Fusion Engineering and design ELSEVIER Fusion Engineering and Design 51-52(2000)159-16 www.elsevier.com/locate/fusengdes Sic-SiC Cmc manufacturing by hybrid CVI-PIP techniques: process optimisation A. Ortona a, * A. Donato b, G. Filacchioni, U. De Angelis, A. La Barbera, C A. Nannettic. B. Riccardi b.. Yeatman d FN S.P.A., 15062 Bosco Marengo, AL, italy b Associazione EURATOM, ENEA, CP 65-00044 Frascati, Rome, Italy ENEA INN NUMA. Casaccia, S. Maria di galeria, 00060 Rome, italy d archer Techmnicoat Ltd, High Wycombe Bucks. HP12 4D, UK Abstract Sic-Sicr ceramic matrix composites( CMC)are candidate structural material for fusion power reactor applications because of their favourable thermo-mechanical and low-activation properties. Among their different manufacturing techniques, present, the most employed ones are chemical vapour infiltration(CVI)and polymer infiltration and pyrolysis(PIP). These two techniques are based on the common principle of filling the porosity among the fibres with Sic resulting from precursor decomposition. Cvi process deposits high purity crystalline Sic with good properties to fibres whereas PIP leaves lower characteristic amorphous Sic with traces of oxygen between fibres. PIP, on the other hand, seems to be much more industrially effective than CVi. In the attempt to maximise the properties and reduce costs, some work has been done on the so called ' hybrid techniques' in which CVI and PIP are both employed. The work performed by ENEA and FN S.P.A. consists of a series of combined CVi-PiP process cycles and the subsequent product characterisation. o 2000 Elsevier Science B V. All rights reserved Keywords: SiC-SiC ceramic matrix composites; Chemical vapour infiltration(CVn): Polymer infiltration and pyrolysis (PIP) Process optimisation 1. Introduction sion reactor first wall(FW)and blanket systems [1-3] because of their good mechanical properties SiC-SiCr ceramic matrix composites(CMC) at high temperature, low chemical sputtering, high are innovative materials originally developed for oxygen gettering and very low short- and aerospace and energy application; recently, they medium-term activation. These composites, con- have been proposed as structural material of fu ceived with the aim of reducing the intrinsic brit- tle behaviour of the monolithic silicon carbide are produced by infiltrating a SiC matrix in a Sic Corresponding author. Tel: 39-.131-297338: fax:+39. fibre woven fabric(preform). An interphase mate l31-297250 rial (generally carbon) is deposited on the fibre E-mail address: alortona(a tin. it(A. Ortona). prior to the Sic infiltration, aiming for stopping 0920-3796/00/S- see front matter c 2000 Elsevier Science B.v. All rights reserved. PI:s0920-3796(00)00310-0
Fusion Engineering and Design 51–52 (2000) 159–163 SiC–SiCf CMC manufacturing by hybrid CVI–PIP techniques: process optimisation A. Ortona a,*, A. Donato b , G. Filacchioni c , U. De Angelis c , A. La Barbera c , C.A. Nannetti c , B. Riccardi b , J. Yeatman d a FN S.p.A., 15062 Bosco Marengo, AL, Italy b Associazione EURATOM, ENEA, CP 65-00044 Frascati, Rome, Italy c ENEA INN.NUMA. Casaccia, S.Maria di Galeria, 00060 Rome, Italy d Archer Technicoat Ltd., High Wycombe Bucks. HP12 4JD, UK Abstract SiC–SiCf ceramic matrix composites (CMC) are candidate structural material for fusion power reactor applications because of their favourable thermo-mechanical and low-activation properties. Among their different manufacturing techniques, present, the most employed ones are chemical vapour infiltration (CVI) and polymer infiltration and pyrolysis (PIP). These two techniques are based on the common principle of filling the porosity among the fibres with SiC resulting from precursor decomposition. CVI process deposits high purity crystalline SiC with good properties onto fibres whereas PIP leaves lower characteristic amorphous SiC with traces of oxygen between fibres. PIP, on the other hand, seems to be much more industrially effective than CVI. In the attempt to maximise the properties and reduce costs, some work has been done on the so called ‘hybrid techniques’ in which CVI and PIP are both employed. The work performed by ENEA and FN S.p.A. consists of a series of combined CVI–PIP process cycles and the subsequent product characterisation. © 2000 Elsevier Science B.V. All rights reserved. Keywords: SiC–SiCf ceramic matrix composites; Chemical vapour infiltration (CVI); Polymer infiltration and pyrolysis (PIP); Process optimisation www.elsevier.com/locate/fusengdes 1. Introduction SiC–SiCf ceramic matrix composites (CMC) are innovative materials originally developed for aerospace and energy application; recently, they have been proposed as structural material of fusion reactor first wall (FW) and blanket systems [1–3] because of their good mechanical properties at high temperature, low chemical sputtering, high oxygen gettering and very low short- and medium-term activation. These composites, conceived with the aim of reducing the intrinsic brittle behaviour of the monolithic silicon carbide, are produced by infiltrating a SiC matrix in a SiC fibre woven fabric (preform). An interphase material (generally carbon) is deposited on the fibre prior to the SiC infiltration, aiming for stopping * Corresponding author. Tel.: +39-131-297338; fax: +39- 131-297250. E-mail address: alortona@tin.it (A. Ortona). 0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0920-3796(00)00310-0
A. Ortona et al./Fusion Engineering and Design 51-52(2000)159-16 or deflecting the cracks that appear in the matrix was carried from the idea of investigating the and for increasing the composite toughness. possibility to produce high density thick panels by Moreover, the availability of three-dimensional combining CVI and PIP techniques in order to (3-D)SiC fibre preforms leads to composites with understand the effects of different ratios of CVI higher inter-laminar shear strength and thermal duration process and PlP cycles number on com- conductivity and, consequently, more attractive posite density and mechanical strength. The pre for their use as structural material of fusion liminary results, obtained during the above-mentioned manufacturing campaign, are Different techniques are available for densifying discussed in the paper e preforms and producing SiC-SiCr composites they include chemical vapour infiltration(CVn [5]. polymer infiltration and pyrolysis(PIP)[6-8. 2. Experimental These techniques make the manufacturing of large shaped components or half-finished products pos 2. 1. Materials employed sible, which in principle, can be assembled to parts of fusion reactors. In particular, CVI tech The constituent materials employed in the nique leads to high purity crystalline Sic matrices resent processing tests were kept similar to previ- but this technique has some disadvantages con- ous experiments for comparison [6-8 nected with the long production time, conse- Fibres used were NL 207 Nippon Carbon (14 quently, high cost and to the difficulty to infiltrate um in diameter)with a density of 2.53 g/cm and thick preforms(>4 mm). Nevertheless, a thick- a chemical composition of Si, 56.2%, C, 31.8%: ness up to 10 mm may be requested for FW-blan- and O, 12%. ket system in order to withstand the The fibre preforms were obtained stitching to- thermo-mechanical load expected during the reac- gether layers of fabric(satin, 5 h). That was done tor operation. Generally, the increase of preform to introduce the fibres in a third direction(thick- thickness makes the composites densification ness) and to increase toughness. After stitching rather difficult because of the occurrence of a fibre, percentages in the tree directions were density gradient across the thickness with a pro- warp, 45%; weft, 45%, and thickness, 10%. For gressive sealing effect appearing on the external the PIP processing, polycarbosylane(PCS) poly surfaces of the composite, which reduces the mer from Nippon Carbon was used. infiltration efficiency itself. CVI process has been At the first PIP cycle, preforms were infiltrated assessed to be strongly affected by this problem, with a slurry of PCS and B-crystalline SiC nano- which seems to play a minor role for PIP powders produced by LASEr assisted synthesis technique. from gaseous precursor [4]. Powder characteristics The activity illustrated in this paper was con- are, surface area, 60 m/g: oxygen content, 1.0%: eived with the aim to investigate the possibility total carbon, 30%(by weight); and density, 3.2 of producing panels of relevant thickness using g/cm PIP technique applied on thick preform(7 and mm). As well-known, PIP technique, more eco- 2. 2. Chemical vapour infiltration(CVI nomically attractive, leads to lower purity Sic matrix with the presence of an amorphous glassy The preforms were clamped in graphite tooling phase and consequently, to composites with lower keep their shape and flatness. The tooling thermal-mechanical properties than CVI com- consisted of perforated graphite plates spaced by posites. Nevertheless, the availability of high pu- graphite bushes. A stack of six tooled panels was rity, almost stoichiometric fibres and new compressed in a hydraulic press until the desired preceramic polymers, which allow higher pyrolysis preform thickness was achieved. The height of the temperatures in the PIP process, will lead to more stack was then fixed using carbon /carbon com- crystalline SiC matrices. The presented activity posite fasteners. Both C and SiC CVI were
160 A. Ortona et al. / Fusion Engineering and Design 51–52 (2000) 159–163 or deflecting the cracks that appear in the matrix and for increasing the composite toughness. Moreover, the availability of three-dimensional (3-D) SiC fibre preforms leads to composites with higher inter-laminar shear strength and thermal conductivity and, consequently, more attractive for their use as structural material of fusion reactors. Different techniques are available for densifying the preforms and producing SiC–SiCf composites, they include chemical vapour infiltration (CVI) [5], polymer infiltration and pyrolysis (PIP) [6–8]. These techniques make the manufacturing of large shaped components or half-finished products possible, which in principle, can be assembled to parts of fusion reactors. In particular, CVI technique leads to high purity crystalline SiC matrices, but this technique has some disadvantages connected with the long production time, consequently, high cost and to the difficulty to infiltrate thick preforms (]4 mm). Nevertheless, a thickness up to 10 mm may be requested for FW-blanket system in order to withstand the thermo-mechanical load expected during the reactor operation. Generally, the increase of preform thickness makes the composites densification rather difficult because of the occurrence of a density gradient across the thickness with a progressive sealing effect appearing on the external surfaces of the composite, which reduces the infiltration efficiency itself. CVI process has been assessed to be strongly affected by this problem, which seems to play a minor role for PIP technique. The activity illustrated in this paper was conceived with the aim to investigate the possibility of producing panels of relevant thickness using PIP technique applied on thick preform (7 and 10 mm). As well-known, PIP technique, more economically attractive, leads to lower purity SiC matrix with the presence of an amorphous glassy phase and consequently, to composites with lower thermal–mechanical properties than CVI composites. Nevertheless, the availability of high purity, almost stoichiometric fibres and new preceramic polymers, which allow higher pyrolysis temperatures in the PIP process, will lead to more crystalline SiC matrices. The presented activity was carried from the idea of investigating the possibility to produce high density thick panels by combining CVI and PIP techniques in order to understand the effects of different ratios of CVI duration process and PIP cycles number on composite density and mechanical strength. The preliminary results, obtained during the above-mentioned manufacturing campaign, are discussed in the paper. 2. Experimental 2.1. Materials employed The constituent materials employed in the present processing tests were kept similar to previous experiments for comparison [6–8]. Fibres used were NL 207 Nippon Carbon (14 mm in diameter) with a density of 2.53 g/cm3 and a chemical composition of Si, 56.2%; C, 31.8%; and O, 12%. The fibre preforms were obtained stitching together layers of fabric (satin, 5 h). That was done to introduce the fibres in a third direction (thickness) and to increase toughness. After stitching fibre, percentages in the tree directions were, warp, 45%; weft, 45%, and thickness, 10%. For the PIP processing, polycarbosylane (PCS) polymer from Nippon Carbon was used. At the first PIP cycle, preforms were infiltrated with a slurry of PCS and b-crystalline SiC nanopowders produced by LASER assisted synthesis from gaseous precursor [4]. Powder characteristics are, surface area, 60 m2 /g; oxygen content, 1.0%; total carbon, 30% (by weight); and density, 3.2 g/cm3 . 2.2. Chemical 6apour infiltration (CVI) The preforms were clamped in graphite tooling to keep their shape and flatness. The tooling consisted of perforated graphite plates spaced by graphite bushes. A stack of six tooled panels was compressed in a hydraulic press until the desired preform thickness was achieved. The height of the stack was then fixed using carbon/carbon composite fasteners. Both C and SiC CVI were per-
A. Ortona et al./Fusion Engineering and Design 51-52(2000)159-16 Table I Weight gain for each sample after each manufacturing phase Sample name CVI Total weight gain(g) Duration(h) Weight gain(g) Number of cycles Weight gain(g) 6=20-2 c05 219 25.0 433 formed in an ATL VHT1214 multi-purpose CVI/. three point bending strength M.O.R.(span, 40 Cvd development reactor at Archer Technicoat mm;cross head speed, I mm/min); and Ltd interlaminar shear strength (ISS, short-span Carbon CVi was performed using a mixture of method)(span, 30 mm; and cross head speed I N2 and CH, at a temperature of 1200oC and a mm/min) of I mbar. a deposition rate of api Samples were cut with a diamond saw. For mately 0.01 um/h was achieved ISS(short-span test) they were left at their as Silicon carbide CVi was performed using a prepared thickness; for bending strength, they mixture of methyl-trichloro-silane (MTS, were milled till reaching 3-mm-thickness. CH3SiCIy)and H2 at a temperature of 950C and a pressure of I mbar. a deposition rate of ap- proximately 0.01 um/h was achieved 3. Results and discussion A total of 12 panels of 7-mm-thick were pro- cessed. Each panel had 10 h of carbon CVI fol- Mean data values on the above mentioned me- lowed by 10 h of Sic CvI whilst fixed in tooling. chanical tests are collected in Table 2 Thereafter, the panels were sufficiently stiff to be Fig. I shows how bending stress curve changes processed without tooling SiC CVI coating cycles with increasing PIP cycles while decreasing CVI (10 h per cycle)were performed on each set of cycles. A similar behaviour can be noticed in panel for 20, 30, 40, 50 and 60 h short span tests The strengthening effect of the 3-D texture is 2.3. Polymer infiltration and pyrolysis(PIP) clearly shown in Fig. 2. It presents bending curves for panels with similar density (e.g. 1.8-1.9 g/cm) PIP densification procedure has been described but with different fibre architecture. The grey Isewhere [6-8 curve corresponds to composite with fibres in 2-D After the first infiltration and pyrolysis, lami- (30 h CVI 7 PIP from pre ates exhibited porosity values around 40-50%0 To increase their density and mechanical strength Table 2 they needed further infiltration ons and pyrolysis Mean mechanical test data In order to study the efficiency of the PIP cycles versus CVl, five sets of samples were produced Sample name Density M.O.R. LS.S (MPa) (MPa) employing different number of CVI and PIP cy- cles as per table I 6-20-2 247.03 6-30-3 1.74 216.40 2.4. Mechanical characterisation 6-505 152 66.56 6-60-6 1.5 Two mechanical tests were performed
A. Ortona et al. / Fusion Engineering and Design 51–52 (2000) 159–163 161 Table 1 Weight gain for each sample after each manufacturing phase Sample name CVI PIP Total weight gain (g) Duration (h) Number of cycles Weight gain (g) Weight gain (g) 6-20-2 20 52.0 8.6 7 60.6 6-30-3 13.6 6 51.0 64.6 30 6-40-4 17.4 4 48.3 65.7 40 6-50-5 50 28.6 21.9 3 50.5 6-60-6 60 25.0 3 25.7 50.7 formed in an ATL VHT1214 multi-purpose CVI/ CVD development reactor at Archer Technicoat Ltd. Carbon CVI was performed using a mixture of N2 and CH4 at a temperature of 1200°C and a pressure of 1 mbar. A deposition rate of approximately 0.01 mm/h was achieved. Silicon carbide CVI was performed using a mixture of methyl-trichloro-silane (MTS, CH3SiCl3) and H2 at a temperature of 950°C and a pressure of 1 mbar. A deposition rate of approximately 0.01 mm/h was achieved. A total of 12 panels of 7-mm-thick were processed. Each panel had 10 h of carbon CVI followed by 10 h of SiC CVI whilst fixed in tooling. Thereafter, the panels were sufficiently stiff to be processed without tooling. SiC CVI coating cycles (10 h per cycle) were performed on each set of panel for 20, 30, 40, 50 and 60 h. 2.3. Polymer infiltration and pyrolysis (PIP) PIP densification procedure has been described elsewhere [6–8]. After the first infiltration and pyrolysis, laminates exhibited porosity values around 40–50%. To increase their density and mechanical strength they needed further infiltrations and pyrolysis. In order to study the efficiency of the PIP cycles versus CVI, five sets of samples were produced employing different number of CVI and PIP cycles as per Table 1. 2.4. Mechanical characterisation Two mechanical tests were performed: three point bending strength M.O.R. (span, 40 mm; cross head speed, 1 mm/min); and interlaminar shear strength (I.S.S., short-span method) (span, 30 mm; and cross head speed 1 mm/min). Samples were cut with a diamond saw. For I.S.S. (short-span test) they were left at their ‘as prepared’ thickness; for bending strength, they were milled till reaching 3-mm-thickness. 3. Results and discussion Mean data values on the above mentioned mechanical tests are collected in Table 2. Fig. 1 shows how bending stress curve changes with increasing PIP cycles while decreasing CVI cycles. A similar behaviour can be noticed in short span tests. The strengthening effect of the 3-D texture is clearly shown in Fig. 2. It presents bending curves for panels with similar density (e.g. 1.8–1.9 g/cm3 ) but with different fibre architecture. The grey curve corresponds to composite with fibres in 2-D (30 h CVI 7 PIP from previous experiments), and Table 2 Mean mechanical test data Sample name I.S.S. Density M.O.R. (cm (MPa) (MPa) 3 ) 6-20-2 1.8 247.03 21.3 6-30-3 216.40 20.96 1.74 6-40-4 190.43 1.73 14.2 6-50-5 1.52 66.56 5.76 6-60-6 67.43 5.87 1.58
nto account masses and respective densities of each component assuming volumes additivity he results are collected in Table 3 The first ed is that the initial porosity of 3-D specimens is rather high i.e. the olume fraction of fibres for all the specimens is ther low(32-33%)in com to the fibre typical 2-D specimens 40%o). This is possibly due to the stitching 00D0204a 11310L0 operation which does not allow tight packing of the fabrics in the Z-direction. due to this high isplacement bending curves for different initial porosity, at least partly due fibres architecture and to the high thickness of the preforms, even the maximum number of PIP ()were not enough for a satisfactory densifica- tion of the specimens. Work in progress is targeted to optimise the polymer infiltration pro cedures for this type of preforms Concerning CVI, the decreasing trend of the residual porosity with the duration of the process is evident: the gain in densification(porosity re- duction) obtained with the longest CVI cycle seems to be lost after three or. in the worst cases four PIP steps. This phenomenon could be related 00304008I dkptnetay 140 160 180 20 to the formation of some closed porosity in the specimens subjected to long CVI cycles thus de Fig.2.Stress vs displacement bending curves for 2-D and 3-D creasing the efficiency of the further infiltration and pyrolysis steps. This hypothesis is supported by microscope observation(Fig. 3)of the mi- crostructure of the two manufacturing extremes the black curve to the 3-D(20 h CVI 7 PIP)panel i.e. the sample subjected to the shortest CVI cycle of the present res and the highest number of PIP one The evolution of the total porosity has been with the longest CVI cycle and only three PIP evaluated at various stages of densification trying steps. In the latter one(micrograph B), the CVi to show the effects of different CVI-PIP combi- Sic is clearly visible around the fibres section nations. The porosity has been calculated taking (white phase)while the almost complete absence Composite residual porosity after each process Residual porosity (% Specimen Preform After CVi After three PIP After four PIP After five PIP After six PIP After seven PIP 59.8 38.5 33.1 28.0 6=30-368.2 6-40-4 35.7 6=50-5 56.4 6-60-6 68.3 39.3
162 A. Ortona et al. / Fusion Engineering and Design 51–52 (2000) 159–163 Fig. 1. Stress vs. displacement bending curves for different CVI–PIP combinations. into account masses and respective densities of each component assuming volumes additivity. The results are collected in Table 3. The first point to be noticed is that the initial porosity of 3-D specimens is rather high i.e. the volume fraction of fibres for all the specimens is rather low (32–33%) in comparison to the fibres volume fraction of our typical 2-D specimens (]40%). This is possibly due to the stitching operation. which does not allow tight packing of the fabrics in the Z-direction. Due to this high initial porosity, at least partly due to the 3-D fibres architecture and to the high thickness of the preforms, even the maximum number of PIP steps (7) were not enough for a satisfactory densification of the specimens. Work in progress is targeted to optimise the polymer infiltration procedures for this type of preforms. Concerning CVI, the decreasing trend of the residual porosity with the duration of the process is evident; the gain in densification (porosity reduction) obtained with the longest CVI cycle seems to be lost after three or, in the worst cases, four PIP steps. This phenomenon could be related to the formation of some closed porosity in the specimens subjected to long CVI cycles thus decreasing the efficiency of the further infiltration and pyrolysis steps. This hypothesis is supported by microscope observation (Fig. 3) of the microstructures of the two manufacturing extremes i.e. the sample subjected to the shortest CVI cycle and the highest number of PIP steps, and the one with the longest CVI cycle and only three PIP steps. In the latter one (micrograph B), the CVI SiC is clearly visible around the fibres section (white phase) while the almost complete absence Fig. 2. Stress vs. displacement bending curves for 2-D and 3-D composite. the black curve to the 3-D (20 h CVI 7 PIP) panel of the present research. The evolution of the total porosity has been evaluated at various stages of densification trying to show the effects of different CVI–PIP combinations. The porosity has been calculated taking Table 3 Composite residual porosity after each process Residual porosity (%) Specimen Preform After CVI After three PIP After four PIP After five PIP After six PIP After seven PIP 6-20-2 38.5 28.0 66.4 43.4 59.8 33.1 30.5 6-30-3 33.2 68.2 59.3 40.6 36.3 30.6 6-40-4 67.7 56.9 35.7 31.0 6-50-5 41.4 68.1 56.4 6-60-6 68.3 54.7 39.3
A. Ortona et al./Fusion Engineering and Design 51-52(2000)159-16 Fig. 3. Micrograph(300 x ) showing the influence of CVI and PlP cycles on the composite morphology. of Sic from PIP is partly due to artefacts of by repeated PIP cycles and a final chemical va polishing samples with high porosity, but also pour deposition due to the presence of closed porosity inside many fibres pockets On the other hand, the SiC from PIP(medium grey) is clearly visible References many small pockets within the fibres of the sam- ple since the short CVI cycle did not close any [s. Sharafat, F. Najmabadi. C. P.C. Wong, the ARIES ity(micrograph A) R)SU des. 18(1991)215021 r core engineering, Fusio Team. ARIES-I fusion Ueda, S. Nishio. Y. Seki, R. Kurihara, J. Adachi, S. Yamazaki, DREAM design team, A fusion power reactor concept using SiC/SiC composites, J. Nucl. Mater. 258 4. Conclusions 263(1998)1589-1593. B] A.S. Peres Ramirez, A Caso, L. Gi N. Le Bars. G The activity carried out has shown that Chaumat, J F. Salavy, J. Szczepans nsk. tAU LURO: a ceram VI can deposit high purity B-crystalline Sic composite Pb-I7Li breeder between the fibres, long CVi seems to be not blanket concept, J. Nucl. Mater. 233-237( 1996)1257. effective for a high densification by further PIP (4)E. Borsella, S Botti, M.C. Cesile, SMartelli,R.Alexan drescu, R. Giorgi, C.A. Nannetti, S. Turtu, G. Zappa, On the other hand, CVi is an unequalled pro- Nanostruct. Mater. 6(1995)321 cess for inter-phase deposition and to make preform stiff enough to be densified by PIP Yoposites.Ht-cMcIl,Ceram.Trans.58(1995)/.com- [5 T.M. Besmann, CVI processing of ceramic matri A. Donato, A. Ortona, C.A. Nannetti, S Casadio, SiC/SiC CVI indeed prevents a preform from swelling fibre ceramic composites for fusion application: a new during the early cycles of polymer infiltration manufacturing process, Procedings of the 19th Symposium and pyrolysis. After PIP cycles, chemical vapour on Fusion Technology (SOFT), Lisbon, Portugal, Septem deposition(CVD) for depositing a Sic coating ber 1996 on the panels has shown to increase the final [7 s. Casadio, A. Donato, A. Nannetti, A. Ortona, Rescio, Liquid infiltration and pyrolysis of Sic com density and mechanical properties of the Sicr materials for structural applications, HT-CMC 2, Ce Sic composites [8 Transactions 58(1995)193-198 Santa Barbara, CA, USA The conclusion that can be drawn at the pre liminary stage is that, while using an hybrid 8 Licciulli, A. Ortona, C.A. Nannetti, s an characterisation of SiC-SiC composi CVI-PlP technique the best combination is to brid we vapour processing. Proceedings of CM San Se. have a short lasting initial CVI phase followed bastian, Spain, September 1996
A. Ortona et al. / Fusion Engineering and Design 51–52 (2000) 159–163 163 Fig. 3. Micrograph (300×) showing the influence of CVI and PIP cycles on the composite morphology. of SiC from PIP is partly due to artefacts of polishing samples with high porosity, but also due to the presence of closed porosity inside many fibres pockets. On the other hand, the SiC from PIP (medium grey) is clearly visible in many small pockets within the fibres of the sample since the short CVI cycle did not close any porosity (micrograph A). 4. Conclusions The activity carried out has shown that while CVI can deposit high purity b-crystalline SiC between the fibres, long CVI seems to be not effective for a high densification by further PIP cycles. On the other hand, CVI is an unequalled process for inter-phase deposition and to make a preform stiff enough to be densified by PIP. CVI indeed prevents a preform from swelling during the early cycles of polymer infiltration and pyrolysis. After PIP cycles, chemical vapour deposition (CVD) for depositing a SiC coating on the panels has shown to increase the final density and mechanical properties of the SiCf / SiC composites [8]. The conclusion that can be drawn at the preliminary stage is that, while using an hybrid CVI–PIP technique, the best combination is to have a short lasting initial CVI phase followed by repeated PIP cycles and a final chemical vapour deposition. References [1] S. Sharafat, F. Najmabadi, C.P.C. Wong, the ARIES Team, ARIES-I fusion power core engineering, Fusion Eng. Des. 18 (1991) 215–222. [2] S.Ueda, S. Nishio, Y. Seki, R. Kurihara, J. Adachi, S. Yamazaki, DREAM design team, A fusion power reactor concept using SiC/SiC composites, J. Nucl. Mater. 258– 263 (1998) 1589–1593. [3] A.S. Peres Ramirez, A. Caso, L. Giancarli, N. Le Bars, G. Chaumat, J.F. Salavy, J. Szczepanski, TAURO: a ceramic composite structural material self cooled Pb–17Li breeder blanket concept, J. Nucl. Mater. 233–237 (1996) 1257– 1261. [4] E. Borsella, S. Botti, M.C. Cesile, S. Martelli, R. Alexandrescu, R. Giorgi, C.A. Nannetti, S. Turtu`, G. Zappa, Nanostruct. Mater. 6 (1995) 321. [5] T.M. Besmann, CVI processing of ceramic matrix composites. HT-CMC II, Ceram. Trans. 58 (1995) 1. [6] A. Donato, A. Ortona, C.A. Nannetti, S. Casadio, SiC/SiC fibre ceramic composites for fusion application: a new manufacturing process, Procedings of the 19th Symposium on Fusion Technology (SOFT), Lisbon, Portugal, September 1996. [7] S. Casadio, A. Donato, A. Nannetti, A. Ortona, M.R. Rescio, Liquid infiltration and pyrolysis of SiC composite materials for structural applications, HT-CMC 2, Cermaic Transactions 58 (1995) 193–198 Santa Barbara, CA, USA August 1995. [8] Licciulli, A. Ortona, C.A. Nannetti, S. Botti, Preparation an characterisation of SiC–SiC composite by hybrid wet/ vapour processing, Proceedings of CMMC 1996, San Sebastian, Spain, September 1996.