Multi-Walled Carbon Nanotube-Aluminum Matrix Composites ...

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Sep 25, 2011 - Yohida, L. Licea-Jimenez, S. A. Perez-Garcia and R. Martinez-. Sanchez: Mater. Sci. Eng. A 502 ... 59 (2008) 703–705. 18) M. Estili and A.
Materials Transactions, Vol. 52, No. 10 (2011) pp. 1960 to 1965 #2011 The Japan Institute of Metals

Multi-Walled Carbon Nanotube-Aluminum Matrix Composites Prepared by Combination of Hetero-Agglomeration Method, Spark Plasma Sintering and Hot Extrusion*1 Hiroki Kurita1; *2 , Hansang Kwon2; *3 , Mehdi Estili3 and Akira Kawasaki1 1

Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan Dongnam Regional Division, R&D Department Convergence Component Material Research Group, Korea Institute of Industrial Technology (KITECH), Busan 618-230, Korea 3 Fine Particle Processing Group, Nano Ceramics Center National Institute for Materials Science (NIMS), Tsukuba 305-0047, Japan 2

Multi-walled carbon nanotubes (MWCNTs) with outstanding mechanical properties are considered as an ultimate reinforcement for conventional metals such as aluminum (Al), though they require uniform dispersion and intimate contacts with the host metal matrix. However, bundled structure of chemically stable, pristine MWCNTs has been the main challenge to achieving the expected structural reinforcement. In addition, the sole contribution of MWCNTs in strengthening the metal matrix has not been recognized yet, due to the concurrent strengthening mechanisms such as metal’s work hardening, grain refinement, etc. Here, we prepared MWCNT-Al matrix composite powders with uniform MWCNT dispersion by using hetero-agglomeration principle, and fabricated fully dense 1.0 vol% MWCNT-Al matrix composite bulk by using spark plasma sintering (SPS) and a subsequent hot extrusion process, with a 40% improved tensile strength and an elongation to failure of 27.3% similar to that of cast pure Al. During the SPS process, the nanoscale surface defects of MWCNTs were infiltrated by momentarily formed liquid Al and an intimate Al4 C3 -free interface was formed between MWCNTs and bare Al matrix. After the hot extrusion process, straight MWCNTs aligned in the extrusion axis found intimate contacts with the dynamically recovered, crack-free Al matrix. Our study suggests that the tensile improvement realized in our extruded/SPSed composites is directly originated from an effective load transfer at the MWCNT/Al interface, because no evidence of Al work hardening or the formation of interfacial Al4 C3 crystals was detected. [doi:10.2320/matertrans.M2011146] (Received May 16, 2011; Accepted July 22, 2011; Published September 25, 2011) Keywords: powder metallurgy, spark plasma sintering, carbon nanotubes, metal matrix composite, tensile strength

1.

Introduction

Multi-walled Carbon nanotubes (MWCNTs) with outstanding axial elastic modulus (1 TPa) and tensile strength can be considered as an ultimate reinforcement for conventional metals such as aluminum (Al), which is widely used in the industry as a light and inexpensive structural material.1–3) A vital prerequisite for the MWCNT-assisted structural reinforcement of Al is the uniform dispersion of individual MWCNTs within the Al matrix with intimate interfacial contacts.4) However, bundled structure of chemically stable, pristine MWCNTs has been the main challenge to achieving the expected structural reinforcement.4) To address this challenge, the mechanical alloying5–8) approach followed by hot rolling,9) hot extrusion5) and plasma spraying10,11) were previously employed and some tensile improvements were reported, though the contribution of responsible strengthening mechanisms (effects of MWCNT itself, MWCNT-induced prohibition of Al grain growth, work hardening and grain refinement of Al particles during ball milling, etc.) were not clearly recognized and no solid evidence of the uniform MWCNT dispersion within the matrix was provided.5–11) Therefore, the sole contribution of MWCNTs in the strengthening of the Al matrix could not be understood. In these processing methods, the issue of damaging MWCNTs during ball milling and high-temperature consolidation also needs to be carefully considered. *1This

Paper was Originally Published in Japanese in J. Japan Inst. Metals 75 (2011) 259–264. *2Graduate Student, Tohoku University *3Present address: Advanced composite materials processing, RIPSResearch Institute of Peace Studies, Seoul 135-895, Korea

Recently, Kwon et al. reported on the tensile improvements realized in MWCNT-Al matrix composites fabricated by a nanoscale dispersion method providing a uniform dispersion of MWCNTs within the Al powders12) followed by consolidation using spark plasma sintering (SPS) and a subsequent hot extrusion process.13) In their study, formation of Al4 C3 at the interface, believed to favor the load transfer from the matrix to MWCNTs was attributed to the reaction between the liquid Al phase momentarily appeared during SPS and surface defects of MWCNTs.13,14) However, due to the poor quality of starting MWCNTs used in their experiments, the MWCNTs strengthening contribution could not be exactly determined. Here, we prepared the composite powders with uniform MWCNT dispersion by mixing the highly crystalline, surface-functionalized MWCNTs and the Al powders using hetero-agglomeration principle,15–18) and fabricated fully dense MWCNT-Al matrix composite bulks by using the SPS and a subsequent hot extrusion process. 2.

Materials and Methods

Pristine MWCNTs (Nano Carbon Technology Co. Ltd.) with an average diameter of 50 nm and average length of 10 mm, and gas atomized Al powder (Ecka granule Japan Co. Ltd.) with 99.85% purity and an average particle size of 6 mm were used as starting materials in our study. A schematic illustration of our processing strategy is shown in Fig. 1. At first, a controlled surface modification process (acid treatment in an ultrasonicated mixture of H2 SO4 98%/HNO3 68% (3 : 1 v/v) at 50 C for 24 h) was performed not only to breaking the strong agglomerates of

Multi-Walled Carbon Nanotube-Aluminum Matrix Composites Prepared by Combination of Hetero-Agglomeration Method

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Fig. 1 Schematic illustration of the processing method employed to fabricate fully dense MWCNT-Al matrix composite bulks with uniform dispersion of MWCNTs and intimate interfacial contacts.

pristine MWCNTs but also to attach oxygen-containing functional groups to the MWCNT surface for enhancing the chemical compatibility with the Al matrix. The surfacetreated MWCNTs were then thoroughly washed with distilled water before being dispersed in ethanol under ultrasonication. Next, the MWCNT/ethanol suspension was mixed with an ultrasonicated Al particle/ethanol suspension and the MWCNT-decorated Al particles were then obtained due to the hetero-agglomeration of oppositely charged MWCNTs and Al particles.15) The dried composite powders were consolidated into fully dense composite bulks (1.0 and 5.0 vol% MWCNT) by SPS (Dr. Sinter S511, SPS Sintex Inc.) using carbon die and punches at 600 C, 50 MPa applied stress, 20 min holding time and a heating rate of 40 C/min. Finally, the SPSed composite bulks (15 mm diameter and 30 mm height) were extruded in 60 conical die at 550 C with the applied pressure of 500kN (UH-500kN1, Shimadzu corporation, Japan). The extrusion experiments were performed in a rate of 1 mm/min and a ratio of 20. Crystalline of pristine and surface-functionalized MWCNTs was evaluated by Raman spectroscopy (SOLAR TII Nanofinder, Tokyo Instruments Co. Ltd., Japan). Microstructural characterization of MWCNTs, the composite powders, and the SPSed and extruded composite bulks were performed by field emission scanning electron microscope (FE-SEM; JSM-6500F, JEOL, Japan) and highresolution transmission electron microscope (HR-TEM; HF-2000EDX, Hitachi, Japan). MicroVickers hardness of the MWCNT-Al matrix composite bulks were measured by Vickers hardness tester (MVK-Type G7, Akashi Co. Ltd., Japan) using 1.96N in 15 s. The extruded composite bulks were machined for tensile testing in accordance with JIS Z 2201. The tensile tests were performed using the universal testing machine (AUTOGRAPH AG-I 50kN, Shimadzu Co. Ltd., Japan). 3. 3.1

Results and Discussion

Dispersion of surface-functionalized MWCNTs within Al powders Molecular vibrational frequencies of pristine and acidtreated surface-functionalized MWCNTs, which can be used to evaluate the degree of disorders/defects of MWCNTs after the acid treatment process are appeared in Fig. 2. Two features in the first-order Raman spectra are a G-band at

Fig. 2 Raman spectra of pristine and acid-treated surface-functionalized MWCNTs.

1580 cm 1 revealing the two-dimensional graphitic ordering of nested graphene layers of MWCNTs, and a D-band at 1350 cm 1 which is highly responsive to non-planar atomic distortions, amorphous carbon, MWCNT curvature and other carbon impurities.19,20) Although the ID =IG ratio shows increase for acid-treated MWCNTs due to formation of defects or amorphous carbons (see Fig. 3); however, still a considerable graphitic ordering exist in the acid-treated MWCNTs looking at the absolute ID =IG ratio of the treated MWCNTs; i.e. the crystalline integrity of pristine MWCNTs is considerably preserved after our controlled acid treatment process. Figure 4 shows the FE-SEM images of two types of 1.0 vol% MWCNT-Al matrix composite powders made from pristine (Fig. 4(a)) and surface-functionalized MWCNTs (Fig. 4(b)). While the pristine MWCNTs were failed to be dispersed within the Al particles (see the big, porous MWCNT agglomerates in Fig. 4(a)), the long and straight surface-functionalized MWCNTs were individually and uniformly dispersed within the Al powders. At first, the appearance of individual, long and straight MWCNTs in Fig. 4(b) shows the success of our controlled acid-treatment process in breaking the MWCNT agglomerates without considerable destruction of MWCNTs integrity, which is well consistent with the Raman spectroscopy results shown in

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Fig. 2. On the other hand, our results suggest that the heteroagglomeration method, in which two oppositely charged, well-dispersed species in a liquid media attract each other by electrostatic forces can be used to prepare homogeneous MWCNT-Al matrix composite powders (see the uniform MWCNT dispersion in Fig. 4(b)). The acid-treated MWCNTs having oxygen-containing surface functional groups (e.g., Carboxylic, etc.) are believed to be negatively charged, while Al particles with their thin alumina layers believed to be positively charged in ethanol. Thus, it is highly possible for the well-dispersed Al particles to be decorated with the oppositely charged, long and straight individual MWCNTs during mixing in ethanol. Although the heteroagglomeration method was mainly used for preparation of

Fig. 3 HR-TEM image of a common nanoscale surface defect of acidtreated functionalized MWCNTs.

MWCNT-ceramic matrix composite powders,15–18) our study confirms the effectiveness of this method in ethanol media for preparation of homogeneous MWCNT-Al metal matrix composite powders, though some MWCNT agglomerates were observed in higher MWCNT concentrations (e.g., 5.0 vol%). In this method, undesirable issues of MWCNT damage, work hardening and grain refinement of Al powders occurred in ball milling-assisted preparation method,5–8) and formation of impurities during binder removal of composite powders happened in nanoscale dispersion method12) are successfully resolved. 3.2 SPS of MWCNT-Al matrix composite powders Figure 5 shows TEM images of SPSed 5.0 vol% MWCNT-Al matrix composite bulk with relative density of 99.5%. The image reveals that our SPS process conducted at 600 C, which is below the melting temperature of Al surprisingly led to the infiltration of nanoscale surface defects (Fig. 5(b) and Fig. 3) of MWCNT with the Al phase and formation of an intimate interfacial contact between MWCNT and bare Al matrix (see defect) with no intermediate interfacial compounds such as Al4 C3 . This suggests that the liquid Al could have momentarily formed during SPS and that the formation of Al4 C3 was prohibited mainly due to the high quality of our surface-functionalized MWCNTs and a SPS holding time inadequate for the chemical reaction of liquid Al and MWCNTs to occur. Similar interfacial phenomena excluding the lack of interfacial Al4 C3 crystals were previously observed in SPSed samples of Kwon et al. containing curled MWCNTs with poor quality.14) Formation of such an intermediate interfacial compound was also reported by Ci et al.21) claiming the formation of Al4 C3 at amorphous carbon layer of MWCNT surface, open-ends and surface defects of MWCNTs during heat treatment at 650 C. They also found no evidence of chemical reaction between the liquid Al and the inner walls of MWCNTs even at 800 C, but only at the MWCNT surface and the openends.21)

Fig. 4 FE-SEM images of 1.0 vol% MWCNT-Al matrix composite powders. (a): Agglomerates of pristine MWCNTs within Al powders, (b): Uniform dispersion of individual long and straight surface-functionalized MWCNTs within Al powders.

Multi-Walled Carbon Nanotube-Aluminum Matrix Composites Prepared by Combination of Hetero-Agglomeration Method

Fig. 5

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TEM images of MWCNT/Al interface in SPSed 5.0 vol% MWCNT-Al matrix composite bulk. (a): Low, (b): High magnification.

temperatures and longer holding times, which needs to be investigated. 3.3

Hot extrusion of the SPSed MWCNT-Al matrix composite bulk Figure 7 shows the TEM images of the SPSed 5.0 vol% MWCNT-Al matrix composite bulk after the hot extrusion process, in which individual, straight MWCNTs aligned in the extrusion axis are intimately embedded in a crack-free Al matrix. However, a few MWCNTs were failed or severely deflected. No Fresnel fringes generally appear near a crack in the low-magnification TEM images was found here, which confirms that the extrusion process did not cause any interfacial cracks; i.e., an intimate MWCNT/Al interface still exists after the extrusion process. The 99.9% relative density of the extruded composite further confirms the existence of the intimate MWCNT/Al interface. As shown in Fig. 8, the extrusion process had almost a negligible effect on the micro Vickers hardness mainly due to the dynamic recovery of the Al matrix during the hightemperature extrusion (550 C). Perhaps, a more reliable investigation can be made looking at the smaller test areas using Vickers nano indentation method.

Fig. 6 TEM image of ACF/Al interface in SPSed 5.0 vol% ACF-Al matrix composite bulk. The inset shows the selected area electron diffraction pattern of the interfacial Al4 C3 crystals.

We further investigated the formation mechanism of interfacial Al4 C3 crystals by preparing amorphous carbon fiber (ACF)-Al matrix composites SPSed at the same condition as our composites. Figure 6 reveals the formation of needle-like Al4 C3 crystals which are indicated with white arrows at the ACF/Al interface after SPS. Thus, it can be concluded that the formation of Al4 C3 crystals during SPS strongly depends on the crystallinity and purity of MWCNTs dispersed within the Al powders. Perhaps, the formation of Al4 C3 in our composites can be favorable in higher SPS

3.4 Tensile strength and strengthening mechanisms Figure 9 shows the stress-strain responses of the extruded/ SPSed MWCNT-Al matrix composites, the extruded/SPSed Al and the cast pure Al. Ultimate tensile strengths of MWCNT-Al matrix composite bulks were increased by about 40% (from 115 MPa to 160 MPa) compared with the Al consolidated by the same process. The addition of 1.0 and 5.0 vol% MWCNTs to the Al matrix led to a similar improvement of the tensile strength. Tensile strength of composite can be increased with MWCNT content. It was reported that a cluster of MWCNTs was revealed in MWCNT5.0 wt%-Al composite by Esawi et al.,22) it is possible that the effect of MWCNT content is canceled by MWCNT agglomeration in MWCNT5.0 vol%-Al com-

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Fig. 7

TEM images of the SPSed 5.0 vol% MWCNT-Al matrix composite bulk after the hot extrusion process at 550 C.

Fig. 8 Effect of the extrusion process on the microVickers hardness of the SPSed 5.0 vol% MWCNT-Al matrix composite bulk.

posite. Thus it can be seemed tensile strength is constant. Surprisingly, elongation to failure of the 1.0 vol% MWCNTAl matrix composite reached about 27.3% (similar to that of cast pure Al) despite the 40% improvement in the ultimate tensile strength. It can be concluded that the tensile improvement realized in our extruded/SPSed composites is directly originated from an effective load transfer at the MWCNT/Al interface, because no evidence of Al work hardening (e.g., less dislocations (Fig. 7) and dynamic recovery (Fig. 8)) or the formation of interfacial Al4 C3 crystals was detected. From this point of view, tensile strength of composite should be increased with MWCNT volume fraction. 4.

Conclusion

We prepared homogeneous MWCNT-Al matrix composite powders with uniform MWCNT dispersion by mixing the highly crystalline, surface-functionalized MWCNTs and the Al powders using the hetero-agglomeration principle, and fabricated fully dense 1.0 vol% MWCNT-Al matrix composite bulk by using the SPS and a subsequent hot extrusion process, with a 40% improved tensile strength and an

Fig. 9 Nominal stress-strain responses of the extruded/SPSed 1.0 and 5.0 vol% MWCNT Al matrix composites, extruded/SPSed Al and the cast pure Al.

elongation to failure of 27.3% similar to that of cast pure Al. During the SPS process, the nanoscale surface defects of MWCNTs were infiltrated by momentarily formed liquid Al and an intimate Al4 C3 -free interface was formed between MWCNTs and bare Al matrix. After the hot extrusion process, straight MWCNTs aligned in the extrusion axis found intimate contacts with the dynamically recovered, crack-free Al matrix. It can be concluded that the tensile improvement realized in our extruded/SPSed composites is directly originated from an effective load transfer at the MWCNT/Al interface, because no evidence of Al work hardening or the formation of interfacial Al4 C3 crystals was detected. Acknowledgment A financial grant of Global COE program, Materials integration international center of education and research (TOHOKU univ.) is gratefully acknowledged.

Multi-Walled Carbon Nanotube-Aluminum Matrix Composites Prepared by Combination of Hetero-Agglomeration Method

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