In the present investigation the possibility of using exfoliated graphite nanoplatelets (xGnP) as reinforcement in order to enhance the mechanical properties of Cu-based metal matrix composites is explored. Cu-based metal matrix composites reinforced with different amounts of xGnP were fabricated by powder metallurgy route. The microstructure, sliding wear behaviour and mechanical properties of the Cu-xGnP composites were investigated. xGnP has been synthesized from the graphite intercalation compounds (GIC) through rapid evaporation of the intercalant at an elevated temperature. The thermally exfoliated graphite was later sonicated for a period of 5 h in acetone in order to achieve further exfoliation. The xGnP synthesized was characterized using SEM, HRTEM, X-ray diffraction, Raman spectroscopy and Fourier transform infrared spectroscopy. The Cu and xGnP powder mixtures were consolidated under a load of 565 MPa followed by sintering at 850°C for 2 h in inert atmosphere. Cu-1, 2, 3 and 5 wt% xGnP composites were developed. Results of the wear test show that there is a significant improvement in the wear resistance of the composites up to addition of 2 wt% of xGnP. Hardness, tensile strength and strain at failure of the various Cu-xGnP composites also show improvement upto the addition of 2 wt% xGnP beyond which there is a decrease in these properties. The density of the composites decreases with the addition of higher wt% of xGnP although addition of higher wt% of xGnP leads to higher sinterability and densification of the composites, resulting in higher relative density values. The nature of fracture in the pure Cu as well as the various Cu-xGnP composites was found to be ductile. Nanoplatelets of graphite were found firmly embedded in the Cu matrix in case of Cu-xGnP composites containing low wt% of xGnP.
Over the last several decades, there has been a considerable interest in the use of Cu-based metal matrix composites (MMCs). Cu has a wide range of excellent properties like high electrical conductivity (5.98 × 106 S/m), thermal conductivity (401 W∙m−1∙K−1) and corrosion resistance. Cu has a melting point of 1083.4˚C and its density is 8.96 gm/cc. Its Young’s modulus is 130 GPa. Its yield strength is 117 MPa while its tensile strength is 210 MPa. The main drawback associated with Cu is its relatively low strength. For many applications pure Cu cannot be used because of its low strength. Therefore, improvement in the properties of Cu has become essential for its use in cutting-edge technological applications. Cu is used as a structural material for cooling as it has high thermal conductivity. In order to increase its high temperature properties, different reinforcements are being used. In recent years, graphene has attracted considerable research interest due to its unique properties. Graphene is a two-dimensional platelet made of carbon atoms. It is a one atom thick material and is a promising nanofiller that could improve the mechanical, electrical and thermal properties of the composites. Its excellent mechanical properties make it an ideal reinforcement for developing nanocomposites. It has a modulus of elasticity of 1 TPa and a fracture strength is 125 GPa. The electrical resistivity of graphene is 10−6 Ω∙cm and its thermal conductivity is ~3080 - 5300 W/m∙K. One possible way of harnessing the extraordinary properties of graphene is by dispersing graphene in the polymer, metal or ceramic matrix [
This work explores the potential of using xGnP as a reinforcement in Cu-based metal matrix composites. Cu-xGnP nanocomposites containing different wt% of xGnP were fabricated using powder metallurgy route. Here an expandable graphite also known as the graphite intercalation compound (GIC) is developed first. It is prepared from natural flake graphite using acid intercalation in the presence of an oxidizing agent. Sulfuric acid (H2SO4) has been used as the acid intercalant and hydrogen peroxide (H2O2) has been used as an oxidizing agent. The GIC was later given a thermal treatment to produce exfoliated graphite. The expandable graphite prepared using the acid intercalation process can expand several times its original volume when heated to high temperatures [
Elemental Cu powder having purity > 99% and average particle size of ~27 µm and natural flake graphite (NFG) having 98% purity and mesh size of ~60 were procured form Loba Chemie. Sulfuric acid (H2SO4) having 98% purity and hydrogen peroxide (H2O2) having 30 vol% concentration was obtained from Merck India. All reagents used were of analytical grade and had the highest commercially available purity. Expandable graphite was prepared at room temperature from the natural flake graphite by mixing 16 ml of H2SO4 (98%) as an intercalant and 1.5 ml H2O2 (30%) as an oxidant with 6 gm of natural flake graphite under vigorous stirring. The mixture was placed for 1 h 30 min in a magnetic stirrer. The reaction mixture which was prepared after stirring was repeatedly washed and filtered in distilled water in order to achieve an aqueous mixture solution having pH in the range of 5 - 7. Once the acidic impurities were removed and the prepared mixture was obtained through sedimentation it was then dried at 60˚C for 30 h before further heat treatment. Finally the expandable graphite powder was subjected to a thermal shock in a muffle furnace at a temperature of 1000˚C for 30 s in an air atmosphere resulting in the formation of thermally exfoliated graphite. The thermally exfoliated graphite was then dispersed in acetone in a magnetic stirrer for 30 minutes and thereafter it was dispersed in an ultrasonicator for 5 h in acetone in order to achieve further exfoliation.
After proper mixing of the Cu and xGnP powders the Cu-xGnP samples were developed by uniaxial cold compaction of the powder under a load of 565 MPa followed by sintering at 850˚C for 2 h in inert atmosphere. Cu-1, 2, 3, 5 wt% xGnP composites were developed. X-ray diffraction (XRD) of the various powders and composites were done in a Phillips PANalytical diffractometer using Cu Kα radiation (λ = 0.15406 nm).
The microstructure of the composites were characterized using a JEOL JSM-6480LV scanning electron microscope (SEM) equipped with an Oxford Instrument INCAPentaFET-x3 energy dispersive X-ray spectroscopy (EDS) microanalysis system. A FEI NOVA NANO SEM 450(FEG) field-emission scanning electron microscope (FESEM) equipped with EDS (Bruker) was also used for analyzing the microstructure. A JEM-2100 JEOL HRTEM with a point to point resolution of 0.194 nm was used for analyzing the structure of graphite nanoplatelets. Atomic force microscopy (AFM) analysis was conducted by deposition of the graphite nanoplatelets on a mica substrate. A Park XE 70 AFM with a 100 × 100 µm XY-scanner and 10 µm Z-scanner having a minimum lateral resolution of the order of 1 nm was used to analyze the samples using the non-contact mode. The non-contact tapping mode allows us nanometer resolution without disturbing the surface of the substrate. X-ray photoelectron spectroscopy (XPS) data was collected using a PHI 5000 VersaProbe II spectrometer with a monochromatized Al Kα X-ray source (1486.6 eV). All multiplex scans used pass energy of 23.5 eV with a scan step of 0.1 eV. The number of sweeps per peak was 10 and the scan range for binding energy was 0 - 1200 eV. Each XPS core level, i.e., C1s and O1s, considered in this study was analyzed using Shirley background subtraction and resolution into components via a curve fitting procedure using Lorentz peak fitting. A Shimadzu IR Prestige 21 Fourier transform infrared spectroscopy (FTIR) and a JobinYvon Horiba T64000 Raman spectrometer was used for spectroscopic analysis. The density of the samples was measured by Archimedes’ principle. A Ducom ball-on-plate tribometer was used for the wear test of the various samples. Wear test was done under a load of 15 N using a diamond indenter. A Leeco Vickers microhardness tester was used to determine the hardness of the various samples. The tensile tests of the various samples were performed in an Instron 1195 computerized universal testing machine (UTM).
The X-ray diffraction spectra of the as-received natural flake graphite (NFG) and the exfoliated graphite nanoplatelets (xGnP) are shown in
The exfoliation of intercalated graphite is a phase transition process which involves the vapourization of the
intercalate in the graphite lattice. The xGnP formed by subjecting the GIC to a thermal shock at 1000˚C for 30 s followed by sonication in acetone in order to achieve further exfoliation is found to consist of several sheets of graphene stacked together. The diameter of the platelets range from submicrons to more than 100 µm. The xGnP has been synthesized here from the GIC through rapid evaporation of the intercalant at an elevated temperature. The thermal exfoliation of the acid-intercalated graphite is known to produce few layer graphene very successfully [
The HRTEM micrographs of the xGnP synthesized from the NFG in Figures 6(a)-(d) show its morphology. The HRTEM images show that the xGnP are composed of several layers of graphene stacked together. These images suggest that the xGnP are highly transparent and have folded edges. In the HRTEM images in Figures 6(a)-(c) wrinkles could also be seen at the edges of the xGnP. A large number of broken graphite platelets could be seen in the HRTEM images. The xGnP have a large surface area with diameter of several microns. Due to their high surface area, the xGnP have a high tendency to coalesce into overlapped structures.
The AFM analysis of the xGnP is shown in
The synthesis of xGnP was further confirmed by using Raman spectroscopy. Using this technique it is possible to identify graphene from graphite and few-layers graphene. Raman spectroscopy is able to probe disorder in graphene through defect-activated peaks. The Raman spectra in
Graphene is a planar sheet of sp2 bonded carbon atoms arranged in a hexagonal lattice. Disorder plays an important role on graphene properties. The formation of the GIC from the NFG led to the increased fictionalization and covalent bond formation. The intensity ratio of the D band at 1350 cm−1 to G band at 1580 cm−1 (ID/IG) can serve as a convenient measurement of the amount of defects in the graphitic materials. The ID/IG ratio for the NFG was found to be 0.10 whereas the ID/IG ratio increased slightly after the thermal exfoliation of the GIC at 1000˚C for 30 seconds and was found to be 0.12. The ID/IG ratio shows even further increase for the xGnP obtained by ultrasonication of the thermally exfoliated graphite and was found to be nearly equal to 0.29. This ratio indicates that the crystal structure of xGnP has been preserved and is well ordered. However, the slight rise
in the value of ID/IG ratio can be explained by the destruction of the C=C aromacity and the disappearance of the sp2 carbon hexagonal structure. These results suggest that the exfoliation process was effective [
So far only few reports have been published on the use of graphite nanoplatelets to improve the mechanical properties of metal matrix composites. Here in our work the graphite nanoplatelets developed were subsequently used for the development of the Cu-xGnP composites. The Cu and xGnP powder mixtures were blended and consolidated under a load of 565 MPa followed by sintering at 850˚C for 2 h in inert atmosphere. Cu-1, 2, 3 and 5 wt% xGnP composites were developed. X-ray diffraction analysis was carried out for pure Cu and Cu-xGnP
composites in the range of 2θ = 20˚ - 100˚ as shown in
The various samples of Cu-xGnP composites were observed under an optical microscope. The optical micrograph of Cu-1 and 2 wt% xGnP composite in
Additional characterization by SEM along with EDS of the samples were done for finding out the distribution of xGnP in the various composites. Similar results as seen in the optical micrographs could also be seen in the SEM images of the various Cu-xGnP composites. Cu-1 wt% xGnP nanocomposite shows a very uniform distribution of xGnP in the Cu matrix in
SAD pattern from the sample. The SEM and HRTEM analysis suggests that there is a close contact between the Cu matrix and the xGnP in the Cu-xGnP composites which contributes in enhancing the mechanical properties of the composites. Graphite nanoplatelets could be found in the grain boundary junctions [
The theoretical density of the various Cu-xGnP composites was calculated using the rule of mixtures where the density of xGnP was 2.1 gm/cc and the density of Cu was 8.9 gm/cc. It is clear from the plot of the theoretical and experimental density in
Vickers microhardness was used to determine the hardness of both pure Cu and Cu-xGnP composites. The hardness values were measured on the polished surface of the various composites. It can be seen from the plot in
In order to find out the effect of the addition of xGnPs on the tensile properties of the Cu-xGnP composites the tensile tests of the various composites were done.
prevent the rupture and shearing of the Cu matrix. The graphite nanoplatelets restricted the propagation of the dislocations across the interface. This results in high strength of the composites. The strengthening that has been gained in the Cu-xGnP composites through the homogeneous dispersion of the graphite nanoplatelets in the Cu matrix can be attributed to the Orowan strengthening mechanism [
Cu-based metal matrix composites are used in a wide range of applications like heat exchangers, structural parts, electrical connectors, in contacts like brushes, frictional parts of machines like bearings and bushings etc. Relative motion between the surfaces results in friction, as well as progressive loss of the material. This is why the study of the wear properties of the Cu-xGnP composites is a very important. In order to understand how the Cu-xGnP composites behave while in contact with a hard material the wear properties of the Cu-xGnP composites were studied in unlubricated ball-on-plate experiments using a tribometer. The samples were in the form of polished disks having 15 mm diameter and 3 mm thickness. The specimens were tested normal to the major surface with a load of 15 N using a diamond indenter.
During sliding, the friction between the diamond indenter and the Cu-xGnP composites is less due to the multilayer structure of xGnP which provides a lubricating effect to the Cu matrix, resulting in a reduced coefficient of friction. However, Cu-3 and 5 wt% xGnP composites exhibit poor wear resistance leading to an increase in wear depth. The reduction in the wear resistance of the Cu-xGnP composites when the content of xGnP in the composites is beyond 2 wt% is possibly due to the effect of agglomeration of xGnP in the composites.
Figures 17(a)-(e) show the SEM images of the wear track of pure Cu and Cu-1, 2, 3 and 5 wt% xGnP composites. The width of the wear track is minimum in the case of Cu-2 wt% xGnP and is found to be around 520 µm. The smallest amount of wear debris is also observed in the case of Cu-2 wt% xGnP composite and the surface of the wear track of the composite is also much smoother in comparison to other samples. The increase in the wear resistance of Cu-2 wt% xGnP nanocomposite is due to the formation of an interconnected network of xGnP inside the Cu matrix. The interconnected network of xGnP gives a lubricating effect that helps in reducing the removal of material from the Cu matrix. The two-dimensional geometry of the xGnP forms a lubricating layer on the surface of Cu limiting the damage during the wear test. The increased wear resistance of the Cu-xGnP composites is also linked to its improved mechanical properties. Addition of xGnP beyond 2 wt% increased the width of the wear track significantly. This is possibly due to the agglomeration of xGnP when a
large amount of xGnP is added in the Cu-xGnP composites. The width of the wear track was largest in the case Cu-3 wt% xGnP and was found to be about 1.23 mm. The SEM image in
The XRD analysis of the wear debris from both Cu-2 and 5 wt% xGnP composites is shown in
The SEM images of the wear debris from the wear track of Cu-5 wt% xGnP in
thereby oxidizing Cu to Cu+ and Cu2+ forming a film of Cu2O or CuO on the surface. However, O− and O2− could also react with the xGnP first instead of the Cu matrix as carbon is easier to oxidize compared to Cu resulting in the formation of CO and CO2. As a result the formation of CuO and Cu2O during the wear test was retarded. Thus the xGnP not only acts as a lubricant but it also prevents the Cu matrix from oxidizing. Due to this the weight loss of the composite is reduced [
The fracture surfaces of the various samples of Cu-xGnP composites fractured by tensile test were analyzed in SEM. The fracture behavior of both the monolithic Cu and the various Cu-xGnP composites are shown in Figures 23(a)-(h). Dimples could be seen in the fracture surfaces of all the samples.
nanocomposites xGnP could be seen embedded in the Cu matrix. The SEM image in
The elemental maps of Cu, C and O in the fracture surface of Cu-1 wt% xGnP sample were acquired in order to determine the distribution of these elements in the fracture surface of the sample. The elemental maps of Cu, C and O in the fracture surface of Cu-1 wt% xGnP sample are shown in Figures 24(b)-(d) respectively. From the elemental map of C in
In summary, Cu-xGnP composites were developed via powder metallurgy route and the effect of the addition of xGnP to Cu matrix was determined.
1) Exfoliated graphite nanoplatelets (xGnP) were successfully synthesized by the ultrasonication of thermally exfoliated graphite. Raman spectroscopy analysis and HRTEM results confirm that the structural properties of the graphite nanoplatelets approach those of multilayer graphene. The exfoliation process was very effective.
2) The density of the Cu-xGnP composites shows a decrease with the increase in the xGnP content due to the addition of xGnP which has a low density. However the relative density of the Cu-xGnP increases with the increase in xGnP content. xGnP leads to a better sinterability and densification of the composite.
3) Cu-xGnP nanocomposites fabricated by powder metallurgy route show an increase in hardness and wear
resistance upto 2 wt% addition of xGnP. Beyond 2 wt% of xGnP there is a decrease in both the hardness and the wear resistance properties of the composites. Higher addition of xGnP leads to agglomeration of xGnP and this results in the decrease of both hardness and wear properties of the Cu-xGnP composites.
4) The tensile strength and the strain to failure of the Cu-xGnP composites also show a similar trend. Both the tensile strength and the strain to failure of the Cu-xGnP composites increase upto the addition of 2 wt% xGnP. Addition of xGnP beyond 2 wt% leads to the decrease of the tensile strength of the composites due to the agglomeration of xGnP.
5) The nature of fracture in pure Cu as well as the various Cu-xGnP composites was found to be ductile. Nanoplatelets of graphite were found firmly embedded in the Cu matrix in the case of Cu-xGnP composites containing low wt% of xGnP which was responsible for the improvement of tensile strength, strain to failure, hardness and wear properties.
We gratefully acknowledge the support provided by the XRD, SEM and thermal analysis laboratories of Metallurgical and Materials Engineering Department and the FESEM laboratory of the Ceramic Engineering Department, NIT Rourkela. We also thank the support provided by Chemistry Department, NIT Rourkela and the Central Research Facility, IIT Kharagpur.
Syed NasimulAlam,LaileshKumar,NidhiSharma, (2015) Development of Cu-Exfoliated Graphite Nanoplatelets (xGnP) Metal Matrix Composite by Powder Metallurgy Route. Graphene,04,91-111. doi: 10.4236/graphene.2015.44010