- Email: [email protected]
Functionally Graded Dual-nanoparticulate-reinforced Aluminium Matrix Bulk Materials Fabricated by Spark Plasma Sintering
Available online at ScienceDirect
ScienceDirect J. Mater. Sci. Technol., 2014,
-(-),
1e7
Functionally Graded Dual-nanoparticulate-reinforced Aluminium Matrix Bulk Materials Fabricated by Spark Plasma Sintering Hansang Kwon1,2)*, Marc Leparoux2), Akira Kawasaki3) 1) Department of Materials System Engineering, Pukyong National University, 365 Sinsenro, 608-759 Busan, Korea 2) Empa, Swiss Federal Laboratories for Materials Science and Technology, Advanced Materials Processing, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland 3) Department of Materials Processing Engineering, Graduate School of Engineering, Tohoku University, 980-8579 Sendai, Japan [Manuscript received April 28, 2013, in revised form June 18, 2013, Available online xxx]
Functionally graded (FG) carbon nanotubes (CNT) and nano-silicon carbide (nSiC) reinforced aluminium (Al) matrix composites have been successfully fabricated using high-energy ball milling followed by solid-state spark plasma sintering processes. The CNTs were well-dispersed in the Al particles using the nSiC as a solid mixing agent. Two different types of multi-walled CNTs were used to add different amounts of CNTs in the same volume. The ball milled AleCNTenSiC and AleCNT powder mixtures were fully densified and demonstrated good adhesion with no serious microcracks and pores within an FG multilayer composite. Each layer contained different amounts of the CNTs, and the nSiC additions showed different microstructures and hardness. It is possible to control the characteristics of the FG multilayer composite through the efficient design of an AleCNTenSiC gradient layer. This concept offers a feasible approach for fabricating the dualnanoparticulate-reinforced Al matrix nanocomposites and can be applied to other scenarios such as polymer and ceramic systems. KEY WORDS: Carbon nanotubes (CNT); Silicon carbide; High-energy ball milling; Spark plasma sintering (SPS); Functionally graded materials (FGM)
1. Introduction The concept of functionally graded materials (FGM) was suggested in the early 1980s from Japan[1e3]. This concept states that it is possible to control the different properties at each layer within a bulk material by varying the composition design. The coating process can also control the surface properties of materials, but generally, the final coat must be within the micrometer range[4]. One of the common problems with coated materials is that the coating layer and matrix easily undergo delamination under the stress that arises from the environment due to their difference in physical and chemical properties[5,6]. In the case of FGMs, it is possible to reduce delamination because each layer of different compositions within a bulk can be recognised as the same unit by efficient and smooth control of the composition over a range of dozens of centimetres. Since the discovery of Corresponding author. Ph.D; Tel.: þ82 51 629 6383; Fax: þ82 51 629 6373; E-mail address: [email protected] (H. Kwon). 1005-0302/$ e see front matter Copyright Ó 2014, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2014.03.003 *
carbon nanotubes (CNTs) in 1991[7], they have received significant attention due to their remarkable physical and chemical properties[8e10]. However, production of CNTs reinforced materials for commercialised industrial parts remains distant due to the lack of a suitable fabrication process, control of the interface properties between the CNT and matrix, and homogeneous dispersion of the CNT in the matrix materials[11e14]. Many methods have been applied to overcome these problems, and some solutions are being discovered[15e20]. Recently, an efficient method for CNT dispersion in aluminium (Al) powders was proposed, using a nano-SiC (nSiC) particle as a mixing agent during a ball milling process[21]. It seemed to be physically easy for the nearly spherically shaped nSiC to infiltrate the agglomerated line shape CNTs in the Al powder, resulting in better dispersity. In this study, we fabricated four functionally graded (FG), AleCNTenSiC multilayer reinforced bulk materials with various composition layers dispersed using nSiC particles. In particular, the use of nSiC as the dispersion agent was expected to yield synergistic effects due to its nanosize particle distribution, such as fine particle dispersion strengthening. Spark plasma sintering (SPS) was utilised for fabrication of the FG dualnanoparticulate-reinforced multilayer bulk materials, and
Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
2
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
samples obtained from SPS were analysed based on their microstructure and micro-hardness properties. 2. Experimental Two types of multi-walled CNTs (Baytubes C150P, Bayer Material Science, purity 99.5%, mean diameter of 20 nm, length of 10 mm and Hodogaya Chemical Co. Ltd. purity 99.5%, mean diameter of 100 nm, length of 20 mm), gas-atomised pure Al powder (ECKA Granules, purity 99.5%, particle size below 63 mm) and nano-silicon carbide (nSiC) were used as starting materials. The smaller size CNT (20 nm) was selected in order to add more CNTs in a large volume per unit. The nSiC particles used as the mixing agent were produced in an inductively coupled plasma (ICP) reactor by a process described in detail elsewhere[22], producing an average particle size between 20 and 30 nm. The Al powder and average size (100 nm) CNTs with an nSiC mixing agent were mixed in a planetary ball mill (Retsch GmbH, PM400) for 3 h under an argon atmosphere at 360 r/min using Ø10 mm balls at a 10:1 ball to powder weight ratio and 20 wt% heptane as a process control agent. Two compositions of Ale10 vol.% CNTe30 vol.% nSiC and Ale30 vol.% CNTe 10 vol.% nSiC were prepared. A fixed 10 vol.% of CNTs with a mean diameter of 20 nm was also prepared under the same conditions. At the end of the process, the powder blend was transferred to a glove-box with a controlled, inert atmosphere of argon. After passivation in the glove-box, the powders were assembled in a layered structure inside a die of 15 mm in diameter, with compositions ranging from 10 vol.% CNT, pure Al, Ale10 vol.% CNT with 30 vol.% nSiC and Ale30 vol.% CNT with 10 vol.% nSiC, followed by treatment with a spark plasma sintering device (SPS-S515) manufactured by Sumitomo Coal Mining Co. Ltd.
-(-),
1e7
The sintering conditions were: a maximum temperature of 600 C, a holding time of 20 min, a heating rate of 40 C/min, and a pressure of 50 MPa. The resulting functionally graded (FG) composites had a diameter of 15 mm and a thickness of approximately 10e20 mm depending on the number of layers. The density of the FG-composites was measured by the Archimedes’ principle according to ISO 3369:1975. The microVickers hardness of the FG-composites was measured according to EN ISO 6507-1 with loads of 20 and 0.02 kg for 15 s (220, GNEHM Härteprüfer AG and Paar MTH4 microhardness-tester). At least five measurements were made per sample. The microstructure of the composites was observed by highresolution cold field emission scanning electron microscopy (Hitachi, HRCFE-SEM S-4800) and high-resolution transmission electron microscopy (HR-TEM, Hitachi, Japan) using selected-area diffraction patterns (SADP). X-ray diffraction (XRD) patterns were measured using an X’Pert Pro diffractometer (PANAlytical) with a Cu-Ka radiation source (l ¼ 0.15148 nm, 35 kV and 40 mA) in the 2q range of 20 e80 using a linear detector (X’Celerator). A step size of 0.02 and a scan rate of 0.05 /s were used. The crystallite size was calculated by the Scherrer equation[23]. Raman spectroscopy was performed using a red He-Ne ion laser with a wavelength of 633 nm (Leica) to evaluate the disorder in the CNTs. 3. Results and Discussion FE-SEM micrograph of the raw Al particles shows irregular shape and several size distributions (Fig. 1(a)). The nSiC mixing agent shows an aspect ratio close to 1 and an overall spherical morphology (Fig. 1(b)). The CNTs of 20 nm in mean diameter were extremely agglomerated, and the thickness of one-side of the multi-wall was approximately 2 nm, as shown in Fig. 1(c).
Fig. 1 FE-SEM micrographs of (a) raw Al particles, (b) nSiC, and CNTs with a mean diameter of (c) 20 nm and (d) 100 nm. Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
-(-),
1e7
3
Fig. 2 FE-SEM micrographs of the ball milled (aec) Ale10 vol.% CNT, (def) Ale10 vol.% CNTe30 vol.% nSiC, and (gei) Ale30 vol.% CNTe 10 vol.% nSiC.
The surfaces of several CNTs were covered by an amorphous impurity (black arrow in Fig. 1(c)). The CNTs with a mean diameter of 100 nm had a highly crystalline, thick-walled structure with several different sizes, but some parts showed disordered regions (black arrow) as shown in Fig. 1(d). The ball milled composite powders showed several particle size distributions, as indicated in Fig. 2. Some of the coarse Al particles were formed by re-agglomeration of the fine milled particles (Fig. 2(a) and (b)). Overall, it was very difficult to observe the CNTs after the ball milling process in the mixture powder containing 10 vol.% Al and CNTs with a mean diameter of 20 nm, but some of the cut and trapped CNTs were also observed on the Al particle, as shown by the white arrow in Fig. 2(c). In the case of the Ale10 vol.% CNTe30 vol.% nSiC composite powder, the Al particles were finer after the ball milling process and the CNTs (mean diameter of 100 nm) were welldispersed on Al particles with the nSiC mixing agent, as shown in Fig. 2(def) (white arrow). It is estimated that the milling energy and lubrication effect were higher than the composite powder with only CNTs added. Many larger Al particles were observed in the cases of high and low amounts of CNTs, and nSiC added composite powders when compared to the raw Al particles, Ale10 vol.% CNT and Ale10 vol.% CNT with 30 vol.% nSiC composite powders, as shown in Fig. 2(g). Carbon allotropes are often used as lubricant (solid, liquid, and pastes) and CNT is one of the carbon allotrope[24]. Because of this reason, the lubricant effect could increase with increasing the CNTs additions and maybe produce better quality of fine particles. However, despite expectation of fine particles size, the
high amount of CNT added composite particles were rather bigger than the small amount of CNT added one. This phenomenon occurred by re-agglomeration of the produced fine particles during the ball milling process, which means that the lubrication effect by the CNT also functioned as an efficient process control agent leading to fine particle formation. However, the lubrication effect of CNT during the ball milling process should be carefully investigated. Moreover, cold-welding also partially contributed to grain growth in spite of using process control agent of heptane. Most of the Al particle was fully surrounded by well-dispersed CNTs, and the CNTs were very easy to observe, almost retaining their original morphology (Fig. 2(h) and (i)). However, the CNTs were well-dispersed regardless of the amount of nSiC added (at least in the AleCNTenSiC system under the tested ball milling condition). Upon further examination with TEM, we could more clearly observe the CNTs in the Ale10 vol.% CNT composite powder, as shown by the black arrow in Fig. 3(a). Some of the observed CNTs became shorter in length than the raw CNT due to breakdown and became partially implanted onto soft Al particles during the ball milling process. A large amount of the CNTs were well-dispersed in the Al particles with the nSiC (Fig. 3(b) and (c)), despite the existence of several nSiC clusters (approximately 300 nm) as shown in the white circle in Fig. 3(b). In the case of the 10 vol.% nSiC added composite powder, very few of the nSiC clusters were observed (Fig. 3(c)). We believe that the nSiC infiltrated the CNT agglomerated during the ball milling process and the nSiC acted as a physical nano-ball, resulting in increased dispersity. However, it is necessary to
Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
4
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
-(-),
1e7
Fig. 3 TEM micrographs of the ball milled (a) Ale10 vol.% CNT, (b) Ale10 vol.% CNTe30 vol.% nSiC, and (c) Ale30 vol.% CNTe10 vol.% nSiC.
carefully investigate the effect of nSiC particles in order to use them as a mixing agent for CNT dispersion. The composite powders were fully densified after the spark plasma sintering (SPS) process, regardless of the composition as indicated in Table 1. The Vickers hardness of the composite increased up to approximately four times higher than that of pure Al (Table 1). The nSiC was the main contributor to increasing the hardness when compared with the CNT because the 30 vol.% nSiC added sample had the highest hardness value. This finding suggested that the nSiC not only functioned as a dispersion medium but also played an important role in hardness. Fig. 4 shows a digital image of the dual-nanoparticulatereinforced FG multilayer composite materials with various layer designs. The ball milled composite powders were well synthesised with 2, 3, and 4 layers. There was no macro pores found in the FG composite materials. We believe that the FG composite can be designed and fabricated using many types of composition and layers in our suggested system. However, the concept of the FG multilayer composite materials may not be restricted to the Al matrix and could utilise other metal matrix systems as well. The interfaces of FG multilayers were very tightly bonded, regardless of their composition, as shown in Fig. 5(aec). Each of the layers showed different microstructures (Fig. 5(deg)). Some black dots were observed at several regions on the surface of the multilayer, but these were not pores. Because these features were not observed before the chemical etching, they were likely created during the chemical etching process. The measured density values before the etching process also supported this hypothesis, as indicated in Table 1. The FG multilayer showed different colours due to the different deflection of light. The dual reinforced multilayer had an aligned microstructure from the direction perpendicular to the pressure, as shown in Fig. 5(b) and (c). This aligned microstructure originally came from the morphology of the ball milled composite powder with a high aspect ratio of the CNTs (Fig. 3(b) and (c)). Some of the coarse
SiC particles were also observed in limited area and were derived from the SiC precursor used for production of the nSiC particles[22]. However, the interfaces between the FG multilayers were well-bonded with no serious micro-crack and pores. In other words, the SPS process is a suitable method for fabricating such multilayer bulk materials. The FG multilayer was analysed by TEM to observe nanolevel porosity and bonding. The interface between Al and Ale10 vol.% CNT showed clearly different microstructures that were tightly bonded without nano-cracks, as shown in Fig. 6. Some pores were observed in the interface between the Al and Ale10 vol.% CNT (Fig. 6(c) and (d)). Those pores were likely created during sample preparation by ion milling because they were observed near the ion-milled region and their shape showed a general, ion-milled morphology (black arrow in Fig. 6(c) and (d)). Most of the interfaces between the Al and Ale10 vol.% were covered with very thin and amorphous Al oxide films that seemed to also bond well (Fig. 6(e)). In the Ale10 vol.% CNT matrix, we observed many dispersed CNTs onto the Al matrix as well as unzipped or broken CNTs (Fig. 6(f)). Some Al carbides (Al4C3) were also observed with a similar size range of raw CNT, as shown in Fig. 6(f). It is hypothesized that the Al4C3 formed during the SPS process because the ball milled powder was in a highly unstable energy state (having high reaction potential). The temperature during SPS offered enough driving force for the reaction to occur between the Al and some of the disordered CNTs, resulting in the production of nanosized Al4C3. In general, Al4C3 is an undesirable compound in structural material because of their brittleness and hygroscopic properties[25,26]. However, if the Al4C3 is consistently nanosized with good dispersity of a small amount in the matrix, they can act as a load transfer medium between the Al matrix and the reinforcement through the chemically bonded interface[11,12]. Cho et al. also explained the effective load transfer through carbide[27], but this load transfer effect should be cautiously implemented.
Table 1 Properties of pure Al and AleCNTenSiC in functionally graded composite materials Sample
Pure Al bulk Ale10 vol.% CNT Ale10 vol.% CNTe30 vol.% nSiC Ale30 vol.% CNTe10 vol.% nSiC
Density (g/cm3) 0.01 Theoretical
Experimental
2.70 2.63 2.78 2.54
2.70 2.63 2.78 2.54
Vickers hardness (HV20) 1.2
Raman spectra ID/IG ratio
42.1 102.0 154.3 132.6
e 1.19 (Raw 20 nm CNT: 1.18) 0.94 (Raw 100 nm CNT: 0.23) 0.41
Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
-(-),
1e7
5
Fig. 4 Functionally graded dual-nanoparticulate-reinforced aluminium bulk materials with several layers.
The interface between the Al and the Ale10 vol.% CNTe 30 vol.% nSiC showed a similar behaviour as the interface between the Al and Ale10 vol.% CNT. However, the interface between dual-nanoparticulate-reinforced layers showed a more interesting microstructure. It was difficult to distinguish each side of the interface because of their perfect bonding, meaning that they were recognised as one body (Fig. 7(a)). The edge region of the interface showed a general, ion-milled sample pattern, providing evidence that supports strong interface bonding or similar properties, as shown in Fig. 7(a). Several transformed nanosized Al4C3 was observed on the Al particle (Fig. 7(b)). Some Al4C3 was also embedded across the Al matrix and connected each boundary (Fig. 7(c)). The nanotie-rod structure of Al4C3 may provide the efficient load transfer between the matrix and the reinforcement. Therefore, we investigated the unique nano-tie-rod structure effect. The CNT and nSiC mixture was located between the Al particles regardless of the composition of bulk materials, but only some nSiC regions were observed with a nanometre distribution at the boundary (Fig. 7(d) and (e)). Some surfaces of the Al particles were covered by nSiC at a thickness of 100e200 nm, as shown in the area bounded by the white dotted line in Fig. 7(e). The nSiC may have played an important role in preventing direct contact between the disordered CNT and Al particles, resulting in reduced potential reactibity, which may explain why
we could observe only a small amount of Al4C3, despite the high carbon source environment (10 and 30 vol.% CNT). Most of the CNTs at boundaries were well-dispersed with the nSiC particles, which suggest that the microstructure of the SPS layer followed the same behaviour as the composite powders. Highly deformed and nondeformed CNTs were observed in the Al matrix as shown by the black arrow in Fig. 7(f). However, the interfaces of dual-nanoparticulate-reinforced layers presented mainly the AlnSiC and CNT mixtureeAl, the AlenSiCeAl, and the Ale nSiCenSiC and CNT mixtureeAl. Defect (D) induced and graphitic (G) peaks of the raw CNT and each multilayer of the FG composite were investigated by Raman spectroscopy. The 100 nm CNT has a lower defect peak intensity compared to the 20 nm CNT, as shown in Fig. 8(a) and (b). In general, the G peak is shown in all graphite materials, and the D peak is easy to detect in defective graphite or defective CNTs. It is well known that the intensity of the D (ID)/intensity of the G (IG) ratio is a direct measure of the quality of the CNT[28]. The ID/IG ratio of 100 nm and 20 nm mean diameter CNTs were 0.23 and 1.18, respectively, as indicated in Table 1. This finding indicated that the 100 nm mean diameter CNT is of better quality than the 20 nm CNT. The Raman spectra of the FG multilayer composites were shown to have similar patterns with their raw CNTs, except in the case of a high amount of nSiC (Ale10 vol.% CNTe30 vol.%
Fig. 5 FE-SEM micrograph of the interfaces of functionally graded multilayers with various compositions. Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
6
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
-(-),
1e7
Fig. 6 TEM micrograph of the interface between Al and Ale10 vol.% CNT.
nSiC), as shown in Fig. 8(d). This may be explained by two reasons. First, the high strength of the nSiC offered relatively efficient impact energy transfer to the CNTs during the milling process when compared to samples with lower amounts of nSiC, resulting in destruction of many graphic regions on the CNT and the production of amorphous carbon impurities, which would explain why we could observe a relatively low intensity G peak (Fig. 8(d)). The second possibility is that the relative intensity of the G peak in the CNT was less exposed by the Raman laser due to the high amount of nSiC particles.
We believe that both scenarios were affected together because the ID/IG ratio also showed the same behaviour (Table 1) and these values can be explained by the aforementioned reasons. The ID/IG ratio of the 30 vol.% CNT (Fig. 8(e)) sample showed a two-fold lower value when compared to the 10 vol.% CNT sample (Fig. 8(d)), as indicated in Table 1. The G peak of Ale 10 vol.% CNT sample was slightly shifted (Fig. 8(c)) because of an existing high residual stress. According to the ID/IG ratio, the employed milling conditions were not seriously affected to the extent of defect in the 20 nm CNT, but the 100 nm mean
Fig. 7 TEM micrograph of the interface between Ale10 vol.% CNTe30 vol.% nSiC and Ale30 vol.% CNTe10 vol.% nSiC. Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
-(-),
1e7
7
the maximum hardness value was four times higher than that of pure Al when a high amount of nSiC was added to the sample. Based on the Raman analysis, the CNT functioned both as a reinforcement and as a lubricant during the ball milling process. We believe that this FG multilayer concept is not only applicable to dual-nanoparticulate-reinforced Al matrix nanocomposites but also to other scenarios, such as polymer and ceramic systems. Acknowledgements The authors acknowledge Dr. Miyazaki Takamichi and Dr. Songhak Yoon at Tohoku University, and EMPA for their technical support.
Fig. 8 Raman spectra of raw CNT and multilayer composite samples.
Fig. 9 XRD of multilayer composite materials.
diameter CNT was influenced much more. Nonetheless, the ID/IG ratios of the 100 nm CNT added samples were lower than those of the 20 nm CNT samples. The formed Al4C3 was not detected under the XRD diffraction pattern (Fig. 9) despite being observed in the TEM micrograph (Figs. 6 and 7). This is directly related to the limited resolution of the equipment because the formed Al4C3 was nanosized, and there was only a relatively small amount. On the other hand, clear Al and SiC peaks were detected, as well as a C peak, and these peaks increased proportionally with the amount of CNTs added. 4. Conclusion The fully densified functionally graded multilayer composite was successfully fabricated by ball milling and spark plasma sintering processes. The CNTs were well-dispersed onto the Al particles when using nSiC as the mixing agent, independent of the composition. Each layer was tightly bonded without any serious microcracks and pores. Some nanosized Al4C3 particles were observed in the CNT layer, but they could not be detected by XRD due to their nanosized distribution and small concentration in a limited area. Moreover, some of the formed Al4C3 was embedded across the Al matrix and connected between each boundary. This nano-tie-rod Al4C3 structure may be helpful in strengthening materials. The Vickers hardness of the FG multilayer showed different values depending on the composition, and
REFERENCES [1] M. Koizumi, Compos. Part B: Eng. 28 (1997) 1e4. [2] A. Kawasaki, R. Watanabe, Eng. Frac. Mech. 69 (2002) 1713e1728. [3] H. Kwon, C.R. Bradbury, M. Leparoux, Adv. Eng. Mater. 13 (2011) 325e329. [4] D. Pinheiro, M.T. Vieira, M.A. Djouadi, Surf. Coat. Technol. 203 (2009) 3197e3205. [5] A.G. Evans, J.W. Hutchinson, Surf. Coat. Technol. 201 (2007) 7905e7916. [6] U. Wiklund, J. Gunnars, S. Hogmark, Wear 232 (1999) 262e269. [7] S. Ijima, Nature 354 (1991) 56e58. [8] K.P. So, C. Biswas, S.C. Lim, K.H. An, Y.H. Lee, Synth. Met. 161 (2011) 208e212. [9] J. Tersoff, R.S. Ruoff, Phys. Rev. Lett. 73 (1994) 676e679. [10] A. Mathur, S. Wadhwa, M. Tweedie, K.S. Hazra, C. Dickinson, S.S. Roy, S.K. Mitra, D.S. Misra, J.A. McLaughlin, Physica E: Low-dimen. Syst. Nanostruc. 44 (2011) 29e36. [11] H. Kwon, M. Estili, K. Takagi, T. Miyazaki, A. Kawasaki, Carbon 47 (2009) 570e577. [12] H. Kwon, D. Park, J.F. Silvain, A. Kawasaki, Compos. Sci. Technol. 70 (2010) 546e550. [13] H. Kwon, A. Kawasaki, J. Nanosci. Nanotechnol. 9 (2009) 6542e6548. [14] A.M.K. Esawi, K. Morsi, Compos. Part A 38 (2007) 646e650. [15] H. Kurita, H. Kwon, M. Estili, A. Kawasaki, Mater. Trans. 52 (2011) 1960e1965. [16] A.M.K. Esawi, K. Morsi, A. Sayed, M. Taher, S. Lanka, Compos. Sci. Technol. 70 (2010) 2237e2241. [17] T. Laha, S. Kuchibhatla, S. Seal, W. Li, A. Agarwal, Acta Mater. 55 (2007) 1059e1066. [18] S.R. Bakshi, D. Lahiri, A. Agarwal, Int. Mater. Rev. 55 (2010) 41e64. [19] C.F. Deng, D.Z. Wang, X.X. Zhang, A.B. Li, Mater. Sci. Eng. A 444 (2007) 138e145. [20] S. Cho, K. Takagi, H. Kwon, D. Seo, K. Ogawa, K. Kikuchi, A. Kawasaki, Surf. Coat. Technol. 206 (2012) 3488e3494. [21] H. Kwon, S. Cho, M. Leparoux, A. Kawasaki, Nanotechnology 23 (2012) 225704. [22] Y. Leconte, M. Leparoux, X. Portier, N. Herlin-Boime, Plasma Chem. Plasma Process. 28 (2008) 233e248. [23] A.W. Burton, K. Ong, T. Rea, I.Y. Chan, Micropor. Mesopor. Mater. 117 (2009) 75e90. [24] C. Espejo, F.J. Carrion, D. Martinez, M.D. Bermudez, Tribol. Lett. 50 (2013) 127e136. [25] T. Etter, P. Schulz, M. Weber, J. Metz, M. Wimmler, J.F. Löffler, P.J. Uggowitzer, Mater. Sci. Eng. A 448 (2007) 1e6. [26] V.L. Solozhenko, O.O. Kurakevych, Solid State Commun. 133 (2005) 385e388. [27] S. Cho, K. Kikuchi, A. Kawasaki, H. Kwon, Y. Kim, Nanotechnology 33 (2012) 315705. [28] M.M. Lucchese, F. Stavale, E.H. Martins-Ferreira, C. Vilani, M. V.O. Moutinho, R.B. Capaz, C.A. Achete, A. Jorio, Carbon 48 (2010) 1592e1597.
Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
ScienceDirect J. Mater. Sci. Technol., 2014,
-(-),
1e7
Functionally Graded Dual-nanoparticulate-reinforced Aluminium Matrix Bulk Materials Fabricated by Spark Plasma Sintering Hansang Kwon1,2)*, Marc Leparoux2), Akira Kawasaki3) 1) Department of Materials System Engineering, Pukyong National University, 365 Sinsenro, 608-759 Busan, Korea 2) Empa, Swiss Federal Laboratories for Materials Science and Technology, Advanced Materials Processing, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland 3) Department of Materials Processing Engineering, Graduate School of Engineering, Tohoku University, 980-8579 Sendai, Japan [Manuscript received April 28, 2013, in revised form June 18, 2013, Available online xxx]
Functionally graded (FG) carbon nanotubes (CNT) and nano-silicon carbide (nSiC) reinforced aluminium (Al) matrix composites have been successfully fabricated using high-energy ball milling followed by solid-state spark plasma sintering processes. The CNTs were well-dispersed in the Al particles using the nSiC as a solid mixing agent. Two different types of multi-walled CNTs were used to add different amounts of CNTs in the same volume. The ball milled AleCNTenSiC and AleCNT powder mixtures were fully densified and demonstrated good adhesion with no serious microcracks and pores within an FG multilayer composite. Each layer contained different amounts of the CNTs, and the nSiC additions showed different microstructures and hardness. It is possible to control the characteristics of the FG multilayer composite through the efficient design of an AleCNTenSiC gradient layer. This concept offers a feasible approach for fabricating the dualnanoparticulate-reinforced Al matrix nanocomposites and can be applied to other scenarios such as polymer and ceramic systems. KEY WORDS: Carbon nanotubes (CNT); Silicon carbide; High-energy ball milling; Spark plasma sintering (SPS); Functionally graded materials (FGM)
1. Introduction The concept of functionally graded materials (FGM) was suggested in the early 1980s from Japan[1e3]. This concept states that it is possible to control the different properties at each layer within a bulk material by varying the composition design. The coating process can also control the surface properties of materials, but generally, the final coat must be within the micrometer range[4]. One of the common problems with coated materials is that the coating layer and matrix easily undergo delamination under the stress that arises from the environment due to their difference in physical and chemical properties[5,6]. In the case of FGMs, it is possible to reduce delamination because each layer of different compositions within a bulk can be recognised as the same unit by efficient and smooth control of the composition over a range of dozens of centimetres. Since the discovery of Corresponding author. Ph.D; Tel.: þ82 51 629 6383; Fax: þ82 51 629 6373; E-mail address: [email protected] (H. Kwon). 1005-0302/$ e see front matter Copyright Ó 2014, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2014.03.003 *
carbon nanotubes (CNTs) in 1991[7], they have received significant attention due to their remarkable physical and chemical properties[8e10]. However, production of CNTs reinforced materials for commercialised industrial parts remains distant due to the lack of a suitable fabrication process, control of the interface properties between the CNT and matrix, and homogeneous dispersion of the CNT in the matrix materials[11e14]. Many methods have been applied to overcome these problems, and some solutions are being discovered[15e20]. Recently, an efficient method for CNT dispersion in aluminium (Al) powders was proposed, using a nano-SiC (nSiC) particle as a mixing agent during a ball milling process[21]. It seemed to be physically easy for the nearly spherically shaped nSiC to infiltrate the agglomerated line shape CNTs in the Al powder, resulting in better dispersity. In this study, we fabricated four functionally graded (FG), AleCNTenSiC multilayer reinforced bulk materials with various composition layers dispersed using nSiC particles. In particular, the use of nSiC as the dispersion agent was expected to yield synergistic effects due to its nanosize particle distribution, such as fine particle dispersion strengthening. Spark plasma sintering (SPS) was utilised for fabrication of the FG dualnanoparticulate-reinforced multilayer bulk materials, and
Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
2
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
samples obtained from SPS were analysed based on their microstructure and micro-hardness properties. 2. Experimental Two types of multi-walled CNTs (Baytubes C150P, Bayer Material Science, purity 99.5%, mean diameter of 20 nm, length of 10 mm and Hodogaya Chemical Co. Ltd. purity 99.5%, mean diameter of 100 nm, length of 20 mm), gas-atomised pure Al powder (ECKA Granules, purity 99.5%, particle size below 63 mm) and nano-silicon carbide (nSiC) were used as starting materials. The smaller size CNT (20 nm) was selected in order to add more CNTs in a large volume per unit. The nSiC particles used as the mixing agent were produced in an inductively coupled plasma (ICP) reactor by a process described in detail elsewhere[22], producing an average particle size between 20 and 30 nm. The Al powder and average size (100 nm) CNTs with an nSiC mixing agent were mixed in a planetary ball mill (Retsch GmbH, PM400) for 3 h under an argon atmosphere at 360 r/min using Ø10 mm balls at a 10:1 ball to powder weight ratio and 20 wt% heptane as a process control agent. Two compositions of Ale10 vol.% CNTe30 vol.% nSiC and Ale30 vol.% CNTe 10 vol.% nSiC were prepared. A fixed 10 vol.% of CNTs with a mean diameter of 20 nm was also prepared under the same conditions. At the end of the process, the powder blend was transferred to a glove-box with a controlled, inert atmosphere of argon. After passivation in the glove-box, the powders were assembled in a layered structure inside a die of 15 mm in diameter, with compositions ranging from 10 vol.% CNT, pure Al, Ale10 vol.% CNT with 30 vol.% nSiC and Ale30 vol.% CNT with 10 vol.% nSiC, followed by treatment with a spark plasma sintering device (SPS-S515) manufactured by Sumitomo Coal Mining Co. Ltd.
-(-),
1e7
The sintering conditions were: a maximum temperature of 600 C, a holding time of 20 min, a heating rate of 40 C/min, and a pressure of 50 MPa. The resulting functionally graded (FG) composites had a diameter of 15 mm and a thickness of approximately 10e20 mm depending on the number of layers. The density of the FG-composites was measured by the Archimedes’ principle according to ISO 3369:1975. The microVickers hardness of the FG-composites was measured according to EN ISO 6507-1 with loads of 20 and 0.02 kg for 15 s (220, GNEHM Härteprüfer AG and Paar MTH4 microhardness-tester). At least five measurements were made per sample. The microstructure of the composites was observed by highresolution cold field emission scanning electron microscopy (Hitachi, HRCFE-SEM S-4800) and high-resolution transmission electron microscopy (HR-TEM, Hitachi, Japan) using selected-area diffraction patterns (SADP). X-ray diffraction (XRD) patterns were measured using an X’Pert Pro diffractometer (PANAlytical) with a Cu-Ka radiation source (l ¼ 0.15148 nm, 35 kV and 40 mA) in the 2q range of 20 e80 using a linear detector (X’Celerator). A step size of 0.02 and a scan rate of 0.05 /s were used. The crystallite size was calculated by the Scherrer equation[23]. Raman spectroscopy was performed using a red He-Ne ion laser with a wavelength of 633 nm (Leica) to evaluate the disorder in the CNTs. 3. Results and Discussion FE-SEM micrograph of the raw Al particles shows irregular shape and several size distributions (Fig. 1(a)). The nSiC mixing agent shows an aspect ratio close to 1 and an overall spherical morphology (Fig. 1(b)). The CNTs of 20 nm in mean diameter were extremely agglomerated, and the thickness of one-side of the multi-wall was approximately 2 nm, as shown in Fig. 1(c).
Fig. 1 FE-SEM micrographs of (a) raw Al particles, (b) nSiC, and CNTs with a mean diameter of (c) 20 nm and (d) 100 nm. Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
-(-),
1e7
3
Fig. 2 FE-SEM micrographs of the ball milled (aec) Ale10 vol.% CNT, (def) Ale10 vol.% CNTe30 vol.% nSiC, and (gei) Ale30 vol.% CNTe 10 vol.% nSiC.
The surfaces of several CNTs were covered by an amorphous impurity (black arrow in Fig. 1(c)). The CNTs with a mean diameter of 100 nm had a highly crystalline, thick-walled structure with several different sizes, but some parts showed disordered regions (black arrow) as shown in Fig. 1(d). The ball milled composite powders showed several particle size distributions, as indicated in Fig. 2. Some of the coarse Al particles were formed by re-agglomeration of the fine milled particles (Fig. 2(a) and (b)). Overall, it was very difficult to observe the CNTs after the ball milling process in the mixture powder containing 10 vol.% Al and CNTs with a mean diameter of 20 nm, but some of the cut and trapped CNTs were also observed on the Al particle, as shown by the white arrow in Fig. 2(c). In the case of the Ale10 vol.% CNTe30 vol.% nSiC composite powder, the Al particles were finer after the ball milling process and the CNTs (mean diameter of 100 nm) were welldispersed on Al particles with the nSiC mixing agent, as shown in Fig. 2(def) (white arrow). It is estimated that the milling energy and lubrication effect were higher than the composite powder with only CNTs added. Many larger Al particles were observed in the cases of high and low amounts of CNTs, and nSiC added composite powders when compared to the raw Al particles, Ale10 vol.% CNT and Ale10 vol.% CNT with 30 vol.% nSiC composite powders, as shown in Fig. 2(g). Carbon allotropes are often used as lubricant (solid, liquid, and pastes) and CNT is one of the carbon allotrope[24]. Because of this reason, the lubricant effect could increase with increasing the CNTs additions and maybe produce better quality of fine particles. However, despite expectation of fine particles size, the
high amount of CNT added composite particles were rather bigger than the small amount of CNT added one. This phenomenon occurred by re-agglomeration of the produced fine particles during the ball milling process, which means that the lubrication effect by the CNT also functioned as an efficient process control agent leading to fine particle formation. However, the lubrication effect of CNT during the ball milling process should be carefully investigated. Moreover, cold-welding also partially contributed to grain growth in spite of using process control agent of heptane. Most of the Al particle was fully surrounded by well-dispersed CNTs, and the CNTs were very easy to observe, almost retaining their original morphology (Fig. 2(h) and (i)). However, the CNTs were well-dispersed regardless of the amount of nSiC added (at least in the AleCNTenSiC system under the tested ball milling condition). Upon further examination with TEM, we could more clearly observe the CNTs in the Ale10 vol.% CNT composite powder, as shown by the black arrow in Fig. 3(a). Some of the observed CNTs became shorter in length than the raw CNT due to breakdown and became partially implanted onto soft Al particles during the ball milling process. A large amount of the CNTs were well-dispersed in the Al particles with the nSiC (Fig. 3(b) and (c)), despite the existence of several nSiC clusters (approximately 300 nm) as shown in the white circle in Fig. 3(b). In the case of the 10 vol.% nSiC added composite powder, very few of the nSiC clusters were observed (Fig. 3(c)). We believe that the nSiC infiltrated the CNT agglomerated during the ball milling process and the nSiC acted as a physical nano-ball, resulting in increased dispersity. However, it is necessary to
Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
4
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
-(-),
1e7
Fig. 3 TEM micrographs of the ball milled (a) Ale10 vol.% CNT, (b) Ale10 vol.% CNTe30 vol.% nSiC, and (c) Ale30 vol.% CNTe10 vol.% nSiC.
carefully investigate the effect of nSiC particles in order to use them as a mixing agent for CNT dispersion. The composite powders were fully densified after the spark plasma sintering (SPS) process, regardless of the composition as indicated in Table 1. The Vickers hardness of the composite increased up to approximately four times higher than that of pure Al (Table 1). The nSiC was the main contributor to increasing the hardness when compared with the CNT because the 30 vol.% nSiC added sample had the highest hardness value. This finding suggested that the nSiC not only functioned as a dispersion medium but also played an important role in hardness. Fig. 4 shows a digital image of the dual-nanoparticulatereinforced FG multilayer composite materials with various layer designs. The ball milled composite powders were well synthesised with 2, 3, and 4 layers. There was no macro pores found in the FG composite materials. We believe that the FG composite can be designed and fabricated using many types of composition and layers in our suggested system. However, the concept of the FG multilayer composite materials may not be restricted to the Al matrix and could utilise other metal matrix systems as well. The interfaces of FG multilayers were very tightly bonded, regardless of their composition, as shown in Fig. 5(aec). Each of the layers showed different microstructures (Fig. 5(deg)). Some black dots were observed at several regions on the surface of the multilayer, but these were not pores. Because these features were not observed before the chemical etching, they were likely created during the chemical etching process. The measured density values before the etching process also supported this hypothesis, as indicated in Table 1. The FG multilayer showed different colours due to the different deflection of light. The dual reinforced multilayer had an aligned microstructure from the direction perpendicular to the pressure, as shown in Fig. 5(b) and (c). This aligned microstructure originally came from the morphology of the ball milled composite powder with a high aspect ratio of the CNTs (Fig. 3(b) and (c)). Some of the coarse
SiC particles were also observed in limited area and were derived from the SiC precursor used for production of the nSiC particles[22]. However, the interfaces between the FG multilayers were well-bonded with no serious micro-crack and pores. In other words, the SPS process is a suitable method for fabricating such multilayer bulk materials. The FG multilayer was analysed by TEM to observe nanolevel porosity and bonding. The interface between Al and Ale10 vol.% CNT showed clearly different microstructures that were tightly bonded without nano-cracks, as shown in Fig. 6. Some pores were observed in the interface between the Al and Ale10 vol.% CNT (Fig. 6(c) and (d)). Those pores were likely created during sample preparation by ion milling because they were observed near the ion-milled region and their shape showed a general, ion-milled morphology (black arrow in Fig. 6(c) and (d)). Most of the interfaces between the Al and Ale10 vol.% were covered with very thin and amorphous Al oxide films that seemed to also bond well (Fig. 6(e)). In the Ale10 vol.% CNT matrix, we observed many dispersed CNTs onto the Al matrix as well as unzipped or broken CNTs (Fig. 6(f)). Some Al carbides (Al4C3) were also observed with a similar size range of raw CNT, as shown in Fig. 6(f). It is hypothesized that the Al4C3 formed during the SPS process because the ball milled powder was in a highly unstable energy state (having high reaction potential). The temperature during SPS offered enough driving force for the reaction to occur between the Al and some of the disordered CNTs, resulting in the production of nanosized Al4C3. In general, Al4C3 is an undesirable compound in structural material because of their brittleness and hygroscopic properties[25,26]. However, if the Al4C3 is consistently nanosized with good dispersity of a small amount in the matrix, they can act as a load transfer medium between the Al matrix and the reinforcement through the chemically bonded interface[11,12]. Cho et al. also explained the effective load transfer through carbide[27], but this load transfer effect should be cautiously implemented.
Table 1 Properties of pure Al and AleCNTenSiC in functionally graded composite materials Sample
Pure Al bulk Ale10 vol.% CNT Ale10 vol.% CNTe30 vol.% nSiC Ale30 vol.% CNTe10 vol.% nSiC
Density (g/cm3) 0.01 Theoretical
Experimental
2.70 2.63 2.78 2.54
2.70 2.63 2.78 2.54
Vickers hardness (HV20) 1.2
Raman spectra ID/IG ratio
42.1 102.0 154.3 132.6
e 1.19 (Raw 20 nm CNT: 1.18) 0.94 (Raw 100 nm CNT: 0.23) 0.41
Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
-(-),
1e7
5
Fig. 4 Functionally graded dual-nanoparticulate-reinforced aluminium bulk materials with several layers.
The interface between the Al and the Ale10 vol.% CNTe 30 vol.% nSiC showed a similar behaviour as the interface between the Al and Ale10 vol.% CNT. However, the interface between dual-nanoparticulate-reinforced layers showed a more interesting microstructure. It was difficult to distinguish each side of the interface because of their perfect bonding, meaning that they were recognised as one body (Fig. 7(a)). The edge region of the interface showed a general, ion-milled sample pattern, providing evidence that supports strong interface bonding or similar properties, as shown in Fig. 7(a). Several transformed nanosized Al4C3 was observed on the Al particle (Fig. 7(b)). Some Al4C3 was also embedded across the Al matrix and connected each boundary (Fig. 7(c)). The nanotie-rod structure of Al4C3 may provide the efficient load transfer between the matrix and the reinforcement. Therefore, we investigated the unique nano-tie-rod structure effect. The CNT and nSiC mixture was located between the Al particles regardless of the composition of bulk materials, but only some nSiC regions were observed with a nanometre distribution at the boundary (Fig. 7(d) and (e)). Some surfaces of the Al particles were covered by nSiC at a thickness of 100e200 nm, as shown in the area bounded by the white dotted line in Fig. 7(e). The nSiC may have played an important role in preventing direct contact between the disordered CNT and Al particles, resulting in reduced potential reactibity, which may explain why
we could observe only a small amount of Al4C3, despite the high carbon source environment (10 and 30 vol.% CNT). Most of the CNTs at boundaries were well-dispersed with the nSiC particles, which suggest that the microstructure of the SPS layer followed the same behaviour as the composite powders. Highly deformed and nondeformed CNTs were observed in the Al matrix as shown by the black arrow in Fig. 7(f). However, the interfaces of dual-nanoparticulate-reinforced layers presented mainly the AlnSiC and CNT mixtureeAl, the AlenSiCeAl, and the Ale nSiCenSiC and CNT mixtureeAl. Defect (D) induced and graphitic (G) peaks of the raw CNT and each multilayer of the FG composite were investigated by Raman spectroscopy. The 100 nm CNT has a lower defect peak intensity compared to the 20 nm CNT, as shown in Fig. 8(a) and (b). In general, the G peak is shown in all graphite materials, and the D peak is easy to detect in defective graphite or defective CNTs. It is well known that the intensity of the D (ID)/intensity of the G (IG) ratio is a direct measure of the quality of the CNT[28]. The ID/IG ratio of 100 nm and 20 nm mean diameter CNTs were 0.23 and 1.18, respectively, as indicated in Table 1. This finding indicated that the 100 nm mean diameter CNT is of better quality than the 20 nm CNT. The Raman spectra of the FG multilayer composites were shown to have similar patterns with their raw CNTs, except in the case of a high amount of nSiC (Ale10 vol.% CNTe30 vol.%
Fig. 5 FE-SEM micrograph of the interfaces of functionally graded multilayers with various compositions. Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
6
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
-(-),
1e7
Fig. 6 TEM micrograph of the interface between Al and Ale10 vol.% CNT.
nSiC), as shown in Fig. 8(d). This may be explained by two reasons. First, the high strength of the nSiC offered relatively efficient impact energy transfer to the CNTs during the milling process when compared to samples with lower amounts of nSiC, resulting in destruction of many graphic regions on the CNT and the production of amorphous carbon impurities, which would explain why we could observe a relatively low intensity G peak (Fig. 8(d)). The second possibility is that the relative intensity of the G peak in the CNT was less exposed by the Raman laser due to the high amount of nSiC particles.
We believe that both scenarios were affected together because the ID/IG ratio also showed the same behaviour (Table 1) and these values can be explained by the aforementioned reasons. The ID/IG ratio of the 30 vol.% CNT (Fig. 8(e)) sample showed a two-fold lower value when compared to the 10 vol.% CNT sample (Fig. 8(d)), as indicated in Table 1. The G peak of Ale 10 vol.% CNT sample was slightly shifted (Fig. 8(c)) because of an existing high residual stress. According to the ID/IG ratio, the employed milling conditions were not seriously affected to the extent of defect in the 20 nm CNT, but the 100 nm mean
Fig. 7 TEM micrograph of the interface between Ale10 vol.% CNTe30 vol.% nSiC and Ale30 vol.% CNTe10 vol.% nSiC. Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003
H. Kwon et al.: J. Mater. Sci. Technol., 2014,
-(-),
1e7
7
the maximum hardness value was four times higher than that of pure Al when a high amount of nSiC was added to the sample. Based on the Raman analysis, the CNT functioned both as a reinforcement and as a lubricant during the ball milling process. We believe that this FG multilayer concept is not only applicable to dual-nanoparticulate-reinforced Al matrix nanocomposites but also to other scenarios, such as polymer and ceramic systems. Acknowledgements The authors acknowledge Dr. Miyazaki Takamichi and Dr. Songhak Yoon at Tohoku University, and EMPA for their technical support.
Fig. 8 Raman spectra of raw CNT and multilayer composite samples.
Fig. 9 XRD of multilayer composite materials.
diameter CNT was influenced much more. Nonetheless, the ID/IG ratios of the 100 nm CNT added samples were lower than those of the 20 nm CNT samples. The formed Al4C3 was not detected under the XRD diffraction pattern (Fig. 9) despite being observed in the TEM micrograph (Figs. 6 and 7). This is directly related to the limited resolution of the equipment because the formed Al4C3 was nanosized, and there was only a relatively small amount. On the other hand, clear Al and SiC peaks were detected, as well as a C peak, and these peaks increased proportionally with the amount of CNTs added. 4. Conclusion The fully densified functionally graded multilayer composite was successfully fabricated by ball milling and spark plasma sintering processes. The CNTs were well-dispersed onto the Al particles when using nSiC as the mixing agent, independent of the composition. Each layer was tightly bonded without any serious microcracks and pores. Some nanosized Al4C3 particles were observed in the CNT layer, but they could not be detected by XRD due to their nanosized distribution and small concentration in a limited area. Moreover, some of the formed Al4C3 was embedded across the Al matrix and connected between each boundary. This nano-tie-rod Al4C3 structure may be helpful in strengthening materials. The Vickers hardness of the FG multilayer showed different values depending on the composition, and
REFERENCES [1] M. Koizumi, Compos. Part B: Eng. 28 (1997) 1e4. [2] A. Kawasaki, R. Watanabe, Eng. Frac. Mech. 69 (2002) 1713e1728. [3] H. Kwon, C.R. Bradbury, M. Leparoux, Adv. Eng. Mater. 13 (2011) 325e329. [4] D. Pinheiro, M.T. Vieira, M.A. Djouadi, Surf. Coat. Technol. 203 (2009) 3197e3205. [5] A.G. Evans, J.W. Hutchinson, Surf. Coat. Technol. 201 (2007) 7905e7916. [6] U. Wiklund, J. Gunnars, S. Hogmark, Wear 232 (1999) 262e269. [7] S. Ijima, Nature 354 (1991) 56e58. [8] K.P. So, C. Biswas, S.C. Lim, K.H. An, Y.H. Lee, Synth. Met. 161 (2011) 208e212. [9] J. Tersoff, R.S. Ruoff, Phys. Rev. Lett. 73 (1994) 676e679. [10] A. Mathur, S. Wadhwa, M. Tweedie, K.S. Hazra, C. Dickinson, S.S. Roy, S.K. Mitra, D.S. Misra, J.A. McLaughlin, Physica E: Low-dimen. Syst. Nanostruc. 44 (2011) 29e36. [11] H. Kwon, M. Estili, K. Takagi, T. Miyazaki, A. Kawasaki, Carbon 47 (2009) 570e577. [12] H. Kwon, D. Park, J.F. Silvain, A. Kawasaki, Compos. Sci. Technol. 70 (2010) 546e550. [13] H. Kwon, A. Kawasaki, J. Nanosci. Nanotechnol. 9 (2009) 6542e6548. [14] A.M.K. Esawi, K. Morsi, Compos. Part A 38 (2007) 646e650. [15] H. Kurita, H. Kwon, M. Estili, A. Kawasaki, Mater. Trans. 52 (2011) 1960e1965. [16] A.M.K. Esawi, K. Morsi, A. Sayed, M. Taher, S. Lanka, Compos. Sci. Technol. 70 (2010) 2237e2241. [17] T. Laha, S. Kuchibhatla, S. Seal, W. Li, A. Agarwal, Acta Mater. 55 (2007) 1059e1066. [18] S.R. Bakshi, D. Lahiri, A. Agarwal, Int. Mater. Rev. 55 (2010) 41e64. [19] C.F. Deng, D.Z. Wang, X.X. Zhang, A.B. Li, Mater. Sci. Eng. A 444 (2007) 138e145. [20] S. Cho, K. Takagi, H. Kwon, D. Seo, K. Ogawa, K. Kikuchi, A. Kawasaki, Surf. Coat. Technol. 206 (2012) 3488e3494. [21] H. Kwon, S. Cho, M. Leparoux, A. Kawasaki, Nanotechnology 23 (2012) 225704. [22] Y. Leconte, M. Leparoux, X. Portier, N. Herlin-Boime, Plasma Chem. Plasma Process. 28 (2008) 233e248. [23] A.W. Burton, K. Ong, T. Rea, I.Y. Chan, Micropor. Mesopor. Mater. 117 (2009) 75e90. [24] C. Espejo, F.J. Carrion, D. Martinez, M.D. Bermudez, Tribol. Lett. 50 (2013) 127e136. [25] T. Etter, P. Schulz, M. Weber, J. Metz, M. Wimmler, J.F. Löffler, P.J. Uggowitzer, Mater. Sci. Eng. A 448 (2007) 1e6. [26] V.L. Solozhenko, O.O. Kurakevych, Solid State Commun. 133 (2005) 385e388. [27] S. Cho, K. Kikuchi, A. Kawasaki, H. Kwon, Y. Kim, Nanotechnology 33 (2012) 315705. [28] M.M. Lucchese, F. Stavale, E.H. Martins-Ferreira, C. Vilani, M. V.O. Moutinho, R.B. Capaz, C.A. Achete, A. Jorio, Carbon 48 (2010) 1592e1597.
Please cite this article in press as: H. Kwon, et al., Journal of Materials Science & Technology (2014), http://dx.doi.org/10.1016/j.jmst.2014.03.003