Journal of the American Ceramic Society-Bertrand et al. Vol. 82. No 9 dinal Young s modulus(EL). The Poisson's ratio(v) has been assumed to be equal to that estimated for the Nicalon fiber. 9 The shear modulus (G)has been estimated from E and v. The statistical parameters have been derived from the distribution of strength data, using the maximum-likelihood estimator. 20 coho The longitudinal coefficient of thermal expansion(a) has been determined from the comparison of the expansion of a single filament to that of a reference sintered SiC tube at tem- peratures of 293-1373 K. The filament and the tube were mounted in parallel in a furnace that was located in the cham- ber of a scanning electron microscopy(SEM) system(Mod 840, JEOL, Tokyo, Japan). 21 The fiber has an isotropic micro- structure, as already mentioned; thus, the transverse coefficient of thermal expansion(ar) is assumed to be equal to or. B cause the microstructures of the sic Hi-Nicalon fiber and the SiC interphase sublayer are similar 4, 18(they both consist of M SiC nanocrystallites and free carbon), the properties of the SiC interphase are assumed to be equal to those of Sic Hi-nicalon fiber. The statistical parameters that are pertinent to the p-Cv o SiC matrix have been derived from the statistical distributions of the matrix fragmentation stresses that were measured on minicomposites, using a tensile device in the chamber of the SEM equipment pq· Shut-off valve (2) Microstructural Characterization Mass flowmeter The chemical characterization of the sic hi-Nicalon fibers has been reported elsewhere. 8 The surface analysis of the Hi- Nicalon fibers has been performed using Auger electron spe Fig. 1. Schematic diagram showing the P-CV troscopy(AES)(Model 310F, VG Microscopes, West Sussex process the SiC brous preform; nace;“3," source gase , fi: vacuum at 1223 k in the reaction chamber. to reproduce the heated tank,“6,” conditions that were experienced by the fiber prior to inter ase deposition. The microstructure of PyC and SiC in the interphase has been assessed using(1) optical microscopy (Model MeF3, Reichert-Jung) in polarized light, on polished The(PyC/SiC)n multilayered interphases exhibited the fol- cross sections of minicomposites, to measure the extinction wing features: (i)the PyC and SiC sublayers had a uniform angle(Ae, which characterizes the PyC anisotropy (only per- hickness; (ii) the first sublayer that was deposited on the fiber formed on minicomposites with large interphase size of always consisted of PyC; (iii)the number of(Py C/SiC)bilayers C)),3(ii)X-ray diffractometry(XRD)(Model D5000, Sie- (n) was varied in the range of 10-30;(iv) the thickness of mens, Karlsruhe, Germany) to determine the apparent mean the PyC and SiC sublayers(respectively noted e(Pvc) and and grain size(noted as Lu) of the b-sic crystalline phase, and e(sic) was 3-20 nm and 10-50 nm, respectively; and (v) the (iii) SEM(Model S-4500, Hitachi, Tokyo, Japan) and trans- interphase thickness was 100-1800 nm. For comparison pur mission electronic microscopy(TEM)(Model CM30, Philips poses, a batch of reference minicomposites with a single Research Laboratories, Eindhoven, The Netherlands). For op nte (100 nm thick)also was prepared (Table I). The a microscopy studies, the minicomposite cross sections ber volume fraction in the minicomposites was%(+5% were polished using standard metallographic techniques. For The properties of the minicomposites constituents are listed the SEM observations, certain minicomposites were etched us- n Table Il. The mechanical properties of the Hi-Nicalon fibers Murakami's reagent, to get a selective attack of the SiC on single filaments(gauge lengths of 10, 25, and 65 mm). The palladium or carbon. The TEM analyses have been performed fiber has an isotropic microstructure; therefore, the transverse on cross sections of the minicomposites. The preparation of the Youngs modulus(Er) is assumed to be equal to the longitu- thin foils is detailed elsewhere Table L. Batches and Mechanical Properties of the SiC/(PyC/SiC)/SiC Minicomposites Mechanical properties nterfacial shear stress, T(MPa Fq(4) (10001100 41342710.091710.6911618 95 (3/10)1 810.17 1684 38 1041345820.131810.9 7535 10101040349730.121660.8157 343870.161530. 26710 37 560.111680 2875011036 0501039351880.121950.832085017535 (1050)30 PyC sut he stress 温邮你叫The (PyC/SiC)n multilayered interphases exhibited the fol￾lowing features: (i) the PyC and SiC sublayers had a uniform thickness; (ii) the first sublayer that was deposited on the fiber always consisted of PyC; (iii) the number of (PyC/SiC) bilayers (n) was varied in the range of 10–30; (iv) the thickness of the PyC and SiC sublayers (respectively noted e (P yC) and e(SiC)) was 3–20 nm and 10–50 nm, respectively; and (v) the interphase thickness was 100–1800 nm. For comparison pur￾poses, a batch of reference minicomposites with a single PyC interphase (100 nm thick) also was prepared (Table I). The fiber volume fraction in the minicomposites was ∼40% (±5%). The properties of the minicomposites constituents are listed in Table II. The mechanical properties of the Hi-Nicalon fibers have been measured using tensile tests, at ambient temperature, on single filaments (gauge lengths of 10, 25, and 65 mm). The fiber has an isotropic microstructure;18 therefore, the transverse Young’s modulus (ET) is assumed to be equal to the longitu￾dinal Young’s modulus (EL). The Poisson’s ratio (n) has been assumed to be equal to that estimated for the Nicalon fiber.19 The shear modulus (G) has been estimated from E and n. The statistical parameters have been derived from the distribution of strength data, using the maximum-likelihood estimator.20 The longitudinal coefficient of thermal expansion (aL) has been determined from the comparison of the expansion of a single filament to that of a reference sintered SiC tube at tem￾peratures of 293–1373 K. The filament and the tube were mounted in parallel in a furnace that was located in the cham￾ber of a scanning electron microscopy (SEM) system (Model 840, JEOL, Tokyo, Japan).21 The fiber has an isotropic micro￾structure, as already mentioned; thus, the transverse coefficient of thermal expansion (aT) is assumed to be equal to aL. Be￾cause the microstructures of the SiC Hi-Nicalon fiber and the SiC interphase sublayer are similar14,18 (they both consist of SiC nanocrystallites and free carbon), the properties of the SiC interphase are assumed to be equal to those of SiC Hi-Nicalon fiber. The statistical parameters that are pertinent to the P-CVI SiC matrix have been derived from the statistical distributions of the matrix fragmentation stresses that were measured on minicomposites, using a tensile device in the chamber of the SEM equipment.22 (2) Microstructural Characterization The chemical characterization of the SiC Hi-Nicalon fibers has been reported elsewhere.18 The surface analysis of the Hi￾Nicalon fibers has been performed using Auger electron spec￾troscopy (AES) (Model 310F, VG Microscopes, West Sussex, U.K.). The as-received Hi-Nicalon fibers were heated under vacuum at 1223 K in the reaction chamber, to reproduce the conditions that were experienced by the fiber prior to inter￾phase deposition. The microstructure of PyC and SiC in the interphase has been assessed using (i) optical microscopy (Model MeF3, Reichert–Jung) in polarized light, on polished cross sections of minicomposites, to measure the extinction angle (Ae, which characterizes the PyC anisotropy (only per￾formed on minicomposites with large interphase size of PyC)),23 (ii) X-ray diffractometry (XRD) (Model D5000, Sie￾mens, Karlsruhe, Germany) to determine the apparent mean grain size (noted as L111) of the b-SiC crystalline phase, and (iii) SEM (Model S-4500, Hitachi, Tokyo, Japan) and trans￾mission electronic microscopy (TEM) (Model CM30, Philips Research Laboratories, Eindhoven, The Netherlands). For op￾tical microscopy studies, the minicomposite cross sections were polished using standard metallographic techniques. For the SEM observations, certain minicomposites were etched us￾ing Murakami’s reagent, to get a selective attack of the SiC layers, and then they were coated with a thin layer of gold– palladium or carbon. The TEM analyses have been performed on cross sections of the minicomposites. The preparation of the thin foils is detailed elsewhere.24 Fig. 1. Schematic diagram showing the P-CVI apparatus used to process the SiC/SiC minicomposites. Legend is as follows: “1,” fi￾brous preform; “2,” furnace; “3,” source gases; “4,” drying oven; “5,” heated tank; “6,” liquid nitrogen trap; and “7,” vacuum pump. Table I. Batches and Mechanical Properties of the SiC/(PyC/SiC)n/SiC Minicomposites Batch Interphase parameters† Mechanical properties‡ Interfacial shear stress, t (MPa) e(PyC) (nm) e(SiC) (nm) n Vf (%) Ec (GPa) Fe (N) ee (%) FR (N) eR (%) DlRT (mm) sS (MPa) lS (mm) Eq. (1) Eq. (3) Eq. (4) (100/0)1 § 100 0 1 41 342 71 0.09 171 0.69 11 618 95 50 34 29 (3/10)10 3 10 10 52 329 81 0.17 164 0.69 16 840 90 40 38 32 (3/30)10 3 30 10 41 355 95 0.14 158 0.64 17 678 107 55 39 35 (3/50)10 3 50 10 41 345 82 0.13 181 0.92 24 615 75 35 49 44 (10/10)10 10 10 10 40 349 73 0.12 166 0.81 57 600 185 32 19 17 (10/50)10 10 50 10 45 343 87 0.16 153 0.72 26 710 98 37 34 29 (20/10)10 20 10 10 44 336 56 0.11 168 0.92 28 750 110 36 32 28 (20/30)10 20 30 10 40 350 69 0.12 173 0.77 22 95 (20/50)10 20 50 10 39 351 88 0.12 195 0.83 20 850 175 35 25 22 (10/50)30 10 50 30 41 346 76 0.14 165 0.77 22 817 125 70 44 40 † Interphase parameters include the thickness of each PyC sublayer (e(PyC)), the thickness of each SiC sublayer (e(SiC)), and the number of bilayers (n). ‡ Mechanical properties include the fiber volume fraction (Vf ), Young’s modulus (Ec), the force and strain at the proportional limit (Fe and ee, respectively) and at failure (FR and eR, respectively), the maximum permanent elongation at zero load (DlRT), the stress at saturation (sS), and the matrix spacing distance (lS). § Reference interphase. 2466 Journal of the American Ceramic Society—Bertrand et al. Vol. 82, No. 9