Investigation of graphene-reinforced magnesium metal

Bull. Mater. Sci. (2019) 42:39 © Indian Academy of Scienceshttps://doi.org/10.1007/s12034-018-1720-1

Investigation of graphene-reinforced magnesium metal matrixcomposites processed through a solvent-based powder metallurgyroute

V KAVIMANI1,∗, K SOORYA PRAKASH1 and TITUS THANKACHAN2

1Department of Mechanical Engineering, Anna University Regional Campus, Coimbatore 641 046, India2Department of Mechanical Engineering, Karunya Institute of Technology and Sciences, Coimbatore 641 114, India∗Author for correspondence ([email protected])

MS received 21 November 2017; accepted 26 June 2018; published online 7 February 2019

Abstract. In the present investigation, AZ31 alloy is homogeneously reinforced with 0.2, 0.3, 0.4 and 0.5 wt% of reducedgraphene oxide (r-GO) nanosheets for the first time through a series of methodologies involving solvent processing, mechan-ical alloying, cold pressing and finally sintering under argon atmosphere at 560◦C. Scanning Electron Microscopy (SEM)assisted with energy-dispersive X-ray analysis revealed that this inventive fabrication route is useful to easily disperse r-GOinto the matrix material and thereby attain methodical homogeneity with a uniform particle size. The attained results showthat amongst the others, addition of 0.4 wt% r-GO have obviously improved the hardness up to 64.6 HV and also yieldeda better inhibition efficiency of 84% on corrosion. Any further increase of r-GO content resulted for significant decrease inthe wear rate up to the level of 2.6 mm3 Nm−1.

Keywords. Metal matrix composites (MMCs); reinforcement synthesis; solvent-based powder metallurgy (PM); materialcharacterization.

1. Introduction

Graphene has great potential to be used as areinforcement material in polymers and metals and thusimproves the performance of composites owing to its inher-ent strong mechanical properties, chemical inertness, thermalstability and good electrical properties [1–4]. Nowadays,magnesium (Mg)-based alloys are of great interest in indus-trial sectors due to their low density which supports weightreduction in the transport industry thereby increasing fuel effi-ciency [5–9]. Carbon nanomaterial has attracted attention as apositive reinforcement for Mg [10] and its alloys over the lasttwo decades because of its capability of improving strengthin structural applications [11]. Soorya Prakash et al devel-oped Mg-based Mg matrix composites (MMCs) using powdermetallurgy (PM) route with silicon carbide- and graphite-based reinforcements. They studied the wear behaviour ofMg MMCs and observed that in certain forms, addition ofgraphite also enhances the inherent tendency of altering thewear rate [12].

Many interesting studies were conducted ongraphene-reinforced MMCs but a high concentration ofgraphene in a matrix material has not yet been openly dis-cussed mainly because of two major difficulties: (i) agglomer-ation and (ii) non-uniform dispersion in matrix material [13–15]. From the research point of view, the above abnormalitiescan be eliminated only by way of selecting appropriate fab-rication methodologies with due considerations of numerous

material- and process-oriented engineering strategies. Spearheaded evaluation over the available composite developmentmethods reveals that stir casting is not suitable for graphene-based composite fabrication due to its non-homogeneousdistribution of graphene within the matrix material [16].In order to improve the dispersion of graphene and toobtain homogeneous mixtures of matrix and reinforcement,PM was one of the techniques usually preferred widely[11,17]. Also, usage of simple conventional PM, compactionand sintering techniques will be sufficient instead of usingmore expensive hot extrusion, vacuum hot pressing, andhot rolling processes. If achievable, this would allow andpave a newer way for conventional PM processes to bemore extensively used by industry and academia for nano-reinforced metal matrix composite production in large-scalecircumstances.

A thorough insight into the available literature gives aclear picture that the influence of reduced graphene oxide(r-GO) content on the mechanical properties of Mg-basedcomposites has been hardly reported so far. In the presentwork, a solvent-based powder processing technique was pro-posed for the development of novel composites. It is aninevitable fact that ball milling is not suitable for Mg-basedmaterials without an argon environment since it catches fireeasily. Based on these formulations, in the first step r-GO andAZ31 alloy powder were dispersed in an organic solvent sep-arately and then uniformly mixed by means of mechanicalstirring. The powder mixture is semi-dried, ball milled, cold

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pressed and then sintered at 560◦C. As a research outcome, thecomposite specimens developed under the hood of integrationof the existing schemes for underpinning manufacturingnovelty were investigated for their density, mechanical prop-erties, wear and corrosion properties.

2. Materials

2.1 Experimental procedure

2.1a Materials: Commercially available AZ31 alloy metalpowder was used as the base matrix material; its particlesize was measured by using scanning electron microscopy(SEM) (linear interpolation) and was found to be approxi-mately 400 µm. Table 1 shows the chemical proportion of theas-procured alloy powder. r-GO synthesized (up to ∼5.8 nmthickness) by adopting a modified Hummers method [17] forvarying wt% (0.2, 0.3, 0.4, 0.5) was used as reinforcementto improvise the basic and functional properties of the AZ31alloy. SEM micrographs of AZ31 alloy powder and r-GO areas shown in figure 1a and b.

2.1b Fabrication of AZ31/r-GO nanocomposites: AZ31alloy powder and r-GO dispersion into an organic solvent wasperformed in parallel and consequently ultrasonicated for 2 hduration. After this, the AZ31 alloy powder was stirred at900 rpm for 1 h followed by dropwise addition of r-GO intothe solution; the so-obtained slurry mixture was then stirredvigorously at 1600 rpm speed for 2 h. Afterwards, as perthe research postulate, under wet conditions the compositepowder was ball milled using sphere-shaped stainless steel

Table 1. Chemical proportion of AZ31 alloy metal powder.

Elements Al Zn Mn Fe Cu Si Ni Mgwt% 3.5 1.4 0.3 0.003 0.008 1.2 0.001 Balanced

balls (commonly conserved at a 1:10 powder ratio). Theball-milled composite powder was dried at 80◦C under anargon atmosphere. The dried composite powder was then pru-dently filled into a hollow cylinder of size 40 mm diameter; theinner surface of the cylinder was lined with graphite powderfor easier removal of product after the compaction process.After that, the filled composite powder was pressed at a com-paction pressure of 580 MPa at room temperature so as toachieve the green compacted composite specimen. These fab-ricated specimens were concealed in silica sand and furthersintered at a temperature of 560◦C in an argon environment[18]. Samples of these synthesized r-GO and composite spec-imens completely fabricated as per the project sequence areshown in figure 2a and b.

2.1c Material characterization: The presence ofreinforcement particles was confirmed by using an X-raydiffractometer (XRD) and SEM-assisted with energy dis-persive X-ray analysis (SEM/EDS). Vickers hardness of thedeveloped MMC is measured using a ZWICK Micro-hardnesstester in accordance with the ASTM standard E384-99.Archimedes’ principle was utilized to calculate the density ofthe developed MMC and the theoretical density of the sampleswere calculated by using the rule of mixture. The differencesin value between the experimental and theoretical outcomesfurnish the porosity value. A DUCOM TR-20 M-106 tri-bometer was used to measure the wear rate and coefficientof friction with constant control factors, i.e., at 5 N of appliedload, 1 m s−1 of sliding velocity and 500 m as the sliding dis-tance [12] based on ASTM: G99-05 standards. AUTOLAB,AUT85670 electrochemical workstation instrument was usedto study the corrosion behaviour of the fabricated MMC in0.1 M Na2SO4 electrolyte. Evaluation of corrosion rate iscarried out by

CR = KicorrEW

d

Figure 1. SEM micrograph of: (a) AZ31 alloy powder and (b) r-GO nanosheets.

Bull. Mater. Sci. (2019) 42:39 Page 3 of 9 39

Figure 2. Pictorial view of: (a) synthesized r-GO nanosheets and (b) sintered AZ31/r-GO composite.

where K is the corrosion rate constant (milli-inch per year),icorr is corrosion current density, EW is the equivalent weightof Mg, d is the material density, and polarization resistance(Rp) is calculated using the Stern–Geary equation [17].

3. Results and discussion

3.1 Microstructural characterization and XRD of thefabricated composite

It can be visualized from figure 3b that the microstructureexhibits a clear surface with a few micro-voids. This obser-vance of minimal micro-voids and cracks (figure 3aand c) may be mainly due to the variation in optimizedcompaction pressure applied during the PM process. A largenumber of cracks throughout the scanned surface could eas-ily be observed in figure 3c. Agglomeration of reinforcementparticles was not detected even at higher level of reinforce-ment; the development of micro-cracks may be owing to theconsequence of weaker bonding between the matrix and itsreinforcement particles. However, these facets persist evenfor increased reinforcement percentages, i.e., over and above0.5 wt% and hence this untried composite will obviouslydecrease the mechanical bonding that principally existswithin. This substantial happening leads to crack initiationswithin the developed composites. The XRD of the sinteredspecimen is depicted in figure 3d. Peaks consonant to r-GOnanosheets was immediate (24.54◦). The existence of alu-minium [11] was endorsed by the peak at 50◦ and other residuepeaks are the relevant peaks of Mg (JCPDS no. 35-0821).Addition of r-GO into the alloy powder results in appearanceof additional peaks for the MMC. Intensity of these peaks isvery low and only few peaks appear in the MMC which pos-sibly support a low-weight fraction of reinforcements in theMg matrix. Authentication of reinforcement and its disper-sion was conceded with the guidance of elementary mappingand the attained results are as illustrated in figure 4.

The existence of reinforcement in the matrix was confirmedwith the EDS image shown in figure 4d. Herein, traces of inter-metallic phase (i.e., carbide) formation could not be observedin the developed composite. In general, Mg-alloy contains

aluminium particles that have the tendency to form carbideduring sintering in the presence of carbon material [19]. Yetagain, it can be observed that carbon particles were uniformlydispersed in the base matrix. Hence, it is suppositious to statethat the solvent-based PM method has the capability to manu-facture nanoparticle-reinforced MMCs with uniform particledispersion.

3.2 Basic characterization of r-GO-reinforced MMC

As of now, no literature exists on reinforcing r-GO intothe AZ31 alloy matrix which acts as the driving task forresearchers to optimize the weight percentage of r-GO thatcan possibly be added into Mg alloys so as to attain bet-ter mechanical, corrosive and tribological properties. Basedon a trial and error method and out of expertise acquaintedwith such relevant line of investigations, the wt% of r-GOwas varied up to the level of 0.9%. It was assumed thatthe introduction of r-GO increased the porosity of thedeveloped specimens mainly because of lower mechanicalbonding between the matrix and the particles; such happen-ings obviously results in material failure. In view of thefact from figure 5a, the value of porosity exhibited by thespecimens with 0.6% r-GO was observed to be extremelyhigh and hence it can be explained that, as and whenthe weight percentage of r-GO increases anywhere beyond0.5 wt% the bonding between the particles might get weak-ened; and thereby reduce the properties of the developedspecimens. Thus, it was decided to study the properties indetail for the specimens with r-GO weight percentage wellbelow 0.6% and in that way monitor the effect of the sameon mechanical, corrosive and tribological properties of thedeveloped composite specimens. It can be visualized from fig-ure 5b that density of the developed composite reduces withrespect to accumulation of reinforcement. These facets can beendorsed to the inference of incorporation of reinforcementsinto the base matrix material that turn out to reduce densityvalues.

Yet another reason for this reduction in density can beexplained by means of a relative increase in the porosity levelas presented in figure 5b. This increase in porosity competes


Figure 3. SEM micrographs of: (a) AZ31/0.3 wt% r-GO, (b) AZ31/0.4 wt% r-GO, (c) AZ31/0.5 wt% r-GO and (d) XRD ofthe developed MMC.

Figure 4. X-ray pattern of: (a) Mg, (b) oxygen, (c) carbon and (d) EDS of 0.4 wt% r-GO.


Figure 5. Effect of r-GO wt% on (a) porosity, (b) density and (c) hardness.

primarily with the inefficient pressure provided during thecompaction process and/or possibly due to deficient bondingbetween the r-GO and the Mg matrix. It can be easilyvisualized from figure 5b that the 0.5 wt% r-GO-reinforcedMg composite exhibits more porosity than all other consider-ations. This was one of the prime obstacles that accompaniedthe fabrication of MMCs using ball milling and may con-ceivably be because of massive size variations between AZ31metal chips and r-GO nanosheets. Increment in r-GO additionhighlights the enormous distribution in the obtained data thatpredominantly owes to porosity.

Whatsoever, even mild increments of reinforcementsubstances added to the MMC besides condensing the basematrix density indicates that they are the effective sources forporosity increment when composite structure is considered.The micro-hardness results (figure 5c) revealed that incorpo-ration of reinforcement into the base matrix material helpsto reform the micro-hardness of the developed compositesup to 64.6 HV. Homogeneous distribution of strong r-GOcontrols the happening of localized deformation at some stageof indentation as well as encumbers the grain growth; this hap-pening could be recognized for eventual increase in compositehardness. Whatsoever, r-GO has better toughness and

hardness and thus for sure will increase the work hardeningbehaviour of the matrix.

Figure 5c illustrates that the developed sample with0.5 wt% r-GO is a sign of maximum hardness despite thefact that the same sample also accounts for lower increment.Any such observable fact perhaps is the subsequent effect thattakes place due to the inclusions of stronger r-GO particlesinto the Mg matrix.

3.3 Functional characterization of r-GO-reinforced MMC

3.3a Wear characterization: The wear rate of the fabricatedcomposites decreases with a considerable increase in the wt%of r-GO as seen from figure 6. The wear test measurementscarried out concludes in general that the wear rate of the basealloy specimen decreases until 0.4 wt% additions, owing tothe fact of r-GO presence. In the case of 0.3 and 0.4 wt% ofr-GO content, the wear rate is lower than that of the basealloy, whereas the wear rate is relatively high in the caseof 0.2 wt%. This noticeable reality may be due to the weakcoherence between the subsidized components of the newercomposite specimen. The hard r-GO particles reduce the wearrate to a great extent just by the hardness that arises with


Figure 6. Influence of r-GO wt% on wear rate and frictioncoefficient.

the composite; and at the same time r-GO being a part ofgraphite exhibits a self-lubricating property in turn leadingto the reduction of the wear rate. It can be witnessed that0.5 wt% r-GO exhibits a maximum wear rate which againpoints to the weak mechanical bonding that still existsbetween the base Mg alloy and r-GO. Due to weak bonding,the particles along with Mg powder get ploughed away in theintent of load application and velocity leading to an intensewear of the specimen. It can be seen from figure 6 that thecoefficient of friction value decreases with increase in r-GOwhich can further be accredited to the self-lubricating prop-erty of r-GO layers [20]. Beyond this, it continues to providea self-lubricating layer between the pin–disc interfaces andthereby reduces the friction created by the pin over the disccounter-face and by this means also reduces the coefficientof friction. But the friction coefficient of Mg composite rein-forced with 0.5 wt% of r-GO was seen to experience a slightincrease which may be due to the loosening out of particlesthat gravitate to tear away the developed self-lubricating layerthat exists between the pin–disc interfaces.

The wear out surface morphology of the base matrixmaterial as demonstrated in figure 7a gives further detailsabout the occurrence of both adhesive and abrasive wearmechanisms [21]. Ploughing out of the matrix material owingto abrasive wear was tremendously higher along the slid-ing distance. This incidence could easily be attributed tothe micro-void existence in samples besides their soft mate-rial properties. Evidence of plastic deformation caused byadhesive-based wear was noticed in figure 7b. These foremostexistences are predominantly due to the thermal softeningbehaviour of matrix material. If these were to be accurate, itcould well be said that the uniform dispersion of reinforce-ment particles into the base matrix material has extremelydiminished the plastic deformation of the fabricated MMC.Uniform dispersion of reinforcement particles was preferredfor succeeding enhanced wear behaviour as well as better

mechanical properties were hauled off by adopting solvent-based mixing pursued by a mechanical alloying approach.From figure 7d it could be visualized that plough out causedby abrasive wear was minimal; this might be owing to theuniform dispersion of reinforcement into the matrix material.Because addition of strong r-GO improvises the hardness ofMMC, these reinforcements carry away the load applied overthe specimens thus leading to decrement in their wear rate.Formation of tribolayer is seen in figure 7c. It can also bestated that the self-lubricating effect of graphene layers reducethe wear rate to a great extent [16]. Figure 7d shows the wornout surface of the AZ31 matrix reinforced with 0.5 wt% ofr-GO, and demonstrates an incremental wear rate which wasmostly because of abrasive wear only. These incidents accountfor the influence of low level of bonding between Mg and r-GO thereby leading to loosening of the sintered particles.

3.3b Corrosion behaviour: Under saline conditions, pureMg undergoes the chemical reaction as given below [22]:

Mg(s) → Mg2+(aq) + 2e− (anodic corrosion reaction) (1)

2H2O + 2e− → H2(g) + OH−(aq)

(cathodic corrosion reaction) (2)

Mg2+(aq) + 2OH−

(aq) → Mg(OH)2 (corrosion product) (3)

The equilibrium of material in the electrolyte can be observedusing open circuit potential (OCP) and the same measured forthe fabricated samples are tabulated in table 2. The values indi-cate that the OCP of AZ31 alloy becomes negative (−1.53 V)as compared with that of the composite material. However,the standard potential of the Mg electrode is 2.37 V, but mea-surement of their OCP as −1.53 V indicates the formation ofMg(OH)2 [23]. Increasing the value of OCP shows the pres-ence of the used metals for not as much of dissolving nature inthe electrolyte. Incidence of more negative OCP for Mg metalpoint towards their active behaviour which might possibly bethe reason that their oxidation into Mg2+ ion changes equilib-rium potential into more negative cathodic nature of Mg [24].

Tafel plots presented in figure 8 show that the polarizationcurve for 0.5 wt% gets overlapped above 0.3 wt%, enlighten-ing that corrosion resistance of the Mg composites tends todecrease with further increase in r-GO wt%.

In the Tafel polarization method, the anodic Tafel plotconstant (βa) entails the metal disbanding and a cathodic Tafelplot constant denotes the evolution of hydrogen ions duringthe corrosion process. If the hydrogen progression is high, thecorrosion potential gets shifted towards the cathodic region;likewise metal oxidation would be identified by shifting ofcorrosion potential towards the anodic region [25].

When the alloy strip was immersed in 0.1 MNa2SO4

solution, the corrosion potential was observed to be −1.416 V,corrosion current density value was 371.54 µA cm−2 andthe polarization resistance (Rp) was 108.6 � as tabulated intable 2. Whilst 0.2 wt% r-GO composite strip was immersed


Figure 7. SEM of worn out surface morphology of: (a) AZ31 alloy, (b) AZ31/0.3 wt% r-GO, (c) AZ31/0.4 wt% r-GO and(d) AZ31/0.5 wt% r-GO.

Table 2. The fitted electrochemical parameters of pure AZ31 alloy and its composites.

Samples OCP (V)

Cathodic Tafelconstant, βc(mV dec−1)

Anodic Tafelconstant, βa(mV dec−1)

Corrosionpotential, Ecorr

(V)

Corrosion currentdensity, Icorr(µA cm−2)

Polarizationresistance, Rp

(�)

Pure −1.53 271.4 141.3 −1.416 371.54 108.60.2 wt% −1.59 313.04 161.1 −1.456 992.08 465.70.3 wt% −1.57 419.6 255.9 −1.464 207.25 7550.4 wt% −1.52 319.3 127.3 −1.328 61.21 221.40.5 wt% −1.58 420.3 254.1 −1.464 207.41 754

into Na2SO4 solution, the corrosion potential shifts from−1.416 to −1.456 V towards the cathodic region, corrosionrate increases from 4.31 to 11.52 mpy and Rp value suddenlyincreases from 108.6 to 465.7 � [26]. After hosting 0.3%r-GO-coated Mg strip in the Na2SO4 solution, its Ecorr getsshifted from −1.456 to −1.464 V towards the cathodic region,whereas the corrosion rate drops down to 2.18 mpy and Rp

value increases to 755 �. Because of the chemical inertnessof r-GO, it acts as the ideal material to inhibit the corrosionrate of the metal. As presented in figure 8, when 0.4 wt% Mgstrip was immersed in the Na2SO4 solution, the corrosionpotential shifts to the tune of −1.328 V towards the anodicregion, the corrosion rate decreases to 0.71 mpy and Rp valueapproximates to 221.4 �. When 0.5 wt% MMC strip was

dipped in the Na2SO4 solution its Ecorr values get shiftedto −1.464 V towards the cathodic region, corrosion rateincreases to 2.08 mpy and Rp value increases to 754 �. Theabove detailed attributes of corrosion highly correlates withthe outcomes of the existing literature in this field and at thisjuncture it could be well defined that the r-GO has betterchemical stability which in turn diminishes the influence ofthe corrosive electrolyte; it also reduces the metal dissolutionby initiating the cathodic reaction in the composite thus result-ing in high corrosion resistance. However further increase inthe wt% of r-GO sheet results in cluster formation within andforms a graphite structure. This increases the electron flowbecause of which the corrosion current density increases thusincreasing the corrosion rate; it can also be observed from


Figure 8. Potentiodynamic polarization curves for the unrein-forced alloy and the composites with 0.2, 0.3, 0.4 and 0.5 wt% r-GO.

Figure 9. Effect of r-GO wt% on the corrosion rate of thecomposites.

figure 9 that 0.2 wt% exhibits a higher corrosion rate and thismight be due to the lower addition of reinforcement that hasthe potential to cause poor mechanical bonding between thematrix and reinforcement particles [18,27].

Inhibiting the efficiency of the developed MMC wascalculated based on the CR as illustrated in equation (4) [28]

η = CR (alloy) − CR (composite)

CR (alloy)× 100 (4)

From the above equation the inhibiting efficiency observedexhibits a negative value. On the other hand, 0.3 wt% r-GOcomposite has 45% and the composite containing 0.4 wt%r-GO has high-inhibiting efficiency (84%). From figure 9 itcould easily be confirmed that 0.4 wt% strip possesses

reasonable increment in corrosion resistance for 0.1 M ofNa2SO4 aqueous solution.

4. Conclusion

The conclusions of this study conducted through asolution-based PM technique for fabricating the AZ31/r-GOcomposite, with varying (between 0.2 and 0.5 wt%) r-GOcontent are as follows:

1. The combination of solvent-based powder mixingalong with mechanical ball milling, cold pressing andsintering that steps up for obtaining a new r-GO-reinforced Mg composite has provided a distinctiveopportunity to produce lightweight metal matrix com-posites.

2. Microstructure characterization studies show that r-GO nanosheets were uniformly distributed with goodbonding strength since no cracks were viewed in themicrograph. A few micro-voids were observed due todecrement in density.

3. Addition of r-GO (0.4 wt%) to the base alloy improvedthe hardness, wear and corrosion inhibition efficiencyup to 84%; however, further increase in the r-GOcontent decreased the wear rate. These newer andinnovatively processed composites are suitable forengineering applications such as brake shoes etc., andin certain other places where high-corrosion resistanceand wear resistance are most needed.

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Investigation of graphene-reinforced magnesium metal