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The effect of milling and sintering techniques on mechanical properties of Cu–graphite metal matrix composite prepared by powder metallurgy route
Journal of Alloys and Compounds 569 (2013) 95–101
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
The effect of milling and sintering techniques on mechanical properties of Cu–graphite metal matrix composite prepared by powder metallurgy route C.P. Samal, J.S. Parihar, D. Chaira ⇑ Department of Metallurgical and Materials Engineering, National Institution of Technology Rourkela, Rourkela 769 008, Orissa, India
a r t i c l e
i n f o
Article history: Received 4 February 2013 Received in revised form 10 March 2013 Accepted 12 March 2013 Available online 27 March 2013 Keywords: Cu–graphite Metal matrix composites Powder metallurgy Microstructure Mechanical property
a b s t r a c t When copper is used as electrical sliding contacts, a large contact force is desirable to maintain effective current transfer, whilst on other hand it is advisable to have as small as contact force as possible in order to reduce the wear of the sliding components. In the present investigation Cu–graphite metal matrix composites (MMCs) were prepared by conventional powder metallurgy route using conventional and spark plasma sintering (SPS) techniques. It has been found that addition of graphite into copper does not result in much improvement on the hardness due to the soft nature of graphite. However, 90% and 97% of relative density have been obtained for conventional sintered and SPS samples respectively. Maximum Vickers hardness of around 100 has been achieved for Cu–1 vol.% graphite MMC when it is fabricated by SPS due to the combined effect of pressure and spark plasma effect. However, a hardness value of 65 has been obtained for the same composite when it is fabricated by conventional sintering at 900 °C for 1 h. It has been found that density and hardness of MMCs decreases up to 20 h of milling due to flake formation and increase in size and after that these values increases as particle shape changes to irregular and size reduction takes place. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Metal matrix composites (MMCs) have recently attracted attention due to their unusual properties like specific strength, specific stiffness, wear resistance, corrosion resistance and elastic modulus. Copper is widely used as electrical contacts and resistance welding electrodes as it has very good electrical and thermal conductivity where one is faced with the problem of transferring electric current from a stationary conductor to another conductor moving relative to it. In such type of electrical contacts a large contact force is desirable to maintain effective current transfer, whilst on other hand it is advisable to have as small as contact force as possible in order to reduce the wear of the sliding components. These two requirements cannot be treated in isolation from one another. So, the mechanical behavior of copper is most important which ultimately tells us that whether copper can be used smoothly for a certain electrical contact application. Copper–graphite particulate composites possess the properties of copper, i.e. excellent thermal and electrical conductivities, and properties of graphite, i.e. solid lubricating and small thermal expansion coefficient. Copper matrix containing graphite is widely used as brushes, electrical contacts and bearing materials in many ⇑ Corresponding author. Tel.: +91 9438370956; fax: +91 661 2462999. E-mail address: [email protected] (D. Chaira). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.122
applications due to the excellent thermal and electrical conductivities, and the favorable self-lubricating performance [1]. It has been reported that the addition of solid lubricant particles into a metal matrix improves not only the anti-friction properties, but also wear and friction properties. Most of the bearing alloys that are presently used contain a soft phase like lead, which gives the required anti-friction property. Due to its harmful effects, restrictions have been imposed on the use of lead. This has prompted researchers to find alternative materials, which impart tribological properties similar to those of lead. Certain metal matrix composites (MMCs) containing soft particles have been investigated for tribological properties. These MMCs have not only reduced friction but also lead to reduced wear of the counterface. Faced with all these challenges like low mechanical properties of pure Cu, Cu based alloys and harmful effects of Pb, a suitable alternative material copper–graphite metal matrix composite has been developed in the present investigation. Although certain metals, metal oxides and non-metallic materials have desirable characteristics for contact applications, such as erosion and welding resistance, but are of low conductivity. Combination of these with copper, to give acceptable conductivity, should therefore produce a material with optimum properties. The only viable manufacturing procedure at room temperature to produce such combinations is powder metallurgy. The powder metallurgy is one of the popular solid state methods used in pro-
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duction of metal matrix composites. Powder processing involves cold pressing and sintering, or hot pressing to fabricate primarily particle- or whisker-reinforced MMCs. The matrix and the reinforcement powders are blended to produce a homogeneous distribution [2–4]. Several researchers have fabricated Cu–graphite MMC by powder metallurgy route. Moustafa et al. [5] showed that density, yield and compression strengths of Cu coated composites are superior to those of uncoated ones. Ma et al. [6] proposed that the wear loss of Cu–graphite increases with increasing normal stress and electrical current. Adhesive wear, abrasive wear and electrical erosion wear are the dominant wear mechanisms during the electrical sliding wear processes. Seah et al. [7] prepared cast ZA-27/graphite composite and reported that the increase in compressive strength is due to graphite particle acting as barriers to the dislocations in the microstructure and with increasing graphite content within the ZA-27 matrix results in significant increases in the ductility, UTS, compressive strength and Young’s modulus, but a decrease in the hardness. Ramesh et al. [8] suggested that microhardness and tensile strength of copper–SiC–graphite hybrid composites are higher as compared to the matrix copper. Higher amount of hard reinforcement in the hybrid composites leads to enhancement in microhardness and strength of the hybrid composites, however, ductility decreases. Rajkumar et al. [9] noticed that copper–graphite composites were effectively sintered using microwave hybrid heating without any crack. The finer microstructure with relatively smaller and round pores, resulted due to microwave heating, enhances the performance of the composite. Yeoh et al. [10] illustrated that the expansion of the cylindrical specimens is observed in both the longitudinal and lateral dimensions with the greatest expansions measured for those composites in the 50 vol.% copper–50 vol.% graphite ranges. Gautam et al. [11] studied the dry sliding wear behavior of hot forged and annealed Cu–Cr–graphite in situ metal matrix composites. They showed that average coefficient of friction decreases with increasing the normal load and the graphite containing composite shows the lower average coefficient of friction than the other materials. They also pointed out that worn surface of the pin specimen show significant formation of transfer layer for hot forged and annealed Cu–4Cr–4G composite. Chen et al. [12] prepared Cu based MMC containing solid lubricants like graphite and BN by powder metallurgy hot press method. Zhang et al. [13] showed that Cu/Zr2Al3C4 composite possesses comparable electrical conductivity and better mechanical properties than those of Cu/graphite composite, which was widely accepted as the electro-contacting material. Yang et al. [14] observed that bending strength and micro-hardness increases, whereas wear rate decreases with increasing the pitch coke content in copper–carbon composite. Queipo et al. [15] prepared pitch based Cu–C composites by powder metallurgy route and they found that the incorporation of copper into the optimized pitch/ graphite mixtures favors the compressibility of the system and flexural strength and electrical conductivity of carbon–copper composites are improved with respect to carbon composites. It has been observed that there is a plenty of literatures available on wear studies of Cu–graphite composites. However, there is a scarcity of literatures on microstructure and mechanical properties of Cu–graphite composites, which ultimately affects the properties of Cu–graphite MMC that can be used for electrical contact applications. So, we have decided to give a fresh look on microstructure and mechanical properties of Cu–graphite composite. In the present investigation, our main aim is to fabricate Cu–graphite MMCs by conventional powder metallurgy route using conventional and spark plasma sintering techniques to see the physical, mechanical attributes and microstructures of MMCs. The effect of milling of the composite powder mixture before sintering on final properties and microstructure has also been studied.
Table 1 Used milled parameters. Mill type Milling time Wet milling Milling speed
Pulverisette-5 planetary ball mill 2, 4, 8, 20 and 40 h Toluene 300 rpm
Grinding media Type Ball size (diameter) Ball weight Ball to power ratio by weight
Ceramic (Zirconia) 15 mm 125 g/each 5:1
Jar capacity
250 ml
2. Experimental Starting materials Cu (electrolyte grade, purity 99%) and graphite powder having purity 95% were used. Cu powders were mixed with graphite to prepare composite powder mixture of 1, 3, 5 and 10 vol.% of graphite. Here, matrix and reinforcements are taken into vol.% so that various properties of composites can be calculated using rule of mixtures formula. The composite powder mixtures were then cold compacted at a pressure of 700 MPa for 2 min and sintered in tubular furnace at 900 °C using argon gas for 1 h. In another set of experiment, Cu–1 vol.% graphite and Cu–5 vol.% graphite powder mixture were sintered by spark plasma sintering (DR SINTER LAB SPS Syntex INC, model: SPS-515S, Kanagawa, Japan) at a temperature and pressure of 700 °C and 50 MPa respectively for 5 min under vacuum at a heating rate of 80 °C/min. Spark plasma sintering (SPS) uses high amperage, low voltage, pulse DC current and uni-axial pressure to consolidate powders. To study the effect of milling, Cu–1 and 5 vol.% of graphite powder mixture were milled separately for 40 h. Milling was conducted in Pulverisette-5 planetary ball mill under toluene to prevent oxidation. Amount of toluene was taken before milling such that all the balls and powders were immersed into toluene. The various milling parameters are given in Table 1. The fabricated composites were characterized by X-ray diffractometer (Philips X-Pert) using Cu Ka target (k ¼ 1:542Å), SEM (JEOL JSM-6480 LV) using suitable accelerating voltage and imaging modes and field emission scanning electron microscopy (ZEISS). Sintered densities of the composites were measured by using Archimedes principle. Vickers hardness values of all the sintered specimens were measured by using microhardness tester LM248AT under 0.3 kg load and 5 s dwell time. For each specimen at least five measurements were taken at equivalent positions. Samples for compression test were prepared after cold compaction and sintering while maintaining L/D ratio >0.8. Compression test was carried out on SATEC, 600KN INSTRON at a strain rate of 102 s1. Samples for transverse rupture strength (TRS) were prepared by making standard transverse rupture strength test blocks (6.35 12.7 31.7 mm) according to ASTM B 312. After sintering for 1 h at 900 °C, TRS test was carried out on Instron-1195 using standard 3-point bend test. It should be mentioned here that spark plasma sintering shows higher densification and hardening response than conventional sintering. But compression test and transverse rupture test were carried out on conventional sintered samples as such type of sample preparation is not feasible in SPS process in the present condition. It is also mentioned here that samples for compression strength and TRS could not be prepared for 20 and 40 h milled powder, only density and hardness values are reported here for those samples.
3. Results and discussions 3.1. X-ray diffraction (XRD) study XRD analysis of copper–graphite MMC with different volume percentage of graphite was well conducted. The XRD spectra of conventional sintered and spark plasma sintered composites are shown in Fig. 1a and b. Fig. 1a shows the presence of strong peaks of Cu and very weak peaks of graphite in case of conventional sintering. A very weak peak of graphite is seen due to the occurrence of less amount of graphite. Some amount of copper oxide is developed due to the reaction between residual atmospheric oxygen present in the furnace with Cu as clear from XRD spectra of pure Cu sample. But in case of Cu–graphite MMCs, very less/no copper oxide peak is present as evident from X-ray peaks. The reason may be that copper oxide which is formed is getting reduced to Cu by graphite in case of Cu–graphite MMCs. It can be concluded from XRD spectra that during fabrication of composites, no reaction between copper and graphite takes place. If new phase forma-
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3.2. Microstructure study Fig. 2 shows the representative FESEM micrograph of Cu– 10 vol.% graphite MMC conventionally sintered at 900 °C for 1 h. The micrograph shows the distribution of graphite reinforcement (dark portion) into copper matrix (white portions). It is also observed that agglomeration of graphite into Cu matrix cannot be avoided completely. It has been noticed that some graphite particles float on the surface as graphite is very light as compared to copper. So, proper mixing of Cu and graphite is a challenge. The EDS spectra, which was captured from whole micrograph shows the peaks of Cu and graphite. The quantitative values of Cu and graphite are shown in the table. To study the effect of milling on the mechanical property of Cu– graphite MMC, milling was conducted. It is also aimed to study the effect of metal-coated graphite (MCG) on the mechanical properties (strength and hardness) of consolidated and sintered composite materials. Fig. 3 shows the SEM micrographs of Cu–1 vol.% graphite composite powder mixture milled for different periods in planetary mill. It is clear from the micrographs that particle size increases up to 20 h of milling. As copper is ductile, particle size increases in the initial milling period due to flake formation. It is also observed from the micrographs that after 20 h of milling, size reduction takes place due to strain hardening during milling. It has been noticed that particle shape also changes from flake shape to irregular shape in this stage. Guel et al. [16] also studied the effect of metallic addition on mechanical properties in an aluminum–graphite composite synthesized by means of mechanical milling. They observed that after 8 h of milling, particles are smaller and rounded and homogeneously distributed into the layers of aluminum matrix. Fig. 4 shows the SEM micrographs of Cu–5 vol.% graphite reinforced composite powder mixture milled for different time periods and then sintered at 900 °C for 1 h. The micrographs show the presence of large amount of pores in initial milling period. As the particles being flaky, compaction and sintering of such powder leads to generation of large amount of porosity. But after 20 h of milling, porosity decreases drastically and it is clearly visible from the micrographs. After 20 h of milling, particle size changes from flaky to irregular shape and eventually leads to decrease in porosity.
3.3. Density measurement
Fig. 1. XRD spectra of Cu–graphite MMCs (a) conventionally sintered at 900 °C for 1 h (b) spark plasma sintered at 700 °C for 5 min and (c) Cu–5 vol.% graphite powder mixture milled in planetary mill for different periods.
tion is there, amount is less than 5% (wt/wt), which cannot be detected by X-ray diffraction. Fig. 1b shows the various peaks of Cu for spark plasma sintering. As SPS was carried out in vacuum no copper oxide was formed. Fig. 1c shows the XRD patterns of Cu– 5 vol.% graphite composite powder mixture milled for different times in planetary mill. It can be seen that peak width increases with increasing milling time. During milling powder particles are plastically deformed due to the collision between balls and powders. This indicates that grain refinement takes place and lattice strain increases as milling progresses.
The variation of density with increase in graphite volume percentage is shown in Fig. 5a. It has been found a density value of around 90% for conventional sintering and 96% for spark plasma sintering. The reason is improved bonding between Cu and graphite due to the simultaneous applications of pressure and pulsed continuous current during SPS process. A very high temperature may be attained in the contact area, over the melting temperature of the material, leading to localized melting, which enhances interparticle bonding. It is also noticed that density value increases with graphite content. The reason is that when pressure is applied graphite particles are also deformed with Cu particles due to soft nature and which helps in plastic flow and eventually density value goes up. This also can be attributed to the relationship of the relative density (sintered density/theoretical density). The higher the graphite content, the lower the theoretical and measured density of the composites. Fig. 5b shows the variation of density with milling. It is found that density decreases up to 20 h of milling and after that density increases. The presence of porosity leads to decrease in density up to 20 h, after that porosity decreases and density goes up. SEM micrographs (Fig. 4) also support the same observations.
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Fig. 2. Representative FESEM micrographs of Cu–10 vol.% graphite MMC conventionally sintered at 900 °C for 1 h. EDS spectra and elemental compositions are also shown in this figure.
Fig. 3. SEM micrographs of Cu–1 vol.% graphite composite powder mixture milled for different periods in planetary mill.
3.4. Hardness study The variation in hardness with graphite reinforcements is shown in Fig. 6a. The general hardness trend is that it decreases with increase in graphite percentage due to the soft nature of graphite. The maximum Vickers hardness value of around 70 is achieved for 1 vol.% graphite reinforced conventional sintered composite. Whereas higher hardness value of around 100 is obtained for spark plasma sintered composites due to fine grain size in case of SPS as it takes only 5 min for densification as compared to 1 h for conventional sintering. It is also to be noted that SPS samples show higher density than conventional sintered samples. The good bonding between matrix and reinforcements is also responsible for improvement on the hardness in case of SPS. The dispersion strengthening and grain size refinement are the main strengthening mechanism which is responsible for increase in hardness of Cu–graphite composites. Fig. 6b shows the variation in hardness with milling time. In the initial milling period (up to 10 h) hardness
trend is decreasing but after that hardness value goes up. The reason is large porosity and less density in the initial milled samples and later porosity decreases and density increases. After 40 h of milling, the particle becomes fine and more closely bound and particle–particle contact increases with increase in fineness due to increase in surface area of the milled powder particle. Another reason of increasing hardness is that Cu powder particles are coated with graphite powder to some extent and improves the bonding between Cu and graphite. Guel et al. [16] studied the effect of metallic addition on mechanical properties of Al–graphite MMC by mechanical milling. Similar observations were made by Yusoff et al. [17]. 3.5. Compressive strength study Compression strengths plots are shown in Fig. 7a and b. It is found from Fig. 7a that maximum compression strength (875 MPa) is found in 1 vol.% of graphite reinforced MMC and the
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Fig. 4. SEM micrographs of Cu–5 vol.% graphite composite powder mixture milled for different periods and then conventionally sintered at 900 °C for 1 h.
Fig. 5. (a) Variation in density with graphite content of MMCs conventionally sintered at 900 °C for 1 h and spark plasma sintered at 700 °C for 5 min and (b) variation in density with milling time of MMCs (Cu–1 vol.% and Cu–5 vol.% graphite) conventionally sintered at 900 °C for 1 h.
Fig. 6. (a) Variation of hardness with graphite content for Cu–graphite MMCs fabricated by conventional sintering at 900 °C for 1 h and spark plasma sintered at 700 °C for 5 min; and (b) variation in hardness with milling time for Cu–graphite MMCs fabricated by conventional sintering at 900 °C for 1 h.
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Then have shown that compressive strength decreases from 600 to 475 MPa when the amount of graphite increases from 2.5 to 5 wt.%. Fig. 7b shows variation in compressive strength with milling time for Cu–5 vol.% graphite reinforced MMC. It is found that compressive strength increases with milling due to introduction of large number of defects, improved dispersion during milling and strain hardening effect. Seah et al. [7] prepared cast ZA-27/graphite composite and reported that the increase in compressive strength is due to graphite particle acting as barriers to the dislocations in the microstructure and with increasing the graphite content within the ZA-27 matrix results in significant increases in the ductility, UTS, compressive strength and Young’s modulus, but a decrease in the hardness. The main strengthening mechanism is proposed for high compression strength of the MMCs is dispersion hardening effects. Table 2 shows the summary of mechanical properties of Cu–graphite MMCs obtained from literatures and present investigation.
value is almost close for 5 vol.% graphite also. Dispersion of up to 5 vol.% graphite increases compressive strength due to positive dispersion strengthening. However, beyond 5 vol.%, further addition of graphite reduces compressive strength due to agglomeration of brittle graphite. At 10 vol.% graphite addition compressive strength reaches minimum to 450 MPa due to brittle nature of composite. Dewidar et al. [18] also studied the effect of graphite content on the compressive strength of Cu–graphite composite.
3.6. Transverse rupture strength (TRS) study The flexural strength graph of Cu with 0, 1, 3, 5 and 10 vol.% of graphite composite fabricated by conventional sintering method for 1 h are given in Fig. 8. From this graph, it is seen that maximum elastic modulus (14.5 GPa) is found for 1 vol.% graphite composite. After that elastic modulus decreases continuously up to 5 vol.%. The reason may be due to the agglomeration of graphite particles
Fig. 7. (a) Compressive stress vs. strain curves for Cu–1 vol.%, Cu–5 vol.% and Cu– 10 vol.% graphite reinforced MMC; and (b) compressive stress vs. strain curves for various milling time of Cu–5 vol.% graphite reinforced MMC.
Fig. 8. Plot of flexural strength and elastic modulus with graphite content.
Table 2 Summary of some mechanical properties of Cu–graphite MMCs available from literatures. References
System
% of relative density
Hardness
Rajkumar et al. [9] Moustafa et al. [5]
94 95
100 VHN 21.6 BHN
Yang et al. [14] Chen et al. [12] Zhao et al. [19] Kumar et al. [20]
Cu–graphite MMC by microwave sintering Cu–graphite composites with Cu-coated and uncoated graphite powder Cu–pitch coke–graphite composite Cu–BN–SiC–graphite–Sn–Al–Fe composite Cu–graphite composite by electroforming technique Pitch based Cu–C composite
Present study
Cu–graphite composite by conventional sintering & SPS
90 (conventional) 97 (SPS)
102 VHN 122 VHN 81 VHN 88 VHN 87 VHN (conventional) 105 VHN (SPS)
Max. bending strength (MPa)
Max. compressive strength (MPa)
% Elongation
376
43
875 (conventional)
50
64
150 275 (conventional)
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and improper dispersion of reinforcements. However, at 10 vol.% graphite elastic modulus again increases. The reason is not clear to us. The flexural strength continuously increases up to 5 vol.% and then it decreases. The maximum bending strength (275 MPa) is found in 5 vol.% of graphite reinforced composite. Flexural strength decreases after addition of 5 vol.% graphite due to increase in brittleness of the composites. So, it can be concluded that bending strength increases due to increase in strength arises from dispersion strengthening.
4. Conclusions The following conclusions can be drawn from the present investigation: 1. The copper–graphite metal matrix composites were successfully fabricated by conventional as well as spark plasma sintering techniques. Spark plasma sintering shows better response to densification and hardening than conventional sintering. 2. A density value of around 90% of relative density for conventional sintering and 96% of relative density has been achieved for spark plasma sintering. 3. The maximum Vickers hardness value of around 70 and 100 are achieved for Cu–1 vol.% graphite reinforced MMC fabricated by conventional and spark plasma sintering respectively. 4. Milling of composite powder mixture results in increase in size and flake formation up to 20 h of milling and so sintering of such powders results in reduction in density and hardness due to high porosity. However, after 20 h of milling, size reduction takes place and results in irregular shape of powder particles due to strain hardening. Sintering of such powder shows improvement in density and hardness due to less porosity. 5. The transverse rupture strength increases up to 5 vol.% of graphite reinforcing and after that it decreases.
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6. Compressive strength is maximum at 5 vol.% of graphite reinforced MMC and then further addition of graphite leads to decrease in compressive strength.
Acknowledgements Financial support for this work from the Department of Science and Technology (DST), India (Grant No. SR/FTP/ETA-100/2010) is gratefully acknowledged. The authors are also grateful to Prof. S.K. Karak for conducting FESEM experiments. References [1] M.M. Dewidar, J.K. Lim, J. Compos. Mater. 41 (2007) 2183–2194. [2] X.U. Wei, H.U. Rui, L. Jin-Shan, F. Heng-zhi, Trans. Nonferrous Met. Soc. China 21 (2011) 2237–2241. [3] H. Kaftelen, N. Unlu, G. Goller, M.L. Ovecoglu, H. Henein, Composites: Part A 42 (2011) 812–824. [4] S. Dorfman, D. Fuks, Sens. Actuators A 51 (1995) 13–16. [5] S.F. Moustafa, S.A. El-Badry, A.M. Sanad, B. Kieback, Wear 253 (2002) 699–710. [6] X.C. Ma, G.Q. He, D.H. He, C.S. Chen, Z.F. Hu, Wear 265 (2008) 1087–1092. [7] K.H.W. Seah, S.C. Sharma, B.M. Girish, Mater. Des. 16 (1995) 271–275. [8] C.S. Ramesh, R.N. Ahmed, M.A. Mujeebu, M.Z. Abdullah, Mater. Des. 30 (2009) 1957–1965. [9] K. Rajkumar, S. Aravindan, J. Mater. Process. Technol. 209 (2009) 5601–5605. [10] A. Yeoh, C. Persad, Z. Eliezer, Scr. Mater. 37 (1997) 271–277. [11] R.K. Gautam, S. Ray, S.C. Sharma, S.C. Jain, R. Tyagi, Wear 271 (2011) 658–664. [12] B. Chen, Q. Bi, J. Yang, Y. Xia, J. Hao, Tribol. Int. 41 (2008) 1145–1152. [13] J. Zhang, L. Hea, Y. Zhou, Scr. Mater. 60 (2009) 976–979. [14] H. Yang, R. Luo, S. Han, M. Li, Wear 268 (2010) 1337–1341. [15] P. Queipo, M. Granda, R. Santamarı, R. Mene´ndez, Fuel 83 (2004) 1625–1634. [16] E. Guel, C.C. Gallardo, J.L.C. Cortés, E.R. Rangel, J.M.H. Ramírez, R.M. Sánchez, J. Alloys Comp. 495 (2010) 403–407. [17] Mahani Yusoff, Radzali Othman, Zuhailawati Hussain, Mater. Des. 32 (2011) 3293–3298. [18] M. Dewidar, G.T. Abdel-Jaber, M. Bakrey, H. Badry, Int. J. Mech. Mechatron. 10 (2010) 25–40. [19] H. Zhao, L. Liu, Y. Wu, W. Hu, Compos. Sci. Technol. 67 (2007) 1210–1217. [20] A. Kumar, M. Kaur, R. Kumar, P.R. Sengupta, V. Raman, G. Bhatia, K.N. Sood, J. Mater. Sci. (2010) 1393–1400.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
The effect of milling and sintering techniques on mechanical properties of Cu–graphite metal matrix composite prepared by powder metallurgy route C.P. Samal, J.S. Parihar, D. Chaira ⇑ Department of Metallurgical and Materials Engineering, National Institution of Technology Rourkela, Rourkela 769 008, Orissa, India
a r t i c l e
i n f o
Article history: Received 4 February 2013 Received in revised form 10 March 2013 Accepted 12 March 2013 Available online 27 March 2013 Keywords: Cu–graphite Metal matrix composites Powder metallurgy Microstructure Mechanical property
a b s t r a c t When copper is used as electrical sliding contacts, a large contact force is desirable to maintain effective current transfer, whilst on other hand it is advisable to have as small as contact force as possible in order to reduce the wear of the sliding components. In the present investigation Cu–graphite metal matrix composites (MMCs) were prepared by conventional powder metallurgy route using conventional and spark plasma sintering (SPS) techniques. It has been found that addition of graphite into copper does not result in much improvement on the hardness due to the soft nature of graphite. However, 90% and 97% of relative density have been obtained for conventional sintered and SPS samples respectively. Maximum Vickers hardness of around 100 has been achieved for Cu–1 vol.% graphite MMC when it is fabricated by SPS due to the combined effect of pressure and spark plasma effect. However, a hardness value of 65 has been obtained for the same composite when it is fabricated by conventional sintering at 900 °C for 1 h. It has been found that density and hardness of MMCs decreases up to 20 h of milling due to flake formation and increase in size and after that these values increases as particle shape changes to irregular and size reduction takes place. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Metal matrix composites (MMCs) have recently attracted attention due to their unusual properties like specific strength, specific stiffness, wear resistance, corrosion resistance and elastic modulus. Copper is widely used as electrical contacts and resistance welding electrodes as it has very good electrical and thermal conductivity where one is faced with the problem of transferring electric current from a stationary conductor to another conductor moving relative to it. In such type of electrical contacts a large contact force is desirable to maintain effective current transfer, whilst on other hand it is advisable to have as small as contact force as possible in order to reduce the wear of the sliding components. These two requirements cannot be treated in isolation from one another. So, the mechanical behavior of copper is most important which ultimately tells us that whether copper can be used smoothly for a certain electrical contact application. Copper–graphite particulate composites possess the properties of copper, i.e. excellent thermal and electrical conductivities, and properties of graphite, i.e. solid lubricating and small thermal expansion coefficient. Copper matrix containing graphite is widely used as brushes, electrical contacts and bearing materials in many ⇑ Corresponding author. Tel.: +91 9438370956; fax: +91 661 2462999. E-mail address: [email protected] (D. Chaira). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.122
applications due to the excellent thermal and electrical conductivities, and the favorable self-lubricating performance [1]. It has been reported that the addition of solid lubricant particles into a metal matrix improves not only the anti-friction properties, but also wear and friction properties. Most of the bearing alloys that are presently used contain a soft phase like lead, which gives the required anti-friction property. Due to its harmful effects, restrictions have been imposed on the use of lead. This has prompted researchers to find alternative materials, which impart tribological properties similar to those of lead. Certain metal matrix composites (MMCs) containing soft particles have been investigated for tribological properties. These MMCs have not only reduced friction but also lead to reduced wear of the counterface. Faced with all these challenges like low mechanical properties of pure Cu, Cu based alloys and harmful effects of Pb, a suitable alternative material copper–graphite metal matrix composite has been developed in the present investigation. Although certain metals, metal oxides and non-metallic materials have desirable characteristics for contact applications, such as erosion and welding resistance, but are of low conductivity. Combination of these with copper, to give acceptable conductivity, should therefore produce a material with optimum properties. The only viable manufacturing procedure at room temperature to produce such combinations is powder metallurgy. The powder metallurgy is one of the popular solid state methods used in pro-
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duction of metal matrix composites. Powder processing involves cold pressing and sintering, or hot pressing to fabricate primarily particle- or whisker-reinforced MMCs. The matrix and the reinforcement powders are blended to produce a homogeneous distribution [2–4]. Several researchers have fabricated Cu–graphite MMC by powder metallurgy route. Moustafa et al. [5] showed that density, yield and compression strengths of Cu coated composites are superior to those of uncoated ones. Ma et al. [6] proposed that the wear loss of Cu–graphite increases with increasing normal stress and electrical current. Adhesive wear, abrasive wear and electrical erosion wear are the dominant wear mechanisms during the electrical sliding wear processes. Seah et al. [7] prepared cast ZA-27/graphite composite and reported that the increase in compressive strength is due to graphite particle acting as barriers to the dislocations in the microstructure and with increasing graphite content within the ZA-27 matrix results in significant increases in the ductility, UTS, compressive strength and Young’s modulus, but a decrease in the hardness. Ramesh et al. [8] suggested that microhardness and tensile strength of copper–SiC–graphite hybrid composites are higher as compared to the matrix copper. Higher amount of hard reinforcement in the hybrid composites leads to enhancement in microhardness and strength of the hybrid composites, however, ductility decreases. Rajkumar et al. [9] noticed that copper–graphite composites were effectively sintered using microwave hybrid heating without any crack. The finer microstructure with relatively smaller and round pores, resulted due to microwave heating, enhances the performance of the composite. Yeoh et al. [10] illustrated that the expansion of the cylindrical specimens is observed in both the longitudinal and lateral dimensions with the greatest expansions measured for those composites in the 50 vol.% copper–50 vol.% graphite ranges. Gautam et al. [11] studied the dry sliding wear behavior of hot forged and annealed Cu–Cr–graphite in situ metal matrix composites. They showed that average coefficient of friction decreases with increasing the normal load and the graphite containing composite shows the lower average coefficient of friction than the other materials. They also pointed out that worn surface of the pin specimen show significant formation of transfer layer for hot forged and annealed Cu–4Cr–4G composite. Chen et al. [12] prepared Cu based MMC containing solid lubricants like graphite and BN by powder metallurgy hot press method. Zhang et al. [13] showed that Cu/Zr2Al3C4 composite possesses comparable electrical conductivity and better mechanical properties than those of Cu/graphite composite, which was widely accepted as the electro-contacting material. Yang et al. [14] observed that bending strength and micro-hardness increases, whereas wear rate decreases with increasing the pitch coke content in copper–carbon composite. Queipo et al. [15] prepared pitch based Cu–C composites by powder metallurgy route and they found that the incorporation of copper into the optimized pitch/ graphite mixtures favors the compressibility of the system and flexural strength and electrical conductivity of carbon–copper composites are improved with respect to carbon composites. It has been observed that there is a plenty of literatures available on wear studies of Cu–graphite composites. However, there is a scarcity of literatures on microstructure and mechanical properties of Cu–graphite composites, which ultimately affects the properties of Cu–graphite MMC that can be used for electrical contact applications. So, we have decided to give a fresh look on microstructure and mechanical properties of Cu–graphite composite. In the present investigation, our main aim is to fabricate Cu–graphite MMCs by conventional powder metallurgy route using conventional and spark plasma sintering techniques to see the physical, mechanical attributes and microstructures of MMCs. The effect of milling of the composite powder mixture before sintering on final properties and microstructure has also been studied.
Table 1 Used milled parameters. Mill type Milling time Wet milling Milling speed
Pulverisette-5 planetary ball mill 2, 4, 8, 20 and 40 h Toluene 300 rpm
Grinding media Type Ball size (diameter) Ball weight Ball to power ratio by weight
Ceramic (Zirconia) 15 mm 125 g/each 5:1
Jar capacity
250 ml
2. Experimental Starting materials Cu (electrolyte grade, purity 99%) and graphite powder having purity 95% were used. Cu powders were mixed with graphite to prepare composite powder mixture of 1, 3, 5 and 10 vol.% of graphite. Here, matrix and reinforcements are taken into vol.% so that various properties of composites can be calculated using rule of mixtures formula. The composite powder mixtures were then cold compacted at a pressure of 700 MPa for 2 min and sintered in tubular furnace at 900 °C using argon gas for 1 h. In another set of experiment, Cu–1 vol.% graphite and Cu–5 vol.% graphite powder mixture were sintered by spark plasma sintering (DR SINTER LAB SPS Syntex INC, model: SPS-515S, Kanagawa, Japan) at a temperature and pressure of 700 °C and 50 MPa respectively for 5 min under vacuum at a heating rate of 80 °C/min. Spark plasma sintering (SPS) uses high amperage, low voltage, pulse DC current and uni-axial pressure to consolidate powders. To study the effect of milling, Cu–1 and 5 vol.% of graphite powder mixture were milled separately for 40 h. Milling was conducted in Pulverisette-5 planetary ball mill under toluene to prevent oxidation. Amount of toluene was taken before milling such that all the balls and powders were immersed into toluene. The various milling parameters are given in Table 1. The fabricated composites were characterized by X-ray diffractometer (Philips X-Pert) using Cu Ka target (k ¼ 1:542Å), SEM (JEOL JSM-6480 LV) using suitable accelerating voltage and imaging modes and field emission scanning electron microscopy (ZEISS). Sintered densities of the composites were measured by using Archimedes principle. Vickers hardness values of all the sintered specimens were measured by using microhardness tester LM248AT under 0.3 kg load and 5 s dwell time. For each specimen at least five measurements were taken at equivalent positions. Samples for compression test were prepared after cold compaction and sintering while maintaining L/D ratio >0.8. Compression test was carried out on SATEC, 600KN INSTRON at a strain rate of 102 s1. Samples for transverse rupture strength (TRS) were prepared by making standard transverse rupture strength test blocks (6.35 12.7 31.7 mm) according to ASTM B 312. After sintering for 1 h at 900 °C, TRS test was carried out on Instron-1195 using standard 3-point bend test. It should be mentioned here that spark plasma sintering shows higher densification and hardening response than conventional sintering. But compression test and transverse rupture test were carried out on conventional sintered samples as such type of sample preparation is not feasible in SPS process in the present condition. It is also mentioned here that samples for compression strength and TRS could not be prepared for 20 and 40 h milled powder, only density and hardness values are reported here for those samples.
3. Results and discussions 3.1. X-ray diffraction (XRD) study XRD analysis of copper–graphite MMC with different volume percentage of graphite was well conducted. The XRD spectra of conventional sintered and spark plasma sintered composites are shown in Fig. 1a and b. Fig. 1a shows the presence of strong peaks of Cu and very weak peaks of graphite in case of conventional sintering. A very weak peak of graphite is seen due to the occurrence of less amount of graphite. Some amount of copper oxide is developed due to the reaction between residual atmospheric oxygen present in the furnace with Cu as clear from XRD spectra of pure Cu sample. But in case of Cu–graphite MMCs, very less/no copper oxide peak is present as evident from X-ray peaks. The reason may be that copper oxide which is formed is getting reduced to Cu by graphite in case of Cu–graphite MMCs. It can be concluded from XRD spectra that during fabrication of composites, no reaction between copper and graphite takes place. If new phase forma-
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3.2. Microstructure study Fig. 2 shows the representative FESEM micrograph of Cu– 10 vol.% graphite MMC conventionally sintered at 900 °C for 1 h. The micrograph shows the distribution of graphite reinforcement (dark portion) into copper matrix (white portions). It is also observed that agglomeration of graphite into Cu matrix cannot be avoided completely. It has been noticed that some graphite particles float on the surface as graphite is very light as compared to copper. So, proper mixing of Cu and graphite is a challenge. The EDS spectra, which was captured from whole micrograph shows the peaks of Cu and graphite. The quantitative values of Cu and graphite are shown in the table. To study the effect of milling on the mechanical property of Cu– graphite MMC, milling was conducted. It is also aimed to study the effect of metal-coated graphite (MCG) on the mechanical properties (strength and hardness) of consolidated and sintered composite materials. Fig. 3 shows the SEM micrographs of Cu–1 vol.% graphite composite powder mixture milled for different periods in planetary mill. It is clear from the micrographs that particle size increases up to 20 h of milling. As copper is ductile, particle size increases in the initial milling period due to flake formation. It is also observed from the micrographs that after 20 h of milling, size reduction takes place due to strain hardening during milling. It has been noticed that particle shape also changes from flake shape to irregular shape in this stage. Guel et al. [16] also studied the effect of metallic addition on mechanical properties in an aluminum–graphite composite synthesized by means of mechanical milling. They observed that after 8 h of milling, particles are smaller and rounded and homogeneously distributed into the layers of aluminum matrix. Fig. 4 shows the SEM micrographs of Cu–5 vol.% graphite reinforced composite powder mixture milled for different time periods and then sintered at 900 °C for 1 h. The micrographs show the presence of large amount of pores in initial milling period. As the particles being flaky, compaction and sintering of such powder leads to generation of large amount of porosity. But after 20 h of milling, porosity decreases drastically and it is clearly visible from the micrographs. After 20 h of milling, particle size changes from flaky to irregular shape and eventually leads to decrease in porosity.
3.3. Density measurement
Fig. 1. XRD spectra of Cu–graphite MMCs (a) conventionally sintered at 900 °C for 1 h (b) spark plasma sintered at 700 °C for 5 min and (c) Cu–5 vol.% graphite powder mixture milled in planetary mill for different periods.
tion is there, amount is less than 5% (wt/wt), which cannot be detected by X-ray diffraction. Fig. 1b shows the various peaks of Cu for spark plasma sintering. As SPS was carried out in vacuum no copper oxide was formed. Fig. 1c shows the XRD patterns of Cu– 5 vol.% graphite composite powder mixture milled for different times in planetary mill. It can be seen that peak width increases with increasing milling time. During milling powder particles are plastically deformed due to the collision between balls and powders. This indicates that grain refinement takes place and lattice strain increases as milling progresses.
The variation of density with increase in graphite volume percentage is shown in Fig. 5a. It has been found a density value of around 90% for conventional sintering and 96% for spark plasma sintering. The reason is improved bonding between Cu and graphite due to the simultaneous applications of pressure and pulsed continuous current during SPS process. A very high temperature may be attained in the contact area, over the melting temperature of the material, leading to localized melting, which enhances interparticle bonding. It is also noticed that density value increases with graphite content. The reason is that when pressure is applied graphite particles are also deformed with Cu particles due to soft nature and which helps in plastic flow and eventually density value goes up. This also can be attributed to the relationship of the relative density (sintered density/theoretical density). The higher the graphite content, the lower the theoretical and measured density of the composites. Fig. 5b shows the variation of density with milling. It is found that density decreases up to 20 h of milling and after that density increases. The presence of porosity leads to decrease in density up to 20 h, after that porosity decreases and density goes up. SEM micrographs (Fig. 4) also support the same observations.
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Fig. 2. Representative FESEM micrographs of Cu–10 vol.% graphite MMC conventionally sintered at 900 °C for 1 h. EDS spectra and elemental compositions are also shown in this figure.
Fig. 3. SEM micrographs of Cu–1 vol.% graphite composite powder mixture milled for different periods in planetary mill.
3.4. Hardness study The variation in hardness with graphite reinforcements is shown in Fig. 6a. The general hardness trend is that it decreases with increase in graphite percentage due to the soft nature of graphite. The maximum Vickers hardness value of around 70 is achieved for 1 vol.% graphite reinforced conventional sintered composite. Whereas higher hardness value of around 100 is obtained for spark plasma sintered composites due to fine grain size in case of SPS as it takes only 5 min for densification as compared to 1 h for conventional sintering. It is also to be noted that SPS samples show higher density than conventional sintered samples. The good bonding between matrix and reinforcements is also responsible for improvement on the hardness in case of SPS. The dispersion strengthening and grain size refinement are the main strengthening mechanism which is responsible for increase in hardness of Cu–graphite composites. Fig. 6b shows the variation in hardness with milling time. In the initial milling period (up to 10 h) hardness
trend is decreasing but after that hardness value goes up. The reason is large porosity and less density in the initial milled samples and later porosity decreases and density increases. After 40 h of milling, the particle becomes fine and more closely bound and particle–particle contact increases with increase in fineness due to increase in surface area of the milled powder particle. Another reason of increasing hardness is that Cu powder particles are coated with graphite powder to some extent and improves the bonding between Cu and graphite. Guel et al. [16] studied the effect of metallic addition on mechanical properties of Al–graphite MMC by mechanical milling. Similar observations were made by Yusoff et al. [17]. 3.5. Compressive strength study Compression strengths plots are shown in Fig. 7a and b. It is found from Fig. 7a that maximum compression strength (875 MPa) is found in 1 vol.% of graphite reinforced MMC and the
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Fig. 4. SEM micrographs of Cu–5 vol.% graphite composite powder mixture milled for different periods and then conventionally sintered at 900 °C for 1 h.
Fig. 5. (a) Variation in density with graphite content of MMCs conventionally sintered at 900 °C for 1 h and spark plasma sintered at 700 °C for 5 min and (b) variation in density with milling time of MMCs (Cu–1 vol.% and Cu–5 vol.% graphite) conventionally sintered at 900 °C for 1 h.
Fig. 6. (a) Variation of hardness with graphite content for Cu–graphite MMCs fabricated by conventional sintering at 900 °C for 1 h and spark plasma sintered at 700 °C for 5 min; and (b) variation in hardness with milling time for Cu–graphite MMCs fabricated by conventional sintering at 900 °C for 1 h.
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Then have shown that compressive strength decreases from 600 to 475 MPa when the amount of graphite increases from 2.5 to 5 wt.%. Fig. 7b shows variation in compressive strength with milling time for Cu–5 vol.% graphite reinforced MMC. It is found that compressive strength increases with milling due to introduction of large number of defects, improved dispersion during milling and strain hardening effect. Seah et al. [7] prepared cast ZA-27/graphite composite and reported that the increase in compressive strength is due to graphite particle acting as barriers to the dislocations in the microstructure and with increasing the graphite content within the ZA-27 matrix results in significant increases in the ductility, UTS, compressive strength and Young’s modulus, but a decrease in the hardness. The main strengthening mechanism is proposed for high compression strength of the MMCs is dispersion hardening effects. Table 2 shows the summary of mechanical properties of Cu–graphite MMCs obtained from literatures and present investigation.
value is almost close for 5 vol.% graphite also. Dispersion of up to 5 vol.% graphite increases compressive strength due to positive dispersion strengthening. However, beyond 5 vol.%, further addition of graphite reduces compressive strength due to agglomeration of brittle graphite. At 10 vol.% graphite addition compressive strength reaches minimum to 450 MPa due to brittle nature of composite. Dewidar et al. [18] also studied the effect of graphite content on the compressive strength of Cu–graphite composite.
3.6. Transverse rupture strength (TRS) study The flexural strength graph of Cu with 0, 1, 3, 5 and 10 vol.% of graphite composite fabricated by conventional sintering method for 1 h are given in Fig. 8. From this graph, it is seen that maximum elastic modulus (14.5 GPa) is found for 1 vol.% graphite composite. After that elastic modulus decreases continuously up to 5 vol.%. The reason may be due to the agglomeration of graphite particles
Fig. 7. (a) Compressive stress vs. strain curves for Cu–1 vol.%, Cu–5 vol.% and Cu– 10 vol.% graphite reinforced MMC; and (b) compressive stress vs. strain curves for various milling time of Cu–5 vol.% graphite reinforced MMC.
Fig. 8. Plot of flexural strength and elastic modulus with graphite content.
Table 2 Summary of some mechanical properties of Cu–graphite MMCs available from literatures. References
System
% of relative density
Hardness
Rajkumar et al. [9] Moustafa et al. [5]
94 95
100 VHN 21.6 BHN
Yang et al. [14] Chen et al. [12] Zhao et al. [19] Kumar et al. [20]
Cu–graphite MMC by microwave sintering Cu–graphite composites with Cu-coated and uncoated graphite powder Cu–pitch coke–graphite composite Cu–BN–SiC–graphite–Sn–Al–Fe composite Cu–graphite composite by electroforming technique Pitch based Cu–C composite
Present study
Cu–graphite composite by conventional sintering & SPS
90 (conventional) 97 (SPS)
102 VHN 122 VHN 81 VHN 88 VHN 87 VHN (conventional) 105 VHN (SPS)
Max. bending strength (MPa)
Max. compressive strength (MPa)
% Elongation
376
43
875 (conventional)
50
64
150 275 (conventional)
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and improper dispersion of reinforcements. However, at 10 vol.% graphite elastic modulus again increases. The reason is not clear to us. The flexural strength continuously increases up to 5 vol.% and then it decreases. The maximum bending strength (275 MPa) is found in 5 vol.% of graphite reinforced composite. Flexural strength decreases after addition of 5 vol.% graphite due to increase in brittleness of the composites. So, it can be concluded that bending strength increases due to increase in strength arises from dispersion strengthening.
4. Conclusions The following conclusions can be drawn from the present investigation: 1. The copper–graphite metal matrix composites were successfully fabricated by conventional as well as spark plasma sintering techniques. Spark plasma sintering shows better response to densification and hardening than conventional sintering. 2. A density value of around 90% of relative density for conventional sintering and 96% of relative density has been achieved for spark plasma sintering. 3. The maximum Vickers hardness value of around 70 and 100 are achieved for Cu–1 vol.% graphite reinforced MMC fabricated by conventional and spark plasma sintering respectively. 4. Milling of composite powder mixture results in increase in size and flake formation up to 20 h of milling and so sintering of such powders results in reduction in density and hardness due to high porosity. However, after 20 h of milling, size reduction takes place and results in irregular shape of powder particles due to strain hardening. Sintering of such powder shows improvement in density and hardness due to less porosity. 5. The transverse rupture strength increases up to 5 vol.% of graphite reinforcing and after that it decreases.
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6. Compressive strength is maximum at 5 vol.% of graphite reinforced MMC and then further addition of graphite leads to decrease in compressive strength.
Acknowledgements Financial support for this work from the Department of Science and Technology (DST), India (Grant No. SR/FTP/ETA-100/2010) is gratefully acknowledged. The authors are also grateful to Prof. S.K. Karak for conducting FESEM experiments. References [1] M.M. Dewidar, J.K. Lim, J. Compos. Mater. 41 (2007) 2183–2194. [2] X.U. Wei, H.U. Rui, L. Jin-Shan, F. Heng-zhi, Trans. Nonferrous Met. Soc. China 21 (2011) 2237–2241. [3] H. Kaftelen, N. Unlu, G. Goller, M.L. Ovecoglu, H. Henein, Composites: Part A 42 (2011) 812–824. [4] S. Dorfman, D. Fuks, Sens. Actuators A 51 (1995) 13–16. [5] S.F. Moustafa, S.A. El-Badry, A.M. Sanad, B. Kieback, Wear 253 (2002) 699–710. [6] X.C. Ma, G.Q. He, D.H. He, C.S. Chen, Z.F. Hu, Wear 265 (2008) 1087–1092. [7] K.H.W. Seah, S.C. Sharma, B.M. Girish, Mater. Des. 16 (1995) 271–275. [8] C.S. Ramesh, R.N. Ahmed, M.A. Mujeebu, M.Z. Abdullah, Mater. Des. 30 (2009) 1957–1965. [9] K. Rajkumar, S. Aravindan, J. Mater. Process. Technol. 209 (2009) 5601–5605. [10] A. Yeoh, C. Persad, Z. Eliezer, Scr. Mater. 37 (1997) 271–277. [11] R.K. Gautam, S. Ray, S.C. Sharma, S.C. Jain, R. Tyagi, Wear 271 (2011) 658–664. [12] B. Chen, Q. Bi, J. Yang, Y. Xia, J. Hao, Tribol. Int. 41 (2008) 1145–1152. [13] J. Zhang, L. Hea, Y. Zhou, Scr. Mater. 60 (2009) 976–979. [14] H. Yang, R. Luo, S. Han, M. Li, Wear 268 (2010) 1337–1341. [15] P. Queipo, M. Granda, R. Santamarı, R. Mene´ndez, Fuel 83 (2004) 1625–1634. [16] E. Guel, C.C. Gallardo, J.L.C. Cortés, E.R. Rangel, J.M.H. Ramírez, R.M. Sánchez, J. Alloys Comp. 495 (2010) 403–407. [17] Mahani Yusoff, Radzali Othman, Zuhailawati Hussain, Mater. Des. 32 (2011) 3293–3298. [18] M. Dewidar, G.T. Abdel-Jaber, M. Bakrey, H. Badry, Int. J. Mech. Mechatron. 10 (2010) 25–40. [19] H. Zhao, L. Liu, Y. Wu, W. Hu, Compos. Sci. Technol. 67 (2007) 1210–1217. [20] A. Kumar, M. Kaur, R. Kumar, P.R. Sengupta, V. Raman, G. Bhatia, K.N. Sood, J. Mater. Sci. (2010) 1393–1400.