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Reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal
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Reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal Jian Yanga,∗, Haodong Lia, Xuanwei Leib, Haifeng Xua, Jihua Huanga, Shuhai Chena, Yue Zhaoc a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China Faculty of Materials, Metallurgy and Chemistry, Jiangxi University of Science and Technology, Ganzhou, 341000, China c Department of Vehicle Engineering, Army Academy of Armored Forces, Beijing, 100072, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: AlN ceramic Active brazing Reactive wetting Surface tension Interface reaction
In order to propel the application of the developed CuNi-Xwt%Ti active filler metal in AlN brazing and get the universal reactive wetting mechanism between liquid metal and solid ceramic, the reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal were investigated. The results indicate that, with the increasing Ti content, surface tension for liquid CuNi-Xwt%Ti filler metal increases at low-temperature interval, but very similar at high-temperature interval, which influence the wetting behavior on AlN ceramic obviously. CuNi/AlN is the typical non-reactive wetting system, the wetting process including rapid wetting stage and stable stage. The wettability is depended on surface tension of the liquid CuNi filler metal completely. However, the wetting process of CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system is composed by three stages, which are rapid wetting stage decided by surface tension, slow wetting stage caused by interfacial reaction and stable stage. For CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system, although the surface tension of liquid filler metal is the only factor to influence the instant wetting angle θ0 at rapid wetting stage, the reduced free energy caused by interfacial reaction at slow wetting stage plays the decisive role in influencing the final wettability.
1. Introduction Because of the high hardness, remarkable wear resistance and excellent oxidation resistance, AlN ceramic has been applied in aerospace engineering, military engineering and mechanical manufacturing widely [1–3]. However, due to the high brittleness, AlN ceramic is difficult to be processed into complex component directly, so the selection of suitable method to bond AlN ceramic with itself and metals is the precondition on promoting its further application [4]. Considering the high melting point and electrical insulation characteristic of AlN ceramic, it is difficult to obtain the high performance AlN joints using fusion welding [5]. Although diffusion bonding has been developed for AlN jointing, it shows the poor structural adaptability considering that the appearance and dimension for the components to be bonded must be controlled strictly [6,7]. It is no doubt that brazing (especially active brazing) is the most suitable method for jointing ceramic to ceramic and ceramic to metal because of the high structural adaptability and strong potential for mass production [8–10]. In the initial research stage of AlN brazing, the researchers adopted Ag–Cu alloy and Ag–In alloy as the filler metals. Huh [11] used
∗
InAg19Ti2 alloy film as filler metal to braze AlN ceramics, and showed that the dense joint can be obtained in vacuum at temperatures of 750 °C. Kara-Slimane [12] brazed AlN ceramic with steel using Ag 62.95Cu35.4Ti1.66 alloy at 850 °C for 5 min, and indicated the shear strength of the joint was 50 MPa. Zhu [13] employed AgCu19.5Ti3In5 filler metal to braze AlN ceramic with Cu and FeNi42 metals, and the mechanical strength of joints are 127 MPa and 176 MPa. However, due to the low melting point of Ag–Cu and Ag–In alloys, these joints are difficult to apply in high temperature environments. After realizing this problem, Koltsov [14] used Si-17 at.% Pr eutectic alloy to braze AlN ceramic and SiC ceramic, and indicated that the temperature resistance of the joint can reach 1212 °C. Xiong [15] brazed AlN ceramics using Au–Pd–Co–Ni–V brazing alloy, and indicated that the sound AlN/AlN joints with temperature resistance above 1100 °C were achieved at 1170 °C for 10 min. However, the joint brazed using high temperature resistant filler metal usually shows the low mechanical strength due to the terrible brittleness of the connection layer [14,15]. The novel CuNiTi active filler metal (Cu86Ni14-Xwt.%Ti) is the promising material for achieving the high strength and high temperature resistance AlN ceramic brazing joint because of two reasons: (i)
Corresponding author. E-mail address: [email protected] (J. Yang).
https://doi.org/10.1016/j.ceramint.2019.10.150 Received 6 October 2019; Received in revised form 12 October 2019; Accepted 16 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Jian Yang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.150
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Fig. 1. Schematic diagram of high temperature wetting angle measuring instrument.
Fig. 2. Microstructure of CuNi filler metal as well as energy spectrum results: (a) low magnification microstructure; (b) high magnification microstructure; (c) energy spectrum of point A; (d) energy spectrum of point B.
The large component of Cu and Ni in filler metal are generated the Cu–Ni solid solution (Cu,Ni)ss substrate, which shows both high melting point and excellent plasticity [16]. (ii) The small component of Ti not only reacts with AlN ceramic and improves the interfacial bonding property, but also reacts with elements Cu and Ni and generates Cu–Ni–Ti intermetallic compounds in the filler metal, which is beneficial to improve the strength of connection layer as the reinforcing phase [17]. Actually, as the novel brazing material, the wettability of CuNiTi alloy on AlN ceramic determines the interfacial bonding property directly, and should be investigated preferentially. Moreover, in the research field of Liquid metal/Solid ceramic reactive wetting, it had been confirmed that the interfacial reaction plays the important role on influencing reactive wettability. However, there are still some important problems, such as the reactive wetting process, the effect of surface tension of liquid metal on reactive wettability, as well as the relationship between liquid metal surface tension and microstructure, are waiting to be solved [18–20]. In this work, the Cu86Ni14-XTi (X = 0 wt%, 8 wt% and 16 wt%) active filler metal was prepared, and the microstructure evolution
behavior due to the increasing Ti content was researched initially. Subsequently, the effect of temperature on viscosity and surface tension of CuNiTi liquid filler metal was studied. Finally, the wetting behavior of CuNiTi filler metal on AlN ceramic was investigated, and the reactive wetting process as well as the wettability influence mechanism was discussed.
2. Experimental The CuNiTi active filler metal was obtained using multiply melting method with the raw materials of high-purity copper, nickel and titanium ingots. For researching the relationship between Ti content in CuNiTi filler metal and its wetting behavior on AlN ceramic, three kinds of CuNiTi filler metals with the same weight percent ratio of Cu and Ni but different Ti weight percent were designed. The chemical compositions (wt.%) of the prepared CuNiTi filler metals were Cu86Ni14, (Cu86Ni14)92Ti8 and (Cu86Ni14)84Ti16, respectively, which was abbreviated as CuNi-Xwt.%Ti. The microstructure of the CuNiTi filler metal was researched using EM-30 PLUS scanning electron microscope, 2
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Fig. 3. Microstructure of CuNi-8wt.%Ti filler metal as well as energy spectrum results: (a) low magnification microstructure; (b) high magnification microstructure; (c) energy spectrum of point C; (d) energy spectrum of point D; (e) energy spectrum of point E.
The same conclusion also be given in the XRD analysis result, as shown in Fig. 4(a). While for CuNi-8wt.%Ti filler metal, the microstructure consists of the gray filamentous phase, white sheet phase and black block phase. The EDS results show that in the gray filamentous phase, the atomic ratio of Cu:Ni:Ti is 1:2:1, indicating that the gray strip phase is CuNi2Ti intermetallic compound. In addition, white sheet phase consists of the large amount of Cu and Ni with a small amount of Ti, indicating that it is the Cu–Ni–Ti ternary solid solution (CuNiTi)ss.While for the black block phase distributed in (CuNiTi)ss, the atomic ratio of Cu:Ni:Ti is near 1:1:1, combined with the XRD analysis, it can be found that the black block phase is (CuxNi2-x)Ti (τ1 phase). From Figs. 3 and 4(c), CuNi-16 wt%Ti filler metal consists of the white (CuNiTi)ss and the black strip phase with the Cu:Ni:Ti atomic ratio near 4:2:3, which is also the (CuxNi2-x)Ti (τ1 phase). The microstructure evolution behavior of CuNi-Xwt.%Ti filler metals with the increasing Ti content can be explained using Cu–Ni–Ti ternary alloy phase diagram (isothermal cross-section at 870 °C) [21], as shown in Fig. 6. In Fig. 6, the chemical compositions of CuNi alloy, CuNi-8wt.%Ti alloy and CuNi-16 wt%Ti alloys are labeled as points H, I and J, respectively. For CuNi alloy, it is obvious that it is situated at γ single phase zone, so only (CuNi)ss was formed because of the infinite mutual solubility between Cu and Ni. While for CuNi-8wt.%Ti alloy and CuNi-16 wt%Ti alloy, it can be found that they are located at γ+τ1+TiNi2Cu ternary phase zone and γ+τ1 binary phase zone. Therefore, their microstructures are (CuNiTi)ss + TiNi2Cu + (CuxNi2x)Ti and (CuNiTi)ss + (CuxNi2-x)Ti, respectively. Moreover, the chemical compositions of (CuxNi2-x)Ti (τ1 phase) in CuNi-8wt.%Ti alloy and CuNi-16 wt%Ti alloy can be achieved using leverage method, and labeled as points K and L. The theoretical atomic ratio of Cu:Ni:Ti in (CuxNi2-x)Ti for CuNi-8wt.%Ti alloy is 30:37:33 while that for CuNi16 wt%Ti alloy is 40:27:33, which are both consistent with the EDS results (33:35:32 and 41:25:34). DSC curve for CuNi-Xwt.%Ti filler metal is given in Fig. 7. For CuNi
and the phase structure was determined by AXS Bruker X-ray diffraction. The phase transition temperature and melting point was during heating process was investigated using Mettler-Toledo differential scanning calorimeter. The viscosity of liquid CuNiTi filler metal was determined using RHEOTRONIC VIII rotary vibrating melt viscometer. The sample was placed in an Al2O3 container, and installed in the sample chamber followed by evacuated to 1 × 10−4 Pa. The viscosity test was carried out using cooling measure methods, in which, the sample was heated to the highest experimental temperature at 4 °C/min, and the viscosity value was determined after 60 min insulation. Subsequently, the melt was cooled to the next experimental temperature, and the viscosity measurement was started after 30 min insulation. At each temperature point, the viscosity was measured 8 times repeatedly and the average value was adopted. The wetting experiment was carried out using SCI-1700 High temperature wetting angle measuring instrument, as shown in Fig. 1. The tungsten plate and AlN ceramic were the substrate metals for non-reactive wetting and reactive wetting experiments, respectively. The detailed description of the experimental procedure was given in our preliminary work [19]. After the wetting experiments, the microstructure of wetting interface was researched using scanning electron microscope. 3. Results 3.1. Material characterization The microstructures and EDS analysis results of CuNi, CuNi-8wt.%Ti and CuNi-16 wt%Ti filler metals are shown in Figs. 2–4, and the XRD patterns are given in Fig. 5. From Fig. 2, CuNi filler metal is composed by the uniform distributed gray Cu–Ni solid solution (CuNi)ss completely, and the mass ratio (wt.%) of Cu and Ni is close to 85:15, which is consistent with the chemical composition of the designed CuNi alloy. 3
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Fig. 4. Microstructure of CuNi-16 wt%Ti filler metal as well as energy spectrum results: (a) low magnification microstructure; (b) high magnification microstructure; (c) energy spectrum of point F; (d) energy spectrum of point G.
Fig. 5. XRD patterns of (a) CuNi (b) CuNi-8wt.%Ti and (c) CuNi-16 wt%Ti filler metals.
filler metal, there is only one endothermic peak located at 1146.87 °C, as shown in Fig. 7(a). Considering that CuNi alloy is composed by (CuNi)ss completely, it can be concluded that this endothermic peak is caused by the melting of filler metal. While from Fig. 7(b) and (c), there are three endothermic peaks (1019.32 °C, 1077.40 °C and 1133.49 °C) for CuNi-8wt.%Ti filler metal and two endothermic peaks (1077.58 °C and 1116.38 °C) for CuNi-16 wt%Ti filler metal. From the polythermal section diagram of CuNiTi alloy shown in Fig. 8, which proposed by Yakushiji [22] and Alisova [23], the eutectic reaction γ + τ1 → L [(CuNiTi)ss + (CuxNi2-x)Ti → L] can be occurred at 1070 °C. Therefore, it can be concluded that 1077.40 °C and 1077.58 °C are the initial melting temperature for CuNi-8wt.%Ti filler metal and CuNi-16 wt%Ti filler metal, respectively. The subsequent weak endothermic peaks for the two filler metals located at 1133.49 °C and 1116.58 °C are caused by the melting of
residual (CuNiTi)ss, because that the temperature can reach melting point of (CuNiTi)ss before it involves in the eutectic reaction completely considering the large heating rate during the DSC process. This phenomenon was also discovered by other researchers [24–26]. The difference of the melting points for the (CuNiTi)ss in the two CuNi-Xwt. %Ti filler metals is caused by the various Ti content. Moreover, it should be noticed that there is one endothermic peak before the initial melting temperature in CuNi-8wt.%Ti filler metal but in CuNi-16 wt%Ti filler metal. Considering that the TiNi2Cu intermetallic compound is only existed in CuNi-8wt.%Ti filler metal, it can be speculated that this endothermic reaction (phase transition) is must related with the TiNi2Cu intermetallic compound, which has not been reported before and will be investigated systemically in the following work.
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Fig. 6. CuNiTi ternary alloy phase diagram (isothermal cross-section at 870 °C) [21].
3.2. Liquid metal structure and surface tension
γ=
The relationship between viscosity of CuNi-Xwt.%Ti filler metal and temperature is shown in Fig. 9, in which, with the increase of temperature, the viscosities for three CuNi-Xwt.%Ti filler metals are all decreased. It should be noticed that for CuNi filler metal, the viscosity decreases smoothly in the whole temperature range. While for CuNi8wt.%Ti filler metal and CuNi-16 wt%Ti filler metal, although the viscosity also decreases smoothly in range of 1150 °C–1300 °C and 1330 °C–1420 °C, it shows step mutation between 1300 °C to 1330 °C. Therefore, divided by the viscosity step mutation temperature, the viscosity of CuNi-Xwt.%Ti filler metal was split into low-temperature interval and high-temperature interval, which are 1150 °C–1300 °C and 1330 °C–1420 °C, respectively. In addition, as the Ti content increasing, viscosity of CuNi-Xwt.%Ti filler metal increases at low-temperature interval, but decreases at high-temperature interval. The surface tension of liquid CuNiTi filler metal can be calculated according to Eq (1), which is deduced by Guthrie equation [27] and Abtew equation [28],
15 η kT 16 m
(1)
in which, η is the viscosity, m is the average atomic mass, k is the Boltzmann constant, and T is the absolute temperature. The surface tensions of liquid CuNi-Xwt%Ti filler metals from 1150 °C to 1420 °C are shown in Fig. 10. From Fig. 10, with the temperature increasing, surface tension of the three CuNi-Xwt%Ti filler metals all decrease firstly (1150 °C–1330 °C) and then increases (1330 °C–1420 °C). Besides that, with the increase of Ti content, the surface tension of CuNi-Xwt%Ti filler metal increases at low-temperature interval, but very similar at high-temperature interval. The wettability of CuNi-Xwt%Ti filler metal on W plate at 1300 °C and 1360 °C is given in Fig. 11. Because that there is no chemical reaction between filler metal and substrate, the wetting behavior is the non-reactive wetting, which can be analyzed using Young-Dupre formula [29]. From Fig. 11(a), (b) and (c), the wetting angle of CuNi filler metal, CuNi-8wt.%Ti filler metal and CuNi-16 wt%Ti filler metal on W plate are 38.4°, 66.3° and 71.5° at 1300 °C. This phenomenon is not difficult to be understood considering that surface tension for CuNi-Xwt %Ti filler metal is increased with the increasing Ti content at low5
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Fig. 7. DSC curves of (a) CuNi (b) CuNi-8wt.%Ti and (c) CuNi-16 wt%Ti filler metals.
angle in the non-reactive wetting stage (Stage I) following the order of CuNi-16 wt%Ti/AlN (112.7°) > CuNi-8wt%Ti/AlN (98.3°) > CuNi/AlN (88.6°). The reason is that at the non-reactive wetting stage, the wetting angle is depended on surface tension of the liquid CuNi-Xwt%Ti filler metal completely. However, although the initial wetting angle of CuNi8wt%Ti/AlN is smaller than that of CuNi-16 wt%Ti/AlN at stage I, the final wetting angle of CuNi-8wt%Ti/AlN (53.4°) is larger than that of CuNi-16 wt%Ti/AlN (32.8°). Moreover, compared the wetting process, it can be found that the reactive wetting stage (stage II) of CuNi-16 wt %Ti/AlN system is longer than that of CuNi-8wt%Ti/AlN system, indicating that the interface chemical reaction of the former shows longer duration. For further researching the effect of interfacial reaction on wettability of CuNi-Xwt%Ti/AlN, the wetting interfaces of the two reactive wetting systems (CuNi-8wt%Ti/AlN and CuNi-16 wt%Ti/AlN) were investigated, as shown in Fig. 13. From Fig. 13(a) and (d), it can be found that the gray compounds are formed at the wetting interfaces, and the wetting angle of CuNi-8wt%Ti/AlN and CuNi-16 wt%Ti/AlN are 56.5° and 34.6°, which are slightly larger than the results shown in Fig. 12. This phenomenon is not worth questioning because that the liquid filler metal will contract at the solidification stage and causing the slightly increase in wetting angle inevitably. The boundary region and central region of wetting interfaces for CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN wetting systems are shown in Fig. 13 (b), (c), (e) and (f). From them, for CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN, the generated compounds are continuously distributed at the wetting interface of both central region (areas M and O) and boundary region (areas N and P), and the precursor film can be found at triple junction. Moreover, it can be found that although the thicknesses of the formed compounds at the two interfaces are similar, the acreage of the compounds at CuNi-16 wt%Ti/AlN is much larger
temperature interval. However, when wetting temperature is 1360 °C, as shown in Fig. 11(d), (e) and (f), the wetting angles of the three wetting systems are the approximate 45.3°, 46.1° and 45.7° due to the similar surface tension at high-temperature interval. It is obvious that the calculated surface tension is consistent with the non-reactive wetting experimental result well. 3.3. Reactive wetting behavior of CuNiTi filler metal on AlN ceramic The wetting and spreading behavior of CuNi-Xwt%Ti filler metal on AlN ceramic at 1150 °C is shown in Fig. 12, in which, 0s is the complete liquid time of the filler metal. From Fig. 12, it is can be found there is obvious difference between the wetting behavior of CuNi filler metal on AlN ceramic and those of CuNi-8wt%Ti and CuNi-16 wt%Ti filler metals. For CuNi/AlN wetting system, the wetting process consists of two stages, within the initial 1.5 s (Stage I), the wetting angle decreases to 88.6° instantly, and then maintains constant (Stage II). While for CuNi8wt%Ti and CuNi-16 wt%Ti filler metals, the wetting process is composed by three stages. In which, at the initial 1.5 s (Stage I), the wetting angle decreases to 98.3° and 112.7° quickly. After that, with the increasing wetting time, the wetting angle decreases to 53.4° and 32.8° slowly (Stage II), and then remains stable (Stage III). The reason is that, the CuNi/AlN is the non-reactive wetting system due to the lack of active element Ti. Therefore, the wetting balance can be achieved in a short period of time [30,31]. While CuNi-8wt%Ti/AlN and CuNi-16 wt%Ti/AlN are both the reactive wetting systems, therefore, the wetting process contains not only the non-reactive wetting stage (stage I), but also the reactive wetting stage (stage II), which is caused by the interfacial reaction between element Ti and AlN substrate, and continues the longer time. It should be noticed that for the three wetting systems, the wetting 6
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Fig. 8. Polythermal section diagram of CuNiTi alloy [22,23].
Fig. 10. Surface tension for melting CuNi-Xwt%Ti filler metal.
Fig. 9. Viscosities of CuNi-Xwt.%Ti filler metals at different temperatures.
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Fig. 11. Wetting morphology of CuNi-Xwt%Ti filler metal on W plate: (a) CuNi, 1300 °C (b) CuNi-8wt.%Ti, 1300 °C (c) CuNi-16 wt%Ti, 1300 °C(d) CuNi, 1360 °C (e) CuNi-8wt.%Ti, 1360 °C (f) CuNi-16 wt%Ti, 1360 °C.
wetting. At the end moment of this stage, the three-force (γgs = γls+γglcosθ0) balance at the triple junction can be achieved, and the instant wettability (θ0) is decided by the surface tension of CuNiXwt.%Ti liquid filler metal completely. Subsequently, in stage II, the active element Ti in the liquid filler metal diffuses to the wetting interface drastically, and promotes the violent chemical reaction with AlN ceramic:
1 1 AlN + [Ti] ⇔ Ti2 AlN 2 2
(2)
Therefore, the interfacial reaction compound Ti2AlN is spread on the interface between CuNi-Xwt.%Ti liquid filler metal and AlN ceramic, and the precursor film with the same chemical composition was formed at the wetting front because of the adsorption behavior for AlN ceramic to active element Ti. As the interfacial reaction compound and precursor film were formed at the interface, the three-force balance at the triple junction was broken. In which, γgs shows little change although the solid substrate has been changed from AlN ceramic to Ti2AlN compound. The reason is that, at the vacuum condition, γgs is almost stable not matter the solid phase is [32]. However, γls is decreased because that the L/S interface changed from CuNiTi/AlN to CuNiTi/Ti2AlN, and the chemical properties of Ti2AlN compound is more similar with that of CuNiTi alloy than AlN ceramic. Moreover, it should be noticed that during this processing, the element Ti in the liquid filler metal was consumed, and causing the decrease of γls, as shown in Fig. 10. Therefore, the force situation for the wetting system is γgs > γls + γglcosθ0, causing the triple junction moves forward, in other words, the wetting angle was decreased. At the same time, the precursor film will be formed continuously before the wetting triple junction. When the wetting angle decreases to θmin, the three-force balance condition (γgs = γls+γglcosθ0) at the triple junction can be achieved again, so the triple junction will not move anymore with the increasing time (stage III), and the wetting behavior reaches the final equilibrium. Actually, for the CuNi-Xwt.%Ti/AlN reactive wetting system, the wettability can be analyzed using the minimum contact angle formula [33],
Fig. 12. Relationship between wetting angle and time for CuNi-Xwt%Ti filler metal reactive wetting on AlN ceramic at 1150 °C.
than that at CuNi-8wt.%Ti/AlN, indicating that the more compounds were generated at CuNi-16 wt%Ti/AlN wetting interface. The formed compound at the wetting interface was characterized, as given in Fig. 14. The energy spectrum analysis shows the interface compound is composed by element Ti, Al and N with the atomic ratio of 56:23:24, indicating it is Ti2AlN compound. Moreover, the line scans energy spectrum shown in Fig. 14(c) shows that the elements Cu and Ni are concentrated in liquid filler metal while element Ti is concentrated in interface compound. This phenomenon indicates that, during the wetting process, element Ti in liquid filler metal diffuses to wetting interface drastically to generate Ti2AlN compound, which leading to the lack of Ti in liquid filler metal. Fig. 14(d) shows the map scans energy spectrum about element Ti, it also can be found that Ti is concentrated in interface compound and shows the very low content in the filler metal, which can confirm the speculation in Fig. 14(c). 4. Discussion
cosθmin = cosθ0 − Based on the analysis results of wetting angle-time curve and wetting interface, the mechanism for CuNi-Xwt.%Ti liquid filler metal wetting on AlN ceramic can be achieved, and the schematic diagram is shown in Fig. 15. From Fig. 15, in stage I, at the first few seconds (0–1.5s) after the filler metal melting, the interface chemical reaction between CuNi-Xwt. %Ti and AlN has not yet occurred, so this stage is the non-reactive
ΔGr Δδr − δlv δlv
(3)
in which, θmin is the final wetting angle, θ0 is the instant wetting angle before interfacial reaction (at end moment of stage I), δlv is the surface tension of the liquid filler metal, △Gr and △δr are the reduced free energy (negative value) and change of interface energy caused by the interfacial reaction, respectively. For CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN, it is can be found 8
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Fig. 13. Cross-section morphologies for the CuNi-Xwt.%Ti filler metals wetting on AlN ceramics: (a) CuNi-8wt.%Ti/AlN low magnification morphology (b) High magnification morphology of area M (c) High magnification morphology of area N (d) CuNi-16 wt%Ti/AlN low magnification morphology (e) High magnification morphology of area O (f) High magnification morphology of area P.
Xwt%Ti filler metals all decreases firstly (1150 °C–1330 °C) and then increases (1330 °C–1420 °C). Moreover, with the increase of Ti content, the surface tension of the CuNi-Xwt%Ti filler metal increases gradually at the low-temperature interval, but very similar at high-temperature interval. 4. CuNi/AlN is the non-reactive wetting system, and the wetting process consists of two stages, within the initial 1.5 s (Stage I), the wetting angle decreases to 88.6° instantly, and then maintains constant (Stage II). The wettability is depended on surface tension for the liquid CuNi filler metal completely. 5. The wetting process of CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system is composed by three stages. In which, at the initial 1.5 s (Stage I), the wetting angle decreases to 98.3° and 112.7° instantly. After that, with the increasing wetting time, the wetting angle decreases to 53.4° and 32.8° slowly because of the interfacial reaction (Stage II), and then remains stable (Stage III). 6. For the CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system, although the surface tension of liquid filler metal is the only factor to influence the instant wetting angle θ0 at stage I, the reduced free energy caused by interfacial reaction at stage II plays the decisive role in influencing the final wettability.
the interface chemical reactions are the same, and the wetting interfaces both transformed from CuNiTi/AlN to CuNiTi/Ti2AlN. Therefore, interface energy change △δr for the two wetting systems are the equivalent. However, because of the large Ti content, the interfacial reaction severity of CuNi-16 wt%Ti/AlN is stronger than that of CuNi8wt.%Ti/AlN, which expressed as the generation of more Ti2AlN compound. Therefore, the absolute value of the reduced free energy △Gr for CuNi-16 wt%Ti/AlN is larger than that of CuNi-8wt.%Ti/AlN, which leading that the reactive wettability of CuNi-16 wt%Ti/AlN is more excellent than that of CuNi-8wt.%Ti/AlN. Moreover, considering that the instant wetting angle θ0 for CuNi-8wt.%Ti/AlN is smaller than CuNi-16 wt%Ti/AlN, while the final wetting angle θmin shows the opposite rule. Therefore, for the reactive wetting system, although the surface tension is the only factor to influence the instant wetting angle θ0 at stage I, the reduced free energy caused by interfacial reaction at stage II plays the decisive role in influencing the final wettability. 5. Conclusion 1. The microstructure of CuNi filler metal is composed by Cu–Ni solid solution (CuNi)ss completely, while for CuNi-8wt.%Ti filler metal and CuNi-16 wt%Ti filler metal, the microstructures are (CuNiTi)ss + TiNi2Cu + (CuxNi2-x)Ti and (CuNiTi)ss + (CuxNi2-x)Ti, respectively. 2. The viscosity of CuNi-Xwt.%Ti filler metal was split into low-temperature interval and high-temperature interval, which are 1150 °C–1300 °C and 1330 °C–1420 °C, respectively. As the Ti content increasing, viscosity of CuNi-Xwt.%Ti filler metal increases at low-temperature interval, but decreases at high-temperature interval. 3. With the temperature increasing, surface tension for the three CuNi-
Declaration of competing interest We wish to draw the attention of editor to the following facts which may be considered as potential conflicts of interest this work. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have given due consideration to the protection 9
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Fig. 14. Microstructure and energy spectrum analysis result for CuNi-16 wt%Ti/AlN wetting interface (a) Interface microstructure (b) Energy spectrum of point R (c) line scans energy spectrum (d) map scans energy spectrum about element Ti.
Fig. 15. Mechanism schematic diagram for CuNi-Xwt.%Ti melting filler metal wetting on AlN ceramic.
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of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication. We further confirm that we have followed the regulations of our institutions concerning intellectual property. We understand that the corresponding author is the sole contact for the editorial process. He is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs.
[12] A. Kara-Slimane, D. Juve, E. Leblond, D. Treheux, Joining of AlN with metals and alloys, J. Eur. Ceram. Soc. 20 (2000) 1829–1836. [13] S. Zhu, W. Włosiński, Joining of AlN ceramic to metals using sputtered Al or Ti film, J. Mater. Process. Technol. 109 (2001) 277–282. [14] A. Koltsov, F. Hodaj, N. Eustathopoulos, Brazing of AlN to SiC by a Pr silicide: physicochemical aspects, Mater. Sci. Eng., A 495 (2008) 259–264. [15] B. Chen, H.P. Xiong, Y.Y. Cheng, W. Mao, S.B. Wu, Microstructure and property of AlN joint brazed with Au-Pd-Co-Ni-V brazing filler, J. Mater. Sci. Technol. 31 (2015) 1034–1038. [16] R.O. Apaydın, B. Ebin, S. Gürmen, Single-step production of nanostructured coppernickel (CuNi) and copper-nickel-indium (CuNiIn) alloy particles, Metall. Mater. Trans. A 47 (2016) 3744–3752. [17] Y.L. Wang, W.L. Wang, J.H. Huang, R.H. Yu, J. Yang, S.H. Chen, Reactive composite brazing of C/C composite and GH3044 with Ag-Ti mixed powder filler material, Mater. Sci. Eng., A 759 (2019) 303–312. [18] N. Eustathopoulos, Dynamics of wetting in reactive metal/ceramic systems, Acta Mater. 46 (1998) 2319–2327. [19] J. Yang, J.H. Huang, Z. Ye, S.H. Chen, Y. Zhao, Influence of interfacial reaction on reactive wettability of molten Ag-Cu-Xwt.%Ti filler metal on SiC ceramic substrate and mechanism analysis, Appl. Surf. Sci. 436 (2017) 768–778. [20] H. Bian, Y.Y. Song, D. Liu, Y.Z. Lei, X.G. Song, J. Cao, Joining of SiO2 ceramic and TC4 alloy by nanoparticles modified brazing filler metal, Chin. J. Aeronaut. (2019), https://doi.org/10.1016/j.cja.2019.03.040. [21] F.J.J. Van Loo, G.F. Bastin, A.J.H. Leenen, Phase relations in the ternary Ti-Ni-Cu system at 800 and 870 °C, J. Less Common Met. 57 (1978) 111–121. [22] M. Yakushiji, Y. Kondo, K. Kamei, Phase equilibria in the Ti-rich region of a Ti-NiCu system, J. Jpn. Inst. Metals 46 (1982) 571–577. [23] S.P. Alisova, N.V. Volynskaya, P.B. Budberg, A.N. Kobylkin, Phase equilibria in alloys of the TiCu-TiNi-TiCuNi system, Russ. Metall. 5 (1986) 207–209. [24] P.P. Jana, J. Das, Precise estimation of glass transition and crystallization temperatures of Zr55Cu30Ni5Al10 metallic glass using step-scan modulated temperature differential scanning calorimeter, Thermochim. Acta 660 (2018) 18–22. [25] Z.K. Shen, Y.Q. Ding, A.P. Gerlich, Advances in friction stir spot welding, Crit. Rev. Solid State Mater. Sci. (2019) 1–78. [26] J. Orava, D.W. Hewak, A.L. Greer, Fragile-to-Strong crossover in supercooled liquid Ag-In-Sb-Te studied by ultrafast calorimetry, Adv. Funct. Mater. 25 (2015) 4851–4858. [27] T. Iida, R.I. Guthrie, The Physical Properties of Liquid Metals, Clarendon Press, Walton Street, Oxford OX 26 DP, UK, 1988. [28] M. Abtew, G. Selvaduray, Lead-free solders in microelectronics, Mater. Sci. Eng. R 27 (2000) 95–141. [29] P.G. De Gennes, Wetting: statics and dynamics, Rev. Mod. Phys. 57 (1985) 827–863. [30] P. Nautiyal, A. Gupta, S. Seal, B. Boesl, A. Agarwal, Reactive wetting and filling of boron nitride nanotubes by molten aluminum during equilibrium solidification, Acta Mater. 126 (2017) 124–131. [31] L.X. Xi, I. Kaban, R. Nowak, G. Bruzda, N. Sobczak, J. Eckert, Wetting, reactivity, and phase formation at interfaces between Ni-Al melts and TiB2 ultrahigh-temperature ceramic, J. Am. Ceram. Soc. 101 (2018) 911–918. [32] A. Fortini, A.P. Hynninen, M. Dijkstra, Gas-liquid phase separation in oppositely charged colloids: stability and interfacial tension, J. Chem. Phys. 125 (2006) 094502. [33] V. Laurent, D. Chatain, N. Eustathopoulos, Wettability of SiO2 and oxidized SiC by aluminium, Mater. Sci. Eng. A 135 (1991) 89–94.
Acknowledgments The authors would like to express their gratitude for projects funded by the National Natural Science Foundation of China (51605027), National Defense Pre-Research Foundation of China (61409230503) and Fundamental Research Funds for the Central Universities (FRF-GF18-003B). References [1] T. Kusunose, T. Sekino, Improvement in fracture strength in electrically conductive AlN ceramics with high thermal conductivity, Ceram. Int. 42 (2016) 13183–13189. [2] S.S. Zhang, L.C. Yan, K.W. Gao, H.S. Yang, L.H. Yang, Y.B. Wang, X.L. Wan, Thermal ratcheting effect of AMB-AlN ceramic substrate: experiments and calculations, Ceram. Int. 45 (2019) 14669–14674. [3] H.W. Son, B.N. Kim, T.S. Suzuki, Y. Suzuki, Fabrication of translucent AlN ceramics with MgF2 additive by spark plasma sintering, J. Am. Ceram. Soc. 101 (2018) 4430–4433. [4] D.G. Pahinkar, W. Puckett, S. Graham, L. Boteler, D. Ibitayo, S. Narumanchi, P. Paret, D. DeVoto, J. Major, Transient liquid phase bonding of AlN to AlSiC for durable power electronic packages, Adv. Eng. Mater. 20 (2018) 1800039. [5] D.S. King, G. Hilmas, W. Fahrenholtz, Mechanical behavior and applications of plasma arc welded ceramics, Int. J. Appl. Ceram. Technol. 13 (2016) 41–49. [6] M. Barlak, W. Olesinska, J. Piekoszewski, Z. Werner, M. Chmielewski, J. Jagielski, D. Kalinski, B. Sartowska, K. Borkowska, Ion beam modification of ceramic component prior to formation of AlN-Cu joints by direct bonding process, Surf. Coat. Technol. 201 (2007) 8317–8321. [7] M. Fei, R.L. Fu, S. Agathopoulos, J. Fang, C.X. Wang, H.T. Zhu, A preparation method for Al/AlN ceramics substrates by using a CuO interlayer, Mater. Des. 130 (2017) 373–380. [8] B. Hong, X.G. Song, S.P. Hu, Y.Z. Lei, Y.D. Jiao, S.T. Duan, J.C. Feng, W.M. Long, Microstructure evolution and mechanical properties of titanium/alumina brazed joints for medical implants, Metals 9 (2019) 644. [9] J.M. Fernandez, R. Asthana, M. Singh, F.M. Valera, Active metal brazing of silicon nitride ceramics using a Cu-based alloy and refractory metal interlayers, Ceram. Int. 42 (2016) 5447–5454. [10] X.Y. Dai, J. Cao, Z. Chen, X.G. Song, J.C. Feng, Brazing SiC ceramic using novel B4C reinforced Ag-Cu-Ti composite filler, Ceram. Int. 42 (2016) 6319–6328. [11] D. Huh, D.H. Kim, Joining of AlN to Cu using In-base active brazing fillers, J. Mater. Res. 12 (1997) 1048–1055.
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Reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal Jian Yanga,∗, Haodong Lia, Xuanwei Leib, Haifeng Xua, Jihua Huanga, Shuhai Chena, Yue Zhaoc a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China Faculty of Materials, Metallurgy and Chemistry, Jiangxi University of Science and Technology, Ganzhou, 341000, China c Department of Vehicle Engineering, Army Academy of Armored Forces, Beijing, 100072, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: AlN ceramic Active brazing Reactive wetting Surface tension Interface reaction
In order to propel the application of the developed CuNi-Xwt%Ti active filler metal in AlN brazing and get the universal reactive wetting mechanism between liquid metal and solid ceramic, the reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal were investigated. The results indicate that, with the increasing Ti content, surface tension for liquid CuNi-Xwt%Ti filler metal increases at low-temperature interval, but very similar at high-temperature interval, which influence the wetting behavior on AlN ceramic obviously. CuNi/AlN is the typical non-reactive wetting system, the wetting process including rapid wetting stage and stable stage. The wettability is depended on surface tension of the liquid CuNi filler metal completely. However, the wetting process of CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system is composed by three stages, which are rapid wetting stage decided by surface tension, slow wetting stage caused by interfacial reaction and stable stage. For CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system, although the surface tension of liquid filler metal is the only factor to influence the instant wetting angle θ0 at rapid wetting stage, the reduced free energy caused by interfacial reaction at slow wetting stage plays the decisive role in influencing the final wettability.
1. Introduction Because of the high hardness, remarkable wear resistance and excellent oxidation resistance, AlN ceramic has been applied in aerospace engineering, military engineering and mechanical manufacturing widely [1–3]. However, due to the high brittleness, AlN ceramic is difficult to be processed into complex component directly, so the selection of suitable method to bond AlN ceramic with itself and metals is the precondition on promoting its further application [4]. Considering the high melting point and electrical insulation characteristic of AlN ceramic, it is difficult to obtain the high performance AlN joints using fusion welding [5]. Although diffusion bonding has been developed for AlN jointing, it shows the poor structural adaptability considering that the appearance and dimension for the components to be bonded must be controlled strictly [6,7]. It is no doubt that brazing (especially active brazing) is the most suitable method for jointing ceramic to ceramic and ceramic to metal because of the high structural adaptability and strong potential for mass production [8–10]. In the initial research stage of AlN brazing, the researchers adopted Ag–Cu alloy and Ag–In alloy as the filler metals. Huh [11] used
∗
InAg19Ti2 alloy film as filler metal to braze AlN ceramics, and showed that the dense joint can be obtained in vacuum at temperatures of 750 °C. Kara-Slimane [12] brazed AlN ceramic with steel using Ag 62.95Cu35.4Ti1.66 alloy at 850 °C for 5 min, and indicated the shear strength of the joint was 50 MPa. Zhu [13] employed AgCu19.5Ti3In5 filler metal to braze AlN ceramic with Cu and FeNi42 metals, and the mechanical strength of joints are 127 MPa and 176 MPa. However, due to the low melting point of Ag–Cu and Ag–In alloys, these joints are difficult to apply in high temperature environments. After realizing this problem, Koltsov [14] used Si-17 at.% Pr eutectic alloy to braze AlN ceramic and SiC ceramic, and indicated that the temperature resistance of the joint can reach 1212 °C. Xiong [15] brazed AlN ceramics using Au–Pd–Co–Ni–V brazing alloy, and indicated that the sound AlN/AlN joints with temperature resistance above 1100 °C were achieved at 1170 °C for 10 min. However, the joint brazed using high temperature resistant filler metal usually shows the low mechanical strength due to the terrible brittleness of the connection layer [14,15]. The novel CuNiTi active filler metal (Cu86Ni14-Xwt.%Ti) is the promising material for achieving the high strength and high temperature resistance AlN ceramic brazing joint because of two reasons: (i)
Corresponding author. E-mail address: [email protected] (J. Yang).
https://doi.org/10.1016/j.ceramint.2019.10.150 Received 6 October 2019; Received in revised form 12 October 2019; Accepted 16 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Jian Yang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.150
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Fig. 1. Schematic diagram of high temperature wetting angle measuring instrument.
Fig. 2. Microstructure of CuNi filler metal as well as energy spectrum results: (a) low magnification microstructure; (b) high magnification microstructure; (c) energy spectrum of point A; (d) energy spectrum of point B.
The large component of Cu and Ni in filler metal are generated the Cu–Ni solid solution (Cu,Ni)ss substrate, which shows both high melting point and excellent plasticity [16]. (ii) The small component of Ti not only reacts with AlN ceramic and improves the interfacial bonding property, but also reacts with elements Cu and Ni and generates Cu–Ni–Ti intermetallic compounds in the filler metal, which is beneficial to improve the strength of connection layer as the reinforcing phase [17]. Actually, as the novel brazing material, the wettability of CuNiTi alloy on AlN ceramic determines the interfacial bonding property directly, and should be investigated preferentially. Moreover, in the research field of Liquid metal/Solid ceramic reactive wetting, it had been confirmed that the interfacial reaction plays the important role on influencing reactive wettability. However, there are still some important problems, such as the reactive wetting process, the effect of surface tension of liquid metal on reactive wettability, as well as the relationship between liquid metal surface tension and microstructure, are waiting to be solved [18–20]. In this work, the Cu86Ni14-XTi (X = 0 wt%, 8 wt% and 16 wt%) active filler metal was prepared, and the microstructure evolution
behavior due to the increasing Ti content was researched initially. Subsequently, the effect of temperature on viscosity and surface tension of CuNiTi liquid filler metal was studied. Finally, the wetting behavior of CuNiTi filler metal on AlN ceramic was investigated, and the reactive wetting process as well as the wettability influence mechanism was discussed.
2. Experimental The CuNiTi active filler metal was obtained using multiply melting method with the raw materials of high-purity copper, nickel and titanium ingots. For researching the relationship between Ti content in CuNiTi filler metal and its wetting behavior on AlN ceramic, three kinds of CuNiTi filler metals with the same weight percent ratio of Cu and Ni but different Ti weight percent were designed. The chemical compositions (wt.%) of the prepared CuNiTi filler metals were Cu86Ni14, (Cu86Ni14)92Ti8 and (Cu86Ni14)84Ti16, respectively, which was abbreviated as CuNi-Xwt.%Ti. The microstructure of the CuNiTi filler metal was researched using EM-30 PLUS scanning electron microscope, 2
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Fig. 3. Microstructure of CuNi-8wt.%Ti filler metal as well as energy spectrum results: (a) low magnification microstructure; (b) high magnification microstructure; (c) energy spectrum of point C; (d) energy spectrum of point D; (e) energy spectrum of point E.
The same conclusion also be given in the XRD analysis result, as shown in Fig. 4(a). While for CuNi-8wt.%Ti filler metal, the microstructure consists of the gray filamentous phase, white sheet phase and black block phase. The EDS results show that in the gray filamentous phase, the atomic ratio of Cu:Ni:Ti is 1:2:1, indicating that the gray strip phase is CuNi2Ti intermetallic compound. In addition, white sheet phase consists of the large amount of Cu and Ni with a small amount of Ti, indicating that it is the Cu–Ni–Ti ternary solid solution (CuNiTi)ss.While for the black block phase distributed in (CuNiTi)ss, the atomic ratio of Cu:Ni:Ti is near 1:1:1, combined with the XRD analysis, it can be found that the black block phase is (CuxNi2-x)Ti (τ1 phase). From Figs. 3 and 4(c), CuNi-16 wt%Ti filler metal consists of the white (CuNiTi)ss and the black strip phase with the Cu:Ni:Ti atomic ratio near 4:2:3, which is also the (CuxNi2-x)Ti (τ1 phase). The microstructure evolution behavior of CuNi-Xwt.%Ti filler metals with the increasing Ti content can be explained using Cu–Ni–Ti ternary alloy phase diagram (isothermal cross-section at 870 °C) [21], as shown in Fig. 6. In Fig. 6, the chemical compositions of CuNi alloy, CuNi-8wt.%Ti alloy and CuNi-16 wt%Ti alloys are labeled as points H, I and J, respectively. For CuNi alloy, it is obvious that it is situated at γ single phase zone, so only (CuNi)ss was formed because of the infinite mutual solubility between Cu and Ni. While for CuNi-8wt.%Ti alloy and CuNi-16 wt%Ti alloy, it can be found that they are located at γ+τ1+TiNi2Cu ternary phase zone and γ+τ1 binary phase zone. Therefore, their microstructures are (CuNiTi)ss + TiNi2Cu + (CuxNi2x)Ti and (CuNiTi)ss + (CuxNi2-x)Ti, respectively. Moreover, the chemical compositions of (CuxNi2-x)Ti (τ1 phase) in CuNi-8wt.%Ti alloy and CuNi-16 wt%Ti alloy can be achieved using leverage method, and labeled as points K and L. The theoretical atomic ratio of Cu:Ni:Ti in (CuxNi2-x)Ti for CuNi-8wt.%Ti alloy is 30:37:33 while that for CuNi16 wt%Ti alloy is 40:27:33, which are both consistent with the EDS results (33:35:32 and 41:25:34). DSC curve for CuNi-Xwt.%Ti filler metal is given in Fig. 7. For CuNi
and the phase structure was determined by AXS Bruker X-ray diffraction. The phase transition temperature and melting point was during heating process was investigated using Mettler-Toledo differential scanning calorimeter. The viscosity of liquid CuNiTi filler metal was determined using RHEOTRONIC VIII rotary vibrating melt viscometer. The sample was placed in an Al2O3 container, and installed in the sample chamber followed by evacuated to 1 × 10−4 Pa. The viscosity test was carried out using cooling measure methods, in which, the sample was heated to the highest experimental temperature at 4 °C/min, and the viscosity value was determined after 60 min insulation. Subsequently, the melt was cooled to the next experimental temperature, and the viscosity measurement was started after 30 min insulation. At each temperature point, the viscosity was measured 8 times repeatedly and the average value was adopted. The wetting experiment was carried out using SCI-1700 High temperature wetting angle measuring instrument, as shown in Fig. 1. The tungsten plate and AlN ceramic were the substrate metals for non-reactive wetting and reactive wetting experiments, respectively. The detailed description of the experimental procedure was given in our preliminary work [19]. After the wetting experiments, the microstructure of wetting interface was researched using scanning electron microscope. 3. Results 3.1. Material characterization The microstructures and EDS analysis results of CuNi, CuNi-8wt.%Ti and CuNi-16 wt%Ti filler metals are shown in Figs. 2–4, and the XRD patterns are given in Fig. 5. From Fig. 2, CuNi filler metal is composed by the uniform distributed gray Cu–Ni solid solution (CuNi)ss completely, and the mass ratio (wt.%) of Cu and Ni is close to 85:15, which is consistent with the chemical composition of the designed CuNi alloy. 3
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Fig. 4. Microstructure of CuNi-16 wt%Ti filler metal as well as energy spectrum results: (a) low magnification microstructure; (b) high magnification microstructure; (c) energy spectrum of point F; (d) energy spectrum of point G.
Fig. 5. XRD patterns of (a) CuNi (b) CuNi-8wt.%Ti and (c) CuNi-16 wt%Ti filler metals.
filler metal, there is only one endothermic peak located at 1146.87 °C, as shown in Fig. 7(a). Considering that CuNi alloy is composed by (CuNi)ss completely, it can be concluded that this endothermic peak is caused by the melting of filler metal. While from Fig. 7(b) and (c), there are three endothermic peaks (1019.32 °C, 1077.40 °C and 1133.49 °C) for CuNi-8wt.%Ti filler metal and two endothermic peaks (1077.58 °C and 1116.38 °C) for CuNi-16 wt%Ti filler metal. From the polythermal section diagram of CuNiTi alloy shown in Fig. 8, which proposed by Yakushiji [22] and Alisova [23], the eutectic reaction γ + τ1 → L [(CuNiTi)ss + (CuxNi2-x)Ti → L] can be occurred at 1070 °C. Therefore, it can be concluded that 1077.40 °C and 1077.58 °C are the initial melting temperature for CuNi-8wt.%Ti filler metal and CuNi-16 wt%Ti filler metal, respectively. The subsequent weak endothermic peaks for the two filler metals located at 1133.49 °C and 1116.58 °C are caused by the melting of
residual (CuNiTi)ss, because that the temperature can reach melting point of (CuNiTi)ss before it involves in the eutectic reaction completely considering the large heating rate during the DSC process. This phenomenon was also discovered by other researchers [24–26]. The difference of the melting points for the (CuNiTi)ss in the two CuNi-Xwt. %Ti filler metals is caused by the various Ti content. Moreover, it should be noticed that there is one endothermic peak before the initial melting temperature in CuNi-8wt.%Ti filler metal but in CuNi-16 wt%Ti filler metal. Considering that the TiNi2Cu intermetallic compound is only existed in CuNi-8wt.%Ti filler metal, it can be speculated that this endothermic reaction (phase transition) is must related with the TiNi2Cu intermetallic compound, which has not been reported before and will be investigated systemically in the following work.
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Fig. 6. CuNiTi ternary alloy phase diagram (isothermal cross-section at 870 °C) [21].
3.2. Liquid metal structure and surface tension
γ=
The relationship between viscosity of CuNi-Xwt.%Ti filler metal and temperature is shown in Fig. 9, in which, with the increase of temperature, the viscosities for three CuNi-Xwt.%Ti filler metals are all decreased. It should be noticed that for CuNi filler metal, the viscosity decreases smoothly in the whole temperature range. While for CuNi8wt.%Ti filler metal and CuNi-16 wt%Ti filler metal, although the viscosity also decreases smoothly in range of 1150 °C–1300 °C and 1330 °C–1420 °C, it shows step mutation between 1300 °C to 1330 °C. Therefore, divided by the viscosity step mutation temperature, the viscosity of CuNi-Xwt.%Ti filler metal was split into low-temperature interval and high-temperature interval, which are 1150 °C–1300 °C and 1330 °C–1420 °C, respectively. In addition, as the Ti content increasing, viscosity of CuNi-Xwt.%Ti filler metal increases at low-temperature interval, but decreases at high-temperature interval. The surface tension of liquid CuNiTi filler metal can be calculated according to Eq (1), which is deduced by Guthrie equation [27] and Abtew equation [28],
15 η kT 16 m
(1)
in which, η is the viscosity, m is the average atomic mass, k is the Boltzmann constant, and T is the absolute temperature. The surface tensions of liquid CuNi-Xwt%Ti filler metals from 1150 °C to 1420 °C are shown in Fig. 10. From Fig. 10, with the temperature increasing, surface tension of the three CuNi-Xwt%Ti filler metals all decrease firstly (1150 °C–1330 °C) and then increases (1330 °C–1420 °C). Besides that, with the increase of Ti content, the surface tension of CuNi-Xwt%Ti filler metal increases at low-temperature interval, but very similar at high-temperature interval. The wettability of CuNi-Xwt%Ti filler metal on W plate at 1300 °C and 1360 °C is given in Fig. 11. Because that there is no chemical reaction between filler metal and substrate, the wetting behavior is the non-reactive wetting, which can be analyzed using Young-Dupre formula [29]. From Fig. 11(a), (b) and (c), the wetting angle of CuNi filler metal, CuNi-8wt.%Ti filler metal and CuNi-16 wt%Ti filler metal on W plate are 38.4°, 66.3° and 71.5° at 1300 °C. This phenomenon is not difficult to be understood considering that surface tension for CuNi-Xwt %Ti filler metal is increased with the increasing Ti content at low5
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Fig. 7. DSC curves of (a) CuNi (b) CuNi-8wt.%Ti and (c) CuNi-16 wt%Ti filler metals.
angle in the non-reactive wetting stage (Stage I) following the order of CuNi-16 wt%Ti/AlN (112.7°) > CuNi-8wt%Ti/AlN (98.3°) > CuNi/AlN (88.6°). The reason is that at the non-reactive wetting stage, the wetting angle is depended on surface tension of the liquid CuNi-Xwt%Ti filler metal completely. However, although the initial wetting angle of CuNi8wt%Ti/AlN is smaller than that of CuNi-16 wt%Ti/AlN at stage I, the final wetting angle of CuNi-8wt%Ti/AlN (53.4°) is larger than that of CuNi-16 wt%Ti/AlN (32.8°). Moreover, compared the wetting process, it can be found that the reactive wetting stage (stage II) of CuNi-16 wt %Ti/AlN system is longer than that of CuNi-8wt%Ti/AlN system, indicating that the interface chemical reaction of the former shows longer duration. For further researching the effect of interfacial reaction on wettability of CuNi-Xwt%Ti/AlN, the wetting interfaces of the two reactive wetting systems (CuNi-8wt%Ti/AlN and CuNi-16 wt%Ti/AlN) were investigated, as shown in Fig. 13. From Fig. 13(a) and (d), it can be found that the gray compounds are formed at the wetting interfaces, and the wetting angle of CuNi-8wt%Ti/AlN and CuNi-16 wt%Ti/AlN are 56.5° and 34.6°, which are slightly larger than the results shown in Fig. 12. This phenomenon is not worth questioning because that the liquid filler metal will contract at the solidification stage and causing the slightly increase in wetting angle inevitably. The boundary region and central region of wetting interfaces for CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN wetting systems are shown in Fig. 13 (b), (c), (e) and (f). From them, for CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN, the generated compounds are continuously distributed at the wetting interface of both central region (areas M and O) and boundary region (areas N and P), and the precursor film can be found at triple junction. Moreover, it can be found that although the thicknesses of the formed compounds at the two interfaces are similar, the acreage of the compounds at CuNi-16 wt%Ti/AlN is much larger
temperature interval. However, when wetting temperature is 1360 °C, as shown in Fig. 11(d), (e) and (f), the wetting angles of the three wetting systems are the approximate 45.3°, 46.1° and 45.7° due to the similar surface tension at high-temperature interval. It is obvious that the calculated surface tension is consistent with the non-reactive wetting experimental result well. 3.3. Reactive wetting behavior of CuNiTi filler metal on AlN ceramic The wetting and spreading behavior of CuNi-Xwt%Ti filler metal on AlN ceramic at 1150 °C is shown in Fig. 12, in which, 0s is the complete liquid time of the filler metal. From Fig. 12, it is can be found there is obvious difference between the wetting behavior of CuNi filler metal on AlN ceramic and those of CuNi-8wt%Ti and CuNi-16 wt%Ti filler metals. For CuNi/AlN wetting system, the wetting process consists of two stages, within the initial 1.5 s (Stage I), the wetting angle decreases to 88.6° instantly, and then maintains constant (Stage II). While for CuNi8wt%Ti and CuNi-16 wt%Ti filler metals, the wetting process is composed by three stages. In which, at the initial 1.5 s (Stage I), the wetting angle decreases to 98.3° and 112.7° quickly. After that, with the increasing wetting time, the wetting angle decreases to 53.4° and 32.8° slowly (Stage II), and then remains stable (Stage III). The reason is that, the CuNi/AlN is the non-reactive wetting system due to the lack of active element Ti. Therefore, the wetting balance can be achieved in a short period of time [30,31]. While CuNi-8wt%Ti/AlN and CuNi-16 wt%Ti/AlN are both the reactive wetting systems, therefore, the wetting process contains not only the non-reactive wetting stage (stage I), but also the reactive wetting stage (stage II), which is caused by the interfacial reaction between element Ti and AlN substrate, and continues the longer time. It should be noticed that for the three wetting systems, the wetting 6
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Fig. 8. Polythermal section diagram of CuNiTi alloy [22,23].
Fig. 10. Surface tension for melting CuNi-Xwt%Ti filler metal.
Fig. 9. Viscosities of CuNi-Xwt.%Ti filler metals at different temperatures.
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Fig. 11. Wetting morphology of CuNi-Xwt%Ti filler metal on W plate: (a) CuNi, 1300 °C (b) CuNi-8wt.%Ti, 1300 °C (c) CuNi-16 wt%Ti, 1300 °C(d) CuNi, 1360 °C (e) CuNi-8wt.%Ti, 1360 °C (f) CuNi-16 wt%Ti, 1360 °C.
wetting. At the end moment of this stage, the three-force (γgs = γls+γglcosθ0) balance at the triple junction can be achieved, and the instant wettability (θ0) is decided by the surface tension of CuNiXwt.%Ti liquid filler metal completely. Subsequently, in stage II, the active element Ti in the liquid filler metal diffuses to the wetting interface drastically, and promotes the violent chemical reaction with AlN ceramic:
1 1 AlN + [Ti] ⇔ Ti2 AlN 2 2
(2)
Therefore, the interfacial reaction compound Ti2AlN is spread on the interface between CuNi-Xwt.%Ti liquid filler metal and AlN ceramic, and the precursor film with the same chemical composition was formed at the wetting front because of the adsorption behavior for AlN ceramic to active element Ti. As the interfacial reaction compound and precursor film were formed at the interface, the three-force balance at the triple junction was broken. In which, γgs shows little change although the solid substrate has been changed from AlN ceramic to Ti2AlN compound. The reason is that, at the vacuum condition, γgs is almost stable not matter the solid phase is [32]. However, γls is decreased because that the L/S interface changed from CuNiTi/AlN to CuNiTi/Ti2AlN, and the chemical properties of Ti2AlN compound is more similar with that of CuNiTi alloy than AlN ceramic. Moreover, it should be noticed that during this processing, the element Ti in the liquid filler metal was consumed, and causing the decrease of γls, as shown in Fig. 10. Therefore, the force situation for the wetting system is γgs > γls + γglcosθ0, causing the triple junction moves forward, in other words, the wetting angle was decreased. At the same time, the precursor film will be formed continuously before the wetting triple junction. When the wetting angle decreases to θmin, the three-force balance condition (γgs = γls+γglcosθ0) at the triple junction can be achieved again, so the triple junction will not move anymore with the increasing time (stage III), and the wetting behavior reaches the final equilibrium. Actually, for the CuNi-Xwt.%Ti/AlN reactive wetting system, the wettability can be analyzed using the minimum contact angle formula [33],
Fig. 12. Relationship between wetting angle and time for CuNi-Xwt%Ti filler metal reactive wetting on AlN ceramic at 1150 °C.
than that at CuNi-8wt.%Ti/AlN, indicating that the more compounds were generated at CuNi-16 wt%Ti/AlN wetting interface. The formed compound at the wetting interface was characterized, as given in Fig. 14. The energy spectrum analysis shows the interface compound is composed by element Ti, Al and N with the atomic ratio of 56:23:24, indicating it is Ti2AlN compound. Moreover, the line scans energy spectrum shown in Fig. 14(c) shows that the elements Cu and Ni are concentrated in liquid filler metal while element Ti is concentrated in interface compound. This phenomenon indicates that, during the wetting process, element Ti in liquid filler metal diffuses to wetting interface drastically to generate Ti2AlN compound, which leading to the lack of Ti in liquid filler metal. Fig. 14(d) shows the map scans energy spectrum about element Ti, it also can be found that Ti is concentrated in interface compound and shows the very low content in the filler metal, which can confirm the speculation in Fig. 14(c). 4. Discussion
cosθmin = cosθ0 − Based on the analysis results of wetting angle-time curve and wetting interface, the mechanism for CuNi-Xwt.%Ti liquid filler metal wetting on AlN ceramic can be achieved, and the schematic diagram is shown in Fig. 15. From Fig. 15, in stage I, at the first few seconds (0–1.5s) after the filler metal melting, the interface chemical reaction between CuNi-Xwt. %Ti and AlN has not yet occurred, so this stage is the non-reactive
ΔGr Δδr − δlv δlv
(3)
in which, θmin is the final wetting angle, θ0 is the instant wetting angle before interfacial reaction (at end moment of stage I), δlv is the surface tension of the liquid filler metal, △Gr and △δr are the reduced free energy (negative value) and change of interface energy caused by the interfacial reaction, respectively. For CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN, it is can be found 8
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Fig. 13. Cross-section morphologies for the CuNi-Xwt.%Ti filler metals wetting on AlN ceramics: (a) CuNi-8wt.%Ti/AlN low magnification morphology (b) High magnification morphology of area M (c) High magnification morphology of area N (d) CuNi-16 wt%Ti/AlN low magnification morphology (e) High magnification morphology of area O (f) High magnification morphology of area P.
Xwt%Ti filler metals all decreases firstly (1150 °C–1330 °C) and then increases (1330 °C–1420 °C). Moreover, with the increase of Ti content, the surface tension of the CuNi-Xwt%Ti filler metal increases gradually at the low-temperature interval, but very similar at high-temperature interval. 4. CuNi/AlN is the non-reactive wetting system, and the wetting process consists of two stages, within the initial 1.5 s (Stage I), the wetting angle decreases to 88.6° instantly, and then maintains constant (Stage II). The wettability is depended on surface tension for the liquid CuNi filler metal completely. 5. The wetting process of CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system is composed by three stages. In which, at the initial 1.5 s (Stage I), the wetting angle decreases to 98.3° and 112.7° instantly. After that, with the increasing wetting time, the wetting angle decreases to 53.4° and 32.8° slowly because of the interfacial reaction (Stage II), and then remains stable (Stage III). 6. For the CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system, although the surface tension of liquid filler metal is the only factor to influence the instant wetting angle θ0 at stage I, the reduced free energy caused by interfacial reaction at stage II plays the decisive role in influencing the final wettability.
the interface chemical reactions are the same, and the wetting interfaces both transformed from CuNiTi/AlN to CuNiTi/Ti2AlN. Therefore, interface energy change △δr for the two wetting systems are the equivalent. However, because of the large Ti content, the interfacial reaction severity of CuNi-16 wt%Ti/AlN is stronger than that of CuNi8wt.%Ti/AlN, which expressed as the generation of more Ti2AlN compound. Therefore, the absolute value of the reduced free energy △Gr for CuNi-16 wt%Ti/AlN is larger than that of CuNi-8wt.%Ti/AlN, which leading that the reactive wettability of CuNi-16 wt%Ti/AlN is more excellent than that of CuNi-8wt.%Ti/AlN. Moreover, considering that the instant wetting angle θ0 for CuNi-8wt.%Ti/AlN is smaller than CuNi-16 wt%Ti/AlN, while the final wetting angle θmin shows the opposite rule. Therefore, for the reactive wetting system, although the surface tension is the only factor to influence the instant wetting angle θ0 at stage I, the reduced free energy caused by interfacial reaction at stage II plays the decisive role in influencing the final wettability. 5. Conclusion 1. The microstructure of CuNi filler metal is composed by Cu–Ni solid solution (CuNi)ss completely, while for CuNi-8wt.%Ti filler metal and CuNi-16 wt%Ti filler metal, the microstructures are (CuNiTi)ss + TiNi2Cu + (CuxNi2-x)Ti and (CuNiTi)ss + (CuxNi2-x)Ti, respectively. 2. The viscosity of CuNi-Xwt.%Ti filler metal was split into low-temperature interval and high-temperature interval, which are 1150 °C–1300 °C and 1330 °C–1420 °C, respectively. As the Ti content increasing, viscosity of CuNi-Xwt.%Ti filler metal increases at low-temperature interval, but decreases at high-temperature interval. 3. With the temperature increasing, surface tension for the three CuNi-
Declaration of competing interest We wish to draw the attention of editor to the following facts which may be considered as potential conflicts of interest this work. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have given due consideration to the protection 9
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Fig. 14. Microstructure and energy spectrum analysis result for CuNi-16 wt%Ti/AlN wetting interface (a) Interface microstructure (b) Energy spectrum of point R (c) line scans energy spectrum (d) map scans energy spectrum about element Ti.
Fig. 15. Mechanism schematic diagram for CuNi-Xwt.%Ti melting filler metal wetting on AlN ceramic.
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of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication. We further confirm that we have followed the regulations of our institutions concerning intellectual property. We understand that the corresponding author is the sole contact for the editorial process. He is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs.
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