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Effects of surface microstructure on the active element content and wetting behavior of brazing filler metal during brazing Ti3SiC2 ceramic and Cu
Vacuum 156 (2018) 256–263
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Effects of surface microstructure on the active element content and wetting behavior of brazing filler metal during brazing Ti3SiC2 ceramic and Cu
T
H.Y. Chena,b, X.C. Wangb, L. Fua,b,∗, M.Y. Fengb a b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi'an, 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ti3SiC2 ceramic Surface microstructure Wetting Brazing filler metal
The wetting behavior of Ag-Cu-xTi (x = 0, 0.5, 1, 2, 3, 4.5 wt. %) brazing filler metals on Ti3SiC2 ceramic was investigated. Moreover, two methods, the addition of reactive element Ti in filler metals and surface microstructure on Ti3SiC2, were used to decrease the wetting angle of filler metals. When employing the addition of Ti, the wetting angle of Ag-Cu-xTi gradually decreased with increasing the content of Ti from 0 to 4.5 wt. %, while the interface brittle compounds increased during the same course. According to wetting theory, a square convex platform and groove microstructure was designed to promote the wetting of Ag-Cu-2Ti. The microstructure was processed on Ti3SiC2 surfaces by laser machine, the effects of microstructures on wetting behavior were verified experimentally. The results show that the wetting angle of Ag-Cu-2Ti on Ti3SiC2 with surface microstructure decreased effectively compared with that on smooth Ti3SiC2. Meanwhile, the same wetting behavior and less interface brittle compounds could be achieved by using surface microstructure and filler metal with less Ti. The experimental results had a sound fit with theoretical calculation. The novel method provided a feasible route for optimizing filler metal composition and promoting the application of filler metals with less reactive element in brazing field.
1. Introduction A group of materials called MAX phases where M is an early transition metal [1], A is a group 13 to 16 element, and X is C and/or N, which are also known as metallic ceramics, have the dual properties of both the metals and ceramics [2]. MAX phases have been applied in many fields like aerospace and electronics owing to their remarkable properties. In recent years, there have been a series of interesting discoveries about noble metal base MAX phases especially Ti2AuC2, Ti3Au2C2 [3,4], which can be obtained by a substitutional solid-state reaction of Au into Ti3SiC2 single-crystal thin films with simultaneous out-diffusion of Si. Such new MAX phases exhibit unique electrical properties [4]. Ti3SiC2 ceramic is one of the most typical MAX phases [5], its compressive strength is 600 MPa at room temperature, and drop to 260 MPa at1300 °C in air. Although the room-temperature failure mode of Ti3SiC2 ceramic is brittle, the high-temperature load-displacement curve shows the significant plastic behavior [6]. Ti3SiC2 ceramic also has good electrical conductivity and thermal conductivity, and it is as readily machinable as graphite. Thus, Ti3SiC2 ceramic is used in electric conductive components, corrosion-resistant electrodes widely [7,8]. In generally, Ti3SiC2 ceramic is used by joining them to
∗
ceramics or metals due to the difficulty in manufacturing large-scale or complex shaped components [9,10]. Due to the different chemical bonds and physical and chemical properties between ceramics and metals, there are many difficulties in the joining of ceramics and metals [11,12]. The traditional method of connecting ceramics to metals is mechanical connection, but it is difficult for that to achieve high stability and contact resistance requirements. Compared with mechanical connection, welded joint has higher strength, better high-temperature properties and air tightness [13]. Thus, welding has become the most promising method in connecting ceramics to metals at present. In the past decades, a variety of welding methods have been used to join ceramics and metals, and the mature welding methods are diffusion bonding and brazing [14,15]. Liu [16] joined TiAl and Ti3SiC2 ceramic by diffusion bonding, but the long holding time caused the distortion of TiAl/Ti3SiC2 joints, which depraved the precision of the components. Compared with diffusion bonding, brazing is a suitable method to join Ti3SiC2 ceramic to other metals because of its characteristics of high dimension precision, cost effectiveness and high-quality process [17]. Pure copper and copper alloys are widely used in electronic information industry owing to their excellent electrical properties. However, the applications of copper and
Corresponding author. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China. E-mail address: [email protected] (L. Fu).
https://doi.org/10.1016/j.vacuum.2018.07.043 Received 6 June 2018; Received in revised form 27 July 2018; Accepted 28 July 2018 Available online 29 July 2018 0042-207X/ © 2018 Published by Elsevier Ltd.
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ray diffraction spectrum of Ti3SiC2 ceramic, and only diffraction peaks of Ti3SiC2 ceramic was observed. Ag-Cu-xTi brazing filler metals in foil form with a thickness of 80 μm were used to join Ti3SiC2 ceramic and Cu. All Ti3SiC2 ceramic and Cu were cut into blocks with the dimension of 10 mm × 10 mm × 2 mm by wire cutting machine. The surfaces of all specimens were ground by 2000# SiC sandpaper and degreased by ultrasonic cleaner for 15 min. Ag-Cu-xTi foils were placed between Ti3SiC2 ceramic and Cu. In order to ensure the good fluidity of all brazing filler metals while avoiding cracks and excessive brittle compounds in the joints due to excessive heat input, brazing was carried out at 840 °C for 20min in the vacuum furnace under 1.3 × 10−3Pa. After brazing, all the specimens were furnace-cooled to room temperature. Surface microstructures were processed on Ti3SiC2 ceramic specimens by HSGQ fiber femtosecond laser machine. After processing the microstructure, all Ti3SiC2 ceramic specimens were cleaned by ultrasonic cleaner for 10min again. The interfacial microstructures were characterized by GX71OLYMPUS optical microscope and the microstructure of the joints were characterized by scanning electrons microscope (SEM, Xmax20/INCA250). The wetting experiments were carried out by O2C wetting angle measuring system. First, the Ag-Cu-xTi foils were compressed into spheres. Then, the spheres were placed on the surfaces of Ti3SiC2 ceramic specimens. At last, the specimens were heated with a rate of 30 °C/min to 840 °C in vacuum (< 1.8 × 10−3Pa) and holding for 20min. The wetting angles were measured and recorded by optical photography and data acquisition system.
copper alloys have a great limitation because of their low strength and poor wear properties. In the last few decades, copper and Ti3SiC2 ceramic components have been used in high-speed rail manufacturing [18,19]. Thus, it is of great theoretical significance and engineering value to study the brazed joint of copper and Ti3SiC2 ceramic. At present, Ag-Cu-4.5Ti (wt. %) active brazing filler metal has been widely used to braze ceramics owing to its good wettability [20]. However, a large number of brittle products are formed in the interface, which had a disadvantageous influence on the mechanical and electrical properties of pantograph component [21,22]. Reducing the Ti content in the Ag-Cu-4.5Ti brazing filler metal can fundamentally suppress the formation of brittle products [23]. Ag-28Cu brazing filler metal without Ti element is another option to braze ceramics, and there are almost no brittle products in the brazing joints of Ti3SiC2 ceramic and copper [21]. However, the wettability of Ag-28Cu brazing filler metal on ceramic surface is poor, which leads to the void defects and weak bonding in the interface of joints [24]. A potential method for improving the wettability of brazing filler metal is the using of surface microstructure. At present, surface microstructure has been widely used in the manufacture of hydrophobic and hydrophilic materials [25–28]. Barthlott and Neihuis [29] found the “lotus effect” which showed that the rough lotus leaves have a great influence on the wettability of water on their surfaces. Wang [30] prepared different microstructures on SiC surfaces by laser beam, and they found that the circle-dimpled texture enhanced hydrophobicity of the SiC surfaces. At present, many waterproof materials based on surface microstructure have been widely used [31]. However, in the field of brazing and soldering, there are few reports on the application of regular surface microstructure. Moreover, laser processing has become an important method for manufacturing surface microstructure owing to its high precision [32]. In this study, the regular surface microstructures were manufactured by fiber femtosecond laser machining. In order to obtained good wettability and sound brazing joints, the surface microstructure which can improve the wettability of Ag-Cu-2Ti brazing filler metal was designed by theoretical calculation. The wetting experiments of Ag-Cu-xTi (x = 0, 0.5, 1, 2, 3, 4.5 wt.%) brazing filler metals on Ti3SiC2 ceramic surfaces were carried out and the wetting angles were measured by wetting angle measuring system.
3. Results and discussion 3.1. Microstructural characterization of the joints Fig. 2 shows the secondary electrons images of Cu/Ti3SiC2 joints obtained by using Ag-Cu-xTi brazing filler metals at 840 °C for 20 min. It can be seen that no defects were observed in the six different interfaces of joints, indicating that it is possible to braze Ti3SiC2 ceramic and Cu by all of this Ag-Cu-xTi brazing filler metals with such brazing parameters. Fig. 3 shows the changing curve of diffusion-reaction layer width in brazing joints with the Ti content in Ag-Cu-Ti brazing filler metal. There are two ways to realize metallurgical bonding, reaction and diffusion, both of which have a great influence on the performance of brazing joint. It can be seen from Figs. 2 and 3 that with the increasing of active element Ti, more and more reactants were generated, and the diffusion-reaction layer along the edge of the Ti3SiC2 substrate became narrower and narrower. The magnified microstructure of Cu/ Ag-Cu-4.5Ti/Ti3SiC2 joint and Cu/Ag-Cu/Ti3SiC2 joint are displayed in Fig. 4. According to previous research [21], active element Ti reacted with the Ti3SiC2 ceramic and formed brittle phases in Cu/Ag-Cu-4.5Ti/ Ti3SiC2 joint such as TiCu, TiCu4 and Ti5Si3Cx, such reaction promoted
2. Materials and experimental procedures Ti3SiC2 ceramic used in this study was produced by self-propagating high-temperature synthesis (SHS) [33]. The surface morphology of Ti3SiC2 ceramic sample was studied by scanning electron microscope (SEM) and the crystal structure of the sample was examined by X-ray diffraction (XRD). Fig. 1(a) shows the surface morphology of Ti3SiC2 ceramic sample in secondary electrons mode. It can be seen that there are some voids about 3–7 μm in Ti3SiC2 ceramic. Fig. 1(b) shows the X-
Fig. 1. (a) Microstructure and (b) XRD pattern of Ti3SiC2 ceramic. 257
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Fig. 2. The interfacial morphology of the Ti3SiC2/Cu joints. (a) Ti3SiC2/Ag-Cu/Cu joint. (b) Ti3SiC2/Ag-Cu-0.5Ti/Cu joint. (c) Ti3SiC2/Ag-Cu-1Ti/Cu joint. (d) Ti3SiC2/Ag-Cu-2Ti/Cu joint. (e) Ti3SiC2/Ag-Cu-3Ti/Cu joint. (f) Ti3SiC2/Ag-Cu-4.5Ti/Cu joint.
the spreading of liquid brazing filler metal on Ti3SiC2 substrate [34]. At the same time, Cu atoms diffused toward Ti3SiC2 ceramic due to the diffusion driving force resulting from the concentration gradient, and reacted with the Ti3SiC2 ceramic to form the diffusion-reaction zone along the edge of the Ti3SiC2 substrate, which was determined as Ti47Cu8Si13C32. Compared with Cu/Ag-Cu-4.5Ti/Ti3SiC2 joint, the interfacial microstructure of Cu/Ag-Cu/Ti3SiC2 joint was much simpler, as illustrated in Fig. 4(b), there were no continuous brittle product layers in the interface of Cu/Ag-Cu/Ti3SiC2 joint. Moreover, the interfacial structure of Cu/Ag-Cu/Ti3SiC2 joint instead was the eutectic structure, and the diffusion-reaction zone along the edge of the Ti3SiC2 substrate was also determined as Ti47Cu8Si13C32. However, the thickness of the diffusion-reaction zone adjacent to the Ti3SiC2 substrate in Cu/Ag-Cu/Ti3SiC2 joint was wider than that in Cu/Ag-Cu-4.5Ti/Ti3SiC2 joint. The differences on the width of diffusion-reaction zone can be attributed to the forming of brittle phases like TiCu, TiCu4 and Ti5Si3Cx, which consumed the amount of Cu atoms and further inhibited the diffusion of Cu atoms. The narrower and narrower diffusion-reaction zone with increasing Ti content reveals that the continuous brittle phases affected the positive role of diffusion mechanism. In the meantime, the continuous and thick brittle compound layer, including TiCu, Ti5Si3Cx and so on, weakened the shear strength of Ti3SiC2/Ag-Cu-xTi/ Cu joint. While brazing Ti3SiC2 ceramic and Cu by Ag-Cu brazing filler metal, no continuous brittle compound layer was formed in the interface of joint. Thus, Cu atoms could be easy to diffuse into Ti3SiC2 ceramic substrate and to form Ti47Cu8Si13C32, which further promoted
Fig. 3. The changing curve of diffusion layer width in brazing joints with the Ti content in Ag-Cu-Ti brazing filler metal.
Fig. 4. Magnified microstructure of (a) Ti3SiC2/Ag-Cu-4.5Ti/Cu joint and (b) Ti3SiC2/Ag-Cu/Cu joint. 258
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Fig. 5. Static wetting angle measurement of Ag-Cu-xTi brazing filler metals on smooth Ti3SiC2 ceramic surfaces (a) Ag-Cu (b) Ag-Cu-0.5Ti (c) Ag-Cu-1Ti (d) Ag-Cu2Ti (e) Ag-Cu-3Ti (f) Ag-Cu-4.5Ti.
is much faster and more intense than the diffusion process of Cu atoms. Therefore, the Ti element has a major influence on the wettability of the brazing filler metals. The good wettability of brazing filler metal is the precondition of excellent brazing joint. In order to reduce the brittle products in the interface of brazed joints, a square convex platform and groove surface microstructure was designed and fabricated to promote the wetting of brazing filler with low Ti content on Ti3SiC2 ceramic surface.
the diffusion of Cu atoms into Ti3SiC2 ceramic substrate. As a bridging compound, Ti47Cu8Si13C32 was compatible with both Ti3SiC2 ceramic and Cu, which improved the quality of Ti3SiC2/Ag-Cu/Cu joint [21]. Thus, reaction mechanism and diffusion mechanism complement each other during the brazing and soldering. Atoms diffusion plays an important role in improving the quality of joint. However, it is difficult to realize the sufficient diffusion during brazing and soldering owing to the short brazing period. On the other hand, the interface reaction promotes atomic bonding during the initial stage of brazing and soldering, but the brittle reaction compounds have the natural limitation in improving joint quality. Meanwhile, the continuous compound layer inhabits atoms diffusion, which affects the improvement of joint quality.
3.3. Design and calculation of surface microstructure Lasers have been widely used for submicron processing because of their unique advantages of being a non-contact process, capability of generating the complicated structures without the need of photo mask and ability to work in air. Considering the capillary action mechanism of surface microstructure, the surface microstructure used to improve the wetting behavior was designed as square convex platform and groove, as shown in Fig. 6, where a is the width of square convex platform, b is the width of the groove, and h is the height of square convex platform. According to our previous research, if the wetting angle of brazing filler metal on smooth Ti3SiC2 ceramic surface is lower than 90°, the wetting form of brazing filler metal on microstructure surface is noncompound wetting, which means that the brazing filler metal will fully fill the microstructural groove. To analyze the influence of the texture on the wettability further, some theoretical calculations were done. Fig. 7(a) shows the initial state of wetting (state 1), and Fig. 7(b) shows the final state of wetting (state 2). In this calculation process, the upper surface of protruding platforms is set as zero potential energy surface, and the total system energy is surface energy and gravitational potential energy. In order to facilitate the calculation, we set the state 1 as the standard energy. So, in state 2, the total system energy E can be expressed as:
3.2. Wetting experiments on smooth Ti3SiC2 ceramic substrate Fig. 5 shows the static wetting angle measurements of Ag-Cu-xTi brazing filler metal spheres on the smooth Ti3SiC2 ceramic surfaces. The wetting angle data was taken at the end of holding stage. From Fig. 5, it can be seen clearly that the wetting angle of Ag-Cu-xTi brazing filler metals on the smooth Ti3SiC2 ceramic surfaces decreased with the increase of Ti content in Ag-Cu-Ti brazing filler metal. The wetting angle of Ag-Cu brazing filler metal without Ti on smooth Ti3SiC2 ceramic surface was about 129°, indicating that it is difficult for Ag-Cu brazing filler metal to wet Ti3SiC2 ceramic. The wetting angle reduced dramatically while a small amount of Ti was added to the Ag-Cu brazing filler metal. The wetting angle of Ag-Cu-2Ti brazing filler metal on smooth Ti3SiC2 ceramic surface was about 59.6°, and the wetting angle was 17.3° when there was 4.5% Ti in Ag-Cu brazing filler metal. The changing trend of wetting angle can be attributed to the differences of Ti content in brazing filler metals. For the wetting process of Ag-Cu filler on Ti3SiC2 ceramic, it can be considered as inert system without reaction and the wetting kinetics can be determined by liquid filler surface tension and viscosity [35]. However, the active element Ti can react with Cu and Ti3SiC2 ceramic to form intermetallic compounds like TiCu, Ti5Si3Cx in the wetting interface. Such intermetallic compounds reduce the surface energy of Ti3SiC2 ceramic and reactive wetting mechanism plays an important role in promoting the wetting and spreading of brazing filler metals. Moreover, with the increase of Ti content in brazing filler metals, the effects of reactive wetting gradually increase, so the wetting angle of brazing filler metal becomes smaller. For Ag and Cu in the brazing filler metals, as the matrix of brazing filler metals, Ag has little effect on the wettability of brazing fillers, and part of Cu atoms diffuse into ceramic, react with Ti3SiC2 ceramic to form Ti47Cu8Si13C32, which has a little positive effect on the wettability. However, the reaction process between Ti and Ti3SiC2 ceramic and Cu
E = γlv Slv2 + γsl Ssl2 + Ep2 − γsv Ssv1 − γlv Slv1 − Ep1
(1)
where Slv2 and Ssl2 are the contact areas of liquid-vapor and solid-vapor in state 2, respectively. γlv , γsl and γsv are the surface tensions of liquidvapor, solid-liquid and solid-vapor contacts, respectively. Ssv1 and Slv1 are the contact areas of solid-vapor and liquid-vapor in state 1, respectively. Ep2 is the gravitational potential energy of state 2, and Ep1 is the gravitational potential energy of state 1. According to Young's equation and geometry, E can be given by
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Fig. 6. The morphology schematic of surface microstructure (a) top view (b) stereogram.
cos θ is a function of a, b and h, and it can be expressed as:
4πahr22 ⎞ 4 4 2 2⎤ ⎛ 2 2 E = γlv ⎡ ⎢π (r2 + H ) − cos θY ⎜πr2 + (a + b)2 ⎟ − 4πr1 ⎥ − 3 πρgr1 ⎝ ⎠ ⎣ ⎦ 2 R2 H2 + πρgH 2 ⎡ (1 − cos 2θ) − + RH cos θ ⎤ ⎢2 ⎥ 4 3 ⎣ ⎦ − πρg
r22 h (b2h + 2abh) 2(a + b)2
cos θ = φ (a, b, h)
In this wetting system, we take γlv = 1 N/m, θY = 60°, r1 = 0.0015 m, ρ = 10000 kg/m3, g = 9.8 N/kg. The calculation process was completed by Matlab. Fig. 8 shows the images of function φ (a, b, h) , and the color of the image represents the value of cos θ . In Fig. 8(a), when the value of h are 10 μm, 20 μm and 30 μm, the a and b which can effectively improve the wettability of liquid brazing filler metal are banded distribution, and with the value of h increasing from 10 μm to 30 μm, the band region that can effectively improve the wettability of liquid brazing filler metal gradually shifts. And the influence of a and b on function φ (a, b, h) is similar to that of h, as shown in Fig. 8(b) and (c). However, the depth range of the grooves processed by laser on Ti3SiC2 ceramic surface is 5–20 μm, and within this scope, when a = 30 μm, b = 20 μm, h = 20 μm, cos θ gets the maximum value of 0.9470, which means that θ get the minimum value of 18.73°, as shown in Fig. 8(d). According to the above theoretical calculation, if the wetting angle of brazing filler metal on Ti3SiC2 ceramic surface is lower than 90°, the microstructure with specific a, b and h values can effectively improve the wettability of brazing filler metal. In order to verify the actual effect of surface microstructure, three kinds of surface microstructures with different sizes were designed. Considering the accuracy of laser processing, the value of surface microstructure a is 30 μm, the value of h is 20 μm and the values of b are 10 μm, 20 μm and 30 μm, respectively.
(2)
where r2 is the radius of the projection area of the wetting interface in state 2. H is the height of the droplet in state 2. θY is the wetting angle of the droplet on the smooth substrate surface. r1 is the radius of the droplet in state 1. ρ is the density of liquid brazing filler metal. g is gravitational acceleration. R is the radius of the hemispherical droplet in state 2, and θ is the wetting angle in state 2. According to geometry, some equations can be given:
πr22 4 3 1 [(a + b)2h − a2h] πr1 = πH (3r22 + H 2) + 3 6 (a + b)2
(3)
r2 = R sin θ
(4)
H = R (1 − cos θ)
(5)
For a given wetting system, the γlv , θY , r1, ρ , g, a, b and h are constants, combine and solve the equations (2)–(5), E can be expressed as a function of θ :
E = f (θ)
(8)
(6)
The partial derivative of E to θ is
dE = f ′ (θ) dθ
3.4. Wetting experiments on Ti3SiC2 ceramic with different surface microstructures
(7)
Fig. 9 shows the optical images of different surface microstructures on Ti3SiC2 ceramic. It can be seen that the sides of all platforms are 30 μm, and the widths of grooves are 10 μm, 20 μm and 30 μm, respectively. To further investigate the effects of different surface
Upon examination, there exists θ1, so that when θ = θ1, there is f ′ (θ) = 0 , and E is minimized. In order to facilitate calculation, θ1 is represented by θ in the following calculations. For a changing surface topography, when E takes the minimum,
Fig. 7. The wetting states (a) initial wetting state (state 1) (b) final wetting state (state 2). 260
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Fig. 8. The images of function φ (a, b, h) (a) h = 10 μm, 20 μm, 30 μm (b) b = 10 μm, 20 μm, 30 μm (c) a = 20 μm, 30 μm, 40 μm (d) h = 20 μm, b = 20 μm, a = 30 μm.
microstructures on the wetting behavior of brazing filler, the wetting experiments of Ag-Cu-2Ti brazing filler on Ti3SiC2 ceramic substrate with different surface microstructures were carried out at 840 °C. Fig. 10 shows the static wetting angle measurements of Ag-Cu-2Ti brazing filler metal on Ti3SiC2 ceramic substrate with different surface microstructures at 840 °C. The wetting angle data was taken at the end of holding stage. Compared with the wetting angle of Ag-Cu-2Ti brazing filler metal on smooth Ti3SiC2 ceramic substrate, all the wetting angles on Ti3SiC2 ceramic substrate with different surface microstructures reduced dramatically. In addition, the minimum wetting angle was achieved when the width of groove was 20 μm, which had a sound fit with the theoretical calculation. The results indicated that the surface microstructure can further improve the wettability of the brazing filler metal without increasing the Ti content. Moreover, the surface microstructure has a positive effect on improving the interface microstructure and mechanical properties of brazing joints.
3.5. The brazing experiments of Ti3SiC2 ceramic with surface microstructure and Cu Fig. 11 shows the interfacial morphology of the Ti3SiC2 ceramic with different microstructures and Cu joints obtained by using Ag-Cu2Ti brazing filler metal. It can be seen that the prefabricated surface microstructures were basically complete after brazing. Moreover, all grooves were filled by Ag-Cu-2Ti brazing filler metal, which means that the assumption put forward in the theoretical calculation is reasonable. Compare Figs. 11 and 2 (d), we can see that the diffusion layer width along the edge of the Ti3SiC2 substrate with surface microstructure were bigger than that with smooth surface, indicating that the grooves can promote the diffusion of Ag atoms and Cu atoms into Ti3SiC2 ceramic, which means that surface microstructure can improve the role of diffusion mechanism in brazing process. According to the experimental results and theoretical calculation, it is easy to draw the conclusion that the suitable surface microstructure can reduce the wetting angle and improve the wetting behavior of AgCu-2Ti brazing filler metal, and then a sound brazing joint can be obtained. The method of using surface microstructure provides a feasible
Fig. 9. Optical images of laser processed microstructures (a) b = 10 μm (b) b = 20 μm (c) b = 30 μm. 261
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Fig. 10. Wetting angles of Ag-Cu-2Ti brazing filler metal on Ti3SiC2 ceramic with surface microstructure (a) b = 10 μm (b) b = 20 μm (c) b = 30 μm.
Fig. 11. The interfacial morphology of the Ti3SiC2 ceramic with surface microstructure and Cu joints (a) b = 10 μm (b) b = 20 μm (c) b = 30 μm.
route for optimizing brazing filler metals system and promoting the application of less Ti filler metals in the brazing field.
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4. Conclusions In summary, the suitable laser-textured surface microstructure can improve the wettability of Ag-Cu-2Ti brazing filler metal, which has a greater promotion to the application of less Ti filler metals in brazing field. The conclusions drawn in this paper are as follows: (1) In theory, the alternative structure of convex platform and groove is benefit for the improvement of wetting behavior of brazing filler metal. For the wetting of Ag-Cu-2Ti brazing filler metal on Ti3SiC2 ceramic, the optimal size of the designated surface microstructure is a = 30 μm, b = 20 μm, h = 20 μm. (2) The experimental results indicated that the microstructure surface with optimal size can reduce the wetting angle of Ag-Cu-2Ti brazing filler metal on Ti3SiC2 ceramic plate from 59.6° to 25.7°. The changing tendency of wetting angle had a sound fit with the theoretical calculation. (3) The prefabricated surface microstructure was basically complete after brazing, and all the grooves of microstructure were filled by Ag-Cu-2Ti brazing filler metal. The wetting form of Ag-Cu-2Ti brazing filler metal on microstructure surface was “non-composite surface”, which agreed with the theoretical calculation. Acknowledgment Authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No.51405390 and 51775442), Project of Key areas of innovation team in Shaanxi Province (No. 2014KCT-12), the fund of the State Key Laboratory of Solidification Processing in NWPU (113-QP-2014) and Aeronautical Science Foundation of China (2016ZE53040). References [1] M.W. Barsoum, T. El-Raghy, The MAX phases: unique new carbide and nitride materials: ternary ceramics turn out to be surprisingly soft and machinable, yet also heat-tolerant, strong and lightweight, Am. Sci. 89 (2001) 334–343.
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(2016) 56–62. [22] O. Dezellus, B. Gardiola, J. Andrieux, S. Lay, Experimental evidence of copper insertion in a crystallographic structure of Ti3SiC2, MAX phase, Scr. Mater 104 (2015) 17–20. [23] X.L. Liu, H.B. Zhang, Y. Jiang, Y.H. He, Characterization and application of porous Ti3SiC2, ceramic prepared through reactive synthesis, Mater. Des. 79 (2015) 94–98. [24] O. Dezellus, R. Voytovych, A.P.H. Li, G. Constantin, F. Bosselet, J.C. Viala, Wettability of Ti3SiC2 by Ag-Cu and Ag-Cu-Ti melts, J. Mater. Sci. 45 (2010) 2080–2084. [25] B. He, N.A. Patankar, J. Lee, Multiple equilibrium droplet shapes and design criterion for rough hydrophobic surfaces, Langmuir 19 (2003) 4999–5003. [26] C.W. Extrand, S.I. Moon, A.P. Hall, D. Schmidt, Superwetting of structured surfaces, Langmuir 23 (2007) 8882–8890. [27] C.W. Yao, J.L. Alvarado, C.P. Marsh, B.G. Jones, M.K. Collins, Wetting behavior on hybrid surfaces with hydrophobic and hydrophilic properties, Appl. Surf. Sci. 290 (2014) 59–65. [28] W. Li, A. Amirfazli, Microtextured superhydrophobic surfaces: a thermodynamic
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effects of surface microstructure on the active element content and wetting behavior of brazing filler metal during brazing Ti3SiC2 ceramic and Cu
T
H.Y. Chena,b, X.C. Wangb, L. Fua,b,∗, M.Y. Fengb a b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi'an, 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ti3SiC2 ceramic Surface microstructure Wetting Brazing filler metal
The wetting behavior of Ag-Cu-xTi (x = 0, 0.5, 1, 2, 3, 4.5 wt. %) brazing filler metals on Ti3SiC2 ceramic was investigated. Moreover, two methods, the addition of reactive element Ti in filler metals and surface microstructure on Ti3SiC2, were used to decrease the wetting angle of filler metals. When employing the addition of Ti, the wetting angle of Ag-Cu-xTi gradually decreased with increasing the content of Ti from 0 to 4.5 wt. %, while the interface brittle compounds increased during the same course. According to wetting theory, a square convex platform and groove microstructure was designed to promote the wetting of Ag-Cu-2Ti. The microstructure was processed on Ti3SiC2 surfaces by laser machine, the effects of microstructures on wetting behavior were verified experimentally. The results show that the wetting angle of Ag-Cu-2Ti on Ti3SiC2 with surface microstructure decreased effectively compared with that on smooth Ti3SiC2. Meanwhile, the same wetting behavior and less interface brittle compounds could be achieved by using surface microstructure and filler metal with less Ti. The experimental results had a sound fit with theoretical calculation. The novel method provided a feasible route for optimizing filler metal composition and promoting the application of filler metals with less reactive element in brazing field.
1. Introduction A group of materials called MAX phases where M is an early transition metal [1], A is a group 13 to 16 element, and X is C and/or N, which are also known as metallic ceramics, have the dual properties of both the metals and ceramics [2]. MAX phases have been applied in many fields like aerospace and electronics owing to their remarkable properties. In recent years, there have been a series of interesting discoveries about noble metal base MAX phases especially Ti2AuC2, Ti3Au2C2 [3,4], which can be obtained by a substitutional solid-state reaction of Au into Ti3SiC2 single-crystal thin films with simultaneous out-diffusion of Si. Such new MAX phases exhibit unique electrical properties [4]. Ti3SiC2 ceramic is one of the most typical MAX phases [5], its compressive strength is 600 MPa at room temperature, and drop to 260 MPa at1300 °C in air. Although the room-temperature failure mode of Ti3SiC2 ceramic is brittle, the high-temperature load-displacement curve shows the significant plastic behavior [6]. Ti3SiC2 ceramic also has good electrical conductivity and thermal conductivity, and it is as readily machinable as graphite. Thus, Ti3SiC2 ceramic is used in electric conductive components, corrosion-resistant electrodes widely [7,8]. In generally, Ti3SiC2 ceramic is used by joining them to
∗
ceramics or metals due to the difficulty in manufacturing large-scale or complex shaped components [9,10]. Due to the different chemical bonds and physical and chemical properties between ceramics and metals, there are many difficulties in the joining of ceramics and metals [11,12]. The traditional method of connecting ceramics to metals is mechanical connection, but it is difficult for that to achieve high stability and contact resistance requirements. Compared with mechanical connection, welded joint has higher strength, better high-temperature properties and air tightness [13]. Thus, welding has become the most promising method in connecting ceramics to metals at present. In the past decades, a variety of welding methods have been used to join ceramics and metals, and the mature welding methods are diffusion bonding and brazing [14,15]. Liu [16] joined TiAl and Ti3SiC2 ceramic by diffusion bonding, but the long holding time caused the distortion of TiAl/Ti3SiC2 joints, which depraved the precision of the components. Compared with diffusion bonding, brazing is a suitable method to join Ti3SiC2 ceramic to other metals because of its characteristics of high dimension precision, cost effectiveness and high-quality process [17]. Pure copper and copper alloys are widely used in electronic information industry owing to their excellent electrical properties. However, the applications of copper and
Corresponding author. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China. E-mail address: [email protected] (L. Fu).
https://doi.org/10.1016/j.vacuum.2018.07.043 Received 6 June 2018; Received in revised form 27 July 2018; Accepted 28 July 2018 Available online 29 July 2018 0042-207X/ © 2018 Published by Elsevier Ltd.
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ray diffraction spectrum of Ti3SiC2 ceramic, and only diffraction peaks of Ti3SiC2 ceramic was observed. Ag-Cu-xTi brazing filler metals in foil form with a thickness of 80 μm were used to join Ti3SiC2 ceramic and Cu. All Ti3SiC2 ceramic and Cu were cut into blocks with the dimension of 10 mm × 10 mm × 2 mm by wire cutting machine. The surfaces of all specimens were ground by 2000# SiC sandpaper and degreased by ultrasonic cleaner for 15 min. Ag-Cu-xTi foils were placed between Ti3SiC2 ceramic and Cu. In order to ensure the good fluidity of all brazing filler metals while avoiding cracks and excessive brittle compounds in the joints due to excessive heat input, brazing was carried out at 840 °C for 20min in the vacuum furnace under 1.3 × 10−3Pa. After brazing, all the specimens were furnace-cooled to room temperature. Surface microstructures were processed on Ti3SiC2 ceramic specimens by HSGQ fiber femtosecond laser machine. After processing the microstructure, all Ti3SiC2 ceramic specimens were cleaned by ultrasonic cleaner for 10min again. The interfacial microstructures were characterized by GX71OLYMPUS optical microscope and the microstructure of the joints were characterized by scanning electrons microscope (SEM, Xmax20/INCA250). The wetting experiments were carried out by O2C wetting angle measuring system. First, the Ag-Cu-xTi foils were compressed into spheres. Then, the spheres were placed on the surfaces of Ti3SiC2 ceramic specimens. At last, the specimens were heated with a rate of 30 °C/min to 840 °C in vacuum (< 1.8 × 10−3Pa) and holding for 20min. The wetting angles were measured and recorded by optical photography and data acquisition system.
copper alloys have a great limitation because of their low strength and poor wear properties. In the last few decades, copper and Ti3SiC2 ceramic components have been used in high-speed rail manufacturing [18,19]. Thus, it is of great theoretical significance and engineering value to study the brazed joint of copper and Ti3SiC2 ceramic. At present, Ag-Cu-4.5Ti (wt. %) active brazing filler metal has been widely used to braze ceramics owing to its good wettability [20]. However, a large number of brittle products are formed in the interface, which had a disadvantageous influence on the mechanical and electrical properties of pantograph component [21,22]. Reducing the Ti content in the Ag-Cu-4.5Ti brazing filler metal can fundamentally suppress the formation of brittle products [23]. Ag-28Cu brazing filler metal without Ti element is another option to braze ceramics, and there are almost no brittle products in the brazing joints of Ti3SiC2 ceramic and copper [21]. However, the wettability of Ag-28Cu brazing filler metal on ceramic surface is poor, which leads to the void defects and weak bonding in the interface of joints [24]. A potential method for improving the wettability of brazing filler metal is the using of surface microstructure. At present, surface microstructure has been widely used in the manufacture of hydrophobic and hydrophilic materials [25–28]. Barthlott and Neihuis [29] found the “lotus effect” which showed that the rough lotus leaves have a great influence on the wettability of water on their surfaces. Wang [30] prepared different microstructures on SiC surfaces by laser beam, and they found that the circle-dimpled texture enhanced hydrophobicity of the SiC surfaces. At present, many waterproof materials based on surface microstructure have been widely used [31]. However, in the field of brazing and soldering, there are few reports on the application of regular surface microstructure. Moreover, laser processing has become an important method for manufacturing surface microstructure owing to its high precision [32]. In this study, the regular surface microstructures were manufactured by fiber femtosecond laser machining. In order to obtained good wettability and sound brazing joints, the surface microstructure which can improve the wettability of Ag-Cu-2Ti brazing filler metal was designed by theoretical calculation. The wetting experiments of Ag-Cu-xTi (x = 0, 0.5, 1, 2, 3, 4.5 wt.%) brazing filler metals on Ti3SiC2 ceramic surfaces were carried out and the wetting angles were measured by wetting angle measuring system.
3. Results and discussion 3.1. Microstructural characterization of the joints Fig. 2 shows the secondary electrons images of Cu/Ti3SiC2 joints obtained by using Ag-Cu-xTi brazing filler metals at 840 °C for 20 min. It can be seen that no defects were observed in the six different interfaces of joints, indicating that it is possible to braze Ti3SiC2 ceramic and Cu by all of this Ag-Cu-xTi brazing filler metals with such brazing parameters. Fig. 3 shows the changing curve of diffusion-reaction layer width in brazing joints with the Ti content in Ag-Cu-Ti brazing filler metal. There are two ways to realize metallurgical bonding, reaction and diffusion, both of which have a great influence on the performance of brazing joint. It can be seen from Figs. 2 and 3 that with the increasing of active element Ti, more and more reactants were generated, and the diffusion-reaction layer along the edge of the Ti3SiC2 substrate became narrower and narrower. The magnified microstructure of Cu/ Ag-Cu-4.5Ti/Ti3SiC2 joint and Cu/Ag-Cu/Ti3SiC2 joint are displayed in Fig. 4. According to previous research [21], active element Ti reacted with the Ti3SiC2 ceramic and formed brittle phases in Cu/Ag-Cu-4.5Ti/ Ti3SiC2 joint such as TiCu, TiCu4 and Ti5Si3Cx, such reaction promoted
2. Materials and experimental procedures Ti3SiC2 ceramic used in this study was produced by self-propagating high-temperature synthesis (SHS) [33]. The surface morphology of Ti3SiC2 ceramic sample was studied by scanning electron microscope (SEM) and the crystal structure of the sample was examined by X-ray diffraction (XRD). Fig. 1(a) shows the surface morphology of Ti3SiC2 ceramic sample in secondary electrons mode. It can be seen that there are some voids about 3–7 μm in Ti3SiC2 ceramic. Fig. 1(b) shows the X-
Fig. 1. (a) Microstructure and (b) XRD pattern of Ti3SiC2 ceramic. 257
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Fig. 2. The interfacial morphology of the Ti3SiC2/Cu joints. (a) Ti3SiC2/Ag-Cu/Cu joint. (b) Ti3SiC2/Ag-Cu-0.5Ti/Cu joint. (c) Ti3SiC2/Ag-Cu-1Ti/Cu joint. (d) Ti3SiC2/Ag-Cu-2Ti/Cu joint. (e) Ti3SiC2/Ag-Cu-3Ti/Cu joint. (f) Ti3SiC2/Ag-Cu-4.5Ti/Cu joint.
the spreading of liquid brazing filler metal on Ti3SiC2 substrate [34]. At the same time, Cu atoms diffused toward Ti3SiC2 ceramic due to the diffusion driving force resulting from the concentration gradient, and reacted with the Ti3SiC2 ceramic to form the diffusion-reaction zone along the edge of the Ti3SiC2 substrate, which was determined as Ti47Cu8Si13C32. Compared with Cu/Ag-Cu-4.5Ti/Ti3SiC2 joint, the interfacial microstructure of Cu/Ag-Cu/Ti3SiC2 joint was much simpler, as illustrated in Fig. 4(b), there were no continuous brittle product layers in the interface of Cu/Ag-Cu/Ti3SiC2 joint. Moreover, the interfacial structure of Cu/Ag-Cu/Ti3SiC2 joint instead was the eutectic structure, and the diffusion-reaction zone along the edge of the Ti3SiC2 substrate was also determined as Ti47Cu8Si13C32. However, the thickness of the diffusion-reaction zone adjacent to the Ti3SiC2 substrate in Cu/Ag-Cu/Ti3SiC2 joint was wider than that in Cu/Ag-Cu-4.5Ti/Ti3SiC2 joint. The differences on the width of diffusion-reaction zone can be attributed to the forming of brittle phases like TiCu, TiCu4 and Ti5Si3Cx, which consumed the amount of Cu atoms and further inhibited the diffusion of Cu atoms. The narrower and narrower diffusion-reaction zone with increasing Ti content reveals that the continuous brittle phases affected the positive role of diffusion mechanism. In the meantime, the continuous and thick brittle compound layer, including TiCu, Ti5Si3Cx and so on, weakened the shear strength of Ti3SiC2/Ag-Cu-xTi/ Cu joint. While brazing Ti3SiC2 ceramic and Cu by Ag-Cu brazing filler metal, no continuous brittle compound layer was formed in the interface of joint. Thus, Cu atoms could be easy to diffuse into Ti3SiC2 ceramic substrate and to form Ti47Cu8Si13C32, which further promoted
Fig. 3. The changing curve of diffusion layer width in brazing joints with the Ti content in Ag-Cu-Ti brazing filler metal.
Fig. 4. Magnified microstructure of (a) Ti3SiC2/Ag-Cu-4.5Ti/Cu joint and (b) Ti3SiC2/Ag-Cu/Cu joint. 258
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Fig. 5. Static wetting angle measurement of Ag-Cu-xTi brazing filler metals on smooth Ti3SiC2 ceramic surfaces (a) Ag-Cu (b) Ag-Cu-0.5Ti (c) Ag-Cu-1Ti (d) Ag-Cu2Ti (e) Ag-Cu-3Ti (f) Ag-Cu-4.5Ti.
is much faster and more intense than the diffusion process of Cu atoms. Therefore, the Ti element has a major influence on the wettability of the brazing filler metals. The good wettability of brazing filler metal is the precondition of excellent brazing joint. In order to reduce the brittle products in the interface of brazed joints, a square convex platform and groove surface microstructure was designed and fabricated to promote the wetting of brazing filler with low Ti content on Ti3SiC2 ceramic surface.
the diffusion of Cu atoms into Ti3SiC2 ceramic substrate. As a bridging compound, Ti47Cu8Si13C32 was compatible with both Ti3SiC2 ceramic and Cu, which improved the quality of Ti3SiC2/Ag-Cu/Cu joint [21]. Thus, reaction mechanism and diffusion mechanism complement each other during the brazing and soldering. Atoms diffusion plays an important role in improving the quality of joint. However, it is difficult to realize the sufficient diffusion during brazing and soldering owing to the short brazing period. On the other hand, the interface reaction promotes atomic bonding during the initial stage of brazing and soldering, but the brittle reaction compounds have the natural limitation in improving joint quality. Meanwhile, the continuous compound layer inhabits atoms diffusion, which affects the improvement of joint quality.
3.3. Design and calculation of surface microstructure Lasers have been widely used for submicron processing because of their unique advantages of being a non-contact process, capability of generating the complicated structures without the need of photo mask and ability to work in air. Considering the capillary action mechanism of surface microstructure, the surface microstructure used to improve the wetting behavior was designed as square convex platform and groove, as shown in Fig. 6, where a is the width of square convex platform, b is the width of the groove, and h is the height of square convex platform. According to our previous research, if the wetting angle of brazing filler metal on smooth Ti3SiC2 ceramic surface is lower than 90°, the wetting form of brazing filler metal on microstructure surface is noncompound wetting, which means that the brazing filler metal will fully fill the microstructural groove. To analyze the influence of the texture on the wettability further, some theoretical calculations were done. Fig. 7(a) shows the initial state of wetting (state 1), and Fig. 7(b) shows the final state of wetting (state 2). In this calculation process, the upper surface of protruding platforms is set as zero potential energy surface, and the total system energy is surface energy and gravitational potential energy. In order to facilitate the calculation, we set the state 1 as the standard energy. So, in state 2, the total system energy E can be expressed as:
3.2. Wetting experiments on smooth Ti3SiC2 ceramic substrate Fig. 5 shows the static wetting angle measurements of Ag-Cu-xTi brazing filler metal spheres on the smooth Ti3SiC2 ceramic surfaces. The wetting angle data was taken at the end of holding stage. From Fig. 5, it can be seen clearly that the wetting angle of Ag-Cu-xTi brazing filler metals on the smooth Ti3SiC2 ceramic surfaces decreased with the increase of Ti content in Ag-Cu-Ti brazing filler metal. The wetting angle of Ag-Cu brazing filler metal without Ti on smooth Ti3SiC2 ceramic surface was about 129°, indicating that it is difficult for Ag-Cu brazing filler metal to wet Ti3SiC2 ceramic. The wetting angle reduced dramatically while a small amount of Ti was added to the Ag-Cu brazing filler metal. The wetting angle of Ag-Cu-2Ti brazing filler metal on smooth Ti3SiC2 ceramic surface was about 59.6°, and the wetting angle was 17.3° when there was 4.5% Ti in Ag-Cu brazing filler metal. The changing trend of wetting angle can be attributed to the differences of Ti content in brazing filler metals. For the wetting process of Ag-Cu filler on Ti3SiC2 ceramic, it can be considered as inert system without reaction and the wetting kinetics can be determined by liquid filler surface tension and viscosity [35]. However, the active element Ti can react with Cu and Ti3SiC2 ceramic to form intermetallic compounds like TiCu, Ti5Si3Cx in the wetting interface. Such intermetallic compounds reduce the surface energy of Ti3SiC2 ceramic and reactive wetting mechanism plays an important role in promoting the wetting and spreading of brazing filler metals. Moreover, with the increase of Ti content in brazing filler metals, the effects of reactive wetting gradually increase, so the wetting angle of brazing filler metal becomes smaller. For Ag and Cu in the brazing filler metals, as the matrix of brazing filler metals, Ag has little effect on the wettability of brazing fillers, and part of Cu atoms diffuse into ceramic, react with Ti3SiC2 ceramic to form Ti47Cu8Si13C32, which has a little positive effect on the wettability. However, the reaction process between Ti and Ti3SiC2 ceramic and Cu
E = γlv Slv2 + γsl Ssl2 + Ep2 − γsv Ssv1 − γlv Slv1 − Ep1
(1)
where Slv2 and Ssl2 are the contact areas of liquid-vapor and solid-vapor in state 2, respectively. γlv , γsl and γsv are the surface tensions of liquidvapor, solid-liquid and solid-vapor contacts, respectively. Ssv1 and Slv1 are the contact areas of solid-vapor and liquid-vapor in state 1, respectively. Ep2 is the gravitational potential energy of state 2, and Ep1 is the gravitational potential energy of state 1. According to Young's equation and geometry, E can be given by
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Fig. 6. The morphology schematic of surface microstructure (a) top view (b) stereogram.
cos θ is a function of a, b and h, and it can be expressed as:
4πahr22 ⎞ 4 4 2 2⎤ ⎛ 2 2 E = γlv ⎡ ⎢π (r2 + H ) − cos θY ⎜πr2 + (a + b)2 ⎟ − 4πr1 ⎥ − 3 πρgr1 ⎝ ⎠ ⎣ ⎦ 2 R2 H2 + πρgH 2 ⎡ (1 − cos 2θ) − + RH cos θ ⎤ ⎢2 ⎥ 4 3 ⎣ ⎦ − πρg
r22 h (b2h + 2abh) 2(a + b)2
cos θ = φ (a, b, h)
In this wetting system, we take γlv = 1 N/m, θY = 60°, r1 = 0.0015 m, ρ = 10000 kg/m3, g = 9.8 N/kg. The calculation process was completed by Matlab. Fig. 8 shows the images of function φ (a, b, h) , and the color of the image represents the value of cos θ . In Fig. 8(a), when the value of h are 10 μm, 20 μm and 30 μm, the a and b which can effectively improve the wettability of liquid brazing filler metal are banded distribution, and with the value of h increasing from 10 μm to 30 μm, the band region that can effectively improve the wettability of liquid brazing filler metal gradually shifts. And the influence of a and b on function φ (a, b, h) is similar to that of h, as shown in Fig. 8(b) and (c). However, the depth range of the grooves processed by laser on Ti3SiC2 ceramic surface is 5–20 μm, and within this scope, when a = 30 μm, b = 20 μm, h = 20 μm, cos θ gets the maximum value of 0.9470, which means that θ get the minimum value of 18.73°, as shown in Fig. 8(d). According to the above theoretical calculation, if the wetting angle of brazing filler metal on Ti3SiC2 ceramic surface is lower than 90°, the microstructure with specific a, b and h values can effectively improve the wettability of brazing filler metal. In order to verify the actual effect of surface microstructure, three kinds of surface microstructures with different sizes were designed. Considering the accuracy of laser processing, the value of surface microstructure a is 30 μm, the value of h is 20 μm and the values of b are 10 μm, 20 μm and 30 μm, respectively.
(2)
where r2 is the radius of the projection area of the wetting interface in state 2. H is the height of the droplet in state 2. θY is the wetting angle of the droplet on the smooth substrate surface. r1 is the radius of the droplet in state 1. ρ is the density of liquid brazing filler metal. g is gravitational acceleration. R is the radius of the hemispherical droplet in state 2, and θ is the wetting angle in state 2. According to geometry, some equations can be given:
πr22 4 3 1 [(a + b)2h − a2h] πr1 = πH (3r22 + H 2) + 3 6 (a + b)2
(3)
r2 = R sin θ
(4)
H = R (1 − cos θ)
(5)
For a given wetting system, the γlv , θY , r1, ρ , g, a, b and h are constants, combine and solve the equations (2)–(5), E can be expressed as a function of θ :
E = f (θ)
(8)
(6)
The partial derivative of E to θ is
dE = f ′ (θ) dθ
3.4. Wetting experiments on Ti3SiC2 ceramic with different surface microstructures
(7)
Fig. 9 shows the optical images of different surface microstructures on Ti3SiC2 ceramic. It can be seen that the sides of all platforms are 30 μm, and the widths of grooves are 10 μm, 20 μm and 30 μm, respectively. To further investigate the effects of different surface
Upon examination, there exists θ1, so that when θ = θ1, there is f ′ (θ) = 0 , and E is minimized. In order to facilitate calculation, θ1 is represented by θ in the following calculations. For a changing surface topography, when E takes the minimum,
Fig. 7. The wetting states (a) initial wetting state (state 1) (b) final wetting state (state 2). 260
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Fig. 8. The images of function φ (a, b, h) (a) h = 10 μm, 20 μm, 30 μm (b) b = 10 μm, 20 μm, 30 μm (c) a = 20 μm, 30 μm, 40 μm (d) h = 20 μm, b = 20 μm, a = 30 μm.
microstructures on the wetting behavior of brazing filler, the wetting experiments of Ag-Cu-2Ti brazing filler on Ti3SiC2 ceramic substrate with different surface microstructures were carried out at 840 °C. Fig. 10 shows the static wetting angle measurements of Ag-Cu-2Ti brazing filler metal on Ti3SiC2 ceramic substrate with different surface microstructures at 840 °C. The wetting angle data was taken at the end of holding stage. Compared with the wetting angle of Ag-Cu-2Ti brazing filler metal on smooth Ti3SiC2 ceramic substrate, all the wetting angles on Ti3SiC2 ceramic substrate with different surface microstructures reduced dramatically. In addition, the minimum wetting angle was achieved when the width of groove was 20 μm, which had a sound fit with the theoretical calculation. The results indicated that the surface microstructure can further improve the wettability of the brazing filler metal without increasing the Ti content. Moreover, the surface microstructure has a positive effect on improving the interface microstructure and mechanical properties of brazing joints.
3.5. The brazing experiments of Ti3SiC2 ceramic with surface microstructure and Cu Fig. 11 shows the interfacial morphology of the Ti3SiC2 ceramic with different microstructures and Cu joints obtained by using Ag-Cu2Ti brazing filler metal. It can be seen that the prefabricated surface microstructures were basically complete after brazing. Moreover, all grooves were filled by Ag-Cu-2Ti brazing filler metal, which means that the assumption put forward in the theoretical calculation is reasonable. Compare Figs. 11 and 2 (d), we can see that the diffusion layer width along the edge of the Ti3SiC2 substrate with surface microstructure were bigger than that with smooth surface, indicating that the grooves can promote the diffusion of Ag atoms and Cu atoms into Ti3SiC2 ceramic, which means that surface microstructure can improve the role of diffusion mechanism in brazing process. According to the experimental results and theoretical calculation, it is easy to draw the conclusion that the suitable surface microstructure can reduce the wetting angle and improve the wetting behavior of AgCu-2Ti brazing filler metal, and then a sound brazing joint can be obtained. The method of using surface microstructure provides a feasible
Fig. 9. Optical images of laser processed microstructures (a) b = 10 μm (b) b = 20 μm (c) b = 30 μm. 261
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Fig. 10. Wetting angles of Ag-Cu-2Ti brazing filler metal on Ti3SiC2 ceramic with surface microstructure (a) b = 10 μm (b) b = 20 μm (c) b = 30 μm.
Fig. 11. The interfacial morphology of the Ti3SiC2 ceramic with surface microstructure and Cu joints (a) b = 10 μm (b) b = 20 μm (c) b = 30 μm.
route for optimizing brazing filler metals system and promoting the application of less Ti filler metals in the brazing field.
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4. Conclusions In summary, the suitable laser-textured surface microstructure can improve the wettability of Ag-Cu-2Ti brazing filler metal, which has a greater promotion to the application of less Ti filler metals in brazing field. The conclusions drawn in this paper are as follows: (1) In theory, the alternative structure of convex platform and groove is benefit for the improvement of wetting behavior of brazing filler metal. For the wetting of Ag-Cu-2Ti brazing filler metal on Ti3SiC2 ceramic, the optimal size of the designated surface microstructure is a = 30 μm, b = 20 μm, h = 20 μm. (2) The experimental results indicated that the microstructure surface with optimal size can reduce the wetting angle of Ag-Cu-2Ti brazing filler metal on Ti3SiC2 ceramic plate from 59.6° to 25.7°. The changing tendency of wetting angle had a sound fit with the theoretical calculation. (3) The prefabricated surface microstructure was basically complete after brazing, and all the grooves of microstructure were filled by Ag-Cu-2Ti brazing filler metal. The wetting form of Ag-Cu-2Ti brazing filler metal on microstructure surface was “non-composite surface”, which agreed with the theoretical calculation. Acknowledgment Authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No.51405390 and 51775442), Project of Key areas of innovation team in Shaanxi Province (No. 2014KCT-12), the fund of the State Key Laboratory of Solidification Processing in NWPU (113-QP-2014) and Aeronautical Science Foundation of China (2016ZE53040). References [1] M.W. Barsoum, T. El-Raghy, The MAX phases: unique new carbide and nitride materials: ternary ceramics turn out to be surprisingly soft and machinable, yet also heat-tolerant, strong and lightweight, Am. Sci. 89 (2001) 334–343.
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