materials Article

Tribological Properties of Aluminium Alloy Composites Reinforced with Multi-Layer Graphene—The Influence of Spark Plasma Texturing Process Marek Kostecki *, Jarosław Wo´zniak, Tomasz Cygan, Mateusz Petrus and Andrzej Olszyna Faculty of Materials Science and Engineering, Warsaw University of Technology, Wołoska St. 141, 02-507 Warsaw, Poland; [email protected] (J.W.); [email protected] (T.C.); [email protected] (M.P.); [email protected] (A.O.) * Correspondence: [email protected]; Tel.: +48-22 234-74-49 Received: 28 June 2017; Accepted: 5 August 2017; Published: 10 August 2017

Abstract: Self-lubricating composites are designed to obtain materials that reduce energy consumption, improve heat dissipation between moving bodies, and eliminate the need for external lubricants. The use of a solid lubricant in bulk composite material always involves a significant reduction in its mechanical properties, which is usually not an optimal solution. The growing interest in multilayer graphene (MLG), characterised by interesting properties as a component of composites, encouraged the authors to use it as an alternative solid lubricant in aluminium matrix composites instead of graphite. Aluminium alloy 6061 matrix composite reinforced with 2–15 vol % of MLG were synthesised by the spark plasma sintering process (SPS) and its modification, spark plasma texturing (SPT), involving deformation of the pre-sintered body in a larger diameter matrix. It was found that the application of the SPT method improves the density and hardness of the composites, resulting in improved tribological properties, particularly in the higher load regime. Keywords: aluminium matrix composites; multilayer graphene; SPS sintering; tribological properties

1. Introduction Composites based on aluminium and its alloys, compared to unreinforced material, offer a number of attractive properties, such as greater strength, improved stiffness, reduced density (weight), and improved temperature properties (creep resistance). As such, engineers are considering them more and more often, and the number of commercially available materials is constantly increasing [1]. The most important applications are related to the automotive and aerospace industries, where a significant increase in durability is needed while maintaining low weight for structural components [2]. Composites based on boron fibre–reinforced aluminium alloys have been known since the 1960s, when they were first used in aerial construction [3]. Since then, a number of materials, mainly ceramic, such as SiC, Al2 O3 , B4 C, Si3 N4 , AlN, TiC, TiB2 , and TiO2 , have been proposed to alter the properties of the aluminium matrix, often with very different morphology, ranging from equilibrium particles to whiskers or fibre forms [4]. Initially, the graphite fibres were used to reinforce polymer matrix composites because of the reactivity of carbon with magnesium and aluminium. In the 90s, hybrid composites emerged that, besides increased strength or creep resistance due to additional components, were characterised by increased tribological properties resulting from the addition of graphite solid lubricant [5,6]. The success achieved in these materials involved the use of completely new manufacturing methods to which we can include powder metallurgy [7,8]. The use of solid lubricants as an integral component of the composite allows the composite to achieve a self-lubricating effect. This process is the result of the formation of a thin film between Materials 2017, 10, 928; doi:10.3390/ma10080928

www.mdpi.com/journal/materials

Materials 2017, 10, 928

2 of 14

the surface of the material and the parts cooperating with it. This allows for significant reduction in the coefficient of friction (COF) due to the replacement of the external friction of the solids by the internal friction of the particulate lubricants [9]. Rohatgi et al. [10] first introduced graphite as a solid lubricant in aluminium matrices by casting routes. Much work has been done to modify and optimise the properties of such composites. Unfortunately, soft graphite or MoS2 particles commonly used in micron size are responsible for the simultaneous deterioration of mechanical properties such as hardness, modulus, and strength. Moreover, those particles are susceptible to the formation of defects and cracks, which results in premature failure in mechanical testing [11]. The single layer graphene cannot be the proper reinforcement for self-lubricating composites because of the absence of a layered structure. Therefore, the use of multilayer graphene may be the best way to modify the composite structure [12]. The presence of several to dozens of layers allows the film to be made, while thin flakes will not significantly reduce mechanical properties. A number of authors have attempted to produce such composites with good results. Although different authors have used morphologically differentiated multilayer graphene, in each of these cases, the process has improved mechanical properties such as tensile strength, [13–16] yield strength, [17–19], and compressive strength [20]. Moreover, the improvement in performance is even better than in the case of previously used carbon nanotubes [11]. The phenomenon can be explained as a results of 2D crystals’ good adhesion to the matrix and increased interfacial surface area, which restrains dislocation and crack propagation during tensile tests. An important fact confirmed experimentally is the small volume fraction of flakes required for improvement of these properties, which in most cases does not exceed 1%. After exceeding the barrier of 1% in weight, as in case of large particles, there is a decrease in mechanical properties related with strong agglomeration of flakes into 3D forms. This is a typical tendency of powder composites. Similar dependencies are associated with agglomeration with high MLG volume and were observed by authors for Al2 O3 ceramics composites [21]. Far fewer literary references can be found about the tribological properties of aluminium matrix composites with MLG participation [18,19]. However they point to the possibility of improving tribological properties and are difficult to compare because of different way of manufacture and different testing methods. Additionally for all the studied materials, graphene content does not exceed 3% by volume. Research conducted in the 70s’ [22] denote that the use of specific morphology of solid lubricant particles may be important for the friction process. Considerations concerning the geometry of the grease indicate that the ratio of the surface area of the basal planes to the edges of the particles is significant. Increasing this coefficient allows for a significant reduction in friction resistance. The expected effect of the presence of the MLG in the composite structure is a decrease in the coefficient of friction and a positive impact on the mechanical properties. It can be inferred that this will also contribute to a reduction in wear. However, the mechanisms responsible for wear processes are more complex. Authors in previous studies [23] have attempted to explain the effect of both the type of solid lubricant (multilayer graphene and molybdenum disulphide) and the amount of lubricant needed to produce a friction film and showed that the required amount of MLG needed to improve the tribological properties (COF and wear) is about 10%. This is far beyond the scope of the guarantee of increased mechanical properties. At the same time, it was indicated that the geometry of the flakes and aluminium powder particles can influence their specific orientation in the microstructure, which indirectly causes the applied axial pressure synthesis method. Based on previous research, we have attempted to change the manufacturing parameters by modifying the SPS process in the current considerations. The proposed change aims to obtain a composite characterised by better dispersion of MLG and their further orientation and improved mechanical properties. The applied modification is a process previously performed for other types of materials [24,25] and has been called Spark Plasma Texturing (SPT) because of the specific microstructure characterised by its possible texture. The purpose of the SPT process is to perform

Materials 2017, 10, 928 Materials 2017, 10, 928 Materials 2017, 10, 928

3 of 14 3 of 14 3 of 14

sintering with with a simultaneous dimension change, which which consistsconsists of plastic of the preSPS sintering a simultaneous dimension change, of deformation plastic deformation of sintering with in a simultaneous change, which consists of plastic deformation of the presintered body a die of diameter the compact. As a result, we aget a sinter the pre-sintered body in larger a diedimension of larger than diameter than the compact. As result, wewith get a larger sinter sintered body in a die ofand larger than the As a result, weinfluence get a sinter a larger diameter and diameter less height. The aim of the authors examine of with the obtained with a larger lessdiameter height. The aimcompact. ofwas the to authors wasthe to examine the influence of diameter and less height. The aim of the authors was to examine the influence of the obtained microstructure of composites multilayer ongraphene the tribological the AA6061 the obtained microstructure ofwith composites withgraphene multilayer on the properties tribologicalofproperties of microstructure of composites with multilayer graphene on the tribological properties of the AA6061 matrix. the AA6061 matrix. matrix. 2. Materials and Methods 2. Materials and Methods In order aluminium powder was used as order to toproduce producethe thecomposites, composites,Alpoco AlpocoAA6061 AA6061commercial commercial aluminium powder was used In order to produce the composites, Alpoco AA6061 commercial aluminium powder was used the matrix and multi-layered graphene flakes asas a asolid as the matrix and multi-layered graphene flakes solidlubricant. lubricant.Graphene Graphenepowder powderwith withthe the trade trade as the matrix multi-layered graphene flakes as(Calverton, a solid lubricant. Graphene powder the trade name Gn(12) and comes from Graphene NY, USA). Thewith morphology Supermarket New York, NY, name comes from Graphene (Calverton, New York, NY,below, USA). of the aluminium powder particles inin Figure 1. Based on on the image itThe can be that theGn(12) aluminium powder particlesisSupermarket isshown shown Figure 1. Based the image below, itmorphology canseen be seen of the aluminium powder particles is shown in Figure 1. Based on the image below, it can be seen the havehave different shapes. The The mean diameter of a of particle (d2 )(d is2)10.9 µm.μm. TheThe coefficient of thatparticles the particles different shapes. mean diameter a particle is 10.9 coefficient that the particles different shapes. The mean diameter of a particle (d2from ) isfrom 10.9 μm. The coefficient elongation (µ)(μ) forhave thethe particles is 1.46, which indicates a certain deviation thethe spherical shape. of elongation for particles is 1.46, which indicates a certain deviation spherical shape. of elongation (μ) for the particles is 1.46, which indicates a certain deviation from the spherical shape.

Figure 1. 1. Morphology Morphology of of AA6061 AA6061 powder. powder. Figure Figure 1. Morphology of AA6061 powder.

Figure 2 shows the morphology and transmission image of a cross section of the flake. The name Figure 22 shows shows the morphology morphology and and transmission transmission image of aa cross cross section section of of the flake. flake. The name name Figure of (Figure Gn(12) denotes that the the average thickness of one flake image is 12 nm 2b), aboutthe 30 singleThe graphene Gn(12) denotes that the average thickness of one flake is 12 nm (Figure 2b), about 30 single graphene Gn(12) that the average of onesize flake 12 nm (Figure4.5 2b), about 30 single graphene layers. denotes It was examined that the thickness average lateral is is approximately μm. layers. It was examined that the average lateral size is approximately 4.5 µm. layers. It was examined that the average lateral size is approximately 4.5 μm.

(a) (a)

(b) (b)

Figure 2. (a) STEM and (b) HRTEM micrographs of Gn(12) multilayer graphene (MLG) powder. Figure 2. (a) STEM and (b) HRTEM micrographs of Gn(12) multilayer graphene (MLG) powder. Figure 2. (a) STEM and (b) HRTEM micrographs of Gn(12) multilayer graphene (MLG) powder.

The Raman spectra of the graphene is presented. Based on the spectrum (Figure 3), the I2D/IG The spectra of the graphene is presented. Basedison the spectrum (Figuregraphene 3), the I2D /IG intensity Raman ratio is set at about 0.60. This value and peak shape typical for multi-layer [26]. The Raman spectra of the graphene is presented. Based on the spectrum (Figure 3), the I /I 2D G intensity ratio is set at about 0.60. This value and peak shape is typical for multi-layer graphene [26]. intensity ratio is set at about 0.60. This value and peak shape is typical for multi-layer graphene [26].

Materials 2017, 2017, 10, 10, 928 928

4 of 14

Materials 2017, 10, 928

4 of 14

Figure 3. Raman spectra of Gn(12) MLG powder. Figure 3. Raman spectra of Gn(12) MLG powder.

The first step in the manufacturing process was to prepare the powders’ specific chemical The first step in the manufacturing to MLG prepare the powders’ specific chemical Figure 3. Ramanprocess spectra ofwas Gn(12) powder. composition i.e., (0, 2, 5, 10, 15) vol % of MLG. Graphene content was identical for samples prepared in composition i.e., (0, 2, 5, 10, 15) vol % of MLG. Graphene content was identical for samples prepared the SPS and Then powders were mixed horizontal mill.specific The homogenizing TheSPT firstmethods. step the manufacturing process wasintoamixed prepareinthearotary powders’ chemical in the SPS and SPT inmethods. Then powders were horizontal rotary mill. The parameters used were: mixing time 12 h and rotational speed 300 rpm were previously optimized for composition i.e., (0, 2, 5, used 10, 15) vol % of MLG. Graphene content was identical for samples prepared homogenizing parameters were: mixing time 12 h and rotational speed 300 rpm were previously minimizing agglomeration. ZirconiaThen beads (2 mm in diameter) in a ratio of 3:1 rotary powder weight was in thefor SPS and SPT methods. powders horizontal optimized minimizing agglomeration. Zirconia were beadsmixed (2 mminin adiameter) in a ratio mill. of 3:1The powder homogenizing parameters used were: mixing time 12alcohol. h and rotational speed 300 rpm were previously used. Mixes were held in a suspension of isopropyl The resulting slurry was dried and then weight was used. Mixes were held in a suspension of isopropyl alcohol. The resulting slurry was optimized minimizing agglomeration. Zirconia beads (2 mm in diameter) in a ratio of 3:1 powder granulated on #for 0.3 mm sieves. dried and then granulated on # 0.3 mm sieves. weight was used. Mixes were held in of a suspension of isopropyl alcohol. The resulting slurrysecond was The last step was the the The last step was the consolidation consolidation of the powder powder -- in in the the first first by by the the SPS SPS method, method, the the second in in dried and then granulated on # 0.3 mm sieves. the in two stages. The parameters ofofthe SPS and SPT process were selected onon thethe basis of the SPT SPT method method in two stages. The parameters the SPS and SPT process were selected basis The last step was the consolidation of the powder - in the first by the SPS method, the second in optimisation work carried out for the Al/10Al/10 vol % MLG composition and pure Alpure alloy Al powder. of optimisation work carried outThe forselected the selected MLG composition and alloy the SPT method in two stages. parameters of the SPS vol and % SPT process were selected on the basis The intention of the authors was to obtain the obtain highestthe relative density of composites. These parameters powder. The intention ofcarried the authors highest relative density and of composites. of optimisation work out forwas the to selected Al/10 vol % MLG composition pure Al alloyThese are shown inare Table 1. Figure 4 shows schematically the SPT process and its parameters. powder. The intention of the authors was to obtain the highest relative density of composites. These parameters shown in Table 1. Figure 4 shows schematically the SPT process and its parameters. parameters are shown in Table 1. Figure 4 shows schematically the SPT process and its parameters. Table 1. Spark plasma sintering (SPS) and spark plasma texturing (SPT) process parameters. Table 1. Spark plasma sintering (SPS) and spark plasma texturing (SPT) process parameters. Table 1. Spark plasma sintering (SPS) and spark plasma texturing (SPT) process parameters.

SPS SPS

SPT SPT

SPT SPS Stage 11 Stage 2 2 Stage Stage 1 StageStage 2 Temperature (°C) 550 300 550 ◦ Temperature 550 300 550 550 Temperature ( C) (°C) 550 300 Pressing force 35 3 Pressing force (kN) 33 35 35 35 Pressing force (kN)(kN) 3535 Pressing time (min) 2 2 Pressing time (min) 2 2 8 8 8 Pressing time (min) 2 2 Atmosphere Argon Argon Argon Argon Atmosphere Argon Argon Atmosphere Argon Argon Argon Heating (°C/min) 150 150 150 100100100 Heating (°C/min) 150 Heating raterate (◦rate C/min) 150 (mm) Die diameter 2020 30 30 30 DieDie diameter (mm) 3030 (mm) diameter 30

Figure4.4.SPT SPT processing processing scheme. Figure scheme.

Figure 4. SPT processing scheme.

Materials 2017, 10, 928

5 of 14

The obtained sinters were mechanically grinded and polished down to a grit size of 0.2 mm and subjected to further investigations. Fundamental properties of the obtained materials, such as density (Ultrapycnometer 1000 helium pycnometer Quantachrome Instruments), and hardness (Vickers Hardness Tester FV-700e, Future-Tech, Kawasaki-City, Japan) were measured. XRD analysis was performed to determine the orientation of graphene flakes. These tests were performed with the same parameters on two identical volumes of composites with 10 vol % Gn(12) after the SPS and SPT process. For this purpose, a Bruker D8 Discover (Germany) X-ray diffractometer was used, working with a 0.1540 nm wavelength Cu tube. The test was carried out at room temperature. The angular range was 10◦ –120◦ , and the counting time was 5 s. Quantitative texture analysis was based on three incomplete pole figures: (111), (200), and (220). The measurements were made using a Bruker D8 Discover X-ray diffractometer filtered Co Kα radiation. Based on the measured pole figures for each sample, the orientation distribution functions were obtained and quantitative ratios of the major texture components were calculated. Tribological tests were performed by a ball-on-disc type tribometer (T-21 tribotester, ITeE-PIB Radom, Poland) at room temperature (23 ◦ C) and humidity 35%, using 0.1 m/s sliding speed and normal loads from 1 to 10 N. The tests were carried out for 1000 revolutions. As a counterpart, 100Cr6 steel (10 mm in diameter) balls were selected. The coefficient of friction and wear ratio were determined according to the requirements defined in ASTM G 99-05 [27]. Each test was repeated at least three times per sample, and the average measurements value is shown in the graph. The worn surfaces, microstructure of composites, and substrates morphology were observed using scanning electron microscopes (Hitachi S3500 and S5500, Japan). Raman analysis was made using a Nicolet Almega XRD dispersive Raman spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) (laser with an excitation wavelength of 532 nm), and at least three scans for each sample were performed. 3. Results and Discussion The most important differences in materials produced by SPS and SPT methods should relate to the dispersion of MLG particles in the composite volume. Figure 5 shows images of microstructures of composites obtained in various processes, indicating the cross sections parallel to the direction of acting force (perpendicular to the surface of the die stamp). Dark areas are places of occurrence of carbon (MLG) agglomerate, also thin aluminium powder particle boundaries are partly visible. Many of the fine pores and phases of Mgx Siy identified earlier in the XRD study are visible as dispersed dark objects in the middle of light areas (aluminium powder particles). The apparently fibrous structure of graphene flakes indicates their orientation in the volume of the composite. They are arranged in planes perpendicular to the direction of the operating force of compression, in a manner typical of powdered composites with particles of morphology of the flakes. At the homogenization stage of the suspension, the graphene flakes “stick” to slightly flattened aluminium powder particles (Figure 6). This causes a specific orientation of the graphene in the composite fixed by axial pressure in the SPS process. When some changes in the deformation scheme occur in the SPT process, areas rich in agglomerates of multi-layered graphene flakes become displacing and delaminating. Stereological analysis on cross sections indicates a significant reduction in thickness of graphene flakes agglomerates. In the composites with 15% volume, the average thickness of agglomerates changes from 5.4 µm (SPS) to 3.6 µm (SPT). Present in SPS composites, the large pores caused by the presence of MLG agglomerates (Figure 5D1) disappear almost completely in SPT composites. At the same time, this process causes the changes of the graphene plane’s orientation from an axis perpendicular to the force direction, as a result of the free movement of the particles in the larger matrix until the volume is completely filled. Only small amounts of pores resulting from non-uniform delamination of MLG flakes remain in the structure, as shown in Figure 7.

Materials 2017, 10, 928

6 of 14

Figure 5. Microstructure of AA6061/MLG composites obtain in SPS (A1—2 vol %; B1—5 vol %; C1—10 vol %; D1—15 vol %) and SPT (A2—2 vol %; B2—5 vol %; C2—10 vol %; D2—15 vol %) process.

Due to the expected and observed displacements of MLG during the texturing process, diffraction tests for standard and textured samples were performed. A comparison of the intensity of certain carbon peaks should indicate a change in the orientation of the flakes in the volume of the composite. In order to ensure comparable measurement conditions, two sections were prepared for each 1 volume were cut. Figure 8 presents diffraction composition, and samples of identical geometry and patterns for SPS and SPT samples recorded on similar cross sections. The record indicates the significant conformity of the identified carbon to the graphite. There was no difference in peak intensity from plane

Materials 2017, 10, 928

7 of 14

Materials 2017, 10, 928 Materials 2017, 10, 928

7 of 14 7 of 14

(002), 2θ = 26.67, indicating a similar orientation of the flakes on the examined sections. Diffraction sections. Diffraction analysis also confirms the absence of aluminium carbides that could form during analysis also confirms the absence of aluminium carbides could form during thecould sintering sections. Diffraction also confirms the absence of that aluminium carbides that formprocess. during the sintering process.analysis the sintering process.

Figure 6. Morphology of AA6061/MLG powder mixture after homogenization. Figure powder mixture mixture after after homogenization. homogenization. Figure 6. 6. Morphology Morphology of of AA6061/MLG AA6061/MLG powder

Figure 7. Microstructure of AA6061/10 vol % MLG composite obtain in SPT process. Figure 7. 7. Microstructure Microstructure of of AA6061/10 AA6061/10 vol Figure vol % % MLG MLG composite composite obtain obtain in in SPT SPT process. process.

Figure 8. 8. Diffraction Diffraction pattern pattern of of SPS SPS (red (red line) line) and and SPT SPT (black (black line) vol % % MLG MLG composites. composites. Figure line) AA6061/10 AA6061/10 vol Figure 8. Diffraction pattern of SPS (red line) and SPT (black line) AA6061/10 vol % MLG composites.

Texture analysis for for aluminium aluminium grain grain was was made made for Texture analysis for four four samples samples with with aa varying varying volume volume content content Texture analysis for aluminium grain was made for four samples with a varying volume content of of multilayer multilayer graphene graphene at at their their midpoint midpoint and and at at the the edges. edges. The The Figure Figure 99 shows shows the the exemplary exemplary pole pole of multilayer grapheneand at their midpoint and at the edges. The Figure 9 shows the exemplary pole figures (experimental computational) for the Al/10 vol % MLG composite. On the basis of figures (experimental and computational) for the Al/10 vol % MLG composite. On the basis of the the figures (experimental andthat computational) for resulting the Al/10 texture vol % MLG On the basis the research, it can be stated in all cases, the for Alcomposite. is poorly shaped, whichofmay research, it can be stated that in all cases, the resulting texture for Al is poorly shaped, which may

Materials 2017, 10, 928

8 of 14

Materials 2017, 10, 928 Materials 2017, 10, 928

8 of 14 8 of 14

research, it can be stated that in all cases, the resulting texture for Al is poorly shaped, which may prove to be low value on the scale. It is also not possible to find the correlation of changes in texture prove low on It also to the correlation of changes in texture prove to to be beintensity low value value on the the scale. scale. It is iscontribution also not not possible possible to find find in thethe correlation component depending on the of graphene matrix. of changes in texture component component intensity intensity depending depending on on the the contribution contribution of of graphene graphene in in the the matrix. matrix.

Figure9.9.Pole Polefigures: figures:calculated calculated(a) (a)and andexperimental experimental(b)—(111), (b)—(111),(200), (200),(220) (220)for forAA6061/10 AA6061/10vol vol%% Figure Figure 9. Pole figures: calculated (a) and experimental (b)—(111), (200), (220) for AA6061/10 vol % MLG composites obtain in SPT process. MLG composites obtain in SPT process. MLG composites obtain in SPT process.

The Thepreviously previouslyobserved observedabsence absenceofoflarge largepores poreswithin withinthe theagglomerates agglomerateson onthe themicrostructure microstructure The previously observed absence of large pores within the agglomerates on the microstructure after afterthe theSPT SPTprocess processhas hassome someconsequences consequencesininaasignificant significantincrease increasein indensity. density. Density Densityincreases increases after the SPT process has some consequences in acase significant increasegraphene, in density. Density increases throughout throughoutthe therange rangeofofcompositions, compositions,and andininthe the caseofof10 10vol vol%%of of graphene,we wecan canobserve observean an throughout the range of compositions, and in the case of 10 vol % of graphene, we can observe an increase in relative density by 3.3%. (Figure 10). increase in relative density by 3.3%. (Figure 10). increase in relative density by 3.3%. (Figure 10).

Figure10. 10.Relative Relativedensity densityofofAA6061/MLG AA6061/MLGcomposites compositesobtain obtainininSPS SPSand andSPT SPTprocess. process. Figure Figure 10. Relative density of AA6061/MLG composites obtain in SPS and SPT process.

The with the hardness test results. However, thethe hardness increase is Thedensity densityfactor factorisiscorrelated correlated with the hardness test results. However, hardness increase The density factor is correlated with the hardness test results. However, the hardness increase is not spectacular. SPT composites with a small volume fraction is not spectacular. SPT composites with a small volume fractionofofmultilayer multilayergraphene graphene(2%) (2%)have haveaa not spectacular. SPT composites with a small graphene volume fraction of (Figure multilayer graphene (2%) have a slightly slightlyhigher higherhardness hardnessthan thansamples sampleswithout without grapheneaddition addition (Figure11). 11).With Withthe theincrease increaseinin slightly higher hardness than samples without graphene addition (Figure 11). With the increase in the theamount amountofofflakes, flakes,the thecomposite compositehardness hardnessdecreases decreasestoto37 37HV1 HV1for forthe thecomposite compositewith with1515vol vol%% the amount of flakes, the composite hardness decreases to 37 HV1 for the composite with 15 vol % MLG MLGversus versus56 56HV1 HV1for forthe thecomposite compositewith with22vol vol%%MLG, MLG,but butthe thehigher higherhardness hardnessofofthe thecomposites composites MLG versus 56 HV1 for the composite with 2 vol % MLG, but the higher hardness of the composites produced producedin inthe theSPT SPTprocess processisisvisible visiblein inthe thewhole wholerange rangeof ofcompositions. compositions. produced in the SPT process is visible in the whole range of compositions.

Materials 2017, 10, 928 Materials2017, 2017,10, 10,928 928 Materials

9 of 14 9 9ofof1414

Figure Figure 11. 11. Vickers Vickers hardness hardness of of AA6061/MLG AA6061/MLGcomposites compositesobtain obtainin inSPS SPSand andSPT SPTprocess. process. Figure 11. Vickers hardness of AA6061/MLG composites obtain in SPS and SPT process.

An analysis of the tribological properties was carried out, taking into account the coefficient of Ananalysis analysisofofthe the tribological properties was carried out, taking account the coefficient An was carried out, taking intointo account theof coefficient of friction, wear rate, andtribological the nature properties of worn surface. Figure 12a,b shows the results the friction of friction, wear rate, and the nature of worn surface. Figure 12a,b shows the results of the friction friction, wear rate,toand nature of worn surface. Figure 12a,b shows the results of the friction coefficient related thethe friction load. coefficientrelated relatedto tothe thefriction frictionload. load. coefficient

(a) (a)

(b) (b)

Figure Figure 12. 12. Coefficient Coefficient of of friction friction of of AA6061/MLG AA6061/MLGcomposites compositesobtain obtainin inSPS SPS(a) (a)and andSPT SPT(b) (b)process. process. Figure 12. Coefficient of friction of AA6061/MLG composites obtain in SPS (a) and SPT (b) process.

Comparing the diagrams diagrams(Figure (Figure 12), say changes that changes the consolidation Comparing the 12), wewe cancan say that in the in consolidation process process resulted Comparing the diagrams (Figure 12), we can say that changes in the consolidation resulted in a relatively large reduction in coefficient friction coefficient for materials withvolume a small volume fraction in a relatively large reduction in friction for materials with a small fractionprocess of MLG. resulted inThe a relatively large reduction in materials within a small volume fraction of MLG. most likely mechanism of friction improving COF isfor associated with an increase in hardness. The most likely mechanism of improving COF iscoefficient associated with an increase hardness. For example of MLG. The COF mostof likely of COF is1.3 associated anHardness increase in hardness. For example composites a 2improving vol % of MLG decreased 1.3 (SPS) to 1 (SPT). Hardness COF of composites with amechanism 2 vol %with of MLG decreased from (SPS)from to 1with (SPT). has a stronger For example COF of composites with a 2 vol % of MLG decreased from 1.3 (SPS) to 1 (SPT). Hardness has a stronger effect onofthe decrease COFofat1low of 1 andit3N it is also effect on the decrease COF at lowof loads andloads 3N although is although also noticeable fornoticeable 5 N load. for has stronger on the decreasethat of COF at low of loads 1 and 3N although it is also noticeable forof 5 Naload. It shouldeffect be noted, however, regardless the of consolidation method, the small addition 5MLG N load. It composites should be noted, that regardless ofunreinforced the consolidation method, the small addition of (2 vol however, %) has a higher COF than sinters. The thinnest agglomerates It composites should be noted, thatcontribute regardless the consolidation method, the small addition of MLG (2 vol however, %) has higher COF than unreinforced sinters. The agglomerates observed in these composites doanot toof create lasting tribofilm on thethinnest surface. Furthermore, MLG vol %)there has higher COF than unreinforced sinters. The thinnest agglomerates observed in these composites doaisnot contribute totocreate lasting tribofilm on the surface. Furthermore, in thecomposites shear stress(2 regime, a good chance form monolayer graphene that, due to the adhesive observed in these composites do not contribute to create lasting tribofilm on the surface. Furthermore, in the shear stress regime,surface, there iswill a good to form of monolayer graphene that,Asdue to the bonding to the aluminium form chance a conglomerate considerable hardness. a result of in the shear stress to regime, there is asurface, good chance to aform monolayerofgraphene that,hardness. dueand, to the adhesive bonding aluminium will form conglomerate considerable Asa the cyclical impact ofthe contact stresses (surface fatigue), the conglomerate will be detached as adhesive the aluminium surface, will form a conglomerate of wear considerable hardness. As ahard result ofbonding the cyclical impact of contact stresses fatigue), the conglomerate be detached particle, willto increase friction resistance and(surface contribute to increased of thewill composite. This aand, result of the cyclical impact of contact stresses (surface fatigue), the conglomerate will be detached as a hard will increase frictionlater resistance and contribute to increased wear of the is confirmed by particle, the results of wear presented in the article. and, as a hard particle, will increase friction resistance and contribute tointeresting increased effect wear of of load the composite. This is confirmed by the results of wear presented later in the For composites with high volume fraction 10–15 vol % MLG, anarticle. composite. This is confirmed by the results of wear presented later in the article. For composites with was highobtained, volume and fraction 10–15 vol has % MLG, an interesting effect load independence on the COF the SPT process only slightly intensified this of relation. For composites high volume fraction 10–15 vol % MLG, an interesting effect of load independence was obtained, the SPT process has only slightly intensified this In the range of 1on to the 10 with N,COF COF is maintained atand a constant value close to 0.2. The dominant mechanism of independence on the COF was obtained, and the SPT process has only slightly intensified this relation. In the range of 1 to 10 N, COF is maintained at a constant value close to 0.2. The dominant COF improvement is due to the presence of self-healing tribofilm. The general tendency of the decrease relation. In the 1 to an 10 increasing N, COF is maintained at a constant value closetribofilm. togiven 0.2. The dominant mechanism of range COF of improvement is due to was the retained presence of all self-healing The general in frictional resistance with load for composites in conditions. mechanism of COF improvement is due to the presence of self-healing tribofilm. The tendency the decrease in frictional resistance annature increasing for all In theoffollowing images (Figure 13), we can with see the of theload wearwas for retained AA6061general sinters. tendency of the decrease in frictional resistance with an increasing load was retained for all composites in given conditions. The type of wear can be described as adhesion with significant plastic deformation in the contact composites in given conditions.

Materials 2017, 10, 928

10 of 14

Materials 2017, 10, 928 Materials 10, 928 In 2017, the following

10 of 14

10 of 14 images (Figure 13), we can see the nature of the wear for AA6061 sinters. The type In of the wear can be described as adhesion plastic deformation in the contact following images (Figure 13), wewith can significant see the nature of the wear for AA6061 sinters.zone. The zone.nature The nature the friction process does not significantly for samples produced by the thezone. SPT In the following images (Figure 13), wewith canchange see significantly the nature offor the wear forproduced AA6061 sinters. The The ofcan theof friction process does not change samples by SPT type of wear be described as adhesion significant plastic deformation in the contact method, although much less plastic deformation is visible. type of wear described as adhesion significant plasticfor deformation in the contact method, although much less plastic deformation is visible. The nature ofcan thebe friction process does notwith change significantly samples produced by thezone. SPT An addition of 2 vol % MLG changes the nature of the wear process (Figure 14). Numerous The nature of the friction process does not change significantly for samples produced by the SPT An addition 2 vol % plastic MLG changes the nature of the wear process (Figure 14). Numerous method, although of much less deformation is visible. grooves, delamination, and a large number of flakes and round particles (wear products) are evident, method, although much less plastic deformation is visible. grooves, delamination, and%a large flakes and of round particles (wear(Figure products) evident, An addition of 2 vol MLGnumber changesofthe nature the wear process 14).are Numerous suggesting participation of abrasive wear mechanism (Figure 15). An addition of 2 vol % MLG changes the nature of the wear process (Figure 14). Numerous suggesting participation of abrasive wear mechanism (Figure 15). grooves, delamination, and a large number of flakes and round particles (wear products) are evident, grooves, delamination, a large number of flakes and round15). particles (wear products) are evident, suggesting participationand of abrasive wear mechanism (Figure suggesting participation of abrasive wear mechanism (Figure 15).

Figure 13. Worn Surface morphology for AA6061 load 1N (a) SPS and (b) SPT. Figure 13. Worn Surface morphology for AA6061 load 1N (a) SPS and (b) SPT. Figure 13. Worn Surface morphology for AA6061 load 1N (a) SPS and (b) SPT. Figure 13. Worn Surface morphology for AA6061 load 1N (a) SPS and (b) SPT.

Figure 14. Worn Surface morphology for AA6061 + 2 vol % MLG load 1N (a) SPS and (b) SPT. Figure 14. Worn Surface morphology for AA6061 + 2 vol % MLG load 1N (a) SPS and (b) SPT. Figure 14. 14. Worn Worn Surface Surface morphology morphology for vol % % MLG MLG load load 1N 1N (a) (a) SPS SPS and and (b) (b) SPT. SPT. Figure for AA6061 AA6061 + + 22 vol

Figure 15. Worn Surface morphology for SPS AA6061+2 vol % MLG load 5N. Figure 15. Worn Surface morphology for SPS AA6061+2 vol % MLG load 5N. Figure Worn Surface morphology for SPS SPS AA6061+2 vol % MLG load 5N. of increasing For all composites, observe typical situations of AA6061+2 increased wear as theload function Figure 15. 15.we Worn Surface morphology for vol % MLG 5N. load For (Figure 16). A high we increase in the wear for composites with additions of function 2% and 5ofvol % MLG all composites, observe typical situations of increased wear as the increasing For all composites, we observe typical situations of increased wear as the function of increasing confirms thecomposites, previously proposed ofsituations the presence of with Al/graphene conglomerates. Figure 15 load (Figure 16). A high we increase intheory the wear for composites additions of function 2% and 5ofvol % MLG For all observe typical of increased wear as the increasing load (Figure 16). A high increase intheory the wear for composites with additions of 2% and 5covering vol % MLG shows the presence of fine particles of approximately 5–15 μm in average, uniformly the confirms the previously proposed of the presence of Al/graphene conglomerates. Figure 15 load (Figure 16). A high increase in the wear for composites with additions of 2% and 5 vol % MLG confirms the previously proposed theory of the presence ofμm Al/graphene conglomerates. Figurethe 15 worn surface. shows the presence of fine particles of approximately 5–15 in average, uniformly covering confirms the previously proposed theory of the presence of Al/graphene conglomerates. Figure 15 shows the presence of fine particles of approximately 5–15 μm in average, uniformly covering the worn surface. worn surface.

Materials 2017, 10, 928

11 of 14

Materials 2017, 10,10, 928 1114 of 14 Materials 2017, 928 of fine particles of approximately 5–15 µm in average, uniformly covering 11 of shows the presence the Materials 2017, 10, 928 11 of 14 worn surface.

(a) (a) (a)

(b) (b) (b)

Figure 16. Wear rate of AA6061/MLG composites obtain in SPS (a) and SPT (b) process. Figure 16. Wear rate of AA6061/MLG composites obtain in SPS (a) and SPT (b) process. Figure Wearrate rateofofAA6061/MLG AA6061/MLG composites (b)(b) process. Figure 16.16. Wear compositesobtain obtainininSPS SPS(a) (a)and andSPT SPT process.

However, when analysing the wear test results, particular attention should be paid to However, when analysingthethe wear test results, particular attention should However, when analysing wear test results, particular should be paid topaid composites However, when the wear test results, particular attention should bebepaid to to composites subject to analysing wear processes at higher loads. In theattention case of the SPS process, wear composites subject to wear processes at higher loads. In the case of the SPS process, wear subject to wear processes at higher loads.at Inhigher the case of only the wear reduction compared to composites subject totowear processes loads. In SPS the case of the SPS process, wear reduction compared AA6061 sinter was observed for process, composites with high volume reduction compared to AA6061 sinter was observed only for composites with high volume reduction compared to AA6061 sinter was observed only for composites with high volume AA6061 sinter was(10% observed onlyand for only composites volume fraction MLGindicates (10% and 15%) fraction of MLG and 15%) for low with loads.high Analysis of wear traceofmarks fraction of MLG (10% and 15%) only for low loads. Analysis wear trace marks indicates fraction of MLG (10%of and 15%)and and only forprocess low loads. Analysis ofof wear trace marks indicates and only for low loads. Analysis of wear trace marks indicates the significant effect porosity on the the significant effect porosity on the wear (Figures 17 and 18). We can seeof numerous the significant effect of porosity on the wear process (Figures 17 and 18). We can see numerous the significant effect of porosity on 17 and 18). We canlarger see numerous cracks beginning at the edge18). of the pores that initiate(Figures the process of peeling off pieces of that wear process (Figures 17 and Wethe canwear see numerous cracks beginning at the edge of the pores cracks beginning atof edge of pores that initiate the process process peeling offlarger larger pieces cracks beginning atthe the edge ofthe the pores thatof initiate the ofofpeeling pieces of of material in mechanisms of fatigue and abrasion. initiate the process peeling off larger pieces material in mechanisms of off fatigue and abrasion. material inin mechanisms material mechanismsofoffatigue fatigueand and abrasion. abrasion.

(a) (a)

(b) (b)

Figure 17. Worn Surface morphology for AA 6061 + 10 vol % MLG load 10N SPS. (a) Wear trace image; Figure 17. Worn Surface(a) morphology for AA 6061 + 10 vol % MLG load 10N(b) SPS. (a) Wear trace image; Figure Worn Surface and (b)17. close-up view. morphology for AA 6061 + 10 vol % MLG load 10N SPS. (a) Wear trace image; and (b)17. close-up view. morphology for AA 6061 + 10 vol % MLG load 10N SPS. (a) Wear trace image; Figure Worn Surface and (b) close-up view. and (b) close-up view.

(a) (b) (a) (b) Figure 18. Worn Surface morphology for AA 6061 + 15 vol % MLG load 10N SPS. (a) Wear trace image; Figure 18. Worn Surface morphology for AA 6061 + 15 vol % MLG load 10N SPS. (a) Wear trace image; and (b) view.(a) Figure 18.close-up Worn Surface morphology for AA 6061 + 15 vol % MLG load 10N(b) SPS. (a) Wear trace image; and (b) close-up view. and (b)18. close-up view. morphology for AA 6061 + 15 vol % MLG load 10N SPS. (a) Wear trace image; Figure Worn Surface

and (b) close-up view.

Materials 2017, 10, 928

12 of 14

Materials 2017, 10, 928 Materials 2017, 10, 928

12 of 14 12 of 14

The use use of of the the SPT reduced the the thickness of MLG and the the number number of of The SPT technique technique reduced thickness of MLG agglomerates agglomerates and pores within these agglomerates. At the same time, the total surface of particle boundaries occupied use these of theagglomerates. SPT techniqueAtreduced the thickness of surface MLG agglomerates and the number of poresThe within the same time, the total of particle boundaries occupied by the solid lubricant increased. This makes it easier to access the carbon flakes during the friction pores within these agglomerates. At the same time, the total surface of particle boundaries occupied by the solid lubricant increased. This makes it easier to access the carbon flakes during the friction process and lubricant to the creating aa tribofilm. The observed wornduring surfaces are more by the solid increased. Thisof makes it easier to access theobserved carbon flakes the friction process and to maintain maintain the process process of creating tribofilm. The worn surfaces are more homogeneous. Direct abrasive damage is not visible in the plastic deformation zone. We can observe process and to Direct maintain the process a tribofilm. The deformation observed worn surfaces more homogeneous. abrasive damageofiscreating not visible in the plastic zone. We canare observe spalling with local interruption of the lubricant film especially for composites containing 10 and 15 homogeneous. Direct abrasive damage is not visible in the plastic deformation zone. We can observe spalling with local interruption of the lubricant film especially for composites containing 10 andvol 15 % of%MLG (Figures 19 and 20). 20). spalling local interruption of the lubricant film especially for composites containing 10 and 15 vol of with MLG (Figures 19 and vol % of MLG (Figures 19 and 20).

(a) (b) (a) (b) Figure 19. Worn Surface morphology for AA 6061 + 10 vol % MLG—load 10N SPT. (a) Wear trace Figure 19. Worn Surface morphology for AA 6061 + 10 vol % MLG—load 10N SPT. (a) Wear trace Figure and 19. Worn Surface morphology for AA 6061 + 10 vol % MLG—load 10N SPT. (a) Wear trace image; image; and (b) (b) close-up close-up view. view. image; and (b) close-up view.

(a) (b) (a) (b) Figure 20. Worn Surface morphology for AA 6061 + 15 vol % MLG—load 10N SPT. (a) Wear trace Figure 20. Worn Surface morphology image; (b) close-up Figure and 20. Worn Surfaceview. morphology for for AA AA 6061 6061 ++ 15 15 vol vol % % MLG—load MLG—load 10N 10N SPT. SPT. (a) (a) Wear Wear trace trace image; and (b) close-up view. image; and (b) close-up view.

4. Conclusions 4. Conclusions 4. Conclusions The addition of above 2 vol % of multilayer graphene to an aluminium alloy matrix causes The addition above 2 vol %inoftheir multilayer graphene to an aluminium alloy matrix causes composite porosityof a decrease mechanical properties as hardness) as a result of The addition ofand above 2 vol % of multilayer graphene to an(such aluminium alloy matrix causes composite porosity and a decrease in their mechanical properties (such aslateral hardness) as a result of strong agglomeration in the form of relatively large flakes in thickness and size. In the case of composite porosity and a decrease in their mechanical properties (such as hardness) as a result strong agglomeration in the form of relatively large flakes in thickness and lateral size. In the case of astrong smallagglomeration volume fraction we observe a small in hardness thatsize. can be caused in of thegraphene, form of relatively large flakesincrease in thickness and lateral In the caseby of a small volume fraction of graphene, we observe athe small increase in hardness that canofbe caused by two factors: limitation of grain size growth during sintering process, and blocking dislocations a small volume fraction of graphene, we observe a small increase in hardness that can be caused by two factors: limitation of grain size growth during sintering and blocking of dislocations motion at the boundaries of aluminium grains. Thethe use of MLGprocess, as an additive in aluminium matrix two factors: limitation of grain size growth during the sintering process, and blocking of dislocations motion at the boundaries of aluminium grains. The use of MLG as an additive in aluminium matrix composites significantly their grains. tribological properties, friction coefficients, and wear rate, motion at the boundariesimproves of aluminium The use of MLG as an additive in aluminium matrix composites significantly improves their tribological properties, friction coefficients, and wear rate, especially for composites with 10 vol % and 15 vol % graphene. With the increase of graphene content composites significantly improves their tribological properties, friction coefficients, and wear rate, especially for composites with 10wear vol % and 15 volis%observed. graphene. With the increase content in composites, the changewith in the10 mechanism abrasive natureof ofgraphene wear disappears especially for composites vol % and 15 vol % graphene.The With the increase of graphene content in composites, the change in the wear mechanism is observed. The abrasive nature of wear disappears after the addition of about 10 vol % MLG and self-healing tribofilm appears. in composites, the change in the wear mechanism is observed. The abrasive nature of wear disappears after The the addition about 10 vol % MLG and self-healing tribofilm appears. use of theof method it possible to produce composites with higher relative density after the addition ofSPT about 10 volmakes % MLG and self-healing tribofilm appears. The use of the SPT method makes it possible to produce composites with higher relative compared to SPS materials. The SPT process results in a more homogeneous structure anddensity better compared to SPS materials. The SPT process results in a more homogeneous structure and better

Materials 2017, 10, 928

13 of 14

The use of the SPT method makes it possible to produce composites with higher relative density compared to SPS materials. The SPT process results in a more homogeneous structure and better dispersion of graphene, which results in a reduction in the number of pores in the agglomerates and an increase in the surface covered with flakes on which a tribofilm is formed. The SPT method may be a compromise between improving tribological properties and retaining the good mechanical properties of the composites. The process of optimisation of the composite properties proposed in this paper should take into account further work to ensure the anisotropic microstructure, the directional dispersion of the flakes, and the reduction of their thickness. This can be achieved using a higher degree of processing in the SPT method. Acknowledgments: Financial support from the National Science Centre (Grant No. UMO-2014/15/D/ST8/02643) is gratefully acknowledged. The research funding organization agrees with open access publication and provides adequate funding for this purpose. Author Contributions: Marek Kostecki was responsible for planning the experiments, writing the article, and data analysis. Jarosław Wo´zniak performed the SPS sintering experiments. Tomasz Cygan and Mateusz Petrus contributed material preparation and SEM observations. Andrzej Olszyna was responsible for data analysis and final corrections. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5.

6. 7. 8.

9.

10. 11.

12.

13.

14.

Miracle, D.B. Metal matrix composites—From science to technological significance. Compos. Sci. Technol. 2005, 65, 2526–2540. [CrossRef] Fundamentals of Metal-Matrix Composites; Suresh, S., Ed.; Elsevier: Amsterdam, The Netherlands, 2013. Rawal, S. Metal-Matrix Composites for Space Applications. J. Miner. Met. Mater. Soc. 2001, 53, 14–17. [CrossRef] Das, D.K.; Mishra, P.C.; Singh, S.; Thakur, R.K. Properties of ceramic-reinforced aluminium matrix composites—A review. Int. J. Mech. Mater. Eng. 2014, 9, 12. [CrossRef] Bodunrina, M.O.; Alaneme, K.K.; Chown, L.H. Aluminium matrix hybrid composites: A review of reinforcement philosophies; mechanical, corrosion and tribological characteristics. J. Mater. Res. Technol. 2015, 4, 434–445. [CrossRef] Suresha, S.; Sridhara, B.K. Wear characteristics of hybrid aluminium matrix composites reinforced with graphite and silicon carbide particulates. Compos. Sci. Technol. 2010, 70, 1652–1659. [CrossRef] Torralbaa, J.M.; Da Costab, C.E.; Velascoa, F. P/M aluminum matrix composites: An overview. J. Mater. Process. Technol. 2003, 133, 203–206. [CrossRef] Ravindran, P.; Manisekar, K.; Rathika, P.; Narayanasamy, P. Tribological properties of powder metallurgy —Processed aluminium self-lubricating hybrid composites with SiC additions. Mater. Des. 2013, 45, 561–570. [CrossRef] Omrani, E.; Moghadam, A.D.; Menezes, P.L.; Rohatgi, P.K. Influences of graphite reinforcement on the tribological properties of self-lubricating aluminum matrix composites for green tribology, sustainability, and energy efficiency—A review. Int. J. Adv. Manuf. Technol. 2016, 83, 325–346. [CrossRef] Rohatgi, P.K.; Pai, B.C. Seizure resistance of cast aluminium alloys containing dispersed graphite particles of various sizes. Wear 1980, 32, 148–152. [CrossRef] Moghadam, A.D.; Omrani, E.; Menezes, P.L.; Rohatgi, P.K. Mechanical and tribological properties of self-lubricating metal matrix nanocomposites reinforced by carbon nanotubes (CNTs) and grapheme—A review. Compos. Part B 2015, 77, 402–420. [CrossRef] Bianco, A.; Cheng, H.M.; Enoki, T.; Gogotsi, Y.; Hurt, R.H.; Koratkar, N.; Kyotani, T.; Monthioux, M.; Park, C.R.; Tascon, J.M.D.; Zhang, J. All in the graphene family—A recommended nomenclature for two-dimensional carbon materials. Carbon 2013, 65, 1–6. [CrossRef] Li, J.L.; Xiong, Y.C.; Wang, X.D.; Yan, S.J.; He, W.W.; Chen, J.Z.; Wang, S.Q.; Zhang, X.Y.; Dai, S.L. Microstructure and tensile properties of bulk nanostructured aluminum/graphene composites prepared via cryomilling. Mater. Sci. Eng. A 2015, 626, 400–405. [CrossRef] Rashad, M.; Pan, F.; Tang, A.; Asif, M. Effect of Graphene Nanoplatelest addition on mechanical properties of pure aluminum using a semi-powder method. Prog. Nat. Sci. Mater. Int. 2014, 24, 101–108. [CrossRef]

Materials 2017, 10, 928

15. 16. 17.

18.

19.

20.

21.

22. 23. 24. 25. 26.

27.

14 of 14

Bartolucci, S.F.; Paras, J.; Rafiee, M.A.; Rafiee, J.; Lee, S.; Kapoor, D.; Koratkar, N. Graphene-aluminum nanocomposites. Mater. Sci. Eng. A 2011, 528, 7933–7937. [CrossRef] Wang, J.; Li, Z.; Fan, G.; Pan, H.; Chenb, Z. Reinforcement with graphene nanosheets in aluminum matrix composites. Scr. Mater. 2012, 66, 594–597. [CrossRef] Latief, F.H.; Sherif, E.S.M.; Almajid, A.A.; Junaedi, H. Fabrication of exfoliated graphite nanoplatelets-reinforced aluminum composites and evaluating their mechanical properties and corrosion behavior. J. Anal. Appl. Pyrolysis 2011, 92, 485–492. [CrossRef] Ghazaly, A.; Seif, B.; Salem, H.G. Mechanical and Tribological Properties of AA2124-Graphene Self Lubricating Nanocomposite. In Light Metals 2013; Sadler, B.A., Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2013; pp. 411–415. Tabandeh-Khorshid, M.; Omrani, E.; Menezes, P.L.; Rohatgi, P.K. Tribological performance of self-lubricating aluminum matrix nanocomposites: Role of graphene nanoplatelets. Eng. Sci. Technol. Int. J. 2016, 19, 463–469. [CrossRef] Liu, J.; Khan, U.; Coleman, J.; Fernandez, B.; Rodriguez, P.; Naher, S.; Brabazon, D. Graphene oxide and graphene nanosheet reinforced aluminium matrix composites: Powder synthesis and prepared composite characteristic. Mater. Des. 2016, 94, 87–94. [CrossRef] Kostecki, M.; Grybczuk, M.; Klimczyk, P.; Cygan, T.; Wo´zniak, J.; Wejrzanowski, T.; Jaworska, L.; Morgiel, J.; Olszyna, A. Structural and mechanical aspects of multilayer graphene addition in alumina matrix composites–validation of computer simulation model. J. Eur. Ceram. Soc. 2016, 36, 4171–4179. [CrossRef] Groszek, A.J.; Witheridge, R.E. Surface Prosperities and Lubricating Action of Graphite and MoS2 . ASLE Trans. 1971, 14, 254–266. [CrossRef] Wo´zniak, J.; Kostecki, M.; Cygan, T.; Buczek, M.; Olszyna, A. Self-lubricating aluminium matrix composites reinforced with 2D crystals. Compos. Part B 2017, 111, 1–9. [CrossRef] Noudem, J.G.; Kenfaui, D.; Chateigner, D.; Gomina, M. Toward the enhancement of thermoelectric properties of lamellar Ca3Co4O9 by edge-free spark plasma texturing. Scr. Mater. 2012, 66, 258–260. [CrossRef] Noudem, J.G.; Retoux, R.; Lanfredi, S. Spark plasma texturing (SPT) of p-type [Ca2CoO3]0.62[CoO2] thermoelectric oxide. J. Alloys Compd. 2016, 676, 499–504. [CrossRef] Ferrari, A.C.; Meyer, J.C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K.S.; Roth, S.; et al. Raman spectrum of graphene and graphene layers. Phys. Rev. Lett. 2006, 97, 187401. [CrossRef] [PubMed] ASTM G99-05, Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus; ASTM International: West Conshohocken, PA, USA, 2005. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).