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Microstructural and morphological changes during ball milling of Copper-Silver-Graphite flake mixtures
Advanced Powder Technology xxx (xxxx) xxx
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Original Research Paper
Microstructural and morphological changes during ball milling of Copper-Silver-Graphite flake mixtures A. Pragatheeswaran, Rahul Ravi, Srinivasa Rao Bakshi ⇑ Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Chennai 600036, India
a r t i c l e
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Article history: Received 23 April 2019 Received in revised form 12 August 2019 Accepted 19 August 2019 Available online xxxx Keywords: Copper Graphene Graphite flake Ball milling Raman spectra
a b s t r a c t The present study reports the microstructural and morphological changes during high energy ball milling of Cu with Ag and Graphite flakes. XRD patterns of ball milled Cu-Ag showed a reduction in the intensity of Ag peaks (1 1 1) and an increase in the lattice parameter of Cu. With an increase in milling time, the formation of metastable Cu-Ag solid solution was observed. Lattice parameter values for 40 h milled Cu (3.6169 Å) and Cu-GF composites (3.6166 Å) indicated that C does not dissolve in Cu. The lattice parameter of Cu in milled Cu-Ag-graphite flake was lower compared to milled Cu-Ag mixture indicating that graphite flakes inhibit solid solution formation. Raman spectra revealed that graphite flakes were converted into multilayer graphene after 10 h of milling. The crystallite size of Cu in the milled powders decreased with increase in milling time and reached a value of 25 nm after 35 h of milling. The lattice strain also increased with milling time. The D10, D50 and D90 size reduced appreciably after 5 h of milling. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Copper and its alloys are conventional materials widely used in industrial applications for their excellent electrical and thermal conductivity, low cost of fabrication, and high resistance to corrosion and fatigue [1]. The basic properties needed for conductor and contact materials are high electrical conductivity, good mechanical strength and wear resistance, low weight, and low cost. The mechanical strength of copper can be enhanced by alloying it with other elements or by the addition of fine particles of the second phase in its matrix [2–4]. Many investigations have shown improvement in mechanical properties, however, it is well reported that introducing any second phase into copper reduces its electrical conductivity [5–7]. Hence, the selection of materials for reinforcement is very important. Carbon nanomaterials like carbon nanotubes (CNT) and graphene have been used in Cubased composites for thermal and electrical applications [8,9]. CNT/graphene hinder the dislocation motion in metal-CNT/ graphene composites resulting in high strength [10–13]. However, homogeneous distribution of the reinforcement throughout the sample and high cost of graphene/CNT are major problems. Powder metallurgy (PM) has been used in manufacturing precision components/composite materials for various industries such ⇑ Corresponding author.
as aerospace, electrical, oil and gas, automotive and healthcare industries [14,15]. Mechanical alloying (MA) is one of the methods used for synthesizing materials with metastable phases, such as supersaturated solid solutions, nonequilibrium crystalline or quasicrystalline intermediate phases, and amorphous alloys [16]. In the MA powder processing technique, the powder particles are repeatedly deformed, cold-welded and fractured under the impact of hard balls. Homogeneous distribution of particles and alloying can be achieved using this method [17,18]. Additionally, reduction of particle and grain size happens during milling leading to enhanced diffusivity [19]. All these effects lead to alloying of the elemental powders. Cu-Graphite PM products have been used for electrical contact applications. Powder metallurgy is the preferred choice of manufacturing process as compared to casting methods since graphite has very low density (2.3 g/cc) compared to Cu (8.9 g/cc) and also does not wet liquid copper [20,21]. Graphite is a layered structure in which carbon atoms are packed in a hexagonal lattice. Extremely weak van der Waals force holds the adjacent layers of graphite [22]. The exfoliation of graphite layers to multilayered graphene can happen during ball milling. Mechanical milling has been used to prepare exfoliated graphite or graphene using different milling mediums such as an organic solvent in the presence of dry ice, NaCl, and melamine [23–26]. The aim of the present study is to investigate the possibility of producing Cu-Ag-Graphene composite powder using high
E-mail address: [email protected] (S.R. Bakshi). https://doi.org/10.1016/j.apt.2019.08.023 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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energy ball milling of Cu-Ag-Graphite flake mixtures. The effect of ball milling time on the phase evolution, lattice parameter, microstructure, and particle size of the powders was investigated. 2. Experimental 2.1. Ball milling The SEM images of the powders used in the study are shown in Fig. 1. The Copper and silver powders were obtained from Alpha Aeser having a purity of 99.9%. The particle size of Copper is found to be in the range of 10–20 mm. The silver particles were in the form of agglomerates of fine sub-micron sized particles as seen in the inset image of Fig. 1b. Graphite flake was obtained from Alpha Aeser having a purity of 99% and the average particle size of 10 mm as shown in Fig. 1c. Copper (named Bm-Cu), Copper2 wt% Graphite flake (named Cu-GF), Copper-5 wt% Silver (named Cu-Ag), Copper-5 wt% Silver-2 wt% Graphite flake (named Cu-AgGF) mixtures were subjected to ball milling for 40 h. In addition, the copper powder was also milled for comparison. Tungsten carbide lined vials and balls were used for milling and a ball to powder ratio of 10:1 was used for all the mixtures. The ball milling was performed in toluene medium with a rotation speed of 300 rpm using a Fritsch Pulverisette 5 planetary ball mill. The milling was stopped for 30 min after every 1 h of milling, to prevent heating of the powder. 2.2. Powder characterization Powder X-Ray diffraction (XRD) patterns were recorded using a PANalytical(X’pert Pro) with Cu-Ka X-rays. Raman spectroscopy
measurements were performed by a laser Raman spectrometer (LabRAM HR, France) with a He-Ne laser (514 nm wavelength) as the excitation source. The microstructure and morphology of milled powders were observed by scanning electron microscopy (Quanta 400, FEI, Hillsboro, USA). The milled powders were mounted in cold setting epoxy resin and polished using various emery papers and final polishing was done with diamond paste (up to 0.25 mm size). The particle size distribution was measured using a laser particle size analyser (Microtrac s3500). The test was repeated on 3 samples and average D10, D50 and D90 values were reported.
3. Results and discussion 3.1. Phase evolution of milled powders Fig. 2 shows the XRD patterns of the milled powders of Cu and Cu-GF mixture up to 40 hrs of milling. Peaks corresponding to copper were observed in starting copper powder and there were no oxide peaks. Peak broadening was observed with increasing ball milling time as seen in Fig. 2a. Formation of Cu2O was found to increase with milling time. This indicates that the reduction in the particle size due to prolonged milling time had severe effects on oxidation during milling and drying thereafter. As shown in Fig. 2b, the XRD patterns of milled Cu-2 wt% graphite flake mixture shows a peak corresponding to (0 0 2) reflections of graphite at 26.7°. The Lattice parameter of the Cu in CuGF mixture is found to be 3.6103 Å from the XRD pattern, which is similar to that of Cu (3.610 Å as per JCPDS card no. 04-0836). The deviation from the JCDPS card value is due to dissolved impurities in the powder. Thus, the milling did not result in alloying of
Fig. 1. SEM images of starting powders (a) Copper, (b) Silver, and (c) Graphite flakes.
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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Fig. 2. X-ray diffraction patterns of powders milled for different times for (a) Cu, and (b) Cu-GF mixture.
Fig. 3. (a) X-ray diffraction patterns of Cu-Ag mixtures milled for various times, and (b) XRD patterns of ball milled Cu-Ag on a magnified scale.
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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Cu and C. After milling for 40 h, the peak of graphite disappeared and the presence of Cu2O was observed. The disappearance of the graphite peak could be due to the formation of multi-layered graphene and the dispersion within the matrix. Chen et al. [24] have reported that graphite can be exfoliated under the shear force during BM resulting in the formation of multilayered graphene. Hence, the disappearance of the graphite diffraction peak may indicate the exfoliation of the graphite layers. Several authors have observed disappearance of C peaks after ball milling. Rajendra Kumar et al. [27] have reported that 20 h of ball milling converted graphite to graphene and observed that the graphite peaks intensity was substantially decreased with milling time due to the formation of few layered graphene structure. Nayan et al. [28] have also shown that peaks corresponding to carbon nanotube disappear after milling for a few hours in copper. It is noted that the disappearance of graphite peak is not due to dissolution in carbon as discussed later using lattice parameter calculations. Fig. 3(a) shows the XRD patterns of Cu-Ag ball milled powder up to 40 h. The Ag peaks disappear after 5 h and the Cu peaks shift towards lower angles as seen in Fig. 3b. It is known from the CuAg phase diagram that the maximum solubility of Ag in Cu is 8% at 779 °C, however, there is no solubility at room temperature [29]. It is also known that BM leads to mechanical alloying and formation of metastable solid solution [16]. The dissolution of Ag into Cu results in an increase of the lattice parameter of Cu. This also results in a peak shift towards lower angles. The XRD patterns of ball milled Cu-Ag-GF powders are shown in Fig. 4. It is observed that with graphite flake addition, the Ag peak intensity is reduced to very low values only after 15 h of milling. This indicates that graphite flakes inhibit the mechanical alloying Fig. 5. XRD patterns of Cu (1 1 1) peak for different ball milled powders.
process. It is well known that during ball milling, the powders are flattened into flakes and cold welding of flakes take place due to the impact of the balls. This repeated flattening and coldwelding leads to alloying. Presence of graphite flakes at the interface is expected to prevent cold welding and alloying of Cu with Ag. Fig. 5 shows the (1 1 1) peak of Cu in all the 40 h milled powders. The peak shift in Cu-Ag is clearly seen in Fig. 5. It is to be noted that all the XRD patterns do not show any tungsten carbide peaks which could be generated from the wear of milling balls or vials.
Fig. 4. X-ray diffraction patterns of as-blended Cu-Ag-GF and ball milled Cu-Ag-GF with an interval time period of 5 h.
Fig. 6. Lattice parameter variation of Cu with ball milling time.
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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Fig. 7. Variation of the (a) crystallite size, and (b) lattice strain of copper with milling time.
3.2. Lattice parameter, crystalline size and lattice strain variation Fig. 6 shows the variation of the lattice parameter of Cu in different powders as a function of milling time. The lattice parameters of the as received Cu powder and the mixtures were measured by Nelson Riley function from powder XRD patterns. The lattice parameter of ball milled Cu and Cu-GF increases by 0.16% after 5 h of milling. There is no variation in the lattice parameter with further increasing milling time. This could be due to the dissolution of oxides or impurities present in the powder. It is noted that the maximum solubility of C in Cu is a maximum of 0.0076 wt% at 1100 °C [30]. The lattice parameter values of the 40 h milled Cu and Cu-GF are very close which indicates that no dissolution of C occurs in Cu due to mechanical milling. It is observed that in the
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case of Cu-Ag mixture, the lattice parameter of copper increased from 3.610 to 3.630 Å after 20 h milling and further increasing the milling time does not change lattice constant significantly (3.632 Å for 40 h powder). Thus, almost all the silver is dissolved after 20 h of milling. In case of Cu-Ag-GF mixtures, the lattice parameter in Cu increases gradually up to 40 h of milling. The lower values of lattice parameter of Cu in Cu-Ag-GF powder compared to Cu-Ag powders indicate that complete alloying does not take place even after 40 h in the presence of graphite flakes. This indicates that the graphite flakes inhibit the alloying process and acts like a process control agent. Fig. 7 shows the crystallite size and lattice strain variation of Cu in ball milled Cu, Cu-Ag, Cu-GF and Cu-Ag-GF for various milling times. The crystallite size and lattice strain were estimated by the Williamson-Hall method. The crystallite size decreases exponentially with time and reaches 25 nm after 35 h. The crystallite size increase after 40 h of milling for Cu-Ag could be due to recrystallization. This increase in crystallite size was confirmed through repeated XRD experiments and needs to be looked into in further detail. The 40 h milled Cu shows further reduction in crystallite size. Severe plastic deformation undergone by the particles during ball milling leads to refinement of crystalline size due to crystal defects such as dislocations, vacancies and increased number of grain boundaries [16,31]. The rate of decrease of crystallite size in Cu-Ag is higher compared to Cu and reaches the lowest value in about 20 h. The Cu-Ag mixture starts alloying after 15 h and the alloy gets easily work hardened and fractures more leading to finer flakes. The extent of dissolution up to 15 h of milling time of Ag in Cu is less as indicated by the lower peak shifts in Fig. 3b and small increase in lattice parameter in Fig. 6. It has been reported that the solid solubility limits increases as the crystallite size reduces in the nano-range [32,33]. The reduced crystallite size in Cu-Ag powder leads to high diffusivity and reactivity. The reduction of crystallite size after 20 h of milling increases the rate of dissolution significantly. After 5 h of milling, the crystallite size of Cu in ball milled Cu and Cu–Ag reduced to 175 nm and 150 nm respectively, however, in case of Cu-GF and Cu-Ag-GF powders it reduced to 50 nm and 75 nm, respectively. Thus, addition of graphite reduces the crystallite size at a faster rate. Graphite addition hinders the cold-welding and enhances the work-hardening rate resulting in larger reduction of crystallite size. The lattice strain of Cu in ball milled samples is found to increase with increase in milling time as seen in Fig. 7b. It is observed that addition of Ag
Fig. 8. (a)-Raman spectra of starting GF, ball milled Cu-GF, (b)-ball milled Cu-GF-Ag, (c) -ID/IG ratio of GF addition powders with various time intervals.
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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Fig. 9. Backscattered scanning electron microscopy images of Cu-Ag powder milled for 5 h along with the EDS elemental distribution map (a) and after 40 h of milling (b).
increases the lattice strain which could be due to the alloying effect. Also the addition of graphite results in further increase in the lattice strain. This could be due to the higher deformation of the Cu flakes due to the lubricating effect of graphite. Exfoliation of graphite leading to formation of multi-layer graphene was confirmed by Raman spectra measurements. 3.3. Raman spectra of milled powders Raman scattering is a good tool to get information on the surface structure of graphene and its defects density [34]. Fig. 8(a and b) shows the Raman spectra of as-received graphite flake and ball milled Cu-GF and Cu-Ag-GF for after 5, 10, 20 and 40 h of milling. Fig. 8(a and b) shows the Raman spectrum of graphite flake consists of three characteristic bands. The most remarkable
features of the spectrum are the G band at 1563 cm 1 which relates to E2g mode [35,36]. The second peak at 1334 cm 1, called the D peak, is usually associated with the presence of lattice disorders of the graphite flake. Another important peak observed at 2683 cm 1, named as 2D peak or second order graphite peak. For graphite, the 2D mode has a lower intensity and is wider as compared to G mode (Fig. 8). This suggests the presence of a layered structure in case of the graphite flake [37,38]. The G band in case of the ball milled powders decreases in intensity with an increase in milling time. The broadening and less intensity of G peak indicate that graphite layers were exfoliated upon ball milling for 10 h. The G band shifts towards higher wavenumbers with increasing ball-milling time up to 10 h, and thereafter starts shifting towards lower wave number. This result can be attributed to decreasing graphite layers with increasing
Fig. 10. Backscattered scanning electron microscopy images of polished cross section of Cu-GF powder milled for 5 h (a) and 40 h (b) and Cu-Ag-GF powder milled for 5 h (c) and 40 h (d).
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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ball-milling time. The D band intensity depends on the atomic structure at the edge of graphite. The lower intensity of D band in graphite flake indicates that the graphite sample was defect free and had a highly oriented structure [39]. The intensity ratios of D to G peak (ID/IG) for ball milled samples of Cu-GF and Cu-Ag-GF composites are presented in Fig. 6c. It is observed that the ID/IG ratio gradually increases with increase in ball-milling time. The ratio of ID/IG in both the mixtures is found to be 0.15 after 5 h of milling and it increases to 0.91 upon 40 h of milling. This result shows the structural distortion and size reduction of the in-plane sp2 domain caused due to their processing through ball-milling route. The ratio of IG/ID suggests two important results. Firstly, the initial graphite layers are exfoliated and converted as multilayered graphene after about 10 h of milling, and secondly, the ball milling at a prolonged time generated structural disorder in the graphene. The results indicate that inexpensive graphite flakes can be used in place of expensive graphene nano platelet for preparation of the composites. 3.4. Microstructure of the milled powders Fig. 9a shows the backscattered SEM images of the cross section with 5 h milled Cu-Ag powder. The lamellar structure can be clearly observed due to cold welding of the layers. SEM EDS maps indicate a uniform dispersion of Ag indicating the dissolution of Ag into Cu matrix. It is observed that ball milling for 5 h changes results in conversion of fine spherical starting powders into large
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flakes with thickness up to 40 mm and length up to 80 mm. Very fine flakes are also observed. A substantial decrease in the particle size is observed for the 40 h milled Cu-Ag powder as shown in Fig. 9b. The impact of the balls imparts large mechanical energy and strain on particles during milling which leads to severe work hardening that fracture of large particles into smaller powder particles [40]. Fig. 10a shows the morphology of 5 h milled Cu-GF mixture and is observed to be different from Cu-Ag mixture. Formation of flake like structure in ball milled ductile materials is well known and is used to prepare flaky powders. It is also known that long duration of milling can also lead to cold welding and formation of large spherical powders as is in the case of Al powders [41]. Finer flakes are observed when graphite flake are added to copper powder since the graphite exfoliates and forms multilayer graphene which is expected to coat surface of the copper particles and inhibit the cold welding during impact. Also, exfoliated graphite is expected to act like a solid lubricant and result in enhanced deformation of the Cu particles due to reduced friction between the impacting WC balls and copper surface. Hence, with the increase in ball milling time, the composite powder particles are fragmented into fine powders as shown in Fig. 10b. The cross-section of 5 h milled Cu-Ag-GF mixture shown in Fig. 10c indicates fine sized lamellar structure. Un-dissolved Ag lamella is observed to be distributed inside the copper matrix as seen in the inset in Fig. 10c. The EDS spectra from the bright lamella confirmed them to be Ag. The 40 h milled Cu-Ag-GF mixture is observed to be flake like. Each of the flakes seems to be made of several smaller flakes which
Fig. 11. The variation of D10 (a), D50 (b) and D90 (c) values with milling time for the different powders.
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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are either mechanically bonded or cold welded to each other. The presence of graphite is expected to inhibit cold welding resulting in the unique structure of the flakes. 3.5. Particle size distribution of milled powders The D10, D50 and D90 values obtained from particle size analysis are plotted as a function of milling time in Fig. 11. The D10, D50 and D90 values for 5 h milled Cu were 18, 65 and 190 mm and for 5 h milled Cu-Ag powder were 28, 62 and 135 mm. The lower particle size for Cu-Ag powders is due to alloying of Ag in Cu (as confirmed by Fig. 9) which is expected to make it harder and less ductile resulting in fracture upon impact by the balls. The D10 value of Cu-Ag is higher, which could be due to the fact that Ag is more ductile than Cu. The particle size of 5 h milled Cu-GF and Cu-Ag-GF is found to be smaller than Cu and Cu-Ag mixtures. The presence of graphite inhibits cold welding and results in drastic reduction of the particle size upon milling as compared to gradual reduction observed in Cu and Cu-Ag. The D50 values indicate that the 40 h milled Cu-GF powders have a slightly larger size compared to other powders. This could be due to the lubricating effect of the graphene layers which results in higher flattening of Cu during impacts. The presence of GF and higher work hardening rate and fracture of the Cu-Ag solid solution results in lowest sizes for Cu-Ag-GF powder. However, it is observed that beyond 20 h of milling, there is no significant reduction in the size of the powders. The lattice parameter variation (Fig. 6) indicates that there is no increase in alloying for Cu-GF or Cu-Ag beyond 20 h of milling. However, dissolution of Ag continues in case of Cu-Ag-GF powders up to 40 h of milling. 4. Conclusions The following conclusions are drawn from the present work: Ball milling of Cu, Cu-Ag, Cu-GF & Cu-Ag-GF powders resulted in flaky particles with lamellar structure. With the addition of Ag, the lattice parameter of Cu increased with milling time. The dissolution of Ag was complete by 20 h of milling as noticed by the constant lattice parameter beyond 20 h of milling. There is no change in lattice parameter of Cu upon graphite addition indicating that alloying of Cu and carbon does not take place. Graphite addition also hinders the solubility of Ag in Cu as is seen from the undissolved Ag layers in the 5 h ball milled powder of Cu-Ag-GF and the lower lattice parameter compared to Cu-Ag powder. Upon ball milling, graphite addition drastically reduces the crystallite size of Cu and Cu-Ag powders initially. The crystallite size becomes more or less same for all powders at higher ball milling durations. A similar phenomenon is also observed for particle size. Raman spectra indicated that the graphite flakes exfoliated into multi-layered graphene in Cu-GF and Cu-Ag-GF powders upon 10 h of ball milling. The Id/Ig ratio increases for all the graphite added powders indicating defect accumulation.
Acknowledgments A. Pragatheeswaran acknowledges funding from Institute Post Doctoral Fellowship from IIT Madras for carrying out the work. Authors thank Nanotechnology Laboratory of Dept. of Metallurgical and Matreials Engineering, IIT Madras, for extending the facilities.
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Original Research Paper
Microstructural and morphological changes during ball milling of Copper-Silver-Graphite flake mixtures A. Pragatheeswaran, Rahul Ravi, Srinivasa Rao Bakshi ⇑ Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Chennai 600036, India
a r t i c l e
i n f o
Article history: Received 23 April 2019 Received in revised form 12 August 2019 Accepted 19 August 2019 Available online xxxx Keywords: Copper Graphene Graphite flake Ball milling Raman spectra
a b s t r a c t The present study reports the microstructural and morphological changes during high energy ball milling of Cu with Ag and Graphite flakes. XRD patterns of ball milled Cu-Ag showed a reduction in the intensity of Ag peaks (1 1 1) and an increase in the lattice parameter of Cu. With an increase in milling time, the formation of metastable Cu-Ag solid solution was observed. Lattice parameter values for 40 h milled Cu (3.6169 Å) and Cu-GF composites (3.6166 Å) indicated that C does not dissolve in Cu. The lattice parameter of Cu in milled Cu-Ag-graphite flake was lower compared to milled Cu-Ag mixture indicating that graphite flakes inhibit solid solution formation. Raman spectra revealed that graphite flakes were converted into multilayer graphene after 10 h of milling. The crystallite size of Cu in the milled powders decreased with increase in milling time and reached a value of 25 nm after 35 h of milling. The lattice strain also increased with milling time. The D10, D50 and D90 size reduced appreciably after 5 h of milling. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Copper and its alloys are conventional materials widely used in industrial applications for their excellent electrical and thermal conductivity, low cost of fabrication, and high resistance to corrosion and fatigue [1]. The basic properties needed for conductor and contact materials are high electrical conductivity, good mechanical strength and wear resistance, low weight, and low cost. The mechanical strength of copper can be enhanced by alloying it with other elements or by the addition of fine particles of the second phase in its matrix [2–4]. Many investigations have shown improvement in mechanical properties, however, it is well reported that introducing any second phase into copper reduces its electrical conductivity [5–7]. Hence, the selection of materials for reinforcement is very important. Carbon nanomaterials like carbon nanotubes (CNT) and graphene have been used in Cubased composites for thermal and electrical applications [8,9]. CNT/graphene hinder the dislocation motion in metal-CNT/ graphene composites resulting in high strength [10–13]. However, homogeneous distribution of the reinforcement throughout the sample and high cost of graphene/CNT are major problems. Powder metallurgy (PM) has been used in manufacturing precision components/composite materials for various industries such ⇑ Corresponding author.
as aerospace, electrical, oil and gas, automotive and healthcare industries [14,15]. Mechanical alloying (MA) is one of the methods used for synthesizing materials with metastable phases, such as supersaturated solid solutions, nonequilibrium crystalline or quasicrystalline intermediate phases, and amorphous alloys [16]. In the MA powder processing technique, the powder particles are repeatedly deformed, cold-welded and fractured under the impact of hard balls. Homogeneous distribution of particles and alloying can be achieved using this method [17,18]. Additionally, reduction of particle and grain size happens during milling leading to enhanced diffusivity [19]. All these effects lead to alloying of the elemental powders. Cu-Graphite PM products have been used for electrical contact applications. Powder metallurgy is the preferred choice of manufacturing process as compared to casting methods since graphite has very low density (2.3 g/cc) compared to Cu (8.9 g/cc) and also does not wet liquid copper [20,21]. Graphite is a layered structure in which carbon atoms are packed in a hexagonal lattice. Extremely weak van der Waals force holds the adjacent layers of graphite [22]. The exfoliation of graphite layers to multilayered graphene can happen during ball milling. Mechanical milling has been used to prepare exfoliated graphite or graphene using different milling mediums such as an organic solvent in the presence of dry ice, NaCl, and melamine [23–26]. The aim of the present study is to investigate the possibility of producing Cu-Ag-Graphene composite powder using high
E-mail address: [email protected] (S.R. Bakshi). https://doi.org/10.1016/j.apt.2019.08.023 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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energy ball milling of Cu-Ag-Graphite flake mixtures. The effect of ball milling time on the phase evolution, lattice parameter, microstructure, and particle size of the powders was investigated. 2. Experimental 2.1. Ball milling The SEM images of the powders used in the study are shown in Fig. 1. The Copper and silver powders were obtained from Alpha Aeser having a purity of 99.9%. The particle size of Copper is found to be in the range of 10–20 mm. The silver particles were in the form of agglomerates of fine sub-micron sized particles as seen in the inset image of Fig. 1b. Graphite flake was obtained from Alpha Aeser having a purity of 99% and the average particle size of 10 mm as shown in Fig. 1c. Copper (named Bm-Cu), Copper2 wt% Graphite flake (named Cu-GF), Copper-5 wt% Silver (named Cu-Ag), Copper-5 wt% Silver-2 wt% Graphite flake (named Cu-AgGF) mixtures were subjected to ball milling for 40 h. In addition, the copper powder was also milled for comparison. Tungsten carbide lined vials and balls were used for milling and a ball to powder ratio of 10:1 was used for all the mixtures. The ball milling was performed in toluene medium with a rotation speed of 300 rpm using a Fritsch Pulverisette 5 planetary ball mill. The milling was stopped for 30 min after every 1 h of milling, to prevent heating of the powder. 2.2. Powder characterization Powder X-Ray diffraction (XRD) patterns were recorded using a PANalytical(X’pert Pro) with Cu-Ka X-rays. Raman spectroscopy
measurements were performed by a laser Raman spectrometer (LabRAM HR, France) with a He-Ne laser (514 nm wavelength) as the excitation source. The microstructure and morphology of milled powders were observed by scanning electron microscopy (Quanta 400, FEI, Hillsboro, USA). The milled powders were mounted in cold setting epoxy resin and polished using various emery papers and final polishing was done with diamond paste (up to 0.25 mm size). The particle size distribution was measured using a laser particle size analyser (Microtrac s3500). The test was repeated on 3 samples and average D10, D50 and D90 values were reported.
3. Results and discussion 3.1. Phase evolution of milled powders Fig. 2 shows the XRD patterns of the milled powders of Cu and Cu-GF mixture up to 40 hrs of milling. Peaks corresponding to copper were observed in starting copper powder and there were no oxide peaks. Peak broadening was observed with increasing ball milling time as seen in Fig. 2a. Formation of Cu2O was found to increase with milling time. This indicates that the reduction in the particle size due to prolonged milling time had severe effects on oxidation during milling and drying thereafter. As shown in Fig. 2b, the XRD patterns of milled Cu-2 wt% graphite flake mixture shows a peak corresponding to (0 0 2) reflections of graphite at 26.7°. The Lattice parameter of the Cu in CuGF mixture is found to be 3.6103 Å from the XRD pattern, which is similar to that of Cu (3.610 Å as per JCPDS card no. 04-0836). The deviation from the JCDPS card value is due to dissolved impurities in the powder. Thus, the milling did not result in alloying of
Fig. 1. SEM images of starting powders (a) Copper, (b) Silver, and (c) Graphite flakes.
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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Fig. 2. X-ray diffraction patterns of powders milled for different times for (a) Cu, and (b) Cu-GF mixture.
Fig. 3. (a) X-ray diffraction patterns of Cu-Ag mixtures milled for various times, and (b) XRD patterns of ball milled Cu-Ag on a magnified scale.
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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Cu and C. After milling for 40 h, the peak of graphite disappeared and the presence of Cu2O was observed. The disappearance of the graphite peak could be due to the formation of multi-layered graphene and the dispersion within the matrix. Chen et al. [24] have reported that graphite can be exfoliated under the shear force during BM resulting in the formation of multilayered graphene. Hence, the disappearance of the graphite diffraction peak may indicate the exfoliation of the graphite layers. Several authors have observed disappearance of C peaks after ball milling. Rajendra Kumar et al. [27] have reported that 20 h of ball milling converted graphite to graphene and observed that the graphite peaks intensity was substantially decreased with milling time due to the formation of few layered graphene structure. Nayan et al. [28] have also shown that peaks corresponding to carbon nanotube disappear after milling for a few hours in copper. It is noted that the disappearance of graphite peak is not due to dissolution in carbon as discussed later using lattice parameter calculations. Fig. 3(a) shows the XRD patterns of Cu-Ag ball milled powder up to 40 h. The Ag peaks disappear after 5 h and the Cu peaks shift towards lower angles as seen in Fig. 3b. It is known from the CuAg phase diagram that the maximum solubility of Ag in Cu is 8% at 779 °C, however, there is no solubility at room temperature [29]. It is also known that BM leads to mechanical alloying and formation of metastable solid solution [16]. The dissolution of Ag into Cu results in an increase of the lattice parameter of Cu. This also results in a peak shift towards lower angles. The XRD patterns of ball milled Cu-Ag-GF powders are shown in Fig. 4. It is observed that with graphite flake addition, the Ag peak intensity is reduced to very low values only after 15 h of milling. This indicates that graphite flakes inhibit the mechanical alloying Fig. 5. XRD patterns of Cu (1 1 1) peak for different ball milled powders.
process. It is well known that during ball milling, the powders are flattened into flakes and cold welding of flakes take place due to the impact of the balls. This repeated flattening and coldwelding leads to alloying. Presence of graphite flakes at the interface is expected to prevent cold welding and alloying of Cu with Ag. Fig. 5 shows the (1 1 1) peak of Cu in all the 40 h milled powders. The peak shift in Cu-Ag is clearly seen in Fig. 5. It is to be noted that all the XRD patterns do not show any tungsten carbide peaks which could be generated from the wear of milling balls or vials.
Fig. 4. X-ray diffraction patterns of as-blended Cu-Ag-GF and ball milled Cu-Ag-GF with an interval time period of 5 h.
Fig. 6. Lattice parameter variation of Cu with ball milling time.
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Fig. 7. Variation of the (a) crystallite size, and (b) lattice strain of copper with milling time.
3.2. Lattice parameter, crystalline size and lattice strain variation Fig. 6 shows the variation of the lattice parameter of Cu in different powders as a function of milling time. The lattice parameters of the as received Cu powder and the mixtures were measured by Nelson Riley function from powder XRD patterns. The lattice parameter of ball milled Cu and Cu-GF increases by 0.16% after 5 h of milling. There is no variation in the lattice parameter with further increasing milling time. This could be due to the dissolution of oxides or impurities present in the powder. It is noted that the maximum solubility of C in Cu is a maximum of 0.0076 wt% at 1100 °C [30]. The lattice parameter values of the 40 h milled Cu and Cu-GF are very close which indicates that no dissolution of C occurs in Cu due to mechanical milling. It is observed that in the
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case of Cu-Ag mixture, the lattice parameter of copper increased from 3.610 to 3.630 Å after 20 h milling and further increasing the milling time does not change lattice constant significantly (3.632 Å for 40 h powder). Thus, almost all the silver is dissolved after 20 h of milling. In case of Cu-Ag-GF mixtures, the lattice parameter in Cu increases gradually up to 40 h of milling. The lower values of lattice parameter of Cu in Cu-Ag-GF powder compared to Cu-Ag powders indicate that complete alloying does not take place even after 40 h in the presence of graphite flakes. This indicates that the graphite flakes inhibit the alloying process and acts like a process control agent. Fig. 7 shows the crystallite size and lattice strain variation of Cu in ball milled Cu, Cu-Ag, Cu-GF and Cu-Ag-GF for various milling times. The crystallite size and lattice strain were estimated by the Williamson-Hall method. The crystallite size decreases exponentially with time and reaches 25 nm after 35 h. The crystallite size increase after 40 h of milling for Cu-Ag could be due to recrystallization. This increase in crystallite size was confirmed through repeated XRD experiments and needs to be looked into in further detail. The 40 h milled Cu shows further reduction in crystallite size. Severe plastic deformation undergone by the particles during ball milling leads to refinement of crystalline size due to crystal defects such as dislocations, vacancies and increased number of grain boundaries [16,31]. The rate of decrease of crystallite size in Cu-Ag is higher compared to Cu and reaches the lowest value in about 20 h. The Cu-Ag mixture starts alloying after 15 h and the alloy gets easily work hardened and fractures more leading to finer flakes. The extent of dissolution up to 15 h of milling time of Ag in Cu is less as indicated by the lower peak shifts in Fig. 3b and small increase in lattice parameter in Fig. 6. It has been reported that the solid solubility limits increases as the crystallite size reduces in the nano-range [32,33]. The reduced crystallite size in Cu-Ag powder leads to high diffusivity and reactivity. The reduction of crystallite size after 20 h of milling increases the rate of dissolution significantly. After 5 h of milling, the crystallite size of Cu in ball milled Cu and Cu–Ag reduced to 175 nm and 150 nm respectively, however, in case of Cu-GF and Cu-Ag-GF powders it reduced to 50 nm and 75 nm, respectively. Thus, addition of graphite reduces the crystallite size at a faster rate. Graphite addition hinders the cold-welding and enhances the work-hardening rate resulting in larger reduction of crystallite size. The lattice strain of Cu in ball milled samples is found to increase with increase in milling time as seen in Fig. 7b. It is observed that addition of Ag
Fig. 8. (a)-Raman spectra of starting GF, ball milled Cu-GF, (b)-ball milled Cu-GF-Ag, (c) -ID/IG ratio of GF addition powders with various time intervals.
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Fig. 9. Backscattered scanning electron microscopy images of Cu-Ag powder milled for 5 h along with the EDS elemental distribution map (a) and after 40 h of milling (b).
increases the lattice strain which could be due to the alloying effect. Also the addition of graphite results in further increase in the lattice strain. This could be due to the higher deformation of the Cu flakes due to the lubricating effect of graphite. Exfoliation of graphite leading to formation of multi-layer graphene was confirmed by Raman spectra measurements. 3.3. Raman spectra of milled powders Raman scattering is a good tool to get information on the surface structure of graphene and its defects density [34]. Fig. 8(a and b) shows the Raman spectra of as-received graphite flake and ball milled Cu-GF and Cu-Ag-GF for after 5, 10, 20 and 40 h of milling. Fig. 8(a and b) shows the Raman spectrum of graphite flake consists of three characteristic bands. The most remarkable
features of the spectrum are the G band at 1563 cm 1 which relates to E2g mode [35,36]. The second peak at 1334 cm 1, called the D peak, is usually associated with the presence of lattice disorders of the graphite flake. Another important peak observed at 2683 cm 1, named as 2D peak or second order graphite peak. For graphite, the 2D mode has a lower intensity and is wider as compared to G mode (Fig. 8). This suggests the presence of a layered structure in case of the graphite flake [37,38]. The G band in case of the ball milled powders decreases in intensity with an increase in milling time. The broadening and less intensity of G peak indicate that graphite layers were exfoliated upon ball milling for 10 h. The G band shifts towards higher wavenumbers with increasing ball-milling time up to 10 h, and thereafter starts shifting towards lower wave number. This result can be attributed to decreasing graphite layers with increasing
Fig. 10. Backscattered scanning electron microscopy images of polished cross section of Cu-GF powder milled for 5 h (a) and 40 h (b) and Cu-Ag-GF powder milled for 5 h (c) and 40 h (d).
Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023
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ball-milling time. The D band intensity depends on the atomic structure at the edge of graphite. The lower intensity of D band in graphite flake indicates that the graphite sample was defect free and had a highly oriented structure [39]. The intensity ratios of D to G peak (ID/IG) for ball milled samples of Cu-GF and Cu-Ag-GF composites are presented in Fig. 6c. It is observed that the ID/IG ratio gradually increases with increase in ball-milling time. The ratio of ID/IG in both the mixtures is found to be 0.15 after 5 h of milling and it increases to 0.91 upon 40 h of milling. This result shows the structural distortion and size reduction of the in-plane sp2 domain caused due to their processing through ball-milling route. The ratio of IG/ID suggests two important results. Firstly, the initial graphite layers are exfoliated and converted as multilayered graphene after about 10 h of milling, and secondly, the ball milling at a prolonged time generated structural disorder in the graphene. The results indicate that inexpensive graphite flakes can be used in place of expensive graphene nano platelet for preparation of the composites. 3.4. Microstructure of the milled powders Fig. 9a shows the backscattered SEM images of the cross section with 5 h milled Cu-Ag powder. The lamellar structure can be clearly observed due to cold welding of the layers. SEM EDS maps indicate a uniform dispersion of Ag indicating the dissolution of Ag into Cu matrix. It is observed that ball milling for 5 h changes results in conversion of fine spherical starting powders into large
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flakes with thickness up to 40 mm and length up to 80 mm. Very fine flakes are also observed. A substantial decrease in the particle size is observed for the 40 h milled Cu-Ag powder as shown in Fig. 9b. The impact of the balls imparts large mechanical energy and strain on particles during milling which leads to severe work hardening that fracture of large particles into smaller powder particles [40]. Fig. 10a shows the morphology of 5 h milled Cu-GF mixture and is observed to be different from Cu-Ag mixture. Formation of flake like structure in ball milled ductile materials is well known and is used to prepare flaky powders. It is also known that long duration of milling can also lead to cold welding and formation of large spherical powders as is in the case of Al powders [41]. Finer flakes are observed when graphite flake are added to copper powder since the graphite exfoliates and forms multilayer graphene which is expected to coat surface of the copper particles and inhibit the cold welding during impact. Also, exfoliated graphite is expected to act like a solid lubricant and result in enhanced deformation of the Cu particles due to reduced friction between the impacting WC balls and copper surface. Hence, with the increase in ball milling time, the composite powder particles are fragmented into fine powders as shown in Fig. 10b. The cross-section of 5 h milled Cu-Ag-GF mixture shown in Fig. 10c indicates fine sized lamellar structure. Un-dissolved Ag lamella is observed to be distributed inside the copper matrix as seen in the inset in Fig. 10c. The EDS spectra from the bright lamella confirmed them to be Ag. The 40 h milled Cu-Ag-GF mixture is observed to be flake like. Each of the flakes seems to be made of several smaller flakes which
Fig. 11. The variation of D10 (a), D50 (b) and D90 (c) values with milling time for the different powders.
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are either mechanically bonded or cold welded to each other. The presence of graphite is expected to inhibit cold welding resulting in the unique structure of the flakes. 3.5. Particle size distribution of milled powders The D10, D50 and D90 values obtained from particle size analysis are plotted as a function of milling time in Fig. 11. The D10, D50 and D90 values for 5 h milled Cu were 18, 65 and 190 mm and for 5 h milled Cu-Ag powder were 28, 62 and 135 mm. The lower particle size for Cu-Ag powders is due to alloying of Ag in Cu (as confirmed by Fig. 9) which is expected to make it harder and less ductile resulting in fracture upon impact by the balls. The D10 value of Cu-Ag is higher, which could be due to the fact that Ag is more ductile than Cu. The particle size of 5 h milled Cu-GF and Cu-Ag-GF is found to be smaller than Cu and Cu-Ag mixtures. The presence of graphite inhibits cold welding and results in drastic reduction of the particle size upon milling as compared to gradual reduction observed in Cu and Cu-Ag. The D50 values indicate that the 40 h milled Cu-GF powders have a slightly larger size compared to other powders. This could be due to the lubricating effect of the graphene layers which results in higher flattening of Cu during impacts. The presence of GF and higher work hardening rate and fracture of the Cu-Ag solid solution results in lowest sizes for Cu-Ag-GF powder. However, it is observed that beyond 20 h of milling, there is no significant reduction in the size of the powders. The lattice parameter variation (Fig. 6) indicates that there is no increase in alloying for Cu-GF or Cu-Ag beyond 20 h of milling. However, dissolution of Ag continues in case of Cu-Ag-GF powders up to 40 h of milling. 4. Conclusions The following conclusions are drawn from the present work: Ball milling of Cu, Cu-Ag, Cu-GF & Cu-Ag-GF powders resulted in flaky particles with lamellar structure. With the addition of Ag, the lattice parameter of Cu increased with milling time. The dissolution of Ag was complete by 20 h of milling as noticed by the constant lattice parameter beyond 20 h of milling. There is no change in lattice parameter of Cu upon graphite addition indicating that alloying of Cu and carbon does not take place. Graphite addition also hinders the solubility of Ag in Cu as is seen from the undissolved Ag layers in the 5 h ball milled powder of Cu-Ag-GF and the lower lattice parameter compared to Cu-Ag powder. Upon ball milling, graphite addition drastically reduces the crystallite size of Cu and Cu-Ag powders initially. The crystallite size becomes more or less same for all powders at higher ball milling durations. A similar phenomenon is also observed for particle size. Raman spectra indicated that the graphite flakes exfoliated into multi-layered graphene in Cu-GF and Cu-Ag-GF powders upon 10 h of ball milling. The Id/Ig ratio increases for all the graphite added powders indicating defect accumulation.
Acknowledgments A. Pragatheeswaran acknowledges funding from Institute Post Doctoral Fellowship from IIT Madras for carrying out the work. Authors thank Nanotechnology Laboratory of Dept. of Metallurgical and Matreials Engineering, IIT Madras, for extending the facilities.
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Please cite this article as: A. Pragatheeswaran, R. Ravi and S. R. Bakshi, Microstructural and morphological changes during ball milling of Copper-SilverGraphite flake mixtures, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.08.023