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SiO2 composites for brazing with Nb using AgCuTi
Journal of Manufacturing Processes 46 (2019) 26–33
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Magnesiothermic reduction of SiO2f/SiO2 composites for brazing with Nb using AgCuTi
T
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Jin Ba1, Hang Li1, Jinghuang Lin, Boxu Ren, Xiaohang Zheng, Junlei Qi , Jian Cao, Jicai Feng State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnesiothermic reduction SiO2f/SiO2 Wettability Brazing
Brazing SiO2f/SiO2 composites is always associated with the problem of poor wettability, and directly brazing SiO2f/SiO2 to Nb with AgCuTi cannot obtain the high strength joints. To overcome the problem, strong binding Si layer was synthesized on SiO2f/SiO2 surface by magnesiothermic reduction method. Results show that the content of Si raised to twice higher than original composites. The contact angle of AgCuTi on SiO2f/SiO2 composites was decreased from 95.9° to 34.7° via reducing SiO2 into Si. Due to the reaction of Si and Mg, SiO2 matrix were corroded to some extent inducing the zigzag reaction layer in the joints. With the increasing of Si content, O contained compounds in reaction layer were replaced by Ti-Si phase. And low thermal expansion Ti5Si3 particles would disperse near the reaction layer to maintain the stability of reaction layer. The highest shear strength reached 38.3 MPa with 4 h reducing treatment.
1. Introduction SiO2f/SiO2 (quartz fiber reinforced silica) composites have been widely applied to the radome, combustor and many thermal resistance components due to its low density, high strength, good wave permeability and thermal stability [1,2]. And quartz fibers reinforcement provides higher strength and fracture toughness than pure SiO2 bulk [3,4]. For the practical applications, SiO2f/SiO2 composites are always joined to refractory metal, such as niobium. Compared to other joining technology, active brazing is a feasible and simple method for joining ceramic-based composites and metal. However, joining SiO2f/SiO2 is often facing one major problem. Due to the loose textures of SiO2f/SiO2, even active brazing alloy has poor wettability with SiO2f/SiO2 [5,6]. Although the brazing alloy can be spread via squeezing of SiO2f/SiO2Nb assembly, the brazing alloy cannot fill and infiltrate into grooves and pores of quartz textures. Thus, it is necessary to improve the wettability of brazing alloy on SiO2f/SiO2 to eliminate the voids and defect on the reaction interface of SiO2f/SiO2. To overcome the problem above, many surface treatments have been reported to improve the wettability of SiO2f/SiO2. Nickel coating [7], CaCO3 micro-grains [3], carbon [8], graphene [9], carbon nanotubes [10] were often adopted to modify the poor wettability of ceramics. The methods above took effect utilizing the better wettability or good reaction between coatings and brazing alloy. But poor bonding
between ceramics and coatings limited improvement of these methods. On one hand, the spreading of brazing alloy will push away or dissolve the coatings, leading to the contact of brazing alloy of original ceramics surface. This spread mode is just like the squeezing effect of ceramicsmetal assembly, which did not change the binding energy between ceramics and liquid alloy. On the other hand, these coatings will consume the active elements in the brazing alloy, which will weaken the reaction and joining between the brazing alloy and ceramics. So strong interface and higher binding energy are new requirements for coatings to improve the wettability of SiO2f/SiO2 effectively. Many researchers have reported the harm of O element in the field of joining and the wettability mechanism of O-contained ceramics/ AgCuTi alloy [11–14]. The reaction product between Ti and O, such as TiOx, has poor wettability with brazing alloy. For AgCuTi alloy, Cu3Ti3O was always formed covering TiOx layer to promote the reaction and spread of brazing alloy [9]. But for SiO2f/SiO2, Cu3Ti3O cannot form continuously due to the loose textures. Moreover, forming one mole Cu3Ti3O needs three mole Ti, and the Cu3Ti3O layer was much thicker, which will reduce the activity of brazing alloy rapidly. So the deoxidization on the SiO2f/SiO2 seems to be a better way, and it is well known that Si has good wettability with AgCuTi. To obtain the strong interface of Si coating discussed above, in situ reduction of SiO2 can be adopted. Some researchers have reported the reduction methods of SiO2 to Si, such as carbothermal reduction [15], electrochemical reduction
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Corresponding author. E-mail address: [email protected] (J. Qi). 1 These authors contributed equally. https://doi.org/10.1016/j.jmapro.2019.08.024 Received 13 June 2019; Received in revised form 20 August 2019; Accepted 22 August 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic diagrams of (a) magnesiothermic reduction for SiO2f/SiO2 in tubular furnace, (a) brazing joints assembly and (b) shear test.
constant speed of 0.5 mm/min, which follows the modification of ASTM D905-03 test procedure. The fixture of shear test was shown in Fig. 1c. The fixture supported Nb plate via contact roller to maintain the force parallel to brazing seam and eliminate the friction effect on shear strength. For each condition, at least five specimens were tested to obtain average value.
[16] and magnesiothermic reduction [17]. These methods can synthesize Si layer effectively and maintain the strong bonding between Si and SiO2. Considering the structure stability of SiO2f/SiO2, magnesiothermic reduction was adopted to reduce SiO2 at lower temperature. In this paper, Si layer was synthesized via magnesiothermic reduction to obtain the high strength SiO2f/SiO2-Nb joints brazed with AgCuTi. Microstructure and component of the Si-SiO2f/SiO2 was investigated in detail. The mechanism and process of magnesiothermic reduction for SiO2f/SiO2 were discussed. Additionally, the effect of reducing time on the wettability of AgCuTi and microstructure of joints were performed, in order to analyze the strengthening mechanism.
3. Results and discussion The component change of SiO2f/SiO2 surface during reduction process is presented in Fig. 2. Fig. 2a shows the XRD pattern of pure SiO2f/SiO2, which only exhibits the amorphous silica peak. When the SiO2f/SiO2 was reduced for 4 h as shown in Fig. 2b, the peak of MgO and Mg2Si emerged. Their intensity is too high, and no obvious Si peaks can be found. The color of reduced SiO2f/SiO2 without pickling (marked II) was blue as shown in Fig. 1c, which agrees with the color of Mg2Si. It is fortunate that MgO and Mg2Si can dissolve in HCl solution easily, the obtained Si-SiO2f/SiO2 reduced for 4 h (marked III) has been shown in Figs. 2c and 2 d shows the XRD pattern of acid cleaned SiO2f/SiO2 with various reducing time (short for Xh-SiO2f/SiO2). The peaks of Mg2Si and MgO disappeared. When reducing time was 2 h, the peaks of Si emerged and the amorphous peak of SiO2 was still strong. As reduction time increased to 4 h, the intensity of Si peaks increased and that of SiO2 decreased correspondingly. When the reducing time lasted to 6 h, the Si peaks were so high and SiO2 peaks could not be observed. The peak intensity increased with the reducing time lasting, indicating the crystallinity of Si increasing. Fig. 3 presents the surface of SiO2f/SiO2 composites reduced with various time. As reducing time increasing from 0 h to 4 h, SiO2 matrix was eroded gradually and many holes appeared. It was obvious that the top of vertical quartz fibers emerged. It is beneficial for brazing to increase the joining area and form the zigzag reaction layer. But for 6 h reducing time, excessive reaction leaded to the crack and exfoliation of the composites surface, which threatened the joining stability and strength. To explain the phenomenon of fiber top emerging, the reduction process was shown in Fig. 4. At first, Mg gas deposed on the surface of SiO2f/SiO2 as shown in Fig. 4a. Then the reaction of Mg and Si mainly followed reaction (1), and Si layer began to form as shown in Fig. 4b. Meanwhile, Mg intended to react with new obtained Si as lacking of O followed reaction (2). It is accordance with Mg-Si phase diagram [20] and some reports [21–23]. Some reporters explained that Mg2Si was intermediate phase and would disappear followed the reaction (3) [21,24]. But in this study, the SiO2f/SiO2 with 6 h reducing time was still blue. Due to the reaction (3) requiring SiO2, Mg2Si among the skin surface cannot transform into Si. As shown in Fig. 4c, O element diffused towards Mg side constantly and Mg tended to react with Si and the diffused O element. There was the dynamic balance between reaction (1) and (2), and Si tends to react with Mg with lacking of O. Thus, the erosion of SiO2 matrix was the result from the reaction
2. Experiment The magnesiothermic reduction was carried out as shown in Fig. 1a. The SiO2f/SiO2 composites were cut into 5 mm × 5 mm × 5 mm and polished to obtain the plane and clean surface. Then the SiO2f/SiO2 composites and magnesium foil were placed in the crucible with lid, avoiding the leakage of gas magnesium during heating process. The ratio of the mass of magnesium and the area of crucible was 0.2 g/cm2. Next, the crucible was put into tubular furnace with argon atmosphere. The furnace was heated to 700 ℃ at the rate of 10 ℃/min. 2 h, 4 h and 6 h holding time was adopted to investigate the effect of reducing time on wettability and strength improvement. Final, the reduced SiO2f/SiO2 composites were soaked in 0.5 M HCl solution for 20 min and then cleaned by ethanol. Nb were sectioned into 10 mm × 10 mm × 4 mm. 200 μm thickness of Ag-26.7Cu-4.5Ti (wt%) foil was adopted, and the solidus and liquidus temperatures were 830 ℃ and 850 ℃, respectively [18,19]. Prior to brazing, all joining surfaces of Nb and AgCuTi were ground to 2000 girt and then cleaned in ethanol for 10 min. Then the AgCuTi foil was sandwiched by SiO2f/SiO2 and Nb to form assemblies as shown in Fig. 1b. All samples adopted for characterizing the microstructure, component and mechanical strength were brazed in this dimension. The assemblies were brazed at 880 °C for 10 min at the heating rate of 10 °C/min under 2 × 10−3 Pa. This optimal parameter was established in Fig. S1 and S2. And the cooling process was carried out at 5 °C /min. The wettability test was carried out by placing AgCuTi sphere on SiO2f/ SiO2 at the same parameter. Scanning electron microscopy (SEM, HELIOS NanoLab 600i) equipped with electron dispersive spectroscopy (EDS) was adopted to analyze the microstructure and component of brazing joints. X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi) and X-ray diffraction system (XRD; JDX-3530 M) were utilized to investigate the component of the obtained samples. For micro-zone XRD test, brazing samples were ground parallel to the brazing seam layer by layer until the reaction layer revealed. Then micro-zone XRD was carried out to characterize the microscale zone of reaction layer. Shear strengths at room temperature were tested by Instron-1186 testing machine at a 27
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Fig. 2. XRD pattern of (a) pure SiO2f/SiO2 and (b) 4 h-SiO2f/SiO2 without pickling. (c) Reduction process of SiO2f/SiO2, I: original SiO2f/SiO2, II: 4 h-SiO2f/SiO2 without pickling and III: 4 h-SiO2f/SiO2 with pickling. (d) XRD pattern of pickled composites with various reducing time.
Si layer hindered the diffusion of O, the SiO2 matrix was eroded by Mg via the reaction of Mg and Si.
between Si and Mg as shown in Fig. 4d. Compared with dense quartz fibers, SiO2 matrix was looser because of the immersion fabrication, which is beneficial for the exchange of O. So the reduction of SiO2 matrix can synthesize more Si than quartz fibers. Meanwhile too thicker
2 Mg + SiO2 → 2 MgO + Si
Fig. 3. SEM images of SiO2f/SiO2 surface morphology with (a) 0 h, (b) 2 h, (c) 4 h and (d) 6 h reducing time. 28
(1)
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Fig. 4. Schematic illustration of the formation of Si layer and corrosion of SiO2 matrix.
2 Mg + Si → Mg2Si
(2)
Mg2Si + SiO2 → 2Si +2 MgO
(3)
between Si and SiO2 [26,27]. It was formed by the diffusion of O, which maintained the good bonding between Si and SiO2. The peaks of SiO2180, SiO2-144, SiO2-120, Si2O3, SiO, Si2O and Si are located at 104.1 eV, 103.7 eV, 103.1 eV, 102.4 eV, 101.4 eV, 100.3 eV and 99.4 eV, respectively [19]. The peak area was shown in Table 1 to calculate the content of each phase. Fig. 5a shows the 0 h-SiO2f/SiO2, and only the peaks of SiO2 can be found. When the reducing time increased to 2 h as shown in Fig. 5b, the peaks of Si and intermediate oxidation emerged. As the reducing time increasing, the area of SiO2 and intermediate oxidation decreased. Si become the main component of the surface, and the content of Si was 60.8%, 67.0% and 76.1% with
To further investigate the component and valence state of SiO2f/ SiO2, the component of SiO2f/SiO2 was characterized XPS to ensure the effect of reducing time as shown in Fig. 5. The crystal structure of SiO2 was complex, and the actual charge transfer in Si-O-Si bridging bond is determined by the bond angle. 144° bond angles made up the bulk amorphous oxide, and the lattice mismatch among interface induced the formation of 120° bond angles [25]. Some work reported that a monolayer of intermediate oxidation (Si2O3, SiO and Si2O) was found
Fig. 5. The XPS spectra of the Si-2p electron states for SiO2f/SiO2 with (a) 0 h, (b) 2 h, (c) 4 h and (d) 6 h reducing time. 29
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increased the joining area to some degree. It will reinforce the joining strength between SiO2f/SiO2 and brazing alloy. To analyze the effect of Si content on the reaction layer component, EDS was carried out in layer II to certain the Si content (layer I was too thin to characterize). The result in Table 2 shows that Si content raises with the increase of reducing time, and the component of layer II was transformed from Cu3Ti3O to Cu3Ti3O + TixSiy. Besides, the width of layer I in 4 h-SiO2f/SiO2 increased obviously. To confirm the component change of the reaction layer with or without Si, micro-zone XRD was adopted to analyze 0 h-SiO2f/SiO2 and 4 h-SiO2f/SiO2 joints as shown in Fig. 8. The reaction layer component of 0 h-SiO2f/SiO2 is composed of TiO + Ti5Si3 + Cu3Ti3O, and the intensity of Ti5Si3 was too weak. When 4 h-SiO2f/SiO2 was brazed as shown in Fig. 8b, the intensity of Ti5Si3 increased and the peaks of Ti5Si4 emerged. The increase of Ti-Si phases content is the reason of good wettability. The formation of Ti-Si phases will consume less Ti compared to Ti-O and Cu3Ti3O, which will maintain the high activity of brazing alloy. Moreover, diffusive nano phase (point C) emerged among reaction layer, which can be detected as Ti5Si3 by EDS shown in Table 2. These particles can reduce the coefficient of thermal expansion (CTE) mismatch between SiO2f/SiO2 and brazing alloy (the CTE of Ti5Si3 is about 2.76 × 10−6 K-1) [28,29]. It will release the thermal residual stress to maintain the stability of reaction layer [30,31]. Nevertheless, too long reducing time will cause the crack of SiO2f/SiO2 surface, and the reaction layer tended to break and become imperfect as show in Fig. 7h. The joining strength of SiO2f/SiO2 brazed joints with different reducing time was tested as shown in Fig. 9. The shear strength of 0 hSiO2f/SiO2 was only 13.6 MPa due to poor wettability. 4 h-SiO2f/SiO2 joints owned the highest shear strength, 38.3 MPa. It was because of good wettability, larger joining area and less residual stress. When the reducing time among 2 h and 4 h, the strength increased above 30 MPa rapidly, which was 2 times than the joints without Si layer. It exhibited the good applicability and wider parameter zone of magnesiothermic reduction. And in-situ synthesized Si layer provided more advantages on wettability and microstructure of joints compared to traditional coating. Fig. 10 shows the fracture morphology of the joints brazed with SiO2f/SiO2 reduced for 0 h and 4 h. The joining surface of SiO2f/SiO2 composites contained parallel and vertical fibers. The parallel fibers
Table 1 XPS area and Si content in Si 2p level spectra for reduced SiO2f/SiO2 composites with different reducing time. Sample
0h 2h 4h 6h
Area
Si content (at%)
SiO2 −180
SiO2 −144
SiO2 −120
Si2O3
SiO
Si2O
Si
3650 1070 879 567
3245 1250 980 548
2793 1086 925 689
– 1242 574 284
– 1361 811 202
– 1727 1495 608
– 3171 3743 4117
33.3% 60.8% 67.0% 76.1%
2 h, 4 h and 6 h reduction treatment, which was twice higher than that of pure SiO2f/SiO2. As the same test depth, the increase of Si content indicated the increase of Si layer width. To explore the effect of Si content on the wettability of AgCuTi, wettability test was carried out at 880 ℃ for 10 min as shown in Fig. 6. Due to its loose texture structure, the contact angle of 0 h-SiO2f/SiO2 was 95.9°. It was obvious that the contact angle decreased with the content of Si increasing. All contact angle on reduced SiO2f/SiO2 were between 60° and 20° meaning good wettability rather than excessive spreading. Moreover, pure Si was adopted to determine the wettability of AgCuTi as shown in Fig. 6e, and the contact angle is 25.8°. The contact angle of 6 h-SiO2f/SiO2 (34.7°) was close to that of Si. Due to high diffusion rate of O, the reaction of Ti and O cannot be eliminated, which induced the contact angle of reduced SiO2f/SiO2 was higher. To investigate the effect of Si on the microstructure of joints and strength, SiO2f/SiO2 composites with various reducing time were brazed to Nb with AgCuTi as shown in Fig. 7. When 0 h-SiO2f/SiO2 composites were brazed, the reaction layer near SiO2f/SiO2 can be divided into two layers. Layer I consisted of TiO + Ti5Si3, which is too thin to observe. Layer II was mainly composed of Cu3Ti3O. The reaction layer maintained straight and no voids can be found. The brazing seam is filled with Ag-Cu solid solution, and the joining of Nb was achieved by the diffusion between Nb and brazing alloy. While reduced SiO2f/ SiO2 composites were brazed, reaction layer changed into zig-zag mode due to the infiltration of AgCuTi into uneven composites surface. The infiltration depth of 4 h-SiO2f/SiO2 reached about 4 μm, which
Fig. 6. The contact angle of AgCuTi on SiO2f/SiO2 with (a) 0 h, (b) 2 h, (c) 4 h and (d) 6 h reducing time. (e)The contact angle of AgCuTi on pure Si flake. (f) The effect of Si content on the contact angle. 30
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Fig. 7. Microstructure of brazing SiO2f/SiO2 with (a)(e) 0 h, (b)(f) 2 h, (c)(g) 4 h and (d)(h) 6 h reducing time to Nb using AgCuTi at 880 ℃ for 10 min. Table 2 EDS compositional analysis of phased marked in Fig. 7. Position
A B C D E
Composition (at.%)
Phase
Ag
Cu
Ti
Si
O
0.31 0.06 37.51 0.73 1.03
36.58 24.70 16.05 18.91 16.55
47.57 46.95 30.42 46.94 42.92
1.12 18.73 14.99 25.94 33.55
14.42 9.56 1.03 7.48 5.95
Cu3Ti3O Cu3Ti3O + Ti5Si3 Ag(s,s)+Ti5Si3 Cu3Ti3O + TixSiy Cu3Ti3O + TixSiy
only connected with SiO2f/SiO2 via SiO2 matrix, which was easy to break and leave on the reaction layer. The vertical fibers were caught by the whole composites, and their bonding strength with brazing alloy mainly affected the shear strength of the joints. So the fracture could be divided into two kind area correspondingly. The fracture morphology of original SiO2f/SiO2 was shown in Fig. 10a. Some white stripe can be seen, which may be the parallel fibers. Most area of fracture was black, indicating the break mostly happened at the reaction layer. As shown in Fig. 10b, the brittle crack happened at the reaction layer and no vertical fibers fracture could be found. It is due to the weak reaction between SiO2 and brazing alloy. The brazing alloy had no ability to capture vertical fibers. Under high stress condition, the weak reaction layer would break directly. When 4 h-SiO2f/SiO2 was brazed as shown in Fig. 10c, it was obvious that many parallel fibers were ripped off. The fracture color mostly seemed white, suggesting the break of SiO2 matrix or vertical fibers. The magnified morphology in Fig. 10d shows that many vertical fibers were broken or pull out during the shear test. And
Fig. 9. Shear strength of SiO2f/SiO2-Nb joints with various reducing time.
the fracture of vertical was uneven, indicating the crack happened to deflect. It was sensible that the bonding between vertical fibers and brazing alloy was so strong that fulfilled the fracture advantages of SiO2f/SiO2 composites. And this phenomenon was contributed by good interfacial reaction, pinning effect and the decease of residual stress. 4. Conclusions Si layer synthesized on SiO2f/SiO2 surface via magnesiothermic reduction was developed successfully to satisfy the high strength joining
Fig. 8. XRD patterns of the reaction layer of (a) 0 h-SiO2f/SiO2-Nb joints and (b) 4 h-SiO2f/SiO2-Nb joints. 31
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Fig. 10. Fracture morphology of SiO2f/SiO2-Nb joints with (a)(b) 0 h and (c)(d) 4 h reduction time.
Acknowledgements
of SiO2f/SiO2 and Nb. Microstructure evolution and mechanical properties improvement were concluded as follows:
The support from the National Natural Science Foundation of China (Grant Nos 51575135, 51501049 and U1537206), is highly appreciated.
(1) The contact angle was decreased from 95.9° to 34.7°, which was close to the contact angle of AgCuTi on pure Si 25.8°. It will guarantee the reaction and infiltration of brazing alloy and SiO2f/ SiO2. (2) Due the reaction of Si and Mg, quartz fibers emerged at composites surface inducing the formation of zigzag reaction layer. It will increase the joining area to maintain the high strength bonding between SiO2f/SiO2 and brazing alloy. (3) The typical component of reaction layer changed from TiO + Ti5Si3/Cu3Ti3O to TiO + Ti5Si3/Cu3Ti3O + TixSiy. Ti-Si phase became the main component of reaction layer, which consumed less Ti to maintain high activity of brazing alloy. Low CTE Ti5Si3 particles dispersed among reaction layer, which can modify the CTE mismatch between composites and alloy. (4) The highest shear strength of joints reached 38.3 MPa with 4 h reduction, which was twice higher than original joints. And appropriate reduction ensures the strength above 30 MPa, which satisfies various application. Magnesiothermic reduction method reveals good parameter zone and adaptability.
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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. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Magnesiothermic reduction of SiO2f/SiO2 composites for brazing with Nb using AgCuTi
T
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Jin Ba1, Hang Li1, Jinghuang Lin, Boxu Ren, Xiaohang Zheng, Junlei Qi , Jian Cao, Jicai Feng State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnesiothermic reduction SiO2f/SiO2 Wettability Brazing
Brazing SiO2f/SiO2 composites is always associated with the problem of poor wettability, and directly brazing SiO2f/SiO2 to Nb with AgCuTi cannot obtain the high strength joints. To overcome the problem, strong binding Si layer was synthesized on SiO2f/SiO2 surface by magnesiothermic reduction method. Results show that the content of Si raised to twice higher than original composites. The contact angle of AgCuTi on SiO2f/SiO2 composites was decreased from 95.9° to 34.7° via reducing SiO2 into Si. Due to the reaction of Si and Mg, SiO2 matrix were corroded to some extent inducing the zigzag reaction layer in the joints. With the increasing of Si content, O contained compounds in reaction layer were replaced by Ti-Si phase. And low thermal expansion Ti5Si3 particles would disperse near the reaction layer to maintain the stability of reaction layer. The highest shear strength reached 38.3 MPa with 4 h reducing treatment.
1. Introduction SiO2f/SiO2 (quartz fiber reinforced silica) composites have been widely applied to the radome, combustor and many thermal resistance components due to its low density, high strength, good wave permeability and thermal stability [1,2]. And quartz fibers reinforcement provides higher strength and fracture toughness than pure SiO2 bulk [3,4]. For the practical applications, SiO2f/SiO2 composites are always joined to refractory metal, such as niobium. Compared to other joining technology, active brazing is a feasible and simple method for joining ceramic-based composites and metal. However, joining SiO2f/SiO2 is often facing one major problem. Due to the loose textures of SiO2f/SiO2, even active brazing alloy has poor wettability with SiO2f/SiO2 [5,6]. Although the brazing alloy can be spread via squeezing of SiO2f/SiO2Nb assembly, the brazing alloy cannot fill and infiltrate into grooves and pores of quartz textures. Thus, it is necessary to improve the wettability of brazing alloy on SiO2f/SiO2 to eliminate the voids and defect on the reaction interface of SiO2f/SiO2. To overcome the problem above, many surface treatments have been reported to improve the wettability of SiO2f/SiO2. Nickel coating [7], CaCO3 micro-grains [3], carbon [8], graphene [9], carbon nanotubes [10] were often adopted to modify the poor wettability of ceramics. The methods above took effect utilizing the better wettability or good reaction between coatings and brazing alloy. But poor bonding
between ceramics and coatings limited improvement of these methods. On one hand, the spreading of brazing alloy will push away or dissolve the coatings, leading to the contact of brazing alloy of original ceramics surface. This spread mode is just like the squeezing effect of ceramicsmetal assembly, which did not change the binding energy between ceramics and liquid alloy. On the other hand, these coatings will consume the active elements in the brazing alloy, which will weaken the reaction and joining between the brazing alloy and ceramics. So strong interface and higher binding energy are new requirements for coatings to improve the wettability of SiO2f/SiO2 effectively. Many researchers have reported the harm of O element in the field of joining and the wettability mechanism of O-contained ceramics/ AgCuTi alloy [11–14]. The reaction product between Ti and O, such as TiOx, has poor wettability with brazing alloy. For AgCuTi alloy, Cu3Ti3O was always formed covering TiOx layer to promote the reaction and spread of brazing alloy [9]. But for SiO2f/SiO2, Cu3Ti3O cannot form continuously due to the loose textures. Moreover, forming one mole Cu3Ti3O needs three mole Ti, and the Cu3Ti3O layer was much thicker, which will reduce the activity of brazing alloy rapidly. So the deoxidization on the SiO2f/SiO2 seems to be a better way, and it is well known that Si has good wettability with AgCuTi. To obtain the strong interface of Si coating discussed above, in situ reduction of SiO2 can be adopted. Some researchers have reported the reduction methods of SiO2 to Si, such as carbothermal reduction [15], electrochemical reduction
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Corresponding author. E-mail address: [email protected] (J. Qi). 1 These authors contributed equally. https://doi.org/10.1016/j.jmapro.2019.08.024 Received 13 June 2019; Received in revised form 20 August 2019; Accepted 22 August 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic diagrams of (a) magnesiothermic reduction for SiO2f/SiO2 in tubular furnace, (a) brazing joints assembly and (b) shear test.
constant speed of 0.5 mm/min, which follows the modification of ASTM D905-03 test procedure. The fixture of shear test was shown in Fig. 1c. The fixture supported Nb plate via contact roller to maintain the force parallel to brazing seam and eliminate the friction effect on shear strength. For each condition, at least five specimens were tested to obtain average value.
[16] and magnesiothermic reduction [17]. These methods can synthesize Si layer effectively and maintain the strong bonding between Si and SiO2. Considering the structure stability of SiO2f/SiO2, magnesiothermic reduction was adopted to reduce SiO2 at lower temperature. In this paper, Si layer was synthesized via magnesiothermic reduction to obtain the high strength SiO2f/SiO2-Nb joints brazed with AgCuTi. Microstructure and component of the Si-SiO2f/SiO2 was investigated in detail. The mechanism and process of magnesiothermic reduction for SiO2f/SiO2 were discussed. Additionally, the effect of reducing time on the wettability of AgCuTi and microstructure of joints were performed, in order to analyze the strengthening mechanism.
3. Results and discussion The component change of SiO2f/SiO2 surface during reduction process is presented in Fig. 2. Fig. 2a shows the XRD pattern of pure SiO2f/SiO2, which only exhibits the amorphous silica peak. When the SiO2f/SiO2 was reduced for 4 h as shown in Fig. 2b, the peak of MgO and Mg2Si emerged. Their intensity is too high, and no obvious Si peaks can be found. The color of reduced SiO2f/SiO2 without pickling (marked II) was blue as shown in Fig. 1c, which agrees with the color of Mg2Si. It is fortunate that MgO and Mg2Si can dissolve in HCl solution easily, the obtained Si-SiO2f/SiO2 reduced for 4 h (marked III) has been shown in Figs. 2c and 2 d shows the XRD pattern of acid cleaned SiO2f/SiO2 with various reducing time (short for Xh-SiO2f/SiO2). The peaks of Mg2Si and MgO disappeared. When reducing time was 2 h, the peaks of Si emerged and the amorphous peak of SiO2 was still strong. As reduction time increased to 4 h, the intensity of Si peaks increased and that of SiO2 decreased correspondingly. When the reducing time lasted to 6 h, the Si peaks were so high and SiO2 peaks could not be observed. The peak intensity increased with the reducing time lasting, indicating the crystallinity of Si increasing. Fig. 3 presents the surface of SiO2f/SiO2 composites reduced with various time. As reducing time increasing from 0 h to 4 h, SiO2 matrix was eroded gradually and many holes appeared. It was obvious that the top of vertical quartz fibers emerged. It is beneficial for brazing to increase the joining area and form the zigzag reaction layer. But for 6 h reducing time, excessive reaction leaded to the crack and exfoliation of the composites surface, which threatened the joining stability and strength. To explain the phenomenon of fiber top emerging, the reduction process was shown in Fig. 4. At first, Mg gas deposed on the surface of SiO2f/SiO2 as shown in Fig. 4a. Then the reaction of Mg and Si mainly followed reaction (1), and Si layer began to form as shown in Fig. 4b. Meanwhile, Mg intended to react with new obtained Si as lacking of O followed reaction (2). It is accordance with Mg-Si phase diagram [20] and some reports [21–23]. Some reporters explained that Mg2Si was intermediate phase and would disappear followed the reaction (3) [21,24]. But in this study, the SiO2f/SiO2 with 6 h reducing time was still blue. Due to the reaction (3) requiring SiO2, Mg2Si among the skin surface cannot transform into Si. As shown in Fig. 4c, O element diffused towards Mg side constantly and Mg tended to react with Si and the diffused O element. There was the dynamic balance between reaction (1) and (2), and Si tends to react with Mg with lacking of O. Thus, the erosion of SiO2 matrix was the result from the reaction
2. Experiment The magnesiothermic reduction was carried out as shown in Fig. 1a. The SiO2f/SiO2 composites were cut into 5 mm × 5 mm × 5 mm and polished to obtain the plane and clean surface. Then the SiO2f/SiO2 composites and magnesium foil were placed in the crucible with lid, avoiding the leakage of gas magnesium during heating process. The ratio of the mass of magnesium and the area of crucible was 0.2 g/cm2. Next, the crucible was put into tubular furnace with argon atmosphere. The furnace was heated to 700 ℃ at the rate of 10 ℃/min. 2 h, 4 h and 6 h holding time was adopted to investigate the effect of reducing time on wettability and strength improvement. Final, the reduced SiO2f/SiO2 composites were soaked in 0.5 M HCl solution for 20 min and then cleaned by ethanol. Nb were sectioned into 10 mm × 10 mm × 4 mm. 200 μm thickness of Ag-26.7Cu-4.5Ti (wt%) foil was adopted, and the solidus and liquidus temperatures were 830 ℃ and 850 ℃, respectively [18,19]. Prior to brazing, all joining surfaces of Nb and AgCuTi were ground to 2000 girt and then cleaned in ethanol for 10 min. Then the AgCuTi foil was sandwiched by SiO2f/SiO2 and Nb to form assemblies as shown in Fig. 1b. All samples adopted for characterizing the microstructure, component and mechanical strength were brazed in this dimension. The assemblies were brazed at 880 °C for 10 min at the heating rate of 10 °C/min under 2 × 10−3 Pa. This optimal parameter was established in Fig. S1 and S2. And the cooling process was carried out at 5 °C /min. The wettability test was carried out by placing AgCuTi sphere on SiO2f/ SiO2 at the same parameter. Scanning electron microscopy (SEM, HELIOS NanoLab 600i) equipped with electron dispersive spectroscopy (EDS) was adopted to analyze the microstructure and component of brazing joints. X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi) and X-ray diffraction system (XRD; JDX-3530 M) were utilized to investigate the component of the obtained samples. For micro-zone XRD test, brazing samples were ground parallel to the brazing seam layer by layer until the reaction layer revealed. Then micro-zone XRD was carried out to characterize the microscale zone of reaction layer. Shear strengths at room temperature were tested by Instron-1186 testing machine at a 27
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Fig. 2. XRD pattern of (a) pure SiO2f/SiO2 and (b) 4 h-SiO2f/SiO2 without pickling. (c) Reduction process of SiO2f/SiO2, I: original SiO2f/SiO2, II: 4 h-SiO2f/SiO2 without pickling and III: 4 h-SiO2f/SiO2 with pickling. (d) XRD pattern of pickled composites with various reducing time.
Si layer hindered the diffusion of O, the SiO2 matrix was eroded by Mg via the reaction of Mg and Si.
between Si and Mg as shown in Fig. 4d. Compared with dense quartz fibers, SiO2 matrix was looser because of the immersion fabrication, which is beneficial for the exchange of O. So the reduction of SiO2 matrix can synthesize more Si than quartz fibers. Meanwhile too thicker
2 Mg + SiO2 → 2 MgO + Si
Fig. 3. SEM images of SiO2f/SiO2 surface morphology with (a) 0 h, (b) 2 h, (c) 4 h and (d) 6 h reducing time. 28
(1)
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Fig. 4. Schematic illustration of the formation of Si layer and corrosion of SiO2 matrix.
2 Mg + Si → Mg2Si
(2)
Mg2Si + SiO2 → 2Si +2 MgO
(3)
between Si and SiO2 [26,27]. It was formed by the diffusion of O, which maintained the good bonding between Si and SiO2. The peaks of SiO2180, SiO2-144, SiO2-120, Si2O3, SiO, Si2O and Si are located at 104.1 eV, 103.7 eV, 103.1 eV, 102.4 eV, 101.4 eV, 100.3 eV and 99.4 eV, respectively [19]. The peak area was shown in Table 1 to calculate the content of each phase. Fig. 5a shows the 0 h-SiO2f/SiO2, and only the peaks of SiO2 can be found. When the reducing time increased to 2 h as shown in Fig. 5b, the peaks of Si and intermediate oxidation emerged. As the reducing time increasing, the area of SiO2 and intermediate oxidation decreased. Si become the main component of the surface, and the content of Si was 60.8%, 67.0% and 76.1% with
To further investigate the component and valence state of SiO2f/ SiO2, the component of SiO2f/SiO2 was characterized XPS to ensure the effect of reducing time as shown in Fig. 5. The crystal structure of SiO2 was complex, and the actual charge transfer in Si-O-Si bridging bond is determined by the bond angle. 144° bond angles made up the bulk amorphous oxide, and the lattice mismatch among interface induced the formation of 120° bond angles [25]. Some work reported that a monolayer of intermediate oxidation (Si2O3, SiO and Si2O) was found
Fig. 5. The XPS spectra of the Si-2p electron states for SiO2f/SiO2 with (a) 0 h, (b) 2 h, (c) 4 h and (d) 6 h reducing time. 29
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increased the joining area to some degree. It will reinforce the joining strength between SiO2f/SiO2 and brazing alloy. To analyze the effect of Si content on the reaction layer component, EDS was carried out in layer II to certain the Si content (layer I was too thin to characterize). The result in Table 2 shows that Si content raises with the increase of reducing time, and the component of layer II was transformed from Cu3Ti3O to Cu3Ti3O + TixSiy. Besides, the width of layer I in 4 h-SiO2f/SiO2 increased obviously. To confirm the component change of the reaction layer with or without Si, micro-zone XRD was adopted to analyze 0 h-SiO2f/SiO2 and 4 h-SiO2f/SiO2 joints as shown in Fig. 8. The reaction layer component of 0 h-SiO2f/SiO2 is composed of TiO + Ti5Si3 + Cu3Ti3O, and the intensity of Ti5Si3 was too weak. When 4 h-SiO2f/SiO2 was brazed as shown in Fig. 8b, the intensity of Ti5Si3 increased and the peaks of Ti5Si4 emerged. The increase of Ti-Si phases content is the reason of good wettability. The formation of Ti-Si phases will consume less Ti compared to Ti-O and Cu3Ti3O, which will maintain the high activity of brazing alloy. Moreover, diffusive nano phase (point C) emerged among reaction layer, which can be detected as Ti5Si3 by EDS shown in Table 2. These particles can reduce the coefficient of thermal expansion (CTE) mismatch between SiO2f/SiO2 and brazing alloy (the CTE of Ti5Si3 is about 2.76 × 10−6 K-1) [28,29]. It will release the thermal residual stress to maintain the stability of reaction layer [30,31]. Nevertheless, too long reducing time will cause the crack of SiO2f/SiO2 surface, and the reaction layer tended to break and become imperfect as show in Fig. 7h. The joining strength of SiO2f/SiO2 brazed joints with different reducing time was tested as shown in Fig. 9. The shear strength of 0 hSiO2f/SiO2 was only 13.6 MPa due to poor wettability. 4 h-SiO2f/SiO2 joints owned the highest shear strength, 38.3 MPa. It was because of good wettability, larger joining area and less residual stress. When the reducing time among 2 h and 4 h, the strength increased above 30 MPa rapidly, which was 2 times than the joints without Si layer. It exhibited the good applicability and wider parameter zone of magnesiothermic reduction. And in-situ synthesized Si layer provided more advantages on wettability and microstructure of joints compared to traditional coating. Fig. 10 shows the fracture morphology of the joints brazed with SiO2f/SiO2 reduced for 0 h and 4 h. The joining surface of SiO2f/SiO2 composites contained parallel and vertical fibers. The parallel fibers
Table 1 XPS area and Si content in Si 2p level spectra for reduced SiO2f/SiO2 composites with different reducing time. Sample
0h 2h 4h 6h
Area
Si content (at%)
SiO2 −180
SiO2 −144
SiO2 −120
Si2O3
SiO
Si2O
Si
3650 1070 879 567
3245 1250 980 548
2793 1086 925 689
– 1242 574 284
– 1361 811 202
– 1727 1495 608
– 3171 3743 4117
33.3% 60.8% 67.0% 76.1%
2 h, 4 h and 6 h reduction treatment, which was twice higher than that of pure SiO2f/SiO2. As the same test depth, the increase of Si content indicated the increase of Si layer width. To explore the effect of Si content on the wettability of AgCuTi, wettability test was carried out at 880 ℃ for 10 min as shown in Fig. 6. Due to its loose texture structure, the contact angle of 0 h-SiO2f/SiO2 was 95.9°. It was obvious that the contact angle decreased with the content of Si increasing. All contact angle on reduced SiO2f/SiO2 were between 60° and 20° meaning good wettability rather than excessive spreading. Moreover, pure Si was adopted to determine the wettability of AgCuTi as shown in Fig. 6e, and the contact angle is 25.8°. The contact angle of 6 h-SiO2f/SiO2 (34.7°) was close to that of Si. Due to high diffusion rate of O, the reaction of Ti and O cannot be eliminated, which induced the contact angle of reduced SiO2f/SiO2 was higher. To investigate the effect of Si on the microstructure of joints and strength, SiO2f/SiO2 composites with various reducing time were brazed to Nb with AgCuTi as shown in Fig. 7. When 0 h-SiO2f/SiO2 composites were brazed, the reaction layer near SiO2f/SiO2 can be divided into two layers. Layer I consisted of TiO + Ti5Si3, which is too thin to observe. Layer II was mainly composed of Cu3Ti3O. The reaction layer maintained straight and no voids can be found. The brazing seam is filled with Ag-Cu solid solution, and the joining of Nb was achieved by the diffusion between Nb and brazing alloy. While reduced SiO2f/ SiO2 composites were brazed, reaction layer changed into zig-zag mode due to the infiltration of AgCuTi into uneven composites surface. The infiltration depth of 4 h-SiO2f/SiO2 reached about 4 μm, which
Fig. 6. The contact angle of AgCuTi on SiO2f/SiO2 with (a) 0 h, (b) 2 h, (c) 4 h and (d) 6 h reducing time. (e)The contact angle of AgCuTi on pure Si flake. (f) The effect of Si content on the contact angle. 30
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Fig. 7. Microstructure of brazing SiO2f/SiO2 with (a)(e) 0 h, (b)(f) 2 h, (c)(g) 4 h and (d)(h) 6 h reducing time to Nb using AgCuTi at 880 ℃ for 10 min. Table 2 EDS compositional analysis of phased marked in Fig. 7. Position
A B C D E
Composition (at.%)
Phase
Ag
Cu
Ti
Si
O
0.31 0.06 37.51 0.73 1.03
36.58 24.70 16.05 18.91 16.55
47.57 46.95 30.42 46.94 42.92
1.12 18.73 14.99 25.94 33.55
14.42 9.56 1.03 7.48 5.95
Cu3Ti3O Cu3Ti3O + Ti5Si3 Ag(s,s)+Ti5Si3 Cu3Ti3O + TixSiy Cu3Ti3O + TixSiy
only connected with SiO2f/SiO2 via SiO2 matrix, which was easy to break and leave on the reaction layer. The vertical fibers were caught by the whole composites, and their bonding strength with brazing alloy mainly affected the shear strength of the joints. So the fracture could be divided into two kind area correspondingly. The fracture morphology of original SiO2f/SiO2 was shown in Fig. 10a. Some white stripe can be seen, which may be the parallel fibers. Most area of fracture was black, indicating the break mostly happened at the reaction layer. As shown in Fig. 10b, the brittle crack happened at the reaction layer and no vertical fibers fracture could be found. It is due to the weak reaction between SiO2 and brazing alloy. The brazing alloy had no ability to capture vertical fibers. Under high stress condition, the weak reaction layer would break directly. When 4 h-SiO2f/SiO2 was brazed as shown in Fig. 10c, it was obvious that many parallel fibers were ripped off. The fracture color mostly seemed white, suggesting the break of SiO2 matrix or vertical fibers. The magnified morphology in Fig. 10d shows that many vertical fibers were broken or pull out during the shear test. And
Fig. 9. Shear strength of SiO2f/SiO2-Nb joints with various reducing time.
the fracture of vertical was uneven, indicating the crack happened to deflect. It was sensible that the bonding between vertical fibers and brazing alloy was so strong that fulfilled the fracture advantages of SiO2f/SiO2 composites. And this phenomenon was contributed by good interfacial reaction, pinning effect and the decease of residual stress. 4. Conclusions Si layer synthesized on SiO2f/SiO2 surface via magnesiothermic reduction was developed successfully to satisfy the high strength joining
Fig. 8. XRD patterns of the reaction layer of (a) 0 h-SiO2f/SiO2-Nb joints and (b) 4 h-SiO2f/SiO2-Nb joints. 31
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Fig. 10. Fracture morphology of SiO2f/SiO2-Nb joints with (a)(b) 0 h and (c)(d) 4 h reduction time.
Acknowledgements
of SiO2f/SiO2 and Nb. Microstructure evolution and mechanical properties improvement were concluded as follows:
The support from the National Natural Science Foundation of China (Grant Nos 51575135, 51501049 and U1537206), is highly appreciated.
(1) The contact angle was decreased from 95.9° to 34.7°, which was close to the contact angle of AgCuTi on pure Si 25.8°. It will guarantee the reaction and infiltration of brazing alloy and SiO2f/ SiO2. (2) Due the reaction of Si and Mg, quartz fibers emerged at composites surface inducing the formation of zigzag reaction layer. It will increase the joining area to maintain the high strength bonding between SiO2f/SiO2 and brazing alloy. (3) The typical component of reaction layer changed from TiO + Ti5Si3/Cu3Ti3O to TiO + Ti5Si3/Cu3Ti3O + TixSiy. Ti-Si phase became the main component of reaction layer, which consumed less Ti to maintain high activity of brazing alloy. Low CTE Ti5Si3 particles dispersed among reaction layer, which can modify the CTE mismatch between composites and alloy. (4) The highest shear strength of joints reached 38.3 MPa with 4 h reduction, which was twice higher than original joints. And appropriate reduction ensures the strength above 30 MPa, which satisfies various application. Magnesiothermic reduction method reveals good parameter zone and adaptability.
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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. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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