- Email: [email protected]
Fabrication of aluminum matrix composite reinforced with carbon nanotubes
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RARE METALS Vol. 26, No. 5, Oct 2007, p . 450 E-mail: [email protected]
ScienceDirect
Fabrication of aluminum matrix composite reinforced with carbon nanotubes DENG Chunfeng"2),ZHANG Xuexi", MA Yanxia2),and WANG Dezun') 1) School of Materials Science and Engineering,Harbin Institute of Technology,Harbin 15ooO1, China 2) Luoyang Ship Material Research Institute, Luoyang 471039, China (Received 2006-05-12)
Abstract: 1.0wt.% carbon nanotube (CNT) reinforced 2024Al matrix composite was fabricated by cold isostatic press and subsequent hot extrusion techniques. The mechanical properties of the composite were measured by a tensile test. Meanwhile, the fracture surfaces were examined using field emission scanning electron microscopy. The experimental results show that CNTs are dispersed homogeneously in the composite and that the interfaces of the A1 matrix and the CNT bond well. Although the tensile strength and the Young's modulus of the composite are enhanced markedly, the elongation does not decrease when compared with the matrix material fabricated under the same process. The reasons for the increments may be the extraordinary mechanical properties of CNTs, and the bridging and pulling-out role of CNTs in the Al matrix composite. Key words: composites; carbon nanotube composite; cold isostatic press; hot extrusion; fracture surface
1. Introduction Carbon nanotubes (CNTs) probably offer a kind of nanosize reinforcement with lightweight, hollow core, and immense aspect ratio, which exhibit remarkable mechanical, electrical, and thermal properties [l]. Moreover, CNTs have been proved to be stable under temperature as high as 2700 K in an argon atmosphere or under vacuum [2]. In recent years, incorporating CNTs to develop advanced engineering composites has become an interesting concept; a larger portion of the researches have focused on the development of CNT reinforced polymer [3] and ceramic based composites [4], and only a few studies have been concerned with the manufacture of CNT reinforced metal matrix composites [5-81. A very limited research has been done in the field of CNT reinforced metal matrix composites because the interfacial reaction between CNTs and metal matrices is complicated, and a suitable synthesis technique is scarce. It has been found from the Corresponding author: DENG Chunfeng
above investigation that under proper processing condition, the CNTs can be dispersed in the metal matrix and can maintain their good multi-walled structure. Meanwhile, the properties of composites have been improved relative to the metal matrices. In this attempt, a novel processing approach has been undertaken to fabricate a CNT-A1 nanostructurd composite by cold isostatic press and subsequent hot extrusion techniques. The investigation of the microstructure and the measurement of the mechanical properties of composites are reported.
2. Experimental 2.1. Preparation of the powder Multi-walled carbon nanotubes (-20 nm) with a purity of about 95% were provided by Shenzhen NanoPort Company, and the TEM image of the as-received CNTs is shown in Fig. l(a). The as-received CNTs tangle together into a network around impurities, which is difficult for the disper-
E-mail: [email protected]
Deng C.F. et aL, Fabrication of aluminum matrix composite reinforced with carbon nanotubes
sion owing to the existence of the impurities and the extraordinary aspect ratio of the CNTs. To remove the impurities among CNTs and to reduce the aspect ratio of CNTs, the raw CNTs were purified by the circumfluence in 68 wt.% nitric acid solution for 10 h at 393 K. Then, the extra acid was removed and the deposit was washed with distilled water for four times. Finally, they were added to the alcohol to retain uniform distribution. The image of the purified CNTs is shown in Fig. l(b). It is clearly seen that CNTs are dispersed homogeneously and rarely tangle together after the circumfluence. Meanwhile, the aspect ratio of the CNTs decreases greatly and no other forms of carbon and metal catalysts are identified during the TEM observations. The gas atomized 2024Al (4.20 wt.% Cu, 1.47 wt.% Mg, 0.56 wt.% Mn, 0.02 wt.% Zr, 0.40 wt.% Fe, 0.27 wt.% Si) powders (-50 pm) were used as
451
the matrix; 2024A1 powders were added to CNTs in alcohol, and the mixed powders were dispersed with mechanical stirring assisting ultrasonic shaker in alcohol at 353 K. Finally, the mixed powders were broken up using ball milling for 10 min. For this study, 1 wt.% CNTs have been mixed with 2024A1 powders. Fig. 2 illustrates the images of the mixed powders of 2024Al powders and CNTs. It can be seen from Fig. 2(a) that 2024A1 powders after mixing with CNTs still maintain the almost spherical shape, which is helpful for plastic forming of the powders. Meanwhile, it can also be seen from Fig. 2(b) that CNTs are distributed uniformly on the surfaces of the particles of 2024A1 powders, which can be attributed to the remarkable difference of the specific surface area of 2024Al powders and CNTs in the alcohol and several >c=O and -OH functional groups connected to CNTs after the circumfluence
Fig. 2. (a) SEM micrographs of the homogeneously mixed powders of 2Q24Aland CNTs; (b) local area magnified in Fig. Z(a).
452
of carbon nanotubes [9].
2.2. Measurement of the mechanical properties and the microstructure observation To obtain fully dense composite, to retain the excellent structure of gas-atomized powders, and to avoid the reaction of CNT-Al, cold isostatic press (CIP) and hot extrusion techniques were used. The CIP technique produced a higher and more even density throughout the sample when compared with mechanical press, since the pressure was applied isostatically [lo]. To avoid the oxidation of 2024A1 powders during hot extrusion in air atmosphere, the mixed powders were put into a pure A1 package (62 mm in diameter) and then into a rubber bag, and then the degassing was performed using a mechanical vacuum pump down to 0.01 Pa, for at least 20 min before the bag was sealed. The mixed powders were densified by cold isostatic press at 300 MPa for 10 min. After cold press, the billets were extruded at 723 K into a rod with 12 mm diameter (extrusion ratio = 25: 1). The morphology of CNTs before and after the circumfluence was observed by transmission electron microscopy ( E M CM12, Philips). The microstructure observation of the polished samples was performed using optical microscopy. The relative density of the composite was measured by the Archimedes method, using water immersion. The hardness was measured by the Vicker’s hardness test under a load of 5 kg. The mechanical properties of the composite were tested using a tensile machine with a speed of 0.5 mm/min. Each tensile test datum was obtained from an average of at least three specimens. In addition, to investigate the distribution of CNTs in the composite and the interfacial property between the CNTs and the A1 matrix, field emission scanning electron microscopy (FESEM) was used to examine the fracture surfaces of the tensile specimens.
3. Results and discussion Fig. 3(a) shows the optical micrograph of the mixed powders by isostatic press. It can be obvi-
RARE METALS, Vol. 26, No. 5, Oct 2007
ously found that the holes in the compacts are very uniform, and the morphology of the A1 powders does not change relative to that of the raw powders owing to the equivalent pressure being applied around compacts, which is helpful for the plastic forming of the powders to reach full density. Fig. 3(b) indicates the image of the Al-CNT composite after hot extrusion. The black spots on the surface signify the presence of CNTs in the composite structure. The black carbon phases are evenly distributed throughout the A1 matrix. The pores are not visible apparently. However, some of the black spots may denote some degree of porosity in the composite structure.
Fig. 3. Optical micrographs of the mixed powders of 2024A1 and CNTs after isostatic press (a) and Al-CNT composite after hot extrusion @).
Young’s modulus, tensile strength, and elongation were obtained using the average of three values obtained from the stress-strain curves for each case. The experimental mechanical properties of 1 wt.% CNTl2024Al composite are listed in Table 1. The
Deng C.F. et al., Fabrication of aluminum matrix composite reinforced with carbon nanotubes
hardness, Young’s modulus, and the tensile strength of the A1-CNT composite were enhanced by 30.8%, 41.3%, and 35.7% relative to the 2024Al matrix, respectively. It is interesting that the high strength obtained is at the expense of the plasticity of the matrix for the traditional fibers reinforced composites. However, carbon nanotube reinforced composites do not comply with the law. The elongation of the composite reaches 17.9%. It is equivalent to that of the matrix fabricated under the same process, suggesting good bonding between the CNTs and the A1
453
matrix. These can be due to the factors that CNTs can increase the toughness of composites by absorbing energy because of their highly flexible elastic behavior during loading [l 11, as is different from the traditional fibers or whiskers. Furthermore, the A1 matrix integrality is not destroyed when small amount of CNTs are introduced into the composite. Nevertheless, the relative density is not perfect to both materials. The further increment of the relative density of the composites will be carried out by subsequent hot rolling technique [ 121.
Table 1. Comparative study of the properties of 1.0 wt.% CNT composite and the 2024A1 matrix Composition
Relative density / % Young’s modulus I GPa Tensile strength I MPa Elongation I % Hardness / MPa
2024A1
98.8
72.3
384.5
18.8
104k5
Al-CNT comtxite
98.1
102.2
521.7
17.9
136 ? 5
It is generally accepted that the mechanical properties of composites are dominated not only by the reinforcement and the matrix but also by the interfacial bonding status between them [13-141. It is necessary to study the materials fracture behavior to understand the load transfer between the matrix and the reinforcements. The fracture surfaces of the composite were examined by FESEM as shown in Fig. 4. Fig. 4(a) shows the tear-ridge and the shallow dimple morphologies of the composite, which are responsible for good plasticity of the composite. Improvements in the strength of CNT/Al are largely attributable to suficient load transfer from the matrix to CNTs through the interface. To ensure a good load transfer, the composite must maintain a medium strong interfacial bonding. Some CNTs with obviously tubular structure can be observed, which suggests that CNTs have been embedded into the Al matrix. Meanwhile, CNTs are distributed homogeneously in the A1 matrix, and some are pulled out, as shown Fig. 4(b). The pits formed because of the pulling-out of CNTs show that most CNTs are pulled out almost perpendicularly to the fracture surface, which indicates the stress transfer by shearing between the CNT and the A1 matrix. This indicates that high bonding strength between the CNTs and the A1 matrix is achieved. Fig. 4(c) shows the typical tubular morphology of the as-prepared CNTs.
A CNT bridges between the fractural surfaces of the A1 matrix, which is responsible for transferring the stress in the composite to enhance the mechanical properties of the composites. Noguchi [7] also reported similar results by introducing CNTs into the A1 matrix. With the addition of a very small amount of CNTs, the mechanical properties of the composite were largely enhanced. We assume it to be related to the following factors. Firstly, there are extraordinary mechanical properties. Young’s modulus and the tensile strength of CNTs are as high as 1 TPa and 200 GPa [15], respectively. Moreover, the hot extrusion technique has been used in processing the composite at a lower temperature and a shorter hold time when compared with those of conventional sintering techniques. The grains of the matrix in the composite are very fine since these retain the excellent structure of gas-atomized powders: moreover, CNTs hinder the growth of matrix grains [16], in which CNTs are undamaged. Finally, the bonding to the A1 matrix and the CNTs in the “bridging” and the “pulling-out’’ manner increases the interface strength of Al-CNTs and raises the fracture energy of the composites [17]. Further studies on the reinforced mechanism of CNTs in the A1 matrix composites will be summarized in our next study.
454
RARE METALS, Vol. 26, No. 5, Oct 2007
Fig. 4. SEM images of the fracture surfaces of the composite: (a) showing the shallow dimples of the matrix; (b) showing the pulling-out of CNTs; (c) showing the bridging of CNTs.
4. Conclusions (1) 1.0 wt.% CNT reinforced aluminum matrix composite was successfully fabricated by isostatic press and subsequent hot extrusion techniques. (2) CNTs were retained and dispersed homogeneously in composite structure. The interfaces between CNTs and the Al matrix bonded well. (3) The hardness, Young’s modulus, and tensile strength of the AI-CNT composite were enhanced by 30.8%, 41.3%, and 35.7% relative to the 2024Al matrix, respectively. Meanwhile, the elongation of the composite was maintained at 17.9%, which is equivalent to that of the matrix fabricated under the same process. (4) The fracture manners of the composites involved in “bridging” and “pulling-out” of CNTs in fracture surfaces.
References [l] Iijima S., Helical microtubes of graphitic carbon, Nature, 1991,354:56.
Ma R.Z., Wu J, Wei B.Q., Liang J., and Wu D.H., Processing and properties of carbon nanotubes-nanoS i c ceramic, J. Muter. Sci., 1998,33:5243. Yeh M.K., Tai N.H., and Liu J.H., Mechanical behavior of phenolic-based composites reinforced with multi-walled carbon nanotubes, Carbon, 2006, 44: 1. Peigney A., Flahaut E., Laurent C., Marliere C., Chaste1 F., and Rousset A., Aligned carbon nanotubes in ceramic-matrix nanocomposites prepared by high-temperature extrusion, Chem. Phys. Lea,2002.24: 20. Kuzumaki T., Miyazawa K., Ichinose H., and It0 K., Processing of carbon nanotube reinforced aluminum composite,J. Muter. Rex, 1998,13: 2445. Xu C.L., Wei B.Q., Ma R.Z., Liang J., Ma X.K., and Wu D.H., Fabrication of aluminum-carbon nanotube composites and their electrical properties, Carbon, 1999,37: 855. Noguchi T., Magario A., Fukazawa S., Shimizu S., Beppu J., and Seki M., Carbon nanotube/aluminium composites with uniform dispersion, M a w . Trans., 2004,45: 602. Cha S.I., Kim K.T., Arshad S.N.,Mo C.B., and Hong S.H., Extraordinary strengthening effect of carbon
Deng C.F. et aZ., Fabrication of aluminum matrix composite reinforced with carbon nanotubes
455
Kubokawa H., Sawai Y., and Kishi T., Effects of the nanotubes in metal-matrix nanocomposites processed SiC/AI interface reaction on fracture behavior of a by molecular-level mixing, A h . Mater., 2005, 17: composite conductor using S i c fiber reinforced 1377. aluminum for next generation power equipment, J. [9] Jia Z.J. and Wang Z.Y., Production of short Mater. Sci., 1999,34: 1583. multi-walled carbon nanotubes, Carbon, 1999, 37: [141 Bi G., Rao Q.L., and Xi F., Controlling the interface 903. reaction in Al18B4O33w/AC8Acomposite by nitrided [lo] LeeH.F.,BceyF.,KhorK.A.,TanM.J.,GanJ.,and nano-coating, Mater. Lett., 2004,58: 5. Loh N.L., The production of aluminium alloy [ 151 Jaroenapibal P., Luzzi D.E., Evoy S., and Arepalli S., composites using a cold isostatic press and extrusion Transmission-electron-microscopic studies of approach, J. Mater. Process. Technol., 1992,29: 245. mechanical properties of single-waited carbon [ l l ] Zhan G.D., Kuntz J.D., Wan J., and Mukherhee nanotube bundles, Appl. Phys. Lett., 2004, 85 (19): A.K., Single-wall carbon nanotubes as attractive 4328. toughening agents in alumina based nanocomposites, [16] Sun J., Gao L., Lwasa M., Nakayama T., and Nat. Mater., 2003,2: 42. Niihara K., Failure investigation of carbon nanotubel [12] Qu S.J., Geng L., Meng Q.C., Feng A.H., and Lei 3Y-TZP nanocomposites, Ceram. Int., 2005, 31: T.Q., Effects of hot rolling on microstructure and 1131. properties of a 20 vol.% SiCdAl composite, Rare [17] Zhang F.M., Shen J., and Sun J.F., Processing and Met., 2005,24 (1): 95. properties of carbon nanotubes-nano-WC-Co com[13] Yasutomi Y., Sawada J., &chi T., Nakamura K., posites, Mater. Sci. Eng. A., 2004,381: 86. Manabe Y., Nagano K., Kuroda H., Sumi T.,
RARE METALS Vol. 26, No. 5, Oct 2007, p . 450 E-mail: [email protected]
ScienceDirect
Fabrication of aluminum matrix composite reinforced with carbon nanotubes DENG Chunfeng"2),ZHANG Xuexi", MA Yanxia2),and WANG Dezun') 1) School of Materials Science and Engineering,Harbin Institute of Technology,Harbin 15ooO1, China 2) Luoyang Ship Material Research Institute, Luoyang 471039, China (Received 2006-05-12)
Abstract: 1.0wt.% carbon nanotube (CNT) reinforced 2024Al matrix composite was fabricated by cold isostatic press and subsequent hot extrusion techniques. The mechanical properties of the composite were measured by a tensile test. Meanwhile, the fracture surfaces were examined using field emission scanning electron microscopy. The experimental results show that CNTs are dispersed homogeneously in the composite and that the interfaces of the A1 matrix and the CNT bond well. Although the tensile strength and the Young's modulus of the composite are enhanced markedly, the elongation does not decrease when compared with the matrix material fabricated under the same process. The reasons for the increments may be the extraordinary mechanical properties of CNTs, and the bridging and pulling-out role of CNTs in the Al matrix composite. Key words: composites; carbon nanotube composite; cold isostatic press; hot extrusion; fracture surface
1. Introduction Carbon nanotubes (CNTs) probably offer a kind of nanosize reinforcement with lightweight, hollow core, and immense aspect ratio, which exhibit remarkable mechanical, electrical, and thermal properties [l]. Moreover, CNTs have been proved to be stable under temperature as high as 2700 K in an argon atmosphere or under vacuum [2]. In recent years, incorporating CNTs to develop advanced engineering composites has become an interesting concept; a larger portion of the researches have focused on the development of CNT reinforced polymer [3] and ceramic based composites [4], and only a few studies have been concerned with the manufacture of CNT reinforced metal matrix composites [5-81. A very limited research has been done in the field of CNT reinforced metal matrix composites because the interfacial reaction between CNTs and metal matrices is complicated, and a suitable synthesis technique is scarce. It has been found from the Corresponding author: DENG Chunfeng
above investigation that under proper processing condition, the CNTs can be dispersed in the metal matrix and can maintain their good multi-walled structure. Meanwhile, the properties of composites have been improved relative to the metal matrices. In this attempt, a novel processing approach has been undertaken to fabricate a CNT-A1 nanostructurd composite by cold isostatic press and subsequent hot extrusion techniques. The investigation of the microstructure and the measurement of the mechanical properties of composites are reported.
2. Experimental 2.1. Preparation of the powder Multi-walled carbon nanotubes (-20 nm) with a purity of about 95% were provided by Shenzhen NanoPort Company, and the TEM image of the as-received CNTs is shown in Fig. l(a). The as-received CNTs tangle together into a network around impurities, which is difficult for the disper-
E-mail: [email protected]
Deng C.F. et aL, Fabrication of aluminum matrix composite reinforced with carbon nanotubes
sion owing to the existence of the impurities and the extraordinary aspect ratio of the CNTs. To remove the impurities among CNTs and to reduce the aspect ratio of CNTs, the raw CNTs were purified by the circumfluence in 68 wt.% nitric acid solution for 10 h at 393 K. Then, the extra acid was removed and the deposit was washed with distilled water for four times. Finally, they were added to the alcohol to retain uniform distribution. The image of the purified CNTs is shown in Fig. l(b). It is clearly seen that CNTs are dispersed homogeneously and rarely tangle together after the circumfluence. Meanwhile, the aspect ratio of the CNTs decreases greatly and no other forms of carbon and metal catalysts are identified during the TEM observations. The gas atomized 2024Al (4.20 wt.% Cu, 1.47 wt.% Mg, 0.56 wt.% Mn, 0.02 wt.% Zr, 0.40 wt.% Fe, 0.27 wt.% Si) powders (-50 pm) were used as
451
the matrix; 2024A1 powders were added to CNTs in alcohol, and the mixed powders were dispersed with mechanical stirring assisting ultrasonic shaker in alcohol at 353 K. Finally, the mixed powders were broken up using ball milling for 10 min. For this study, 1 wt.% CNTs have been mixed with 2024A1 powders. Fig. 2 illustrates the images of the mixed powders of 2024Al powders and CNTs. It can be seen from Fig. 2(a) that 2024A1 powders after mixing with CNTs still maintain the almost spherical shape, which is helpful for plastic forming of the powders. Meanwhile, it can also be seen from Fig. 2(b) that CNTs are distributed uniformly on the surfaces of the particles of 2024A1 powders, which can be attributed to the remarkable difference of the specific surface area of 2024Al powders and CNTs in the alcohol and several >c=O and -OH functional groups connected to CNTs after the circumfluence
Fig. 2. (a) SEM micrographs of the homogeneously mixed powders of 2Q24Aland CNTs; (b) local area magnified in Fig. Z(a).
452
of carbon nanotubes [9].
2.2. Measurement of the mechanical properties and the microstructure observation To obtain fully dense composite, to retain the excellent structure of gas-atomized powders, and to avoid the reaction of CNT-Al, cold isostatic press (CIP) and hot extrusion techniques were used. The CIP technique produced a higher and more even density throughout the sample when compared with mechanical press, since the pressure was applied isostatically [lo]. To avoid the oxidation of 2024A1 powders during hot extrusion in air atmosphere, the mixed powders were put into a pure A1 package (62 mm in diameter) and then into a rubber bag, and then the degassing was performed using a mechanical vacuum pump down to 0.01 Pa, for at least 20 min before the bag was sealed. The mixed powders were densified by cold isostatic press at 300 MPa for 10 min. After cold press, the billets were extruded at 723 K into a rod with 12 mm diameter (extrusion ratio = 25: 1). The morphology of CNTs before and after the circumfluence was observed by transmission electron microscopy ( E M CM12, Philips). The microstructure observation of the polished samples was performed using optical microscopy. The relative density of the composite was measured by the Archimedes method, using water immersion. The hardness was measured by the Vicker’s hardness test under a load of 5 kg. The mechanical properties of the composite were tested using a tensile machine with a speed of 0.5 mm/min. Each tensile test datum was obtained from an average of at least three specimens. In addition, to investigate the distribution of CNTs in the composite and the interfacial property between the CNTs and the A1 matrix, field emission scanning electron microscopy (FESEM) was used to examine the fracture surfaces of the tensile specimens.
3. Results and discussion Fig. 3(a) shows the optical micrograph of the mixed powders by isostatic press. It can be obvi-
RARE METALS, Vol. 26, No. 5, Oct 2007
ously found that the holes in the compacts are very uniform, and the morphology of the A1 powders does not change relative to that of the raw powders owing to the equivalent pressure being applied around compacts, which is helpful for the plastic forming of the powders to reach full density. Fig. 3(b) indicates the image of the Al-CNT composite after hot extrusion. The black spots on the surface signify the presence of CNTs in the composite structure. The black carbon phases are evenly distributed throughout the A1 matrix. The pores are not visible apparently. However, some of the black spots may denote some degree of porosity in the composite structure.
Fig. 3. Optical micrographs of the mixed powders of 2024A1 and CNTs after isostatic press (a) and Al-CNT composite after hot extrusion @).
Young’s modulus, tensile strength, and elongation were obtained using the average of three values obtained from the stress-strain curves for each case. The experimental mechanical properties of 1 wt.% CNTl2024Al composite are listed in Table 1. The
Deng C.F. et al., Fabrication of aluminum matrix composite reinforced with carbon nanotubes
hardness, Young’s modulus, and the tensile strength of the A1-CNT composite were enhanced by 30.8%, 41.3%, and 35.7% relative to the 2024Al matrix, respectively. It is interesting that the high strength obtained is at the expense of the plasticity of the matrix for the traditional fibers reinforced composites. However, carbon nanotube reinforced composites do not comply with the law. The elongation of the composite reaches 17.9%. It is equivalent to that of the matrix fabricated under the same process, suggesting good bonding between the CNTs and the A1
453
matrix. These can be due to the factors that CNTs can increase the toughness of composites by absorbing energy because of their highly flexible elastic behavior during loading [l 11, as is different from the traditional fibers or whiskers. Furthermore, the A1 matrix integrality is not destroyed when small amount of CNTs are introduced into the composite. Nevertheless, the relative density is not perfect to both materials. The further increment of the relative density of the composites will be carried out by subsequent hot rolling technique [ 121.
Table 1. Comparative study of the properties of 1.0 wt.% CNT composite and the 2024A1 matrix Composition
Relative density / % Young’s modulus I GPa Tensile strength I MPa Elongation I % Hardness / MPa
2024A1
98.8
72.3
384.5
18.8
104k5
Al-CNT comtxite
98.1
102.2
521.7
17.9
136 ? 5
It is generally accepted that the mechanical properties of composites are dominated not only by the reinforcement and the matrix but also by the interfacial bonding status between them [13-141. It is necessary to study the materials fracture behavior to understand the load transfer between the matrix and the reinforcements. The fracture surfaces of the composite were examined by FESEM as shown in Fig. 4. Fig. 4(a) shows the tear-ridge and the shallow dimple morphologies of the composite, which are responsible for good plasticity of the composite. Improvements in the strength of CNT/Al are largely attributable to suficient load transfer from the matrix to CNTs through the interface. To ensure a good load transfer, the composite must maintain a medium strong interfacial bonding. Some CNTs with obviously tubular structure can be observed, which suggests that CNTs have been embedded into the Al matrix. Meanwhile, CNTs are distributed homogeneously in the A1 matrix, and some are pulled out, as shown Fig. 4(b). The pits formed because of the pulling-out of CNTs show that most CNTs are pulled out almost perpendicularly to the fracture surface, which indicates the stress transfer by shearing between the CNT and the A1 matrix. This indicates that high bonding strength between the CNTs and the A1 matrix is achieved. Fig. 4(c) shows the typical tubular morphology of the as-prepared CNTs.
A CNT bridges between the fractural surfaces of the A1 matrix, which is responsible for transferring the stress in the composite to enhance the mechanical properties of the composites. Noguchi [7] also reported similar results by introducing CNTs into the A1 matrix. With the addition of a very small amount of CNTs, the mechanical properties of the composite were largely enhanced. We assume it to be related to the following factors. Firstly, there are extraordinary mechanical properties. Young’s modulus and the tensile strength of CNTs are as high as 1 TPa and 200 GPa [15], respectively. Moreover, the hot extrusion technique has been used in processing the composite at a lower temperature and a shorter hold time when compared with those of conventional sintering techniques. The grains of the matrix in the composite are very fine since these retain the excellent structure of gas-atomized powders: moreover, CNTs hinder the growth of matrix grains [16], in which CNTs are undamaged. Finally, the bonding to the A1 matrix and the CNTs in the “bridging” and the “pulling-out’’ manner increases the interface strength of Al-CNTs and raises the fracture energy of the composites [17]. Further studies on the reinforced mechanism of CNTs in the A1 matrix composites will be summarized in our next study.
454
RARE METALS, Vol. 26, No. 5, Oct 2007
Fig. 4. SEM images of the fracture surfaces of the composite: (a) showing the shallow dimples of the matrix; (b) showing the pulling-out of CNTs; (c) showing the bridging of CNTs.
4. Conclusions (1) 1.0 wt.% CNT reinforced aluminum matrix composite was successfully fabricated by isostatic press and subsequent hot extrusion techniques. (2) CNTs were retained and dispersed homogeneously in composite structure. The interfaces between CNTs and the Al matrix bonded well. (3) The hardness, Young’s modulus, and tensile strength of the AI-CNT composite were enhanced by 30.8%, 41.3%, and 35.7% relative to the 2024Al matrix, respectively. Meanwhile, the elongation of the composite was maintained at 17.9%, which is equivalent to that of the matrix fabricated under the same process. (4) The fracture manners of the composites involved in “bridging” and “pulling-out” of CNTs in fracture surfaces.
References [l] Iijima S., Helical microtubes of graphitic carbon, Nature, 1991,354:56.
Ma R.Z., Wu J, Wei B.Q., Liang J., and Wu D.H., Processing and properties of carbon nanotubes-nanoS i c ceramic, J. Muter. Sci., 1998,33:5243. Yeh M.K., Tai N.H., and Liu J.H., Mechanical behavior of phenolic-based composites reinforced with multi-walled carbon nanotubes, Carbon, 2006, 44: 1. Peigney A., Flahaut E., Laurent C., Marliere C., Chaste1 F., and Rousset A., Aligned carbon nanotubes in ceramic-matrix nanocomposites prepared by high-temperature extrusion, Chem. Phys. Lea,2002.24: 20. Kuzumaki T., Miyazawa K., Ichinose H., and It0 K., Processing of carbon nanotube reinforced aluminum composite,J. Muter. Rex, 1998,13: 2445. Xu C.L., Wei B.Q., Ma R.Z., Liang J., Ma X.K., and Wu D.H., Fabrication of aluminum-carbon nanotube composites and their electrical properties, Carbon, 1999,37: 855. Noguchi T., Magario A., Fukazawa S., Shimizu S., Beppu J., and Seki M., Carbon nanotube/aluminium composites with uniform dispersion, M a w . Trans., 2004,45: 602. Cha S.I., Kim K.T., Arshad S.N.,Mo C.B., and Hong S.H., Extraordinary strengthening effect of carbon
Deng C.F. et aZ., Fabrication of aluminum matrix composite reinforced with carbon nanotubes
455
Kubokawa H., Sawai Y., and Kishi T., Effects of the nanotubes in metal-matrix nanocomposites processed SiC/AI interface reaction on fracture behavior of a by molecular-level mixing, A h . Mater., 2005, 17: composite conductor using S i c fiber reinforced 1377. aluminum for next generation power equipment, J. [9] Jia Z.J. and Wang Z.Y., Production of short Mater. Sci., 1999,34: 1583. multi-walled carbon nanotubes, Carbon, 1999, 37: [141 Bi G., Rao Q.L., and Xi F., Controlling the interface 903. reaction in Al18B4O33w/AC8Acomposite by nitrided [lo] LeeH.F.,BceyF.,KhorK.A.,TanM.J.,GanJ.,and nano-coating, Mater. Lett., 2004,58: 5. Loh N.L., The production of aluminium alloy [ 151 Jaroenapibal P., Luzzi D.E., Evoy S., and Arepalli S., composites using a cold isostatic press and extrusion Transmission-electron-microscopic studies of approach, J. Mater. Process. Technol., 1992,29: 245. mechanical properties of single-waited carbon [ l l ] Zhan G.D., Kuntz J.D., Wan J., and Mukherhee nanotube bundles, Appl. Phys. Lett., 2004, 85 (19): A.K., Single-wall carbon nanotubes as attractive 4328. toughening agents in alumina based nanocomposites, [16] Sun J., Gao L., Lwasa M., Nakayama T., and Nat. Mater., 2003,2: 42. Niihara K., Failure investigation of carbon nanotubel [12] Qu S.J., Geng L., Meng Q.C., Feng A.H., and Lei 3Y-TZP nanocomposites, Ceram. Int., 2005, 31: T.Q., Effects of hot rolling on microstructure and 1131. properties of a 20 vol.% SiCdAl composite, Rare [17] Zhang F.M., Shen J., and Sun J.F., Processing and Met., 2005,24 (1): 95. properties of carbon nanotubes-nano-WC-Co com[13] Yasutomi Y., Sawada J., &chi T., Nakamura K., posites, Mater. Sci. Eng. A., 2004,381: 86. Manabe Y., Nagano K., Kuroda H., Sumi T.,