An Cermet.So.87m11205-129(204) urna Manufacturing SiC-Fiber-Reinforced SiC Matrix Composites by Improved CVI/Slurry Infiltration/Polymer Impregnation and Pyrolysis Carlo Alberto Nannetti ENEA, Ente per le Nuove Tecnologie. I' Energia e I'Ambiente, C. R. Casaccia, 00060 Rome, Italy Alberto Ortona FN S p.A., 15062 Bosco Marengo(AL), Italy Dario a. de pinto ENEA, Ente per le Nuove Tecnologie, I'Energia e I'Ambiente. C. R. Brindisi, 72100 Brindisi, Italy Bruno riccardi Associazione EURATOM. ENEA, CP 65-000-44 Frascati(Rome), Italy Two- and three-dimensional SiC/SiC composites have been SiC /SiC composites, even with large and complex shapes. prepared starting from Tyranno Sa fiber preforms. Pre be made by impregnating a fibrous preform with a SiC-precursor form densification has been performed by a modified prece- polymer(PIP): in this case several impregnation-pyrolysis cycles ramic polymer impregnation and pyrolysis(PIP)process con- are necessary to achieve an acceptable densification sisting of filling the preform large interbundle voids with SiC To reduce the number of impregnation-pyrolysis cycles, Sic powder before the PIP process. This step was accomplished by powder is sometimes added to the polymer before preform low-pressure infiltration of a SiC powder dilute slurry through regnation: 7-5 Only a limited SiC powder amount can be the preform thickness. Specimens were further processed with dded to the polymer, as observed in our previous sudies. because polymer impregnation and pyrolysis to determine the effects on structural, thermal, and mechanical properties of the impregnation efficiency. Moreover, during infiltration, the preform which promoted polymer derived SiC matrix crystallization, may act as a filter to the incoming Sic-polymer suspension, often markedly increased thermal diffusivity leading to inhomogeneous distribution of the filler inside the refo So far, the lack of stoichiometry and the relatively low crystal L. Introduction linty of the SiC matrix formed by conversion of precursor polymers represent the major limitations for PIP, particularly for Cs ONTINUouS SiC-fiber-reinforced SiC matrix composites (SiC more advanced applications such as the nuclear ones, because SiC) are one of the most investigated materials for higl foreign phases and poor matrix crystallization significantly lower temperature structural applications because of their high strength composite thermal and mechanical properties and wear and corrosion resistance under severe conditions. In The recent availability of new high-yield polymers, which after addition, stability under neutron irradiation makes SiC/Sic a pyrolysis turn into almost stoichiometric SiC, and the devel promising candidate for future nuclear fusion reactors. Compos- ment of high-purity SiC fibers, which also retain their physical ites with two- and three-dimensional fiber texture are usually produced building a Sic matrix inside fibrous preforms by either amorphous polymer-derived matrix), make the PIP process a viable alternative to CVI in achieving a pure SiC matrix compos reaction bonding(RB) CVI, acting on fiber surfaces within the tow, is particularly ite.The PIP process. however, is not able to fill large interbundle suitable for depositing thin debonding interlayers between fiber increase and shrinkage in the precursor polymer leave a significant carbon(PyC) thin layer deposition onto fibers is the simplest CVI amount of residual porosity (10%-20%), unless many infiltration provides poor oxidation resistance. Concerning the matrix, CVI In the present work, we followed our previous experience in aid te needs long processing times, is not effective in completely filling ptmormed slurry infiltration with a fine SiC powder into carbon large interbundle voids, and suffers from untimely closure of small fiber performs followed by PIP and pulsed CVI, without analyzing pore access, thus leaving an unavoidable final porosity the effects of the slurry infiltration. A new combined CVI/slurry infiltration/PIP process was investigated with the overall aim to reduce the previously reported disadvantages on CVI and PIP. An N. S. Jacobson--contnbuting editor effective way of filling fiber preform large interbundle voids was also investigated. Further polymer-derived SiC matrix crystalliza- tion was pursued to increase the thermal conductivity. The process described here provides a promising way to fabricate SiC/SiC and Manuseript No, 186513. Received December 3, 2002: approved January 16, 2004 other CMCs with a low final porosity, reducing time process and Member. American Ceramic Soc
Journal of the American Ceramic Society-Nannetti et al Vol. 87. No. 7 Table I. Composites Densification Behavior at Various Stages of Preparation After CvI After Slurry Filtration arent densit Slurry int, PIP cycles Porosity Apparent density 153 20 153 0 197 14 1.4 1.41 2.0 10 1.48 1.93 2.63 2-D: composite made of stacked fabric, 3-D composite with fibers placed along three dimensions: CP: coarse SiC powder, FP: fine SiC powder. II. Experimental Procedure IlL. Results and Discussion (1) Composite Manufacturing (1) Densification Behavior Two-dimensional preforms were obtained stacking and pressing The typical densification behavior of the 2-D and 3-D compos- yranno SA(Ube Industries, Ube, Japan) plain woven cloths in a ites, measured at various stages of preparation, is summarized in T perforated graphite holder until the desired volume fraction Table L. The specimens related to both 2-D and 3-D composites are (-40%)was achieved. The 3-D preform architecture had a relative identified by the number of PIP cycles and by the Sic powder tiber distribution of 40: 40: 20 in the r y z directions, respectively, slurry infiltration step before PIP, using coarse powder(CP and a 36% fiber volume fraction pecimens)and fine powder(FP specimens). The slurry infiltration A thin pyrolytic carbon layer (-0.2 um) was deposited onto permitted homogeneous filling with SiC powder-15%-19% 2-D and 3-D-preforms fibers by isothermal chemical vapor infil (depending on initial preform porosity and SiC powder particle tration (I-CVI) using methane(CH,) precursor. A SiC thin layer ze)of the preform geometrical volume, roughly corresponding to (-0.2 um) was also deposited by I-CVI using methyltrichlorosi one-third of the composite total porosity after CVI lane(CH, SiCI,). The purpose was to increase preform stiffness and to prevent damages during subsequent processing A The porosity decrease versus PIP cycles number is shown in g I for two typical 3-D preforms. It is interesting to note that the To fill the large interbundle porosity inside the preforms and to SiC powder introduced through the slurry infiltration did not seem reduce the number of steps required by the typical PIP process, to affect significantly the subsequent PIP steps efficiency. This some samples were subjected to low-pressure infiltration, through effect can be clearly observed in Fig. I where, especially for the the preform thickness, of a dilute SiC powder aqueous slurry (6 early cycles, similar porosity trends versus PIP cycles are shown for 3-D preforms infiltrated with and without the powder infiltra- average grain size of -5 um (F 1000, Logitech, Glasgow tion step(3-D-CP-7 PIP and 3-D-14 PIP specimens, respectively. Scotland) and a finer one, with average grain size of-2 um(Dow Table D). The Sic powder infiltration step proved to sensibly Ceram, Dortmund, Germany ) Some preforms were also processed increase the preforms density, allowing final lower porosity levels without performing the SiC powder infiltration step with a reduced number of PIP cycles) than the samples produced The samples underwent several polymer impregnation/pyrolysis ly by CVI and PIP (Table 1). Malta, NY)as SiC-precursor, polymer pyrolysis was con- with powder infiltration and those densified only by PIP evidences alternatively at 1 100 and 1300 C in an argon atmospl the Sic powder filling efficiency in preforms large interbundle After densification, some composites were heated up to 1700. to voids. Figure 2 shows a polished cross section of a typical 3-D crystallize the PIP-derived SiC matrix, as performed by Berbon et specimen where a large void, delimited by three fiber bundles. al. for C/SiC composites. appears homogeneously filled with SiC particles bound by the polymer derived SiC. In addition, the presence of a continuous network of SiC powder greatly reduced the length and the size of (2) Characterization the cracks induced by the polymer shrinkage on pyrolisis.These are clearly evident in Fig. 3, which shows the cross section of a Composite apparent density values were computed by mass and geometric volume. The reported porosity values refer to total 2-D composite processed without the slurry infiltration step and porosity and were calculated assuming volume additivity for each phase present in the composite. Fiber density was assumed to be 3.02 g/cm, SiC powder density 3.20 g/cm, while the polymer derived SiC density, measured by helium picnometry, was 2.51 g/em(after pyrolysis at 1300C) -3D-14PIP The modulus of rupture(MOR) was measured by three-point 8-3D-CP-7PIP nding test at room temperature (span 40 mm, crosshead speed 0.5 mm/min) using 3 mm x 6 mm X 50 mm specimens. Elastic constants were measured by the longitudinal and transverse reso- nant frequencies using an e-meter resonant frequencies tester James Instruments, Chicago, IL). Thermal diffusivity was mea sured by the laser flash technique under vacuum. The specific heat 王王10% value of the finished composites was determined by differential 章8% scanning calorimetry (DSC 7, Perkin-Elmer, Norwalk, CT)at nea room temperature. An estimation of the thermal conductivity (TC) 01234567891011121314 was thus obtained by multiplying the thermal diffusivity data by parent density and specific heat. The composite microstructure was investigated by optical microscopy and scanning electron microscopy (SEM), performed on polished cross sections. Fig. I. Porosity versus number of PIP cycles for 3-D composite cessed with and without SiC slurry infiltration
July 2004 anufacturing SiC-Fiber-Reinforced SiC Matrix Composites 1207 50 um Fig. 2. Polished cross section of a 3-D composite processed with the Sic Fig. 4. Polished cross section of a 2-D composite processed with the SiC seven PIP cycles. The mage in Fig. 4, taken at low slurry infiltration at low magnification showing homogeneity of matrix magnification, indicates mogeneity of the densification densification (SEM). process and the almost absence of macroporosity of a 3-D composite processed SiC powder filtration step( CP-7 PIP specimen. Table D) es, presenting the same level of porosity. Moreover, the presence and grain size of SiC powder in the matrix affected (2) Thermal and Mechanical Properties neither the elastic modulus values of the posites, nor the bending strength, which is mainly influenced by the fiber content and 3-D composites: 2-D composites processed only by PIP had a and architecture. Two typical stress-deflection curves are illus. low thermal diffusivity in the thickness direction(<0.030 cm"/s) and significant strength retention after initial failure the corresponding thermal diffusivity values were doubled. The 3-D composites thermal diffusivity densified only by PIP was (3) High-Temperature Treatment significantly higher than 2-D composites because of the fraction of The specimens characterized by the highest thermal diffusivity fibers in the thickness direction. In this case. the SiC powder values were subjected to a further high temperatur infiltration step induced a further increase of thermal diffusivity up treatment(700C, 10 min) with the aim to induce polymer to-0.075cm2/s derived SiC matrix crystallization, and to investigate the effects of Concerning mechanical properties, MOR values of the polymer this treatment on structural, thermal, and mechanical properties. filtrated samples were improved from -400-480 MPa in both Polymer-derived SiC was subjected to the same treatment to 2-D and 3-D composites increasing PIP cycles from 7 to 14 follow the structural evolution of the material. X-ray diffraction because of the porosity decrease. Composites obtained by slurry attern evidenced a significant peak sharpening indicating growth infiltration and only 7 PIP cycles were characterized by a MOR of crystallite size up to 20-30 nm in agreement with the results 500 MPa)slightly higher than in samples processed with 14 PIP obtained by Berbon et al. In addition, the high-temperature treatment of the SiC from polymer pyrolysis increased the densit measured by He-picnometry, from 2.51 to 2.71 g/em and induced a further 5% mass loss. Composite density decreased following the high-temperature treatment with a concurrent porosity and pore subjected to two more PIP cycles with ing. Some specimens were before characterization. The main properties measured after the high-temperature treatment and after the two additional PIP cycles ollected in Table Ill The most significant effect of the high-temperature treatment was the increase in thermal diffusivity which almost doubled (and more than doubled for 3-D composites containing SiC fine powder). If high-thermal conductivity is of prime importance for material designers, polymer-derived SiC matrix crystallization seems unavoidable for PIP-processed composites Two additional PIP cycles seemed to be effective in regaining e initial density with a concurrent slight further increase in hermal conductivity. On the contrary, mechanical properties reouce d possibly indicating that Tyranno SA fibers or the interphase or the fiber-carbon interface are not completely um fected by the thermal treatment at 1700"C Figure 6 shows a polished cross section of a 3-D specimen processed with the SiC powder infiltration step, treated at high temperature and subjected to two additional PIP cycles: it evi Fig 3. Polished cross section of a 2-D composite processed only by PIP. dences dense matrix material in which the SiC powder is bound by
Journal of the American Ceramic Society-Nannetti et al Vol. 87. No. 7 Table Il. Composite Thermal and Mechanical Properties diffusivity modulus Mean mor Slurry inf PIP cycle 0.026 4.2 2.36 0.042 7.6 0.04 2.48 0.005 41 127 13.9 13.8 124 2-D: composite made of stacked fabric, 3-D composite with fibers placed along three dimensions: CP: coarse SiC powder: FP: fine Sic powder Table Ill. Composite Properties after Heat Treatment Before Heat Treatment After Heat Treatment After 2 additional PIP cycles Slurry inf. PIP cycles Apparent density Ther cond. Mean MOR Apparent density Ther cond. Apparent density Ther cond. Mean MOR 2.54 17.3 2.60 13.9 2.52 33.2 2.59 2.6 13.8 26.5 2.63 28.4 2-D: composite made of stacked fabric, 3-D composite with fibers placed along three dimensions: CP: course SiC powder. FP: fine SiC he polymer-derived SiC with a limited amount of pores. defects. IV. Conclusions 2D-CP-7PIP SiC-fiber-reinforced SiC matrix composites were prep 2D-7PIP starting from 2-D and 3-D Tyranno SA fiber preforms. Following CVI, densification was performed by an improved preceramic polymer impregnation and pyrolysis(PIP) process consisting of filling the interbundle voids with SiC powder through a slurry infiltration step before PIP. Approximately one-third of the total preform porosity was filled, indicating a homogeneous and tight 1.5 packing of the SiC particles inside the macro- and mesoporosity Consequently, a limited number of preceramic polymer(allylhy dridopolycarbosilane) impregnation and pyrolysis cycles were Fig. 5. Stress-deflection curve under flexural loading for typical 2-D ssed with and without the Sic polymer-derived SiC, bound to the SiC powder, lead to a compos- ite matrix with a limited amount of pores, def fects. and micro cracks Sic particles presence did not show remarkable effects on the elastic and mechanical properties, which were, as a whole, rather satisfactory(see Refs. 4, 7, and 12)and were dictated mainly by the degree of densification and the fiber volume fraction and architecture. The through-the-thickness thermal diffusivity almost doubled for 2-D composites and significantly increased for 3-D ones A final thermal treatment at high temperature(1700 C, 10 min) promoted polymer-derived SiC matrix crystallization, markedly increasing thermal diffusivity from -0.075 up to 0. 180 cm/s Flexural strength decreased from--500 to 350 MPa, indicating that the Tyranno SA fibers or the interface between fibers and carbon-SiC interlayers are not completely stable at 1700.C even Acknowledgments The authors wish to thank Mr Mesto Scafe for thermal diffusivity measure 50 nts, Mr, Franco Padella for XRD and specifie heat measurements, and Mrs Federica De Riccardis for SEM observations References Fig. 6, Polished cross section of a 3-D composite processed with the SiC R H Jones, D. Steiner, H. L. Heinisch, G. A. Newsome, and H. M. Kerch slurry infiltration step after high-temperature(1700 C) treatment ad two additional PIP cycles Radiation Resistant Ceramic Matrix Composites."J. NucL Mater. 245. 87-107
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