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Tungsten cladding of new carbon composite with AlN ceramics
Journal of Nuclear Materials 420 (2012) 136–140
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Tungsten cladding of new carbon composite with AlN ceramics Weiwu Chen a,⇑, Tetsuro Tojo a,b, Yoshinari Miyamoto a,b a b
Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan Advanced Carbon Technology Center, Toyo Tanso. Co., Ltd. 5-7-12, Takeshima, Nishiyodogawa-Ku, Osaka 555-0011, Japan
a r t i c l e
i n f o
Article history: Received 14 July 2011 Accepted 21 September 2011 Available online 5 October 2011
a b s t r a c t A new carbon composite called ceramic bonded carbon (CBC) is composed of separated carbon particles bonded with thin ceramic boundaries. To fabricate reliable plasma facing components, AlN ceramic bonded carbon (AlN/CBC, 20 vol.% AlN) was directly cladded with tungsten (W) at 1700 °C by spark plasma sintering. The cladding shows strong bonding, which is attributed to the formation of interface layer containing mixed WC/W2C and AlN ceramic. The interface between W and AlN/CBC was analyzed and the cladding mechanism was discussed. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Tungsten (W) is extensively used as X-ray target, high temperature armor due to its high melting point, high strength and some other unique physical/chemical properties. It is also a prime candidate for the plasma facing components, because of its low sputtering yield, tritium retention, and good thermal properties [1,2]. In order to sustain steady state operation at high temperature, W components normally need to be joined with a heat sink, such as Cu or graphite substrates [3–5]. However, W is heavy and difficult to machine and weld. W/Cu joints have been extensively studied focusing on relaxation of mismatches in coefficients of thermal expansion (CTE) (aCu 4aW) and elastic moduli (ECu 0.2EW). Carbon materials have high thermal conductivity and matched CTE value with W [6]. However, the lower strength and poor joining capability limit the development of W/C joints by the conventional contact bonding method. Carbon fiber composites (CFC) and carbon blocks were successfully coated with W by low pressure plasma spray or physical vapor deposition (PVD) route [5,7–9]. However, the plasma spray coating is usually porous and weak in adhesion strength. Thick coating over millimeter is difficult by PVD. Recently, we proposed a novel carbon-based composite, called ceramic bonded carbon (CBC) [10,11]. CBC has a unique microstructure consisting of carbon particles and a ceramic boundary. In CBCs, the ceramic network bonds carbon particles together and provides high strength and other functional properties as required. AlN/CBC (20 vol.% AlN) provides lightweight (2.34 g/cm3), high bending strength (100 MPa), high thermal conductivity (170 W/mK). Moreover, CBCs can be joined or cladded with ceramics and metals by sintering method due to the ceramic network structure in carbon. ⇑ Corresponding author. E-mail address: [email protected] (W. Chen). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.09.022
For the tungsten cladding of AlN/CBC by powder metallurgy method, it has some unique performances compared to conventional W/graphite components prepared by coating methods. For example, (1) The composition, size and thickness of W layer is easily controlled; (2) The bonding between W and AlN/CBC may be stronger due to the new interlayer formation containing WCx and AlN–Y–Al–O ceramics; (3) The AlN/CBC has higher relative density, strength and thermal conductivity than the conventional graphite bulk. In this work, as an example, W cladding on AlN/CBC was prepared by sintering the stacked W powder on AlN/CBC using spark plasma sintering (SPS). The cladding structure and mechanism were investigated. 2. Experimental 2.1. Starting materials and processing conditions The starting materials used are an AlN/CBC disk (U25 mm, laboratory made) and W powders (0.6 lm, Kojundo Chem. Co. Ltd., Japan). The preparation method and properties of AlN/CBC are previously discussed [10,11] and shown in Table 1. To enhance the densification of AlN/CBC, a sintering additive of 5 wt% Y2O3 was mixed with AlN before AlN/CBC preparation. To prepare the W cladding on AlN/CBC, firstly, the AlN/CBC disk was finished by a #80 SiC paper and then loaded into 25-mm diameter graphite dies. To form a 0.5 mm-thick W layer, 4.8 g of W powder was stacked onto the AlN/CBC disk, and then sintered in vacuum at 1700 °C at a heating rate of 100 °C/min and a uniaxial pressure of 30 MPa for 5 min by SPS using a Dr. 1050 SPS apparatus (Sumitomo Coal Co. Ltd., Japan). For comparison, a sample of W cladding on carbon was fabricated at the same condition using a graphite disk (25-mm diameter, ISO-88, Toyo Tanso. Co. Ltd., Japan) and W powders.
137
W. Chen et al. / Journal of Nuclear Materials 420 (2012) 136–140 Table 1 Characteristics of AlN/CBC, monolithic W, and W cladding on AlN/CBC. Samples
Sintering temperature (°C)
Density (g/cm3)
Relative density (%)
Strength (MPa)
Thermal conductivity (W/mK)
AlN/CBC Sintered W Cladding joint
1900 1700 1700
2.34 18.5 –
98 96
100 1094 170
170 – –
(a)
(b)
Carbon
Intensity
14000
W
∇ ♦ : WC Δ : W2C ∇: C
12000
♦
4000
∇∇ ♦
2000 0
∇
∇
♦ Δ
Δ
♦ ♦
Δ
C side
♦ Δ ♦♦
W side
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2-theta Fig. 1. W and carbon disks separated without bonding (a), and the XRD patterns of the interfaces (b).
(a)
(b) 7000 6000
∇
∇: W ♦: C ∇ ∗ : AlN Ο : WC + : Y2O3
∇
♦
5000
Intensity
W
Interface
• : Al2Y4O9
4000
W 3000
∗
2000
∗ • Ο Ο
1000 +
AlN/CBC
0
∗
∇
∗ ♦
•
∇
∗
Ο ∗
∇ ∗
♦
♦
+
+
∗ ∗
Interface ♦
AlN/CBC
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2-theta Fig. 2. Cross-sectional view of W cladding with AlN/CBC (a), and XRD patterns of W, interface, and AlN/CBC layers (b).
2.2. Characterizations 120
Cladding of W and CBC
110
Microstructure characterization was carried out using a field emission scanning electron microscope (FE-SEM, ERA-8800, Elionix). X-ray diffraction (XRD, JDX-3530 M, JEOL) was employed to characterize the samples’ compositions. Rectangular bars of the claddings, 3 2–3 20 mm3, were prepared to measure the three-point bending strength using a table-top universal tester (EZ-Test Type S, Shimadzu) with a span of 15 mm at room temperature. The speed of the crosshead displacement was 0.5 mm/min.
100 90
Load (N)
80 70 60 50
CBC
40 30
3. Results and discussion
20 10
3.1. Properties of W claddings on carbon and AlN/CBC
0 -10 -0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
Displacement (mm) Fig. 3. Load–displacement of AlN/CBC and W cladding on AlN/CBC.
After sintering at 1700 °C, the W reached 96% theoretical density, as shown in Table 1. The claddings showed that the sintered W layer was bonded strongly with AlN/CBC. In case of the W cladding on a carbon disk, the W layer and carbon disk separated immediately after processing.
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W. Chen et al. / Journal of Nuclear Materials 420 (2012) 136–140
Fig. 4. Microstructure (a), and corresponding element distributions of Al (b), Y (c) and W (d) at the interface area of W cladding with AlN/CBC.
(a)
W
(b)
WC/WC2
W
AlN/CBC
(c)
(d)
WC/W2C
Fig. 5. Fractured surfaces of the interface area of W cladding with AlN/CBC (a), and enlarged images of W layer (b), WC/W2C layer (c), and ceramic layer (d).
3.1.1. Bonding of W layer and carbon The interfaces of separated W and carbon (Fig. 1a) were examined by XRD. As seen in Fig. 1b, the carbides of WC and W2C were formed at the W interface. At the carbon interface, only the graphite phase exists. Although reactions between W and carbon result in an interface containing WC/W2C, the interface layer is only
bonded to W, and separates easily from the carbon disk. This result suggests a weak chemical bonding between WC/W2C and carbon. 3.1.2. Bonding of W layer and AlN/CBC The side view of the polished cladding is shown in Fig. 2a. The W layer, around 0.3 mm in thickness, is strongly bonded to AlN/
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W. Chen et al. / Journal of Nuclear Materials 420 (2012) 136–140
(a)
W powder
(b)
W layer WC/W2C Ceramic
AlN Y-rich phase
AlN/CBC
AlN/CBC
AlN
Carbon Fig. 6. Schematic illustrations of the interface structure of W cladding with AlN/CBC: (a) before cladding and (b) after cladding.
CBC. No void was observed at the interface. Fig. 2b shows XRD patterns of W layer, AlN/CBC, and the interface areas. In the AlN/CBC area, besides the peaks of graphite and AlN, small amounts of Y2O3 and Al2Y4O9 were detected because the AlN in AlN/CBC contains 5 wt% Y2O3 as a sintering additive. In the interface area, small WC peaks are observed besides the peaks appeared in W and AlN/ CBC layers. No reaction occurs between W and AlN as previously reported [12]. The measured properties of W, AlN/CBC, and the cladding of W and AlN/CBC are listed in Table 1. The W prepared at the same condition as the cladding sample has a 96% theoretical density and bending strength of 1094 MPa. AlN/CBC has a bending strength of 100 MPa and thermal conductivity of 170 W/mK. The cladding sample (0.3-mm thick W and 2.2-mm thick AlN/CBC layer) exhibits an average bending strength of 172 MPa. The W layer side was placed on the tensile side in bending test. Load–displacement responses of the AlN/CBC and the cladding after the bending test are illustrated in Fig. 3. The cladding shows a typical response of linear elasticity up to fracture, similar to monolithic AlN/CBC. If it is assumed that a crack starts from the W layer, the flexural response of the cladding may mean that the crack produced in the W layer propagates readily into the AlN/ CBC layer. Therefore, the bonding between W layer and AlN/CBC layer is considered strong [13]. 3.2. Analysis of interface Fig. 4 shows the microstructure and the selected element distributions of the polished interface of the W-AlN/CBC cladding. Between W and AlN/CBC layers, a new layer of around 10 lm in thickness is observed. EDS analysis shows that this new layer contains large size AlN grains and a Y-rich phase, which is called the ceramic layer in the subsequent discussion. The Y-rich phase may be a Y–Al–O glass. W is not detected in this Y-rich phase, suggesting that the ceramic layer is only physically bonded to the W layer. The fractured surface of the W cladding on AlN/CBC was observed, as shown in Fig. 5. The phase compositions change gradually from W to the AlN/CBC side in the following sequence: W layer, WC/ W2C layer, ceramic layer, and AlN/CBC. The W layer contains grains with a size around 5–10 lm, while WC/W2C grains have an average size around 30 lm. The thickness of the WC/W2C layer is around 70 lm. Some dark spots, as denoted by arrows in Fig. 5c, are noticed in the WC/W2C layer, but it is difficult to identify their compositions and those of the surrounding gray areas from EDS analysis. The composition of W and carbon may be slightly different locally because of the fast reaction. The ceramic layer is around 10 lm in thickness, and tightly bonded with both WC/W2C and CBC layers.
3.3. Cladding mechanism Based on the above results, the development of phase composition and microstructure before and after W cladding on AlN/CBC by sintering is illustrated in Fig. 6. During sintering of W powders there is accompanying grain growth, whereas there is no change in AlN/CBC because it has been pre-sintered at a higher temperature of 1900 °C than the cladding temperature (1700 °C). Between the W and AlN/CBC layers, W will react with carbon and form WC and W2C, but will not react with AlN boundaries that cover carbon particles. For the reaction between W and carbon, carbon will diffuse to the W metal side, and at the same time, W may also diffuse to the CBC side. But it is difficult to identify which one is faster. The left AlN boundary layer that contains AlN and 5 wt% Y2O3 may separate and aggregate from the W–C reaction layer gradually at high temperature, and then form a new ceramic layer. The AlN grains in the new ceramic layer is much bigger, around 5 lm. compared to the AlN grains (0.5–2.5 lm in size) in AlN/CBC. In this new layer, AlN grains can grow more easily because the constraint from carbon particles in AlN/CBC is released. The newly formed ceramic layer is strongly connected with the AlN/CBC by the ceramic–ceramic bonding between them. The upper region of the ceramic layer, as illustrated in Fig. 6b, is bonded tightly with the WC/W2C layer probably due to the interlocking of the ceramic and WC/W2C grains at the interface. Therefore, the unique interface layer containing WC/W2C and ceramic layers result in the strong cladding of W and AlN/CBC. The formation of this interface structure is related to the unique microstructure of the AlN/CBC, which is composed of carbon particles and AlN ceramic boundaries. 4. Conclusions W cladding on AlN/CBC has been performed at 1700 °C by spark plasma sintering. The cladding shows higher strength than the base material of AlN/CBC (100 MPa). The interface consists of WC/W2C and ceramic layers resulting from the reaction between W and AlN/CBC. The ceramic layer is formed by large AlN grains and Y-rich phase. The formation of this unique interface is due to both the composition and structure of AlN/CBC. References [1] V. Barabash, M. Akiba, I. Mazul, M. Ulrickson, G. Vieider, Journal of Nuclear Materials 233–237 (1996) 718–723. [2] R. Mitteau, J. Missiaen, P. Brustolin, O. Ozer, A. Durocher, C. Ruset, C. Lungu, X. Courtois, C. Domonicy, H. Maier, C. Grisolia, G. Piazza, P. Chappuis, Fusion and Engineering and Design 82 (2007) 1700–1705.
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[3] I. Smid, M. Akiba, G. Vieider, L. Plochl, Journal of Nuclear Materials 258–263 (1998) 160–172. [4] O. Ozer, J.-M. Missiaen, S. Lay, R. Mitteau, Material Science and Engineering A 460–461 (2007) 525–531. [5] K. Tokunaga, T. Matsubara, Y. Miyamoto, Y. Takao, N. Yoshida, N. Noda, Y. Kubota, T. Sogabe, T. Kato, L. Plochl, Journal of Nuclear Materials 283–287 (2000) 1121–1127. [6] M. Inagaki, F.Y. Kang, Carbon materials science and engineering-from fundamentals to applications, Tsinghua University Press, Beijing, 2006. pp. 3–6. [7] Y. Zhang, Z. Chen, L. Wang, B. Yan, C. Li, D. Fang, Fusion Engineering and Design 84 (2009) 15–18.
[8] K. Tokunaga, N. Yoshida, N. Noda, Y. Kubota, S. Inagaki, R. Sakamoto, T. Sogabe, L. Plochl, Journal of Nuclear Materials 266–269 (1999) 1224–1229. [9] H. Maier, J. Luthin, M. Balden, S. Lindig, J. Linke, V. Rohde, H. Bolt, Journal of Nuclear Materials 307–311 (2002) 116–120. [10] W.W. Chen, Y. Miyamoto, T. Matsumoto, T. Tojo, Carbon 48 (2010) 3399–3404. [11] W.W. Chen, Y. Miyamoto, T. Tojo, M. Naito, Densification and microstructure development of AlN ceramic bonded carbon, Journal of the European Ceramic Society 32 (2012) 245–250. [12] Y. Kurokawa, C. Toy, W. Scott, Journal of the American Ceramic Society 72 (1989) 612–616. [13] C.A. Folsom, F.W. Zok, F.F. Lange, Journal of the American Ceramic Society 77 (1994) 689–696.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Tungsten cladding of new carbon composite with AlN ceramics Weiwu Chen a,⇑, Tetsuro Tojo a,b, Yoshinari Miyamoto a,b a b
Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan Advanced Carbon Technology Center, Toyo Tanso. Co., Ltd. 5-7-12, Takeshima, Nishiyodogawa-Ku, Osaka 555-0011, Japan
a r t i c l e
i n f o
Article history: Received 14 July 2011 Accepted 21 September 2011 Available online 5 October 2011
a b s t r a c t A new carbon composite called ceramic bonded carbon (CBC) is composed of separated carbon particles bonded with thin ceramic boundaries. To fabricate reliable plasma facing components, AlN ceramic bonded carbon (AlN/CBC, 20 vol.% AlN) was directly cladded with tungsten (W) at 1700 °C by spark plasma sintering. The cladding shows strong bonding, which is attributed to the formation of interface layer containing mixed WC/W2C and AlN ceramic. The interface between W and AlN/CBC was analyzed and the cladding mechanism was discussed. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Tungsten (W) is extensively used as X-ray target, high temperature armor due to its high melting point, high strength and some other unique physical/chemical properties. It is also a prime candidate for the plasma facing components, because of its low sputtering yield, tritium retention, and good thermal properties [1,2]. In order to sustain steady state operation at high temperature, W components normally need to be joined with a heat sink, such as Cu or graphite substrates [3–5]. However, W is heavy and difficult to machine and weld. W/Cu joints have been extensively studied focusing on relaxation of mismatches in coefficients of thermal expansion (CTE) (aCu 4aW) and elastic moduli (ECu 0.2EW). Carbon materials have high thermal conductivity and matched CTE value with W [6]. However, the lower strength and poor joining capability limit the development of W/C joints by the conventional contact bonding method. Carbon fiber composites (CFC) and carbon blocks were successfully coated with W by low pressure plasma spray or physical vapor deposition (PVD) route [5,7–9]. However, the plasma spray coating is usually porous and weak in adhesion strength. Thick coating over millimeter is difficult by PVD. Recently, we proposed a novel carbon-based composite, called ceramic bonded carbon (CBC) [10,11]. CBC has a unique microstructure consisting of carbon particles and a ceramic boundary. In CBCs, the ceramic network bonds carbon particles together and provides high strength and other functional properties as required. AlN/CBC (20 vol.% AlN) provides lightweight (2.34 g/cm3), high bending strength (100 MPa), high thermal conductivity (170 W/mK). Moreover, CBCs can be joined or cladded with ceramics and metals by sintering method due to the ceramic network structure in carbon. ⇑ Corresponding author. E-mail address: [email protected] (W. Chen). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.09.022
For the tungsten cladding of AlN/CBC by powder metallurgy method, it has some unique performances compared to conventional W/graphite components prepared by coating methods. For example, (1) The composition, size and thickness of W layer is easily controlled; (2) The bonding between W and AlN/CBC may be stronger due to the new interlayer formation containing WCx and AlN–Y–Al–O ceramics; (3) The AlN/CBC has higher relative density, strength and thermal conductivity than the conventional graphite bulk. In this work, as an example, W cladding on AlN/CBC was prepared by sintering the stacked W powder on AlN/CBC using spark plasma sintering (SPS). The cladding structure and mechanism were investigated. 2. Experimental 2.1. Starting materials and processing conditions The starting materials used are an AlN/CBC disk (U25 mm, laboratory made) and W powders (0.6 lm, Kojundo Chem. Co. Ltd., Japan). The preparation method and properties of AlN/CBC are previously discussed [10,11] and shown in Table 1. To enhance the densification of AlN/CBC, a sintering additive of 5 wt% Y2O3 was mixed with AlN before AlN/CBC preparation. To prepare the W cladding on AlN/CBC, firstly, the AlN/CBC disk was finished by a #80 SiC paper and then loaded into 25-mm diameter graphite dies. To form a 0.5 mm-thick W layer, 4.8 g of W powder was stacked onto the AlN/CBC disk, and then sintered in vacuum at 1700 °C at a heating rate of 100 °C/min and a uniaxial pressure of 30 MPa for 5 min by SPS using a Dr. 1050 SPS apparatus (Sumitomo Coal Co. Ltd., Japan). For comparison, a sample of W cladding on carbon was fabricated at the same condition using a graphite disk (25-mm diameter, ISO-88, Toyo Tanso. Co. Ltd., Japan) and W powders.
137
W. Chen et al. / Journal of Nuclear Materials 420 (2012) 136–140 Table 1 Characteristics of AlN/CBC, monolithic W, and W cladding on AlN/CBC. Samples
Sintering temperature (°C)
Density (g/cm3)
Relative density (%)
Strength (MPa)
Thermal conductivity (W/mK)
AlN/CBC Sintered W Cladding joint
1900 1700 1700
2.34 18.5 –
98 96
100 1094 170
170 – –
(a)
(b)
Carbon
Intensity
14000
W
∇ ♦ : WC Δ : W2C ∇: C
12000
♦
4000
∇∇ ♦
2000 0
∇
∇
♦ Δ
Δ
♦ ♦
Δ
C side
♦ Δ ♦♦
W side
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2-theta Fig. 1. W and carbon disks separated without bonding (a), and the XRD patterns of the interfaces (b).
(a)
(b) 7000 6000
∇
∇: W ♦: C ∇ ∗ : AlN Ο : WC + : Y2O3
∇
♦
5000
Intensity
W
Interface
• : Al2Y4O9
4000
W 3000
∗
2000
∗ • Ο Ο
1000 +
AlN/CBC
0
∗
∇
∗ ♦
•
∇
∗
Ο ∗
∇ ∗
♦
♦
+
+
∗ ∗
Interface ♦
AlN/CBC
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2-theta Fig. 2. Cross-sectional view of W cladding with AlN/CBC (a), and XRD patterns of W, interface, and AlN/CBC layers (b).
2.2. Characterizations 120
Cladding of W and CBC
110
Microstructure characterization was carried out using a field emission scanning electron microscope (FE-SEM, ERA-8800, Elionix). X-ray diffraction (XRD, JDX-3530 M, JEOL) was employed to characterize the samples’ compositions. Rectangular bars of the claddings, 3 2–3 20 mm3, were prepared to measure the three-point bending strength using a table-top universal tester (EZ-Test Type S, Shimadzu) with a span of 15 mm at room temperature. The speed of the crosshead displacement was 0.5 mm/min.
100 90
Load (N)
80 70 60 50
CBC
40 30
3. Results and discussion
20 10
3.1. Properties of W claddings on carbon and AlN/CBC
0 -10 -0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
Displacement (mm) Fig. 3. Load–displacement of AlN/CBC and W cladding on AlN/CBC.
After sintering at 1700 °C, the W reached 96% theoretical density, as shown in Table 1. The claddings showed that the sintered W layer was bonded strongly with AlN/CBC. In case of the W cladding on a carbon disk, the W layer and carbon disk separated immediately after processing.
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W. Chen et al. / Journal of Nuclear Materials 420 (2012) 136–140
Fig. 4. Microstructure (a), and corresponding element distributions of Al (b), Y (c) and W (d) at the interface area of W cladding with AlN/CBC.
(a)
W
(b)
WC/WC2
W
AlN/CBC
(c)
(d)
WC/W2C
Fig. 5. Fractured surfaces of the interface area of W cladding with AlN/CBC (a), and enlarged images of W layer (b), WC/W2C layer (c), and ceramic layer (d).
3.1.1. Bonding of W layer and carbon The interfaces of separated W and carbon (Fig. 1a) were examined by XRD. As seen in Fig. 1b, the carbides of WC and W2C were formed at the W interface. At the carbon interface, only the graphite phase exists. Although reactions between W and carbon result in an interface containing WC/W2C, the interface layer is only
bonded to W, and separates easily from the carbon disk. This result suggests a weak chemical bonding between WC/W2C and carbon. 3.1.2. Bonding of W layer and AlN/CBC The side view of the polished cladding is shown in Fig. 2a. The W layer, around 0.3 mm in thickness, is strongly bonded to AlN/
139
W. Chen et al. / Journal of Nuclear Materials 420 (2012) 136–140
(a)
W powder
(b)
W layer WC/W2C Ceramic
AlN Y-rich phase
AlN/CBC
AlN/CBC
AlN
Carbon Fig. 6. Schematic illustrations of the interface structure of W cladding with AlN/CBC: (a) before cladding and (b) after cladding.
CBC. No void was observed at the interface. Fig. 2b shows XRD patterns of W layer, AlN/CBC, and the interface areas. In the AlN/CBC area, besides the peaks of graphite and AlN, small amounts of Y2O3 and Al2Y4O9 were detected because the AlN in AlN/CBC contains 5 wt% Y2O3 as a sintering additive. In the interface area, small WC peaks are observed besides the peaks appeared in W and AlN/ CBC layers. No reaction occurs between W and AlN as previously reported [12]. The measured properties of W, AlN/CBC, and the cladding of W and AlN/CBC are listed in Table 1. The W prepared at the same condition as the cladding sample has a 96% theoretical density and bending strength of 1094 MPa. AlN/CBC has a bending strength of 100 MPa and thermal conductivity of 170 W/mK. The cladding sample (0.3-mm thick W and 2.2-mm thick AlN/CBC layer) exhibits an average bending strength of 172 MPa. The W layer side was placed on the tensile side in bending test. Load–displacement responses of the AlN/CBC and the cladding after the bending test are illustrated in Fig. 3. The cladding shows a typical response of linear elasticity up to fracture, similar to monolithic AlN/CBC. If it is assumed that a crack starts from the W layer, the flexural response of the cladding may mean that the crack produced in the W layer propagates readily into the AlN/ CBC layer. Therefore, the bonding between W layer and AlN/CBC layer is considered strong [13]. 3.2. Analysis of interface Fig. 4 shows the microstructure and the selected element distributions of the polished interface of the W-AlN/CBC cladding. Between W and AlN/CBC layers, a new layer of around 10 lm in thickness is observed. EDS analysis shows that this new layer contains large size AlN grains and a Y-rich phase, which is called the ceramic layer in the subsequent discussion. The Y-rich phase may be a Y–Al–O glass. W is not detected in this Y-rich phase, suggesting that the ceramic layer is only physically bonded to the W layer. The fractured surface of the W cladding on AlN/CBC was observed, as shown in Fig. 5. The phase compositions change gradually from W to the AlN/CBC side in the following sequence: W layer, WC/ W2C layer, ceramic layer, and AlN/CBC. The W layer contains grains with a size around 5–10 lm, while WC/W2C grains have an average size around 30 lm. The thickness of the WC/W2C layer is around 70 lm. Some dark spots, as denoted by arrows in Fig. 5c, are noticed in the WC/W2C layer, but it is difficult to identify their compositions and those of the surrounding gray areas from EDS analysis. The composition of W and carbon may be slightly different locally because of the fast reaction. The ceramic layer is around 10 lm in thickness, and tightly bonded with both WC/W2C and CBC layers.
3.3. Cladding mechanism Based on the above results, the development of phase composition and microstructure before and after W cladding on AlN/CBC by sintering is illustrated in Fig. 6. During sintering of W powders there is accompanying grain growth, whereas there is no change in AlN/CBC because it has been pre-sintered at a higher temperature of 1900 °C than the cladding temperature (1700 °C). Between the W and AlN/CBC layers, W will react with carbon and form WC and W2C, but will not react with AlN boundaries that cover carbon particles. For the reaction between W and carbon, carbon will diffuse to the W metal side, and at the same time, W may also diffuse to the CBC side. But it is difficult to identify which one is faster. The left AlN boundary layer that contains AlN and 5 wt% Y2O3 may separate and aggregate from the W–C reaction layer gradually at high temperature, and then form a new ceramic layer. The AlN grains in the new ceramic layer is much bigger, around 5 lm. compared to the AlN grains (0.5–2.5 lm in size) in AlN/CBC. In this new layer, AlN grains can grow more easily because the constraint from carbon particles in AlN/CBC is released. The newly formed ceramic layer is strongly connected with the AlN/CBC by the ceramic–ceramic bonding between them. The upper region of the ceramic layer, as illustrated in Fig. 6b, is bonded tightly with the WC/W2C layer probably due to the interlocking of the ceramic and WC/W2C grains at the interface. Therefore, the unique interface layer containing WC/W2C and ceramic layers result in the strong cladding of W and AlN/CBC. The formation of this interface structure is related to the unique microstructure of the AlN/CBC, which is composed of carbon particles and AlN ceramic boundaries. 4. Conclusions W cladding on AlN/CBC has been performed at 1700 °C by spark plasma sintering. The cladding shows higher strength than the base material of AlN/CBC (100 MPa). The interface consists of WC/W2C and ceramic layers resulting from the reaction between W and AlN/CBC. The ceramic layer is formed by large AlN grains and Y-rich phase. The formation of this unique interface is due to both the composition and structure of AlN/CBC. References [1] V. Barabash, M. Akiba, I. Mazul, M. Ulrickson, G. Vieider, Journal of Nuclear Materials 233–237 (1996) 718–723. [2] R. Mitteau, J. Missiaen, P. Brustolin, O. Ozer, A. Durocher, C. Ruset, C. Lungu, X. Courtois, C. Domonicy, H. Maier, C. Grisolia, G. Piazza, P. Chappuis, Fusion and Engineering and Design 82 (2007) 1700–1705.
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