composites Part A: applied science and manufacturing ELSEVIER Composites: Part A 32 (2001)1021-1029 www.elsevier. com/locate/compositesa Fabrication of fiber-reinforced celsian matrix composites N.p. Bansal.,j.a. . Setlock bDepartment of Materials Science and Engineering Case Western Resenve Universiry, Cleveland, OH 44106,USA Abstract A method has been developed for the fabrication of small diameter, multifilament tow, fiber-reinforced ceramic matrix composites. Its application has been successfully demo emonstrated for the Hi-Nicalon/celsian system. Strong and tough celsian matrix composites, reinforce d cutting and stacking of eg tapes in the desired orientation, and hot pressing The monoclinic celsian phase in the matrix was produced in s -2Si02 mixed precursor synthesized by solid state reaction from metal oxides. Hot pressing resulted e fiber-reinforced composites. The unidirectional composites having ~42 vol. of fibers exhibited graceful tests at room temperature. Values of yield stress and strain were435±35 f900±60 MPa were observed. Young's' modulus of the composites Keywords: A. Cer 1. Introductio sian, and the orthorhombic phase are found only in synthetic products. The hexacelsian phase is thermodynamically Monoclinic celsian BaAlSi2O8 (BAS)and SrAl2S stable at temperatures between 1590C and the melting (SAS) are refractory materials having melting points higher point whereas the celsian phase is stable at temperatures than 1700 xist as a meta- reduction a ance to alkali stable phase at all temperatures from 1590C to room atta goes a rapio he orthorhom- ge volume change of ~3% is always treatment at ansformation into and very h SAS turbine er also being forced com application Asexist phase, common and the multifilament ring phase. The h small diameter Nicalon [11] and HPZ 12] fibers have ve dy was to Corresponding a develop the fabrication of small diameter, multifilament tow E-mail address: nar fiber-reinforced celsian matrix composites. Microstructures -835X( ront matter 1359-835x//s psi359-835xo0)00169-
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N P Bansal, J.A. Setlock/Composites: Part A 32(2001)1021-1029 Fiber spool Cut tap Take-up drum Hot press Binder burnout Stack tapes arm bres Fig. 1 Schematic of the set-up used for fabrication of small diameter, multifilament fiber tow reinforced ceramic composites by matrix slurry infiltration and hot pressing of the resulting FRCs were characterized and room tempera- were BacO,(Alfa Products), SrCO3(Alfa Products), AlO3 ture mechanical properties were measured. Strong, tough, (Baikowski International Corp, high purity CR 30), and nd almost fully dense composites reinforced with BN/SiC- SiOz( Cerac Inc,99.9% purity,-325 mesh) powders coated. Hi-Nicalon fibers have been obtained Appropriate quantities of various powders were slurry mixed in acetone and ball milled for -24 h using alumina milling media. Acetone was then evaporated and a part of 2. Materials and experimental procedure the mixture was subjected to thermogravimetric analysis (TGA)in air. The oxide mixed powder was calcined at The matrix of BSAs composition was synthesized by -900-920C for decomposition of the carbonates into olid-state reaction method [13]. The starting materials used oxides, followed by cooling to room temperature and Fig. 2. TGA curve of 0.75BaCO3-0 25SrCO3-Al2O3-2SiO2 mixed powder at a heating rate of 5.C/min in
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P. Bansal, J.A. Setlock/Composites: Part A 32(2001)1021-1029 A- BaAl2O4 2500 B-Ba2SiO4 C-Ba Sr2 A207 500 B BAcA 20, deg Fig 3. Powder XRD pattern of the mixed 0.75BaCO3-0 25SrCO3 -Al2O3-2Sio powder calcined at 915'C for 20 h in air. grinding. A small part of the calcined powder was loaded dual surface layer of BN overcoated with Sic were used in into a graphite die and hot pressed at 1300C for 2-3 h the present study. The fiber coatings were applied by a under 27. 5 MPa (4 ksi) pressure. commercial vendor using a continuous chemical vapor Polymer derived Hi-Nicalon fiber tows(1800 denier, 500 deposition(CVD)reactor. The Bn coating was deposited filaments/tow) with low oxygen content produced by at -1000C utilizing a proprietary precursor and was amor- Nippon Carbon Co were used as the reinforcement. These phous to partly turbostratic in nature. A thin overcoating of fibers have an average diameter of -14 um; a report Sic was also deposited by CVD onto the BN-coated fibers 14, 15] chemical composition(wt %)of 62.4% Si, 37 The SiC layer was crystalline. The nominal coating thick- C, and 0.5%O; and C/Si atomic ratio of -1.39. The Hi- nesses were 0. 4 um for BN and 0.3 um for SiC. The BN Nicalon fibers mainly consist of Sic microcrystals with an interfacial layer acts as a weak, crack-deflecting phase average grain size of 4 nm and amorphous carbon. These while the Sic overcoat acts as a barrier to diffusion of fibers have a density of 2.7 g/cm, room temperature tensile boron from bn into the oxide matrix and also prevents strength of -2.8 GPa, elastic modulus of 270 GPa, and an diffusion of matrix elements into the fiber. average coefficient of thermal expansion of 3.5 x 10 /C A sketch of the set-up used for infiltration of the matrix from room temperature to 500"C. Hi-Nicalon fibers having a slurry into the fiber tows is shown in Fig. 1. This is similar to 1000 25 eg Fig. 4. XRD spectra from the surface of a hot pressed plate of Bao.7sSro25Al2Si2Os. All the diffraction peaks match with the monoclinic celsian phase
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N P Bansal, J.A. Setlock/Composites: Part A 32(2001)1021-1029 vW014 10Nn WD19 Fig 5 SEM micrographs showing surface and cross-section of BNSiC coated Hi-Nicalon fibers. the set-up reported earlier by Prewo [16]. BSAS powder that slurry was squeezed out of the fiber tow before winding at had been calcined at 900-920C for 20-24 h was made into 0.977 mm tow spacing (26 fiber tows/inch) on a rotatin a slurry by dispersing it in methyl ethyl ketone along with drum. After drying, the prepreg tape was cut to size. Uni- organic additives as binder, surfactant, deflocculant and directional fiber-reinforced composites were prepared by plasticizer followed by ball milling. Tows of BN/SiC-coated tape lay up (12 plies) followed by warm pressing at Hi-Nicalon fibers were spread using rollers and coated with 150.C to form a green'composite. The fugitive organics the matrix precursor by passing through the slurry. Excess were slowly burned out of the sample in air at -500"C, N Fig. 6. SEM micrographs showing polished cross-sections of Hi-Nicalon fibers with a duplex CVD BN/SiC coating Table 1 Hi-Nicalon fiber-reinforced Bao.75 Si2Os celsian composites(Unidirectional; 12 Plies) Sample #f Fiber coating Fiber content, Ve Density, p(g/cm) Phase from XRD HI-NIC-BSAS-I- 0.42 HI-NIC-BSAS-1-3 BN/SiC Monoclinic celsian
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N P. Bansal, J.A. Setlock /Composites: Part A 32(2001)1021-1029 99818Fn 888225KV Fig. 7. SEM micrographs showing polished cross-section of a unidirectional Hi-Nicalon/BN/SIC/BSAS cor followed by hot pressing under vacuum in a graphite die. curves recorded in three-point flexure using an Instron The hot pressed fiber-reinforced composite panel was 4505 universal testing instrument at a crosshead speed of surface polished and sliced into test bars(50.4 mm 1. 27 mm/min (0.05 in /min) and support span (L)of x6.4 mm x1.9 mm) for mechanical testing. 40 mm. Strain gauges were glued to the tensile surfaces TGA of the calcination process was carried out at a of the flexure test bars. Stress was calculated using beam heating rate of 5C/min under flowing air(60 ml/min) theory. The yield stress was calculated from the stress from room temperature to 1500C using a Perkin-Elmer strain curves from the point where the curve deviates TGA-7 system which was interfaced with a computerized from linearity Elastic modulus of the composite was deter data acquisition and analysis system. X-ray diffraction mined from the linear portion of the stress-strain curve (XRD) patterns were recorded at room temperature using to the yield point using linear interpolation a step scan procedure (0.02/26 step, time per step 0.5 or I s)on a Philips ADP-3600 automated diffractometer equipped with a crystal monochromator employing CuK a 3 Results and discussion adiation. density was measured from dimensions and mass as well as by the archimedes method. Microstruc The TGA curve of the mixed BSAS powder consisting of tures of the polished cross-sections and fracture surfaces metal carbonates and oxides is shown in Fig. 2. Minor were observed in an optical microscope as well as in a weight loss near room temperature is due to evaporation JEOL JSM-840A scanning electron microscope (SEM). of the residual moisture and acetone. A major event showing For transmission electron microscopy (TEM), thin foils a large weight loss, due to the decomposition of barium and of the composite samples were prepared by slicing, polish- strontium carbonates into oxides, is observed between -750 ing, dimple grinding, and argon ion beam milling. A thin and 1000.C. A calcination temperature of 900-920C was carbon coating was evaporated onto the TEM thin foils and chosen for decomposition of the carbonates. The mixed the SEM specimens for electrical conductivity prior to powder was calcined at this temperature for 20-24 h analysis. The thin foils were examined in a Philips air TGA analysis of this calcined powder showed no further EM400T operating at 120 keV. X-ray elemental analyses weight loss indicating complete decomposition of the metal on the tEM were acquired using a Kevex thin window carbonates during the calcination step. energy dispersive spectrometer(EDS) and analyzer. The XRD pattern of the mixed powder, calcined at Mechanical properties were determined from stress-strain -915C for 20 h in air, is presented in Fig. 3. SiO2
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