Mechanical properties and interfacial phenomena in aluminum reinforced with carbon nanotubes manufactured by the sandwich technique

Abstract

Recently, a new manufacturing process for the production of metallic matrix composite materials reinforced with carbon nanotubes, known as sandwich technique has been proposed. This technique produces a material comprised of a metallic matrix and a banded structures-layers of multi-walled carbon nanotubes. However, among other issues, the matrix-reinforcement interface and the reinforcement dispersion degree are still open questions. The present study uses field emission scanning electron microscopy and high resolution transmission electron microscopy to probe that the method is capable to achieve a good dispersion of the multi-walled carbon nanotubes with no evidence of carbon nanotubes’ damage. The mechanical properties were measured by tensile and nanoindentation tests; improvements in the elastic modulus, yield and ultimate strengths were found, with respect to the unreinforced material.

Keywords

Metal matrix composites interfacial phenomena carbon nanotubes mechanical properties

Introduction

The need to find high-strength lightweight materials in transportation industries has driven the development of new materials and new manufacturing methods to reduce CO₂ emissions into the atmosphere, this means to develop resistant and lightweight materials. Since 1991, carbon nanotubes (CNTs)¹ have reached the attention of the scientific community. They are composed of graphene layers arranged concentrically in form of a tube with very small dimensions, which have potential applications as a reinforcing material for metals,^2–5 polymers,^6–10 and ceramics^11–13; they have also generated a great deal of interest in recent years, because of their mechanical and physical properties, such as elastic modulus, high strength, and high thermal conductivity,^14–17 which, combined with their low weight, make them an ideal reinforcement material. However, their incorporation and dispersion in a metal matrix is not simple, because CNTs have very large surface area to volume ratios and unsatisfied bonds try to minimize their energy joining, which causes the particle agglomeration. Recent developments on this field seek for processes that promote a good dispersion of CNTs in the matrix without CNTs damage, along with high CNTs density and effective load transfer between reinforcement and metal matrix.¹⁸ The mechanical properties of the composite depend on the chemistry, atomic structure, and the bonding at the interface because this is the region where the mechanical load transfer from the matrix to the reinforcement occurs.^19,20

The manufacturing techniques, including powder metallurgy, melt processing, and thermal spray,^2,21–25 are generally followed by cold or hot deformation processes to increase the mechanical properties.^26,27 However, techniques like powder metallurgy in some cases do not lead to a major increase of the mechanical properties due to the agglomeration of CNTs in the metal matrix, which allows the formation of aluminum carbides that inhibit the effective load transfer because of their brittleness²⁸; in addition, these phases are induced by high temperatures involved in the process.^5,18,21

A new manufacturing technique described elsewhere²⁹ promises to overcome some of the problems found during the metallic matrix composite materials (MMCs) processing; in that work, indirect verification of good dispersion and alignment has been reported. However, high temperatures were still needed, so that some harmful phases could be produced. In the present work, field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM) analyses were used to study the Al/CNTs interface of a composite produced by this sandwich technique; the effect of the multi-walled carbon nanotubes (MWCNTs) on the mechanical properties was also studied.

Materials and methods

MWCNTs were supplied by Nanostructured & Amorphous Materials, Inc. Sheets of aluminum (purity of 99.5%, 400 microns in thickness) were utilized. For the synthesis of the polymer matrix composite (Figure 1), pellets of low-density polyvinyl alcohol (PVA) were diluted in hot distilled water to produce a solution of 4 wt.% of PVA. MWCNTs were introduced into the PVA solution in percentages of 0.5 and 2 wt.%. The reinforcement of the composite was dispersed by magnetic stirring during 1 h at an average speed of 900 rpm followed by a sonication in a Vibra Cell series CLC equipment, which was set to a power of 100 W and an amplitude of the probe of 20%; the dispersion maximum energy was 60 kJ to prevent the damage of the CNTs.^30,31 The polymer solution was poured into a rectangular mold and dried during 8 days at room temperature for allowing the polymer to cure, producing a composite with a thickness of about 300 µm. Then, sheets of this composite were longitudinally stretched using a Monsanto tensile machine at a speed of 2 mm/min and temperature of 80℃ until reaching a final thickness of 100–150 µm.

Two composite sheets of PVA/MWCNTs were alternately stacked with three aluminum sheets. Finally, the stacks were hot compacted in a die using an argon atmosphere to prevent the MWCNTs oxidation during the process. Thermo-gravimetric analysis, performed in a previous work,³² showed that when an argon atmosphere is used the minimum temperature to evaporate the PVA and to leave the CNTs is about 500℃. However, both the PVA evaporation rate and aluminum diffusion rate depend on time and temperature, which is why in this work the processing temperature was set near the aluminum melting point, in order to increase both rates. Thereby, the temperature was gradually raised during 1.5 h until 650℃ were reached; the pressure was also gradually raised up to 40 MPa. This was followed by a holding period of 30 min for allowing the PVA to evaporate and the aluminum to diffuse between the sheets, to finally accomplish the composite consolidation. The final thickness of the composite was approximately 300–400 µm. Figure 2(a) outlines the composite sections studied. In Figure 2(b), a SEM image shows a cross-section of one Al-MWCNTs composite, where some pores can be clearly seen, as a result of, probably, PVA remnants trapped among aluminum sheets.

Some samples were extracted from the metal matrix composites for metallographic analysis. Dog-bone type samples were cut along the longitudinal direction by using a controlled water jet machine; their dimensions were 23 mm in gage length, 10 mm in width, and 300–400 µm in thickness.

Transversal sections of some composites samples were prepared by a focused ion beam system (JEOL JEM 9320FIB) to analyze the interface between CNTs and aluminum; the images of these zones were obtained by HRTEM in a JEOL JEM2200FS. Microstructural characterization was done in a FESEM JEOL JSM-7401F. The densities of the composites were measured by Archimedes method using isopropyl alcohol. Tensile samples were tested in an AG Shimadzu model GX testing machine with a speed of 0.5 mm/min.

Samples for nanoindentation tests were metallographically polished with a final step using a suspension of 0.025 µm of silica colloidal. This last step was included to diminish the surface hardening due to the metallographic polishing. The nanoindentation tests were carried out using an IBIS Authority Fischer—Cripps nanoindenter with a diamond Berkovich indenter tip. A peak load of 1 mN was held during 5 s to reduce the creep effects during the final unloading step. The data were acquired in open loop mode to produce quick tests and minimize the thermal drift effects. Loading–unloading cycles last 20 s. A matrix of 10 × 10 indentations was set, being the lines parallel to the interface. The distance between the indentations was 3 µm. The hardness and elastic modulus were calculated by the Oliver and Pharr method; average values were reported.

Results and discussion

MWCNTs characterization

The characteristics of MWCNTs such as morphology and diameter are essential for their dispersion into the matrix. MWCNTs with a very large diameter have an small tendency to re-agglomerate after a dispersion process.³³ Contrarily, a larger aspect ratio relationship promotes the CNTs tendency to agglomerate because of its large surface energy,³⁴ allowing the formation of clusters which in the composite causes an inefficient load transfer from the matrix to the reinforcement.

Figures 3 and 4, respectively, show FESEM and HERTEM images of the as-received MWCNTs, as well as their outer and inner diameter distribution. Outer diameters of 10–40 nm and inner diameters of 10–20 nm were found; the presence of MWCNTs was confirmed.

The defects showed in Figure 4(b) include variable numbers of carbon layers and partial interior filling, amorphous carbon, and “bamboo” defects, which consist of several transverse, internal walls segmenting the interior of a MWCNTs into independent pods or isolated volumes. Bamboo defects are often quasi-periodic in a MWCNT.³⁵ Some nickel impurities were also seen. In Figure 4(c), the presence of 21 walls in a CNT was observed. Although inner diameters of the MWCNTs have no direct influence on the dispersion of the CNTs in a material, outer diameters of the MWCNTs used in this work are small, which could increase the risk of re-agglomeration. However, the use of the PVA solution allowed to have a very good dispersion of MWCNTs, as well as to increase the MWCNTs percentage in the polymeric matrix. The dispersing effect of the polymer continued even after the polymerization and curing steps, whereby the CNTs re-agglomeration was avoided, as reported by Olayo et al.³⁶

Microstructural analysis of Al/MWCNTs

Figures 5 and 6 show images of the aluminum reinforced with 0.5 wt.% and 2 wt.% of MWCNTs, respectively, and close up views of the interface zones (see Figure 2(b)). These FESEM images show that the MWCNTs appear to be isolated, which means that a good dispersion of them near the interface between aluminum sheets was achieved. Figure 5(b) reveals that most of the MWCNTs are aligned in the stretching direction (indicated by an arrow). This fact allows to asseverate that the MWCNTs were well aligned in the PVA matrix and that such an alignment was retained during the subsequent manufacturing process.

In the case of the composite with 2 wt.% of MWCNTs (Figure 6(b)), the alignment was not so good. As explained elsewhere,²⁹ this poor alignment is probably a result of the low deformability of the reinforced PVA layer during the stretching process, which in turn is due to the high content of MWCNTs.

To verify the dispersion and alignment degree of the MWCNTs for each MWCNTs content, a layer of aluminum was peeled-off. The exposed surfaces are shown in Figure 7, where it is revealed that, again, most of the MWCNTs are aligned in the composite with lower percentage of MWNTCs (Figure 7(a)), but not in the composite with higher percentage (Figure 7(b)). During the compaction step, due to the shape of the punch-die set used (Figure 1), the die produces a maximum pressure at the center of the sample and zero pressure at the edges. Then the edges are not well consolidated and there is a poor diffusion. Taking advantage of this, it was just in this region where a sharp blade was introduced to induce a crack. After that, a peeling-off of the layer could be performed owing to the low fracture toughness of the aluminum (K_1c ∼ 30 MPa m^0.5). At this point, it is not clear if the CNTs improve or not the fracture toughness, but this is not the unique property that controls the crack growing; the fact that the elastic modulus is increased means that the energy release rate (GαK_1c²/E) could probably be decreased if fracture toughness did not have an enough increasing, notwithstanding that the presence of CNTs in metallic materials has been previously reported to produce an increase in the fracture toughness of the composite.³⁷

Interfacial analysis

The interface between the aluminum matrix and MWCNTs was examined by TEM and HRTEM. TEM images (Figure 8) revealed that the MWCNTs are embedded into the aluminum matrix, with no evidence of cluster formations. As it is already known,^18,21,33 these features are essential for a good load transfer and thereby to improve the mechanical properties of the metal matrix composites. TEM and HRTEM images did not show evidence of MWCNTs damage. These findings clearly demonstrate that the sandwich technique is much less prone to induce damage in the reinforcement, since it does not use high mechanical energies compared with conventional composite manufacturing processes derived from powder metallurgy.^4,21,25,26

Figure 8(c) presents images of the Al matrix-MWCNTs composite for both the matrix and reinforcement. Image (a) from Figure 8(c) shows that the interlayer spacing of the MWCNT is about 0.33 nm and image (b) shows that the interplanar distance of the aluminum is about 0.23 nm, which corresponds to the plane (1 1 1) of the aluminum matrix (data from ICDD database PDF-2/2003).

In the evaluated zones, the presence of any aluminum carbide phase was not detected, which confirms the findings of micro X-ray diffraction previously reported.²⁹ However, several studies have shown the formation of aluminum carbides in this kind of composite materials. In addition, the mechanisms of phase growth when the composites are manufactured at temperatures above their melting point have also been studied. The growth of these phases occurs in some predetermined directions, with no evidence of epitaxial growth.³⁸ Nevertheless, another studies have shown the formation of carbides at low temperatures,⁵ but in a few percentages. Although it is likely the formation of some aluminum carbides in the composites produced in the present investigation, they were not detected by XRD analysis.

Mechanical properties

The nanoindentation technique was chosen because it is able to measure mechanical properties of the Al/MWCNTs layers and their surroundings, whose size is just a few microns. However, the macromechanical behavior of the composite is ruled by the stacked materials: aluminum, aluminum/MWCNTs, and defects on the interface (either pores or remnants). The defects can act as stress raisers, decreasing the ultimate and yield stresses.

Results of the tensile tests are shown in Figure 9, they correspond to the average of five tests for each composite. It is observed that both the yield and tensile strengths increase with the content of MWCNTs, compared with the aluminum sheets compacted without MWCNTs (Al 0 wt.%). Despite the fact that during the manufacturing process, some pores were identified in the composites (Figure 2(b)), which is in agreement with a density decrement with the increment of the MWCNTs content (Figure 10), the mechanical properties of the composites always increased with the addition of the MWCNTs. This shows the efficiency of the sandwich technique for producing metal matrix composites reinforced with CNTs. Problems related to pores and low density may be solved by post-processing operations such as sheet rolling.

Figure 11 shows the values of the elastic modulus and hardness measured by nanoindentation tests in the vicinity of the interfaces between aluminum sheets (Figure 11(a) and (c)). It is clearly seen that both the modulus and hardness are significantly increased close to the interface, which is evidenced by the curves inserted in Figure 11(a) and (c), especially where the MWCNTs content is higher. For example, in the case of the composite reinforced with 2 wt.% of MWCNTs, the average of the highest peak of the elastic modulus represents an increment of ∼70% (120 GPa) with respect to the material without reinforcement, while for the composite reinforced with 0.5 wt.% of MWCNTs, it represents an increase of 50% (100 GPa). The average increase of the elastic modulus in the materials is 27 and 34% for the composites reinforced with 0.5 and 2 wt.%, respectively (Figure 11(b) and (d)).

The hardness obtained by nanoindentation tests in the reinforced layer showed an increase with the increase of the MWCNTs percentage. A mean value for this layer was calculated as the mean integral under the curve in the area of interest. On the other hand, the yield (and also the ultimate tensile strength) is proportional to the hardness ( $H = K σ_{Y}$ ). This equation is called the Tabor’s relationship,³⁹ where the constant K has not yet been established for the Al pure reinforced with CNTs. However, for the reinforced layers, assuming a value of 2.71 for 1100 aluminum alloys, the mean yield stress can be estimated.

To estimate the mechanical properties of the composites, the rule of mixtures was utilized. For this purpose, the next parameters for aluminum layers were used: Volumetric fraction of aluminum layers was 0.92, elastic modulus of aluminum was 69 GPa, and yield strength of aluminum was 38 MPa. On the other hand, for the Al/MWCNTs layers, the mean values obtained from nanoindentation tests and Tabor’s relationship were utilized.

The predicted values for the yield strength were 40 MPa and 42 MPa for the composites reinforced with 0.5 and 2 wt.% of MWCNTs, respectively. It should be noted that the value of the yield strength measured by tensile tests is quite higher than that estimated by the rule of mixtures, which is probably due to mechanisms associated with the increment in mechanical properties: (a) possible formation of interfaces, (b) dislocation stacking, and (c) recrystallization in nearby areas between the reinforcement and the matrix, among others. Clearly, the rule of mixtures cannot be applied to this type of composites for the prediction of strength properties.

On the other hand, the estimated values for the elastic modulus were 71 GPa and 74 GPa for the composites reinforced with 0.5 and 2 wt.% of MWCNTs, respectively, in this case, the elastic modulus was not measured in the tensile tests due to technical limitations of the equipment. Thus, a comparison with the predictions of the rule of mixtures was not possible. However, based on the results of yield strength, it can be said that the rule of mixtures could probably also underestimate the elastic properties for this type of composite.

Based on these results, an additional advantage of the sandwich technique is that, it allows to design the overall behavior of this kind material, controlling both the aluminum thickness and the CNTs content. The increase in the mechanical properties with the addition of MWCNTs, at both bulk (Figure 9) and small-scale (Figure 11) levels, demonstrates once again the effectiveness of the sandwich technique for manufacturing metal matrix composites reinforced with CNTs.

Conclusions

The alternative technique developed for manufacturing aluminum composites reinforced with CNTs is effective and shows an enhancement of the mechanical properties evaluated by tensile and nanoindentation tests. This study will allow further research on lightweight materials for applications in aeronautics, aerospace, and automotive industries.

Morphological characterization revealed that the MWCNTs used were wavy and entangled. HRTEM observations showed some defects like bamboo structure and some impurities like nickel. However, these defects do not promote the carbide phase formation during the process. MWCNTs did not present morphological changes, such as in diameter and length, due to the dispersion process. The dispersion was effective using ultrasonication in ranges between 40 and 60 kJ.

FESEM, TEM, and HRTEM observations showed that the MWCNTs are well dispersed and well embedded into the matrix, with no evidence of carbide formation as it was shown in the interface images. Furthermore, a crystal lattice non-coherence between MWCNTs and the aluminum was observed, which allowed a very good load transfer, and this in turn favored the mechanical properties enhancement in the metal matrix composites.

The nanoindentation tests showed the mechanical properties in small volumes close to the interfaces between the aluminum sheets, but they do not represent the mechanical properties of the bulk material. However, in both the nanoindentation tests and the tensile tests, the mechanical properties increased. In nanoindentation tests, the aluminum reinforced with 0.5 wt.% of MWCNTs had a rise of 50% in the elastic modulus, while the reinforced with 2 wt.% of MWCNTs had an increment of ∼70%, with respect to the material without reinforcement. This peak increment measured near the interface between aluminum layers must have been the responsible of the mechanical properties enhancement in the bulk materials, measured by tension tests; in this case, the increments were about 27 and 34% for the composites reinforced with 0.5 and 2 wt.% of MWCNTs, respectively. However, the mechanical properties could increase if the aluminum sheets could be thinner (50–100 µm), since the reinforcement would be better distribute in the bulk material.

The rule of mixtures highly underestimate the mechanical strength properties of the composite. A further investigation in reason why this happens must be conducted. The sandwich technique is particularly interesting, because it produces a substantial increase in tensile strength and elastic modulus, allows the addition, dispersion, and alignment of CNTs in a simple manner and can potentially produce less damage to the reinforcing material, when compared with other techniques previously reported in the literature.

Footnotes

Acknowledgments

The technical assistance of C.E. Ornelas-Gutiérrez, W. Antúnez-Flores, and O. Solís-Canto are greatly appreciated.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Universidad Nacional de Colombia at Medellín through the project to support graduate students and Centro de Investigación en Materiales Avanzados (CIMAV-Mexico).

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