Indian Journal of Engineering & Materials Sciences Vol. 16, August 2009, pp. 281-287

Nanostructured silver-graphite electrical contact materials processed by mechanical milling†

Bharati R Rehania, P B Joshia* & V K Kaushikb aMetallurgical and Materials Engineering Department, Faculty of Technology and Engineering,

M S University of Baroda, Vadodara 390 001, India bReliance Industries Limited, P.O. Petrochemicals, Vadodara 391 346, India

Received 14 July 2008, accepted 30 June 2009

In last few decades there has been a growing interest in nanostructured materials in view of their improved mechanical, physical and chemical properties. A wide variety of techniques have been developed to synthesize nanopowders including high-energy milling or mechanical alloying (MA) for producing bulk nanostructures. High-energy milling is used for preparation of elemental and composite powders of different metals and non-metals. Literature reports the synthesis of ductile-ductile composites such as aluminium-graphite and copper-graphite by MA owing to its potential benefits in terms of improved mechanical properties. Reports are also there on the application of this technology for processing of silver-graphite contact materials offering improved switching properties.

The present investigation deals with a comparative study on synthesis of silver-graphite nanocomposite powders in a planetary ball mill using two different approaches, namely milling of a blend of silver and graphite powder particles in one case and milling of only elemental silver powder in the other. The study revealed that somewhat inferior properties are obtained for the AgC bulk solids prepared from powders synthesized by the former approach. This is in view of the tendency of graphite to coat silver during milling, leading to very poor silver-silver interparticle contacts and resultant bulk solid with high porosity. However, an alternative approach of milling silver alone in the planetary mill is found to offer not only improved properties but also reduction in sintering temperature.

Keywords: Silver-graphite contact materials, High-energy milling, Nanocomposite powders, Powder metallurgy process

Silver-base composites are developed as electrical contact materials for low- as well as high-voltage switchgear devices owing to their superior electrical properties compared to silver-base alloys1,2. The silver-graphite (AgC) contacts, an important member of this group, are the composites having a fine and uniform dispersion of graphite particles in silver matrix. They are known for their excellent behaviour in regard to antiwelding characteristics and low contact resistance under steady state current. The AgC contacts therefore find widespread applications in various protective switchgears, such as low voltage circuit breakers, earth leakage circuit breakers, circuit protective switches, fault current protective switches, railway signal relays and motor protection switches3

.

Due to the fact that silver and graphite are insoluble in each other, the technique for production

of AgC contacts is restricted to powder metallurgical methods only. Commercially AgC contacts are therefore either produced by press-sinter-repress route or press-sinter-extrude route. The conventional method to produce composite powders of silver and graphite as starting material for either of these routes is blending/mixing or ball milling. However, the major drawback of this method is high tendency of segregation because of large difference between the densities of the constituent powder particles and eventually heterogeneous microstructure. As a result, a few alternative methods are developed or under development to produce silver-graphite composite powders with uniform dispersion of graphite in silver matrix namely the electroless coating, microemulsion process and high-energy milling4,5. Of all these methods, high-energy milling has emerged as a proven top-down approach for synthesis of nanocomposite powders and the nanostructured materials there from. Recently, there are some reports in the literature on synthesis of Ag-C composite powders by this method6.

—————— *For correspondence (E-mail: drpbjoshi@yahoo.com) †Paper presented at the One-day Workshop on “Synthesis and Characterization of Nanostructrured Materials (SCNM -08), organised by Applied Physics Department of M S University of Baroda, Vadodara, on March 30, 2008

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High-energy milling or mechanical alloying (MA) was originally developed by Benjamin at INCO Alloys International in the late 1960’s7 and has traditionally been used for the production of dispersion-strengthened alloys for high-temperature structural applications, e.g., oxide dispersion-strengthened (ODS) superalloys based on Al, Ni, Fe and Ti8,9. Since early 1980’s, it has been also recognized that high-energy milling or MA of crystalline elemental powder mixtures can produce a variety of equilibrium as well as metastable alloy phases, including intermetallic compounds, supersaturated solid solutions, dispersion strengthened metal-matrix composites and amorphous alloys10,11. MA is a method that involves the flattening, fracturing, and re-welding of a mixture of powder particles so as to produce a controlled and extremely fine microstructure12. If two metals form a solid solution, MA technique provides fine structured alloy powder. Conversely, if the two materials are insoluble in each other in liquid or solid state, an extremely fine dispersion of one of them in the other can be accomplished. The advantage of this method is that on subsequent consolidation of the milled powder, bulk nanostructured material with close to theoretical density can be produced. Besides this, there are the potential benefits in terms of enhanced sinterability of such nanocrystalline powders, reduced sintering temperatures and improved mechanical properties of the resultant nanostructured materials, viz., strength and hardness.

Haifeng et al.6 carried out a novel study on application of nanotechnology in developing silver-graphite contact material and optimization of its physical and mechanical properties. In this investigation, 50-60 nm size graphite flakes were first subjected to surface activation by high-energy ball milling in a planetary ball mill and subsequently coated with silver by liquid spraying-coating method. The silver as a coated phase was obtained from the reaction of silver nitrate (AgNO3) with hydrazine (H2N-NH2-H2O) as reducing agent. The electroless coated Ag-C powders were cold compacted to 90% theoretical density, sintered in hydrogen atmosphere and repressed to final shape.

Bostan and co-workers13 and Michal Besterci14-16 have done in depth studies on dispersion-strengthened aluminium prepared by mechanical alloying and have found that a considerable improvement in mechanical properties is attainable in high-energy milled

Al-graphite system due to formation of Al4C3 nanosize particles in aluminum matrix after sintering. Likewise, similar studies by Saji et al.17 and Dewidar and Lim18 on processing of copper-graphite composites by mechanical alloying have revealed that the method offers enhanced solubility of graphite in copper along with high density levels after sintering.

It is thus customary to subject both the constituent powder particles to milling so as to produce nanocomposite powders. However, contrary to this, in the present investigation use of high-energy milled silver-graphite nanocomposite powders to produce high density AgC nanocomposites have shown an adverse behaviour. The AgC bulk solids after sintering and repressing have relatively poor density and electrical conductivity compared to conventionally processed material.

Hence, an alternative milling procedure has been developed during this study to overcome this problem by subjecting only elemental silver powder to milling in a planetary ball mill followed by thorough mixing of the stoichiometric amount of graphite with it, instead of milling of silver and graphite powder particles together. The following discussion deals with the experimental procedures and the results for both these routes.

Experimental Procedure

The material composition selected for this investigation was Ag-3wt.%C composite. The raw materials used were 99.9 % purity silver powder supplied by M/s Modison Pvt. Ltd. and high purity electrolytic graphite powder. The Ag-3wt.%C nanocomposite powders were synthesized by high-energy milling in a planetary ball mill using two different approaches. In the first case, the blend of silver and graphite powder particles was subjected to high- energy milling (called as Route-A) whereas in the second case, only elemental silver powder is subjected to milling followed by ultrasonic mixing of stoichiometric amount of graphite with it (called as Route-B). For both the milling routes a 250 mL capacity twin- bowl type planetary ball mill of M/s. Insmart Systems, Hyderabad was used. The milling was carried out in tungsten carbide lined stainless steel vial using tungsten carbide balls of 10 mm and 16 mm diameters as grinding media. The mill was operated at a speed of 300 rpm and at a ball to charge ratio of 10:1. The powder blend of Route-A was subjected to milling without using any Process Control Agent (PCA) whereas 2 wt% of stearic acid

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was used as PCA in case of Route-B so as to avoid excessive cold welding between the silver powder particles. The powder samples were drawn in each case during the course of milling at periodic time intervals for the purpose of their characterization using techniques like scanning electron microscopy (SEM), electron spectroscopy for chemical analysis (ESCA) and X-ray diffraction (XRD).

Morphological investigation of powder samples was done using scanning electron microscope model JEOL JSM-5610 LV. The SEM images were taken at various magnifications in compositional and topographic mode.

Highly surface sensitive technique, electron spectroscopy for chemical analysis (ESCA), was used to observe the changes occurring on the surface of the powder particles of Route-A, during the course of milling. A Vacuum Generator’s ESCALAB MK II spectrometer equipped with twin (aluminium and magnesium) X-ray source was used for recording XPS spectra. The anode of Mg source used in present study was operated at 10 kV and 10 mA and the vacuum in the analysis chamber was maintained better than 5×10-6 Pa during analysis. The spectrometer was calibrated using Cu (2p

3/2) photoelectron line at 932.7 eV. Powder samples were placed onto double-side adhesive tape and mounted on the sample holder. The samples were kept overnight in the preparation chamber before being transferred to analysis chamber for recording of spectra. Data were collected and analyzed on a computer interfacing the spectrometer.

The elemental and composite powder samples drawn during the course of milling were subjected to phase identification and crystallite size estimation using X-ray diffraction method (XRD). The powder diffraction profiles were obtained using Rigaku Gieger Flex D-max model (Japanese make) of X-ray diffractometer within the 2θ range of 20º - 80º at a scan speed of 3 deg/min using Cu target and Cu-Kα radiation of 1.5406 Å wavelength at a power rating of 30 kV and 25 mA.

For making the bulk solid samples, AgC nanocomposite powders of both the processing routes were pressed on a 100 ton capacity hydraulic press at 300 MPa pressure in single action die compaction mode. In order to examine the effect of sintering temperature on properties of AgC material, the green compacts produced from the silver-graphite nanocomposite powders of both the routes were then subjected to sintering at temperatures ranging from

500oC to 800oC for 1 h under a vacuum of 10-2 torr. A microprocessor-based PID type temperature programmer/controller model WEST-2050 of M/s. Toshniwal Brothers (Mumbai) Pvt. Ltd. was used to monitor the temperature during the course of sintering. The heating rate during sintering cycles was maintained at 6-7oC/min. The sintered compacts were repressed at a pressure of 900 MPa using the same die-set to produce high density compacts. The bulk solid compacts were evaluated for their density and electrical conductivity values after sintering as well as repressing. The electrical conductivity was measured on ground and polished samples with the help of an electrical conductivity meter Type 979 of M/s. Technofour, India. SEM of the compacts was done in unetched condition using JEOL JSM-5610 LV scanning electron microscope.

Results and Discussion The SEM micrograph as shown in Fig. 1a indicates

heterogeneously dispersed grey coloured particles of graphite in silver matrix for Ag-3wt%C as-blended powder (Route- A). The high-energy milling of Ag-3wt%C powder in planetary ball mill results in two major effects, viz., the flattening of the matrix, i.e., silver particles in view of their ductile nature and the smearing and spreading over of the second phase, i.e., graphite over the silver particles with increasing milling time. The changes occurring in the shape and size of silver particle as well as the extent of coating of graphite on the surface of the silver particle with progressive milling are displayed by the SEM micrographs (Figs1b-1d).

More or less similar behaviour is exhibited by the elemental silver powder (Route- B) as regards the powder particle morphology when subjected to high energy milling in the presence of PCA. The silver particles undergo flattening in this case also (Figs 2a-2d). For instance, the SEM micrograph for the powder milled for 360 min shows highly coarse and flattened silver particles (Fig. 2d).

Figures 3a and 3b show the XPS (X-ray photoelectron spectrum) survey scan spectra for Ag-3wt%C powder sample (Route-A) indicating photoelectron lines for silver and carbon, after 45 min and 180 min of milling, respectively. As per these figures there is a considerable change in the relative intensity ratio of silver to carbon with increasing milling time. As the milling time increases the intensity of peak Ag (3d) line decreases whereas that of C (1s) increases. In other words the relative

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intensity ratio of Ag to C decreases with progressive milling. This means that the graphite tends to coat silver with more and more milling so that silver

particles buried below graphite particle layers do not contribute to any signals from them. This eventually prohibits silver to silver interparticle contacts during

Fig. 1 — SEM micrographs of as-blended and milled Ag-3wt%C composite powders of Route-A; (a) as-blended (b) 45 min milled (c) 90 min milled and (d) 180 min milled

Fig. 2 — SEM micrographs of elemental silver powders of Route-B milled for different milling durations; (a) 30 min (b) 45 min (c) 90 min and (d) 180 min

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sintering. The multiscan data were also collected for these two elements and the corresponding relative intensities obtained for silver Ag (3d) and carbon C (1s) lines for 45 min and 180 min milling conditions are shown in Fig. 4. The photoelectron lines for Ag (3d) and C (1s) corresponding to 45 min of milling are given as full lines whereas those for 180 min of milling are shown as dotted lines. This figure also confirms that with the increase in milling time the intensity of Ag (3d) lines decreases whereas that of C (1s) line increases and thus confirming the tendency of graphite to coat silver with progressive milling.

The XRD analysis of the powder samples of Route-A showed the diffraction peaks corresponding to pure silver and graphite which is obvious (Fig. 5). There was no indication of formation of any new phase like silver carbide (Ag4C3) unlike Al4C3 found in case of high-energy milled Al-C powders. The high-energy milling of powder normally leads to decrease in the crystallite size of the material. Figure 6 shows the variation in crystallite size of the milled powders of Route-A and Route-B obtained using peak broadening data from XRD profiles and the calculations based on Scherrer equation. As expected, the crystallite size, by and large, decreases with increasing milling time for both the powders. The crystallite size of the powders of Route-A and Route-B after 90 min of milling is 36 nm and 28 nm, respectively.

Fig. 3— XPS survey scan spectra for Ag-3wt%C powder sample (Route-A) milled for (a) 45 min and (b) 180 min

Fig. 4 — Multiscan data for Ag and C and the corresponding relative intensities for silver Ag (3d) and carbon C (1s) lines for 45 min and 180 min milling

Fig. 5 — Multiple XRD profiles for Ag-3wt.%C high-energy milled composite powders of Route-A for different milling durations

Fig. 6 — Variation of crystallite size with milling duration for powders of Route-A and Route-B

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A set of bulk solid samples were prepared from Ag-3wt%C composite powders synthesized by Route-A to examine the effect of milling on the electrical conductivity of the resultant bulk solid compacts. The electrical conductivity for the bulk solid samples produced from the composite powders of Route-A, milled for different milling durations gradually decreases as the milling duration increases. This is in view of more and more smearing of graphite and its subsequent coating on silver particle surfaces with increasing milling duration. The bulk solid compacts prepared from such powders have lesser and lesser silver to silver interparticle contacts and hence higher porosity levels which lead to drop in electrical conductivity. A close look at the comparative data given in Table 1 for the density and electrical conductivity in as-sintered condition and after repressing for bulk solid compacts of Route-A and Route-B indicates that it is possible to attain enhanced density levels (close to theoretical density) and in turn improved electrical conductivity for the compacts of Route-B. Whereas the results for the corresponding compacts of Route-A are not so encouraging. The microstructures for the bulk solid compact of these two routes shown in Figs 7a and 7b explain this difference. The bulk solid compact of Route-A shows higher porosity, accounting for the lower density and electrical conductivity. The nanocrystalline grains of

silver (grain size 28 nm) in powders synthesized by Route-B enable the sintering of resultant Ag-C nanocmposites at a temperature as low as 500oC.

Conclusions Conventional method of high-energy milling of a

blend of silver and graphite powder particles to produce AgC bulk solids for electrical contact application appears to be unsuitable. Alternatively, high-energy milling of elemental silver followed by addition of stoichiometric amount of graphite seems to be a better option. The latter approach offers the benefits like enhanced sinterability, reduced sintering temperature, improved sintered density and electrical conductivity.

Acknowledgement The authors acknowledge the financial assistance

provided by the All India Council of Technical Education (AICTE), New Delhi under the research

Table 1 — Sintered density and electrical conductivity of Ag-3wt%C bulk solid compacts produced by Route-A and Route-B

Route-A

s- sintered

Repressed

Sintering temperature

oC Density

(%) Electrical

conductivity ( % IACS)

Density (%)

Electrical conductivity ( % IACS)

800 90.3 48 92.6 48 700 90.4 45 93.1 46 600 90.2 46 92.6 47 500 90.1 45 92.1 46

Route-B

As-sintered

Repressed

Sintering

temperature oC

Density (%)

Electrical conductivity ( % IACS)

Density (%)

Electrical conductivity ( % IACS)

800 91.0 64 98.0 73 700 91.0 62 98.9 69 600 91.0 63 99.1 69 500 91.0 62 99.0 68

Fig 7— SEM micrograph of Ag-3wt%C bulk solid compact of Route-A at 250× magnification showing (a) high porosity and (b) uniformly dispersed graphite particles in silver matrix

(a)

(b)

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promotion scheme File No. 8023/BOR/RPS-192/2006-07.

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