Effects of nickel addition on properties of Ag W electrical contact materials

Transcription

1 Effects of nickel addition on properties of Ag W electrical contact materials Z. Aslanoǧlu, Y. Karakaş, M.L.Öveçoǧlu, and B. Özkal Dr Aslanoǧlu and Dr Karakaş are in the Department of Metallurgical and Materials Engineering, Faculty of Engineering, Sakarya University, Adapazari, Turkey and Dr Öveçoǧlu, and Dr Özkal are in the Department of Metallurgical and Materials Engineering, Faculty of Chemistry Metallurgy, Istanbul Technical University, Maslak, Istanbul, Turkey. Manuscript received 18 April 2000; accepted 22 August IoM Communications Ltd. Tungsten and silver powders doped with small amounts of nickel were milled in a heavy duty attritor for 150 min to constitute the W 35 wt-%ag compos- ition. Milled powders were compacted, sintered, furnace cooled, and re-sintered using industrial conditions. Scanning electron microscopy (SEM) investigations revealed homogeneous dispersions of silver and tungsten regions both in the as milled pow- ders and the as milled compacts. The X-ray results taken from the milled powders and the as milled com- pacts showed the presence of the characteristic W and Ag peaks. Bulk hardness values of the sintered com- pacts increased with increases in nickel additions. The arc erosion characteristics of the W 35 wt-%ag com- pacts indicated that the erosion rate of the contact material declined with nickel additions. Contact life expectancy increases about 10 fold for the W 35 wt-%ag compact containing 1 wt-%ni from the one with no Ni content. PM/0910 and cobalt, are used as a chemical activators for tungsten.7 The effect of activated sintering for tungsten powders with the addition of nickel was first discovered at the end of 1950s.8 The improved sinterability has been attributed to the beneficial effects of Ni on the wetting characteristics between liquid and solid phases and also its possible role as an activator in the solid state sintering of W.9 It has been suggested that the sinterability may depend on the method adopted to add the nickel to the tungsten. It was implied that the behaviour nickel during the initial stage of sintering might be important in the sintering mechanism and initial stage of nickel, i.e. its initial location and size, because the physical state of activator appears to show a dependence on the method used to add it to the tungsten. Kim et al.10 reported that both the nickel doping method and the particle size affect the sinterability of tungsten. Furthermore, the smaller the particle size of nickel added to tungsten, the higher the sinterability observed. Nickel can be added using different processes, such as a nickel salt solution and impregnation of nickel salt solution and in the powder form.7 12 The aim of this study was to investigate the effect of adding the activator on the activated sintering and arc erosion behaviour of silver tungsten compacts. On this basis, nickel was added to silver powder during the production of silver by chemical co-precipitation method. A maximum of 1 wt-%ni is normally used as a sintering aid, the upper limit corresponding to a decrease in the electrical conductivity of the composite. INTRODUCTION Owing to their high hardness, good thermal and electrical conductivity, and improved wear resistance, electrical contact materials have found a wide variety of applications in circuit breakers and switchgears.1 4 In addition to these properties, in any electrical switch, electrical contacts should have a good service life and should not suffer any alterations in the mode of operation after a large number of make break cycles.5 In many cases the desired combi- nation of properties can be realised only by utilising refractory metals (tungsten, molybdenum, etc.) with high conductivity metals, such as silver or copper. Silver tungsten (Ag W) based contact materials are ideally suited for applications as circuit breakers and switchgears where high voltages generate extensive arcing. Owing to mutual insolubility of W and Ag and the large differences in their melting points, the W Ag contact materials can only be fabricated via powder metallurgy routes. Tungsten has a slow sintering rate at high temperatures due to a high interatomic bond strength and high activation energy for diffusion. To obtain high densification, sintering must be activated. Several chemical and physical methods are commonly used for the activation sintering of tungsten6 and various additives, such as nickel EXPERIMENTAL PROCEDURE Commercial grade powders of elemental tungsten (2 6 mm, >99 7% purity) and silver (1 2 mm, >99% purity) were used. Powder characteristics, including purity are listed in Table 1. Nickel was doped with silver in small quantities varying from 0 25 to 1 wt-% using chemical co-precipitation methods,13 in accordance with the steps disclosed in Fig. 1. The solid silver (99 5% purity) was dissolved in nitric acid aqueous solution under heating to obtain a homogeneous solution and nickel nitrate was dissolved in a dilute nitric acid aqueous solution. The two resulting homogeneous solutions were mixed. The aqueous solution containing reductant was added to the mixed nitric acid solution for reducing the silver and nickel ions to their corresponding precipitates. The precipitated powders were collected by filtration, washed with purified water 10 times, and then dried in an oven at 120 C for 12 h. Mixed hydrates of silver and nickel were calcinated at 550 C for 1 h and then reduced at 800 C for 1 h under a hydrogen atmosphere. Nickel containing silver, and tungsten powders were mechanically blended into 300 g batches to form the W 35 wt-%ag (referred to hereafter as W 35Ag) compos- ition. Mechanical milling (MM) was carried out on powder batches using a Union Process 01 HD attritor for 150 min. A shaft speed of 500 rev min 1 was used and the outer jacket of the attritor was chilled by running cold water through cooling coils during milling. The grinding medium consisted of 6 35 mm diameter WC balls with a ball/powder ISSN

2 78 Aslanoǧlu et al. Effects of Ni addition on properties of Ag W electrical contact materials 2 Representative SEM micrograph of W 35Ag powders doped 0 25 wt-%ni 1 Schematic diagram of Ni doped Ag powder production process ratio (BPR) of 5 : 1. All milling investigations were per- formed in air using a process control (PCA) agent. Consolidation of the as blended and the as milled powders was carried out in a laboratory scale uniaxial press at 300 MPa. Following consolidation, the green compacts were sintered in a tube furnace at 870 C for 2 h under a hydrogen atmosphere. After sintering, the compacts were furnace cooled to room temperature again under the hydrogen atmosphere. After the re-pressing step of the sintered compacts at the 600 MPa, re-sintering was performed at 850 C for 1 h, using the same conditions for all RESULTS compacts. Mixed powders X-ray diffraction investigations were carried out on the Figure 2 is a representative SEM micrograph taken from milled powders and the sintered compacts using Cu K the milled W 35Ag powders containing 0 25 wt-%ni. As a radiation at 50 kv/30 ma in a Philips PW 1730 X-ray seen in Fig. 2, a large range of powder particle sizes diffractometer. Morphological and microstructural characterisation of the as blended and milled powders and during mechanical milling. EDS analysis on powders with (2 13 mm) and a variety of powder shapes are produced sintered compacts were performed by using a CAMSCAN different characteristic shapes revealed that the polygonal 2 A scanning electron microscope (SEM) operated at 20 kv. aggregates with sharp edges are W rich particles (region 1), Density measurements of the green and sintered compacts whereas irregular flaky aggregates with light contrast were carried out using the Archimedes principles by the (region 2) comprised both W and Ag. The latter are use of a Metler Toledo density measurement kit. Ten composite in nature and are made up of Ag flattened specimens from each compact were used for density measurement and the mean value of the all measurements was taken as the accepted value. Hardness measurements of the sintered compacts were performed using a Brinell test procedure in a Wolpert test machine. Cylindrical shaped contacts, 7 mm in diameter and 2 mm in height, were prepared from the sintered compacts which were torch brazed to brass studs for electrical testing. The contact surface was polished with a 1200 grit silicon carbide paper to a uniform surface finish, followed by ultrasonic cleaning in alcohol before testing. The electrical arc erosion test was carried out on the sintered contacts using a computer controlled switching machine for up to make and break operations with a resistance load at 15 A alternative current (ac) and 220 V. A motor cam drive energised a cylinder to operate the switches sequentially at a total cycle time 4 6 s. The contacts were also removed from the switch every 2000 cycles, cleaned ultrasonically in alcohol, dried, and measured for weight loss. They were then reassembled with the same alignment as before and the test continued. At the end of make and break operations, the eroded surface of the contacts were examined via scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques. A math- ematical approach which utilises the contact erosion curve was used to determine the contact service life. The standard contact dimensions were taken as mm. It has been accepted that only 25% volume erosion can be tolerated in the contact service life for safety. Table 1 Characteristics of starting powders Production Powder Mean particle Purity, Manufacturer method shape size, mm % Silver In house laboratory Chemical Round >2 99 scale production precipitation Tungsten Hermann C. Starck Reduction Polygonal 2 10 >99

3 Aslanoǧlu et al. Effects of Ni addition on properties of Ag W electrical contact materials 79 4 Representative SEM micrograph of sintered W 35Ag compacts doped with 0 25 wt-%ni sintered and re-sintered densities and these values increased with increased Ni contents, as expected. The highest re-sintered density was obtained for the compact having 1 wt-%ni, which exhibited about 5% porosity. The hard- 3 X-ray diffraction pattern of W 35Ag powders ness values also increased with increasing Ni contents, thus, the compacts containing 1 wt-%ni have the highest bulk hardness values of 112 HB whereas those with no Ni particle remnants which coated on the fragmented content have bulk hardness values of 50 HB. W particles during the milling process. Similarly, W and Figure 4 is a representative SEM micrograph of the Ag contents were found on the other light flaky aggregates W 35Ag compact containing 0 25 wt-%ni. As seen in indicating that mechanical milling of elemental tungsten Fig. 4, microstructural homogeneity was obtained. Silver and silver powders resulted in the formation of homoagglomeration did not occur during the sintering. There is geneously mixed composite powders. no obvious effect of nickel addition on the homogeneity of Figure 3 shows the X-ray diffraction pattern of the milled the microstructure of compacts. W 35Ag powders containing 0 25 wt-%ni and of the Erosion weight loss of sintered compacts in the same compact comprising 0 25 wt-%ni prepared from the milled switching test is shown in Fig. 5. In the beginning of the powders sintered in the tube furnace under a hydrogen switching test, an intensive arc occurred at the surface of atmosphere. The characteristic diffraction peaks belonging compact and caused excess erosion of the compact. This to W and Ag can be clearly seen both in the as milled can be attributed to the finding of Rieder et al.14 who powder and the sintered compact. It is evident from Fig. 3 reported that the initial arc progresses slowly as a result of that no oxide formation occurred during milling and the intensive erosion and evaporation from the surface of the WO amount in the sintered compact is quite negligible. 2 metals. The later arcs rapidly progressed using the same arc path and caused a low erosion rate, therefore, the Sintered compacts erosion rate slowly decreased with the increasing make Table 2 gives the fractional densities of the MM W 35Ag break operations. There was a higher erosion rate of the powders doped with different Ni contents following uniaxial nickel undoped sintered compacts than the other nickel pressing, sintering, re-pressing, re-sintering and furnace doped compacts. Erosion rates of contact materials cooling, and bulk hardness values of those in the re-sintered decreased with increase in nickel content. Weight loss of condition. The compact without Ni has the lowest sintered density values both in the sintered and re-sintered states. On the other hand, Ni doped compacts exhibited higher contacts after make break operations decreased rapidly from 74 5 to mg cm 2 for the compact containing 0 75 wt-%ni. Table 2 Processing conditions, density, and hardness of nickel doped W-35Ag compacts Relative density,* % Bulk Processing hardness Nickel content, Re-press and (HB), % Pressing Sintering Re-pressing Re-sintering Green Sinter re-sinter Porosity kg mm 2 0 (as-blended) Uniaxial, 870 C/2 h in Uniaxial pressing 850 C/2 h in ± Uniaxial, 870 C/2 h in Uniaxial pressing 850 C/2 h in ± Uniaxial, 870 C/2 h in Uniaxial pressing 850 C/2 h in ± Uniaxial, 870 C/2 h in Uniaxial pressing 850 C/2 h in ±33 1 Uniaxial, 870 C/2 h in Uniaxial pressing 850 C/2 h in ±01 * Density values listed are the mean values for 10 specimens of each compact measured using the Archimedes principles. Hardness values listed are based upon Brinell measurements from the more than 15 regions of each compact.

4 80 Aslanoǧlu et al. Effects of Ni addition on properties of Ag W electrical contact materials a 5 Weight loss versus nickel contents during arc erosion testing at 15 A and 220 V (ac) Figure 6a c shows a series of SEM micrographs depicting the eroded contact surfaces taken from the undoped W 35Ag contact, the W 35Ag doped with 0 5 wt-%ni and the W 35Ag contact doped with 1 wt-%ni, respectively. Switching of electrical current resulted in radical changes in the contact surfaces. All surfaces were roughened and had silver droplets, tungsten cones, pores, craters, and re-solidified W Ag agglomerates. After erosion testing of nickel undoped compacts, the eroded surfaces had cracks crossing the whole surface and blue and brown tungsten oxides were observed, because of the oxidising effect of the arc heat (Fig. 6a). Nickel doped sintered compacts were observed to have pores, re-solidified W Ag, and a porous tungsten skeleton. Nickel doped contacts had microcracks on the re-solidified surfaces owing to thermal shock of contacts (Fig. 6b). On the contrary, the 0 5 wt-%ni doped compact had weak bonded and needlelike tungsten oxide phases because of the oxidation of the eroded surface (Fig. 6c). The cross-section microexamination of the eroded zone is shown in Fig. 7. A microsection of contact surface after operations showed that no cracks occurred at the cross-section of the contact. The arc affected zone was mm thick and homogeneous erosion occurred. The total arc affected layer was more uniform in thickness. A calculation of the service life of the contacts is given in Table 3. The service life of the contacts were increased with the increasing nickel doping. However, the nickel undoped compact with make break operations and the 1 wt-%ni doped compact showed approximately a W 35Ag; b W 35Ag 0 25Ni; c W 35Ag 0 5Ni 6 Eroded surface of W 35Ag contacts b c Table 3 Estimated life cycle of Ag W contacts versus nickel concentrations Nickel concentration, wt-% Service life, Micrograph of cross-section of W 35Ag 1Ni electrical contact make break operations in safety, however, commercial contacts produced from Federal Electric Corporation show approximately make break operations.

5 Aslanoǧlu et al. Effects of Ni addition on properties of Ag W electrical contact materials 81 DISCUSSION contact service life. On the contrary, low sintered density In this investigation, nickel was used as a sintering aid and hardness, and high microstructural homogeneity (activator) to enhance sintering and to improve the erosion increased the contact erosion rate and service life. properties of the silver tungsten compacts. Rapid increases in the sinter densities were achieved with nickel additions. CONCLUSIONS In addition, the hardness of the W 35Ag compacts were improved with nickel addition. Effects of nickel concenof nickel addition on the W 35Ag contacts. Based on this In this study, an attempt was made to understand the effect tration and the doping methods in W Ag contacts materials have been studied by several researchers. Samsanov et al. results, the following conclusions can be drawn. and Lee15 reported an effective Ni concentration between 1. Nickel doping can be processed by the wt-%ni. However, Walkden16 showed that the co-precipitation method. addition of wt-%ni was more effective than small 2. Contact erosion, in a low current switching test, Ni concentrations. In this study, even a small amount of showed increasing erosion rates with nickel addition wt-%ni had an important effect on the sintered density 3. The mechanical and morphophysical properties of the and arc erosion rate. It is believed that the chemical contacts are dependent on nickel concentration and co-precipitation of nickel and silver had an additional effect distribution in matrix. on the activated sintering of the doped W 35Ag compacts 4. W 35Ag doped with 0 75 wt-%ni has the highest owing to rapid transportation of nickel to tungsten powder erosion resistance. surfaces as a result of small particle sizes and homogeneous mixing of Ni.11,17 ACKNOWLEDGEMENTS Several factors may influence the erosion behaviour The authors would like to acknowledge the financial of the electrical contacts. Holm18 reported a relationship support of this investigation by the DPT (Government between the hardness and the contact arc erosion behaviour, Planning Institute) and wish to thank Dr F. Findik and as increasing the hardness value of the contacts increased G. Düzenli for their help and contribution during the the erosion resistance. This observation is also valid for course of this investigation. this investigation in that compacts with high sintered density and high hardness showed higher erosion resistance. Contrary to the undoped contact, nickel addition increased REFERENCES the switching performance of contacts. As reported by 1. P. 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