Investigation of mechanical properties of Copper-Graphene composites in terms of production methods

46.0. GPa

bonds with the 3 closest atoms [26–28]. The carbon atoms that make up the structure of graphene are connected by covalent bonds and π bonds. Covalent bonds increase the mechanical properties of graphene, while π bonds contribute to electron conduction [29]. Graphene, like copper, has very good thermal and electrical properties. An important advantage of graphene is its large surface area and its contribution to mechanical features [30–32]. Particles up to 100 nm in size in all dimensions are called nanomaterials. Nanomaterials show different properties compared to other combinations formed by the basic component. Therefore, it draws a lot of attention in terms of production and use. On the other hand, nanocomposites are composites where one of their components is at the nanoscale and can have different dimensions [33–35]. Powder metallurgy (PM) is the technique of producing new and superior materials using powdered metals through various processes. Metal powders of different sizes produced by various techniques in classical powder metallurgy are subjected to a mixing process. The powder mixture, which is made as homogeneous as possible, is compressed by applying high pressure in a prepared mold. Finally, the material is obtained by applying sintering. All stages of this process can change depending on many different variables of the powders used which are particle shape, size, and amount of combination [36]. Graphene shows weak proximity to copper. This prevents an effective interface bonding to be established. Moreover, graphene agglomerates due to Van der Waals forces and powerful π-π bonds. As a result, it is difficult to distribute graphene uniformly in metal matrix composites [37]. Therefore, choosing the mixing method according to the sintering type is of great importance. Magnetic stirring and sonication mixing methods are chosen in most of the studies in which graphene is subjected to various chemical treatments [37–42]. The ball mill method is used in studies where copper and graphene powders are generally formed with flakes and formed with each other [43–53]. Furthermore, mechanical alloying methods also take place in the literature [54,55]. While applying these mixing methods, some chemicals such as stearic acid or ether can be used to prevent agglomeration [30,56]. In some studies, mixing can be carried out in an atmosphere of hydrogen or argon gases to prevent the oxidation of copper [30]. There are many different consolidation methods in powder metallurgy. In the hot-pressing method, heating and compression are carried out simultaneously. Metal powders, which are liquefied by heating, are sintered under a certain pressure applied uniaxially [37,55,57–60]. The spark plasma sintering method is the consolidation of the material in a graphite mold in a certain vacuum environment by applying pulsed DC or AC. In this method, when the current flows directly over the material to be applied, a spark discharge will form and this will cause rapid heating. Therefore grain growth may occur [61–67]. Electrodeposition is based on the principle of depositing

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the desired metal on the cathode electrode by applying DC or pulsed DC between two electrodes in an environment containing ions or chemical solution of the desired metal using an electrical source [68]. Although graphene provides many positive contributions to copper composites when used as an additive material, some problems have also been encountered. These problems are usually caused by the difference in some physical and chemical properties between copper and graphene. Copper and graphene are not very compatible in terms of interfacial wettability. In addition, due to the weak affinity between graphene and copper, strong interfacial bonding is very difficult. The density difference between the two materials is also one of these problems. Another issue that may occur during the production of composite is that the graphene may be damaged. For these reasons, it is very difficult for copper and graphene to form covalent bonds and graphene may show an agglomeration tendency during metal composite production. Some articles in the literature have suggested various methods to overcome these problems [28,30,50,69]. Ali et al. [70], have investigated the factors affecting the thermal conductivity of Cu-GNPs composites and the improvement of thermal conductivity by changing these factors. Hidalgo-Manrique et al. [30], have explained the mechanical, electrical, thermal and tribological properties of graphene-reinforced copper composites in detail by revealing the effects of different production methods. Güler et al. [32], have studied the mechanical properties of graphene reinforcement in metal matrix composites and the factors influencing these properties. Zhao et al. [71], have

demonstrated the mechanical and functional properties of graphene composites in a comprehensive study. Iqbal et al. [72], have examined graphene synthesis techniques and their advantages. Furthermore, they have demonstrated the effects of graphene reinforcement in the production of nanocomposites of various structures. Most of the studies have focused on the production of materials with better properties. Additionally, related studies have been presented in detail in the literature review. The objective of this research, considering the results of the studies in the literature, is to determine the production methods of the Cu-GNP composites produced with the lowest additive ratio, which have the highest yield strength, hardness and tensile strength properties. In addition, the effects of different production and mixing methods in terms of these properties are discussed based on the studies examined for each mechanical feature. Thus, the importance of a cost-effective perspective has been emphasized to researchers interested in the subject.

1.0. vol% GN

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Table 2. Results of studies on GNP reinforced copper composites No

0.4. g/L GO

Hardness =3.32 GPa (~338.5HV) Young’s modulus=201.57 GPa Ultimate tensile strength=280 MPa

0.70. wt.% Gr

Electric field-activated pressure-assisted synthesis (FAPAS)

0.3. wt.% GNPs+2.0 wt.% Yield strength =295.6 MPa

According to many studies examined, it can be concluded that the factors that have the greatest impact on the results are the difference in material selection and production method. Therefore, it will be useful to mention the production methods in some studies. Chu et al. [76], investigated the mechanical properties of Cu-GNPs composites and the details of the production method. They obtained the graphene which they used in their studies according to the modified Brodie’s method [93]. They used the ball milling mixing method for the homogeneous distribution of graphene. Samples were produced in various proportions by volume. The weight ratio of the produced samples to the balls is 1:5. The powders were mixed in an argon atmosphere for 3 hours at a rotational speed of 1200 rpm and ether was used for process control. Bulk composite powders were sintered at 800 °C for 15 minutes. This process was carried out by applying a pressure of 40 MPa with a heating rate of 500 °C min1 . Chen et al. [84], presented a new method in which they obtained graphene on copper powders and used PMMA (Polymethyl methacrylate) as the graphene source. In their work, they focused especially on energy efficiency and structural durability. Three separate samples with copper and PMMA mass ratios of 10:0.1, 10:0.2 and 10:0.3 were produced. First, copper powder and PMAA powder were mixed in a 150 g stainless steel ball mill with Argon (Ar) gas at 400 rpm for 2 hours. PMMA/Cu composite powders were calcined for 10 minutes in a quartz tube furnace containing Ar (200 ml/min) and H2 (100 ml/min) gases at 800 °C and cooled to room temperature rapidly. Composite powders were produced by the hot pressing sintering method for one hour by applying 50 MPa pressure

at 800 °C under a vacuum (10-4 MPa) in a graphite mold. Inspired by the natural mother-of-pearl structure, Cao et al. [74], developed a new method to improve the decreasing ductility and electrical conductivity properties after the hardening process. In their studies, they emphasized the importance of architectural design in improving the structural properties of metal composites. There is a 1:20 mass ratio between copper powders and stainless-steel balls. Copper powders were mixed in ethanol for 5 hours by ball milling method. Copper powders flaked in this way were mixed with 0.05-0.5 wt.% PMMA anisole solution and turned into a slurry. This slurry was stirred for 12 hours and then centrifuged at 4000 rpm for 10 minutes. The PMMA/Cu flakes obtained by this method were dried in the oven at 85 °C for 2 hours and removed from the solvent. The produced composite powders were obtained by heating for one hour in a quartz tube furnace containing Ar (400 sccm) and H2 (100 sccm) gases at 900 °C and were cooled to room temperature rapidly. Composite powders were produced by the hot pressing sintering method for 20 minutes at a heating rate of 15 °C /min by applying 50 MPa pressure at 900 °C in a graphite mold under Ar atmosphere. Hot pressed composites were hot rolled at 850 °C. Shengcheng et al. [86], recommended using a nanocellulose gel (NCG) assisted production method for the homogeneous dispersion of graphene on copper. Because this method is simple, inexpensive, and efficient, it can be used in the mass production of copper-graphene composite material. 20 g of copper powder and 2 g of 0.65% NCG were mixed with a high-speed mixer at a rotational speed of 3500 r/min. Thus, copper powders were homogeneously coated with gel. 0.5 g of GNPs was also added to

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the mixture by a similar method. Thanks to NCG, GNPs were able to adhere to copper particles. After the water at 60 °C was evaporated in 2 hours by applying vacuum, the produced composite powders were heated at a heating rate of 15 °C min-1 in a quartz tube furnace containing Ar (100 sccm) and H2 (30 sccm) gases and the temperature of which was at 800 °C. Then, the annealing process was carried out under similar atmospheric conditions at 850 °C for 2 hours by keeping the temperature constant. After that, the prepared particles were cooled at a cooling rate of 35 °C min-1. Composite powders were produced by the hot pressing sintering method for 20 minutes at a heating rate of 25 °C min-1 by applying 40 MPa pressure at 850 °C in a graphite mold under an Ar atmosphere. Fanyan et al. [38], formed composites using molecular level mixing process and spark plasma sintering methods by preventing agglomeration of graphene. They also examined the effect of graphene content on various material properties. To suspend graphene in the solution containing copper ions, it was stirred in the alcohol solution. Then, sonication was continued by adding liquid C6 H12 O6 and liquid NaOH, respectively. The resulting mixture was kept in the oven for 4 hours. Thus, precipitate powders were obtained. These were washed with 50 vol.% ethanol solution, filtered and then dried. Finally, the obtained powders were reduced to 300 °C for 3 hours. Composite powders containing graphene in different proportions were put into molds and sintered at 700 °C in a vacuum environment by spark plasma sintering (SPS) method. The consolidation pressure was initially applied at 40 MPa and then at 50 MPa in the cooling section. Ke et al. [85], used electrostatic self-assembly and electroless plating methods together in this study. Thanks to the electrostatic self-assembly method, GNPs were homogeneously absorbed by the copper powders. Thus, the interfacial strength was increased by homogeneously dispersing the graphene in

the prepared composite. Modified Hummer’s method was used to obtain GO liquid solution. The liquid GO solution was mixed first with SnCl2 and then with PdCl2, and the pre-treated GO solution was prepared. After the copper powders were ball milled at 300 rpm for 4 hours, they were placed in 0.5 wt.% cetyltrimethyl ammonium bromide (CTAB) solution for positive electrical charge. Thus, a pretreated Cu slurry was obtained. The pre-treated GO and Cu slurry obtained were mixed. This mixture CuSO4.5H2O was added to the solution and mixed in a water bath at 80 °C for 30 minutes. Finally, composite powders were obtained by heating the obtained powders at 500 °C for 2 hours under H2 and Ar atmosphere. Composite powders were sintered by spark plasma sintering method by applying at 35 MPa pressure at 600 °C. Ajay et al. [11], used the electro-co-deposition method to obtain Cu-GNPs composite powders in this study. Classical powder metallurgy methods were used for bulk material production. Apart from the main components of the study Cu and GNPs, there are also copper sulfate pentahydrate (CuSO4.5H2O) and sulfuric acid (H2SO4). First, GNPs and CuSO4.5H2O aqueous solution were prepared, then H2SO4 was added to the solution for pH balancing. The prepared solution was mixed both ultrasonically and magnetically to prevent aggregation and to distribute GNPs homogeneously. As a result of the process, the composite powders accumulated at the end of the cathode electrode. The powders were filtered and dried in an inert environment. The obtained composition was compressed in the mold using a hydraulic press. The compacted samples obtained were sintered in the Ar environment in the furnace at 950 °C for 2 hours and cooled at room temperature. The structural properties of the powders preferred in the above-mentioned studies and the chemicals used in the production process are shown in Table 3.

Cu powders are 400 mesh and PMMA powders are about 80 PMMA (Polymethyl methacrylate) µm in diameter and both are 99.9% pure.

The average particle size of Cu powders is 40 µm and they have a purity of 99.9%. PMMA (Polymethyl methacrylate) was used as carbon source.

Copper particles are 20 µm in diameter. 0.65% nanocellulose None gel (NCG) was used.

The alcohol solution of Cupric nitrate trihydrate [38] (Cu(NO3)2.3H2O) and C6H12O6 aqueous liquid solution.

Graphene oxide (GO) aqueous solution, Tin (II) chloride (SnCl2) and Copper (II) sulfate pentahydrate (CuSO4.5H2O)

Sulfuric acid (H2SO4) and Copper (II) sulfate pentahydrate (CuSO4.5H2O)

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0.3. wt.% GNPs+2.0

bonds in Cu-GNPs composite. The study claims that a rise in the grain boundary is seen because grain refinement is brought on by the input of GNPs. This increase might stop the movement of grain dislocation. As a result, the hardness rises. Pavithra et al. [79], using 1.3 wt.% GNPs additive, acquired a 54% increase in hardness value in the sintered sample by pulsed reverse electrodeposition. In this method, if the pulse parameters and current density are well designed, the graphene is well distributed in Cu-GNPs composite and this makes a significant contribution to the hardness value. Adjusting the current density from the electrolysis parameters affected the hardness values. Especially creating a forward pulse or reverse pulse current and optimizing the on-off times increased the hardness values. Although the highest increase rate and hardness value are in study 1, it has a high contribution rate with 10% graphene contribution. In Study 3, 1.75 GPa is reached with the lowest GNPs contribution ratio compared to the others. Although the lowest increase rate is in Study 5, it has the second highest hardness value. Tensile Strength Properties Among the studies in Table 2, the 5 studies with the best tensile strength values are shown in Table 5. According to the values in Table 5 and Figure 2, Pingale et al. [11], achieved an 110% increase in the tensile strength value of the sample sintered by Electroco-deposition and powder metallurgy, using 2.57 wt.% GNPs. The most important effect on the increase in the tensile strength value in this study is related to the homogeneous distribution of graphene in the composite and the strength of the interface bond in the composite [30]. Enhanced interfacial interaction increases load transfer and tensile strength value. Further, the addition of GNPs to the composite causes grain refinement. Thus, the grain

Figure 2. Rates of increase in tensile strength values. boundaries increase. Grain boundaries can also prevent the movement of dislocations. Zhao et al. [75], achieved a 107% increase in the tensile strength value of the sample sintered by spark plasma sintering (SPS), using 1.3 %wt. GNPs. The ball milling process was not used in this investigation. Graphene can be damaged during ball milling which can impair its mechanical qualities. Furthermore, the link established between the copper and carbon atoms via oxygen increased the interface bonding. As a result, improved load transfer was accomplished. Shi et al. [92], obtained a 100% increase in the tensile strength value of the sample sintered by spark plasma sintering when they used 0.3 wt.% GNPs+2.0 wt.% Ti. The use of Ti-GNPS as an additive in Cu metal matrix composites can be explained by different mechanisms to increase the mechanical properties. There is poor interface affinity between copper and GNPs. The better interface bond of the Ti interface layer naturally strengthens the interfacial shear stress. Thus, the load transfer will be increased. Grain refinement is another factor. The composite will be more refined when

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0.3. wt.% GNPs+2.0 wt.% Ti Yield strength =295.6

the Ti interlayer and GNPs surround the copper grains. Cao et al. [74], accomplished a 73% increase in the tensile strength value of the sample sintered by hot pressing and hot rolling, using 2.5 vol% Gr also they had interfacial bonding between Cu and graphene was increased. A new method has been developed, inspired by the nacreous part of seashells. A building with a brick-mortar architecture greatly increases properties such as toughness and ductility. The produced copper-graphene flakes were combined in the form of bricks. Instead of the protein layer in seashells, a graphene layer was formed. Si et al. [80], have seen a 56% increase in the tensile strength value of the sample sintered by spark plasma sintering when they used 0.5 wt.% GNPs and TiC. Carbide coating was formed on graphene nanoplatelets (GNPs) using the molten salt treatment method to strengthen the interfacial bond. Thus, the interfacial bonding is strengthened by forming a carbide layer in the gaps between Cu and GNPs. Because early transition metals are very suitable for forming interlayers in terms of bond structures. Although the highest increase rate of tensile strength is in study 1, the highest tensile strength value is observed in study 2. At the same time, the least graphene contribution is obtained in study 5 by the spark plasma sintering method. The spark plasma sintering method reaches the highest two tensile strength values. The contribution of GNPs and TiC or Ti increases the tensile strength value and increases the contribution efficiency. Yield Strength Properties The best five yield strength values in Table 2 are seen in Table 6. According to the values in Table 6 and Figure 3, Shi et al. [92], acquired a 239% increase in the yield strength value of the sample sintered by spark plasma sintering, using 0.3 wt.% GNPs+2.0 wt.% Ti. Interaction between dislocations and additives is one of the mechanical strengthening factors. Ti interlayer prevents the dislocations from moving freely and allows them to agglomerate in their borders.

Figure 3. Rates of increase in yield strength values. In this way, it causes an increase in the densities of dislocations within the grains. Strengthening dislocations is a very important parameter in terms of yield strength. The other important reason for the strengthening effect is that the Ti transition layer creates a strong interface bonding between Cu and GNPs. Thus, a more efficient load transfer reinforcement can be created. Grain refinement has been made possible to the Ti transition layer and GNPs wrapping around the grains. Chen et al. [73], achieved a 233% increase in the yield strength value of the sample sintered by the hot-pressing method, using 0.5 %wt. %3D-graphene. The use of 3D graphene has increased the yield strength property. The number of bonds in the Cu matrix of 3D graphene can improve the toughening property of the composite. In this study, High-ratio Differential Speed Rolling (HRTEM) mixing method was applied to increase the number of these bonds and to distribute the graphene dispersion in the Cu matrix without agglomeration. Another important effect is the interface bonding of the materials that make up the composite. It could be crucial for the interface to be empty and pure to increase this impact. A pure interface was made in this investigation. In this work, 3D graphene served as a

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barrier to prevent dislocations from spreading. Cao et al. [74], have seen a 178% increase in the yield strength value of the sample sintered by hot pressing and hot rolling when they used 2.5 vol% graphenes. Graphene grown in-situ enhances interface bonding and mechanical properties. The combination of the nanolamination technique and the graphene additive has given a good result in terms of the yield strength property of composite materials. Moreover, the in situ catalytic growth method offers good structural quality and improved interface binding strength. Zhao et al. [75], obtain a 133% increase in the yield strength value of the sample sintered by spark plasma sintering when they used 1.3 wt.% GNPs. According to the study, there is a large difference in thermal expansion rates between copper and graphene. Therefore, a plastic zone formation is observed in Cu-GNPs composites. As the grain size decreases, the grain boundary increases. Thus, a resistance to the dislocation motion is created. Pingale et al. [11], achieved a 129% increase in the yield strength value of the sample sintered by the electro-co-deposition and powder metallurgy, using

2.57. wt.% GNPs.

In all articles in Table 6, the increase rate in the yield strength feature is higher compared to the increase rate in hardness and tensile strength properties. Study 2 has the lowest contribution rate and the second highest increase rate.

with the contribution of 0.6 vol% GNPs with the spark plasma sintering method. The use of certain tensile stress and pressure during consolidation may be one of the significant factors that improve the hardness characteristic. In addition, the excess amount of graphene additive can affect the rise in hardness. Interface bonding and strengthening dislocations are efficient causes for increasing the tensile strength feature. When graphene additive and increase rate are evaluated together in Table 5, the spark plasma sintering method was obtained with the best tensile strength value, 1.3 wt.% GNPs contribution and 107% increase rate. However, the lowest contribution rate of 0.5 wt.% GNPs was similarly achieved by the spark plasma sintering method. Considering the percentage increase rates in Table 4-6, the highest increase rates were seen in the yield strength feature, although the graphene additive caused an increase in all mechanical properties regardless of the production method. When the studies in Table 5 are evaluated, the best percentage increase value was reached by the spark plasma sintering method for the others. Therefore, the spark plasma sintering method can provide more reliable results for a percentage increase rate in tensile strength. When the studies in Table 6 are evaluated, the studies showing the best percentage increase value were hot pressing sintering and spark plasma sintering methods. In addition, the lowest additive ratio (0.5% by weight of 3D-graphene) was used in the hot pressing method. The hot pressing method may be considered the most successful technique, and 3D-graphene can be accepted as the most effective additive material because the first research includes extra Ti contribution. The use of 3D graphene in Cu-GNP composites reduces the graphene additive rate and also increases the yield strength percentage increase rate. However, the falling graphene amount causes a decrease in the value of the yield strength. The highest yield strength (363 MPa) and tensile strength (485 MPa) values were reached by the spark plasma sintering method. The study using the electro-co-deposition method [11] is available in the hardness, tensile strength and yield strength tables. Therefore, the electro-co-deposition method is the best method to increase all property values. Mixing methods are an important part of the process. There is a mixing method suitable for each production method. The molecular level mixing method is preferred in most studies using the spark plasma sintering method. Ball milling was most used in the hot press sintering method.