Effects of graphene nanoplatelets and graphene nanosheets onfracture toughness of epoxy nanocomposites

M. M. SHOKRIEH1, S. M. GHOREISHI1, M. ESMKHANI1 and Z. ZHAO2

1Composites Research Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University ofScience and Technology Tehran, 16846-13114, Iran, 2School of Chemical Engineering, Shandong University of Technology, 255049, Zibo, Shandong, China

Received Date: 21 January 2014; Accepted Date: 8 March 2014; Published Online: 21 April 2014

ABSTRACT The effects of graphene nanoplatelets (GPLs) and graphene nanosheets (GNSs) onfracture toughness and tensile properties of epoxy resin have been studied. A newtechnique for synthesis of GPLs based on changing magnetic field is developed. Thetransmission-electron microscopy and the Raman spectroscopy were employed tocharacterize the size and chemical structure of the synthesized graphene platelets. Thecritical stress intensity factor and tensile properties of epoxy matrix filled with GPL andGNS particles were measured. Influence of filler content, filler size and dispersion statewas examined. It was found that the GPLs have greater impact on both fracture toughnessand tensile strength of nanocomposites compared with the GNSs. For instance, fracturetoughness increased by 39% using 0.5wt% GPLs and 16% for 0.5wt% GNSs.

Keywords fracture toughness; graphene nanoplatelet; graphene nanosheets; mechanicalproperties; nanocomposites.

NOMENCLATURE a = crack lengthB = single-edge-notch bending specimen thicknessKc = critical stress intensity factor

KIC = mode I critical stress intensity factorGIC = mode I fracture energyPQ = applied loadW = specimen width

Wt.% = weight per centCTOD = crack-tip opening displacement

E = Young’s modulusσy = tensile strengthυ = Poisson’s ratio

INTRODUCT ION

Graphene is a two-dimensional flat sheet of sp2-hybridized carbon atoms, which is the basic buildingblock of other important carbon allotropes and has beenattracted, because of its exceptional electrical properties.However, after introducing new methods capable of pro-ducing graphene in large quantities, growing attentionhas been paid to mechanical properties of these planarcarbon structures.1,2 Measured mechanical properties of asingle graphene layer proved that graphene is one of thestiffest known materials. The in-plane elastic modulus ofmonolayer graphene membrane is about 1.1TPa.3

Recently, the usage of different carbon nanostructuresfor manufacturing of epoxy-based nanocomposites istaken into account as an exciting area for experimental-ists.4–8 Epoxy resins due to their excellent mechanicalproperties and chemical resistance have considerablybecome useful in load bearing structures. Consideringthe brittle fracture behaviour of epoxy-based compo-nents, especially those with pre-existing cracks anddefects that are created most often during the process ofmanufacturing or preparation, many scientific investiga-tions have been performed with the aim of improving thefracture resistance of epoxy matrices. Fracture toughnessof epoxy has been substantially improved by incorporationof liquid rubber, rubber or thermoplastic particles.9,10 But,Correspondence: M. M. Shokrieh. E-mail: Shokrieh@iust.ac.ir

© 2014 Wiley Publishing Ltd. Fatigue Fract Engng Mater Struct, 2014, 37, 1116–11231116

doi: 10.1111/ffe.12191

these additives reduce composite modulus and its thermalproperties, undesirably. Other conventional tougheningfillers such as silica, alumina and clay platelets couldsuccessfully enhance both elastic modulus and fracturetoughness.11–13 In addition, many studies concerning theeffects of carbon nanostructures on mechanical andfracture behaviour of thermosetting polymers had reporteda notable change in fracture resistance of nanocompositeswith lower filler contents in comparison with silica orclay.14–18 Modification of polymer matrices with grapheneplatelets (GPs) as reinforcement has opened a new pathtowards manufacturing advanced materials.19,20 A GPconsists of several single graphene layers with a usual stackthickness around 3 to 80nm.20,21 In a recent study, theeffects of two types of graphene nanoparticles on mechan-ical properties of epoxy matrix were investigated byShokrieh et al.22 In this research, the thick grapheneparticles with a range of thickness from 20 to 30nm andthin particles are named as graphite nanoplatelets (GNPs)and graphene nanosheets (GNSs), respectively. Significantimprovements in mechanical properties of epoxy resinwere achieved by adding 0.1wt% additives for both theGNSs and the GNPs. It has been reported22 that theGNSs with an average thickness lower than 10nm havegreater effect compared with GNPs on Young’s modulusand tensile strength of epoxy resin. Meanwhile, very fewreports have been reviewed considering the effects of these2D carbon nanostructures on fracture resistance of poly-mer matrices. Rafiee et al.23 studied the fracture and fatiguebehaviour of epoxy matrix with various weight fractions offunctionalized graphene sheets (FGSs). Nanoparticles wereprepared by rapid thermal expansion of completely oxidizedgraphite oxide.24,25 Their experimental results on lowcontents for fracture toughness showed that addition of0.125wt% of FGS can increase the value of mode I fracturetoughness for nearly 65%. In a later comprehensiveapproach, they worked on mechanical and fracture proper-ties of epoxy systems using 0.1wt% graphene nanoplatelets(GPLs), single-walled carbon nanotube (SWCNT) andmulti-walled carbon nanotube (MWCNT) as additives.26

It had been concluded thatGPLswere capable of improvingfracture toughness about 53%, which was a much highervalue than the case for two other nanoparticles (i.e. 12%for SWCNT and 20% for MWCNT). Qiu and Wang27

reported 41% rise in fracture toughness of an epoxy systemmodified by 0.54wt% of monolayer graphene oxide (GO)produced by a modified Brodie’s method.28 Zaman et al.29

performed an investigation in order to understand the effectof interface strength on the fracture toughness of agraphene/epoxy material. They used two types of GPs withor without surface chemical modification. Their results for4wt% of GPs indicated that surface modification caused85% improvement in comparison with 42% in the case ofunmodified fillers. Bortz et al.30 performed an investigation

on fracture and fatigue life of a modified epoxy matrix anddispersed GO additives using calendaring technique; here,110% enhancement in fracture energy at 1wt% filler wasachieved. Shadlou et al.31 utilized GO, carbon nanofiberand nanodiamond for a comprehensive study on mixedmode fracture toughness of polymeric matrices. Accordingto their reports for pure mode Ι, the highest fracturetoughness value was obtained for GO/epoxy specimens.Chandrasekaran et al.32 studied the influence of GNPs onmechanical and electrical properties of an epoxy matrix.The fracture toughness measurements show a maximumimprovement of 43% in 1wt% GNP/epoxy specimens.

In this study, a new method was developed to producelow thickness graphene plates, which are referred to asGPLs. Effects of addition of GPL nanoparticles on thefirst mode of fracture toughness of an epoxy system wereinvestigated by measuring the critical stress intensityfactor. The uni-axial tensile tests were performed inorder to measure the Young’s modulus and tensile strengthof composite specimens with different nanoparticle weightfractions. Minimum of five specimens has been tested foreach case. Finally, to examine the role of the geometricalfactors of GP on fracture and tensile properties of epoxyresin, a second type of GP, as GNS with different diagonaldiameter and thickness, was employed for manufacturingof 0.5wt% nanocomposites.

EXPER IMENTS

Graphene nanoplatelet synthesis

There are different methods for production of graphenenanoparticles.1,2,33–35 In the present study, GPL particlesare synthesized with a stirring grinding driven by changingthemagnetic field as shown in Fig. 1. The steel needles withweak magnetism are used as grinding media, and fourNdFeB permanent magnets are inserted into a motor-driven disc (Fig. 1). When the disc is made of steel, themagnetic stainless steel needles are attracted by the perma-nent magnets (Fig. 1a and b). By increasing the rotationalspeed, the magnetic stainless steel needles fly up and collidewith each other with a high frequency under the changingattraction and repulsion forces of the high-speed rotatingpermanent magnets (Fig. 1c). When a rigid grinding cham-ber filled with a certain amount of graphite powder is set onthe disc, there are high frequent collisions and shearsbetween the grinding chamber and the magnetic stainlesssteel needles, which can finally result in a strong collisionand shear forces. Graphite in the chamber will be crushedinto ultra-fine powder under the action of these strongforces, and then, the powder will be prepared efficiently.The synthesized graphene powders have an average thick-ness of approximately 3–5nm and a typical surface weight

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© 2014 Wiley Publishing Ltd. Fatigue Fract Engng Mater Struct, 2014, 37, 1116–1123

of 500m2g�1 obtained fromBrunauer–Emmett–Teller testof nanoparticle powders. The average GPLs diameter isaround 80nm. The transmission-electron microscopyimage of the synthesized GPL powder has been shown inFig. 2a. The D, G and 2D bands of Raman spectra of thesynthesized GPLs powder are demonstrated in Fig. 3.

Materials specification

ML-526 (Bisphenol-A) epoxy resin was selected becauseof its low viscosity and extensive industrial applications.

The low viscosity of the matrix, 1190 cP at 25 °C, makesthe dispersion of additives easier. The curing agentwas HA-11 polyamine. The ML-526 epoxy resin andHA-11 polyamine hardener were supplied by MokarrarEngineering Materials Company, Tehran, Iran. TheGNSs were supplied by XG Science Inc, Lansing, MI,United States.* These particles have an average thicknessof approximately 6–8 nm and a typical surface weight of120–150m2 g�1 (Fig. 2b). The geometrical characteris-tics of GPL and GNS particles are presented in Table 1.

Raman spectroscopy

The quality of nanoparticles can be investigated byconsidering the Raman spectra of the synthesized GPL

Fig. 2 Different types of nanoparticles used in the present work. (a) Transmission-electron microscopy of the synthesized graphenenanoplatelet powder; (b) scanning electron microscope picture of graphene nanosheet powder.16

Fig. 3 Raman spectra comparison of synthesized graphenenanoplatelet, D, G and 2D bands with those of graphene nanosheet.

Table 1 Different types of GPL and GNS used in the present work

Category Diameter Thickness (nm)Specific surfacearea. (m2 g�1)

GPL 40–120 nm 3–5 500GNS 5–10μ 6–8 120–150

GPL, graphene nanoplatelet; GNS, graphene nanosheet.

*www.XGSciences.com.

N

S

N

S

NS

N

N

S

N

S

S

(a) (b) (c)

A

E

D

C

B

Fig. 1 Graphene nanoplatelet synthesis method. (a, b) Still condition; (c) moving condition.

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andGNS. The Raman spectroscopy is a spectroscopic tech-nique and one of the most comprehensive tools to identifythe type and quality of unknown materials. In this method,a sample is illuminated with a laser beam, and the light fromthe illuminated spot is collected with a lens and sent througha monochromator. The laser light interacts with the mole-cule in the system, resulting in the energy of the laser pho-tons being shifted up or down. The pick in intensityversus wave number curve gives information about thestructure of nanoparticles. As shown in Fig. 3, it is clearlyobserved that wave numbers that show the composition ofGNS and GPL materials are almost at the same valuesand confirm a similar base material of these nanoparticles.

Specimen preparation

Polymer matrices reinforced with 0.05, 0.1, 0.25, 0.5and 1wt% GPL and 0.5wt% GNS were prepared asfollows. First, the epoxy resin was vacuumed at 60 °C for24h to minimize any possible diluent or solvent effect.Then, it was mixed with the nanoparticles and stirred for10min at 2000 rpm, and then, the mixtures were sonicatedwith 14mm diameter probe sonicator (Hielscher UP400S)(Hielscher Ultrasonics GmbH, Germany) at an outputpower of 200W. It is good to mention that during the son-ication process with intervals, the mixture container washeld in an ice bath to prevent overheating of the suspensionand to keep the temperature around 40 °C.

After sonication, the hardener was added to the mix-ture at a ratio of 15:100 and stirred gently for 5min.Then, it was vacuumed at 1mbar for 10min to removeany trapped air. After molding, all samples were curedat room temperature followed by a post curing processfor 2 h at 80 °C and 1 h at 110 °C.

Test instruments and standards

The Santam universal testing machine (SANTAM Engi-neering Design Co. Ltd, Iran) (STM-150) was utilized to

perform tensile tests conducted on the dog bone-shapedspecimen according to the ASTM D638. The cross-headspeed was set at 2mmmin�1, and an extensometer of50mm gauge length was used to measure the strain.

According to ASTM D5045, the single-edge-notchbending (SENB) geometry was utilized to conduct frac-ture tests. The width of the notch was 1mm located atthe mid-span of 9� 18� 88mm specimens using a thinsaw machine. Furthermore, a primary crack was initiatedby tapping a fresh razor blade in the notch. The SENBtests of the specimens with a crack length of a = 9mmwere performed and shown in Fig. 4. The appliedcross-head speed was set at 1mmmin�1.

The mode I fracture toughness of the neat and filledepoxy with GPL and GNS was determined from SENBfracture tests. The critical stress intensity factor, KC,was calculated using the following equation:

KC ¼ PQ

B�W 1=2

� �� f xð Þ (1)

where PQ is the applied load on the specimen, B is thespecimen thickness,W is the specimen width and a standsfor the crack length. The f(x) as a non-dimensionalgeometry factor is given by

f xð Þ ¼ 6x1=21=99� x 1� xð Þ 2=15� 3=93xþ 2=7x2

� �� �1þ 2xð Þ 1� xð Þ3=2

(2)

Because almost no nonlinearity before failure has beennoticed in the load–displacement curves, proving the factof being in linear elastic fracture mechanics domain, thepeak load of each load–displacement diagram was usedin order to calculate the critical stress intensity factor.Fracture energy is also determined using the integrationof load–displacement curves.

Fig. 4 The single-edge-notch bending test specimen under testing.

2.4

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2.75

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2.85

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You

ng's

Mod

ulus

(G

Pa)

Filler Content (wt%)

GPL epoxy nanocomposites

Fig. 5 Young’s modulus (GPa) of composites versus fillercontent (wt%).

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RESULTS AND DISCUSS ION

Tensile properties

A schematic representation of the Young’s modulus andtensile strength is shown in Figs 5 and 6. Tensile modu-lus of the neat epoxy enhanced by ≈4.5% from 2.5 to2.61GPa with addition of only 0.1wt% GPL. In higherfiller fractions, development intensity was reduced, andthe highest magnitude for the modulus was achieved at0.5wt% of GPLs (2.76GPa) showing nearly 10% im-provement. Note that in 1wt% GPL/epoxy specimens,despite doubling the amount of filler, the modulus didnot show noticeable improvement. Unlike the tensilemodulus, the maximum of the tensile strength is foundat 0.25wt% of GPLs (73.9MPa) showing a rise of 23%in comparison with the neat polymer (60MPa). A majorreason for decreasing trend of the strength in higher fillercontents, that is, 0.5 and 1wt%, can be attributed toincreasing stress concentrations in un-dispersed or re-agglomerated GPLs. Agglomerates reduce the level of

stress transfer from matrix to individual platelets andintroduce larger stress concentration regions.

Fracture toughness

In order to characterize the first mode fracture behaviourof low content GPL/epoxy and determine the mostefficient amount of nanofiller, test specimens based ondifferent filler contents, including 0.05, 0.1, 0.25, 0.5and 1wt% of GPLs, were prepared. The results of thefracture tests are presented in Fig. 7. KIC values ofnanocomposites are generally increased by addition ofGPLs in comparison with the neat epoxy. The highestfracture toughness was achieved at 0.5wt% of GPLswith an average improvement of ≈39% (from 0.98to 1.36MPa.m0.5) while composites containing 1wt%GPLs showed lower toughening increment. The reasonsof this trend have been discussed based on fracturesurface morphology in the following section.

Fracture surface morphology

Fracture surface of the specimens has been investigatedusing the Tescan Vega II scanning electron microscope(TESCAN ORSAY HOLDING, Czech Republic). InFig. 8, the fracture surfaces of the specimens containingdifferent amounts of filler, that is, 0, 0.1 and 0.5wt%,are compared. As it was expected, the neat polymershowed a mirror-like surface confirming a brittle fracturebehaviour (Fig. 8a). In other cases, fracture surfacesexhibited rough texture near the crack propagationzones. Note that the surface roughness increases rela-tively by increasing the amount of filler. The crackgrowth directions are from the bottom to the top in allimages. Fig. 8c and d shows a closer view of a smallGPL cluster with different magnification levels. As itcan be seen, inelastic matrix deformation in shear bandsand void formation initiated by particle–matrixdebonding are partly responsible for toughening incre-ment.4,36 The crack pinning has also been detected atsome small GPL clusters.37,38 Facing these impenetrableareas, the propagating crack front is forced to bow outbetween these obstacles so that the crack length increasesand more energy is dissipated. Calculation of crack-tipopening displacement for the pure and modified epoxyon the basis of formulation introduced by Kinloch andYoung39 results in a range from 5.6 to ≈10μm.

CTOD ¼ K2IC

Eσ y1� v2� �

(3)

Compared with the high magnified GPL clustershown in Fig. 8c with a measured span of 3.26μm,particle size-dependent fracture mechanisms like crackpinning or crack deflection40 are likely to contribute to

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Ten

sile

Str

engt

h (M

Pa)

Filler Content (wt%)

GPL epoxy nanocomposites

Fig. 6Tensile strength (MPa) of composites versusfiller content (wt%).

Fig. 7 Effect of different graphene nanoplatelet contents on fracturetoughness and fracture energy of nanocomposites.

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toughness enhancement in nanocomposites with higherfiller weight fractions.

For 1wt% of GPL/epoxy despite experiencingmore sonication time, the state of dispersion was poor,and a plethora of large agglomerations were observed.It was concluded that sonication times longer than60min did not greatly influence the dispersion ofGPLs. In addition, excessive sonication energy from aprobe sonicator can affect the structural continuity ofthe nanostructure.41 By increasing the numbers of largeagglomeration zones (bigger than 10μm), the stressconcentration regions are the main reason for decreasingthe fracture toughness increment.

Fracture behaviour of graphene nanoplatelet andgraphene nanosheet epoxy nanocomposites

The GPL diameter is very small compared with conven-tional GPs or GOs (Table 1). According to extensive

literature on toughening mechanisms,13,42 local matrixplastic deformation and void creation could supposedlybe responsible for the toughness enhancement with suchsmall nanofillers. Meanwhile, for 0.25 and 0.5 wt%, K C

value increased from 1.18 to 1.36MPa.m0.5, which repre-sents a higher toughening increment than the case for 0.1and 0.25wt% (1.11 and 1.18MPa.m0.5). This behaviourexplains that small GPL clusters have a superior effecton the fracture energy. These clusters are large enoughso that other toughening mechanisms, such as crack pin-ning and crack deflection, participate. Also, penetrationof polymer through GPL clusters causes extensive localshear banding and debonding during the fracture pro-cess. Therefore, different fracture behaviours wereexpected for epoxy modified with micron-sized diameterGPs. For example, toughness enhancement in GO andGNP modified polymers is mainly related to crack de-flection and crack pinning.30,32 In order to clarifywhether the influence of plate diameter dominates or

Fig. 8 Fracture surface of graphene nanoplatelet (GPL)/epoxy composites. (a) Neat epoxy; (b) 0.1wt% GPL/epoxy; (c) 0.5wt% GPL/epoxy.Evidence of crack tip pinning is indicated by arrows. A closer view at surface texture near small GPL clusters at �3000 and �20000 scanningelectron microscope MAG. (d) Particle matrix debonding.

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its thickness, a number of 0.5 wt% GNS/epoxy SENBspecimens were prepared and tested. Tensile experimentaldata for these nanocomposites were extracted from ourprevious work.22 These measurements were comparedwith those for 0.5wt% GPL/epoxy and showed differentresults on each property, as illustrated in Fig. 9.

Figure 9 indicates that 0.5wt% GPL/epoxy has higherfracture toughness and tensile strength than 0.5wt%GNS/epoxy while the GNS/epoxy is stiffer than theGPL/epoxy (≈3GPa compared with ≈2.75GPa). Thefracture toughness of GNS/epoxy improved by only 16%(1.14MPa.m0.5), which was much lower than that ofGPL/epoxy composites. With a similar base material(Fig. 3), the GNSs have a thickness almost twice the GPLand a diameter nearly hundred times larger (Table 1). Onthe other hand, because of larger diameter of GNS, tough-ening as a result of the crack pinning, crack deflection andeven crack bridging is expectable for GNS/epoxy compos-ites. Lower fracture toughness of GNS/epoxy comparedwith GPL/epoxy clearly shows that these three tougheningmechanisms have little contribution.

CONCLUS IONS

In the present study, the effect of graphene nanofillers hav-ing a planar geometric shape on fracture toughness of athermosetting epoxy system has been investigated undermode I loading conditions. First, a newmethod for synthe-sizing GPLs from graphite powder was employed using a

changing magnetic field to achieve strong shear forcesand obtain low thickness graphene particles. Later, tensileand fracture experiments of nanocomposites with differentcontents of GPL particles, that is, 0.05, 0.1, 0.25, 0.5 and1wt% dispersed in epoxy, have been performed. Theresults showed that addition of very low content of GPLcan have a remarkable impact on the fracture resistanceas well as tensile properties. Experimental observationsshowed optimal improvement in critical stress intensityfactor and Young’s modulus for 0.5wt% GPL/epoxy withan average growth compared with neat epoxy of about 39%and 15%, respectively.

During fractography, a relative increase in fracture sur-face roughness was detected. It was concluded that inelasticmatrix deformation and creation of voids were the majorreasons for considerable fracture toughness improvement.

Finally, the effects of GNSs with bigger thickness andlower specific surface area on tensile properties and firstmode fracture toughness of epoxy resin were comparedwith those of 0.5wt% GPL/epoxy. It was found that theeffect of GPL particles in increasing the fracture resistance,compared with that of GNS particles, was more desirable.

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