Materials Science and Engineering A 380 (2004) 394–401

Interfacial microstructure and strength of partial transient liquid-phasebonding of silicon nitride with Ti/Ni multi-interlayer

Zheng Chena,∗,1, M.S. Caob, Q.Z. Zhaoa, J.S. Zoua

a Department of Materials Science and Engineering, East China Shipbuilding Institute, Zhenjiang 312003, PR Chinab School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China

Received 31 July 2003; received in revised form 1 April 2004

Abstract

Partial transient liquid-phase bonding (PTLP bonding) of silicon nitride (Si3N4) ceramic has been performed using Ti/Ni multi-interlayerin vacuum at 1273–1423 K. Interfacial microstructures were examined by scanning electron microscope, electron probe micro-analysis,and X-ray diffraction. The joint strength has been measured by four-point bending tests from room temperature up to 1000◦C. Interfacialstructure of Si3N4/TiN/Ti 5Si3 + Ti5Si4 + Ni3Si/(NiTi)/Ni 3Ti/Ni is formed after bonding process. The NiTi layer is gradually consumed withsimultaneous growth of the reaction layer and the Ni3Ti layer. The room temperature joint strength is significantly affected by the reactionlayer thickness, whereas the elevated temperature joint strength significantly depends on whether the low melting point NiTi layer exists inthe joint. The joint strength of more than 100 MPa is retained up to 800◦C as the NiTi layer is completely consumed. A model is proposed tooptimize the PTLP bonding parameters for optimizing joint strength at both room temperature and elevated temperature.© 2004 Elsevier B.V. All rights reserved.

Keywords: Silicon nitride; Transient liquid-phase; Diffusion bonding; Interfacial microstructure; Joint strength

1. Introduction

Silicon nitride (Si3N4) features superior mechanical be-havior at high temperature[1]. However, in most applica-tions, it is necessary to join Si3N4 ceramic to metals becauseof its brittleness and to itself due to difficulty in fabricationof complex shaped and large sized ceramic components.Therefore, it is significant to develop techniques for fabri-cation of Si3N4–metal or Si3N4–Si3N4 joints, which canwithstand high temperature. Various techniques have beendeveloped to make Si3N4–Si3N4 and Si3N4–metal joints inlast decade[2–20]. Among all the joining methods, thoseinvolving liquid-phase formations such as active metalbrazing [9–15] and partial transient liquid-phase bonding

∗ Corresponding author. Present address: Department of MechanicalEngineering, University of Waterloo, 200 University Avenue West, Warter-loo, Ont., Canada N2L 3G1. Tel.:+1 519 888 4567x7142; fax:+1 519885 5862.

E-mail addresses: chenz31@yahoo.com.cn, zhc8@pitt.edu (Z. Chen).1 Currently working at the Department of Mechanical Engineering,

University of Pittsburgh, Pittsburgh, PA 15261, USA, as a visiting pro-fessor.

(PTLP bonding)[16–20] are receiving more attention be-cause complex shaped parts can be fabricated using simpletooling and mating surface preparation. Although activebrazing is widely used to join ceramics and consideredas the most effective method, the maximum service tem-perature of the joint brazed with Ag–Cu–Ti fillers is only400–500◦C [11–13,16]. In addition, the brazes with highliquidus temperature such as an iron-based IncoloyTM 909alloy (Fe–38Ni–13Co–5Nb–1.5Ti, in wt.%)[21] pose prob-lems because Si3N4 ceramic decomposes at temperaturesabove 1200◦C [15].

Recent investigations[16–20] show that partial transientliquid-phase bonding is a low temperature bonding processthat can produce a heat-resistant joint. PTLP bonding, whichis an extension of the conventional transient liquid-phase(TLP) bonding, uses multilayer metals composed of a thininsert outer layer with a lower melting point on a thick coreof refractory metal. A liquid is formed at the bonding tem-perature either resulting from the melting of outer layer orfrom the eutectic reaction between the core and the outerlayer. This allows the ceramic–metal interface formation bya process at relative low bonding temperature that is similarto brazing. However, the core remains solid at the bonding

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2004.04.012

Z. Chen et al. / Materials Science and Engineering A 380 (2004) 394–401 395

temperature. The liquid wets the solid ceramic surface andinterdiffuses with the core materials to transform into a morerefractory solid material through re-solidification process.Ultimately, a homogeneous and refractory joint is formed,which is similar to that produced by solid-state diffusionbonding. As discussed above, PTLP bonding is combina-tion of brazing and solid-state diffusion bonding processes.However, few attempts were made to select bonding param-eters to optimize the joint strength of PTLP bonding[16],although much attention has been paid to the microstructuredeveloped during bonding[17–20].

When silicon nitride is bonded to a metal, high residualstresses at the interface will be developed due to the thermalexpansion coefficient mismatch between the ceramic and themetal. Previous studies have shown that Ni can be used asthe bonding interlayer for self-bonding of silicon nitride be-cause its relative ductility can reduce the level of thermalstresses at the interface[22]. PTLP bondings of Si3N4 ce-ramic with NiCr/Au [23], NiCr/Cu [23], and Au/Ni–22Cr[16] interlayer have also been investigated. In those cases,Cr acts as an active element and is alloyed into the core ofNiCr alloy. On the other hand, Ti/Cu/Ni[18], CuTi/Pd, andTi/Cu/Ti multilayer [17,20] have been used as interlayer inPTLP bonding of Si3N4 ceramic because Ti is the most at-tractive active element for almost all the structure ceramics[24]. The above-mentioned investigations on PTLP bond-ing, however, have paid little attention to the relationshipsamong the bonding parameters, the microstructure formed,interfaces migration, and the mechanical properties of thejoint.

Because a low melting point liquid-phase can be formedabove 942◦C between Ni and Ti according to the Ni–Tiphase diagram (Fig. 1) [25], the present study deals withPTLP bonding of silicon nitride ceramic composite usingthe Ti/Ni multi-interlayer under a low pressure of about0.1 MPa with emphasis on the effects of interfacial reac-

Fig. 1. Ti–Ni phase diagram.

tion and interface migration on joint strength during PTLPbonding.

2. Experimental procedure

Hot-pressure-sintered silicon nitride reinforced with TiCparticles was used in this study, which was fabricated in Qs-inghua University, China. The Si3N4 specimens were sup-plied as blocks of 19 mm× 19 mm× 8 mm. A 0.8 mm thickNi sheet (99.98 wt.% pure) was used as the core material. A20�m thick Ti foil (99.99 wt.% pure) was used as the outerlayer. The surfaces of Si3N4 and Ni sheet were polishedwith 1�m diamond paste and then ultrasonically cleanedin acetone and ethanol for about 20 min, respectively. Asandwich-like bonding couple of Si3N4/Ti/Ni/Ti/Si3N4 wasplaced in an alumina jig. An uniaxial pressure of about0.1 MPa was applied to the bonding couple to maintain thealignment and intimate contact of the samples during thethermal cycle. PTLP bonding was carried out in a furnacewith a vacuum of about 6.67 mPa under various bondingtemperature (1273–1423 K) and bonding times (0.9–7.2 ks),with heating and cooling rates of 5 K/min.

After PTLP bonding, the specimens with dimension of3 mm× 4 mm× 38 mm, which were used for bending tests,were cut from the joints perpendicular to the bonded inter-face. The four-point bending test was carried out from roomtemperature up to 1273 K with a Model-8500 INSTRONTesting System with a displacement rate of 0.5 mm/min. Thehigh temperature tests were done in air with a heating rateof 10◦C/min. The specimens were kept at the testing tem-perature for 0.5 h before being loaded. For each set of ex-perimental data, at least five samples were used to averagethe joint strength.

The fracture surfaces of tested samples were examinedby X-ray diffraction (XRD) with Cu K� radiation to iden-tify the interfacial reaction products. The microstructureacross the ceramic–metal interface was characterized withthe JXA-840A scanning electron microscope (SEM) and anelectron probe micro-analysis (EPMA) system, respectively.The cross-section specimens were polished with 0.5�m di-amond paste prior to SEM observation.

3. Results

3.1. Microstructure and elemental distribution

Fig. 2 shows SEM backscattered images obtained fromthe interface of joints bonded at 1323 K for different times.The line-scanning of the elements Ti, Si, and Ni across theinterface are shown inFig. 3. It can be seen that Ti-rich re-action layer had been formed adjacent to the ceramic evenin the joint bonded only for 0.9 ks (Fig. 2(a)). Moreover,two reaction layers can be clearly seen in the joint that wasbonded for more than 1.8 ks. Besides the formation of the

396 Z. Chen et al. / Materials Science and Engineering A 380 (2004) 394–401

Fig. 2. SEM backscattered images showing interfacial microstructure of joints bonded at 1323 K for 0.9 ks (a), 1.8 ks (b), 3.6 ks (c), and 7.2 ks (d).

0 5 10 15 20 25 30 35

Distance from the interface , µ, µm

0

20

40

60

80

100

Co

nce

ntr

atio

n,

at%

(a)

CeramicReaction layer

3Ni TiNiTi

0 5 10 15 20 25 30 35

distance from the interface, µm

0

20

40

60

80

100

Co

nce

ntr

atio

n, a

t%

(b)

CeramicReactionlayer

3Ni TiNiTi

0 5 10 15 20 25 30 35

Distance from the interface,µm

0

20

40

60

80

100

Co

nce

ntr

atio

n, a

t%

(c)

Reactionlayer

3Ni TiNiTi

0 5 10 15 20 25 30 35

Distance from the interface, µm

0

20

40

60

80

100

Co

nce

ntr

atio

n, a

t%

(d)

Reactionlayer

3Ni Ti

Fig. 3. Quantitative EPMA elemental profiles across joint interfacial region corresponding toFig. 1, joints were bonded at 1323 K for different bondingtime of (a) 0.9 ks, (b) 1.8 ks, (c) 3.6ks, and (d) 7.2 ks. (�, Ti; �, Ni; �, Si).

Z. Chen et al. / Materials Science and Engineering A 380 (2004) 394–401 397

reaction layer, intermetallic NiTi and Ni3Ti layers were alsoformed during the PTLP bonding. The formation of Ni–Tiintermetallic compounds is consistent with the Ni–Ti phasediagram, as shown inFig. 1. These results demonstrated thatthe inserted active Ti foil had been molten and consumedduring the formation of the reaction layer and the inter-metallic compound layers leading to the presence of the in-terfacial structure of Si3N4/reaction layer/(NiTi)/Ni3Ti/Ni.As the bonding time increased, the reaction layers betweenceramic and metal became thick. In addition, intermetallicNi3Ti layer grew towards both the NiTi and Ni sides, whileNiTi layer was gradually consumed with the growth of boththe reaction layer and the Ni3Ti layer with increasing theholding time. The arrows shown inFig. 3 indicated the mi-gration direction of the reaction layer/NiTi, NiTi/Ni3Ti, andNi3Ti/Ni interfaces. Consequently, the NiTi layer near thereaction layer eventually disappeared when the holding timewas prolonged to 7.2 ks at 1323 K (Figs. 2(d) and 3(d)).

Fig. 4reveals the SEM images obtained from cross-sectionsof the joints bonded at different temperatures for 3.6 ks.Increasing the bonding temperature had more significant

Fig. 4. Cross-section SEM backscattered images taken from joints bonded for constant time of 3.6 ks at different temperature of 1273 K (a), 1323 K (b),1373 K (c), and 1423 K (d).

effect on the growth of reaction layer and the migration ofvarious interfaces than increasing the bonding time did. Forexample, the reaction layer was 6�m thick after bondingat 1323 K for 7.2 ks. In contrast, the reaction layer was10�m thick after bonding at 1423 K for 3.6 ks. In addition,the NiTi compound layer was completely consumed afterbonding at 1423 K for 3.6 ks.

3.2. XRD analysis of reaction products

The samples bonded at 1323 K for 0.9 and 1.8 ks werefractured along the ceramic/intermediate interface uponbend loading. The fractured surface of the intermediatelayer side was golden yellow, and its XRD pattern (patternA in Fig. 5) shows that the reaction products containedTiN, Ti5Si3, Ti5Si4, and Ni3Si. None of those productscould be found in XRD pattern obtained from the fracturedsurface of the ceramics side in which only the XRD pat-tern corresponded to the parent ceramic was observed. Thisconfirmed that the fracture occurred at the ceramic/reactionlayer interface.

398 Z. Chen et al. / Materials Science and Engineering A 380 (2004) 394–401

Fig. 5. XRD patterns of interfacial reaction layers taken from fracturedsurface of metal side of the joint produced at 1323 K for 1.8 ks. (a)As-fractured surface of reaction layers; (b) part of reaction layers beingpolished; and (c) after further polishing.

In order to clarify the interfacial reaction layers sequence,further XRD analysis were carried out after removing ofthe reaction layers by polishing. After part of the reac-tion layer had been removed (the fracture surface changedfrom golden yellow to shallow golden yellow), the inten-sity of the TiN spectral peaks decreased, while those ofTi5Si3, Ti5Si4, and Ni3Si3 spectral peaks increased (pat-tern B inFig. 5). After further polishing, the spectral peaksof TiN disappeared and the intensity of the Ti5Si spectralpeaks also decreased remarkably, whereas those of the Ti5Si4and Ni3Si spectral peaks further increased (pattern C inFig. 5).

The XRD analyses combined with SEM and EPMAresults indicated that the reaction layer I adjacent to theSi3N4 ceramic was composed of only TiN (as shown inFig. 2); and the reaction layer II was a mixture of Ti5Si3,Ti5Si4, and Ni3Si in which Ti5Si3 was adjacent to the TiNlayer. The interfacial structure after PTLP bonding usingTi/Ni multi-interlayer thus exhibited an array sequenceof Si3N4/TiN/Ti5Si3 + Ti5Si4 + Ni3Si/(NiTi)/Ni3Ti/Ni,in which NiTi layer was a transient layer that could beconsumed by long-time PTLP bonding.

3.3. Growth of the reaction layers and Ni–Ticompounds layers

Fig. 6 shows the variation of reaction layer (TiN/Ti5Si3+ Ti5Si4 + Ni3Si) thickness and the intermetallic com-pound layers thickness as a function of the bonding time.It was found that the growth of these layers followed theparabolic law and the thickness (x) can be described by

x = k√

Dt (1)

wherek is the materials constant,D is the diffusion coeffi-cient, andt is the bonding time. At a definite bonding tem-

Fig. 6. The effect of bonding time on thickness of reaction layer as well asintermetallic compound layers of NiTi and Ni3Ti at bonding temperatureof 1323 K.

perature,D is a constant. Therefore,Eq. (1)can be simpli-fied as

x = kp√

t (2)

wherekp is the reaction layer growth coefficient. After the re-gression treatment of the experimental data by means of theleast square method, the growth coefficients of the reactionlayer, the Ni3Ti layer, and the NiTi layer were obtained asfollows: kp (reaction layer)= 8.7 × 10−8 m/s1/2, kp (Ni3Tilayer)= 8.7× 10−7 m/s1/2 andkp (NiTi layer) = −2.57×10−7 m/s1/2, respectively (the minus quantity indicated thatthe NiTi layer was gradually consumed during bonding). Ifthe growth of reaction layer to the ceramic side was ignored,a growth rate of 1.80× 10−7 m/s1/2 for the Ni3Ti layer tothe ceramic side and a growth rate of 0.02× 10−7 m/s1/2

for the Ni3Ti layer to the Ni side at 1323 K could be de-termined. Based upon the above calculations, it is deducedthat the intermetallic Ni3Ti layer grew much faster than thereaction layers did. On the other hand, the Ni3Ti layer grewtoward the ceramics side much faster than did it toward theNi side, owing to the diffusion of Ti atoms towards ceramicto react with ceramic to form reaction layers.

For a given bonding time,t, the effect of bonding temper-ature on the reaction layer thickness can be described by

x2 = k0 exp

(−Q

RT

)t (3)

or

ln x2 = − Q

RT+ ln K (4)

wherek0 is the pre-exponential factor,Q is the activationenergy for the growth of reaction layer,R is the universe gasconstant,T is the bonding temperature, andK is a constant.

The relationship between the reaction layer (TiN/Ti5Si3+ Ti5Si4 + Ni3Si) thickness and the bonding temperatureis presented inFig. 7. The reaction layers thickened withincreasing the bonding temperature. However, the bonding

Z. Chen et al. / Materials Science and Engineering A 380 (2004) 394–401 399

Fig. 7. The effect of bonding temperature on the growth of reaction layerand intermetallic compound Ni3Ti layer at joint interfaces bonded for aconstant bonding time of 3.6 ks.

temperature showed much less influence on the reactionlayer thickness at higher bonding temperature (>1373 K).This was due to the low activity of Ti atoms in the solidNi3Ti compound and the slow diffusion rate of Ti atomscross the reaction layers towards the ceramic/reaction layerinterface. When the bonding temperature was high enough,a plateau was reached and the maximum thickness (xmax)was observed.

The apparent activation energy for the growth of reactionlayer, which was calculated from the slopes of the lnx2 ver-sus 1/T lines, was determined to be 540 kJ/mol. This acti-vation energy value was obviously higher than 210 kJ/molfor the growth of reaction layer in brazing Si3N4 ceramicswith Ag–Cu–Ti filler that was reported by Tillmann[10].The difference in activation energy is ascribed to the fol-lowing aspects. During the PTLP bonding, the isothermalsolidification of the liquid phase leads to the transformationof the interfacial reaction from a solid–liquid reaction to a

Table 1Effects of bonding temperature and time on the joint strength (strength of base Si3N4 ceramic is also involved for comparison)

Conditions Thickness (�m) Average joint strength (MPa) at testing temperature (K)

Reaction layer NiTi Ni3Ti 293 673 873 1073 1273

Base Si3N4 720 632 567 516 4751323 K 0.9 ks 1.50 14.4 12.5 25.6a

1.8 ks 2.02 7.5 18.8 69.4a

3.6 ks 3.02 6.25 21.9 139b 138 110 51 347.2 ks 6.32 0.3 25 126c 131 118 88 63

3.6 ks 1273 K 1.07 14.9 13.9 56.6a

1323 K 3.02 6.25 21.9 139b 138 110 51 341373 K 10.7 2.6 22.7 120c 126 120 56 411423 K 10.8 0 23.5 124c 135 128 102 71

a Fracture at the interface between ceramic and the reaction layer.b Mixed fracture from interface to ceramic.c Fracture in the ceramic near the interface.

solid–solid reaction. When the bonding time exceeded 0.9 ksat the temperature of 1323 K, the interface structure changedto Si3N4/reaction layer/NiTi/Ni3Ti/Ni (Fig. 2). The furtherreaction required the migration of the Ti atoms in the Ni–Ticompounds toward the reaction layer/NiTi interface and sub-sequent diffusion cross the reaction layer. Because the diffu-sion of the Ti atoms in Ni–Ti compounds was in solid state,the corresponding activation energy for the growth of reac-tion layer was high. Similarly, Ceccone et al.[16] also foundthat in the PTLP bonding of Si3N4/Au/Ni–22Cr, the growthactivation energy of the reaction layer was almost the sameas that of solid-state diffusion bonding of Si3N4/Ni–20Cr.

In addition, the apparent activation energy for the growthof Ni3Ti layer could also be estimated to be 162 kJ/mol fromFig. 7.

3.4. Joint strength

Table 1demonstrates the effects of the bonding temper-ature and the bonding time on the joint strength and thefracture path. The thickness of the reaction layer and theintermetallic NiTi and Ni3Ti layers are also listed inTable 1in order to discuss the dependence of the joint strengthupon the interfacial microstructure.

Table 1reveals that the thickness of reaction layer hada significant effect on the room temperature joint strength,which is consistent with the previous studies[5,8,10,16].Lower bonding temperature or shorter bonding time resultedin an insufficient interfacial reaction leading to a relativeweak interfacial strength. As a result, fracture occurred alongthe ceramic/reaction layer interface upon bending. On theother hand, if the reaction layer was too thick, a much higherresidual stress could be generated in the ceramics adjacentto the interface. This is believed to be due to the fact thatthe reaction layers were too brittle to relieve the residualstress. Hence, the residual stress would cause fracture inthe ceramic under a much low stress value compared withthe strength of base Si3N4 ceramic as shown inTable 1. In

400 Z. Chen et al. / Materials Science and Engineering A 380 (2004) 394–401

short, the maximum joint strength could be obtained only inthe medium range of the bonding temperature and time be-cause a reaction layer with an appropriate thickness resultedfrom such a condition. In this case, the sufficient interfacialstrength of joint and minor residual stress in the ceramicscan be obtained, resulting in a mixed fracture path from theinterface to the ceramic near the interface.

In the present work, the high temperature bending testshave shown that the joint strength of about 100 MPa wasretained up to 800◦C for the PTLP bonded joint in con-trast to the behavior of the Ag–Cu–Ti brazed Si3N4 joints(less than 10 MPa at 800◦C, having a maximum servicetemperature of only 400–500◦C) [16]. As compared to theroom temperature joint strength, however, the high temper-ature joint strength was dependent upon not only the reac-tion layer thickness but also the presence of the low meltingpoint intermetallic NiTi compound in the interfacial regionafter bonding. For example, the joints bonded at 1423 K for3.6 ks and at 1323 K for 7.2 ks exhibited lower room tem-perature strength than one bonded at 1323 K for 3.6 ks. Incontrast, they showed higher high temperature strength dueto the absence of the NiTi layer in the joint. Similarly, thejoint bonded at 1423 K for 3.6 ks had almost the same roomtemperature strength with one bonded at 1373 K for 3.6 ksbecause there was the approximate reaction layer thicknessin the both samples. However, the former had higher hightemperature strength than the latter.

4. Discussion

Transient liquid-phase bonding process can be split intofour stages[26]: (1) dissolution and melting of the inter-layer (the Ti foil in the current work); (2) homogenizationand widening of the liquid layer (the Ni–Ti liquid phase);(3) isothermal solidification of the liquid zone; and (4) ho-mogenization of the solidified bond region. It has also beenproposed that stages (1) and (2) are relatively fast processesbecause they are controlled by the diffusion of atoms in theliquid phase[26,27]. The maximum width of liquid zone canbe calculated from mass balance constrains of Ti[18] andwas estimated to be about 27�m at 1323 K (1.33W0, W0 isthe thickness of Ti foil). Our results showed that isothermalsolidification of the Ni–Ti liquid phase layer had been com-pleted after PTLP bonding at 1323 K for 0.9 ks resulting inthe formation of the Si3N4/reaction layer/NiTi/Ni3Ti/Ni in-terfacial structure. Prolonged bonding time or higher bond-ing temperature homogenized the solidified bond region,leading to disappearance of the “transient” NiTi layer.Fig. 3clearly shows that the “reaction layer/NiTi” interface andthe “NiTi/Ni3Ti” interface migrated simultaneously duringbonding (the migration directions indicated by the arrows inFig. 3).

The most important features of the PTLP bonding, whichis significantly different from the brazing processes, areisothermal solidification of the liquid zone and homoge-

nization of the solidified bond region. These two processeseventually eliminate the low melting point materials in thejoint via the long-time interdiffusion of elements. As aresult, PTLP bonding can produce the joints that can with-stand high temperature service owing to the absence of lowmelting point materials that are usually present in a brazedjoint especially when low melting point filler metal is em-ployed. However, the problem associated with the PTLPbonding of ceramic materials is that the reaction layer,which significantly affects the joint strength, can be thick-ened during isothermal solidification and homogenization ofthe solidified bond region during long-time PTLP bonding.Therefore, as compared with the brazing or the solid-statediffusion bonding, PTLP bonding process is more compli-cated and difficult in determining the bonding parameters,with which the maximum joint strength can be obtained notonly at room temperature but also at elevated temperature.

As can be seen from the experimental results (Table 1), thejoint with the 3�m thick reaction layers had a high strength.Further thickening of the reaction layer reduced the roomtemperature joint strength. On the other hand, the joint, inwhich the NiTi layer was completely consumed, had higherhigh temperature strength than those with the presence of theNiTi layer. Therefore, it is necessary to know how to adjustthe bonding parameters in order to obtain an acceptable jointwith the optimal reaction layer thickness but without the lowmelting point NiTi “transient” layer.

The results obtained show that formation and growth ofthe reaction layer and the intermetallic Ni3Ti compoundlayer are simultaneous processes, and the growth rates forboth the layers follow the parabolic law as shown inEqs. (2)and (3). From Eq. (3), the time,t, to form a reaction layerwith the thickness,x, can be determined by

ln t = 2lnx − ln k0 + Q

RT. (5)

The time,t, to form the reaction layer can be plotted as afunction of temperature as shown inFig. 8. The time to ob-tain certain reaction layer thickness decreases with increasein the temperature. Similarly, the time to form the Ni3Ticompound layer can also be plotted in the same figure withsmaller activation energy than that of the reaction layer. Thetwo sets of parallel lines with different slopes can be seenin Fig. 8. Given an optimal reaction layer thickness,x2, thethickness of Ni3Ti compound layer can be adjusted fromy1to yi by changing temperature and time simultaneously. Itis worthy noting that PTLP bonding at lower bonding tem-perature with longer bonding time is favorable for obtain-ing thicker Ni3Ti compound layer, meanwhile the reactionlayer thickness can be kept atx2. When the thickness ofNi3Ti compound layer exceeds a certain value (as shown inFigs. 2 and 4), the NiTi layer can be completely consumed.Consequently, the joint can achieve high strength not onlyat room temperature also at elevated temperature. After theNiTi layer is completely consumed, the thickness of Ni3Ticompound layer will decrease due to the diffusion of Ni

Z. Chen et al. / Materials Science and Engineering A 380 (2004) 394–401 401

Fig. 8. Conceptual current PTLP bonding process controlling as a functionof the time of reaction layer and intermetallic Ni3Ti layer formation as wellas PTLP bonding temperature. For an optimal reaction layer thickness,x2, bonding at relative low temperature for relative long time is in favorfor obtaining thicker Ni3Ti compound layer (lntR is the time to formreaction layer and lntM is the time to form intermetallic Ni3Ti layer).

into Ni3Ti causing the migration of Ni3Ti/Ni interface to-wards the ceramic side. At this stage, however, the reactionlayer may grow much more slowly due to lower diffusionrate of Ti atoms in the solid Ni3Ti compound and in the re-action layers that have been formed as indicated inFig. 4andTable 1. Subsequently, a stable reaction layer thicknessis responsible for maintaining a stable joint strength at hightemperature.

5. Conclusion

Interfacial structure of Si3N4/TiN/Ti5Si3 + Ti5Si4+ Ni3Si/(NiTi)/Ni3Ti/Ni was formed after PTLP bondingof Si3N4 using the Ni/Ti multi-layers. The reaction layersadjacent to Si3N4 consisted of TiN, Ti5Si3, Ti5Si4, andNi3Si, which grew following the parabolic law with the ac-tivation energy of 540 kJ/mol. Besides the reaction layers,the intermetallic compound layers of NiTi and Ni3Ti werealso formed. The Ni3Ti layer grew towards to both the Niand ceramic sides, and the growth rate was controlled by theparabolic law with the activation energy of 162 kJ/mol. TheNiTi layer was gradually consumed with the simultaneousgrowth of the reaction layers and the Ni3Ti layer.

The room temperature joint strength was significantly af-fected by the thickness of reaction layer. The maximum jointstrength of 160 MPa was obtained when the reaction layerwas 3�m thick. In contrast, the elevated temperature jointstrength was independent upon the reaction layer thickness,while it is dependent upon whether the low melting pointNiTi layer was present in the joint. For the joint after 3.6 ksof bonding at 1423 K, the joint strength of 100 MPa was re-tained up to 800◦C, since the NiTi layer was completelyconsumed. However, its room temperature joint strength

was relatively low due to the presence of thick reactionlayers.

A model was proposed to optimize the PTLP bondingparameters based on the experimental results. The proposedmodel indicated that the high joint strength at both roomtemperature and elevated temperature could be obtained bythe PTLP bonding at relative low bonding temperature withprolonged holding time.

Acknowledgements

The author gratefully acknowledges Dr. Nianqiang Wufor his valuable discussion and suggestions. This work wassupported by Jiangsu Province Natural Foundation of China(project number is BK2002602).

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