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HIGH TEMPERATURE BRAZING FOR SiC AND SiC F /SiC CERAMIC MATRIX COMPOSITES B.Riccardi Associazione EURATOM-ENEA, ENEA CR Frascati, PB 65- 00044 Frascati (Rome), Italy C.A.Nannetti ENEA CR Casaccia, 00060 S.Maria di Galeria (Rome), Italy J.Woltersdorf and E.Pippel Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle/Saale, Germany T.Petrisor Technical University of Cluj-Napoca, Romania, ENEA Consultant ABSTRACT The paper presents the results of the development of a brazing technique for monolithic SiC and SiC f /SiC composites. This brazing technique is based on the use of Si-16Ti (at. %) and Si-18Cr (at %) eutectic alloys. The brazing temperature of the used alloys allows to avoid the degradation of the fibre/matrix- interfaces in the composite materials at least for advanced stoichiometric SiC fibre composites. All the joints showed excellent adhesion and no discontinuities and defects at the interface, while the brazing layer revealed a fine eutectic structure. In particular, in the composite joints the brazing layer appeared well adherent both to the matrix, the fibres and the fibre-matrix interphase, and the brazing alloy infiltration looked sufficiently controlled. The brazing alloys were characterized by X ray diffraction and scanning electron microscopy (SEM). All the brazed joints were analysed by SEM and the Si-16Ti joints were also characterized by high resolution transmission electron microscopy investigations of the microstructure and of the nanochemistry (HREM, EELS, esp. ELNES). These analyses revealed atomically sharp interfaces without interdiffusion or phase formation at the interface leading to the conclusion that direct chemical bonds are responsible for the adhesion.
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HIGH TEMPERATURE BRAZING FOR SiC AND SiCF/SiC CERAMICMATRIX COMPOSITES

B.Riccardi

Associazione EURATOM-ENEA, ENEA CR Frascati, PB 65- 00044 Frascati(Rome), Italy

C.A.NannettiENEA CR Casaccia, 00060 S.Maria di Galeria (Rome), Italy

J.Woltersdorf and E.PippelMax-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle/Saale,Germany

T.PetrisorTechnical University of Cluj-Napoca, Romania, ENEA Consultant

ABSTRACTThe paper presents the results of the development of a brazing technique

for monolithic SiC and SiCf/SiC composites. This brazing technique is based onthe use of Si-16Ti (at. %) and Si-18Cr (at %) eutectic alloys. The brazingtemperature of the used alloys allows to avoid the degradation of the fibre/matrix-interfaces in the composite materials at least for advanced stoichiometric SiC fibrecomposites. All the joints showed excellent adhesion and no discontinuities anddefects at the interface, while the brazing layer revealed a fine eutectic structure.In particular, in the composite joints the brazing layer appeared well adherent bothto the matrix, the fibres and the fibre-matrix interphase, and the brazing alloyinfiltration looked sufficiently controlled. The brazing alloys were characterizedby X ray diffraction and scanning electron microscopy (SEM). All the brazedjoints were analysed by SEM and the Si-16Ti joints were also characterized byhigh resolution transmission electron microscopy investigations of themicrostructure and of the nanochemistry (HREM, EELS, esp. ELNES). Theseanalyses revealed atomically sharp interfaces without interdiffusion or phaseformation at the interface leading to the conclusion that direct chemical bonds areresponsible for the adhesion.

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Shear tests performed at room temperature and 600°C on lap joint specimens gaveremarkable results: the samples manufactured with monolithic SiC exhibit a highshear stress level and mainly cracked in the SiC bulk (150 MPa at RT for Si-18Crjoints), while the composite samples exhibited a RT shear strength up to 70 MPafor Si-16Ti and up to 80 MPa for Si-18 Cr with failure occurring mainly in thebase material.

INTRODUCTIONSilicon carbide and SiCf/SiC ceramic matrix composites are attractive

materials for energy application because of their chemical stability andmechanical properties at high temperature [1]. Nevertheless, in order tomanufacture complex components the availability of suitable joining techniquesis necessary. Among several joining techniques under development [2,3,4,5,6],the brazing is one of the most promising[7]. The requirements of a suitablebrazing material are: chemical compatibility and wettability with SiC substrate,thermal expansion coefficient similar to that of the SiC substrate, high shearstrength, and for the composite joints a brazing temperature low enough to avoid adegradation of the fibres and the fibre-matrix interface. The possibility to use pure silicon without active metal filler as braze for siliconcarbide has been assessed in previous works [8]. Characteristics of Si are a goodchemical compatibility and wettability with silicon carbide, in particular at1480°C the contact angle between liquid Si and solid SiC is 38° [9]. Moreover thethermal expansion coefficient α is similar to that of silicon carbide: α Si(RT) =3.0x10-6 K-1 and α SiC(RT) =4.0 x10-6 K-1. Unfortunately the use of pure silicon leadsto serious problems because of the high melting point (1410°C) that may degradefibres or fibre-matrix interfaces. This aspect represents the main drawback also inthe case of joints performed by reaction forming techniques employing infiltrationof molten silicon into joining parts interspaced by carbon or carbon-SiC mixturesto be converted into SiC by the infiltrating silicon.In this paper, a recently developed silicon carbide brazing technique is reviewedand discussed. The alloys used are based on eutectic compositions of silicon-titanium and silicon-chromium.

EXPERIMENTALThe use of Si-Ti and Si-Cr eutectic alloys was proposed in order to take

advantage of the lower melting point with respect to pure Si, and the presence oftitanium and chromium which behave as active elements. The Si-Ti and Si-Crphase diagrams [10] show the presence of two eutectics of interest (Si-16Ti at.%and Si-18Cr at.%) with melting points of 1330°C and 1305°C respectively. TheSi-16Ti eutectic is composed of free silicon and TiSi2, while the Si-18Ti iscomposed of free Si and SiCr2 . The joining cannot be performed simply bymixing Si-Ti or Si-Cr powders because in this way it is not possible to get theabove-mentioned eutectics. Therefore, the alloys have to be prepared previously,

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by means of a melting procedure, able to produce a fine eutectic structure [11]; inparticular, Si-Ti and Si-Cr mixtures were melted by a plasma torch and then re-melted several times by electron beam.

Fig. 1: Micrography of a Si-16Ti alloy before the joining process: grey zones =Si ; white zones=TiSi2

Afterwards, the obtained ingots were reduced to powders by crushing and millingand finally used for the brazing experiments. Fig. 1 shows a SEM picture of theSi-16Ti alloy prior to brazing; evidencing a fine and homogeneous microstructurecomposed of Si and TiSi2 . X-ray diffraction confirmed that no other phases thanSi and TiSi2 and Si and SiCr2 were detected in the proposed brazing alloys (Fig.2and 3).

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Fig. 2: XRD pattern of Si-16Ti alloy (SH=sample holder)

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Fig. 3: XRD pattern of Si-18Cr alloy

The joining was carried out by using monolithic polycrystalline α-SiC (Hexoloy-SA Carborundum) and a SiCf/SiC composite produced by SNECMA(CERASEP N3-1), with the latter consisting of a pseudo tri-dimensional weaveof NicalonTM CG fibres, densified by chemical vapour infiltration (CVI) andfinally SiC coated by chemical vapour deposition (CVD). The above materialshave the necessary chemical stability at the brazing temperature. The typicalproperties of the monolithic and composite joining parts to be joined used arereported in tables 1 and 2 [12,13].

Table I. Main properties of Hexoloy-SA CarborundumProperty Temperature (°C)Density 20 3.07 g/cm3

Apparent porosity 20 0 %Young’s modulus 20 350 GPaThermal expansion coefficient 20-1000 4.02 10-6 1/KModulus of Rupture (MOR) 20-1600 380-410 MPa

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Table II. Main properties of SNECMA-CERASEP N3-1 composite

TemperatureProperty 20°C 1000°CDensity 2.4 – 2.5 g/cm3 --Thermal expansion coefficient 4.0 10-6 1/K --Tensile strength (in plane) 300 MPa --Tensile strain (in plane) _ 0.6 % 0.3-0.4 %Trans laminar shear strength (200 _ 20) MPa --Inter-laminar shear strength 40 MPa 30 MPa4 points bending strength 600 MPa --

The used samples were 12 x 10 x 3 mm 3 plates both of monolithic SiC andSiCf/SiC composite. Bulk SiC samples didn’t need any surface treatment sincetheir roughness was in the order of 1 µm.The composite specimens were groundin order to reach a surface roughness in the order of a few microns. Thementioned CVD coating (> 100 µm) was partially removed by surfacepreparation, thus some fibres remained uncoated and thus exposed to the brazingalloy.After ultrasonic cleaning in acetone and the application of the brazing alloybetween the two pieces to be joined, the samples were inserted in the oven andkept in contact during thermal cycle with a 1 N load. The first joinings werecarried out in inert atmosphere (Ar+3% H2) but then they were always performedin a vacuum furnace (10-6 mbar) because that facility allowed a better control ofthe thermal cycle. The samples were heated up to the eutectic temperature with aheating rate of 10 °C/min; the hold time at melting temperature was about 10min; cooling down to 600°C was performed at 20°C/min followed by naturalcooling down to room temperature.

MICROSTRUCTURE AND NANOCHEMISTRY All the joints were examined to detect macroscopic defects or cracks.Afterwards cross-sections of monolithic SiC and SiCf/SiC composite joints wereexamined by SEM equipped with Energy Dispersive X ray spectroscopy (EDX).For both alloys, the joint thickness of composites was in the range 20-30 µm, buta local variation in the thickness could be observed depending on the surfaceroughness . The joint thickness of monolithic specimens was generally higher dueto the absence of infiltration in the impervious SiC; the values ranged from fewtenth of microns up to 100 µm depending on the amount of braze depositedbetween the pieces to be jointed. SEM micrographs showed no discontinuitiesand defects at the joint interface and no unmelted particles. Moreover the eutecticstructure showed a morphology comparable with that of the starting powder

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(Fig.4), the Si-18Cr was generally finer than that of Si-16Ti. The duration of thebrazing cycle allowed to control sufficiently the infiltration of the joints ofcomposites. Sometimes infiltration was observed but it was limited to no morethan a couple of fabric layers close to the joint interface.

Fig. 4 SEM images of a joint performed between SiCf/SiC composites (Si-18Cr)

EDX maps (Fig.5 and 6) showed absence of Ti and Cr within SiC (close to thejoin interface). The oxigen maps evidenced only a very slightly higher oxygencontent in the brazing alloy with respect to the bulk SiC. No macroscopic reactionlayers were visible at the interface.In order to study the interface structure and the nature of the bonds between theSi-16Ti eutectic alloy and the SiC and SiCf/SiC composites, investigations wereperformed by transmission electron microscopy (TEM) including high resolutionor atomic plane imaging (HREM), and electron energy-loss spectroscopy (EELS)for chemical analysis (cf., e.g., [14,15]). The EELS method allowed to estimatethe kind and concentration of the chemical elements with a spatial resolution inthe order of 1-2 nm. In particular, the analysis of the near-edge fine structures(ELNES) of the relevant ionisation edges allowed to characterise the chemicalbonding state of individual elements with the same resolution. Also with theseanalyses, no Ti diffusion into SiC and no new phases were found in the relatedinterfaces. In some TEM images strain-contrast contours have formed along theinterface (cf. Fig.7). This mechanical strain could result from the thermalexpansion mismatch between the SiC bodies and the brazing alloy and hints atstrong bonding in the interface.

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Fig. 5: Interface between Si-16Ti joint and bulk SiC (a) and O (b), Si (c) and Ti(d)mapping (EDX)

Fig. 6: Interface between Si-18Cr joint and bulk SiC and O (b), Si (c) and Cr (d)mapping (EDX)

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As results both from EELS measurements and HREM images, all the analysedinterfaces between the substrate and the brazing alloy could be proved to benearly atomically sharp, i.e., there is no detectable interdiffusion or formation ofnew phases. Thus, the high strength macroscopically measured in these joiningsystems must be attributed to direct chemical Si-Si and Si-C bonds in the case of aSi/SiC interface with additional Si-Ti or Ti-C bonds at TiSi2/SiC interfaces. Theseconsiderations hold for SiCf/SiC and monolithic SiC joints as well.

Fig. 7 TEM image of the interface area between SiC/SiCf andSi-16Ti brazing,

SHEAR STRENGTHAmong all the mechanical features of the joints, the shear strength is one

of the leading properties to assess the reliability of any joint technique. Severalmethods to test the shear strength of joints [16] have been proposed. In this work,the shear tests were performed following an ”ad hoc” modification of the ASTMD905-89 test procedure [17]. Although a pure shear strain field cannot be assuredby this procedure, this method is, however, a simple one suitable to obtain arather good estimation of shear strength and a good way for a comparativeevaluation. Tests have been performed at RT (Fig. 8a) and 600°C (Fig. 8b) forSi-16Ti joints and only at RT for Si-18Cr joints. The crosshead speed was 0.6mm/min.

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a) b)Fig.8: Scheme of the shear testarrangement at RT( a) and 600°C ( b)

Fig.9: A shear load displacement curve(composite sample)

The results can be summarised as follows. The samples joined by using the Si-16Ti alloy and manufactured with the composite samples exhibited a shearstrength of 71 ± 10 MPa at RT and up to 70 MPa at 600°C. The samples joined byusing the Si-18Cr alloy and manufactured with monolithic SiC cracked at 150MPa at RT, while the composite samples exhibited a shear strength of 80 ± 10MPa at RT. The shear strength level measured are similar to those obtained byhigh performance reaction forming technique but tested with a different method[18]. Moreover, all the tests, that were carried out at least on five specimens foreach typology of brazed joints, gave sufficiently reproducible results with alimited scattering. A typical shear load-displacement curve for composite samplesis shown in Fig. 9. With the exception of an initial adjusting phase, the trendappears practically linear up to failure with a very limited toughness.The jointstrength was slightly affected by the residual roughness and open porosity of thecomposite substrate. Observation of the fracture surfaces revealed that failure wasalways cohesive in all tested specimens. In monolithic samples, the failureoccurred mainly in the substrate but sometimes started at the joint interface andpropagated in the bulk SiC. Since the compressive strength of Hexoloy should behigher than 1 Gpa, failure of the substrate is likely due to not pure compressiveloading and to some local stress concentrations. Consequently the joints strengthis likely to be even higher than that measured.In the composite samples, the failure always occurred in the composites (Fig. 10),leading to the conclusion that the limiting parameter of the performance was the

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shear strength of the composite itself that is locally further increased by thebrazing alloy infiltration.

Fig.10:Typical composite specimen failure image (arrow)

CONCLUSIONSThe proposed joining technique, which employs the eutectic Si-Ti and Si-Cr

alloys, appears suitable for joining of SiC and SiCf/SiC composites. Joints withlow residual strains and satisfactory mechanical features were obtained.Following SEM analysis all the joints investigated did not show any defects in thebrazing layer which maintained, even after joining, a fine eutectic structure.Moreover, concerning composites, the brazing alloy infiltration lookedsufficiently controlled. Systematic investigations of the microstructure and of thenanochemistry (HREM, EELS, esp. ELNES) of Si-Ti joints led to the conclusionthat direct chemical bonds are responsible for the adhesion. Shear tests of thejoints of SiCf/SiC composites showed remarkable values of the bonding strength(about 70-80 MPa) which were scarcely influenced by the testing temperature atleast up to 600 °C and in the experimental conditions (quite high cross head

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speed) employed, while joints of monolithic SiC (Si-18Cr) exhibited up to 140MPa.The technique presents some disadvantages, such as need of grinding the surfacesto joint in order to get tight tolerances and a tight joining thermal cycle, but thereis no evidence of unsuitability for the joining of large pieces.In order to assess the suitability of the alloy for energy conversion application, thechemical behaviour of the joints with respect to oxidation will be investigated inthe near future.

REFERENCES1 R. Naslain, Materials design and processing of high temperature ceramic

matrix composites: State of the art and future trends. Advanced compositematerials Vol. 8, n.1, pp 3-16.

2 M.Ortelt, F.Ruehle, H.Hald, H.Weihs, J.Greenwood, A.Pradier in “HighTemeperature Ceramic Matric Composites” p. 760, Edited by W.Krenkel,R.Naslain H.Schneider, WILEY-VCH Weinheim (Germany) 2001.

3 O.M.Akselsen, J.Mat.Sci., 27, 569-579 (1992)4 T.J.Moore, J.Am.Ceram.Soc, 68 [6], C151-C153(1985).5M.Salvo, M.Ferraris, P.Lemoine, M.Appendino Montorsi, M.Merola,

J.Nucl.Mat., 212-215, 1613-1616 (1994)6 J.Martinez Fernandez, A.Munoz, F.M.Varela-Feria, M.Sing. Journal

European Ceramic Society Vol 20 [14-15] (2000) 2641-26487 M. M.Schwartz, Ceramic joining. ASM International (1990)8 M. Ferraris, C. Badini, M. Montorsi, P. Appendino, H. W. Scholz, J. Nucl.

Mat. 212-215 (1994) 1613-16169 J. G. Li, H. Husner, Journal of Material Science Letters 10 (1991) 1275-127610 ASM Handbook. Volume 3 : Alloy phase diagrams. ASM International

(1992)11 ENEA Patent 478 (2001) –RM2001A00010112 Hexoloy-SA .Technical specification. Carborundum Italia s.r.l. (1998).13 A.La Barbera, B.Riccardi, C.A.Nannetti, A.Donato, L.F.Moreschi, J.Nucl.

Mat 294 (2001) 223-23114E. Pippel, J. Woltersdorf, G. Pöckel, G. Lichtenegger, Microstructure and

Nano-chemistry of Carbide Precipitates, Mater. Charact. 43, 41-55 (1999)

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15R. Schneider, O. Lichtenberger, J. Woltersdorf, Phase identificationin incomposite materials by EELS fine structure analysis, J. Microsc. 183, 39-51(1996)

16S. Cordeau, C. Taffarel, Characterisation de la resistance au cisaillement dejonctions, Technical Note CEA-CEREM Grenoble (France) DEM n.38/97(1997).

17 B. Riccardi, A. Donato, P. Colombo, G. Scarinci : Development ofhomogeneous joining techniques for SiC/SiCf composites. R.Beaumont,P.Libeure, B.de Gentile, G.Tonon (Eds) Fusion Technology 1998- 20th SOFTMarseille

18 M.Singh, E.Lara Curzio, Transactions of the ASME. Vol 123 (2001)pp288-292.


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