December 2000 eramic Composites with Multilayer Interfa c Bertrand et al. tested SiC-matrix minicomposites containing Honeywell, Morristown, NJ). Before deposition, the fibers expe- lin C/SiC- interface layers(10 sets of layers each containing a 20 rienced a heating cycle to 950C for 1 h, which removed the nm C-and a 50 nm SiC-thickness layer) on untreated Hi-Nicalon protective sizing from the fiber surface. The conditions for and fibers treated with a proprietary surface modification process. depositing all of the interface layers and chemical vapor infiltra- Samples were tested in static fatigue in an oxidizing atmosphere at tion(CVi) of the matrix are given in Table I 700C under a load 10% above the proportional limit to assure matrix cracking. The results indicated a factor of ll increase in life for minicomposites with multilayer C/SiC interfaces on untreated (3) Plate Fabrication fibers(2 vs 22 h) and a 50% increase for minicomposites prepared The fabricated plates measured 152 mm on a side and were with treated fibers(100 vs 150 h)over those with equivalent total prepared using FCVI. A total of 18 layers of cloth with a fabric thickness single carbon layer interfaces. The study thus supports layer orientation of (-30%/0 /30)6 were restrained in a graphite the concept that thin(<100 nm)carbon layers are required to fixture. The protective sizing on the cloth was removed by washing allow in situ sealing of the exposed interlayer by silica growth vith acetone while in the graphite fixture. A uniform pressure(6.9 The work reported here was undertaken to study the fabrication MPa) was then applied to the surface of the preform holder to by forced-flow, thermal-gradient chemical vapor infiltration compress the cloth sufficiently(to 4.76 mm thickness)to obtain (FCvIo of multilayer interfaces in composites containing either the desired -40 vol% fiber loading. The fixture was then placed ir ceramic-grade Nicalon or Hi-Nicalon fibers within a SiC matrix. an FCVI unit capable of fabricating 300-mm-diameter, 25-mm- Only Lackey and co-workers have previously prepared layered thick components. After infiltration the parts were ground to the atrix structures by FCvi. The intent of the current effort was to desired thickness (3. 18 mm) demonstrate FCVI fabrication, to verify the previous conclusions It was necessary to identify the deposition times for each type of with regard to composite contal coating for the alternating C/SiC-interface layers. Times were multilayer C/SiC interfaces, and to determine whether the use of determined by calibration runs and from previous experience with Hi-Nicalon fibers with their improved properties would differ from carbon deposition. Control of interface coating thicknesses in an those observed in the previous studies. FCVI system is difficult because of the changing conditions Minicomposites were used in the initial work to obtai data and experience with the multilayer interface system n through the thickness of the preform. The single-layer carbon interface coating was deposited in the FCVI system at a hot- composites require significantly less labor, time, and materials surface temperature of 1100.C and at 12 kPa total pressure with (including fibers, graphite fixturing, and precursor gases)to flow rates of 150 cm/min C3 Hs and 4400 cm /min argon for 3 h process than do composite plates. In addition, no cutting or The individual carbon layers of the C/SiC-multilayer interfaces rinding steps are required to prepare samples for tensile testing. were deposited under the same conditions; however, each carbon Results of the work with minicomposites were used to develop the layer was deposited for 42 min, Each SiC layer was deposited at process for depositing multilayers on fabric preforms for preparing hot-surface temperature of 1100C and at atmospheric pressure omposite plates. Specimens from the plates were tested in tension with an MTS flow rate of 540 cm /min carried by 5000 cm/min as fabricated and after exposure to ambient air at 950 C for 100 h of hydrogen for 2 min. Carbon was deposited initially, and the coatings were alternated to make six independent carbon laye and five independent SiC layers. IL. Experimental Details After the interface layers were deposited, the matrix was (1 Materials filtrated at a hot-surface temperature of 1100oC at atmospher pressure with an MTS flow rate of 270 cm/min carried by 3000 Two types of fibers were used: ceramic-grade Nicalon and cm/min of hydrogen. The exception was sample No. CVI-1172, Hi-Nicalon(Nippon Carbon, Tokyo, Japan). The tows for which a 1200C hot-surface temperature was used. (It was minicomposites contained 500 filaments. The cloth used determined from sample No. CVI-1172 that 1200C was too high composite plates was 600 X 600 tows/m, plain-weave fabric for effective infiltration of these relatively thin samples. whose tows also each contained 500 filaments The precursor for carbon deposition was 99% purity C3H6 (propylene, Matheson, Morrow, GA)and for Sic deposition, (4 Mechanical Property Testing technical-grade CH3 SiCl, [methyltrichlorosilane(MTS), Gelest, The minicomposites were evaluated in a testing machine that nc,Tullytown, PA]. In addition, 99.997% argon and 99.999% was developed in-house. Samples were loaded in tension until hydrogen(Air Liquide, Houston, TX) were used. failure in ambient air under a constant crosshead displacement rate of I um/s. The samples were gripped by a technique (also developed in-house)in which the ends of the minicomposite (2 Incomposite Fabrication specimens were embedded in epoxy inside aluminum rivets. The The fiber tows were supported in a graphite fixture that allowed load was transferred from the machine to the specimens through the fibers to be kept relatively straight. The fixture was set the rivets, which were held by a pair of specially designed grips vertically in a resistance-heated graphite furnace Gas flowed from The grips were connected to the load train by a self-aligni the bottom to the top of the reactor, and temperatures were mechanism. The resolution of the load cell was one part in 2. The measured with an optical pyrometer (Leeds and Northrup 8627, maximum capacity at 10 V is 1. 5kN (e, the resolution is 0. 2 N) Table I. Interface Layer Deposition Conditions and Infiltration Conditions for Minicomposites Interface coatings Infiltration SiC emperature°C) 0-1000 900-1000 Gas flow(cm/min unless Ar, 250-500 noted otherwise) C3H6, 12-25 CH3 SICl,(MTS), MTS, 0.3 g/min 0.15-0.3g Pressure(kPa) 0.67 0.67 5-60 min 2-30 min 8 hBertrand et al.19 tested SiC-matrix minicomposites containing thin C/SiC-interface layers (10 sets of layers each containing a 20 nm C- and a 50 nm SiC-thickness layer) on untreated Hi-Nicalon and fibers treated with a proprietary surface modification process. Samples were tested in static fatigue in an oxidizing atmosphere at 700°C under a load 10% above the proportional limit to assure matrix cracking. The results indicated a factor of 11 increase in life for minicomposites with multilayer C/SiC interfaces on untreated fibers (2 vs 22 h) and a 50% increase for minicomposites prepared with treated fibers (100 vs 150 h) over those with equivalent total thickness single carbon layer interfaces. The study thus supports the concept that thin (,100 nm) carbon layers are required to allow in situ sealing of the exposed interlayer by silica growth. The work reported here was undertaken to study the fabrication by forced-flow, thermal-gradient chemical vapor infiltration (FCVI)20 of multilayer interfaces in composites containing either ceramic-grade Nicalon or Hi-Nicalon fibers within a SiC matrix. Only Lackey and co-workers16 have previously prepared layered matrix structures by FCVI. The intent of the current effort was to demonstrate FCVI fabrication, to verify the previous conclusions with regard to composites containing ceramic-grade Nicalon with multilayer C/SiC interfaces, and to determine whether the use of Hi-Nicalon fibers with their improved properties would differ from those observed in the previous studies. Minicomposites were used in the initial work to obtain some data and experience with the multilayer interface system. Mini￾composites require significantly less labor, time, and materials (including fibers, graphite fixturing, and precursor gases) to process than do composite plates.21,22 In addition, no cutting or grinding steps are required to prepare samples for tensile testing. Results of the work with minicomposites were used to develop the process for depositing multilayers on fabric preforms for preparing composite plates. Specimens from the plates were tested in tension as fabricated and after exposure to ambient air at 950°C for 100 h. II. Experimental Details (1) Materials Two types of fibers were used: ceramic-grade Nicalon and Hi-Nicalon (Nippon Carbon, Tokyo, Japan). The tows for the minicomposites contained 500 filaments. The cloth used for the composite plates was 600 3 600 tows/m, plain-weave fabric whose tows also each contained 500 filaments. The precursor for carbon deposition was 99% purity C3H6 (propylene, Matheson, Morrow, GA) and for SiC deposition, technical-grade CH3SiCl3 [methyltrichlorosilane (MTS), Gelest, Inc., Tullytown, PA]. In addition, 99.997% argon and 99.999% hydrogen (Air Liquide, Houston, TX) were used. (2) Minicomposite Fabrication The fiber tows were supported in a graphite fixture that allowed the fibers to be kept relatively straight. The fixture was set vertically in a resistance-heated graphite furnace. Gas flowed from the bottom to the top of the reactor, and temperatures were measured with an optical pyrometer (Leeds and Northrup 8627, Honeywell, Morristown, NJ). Before deposition, the fibers expe￾rienced a heating cycle to 950°C for 1 h, which removed the protective sizing from the fiber surface. The conditions for depositing all of the interface layers and chemical vapor infiltra￾tion (CVI) of the matrix are given in Table I. (3) Plate Fabrication The fabricated plates measured 152 mm on a side and were prepared using FCVI. A total of 18 layers of cloth with a fabric layer orientation of (230°/0°/30°)6 were restrained in a graphite fixture. The protective sizing on the cloth was removed by washing with acetone while in the graphite fixture. A uniform pressure (6.9 MPa) was then applied to the surface of the preform holder to compress the cloth sufficiently (to 4.76 mm thickness) to obtain the desired ;40 vol% fiber loading. The fixture was then placed in an FCVI unit capable of fabricating 300-mm-diameter, 25-mm￾thick components. After infiltration the parts were ground to the desired thickness (3.18 mm). It was necessary to identify the deposition times for each type of coating for the alternating C/SiC-interface layers. Times were determined by calibration runs and from previous experience with carbon deposition. Control of interface coating thicknesses in an FCVI system is difficult because of the changing conditions through the thickness of the preform. The single-layer carbon interface coating was deposited in the FCVI system at a hot￾surface temperature of 1100°C and at 12 kPa total pressure with flow rates of 150 cm3 /min C3H6 and 4400 cm3 /min argon for 3 h. The individual carbon layers of the C/SiC-multilayer interfaces were deposited under the same conditions; however, each carbon layer was deposited for 42 min. Each SiC layer was deposited at a hot-surface temperature of 1100°C and at atmospheric pressure with an MTS flow rate of 540 cm3 /min carried by 5000 cm3 /min of hydrogen for 2 min. Carbon was deposited initially, and the coatings were alternated to make six independent carbon layers and five independent SiC layers. After the interface layers were deposited, the matrix was infiltrated at a hot-surface temperature of 1100°C at atmospheric pressure with an MTS flow rate of 270 cm3 /min carried by 3000 cm3 /min of hydrogen. The exception was sample No. CVI-1172, for which a 1200°C hot-surface temperature was used. (It was determined from sample No. CVI-1172 that 1200°C was too high for effective infiltration of these relatively thin samples.) (4) Mechanical Property Testing The minicomposites were evaluated in a testing machine that was developed in-house. Samples were loaded in tension until failure in ambient air under a constant crosshead displacement rate of 1 mm/s. The samples were gripped by a technique (also developed in-house) in which the ends of the minicomposite specimens were embedded in epoxy inside aluminum rivets. The load was transferred from the machine to the specimens through the rivets, which were held by a pair of specially designed grips. The grips were connected to the load train by a self-aligning mechanism. The resolution of the load cell was one part in 2.16 The maximum capacity at 10 V is 1.5 kN (i.e., the resolution is 0.2 N). Table I. Interface Layer Deposition Conditions and Infiltration Conditions for Minicomposites Interface coatings Carbon SiC Infiltration SiC Temperature (°C) 900–1000 900–1000 900–1000 Gas flow (cm3 /min unless noted otherwise) Ar, 250–500 H2, 250–500 H2, 500 C3H6, 12–25 CH3SiCl3 (MTS), 0.15–0.3 g/min MTS, 0.3 g/min Pressure (kPa) 0.67 0.67 0.67 Time 5–60 min 2–30 min 8 h December 2000 Ceramic Composites with Multilayer Interface Coatings 3015