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Reactive wetting of binary SnCr alloy on polycrystalline chemical vapour deposited diamond at relatively low temperatures
Diamond & Related Materials 92 (2019) 92–99
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Reactive wetting of binary SneCr alloy on polycrystalline chemical vapour deposited diamond at relatively low temperatures
T
⁎
Jinchang Chena, Xinjiang Liaoa, Qiaoli Linb, Dekui Mua, , Hui Huanga, Xipeng Xua, Han Huangc a
Fujian Engineering Research Centre of Intelligent Manufacturing for Brittle Materials, Institute of Manufacturing Engineering, Huaqiao University, Xiamen 361021, China School of Materials Science and Engineering, Lanzhou University of Science and Technology, Lanzhou 730000, China c School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: CVD diamond Reactive wetting Interface reaction Chromium carbides
Synthetic diamond has excellent mechanical, thermal, and electrical properties, which makes it an ideal material in a wide range applications from abrasive grinding tools to modern electronic devices. Hence, understanding the wettability of metals on the synthetic diamond is of great importance for the development of diamondrelated materials and devices. In this study, the wettability and spreading kinetics of binary SneCr alloy on chemical vapour deposed (CVD) polycrystalline diamond compacts were investigated using a sessile drop method. In situ observation of contact angle at elevating temperatures indicated trace addition of Cr dramatically improved the wettability of Sn on CVD diamond, and the SneCr alloy started to wet CVD diamond at approximately 750 °C. Isothermal spreading kinetic analysis revealed that the spreading of SneCr alloy on CVD diamond was controlled by the kinetics of chemical reaction at advancing triple line. Microstructure characterization indicated that the formation of nano-sized scallop-like Cr7C3 grains was responsible for the improved wettability of SneCr alloy on CVD diamond substrate. The wetting temperature was found to play a determinant role in the interfacial carbide formation, and hence the reactive wetting of SneCr alloy on CVD diamond at temperatures from 700 to 900 °C.
1. Introduction Due to its ultra-high hardness, synthetic diamond has been used as cutting abrasives for decades [1–5], which is of special importance for machining hard and/or brittle materials [6,7]. Moreover, synthetic diamond has found increasingly more applications in modern electronic devices because of its superior thermal conductivity [8], optical and electric properties, as well as excellent bio-compatibility [9]. For aforementioned applications, it is always desirable that synthetic diamond should be properly wetted by bonding metals [10]. Among existing bonding techniques, the active brazing using NieCr [1,3,11–15], CueSn [16–19], and AgeCu [20] filler alloys containing carbideforming elements (Ti, Cr, or V) has been recently recognized as a promising one to improve the bonding strength of synthetic diamond. However, synthetic diamond is metastable, graphitizing at approximately 700 °C at the presence of catalysing elements (Ni, Fe, and Co) [21,22]. Because brazing temperatures are often well above the graphitization temperature of synthetic diamond, undesirable thermal damage cannot be avoided. This is especially true for the synthetic diamond brazed by Cr-containing Ni-based filler alloys, as severe
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graphitization of diamond was regularly reported [3,11,14,15]. Thus, understanding the effect of temperature on the wettability and interfacial carbide formation on synthetic diamond is essential for the development of diamond bonding technique [10]. Naidich and Kolesnichenko investigated the final contact angle of CueTi and CueCr alloys on synthetic diamond powders at 1150 °C and 1250 °C, respectively [23]. Scott and Nicholas reported a compromise between bonding strength and wettability of synthetic diamond brazed by Cu alloys containing Ti, Cr, and V at 1150 °C [24]. Palavra et al. reported that the CVD diamond can be well wetted by near-eutectic Ag-Cu-Ti alloy at 810 to 850 °C [25]. Unfortunately, the wettability of diamond was still studied at temperatures much higher than the graphitization temperature of synthetic diamond. Recently, Yang et al. [26] and Hu et al. [27] showed that Cr could exert a stronger effect on the improvement of the wettability of CueCr and SnAgCu-Cr alloys on graphite. However, whether Cr has the same effect on the wettability of the synthetic diamond is still to be uncovered, especially at relatively low temperatures. Previous studies focused more on the wettability indicated by equilibrium contact angles [23–25]. It is well known that the carbide-
Corresponding author. E-mail address: [email protected] (D. Mu).
https://doi.org/10.1016/j.diamond.2018.12.022 Received 1 November 2018; Received in revised form 24 December 2018; Accepted 25 December 2018 Available online 26 December 2018 0925-9635/ © 2019 Elsevier B.V. All rights reserved.
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middle of the CVD diamond substrate, as shown in Fig. 1(b). The wetting experiments were carried out using the sessile drop method in a vacuum better than 1 × 10−3 Pa. The assembled sample was subsequently placed into a quartz tube, aligned to a horizontal position, and heated in an electrical resistance tube furnace. For wetting experiment at elevating temperatures, the sample in the quartz tube was firstly heated to 400 °C at a rate of 30 °C/min and kept for 30 min, in order to obtain a homogeneously heated liquid droplet. Then the temperature was continuously increased to 950 °C at a heating rate of 2 °C/min. For isothermal wetting experiments, the sample was heated from 400 °C to 700, 750, 800 and 900 °C at a rate of 30 °C/min and isothermally held for 2 h. Both wetting and spreading behaviours of SneCr drops were recorded by a digital camera, and the contact angle was obtained using a drop shape analysis software with an accuracy of ± 3 degrees. After wetting, the sample was withdrawn from the tube furnace and cooled to room temperature in vacuum. The detailed experimental information can be found elsewhere [35]. The wetted samples were then cut in a direction perpendicular to the CVD diamond substrate using a lowspeed diamond saw, mounted in epoxy resin, and mechanically polished. To examine surface morphologies of the interfacial reaction products, some of the samples were deeply etched in 20 wt% HNO3 solution to remove the remained SneCr alloys. A Phenom ProX scanning electron microscope (SEM) and a Sigma 500 field emission SEM installed with an energy dispersive spectrometer (EDS) and an XPert Pro X-ray diffractometer (XRD, PANalytical B.V.) were used for microstructure characterization.
forming element and synthetic diamond compose a typical reactive wetting system, in which the interface reaction at advancing triple line affects not only the equilibrium contact angle, but also the spreading kinetics [28,29]. Eustathopoulos et al. conducted a series of studies of reactive wetting on ceramics and carbonuous substrates [30,31] and proposed the spreading of reactive metals can be controlled by either the chemical kinetics at advancing triple line [30], or the diffusion of reactant elements from molten metal droplet to advancing triple line [31]. In other words, the carbides formation at the triple line influences both final contact angle and spreading kinetic of a molten metallic droplet. Voitovitcha et al. [32] and Hodaj et al. [33] reported that the spreading kinetics of CueCr alloys on vitreous carbon was controlled by Cr diffusion from bulk liquid to the advancing triple line, where Cr7C3 was formed at 1100 °C and 1150 °C. Devincent et al. investigated the wettability of CueCr alloys on a commercial H-490 graphite substrate in Ar atmosphere at 1130 °C [34], and their results showed that the wettability increased with increased Cr content and Cr3C2 should be the most stable phase at the interface. Yang et al. studied the isothermal wetting of CueCr on graphite at 1100 °C in Ar atmosphere and both Cr3C2 and Cr7C3 carbides were detected at reaction interface [26]. When Cr content was increased, a preferential formation of more metallic-like Cr7C3 carbide significantly improved the wettability of CueCr alloy on graphite [26]. Meanwhile, the spreading kinetics was firstly controlled by the chemical reaction, and then by the diffusion of Cr to the advancing triple line [26]. Obviously, the isothermal spreading behaviours of several Cr-containing alloys were extensively studied on various carboneous materials (graphite or vitreous carbon) at high temperatures (above 1000 °C). However, the fundamental kinetics being involved such as spreading mechanism and interfacial chromium carbide formation are still ambiguous. Moreover, the spreading mechanism of Cr-containing alloys on the synthetic diamond are lacking. In this work, the reactive wetting of the SneCr binary alloy on polycrystalline CVD diamond was systematically investigated, aimed to reveal the effects of temperature on the wetting behaviour of the SneCr alloy with Cr contents of 0, 1, 2, 4, 6 wt%. The temperature-dependent wettability of SneCr alloys on CVD diamond was experimentally obtained via in situ observation of the contact angle at continuously elevated temperature. In addition, isothermal spreading kinetics of the SneCr alloy was analysed to identify the effect of temperature on its wetting mechanism. The microstructure of interfacial Cr7C3 carbides formed during reactive wetting was characterized using x-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Thermodynamic analysis was conducted to elucidate the role of wetting temperature in the formation of interfacial chromium carbides, and hence the wetting behaviour of SneCr alloys on CVD diamond.
3. Results 3.1. Wettability and spreading kinetics Fig. 2(a) shows the variation of the contact angle of the Sn-xCr (x = 0, 1, 2, 4, 6 wt%) alloys on a polycrystalline CVD diamond compact when the wetting temperature was elevated from 400 to 950 °C. Clearly, pure Sn poorly wetted CVD diamond with a final contact angle more than 120° after wetting at 950 °C. However, the Cr addition into SneCr alloys dramatically decreased the final contact angle to < 30 degrees at 950 °C. The wetting process of those Cr-containing SneCr alloys can be divided into three stages according to the wetting temperature: 1) a non-wetting stage at temperatures of below 750 °C, during which the contact angle was almost constant; 2) a rapid wetting stage at temperatures from 750 °C and 880 °C, during which the contact angle rapidly decreased as temperature being increased; and 3) an equilibrium wetting stage at temperature from 880 to 950 °C, during which the SneCr alloy well wetted CVD diamond but the change of contact angle was sluggish. Fig. 2(b) shows the relationship between the final contact angles and the nominal Cr contents of SneCr alloys after wetting at elevating temperatures up to 950 °C. It can be seen that the addition of 1 wt% Cr into Sn was sufficient to decrease the final contact angle from 124.5° to 22.0°. The increase in Cr content from 1 to 4 wt% further decreased the contact angle of Sne4Cr to 6.5°; however, as Cr contents increased to 6 wt%, the final contact angle conversely increased to 13.2°, and the concentration of Cr-rich particles can be observed in solidified Sne6Cr droplets, as shown in the SEM micrographs embedded in Fig. 2(b). Clearly, the wetting temperature had a prominent influence on the wettability of SneCr alloy on CVD diamond. In particular, the initiation wetting temperature of all Cr containing SneCr alloys on CVD diamond compacts was experimentally identified as 750 °C, which is significantly lower than that implied in the previous studies on wettability [23,24] and brazing of diamond by Cr-containing alloys [3,11,13,14,26]. From the wetting results obtained at elevating temperatures, 2 wt% addition of Cr into Sn is sufficient to well wet CVD diamond, without remarkable formation of Cr-rich solid particles in solidified SneCr droplets (see Fig. 2(b)). Isothermal wetting experiments were therefore conducted using Sne2Cr alloy at 700, 750, 800, and 900 °C, in order to
2. Experiment Polycrystalline CVD diamond compacts (supplied by Suzhou Jiaozuan Superhard Nanocoatings Co. Ltd., China), prepared by chemical vapour depositing a CVD diamond film of about 10 μm thick on a tungsten carbide (WC) substrate with a dimension of 15 mm in diameter and 5 mm in thickness, were used as the substrate. According to the XRD pattern and SEM micrograph of the CVD diamond in Fig. 1, the CVD diamond compact mainly consists of CVD diamond in a grain size of approximately 5 μm, although strong signal from the WC substrate can also be detected due to the low absorption of X-ray by diamond. The binary Sn-xCr (in wt% unless mentioned elsewhere, x = 1, 2, 4, and 6) powder alloys were prepared by mechanically mixing balanced amount of Sn (purity of 99.99 wt%, size in ~50 μm) and Cr (purity of 99.7 wt%, size in ~20 μm) powders (Changsha Tianjiu Co., Ltd., China) in a ball milling machine for 4 h. Approximate 140 mg of mixed powders were cold pressed using a cylindrical die in a diameter of 3 mm under a pressure of 400 MPa. Prior to wetting, both the CVD diamond compact and the SneCr alloy ingot were cleaned in an ultrasonic alcohol bath. After cleaning, the SneCr alloy ingot was placed in the 93
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Fig. 1. (a) XRD pattern and surface morphology of original CVD diamond; (b) Schematic illustration of sample assembling.
patterns and surface morphologies of the interfacial chromium carbides formed during reactive wetting of Sn-0/1/4/6Cr alloys on CVD diamond at the elevating temperatures from 400 to 950 °C. As indicated by the XRD patterns given in Fig. 4(a), there is no reaction products formed during the wetting of pure Sn at temperatures up to 950 °C, and after etching only residual Sn can be observed on the CVD diamond surface, as shown in Fig. 4(b). Hence, the poor wettability of pure Sn on CVD diamond can be attributed to the weak Van Der Waals attraction between Sn and C [36]. Moreover, only Cr7C3 was formed at the interface between polycrystalline CVD diamond and the SneCr alloys with Cr content ranged from 1 to 6 wt%. As shown in Fig. 4(c), the addition of 1 wt% Cr was sufficient for the formation of a continuous layer of nano-sized scallop-like Cr7C3. Fig. 4(d) and (e) indicated that increasing Cr content to 4 and 6 wt% had negligible influence on the morphology of interfacial Cr7C3. Fig. 5 shows the XRD patterns and surface morphologies of the interfacial chromium carbides formed during isothermal wetting of Sne2Cr alloy on CVD diamond at 700, 750, 800, and 900 °C. As shown in Fig. 5(a), only Cr7C3 was formed during isothermal wetting experiments at 700, 750, 800 and 900 °C. As can be seen from the SEM images in Fig. 5(b)–(e), the Cr7C3 grains remained in a scallop-like morphology with the average grain size slightly increased from ~200 nm to ~500 nm, when isothermal wetting temperature was increased from 700 to 900 °C. The EDS spot analysis revealed these grains were in a chemical composition of 69.67 at.% of Cr, 30.23 at.% of C, and 0.1 at.% of Sn, which also corresponds to the Cr7C3 phase. Thus, it can be concluded that a layer of nano-sized Cr7C3 scallops was formed during the reactive wetting of all Cr-containing SneCr alloys on CVD diamond, and the original Cr contents in SneCr alloys had limited influence on the growth morphology of interfacial Cr7C3.
reveal the effect of temperature on the reactive spreading of the SneCr alloy. The variation of contact angle and droplet radius against the dwell time at each isothermal wetting temperature was given in Fig. 3(a) and (b), respectively. As shown in Fig. 3(a), at 700 °C, the final contact angle of Sne2Cr alloys on CVD diamond compacts decreased from 125.5° to around 99.5°, which corresponds to the partial wetting of Sne2Cr alloy on CVD diamond (θ > 90°). However, at 750 °C, the Sne2Cr alloy well wetted CVD diamond with an equilibrium final contact angle < 50°. When the wetting temperature increased to 800 °C, the equilibrium final contact angle further decreased to 25.5°. This result once again confirms that CVD diamond can be well wetted at a temperature significantly lower than the wetting or brazing temperature using these Cr-containing alloys [3,11,13,14,26]. Moreover, it should be noted that increasing temperature from 800 to 900 °C had an only minor influence on the equilibrium final contact angle, but the spreading kinetics was significantly accelerated as the time to reach the equilibrium final contact angle decreased from 5000 s to < 1000 s. As shown in Fig. 3(b), as the final contact angle decreased, the molten droplet radius increased because of the spreading of Sne2Cr alloy during isothermal wetting, with the maximum radius obtained at 900 °C. Obviously, the wetting temperature affected not only the equilibrium contact angle but also the spreading kinetics of the Sne2Cr alloy on CVD diamond. 3.2. Interface microstructures To examine the phase formation and the morphology of the interfacial chromium carbides formed between SneCr alloys and CVD diamond, the remained SneCr alloy was removed through deep etching of the wetted samples in a 20 wt%HNO3 solution. Fig. 4 shows the XRD
Fig. 2. (a) Variation of contact angle of Sn-xCr (x = 1, 2, 4, 6 wt%) alloys plotted as a function of wetting temperature; (b) final contact angles and projective images Sn-xCr (x = 1, 2, 4, 6 wt%) alloys after reactive wetting at elevating temperature up to 950 °C, together with cross-sectional microstructures of the Sn-2/4/6Cr alloys. 94
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Fig. 3. Time histories of (a) contact angle and (b) droplet radius during isothermal wetting of Sne2Cr at different wetting temperatures.
Thermodynamically, at the interface between SneCr alloy and CVD diamond, two reactions likely occur as:
Fig. 6 shows the surface morphologies and cross-sectional microstructures of interfacial Cr7C3 at low magnification. As shown in Fig. 6(a), the Cr7C3 layer is discontinuous after isothermal wetting at 700 °C for two hours, indicating the growth of Cr7C3 was sluggish. As shown in Fig. 6(b), when the wetting temperature was increased to 750 °C, a continuous Cr7C3 layer was formed and the wettability of Sne2Cr alloy was substantially enhanced at this temperature. As shown in Fig. 6(c–e), a further increase in isothermal wetting temperature from 750 to 900 °C led to the increase of the average Cr7C3 thickness from 0.85 to 1.96 μm in the vicinity of wetting triple line. This accelerated growth of metallic-like Cr7C3 phase consequently reduced the final contact angle from 49.0° at 750 °C to 19.7° at 900 °C. It should be note that, after the isothermal wetting temperature was increased from 800 to 900 °C, the Cr7C3 thickness was only slightly increased from 1.55 to 1.96 μm, which agrees well with the wetting result that almost equal contact angles were achieved at 800 and 900 °C.
[Cr]Sn +
2 1 C ↔ Cr3C2 3 3
(1)
[Cr]Sn +
3 1 C ↔ Cr7C3 7 7
(2)
Assuming the activity of C in diamond as constant and the interfacial chromium as a pure substance, the changes in the Gibbs free energy of the reactions (1) and (2) could be expressed as:
ΔGr (Cr3 C2) =
1 Sn × ΔGf0 (Cr3 C2) − RT ln (αCr ) 3
(3)
ΔGr (Cr7 C3) =
1 Sn × ΔGf0 (Cr7 C3) − RT ln (αCr ) 7
(4)
where ΔG0 f stands for the standard Gibbs free energy for the formation, which equals −95.03 kJ/mol and − 98.76 kJ/mol for Cr3C2 at 700 °C and 900 °C, and − 184.29 kJ/mol and − 195.91 kJ/mol for Cr7C3 at 700 °C and 900 °C, respectively. T is the temperature, R is Gas constant, and αSn Cr is the activity of solute Cr in solvent Sn, which can be calculated as:
4. Discussion 4.1. Formation of Cr7C3 It can be clearly seen from the XRD and SEM results shown in Figs. 4, 5 and 6, Cr7C3 carbides was formed at the SneCr alloy/CVD diamond interface, regardless of wetting temperature and Cr contents in SneCr alloys. This is different from the previous studies that both Cr3C2 and Cr7C3 chromium carbides were formed at interfaces of Cu-Cr/graphite, Ni-Cr/synthetic diamond [1,3,11,14,15], and Cu-Cr/vitreous carbon [32,33] and graphite [26,27], at temperatures around 1000 °C.
Sn Sn α Cr = γCr × χ Cr
(5)
where γSn Cr is the activity coefficient of solute Cr in solvent Sn, χ Cr is the real atomic content of Cr in liquid Sn. Assuming the SneCr alloy as an infinitely dilute regular solution, γSn Cr can be estimated as:
Fig. 4. XRD patterns and surface morphologies of Cr7C3 formed at Sn-xCr alloy/CVD diamond interface after reactive wetting at elevating temperature up to 950 °C. Note that the remained SneCr alloy was removed. 95
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Fig. 5. XRD patterns and surface morphologies of Cr7C3 formed at Sn-2Cr/CVD diamond interface during isothermal wetting at 700, 750, 800 and 900 °C, after the remained SneCr alloy was removed.
Fig. 6. Surface morphologies formed at (a) 700 °C and (b) 750 °C, and cross-sectional microstructures formed at (c) 750 °C, (d) 800 °C, and (e) 900 °C of Cr7C3 carbide at Sn-2Cr/CVD diamond interface after isothermal wetting. Table 1 Calculation of the change of Gibbs free energy for the formation of Cr7C3 and Cr3C2 carbides at 700 and 900 °C. Wetting temperature
700 °C
Composition of SneCr alloy (in wt%) Atomic Cr content (δSn Cr, in at.%) Cr solubility in liquid Sn (χ Cr, in at.%) ΔGr(Cr3C2), (in KJ/mol) ΔGr(Cr7C3), (in KJ/mol)
Sn-1Cr 2.25 1.00 −26.42 −21.08
Sn γCr = exp ⎛⎜ ⎝
RT
⎟
⎠
(6)
where, ΔH∞ Cr(Sn) is the mixing enthalpy of the solution, which equals 32 kJ/mol for the SneCr alloy [36]. From Eqs. (5) and (6), it can be seen that the activity of solute Cr in solvent Sn is fully dependent on the real atomic content of Cr in liquid Sn. If the maximum solubility of solute Cr in solvent Sn is higher than the nominal concentration of Cr content in SneCr alloys, the nominal Cr concentration is the real atomic content of Cr in liquid Sn. Otherwise, the real atomic Cr content in the SneCr alloy is dependent on the solubility of Cr in liquid Sn. Because the nominal concentration of Cr content in SneCr alloys is given, the only variable is the temperature-dependent solubility of Cr in liquid Sn.
900 °C Sn-1Cr 2.25 2.25 −28.93 l −24.02
∆H∞ Cr(Sn) ⎞
Sn-2Cr 4.45 4.40 −34.44 −29.53
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apparent activation energy for the reaction-limited spreading was therefore calculated as 158.69 KJ/mol according to the Arrhenius law [24]. According to the spreading kinetics of AleSi and CueSi alloys on vitreous carbon [28], the composition of wetting liquid would only slightly affect the activation energy, which was mainly determined by the rupture of CeC bonds at the substrate surface. The apparent activation energy obtained in this study is considerably smaller than the CeC bonding energy in graphite or diamond (approximately 340 kJ/ mol) [28]. This reduced apparent activation energy is highly likely due to the combined effects resultant from the Cr adsorption prior to the occurrence of interfacial carbides formation, the strong interaction between the d orbital of Cr and C elements [10], and the presence of dense defects in the surface of polycrystalline CVD diamond. Based on the thermodynamic and spreading kinetic analysis, the reactive wetting behaviour of the SneCr alloys on CVD diamond can be readily understood. Fig. 8 schematically shows the wetting process of SneCr alloys on CVD diamond compacts. As shown in Fig. 8(a), the SneCr alloy was compacted in a cylindrical shape prior to wetting and solid Cr particles were evenly disturbed in the Sn matrix. As shown in Fig. 8(b), when the temperature increased to 232.2 °C [37], Sn melted and started to dissolve Cr into a liquid phase. According to the Sn-rich side of SneCr phase diagram [37], the solubility of Cr in liquid Sn was negligible at 232.2 °C. As demonstrated in Fig. 8(c), the solubility of Cr in liquid Sn gradually increased with temperature and thermodynamically, the Cr solute was absorbed on the surface of CVD diamond. As shown in Fig. 8(d), as the temperature further increased, C atoms were detached from CVD diamond to establish a Cr and C supersaturated zone, in which the high Cr concentration favoured the precipitation of a Cr-rich phase. As a result, a discontinuous Cr7C3 layer was precipitated at the early stage of interface reaction and resulted in a partially wetting of SneCr alloys on CVD diamond. This inference is supported by the interface microstructure between Sne2Cr alloy and CVD diamond at 700 °C (see Fig. 6(a)). As indicated by the thickness of Cr7C3 formed during the isothermal experiment shown in Fig. 6(c–e) and the dynamic constant value (see Fig. 7(a)), the growth kinetics of interfacial Cr7C3 appears very sensitive to wetting temperature. Also illustrated in Fig. 8(e), when temperature increased from 700 to 750 °C, the growth of Cr7C3 was accelerated and resulted in the formation of a continuous metallic-like Cr7C3 layer, which significantly improved the wettability and induced reactive spreading of the Sne2Cr alloy on CVD diamond. The increasing of wetting temperature from 750 to 900 °C further increased the growth kinetics of interfacial Cr7C3, which consequently reduced the final contact angle and increased the spreading kinetics of SneCr alloy. Because the critical concentration for the Cr7C3 formation is significantly lower than the solubility of Cr in liquid Sn at a temperature above 700 °C, sufficient Cr solute was readily dissolved in liquid Sn solvent to support the interface reaction at advancing triple
According to the Sn-rich side of SneCr phase diagram [37], the solubility of Cr in liquid Sn can be estimated at a certain temperature and the changes in the Gibbs free energy of reactions (1) and (2) can be calculated as given in Table 1. Clearly, the change of Gibbs free energy suggests a preferential Cr3C2 formation to Cr7C3, at equilibrium. Hence, the formation of Cr7C3 carbides could be better justified by taking the local Cr concentration into account. Yang et al. explained the preferential formation of Cr3C2 was resultant from the higher critical concentration for the formation of Cr7C3 [26]. However, the equilibrium solubility of Cr in liquid Sn at 700 °C is approximately 1 at.% [37], which is much higher than the critical concentration for the formation of both Cr7C3 (8.97 × 10−4 at.%) and Cr3C2 (5.75 × 10−4 at. %). Prior to the occurrence of interface reaction, the local Cr concentration at diamond interface could be several times of the equilibrium solubility of Cr in liquid Sn, which would further favour the formation of Cr-rich Cr7C3 carbide. In addition, the Cr solubility in liquid Sn is sufficient to induce the formation of interfacial Cr7C3 at temperature significantly lower than the melting point of Ni-based or Cu-based alloys, which consequently improves the wettability of SneCr alloys on CVD substrate at relatively low temperature, i.e. 750 °C in this study. 4.2. Wetting mechanism At the advancing triple line of a reactive wetting system, the growth kinetics of interface product could control the spreading kinetics of a droplet [28,30–32]. The reaction-limited spreading can be described as [28]:
cos θe − cos θe = (cos θe − cos θ0 ) ∙exp(−kt)
(7)
where θe, θd and θ0, are the equilibrium, dynamic and initial contact angles, respectively. k is a dynamic constant with unit of s−1, which is dependent on both wetting temperature and alloy composition. Fig. 7(a) shows the values of ln(cos θe - cos θd) plotted against the dwell time of Sne2Cr alloy at 750 °C, 800 °C and 900 °C. Note that the Sne2Cr alloy did not fully wet CVD diamond at 700 °C and the variation of contact angle at this temperature was not used for spreading kinetic analysis. As shown in Fig. 7(a), during the rapid spreading stage of the reactive wetting, the ln(cos θe - cos θd) value was in a good linear relationship with dwell time, which indicates a reaction-limited spreading. The values of dynamic constant k that represents the kinetics of interface reaction at advancing triple line were calculated as 5.22 × 10−3 s−1, 1.13 × 10−3 s−1, and 0.48 × 10−3 s−1 at 900, 800, and 750 °C, respectively. It is well known that the interface reaction kinetics is affected by the reaction temperature, which can be related to the activation energy. The logarithmic values of k were thus plotted as a function of 1/T in Fig. 7(b), showing a linear relationship. The average
Fig. 7. (a) Napoerian logarithm of (cos θe -cos θd) versus dwell time for Sne2Cr at 750, 800, and 900 °C, (b) Arrhenius plotting of the kinetic constant. 97
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Fig. 8. Schematic illustration of reactive wetting of SneCr alloy on CVD diamond: (a) prior to wetting, (b) melting of Sn at relatively low temperature, (c) dissolution and interfacial concentration of Cr solute in liquid Sn as temperature increased (d) formation of Cr-rich Cr7C3 carbide at the early stage of interface reaction; (e) improved wettability of Sn-1/2/4Cr molten droplet through the formation of a continuous Cr7C3 layer, (f) impeded spreading of the Sne6Cr alloy by concentrated Crrich particles in molten droplet.
Acknowledgements
line. As a result, the isothermal spreading of Sne2Cr alloy was mainly controlled by the chemical reaction at the triple line. However, it can be seen in Table 1 that the equilibrium solubility of Cr in liquid Sn is around 4.4 at.% at 950 °C. This means a considerable amount of solid Cr particles was present during the wetting experiments of Sne6Cr alloys, which would impede the fluidity of molten droplet. As a result, the concentration of Cr-rich particles can be observed in the middle of solidified Sne6Cr alloys as and the final contact angle of Sne6Cr alloy was increased as shown in Fig. 2(b) and illustrated in Fig. 8(f). In summary, wetting temperature would affect not only the solubility of Cr in liquid Sn, but also the growth kinetics of interfacial Cr7C3, which in turn determine the reactive wetting behaviours of SneCr alloys on CVD diamond. During isothermal wetting experiments, the equilibrium solubility of Cr in liquid Sn is much higher than the critical concentration for the formation of Cr7C3, and the isothermal spreading of Sne2Cr alloy on CVD diamond was mainly controlled by the kinetics of chemical reaction at advancing triple line.
The authors would like to thank the financial support from National Natural Science Foundation of China (Grant Nos.: 51675191, 51835004). D. K. Mu would acknowledge the financial support from the Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University. References [1] A.K. Chattopadhyay, L. Chollet, H.E. Hintermann, Experimental investigation on induction brazing of diamond with Ni-Cr hardfacing alloy under argon atmosphere, J. Mater. Sci. 26 (1991) 5093–5100. [2] J.C. Sung, M. Sung, The brazing of diamond, Int. J. Refract. Met. Hard Mater. 27 (2009) 382–393. [3] A.K. Chattopadhyay, L. Chollet, H. Hintermann, On performance of brazed bonded monolayer diamond grinding wheel, CIRP Ann. Manuf. Technol. 40 (1991) 347–350. [4] J.T. Lowder, E.M. Tausch, Method of manufacturing diamond abrasive tools, American Patents, 1975. [5] M. Mcclymont, Polycrystalline chemical vapour deposited diamond tool parts and methods of fabricating, mounting, and using the same, American Patents, 2018. [6] H.N. Li, D. Axinte, Textured grinding wheels: a review, I a review, Int. J. Mach. Tools Manuf. 109 (Supplement C) (2016) 8–35. [7] H. Huang, Y. Liu, Experimental investigations of machining characteristics and removal mechanisms of advanced ceramics in high speed deep grinding, Int J Mach Tool Manu 43 (2003) 811–823. [8] J. Li, X. Wang, Y. Qiao, Y. Zhang, Z. He, H. Zhang, High thermal conductivity through interfacial layer optimization in diamond particles dispersed Zr-alloyed Cu matrix composites, Scr. Mater. 109 (2015) 72–75. [9] S.G. Lichter, M.C. Escudié, D.A. Stacey, K. Ganesan, K. Fox, A. Ahnood, N.V. Apollo, D.C. Kua, A.Z. Lee, C. McGowan, A.L. Saunders, O. Burns, D.A.X. Nayagam, R.A. Williams, D.J. Garrett, H. Meffin, S. Prawer, Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys, Biomaterials 53 (2015) 464–474. [10] C. Artini, M.L. Muolo, A. Passerone, Diamond–metal interfaces in cutting tools: a review, J. Mater. Sci. 47 (2012) 3252–3264. [11] J.C. Chen, D.K. Mu, X.J. Liao, G.Q. Huang, H. Huang, X.P. Xu, H. Huang, Interfacial microstructure and mechanical properties of synthetic diamond brazed by Ni-Cr-P filler alloy, Int. J. Refract. Met. Hard Mater. 74 (2018) 52–60. [12] Y. Chen, Y. Fu, H. Su, J. Xu, H. Xu, The effects of solder alloys on the morphologies and mechanical properties of brazed diamond grits, Int. J. Refract. Met. Hard Mater. 42 (2014) 23–39. [13] P. Mukhopadhyay, D.R. Simhan, A. Ghosh, Challenges in brazing large synthetic diamond grit by Ni-based filler alloy, J. Mater. Process. Technol. 250 (2017) 390–400. [14] C.Y. Wang, Y.M. Zhou, F.L. Zhang, Z.C. Xu, Interfacial microstructure and performance of brazed diamond grits with Ni-Cr-P alloy, J. Alloys Compd. 476 (2009) 884–888. [15] S.F. Huang, H.L. Tsai, S.-T. Lin, Effects of brazing route and brazing alloy on the interfacial structure between diamond and bonding matrix, Mater. Chem. Phys. 84 (2004) 251–258. [16] X. Liao, D. Mu, J. Wang, G. Huang, H. Huang, X. Xu, H. Huang, Formation of TiC via interface reaction between diamond grits and Sn-Ti alloys at relatively low
5. Conclusions In this study, the reactive wetting behaviour of binary SneCr alloys on polycrystalline CVD diamond substrates was systematically investigated using a sessile drop method. From the experiment results, the conclusions can be drawn as follows. 1. The addition of 1 wt% Cr into Sn substantially improved the wettability of Sn on CVD diamond. The minimum final contact angle after wetting at 950 °C was achieved for the SneCr alloy with 4 wt% Cr content. The addition of 6 wt% Cr resulted in concentration of solid Cr particles, which would impede fluidity, and hence reduce wettability of the SneCr alloys on the CVD diamond substrate. 2. The wetting temperature played a dominant role in the wettability of SneCr alloys on CVD diamond. Isothermal wetting experiments showed that the Sne2Cr alloy well wetted with CVD diamond at 750 °C. When the wetting temperature was in the range from 750 to 900 °C, the spreading of the Sne2Cr alloy was mainly controlled by the chemical reaction at advancing triple line on the polycrystalline CVD diamond substrate. 3. A layer of nano-sized Cr7C3 grains was observed at the interface between SneCr alloys and CVD diamond after wetting at both isothermal and elevating temperatures. The wetting temperature affected the growth kinetics of Cr7C3, and hence the wettability and the spreading kinetics of SneCr alloys on CVD diamond substrates.
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system, Appl. Surf. Sci. 257 (2011) 6276–6281. [27] S. Hu, Y.Z. Lei, X.G. Song, J.R. Kang, Y.X. Zhao, J. Cao, D.Y. Tang, Wettability of SnAgCu–Cr alloys on graphite with different Cr contents, J. Mater. Sci. 53 (2018) 6864–6871. [28] O. Dezellus, F. Hodaj, N. Eustathopoulos, Chemical reaction-limited spreading: the triple line velocity versus contact angle relation, Acta Mater. 50 (2002) 4741–4753. [29] A. Mortensen, B. Drevet, N. Eustathopoulos, Kinetics of diffusion-limited spreading of sessile drops in reactive wetting, Scr. Mater. 36 (1997) 645–651. [30] N. Eustathopoulos, Dynamics of wetting in reactive metal/ceramic systems, Acta Mater. 46 (1998) 2319–2327. [31] K. Landry, N. Eustathopoulos, Dynamics of wetting in reactive metal/ceramic systems: linear spreading, Acta Mater. 44 (1996) 3923–3932. [32] R. Voitovitch, A. Mortensen, F. Hodaj, N. Eustathopoulos, Diffusion-limited reactive wetting: study of spreading kinetics of Cu–Cr alloys on carbon substrates, Acta Mater. 47 (1999) 1117–1128. [33] F. Hodaj, O. Dezellus, J.N. Barbier, A. Mortensen, N. Eustathopoulos, Diffusionlimited reactive wetting: effect of interfacial reaction behind the advancing triple line, J. Mater. Sci. 42 (19) (2007) 8071–8082. [34] S.M. Devincent, G.M. Michal, Reaction layer formation at the graphite/copperchromium alloy interface, Metall. Trans. A. 24 (1) (1993) 53–60. [35] Q. Lin, W. Zhong, F. Li, W. Yu, Reactive wetting of tin/steel and tin/aluminum at 350°C-450°C, J. Alloys Compd. 716 (2017) 73–80. [36] N. Eustathopoulos, M.G. Nicholas, B. Drevet, Wettability at High Temperatures, Elsevier, 3, (1999). [37] J. Darby, D. Jugle, Solubility of several first-long-period transition elements in liquid tin, Trans. Metall. Soc. AIME 245 (1969) 2515–2518.
temperatures, Int. J. Refract. Met. Hard Mater. 66 (2017) 252–257. [17] F.A. Khalid, U.E. Klotz, H.-R. Elsener, B. Zigerlig, P. Gasser, On the interfacial nanostructure of brazed diamond grits, Scr. Mater. 50 (2004) 1139–1143. [18] S. Buhl, C. Leinenbach, R. Spolenak, K. Wegener, Microstructure, residual stresses and shear strength of diamond-steel-joints brazed with a cu-Sn-based active filler alloy, Int. J. Refract. Met. Hard Mater. 30 (2012) 16–24. [19] U.E. Klotz, C. Liu, F.A. Khalid, H.R. Elsener, Influence of brazing parameters and alloy composition on interface morphology of brazed diamond, Mater. Sci. Eng. A 495 (2008) 265–270. [20] U.E. Klotz, F.A. Khalid, H.-R. Elsener, Nanocrystalline phases and epitaxial interface reactions during brazing of diamond grits with silver based Incusil-ABA alloy, Diam. Relat. Mater. 15 (2006) 1520–1524. [21] M. Yudasaka, K. Tasaka, R. Kikuchi, Y. Ohki, S. Yoshimura, Influence of chemical bond of carbon on Ni catalyzed graphitization, J. Appl. Physiol. 81 (1997) 7623–7629. [22] M. Yudasaka, R. Kikuchi, Graphitization of Carbonaceous Materials by Ni, Co and Fe, 33 Springer Berlin Heidelberg, 1998, pp. 99–105. [23] Y.V. Naidich, G.A. Kolesnichenko, Investigation of the wetting of and adhesion to graphite and diamond by liquid metals, Surface Phenomena in Metallurgical Processes, 1965, pp. 218–223. [24] P. Scott, M. Nicholas, B. Dewar, The wetting and bonding of diamonds by copperbase binary alloys, J. Mater. Sci. 10 (1975) 1833–1840. [25] A. Palavra, A.J.S. Fernandes, C. Serra, F.M. Costa, L.A. Rocha, R.F. Silva, The wetting and bonding of diamonds by copper-base binary alloys, Diam. Relat. Mater. 10 (2001) 775–780. [26] L. Yang, P. Shen, Q. Lin, F. Qiu, Q. Jiang, Effect of Cr on the wetting in cu/graphite
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Reactive wetting of binary SneCr alloy on polycrystalline chemical vapour deposited diamond at relatively low temperatures
T
⁎
Jinchang Chena, Xinjiang Liaoa, Qiaoli Linb, Dekui Mua, , Hui Huanga, Xipeng Xua, Han Huangc a
Fujian Engineering Research Centre of Intelligent Manufacturing for Brittle Materials, Institute of Manufacturing Engineering, Huaqiao University, Xiamen 361021, China School of Materials Science and Engineering, Lanzhou University of Science and Technology, Lanzhou 730000, China c School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: CVD diamond Reactive wetting Interface reaction Chromium carbides
Synthetic diamond has excellent mechanical, thermal, and electrical properties, which makes it an ideal material in a wide range applications from abrasive grinding tools to modern electronic devices. Hence, understanding the wettability of metals on the synthetic diamond is of great importance for the development of diamondrelated materials and devices. In this study, the wettability and spreading kinetics of binary SneCr alloy on chemical vapour deposed (CVD) polycrystalline diamond compacts were investigated using a sessile drop method. In situ observation of contact angle at elevating temperatures indicated trace addition of Cr dramatically improved the wettability of Sn on CVD diamond, and the SneCr alloy started to wet CVD diamond at approximately 750 °C. Isothermal spreading kinetic analysis revealed that the spreading of SneCr alloy on CVD diamond was controlled by the kinetics of chemical reaction at advancing triple line. Microstructure characterization indicated that the formation of nano-sized scallop-like Cr7C3 grains was responsible for the improved wettability of SneCr alloy on CVD diamond substrate. The wetting temperature was found to play a determinant role in the interfacial carbide formation, and hence the reactive wetting of SneCr alloy on CVD diamond at temperatures from 700 to 900 °C.
1. Introduction Due to its ultra-high hardness, synthetic diamond has been used as cutting abrasives for decades [1–5], which is of special importance for machining hard and/or brittle materials [6,7]. Moreover, synthetic diamond has found increasingly more applications in modern electronic devices because of its superior thermal conductivity [8], optical and electric properties, as well as excellent bio-compatibility [9]. For aforementioned applications, it is always desirable that synthetic diamond should be properly wetted by bonding metals [10]. Among existing bonding techniques, the active brazing using NieCr [1,3,11–15], CueSn [16–19], and AgeCu [20] filler alloys containing carbideforming elements (Ti, Cr, or V) has been recently recognized as a promising one to improve the bonding strength of synthetic diamond. However, synthetic diamond is metastable, graphitizing at approximately 700 °C at the presence of catalysing elements (Ni, Fe, and Co) [21,22]. Because brazing temperatures are often well above the graphitization temperature of synthetic diamond, undesirable thermal damage cannot be avoided. This is especially true for the synthetic diamond brazed by Cr-containing Ni-based filler alloys, as severe
⁎
graphitization of diamond was regularly reported [3,11,14,15]. Thus, understanding the effect of temperature on the wettability and interfacial carbide formation on synthetic diamond is essential for the development of diamond bonding technique [10]. Naidich and Kolesnichenko investigated the final contact angle of CueTi and CueCr alloys on synthetic diamond powders at 1150 °C and 1250 °C, respectively [23]. Scott and Nicholas reported a compromise between bonding strength and wettability of synthetic diamond brazed by Cu alloys containing Ti, Cr, and V at 1150 °C [24]. Palavra et al. reported that the CVD diamond can be well wetted by near-eutectic Ag-Cu-Ti alloy at 810 to 850 °C [25]. Unfortunately, the wettability of diamond was still studied at temperatures much higher than the graphitization temperature of synthetic diamond. Recently, Yang et al. [26] and Hu et al. [27] showed that Cr could exert a stronger effect on the improvement of the wettability of CueCr and SnAgCu-Cr alloys on graphite. However, whether Cr has the same effect on the wettability of the synthetic diamond is still to be uncovered, especially at relatively low temperatures. Previous studies focused more on the wettability indicated by equilibrium contact angles [23–25]. It is well known that the carbide-
Corresponding author. E-mail address: [email protected] (D. Mu).
https://doi.org/10.1016/j.diamond.2018.12.022 Received 1 November 2018; Received in revised form 24 December 2018; Accepted 25 December 2018 Available online 26 December 2018 0925-9635/ © 2019 Elsevier B.V. All rights reserved.
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middle of the CVD diamond substrate, as shown in Fig. 1(b). The wetting experiments were carried out using the sessile drop method in a vacuum better than 1 × 10−3 Pa. The assembled sample was subsequently placed into a quartz tube, aligned to a horizontal position, and heated in an electrical resistance tube furnace. For wetting experiment at elevating temperatures, the sample in the quartz tube was firstly heated to 400 °C at a rate of 30 °C/min and kept for 30 min, in order to obtain a homogeneously heated liquid droplet. Then the temperature was continuously increased to 950 °C at a heating rate of 2 °C/min. For isothermal wetting experiments, the sample was heated from 400 °C to 700, 750, 800 and 900 °C at a rate of 30 °C/min and isothermally held for 2 h. Both wetting and spreading behaviours of SneCr drops were recorded by a digital camera, and the contact angle was obtained using a drop shape analysis software with an accuracy of ± 3 degrees. After wetting, the sample was withdrawn from the tube furnace and cooled to room temperature in vacuum. The detailed experimental information can be found elsewhere [35]. The wetted samples were then cut in a direction perpendicular to the CVD diamond substrate using a lowspeed diamond saw, mounted in epoxy resin, and mechanically polished. To examine surface morphologies of the interfacial reaction products, some of the samples were deeply etched in 20 wt% HNO3 solution to remove the remained SneCr alloys. A Phenom ProX scanning electron microscope (SEM) and a Sigma 500 field emission SEM installed with an energy dispersive spectrometer (EDS) and an XPert Pro X-ray diffractometer (XRD, PANalytical B.V.) were used for microstructure characterization.
forming element and synthetic diamond compose a typical reactive wetting system, in which the interface reaction at advancing triple line affects not only the equilibrium contact angle, but also the spreading kinetics [28,29]. Eustathopoulos et al. conducted a series of studies of reactive wetting on ceramics and carbonuous substrates [30,31] and proposed the spreading of reactive metals can be controlled by either the chemical kinetics at advancing triple line [30], or the diffusion of reactant elements from molten metal droplet to advancing triple line [31]. In other words, the carbides formation at the triple line influences both final contact angle and spreading kinetic of a molten metallic droplet. Voitovitcha et al. [32] and Hodaj et al. [33] reported that the spreading kinetics of CueCr alloys on vitreous carbon was controlled by Cr diffusion from bulk liquid to the advancing triple line, where Cr7C3 was formed at 1100 °C and 1150 °C. Devincent et al. investigated the wettability of CueCr alloys on a commercial H-490 graphite substrate in Ar atmosphere at 1130 °C [34], and their results showed that the wettability increased with increased Cr content and Cr3C2 should be the most stable phase at the interface. Yang et al. studied the isothermal wetting of CueCr on graphite at 1100 °C in Ar atmosphere and both Cr3C2 and Cr7C3 carbides were detected at reaction interface [26]. When Cr content was increased, a preferential formation of more metallic-like Cr7C3 carbide significantly improved the wettability of CueCr alloy on graphite [26]. Meanwhile, the spreading kinetics was firstly controlled by the chemical reaction, and then by the diffusion of Cr to the advancing triple line [26]. Obviously, the isothermal spreading behaviours of several Cr-containing alloys were extensively studied on various carboneous materials (graphite or vitreous carbon) at high temperatures (above 1000 °C). However, the fundamental kinetics being involved such as spreading mechanism and interfacial chromium carbide formation are still ambiguous. Moreover, the spreading mechanism of Cr-containing alloys on the synthetic diamond are lacking. In this work, the reactive wetting of the SneCr binary alloy on polycrystalline CVD diamond was systematically investigated, aimed to reveal the effects of temperature on the wetting behaviour of the SneCr alloy with Cr contents of 0, 1, 2, 4, 6 wt%. The temperature-dependent wettability of SneCr alloys on CVD diamond was experimentally obtained via in situ observation of the contact angle at continuously elevated temperature. In addition, isothermal spreading kinetics of the SneCr alloy was analysed to identify the effect of temperature on its wetting mechanism. The microstructure of interfacial Cr7C3 carbides formed during reactive wetting was characterized using x-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Thermodynamic analysis was conducted to elucidate the role of wetting temperature in the formation of interfacial chromium carbides, and hence the wetting behaviour of SneCr alloys on CVD diamond.
3. Results 3.1. Wettability and spreading kinetics Fig. 2(a) shows the variation of the contact angle of the Sn-xCr (x = 0, 1, 2, 4, 6 wt%) alloys on a polycrystalline CVD diamond compact when the wetting temperature was elevated from 400 to 950 °C. Clearly, pure Sn poorly wetted CVD diamond with a final contact angle more than 120° after wetting at 950 °C. However, the Cr addition into SneCr alloys dramatically decreased the final contact angle to < 30 degrees at 950 °C. The wetting process of those Cr-containing SneCr alloys can be divided into three stages according to the wetting temperature: 1) a non-wetting stage at temperatures of below 750 °C, during which the contact angle was almost constant; 2) a rapid wetting stage at temperatures from 750 °C and 880 °C, during which the contact angle rapidly decreased as temperature being increased; and 3) an equilibrium wetting stage at temperature from 880 to 950 °C, during which the SneCr alloy well wetted CVD diamond but the change of contact angle was sluggish. Fig. 2(b) shows the relationship between the final contact angles and the nominal Cr contents of SneCr alloys after wetting at elevating temperatures up to 950 °C. It can be seen that the addition of 1 wt% Cr into Sn was sufficient to decrease the final contact angle from 124.5° to 22.0°. The increase in Cr content from 1 to 4 wt% further decreased the contact angle of Sne4Cr to 6.5°; however, as Cr contents increased to 6 wt%, the final contact angle conversely increased to 13.2°, and the concentration of Cr-rich particles can be observed in solidified Sne6Cr droplets, as shown in the SEM micrographs embedded in Fig. 2(b). Clearly, the wetting temperature had a prominent influence on the wettability of SneCr alloy on CVD diamond. In particular, the initiation wetting temperature of all Cr containing SneCr alloys on CVD diamond compacts was experimentally identified as 750 °C, which is significantly lower than that implied in the previous studies on wettability [23,24] and brazing of diamond by Cr-containing alloys [3,11,13,14,26]. From the wetting results obtained at elevating temperatures, 2 wt% addition of Cr into Sn is sufficient to well wet CVD diamond, without remarkable formation of Cr-rich solid particles in solidified SneCr droplets (see Fig. 2(b)). Isothermal wetting experiments were therefore conducted using Sne2Cr alloy at 700, 750, 800, and 900 °C, in order to
2. Experiment Polycrystalline CVD diamond compacts (supplied by Suzhou Jiaozuan Superhard Nanocoatings Co. Ltd., China), prepared by chemical vapour depositing a CVD diamond film of about 10 μm thick on a tungsten carbide (WC) substrate with a dimension of 15 mm in diameter and 5 mm in thickness, were used as the substrate. According to the XRD pattern and SEM micrograph of the CVD diamond in Fig. 1, the CVD diamond compact mainly consists of CVD diamond in a grain size of approximately 5 μm, although strong signal from the WC substrate can also be detected due to the low absorption of X-ray by diamond. The binary Sn-xCr (in wt% unless mentioned elsewhere, x = 1, 2, 4, and 6) powder alloys were prepared by mechanically mixing balanced amount of Sn (purity of 99.99 wt%, size in ~50 μm) and Cr (purity of 99.7 wt%, size in ~20 μm) powders (Changsha Tianjiu Co., Ltd., China) in a ball milling machine for 4 h. Approximate 140 mg of mixed powders were cold pressed using a cylindrical die in a diameter of 3 mm under a pressure of 400 MPa. Prior to wetting, both the CVD diamond compact and the SneCr alloy ingot were cleaned in an ultrasonic alcohol bath. After cleaning, the SneCr alloy ingot was placed in the 93
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Fig. 1. (a) XRD pattern and surface morphology of original CVD diamond; (b) Schematic illustration of sample assembling.
patterns and surface morphologies of the interfacial chromium carbides formed during reactive wetting of Sn-0/1/4/6Cr alloys on CVD diamond at the elevating temperatures from 400 to 950 °C. As indicated by the XRD patterns given in Fig. 4(a), there is no reaction products formed during the wetting of pure Sn at temperatures up to 950 °C, and after etching only residual Sn can be observed on the CVD diamond surface, as shown in Fig. 4(b). Hence, the poor wettability of pure Sn on CVD diamond can be attributed to the weak Van Der Waals attraction between Sn and C [36]. Moreover, only Cr7C3 was formed at the interface between polycrystalline CVD diamond and the SneCr alloys with Cr content ranged from 1 to 6 wt%. As shown in Fig. 4(c), the addition of 1 wt% Cr was sufficient for the formation of a continuous layer of nano-sized scallop-like Cr7C3. Fig. 4(d) and (e) indicated that increasing Cr content to 4 and 6 wt% had negligible influence on the morphology of interfacial Cr7C3. Fig. 5 shows the XRD patterns and surface morphologies of the interfacial chromium carbides formed during isothermal wetting of Sne2Cr alloy on CVD diamond at 700, 750, 800, and 900 °C. As shown in Fig. 5(a), only Cr7C3 was formed during isothermal wetting experiments at 700, 750, 800 and 900 °C. As can be seen from the SEM images in Fig. 5(b)–(e), the Cr7C3 grains remained in a scallop-like morphology with the average grain size slightly increased from ~200 nm to ~500 nm, when isothermal wetting temperature was increased from 700 to 900 °C. The EDS spot analysis revealed these grains were in a chemical composition of 69.67 at.% of Cr, 30.23 at.% of C, and 0.1 at.% of Sn, which also corresponds to the Cr7C3 phase. Thus, it can be concluded that a layer of nano-sized Cr7C3 scallops was formed during the reactive wetting of all Cr-containing SneCr alloys on CVD diamond, and the original Cr contents in SneCr alloys had limited influence on the growth morphology of interfacial Cr7C3.
reveal the effect of temperature on the reactive spreading of the SneCr alloy. The variation of contact angle and droplet radius against the dwell time at each isothermal wetting temperature was given in Fig. 3(a) and (b), respectively. As shown in Fig. 3(a), at 700 °C, the final contact angle of Sne2Cr alloys on CVD diamond compacts decreased from 125.5° to around 99.5°, which corresponds to the partial wetting of Sne2Cr alloy on CVD diamond (θ > 90°). However, at 750 °C, the Sne2Cr alloy well wetted CVD diamond with an equilibrium final contact angle < 50°. When the wetting temperature increased to 800 °C, the equilibrium final contact angle further decreased to 25.5°. This result once again confirms that CVD diamond can be well wetted at a temperature significantly lower than the wetting or brazing temperature using these Cr-containing alloys [3,11,13,14,26]. Moreover, it should be noted that increasing temperature from 800 to 900 °C had an only minor influence on the equilibrium final contact angle, but the spreading kinetics was significantly accelerated as the time to reach the equilibrium final contact angle decreased from 5000 s to < 1000 s. As shown in Fig. 3(b), as the final contact angle decreased, the molten droplet radius increased because of the spreading of Sne2Cr alloy during isothermal wetting, with the maximum radius obtained at 900 °C. Obviously, the wetting temperature affected not only the equilibrium contact angle but also the spreading kinetics of the Sne2Cr alloy on CVD diamond. 3.2. Interface microstructures To examine the phase formation and the morphology of the interfacial chromium carbides formed between SneCr alloys and CVD diamond, the remained SneCr alloy was removed through deep etching of the wetted samples in a 20 wt%HNO3 solution. Fig. 4 shows the XRD
Fig. 2. (a) Variation of contact angle of Sn-xCr (x = 1, 2, 4, 6 wt%) alloys plotted as a function of wetting temperature; (b) final contact angles and projective images Sn-xCr (x = 1, 2, 4, 6 wt%) alloys after reactive wetting at elevating temperature up to 950 °C, together with cross-sectional microstructures of the Sn-2/4/6Cr alloys. 94
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Fig. 3. Time histories of (a) contact angle and (b) droplet radius during isothermal wetting of Sne2Cr at different wetting temperatures.
Thermodynamically, at the interface between SneCr alloy and CVD diamond, two reactions likely occur as:
Fig. 6 shows the surface morphologies and cross-sectional microstructures of interfacial Cr7C3 at low magnification. As shown in Fig. 6(a), the Cr7C3 layer is discontinuous after isothermal wetting at 700 °C for two hours, indicating the growth of Cr7C3 was sluggish. As shown in Fig. 6(b), when the wetting temperature was increased to 750 °C, a continuous Cr7C3 layer was formed and the wettability of Sne2Cr alloy was substantially enhanced at this temperature. As shown in Fig. 6(c–e), a further increase in isothermal wetting temperature from 750 to 900 °C led to the increase of the average Cr7C3 thickness from 0.85 to 1.96 μm in the vicinity of wetting triple line. This accelerated growth of metallic-like Cr7C3 phase consequently reduced the final contact angle from 49.0° at 750 °C to 19.7° at 900 °C. It should be note that, after the isothermal wetting temperature was increased from 800 to 900 °C, the Cr7C3 thickness was only slightly increased from 1.55 to 1.96 μm, which agrees well with the wetting result that almost equal contact angles were achieved at 800 and 900 °C.
[Cr]Sn +
2 1 C ↔ Cr3C2 3 3
(1)
[Cr]Sn +
3 1 C ↔ Cr7C3 7 7
(2)
Assuming the activity of C in diamond as constant and the interfacial chromium as a pure substance, the changes in the Gibbs free energy of the reactions (1) and (2) could be expressed as:
ΔGr (Cr3 C2) =
1 Sn × ΔGf0 (Cr3 C2) − RT ln (αCr ) 3
(3)
ΔGr (Cr7 C3) =
1 Sn × ΔGf0 (Cr7 C3) − RT ln (αCr ) 7
(4)
where ΔG0 f stands for the standard Gibbs free energy for the formation, which equals −95.03 kJ/mol and − 98.76 kJ/mol for Cr3C2 at 700 °C and 900 °C, and − 184.29 kJ/mol and − 195.91 kJ/mol for Cr7C3 at 700 °C and 900 °C, respectively. T is the temperature, R is Gas constant, and αSn Cr is the activity of solute Cr in solvent Sn, which can be calculated as:
4. Discussion 4.1. Formation of Cr7C3 It can be clearly seen from the XRD and SEM results shown in Figs. 4, 5 and 6, Cr7C3 carbides was formed at the SneCr alloy/CVD diamond interface, regardless of wetting temperature and Cr contents in SneCr alloys. This is different from the previous studies that both Cr3C2 and Cr7C3 chromium carbides were formed at interfaces of Cu-Cr/graphite, Ni-Cr/synthetic diamond [1,3,11,14,15], and Cu-Cr/vitreous carbon [32,33] and graphite [26,27], at temperatures around 1000 °C.
Sn Sn α Cr = γCr × χ Cr
(5)
where γSn Cr is the activity coefficient of solute Cr in solvent Sn, χ Cr is the real atomic content of Cr in liquid Sn. Assuming the SneCr alloy as an infinitely dilute regular solution, γSn Cr can be estimated as:
Fig. 4. XRD patterns and surface morphologies of Cr7C3 formed at Sn-xCr alloy/CVD diamond interface after reactive wetting at elevating temperature up to 950 °C. Note that the remained SneCr alloy was removed. 95
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Fig. 5. XRD patterns and surface morphologies of Cr7C3 formed at Sn-2Cr/CVD diamond interface during isothermal wetting at 700, 750, 800 and 900 °C, after the remained SneCr alloy was removed.
Fig. 6. Surface morphologies formed at (a) 700 °C and (b) 750 °C, and cross-sectional microstructures formed at (c) 750 °C, (d) 800 °C, and (e) 900 °C of Cr7C3 carbide at Sn-2Cr/CVD diamond interface after isothermal wetting. Table 1 Calculation of the change of Gibbs free energy for the formation of Cr7C3 and Cr3C2 carbides at 700 and 900 °C. Wetting temperature
700 °C
Composition of SneCr alloy (in wt%) Atomic Cr content (δSn Cr, in at.%) Cr solubility in liquid Sn (χ Cr, in at.%) ΔGr(Cr3C2), (in KJ/mol) ΔGr(Cr7C3), (in KJ/mol)
Sn-1Cr 2.25 1.00 −26.42 −21.08
Sn γCr = exp ⎛⎜ ⎝
RT
⎟
⎠
(6)
where, ΔH∞ Cr(Sn) is the mixing enthalpy of the solution, which equals 32 kJ/mol for the SneCr alloy [36]. From Eqs. (5) and (6), it can be seen that the activity of solute Cr in solvent Sn is fully dependent on the real atomic content of Cr in liquid Sn. If the maximum solubility of solute Cr in solvent Sn is higher than the nominal concentration of Cr content in SneCr alloys, the nominal Cr concentration is the real atomic content of Cr in liquid Sn. Otherwise, the real atomic Cr content in the SneCr alloy is dependent on the solubility of Cr in liquid Sn. Because the nominal concentration of Cr content in SneCr alloys is given, the only variable is the temperature-dependent solubility of Cr in liquid Sn.
900 °C Sn-1Cr 2.25 2.25 −28.93 l −24.02
∆H∞ Cr(Sn) ⎞
Sn-2Cr 4.45 4.40 −34.44 −29.53
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apparent activation energy for the reaction-limited spreading was therefore calculated as 158.69 KJ/mol according to the Arrhenius law [24]. According to the spreading kinetics of AleSi and CueSi alloys on vitreous carbon [28], the composition of wetting liquid would only slightly affect the activation energy, which was mainly determined by the rupture of CeC bonds at the substrate surface. The apparent activation energy obtained in this study is considerably smaller than the CeC bonding energy in graphite or diamond (approximately 340 kJ/ mol) [28]. This reduced apparent activation energy is highly likely due to the combined effects resultant from the Cr adsorption prior to the occurrence of interfacial carbides formation, the strong interaction between the d orbital of Cr and C elements [10], and the presence of dense defects in the surface of polycrystalline CVD diamond. Based on the thermodynamic and spreading kinetic analysis, the reactive wetting behaviour of the SneCr alloys on CVD diamond can be readily understood. Fig. 8 schematically shows the wetting process of SneCr alloys on CVD diamond compacts. As shown in Fig. 8(a), the SneCr alloy was compacted in a cylindrical shape prior to wetting and solid Cr particles were evenly disturbed in the Sn matrix. As shown in Fig. 8(b), when the temperature increased to 232.2 °C [37], Sn melted and started to dissolve Cr into a liquid phase. According to the Sn-rich side of SneCr phase diagram [37], the solubility of Cr in liquid Sn was negligible at 232.2 °C. As demonstrated in Fig. 8(c), the solubility of Cr in liquid Sn gradually increased with temperature and thermodynamically, the Cr solute was absorbed on the surface of CVD diamond. As shown in Fig. 8(d), as the temperature further increased, C atoms were detached from CVD diamond to establish a Cr and C supersaturated zone, in which the high Cr concentration favoured the precipitation of a Cr-rich phase. As a result, a discontinuous Cr7C3 layer was precipitated at the early stage of interface reaction and resulted in a partially wetting of SneCr alloys on CVD diamond. This inference is supported by the interface microstructure between Sne2Cr alloy and CVD diamond at 700 °C (see Fig. 6(a)). As indicated by the thickness of Cr7C3 formed during the isothermal experiment shown in Fig. 6(c–e) and the dynamic constant value (see Fig. 7(a)), the growth kinetics of interfacial Cr7C3 appears very sensitive to wetting temperature. Also illustrated in Fig. 8(e), when temperature increased from 700 to 750 °C, the growth of Cr7C3 was accelerated and resulted in the formation of a continuous metallic-like Cr7C3 layer, which significantly improved the wettability and induced reactive spreading of the Sne2Cr alloy on CVD diamond. The increasing of wetting temperature from 750 to 900 °C further increased the growth kinetics of interfacial Cr7C3, which consequently reduced the final contact angle and increased the spreading kinetics of SneCr alloy. Because the critical concentration for the Cr7C3 formation is significantly lower than the solubility of Cr in liquid Sn at a temperature above 700 °C, sufficient Cr solute was readily dissolved in liquid Sn solvent to support the interface reaction at advancing triple
According to the Sn-rich side of SneCr phase diagram [37], the solubility of Cr in liquid Sn can be estimated at a certain temperature and the changes in the Gibbs free energy of reactions (1) and (2) can be calculated as given in Table 1. Clearly, the change of Gibbs free energy suggests a preferential Cr3C2 formation to Cr7C3, at equilibrium. Hence, the formation of Cr7C3 carbides could be better justified by taking the local Cr concentration into account. Yang et al. explained the preferential formation of Cr3C2 was resultant from the higher critical concentration for the formation of Cr7C3 [26]. However, the equilibrium solubility of Cr in liquid Sn at 700 °C is approximately 1 at.% [37], which is much higher than the critical concentration for the formation of both Cr7C3 (8.97 × 10−4 at.%) and Cr3C2 (5.75 × 10−4 at. %). Prior to the occurrence of interface reaction, the local Cr concentration at diamond interface could be several times of the equilibrium solubility of Cr in liquid Sn, which would further favour the formation of Cr-rich Cr7C3 carbide. In addition, the Cr solubility in liquid Sn is sufficient to induce the formation of interfacial Cr7C3 at temperature significantly lower than the melting point of Ni-based or Cu-based alloys, which consequently improves the wettability of SneCr alloys on CVD substrate at relatively low temperature, i.e. 750 °C in this study. 4.2. Wetting mechanism At the advancing triple line of a reactive wetting system, the growth kinetics of interface product could control the spreading kinetics of a droplet [28,30–32]. The reaction-limited spreading can be described as [28]:
cos θe − cos θe = (cos θe − cos θ0 ) ∙exp(−kt)
(7)
where θe, θd and θ0, are the equilibrium, dynamic and initial contact angles, respectively. k is a dynamic constant with unit of s−1, which is dependent on both wetting temperature and alloy composition. Fig. 7(a) shows the values of ln(cos θe - cos θd) plotted against the dwell time of Sne2Cr alloy at 750 °C, 800 °C and 900 °C. Note that the Sne2Cr alloy did not fully wet CVD diamond at 700 °C and the variation of contact angle at this temperature was not used for spreading kinetic analysis. As shown in Fig. 7(a), during the rapid spreading stage of the reactive wetting, the ln(cos θe - cos θd) value was in a good linear relationship with dwell time, which indicates a reaction-limited spreading. The values of dynamic constant k that represents the kinetics of interface reaction at advancing triple line were calculated as 5.22 × 10−3 s−1, 1.13 × 10−3 s−1, and 0.48 × 10−3 s−1 at 900, 800, and 750 °C, respectively. It is well known that the interface reaction kinetics is affected by the reaction temperature, which can be related to the activation energy. The logarithmic values of k were thus plotted as a function of 1/T in Fig. 7(b), showing a linear relationship. The average
Fig. 7. (a) Napoerian logarithm of (cos θe -cos θd) versus dwell time for Sne2Cr at 750, 800, and 900 °C, (b) Arrhenius plotting of the kinetic constant. 97
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Fig. 8. Schematic illustration of reactive wetting of SneCr alloy on CVD diamond: (a) prior to wetting, (b) melting of Sn at relatively low temperature, (c) dissolution and interfacial concentration of Cr solute in liquid Sn as temperature increased (d) formation of Cr-rich Cr7C3 carbide at the early stage of interface reaction; (e) improved wettability of Sn-1/2/4Cr molten droplet through the formation of a continuous Cr7C3 layer, (f) impeded spreading of the Sne6Cr alloy by concentrated Crrich particles in molten droplet.
Acknowledgements
line. As a result, the isothermal spreading of Sne2Cr alloy was mainly controlled by the chemical reaction at the triple line. However, it can be seen in Table 1 that the equilibrium solubility of Cr in liquid Sn is around 4.4 at.% at 950 °C. This means a considerable amount of solid Cr particles was present during the wetting experiments of Sne6Cr alloys, which would impede the fluidity of molten droplet. As a result, the concentration of Cr-rich particles can be observed in the middle of solidified Sne6Cr alloys as and the final contact angle of Sne6Cr alloy was increased as shown in Fig. 2(b) and illustrated in Fig. 8(f). In summary, wetting temperature would affect not only the solubility of Cr in liquid Sn, but also the growth kinetics of interfacial Cr7C3, which in turn determine the reactive wetting behaviours of SneCr alloys on CVD diamond. During isothermal wetting experiments, the equilibrium solubility of Cr in liquid Sn is much higher than the critical concentration for the formation of Cr7C3, and the isothermal spreading of Sne2Cr alloy on CVD diamond was mainly controlled by the kinetics of chemical reaction at advancing triple line.
The authors would like to thank the financial support from National Natural Science Foundation of China (Grant Nos.: 51675191, 51835004). D. K. Mu would acknowledge the financial support from the Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University. References [1] A.K. Chattopadhyay, L. Chollet, H.E. Hintermann, Experimental investigation on induction brazing of diamond with Ni-Cr hardfacing alloy under argon atmosphere, J. Mater. Sci. 26 (1991) 5093–5100. [2] J.C. Sung, M. Sung, The brazing of diamond, Int. J. Refract. Met. Hard Mater. 27 (2009) 382–393. [3] A.K. Chattopadhyay, L. Chollet, H. Hintermann, On performance of brazed bonded monolayer diamond grinding wheel, CIRP Ann. Manuf. Technol. 40 (1991) 347–350. [4] J.T. Lowder, E.M. Tausch, Method of manufacturing diamond abrasive tools, American Patents, 1975. [5] M. Mcclymont, Polycrystalline chemical vapour deposited diamond tool parts and methods of fabricating, mounting, and using the same, American Patents, 2018. [6] H.N. Li, D. Axinte, Textured grinding wheels: a review, I a review, Int. J. Mach. Tools Manuf. 109 (Supplement C) (2016) 8–35. [7] H. Huang, Y. Liu, Experimental investigations of machining characteristics and removal mechanisms of advanced ceramics in high speed deep grinding, Int J Mach Tool Manu 43 (2003) 811–823. [8] J. Li, X. Wang, Y. Qiao, Y. Zhang, Z. He, H. Zhang, High thermal conductivity through interfacial layer optimization in diamond particles dispersed Zr-alloyed Cu matrix composites, Scr. Mater. 109 (2015) 72–75. [9] S.G. Lichter, M.C. Escudié, D.A. Stacey, K. Ganesan, K. Fox, A. Ahnood, N.V. Apollo, D.C. Kua, A.Z. Lee, C. McGowan, A.L. Saunders, O. Burns, D.A.X. Nayagam, R.A. Williams, D.J. Garrett, H. Meffin, S. Prawer, Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys, Biomaterials 53 (2015) 464–474. [10] C. Artini, M.L. Muolo, A. Passerone, Diamond–metal interfaces in cutting tools: a review, J. Mater. Sci. 47 (2012) 3252–3264. [11] J.C. Chen, D.K. Mu, X.J. Liao, G.Q. Huang, H. Huang, X.P. Xu, H. Huang, Interfacial microstructure and mechanical properties of synthetic diamond brazed by Ni-Cr-P filler alloy, Int. J. Refract. Met. Hard Mater. 74 (2018) 52–60. [12] Y. Chen, Y. Fu, H. Su, J. Xu, H. Xu, The effects of solder alloys on the morphologies and mechanical properties of brazed diamond grits, Int. J. Refract. Met. Hard Mater. 42 (2014) 23–39. [13] P. Mukhopadhyay, D.R. Simhan, A. Ghosh, Challenges in brazing large synthetic diamond grit by Ni-based filler alloy, J. Mater. Process. Technol. 250 (2017) 390–400. [14] C.Y. Wang, Y.M. Zhou, F.L. Zhang, Z.C. Xu, Interfacial microstructure and performance of brazed diamond grits with Ni-Cr-P alloy, J. Alloys Compd. 476 (2009) 884–888. [15] S.F. Huang, H.L. Tsai, S.-T. Lin, Effects of brazing route and brazing alloy on the interfacial structure between diamond and bonding matrix, Mater. Chem. Phys. 84 (2004) 251–258. [16] X. Liao, D. Mu, J. Wang, G. Huang, H. Huang, X. Xu, H. Huang, Formation of TiC via interface reaction between diamond grits and Sn-Ti alloys at relatively low
5. Conclusions In this study, the reactive wetting behaviour of binary SneCr alloys on polycrystalline CVD diamond substrates was systematically investigated using a sessile drop method. From the experiment results, the conclusions can be drawn as follows. 1. The addition of 1 wt% Cr into Sn substantially improved the wettability of Sn on CVD diamond. The minimum final contact angle after wetting at 950 °C was achieved for the SneCr alloy with 4 wt% Cr content. The addition of 6 wt% Cr resulted in concentration of solid Cr particles, which would impede fluidity, and hence reduce wettability of the SneCr alloys on the CVD diamond substrate. 2. The wetting temperature played a dominant role in the wettability of SneCr alloys on CVD diamond. Isothermal wetting experiments showed that the Sne2Cr alloy well wetted with CVD diamond at 750 °C. When the wetting temperature was in the range from 750 to 900 °C, the spreading of the Sne2Cr alloy was mainly controlled by the chemical reaction at advancing triple line on the polycrystalline CVD diamond substrate. 3. A layer of nano-sized Cr7C3 grains was observed at the interface between SneCr alloys and CVD diamond after wetting at both isothermal and elevating temperatures. The wetting temperature affected the growth kinetics of Cr7C3, and hence the wettability and the spreading kinetics of SneCr alloys on CVD diamond substrates.
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