Materials and Design 54 (2014) 375–381

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Materials and Design

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Vacuum brazing of sapphire with Inconel 600 using Cu/Ni porouscomposite interlayer for gas pressure sensor application

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.08.046

⇑ Corresponding author. Address: Center of Advanced Manufacturing and Mate-rial Processing, Department of Mechanical Engineering, Faculty of Engineering,University of Malaya, 50603 Kuala Lumpur, Malaysia. Tel.: +60 3 79677633; fax:+60 3 79675330; H/P # +60 162470517.

E-mail addresses: tzaharinie@um.edu.my (T. Zaharinie), raza_moshwan@sis-wa.um.edu.my (R. Moshwan), farazila@um.edu.my (F. Yusof), hamdi@um.edu.my(M. Hamdi), ttariga@keyaki.cc.u-tokai.ac.jp (T. Ariga).

Tuan Zaharinie a,b, Raza Moshwan a,b,⇑, Farazila Yusof a,b, M. Hamdi a,b, Tadashi Ariga c

a Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Centre of Advanced Manufacturing & Material Processing (AMMP Centre), University of Malaya, 50603 Kuala Lumpur, Malaysiac Department of Materials Science, School of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka-shi, Kanagawa 259-1292, Japan

a r t i c l e i n f o

Article history:Received 22 January 2013Accepted 14 August 2013Available online 27 August 2013

Keywords:BrazingSapphireInconel 600Cu/Ni porous compositeGas pressure sensor

a b s t r a c t

In this research, sapphire as a ceramic was brazed to Inconel 600 as a metal with a commercially availableCusil ABA (63Ag–1.75Ti–35.25Cu) filler foil as braze alloy where Cu/Ni porous composite introduced asan interlayer so it could be used in a particular gas pressure sensor application. Several brazing processeswere carried out in a high vacuum furnace in order to investigate the effects of brazing parameters on thejoint interface and mechanical properties. The common brazing temperature and time were in the rangesof 830–900 �C and 15–30 min respectively, while vacuum pressure was remained constant at1 � 10�4 Pa. SEM-EDS and XRD analyses of the joint microstructure and interface composition revealedfive distinct phases; Ni3Ti, AlNi, Cr1.97Ti1.07, Fe0.2Ni4.8Ti5, (TiO1.06)3.32. The brazing area formed two‘‘ocean’’ structures near to Inconel and sapphire interfaces whereas a reaction layer was developed atthe surface of Inconel 600. Under the mechanical property analyses the brazed joint at 900 �C for30 min obtained the maximum shear strength of 58.5 MPa which is adequate for particular gas pressuresensor application.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Joining process for ceramic based product in recent years, be-come popular due to its excellent physical and mechanical proper-ties that allowed them to be used significantly in the automobile,aerospace and electronics fields. The capability to join the ceramicand metal become a noteworthy advantage since both propertiesof material could be utilized in a single product [1]. Artificial sap-phire is a single crystal material that consists of Al2O3 which istransparent and non-colored [2] and it is one of the ceramic thatpossesses a very high hardness of (2300 HV) which increases theability to resist scratch and wear. These attractive properties ofceramic are meeting the requirement with some applications espe-cially in aerospace, bioengineering, optics and electronics indus-trial fields [3]. In particular, the applications of gas pressuresensor nowadays require such properties that can withstand hightemperatures and corrosion in harsh environment of sensingmedium.

Therefore, the development of ceramic pressure sensor is ademanding issue since it is excellent in elastic properties, corrosionresistance and heat resistance [4,5]. Fig. 1 is presenting the sche-matic diagram of packaging technology of sapphire based capaci-tance diaphragm gauge. However, the usage of metal body of thepressure sensor is necessary in this application. To address thischallenge, the selection of Inconel 600 for the metal body of pres-sure sensor is vital since Inconel is a Ni-based alloy that fulfills allthe requirements like as corrosion resistance and high strength inhigh temperature applications [6–12]. Due to the interesting appli-cation of sapphire with Inconel 600 for gas pressure sensor, it isworth to write that joining these materials is essential especiallyby brazing techniques in order to utilize both materials properties[13,14].

It has been postulated that, these materials have been joined bydiffusion brazing techniques [15] and the techniques is preferabledue to its susceptible to corrosion and stable at high temperature.However, it is relatively expensive, time consuming and less effec-tive comparing with vacuum brazing that using active brazing al-loy [16]. It is well reported that, the use of active brazing alloycontaining titanium in vacuum brazing is preferable that tends toreact with ceramic surfaces and changes the chemistry of its sur-face [17].

Hence, the aim of this study is to evaluate the implementationof vacuum brazing technique by investigating the morphologymicrostructure of sapphire with Inconel 600 brazed joint that using

Table 1Summaries of specimens used.

Specimen/material

Function Nominal composition

Sapphire Ceramic base Single crystal Al2O3 with purity 99.999%Inconel 600 Metal base 72Ni–15.5Cr–8Fe (mass%)Cusil ABA alloy Filler metal 63Ag–35.25Cu–1.75Ti (mass%)

Fig. 1. Schematic diagram of packaging technology of sapphire based capacitancediaphragm gauge.

376 T. Zaharinie et al. / Materials and Design 54 (2014) 375–381

interlayer of Cu/Ni porous composite and active filler metal of CusilABA through SEM-EDS analysis. Besides, their relations with thebonding strength of new joining technique were investigated aswell. It is notable that, there are some particular interests to usinginterlayers in brazing ceramics to metal, with most research on theutilization of a copper interlayer recognizing the capability ofresidual stress absorption during brazing [18]. However, the diffu-sion of Cu is high during heating, something that can lead to theformation of micro-voids in the joint after brazing. Micro-voidsmay reduce joint strength due to void propagation within joints[19]. Therefore, the use of a porous Ni interlayer together with por-ous Cu is expected to reduce the residual stress occurring duringcooling form brazing temperature. Using a Ni interlayer is prefera-ble also owing to its satisfactory corrosion resistance performancein high temperature applications [19].

2. Experimental details

In the present work, the sapphire was brazed with Inconel 600using Cu/Ni porous composite as interlayer for gas pressure sensor

Table 2List of properties and the related brazing materials.

Metal Density(103 kg/m3)

Meltingrange (�C)

Specific h(J/kg � K)

Al (99%) 2.71 643–657 904Cu, oxygen free (99.95%) 8.89–8.94 1083 385Ni (pure) 8.91 1453 461Inconel 718 8.22 1335 430Inconel 600a 8.44 1290–1350 410Silver (pure), cold rolled (Hard temper) 10.49 961 234Gold (pure) 19.32 1063 130Titanium (Ti-6Al-4 V) 4.43Alumina 3.96 2045 1013Sapphire –Al2O3

b 3.98 2030Cusil ABAc 9.8 815

Ref: Brazing Handbook (5th ed), American Welding Society (1991), Miami FL. (All in thea http://www.aviationmetals.net/inconel.php.b http://www.redoptronics.com/sapphire-optical-material.html 30/9/2010.c Morgan Technical Ceramics, Wesgo Metals, USA.

application. Sapphire with high purity (99.999%) was used as aceramic material to be brazed with Inconel 600 alloy. The sapphirewas supplied by Azbil-Yamatake Corporation, Japan while theInconel 600 alloy was supplied by Huntington Alloy, USA. Whereas,the 50 lm thickness of Cusil ABA (Supplied by Wesgo Metals, USA)having chemical composition of 63Ag-1.75Ti-35.25Cu was used asan active filler metal. The pure copper (Cu) porous and pure nickel(Ni) porous composite were used as an interlayer between sap-phire and Inconel 600 alloy after it was rolled together to the thick-ness of 400 lm. The summaries of specimens used were presentedin Table 1. A list of the properties of related brazing materials werepresented in Table 2.

The specimens were arranged in form of sandwich configura-tion for microstructural and shear test analysis as can be seen inFig. 2. In order to braze sapphire with inconel 600 and to ensurethe filler infiltrate the Cu/Ni porous composite during brazing,the same size of filler and porous composite was cut to15 mm � 15 mm. The Inconel 600 alloy size of 25 mm � 25 mmused as a based metal. The thicknesses of sapphire with Inconel600 were varied in order to conduct the analysis precisely. Forthe microstructural analysis, quarter size of 20 mm diameter with0.7 mm thickness of sapphire was used to 0.05 mm thickness ofInconel 600 alloy. While for shear test analysis, quarter size of20 mm diameter with 2 mm thickness of sapphire was used to4 mm thickness of Inconel 600.

Brazing was conducted in a high vacuum furnace (Tokyo Vac-uum) with 1 � 10�4 Pa vacuum pressure to avoid any unwantedconstituents from contaminating the sample during experiment.The heating chamber is made of stainless steel and the vacuumpressure was controlled by two pumps; rotary and diffusion pump.The heating were conducted in 830, 865 and 900 �C in 15 and30 min brazing time with the heating and cooling rate were setat 5 �C/min and 3 �C/min, respectively. During cooling, from300 �C and below, the specimen was ensured to furnace cool.

After brazing, the specimen was prepared in cross-section forthe microstructural analysis by using standard metallographictechniques. It was analyzed by using an optical microscope(model: Olympus) followed by Scanning Electron Microscope(SEM – Model: Quanta 2000) and Energy Dispersive Spectros-copy (EDS) that equipped with INCA software. The phases inthe brazed joints were analyzed by X-ray diffraction (Siemens,D5000). On the other hand, shear test was carried out by usingINSTRON Universal Testing Machine with a load cell system(model 3369). A diagram of the shear test clamping system isshown in Fig. 3, and the crosshead speed was set at 0.5 mm/min. The shear test was repeated three (3) times to ensure thereliability of the joined samples.

eat Coefficient of thermal expansion(CTE) (10�6 K�1)

Shear modulus(GPa)

Tensile moduluselasticity (GPa)

23.6 25.9 6917 44.1 11713.4 20714.4 20012.819.7 32 7414.2 31 774.8 42 1146 163 3865.6 148 33518.5

table except superscript 1, 2 and 3.)

Fig. 4. SEM micrograph of bonding and remaining of brazing gap after brazingsapphire with Inconel 600 using Cu/Ni porous composite.

Fig. 3. Diagram of shear test clamping system.

Fig. 2. The specimen arrangement in the form of sandwich configuration formicrostructural and shear test analysis in specific clamp.

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In order to evaluate the relation of hardness in the brazed area,some indentations were introduced at the joined specimen. Theindentation were carried out with a Vickers micro-indentation(Shimadzu, HMV 2T E) using 0.0981 N load with 10 s holding time.The ASTM: E384-11e1 standard was used to measuremicrohardness.

Fig. 5. The bonding at brazing interface of sapphire/brazed area at 900 �C and30 min brazing time.

3. Results and discussions

3.1. Effect of brazing parameters on microstructure of the brazed joint

The SEM micrograph of brazed joint is shown in Fig. 4. Thethickness of brazing area was preserved at 400 lm before and afterbrazing. This behavior has proven that, the gap of brazing could becontrolled using Cu/Ni porous composite. It can be noted that, ifthe material was joined by direct brazing the brazing filler willexperience overflow of silver and reduction of copper during braz-ing. This phenomenon would affect the bonding strength and in-crease the residual stress left in the brazing area. Latter test onthe effect of using silver based brazing filler to the ceramic showsthe significant role of controlling brazing gap thickness of Ag–Cu. Itis important to control the brazing gap thickness of Ag–Cu sincethe thickness of filler could influence the reduction of the thermalexpansion mismatch induced during cooling from brazing temper-ature [20].

According to this theory, the joining of ceramic to metal weredepended on the ratio of the thickness of ceramic to metal dueto the reduction of metal thickness will increase high tensile resid-ual stress on ceramic surface [21]. Xu [21], in his thesis calculatedand proved that the ratio of the ceramic to metal is important toavoid the residual stress arise near to the ceramic interface duringbrazing and suggest that the use of thick metal is more stable com-pared with the thin metal. However in this research, attempts weremade to braze a thin sapphire to a thin Inconel 600 by the variationof temperature ranging from 830–900 �C for brazing time 15–30 min

according to literatures [22,23]. For low brazing temperature(830 �C), the brazing was failed due to high residual stress occurredduring brazing which is similar with the studies conducted by Xu[21] for silicon nitride with metal. However, at high temperature(900 �C) for both holding time, the specimen could preserve andsuccessful joining was obtained. Therefore, further analysis anddiscussion focus on the specimen of 900 �C with 30 min. The cool-ing rate of 3 �C/min was identified to facilitates the reducing of thethermal stress that provided a successful joining. Adopting of Cu/Niporous composite as an interlayer is significant to absorb the resid-ual stress left during cooling. The orientation of Cu/Ni porous com-posite should match such as the copper porous side should beplaced next to the sapphire to ensure the efficient adsorption ofthe deformation. This phenomenon is due to lowering of residualstress during cooling from brazing temperature by the ductilityof the pure copper porous interlayer [24].

The bonding at brazing interface of sapphire/brazed area at900 �C and 30 min brazing time is shown in Fig. 5. While, Fig. 6is presenting the microstructure of thin reaction layer near to thesapphire interface. It is appeared that the reaction layers is absentunder the magnified image. This finding is agreeable to the re-search by Fang [18] on brazing silicon nitride to ferrous using Cuand Ni interlayer which highlighting that the reaction layer is notnecessarily related to the joint strength [18]. However in this re-search, there is a thin reaction layer observed after shear test that

Fig. 6. Microstructure of thin reaction layer near to the sapphire surface.

Fig. 7. XRD analysis of element present on the sapphire surface after shear test.

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close to the sapphire with the microstructure of the reaction layercould be observed in Fig. 6. These thin reaction layers appear inblack color attached to the sapphire surface in most area. This sit-uation is in conjunction with a recent literature that identified thethin black layer is TiOx together with aluminum, but with absenceof copper that occurred due to the reaction between Ti from activebrazing filler [25]. From Fig. 6, it could be observed that the micro-structure consists of small spherical due to the structure from por-ous interlayer.

Fig. 8. Mapping of eleme

Further investigation was conducted by an XRD analysis on thedetached sapphire to identify the element present on the sapphiresurface after shear test (Fig. 7). It could be seen that copper ele-ment was absent and five phases were identified due to the reac-tion from the active element of titanium from Cusil ABA brazingfiller with Inconel 600 and sapphire. These existing phases weredetected as nickel titanium (Ni3Ti), aluminum nickel (AlNi), chro-mium titanium (Cr1.97Ti1.07), iron nickel titanium (Fe0.2Ni4.8Ti5)and titanium oxide (TiO1.06)3.32. This data is in agreement with Val-ette [25] that the only elements exist on the surface for the bon-dings are Ti, Ni, Al, O, Cr and Fe. Also, purposes of the analysis(XRD) on the surface are identification of the layer which relatesto the shear and identification of the compound layer formed inthe sapphire surface. It was confirmed that compound layer hadbeen formed in the sapphire surface from result of analysis. Thesechemical compounds are formed by the reaction of Ti with the sap-phire surface. In this fact, it is shown that the joining was formed,as the brazing filler metal got wet on the sapphire surface. Poten-tial reactions in brazing can be written as follows.

Al2O3 þ 3Ti � 3TiOþ 2Al ð1Þ

3Niþ Ti � Ni3Ti ð2Þ

Alþ Ni � AlNi ð3Þ

Fig. 8 shows the mapping of elements by SEM analysis where Ti ele-ment was greatly influence the bonding since the element is segre-gated closely to the both interfaces of sapphire and Inconel 600. Itcould be observed that the distribution of Ti formed an ‘‘ocean’’structure to the interface. Meanwhile in mapping of Ti element, athin reaction layer was formed near to the interface of brazedarea/Inconel 600. However, the reaction layer of brazed area/sap-phire interface in this analysis could not be observed clearly. More-over, the Ni element shows almost same ocean structure. In thiscase, copper is absent in the same behavior and Cu mostly segre-gated in copper island together with Cu/Ni porous composite;mostly in the middle of the brazed area. The influence of silver torelease titanium could be seen by the microstructure as shown inFig. 8 [20]. From the mapping elements in brazing of sapphire withInconel 600 using Cu/Ni porous composites, the titanium, nickel and

nts by SEM analysis.

Fig. 9. Variation of hardness in the brazing of sapphire with Inconel 600.

Fig. 10. Load-extension curve obtain during shear test of sapphire with Inconel 600alloy using Cu/Ni porous composite.

T. Zaharinie et al. / Materials and Design 54 (2014) 375–381 379

silver elements tends to form an ‘‘ocean’’ structure near to the inter-face of sapphire with Inconel 600 rather than forming a reactionlayer near to the sapphire surface. This phenomenon is in agree-ment with recent literature that the reaction between Ni and mol-ten CuTi lead to a strong Ni–Ti interaction due to decreasing of Tithermodynamics activity [25]. However, it is noted that, the micro-structure of Cu and Ni porous composite were remained in porousproperties with pore sizes ranging from 100 to 200 lm.

Fig. 11. Shear strength of brazing sapphire with Inconel 600 for three (3) brazingtemperatures and two (2) brazing times.

3.2. Effect of brazing parameters on mechanical properties of thebrazed joint

3.2.1. Hardness analysisIn order to investigate the hardness variation in the joining, the

hardness were measured from the middle of Inconel 600 (0.5 mmthickness to the brazed area/sapphire interface). Fig. 9 is present-ing the variation of hardness in the brazing of sapphire with Inco-nel 600. The hardness on the border of Inconel 600/brazed areashows a reduction compared to the hardness on Inconel 600 andsmall fluctuation of hardness was identified in the ‘‘ocean’’ struc-ture near to the Inconel 600 interface. Besides, it is observed thatthe hardness in between the two ‘‘ocean’’ structure (near to Inconel600 and sapphire interface) is consistent in the range of 69–76 HVshowing that the area is not influenced by any residual stress[26,27]. However, in the area of ‘‘ocean’’ structure near to the sap-phire interface, a very high of hardness of 141 HV suddenly wasidentified at a distance of 399.18 lm (from Inconel 600 side). Thisphenomenon is influenced by micro residual stress left due to thecoefficient of thermal expansion (CTE) between sapphire with

Inconel 600. Due to the relatively low coefficient of thermal expan-sion of sapphire compared to Inconel 600, the residual stresses wasidentified to be in tensile. This finding was proven by analyticaland experimental studies by several researchers on ceramic to me-tal joining [28–30].

3.2.2. Strength analysisFig. 10 shows load–extension curve obtained during shear test

of brazed joint. From Fig. 10, the curve for brazing sapphire withInconel 600 shows almost same pattern which involved of two ormore peaks before failure. Due to the weakest area of the sapphirenear to the brazed area [20], the crack initiate in the sapphire areabefore propagated in the bonding between sapphire/brazed area.During propagation of crack in the bonding, the fracture behaviorbegin to show a brittle failure until it reach the first peak. The brit-tle failure was identified by the influence of strong Ni–Ti interac-tion area. The strong Ni–Ti interaction during joining hasmodified the thermodynamics activity of Ti and hence, changethe composition of the reaction layer produced by titanium byreducing the thickness of the reaction layer [25]. However, thecracked again start to initiate in the sapphire region due to theinfluence by some other phases such as TiO and Ni3Al that rela-tively stable and ductile intermetallic compound [31–33]. Thecrack was propogated in the sapphire area until it break at secondpeak by ductile behavior. In this case, the second peak is known asa qualify bonding due to the mix-mode of adhesive and cohesivefailure occurred in the bonding between sapphire/brazed area byconsidering that, the bonding is not fully broken at the first peak.The value of the maximum load for the bonding was identified at529.91 N which is quite enough for the gas pressure sensor appli-cation. After shear test, it was identified that the shear strength forthe bonding is 58.5 MPa which is acceptable for particular applica-tion due the real stress occurred in gas pressure sensor applicationmostly in perpendicular direction. In this research, the value obtainis comparable with alumina to Inconel 600 joining using Au–Nibrazing alloy developed for neutron sensor application [26].

Fig. 11 shows the shear strength of brazing sapphire with Inco-nel 600 for three different brazing temperatures and two differentbrazing times. From this figure, it can be seen that a gradualincreasing of shear strength occurred by increasing brazing tem-peratures and brazing times. The bonding strength varies in theranges of 33.43–61.78 MPa. The graphical shear strength plot indi-cated that the brazing temperature was greatly influence the bond-ing strength while brazing time was not significantly affected theperformance of bonding strength. The values obtained were com-parable with However, for particular application of sapphire basedgas pressure sensor, the sufficient bonding strength is achieved.

Fig. 12. The thickness of ocean structure near to sapphire with Inconel 600 interfaces at different brazing parameters.

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3.2.3. Thickness analysisThe thickness of ‘‘ocean’’ structure was measured and plotted in

order to investigate the relationship with the shear strength.Fig. 12 is presenting the thickness of ocean structure near to sap-phire with Inconel 600 interfaces at different brazing parameters.As could be seen in Fig. 12, the thickness of ocean structure will in-crease when the temperature and time increased. The bondingstrength behavior is proportional with the thickness of oceanstructure, particularly for the ocean structure that near to the sap-phire interfaces. On the other words, this behavior is linear withthe shear strength. However, the thickening process of ocean struc-ture near to the sapphire side is faster than the ocean structurenear to the interface of Inconel 600. According to the analysis bySEM–EDS, the ocean structure would lead to formation of brittlearea and harmful to the bonding strength due to the high interac-tion of Ni and Ti. These could be seen from the hardness data that amicro residual stress occurred in this area. In this respect, thethickening of ocean structure is important to give time for thestress to relax. Therefore, the narrow thickness of ocean structurenear to sapphire side is unpreferable since it would lead to theincreasing of the micro residual stress. However, with increasingbrazing time and temperature, the thickening of the ocean struc-ture attribute time to the relaxation of micro residual stress espe-cially for the ocean structure near to sapphire interface. Thisphenomenon lead to the gradual increasing of shear strength athigh temperature of 900 �C with 30 min brazing time comparingto 830 �C and 15 min brazing time.

3.3. Residual stresses

For residual stress calculation here an attempt has been takenby considering thermal residual stresses. As mentioned earlier,the bonding strength behavior is proportional with the thicknessof ocean structure, particularly for the ocean structure that is nearto the sapphire interfaces. Thus, sapphire could be considered asthe main determinant for residual stresses that is developed inbrazing. The residual stresses which is associated with sapphirecould be calculated as follows by considering thermal residualstresses.

As it is well known that, thermal residual stresses are the basisfor the evolution of residual stresses in addition to volume changesof phase in the filler alloy layer due to phase transformation as wellas the relaxation of stresses due to creep. The first form as a resultof the different coefficients of thermal expansion (CTE) of the Inco-nel 600/filler alloy ðainconel600 ¼ 12:8� 10�6 K�1Þ and the sapphireðasapphire ¼ 5:6� 10�6 K�1Þ The thermal residual stresses can be

determined from the thermal strain eth, which can be calculatedaccording to following equation:

eth;sapphire ¼ Da � DT ¼ ðasapphire � ainconel600Þ � DT ð4Þ

where Da ¼ ðasapphire � ainconel600Þ is the mismatch in thermal expan-sion coefficients of sapphire (asapphire) and of the Inconel 600 (ainco-

nel600) and DT is the difference between brazing temperature androom temperature.

Considering room temperature 25 �C, at brazing temperature830 �C, the value of thermal strain,

eth;sapphire ð830�CÞ ¼ �0:0058 ð5Þ

For brazing temperature at 900 �C, the value of thermal strain,

eth;sapphire ð900�CÞ ¼ �0:0063 ð6Þ

Using young modulus of Esapphire = 335GPa and considering onlyelastic deformation, the thermal residual stress in the sapphirewould be approximately at 830 �C,

rth;sapphire ð830�CÞ ¼ Esapphire � eth;sapphire ð830

�CÞ

¼ �1:943GPa ð7Þ

At 900 �C,

rth;sapphire ð900�CÞ ¼ Esapphire � eth;sapphire ð900

�CÞ

¼ �2:111GPa ð8Þ

From Eqs. (3) and (4) it can be seen that the value of thermal resid-ual stress rth;sapphire ð830 �CÞ > rth;sapphire ð900 �CÞ. Since the thermalresidual stresses are the basis of residual stresses in addition to vol-ume changes of phase in the filler alloy layer due to phase transfor-mation as well as the relaxation of stresses due to creep thus, it canbe concluded that residual stress after brazing at 830 �C was higherthan that brazing at 900 �C for this particular research.

4. Conclusions

Sapphire with Inconel 600 alloy with different thickness wasbrazed using Cu/Ni porous composites at 900 �C for 30 min. Themicrostructural analysis shows that there is a reaction layer nearto the Inconel 600/brazed area interface. However, the reactionlayer near to the sapphire side was difficult to observe under themagnified image. This is due to the reduction of the thickness ofreaction layer by strong Ni–Ti interaction. Mechanical propertyanalysis concludes that hardness evaluation shows some variationin the ‘‘ocean’’ structure near to the sapphire surface that influ-enced by some amount of residual stress in the brazing area. Thethickening of ‘‘ocean’’ structure specifically near to sapphire is

T. Zaharinie et al. / Materials and Design 54 (2014) 375–381 381

preferable due to the increasing of time and temperature that en-ough to alleviate the residual stresses. On the other hand, the aver-age brazing joint bonding strength of 58.5 MPa was obtained byshear test and it is adequate for gas pressure sensor application.

Acknowledgements

The authors greatly acknowledge the members of Azbil-Yama-take Corporation for supplying the Inconel 600 alloy and sapphirefor investigation in this research and the University of Malaya forproviding the necessary facilities and resources for this research.This research is supported by High Impact Research Grant (UM/MOHEUM.C/625/1/HIR/MOHE/H16001-D000001) from the Minis-try of Higher Education Malaysia.

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