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Ni–51Cr system
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Scripta Materialia 64 (2011) 1087–1090 www.elsevier.com/locate/scriptamat
Wettability of Ni–Cr filler on SiC ceramic and interfacial reactions for the SiC/Ni–51Cr system Yangwu Mao,⇑ Domenico Mombello and Chiara Baroni Department of Materials Science and Chemical Engineering, Politecnico di Torino, Torino 10129, Italy Received 10 February 2011; revised 19 February 2011; accepted 19 February 2011 Available online 26 February 2011
The chromium addition remarkably improves the wettability of SiC/Ni–Cr systems within the Cr content range of 10–60 wt.%. For the SiC/Ni–51Cr system, a reaction layer about 100 lm thick exists between the SiC ceramic and the filler layer and is composed mainly of Ni2Si and graphite with a little Cr23C6, whereas the filler layer consists mainly of Cr23C6 and Ni2Si. Both the interfacial reactions and the formation of reaction products contribute to the improvement of the wetting of SiC/Ni–Cr systems. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Joining; Ceramics; Interfaces; Microstructure
It is well known that the wettability of ceramics by liquid metals plays an important role in many technological processes, such as fabrication of metal– ceramic composite materials and metal–ceramic or ceramic–ceramic joining by brazing alloys [1–3]. The wettability of SiC by pure liquid Ni has been investigated in Refs. [4–7]. Melting of pure Ni on SiC starts at the interface at about 1273 K and is completed at 1533 K (lower than the melting point of Ni, 1728 K) owing to the Si and C dissolving in the Ni. The deep dissolution of SiC and formation of large graphite precipitates are observed at the interface after cooling [4]. Nogi and Ogino [5] determined the contact angle of Ni on SiC to be between 65° and 75° at 1773 K in a vacuum. They found the contact angle to be affected by the kind of SiC used and remarkably affected by the dissolution reaction of Si and C into liquid metal and the carbide formation reaction at the interface. Nikolopoulos et al. [6] reported a contact angle of 74° for a b-SiC/Ni system at 1770 K in a purified argon atmosphere and Li et al. [7] reported one of 86° for a crystallized SiC/Ni system at 1623 K in a vacuum, respectively. With regard to joining of SiC, metallic fillers containing active metals such as Ti and Cr are commonly used [8,9]. It has been found that the addition of chromium can improve the wettability of Al2O3/Ni-based alloys and Si3N4/Cu-based alloys, as well as SiC/Cu-based al-
⇑ Corresponding
author. Tel.: +39 011 564 4702; fax: +39 011 564 4699; e-mail: [email protected]
loys [10–12]. The beneficial effects of a reactive addition on wettability have been linked to the reduction of interface energy contributed by the negative free energy of the reactions between the reactive element and the substrate and by the formation of a reaction product in the interface [13]. However, there is still uncertainty in the scientific literature about the effect of interfacial reaction on wetting. Naidich [14] considered that the reaction represents the predominant factor for wetting improvement, meaning that in order to obtain a good wetting of a liquid on a solid an intense reaction is required. Ref. [15] demonstrates that the physico-chemical properties of the resulting interfaces are the key factor in reactive wetting. In this paper, the wetting of Cr addition in Ni–Cr filler on SiC ceramic has been investigated at high temperature in a vacuum, and the factors affecting the wettability of SiC/Ni–Cr are also discussed. Commercial pressureless sintered SiC ceramic (Weifang Astek Ceramic Co., Ltd) with the density of 3.10 ± 0.03 g cm3 was used as the substrate. The ceramic surface (diameter of / 10 mm) was prepared for the wetting test by polishing with 1.0 lm diamond paste. The areal roughness parameter (Sa) is used to specify the surface roughness of SiC substrate. The surface roughness Sa of polished SiC is 0.46 lm, with a detection area of 1 mm 1 mm. The SiC pieces were then washed in alcohol with an ultrasonic bath for 20 min. High-purity (>99.97%) powders (300 mesh) of Ni and Cr were mixed at the designed ratio of the filler in anhydrous alcohol. After drying, the mixture was cold
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.02.027
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Y. Mao et al. / Scripta Materialia 64 (2011) 1087–1090 Table 1. Wettability of Ni–Cr filler on SiC ceramic in a vacuum.
a
Filler
Test conditions
Contact angle/°
Interaction of substrate and filler
Ni–10Cra Ni–20Cr Ni–30Cr Ni–40Cr Ni–51Cr Ni–60Cr
1573 K, 1573 K, 1573 K, 1593 K, 1593 K, 1673 K,
61 23 20 19 17 16
Large crack in the substrate Large crack in the substrate Little crack generated in the substrate Good Good Good
10 min 10 min 10 min 10 min 10 min 10 min
Ni–10Cr denotes that the Cr content of the filler is 10 wt.%.
compacted to form thin compacts with a size of 5 mm 3.6 mm 1 mm. The compact was put on the polished surface of the SiC sample and then the combination was loaded into the horizontal alumina tube of a home-made high-temperature furnace. When the vacuum reached 7 103 Pa, the specimen was heated up to the defined temperature at a heating rate of 15 K min1. The shape of the metallic drop was observed and photographs were taken with a digital camera. The time-dependent variation in contact angle during isothermal holds at fixed temperature was monitored. The contact angles were calculated from those droplet images with an accuracy of ±2°. A JEOL-5600LV type scanning electron microscope was used for microstructure observation of the interface between the filler and the SiC ceramic, and an Oxford Inca type X-ray energy-dispersive spectrometer was used for composition analysis of the interfacial area. Furthermore, the phases of the interfacial area were determined by X-ray diffraction (XRD). The results of the wettability of Ni–Cr filler on SiC ceramic in a vacuum are given in Table 1. The contact angle decreases from 61° to 16° when the chromium addition in the Ni–Cr filler increases from 10 to 60 wt.%, which illustrates that the wettability of Ni–Cr filler on SiC ceramic is remarkably improved with increasing Cr content within the test range. Figure 1 shows the scanning electron microscopy (SEM) image of a cross-section of SiC/Ni–10Cr specimen obtained at 1573 K for 10 min. The dissolution of SiC in the liquid Ni–10Cr alloy can be observed in the interaction between the substrate and the filler. This phenomenon is attributed to the dissolution and interfacial reaction of SiC with the liquid metal [4]. Furthermore, the presence of a big crack in the SiC substrate in Figure 1 may be caused by the large residual stresses
Figure 1. SEM image of a cross-section of the SiC/Ni–10Cr system.
generated due to the mismatch of thermal expansion coefficients between the alloy and SiC ceramic [16]. Figure 2 shows the contact angle as a function of time for the SiC/Ni–30Cr system at 1573 K and the SiC/ Ni–51Cr system at 1593 K. The contact angle for both systems decreases rapidly within the first 600 s, then remains nearly constant with further time. These experimental results demonstrate that the wetting behavior of both the SiC/Ni–30Cr and SiC/Ni–51Cr systems is reactive wetting. In the initial wetting, the reactions are in progress, which causes the contact angle to be sensitive to time. When the interfacial reactions reach an equilibrium state, the contact angle remains stable. Figure 3 shows the SEM micrograph and areal distribution of C, Si, Cr and Ni elements in the interfacial area for the SiC/Ni–51Cr system obtained at 1593 K for 10 min. As can be seen from the SEM micrograph, a reaction layer about 100 lm thick exists between the filler layer and the SiC ceramic. The areal distribution of the elements demonstrates that the reaction layer mainly consists of Ni and C with a little Cr and Si, and the filler layer mainly consists of Ni and Cr with a little Si and C. These results indicate that during the wetting process both Ni and Cr penetrate from the filler into the SiC ceramic. At the same time, both Si and C dissolve from the SiC ceramic into the filler. The areal distribution also indicates that more Ni penetrates into the SiC ceramic than Cr, which leads to more Ni existing in the reaction layer than Cr. The compositions of reaction layer (microzone A in the grey area and B in the black area) and filler layer (microzone E in the dark grey area and microzone F
Figure 2. Effect of time on the contact angle of Ni–Cr filler on SiC ceramic.
Y. Mao et al. / Scripta Materialia 64 (2011) 1087–1090
Figure 3. SEM micrograph and areal distribution of elements in the interfacial area of the SiC/Ni–51Cr system obtained at 1593 K for 10 min.
in the grey area) analyzed by energy-dispersive spectrometry (EDS) are listed in Table 2. The results show that microzone A consists mainly of Ni (60.08 at.%) and Si (30.57 at.%), with a small amount of Cr and C. The atomic composition of Ni and Si is around 2:1, which is consistent with Ni2Si. The EDS result of microzone B indicates the black area of the reaction layer is composed mainly of C. It can be inferred that the possible main phases of the reaction layer are
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Ni2Si and C, with a little Cr23C6. The composition of reaction layer is in agreement with results for the SiC/ Ni system in Ref. [6], which presents the formation of a layer composed of Ni–Si compounds and graphite between the SiC substrate and an Ni drop. The EDS results of microzones E and F in the filler layer reveal that the main phase of the dark grey is possibly Cr23C6, while the grey area is composed mainly of Ni2Si. It can be predicted that the filler layer is composed mainly of Cr23C6 and Ni2Si. The formation of Cr23C6 is in accordance with the description of Ref. [1], which points out that for the SiC/Cr system six interfacial reaction compounds (Cr23C6, Cr7C3, Cr3C2, Cr3Si, CrSi and Cr5Si3C) are detected, but the Cr23C6 phase is the one most commonly detected due to its lowest Gibbs free energy of formation. In order to identify the interfacial reaction products, the SiC/Ni–51Cr specimen was ground from the drop surface to remove a layer with a thickness of about 200 lm and the obtained plane A was analyzed by XRD. The XRD patterns presented in Figure 4 show that plane A is composed mainly of Cr23C6 and Ni2Si. Taking into account the results of the SEM and EDS analyses, which suggest that the main phases of the filler layer are possibly Cr23C6 and Ni2Si, one can conclude that plane A is located in the filler layer, which is composed mainly of Cr23C6 and Ni2Si. Plane A was ground to remove a layer with a thickness of about 200 lm to give plane B obtained. The XRD analysis of plane B indicates the existence of Ni2Si and C with a little Cr23C6 and SiC. Considering the results of the SEM and EDS analyses, which show that the reaction layer is composed mainly of Ni2Si and C with a little Cr23C6, plane B can be identified as being located in the reaction layer, which mainly contains Ni2Si and C with a little Cr23C6. The presence of the small amount of SiC ceramic in plane B could be attributed to the substrate. According to the experimental results, it is evident that interfacial reactions occur between SiC and Ni–Cr filler during the wetting process. Ni reacts with SiC to form Ni2Si and C [17,18]. Meanwhile, active chromium reacts with carbon, which is generated from the dissociation of SiC at high temperatures as well as from one of the reaction products of SiC and Ni, to form Cr23C6 [9]. In reactive systems, the minimum contact angle according to Laurent et al. [19] is given by: cos hmin ¼ cos h0
Drr DGr rlv rlv
ð1Þ
where h0 is the contact angle of the system in the absence of any reactions, rlv is the surface tension of the liquid
Table 2. The compositions of microzones in the reaction layer and filler layer in Figure 3. Microzone
A B E F
Composition (at.%)
Possible main phases
Ni
Cr
Si
C
Total
60.08 3.52 10.25 59.97
7.16 0.95 65.09 5.40
30.57 3.72 4.12 34.63
2.19 91.81 20.54 –
100.00 100.00 100.00 100.00
Ni2Si with a little Cr23C6 C Cr23C6 Ni2Si
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Y. Mao et al. / Scripta Materialia 64 (2011) 1087–1090
Figure 4. XRD patterns of the interfacial areas of the SiC/Ni–51Cr system obtained at 1593 K for 10 min.
alloy, Drr is the change of interfacial tensions caused by the interfacial reactions and DGr is the change of Gibbs energy per unit area released by the reactions. It can be seen from Eq. (1) that the wettability of reactive systems can be influenced by the interfacial reactions and the formation of reaction products. As proposed in Ref. [13], if the volume of the ceramic increases after reaction and a dense layer of reacted ceramic is formed at the interface, the reactions would occur at the contact triple line and the contact angle is given by the dynamic balance between three interface tensions at that point. Thus, the wettability can be improved significantly by the interfacial reactions. For the SiC/Ni–Cr system, the following interfacial reactions occur: 2 Ni + SiC = Ni2Si + C and 23 Cr + 6 C = Cr23C6. The interfacial reactions decrease the free energy of the system, and consequently improve its wettability [7]. Meanwhile, the volume of ceramics increases after the interfacial reactions and the dense reaction layer is formed. Therefore, the wettability of the SiC/ Ni–Cr systems can be improved by the interfacial reactions. On the other hand, for both the SiC/Ni and SiC/ Ni–Cr systems, a reaction layer containing Ni2Si and graphite is formed, resulting in the change of the wetting interface from the initial SiC/Ni–Cr interface to the reaction products/Ni–Cr interface. It has been found that Cr can react with C to form carbides and wets graphite well (a contact angle of 40° at 2058 K) [6]. On the contrary, Ni and C do not form stable carbides (the contact angle of Ni on C is between 80° and 140° at 1523 K) [6]. As a consequence, Cr addition may improve the wettability of the SiC/Ni–Cr systems because of the existence of graphite in the reaction layer, as Cr wets graphite better than Ni. The chromium addition in the Ni–Cr filler may also change the surface tension
of the liquid alloy. This can also be interpreted as the chromium addition improving the wettability of the SiC/Ni–Cr systems. This favorable effect becomes more pronounced with increasing Cr content and time at high temperatures, as more reaction products are formed. In conclusion, the wettability of Ni–Cr filler on SiC ceramic is improved by adding chromium to the metallic filler. The contact angles of Ni–Cr filler on SiC ceramic decrease with increasing Cr content within the Cr content range of 10–60 wt.%. Interfacial reactions occur during the wetting process in the SiC/Ni–51Cr system. A reaction layer about 100 lm thick exists in the interfacial area between the SiC ceramic and the filler layer. This reaction layer is composed mainly of Ni2Si and graphite with a little Cr23C6, while the filler layer mainly consists of Cr23C6 and Ni2Si. Both the interfacial reactions and the formation of reaction products contribute to the improvement in wetting of the SiC/Ni–Cr systems. The authors are grateful for the financial support of the China Postdoctoral Science Foundation (Grant No. 20070420282). [1] G.W. Liu, M.L. Muolo, F. Valenza, A. Passerone, Ceram. Int. 36 (2010) 1177. [2] K. Nogi, Scripta. Mater. 62 (2010) 945. [3] V. Bissig, M. Galli, J. Janczak-Rusch, Adv. Eng. Mater. 8 (2006) 191. [4] N. Eustathopoulos, M.G. Nicholas, B. Drevet, Wettability at High Temperatures, Elsevier, Kidlington, 1999. [5] K. Nogi, K. Ogino, Trans. Jpn. Inst. Met. 29 (1988) 742. [6] P. Nikolopoulos, S. Agathopoulos, G.N. Angelopoulos, A. Naoumidis, H. Grubmeier, J. Mater. Sci. 27 (1992) 139. [7] S. Li, Y. Zhou, H. Duan, J. Mater. Sci. 37 (2002) 2575. [8] M. Naka, H. Taniguchi, I. Okamoto, Trans. JWRI. 19 (1990) 25. [9] Y. Mao, S. Li, L. Yan, Mater. Sci. Eng. A. 491 (2008) 304. [10] P. Kritsalis, V. Merlin, L. Coudurier, N. Eustathopoulos, Acta Metall. Mater. 40 (1992) 1167. [11] P. Xiao, B. Derby, J. Mater. Sci. 30 (1995) 5915. [12] P. Xiao, B. Derby, Acta Mater. 46 (1998) 3491. [13] X.B. Zhou, J. Th. M. De Hosson, Acta Mater. 44 (1996) 421. [14] V. Naidich, Prog. Surf. Membr. Sci. 14 (1981) 353. [15] S. Kalogeropoulou, L. Baud, N. Eustathopoulos, Acta. Metall. Mater. 43 (1995) 907. [16] S. Li, Y. Zhou, H. Duan, J. Qiu, Y. Zhang, J. Mater. Sci. 38 (2003) 4065. [17] S.A. Perez-Garc, L. Nyborg, Surf. Interface Anal. 38 (2006) 859. [18] K. Bhanumurthy, R. Schmid-Fetzer, Compos. Part A 32 (2001) 569. [19] V. Laurent, D. Chatain, N. Eustathopoulos, Mater. Sci. Eng. A 135 (1991) 89.
Scripta Materialia 64 (2011) 1087–1090 www.elsevier.com/locate/scriptamat
Wettability of Ni–Cr filler on SiC ceramic and interfacial reactions for the SiC/Ni–51Cr system Yangwu Mao,⇑ Domenico Mombello and Chiara Baroni Department of Materials Science and Chemical Engineering, Politecnico di Torino, Torino 10129, Italy Received 10 February 2011; revised 19 February 2011; accepted 19 February 2011 Available online 26 February 2011
The chromium addition remarkably improves the wettability of SiC/Ni–Cr systems within the Cr content range of 10–60 wt.%. For the SiC/Ni–51Cr system, a reaction layer about 100 lm thick exists between the SiC ceramic and the filler layer and is composed mainly of Ni2Si and graphite with a little Cr23C6, whereas the filler layer consists mainly of Cr23C6 and Ni2Si. Both the interfacial reactions and the formation of reaction products contribute to the improvement of the wetting of SiC/Ni–Cr systems. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Joining; Ceramics; Interfaces; Microstructure
It is well known that the wettability of ceramics by liquid metals plays an important role in many technological processes, such as fabrication of metal– ceramic composite materials and metal–ceramic or ceramic–ceramic joining by brazing alloys [1–3]. The wettability of SiC by pure liquid Ni has been investigated in Refs. [4–7]. Melting of pure Ni on SiC starts at the interface at about 1273 K and is completed at 1533 K (lower than the melting point of Ni, 1728 K) owing to the Si and C dissolving in the Ni. The deep dissolution of SiC and formation of large graphite precipitates are observed at the interface after cooling [4]. Nogi and Ogino [5] determined the contact angle of Ni on SiC to be between 65° and 75° at 1773 K in a vacuum. They found the contact angle to be affected by the kind of SiC used and remarkably affected by the dissolution reaction of Si and C into liquid metal and the carbide formation reaction at the interface. Nikolopoulos et al. [6] reported a contact angle of 74° for a b-SiC/Ni system at 1770 K in a purified argon atmosphere and Li et al. [7] reported one of 86° for a crystallized SiC/Ni system at 1623 K in a vacuum, respectively. With regard to joining of SiC, metallic fillers containing active metals such as Ti and Cr are commonly used [8,9]. It has been found that the addition of chromium can improve the wettability of Al2O3/Ni-based alloys and Si3N4/Cu-based alloys, as well as SiC/Cu-based al-
⇑ Corresponding
author. Tel.: +39 011 564 4702; fax: +39 011 564 4699; e-mail: [email protected]
loys [10–12]. The beneficial effects of a reactive addition on wettability have been linked to the reduction of interface energy contributed by the negative free energy of the reactions between the reactive element and the substrate and by the formation of a reaction product in the interface [13]. However, there is still uncertainty in the scientific literature about the effect of interfacial reaction on wetting. Naidich [14] considered that the reaction represents the predominant factor for wetting improvement, meaning that in order to obtain a good wetting of a liquid on a solid an intense reaction is required. Ref. [15] demonstrates that the physico-chemical properties of the resulting interfaces are the key factor in reactive wetting. In this paper, the wetting of Cr addition in Ni–Cr filler on SiC ceramic has been investigated at high temperature in a vacuum, and the factors affecting the wettability of SiC/Ni–Cr are also discussed. Commercial pressureless sintered SiC ceramic (Weifang Astek Ceramic Co., Ltd) with the density of 3.10 ± 0.03 g cm3 was used as the substrate. The ceramic surface (diameter of / 10 mm) was prepared for the wetting test by polishing with 1.0 lm diamond paste. The areal roughness parameter (Sa) is used to specify the surface roughness of SiC substrate. The surface roughness Sa of polished SiC is 0.46 lm, with a detection area of 1 mm 1 mm. The SiC pieces were then washed in alcohol with an ultrasonic bath for 20 min. High-purity (>99.97%) powders (300 mesh) of Ni and Cr were mixed at the designed ratio of the filler in anhydrous alcohol. After drying, the mixture was cold
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.02.027
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Y. Mao et al. / Scripta Materialia 64 (2011) 1087–1090 Table 1. Wettability of Ni–Cr filler on SiC ceramic in a vacuum.
a
Filler
Test conditions
Contact angle/°
Interaction of substrate and filler
Ni–10Cra Ni–20Cr Ni–30Cr Ni–40Cr Ni–51Cr Ni–60Cr
1573 K, 1573 K, 1573 K, 1593 K, 1593 K, 1673 K,
61 23 20 19 17 16
Large crack in the substrate Large crack in the substrate Little crack generated in the substrate Good Good Good
10 min 10 min 10 min 10 min 10 min 10 min
Ni–10Cr denotes that the Cr content of the filler is 10 wt.%.
compacted to form thin compacts with a size of 5 mm 3.6 mm 1 mm. The compact was put on the polished surface of the SiC sample and then the combination was loaded into the horizontal alumina tube of a home-made high-temperature furnace. When the vacuum reached 7 103 Pa, the specimen was heated up to the defined temperature at a heating rate of 15 K min1. The shape of the metallic drop was observed and photographs were taken with a digital camera. The time-dependent variation in contact angle during isothermal holds at fixed temperature was monitored. The contact angles were calculated from those droplet images with an accuracy of ±2°. A JEOL-5600LV type scanning electron microscope was used for microstructure observation of the interface between the filler and the SiC ceramic, and an Oxford Inca type X-ray energy-dispersive spectrometer was used for composition analysis of the interfacial area. Furthermore, the phases of the interfacial area were determined by X-ray diffraction (XRD). The results of the wettability of Ni–Cr filler on SiC ceramic in a vacuum are given in Table 1. The contact angle decreases from 61° to 16° when the chromium addition in the Ni–Cr filler increases from 10 to 60 wt.%, which illustrates that the wettability of Ni–Cr filler on SiC ceramic is remarkably improved with increasing Cr content within the test range. Figure 1 shows the scanning electron microscopy (SEM) image of a cross-section of SiC/Ni–10Cr specimen obtained at 1573 K for 10 min. The dissolution of SiC in the liquid Ni–10Cr alloy can be observed in the interaction between the substrate and the filler. This phenomenon is attributed to the dissolution and interfacial reaction of SiC with the liquid metal [4]. Furthermore, the presence of a big crack in the SiC substrate in Figure 1 may be caused by the large residual stresses
Figure 1. SEM image of a cross-section of the SiC/Ni–10Cr system.
generated due to the mismatch of thermal expansion coefficients between the alloy and SiC ceramic [16]. Figure 2 shows the contact angle as a function of time for the SiC/Ni–30Cr system at 1573 K and the SiC/ Ni–51Cr system at 1593 K. The contact angle for both systems decreases rapidly within the first 600 s, then remains nearly constant with further time. These experimental results demonstrate that the wetting behavior of both the SiC/Ni–30Cr and SiC/Ni–51Cr systems is reactive wetting. In the initial wetting, the reactions are in progress, which causes the contact angle to be sensitive to time. When the interfacial reactions reach an equilibrium state, the contact angle remains stable. Figure 3 shows the SEM micrograph and areal distribution of C, Si, Cr and Ni elements in the interfacial area for the SiC/Ni–51Cr system obtained at 1593 K for 10 min. As can be seen from the SEM micrograph, a reaction layer about 100 lm thick exists between the filler layer and the SiC ceramic. The areal distribution of the elements demonstrates that the reaction layer mainly consists of Ni and C with a little Cr and Si, and the filler layer mainly consists of Ni and Cr with a little Si and C. These results indicate that during the wetting process both Ni and Cr penetrate from the filler into the SiC ceramic. At the same time, both Si and C dissolve from the SiC ceramic into the filler. The areal distribution also indicates that more Ni penetrates into the SiC ceramic than Cr, which leads to more Ni existing in the reaction layer than Cr. The compositions of reaction layer (microzone A in the grey area and B in the black area) and filler layer (microzone E in the dark grey area and microzone F
Figure 2. Effect of time on the contact angle of Ni–Cr filler on SiC ceramic.
Y. Mao et al. / Scripta Materialia 64 (2011) 1087–1090
Figure 3. SEM micrograph and areal distribution of elements in the interfacial area of the SiC/Ni–51Cr system obtained at 1593 K for 10 min.
in the grey area) analyzed by energy-dispersive spectrometry (EDS) are listed in Table 2. The results show that microzone A consists mainly of Ni (60.08 at.%) and Si (30.57 at.%), with a small amount of Cr and C. The atomic composition of Ni and Si is around 2:1, which is consistent with Ni2Si. The EDS result of microzone B indicates the black area of the reaction layer is composed mainly of C. It can be inferred that the possible main phases of the reaction layer are
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Ni2Si and C, with a little Cr23C6. The composition of reaction layer is in agreement with results for the SiC/ Ni system in Ref. [6], which presents the formation of a layer composed of Ni–Si compounds and graphite between the SiC substrate and an Ni drop. The EDS results of microzones E and F in the filler layer reveal that the main phase of the dark grey is possibly Cr23C6, while the grey area is composed mainly of Ni2Si. It can be predicted that the filler layer is composed mainly of Cr23C6 and Ni2Si. The formation of Cr23C6 is in accordance with the description of Ref. [1], which points out that for the SiC/Cr system six interfacial reaction compounds (Cr23C6, Cr7C3, Cr3C2, Cr3Si, CrSi and Cr5Si3C) are detected, but the Cr23C6 phase is the one most commonly detected due to its lowest Gibbs free energy of formation. In order to identify the interfacial reaction products, the SiC/Ni–51Cr specimen was ground from the drop surface to remove a layer with a thickness of about 200 lm and the obtained plane A was analyzed by XRD. The XRD patterns presented in Figure 4 show that plane A is composed mainly of Cr23C6 and Ni2Si. Taking into account the results of the SEM and EDS analyses, which suggest that the main phases of the filler layer are possibly Cr23C6 and Ni2Si, one can conclude that plane A is located in the filler layer, which is composed mainly of Cr23C6 and Ni2Si. Plane A was ground to remove a layer with a thickness of about 200 lm to give plane B obtained. The XRD analysis of plane B indicates the existence of Ni2Si and C with a little Cr23C6 and SiC. Considering the results of the SEM and EDS analyses, which show that the reaction layer is composed mainly of Ni2Si and C with a little Cr23C6, plane B can be identified as being located in the reaction layer, which mainly contains Ni2Si and C with a little Cr23C6. The presence of the small amount of SiC ceramic in plane B could be attributed to the substrate. According to the experimental results, it is evident that interfacial reactions occur between SiC and Ni–Cr filler during the wetting process. Ni reacts with SiC to form Ni2Si and C [17,18]. Meanwhile, active chromium reacts with carbon, which is generated from the dissociation of SiC at high temperatures as well as from one of the reaction products of SiC and Ni, to form Cr23C6 [9]. In reactive systems, the minimum contact angle according to Laurent et al. [19] is given by: cos hmin ¼ cos h0
Drr DGr rlv rlv
ð1Þ
where h0 is the contact angle of the system in the absence of any reactions, rlv is the surface tension of the liquid
Table 2. The compositions of microzones in the reaction layer and filler layer in Figure 3. Microzone
A B E F
Composition (at.%)
Possible main phases
Ni
Cr
Si
C
Total
60.08 3.52 10.25 59.97
7.16 0.95 65.09 5.40
30.57 3.72 4.12 34.63
2.19 91.81 20.54 –
100.00 100.00 100.00 100.00
Ni2Si with a little Cr23C6 C Cr23C6 Ni2Si
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Y. Mao et al. / Scripta Materialia 64 (2011) 1087–1090
Figure 4. XRD patterns of the interfacial areas of the SiC/Ni–51Cr system obtained at 1593 K for 10 min.
alloy, Drr is the change of interfacial tensions caused by the interfacial reactions and DGr is the change of Gibbs energy per unit area released by the reactions. It can be seen from Eq. (1) that the wettability of reactive systems can be influenced by the interfacial reactions and the formation of reaction products. As proposed in Ref. [13], if the volume of the ceramic increases after reaction and a dense layer of reacted ceramic is formed at the interface, the reactions would occur at the contact triple line and the contact angle is given by the dynamic balance between three interface tensions at that point. Thus, the wettability can be improved significantly by the interfacial reactions. For the SiC/Ni–Cr system, the following interfacial reactions occur: 2 Ni + SiC = Ni2Si + C and 23 Cr + 6 C = Cr23C6. The interfacial reactions decrease the free energy of the system, and consequently improve its wettability [7]. Meanwhile, the volume of ceramics increases after the interfacial reactions and the dense reaction layer is formed. Therefore, the wettability of the SiC/ Ni–Cr systems can be improved by the interfacial reactions. On the other hand, for both the SiC/Ni and SiC/ Ni–Cr systems, a reaction layer containing Ni2Si and graphite is formed, resulting in the change of the wetting interface from the initial SiC/Ni–Cr interface to the reaction products/Ni–Cr interface. It has been found that Cr can react with C to form carbides and wets graphite well (a contact angle of 40° at 2058 K) [6]. On the contrary, Ni and C do not form stable carbides (the contact angle of Ni on C is between 80° and 140° at 1523 K) [6]. As a consequence, Cr addition may improve the wettability of the SiC/Ni–Cr systems because of the existence of graphite in the reaction layer, as Cr wets graphite better than Ni. The chromium addition in the Ni–Cr filler may also change the surface tension
of the liquid alloy. This can also be interpreted as the chromium addition improving the wettability of the SiC/Ni–Cr systems. This favorable effect becomes more pronounced with increasing Cr content and time at high temperatures, as more reaction products are formed. In conclusion, the wettability of Ni–Cr filler on SiC ceramic is improved by adding chromium to the metallic filler. The contact angles of Ni–Cr filler on SiC ceramic decrease with increasing Cr content within the Cr content range of 10–60 wt.%. Interfacial reactions occur during the wetting process in the SiC/Ni–51Cr system. A reaction layer about 100 lm thick exists in the interfacial area between the SiC ceramic and the filler layer. This reaction layer is composed mainly of Ni2Si and graphite with a little Cr23C6, while the filler layer mainly consists of Cr23C6 and Ni2Si. Both the interfacial reactions and the formation of reaction products contribute to the improvement in wetting of the SiC/Ni–Cr systems. The authors are grateful for the financial support of the China Postdoctoral Science Foundation (Grant No. 20070420282). [1] G.W. Liu, M.L. Muolo, F. Valenza, A. Passerone, Ceram. Int. 36 (2010) 1177. [2] K. Nogi, Scripta. Mater. 62 (2010) 945. [3] V. Bissig, M. Galli, J. Janczak-Rusch, Adv. Eng. Mater. 8 (2006) 191. [4] N. Eustathopoulos, M.G. Nicholas, B. Drevet, Wettability at High Temperatures, Elsevier, Kidlington, 1999. [5] K. Nogi, K. Ogino, Trans. Jpn. Inst. Met. 29 (1988) 742. [6] P. Nikolopoulos, S. Agathopoulos, G.N. Angelopoulos, A. Naoumidis, H. Grubmeier, J. Mater. Sci. 27 (1992) 139. [7] S. Li, Y. Zhou, H. Duan, J. Mater. Sci. 37 (2002) 2575. [8] M. Naka, H. Taniguchi, I. Okamoto, Trans. JWRI. 19 (1990) 25. [9] Y. Mao, S. Li, L. Yan, Mater. Sci. Eng. A. 491 (2008) 304. [10] P. Kritsalis, V. Merlin, L. Coudurier, N. Eustathopoulos, Acta Metall. Mater. 40 (1992) 1167. [11] P. Xiao, B. Derby, J. Mater. Sci. 30 (1995) 5915. [12] P. Xiao, B. Derby, Acta Mater. 46 (1998) 3491. [13] X.B. Zhou, J. Th. M. De Hosson, Acta Mater. 44 (1996) 421. [14] V. Naidich, Prog. Surf. Membr. Sci. 14 (1981) 353. [15] S. Kalogeropoulou, L. Baud, N. Eustathopoulos, Acta. Metall. Mater. 43 (1995) 907. [16] S. Li, Y. Zhou, H. Duan, J. Qiu, Y. Zhang, J. Mater. Sci. 38 (2003) 4065. [17] S.A. Perez-Garc, L. Nyborg, Surf. Interface Anal. 38 (2006) 859. [18] K. Bhanumurthy, R. Schmid-Fetzer, Compos. Part A 32 (2001) 569. [19] V. Laurent, D. Chatain, N. Eustathopoulos, Mater. Sci. Eng. A 135 (1991) 89.