Anisotropic mechanical properties of graphene/copper composites with aligned graphene
Introduction
Graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (RGO), have drawn tremendous attention for a wide range of applications owing to their unique two dimensional (2D) thin-layered structure and outstanding electrical, thermal and mechanical properties [1]. These fascinating features make graphene an ideal nanofiller for the preparation of high-performance composites [2]. Along with the extensive studies on graphene/polymer composites [3], [4], the investigations of graphene in metal matrix composites (MMCs) have recently attracted a great deal of interest [5], [6]. Specifically, the pioneering work from Zhang's group reported a 62% improvement of tensile strength in 0.3 wt% RGO/Al composites [7], which stimulated a large body of research on the development of a variety of graphene/metal composites with prominent properties [6]. Compared to other graphene/metal materials, such as graphene/Al, graphene/Cu composites have recently attracted growing attention because of their prominent combination of fascinating electrical, thermal and mechanical properties that make them promising materials for a broad range of applications with functional-structural integration [8], [9], [10], [11], [12].
In regard to the mechanical properties of graphene/metal composites, there are several technical challenges that still hinder the full exploration of exceptional mechanical properties of graphene to strengthen metals, including uniform graphene dispersion, strong graphene-metal interfacial bonding and good graphene alignment. The graphene dispersion is a basic factor affecting the composite mechanical properties. The poor distribution can yield a large amount of graphene agglomerates that are intrinsically rather weak and facilitate the crack initiation and propagation in the composites [13], substantially compromising the reinforcing efficiency of the graphene. A large number of innovative strategies have been developed to improve the graphene distribution in metals, including flake powder metallurgy [7], [14], molecular level mixing [8], [15], in situ chemical vapor deposition (CVD)[16], [17], electroless plating [9], [18], bioinspired strategies [19], [20], etc. leading to the dramatically enhanced mechanical properties of the composites at very low graphene loading. In addition, interface is also critical to determine the stress transfer efficiency in the composites. The weak interface considerably lowers the efficiency of stress transfer from matrix to graphene, which is detrimental to the mechanical properties of the composites [10]. The interfacial bonding between the graphene and most metals are generally insufficient due to their non-wetting nature. The improved interfacial bonding can be attained by inhibiting the undesired interfacial reaction [21], [22], modifying the graphene surface by Ni decoration [9], [23], and establishing the oxygen-coordinated bonding between the metal matrix and RGO [24], [25].
In strong contrast to the great efforts made on solving the graphene dispersion and interface problems, the effect of graphene alignment on the mechanical properties of graphene/metal composites has rarely been studied due possibly to the lack of an efficient approach to realize a good alignment of graphene in the metal matrix. As a representative example, Cao et al. [20] recently reported a bioinspired method to fabricate the graphene/Cu composites that exhibited a nacre-inspired nanolaminated structure with well aligned graphene. Such fascinating structure resulted in a significantly enhanced mechanical strength with simultaneously maintaining the extraordinary ductility and electrical conductivity of Cu matrix. Chen et al. [26] employed a molecular-level mixing to prepare the graphene/Cu composites that displayed the preferred graphene alignment perpendicular to the pressing direction in the composites at graphene content of above 2.0 vol%. Nevertheless, they demonstrated that such pressure-assisted alignment reduced the strength of the composites by aggravating the interface debonding due to generating the same direction between the tensile force and shear force of the interface. Hence, the alignment-dependent mechanical properties of graphene/metal composites are still uncertain and remain to be fully elucidated. In addition, the above alignment reports only focused on the in-plane mechanical properties of the composites at very low graphene contents of 0.2–4 vol%. The investigation on the anisotropic mechanical properties of the composites in both in-plane and through-plane directions, especially at relatively high graphene contents, i.e., > 10 vol%, to the best of our knowledge, has not been reported yet.
In this study, a good alignment of graphene nanoplatelets (GNPs) in the Cu matrix was realized by the combined vacuum filtration and followed spark plasma sintering (SPS) [11], as illustrated in Fig. 1. The microstructures and anisotropic tensile properties of GNP/Cu composites with high GNP contents of 10 and 20 vol% were evaluated. It was found that the composites exhibited a laminated structure with highly aligned GNPs, which resulted in the prominent anisotropic mechanical properties with in-plane strength and elongation superior to through-plane ones. Nonetheless, the composites only presented a moderate in-plane strength enhancement that was further compromised with increasing GNP content due to the weak GNP-Cu interfacial bonding strength. The associated strengthening mechanisms and interface failure modes were investigated to interpret the anisotropic tensile behavior and provide new insights into the anisotropic mechanical properties of the composites with aligned graphene.
Section snippets
Experimental procedure
Multi-layer GNPs with a lateral size of 5–30 µm and a thickness of 5–10 nm were purchased from Tanfeng Tech Co. Ltd. The as-received GNPs were further annealed at 2000 °C for 30 min in an argon atmosphere to restore the graphene defects. Atomized Cu powder with a particle size of 0.5–2 µm was purchased from Xingye Metal Materials, Co. Ltd. All the chemicals were used as received without further treatment.
As schematically shown in Fig. 1, the as-annealed GNPs were dispersed in ethanol under
Results and discussion
The characterization of as-annealed GNPs is shown in Fig. 2. The SEM (Fig. 2a) and TEM (Fig. 2b) images show that the GNPs display a silk-like structure with crumpled and wrinkled surfaces, which are similar to the previously reported graphene materials [27], [28]. The corresponding SAED pattern (Fig. 2c) shows several sets of diffraction spots with a distinct hexagonal symmetry, demonstrating the multiple layers of graphene with a high crystallinity [29]. Raman spectra (Fig. 2d) of GNPs show
Conclusions
In summary, the GNP/Cu composites with highly aligned GNPs were fabricated by the combined vacuum infiltration and SPS. TEM study revealed that the interface of GNP/Cu composite was bonded by the mechanical interlocking. The composites showed notably anisotropic mechanical properties with both strength and elongation in the in-plane direction being largely higher than those along the through-plane direction. The composites with 10 vol% GNPs exhibited a moderate 26% in-plane strength enhancement
Acknowledgement
This work is supported by National Natural Science Foundation of China (51501083, 51761024) and Natural Science Foundation of Gansu Province (1606RJZA097).
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