Synthesis, structural analysis and in situ transmission electron microscopy mechanical tests on individual aluminum matrix/boron nitride nanotube nanohybrids
Introduction
Among all structural metals, low density, lightweight aluminum and its alloys are one of the most important constituents of functional components used in automotive and aerospace applications. Their light weight is the key for economic usage of fuel, saving energy and decreasing environmental pollution. However, such materials are relatively soft. For example, pure Al has a very modest values of Young’s modulus and ultimate tensile strength, ∼70 GPa and ∼40 MPa [1], respectively. These further decrease several times under heating to moderate temperatures of only 200–300 °C. Making Al tougher and stiffer would lead to significantly minimized demands for structural component sizes and their thicknesses, and in turn further reduce the overall construction weight.
Conventional multiwalled nanotubes made of carbon (CNTs) [2] possess a Young’s modulus of up to 1 TPa, which rivals that of diamond [3], and a strength of the order of 100 GPa [4], [5], [6], [7], [8], thus ∼100 times of that of a typical steel, while being just one-fifth of its weight [9]. Thus, at present, the nanotubes are likely the strongest one-dimensional reinforcing fibers known to mankind. And, not surprisingly, very recently, nanotube-reinforced metal matrix composites have received increased attention from materials scientists and engineers [10]. However, the applications of the most abundant multiwalled CNTs grown from a vapor phase (through a so-called chemical vapor deposition (CVD) process) for metal matrix composites have evidenced severe fundamental problems due to CNT specific and unfavorable morphologies. That is, due to weakly bonded individual multishells, the standard CNTs are normally curled and waved, and form numerous buckles and entangled ensembles [9]. This kills the possibilities for their homogeneous distribution in a given matrix, creates macrovoids in the composite sheets, fibers and stripes which, in turn, serve as stress concentrators, and weaken matrix-reinforcing fiber interface, and, in the end, lead to a material catastrophic brittle failure.
However, there is another nanotube in nature that is structurally very similar to CNTs, but functionally very different – such nanotube is made of a layered boron nitride (BN). Boron nitride nanotube (BNNT) [11] – is an amazing material whose rich potential has yet been properly and fully understood. It is extremely thermally stable, chemically inert, and does not dissolve in acids and molten salts. Whereas CVD C nanotubes start to completely disintegrate at only ∼500 °C in air via oxidation, BNNTs withstand temperatures in excess of 900 °C without any traces of structural degradation or chemical modification [11].
Over the past years the present authors performed the first ever direct bending [12] and nanotensile tests [13] on multiwalled BN tubes inside a transmission electron microscope and realized that, similarly to C nanotubes, they possess a huge Young’s modulus of ∼1 TPa and an ultimate tensile strength exceeding 30 GPa. Thanks to a specific ionic-type B–N chemical bonding in multiwalled BN nanotubes and resultant strong intra-layer coupling (so-called “lip-lip” interactions), such tubes crystallize in peculiar straight needle-like morphologies (as opposed to commonly bundled and buckled standard CVD C nanotubes). Due to this feature such tubes possess characteristic multishell breakage that allow them to carry a load several times exceeding that natural for multiwalled C nanotubes at the same loading conditions [13].
Intuitively, needle-like shapes of BN tubes look remarkably preferable for their perfect distribution and even texturing/networking in any given matrix, including metals. In fact, this BN tube advantage was experimentally verified by us over the last years in respect of BN nanotube–polymeric composites [14] and BN nanotube–ceramic matrix composites [15]. The strength, toughness, elasticity and thermal conductivity of polymers and engineering ceramics have dramatically been enhanced, up to 200–300%, by adding marginal amounts of multi-walled BN tubes (only few wt.%).
We also measured the true density of BN nanotubes as only ∼1.4 g cm−3 [16]. Therefore, a possible increase of strength and elasticity of an Al matrix by such low density, ultralight, but superstrong reinforcing fibers would allow one to dramatically reduce the carrying-load component dimensions and further dramatically reduce the overall structure weight, decrease fuel consumption, save energy, and reduce pollution effects if such new hybrid material may be utilized for the new generation of automobiles, aircrafts and/or spacecrafts.
However, to date BN nanotube–metal matrix composites have been given very little attention due to difficulties involved in BNNT high yield fabrication (needed for a realistic metal composite research) compared with the relative ease in making standard CVD C nanotubes. It is only during the last few years that our group at NIMS has achieved technologically meaningful amounts of multi-walled BNNTs [11]. Thus, the expected high-quality BN nanotube-reinforced metal matrix composite studies have just become feasible and timely.
Therefore, the ultimate target of the present work is the design, development and fabrication of a new prototype “dream” structural material which would be ultralight (density of only 2.5 g cm−3 or less), superelastic and superstrong (tensile strength in the GPa range). We are predicting that such a material should be based on a composite hybrid between an Al-based matrix and reinforcing superstrong, ultralight fibers made of straight needle-like multiwalled BN nanotubes.
In order to elucidate the regarded working principle of the proposed reinforcement approach, in this paper we for the first time fabricated, bent and tensioned the individual BN nanotube-reinforced aluminum-based nanohybrids in a transmission electron microscope equipped with an atomic-force microscopy (AFM) sensor.
Section snippets
Experimental methods
Pure “snow-white” multi-walled BNNTs were synthesized at a high yield through the so-called boron oxide-assisted CVD (BOCVD) method, as was reported in our previous publications [11], [12], [13], [14]. After subsequent purification, the nanotubes were dispersed in ethanol. Their average external diameter was ∼40–50 nm.
BNNT/Al nanocomposites were fabricated at room temperature using magnetron sputtering (CFS-4EP-LL, Shibaura Mechatronics Corp.) of pure Al onto dispersed BN multiwalled nanotubes.
Structural and chemical characterizations of composite BNNT–Al nanohybrids
The synthesized BNNT–Al nanocomposites entirely preserved a one-dimensional straight morphology peculiar to the starting multiwalled BNNT templates. The thickness of the Al coating became larger in proportion to increased sputtering time. X-ray diffraction patterns of the BNNT–Al nanocomposites with decently thick (∼200 nm) Al coatings revealed the clear peaks which were identified to a hexagonal BN (0 0 2) plane (ICSD #: 027987), and Al (1 1 1), Al (2 0 0) and Al (2 2 0) reflections (ICSD #: 044321).
Conclusions
We fabricated BNNT–Al composite nanohybrids with a varying Al coating thickness, from 5 nm to 200 nm, by magnetron sputtering. XRD, SEM, TEM, XPS and in situ TEM bending and tensile tests on individual nanohybrids shed a new light on their micro- and atomic structures, chemical status and mechanical properties. We demonstrated that the applied synthetic route is an effective method for fabricating novel ultralight, superstrong BNNT–Al hybrid nanomaterials. Nanocrystalline Al coatings were
Acknowledgements
This work was supported by a Grant-in-aid No. 23310082, kakenhi (MEXT), and the World Premier International (WPI) Center for Materials Nanoarchitectonics (MANA) tenable at the National Institute for Materials Science (NIMS), Tsukuba, Japan. D.G. also acknowledges a funding “Mega-Grant” award for leading scientists tenable at the National University of Science and Technology “MISIS”, Moscow, Russian Federation, under the Agreement No. 11.G34.31.0061. The authors thank Drs I. Yamada, M. Mitome
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