An investigation on the effect of sintering mode on ... · Composites Powder processing Electrical properties Mechanical properties Microstructure Optical microscopy abstract The

Advanced Powder Technology 28 (2017) 1760–1768

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Advanced Powder Technology

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Original Research Paper

An investigation on the effect of sintering mode on various properties ofcopper-graphene metal matrix composite

http://dx.doi.org/10.1016/j.apt.2017.04.0130921-8831/� 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

⇑ Corresponding author.E-mail address: [email protected] (A. Raja Annamalai).

C. Ayyappadas a, A. Muthuchamy a, A. Raja Annamalai b,⇑, Dinesh K. Agrawal c

a School of Mechanical Engineering, VIT University, Vellore 632 014, Tamil Nadu, IndiabCentre for Innovative Manufacturing Research, VIT University, Vellore 632 014, Tamil Nadu, IndiacMaterials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA

a r t i c l e i n f o

Article history:Received 7 October 2016Received in revised form 22 February 2017Accepted 20 April 2017Available online 3 May 2017

Keywords:CompositesPowder processingElectrical propertiesMechanical propertiesMicrostructureOptical microscopy

a b s t r a c t

The present work investigates the effect of sintering mode and graphene addition on the microstructural,mechanical and electrical properties of copper–graphene metal matrix composites reinforced with vary-ing amounts (0.9, 1.8, 2.7 and 3.6 vol%) of graphene particles fabricated through powder metallurgyroute. Sintering was carried out at 900 �C in 95%N2-5%H2 (forming gas) atmosphere with a heating rateof 5�/min for conventional and 20 �C/min for microwave with a holding time of 60 min in both cases.All the composites were found to couple well with microwave field and had resulted in 63% reductionin the processing cycle time as compared to the conventional process. Micro-structural analysis revealedthe homogeneous distribution of graphene in copper matrix. Copper-graphene composites exhibitedexcellent wear resistance due to higher hardness and excellent lubricating nature of graphene. It wasobserved that porosity has a significant effect on the electrical conductivity values.� 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction

Copper metal powder has been widely used in engineeringapplications because of its high electrical and thermal conductivi-ties combined with excellent corrosion resistance and ease of fab-rication [1,2]. Pure copper has wide variety of applications such aselectrical contacts in relays, magnetrons in microwaves, electro-magnets, vacuum tubes, heat sinks, welding electrodes, semi-conductors, microchips, piping systems, automobiles, etc. [3–5].Copper is not considered as a good material for structural applica-tions due to its poor mechanical properties especially at elevatedtemperature [6]. Therefore the researchers have been investigatingthe effect of different ways to improve its mechanical properties.The most effective method is making a Cu based composite byreinforcement addition of a suitable material [7]. Graphene is a2-D allotrope of carbon and is considered as the strongest materialever tested with a tensile strength of 130 GPa, Young’s modulus of1 TPa and excellent lubricating properties in both dry and humidconditions [8–11]. It is considered an excellent reinforcementmaterial because of its excellent mechanical strength, thermal con-ductivity (5300W/mK), electrical conductivity, and large specificsurface area (SSA – 500–1200 m2/g) [12]. The dispersion of gra-

phene in copper matrix is challenging due to the relatively highdifference in densities of the matrix and reinforcement phase.The high interfacial contact area of graphene and the mis-matchin thermal conductivities result in complex behavior of the resul-tant composite. An investigation to all these aspects is still in itsinfancy.

Researchers at the Korean Advanced Institute of Science andTechnology recently reported that graphene reinforced compositematerials exhibited up to 500 times tensile strength than the rawor monolithic material. Rashad et al. investigated the effect of gra-phene on magnesium based metal matrix nano composites andreported uniform dispersion of multilayer graphene in the matrixthat led to enhanced mechanical properties [13]. Varol et al. fabri-cated multilayer graphene copper nano composites by employingflake powder metallurgy and conventional sintering process. Theyreported that the composite resulted in reduced density, improvedhardness and electrical conductivity due to non-homogeneous dis-tribution of multilayer graphene in the copper matrix [14]. Zhanget al. studied the strengthening effect of graphene nano plateletsand reduced graphene oxide in copper matrix through a modifiedmolecular level mixing process and spark plasma sintering process[15]. Kim et al. fabricated Cu and Ni graphene nano layered com-posites with layer thickness of 70 nm and 100 nm for Cu-Gn andNi-Gn nano composites respectively. The Cu-Gn and Ni-Gn nanocomposites exhibited strength of 1.5 and 4 GPa respectively. These

Fig. 1. Scanning electron micrographs of as received (a) copper powder and (b) graphene powder.

Table 1Powder characteristics of the as received powders.

Characteristics Copper Graphene

Apparent density (g/cc) 4.5 0.015Tap density (g/cm) 5.3 0.12Flow rate in secs, 50 g 30 Non flowingParticle size (lm)D10 13.82 3.90D50 30.53 8.84D90 62.02 17.77

Theoretical density, g/cm3 8.96 2.2Surface area, m2/g 0.246 0.842Shape Spherical FlakyPurity >99.5% 99.99%

Fig. 2. Comparison of the temperature profile for conventional and microwavesintering modes.

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values were the highest values reported for any MMCs ever fabri-cated [9]. Literature presented here mainly depicts the improve-ment in properties of the composites as compared to themonolithic material as a result of graphene addition. The majorstrengthening mechanisms in Cu-Gn composites are alsoidentified.

Copper graphene composites in general exhibit superior electri-cal and thermal conductivity with low coefficient of thermalexpansion (CTE), good lubricating properties and improvedmechanical properties [16–18]. Copper-graphene composites findapplication in bearing materials, heat spreaders, electro-frictionmaterials and a variety of applications which require low thermalexpansion coefficient, high electrical and thermal conductivitywith higher hardness values [19]. The challenge to develop gra-phene based metal matrix composites is to address the agglomer-ation tendency and poor wettability of graphene and metal matrix[20]. Microwave heating is a rapid sintering technique for the con-solidation of various materials. The major advantage of microwavesintering over conventional sintering is that it provides rapid heat-ing resulting into much finer microstructure [21,22]. Material getheated up as a result of the coupling reaction between samplesand the electromagnetic wave. In addition to this, the grain bound-ary diffusion is promoted by the decrease in the activation energyfor sintering [23,24]. Microstructural homogeneity is preservedeven though the heating rates are high. Though sintering processare many, microwave sintering is projected as environmentfriendly and energy efficient technique [24–30]. Zheng et al.reported that microwave sintering could result in enhanced densi-fication, smaller grain size due to faster heating rate and lower sin-tering temperature [31]. Limited numbers of researches arereported on metal based composite materials processed by powdermetallurgy route [32–35]. Gupta et al. synthesized aluminiumbased composites by employing microwave sintering. It wasreported that the process parameters used in powder metallurgyroute coupled with microwave sintering which was suitable in pro-ducing hybrid composite materials with improved microstructure,

good distribution of reinforcement and mechanical characteristics[36]. Goldstein et al. conducted microwave sintering on ceramicmixtures. Results showed a dense composite with high hardness[27]. Molinari et al. synthesised M type hexaferrite in a singlemode microwave cavity and reported that the precursors couplewell with microwave mode without any susceptor. The magneticproperties of samples heated in microwave cavity for 30 min weresimilar to those exhibited by conventionally sintered samples for atime period of 12 h [37]. Yoon et al. made comparisons betweenmicrowave and conventional pressure-less sintering. Relative den-sity of microwave sintered samples crossed 99% and phase trans-formation rate achieved was 100% at a temperature of 1600 �Cwhereas for conventional pressure less sintering such high densityand phase transformation rate could not be achieved even at sin-tering temperature of 1850 �C [38]. Xu et al. studied the character-istics of microwave melting of copper powder. Changes inmicrostructure and densification were analyzed. Results showedthat copper powder could be quickly heated to melting. Microwaveheating efficiency and the heating rate were found to be higherwith the decrease in particle size and increase in microwavepower. Heating rate had a linear relationship with the reciprocalof the particle size. Microstructure and density indicates that thedensification process accelerates when the temperature is above900 �C. At lower temperature the migration of the matter for cop-per particles is mainly internal diffusion [28]. Xu studied the effectof kinds and contents of additives on mechanical properties, phasetransformation and microstructure behavior of Si3N4 ceramicmaterials processed by microwave sintering [39].

Literature points out that addition of even a small amount ofgraphene to the monolithic material can improve the properties

Table 2Effect of varying graphene addition and heating mode (conventional versus microwave) on the densification response of copper powder compacts.

Composition Green density Sintered density Densificationparameter

% Porosity

Sample1 Sample 2 CON MWS CON MWS CON MWS

Pure copper 7.43 (83%) 7.43 (83%) 7.71 (86%) 7.97 (89%) 0.183 0.35 14 11Cu- 0.9 Vol%Gn 7.53 (84.2%) 7.55 (84.4%) 7.87 (88%) 8.23 (92%) 0.24 0.49 12 8Cu-1.8 Vol%Gn 7.48 (83.8%) 7.50 (84%) 7.81 (87.43%) 8.06 (90%) 0.23 0.39 12.57 10Cu-2.7 Vol%Gn 7.37 (82.6%) 7.40 (83%) 7.58 (85%) 7.94 (89%) 0.135 0.36 15 11Cu-3.6 Vol%Gn 7.21 (81%) 7.23 (81.2%) 7.52 (84.39%) 7.84 (88%) 0.183 0.36 15.61 12

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to a considerable extent [40,41]. In this work composites of copperreinforced with 0.9, 1.8, 2.7 and 3.6 Vol% of graphene were fabri-cated through powder metallurgy route by employing conven-tional and microwave sintering processes. A comparison ofproperties of the Cu-Gn composites based on the two heatingmodes has been made. In this investigation, the microstructuralcharacterization, hardness, wear and electrical conductivity stud-ies were performed on the composites following the standardprocedures.

2. Experimental procedure

SEM micrographs of the as received copper (Sigma Aldrich, USA– powder size < 425 mm, 99.5%) and graphene (Angstron Materials,USA – Average Z Dimension (nm): 50–100, Average X & Y Dimen-sions (lm): �10, Specific Surface Area (m2/gm): 20–40, True Den-sity (gm/cm3): �2.20, Carbon (%): �97.00 Oxygen (%): �1.00,Hydrogen (%): �1.00 Ash (%): �2.50) powders are shown in Fig. 1and their properties are listed in Table 1. Compositions with 0.9,1.8, 2.7 and 3.6 vol% graphene and copper powder were accuratelyweighed and mixed thoroughly using a pestle and mortar for 30–45 min to ensure uniform mixing. The blended powders were coldcompacted using a closed cylindrical die in a hydraulic press with auniaxial pressure of 600 MPa. The cylindrical pellets fabricated hada diameter of 16 mm and height of 5–6 mm. For easy ejection ofthe green compact from the die after compaction a small amountof zinc stearate powder was applied to the die wall and punch.The green compacts were sintered at 900 �C in forming gas (95%N2-5%H2) by conventional and microwave sintering processes.Conventional sintering was carried out in a tubular furnace (Sup-plier: Indfurr superheat furnace, Chennai) with a heating rate of5 �C/min and holding time of 60 min. Intermediate holding timeof 20 min was provided to burn out the lubricant. Microwave sin-tering was performed in a 2.45 GHz, 10 kW microwave furnace(supplier: VB ceramic consultants, Chennai) with a heating rateof 20 �C/min and a holding time of 60 min. The samples wereallowed to cool naturally in the furnace itself after the powerwas turned off. The sintered densities of the samples were deter-mined using a method based on Archimedes principle. The densifi-cation parameter gives an insight to the degree of densificationoccurred during sintering process is expressed as shown in formula(1).

Densification parameter ¼ Sintered density� green densityTheoritical density� green density

ð1Þ

Density ðqÞ ¼ ðWa � dÞ=ðWa �WdÞ ð2ÞFor obtaining the optical micrographs of the composites con-

ventional metallographic preparation was used. The samples werepolished with SiC emery sheets of different grades (220, 400, 600,800, 1000, 1200, 1500 and 2000). In order to obtain a mirror finishon the samples, disk polishing of the samples were performed with

an aqueous alumina medium with alumina particle size of 1 mmsuspended in water. The etching of the samples was done as perASTM E407 standard with a solution containing 5 g FeCl3, 10 mlHCl, 30 ml water and 50 ml glycerol [42]. The polished sampleswere etched by swabbing using cotton soaked with etchant for atime period of 15–60 s. The optical micrographs of the compositeswere taken using an Optical microscope (Model: Axioscop A40,Zeiss, Germany). Grain size measurement was done using the lin-ear intercept method. The measurements were done on 5 micro-graphs captured at different portions of each sample at 500�magnification and on each micrograph 7–10 lines (200 lm length)oriented in various directions were considered. The Scanning elec-tron micrographs of the samples were obtained using a Scanningelectron microscope (Model: ZEISS EVO MA10, Zeiss, Germany).Vickers micro hardness tester (Model: Matsuzawa MMT-X7, Japan,Supplier: Chennai Metco, Chennai) was employed to performmicro hardness test on all the pellet samples with a load of 100 gand a dwell time of 10 s. Five readings were taken on each of thesamples by carefully picking a point which includes both thematrix and the reinforcement homogeneously. Dry wear test wasperformed on a pin-on-disc wear testing machine (Model: TR-201LE, DUCOM, Disc Material: EN-31 Steel, Hardness: 60HRC) asper ASTM G99 standard [43]. Pin-on-disc is a method to character-ize the coefficient of friction (COF), wear rate and frictional force ofthe material which forms the pin of the system. The pin is kept sta-tionary and a steel hardened disk rotates against the material caus-ing it to wear. The value of COF can be directly read from theinterface. A square cross section was cut from the rectangular sam-ple for performing the wear test. The dimensions of the sampleused for wear test are 31.7 ⁄ 3.3 ⁄ 3.3 mm. A constant sliding dis-tance of 650 mm was maintained for all the test samples. A discrotation speed of 500 rpm, and a wear track diameter of 50 mmwas kept constant through the experiment with applied loads of10, 20 and 30 N. The electrical resistivity of the samples was mea-sured using an electrical resistivity measuring instrument (Model:DDC-8, Chongqing, China) as per ASTM B193. The electrical con-ductivity (r) is the reciprocal of the resistivity (m). For the purposeof reproducibility five readings were taken on each samples. Thetwo probes of the instrument were kept on the sample till thereading became stable. An average value was taken and its recipro-cal was found to be the conductivity of the composite material.

3. Results and discussion

Fig. 1 shows the SEM micrograph of as-received powders. Thespheroidal nature of the copper particles can be observed inFig. 1a. Fig. 1b is the SEM image of multilayered graphene usedin this study. It shows clearly layered graphene particles stackedupon each other. The graphene flakes were observed to have anaverage thickness of 20–22 nm and an average platelet dimensionof 10 mm. Fig. 2 compares the temperature profiles of two heatingmodes. There are two intermediate holding times in case of con-ventional heating. Temperature was monitored throughout thesintering process by means of an infrared thermal sensor. In order

Conventional Microwave

Fig. 3. Optical micrographs of Cu-Gn composites sintered by conventional (left) and microwave (right) sintering modes.

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to avoid the thermal shock in the heating elements and furnacecomponents the heating rate in conventional sintering processwas limited to maximum 5 �C/min. But in microwave process a

much higher heating rate of 20 �C/min was employed. A holdingtime of 1 h at the final sintering temperature was maintained forboth the processes. The sintered samples are allowed to have fur-

Fig. 4. EDS analysis of Cu-0.9 vol%Gn and Cu-3.6 vol%Gn composites by conventional (left) and microwave (right) sintering modes.

Table 3Variation of Vicker’s micro hardness & Grain size of the Cu-Gn composites sintered by conventional and microwave process.

Composition Sintering mode? Grain size measurement (mm)

Conventional Microwave Conventional Microwave

Pure copper 43 ± 2.6 HV100 46 ± 2.8 HV100 50.27 43.9Cu-0.9 Vol%Gn 45 ± 2 HV100 52 ± 2 HV100 47.7 42.8Cu-1.8 Vol%Gn 56 ± 2.2 HV100 60 ± 2 HV100 44.9 42.26Cu-2.7 Vol%Gn 68 ± 1.8 HV100 74 ± 2.2 HV100 53.09 48.43Cu-3.6 Vol%Gn 82 ± 3 HV100 89 ± 2.8 HV100 46.19 40

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Conventional Microwave

Fig. 5. SEM micrographs of the worn surface.

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Table 4Wear rate of the Cu-Gn composites performed on a pin-on-disc wear testing machine.

Composition Sintering mode? Conventional Microwave 900 �CWear rate, g/m

10 N 20 N 30 N

CON MWS CON MWS CON MWS

Pure copper 3.07 2.2 5.53 4.6 9.31 7.9Cu-0.9 Vol%Gn 1.21 1.02 2.56 2.1 3.31 2.9Cu-1.8 Vol%Gn 0.58 0.45 1.18 0.96 1.69 1.1Cu-2.7 Vol%Gn 0.24 0.18 0.46 0.34 0.61 0.5Cu-3.6 Vol%Gn 0.13 0.10 0.26 0.18 0.40 0.28

Table 5Variation of the coefficient of friction (COF) of the Cu-Gn composites with different loads in a pin-on-disc tear testing machine.

Composition Coefficient of friction (COF)

10 N 20 N 30 N

CON MWS CON MWS CON MWS

Pure copper 0.52 0.51 0.54 0.52 0.59 0.54Cu-0.9 Vol%Gn 0.36 0.35 0.38 0.39 0.41 0.39Cu-1.8 Vol%Gn 0.34 0.33 0.35 0.33 0.38 0.36Cu-2.7 Vol%Gn 0.29 0.30 0.35 0.36 0.38 0.39Cu-3.6 Vol%Gn 0.22 0.21 0.24 0.22 0.27 0.26

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nace cooling after the completion of the sintering process. Thisresulted in rapid processing of the composites in the microwavefield, reducing the processing time by �63% as compared to con-ventional process. Similar trends were reported by other research-ers [21,22,44] in other materials.

4. Densification response

The effect of heating mode and graphene addition on the densi-fication response of the composites is shown in Table 2. A maxi-mum sintered density for the Cu-0.9 vol% Gn composite was 88and 92% of theoretical for conventional and microwave processes,respectively. A comparatively higher density was observed for Cu-0.9 vol% Gn composite with respect to pure copper despite thelighter nature of graphene. This may be due to the fact that gra-phene is acting as a lubricant during cold pressing. The sintereddensity values of Cu-Gn composites were much closer to the pureCu density as the weight percentage of graphene particles wasbelow 1 wt%. The density of graphene is 2.2 g/cm3 which was muchlower than that of copper (8.96 g/cm3). Hence the equivalent den-sity of the composite was found to be lower as per the rule of mix-tures [45]. Porosity was observed in all the composites sintered byconventional process. The decrease in density is attributed to theincreased addition of graphene. All the compositions with increas-ing additions of graphene found to couple well with the microwavefield. The decrease in the density values with the addition of gra-phene can also be correlated with the establishment of diffusionbarrier offered to copper particles. Hence with more addition ofgraphene copper diffusion becomes more difficult and henceresults in little less densification [14]. Densification parameter forthe microwave sintered samples was higher as compared to con-ventional counterparts.

5. Microstructure and SEM-EDS analysis

Optical micrographs of the Cu-Gn composites show homoge-neously distributed graphene particles within the copper matrixfor the range of the study (Fig. 3). Copper matrix and grapheneparticles could be clearly distinguished from the micrographs. The

dark areas indicate the uniformly distributed graphene particlesand the bright areas indicate the copper matrix (Fig. 3d). Some ofthe graphene particles were embedded in the copper matrix whilethe rest of the particles were observed at the grain boundary of thecopper matrix. The embedded particles do not have a re-arrangingability. With the increasing amount of graphene the clustering wasobserved at the grain boundary of copper. It is to be noted that rein-forced particles can agglomerate very easily in the matrix when thereinforcement size is smaller than thematrix grain size [14]. Amorehomogeneous microstructure was observed in the microwave sin-tered samples. Though the mixing and compaction parametersare identical, but the heating rates employed in two sintering pro-cesses are different. As compared to conventional sintering processthe heating rate employed in microwave sintering process is rela-tively high. It was observed that rapid heating results in a refinedand more homogeneous microstructure. Crystal twinning isobserved in the Cu-Gn composite sintered by microwave sintering(Fig. 3d and f). The regions on both sides of the twin boundary getattacked differently because of the difference in atomic configura-tion when the polished surface of the composite material is sub-jected to the etching process. These areas under microscopeshows dark and bright parallel regions called as twin boundarieswithin each grain. Rapid thermal expansion and contraction mayalso cause twinning [46]. Due to the smaller particle size, grapheneparticles were observed in the grain boundaries of the coppermatrix and no other phases or intermetallics were observed. XinGao et al. and Kim et al. reported that graphene exhibited strength-ening effect in the MMC due to the strong adhesion betweengraphene and copper. Graphene strengthens the material whilemaintaining the structural integrity. Zhang et al. reported theadsorption phenomenon of graphene derivatives on copper surface.Raman spectroscopy and FTIR spectroscopy showed that the gra-phene derivatives with oxygen content is better absorbed on toCu surface due to Cu-O-C chemical bonding. Presence of any otherreaction products were not reported by the authors. Porosity wasobserved in the SEM micrographs of the composites sintered byconventional process as shown in Fig. 3(e, g). The SEM-EDS analysesof the composites are shown in Fig. 4. Presence of oxygen wasdetected in the analysis. The EDS results show the presence of

Table 6Variation of electrical conductivity of the Cu-Gn composites sintered by conventionaland microwave process.

Composition Sintering mode? Conventional MicrowaveElectrical conductivity, %IACS

Pure copper 89%IACS 92%IACSCu-0.9 Vol%Gn 92%IACS 94%IACSCu-1.8 vol%Gn 91%IACS 92%IACSCu-2.7 Vol%Gn 88%IACS 89%IACSCu-3.6 Vol%Gn 84%IACS 86%IACS

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copper and carbon only. Presence of any interfacial products wasnot observed from the microstructure which confirms that no reac-tion takes place between copper and graphene during the sinteringprocess. Graphene is induced into the matrix due to its soft nature.Due to the low solubility of carbon in copper onlymechanical bond-ing between two phases occurs.

6. Mechanical and electrical properties characterization

Higher hardness values were observed for Cu-Gn composites forthe entire range as compared to pure copper due to the superiormechanical properties of graphene. As there is not much changein the density values with graphene addition for each heatingmodes the effect of graphene content was attributed to be thekey factor in imparting high hardness values to the composites.Highest value of 89 ± 2.4 HV100 was observed for Cu-3.6 vol% Gn,microwave sintered and 82 ± 2.2 HV100 for the conventional sin-tered sample (Table 3). It was observed that a very small weightpercentage of graphene particles added to the matrix couldimprove the hardness values substantially. Varol et al. and XinGao et al. reported that graphene reinforced MMC’s possess supe-rior hardness. The reduction in hardness reported by Varol et al.was due to agglomeration of graphene at copper grain boundaries.The distribution of graphene was observed to be homogeneous inthe present study. Dispersion strengthening is the main strength-ening mechanism which is responsible for the improvement inthe hardness values of these composites [14]. The increase in hard-ness values can be attributed to the strong mechanical bondingbetween the copper and graphene particles for the entire rangeof reinforcement addition. The presence of twin boundaries in themicrowave sintered samples changes the crystal orientation acrossthe interface which results in the discontinuity of slip systems andstrengthens the material [29]. Microwave sintered samples exhib-ited higher hardness compared to conventional counterparts.

The homogeneous microstructure observed in the microwavesintered samples also contributes in improving the mechanicalproperties in general. As expected from the hardness values, Cu-Gn composites exhibited excellent wear resistance due to thesuperior mechanical and lubricating properties of graphene parti-cles. With a small addition of graphene a drastic change in wearrate can be obtained as indicated by the results shown in Table 4.The lowest wear loss was observed for Cu-3.6 vol% Gn compositewith a wear rate of 0.28 ⁄ 10�5 g/m with highest applied load of30 N. The results obtained were in agreement with Archard’s wearequation, where an increase in hardness resulted in an improve-ment in wear resistance [20]. The wear rate was observed to beproportional to the load as stated by the wear equation. A linearrelationship between wear rate and normal load was observedwhere an increase in load resulted in a high wear rate. With theaddition of more graphite, wear rate was found to decline drasti-cally as compared to pure copper sample. The major mechanismwhich reduces wear rate is the interlayer shearing in graphene.The removed layer acts as a solid lubricant and prevents the fur-ther removal of the material. Graphene layer prevents the metalto metal contact resulting in low wear rate. The coefficient of fric-

tion (COF) of pure copper in dry sliding wear against the hardenedsteel disc was found to be very high (0.5–0.8) as shown in Table 5.With the addition of graphene the COF values were considerablyreduced. This reduction in the frictional coefficient can be corre-lated with the interlayer shearing of graphene which forms alubricative coating on the hardened disc, which lowers the wearrate and frictional coefficient subsequently [11]. Coefficient offriction is nearly constant, as it is not affected by loading condi-tions. Delamination surface cracks and macro grooves wereobserved in the SEM images of worn out pure copper sample asshown in Fig. 5a and b. Fig. 5c-f clearly indicates that the compositespecimen experiences a lower wear loss as compared to pure cop-per. The excessive delamination of surface layers of copper leads toa high wear loss. Delamination surface cracks were not observed inthe composite specimens. Self-lubricating carbon and oxide filmformations are helpful to extend the life of copper graphene com-posite during wear test, as confirmed by SEM-EDS studies. Microand macro grooves were observed in the SEM micrographs of theworn surface of composite specimens as seen in Fig. 5. With theaddition of more graphene the groove size is reduced (Fig. 5f).

The electrical conductivity of graphene is reported to be superiorto that of copper [40]. But the presence of porosity in the samplescaused a reduction in the electrical conductivity values. The Cu-0.9 vol% Gn composite with the highest density value showed anelectrical conductivity of 92% and 94% IACS for conventional andmicrowave sintered samples respectively, as illustrated in Table 6.The lowest value was observed for the Cu-3.6 vol% Gn sample with84% IACS, conventionally sinteredwhich is found in good agreementwith values reported by Varol et al. [14]. Presence of a conductivenetwork is essential for attaining high electrical conductivity values.Particle-particle contact establishes the network of conductive pathwithin the composite.With addition of higher volume percentage ofgraphene, porosity was introduced in the composite samples whichbroke the continuity in the conductivity path. The electrical conduc-tivity of pure copper sample was observed to be 89% and 92% IACSfor conventional and microwave process, respectively. The higherconductivity attained in the Cu-0.9 vol% Gn composite is due tothe homogeneous distribution of graphene conductive network inthe copper matrix. Presence of porosity resulted in increasing theinterfacial area followed by decrease in the conductivity values forthe higher amount of graphene present in the composite.

7. Conclusions

1. Copper matrix composites reinforced with varying amounts(0.9, 1.8, 2.7 and 3.6 vol%) of graphene particles were fabricatedusing microwave and conventionally sintered processes.

2. The density of microwave sintered samples was observed to behigher than the conventional sintered counterparts. Sintereddensity of pure copper was determined as 89% for microwaveand 86% for conventional sintering process.

3. Pure copper exhibited high wear loss under applied loads. Gra-phene reinforced composites exhibited excellent wear resis-tance. The COF values of Cu-Gn composites were observed tocome down with the addition of graphene, which supports gra-phene as the best solid lubricant in dry conditions.

4. The electrical conductivity values of microwave sintered purecopper sample was estimated as 92% IACS, and 89% IACS forthe conventional sample. Highest electrical conductivity of94% IACS was observed for Cu-0.9 vol% Gn composite. Thereduction in electrical conductivity values of Cu-Gn compositesare due to the presence of porosity observed in the samples.

5. It is concluded that microwave sintered composites exhibitssuperior electrical and mechanical properties to conventionalcounterparts owing to the homogeneous microstructure andhigh densification response of these composites.

der Technology 28 (2017) 1760–1768

Acknowledgements

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The authors would like to acknowledge the RGEMS seed fundfrom VIT University, Vellore for support of this research work. Also,the authors acknowledge the DST-FIST available at VIT University,Vellore.

References

[1] Jan Dutkiewicz, Piotr Ozga, Wojciech Maziarz, et al., Microstructure andproperties of bulk copper matrix composites strengthened with various kindsof graphene nano platelets, Mater. Sci. Eng. A628 (2015) 124–134.

[2] G. Celebi Efe, S. Zeytin, C. Bindal, The effect of SiC particle size on the propertiesof Cu–SiC composites, Mater. Des. 36 (2012) 633–639.

[3] G. Celebi Efe, M. Ipek, S. Zeytin, C. Bindal, An investigation of the effect of SiCparticle size on Cu–SiC composites, Composites: Part B 43 (2012) 1813–1822.

[4] J. Kovacik, S. Emmer, J. Bielek, Thermal conductivity of Cu-graphite composites,Int. J. Therm. Sci. 90 (2015) 298–302.

[5] K. Rajkumar, S. Aravindan, M.S. Kulkarni, Wear and life characteristics ofmicrowave-sintered copper-graphite composite, JMEPEG 21 (2012) 2389–2397 (ASM International).

[6] G.F. Celebi Efe, I. Altinsoy, M. Ipek, S. Zeytin, C. Bindal, Some properties of Cu–SiC composites produced by powder metallurgy method, Kovove Materialy 49(2) (2011) 131–136.

[7] G.S. Upadhyaya, Sintered Metallic and Ceramic Materials preparation,Properties and Applications, vol. 1, J. Wiley & Sons Inc., New York, NY, USA,1999.

[8] G. Van Lier, C. Van Alsenoy, V. Van Doren, et al., Study of the elastic propertiesof single-walled carbon nanotubes and graphene, Chem. Phys. Lett. 326 (1–2)(2000) 181–185.

[9] I.A. Ovidko, Metal graphene nano composites with enhanced mechanicalproperties; a review, Rev. Adv. Mater. Sci. 38 (2014) 190–200.

[10] Jinghang Liu, Umar Khan, Jonathan Coleman, et al., Graphene oxide andgraphene nanosheet reinforced aluminium matrix composites: powdersynthesis and prepared composite characteristics, Mater. Des. 94 (2016) 87–94.

[11] Diana Berman, Ali Erdemir, Anirudha V. Sumant, Graphene: a new emerginglubricant, Mater. Today 17 (1) (2014) 31–42.

[12] A.A. Balandin, S. Ghosh, W.Z. Bao, I. Calizo, D. Teweldebrhan, F. Miao, et al.,Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (3) (2008)902–907.

[13] Muhammad Rashad, Fusheng Pan, Hu Huanhuan, et al., Enhanced tensileproperties of magnesium composites reinforced with graphene nanoplatelets,Mater. Sci. Eng., A 630 (2015) 36–44.

[14] T. Varol, A. Canakci, Microstructure, electrical conductivity and hardness ofmultilayer graphene/copper nanocomposites synthesized by flake powdermetallurgy, Met. Mater. Int. 21 (4) (July 2015) 704–712.

[15] Dandan Zhang, Zaiji Zhan, Strengthening effect of graphene derivatives incopper matrix composites, J. Alloy. Compd. 654 (2016) 226–233.

[16] Bunyod Allabergenov, Sungjin Kim, Investigation of electrophysical andmechanical characteristics of porous copper-carbon composite materialsprepared by spark plasma sintering, Int. J. Precis. Eng. Manuf. 14 (7) (2013)1177–1183.

[17] Xin Gao, Hongyan Yue, Erjun Guo, Hong Zhang, et al., Preparation and tensileproperties of homogeneously dispersed graphene reinforced aluminummatrixcomposites, Mater. Des. 94 (2016) 54–60.

[18] P. Bharathidasan, Dong-Won Kim, S. Devaraj, S.R. Sivakkumar, et al.,Supercapacitive characteristics of carbon-based graphene composites,Electrochim. Acta 204 (2016) 146–153.

[19] Weiping Li, Delong Li, Qiang Fu, Chunxu Pan, Conductive enhancement ofcopper/graphene composites based on high-quality graphene, RSC Adv. 5(2015) 80428–80433.

[20] H.G. Prashantha Kumar, M. Anthony Xavior, Fatigue and wear behavior ofAl6061–graphene composites synthesized by powder metallurgy, Trans.Indian Inst. Met. 69 (2) (2016) 415–419.

[21] Avijit Mondal, Anish Upadhyaya, Dinesh Agrawal, Effect of heating mode andcopper content on the densification of W-Cu alloys, Indian J. Mater. Sci. 2013(2013) 7 pages Article ID 603791.

[22] R. Roy, D. Agrawal, J. Cheng, S. Gedevanishvili, Full sintering of powdered-metal bodies in a microwave field, Nature 399 (1999) 668–670.

[23] Yu Cheng, Yong Zhang, Taoyu Wan, et al., Mechanical properties andtoughening mechanisms of graphene platelets reinforced Al2O3/TiCcomposite ceramic tool materials by microwave sintering, Mater. Sci. Eng., A680 (2017) 190–196.

[24] S. Nawathe, W.L.E. Wong, M. Gupta, Using microwaves to synthesize purealuminum and metastable Al/Cu nanocomposites with superior properties, J.Mater. Process. Technol. 209 (2009) 4890–4895.

[25] M. Gupta, W.L.E. Wong, Scr. Mater. 52 (2005) 479–483.[26] W.L.E. Wong, M. Gupta, Development of Mg/Cu nanocomposites using

microwave assisted rapid sintering, Compos. Sci. Technol. 67 (2007) 1541–1552.

[27] Adrian Goldstein, Roman Ruginets, Liana Geifman, Carbide matrix compositesby fast MW reaction-sintering in air of B4C–SiC–Al mixtures, Ceram. Int. 35(2009) 1297–1300.

[28] Lei Xu, C. Srinivasakannan, Jinhui Peng, Study on characteristics of microwavemelting of copper powder, J. Alloy. Compd. 701 (2017) 236e243.

[29] M.G. Ananda Kumar, S. Seetharamu, Jagannath Nayak, A study on thermalbehavior of aluminum cenosphere powder metallurgy composites sintered inmicrowave, Procedia, Mater. Sci. 5 (2014) 1066–1074.

[30] Dmytro Demirskyi, Oleg Vasylkiv, Microstructure and mechanical propertiesof boron suboxide ceramics prepared by pressure less microwave sintering,Ceram. Int. 42 (2016) 14282–14286.

[31] R.R. Zheng, Y. Wu, S.L. Liao, et al., Microstructure and mechanical properties ofAl/(Ti, W)C composites prepared by microwave sintering, J. Alloy. Compd. 590(2014) 168–175.

[32] Z. Xie, C. Wang, X. Fan, et al., Mater. Lett. 38 (1999) 190–196.[33] K. Saitou, Scr. Mater. 54 (2006) 875–879.[34] E. Breval, J.P. Cheng, D.K. Agrawal, et al., Material. Sci. Eng. A 391 (2005) 285–

295.[35] D.K. Agrawal, Microwave processing of ceramics, Curr. Opin. Solid State Mater.

Sci. 3 (1998) 480–486.[36] Sanjay Kumar Thakur, Tung Siew Kong, Manoj Gupta, Microwave synthesis

and characterization of metastable (Al/Ti) and hybrid (Al/Ti + SiC) composites,Mater. Sci. Eng., A 452–453 (2007) 61–69.

[37] Flora Molinari, Antoine Maignan, Sylvain Marinela, Fast synthesis ofSrFe12O19 hexaferrite in a single-mode microwave cavity, Ceram. Int. 43(2017) 4229–4234.

[38] C.K. Yoon, H.K. Chong, K.K. Do, Effect of microwave heating on densificationand a-b phase transformation of silicon nitride, J. Eur. Ceram. Soc. 17 (1997)1625–1630.

[39] Weiwei Xu, Zengbin Yin, Juntang Yuan, et al., Effects of sintering additives onmechanical properties and microstructure of Si3N4 ceramics by microwavesintering, Mater. Sci. Eng., A 684 (2017) 127–134.

[40] K. Jagannadham, Volume fraction of graphene platelets in copper-graphenecomposites, Metall. Mater. Trans. A 44 (1) (2013) 552–559.

[41] Rongrong Jiang, Xufeng Zhou, Qile Fang, Zhaoping Liu, Copper–graphene bulkcomposites with homogeneous graphene dispersion and enhanced mechanicalproperties, Mater. Sci. Eng., A 654 (2016) 124–130.

[42] Standard Practice for Micro-etching Metals and Alloys, ASTM International,Designation: E407-07, 2015.

[43] Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus, ASTMInternational, Designation: G99-05, 2010.

[44] A. Muthuchamy, A. Raja Annamalai, Dinesh Agrawal, Anish Upadhyaya, Aninvestigation on effect of heating mode and temperature on sintering of Fe-Palloys, Mater. Charact. 114 (April) (2016) 122–135.

[45] S.C. Sharma, Equation for the density of particle-reinforced metal matrixcomposites: a new approach, ASM Int., JMEPEG 12 (2003) 324–330.

[46] J. Wang, I.J. Beyerlein, N. Mara, A. Misra, C.N. Tome, Deformation twinningmechanisms in FCC and HCP metals, advances in heterogeneous materialmechanics, in: 3rd International Conference on Heterogeneous MaterialMechanics (ICHMM-2011), Shanghai (Chong Ming Island), China.