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Kovar braze joints
Journal of Alloys and Compounds xxx (xxxx) xxx
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The microstructural evolution and formation mechanism in Si3N4/ AgCuTi/Kovar braze joints Chenglai Xin a, Jiazhen Yan b, *, Qingyuan Wang a, Wei Feng a, Chengyun Xin c a
School of Mechanical Engineering, Chengdu University, Chengdu, 610065, PR China School of Manufacturing Science and Engineering, Sichuan University, Chengdu, 610065, PR China c School of Electric Power Engineering, China University of Mining and Technology, Xuzhou, 221116, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2019 Received in revised form 25 November 2019 Accepted 25 November 2019 Available online xxx
The braze joints of Si3N4-Kovar using AgCuTi braze alloys are investigated systematically with the brazing temperatures between 860 C and 950 C for 2e30min. The correlation between the joint strength and the microstructures of the braze joints is discussed. These results show that the reaction layer plays an important role in interface bonding. Growth of the reaction layer is a reaction-diffusion process, which mainly depends on the diffusion of Ti. The diffusion activation energy (Q) is estimated as 170.9e248.7 kJ/ mol in this system, which may provide data support for Si3N4/metal braze joints. As the holding time is further prolonged (in the range of 10e30min), intermetallic compounds (IMCs) forms in the braze seam, which will prevent Ag solid solution from alleviating interfacial thermal stress. With the increasing of the brazing temperature, the residual thermal stresses increases. As a results, larger residual thermal stress is produced in the braze joints at higher braze temperature, which result in a weaker bonding. © 2019 Elsevier B.V. All rights reserved.
Keywords: Si3N4 Kovar alloy Microstructural evolution Braze joints
1. Introduction Silicon nitride ceramics (Si3N4) are often joined to metals to form reliable assemblies in a wide variety of fields including aviation, aerospace and electronics industry, which have been received extensive attentions [1e3]. In that case, strong and reliable joints of Si3N4-metal have to be obtained for engineering applications. Braze joint of Si3N4-metal is difficult due to the poor wettability and the residual stresses in the joint generated by mismatch of the coefficients of thermal expansion (CTE) [4,5]. To solve these problems, various joint methods including active metal brazing bonding (AMB), diffusion bonding, and partial transient-liquid-phase (TLP) bonding have been developed. Among these methods, active metal brazing is one of the most important methods for fabricating assemblies [1e5], and the joints of Si3N4-metal have been studied extensively in recent years. Active metal brazing is usually performed using AgCu eutectic alloys containing a small amount of Ti, an element that promotes wetting and adhesion [4,6]. It is proved that the active element Ti always migrates to Si3N4 sides to form a reaction layer during the brazing process [7e9]. Electron microscopy investigations on the
* Corresponding author. E-mail address: [email protected] (J. Yan).
interfacial reaction at AgCuTi/Si3N4 interfaces show that the reaction products consisted of upper Ti5Si3 and lower TiN layers [7e9]. Zhang et al. [7,8] study the interfacial microstructure in Si3N4/ 42CrMo joints and their results show that a better interfacial microstructure can improve the interfacial bonding strength [10]. Yang et al. [11,12] prove that the formation of soft solid solution microstructure (e.g., Cu solid solution) in braze seam can relieve residual thermal stress by plastic deformation, and therefore the bonding strength of the joints are improved. The research results of Zhang et al. [7,13] show that the addition of TiN particles or Ni reduced the CTE of the braze seam and played a significant role in strengthening the braze joints. Most of these researches [1,10,14e16] consider that the microstructure of the braze seam can affect the mechanical property of brazed joint and plays an important role in alleviating residual thermal by stress relaxation or creep deformation. Nevertheless, little study is focused on the microstructural evolution of braze seams and formation mechanism of the interfacial reaction layer in Si3N4-metal joint systems. Through the above reviews, it is very necessary to study the microstructural evolution and formation mechanism on the interfacial reaction layer of the Si3N4-metal joints, and then to study the effects of the microstructures on relieving residual thermal stress. In this study, Si3N4 ceramics were brazed to Kovar alloys (a lowexpansion FeeCoeNi alloy used extensively in electronic
https://doi.org/10.1016/j.jallcom.2019.153189 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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applications [17]) using AgCuTi alloys. The joint strength and microstructures of the braze joints were investigated systematically. Through this study, the microstructural evolution and formation mechanism of the interfacial reaction layer in Si3N4-Kovar joint systems will be explored, which is of great significance for the knowledge enrichment of the formation mechanism of braze joints. 2. Materials and experimental procedure Silicon nitride ceramics (Si3N4) with a purity of >98 wt % are used as the substrate material. The average surface roughness of Si3N4 measured by surface roughness detector is 3.5 mm. The dimensions of Si3N4 are shown in Fig. 1 (a). 4J33-Kovar alloys (Fe33 wt %Ni-17 wt %Co) are used as the base material for braze joint. The 4J33-Kovar alloy is in the form of ring with an outer diameter of 16 mm, an inner diameter of 9.8 mm and a thickness of 1 mm. The braze alloy is commercial Ag-28 wt% Cu eutectic alloy with an addition of 4 wt% Ti (AgCu4Ti). The test method for tensile strength is shown in Fig. 1. The substrates and Kovar alloys were ultrasonically cleaned in alcohol for 10min, and then were coated by the braze alloys on the contact interface. The braze experiments were performed in vacuum brazing furnace with a vacuum degree of 3.5 103 Pa under the effect of the diffusion pump chamber. The braze process parameters (see Fig. 2) was set at different brazing temperatures (Tb) with varied holding time (t). In order to test tensile strength of the joints, the braze specimen were assembled in the well-designed ‘sandwich’ configuration. Microstructural characterization of the braze joints are analyzed by the X-ray diffraction (XRD, X’Pert PRO) with Cu Ka radiation, the scanning electron microscopy (SEM, HITACHI S-4800) equipped with an energy dispersive X-ray (EDX) analyzer and transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN). The thin foils for transmission electron microscopy (TEM) observation of the evolved microstructures in the brazing seams are prepared using focused ion beam (FIB) technique (HELIOS Nanolab600i, FEI, USA). The tension tests are performed using electronic universal testing machine (RGX-M300) at a speed of 0.2 mm/min, in which the value of the tensile specimen in each group is measured five times repeatedly and then the average value is obtained in order to ensure the accuracy. 3. Results 3.1. The microstructures in the braze joints The microstructures of the braze joints prepared with AgCu4Ti
Fig. 2. The heating cycling curve for braze joints.
alloys at varied temperature of 860 C, 890 C, 920 C for 5min are shown in Fig. 3. The microstructures of the braze joints at a temperature of 890 C for a varied holding time of 2min, 5min, 10min, 30min are shown in Fig. 4. These images show that the joints are mainly composed of three parts: a continuous interfacial reaction layer formed at AgCu4Ti/Si3N4 is referred to I, an interfacial reaction layer formed between Kovar and AgCu4Ti is referred to III, and the braze zone between I and III is referred to II. These results reveal significant changes to the interface reaction I, particularly as a function of the holding time. The thickness of interface reaction layer I increased from 190e260 nm to 230e300 nm and 290e370 nm (see Table 1), as the braze temperature increased from 860 C to 890 C and 920 C; the thickness of interface reaction I increased from 60e130 nm to 230e300 nm, 320e560 nm and 480e750 nm (see Table 1) by prolonging the t from 2min to 30min. It has been discussed that the interface reaction layer I (Hereinafter referred to as reaction layer) is composed of TiN and Ti5Si3 [1,7e9]. In this paper, it is speculated that the reaction layer may be TiN and Ti5Si3, and the XRD patterns give the evidences that the TiN and Ti5Si3 are emerged in the reaction layer. The reaction layer III is thicker than the reaction layer I. The thicker reaction layer III implies an adequate interfacial reaction between AgCuTi and Kovar, and has been discussed in our previous study [15]. These images indicate braze seams II mainly consist of white substrate structures and grey bulk structures (see Fig.3). The composition analysis of Table 2 show that the white substrate
Fig. 1. Schematic diagrams of experiment: (a) the dimensions of Si3N4 ceramic and the brazing assembling; (b) the test method for tensile strength.
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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3.2. The bonding properties of the joints The tensile test was used to evaluate the bonding properties of the joints brazed with AgeCueTi braze alloys. Fig. 6 shows the effects of braze temperature Tb and holding time t on the joint strength of the brazed joints. These results prove that both Tb and t has great effect on the joint strength. The tensile strength of these joints increase as Tb increased from 860 C to 890 C, and maximal tensile strength of 102.0 ± 15.8 MPa is obtained at a Tb of 890 C for 5 min, whereas it decreased to 69.9 ± 4.5 MPa when Tb increased to 950 C. In the other hand, the tensile strength increased from 74.2 ± 5.6 MPa to 102.0 ± 15.8 MPa by appropriate prolonging holding time from 2min to 5min, then the tensile strength decreased by extending the holding time from 5min to 50min. From the microstructural point of view, the significant changes of the joint strength are strongly dependent on the microstructure of braze joints [16,18]. Specifically, both the reaction and the braze seam are the essential factors in determining the joint strength. In addition, the residual stress caused by CTE mismatch between Si3N4 and Kovar may deteriorate the joint strength. 4. Discussion 4.1. The growth of interfacial reaction layer The formation of reaction layer is strongly depended on the holding time t. Fig. 7 shows the relationship between the square (X2) of the thickness of reaction layer and the modified holding time (t). From Fig. 7, the line relationship between X2 and t can be written as [19]. X2 ¼ k t
Fig. 3. The microstructures of the braze joints prepared with AgCu4Ti alloys at varied temperature of 860 C (a), 890 C (b), 920 C (c) for 5 min.
structures is Ag(s, s) and the grey bulk structures is mainly (Ni, Cu) (s, s). The microstructures in the braze seam present significant changes, particularly as a function of the holding time. As shown in Fig. 4(a), only small quantities of grey bulk structures are found. As the holding time increases, the quantities of grey bulk structures increase. By prolonging the holding time to 30min, the grey bulk structures begin dividing into dark grey structures and grey structures, and big quantities of particles are found. These results indicate that the composition of the grey structures changes, which results in the formation of some other phases. According to the composition analysis of Table 3, for the holding time of 30min, the grey structures is mainly composited of (Ni, Cu) (s, s), Ni3Ti and NiTi; the dark particles observed in the vicinity of the reaction layer is (Ni3Ti þ TiSi þ NiSi). These results are also proved by XRD analysis, shown in Fig. 5. From the results of XRD, the relatively stronger diffraction peaks of 3 and 4 in Fig. 5 indicate that larger quantities of TiSi, NiSi compounds are emerged near the reaction layer. By prolonging the holding time to 30min, the quantities of dark particles increased significantly.
(1)
where k is the constant of growth of reaction layer. According to Fig. 7, k1, k2 and k3 can be estimated as 183 ± 8 nm2/s, 362 ± 22 nm2/ s and 564 ± 21 nm2/s at 860 C, 890 C, 920 C, respectively. These results indicate that the formation of reaction layer is controlled by the diffusion with t ranging from 2min (60s) to 10min (540s). The parabolic growth relationship between the thickness X of reaction layer and t indicates that the formation rate of the interfacial reaction layer is determined by the diffusion rate of the atoms. It is well known that the formation of reaction layer is benefited from the diffusion of Ti from the molten AgCuTi to react with Si3N4 [1e3,7e9]. However, once the reaction layer form, Ti atoms have to diffuse through the reaction layer to react with Si3N4 to form a new reaction layer. In this case, the initial reaction layer adjacent to the AgCuTi braze seam became a barrier to block the further diffusion of Ti atoms. Therefore, the Ti atoms have to diffuse through the initial reaction layer by vacancy diffusion mechanism and the growth of the reaction layer is a reaction-diffusion process. Considering the progress of reaction diffusion, there must be adequate energy to overcome the diffusion activation energy (Q) provided for Ti atoms diffusing through the reaction layer. As for the AgCu4Ti braze system, the value of Q can be calculated from the formula [19]. InD ¼ InD0 e Q/(RTb)
(2)
Where D 0 is the diffusion constant, R is gas constant and D is diffusion coefficient. In this system, the growth rate of the reaction layer lies on the diffusion rate of Ti atoms through initial reaction layer, therefore, the value of D is approximately equal to the value of k in equation (1). Therefore, Q is estimated as 170.9e248.7 kJ/mol and D 0 is about 9.62 10 7 m 2 /s based on above analysis (see Fig. 7). Through this research, the
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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Fig. 4. The microstructures of the braze joints prepared with AgCu4Ti alloys at a temperature of 890 C for a varied holding time: (a) 2min, (b) 5min, (c) 10min, (d) 30min.
Table 1 The thickness of reaction layer at varied temperatures for different holding time. The holding time/min
The modified time/s
2 5 10 30
The braze temperature
60s 240s 540s 1740s
860 C
890 C
920 C
50e100 nm 190e260 nm 180e420 nm 390e510 nm
60e130 nm 230e300 nm 320e560 nm 480e750 nm
70e140 nm 290e370 nm 390e630 nm 650e900 nm
Table 2 Compositions (at. %) and possible phases in different positions shown in Fig. 3 (b). EDS
Fe
Co
Ni
Ag
Cu
Ti
Si
N
possible phases
A B C
e 1.66 0.85
e 1.05 0.50
e 11.81 0.71
2.17 12.30 67.43
1.85 65.85 30.01
6.82 4.39 0.50
25.41 2.93 e
63.75 0.02 e
TiN þ Ti5Si3 (Ni,Cu) (s, s)þ trace IMCs Ag(s, s)
Table 3 Compositions (at. %) and possible phases in point B in part II. EDS
Fe
Co
Ni
Ag
Cu
Ti
Si
N
possible phases
B B1 B2 B3
1.66 6.03 8.32 4.08
1.05 5.63 5.78 e
11.81 39.27 53.51 52.01
12.30 e 0.03 1.13
65.85 30.45 8.93 8.47
4.39 18.62 23.43 18.87
2.93 e e 15.44
0.02 e e e
(Ni, Cu) (s, s)þ trace IMCs (Ni, Cu) (s, s)þ Ni3Ti þ NiTi (Ni, Cu) (s, s)þ Ni3Ti þ NiTi Ni3Ti þ TiSi þ NiSi
growth of interfacial reaction layer can be described as followings In D ¼ (2.056~-2.992) 104 1/Tb 13.854 (3)
When t is in the range of 10e30min, some amounts of Si and Ni is found in the braze seam, mainly distributed in IMCs zone, which indicate the diffusion of Si and Ni to the braze seam (see Fig. 8). Based on above analysis, the present of Si and Ni in the position of
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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Fig. 5. The XRD patterns in the braze joint at a temperature of 890 C for 30min: 1, 2, 3, 4 is the patterns of parallel planes of the braze seam after layer by layer polishing from the left side to the right side of the braze seam.
Fig. 7. The formation of interfacial reaction layer is strongly depended on diffusion: (a) the relationship between the thickness (X2) and the holding time, (b) the relationship between Natural logarithm of diffusion coefficient (In D) and the reciprocal of brazing temperature (T1 b ).
the IMCs suggest the formation of (NiSi þ Ni3Ti þ TiSi þ FeSi2) compounds. Formation of IMCs may decrease the activity of Ti, resulting in the decrease of diffusion coefficient of Ti in the reaction layer. Therefore, the growth rate of the reaction layer dropped in the range of 10e30min.
4.2. The formation mechanism of the joints The reaction layer between AgCuTi and Si3N4 consists of two layers: the inner layer is TiN; the outer layer is Ti5Si3 (see Fig. 8), which agrees well with others’ investigations in AgCuTi/Si3N4 system [7e9,20]. The formation of reaction layers is benefited from diffusion of Ti. The driving force for the diffusion is from reduction of systemic chemical potential energy [21]. At the initial stage of brazing process, adequate quantity of Ti is enriched at AgCuTi/Si3N4 interface, and the following reaction occurred. 4Ti þ Si3N4 / 4TiN þ 3Si
Fig. 6. Effects of braze temperature Tb and holding time t on the joint strength of the braze joints.
(4)
The formation of TiN is feasible thermodynamically, which has been proved by Song et al. [2] and Zhang et al. [9]. As the reaction proceeds, the TiN reaction layer forms and the Si atoms diffuse through the TiN layer into the molten braze alloys. Therefore, the Ti 5 Si 3 compounds form adjacent to TiN surface. As t is prolonged, large quantities of Ni, Fe atoms will
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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Fig. 9. Gibbs formation free energy of TiSi, Ti5Si3 and NiSi compounds.
4.3. The fracture mechanism of the joint The residual stresses in the joints arise from the shrinkage of metals on cooling from Tb. Considering the relationship between stress and deformation, the residual thermal stress s in ceramic side can be estimated by the following formula [22].
s¼ε E ¼
Fig. 8. The cross-sectional TEM micrographs of interface between braze seam and Si3N4 at Tb of 890 C for 30min: (a) is a HAADF image; (b) is the corresponding electoral energy mapping; (c), (d) is the electron-diffraction patterns from zone C, D in figure (a), respectively.
Da$DT$ Em$Ec$tm $ ð1 nÞ ðEmtm þ EctcÞ
(8)
where Da is the difference of thermal expansion coefficients, DT is the temperature change from the braze temperature to room temperature, E and t represent the Young’s modulus and thickness of ceramic or metal layer, n is the Poisson’s ratio of ceramic. From this estimation formula, the residual thermal stress s is related to Tb. The higher Tb is, the greater the residual thermal stress is. Thus, with Tb increasing, the residual thermal stress s increases. The stress dependence of the Raman bands of silicon nitride [23] also proved the increasing of residual thermal stress with the increasing of Tb (see Fig. 10). Increasing of residual thermal stresses will greatly affect the bonding strength of the joint. At higher brazing temperature, crack may be induced due to the huge residual thermal
dissolve in the molten braze alloys. In this case, Ni, Ti will react with Si to form NiSi þ TiSi þ Ti 5 Si 3 compounds through the following reaction Ni þ Si / NiSi
(5)
Ti þ Si / TiSi
(6)
5/3Ti þ Si /1/3Ti5Si3
(7)
The Gibbs formation free energies of these compounds in the range of 298 Ke1200 K are calculated (see Fig. 9) [21], these results predict the possibility of these compounds. The present of black particle phase in Fig. 4(d) demonstrated the formation of these compounds, which agree well with the analysis of XRD (Fig. 5) and the observation of TEM (Fig. 8). The formation of TieSi compounds reduce the Ti concentration in the molten braze alloys, therefore the diffusion capacity of Ti decrease and the growth rate of the reaction layer decline.
Fig. 10. Raman spectra of the Si3N4 material at the outer edge of Si3N4ceramics and adjacent to the interface: the stresses dependence of the band at 862 cm1 indicates that with Tb increasing (see Ref. [23]), the residual thermal stress s increases.
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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huge residual thermal stresses is usually in ceramic interior, while fracture induced by IMCs is always in braze seam. Therefore, the macro fracture is always different from the arc-shaped profile (see Fig. 13). When the holding time is short, the interfacial reaction is inadequate leading to a weaker interfacial reaction layer, which means that the crack germinates at the interfacial reaction layer. Properly prolonging the holding time can improve the interfacial bonding strength. However, excessive prolongation of holding time can weaken the bonding strength because the quantity of IMCs increase as the holding time is prolonged. When Tb is lower, thermal stress is small and has little effect on tensile failure. However, when the Tb is higher, thermal stress become bigger and micro cracks may form in ceramic interior.
Fig. 11. Larger-range lattice strain caused by residual thermal stress in Ag (s, s) zone of Fig. 8
stresses, which results in a weaker bonding (see the joint strength in Fig. 6(a)). Residual thermal stress generate during cooling from Tb because of the mismatch of CTE, thus holding times have slight influence on the generation of residual thermal stress. It has been generally accepted the braze alloy (Ag) in the seam is expected to help mitigate the mismatch in CTE by stresses relaxation or the creep deformation [10]. In this paper, the high resolution image of Ag(s,s) presents larger-range lattice strain, which proves that the effect of Ag on relieving thermal stress (see Fig. 11). However, the IMCs will form in the braze seam by prolonging the holding time. The structural characteristics of this compound suggest that it would be difficult to deform. The micro cracks in Fig. 8 may provide the evidence that IMCs can induce the crack propagation. In this case, the micro cracks will form and propagate. Therefore, prolonging the holding time can increase the quantities of IMCs in the braze zone and deteriorate the joint strength (see the joint strength in Fig. 6(b)). The residual thermal stresses in Si3N4 ceramics are tensile conditions. The maximum value of smax is at the outer edge of Si3N4ceramics and adjacent to the interface (see Fig. 12). Significant stress concentrates on this position under the effect of residual thermal stress (smax) and tensile tests (stest), which will result in crack nucleation. As a result, the direction of crack propagation is at a certain angle to the metal-ceramic interface, which can be defined as radial angle (q) to describe the direction of crack propagation (Fig. 12). Under normal circumstances, the crack growth profile is arc-shaped (Fig. 12). However, the position of fracture induced by
5. Conclusions The braze joints of Si3N4-Kovar using AgCuTi braze alloys are investigated systematically. The conclusions drawn in this study are as follows. (1) A continuous interfacial reaction layer formed at AgCu4Ti/ Si3N4 interface, which played an essential role in determining the joint strength. The formation of reaction layer benefited from the diffusion of Ti from the molten AgCuTi to react with Si3N4. Growth of the reaction layer is a reactiondiffusion process. Once the reaction layer form, Ti atoms have to diffuse through the reaction layer to react with Si3N4 to form a new reaction layer. The diffusion activation energy (Q) provided for Ti atoms diffusing through the reaction layer is estimated as 170.5e246.9 kJ/mol. However, as the holding time was further prolonged (in the range of 10e30min), IMCs forms in the braze seam, which decreases the activity of Ti and leads to the decrease of the diffusion coefficient of Ti in the reaction layer. Therefore, the growth rate of the reaction layer dropped in the range of 10e30min. (2) Residual thermal stresses increase as the braze temperatures increase, which will greatly affect the bonding strength of the joint. At higher brazing temperature, crack initiation is mainly due to the huge residual thermal stresses, which results in a weaker bonding. Prolonging the holding time can increase the quantities of IMCs in the braze zone. The braze alloy (Ag) in the seam is expected to help mitigate the mismatch in CTE. With the increasing of IMCs in the braze zone, the ability of braze seam to relieve interfacial stress becomes weaker, therefore, prolonging the holding time can decrease the bonding strength of the joint.
Fig. 12. (a) Vectors summation of residual thermal stress and tensile tests at interface; (b) crack propagation direction.
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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[2] [3]
[4]
[5]
[6]
Fig. 13. Macro fracture morphology analysis: (a) fracture induced from huge residual thermal stresses from typical sample brazed at 950 C for 5min; (b) fracture induced from IMCs from sample brazed at 890 C for 50min.
Author contribution Chenglai Xin: Data curation, Writing- Original draft preparation, Conceptualization. Jiazhen Yan: Visualization, Investigation, Conceptualization. Qingyuan Wang: Software, Validation. Wei Feng: Supervision, Validation. Chengyun Xin: Writing - Review & Editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[7]
[8]
[9]
[10]
[11]
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[14]
[15]
Acknowledgements [16]
This work was mainly supported financially by the National Natural Science Foundation of China (No. 51905050). This work was also supported financially by the other National Natural Science Foundation of China (No. 11832007) and the Applied Basic Research Programs of Sichuan province (Grant No. 2018JY0062).
[17]
[18]
Appendix A. Supplementary data
[19] [20]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.153189.
[21]
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Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The microstructural evolution and formation mechanism in Si3N4/ AgCuTi/Kovar braze joints Chenglai Xin a, Jiazhen Yan b, *, Qingyuan Wang a, Wei Feng a, Chengyun Xin c a
School of Mechanical Engineering, Chengdu University, Chengdu, 610065, PR China School of Manufacturing Science and Engineering, Sichuan University, Chengdu, 610065, PR China c School of Electric Power Engineering, China University of Mining and Technology, Xuzhou, 221116, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2019 Received in revised form 25 November 2019 Accepted 25 November 2019 Available online xxx
The braze joints of Si3N4-Kovar using AgCuTi braze alloys are investigated systematically with the brazing temperatures between 860 C and 950 C for 2e30min. The correlation between the joint strength and the microstructures of the braze joints is discussed. These results show that the reaction layer plays an important role in interface bonding. Growth of the reaction layer is a reaction-diffusion process, which mainly depends on the diffusion of Ti. The diffusion activation energy (Q) is estimated as 170.9e248.7 kJ/ mol in this system, which may provide data support for Si3N4/metal braze joints. As the holding time is further prolonged (in the range of 10e30min), intermetallic compounds (IMCs) forms in the braze seam, which will prevent Ag solid solution from alleviating interfacial thermal stress. With the increasing of the brazing temperature, the residual thermal stresses increases. As a results, larger residual thermal stress is produced in the braze joints at higher braze temperature, which result in a weaker bonding. © 2019 Elsevier B.V. All rights reserved.
Keywords: Si3N4 Kovar alloy Microstructural evolution Braze joints
1. Introduction Silicon nitride ceramics (Si3N4) are often joined to metals to form reliable assemblies in a wide variety of fields including aviation, aerospace and electronics industry, which have been received extensive attentions [1e3]. In that case, strong and reliable joints of Si3N4-metal have to be obtained for engineering applications. Braze joint of Si3N4-metal is difficult due to the poor wettability and the residual stresses in the joint generated by mismatch of the coefficients of thermal expansion (CTE) [4,5]. To solve these problems, various joint methods including active metal brazing bonding (AMB), diffusion bonding, and partial transient-liquid-phase (TLP) bonding have been developed. Among these methods, active metal brazing is one of the most important methods for fabricating assemblies [1e5], and the joints of Si3N4-metal have been studied extensively in recent years. Active metal brazing is usually performed using AgCu eutectic alloys containing a small amount of Ti, an element that promotes wetting and adhesion [4,6]. It is proved that the active element Ti always migrates to Si3N4 sides to form a reaction layer during the brazing process [7e9]. Electron microscopy investigations on the
* Corresponding author. E-mail address: [email protected] (J. Yan).
interfacial reaction at AgCuTi/Si3N4 interfaces show that the reaction products consisted of upper Ti5Si3 and lower TiN layers [7e9]. Zhang et al. [7,8] study the interfacial microstructure in Si3N4/ 42CrMo joints and their results show that a better interfacial microstructure can improve the interfacial bonding strength [10]. Yang et al. [11,12] prove that the formation of soft solid solution microstructure (e.g., Cu solid solution) in braze seam can relieve residual thermal stress by plastic deformation, and therefore the bonding strength of the joints are improved. The research results of Zhang et al. [7,13] show that the addition of TiN particles or Ni reduced the CTE of the braze seam and played a significant role in strengthening the braze joints. Most of these researches [1,10,14e16] consider that the microstructure of the braze seam can affect the mechanical property of brazed joint and plays an important role in alleviating residual thermal by stress relaxation or creep deformation. Nevertheless, little study is focused on the microstructural evolution of braze seams and formation mechanism of the interfacial reaction layer in Si3N4-metal joint systems. Through the above reviews, it is very necessary to study the microstructural evolution and formation mechanism on the interfacial reaction layer of the Si3N4-metal joints, and then to study the effects of the microstructures on relieving residual thermal stress. In this study, Si3N4 ceramics were brazed to Kovar alloys (a lowexpansion FeeCoeNi alloy used extensively in electronic
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Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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applications [17]) using AgCuTi alloys. The joint strength and microstructures of the braze joints were investigated systematically. Through this study, the microstructural evolution and formation mechanism of the interfacial reaction layer in Si3N4-Kovar joint systems will be explored, which is of great significance for the knowledge enrichment of the formation mechanism of braze joints. 2. Materials and experimental procedure Silicon nitride ceramics (Si3N4) with a purity of >98 wt % are used as the substrate material. The average surface roughness of Si3N4 measured by surface roughness detector is 3.5 mm. The dimensions of Si3N4 are shown in Fig. 1 (a). 4J33-Kovar alloys (Fe33 wt %Ni-17 wt %Co) are used as the base material for braze joint. The 4J33-Kovar alloy is in the form of ring with an outer diameter of 16 mm, an inner diameter of 9.8 mm and a thickness of 1 mm. The braze alloy is commercial Ag-28 wt% Cu eutectic alloy with an addition of 4 wt% Ti (AgCu4Ti). The test method for tensile strength is shown in Fig. 1. The substrates and Kovar alloys were ultrasonically cleaned in alcohol for 10min, and then were coated by the braze alloys on the contact interface. The braze experiments were performed in vacuum brazing furnace with a vacuum degree of 3.5 103 Pa under the effect of the diffusion pump chamber. The braze process parameters (see Fig. 2) was set at different brazing temperatures (Tb) with varied holding time (t). In order to test tensile strength of the joints, the braze specimen were assembled in the well-designed ‘sandwich’ configuration. Microstructural characterization of the braze joints are analyzed by the X-ray diffraction (XRD, X’Pert PRO) with Cu Ka radiation, the scanning electron microscopy (SEM, HITACHI S-4800) equipped with an energy dispersive X-ray (EDX) analyzer and transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN). The thin foils for transmission electron microscopy (TEM) observation of the evolved microstructures in the brazing seams are prepared using focused ion beam (FIB) technique (HELIOS Nanolab600i, FEI, USA). The tension tests are performed using electronic universal testing machine (RGX-M300) at a speed of 0.2 mm/min, in which the value of the tensile specimen in each group is measured five times repeatedly and then the average value is obtained in order to ensure the accuracy. 3. Results 3.1. The microstructures in the braze joints The microstructures of the braze joints prepared with AgCu4Ti
Fig. 2. The heating cycling curve for braze joints.
alloys at varied temperature of 860 C, 890 C, 920 C for 5min are shown in Fig. 3. The microstructures of the braze joints at a temperature of 890 C for a varied holding time of 2min, 5min, 10min, 30min are shown in Fig. 4. These images show that the joints are mainly composed of three parts: a continuous interfacial reaction layer formed at AgCu4Ti/Si3N4 is referred to I, an interfacial reaction layer formed between Kovar and AgCu4Ti is referred to III, and the braze zone between I and III is referred to II. These results reveal significant changes to the interface reaction I, particularly as a function of the holding time. The thickness of interface reaction layer I increased from 190e260 nm to 230e300 nm and 290e370 nm (see Table 1), as the braze temperature increased from 860 C to 890 C and 920 C; the thickness of interface reaction I increased from 60e130 nm to 230e300 nm, 320e560 nm and 480e750 nm (see Table 1) by prolonging the t from 2min to 30min. It has been discussed that the interface reaction layer I (Hereinafter referred to as reaction layer) is composed of TiN and Ti5Si3 [1,7e9]. In this paper, it is speculated that the reaction layer may be TiN and Ti5Si3, and the XRD patterns give the evidences that the TiN and Ti5Si3 are emerged in the reaction layer. The reaction layer III is thicker than the reaction layer I. The thicker reaction layer III implies an adequate interfacial reaction between AgCuTi and Kovar, and has been discussed in our previous study [15]. These images indicate braze seams II mainly consist of white substrate structures and grey bulk structures (see Fig.3). The composition analysis of Table 2 show that the white substrate
Fig. 1. Schematic diagrams of experiment: (a) the dimensions of Si3N4 ceramic and the brazing assembling; (b) the test method for tensile strength.
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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3.2. The bonding properties of the joints The tensile test was used to evaluate the bonding properties of the joints brazed with AgeCueTi braze alloys. Fig. 6 shows the effects of braze temperature Tb and holding time t on the joint strength of the brazed joints. These results prove that both Tb and t has great effect on the joint strength. The tensile strength of these joints increase as Tb increased from 860 C to 890 C, and maximal tensile strength of 102.0 ± 15.8 MPa is obtained at a Tb of 890 C for 5 min, whereas it decreased to 69.9 ± 4.5 MPa when Tb increased to 950 C. In the other hand, the tensile strength increased from 74.2 ± 5.6 MPa to 102.0 ± 15.8 MPa by appropriate prolonging holding time from 2min to 5min, then the tensile strength decreased by extending the holding time from 5min to 50min. From the microstructural point of view, the significant changes of the joint strength are strongly dependent on the microstructure of braze joints [16,18]. Specifically, both the reaction and the braze seam are the essential factors in determining the joint strength. In addition, the residual stress caused by CTE mismatch between Si3N4 and Kovar may deteriorate the joint strength. 4. Discussion 4.1. The growth of interfacial reaction layer The formation of reaction layer is strongly depended on the holding time t. Fig. 7 shows the relationship between the square (X2) of the thickness of reaction layer and the modified holding time (t). From Fig. 7, the line relationship between X2 and t can be written as [19]. X2 ¼ k t
Fig. 3. The microstructures of the braze joints prepared with AgCu4Ti alloys at varied temperature of 860 C (a), 890 C (b), 920 C (c) for 5 min.
structures is Ag(s, s) and the grey bulk structures is mainly (Ni, Cu) (s, s). The microstructures in the braze seam present significant changes, particularly as a function of the holding time. As shown in Fig. 4(a), only small quantities of grey bulk structures are found. As the holding time increases, the quantities of grey bulk structures increase. By prolonging the holding time to 30min, the grey bulk structures begin dividing into dark grey structures and grey structures, and big quantities of particles are found. These results indicate that the composition of the grey structures changes, which results in the formation of some other phases. According to the composition analysis of Table 3, for the holding time of 30min, the grey structures is mainly composited of (Ni, Cu) (s, s), Ni3Ti and NiTi; the dark particles observed in the vicinity of the reaction layer is (Ni3Ti þ TiSi þ NiSi). These results are also proved by XRD analysis, shown in Fig. 5. From the results of XRD, the relatively stronger diffraction peaks of 3 and 4 in Fig. 5 indicate that larger quantities of TiSi, NiSi compounds are emerged near the reaction layer. By prolonging the holding time to 30min, the quantities of dark particles increased significantly.
(1)
where k is the constant of growth of reaction layer. According to Fig. 7, k1, k2 and k3 can be estimated as 183 ± 8 nm2/s, 362 ± 22 nm2/ s and 564 ± 21 nm2/s at 860 C, 890 C, 920 C, respectively. These results indicate that the formation of reaction layer is controlled by the diffusion with t ranging from 2min (60s) to 10min (540s). The parabolic growth relationship between the thickness X of reaction layer and t indicates that the formation rate of the interfacial reaction layer is determined by the diffusion rate of the atoms. It is well known that the formation of reaction layer is benefited from the diffusion of Ti from the molten AgCuTi to react with Si3N4 [1e3,7e9]. However, once the reaction layer form, Ti atoms have to diffuse through the reaction layer to react with Si3N4 to form a new reaction layer. In this case, the initial reaction layer adjacent to the AgCuTi braze seam became a barrier to block the further diffusion of Ti atoms. Therefore, the Ti atoms have to diffuse through the initial reaction layer by vacancy diffusion mechanism and the growth of the reaction layer is a reaction-diffusion process. Considering the progress of reaction diffusion, there must be adequate energy to overcome the diffusion activation energy (Q) provided for Ti atoms diffusing through the reaction layer. As for the AgCu4Ti braze system, the value of Q can be calculated from the formula [19]. InD ¼ InD0 e Q/(RTb)
(2)
Where D 0 is the diffusion constant, R is gas constant and D is diffusion coefficient. In this system, the growth rate of the reaction layer lies on the diffusion rate of Ti atoms through initial reaction layer, therefore, the value of D is approximately equal to the value of k in equation (1). Therefore, Q is estimated as 170.9e248.7 kJ/mol and D 0 is about 9.62 10 7 m 2 /s based on above analysis (see Fig. 7). Through this research, the
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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Fig. 4. The microstructures of the braze joints prepared with AgCu4Ti alloys at a temperature of 890 C for a varied holding time: (a) 2min, (b) 5min, (c) 10min, (d) 30min.
Table 1 The thickness of reaction layer at varied temperatures for different holding time. The holding time/min
The modified time/s
2 5 10 30
The braze temperature
60s 240s 540s 1740s
860 C
890 C
920 C
50e100 nm 190e260 nm 180e420 nm 390e510 nm
60e130 nm 230e300 nm 320e560 nm 480e750 nm
70e140 nm 290e370 nm 390e630 nm 650e900 nm
Table 2 Compositions (at. %) and possible phases in different positions shown in Fig. 3 (b). EDS
Fe
Co
Ni
Ag
Cu
Ti
Si
N
possible phases
A B C
e 1.66 0.85
e 1.05 0.50
e 11.81 0.71
2.17 12.30 67.43
1.85 65.85 30.01
6.82 4.39 0.50
25.41 2.93 e
63.75 0.02 e
TiN þ Ti5Si3 (Ni,Cu) (s, s)þ trace IMCs Ag(s, s)
Table 3 Compositions (at. %) and possible phases in point B in part II. EDS
Fe
Co
Ni
Ag
Cu
Ti
Si
N
possible phases
B B1 B2 B3
1.66 6.03 8.32 4.08
1.05 5.63 5.78 e
11.81 39.27 53.51 52.01
12.30 e 0.03 1.13
65.85 30.45 8.93 8.47
4.39 18.62 23.43 18.87
2.93 e e 15.44
0.02 e e e
(Ni, Cu) (s, s)þ trace IMCs (Ni, Cu) (s, s)þ Ni3Ti þ NiTi (Ni, Cu) (s, s)þ Ni3Ti þ NiTi Ni3Ti þ TiSi þ NiSi
growth of interfacial reaction layer can be described as followings In D ¼ (2.056~-2.992) 104 1/Tb 13.854 (3)
When t is in the range of 10e30min, some amounts of Si and Ni is found in the braze seam, mainly distributed in IMCs zone, which indicate the diffusion of Si and Ni to the braze seam (see Fig. 8). Based on above analysis, the present of Si and Ni in the position of
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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Fig. 5. The XRD patterns in the braze joint at a temperature of 890 C for 30min: 1, 2, 3, 4 is the patterns of parallel planes of the braze seam after layer by layer polishing from the left side to the right side of the braze seam.
Fig. 7. The formation of interfacial reaction layer is strongly depended on diffusion: (a) the relationship between the thickness (X2) and the holding time, (b) the relationship between Natural logarithm of diffusion coefficient (In D) and the reciprocal of brazing temperature (T1 b ).
the IMCs suggest the formation of (NiSi þ Ni3Ti þ TiSi þ FeSi2) compounds. Formation of IMCs may decrease the activity of Ti, resulting in the decrease of diffusion coefficient of Ti in the reaction layer. Therefore, the growth rate of the reaction layer dropped in the range of 10e30min.
4.2. The formation mechanism of the joints The reaction layer between AgCuTi and Si3N4 consists of two layers: the inner layer is TiN; the outer layer is Ti5Si3 (see Fig. 8), which agrees well with others’ investigations in AgCuTi/Si3N4 system [7e9,20]. The formation of reaction layers is benefited from diffusion of Ti. The driving force for the diffusion is from reduction of systemic chemical potential energy [21]. At the initial stage of brazing process, adequate quantity of Ti is enriched at AgCuTi/Si3N4 interface, and the following reaction occurred. 4Ti þ Si3N4 / 4TiN þ 3Si
Fig. 6. Effects of braze temperature Tb and holding time t on the joint strength of the braze joints.
(4)
The formation of TiN is feasible thermodynamically, which has been proved by Song et al. [2] and Zhang et al. [9]. As the reaction proceeds, the TiN reaction layer forms and the Si atoms diffuse through the TiN layer into the molten braze alloys. Therefore, the Ti 5 Si 3 compounds form adjacent to TiN surface. As t is prolonged, large quantities of Ni, Fe atoms will
Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189
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Fig. 9. Gibbs formation free energy of TiSi, Ti5Si3 and NiSi compounds.
4.3. The fracture mechanism of the joint The residual stresses in the joints arise from the shrinkage of metals on cooling from Tb. Considering the relationship between stress and deformation, the residual thermal stress s in ceramic side can be estimated by the following formula [22].
s¼ε E ¼
Fig. 8. The cross-sectional TEM micrographs of interface between braze seam and Si3N4 at Tb of 890 C for 30min: (a) is a HAADF image; (b) is the corresponding electoral energy mapping; (c), (d) is the electron-diffraction patterns from zone C, D in figure (a), respectively.
Da$DT$ Em$Ec$tm $ ð1 nÞ ðEmtm þ EctcÞ
(8)
where Da is the difference of thermal expansion coefficients, DT is the temperature change from the braze temperature to room temperature, E and t represent the Young’s modulus and thickness of ceramic or metal layer, n is the Poisson’s ratio of ceramic. From this estimation formula, the residual thermal stress s is related to Tb. The higher Tb is, the greater the residual thermal stress is. Thus, with Tb increasing, the residual thermal stress s increases. The stress dependence of the Raman bands of silicon nitride [23] also proved the increasing of residual thermal stress with the increasing of Tb (see Fig. 10). Increasing of residual thermal stresses will greatly affect the bonding strength of the joint. At higher brazing temperature, crack may be induced due to the huge residual thermal
dissolve in the molten braze alloys. In this case, Ni, Ti will react with Si to form NiSi þ TiSi þ Ti 5 Si 3 compounds through the following reaction Ni þ Si / NiSi
(5)
Ti þ Si / TiSi
(6)
5/3Ti þ Si /1/3Ti5Si3
(7)
The Gibbs formation free energies of these compounds in the range of 298 Ke1200 K are calculated (see Fig. 9) [21], these results predict the possibility of these compounds. The present of black particle phase in Fig. 4(d) demonstrated the formation of these compounds, which agree well with the analysis of XRD (Fig. 5) and the observation of TEM (Fig. 8). The formation of TieSi compounds reduce the Ti concentration in the molten braze alloys, therefore the diffusion capacity of Ti decrease and the growth rate of the reaction layer decline.
Fig. 10. Raman spectra of the Si3N4 material at the outer edge of Si3N4ceramics and adjacent to the interface: the stresses dependence of the band at 862 cm1 indicates that with Tb increasing (see Ref. [23]), the residual thermal stress s increases.
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huge residual thermal stresses is usually in ceramic interior, while fracture induced by IMCs is always in braze seam. Therefore, the macro fracture is always different from the arc-shaped profile (see Fig. 13). When the holding time is short, the interfacial reaction is inadequate leading to a weaker interfacial reaction layer, which means that the crack germinates at the interfacial reaction layer. Properly prolonging the holding time can improve the interfacial bonding strength. However, excessive prolongation of holding time can weaken the bonding strength because the quantity of IMCs increase as the holding time is prolonged. When Tb is lower, thermal stress is small and has little effect on tensile failure. However, when the Tb is higher, thermal stress become bigger and micro cracks may form in ceramic interior.
Fig. 11. Larger-range lattice strain caused by residual thermal stress in Ag (s, s) zone of Fig. 8
stresses, which results in a weaker bonding (see the joint strength in Fig. 6(a)). Residual thermal stress generate during cooling from Tb because of the mismatch of CTE, thus holding times have slight influence on the generation of residual thermal stress. It has been generally accepted the braze alloy (Ag) in the seam is expected to help mitigate the mismatch in CTE by stresses relaxation or the creep deformation [10]. In this paper, the high resolution image of Ag(s,s) presents larger-range lattice strain, which proves that the effect of Ag on relieving thermal stress (see Fig. 11). However, the IMCs will form in the braze seam by prolonging the holding time. The structural characteristics of this compound suggest that it would be difficult to deform. The micro cracks in Fig. 8 may provide the evidence that IMCs can induce the crack propagation. In this case, the micro cracks will form and propagate. Therefore, prolonging the holding time can increase the quantities of IMCs in the braze zone and deteriorate the joint strength (see the joint strength in Fig. 6(b)). The residual thermal stresses in Si3N4 ceramics are tensile conditions. The maximum value of smax is at the outer edge of Si3N4ceramics and adjacent to the interface (see Fig. 12). Significant stress concentrates on this position under the effect of residual thermal stress (smax) and tensile tests (stest), which will result in crack nucleation. As a result, the direction of crack propagation is at a certain angle to the metal-ceramic interface, which can be defined as radial angle (q) to describe the direction of crack propagation (Fig. 12). Under normal circumstances, the crack growth profile is arc-shaped (Fig. 12). However, the position of fracture induced by
5. Conclusions The braze joints of Si3N4-Kovar using AgCuTi braze alloys are investigated systematically. The conclusions drawn in this study are as follows. (1) A continuous interfacial reaction layer formed at AgCu4Ti/ Si3N4 interface, which played an essential role in determining the joint strength. The formation of reaction layer benefited from the diffusion of Ti from the molten AgCuTi to react with Si3N4. Growth of the reaction layer is a reactiondiffusion process. Once the reaction layer form, Ti atoms have to diffuse through the reaction layer to react with Si3N4 to form a new reaction layer. The diffusion activation energy (Q) provided for Ti atoms diffusing through the reaction layer is estimated as 170.5e246.9 kJ/mol. However, as the holding time was further prolonged (in the range of 10e30min), IMCs forms in the braze seam, which decreases the activity of Ti and leads to the decrease of the diffusion coefficient of Ti in the reaction layer. Therefore, the growth rate of the reaction layer dropped in the range of 10e30min. (2) Residual thermal stresses increase as the braze temperatures increase, which will greatly affect the bonding strength of the joint. At higher brazing temperature, crack initiation is mainly due to the huge residual thermal stresses, which results in a weaker bonding. Prolonging the holding time can increase the quantities of IMCs in the braze zone. The braze alloy (Ag) in the seam is expected to help mitigate the mismatch in CTE. With the increasing of IMCs in the braze zone, the ability of braze seam to relieve interfacial stress becomes weaker, therefore, prolonging the holding time can decrease the bonding strength of the joint.
Fig. 12. (a) Vectors summation of residual thermal stress and tensile tests at interface; (b) crack propagation direction.
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Fig. 13. Macro fracture morphology analysis: (a) fracture induced from huge residual thermal stresses from typical sample brazed at 950 C for 5min; (b) fracture induced from IMCs from sample brazed at 890 C for 50min.
Author contribution Chenglai Xin: Data curation, Writing- Original draft preparation, Conceptualization. Jiazhen Yan: Visualization, Investigation, Conceptualization. Qingyuan Wang: Software, Validation. Wei Feng: Supervision, Validation. Chengyun Xin: Writing - Review & Editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements [16]
This work was mainly supported financially by the National Natural Science Foundation of China (No. 51905050). This work was also supported financially by the other National Natural Science Foundation of China (No. 11832007) and the Applied Basic Research Programs of Sichuan province (Grant No. 2018JY0062).
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.153189.
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Please cite this article as: C. Xin et al., The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153189