190 I.A. Ovid’ko

© 2014 Advanced Study Center Co% Ltd%

Rev. Adv. Mater. Sci. 38 (2014) 190-200

Corresponding author: I.A. Ovid'ko, e-mail: ovidko@nano.ipme.ru

METAL-GRAPHENE NANOCOMPOSITES WITH ENHANCEDMECHANICAL PROPERTIES: A REVIEW

I A Ovid’ko1,2

1Research Laboratory for Mechanics of New Nanomaterials, St. Petersburg State Polytechnical University,St. Petersburg 195251, Russia

2Department of Mathematics and Mechanics, St. Petersburg State University, St. Petersburg 198504, Russia

Received: August 21, 2014

Abstract. This paper presents an overview of research efforts focused on both fabrication andmechanical properties of metal-matrix nanocomposites containing graphene inclusions. A par-ticular attention is devoted to experimental data giving evidence for enhancement of strength,microhardness and Young modulus (due to the effects of graphene inclusions) of suchnanocomposites, as compared to both unreinforced metals and metal-matrix nanocompositesreinforced by non-graphene inclusions. Key factors are discussed which influence the role ofgraphene nanosheets and nanoplatelets in remarkable enhancement of the mechanical char-acteristics of metal-graphene nanocomposites.

1. INTRODUCTION

Graphene is a new two-dimensional carbon mate-rial showing unique mechanical, transport and ther-mal properties; see, e.g., reviews [1-7]. In particu-lar, graphene is specified by both superior tensilestrength of 130 GPa and Young modulus of 1TPa [7,8]. These excellent mechanical characteris-tics cause a great potential in using graphene in-clusions as strengthening elements in polymer-,ceramic- and metal-matrix composites for struc-tural and functional applications; see, e.g., [9-28].In particular, in recent years, several research groupshave fabricated metal-matrix nanocomposites rein-forced by graphene inclusions [21-28]. Suchnanocomposites exhibit enhanced mechanical char-acteristics (due to the effects of graphene fillers),as compared to both unreinforced metals and metal-matrix nanocomposites reinforced by non-grapheneinclusions. The main aim of this paper is to give anoverview of research efforts focused on fabrication

and high mechanical properties of new metal-graphene nanocomposites.

2. GENERAL ASPECTS

In general, the idea to enhance strength and othermechanical characteristics of metallic materialsthrough reinforcement of metallic matrices bynanoinclusions of the second phase has been ef-fectively exploited by many research groups in overthe world; see, e.g., [29-31]. For instance, withinthe approach in question, ceramic nanoparticles andnanofibers, metallic nanoparticles, and carbonnanotubes have been effectively exploited as rein-forcing nanoinclusions. Although the results in thisarea are impressive, the development of strong,durable and cost-efficient metal-matrix compositematerials for current and future technologies is stillan unsolved problem. In this context, search for novelmaterials as reinforcing fillers in metal-matrix com-posites is of crucial significance. In search for new

191Metal-graphene nanocomposites with enhanced mechanical properties: a review

fillers, one can distinguish the following key factorsthat crucially influence the role of nanoinclusionsas reinforcing structural elements in metal-matrixnanocomposites:(i) Reinforcing inclusions should have high mechani-cal characteristics such as strength and Youngmodulus.(ii) Geometry of reinforcing inclusions should becharacterized by high aspect ratio and high surfacearea.(iii) Strong binding between inclusions and metalmatrix should exist.(iv) Reinforcing inclusions should be homogeneouslydispersed, and their agglomeration should be pre-vented.

In addition, from a practical viewpoint, fabrica-tion of metal-matrix nanocomposites is desired tobe a cost-efficient procedure.

It is a formidable challenge to simultaneouslymeet the conditions (i)-(iv), in parallel with cost effi-ciency. In these circumstances, fabrication of metal-matrix nanocomposites with enhanced mechanicalcharacteristics and search for novel materials asreinforcing fillers in such composites represent thesubjects of intense research efforts in nanoscienceand nanotechnology.

Graphene specified by superior values of bothtensile strength 130 GPa [7,8] and Young modu-lus Y 1 TPa [7,8] is a very promising candidate toserve as a reinforcing material in metal-matrixnanocomposites. In addition, production of graphene(at cost of 5 EUR/kg [32]) is cheap compared tothat of carbon nanotubes having similar mechanicalcharacteristics. Besides, 2D geometry of graphenenanosheets and nanoplatelets is responsible formaximum value of their surface-to-volume ratio andcan provide very high aspect ratios. These high geo-metric parameters as well as superior mechanicalcharacteristics of graphene nanoinclusions are fa-vorable for enhancement of mechanical propertiesof graphene-reinforced nanocomposites.

Thus, the conditions (i) and (ii), along with thecost efficiency condition, are in fact automaticallysatisfied for graphene fillers utilized in metal-matrixnanocomposites. In these circumstances, it is im-portant to devote a special attention to the condi-tions/factors (iii) and (iv) in development of metal-graphene nanocomposites with enhanced mechani-cal characteristics. In next sections, we will dis-cuss examples of such nanocomposites and con-sider factors responsible for enhancement of theirmechanical properties.

3. METAL-GRAPHENENANOCOMPOSITES WITHENHANCED MECHANICALPROPERTIES: FABRICATION ANDMECHANICAL TESTS

In general, fabrication of metal-matrix compositesreinforced by graphene inclusions (sheets,nanoplatelets) represents a rather difficult techno-logical problem. As a corollary, successful studiesin this area are very limited [21-28]. Below we willconsider experiments focused on fabrication of metal-graphene nanocomposites with enhanced mechani-cal properties.

3.1. Al-graphene nanocomposites

Wang with co-workers [21] fabricated Al-graphenenanocomposites showing a dramatic enhancementin strength, as compared to their graphene-freecounterpart. In doing so, they utilized the novel ap-proach based on flake powder metallurgy. More pre-cisely, following Ref. [21], there are the followingkey steps of the fabrication of Al-matrixnanocomposites reinforced by graphenenanosheets:(1) Aqueous solution of well dispersed nanosheetsof graphene oxide is prepared. The nanosheets arespecified by thicknesses being less than 1.5 nm,that is, no more than 5 graphene oxide layers.(2) Al flakes are prepared with surfaces being cov-ered by hydrophilic PVA membranes. The flakestypically have a 2D planar morphology with an aver-age thickness of around 2 micrometers.(3) Graphene oxide nanosheets are absorbed atsurfaces of Al flakes and then reduced (by heatingtreatment) to graphene nanosheets. As a result, oneobtains graphene-Al composite powders, that is, Alflakes with graphene nanosheets homogeneouslydispersed at their surfaces.(4) Graphene-Al composite powders are compactedand consolidated to fabricate bulk nanocompositeconsisting of Al matrix reinforced by homogeneouslydispersed graphene nanosheets. The consolidationoccurs by sintering of compacted billets of graphene-Al composite powders in Ar atmosphere at 580 °Cfor 2 hours and consequent hot extrusion at 440 °Cwith an extrusion ratio of 20:1.

Wang with co-workers performed tensile testswith specimens of 5 mm diameter and 25 mm gaugelength machined from extruded rods consisting ofAl-matrix reinforced by 0.3 wt.% graphenenanosheets [21]. In the tests, the Al-graphene

192 I.A. Ovid’ko

Fig. 1. (color online) (a) Tensile properties of 0.3wt.% graphene-Al composite and the correspond-ing flaky Al specimen; (b) fracture surface of 0.3wt.% graphene-Al composite; the inset showsgraphene nanosheets pulled out. Reprinted fromScripta Materialia, Vol 66, J. Wang, Z. Li, G. Fan,H. Pan, Z. Chen, D. Zhang, Reinforcement withgraphene nanosheets in aluminium matrix compos-ites, pages 594-597, Copyright (2012), with permis-sion from Elsevier.

nanocomposite showed both a tensile strength of256 MPa and 13% elongation (Fig. 1a) which areby 62% higher and around 2 times lower than thestrength (154 MPa) and elongation (27%) of pure(unreinforced) Al matrix, respectively. Thus, thenanocomposite exhibits a very high strength of 256MPa, along with functional ductility characterizedby 13% elongation exceeding the 5% standard forengineering applications. Following Wang et al. [21],the reported enhancement in strength due to inser-tion of 0.3 wt.% graphene nanosheets is most ef-fective, as compared to that of any other reinforcinginclusions in Al. At the same time, the reported en-hancement by 62% is much lower than that ex-

pected from theoretical estimates of the potentialeffects caused by graphene nanosheets on themechanical properties of Al-graphenenanocomposites. As a corollary, there are reason-able expectations in reaching further large progressin dramatic improvement of strength and other me-chanical characteristics exhibited by Al-matrixnanocomposites due to their strengthening bygraphene nanosheets [21]. Based on their results,Wang with co-coworkers concluded that reinforce-ment by graphene nanosheets is most effective forAl-matrix materials and have a huge potential forapplications.

The key effects of graphene nanosheets are asfollows: grain refinement, dislocation strengthening,and stress transfer. First, since graphenenanosheets hamper grain boundary migration, graingrowth is hindered in metal-graphenenanocomposites which thereby tend to have lowergrain sizes than their metal counterparts free fromgraphene. At the same time, following the classicalHall-Petch relationship =

o + k d-1/2 (where is

the strength, o is the dislocation friction stress, k

is a material constant, and d is the mean grain size),the strength of materials increases with decreasingthe grain size. This factor, however, does not playany significant role in the examined Al-matrixnanocomposites with 0.3% wt. graphene inclusions,because of negligibly low fraction of grain bound-aries hampered by so low amount of graphenenanosheets.

Second, such nanosheets stop lattice disloca-tions whose slip represents the dominant deforma-tion mode in metals. Suppression of lattice disloca-tion slip by graphene nanosheets significantly con-tributes to strengthening of Al-graphenenanocomposites.

The third strengthening mechanism is the stresstransfer. In the case under consideration, the thirdmechanism effectively contributes to the experimen-tally documented dramatic strengthening of Al-graphene nanocomposites [21]. This statement issupported by experimental observation of pro-nounced dimples at fracture surfaces (Fig. 1b). Suchdimples are elongated along the tensile load direc-tion, with some graphene nanosheets being pulledout at the edges of the dimples (Fig. 1b).

3.2. Mg-graphene nanocomposites

Chen with co-workers [22] fabricated Mg-matrixnanocomposites reinforced by graphene platelets,using a novel nanoprocessing method that involvesliquid state ultrasonic processing and solid state

193Metal-graphene nanocomposites with enhanced mechanical properties: a review

stirring. In this study, each platelet typically con-sists of several graphene layers and has thicknessof approximately 10-20 nm, while its length andwidth are lower than 14 microns. Although in-planestrength of graphene platelets is lower than that ofsingle-layer graphene sheets, chemical stability ofplatelets and comparatively easy manipulation withthem make such platelets to be attractive for useas reinforcing elements in metal-matrixnanocomposites.

At the first stage of the Mg-graphenenanocomposite fabrication [22], graphene plateletsare fed into the melt of Mg at 700 oC in parallel withultrasonic treatment. In doing so, a rather homoge-neous dispersion of graphene platelets in the liquidMg is provided by a high-power ultrasonic probeaction; for details, see [22]. Also, the ultrasonic treat-ment was applied to the melt of Mg for 15 minutesafter its feeding by graphene inclusions. The result-ant composite system was cast to a plane mold.

Characterization of the Mg-graphene mold struc-ture revealed that, despite of a rather good disper-sion of graphene platelets, there are micrometer-sized aggregates of such platelets in thenanocomposite [22]. This aspect motivates the sec-ond stage of processing in order to achieve a morehomogeneous dispersion of reinforcingnanoinclusions. At the second stage, solid statestirring treatment is applied to the nanocomposite.This treatment serves as an effective method forhomogenization of inclusions in composites in theirsolid state and is realized using a specific, noncon-sumable rotating tool [22]. With the solid state stir-ring, Chen with co-workers [22] fabricated the Mg-matrix nanocomposite containing uniformly distrib-uted and well dispersed graphene platelets. In addi-

Fig. 2. (color online) (a) Hardness increase and (b) strengthening efficiency of graphene nanoplatelet andother nano-scale reinforcements in Mg-based metal-matrix nanocomposites. Reprinted from Scripta Materialia,Vol 67, L.-Y. Chen, H. Konishi, A. Fehrenbacher, C. Ma, G.-Q Hu, H. Choi, H.-F. Hu, F.E. Pfefferkorn, X.-C.Li, Novel nanoprocessing route for bulk graphene nanoplatelets reinforced metal matrix nanocomposites,pages 29-32, Copyright (2012), with permission from Elsevier.

tion, as it has been illustrated in a representativeexample in Ref. [22], bonding between platelets andthe Mg matrix is good in the nanocomposite, andplatelets are typically located inside Mg grains.

In mechanical test, microhardness was mea-sured [22]. It was found that the microhardness (=66 kg mm-2) of the Mg-matrix nanocomposite rein-forced by 1.2% graphene platelets is by 78% higherthan the microhardness (= 37 kg mm-2) of pure Mgfabricated at the same conditions. With both thesedata and analysis of the literature presenting resultsof other experiments concerning Mg-matrix com-posites, Chen with co-workers concluded that rein-forcement of Mg by graphene platelets is the mosteffective method for strengthening, as compared toreinforcement by either carbon nanotubes or (ce-ramic or metallic) nanoparticles. The discussedadvantages of graphene platelets as reinforcingstructural elements in Mg-matrix nanocompositesare illustrated by comparison in both magnitude andefficiency of the strengthening between the Mg-graphene nanocomposite and Mg-matrixnanocomposites reinforced by carbon nanotubes andnanoparticles (Fig. 2). In doing so, the strengthen-ing efficiency R is given as:

c m mR H H VH/ , (1)

where Hc is the microhardness of a Mg-matrix com-

posite, Hm

is the microhardness of pure Mg, and Vis the volume fraction occupied by reinforcing struc-tural elements. With data presented in Fig. 2, Chenwith co-workers [22] noted that metal-matrix com-posites reinforced by graphene represent a newclass of nanocomposites with uniquely enhancedmechanical properties.

194 I.A. Ovid’ko

Fig. 3. (color online) Strength characteristics ofcopper-based composites. Reprinted from ActaMaterialia, Vol 61, A.G. Nasibulin, T.S. Koltsova,L.I. Nasibulina, I.V. Anoshkin, A. Semencha, O.V.Tolochko, E.I. Kauppinen, A novel approach to com-posite preparation by direct synthesis of carbonnanomaterial on matrix or filler particles, pages1862-1871, Copyright (2013), with permission fromElsevier.

The basic micromechanisms responsible for aremarkable strengthening effect in graphene-Mgnanocomposites are identified as effective load bear-ing and a pronounced role of graphene platelets asobstacles for lattice dislocation slip [22]. The effec-tive load bearing is related to good bonding betweengraphene and Mg matrix. The second factor comesinto play due to both plane-like geometry of grapheneplatelets and their superior strength characteristics,in which case free dislocation path is dramaticallyreduced by strong platelets in graphene-metalnanocomposites.

3.3. Cu-graphene nanocompositesfabricated by two-stageprocedure involving synthesis ofcopper powders covered bygraphene coatings andmechanical compaction

Papers [23,24] presented results of research fo-cused on mechanical behavior of new Cu-graphenenanocomposites fabricated in a two-stage proce-dure. At the first stage, Nasibulin with co-workers[23,24] exploited chemical vapor deposition tech-nique in order to synthesize n-layered graphenecoatings on micron-sized powders of copper. Trans-mission electron microscopy analysis of copper-graphene composite powders showed the numbern of graphene layers synthesized at optimum con-

ditions (in particular, at temperature T = 940 °C) tobe in the range from 8 to 12. At the second stage ofthe fabrication procedure, copper-graphene compos-ite powders were compacted by cold rolling in alaboratory double cold rolling mill in two steps. Afterthe first-step rolling, compacted specimens wereannealed at T = 900 °C in a hydrogen atmosphereduring one hour. Then the second-step rolling wasperformed. Finally, the copper-graphene compositespecimens were annealed at T = 420 °C in a hydro-gen atmosphere during one hour.

The resultant specimens of the copper-graphenecomposite were mechanically tested to measuretheir elongation-to-fracture (in three-point bendingtest) and microhardness. The experimentally docu-mented mechanical characteristics of the copper-graphene composite were compared with those ofpure copper specimens and the previously [33] re-vealed mechanical characteristics of copper-matrixcomposite with graphite inclusions and copper-ma-trix composites containing carbon nanofibers(CNFs), in the case of Cu-3%C composition for allthe three composites (Fig. 3). First of all, the ex-periments [23,24] show a pronounced (39%) in-crease in the microhardness (characterizingstrength) of the copper-graphene composite, ascompared to its pure copper counterpart. This in-crease in the microhardness occurs at expenses ofa dramatic decrease in elongation-to-failure (char-acterizing ductility) from value of 55%, for pure cop-per, down to value of 17%, for the copper-graphenecomposite.

Also, comparison of the mechanical character-istics of copper-matrix composites fabricated by thesame methods [23,24,33] and reinforced by variouscarbon inclusions (graphene, graphite, CNFs) re-vealed advantage of copper-matrix composites con-taining CNFs. In particular, these composites ex-hibit 70% increase in the microhardness, as com-pared to pure copper (Fig. 3), whereas copper-ma-trix composites containing graphene and graphiteinclusions are specified by 39% and 10% improve-ments in strength, respectively (Fig. 3). Also, forcopper-CNF composites, elongation-to-failure hasvalue of around 47%. This value significantly exceedselongation-to-failure values of 18% and 17%, forcopper-graphite and copper-graphene composites,respectively.

Following Nasibulin with co-workers [23,24,33],in addition to the “pure” effects of CNFs, the matrixmicrostructure of copper-CNF composites is alsoresponsible for their higher mechanical characteris-tics, as compared to those exhibited by copper-

195Metal-graphene nanocomposites with enhanced mechanical properties: a review

graphite and copper-graphene composites. The factis that the matrix microstructure of copper-CNFcomposites is specified by an average grain sized = 4 m, whereas copper-graphite and copper-graphene composites are characterized by d =10 m and 7 m, respectively [23,24,33]. Metalswith smaller grain size typically show higher strengthand hardness, as compared to its counterparts withcomparatively large grains; see, e.g., [34]. (Thistrend is violated in the only case of nanomaterialshaving ultrasmall grain sizes < 15-30 nm; see, e.g.,[34-36].) Besides, since graphene inclusions aretypically located at grain boundaries, they tend tobe homogeneously dispersed in metals with smalland ultrasmall grains (because they have a lot ofpotential sites) and are effective in grain growth in-hibition in such metals.

3.4. Cu- and Ni-graphene nanolayeredcomposites

Kim with co-workers [25] fabricated Cu- and Ni-graphene nanolayered composites showing supe-rior strength characteristics. So, nanolayered com-posites consisting of alternating layers of metal (Cuor Ni) and monolayer graphene were synthesizedby chemical vapor deposition method. Cu-graphenecomposites with repeat layer thickness = 70 nmand Ni-graphene composites with = 100 nm ex-hibit strengths of 1.5 and 4.0 GPa, respectively [25].These values are highest for ever fabricated metal-graphene composites.

Cu-graphene nanolayered composites with =70, 125 and 200 nm as well as Ni-graphenenanolayered composites with = 100, 150, and 300nm were synthesized by chemical vapor depositionmethod described in detail in Ref. [37]. This methodallows one to fabricate metallic nanolayers withgraphene interfaces having mostly a single (85%)atomic layer structure and, in some part, a bi-layerstructure (Fig. 4a). For examination of mechanicalcharacteristics, Kim with co-workers utilizednanopillar compression tests (developed and effec-tively exploited in mechanics of metallic nanopillars;see, e.g., [38,39]). In doing so, nanolayered metal-graphene nanopillars with a diameter of 200 nm andheight of 400 – 600 nm were fabricated by focusedion milling method and then mechanically com-pressed (Figs. 4 (b and c) and 5 (a and b)). Typicalstress-strain curves for Ni- and Cu-graphenenanolayered nanopillars with various values of therepeat layer spacing are shown in Figs. 5c and5d. The flow stress at 5% plastic strain as a func-tion of , for Ni- and Cu-graphene nanolayered

nanopillars, is presented in Figs. 5e and 5f, respec-tively. These figures show the trend that the flowstress increases in an approximately linear way withdiminishing the metal layer thickness . In particu-lar, the flow stress has its maximum values of 1.5and 4.0 GPa, for Cu-graphene composite with =70 nm and Ni-graphene composites with = 100nm, respectively [25]. The latter value is around 52%of the theoretical strength for Ni.

Note that, in parallel with graphene interfaces,both the nanolayered structure and the nanopillargeometry can significantly increase the flow stressof the composite materials under examination. Kimwith co-workers [25] estimated the strengtheningeffects of the nanolayered structure and thenanopillar geometry in the case of metal-graphenenanolayered composites on the basis of the corre-sponding experimental data reported in the litera-ture. With this consideration, it was concluded thatthe strengthening effect of graphene interfaces inthe metal-graphene composites is much more sig-nificant than the combined effects of the nanolayeredstructure and the nanopillar geometry.

The key factor responsible for superior strengthof metal-graphene nanolayered composites repre-sents the role of graphene interfaces as highly ef-fective obstacles for lattice dislocation slip [25]. Thisstatement was illustrated by experimental evidencefor large amounts of dislocations that are present ina plastically deformed layer and do not penetrateacross a graphene interface between this layer andits neighboring layer free from dislocations in a Cu-graphene nanolayered nanopillar (Fig. 4c).

3.5. Cu-graphene nanocompositessynthesized by molecular-levelmixing and spark plasmasintering

Hwang with co-workers [26] suggested a newmethod for fabrication of metal-graphenenanocomposites, the namely two-step procedureinvolving both a molecular-level mixing process andspark plasma sintering. The molecular-level mixingprocess consists of attaching functional groups ontographene flakes and making chemical bonding be-tween graphene and metal powders. The sparkplasma sintering consolidates metal powders intoa polycrystalline solid through local joule heatingand spark plasma generated between neighboringpowders. This method allows one to provide gooddispersion of reduced graphene oxide (RGO) flakeswithin a metal matrix and high adhesion energybetween graphene and metal [26].

196 I.A. Ovid’ko

Fig. 4. (color online) Transmission electron microscopy (TEM) analysis of the Cu-graphene nanolayeredcomposite. (a) TEM image of a metal-graphene interface that shows mostly single layers with some doublelayers. Scale bar, 5 nm. (b) TEM image of a Cu-graphene nanopillar with 125-nm repeat layer spacing at alow magnification after deformation. Scale bar, 100 nm. (c) TEM image of a Cu-graphene nanopillar afterdeformation that shows a higher density of dislocations above the graphene interface. Scale bar, 50 nm.Reprinted by permission from Macmillan Publishers Ltd: Nature Communications, Vol 4, Y. Kim, J. Lee,M.S. Yeom, J.W. Shin, H. Kim, Y. Cui, J.W. Kysar, J. Hone, Y. Jung, S. Jeon and S.M. Yan, Strengtheningeffect of single-atomic-layer graphene in metal-graphene nanolayered composites, article 2114, copyright(2013).

In the experiment [26], specimens of Cu-matrixnanocomposite containing RGO flakes were syn-thesized by the two-step procedure under discus-sion. These specimens were mechanically testedunder tensile deformation, and the characteristicadhesion energy between copper and graphene wasmeasured. In particular, following Hwang with co-workers [26], specimens of Cu-matrixnanocomposite containing 2.5 vol.% RGO flakesshowed 80% increase in the yield strength (from160 to 284 MPa) and 30% increase in the ulti-

mate tensile strength (from 255 to 335 MPa), ascompared to pure copper. The strengthening effectof RGO flakes is attributed to the two key factors:the role of RGO flakes as effective obstacles fordislocation slip and high values of adhesion energybetween copper matrix and graphene. For instance,in the case under examination, Cu-graphenenanocomposite is specified by the adhesion energyof 165 J m-2, being much higher than that ( 0.72J m-2) for as-grown graphene on Cu substrate [26].

197Metal-graphene nanocomposites with enhanced mechanical properties: a review

3.6. Cu-graphene nanocomposite foilssynthesized by electrodeposition

Pavithra with co-workers [28] focused their researchon fabrication and mechanical testing of Cu-graphene nanocomposite foils. The Cu-graphenenanocomposites with Cu-matrix having an averagegrain size of 1.2 0.4 m were fabricated by pulsereverse electrodeposition method. Graphene and/or GO sheets with 1-5 atomic layers are located atgrain boundaries and serve as effective inhibitors ofgrain growth in the Cu-graphene nanocomposites.

Fig. 5. (color online) Results of nanopillar compression test. Scanning electron microscopy image of a Cu-graphene nanopillar with 125-nm repeat layer spacing (a) before and (b) after compression testing. Scalebar, 200 nm. Stress versus strain curves for (c) Ni-graphene and (d) Cu-graphene of various repeat metallayer spacings. The flow stresses at 5% plastic strain versus repeat metal layer spacing plots for (e) Ni-graphene and (f) Cu-graphene nanolayered composites. Reprinted by permission from Macmillan Publish-ers Ltd: Nature Communications, Vol 4, Y. Kim, J. Lee, M.S. Yeom, J.W. Shin, H. Kim, Y. Cui, J.W. Kysar,J. Hone, Y. Jung, S. Jeon and S.M. Yan, Strengthening effect of single-atomic-layer graphene in metal-graphene nanolayered composites, article 2114, copyright (2013).

Hardness and an elastic modulus of the Cu-graphene nanocomposites were measured innanoindentation test and compared with the corre-sponding mechanical characteristics of pure Cu foils(with an average grain size of 1.3 0.3 m) fabricatedby pulse reverse electrodeposition method. In do-ing so, it was experimentally revealed that the hard-ness and the elastic modulus of the Cu-graphenecomposite have values of 2.5 GPa and 137 GPa,respectively, whereas pure electrodeposited Cuspecimens are specified by the hardness of 1.2GPa and the elastic modulus of 116 GPa [28].

198 I.A. Ovid’ko

Also, Pavithra with co-workers [28] examinedgrain growth processes in the pure Cu and Cu-graphene nanocomposite specimens under 30 minannealing in argon atmosphere at 300 °C. Theyfound that grain size in the Cu-graphene compositein practice does not change after the annealing treat-ment, whereas grain size of pure Cu exhibits in-crease from 1.3 up to around 10 m. These experi-mental data are indicative of the drastic hamperingeffect of graphene on grain growth in metal-matrixnanocomposites. In these circumstances, it is logi-cal to conclude that metal matrices in metal-graphene nanocomposites typically have lower grainsize and thereby higher strength, as compared tocorresponding characteristics of pure metals.

3.7. Ni-graphene nanocompositessynthesized by electrodeposition

Kuang with co-workers [27] exploited electrodepo-sition method to fabricate Ni-graphenenanocomposite films. The nanocomposite films arecharacterized by 0.12 wt.% graphene fraction withgraphene nanoinclusions well dispersed in the formof multilayered sheets in Ni-matrix. Also, transmis-sion electron microscopy studies revealed thatgraphene sheets in Ni-matrix tend to be curled andentangled together [27].

The hardness and Young modulus of the Ni-graphene composite specimens were measured innanoindention tests. These mechanical character-istics of Ni-graphene composites were found to bedramatically enhanced as compared to those of pureelectrodeposited Ni. So, the hardness and Youngmodulus of the Ni-graphene composite have valuesof 6.85 GPa and 252.76 GPa, respectively, whereaspure electrodeposited Ni is specified by the hard-ness of 1.81 GPa and Young modulus of 166.70GPa. Following Kuang with co-workers [27], theexperimentally revealed remarkable ( 4-fold) in-crease in the hardness is mostly related to the roleof graphene inclusions as effective stoppers of lat-tice dislocation slip.

4. Concluding remarks

Thus, the newest and very effective approach to cre-ate metallic materials with high strength, hardnessand Young modulus is to implant graphene plate-lets and few-layers sheets in metallic matrices. Inparticular, metal-graphene nanocomposites with lowvolume/weight fractions of graphene inclusions ex-hibit dramatically enhanced strength and hardness.For instance, in the experiment [21], the Al-graphene

nanocomposite (consisting of Al-matrix reinforcedby 0.3 wt.% graphene nanosheets) showed both atensile strength of 256 MPa which is by 62% higherthan the strength (154 MPa) of pure Al. The reportedenhancement in strength due to insertion of 0.3 wt.%graphene nanosheets is most effective, as comparedto that of any other reinforcing inclusions specifiedby 0.3 wt.% fraction in Al.

Kim with co-workers [25] fabricated Cu- and Ni-graphene nanolayered composites showing supe-rior strength characteristics. So, Cu-graphene com-posites with repeat layer thickness = 70 nm andNi-graphene composites with = 100 nm exhibitstrengths of 1.5 and 4.0 GPa, respectively [25].These values are highest for ever fabricated metal-graphene composites.

Kuang with co-workers [27] fabricated Ni-graphene nanocomposite films characterized by0.12 wt.% graphene fraction with graphenenanoinclusions well dispersed in the form of multi-layered sheets in Ni-matrix. The hardness of the Ni-graphene composite has value of 6.85 GPa. Thisvalue is by 4 times larger than the hardness (1.81GPa) of pure electrodeposited Ni [27].

The key effects of graphene nanosheets andnanoplatelets on mechanical characteristics ofmetal-graphene nanocomposites are as follows: dis-location strengthening, stress transfer and grainrefinement in metallic matrices. First, graphenenanoinclusions effectively hinder lattice dislocationslip being the dominant deformation mode in met-als. This effect comes into play in all the metal-graphene nanocomposites under consideration [21-28] and most probably causes the dominant contri-bution to their strengthening. Suppression of latticedislocation slip by graphene nanosheets is confirmedby computer simulations of deformation behaviorexhibited by Al-graphene nanocomposites [40].

The second strengthening mechanism in metal-graphene nanocomposites is the stress transfer. Ingeneral, effect of this mechanism on both strengthand elastic properties of a nanocomposite is sensi-tive to chemical binding between graphene and ametallic matrix. In addition, there are geometric fac-tors that enhance the stress transfer in metal-graphene nanocomposites. The fact is that, withcurved shape, large values of aspect ratio, large widthand length of graphene platelets/sheets, thesegraphene nanoinclusions are bent, anchored andoften wrapped around metallic grains at grain bound-aries in metal-graphene nanocomposites. As a cor-ollary, graphene nanoinclusions have large contactarea with a metallic matrix and thus provide effec-

199Metal-graphene nanocomposites with enhanced mechanical properties: a review

tive load transfer. Besides, their geometry highlyprevents their pull-out, leading to pronounced tough-ening effects in pre-cracked nanocomposites.

Third, since graphene nanosheets hamper grainboundary migration, grain growth is hindered inmetal-graphene nanocomposites at the stage oftheir fabrication and in the course of their plasticdeformation. As a result, metal-graphenenanocomposites tend to have lower grain sizes, ascompared to their pure metal counterparts. At thesame time, following the classical Hall-Petch rela-tionship =

o + k d-1/2 (where is the strength,

o

is the dislocation friction stress, k is a material con-stant, and d is the mean grain size), the strength ofmaterials increases with decreasing the grain size.This factor – suppression of grain growth in meta]matrices by graphene nanoinc]usions – can signifi-cantly contribute to strengthening of metal-graphenenanocomposites.

Finally, note that fabrication of metal-graphenenanocomposites for high strength, hardness andelastic properties is in its infancy. In the near future,it is logical to expect explosive progress in funda-mental science, fabrication and applications of suchnanomposites whose potential to transform so manytechnologies is tremendous.

ACKNOWLEDGEMENTS

This work was supported by the Russian ScienceFoundation (Research Project 14-29-00199).

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