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Study of ion bombardment of SiC ceramics: Surface and interfacial reaction modification
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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Study of ion bombardment of SiC ceramics: Surface and interfacial reaction modification Z.B. Chena,c, S.P. Hua,c,*, X.K. Duanc, M. Danb, X.G. Songa,c,*, W.M. Longd, J.C. Fenga,c a
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China Southwestern Institute of Physics, Chengdu, 610041, China c Shandong Provincial Key Lab of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai, 264209, China d State Key Laboratory of Advanced Brazing Filler Metals and Technology, Zhengzhou Research Institute of Mechanical Engineering, Zhengzhou, 450001, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ion bombardment Microstructure Brazing Ceramics
Ar ion bombardment was conducted to modify the SiC surface microstructures, which had a vital effect on the interfacial microstructure and shear property of brazing joints. The amorphous layer with thickness of ∼120 nm was formed on the bombarded surface, accompanied with plenty of dislocations and twins beneath the amorphous layer. Reliable SiC/AgCu-Ti/SiC joints were brazed in vacuum at 900 °C for 10 min, and the interfacial microstructure was investigated by SEM, EDS and TEM in detail. When the ion bombarded SiC was used as substrates, the microstructure of brazing beam was optimized as SiC / Ti5Si3 + TiC mixed layer / Ag(s,s) + Cu (s,s) containing TiCu / Ti5Si3 + TiC mixed layer / SiC, in which the interfacial stratification was eliminated compared to the conventional SiC brazing. The shear strength was improved to 30.9 MPa with ion bombardment, which was ∼72.6 % higher than that of the original SiC joints without ion bombardment. The proposed Ar ion bombardment method provides a novel way to modify the brazability of ceramics.
1. Introduction Recently, SiC ceramics with its excellent mechanical properties, good thermal conductivity and low thermal expansion coefficient, has attracted increasingly attention for applications in the industry fields of aerospace, automotive, energy, electronics, nuclear and optical devices and so on [1–4]. In practical applications, SiC ceramics must be firmly joined to themselves or to metals. Therefore, reliable joining of SiC ceramics is absolutely necessary to broaden its application range [5]. Brazing, as a simple, economic and precision joining method, is the most commonly used to join ceramics among various join methods [6–8]. However, the poor wettability of ceramics often restricts the effective brazing of ceramics. At present, indirect brazing with premetallization and direct brazing using active filler are usually used to realize ceramics joining. It is well known that the surface state has a vital effect on the wetting of the filler metal on ceramics surface and the properties of brazing joints. Recently, ion bombardment is considered a favored technology to modify the surface and near-surface of materials [9–13], which is a great of influence on the wetting and bonding of molten filler on ceramics. Howlader et al. [14,15] used sequential plasma activation containing reactive ion etching followed by microwave radicals to treat
⁎
the surfaces of silicon and glass, then the void-free and strong silicon/ glass boing at low temperature was achieved. They pointed out the improved results were contributed to the highly reactive and clean surfaces enhancing the mobility of alkaline cations due to the surface activation. Wang et al. [16] studied the influence of Ar mixed with 5 % H2 plasma pretreatment on the surface properties of electro-plated Cu for Cu-Sn low temperature bonding. The results demonstrated that the plasma treatment could effectively increase the surface activity via reducing the O content of the Cu surface. Park et al. [17] investigated the effects of Ar plasma conditions on the Cu surface and Cu-Cu bonding. They indicated that the plasma treatment time strongly affected the Cu surface morphology, while the contact angle and grain size were affected by pressure and rf power. In addition, plasma using different gases, such as He, N2 and O2, were also conducted to treat the substrate surface for bonding [18–20]. In general, plasma treatment is an effective method to remove oxide and improve surface activity of base material for microelectromechanical systems (MEMS) applications. However, no relevant literatures study the plasma treatment or ion bombardment for ceramics bonding. The object of this work was to explore new surface modification treatment to activate ceramics surfaces, which would improve the brazing behavior between molten filler and activated substrates. We
Corresponding authors at: State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China. E-mail addresses: [email protected] (S.P. Hu), [email protected] (X.G. Song).
https://doi.org/10.1016/j.jeurceramsoc.2019.11.008 Received 25 August 2019; Received in revised form 31 October 2019; Accepted 4 November 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Z.B. Chen, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.11.008
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Fig. 1. (a) SEM image and (b) XRD pattern of SiC substrate.
degree less than 3.0 × 10−3Pa. The whole brazing process can be divided into three steps. Firstly, all the assemblies were heated to 740 °C at a rate of 20 °C/min and remained for 10 min to keep a uniform temperature distribution. Secondly, the assemblies were heated to the brazing temperature, i.e. 900 °C, at a rate of 10 °C/min and then held for 10 min. Lastly, the specimens were cooled down to 300 °C at 5 °C/ min. Surface characteristics of bombarded SiC ceramics were studied by scanning electron microscopy (SEM, MERLIN Compact, ZEISS), glancing X-ray diffraction (GIXRD) and transmission electron microscopy (TEM, FEI TECNAI F30). The structure of as-bombarded sample was determined by Raman spectra (Renishaw, RM-1000) using a 532 nm laser. The morphology and reaction phases were studied by TEM and SEM equipped with energy dispersive spectrum analysis (EDS, OCTANE PLUS, EDAX). The samples for TEM observation were fabricated by focused ion beam (FIB). Moreover, the room-temperature shear strengths of brazed joints were tested at a constant speed of 0.5 mm/ min by a universal testing machine (Instron 5967), and the schematic diagram of shear test was shown in ref. [21]. Each shear test under different conditions was performed three times and then the averaged value was taken for this work.
presented a study of ion bombardment on surface microstructure modification, subsequent wetting and brazing behaviors on SiC ceramics. Such design aimed to modify the SiC ceramics surface obtaining metastable state or “activation” state. The results showed that the ion bombardment was an effective method for surface activation of SiC ceramics, which was of great significance for the subsequent joining of ceramics.
2. Experimental procedure The substrate materials used in the experiment were SiC ceramics sintered by 99 wt.% SiC powder, 0.5 wt.% C and 0.5 wt.% B, which were supplied by Shanghai Unite Technology Co., Ltd. Shanghai, China. The micro-characterization of SiC ceramics was shown in Fig. 1. It revealed that SiC ceramics mainly consisted of SiC phase while C and B were not detected which may due to the low content. The dimensions of SiC substrates used for brazing experiment were about 17 mm × 8 mm × 5 mm and 5 mm × 5 mm × 5 mm. All the samples were ultrasonically cleaned for 15 min in acetone and dried by air blowing prior to the experiment. Then Ar ion bombardment was performed on the cleaned samples with the acceleration voltage of 50 keV and ion bombardment fluence of 1 × 1017ions/cm2. The temperature of the SiC ceramics was about 200 °C and the vacuum was better than 1 × 10−3Pa during Ar ion bombardment. The AgCu-1Ti (wt.%) filler metal with the melting point of 780 °C was used for brazing experiments. Before brazing experiment, AgCu-Ti filler was fixed between two pieces of SiC ceramics to assemble the sandwich structure, as shown in ref. [21], and a slight pressure of 60 kPa was applied on each assembly to maintain proper contact. The brazing process was conducted in a vacuum furnace with vacuum
3. Results and discussion 3.1. Ion bombardment induced surface modification of SiC ceramics After Ar ion bombardment, the bombarded SiC ceramics surface was characterized by SEM and GIXRD, as shown in Fig. 2. It was worth noting that the surface was smooth compared with the original sample shown in Fig. 1 and no new diffraction peaks were detected in the
Fig. 2. (a) SEM image and (b) GIXRD pattern of bombarded SiC surface. 2
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Surface microstructure of bombarded SiC were analyzed by TEM and the results were shown in Fig. 4. As shown in Fig. 4(a), the surface microstructure of SiC substrate was changed significantly. The bubblelike layer was packed tightly at the surface while a flat and clean layer existed inside. Then the black layer containing a large number of dislocations existed in the substrate. Four regions marked as area I, II, III and IV could be divided by morphology. The results of Fig. 4(b)–(e) showed that an amorphous layer with thickness of about 119.2 nm was formed at the surface composed of area I and area II. Bubble formation were observed at area I. It can be seen that an obvious interface between area I and area II were existed (Fig. 4(f)), which may be caused by the inconsistent degree of Ar ion bombardment in the two areas. Many literatures also had indicated that crystal materials, especially SiC, can be amorphized under electron, neutron, and ion irradiation [26–30]. Jones et al. [31,32] indicated that SiC can be amorphized upon ions irradiation below 300 °C and the amorphization of SiC under irradiation occurred by the accumulation of point defects produced by atomic displacement. In our study, large numbers of point defects composed of C and Si Frenkel pairs and antisite defects were produced at SiC surface by Ar ions bombardment [25]. These defects gradually accumulated to form defects cluster, then amorphous domains or larger cluster were generated during continued cascade overlap, the coalescence of amorphous domain with small or larger clusters gives rise to the formation of larger amorphous regions [33,34]. Thus, amorphous layer was formed at SiC surface. As a superstructure, the diffraction pattern from SiC substrate contains six subsets of intensity in Fig. 4(g) and (i). One is from the primary SiC sub-cell; the other is from the supercell of ordered SiC induced by ion bombardment. From Fig. 4(g)–(j), it revealed that twin crystal was formed in SiC induced by ion bombardment which were corresponded to area III and IV. Most twin boundaries were perfectly coherent and atomically sharp (Fig. 4(h) and (j)). From a thermodynamic point of view, the formation of twins could lower strain of the sample by relaxation, because the excess energy for coherent twin boundaries is much smaller than that for conventional high-angle grain boundaries [35]. The twinning occurred could be attributed to numerous point defects induced by Ar ions bombardment [36]. Under continuous
Fig. 3. Raman spectra of SiC before and after ion bombardment.
bombarded layer, which meant that the new phase might exist in amorphous or highly dispersed form if any new phase was formed [22]. The results showed that the basic line of SiC structure shifted left slightly to lower diffraction angle with the shift value of 0.02°, indicating the existing lattice expansion after Ar ion bombardment [23]. Fig. 3 showed the Raman spectra results of SiC before and after ion bombardment. It was clearly observed that two evident peaks at around 800 and 900 cm−1 are characteristic for the SieC vibration [24] in both samples. The intensity of the characteristic peaks decreased strongly after ion bombardment but maintained the shape of the original sample. These results indicated that the ion bombardment caused serious damage to the matrix, decreased the crystallization and formed the long-range disordered structure. The Raman characteristic peaks of SiC did not shift after bombardment, which indicated that the lattice stress caused by bombardment was not enough to cause the Raman peak shifting. Generally, the breaking of SieC bond leaded to the atomic displacement, and the accumulation of Frenkel pairs and antisite defects [25], resulted in the attenuation of Raman polarization tensor.
Fig. 4. TEM, SAED and HRTEM images from the cross-sectional bombarded SiC sample. 3
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(Figs. 5 and 6), which was composed of Ag(s,s) and Cu(s,s) with some TiCu compounds (Table 1). It is notable that the reactions between SiC and active filler metal containing Ti element could form types of products, such as TiC [37–40], Ti-Si [37,38,40,41], Cu-Ti [39] and Ti-Si-C compounds [42] during brazing. To have a further identification on reaction products and stratification phenomenon, TEM and corresponding SAED, HRTEM were conducted on the SiC/AgCu-Ti interface with original and ion bombarded SiC ceramics, respectively. Fig. 7 showed the cross-sectional TEM image and corresponding SAED and HRTEM patterns of original SiC/AgCu-Ti interface. The neat crack bands existed at the whole interface of SiC and brazing seam, which might be caused by excessive stress during brazing. The reaction layer was composed of two sublayers, i.e., TiC sublayer (Spot A) containing some Ti5Si3 particles (Spot B) near SiC substrate and Ti5Si3 sublayer of coarse grain (shown as Spot C) adjacent to the TiC sublayer. TiC was nucleated firstly and then grew along SiC surface to form a continuous TiC sublayer. At last, Ti5Si3 crystal grains were generated and distributed on the TiC sublayer. This is consistent with the EDS results above, indicating that the interfacial reaction layer of the original SiC is composed of two sublayers named TiC sublayer and Ti5Si3 sublayer, and TiC is adjacent to the SiC substrate. The HRTEM analysis of TiC/Ti5Si3 interface shown in Fig. 7 illustrated that (200) TiC with the spacing of 0.226 nm were compacted with (002) Ti5Si3 with the spacing of 0.251 nm. In order to analysis the lattice mismatch between TiC sublayer and Ti5Si3 sublayer, the matching between crystals can be evaluated according to the Eq. [43]:
bombardment, surviving vacancies could accumulate into plates, thereby causing atomic collapse. High-density bombardment-induced dislocations were formed in area II, as shown in Fig. 4(a). Thus, twins were formed due to the coalescence of dislocation loops facilitating the growth of stacking faults where the twins nucleate, which were used to absorb point defects induced by bombardment. The twinning was occurred beneath the amorphous layer formed at the surface, which may be caused by the difference of bombardment degree with depth increased. In summary, surface microstructure of SiC ceramics was modified dramatically by ion bombardment. An amorphous layer with thickness of ∼120 nm was obtained for ion bombardment fluence of 1 × 1017ions/cm2 at 50 keV. After ion bombardment, a large number of dislocations existed and twinning were occurred in the substrate beneath the amorphous layer. 3.2. Brazing of SiC ceramics modified by ion bombardment The bombarded microstructure of SiC ceramics surface was considered to have a significant impact on the brazing of SiC/AgCu-Ti system. Thus, brazing experiments of the ion bombarded and original SiC ceramics were adopted to evaluate the effect of surface modification induced by ion bombardment on brazing behavior. The brazing of SiC ceramics was conducted under 900 °C for 10 min for evaluating the effect of ion bombardment on the microstructure and shear property of brazed joint. Fig. 5 showed the interfacial microstructure of SiC/AgCu-Ti/SiC brazed joint and corresponding EDS map analyses. Sound joints without any cracks and pores were obtained and reaction layer containing Ti element at SiC/brazing seam interface were formed for both SiC states. As shown in Fig. 6, the thickness of reaction layer decreased from 1.07 μm to 0.6 μm when SiC was pre-treated by ion bombardment. It was clearly seen that the reaction layer in original SiC joints was composed of two sublayers marked as I and II in Fig. 6(a), while no stratification was occurred in bombarded SiC joint sample. On the basis of the EDS results shown in Table 1, the reaction layer in both of the SiC joints were Ti-C and Ti-Si compounds. After ion bombardment, the microstructure of brazing seam was homogenized and refined
F = 2(dI - dII) / (dI + dII)
(1)
where dI and dII stand for the corresponding d-spacings of two contacted reaction layers, respectively. The lattice mismatch to be 10.48 % demonstrated the incoherent interface with high strain energy was formed, which indicated the match of TiC sublayer and Ti5Si3 sublayer was not so good. Based on the above analysis, the whole interfacial reaction process between AgCu-Ti filler metal and original SiC without ion bombardment can be divided into two steps. Firstly, the AgCu-Ti filler metal
Fig. 5. Interfacial microstructure of the brazed joint and corresponding EDS map analyses: (a) (b) original SiC ceramics; (c) (d) ion bombarded SiC ceramics. 4
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Fig. 6. Interfacial microstructure of the brazed joint: (a) original SiC ceramics; (b) ion bombarded SiC ceramics.
5/3Ti+[Si] = 1/3Ti5Si3, ΔG=-220.4 + 26.5 × 10−3T(kJ/mol)
Table 1 EDS results and possible phases of each spot labeled in Fig. 6. (at.%). Spot
Ag
Cu
Ti
Si
C
Possible phase
A B C D E
4.9 86.4 18.6 2.8 16.9
2.7 13.4 35.5 94.7 15.3
34.0 0.2 28.2 2.5 28.7
26.9 0 12.3 0 13.6
31.5 0 5.4 0 25.5
Ti-C + Ti-Si Ag(s,s) TiCu Cu(s,s) Ti-C + Ti-Si
Where T denotes to temperature, K. The Gibbs free energies of reactions (2) and (3) are about −125.76 kJ/mol and −189.32 kJ/mol at 1173 K (900 °C), respectively. This suggests that the formation of TiC and Ti5Si are favored. Thus, a reaction layer composed of two sublayers, i.e., TiC and Ti5Si3, was formed at the interface of SiC/AgCu-Ti/SiC joint without ion bombardment. The TEM and SEM characteristics demonstrated the microstructure of original SiC/AgCu-Ti/SiC joint was SiC/ TiC/Ti5Si3/Ag(s,s)+Cu(s,s) containing TiCu/Ti5Si3/TiC/SiC. Fig. 8 showed the TEM image and SAED pattern of SiC/AgCu-Ti interface pre-treated by ion bombardment. Relatively fine TiC and Ti5Si3 particles were distributed randomly and uniformly at reaction layer adjacent to SiC substrate. It can be noticed that no stratification occurred differing from the results of original SiC sample. As TiC and Ti5Si3 particles could act as the nucleation cores, TiCu compounds with thickness of 1∼2 μm was formed near the interfacial reaction layer via the reaction of Ti and Cu in the filler metal under high temperature. Ag (s,s) and Cu(s,s) with micron size were also detected contributed to
converts into liquid when the brazing temperature exceeds melting point of filler metal, then Ti atoms enriched at interface react with SiC, forming TiC and [Si], revealed as Eq. (2) [37,38,44]. The brackets indicate that Si elements are dissolved in liquid filler. Then, the free Si atoms react with Ti atoms producing Ti5Si3, illustrated as Eq.(3) [37,38,44]. Ti + SiC = TiC+[Si], ΔG=-136.9 + 9.5 × 10−3T(kJ/mol)
(3)
(2)
Fig. 7. TEM characteristic of original SiC/AgCu-Ti interface. 5
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Fig. 8. TEM characteristic of SiC/AgCu-Ti interface with ion bombardment.
crystal face of TiC and (204) crystal face of SiC, respectively. TiC and Ti5Si3 phases were tightly embedded into SiC substrate. The interfacial misfit could be calculated by the equations as follow [43,45,46]:
brazing seam. Fig. 9 showed the HRTEM images at bombarded SiC/brazing seam interface. As shown in Fig. 9(a), the spacing of 0.248 nm, 0.232 nm and 0.212 nm were matched with the (210) crystal face of Ti5Si3, (200)
Fig. 9. HRTEM characteristic of bombarded SiC joint interface from Fig.12. 6
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Fig. 10. (a) Shear strength and fracture surfaces of brazed joints using (b) (c) original SiC ceramics, (d) (e) ion bombarded SiC ceramics.
F = 2(mdII - ndI) / (mdI + ndII)
(4)
dII/dI=n/m
(5)
layer /Ag(s,s)+Cu(s,s) containing TiCu/ Ti5Si3 + TiC mixed layer /SiC. Based on the above analysis, the interfacial reaction layer was formed by one-step chemical reaction when SiC was pre-treated by ion bombardment, which was different from that on original sample. When SiC was ion bombarded, the SieC bond of the substrate surface were broken forming an amorphous layer composed of Frenkel pairs and antisite defects [25]. During brazing, these Frenkel pairs and antisite defects could react with Ti directly with the reaction equation shown below:
where dI and dII represent the corresponding d-spacings of SiC, TiC and Ti5Si3, respectively, and m and n are integers, then a fraction of the interfacial atoms in I lie in atomic sites on the surface of II. For the Ti5Si3/SiC interface, dTi5Si3 (210) =0.248 nm and dSiC (204) =0.212 nm, so m, n equal to 7 and 6, respectively, i.e. six (210) planes of Ti5Si3 match seven (204) planes of SiC. By the calculating results, the lattice mismatch of Ti5Si3/SiC interface was 0.27 %. Similarly, the lattice mismatch of TiC/SiC interfaces was calculated to be 0.31 % according to Eqs. (4) and (5). Thus, both of Ti5Si3/SiC and TiC/SiC structures were semi-coherent interfaces with low strain energy. The lattice mismatch of TiC/Ti5Si3 interfaces was calculated to be 6.7 % according to Eq. (3), which indicated the match of TiC layer and Ti5Si3 layer was better than that of the original sample shown in Fig. 7. Fig. 9(b) showed the spacing of 0.237 nm and 0.202 nm matched with the (200) crystal face of TiC and (102) crystal face of TiCu, and the misfit of TiC/TiCu interface was 15.9 % indicated the incoherent interface with high strain energy was formed. Fig. 9(c) and (d) showed the HRTEM images of TiCu/Cu and Ag/Cu interfaces, respectively. The mismatch of each interface was calculated as 2.43 % and 12.13 %. Based on the TEM and SEM characteristics, microstructure of SiC/ AgCu-Ti/SiC joint with ion bombardment was SiC/ Ti5Si3 + TiC mixed
6Ti+3[Si]+[C]=TiC + Ti5Si3, ΔG=-787.3 + 22.1 × 10−3T(KJ/mol) (6) Where T denotes to temperature ranges from 1123 K to 1473 K. The ΔG values of forming solid-state TiC and Ti5Si3 simultaneously are −761.38 kJ/mol at brazing temperature of 1173 K (900 °C), indicating that the reaction (6) is exergonic and proceed spontaneously. Therefore, a mixed reaction layer containing TiC and Ti5Si3 particles was formed at the interface of SiC/AgCu-Ti/SiC joint with pre-treatment by ion bombardment. Besides, the active Ti reacts with liquid filler forming TiCu intermetallic compounds, and the reaction between Ti and Cu are listed as Eq. (7) [40]. The ΔG values of forming TiCu are −11.34 kJ/mol at 1173 K (900 °C), the negative value of ΔG indicated that the TiCu was favor to form at brazing seam. 7
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Ti + Cu = TiCu, ΔG = −17.069 + 4.887 × 10−3T(KJ/mol)
Nanyang Technological University (NTU) in Singapore as an exchange student.
(7)
The room-temperature shear strength of SiC/AgCu-Ti/SiC joints brazing at 900 °C for 10 min were tested and the results were shown in Fig. 10(a). The results indicated that the ion bombardment influenced the shear strength obviously. The shear strength of the joints improved with Ar ion bombardment and the optimal shear strength of 30.9 MPa was reached, which was ∼72.6 % higher than that of the SiC brazed joints without ion bombardment. The appropriate thickness of TiC + Ti5Si3 mixed reaction layer could contribute to the optimal shear strength. Fig. 10(b)–(e) showed the fracture surface morphology of SiC/ AgCu-Ti/SiC joints with original and ion bombarded SiC ceramics, respectively. For original sample, the joint fractured at SiC/brazing seam interface after shear test as shown in Fig. 10(b). It was shown that SiC and reaction layer, i.e., TiC and Ti5Si3 compounds, appeared alternately on the fracture surface in Fig. 10(c). In fact, pores and cracks were existed at the SiC/ interfacial reaction layer for the original brazing sample shown in Fig. 7, indicating large residual stress existed in the joint. For the bombarded sample, the joint cracked from one side of SiC substrate, ruptured through the brazing seam, then extended to the other SiC side, as illustrated in Fig. 10(d). Fig. 10(e) showed the SEM fracture surface of area I and II depicted in Fig. 10(d). In area I, no obvious plastic deformation was occurred, and the fracture was flat and granular, exhibiting a typical brittle cleavage fracture in SiC substrate. A large number of tear ridges and flat fracture surface were observed at area II, indicating that the fracture type was also brittle model. The fracture path showed that ion bombardment effectively improved the bonding between brazing seam and substrate, thus, the shear property of brazed joints was enhanced obviously.
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Gao, W.J. Weber, Amorphization of silicon carbide by carbon displacement, Appl. Phys. Lett. 84 (19) (2004) 3909–3911. [26] K.E. Sickafus, L. Minervini, R.W. Grimes, J.A. Valdez, M. Ishimaru, F. Li, K.J. Mcclellan, T. Hartmann, Radiation tolerance of complex oxides, Science 289 (5480) (2000) 748–751. [27] H. Inui, H. Mori, A. Suzuki, H. Fujita, Electron-irradiation-induced crystalline-toamorphous transition in β-SiC single crystals, Philos. Mag. B 65 (1) (1992) 1–14. [28] M. Ishimaru, I. Bae, Y. Hirotsu, Electron-beam-induced amorphization in SiC, Phys. Rev. B 68 (14) (2003) 144102.
4. Conclusions The surface of SiC ceramics was modified by Ar ion bombardment at 50 keV with the fluence of 1 × 1017ions/cm2. The brazing behavior of SiC ceramics using AgCu-Ti filler were also studied. Primary conclusions were summarized as follows: (1) SieC bonds were broken, and lots of Frenkel pairs and antisite defects were formed in ion bombardment, resulting in the formation of amorphous layer at the surface. The thickness of amorphous layer was ∼120 nm when the ion bombardment was performed at 50 keV with the fluence of 1 × 1017ions/cm2. In addition, more dislocation and twins were formed beneath the amorphous layer after bombardment. (2) The interfacial microstructure of SiC/AgCu-Ti/SiC brazed joint was SiC/Ti5Si3+TiC mixed layer/Ag(s,s)+Cu(s,s) containing TiCu/ Ti5Si3+TiC mixed layer/SiC when ion bombardment was employed. In addition, the interfacial stratification, which was observed in the original SiC joint without ion bombardment, was eliminated using bombarded SiC. (3) The shear strength of SiC brazed joints was improved with Ar ion bombardment, and the optimal shear strength was 30.9 MPa, which was ∼72.6 % higher than that of the joints without ion bombarded. Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 51775138 and U1737205), the Natural Science Foundation of Shandong Province (No. ZR2019BEE031), the Key Research & Development program of Shandong Province (No. 2017GGX40103) and Science and Technology Support program of Sichuan Province (No. 2016NZ0106). Z.B. Chen acknowledges the support from the China Scholarship Council (CSC) for one-year studying at School of Materials Science and Engineering, 8
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[38] M. Li, X. Song, S. Hu, Z. Chen, Y. Song, C. Niu, Effect of Ti addition on the wetting and brazing of Sn0.3Ag0.7Cu filler on SiC ceramic, J. Am. Ceram. Soc. 102 (6) (2019) 3318–3328. [39] J.M. Shi, J.C. Feng, X.Y. Tian, H. Liu, L.X. Zhang, Interfacial microstructure and mechanical property of ZrC-SiC ceramic and Ti6Al4V joint brazed with AgCuTi alloy, J. Eur. Ceram. Soc. 37 (8) (2017) 2769–2778. [40] Z. Chen, H. Bian, C. Niu, X. Song, X. Duan, J. Cao, J. Feng, Titanium-deposition assisted brazing of SiC ceramics using inactive AgCu filler, Mater. Charact. 142 (2018) 219–222. [41] J. Yang, J. Huang, Z. Ye, S. Chen, R. Ji, Y. Zhao, Influence of interfacial reaction on reactive wettability of molten Ag-Cu-X wt.%Ti filler metal on SiC ceramic substrate and mechanism analysis, Appl. Surf. Sci. 436 (2018) 768–778. [42] Y. He, Y. Sun, J. Zhang, X. Li, An analysis of deformation mechanism in the Si3N4–AgCuTi+SiCp–Si3N4 joints by digital image correlation, J. Eur. Ceram. Soc. 33 (1) (2013) 157–164. [43] L. Zheng, F. Li, Y. Zhou, Preparation, Microstructure, and Mechanical Properties of TiB2 Using Ti3AlC2 as a Sintering Aid, J. Am. Ceram. Soc. 95 (6) (2012) 2028–2034. [44] S.N. Li, H.P. Xiong, N. Li, B.Q. Chen, C. Gao, W.J. Zou, H.S. Ren, Mechanical properties and formation mechanism of Ti/SiC system gradient materials fabricated by in-situ reaction laser cladding, Ceram. Int. 43 (1) (2017) 961–967. [45] Y. Wang, J. Feng, Z. Chen, X. Song, J. Cao. Wetting of microstructured alumina fabricated by epitaxial growth of Al4B2O9 whiskers, Appl. Surf. Sci. 357 (2015) 766–770. [46] F.R.N. Nabarro, Dislocations in Crystals, Oxford, North‐Holland, 1979 p. 466.
[29] L.L. Snead, J.C. Hay, Neutron irradiation induced amorphization of silicon carbide, J. Nucl. Phys. Mater. Sci. Radiat. Appl. 273 (2) (1999) 213–220. [30] W.J. Weber, L.M. Wang, The temperature dependence of ion-beam-induced amorphization in β-SiC, Nucl. Instrum. Methods Phys. Res. B 106 (1) (1995) 298–302. [31] A. Hasegawa, A. Kohyama, R.H. Jones, L.L. Snead, B. Riccardi, P. Fenici, Critical issues and current status of SiC/SiC composites for fusion, J. Nucl. Phys. Mater. Sci. Radiat. Appl. 283 (2000) 128–137. [32] R.H. Jones, L. Giancarli, A. Hasegawa, Y. Katoh, A. Kohyama, B. Riccardi, L.L. Snead, W.J. Weber, Promise and challenges of SiCf/SiC composites for fusion energy applications, J. Nucl. Phys. Mater. Sci. Radiat. Appl. 307–311 (2002) 1057–1072. [33] M. Jiang, H.Y. Xiao, H.B. Zhang, S. Peng, C.H. Xu, Z.J. Liu, X.T. Zu, A comparative study of low energy radiation responses of SiC, TiC and ZrC, Acta Mater. 110 (2016) 192–199. [34] F. Gao, W.J. Weber, R. Devanathan, Defect Production, Multiple Ion-Solid Interactions and Amorphization in SiC, Nucl. Instrum. Methods Phys. Res. B 191 (1) (2002) 487–496. [35] L. Lu, Y. Shen, X. Chen, L. Qian, K. Lu, Ultrahigh Strength and High Electrical Conductivity in Copper, Science 304 (5669) (2004) 422. [36] J. Ding, S. Yang, B. Zhu, H. Liu, G. Liu, L. Zhou, Q. Zhan, F. Wan, Twins induced by high-temperature ion irradiation in body-centered cubic V-4Cr-4Ti alloy, Scripta Mater. 162 (2019) 377–381. [37] X. Song, A. Passerone, W. Fu, S. Hu, C. Niu, Y. Zhao, M. Wang, F. Valenzab, Wetting and spreading behavior of Sn–Ti alloys on SiC, Materialia 3 (2018) 57–63.
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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Study of ion bombardment of SiC ceramics: Surface and interfacial reaction modification Z.B. Chena,c, S.P. Hua,c,*, X.K. Duanc, M. Danb, X.G. Songa,c,*, W.M. Longd, J.C. Fenga,c a
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China Southwestern Institute of Physics, Chengdu, 610041, China c Shandong Provincial Key Lab of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai, 264209, China d State Key Laboratory of Advanced Brazing Filler Metals and Technology, Zhengzhou Research Institute of Mechanical Engineering, Zhengzhou, 450001, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ion bombardment Microstructure Brazing Ceramics
Ar ion bombardment was conducted to modify the SiC surface microstructures, which had a vital effect on the interfacial microstructure and shear property of brazing joints. The amorphous layer with thickness of ∼120 nm was formed on the bombarded surface, accompanied with plenty of dislocations and twins beneath the amorphous layer. Reliable SiC/AgCu-Ti/SiC joints were brazed in vacuum at 900 °C for 10 min, and the interfacial microstructure was investigated by SEM, EDS and TEM in detail. When the ion bombarded SiC was used as substrates, the microstructure of brazing beam was optimized as SiC / Ti5Si3 + TiC mixed layer / Ag(s,s) + Cu (s,s) containing TiCu / Ti5Si3 + TiC mixed layer / SiC, in which the interfacial stratification was eliminated compared to the conventional SiC brazing. The shear strength was improved to 30.9 MPa with ion bombardment, which was ∼72.6 % higher than that of the original SiC joints without ion bombardment. The proposed Ar ion bombardment method provides a novel way to modify the brazability of ceramics.
1. Introduction Recently, SiC ceramics with its excellent mechanical properties, good thermal conductivity and low thermal expansion coefficient, has attracted increasingly attention for applications in the industry fields of aerospace, automotive, energy, electronics, nuclear and optical devices and so on [1–4]. In practical applications, SiC ceramics must be firmly joined to themselves or to metals. Therefore, reliable joining of SiC ceramics is absolutely necessary to broaden its application range [5]. Brazing, as a simple, economic and precision joining method, is the most commonly used to join ceramics among various join methods [6–8]. However, the poor wettability of ceramics often restricts the effective brazing of ceramics. At present, indirect brazing with premetallization and direct brazing using active filler are usually used to realize ceramics joining. It is well known that the surface state has a vital effect on the wetting of the filler metal on ceramics surface and the properties of brazing joints. Recently, ion bombardment is considered a favored technology to modify the surface and near-surface of materials [9–13], which is a great of influence on the wetting and bonding of molten filler on ceramics. Howlader et al. [14,15] used sequential plasma activation containing reactive ion etching followed by microwave radicals to treat
⁎
the surfaces of silicon and glass, then the void-free and strong silicon/ glass boing at low temperature was achieved. They pointed out the improved results were contributed to the highly reactive and clean surfaces enhancing the mobility of alkaline cations due to the surface activation. Wang et al. [16] studied the influence of Ar mixed with 5 % H2 plasma pretreatment on the surface properties of electro-plated Cu for Cu-Sn low temperature bonding. The results demonstrated that the plasma treatment could effectively increase the surface activity via reducing the O content of the Cu surface. Park et al. [17] investigated the effects of Ar plasma conditions on the Cu surface and Cu-Cu bonding. They indicated that the plasma treatment time strongly affected the Cu surface morphology, while the contact angle and grain size were affected by pressure and rf power. In addition, plasma using different gases, such as He, N2 and O2, were also conducted to treat the substrate surface for bonding [18–20]. In general, plasma treatment is an effective method to remove oxide and improve surface activity of base material for microelectromechanical systems (MEMS) applications. However, no relevant literatures study the plasma treatment or ion bombardment for ceramics bonding. The object of this work was to explore new surface modification treatment to activate ceramics surfaces, which would improve the brazing behavior between molten filler and activated substrates. We
Corresponding authors at: State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China. E-mail addresses: [email protected] (S.P. Hu), [email protected] (X.G. Song).
https://doi.org/10.1016/j.jeurceramsoc.2019.11.008 Received 25 August 2019; Received in revised form 31 October 2019; Accepted 4 November 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Z.B. Chen, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.11.008
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Fig. 1. (a) SEM image and (b) XRD pattern of SiC substrate.
degree less than 3.0 × 10−3Pa. The whole brazing process can be divided into three steps. Firstly, all the assemblies were heated to 740 °C at a rate of 20 °C/min and remained for 10 min to keep a uniform temperature distribution. Secondly, the assemblies were heated to the brazing temperature, i.e. 900 °C, at a rate of 10 °C/min and then held for 10 min. Lastly, the specimens were cooled down to 300 °C at 5 °C/ min. Surface characteristics of bombarded SiC ceramics were studied by scanning electron microscopy (SEM, MERLIN Compact, ZEISS), glancing X-ray diffraction (GIXRD) and transmission electron microscopy (TEM, FEI TECNAI F30). The structure of as-bombarded sample was determined by Raman spectra (Renishaw, RM-1000) using a 532 nm laser. The morphology and reaction phases were studied by TEM and SEM equipped with energy dispersive spectrum analysis (EDS, OCTANE PLUS, EDAX). The samples for TEM observation were fabricated by focused ion beam (FIB). Moreover, the room-temperature shear strengths of brazed joints were tested at a constant speed of 0.5 mm/ min by a universal testing machine (Instron 5967), and the schematic diagram of shear test was shown in ref. [21]. Each shear test under different conditions was performed three times and then the averaged value was taken for this work.
presented a study of ion bombardment on surface microstructure modification, subsequent wetting and brazing behaviors on SiC ceramics. Such design aimed to modify the SiC ceramics surface obtaining metastable state or “activation” state. The results showed that the ion bombardment was an effective method for surface activation of SiC ceramics, which was of great significance for the subsequent joining of ceramics.
2. Experimental procedure The substrate materials used in the experiment were SiC ceramics sintered by 99 wt.% SiC powder, 0.5 wt.% C and 0.5 wt.% B, which were supplied by Shanghai Unite Technology Co., Ltd. Shanghai, China. The micro-characterization of SiC ceramics was shown in Fig. 1. It revealed that SiC ceramics mainly consisted of SiC phase while C and B were not detected which may due to the low content. The dimensions of SiC substrates used for brazing experiment were about 17 mm × 8 mm × 5 mm and 5 mm × 5 mm × 5 mm. All the samples were ultrasonically cleaned for 15 min in acetone and dried by air blowing prior to the experiment. Then Ar ion bombardment was performed on the cleaned samples with the acceleration voltage of 50 keV and ion bombardment fluence of 1 × 1017ions/cm2. The temperature of the SiC ceramics was about 200 °C and the vacuum was better than 1 × 10−3Pa during Ar ion bombardment. The AgCu-1Ti (wt.%) filler metal with the melting point of 780 °C was used for brazing experiments. Before brazing experiment, AgCu-Ti filler was fixed between two pieces of SiC ceramics to assemble the sandwich structure, as shown in ref. [21], and a slight pressure of 60 kPa was applied on each assembly to maintain proper contact. The brazing process was conducted in a vacuum furnace with vacuum
3. Results and discussion 3.1. Ion bombardment induced surface modification of SiC ceramics After Ar ion bombardment, the bombarded SiC ceramics surface was characterized by SEM and GIXRD, as shown in Fig. 2. It was worth noting that the surface was smooth compared with the original sample shown in Fig. 1 and no new diffraction peaks were detected in the
Fig. 2. (a) SEM image and (b) GIXRD pattern of bombarded SiC surface. 2
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Surface microstructure of bombarded SiC were analyzed by TEM and the results were shown in Fig. 4. As shown in Fig. 4(a), the surface microstructure of SiC substrate was changed significantly. The bubblelike layer was packed tightly at the surface while a flat and clean layer existed inside. Then the black layer containing a large number of dislocations existed in the substrate. Four regions marked as area I, II, III and IV could be divided by morphology. The results of Fig. 4(b)–(e) showed that an amorphous layer with thickness of about 119.2 nm was formed at the surface composed of area I and area II. Bubble formation were observed at area I. It can be seen that an obvious interface between area I and area II were existed (Fig. 4(f)), which may be caused by the inconsistent degree of Ar ion bombardment in the two areas. Many literatures also had indicated that crystal materials, especially SiC, can be amorphized under electron, neutron, and ion irradiation [26–30]. Jones et al. [31,32] indicated that SiC can be amorphized upon ions irradiation below 300 °C and the amorphization of SiC under irradiation occurred by the accumulation of point defects produced by atomic displacement. In our study, large numbers of point defects composed of C and Si Frenkel pairs and antisite defects were produced at SiC surface by Ar ions bombardment [25]. These defects gradually accumulated to form defects cluster, then amorphous domains or larger cluster were generated during continued cascade overlap, the coalescence of amorphous domain with small or larger clusters gives rise to the formation of larger amorphous regions [33,34]. Thus, amorphous layer was formed at SiC surface. As a superstructure, the diffraction pattern from SiC substrate contains six subsets of intensity in Fig. 4(g) and (i). One is from the primary SiC sub-cell; the other is from the supercell of ordered SiC induced by ion bombardment. From Fig. 4(g)–(j), it revealed that twin crystal was formed in SiC induced by ion bombardment which were corresponded to area III and IV. Most twin boundaries were perfectly coherent and atomically sharp (Fig. 4(h) and (j)). From a thermodynamic point of view, the formation of twins could lower strain of the sample by relaxation, because the excess energy for coherent twin boundaries is much smaller than that for conventional high-angle grain boundaries [35]. The twinning occurred could be attributed to numerous point defects induced by Ar ions bombardment [36]. Under continuous
Fig. 3. Raman spectra of SiC before and after ion bombardment.
bombarded layer, which meant that the new phase might exist in amorphous or highly dispersed form if any new phase was formed [22]. The results showed that the basic line of SiC structure shifted left slightly to lower diffraction angle with the shift value of 0.02°, indicating the existing lattice expansion after Ar ion bombardment [23]. Fig. 3 showed the Raman spectra results of SiC before and after ion bombardment. It was clearly observed that two evident peaks at around 800 and 900 cm−1 are characteristic for the SieC vibration [24] in both samples. The intensity of the characteristic peaks decreased strongly after ion bombardment but maintained the shape of the original sample. These results indicated that the ion bombardment caused serious damage to the matrix, decreased the crystallization and formed the long-range disordered structure. The Raman characteristic peaks of SiC did not shift after bombardment, which indicated that the lattice stress caused by bombardment was not enough to cause the Raman peak shifting. Generally, the breaking of SieC bond leaded to the atomic displacement, and the accumulation of Frenkel pairs and antisite defects [25], resulted in the attenuation of Raman polarization tensor.
Fig. 4. TEM, SAED and HRTEM images from the cross-sectional bombarded SiC sample. 3
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(Figs. 5 and 6), which was composed of Ag(s,s) and Cu(s,s) with some TiCu compounds (Table 1). It is notable that the reactions between SiC and active filler metal containing Ti element could form types of products, such as TiC [37–40], Ti-Si [37,38,40,41], Cu-Ti [39] and Ti-Si-C compounds [42] during brazing. To have a further identification on reaction products and stratification phenomenon, TEM and corresponding SAED, HRTEM were conducted on the SiC/AgCu-Ti interface with original and ion bombarded SiC ceramics, respectively. Fig. 7 showed the cross-sectional TEM image and corresponding SAED and HRTEM patterns of original SiC/AgCu-Ti interface. The neat crack bands existed at the whole interface of SiC and brazing seam, which might be caused by excessive stress during brazing. The reaction layer was composed of two sublayers, i.e., TiC sublayer (Spot A) containing some Ti5Si3 particles (Spot B) near SiC substrate and Ti5Si3 sublayer of coarse grain (shown as Spot C) adjacent to the TiC sublayer. TiC was nucleated firstly and then grew along SiC surface to form a continuous TiC sublayer. At last, Ti5Si3 crystal grains were generated and distributed on the TiC sublayer. This is consistent with the EDS results above, indicating that the interfacial reaction layer of the original SiC is composed of two sublayers named TiC sublayer and Ti5Si3 sublayer, and TiC is adjacent to the SiC substrate. The HRTEM analysis of TiC/Ti5Si3 interface shown in Fig. 7 illustrated that (200) TiC with the spacing of 0.226 nm were compacted with (002) Ti5Si3 with the spacing of 0.251 nm. In order to analysis the lattice mismatch between TiC sublayer and Ti5Si3 sublayer, the matching between crystals can be evaluated according to the Eq. [43]:
bombardment, surviving vacancies could accumulate into plates, thereby causing atomic collapse. High-density bombardment-induced dislocations were formed in area II, as shown in Fig. 4(a). Thus, twins were formed due to the coalescence of dislocation loops facilitating the growth of stacking faults where the twins nucleate, which were used to absorb point defects induced by bombardment. The twinning was occurred beneath the amorphous layer formed at the surface, which may be caused by the difference of bombardment degree with depth increased. In summary, surface microstructure of SiC ceramics was modified dramatically by ion bombardment. An amorphous layer with thickness of ∼120 nm was obtained for ion bombardment fluence of 1 × 1017ions/cm2 at 50 keV. After ion bombardment, a large number of dislocations existed and twinning were occurred in the substrate beneath the amorphous layer. 3.2. Brazing of SiC ceramics modified by ion bombardment The bombarded microstructure of SiC ceramics surface was considered to have a significant impact on the brazing of SiC/AgCu-Ti system. Thus, brazing experiments of the ion bombarded and original SiC ceramics were adopted to evaluate the effect of surface modification induced by ion bombardment on brazing behavior. The brazing of SiC ceramics was conducted under 900 °C for 10 min for evaluating the effect of ion bombardment on the microstructure and shear property of brazed joint. Fig. 5 showed the interfacial microstructure of SiC/AgCu-Ti/SiC brazed joint and corresponding EDS map analyses. Sound joints without any cracks and pores were obtained and reaction layer containing Ti element at SiC/brazing seam interface were formed for both SiC states. As shown in Fig. 6, the thickness of reaction layer decreased from 1.07 μm to 0.6 μm when SiC was pre-treated by ion bombardment. It was clearly seen that the reaction layer in original SiC joints was composed of two sublayers marked as I and II in Fig. 6(a), while no stratification was occurred in bombarded SiC joint sample. On the basis of the EDS results shown in Table 1, the reaction layer in both of the SiC joints were Ti-C and Ti-Si compounds. After ion bombardment, the microstructure of brazing seam was homogenized and refined
F = 2(dI - dII) / (dI + dII)
(1)
where dI and dII stand for the corresponding d-spacings of two contacted reaction layers, respectively. The lattice mismatch to be 10.48 % demonstrated the incoherent interface with high strain energy was formed, which indicated the match of TiC sublayer and Ti5Si3 sublayer was not so good. Based on the above analysis, the whole interfacial reaction process between AgCu-Ti filler metal and original SiC without ion bombardment can be divided into two steps. Firstly, the AgCu-Ti filler metal
Fig. 5. Interfacial microstructure of the brazed joint and corresponding EDS map analyses: (a) (b) original SiC ceramics; (c) (d) ion bombarded SiC ceramics. 4
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Fig. 6. Interfacial microstructure of the brazed joint: (a) original SiC ceramics; (b) ion bombarded SiC ceramics.
5/3Ti+[Si] = 1/3Ti5Si3, ΔG=-220.4 + 26.5 × 10−3T(kJ/mol)
Table 1 EDS results and possible phases of each spot labeled in Fig. 6. (at.%). Spot
Ag
Cu
Ti
Si
C
Possible phase
A B C D E
4.9 86.4 18.6 2.8 16.9
2.7 13.4 35.5 94.7 15.3
34.0 0.2 28.2 2.5 28.7
26.9 0 12.3 0 13.6
31.5 0 5.4 0 25.5
Ti-C + Ti-Si Ag(s,s) TiCu Cu(s,s) Ti-C + Ti-Si
Where T denotes to temperature, K. The Gibbs free energies of reactions (2) and (3) are about −125.76 kJ/mol and −189.32 kJ/mol at 1173 K (900 °C), respectively. This suggests that the formation of TiC and Ti5Si are favored. Thus, a reaction layer composed of two sublayers, i.e., TiC and Ti5Si3, was formed at the interface of SiC/AgCu-Ti/SiC joint without ion bombardment. The TEM and SEM characteristics demonstrated the microstructure of original SiC/AgCu-Ti/SiC joint was SiC/ TiC/Ti5Si3/Ag(s,s)+Cu(s,s) containing TiCu/Ti5Si3/TiC/SiC. Fig. 8 showed the TEM image and SAED pattern of SiC/AgCu-Ti interface pre-treated by ion bombardment. Relatively fine TiC and Ti5Si3 particles were distributed randomly and uniformly at reaction layer adjacent to SiC substrate. It can be noticed that no stratification occurred differing from the results of original SiC sample. As TiC and Ti5Si3 particles could act as the nucleation cores, TiCu compounds with thickness of 1∼2 μm was formed near the interfacial reaction layer via the reaction of Ti and Cu in the filler metal under high temperature. Ag (s,s) and Cu(s,s) with micron size were also detected contributed to
converts into liquid when the brazing temperature exceeds melting point of filler metal, then Ti atoms enriched at interface react with SiC, forming TiC and [Si], revealed as Eq. (2) [37,38,44]. The brackets indicate that Si elements are dissolved in liquid filler. Then, the free Si atoms react with Ti atoms producing Ti5Si3, illustrated as Eq.(3) [37,38,44]. Ti + SiC = TiC+[Si], ΔG=-136.9 + 9.5 × 10−3T(kJ/mol)
(3)
(2)
Fig. 7. TEM characteristic of original SiC/AgCu-Ti interface. 5
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Fig. 8. TEM characteristic of SiC/AgCu-Ti interface with ion bombardment.
crystal face of TiC and (204) crystal face of SiC, respectively. TiC and Ti5Si3 phases were tightly embedded into SiC substrate. The interfacial misfit could be calculated by the equations as follow [43,45,46]:
brazing seam. Fig. 9 showed the HRTEM images at bombarded SiC/brazing seam interface. As shown in Fig. 9(a), the spacing of 0.248 nm, 0.232 nm and 0.212 nm were matched with the (210) crystal face of Ti5Si3, (200)
Fig. 9. HRTEM characteristic of bombarded SiC joint interface from Fig.12. 6
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Fig. 10. (a) Shear strength and fracture surfaces of brazed joints using (b) (c) original SiC ceramics, (d) (e) ion bombarded SiC ceramics.
F = 2(mdII - ndI) / (mdI + ndII)
(4)
dII/dI=n/m
(5)
layer /Ag(s,s)+Cu(s,s) containing TiCu/ Ti5Si3 + TiC mixed layer /SiC. Based on the above analysis, the interfacial reaction layer was formed by one-step chemical reaction when SiC was pre-treated by ion bombardment, which was different from that on original sample. When SiC was ion bombarded, the SieC bond of the substrate surface were broken forming an amorphous layer composed of Frenkel pairs and antisite defects [25]. During brazing, these Frenkel pairs and antisite defects could react with Ti directly with the reaction equation shown below:
where dI and dII represent the corresponding d-spacings of SiC, TiC and Ti5Si3, respectively, and m and n are integers, then a fraction of the interfacial atoms in I lie in atomic sites on the surface of II. For the Ti5Si3/SiC interface, dTi5Si3 (210) =0.248 nm and dSiC (204) =0.212 nm, so m, n equal to 7 and 6, respectively, i.e. six (210) planes of Ti5Si3 match seven (204) planes of SiC. By the calculating results, the lattice mismatch of Ti5Si3/SiC interface was 0.27 %. Similarly, the lattice mismatch of TiC/SiC interfaces was calculated to be 0.31 % according to Eqs. (4) and (5). Thus, both of Ti5Si3/SiC and TiC/SiC structures were semi-coherent interfaces with low strain energy. The lattice mismatch of TiC/Ti5Si3 interfaces was calculated to be 6.7 % according to Eq. (3), which indicated the match of TiC layer and Ti5Si3 layer was better than that of the original sample shown in Fig. 7. Fig. 9(b) showed the spacing of 0.237 nm and 0.202 nm matched with the (200) crystal face of TiC and (102) crystal face of TiCu, and the misfit of TiC/TiCu interface was 15.9 % indicated the incoherent interface with high strain energy was formed. Fig. 9(c) and (d) showed the HRTEM images of TiCu/Cu and Ag/Cu interfaces, respectively. The mismatch of each interface was calculated as 2.43 % and 12.13 %. Based on the TEM and SEM characteristics, microstructure of SiC/ AgCu-Ti/SiC joint with ion bombardment was SiC/ Ti5Si3 + TiC mixed
6Ti+3[Si]+[C]=TiC + Ti5Si3, ΔG=-787.3 + 22.1 × 10−3T(KJ/mol) (6) Where T denotes to temperature ranges from 1123 K to 1473 K. The ΔG values of forming solid-state TiC and Ti5Si3 simultaneously are −761.38 kJ/mol at brazing temperature of 1173 K (900 °C), indicating that the reaction (6) is exergonic and proceed spontaneously. Therefore, a mixed reaction layer containing TiC and Ti5Si3 particles was formed at the interface of SiC/AgCu-Ti/SiC joint with pre-treatment by ion bombardment. Besides, the active Ti reacts with liquid filler forming TiCu intermetallic compounds, and the reaction between Ti and Cu are listed as Eq. (7) [40]. The ΔG values of forming TiCu are −11.34 kJ/mol at 1173 K (900 °C), the negative value of ΔG indicated that the TiCu was favor to form at brazing seam. 7
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Ti + Cu = TiCu, ΔG = −17.069 + 4.887 × 10−3T(KJ/mol)
Nanyang Technological University (NTU) in Singapore as an exchange student.
(7)
The room-temperature shear strength of SiC/AgCu-Ti/SiC joints brazing at 900 °C for 10 min were tested and the results were shown in Fig. 10(a). The results indicated that the ion bombardment influenced the shear strength obviously. The shear strength of the joints improved with Ar ion bombardment and the optimal shear strength of 30.9 MPa was reached, which was ∼72.6 % higher than that of the SiC brazed joints without ion bombardment. The appropriate thickness of TiC + Ti5Si3 mixed reaction layer could contribute to the optimal shear strength. Fig. 10(b)–(e) showed the fracture surface morphology of SiC/ AgCu-Ti/SiC joints with original and ion bombarded SiC ceramics, respectively. For original sample, the joint fractured at SiC/brazing seam interface after shear test as shown in Fig. 10(b). It was shown that SiC and reaction layer, i.e., TiC and Ti5Si3 compounds, appeared alternately on the fracture surface in Fig. 10(c). In fact, pores and cracks were existed at the SiC/ interfacial reaction layer for the original brazing sample shown in Fig. 7, indicating large residual stress existed in the joint. For the bombarded sample, the joint cracked from one side of SiC substrate, ruptured through the brazing seam, then extended to the other SiC side, as illustrated in Fig. 10(d). Fig. 10(e) showed the SEM fracture surface of area I and II depicted in Fig. 10(d). In area I, no obvious plastic deformation was occurred, and the fracture was flat and granular, exhibiting a typical brittle cleavage fracture in SiC substrate. A large number of tear ridges and flat fracture surface were observed at area II, indicating that the fracture type was also brittle model. The fracture path showed that ion bombardment effectively improved the bonding between brazing seam and substrate, thus, the shear property of brazed joints was enhanced obviously.
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4. Conclusions The surface of SiC ceramics was modified by Ar ion bombardment at 50 keV with the fluence of 1 × 1017ions/cm2. The brazing behavior of SiC ceramics using AgCu-Ti filler were also studied. Primary conclusions were summarized as follows: (1) SieC bonds were broken, and lots of Frenkel pairs and antisite defects were formed in ion bombardment, resulting in the formation of amorphous layer at the surface. The thickness of amorphous layer was ∼120 nm when the ion bombardment was performed at 50 keV with the fluence of 1 × 1017ions/cm2. In addition, more dislocation and twins were formed beneath the amorphous layer after bombardment. (2) The interfacial microstructure of SiC/AgCu-Ti/SiC brazed joint was SiC/Ti5Si3+TiC mixed layer/Ag(s,s)+Cu(s,s) containing TiCu/ Ti5Si3+TiC mixed layer/SiC when ion bombardment was employed. In addition, the interfacial stratification, which was observed in the original SiC joint without ion bombardment, was eliminated using bombarded SiC. (3) The shear strength of SiC brazed joints was improved with Ar ion bombardment, and the optimal shear strength was 30.9 MPa, which was ∼72.6 % higher than that of the joints without ion bombarded. Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 51775138 and U1737205), the Natural Science Foundation of Shandong Province (No. ZR2019BEE031), the Key Research & Development program of Shandong Province (No. 2017GGX40103) and Science and Technology Support program of Sichuan Province (No. 2016NZ0106). Z.B. Chen acknowledges the support from the China Scholarship Council (CSC) for one-year studying at School of Materials Science and Engineering, 8
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