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Available online at www.sciencedirect.com

ScienceDirect

Journal of the European Ceramic Society 34 (2014) 1681–1686

Joining of �-SiC by spark plasma sintering

Salvatore Grasso a,∗, Peter Tatarko b,1, Stefano Rizzo c, Harshit Porwal a, Chunfeng Hu d,Yutai Katoh e, Milena Salvo c, Michael J. Reece a, Monica Ferraris c

a School of Engineering & Materials Science and Nanoforce Technology Ltd., Queen Mary University of London, Mile End Road, London E1 4NS, UKb Institute of Physics of Materials, Academy of Sciences of the Czech Republic, 616 62 Brno, Czech Republic

c Department of Applied Science and Technology, Institute of Materials Physics and Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24,10129 Torino, Italy

d Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences (CAS), Ningbo 315201, Chinae Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6138, USA

Received 30 September 2013; received in revised form 10 December 2013; accepted 14 December 2013Available online 29 January 2014

bstract

park plasma sintering (SPS) was employed to join monolithic �-SiC with or without titanium as intermediate joining material. Both the localizednd rapid heating contributed to the inherent energy saving of electric current assisted joining technique. The effects of uniaxial pressure and surfacereparation were analyzed independently with respect to the flexural strength and the morphology of the joints. In particular samples polished

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own to 1 �m and joined at 1900 C for 5 min achieved the strength of the as received material. The failure occurred outside the joining interface,onfirming the optimum quality of the joint. Pressure in combination with surface preparation was necessary to achieve perfect adhesion and poreree direct joining of SiC. The use of Ti foil as a joining material and pressure allowed joining of unpolished SiC.rown Copyright © 2013 Published by Elsevier Ltd. All rights reserved.

eywords: �-SiC; Joining; Spark plasma sintering

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. Introduction

Silicon carbide based materials have been traditionallyonsidered as the ideal choice for several applications inxtreme environments due to their unique combination ofigh-temperature mechanical properties and good corrosionesistance.1 In the nuclear field, SiC is of particular interestue to its low cross-section neutron absorption and good stabil-ty under high temperature neutron irradiation. Moreover, SiChows low neutron induced radioactivity thus offering econom-

1

cal waste disposal and maintenance benefits. Among siliconarbide based materials, continuous SiC fibre-reinforced SiC-atrix composites offer higher flaw tolerance if compared to

∗ Corresponding author at: Nanoforce Technology Limited, Joseph Priestleyuilding, Queen Mary University of London, Mile End Road, London E1 4NS,K. Tel.: +44 020 7882 2773; fax: +44 020 7882 2773.

E-mail addresses: s.grasso@nanoforce.co.uk, salpolito@hotmail.itS. Grasso).1 Currently working at Institute of Materials Research, Slovak Academy ofciences, Watsonova 47, 04001 Kosice, Slovak Republic.

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955-2219/$ – see front matter. Crown Copyright © 2013 Published by Elsevier Ltd.ttp://dx.doi.org/10.1016/j.jeurceramsoc.2013.12.023

onolithic SiC, they are being considered for fusion blankettructures, nuclear core and fuel structures.

Joining of SiC-based materials is a crucial issue2,3 and mostf the work on joining of SiC based materials available in theiterature has been performed on �- or �-SiC bulk substratesecause SiC is less expensive than SiCf/SiC, but also becauseiCf/SiC composites are usually coated by a Chemical Vapoureposition (CVD) SiC layer. Thus, joining technology suitable

or CVD SiC may in principle be transferred to SiCf/SiC. Theabrication route of SiC by CVD produces highly crystalline,ully dense (3.21 g/cm3) stoichiometric and high-purity �-iC.4–6 In particular CVD SiC exhibits the highest elastic mod-lus purity and superior irradiation resistance among SiC basedaterials. The direct joining (i.e. absence of joining material) of

ilicon carbide parts has been attempted in the past.7 Ideally itight be the perfect type of joining for a wide range of applica-

ions since the absence of joining material leaves unchanged theull set of properties of the material to be joined. The absence

f joining material might be particularly important to mini-ize corrosion/oxidation degradation, residual stresses resulting

rom working conditions and differential thermal expansion or

All rights reserved.

1 ean Ceramic Society 34 (2014) 1681–1686

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Fig. 1. Schematic of spark plasma sintering (SPS) joining set up and samplepp

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ifferential swelling induced by neutron irradiation. In the-ry, directly joined SiC based materials might find applicationsn extreme environments with temperature exceeding 1600 ◦C,here most of the other type of joints (including NITE, metallicrazes and glass ceramics) cannot withstand operation condi-ions.

Moore et al.8 attempted to join SiC to SiC by polishingommercially sintered alpha-SiC blocks (theoretical density8%) and joining them by hot isostatic pressing at 1950 ◦Cnd 138 MPa for 2 h. However, no joining took place, and thisas correlated to difficulty to sinter pure silicon carbides in

bsence of sintering aids. Gupta et al.9 joined polished siliconarbide blocks using 8% excess silicon on at least one of the sur-aces to be joined, thus achieving sound joints under pressuresf 0.1–1 MPa at temperatures of 1500–1800 ◦C. However its well known that the presence of residual silicon inside theoint might significantly deteriorate its response at temperaturesbove 1200 ◦C.10 Rossi et al.7 succeeded in producing a directiC to SiC joint by mirror-finishing, and then subjecting the

oined structure to hot isostatic pressing. However they required surface polishing down to 50 nm, which is hard to be obtainedn an industrial level.

This work is focused on Spark Plasma Sintering (SPS) join-ng of CVD �-SiC: SPS is proposed here as localized heatingechnique to obtain sound joints for SiC based materials, suit-ble for nuclear and generally for high demanding applications.he origins of joining under electric field are back to the 1933hen Taylor patented a method for sintering-bonding dissimi-

ar electric conductive materials.11 The joints and intermediateowders were placed in an electrically insulating mould andelt/sintered by a rapid (i.e. fraction of a second) electric dis-

harge passing through the joining materials. Some work haseen published on the electric field assisted joining of dissimilaretals,12 metals to glasses,13 and metal to ceramics14 with orithout the use of intermediate layers. More recently, SPS appa-

atuses have been proposed as an effective joining technique forltra-high temperature ceramics.15

However, at present there is no systematic investigation onhe SPS joining of SiC. In our work, special emphasis is giveno surface preparation, presence or absence of joining materialnd role of pressure. Titanium foils were selected as interme-iate joining materials due to their excellent wettability on SiCubstrates.16,17

. Experimental procedure

The starting material was the Dow Chemical Company CVD-iC (high resistivity grade, 99.9995% purity, and fully dense3.21 g/cm3)). The SPS joint preparation and mechanical testsre shown schematically in Fig. 1: growth direction refers tohe CVD SiC growth. At first 15 × 6 × 25 mm3 componentsere cut from the initial SiC disc (180 mm diameter and 7 mm

hickness). The samples were joined along the 15 × 6 mm2 cross

ection. All the surfaces were cleaned to remove dust particles,rease and other contaminants. Before joining the polished sam-les were lapped down to 1 �m diamond slurry (DP Struers,enmark). As received, diamond blade cut and unpolished SiC

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reparation for bend testing. All the samples were tested with load appliederpendicularly to the CVD growth direction.

as also used (Table 1). The samples were inserted in the SPSquipment (FCT HP D 25, FCT Systeme GmbH, Germany)) in

cylindrical mould and heated under pulsed current at a rate of0 ◦C/min with a dwell time of 5 min. The optimized pressureas 60 MPa. All the samples were joined in the temperature

ange 1700–2100 ◦C as detailed in Table 1. The initial thick-ess of the Titanium foil was 30 �m. After joining the samples,hey were cut as shown in Fig. 1 and the polished surfaces werebserved by a field emission scanning electron microscopy (JeolSM-6300, Tokyo, Japan). Indentation tests were performed onhe joined area using a microindenter with 200 g applied load.

The joint strength was determined by four-point bending testsn air at room temperature. The joined SiC parts as well as theeference un-joined SiC with dimensions of 6 × 15 × 50 mm3

ere cut into three test beams (4 × 6 × 50 mm3) according toig. 1. The beam surface was polished to mirror quality anddges on the tensile side of the specimen were chamfered. Theoad was applied in direction perpendicular to the CVD SiCrowth (Fig. 1). All bending tests were performed under 4-pointend loading with an inner span of 20 mm and an outer span of0 mm in accordance with EN 843-1:2006.18 A universal electroechanical testing system (Instron 8862, USA) equipped with a

recision load cell calibrated to 5 kN force was used to apply theoading. The cross head speed for all of the measurements was.5 mm/min. The deflection was recorded continuously duringhe loading in the centre of the inner span (middle of the spec-men) using a linear voltage differential transformer (LVDT).he bending strength for each test beam was determined using

he standard formula for four-point bending:18

= 3P(l1 − l2)

4ab2 (1)

here P is the applied load in N, l1 and l2 are the outer and innerpan, respectively, (20 mm), a and b are width and thickness ofhe specimen in mm, respectively. For each type of joining ateast 3 samples were tested.

Young’s modulus of the investigated materials has been deter-ined from the slope of the load–displacement curve according

o the standard19 using the following equation:

= 3(P2 − P1)d1d22

ab3dt

(2)

S. Grasso et al. / Journal of the European Ceramic Society 34 (2014) 1681–1686 1683

Table 1Joining process and mechanical properties of the as received and joined SiC.

SPS Joining T [◦C], P (MPa) E [GPa] σ [MPa] Joint thickness [�m] Failure modea

As received Unbonded 448 ± 4 176 ± 10 – –Directly joined polished 1900, 60 436 ± 1 193 ± 21 ≈0.20 OUTDirectly joined polished, pressure-less 2100, 0 – – ≈20 –Directly joined not polished 2000, 60 408 ± 2 68 ± 9 ≈10–20 INTi foil not polished 1600, 60 – – – –Ti foil not polished 1700, 60 421 ± 3 126 ± 16 ≈18 MIX

a OUT/IN/MIX means failure occurred outside/inside/and in a mixed mode respect to the joined region.

Fig. 2. Scanning electron microscope (SEM) images of polished SiC samplejoined by SPS, samples named directly joined polished in Table 1. Before join-ing the surfaces of the samples were polished down to 1 um with diamond slurry.Figure refers to (a) low and (b) high magnification of joined sample, the join-im

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Fig. 3. Morphology of directly joined polished SiC to SiC without the appli-cation of any pressure by SPS. The sample was polished down to 1 �m beforebeing joined at 2100 ◦C for 5 min. As evident, even if the sample was polished,there was limited adhesion between the two joining surfaces due to absence ofa

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ng line can hardly be seen between the dashed lines and between the slightlyisaligned sample edges as seen in the left side of figure (a).

here P1 and P2 are lower and upper load level selected fromecordings expressed in N, respectively; d1 is the test jig inneroller to outer roller spacing in four-bending expressed in mm;2 is one half of the test jig inner span in four-point bending in

and dt is transducer displacement recorded between the testiece centre and the inner loading point in four-point bendingver the selected load range expressed in mm. In the presentase, d1 = d2.19

. Results and discussion

.1. Processing and morphology of the SPS joints

The experimental conditions of the samples joined by SPS areummarized in Table 1. The joining temperature was indicated,he heating rate was kept at 50 ◦C/min, the pressure was 60 MPa,nd the dwell time was 5 min. Hereinafter, the samples are nameds listed in the first column of Table 1.

Fig. 2 illustrates the SEM morphology of directly joinedolished SiC. At low and high magnification of Fig. 2(a) andb), it was hard to distinguish the joining line. As confirmed

y the slight misalignment at the sample edges (Fig. 2(a)), theoining line can be visualized between the two dashed lines. No

acro defect could be observed along the joining line. At high

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compressive stress.

agnification, it is also difficult to localize the joining line.owever, it can be identified by looking at the small poresointed by five arrows in Fig. 2(b). The fine porosity isistributed along the joining line. The diameter of the pores isell below 1 �m and typically it is around 150–200 nm. Theresence of such not interconnected fine porosity is expected toe not critical for the bending strength of the joined materialss discussed in reference.20

In order to better understand the mechanism involved inirect joining, the effects of pressure and surface preparationere investigated independently. Fig. 3 shows an SEM micro-raph of an SPS directly joined polished sample preparednder pressure-less conditions at 2100 ◦C for 5 min. Theample did not possess enough mechanical strength so it couldot be characterized by 4 points bending test. Even if therocessing temperature was 200 ◦C higher than for the directlyoined polished sample discussed above, the microstructuren Fig. 3 shows the presence of large interconnected voidsxisting between the joined surfaces. A clear interlayer can beistinguished, and the bonding is achieved only at a few contactoints. By comparing Figs. 2 and 3, where joining is achievedith or without pressure in presence of polished surfaces, it is

ossible to understand the significant role played by the pressureuring the SPS processing. As reported by Carter et al.,21 atemperatures above 1650 ◦C pressure promoted deformation

1684 S. Grasso et al. / Journal of the European C

Fig. 4. Morphology of directly joined not polished sample. The as receivedsp

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urface after machining by a diamond cutting wheel was responsible for theresence of large defects at the joint interface.

y creep. In severe loading condition, primary creep strain inVD SiC could be as high as 1%, the steady state creep rate inompression was of the order of 10−9 s−1 under a pressure of00 MPa. In our case, even if the 1 �m polishing seems still quiteough for a direct bonding process, the applied pressure mightive an effective in contribution to the near full elimination ofesidual porosity through primary and steady creep deformation.

The effect of surface preparation with respect to the qualityf the joint can be also appreciated by comparing Figs. 2 and 4Table 1). A directly joined not polished sample (Fig. 4) wasoined without any surface preparation. The surfaces were putogether after being cut with a diamond cutting wheel. As evidentn Fig. 4, the sample joined with pressure but without surfaceolishing has elongated voids in the direction perpendicular tohe applied load. The presence of such voids is attributed tohe irregular surface of the starting samples. The porosity isxtended over an area of larger than 30 �m (Fig. 4 b), which istill comparable with the roughness deriving from the diamondutting wheel.

In summary, in order to achieve a good SiC direct joining byPS two conditions have to be satisfied, namely surface prepara-

ion and pressure, but pressure plays the most significant role. Inomparison with the state of the art, SPS allowed direct joining atelatively low pressure (60 MPa), temperature (1900–2000 ◦C)nd short dwelling time (5 min) with surface polishing down to

�m.In order to give a more comprehensive overview of SPS as a

oining technique for SiC, we employed a titanium foil as a join-ng material. Ti foil with initial thickness of 30 �m was used tooin SiC at 1600 and 1700 ◦C, below and above Ti melting tem-erature, respectively, with 60 MPa of applied pressure, resulting

n a thickness of the joining surface of 26 and 17 �m respec-ively. However the samples joined at 1600 ◦C were not so goodnd they will not be described in further details, since typically

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eramic Society 34 (2014) 1681–1686

ailure of the joints occurred during sample cutting. Fig. 5 showshe cross-section of a Ti joined SiC sample at 1700 ◦C. Theoining material is dense, pore free and interface is continuous.

By looking at the EDS maps shown in Fig. 5 there was for-ation of a titanium- carbon rich phase in the joined region with

limited Ti/Si interdiffusion. Point line EDS mapping showedhat the interlayer between the SiC and TiC rich phases was about

�m thickness. Micro indentation tests nearby the joining inter-aces shows that the crack propagates in the joint material andot along the interface, demonstrating a strong SiC/joint inter-ace as shown in the inset of Fig. 5(a). It was assumed that thereere possibly Ti–Si phases and Ti–Si–C phases existing, such asiSi2 and Ti5Si3Cx. Here, the main detected phase in the joiningrea was TiCx while silicon contain phases were not identified.

.2. Mechanical characterization

The four points bending strength (σ) of as received SiCas 176 ± 10 MPa. The samples directly joined with and with-ut polishing exhibited strengths of 193 ± 21 and 68 ± 9 MPa,espectively. In the case of directly joined polished samples, asvidenced in Fig. 6(a), the fracture occurred outside the joiningrea, thus further confirming that the stress failure of the joinedurface was comparable to the strength of the bulk material. Onhe contrary, in the case of the directly joined samples withoutolishing the fracture clearly occurred along the joined surface,s also confirmed by the significant presence of distributed largeefects along the joint as seen in Fig. 4.

It should be noted that the measured flexural strength val-es for as received SiC are well below that reported by theupplier. However, typical values of bending strength from 200o 500 MPa have been reported for �-SiC.22 The actual failureehaviour for �-SiC is quite complicated because the flaw dis-ribution is substantially more complex than that produced by aingle flaw. Byun et al.23 estimated, using a finite element model,

Weibull characteristic strength in the range of 220–320 MPa.he room temperature failure strength variation reported in the

iterature for �-SiC is due primarily to the specimen size effectelated to the Weibull nature of failure. In fact, as shown in theractograph there are fine notches or flaw initiation sites thatesult in a significantly lower strength as pointed by arrow inig. 6(a). Those may exist in the as produced samples as resultsf residual stress24 or introduced during machining. For fouroint bending, edge flaws can be relevant. Large grains 50 to00 microns can also act as flaws, since the single crystal cleav-ge toughness. In some fractographs in Fig. 6(a), as pointed byhe arrow, it can be seen near the tensile surfaces the presencef large grains might lead to cleavage fracture flaws.

The bending strength along with the fracture mode suggestshat directly joined polished samples exhibited strength and elas-ic modulus comparable to the starting material. The strength ofirectly joined not polished samples was largely affected by theresence of elongated defects which affected significantly the

Most of the four point bending strength data available in liter-ture refers to joining of �-SiC,7,8,12 thus it is difficult to comparePS joints bending strength data with the ones reported in

S. Grasso et al. / Journal of the European Ceramic Society 34 (2014) 1681–1686 1685

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ig. 5. EDAX mapping of SiC joined to SiC by SPS with a Ti foil (30 �m), saas 60 MPa and the dwell time was 5 min at 1700 ◦C. Figure (a) shows backsc

lemental mapping. The inset of Figure (a) refers to the Vickers indentation.

iterature. However, the room temperature bending strengthalue of the directly joined polished SiC obtained by SPS areomparable with other techniques such as reaction-formed

18

ilicon carbide for joining �-SiC. The strength of the directlyoined polished sample is significantly higher if compared to-SiC joined using allyl-hydrido-polycarbosilane (a SiC pre-ursor) where the strength was 85 MPa at room temperature.19

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ig. 6. Directly joined polished, directly joined not polished and Ti foil samples are

ending test samples before mechanical testing. Second and third columns show lowhe up side of the micrograph.

named as Ti foil in Table 1. The SiC samples were not polished, the pressured electron images, while figures (b), (c), (d) are Carbon, Silicon and Titanium

In the case of Ti foil joined not polished SiC, a bendingtrength of 126 ± 16 MPa was measured, with a modest decreasen the elastic modulus. As confirmed by Fig. 6 (c), back-scattered

lectrons revealed the presence of light grey colour titanium ori compounds, unfortunately not detectable by micro-XRD due

o their small size. It is evidenced the crack propagation insidend outside the joining area, defined as mixed mode in Table 1.

shown from the top to the bottom. First column is the side view of the 4 point and high magnification of the sample fracture surface, the tensile surface is on

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At present SPS machine allows to process samples with sizep to 40 cm in diameter,25 the possibility of heating relativelyarge components might be attractive to join �-SiC for a wideange of industrial applications. In particular, �-SiC exhibitsufficient electrical conductivity to be heated by direct Jouleffects from room temperature, as result, components with largeimension and complex shapes might be joined by joule heatingven in absence of graphite mouldings.

. Conclusions

In order to achieve SPS joining of SiC, pressure and surfaceolishing played a crucial role. In particular, SPS allowed thenique possibility of direct joining of SiC while retaining the fulltrength of the starting material. Bending strength on these sam-les suggested that the failure occurred outside the joining area,hus confirming the good quality of the joint. In the case of Ti foiloined SiC, a bending strength of 126 ± 16 MPa was measuredfter SPS joining at 1700 ◦C under a pressure of 60 MPa.

cknowledgement

The research leading to these results was supportedy the European Union’s Seventh Framework Pro-ramme managed by REA-Research Executive Agencyhttp://www.ec.europa.eu/research/rea) (Marie Curie Action,laCERCo GA 264526).

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