Fabrication and arc erosion behaviors of AgTiB2 contact materials

Powder Technology 256 (2014) 20–24

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Fabrication and arc erosion behaviors of AgTiB2 contact materials Xianhui Wang ⁎, Hao Yang, Mei Chen, Juntao Zou, Shuhua Liang School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, PR China

a r t i c l e

i n f o

Article history: Received 16 August 2013 Received in revised form 19 January 2014 Accepted 24 January 2014 Available online 4 February 2014 Keywords: Reactive ball milling TiB2 particle size Electrical contact material Arc erosion

a b s t r a c t To investigate the effect of TiB2 particle size on the arc erosion behaviors of Ag–4wt.% TiB2 contact materials, ultrafine TiB2 powders were prepared by reactive ball milling with the mixture of Mg, B2O3 and TiO2 powders followed by acid wash. The phase constituents, size and morphology of the powders prepared were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Two kinds of Ag–4wt.% TiB2 contact materials were subsequently consolidated by powder metallurgy using ultrafine TiB2 powders and coarse TiB2 powders. The microstructure of Ag–4wt.% TiB2 contact materials was characterized, and the relative density, hardness and electrical conductivity of AgTiB2 contact materials were determined utilizing Archimedes method, a hardness analyzer and an eddy electrical conductivity gage, respectively. The arc erosion of Ag–4wt.% TiB2 contact materials was tested, the surface morphologies of Ag– 4wt.% TiB2 contact materials after arc erosion were characterized by scanning electron microscopy, the spatial distribution of TiB2 particles in Ag matrix was evaluated, and the arc duration and mass loss before and after arc erosion were determined as well. The results show that the relative density, electrical conductivity and hardness of Ag–4wt.% TiB2 contact materials consolidated using ultrafine TiB2 powders are 85.45%, 55.17% International Annealed Copper Standard and 49.20 Brinell Hardness, respectively. Compared with the Ag–4wt.% TiB2 contact material consolidated using coarse TiB2 powders, the Ag–4wt.% TiB2 contact material prepared by ultrafine TiB2 powders presents shallower arc erosion pits, larger erosion area, more uniform dispersion and shorter arc duration. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The electrical contact materials are widely used in the different low voltage switch devices, such as relays, contactors, circuit breakers and switches, and their properties are of importance to the switching capacity, reliability, stability and service life of integral electrical systems [1,2]. Though conventional AgCdO materials have good properties, especially for excellent arc extinction, their applications have been progressively restricted because they are harmful to the environment and human health [3,4]. Hence, it is a challenge for material researchers to develop a new material instead of AgCdO contact materials. So far, many silverbased contact materials, such as AgMeO, AgNi, AgC and AgWC have been developed. Among of those novel materials, SnO2 reinforced silver matrix composites have excellent electrical properties which is comparable to the AgCdO contact materials. However, during the long-term service, it is found that AgSnO2 contact materials present larger contact resistance and higher temperature rise [5], which give rise to significantly negative effects on the stability and reliability of electric system. Subsequently, it is of significance to develop a novel Ag base contact materials. The titanium diboride exhibits high melting point (2900 °C), high hardness (30 GPa), and good thermal conductivity (25 J/m.s.K). The most important is that it has the lowest resistivity among superhard ⁎ Corresponding author. Tel.: +86 29 82312185; fax: +86 29 82312181. E-mail address: [email protected] (X. Wang). 0032-5910/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2014.01.068

ceramic materials (9 μΩ cm2). It is a promising ceramic reinforced phase for Ag matrix contact materials to minimize the arc erosion and contact resistance and improve the stability and reliability of electric system [6]. Ciuman-Krzemien et al. [7] fabricated Ag–TiB2 composite by powder metallurgy, in which Ag powders were obtained by cathodic reduction of Ag+ ions from AgCl in a Zn|0.5 M H2SO4, AgCl|Ag cell. The present authors of the investigation [8–12] have prepared the AgTiB2 composites by high energy milling and powder metallurgy. Up to now, some literatures reported show that the fine particle can enhance the arc erosion resistance of electrical contact materials. Gan and Li [13] and Wang et al. [14] believed that the fine SnO2 particle is beneficial for the improvement of electrical properties, especially for the arc erosion resistance of AgSnO2 contact materials. Zhao et al. [15] reported that the erosion spots on the surface of nanocrystalline CuCr materials are more uniform and dispersive compared to that of microcrystalline CuCr. Yang et al. [16] found that the arc behavior of microcrystal CuCr materials is quite different from that of amorphous and nanometer materials, and the arc motion changes from random hopping to continuous movement. Wang et al. [17–19] thought that arc formation, distribution, motion, size and shape of cathode spots and arc erosion of Cu/Al2O3 composites are closely related to the Al2O3 particle size, content, spatial distribution and interface characteristic. As the aforementioned, the grain size has a large effect on the arc erosion of electrical contact materials, but no literature has reported on the arc erosion behavior of the AgTiB2 composite so far. Hence, it is important to understand the arc

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erosion behavior for the AgTiB2 composite. In the investigation, ultrafine TiB2 powders were firstly prepared by reactive ball milling [20], and two kinds of Ag–4wt.% TiB2 contact materials were subsequently fabricated by powder metallurgy using ultrafine TiB2 powders and commercially coarse TiB2 powders. The effect of TiB2 particle size and spatial distribution of TiB2 particles on the microstructure and properties of Ag–4wt.% TiB2 contact materials were comparatively investigated. The purpose is to clarify the relationship between the TiB2 particle size and the arc erosion of Ag–TiB2 contact materials, and the research can provide a basis for the design of metal base contact materials and the manipulation of arc control. 2. Experimental The boron oxide (purity ≥ 99.5%), titanium dioxide (purity ≥ 99.0%), magnesium (purity ≥ 99.5%) were used as raw materials to prepare ultrafine TiB2 powders. The mixture of boron oxide, titanium dioxide and magnesium at the molar ratio of 1:1:5 were milled for 45 min in a KQM-YB/B planetary ball mill containing stainless steel balls of 8–13 mm diameter under the protection of argon gas at the ball-to-powder mass ratio of 40:1 and the rotational speed of 350–400 rpm. The chemical reaction during ball milling can be expressed as follows: TiO2 þ B2 O3 þ 5Mg ¼ TiB2 þ 5MgO:

ð1Þ

Fig. 2. XRD pattern of the mixed powders milled at 400 rpm for 45 min.

conductivity gage and the hardness was tested on a HB-3000 hardness analyzer under the load of 500 kg, holding for 30 s, and the mean values were the average of five measured results. The spatial distribution of TiB2 particles in Ag matrix was evaluated by the enumeration method proposed by Xie et al. [21]. This method can be described as follows. First a given area is photographed and copied, then the area can be divided into N micro-areas, the number of particles in each micro-area (Zi) and the average number of particles in unit micro-area (Z) can be obtained. Hence, the relative standard deviation of particles in the micro-area (Srel′) can be expressed as the Eq. (2):

MgO was removed by acid wash in a 3 M HCl for 12 h at room temperature, followed by the dilution with distilled water. The residual powders were characterized by a XRD-7000 X-ray diffractometer, a JSM-6700F scanning electron microscopy and a JEM-3010 transmission electron microscopy. The powders of Ag and TiB2 powders were mixed in a custom made vibrating mill containing agate balls of 8–13 mm diameter under argon gas for 12 h at a ball-to-powder mass ratio of 40:1 and a rotational speed of 150 rpm. During mixing, the absolute ethyl alcohol was adopted as dispersant. The mixed powders were subsequently pressed in a closed die on a WE-600 hydraulic press under a pressure of 200 MPa for 40 s to obtain a compact with a cylindrical shape, 15 mm in diameter and 7 mm in thickness, followed by sintering in a XP-80B hot press furnace at 700 °C for 2 h under nitrogen gas. Similarly, the same procedures were also used to prepare Ag–4wt.% TiB2 contact material by the commercial TiB2 powders (5 μm) for the comparative study. The microstructure of Ag–4wt.% TiB2 contact materials was characterized by a JSM6700F SEM. The density was measured utilizing Archimedes method, the electrical conductivity test was performed on a 7501 eddy electrical

The number of reinforced particles can be accomplished in an automatic image analyzer with Image-pro plus 6.0 software, and the calculated Srel′ can be used to characterize the distribution of reinforced particles in different micro-areas. The arc erosion of Ag–4wt.% TiB2 contact materials was tested on a modified TDR240A single crystal furnace. After polishing, the sample as a cathode was put in a Cu platform, which can move vertically in the vacuum chamber. Above the cathode there was a pure tungsten rod with a radius of 5 mm and a tip radius of 1 mm as the anode.

Fig. 1. XRD pattern of the mixed powders milled at 350 rpm for 45 min.

Fig. 3. XRD pattern of the powder product derived from leaching.

′

Srel ¼

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N  u 1 X 2 u Z i −Z uðN−1Þ t i¼1 Z

 100%:

ð2Þ

22

X. Wang et al. / Powder Technology 256 (2014) 20–24 Table 1 The number of TiB2 particles in each micro-area of AgTiB2 composites consolidated using coarse and ultrafine TiB2 powders. Number

01

02

03

04

05

06

07

08

09

Srel′

5 μm 300 nm

91 175

117 202

129 183

95 189

114 191

123 181

95 184

111 183

115 208

12.53 9.93

dilution with distilled water. The reaction process can be given as follows: MgO þ 2HCl ¼ MgCl2 þ H2 O:

Fig. 4. TEM image of the powder product derived from leaching.

When the chamber was evacuated to 5.0 × 10−3 Pa and the capacitor of 120 μF was charged at the voltage of 4 kV, the lower cathode moved upward at a velocity of 0.2 mm/min until the gap was broken down. The arc duration can be collected from the discharged waveform recorded by a Tektronix TDS-2014 dual channel digital memory oscilloscope (200 MHz). The operation was repeated 30 times. Considering no full discharge of the capacitor, the residual charge was artificially released to zero, and then the capacitor was charged at the voltage of 4 kV each time. Hence, each arc erosion test can be carried at the same conditions. The mass loss of Ag–4wt.% TiB2 contact materials before and after arc erosion was determined by a TG328A photoelectric analytical balance. The morphologies of Ag–4wt.% TiB2 contact materials after vacuum arc erosion were characterized by a JSM-6700F SEM. 3. Results and discussion 3.1. Preparation of ultrafine TiB2 powders Fig. 1 is the XRD pattern of the mixture of TiO2, Mg and B2O3 powders milled at 350 rpm for 45 min. It is evident that the phase constituents are still consisted of TiO2, Mg and B2O3, suggesting that no reaction occurs during milling process. Fig. 2 is the XRD pattern of TiO2, Mg and B2O3 mixed powders milled at 400 rpm for 45 min. As seen from Fig. 2, MgO and TiB2 phase form in the as-milled powders, indicating that the large rotational speed can promote the reaction of TiO2, Mg and B2O3 mixed powders. As TiB2 has good chemical stability, indissolubility in water and no reaction with acid, the as-milled powders were washed in a 3 M HCl acid solution for 12 h at room temperature, followed by the repeated

ð3Þ

Fig. 3 is the XRD pattern of powder product derived from leaching. It is obvious that that only a single TiB2 phase presents after leaching. The powder product derived from leaching was characterized by TEM, as shown in Fig. 4. It can be seen that the size of TiB2 particles prepared is in the range of 100–500 nm, and the average size is 300 nm. 3.2. Microstructure of Ag–4wt.% TiB2 contact materials The SEM micrographs of Ag–4wt.% TiB2 contact materials consolidated using coarse and ultrafine TiB2 powders are shown in Fig. 5a and b, respectively. The black regions are the TiB2 phase, while the gray regions are the Ag matrix. It is obvious from Fig. 5a that the serious agglomeration of TiB2 particles and numbers of non-uniform pores present in the Ag–4wt.% TiB2 contact material consolidated using coarse TiB2 powders. However, as seen from Fig. 5b, TiB2 particles are well dispersed in the Ag matrix, and the Ag–4wt.% TiB2 contact material consolidated using ultrafine TiB2 powders has much better densification than that consolidated using coarse TiB2 powders. 3.3. Spatial distribution of TiB2 particles The distribution of TiB2 particles in the Ag–4wt.% TiB2 contact material consolidated using ultrafine TiB2 powders was quantitatively evaluated by the enumeration method [21]. The number of TiB2 particles in each micro-area is listed in Table 1. According to Eq. (2), then Srel′ = 9.93. Similarly, another Srel′ can also be calculated utilizing the above method, and the results are also given in Table 1. It suggests that the ultrafine TiB2 particles have better spatial distribution in the Ag–4wt.% TiB2 contact material. This is in good accordance with the SEM results. 3.4. Properties of Ag–4wt.% TiB2 contact materials The relative density, hardness and electrical conductivity of Ag– 4wt.% TiB2 contact materials consolidated using coarse and ultrafine TiB2 powders are given in Table 2. As seen from Table 2, the relative density of the Ag–4wt.% TiB2 contact material consolidated using

Fig. 5. SEM photographs of Ag–4wt.% TiB2 contact materials consolidated using coarse (a) and ultrafine (b) TiB2 powders.

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Table 2 Relative density, hardness and electrical conductivity of AgTiB2 contact materials consolidated using coarse and ultrafine TiB2 powders. TiB2 particle size

Relative density (%)

Hardness (HB)

Electrical conductivity (% IACS)

5 μm 300 nm

72.98 85.45

76.30 49.20

31.78 55.17

ultrafine TiB2 powders is 85.45%, which is increased by 17.09% compared to that of the Ag–4wt.% TiB2 contact material consolidated using coarse TiB2 powders. However, it is interesting to notice that the hardness number is decreased by 35.44% compared to that of Ag–4wt.% TiB2 contact material consolidated using coarse TiB2 powders. It is possible due to the presence of the softening phenomenon arising from the Coble creep if the grain size is below a certain value [22]. Although several references [23,24] support the evidence of the phenomenon, more work is necessary to get a further understanding. Compared to the Ag–4wt.% TiB2 contact material consolidated using coarse TiB2 powders, the electrical conductivity of Ag–4wt.% TiB2 contact material consolidated using ultrafine TiB2 powders is increased by 73.60%. This can be ascribed to the contribution of fine TiB2 particles to the improvement on the densification for the Ag–4wt.% TiB2 contact material. Based on the electron conductivity theory, less porosity decreases electron scattering, thus improving the electrical conductivity.

3.5. Effect of TiB2 particle size on the arc erosion resistance for Ag–4wt.% TiB2 contact materials Fig. 6 is the change of arc duration with operation times for Ag– 4wt.% TiB2 contact materials consolidated using coarse and ultrafine TiB2 powders. For the Ag–4wt.% TiB2 contact material consolidated using coarse TiB2 powders, the arc duration has a large fluctuation at the first 20 times and has a less fluctuation at the last 10 times, and the average arc duration is 27.03 ms with a standard deviation of 3.24. However, for the Ag–4wt.% TiB2 contact material consolidated using ultrafine TiB2 powders, the arc duration has a less fluctuation, and the average arc duration is 18.31 ms with a standard deviation of 1.14. This indicates that ultrafine TiB2 particle is favorable for the improvement on the arc erosion resistance. Table 3 lists the mass losses of Ag–4wt.% TiB2 contact materials before and after arc erosion. It is obvious that the Ag–4wt.% TiB2 contact material consolidated using coarse TiB2 powders has a more mass loss than that consolidated using ultrafine TiB2 powders. The result is in a good agreement with short arc duration. This can be explained as

follows. Wang et al. [25] suggested that the phase with a low electron work function is the dielectric weak phase, which can emit limited electrons. For Ag/TiB2 composites, the electron work functions of Ag and TiB2 are 4.70 eV and 5.08 eV, respectively [26]. In addition, Ag has the characteristics of good electrical conductivity and poor thermal stability, the arc is more easily focused on the area surrounding TiB2 particles, and results in higher thermal energy in the area surrounding TiB2 particles. Hence, the arc is easy to concentrate on the area surrounding TiB2 particles. With increase of operation times, the surface of the Ag–4wt.% TiB2 contact material consolidated using coarse TiB2 powders becomes obscure and uneven, which further enhance the ion bombardment and splash of metal drops, thus resulting in more mass loss, When the fine TiB2 particles are uniformly distributed in the Ag/TiB2 composite, the spacing between the preferred erosion areas is decreased. Subsequently, arc will not be concentrated in a fixed site and will generate in the electrode material simultaneously, and, thus, the fine and well dispersed TiB2 particles in the Ag matrix improve the molten viscosity during arcing, and, thus, decrease the splash of molten Ag. Fig. 7a and b are the surface erosion morphologies of Ag–4wt.% TiB2 contact materials consolidated using coarse and ultrafine TiB2 powders after operation 30 times, respectively, and Fig. 7a1 and b1 are the central erosion morphologies of the corresponding samples at higher magnification. As seen from Fig. 7a, large and deep erosion pits form on the contact surface and arc erosion concentrates on a small region. It can be seen from Fig. 7a1 that a number of holes present in the central zone. This is because coarse TiB2 particles are migrated from the primary position by splashing of molten Ag, which result in the formation of small pores, and further promote the formation of deep cavities after operation 30 times. However, for Ag–4wt.% TiB2 contact material consolidated using ultrafine TiB2 powders, the erosion pits become shallower, and the erosion pits are well-dispersed as well. Furthermore, the boundaries between the erosion pits become obscure, see Fig. 7b. In addition, no obvious splash and solidifications of molten silver occur, as shown in Fig. 7b1. Based on the above results, it is believed that the TiB2 particle size and their spatial distributions have remarkable effects on the arc erosion resistance. Fine and well-dispersed TiB2 particles are favorable for the improvement in arc erosion resistance and the decrease in arc erosion rate, while the coarse TiB2 particles easily cause serious concentrated erosion, and, thus, bring about serious Ag splash and more mass loss. 4. Conclusions (1) The relative density, electrical conductivity and hardness number of Ag–4wt.% TiB2 contact material consolidated using ultrafine TiB2 powders are 85.45%, 55.17% IACS and 49.20 HB, respectively. Compared with the Ag–4wt.% TiB2 contact material consolidated using coarse TiB2 powders, the relative density and electrical Table 3 Mass losses of Ag–4wt.% TiB2 contact materials consolidated using coarse and ultrafine TiB2 powders after operation 30 times.

Fig. 6. The change of arc duration with operation times for Ag–4wt.% TiB2 contact materials consolidated using coarse (a) and ultrafine (b) TiB2 powders.

TiB2 particle size

Mbefore (g)

Mafter (g)

ΔM (mg)

Mass loss rate (%)

5 μm 5 μm 5 μm 300 nm 300 nm 300 nm

10.3054 10.2018 10.2746 9.4903 9.5212 9.5187

10.3015 10.1983 10.2709 9.4895 9.5203 9.5179

−3.9 −3.5 −3.7 −0.8 −0.9 −0.8

0.038 0.034 0.036 0.008 0.009 0.008

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X. Wang et al. / Powder Technology 256 (2014) 20–24

Fig. 7. Surface erosion morphologies of Ag–4wt.% TiB2 contact materials consolidated using coarse (a) and ultrafine (b) TiB2 powders, and a1–b1 are the morphologies of central erosion regions of a–b at high magnification.

conductivity are increased by 17.09% and 73.60%, respectively, while the hardness number is decreased by 35.52%. (2) Ultrafine TiB2 particle is more favorable for shorter arc duration and less arc fluctuation. The average arc duration of the Ag– 4wt.% TiB2 contact material consolidated using ultrafine TiB2 powders is 18.31 ms. (3) Fine and well-dispersed TiB2 particles are favorable for the decreased arc erosion rate and the improved arc erosion resistance.

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