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Al composites with three dimensional interpenetrating network structure
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 7539–7544 www.elsevier.com/locate/ceramint
Thermophysical properties of SiC/Al composites with three dimensional interpenetrating network structure Shun Lin, Degan Xiong, Meng Liu, Shuxin Bai, Xun Zhao Department of Materials Sicence and Engineering, National University of Defense Technology, Changsha 410073, China Received 25 November 2013; received in revised form 18 December 2013; accepted 23 December 2013 Available online 31 December 2013
Abstract Silicon carbide (SiC) reinforced aluminum composites with three dimensional interpenetrating network structure (3D-SiC/Al) were fabricated by the gas pressure infiltration method, and their thermophysical properties were investigated. The results show that the geometry of SiC reinforcement has a significant impact on the thermophysical properties of the composites. Continuous SiC reinforced aluminum composites (3DSiC/Al) have higher thermal conductivities and lower coefficients of thermal expansion (CETs) than those of particulate SiC reinforced aluminum composites (SiCp /Al) with the same SiC volume fraction. The co-continuous structures of both the SiC reinforcement and the Al matrix in 3D-SiC/Al can be the reason behind this phenomena. As a result, when SiC volume fraction values were the same, 3D-SiC/Al composites will be more suitable for electronic packaging applications comparing with SiCp/Al composite. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Metal matrix composites; Thermophysical properties; Interpenetrating structure; Electronic packaging
1. Introduction SiC reinforced aluminum and copper metal matrix composites have attracted broad interests for their tailored mechanical properties, designed thermophysical properties, good dimensional stability and high wear resistance, in comparison with the individual components [1–3]. These properties can make them more suitable for potential electronic packaging applications. Such types of composites can achieve high specific strength and stiffness properties, when the volume fraction of SiC is lower than 30%, allowing applications such as light weight high performance structural materials [4,5]. When the volume fraction of SiC is higher than 30%, composites can exhibit high thermal conductivity and low thermal expansion characteristics, which will be advantageous in electronic packaging applications [1,3]. Meanwhile, varying the volume fraction of SiC can allow manipulation of the thermophysical properties including CTE and thermal conductivity for the composites. Usually, particulate SiC reinforcements are used to produce these composites. n
Corresponding author. Tel.: þ86 731 84576412. E-mail address: [email protected] (S. Li).
In recent years, a new type of ceramic reinforced metal matrix composite, with a three dimensional interpenetrating network structure, has been developed due to its excellent strength, improved fracture toughness, good wear resistance, as well as high thermal shock resistance [6–9]. Unlike the particle reinforced, fiber reinforced and whisker reinforced metal matrix composites, in this kind of composites, both the reinforcement and the metal matrix exhibit a continuous structure. Consequently, the characteristics of the reinforcement and the metal matrix can be preserved simultaneously, leading to composites with the best overall performance. Since both the ceramic phase and metal phase in metal matrix composites with three dimensional interpenetrating network structure are continuous, the interfacial area between these two phases are relatively smaller than that of particle reinforced metal matrix composites with the same ceramic volume fraction. The interface has an important influence on the thermal conductive properties of the composites. The more interface there is, the larger the interfacial thermal resistance will be. For this reason, the effect of interface on the thermal conductive properties of the 3D-SiC/Al will be much weaker than that of SiCp /Al composites. Furthermore, the continuous SiC phase and the continuous Al phase can provide two
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.12.105
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S. Li et al. / Ceramics International 40 (2014) 7539–7544
Table 1 Physical properties of 3D-SiC and SiCp reinforcements with different porosity. No.
Type
Porosity (%)
Open pore ratio (%)
Bulk density (g/cm3)
Flexural strength (MPa)
CTE ( 10 6/1C) (20–1000 1C)
Thermal conductivity (W/(m K)) (30 1C)
1
P 3D P 3D P 3D P 3D
54 52 46 43 37 35 30 29
98.2 97.8 98.5 97.2 98.3 97.6 98.5 97.5
1.48 1.49 1.81 1.83 2.02 2.05 2.25 2.28
14.4 24.3 16.3 29.6 20.2 34.5 24.8 50.6
4.82 4.73 4.75 4.63 4.72 4.61 4.64 4.57
1.82 15.2 4.53 47.8 5.25 58.1 6.32 68.2
2 3 4
individual thermal conductive passages. Therefore, 3D-SiC/Al composites will show a higher thermal conductivity. Literatures about 3D-SiC reinforced metallic matrix composites have been reported. However, the porosity of 3D-SiC reinforcement in previous studies was always higher than 70% [6,10]. The CTEs of such composites are too large for electronic packaging materials, which always require low CTEs and high thermal conductivities to meet the requirements of high power electronic components, and the porosity of 3DSiC reinforcements meeting such requirements must be far smaller than 70% [11]. However, it is difficult to achieve 3DSiC reinforcements with a lower porosity whilst keeping a larger open pore ratio. In addition, the physical properties of 3D-SiC reinforcements alone have not been well explored previously. In this paper, we prepared 3D-SiC reinforcements with porosity lower than 45% whilst open pore ratio was larger than 97% with a relatively simple method. Following this, we employed a gas pressure infiltration method, by infiltrating the molten aluminum into the 3D-SiC reinforcements, to manufacture 3D-SiC/Al composites with different SiC volume fraction. At last, we investigated the thermophysical properties of both 3D-SiC reinforcements and 3D-SiC/Al composites, and as a contrast, the thermophysical properties of both SiCp reinforcements and SiCp/Al composites were also investigated. 2. Experimental 3D-SiC and particulate SiC reinforcements (SiCp) were prepared by sintering the green SiC bodies up to 2400 1C and 900 1C, respectively. The green SiC bodies were all prepared by pressing different grain size SiC mixtures in a mold using Polycarbosilane (PCS) and wax as binding agents. The weight proportion of PCS was about 2.5%. This technique allows changing the porosity of 3D-SiC and SiCp reinforcements to flexible values by altering either the size and proportion of SiC particles or the content of wax. 3D-SiC/Al and SiCp/Al composites were produced by the gas pressure infiltration method through infiltrating the molten aluminum (6063Al) alloy into the pores of 3D-SiC and SiCp reinforcements. During the infiltration process, the temperature of molten 6063 aluminum alloy was controlled at around 750 1C and the gas pressure was approximately 10 MPa.
CTEs of the reinforcements and the composites were examined with a heating rate of 5 1C/min using a DIL 402EP dilatometer system. Thermal conductivities of the reinforcements and composites (30 1C) were measured by the laser flash method with a Netzsch LFA447 thermal analysis. A Netzsch STA449C differential scanning calorimeter (DSC) was applied to determine the heat capacities of the composites at a heating rate of 10 1C/min. Microstructures of the composites were investigated with a scanning electron microscope (SEM). Bulk densities of the composites were measured based on Archimedes0 principle. 3. Results and discussion 3.1. Physical properties of 3D-SiC reinforcements Firstly, SiC powders with different grain sizes were mixed and then molded to SiC green bodies using PCS and wax as binding agents. After sintering up to different temperatures, 3D-SiC reinforcements and SiCp reinforcements with different porosities have been achieved, and Table 1 lists their physical properties. The two types of SiC reinforcements, made from the same green SiC bodies at 900 1C and 2400 1C respectively, show clear difference in terms of physical characteristics. Comparing with particulate SiCp reinforcements, the CTEs of 3D-SiC reinforcements are slightly lower but are all greatly improved both in the flexural strengths and thermal conductivities. The thermal conductivities of 3D-SiC are approximately ten times higher than that of SiCp. The open pore ratios of 3DSiC reinforcements and SiCp reinforcements with different porosities are both larger than 97%, which indicates that they will be suitable for the preparation of SiC reinforced aluminum metal matrix composites. When SiC particles are sintered at high temperature up to 1750 1C, there is a tendency for material transfer because of the differences in vapour pressure at various parts of the SiC particles [12]. The SiC, lying at the convex surface of the particles, is vaporized and the vapour is then condensed at the concave and flat surfaces of the particles. This process is the so called evaporation–condensation process. During such process, growth and consolidation of SiC components will occur, resulting in the subsequent growth of necks at the contact place of SiC particles, especially when fine and coarse particles
S. Li et al. / Ceramics International 40 (2014) 7539–7544
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Fig. 2. Pore size distribution of 3D-SiC reinforcements.
diameters and has irregular shapes. These particles are bonded together by a thin layer of SiC, generated through the decomposition of PCS. On the other hand, for 3D-SiC reinforcements prepared by sintering at 2400 1C (Fig. 1(b)), smaller SiC particles almost disappear, and the edge of the SiC particles become smooth. The particles in this case are connected during the vaporization–condensation process. As a result, SiC reinforcements with continuous network structure are achieved (Fig. 1(c)). The pore size distribution of 3D-SiC reinforcements with porosity of 44%, 37% and 30% have been tested with the help of mercury porosimeter, and the results are shown in Fig. 2 respectively. As shown in Fig. 2, the average pore sizes of 3DSiC reinforcements decrease with the reduction of porosity. The average pore sizes of 3D-SiC reinforcements with porosity of 44%, 37% and 30% are about 10 μm, 7.1 μm and 2.5 μm respectively.
3.2. Thermal conductivities of 3D-SiC/Al and SiCp /Al composites
Fig. 1. SEM micrographs of SiCp and 3D-SiC reinforcements: SiCp reinforcement (a) and 3D-SiC reinforcement (b) and (c).
are both presented in the green body [13,14]. This strongly connected SiC will show a continuous network structure (3DSiC), which is known as recrystallized silicon carbide [12]. Scanning electron micrographs of SiCp and 3D-SiC reinforcements are shown in Fig. 1 (samples No.2 in Table 1). As it can be seen from Fig. 1(a) clearly, SiCp reinforcements, obtained by sintering at 900 1C, is composed of SiC particles with different
3D-SiC and SiCp reinforcements with the same porosity have been prepared by adjusting the process of preparation. 3D-SiC/Al and SiCp/Al metal matrix composites with SiC volume fraction of 46%, 54%, 63% and 70% have also been prepared using the gas pressure infiltration method mentioned earlier. Fig. 3 shows the thermal conductivities of 3D-SiC/Al and SiCp/Al composites with different SiC volume fractions. As illustrated in Fig. 3, the thermal conductivity of the composite increases initially and then drops with the increasing of SiC volume fraction. 3D-SiC/Al composites show higher thermal conductivities than SiCp/Al composites with the same SiC volume fraction. The difference becomes more obvious when the volume fraction of SiC is 54% and 63%, at which 3D-SiC/Al composites with thermal conductivities of 248.0 W/(m K) and 250.5 W/(m K) respectively are obtained. Comparing with SiCp/ Al composites with the same SiC volume fraction, significant
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Fig. 3. Thermal conductivities of 3D-SiC/Al and SiCp/Al composites with different SiC volume fractions.
improvements of about 70.2 W/(m K) and 63.7 W/(m K) are achieved. According to their attributes, the conduction of thermal energy of different materials occurs through different mechanisms. For materials such as metals and alloys, in which free electrons exist, the conduction of thermal energy occurs through the flow of free electrons. For materials in which no free electrons exist, the conduction of thermal energy occurs through the flow of phonons [15]. For SiC reinforced aluminum matrix composites, the matrix is aluminum alloy and the reinforcement is non-metallic SiC ceramic. As a result, the conduction of thermal energy occurs through the flow of both free electrons and phonons, more specifically, flow of free electrons in metal matrix and flow of phonons in non-metallic ceramic reinforcement. However, the interface between SiC and Al can scatter the free electrons and phonons during heat conduction. Generally, the larger the interfacial area there is, the greater the scattering effect will be. Therefore, the interfacial area has significant impact on the thermal conductivity of SiC reinforced aluminum matrix composites. Because the specific surface area of 3D-SiC reinforcement is smaller than that of SiCp reinforcement with the same porosity. 3D-SiC/Al composite has less interfacial area between Al and SiC, which will lead to a smaller scattering effect during the flow of free electrons and phonons when compared with SiCp/Al composite, resulting in a greater improvement of thermal conductivity. Fig. 4 shows the scanning electron micrographs of SiCp/Al and 3D-SiC/Al composites. As can be seen, only Al matrix exists as a continuous phase in SiCp/Al composites. But in 3DSiC/Al composites, both SiC and Al exist as continuous phases which means two relatively continuous channels for the conduction of thermal energy, resulting in further improvement of thermal conductivity to a certain extent. However, as the volume fraction of 3D-SiC reinforcement rises, the pore size of reinforcement declines gradually, which will increase the difficulty of infiltrating molten Al liquid into
Fig. 4. SEM micrographs of SiCp/Al and 3D-SiC/Al composites: SiCp/Al (a) and 3D-SiC/Al (b).
the reinforcements. This can reduce the continuity of the matrix Al in the composites, resulting in the reduction of thermal conductivity. 3.3. CTEs of 3D-SiC/Al and SiCp/Al composites Fig. 5 shows the CETs of 3D-SiC/Al and SiCp/Al composites with different SiC volume fractions. For both composites, with the increase of SiC volume fraction, the CTEs of the composites decrease. 3D-SiC/Al composites show relatively lower CTEs than those of SiCp/Al composites with the same SiC volume fraction. The CTEs of 3D-SiC/Al composites are 9.0 10 6/1C, 7.3 10 6/1C, 6.0 10 6/1C and 5.1 10 6/1C, when the volume fractions of 3D-SiC are at 46%, 54%, 63% and 70%, which are about 1.3 10 6/1C, 1.4 10 6/1C, 1.2 10 6/1C and 1.2 10 6/1C lower than those of SiCp/Al composites. There are mainly two major factors that can affect the overall CTE values of SiC reinforced Al metal matrix composites. The first one is the intrinsic thermal expansion properties of the SiC and Al matrix. The second one is the geometry of the SiC reinforcement. The mean CTE of SiC is about 4.7 10 6/1C
S. Li et al. / Ceramics International 40 (2014) 7539–7544
Fig. 5. CTEs of 3D-SiC/Al and SiCp/Al composites with different SiC volume fractions.
(0–1000 1C) [16], which is far smaller than that of Al (about 25.3 10 6/1C) [17]. Therefore, the introduction of low expansion SiC phase will reduce the overall CTEs of SiC reinforced Al metal matrix composites. The more volume fraction of SiC reinforcement there is, the lower the CTE of the composite will be. Since SiC particles are hard to deform, the contact surface between the particles after pressing is relatively small, resulting in a low meshing force between them. Therefore, the SiC particles are mostly bonded together by SiC layer splitting from PCS in SiCp reinforcements. However, due to a smaller contact surface, the adhesion among the particles is not strong. For this reason, the SiCp reinforcement is not continuous in nature. The Al matrix is relatively less constrained by the ceramic SiC phase in SiCp/Al composites because of the relatively weak interaction among the particles [9,11,18,19]. The SiC particles will move together with the Al matrix during thermal expansion. But for 3D-SiC/Al composites with a continuous structure, the ceramic phase can severely constrain the Al matrix [9,11,18,19]. The continuous 3D-SiC phase will restrain the movement of Al matrix during thermal expansion to a certain extent. Therefore, 3D-SiC/Al composites have relatively lower CTEs, comparing with SiCp/Al composites with the same SiC volume fraction. Studies have also illustrated that the microvoids in ceramic reinforced metal matrix composites also had a certain influence on the thermal expansion behavior of the composites [11,17,20]. In 3D-SiC/Al and SiCp/Al composites, there are three specific types of microvoids. Firstly, incomplete infiltration can cause residual microvoids, especially in the very narrow pore channels due to poor wettability between molten Al matrix and SiC reinforcement. The second type is the interfacial debonding microvoids existing at the concave surface regions of the SiC reinforcement, due to the differential shrinkage of Al matrix cooled from high processing temperature. The last type is microvoids form in the ductile Al matrix because of the residual thermal tensile stress, induced by the
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larger thermal expansion mismatch between Al matrix and SiC reinforcement during cooling down from higher processing temperature, is larger than the critical strength of Al matrix [11,17]. During thermal expansion, Al matrix will yield plastic deformation around the microvoids surface. The microvoids will relieve the local constraint and enable plastic deformation to compensate for the thermal strain mismatch between the matrix and reinforcement, thus reducing the CTEs of the composites to a certain extent. However, for SiCp/Al composites, SiC particles will move together with Al matrix during thermal expansion, so the thermal expansion is relatively insensitive to microvoids. On the other hand, for 3D-SiC/Al composites, due to the constraint of Al matrix from a continuous 3D-SiC phase, it will be more difficult for Al matrix to move during thermal expansion, so the thermal expansion is sensitive to microvoids [20]. Therefore, 3D-SiC/Al composites exhibit relatively lower CTEs than SiCp/Al composites with the same SiC volume fraction, which are more suitable for electronic packaging applications. 4. Conclusions In comparison with SiCp/Al composites with the same volume fraction of SiC, remarkable improvements of thermal conductivity and CTE are obtained in 3D-SiC/Al composites. This observation implies that the geometry of SiC reinforcement has greatly influenced the thermophysical properties of the composites. The improved thermophysical properties are attributed to the co-continuous structure of SiC reinforcement and Al matrix in 3D-SiC/Al composites. Larger thermal conductivities and lower CTEs have been achieved, suggesting 3D-SiC/Al composites will be more suitable for electronic packaging applications compared to the SiCp/Al composite with the same SiC volume fraction. References [1] Q. Zhang, X.Y. Ma, G.H. Wu, Interfacial microstructure of SiCp/Al composite produced by the pressureless infiltration technique, Ceram. Int. 39 (2013) 4893–4897. [2] M. Rodriguez Reyes, M.I. Pech Canul, J.C. Rendón Angeles, J. López Cuevas, Limiting the development of Al4C3 to prevent degradation of Al/ SiCp composites processed by pressureless infiltration, Compos. Sci. Technol. 66 (2006) 1056–1062. [3] Th. Schubert, A. Brendel, K. Schmid, Th. Koeck, Ł. Ciupiński, W. Zieliński, T. Weißgärber, B. Kieback, Interfacial design of Cu/SiC composites prepared by powder metallurgy for heat sink applications, Compos. Part A 38 (2007) 2398–2403. [4] A.A. El-Daly, M. Abdelhameed, M. Hashish, A.M. Eid, Synthesis of Al/ SiC nanocomposite and evaluation of its mechanical properties using pulse echo overlap method, J. Alloys Compd. 542 (2012) 51–58. [5] J.C. Romero, L. Wang, R.J. Arsenault, Interfacial structure of a SiC/Al composite, Mater. Sci. Eng. A 212 (1996) 1–5. [6] J.C. Chen, C.Y. Hao, J.S. Zhang, Fabrication of 3D-SiC network reinforced aluminum-matrix composites by pressureless infiltration, Mater. Lett. 60 (2006) 2489–2492. [7] A. Mattern, B. Huchler, D. Staudenecker, R. Oberacker, A. Nagel, M.J. Hoffmann, Preparation of interpenetrating ceramic–metal composites, J. Eur. Ceram. Soc. 24 (2004) 3399–3408. [8] M.L. Young, R. Rao, J.D. Almer, D.R. Haeffner, J.A. Lewis, D.C. Dunand, Effect of ceramic preform geometry on load partitioning
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[15] R.E. Hummel, Electronic Properties of Materials, 4th edition, Springer Scienceþ Business Media, New York, 2011, p. 406. [16] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to ceramics, 2nd ed., Wiley-Interscience, New York; Chichester, 1976, p. 595. [17] D.K. Balch, T.J. Fitzgerald, V.J. Michaud, A. Mortensen, Y.L. Shen, S. Suresh, Thermal expansion of metals reinforced with ceramic particles and microcellular foams, Metall. Mater. Trans. A 27 (1996) 3700–3717. [18] S. Skirl, M. Hoffman, K. Bowman, S. Wiederhorn, J. Rodel, Thermal expansion behavior and macrostrain of Al2O3/Al composites with interpenetrating networks, Acta Mater. 46 (1998) 2493–2499. [19] P. Agrawal, K. Conlon, K.J. Bowman, C.T. Sun, F.R. Cichocki Jr, K.P. Trumble, Thermal residual stresses in co-continuous composites, Acta Mater. 51 (2003) 1143–1156. [20] Y.L. Shen, Combined effects of microvoids and phase contiguity on the thermal expansion of metal–ceramic composites, Mater. Sci. Eng. A 237 (1997) 102–108.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 7539–7544 www.elsevier.com/locate/ceramint
Thermophysical properties of SiC/Al composites with three dimensional interpenetrating network structure Shun Lin, Degan Xiong, Meng Liu, Shuxin Bai, Xun Zhao Department of Materials Sicence and Engineering, National University of Defense Technology, Changsha 410073, China Received 25 November 2013; received in revised form 18 December 2013; accepted 23 December 2013 Available online 31 December 2013
Abstract Silicon carbide (SiC) reinforced aluminum composites with three dimensional interpenetrating network structure (3D-SiC/Al) were fabricated by the gas pressure infiltration method, and their thermophysical properties were investigated. The results show that the geometry of SiC reinforcement has a significant impact on the thermophysical properties of the composites. Continuous SiC reinforced aluminum composites (3DSiC/Al) have higher thermal conductivities and lower coefficients of thermal expansion (CETs) than those of particulate SiC reinforced aluminum composites (SiCp /Al) with the same SiC volume fraction. The co-continuous structures of both the SiC reinforcement and the Al matrix in 3D-SiC/Al can be the reason behind this phenomena. As a result, when SiC volume fraction values were the same, 3D-SiC/Al composites will be more suitable for electronic packaging applications comparing with SiCp/Al composite. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Metal matrix composites; Thermophysical properties; Interpenetrating structure; Electronic packaging
1. Introduction SiC reinforced aluminum and copper metal matrix composites have attracted broad interests for their tailored mechanical properties, designed thermophysical properties, good dimensional stability and high wear resistance, in comparison with the individual components [1–3]. These properties can make them more suitable for potential electronic packaging applications. Such types of composites can achieve high specific strength and stiffness properties, when the volume fraction of SiC is lower than 30%, allowing applications such as light weight high performance structural materials [4,5]. When the volume fraction of SiC is higher than 30%, composites can exhibit high thermal conductivity and low thermal expansion characteristics, which will be advantageous in electronic packaging applications [1,3]. Meanwhile, varying the volume fraction of SiC can allow manipulation of the thermophysical properties including CTE and thermal conductivity for the composites. Usually, particulate SiC reinforcements are used to produce these composites. n
Corresponding author. Tel.: þ86 731 84576412. E-mail address: [email protected] (S. Li).
In recent years, a new type of ceramic reinforced metal matrix composite, with a three dimensional interpenetrating network structure, has been developed due to its excellent strength, improved fracture toughness, good wear resistance, as well as high thermal shock resistance [6–9]. Unlike the particle reinforced, fiber reinforced and whisker reinforced metal matrix composites, in this kind of composites, both the reinforcement and the metal matrix exhibit a continuous structure. Consequently, the characteristics of the reinforcement and the metal matrix can be preserved simultaneously, leading to composites with the best overall performance. Since both the ceramic phase and metal phase in metal matrix composites with three dimensional interpenetrating network structure are continuous, the interfacial area between these two phases are relatively smaller than that of particle reinforced metal matrix composites with the same ceramic volume fraction. The interface has an important influence on the thermal conductive properties of the composites. The more interface there is, the larger the interfacial thermal resistance will be. For this reason, the effect of interface on the thermal conductive properties of the 3D-SiC/Al will be much weaker than that of SiCp /Al composites. Furthermore, the continuous SiC phase and the continuous Al phase can provide two
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.12.105
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S. Li et al. / Ceramics International 40 (2014) 7539–7544
Table 1 Physical properties of 3D-SiC and SiCp reinforcements with different porosity. No.
Type
Porosity (%)
Open pore ratio (%)
Bulk density (g/cm3)
Flexural strength (MPa)
CTE ( 10 6/1C) (20–1000 1C)
Thermal conductivity (W/(m K)) (30 1C)
1
P 3D P 3D P 3D P 3D
54 52 46 43 37 35 30 29
98.2 97.8 98.5 97.2 98.3 97.6 98.5 97.5
1.48 1.49 1.81 1.83 2.02 2.05 2.25 2.28
14.4 24.3 16.3 29.6 20.2 34.5 24.8 50.6
4.82 4.73 4.75 4.63 4.72 4.61 4.64 4.57
1.82 15.2 4.53 47.8 5.25 58.1 6.32 68.2
2 3 4
individual thermal conductive passages. Therefore, 3D-SiC/Al composites will show a higher thermal conductivity. Literatures about 3D-SiC reinforced metallic matrix composites have been reported. However, the porosity of 3D-SiC reinforcement in previous studies was always higher than 70% [6,10]. The CTEs of such composites are too large for electronic packaging materials, which always require low CTEs and high thermal conductivities to meet the requirements of high power electronic components, and the porosity of 3DSiC reinforcements meeting such requirements must be far smaller than 70% [11]. However, it is difficult to achieve 3DSiC reinforcements with a lower porosity whilst keeping a larger open pore ratio. In addition, the physical properties of 3D-SiC reinforcements alone have not been well explored previously. In this paper, we prepared 3D-SiC reinforcements with porosity lower than 45% whilst open pore ratio was larger than 97% with a relatively simple method. Following this, we employed a gas pressure infiltration method, by infiltrating the molten aluminum into the 3D-SiC reinforcements, to manufacture 3D-SiC/Al composites with different SiC volume fraction. At last, we investigated the thermophysical properties of both 3D-SiC reinforcements and 3D-SiC/Al composites, and as a contrast, the thermophysical properties of both SiCp reinforcements and SiCp/Al composites were also investigated. 2. Experimental 3D-SiC and particulate SiC reinforcements (SiCp) were prepared by sintering the green SiC bodies up to 2400 1C and 900 1C, respectively. The green SiC bodies were all prepared by pressing different grain size SiC mixtures in a mold using Polycarbosilane (PCS) and wax as binding agents. The weight proportion of PCS was about 2.5%. This technique allows changing the porosity of 3D-SiC and SiCp reinforcements to flexible values by altering either the size and proportion of SiC particles or the content of wax. 3D-SiC/Al and SiCp/Al composites were produced by the gas pressure infiltration method through infiltrating the molten aluminum (6063Al) alloy into the pores of 3D-SiC and SiCp reinforcements. During the infiltration process, the temperature of molten 6063 aluminum alloy was controlled at around 750 1C and the gas pressure was approximately 10 MPa.
CTEs of the reinforcements and the composites were examined with a heating rate of 5 1C/min using a DIL 402EP dilatometer system. Thermal conductivities of the reinforcements and composites (30 1C) were measured by the laser flash method with a Netzsch LFA447 thermal analysis. A Netzsch STA449C differential scanning calorimeter (DSC) was applied to determine the heat capacities of the composites at a heating rate of 10 1C/min. Microstructures of the composites were investigated with a scanning electron microscope (SEM). Bulk densities of the composites were measured based on Archimedes0 principle. 3. Results and discussion 3.1. Physical properties of 3D-SiC reinforcements Firstly, SiC powders with different grain sizes were mixed and then molded to SiC green bodies using PCS and wax as binding agents. After sintering up to different temperatures, 3D-SiC reinforcements and SiCp reinforcements with different porosities have been achieved, and Table 1 lists their physical properties. The two types of SiC reinforcements, made from the same green SiC bodies at 900 1C and 2400 1C respectively, show clear difference in terms of physical characteristics. Comparing with particulate SiCp reinforcements, the CTEs of 3D-SiC reinforcements are slightly lower but are all greatly improved both in the flexural strengths and thermal conductivities. The thermal conductivities of 3D-SiC are approximately ten times higher than that of SiCp. The open pore ratios of 3DSiC reinforcements and SiCp reinforcements with different porosities are both larger than 97%, which indicates that they will be suitable for the preparation of SiC reinforced aluminum metal matrix composites. When SiC particles are sintered at high temperature up to 1750 1C, there is a tendency for material transfer because of the differences in vapour pressure at various parts of the SiC particles [12]. The SiC, lying at the convex surface of the particles, is vaporized and the vapour is then condensed at the concave and flat surfaces of the particles. This process is the so called evaporation–condensation process. During such process, growth and consolidation of SiC components will occur, resulting in the subsequent growth of necks at the contact place of SiC particles, especially when fine and coarse particles
S. Li et al. / Ceramics International 40 (2014) 7539–7544
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Fig. 2. Pore size distribution of 3D-SiC reinforcements.
diameters and has irregular shapes. These particles are bonded together by a thin layer of SiC, generated through the decomposition of PCS. On the other hand, for 3D-SiC reinforcements prepared by sintering at 2400 1C (Fig. 1(b)), smaller SiC particles almost disappear, and the edge of the SiC particles become smooth. The particles in this case are connected during the vaporization–condensation process. As a result, SiC reinforcements with continuous network structure are achieved (Fig. 1(c)). The pore size distribution of 3D-SiC reinforcements with porosity of 44%, 37% and 30% have been tested with the help of mercury porosimeter, and the results are shown in Fig. 2 respectively. As shown in Fig. 2, the average pore sizes of 3DSiC reinforcements decrease with the reduction of porosity. The average pore sizes of 3D-SiC reinforcements with porosity of 44%, 37% and 30% are about 10 μm, 7.1 μm and 2.5 μm respectively.
3.2. Thermal conductivities of 3D-SiC/Al and SiCp /Al composites
Fig. 1. SEM micrographs of SiCp and 3D-SiC reinforcements: SiCp reinforcement (a) and 3D-SiC reinforcement (b) and (c).
are both presented in the green body [13,14]. This strongly connected SiC will show a continuous network structure (3DSiC), which is known as recrystallized silicon carbide [12]. Scanning electron micrographs of SiCp and 3D-SiC reinforcements are shown in Fig. 1 (samples No.2 in Table 1). As it can be seen from Fig. 1(a) clearly, SiCp reinforcements, obtained by sintering at 900 1C, is composed of SiC particles with different
3D-SiC and SiCp reinforcements with the same porosity have been prepared by adjusting the process of preparation. 3D-SiC/Al and SiCp/Al metal matrix composites with SiC volume fraction of 46%, 54%, 63% and 70% have also been prepared using the gas pressure infiltration method mentioned earlier. Fig. 3 shows the thermal conductivities of 3D-SiC/Al and SiCp/Al composites with different SiC volume fractions. As illustrated in Fig. 3, the thermal conductivity of the composite increases initially and then drops with the increasing of SiC volume fraction. 3D-SiC/Al composites show higher thermal conductivities than SiCp/Al composites with the same SiC volume fraction. The difference becomes more obvious when the volume fraction of SiC is 54% and 63%, at which 3D-SiC/Al composites with thermal conductivities of 248.0 W/(m K) and 250.5 W/(m K) respectively are obtained. Comparing with SiCp/ Al composites with the same SiC volume fraction, significant
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Fig. 3. Thermal conductivities of 3D-SiC/Al and SiCp/Al composites with different SiC volume fractions.
improvements of about 70.2 W/(m K) and 63.7 W/(m K) are achieved. According to their attributes, the conduction of thermal energy of different materials occurs through different mechanisms. For materials such as metals and alloys, in which free electrons exist, the conduction of thermal energy occurs through the flow of free electrons. For materials in which no free electrons exist, the conduction of thermal energy occurs through the flow of phonons [15]. For SiC reinforced aluminum matrix composites, the matrix is aluminum alloy and the reinforcement is non-metallic SiC ceramic. As a result, the conduction of thermal energy occurs through the flow of both free electrons and phonons, more specifically, flow of free electrons in metal matrix and flow of phonons in non-metallic ceramic reinforcement. However, the interface between SiC and Al can scatter the free electrons and phonons during heat conduction. Generally, the larger the interfacial area there is, the greater the scattering effect will be. Therefore, the interfacial area has significant impact on the thermal conductivity of SiC reinforced aluminum matrix composites. Because the specific surface area of 3D-SiC reinforcement is smaller than that of SiCp reinforcement with the same porosity. 3D-SiC/Al composite has less interfacial area between Al and SiC, which will lead to a smaller scattering effect during the flow of free electrons and phonons when compared with SiCp/Al composite, resulting in a greater improvement of thermal conductivity. Fig. 4 shows the scanning electron micrographs of SiCp/Al and 3D-SiC/Al composites. As can be seen, only Al matrix exists as a continuous phase in SiCp/Al composites. But in 3DSiC/Al composites, both SiC and Al exist as continuous phases which means two relatively continuous channels for the conduction of thermal energy, resulting in further improvement of thermal conductivity to a certain extent. However, as the volume fraction of 3D-SiC reinforcement rises, the pore size of reinforcement declines gradually, which will increase the difficulty of infiltrating molten Al liquid into
Fig. 4. SEM micrographs of SiCp/Al and 3D-SiC/Al composites: SiCp/Al (a) and 3D-SiC/Al (b).
the reinforcements. This can reduce the continuity of the matrix Al in the composites, resulting in the reduction of thermal conductivity. 3.3. CTEs of 3D-SiC/Al and SiCp/Al composites Fig. 5 shows the CETs of 3D-SiC/Al and SiCp/Al composites with different SiC volume fractions. For both composites, with the increase of SiC volume fraction, the CTEs of the composites decrease. 3D-SiC/Al composites show relatively lower CTEs than those of SiCp/Al composites with the same SiC volume fraction. The CTEs of 3D-SiC/Al composites are 9.0 10 6/1C, 7.3 10 6/1C, 6.0 10 6/1C and 5.1 10 6/1C, when the volume fractions of 3D-SiC are at 46%, 54%, 63% and 70%, which are about 1.3 10 6/1C, 1.4 10 6/1C, 1.2 10 6/1C and 1.2 10 6/1C lower than those of SiCp/Al composites. There are mainly two major factors that can affect the overall CTE values of SiC reinforced Al metal matrix composites. The first one is the intrinsic thermal expansion properties of the SiC and Al matrix. The second one is the geometry of the SiC reinforcement. The mean CTE of SiC is about 4.7 10 6/1C
S. Li et al. / Ceramics International 40 (2014) 7539–7544
Fig. 5. CTEs of 3D-SiC/Al and SiCp/Al composites with different SiC volume fractions.
(0–1000 1C) [16], which is far smaller than that of Al (about 25.3 10 6/1C) [17]. Therefore, the introduction of low expansion SiC phase will reduce the overall CTEs of SiC reinforced Al metal matrix composites. The more volume fraction of SiC reinforcement there is, the lower the CTE of the composite will be. Since SiC particles are hard to deform, the contact surface between the particles after pressing is relatively small, resulting in a low meshing force between them. Therefore, the SiC particles are mostly bonded together by SiC layer splitting from PCS in SiCp reinforcements. However, due to a smaller contact surface, the adhesion among the particles is not strong. For this reason, the SiCp reinforcement is not continuous in nature. The Al matrix is relatively less constrained by the ceramic SiC phase in SiCp/Al composites because of the relatively weak interaction among the particles [9,11,18,19]. The SiC particles will move together with the Al matrix during thermal expansion. But for 3D-SiC/Al composites with a continuous structure, the ceramic phase can severely constrain the Al matrix [9,11,18,19]. The continuous 3D-SiC phase will restrain the movement of Al matrix during thermal expansion to a certain extent. Therefore, 3D-SiC/Al composites have relatively lower CTEs, comparing with SiCp/Al composites with the same SiC volume fraction. Studies have also illustrated that the microvoids in ceramic reinforced metal matrix composites also had a certain influence on the thermal expansion behavior of the composites [11,17,20]. In 3D-SiC/Al and SiCp/Al composites, there are three specific types of microvoids. Firstly, incomplete infiltration can cause residual microvoids, especially in the very narrow pore channels due to poor wettability between molten Al matrix and SiC reinforcement. The second type is the interfacial debonding microvoids existing at the concave surface regions of the SiC reinforcement, due to the differential shrinkage of Al matrix cooled from high processing temperature. The last type is microvoids form in the ductile Al matrix because of the residual thermal tensile stress, induced by the
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larger thermal expansion mismatch between Al matrix and SiC reinforcement during cooling down from higher processing temperature, is larger than the critical strength of Al matrix [11,17]. During thermal expansion, Al matrix will yield plastic deformation around the microvoids surface. The microvoids will relieve the local constraint and enable plastic deformation to compensate for the thermal strain mismatch between the matrix and reinforcement, thus reducing the CTEs of the composites to a certain extent. However, for SiCp/Al composites, SiC particles will move together with Al matrix during thermal expansion, so the thermal expansion is relatively insensitive to microvoids. On the other hand, for 3D-SiC/Al composites, due to the constraint of Al matrix from a continuous 3D-SiC phase, it will be more difficult for Al matrix to move during thermal expansion, so the thermal expansion is sensitive to microvoids [20]. Therefore, 3D-SiC/Al composites exhibit relatively lower CTEs than SiCp/Al composites with the same SiC volume fraction, which are more suitable for electronic packaging applications. 4. Conclusions In comparison with SiCp/Al composites with the same volume fraction of SiC, remarkable improvements of thermal conductivity and CTE are obtained in 3D-SiC/Al composites. This observation implies that the geometry of SiC reinforcement has greatly influenced the thermophysical properties of the composites. The improved thermophysical properties are attributed to the co-continuous structure of SiC reinforcement and Al matrix in 3D-SiC/Al composites. Larger thermal conductivities and lower CTEs have been achieved, suggesting 3D-SiC/Al composites will be more suitable for electronic packaging applications compared to the SiCp/Al composite with the same SiC volume fraction. References [1] Q. Zhang, X.Y. Ma, G.H. Wu, Interfacial microstructure of SiCp/Al composite produced by the pressureless infiltration technique, Ceram. Int. 39 (2013) 4893–4897. [2] M. Rodriguez Reyes, M.I. Pech Canul, J.C. Rendón Angeles, J. López Cuevas, Limiting the development of Al4C3 to prevent degradation of Al/ SiCp composites processed by pressureless infiltration, Compos. Sci. Technol. 66 (2006) 1056–1062. [3] Th. Schubert, A. Brendel, K. Schmid, Th. Koeck, Ł. Ciupiński, W. Zieliński, T. Weißgärber, B. Kieback, Interfacial design of Cu/SiC composites prepared by powder metallurgy for heat sink applications, Compos. Part A 38 (2007) 2398–2403. [4] A.A. El-Daly, M. Abdelhameed, M. Hashish, A.M. Eid, Synthesis of Al/ SiC nanocomposite and evaluation of its mechanical properties using pulse echo overlap method, J. Alloys Compd. 542 (2012) 51–58. [5] J.C. Romero, L. Wang, R.J. Arsenault, Interfacial structure of a SiC/Al composite, Mater. Sci. Eng. A 212 (1996) 1–5. [6] J.C. Chen, C.Y. Hao, J.S. Zhang, Fabrication of 3D-SiC network reinforced aluminum-matrix composites by pressureless infiltration, Mater. Lett. 60 (2006) 2489–2492. [7] A. Mattern, B. Huchler, D. Staudenecker, R. Oberacker, A. Nagel, M.J. Hoffmann, Preparation of interpenetrating ceramic–metal composites, J. Eur. Ceram. Soc. 24 (2004) 3399–3408. [8] M.L. Young, R. Rao, J.D. Almer, D.R. Haeffner, J.A. Lewis, D.C. Dunand, Effect of ceramic preform geometry on load partitioning
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