1820 Journal of the American Ceramic Society-Sato et al. Vol. 85. No. 7 Table IV. Flexural Strength of Composites after Oxidation EDS, which was applied simultaneously with TEM, was insuffI After oxidation at 1523 K for After oxidation at 1523 K for cient for lightweight elements, such as boron. If it is confirmed that the fiber-matrix interface became coated with almost no change Flexural strength during the PIP composite fabrication, the chemical composition of (GPa) the coating applies to the fiber-matrix interface. To confirm the Retention Coating Average SD rate(%) Average SD rate ( Retention difference between the coating and the fiber-matrix interface, the AES result of the coating was compared with that of the fracture surface of the fiber-matrix interface as follows First. the boron- Ⅱ0.680.13610.670.0260 containing sublayer, the second sublayer of the coating, was found I0.590.03550 on the pullout fiber only. Second, the carbon-rich layer, the third sublayer of the coating, was found on the pullout fiber and the tSD is standard deviation bRittle fracture was shown fractured matrix surface. These results showed that(i)the layered structure of the coating changed little during the pip process and that(ii) the fracture of the fiber-matrix interface proceeded in the IV Discussion outer la of the interface, which corresponded to the third (I) Structure of Coatings and Fiber-Matrix Interface ublayer of the coating. The d terence between the coati g the CVd reactor. in which thickness of 40-120 nm(Table D), revealed by AEs analysis, an fibers were fed continuously Therefore. it was expected that the the interface thickness of 30-50 nm(Fig. 4), obtained from TEM elemental depth profile of the coating would roughly agree with observation, would present a problem if the coating were to the chemical composition of the deposit, which changed along the became the fiber-matrix interface with no changes. However, th horizontal direction of the CVD reactor. Actually, the depth profile thickness obtained using two te observations was considered within the range of eight measurements using AES deposit(Fig. 3)on the concentration change of boron and carbon Therefore, the sublayers of the coating and the fiber-matrix AES and EPMA results showed coating I consisted of three interface corresponded to each other as follows. L3 was the third sublayers(from the fiber side): (i) first sublayer without boron; (i) sublayer of the coating, a carbon layer with a small amount of second sublayer of boron, silicon, and carbon; and(iii) third silicon and a graphitelike structure. L2 was the second sublayer of sublayer mainly of carbon. The weak peaks of B.C and Sic, the coating, composed of crystallites, with a 0. 26 nm lattice detected using XRD on the deposit, corresponded to the second interlayer spacing, and it consisted of carbon, boron, and silicon sublayer therefore, boron and silicon were determined to be The L2 interlayer spacing was similar to the 0.25 nm of the closest ontained as B.c and Sic. packing plane of a-or B-SiC, the 0.26 nm of the(104) plane of TEM observation of the fiber-matrix interface of composite I B.C, and the 0.24 nm of the(021) plane of B. C. BC, and Sic that revealed a three-layered structure. However, the chemical compo- were detected using XRD on the deposits in the coat sition of each sublayer was uncertain, because the sensitivity of as mentioned before. Therefore, the crystallite seemed to be Sic Z=.5 4z=05 4z=02 42e03 (d) Fig. 6. Oxygen distribution maps of (b) composite R(carbon coating), (c)composite I, and(d)composite Il using EPMA. (a) SEM image of the composite R at the same position as(b).(e) Distance from the surface of the composites.IV. Discussion (1) Structure of Coatings and Fiber–Matrix Interface The coating was prepared using the CVD reactor, in which fibers were fed continuously. Therefore, it was expected that the elemental depth profile of the coating would roughly agree with the chemical composition of the deposit, which changed along the horizontal direction of the CVD reactor. Actually, the depth profile of coating I (Fig. 2) agreed generally with the EPMA result of the deposit (Fig. 3) on the concentration change of boron and carbon. AES and EPMA results showed coating I consisted of three sublayers (from the fiber side): (i) first sublayer without boron; (ii) second sublayer of boron, silicon, and carbon; and (iii) third sublayer mainly of carbon. The weak peaks of B4C and SiC, detected using XRD on the deposit, corresponded to the second sublayer; therefore, boron and silicon were determined to be contained as B4C and SiC. TEM observation of the fiber–matrix interface of composite I revealed a three-layered structure. However, the chemical compo￾sition of each sublayer was uncertain, because the sensitivity of EDS, which was applied simultaneously with TEM, was insuffi￾cient for lightweight elements, such as boron. If it is confirmed that the fiber–matrix interface became coated with almost no change during the PIP composite fabrication, the chemical composition of the coating applies to the fiber–matrix interface. To confirm the difference between the coating and the fiber–matrix interface, the AES result of the coating was compared with that of the fracture surface of the fiber–matrix interface as follows. First, the boron￾containing sublayer, the second sublayer of the coating, was found on the pullout fiber only. Second, the carbon-rich layer, the third sublayer of the coating, was found on the pullout fiber and the fractured matrix surface. These results showed that (i) the layered structure of the coating changed little during the PIP process and that (ii) the fracture of the fiber–matrix interface proceeded in the outer layer of the interface, which corresponded to the third sublayer of the coating. The difference between the coating thickness of 40–120 nm (Table I), revealed by AES analysis, and the interface thickness of 30–50 nm (Fig. 4), obtained from TEM observation, would present a problem if the coating were to became the fiber–matrix interface with no changes. However, the thickness obtained using two TEM observations was considered within the range of eight measurements using AES. Therefore, the sublayers of the coating and the fiber–matrix interface corresponded to each other as follows. L3 was the third sublayer of the coating, a carbon layer with a small amount of silicon and a graphitelike structure. L2 was the second sublayer of the coating, composed of crystallites, with a 0.26 nm lattice interlayer spacing, and it consisted of carbon, boron, and silicon. The L2 interlayer spacing was similar to the 0.25 nm of the closest packing plane of - or -SiC, the 0.26 nm of the 104 plane of B4C, and the 0.24 nm of the (021) plane of B4C. B4C, and SiC that were detected using XRD on the deposits in the coating apparatus as mentioned before. Therefore, the crystallite seemed to be SiC Table IV. Flexural Strength of Composites after Oxidation Fiber Coating After oxidation at 1523 K for 10 h After oxidation at 1523 K for 100 h Flexural strength (GPa) Retention rate (%) Flexural strength (GPa) Retention Average SD rate (%) † Average SD† Si3N4 I 0.92 0.15 86 0.83 0.08 77 Si3N4 II 0.68 0.13 61 0.67 0.02 60 Si3N4 R 0.27‡ 0.05 25 0.20‡ 0.03 18 SiC I 0.59 0.03 55 0.51 0.03 48 † SD is standard deviation. ‡ Brittle fracture was shown. Fig. 6. Oxygen distribution maps of (b) composite R (carbon coating), (c) composite I, and (d) composite II using EPMA. (a) SEM image of the composite R at the same position as (b). (e) Distance z from the surface of the composites. 1820 Journal of the American Ceramic Society—Sato et al. Vol. 85, No. 7