Elsevier

Mechanics of Materials

Volume 141, January 2020, 103270
Mechanics of Materials

Effects of Cu/graphene interface on the mechanical properties of multilayer Cu/graphene composites

https://doi.org/10.1016/j.mechmat.2019.103270Get rights and content

Highlights

  • Cu/graphene interface can act as a resource of dislocation emission.

  • Dislocation propagations are confined by the impenetrable Cu/graphene interface.

  • Effects of Cu/graphene interface on dislocation evolutions are discussed deeply.

Abstract

Molecular dynamics simulations are performed to investigate the effects of graphene on the mechanical properties in multilayer Cu/graphene composites under uniaxial tension. It is found that both of zigzag and armchair graphene can improve the mechanical strength of multilayer Cu/graphene composites. The enhanced efficiency is concerned with chirality and interlayer thickness of graphene. The Cu/graphene interface has a great effect on the dislocation nucleation and propagation in the plastic deformation. Firstly, the interface can act as a resource of dislocation emission. This is due to the high stress concentrated on the interface caused by lattice mismatch and shear modulus mismatch between Cu and graphene, which can reduce the energy of nucleation. The interface stress of armchair graphene is more evident than the zigzag graphene. Secondly, the dislocations are confined by the impenetrable interface during the propagation process, which leads to intense interaction between dislocations and interface. Both the confinements and interactions are responsible for high stress required during the propagation process. A confined layer slip (CLS) model is established to predict the strength of multilayer composites in quantification. After the fracture of graphene, the dislocations penetrate through the interface of Cu and graphene and the composites would neck and fracture around the region.

Introduction

Graphene (Geim, 2009), a two-dimensional monolayer of sp2-hybridized carbon atoms packed into a honeycomb lattice, has attracted tremendous attention in recent years owing to its giant electron mobility (Pavithra et al., 2014; Peres, 2010), superior thermal conductivity (Balandin et al., 2008) and extraordinary mechanical properties (Geim, 2009). These excellent properties offer a wide range of possibilities to utilize graphene in various systems such as photocatalysts, energy storage, nanoelectronics and batteries (Bartolucci et al., 2011; Fang et al., 2015). However, it is still quite difficult to directly apply graphene as a structural material. This drawback is mainly because the aggregation or restacking of graphene sheets significantly (Chen et al., 2014) affects their intrinsic properties owing to the strong van der Waals forces as a consequence of high surface area, high aspect ratio and interfacial instability of graphene (Lavanya and Gomathi, 2016).

Consequently, a variety of methods have been employed for the utilization of graphene as a reinforced phase in diverse composites to achieve better mechanical properties for various potential applications (Bartolucci et al., 2011; Wang et al., 2012). Recently, some researches investigate metal-matrix composites reinforced by graphene, which can reveal higher strength compared to original materials without graphene. Kim et al. (2013) design a new nanolayered composite material consisting of repeating layers of metal and monolayer graphene, which obtains ultra-high strength characteristics of 1.5 and 4.0 GPa for Cu/graphene and Ni/graphene composites respectively. Hwang et al. (2013) utilize a new method of a molecular-level mixing process and spark plasma sintering to fabricate the Cu-graphene nanocomposites which exhibit great improvement in the yield and tensile strengths as compared to pure Cu. Similar enhancements in the tensile strength can also be found in aluminum composites reinforced with graphene nanosheets reported by Wang et al. (2012). Moreover, molecular dynamics (MD) simulations are carried out to investigate the relationship between mechanical properties and microstructures of metal/graphene systems owing to their great advantages to observe the structures evolution at the atomic scale. Great improvements are reflected in shock strength (Long et al., 2016), shear (Liu et al., 2016), compressive (Weng et al., 2018) and tensile (He et al., 2017) properties and load-bearing capacity (He et al., 2018) of metal/graphene nanocomposites with the MD method.

It is acceptable that the key strengthening mechanisms of graphene on the mechanical properties of metal/graphene systems can be attributed to the blocking of propagation of dislocation by interface between graphene and metal (Kim et al., 2013; Wang et al., 2012). Accordingly, the interface of metal/graphene plays a crucial role in deformation and mechanical response of composites. The laminated structures of metal/graphene are established by some researchers to further investigate the interfacial effects of metal/graphene. Liu et al. (2016) demonstrated an interface strengthening in Cu/graphene nanolayered composites under shear deformation and found that the interface constraining effect between graphene and Cu layer improves the shear strength and toughness of composites. Weng et al. (2018) studied the strengthening mechanism of interface between metal and graphene under compression. They showed that the interfacial thickness has a great effect on the mechanical properties of Cu-graphene composites and related strengthening mechanisms depend on the deformation stages. He et al. (2017) investigated the effects of the number of graphene layers on the mechanical properties of the Cu/graphene composites. Although many contributions have been published for metal/graphene composites, the role of the metal/graphene interface played in the deformation of laminated composites needs to be further explored.

In this work, multilayer Cu/graphene composites have been conducted under uniaxial tension to investigate effects of graphene on the mechanical properties and dislocation evolutions using molecular dynamics simulations. The choice of Cu as the representing metal in this work is motivated by the fact that dislocation propagation dominates the plastic deformation process for the lower fault energy of Cu, which benefits to observing interaction between dislocation and interface. The multilayer Cu/graphene structures aims at focusing on the effects of interface. The effects of the interface on the nucleation and propagation of dislocation are discussed at length in this work. Moreover, the orientations of graphene with zigzag and armchair directions are investigated as well.

Section snippets

Modeling and simulation details

In our work, all simulations are performed using a large-scale atomic/molecular massively parallel simulator (LAMMPS) (Plimpton, 1995). As shown in Fig. 1, the initial model is composed of crystal Cu substrate and different graphene layers (1–4 layers). The coordinate systems of all the models along X, Y and Z axis direction are defined by the orientations of Cu crystallographic with [112¯], [1¯10] and [111], respectively. The total length of all the model along X, Y and Z axis directions are

Stress-strain curves of multilayer Cu/graphene systems

The typical stress-strain curves of Cu/graphene systems under uniaxial tension with zigzag and armchair graphene are depicted in Fig. 2. In general, all the curves of multilayer Cu/graphene systems can be divided into three fundamental parts including elastic stage Ⅰ, plastic strengthening stage Ⅱ and fracture stage Ⅲ while the curve of pure Cu excludes the stage Ⅱ. For the CuZig models in Fig. 2(a), two points marked with ax and bx of the peak stress value are described in the curve and the

Effects of Cu/interface on the dislocation nucleation

As concluded in Section 3.1, the Cu/graphene interface can act as a source for the dislocation nucleation. The armchair graphene layers can reduce the stress for the dislocation nucleation while the zigzag graphene layers improve the nucleation stress. In this section, this phenomenon would be discussed in detail.

To understand the effects of the Cu/graphene interface on the nucleation stress, the cross-section of von-Mises stress distribution for the dislocation nucleation is depicted in Fig. 10

Conclusion

In summary, the multilayer Cu/graphene composites under uniaxial tension are carried out to investigate the effects of chirality and interlayer thickness on the mechanical properties and dislocation evolution using molecular dynamics simulations.

Firstly, the stress-strain curves of Cu/graphene systems under uniaxial tension can be divided into elastic stage Ⅰ, plastic strengthening stage Ⅱ and fracture stage Ⅲ. The graphene can improve the mechanical strength of multilayer Cu/graphene

Declaration of Competing Interest

There are no conflicts between the authors. Kun Sun conceived and designed the experiments; Weixiang Peng performed the experiments and wrote the paper.

Acknowledgments

The present authors are appreciated to the financial support from the National Natural Science Foundations of China (Grant no. 51475359).

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