Production and properties of aluminum-based composites modified with carbon nanotubes

Materials Today: Proceedings xxx (xxxx) xxx

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Production and properties of aluminum-based composites modified with carbon nanotubes M.P. Kuz’min, M.Yu. Kuz’mina, A.S. Kuz’mina Irkutsk National Research Technical University, 83 Lermontov St., Irkutsk 664074, Russian Federation

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Article history: Received 27 April 2019 Accepted 2 July 2019 Available online xxxx Keywords: Aluminum Composite materials CNT Carbon nanotubes Nano-modifiers Hot pressing

a b s t r a c t A hot pressing method is developed for aluminum powder with preliminary sample and die heating to 600 °C. The effect of modifying aluminum with carbon nanotubes in an amount of 0.01–1 wt% is studied. It is established that the best uniformity and mechanical strength applies to aluminum specimens containing small additions of nanotubes, i.e., 0.01–0.05 wt. (elasticity modulus – 2346 N/mm2; tensile load – 2829 N; tensile elongation – 1.454 mm). The electrical resistance values of composite samples obtained by hot pressing of aluminum powder are close to cast aluminum values (0.3 Ohm). Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment 2019.

1. Introduction The development of composite materials, consisting of the metal matrix and reinforcing elements distributed in it, is one of the most priority directions for the development of modern metallurgy and materials science. In some cases, only composite materials can meet the requirements of new technologies characterized with tougher operating conditions: increased loads, higher speeds and temperatures, more aggressive environments and weight loss [1–4]. Moreover, metal-matrix nanocomposites have a significant advantage over conventional composites because the strengthening effects are highly enhanced by reducing the size of reinforcing agents from micro to nanometer-scale [5]. Numerous researches have been focused on the developments of metal matrix composites in various applications [5–12]. Among nanocomposites, aluminum-matrix composites have developed greatly and are widely used in many industrial fields. This is due to the fact that aluminum matrix composites can have increased strength, low specific gravity as well as a favorable combination of a number of mechanical and operational properties. Carbon nanotubes (CNT) and their mechanical properties have been closely investigated since the late 1990s [1,9]. Theoretical work [1] and experimental results [5–9] have indicated their extraordinary stiffness and strength, which are unparalleled by any other material available today. It is known that carbon

nanotubes (CNTs) have unique physical and chemical properties (tensile strength of 10–110 GPa, Young’s modulus of 0.62– 1.2 TPa, large aspect ratio (>100), thermal expansion coefficient (CTE 0), specific gravity of 1.2–2.1 g/cm3), which makes their use as nanomodifiers promising [13]. Their introduction into aluminum and its alloys can significantly improve physical and mechanical properties (hardness, wear resistance, crack resistance, etc.) of finished products. For these reasons, aluminum-matrix composites reinforced by carbon nanotubes draw particular interest as promising materials in aerospace and automobile industries because of their light weight, high strength and anti-corrosive properties [14]. In this regard, many researchers have attempted to fabricate metal-matrix composites reinforced with CNTs [12– 18]. However, despite this enthusiastic field of research, CNTreinforced Al-matrix composites are still far away from being commercialized, and improvement of their mechanical properties is not proportional to the significant properties of CNTs because of some difficulties especially associated with CNT dispersion and controlling of Al/CNT interface. The main problem of creating ‘‘metal-CNT” composite materials is the high tendency of nanotubes to agglomeration, while their uniform distribution in the metal matrix volume is required to obtain high values of mechanical properties [2,17]. At present, there are many methods for obtaining metal matrix composites using CNTs: introduction of nanostructures into

https://doi.org/10.1016/j.matpr.2019.07.021 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment 2019.

Please cite this article as: M. P. Kuz’min, M. Y. Kuz’mina and A. S. Kuz’mina, Production and properties of aluminum-based composites modified with carbon nanotubes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.021

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metallic melt; hot spraying (plasma spraying, cold spraying); electrochemical methods (electrodeposition, chemical deposition); laser spraying; mixing of powders at the molecular level; nanoscale dispersion [1,5–12]. However, the most promising way for obtaining aluminum-carbon nanotube composites is the powder metallurgical technique. Its prospects are due to the rapid development of powder metallurgy, creation of new powder materials with unique properties, as well as the possibility for industrial use. The essence of the technology consists in mixing of aluminum powder particles with CNTs and their sintering during subsequent or simultaneous pressing. 2. Materials and research methods In order to produce the metal based composite material, the powder of PAP-2 technical aluminum (GOST 5494-95), which was obtained at SUAL-PM of UC RUSAL, was used. The research of the original aluminum powder using the method of scanning electron microscopy with the JIB-4500 Multibeam microscope manufactured by JEOL, made it possible to establish that it consisted of spherical particles of 0.2–4 lm (Fig. 1a). To strengthen the aluminum matrix, the multilayer nanotubes manufactured by NanoTechCenter LLC were chosen (carbon nanomaterial ‘‘Taunit”). The used CNTs were obtained using the chemical vapor deposition (CVD) method. The research provides the results of X-ray phase and electron microprobe analyses [5]. According to the manufacturing company, the internal diameter of nanotubes is 5–10 nm, the outer diameter is 20–40 nm, the length of nanotubes is in the range of 2–20 lm. In their original form, the nanotubes are in the agglomerated state (Fig. 1b). To separate the nanotubes in agglomerates, to prevent the subsequent agglomeration and to improve the interaction with aluminum powder particles, the original CNTs were functionalized in the mixture of concentrated sulfuric and nitric acids (in 3:1 volume ratio) during 90 minutes at 90 °C [6]. The CNTs in the amount of 1; 0.5; 0.1; 0.05; 0.01 wt% were mixed with the aluminum powder. The mixtures were prepared out in a jasper mortar for 20 minutes [11]. Then, using ultrasonic intensive mixing, the resulting mixture was averaged to get a uniform distribution of nanotube particles in the volume of aluminum powder (Fig. 2). Furthermore, the sample weight of each mixture was put into a ‘‘piston-cylinder” mold made of HRC 62-64 tool steel. The mold was placed in a muffle shaft furnace preheated to 600 °C, then it was kept in the furnace for 10 min with the subsequent pressing

Fig. 2. Schematic representation of CNT distribution in the volume of aluminum powder.

of the sample weight. The prepared composition was pressed on the P6324B UHL 4.1 hydraulic press manufactured by the Tambov Plant for technological equipment with the maximum force of 250 kN. The obtained samples were ‘‘tablets” with a diameter of 25 mm and thickness of 4 mm The hardness tests were performed using the Brinell method with a hard-alloy ball with a diameter of 5 mm at the load of 98 N (the Zwick/Roell ZHU 250 universal hardness tester). The Shimadzu AG-X series testing machine with the maximum load of 500 kN was used to measure the elastic modulus, tensile load and breaking elongation of samples. Electrical conductivity of the samples was measured at an ambient temperature using the four-point probe technique (the Van der Pauw method) with the help of the HL5500PC Hall Effect measurement system. To study the phase composition of the composite samples, the X-ray structure analysis was applied using the XRD-7000 X-ray diffractometer manufactured by Shimadzu. The samples were investigated in the 2h-range from 10 to 70°. 3. Results and discussion It is known that the production of high-quality composite materials Al-CNT is complicated with a number of factors [1,2,11]: – high tendency of nanotubes to agglomeration prevents their uniform distribution in the aluminum matrix; – high surface energy of nanoparticles prevents the formation of quality monolithic samples.

Fig. 1. SEM images of: (a) PAP-2 aluminum powder; (b) multilayered carbon nanotubes.

Please cite this article as: M. P. Kuz’min, M. Y. Kuz’mina and A. S. Kuz’mina, Production and properties of aluminum-based composites modified with carbon nanotubes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.021

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of aluminum samples obtained without introduction of nanomodifiers was 1380 N, and for the cast samples it was 1750 N. The obtained results show a significant improvement of the mechanical properties of composite materials of aluminumcarbon nanotubes when the content of the latter in the aluminum matrix is 0.05–0.01 wt%. However, the modification of aluminum with CNT does not allow to significantly increase the plastic properties of composites in comparison with the samples obtained without nanomodifiers. When the content of nanotubes is 0.01 wt%, the tensile elongation is 1.454 mm, and without application of nanotubes it is 0.490 mm (Fig. 3c). In this case, the tensile elongation of the cast sample is 2.164 mm. The electrical resistance of aluminum samples modified with carbon nanotubes decreases with the decrease in nanotubes concentration. When the concentration of nanotubes in the aluminum matrix is 1–0.5 wt%, the electrical resistance of the samples is 0.6 Ohm, when the concentration is 0.1–0.01 wt%, its values are reduced to 0.3 Ohm. The microstructure of compacted aluminum modified with CNTs was studied using the method of light microscopy (EM) and scanning electron microscopy (SEM). Fig. 4 shows the structure of the obtained composite materials with different content of nanotubes. In the sample with CNT content of 0.01 wt%, due to the low concentration, only individual nanotubes located in the intergrain space were found among aluminum particles. This indirectly indicates their uniform distribution in the volume of the aluminum matrix. The samples are characterized by a lack of porosity. In the sample, containing nanotubes in the amount of 0.1 wt%, it is also possible to observe a fairly uniform distribution of carbon nanotubes. However, in these samples, the porosity development is observed. In the composite containing 1 wt%, it is possible to observe an increased development of porosity associated with the formation of large agglomerates. This circumstance is the main reason for the violent decrease in the strength properties of the samples. To determine the effect of adding CNTs in the amount of 0.01 wt % on the structure of the obtained composite (the presence of carbide and aluminum oxide, which can have a negative effect on its

When carbon nanotubes were mixed with aluminum powder, such problems were solved through: – introduction of carbon nanotubes into aluminum powder in the amount less than 1 wt%; – consolidation of nanotubes in a jasper mortar for 20 min; – ultrasonic mixing; – preliminary temperature exposure (the aluminum powder is not brought to the melting point of 660 °C and the carbon nanotubes are not brought to the carbonation stage – 720 °C [10]).

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In the course of the study, aluminum samples with CNT content in the amount of 1; 0.5; 0.1; 0.05; 0.01 wt% were prepared. In addition, the reference samples without carbon nanotubes were produced. Mechanical properties of the samples were evaluated based on determination of their strength (elasticity modulus, tensile load) and plasticity (breaking elongation) indices. The results of mechanical tests of aluminum samples modified with CNTs, which were obtained during hot pressing at a preheating temperature of 600 °C, are shown in Fig. 3. The values of the elasticity modulus indicate that the strength of composite materials increases with the decrease in quantity of the carbon nanotubes introduced (Fig. 3a). When the content of CNT is in the amount of 0.01 wt%, the elasticity modulus was 2346 N/mm2, which is two and a half times more than 2 times higher than the value of the elasticity modulus when the nanotube content is 1 wt%. The elasticity modulus of the aluminum samples, obtained without introduction of nanomodifiers, was 1300 N/mm2. This indicates that the introduction of CNT into aluminum in the amount of more than 0.5 wt% does not result in the increase in the elasticity modulus. However, when the concentration of carbon nanotubes is 0.01 wt%, the elasticity modulus of the samples violently increases up to 2346 N/mm2, which exceeds its value for a cast sample (1350–1500 N/mm2). Measurements of the tensile load of the samples showed that with a decrease in the number of CNTs introduced, its values increase (Fig. 3b). When the content of carbon nanotubes is 0.01 wt%, the tensile load reaches the value of 2829 N. The tensile load

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c Fig. 3. Mechanical properties of aluminum-based composites: (a) elasticity modulus; (b) tensile load; (c) breaking elongation.

Please cite this article as: M. P. Kuz’min, M. Y. Kuz’mina and A. S. Kuz’mina, Production and properties of aluminum-based composites modified with carbon nanotubes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.021

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Fig. 4. Microstructure of the obtained composites with different content of CNTs: (a) 0.01 wt%; (b) 0.1 wt. %; (c) 1 wt%

properties), the samples were examined using the X-ray phase analysis (Fig. 5). The peaks of metallic aluminum (38.5, 44.7, 65.1° 2h) with a cubic crystal lattice (a = 4.05 Å) have the highest intensity. In the range of 25–45° 2h there are peaks belonging to carbon with a hexagonal crystal lattice (a = 2.47 Å, c = 6.79 Å). Peaks with values of 26.3; 42.2; 44.34° 2h have the highest intensity. There are no peaks of alumina in the diffractogram, which may indicate the partial destruction of oxide films on the surface of aluminum particles during their compaction. Al4C3 peaks are not detected either. This indicates that the reaction of carbide formation does not take place at selected values of temperature of hot pressing and CNTs concentration. The performed research showed that aluminum samples containing small additions of nanoparticles (0.01–0.05 wt% of CNTs) are characterized by the greatest homogeneity. Carbon nanotubes contained in the amount of more than 0.1 wt% cause the formation of heterogeneities due to the agglomerates formed by nanotubes. This feature has a decisive influence on the reduction of mechanical properties of the resulting composite materials.

Fig. 5. Diffractogram of the aluminum sample with CNTs in the amount of 0.01 wt% compacted at t = 600 °C (e – peaks of Al, D – peaks of C).

Please cite this article as: M. P. Kuz’min, M. Y. Kuz’mina and A. S. Kuz’mina, Production and properties of aluminum-based composites modified with carbon nanotubes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.021

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4. Conclusion The obtained results indicate that the introduction of carbon nanotubes into the aluminum structure can significantly improve its properties. An increased level of mechanical strength has been recorded for aluminum in the structure of which there is 0.01– 0.05 wt% of CNTs. Carbon nanotubes (more than 0.1 wt%) cause the formation of inhomogeneities due to formation of agglomerates by nanotubes. The electrical resistance values of composite samples obtained by hot pressing of aluminum powder are close to cast aluminum values. The only indicator that composite materials of aluminum-carbon nanotubes are inferior to cast items is their plasticity. One of the possible solutions of this problem is the application of deformation of large degrees (the accumulated rolling method), which allows to completely destroy oxide films along the aluminum particle boundaries, and also to reach the matrix material welding around CNTs. In this case, higher values of strength, wear resistance and plasticity of the material can be obtained. Acknowledgment The research was supported by the grant for the financial support of scientific and pedagogical collectives of Irkutsk National Research Technical University (project № 02-fpk-19). References [1] Sunil Kumar, Tiwari Harpreetsingh, Anil Midathada, Sumit Sharma, Uday Krishna Ravella, Study of fabrication processes and properties of Al-CNT composites reinforced by carbon nano tubes – a review, Mater. Today: Proc. 5 (14) (2018) 28262–28270. [2] M.P. Kuz’min, N.A. Ivanov, V.V. Kondrat’ev, M.Yu Kuz’mina, A.I. Begunov, A.S. Kuz’mina, N.N. Ivanchik, Preparation of aluminum–carbon nanotubes composite material by hot pressing, Metallurgist 61 (2018) 815–821. [3] M.P. Kuz’min, V.V. Kondratiev, L.M. Larionov, Production of Al-Si alloys by the direct silicon reduction from the amorphous microsilica, Solid State Phenom. 284 (2018) 647–652.

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Please cite this article as: M. P. Kuz’min, M. Y. Kuz’mina and A. S. Kuz’mina, Production and properties of aluminum-based composites modified with carbon nanotubes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.07.021