N P Bansal/ Joumal of the European Ceramic Society 29(2009)525-535 barrier to diffusion of boron from bn into the oxide matrix and also prevents diffusion of matrix elements into the fiber. c8Eo 9000000 The matrix of 0.75 Ba0-0.25Sr0-Al2O3-2SiO2(BSAs)com- position was synthesized by a solid-state reaction method as described earlier. 0 The advantage of BSAS over BAS as matrix has been explained earlier. 1.20 Briefly speaking, hexacelsian is the first phase to form in both BAS and SAS systems On heat treatment at -1200C or higher temperatures, transformation of hexacelsian to monoclinic celsian phase is very sluggish in 20 BAS and very rapid in SAs. However, it is known that substi 10 tution of about 25 mol% of Bao with Sro in bAs accelerates the transformation of hexacelsian to the desired monoclinic celsian phase. The experimental setup and the procedure used for fabrica g 1. Scanning Auger microprobe depth profiles of various elements for Hi- Nicalon fibers having a duplex "BN/SiC surface coating deposited by CVD tion of the fiber-reinforced celsian matrix CMC were essentially the same as described earlier. The matrix precursor powder was made into a slurry by dispersing in an organic solvent along 2. Materials and experimental methods with organic additives as binder. surfactant, deflocculant and plasticizer followed by ball milling. Tows of BN/SiC-coated Polymer derived Hi-Nicalon fiber tows(1800 denier, 500 Hi-Nicalon fibers were coated with the matrix precursor by filaments/tow) with low oxygen content from Nippon Carbon passing through the slurry and winding on a rotating drum. Co. were used as the reinforcement. 8, 19 A duplex surface After drying, the prepreg tape was cut to size. Unidirectional layer of boron nitride(Bn) over coated with silicon carbide fiber-reinforced composites were prepared by tape lay-up(12 was applied on the fibers by a commercial vendor using a plies) followed by warm pressing to form a"green"com- continuous chemical vapor deposition(CVD)reactor. The Bn posite. The fugitive organics were slowly burned out of the ating was deposited at 1000C utilizing a proprietary pre- sample in air, followed by hot pressing under vacuum in a cursor and was amorphous to partly turbostratic in nature. a graphite die to yield dense composites. The oxide precursor thin overcoating of SiC was also deposited by CVD onto the was converted into the desired monoclinic celsian phase in situ BN-coated fibers. The SiC layer was crystalline. The nomi- during hot pressing as was confirmed from X-ray diffraction nal coating thicknesses were 0.4 um for BN, and 0.3 um for The hot pressed CMC panel -ll.I cm x 5cm(4.5 in. x 2 in. SiC. The Bn interfacial layer was intended to be a weak, was annealed in argon at 1100C for 2 h and machined into crack deflecting phase, while the SiC overcoat was used as a test bars(50 mm x 0.625 mm x 2.4 mm) for high-temperature ·b 回沙100m m Fig. 2. SEM micrographs at different magnifications showing polished cross-section of a unidirectional Hi-Nicalon/BNSIC/BSAS composite526 N.P. Bansal / Journal of the European Ceramic Society 29 (2009) 525–535 Fig. 1. Scanning Auger microprobe depth profiles of various elements for Hi￾Nicalon fibers having a duplex “BN/SiC” surface coating deposited by CVD. 2. Materials and experimental methods Polymer derived Hi-Nicalon fiber tows (1800 denier, 500 filaments/tow) with low oxygen content from Nippon Carbon Co. were used as the reinforcement.18,19 A duplex surface layer of boron nitride (BN) over coated with silicon carbide was applied on the fibers by a commercial vendor using a continuous chemical vapor deposition (CVD) reactor. The BN coating was deposited at ∼1000 ◦C utilizing a proprietary pre￾cursor and was amorphous to partly turbostratic in nature. A thin overcoating of SiC was also deposited by CVD onto the BN-coated fibers. The SiC layer was crystalline. The nomi￾nal coating thicknesses were 0.4 m for BN, and 0.3 m for SiC. The BN interfacial layer was intended to be a weak, crack deflecting phase, while the SiC overcoat was used as a barrier to diffusion of boron from BN into the oxide matrix and also prevents diffusion of matrix elements into the fiber. The matrix of 0.75BaO–0.25SrO–Al2O3–2SiO2 (BSAS) com￾position was synthesized by a solid-state reaction method as described earlier.20 The advantage of BSAS over BAS as matrix has been explained earlier.11,20 Briefly speaking, hexacelsian is the first phase to form in both BAS and SAS systems. On heat treatment at ∼1200 ◦C or higher temperatures, transformation of hexacelsian to monoclinic celsian phase is very sluggish in BAS and very rapid in SAS.8 However, it is known that substi￾tution of about 25 mol% of BaO with SrO in BAS accelerates the transformation23 of hexacelsian to the desired monoclinic celsian phase. The experimental setup and the procedure used for fabrica￾tion of the fiber-reinforced celsian matrix CMC were essentially the same as described earlier.10,11 The matrix precursor powder was made into a slurry by dispersing in an organic solvent along with organic additives as binder, surfactant, deflocculant and plasticizer followed by ball milling. Tows of BN/SiC-coated Hi-Nicalon fibers were coated with the matrix precursor by passing through the slurry and winding on a rotating drum. After drying, the prepreg tape was cut to size. Unidirectional fiber-reinforced composites were prepared by tape lay-up (12 plies) followed by warm pressing to form a “green” com￾posite. The fugitive organics were slowly burned out of the sample in air, followed by hot pressing under vacuum in a graphite die to yield dense composites. The oxide precursor was converted into the desired monoclinic celsian phase in situ during hot pressing as was confirmed from X-ray diffraction. The hot pressed CMC panel ∼11.1 cm × 5 cm (4.5 in. × 2 in.) was annealed in argon at 1100 ◦C for 2 h and machined into test bars (∼50 mm × 0.625 mm × 2.4 mm) for high-temperature Fig. 2. SEM micrographs at different magnifications showing polished cross-section of a unidirectional Hi-Nicalon/BN/SiC/BSAS composite.