Carbon Nanotube Reinforced Aluminum Composites Research Articles

An investigation on the effect of Al4C3 on microstructure and mechanical properties of carbon nanotube reinforced aluminum composite

In carbon nanotube (CNT) reinforced metal-matrix composites, a strong interface between CNTs and the matrix is essential for an effective load transfer. Otherwise, the excellent qualities of CNTs would virtually remain unexploited. The aim of this study is to investigate the effect of Al4C3 formation on the mechanical properties of CNT/Al composites. Composite powders were produced through a three-stage process; First, CNTs were sufficiently crushed via high-energy milling of 20 h. Then, the crushed CNTs were ultrasonically mixed with raw CNTs (in an ethanol medium) followed by a wet milling process. Finally, different amounts of the obtained reinforcing agent (0.5, 1, 1.5, 2, and 3 wt.%) were milled with aluminum powders. Composite powders were subsequently consolidated using spark plasma sintering (SPS) process at two different temperatures (500 and 550 °C) For each sample, the amount of carbide was calculated in a semi-quantitative manner using Raman spectroscopy. The results of phase analysis and mechanical-behavior evaluations showed that Al4C3 formation, due to the consumption of highly-activated crushed CNTs, could improve the interface bonding state while maintaining CNTs structural integrity. Moreover, mechanical and wear tests revealed that the optimum properties were obtained for the composite reinforced by 1wt.% processed carbon nanotubes (PCNTs) and SPSed at 550 °C for 10 min. These findings can open up a window toward the realm of design and processing of CNT/Al composites.

Preparation and properties of multi-walled carbon nanotube reinforced alumina composites by sol- spray method

The spherical composite powders of multi-walled carbon nanotubes reinforced alumina are prepared by sol- spray. The results show that the multi-walled carbon nanotubes are well dispersed in the composites. The analyses of the composite properties show that most of the multi-walled carbon nanotubes are distributed in a network at the grain boundaries, and a small number of them are distributed in the grains. When the mass fraction of multi-walled carbon nanotubes accounts for 0.5%, the Vickers hardness of the composite increases by 32.6% relative to pure alumina; the thermal diffusion coefficient increased averagely by 27.2% with respect to pure alumina at different temperatures. The composites are conductive at 0.5% of multi-walled carbon nanotubes, and the percolation threshold of the composites prepared by this method is 0.32<i>wt.</i>% based on the fitting of the percolation conductivity theory, indicating that the multi-walled carbon nanotubes are well dispersed in the alumina matrix.

Properties of Multiwalled Carbon Nanotube-Reinforced Alumina Composites Produced Following the Sol-Spray Technique

Properties of Multiwalled Carbon Nanotube-Reinforced Alumina Composites Produced Following the Sol-Spray Technique

Mechanical and Tribological Attributes of Al-CNT-Sn Composites Prepared by Press and Sintering

Carbon nanotubes (CNTs) have shown tremendous progress during the past two decades due to their extraordinary properties. With CNTs added as an alloying element, various engineering materials exhibit better mechanical properties. Multi-walled carbon nanotubes (MWCNT) were synthesized in-house by chemical vapor deposition process. Carbon nanotube-reinforced aluminum composites were prepared by cold pressing (or compaction) and sintering using different fractions (0.5, 1.0, 1.5, and 2.0 weight percent) of MWCNTs. The Al-CNT composites consists of tin (Sn) at 1.0 wt. % in each composition. Tin promotes the sintering of aluminum matrix composite. The effect of CNT on the density, hardness, and wear behavior of the composites were studied. Wear tests were performed to determine friction and wear under dry, wet, and hot conditions under varying loads from 5 N to 20 N. X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy techniques were used for the characterization. This investigation shows that increased CNT content significantly improves the hardness and wear resistance of the composites. The friction and wear were found to increase with operating temperature. A significant reduction in coefficient of friction and wear rate was observed with the application of oil during the wear test.

Investigation of the main strengthening mechanism of carbon nanotube reinforced aluminum composites

The effects of carbon nanotube aspect ratio ( R ) on mechanical properties of CNT/Al composites containing 0–2.0 wt% CNTs are studied. The optimum carbon nanotube (CNT) reinforced aluminum (Al) composites are fabricated at 580 °C for 1 h under 60 MPa, and subsequently hot extruded at 480 °C. CNT addition within 0.5 wt% can increase the strength significantly without plasticity loss, which overcomes the dilemma of strength and toughness. Strength and hardness increase significantly due to the uniformly distributed CNTs in Al matrix within a content of 1.0 wt%. Quantitatively fitting results also show that the main strengthening mechanism of CNT/Al composites should be dispersion strengthening rather than load transfer. The R , which is very crucial for load transfer theory, has only small effects on the mechanical properties of CNT/Al composites. The calculated CNTs diameters are close to the experimental used ones. • CNT aspect ratio has only small effects on the strength of CNT/Al. • Reinforcement of CNT/Al is dominated by dispersion strengthening. • CNT addition within 0.5 wt% increases strength markedly without plasticity loss. • The calculated CNTs diameters are close to the experimental used ones.

Preparation and properties of multi-walled carbon nanotube reinforced alumina composites by sol- spray method

Preparation and properties of multi-walled carbon nanotube reinforced alumina composites by sol- spray method

Enhanced load transfer by designing mechanical interfacial bonding in carbon nanotube reinforced aluminum composites

The interfacial native oxide layer always acts as barriers preventing effective interfacial bonding and load transfer in carbon nanotube reinforced aluminum (CNT/Al) composites. In this work, annealing-controlled reaction between the interfacial native oxide layer and Mg element was attempted to improve interfacial bonding in CNT/AlMg composite, and mechanical interfacial bonding featured with direct CNT-Al contact and well-reserved CNTs was achieved in the 1 h annealed composite. For the 2 h annealed composite, chemical interfacial bonding featured with interfacial aluminum carbide (Al4C3) was established due to progressive interfacial reaction. Further tensile tests and numerical analysis revealed that, owing to barrier-free load path and increased interfacial friction stress, the mechanical interfacial bonding significantly enhanced CNT load transfer effect from 30.9 MPa to 59.4 MPa, which was close to shear-lag model prediction of 59.9 MPa. While, the chemical interfacial bonding led to a combined strengthening effect of 58.8 MPa from CNT-Al4C3 hybrid as a comprised outcome of Al4C3 strengthening and CNT damage. Thus, the design of mechanical interfacial bonding should be a superior strategy for improving interfacial bonding and enhancing mechanical performance in CNT/Al composites, considering its effective protection of CNT structure integrity and suppressed formation of hydrolysable Al4C3 phase.

On the limiting grain sizes in carbon nanotube nanocomposites

Despite their continuing popularity over the past two decades, the influence of carbon nanotubes on grain growth is not understood clearly and often a Zener analysis is applied erroneously. The role of second phase particle drag on grain boundaries and the Zener limiting grain size behavior is well documented for spherical particles. Surprisingly, no such reports of limiting grain size in carbon nanotube composites are available in the literature, which may be related potentially to issues with poor dispersion. Systematic grain growth studies in well dispersed multiwall carbon nanotube reinforced alumina composites show for the first time a limiting grain growth behavior. The drag force exerted by cylindrical nanotubes on grain boundaries was calculated and compared with spherical particles. Due to nanometric dimensions, radial flexibility and the unique tendency of nanotubes to be present only at the grain boundaries, a Zener analysis is inapplicable for analyzing grain growth in these composites. Therefore, a simple geometrical analytical model was developed that rationalizes the experimentally observed limiting grain growth behavior. The influence of grain size distributions on the limiting grain size was determined, showing that the limiting grain size is reduced with an increase in the standard deviation. The geometric grain boundary coverage model proposed here can be used as a guideline for tailoring microstructures based on size, shape and volume fraction of nanometric carbon-based reinforcements, for exploring interesting properties including superplasticity.

Creep in alumina and carbon nanotube reinforced alumina composites

Despite numerous studies over five decades, the mechanism controlling creep in fine grained alumina has not yet been identified unambiguously. Compression creep experiments in alumina and carbon nanotubes (CNTs) reinforced alumina composites reveal that the creep at high stresses is not influenced by the presence of CNTs at grain boundaries. All data at high stresses are consistent with a Coble diffusion creep mechanism and a stress exponent of one, and an interface controlled diffusion creep process at lower stresses with a stress exponent greater than one. This is similar to earlier observations in fine grained yttria stabilized tetragonal zirconia. Based on a recent study on Al tracer grain boundary diffusion, analysis of the data in terms of cation and anion diffusion along grain boundaries suggests that creep is controlled by the slower O diffusion along grain boundaries.

Design of an efficient flake powder metallurgy route to fabricate CNT/6061Al composites

A flake powder metallurgy route that combines high-shear pre-dispersion process with shift-speed ball milling was designed to fabricate carbon nanotube reinforced aluminum composites. In order to mitigate the dispersion responsibility of ball milling and improve the dispersion efficiency, high-shear pre-dispersion process utilizing the mechanism of dry particle coating was firstly applied to CNTs and 6061Al powder mixtures for a few minutes to de-agglomerate CNTs and uniformly coat them onto surface of 6061Al powder. It was demonstrated that, for the pre-dispersed powders, more rapid and uniform dispersion of CNTs was achieved during following shift-speed ball milling process, which in turn protected CNTs from additional damage and led to better interfacial bonding and higher reinforcing efficiency. As results, compared to counterpart without pre-dispersion, the as-extruded bulk 1.5 wt% CNT/6061Al composites exhibited simultaneous enhancement in Young's modulus, tensile strength and ductility.

Superplastic behavior of carbon nanotube reinforced aluminum composites fabricated by flake powder metallurgy

Superplastic deformation of a carbon nanotube reinforced aluminum alloy matrix composite (CNT/6061 Al) fabricated through Flake powder metallurgy (Flake PM) was investigated using uniaxial tensile tests under different temperatures and different strain rates. The result showed that the maximum elongation at 853K was 106% at strain rate of 4.17E−1s−1, while the maximum elongation at 673K was 89% obtained at 4.17E−1s−1. Threshold stress was evident at lower temperature but insignificant at higher temperature. Strain rate sensitivity and apparent activation energy was determined to be 0.33 and 87.3kJ/mol, respectively, indicating the grain boundary diffusion controlled mechanism was dominant for this material. With the increase of temperature, partial liquid phase appeared and the reaction of CNT was intensified, which indicated the superplastic deformation of CNT/6061 Al should be conducted at lower temperature (673K).

Deformation and strengthening mechanisms of a carbon nanotube reinforced aluminum composite

The objective of this study was to characterize microstructure, texture, and deformation behavior of a carbon nanotube (CNT) reinforced aluminum composite via electron backscatter diffraction (EBSD), X-ray diffraction (XRD), scanning and transmission electron microscopy. The addition of 2.0 wt.% CNTs in a 2024Al alloy led to considerable grain refinement, with a bi-modal distribution of grain misorientation angles positioned at ∼7° for low-angle grain boundaries and ∼50° for high-angle grain boundaries. The CNTs were observed to be uniformly dispersed in the matrix while some CNT shortening occurred during ball milling. The grain size reduction and compressive micro-strains in the composite broadened X-ray peaks and resulted in lattice shortening. The macro-texture determined via XRD and micro-texture via EBSD were in good agreement, revealing that the composite contained stronger {112} <111> Copper texture and {110} <001> Goss texture in comparison with 2024Al base alloy. The deformation resistance of the composite at both room and elevated temperatures was effectively enhanced due to the CNT addition. This was mainly attributed to the role of underlying strengthening mechanisms: Hall-Petch strengthening (significant grain refinement involving the Zener pinning effect of CNTs), and composite strengthening including load transfer, thermal mismatch and Orowan looping.

Strengthening mechanisms in carbon nanotube-reinforced aluminum composites

Although carbon nanotubes (CNTs) are used to reinforce aluminum alloys, the underlying strengthening mechanisms have not yet been clarified. Here, we focus on elucidating several of these mechanisms including load transfer, generation of dislocations from thermal mismatch, and the Orowan looping system to explain the enhanced mechanical properties of CNT-reinforced Al. The sample preparation procedure involves mechanically pulverizing Al in paraffin oil mixed with CNTs dispersed in dichloroethane. The mixture was then sintered and melt-blended to produce solid samples. The formation of covalent bonds (Al4C3) between Al and CNT walls was confirmed by X-ray diffraction, Raman spectroscopy, and transmission electron microscope measurements. The yield strength and tensile strength were improved by 60% and 23%, respectively, with the addition of 0.2 wt% CNTs. These enhancements corroborate well with the predicted values from the multiple strengthening mechanisms.

Relationship between microstructure and mechanical properties in acid-treated carbon nanotube-reinforced alumina composites

Alumina composites reinforced with multiwalled carbon nanotubes (MWCNTs) at up to 3.7 vol% are prepared by a precursor method followed by a spark plasma sintering. We systematically and quantitatively investigate the effects of acid-treatment time of the MWCNTs on not only bending strength and fracture toughness of the composites but also on the mechanical strength and dispersibility of the MWCNTs, the grain size of the alumina matrix, and the interfacial strength between MWCNT and alumina. The main objective of this study is to evaluate how these parameters influence the mechanical properties with the aid of multiple regression analysis. We demonstrate that the matrix grain size, the mechanical strength of the MWCNTs, and the interfacial strength have little impact on the mechanical properties for the composites prepared in this study. On the other hand, the dispersibility of MWCNTs has the significant influence on the mechanical properties. Both the dispersibility of the MWCNTs and the mechanical properties of the composites increase as the acid-treatment time increases up to 2 h at low MWCNT content (0.9 vol%). Conversely, at a higher amount of MWCNTs, the degradation in the mechanical properties is shown to be associated with the deterioration of MWCNTs’ dispersibility. As MWCNT agglomerates are anticipated to act as imperfections, they may override the effects of the strength of MWCNTs, matrix grain size, and interfacial strength. By means of the multiple regression analysis, we quantitatively show that improving MWCNTs’ dispersibility is one of the most important factors in enhancing the mechanical properties of MWCNT/alumina composites.

Vickers Hardness of Multi-Wall Carbon Nanotube Reinforced Alumina

Reliability of indentation method in carbon nanotube reinforced ceramics had been proposed in some works, especially of indentation toughness. In this work the traditional indentation hardness of the Multi-wall carbon Nanotube reinforced alumina composites was studied. It is found that the Vickers hardness and nanoindentation hardness of MWNT reinforced alumina fabricated by SPS decreased from about 20GPa to 16GPa at a critical load force about 1kg. While pure alumina showed a Vickers hardness of 20.6GPa in the whole load force range. When the load force or displacement into surface increasing, continuous recorded hardness, contact stiffness and modulus of the MWNT reinforced alumina showed quite different behavior compared with the pure alumina, especially at middle period of indentation. It was believed that different properties in vertical and horizontal directions of the MWNT were the key. MWNT was compressed and deformed as a soften component in the whole indentation processing. And tensile stress caused by MWNT in composite became significant until an enough strain attained and decreased when a critical stress was over. The results mean the definition of hardness or similar ones seemed need deep description in this type of material.

Improved Tensile Strength of Carbon Nanotube Reinforced Aluminum Composites Processed by Powder Metallurgy

Carbon nanotube (CNT) reinforced aluminum (Al) composites were synthesized using the powder metallurgy (P/M) technique, combined with hot extrusion and hot rolling. 0-2.0wt.% of CNTs were added as reinforcements. The effect of CNTs on the mechanical properties of Al was investigated and a significant enhancement in tensile strength was obtained compared with the pure matrix. The improved strength was analyzed based on (i) Orowan strengthening, (ii) thermal mismatch between CNTs and matrix, and (iii) load partition effect due to the CNTs.

A comparison of mechanical and wear properties of plasma sprayed carbon nanotube reinforced aluminum composites at nano and macro scale

The macro- and nano-scale mechanical and wear properties of carbon nanotube (CNT) reinforced Al–Si composite coatings prepared by plasma spraying have been compared in this paper. The composite coatings show a two phase microstructure; one phase being the Al–Si matrix with well dispersed CNTs and the other being the CNT clusters. Nanoindentation testing on the matrix portion with dispersed CNTs indicated an increase in the elastic modulus by 19% and 39% and an increase in the yield strength by 17.5% and 27% by the addition of 5wt.% and 10wt.% CNTs respectively. Macro-scale compression tests indicated no improvement in the elastic modulus but an increases in the compressive yield strength by 27% and 77% respectively, by addition of 5wt.% and 10wt.% CNTs. Nanoscratch testing carried out on the Al–Si matrix with dispersed CNTs indicated a decrease in scratch volume by 34% and 71% by addition of 5wt.% and 10wt.% CNTs respectively. Macro-scale wear tests indicated a decrease in the wear volume by 68% in case of 5wt.% CNT coatings but an increase in the wear volume by 15% for the 10wt.% CNT coating. The differences in mechanical and wear properties at nano and macro scales are explained in terms of the bimodal CNT dispersion (well dispersed and clusters) in Al–Si matrix, CNT cluster size and fraction and carbide formation.

カーボンナノチューブ/アルミナ複合材料の作製と破壊機構の解明に関する研究

カーボンナノチューブ/アルミナ複合材料の作製と破壊機構の解明に関する研究

An analysis of the factors affecting strengthening in carbon nanotube reinforced aluminum composites

Tensile strength data of Al/CNT composites from the literature is analyzed to understand the effects of CNT dispersion, processing technique, degree of deformation and CNT–matrix interface on the elastic modulus, strength and toughness of composites. Strengthening can be divided in three regimes which show decreasing strengthening effect with an increasing CNT content. The strengthening is highest for CNT content less than 2 vol.%. The applicability of the micromechanics models in predicting the strength and elastic modulus of CNT reinforced metal matrix composites is also analyzed. The rule of mixtures is effective in predicting the elastic modulus of the Al/CNT composites for low CNT content (<2 vol.%) whereas Halpin–Tsai and combined Voigt–Reuss models are better at intermediate CNT content (2–5 vol.%). Effect of degree of deformation such as extrusion ratio during processing on the load transfer to CNT and resulting strengthening is also discussed. Tensile data on Cu/CNT and Mg/CNT composites is compared with Al/CNT to show that strengthening is not effective when there is no chemical interaction between metal matrix and CNT. The analysis presented here would be very helpful in the future design of high strength CNT/metal matrix composites.

Thermal conductivity of carbon nanotube reinforced aluminum composites: A multi-scale study using object oriented finite element method

In this study, object oriented finite element method (OOF) has been utilized to compute the thermal conductivity of plasma sprayed Al-12 wt.% Si containing 10 wt.% multiwall carbon nanotubes (CNTs). The computations have been made at micro- and macro-length scales which highlight the effect of CNT dispersion on thermal conductivity. Experimentally measured values at 50 °C indicate that CNT addition reduced the thermal conductivity of Al–Si matrix from 73 W m −1 K −1 to 25.4 W m −1 K −1 which is attributed to the presence of CNT clusters. OOF calculations at micro-length scale predicted an 81% increase in the conductivity of Al–Si matrix due to presence of well dispersed CNTs inside the matrix. At larger lengths scale, the decrease in the overall conductivity is related to the extremely low conductivity of CNT clusters. Thermal conductivity of CNT clusters could be up to three orders of magnitude lower than individual CNTs. OOF computed values match well with experimental results. OOF compute thermal conductivity of Al–CNT composite is also compared with theoretical two-phase models for CNT-composites at different length scales.