arrow only Fig. 7c . The third Ž .Ž . sequence coats a group of fibers double arrow and, Ž . finally, the last sequence S acts as a multilayered Ž . 4 seal-coating for the whole fiber tow triple arrow . Ž . The lifetime of the minicomposite tensile tests in air Ž at 7008C under a constant 250-MPa stress , i.e. . )70 h, was considerably improved with respect to that 1Ž h of a conventional C . rPyCrSiC minicomposite with one single pyrocarbon interphase. Similar re￾sults were obtained for real composites fabricated with a complex 2.5D fiber architecture 6 . These w x data show again how flexible and effective the P-CVI process is to engineer non-oxide ceramic matrices in CMCs. In these matrices, oxygen diffusing along extremely tortuous paths tortuosity resulting from Ž the occurrence of multiple crack deflections is en- . trapped by forming liquidrglass oxide phases with