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  • Mechanical Properties of Chemically Vapor-Infiltrated

    Silicon Carbide Structural Composites with Thin Carbon Interphases

    for Fusion and Advanced Fission Applications

    Yutai Katoh1, Lance L. Snead1, Takashi Nozawa1, Tatsuya Hinoki2,Akira Kohyama2, Naoki Igawa3 and Tomitsugu Taguchi3

    1Metals and Ceramics Division, Oak Ridge National Laboratory, P.O.Box 2008, Oak Ridge, TN 37831, USA2Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan3Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1195, Japan

    Fast fracture properties of chemically vapor-infiltrated silicon carbide matrix composites with Hi-Nicalon Type-S near-stoichiometricsilicon carbide fiber reinforcements and thin pyrolytic carbon interphase were studied. The primary emphasis was on preliminary assessment ofthe applicability of a very thin pyrolytic carbon interphase between fibers and matrices of silicon carbide composites for use in nuclearenvironments. It appears that the mechanical properties of the present composite system are not subject to strong interphase thickness effects, incontrast to those in conventional non-stoichiometric silicon carbide-based fiber composites. The interphase thickness effects are discussed fromthe viewpoints of residual thermal stress, fiber damage, and interfacial friction. A preliminary conclusion is that a thin pyrolytic carboninterphase is beneficial for fast fracture properties of stoichiometric silicon carbide composites.

    (Received October 12, 2004; Accepted January 31, 2005)

    Keywords: ceramic matrix composite (CMC), SiC/SiC composite, chemical vapor infiltration, interphase effect, mechanical property, fast

    fracture strength

    1. Introduction

    Silicon carbide (SiC) is a unique material that maintainshigh strength and corrosion resistance at temperatures wellbeyond the typical high temperature limits for superalloys.More importantly, the excellent thermo-mechanical proper-ties of SiC are maintained after neutron irradiation tomedium-to-high fluences at elevated temperatures.1) Contin-uous SiC fiber-reinforced SiC matrix composites (SiC/SiCcomposites) can be used as structural materials, since theypossess pseudo-ductile, predictable, and tailorable fractureproperties in addition to the unique merits of monolithic SiC.

    The use of SiC/SiC composites in nuclear fusion andadvanced fission reactors has been considered for a fewdecades.2) Early generations of SiC/SiC composites failed todemonstrate neutron tolerance due to a rapid irradiation-induced densification of SiC-based non-stoichiometricfibers.35) However, as improved SiC fibers became available,experiments started to demonstrate good tolerance of SiC/SiC composites to neutron irradiation.57) Those SiC fibers,which consist primarily of a polycrystalline form of cubicSiC and are termed Generation-III SiC fibers,8) are nowcommercially available as Hi-Nicalon Type-S (NipponCarbon Co., Tokyo, Japan)9) and Tyranno-SA (UbeIndustries, Ltd., Ube, Japan).10) The Generation-III SiC fibercomposites are considered for application as in-core compo-nents of advanced gas thermal reactors11) and gas fastreactors.12) Fusion power reactor design studies assume thatSiC/SiC composites can be used in gas-cooled solid-breeding blankets1316) and helium/Pb-Li dual-coolant blan-kets.17,18) Some of the dual-coolant blanket concepts,including the US-proposed Test Blanket Module (TBM) forthe International Thermonuclear Experimental Reactor(ITER), utilize channel inserts made of SiC or SiC/SiC asan electrical and thermal insulator.19)

    Among various processing techniques for matrix densifi-cation of SiC/SiC composites, chemical vapor infiltration(CVI) is the technique that produces reference materials fornuclear applications.8) The CVI process is essentially achemical vapor deposition (CVD) of the matrix material on(coated) fiber surfaces as the substrate. A high purity,stoichiometric, and polycrystalline matrix for SiC/SiCcomposites can not be efficiently produced by any otherindustrialized processes. Non-stoichiometric and/or nano-crystalline SiC matrices in melt-infiltrated or polymer-impregnated and pyrolyzed composites are usually prone todeteriorate during irradiation.20,21) The CVI process is notonly effective in producing high strength composites forprimary structure applications but is also appropriate fortailoring of trans-thickness thermal and electrical conductiv-ity for the channel insert application, by controlling porosityand inter-fiber spacing.

    Another advantage of the CVI technique is that thedeposition of fiber-matrix interphases as a CVD coating onthe fibers can be incorporated in the matrix densificationprocess. Properties of the interphases are highly tailorable inCVI SiC/SiC composites.22)

    The objective of this work is to evaluate the non-irradiatedmechanical properties of Generation III SiC fiber-reinforcedCVI SiC-matrix composites with a tailored pyrolytic carbon(PyC) interphase. The primary focus was on the influence ofinterphase thickness, which is a key parameter that controlsmechanical properties of ceramic composites. For conven-tional CVI-SiC/SiC composite systems, an optimum PyCinterphase thickness range of 150300 nm is reported for thebest fracture behavior.2326) However, for application infusion blankets, the minimum use of carbon constituents ispreferred to optimize irradiation stability, chemical compat-ibility with coolants and/or breeding materials, tritiumpermeability, and electrical conductivity. The Generation

    Materials Transactions, Vol. 46, No. 3 (2005) pp. 527 to 535Special Issue on Fusion Blanket Structural Materials R&D in Japan#2005 The Japan Institute of Metals

  • III SiC fibers are significantly different from conventionalSiC fibers in physical (coefficient of thermal expansion,elastic modulus), mechanical (fracture strain), chemical(reactivity and bonding at the surface), and topographical(surface roughness) properties. These differences can causedifference in the effect of interphase characteristics oncomposite properties, so the effects must be quantified.

    2. Experimental Procedure

    The materials studied were Hi-Nicalon Type-S fiber-reinforced, PyC interphase, CVI SiC-matrix (Hi-NicalonType-S/PyC/CVI-SiC) composites produced at Oak RidgeNational Laboratory. Composites with three PyC interphasethicknesses were produced with 2D plain-weave fabric froman early lot of Hi-Nicalon Type-S fibers (Lot# 298201,produced in 1998) with a [0/30/60] stacking sequence(0/30/60, hereafter).27) The average interphase thicknesseswere 80, 130 and 270 nm. Additionally, one composite wasproduced with 2D plain-weave fabric of a newer lot of fibers(Lot# 320203, 2002) using a [0/90] stacking sequence (0/90). The newer Hi-Nicalon Type-S fibers were produced byimproved factory line, including spinning, electron beamcuring, and decarbonization. The specification sheet providedby the manufacturer shows that the newer fibers possessslightly higher nominal tensile strength and very slightlylower tensile modulus than the older fibers. The propertydifferences between the two generations of Hi-Nicalon Type-S fibers are seen in Table 1, along with the summary ofproperties of the composites fabricated in this study.

    The interphase deposition and matrix densification werecarried out in isothermal and temperature gradient28,29)

    configurations, respectively, at the High Temperature Mate-rials Laboratory, Oak Ridge National Laboratory (ORNL).Stacks of 60 fabric sheets of 76mm-diameters were tightlyheld in graphite fixtures during the infiltration. The PyCinterphase was deposited from propylene precursor (50 cm3/min) diluted with argon (1000 cm3/min) at 1373K and 5 kPatotal pressure. The interphase thickness was controlled byadjusting the time of deposition, since the deposition rate wasfairly constant during the coating process. The SiC matrixwas infiltrated using methyltrichlorosilane (MTS, Gelest Inc.,Morrisville, PA) at a hot surface temperature of 13731473Kand back pressure of 100 kPa. The liquid MTS precursor wascarried by hydrogen bubbled through it at a flow rate of 0.30.5 g/min. The deposition rate was approximately 30 nm/min.

    Tensile specimens were machined from the compositediscs with the longitudinal direction parallel to one of thefiber directions. Miniature tensile specimen geometry thathad been developed for neutron irradiation studies of ceramiccomposites was employed.30) The specimen geometry anddimensions are given in Fig. 1. The gauge length of 15mmand the width of 3mm (corresponding to two threadintervals) are within a range where a systematic gauge sizeeffect is observed.31) The gauge thickness of 2:3mmaccommodates approximately 10 fabric layers. The tensiletesting procedure followed general guidelines of ASTMstandards C1275-00 and C1359-96. The tensile test incorpo-rated several unloading/reloading sequences in order to

    Table 1 Summary of properties of Hi-Nicalon Type-S/PyC/CVI-SiC composites studied.

    Material ID CVI-1258 CVI-1257 CVI-1259 CVI-1272

    Reinforcement

    Fiber lot Hi-Nicalon Type-S Lot 298201 Lot 320203Tensile strength 2.60GPa 2.80GPa

    Tensile modulus 390GPa 380GPa

    Fabric lot NCS-9902 SS0201

    Architecture 2D-PW 2D-PW

    Thread count 0.63/mm (16/inch) 0.71/mm (18/inch)

    Lay-up orientation [0/30/60] [0/90]

    Fiber volume fraction (%) 36 33 35 41

    PyC Interphase thickness

    Measured average (nm) 80 130 270 50

    Range of scatter (nm) 5595 80180 200390 3565

    Composite

    Density* (g/cm3) 2.63 (0.07) 2.39 (0.16) 2.73 (0.02) 2.60 (0.06)

    Porosity* (%) 16.2 (2.4) 24.0 (5.1) 13.1 (0.7) 17.3 (2.0)

    Tensile properties at RT

    Number of valid tests 4 5 4 4

    UTS* (MPa) 164 (19) 161 (25) 182 (27) 239 (33)

    PLS* (MPa) 35 (8) 34 (3) 38 (5) 74 (3)

    Modulus* (GPa) 233 (23) 202 (6) 206 (9) 230 (11)

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