l816 Journal of the American Ceramic Society-Sato et al. Vol. 85. No. 7 Bobbin Exhaust Precursor gas Exhaust Oiling roller chamber Deposition chamber Winder N2 Bobbin Fig. 1. Schematic drawing of the apparatus for fiber coating resistance of the composites Properties of the SiC-fiber-reinforced reinforcement and methylhydrosilazane(MHS; NN710, Tonen composite produced when a C-B-Si layer is applied to a Sic fiber Corp, Tokyo, Japan)as a matrix precursor. Details of the are evaluated fabrication process and properties of MHS are described else where 30, MHS was a random copolymer mposed of -SiH,NHI and-SiMeHNHH units and converted to an amor- IL. Experimental Procedure phous Si-N-C by pyrolysis at 1200 K in a nitrogen-gas or inert (I Preparation and Analysis of coatings atmosphere. The chemical composition of the pyrolyzed MHS was Two fibers were used as reinforcements in the present study: () (in mol%)43 silicon, 38 nitrogen, 18 carbon, and 1 oxygen, with an amorphous Si,Na fib developed by Tonen Corporation a 1: 1 ratio of -SiH,NH-ISiMeHNHh and pyrolysis was and(ii) a commercial SiC fiber(Hinicalon, Nippon Carbon C performed in nitrogen gas at 1623 K 30, Unidirectionally rein- Tokyo, Japan). The Si3 N4 fiber was a strand composed of 1000 forced composites were fabricated by the following process: () fabrication of unidirectionally fiber-aligned prepreg; (ii) stacking single filaments. The diameter of the filament was 10 um. The and curing of the prepreg sheets: (i) pyrolysis of a cured sample chemical composition of the fiber was(in mass%)60 silicon, 37 at 1623 K: and (iv) densification of the sample by seven cycles of nitrogen, <1 carbon, and <3 oxygen, The fabrication method and basic character of the fiber are described elsewhere 27,28 impregnation and pyrolysis. Details of the process are described The C-B-Si coating was formed using the CVD apparatus elsewhere 30-32 described in Fig. 1, which consisted of a strand feeder, cleaning The mechanical properties of the composites were evaluated chamber, deposition chamber, and strand winder. The cleaning a terms of flexural strength and interlaminar shear strength (ILSS)at deposition chambers were tubular fumaces with an inner diameter room temperature. The test pieces, which were cut from the of 60 mm and lengths of I and 3 m, respectively. A strand of fiber composite panels, measured 4 mm X 40 mm X 3 mm for the was fed from a bobbin to the cleaning chamber at 33 mm/s and flexural test and 4 mm x 12 mm x 2 mm for the ilss test. the heated to 1073 K under a nitrogen-gas atmosphere, where the longitudinal direction of the test pieces was aligned with the fiber sizing on the fiber thermally decomposed. The desized strand was orientation. The flexural strength was measured using the three. oated with C-B-Si by the reaction of BCl3, SiCla, CHa, and H under atmospheric pressure, in the deposition chamber. Nitroge The span and testing speed of the ILSs test were 8 mm and 8.3 was used as the carrier gas. Finally, the coated strand was resized um/s, respectively. Both tests were completed five times. The with a polyether and wound onto another bobbin in the winder volume content of fiber (e was calculated from the dimensions of The single-filament strengths of the uncoated fiber and the the composite panel and the amount of fiber used. The bulk density oated fiber were tested. according to ASTM standard. 1 at room was calculated from the weight and dimensions of the test piece temperature(25 mm gauge length, 8.3 um/s testing speed, 25 used for the flexural-strength test. The true density of the compos- tests). Cross-section areas of the filaments, needed for strength ite was measured using a pycnometer at 303 K, using n-butanol a calculations. were measured from observations of the fracture the medium, on a sample crushed under No 80 mesh. surface using scanning electron microscopy (SEM). Auger elec- For the oxidation test on the composites, the test pieces measuring tron spectroscopy (AES, Model No. PHI650, ULVAC, Ltd 4 mm X 40 mm x 3 mm were placed inside a tube fumace, heated okyo, Japan), with accelerating voltage of 3 kv and sample to a given temperature at a rate of 0.167 K/s, and maintained at that current of 3 nA, was used to examine the elemental depth profiles temperature for a given time under a dry-air flow of 74 mmol/s. Some of the coatings. The etching rate, determined using argon sputter samples were exposed at the same temperature and time, under a ing, was 0.27 nm/s, with SiO, as a standard nitrogen-gas flow of 74 as references The deposits in the deposition chamber were analyzed to The microstructure of the fiber-matrix interface was investi investigate the synthesis mechanism of the multilayered coating ated using transmission electron microscopy(TEM; Model No. Graphite sheets were placed inside the chamber, along its tubular JEM3010, JEOL) using the following techniques: (i)observation wall, before the coating operation. The graphite sheets were on a bright-field image; (ii)crystal-structure analysis by nanobeam emoved from the furnace after the operation was complete and electron diffraction (NBED)with a 5 nm beam, and (iii)elemental analyzed using electron probe microanalysis(EPMA; Model No analysis by energy dispersive spectroscopy(EDS), with a 5 nm JXA8600MX, JEOL, Tokyo, Japan) and X-ray diffractometry beam. aes depth profiles were investigated on the surfaces of a (XRD; Model No. RINT 1400, Rigaku Co, Ltd, Tokyo, Japan) ullout fiber and on the matrices from which a fiber had debonded after fracture. The measuring conditions for aes were the same as those for the coated fiber (2) Preparation and Analysis of the Composite Composite panels measuring 100 mm X 100 mm x 3 mm were fabricated using the PIP process, with the coated fiber as a IlL. Results Modulus Single-Filam Materials, ASTM Designation D-3379- Society for Testing and Materials, West Conshohocken, P. deposition conditions of the coatings are shown in Table I. Aresistance of the composites. Properties of the SiC-fiber-reinforced composite produced when a C-B-Si layer is applied to a SiC fiber are evaluated. II. Experimental Procedure (1) Preparation and Analysis of Coatings Two fibers were used as reinforcements in the present study: (i) an amorphous Si3N4 fiber27,28 developed by Tonen Corporation; and (ii) a commercial SiC fiber29 (Hinicalon, Nippon Carbon Co., Tokyo, Japan). The Si3N4 fiber was a strand composed of 1000 single filaments. The diameter of the filament was 10 m. The chemical composition of the fiber was (in mass%) 60 silicon, 37 nitrogen, 1 carbon, and 3 oxygen. The fabrication method and basic character of the fiber are described elsewhere.27,28 The C-B-Si coating was formed using the CVD apparatus described in Fig. 1, which consisted of a strand feeder, cleaning chamber, deposition chamber, and strand winder. The cleaning and deposition chambers were tubular furnaces with an inner diameter of 60 mm and lengths of 1 and 3 m, respectively. A strand of fiber was fed from a bobbin to the cleaning chamber at 33 mm/s and heated to 1073 K under a nitrogen-gas atmosphere, where the sizing on the fiber thermally decomposed. The desized strand was coated with C-B-Si by the reaction of BCl3, SiCl4, CH4, and H2, under atmospheric pressure, in the deposition chamber. Nitrogen was used as the carrier gas. Finally, the coated strand was resized with a polyether and wound onto another bobbin in the winder. The single-filament strengths of the uncoated fiber and the coated fiber were tested, according to ASTM standard,¶ at room temperature (25 mm gauge length, 8.3 m/s testing speed, 25 tests). Cross-section areas of the filaments, needed for strength calculations, were measured from observations of the fracture surface using scanning electron microscopy (SEM). Auger elec￾tron spectroscopy (AES; Model No. PHI650, ULVAC, Ltd., Tokyo, Japan), with accelerating voltage of 3 kV and sample current of 3 nA, was used to examine the elemental depth profiles of the coatings. The etching rate, determined using argon sputter￾ing, was 0.27 nm/s, with SiO2 as a standard. The deposits in the deposition chamber were analyzed to investigate the synthesis mechanism of the multilayered coating. Graphite sheets were placed inside the chamber, along its tubular wall, before the coating operation. The graphite sheets were removed from the furnace after the operation was complete and analyzed using electron probe microanalysis (EPMA; Model No. JXA8600MX, JEOL, Tokyo, Japan) and X-ray diffractometry (XRD; Model No. RINT 1400, Rigaku Co., Ltd., Tokyo, Japan). (2) Preparation and Analysis of the Composite Composite panels measuring 100 mm  100 mm  3 mm were fabricated using the PIP process, with the coated fiber as a reinforcement and methylhydrosilazane (MHS; NN710, Tonen Corp., Tokyo, Japan) as a matrix precursor. Details of the fabrication process and properties of MHS are described else￾where.30,31 MHS was a random copolymer, composed of –[SiH2NH]– and –[SiMeHNH]– units and converted to an amor￾phous Si-N-C by pyrolysis at 1200 K in a nitrogen-gas or inert atmosphere. The chemical composition of the pyrolyzed MHS was (in mol%) 43 silicon, 38 nitrogen, 18 carbon, and 1 oxygen, with a 1:1 ratio of –[SiH2NH]–:–[SiMeHNH]–, and pyrolysis was performed in nitrogen gas at 1623 K.30,31 Unidirectionally rein￾forced composites were fabricated by the following process: (i) fabrication of unidirectionally fiber-aligned prepreg; (ii) stacking and curing of the prepreg sheets; (iii) pyrolysis of a cured sample at 1623 K; and (iv) densification of the sample by seven cycles of impregnation and pyrolysis. Details of the process are described elsewhere.30–32 The mechanical properties of the composites were evaluated in terms of flexural strength and interlaminar shear strength (ILSS) at room temperature. The test pieces, which were cut from the composite panels, measured 4 mm  40 mm  3 mm for the flexural test and 4 mm  12 mm  2 mm for the ILSS test. The longitudinal direction of the test pieces was aligned with the fiber orientation. The flexural strength was measured using the three￾point bend test, with a 30 mm span and a 8.3 m/s testing speed. The span and testing speed of the ILSS test were 8 mm and 8.3 m/s, respectively. Both tests were completed five times. The volume content of fiber (Vf ) was calculated from the dimensions of the composite panel and the amount of fiber used. The bulk density was calculated from the weight and dimensions of the test piece used for the flexural-strength test. The true density of the compos￾ite was measured using a pycnometer at 303 K, using n-butanol as the medium, on a sample crushed under No. 80 mesh. For the oxidation test on the composites, the test pieces measuring 4 mm  40 mm  3 mm were placed inside a tube furnace, heated to a given temperature at a rate of 0.167 K/s, and maintained at that temperature for a given time under a dry-air flow of 74 mmol/s. Some samples were exposed at the same temperature and time, under a nitrogen-gas flow of 74 mmol/s, as references. The microstructure of the fiber–matrix interface was investi￾gated using transmission electron microscopy (TEM; Model No. JEM3010, JEOL) using the following techniques: (i) observation on a bright-field image; (ii) crystal-structure analysis by nanobeam electron diffraction (NBED) with a 5 nm beam; and (iii) elemental analysis by energy dispersive spectroscopy (EDS), with a 5 nm beam. AES depth profiles were investigated on the surfaces of a pullout fiber and on the matrices from which a fiber had debonded after fracture. The measuring conditions for AES were the same as those for the coated fiber. III. Results (1) Coatings Two types of fiber coating, coating I and II, were effective in the improvement of the oxidation resistance of composites. The deposition conditions of the coatings are shown in Table I. A ¶ “Standard Test Method for Tensile Strength and Young’s Modulus for High￾Modulus Single-Filament Materials,” ASTM Designation D-3379–75. American Society for Testing and Materials, West Conshohocken, PA. Fig. 1. Schematic drawing of the apparatus for fiber coating. 1816 Journal of the American Ceramic Society—Sato et al. Vol. 85, No. 7