Metal to ceramic joining via a metallic interlayer bonding technique

Journal of Materials Processing Technology 152(2004)

257–265

Metal to ceramic joining via a metallic interlayer bonding technique

A.K.Jadoon a ,?,

B.Ralph a ,P.R.Hornsby b

a

Department of Mechanical Engineering,Brunel University,Uxbridge,Middlesex UB83PH,UK b

The Wolfson Centre for Material Processing,Brunel University,Uxbridge,Middlesex UB83PH,UK

Accepted 15October 2003

Abstract

The lack of suitable joining techniques for dissimilar materials such as metal and ceramic for high-temperature applications and devices is a major hindrance to the utilization of their outstanding properties and lower costs.

This paper presents the results of joining between the high-temperature and corrosion resistant iron–chromium–aluminium alloy (Fe-cralloy)with silicon nitride by use of metallic interlayer.The primary objectives were to produce joints for high-temperature applications (above 500?C)and to observe the effects of processing parameters with regards to joint strength.

Joining by use of a thin (12.5?m)and non-remaining copper foil produced joints with an average maximum shear strength of 67.5MPa (±5)at 1100?C,30min dwell time at 9.5MPa,under vacuum.The processing conditions were found to play a very important role in the joining process and effected joint strength.The results suggest that a thin aluminium nitride,AlN,reaction product layer was present on the silicon nitride interface in the joined samples.This is an indication that reactive wetting was occurring.Iron,chromium,aluminium and copper were found to in?ltrate the silicon nitride,whilst silicon and nitrogen diffused into the Fecralloy.

Joining was also attempted using a Ti/Cu/Ti multilayer and its variations.Joining was not possible due to the formation of a brittle reaction product layer,highlighting that wetting and joining are not synonymous.?2003Elsevier B.V .All rights reserved.

Keywords:Metal–ceramic joining;Interfacial melting;In?ltration;Process parameters

1.Introduction

Silicon nitride is an attractive engineering ceramic due to its high strength and toughness,good thermal properties and chemical resistance at elevated temperatures [1].It ?nds use in a variety of applications including spark plugs,piston heads and rocker arms in automobile engines [2].Due to the brittle nature of ceramics,machining holes for riveting or bolting is not advisable and the use of bonding adhesives is avoided where service temperatures rise above 100?C.The popular choice for metal to ceramic joining tends to be active metal brazing.Joining of heat resistant alloys to silicon nitride using this method has been extensively studied and developed over the last decade [3–5].

There are numerous obstacles for successful metal–ceramic joining,the most important of which is the relative inert-ness of the ceramic and the coef?cient of thermal expansion (CTE)mismatch.The latter tends to be a major problem when joining any dissimilar material.Temperature changes induced during cooling from the joining temperature and

?

Corresponding author.

during subsequent service can generate high internal stresses due to the CTE mismatch and lead to poor joint strength or failure [6].It has been proposed that the use of a remaining ductile interlayer,such as in active metal brazing,can over-come or reduce the residual stress build up by deformation [7].

Like all techniques active metal brazing has its limita-tions and cannot join all combinations of metal–ceramic.More importantly the joints formed cannot be used for high-temperature applications with a ceiling limit of around 400?C [8].Thus,a need has been identi?ed to develop and study a technique(s)that can join metal–ceramic with a large CTE mismatch for potential high-temperature applications (above 500?C)by a simple process.

This study deals with the joining of silicon nitride to the high-temperature iron–chromium–aluminium alloy (com-mercially known as ‘Fecralloy’)using copper (Cu)and titanium (Ti)/Cu variant interlayers.Joining was performed under varying processing conditions and the optical pro-cessing conditions were determined by measuring the shear strength of the joints obtained.

Joining of Fecralloy–silicon nitride by another technique has been achieved and is reported elsewhere [9].

0924-0136/$–see front matter ?2003Elsevier B.V .All rights reserved.doi:10.1016/j.jmatprotec.2003.10.005

258 A.K.Jadoon et al./Journal of Materials Processing Technology152(2004)257–265

2.Experimental

2.1.Materials

The materials to be joined were Fecralloy(72.3Fe,22.0Cr, 5.0Al,0.3Si,0.02C,0.2Mn,0.1Y,0.1Zr,wt.%)and reaction bonded silicon nitride.They were supplied by Goodfellow Ltd.,UK in sheet form100mm×100mm×https://www.360docs.net/doc/ab18550051.html,ing a low speed diamond wheel they were cut into samples mea-suring10mm×10mm×5mm.The joining surface was manually ground to a planar1200grit?nish.Polishing to a 1?m?nish using6?m and then1?m diamond paste fol-lowed this.

The physical properties of the reaction bonded silicon nitride and Fecralloy are given in Table1.Foils of technically pure Cu and Ti were used for interlayer.The Cu foil(12.5 and50?m thickness)and Ti foil(12.5?m thickness)were cut into10mm×10mm squares.After polishing,the silicon nitride and Fecralloy substrates along with the Cu and Ti foil were all cleaned ultrasonically and stored in acetone until use.

2.2.Joining procedure

Joining was carried out in a purpose built high-temperature vacuum furnace,where temperatures of1200?C under a vacuum of5×10?4Torr were possible.Greater experimen-tal detail,including that of the high-temperature vacuum furnace has been reported previously[9].

The Cu foil was placed between the Fecralloy and silicon nitride in a butt joint con?guration.This con?guration was then placed into the high-temperature vacuum furnace using a graphite sample holder.

Joining trials were carried out between850and1140?C, dwell time0–60min and applied uni-axial pressure 0–9.5MPa.The optimum heating and cooling rate was found to be5K/min.A higher cooling rate was found to cause cracking after processing.Joining was attempted using a Cu foil and a Ti/Cu/Ti multilayer and its variations. Table1

Properties of Fecralloy and silicon nitride(courtesy of Goodfellow Ltd., UK)

Property RB silicon nitride Fecralloy Apparent porosity15–23%0

Density 2.4g cm?37.2g cm?3 Thermal conductivity

at20?C

10–16W m?1K?116W m?1K?1

Thermal expansivity,

20–1000?C

3.3×10?6K?111.1K?1

Upper continuous use

temperature

1200–1500?C1100–1300?C

Compressive strength550–650MPa Not given Hardness(Vickers)800–1000kgf mm?2Not given Shear strength190–240MPa Not given Tensile modulus170–220GPa330GPa 2.3.Analysis

Analysis of the metal–ceramic interface is important as trends or relationships can be drawn from the interfacial microstructure with regards to the processing parameters. The transversely cross-sectioned metal–ceramic joint was initially examined using a Leitz optical microscope.The in-terfacial microstructure was examined in greater detail us-ing a JEOL840scanning electron microscope(SEM).En-ergy dispersive X-ray(EDX)analysis was used to iden-tify formation of any precipitates.All image and analysis work was carried out using secondary electron imaging/ analysis.

2.4.Mechanical testing

Joint strength was quanti?ed by means of shear strength testing.In order to ensure greater accuracy three samples were tested for each joining condition.A designed?xture was used such that the shear force was applied to the bond line and sample movement was prevented.

While shear strength measurements quantify joint strength,it does not re?ect upon how the joint would be-have under possible service like conditions.Hence,thermal cycling was used to assess the effects of temperature and thermal cycling upon joint integrity.The joined sample was placed in an air furnace and heated to850?C with a dwell time of4h(heating/cooling rate of50K/min).Upon cool-ing the joint would be examined visually and if still intact the rigorous thermal cycling repeated until joint failure. 3.Results and discussion

3.1.Cu interlayer

Joining of Fecralloy–silicon nitride was investigated by use of pure Cu foil of different thickness(12.5and50?m). Due to Cu poor oxidation resistance,a remaining Cu inter-layer was not desirable and so thin Cu foil was used,as it would not remain after processing.

3.1.1.Interfacial microstructure

On viewing the micrographs shown in Figs.1–8,good interfacial bonding with no remaining Cu foil and no inter-facial reaction products are apparent for the joined samples. Joining carried out under low applied pressure of1.5MPa (Fig.1)was found to have considerable open porosity at or close to the joint interface with no visible interfacial reaction products and no in?ltration/diffusion zones in the ceramic and metal.

However,a slight increase in temperature resulted in the formation of numerous precipitates.The EDX elemental map of Fig.2shows the presence of these precipitates along with the elemental distribution.Our EDX spot analysis re-sults suggest that the needles like precipitates are chromium

A.K.Jadoon et al./Journal of Materials Processing Technology152(2004)257–265259 Fig.1.Micrograph of a Fecralloy–silicon nitride joint(1100?C,30min,1.5MPa).Good interfacial bonding is apparent with signs of reactive wetting (uneven joint interface).

nitride(Cr2N),while the smaller and more numerous pre-cipitates are iron silicide(Fe3Si).

The elemental maps also establish the presence of a thin reaction product layer at the silicon nitride joint interface for all the joined samples(average thickness of8?m).From the results obtained,this is most likely to be aluminium nitride, AlN.

The castrotropic effects of a high cooling rate can be seen in Fig.3,where interfacial cracking is apparent.The most likely reason for this is the large residual stresses build up due to the CTE mismatch.

Joining carried out using greater applied pressures of 5.5–9.5MPa all have a diffusion and in?ltration zone,as shown in Figs.4and5.The?rst diffusion zone at the Fe-cralloy interface is that of Si,where precipitates of Fe3Si are present.The second diffusion zone is that of N,where the needles like Cr2N precipitates exist.The chemical compo-sition of the in?ltrated compound in the silicon nitride was found to be similar to that of Fecralloy,in addition to small amounts of Cu,Si and N(<10wt.%).The size of the dif-fusion and in?ltration zone was found to vary according to the processing temperature and not pressure or time.This is because they are thermally assisted processes.

The use of a thicker Cu foil did not result in joining, as can be seen in Fig.6.The foil remained after process-ing adhering to the Fecralloy only.Failure was always at the ceramic–interlayer interface.The reason for this is be-lieved to be due to the formation of brittle phases at the ceramic–interlayer interface(mostly copper silicides).

The morphology of the metal–ceramic interface depends upon the type of interaction that has occurred.If only phys-ical interaction has occurred,the structure of the metal and ceramic is unchanged.However,if a chemical reaction oc-curs,the morphology is effected depending upon whether solid–solid or solid–liquid reactions occur and if new in-terfacial phases are formed.Formation of new interfacial phases not only alters the microstructure but also the phys-ical and mechanical properties.These interfacial phases or reaction product layers are a consequence of the reactions needed to cause wetting of the ceramic.One can consider the reaction product layer as chemical bridges between the metal and ceramic.

It is believed that the advantages or disadvantages of a re-action product layer vary from system to system and general-ization would be dif?cult.However,a thick reaction product layer tends to weaken the interface due to excessive growth stresses and brittle nature.Opinion is varied about the pres-ence of a reaction layer product.Some researchers believe they are very detrimental to joint strength[10],while oth-ers attribute interfacial failure due to a thin reaction layer product[11].

3.1.2.Effects of processing conditions

The primary variables in the joining process are pressure, temperature and time.These variables are not independent from each other and the effect of each variable is discussed outlining the important considerations associated with each variable.Table2presents the shear strength results for the varying processing conditions.

3.1.2.1.Pressure.In order to determine the optimum pro-cess conditions,the in?uence of applied bonding pressure on joint integrity was studied.Experiments were carried out with different pressures varying between0and9.5MPa.Fig. 7shows the plot of average shear strength(±5MPa)versus bonding pressure at1100and1140?C.

260 A.K.Jadoon et al./Journal of Materials Processing Technology152(2004)257–265 Fig.2.EDX elemental map showing presence of AlN reaction product layer and needle like Cr2N precipitates(1140?C,30min,1.5MPa).

It is clear that the applied pressure affects joint strength. At high-temperature and applied pressure the yield point of Fecralloy is lowered.The applied pressure causes partial de-formation of the Fecralloy at the joining interface,which is suf?cient to bring the Fecralloy into full contact with the sil-icon nitride on an atomic scale.Once full contact is made,further increase of pressure aids the diffusion/in?ltration process that is occurring at the interface.The liquid metal that in?ltrates the silicon nitride holds the substrate together by mechanical interlocking upon cooling.It follows that the more liquid metal that in?ltrates the silicon nitride,the stronger the joint.Of course there must be an applied pres-

A.K.Jadoon et al./Journal of Materials Processing Technology152(2004)257–265261 Fig.3.Micrograph of a failed joint due to high cooling rate(1100?C,30min,1.5MPa).

sure after which detrimental effects occur such as bulk de-formation of the metal,expulsion of the liquid metal,etc. This was not identi?ed in the present work.

The applied pressure also helps to break up surface oxides present at the joining interface.Fecralloy has a protective iron–chromium–aluminium oxide spinnel and a silicon ox-ide layer covers silicon nitride.These oxide layers present a barrier for wetting and also the diffusion/in?ltration pro-cess,giving rise to an incubation period for the interfacial reactions to begin.Quantitative data about the applied pres-sure required to promote the removal of these oxide layers is not available.

3.1.2.2.Temperature.This is an important variable and the dependence of strength on temperature can be explained by Fig.

4.Micrograph of Fecralloy–silicon nitride joint(1100?C,30min, 7.5MPa).considering the behaviour of the materials in the vicinity of both the metal and ceramic interface.Temperature increases interaction across the metal–ceramic interface by increasing the mobility of the atoms and also the mobility of disloca-tions in the metal during joining.

Diffusion and in?ltration are thermally assisted processes and so an increase in temperature is followed by an increase in their respective rates.

Increasing temperature can cause a non-wetting-to-wetting transition for liquid metals in contact with ceramics,usually at1100?C.This was found to be the case for an Al drop on silicon nitride[12].At low temperatures,large non-wetting angles were observed as both Al and silicon nitride had sur-face oxide layers.At higher temperatures(1100?C)contact angles of90?(i.e.wetting was occurring)were observed Fig.5.Micrograph of Fecralloy–silicon nitride joint(1140?C,30min, 7.5MPa).

262 A.K.Jadoon et al./Journal of Materials Processing Technology152(2004)257–265 Fig.6.Micrograph showing failed Fecralloy–silicon nitride joint using a

thicker Cu foil(1100?C,30min,1.5MPa).

and believed to be due to the breaking down of surface oxide layers and lowering of free surface energy.This is believed to be the case with our joining system,as joining was not possible below1100?C.

3.1.2.3.Time.The dwell time at temperatures of1100?C or above effects the interfacial reactions and size of dif-fusion/in?ltration zones.At1100and1140?C we see the same pattern where there is a minimum time to initiate the reactions in order to produce a joint(10min),an optimum time that produces joints of maximum strength(30min) and a detrimental time that produces weak joints/no joints (60min).From this it is apparent that there is a minimum time to initiate the joining process and this is due to the breaking up of surface oxides and wetting of ceramic in-Fig.7.Graph showing dependence of average shear strength of joints with applied pressure(Cu interlayer).Fig.8.Micrograph of a failed sample during thermal cycling(Cu inter-layer).

terface.There is an optimum time,where suf?cient wet-ting/interfacial reactions have occurred that produce a thin reaction product layer and elemental diffusion/in?ltration to produce a good joint.Then there is a detrimental excess Table2

Results of joining trials and thermal cycling using Cu foil interlayer Temperature

(?C)

Dwell

time

(min)

Applied

pressure

(MPa)

Average shear

strength

(±5MPa)

No.of thermal

cycles till joint

failure 100010 1.500

100030 1.500

100060 1.500

110010000

110030000

110060000

110010 1.512.71

110030 1.549.63

110060 1.532.32

110030 3.556.10

110030 5.562.33

1100307.566.83

1100309.567.53

114010000

114030000

114060000

114010 1.516.11

114030 1.534.32

114060 1.511.81

114030 3.548.92

114030 5.557.42

1140307.563.83

1140309.562.23

A.K.Jadoon et al./Journal of Materials Processing Technology152(2004)257–265263

time,where the reactions proceed to form brittle products, usually silicides that cause a weak joint.

3.1.3.Mechanical testing

3.1.3.1.Shear testing.Table2presents the data for the average shear strength of the joined samples.Shear strength ranged from11.8to67.5MPa and are modest compared to those of metal–metal and ceramic–ceramic joining. Metal–ceramic joints tend to have inherently low shear strength values.The two main reasons for this are the resid-ual thermal stress due to the CTE mismatch and formation of brittle phases at the joint interface.Our shear strength values compare well to other metal–silicon nitride joints. Using an active metal braze,Lee[13]presented shear strength values of46.1–70.4MPa for Inconnel718–silicon nitride joints.Work by Vegeter and Den Ouden[14]pro-duced zirconia–silicon nitride joints using a Ni interlayer that had a maximum shear strength of57MPa.

The modest shear strength values obtained in this work can be attributed to the mentioned reasons along with the presence of interfacial defects such as cracks and pores. Processing conditions do affect the shear strength of the joint and the results reveal the following trends:

1.At a given joining time and pressure,the shear strength

increases with joining temperature,reaching a maximum and then decreasing with further temperature increase.

2.At a given joining temperature and pressure,the shear

strength increases with joining time,reaching a maxi-mum and then decreasing with further dwell time in-crease.

3.At a given joining time and temperature,the shear

strength increases with pressure.This work did not es-tablish the maximum pressure after which shear strength decreases.

These results establish that the optimum processing con-ditions to produce a Fecralloy–silicon nitride joint with the highest shear strength is1100?C,9.5MPa and30min.Dwell times greater than30min and temperatures greater than 1100?C,make the formation of brittle silicides at the ce-ramic interface likely and lower shear strength.

Lee and Kim[15]studied the residual stress in various metal–ceramic joints using an X-ray diffraction technique and compared these with analytical calculations of the resid-ual stress.They put forth a?gure of700MPa for a di-rectly joined316stainless steel–silicon nitride joint.This value is believed to be similar for Fecralloy–silicon nitride joints.While the exact?gure of residual stress is unsure, it is clear that it is considerably high.This explains why metal–ceramic joints tend to have low shear strength values.

3.1.3.2.Thermal cycling.Total reliance upon joint strength measurements to characterize the mechanical be-haviour of a metal–ceramic joint in service like conditions is incorrect.As mentioned in the previous section failure is mainly attributed to residual thermal stress and formation of brittle interfacial phases.Interfacial defects such as cracks and pores and the local variation of stresses governed by the geometry of the joint,the elastic and plastic properties of the metal/ceramic and loading conditions all in?uence joint failure.

It is therefore important to characterize mechanical be-haviour of the joint under service like conditions.One op-tion is to measure joint strength at temperature and this al-ways shows that joint strength decreases with temperature [16].The main reason for this tends to be the low melt-ing temperature of the active metal braze and an annealing effect at lower temperatures.Another option is thermal cy-cling,which was used in this work.This gives an indication to the durability of the joint to thermal cycling.Provided all the test conditions are kept the same,the results are reliable enough to serve as a means of comparison.

The temperature of850?C was chosen as it higher than the service temperature of most active metal braze joints. From Table2it can be seen that the thermal cycling results are quite encouraging.They can be directly related to the shear strength values,as the stronger joints tend to be able to withstand more thermal cycles.

It is the residual stress factor that comes into play leading to joint failure during thermal cycling.The stresses parallel to the interface are tensile in metals and compressive in ceramics showing a jump at the interface.Maximum tension in ceramics occurs near the free edge of the interface,where the stresses are singular.It is this tensile stress developed in the ceramic adherent that is thought to be the most important factor effecting joint failure.Fig.8shows the micrograph of a sample that failed after thermal cycling.Extensive cracking in the ceramic where the cracks initiate and propagate from the side can be seen.Failure was due to the excessive thermal stresses in the joint.

3.2.Ti/Cu/Ti multi-interlayer

Partial transient liquid phase(PTLP)bonding has been explored for ceramic–ceramic and metal–ceramic joining in recent years[17].This process offers the possibility of pro-ducing an interlayer with a high melting temperature and hence a joint that can possibly withstand high-temperatures. Iino[18]used this process to join silicon nitride with Ni us-ing a Ti foil.Zhang et al.[7]further increased the strength of this joint(four-point bend test)by the insertion of a Cu foil between the Ni metal and Ti foil.

The joining of Fecralloy–silicon nitride was investigated by use of a Ti/Cu/Ti multi-interlayer and its variations.The Ti/Cu/Ti system was selected as it has a eutectic point and a reaction between the inner insert(Cu)and outer inserts (Ti)would occur with the formation of a transient liquid. At1050?C the liquid phase formed at the Ti/Cu interface would disappear as the Cu diffused into the Ti.If the Cu concentration in the Ti is below30wt.%,this interlayer has a melting temperature of approximately1200?C,producing a high-temperature ductile interlayer.This system of a re-

264 A.K.Jadoon et al./Journal of Materials Processing Technology 152(2004)257–265

Table 3

Results of the joining trials using multi-interlayer foil system (no joining)Multilayer system Temperature (?C)Dwell time (min)Applied pressure (MPa)Ti/Cu/Ti

950302100030 1.5100030 3.5100060 1.5110010 1.5110030 3.5110030 5.5110060 3.5114030 1.5Cu/Ti/Cu

100030 1.51100

30

1.5

maining ductile interlayer would be compared to that of the previous section where the Cu interlayer does not remain.3.2.1.Results and discussion

3.2.1.1.Microstructure of interface.Table 3presents the results of the joining trials using the multi-interlayer sys-tems.Although varying processing conditions were used,joining was not possible at all as weak joints were produced that broke under handling.

Figs.9and 10show the trials carried out at 1100and 1140?C.From the Cu–Ti phase diagram [19]it is established that a eutectic reaction occurs at 1050?C,while at 1100?C,the Cu foil completely melts.The rough silicon nitride sur-face indicates that reactive wetting occurred during process-ing.EDX analysis of the interlayer found it to be mostly TiCu and Ti 2Cu 3both of which had dissolved Si.There were also small areas where pure Cu or Ti existed.EDX analy-sis of the silicon nitride interface determined the presence of two products.The ?rst was TiN,from the reaction of Ti with the silicon nitride.The second product contained Ti,Cu and Si with small amounts of N.This is believed to be

a

Fig.9.Micrograph of failed Fecralloy–silicon nitride joint using Ti/Cu/Ti interlayer (1100?C,30min,1.5MPa).

Ti–Cu Si–N compound and its presence along with that of TiN was also con?rmed by Suganuma et al.[20]in the join-ing of silicon nitride to itself using a Ti–Ag–Cu braze.From this it is apparent that reactive wetting and dissolution of the ceramic occurs,with small amounts of reactive in?ltration into the silicon nitride (Ti and Cu)and diffusion into the Fecralloy (Ti,Cu,Si and N).Joint failure can be attributed to the two reaction products that form in all the joints with varied processing conditions.This can be due to their brit-tle nature or excessive residual stresses that accompany the volume change with the formation of the reaction product.In order to overcome this problem and achieve joining two different methods were tried.The ?rst used a thinner Ti foil (12.5?m),as the thickness of the Ti foil determines the amount of liquid present.Weak joints were produced that broke under handling.EDX analysis identi?ed the presence of the Ti–Cu–Si–N compound,which is believed to cause failure.However,it should be noted that although these joints also broke under handling,considerably more force was re-quired with the Ti/Cu/Ti joints.From this it is apparent that as little Ti as possible should be used due to the formation of the brittle compound.

The second approach was a reversal of the multilayer,in that a Cu/Ti/Cu multilayer was used (12.5?m/20?m/12.5?m).Once again joining was not possible as the joints broke under handling but showed signs of reactive wetting at the silicon nitride interface.At the processing tempera-tures of 1050or 1100?C,the Cu foil would react with the free Si on the silicon nitride surface forming a Cu–Si liquid and would also react with the Ti foil forming a transient liquid.It is believed that a Ti–Cu–Si liquid was present at the silicon nitride interface,which is responsible for the ex-tensive reactive wetting (cratoring).Again the Ti–Cu–Si–N compound was present at the silicon nitride interface.One can conclusively attribute the presence of this compound to failure highlighting the dif?culties of metal–ceramic joining.

A.K.Jadoon et al./Journal of Materials Processing Technology152(2004)257–265265 Fig.10.Micrograph of failed Fecralloy–silicon nitride joint using Ti/Cu/Ti interlayer(1140?C,30min,1.5MPa).

4.Conclusions

It has been possible to join Fecralloy with silicon nitride using a thin Cu interlayer foil.Good interfacial bonding is achieved and mechanical interlocking forces hold the joints together that exhibit the highest shear strength.Joining re-sults in a thin reaction product layer at the silicon nitride interface(results suggest AlN),diffusion of Cu,Si and N into the Fecralloy and in?ltration of Fe,Cr,Al and Cu into the silicon nitride.

The shear strength values are very modest but compare well to other metal–ceramic joints,where the CTE mismatch is also large.The thermal stresses that are induced in the silicon nitride during the process cooling are believed to be the main cause of the modest shear strength values and reason for failure during rigorous thermal cycling.

To overcome this problem,the use of a remaining and high-temperature ductile interlayer was looked in to by use of Ti/Cu/Ti multi-interlayer and its variations.However, joining was not possible and this was attributed to the pres-ence of a Ti–Cu–Si–N brittle compound.Although Ti is an active element aiding the wetting of ceramics,in large concentrations it tends to be detrimental as it can form brittle compounds.This is one of the main reasons why its concentration is low in active metal brazes.

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