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The investigation of contact performance of oxide reinforced copper composite via mechanical alloying
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1286–1290
journal homepage: www.elsevier.com/locate/jmatprotec
The investigation of contact performance of oxide reinforced copper composite via mechanical alloying ¨ ¨ Omer Guler, Ertan Evin ∗ ˘ Turkey Fırat University, Department of Metallurgical and Materials Engineering, Elazıg,
a r t i c l e
i n f o
a b s t r a c t
Article history:
In this study, the electrical performance of ODS Cu-based contact materials produced by
Received 24 August 2007
mechanical alloying (MA) was investigated. Cu-based powder mixtures that contain vari-
Received in revised form
ous oxides at different proportions were milled by the high energy planetary ball mill for
13 February 2008
5 h. Mechanically alloyed powder mixtures containing ZnO, Al2 O3 and Y2 O3 at the ratios
Accepted 24 March 2008
of 1, 2, 4 and 6 wt.% were pressed and sintered at 800 ◦ C in vacuum environment. These compacts were then forged axially at the ratio of 75% at 650 ◦ C so as to increase the density. At the first step, electrical conductivity experiments were applied to these samples to
Keywords:
determine the best conductivity. Results showed that, reinforced Cu samples containing 4%
Mechanical alloying
oxides exhibited the best. At the second step, contact count experiments were made with
High energy ball milling
these samples for determining contact performance for the counts of 3000, 6000 and 9000
ODS copper
turn on/off. The samples of 4 wt.% ZnO, 4 wt.% Al2 O3 , 4 wt.% Y2 O3 reinforced Cu materials,
Electrical properties
respectively, exhibited both the best conductivity and the best contact performances. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Electrical contact materials are used in a variety of applications, such as electrical switches, contactors, circuit breakers, voltage regulators, arcing tips, switch gears and relays (ASM Handbook, 1997; Lenel, 1980). Materials used as electrical contacts in these applications must have a good combination of electrical conductivity, wearing qualities, and resistance to erosion and welding. Otherwise, the contacts will erode, causing poor contact and arcing. Arcing takes place when contacts are in the process of establishing a current flow or interrupting the flow of current. Arc is characterized by high temperature and a high current density in the arc column. Because of the high temperature and mass flow, the contact material surface is severely corroded and eroded, which results in erratic contact resistance and material loss. Therefore a contact material should have high electrical and thermal conductivity, high melting point, and high resistance to environmental reaction,
∗
as well as high arc erosion to maintain contact integrity (Chen et al., 2006). Contacts are made of elemental metals, composites, or alloys which are made by the melt-cast method. Because of low solubility required for the reinforcing phases, making these alloys by ingot metallurgy would lead to very coarse particles that would degrade the toughness significantly. This limitation can be avoided using the powder metallurgy (PM) process. Mechanical attrition of copper powders with ceramic particles has allowed the uniform introduction of small strengthening phases and also promotes a microstructural grain refinement (Lopez et al., 2007). As a way to improve the mechanical properties at low temperatures, the matrix must be strengthened with very low solubility particles, which have a low diffusivity in copper by means of mechanical alloying equilibrium with the electrical properties obtained (Lopez et al., 2005). The ideal metal or metal combination that can function as the perfect contact material under all conditions
Corresponding author. Tel.: +90 424 2370000/5442; fax: +90 424 2415526. E-mail address: eevin@firat.edu.tr (E. Evin). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.03.034
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1286–1290
does not exist. A thorough evaluation and understanding of the operating conditions of an electrical contact device, as well as consideration of economy, is necessary before selecting the most suitable contact material. Mechanical alloying (MA) is a solid-state powder processing technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. MA is a complex process and type of mill, milling time, type, size, and size distribution of the grinding medium, ball-to-powder weight ratio, temperature of milling are some of the important process variables. The ratio of the weight of the balls to the powder (BPR) has been varied by different researchers from a value of 1:1 to 220:1 (Suryanarayana, 2001). The optimum milling time depends on the type of mill, size of the grinding medium, temperature of milling, ball-to-powder ratio and desired phase formation (Ovecoglu and Ozkal, 2004). In this study, the electrical performance of ODS Cu-based contact materials produced by mechanical alloying was investigated. At the first step, electrical conductivity experiments were applied to these samples to determine the best conductivity. At the second step, contact count experiments were conducted with these samples for determining contact performance for the counts of 3000, 6000 and 9000 turn on/off.
2.
Experimental
As starting material, commercial (Merck kGA, Germany) elemental Cu (99.5%, 63 m), ZnO (99%, 7 m), Y2 O3 (99%, 6 m), Al2 O3 (99.5%, 63 m) powders were used for mechanical alloying experiments. Oxides were added at rates of 1, 2, 4 and 6 wt.% to the raw Cu powder. Meanwhile, Zn–stearate was added to composition for avoiding agglomeration of powder particles. MA was carried out in a planetary ball mill using a hardened steel vial and Ø8 mm balls with a speed of 200 rpm for 5 h. Ball-to-powder weight ratio (BPR) was 12:1. The vials were purged by argon before MA to prevent the oxidation of powder during the milling process. Milled powders and asreceived Cu powders (P/M Cu) were pressed at 200 MPa and sintered under 10−3 Torr vacuum atmosphere at 800 ◦ C for 30 min. Then, the samples were forged at the ratio of 75% axially at 650 ◦ C so as to increase density. Contact surfaces were cleaned with 1200 mesh SiC grinder down to 1 mm. Electrical conductivity of these samples was investigated so as to determine the best electrical conductivity. For this aim Hioki LCR meter was used. After that, contact count experiments were made with these samples which exhibited the best conductivity for counts of 3000, 6000 and 9000 turn on/off. Experimental setup was consisted of a square wave oscillator (f = 1 Hz) with turn on/off the contactor, a counter and a contactor on which oxide reinforced Cu samples were mounted. Contact samples were made the same material for each experiment group of the contact count as shown in Fig. 1. The contact count was adjusted to desired count number (3000, 6000 and 9000) before the experiment then when it was reached the count number, operation was automatically ended over the counter electrical load was 1600 W (at 220 VAC) for all experiments. Experimental setup which was used in this study is shown in Fig. 1. Weight loss of the samples was determined by a precision (10−5 g) weighing scale. Subsequently, surfaces and oxide evaluations
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Fig. 1 – Schematic of experimental setup.
of samples were investigated by Zeiss Evo 40 model SEM. XRD investigation was applied to 4 wt.% ZnO containing sample which has the highest conductivity via Rigaku RAD-B Geigerflex model equipment by Cu K␣ radiation. P/M Cu sample was used as a comparing sample so as to determine the effect of MA on conductivity.
3.
Results
Fig. 2 shows the relation between oxide ratio and electrical conductivity in ZnO, Y2 O3 and Al2 O3 reinforced Cu samples. It is seen that conductivity increases with increasing oxide ratio up to 4%. After this ratio a sharp decrease occurs despite of the increasing oxide ratio. It is known that, both matrix particle size and oxide size decrease because of the nature of MA (Eskandarany, 2001; Evin, 2003). Consequently, average-free way of electrons decreases. Lower oxide particle size causes a decrease in electrical conductivity. However, MA causes less decrease in electrical conductivity than the other strengthening methods. Especially, conductivity less affected in 4 wt.% ZnO reinforced Cu. Because of ZnO has a higher electrical conductivity (1 × 10−7 MS/m) then other. Optimal oxide rate was obtained in 4 wt.% for whole samples. It can be said that a balance occurred between oxide ratio and conductivity. Also in Fig. 2, electrical conductivity of all samples is shown as International Annealed Copper Standard (IACS%, 100% IACS = 58 MS/m (Rajkovic et al., 2006; Lopez et al., 2005)). 87% IACS was obtained form P/M Cu sample which higher then other. This can be explaining by when the particle size is
Fig. 2 – Oxides ratio vs. conductivity changes.
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1286–1290
Fig. 3 – Turn on/off counts vs. % weight loss.
decrease with prolonged MA time according to raw Cu powders; grain size is finer after sintering. Therefore, this causes a decrease in electrical conductivity (Verma, 1990). Relatively increased grain boundary existence occur considerably interfacial barrier for electrons. The best conductivity was obtained from 4 wt.% oxides reinforced samples. Thus, contact count experiments were carried out by these samples. Fig. 3 is plotted according to weight loss. The best conductivity and the least % weight loss were obtained from 4 wt.% ZnO reinforced Cu sample as can be seen in Fig. 3. Generally, weight loss increased with increasing contact count. Results in Fig. 3 show that weight loss is highly affected by the conductivity of samples. Low limit requirement of conductivity for copper-based alloys for higher temperature applications is 50% IACS (Rajkovic et al., 2006). Thus, 4 wt.% ZnO reinforced Cu material conductivity is significantly over the applicable limit. SEM micrograph of 4 wt.% ZnO reinforced Cu sample which gave the best conductivity, after the counts of 3000 and 9000 turn on/off is shown in Fig. 4a and b. Surface evaluations show that oxide increases when turn on/off counts increase and arc formation gets easier. It can be said that this situation has a feed-back mechanism which causes the surface of reinforced Cu to be melted and/or oxidized. Contact materials heat up during turn on/off because of arc or high resistance. Brittle oxide layer formed on contact material is broken by impact force and removed from surface. XRD result showed that some oxide formations occurred on the surface of sample. This causes mass loss. When the contact counts were increased, particles on the surface got coarser. Craters which can be seen in Fig. 4a were disappeared in Fig. 4b. This can be explained by increased arc formation which caused oxidation on the surface. In Fig. 4b, it is also supposed that ZnO oxide particles were dissolved and alloyed to Cu matrix by absorbing the heat generated by arc. By this way, localized Cu–Zn alloy which has a higher melting point occurred on the surface of the sample. XRD result is including both some oxide types, e.g. CuO, Cu2 O and Cu–Zn alloy that they formed on the surface of the sample. XRD analysis is shown in Fig. 5 which taken from 4 wt.% ZnO containing sample.
Fig. 4 – SEM micrograph of 4 wt.% ZnO reinforced Cu after counts of (a) 3000 turn on/off and (b) 9000 turn on/off.
It can be thought that arc occurred especially on pores or close to pores caused severe oxidation of the surface of oxide reinforced Cu. Complex oxides which contain Zn and Cu caused the formation of a protective layer on the surface of the 4 wt.% ZnO reinforced Cu. Since weight loss decreases with increasing layer strength, the strength of this protective layer is important in weight loss. For this reason, less weight loss was determined for 4 wt.% ZnO reinforced Cu (Fig. 3). There did not generate complex oxide on the contact surface of Cu samples that contain 4 wt.%. Al2 O3 and 4 wt.% Y2 O3 . Because
Fig. 5 – XRD analyze result is taken from 4 wt.% ZnO containing sample.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1286–1290
1289
the contact sample is melted and/or oxidized. In this figure, a non-broken oxide layer also surrounds the aforementioned craterlike layer. The particle size of oxides is smaller than 4 wt.% ZnO reinforced Cu sample and these particles are less bonded to main matrix than 4 wt.% ZnO reinforced Cu. In Fig. 6c it can be thought that oxide particles are easily removable from the surface. This is also supported by the highest weight loss determined for 4 wt.% Y2 O3 in Fig. 3.
4.
Conclusions
Mechanical alloyed powder samples that including different oxides were exposed to turn on/off experiments up to 9000 counts. Results showed that 4 wt.% ZnO, 4 wt.% Al2 O3 and 4 wt.% Y2 O3 in order oxides containing samples exhibited the best electrical conductivity relatively. In contrast, Pure P/M Cu sample has the best IACS%. The higher conductivity was obtained from this sample, because no process was applied to P/M Cu sample. Weight loss increased with the increasing turn on/off count in contact count experiments. Matrix hardness increased by dispersoids containing sub-grain boundaries is effective barriers to dislocation motion. The reinforcing particles have incoherent with the copper lattice; dislocations can only pass these obstacles by an Orowan mechanism (Lopez et al., 2007). These particles are decreased weight loss by the way of both barriers to climbing dislocation and absorbing the heat generated by arc at elevated temperatures. It can be thought that ZnO on the contact surface was partially dissolved in the main matrix due to cause of the heat generated by the electric arc, and alloyed the main matrix at the contact surface. So the main matrix turned to the brass. Relatively, brass has higher hardness than copper. Thus, less weight loss was obtained from 4 wt.% ZnO reinforced Cu contact material. Among the oxide reinforced Cu composites prepared via mechanical alloying, 4 wt.% oxide reinforced samples exhibited good electrical conductivity. Both the best electrical conductivity and the lowest weight loss were obtained for 4 wt.% ZnO reinforced sample. Fig. 6 – SEM micrograph of (a) P/M Cu, (b) 4 wt.% Al2 O3 and (c) 4 wt.% Y2 O3 , after 9000 turn/off counts.
Acknowledgement This work has been financially supported by the Firat Unv. Sci. ¨ Res. Fund (FUBAP, Project No. 1099)
Y2 O3 and Al2 O3 are refractory oxides, they melt at elevated temperatures. So, only Cu oxide can be seen on the surface of these samples. Some particles have spherical form in Fig. 4b. When the arc was occurred, some particles which were semibonded to the main matrix melted and returned to a spherical form or were transferred by arc blow. In Fig. 6a–c, SEM micrograph of P/M Cu, 4 wt.% Al2 O3 and 4 wt.% Y2 O3 reinforced samples after 9000 turn on/off counts are shown, respectively. It is seen in Fig. 6a that pure P/M Cu contact sample was oxidized severely and some plateaus (indicated by arrows) could not be broken and removed from the surface by impact force applied by electromagnet of the contactor for closing circuit. A massive oxide layer look like a crater is seen in the middle of the picture in Fig. 6b. This shows that the surface of
references
ASM Handbook, 1997. Powder Metal Technologies and Applications, vol. 7. ASM, USA, pp. 1020–1030. Chen, W., Kang, Z., Shen, H., Ding, B., 2006. Arc erosion behavior of a nanocomposite W–Cu electrical contact material. Rare Met. 25, 37. Eskandarany, M.S.E., 2001. Mechanical alloying for fabrication of advanced engineering materials, NY, p. 12. ˘ p. 92. Evin, E., 2003. Ph.D. Thesis. Fırat University, Elazıg, Lenel, F.V., 1980. Powder Metallurgy Principles and Applications. MPIF Princeton, New Jersey, pp. 549–559. Lopez, M., Corredor, D., Camurri, C., Vergara, V., Jimenez, J., 2005. Performance and characterization of dispersion strengthened
1290
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1286–1290
Cu–Tib2 composite for electrical use. Mater. Charact. 55, 252–262. Lopez, M., Jimenez, J.A., Corredor, D., 2007. Precipitation strengthened high strength-conductivity copper alloys containing ZrC ceramics. Compos.: Part A 38, 272–279. Ovecoglu, M.L., Ozkal, B., 2004. Mechanochemical synthesis of WC powders by mechanical alloying. Key Eng. Mater. 264–268, 89–92.
Rajkovic, V., Bozic, D., Jovanovic, T.M., 2006. Characterization of dispersion strengthened copper with 3 wt.% Al2 O3 by mechanical alloying. Mater. Charact. 57, 94–99. Suryanarayana, C., 2001. Mechanical alloying and milling. Progr. Mater. Sci. 46, 1–184. Verma, A., 1990. Production of Ag–SnO2 electrical contact materials by mechanical alloying. Powder Metall. Sci. Technol. 2, 55.
journal homepage: www.elsevier.com/locate/jmatprotec
The investigation of contact performance of oxide reinforced copper composite via mechanical alloying ¨ ¨ Omer Guler, Ertan Evin ∗ ˘ Turkey Fırat University, Department of Metallurgical and Materials Engineering, Elazıg,
a r t i c l e
i n f o
a b s t r a c t
Article history:
In this study, the electrical performance of ODS Cu-based contact materials produced by
Received 24 August 2007
mechanical alloying (MA) was investigated. Cu-based powder mixtures that contain vari-
Received in revised form
ous oxides at different proportions were milled by the high energy planetary ball mill for
13 February 2008
5 h. Mechanically alloyed powder mixtures containing ZnO, Al2 O3 and Y2 O3 at the ratios
Accepted 24 March 2008
of 1, 2, 4 and 6 wt.% were pressed and sintered at 800 ◦ C in vacuum environment. These compacts were then forged axially at the ratio of 75% at 650 ◦ C so as to increase the density. At the first step, electrical conductivity experiments were applied to these samples to
Keywords:
determine the best conductivity. Results showed that, reinforced Cu samples containing 4%
Mechanical alloying
oxides exhibited the best. At the second step, contact count experiments were made with
High energy ball milling
these samples for determining contact performance for the counts of 3000, 6000 and 9000
ODS copper
turn on/off. The samples of 4 wt.% ZnO, 4 wt.% Al2 O3 , 4 wt.% Y2 O3 reinforced Cu materials,
Electrical properties
respectively, exhibited both the best conductivity and the best contact performances. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Electrical contact materials are used in a variety of applications, such as electrical switches, contactors, circuit breakers, voltage regulators, arcing tips, switch gears and relays (ASM Handbook, 1997; Lenel, 1980). Materials used as electrical contacts in these applications must have a good combination of electrical conductivity, wearing qualities, and resistance to erosion and welding. Otherwise, the contacts will erode, causing poor contact and arcing. Arcing takes place when contacts are in the process of establishing a current flow or interrupting the flow of current. Arc is characterized by high temperature and a high current density in the arc column. Because of the high temperature and mass flow, the contact material surface is severely corroded and eroded, which results in erratic contact resistance and material loss. Therefore a contact material should have high electrical and thermal conductivity, high melting point, and high resistance to environmental reaction,
∗
as well as high arc erosion to maintain contact integrity (Chen et al., 2006). Contacts are made of elemental metals, composites, or alloys which are made by the melt-cast method. Because of low solubility required for the reinforcing phases, making these alloys by ingot metallurgy would lead to very coarse particles that would degrade the toughness significantly. This limitation can be avoided using the powder metallurgy (PM) process. Mechanical attrition of copper powders with ceramic particles has allowed the uniform introduction of small strengthening phases and also promotes a microstructural grain refinement (Lopez et al., 2007). As a way to improve the mechanical properties at low temperatures, the matrix must be strengthened with very low solubility particles, which have a low diffusivity in copper by means of mechanical alloying equilibrium with the electrical properties obtained (Lopez et al., 2005). The ideal metal or metal combination that can function as the perfect contact material under all conditions
Corresponding author. Tel.: +90 424 2370000/5442; fax: +90 424 2415526. E-mail address: eevin@firat.edu.tr (E. Evin). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.03.034
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1286–1290
does not exist. A thorough evaluation and understanding of the operating conditions of an electrical contact device, as well as consideration of economy, is necessary before selecting the most suitable contact material. Mechanical alloying (MA) is a solid-state powder processing technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. MA is a complex process and type of mill, milling time, type, size, and size distribution of the grinding medium, ball-to-powder weight ratio, temperature of milling are some of the important process variables. The ratio of the weight of the balls to the powder (BPR) has been varied by different researchers from a value of 1:1 to 220:1 (Suryanarayana, 2001). The optimum milling time depends on the type of mill, size of the grinding medium, temperature of milling, ball-to-powder ratio and desired phase formation (Ovecoglu and Ozkal, 2004). In this study, the electrical performance of ODS Cu-based contact materials produced by mechanical alloying was investigated. At the first step, electrical conductivity experiments were applied to these samples to determine the best conductivity. At the second step, contact count experiments were conducted with these samples for determining contact performance for the counts of 3000, 6000 and 9000 turn on/off.
2.
Experimental
As starting material, commercial (Merck kGA, Germany) elemental Cu (99.5%, 63 m), ZnO (99%, 7 m), Y2 O3 (99%, 6 m), Al2 O3 (99.5%, 63 m) powders were used for mechanical alloying experiments. Oxides were added at rates of 1, 2, 4 and 6 wt.% to the raw Cu powder. Meanwhile, Zn–stearate was added to composition for avoiding agglomeration of powder particles. MA was carried out in a planetary ball mill using a hardened steel vial and Ø8 mm balls with a speed of 200 rpm for 5 h. Ball-to-powder weight ratio (BPR) was 12:1. The vials were purged by argon before MA to prevent the oxidation of powder during the milling process. Milled powders and asreceived Cu powders (P/M Cu) were pressed at 200 MPa and sintered under 10−3 Torr vacuum atmosphere at 800 ◦ C for 30 min. Then, the samples were forged at the ratio of 75% axially at 650 ◦ C so as to increase density. Contact surfaces were cleaned with 1200 mesh SiC grinder down to 1 mm. Electrical conductivity of these samples was investigated so as to determine the best electrical conductivity. For this aim Hioki LCR meter was used. After that, contact count experiments were made with these samples which exhibited the best conductivity for counts of 3000, 6000 and 9000 turn on/off. Experimental setup was consisted of a square wave oscillator (f = 1 Hz) with turn on/off the contactor, a counter and a contactor on which oxide reinforced Cu samples were mounted. Contact samples were made the same material for each experiment group of the contact count as shown in Fig. 1. The contact count was adjusted to desired count number (3000, 6000 and 9000) before the experiment then when it was reached the count number, operation was automatically ended over the counter electrical load was 1600 W (at 220 VAC) for all experiments. Experimental setup which was used in this study is shown in Fig. 1. Weight loss of the samples was determined by a precision (10−5 g) weighing scale. Subsequently, surfaces and oxide evaluations
1287
Fig. 1 – Schematic of experimental setup.
of samples were investigated by Zeiss Evo 40 model SEM. XRD investigation was applied to 4 wt.% ZnO containing sample which has the highest conductivity via Rigaku RAD-B Geigerflex model equipment by Cu K␣ radiation. P/M Cu sample was used as a comparing sample so as to determine the effect of MA on conductivity.
3.
Results
Fig. 2 shows the relation between oxide ratio and electrical conductivity in ZnO, Y2 O3 and Al2 O3 reinforced Cu samples. It is seen that conductivity increases with increasing oxide ratio up to 4%. After this ratio a sharp decrease occurs despite of the increasing oxide ratio. It is known that, both matrix particle size and oxide size decrease because of the nature of MA (Eskandarany, 2001; Evin, 2003). Consequently, average-free way of electrons decreases. Lower oxide particle size causes a decrease in electrical conductivity. However, MA causes less decrease in electrical conductivity than the other strengthening methods. Especially, conductivity less affected in 4 wt.% ZnO reinforced Cu. Because of ZnO has a higher electrical conductivity (1 × 10−7 MS/m) then other. Optimal oxide rate was obtained in 4 wt.% for whole samples. It can be said that a balance occurred between oxide ratio and conductivity. Also in Fig. 2, electrical conductivity of all samples is shown as International Annealed Copper Standard (IACS%, 100% IACS = 58 MS/m (Rajkovic et al., 2006; Lopez et al., 2005)). 87% IACS was obtained form P/M Cu sample which higher then other. This can be explaining by when the particle size is
Fig. 2 – Oxides ratio vs. conductivity changes.
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1286–1290
Fig. 3 – Turn on/off counts vs. % weight loss.
decrease with prolonged MA time according to raw Cu powders; grain size is finer after sintering. Therefore, this causes a decrease in electrical conductivity (Verma, 1990). Relatively increased grain boundary existence occur considerably interfacial barrier for electrons. The best conductivity was obtained from 4 wt.% oxides reinforced samples. Thus, contact count experiments were carried out by these samples. Fig. 3 is plotted according to weight loss. The best conductivity and the least % weight loss were obtained from 4 wt.% ZnO reinforced Cu sample as can be seen in Fig. 3. Generally, weight loss increased with increasing contact count. Results in Fig. 3 show that weight loss is highly affected by the conductivity of samples. Low limit requirement of conductivity for copper-based alloys for higher temperature applications is 50% IACS (Rajkovic et al., 2006). Thus, 4 wt.% ZnO reinforced Cu material conductivity is significantly over the applicable limit. SEM micrograph of 4 wt.% ZnO reinforced Cu sample which gave the best conductivity, after the counts of 3000 and 9000 turn on/off is shown in Fig. 4a and b. Surface evaluations show that oxide increases when turn on/off counts increase and arc formation gets easier. It can be said that this situation has a feed-back mechanism which causes the surface of reinforced Cu to be melted and/or oxidized. Contact materials heat up during turn on/off because of arc or high resistance. Brittle oxide layer formed on contact material is broken by impact force and removed from surface. XRD result showed that some oxide formations occurred on the surface of sample. This causes mass loss. When the contact counts were increased, particles on the surface got coarser. Craters which can be seen in Fig. 4a were disappeared in Fig. 4b. This can be explained by increased arc formation which caused oxidation on the surface. In Fig. 4b, it is also supposed that ZnO oxide particles were dissolved and alloyed to Cu matrix by absorbing the heat generated by arc. By this way, localized Cu–Zn alloy which has a higher melting point occurred on the surface of the sample. XRD result is including both some oxide types, e.g. CuO, Cu2 O and Cu–Zn alloy that they formed on the surface of the sample. XRD analysis is shown in Fig. 5 which taken from 4 wt.% ZnO containing sample.
Fig. 4 – SEM micrograph of 4 wt.% ZnO reinforced Cu after counts of (a) 3000 turn on/off and (b) 9000 turn on/off.
It can be thought that arc occurred especially on pores or close to pores caused severe oxidation of the surface of oxide reinforced Cu. Complex oxides which contain Zn and Cu caused the formation of a protective layer on the surface of the 4 wt.% ZnO reinforced Cu. Since weight loss decreases with increasing layer strength, the strength of this protective layer is important in weight loss. For this reason, less weight loss was determined for 4 wt.% ZnO reinforced Cu (Fig. 3). There did not generate complex oxide on the contact surface of Cu samples that contain 4 wt.%. Al2 O3 and 4 wt.% Y2 O3 . Because
Fig. 5 – XRD analyze result is taken from 4 wt.% ZnO containing sample.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1286–1290
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the contact sample is melted and/or oxidized. In this figure, a non-broken oxide layer also surrounds the aforementioned craterlike layer. The particle size of oxides is smaller than 4 wt.% ZnO reinforced Cu sample and these particles are less bonded to main matrix than 4 wt.% ZnO reinforced Cu. In Fig. 6c it can be thought that oxide particles are easily removable from the surface. This is also supported by the highest weight loss determined for 4 wt.% Y2 O3 in Fig. 3.
4.
Conclusions
Mechanical alloyed powder samples that including different oxides were exposed to turn on/off experiments up to 9000 counts. Results showed that 4 wt.% ZnO, 4 wt.% Al2 O3 and 4 wt.% Y2 O3 in order oxides containing samples exhibited the best electrical conductivity relatively. In contrast, Pure P/M Cu sample has the best IACS%. The higher conductivity was obtained from this sample, because no process was applied to P/M Cu sample. Weight loss increased with the increasing turn on/off count in contact count experiments. Matrix hardness increased by dispersoids containing sub-grain boundaries is effective barriers to dislocation motion. The reinforcing particles have incoherent with the copper lattice; dislocations can only pass these obstacles by an Orowan mechanism (Lopez et al., 2007). These particles are decreased weight loss by the way of both barriers to climbing dislocation and absorbing the heat generated by arc at elevated temperatures. It can be thought that ZnO on the contact surface was partially dissolved in the main matrix due to cause of the heat generated by the electric arc, and alloyed the main matrix at the contact surface. So the main matrix turned to the brass. Relatively, brass has higher hardness than copper. Thus, less weight loss was obtained from 4 wt.% ZnO reinforced Cu contact material. Among the oxide reinforced Cu composites prepared via mechanical alloying, 4 wt.% oxide reinforced samples exhibited good electrical conductivity. Both the best electrical conductivity and the lowest weight loss were obtained for 4 wt.% ZnO reinforced sample. Fig. 6 – SEM micrograph of (a) P/M Cu, (b) 4 wt.% Al2 O3 and (c) 4 wt.% Y2 O3 , after 9000 turn/off counts.
Acknowledgement This work has been financially supported by the Firat Unv. Sci. ¨ Res. Fund (FUBAP, Project No. 1099)
Y2 O3 and Al2 O3 are refractory oxides, they melt at elevated temperatures. So, only Cu oxide can be seen on the surface of these samples. Some particles have spherical form in Fig. 4b. When the arc was occurred, some particles which were semibonded to the main matrix melted and returned to a spherical form or were transferred by arc blow. In Fig. 6a–c, SEM micrograph of P/M Cu, 4 wt.% Al2 O3 and 4 wt.% Y2 O3 reinforced samples after 9000 turn on/off counts are shown, respectively. It is seen in Fig. 6a that pure P/M Cu contact sample was oxidized severely and some plateaus (indicated by arrows) could not be broken and removed from the surface by impact force applied by electromagnet of the contactor for closing circuit. A massive oxide layer look like a crater is seen in the middle of the picture in Fig. 6b. This shows that the surface of
references
ASM Handbook, 1997. Powder Metal Technologies and Applications, vol. 7. ASM, USA, pp. 1020–1030. Chen, W., Kang, Z., Shen, H., Ding, B., 2006. Arc erosion behavior of a nanocomposite W–Cu electrical contact material. Rare Met. 25, 37. Eskandarany, M.S.E., 2001. Mechanical alloying for fabrication of advanced engineering materials, NY, p. 12. ˘ p. 92. Evin, E., 2003. Ph.D. Thesis. Fırat University, Elazıg, Lenel, F.V., 1980. Powder Metallurgy Principles and Applications. MPIF Princeton, New Jersey, pp. 549–559. Lopez, M., Corredor, D., Camurri, C., Vergara, V., Jimenez, J., 2005. Performance and characterization of dispersion strengthened
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1286–1290
Cu–Tib2 composite for electrical use. Mater. Charact. 55, 252–262. Lopez, M., Jimenez, J.A., Corredor, D., 2007. Precipitation strengthened high strength-conductivity copper alloys containing ZrC ceramics. Compos.: Part A 38, 272–279. Ovecoglu, M.L., Ozkal, B., 2004. Mechanochemical synthesis of WC powders by mechanical alloying. Key Eng. Mater. 264–268, 89–92.
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