Ceramic to Metal Joining Report

High-ReliabilityJoiningof Ceramicto MetalHOWARDMIZUHARA,'E. HUEBEL,and T. OYAMA

WESGODivision,GTEProductCorporation,Belmont,California94002

With the development of new ceramicmaterials, including those for struc-

tural applications, there is an increasingdemand to join ceramic components tometal structures. The most commonly usedmethodis the moly-manganyseprocess.1-3

This process is well-established and pro-duces highly reliable joints between ce-ramics and metals. However, the moly-manganese process requires two process-ing steps: metalization of the ceramic, fol-lowed by brazing. It is, therefore, time-consuming. Furthermore, the metaliza-tion process is conducted at very hightemperature ( -1500°C). In addition, jointproperties are sensitive to process varia-bles, and, hence, precise process controlis required to obtain the reliable joints.As a result, this process is very expensive.

Direct brazing between ceramic andmetal is also possible through active fillermetals.4-'o Because this is a one-step pro-cess, it is simple and economical. Al-though active metal brazing has been in-vestigated since 1940, it has not beenwidely accepted because of inconsistentjoint properties. There are several meth-ods of the active metal brazing. For ex-ample, one method involves a sheet of ti-tanium (the most commonly used activeelement) that can be cladded by two sheetsof the conventional brazing metals.6 An-other example usestitanium hydride pow-ders mixed with powders of conventionalbrazing metals.s However, the most eco-nomical method utilizes a filler metal where

an active element(s) forms a true alloywith the base filler metal. The presentinvestigation centers on a copper-silverfiller metal containing titanium as an ac-tive element (Cusil-ABA)@. * The nominalchemical compositions and several im-portant properties of the Cusil-ABA@ arelisted in Table 1. Because the concentra-tion of the titanium is relatively low (2wt%), titanium dissolves in the matrix(mainly copper-rich phase) as a solid so-lution.

There are a variety of different prop-erties to be considered in the ceramic-metal joints, e.g., mechanical, electrical,thermal properties, etc. Depending on theapplication of the joint, some propertiesare more important than others. However,

'Member, American Ceramic Society.'GTE Product Corp., WESGO Division, Belmont,

CA.

Table I. Chemical Compositions and Properties of Cusil-ABNMFillerMetalNominal composition 63 wt% Ag+35 wt% Cu+2 wt% Ti

Physical properties815°C (I 500°F)780°C (l435°F)9.8 Mg.m-J (5.2 troz.in.-3)180 W.m-I.K-1 (104 Btu.h-'.ft-,.oF-')18.5 X 10-6 K-I (10.3 X 10-6 OF-I)

Liquidus temperatureSolidus temperatureDensityThermal conductivity*Thermal expansion coefficient (RT to 500°C)

Electrical properties44 x 10-9 Q.m23 x 106 Q-I .m-I

Mechanical properties83 GPa (12 x 106 psi)0.36

271 MPa (39300 psi)346 MPa (50200 psi)20%1100 MPa (110 KHN)

Electrical resistivityElectrical conductivity

Young's modulusPoisson's ratio'Yield strengthUltimate tensile strengthElongation (2-in. gage section)Hardness

'Calculated from the electrical conductivity by the Wiedmann-Franz law. 'Estimated by the volumetricaverage of the Poisson's ratios for silver and copper.

the mechanical properties are some of themost important properties for any joint.Joints without any mechanical strengthcan be regarded as unsuccessful joints. Inthe present paper, therefore, the mechan-ical properties of the joints are empha-sized. For the evaluation of the joint prop-erties, it is essential to establish propertesting methods so that effects of the pro-cessing variables can be detected accu-rately and consistently. It is also desirablethat the results of the tests provide mean-ingful engineering parameters for designengineers.Severaltestingmethodsareex-amined.The optimumtestingmethodsforthe evaluation of the ceramic-metal jointproperties are recommended.

One of the major problems of joiningceramic to metal is the thermal expansionmismatch between them. Generally, theceramicmaterialshavelowerthermal ex-pansioncoefficientsthan the metallicma-terials. A list of the thermal expansioncoefficients of several structural ceramicsand common metals is given in Table II.The difference in the thermal expansioncoefficients can lead to very high stress atthe brazed region during cooling from thebrazing temperature to room tempera-ture. The high stress sometimes results injoint failures or unreliable joints. There-fore, the thermal stress problems shouldbe overcome to obtain reliable joints be-tween the ceramic and the metal.

In the present paper, the active metalbrazing process of the ceramic to the metalis examined in detail. Influences of allprocess variables, including joining ma-

terials, engineering joint design, and braz-ing procedures, are evaluated to obtainreliable joints between ceramic and metalwith the active filler metal.

Joining MaterialsCeramic-metal brazing involves three

materials: ceramic, metal, and filler metal.Properties of each material influence theresultant joint properties.

Ceramic ComponentThe ceramic, because of its inherent

brittleness, is the most critical materialfor obtaining reliable joints. The baseproperties of the bulk ceramic memberare essential. When the properties of thebulk ceramic are not sufficient, the ther-mal stress simply fractures the ceramicmember. Furthermore, the surface con-dition of the ceramic is also very impor-tant for the joint reliability.7.'oMizuharaand Mally7 and Mizuhara and Huebel'Odemonstrated that, when the ceramic sur-face is in the as-ground condition, onlymarginal joint properties between the ce-ramic and the metal will be obtained.

When the ceramic material is groundby a metal-bonded diamond wheel, mi-crocracks are introduced at the surfaceof the ceramic. The size of the micro-cracks depends on the diamond grit sizeof the wheel and also on the rate of ma-terial removal. The surface damage caninitiate major cracks in the ceramic bythe thermal stress and, hence, result in anunreliable joint. Therefore, the ceramicsurface should be free of damage to ob-

CERAMIC BULLETIN, VOL. 68, NO.9, 1989 (@ACerS) 1591Reprinted for The American Ceramic Society Bulletin. Vol. 68, No.9, September 1989Copyright @ 1989 by The American Ceramic Society, Inc.

tain high-reliability joints. This conditioncan be met simply by using sintered ce-ramic materials. However, nearly all sin-tered ceramic parts over about 2 cm insize should be ground, because distortionof the parts during the sintering requiresgrinding for dimensional control. Groundceramic materials should be treated fur-ther to obtain a defect-free surface con-dition. This can be performed by a resin-tering or lapping process. In the resin-tering process, the damaged layer is healedthrough sintering. In the case of the lap-ping process, the damaged layer is phys-ically removed. It should be mentionedthat the thickness removed by the lappingmust completely eliminate the surfacedamages. In the present investigation, AL-995* (99.5% AlzOJ)is used as the ceramicmember of the joint. Two surface condi-tions are examined: the as-ground and theground-and-resintered conditions. The re-sintering process is conducted at 1650°Cfor 1 h.

Metal Component

The metal is generally ductile and,hence, does not readily fracture. How-ever, there are several important require-ments to obtain reliable joints.

The metal should exhibit only lowblushing (or surface flow)of the filler metal.The extensive blushing effectively de-pletes the amount of the active elementavailable for wetting the ceramic. Amongthe parameters controlling the blushingbehaviors are the relative properties ofthe metal member and filler metal. Ef-fects of the metal member on the blush-ing behaviors are demonstrated in Fig. 1.In these examples, the Cusil-ABA@foils(50-tLmthick) are placed on different sub-strate metals (and alumina) and meltedunder vacuum at 830°C. As demonstrat-ed in Fig. 1, the Cusil-ABA@shows dif-ferent blushing behaviors depending onthe substrate material. No blushing is ob-served on alumina, 304 stainless steel, orcopper, whereas extensive blushing is ob-served on nickel. These results demon-strate that the blushing of the filler metalcan be minimized by proper selection ofthe metal member. Another method toeliminate the blushing is to apply stop-offpaint on the metal member. It should bealso mentioned that machine marks onthe metal surface promote blushing.

As described earlier, high thermal stresscan be created at the joint because of thethermal expansion mismatch between theceramic and metal members. One of themethods to reduce the thermal stress isto closely match the thermal expansioncoefficients of the metal and the ceramic.For example, molybdenum can be usedto bond with an alumina ceramic (see Ta-ble II). However, there are a limited num-ber of the ceramic-metal combinations to

'Spang Industries, Magnetics Division, Butlep, PA.

Fig. 1. Blushing behaviors of the Cusil-ABA@on different substrates. No blushing is observedon (A) alumina, (B) 304 stainless steel, or (C) copper, whereas extensive blushing is observedon (0) nickel.

Table II. Thermal Expansion Coefficients of StructuralCeramics and Common Metals

Material

Silicon nitrideAlumina (99.5%)MolybdenumKovar@Alloy 42410 stainless steelCopper

Thermal expansioncoefficient(xlO-6°C-I)

3.28.05.7

10.010.214.020.0

satisfy this condition. In the present in-vestigation, alloy 42t (Fe-4lNi), whichalso has a similar thermal expansion coef-ficient to that of the alumina ceramic, isused as the metal member.

Another method to reduce the thermalstress is to use a metal having a low yieldstrength. Plastic deformation of the metalmember accommodates the thermal ex-pansion mismatch, thereby reducing thethermal stress at the joint. In the presentinvestigation, two alloy 42 materials withdifferent strength levels are examined. Onematerial is in the cold-worked % hard (%H) condition and has a hardness of 75 inRockwell B scale (Rb). The other materialhas been fully annealed at 975°C for Ih. The hardness of the annealed materialis 65 in Rockwell B scale.

Filler-MetalComponentThe basic and most important require-

ment for the active filler metal is that thefiller metal should be able to wet and bondstrongly with the ceramic. However, thereare additional requirements on the activefiller metal. As described previously, thefiller metal should not blush over the ce-ramic or the metal member. Figure 2demonstrates the effect of the filler metalon blushing behavior. Two different fillermetals, a 70Ti-15Cu-15Ni alloy and the

Cusil-ABA@,are melted on the aluminasubstrate. The 70Ti-15Cu-15Ni alloyshows extensive blushing because of thehigh titanium concentration. In contrast,the Cusil-ABA@ (2 wt% Ti) shows noblushing. It is also important that the fill-er metal is ductile. The ductile filler metalaccommodates the thermal expansionmismatch to reduce the thermal stress atthe joint as in the case of the ductile metalmember described previously. Also, thefiller metal should have low vapor pres-sure. In addition, the filler metal shouldhave the ability to be tailored for specificapplications that require specific proper-ties such as melting temperature, oxida-tion and/or corrosion resistance, density,thermal expansion, etc.

As described earlier, the copper-silveralloy containing 2 wt% titanium is usedas the filler metal in the present investi-gation. The thickness of the filler metalhas been chosen as one of the process var-iables. Foils with three different thick-nesses (50, 100, and 150 tLm) have beenprepared and the effects of the filler-metalthickness on the joint properties are eval-uated.

Engineering Joint Design

As previously stated, one of the majorproblems of joining ceramic to metal is

f'TN> A l\KIf' UTn T I<',[,T1\1 \TnT. I':R Nn Q 1 QRQ Ira Arpr~)'~nn

the thermal expansion mismatch betweenthe ceramic and the metal. The differenceof the thermal expansion coefficients leadsto the high thermal stress in the joint re-gion. The thermal stress can be reduced1;>ythe selection of the materials, as dis-cussed in the Joining Materials section.As the size of the joint assembly increas-es, however, the thermal stress also in-creases, and it becomes more difficult toovercome the thermal stress by the ma-terials selections. Oftentimes, this diffi-culty can be overcome by engineering jointdesigns.

CompliantJomtDes~nEdge brazing of a metal cylinder to a

ceramic face is a popular form of com-pliant joint. In this case, the thermal ex-pansion mismatch is accommodated bythe concentric distortion of the metal cyl-inder. In addition, the fillet formed at thejoint distributes the thermal stress over alarge surface area of the ceramic, and,hence, the thermal stress on the ceramicis reduced. Figure 3 shows an example ofsuch a design, where a copper-clad 430-stainless-steel cup has been brazed on anend of an alumina tube. Honeycombstructures or Feltmetal@~can also be usedto have compliant joints, where distor-tions in the honeycomb structure or in theFeitmetal@reduce the thermal stress. Oneof the disadvantages of these two designsis that the joints cannot sustain compres-sive loading.

Ductile metal interlayer(s) can be usedin the joint assembly. The ductile inter-layer deforms plastically to reduce thethermal stress. Another method to obtaincompliant joints is to use an interlayer(s)that has a thermal expansion coefficientbetween those of the ceramic and themetal. This distributes the thermal ex-pansion mismatch and reduces the ther-mal stress. Figure 4 shows an example ofsuch a joint, known as a gradient seal. Inthis case, a molybdenum interlayer (ther-mal expansion coefficient of 5.7 x 10-6°C-I) is utilized to braze a silicon nitride(thermal expansion coefficient of 3.2 x10-6 °C-I) disk to a ductile cast-iron(thermal expansioncoefficientof 12 x 10-6°C-I) substrate.

Compression Joint Design

In this joint, the thermal expansionmismatch is utilized as an advantage toobtain the reliable joint. This is achievedby brazing the ceramic member into themetal member. During cooling from thebrazing temperature to room tempera-ture, the metal member (outside) con-tracts more than the ceramic member (in-side). This results in compressive stress inthe ceramic member as well as in the joint,and the joint strength is increased. Anexample of such a joint is shown in Fig.

IEnergy Conservation Systems, Brunswick Technit-ics Division, DeLand, FL.

Fig. 2. Effects of titanium concentration on blushing behaviors of active brazing metals onalumina: (left)extensive blushing is shown for 70Ti-15Cu-15Ni melted at 1050°C and (right)no blushing is observed for the Cusil-ABA@melted at 830°C.

M Copper -cladstainless steel

~ ~ Cusil-ABAfiller metal

Aluminacylinder

.,";Jt' :*'~~~E: ::1-.-.-

Fig.3. Example of the edge brazing where a copper-clad 430-stainless-steel cap has beenbrazed on an alumina cylinder: (left)a schematic cross section of the assembly and (right)the actual brazed sample.

~-~, ,~, ,,~,~, VM V" ,~,.,."" .~"'".,."

Si 3N4 CeramicCusil-ABA. !!!J Molybdenum

Cusil-ABA

m""""" ""'""<..",,,,~""~" """'~"""""""""- Dulile cast iron

Fig.4. Exampleof the gradient seal where a siliconnitridedisk has been brazed on a ductilecast-iron plate with a molybdenum interlayer: (left)a schematic cross section of the seal and(right) the actual brazed seal.

1593CERAMIC BULLETIN, VOL. 68, NO.9, 1989 (@ACerS)

.L-

5, where a silicon nitride rod is brazed to410 stainless steel through the compres-sion joint. It should be noticed that thesilicon nitride rod has a tapered end.Therefore, the stainless steel has a holewith the same taper and the end of thesteel has a knife edge. This is to eliminatesudden changes in the stress that can bedetrimental for the joint properties.

Stress-Distribution Joint Design

A ceramic backup can be used to dis-tribute tensile loading on the metal dia-phragm. Figure 6 shows an example ofthis type joint, where a cupronickel sheethas been brazed on an alumina cylinder.As shown in Fig. 6, a backup ring of alu-mina has also been brazed on top of thecupronickel sheet. The backup ring servestwo purposes. First, the backup ring cre-ates additional interface area (at the cu-pronickel sheet and the backup ring); thus,the thermal stress is distributed at twointerfaces instead of one. Second, thebackup ring changes the stress distribu-tion in the cupronickel sheet so that thereis no peeling moment at the edge of thecupronickel sheet.

Brazing Procedures

Furnace brazing is the best techniquefor ceramic-metal joining by the activefiller metal. Vacuum or inert gas is themost economical and pollution-free op-eration. In the case of vacuum brazing, aleak rate should be controlled to be lessthan 0.005 mm Hg/h. In the case of inert-gas brazing, a dry Ar, He, or H2 atmo-sphere can be used. Ironically, one of themajor problems related to active fillermetal brazing is that the active elementwets nearly all materials. Therefore, careshould be taken in designing the joint as-sembly so that molten filler metal doesnot touch any part of the brazing fixture.

The heating rate 'during the brazing isimportant. Since the metal member hashigher thermal conductivity than the ce-ramic member, the metal can heat up fasterthan the ceramic. In this event, the fillermetal is drawn away by the metal mem-ber. This becomes more important in theapplications where the mass of the metalmember is much smaller than that of theceramic (e.g., the edge brazing). There-fore, the heating rate should be controlledcarefully to minimize the temperature dif-ference between the ceramic and the metal.This can be achieved by initially holdingthe temperature just below the solidus andthen increasing the temperature at a con-trolled rate to the brazing temperature.The cooling rate is also critical for thereliable joints. When the cooling rate istoo fast, there is insufficient time for thefiller metal and/or the metal member todeform plastically and reduce the thermalstress. Therefore, the cooling rate should

§Westinghouse Electric Corp., Pittsburgh, PA.

be controlled to minimize the residualstress. In both the heating and coolingprocesses, the slower the rate of temper-ature change, the better the resultant jointproperties. However, the temperaturechanges should be fast enough to mini-mize the processing cost.

Considering all the factors mentioned,preliminary experiments have been con-ducted to optimize brazing procedures forthe present investigation. The results leadto the temperature-time profile shown inFig. 7. This profile is utilized throughoutthe present investigation unless otherwisementioned.

Testing Methods

There are three major requirements fora testing method to evaluate the mechan-ical properties of the ceramic-metaljoints.First, a testing method should lead to ac-curate and consistent results. Second, atesting method should be able to evaluateeffects of processing variables on the jointproperties. Third, the results of the testsshould provide meaningful engineeringparameters that can be utilized for de-signing the ceramic-metal joints. Thereare many testing methods used for eval-uating the joint properties. Three fre-quently used methods will be reviewed.In addition, a new testing method will beintroduced and examined.

01&I

Copper Tube

II

Si 3N4 C"ami, 410SlaJo'",S'oo'

j

Fig. 6. Example of the stress-distributionjoint. Alloy42 sheets are brazed on bothends of an alumina cylinder with aluminabackup rings: (top) a schematic cross sec-tionof the jointand (bottom)the actual stress-distribution joint.

ined. The detailed descriptions for the peeltest have been given in Ref. 7. The resultsare shown in Fig. 9. A group of six resultsshown on the left side of Fig. 9 is obtainedon the ground-and-resintered surface. Highpeel strengths (=20 lb (90 N)) have beenobtained in all six tests. The results shownon the right side of Fig. 9 have been ob-tained on the ground surface. Again, con-sistent results are obtained. Peel strengthsof the ground surface (=6 lb (27 N)) are

~~~'Hm ~TnT~mnT Hr.T ~O "Tr. '"' ""'0'"' {r?\Af"~_"\

"",ii-ABA fill" m,1aJ

1-- --. ~~-~

Fig. 5. Example of the compression joint.A silicon nitride rod is brazed into a hole ma-chined in a 410 stainless steel with Incusil-

ABA@* (top) a schematic cross section ofthe joint and (bottom) the actual compres-sion joint.

Peel Test

In the peel test, a metal strip is brazedon a ceramic substrate by the active fillermetal. The load required to peel the metalstrip from the ceramic substrate is mea-sured. Therefore, the peel test is relativelysimple and cheap. Examples of the peeltest are shown in Fig. 8, where three Ko-var@§strips have been brazed to aluminasubstrate by Cusil-ABA@ (50-I.LIDthick).Two different surface conditions of thealumina (AL-995) substrate are exam-

.-Alloy42 IIIII!IIIII- IIIIIIIIIIIIIICusil-ABA

I AluminaCylinder

- IIIIIIIIIIIIIICusil-ABA

Alloy 42!IIIIIIIIIIiII III!IIII!iIiII Cusil-ABA

II II Alumina backup ring

considerably lower than those of theground-and-resintered surface. This is dueto surface damages on the ground ceram-ic as described earlier. Figure lO showscross-sectional views of the untested peeltest samples. The interface between theKovar@and the ground-and-resintered ce-ramic is very smooth (Fig. lO(A» andresulted in the high peel strength. Theinterface between the Kovar@ and theground ceramic is not smooth and alsocontains small cracks (Fig. 10(B». As aresult, low peel strength has been ob-tained.

These results demonstrate that the peeltest is a useful method for evaluating jointproperties. It is simple and cheap, and theresults are consistent. Also, effects of theprocessing parameters can be detected.However, one of the disadvantages of thepeel test is that the result of the peel testcannot be translated into meaningful en-gineering parameters for designing of ce-ramic-metal joints.

Tensile TestThe ASTM F19-61 CLM-15 tensile

test]2 is examined for the evaluation ofthe ceramic-metal joint by the activebrazing metal. Figure II shows the testsample assembly, brazed sample, andfractured sample. As shown in Fig. II, aKovar@ring is brazed to two alumina ce-ramic parts with [email protected], twosurface conditions of the ceramic aretested: ground and ground-and-resinteredsurfaces. The results demonstrate that thetensile test can also detect the effect ofthe ceramic surface conditions. The frac-ture strength of the sample with theground-and-resintered ceramic is 75 MPa(10.8 ksi), whereas that of the sample withthe ground ceramic is 49 MPa (7.1 ksi).In the majority of the tensile tests, how-ever, failures have occurred predomi-nantly in the ceramic member instead ofthe joint. This is primarily because theloading mechanism places the ceramic intension. When the joint strength is higherthan the tensile fracture strength of theceramic, the tensile test becomes ineffec-tive for evaluating the joint strength.

25

- 20g-<=

g> 15~1;)

gj 10Q.

5

Ground and resintered Ground

Fig. 9. Results of the peel tests performedon the Cusil-ABA@filler metal. High peelstrengths (20 Ib (90 N)) are obtained on theground-and-resintered alumina. In contrast,the peel strengths for the ground substrateare low (6 Ib (27 N)).

2000

20 min brazing

-Jf- Liquidus (815'C)

Solidus (780'C)

1500

1000 E

500

2 4 6 8

Time (h)

Fig. 7. Temperature cycle for Cusil-ABA@ brazing.

Fig. 8. Sample assembly and brazed samples for the peel test. Alumina substrate, a foil ofan active filler metal (Cusil-ABA@),and three strips of Kovar@are shown at the bottom ofthe figure. At the top-right of the figure is a brazed sample and at the top-left is the samplewhere one Kovar@strip has been peeled off from the substrate.

ShearTest

The shear test provides important in-formation on the mechanical properties ofthe joint, namely shear strength. Figure12 schematically illustrates two common-ly used shear tests. In the test shown onthe left side of Fig. 12, two blocks of thematerials are brazed together and pushedin opposite directions.]) The major prob-lem in this type of shear test is that arotating moment can act on the specimenbecause of the loading mechanism.Therefore, to eliminate the rotation of thesample, a hold-down force (FH) is re-quired. The hold-down force leads to a

compressive stress at the joint, and, hence,the shear stress measured will depend onthe hold-down force. The larger FH, thelarger the shear strength. The fillet at thejoint can also affect the result when thesize of the test blocks are not sufficientlylarge, compared with the fillet size.Therefore, the accurate evaluation of thejoint shear strength cannot be performedin this type of shear test. Another com-mon type of the shear test is shown onthe right side of Fig. 12. In this case, aceramic rod is brazed in a metal ring. Theceramic rod is then pushed and the shearstrength of the joint is determined. How-

CERAMIC BULLETIN, VOL. 68, NO.9, 1989 (@ACerS) 1595

1200

1000

800

S5ill:; 600a;Q;0..E

400

200

0]

0

--

.. - ... -16Gsil~ABBI,-" . I

Fig. 10. Scanning electron micrographs taken at the brazed joints, (A) The interface between the filler metal (Cusil-ABA@)and the ground-and-resintered alumina is very smooth, whereas (B) that between the filler metal and the ground alumina contains small cracks,

ever, because of the thermal expansionmismatch between the ceramic rod andmetal ring, the joint is in compressivestress(as in the case of the compression jointas described previously). The compres-sive stress depends on process variables,e.g., a gap between the ceramic rod andthe metal ring, cooling rate, etc. There-fore, changes in the process variables af-fect both the shear strength of the jointand the compressive stress at the joint.As a result, the effects of the process var-iables on the shear strength cannot beproperly evaluated in this type of test.

Double-Brazed Shear Test

A new type of the shear test, namely adouble-brazed shear (DBS) test, is uti-lized in the present investigation, The ba-sic principle of the DBS test is illustratedschematicallyin Fig. 13.Threeblocksare

Material B

F2

FI-'

Fig, 13. Schematic illustration of the dou-ble-brazed shear test.

Fig, 11, (L-R) Test-specimen assemblies (ceramic-filler metal-metal-filler metal-ceramic andceramic-filler metal-ceramic), a brazed sample, and a tested sample for tensile test. Theassembly shown on the left includes metal-ceramic joints and, hence, is used for the presentinvestigation,

Fs

FH

Fs

Braze

FH

Ceramic Block on Metal Block

Fs

Ceramic rod

Braze

Fs Fs

Ceramic Rod in Metal Ring

F2

Fig, 12, Schematic illustrations of two common types of shear tests on brazed joints,

1596 CERAMIC BULLETIN, VOL. 68, NO.9, 1989 (@ACerS)

- --- -

~~

-."

Fig. 14. (L-R) Test-specimen assembly, a brazed sample, and atested sample for the double-brazed shear test.

.

IFig. 15. Test apparatus for the double-brazed shear test.

butt-brazedtogetherandthe centerblockis pushed against the two end blocks.Therefore, shearstressis applied on twobrazedjoints simultaneously.That is, thepropertiesof two joints can be tested ina single DBS test. It is found, however,that theshearstrengthsof both jointscan-not be determinedby a single test. Thisis because once onejoint (the weaker one)fractures, the loading condition on theother joint becomes undesirable (this pointwill be explained in detail later). Conse-quently, one DBS test provides the fol-lowinginformation:0) it provides the shearstrength of the weaker joint and (ii) it candetermine which joint is weaker. There-fore, when two different materials are usedat the end blocks, one can measure theshear strength of the weaker joint and alsodetermine which material forms thestronger joint with the material used asthe center block. When the same materialis used as the end blocks, the shear strengthof the weakerjoint obtainedby the DBStest provides a safety factor for the designengineer.

Figure 14 shows the test specimen as-sembly, before and after brazing and alsoafter testing. The material for the centerblock is alumina ceramic (AL-995) andthe two end blocks are alloy 42 (Fe-41Ni).All three blocks have the same dimen-sions of 12.4 mm X 12.4 mm x 15.9 mm(0.490 in. X 0.490 in. x 0.625 in.). Cusil-ABA@is used as a filler metal. Brazingis performed at 830°C for 20 min. Afterthe brazing, two opposite surfaces of thebrazed specimen are ground to assure thatthese two surfaces are parallel to eachother for proper loading.

The DBS test apparatus is shown inFig. 15. The brazed sample, also shownin Fig. 15, has been coated by amorphousboron nitride powder to prevent friction.It is then inserted in the test apparatusthrough square holes on two facing side-walls. Therefore, no hold-down load is

~Instron Corp., Canton, MA.

Processing Conditions for Double-Brazed Shear Test SpecimensFiller metal

thickness Cooling rateCeramic Metal (I'm (in.)) (OC/min)

Table II!.

SP'Ccimen

A Ground and resintered*B GroundC Ground and resinteredD Ground and resinteredE Ground and resinteredF Ground and resintered

*1650°Cfor I h. '975°C for I h.

necessary. End covers attached to the wallsprevent fractured fragments from flyingout through the holes. These covers donot touch the specimen, and, hence, noadditional stresses can be applied on thespecimen. After aligning the specimencarefully, a plunger is inserted into themain cavity of the assembly. The bottomof the plunger is flat so that the load isapplied over a whole area of the centerblock. The test assembly with the speci-men is placed in an Instron testing ma-chine.~The plunger is pushed under aconstantcrossheadspeedof 0.25mm/min(0.01 in./min), and the load is recordedby a strip chart recorder.

ProcessVariableEffects

Effects of four major process variableson the joint properties are examined bythe DBS test. They are surface conditionof the ceramic member, strength level ofthe metal member, thickness of the fillermetal, and cooling rate during brazing.Processing conditions are summarized inTable III. For specimen A, the ground-and-resintered ceramic material and an-nealedalloy42 havebeen brazed by a 50-~m-thick foil of the Cusil-ABA@. Thecooling rate is 5°C/min, which is a stan-dard cooling rate as shown in Fig. 7. Thiscondition has been regarded as a basecondition.For each of the remainingfivespecimens, only one of the four variablesis changed and the other three variablesare kept constant so that effects of a sin-gle variable can be determined. For spec-imen B, the surface of the ceramic ma-

Annealed'AnnealedAs-received (3/4 H)AnnealedAnnealedAnnealed

50 (0.002)50 (0.002)50 (0.002)

100 (0.004)150 (0.006)50 (0.002)

555551

terial is in as-ground condition. In thiscase, two shear stress orientations, normaland parallel to the grinding direction, areutilized to investigate effects of shear stressorientation relative to the grinding direc-tion on the shear strength. For specimenC, an as-received alloy 42 is used. Theas-received alloy 42 is in 3/4 H conditionand has a hardness of 75 on the RockwellB scale. In contrast, the annealed alloy42 has a lower hardness of 65 on theRockwell B scale. Specimens D and E arebrazed by thick foils of the Cusil-ABA@,100 ~m and 150 ~m, respectively. A slowcooling rate (1°C/min) is used for braz-ing of specimen F.

In all of the tests, one joint fracturesbefore the other, as expected. Figure 16shows a typical load-displacement curveof the DBS test. (Displacementin Fig. 16indicates the crosshead displacement and,hence, includes displacements of the test

O.OOI-in. displacement'---'4000

-;;; 3000g"0'".3 2000

1000

Fig. 16. Typical load-displacement curveobtained by the double-brazed shear test(specimen D).

1597CERAMIC BULLETIN, VOL. 68, NO.9, 1989 (@ACerS)

IAIoy 42U

tl

Cusil-ABA .',"". l I

Cusil-ABA

IAlloy42r....... L

II

- -- - - - -_J

apparatus and testing machine as well asthe test specimen.) One joint has frac-tured at 1750 Ib (7785 N), and then theother joint has fractured at 3500 Ib (15570N). The load for the second fracture ismuch higher than that for the first frac-ture, because when one brazed joint frac-tures, the specimen is jammed in the maincavity. In most DBS tests, therefore, test-ing is terminated after the first fracture.The shear strength is calculated simplyby dividing the load at the first fractureover a total area of two brazed surfaces.

The results of the DBS tests are sum-

marized in Table IV. In general, the stan-dard deviations are relatively small, in-dicating that the DBS test has producedconsistent results. However, specimen Eshows a large standard deviation. Speci-men E has a thick filler metal (150 /lm).It is observed that molten filler metal has

seeped out from the joining surfaces dur-ing brazing. Therefore, the thickness ofthe filler metal after the brazing is notuniform and varies from one specimen toanother. As a result, the standard devia-tion for specimen E becomes large. It isalso demonstrated in Table IV that theDBS test is sensitive to the process var-iables.

(A),r'~I

relative to the grinding direction are alsoinvestigated. When the shear loading isapplied normal to the grinding direction,all specimens except one break at verylow loads so that the shear strengths cannot be determined accurately. These re-sults indicate that the shear strength nor-mal to the grinding direction is lower thanthat parallel to the grinding direction. Itshould be mentioned that the shearstrength and standard deviation values inTable IV are calculated from the resultsof all of the DBS tests performed on spec-imen B.

StrengthLevelof the MetalThe results for specimen C demon-

strate that the strength level of the metal

joint properties. When the filler metalthickness is increased to 100 /lm (speci-men D) from 50 /lm (specimen A), anincrease in the shear stress' is observed.There are two possible explanations forthese results. First, the thicker filler metalcontains more titanium atoms availableto bond with the ceramic. Second, thethicker filler metal provides more plasticdeformation during the cooling cycle toresult in lower thermal stress at the joint.When the thickness of the filler metal isincreased to 150 /lm, however, the resultsof the DBS tests become more scattered.As described earlier, this is due to the factthat the filler metal has seeped out fromthe joining surface.

Table IV. Results of the Double-Brazed Shear TestsStandarddeviation

MPa psi

2.01.53.61.07.40.8

300220530140

1075111

~...-JI"\:}

I~'

.

'.'rl

I

~ -=- Jlqusil~BA1

- ..,. --,

') ~Usn:-AB~I- ~ "I

Fig. 17. Scanning electron micrographs of the joint interfaces between the filler metal and ceramic member. (A) The interface between thefiller metal and the ground-and-resintered alumina is very smooth, whereas (B) that between the filler metal and the ground alumina containscracks at the interface.

Surface Condition of the Ceramic

Specimen B, with the ground ceramic,has the lowest shear strength (6.2 MPa).This indicates that the surface conditionof the ceramic member is very critical forthe reliable joint. This result agrees withthose obtained in the peel test and tensiletest. The damaged surface layer on theground ceramic leads to the low shearstrength. Figure 17 shows the cross-sec-tional views of the joints for specimens Aand B at a high magnification. SpecimenA shows a defect-free interface (Fig.17(A», whereas specimen B containscracks parallel to the interface (Fig. 17(8)).The effects of the shear stress orientation

member is also important. When the un-annealedalloy 42 (Rb=75) is used, theshear strength of the joint is 9.4 MPa,compared with that obtained in specimenA (20.5 MPa) with the annealed alloy 42(Rb=65). The metal member can deformplastically to reduce the thermal stress.The amount of the plastic deformationdependsonthe strength levelof the metalmember.The lowerthe strength,the morethe amount of plastic deformation. As aresult, the metal member with the lowerstrength level leads to the lower thermalstress at the joint which, in turn, increasesthe joint strength.

Filler-MetalThicknessThe filler-metalthicknessinfluencesthe

30

20coa.6

10

aB C

Specimen

0

Fig. 18. Summary of the OBS tests forspecimens A through O. Bars indicate theaverage shear strength and solid circles in-dicate data points. It is demonstrated thatthe OBS test is sensitive to process varia-bles.

1598 CERAMIC BULLETIN, VOL. 68, NO.9, 1989 (@ACerS)

AverageshearNumber strength

Specimen of tests MPa psi

A 4 20.5 2980B 4 6.2 900C 3 9.4 1360D 2 26.5 3840E 3 19.6 2845F 2 18.9 2740

5000

'iji 4000E,-<:0, 3000cQ)

1;5 2000

mQ)

-<:(j) 1000

0A

Alloy 42

.-

----.

Cooling Rate During Brazing

Finally, the effects of the cooling rateduring brazing are investigated. Twocooling rates, 5°Cjmin and I °Cjmin, areutilized. As demonstrated in Table IV(specimens A and F), there is no detect-able difference in the shear strengths be-tween the two cooling rates.

Experimental Results

Figure 18 shows the results of the DBStests for specimens A through D in agraphical manner. Bars indicate the av-erage shear strengths and solid circles in-dicate data points. As described previ-ously, the effects of the process variableson the shear strength are clearly shown.It should be also mentioned that all of thedata points for each specimen show smallscatters, which indicate consistency in theDBS test.

Figure 19 shows four broken samples(one for each of specimens A through D).In all tests, cracks have initiated at theright side of the samples. This might sug-gest that a small bending moment has beenapplied on the sample and has produceda tensile stress at the right-side surface.Although cracks have initiated at the sameplace, the crack-propagation path variesdepending on the specimen. For speci-mens A and D which show high shearstrengths (see Table IV), the crack ini-tially has propagated at the joint interfaceand then has changed direction by ap-proximately 45° into the ceramic. Forspecimen B, the crack simply propagatedat the joint interface. This is due to thefact that the joint strength is very low (6.2MPa). Specimen C shows that the crackpropagated along the joint, but in the ce-ramic. The high-strength metal in speci-

Fig.19. Broken test samples for specimens Athrough D. Inallcases, the cracks are initiatedat the right side of the sample. However, the crack-propagation paths are shown to bedependent on the conditions of the specimen preparations.

men C does not deform plastically to re-duce the thermal stress during the coolingcycle. As a result, the ceramic in the vi-cinity of the joint might have been dam-aged and provided the path for the crackpropagation.

These results indicate that the DBS testcan be the optimum testing method forthe evaluation of the joint strength be-tween the ceramic and the metal. The DBStest provides consistent results and also issensitive to the process variables. Fur-thermore, the shear strength values whichare obtained by the DBS tests can be useddirectly for the engineering design of ce-ramic-metal joints. However, the resultsof the DBS tests also demonstrate that allprocess variables should be well-definedwhen designing high-reliability joints be-tween ceramics and metals.

Conclusions

(I) High-reliabilityjoining of ceramicto metal requires complete understandingof each and every process variable.

(2) The double-brazed shear test pro-vides a meaningful engineering parame-ter for design purposes.

(3) The double-brazed shear test issensitive to process variables in the braz-ing operation.

(4) Ceramic surface preparation is themost critical parameter to obtain a reli-able ceramic-metal joint.

(5) Use of a metal with a lower yieldstrength results in higher shear strengthat the brazed joint.

(6) Thicker filler metal generally leadsto higher shear strength.

(7) The results of the double-brazedshear tests demonstrate that all processvariables should be clearly defined to op-timize engineering design (e.g., finite-ele-

ment method) of joints between ceramicsand metals.

AcknowledgmentsThe authors are deeply indebted to Mr. J."Rosek

and Mr. R. Spano, both members of WESGO R&D,for their technical support and helpful discussions.

References

'H.Pulfrich, "Ceramic to Metal Seals," U.S. Pat.Nos. 2163407 and 2 163410, June, 1939.

'H. J. Nolte and R. F. Spurch, "Metallizing andCeramic Sealing with Manganese," U.S. Pat. Nos.2667432 and 2667427, Jan. 24, 1954.

JA. J. Chick and L. J. Speck, "Fabrication of Metal-to-Ceramic Seals," U.S. Pat. No.2 708 787, May,1955.

'C. S. Pearsall, "New Brazing Methods for JoiningNon-Metallic Material to Metals," Mater. Methods,30, 61-62 (1942).

'F. C. Kelly, "Metallizing and Bonding Non-Me-tallic Bodies," U.S. Pat. No.2 570 248, Oct. 9, 1955.

'H.Mizuhara, "Process of Making a CompositeBrazing Alloy of Titanium, Copper and Nickel," U.S.Pat. No.3 561 099, Feb. 9, 1971.

'H. Mizuhara and K. Mally, "Ceramic-to-MetalJoining with Active Brazing Metal," Weld. J., 64 [10]27-32 (1985).

'E. Lugscheider, H. Krappits, and H. Mizuhara,"Joining of Non-Metallized Ceramic with Metals byInsert of Ductile Active Brazing"; presented at theSecond International Colloquium, Joining of Ceram-ic, Glass, and Metal. Bad Nauheim, FRG, 1985.

"A. J. Moorhead and H. Keating, "Direct Brazingof Ceramics for Advanced Heavy-Duty Diesels," Weld.J.,65 [10] 17-31 (1986).

"'H. Mizuhara and E. Huebel, "Joining Ceramicto Metal with Ductile Filler Metal," Weld. J., 65 [10]43-51 (1986).

"H. Mizuhara, "Vacuum Brazing Ceramics toMetals,"Adv.Mater. Processes,13 [2]53-55 (1987).

""Tension and Vacuum Testing Metalized CeramicSeals," ASTM Specification F19-64. 1964 AnnualBook of ASTM Standards, Part 8. American Societyfor Testing and Materials, Philadelphia, PA.

1Jw.H. Sutton, "Wetting and Adherence of Ni/NiAlloys to Sapphire," GE Space Sciences Laboratory,Rept.R-64SD44,Philadelphia,PA,1964. .

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