Materials Science & Engineering A 564 (2013) 192–198

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Materials Science & Engineering A

0921-50

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n Corr

E-m

paul.chu

journal homepage: www.elsevier.com/locate/msea

Microstructure evolution and mechanical properties of vacuum-brazedC/C composite with AgCuTi foil

Wei Guo a,b, Ying Zhu a, Lin Wang c, Ping Qu a, Hui Kang a, Paul K. Chu b,n

a School of Mechanical engineering and automation, Beijing University of Aeronautics and Astronautics, Beijing, 100191, Chinab Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong, Chinac School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China

a r t i c l e i n f o

Article history:

Received 10 April 2012

Received in revised form

14 November 2012

Accepted 17 November 2012Available online 23 November 2012

Keywords:

Brazing

Carbon/carbon composite

AgCuTi foil

Microstructure

Fracture

93/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.msea.2012.11.057

esponding author. Tel.: þ852 34427724; fax

ail addresses: gwei@buaa.edu.cn (W. Guo),

@cityu.edu.hk (P.K. Chu).

a b s t r a c t

The microstructure and bonding strength of vacuum-brazed C/C composite and C/C composite with

AgCuTi foil are studied. The interface structure of the brazed joint is C/C composite–TiC–eutectic

structure of AgCu–TiC–C/C composite. The maximum shear strength of the joint is about 20 MPa and

TiC formed at the edge of C/C composite plays a key role in the brazing process. It improves the

wettability of the C/C composite and inhibits diffusion of the Ag and Cu atoms in the filler metal and C

atoms in the C/C composite. The fracture mode of the brazing joint is brittle. The interface evolution in

the brazing process and associated mechanism are discussed.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Carbon/carbon (C/C) composites are attractive materials inthermo-structural applications due to the low density, smallcoefficient of thermal expansion (CTE), high thermal conductivity(better than copper and silver), and especially excellent high-temperature mechanical and thermal shock resistance (highstrength and toughness can be maintained up to more than3000 K). C/C composites have thus been in high temperatureapplications including hypersonic aircraft thermal structures,high thrust–weight ratio turbine engines, rocket nozzles, spaceshuttles, and nuclear reactors [1,2]. However, being an intrinsicbrittle ceramic,it is difficult to fabricate C/C composites intocomponents with a complex shape by conventional methodsthereby hampering wider utilization of the materials. One of thepossible solutions is to employ joining technology.

Several joining techniques have been developed for joining C/Ccomposites, and diffusion bonding, reaction forming [3,4], andbrazing [5–16] appear to be the most attractive methods. Sincediffusion bonding requires a high bonding temperature up to1700 1C, loading pressure, and high-quality matching bondingsurface, the use of this technique tends to be limited. C/Ccomposites can be joined by reaction forming using Ti–Si–C

ll rights reserved.

: þ852 34420542.

at 1823 K, and the transition interlayer is prepared at an evenhigh temperature of 2073–2273 K [3]. Milena Salvo has triedsome possible filler materials, such as Si sheet, aluminum sheet,titanium powder, titanium sheet, Mg2Si powder, SB-glass powderand ZBM-glass powder, to join C/C composites, and the joint withSi sheet filler material brazed at 1693 K for 90 min shows bestshear strength at room temperature. The average shear strengthof the joints is 22 MPa [4]. The high bonding temperature limitsthe use of diffusion bonding and reaction methods. In comparison,vacuum brazing is a more effective process due to its simplicity,low cost, mass production capability, and widespread applica-tions. Active brazing is a conventional method to join ceramicmaterials to metals and applicable to C/C composites [5–16].Brazing of C/C composite to other metals with reactive fillers hasbeen performed by others [7,8,11,12,15]. Ag based filler metalssuch as TiCuSil or Copper ABA have been used because of thefact that a higher strength is obtained with Ag based filler metalsthan Ni- and Ti-based filler metals [17]. Moreover, the brazingmechanism and related interfacial reactions between the carbonsystem (mainly graphite and diamond) and metal has beenproposed by Eustathopoulos et al. [18]. In their work, reactivefillers including Ti or Cr were used to join carbon materials(graphite, diamond and C/C composite). However, C/C compositeis made of carbon fibers with complicated microstructures, andthe joining technology must be further explored. In addition, themicrostructure evolution and strength of the joints in the brazingprocess need more in-depth and systematic studies. Furthermore,

300οC Cooling to room temperaturein the furnace

Maintaining at brazing temperature

Declining to 300°C at 1οC /min rate

Tem

pera

ture

/°C

Time/min

Rising to brazing temperature in 60min

Fig. 2. Brazing temperature versus time curve.

W. Guo et al. / Materials Science & Engineering A 564 (2013) 192–198 193

the metallurgical reaction in the joining interface during thebrazing process is needed further researched and discussed.

The differential CTEs of the joining materials likely createthermal stress during the brazing process and the absence ofthermal stress in the bonding interface reduces the strength of thejoint. Past work tends to focus on the thermal stress in the brazingjoint between the C/C composite and other materials [13]. Severalmeasures such as drilling, wave interface [13], stress reliefinterlayer [11,16], and so on have been taken to accommodatethe thermal stress. However, the thermal stress problem asso-ciated with C/C composite and C/C composite joint has seldombeen investigated. It should be noted that the filler alloy has muchlarger CTE than the C/C composite and the mismatch between theCTE of the C/C composite and filler alloy also creates thermalstress in the joining interface during the brazing cooling process.Hence, fractures across the interface or big holes likely form at theinterface if the filler alloy has poor ductility.

In this study, the microstructures of C/C composite and C/Ccomposite brazed joints with AgCuTi filler are investigated. Theinterface evolution is analyzed and mechanical properties of thejoints are also determined.

Fig. 3. Schematic of the shear test of the joined sample.

2. Experimental details

The semi-3D (three-dimensional) C/C composite was fabri-cated by needled felt and carbon fiber cloth. The C fiber was99.9 wt% pure 12 K acrylic-based activated materials. The three-point flexural strength was about 80–100 MPa, shear strengthwas about 30–40 MPa at room temperature. A density of1.6–1.7 g/cm3 and residual porosity of 10–20% were obtained.The microstructure of the C/C composite is shown in Fig. 1.

A commercial foil of Ag–27Cu–3.5Ti (wt%—nominal composi-tion) with a thickness of about 50 mm was used, and its solidusand liquidus temperatures were about 780 1C and 900 1C. The C/Ccomposite samples were cut into 10 mm�5 mm�5 mm rectan-gular blocks using a wire-cutting machine prior to brazing. Thejoining surfaces were ground by 1000 grit silicon carbide paperand then cleaned ultrasonically with ethanol. The AgCuTi brazingfoil was sandwiched between the C/C composite samples and aspecial jig was used to maintain good contact between the joiningsurfaces. The brazing experiments were carried out in a vacuumfurnace controlled to 71 1C between 850 1C and 940 1C. Brazingwas conducted for 5–60 min at a pressure of less than 5�10�3 Pa

Fig. 1. Microstructure of the C/C composite.

vacuum degree. The brazing temperature–time curve is depictedin Fig. 2.

After brazing, the joints were mounted and polished formicrostructure evaluation. The microstructure was examined ona scanning electron microscope (SEM) equipped with an energydispersive X-ray spectrometer (EDS) and the phase compositionwas determined by X-ray diffraction (XRD). The shear strength ofthe joints was investigated using the GLEEBLE-1500 thermome-chanical simulator machine at room temperature and the fracturecharacteristics were also evaluated. The schematic of the sheartest is shown in Fig. 3.

3. Results and discussion

Fig. 4 presents the microstructures of the joints brazed at 880,900, 910, and 940 1C for 10 min and those of the joints brazed at900 1C for 5, 10, 30, and 60 min are displayed in Fig. 5.

Past results show that the CTE of C/C composites is about 0–1.0�10�6 K�1 in the range of 20–250 1C, and 2.0–4.0�10�6 K�1

from 250 to 2500 1C [19]. However, the CTE of the Ag–Cu–Ti filler(about 18–20�10�6 K�1) is much larger than that of the C/Ccomposite. The mismatch in the CTE between the C/C compositeand filler results in larger thermal stress at the joining interfaceduring the brazing process. However, the small thickness andgood plastic deformation ability of the AgCuTi foil help toaccommodate the thermal stress. According to the macrostruc-ture of the joint, there are no big holes or fractures at the interfaceof the entire joint. Based on the SEM micrographs (Figs. 4 and 5),

C/CC/C

C/C

Infiltration

C/CC/CC/C

C/C C/C

Fig. 4. Microstructures of the joints brazed for 10 min: (a) 880 1C, (b) 900 1C, (c) 910 1C, and (d) 940 1C.

C/CC/CC/C C/C

Infiltration

C/C C/CC/CC/C

Infiltration

Fig. 5. Microstructures of the joints brazed at 900 1C for different time durations: (a) 5 min, (b) 10 min, (c) 30 min, and (d) 60 min.

W. Guo et al. / Materials Science & Engineering A 564 (2013) 192–198194

the interface is joined well and conventional defects such ascracks and holes are not observed. The widths of the brazed jointsvary from 10 mm to 50 mm. The C/C composites possess intrinsicmicro-cracks and pores, and the filler metal infiltrates into the C/Ccomposites along these micro-gaps, as shown in Figs. 4 and 5.

It should be noted that the intrinsic cracks are beneficial to thefatigue properties of C/C composites.

To evaluate the phase composition of the interface, XRD isconducted on the fracture surfaces of joints produced at 880 1C,900 1C, 910 1C and 940 1C for 10 min and the results are given

θ θ

θ

Fig. 6. XRD patterns of the joint: (a) 850 1C for 10 min, (b) 890 1C for 10 min, and (c) 910 1C for 10 min.

Zone I

Gray zone

White zone

Zone II

C/C compositeJoining zone

Fig. 7. High magnification microstructure of the joint.

W. Guo et al. / Materials Science & Engineering A 564 (2013) 192–198 195

in Fig. 6. The interface consists of TiC, Cu(S.S), Ag(S.S), andC phases. The filler metal is composed of Ag, Cu, Ti and C/Ccomposite is made of C, thereby constituting a metallurgicalsystem comprising Ag, Cu, Ti, and C. It is well known that Ag–Cu is a segregating system, immiscible in the solid state, and asingle phase in liquid. The eutectic temperature of the materialswith 28 wt% Cu is about 780 1C. Introduction of Ti to the Ag–Cueutectic system changes the system energetic. It has been shownthat Ti and Cu have negative values of the Gibbs energy of mixingand can form a series of intermetallic compounds such as CuTi2,CuTi, Cu4Ti3, Cu3Ti2, Cu2Ti, Cu4Ti, etc. or CuxTiy in the general termat the brazing temperature [20]. However, only two intermetalliccompounds, AgTi and Ag3Ti (termed AgxTiy) from the Ag–Ti binaryphase diagrams [21] exist in the Ag–Ti binary system at hightemperature. The mutual exclusion between Ag and Ti, Cu and Tiis important. It has been shown that the partial enthalpy of the Tisolution in molten Cu at infinite dilution is �10 kJ/mol, whereasit is 39 kJ/mol in Ag [22]. Hence, the Ag–Ti system has a weakcompound formation tendency compared to the Cu–Ti system.Many intermetallics of CuxTiy come into being during the brazingtemperature (below 940 1C) when the AgCuTi filler is used to joinC/C composites and TC4 [14]. However, XRD does not revealintermetallic phases such as TiCu, Ti3Cu4, Ti2Cu, etc., nor-intermetallic compositions of Ag and Ti in C/C composite andC/C composite joint with AgCuTi.

The Gibbs free energy formation of TiC (TiþC-TiC) is nega-tive. It is from �174 to �169 kJ between 920 and 1050 1C [19],and so formation of Ti and C is thermodynamically possible at thebrazing temperature. On the other hand, the absence of CuxTiy

shows that TiC forms more easily than CuxTiy intermetallicscompounds. Furthermore, CuxTiy compounds produced duringthe brazing process are changed by the following reactions [23]:

CuxTiyþyC-yTiCþxCu

In this metallurgical system, Ti content is small (about 3.5 wt%).Therefore, the reaction between C and Ti occurs adequately due to

diffusion of Ti in the liquid Ag–Cu alloy to the edge of the C/Ccomposite. Most of the Ti is exhausted by C and hence, CuxTiy andAgxTiy cannot be detected by XRD.

To further study metallurgical reaction at the interfacebetween the C/C composite and AgCuTi, a high-magnificationfigure is presented in Fig. 7. A clear layered structure is formed inthe joint and EDS is performed at different regions. In zone I,flakes with a thickness of 0.5 mm form between the AgCuTifiller and C/C composite. The composition is 15.58 wt% Ti,77.77 wt% C, 1.93 wt% Ag, and 4.72 wt% Cu constituting a phaseof TiCþCþCu(S.S). It is noted that TiC phase formation is the keyto the joint formation, which leads to better wettability on theAgCuTi filler [24]. The gray zone in the joint is Cu rich (Cu (S.S))and the white zone is Ag rich (Ag (S.S)), denoted in Fig. 7.

The reaction between Ti and C is not limited to the interfacebetween the AgCuTi and C/C composite and it can be seen that

W. Guo et al. / Materials Science & Engineering A 564 (2013) 192–198196

some fillers infiltrate along the micro-gaps into the C/C compo-site. Similar microstructures [Ti aggregated (TiC Formation)] areproduced in the infiltration filler gaps near the C/C compositeinterface (zone II in Fig. 7). The infiltration increases the strengthof the joint by enlarging the joint area and avoiding stressconcentration.

To evaluate elemental diffusion at the interface of the joint,line scans and elemental maps are obtained by EDS across thejoining zone and the results are depicted in Figs. 8 and 9. The linescans reveal the presence of Ti, Cu, Ag, and C, and Ti aggregates atthe edge of the C/C composite. Similar results are shown by the

C

Cu

Ag

Brazed zone

Ti

C/CC/C

Fig. 8. EDS line scan of the joint.

Fig. 9. EDS elemental maps of the joint: (a) Scann

elemental maps (Fig. 9). Adjacent to the Ti aggregation (TiC)besides the C/C composite (Ti wave crest in Fig. 8), Cu aggregationand Ag aggregation zone (eutectic microstructure of Ag and Cu) isobserved. It is interesting that Cu and Ag are distributed alter-nately in the bonding zone. Another phenomenon observed fromFig. 8 is that Ag and Cu exist in the Ti wave crests but most of C isoutside. Literature shows that if TiC is compact enough, not onlyAg and Cu elements but also small atom C element [25], cannotdiffuse through it due to their lower diffusion ability in TiC.

Based on the data, an interface evolution model is proposedand illustrated in Fig. 10. It comprises the following stages:

(1)

ing

Physical contact with rising temperature: In this stage, the fillerdeforms plastically under the load and makes physical contactas the brazing temperature is increased.

(2)

Diffusion of atom: Here, atom diffusion increases with increas-ing temperature. Ti atom diffuses from the filler metal to C/Ccomposite interface, as shown in Fig. 10a. However, thediffusion rates are small when the temperature is low.

(3)

Further diffusion and interface reaction: Diffusion continuesand increases with the temperature raising. The reactionbetween Ti and C has occurred at the interface of C/C sideand TiC has been produced before the filler melts completely[26], as illustrated in Fig. 10b. In this stage, the reactionproceeds faster as the temperature goes up.

(4)

Growth of reaction layers: The TiC thickness increases withbrazing temperature and holding time. Since TiC layer is thinand discontinuous, C can diffuse through it even at a lowbrazing temperature and short holding time, and react with Tidiffused from filler metal. As TiC layer forms continuously asthe temperature and holding time are raised, the thickness of

position, (b) Cu, (c) Ag, (d) C, and (e) Ti.

T

C

Fig.

W. Guo et al. / Materials Science & Engineering A 564 (2013) 192–198 197

the TiC layer increases, shown in Fig. 10c. Furthermore, TiClayer becomes more compact gradually and so diffusion of Cfrom C/C to filler metal through TiC becomes more difficult.

Joining zone

Ag

Cu

C/C

TiCiC

/C

Ti

TiAg

Ti

Cu

C/CC/C

Cu

Ti

C/C

TiC

C/C

TiC Eutectic structure

(Ag, Cu)

10. Interface evolution of the joint: (a) diffusion, (b) reaction, and (c) freezing.

840 860 880 900 920 940 9600

4

8

12

16

20

24

28

32Holding time: 10min

Temperature/°C

She

ar S

treng

th/M

Pa

Fig. 11. Effects of brazing temperature and holding time on the shear strength

(5)

1

1

2

2

3

She

ar S

teng

th /M

Pa

of th

Freezing of reaction layers and formation of the eutectic structure

of silver, copper: TiC exists in the solid state when the temper-ature is above the melting point of the filler metal due to itshigh melting point. When the temperature begins to drop, thefiller metal begins to freeze and the eutectic structure of silverand copper forms when the brazing temperature dips belowthe freezing point of the filler metal, as shown in Fig. 10c.Finally, the interfacial structure composed of C/C composite –TiC–eutectic structure (AgCu)–TiC–C/C composite forms.

The effects of brazing temperature and holding time on the shearstrength of the joints are shown in Fig. 11, and each data point isobtained by averaging three measurements. When the brazingtemperature is 850 1C, the shear strength of the joint is lower (about15.5 MPa). It is because the atomic energy is smaller at a lowtemperature and the interlayer reaction between the filler alloy andC/C composite is insufficient. As the temperature goes up, the atomenergy increases and diffusion is enhanced leading to faster interfacereactions especially after the filler metal melts. Fig. 11b reveals thatwhen the holding time is 5 min (short), the shear strength of the jointis about 11 MPa. Both element diffusion and interface reactions taketime and 5 min is too short. When the holding time is increased, morediffusion and reactions occur and higher shear strength results. Whenthe brazing temperature is 900 1C and holding time is 10 min, theshear strength reaches the maximum value of about 20 MPa. How-ever, further increase of the brazing temperature and holding timereduces the shear strength due to formation of much more brittleintermetallics in the interlayer. For instance, when the brazingtemperature is raised to 940 1C for 10 min, the shear strength declinesto 12.1 MPa. If the holding time is prolonged to 90 min (900 1C), theshear strength is about 15.4 MPa.

The typical fracture microstructures of the brazed joint areshown in Fig. 12 and a brittle fracture mode is disclosed. Threetypes of fracture morphologies are present in the different zones onthe fracture surface (denoted as A, B, C in Fig. 12a) due to differentdistributions of carbon fibers, namely perpendicular, parallel, andoblique (transition zone) to the fracture surface. Fig. 11b, c, d showsthe magnified microstructures of the three types, correspondingly.Fig. 12b shows the microstructure in zone A and a white interlayerobserved above the circular carbon fibers. Fig. 12c shows thefracture morphology in zone B and Fig. 12d is that of the transitionzone in the joint. It can be observed that the crack propagates alongthe TiC and eutectic structure of Ag Cu.

4. Conclusion

The microstructure and bonding strength of vacuum-brazedC/C composite and C/C composite with AgCuTi foil are experi-mentally studied at 850–940 1C for holding time between 5 and60 min. The maximum shear strength of the joint is about 20 MPa

0 20 40 60 80 1000

5

0

5

0

5

0

Holding Time/ Min

Brazing temperature: 900°C

e brazing joints: (a) temperature effects and (b) holding time effects.

Fig. 12. Fracture morphologies of the brazed joint: (a) Fracture, (b) A, (c) B, and (d) C.

W. Guo et al. / Materials Science & Engineering A 564 (2013) 192–198198

achieved at 900 1C and 10 min. The interface structure is C/Ccomposite–TiC–eutectic structure of AgCu–TiC–C/C composite.TiC forming at the edge of the C/C composite plays a key rolein the brazing process by improving the wettability of the C/Ccomposite and inhibiting diffusion of Ag and Cu atoms in the fillermetal and C in the C/C composite through it. The fracture mode ofthe brazed joint is observed to be brittle. The interface evolutioncomprises of the following 5 stages: (1) physical contact withincreasing temperature, (2) diffusion of atom, (3) further diffusionand interface reaction, (4) growth of the reaction layers, and (5)freezing of reaction layers and formation of the silver–coppereutectic structure.

Acknowledgments

The work was jointly supported by the Fundamental ResearchFunds for the Central Universities and Hong Kong Research GrantsCouncil (RGC) General Research Funds (GRF) no. 112510.

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