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Microwave sintering of copper–graphite composites
Journal of Materials Processing Technology 209 (2009) 5601–5605
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Microwave sintering of copper–graphite composites K. Rajkumar, S. Aravindan ∗ Department of Mechanical Engineering, Indian Institute of Technology, New Delhi, India
a r t i c l e
i n f o
Article history: Received 19 February 2009 Received in revised form 21 April 2009 Accepted 17 May 2009 Keywords: Microwave hybrid heating Copper matrix Graphite Electrical sliding contact
a b s t r a c t Microwave processing is a distinctive and alternative technique when compared with the available processes such as material synthesis, sintering, and processing utilizing the conventional heating sources. Owing to microwave–material molecular interaction, microwave heating is of internal and faster. This results in improved quality of the product with time and energy savings. Metal at its bulk form reflects microwaves; however in its powder form, it couples with microwaves. This work emphasizes on the development of copper–graphite metal matrix composite for electrical sliding contact applications through microwave hybrid heating (2.45 GHz, 3.2 kW). The fabricated composites were tested for their mechanical properties such as porosity, relative density and hardness. Microstructural aspects were studied through SEM. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Metal matrix composites (MMC) are continuously used particularly for tribological applications owing to their improved wear resistance and better properties. Among the available metal matrices, copper received more attention recently due to its inherent properties. Copper and its alloys are used widely where high electrical, thermal conductivity, corrosion resistance and wear resistance are necessitated. Moustfa et al. (2002) reported in their study that copper–graphite composites are having high electrical conductivity and excellent lubrication properties for sliding contacts. Paulo Queipo et al. (2004) explained the manufacturing procedure for copper–graphite/carbon through powder metallurgy route with conventional sintering. Coarser microstructure with larger porosity is obtained by this conventional sintering process. Bocchini (1986) proved that the physical and mechanical properties were affected by the presence of larger number of angular pores and coarse microstructure. The inherent porosity of the conventionally sintered composite decreases the strength, wear resistance as well. This leads to poor tribological performance. Novel processing methodology is always necessary for improving the required properties of the composites. In microwave sintering, heat is generated internally within the material and the sample becomes the source of heat. The direct delivery of energy to the material through the molecular interaction, volumetric heating results in. This leads to many potential advantages for the composites.
∗ Corresponding author. Tel.: +91 1126596350; fax: +91 1126582053. E-mail address: [email protected] (S. Aravindan). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.05.017
Many researchers attempted to sinter metal powder by microwave sintering. Roy et al. (1999) made a comparative evaluation on mechanical properties of sintered metal powders such as Fe–Ni, and Fe–Cu through microwave and conventional methods. Microwave sintered specimens exhibited better mechanical properties. Microwave sintered Fe–Ni exhibited a modulus of rupture which is 60% higher than the conventionally sintered ones. Ankelekar et al. (2001) reported that copper steel alloys could be sintered using microwaves. These alloys exhibited better mechanical properties than conventionally sintered ones. Upadhyaya et al. (2003) investigated microwave sintering response of Cu–Sn. The problem of negative densification (expansion) through conventional sintering is surpassed by microwave sintering. Positive densification (shrinkage) was attributed to faster sintering which hinder the diffusion of tin particles into copper lattice. Gupta and Wong (2005) ascribed the improvement of mechanical properties of microwave sintered Al and Mg, to finer microstructure. Microwave radiation is useful in material processing activities. Microwave post-sintering treatment enhances the properties of WC inserts by Ramkumar et al. (2002). Aravindan and Krishnamurthy (1999) reported that microwaves are used for joining of ceramic and composites. Like lasers, microwaves are used for glazing of alumina–titania composite was described by Sharma et al. (2001). Microwave sintering offers many advantages such as faster heating rate, lower sintering temperature, enhanced densification, smaller average grain size and an apparent reduction in activation energy in sintering. Some negative aspects of conventional sintering such as non-uniform heating, coarser microstructure and larger porosity can be minimized in microwave sintering. The present study uses microwave hybrid heating for the development of metal matrix composites. Microwave hybrid heating comprises simul-
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K. Rajkumar, S. Aravindan / Journal of Materials Processing Technology 209 (2009) 5601–5605
Fig. 1. Powder morphology of (A1) electrolytic copper powder and (A2) graphite powder.
taneous actions of microwave and microwave coupled radiative external heating, to realize the uniform and rapid heating. Though some literature exists in microwave sintering of metal, no literature is available presently in the area of microwave sintering of copper based composites. 2. Experimental procedure 2.1. Materials Copper–graphite composites were manufactured through powder metallurgy (P/M) route. Electrolytic copper powder having average grain size of 12 m was mixed with the graphite powder of 50 m size. Fig. 1(A1) and (A2) shows powder morphology of electrolytic copper powder (A1) and graphite powder (A2) used in this investigation. Copper powder was mixed with various volume fractions of graphite powder 5, 10, 15, 20, 25 and 30% in electric agate pestle mortar for uniform mixing. In order to obtain the proper mixing, rotational speed of agate pestle mortar was controlled to 20 rpm for duration of 2 h. Mixed powders were preheated at 150 ◦ C to evaporate any volatile matter. The preheated powders were uniaxially compacted in a hydraulic press at a pressure of 630 MPa to obtain disc shaped specimens having dimension of 14 mm diameter and 10 mm height. 2.2. Microwave sintering and characterizations A 3.2 kW industrial microwave furnace (2.45 GHz) was used in this study. Hybrid setup was designed to have two layers of elements, i.e. one is transparent to microwave and the other is absorber of microwaves for uniform heating. The alumina wool (transparent to microwave) was used for preserving the heat inside the crucible. SiC fencing was used as a susceptor (microwave absorbing element). SiC fencing not only provides hybrid heating facility but
Fig. 2. Hybrid microwave sintering setup.
also reduces the thermal gradient and this promotes crack free components. Accurate temperature of samples was monitored using a ‘K’ type thermocouple. The schematic diagram of the microwave sintering setup used in the present investigation is shown in Fig. 2. The green compacts were kept inside the hybrid microwave sintering setup. Preliminary sintering trials were performed for obtaining better sintering condition in terms of densification parameter (DP) for copper-5% volume graphite composite. Preliminary sintering trials were conducted for the sintering temperature range of 700–900 ◦ C and isothermal holding time range of 10–30 min. Sintering condition which emerged from the trial experiment has been extended to copper with various volume fractions of graphite composite. Initially microwave easily couples with the SiC fencing and the heat is generated due to its high dielectric loss. SiC fence radiates the heat energy to sample being processed. The microwave simultaneously interact with the green body sample, i.e. couple with green body sample and generates the heat internally due to penetrating feature of microwave in powder compacted samples. This will lead to volumetric heating of samples with short duration and also insulation effectively contains the heat dissipation. Three sets of green samples were sintered for each condition in order to assess the variation in processing and for the reproducibility of final properties. In all the cases, the power of microwave was controlled to 20% of maximum available power and the heating rate was also set within 12 ◦ C/min. After the definite isothermal holding time, the samples were allowed to cool in the furnace. The sinterability of samples was calculated through densification parameter (DP). The DP is calculated from Eq. (1) DP =
sintered density − green density theoretical density − green density
(1)
Positive DP signifies shrinkage where as the negative DP means volumetric expansion of samples during sintering. Microwave sintered copper–graphite samples were studied through Scanning Electron Microscope (SEM with EDAX). Sintered density of the samples was determined by Archimedes’ principle.
Fig. 3. Densification parameter of copper-5% volume graphite composites.
K. Rajkumar, S. Aravindan / Journal of Materials Processing Technology 209 (2009) 5601–5605
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Fig. 4. Representative SEM images of copper–graphite composite, (B1) copper-10% graphite (2.5k×), (B2) copper-20% graphite (2.5k×), (B3) copper-30% graphite (2.5k×), (C1) copper-10% graphite (7.5k×), (C2) copper-20% graphite (7.5k×) and (C3) copper-30% graphite (7.5k×).
The samples were weighed using an electronic balance of having a least count of 0.0001 g. Porosity was calculated as per ASTM standard C1309-85. The sinterability of the sample was assessed through relative density or rated density. The hardness of the composites was evaluated by Vickers’ hardness tester with 10 kg indenting load. 3. Results and discussion The effect of sintering temperature and isothermal holding time on the densification parameter of sintered copper-5% volume graphite is shown in Fig. 3. From Fig. 3, it can be observed that for all the three sintering temperatures and isothermal holding time, microwave sintered samples show positive densification parameter. Positive densification parameter implies the shrinkage. A densification response of copper–graphite with microwave sintering shows no volumetric expansion. It is understood from the figure that the densification parameter increases consistently with the increase in temperature and time. However, the densification parameter for 30 min of isothermal holding time is approximately equal to that of 20 min.
Isothermal holding time beyond 20 min has not shown any observable increase in densification parameter. Initial coupling of copper powder with microwave induces rapid heating, neck growth of copper particles and their coalescence leads to faster densification. After 20 min, there is a possibility of decoupling of phases with microwaves. This results in no improvement in densification parameter (DP) even with prolonged holding time. Also microwaves decouple with the sintered metal parts. Based on the above the better sintering temperature and isothermal holding time are 850 ◦ C and 20 min, respectively. Typical SEM micrographs of microwave sintered samples of copper–graphite are presented in Fig. 4. The graphite particles are uniformly distributed in the matrix phase. It is also observed from Fig. 4(B3) that the graphite particles are agglomerated in the copper matrix. No cracks or fissures are seen in SEM micrographs which confirm the advantages of faster and homogeneous heating. It is understood that the pores seen in micrographs are relatively smaller and more rounded. Near equiaxed grain morphology and finer microstructure can be seen in Fig. 4(C1)–(C3). Thus finer microstructure is achieved in graphite reinforced copper matrix.
Fig. 5. Typical EDX of sintered copper–graphite composites.
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K. Rajkumar, S. Aravindan / Journal of Materials Processing Technology 209 (2009) 5601–5605
Fig. 8. Hypothetical model of microwave heating.
3.1. Proposed microwave heating
Fig. 6. Variation of (a) sintered density and (b) relative density and porosity with % of graphite.
Fig. 5 shows peaks of carbon and copper elements observed from the EDAX analysis. It is evident from Fig. 6, that there is no oxygen reaction to the samples during the sintering process. Variation of sintered density, variation of porosity, and relative density of sintered composites with percentage of graphite is presented Fig. 6. It can be understood that the increase in percentage of graphite in copper matrix influences the sintered density. It can be observed from Fig. 6 that there is a decrease in porosity with the increase in % of graphite. This can be attributed to the closure of pores and selective coupling of microwave interaction with the graphite. When the graphite percentage is increased beyond 25% volume, the increase in porosity is observed. This is due to the formation of larger agglomerates. These agglomerated particles can influence the penetration depth and thereby it decreases the amount of microwave absorbed. It reduces also the heating capability of composite and finally leads to increased porosity. The variation of hardness with increase in volume fraction graphite of copper matrix is shown in Fig. 7. The electrical sliding performance of copper–graphite composite in its applications is strongly influenced by hardness. Graphite is a well-known soft material, and hence reinforcement of graphite to copper matrix leads to reduction in the hardness. This consequence of negative effect on hardness can be resolved through the finer microstructure of microwave sintered composite. It is interesting to observe that microwave sintered samples exhibited finer microstructure with reduced porosity, and increased hardness.
Copper is having high electrical conductivity and it should reflect microwaves when it is in bulk form. The graphite also is a good electrical conductor but relatively at a lower order. The mixture of copper and graphite powders gives different response when they interact with microwaves. Hypothetical model of microwave heating is shown in Fig. 8. The copper and graphite particles are mixed in the compacts, with the pore sites. Since the dielectric loss of air at pores is larger, by way of selective heating pore closure occurs. The heat generation in conducting particles can be related to penetration depth and skin effect. The penetration depth can be expressed by Eq. (2) D = (f)
−1/2
(2)
where D is penetration depth (micron), f is frequency of electromagnetic radiation (GHz), is the electrical conductivity (−1 cm−1 ) and is the magnetic permeability (N/A2 ). Electrical conductivity () of the graphite at room temperature is 103 −1 cm−1 (Meredith, 1998). Eq. (2) yields penetration depth of graphite as 30 m at 2.45 GHz. Graphite particle size used here is in the same order of penetration depth. Electrical conductivity and magnetic permeability of copper at room temperature are 0.85 × 106 −1 cm−1 and 1.2566290 × 10−6 N/A2 , respectively (Meyer and Gundolf, 1991). Penetration depth of copper is equal to 1.4 m at 2.45 GHz frequency. Though the size of the copper particle used is larger than the penetration depth, the possible heating can be attributed to that at least one dimension of particle is less than or equal to the penetration depth. The copper particles have dendritic structure so that one of its dimension is less than or equal to 1.4 m. These fine copper particles can couple with incident microwave radiation and then volumetric heating is possible. The heating of copper and graphite particles takes place through joule effect caused by microwave induced electrical current loss in the particles. It can be understood from the model that the distribution of graphite in copper results in uniform heating.
4. Conclusions The application of microwave sintering of metallic powder is successfully extended to metal matrix composite. Copper–graphite composites were effectively sintered using microwave hybrid heating without any crack. Hypothetical model on microwave sintering is developed. The finer microstructure with relatively smaller and round pores, resulted due to microwave heating, enhances the performance of the composite.
Acknowledgment
Fig. 7. Hardness of copper–graphite composites.
The authors kindly acknowledge DST, India, for funding of this research study (SR/FTP/ETA-41/2005).
K. Rajkumar, S. Aravindan / Journal of Materials Processing Technology 209 (2009) 5601–5605
References Ankelekar, R.M., Agrwal, D.K., Roy, R., 2001. Microwave sintering and mechanical properties of PM copper steel. Powder Metallurgy 44, 355–362. Aravindan, S., Krishnamurthy, R., 1999. Joining of ceramic composites by microwave heating. Materials Letter 38, 245–249. Bocchini, G.F., 1986. The influences of porosity on the characteristics of sintered materials. International Journal Powder Metallurgy l22, 185–202. Gupta, M., Wong, W.L.E., 2005. Enhancing overall mechanical performance of metallic materials using two-directional microwave assisted rapid sintering. Scripta Materialia 52, 479–483. Meredith, R.J., 1998. Engineers Hand Book of Industrial Microwave Heating, IEE., ISBN 0852969163. Meyer, Gundolf, Composite superconductor, US Patent 4,983,574 (01/08/1991).
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Moustfa, S.F., Badry, E.I., Sannad, A.M., 2002. Friction and wear of copper–graphite composite made with Cu-coated and uncoated graphite powders. Wear 253, 699–710. Paulo, Q., Marcos, G., Ricardo, S., Rosa, M., 2004. Preparation of pitch based carbon–copper composites for electrical applications. Fuel 83, 25–34. Ramkumar, J., Aravindan, S., Malhotra, S.K., Krishnamurthy, R., 2002. Enhancing the metallurgical properties of WC insert (K-20). Materials Letter 53, 200–204. Roy, R., Agrawal, D., Cheng, J., Gedevanishvili, S., 1999. Full sintering of powderedmetal bodies in a microwave field. Nature 399, 668–670. Sharma, A.K., Aravindhan, S., Krishnamurthy, R., 2001. Microwave glazing of alumina–titania ceramic composite coatings. Materials Letter 50, 295–301. Upadhyaya, A., Sethi, G., Agrwal, D., 2003. Microwave sintering of Cu–12Sn alloy. In: Sintering 2003 Conference, Pennstate University, Pennsylvania, USA, pp. 15–17.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Microwave sintering of copper–graphite composites K. Rajkumar, S. Aravindan ∗ Department of Mechanical Engineering, Indian Institute of Technology, New Delhi, India
a r t i c l e
i n f o
Article history: Received 19 February 2009 Received in revised form 21 April 2009 Accepted 17 May 2009 Keywords: Microwave hybrid heating Copper matrix Graphite Electrical sliding contact
a b s t r a c t Microwave processing is a distinctive and alternative technique when compared with the available processes such as material synthesis, sintering, and processing utilizing the conventional heating sources. Owing to microwave–material molecular interaction, microwave heating is of internal and faster. This results in improved quality of the product with time and energy savings. Metal at its bulk form reflects microwaves; however in its powder form, it couples with microwaves. This work emphasizes on the development of copper–graphite metal matrix composite for electrical sliding contact applications through microwave hybrid heating (2.45 GHz, 3.2 kW). The fabricated composites were tested for their mechanical properties such as porosity, relative density and hardness. Microstructural aspects were studied through SEM. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Metal matrix composites (MMC) are continuously used particularly for tribological applications owing to their improved wear resistance and better properties. Among the available metal matrices, copper received more attention recently due to its inherent properties. Copper and its alloys are used widely where high electrical, thermal conductivity, corrosion resistance and wear resistance are necessitated. Moustfa et al. (2002) reported in their study that copper–graphite composites are having high electrical conductivity and excellent lubrication properties for sliding contacts. Paulo Queipo et al. (2004) explained the manufacturing procedure for copper–graphite/carbon through powder metallurgy route with conventional sintering. Coarser microstructure with larger porosity is obtained by this conventional sintering process. Bocchini (1986) proved that the physical and mechanical properties were affected by the presence of larger number of angular pores and coarse microstructure. The inherent porosity of the conventionally sintered composite decreases the strength, wear resistance as well. This leads to poor tribological performance. Novel processing methodology is always necessary for improving the required properties of the composites. In microwave sintering, heat is generated internally within the material and the sample becomes the source of heat. The direct delivery of energy to the material through the molecular interaction, volumetric heating results in. This leads to many potential advantages for the composites.
∗ Corresponding author. Tel.: +91 1126596350; fax: +91 1126582053. E-mail address: [email protected] (S. Aravindan). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.05.017
Many researchers attempted to sinter metal powder by microwave sintering. Roy et al. (1999) made a comparative evaluation on mechanical properties of sintered metal powders such as Fe–Ni, and Fe–Cu through microwave and conventional methods. Microwave sintered specimens exhibited better mechanical properties. Microwave sintered Fe–Ni exhibited a modulus of rupture which is 60% higher than the conventionally sintered ones. Ankelekar et al. (2001) reported that copper steel alloys could be sintered using microwaves. These alloys exhibited better mechanical properties than conventionally sintered ones. Upadhyaya et al. (2003) investigated microwave sintering response of Cu–Sn. The problem of negative densification (expansion) through conventional sintering is surpassed by microwave sintering. Positive densification (shrinkage) was attributed to faster sintering which hinder the diffusion of tin particles into copper lattice. Gupta and Wong (2005) ascribed the improvement of mechanical properties of microwave sintered Al and Mg, to finer microstructure. Microwave radiation is useful in material processing activities. Microwave post-sintering treatment enhances the properties of WC inserts by Ramkumar et al. (2002). Aravindan and Krishnamurthy (1999) reported that microwaves are used for joining of ceramic and composites. Like lasers, microwaves are used for glazing of alumina–titania composite was described by Sharma et al. (2001). Microwave sintering offers many advantages such as faster heating rate, lower sintering temperature, enhanced densification, smaller average grain size and an apparent reduction in activation energy in sintering. Some negative aspects of conventional sintering such as non-uniform heating, coarser microstructure and larger porosity can be minimized in microwave sintering. The present study uses microwave hybrid heating for the development of metal matrix composites. Microwave hybrid heating comprises simul-
5602
K. Rajkumar, S. Aravindan / Journal of Materials Processing Technology 209 (2009) 5601–5605
Fig. 1. Powder morphology of (A1) electrolytic copper powder and (A2) graphite powder.
taneous actions of microwave and microwave coupled radiative external heating, to realize the uniform and rapid heating. Though some literature exists in microwave sintering of metal, no literature is available presently in the area of microwave sintering of copper based composites. 2. Experimental procedure 2.1. Materials Copper–graphite composites were manufactured through powder metallurgy (P/M) route. Electrolytic copper powder having average grain size of 12 m was mixed with the graphite powder of 50 m size. Fig. 1(A1) and (A2) shows powder morphology of electrolytic copper powder (A1) and graphite powder (A2) used in this investigation. Copper powder was mixed with various volume fractions of graphite powder 5, 10, 15, 20, 25 and 30% in electric agate pestle mortar for uniform mixing. In order to obtain the proper mixing, rotational speed of agate pestle mortar was controlled to 20 rpm for duration of 2 h. Mixed powders were preheated at 150 ◦ C to evaporate any volatile matter. The preheated powders were uniaxially compacted in a hydraulic press at a pressure of 630 MPa to obtain disc shaped specimens having dimension of 14 mm diameter and 10 mm height. 2.2. Microwave sintering and characterizations A 3.2 kW industrial microwave furnace (2.45 GHz) was used in this study. Hybrid setup was designed to have two layers of elements, i.e. one is transparent to microwave and the other is absorber of microwaves for uniform heating. The alumina wool (transparent to microwave) was used for preserving the heat inside the crucible. SiC fencing was used as a susceptor (microwave absorbing element). SiC fencing not only provides hybrid heating facility but
Fig. 2. Hybrid microwave sintering setup.
also reduces the thermal gradient and this promotes crack free components. Accurate temperature of samples was monitored using a ‘K’ type thermocouple. The schematic diagram of the microwave sintering setup used in the present investigation is shown in Fig. 2. The green compacts were kept inside the hybrid microwave sintering setup. Preliminary sintering trials were performed for obtaining better sintering condition in terms of densification parameter (DP) for copper-5% volume graphite composite. Preliminary sintering trials were conducted for the sintering temperature range of 700–900 ◦ C and isothermal holding time range of 10–30 min. Sintering condition which emerged from the trial experiment has been extended to copper with various volume fractions of graphite composite. Initially microwave easily couples with the SiC fencing and the heat is generated due to its high dielectric loss. SiC fence radiates the heat energy to sample being processed. The microwave simultaneously interact with the green body sample, i.e. couple with green body sample and generates the heat internally due to penetrating feature of microwave in powder compacted samples. This will lead to volumetric heating of samples with short duration and also insulation effectively contains the heat dissipation. Three sets of green samples were sintered for each condition in order to assess the variation in processing and for the reproducibility of final properties. In all the cases, the power of microwave was controlled to 20% of maximum available power and the heating rate was also set within 12 ◦ C/min. After the definite isothermal holding time, the samples were allowed to cool in the furnace. The sinterability of samples was calculated through densification parameter (DP). The DP is calculated from Eq. (1) DP =
sintered density − green density theoretical density − green density
(1)
Positive DP signifies shrinkage where as the negative DP means volumetric expansion of samples during sintering. Microwave sintered copper–graphite samples were studied through Scanning Electron Microscope (SEM with EDAX). Sintered density of the samples was determined by Archimedes’ principle.
Fig. 3. Densification parameter of copper-5% volume graphite composites.
K. Rajkumar, S. Aravindan / Journal of Materials Processing Technology 209 (2009) 5601–5605
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Fig. 4. Representative SEM images of copper–graphite composite, (B1) copper-10% graphite (2.5k×), (B2) copper-20% graphite (2.5k×), (B3) copper-30% graphite (2.5k×), (C1) copper-10% graphite (7.5k×), (C2) copper-20% graphite (7.5k×) and (C3) copper-30% graphite (7.5k×).
The samples were weighed using an electronic balance of having a least count of 0.0001 g. Porosity was calculated as per ASTM standard C1309-85. The sinterability of the sample was assessed through relative density or rated density. The hardness of the composites was evaluated by Vickers’ hardness tester with 10 kg indenting load. 3. Results and discussion The effect of sintering temperature and isothermal holding time on the densification parameter of sintered copper-5% volume graphite is shown in Fig. 3. From Fig. 3, it can be observed that for all the three sintering temperatures and isothermal holding time, microwave sintered samples show positive densification parameter. Positive densification parameter implies the shrinkage. A densification response of copper–graphite with microwave sintering shows no volumetric expansion. It is understood from the figure that the densification parameter increases consistently with the increase in temperature and time. However, the densification parameter for 30 min of isothermal holding time is approximately equal to that of 20 min.
Isothermal holding time beyond 20 min has not shown any observable increase in densification parameter. Initial coupling of copper powder with microwave induces rapid heating, neck growth of copper particles and their coalescence leads to faster densification. After 20 min, there is a possibility of decoupling of phases with microwaves. This results in no improvement in densification parameter (DP) even with prolonged holding time. Also microwaves decouple with the sintered metal parts. Based on the above the better sintering temperature and isothermal holding time are 850 ◦ C and 20 min, respectively. Typical SEM micrographs of microwave sintered samples of copper–graphite are presented in Fig. 4. The graphite particles are uniformly distributed in the matrix phase. It is also observed from Fig. 4(B3) that the graphite particles are agglomerated in the copper matrix. No cracks or fissures are seen in SEM micrographs which confirm the advantages of faster and homogeneous heating. It is understood that the pores seen in micrographs are relatively smaller and more rounded. Near equiaxed grain morphology and finer microstructure can be seen in Fig. 4(C1)–(C3). Thus finer microstructure is achieved in graphite reinforced copper matrix.
Fig. 5. Typical EDX of sintered copper–graphite composites.
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K. Rajkumar, S. Aravindan / Journal of Materials Processing Technology 209 (2009) 5601–5605
Fig. 8. Hypothetical model of microwave heating.
3.1. Proposed microwave heating
Fig. 6. Variation of (a) sintered density and (b) relative density and porosity with % of graphite.
Fig. 5 shows peaks of carbon and copper elements observed from the EDAX analysis. It is evident from Fig. 6, that there is no oxygen reaction to the samples during the sintering process. Variation of sintered density, variation of porosity, and relative density of sintered composites with percentage of graphite is presented Fig. 6. It can be understood that the increase in percentage of graphite in copper matrix influences the sintered density. It can be observed from Fig. 6 that there is a decrease in porosity with the increase in % of graphite. This can be attributed to the closure of pores and selective coupling of microwave interaction with the graphite. When the graphite percentage is increased beyond 25% volume, the increase in porosity is observed. This is due to the formation of larger agglomerates. These agglomerated particles can influence the penetration depth and thereby it decreases the amount of microwave absorbed. It reduces also the heating capability of composite and finally leads to increased porosity. The variation of hardness with increase in volume fraction graphite of copper matrix is shown in Fig. 7. The electrical sliding performance of copper–graphite composite in its applications is strongly influenced by hardness. Graphite is a well-known soft material, and hence reinforcement of graphite to copper matrix leads to reduction in the hardness. This consequence of negative effect on hardness can be resolved through the finer microstructure of microwave sintered composite. It is interesting to observe that microwave sintered samples exhibited finer microstructure with reduced porosity, and increased hardness.
Copper is having high electrical conductivity and it should reflect microwaves when it is in bulk form. The graphite also is a good electrical conductor but relatively at a lower order. The mixture of copper and graphite powders gives different response when they interact with microwaves. Hypothetical model of microwave heating is shown in Fig. 8. The copper and graphite particles are mixed in the compacts, with the pore sites. Since the dielectric loss of air at pores is larger, by way of selective heating pore closure occurs. The heat generation in conducting particles can be related to penetration depth and skin effect. The penetration depth can be expressed by Eq. (2) D = (f)
−1/2
(2)
where D is penetration depth (micron), f is frequency of electromagnetic radiation (GHz), is the electrical conductivity (−1 cm−1 ) and is the magnetic permeability (N/A2 ). Electrical conductivity () of the graphite at room temperature is 103 −1 cm−1 (Meredith, 1998). Eq. (2) yields penetration depth of graphite as 30 m at 2.45 GHz. Graphite particle size used here is in the same order of penetration depth. Electrical conductivity and magnetic permeability of copper at room temperature are 0.85 × 106 −1 cm−1 and 1.2566290 × 10−6 N/A2 , respectively (Meyer and Gundolf, 1991). Penetration depth of copper is equal to 1.4 m at 2.45 GHz frequency. Though the size of the copper particle used is larger than the penetration depth, the possible heating can be attributed to that at least one dimension of particle is less than or equal to the penetration depth. The copper particles have dendritic structure so that one of its dimension is less than or equal to 1.4 m. These fine copper particles can couple with incident microwave radiation and then volumetric heating is possible. The heating of copper and graphite particles takes place through joule effect caused by microwave induced electrical current loss in the particles. It can be understood from the model that the distribution of graphite in copper results in uniform heating.
4. Conclusions The application of microwave sintering of metallic powder is successfully extended to metal matrix composite. Copper–graphite composites were effectively sintered using microwave hybrid heating without any crack. Hypothetical model on microwave sintering is developed. The finer microstructure with relatively smaller and round pores, resulted due to microwave heating, enhances the performance of the composite.
Acknowledgment
Fig. 7. Hardness of copper–graphite composites.
The authors kindly acknowledge DST, India, for funding of this research study (SR/FTP/ETA-41/2005).
K. Rajkumar, S. Aravindan / Journal of Materials Processing Technology 209 (2009) 5601–5605
References Ankelekar, R.M., Agrwal, D.K., Roy, R., 2001. Microwave sintering and mechanical properties of PM copper steel. Powder Metallurgy 44, 355–362. Aravindan, S., Krishnamurthy, R., 1999. Joining of ceramic composites by microwave heating. Materials Letter 38, 245–249. Bocchini, G.F., 1986. The influences of porosity on the characteristics of sintered materials. International Journal Powder Metallurgy l22, 185–202. Gupta, M., Wong, W.L.E., 2005. Enhancing overall mechanical performance of metallic materials using two-directional microwave assisted rapid sintering. Scripta Materialia 52, 479–483. Meredith, R.J., 1998. Engineers Hand Book of Industrial Microwave Heating, IEE., ISBN 0852969163. Meyer, Gundolf, Composite superconductor, US Patent 4,983,574 (01/08/1991).
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