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Joining of AlN and graphite disks using interlayer tapes by spark plasma sintering
Materials and Design 54 (2014) 755–759
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Joining of AlN and graphite disks using interlayer tapes by spark plasma sintering Tomoyuki Okuni a,b, Yoshinari Miyamoto a,b,⇑, Hiroya Abe b, Makio Naito b a b
Advanced Carbon Technology Center, Toyo Tanso Co., Ltd., 5-7-12 Takeshima, Nishiyodogawa-ku, Osaka 555-0011, Japan Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
a r t i c l e
i n f o
Article history: Received 28 March 2013 Accepted 1 September 2013 Available online 10 September 2013 Keywords: Aluminum nitride Graphite Joining Spark plasma sintering
a b s t r a c t AlN and graphite disks were successfully joined using a polymer plasticized ceramic tape as the interlayer by spark plasma sintering (SPS). The tape contains either composite powders of AlN and graphite or AlN powders without graphite. Both tapes contained 5 mass% Y2O3 as the sintering aid of AlN. The joining was carried out at 1700–1900 °C and 30 MPa for 5 min. No other reaction phase except for Al2Y4O9 was identified in the joints. The maximum tensile strength of the joints was obtained when the AlN–graphite composite interlayer tape was used. The joining mechanism is attributed not to the chemical bonding, but to the physical bonding of the Al2Y4O9 phase, which is solidified from the molten Al–Y–O squeezing into the porous graphite under pressure during SPS. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Isotropic graphite has a high temperature resistance and excellent thermal shock and chemical resistance, in addition to its self-lubrication and light weight. These properties make it suitable for many applications, including solar cells and silicon semiconductor manufacturing, continuous casting dies, and electrodes for electric discharge machining. It is typically produced using the following process: after mixing filler coke with binder pitch, the mixture is cold isostatically pressed, baked at approximately 1000 °C, and then graphitized at approximately 3000 °C by the Acheson method [1,2]. The combination of cold isostatic pressing and graphitization results in high quality graphite with isotropic properties that is easy to machine. Large blocks of graphite ranging in size from tens of centimeters to meters can be produced. However, isotropic graphite has lower mechanical strength than ceramics and metals at room temperature, in addition to its poor oxidation resistance at high temperature. Generally, it has a high porosity ranging from 12% to 25% [2,3]. In addition, it cannot be used with certain metals at high temperatures, including iron, cobalt, and tungsten, because carbon atoms diffuse into these metals. When this diffusion occurs, the melting temperature is reduced, and the metals react with the graphite to form carbides.
⇑ Corresponding author at: Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. Tel./fax: +81 6 6879 4373. E-mail address: [email protected] (Y. Miyamoto). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.09.003
Joining graphite with ceramics would be a useful way to overcome these limitations of graphite; many studies have been performed to improve the mechanical properties and oxidation resistance of graphite materials mixed [4–6] and coated with ceramics [7–9]. SiC/graphite joining is performed using brazing metals [10]. Most brazing metals, however, pose difficulties at aggressive heating conditions because of their low melting points and unmatched coefficients of thermal expansion (CTE), which reduces the working temperature of the joints [11]. Ceramic joining of Si3N4 pipes or mullite substrates reportedly performed using glass powders as an interlayer [12,13]. In these joints, the glass phase in the system of Y2O3–Al2O3–SiO2 melts and joins ceramics smoothly because of the good wettability and low thermal expansion mismatch at the interface of the joints [12]. These joints would be stable at high temperature, as opposed to brazing metals. In this study, we joined AlN and graphite disks by inserting a polymer plasticized tape containing either composite powders of AlN and graphite, or AlN powders without graphite as an interlayer. Both tapes include 5 mass% Y2O3 as the sintering aid of AlN. AlN ceramics possess high thermal conductivity, electrical insulation, and a CTE similar to that of graphite. Therefore, AlN/ graphite joints would be applicable to manufacturing electronic packages, heat exchangers, and mechanical parts. Because AlN can prevent carbon diffusion at high temperatures, these joints could be used for a heating tray of metal parts as well. The joining was carried out at 1700–1900 °C and 30 MPa for 5 min by SPS, which is an efficient process for the sintering or joining of ceramics [14–18]. The physical structure and chemical phase at the interface of the AlN/graphite joints were analyzed. The effect of the molten
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phase of Al–Y–O produced in the AlN disk and the interlayer on the joining was investigated. The strength of these joints was evaluated by tensile testing, and a joining mechanism was proposed. 2. Experimental procedure 2.1. Preparation of tapes To prepare the AlN–graphite composite tapes, AlN powders (Tokuyama Co., Ltd.) and spherical graphite (Toyotanso Co., Ltd.) were used as starting materials. The average size of each powder was 0.6 and 25 lm, respectively. The graphite and AlN powders containing 5 mass% Y2O3 as a sintering aid were premixed at a volume ratio of 80/20. Then, a slurry of these mixed powders was prepared with a 66/34 mass% mixture of 2-butanone and ethanol as a solvent. 2-ethylhexyl phosphate, polyvinyl butyral (PVB), and a 50/ 50 mass% mixture of polyethylene glycol and benzyl butyl phthalate were added as the dispersant, binder, and plasticizer, respectively. Thus, a slurry of 32 vol% graphite and AlN mixture was obtained. A polyethylene terephthalate film was coated with the slurry using a doctor blade, and the slurry was dried naturally. The dried AlN–graphite composite tapes peeled from the film were approximately 150 lm in thickness. AlN tapes were also prepared by the same procedure using a 25 vol% AlN slurry. 2.2. Joining the AlN and graphite disks Table 1 lists the physical properties of the graphite disk (IG-12, Toyotanso Co., Ltd.). It has nealy the same CTE value (4.7 106 K1) as AlN (4.5 106 K1) [19]. The relative density (79%) of graphite is comparatively lower than in conventional ceramics. Two commercially available AlN disks (NIHON CERATEC Co., Ltd., size: u25 1 mm) sintered with Y2O3 as sintering aid were then placed on the top and bottom sides of a graphite disk (u25 4 mm) with the AlN–graphite composite tape or AlN tape as an interlayer. Direct joining of the AlN disks on both side of a graphite disk was also conducted. Thus three kinds of joints were prepared. The joint was placed into a cylindrical graphite mold (u = 25 mm). The joining was carried out at 1700–1900 °C and 30 MPa for 5 min in a vacuum by spark plasma sintering (SPS1050, Sumitomo Coal Mining Co., Ltd.). Between the electrode and the mold, graphite spacers and punches were placed. The joining temperature was measured by a pyrometer focusing into a hole 1 mm in diameter at the side of the mold. 2.3. Characterization The obtained AlN/graphite joints were cut and polished for microstructural observation and strength measurements. Microstructural observation and elemental analysis of the joints were carried out by scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDS, ERA-8800FE, ELIONIX Co., Ltd.). The crystalline phases were examined by XRD. The strength of the joints was tested by a testing machine (EZ-L, Shimadzu Corporation) at room temperature. Sub-size specimen (4 mm (W) 4 mm (L) 6 mm (H)) was used, and the crosshead speed was set at 0.5 mm/min. Fig. 1 shows a schematic drawing of the
Table 1 Physical properties of the graphite used in this study. Raw material
Bulk density (Mg/m3)
Bending strength (MPa)
CTE (106/K)
Average pore radius (lm)
Graphite
1.78
41
4.7
1.4
Fig. 1. Schematic drawing of the jigs used for the strength measurements of the joints.
jigs used for the strength measurements. The top and bottom of the joints were adhered to stainless steel jigs using an epoxy resin adhesive (E-60HP, Henkel AG & Co. KGaA) at 80 °C for approximately 24 h. Seven specimens were prepared and subjected to the tensile test. The strength measured by this method was used to discuss the joining mechanism between AlN and graphite. 3. Results 3.1. Interface analysis Fig. 2 shows the cross sectional SEM images and elemental analyses of C, Al, and Y at the interface of the AlN/graphite joint prepared with the AlN–graphite composite tape (a), the AlN tape (b), and without tape (c). The joining temperature was set at 1900 °C. In all the joints, no cracks or delaminations were found at the interfaces. The AlN–graphite composite tape after joining was transformed into a dense AlN–graphite composite layer approximately 60 lm thick, as shown in Fig. 2(a). The graphite particles were covered with a three-dimensional network of thin AlN layers in the composite layer. Although Y2O3 was used as a sintering aid both in the AlN disk and in the AlN–graphite composite tape, Y was distributed in the graphite disk. The distribution of the Al in the graphite almost coincides with that of Y. During the sintering of AlN with Y2O3, a liquid phase in the Al–Y–O system is generally formed at approximately 1800 °C and solidified as the grain boundary phase during cooling [20]. The graphite disk used in this study has an open porosity over 10%. Therefore, as the Al–Y–O compound melts during SPS, the pressure should squeeze it into the open pores of the graphite disk. In ceramic joining using the glass interlayer [12,13], such a penetration of the molten glass phase did not occur because there is little open porosity in ceramics. The characteristic X-ray intensity of Al attributed to the Al–Y–O phase in the graphite is relatively lower than that seen in the AlN layer. On the other hand, there was a small concentration of Y in the AlN disks and the AlN–graphite composite layer, as shown in Fig. 2(a). The scarcity of Y in the AlN–graphite composite layer and the AlN disk suggests that a large part of the molten Al–Y–O phase squeezed into the pores of the graphite, leaving AlN grains sintered at the interface region. In the cases of the joints of AlN and graphite disks using the AlN tape or no tape, Y was also observed in the graphite disk, as can be seen in Fig. 2(b and c). For the joint without tape, Y was condensed in the gap between the graphite and AlN disks, as can be seen in
T. Okuni et al. / Materials and Design 54 (2014) 755–759
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Fig. 2. Cross-sectional SEM images and elemental analyses of C, Al, and Y at the interface of the AlN/graphite joints prepared at 1900 °C: (a) AlN–graphite composite tape, (b) AlN tape and (c) without tape.
Fig. 2(c). This result suggests that there is some difficulty in the direct joining without the interlayer tapes, and the gap is filled with the molten Al–Y–O phase. The AlN–graphite composite tape and the AlN tape containing polymeric binder component can fit both faces between the graphite and AlN disks, resulting in fewer gaps at their interfaces. The XRD patterns of the AlN disk and the interface of the AlN/ graphite joint using the AlN–graphite composite tape prepared at 1900 °C are shown in Fig. 3(a and b), respectively. The interface
region of the AlN/graphite joint was ground with a mortar and subjected to X-ray powder diffraction. In the AlN disk, the crystalline Al2Y4O9 boundary phase was detected. In the joint, graphite, AlN, and Al2Y4O9 were identified. No other phase was observed. The Al2Y4O9 is considered to be solidified from the Al–Y–O melt upon cooling. The XRD results suggest that no chemical bonding occurs between AlN and graphite. 3.2. Tensile strength of the joints Fig. 4 shows the tensile strengths of the AlN/graphite joints as a function of the joining temperature. The average strength value of seven specimens was plotted. The strength increased with an in-
Fig. 3. XRD patterns of the (a) AlN disk and (b) interface region of the AlN/graphite joint using the AlN–graphite composite tape.
Fig. 4. Tensile strengths of the AlN/graphite joints as a function of the joining temperature: AlN–graphite composite tape (s), AlN tape (d), and no tape (N).
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Table 2 Average tensile strengths of the AlN/graphite joints prepared at different temperatures with and without interlayer tapes. Joining temperature (°C)
Average tensile strength (MPa) Composite tape
AlN tape
No tape
1700 1800 1900
1 (1) 10 (3) 16 (4)
7 (4) 9 (6) 14 (6)
3 (1) 8 (5) 10 (6)
Numbers in parentheses denote the standard deviation of the tensile strength.
crease in joining temperature. The joints using the AlN–graphite composite tapes showed the maximum strength of 16 MPa at 1900 °C. On the other hand, the tensile strength of the joints using the AlN tape was 14 MPa, and that of the joints without tape was 10 MPa. In the case of joining at 1800 °C, all the joints showed similar tensile strengths of approximately 9 MPa. At 1700 °C, the joints using the AlN–graphite composite tapes were of poor quality. In this case, the graphite particles acted to prevent the sintering of the AlN. It is clear that a temperature approximately 1900 °C is needed to complete the sintering of the AlN–graphite composite tape and to join the AlN disk with the graphite. In the joints without tape, strong joining could not be achieved even if the gaps were filled with the Al2Y4O9 phase flowed from the AlN disk, because no chemical bonding exists at the AlN/graphite interface. The reported strengths as measured by tensile testing are the average values for seven specimens. The average strengths and standard deviations are listed in Table 2. The electrical resistivity of the joints with and without tape may affect the joining by SPS. Even so, the AlN/graphite joints prepared at 1900 °C using the AlN–graphite composite tape showed the smallest standard deviation.
3.3. Observation of fracture surface For better understanding of the joining mechanism using the AlN–graphite composite tape, SEM-EDS observation was carried out on the fracture surface after measuring the tensile strength of the joints. The SEM images and elemental analyses of C, Al, and Y at the fracture surface are shown in Fig. 5. In the specimen joined at 1700 °C, spherical graphite particles exist at both sides of the graphite (Fig. 5(a)) and AlN disks (Fig. 5(b)). This fractography suggests that the failure occurred in the AlN–graphite composite layer because of the low joining temperature and insufficient sintering of the composite, as mentioned above. On the other hand, in the specimen joined at 1900 °C, graphite disk is clearly observed except for small dots of carbon showing spherical graphite particles surrounded by Al on the graphite side (Fig. 5(c)). The spherical graphite particles surrounded by Al are clearly observed on the AlN disk side (Fig. 5(d)). These observations indicate that the failure occurred near the interface between the AlN–graphite composite layer and graphite disk. This interface would be weak comparing with the other interface between AlN disk and the composite layer. 4. Discussion Based on the above results, the joining mechanism of the AlN and graphite disks prepared at 1900 °C using the AlN–graphite composite tape is proposed as follows: AlN disks and the AlN– graphite composite layer are joined by sintering of their AlN grains through their interface. The AlN–graphite composite layer and graphite are bonded with the Al–Y–O phase, which is solidified from the molten Al–Y–O squeezing into the open pores of the graphite disk under the high pressure during SPS. The AlN–graphite composite tape easily deforms to create good contact with the
Fig. 5. SEM images and elemental analyses at the fracture surface of the joint with the AlN–graphite composite tape: (a) graphite side sintered at 1700 °C, (b) AlN side sintered at 1700 °C, (c) graphite side sintered at 1900 °C and (d) AlN side sintered at 1900 °C.
T. Okuni et al. / Materials and Design 54 (2014) 755–759
irregular surface of the graphite disk. When the joints were prepared using the AlN tape, a similar penetration of the Al–Y–O phase into the graphite would act as the joining mechanism as seen in Fig. 2(b). In the joining with Al–Y–O phase, the pressure required for the penetration of molten glass into porous graphite can be expressed by the following equation [21,22]:
DP ¼ 2cla cos h=R
ð1Þ
where DP is the pressure required for the penetration of molten glass, cla is its surface energy, h is the contact angle between graphite and molten glass, R is the pore radius of graphite. The pressure is inversely proportional to the pore radius when the contact angle is over 90o. In general, the wettability between the mixture of molten oxides, either glasses or slags, and graphite is poor [23,24]. It means that the contact angle is over 90°, and high pressure is required for the achievements of the Al–Y–O liquid phase squeezing into the open pores of graphite according to Eq. (1). The high pressure caused by SPS could satisfy this requirement, thus led to the better bonding between AlN and graphite with Al–Y–O liquid phase. On the contrary, in case of joining with carbon–carbon (C/C) composites categorized as graphite materials and lithium-aluminum–silicate (LAS) glass–ceramics, SiC coating on C/C was actually performed as an interlayer to improve the poor wettability between C/C and LAS glass–ceramics [9]. In addition to the SiC coating, a magnesium–aluminum–silicate (MAS) glass–ceramics interlayer was employed due to their good compatibility with the SiC coating and LAS glass–ceramics. As a result, the joining was conducted at 1200 °C under the pressure of 20 MPa by hot pressing. However, complex interfaces were formed by element inter diffusions and chemical reactions resulting in some cracks due to the thermal stress caused by the mismatch of CTE between SiC and MAS glass–ceramics. For better joining, the reduction of the CTE mismatch would be important. For the practical use of the joints, high temperature strength of the joints should be evaluated. Li and Watanabe measured the flexural strength of a sintered AlN with 2 vol% Y2O3 in argon atmosphere [25]. No apparent decrease in strength of the AlN was found until 1200 °C. For a sintered AlN with 5 mass% Y2O3, the four-point flexural strength measured at 1000 °C in air became 80% for that measured at room temperature, and then remained almost constant up to 1300 °C [26]. The fracture mode of AlN with Y2O3 changed from trasgranular to intergranular at high temperatures (P1000 °C) [26]. It suggests that the fracture is controlled by the mechanical behavior of Al–Y–O phase existing at the grain boundaries when testing temperature is higher than 1000 °C. In the case of AlN/graphite joint with AlN–graphite composite tape, no chemical bonding occurred between AlN and graphite as seen in Fig. 3. Moreover, AlN and graphite have almost similar CTE values. Therefore, it is expected that the AlN/graphite joints using Al– Y–O phase would stable and keep their strength up to around 1300 °C. Further studies including thermal cycle test related with life time are needed to make clear the reliability of the joints at high temperature. 5. Conclusions AlN and graphite disks were successfully joined by AlN–graphite composite tape or AlN tape as an interlayer by SPS. No cracks or delaminations were observed at the interfaces. No other reaction phase except for Al2Y4O9 was identified in the joints by XRD. The joining mechanism of the joints is proposed as follows: when the
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AlN–graphite composite layer is used, AlN disks and the AlN– graphite composite layer are joined by sintering of their AlN grains through their interface. While, the joining of AlN–graphite composite layer and graphite is performed by the Al2Y4O9 phase, which is solidified from the molten Al–Y–O squeezing physically into the open pores of the graphite disk under the high pressure during SPS. A similar penetration of the Al–Y–O phase into the graphite would act as the joining mechanism when the joints are prepared using the AlN tape. The maximum tensile strength of the AlN/graphite joints was obtained when the AlN–graphite composite tape was used. The joints of AlN and graphite using the AlN–graphite composite or AlN tapes are promising for industrial applications at high temperatures. References [1] Nightingale RE. Nuclear graphite. New York: Academic Press; 1962. [2] Inagaki M, Kang F. Carbon materials science and engineering from fundamentals to applications. Beijing: Tsinghua University Press; 2006. [3] Yamashina T, Hino T. Overall evaluation study for isotropic graphite as fusion first wall material in Japan. J Nucl Mater 1989;162–164:841–50. [4] Kobayashi K, Miyazaki K, Ogawa I, Hagio T, Yoshida H. Carbon/ceramics composites-preparation and properties. Mater Des 1988;9(1):10–21. [5] Guo QG, Song JR, Liu L, Zhang BJ. Factors influencing oxidation resistance of B4C/C composites with self-healing properties. Carbon 1998;36(11):1597–601. [6] Kobayashi K, Maeda K, Sano H, Uchiyama Y. Formation and oxidation resistance of the coating formed on carbon material composed of B4C–SiC powders. Carbon 1995;33(4):397–403. [7] Zhu QS, Qiu Xl, Ma CG. Oxidation resistant SiC coating for graphite materials. Carbon 1999;37:1475–84. [8] Zhu D, Hing P, Brown P, Sahai Y. Characterization of silicon carbide coating grown on graphite by chemical vapor deposition. J Mater Process Technol 1995;48:517–23. [9] Li KZ, Wang J, Ren XB, Li HJ, Li W, Li ZQ. The preparation and mechanical properties of carbon–carbon/lithium–aluminum–silicate composite joints. Mater Des 2013;44:346–53. [10] Mao YG, Li SJ, Yan LS. Joining of SiC ceramic to graphite using Ni–Cr–SiC powders as filler. Mater Sci Eng A 2008;491:304–8. [11] Yu MH, Zhou B, Bi DB, Shaw D. Preparation of graded multilayer materials and evaluation of residual stress. Mater Des 2010;31(5):2478–82. [12] Lin YJ, Tu SH. Joining of mullite ceramics with yttrium aluminosilicate glass interlayers. Ceram Int 2009;35:1311–5. [13] Kondo N, Hyuga H, Kita H, Hirao K. Joining of silicon nitride by microwave local heating. J Ceram Soc Japan 2010;118(10):959–62. [14] Angerer P, Yu LG, Khor KA, Korb G, Zalite I. Spark-plasma-sintering (SPS) of nanostructured titanium carbide powders. J Eur Ceram Soc 2005;25:1919–27. [15] Gao L, Hong JS, Miyamoto H, Torre SDDL. Bending strength and microstructure of Al2O3 ceramics densified by spark plasma sintering. J Eur Ceram Soc 2000;20:2149–52. [16] Li W, Gao L. Rapid sintering of nanocrystalline ZrO2(3Y) by spark plasma sintering. J Eur Ceram Soc 2000;20:2441–5. [17] Khor KA, Yu L-G, Chan SH, Chen XJ. Densification of plasma sprayed YSZ electrolytes by spark plasma sintering (SPS). J Eur Ceram Soc 2003;23:1855–63. [18] Xie G, Ohashi O, Yoshioka T, Song M, Mitsuishi K, Yasuda H, et al. Effect of interface behavior between particles on properties of pure Al powder compacts by spark plasma sintering. Mater Trans 2001;42(9):1846–9. [19] Haussonne FJM. Review of the synthesis methods for AlN. Mater Manuf Process 1995;10(4):717–55. [20] Kasori M, Ueno F. Thermal conductivity improvement of YAG added AlN ceramics in the grain boundary elimination process. J Eur Ceram Soc 1995;15:435–43. [21] Young RMK. A liquid metal infiltration model of unidirectional fibre preforms in inert atmospheres. Mater Sci Eng A 1991;135:19–22. [22] Venetti, Antonio C. Progress in materials science research. New York: Nova Science Publishers, Inc.; 2007. [23] Shen P, Fujii H, Nogi K. Wettability of some refractory materials by molten SiO2–MnO–TiO2–FeOx slag. Mater Chem Phys 2009;114:681–6. [24] Eustathopoulos N, Nicholas MG, Drevet B. Wettability at high temperatures. Oxford: Pergamon; 1999. [25] Li J-F, Watanabe R. Pressureless sintering and high-strength of SiC–AlN ceramics. J Ceram Soc Japan 1994;102(8):727–31. [26] Yoshimura HN, Narita NE, Molisani AL, Goldenstein H. High temperature flexural strength and fracture toughness of AlN with Y2O3 ceramic. J Mater Sci 2009;44:5773–80.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Joining of AlN and graphite disks using interlayer tapes by spark plasma sintering Tomoyuki Okuni a,b, Yoshinari Miyamoto a,b,⇑, Hiroya Abe b, Makio Naito b a b
Advanced Carbon Technology Center, Toyo Tanso Co., Ltd., 5-7-12 Takeshima, Nishiyodogawa-ku, Osaka 555-0011, Japan Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
a r t i c l e
i n f o
Article history: Received 28 March 2013 Accepted 1 September 2013 Available online 10 September 2013 Keywords: Aluminum nitride Graphite Joining Spark plasma sintering
a b s t r a c t AlN and graphite disks were successfully joined using a polymer plasticized ceramic tape as the interlayer by spark plasma sintering (SPS). The tape contains either composite powders of AlN and graphite or AlN powders without graphite. Both tapes contained 5 mass% Y2O3 as the sintering aid of AlN. The joining was carried out at 1700–1900 °C and 30 MPa for 5 min. No other reaction phase except for Al2Y4O9 was identified in the joints. The maximum tensile strength of the joints was obtained when the AlN–graphite composite interlayer tape was used. The joining mechanism is attributed not to the chemical bonding, but to the physical bonding of the Al2Y4O9 phase, which is solidified from the molten Al–Y–O squeezing into the porous graphite under pressure during SPS. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Isotropic graphite has a high temperature resistance and excellent thermal shock and chemical resistance, in addition to its self-lubrication and light weight. These properties make it suitable for many applications, including solar cells and silicon semiconductor manufacturing, continuous casting dies, and electrodes for electric discharge machining. It is typically produced using the following process: after mixing filler coke with binder pitch, the mixture is cold isostatically pressed, baked at approximately 1000 °C, and then graphitized at approximately 3000 °C by the Acheson method [1,2]. The combination of cold isostatic pressing and graphitization results in high quality graphite with isotropic properties that is easy to machine. Large blocks of graphite ranging in size from tens of centimeters to meters can be produced. However, isotropic graphite has lower mechanical strength than ceramics and metals at room temperature, in addition to its poor oxidation resistance at high temperature. Generally, it has a high porosity ranging from 12% to 25% [2,3]. In addition, it cannot be used with certain metals at high temperatures, including iron, cobalt, and tungsten, because carbon atoms diffuse into these metals. When this diffusion occurs, the melting temperature is reduced, and the metals react with the graphite to form carbides.
⇑ Corresponding author at: Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. Tel./fax: +81 6 6879 4373. E-mail address: [email protected] (Y. Miyamoto). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.09.003
Joining graphite with ceramics would be a useful way to overcome these limitations of graphite; many studies have been performed to improve the mechanical properties and oxidation resistance of graphite materials mixed [4–6] and coated with ceramics [7–9]. SiC/graphite joining is performed using brazing metals [10]. Most brazing metals, however, pose difficulties at aggressive heating conditions because of their low melting points and unmatched coefficients of thermal expansion (CTE), which reduces the working temperature of the joints [11]. Ceramic joining of Si3N4 pipes or mullite substrates reportedly performed using glass powders as an interlayer [12,13]. In these joints, the glass phase in the system of Y2O3–Al2O3–SiO2 melts and joins ceramics smoothly because of the good wettability and low thermal expansion mismatch at the interface of the joints [12]. These joints would be stable at high temperature, as opposed to brazing metals. In this study, we joined AlN and graphite disks by inserting a polymer plasticized tape containing either composite powders of AlN and graphite, or AlN powders without graphite as an interlayer. Both tapes include 5 mass% Y2O3 as the sintering aid of AlN. AlN ceramics possess high thermal conductivity, electrical insulation, and a CTE similar to that of graphite. Therefore, AlN/ graphite joints would be applicable to manufacturing electronic packages, heat exchangers, and mechanical parts. Because AlN can prevent carbon diffusion at high temperatures, these joints could be used for a heating tray of metal parts as well. The joining was carried out at 1700–1900 °C and 30 MPa for 5 min by SPS, which is an efficient process for the sintering or joining of ceramics [14–18]. The physical structure and chemical phase at the interface of the AlN/graphite joints were analyzed. The effect of the molten
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phase of Al–Y–O produced in the AlN disk and the interlayer on the joining was investigated. The strength of these joints was evaluated by tensile testing, and a joining mechanism was proposed. 2. Experimental procedure 2.1. Preparation of tapes To prepare the AlN–graphite composite tapes, AlN powders (Tokuyama Co., Ltd.) and spherical graphite (Toyotanso Co., Ltd.) were used as starting materials. The average size of each powder was 0.6 and 25 lm, respectively. The graphite and AlN powders containing 5 mass% Y2O3 as a sintering aid were premixed at a volume ratio of 80/20. Then, a slurry of these mixed powders was prepared with a 66/34 mass% mixture of 2-butanone and ethanol as a solvent. 2-ethylhexyl phosphate, polyvinyl butyral (PVB), and a 50/ 50 mass% mixture of polyethylene glycol and benzyl butyl phthalate were added as the dispersant, binder, and plasticizer, respectively. Thus, a slurry of 32 vol% graphite and AlN mixture was obtained. A polyethylene terephthalate film was coated with the slurry using a doctor blade, and the slurry was dried naturally. The dried AlN–graphite composite tapes peeled from the film were approximately 150 lm in thickness. AlN tapes were also prepared by the same procedure using a 25 vol% AlN slurry. 2.2. Joining the AlN and graphite disks Table 1 lists the physical properties of the graphite disk (IG-12, Toyotanso Co., Ltd.). It has nealy the same CTE value (4.7 106 K1) as AlN (4.5 106 K1) [19]. The relative density (79%) of graphite is comparatively lower than in conventional ceramics. Two commercially available AlN disks (NIHON CERATEC Co., Ltd., size: u25 1 mm) sintered with Y2O3 as sintering aid were then placed on the top and bottom sides of a graphite disk (u25 4 mm) with the AlN–graphite composite tape or AlN tape as an interlayer. Direct joining of the AlN disks on both side of a graphite disk was also conducted. Thus three kinds of joints were prepared. The joint was placed into a cylindrical graphite mold (u = 25 mm). The joining was carried out at 1700–1900 °C and 30 MPa for 5 min in a vacuum by spark plasma sintering (SPS1050, Sumitomo Coal Mining Co., Ltd.). Between the electrode and the mold, graphite spacers and punches were placed. The joining temperature was measured by a pyrometer focusing into a hole 1 mm in diameter at the side of the mold. 2.3. Characterization The obtained AlN/graphite joints were cut and polished for microstructural observation and strength measurements. Microstructural observation and elemental analysis of the joints were carried out by scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDS, ERA-8800FE, ELIONIX Co., Ltd.). The crystalline phases were examined by XRD. The strength of the joints was tested by a testing machine (EZ-L, Shimadzu Corporation) at room temperature. Sub-size specimen (4 mm (W) 4 mm (L) 6 mm (H)) was used, and the crosshead speed was set at 0.5 mm/min. Fig. 1 shows a schematic drawing of the
Table 1 Physical properties of the graphite used in this study. Raw material
Bulk density (Mg/m3)
Bending strength (MPa)
CTE (106/K)
Average pore radius (lm)
Graphite
1.78
41
4.7
1.4
Fig. 1. Schematic drawing of the jigs used for the strength measurements of the joints.
jigs used for the strength measurements. The top and bottom of the joints were adhered to stainless steel jigs using an epoxy resin adhesive (E-60HP, Henkel AG & Co. KGaA) at 80 °C for approximately 24 h. Seven specimens were prepared and subjected to the tensile test. The strength measured by this method was used to discuss the joining mechanism between AlN and graphite. 3. Results 3.1. Interface analysis Fig. 2 shows the cross sectional SEM images and elemental analyses of C, Al, and Y at the interface of the AlN/graphite joint prepared with the AlN–graphite composite tape (a), the AlN tape (b), and without tape (c). The joining temperature was set at 1900 °C. In all the joints, no cracks or delaminations were found at the interfaces. The AlN–graphite composite tape after joining was transformed into a dense AlN–graphite composite layer approximately 60 lm thick, as shown in Fig. 2(a). The graphite particles were covered with a three-dimensional network of thin AlN layers in the composite layer. Although Y2O3 was used as a sintering aid both in the AlN disk and in the AlN–graphite composite tape, Y was distributed in the graphite disk. The distribution of the Al in the graphite almost coincides with that of Y. During the sintering of AlN with Y2O3, a liquid phase in the Al–Y–O system is generally formed at approximately 1800 °C and solidified as the grain boundary phase during cooling [20]. The graphite disk used in this study has an open porosity over 10%. Therefore, as the Al–Y–O compound melts during SPS, the pressure should squeeze it into the open pores of the graphite disk. In ceramic joining using the glass interlayer [12,13], such a penetration of the molten glass phase did not occur because there is little open porosity in ceramics. The characteristic X-ray intensity of Al attributed to the Al–Y–O phase in the graphite is relatively lower than that seen in the AlN layer. On the other hand, there was a small concentration of Y in the AlN disks and the AlN–graphite composite layer, as shown in Fig. 2(a). The scarcity of Y in the AlN–graphite composite layer and the AlN disk suggests that a large part of the molten Al–Y–O phase squeezed into the pores of the graphite, leaving AlN grains sintered at the interface region. In the cases of the joints of AlN and graphite disks using the AlN tape or no tape, Y was also observed in the graphite disk, as can be seen in Fig. 2(b and c). For the joint without tape, Y was condensed in the gap between the graphite and AlN disks, as can be seen in
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Fig. 2. Cross-sectional SEM images and elemental analyses of C, Al, and Y at the interface of the AlN/graphite joints prepared at 1900 °C: (a) AlN–graphite composite tape, (b) AlN tape and (c) without tape.
Fig. 2(c). This result suggests that there is some difficulty in the direct joining without the interlayer tapes, and the gap is filled with the molten Al–Y–O phase. The AlN–graphite composite tape and the AlN tape containing polymeric binder component can fit both faces between the graphite and AlN disks, resulting in fewer gaps at their interfaces. The XRD patterns of the AlN disk and the interface of the AlN/ graphite joint using the AlN–graphite composite tape prepared at 1900 °C are shown in Fig. 3(a and b), respectively. The interface
region of the AlN/graphite joint was ground with a mortar and subjected to X-ray powder diffraction. In the AlN disk, the crystalline Al2Y4O9 boundary phase was detected. In the joint, graphite, AlN, and Al2Y4O9 were identified. No other phase was observed. The Al2Y4O9 is considered to be solidified from the Al–Y–O melt upon cooling. The XRD results suggest that no chemical bonding occurs between AlN and graphite. 3.2. Tensile strength of the joints Fig. 4 shows the tensile strengths of the AlN/graphite joints as a function of the joining temperature. The average strength value of seven specimens was plotted. The strength increased with an in-
Fig. 3. XRD patterns of the (a) AlN disk and (b) interface region of the AlN/graphite joint using the AlN–graphite composite tape.
Fig. 4. Tensile strengths of the AlN/graphite joints as a function of the joining temperature: AlN–graphite composite tape (s), AlN tape (d), and no tape (N).
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Table 2 Average tensile strengths of the AlN/graphite joints prepared at different temperatures with and without interlayer tapes. Joining temperature (°C)
Average tensile strength (MPa) Composite tape
AlN tape
No tape
1700 1800 1900
1 (1) 10 (3) 16 (4)
7 (4) 9 (6) 14 (6)
3 (1) 8 (5) 10 (6)
Numbers in parentheses denote the standard deviation of the tensile strength.
crease in joining temperature. The joints using the AlN–graphite composite tapes showed the maximum strength of 16 MPa at 1900 °C. On the other hand, the tensile strength of the joints using the AlN tape was 14 MPa, and that of the joints without tape was 10 MPa. In the case of joining at 1800 °C, all the joints showed similar tensile strengths of approximately 9 MPa. At 1700 °C, the joints using the AlN–graphite composite tapes were of poor quality. In this case, the graphite particles acted to prevent the sintering of the AlN. It is clear that a temperature approximately 1900 °C is needed to complete the sintering of the AlN–graphite composite tape and to join the AlN disk with the graphite. In the joints without tape, strong joining could not be achieved even if the gaps were filled with the Al2Y4O9 phase flowed from the AlN disk, because no chemical bonding exists at the AlN/graphite interface. The reported strengths as measured by tensile testing are the average values for seven specimens. The average strengths and standard deviations are listed in Table 2. The electrical resistivity of the joints with and without tape may affect the joining by SPS. Even so, the AlN/graphite joints prepared at 1900 °C using the AlN–graphite composite tape showed the smallest standard deviation.
3.3. Observation of fracture surface For better understanding of the joining mechanism using the AlN–graphite composite tape, SEM-EDS observation was carried out on the fracture surface after measuring the tensile strength of the joints. The SEM images and elemental analyses of C, Al, and Y at the fracture surface are shown in Fig. 5. In the specimen joined at 1700 °C, spherical graphite particles exist at both sides of the graphite (Fig. 5(a)) and AlN disks (Fig. 5(b)). This fractography suggests that the failure occurred in the AlN–graphite composite layer because of the low joining temperature and insufficient sintering of the composite, as mentioned above. On the other hand, in the specimen joined at 1900 °C, graphite disk is clearly observed except for small dots of carbon showing spherical graphite particles surrounded by Al on the graphite side (Fig. 5(c)). The spherical graphite particles surrounded by Al are clearly observed on the AlN disk side (Fig. 5(d)). These observations indicate that the failure occurred near the interface between the AlN–graphite composite layer and graphite disk. This interface would be weak comparing with the other interface between AlN disk and the composite layer. 4. Discussion Based on the above results, the joining mechanism of the AlN and graphite disks prepared at 1900 °C using the AlN–graphite composite tape is proposed as follows: AlN disks and the AlN– graphite composite layer are joined by sintering of their AlN grains through their interface. The AlN–graphite composite layer and graphite are bonded with the Al–Y–O phase, which is solidified from the molten Al–Y–O squeezing into the open pores of the graphite disk under the high pressure during SPS. The AlN–graphite composite tape easily deforms to create good contact with the
Fig. 5. SEM images and elemental analyses at the fracture surface of the joint with the AlN–graphite composite tape: (a) graphite side sintered at 1700 °C, (b) AlN side sintered at 1700 °C, (c) graphite side sintered at 1900 °C and (d) AlN side sintered at 1900 °C.
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irregular surface of the graphite disk. When the joints were prepared using the AlN tape, a similar penetration of the Al–Y–O phase into the graphite would act as the joining mechanism as seen in Fig. 2(b). In the joining with Al–Y–O phase, the pressure required for the penetration of molten glass into porous graphite can be expressed by the following equation [21,22]:
DP ¼ 2cla cos h=R
ð1Þ
where DP is the pressure required for the penetration of molten glass, cla is its surface energy, h is the contact angle between graphite and molten glass, R is the pore radius of graphite. The pressure is inversely proportional to the pore radius when the contact angle is over 90o. In general, the wettability between the mixture of molten oxides, either glasses or slags, and graphite is poor [23,24]. It means that the contact angle is over 90°, and high pressure is required for the achievements of the Al–Y–O liquid phase squeezing into the open pores of graphite according to Eq. (1). The high pressure caused by SPS could satisfy this requirement, thus led to the better bonding between AlN and graphite with Al–Y–O liquid phase. On the contrary, in case of joining with carbon–carbon (C/C) composites categorized as graphite materials and lithium-aluminum–silicate (LAS) glass–ceramics, SiC coating on C/C was actually performed as an interlayer to improve the poor wettability between C/C and LAS glass–ceramics [9]. In addition to the SiC coating, a magnesium–aluminum–silicate (MAS) glass–ceramics interlayer was employed due to their good compatibility with the SiC coating and LAS glass–ceramics. As a result, the joining was conducted at 1200 °C under the pressure of 20 MPa by hot pressing. However, complex interfaces were formed by element inter diffusions and chemical reactions resulting in some cracks due to the thermal stress caused by the mismatch of CTE between SiC and MAS glass–ceramics. For better joining, the reduction of the CTE mismatch would be important. For the practical use of the joints, high temperature strength of the joints should be evaluated. Li and Watanabe measured the flexural strength of a sintered AlN with 2 vol% Y2O3 in argon atmosphere [25]. No apparent decrease in strength of the AlN was found until 1200 °C. For a sintered AlN with 5 mass% Y2O3, the four-point flexural strength measured at 1000 °C in air became 80% for that measured at room temperature, and then remained almost constant up to 1300 °C [26]. The fracture mode of AlN with Y2O3 changed from trasgranular to intergranular at high temperatures (P1000 °C) [26]. It suggests that the fracture is controlled by the mechanical behavior of Al–Y–O phase existing at the grain boundaries when testing temperature is higher than 1000 °C. In the case of AlN/graphite joint with AlN–graphite composite tape, no chemical bonding occurred between AlN and graphite as seen in Fig. 3. Moreover, AlN and graphite have almost similar CTE values. Therefore, it is expected that the AlN/graphite joints using Al– Y–O phase would stable and keep their strength up to around 1300 °C. Further studies including thermal cycle test related with life time are needed to make clear the reliability of the joints at high temperature. 5. Conclusions AlN and graphite disks were successfully joined by AlN–graphite composite tape or AlN tape as an interlayer by SPS. No cracks or delaminations were observed at the interfaces. No other reaction phase except for Al2Y4O9 was identified in the joints by XRD. The joining mechanism of the joints is proposed as follows: when the
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AlN–graphite composite layer is used, AlN disks and the AlN– graphite composite layer are joined by sintering of their AlN grains through their interface. While, the joining of AlN–graphite composite layer and graphite is performed by the Al2Y4O9 phase, which is solidified from the molten Al–Y–O squeezing physically into the open pores of the graphite disk under the high pressure during SPS. A similar penetration of the Al–Y–O phase into the graphite would act as the joining mechanism when the joints are prepared using the AlN tape. The maximum tensile strength of the AlN/graphite joints was obtained when the AlN–graphite composite tape was used. The joints of AlN and graphite using the AlN–graphite composite or AlN tapes are promising for industrial applications at high temperatures. References [1] Nightingale RE. Nuclear graphite. New York: Academic Press; 1962. [2] Inagaki M, Kang F. Carbon materials science and engineering from fundamentals to applications. Beijing: Tsinghua University Press; 2006. [3] Yamashina T, Hino T. Overall evaluation study for isotropic graphite as fusion first wall material in Japan. J Nucl Mater 1989;162–164:841–50. [4] Kobayashi K, Miyazaki K, Ogawa I, Hagio T, Yoshida H. Carbon/ceramics composites-preparation and properties. Mater Des 1988;9(1):10–21. [5] Guo QG, Song JR, Liu L, Zhang BJ. Factors influencing oxidation resistance of B4C/C composites with self-healing properties. Carbon 1998;36(11):1597–601. [6] Kobayashi K, Maeda K, Sano H, Uchiyama Y. Formation and oxidation resistance of the coating formed on carbon material composed of B4C–SiC powders. Carbon 1995;33(4):397–403. [7] Zhu QS, Qiu Xl, Ma CG. Oxidation resistant SiC coating for graphite materials. Carbon 1999;37:1475–84. [8] Zhu D, Hing P, Brown P, Sahai Y. Characterization of silicon carbide coating grown on graphite by chemical vapor deposition. J Mater Process Technol 1995;48:517–23. [9] Li KZ, Wang J, Ren XB, Li HJ, Li W, Li ZQ. The preparation and mechanical properties of carbon–carbon/lithium–aluminum–silicate composite joints. Mater Des 2013;44:346–53. [10] Mao YG, Li SJ, Yan LS. Joining of SiC ceramic to graphite using Ni–Cr–SiC powders as filler. Mater Sci Eng A 2008;491:304–8. [11] Yu MH, Zhou B, Bi DB, Shaw D. Preparation of graded multilayer materials and evaluation of residual stress. Mater Des 2010;31(5):2478–82. [12] Lin YJ, Tu SH. Joining of mullite ceramics with yttrium aluminosilicate glass interlayers. Ceram Int 2009;35:1311–5. [13] Kondo N, Hyuga H, Kita H, Hirao K. Joining of silicon nitride by microwave local heating. J Ceram Soc Japan 2010;118(10):959–62. [14] Angerer P, Yu LG, Khor KA, Korb G, Zalite I. Spark-plasma-sintering (SPS) of nanostructured titanium carbide powders. J Eur Ceram Soc 2005;25:1919–27. [15] Gao L, Hong JS, Miyamoto H, Torre SDDL. Bending strength and microstructure of Al2O3 ceramics densified by spark plasma sintering. J Eur Ceram Soc 2000;20:2149–52. [16] Li W, Gao L. Rapid sintering of nanocrystalline ZrO2(3Y) by spark plasma sintering. J Eur Ceram Soc 2000;20:2441–5. [17] Khor KA, Yu L-G, Chan SH, Chen XJ. Densification of plasma sprayed YSZ electrolytes by spark plasma sintering (SPS). J Eur Ceram Soc 2003;23:1855–63. [18] Xie G, Ohashi O, Yoshioka T, Song M, Mitsuishi K, Yasuda H, et al. Effect of interface behavior between particles on properties of pure Al powder compacts by spark plasma sintering. Mater Trans 2001;42(9):1846–9. [19] Haussonne FJM. Review of the synthesis methods for AlN. Mater Manuf Process 1995;10(4):717–55. [20] Kasori M, Ueno F. Thermal conductivity improvement of YAG added AlN ceramics in the grain boundary elimination process. J Eur Ceram Soc 1995;15:435–43. [21] Young RMK. A liquid metal infiltration model of unidirectional fibre preforms in inert atmospheres. Mater Sci Eng A 1991;135:19–22. [22] Venetti, Antonio C. Progress in materials science research. New York: Nova Science Publishers, Inc.; 2007. [23] Shen P, Fujii H, Nogi K. Wettability of some refractory materials by molten SiO2–MnO–TiO2–FeOx slag. Mater Chem Phys 2009;114:681–6. [24] Eustathopoulos N, Nicholas MG, Drevet B. Wettability at high temperatures. Oxford: Pergamon; 1999. [25] Li J-F, Watanabe R. Pressureless sintering and high-strength of SiC–AlN ceramics. J Ceram Soc Japan 1994;102(8):727–31. [26] Yoshimura HN, Narita NE, Molisani AL, Goldenstein H. High temperature flexural strength and fracture toughness of AlN with Y2O3 ceramic. J Mater Sci 2009;44:5773–80.