Strengthening behavior of few-layered graphene/aluminum composites

Abstract

Strengthening behavior of composite containing discontinuous reinforcement is strongly related with load transfer at the reinforcement–matrix interface. We selected multi-walled carbon nanotube (MWCNT) and few-layer graphene (FLG) as a reinforcing agent. By varying a volume fraction of the reinforcement, aluminum (Al) matrix composites were produced by a powder metallurgy method. Uniform dispersion and uniaxial alignment of MWCNT and FLG in the Al matrix are evidenced by high-resolution transmission electron microscope analysis. Although the reinforcements have a similar molecular structure, FLG has a 12.8 times larger specific surface area per volume more than MWCNT due to geometric difference. Therefore an increment of a yield stress versus a reinforcement volume fraction for FLG shows 3.5 times higher than that of MWCNT Consequently, for both reinforcements, the composite strength proportionally increases with the specific surface area on the composite, and the composites containing 0.7 vol% FLG exhibit 440 MPa of tensile strength.

Introduction

Graphene has attracted interest as a reinforcing agent for metal matrix composites due to excellent mechanical properties based on the strong sp² CC bonds, which are similar to fullerene and carbon nanotube [1], [2]. Furthermore, it has merits over other carbon-based nano materials, which originate from its inherent two-dimensional (2-D) morphology; the planar structure is more favorable to load transfer as well as to impeding atomic diffusion at high temperatures, as compared to its 0-D and 1-D counterparts. Consequently, it provides superior strength for composites at both room temperature and high temperatures.

In order to transmit the excellent properties of graphene to composites, uniform dispersion of an individually-exfoliated graphene is one key factor. Several processes have been introduced to exfoliate graphite to single-layer or few-layer graphene by mechanical and/or chemical means [3], [4], [5]. Mechanical exfoliation using a tape dispenser [2] or atomic force microscopy (AFM) [6] has exhibited inadequate productivity for large-scale industrial applications. Although large-scale synthesis of graphene by gas phase techniques (e.g., thermal chemical vapor deposition) [7], [8], [9] is actively ongoing, this process is still costly and has restrictions in terms of the selection of the substrate materials. Chemical exfoliation by a solution process has been suggested as a relatively cheap process [10], [11], [12], and yet presents difficulties for scalable synthesis due to complex synthesis steps and the requirement for a large amount of chemicals and acid. Recently, solid phase techniques, combined with ball-milling processes, have enabled production of scalable quantities of carbon-base nano-materials by applying shear forces on pristine agglomerated particles [13], [14], [15].

Even though one has developed a scalable process to produce few-layer graphene (FLG), an additional technical hurdle is in the uniform dispersion of the graphene in the metal matrix, which has closely packed atomic structures. Hence, graphene/metal composites have been seldom investigated compared to polymer matrix composites, although the great strength and light weight features of graphene/metal composites are expected to lead to applications of such composites in the automotive and aerospace industries.

Aluminum (Al) matrix composites reinforced with FLG have recently been produced using different synthesis techniques with powder metallurgy (PM) routes. Table 1 summarizes the fabrication processes and resulting properties of recently developed Al/FLG composites [16], [17], [18], [19], [20], [21]. Similar to other nanostructured composites, dispersion of nanoscale reinforcement is a critical issue in Al/FLG composites. Poorly dispersed FLG may act as defect sites, significantly deteriorating the performance of the final composites [16], [17]. Consolidation processes are also important for Al/FLG composites because a selection of high-processing temperatures to avoid insufficient powder consolidation leads to unfavorable reactions (e.g., transformation of FLG to carbides) [17], [18], [19], [20]. Even though some composites have been produced via cost-ineffective routes or using expensive materials (e.g., graphene oxide), they did not exhibit tensile elongation or they exhibited very limited tensile strength (<300 MPa). Furthermore, the microstructure of Al/FLG composites with an atomic-scale resolution has not been well reported; atomic-scale resolution images may provide detailed information on the morphology of FLG and the interface between Al and FLG [17], [20], [21]. Reinforcement in composites may enhance the strength of the matrix by (i) carrying a great amount of load instead of the matrix and by (ii) interrupting the plastic deformation of the matrix. Hence, two important factors to determine the strengthening efficiency of reinforcement, other than its intrinsic mechanical properties and volume fraction, are (i) how much load can be effectively transmitted from the matrix to the reinforcement and (ii) how much the stress distribution can be altered by the reinforcement. Both are affected by bonding strength between the matrix and reinforcement, morphology/surface area of the reinforcement, and spatial distribution of the reinforcement [22], [23]. Although experimental work on the effect of volume fraction of the reinforcement on the strength of composites [24], [25] and theoretical work on the bonding strength between metal and carbon-based nano materials [26], [27], [28], [29] has been conducted, the role of morphology and surface area on the reinforcement has not been extensively investigated. In particular, a comparison study on the strengthening behavior of reinforcements with a variety of shapes (e.g., fullerene–sphere/0-D, carbon nanotube–tube/1-D, and graphene–planar/2-D), but with similar intrinsic mechanical properties, would be highly interesting.

The primary objective of this work is to produce Al matrix composites with well-dispersed FLG using a scalable powder metallurgy approach. Here, we introduce a favorable route to produce centimeter-scale Al/FLG composites with excellent performances. This work also aims to examine tensile properties of the Al/FLG composite as a function of volume fraction of FLG, so as to compare the strengthening behavior of FLG with that of multi-walled carbon nanotube (MWCNT). The present study also carefully investigates the microstructure of the composites at an atomic scale to provide detailed information on the structure of nano-carbon materials and nano-carbon/Al interface structure. Further, this work quantifies the effect of shape factor (i.e., tube versus sheet) and specific surface area of reinforcement on the composite strength.