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Effect of Ni addition on the arc-erosion behavior of AgTiB2 contact material
Vacuum 161 (2019) 361–370
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Effect of Ni addition on the arc-erosion behavior of AgTiB2 contact material a,b
Hangyu Li a b c
, Xianhui Wang
a,b,∗
, Yong Xi
a,b
, Ting Zhu
a,b
T
c
, Xiuhua Guo
Shaanxi Key Laboratory of Electrical Materials and Infiltration Technology, Xi'an University of Technology, Xi'an, 710048, PR China School of Materials Science and Engineering, Xi'an University of Technology, Xi'an, 710048, PR China School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang, 471003, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ag-based contact material Arc erosion Ni addition Ionization energy
To disclose the electrical characteristics of AgTiB2 contact materials with different Ni additions, the arc-erosion tests were performed 20,000 times at 24 V/16 A DC. The morphology and chemical composition of arc eroded contact surface were characterized by scanning electron microscopy and energy dispersive X-ray spectrometry, the mass change after arc-erosion tests was determined, the arc energy and arc duration at make-arc and breakarc were analyzed, and the arc erosion mechanism was discussed as well. The results show that arc erosion mode changes from anodic erosion to cathodic erosion with Ni addition. The Ag-4 wt.%TiB2 contact material with 2 wt %Ni addition has the optimum performance along with the least mass loss, while especially high arc energy and total mass loss generate at 4 wt% and 8 wt%Ni. Furthermore, the arc erosion mechanism was discussed based on the contact characterization, thermal property and ionization energy of materials. It is believed that excessive Ni addition is not beneficial for the improvement of arc erosion resistance since Ni has large potential to be ionized and the possibility to evaporate due to its little difference of enthalpy change compared with ionization energy.
1. Introduction Due to the excellent electrical conductivity, welding resistance and wear resistance, Ag-based contact materials are widely used in the low and medium voltage electrical systems [1–3]. During service, arc erosion occurs at make and break operations, which not only deteriorates the contact interface, but also results in the material transfer between anode and cathode under the DC load condition. The material transfer can further cause the morphology change or the inhomogeneous mass loss of anode and cathode. If serious, it reduces the reliability and stability of the electrical systems [4]. Currently, several literature have been reported on the effect of processing parameter, microstructure and the chemical and thermodynamic properties of the second phase on the arc erosion behavior for Ag-based contact materials. Rieder et al. [5] believed that the porous eroded surface and loosely adhering splashed particles result in higher mass loss due to the mechanical abrasion. Verma et al. [6] thought that the oxide with moderate stability is more favorable for the improvement on the electrical performance in comparison with extremely stable oxide for the Ag/MeO contact material. Wang et al. [7] thought that fine Al2O3 particles have a lower float velocity than coarse particles in molten pool, thus decreasing the aggregation of strengthening particles at the erosion surface. Biyik et al. [8,9] thought that the electrical
performance of contact materials depends on current load. At present, a lot of investigations reported that the effect of the minor additives on the electrical performances of Ag-based materials (CuO [10], La [11], La3O2 [12], La2Sn2O7 [13], WO3 [14]). Li et al. [10] found that CuO addition results in shallower erosion pits, larger erosion area, and better arc erosion resistance. Wang et al. [11] reported that La doping promotes arc motion and dispersion. Wang et al. [12] found that La3O2 addition significantly improves the wettability between Ag and SnO2. Zhang et al. [13] reported that La2Sn2O7 addition alleviates the damage of eroded surface structure. Furthermore, the material transfer caused by arc erosion is especially complex. So far, a few literature have been reported on the material transfer mechanism. Chen et al. [14] believed that material transfer depends on what kind of ion is dominant to sustain the arc combustion. The metallic ions trigger the material transfer from anode to cathode, whereas the gaseous ions cause the material transfer from cathode to anode. Sone et al. [15] proposed that the material transfer, mass loss and contact electrical resistance are strongly affected by the metallic ion arc. Jemaa et al. [16] thought that material transfer is closely correlated with arc length. At short arc, the material transfer is from anode to cathode. On the contrary, it presents a different mode at long arc. Doublet et al. [17] found that the material transfer mode reverses with increasing current, and the transfer direction is determined
∗
Corresponding author. School of Materials Science and Engineering, Xi'an University of Technology, 5 South Jinhua Road, Xi'an, 710048, PR China. E-mail addresses: [email protected] (H. Li), [email protected] (X. Wang), [email protected] (Y. Xi), [email protected] (T. Zhu), [email protected] (X. Guo). https://doi.org/10.1016/j.vacuum.2019.01.003 Received 12 October 2018; Received in revised form 14 November 2018; Accepted 4 January 2019 Available online 06 January 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. The morphologies of raw powder and milled powder. (a) Ag powder (b) Ni powder (c) TiB2 powder (d) milled powder.
can improve the wettability between Ag and TiB2 and sinterability of AgeTiB2 contact materials. However, the effect of Ni addition on the arc erosion behavior is still unclear. To clarify the effect of Ni addition on the arc erosion behavior of Ag-based contact materials, the Ag-4 wt. %TiB2 contact materials with different Ni contents were prepared. The arc energy, arc duration, eroded morphology, element distribution and the mass changes before and after arc-erosion test were investigated, and the arc erosion mechanism and material transfer mode were discussed as well. The research can enrich the electrical contact theory and provide the reference for the design and manufacture of Ag-based contact materials.
by the mutually competed result of anodic and cathodic arc. Chen et al. [18] confirmed that high voltage results in small critical current transfer from metallic ion arc to gaseous ion arc. Rong et al. [19] reported that the change of material transfer mode depends on the voltage-current characteristics. Morin et al. [20] studied the material transfer of Ag-based contact material under the inductive, resistive and lamp loads, and revealed that the Ag-based contact material has the largest transfer mass at the lamp loads, whereas the least transfer mass occurs at inductive load. Swingler et al. [21] believed that the material transfer of AgCdO and AgSnO2 electrode contacts is derived from cathode evaporation and anode condensation at low current, but liquid droplet splash is the dominant material transfer mechanism at high current, and the AgCdO contact material has a lower erosion rate than the AgSnO2 contact material. Though more understanding can be obtained for the material transfer of Ag-based contact materials from above investigations, the effect of material characteristics on the material transfer behavior is still obscure. Our previous work [22–26] also found that WO3 addition causes the change of material transfer mode for AgTiB2 contact material, and the second phase is beneficial for arc dispersion, resulting in different arc erosion and material transfer behavior for Ag-based contact materials. Furthermore, the influence mechanism of the second phase was also discussed based on the work function and boiling point of material components. However, the arc erosion may be influenced by more physical properties due to the complexity of arc erosion. Hence, it still lacks the deep insights of the underlying arc erosion mechanism so far. Though AgCdO contact material has excellent arc erosion resistance, wear and welding resistance [27], the toxic nature and an increasing environmental awareness limit the application of AgCdO contact material. For the AgSnO2 contact material, the separation of SnO2 particles and Ag particles during long time service causes temperature rise and high values of contact resistance [13]. To tackle this issue, Wang et al. [24] proposed to replace non-conductive SnO2 by conductive TiB2 in the Ag-based contact material. Nevertheless, the poor wettability between TiB2 and Ag influences the sinterability of AgeTiB2 contact materials. Our primary work revealed that Ni addition
2. Experimental The starting materials were Ag powder (purity 99.9 wt%, particle size 72 μm), TiB2 powder (purity 99.9 wt%, particle size 60 nm) and Ni powder (purity 99.9 wt%, particle size 800 nm). As composite contact materials are most frequently prepared using powder metallurgy method, which considers specific chemical composition, and appropriate milling parameters are important for the acquirement of good milling effect and sintering process [29–31], the present authors have conducted on a number of investigations on the fabrication of AgeTiB2 composites with the powder metallurgy method [22–26]. At this work, the process parameters were selected based on our previous work. The AgeTiB2 powder with a mass ratio of 96:4 and AgeNieTiB2 powder with a mass ratio of 94:2:4, 92:4:4 and 88:8:4 were mixed at a planetary ball mill (KQM-YB/B, Xianyang Jinhong Machinery Co. Ltd) at 300 rpm for 4 h and a ball-to-powder ratio of 15:1. The ethyl alcohol and polyvinylpyrrolidone (PVP) (1 wt%) were used as process control agent and dispersant, respectively. The morphologies of raw powder and milled powder are shown in Fig. 1, and the XRD pattern of milled powder is shown in Fig. 2. The mixture was compressed under a pressure of 60 MPa for 30 s to obtain a compact with a diameter of 21 mm and a length of 13 mm, followed by sintering at 750 °C for 2 h at 30 MPa under the protective atmosphere of nitrogen gas in a self-made hotpressing furnace. The relative density was measured utilizing 362
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JF04C arc-erosion test rig. The mass before and after arc-erosion test were determined by an electrical balance (FA1104J, Tianjin Shunnuo Instrument Technology Co. Ltd). The surface morphologies of the arceroded specimens were characterized by a scanning electron microscopy (JSM-6700F, Nippon Electric Company Ltd) and the chemical composition was analyzed on an energy dispersive X-ray spectrometry (Oxford INCA, Oxford Instrument).
3. Results and discussions 3.1. Characterization of Ag-4 wt.%TiB2 contact materials with different Ni additions Fig. 4(a)-(d) are the SEM images of Ag-4 wt.%TiB2 contact materials with different Ni additions. The gray and bright regions are Ag-rich and TiB2/Ni-rich zones, respectively. The results show that Ni and TiB2 are distributed around irregular Ag particles, and there are a few porosities. The irregular Ag particles are derived from deformation during ball milling, pre-compressing and hot pressing. To further determine the distribution of Ni and TiB2, region A marked in Fig. 4(c) is analyzed by elemental mapping, the high magnification image of region A is shown in Fig. 4(e) and the elemental mapping patterns are represented in Fig. 4(f)-(I). This can verify that Ni and TiB2 are distributed along Ag particle boundary. Fig. 5 is the XRD patterns of sintered Ag-4 wt.%TiB2 contact materials with different Ni additions. For the Ag-4 wt.%TiB2 contact material without Ni addition, strong Ag peaks appears along with weak TiB2 peaks. However, for the Ag-4 wt.%TiB2 contact materials with the additions of 2 wt%Ni and 4 wt% Ni, Ni3B peaks appear without the presence of Ni peaks. It indicates that Ni react with TiB2 during sintering process. Furthermore, more Ni3B peaks occur along with weak Ni peaks at 8 wt% Ni. Saqib et al. [32] and Finch et al. [33] reported that the reaction between Ni and TiB2 occurs at the grain boundary. This may hinder Ni and TiB2 to be detected in this work. Furthermore, Ni3B becomes unstable above 1429 K [32], which is much lower than the boiling point of Ag (2435 K). In addition, it is learnt from the XRD analysis that there is very small amount of Ni3B reaction product. Therefore, it is inferred that Ni3B has no apparent influence on arc erosion. Fig. 6 is the TEM image and SAED pattern of the specimen with 4 wt %Ni addition. As learnt from the indexed result, it is verified that the bright particle is Ni phase. During TEM analysis, it cannot get the SAED
Fig. 2. XRD pattern of milled powder.
Archimedean method. The electrical conductivity was performed on an eddy current gauge (Model 7501) and the hardness was tested on a hardness testing equipment (Model HV-120) under the load of 5 kg for 30 s, and the mean values were the average of five measured results. The specimen used for TEM observation was cut from the studied material and then mechanically polished to obtain 50 μm thick slice. Discs of 3 mm in diameter were punched from the slice and thinned in an ion milling machine (Fischione1010) at 4.5 kV. The microstructure was characterized by a transmission electron microscope (Tecnai F30 G2) with an operation voltage of 200 kV. The anode and cathode with a diameter of 3.8 mm and a length of 10 mm were machined from the as-prepared specimens. The contact tests were made on an arc-erosion test rig (JF04C, Kunming Institute of Precious Metals), as schematically shown in Fig. 3. The anode and cathode were corresponded to the movable contact and stationary contact, respectively. Each contact pairs were performed 20000 times under the resistive load with a DC current of 16 A and voltage of 24 V, the operation frequency of 1 Hz, the contact force of 0.4 N and the electrode gap of 1 mm. During arc-erosion tests, the anode moved downward until it contacted the cathode, and then moved upward. The data of arc duration and arc energy were automatically collected by the
Fig. 3. Schematic of arc-erosion test rig. 363
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Fig. 4. SEM images of the Ag-4 wt.%TiB2 contact materials with different Ni additions and the elemental mapping results of Ag-4 wt.%TiB2 contact material with 4 wt%Ni additions. (a) 0 wt% Ni (b) 2 wt%Ni (c) 4 wt% Ni (d) 8 wt% Ni (e) high magnification image of region A (f) Element distribution (g) Ag element (h) Ni element (I) Ti element.
pattern of TiB2 particles. The most possible reason is that Ni3B and TiB2 particles are shed from the specimen during preparation process due to their high hardness. 3.2. Physical and mechanical properties of the Ag-4 wt.%TiB2 contact materials with different Ni additions Table 1 lists the physical and mechanical properties of the Ag-4 wt. %TiB2 contact materials with different Ni additions. As learnt from Table 1, the relative density has no remarkable change with increasing Ni content. However, the electrical conductivity decreases and hardness increases with increasing Ni content. Since the electrical conductivity of Ni is far below than that of Ag, and more Ni addition increase the number of interface, which enhances the electron scattering, and, consequently, reduces the electrical conductivity. Moreover, the interface can block the dislocation slip, and its effect is further enhanced with increasing Ni content, thus enhancing the hardness of the Ag-4 wt.%TiB2 contact material. 3.3. Analysis of make-arc-energy and make-arc-duration for the Ag-4 wt. %TiB2 contact materials with different Ni additions
Fig. 5. XRD patterns of sintered Ag-4 wt.%TiB2 contact materials with different Ni additions.
The cumulative distribution curves of make-arc-energy and makearc-duration for the Ag-4 wt.%TiB2 contact materials with different Ni 364
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Fig. 6. TEM image of the specimen with 4 wt%Ni addition (a), and the SAED pattern (b).
additions are shown in Fig. 7. Obviously, the make-arc-energy mainly distributes in 0–2000 mJ for the Ag-4 wt.%TiB2 contact materials with 0 wt% and 2 wt%Ni additions. However, for the materials with 4 wt% and 8 wt%Ni additions, there are a number of make-arcs with high energy (2000–8000 mJ), as shown in the green rectangle region in Fig. 7. It suggests that more Ni addition causes the large probability for occurrence of the make-arc with high energy. This can also be reflected by the standard deviation, see the inset data in Fig. 7(a). At 0 wt% and 2 wt%Ni, the standard deviation (D) of make-arc-energy is small. Nevertheless, the standard deviation is extremely large at 4 wt% and 8 wt%Ni, suggesting that make-arc-energy has large fluctuations with more Ni addition. Make-arc-duration has a similar distribution to make-arc-energy. At 0 wt% and 2 wt%Ni, the make-arc-duration is distributed in the range of 0–50 ms. However, at 4 wt% and 8 wt%Ni, make-arc-duration has large fluctuations, and the maximum value can reach up to 150 ms. Fig. 7. The cumulative distribution curve of make-arc-energy (a) and make-arcduration (b) for the Ag-4 wt.%TiB2 contact materials with different Ni additions.
3.4. Analysis of break-arc-energy and break-arc-duration for the Ag-4 wt. %TiB2 contact materials with different Ni additions
subsequently keeps around 400 mJ with less fluctuation at 0 wt%Ni. Nevertheless, its fluctuations decrease obviously at 2 wt%Ni. At 4 wt% Ni, the make-arc-energy during first 8000 operations is less than 200 mJ, however, there are large fluctuations along with an overall increased tendency, see Fig. 9(c). Furthermore, its fluctuations are further aggravated at 8 wt%Ni, suggesting that excessive Ni addition gives rise to the dramatic change of the make-arc-energy, and is not favorable for the improvement on the arc erosion resistance. In addition, the break-arc-energy has little fluctuations at 0 wt%Ni, whereas it presents an overall decreased tendency at 2 wt%Ni. Nevertheless, the break-arc-energy at 4 wt% and 8 wt%Ni has large fluctuations, which is similar to the make-arc-energy, see Fig. 9(c) and (d).
The cumulative distribution curves of break-arc-energy and breakarc-duration for the Ag-4 wt.%TiB2 contact materials with different Ni additions are shown in Fig. 8. The distributions of arc energy and arc duration at break arc are similar to those at make arc. The break-arcenergy is mainly distributed in 0–1000 mJ at 0 wt% and 2 wt%Ni, whereas it has a large fluctuation, and the maximum value can reach up to about 5000 mJ at 4 wt% and 8 wt%Ni. 3.5. Variation of arc energy with operation times for the Ag-4wt.%TiB2 contact materials with different Ni additions To get better understanding of the arc energy, the recorded data of make-arc-energy and break-arc-energy every 500 operations were averaged for the Ag-4wt.%TiB2 contact materials with different Ni additions, and variations of make-arc-energy and break-arc-energy with operation times are presented in Fig. 9. As seen from Fig. 9(a) and (b), the make-arc-energy increases during the first 2000 operations, and
Table 1 Physical and mechanical properties of the Ag-4 wt.%TiB2 contact materials with different Ni additions. Ni content (wt.%)
Relative density (%)
Standard deviation
Electrical conductivity (%IACS)
Standard deviation
Hardness (HV)
Standard deviation
0 2 4 8
96.33 95.55 96.05 96.09
0.20 0.31 0.24 0.34
81.44 71.35 69.66 61.06
0.45 0.26 0.48 0.31
57.15 61.83 68.05 82.57
0.40 1.00 0.48 0.68
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particles are more easily to agglomerate on the cathode surface. For the material with 2 wt%Ni addition, Ni content on the surface of anode and cathode is less than 2 wt%. For the Ag-4 wt.%TiB2 contact material with 4 wt%Ni addition, Ni and TiB2 contents in the eroded region increase obviously, and Ag content decreases remarkably. At 8 wt%Ni, TiB2 and Ni agglomerate seriously on the surface of anode and cathode, and Ag content reduces progressively. This can be explained as follows. Firstly, the molten pool at the anode is especially less than that at the cathode because electron mass is far below that of gaseous and metallic cations. At this case, TiB2 and Ni have less floatation on the anode surface than on the cathode surface. Secondly, Ag evaporation reduces the Ag content on the anode surface since Ag has the lowest boiling point. As larger fluctuation of arc energy generates, the fairly high arc energy causes Ag serious evaporation. Therefore, the degree of Ag evaporation is beyond the floatation effect of TiB2 and Ni, thus resulting in agglomeration on the anode surface. Fig. 8. The cumulative distribution curve of break-arc-energy (a) and breakarc-duration (b) of Ag-4 wt.%TiB2 contact materials with different Ni additions.
3.7. Mass change of the eroded anode and cathode for the Ag-4 wt.%TiB2 contact materials with different Ni additions
3.6. Morphologies and chemical compositions of eroded anode and cathode for the Ag-4 wt.%TiB2 contact materials with different Ni additions
Fig. 11 is the mass change of the eroded anode and cathode after the arc-erosion tests for the Ag-4 wt.%TiB2 contact materials with different Ni additions. For the material with 0 wt%Ni addition, the mass loss of the eroded anode and cathode are 3.88 mg and 2.84 mg, respectively. Obviously, the anode has a higher mass loss than the cathode. However, for the materials with different Ni additions, the mass loss of cathode is lower than that of the anode. It implies that the cathodic erosion is dominant, and Ni addition changes the material transfer behavior for the Ag-4 wt.%TiB2 contact material. Furthermore, it should be noticed that the total mass loss is the lowest for the material with 2 wt%Ni addition. At 8 wt%Ni, the mass loss of the eroded anode and cathode are 6.46 mg and 16.12 mg, respectively, and the total mass loss is 22.58 mg, which is the highest among the four contact pairs. Consequently, it is thought that excessive Ni addition is not beneficial for the improvement on arc erosion resistance. In addition, it is necessary to note that the maximum gap between
Fig. 10 shows the morphologies of eroded anode and cathode for the Ag-4 wt.%TiB2 contact materials with different Ni additions. For the materials with 0 wt% and 2 wt%Ni additions, both cathodes of two contact pairs have the flow and solidified trace, and the erosion pits appear at the anodes, indicating that the erosion mechanism of anode is different from that of cathode. Nevertheless, at 4 wt% and 8 wt%Ni, the erosion area, apparently, is larger than that of the first two contact pairs. Moreover, the erosion pits appear on the cathode and anode surface, suggesting that serious arc erosion happens. Table 2 is the EDS analysis results of the eroded contact surface after 20000 operations for the Ag-4 wt.%TiB2 contact materials with different Ni additions. Due to the detection limitation, Ti element was determined to characterize the content of TiB2. Evidently, TiB2 content at the cathode is larger than that at the anode, indicating that TiB2
Fig. 9. Variation of make-arc-energy and break-arc-energy with operation times for the Ag-4wt.%TiB2 contact materials with different Ni additions. (a) 0 wt%Ni (b) 2 wt%Ni (c) 4 wt%Ni (d) 8 wt%Ni. 366
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Fig. 10. Surface morphologies of eroded anode (a–d) and cathode (a1-d1) for the Ag-4 wt.%TiB2 contact materials with different Ni additions.
anode and cathode is only 1 mm. The splashed Ag droplets have little chance to escape from the electrodes because they are hindered by the erosion edge. This was discussed in our previous work, and more details can refer to the literature [26]. Hence, it is believed that the mass loss mainly occurs through the evaporation of electrode materials. The mass loss can be explained based on the change of arc energy (see section 3.3 and 3.4). Compared with the material with 0 wt%Ni addition, the material with 2 wt%Ni addition has less make-arc-energy fluctuations and lower average break-arc-energy. Thus, it has low total mass loss. However, the make arc and break arc with especially high arc energy are much likely to appear for the materials with 4 wt% and 8 wt %Ni addition, resulting in the fairly high mass loss. As a result, the distribution of arc energy is in accordance with the total mass loss for the Ag-4 wt.%TiB2 contact material. 3.8. Discussions Fig. 11. Mass change of the eroded anode and cathode for the Ag-4 wt.%TiB2 contact materials with different Ni additions.
Based on the contact characteristics, thermodynamics and ionization energy, the underlying arc erosion mechanism for the Ag-4 wt. %TiB2 contact materials with different Ni additions is discussed as follows.
nominal contact area is in proportion to the conductive contact area. As learnt from section 3.2, the hardness of the Ag-4 wt.%TiB2 contact material enhances with increasing Ni content. Therefore, the conductive contact area gradually decreases with more Ni addition. It indicates that the current carrying points of contact are reduced to a few points with increasing hardness once two electrodes contact. At this case, the temperature rises for each site, and molten bridge lasts for a shorter time. As a consequence, the break-arc-energy is decreased. This can be verified by the results shown in section 3.4. Another phenomenon is that the make-arc-energy has an opposite tendency compared with the break-arc-energy, i.e., a high make-arc-
3.8.1. Contact characteristics Due to the effect of the contact force and hardness, the conductive contact area generally only occupies a tiny portion of the nominal contact areas, which can be expressed by Eq. (1) [34].
Fc = ξHAa
(1)
where Fc is the contact force, H represents the hardness, and Aa is the nominal contact area. Distinctly, Aa decreases with increasing H. Furthermore, the
Table 2 EDS results of the eroded anode and cathode for the Ag-4 wt.%TiB2 contact materials with different Ni additions (wt.%). Ni content (wt.%)
0
2
4
8
Elements
Ag Ti Ag Ti Ni Ag Ti Ni Ag Ti Ni
Anode
Cathode
1
2
3
Average
Standard deviation
1
2
3
Average
Standard deviation
97.43 2.57 92.61 5.07 2.32 87.87 5.41 6.72 82.44 4.41 13.15
96.52 3.48 98.26 0.64 1.10 82.61 6.06 11.34 84.28 4.68 11.03
98.88 1.12 95.18 3.23 1.59 90.22 3.19 6.59 79.28 6.47 14.25
97.61 2.39 95.35 2.98 1.67 86.90 4.89 8.22 82.00 5.19 12.81
1.19 1.19 2.83 2.23 0.61 3.90 1.50 2.70 2.53 1.12 1.64
94.23 5.77 92.39 5.78 1.83 79.92 10.52 9.57 74.02 9.36 16.62
95.05 4.95 94.32 5.42 0.26 83.23 2.52 14.24 79.60 10.54 9.85
96.06 3.94 91.45 7.85 0.70 72.86 4.94 22.21 69.49 13.03 17.48
95.11 4.89 92.72 6.35 0.93 78.67 5.99 15.34 74.37 10.98 14.65
0.92 0.92 1.46 1.31 0.81 5.30 4.10 6.39 5.07 1.87 4.18
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Table 3 The thermal properties of Ag and Ni [35]. Element
Work function (eV)
Melting point (K)
Heat of fusion (kJ/mol)
Boiling Point (K)
Heat of vaporization (kJ/mol)
Heat capacity (J/mol·K)
Ag Ni
4.70 5.15
1235 1728
11.28 17.48
2435 3003
254 379
25.35 26.07
and Ni atoms prefer to be ionized, and the dominant ions change from gaseous ion to metallic ion. The same viewpoints can also be supported by the literature [37]. This process is schematically shown in Fig. 12. Though there is a little difference for the first ionization energy between Ag and Ni, it is much larger than the enthalpy change from 293 K to 3003 K. The ionization energy plays a major role in arc consumption, and numbers of the gaseous or metallic molecules are ionized accompanying with generation of freedom electrons. As learnt from Table 4, there are only three ionization energy levels for Ag, but Ni can reach up to the 28th ionization energy level, suggesting that Ni has a larger potential to be ionized, see Fig. 12. Based on this viewpoint, the energy fluctuation happens during arc erosion due to arc complexity. If there is more energy, Ni ions can be progressively ionized, resulting in the generation of large amounts of freedom electrons and high valence Ni ions, which can sustain arc combustion for longer time. Simultaneously, the energy imported to the electrode results in the evaporation of electrode materials, giving rise to more difficulty for arc extinction. This can explain why there are a number of make arcs and break arcs with especially high arc energy for the Ag-4 wt.%TiB2 contact materials with 4 wt% and 8 wt%Ni additions. As seen from Fig. 12, at the stage of gaseous arc, the electrons are accelerated under the action of electrical field, resulting in temperature rise and evaporation of anode materials. Simultaneously, the gaseous cations bombard the cathode, causing the splash. Some splashed droplets adhere to the anode, leading to the material transfer from cathode to anode. At this case, those gaseous cations can obtain the electrons on the cathode surface, and subsequently disappear. At this stage, cathode has the net mass loss. However, at the stage of metallic arc, the metallic cations also move towards the cathode. At this case, it is probably to bring about splash. The metallic cations that capture electrons deposit into the cathode. Hence, for the metallic arc, the material transfer depends on which is dominant for deposition and splash. Our previous work [23,25,26] showed that the material characteristics have prominent effect on the arc characteristics, which further results in different material transfer behavior. However, the effect of ionization energy on the arc stability for Ag-based contact materials has not been considered. For the Ag-4 wt.%TiB2 contact material with 2 wt %Ni addition, there are presence of less Ni on the anode and cathode surface, which was confirmed by the XRD and EDS result. Therefore, the high arc energy caused by Ni ionization is not apparent. On the contrary, the arc dispersion decreases temperature rise on the electrode surface and further reduces the evaporation of electrode materials. This can interpret why the material with 2 wt%Ni addition has low average arc energy and small fluctuation (see section 3.3 and 3.4). Furthermore, the mass change can also be explained based on this viewpoint. As mentioned above, 2 wt%Ni addition decreases the evaporation of electrode material, the dominant ion for the arc generation transfers from metallic ion to gaseous ion, so the anodic erosion is serious for the material without Ni addition, whereas the cathodic erosion is dominant for the material with 2 wt%Ni addition. For the materials with 4 wt% and 8 wt%Ni additions, more Ni exists on the electrode surface, which was determined by the TEM and EDS analysis. Hence, the increased Ni content on the electrode surface promotes process instability during arc erosion. Especially, the occurrence of high arc energy has larger probability. At this case, large amounts of electrons promote anode evaporation. Meanwhile, high valence Ni cations can gain more kinetic energy from electrical field, resulting in the increasingly serious evaporation and mass transfer.
energy corresponds with a low break-arc-energy, see Fig. 9(c) and (d). This is because the high make-arc-energy causes the temperature rise of electrode surface, and further shortens the lasting time of molten bridge and break-arc-duration. 3.8.2. Thermodynamics To account for the relation of arc erosion with physical properties, Table 3 lists the thermal properties of Ag and Ni [35]. Our previous work [28] verified that arc prefers to generate at the phase with a low work function. As Ag has a lower work function than Ni, Ni addition increases the integral work function of cathode surface for Ag-4 wt. %TiB2 contact material. Hence, it is more difficult for the electron emission, which further delays arc generation. Furthermore, Ni addition increases the number of interface, which promotes arc motion. Our previous work [28] also revealed that arc tends to generate at the Ag/ TiB2 interface though the TiB2 has a larger work function than Ag, and the existence of TiB2 disperses the arc, thus reducing arc erosion. The similar phenomenon was also be reported by Zhang et al. [36]. Thereby, it is thought that Ni addition can better disperse arc, and reduce the concentrated arc erosion. The energy needed from 293 K to 3003 K for Ag and Ni is calculated by Eq. (2).
ΔH =
∫ C (T ) dT + Lm + Lb
(2)
where ΔH represents the enthalpy change, C(T) is the heat capacity, Lm is the heat of fusion and Lb represents the heat of vaporization. The enthalpy change from 293 K to 3003 K is calculated regardless of the change of heat capacity with temperature. The calculated ΔHAg and ΔHNi are 333.98 kJ/mol and 467.13 kJ/mol, respectively. Apparently, Ni has a larger enthalpy change than Ag. It is in accordance with the cohesive energy of Ag (2.95 eV) and Ni (4.44 eV), which refers to the energy that is needed to remove an atom from solid state to freedom state. Generally, arcing causes the phase transformation of Ag and Ni on the anode and cathode surface. Since this is a transient process, it still has the probability for Ni to evaporate during arcing though more energy is needed for Ni evaporation than Ag. 3.8.3. Ionization energy Table 4 lists the ionization energies of Ag and Ni, and the ionization energies of O and N are also included in Table 4 for the discussion purpose. It is interesting to find that the first ionization energy of O or N is higher than the sum of that of Ag and ΔHAg. However, this does not imply that Ag and Ni prefer to be ionized at the initial stage of make arc. The electron emission occurs on the cathode surface before arc generation, and the electron motion promotes the gas ionization since the cathode and anode materials has not yet been vaporized. Once the electrode surface temperature reaches the boiling point, a large number of metallic atoms generate at the interelectrode gap. At this case, Ag Table 4 The ionization energies of Ag and Ni (kJ/mol). Ionization energy
1st
2nd
3rd
4th
5th
6th
7th
8th
Ag Ni O N
731.0 737.1 1313.9 1402.3
2070 1753 3388 2856
3361 3395 5301 4578
– 5300 7469 7475
– 7339 10989 9445
– 10400 13326 53266
– 12800 71330 64360
– 15600 84078 –
368
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Fig. 12. Schematic diagram of arc erosion mechanism.
Therefore, the mass loss increases for both anode and cathode. Besides these factors, arc dispersion and higher work function of electrode surface also give rise to the dominant gaseous ions in the electric arc, and cathode has a higher mass loss than anode.
of Tailings Resources (2017SKY-WK010) and Special research project of Shaanxi Provincial Department of Education (18JK0722).
4. Conclusions
[1] C.P. Wu, D.Q. Yi, W. Weng, S.H. Li, J.M. Zhou, Influence of alloy components on arc erosion morphology of Ag/MeO electrical contact materials, Trans. Nonferrous Metals Soc. China 26 (2016) 185–195. [2] Z. Wei, L.J. Zhang, H. Yang, T. Shen, T. Chen, Effect of preparing method of ZnO powders on electrical arc erosion behavior of Ag/ZnO electrical contact material, J. Mater. Res. 31 (2016) 468–479. [3] N. Ray, B. Kempf, T. Mutzel, L. Froyen, K. Vanmeensel, J. Vleugels, Effect of WC particle size and Ag volume fraction on electrical contact resistance and thermal conductivity of Ag-WC contact materials, Mater. Des. 85 (2015) 511–519. [4] Y. Xu, Y.C. Guo, F.F. Liu, Y.H. Geng, K.H. Zhang, Study on arc erosion of AgLa2NiO4 contact materials, Precious Met. 3 (2007) 15–19. [5] W. Rieder, V. Weichsler, Make erosion mechanism of AgCdO and AgSnO2 contacts, IEEE Trans. Compon. Hybrids Manuf. Technol. 15 (1992) 332–338. [6] P. Verma, O.P. Pandey, A. Verma, Influence of metal oxides on the arc erosion behavior of silver metal oxides electrical contact materials, J. Mater. Sci. Technol. 1 (2004) 49–52. [7] X.H. Wang, S.H. Liang, P. Yang, Z.K. Fan, Effect of Al2O3 particle size on vacuum breakdown behavior of Al2O3/Cu composite, Vacuum 83 (2009) 1475–1480. [8] S. Biyik, F. Arslan, M. Aydin, Arc-erosion behavior of boric oxide-reinforced silverbased electrical contact materials produced by mechanical alloying, J. Electron. Mater. 44 (2015) 457–466. [9] S. Biyik, M. Aydinb, Investigation of the effect of different current loads on the arcErosion performance of electrical contacts, Acta Phys. Pol., A 129 (2016) 656–660. [10] G.J. Li, H.J. Cui, J. Chen, X.Q. Fang, W.J. Feng, J.X. Liu, Formation and effects of CuO nanoparticles on Ag/SnO2 electrical contact materials, J. Alloy. Comp. 696 (2017) 1228–1234. [11] J.B. Wang, Y. Zhang, M.G. Yang, B.J. Ding, Z.M. Yang, Observation of arc discharging process of nanocomposite Ag–SnO2 and La-doped Ag–SnO2 contact with a highspeed camera, Mater. Sci. Eng. B 131 (2006) 230–234. [12] H.T. Wang, J.Q. Wang, J. Du, F.Q. Meng, Influence of rare earth on the wetting ability of AgSnO2 contact material, Rare Metal Mater. Eng. 43 (2014) 1846–1849. [13] L.J. Zhang, T. Shen, Q.H. Shen, J. Zhang, L. Chen, X.P. Fan, H. Yang, Anti-arc erosion properties of Ag-La2Sn2O7/SnO2 contacts, Rare Metal Mater. Eng. 45 (2016) 1664–1668. [14] Z.K. Chen, S. Koichiro, Effect of arc behavior on material transfer: a Review, IEEE Trans. Compon. Packag. Manuf. Technol. A. 21 (1998) 310–322. [15] H. Sone, T. Takagi, Role of the metallic phase arc discharge on arc erosion in Ag contacts, IEEE. Trans. Comp. Hybrids. Manuf. Technol. 13 (1990) 13–19. [16] N.B. Jemaa, L. Morin, S. Benhenda, L. Nedelec, Anodic to cathodic arc transition according to break arc lengthening, IEEE Trans. Compon. Packag. Manuf. Technol. A. 21 (1998) 599–603. [17] L. Doublet, N.B. Jemaa, F. Hauner, D. Jeannot, Electrical arc phenomena and its interaction on contact material at 42 volts DC for automotive applications, Proc. 50th IEEE Holm Conf. Elect. Contacts, 2004, pp. 8–14. [18] Z.K. Chen, K. Sawa, Particle sputtering and deposition mechanism for material transfer in breaking arcs, J. Appl. Phys. 76 (1994) 3326–3331. [19] M.Z. Rong, Q.P. Wang, Catastrophe models of the direction reverse of material transfer and the transition, J. Xi'an Jiaot. Univ. (Med. Sci.) 24 (1990) 17–22. [20] L. Morin, N.B. Jemaa, D. Jeannot, Make arc erosion and welding in the automotive
References
The effect of Ni addition on the arc erosion behavior of Ag-4 wt. %TiB2 contact material was studied at the present work. It is found that the increasing Ni content decreases the electrical conductivity and increases hardness. Moreover, Ag-4 wt.%TiB2 contact materials with 0 wt % and 2 wt%Ni additions present low make-arc-energy and break-arcenergy, and they also have the little fluctuations with operation times. Nevertheless, at 4 wt% and 8 wt%Ni, the make-arcs and break-arcs with especially high arc energy and long arc duration have large probability to occur, and large fluctuations of arc energy with operation times are presented. This is in accordance with the erosion morphologies and mass change. Moreover, much serious erosion and fairly high mass loss present at 4 wt% and 8 wt%Ni compared with that at 0 wt% and 2 wt% Ni. Among the materials with different Ni contents, the material with 2 wt%Ni addition has the least total mass loss and the optimal arc erosion resistance. In addition, erosion mode changes from anodic erosion to cathodic erosion with more Ni addition. The arc erosion mechanism was discussed based on the contact characterization, thermal property and ionization energy of materials. The increasing hardness with more Ni addition reduces the contact points, which further leads to the temperature rise of each site and the short lasting time of molten bridge and break-arc-duration. Furthermore, Ni has large potential to be ionized and the possibility to evaporate due to its little difference of enthalpy change compared with ionization energy. Though increasing Ni content promotes arc dispersion on a certain degree, the high valence Ni cations with high kinetic energy derived from electrical field results in the increasingly serious evaporation and mass transfer from cathode to anode. Subsequently, excessive Ni addition is not beneficial for the improvement of arc erosion resistance for AgeTiB2 contact material. Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No. 51274163 and 51605146), the China Postdoctoral Science Foundation (Grant No. 2018M632769), the Research Fund of Shaanxi Key Laboratory of Comprehensive Utilization 369
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area, IEEE Trans. Compon. Packag. Technol. 23 (2000) 240–246. [21] J. Swingler, J.W. McBride, The erosion and arc characteristics of AgCdO and AgSnO2 contact materials under DC break conditions, IEEE Trans. Compon. Packag. Manuf. Technol. A. 19 (1996) 404–415. [22] H.Y. Li, X.H. Wang, Y. Xi, Y.F. Liu, X.H. Guo, Influence of WO3 addition on the material transfer behavior of the AgTiB2 contact material, Mater. Des. 121 (2017) 85–91. [23] X.H. Wang, G.J. Li, J.T. Zou, S.H. Liang, Z.K. Fan, Investigation on preparation, microstructure and properties of AgTiB2 composite, J. Compos. Mater. 45 (2011) 1285–1293. [24] G.J. Li, X.H. Wang, J.T. Zou, S.H. Liang, Investigation on arc erosion behavior of AgTiB2 composite, Precious Met. 32 (2011) 36–41. [25] H.Y. Li, X.H. Wang, X.H. Guo, X.H. Yang, S.H. Liang, Material transfer behavior of AgTiB2 and AgSnO2 electrical contact materials under different currents, Mater. Des. 114 (2017) 139–148. [26] H.Y. Li, X.H. Wang, Y.F. Liu, X.H. Guo, Effect of strengthening phase on material transfer behavior of Ag-based contact materials under different voltages, Vacuum 135 (2017) 55–65. [27] V. Cosovic, A. Cosovic, N. Talijan, D. Zivkovic, D. Manasijevic, D. Minic, Improving dispersion of SnO2 nanoparticles in AgSnO2 electrical contact materials using template method, J. Alloy. Comp. 567 (2013) 33–39.
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Effect of Ni addition on the arc-erosion behavior of AgTiB2 contact material a,b
Hangyu Li a b c
, Xianhui Wang
a,b,∗
, Yong Xi
a,b
, Ting Zhu
a,b
T
c
, Xiuhua Guo
Shaanxi Key Laboratory of Electrical Materials and Infiltration Technology, Xi'an University of Technology, Xi'an, 710048, PR China School of Materials Science and Engineering, Xi'an University of Technology, Xi'an, 710048, PR China School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang, 471003, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ag-based contact material Arc erosion Ni addition Ionization energy
To disclose the electrical characteristics of AgTiB2 contact materials with different Ni additions, the arc-erosion tests were performed 20,000 times at 24 V/16 A DC. The morphology and chemical composition of arc eroded contact surface were characterized by scanning electron microscopy and energy dispersive X-ray spectrometry, the mass change after arc-erosion tests was determined, the arc energy and arc duration at make-arc and breakarc were analyzed, and the arc erosion mechanism was discussed as well. The results show that arc erosion mode changes from anodic erosion to cathodic erosion with Ni addition. The Ag-4 wt.%TiB2 contact material with 2 wt %Ni addition has the optimum performance along with the least mass loss, while especially high arc energy and total mass loss generate at 4 wt% and 8 wt%Ni. Furthermore, the arc erosion mechanism was discussed based on the contact characterization, thermal property and ionization energy of materials. It is believed that excessive Ni addition is not beneficial for the improvement of arc erosion resistance since Ni has large potential to be ionized and the possibility to evaporate due to its little difference of enthalpy change compared with ionization energy.
1. Introduction Due to the excellent electrical conductivity, welding resistance and wear resistance, Ag-based contact materials are widely used in the low and medium voltage electrical systems [1–3]. During service, arc erosion occurs at make and break operations, which not only deteriorates the contact interface, but also results in the material transfer between anode and cathode under the DC load condition. The material transfer can further cause the morphology change or the inhomogeneous mass loss of anode and cathode. If serious, it reduces the reliability and stability of the electrical systems [4]. Currently, several literature have been reported on the effect of processing parameter, microstructure and the chemical and thermodynamic properties of the second phase on the arc erosion behavior for Ag-based contact materials. Rieder et al. [5] believed that the porous eroded surface and loosely adhering splashed particles result in higher mass loss due to the mechanical abrasion. Verma et al. [6] thought that the oxide with moderate stability is more favorable for the improvement on the electrical performance in comparison with extremely stable oxide for the Ag/MeO contact material. Wang et al. [7] thought that fine Al2O3 particles have a lower float velocity than coarse particles in molten pool, thus decreasing the aggregation of strengthening particles at the erosion surface. Biyik et al. [8,9] thought that the electrical
performance of contact materials depends on current load. At present, a lot of investigations reported that the effect of the minor additives on the electrical performances of Ag-based materials (CuO [10], La [11], La3O2 [12], La2Sn2O7 [13], WO3 [14]). Li et al. [10] found that CuO addition results in shallower erosion pits, larger erosion area, and better arc erosion resistance. Wang et al. [11] reported that La doping promotes arc motion and dispersion. Wang et al. [12] found that La3O2 addition significantly improves the wettability between Ag and SnO2. Zhang et al. [13] reported that La2Sn2O7 addition alleviates the damage of eroded surface structure. Furthermore, the material transfer caused by arc erosion is especially complex. So far, a few literature have been reported on the material transfer mechanism. Chen et al. [14] believed that material transfer depends on what kind of ion is dominant to sustain the arc combustion. The metallic ions trigger the material transfer from anode to cathode, whereas the gaseous ions cause the material transfer from cathode to anode. Sone et al. [15] proposed that the material transfer, mass loss and contact electrical resistance are strongly affected by the metallic ion arc. Jemaa et al. [16] thought that material transfer is closely correlated with arc length. At short arc, the material transfer is from anode to cathode. On the contrary, it presents a different mode at long arc. Doublet et al. [17] found that the material transfer mode reverses with increasing current, and the transfer direction is determined
∗
Corresponding author. School of Materials Science and Engineering, Xi'an University of Technology, 5 South Jinhua Road, Xi'an, 710048, PR China. E-mail addresses: [email protected] (H. Li), [email protected] (X. Wang), [email protected] (Y. Xi), [email protected] (T. Zhu), [email protected] (X. Guo). https://doi.org/10.1016/j.vacuum.2019.01.003 Received 12 October 2018; Received in revised form 14 November 2018; Accepted 4 January 2019 Available online 06 January 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. The morphologies of raw powder and milled powder. (a) Ag powder (b) Ni powder (c) TiB2 powder (d) milled powder.
can improve the wettability between Ag and TiB2 and sinterability of AgeTiB2 contact materials. However, the effect of Ni addition on the arc erosion behavior is still unclear. To clarify the effect of Ni addition on the arc erosion behavior of Ag-based contact materials, the Ag-4 wt. %TiB2 contact materials with different Ni contents were prepared. The arc energy, arc duration, eroded morphology, element distribution and the mass changes before and after arc-erosion test were investigated, and the arc erosion mechanism and material transfer mode were discussed as well. The research can enrich the electrical contact theory and provide the reference for the design and manufacture of Ag-based contact materials.
by the mutually competed result of anodic and cathodic arc. Chen et al. [18] confirmed that high voltage results in small critical current transfer from metallic ion arc to gaseous ion arc. Rong et al. [19] reported that the change of material transfer mode depends on the voltage-current characteristics. Morin et al. [20] studied the material transfer of Ag-based contact material under the inductive, resistive and lamp loads, and revealed that the Ag-based contact material has the largest transfer mass at the lamp loads, whereas the least transfer mass occurs at inductive load. Swingler et al. [21] believed that the material transfer of AgCdO and AgSnO2 electrode contacts is derived from cathode evaporation and anode condensation at low current, but liquid droplet splash is the dominant material transfer mechanism at high current, and the AgCdO contact material has a lower erosion rate than the AgSnO2 contact material. Though more understanding can be obtained for the material transfer of Ag-based contact materials from above investigations, the effect of material characteristics on the material transfer behavior is still obscure. Our previous work [22–26] also found that WO3 addition causes the change of material transfer mode for AgTiB2 contact material, and the second phase is beneficial for arc dispersion, resulting in different arc erosion and material transfer behavior for Ag-based contact materials. Furthermore, the influence mechanism of the second phase was also discussed based on the work function and boiling point of material components. However, the arc erosion may be influenced by more physical properties due to the complexity of arc erosion. Hence, it still lacks the deep insights of the underlying arc erosion mechanism so far. Though AgCdO contact material has excellent arc erosion resistance, wear and welding resistance [27], the toxic nature and an increasing environmental awareness limit the application of AgCdO contact material. For the AgSnO2 contact material, the separation of SnO2 particles and Ag particles during long time service causes temperature rise and high values of contact resistance [13]. To tackle this issue, Wang et al. [24] proposed to replace non-conductive SnO2 by conductive TiB2 in the Ag-based contact material. Nevertheless, the poor wettability between TiB2 and Ag influences the sinterability of AgeTiB2 contact materials. Our primary work revealed that Ni addition
2. Experimental The starting materials were Ag powder (purity 99.9 wt%, particle size 72 μm), TiB2 powder (purity 99.9 wt%, particle size 60 nm) and Ni powder (purity 99.9 wt%, particle size 800 nm). As composite contact materials are most frequently prepared using powder metallurgy method, which considers specific chemical composition, and appropriate milling parameters are important for the acquirement of good milling effect and sintering process [29–31], the present authors have conducted on a number of investigations on the fabrication of AgeTiB2 composites with the powder metallurgy method [22–26]. At this work, the process parameters were selected based on our previous work. The AgeTiB2 powder with a mass ratio of 96:4 and AgeNieTiB2 powder with a mass ratio of 94:2:4, 92:4:4 and 88:8:4 were mixed at a planetary ball mill (KQM-YB/B, Xianyang Jinhong Machinery Co. Ltd) at 300 rpm for 4 h and a ball-to-powder ratio of 15:1. The ethyl alcohol and polyvinylpyrrolidone (PVP) (1 wt%) were used as process control agent and dispersant, respectively. The morphologies of raw powder and milled powder are shown in Fig. 1, and the XRD pattern of milled powder is shown in Fig. 2. The mixture was compressed under a pressure of 60 MPa for 30 s to obtain a compact with a diameter of 21 mm and a length of 13 mm, followed by sintering at 750 °C for 2 h at 30 MPa under the protective atmosphere of nitrogen gas in a self-made hotpressing furnace. The relative density was measured utilizing 362
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JF04C arc-erosion test rig. The mass before and after arc-erosion test were determined by an electrical balance (FA1104J, Tianjin Shunnuo Instrument Technology Co. Ltd). The surface morphologies of the arceroded specimens were characterized by a scanning electron microscopy (JSM-6700F, Nippon Electric Company Ltd) and the chemical composition was analyzed on an energy dispersive X-ray spectrometry (Oxford INCA, Oxford Instrument).
3. Results and discussions 3.1. Characterization of Ag-4 wt.%TiB2 contact materials with different Ni additions Fig. 4(a)-(d) are the SEM images of Ag-4 wt.%TiB2 contact materials with different Ni additions. The gray and bright regions are Ag-rich and TiB2/Ni-rich zones, respectively. The results show that Ni and TiB2 are distributed around irregular Ag particles, and there are a few porosities. The irregular Ag particles are derived from deformation during ball milling, pre-compressing and hot pressing. To further determine the distribution of Ni and TiB2, region A marked in Fig. 4(c) is analyzed by elemental mapping, the high magnification image of region A is shown in Fig. 4(e) and the elemental mapping patterns are represented in Fig. 4(f)-(I). This can verify that Ni and TiB2 are distributed along Ag particle boundary. Fig. 5 is the XRD patterns of sintered Ag-4 wt.%TiB2 contact materials with different Ni additions. For the Ag-4 wt.%TiB2 contact material without Ni addition, strong Ag peaks appears along with weak TiB2 peaks. However, for the Ag-4 wt.%TiB2 contact materials with the additions of 2 wt%Ni and 4 wt% Ni, Ni3B peaks appear without the presence of Ni peaks. It indicates that Ni react with TiB2 during sintering process. Furthermore, more Ni3B peaks occur along with weak Ni peaks at 8 wt% Ni. Saqib et al. [32] and Finch et al. [33] reported that the reaction between Ni and TiB2 occurs at the grain boundary. This may hinder Ni and TiB2 to be detected in this work. Furthermore, Ni3B becomes unstable above 1429 K [32], which is much lower than the boiling point of Ag (2435 K). In addition, it is learnt from the XRD analysis that there is very small amount of Ni3B reaction product. Therefore, it is inferred that Ni3B has no apparent influence on arc erosion. Fig. 6 is the TEM image and SAED pattern of the specimen with 4 wt %Ni addition. As learnt from the indexed result, it is verified that the bright particle is Ni phase. During TEM analysis, it cannot get the SAED
Fig. 2. XRD pattern of milled powder.
Archimedean method. The electrical conductivity was performed on an eddy current gauge (Model 7501) and the hardness was tested on a hardness testing equipment (Model HV-120) under the load of 5 kg for 30 s, and the mean values were the average of five measured results. The specimen used for TEM observation was cut from the studied material and then mechanically polished to obtain 50 μm thick slice. Discs of 3 mm in diameter were punched from the slice and thinned in an ion milling machine (Fischione1010) at 4.5 kV. The microstructure was characterized by a transmission electron microscope (Tecnai F30 G2) with an operation voltage of 200 kV. The anode and cathode with a diameter of 3.8 mm and a length of 10 mm were machined from the as-prepared specimens. The contact tests were made on an arc-erosion test rig (JF04C, Kunming Institute of Precious Metals), as schematically shown in Fig. 3. The anode and cathode were corresponded to the movable contact and stationary contact, respectively. Each contact pairs were performed 20000 times under the resistive load with a DC current of 16 A and voltage of 24 V, the operation frequency of 1 Hz, the contact force of 0.4 N and the electrode gap of 1 mm. During arc-erosion tests, the anode moved downward until it contacted the cathode, and then moved upward. The data of arc duration and arc energy were automatically collected by the
Fig. 3. Schematic of arc-erosion test rig. 363
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Fig. 4. SEM images of the Ag-4 wt.%TiB2 contact materials with different Ni additions and the elemental mapping results of Ag-4 wt.%TiB2 contact material with 4 wt%Ni additions. (a) 0 wt% Ni (b) 2 wt%Ni (c) 4 wt% Ni (d) 8 wt% Ni (e) high magnification image of region A (f) Element distribution (g) Ag element (h) Ni element (I) Ti element.
pattern of TiB2 particles. The most possible reason is that Ni3B and TiB2 particles are shed from the specimen during preparation process due to their high hardness. 3.2. Physical and mechanical properties of the Ag-4 wt.%TiB2 contact materials with different Ni additions Table 1 lists the physical and mechanical properties of the Ag-4 wt. %TiB2 contact materials with different Ni additions. As learnt from Table 1, the relative density has no remarkable change with increasing Ni content. However, the electrical conductivity decreases and hardness increases with increasing Ni content. Since the electrical conductivity of Ni is far below than that of Ag, and more Ni addition increase the number of interface, which enhances the electron scattering, and, consequently, reduces the electrical conductivity. Moreover, the interface can block the dislocation slip, and its effect is further enhanced with increasing Ni content, thus enhancing the hardness of the Ag-4 wt.%TiB2 contact material. 3.3. Analysis of make-arc-energy and make-arc-duration for the Ag-4 wt. %TiB2 contact materials with different Ni additions
Fig. 5. XRD patterns of sintered Ag-4 wt.%TiB2 contact materials with different Ni additions.
The cumulative distribution curves of make-arc-energy and makearc-duration for the Ag-4 wt.%TiB2 contact materials with different Ni 364
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Fig. 6. TEM image of the specimen with 4 wt%Ni addition (a), and the SAED pattern (b).
additions are shown in Fig. 7. Obviously, the make-arc-energy mainly distributes in 0–2000 mJ for the Ag-4 wt.%TiB2 contact materials with 0 wt% and 2 wt%Ni additions. However, for the materials with 4 wt% and 8 wt%Ni additions, there are a number of make-arcs with high energy (2000–8000 mJ), as shown in the green rectangle region in Fig. 7. It suggests that more Ni addition causes the large probability for occurrence of the make-arc with high energy. This can also be reflected by the standard deviation, see the inset data in Fig. 7(a). At 0 wt% and 2 wt%Ni, the standard deviation (D) of make-arc-energy is small. Nevertheless, the standard deviation is extremely large at 4 wt% and 8 wt%Ni, suggesting that make-arc-energy has large fluctuations with more Ni addition. Make-arc-duration has a similar distribution to make-arc-energy. At 0 wt% and 2 wt%Ni, the make-arc-duration is distributed in the range of 0–50 ms. However, at 4 wt% and 8 wt%Ni, make-arc-duration has large fluctuations, and the maximum value can reach up to 150 ms. Fig. 7. The cumulative distribution curve of make-arc-energy (a) and make-arcduration (b) for the Ag-4 wt.%TiB2 contact materials with different Ni additions.
3.4. Analysis of break-arc-energy and break-arc-duration for the Ag-4 wt. %TiB2 contact materials with different Ni additions
subsequently keeps around 400 mJ with less fluctuation at 0 wt%Ni. Nevertheless, its fluctuations decrease obviously at 2 wt%Ni. At 4 wt% Ni, the make-arc-energy during first 8000 operations is less than 200 mJ, however, there are large fluctuations along with an overall increased tendency, see Fig. 9(c). Furthermore, its fluctuations are further aggravated at 8 wt%Ni, suggesting that excessive Ni addition gives rise to the dramatic change of the make-arc-energy, and is not favorable for the improvement on the arc erosion resistance. In addition, the break-arc-energy has little fluctuations at 0 wt%Ni, whereas it presents an overall decreased tendency at 2 wt%Ni. Nevertheless, the break-arc-energy at 4 wt% and 8 wt%Ni has large fluctuations, which is similar to the make-arc-energy, see Fig. 9(c) and (d).
The cumulative distribution curves of break-arc-energy and breakarc-duration for the Ag-4 wt.%TiB2 contact materials with different Ni additions are shown in Fig. 8. The distributions of arc energy and arc duration at break arc are similar to those at make arc. The break-arcenergy is mainly distributed in 0–1000 mJ at 0 wt% and 2 wt%Ni, whereas it has a large fluctuation, and the maximum value can reach up to about 5000 mJ at 4 wt% and 8 wt%Ni. 3.5. Variation of arc energy with operation times for the Ag-4wt.%TiB2 contact materials with different Ni additions To get better understanding of the arc energy, the recorded data of make-arc-energy and break-arc-energy every 500 operations were averaged for the Ag-4wt.%TiB2 contact materials with different Ni additions, and variations of make-arc-energy and break-arc-energy with operation times are presented in Fig. 9. As seen from Fig. 9(a) and (b), the make-arc-energy increases during the first 2000 operations, and
Table 1 Physical and mechanical properties of the Ag-4 wt.%TiB2 contact materials with different Ni additions. Ni content (wt.%)
Relative density (%)
Standard deviation
Electrical conductivity (%IACS)
Standard deviation
Hardness (HV)
Standard deviation
0 2 4 8
96.33 95.55 96.05 96.09
0.20 0.31 0.24 0.34
81.44 71.35 69.66 61.06
0.45 0.26 0.48 0.31
57.15 61.83 68.05 82.57
0.40 1.00 0.48 0.68
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particles are more easily to agglomerate on the cathode surface. For the material with 2 wt%Ni addition, Ni content on the surface of anode and cathode is less than 2 wt%. For the Ag-4 wt.%TiB2 contact material with 4 wt%Ni addition, Ni and TiB2 contents in the eroded region increase obviously, and Ag content decreases remarkably. At 8 wt%Ni, TiB2 and Ni agglomerate seriously on the surface of anode and cathode, and Ag content reduces progressively. This can be explained as follows. Firstly, the molten pool at the anode is especially less than that at the cathode because electron mass is far below that of gaseous and metallic cations. At this case, TiB2 and Ni have less floatation on the anode surface than on the cathode surface. Secondly, Ag evaporation reduces the Ag content on the anode surface since Ag has the lowest boiling point. As larger fluctuation of arc energy generates, the fairly high arc energy causes Ag serious evaporation. Therefore, the degree of Ag evaporation is beyond the floatation effect of TiB2 and Ni, thus resulting in agglomeration on the anode surface. Fig. 8. The cumulative distribution curve of break-arc-energy (a) and breakarc-duration (b) of Ag-4 wt.%TiB2 contact materials with different Ni additions.
3.7. Mass change of the eroded anode and cathode for the Ag-4 wt.%TiB2 contact materials with different Ni additions
3.6. Morphologies and chemical compositions of eroded anode and cathode for the Ag-4 wt.%TiB2 contact materials with different Ni additions
Fig. 11 is the mass change of the eroded anode and cathode after the arc-erosion tests for the Ag-4 wt.%TiB2 contact materials with different Ni additions. For the material with 0 wt%Ni addition, the mass loss of the eroded anode and cathode are 3.88 mg and 2.84 mg, respectively. Obviously, the anode has a higher mass loss than the cathode. However, for the materials with different Ni additions, the mass loss of cathode is lower than that of the anode. It implies that the cathodic erosion is dominant, and Ni addition changes the material transfer behavior for the Ag-4 wt.%TiB2 contact material. Furthermore, it should be noticed that the total mass loss is the lowest for the material with 2 wt%Ni addition. At 8 wt%Ni, the mass loss of the eroded anode and cathode are 6.46 mg and 16.12 mg, respectively, and the total mass loss is 22.58 mg, which is the highest among the four contact pairs. Consequently, it is thought that excessive Ni addition is not beneficial for the improvement on arc erosion resistance. In addition, it is necessary to note that the maximum gap between
Fig. 10 shows the morphologies of eroded anode and cathode for the Ag-4 wt.%TiB2 contact materials with different Ni additions. For the materials with 0 wt% and 2 wt%Ni additions, both cathodes of two contact pairs have the flow and solidified trace, and the erosion pits appear at the anodes, indicating that the erosion mechanism of anode is different from that of cathode. Nevertheless, at 4 wt% and 8 wt%Ni, the erosion area, apparently, is larger than that of the first two contact pairs. Moreover, the erosion pits appear on the cathode and anode surface, suggesting that serious arc erosion happens. Table 2 is the EDS analysis results of the eroded contact surface after 20000 operations for the Ag-4 wt.%TiB2 contact materials with different Ni additions. Due to the detection limitation, Ti element was determined to characterize the content of TiB2. Evidently, TiB2 content at the cathode is larger than that at the anode, indicating that TiB2
Fig. 9. Variation of make-arc-energy and break-arc-energy with operation times for the Ag-4wt.%TiB2 contact materials with different Ni additions. (a) 0 wt%Ni (b) 2 wt%Ni (c) 4 wt%Ni (d) 8 wt%Ni. 366
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Fig. 10. Surface morphologies of eroded anode (a–d) and cathode (a1-d1) for the Ag-4 wt.%TiB2 contact materials with different Ni additions.
anode and cathode is only 1 mm. The splashed Ag droplets have little chance to escape from the electrodes because they are hindered by the erosion edge. This was discussed in our previous work, and more details can refer to the literature [26]. Hence, it is believed that the mass loss mainly occurs through the evaporation of electrode materials. The mass loss can be explained based on the change of arc energy (see section 3.3 and 3.4). Compared with the material with 0 wt%Ni addition, the material with 2 wt%Ni addition has less make-arc-energy fluctuations and lower average break-arc-energy. Thus, it has low total mass loss. However, the make arc and break arc with especially high arc energy are much likely to appear for the materials with 4 wt% and 8 wt %Ni addition, resulting in the fairly high mass loss. As a result, the distribution of arc energy is in accordance with the total mass loss for the Ag-4 wt.%TiB2 contact material. 3.8. Discussions Fig. 11. Mass change of the eroded anode and cathode for the Ag-4 wt.%TiB2 contact materials with different Ni additions.
Based on the contact characteristics, thermodynamics and ionization energy, the underlying arc erosion mechanism for the Ag-4 wt. %TiB2 contact materials with different Ni additions is discussed as follows.
nominal contact area is in proportion to the conductive contact area. As learnt from section 3.2, the hardness of the Ag-4 wt.%TiB2 contact material enhances with increasing Ni content. Therefore, the conductive contact area gradually decreases with more Ni addition. It indicates that the current carrying points of contact are reduced to a few points with increasing hardness once two electrodes contact. At this case, the temperature rises for each site, and molten bridge lasts for a shorter time. As a consequence, the break-arc-energy is decreased. This can be verified by the results shown in section 3.4. Another phenomenon is that the make-arc-energy has an opposite tendency compared with the break-arc-energy, i.e., a high make-arc-
3.8.1. Contact characteristics Due to the effect of the contact force and hardness, the conductive contact area generally only occupies a tiny portion of the nominal contact areas, which can be expressed by Eq. (1) [34].
Fc = ξHAa
(1)
where Fc is the contact force, H represents the hardness, and Aa is the nominal contact area. Distinctly, Aa decreases with increasing H. Furthermore, the
Table 2 EDS results of the eroded anode and cathode for the Ag-4 wt.%TiB2 contact materials with different Ni additions (wt.%). Ni content (wt.%)
0
2
4
8
Elements
Ag Ti Ag Ti Ni Ag Ti Ni Ag Ti Ni
Anode
Cathode
1
2
3
Average
Standard deviation
1
2
3
Average
Standard deviation
97.43 2.57 92.61 5.07 2.32 87.87 5.41 6.72 82.44 4.41 13.15
96.52 3.48 98.26 0.64 1.10 82.61 6.06 11.34 84.28 4.68 11.03
98.88 1.12 95.18 3.23 1.59 90.22 3.19 6.59 79.28 6.47 14.25
97.61 2.39 95.35 2.98 1.67 86.90 4.89 8.22 82.00 5.19 12.81
1.19 1.19 2.83 2.23 0.61 3.90 1.50 2.70 2.53 1.12 1.64
94.23 5.77 92.39 5.78 1.83 79.92 10.52 9.57 74.02 9.36 16.62
95.05 4.95 94.32 5.42 0.26 83.23 2.52 14.24 79.60 10.54 9.85
96.06 3.94 91.45 7.85 0.70 72.86 4.94 22.21 69.49 13.03 17.48
95.11 4.89 92.72 6.35 0.93 78.67 5.99 15.34 74.37 10.98 14.65
0.92 0.92 1.46 1.31 0.81 5.30 4.10 6.39 5.07 1.87 4.18
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Table 3 The thermal properties of Ag and Ni [35]. Element
Work function (eV)
Melting point (K)
Heat of fusion (kJ/mol)
Boiling Point (K)
Heat of vaporization (kJ/mol)
Heat capacity (J/mol·K)
Ag Ni
4.70 5.15
1235 1728
11.28 17.48
2435 3003
254 379
25.35 26.07
and Ni atoms prefer to be ionized, and the dominant ions change from gaseous ion to metallic ion. The same viewpoints can also be supported by the literature [37]. This process is schematically shown in Fig. 12. Though there is a little difference for the first ionization energy between Ag and Ni, it is much larger than the enthalpy change from 293 K to 3003 K. The ionization energy plays a major role in arc consumption, and numbers of the gaseous or metallic molecules are ionized accompanying with generation of freedom electrons. As learnt from Table 4, there are only three ionization energy levels for Ag, but Ni can reach up to the 28th ionization energy level, suggesting that Ni has a larger potential to be ionized, see Fig. 12. Based on this viewpoint, the energy fluctuation happens during arc erosion due to arc complexity. If there is more energy, Ni ions can be progressively ionized, resulting in the generation of large amounts of freedom electrons and high valence Ni ions, which can sustain arc combustion for longer time. Simultaneously, the energy imported to the electrode results in the evaporation of electrode materials, giving rise to more difficulty for arc extinction. This can explain why there are a number of make arcs and break arcs with especially high arc energy for the Ag-4 wt.%TiB2 contact materials with 4 wt% and 8 wt%Ni additions. As seen from Fig. 12, at the stage of gaseous arc, the electrons are accelerated under the action of electrical field, resulting in temperature rise and evaporation of anode materials. Simultaneously, the gaseous cations bombard the cathode, causing the splash. Some splashed droplets adhere to the anode, leading to the material transfer from cathode to anode. At this case, those gaseous cations can obtain the electrons on the cathode surface, and subsequently disappear. At this stage, cathode has the net mass loss. However, at the stage of metallic arc, the metallic cations also move towards the cathode. At this case, it is probably to bring about splash. The metallic cations that capture electrons deposit into the cathode. Hence, for the metallic arc, the material transfer depends on which is dominant for deposition and splash. Our previous work [23,25,26] showed that the material characteristics have prominent effect on the arc characteristics, which further results in different material transfer behavior. However, the effect of ionization energy on the arc stability for Ag-based contact materials has not been considered. For the Ag-4 wt.%TiB2 contact material with 2 wt %Ni addition, there are presence of less Ni on the anode and cathode surface, which was confirmed by the XRD and EDS result. Therefore, the high arc energy caused by Ni ionization is not apparent. On the contrary, the arc dispersion decreases temperature rise on the electrode surface and further reduces the evaporation of electrode materials. This can interpret why the material with 2 wt%Ni addition has low average arc energy and small fluctuation (see section 3.3 and 3.4). Furthermore, the mass change can also be explained based on this viewpoint. As mentioned above, 2 wt%Ni addition decreases the evaporation of electrode material, the dominant ion for the arc generation transfers from metallic ion to gaseous ion, so the anodic erosion is serious for the material without Ni addition, whereas the cathodic erosion is dominant for the material with 2 wt%Ni addition. For the materials with 4 wt% and 8 wt%Ni additions, more Ni exists on the electrode surface, which was determined by the TEM and EDS analysis. Hence, the increased Ni content on the electrode surface promotes process instability during arc erosion. Especially, the occurrence of high arc energy has larger probability. At this case, large amounts of electrons promote anode evaporation. Meanwhile, high valence Ni cations can gain more kinetic energy from electrical field, resulting in the increasingly serious evaporation and mass transfer.
energy corresponds with a low break-arc-energy, see Fig. 9(c) and (d). This is because the high make-arc-energy causes the temperature rise of electrode surface, and further shortens the lasting time of molten bridge and break-arc-duration. 3.8.2. Thermodynamics To account for the relation of arc erosion with physical properties, Table 3 lists the thermal properties of Ag and Ni [35]. Our previous work [28] verified that arc prefers to generate at the phase with a low work function. As Ag has a lower work function than Ni, Ni addition increases the integral work function of cathode surface for Ag-4 wt. %TiB2 contact material. Hence, it is more difficult for the electron emission, which further delays arc generation. Furthermore, Ni addition increases the number of interface, which promotes arc motion. Our previous work [28] also revealed that arc tends to generate at the Ag/ TiB2 interface though the TiB2 has a larger work function than Ag, and the existence of TiB2 disperses the arc, thus reducing arc erosion. The similar phenomenon was also be reported by Zhang et al. [36]. Thereby, it is thought that Ni addition can better disperse arc, and reduce the concentrated arc erosion. The energy needed from 293 K to 3003 K for Ag and Ni is calculated by Eq. (2).
ΔH =
∫ C (T ) dT + Lm + Lb
(2)
where ΔH represents the enthalpy change, C(T) is the heat capacity, Lm is the heat of fusion and Lb represents the heat of vaporization. The enthalpy change from 293 K to 3003 K is calculated regardless of the change of heat capacity with temperature. The calculated ΔHAg and ΔHNi are 333.98 kJ/mol and 467.13 kJ/mol, respectively. Apparently, Ni has a larger enthalpy change than Ag. It is in accordance with the cohesive energy of Ag (2.95 eV) and Ni (4.44 eV), which refers to the energy that is needed to remove an atom from solid state to freedom state. Generally, arcing causes the phase transformation of Ag and Ni on the anode and cathode surface. Since this is a transient process, it still has the probability for Ni to evaporate during arcing though more energy is needed for Ni evaporation than Ag. 3.8.3. Ionization energy Table 4 lists the ionization energies of Ag and Ni, and the ionization energies of O and N are also included in Table 4 for the discussion purpose. It is interesting to find that the first ionization energy of O or N is higher than the sum of that of Ag and ΔHAg. However, this does not imply that Ag and Ni prefer to be ionized at the initial stage of make arc. The electron emission occurs on the cathode surface before arc generation, and the electron motion promotes the gas ionization since the cathode and anode materials has not yet been vaporized. Once the electrode surface temperature reaches the boiling point, a large number of metallic atoms generate at the interelectrode gap. At this case, Ag Table 4 The ionization energies of Ag and Ni (kJ/mol). Ionization energy
1st
2nd
3rd
4th
5th
6th
7th
8th
Ag Ni O N
731.0 737.1 1313.9 1402.3
2070 1753 3388 2856
3361 3395 5301 4578
– 5300 7469 7475
– 7339 10989 9445
– 10400 13326 53266
– 12800 71330 64360
– 15600 84078 –
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Fig. 12. Schematic diagram of arc erosion mechanism.
Therefore, the mass loss increases for both anode and cathode. Besides these factors, arc dispersion and higher work function of electrode surface also give rise to the dominant gaseous ions in the electric arc, and cathode has a higher mass loss than anode.
of Tailings Resources (2017SKY-WK010) and Special research project of Shaanxi Provincial Department of Education (18JK0722).
4. Conclusions
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