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Active metal brazing of SiO2–BN ceramic and Ti plate with Ag–Cu–Ti + BN composite filler
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Active metal brazing of SiO2 –BN ceramic and Ti plate with Ag–Cu–Ti + BN composite filler Z.W. Yang a,∗ , C.L. Wang a , Y. Wang a , L.X. Zhang b,∗ , D.P. Wang a , J.C. Feng b a b
Tianjin Key Lab of Advanced Joining Technology, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 14 November 2016 Received in revised form 2 February 2017 Accepted 20 February 2017 Available online xxx Keywords: SiO2 –BN ceramic BN particles Microstructure Mechanical properties
a b s t r a c t SiO2 –BN ceramic and Ti plate were joined by active brazing in vacuum with Ag–Cu–Ti + BN composite filler. The effect of BN content, brazing temperature and time on the microstructure and mechanical properties of the brazed joints was investigated. The results showed that a continuous TiN–TiB2 reaction layer formed adjacent to the SiO2 –BN ceramic, whose thickness played a key role in the bonding properties. Four Ti–Cu compound layers, Ti2 Cu, Ti3 Cu4 , TiCu2 and TiCu4 , were observed to border Ti substrate due to the strong affinity of Ti and Cu compared with Ag. The central part of the joint was composed of Ag matrix, over which some fine-grains distributed. The added BN particles reacted with Ti in the liquid filler to form fine TiB whiskers and TiN particles with low coefficients of thermal expansion (CTE), leading to the reduction of detrimental residual stress in the joint, and thus improving the joint strength. The maximum shear strength of 31 MPa was obtained when 3 wt% BN was added in the composite filler, which was 158% higher than that brazed with single Ag–Cu–Ti filler metal. The morphology and thickness of the reaction layer adjacent to the parent materials changed correspondingly with the increase of BN content, brazing temperature and holding time. Based on the correlation between the microstructural evolution and brazing parameters, the bonding mechanism of SiO2 –BN and Ti was discussed. © 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction Fused silica based ceramics exhibit an excellent thermal stability and have increasing use in a wide range of engineering applications. It has also been shown that particle reinforcement of SiO2 ceramics by hexagonal BN (h-BN) particles can lead to a cost effective improvement of thermal shock resistance, ablation resistance, and mechanical and dielectric properties [1–3]. An important engineering application of such a particle-reinforced SiO2 material (SiO2 –BN) is in radar window, where ceramics are used for wave transparency. The assembly of radar window requires joining a SiO2 –BN ceramic window to a metallic holder. Unfortunately, the traditional mechanical fastening causes an additional weight and a high cost. It also limits the vehicle speed because of poor heat resistance of the bonded joint. Thus, active metal brazing is used to join the SiO2 –BN ceramic to a titanium holder in this study.
∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (Z.W. Yang), [email protected] (L.X. Zhang).
Active metal brazing is a well-established method for reliable joining of ceramics to metals [4,5]. Compared to mechanical attachment and adhesives that are suitable for relatively low-temperature applications, this technique is appropriate for high-temperature applications, where strength, air tightness and corrosion resistance are required [6,7]. For active metal brazing, the metallurgical bonding is the result of a chemical reaction between the reactive element in the brazing alloy and the ceramic surfaces. Therefore, the type and concentration of the added active element in the brazing alloy are crucial to the joint strength. Near eutectic Ag–Cu alloy with a few percent of Ti as the active element is the most frequently considered active brazing alloy for the strong bonding of ceramic-metal components. Moreover, previous results have shown that Ti derived from Ag–Cu–Ti system and other systems are characterized as having sufficient thermodynamic driving force to react with SiO2 [8–10] and BN [11–14]. Hence, it can be inferred that the SiO2 –BN ceramic consisting of fused silica and hBN, might be successfully brazed with this kind of Ti-activated filler metal. Recently, commercially available Ag–Cu–Ti brazing alloy has been successfully applied to braze the SiO2 –BN ceramic, and a continuous TiN–TiB2 reaction layer forms at the ceramic surface to
http://dx.doi.org/10.1016/j.jmst.2017.04.005 1005-0302/© 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
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Fig. 1. Morphology of the as-milled Ag–Cu–Ti + 3 wt% BN composite filler and schematic diagram of sample assembly: (a) morphology of composite filler; (b) assembly of brazing sample.
achieve a metallurgical bonding between the ceramic substrate and the brazing alloy [15,16]. But, the significant difference in the thermal expansion coefficients (CTE) between ceramics and metals or brazing alloys can lead to high residual stresses in the joint, which results in reduced strength of the joined components [17,18]. Consequently, it can speculate that the residual stress yield during the cooling process as a result of the CTE mismatch between SiO2 –BN ceramic ((0.5–1.7) × 10−6 K−1 ) and Ti (9.0 × 10−6 K−1 ) or Ag–Cu–Ti brazing alloy (18.2 × 10−6 K−1 ) is large and should be taken seriously. This problem could be alleviated by the addition of low CTE materials (e.g. fibers or particles) into brazing alloys [19–23]. The incidental issue is that the reinforcements are difficult to be distributed uniformly in the matrix due to their bad wetting ability with the liquid brazing alloy. It was reported that the in situ synthesized reinforcements originated from brazing process itself have greater reinforcing effects because of their fine size, uniform distribution, and favorable cohesion with matrix [24–28]. The addition of carbon fibers, SiC, Si3 N4 , TiN, TiC and B powders into the brazing alloy to fabricate composite fillers has been demonstrated to improve the joint strength remarkably. In this work, low CTE reinforcements were in situ synthesized by the addition of h-BN particles in the Ag–Cu–Ti brazing alloy. The effect of BN content, brazing temperature and time on the microstructure and mechanical properties of the brazed SiO2 -BN/Ti joints was investigated. The reinforcing effect caused by the BN additive as well as the brazing mechanism and microstructural evolution of the brazed joint was discussed. 2. Experimental procedures Pure Ti plate with a thickness of 3 mm and SiO2 –BN ceramic fabricated by hot pressing of SiO2 and BN powders were used as parent materials in the brazing experiments. The size of SiO2 –BN ceramic and Ti samples for brazing was 5 mm × 3 mm × 3 mm and 25 mm × 8 mm × 3 mm, respectively. Surfaces to be brazed were firstly ground and then ultrasonically cleaned in acetone for 10 min before brazing. The composite filler used to braze SiO2 –BN and Ti was fabricated by mechanical milling of commercial available Ag–27.5Cu–2.5Ti (wt%) brazing alloy powder and h-BN powder. The weight percentage of h-BN powder in the composite filler was designed as 0%, 1.5%, 3%, and 4.5%, respectively. The average particle diameter of the added h-BN powder was 0.5 m. The initial powders were weighted in the scheduled composition and then the powder mixture was milled for 2 h in an argon atmosphere
using a QM-SB planetary ball mill. The morphology of the as-milled composite filler is shown in Fig. 1(a), from which we can see that fine h-BN powders were homogeneously adhered on the surfaces of Ag–Cu–Ti powders. The as-milled composite filler was mixed with a small amount of cellulose nitrate and octylacetate to make into paste. The brazing paste was sandwiched between the SiO2 –BN ceramic and the Ti sample by coating on the bonded surface of SiO2 –BN ceramic (200–300 m). The joint assembly is schematically illustrated in Fig. 1(b). The assembly was placed in a vacuum furnace and a fixed load of 0.2 kg was kept over the sample to enable a proper contact in the bonding surfaces during brazing. The furnace was heated at a rate of 30 ◦ C/min till it reached 700 ◦ C and kept for 10 min, and then the temperature continued to increase to 850–910 ◦ C at a rate of 10 ◦ C/min. Subsequently, the brazing specimens were held for 5–35 min at the brazing temperature and then cooled down to 400 ◦ C at a rate of 5 ◦ C/min. Finally, the furnace was cooled down spontaneously to room temperature in the furnace. During the brazing process, the vacuum level of the furnace was kept at 3.0 × 10−3 Pa. The cross-sections of the brazed joints were prepared for metallographic examination. The joint microstructure observations as well as quantitative elemental analysis were performed using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). The reaction phases formed in the brazed joints were characterized by X-ray diffraction (XRD) spectrometer with Cu-K˛ radiation. Layer-stripping of the brazed joint by coarse grinding was applied to determine the location of XRD analysis. To evaluate the bonding properties of the brazed joint, shear tests were carried out at a constant displacement rate of 0.5 mm/min using a universal testing machine (Instron1186). At least three samples were tested for each experimental condition. The fracture surface of the brazed joint after shear test was observed with a digital microscope (VHX-1000E). 3. Results and discussion 3.1. Effect of BN content on the microstructure and shear strength of brazed joints Fig. 2 shows the effect of BN content in the composite filler on the interfacial microstructure of Ti/SiO2 –BN joint brazed at 870 ◦ C for 15 min. The EDS analysis of each phase formed in the joint is listed in Table 1. It is noted that Cu in the composite filler had a
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Fig. 2. Microstructure of Ti/SiO2 –BN joints brazed with Ag–Cu–Ti + BN composite filler with different BN content at 870 ◦ C for 15 min: (a) 0; (b) 1.5%; (c) 3%; (d) 4.5%; (e) magnification of brazing seam in (b); (f) magnification of brazing seam in (c).
Table 1 Average chemical compositions of each reactant (at.%) in Fig. 2.
A B C D E F G H
Ti
Cu
Ag
N
B
Si
O
Possible phase
89.90 60.78 42.83 35.41 20.16 1.00 18.04 40.26
8.49 36.50 54.49 63.29 77.08 10.93 80.87 4.38
1.61 2.72 2.51 1.30 1.70 86.31 1.09 1.19
– – – – – – – 20.12
– – – – – – – 24.42
– – 1.07 – 1.06 1.76 – 6.49
– – – – – – – 4.04
␣- Ti Ti2 Cu Ti3 Cu4 TiCu2 TiCu4 Ag(s.s) Cu(s.s) TiN + TiB2
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Fig. 3. XRD analysis of Ti/SiO2 –BN joint brazed with Ag–Cu–Ti + 3 wt% BN composite filler at 870 ◦ C for 15 min: (a) brazing seam; (b) reaction layer adjacent to SiO2 –BN ceramic.
strong tendency to react with Ti substrate, and a series of Ti–Cu compound layers formed adjacent to the Ti substrate. From Ti side to the brazing seam, ␣- phase aggregate and Ti-Cu compound layers including Ti2 Cu, Ti3 Cu4 , TiCu2 and TiCu4 layer, were observed in area I and area II, respectively. The vigorous interactions occurred at the solid/liquid interface during brazing, involving Ti dissolution and atomic diffusion, which were the primary reason for the formation of these characteristic microstructures. The formation of ␣- phase aggregate in area I was ascribed to the diffusion of Cu in Ti substrate. Cu is a strong -Ti stabilizing element and can lower the eutectoid transformation temperature of Ti [29]. Thus, the bright -Ti needles and dark ␣-Ti matrix formed by the decomposition of -Ti, as seen in area I. Meanwhile, the dissolved Ti had a stronger affinity towards Cu than to Ag, because the partial enthalpy of Ti solution in the molten Cu at infinite dilution (–10 kJ/mol) was lower than that in Ag (39 kJ/mol) [30]. As a consequence, a series of Ti–Cu compound layers, with the atomic percentage of Ti/Cu decreased from area I to brazing seam, formed in area II. With the increase of BN content, the thickness of area I and II decreased obviously, as observed in Fig. 2(a–d). Moreover, instead of Ag- and Cu-based solid solution, a large number of fine-grains appeared in the brazing seam when the BN particles were added, resulting in the homogeneous of joint microstructure. The magnification of these black fine-grains is shown in Fig. 2(e) and (f), corresponding to 1.5% and 3 wt% BN addition, respectively. To characterize the phase structure of these fine-grains, the XRD analysis of this zone obtained by layer stripping is given in Fig. 3(a). The results confirmed that TiN, TiB, and three kinds of Ti–Cu phases formed in the brazing seam besides Ag(s.s). It is reasonable that TiN and TiB were formed by the reaction between the active Ti and the BN additive. The calculated Gibbs free energy (GT0 ) of 2Ti + BN → TiB + TiN at the brazing temperature of 870 ◦ C is −236.7 kJ/mol [15], indicating that the formation of TiN and TiB is thermodynamic feasible. The in situ synthesized fine-grains, TiB whisker and TiN particle might act as the nucleation sites for the formation of Ti–Cu compounds, as suggested in Ref. [26,31,32]. Consequently, the presence of TiCu4 , Ti3 Cu4 and TiCu2 were indexed in the XRD pattern. It is noteworthy that the addition of BN particles was beneficial to refining the microstructure of brazing seam. However, the added BN particles were not the more the better. The accumulation of BN was observed in the joint when the BN content reached 4.5 wt%, as seen in Fig. 2(d). On the other hand, partially Ti in the liquid filler was consumed to react with BN additive, in addition to react with Cu to form fine Ti–Cu compounds in the brazing seam. Hence, Ti–Cu compound layers bordering Ti substrate as well as the reaction layer adjacent to the SiO2 –BN ceramic became thin with the increase of BN content. The XRD analysis of the reac-
tion layer in Fig. 3(b) confirmed that TiN, TiB2 , TiCu4 , Ti3 Cu4 and TiCu2 phase formed in this zone. The type of Ti–Cu phases was the same as that found in the brazing seam. Nevertheless, different from the reaction phases of TiN and TiB produced in the brazing seam, TiN and TiB2 formed adjacent to the SiO2 –BN ceramic by the chemical reaction of 3Ti + 2BN → TiB2 + 2TiN. The GT0 for TiN + TiB2 formation at 870 ◦ C was −421.4 kJ/mol [15], which was lower than that of TiN + TiB formation. Compared with TiB phase, theoretically, TiB2 preferred to form in the same brazing condition. However, Ti and TiB2 could further react to form TiB owing to the small negativity of the free energy of Ti + TiB2 → 2TiB. In addition, the reaction formation enthalpy of Ti + TiB2 → 2TiB was also negative [33], indicating that the reaction was exothermic. In terms of Le Chatelier’s principle, if the active Ti in the liquid filler was excessive, the above reaction would be shifted to the right. Consequently, TiB phase could form in the brazing seam as a result of the successive dissolution of Ti. But, for the reaction between Ti and h-BN in the SiO2 –BN ceramic, the diffusion of Ti atoms across the already formed reaction layer was limited, and thus TiN + TiB2 reaction layer was observed adjacent to the SiO2 –BN ceramic. In conclusion, the microstructure of Ti/SiO2 –BN joint changed obviously due to the addition of BN particles. A large number of fine-grains consisting of TiB, TiN and Ti–Cu compounds formed in the brazing seam, which was beneficial to refining the joint microstructure. Meanwhile, with the increase of BN content, the thickness of Ti–Cu compound layers bordering Ti substrate as well as TiN + TiB2 reaction layer adjacent to SiO2 –BN ceramic decreased correspondingly. The variation of interfacial microstructure played a decisive role in determining the mechanical properties of the brazed joints. Fig. 4 shows the effect of BN content in the composite filler on the shear strength of SiO2 –BN/Ti joint brazed at 870 ◦ C for 15 min. It can be seen that the average shear strength of the brazed joint firstly increased and then decreased with the increase of BN content from 0 to 4.5 wt%. The maximum strength reached 31 MPa for the joint brazed with 3.0 wt% BN additive, which was 158% higher than that of joint brazed with single Ag–Cu–Ti filler. The improvement of joint strength caused by the addition of BN was prominent. Moreover, the fracture mode of the brazed joints after shear test changed correspondingly. For the joint brazed with single Ag–Cu–Ti filler, crack propagated in the SiO2 –BN substrate nearby the ceramic/filler interface. This is because a large residual stress yielded in this area due to the CTE mismatch between the parent materials and the brazing seam, forming a weak zone in the joint. The formation of such a large residual stress has been demonstrated by the finite element calculation of a ceramic–metal brazed joint [34]. The brazed joint would be failed under a small shear force owing to
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Fig. 4. Effect of BN content on the shear strength and fracture mode of Ti/SiO2 –BN brazed joints.
the presence of large residual stress, and thus the shear strength of the brazed joint was low. However, a different fracture mode was observed for the joint brazed with composite filler containing 3.0 wt% BN. Crack initiated in the interfacial reaction layer, and then propagated in the ceramic substrate. The crack deflection during shear test would absorb more shear force, and thus a higher joint strength was obtained. The improvement of joint strength was primarily ascribed to the change of microstructure in the brazing seam as well as the alleviation of residual stresses. As analyzed above, a fine particle-reinforced brazing seam was formed as a result of BN additive, which is beneficial to the bonding properties. Moreover, the calculated CTE of the brazing seam was decreased from 19 × 10−6 to 15 × 10−6 K−1 when 3.0 wt% BN particles were added in the composite filler, owing to the formation of TiB whiskers and TiN particles with low CTE values in the brazing seam. Therefore, the CTE mismatch between SiO2 –BN ceramic and brazing seam was decreased, leading to the reduction in detrimental residual stresses generated in the ceramic substrate. Though the CTE value of the brazing seam was further decreased with the increase of BN content in the composite filler, the BN content was not the more the better. When the composite filler with a higher BN content was used to braze Ti and SiO2 –BN, the reaction layer was too thin to bear a large shear force, and cracks were primarily propagated along this reaction layer and partially in the ceramic substrate. Under this circumstance, the joint strength was dropped. 3.2. Effect of brazing temperature on the microstructure of brazed joints Fig. 5 shows the effect of brazing temperature on the interfacial microstructure of Ti/SiO2 –BN joints brazed with Ag–Cu–Ti + 3 wt% BN composite filler for 15 min. This effect was investigated by
Table 2 Average chemical compositions of each reactant (at.%) in Fig. 5.
A B C D E
Ti
Cu
Ag
B
N
Possible phase
43.40 32.57 18.04 2.71 27.72
52.75 65.57 80.87 14.20 7.00
3.85 1.85 1.09 83.09 –
– – – – 65.28
– – – – 0.00
Ti3 Cu4 TiCu2 TiCu4 Ag(s.s) TiB
changing the brazing temperature from 850 to 910 ◦ C, and the joint brazed at 870 ◦ C for 15 min is shown in Fig. 2(c). It can be seen that sound joints were obtained upon increasing the brazing temperature, while the interfacial microstructure of the brazed joints changed markedly. (1) Morphology and thickness of the reaction layer adjacent to SiO2 –BN changed obviously with the increase of brazing temperature. For the joints brazed at a high temperature, in addition to the reaction layer bordering the ceramic substrate, referred to subsequently as layer I, the other discontinuous layer (marked by layer II) appeared, as clearly seen in Fig. 5(d). The discontinuous layer II was not uniform and contained white Ag-rich inclusions. Additionally, the thickness of the reaction zone (including layers I and II) increased from 1.5 to 20 m with increasing the brazing temperature from 850 to 910 ◦ C. Table 2 presents the chemical composition of reaction phase measured by EDS, from which we can see that the phase in layer II mainly consisted of Ti and Cu, while Ti, N and B appeared in layer I. Combined with the microstructure analysis in Section 3.1, it can be concluded that TiN–TiB2 existed in layer I, while Ti–Cu compound appeared in layer II. It should be noted that
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Fig. 5. Effect of brazing temperature on the microstructure of Ti/SiO2 –BN joints brazed with Ag–Cu–Ti + 3 wt% BN for 15 min: (a) 850 ◦ C; (b) 890 ◦ C; (c) 910 ◦ C; (d) magnification of reaction layer in (c); (e) magnification of Zone 1; (f) magnification of Zone 2.
the growth of strip-like Ti–Cu compounds were perpendicular to the SiO2 –BN substrate. (2) The most visible change came in the brazing seam. Fine Ti–Cu compound particles tended to accumulate and grow up with the increase of brazing temperature. For the brazing temperature reached 890 ◦ C, clumpy Ti–Cu compounds appeared in the brazing seam, as seen in Fig. 5(b). From the magnified microstructure of Zone 1 in Fig. 5(e), it can be seen that several blocky Ti–Cu phases formed and connected with each other. The EDS chemical composition analysis of these phases listed in Table 2 showed that TiCu4 , Ti3 Cu4 and TiCu2 phase formed in the brazing seam unambiguously. The result is consistent with
the XRD analysis in Fig. 3(a). Additionally, TiB whiskers were observed clearly in Fig. 5(f), further indicating that the added BN particles were reacted with Ti to form fine TiB and TiN. (3) The thickness of reaction zone bordering Ti substrate, including diffusion-induced ␣- Ti in area I and reaction-formed Ti–Cu compound layers in area II, increased by increasing brazing temperature. As seen in Fig. 5(a–c), the thickness of area I increased from 10 to 30 m with the increase of brazing temperature from 850 to 910 ◦ C. This phenomenon indicated that the diffusion rate of the -Ti stabilizing element Cu in the Ti substrate increased obviously at a high brazing temperature. At the same time, the dissolution of Ti substrate in the liquid
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Fig. 6. Effect of holding time on the microstructure of Ti/SiO2 –BN joints brazed at 870 ◦ C: (a) 5 min; (b) 25 min; (c) 35 min; (d) magnification of brazing seam in (b).
filler was accelerated, and thus enough Ti reacted with Cu to form a thick Ti-Cu compound layer. This microstructural evolution related to the change of elemental composition in the liquid filler. The active Ti in the liquid filler came from two aspects: the original composite filler and dissolved from Ti substrate. The dissolution rate of Ti substrate as well as the diffusion of Cu from the liquid filler into Ti increased at high temperature. As a result, the concentration of Ti in the liquid filler was high enough to react with Cu and SiO2 –BN ceramic to form thick reaction zones. 3.3. Effect of holding time on the microstructure of brazed joints The effect of holding time on the microstructure of Ti/SiO2 –BN joint was studied using four parameters: 15 min, 25 min and 35 min, in addition to 5 min discussed in Section 3.1. The brazing temperature was 870 ◦ C and the BN content was 3 wt%. It could be found that similar interfacial structure was formed in the joint regardless of brazing time. Microstructure observations indicated that the Ti–Cu reaction zone became thick as well as the reaction layer adjacent to the SiO2 –BN ceramic, as shown in Figs. 2 and 6. The second feature was that the distribution of finegrains over Ag matrix phase (area III) became more homogeneous with the increase of holding time. The variation of microstructure was mainly attributed to the following reasons: more Ti dissolved into the liquid filler with the increase of the holding time; and the time for the formation of Ti–Cu compound layers and the reaction layer adjacent to the SiO2 –BN ceramic was prolonged.
Compared with the brazing temperature, the accumulation of Ti–Cu compounds did not appear with the increase of holding time. Besides, there was only a single continuous reaction layer formed adjacent to SiO2 –BN ceramic. So, it can be inferred that the effect of brazing time on the microstructural evolution of brazed joints was much weaker than that of brazing temperature. 3.4. Microstructure evolution mechanism of the brazed joints According to the liquidus surface projection of Ag–Cu–Ti phase diagram [35], when the Ag–Cu–Ti filler metal was completely melted at the brazing temperature, the liquid filler was separated into two immiscible parts, with the compositions of about 20% L1 –6.1Ag–61.3Cu–32.6Ti (at.%) and 80% L2 –64.7Ag–34Cu-2.3Ti (at.%), respectively. The Ti and Cu atomic ratio in L1 was about 1:2, while the composition of L1 was close to the Ag–Cu eutectic at 780 ◦ C. Because the Ti–Cu affinity was stronger than that of Ag–Ti, L1 tended to approach the Ti substrate and L2 moved to SiO2 –BN ceramic side. Although the concentrations of Ti were quite different, the two separated liquids had the same activity of Ti. It is well known that Ti was a very active element, and thus the two separated liquids could wet the substrates well. Meanwhile, the dissolution of Ti substrate in the liquid filler occurred, resulting in the improvement of Ti activity. Then, a concentration gradient of the liquid filler formed between Ti and SiO2 –BN, as schematically shown in Fig. 7(a). Besides the formation of Ti-rich zone abutting the substrates, Ti atoms accumulated nearby the added BN powders. When the Ti activity in the liquid filler was high enough to trigger the chemical reactions at the Ti substrate surface, Ti3 Cu4 phase began to nucleate in L1 and grew as a thin outer layer on
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Fig. 7. Schematic diagram of microstructure evolution of Ti/SiO2 –BN joint brazed with Ag–Cu–Ti + BN composite filler: (a) atomic distribution in the liquid filler; (b) chemical reactions occurred at the interfaces and added B powders; (c) growth of reaction layer and formation of Ti2 Cu; (d) formation of Ti–Cu compounds and particle-reinforced brazing seam.
the Ti substrate. As can be seen in Ti–Cu phase diagram, Ti3 Cu4 phase appeared in the Ti–Cu rich liquid at 879–843 ◦ C. For SiO2 –BN ceramic, the active Ti in L2 reacted with it to form a thin TiN and TiB2 layer. Meanwhile, the chemical reaction between the added BN powders and Ti occurred to form fine TiN particles and TiB whiskers. The formation of reaction products during brazing is shown in Fig. 7(b). As the reactions continued, the thickness of reaction layers adjacent to the bonded substrates became thick. Once the Ti3 Cu4 layer with a certain thickness formed, it acted as a diffusion barrier layer, and thus the concentration of Cu bordering Ti substrate was decreased as well as the Ti in the liquid filler. Meanwhile, interdiffusion took place, and Cu atoms diffused across the Ti3 Cu4 layer and reacted with Ti to form Ti2 Cu between Ti and Ti3 Cu4 . The diffusion rate of Cu atoms across the Ti3 Cu4 layer was faster than that of Ti [35], leading to the formation of residual liquid filler with a low Ti concentration, which was partially transformed into TiCu2 and TiCu4 layers adjacent to Ti3 Cu4 . On the other hand, the thickness of reaction layer adjacent to the SiO2 –BN ceramic increased, which inhibited the diffusion of Ti across it. So, further growth of the reaction layer was slow. The variation of reaction phases during brazing process is schematically shown in Fig. 7(c). It is important to point out that when the dissolution of Ti was dramatic in the liquid filler, three kinds of Ti–Cu compounds, Ti3 Cu4 , TiCu2 and TiCu4
also generated in the brazing seam due to the presence of excessive Ti. As seen in Fig. 7(d), a sound joint formed during the solidification process. A series of Ti–Cu compound layers with a reduced relative Ti and Cu ratio formed abutting Ti substrate, while TiN and TiB2 reaction layer was observed adjacent to SiO2 –BN ceramic, playing a determining role in achieving metallurgical bonding between the ceramic substrate and filler metal. At the same time, a particle and whisker reinforced brazing seam was obtained due to the addition of BN particles.
3.5. Effect of brazing parameters on the mechanical properties of brazed joints Fig. 8 shows the effect of brazing parameters on the shear strength of Ti/SiO2 –BN joints brazed with Ag–Cu–Ti + 3 wt% BN composite filler. The average shear strength of the brazed joints firstly increased and then decreased by increasing brazing temperature and time. The maximum shear strength of 31 MPa was obtained when the Ti and SiO2 –BN ceramic were brazed at 870 ◦ C for 15 min. In addition, compared with the brazing temperature, the variation of joint strength as a function of holding time was dramatic.
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Fig. 8. Effect of (a) brazing temperature and (b) holding time on the shear strength of Ti/SiO2 -BN joints brazed with Ag–Cu–Ti + 3 wt% BN composite filler.
The bonding properties of Ti/SiO2 –BN brazed joint was directly determined by the formation of TiN–TiB2 reaction layer adjacent to the SiO2 –BN ceramic, which played a key role in achieving a metallurgical bonding between the ceramic substrate and the brazing alloy. When a low brazing temperature or time was employed, the dissolution of Ti into the liquid filler alloy was limited. As a result, the interfacial reaction between Ti and SiO2 –BN was insufficient and thus a thin reaction layer formed, which could not bear a large shear force. Under this circumstance, the shear strength of the brazed joints was low. However, for a high brazing temperature or a long holding time, a thick reaction layer formed. The difference between the CTE of reaction layer, SiO2 –BN ceramic and brazing alloy led to the development of residual stresses during cooling process. Such residual stresses reduced the strength of the brazed joint obviously. Therefore, the joint strength began to drop when Ti and SiO2 –BN were brazed at 910 ◦ C for 10 min or 870 ◦ C for 35 min. It is noted that for the joint brazed at a high brazing temperature, a discontinuous reaction layer II with the Ti–Cu grains perpendicular to the ceramic substrate formed in the joint, as seen in Fig. 5(d). This phenomenon was beneficial to reducing the mismatch of CTE between the brazing alloy and TiN–TiB2 reaction layer. Thus, the reduction of joint strength was not obvious though a thick reaction layer was formed at a high brazing temperature. Moreover, the formation of reaction layer II was not obvious with the increase of holding time. This is the reason why the variation of joint strength as a function of holding time was stronger than that of brazing temperature.
4. Conclusions Reliable joints of SiO2 –BN ceramic and Ti were successfully brazed using Ag–Cu–Ti + BN composite filler. Based on the experimental results, primary conclusions are obtained as follows. (1) The added BN particles in the composite filler reacted with Ti to form fine TiB whiskers and TiN particles during brazing, which was beneficial to refining the microstructure of brazing seam. Four Ti–Cu compound layers including Ti2 Cu, Ti3 Cu4 , TiCu2 and TiCu4 bordered Ti substrate while TiN-TiB2 reaction layer formed adjacent to SiO2 -BN ceramic. (2) By increasing BN content, the thickness of Ti–Cu compound layers bordering Ti substrate as well as TiN + TiB2 reaction layer adjacent to SiO2 –BN ceramic decreased slightly, and the shear strength of the brazed joints firstly increased and then decreased. The maximum shear strength of 31 MPa was achieved for the joints brazed with composite filler containing 3 wt% BN, which was 158% higher than that brazed with single Ag–Cu–Ti. The improvement
of joint strength was ascribed to the reduction of CTE mismatch between the bonded materials and brazing seam as well as the variation of microstructure of the brazing seam. (3) The thickness of Ti–Cu compound layers and TiN–TiB2 layer increased with the increase of brazing temperature and time. Meanwhile, blocky Ti–Cu phases formed in the brazing seam, and a directional arrangement of Ti–Cu strips were observed nearby the TiN–TiB2 layer when the brazing temperature reached 890 ◦ C. This microstructural evolution caused by improving brazing temperature and time was related to the dissolved Ti amount in the liquid filler. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51405332). References [1] G. Wen, G.L. Wu, T.Q. Lei, Y. Zhou, Z.X. Guo, J. Eur. Ceram. Soc. 20 (2000) 1923–1928. [2] D.C. Jia, L.Z. Zhou, Z.H. Yang, X.M. Duan, Y. Zhou, J. Am. Ceram. Soc. 94 (2011) 3552–3560. [3] F. Cao, K. Liu, Z.Y. Fang, S.Q. Wang, J. Mater. Sci. Technol. 28 (2012) 956–960. [4] D.M. Jacobson, G. Humpston, Principles of Brazing, second ed., ASM International, Materials Park, OH, USA, 2003, pp. 238. [5] Y.H. Liu, J.D. Hu, Y.P. Zhang, Z.X. Guo, Y. Yang, J. Mater. Sci. Technol. 27 (2011) 653–658. [6] B. Chen, H.P. Xiong, Y.Y. Cheng, W. Mao, S.B. Wu, J. Mater. Sci. Technol. 31 (2015) 1034–1038. [7] H.S. Ren, H.P. Xiong, B. Chen, S.J. Pang, B.Q. Chen, L. Ye, J. Mater. Sci. Technol. 32 (2016) 372–380. [8] L.X. Zhang, L.Z. Wu, D. Liu, J.C. Feng, H.B. Liu, Mater. Sci. Eng. A 496 (2008) 393–398. [9] H.B. Liu, L.X. Zhang, L.Z. Wu, D. Liu, J.C. Feng, Mater. Sci. Eng. A 498 (2008) 321–326. [10] A. Guedes, A. Pinto, M. Vieira, F. Viana, J. Mater. Process. Technol. 92-93 (1999) 102–106. [11] R.J. Miab, A.M. Hadian, Ceram. Int. 40 (2014) 8519–8524. [12] Q. Miao, W.F. Ding, Y.J. Zhu, Z.Z. Chen, J.H. Xu, C.Y. Yang, Ceram. Int. 42 (2016) 13723–13737. [13] W.F. Ding, J.H. Xu, M. Shen, H.H. Su, Y.C. Fu, B. Xiao, Mater. Sci. Eng. A 430 (2006) 301–306. [14] W.F. Ding, J.H. Xu, Z.Z. Chen, Q. Miao, C.Y. Yang, Mater. Sci. Eng. A 559 (2013) 629–634. [15] Z.W. Yang, L.X. Zhang, W. Ren, M. Lei, J.C. Feng, J. Eur. Ceram. Soc. 33 (2013) 759–768. [16] Z.W. Yang, L.X. Zhang, X.Y. Tian, Q. Xue, J.C. Feng, Mater. Sci. Eng. A 556 (2012) 722–727. [17] S.B. Lee, J.H. Kim, J. Mater. Process. Technol. 67 (1997) 167–172. [18] O.M. Akselsen, J. Mater. Sci. 27 (1992) 1989–2000. [19] M.G. Zhu, D.D.L. Chung, J. Mater. Sci. 32 (1997) 5321–5333. [20] Y.M. He, J. Zhang, Y. Sun, C.F. Liu, J. Eur. Ceram. Soc. 30 (2010) 3245–3251. [21] G.B. Lin, J.H. Huang, H. Zhang, H.Y. Liu, Sci. Technol. Weld. Join. 11 (2006) 379–383.
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Active metal brazing of SiO2 –BN ceramic and Ti plate with Ag–Cu–Ti + BN composite filler Z.W. Yang a,∗ , C.L. Wang a , Y. Wang a , L.X. Zhang b,∗ , D.P. Wang a , J.C. Feng b a b
Tianjin Key Lab of Advanced Joining Technology, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 14 November 2016 Received in revised form 2 February 2017 Accepted 20 February 2017 Available online xxx Keywords: SiO2 –BN ceramic BN particles Microstructure Mechanical properties
a b s t r a c t SiO2 –BN ceramic and Ti plate were joined by active brazing in vacuum with Ag–Cu–Ti + BN composite filler. The effect of BN content, brazing temperature and time on the microstructure and mechanical properties of the brazed joints was investigated. The results showed that a continuous TiN–TiB2 reaction layer formed adjacent to the SiO2 –BN ceramic, whose thickness played a key role in the bonding properties. Four Ti–Cu compound layers, Ti2 Cu, Ti3 Cu4 , TiCu2 and TiCu4 , were observed to border Ti substrate due to the strong affinity of Ti and Cu compared with Ag. The central part of the joint was composed of Ag matrix, over which some fine-grains distributed. The added BN particles reacted with Ti in the liquid filler to form fine TiB whiskers and TiN particles with low coefficients of thermal expansion (CTE), leading to the reduction of detrimental residual stress in the joint, and thus improving the joint strength. The maximum shear strength of 31 MPa was obtained when 3 wt% BN was added in the composite filler, which was 158% higher than that brazed with single Ag–Cu–Ti filler metal. The morphology and thickness of the reaction layer adjacent to the parent materials changed correspondingly with the increase of BN content, brazing temperature and holding time. Based on the correlation between the microstructural evolution and brazing parameters, the bonding mechanism of SiO2 –BN and Ti was discussed. © 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction Fused silica based ceramics exhibit an excellent thermal stability and have increasing use in a wide range of engineering applications. It has also been shown that particle reinforcement of SiO2 ceramics by hexagonal BN (h-BN) particles can lead to a cost effective improvement of thermal shock resistance, ablation resistance, and mechanical and dielectric properties [1–3]. An important engineering application of such a particle-reinforced SiO2 material (SiO2 –BN) is in radar window, where ceramics are used for wave transparency. The assembly of radar window requires joining a SiO2 –BN ceramic window to a metallic holder. Unfortunately, the traditional mechanical fastening causes an additional weight and a high cost. It also limits the vehicle speed because of poor heat resistance of the bonded joint. Thus, active metal brazing is used to join the SiO2 –BN ceramic to a titanium holder in this study.
∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (Z.W. Yang), [email protected] (L.X. Zhang).
Active metal brazing is a well-established method for reliable joining of ceramics to metals [4,5]. Compared to mechanical attachment and adhesives that are suitable for relatively low-temperature applications, this technique is appropriate for high-temperature applications, where strength, air tightness and corrosion resistance are required [6,7]. For active metal brazing, the metallurgical bonding is the result of a chemical reaction between the reactive element in the brazing alloy and the ceramic surfaces. Therefore, the type and concentration of the added active element in the brazing alloy are crucial to the joint strength. Near eutectic Ag–Cu alloy with a few percent of Ti as the active element is the most frequently considered active brazing alloy for the strong bonding of ceramic-metal components. Moreover, previous results have shown that Ti derived from Ag–Cu–Ti system and other systems are characterized as having sufficient thermodynamic driving force to react with SiO2 [8–10] and BN [11–14]. Hence, it can be inferred that the SiO2 –BN ceramic consisting of fused silica and hBN, might be successfully brazed with this kind of Ti-activated filler metal. Recently, commercially available Ag–Cu–Ti brazing alloy has been successfully applied to braze the SiO2 –BN ceramic, and a continuous TiN–TiB2 reaction layer forms at the ceramic surface to
http://dx.doi.org/10.1016/j.jmst.2017.04.005 1005-0302/© 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
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Fig. 1. Morphology of the as-milled Ag–Cu–Ti + 3 wt% BN composite filler and schematic diagram of sample assembly: (a) morphology of composite filler; (b) assembly of brazing sample.
achieve a metallurgical bonding between the ceramic substrate and the brazing alloy [15,16]. But, the significant difference in the thermal expansion coefficients (CTE) between ceramics and metals or brazing alloys can lead to high residual stresses in the joint, which results in reduced strength of the joined components [17,18]. Consequently, it can speculate that the residual stress yield during the cooling process as a result of the CTE mismatch between SiO2 –BN ceramic ((0.5–1.7) × 10−6 K−1 ) and Ti (9.0 × 10−6 K−1 ) or Ag–Cu–Ti brazing alloy (18.2 × 10−6 K−1 ) is large and should be taken seriously. This problem could be alleviated by the addition of low CTE materials (e.g. fibers or particles) into brazing alloys [19–23]. The incidental issue is that the reinforcements are difficult to be distributed uniformly in the matrix due to their bad wetting ability with the liquid brazing alloy. It was reported that the in situ synthesized reinforcements originated from brazing process itself have greater reinforcing effects because of their fine size, uniform distribution, and favorable cohesion with matrix [24–28]. The addition of carbon fibers, SiC, Si3 N4 , TiN, TiC and B powders into the brazing alloy to fabricate composite fillers has been demonstrated to improve the joint strength remarkably. In this work, low CTE reinforcements were in situ synthesized by the addition of h-BN particles in the Ag–Cu–Ti brazing alloy. The effect of BN content, brazing temperature and time on the microstructure and mechanical properties of the brazed SiO2 -BN/Ti joints was investigated. The reinforcing effect caused by the BN additive as well as the brazing mechanism and microstructural evolution of the brazed joint was discussed. 2. Experimental procedures Pure Ti plate with a thickness of 3 mm and SiO2 –BN ceramic fabricated by hot pressing of SiO2 and BN powders were used as parent materials in the brazing experiments. The size of SiO2 –BN ceramic and Ti samples for brazing was 5 mm × 3 mm × 3 mm and 25 mm × 8 mm × 3 mm, respectively. Surfaces to be brazed were firstly ground and then ultrasonically cleaned in acetone for 10 min before brazing. The composite filler used to braze SiO2 –BN and Ti was fabricated by mechanical milling of commercial available Ag–27.5Cu–2.5Ti (wt%) brazing alloy powder and h-BN powder. The weight percentage of h-BN powder in the composite filler was designed as 0%, 1.5%, 3%, and 4.5%, respectively. The average particle diameter of the added h-BN powder was 0.5 m. The initial powders were weighted in the scheduled composition and then the powder mixture was milled for 2 h in an argon atmosphere
using a QM-SB planetary ball mill. The morphology of the as-milled composite filler is shown in Fig. 1(a), from which we can see that fine h-BN powders were homogeneously adhered on the surfaces of Ag–Cu–Ti powders. The as-milled composite filler was mixed with a small amount of cellulose nitrate and octylacetate to make into paste. The brazing paste was sandwiched between the SiO2 –BN ceramic and the Ti sample by coating on the bonded surface of SiO2 –BN ceramic (200–300 m). The joint assembly is schematically illustrated in Fig. 1(b). The assembly was placed in a vacuum furnace and a fixed load of 0.2 kg was kept over the sample to enable a proper contact in the bonding surfaces during brazing. The furnace was heated at a rate of 30 ◦ C/min till it reached 700 ◦ C and kept for 10 min, and then the temperature continued to increase to 850–910 ◦ C at a rate of 10 ◦ C/min. Subsequently, the brazing specimens were held for 5–35 min at the brazing temperature and then cooled down to 400 ◦ C at a rate of 5 ◦ C/min. Finally, the furnace was cooled down spontaneously to room temperature in the furnace. During the brazing process, the vacuum level of the furnace was kept at 3.0 × 10−3 Pa. The cross-sections of the brazed joints were prepared for metallographic examination. The joint microstructure observations as well as quantitative elemental analysis were performed using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). The reaction phases formed in the brazed joints were characterized by X-ray diffraction (XRD) spectrometer with Cu-K˛ radiation. Layer-stripping of the brazed joint by coarse grinding was applied to determine the location of XRD analysis. To evaluate the bonding properties of the brazed joint, shear tests were carried out at a constant displacement rate of 0.5 mm/min using a universal testing machine (Instron1186). At least three samples were tested for each experimental condition. The fracture surface of the brazed joint after shear test was observed with a digital microscope (VHX-1000E). 3. Results and discussion 3.1. Effect of BN content on the microstructure and shear strength of brazed joints Fig. 2 shows the effect of BN content in the composite filler on the interfacial microstructure of Ti/SiO2 –BN joint brazed at 870 ◦ C for 15 min. The EDS analysis of each phase formed in the joint is listed in Table 1. It is noted that Cu in the composite filler had a
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Fig. 2. Microstructure of Ti/SiO2 –BN joints brazed with Ag–Cu–Ti + BN composite filler with different BN content at 870 ◦ C for 15 min: (a) 0; (b) 1.5%; (c) 3%; (d) 4.5%; (e) magnification of brazing seam in (b); (f) magnification of brazing seam in (c).
Table 1 Average chemical compositions of each reactant (at.%) in Fig. 2.
A B C D E F G H
Ti
Cu
Ag
N
B
Si
O
Possible phase
89.90 60.78 42.83 35.41 20.16 1.00 18.04 40.26
8.49 36.50 54.49 63.29 77.08 10.93 80.87 4.38
1.61 2.72 2.51 1.30 1.70 86.31 1.09 1.19
– – – – – – – 20.12
– – – – – – – 24.42
– – 1.07 – 1.06 1.76 – 6.49
– – – – – – – 4.04
␣- Ti Ti2 Cu Ti3 Cu4 TiCu2 TiCu4 Ag(s.s) Cu(s.s) TiN + TiB2
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Fig. 3. XRD analysis of Ti/SiO2 –BN joint brazed with Ag–Cu–Ti + 3 wt% BN composite filler at 870 ◦ C for 15 min: (a) brazing seam; (b) reaction layer adjacent to SiO2 –BN ceramic.
strong tendency to react with Ti substrate, and a series of Ti–Cu compound layers formed adjacent to the Ti substrate. From Ti side to the brazing seam, ␣- phase aggregate and Ti-Cu compound layers including Ti2 Cu, Ti3 Cu4 , TiCu2 and TiCu4 layer, were observed in area I and area II, respectively. The vigorous interactions occurred at the solid/liquid interface during brazing, involving Ti dissolution and atomic diffusion, which were the primary reason for the formation of these characteristic microstructures. The formation of ␣- phase aggregate in area I was ascribed to the diffusion of Cu in Ti substrate. Cu is a strong -Ti stabilizing element and can lower the eutectoid transformation temperature of Ti [29]. Thus, the bright -Ti needles and dark ␣-Ti matrix formed by the decomposition of -Ti, as seen in area I. Meanwhile, the dissolved Ti had a stronger affinity towards Cu than to Ag, because the partial enthalpy of Ti solution in the molten Cu at infinite dilution (–10 kJ/mol) was lower than that in Ag (39 kJ/mol) [30]. As a consequence, a series of Ti–Cu compound layers, with the atomic percentage of Ti/Cu decreased from area I to brazing seam, formed in area II. With the increase of BN content, the thickness of area I and II decreased obviously, as observed in Fig. 2(a–d). Moreover, instead of Ag- and Cu-based solid solution, a large number of fine-grains appeared in the brazing seam when the BN particles were added, resulting in the homogeneous of joint microstructure. The magnification of these black fine-grains is shown in Fig. 2(e) and (f), corresponding to 1.5% and 3 wt% BN addition, respectively. To characterize the phase structure of these fine-grains, the XRD analysis of this zone obtained by layer stripping is given in Fig. 3(a). The results confirmed that TiN, TiB, and three kinds of Ti–Cu phases formed in the brazing seam besides Ag(s.s). It is reasonable that TiN and TiB were formed by the reaction between the active Ti and the BN additive. The calculated Gibbs free energy (GT0 ) of 2Ti + BN → TiB + TiN at the brazing temperature of 870 ◦ C is −236.7 kJ/mol [15], indicating that the formation of TiN and TiB is thermodynamic feasible. The in situ synthesized fine-grains, TiB whisker and TiN particle might act as the nucleation sites for the formation of Ti–Cu compounds, as suggested in Ref. [26,31,32]. Consequently, the presence of TiCu4 , Ti3 Cu4 and TiCu2 were indexed in the XRD pattern. It is noteworthy that the addition of BN particles was beneficial to refining the microstructure of brazing seam. However, the added BN particles were not the more the better. The accumulation of BN was observed in the joint when the BN content reached 4.5 wt%, as seen in Fig. 2(d). On the other hand, partially Ti in the liquid filler was consumed to react with BN additive, in addition to react with Cu to form fine Ti–Cu compounds in the brazing seam. Hence, Ti–Cu compound layers bordering Ti substrate as well as the reaction layer adjacent to the SiO2 –BN ceramic became thin with the increase of BN content. The XRD analysis of the reac-
tion layer in Fig. 3(b) confirmed that TiN, TiB2 , TiCu4 , Ti3 Cu4 and TiCu2 phase formed in this zone. The type of Ti–Cu phases was the same as that found in the brazing seam. Nevertheless, different from the reaction phases of TiN and TiB produced in the brazing seam, TiN and TiB2 formed adjacent to the SiO2 –BN ceramic by the chemical reaction of 3Ti + 2BN → TiB2 + 2TiN. The GT0 for TiN + TiB2 formation at 870 ◦ C was −421.4 kJ/mol [15], which was lower than that of TiN + TiB formation. Compared with TiB phase, theoretically, TiB2 preferred to form in the same brazing condition. However, Ti and TiB2 could further react to form TiB owing to the small negativity of the free energy of Ti + TiB2 → 2TiB. In addition, the reaction formation enthalpy of Ti + TiB2 → 2TiB was also negative [33], indicating that the reaction was exothermic. In terms of Le Chatelier’s principle, if the active Ti in the liquid filler was excessive, the above reaction would be shifted to the right. Consequently, TiB phase could form in the brazing seam as a result of the successive dissolution of Ti. But, for the reaction between Ti and h-BN in the SiO2 –BN ceramic, the diffusion of Ti atoms across the already formed reaction layer was limited, and thus TiN + TiB2 reaction layer was observed adjacent to the SiO2 –BN ceramic. In conclusion, the microstructure of Ti/SiO2 –BN joint changed obviously due to the addition of BN particles. A large number of fine-grains consisting of TiB, TiN and Ti–Cu compounds formed in the brazing seam, which was beneficial to refining the joint microstructure. Meanwhile, with the increase of BN content, the thickness of Ti–Cu compound layers bordering Ti substrate as well as TiN + TiB2 reaction layer adjacent to SiO2 –BN ceramic decreased correspondingly. The variation of interfacial microstructure played a decisive role in determining the mechanical properties of the brazed joints. Fig. 4 shows the effect of BN content in the composite filler on the shear strength of SiO2 –BN/Ti joint brazed at 870 ◦ C for 15 min. It can be seen that the average shear strength of the brazed joint firstly increased and then decreased with the increase of BN content from 0 to 4.5 wt%. The maximum strength reached 31 MPa for the joint brazed with 3.0 wt% BN additive, which was 158% higher than that of joint brazed with single Ag–Cu–Ti filler. The improvement of joint strength caused by the addition of BN was prominent. Moreover, the fracture mode of the brazed joints after shear test changed correspondingly. For the joint brazed with single Ag–Cu–Ti filler, crack propagated in the SiO2 –BN substrate nearby the ceramic/filler interface. This is because a large residual stress yielded in this area due to the CTE mismatch between the parent materials and the brazing seam, forming a weak zone in the joint. The formation of such a large residual stress has been demonstrated by the finite element calculation of a ceramic–metal brazed joint [34]. The brazed joint would be failed under a small shear force owing to
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Fig. 4. Effect of BN content on the shear strength and fracture mode of Ti/SiO2 –BN brazed joints.
the presence of large residual stress, and thus the shear strength of the brazed joint was low. However, a different fracture mode was observed for the joint brazed with composite filler containing 3.0 wt% BN. Crack initiated in the interfacial reaction layer, and then propagated in the ceramic substrate. The crack deflection during shear test would absorb more shear force, and thus a higher joint strength was obtained. The improvement of joint strength was primarily ascribed to the change of microstructure in the brazing seam as well as the alleviation of residual stresses. As analyzed above, a fine particle-reinforced brazing seam was formed as a result of BN additive, which is beneficial to the bonding properties. Moreover, the calculated CTE of the brazing seam was decreased from 19 × 10−6 to 15 × 10−6 K−1 when 3.0 wt% BN particles were added in the composite filler, owing to the formation of TiB whiskers and TiN particles with low CTE values in the brazing seam. Therefore, the CTE mismatch between SiO2 –BN ceramic and brazing seam was decreased, leading to the reduction in detrimental residual stresses generated in the ceramic substrate. Though the CTE value of the brazing seam was further decreased with the increase of BN content in the composite filler, the BN content was not the more the better. When the composite filler with a higher BN content was used to braze Ti and SiO2 –BN, the reaction layer was too thin to bear a large shear force, and cracks were primarily propagated along this reaction layer and partially in the ceramic substrate. Under this circumstance, the joint strength was dropped. 3.2. Effect of brazing temperature on the microstructure of brazed joints Fig. 5 shows the effect of brazing temperature on the interfacial microstructure of Ti/SiO2 –BN joints brazed with Ag–Cu–Ti + 3 wt% BN composite filler for 15 min. This effect was investigated by
Table 2 Average chemical compositions of each reactant (at.%) in Fig. 5.
A B C D E
Ti
Cu
Ag
B
N
Possible phase
43.40 32.57 18.04 2.71 27.72
52.75 65.57 80.87 14.20 7.00
3.85 1.85 1.09 83.09 –
– – – – 65.28
– – – – 0.00
Ti3 Cu4 TiCu2 TiCu4 Ag(s.s) TiB
changing the brazing temperature from 850 to 910 ◦ C, and the joint brazed at 870 ◦ C for 15 min is shown in Fig. 2(c). It can be seen that sound joints were obtained upon increasing the brazing temperature, while the interfacial microstructure of the brazed joints changed markedly. (1) Morphology and thickness of the reaction layer adjacent to SiO2 –BN changed obviously with the increase of brazing temperature. For the joints brazed at a high temperature, in addition to the reaction layer bordering the ceramic substrate, referred to subsequently as layer I, the other discontinuous layer (marked by layer II) appeared, as clearly seen in Fig. 5(d). The discontinuous layer II was not uniform and contained white Ag-rich inclusions. Additionally, the thickness of the reaction zone (including layers I and II) increased from 1.5 to 20 m with increasing the brazing temperature from 850 to 910 ◦ C. Table 2 presents the chemical composition of reaction phase measured by EDS, from which we can see that the phase in layer II mainly consisted of Ti and Cu, while Ti, N and B appeared in layer I. Combined with the microstructure analysis in Section 3.1, it can be concluded that TiN–TiB2 existed in layer I, while Ti–Cu compound appeared in layer II. It should be noted that
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Fig. 5. Effect of brazing temperature on the microstructure of Ti/SiO2 –BN joints brazed with Ag–Cu–Ti + 3 wt% BN for 15 min: (a) 850 ◦ C; (b) 890 ◦ C; (c) 910 ◦ C; (d) magnification of reaction layer in (c); (e) magnification of Zone 1; (f) magnification of Zone 2.
the growth of strip-like Ti–Cu compounds were perpendicular to the SiO2 –BN substrate. (2) The most visible change came in the brazing seam. Fine Ti–Cu compound particles tended to accumulate and grow up with the increase of brazing temperature. For the brazing temperature reached 890 ◦ C, clumpy Ti–Cu compounds appeared in the brazing seam, as seen in Fig. 5(b). From the magnified microstructure of Zone 1 in Fig. 5(e), it can be seen that several blocky Ti–Cu phases formed and connected with each other. The EDS chemical composition analysis of these phases listed in Table 2 showed that TiCu4 , Ti3 Cu4 and TiCu2 phase formed in the brazing seam unambiguously. The result is consistent with
the XRD analysis in Fig. 3(a). Additionally, TiB whiskers were observed clearly in Fig. 5(f), further indicating that the added BN particles were reacted with Ti to form fine TiB and TiN. (3) The thickness of reaction zone bordering Ti substrate, including diffusion-induced ␣- Ti in area I and reaction-formed Ti–Cu compound layers in area II, increased by increasing brazing temperature. As seen in Fig. 5(a–c), the thickness of area I increased from 10 to 30 m with the increase of brazing temperature from 850 to 910 ◦ C. This phenomenon indicated that the diffusion rate of the -Ti stabilizing element Cu in the Ti substrate increased obviously at a high brazing temperature. At the same time, the dissolution of Ti substrate in the liquid
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Fig. 6. Effect of holding time on the microstructure of Ti/SiO2 –BN joints brazed at 870 ◦ C: (a) 5 min; (b) 25 min; (c) 35 min; (d) magnification of brazing seam in (b).
filler was accelerated, and thus enough Ti reacted with Cu to form a thick Ti-Cu compound layer. This microstructural evolution related to the change of elemental composition in the liquid filler. The active Ti in the liquid filler came from two aspects: the original composite filler and dissolved from Ti substrate. The dissolution rate of Ti substrate as well as the diffusion of Cu from the liquid filler into Ti increased at high temperature. As a result, the concentration of Ti in the liquid filler was high enough to react with Cu and SiO2 –BN ceramic to form thick reaction zones. 3.3. Effect of holding time on the microstructure of brazed joints The effect of holding time on the microstructure of Ti/SiO2 –BN joint was studied using four parameters: 15 min, 25 min and 35 min, in addition to 5 min discussed in Section 3.1. The brazing temperature was 870 ◦ C and the BN content was 3 wt%. It could be found that similar interfacial structure was formed in the joint regardless of brazing time. Microstructure observations indicated that the Ti–Cu reaction zone became thick as well as the reaction layer adjacent to the SiO2 –BN ceramic, as shown in Figs. 2 and 6. The second feature was that the distribution of finegrains over Ag matrix phase (area III) became more homogeneous with the increase of holding time. The variation of microstructure was mainly attributed to the following reasons: more Ti dissolved into the liquid filler with the increase of the holding time; and the time for the formation of Ti–Cu compound layers and the reaction layer adjacent to the SiO2 –BN ceramic was prolonged.
Compared with the brazing temperature, the accumulation of Ti–Cu compounds did not appear with the increase of holding time. Besides, there was only a single continuous reaction layer formed adjacent to SiO2 –BN ceramic. So, it can be inferred that the effect of brazing time on the microstructural evolution of brazed joints was much weaker than that of brazing temperature. 3.4. Microstructure evolution mechanism of the brazed joints According to the liquidus surface projection of Ag–Cu–Ti phase diagram [35], when the Ag–Cu–Ti filler metal was completely melted at the brazing temperature, the liquid filler was separated into two immiscible parts, with the compositions of about 20% L1 –6.1Ag–61.3Cu–32.6Ti (at.%) and 80% L2 –64.7Ag–34Cu-2.3Ti (at.%), respectively. The Ti and Cu atomic ratio in L1 was about 1:2, while the composition of L1 was close to the Ag–Cu eutectic at 780 ◦ C. Because the Ti–Cu affinity was stronger than that of Ag–Ti, L1 tended to approach the Ti substrate and L2 moved to SiO2 –BN ceramic side. Although the concentrations of Ti were quite different, the two separated liquids had the same activity of Ti. It is well known that Ti was a very active element, and thus the two separated liquids could wet the substrates well. Meanwhile, the dissolution of Ti substrate in the liquid filler occurred, resulting in the improvement of Ti activity. Then, a concentration gradient of the liquid filler formed between Ti and SiO2 –BN, as schematically shown in Fig. 7(a). Besides the formation of Ti-rich zone abutting the substrates, Ti atoms accumulated nearby the added BN powders. When the Ti activity in the liquid filler was high enough to trigger the chemical reactions at the Ti substrate surface, Ti3 Cu4 phase began to nucleate in L1 and grew as a thin outer layer on
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Fig. 7. Schematic diagram of microstructure evolution of Ti/SiO2 –BN joint brazed with Ag–Cu–Ti + BN composite filler: (a) atomic distribution in the liquid filler; (b) chemical reactions occurred at the interfaces and added B powders; (c) growth of reaction layer and formation of Ti2 Cu; (d) formation of Ti–Cu compounds and particle-reinforced brazing seam.
the Ti substrate. As can be seen in Ti–Cu phase diagram, Ti3 Cu4 phase appeared in the Ti–Cu rich liquid at 879–843 ◦ C. For SiO2 –BN ceramic, the active Ti in L2 reacted with it to form a thin TiN and TiB2 layer. Meanwhile, the chemical reaction between the added BN powders and Ti occurred to form fine TiN particles and TiB whiskers. The formation of reaction products during brazing is shown in Fig. 7(b). As the reactions continued, the thickness of reaction layers adjacent to the bonded substrates became thick. Once the Ti3 Cu4 layer with a certain thickness formed, it acted as a diffusion barrier layer, and thus the concentration of Cu bordering Ti substrate was decreased as well as the Ti in the liquid filler. Meanwhile, interdiffusion took place, and Cu atoms diffused across the Ti3 Cu4 layer and reacted with Ti to form Ti2 Cu between Ti and Ti3 Cu4 . The diffusion rate of Cu atoms across the Ti3 Cu4 layer was faster than that of Ti [35], leading to the formation of residual liquid filler with a low Ti concentration, which was partially transformed into TiCu2 and TiCu4 layers adjacent to Ti3 Cu4 . On the other hand, the thickness of reaction layer adjacent to the SiO2 –BN ceramic increased, which inhibited the diffusion of Ti across it. So, further growth of the reaction layer was slow. The variation of reaction phases during brazing process is schematically shown in Fig. 7(c). It is important to point out that when the dissolution of Ti was dramatic in the liquid filler, three kinds of Ti–Cu compounds, Ti3 Cu4 , TiCu2 and TiCu4
also generated in the brazing seam due to the presence of excessive Ti. As seen in Fig. 7(d), a sound joint formed during the solidification process. A series of Ti–Cu compound layers with a reduced relative Ti and Cu ratio formed abutting Ti substrate, while TiN and TiB2 reaction layer was observed adjacent to SiO2 –BN ceramic, playing a determining role in achieving metallurgical bonding between the ceramic substrate and filler metal. At the same time, a particle and whisker reinforced brazing seam was obtained due to the addition of BN particles.
3.5. Effect of brazing parameters on the mechanical properties of brazed joints Fig. 8 shows the effect of brazing parameters on the shear strength of Ti/SiO2 –BN joints brazed with Ag–Cu–Ti + 3 wt% BN composite filler. The average shear strength of the brazed joints firstly increased and then decreased by increasing brazing temperature and time. The maximum shear strength of 31 MPa was obtained when the Ti and SiO2 –BN ceramic were brazed at 870 ◦ C for 15 min. In addition, compared with the brazing temperature, the variation of joint strength as a function of holding time was dramatic.
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Fig. 8. Effect of (a) brazing temperature and (b) holding time on the shear strength of Ti/SiO2 -BN joints brazed with Ag–Cu–Ti + 3 wt% BN composite filler.
The bonding properties of Ti/SiO2 –BN brazed joint was directly determined by the formation of TiN–TiB2 reaction layer adjacent to the SiO2 –BN ceramic, which played a key role in achieving a metallurgical bonding between the ceramic substrate and the brazing alloy. When a low brazing temperature or time was employed, the dissolution of Ti into the liquid filler alloy was limited. As a result, the interfacial reaction between Ti and SiO2 –BN was insufficient and thus a thin reaction layer formed, which could not bear a large shear force. Under this circumstance, the shear strength of the brazed joints was low. However, for a high brazing temperature or a long holding time, a thick reaction layer formed. The difference between the CTE of reaction layer, SiO2 –BN ceramic and brazing alloy led to the development of residual stresses during cooling process. Such residual stresses reduced the strength of the brazed joint obviously. Therefore, the joint strength began to drop when Ti and SiO2 –BN were brazed at 910 ◦ C for 10 min or 870 ◦ C for 35 min. It is noted that for the joint brazed at a high brazing temperature, a discontinuous reaction layer II with the Ti–Cu grains perpendicular to the ceramic substrate formed in the joint, as seen in Fig. 5(d). This phenomenon was beneficial to reducing the mismatch of CTE between the brazing alloy and TiN–TiB2 reaction layer. Thus, the reduction of joint strength was not obvious though a thick reaction layer was formed at a high brazing temperature. Moreover, the formation of reaction layer II was not obvious with the increase of holding time. This is the reason why the variation of joint strength as a function of holding time was stronger than that of brazing temperature.
4. Conclusions Reliable joints of SiO2 –BN ceramic and Ti were successfully brazed using Ag–Cu–Ti + BN composite filler. Based on the experimental results, primary conclusions are obtained as follows. (1) The added BN particles in the composite filler reacted with Ti to form fine TiB whiskers and TiN particles during brazing, which was beneficial to refining the microstructure of brazing seam. Four Ti–Cu compound layers including Ti2 Cu, Ti3 Cu4 , TiCu2 and TiCu4 bordered Ti substrate while TiN-TiB2 reaction layer formed adjacent to SiO2 -BN ceramic. (2) By increasing BN content, the thickness of Ti–Cu compound layers bordering Ti substrate as well as TiN + TiB2 reaction layer adjacent to SiO2 –BN ceramic decreased slightly, and the shear strength of the brazed joints firstly increased and then decreased. The maximum shear strength of 31 MPa was achieved for the joints brazed with composite filler containing 3 wt% BN, which was 158% higher than that brazed with single Ag–Cu–Ti. The improvement
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