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Interface and interfacial reactions in multi-walled carbon nanotube-reinforced aluminum matrix composites
Carbon 96 (2016) 919e928
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Carbon journal homepage: www.elsevier.com/locate/carbon
Interface and interfacial reactions in multi-walled carbon nanotubereinforced aluminum matrix composites Weiwei Zhou a, Sora Bang a, Hiroki Kurita b, Takamichi Miyazaki c, Yuchi Fan a, Akira Kawasaki a, * a b c
Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai, 980-8579, Japan DEN/DANS/DMN/SRMA/LTMEx, CEA Saclay, 91191, GIF sur YVETTE Cedex, France Technical Division, School of Engineering, Tohoku University, Sendai, 980-8579, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2015 Received in revised form 6 October 2015 Accepted 7 October 2015 Available online 22 October 2015
The interface and interfacial reactions in Al-matrix composites reinforced with multi-walled carbon nanotubes (MWCNTs) were thoroughly investigated by high-resolution transmission electron microscopy combined with a precisely controlled heat treatment in the solid state. It was shown that MWCNT (002) formed a coherent interface with the low-index Al planes of Al (111), Al (220), and Al (002), realizing a stable interface. Aluminum carbides (Al4C3) were preferentially formed at the active prism plane edges sited at the open ends and acid treatment-induced surface nanodefects of MWCNTs. The Al4C3 maintained the shape of the pristine MWCNT and showed a typical orientation relationship with the Al matrix, that is, Al (111)//Al4C3 (001). It was suggested that the diffusion of Al atoms through Al4C3 dominated the growth of Al4C3; the Al4C3 originated at the open MWCNT tips may quickly grow in the <100> direction of the MWCNT, while the nanodefect-originated Al4C3 may grow simultaneously in the <110> and <100> directions of the MWCNT. The activation energy of Al4C3 formation, the appearance of twinning in single-crystal Al4C3, and the possible influence of the Al4C3 formation on the enhancement of load transfer at the MWCNT/Al interface were also studied. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recently, carbon nanotubes (CNTs) have been investigated as reinforcements in metal matrix composites (MMCs) for the extraordinary mechanical, thermal, and electrical properties of CNTs [1]. Many researchers have attempted to fabricate CNT-Al matrix composites, which could be used in advanced applications such as aerospace and automotive materials, as well as thin electric wires in which lower weights would save energy [2]. Unfortunately, the fabrication of CNT-Al matrix composites faces two main problems regarding the effective strength enhancement of the composites. One is the poor dispersion of CNTs within the matrix, attributed to strong CNT entanglement caused by Van der Waals forces. The other is the imperfect CNT/Al interfacial bonding, which is caused by the poor wettability between the CNTs and the Al matrix. In recent decades, high-energy ball milling (HEBM) [3],
* Corresponding author. E-mail address: [email protected] (A. Kawasaki). http://dx.doi.org/10.1016/j.carbon.2015.10.016 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
friction stir-processing [4], flake metallurgy [5], and nano-scale dispersion [6] have been used in fabricating CNT-Al composites. However, most studies focused on the problem of CNT dispersion, which sometimes caused damage in the CNTs or left contaminations in the matrix. Less attention has been devoted to the study of the CNT/Al interface, which is key to understand the intrinsic behavior of CNTs in CNT-Al matrix composites [7]. The CNT/metal matrix interface significantly influences the strengthening of MMCs, because applied stresses are transferred at this interface from the matrix to the CNTs [7]. Thus, strong interfacial bonding is a prerequisite for effective load transfer in CNT-Al matrix composites. It is well known that residual compressive stresses could contribute to the tight contact of the CNT/Al interface. The residual compressive stress appears during the fabrication process possibly from the difference in thermal expansion coefficients between the CNTs and metal matrix [8]. However, physical contact at the CNT/Al interface is usually weak and insufficient for efficient load transfer [9,10]. Local interfacial slide between CNTs and the Al matrix may occur during tensile loading, limiting the utilization of the CNT strength and thus
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providing less composite strengthening [9]. To address this problem, the formation of chemical bonds at the CNT/Al interface by favorable and controlled interfacial reactions is promising, but highly challenging. Aluminum carbide (Al4C3) is supposed to be a possible interfacial compound in CNT-Al matrix composites formed during the fabrication process because of the relatively low free energy of formation of the material (196 kJ/mol at 298 K) [11]. From the research of Ci et al. [12], the surface of perfect graphene is chemically stable; no reaction occurs, even in contact with molten Al. They reported that carbide nanostructures were suggested to be preferentially formed [12] at sites of structural disorder, nanodefects, and the ends of multi-walled carbon nanotubes (MWCNTs) in addition to the amorphous carbon layer on the surface of MWCNTs, where dangling carbon atom bonds create chemically active locations. Graphite, meanwhile, has many dangling bonds, and it reacted with an Al melt to easily form a layer of Al4C3 on the graphite surface [13,14]. Hwang et al. reported the formation of nickel carbide nanostructures (~5 nm) at structurally disordered sites in MWCNT-Ni matrix composites fabricated by molecular-level mixing [15]. Cho et al. observed a nanocrystal of chromium carbide at the radially unzipped defects of acid-treated MWCNTs incorporated in a CueCr alloy matrix [10]. Kwon et al. reported that Al4C3, formed in the MWCNTs-Al composite during spark plasma sintering (SPS) at 873 K, may have contributed to the strength enhancement of the matrix [16]. Unfortunately, to the best of our knowledge, clear observation by high-resolution transmission electron microscopy (HRTEM) of nano-carbides on MWCNTs incorporated in metal matrices is lacking. The location, shape, size, and quantity of such carbides have not been clarified. Few studies exist on the crystal orientation relationships among MWCNTs, carbides, and the metal matrix; in general, researchers in the field have poor comprehension of the formation and growth of carbides at MWCNTs. Commonly employed processes seldom obtain direct evidence of the existence of carbides at the MWCNT/metal matrix interface. This is because MWCNTs usually experience degradation and nonuniform dispersion, and thus have poor contact with the matrix. In particular, the HEBM process causes severe damage to MWCNTs. Thus, a plausible first step is the production and control of nanodefects located on the outer walls of MWCNTs. Then the MWCNTs would become individually and tightly embedded in the Al matrix, where dangling carbon bonds could feasibly react with Al matrix. Recently, Zhou et al. reported that the formation of nanodefects on the outer walls of MWCNTs was controllable by mild acid treatment at relatively low temperatures [17]. In addition, we succeeded in fabricating a fully dense MWCNT-Al matrix composite, which has a microstructure free of most strain and dislocations, in which individually dispersed MWCNTs are unidirectionally aligned and form a clean MWCNT/Al interface [18]. This composite allows the evaluation of the MWCNT/Al interface and an understanding of interfacial reactions based on the thermodynamics and kinetics, thereby providing the ability to control carbide formation. The purpose of this work was to investigate the interfacial reactions between the MWCNT reinforcement and the Al matrix by a precisely controlled heat treatment. The formation and morphologies of Al4C3 among the MWCNT, carbides, and the Al matrix were carefully observed by HRTEM and field-emission scanning electron microscopy (FE-SEM). A possible growth mechanism of Al4C3 in MWCNT-Al composites in the solid state was discussed, based on the morphology and crystal orientation of the carbide against the original MWCNTs as well as the Al matrix, combined with the activation energy of formation of Al4C3 in the MWCNT-Al composites.
2. Experimental procedure 2.1. Raw materials The pristine MWCNTs (Hodogaya Chemical Co. Ltd., Japan) having the average diameter of 70 nm with the length of 7.7 mm synthesized by means of the catalytically chemical vapor deposition (CVD), followed by thermally annealed at 2873 K [19], were employed in this study. Al powders (Ecka granule Japan Co. Ltd.) having 99.85% purity with average particle size of 5.5 mm were fabricated by the gas atomization. The sulfuric acid (H2SO4, 97 wt. %) and nitric acid (HNO3, 61 wt. %) were provided by Wako Industries, Japan. 2.2. Acid treatment and preparation of mixed powders The pristine MWCNTs were purified by concentrated HNO3 for 12 h. Subsequently, they were acid treated by a H2SO4/HNO3 mixture (3:1, v/v) in the ultrasonic bath (Powersonic model 50, Yamato Scientific Company, Japan) for 4 h at 323 K [17]. The colloid was scoured with deionized water until pH to be neutral before collecting on the filter paper. The acid-treated MWCNTs were completely dried in an oven at 373 K for 12 h. The MWCNTs and Al powders were individually dispersed in ethanol for 1 h with the ultrasonication, and then the MWCNT suspension was dropwise added into Al suspension. The volume fraction of MWCNTs was changed in the range from 0 to 5.0 vol. %. The mixed suspension was mechanically stirred for 0.5 h, followed by drying in the oven at 343 K for 24 h. 2.3. Consolidation The mixed MWCNT-Al powders were sintered by spark plasma sintering (SPS) method (Dr. Sinter S511, Sumitomo Coal Mining Co. Ltd.) at the sintering temperature of 873 K for the holding time of 0.33 h, at the heating rate of 1200 K/h, and at the pressure of 50 MPa. The SPS bulks were subsequently hot-extruded at 823 K with an applied pressure of 500 kN (UH-500 kN1, Shimadzu Corporation, Japan). The extrusion velocity and extrusion ratio were 0.06 m/h and 20, respectively. 2.4. Heat treatment The precisely controlled heat treatment was carried out in a high frequency induction heating furnace with argon gas flow. The MWCNT-Al composites with MWCNT concentrations of 3.0 vol. % and 5 vol. % were heat-treated at 873 K, 883 K, 893 K, 903 K, 913 K, 923 K, 933 K and for 0.1 h, 1 h, 2 h, respectively. 2.5. Characterizations The grain size distribution of MWCNT-Al composites was determined by the electron back-scattered diffraction (EBSD) (OIM ver. 6, TSL Solutions, Japan). Observation of acid-treated MWCNTs and microstructural characterizations of MWCNT-Al composites were performed by the high-resolution transmission electron microscopy (HRTEM; HF-2000EDX, Hitachi, Japan). TEM samples were prepared by grinding to a thickness of 50 mm, and thinned by the ion milling method (GATAN PISP Model 691, Gatan Inc.) at a voltage of 4 kV. For confirming the formation sites of aluminum carbide, some ion-milled samples of the heat-treated composites were exposed to air at 298 K for 168 h, and observed under the field emission scanning electron microscope (FESEM; JSM-6500F, JEOL, Japan).
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3. Results and discussion 3.1. Nanodefects of acid-treated MWCNTs The evolution of morphology in the pristine and acid-treated MWCNTs was monitored by HRTEM, as shown in Fig. 1. Fig. 1a shows that the pristine MWCNTs are relatively straight. Fig. 1b depicts the (002) basal crystal planes of the pristine MWCNTs. The walls are continuous and parallel to each other along the axial direction, showing the high crystallinity of the structure. Furthermore, small amounts of amorphous carbon of 2e3 nm in thickness
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are observed on the outermost wall. Acid treatment is most frequently employed to remove this amorphous carbon layer, as well as to functionalize the surface of MWCNTs, promoting the uniform dispersion of MWCNTs in ethanol [17,20]. Groove-type defects in Fig. 1d and circumference-type defects in Fig. 1e are observed on the MWCNTs after acid treatment. The mean depth and width of the defects were 5 nm and 12 nm, respectively. Note that the amorphous carbon on the surface of the MWCNTs is completely removed (see Fig. 1dee). The faceted closed tips of the pristine MWCNTs, consisting of five-membered carbon rings [21] (see Fig. S1, Supporting information), become unfolded
Fig. 1. HRTEM characterizations of the pristine MWCNTs (a, b) and acid-treated MWCNTs (c, d, e, f); the white arrows in (c) show nanodefects on the surface of acid-treated MWCNTs.
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into open tips (see Fig. 1f) during the acid treatment. Nanodefects on the sidewalls (Fig. 1cee) and open tips (Fig. 1f) include dangling carbon bonds on the prism plane edges; these can be expected to react with the Al matrix for carbide formation [10]. As confirmed by Raman spectra and X-ray diffraction patterns (Fig. S2, Supporting information), the acid-treated MWCNTs maintained high crystallinity, despite the acid treatment-induced nanodefects on the outer walls and open ends of the MWCNTs. 3.2. MWCNTs incorporated in Al matrix All MWCNT-Al composites were fully dense (relative density
> 99%) after SPS followed by hot extrusion. Fig. 2 shows the typical microstructure of the 1.5 vol. % MWCNT-Al composite. The primary Al powder particles are elongated into spindle-like shapes and oriented along the strong <111> fiber texture of the matrix parallel to the extrusion direction (ED, Fig. 2b). The MWCNTs are mostly aligned with the ED, as shown in Fig. 2e, caused by the plastic flow of the Al matrix during the hot extrusion process. The average grain size in the transversal cross-section, shown in Fig. 2a, is 2.5 mm. This is almost equal to that of pure Al compacts consolidated by the same SPS and extrusion processes [9], indicating negligible variation in the grain size of the matrix by the incorporation of MWCNTs. No MWCNT clustering is observed in the Al matrix. The white
Fig. 2. EBSD mappings and TEM images of (a, c, d) transversal and (b, e, f) longitudinal cross-section of 1.5% MWCNT-Al matrix composite; White arrows in (c) show individually dispersed MWCNTs; ED and TD represent the extrusion and transverse direction, respectively. (A colour version of this figure can be viewed online)
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arrows in Fig. 2c indicate the individually dispersed MWCNTs. The mean distance between adjacent MWCNTs is approximately several hundred nanometers. The interface between the MWCNT and Al matrix is very tight and clean (see Fig. 2d). No Al2O3 layer or Al4C3 is observed at the MWCNT/Al interface. As discussed in a previous report [9], the briefly present liquid Al infiltrates into the boundaries of Al particles from the fractured Al2O3 layer coating the Al particles during SPS. The infiltrated liquid Al directly covers each MWCNT and solidifies in a very short time. This explains the lack of Al4C3 formation. Notably, during hot extrusion, the plastic flow of the Al matrix causes MWCNTs to make contact with fresh Al surfaces. As shown in Fig. 2c and e, the dislocation density in the Al matrix is low. George et al. reported that the mismatch of thermal expansion coefficients between the Al matrix and MWCNTs may cause the prismatic punching of dislocations at the interface [22]. However, this MWCNT-Al composite shows a highly strain-free MWCNT/Al structure. As shown in Fig. 3, most MWCNTs are individually located at the boundaries of primary Al particles (Fig. 3a). We infrequently found MWCNTs embedded inside single Al grains (Fig. 3b). The compressive stress during the hot extrusion polygonally deformed the MWCNTs. Occasionally, this deformation is severe and the structure of the MWCNTs is crushed such that the hollow cores disappearing. The misfit radial compressive stress applied by the Al matrix contributes to the tight contact interface. The corresponding selected area electron diffraction (SAED) patterns in the nanobeam diffraction mode of TEM were taken with an incident electron beam whose zone axis was carefully adjusted for each Al grain. The adjusted zone axis is [001] for the upper Al grain and [011] for the lower Al grain in Fig. 3a. The adjusted zone axis in Fig. 3b is [112]. Macroscopically, Al grains have a strong <111> fiber texture in the direction of hot extrusion, as shown in Fig. 2aeb. However, the microscopic [111] crystal orientation for each grain deviates somewhat from the extrusion direction. Based on the SAED pattern analysis, an apparent relationship of crystal orientations between the MWCNT and Al grains was found, that is, Al (111)//MWCNT (002), Al (220)//MWCNT (002), and Al (002)//MWCNT (002), respectively. Therefore, the SAED patterns clearly show that each faceted MWCNT formed a parallel angle to the crystal planes of the Al grains. These low-index Al planes are characterized by lower surface energies than highereindex planes. Thus, MWCNTs form a coherent and relatively stable interface with the Al matrix.
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3.3. Aluminum carbide formation Fig. 4 shows a typical example of an aluminum carbide produced at the end of a MWCNT incorporated in the Al matrix. The aluminum carbide was identified by the corresponding fast Fouriertrans form (FFT) pattern in this case. A detailed analysis of the crystal structure of the aluminum carbides formed at the ends of MWCNTs is discussed later with Fig. 5. The length of the aluminum carbide observed in Fig. 4 is in the range from 100 nm to 350 nm. The average diameter is almost equal to that of the MWCNTs. An increase in volume during the formation reaction is theoretically expected; the diameter of the aluminum carbide is suggested to be slightly larger than that of the MWCNTs [11]. As mentioned, aluminum carbides are preferentially formed at sites of structural disorder, amorphous carbon layers, nanodefects, and the open ends of MWCNTs where dangling carbon bonds exist [12]. Since the MWCNTs were acid-treated, the amorphous carbon layers were removed and sites of structural disorder were transformed into outer-wall nanodefects. Therefore, nanodefects and open MWCNT ends are expected to be preferential sites for the formation of aluminum carbides. Fig. 4 presents evidence of the formation of aluminum carbides at the ends of MWCNTs. It is considered that the initial reaction occurred at the end of the MWCNT; this observation additionally shows that the aluminum carbide maintained the shape of the pristine MWCNTs. Hence, the aluminum carbide seems to grow along the MWCNT. Fig. 5 shows HRTEM images of the end part of an aluminum carbide. The SAED pattern (inset of Fig. 5a) indicates the Al4C3 structure of the aluminum carbide. The interface between the Al4C3 and the Al matrix is tight and clean. The lattice image shown in Fig. 5c clearly reveals an orientation relationship at the Al4C3/Al matrix interface; that is, Al (111)//Al4C3 (001). This relationship is further confirmed by FFT patterns of the Al matrix (Fig. 5d) and Al4C3 (Fig. 5e) by nanobeam diffraction. As shown in Fig. 3, the MWCNT and Al matrix has the orientation relationship of Al (111)// MWCNT (002). Therefore, the MWCNT reacted with the Al matrix to form Al4C3, which maintained the low-energy interface as well as the shape. Notably, a trace of friction, namely stress contrast, appears at the interface between the Al4C3 and the Al matrix, as indicated by the arrows in Fig. 4b. This stress contrast supports the tight interfacial adhesion. This indicates the availability of effective load transfer at the interface, confirming that the Al4C3 contributes to the enhanced load transfer to the MWCNTs.
Fig. 3. Cross-sectional HRTEM images of faceted MWCNT (a) at Al grain boundaries and (b) in single Al grain; the insets are the selected area electron diffraction (SAED) patterns of Al grains, the white arrows represent the internal compressive stress for MWCNTs. (A colour version of this figure can be viewed online)
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Fig. 4. HRTEM images of the 5 vol. % MWCNT-Al composite after heat treatment (a) at 873 K for 0.1 h and (b) at 883 K for 1 h; The white arrows in (b) indicate the stress contrast. (A colour version of this figure can be viewed online)
Fig. 5. Morphology of the 3.0 vol. % MWCNT-Al composite after heat treatment at 913 K for 2 h (a) HRTEM image of a twined Al4C3 with its SAED pattern (the inset) taken at the blue spot; (b) HRTEM image of the magnified twinning in (a); (c) HRTEM image showing the Al4C3/Al interface acquired from the marked area in (a); (d) and (e) showing the FFT patterns of Al and Al4C3 at the clean interface in (c), respectively. (A colour version of this figure can be viewed online)
The SAED pattern (inset in Fig. 5a) indicates the existence of twin planes in the Al4C3. The mirror plane of the twinning is (001) and the twinning direction is [110]. The presence of twin Al4C3 crystals (clearly shown in Fig. 5b) is detected for the first time in this MWCNT-Al composite. Steffens et al. observed Al4C3 twinning in carbon fiber-reinforced Al composites heat-treated at the temperature of 1003 K [23]. It is well known that twins generally
appear during either crystal growth, crystal deformation, or thermal treatment [24]. Steffens et al. reported that the twinned Al4C3 was introduced after the solidification of liquid-phase Al, thus caused by crystal deformation under compressive stress. The compressive stress was generated during the cooling of the composite to room temperature by means of the different coefficients of thermal expansion between Al (2.5 105 K1) and Al4C3
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(3.6 106 K1) [23]. Therefore, compressive stress is one possible source of the formation of twinned Al4C3 in MWCNT-Al composites. As shown in Fig. 1c, defects in outer walls of MWCNTs are nanoscale and located approximately 1300 nm apart [17]. In the case of the Al4C3 formed at these nanodefects on MWCNTs, a low magnification for a wide view is necessary to view several Al4C3 nanostructures simultaneously. TEM is thus unsuitable for this purpose, and a novel approach is required to perform this observation. It is known that Al4C3 easily absorbs water vapor to form aluminum hydroxide (Al(OH)3) [25]: Al4C3 þ 12 H2O / 4 Al(OH)3 þ 3 CH4
(1)
The Al(OH)3 is amorphous and the reaction is accompanied by volume expansion. If the polished composite surface, under which MWCNTs with nano-Al4C3 exist, is exposed to air, part of the nanoAl(OH)3 may expand to produce nano-projections on the surface. Therefore, the formation of Al(OH)3 is detectable by FE-SEM. Hence, the original Al4C3 sites on the MWCNT could be distinguished under FE-SEM because of the nano-projecting Al(OH)3. Ion-milled samples were prepared and exposed to air at 298 K for 168 h Fig. 6 shows FE-SEM images of the heat-treated 5 vol. % MWCNT-Al composite after exposure to air. The amorphous structures of the island-shaped Al(OH)3 projections (~500 nm in length) are confirmed by the FE-SEM-energy dispersive spectroscopy (EDS) and SAED patterns. These islands are oriented and the average distance between the midpoints of two adjacent Al(OH)3 structures is determined to be 1100 ± 400 nm (Fig. 6a). This average distance agrees well with that of adjacent nanodefects on the outer walls of acid-treated MWCNTs [17]. From this similarity, it was deduced that the islands correspond to the sites of Al4C3 produced at the nanodefects in the outer walls of the MWCNTs. Furthermore, the average length of Al(OH)3 structures is measured to be ~750 nm in the composite heat-treated at 903 K (Fig. 6b), which is slightly longer than that of the composite heat-treated at 873 K, indicating the temperature dependence of the growth of Al4C3. Fig. 7 shows HRTEM micrographs of Al4C3, which perhaps formed at a nanodefect in the outer walls of the MWCNT. The white arrow in Fig. 7a shows the site of the nanodefect. The Al4C3 is confirmed by the corresponding SEAD pattern taken by nanobeam diffraction, as shown in the inset of Fig. 7a. The streaks, indicated by green arrows in the inset along the <001> direction of Al4C3, are
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clearly shown, indicating the high crystallinity of the Al4C3 formed on the outer walls. Interestingly, the inner walls retain the structure of graphene. The MWCNTs used in this study were produced by a chemical vapor deposition (CVD) process; the outer walls were thus partially defective while the inner walls were defect-free by graphitization at very high temperatures for the post-deposition thickening process [26]. The inner walls with nearly perfect graphene surfaces were very stable and effectively restricted the epitaxial growth of Al4C3 along the direction perpendicular to the tubular axis of the MWCNT. If Al4C3 initiated at the end of the MWCNT and grew along the axis, the whole cross-section of the MWCNT would be converted to Al4C3. Fig. 7b presents the continuous and mutually <110>-oriented basal crystal planes of Al4C3. Based on SAED analysis at the MWCNT/Al4C3 interface, the Al4C3 is found to form parallel to the sidewall of the MWCNT. Thus, the typical orientation relationship between the MWCNT and Al4C3 is MWCNT (002)//Al4C3 (001), indicating the epitaxial growth of Al4C3 in the <001> direction. It is thought that nano-Al4C3 crystals were generated at nanodefects on the MWCNT, followed by both axial growth in the <110> direction and radial growth in the <001> direction epitaxially. However, the growth in the <001> direction was restricted by the defect-free inner walls of the MWCNTs. 3.4. Morphology and characteristics of Al4C3 Fig. 8 shows the MWCNTs incorporated in the Al matrix of the 3.0 vol.% MWCNT-Al composite heat-treated at 913 K for 2 h. The SAED pattern (see Fig. 8b) indicates the formation of Al4C3. Under this heat treatment condition, most MWCNTs were reacted and transformed into Al4C3. The Al4C3 is found to be monocrystalline with a rhombohedral structure, confirmed by the strong and periodical SAED pattern. Interestingly, the basal plane of Al4C3 (001) is parallel to the axis of the MWCNT (Fig. 8b). As shown in Fig. 8a, Al4C3 has a rod-like form, similar to the pristine MWCNT, suggesting that MWCNTs directly reacted to form Al4C3 without substantial changes to the original shape of the tubes during the heat treatment. In addition, the Al4C3 has no hollow core. This rod-like Al4C3 formation may support the results of Chen et al., where Al4C3 nanorods were prepared using MWCNTs as templates [27]. Notably, some dislocation lines are observed in the Al4C3 crystal (see the white arrows in Fig. 8a). The dislocation lines are
Fig. 6. FESEM images of the heat-treated 5 vol. % MWCNT-Al composite after exposure to air for 168 h, (a) heat-treated at 873 K for 1 h; (b) heat-treated at 903 K for 1 h. (A colour version of this figure can be viewed online)
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Fig. 7. HRTEM observations of (a) low and (b) high magnification view of the 5 vol. % MWCNT-Al composite after heat treatment at 883 K for 1 h; Inset of (a) showing the SAED pattern of Al4C3. (A colour version of this figure can be viewed online)
Fig. 8. Morphology of the 3.0 vol. % MWCNT-Al composite after heat treatment at 913 K for 2 h (a) A TEM image showing two typical Al4C3 structures; (b) SAED pattern of the Al4C3 acquired from the marked area in (a). (A colour version of this figure can be viewed online)
approximately aligned with the <110> direction, which is ascribed to the rapid growth rate of Al4C3 along the <110> direction and the insufficient release of internal stress during the heat treatment. It seems that the growth of Al4C3 might be fast enough that Al4C3 of several nanometers in the initial stage has never been observed under HRTEM, even with several seconds of heat treatment. 3.5. Formation mechanism of Al4C3 The quantity of Al4C3 was defined as the ratio of the average length of formed Al4C3 and the average length of acid-treated MWCNTs. The average length of each was measured from HRTEM images. Fig. 9a shows the dependence of the quantity of Al4C3 on the heat treatment temperature, as well as the heat treatment time. In the case of heat-treatment for 1 h, the quantity of Al4C3 increases with increasing temperature. The quantity of Al4C3 reaches nearly 100% when the heat-treatment temperature approaches the melting point of Al. For the heat-treatment temperatures of 883 K and 903 K (see Fig. 9b), a parabolic variation is observed and the increase rate of the quantity of Al4C3 decreases as the time increases. To clarify the kinetics of the interfacial phenomenon, the Arrhenius law as Equation (2) was applied to estimate the activation energy of Al4C3 formation [28]:
A ¼ A0exp (eQ/RT)
(2)
where A is the quantity of Al4C3, Q is the activation energy, A0 is the frequency factor, T is the absolute temperature of heat treatment, and R is the gas constant (8.31 J mol1 K1). Fig. 10 shows the resultant linear relation of the plot of ln(A) as a function of (1/T). The activation energy of approximately 195.8 kJ/mol for the formation of Al4C3 is estimated from the slope of Fig. 10. This activation energy is equivalent to the typical activation energy for the formation of Al4C3 in carbon fiber-reinforced Al matrix composites [29]. Lee et al. reported that AleC reactions occurred at the reactive prism plane edges of carbide fibers. The equivalence of the activation energy supports the identity of nanodefects located on the MWCNT surface and open ends of MWCNTs as locations in MWCNT-Al composites where the AleC reaction occurs. Robinson reported that the activation energy for lattice self-diffusion in Al was approximately 140 kJ/mol as evaluated by tracer techniques from 573 to 923 K [30]. This value is lower than the estimated activation energy for Al4C3 formation. Considering the parabolic variation of the Al4C3 quantity with heat treatment time, it is thought that the rate-limiting factor of Al4C3 growth is supposed to be the diffusion of Al atoms through the Al4C3 crystal. From the context above, a possible mechanism of formation and growth of Al4C3 in MWCNT/Al composites in the solid state is
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Fig. 9. The quantity of Al4C3 as a function of (a) temperature heat-treated for 1 h and (b) time heat-treated at 883 K and 903 K. (A colour version of this figure can be viewed online)
Fig. 10. Plot of lnA (A is the quantity of Al4C3) versus 1/T; the estimation of activation energy for the Al4C3 formation in the temperature range from 873 to 933 K. (A colour version of this figure can be viewed online)
proposed as follows. (1) At the initial reaction stage, nano-Al4C3 is preferentially formed at the open ends and nanodefects of MWCNTs by chemicals reaction between dangling carbon bonds and Al atoms. (2) The diffusion of Al atoms through the formed nano-Al4C3 dominates the growth of Al4C3. (3)-1 The open-tip-originated Al4C3 will grow quickly along the <110> axial direction of the MWCNT, whereas the growth rate may gradually decrease with increased length of the Al4C3 crystal because of the longer diffusion path of Al atoms. (3)-2 Meanwhile, the growth of nanodefect-originated Al4C3 will simultaneously occur along the <110> and <001> directions by the diffusion of Al atoms through irregularities in the outer walls of the MWCNT. The axial growth of Al4C3 in the <110> direction should be faster than the radial growth in the <001> direction. The radial growth is restricted by the defect-free inner walls. This is why partially formed Al4C3 was occasionally observed on the outer layers of the MWCNT (see Fig. 7). When two Al4C3 crystals formed at adjacent nanodefects on the outer walls of the MWCNT meet each other during the growth stage, they are supposed to be unified, including twinning, in addition to the compressive stress as previously mentioned. 3.6. Strengthening effect by Al4C3 formation In order to enhance the load transfer at the interface between the MWCNTs and the Al matrix in MWCNT-Al composites, the formation of small amounts of Al4C3 is supposed to be helpful. The realization of partially formed Al4C3 at the open ends and
nanodefects of MWCNTs includes several aspects. (1) The covalent bonds with Al4C3 may bridge the outmost and inner walls of the MWCNT, preventing the peeling of the outmost wall from the inner walls under loading and thus increasing the load-bearing capacity. (2) The nano-Al4C3 formed can enhance the interfacial shear resistance between the MWCNT and Al matrix, preventing the pullout of MWCNTs from the matrix. (3) The diameter of Al4C3 should be slightly larger than that of the original MWCNTs, forming strong anchors for the MWCNTs in the Al matrix and thereby contributing to the load bearing. Notably, the Al4C3 formation is suggested to be controlled by a combination of intentionally induced nanodefects and precisely controlled heat treatment. Insufficient amounts of nanocarbides would not strengthen the chemical bonds between MWCNTs and the matrix for bearing loads, while excess amounts of nanocarbides may reduce the effective length of MWCNTs, leading to poor exploitation of the intrinsic strength of MWCNTs. A study on load transfer from the Al matrix to MWCNTs is now underway by in-situ pullout testing of individual MWCNTs from the Al matrix. The effect of carbide inclusion on the improvement of load transfer should be investigated in future works, including the optimization of the amount of carbide nanostructures formed on MWCNTs. 4. Conclusions In summary, the interfacial reactions between the MWCNT and Al matrix were investigated in a fully dense MWCNT-Al composite by precisely controlled heat treatments at temperatures below the Al melting point. In this composite, the acid-treated, individually dispersed MWCNTs with close interfaces with Al were mostly aligned in the direction of hot extrusion. It was shown that MWCNT (002) formed a coherent interface with low-index Al planes of Al (111), Al (220), and Al (002), all of which created relatively stable interfaces. HRTEM examination and a novel technical observation by FE-SEM clearly revealed that Al4C3 was preferentially formed at the active prism plane edges located at the open ends and acid treatment-induced sidewall nanodefects of the MWCNTs. It was confirmed that Al4C3 was single-crystalline and frequently contained unusual twinning features because of the internal compressive stress and different growth paths of Al4C3 on the MWCNTs. The Al4C3 maintained the shape of the pristine MWCNT and showed a typical orientation relationship with Al matrix, that is, Al (111)//Al4C3 (001). Furthermore, the activation energy of Al4C3 formation, calculated as approximately 195.8 kJ/mol, as well as the parabolic relation between the quantity of Al4C3 and heattreatment time, supported the generation of Al4C3 at the prism plane edges and the growth of Al4C3 by the diffusion of Al atoms through the Al4C3 crystal. These results suggested that the open tip-
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originated Al4C3 may quickly grow in the <100> direction of the MWCNT, while nanodefect-originated Al4C3 may simultaneously grow in the <110> and <100> directions of the MWCNT.
[12]
Acknowledgment
[13]
WW Zhou would like to thank the support of China Scholarship Council. The authors also appreciate the generous helps from Prof. Naoyuki Nomura, Dr. Keiko Kikuchi, Dr. Rui Yamada, Dr. Shihai Sun and Dr. Xin Lu in Tohoku University, and Dr. Mehdi Estili in National Institute for Materials Science.
[14]
[15]
[16]
Appendix A. Supplementary data [17]
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Interface and interfacial reactions in multi-walled carbon nanotubereinforced aluminum matrix composites Weiwei Zhou a, Sora Bang a, Hiroki Kurita b, Takamichi Miyazaki c, Yuchi Fan a, Akira Kawasaki a, * a b c
Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai, 980-8579, Japan DEN/DANS/DMN/SRMA/LTMEx, CEA Saclay, 91191, GIF sur YVETTE Cedex, France Technical Division, School of Engineering, Tohoku University, Sendai, 980-8579, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2015 Received in revised form 6 October 2015 Accepted 7 October 2015 Available online 22 October 2015
The interface and interfacial reactions in Al-matrix composites reinforced with multi-walled carbon nanotubes (MWCNTs) were thoroughly investigated by high-resolution transmission electron microscopy combined with a precisely controlled heat treatment in the solid state. It was shown that MWCNT (002) formed a coherent interface with the low-index Al planes of Al (111), Al (220), and Al (002), realizing a stable interface. Aluminum carbides (Al4C3) were preferentially formed at the active prism plane edges sited at the open ends and acid treatment-induced surface nanodefects of MWCNTs. The Al4C3 maintained the shape of the pristine MWCNT and showed a typical orientation relationship with the Al matrix, that is, Al (111)//Al4C3 (001). It was suggested that the diffusion of Al atoms through Al4C3 dominated the growth of Al4C3; the Al4C3 originated at the open MWCNT tips may quickly grow in the <100> direction of the MWCNT, while the nanodefect-originated Al4C3 may grow simultaneously in the <110> and <100> directions of the MWCNT. The activation energy of Al4C3 formation, the appearance of twinning in single-crystal Al4C3, and the possible influence of the Al4C3 formation on the enhancement of load transfer at the MWCNT/Al interface were also studied. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recently, carbon nanotubes (CNTs) have been investigated as reinforcements in metal matrix composites (MMCs) for the extraordinary mechanical, thermal, and electrical properties of CNTs [1]. Many researchers have attempted to fabricate CNT-Al matrix composites, which could be used in advanced applications such as aerospace and automotive materials, as well as thin electric wires in which lower weights would save energy [2]. Unfortunately, the fabrication of CNT-Al matrix composites faces two main problems regarding the effective strength enhancement of the composites. One is the poor dispersion of CNTs within the matrix, attributed to strong CNT entanglement caused by Van der Waals forces. The other is the imperfect CNT/Al interfacial bonding, which is caused by the poor wettability between the CNTs and the Al matrix. In recent decades, high-energy ball milling (HEBM) [3],
* Corresponding author. E-mail address: [email protected] (A. Kawasaki). http://dx.doi.org/10.1016/j.carbon.2015.10.016 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
friction stir-processing [4], flake metallurgy [5], and nano-scale dispersion [6] have been used in fabricating CNT-Al composites. However, most studies focused on the problem of CNT dispersion, which sometimes caused damage in the CNTs or left contaminations in the matrix. Less attention has been devoted to the study of the CNT/Al interface, which is key to understand the intrinsic behavior of CNTs in CNT-Al matrix composites [7]. The CNT/metal matrix interface significantly influences the strengthening of MMCs, because applied stresses are transferred at this interface from the matrix to the CNTs [7]. Thus, strong interfacial bonding is a prerequisite for effective load transfer in CNT-Al matrix composites. It is well known that residual compressive stresses could contribute to the tight contact of the CNT/Al interface. The residual compressive stress appears during the fabrication process possibly from the difference in thermal expansion coefficients between the CNTs and metal matrix [8]. However, physical contact at the CNT/Al interface is usually weak and insufficient for efficient load transfer [9,10]. Local interfacial slide between CNTs and the Al matrix may occur during tensile loading, limiting the utilization of the CNT strength and thus
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providing less composite strengthening [9]. To address this problem, the formation of chemical bonds at the CNT/Al interface by favorable and controlled interfacial reactions is promising, but highly challenging. Aluminum carbide (Al4C3) is supposed to be a possible interfacial compound in CNT-Al matrix composites formed during the fabrication process because of the relatively low free energy of formation of the material (196 kJ/mol at 298 K) [11]. From the research of Ci et al. [12], the surface of perfect graphene is chemically stable; no reaction occurs, even in contact with molten Al. They reported that carbide nanostructures were suggested to be preferentially formed [12] at sites of structural disorder, nanodefects, and the ends of multi-walled carbon nanotubes (MWCNTs) in addition to the amorphous carbon layer on the surface of MWCNTs, where dangling carbon atom bonds create chemically active locations. Graphite, meanwhile, has many dangling bonds, and it reacted with an Al melt to easily form a layer of Al4C3 on the graphite surface [13,14]. Hwang et al. reported the formation of nickel carbide nanostructures (~5 nm) at structurally disordered sites in MWCNT-Ni matrix composites fabricated by molecular-level mixing [15]. Cho et al. observed a nanocrystal of chromium carbide at the radially unzipped defects of acid-treated MWCNTs incorporated in a CueCr alloy matrix [10]. Kwon et al. reported that Al4C3, formed in the MWCNTs-Al composite during spark plasma sintering (SPS) at 873 K, may have contributed to the strength enhancement of the matrix [16]. Unfortunately, to the best of our knowledge, clear observation by high-resolution transmission electron microscopy (HRTEM) of nano-carbides on MWCNTs incorporated in metal matrices is lacking. The location, shape, size, and quantity of such carbides have not been clarified. Few studies exist on the crystal orientation relationships among MWCNTs, carbides, and the metal matrix; in general, researchers in the field have poor comprehension of the formation and growth of carbides at MWCNTs. Commonly employed processes seldom obtain direct evidence of the existence of carbides at the MWCNT/metal matrix interface. This is because MWCNTs usually experience degradation and nonuniform dispersion, and thus have poor contact with the matrix. In particular, the HEBM process causes severe damage to MWCNTs. Thus, a plausible first step is the production and control of nanodefects located on the outer walls of MWCNTs. Then the MWCNTs would become individually and tightly embedded in the Al matrix, where dangling carbon bonds could feasibly react with Al matrix. Recently, Zhou et al. reported that the formation of nanodefects on the outer walls of MWCNTs was controllable by mild acid treatment at relatively low temperatures [17]. In addition, we succeeded in fabricating a fully dense MWCNT-Al matrix composite, which has a microstructure free of most strain and dislocations, in which individually dispersed MWCNTs are unidirectionally aligned and form a clean MWCNT/Al interface [18]. This composite allows the evaluation of the MWCNT/Al interface and an understanding of interfacial reactions based on the thermodynamics and kinetics, thereby providing the ability to control carbide formation. The purpose of this work was to investigate the interfacial reactions between the MWCNT reinforcement and the Al matrix by a precisely controlled heat treatment. The formation and morphologies of Al4C3 among the MWCNT, carbides, and the Al matrix were carefully observed by HRTEM and field-emission scanning electron microscopy (FE-SEM). A possible growth mechanism of Al4C3 in MWCNT-Al composites in the solid state was discussed, based on the morphology and crystal orientation of the carbide against the original MWCNTs as well as the Al matrix, combined with the activation energy of formation of Al4C3 in the MWCNT-Al composites.
2. Experimental procedure 2.1. Raw materials The pristine MWCNTs (Hodogaya Chemical Co. Ltd., Japan) having the average diameter of 70 nm with the length of 7.7 mm synthesized by means of the catalytically chemical vapor deposition (CVD), followed by thermally annealed at 2873 K [19], were employed in this study. Al powders (Ecka granule Japan Co. Ltd.) having 99.85% purity with average particle size of 5.5 mm were fabricated by the gas atomization. The sulfuric acid (H2SO4, 97 wt. %) and nitric acid (HNO3, 61 wt. %) were provided by Wako Industries, Japan. 2.2. Acid treatment and preparation of mixed powders The pristine MWCNTs were purified by concentrated HNO3 for 12 h. Subsequently, they were acid treated by a H2SO4/HNO3 mixture (3:1, v/v) in the ultrasonic bath (Powersonic model 50, Yamato Scientific Company, Japan) for 4 h at 323 K [17]. The colloid was scoured with deionized water until pH to be neutral before collecting on the filter paper. The acid-treated MWCNTs were completely dried in an oven at 373 K for 12 h. The MWCNTs and Al powders were individually dispersed in ethanol for 1 h with the ultrasonication, and then the MWCNT suspension was dropwise added into Al suspension. The volume fraction of MWCNTs was changed in the range from 0 to 5.0 vol. %. The mixed suspension was mechanically stirred for 0.5 h, followed by drying in the oven at 343 K for 24 h. 2.3. Consolidation The mixed MWCNT-Al powders were sintered by spark plasma sintering (SPS) method (Dr. Sinter S511, Sumitomo Coal Mining Co. Ltd.) at the sintering temperature of 873 K for the holding time of 0.33 h, at the heating rate of 1200 K/h, and at the pressure of 50 MPa. The SPS bulks were subsequently hot-extruded at 823 K with an applied pressure of 500 kN (UH-500 kN1, Shimadzu Corporation, Japan). The extrusion velocity and extrusion ratio were 0.06 m/h and 20, respectively. 2.4. Heat treatment The precisely controlled heat treatment was carried out in a high frequency induction heating furnace with argon gas flow. The MWCNT-Al composites with MWCNT concentrations of 3.0 vol. % and 5 vol. % were heat-treated at 873 K, 883 K, 893 K, 903 K, 913 K, 923 K, 933 K and for 0.1 h, 1 h, 2 h, respectively. 2.5. Characterizations The grain size distribution of MWCNT-Al composites was determined by the electron back-scattered diffraction (EBSD) (OIM ver. 6, TSL Solutions, Japan). Observation of acid-treated MWCNTs and microstructural characterizations of MWCNT-Al composites were performed by the high-resolution transmission electron microscopy (HRTEM; HF-2000EDX, Hitachi, Japan). TEM samples were prepared by grinding to a thickness of 50 mm, and thinned by the ion milling method (GATAN PISP Model 691, Gatan Inc.) at a voltage of 4 kV. For confirming the formation sites of aluminum carbide, some ion-milled samples of the heat-treated composites were exposed to air at 298 K for 168 h, and observed under the field emission scanning electron microscope (FESEM; JSM-6500F, JEOL, Japan).
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3. Results and discussion 3.1. Nanodefects of acid-treated MWCNTs The evolution of morphology in the pristine and acid-treated MWCNTs was monitored by HRTEM, as shown in Fig. 1. Fig. 1a shows that the pristine MWCNTs are relatively straight. Fig. 1b depicts the (002) basal crystal planes of the pristine MWCNTs. The walls are continuous and parallel to each other along the axial direction, showing the high crystallinity of the structure. Furthermore, small amounts of amorphous carbon of 2e3 nm in thickness
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are observed on the outermost wall. Acid treatment is most frequently employed to remove this amorphous carbon layer, as well as to functionalize the surface of MWCNTs, promoting the uniform dispersion of MWCNTs in ethanol [17,20]. Groove-type defects in Fig. 1d and circumference-type defects in Fig. 1e are observed on the MWCNTs after acid treatment. The mean depth and width of the defects were 5 nm and 12 nm, respectively. Note that the amorphous carbon on the surface of the MWCNTs is completely removed (see Fig. 1dee). The faceted closed tips of the pristine MWCNTs, consisting of five-membered carbon rings [21] (see Fig. S1, Supporting information), become unfolded
Fig. 1. HRTEM characterizations of the pristine MWCNTs (a, b) and acid-treated MWCNTs (c, d, e, f); the white arrows in (c) show nanodefects on the surface of acid-treated MWCNTs.
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into open tips (see Fig. 1f) during the acid treatment. Nanodefects on the sidewalls (Fig. 1cee) and open tips (Fig. 1f) include dangling carbon bonds on the prism plane edges; these can be expected to react with the Al matrix for carbide formation [10]. As confirmed by Raman spectra and X-ray diffraction patterns (Fig. S2, Supporting information), the acid-treated MWCNTs maintained high crystallinity, despite the acid treatment-induced nanodefects on the outer walls and open ends of the MWCNTs. 3.2. MWCNTs incorporated in Al matrix All MWCNT-Al composites were fully dense (relative density
> 99%) after SPS followed by hot extrusion. Fig. 2 shows the typical microstructure of the 1.5 vol. % MWCNT-Al composite. The primary Al powder particles are elongated into spindle-like shapes and oriented along the strong <111> fiber texture of the matrix parallel to the extrusion direction (ED, Fig. 2b). The MWCNTs are mostly aligned with the ED, as shown in Fig. 2e, caused by the plastic flow of the Al matrix during the hot extrusion process. The average grain size in the transversal cross-section, shown in Fig. 2a, is 2.5 mm. This is almost equal to that of pure Al compacts consolidated by the same SPS and extrusion processes [9], indicating negligible variation in the grain size of the matrix by the incorporation of MWCNTs. No MWCNT clustering is observed in the Al matrix. The white
Fig. 2. EBSD mappings and TEM images of (a, c, d) transversal and (b, e, f) longitudinal cross-section of 1.5% MWCNT-Al matrix composite; White arrows in (c) show individually dispersed MWCNTs; ED and TD represent the extrusion and transverse direction, respectively. (A colour version of this figure can be viewed online)
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arrows in Fig. 2c indicate the individually dispersed MWCNTs. The mean distance between adjacent MWCNTs is approximately several hundred nanometers. The interface between the MWCNT and Al matrix is very tight and clean (see Fig. 2d). No Al2O3 layer or Al4C3 is observed at the MWCNT/Al interface. As discussed in a previous report [9], the briefly present liquid Al infiltrates into the boundaries of Al particles from the fractured Al2O3 layer coating the Al particles during SPS. The infiltrated liquid Al directly covers each MWCNT and solidifies in a very short time. This explains the lack of Al4C3 formation. Notably, during hot extrusion, the plastic flow of the Al matrix causes MWCNTs to make contact with fresh Al surfaces. As shown in Fig. 2c and e, the dislocation density in the Al matrix is low. George et al. reported that the mismatch of thermal expansion coefficients between the Al matrix and MWCNTs may cause the prismatic punching of dislocations at the interface [22]. However, this MWCNT-Al composite shows a highly strain-free MWCNT/Al structure. As shown in Fig. 3, most MWCNTs are individually located at the boundaries of primary Al particles (Fig. 3a). We infrequently found MWCNTs embedded inside single Al grains (Fig. 3b). The compressive stress during the hot extrusion polygonally deformed the MWCNTs. Occasionally, this deformation is severe and the structure of the MWCNTs is crushed such that the hollow cores disappearing. The misfit radial compressive stress applied by the Al matrix contributes to the tight contact interface. The corresponding selected area electron diffraction (SAED) patterns in the nanobeam diffraction mode of TEM were taken with an incident electron beam whose zone axis was carefully adjusted for each Al grain. The adjusted zone axis is [001] for the upper Al grain and [011] for the lower Al grain in Fig. 3a. The adjusted zone axis in Fig. 3b is [112]. Macroscopically, Al grains have a strong <111> fiber texture in the direction of hot extrusion, as shown in Fig. 2aeb. However, the microscopic [111] crystal orientation for each grain deviates somewhat from the extrusion direction. Based on the SAED pattern analysis, an apparent relationship of crystal orientations between the MWCNT and Al grains was found, that is, Al (111)//MWCNT (002), Al (220)//MWCNT (002), and Al (002)//MWCNT (002), respectively. Therefore, the SAED patterns clearly show that each faceted MWCNT formed a parallel angle to the crystal planes of the Al grains. These low-index Al planes are characterized by lower surface energies than highereindex planes. Thus, MWCNTs form a coherent and relatively stable interface with the Al matrix.
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3.3. Aluminum carbide formation Fig. 4 shows a typical example of an aluminum carbide produced at the end of a MWCNT incorporated in the Al matrix. The aluminum carbide was identified by the corresponding fast Fouriertrans form (FFT) pattern in this case. A detailed analysis of the crystal structure of the aluminum carbides formed at the ends of MWCNTs is discussed later with Fig. 5. The length of the aluminum carbide observed in Fig. 4 is in the range from 100 nm to 350 nm. The average diameter is almost equal to that of the MWCNTs. An increase in volume during the formation reaction is theoretically expected; the diameter of the aluminum carbide is suggested to be slightly larger than that of the MWCNTs [11]. As mentioned, aluminum carbides are preferentially formed at sites of structural disorder, amorphous carbon layers, nanodefects, and the open ends of MWCNTs where dangling carbon bonds exist [12]. Since the MWCNTs were acid-treated, the amorphous carbon layers were removed and sites of structural disorder were transformed into outer-wall nanodefects. Therefore, nanodefects and open MWCNT ends are expected to be preferential sites for the formation of aluminum carbides. Fig. 4 presents evidence of the formation of aluminum carbides at the ends of MWCNTs. It is considered that the initial reaction occurred at the end of the MWCNT; this observation additionally shows that the aluminum carbide maintained the shape of the pristine MWCNTs. Hence, the aluminum carbide seems to grow along the MWCNT. Fig. 5 shows HRTEM images of the end part of an aluminum carbide. The SAED pattern (inset of Fig. 5a) indicates the Al4C3 structure of the aluminum carbide. The interface between the Al4C3 and the Al matrix is tight and clean. The lattice image shown in Fig. 5c clearly reveals an orientation relationship at the Al4C3/Al matrix interface; that is, Al (111)//Al4C3 (001). This relationship is further confirmed by FFT patterns of the Al matrix (Fig. 5d) and Al4C3 (Fig. 5e) by nanobeam diffraction. As shown in Fig. 3, the MWCNT and Al matrix has the orientation relationship of Al (111)// MWCNT (002). Therefore, the MWCNT reacted with the Al matrix to form Al4C3, which maintained the low-energy interface as well as the shape. Notably, a trace of friction, namely stress contrast, appears at the interface between the Al4C3 and the Al matrix, as indicated by the arrows in Fig. 4b. This stress contrast supports the tight interfacial adhesion. This indicates the availability of effective load transfer at the interface, confirming that the Al4C3 contributes to the enhanced load transfer to the MWCNTs.
Fig. 3. Cross-sectional HRTEM images of faceted MWCNT (a) at Al grain boundaries and (b) in single Al grain; the insets are the selected area electron diffraction (SAED) patterns of Al grains, the white arrows represent the internal compressive stress for MWCNTs. (A colour version of this figure can be viewed online)
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Fig. 4. HRTEM images of the 5 vol. % MWCNT-Al composite after heat treatment (a) at 873 K for 0.1 h and (b) at 883 K for 1 h; The white arrows in (b) indicate the stress contrast. (A colour version of this figure can be viewed online)
Fig. 5. Morphology of the 3.0 vol. % MWCNT-Al composite after heat treatment at 913 K for 2 h (a) HRTEM image of a twined Al4C3 with its SAED pattern (the inset) taken at the blue spot; (b) HRTEM image of the magnified twinning in (a); (c) HRTEM image showing the Al4C3/Al interface acquired from the marked area in (a); (d) and (e) showing the FFT patterns of Al and Al4C3 at the clean interface in (c), respectively. (A colour version of this figure can be viewed online)
The SAED pattern (inset in Fig. 5a) indicates the existence of twin planes in the Al4C3. The mirror plane of the twinning is (001) and the twinning direction is [110]. The presence of twin Al4C3 crystals (clearly shown in Fig. 5b) is detected for the first time in this MWCNT-Al composite. Steffens et al. observed Al4C3 twinning in carbon fiber-reinforced Al composites heat-treated at the temperature of 1003 K [23]. It is well known that twins generally
appear during either crystal growth, crystal deformation, or thermal treatment [24]. Steffens et al. reported that the twinned Al4C3 was introduced after the solidification of liquid-phase Al, thus caused by crystal deformation under compressive stress. The compressive stress was generated during the cooling of the composite to room temperature by means of the different coefficients of thermal expansion between Al (2.5 105 K1) and Al4C3
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(3.6 106 K1) [23]. Therefore, compressive stress is one possible source of the formation of twinned Al4C3 in MWCNT-Al composites. As shown in Fig. 1c, defects in outer walls of MWCNTs are nanoscale and located approximately 1300 nm apart [17]. In the case of the Al4C3 formed at these nanodefects on MWCNTs, a low magnification for a wide view is necessary to view several Al4C3 nanostructures simultaneously. TEM is thus unsuitable for this purpose, and a novel approach is required to perform this observation. It is known that Al4C3 easily absorbs water vapor to form aluminum hydroxide (Al(OH)3) [25]: Al4C3 þ 12 H2O / 4 Al(OH)3 þ 3 CH4
(1)
The Al(OH)3 is amorphous and the reaction is accompanied by volume expansion. If the polished composite surface, under which MWCNTs with nano-Al4C3 exist, is exposed to air, part of the nanoAl(OH)3 may expand to produce nano-projections on the surface. Therefore, the formation of Al(OH)3 is detectable by FE-SEM. Hence, the original Al4C3 sites on the MWCNT could be distinguished under FE-SEM because of the nano-projecting Al(OH)3. Ion-milled samples were prepared and exposed to air at 298 K for 168 h Fig. 6 shows FE-SEM images of the heat-treated 5 vol. % MWCNT-Al composite after exposure to air. The amorphous structures of the island-shaped Al(OH)3 projections (~500 nm in length) are confirmed by the FE-SEM-energy dispersive spectroscopy (EDS) and SAED patterns. These islands are oriented and the average distance between the midpoints of two adjacent Al(OH)3 structures is determined to be 1100 ± 400 nm (Fig. 6a). This average distance agrees well with that of adjacent nanodefects on the outer walls of acid-treated MWCNTs [17]. From this similarity, it was deduced that the islands correspond to the sites of Al4C3 produced at the nanodefects in the outer walls of the MWCNTs. Furthermore, the average length of Al(OH)3 structures is measured to be ~750 nm in the composite heat-treated at 903 K (Fig. 6b), which is slightly longer than that of the composite heat-treated at 873 K, indicating the temperature dependence of the growth of Al4C3. Fig. 7 shows HRTEM micrographs of Al4C3, which perhaps formed at a nanodefect in the outer walls of the MWCNT. The white arrow in Fig. 7a shows the site of the nanodefect. The Al4C3 is confirmed by the corresponding SEAD pattern taken by nanobeam diffraction, as shown in the inset of Fig. 7a. The streaks, indicated by green arrows in the inset along the <001> direction of Al4C3, are
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clearly shown, indicating the high crystallinity of the Al4C3 formed on the outer walls. Interestingly, the inner walls retain the structure of graphene. The MWCNTs used in this study were produced by a chemical vapor deposition (CVD) process; the outer walls were thus partially defective while the inner walls were defect-free by graphitization at very high temperatures for the post-deposition thickening process [26]. The inner walls with nearly perfect graphene surfaces were very stable and effectively restricted the epitaxial growth of Al4C3 along the direction perpendicular to the tubular axis of the MWCNT. If Al4C3 initiated at the end of the MWCNT and grew along the axis, the whole cross-section of the MWCNT would be converted to Al4C3. Fig. 7b presents the continuous and mutually <110>-oriented basal crystal planes of Al4C3. Based on SAED analysis at the MWCNT/Al4C3 interface, the Al4C3 is found to form parallel to the sidewall of the MWCNT. Thus, the typical orientation relationship between the MWCNT and Al4C3 is MWCNT (002)//Al4C3 (001), indicating the epitaxial growth of Al4C3 in the <001> direction. It is thought that nano-Al4C3 crystals were generated at nanodefects on the MWCNT, followed by both axial growth in the <110> direction and radial growth in the <001> direction epitaxially. However, the growth in the <001> direction was restricted by the defect-free inner walls of the MWCNTs. 3.4. Morphology and characteristics of Al4C3 Fig. 8 shows the MWCNTs incorporated in the Al matrix of the 3.0 vol.% MWCNT-Al composite heat-treated at 913 K for 2 h. The SAED pattern (see Fig. 8b) indicates the formation of Al4C3. Under this heat treatment condition, most MWCNTs were reacted and transformed into Al4C3. The Al4C3 is found to be monocrystalline with a rhombohedral structure, confirmed by the strong and periodical SAED pattern. Interestingly, the basal plane of Al4C3 (001) is parallel to the axis of the MWCNT (Fig. 8b). As shown in Fig. 8a, Al4C3 has a rod-like form, similar to the pristine MWCNT, suggesting that MWCNTs directly reacted to form Al4C3 without substantial changes to the original shape of the tubes during the heat treatment. In addition, the Al4C3 has no hollow core. This rod-like Al4C3 formation may support the results of Chen et al., where Al4C3 nanorods were prepared using MWCNTs as templates [27]. Notably, some dislocation lines are observed in the Al4C3 crystal (see the white arrows in Fig. 8a). The dislocation lines are
Fig. 6. FESEM images of the heat-treated 5 vol. % MWCNT-Al composite after exposure to air for 168 h, (a) heat-treated at 873 K for 1 h; (b) heat-treated at 903 K for 1 h. (A colour version of this figure can be viewed online)
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Fig. 7. HRTEM observations of (a) low and (b) high magnification view of the 5 vol. % MWCNT-Al composite after heat treatment at 883 K for 1 h; Inset of (a) showing the SAED pattern of Al4C3. (A colour version of this figure can be viewed online)
Fig. 8. Morphology of the 3.0 vol. % MWCNT-Al composite after heat treatment at 913 K for 2 h (a) A TEM image showing two typical Al4C3 structures; (b) SAED pattern of the Al4C3 acquired from the marked area in (a). (A colour version of this figure can be viewed online)
approximately aligned with the <110> direction, which is ascribed to the rapid growth rate of Al4C3 along the <110> direction and the insufficient release of internal stress during the heat treatment. It seems that the growth of Al4C3 might be fast enough that Al4C3 of several nanometers in the initial stage has never been observed under HRTEM, even with several seconds of heat treatment. 3.5. Formation mechanism of Al4C3 The quantity of Al4C3 was defined as the ratio of the average length of formed Al4C3 and the average length of acid-treated MWCNTs. The average length of each was measured from HRTEM images. Fig. 9a shows the dependence of the quantity of Al4C3 on the heat treatment temperature, as well as the heat treatment time. In the case of heat-treatment for 1 h, the quantity of Al4C3 increases with increasing temperature. The quantity of Al4C3 reaches nearly 100% when the heat-treatment temperature approaches the melting point of Al. For the heat-treatment temperatures of 883 K and 903 K (see Fig. 9b), a parabolic variation is observed and the increase rate of the quantity of Al4C3 decreases as the time increases. To clarify the kinetics of the interfacial phenomenon, the Arrhenius law as Equation (2) was applied to estimate the activation energy of Al4C3 formation [28]:
A ¼ A0exp (eQ/RT)
(2)
where A is the quantity of Al4C3, Q is the activation energy, A0 is the frequency factor, T is the absolute temperature of heat treatment, and R is the gas constant (8.31 J mol1 K1). Fig. 10 shows the resultant linear relation of the plot of ln(A) as a function of (1/T). The activation energy of approximately 195.8 kJ/mol for the formation of Al4C3 is estimated from the slope of Fig. 10. This activation energy is equivalent to the typical activation energy for the formation of Al4C3 in carbon fiber-reinforced Al matrix composites [29]. Lee et al. reported that AleC reactions occurred at the reactive prism plane edges of carbide fibers. The equivalence of the activation energy supports the identity of nanodefects located on the MWCNT surface and open ends of MWCNTs as locations in MWCNT-Al composites where the AleC reaction occurs. Robinson reported that the activation energy for lattice self-diffusion in Al was approximately 140 kJ/mol as evaluated by tracer techniques from 573 to 923 K [30]. This value is lower than the estimated activation energy for Al4C3 formation. Considering the parabolic variation of the Al4C3 quantity with heat treatment time, it is thought that the rate-limiting factor of Al4C3 growth is supposed to be the diffusion of Al atoms through the Al4C3 crystal. From the context above, a possible mechanism of formation and growth of Al4C3 in MWCNT/Al composites in the solid state is
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Fig. 9. The quantity of Al4C3 as a function of (a) temperature heat-treated for 1 h and (b) time heat-treated at 883 K and 903 K. (A colour version of this figure can be viewed online)
Fig. 10. Plot of lnA (A is the quantity of Al4C3) versus 1/T; the estimation of activation energy for the Al4C3 formation in the temperature range from 873 to 933 K. (A colour version of this figure can be viewed online)
proposed as follows. (1) At the initial reaction stage, nano-Al4C3 is preferentially formed at the open ends and nanodefects of MWCNTs by chemicals reaction between dangling carbon bonds and Al atoms. (2) The diffusion of Al atoms through the formed nano-Al4C3 dominates the growth of Al4C3. (3)-1 The open-tip-originated Al4C3 will grow quickly along the <110> axial direction of the MWCNT, whereas the growth rate may gradually decrease with increased length of the Al4C3 crystal because of the longer diffusion path of Al atoms. (3)-2 Meanwhile, the growth of nanodefect-originated Al4C3 will simultaneously occur along the <110> and <001> directions by the diffusion of Al atoms through irregularities in the outer walls of the MWCNT. The axial growth of Al4C3 in the <110> direction should be faster than the radial growth in the <001> direction. The radial growth is restricted by the defect-free inner walls. This is why partially formed Al4C3 was occasionally observed on the outer layers of the MWCNT (see Fig. 7). When two Al4C3 crystals formed at adjacent nanodefects on the outer walls of the MWCNT meet each other during the growth stage, they are supposed to be unified, including twinning, in addition to the compressive stress as previously mentioned. 3.6. Strengthening effect by Al4C3 formation In order to enhance the load transfer at the interface between the MWCNTs and the Al matrix in MWCNT-Al composites, the formation of small amounts of Al4C3 is supposed to be helpful. The realization of partially formed Al4C3 at the open ends and
nanodefects of MWCNTs includes several aspects. (1) The covalent bonds with Al4C3 may bridge the outmost and inner walls of the MWCNT, preventing the peeling of the outmost wall from the inner walls under loading and thus increasing the load-bearing capacity. (2) The nano-Al4C3 formed can enhance the interfacial shear resistance between the MWCNT and Al matrix, preventing the pullout of MWCNTs from the matrix. (3) The diameter of Al4C3 should be slightly larger than that of the original MWCNTs, forming strong anchors for the MWCNTs in the Al matrix and thereby contributing to the load bearing. Notably, the Al4C3 formation is suggested to be controlled by a combination of intentionally induced nanodefects and precisely controlled heat treatment. Insufficient amounts of nanocarbides would not strengthen the chemical bonds between MWCNTs and the matrix for bearing loads, while excess amounts of nanocarbides may reduce the effective length of MWCNTs, leading to poor exploitation of the intrinsic strength of MWCNTs. A study on load transfer from the Al matrix to MWCNTs is now underway by in-situ pullout testing of individual MWCNTs from the Al matrix. The effect of carbide inclusion on the improvement of load transfer should be investigated in future works, including the optimization of the amount of carbide nanostructures formed on MWCNTs. 4. Conclusions In summary, the interfacial reactions between the MWCNT and Al matrix were investigated in a fully dense MWCNT-Al composite by precisely controlled heat treatments at temperatures below the Al melting point. In this composite, the acid-treated, individually dispersed MWCNTs with close interfaces with Al were mostly aligned in the direction of hot extrusion. It was shown that MWCNT (002) formed a coherent interface with low-index Al planes of Al (111), Al (220), and Al (002), all of which created relatively stable interfaces. HRTEM examination and a novel technical observation by FE-SEM clearly revealed that Al4C3 was preferentially formed at the active prism plane edges located at the open ends and acid treatment-induced sidewall nanodefects of the MWCNTs. It was confirmed that Al4C3 was single-crystalline and frequently contained unusual twinning features because of the internal compressive stress and different growth paths of Al4C3 on the MWCNTs. The Al4C3 maintained the shape of the pristine MWCNT and showed a typical orientation relationship with Al matrix, that is, Al (111)//Al4C3 (001). Furthermore, the activation energy of Al4C3 formation, calculated as approximately 195.8 kJ/mol, as well as the parabolic relation between the quantity of Al4C3 and heattreatment time, supported the generation of Al4C3 at the prism plane edges and the growth of Al4C3 by the diffusion of Al atoms through the Al4C3 crystal. These results suggested that the open tip-
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originated Al4C3 may quickly grow in the <100> direction of the MWCNT, while nanodefect-originated Al4C3 may simultaneously grow in the <110> and <100> directions of the MWCNT.
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Acknowledgment
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WW Zhou would like to thank the support of China Scholarship Council. The authors also appreciate the generous helps from Prof. Naoyuki Nomura, Dr. Keiko Kikuchi, Dr. Rui Yamada, Dr. Shihai Sun and Dr. Xin Lu in Tohoku University, and Dr. Mehdi Estili in National Institute for Materials Science.
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Appendix A. Supplementary data [17]
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