Study on properties of AgCuInTi filler metal for active bonding of alumina ceramic

doi:10.21203/rs.3.rs-3746001/v1

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Study on properties of AgCuInTi filler metal for active bonding of alumina ceramic

https://doi.org/10.21203/rs.3.rs-3746001/v1

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In this paper, a novel AgCuInTi20-20-4.5 active filler paste for Al₂O₃ bonding was prepared using the route of vacuum smelting and centrifugal atomizing powder production. The aim of the research was to characterize the AgCuInTi filler paste, and to study the direct brazing of Al₂O₃ ceramics and copper. The microstructure and melting properties of AgCuInTi powder was analyzed using scanning electron microscopy (SEM) and differential scanning calorimeter (DSC). The brazing process was carried out on vacuum brazing furnace at 700°C for 10 min. The joint microstructures were studied by means of SEM and EDS. The results show that Ti reacted with Al₂O₃ as the active element at the interface between ceramic and filler metal and finally realizing the metallurgical combination. In participated in the reaction with Ag and Cu. The main products in the brazing seam are TiO₂, Cu₃TiO₄, Cu₄Ti, Ag₃In and Cu₄In based on XRD. The mean value of tensile strength of the brazed joints is approximately 89 MPa. The studied AgCuInTi possesses excellent joining characteristics with several metallic and ceramic materials.

ceramic-copper joint

brazing

AgCuInTi

microstructure

mechanical properties

Alumina ceramics are often joined to metals to form assemblies with a diverse range of applications, such as vacuum feedthrough technology for high voltage and pressure environments, semiconductorhousing and accelerator, plasma and laser technologies ^[1]. There are two main issues affecting the mechanical performance of a ceramic-metal joint: the bond quality, which depends the ability of the filler to wet the ceramic surface, and the residual stresses which develop in the assembly after brazing. During the last decade, various methods for fabricating alumina ceramic to metal joints have been developed, such as diffusion bonding, soldering, and brazing with or without ultrasonic waves ^[2]. As a convenient and economical method, brazing has been employed extensively for alumina/metal joining ^[3]. AgCuInTi active braze alloy is one of the promising alternatives in metal-ceramic joining. Elements indium and titanium were selected because both are active metals. The melting point of AgCuInTi filler is lower than that of traditional Ag-based alloys, and it has been successfully used for brazing ceramics and ceramics or ceramics and metals ^{[4, 5]}. It is reported that the addition of In element can lower the brazing temperature of the filler, enhance the weldability of the filler, and effectively reduce the effect of residual thermal stress of the brazed joints ^[6]. M. Galli et al.^[7] conducted the brazing of ceramic to steel with modified Ag61.5Cu23.5In15 alloy by adding Ti in different amounts: 0.5, 1.0 and 1.5 wt.%. Zhao et al. ^[6] used Ag23.4Cu14.3In3.5Ti filler in C_f/SiC composite and Invar joining and the maximum shear strength of the joint was about 47 MPa at 720 ℃ for 10 min.

In this study, a novel AgCuInTi20-20-4.5 alloy paste for multilevel package was prepared. This alloy is intended for lower temperature brazing, which conventional AgCuInTi alloys (e.g., Ag23.5Cu14.5In3.5Ti and 72.5-19.5-5-3) do not cover ^[8]. The amount of element In used varied between 20 to 26 wt %. The optimum amount of element In in brazed alloy is determined according to the differential thermal analysis results. Meanwhile, 4.5 wt % of Ti was proposed. This AgCuInTi20-20-4.5 alloy was used to for Al₂O₃/Cu brazing at 700°C for 10 min using vacuum brazing furnace. The microstructural evolution, melting characteristic of the alloy and the interfacial microstructures of joints were analyzed. Joint properties and fracture mode were also investigated in detail.

2.1 Materials

The commercial alumina of 99.5% purity in the form of disks Φ15 mm×3 mm and the copper plates with 4N purity of dimensions Φ15 mm×1 mm were used as parent materials for joint interface observation. The samples for tensile strength test were cut into the proper size according to SJ/T 3326 − 2016 ^[9] and assembled with brazing alloy paste as shown in Fig. 1. The ceramic disks were heat treated at 900°C for 2 hours and then cool with the furnace before brazing in order to avoid cracking caused by participating stress. The copper samples were polished progressively from 400–1000 grit with silica paper in order to remove any oxide layer at the surface. The substrates were carefully cleaned in ethanol using an ultrasonic machine before all experiments.

The most well-established active metal brazes are based on the 72Ag-28Cu eutectic alloy, to which 1–5 wt% titanium is added to produce sheets or cored wires^[10–13]. Ti in the Ag-Cu braze improves the wettability of alumina and lowers the contact angle; higher Ti contents yield better Al₂O₃/braze wetting ^[14]. Based on the composition of Ticusil® ^[15], a series of Ag-Cu-Ti base active braze alloys with different In content were prepared as displayed in Table 1. The melting property of brazing alloy is one of the key performance indicators and the brazing temperature should be dependent on the melting temperature. The DSC results of AgCu(In)Ti alloys were also given in Table 1. According to the results, the melting temperature of No.6 sample is similar to that of No.7 sample. The No.6 sample (AgCuInTi20-20-4.5) was chosen because the filler metal with lower indium content costs less. The solidus and liquidus temperatures of AgCuInTi20-20-4.5 are 467°C and 495°C respectively. The AgCuInTi20-20-4.5 powder was improved by gas atomization, the average particle diameter of the gas atomized powders was approximate 35µm. Then the powders were mixed with the nitrocotton solution in a 9 to 1 weight ratio and used as filler metal. In order to ensure the consistency of the experiment, the Al₂O₃ substrate was coated with a 0.3 mm AgCuInTi20-20-4.5 pasty brazing filler by screen print method.

Table 1

The chemical composition Ag-Cu-Ti alloys prepared in the experiment
Specimen number	Nominal compositions/ wt%	Ag	Cu	In	Ti	Soldering temperature/℃	Liquidus temperature/℃
1	AgCuTi26-4.5	Bal.	26.04	-	4.52	783	790
2	AgCuInTi26-2-4.5	Bal.	26.01	1.97	4.48	755	776
3	AgCuInTi25-5-4.5	Bal.	24.99	5.04	4.49	680	750
4	AgCuInTi23-10-4.5	Bal.	23.02	10.01	4.50	644	720
5	AgCuInTi22-15-4.5	Bal.	21.97	14.98	4.51	542	644
6	AgCuInTi20-20-4.5	Bal.	20.04	20.03	4.49	467	495
7	AgCuInTi20-30-4.5	Bal.	19.97	39.98	4.47	466	495

2.2 Brazing processes

The AgCuInTi20-20-4.5 was chosen due to the proper melting temperature and lower cost and assembled with the parent metals. The assembly components were put into a vacuum furnace (P = 3x10^− 3 Pa) at a temperature of 700 ℃ for 10 min and the brazing parameter curve is shown in Fig. 2.

2.3 Characterization

The melting characteristics of AgCu(In)Ti active braze alloys were determined by differential scanning calorimetry (DSC, STA 449 F5) at a heating rate of 10℃/min under nitrogen atmosphere. Wettability of AgCuInTi20-20-4.5 was examined by a wetting experiment on Al₂O₃ according to the China National Standard GB/T 11364 − 2008. The contact angle of AgCuInTi paste on alumina was also measured. The Al₂O₃ surface of 40 mm×40 mm×1 mm in dimension. During the experiments, the braze alloy paste was put on the surface of the Al₂O₃ substrates in vacuum furnace (P = 3x10^− 3 Pa) at a temperature of 700 ℃ for 5min. The spreading area was photographed from the top surface of plate with a digital camera and spreading area value was calculated using software Image-Pro Plus.

The microstructures of AgCuInTi20-20-4.5 brazing powder and brazed joint interfaces were observed by scanning electron microscope (SEM, Hitachi SU-1510) with energy dispersive spectroscopy (EDS). The tensile tests were carried out using a universal testing machine (CMT4105) to investigate the tensile properties of the joints. Three specimens were used for each test and the average value was taken into account.

3.1 Wettability of AgCuInTi20-20-4.5 on Al₂O₃ substrate

The wetting experiments of AgCuInTi20-20-4.5 on Al₂O₃ substrate were conducted at 700 ℃ for 5 min. As given in Fig. 3, the contact angle of AgCuInTi20-20-4.5 on Al₂O₃ substrate after brazing is 22 °, and the wettability parameters were listed in Table 2. It can be seen the AgCuInTi20-20-4.5 alloy has an appropriate spreading coefficient.

Table 2

Wettability of AgCuInTi20-20-4.5 on Al₂O₃
Specimen number	Spreading area /cm²	Spreading coefficient/%
1	1.42	96.5
2	1.43	96.3
3	1.40	96.1

3.3 Microstructure of AgCuInTi20-20-4.5 powder and Al₂O₃/Cu brazed joint

SEM micrograph of the AgCuInTi20-20-4.5 powder is shown in Fig. 4. It can be seen the average particle diameter of the spherical alloy particles is approximate 35 µm.

The Al₂O₃/AgCuInTi20-20-4.5/Cu brazed joint was fabricated at 700 ℃ for 10 min. The microstructure of the brazed joint is shown in Fig. 5. The interface microstructure has a great influence on the mechanical property of the joint. Obviously, there is no micro defects such as voids and cracks on the interface. The microstructures of the interlayer adjoined different substrates are obviously different. It can be seen from the Fig. 5 (a) that the boundary between Al₂O₃ and filler metal layer is more tortuous. According to the EDS result of the brazing joint shown in Fig. 5(b), the majority of Ti element remained in the middle of the filler, some Ti atoms distributed along the Al₂O₃/filler interface. This indicates that the filler metal has active reaction with the Al₂O₃ matrix. Additionally, large white blocks were observed in the bonding area. According to Refs. [16], the white blocks were Ag-based solid solution containing the In element. The area distribution of the In element was in good agreement with that of the Ag element.

To further elucidate the microstructures, the partial enlarged drawings of the areas marked in Fig. 5(a) are shown in Fig. 6(a) and Fig. 6(b). EDS point analysis of the brazed joint was done to determine the chemical composition and identification of individual phases (Table 3). According to previous research ^[17–20], active Ti element would react with the Al₂O₃ matrix and form TiO and TiO₂ during brazing, as given in Eq. (1). In this study, TiO₂ was found at the bonding interface (marked as A). The white block adjacent to Al₂O₃ (marked as B) was Cu₃TiO₄. According to the EDS analysis of the microdomain “C, D and F” shown in Table 3, the light gray phase in the middle of the filler is mainly composed of Cu, Ti, and In, it can be inferred that the light gray phase is Cu₂InTi. The light gray Cu₄In phase was found in the matrix in the middle of the brazing area (marked as E and H). At the same time, a large white silver-rich phase (marked as I) was observed, which is consistent with previous EDS scan results shown as Fig. 5 (b). Around the silver-rich phase, a small group of black particles (marked as J) were observed and the details were explained in Fig. 6(b). Based on the composition of position J, L and M, main phase in this region was Cu₄Ti. During brazing, Ti atoms diffused to the Cu side and formed Cu-Ti phase with Cu atoms. Some short rod-shaped Ag (marked as N) were embedded into matrix. In addition, some small black particles were also observed inlaid on the silver-rich phase matrix (marked as K), which may be Cu₂InTi combined with EDS analysis.

Ti + xAl₂O₃→Ti(O)_3x+2xAl (1)

If x = 1/3, TiO is formed. If x = 2/3, TiO₂ is formed.

Table 3

Chemical analyses at areas shown in Fig. 6
Symbol	Average chemical analyses, at.%						Possible phase
Symbol	O	Al	Ti	Cu	Ag	In	Possible phase
A	33.27	23.52	18.24	12.93	10.01	2.03	TiO₂
B	31.81	14.15	20.45	14.28	16.19	3.12	Cu₃TiO₄
C	10.32	2.08	24.34	44.61	1.10	17.54	Cu₂InTi
D	12.45	2.10	23.34	44.27	1.95	15.88	Cu₂InTi
E	7.88	2.17	24.00	47.20	1.18	17.57	Cu₄In
F	12.58	2.14	22.15	48.02	0.83	14.28	Cu₂InTi
G	19.58	1.90	7.14	10.88	50.99	9.52	Ag₃In
H	5.35	1.05	23.01	51.83	1.96	16.8	Cu₄In
I	4.69	0.56	5.94	9.39	65.67	13.75	Rich Ag
J	7.32	1.89	15.49	65.31	1.30	8.69	Cu₄Ti
K	18.60	1.45	8.16	22.34	42.30	7.15	Ag + Cu₂InTi
L	7.85	2.15	21.62	51.49	0.81	16.08	Cu₄Ti
M	8.91	1.87	18.58	64.88	1.25	4.51	Cu₄Ti
N	12.34	1.85	0.18	8.92	67.14	9.56	Rich Ag

3.4 Joining performance of the Al₂O₃/Cu brazed joints

In order to evaluate the joining performance of the Al₂O₃/Cu joints brazed with AgCuInTi20-20-4.5, shear tests of joints were conducted. The mean shear strength of the joints is 89 MPa. The fractured surfaces of joints were analyzed for more exact identification of the bond formation mechanism. Figure 7 shows the fractured surface at the interface of the joint. The fractured surface, on the side containing Al₂O₃, remained completely covered with filler metal. A ductile fracture occurred within the bonding area. XRD analysis was carried out on this fracture surface, as shown in Fig. 8. The results indicated that except the diffraction peaks of Al₂O₃, Ag, those of TiO₂, Cu₃TiO₄, Cu₄Ti, Cu₄In and Ag₃In intermetallic phases also appeared in the XRD pattern, which was correspond of the EDS analyses. The active Ti element diffused to the base materials on both sides under the driving force of the chemical potential difference, TiO₂ and Cu₄Ti formed. Some undiffused Ti atoms were still in the middle of the filler, which reacted with Cu and In in the AgCuIn eutectic to form the Cu₂InTi phase ^[21].

In the present study, a novel AgCuInTi20-20-4.5 alloy paste was prepared and the microstructure of Al₂O₃/AgCuInTi20-20-4.5/Cu brazed joint was studied. The characteristic of the filler metal and the interfacial microstructure, elemental composition, and tensile strength of the joints were investigated. Primary conclusions are summarized as follows.

(1) The solidus and liquidus temperatures of AgCuInTi20-20-4.5 are 467°C and 495°C respectively. The defect free Al₂O₃/Cu joints were achieved in the case of brazing at 700°C for 10 min.

(2) During the brazing process, active element Ti in the filler metal diffuses into the base materials. Ti reacted with Al₂O₃ as the active element at the interface between ceramic and filler metal, and the main products are TiO₂, Cu₃TiO₄, Cu₄Ti. Element In did not participate in the reaction of Al₂O₃, but formed Ag₃In and Cu₄In with Ag and Cu respectively.

(3) The mean shear strength of Al₂O₃/AgCuInTi20-20-4.5/Cu joints reached 89MPa and fracture analyses indicated that a ductile fracture occurred within the bonding area.

Conflict of Interest

The authors declare that they have no conflict of interest.

Acknowledgements

This study was sponsored by Beijing Nova Program-20230484363.

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You are reading this latest preprint version

Study on properties of AgCuInTi filler metal for active bonding of alumina ceramic

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1 Materials

2.2 Brazing processes

2.3 Characterization

3. Results and discussion

3.1 Wettability of AgCuInTi20-20-4.5 on Al₂O₃ substrate

3.3 Microstructure of AgCuInTi20-20-4.5 powder and Al₂O₃/Cu brazed joint

3.4 Joining performance of the Al₂O₃/Cu brazed joints

4. Conclusions

Declarations

Conflict of Interest

Acknowledgements

References

Status:

Version 1

Study on properties of AgCuInTi filler metal for active bonding of alumina ceramic

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1 Materials

2.2 Brazing processes

2.3 Characterization

3. Results and discussion

3.1 Wettability of AgCuInTi20-20-4.5 on Al2O3 substrate

3.3 Microstructure of AgCuInTi20-20-4.5 powder and Al2O3/Cu brazed joint

3.4 Joining performance of the Al2O3/Cu brazed joints

4. Conclusions

Declarations

Conflict of Interest

Acknowledgements

References

Status:

Version 1

3.1 Wettability of AgCuInTi20-20-4.5 on Al₂O₃ substrate

3.3 Microstructure of AgCuInTi20-20-4.5 powder and Al₂O₃/Cu brazed joint

3.4 Joining performance of the Al₂O₃/Cu brazed joints