l818 Journal of the American Ceramic Society-Sato et al. Vol. 85. No. 7 Table ll. Strength and Elastic Modulus of Row Fibers and Coated Fibers As-fabricated fiber Coated fiber Strength(GPa) E(GPa) Strength(GPa) E(GPa) Fiber 1.71 0.38 0.56 157 2.66 0.38 91 37 2.56 0.65 216 96 TsD is standard deviation Figure 4(b)shows a TEM image of the fiber-matrix interface of carbon Silicon and nitrogen were detected at a depth from 15 nm, non-oxidized composite Il. The interface was a monolayer 20 nm increased slowly, and reached saturation at a depth of 50 nm, where thick, with no crystal lattice, and it exhibited an obscure ring the Si-N-C matrix was located. Seven of eight samples showed result pattern under NBED. Under EDS, the peak intensities of carbon similar to those in Fig. 5(b), the one exception had a boron-containin and silicon on the interface were, respectively, about three times surface layer. In the case of composite Il, the pullout fiber had a and one-third that on the si-N-C matrix; therefore, the interface surface layer 80-110 nm thick, which consisted of carbon and a small consisted of carbon and a small amount of silicon amount of boron. The surface layer of the fractured matrix consisted Figure 4(c) shows the fiber-matrix interface of composite I mainly of carbon and was <15 nm thick. No boron-containing layer oxidized at 1523 K for 10 h The observed point located just under was found on the matrix surface the oxygen-sealing layer is discussed later. The fiber, Ll, and L2 exhibited no change with oxidation. L3 and the matrix showed bvious changes: (i) bright bubbles(B in Fig 4(c), where L3 was (3) Oxidation Resistance of Composites located before oxidation; and (ii) a layer that formed at the fiber The mechanical properties of the as-fabricated composites are side of the matrix(O in Fig 4(c). Under EDS, mainly carbon and shown in Table Ill. All the composites had dense matrices( total silicon were detected on LI and L2. On L3 and O, the peak porosity of <1l vol%), high fiber content(r> 45 vol%), nonbrittle intensity of oxygen was about twice that on LI and L2: on the fracture, and high strength. Table IV shows the strengths of the matrix(M) and the fiber (f), no oxygen was detected. opposites after oxidation Composites I and ll retained high strength, Figure 5 shows the AES spectra of the fractured surfaces of 0.8-0.6 GPa, and maintained their nonbrittle fracture, even after composite I. The AES spectra(Fig. 5)are shown instead of AE xidation at 1523 K for 100 h. The fracture surfaces of oxidized lepth profiles to illustrate the depth distribution of boron, because the composites I and Il had a region 0.1-0.4 mm wide around their aES depth profile cannot show boron content, as explained for Fig. 2 periphery, in which fibers failed along a matrix crack plane. The Figure 5(a) is the typical spectra of the surface of a pullout fiber. The center region of the fracture surface showed many pullout fibers fiber surface consisted mainly of carbon and a small amount of Reference composite r showed a large degradation in strength, 0.2 nitrogen and boron. Silicon was detected from a depth of 15 nm. GPa, and brittle fracture after oxidation. No pullout fiber was Silicon and nitrogen gradually increased, reaching saturation at a observed Heat exposure in a nitrogen-gas flow at 1523 K for 10 h depth of 80 nm because of the proximity of the fiber. Boron was caused no change of the strengths for all composites; therefore, the detected at depth 5 to 80 nm. Four pullout fibers were strength degradation using the oxidation test resulted from the mined, and all revealed similar results. Figure 5(b)is the typic oxidatio AES spectra of the fracture surface of the matrix from which a fiber Figure 6 shows the oxygen-concentration map of a cross section of had pulled out. The surface of the fractured matrix consisted of composites I, Il, and R after oxidation at 1523 K for 100 h. The matrix M B L3 L1 F 2 (a 10nm 5m(c) 10nm Fig 4. TEM image of the fiber-matrix interface of the as-fabricated composites(a)I and(b) ll, and(c)of the oxidized composite I at 1523 K for 10 h. M and F indicate matrix and fiber, respectively. L, LI, L2, and L3 indicate sublayers on the interf and B indicate an oxidized sublayer and a bubbleFigure 4(b) shows a TEM image of the fiber–matrix interface of non-oxidized composite II. The interface was a monolayer 20 nm thick, with no crystal lattice, and it exhibited an obscure ring pattern under NBED. Under EDS, the peak intensities of carbon and silicon on the interface were, respectively, about three times and one-third that on the Si-N-C matrix; therefore, the interface consisted of carbon and a small amount of silicon. Figure 4(c) shows the fiber–matrix interface of composite I oxidized at 1523 K for 10 h. The observed point located just under the oxygen-sealing layer is discussed later. The fiber, L1, and L2 exhibited no change with oxidation. L3 and the matrix showed obvious changes: (i) bright bubbles (B in Fig. 4(c)), where L3 was located before oxidation; and (ii) a layer that formed at the fiber side of the matrix (O in Fig. 4(c)). Under EDS, mainly carbon and silicon were detected on L1 and L2. On L3 and O, the peak intensity of oxygen was about twice that on L1 and L2; on the matrix (M) and the fiber (F), no oxygen was detected. Figure 5 shows the AES spectra of the fractured surfaces of composite I. The AES spectra (Fig. 5) are shown instead of AES depth profiles to illustrate the depth distribution of boron, because the AES depth profile cannot show boron content, as explained for Fig. 2. Figure 5(a) is the typical spectra of the surface of a pullout fiber. The fiber surface consisted mainly of carbon and a small amount of nitrogen and boron. Silicon was detected from a depth of 15 nm. Silicon and nitrogen gradually increased, reaching saturation at a depth of 80 nm because of the proximity of the fiber. Boron was detected at depths from 15 to 80 nm. Four pullout fibers were examined, and all revealed similar results. Figure 5(b) is the typical AES spectra of the fracture surface of the matrix from which a fiber had pulled out. The surface of the fractured matrix consisted of carbon. Silicon and nitrogen were detected at a depth from 15 nm, increased slowly, and reached saturation at a depth of 50 nm, where the Si-N-C matrix was located. Seven of eight samples showed results similar to those in Fig. 5(b); the one exception had a boron-containing surface layer. In the case of composite II, the pullout fiber had a surface layer 80–110 nm thick, which consisted of carbon and a small amount of boron. The surface layer of the fractured matrix consisted mainly of carbon and was 15 nm thick. No boron-containing layer was found on the matrix surface. (3) Oxidation Resistance of Composites The mechanical properties of the as-fabricated composites are shown in Table III. All the composites had dense matrices (total porosity of 11 vol%), high fiber content (Vf 45 vol%), nonbrittle fracture, and high strength. Table IV shows the strengths of the composites after oxidation. Composites I and II retained high strength, 0.8–0.6 GPa, and maintained their nonbrittle fracture, even after oxidation at 1523 K for 100 h. The fracture surfaces of oxidized composites I and II had a region 0.1–0.4 mm wide around their periphery, in which fibers failed along a matrix crack plane. The center region of the fracture surface showed many pullout fibers. Reference composite R showed a large degradation in strength, 0.2 GPa, and brittle fracture after oxidation. No pullout fiber was observed. Heat exposure in a nitrogen-gas flow at 1523 K for 10 h caused no change of the strengths for all composites; therefore, the strength degradation using the oxidation test resulted from the oxidation. Figure 6 shows the oxygen-concentration map of a cross section of composites I, II, and R after oxidation at 1523 K for 100 h. The matrix Table II. Strength and Elastic Modulus of Row Fibers and Coated Fibers Fiber Coating As-fabricated fiber Coated fiber Strength retention rate (%) Strength (GPa) E (GPa) Strength (GPa) E (GPa) Average SD† Average SD† Average SD† Average SD† Si3N4 I 2.07 0.50 182 24 1.59 0.70 175 27 77 Si3N4 II 1.78 0.54 173 16 1.90 0.61 151 10 106 Si3N4 R 1.71 0.38 169 11 2.06 0.56 157 5 121 SiC I 2.66 0.38 291 37 2.56 0.65 216 23 96 † SD is standard deviation. Fig. 4. TEM image of the fiber–matrix interface of the as-fabricated composites (a) I and (b) II, and (c) of the oxidized composite I at 1523 K for 10 h. M and F indicate matrix and fiber, respectively. L, L1, L2, and L3 indicate sublayers on the interface. O and B indicate an oxidized sublayer and a bubble. 1818 Journal of the American Ceramic Society—Sato et al. Vol. 85, No. 7