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Research progress of carbon nanotube reinforced metal matrix composites

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https://www.eduzhai.net International Journal of Composite M aterials 2013, 3(6A): 28-44 DOI: 10.5923/s.cmaterials.201309.04 State-of-the-art Review on Carbon Nanotube Reinforced Metal Matrix Composites Nuno Silvestre Department of M echanical Engineering, Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Abstract The unceasing upgrading of techniques and processes to fabricate high purity carbon nanotubes (CNTs) and the improvement of the available techniques to produce high performance matrix materials, have fostered the way to enhance composite materia ls and their properties, either mechanical, therma l, e lectrica l or magnetic. CNTs reinforce ments have been introduced into polymers, ceramics, cement-based materials and metals. Po ly mers were the first material to be explo ited as matrix material being reinforced by CNTs. Up to now other materials have tentatively been investigated for that purpose, including metals. Today, many applications of CNT reinforced co mposites exist but CNT reinfo rced metals are still scarce and only found in very specific applications. Several reasons can be identified but the still gro wing demand for lighter and stronger metals paved the way to more fundamental research on the topic of CNT reinforced metal matrix co mposites (MMCs). Th is review describes the state-of-the art in this field and highlights the excellent and promising mechanical, thermal, electrical properties of CNT reinforced MM Cs. Keywords Carbon Nanotube, Metal Matrix Co mposites, Mechanical Properties, Stiffness, Strength, Ductility 1. Introduction Since their d iscovery by Ijiima[1], carbon n anotubes (CNTs) have been considered as an ideal reinforced material to improve the mechanical performance of many materials. CNTs are pro mis ing rein forcements for light weight and high strength composites. This is due to their exceptional small d iameters and h igh Young’s modu lus, h igh tensile strength and chemical stability. St ill, the main obstacle is to obtain a homogenous dispersion of the CNTs in the desired material matrix. Several methods and processes have been developed to improve the dispersion of CNTs in poly mer matrices. The state-of-the-art report made by Andrews and Weisenberger[2] on CNT-poly mer co mposites emphasised on the problem o f CNT d ispersion. Particu lar focus was given by these authors to interfacia l bonding between CNTs and po ly mer mat rices , as well as to potent ial top ics of interest at that time. More recently, Chou et al.[3] examined the recent advancements in the science and technology of CNT-based fibers for poly mer co mposites. Their assessment was made according to the hierarchical structural levels of CNTs used in composites, ranging from 1-D to 2-D to 3-D. At the 1-D level, fibers composed of pure CNTs or CNTs emb edded in a po ly meric mat rix p rod uced b y various techniques were revie wed. At the 2-D level, the focuses were * Corresponding author: nsilvestre@ist.utl.pt (Nuno Silvestre) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved on CNT-mod ified advanced fibers, CNT-mod ified interlaminar surfaces and highly oriented CNTs in planar form. At the 3-D level, they examined the mechanical and physical properties CNT-poly mer co mposites, CNT-based damage sensing, and textile assemblies of CNTs. The review by Yan et a l.[4] focused ma inly on functional CNTs and their applications in property enhancement of various polymer composites. Firstly, the general methods for CNT preparation were briefly introduced. Secondly, the functionalization of CNTs, particularly via chemical approaches, was summarized and the application of these functionalized CNTs was discussed. Finally, the interaction of CNTs with various polymers, the formation of CNT-poly mer co mposites, and their property and applications were discussed by Yan et al.[4]. Nowadays, the CNT-reinforced poly mer matrix composites are being intensively investigated. However, scarce to moderate research has been conducted to improve the CNT dispersion in metal matrices. Despite the fact that CNTs are an effective reinforcement to improve the mechanical and thermal responses of metal matrix co mposite (MMCs), very few successful attempts have been made for commercial applications due to the difficult ies of incorporating CNTs in metals. It became obvious that segregation of CNTs due to their strong van der Waals forces often produces material defects, decreasing the material properties. Despite CNT reinfo rced MMCs have received the least attention, they are being thought for use in structural applications because of their high specific strength as well as functional materials for their exciting thermal and electrical International Journal of Composite M aterials 2013, 3(6A): 28-44 29 characteristics. Light MMCs are of great interest due to their potential for reducing CO2 emission through lightweight design. Using the exceptional p roperties of CNT in MMCs for mac roscopic applications still constitutes a big challenge for the scientific and technological co mmunities. In order to base these thoughts, let us look at Figure 1. It shows the variation of the nu mber o f publications in SCI Web of knowledge per year, since 2000. Each curve corresponds to a different co mbination of keywords in searched topic: (i) the curve with circles corresponds to “Carbon nanotube and Composite”, (ii) the curve with squares corresponds to “Carbon nanotube and Composite and Poly mer”, and (iii) the curve with t riangles corresponds to “Carbon nanotube and Composite and Metal”. It is easily seen that the works on CNT-based composites rapidly increased fro m 81 in 2000 to over 7703 in 2012. The increase rate steadily increased fro m 2000 to 2007, but fro m 2007 to 2012 it showed a huge rise main ly due to the advent of nanomaterials. Since 2007, several types of materials fabricated at nanoscale emerged as promising matrices and nanodevices to which CNTs can effectively be added. Figure 1 also shows that the number of publications indexed in ISI Web of Knowledge[5] and focused on CNT-Poly mer composites also increased a lot fro m 2000, with 31 publications, to 2012, with 1593. Regarding the nu mber of publications on CNT-Metal co mposites, it has also increased since 2000 but to a minor extent: 19 publications in 2000 and 468 in 2012. Figure 2 shows the percentage of publications on CNT-Poly mer and CNT-Metal co mposites with respect to CNT composites. Concerning CNT-Po ly mer co mposites, the percentage was more or less uniform t ill 2007, but then showed a huge drop due to the advent of nano-fabricated materials. It is interesting to note that the percentage of published works on CNT-Metal co mposites has been decreased since 2000, despite the total number has been increased (Figure 1). 9000 8000 7000 6000 Carbon nanotube AND Composite Carbon nanotube AND Composite AND polymer Carbon nanotube AND Composite AND Metal 5000 4000 3000 2000 1000 0 Number of publications 2 000 2 001 2 002 2 003 2 004 2 005 2 006 2 007 2 008 2 009 2 010 2 011 2 012 Year Figure 1. Number of publications indexed in Web of Knowledge per year % of number of publications 2 000 2 001 2 002 2 003 2 004 2 005 2 006 2 007 2 008 2 009 2 010 2 011 2 012 50% 45% 40% 35% CNT-Polymer composites 30% CNT-Metal composites 25% 20% 15% 10% 5% 0% Year Figure 2. Percentage of the number of publications per year focused on CNT -Polymer and CNT-Metal composites Since the review by Girot et al[6], few reviews on CNT-reinforced MMCs have been published. In 2004, Curtin et al.[7] reviewed the research on the incorporation of CNTs into ceramic and metal matrices to form co mposite structures. They emphasised the processing methods, mechanical perfo rmance, and prospects for successful applications. They reviewed the literature on the topics of fabrication and properties of ceramic and metal matrix systems. In case of metal matrix materials, they mentioned that enhanced mechanical properties (stiffness, wear, and fatigue resistance) are desirable but a wider range of properties (electrical, magnetic and v ibrational) were also investigated. Li et al.[8] studied the behaviour of CNT reinforced light metal co mposites produced by melt stirring and by high pressure die casting. The light metal co mposites showed significantly imp roved mechanical properties already at s mall CNT contents. The influence of CNT concentration on the composites was also studied by Li et al.[8]. The review by Neubauer et al.[9] g ives an overview and summarises the activities related to CNTs and carbon nanofibers used as a reinforcement in metallic matrix materials. They presented the main challenges and the potential with respect to material properties. Bakshi et al.[10] reviewed and summarised the research work carried out in the field of CNT reinforced MMCs. They focused on the critical issues of CNT reinfo rced MMCs that include processing techniques, nanotube dispersion, interface, strengthening mechanisms and mechanical properties. Processing techniques used for synthesis of the composites were crit ically reviewed with an objective to achieve homogeneous distribution of CNTs in the matrix. The mechanical property improvements achieved by addition of CNTs in various metal matrix systems were summarised by Bakshi et al.[10]. Several factors, such as the structural and chemical stability of CNTs in d ifferent metal matrices and 30 Nuno Silvestre: State-of-the-art Review on Carbon Nanotube Reinforced M etal M atrix Composites the importance of the CNT-metal interface were described. The relevance of CNT d ispersion and its quantification was also highlighted. The purpose of this review is to update the state-of-the-art concerning the use of CNTs for reinforcement of MM Cs. The recent developments are shown separately by each type of material matrix, in alphabetic order: Alu miniu m (A l), Cobalt (Co), Copper (Cu), Iron (Fe), Magnesium (Mg), Nickel (Ni) and Titaniu m (Ti). 2. CNT-Aluminium (Al) Composites Aluminiu m is a chemical element in the boron group with symbol Al and ato mic nu mber 13. It is a silvery white, soft, ductile metal. Al is the third most abundant element (after oxygen and silicon), and the most abundant metal in the Earth's crust. Al is remarkab le for its low density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made fro m A l and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials. The most useful co mpounds of Al, at least on a weight basis, are the oxides and sulfates. A sample of Alu miniu m is shown in Figure 3 and its properties are presented in Table 1. Figure 3. Aluminium (Al) sample Table 1. Al properties Crystal structure Magnetic ordering Electrical resistivity Thermal conductivity Thermal expansion Speed of sound Young's modulus Shear modulus Bulk modulus Poisson ratio Mohs hardness Vickers hardness Brinell hardness Density (near r.t.) Liquid density at m.p. Melting point Boiling point Heat of fusion Heat of vaporization Molar heat capacity face-centered cubic paramagnetic (20°C) 28.2 nΩ.m 237 W.m−1.K−1 (25°C) 23.1 µm .m−1.K−1 (r.t.) (rolled) 5,000 m.s−1 70 GPa 26 GPa 76 GPa 0.35 2.75 167 MPa 245 MPa 2.70 g.cm−3 2.375 g.cm −3 933.5 K, 660.3°C, 1220.6 °F 2792 K,2519°C,4566 °F 10.71 kJ.mol−1 294.0 kJ.mol−1 24.200 J.mol−1.K−1 Al matrix is widely used for CNT reinforced MM Cs. Since the work by Zhong et a l.[11], many achieve ments have been made in the develop ment of CNT-Al co mposites. In this review, 24 papers are described and dedicated to this topic[12-35]. Cha et al.[12] proposed a novel process to fabricate CNT-alu mina nanocomposites, consisting of a mo lecular level mixing process and an in situ spark plas ma sintering process. The CNT/alu mina nanocomposites fabricated by this proposed process showed imp roved hardness due to a load transfer mechanism of the CNTs and increased fracture toughness arising from the bridging mechanis m of CNTs during crack p ropagation. In order to produce optimized co mposites, George et al.[13] studied the strength of CNT-A l composites and investigated the relevant strengthening mechanisms involved in CNT-Al co mposites. Three major mechanis ms were analy zed along with experimental procedure for making CNT/Al co mposites. Using cold isostatic press and subsequent hot extrusion techniques Deng et al.[14] fabricated 1.0 wt.% carbon nanotube (CNT) reinforced 2024Al matrix co mposite, measured the mechanical properties of the co mposite by tensile tests and examined the fracture surfaces using field emission scanning electron microscopy. Their experimental results showed that CNTs are dispersed homogeneously in the composite and that the interfaces of the Al matrix and the CNT bonded well. Deng et al.[15] observed that the tensile strength and the Young's modulus of the co mposite were enhanced markedly, and the elongation didn’t decrease when compared with the matrix materia l fabricated under the same process. The extraordinary mechanical propert ies of CNTs and their pulling-out role in the Al matrix composite are factors that explain this improved behaviour. Deng et al.[15] also investigated the microstructure characteristics and the distribution of CNTs in the Al matrix. They showed that adding a small amount of CNTs to the matrix, the elastic modulus and the tensile strength were increased highly with respect to those of the 2024Al base material. Deng et al.[16] investigated the damping behaviour of 2024A l reinforced with mu lti-walled CNTs. The damping characteristics of the composite were investigated with frequency of 0.5, 1.0, 5.0, 10, 30 Hz, at a temperature of 25– 400 °C. The experimental results showed that the frequency significantly affects the damping capacity of the composite when the temperature is above 230 °C. The damping capacity of the composite with a frequency of 0.5 Hz reached 975×10−3, and the storage modulus is 82.3 GPa when the temperature was 400 °C. This study proved that CNTs can improve h ighly the damping properties of MMCs at elevated temperature without sacrificing their mechanical strength and stiffness. Deng et al.[17] studied the thermal expansion behaviour of Al composite reinfo rced with CNTs. The coefficient of thermal expansion of Al matrix co mposite reinforced with 1.0wt.% mu lti-wall CNTs fabricated by cold isostatic pressing and hot squeeze technique was measured between 25 and 400°C with a high-precision thermo-mechanical analy zer, and compared with those of pure Al and 2024Al matrix fabricated under the same processing. The results by Deng et International Journal of Composite M aterials 2013, 3(6A): 28-44 31 al.[17] showed that the coefficient of thermal expansion of the composite reduces in relation to those of pure Al and 2024Al matrix due to the introduction of CNTs. The addition of 1.0wt.% CNTs to 2024A l matrix decreased the coefficient of thermal expansion by as much as 12% and 11% co mpared with those of pure Al and 2024Al matrix at 50°C, respectively. These evidences indicated that CNT reinforcement of MM C may be a pro mising material with low coeffic ient of therma l e xpansion. Bakshi et al.[18] prepared mu lti-walled CNT rein forced Al coatings using cold gas kinetic spraying in order to obtain a good CNT dispersion in micron-sized gas atomized A l–Si eutectic powders. Spray dried powders containing 5 wt.% CNT were b lended with pure Al powder to give overall nominal CNT co mpositions of 0.5 wt.% and 1 wt.% respectively. They showed that the CNTs were uniformly distributed in the matrix. Nanoindentation led to elastic modulus values between 40 and 229 GPa, and this high scatter was attributed to microstructural heterogeneity of the coatings (includ ing pure Al, Al-Si eutectic, porosity and CNTs ). Using hot e xtrusion of ball-milled powders, Choi et a l.[19] fabricated Al mat rix co mposite rods in which t ightly bonded mu lti-walled CNTs were separately dispersed and uniaxially aligned. They showed that the reinforcing efficiency of CNTs in the composites followed the volume fraction rule of discontinuous fibers in the grain size range down to 70 n m. Pérez-Bustamante et al.[20] produced Al-based nanocomposites reinforced with mu lti-walled CNTs using mechanical milling fo llo wed by pressure-less sintering at 823 K under vacuum. The interface between Al matrix and the mult i-walled CNTs was examined using transmission electron microscopy. This work showed that the mu lti-walled CNTs were not damaged during the preparation of the nanocomposite and that no reaction products were detected after sintering. The mechanical p roperties of sintered nanocomposites specimens were evaluated by a compression test. The yield stress and the maximu m strength obtained were considerably higher than those reported in the literature for pure Al prepared by the same route. The values for yield stress and ultimate strength increased about 100% as the volume fraction of mu lt i-walled CNTs increased from 0 to 0.75 wt.%, for 2h of milling time. They concluded that the milling t ime and the concentration of CNTs had an important effect on the mechanical properties of the n an o co mp os ite. In the paper by Esawi and El Borady[21], a powder rolling technique is used to fabricate CNT-reinforced A l strips. The Al-CNT mixtures were b lended in either a mixer-shaker at a rotary speed of 46 rp m, o r under argon in a p lanetary mill at a rotary speed of 300 rp m, prio r to rolling. The CNT dispersion was shown to be better under the higher energy planetary action. The strength of the rolled strips was evaluated for various wt% CNT samp les. The A l-0.5 wt% co mposite strips exhibited enhanced mechanical properties. The CNT reinforced A l strips were shown to have numerous attractive applications in the aerospace, automotive and electronics industries. Later, Esawi et al.[22] used planetary ball milling to disperse 2 wt% mu lti-walled CNT in Al (A l) powder. Despite the success of ball milling in dispersing CNTs in Al powder, it is often accompanied with considerable strain hardening of the Al powder, a fact that might have far reaching implications on the final properties of the composite. Thus, both un-annealed and annealed Al-2 wt% CNT co mposites were investigated by Esawi et al.[22]. They found that, ball-milled and ext ruded (un-annealed) samples of Al-2 wt% CNT demonstrated high notch - sensitivity and consistently fractured outside the gauge length during tensile testing. In contrast, extruded samples annealed at 400 and at 500°C for 10 h prior to testing, exh ibited more ductile behaviour and no notch sensitivity. Under similar p rocessing conditions, they showed that ball milling for 3 h followed by hot extrusion and annealing at 500°C resulted in enhancements of around 21% in tensile strength compared with pure Al with the same process history. They argued that ball-milling conditions resulted in the creat ion of a nanostructure in all samples produced, a fact that as proved by means of XRD and TEM analysis. The tensile testing fracture surfaces identified by Esawi et al.[22] showed uniform dispersion and alignment of the CNTs in the Al matrix but also showed that CNTs acted as nucleation sites for void formation during tensile testing. This effect contributed to the observation of CNT pullout due to the poor bond between the CNTs and the matrix. Since the uniform dispersion of CNTs in the Al matrix has been identified as being crit ical to the pursuit of enhanced properties, ball milling as a mechanical d ispersion technique has proved its potential. Esawi et al.[23] used ball milling to disperse up to 5 wt.% CNT in Al matrix and investigated the effect of CNT content on the mechanical properties of the composites. Cold compaction and hot extrusion were used to consolidate the ball-milled Al–CNT mixtures. In co mparison with to pure Al, 50% increase in tensile strength and 23% increase in stiffness compared were observed by Esawi et al.[23]. So me carbide formation was observed in the composite containing 5 wt.% CNT. The large aspect ratio of CNTs used by Esawi et al.[23] led to d ifficu lties in CNT d ispersion at CNT wt.% greater than 2. Therefore, the expected improvements in mechanical properties with increase in CNT weight content were not fully realized by these authors. Choi et al.[24] reported a study on the mechanical properties and wear characteristics of ultrafine-grained Al and Al-based composites. In this composite, well dispersed and Al atom-infiltrated mult i-walled CNTs formed a strong interface with the matrix by mechanical interlocking. Wear characteristics, varied according to the grain size and the CNT volu me, were evaluated by Choi et al.[24] under several combinations of applied load and sliding speed. They reported that strength and wear resistance were significantly enhanced and the coefficient of friction was extremely reduced, for grain size decrease and CNT volu me increase. The ultrafine-grained composite containing 4.5 vol. % of CNTs exh ibited more than 600 MPa in yield stress and less than 0.1 in the coefficient of frict ion. Both coefficient of 32 Nuno Silvestre: State-of-the-art Review on Carbon Nanotube Reinforced M etal M atrix Composites friction and wear rate aug mented with increasing load, wh ile they were reduced with increasing the sliding speed. This study demonstrated that CNTs are effect ive reinforcement for enhancing wear characteristics as well as mechanical properties. Choi et al.[25] produced nanocrystalline Al-Si alloy-based composites containing CNTs using hot rolling ball-milled powders. The grain size was effect ively reduced and the Si ele ment was dissolved in the Al matrix during the milling process. Using a thermo-mechanical process, CNTs were gradually dispersed into the Al powders. The composite produced by Choi et al.[25] contained 3 vol.% of CNTs and showed yield strength about 520 MPa and plastic elongation to failure of 5%. Al and Al-based composites containing 4.5 vol.% mu lt i-walled CNTs were fabricated by Choi and Bae[26] using hot-rolling the ball-milled powder. They studied the composite creep behaviour at 523 K and it displayed much enhanced creep resistance at the applied stresses higher than 200 MPa. They also identified a negligible dependency of strain rate on stress for relat ively low applied stress region (stresses below 110 MPa). They e xpla ined this behaviour due to the fact that diffusional flow of the matrix is significantly restricted by multi-walled CNTs . Singhal et al.[27] report the fabricat ion of Al-matrix composites reinforced with amino-functionalized CNTs by means of powder metallurgy process. Functionalizat ion of the CNTs was carried out by ball milling mult i-walled CNTs in the presence of ammon iu m bicarbonate. They found that the mechanical properties of Al-functionalized CNTs composites were much superior to the composites fabricated using non-functionalized or acid functionalized CNTs. Using high-resolution transmission electron microscopy, they attributed the improvement of mechanical properties to (i) homogeneous dispersion of functionalized CNTs in Al matrix, as co mpared to non-functionalized o r acid functionalized CNTs, and (ii) the formation of strong interfacial bonding between functionalized CNTs and Al matrix, leading to efficient load transfer fro m Al matrix to functionalized CNTs. Using measurements of grain size and mechanical property changes upon annealing at various temperatures, Lipecka et al.[28] evaluated the effect of CNTs on the thermal stability of ultrafine grained Al alloy processed by the consolidation of nano-powders obtained by mechanical alloying. They found that the grain size of the samples containing CNTs is stable up to high temperatures, and even after annealing at 450 °C, no evident grain gro wth was observed. This was attributed to the presence of entangled networks of CNTs located at grain boundaries and to the formation of nanoscale particles of carbide A l4C3. They also revealed that CNTs decompose at a relatively low temperature (450°C) and form fine Al4C3 precip itates. Lipecka et al.[28] revealed that this transformat ion did not affect the mechanical propert ies due to the carbide nanoscale s ize. Choi et al.[29] investigated the influence of mu lti-walled CNTs on the strength of Al-based composites with grain sizes ranging fro m 250 to 65 n m. They found that the composite strength was significantly enhanced by the increase of the CNT volu me. The strengthening efficiency of CNTs in ultrafine-grained composites was comparable with that predicted by the discontinuous fiber model. For grain size below 70 n m, the efficiency was half of the theoretical prediction. For nano-grained A l, Choi et al.[29] found that the activities of forest dislocations diminished and dislocations emitted fro m grain boundaries were dynamically annih ilated during the recovery process. This provide weak plastic strain field around CNTs. Their observation gave a basic understanding of the strengthening behaviour of nano-grained MMCs. Jiang et al.[30] used flake powder metallurgy (flake PM) to achieve uniform distribution of CNTs in CNT-Al composites and realized the potential of CNTs for reinforcement. The structural integrity of the CNTs was maintained in the composites since CNTs were protected fro m high energy physics force such as ball-milling. They concluded that a strong and ductile CNT-Al co mposite might be fabricated using this process, leading to tensile strength of 435 MPa and ult imate strain of 6%. These values greatly surpassed the strength values for materials produced by conventional methods. In order to comb ine high tensile strength and ductile behaviour, CNT/Al nanolaminated composites with alternating layers of Al (400 n m) and CNTs (50 n m) were fab ricated by Jiang et al.[31] using flake PM. Co mpared with conventional ho mogeneous nanocomposites composed of the same constituents, they found that the final bulk products with high level o rdered nanolaminates exhibited both high tensile strength of 375 MPa and high strain of 12%. They attributed this enhancement to the fact that they enabled enhanced dislocation storage capability and two-dimensional align ment of CNTs. CNT-reinforced Al matrix co mposite materials were successfully fabricated by Kwon and Leparou x[32] using mechanical ball milling followed by powder hot ext rusion processes. Although only a small quantity of CNTs were added to the composite (1 vol%), they found that the Vic kers hardness and the tensile strength were significantly enhanced, with an up to three-fold increase relative to that of pure Al. The CNT-Al co mposites were not only strengthened by the addition of CNTs but also enhanced by several synergistic effects. The nanoindentation stress–strain curve was successfully constructed by setting the effective zero -load and zero-d isplacement points and was compared with the tensile stress–strain curve. The yield strengths of the Al – CNT co mposites were obtained from the nanoindentation and tensile tests, and then compared. Using powder metallurgy fo llo wed by 4-pass friction stir processing, Liu et al.[33] fabricated 1.5 vol.% and 4.5 vol.% CNT reinforced 2009Al (CNT-2009Al) co mposites with homogeneously dispersed CNTs and refined mat rix g rains. They tested the tensile properties of the co mposites between 293 and 573 K and the coeffic ient of therma l e xpansion from 293 to 473 K. They indicated that load transfer mechanism still takes place at temperatures elevated up to 573 K. Thus, International Journal of Composite M aterials 2013, 3(6A): 28-44 33 the yield strength of the 1.5 vol.% CNT-2009Al co mposite at 423–573 K was enhanced compared with the 2009Al matrix. However, for the 4.5 vol.% CNT-2009Al co mposite, the yield strength at 573 K was even lo wer than that for the matrix, due to the quicker softening of ultrafine-grained matrix. Co mpared with the 2009Al matrix, Liu et al.[33] concluded that the coefficient of thermal expansion of the composites was greatly reduced for the zero thermal expansion and high modulus of the CNTs, and well predicted by the Schapery’s model. Yoo et al.[34] studied the behaviour of CNT-reinforced Al composites fabricated through ball milling co mbined with rolling. The co mposites exhib ited high strength and high strain-hardening ability. During the ball-milling process, CNTs were broken and became low aspect ratio tubes. They observed that the composites had the CNTs (a few dozen nanometers in size) randomly and uniformly dispersed in their grain interiors. They concluded that this type of CNT distribution contributed to work hardening and strengthening by the Orowan mechanism. Yang et al.[35] developed an approach to fabricate CNT-reinforced Al co mposites. This process allowed well d ispersed and deeply embedded CNT reinforcement in the Al powder, forming an effective interface bonding with matrix. They concluded that these CNT-Al co mposites containing 2.5 wt.% CNTs exh ibited ultimate tensile strength of 334 MPa, which was 1.7 times higher than that of unreinforced Al, as well as good ductility of 18% elongation to failure. 3. CNT-Cobalt (Co) Composites Figure 4. Cobalt (Co) sample Cobalt is a chemical element with sy mbol Co and ato mic number 27. It is found naturally only in chemically co mbined form. The free element, p roduced by reductive smelting, is a hard, lustrous, silver-gray metal. Today, some cobalt is produced specifically fro m various metallic-lustered ores, e.g. cobaltite (CoAsS), but the main source of the element is as a by-product of copper and nickel min ing. Cobalt is used in the preparation of magnetic, wear-resistant and high-strength alloys. Cobalt silicate and cobalt(II) alu minate give a distinctive deep blue color to glass, smalt, ceramics, inks, paints and varnishes. A sample of Cobalt is shown in Figure 4 and its properties are presented in Table 2. Table 2. Co properties Crystal structure Magnetic ordering Electrical resistivity Thermal conductivity Thermal expansion Speed of sound Young's modulus Shear modulus Bulk modulus Poisson ratio Mohs hardness Vickers hardness Brinell hardness Density (near r.t.) Liquid density at m.p. Melting point Boiling point Heat of fusion Heat of vaporization Molar heat capacity hexagonal close-packed ferromagnetic (20 °C) 62.4 nΩ.m 100 W.m−1.K−1 (25°C) 13.0 µm .m−1.K−1 (20°C) 4720 m.s−1 209 GPa 75 GPa 180 GPa 0.31 5.0 1043 MPa 700 MPa 8.90 g.cm−3 7.75 g.cm−3 1768 K, 1495°C, 2723 °F 3200 K, 2927°C, 5301 °F 16.06 kJ.mol−1 377 kJ.mol−1 24.81 J.mol−1.K−1 Co matrix is used for CNT reinforced MMCs, but mainly for electroche mica l performance in coatings and batteries. In this review, only 2 papers are dedicated to CNT-Co composites[36,37]. Chen et al.[36] demonstrated that cobalt could be plated onto the surfaces of CNTs by electroless plating. In this manner a layer o f cobalt is formed as nanoparticles on the suface of the CNTs. It was found that the activation process and low deposition rate were critical for getting better coating. Additionally, the heat-treatment of the coated CNTs was found to be a very effective way of improving the deposited coating layer. The results from the study by Chen et al.[36] demonstrated the technical feasibility of electroless plating for the preparation of a one-dimensional nanoscale composite. Recently, Su et al[37] characterized nanocrystalline Co and CNT-Co coatings produced by different electrodeposition techniques. Nanocrystalline Co and multi-walled CNT-Co coatings were synthesized by direct current and pulse reverse current electrodeposition fro m aqueous bath containing cobalt sulfate and mu lt i-walled CNTs. Effect o f the functionalizat ion of CNTs and electrodeposition techniques on the microstructure and properties of these coatings was evaluated. Their results showed that the incorporations of CNTs, particularly the functionalized CNTs, substantially improve the hardness and the resistance to wear and corrosion of the deposited coatings. The functionalizat ion of CNTs favors the co-deposition of CNTs with Co ions, and then improves the hardness and the corrosion and wear resistance of the produced composite coatings. The differences in friction and wear behaviour of these nanocrystalline Co and CNT-Co coatings as a function of treatment of CNTs or electrodeposition techniques were 34 Nuno Silvestre: State-of-the-art Review on Carbon Nanotube Reinforced M etal M atrix Composites attributed to their different hardness, microstructures and the corresponding wear mechanis ms. 4. CNT-Copper (Cu) Composites Copper is a chemical element with the symbol Cu and atomic nu mber 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; a freshly exposed surface has a reddish-orange color. It is used as a conductor of heat and electricity, a building material, and a constituent of various metal alloys. The main areas where copper is found in humans are liver, muscle and bone. In sufficient concentration, Cu compounds are poisonous to higher organisms and are used as bacteriostatic substances, fungicides and wood preservatives. A sample of Copper is shown in Figure 5 and its properties are presented in Table 3. Cu matrix has been fairly used for CNT reinforced MM Cs. In this review, a total of 9 papers are dedicated to CNT-Cu composites[38-46]. Kim et al.[38] investigated the hardness and wear resistance of CNT-reinforced Cu matrix (CNT-Cu) composites. The nanocomposite was fabricated by mo lecular level process, which involved suspending CNTs in solvent by surface functionalization, mixing Cu ions with CNT suspension, drying, calcination and reduction. The hardness and sliding wear resistance of CNT-Cu co mposite were enhanced by two and three times, respectively, compared to those of Cu matrix. They attributed the enhancement of hardness to the effect of homogeneous distribution of CNTs in Cu matrix, good bonding at CNT-Cu interfaces and high relative density of composites. The dispersed CNTs in Cu-matrix co mposite gave significantly enhanced wear resistance by retarding the peeling of Cu grains during sliding wear process. Table 3. Cu properties Crystal structure Magnetic ordering Electrical resistivity Thermal conductivity Thermal expansion Speed of sound Young's modulus Shear modulus Bulk modulus Poisson ratio Mohs hardness Vickers hardness Brinell hardness Density (near r.t.) Liquid density at m.p. Melting point Boiling point Heat of fusion Heat of vaporization Molar heat capacity face-centered cubic diamagnetic (20°C) 16.78 nΩ.m 401 W.m−1.K−1 (25°C) 16.5 µm .m−1.K−1 (r.t.) (annealed) 3810 m.s−1 110–128 GPa 48 GPa 140 GPa 0.34 3.0 369 MPa 35 HB =874 MPa 8.96 g.cm−3 8.02 g.cm−3 1357.8 K, 1084.6°C, 1984.3 °F 2835 K, 2562°C, 4643 °F 13.26 kJ.mol−1 300.4 kJ.mol−1 24.440 J.mol−1.K−1 Figure 5. Copper (Cu) sample Sun et al.[39] applied mo lecular dynamics and continuum mechanics to predict the comp ressive mechanical properties of CNTs encapsulating helica l copper nanowire. The helical structures of the copper nanowires were obtained using the “simu lated annealing” method. The strain energy curves were shown to predict the interaction between the atoms during the co mpressive course. They used the model to determine the mechanical properties (critical buckling load and the Young’s moduli) of CNTs encapsulating helical copper nanowire with different diameters and lengths. Sun et al.[39] found that not all the maximu m strengths of the composites are larger than the corresponding carbon nanotube bases, and they were related with the diameter, length and the CNTs chirality. So me excellent properties of CNTs encapsulating helical copper nanowire were revealed in this study. Trinh et al.[40] reported results on the fabrication and calculation of the frict ion coefficient of Cu matrix co mposite material reinforced by CNTs. CNT-Cu composites were fabricated by powder metallurgy method and mechanica l properties, such as frict ion coefficient, we re evaluated using the rule of mixtures. Trinh et al[40] determined the coefficient of frict ion of the CNTs component in the CNT-Cu co mposites. Uddin et al.[41] investigated the hardness and electrical properties of CNT reinforced Cu and Cu alloy (bronze) composites. They were fabricated by well-established hot-press sintering method of powder metallu rgy. They found that the effect of shape and size of metal particles as well as selection of CNTs influenced significantly the mechanical and electrical properties of the composites. The hardness of CNT-Cu co mposite improved up to 47% compared to that of pure Cu, while the e lectrical conductivity of bronze co mposite increased up to 20% co mpared to that of the pure alloy. They concluded that CNTs imp roved the mechanica l properties of highly-conductive low-strength Cu metals, whereas in low-conductivity high-strength copper alloys the electrical conductivity was improved. Using high power lasers, Bhat et al.[42] fabricated and studied the mechanical and thermal properties of mult i-walled CNT reinforced Cu–10Sn alloy co mposites. Microstructural International Journal of Composite M aterials 2013, 3(6A): 28-44 35 observations showed that CNTs were retained in the composite matrix after laser processing. The addition of CNTs showed improvement in strain hardening, mechanical and thermal properties of Cu–10Sn alloy. Co mposites with 12 vol.% CNTs showed mo re than 80% increase in the Young's modulus and 40% increase in the thermal conductivity of Cu–10Sn alloy. Bhat et al.[42] found that the yield strength estimates obtained from models based on strengthening mechanisms derived fro m the Maxwell – Garnett effective mediu m were shown very accurate. Xu et al.[43] studied the effect of electrical current on tribological properties of Cu matrix co mposite reinforced by CNTs. The CNT-Cu co mposites were reinforced with 10% CNTs and pure Cu bulk were p repared by powder metallurgy techniques under the same consolidation processing condition. They investigated the effect of electrical current on tribological property of the materials using a pin-on-d isk friction and wear tester. The results published by Xu et al.[43] showed that the friction coeffic ient and wear rate of CNT-Cu composite as well as those of pure Cu bulk increased with the electrical current. They identified that the dominant wear mechanis ms were arc erosion wear and plastic flow deformation, and CNTs imp roved tribological properties of Cu co mposites with electrical current. Multi-walled CNT-Cu co mposites, exhibit ing chromiu m (Cr) carb ide nanostructures at the CNT-Cu interface, were prepared by Cho et al.[44] using a carb ide format ion using CuCr alloy powder. The tensile strengths of the CNT-Cu Cr composites increased with increasing CNTs content, while the tensile strength of CNT-Cu composite decreased from that of monolith ic Cu. The enhanced tensile strength of the CNT-Cu Cr co mposites was a result of possible load-transfer mechanis ms of the interfacial Cr carbide nanostructures. They observed the failu re of mu lti-wall of CNTs in the fracture surface of the CNT-Cu Cr co mposites, indicating an improvement in the load-bearing capacity of the CNTs. This result proved that the Cr carbide nanostructures effectively transferred the tensile load to the CNTs during fracture through carbide nanostructure formation in the CNT-Cu composite. Cu mat rix co mposites reinforced with 0.2, 5 and 10 vol% single-walled CNT and 5 and 10 vol% mu lti-walled CNTs were produced by Shukla et al.[45] using high energy milling of pure copper powder with CNTs and subsequent consolidation by vacuum hot pressing. Significant improvement in hardness of single-walled CNT-Cu composite was observed with increase in CNTs content. In case of mu lti-walled CNT-Cu co mposite, hardness reduced for 10 vol% CNT co mposites. Co mpression strength of the single-walled CNT-Cu samples was found by Shukla et al.[45] to be higher than the mult i-walled CNT reinforced s amp les . Tsai and Jeng[46] investigated experimentally and numerically the effect of CNT buckling on the reinforcement of CNT-Cu co mposites. The CNT-Cu co mposites with high strength and good damping were developed using acid treatment, sintering processes and consolidation techniques. In this study, strengthening of the composites, with influence of CNT buckling, was demonstrated by experimental nanoindentation tests and molecular dynamics simulat ions. The experimental results obtained by Tsai and Jeng[46] showed that the buckling behaviour of CNTs in the CNT-Cu composites varies with their slenderness ratio. Their study showed significant buckling behaviour of CNTs in the CNT-Cu co mposites, where the shorter CNTs (with a smaller slenderness ratio) gave rise to a global buckling and the slender CNTs (with a larger slenderness ratio) induced local buckling. The MD simu lation results revealed that the buckling behaviour of the CNTs played a key role in increasing the mechanical strength of CNT-Cu co mposites. 5. CNT-Iron (Fe) Composites Iron is a chemical element with the symbol Fe and ato mic number 26. It is a metal in the first transition series. It is the most common element (by mass) forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust. Iron metal has been used since ancient times, though copper alloys, which have lower melting temperatures, were used first in history. Pure iron is soft (softer than alu miniu m), but is unobtainable by smelt ing. The material is significantly hardened and strengthened by impurities fro m the smelting process, such as carbon. A certain proportion of carbon (between 0.002% and 2.1%) produces steel, which may be up to 1000 times harder than pure iron. Crude iron metal is produced in blast furnaces, where ore is reduced by coke to pig iron, which has a high carbon content. Further refinement with o xygen reduces the carbon content to the correct proportion to make steel. Steels and low carbon iron alloys with other metals (alloy steels) are by far the most co mmon metals in industrial use, due to their great range of desirab le properties and the abundance of iron. A sample of Iron is shown in Figure 6 and its properties are presented in Table 4. Figure 6. Iron (Fe) sample Fe matrix has been fairly used for CNT reinfo rced MMCs. In this section, a total of 8 papers on CNT-Fe co mposites are reviewed[47-54]. Ding et al.[47] emp loyed mechanical alloying to produce Al2O3/Fe, Al2O3/Co and Al2O3/Ni 36 Nuno Silvestre: State-of-the-art Review on Carbon Nanotube Reinforced M etal M atrix Composites nanocomposites and found that high-energy mechanical milling leads not only to drastic refinement but also to well dispersion of catalyst precursors in oxide matrixes. After mechanical milling, they observed solid-state alloying and accelerated substitutional reactions between the parent oxides. These nanocomposites possessed the fine-grained and porous structures and thus high reducibility. Large-scale formation of mult i-walled and single-walled CNTs were achieved by using these mechanical alloying-derived Al2O3/Fe, Al2O3/Co and Al2O3/Ni nanocomposites. Table 4. Fe properties Crystal structure Magnetic ordering Electrical resistivity Thermal conductivity Thermal expansion Speed of sound Young's modulus Shear modulus Bulk modulus Poisson ratio Mohs hardness Vickers hardness Brinell hardness Density (near r.t.) Liquid density at m.p. Melting point Boiling point Heat of fusion Heat of vaporization Molar heat capacity body-centered cubic ferromagnetic (20 °C) 96.1 nΩ.m 80.4 W.m−1.K−1 (25 °C) 11.8 µm.m −1.K−1 (r.t.) (electrolytic) 5120 m·s−1 211 GPa 82 GPa 170 GPa 0.29 4 608 MPa 490 MPa 7.874 g.cm −3 6.98 g.cm−3 1811 K, 1538 °C, 2800 °F 3134 K, 2862 °C, 5182 °F 13.81 kJ.mol−1 340 kJ.mol−1 25.10 J.mol−1.K−1 Atomic structures of single-crystalline iron-based nanowires crystallized inside mu lti-walled CNTs during pyrolysis on silicon substrates with ferrocene as a precursor were analy zed by Go ldberg et al.[48] using high-resolution analytical transmission electron microscopy and electron diffraction. Standard crystal lattices (body-centered cubic – bcc, α-Fe; face-centered cubic - fcc: γ-Fe; orthorhomb ic cementite Fe3C) were all found to form inside the CNTs. Both bcc and fcc nanowires display a wide variety of lattice planes being parallel to the CNT walls, with none of the orientations being preferable. Both long-period and standard cementite nanowires exh ibited well-defined transient zones in the vicinity o f nanowire–CNT interfaces, where perfectly ordered carbide lattice fringes disappeared. The results by Go ldeberg et al.[48] suggested the non-existence of metastable equilibriu m in the nanoscale Fe–C system between carbide and graphite phases during iron crystallization inside graphitic tubular channels. Shpak et al.[49] used X-ray diffraction, transmission electron microscopy, ferro magnetic and electron paramagnetic resonances to investigate the Fe-filled mu lti-walled CNTs. The iron within the CNTs was found to be in three phases: the austenite γ-Fe is located at the top of the CNTs, while the ferrite α-Fe and cementite θ-Fe3C are found close to the substrate. Two ferro magnetic signals were observed by Shpak et al.[49] and identified as those belonging to ferrite and cementite. Ferro magnetic signals revealed a surprising temperature dependence: with decreasing temperature, their integral intensity decreases nearly linearly, and the signals disappear at temperatures below 70 K. Spark plas ma sintering was used by Pang et al.[50] to fabricate dense Fe3Al/CNT co mposite with CNT content of 5.0 vol.%, retaining the integrity of CNT in the matrix. They synthesized samples at pressure of 30 MPa and temperature of 1273 K, and studied the composite structure using X-ray diffraction and transmission electron microscope (TEM). Pang et al.[50] concluded that the composites had very promising mechanical properties, such as microhardness of 8.7 GPa, co mpressive yield strength of 3175 MPa, which is about 95% and 56% higher than monolithic Fe3Al fabricated under the same process. They indicated that CNTs can be considered for effective nanoscale reinforcement of intermetallics matrix co mposites. Pang et al.[51] prepared mult i-walled CNTs-Fe3Al composite using spark plasma sintering and investigated the magnetic properties of the nanocomposite with alternating gradient force magnetometer. They showed that the CNTs-Fe3Al co mposite displays good soft magnetic properties and has a similar magnetic hysteresis loops to that of Fe3Al. The excellent magnetic properties evidenced in their study imply that mult i-walled CNTs-Fe3Al co mposites might have significant potential for applications in electronic-magnetic nanodevices. Pang et al.[52] perfo rmed analyses on the stress in the CNT-Fe3Al co mposites. They mentioned that the biphase interface valence electron structure was established on the basis of Pauling's nature of the chemical bond and that the stress occurs by the huge interface electron density difference, thus blocking the Fe3Al grain agglomeration and growth. They also measured the compressive stress existing in the CNT-Fe3Al interface with the X-ray diffractions, giving a value of 0.38 GPa. They declare that this experimental result verifies that the stress has a positive effect on the enhancement of mechanical properties of composite. Using spark plasma sintering, Pang et al.[53] fabricated iron alu minides (Fe3Al) based composite with different amounts of multi-walled CNTs and studied the influence of CNTs content on the mechanical propert ies of the co mposite. They concluded that the compressive yield strength and fracture toughness of the 3 vol% CNT-Fe3Al co mposite, compared with the monolithic Fe3Al, were enhanced 73.6% and 40%, respectively. The defects and align ment direction of CNTs in the matrix a ffected the mechanica l propert ies of the composite. They also identified that the failure mechanis ms include CNTs pulling-out, crack deflection, bridging and CNTs rupture. Madaleno et al.[54] used catalytic decomposition of ethylene over iron montmorillonite surfaces to synthesize mont morillonite–CNT hybrids. SEM and STEM analyses revealed the presence of CNTs attached to the clay layers and X-ray diffraction results and indicated that sodium International Journal of Composite M aterials 2013, 3(6A): 28-44 37 mont morillonite layers were intercalated with iron species during the ion-exchange processes and further delaminated due to the growth of CNTs. Due to their pre-exfo liated internal structure and the presence of surface CNTs, Madaleno et al.[54] suggested that montmorillon ite–CNT hybrids benefit the enhancement of mechanical p roperties in polymer nanocomposites. 6. CNT-Magnesium (Mg) Composites Magnesium is a chemical element with the symbol Mg and atomic nu mber 12. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole. The free metal burns with a characteristic brilliant wh ite light, making it a useful ingredient in flares. The metal is now mainly obtained by electrolysis of magnesium salts obtained from brine. Co mmercially, the main use for the metal is as an alloying agent to make alu miniu m-magnesium alloys, sometimes called magnaliu m or magneliu m. Since magnesium is less dense than aluminiu m, these alloys are prized for their relative lightness and strength. A sample of Magnesium is shown in Figure 7 and its properties are presented in Table 5. samples failed in uniaxial tension, which revealed the presence of cleavage-like features on the fracture surface indicative of the occurrence of locally brittle fracture mechanis m in the composite microstructure. Table 5. Mg properties Crystal structure Magnetic ordering Electrical resistivity Thermal conductivity Thermal expansion Speed of sound Young's modulus Shear modulus Bulk modulus Poisson ratio Mohs hardness Brinell hardness Density (near r.t.) Liquid density at m.p. Melting point Boiling point Heat of fusion Heat of vaporization Molar heat capacity hexagonal close-packed paramagnetic (20°C) 43.9 nΩ.m 156 W.m−1.K−1 (25°C) 24.8 µm .m−1.K−1 (r.t.) (annealed) 4940 m.s−1 45 GPa 17 GPa 45 GPa 0.290 2.5 260 MPa 1.738 g.cm −3 1.584 g.cm −3 923 K,650°C,1202 °F 1363 K,1091°C,1994 °F 8.48 kJ.m ol−1 128 kJ.mol−1 24.869 J.mol−1.K−1 Figure 7. Magnesium (Mg) sample Mg matrix is generally used for CNT reinforced MM Cs. In recent years, some ach ievements have been made in the development of CNT-Mg co mposites. In this section, 13 papers dedicated to this topic are summarized[55-67]. Thakur et al.[55] investigated the synthesis and mechanical behaviour of CNT– magnesium co mposites hybridized with nanoparticles of alumina. The CNT-Mg co mposites were prepared by powder metallurgy route coupled with rapid micro wave sintering. Nano meter-sized part icles of alu mina were used to hybridize the CNT rein forcement in the Mg matrix and establish the intrinsic influence of hybridization on mechanical behaviour of the composite. The yield strength, tensile strength and strain-to-failure of the CNT–Mg composites were found to increase with the addition of nano meter-sized alu mina part icles to the composite mat rix. They performed scanning electron microscopy observations of the fracture surfaces of the Li et al.[56] applied a t wo-step process to produce CNT-Mg alloy composites. In the first stage, they used a block copoly mer as a dispersion agent to pre-disperse mu lti-walled CNTs on Mg alloy chips. After that, they melted stirred the chips with the dispersed CNTs on their surface. They observed that CNTs were quite successfully dispersed on the surfaces of the Mg alloy chips. The mechanical propert ies of the CNT-Mg co mposites were measured by co mpression testing. They observed that the compression at failure, the co mpressive yield strength and ultimate co mpressive strength improved significantly up to 36% by only adding 0.1 wt% CNTs to the Mg alloy. CNT-Mg and CNT-Mg-Ni (Mg–23.5 wt% Ni) co mposites were processed by Schaller et al.[57] using powder metallurgy, and then charged with hydrogen by annealing at 620 K under a pressure of 0.4 MPa of hydrogen. They performed mechanical spectroscopy and concluded that such a treatment has no effect in the composites with pure Mg matrix. On the other hand, Mg–23.5 wt% Ni alloys, unreinforced as well as reinforced with CNTs, exh ibited mechanical loss spectrum, which was deeply modified by hydrogen charging. Aung et al.[58] mentioned that CNTs may be added to Mg matrix to produce composites of better mechanical properties, but their effect on the corrosion behaviour was not well understood. They studied the corrosion resistance of pure Mg and its composites reinforced with 0.3 and 1.3 wt.% CNTs in 3.5 wt.% NaCl solution using immersion testing and electrochemical measurements. They found that the corrosion rate was increased considerably by the presence of CNTs because of microgalvanic act ion between the cathodic

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