Arc erosion dynamic of island- and skeleton-restricted microstructure evolution modes in Ag–CuO contact materials

Journal of Alloys and Compounds 828 (2020) 154412

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Arc erosion dynamic of island- and skeleton-restricted microstructure evolution modes in AgeCuO contact materials Zhe Wang a, *, Xueshuo Zhang a, Song Jiang b, Yinhu Qu a, Daquan Ou c, Jun Wang a, ** a

School of Materials Science and Engineering, Xi’an Polytechnic University, Xi’an, 710048, PR China School of Resources Engineering, Xi’an University of Architecture and Technology, Xi’an, 710055, PR China c ABB Xinhui Low Voltage Switchgear Company Ltd., Jiangmen, 529100, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 November 2019 Received in revised form 8 February 2020 Accepted 17 February 2020 Available online xxx

Tuning the microstructure to optimize the arc erosion properties of contact materials via the dynamic model is quite important for their application in switches. In this work, we prepared AgeCuOS (with skeleton CuO) and AgeCuOI (with island CuO) materials to investigate the effects of microstructure on arc erosion resistance. In parallel, the three-dimensional models of AgeCuO contacts are reconstructed by phase identification and microstructure analysis, then the arc erosion dynamic of microstructure evolution modes are tracked and studied using computational fluid dynamics (CFD) simulations. Simulation and experiment results both show that the repetitive thermal impact can cause the formation of crater surface and smooth surface in AgeCuOI and AgeCuOS, respectively. The local gap of AgeCuOS contact can work as the driving force to reconstruct CuO skeleton, the newly formed CuO with anisotropic microstructure that can restrict the segregation and evaporation of Ag in molten pool. For the Ag eCuOI contact, the arc erosion merely restructures the initial island of CuO, and the continuous erosion is hardly restricted by this microstructure. Subsequently, we further quantify and evaluate the microstructural continuity of CuO. The results indicate that the CuO microstructure of AgeCuOS shows a lower continuity coefficient (acn ) and a better continuity than that of AgeCuOI during the arc erosion process. © 2020 Elsevier B.V. All rights reserved.

Keywords: Ag-CuO contacts Arc erosion Dynamic microstructure evolution Skeleton reconstruction Continuity coefficient

1. Introduction AgeMeO (silver metal oxide) materials offer a unique set of characteristics, such as low material transfer characteristic, remarkable resistance to welding, good arc erosion resistance, making them well suited for electrical contact applications, especially in the low voltage apparatus [1,2]. AgeMeO contacts including AgeCuO, AgeSnO2, AgeCdO, and AgeZnO materials, meanwhile, AgeCuO material has been more attention to the low voltage switches due to its lower materials transfer characteristic [3]. Particularly, the AgeCuO material with continuous CuO phase can effectively postpone the arc erosion of surface owing to its anisotropy microstructure. However, with advancing arc erosion, the dynamic evolution of CuO microstructure is complicated by the interaction of convection diffusion with flow path. How the evolution and formation of CuO microstructures are influenced by

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Wang), [email protected] (J. Wang). https://doi.org/10.1016/j.jallcom.2020.154412 0925-8388/© 2020 Elsevier B.V. All rights reserved.

turbulence and phase-transition in molten pool that remains unclear. Consequently, the evolution of microstructure plays a key role on the resistance of arc erosion during make-and-break operation for contact materials. The dynamic understanding of the microstructural evolution, originating from metal oxide as well as metal matrix during arc erosion process, has been a focus of substantial research in the last decade [4e6]. To understand the reinforce behavior of CuO deeply, the systematic investigation of arc erosion dynamic in the microstructure of AgeCuO contact is required. Conventional metal oxide (second phase) can hardly restrict the transfer of materials continuously. Thus, arc impact on molten pool throughout cyclic make-and-break operation resulting in the segregation and evaporation of phases, which eventually erode leading to the disintegration of contact surface. More recently, in order to improve the arc erosion resistance of Ag-based contact materials, some studies have investigated the types of addition phases such as WO3 [7], MoO3 [8], Bi2O3 [9], and Cu [10] on the modification of second phase (WC [11], SnO2 [12], CdO [13], ZnO [14], etc.) for the arc erosion characteristics of contacts. They proved that the surface topography of contacts can be regulated by the different addition phases which were added into second phase, and

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increasing the content of addition phase within a certain range could be expected to significantly improve the resistance of arc erosion. Nevertheless, these works are focusing mainly on the influence of addition phase on the arc erosion properties of contacts, only a few have investigated the effect of second phase microstructure on the arc erosion behavior of Ag-based contacts. In fact, the microstructural regulation of second phase can not only improve the arc erosion properties which can hardly be enhanced by different addition phases further, but also reduce the interface effect of excessive addition phase. For example, the AgeSnO2 contact materials with semi-continuous SnO2, which can effectively improve the arc erosion resistance due to its anisotropic microstructure [15]. For the microstructure evolution during arc process, the arc erosion dynamic of contact materials cannot be easily detected by observers, so that the understanding of arc erosion resistance is substantially incomplete. Therefore, in order to observe the arc erosion behavior of contact, using the mathematical computation and simulation has been demonstrated as an effective way to investigate the erosion dynamic of contact materials [16e20]. Zhou and co-workers used a coupled simulating method to estimate the influences of arc current and material properties on the evaporation rate [21]. Subsequently, to study the effect of microstructure on arc erosion, Wang et al. [22] reported the influences of tungsten skeleton and graded structure on the failure mechanism of CuW contact material during the arc erosion experiment. Researchers have also analyzed the inner thermal stresses of molten pool by using finite element method. However, because of the complex interactions between transformation and turbulence of different phases, the arc erosion dynamic of different microstructure evolutions has so far not been undertaken for Ag-based contacts, such that the key details regarding the impact of microstructures on arc erosion resistance in the surface of molten pool, which has yet to be revealed. Our previous experimental studies have found that the AgeCuO contact material with skeletal CuO, which has shown good arc erosion resistance in the vacuum low voltage device [23]. However, the above studies of CuO microstructural evolution were difficult to be tracked by experiments, which leads to the unclear mechanism of arc erosion resistance. Particularly, the dynamic formation of crater morphology in surface and the dynamic reconstruction of microstructure in molten pool should be investigated in details. Therefore, a complete understanding of the different microstructure evolution in the arc erosion process for Agbased contacts, which needs to be addressed both experimentally and theoretically. Motivated by the above issues, having noted the impact of microstructures on the arc erosion process for AgeCuO contacts, the island-restricted microstructure of CuO and the skeleton-restricted microstructure of CuO were prepared by high-energy ball milling following hot-pressing techniques [24e27]. Subsequently, on the basis of microstructural characterization of experiments, the model of AgeCuO contacts with island-CuO and skeleton-CuO that were reconstructed by phase identification and microstructure analysis, respectively. Aiming at understanding the arc erosion dynamic of island- and skeleton-restricted microstructure evolution modes in AgeCuO contact materials, we draw on the CFD (Computational Fluid Dynamics) with UDF (User Defined Function) simulations to obtain detailed insight into the flow evolution of solid, liquid and gas phase within AgeCuO contacts during the process of arc erosion. Especially, the present work focuses on the microstructural reconstruction of CuO in the molten pool, which was analyzed by the calculation of convection diffusion, gas/liquid turbulence and flow phase-transition. Further, to quantitatively understand the effect of island- and skeleton-restricted microstructure on the arc erosion resistance of AgeCuO contacts, the continuity coefficient

(acn ) of CuO phase was calculated and evaluated. Our results reveal previously unreported the dynamic formation of crater morphology in surface and the dynamic reconstruction of CuO skeleton in molten pool during the process of arc erosion, which not only reveal the dynamic evolution of microstructure in the molten pool of contacts, but also provide new perspectives on the microstructure design of contact materials with high arc erosion resistance. 2. Methods 2.1. Experimental methodology The Ag powders were commercially available pure powders that prepared by atomized method (C-wmm, Ltd., Xi’an, China), and the pure CuO powders were prepared by chemical precipitation technique [23]. The Ag with 20 wt% CuO powders (to prepare islandrestricted microstructure) and Ag with 45 wt% CuO powders (to prepare skeleton-restricted microstructure) were high-energy ball milled in the high-energy miller (8000D, SPEX, USA) for 40 min at room temperature, respectively. Where the ball/powder ratio was 20:1, the milling speed was 500 rmp, and the milling media was absolute ethanol. The milled Ag with 20 wt% CuO powders and Ag with 45 wt% CuO powders were first annealed at 673 K for 1.5 h to release milling stress, after that they were compacted under the pressure of 100 MPa into the F35
20 mm billets, respectively. Subsequently, the milled samples of cold-pressure were sintered at 1173 K for 2 h, followed by hot pressed at 973 K under 30 MPa for 40 min. The powders and samples of AgeCuO materials that were determined by an XRD-7000S diffractometer (Shimadzu, Ltd., Kyoto, Japan) using Cu Ka radiation (l ¼ 1.54060 Å) operated at 40 kV and 40 mA. The morphology and microstructures of AgeCuO contacts were characterized by field-emission scanning electron microscopy (SEM, JEOL JSM-7000 F) that equipped with an energy dispersion spectrometer and a backscattered electron detector. The AgeCuO contacts (F 6
3 mm) were cut by EMD (Electrical Discharge Machining) and then fixed into brass holders for 10
103 make-and-break arc erosion testing, then the samples were polished mechanically and put into an in-house developed switching device [28]. The AgeCuO contacts were implemented under the same operating conditions, a circuit voltage of 220 V AC was applied and a discharge current of 20 A was adopted in the vacuum arc testing. The closing and opening frequency was set at 60 times/ min at room temperature, the temperature rise was measured under AC 10 A for 30 min, and the contact force ranges from 10 N to 100 N. 2.2. Numerical model The arc erosion of AgeCuO models was implemented by the ANSYS-FLUENT software (ANSYS Inc., USA) which is a finite element numerical method for tracking convection-diffusion, flow-path and phase-transition [29e33]. The conservation principles of mass, momentum and energy, which were calculated by considering the continuity equation for the incompressible fluid. For the initial model, the meshed strand geometries were generated by the GAMBIT (ANSYS, Inc., USA) which is a preprocessing tool. After functional volume meshes had been produced, the mesh refinement was continued iteratively until numerical flow solutions converged. Then, the ANSYS-FLUENT was used to calculate the arc erosion processes of AgeCuO contacts, which was solved by UDF function (User Defined Function) and SIMPLE algorithm [34,35]. On the basis of the experimental result, the 3D scale models of molten pool for AgeCuO contacts were constructed, and where the surface

Z. Wang et al. / Journal of Alloys and Compounds 828 (2020) 154412

of molten pool can be defined as circular area during this process. In addition, the time step must be determined by the repeatedly empirical simulations for the relatively high accuracy solutions. The implementation of the experimental and simulated procedures was illustrated in Fig. 1. In this simulation process, all calculations of AgeCuO contacts were run within a time step of 0.001s, the maximum iteration per time step was set to 100. The details of species properties [36,37], boundary conditions and model parameters were summarized in Table 1. For the simplicity of the discussion, some parameters were simplified in this case. 3. Results

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Table 1 Species properties, boundary conditions and model parameters. Parameter

CuO

Ag

Gas

Atomic fraction/(%) Melting point/(K) Density/(g$cm3) Specific heat/(J$kg1$K1) Viscosity/(kg$m1$s1) Heat conductivity/(W$m1$K1) Thermal expansion coefficient Minimum temperature/(K) Maximum temperature/(K) Temperature cycles Grid size/(mm) Walls/()

79.5 1600 6.32 564 e 0.20
102 0.93
106 300 1900 10
103 1
103 Adiabatic and

107.9 1234 10.49 283 1.1
103 4.29
102 19.5
106

107.9 e 1.15 1012 1.8
105 2.55
102 3.7
103

non-slip

3.1. Microstructure characterization of experimental and simulated AgeCuO materials For the investigation of the arc erosion process of island- and skeleton-restricted microstructure evolution modes in AgeCuO contacts, the microstructures of AgeCuO materials need to be analyzed with which to reconstruct the 3D models of contacts. Fig. 2 shows the microstructure characterization of experimental and simulated AgeCuO materials, obviously, the AgeCuO with 20 wt% CuO and AgeCuO with 45 wt% CuO materials that show the island and skeleton microstructures of CuO, respectively. Here, we define the AgeCuO with 20 wt% CuO as AgeCuOI materials and the AgeCuO with 45 wt% CuO as AgeCuOS materials. The XRD patterns of AgeCuOI and AgeCuOS materials are shown in Fig. 2c. It is observed that the main phases are Ag and CuO, and no additional intermetallic or compound are detected. In addition, the main peaks for Ag and CuO of AgeCuOS materials move to right compared with that of AgeCuOI materials, and the narrowing of Ag and CuO diffraction peaks when CuO content increased from 20 wt % to 45 wt%. These can be attributed to the increased strain in crystals of Ag and CuO particles after milling process. Fig. 2a and b shows the SEM images of AgeCuOI and AgeCuOS materials, respectively, and the summary composition of material is analyzed by EDS. The results indicate that the light gray zones are Ag-rich zones and the dark gray zones are CuO-rich zones. The CuO-rich zones are uniformly distributed in Ag matrix, and there are no obvious porosities and cracks in them. The CuO-rich zones of AgeCuOS material are interconnected with each other and formed skeleton structures (Fig. 2d), while the CuO-rich zones of AgeCuOI material are separated from each and agglomerated in the form of

isolated structures (Fig. 2a). On the basis of experimental microstructures of AgeCuOI and AgeCuOS materials, the 3D scale models (F 3.5
1.5 mm) of islandand skeleton-restricted microstructures are reconstructed. The initial models of AgeCuO contact materials with homogeneous structure are employed by CFD simulation, and the detailed formation and evolution of AgeCuOI and AgeCuOS materials are calculated by means of the multiphase flow and solidification models. The initial Ag and CuO as liquid phases are restricted in the scale models of contact materials with 1610 K in order to form the isolated and skeletal CuO with homogeneous microstructures. Subsequently, the AgeCuO contact materials with island- and skeleton-restricted microstructures are formed when the temperature drops below 298 K. The initial AgeCuOI and AgeCuOS models of contact materials before arc erosion are shown in Fig. 2e and f, respectively, where the blue regions are Ag-rich phases and gray regions are CuO-rich phases. The AgeCuOS model shows continuous skeleton CuO, while the AgeCuOI model shows isolated island CuO in Ag matrix. It can be seen that the simulation results are in agreement with the experimental observations. Therefore, this AgeCuOS model and AgeCuOI model can be used to evaluate the process of arc erosion as initial models. 3.2. Arc erosion morphology of simulated and experimental AgeCuO contacts In order to study the arc erosion behavior of AgeCuOI and

Fig. 1. Schematic illustration of the experimental and simulated procedures of AgeCuO contact materials.

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Fig. 2. Microstructure characterization of experimental and simulated AgeCuO materials. SEM images of AgeCuOI materials (a), SEM images of AgeCuOS materials (b) and its magnified image (d). XRD spectra of AgeCuO samples (c). The initial 3D models of AgeCuOI contact (e) and AgeCuOS contact (f).

AgeCuOS contacts deeply, the CFD with UDF simulation as a dynamic evolution of microstructure, which is implemented in arc erosion processes. The models of AgeCuOI and AgeCuOS contact materials before arc erosion are carried out based on the experimental results. Nevertheless, the numerical simulations focus mainly on the dynamic microstructural evolution of AgeCuOI and AgeCuOS materials in the arc erosion processes. To track the dynamic evolution of molten pool, especially on the surface of islandand skeleton-restricted microstructures. We merely consider instantaneous temperature changes in the behavior of arc erosion, i.e., the temperature increased and decreased process. Where the one make-and-break operation can be defined as the one-time temperature increased and then decreased simulation. The mass transfer of spattered minute droplet from arc forces is ignored, while the mass loss of contacts by phase transition of liquid Ag and gaseous Ag cannot be ignored. In addition, the temperature difference in the regions of molten pool and vacuum cannot be ignored. We define the distance between the contact and the heat source as 0.1 mm, and the cyclic processes of arc erosion are kept in vacuum. The microstructure of arc erosion is simplified with a focus on the instantaneous temperature increase or decrease in the local area of molten pool. Hence, one make-and-break operation of AgeCuO contact is the rapid temperature change process, i.e., increasing the temperature from 298 K to 1900 K and then cooling the temperature from 1900 K to 298 K. This process is the interaction of multiphase flow with phase transition, which includes the complex turbulence and the phase transition behavior of solidliquid-gas then gas-liquid-solid. Fig. 3a and e shows the 3D models of AgeCuOI and AgeCuOS contacts at the instant of arc erosion, where this behavior is the multiphase transition in temperature rising at 10
103 make-andbreak operation. The gray regions are CuO-rich phases, the blue regions are Ag-rich phases, and the green regions are gaseous Ag phases. It can be seen that the arc erosion of AgeCuOI contact is more serious than that of AgeCuOS contact at 10
103 make-andbreak operation. A large number of Ag phases are evaporated by rapid temperature rise in the surface of AgeCuOI contact, due to its isolated CuO microstructure. The fluid and evaporated Ag are

hardly constrained by the isolated CuO, and the gaseous Ag are easily released because of the large contact area of surface with temperature field. In contrast, the skeletal CuO can provide effective restriction for the flow and evaporation of molten Ag in AgeCuOS contact. To further insights into the microstructure of arc erosive surface in the molten pool of AgeCuO contacts, after 10
103 make-andbreak operation, the morphologies of surface and cross-section are shown in Fig. 3b and f. The surface of AgeCuOI contact presents a relatively rough morphology, this uneven microstructure that includes some craters with a wide range of sizes in the surface. Nevertheless, the AgeCuOS contact with skeleton-restricted CuO shows a relatively smooth surface, and the planeness of AgeCuOS contact is better than that of AgeCuOI contact as seen in the crosssectional models. For comparison, experimentally, Fig. 3d and h shows the surface morphologies of AgeCuOI and AgeCuOS contacts after 10
103 make-and-break testing, respectively. It is clear that some craters are formed in the surface of AgeCuOI contact after arc erosion testing (see the black dotted box of Fig. 3d). Yet, only some voids are found in the surface of AgeCuOS contact, this may be due to the fact that the skeletal CuO can effectively restrain the flow of molten Ag. Subsequently, in order to evaluate the surface characteristic of arc erosion, the transparent area of Fig. 3b and f are cut in the horizontal surface of models, and the magnified image of surface morphologies for experimental and simulated AgeCuO materials are compared in Fig. 3c and g. The surface characteristic of simulation is in agreement with that of experiment, especially focusing on the AgeCuOI surface with crater structures and AgeCuOS surface with smooth structures. The formation of crater structure on contact surface is a common phenomenon, and these excessive structures can lead to the contacts to fail catastrophically before the complete erosion of contacts. However, how the dynamic evolution of surface morphologies after make-and-break testing is influenced by the interplay between turbulence and phase-transition remains unclear. Therefore, the dynamic microstructural evolution of molten pool, which needs to be deeply studied with a focus on the evolution and formation of crater surface and smooth surface.

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Fig. 3. 3D models of AgeCuOI (a) and AgeCuOS (e) contacts at the instant of arc erosion. The surface morphologies of AgeCuOI (b) and AgeCuOS (f) contacts after 10
103 makeand-break simulation. The surface morphologies of AgeCuOI (d) and AgeCuOS (h) contacts after 10
103 make-and-break testing. The cross-section characteristic of simulated result and experimental result for AgeCuOI (c) and AgeCuOS (g) contact.

4. Discussion 4.1. Effect of island- and skeleton-restricted microstructure on concentration evolution Based on the models of AgeCuOI and AgeCuOS contacts, we first discuss the concentration distribution evolution of the profiles in molten pool. The dynamic evolution of Ag and CuO concentrations in the molten pool of AgeCuOI contact at 10
101, 10
102 and 10
103 make-and-break operations are shown in Fig. 4. Where the Ag concentration is exhibited as the color map background, the Xaxis is the CuO concentration and the Y-axis is the vertical direction of contact. Fig. 4a shows the concentration of Ag and CuO at 10
101 make-and-break operation, the CuO and Ag concentration are uniformly distributed in the vertical direction of molten pool. The erosive surface of molten pool is relatively smooth, at this time the arc is hard to erode the contact surface. During temperature rising, the Ag will be melted when temperature reaches its melting point, while CuO phase is still sustain in solid state due to the less times of temperature rise. Thus, the early stage of the arc erosion in molten pool can be restricted by the CuO phases. As arc erosion times increase, after 10
102 make-and-break operation, the surface morphology tends to become rougher

owing to the strong temperature increased and decreased process, as seen in Fig. 4b. The interaction of liquid evaporation and gravity segregation of Ag, which causes the formation of complex flow channels in the molten pool. The CuO is repeatedly remelted and enriched in the surface of contact, such that the CuO phases can no longer provide effective restriction for molten Ag because of the interpenetrating flow-channels formation. Thus, the CuO gradually moves to the surface of molten pool due to its low density, so that the curve of CuO concentration becomes uneven with increasing make-and-break operation. Furthermore, the rough surface can lead to the increase of arc erosive sites, and these bulging structures can be further deteriorated by thermal impact. The Ag and CuO concentrations after 10
103 make-and-break operations are presented in Fig. 4c, it can be seen that the surface roughness continuously increased with an increase in erosive times. The evaporated and declined Ag can lead to the increase of Ag concentration in the bottom of molten pool, and a large number of CuO phases are enriched on the surface. Consequently, the serious deformation has been observed on the surface of molten pool, and the degree of segregation is further increased by thermal impact. More importantly, the more obvious concave and convex surface that are formed by the interaction of evaporated Ag and segregated CuO. Therefore, the island microstructure of AgeCuOI

Fig. 4. The dynamic evolution of Ag and CuO concentrations in the molten pool of AgeCuOI contact at 10
101 (a), 10
102 (b), and 10
103 (c) make-and-break operations.

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contact is quite limited to restrict the evaporated and declined Ag, and this rough surface is easy to be eroded by the repetitive temperature change of make-and-break operation. In parallel, the dynamic evolution of Ag and CuO concentrations in the molten pool of AgeCuOS contact at 10
101, 10
102, and 10
103 make-and-break operations are presented in Fig. 5. For the AgeCuOS contact at 10
101 make-and-break operation, the CuO and Ag concentrations are uniformly distributed in the vertical direction of molten pool (Fig. 5a), which is same as that of AgeCuOI contact material. The molten pool of AgeCuOS contact describes the smooth surface, which indicates that Ag phases can be restricted by the skeletal CuO microstructure at the early stage of arc erosion as well. After 10
102 make-and-break operation, the segregation of Ag and CuO is formed by the increased and decreased temperature as seen in Fig. 5b. However, the surface deformation of AgeCuOS contact is slighter than that of AgeCuOI contact. This result is thought to be due to that the skeleton-restricted microstructure of AgeCuOS contact prevents the Ag phases to be evaporated in the surface of molten pool. This skeleton CuO with multiple directional scales and anisotropy, which may also play a role in restricting the flow channels of ascended and declined Ag, thus the arc erosion of its surface is not as serious as AgeCuOI contact. During the later stage of arc erosion, after 10
103 make-andbreak operation as seen in Fig. 5c, the segregation of Ag and CuO is deteriorated further under the repeated impact of temperature change. Remarkably, based on the analysis of slope of CuO concentration curves, the slope of 10
103 make-and-break operation that is little change in that of 10
102 make-and-break operation. The slope of AgeCuOS contact (0.217, see the purple line of Fig. 5c) is greater than that of AgeCuOI contact (0.143, see the purple line of Fig. 4c), indicating that the uniformity of AgeCuOS contact is better than that of AgeCuOI contact in the vertical section. As a matter of fact, the skeleton-restricted microstructure of CuO may play a role in constraining the liquid and gaseous Ag and slowing down the arc erosion. Additionally, the continuous CuO may be rearranged by the smelting of increased and decreased temperature, so that Ag phases can be separated by the newly formed CuO skeleton. Compared with AgeCuOI contact, the slighter surface deformation can be seen in the molten pool of AgeCuOS contact in this stage. Hence, we suggest that the microstructure of CuO is the important factor to slow arc erosion of AgeCuO contact. In order to study the anti-arc erosion dynamic of island- and skeleton-restricted microstructure, the evolutions of erosive surface in AgeCuOI and AgeCuOS contacts, which needs to be investigated in detail.

4.2. Dynamic micromorphology evolution of erosive surface It is a well-known fact that the crater morphology can cause the rapidly anti-erosion failure of contact materials (Fig. 3d) [38,39]. Based on the analysis of the concentration distribution of Figs. 4 and 5, we focus on the region of Path I and Path II in order to explain the dynamic evolution of surface morphology, especially on the formation mechanism of crater in AgeCuO contacts. However, the dynamic evolution process of contact surface cannot be easily detected by observers, the CFD simulation can be effectively used for tracking the evolution of surface morphology. The dynamic evolution of surface during the 10
102 makeand-break operation for AgeCuOI contact is presented in Fig. 6, particularly the crater formation will be focused upon. Before the arc erosion of AgeCuOI contact, the surface shows a relatively flat morphology. The height of surface is relatively homogeneous (Fig. 6a), at this time the Ag phase shows a relatively smooth surface as seen in Fig. 6d. As temperature rapidly increases, the Ag phase is melted by the action of temperature gradient, such that the complex flow channels can be formed in the molten pool. The Ag phase shows the trend of escape out of surface after being evaporated by instantaneous high temperature (Fig. 6e), and the obvious bulge can be found in the surface at this moment as seen in Fig. 6b. Subsequently, a part of gaseous Ag is able to break away from the restriction of molten pool surface when the evaporative power is beyond the surface tension (Fig. 6f), and the crater morphology can be formed on the surface of molten pool thanks to the mass transfer of Ag phases (Fig. 6c). The formation of crater can be summarized that there is not enough time for Ag phase to distribute evenly on the surface by the surface tension, because of the instantaneous cooling after break operation. It can be seen that the depth of crater is greater than 0.09 mm, this surface morphology is disadvantageous to the resistance of arc erosion for AgeCuOI contact. This is due to the fact that the crater region will preferentially suffer from repeated arc erosion during make-and-break operation. Therefore, the surface deformation will become more and more serious in AgeCuOI contact. For comparison, the dynamic evolution of surface morphology into Path II of Fig. 5 can be seen in Fig. 7. Remarkably, the surface evolution of AgeCuOS contact is very different from that of AgeCuOI contact. For the same surface morphology as AgeCuOI contact, the relatively flat surface and smooth Ag phase can be seen in AgeCuOS contact before the arc erosion of 10
102 make-andbreak operation (Fig. 7a and d). When the temperature increases instantaneously, the surface can be lifted up by the molten and fluid Ag as shown in Fig. 7b. Subsequently, the Ag begins to evaporate when the temperature exceeds its melting point, thus the Ag

Fig. 5. The dynamic evolution of Ag and CuO concentrations in the molten pool of AgeCuOS contact at 10
101 (a), 10
102 (b), and 10
103 (c) make-and-break operations.

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Fig. 6. The dynamic evolution of surface morphology (aec) and Ag phase (def) in 10
102 make-and-break operation for AgeCuOI contact.

Fig. 7. The dynamic evolution of surface morphology (aec) and Ag phase (def) in 10
102 make-and-break operation for AgeCuOS contact.

phases of surface show the trend of escape out of molten pool surface (Fig. 7e). Yet, the flowing gaseous Ag and liquid Ag are obviously restricted by the skeletal CuO microstructure, so that the flow paths of Ag can be extended by the flow channels with multiple directional scales and anisotropy. Thus, the evaporation of Ag is not serious in AgeCuOS contact compared with that in AgeCuOI

contact. For the ultimate stage of this make-and-break operation, the flow paths of Ag are extended and the escape time of Ag from surface that are reduced during the process of temperature decreasing. In addition, the evaporation capacity of Ag in AgeCuOS contact is relatively weaker than in AgeCuOI contact, this is due to the fact that a part of Ag is not enough time to flow out of surface of

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molten pool and to evaporate off (Fig. 7f). After this make-andbreak operation, there is no obvious deformation and crater can be observed in surface as seen in Fig. 7c. Moreover, the surface morphology of AgeCuOS contact can effectively restrict the arc erosion of molten pool because of the delayed failure of surface. Based on above the dynamic microstructural evolution of arc erosive surface for AgeCuOS and AgeCuOI contact analyses, it can be noted that the dynamic evolution of surface morphology is impacted by the CuO microstructures. However, what is the effect mechanism of island- and skeleton-CuO microstructure on the resistance of arc erosion? That need to be discussed deeply.

4.3. Erosion resistance mechanism of island- and skeletonrestricted microstructure As aforementioned, the dynamic micromorphology evolution of erosive surface is strongly influenced by the microstructure of CuO in AgeCuO contacts. In order to study the erosion resistance mechanism of island- and skeleton-restricted CuO, the dynamic evolution of CuO is emphatically focused on the simulation of erosion process. Here, the 3D model of CuO skeleton is selected to be a critical aspect in the resistance of arc erosion for AgeCuOS contact, especially the evolution of skeleton-restricted microstructure in molten pool that needs to be addressed in detail. Fig. 8a shows the AgeCuOS model before arc erosion, where the 3D skeleton of CuO and the vertical section of Ag concentration map are exhibited. As mentioned in section 4.2, the skeletonrestricted microstructure of CuO can provide effective restriction for the evaporation of molten Ag, due to its microstructure with anisotropy and continuity. Moreover, this CuO microstructure has excellent arc erosion resistance for the repetitive thermal impact of make-and-break operations. Therefore, we focus on the dynamic

evolution of CuO and on the restrictive effect of CuO in the molten pool, as a contrast of experiment and simulation. Fig. 8b and c shows the 3D morphology of CuO and the concentration map of Ag in vertical section before and after make-and-break operations, respectively. It can be seen that the CuO presents a continuous skeleton with interconnection network before make-and-break operation, meanwhile, the Ag phase shows a homogenous distribution in the gap of CuO skeleton. At this time, the surface of molten pool shows a lower fluctuation and a smoother interface as seen in Zone III of Fig. 8b. After 10
103 make-and-break operation as seen in Zone IV of Fig. 8c, the continuity of CuO is enhanced by the repetitive reconstruction of CuO, and the Ag phases are enriched in local gap of CuO skeleton (see in the concentration map of Ag). Finally, there are no serious concave and convex of surface that formed in the molten pool of AgeCuOS contact (see in black dotted box). For the experiment of arc erosion, the vertical morphology of AgeCuOS contact after 10
103 make-and-break operation is shown in Fig. 8d. The significant difference can be seen in the arc erosion region and no-arc erosion region. In the vertical section of no-arc erosion region, the CuO exhibits a continuous and uniform microstructure (see in Zone I of Fig. 8d). However, after 10
103 make-and-break experimental operation, the CuO with higher continuity is formed by skeleton reconstruction due to the repetitive thermal impact and, more importantly, the Ag phases are obviously restricted into CuO skeleton (see in Zone II of Fig. 8d). Moreover, the arc erosion region shows a relatively smooth surface as seen in SEM photo, thus the CuO morphology of simulation is in good agreement with that of experiment. For analysis of simulation and experiment results, these can be suggested that the reconstructed CuO skeleton can enhance the continuity of CuO and can promote the restriction of Ag into CuO network.

Fig. 8. 3D structure of CuO in the molten pool of AgeCuOS contact (a). 3D morphology of CuO and the concentration map of Ag in vertical section before (b) and after (c) make-andbreak operations. SEM image of vertical morphology after 10
103 make-and-break operation (d). The evolution of CuO in experiment and simulation before (e) and after (f) makeand-break operations.

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However, how the newly formed CuO skeleton with higher continuity is formed, which remains need to be analyzed deeply. In order to focus attention on the experiment of Zone I and Zone II and on simulation of Zone III and Zone IV, further, the evolution of CuO can be compared and discussed as seen in Fig. 8e and f. The CuO shows a continuous skeleton structure before arc erosion (Fig. 8e), and this CuO skeleton can become more continuity after arc erosion (Fig. 8f). It can be concluded that the local gap of Ag can work as the driving force to reconstruct CuO skeleton. Therefore, the arc erosion merely restructures the initial structure of CuO, but does not disintegrate the connectivity of CuO, i.e., the process of skeleton reconstruction. In addition, a part of agglomerated Ag and evaporated Ag provide the reconstructed space of CuO, so that this process can promote the formation of new skeleton and can enhance the continuity of CuO (see in Fig. 8f). In parallel, the evolution of CuO is also tracked in the process of arc erosion for AgeCuOI contact. In AgeCuOI model before arc erosion, the 3D morphology of CuO microstructure and the vertical section of Ag concentration map are showed in Fig. 9a. Obviously, different from CuO in AgeCuOS model, AgeCuOI model shows the isolated island CuO with a poor continuity. This CuO structure can hardly provide necessary restrictions for the evaporation and flow of Ag based on the above researches. In order to reveal the poor restrictive behavior of CuO in AgeCuOI contact, similarly, the dynamic evolution of CuO in experiment and simulation process that can be contrasted and analyzed in AgeCuOI model. Before arc erosion operation, the CuO is dispersed and disconnected in Ag matrix, and the molten pool shows a relatively smooth surface as seen in the Zone V of Fig. 9b. Subsequently, the isolated CuO is repeatedly remelted by make-and-break operation with thermal impact, so that a part of CuO is gradually enriched and that the bigger one is agglomerated in matrix by the action of flow-diffusion

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and phase-transition. Thus, the flow of liquid and gas in molten pool that leads to the formation of uneven surface as seen in the black dotted box of Zone VI. Further experiments will be conducted to study the formation of isolated CuO in more details, the vertical morphology of AgeCuOI contact after 10
103 make-and-break operation is shown in Fig. 9d. Comparison between arc erosion region and no-arc erosion region for experimental results, it can be seen that the isolated CuO is dispersed in Ag matrix before make-and-break operation, while after 10
103 make-and-break operation, a part of small CuO is agglomerated to form big CuO. Moreover, the local inhomogeneity of CuO can be seen in the arc erosion region of Zone VI in Fig. 9b. Comparison between Figs. 8b and 9b, the AgeCuOI contact shows a more uneven surface and a deeper erosive zone than AgeCuOS contact. In order to clarify the formation of agglomerated CuO, the evolution process of CuO in experiment and simulation is extracted. As seen in Fig. 9e and f, the CuO shows the small and even phase before arc erosion, and shows the big and uneven phase after arc erosion. It can be concluded that the repeated thermal impact can destroy the CuO structure. Under the action of repeated remelting and surface tension, these small CuO phases are fused and agglomerated to form a big CuO phase, while the continuous skeleton CuO cannot be formed by these round-like or ellipse-like CuO phases. Subsequently, these newly formed CuO phases can hardly provide necessary restrictions for the evaporation and flow of Ag during repeatedly remelting processes, thus the local segregation and disintegrate of CuO are aggravated by this process. In summary, the CuO structures play the key role in the formation of surface morphology in molten pool, which can strongly influence the arc erosion resistance of AgeCuO contact, i.e., the more CuO continuity the better erosion resistance. However, how to evaluate

Fig. 9. 3D structure of CuO in the molten pool of AgeCuOI contact (a). 3D morphology of CuO and the concentration map of Ag in vertical section before (b) and after (c) make-andbreak operations. SEM image of vertical morphology after 10
103 make-and-break operation (d). The evolution of CuO in experiment and simulation before (e) and after (f) makeand-break operations.

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the continuity of CuO needs to be quantified by mathematical model further.

4.4. Continuity evaluation of island- and skeleton-restricted microstructure For this purpose, we use the MATLAB image analysis method (MATLAB, Software, USA) to quantify the continuity of CuO structure in AgeCuO contact [24,40,41], and the acn is defined as continuity coefficient of CuO that can be described by the following equation:

acn ¼

m h .  i X 4pSm L2m , ðsd = dÞ

, m

(1)

i¼1

where sd is the standard deviation in the mean nearest-neighbor distance, d is the average of mean near-neighbor distances for all CuO phases, S and L are area and perimeter of a single CuO phase, and m is the ID of a single CuO phase. The value of acn is closer to zero, the more well-continuity of CuO structure shows. Additionally, the 4pSm =L2m is the roundness of phases. To obtain the 4pS= L2 and sd =d of CuO in the AgeCuO contact of experiment and simulation, the roundness and near-neighbor distance of CuO in vertical section that can be identified and calculated by MATLAB. The roundness of CuO phases for simulation and experiment results before arc erosion is shown in Fig. 10. In experimental result as seen in Fig. 10a, the isolated CuO of AgeCuOI contact shows the relatively high value of roundness, and the value of acn is 0.441 that presents a worse continuity of CuO structure. Compared with the experimental results, the simulated AgeCuOI contact has a slightly lower acn value (0.417) due to its coarse CuO structure (Fig. 10b). This is because that the surface passivation of CuO phase and the energy consumption of CuO agglomeration, which are ignored in the process of simulation. Furthermore, for the acn value of AgeCuOS contact before arc erosion, the simulation and experiment results are also displayed and compared. Fig. 10c and d shows the roundness of AgeCuOS contact, and the acn value of simulation (0.322) is also slightly lower than that of experiment (0.348). These results indicated that the continuity coefficient of CuO can be

quantified effectively, and the CuO phase of AgeCuOS contact has better continuity than that of AgeCuOI contact before arc erosion. Subsequently, we calculated and analyzed the acn value of makeand-break operation in vertical section. The acn values decreased with increasing make-and-break operation for simulation and experiment, respectively, which are exhibited in Fig. 11a and b. Fig. 11a shows the acn value of AgeCuOI contact, the acn curve exhibits a rapid decrease and then tends to increase slightly. This is due to the fact that the initial CuO phases are gradually agglomerated by repeatedly arc action, thus its continuity is increased and the acn value is decreased. When the make-and-break operation advances to about 6
103 times, the agglomerated and irregular CuO phase that gradually forms the round-like or ellipse-like CuO phase, due to the action of repeated remelting and surface tension. Therefore, the distribution of CuO becomes uneven in molten pool, and the acn value slightly increases after 6
103 make-and-break operation. Subsequently, due to the fact that the experimental acn value cannot be continuously tracked in the process of arc erosion for one given contact, we thus only calculate the initial and finished acn value within 10
103 make-and-break operation as seen in the red star of Fig. 11a. The acn value of experiment is close to that of simulation, but slightly higher than that of simulation. It can be concluded that the surface passivation of CuO phase and the energy consumption of CuO agglomeration that are ignored in the process of simulation. Therefore, after 10
103 make-and-break operation, the acn value of simulation is less than that of experiment due to its coarse CuO structure. The acn values of AgeCuOS contact of simulation and experiment are shown in Fig. 11b. Unlike the acn value of AgeCuOI contact, the acn value of AgeCuOS contact that is decreased continuously with increasing make-and-break operation, the curve decreases almost linearly and then decreases slowly with increasing make-and-break operation. This is due to the fact that the repetitive remelting and thermal impact of make-and-break operation that can cause the skeleton reconstruction of CuO. In this process, the skeleton reconstruction is accompanied by the flow and evaporation of Ag, which leads to the formation of gap in molten pool, and this process can provide the driving force to reconstruct CuO skeleton. This anisotropy of CuO skeleton can be further improved by these gaps, thus the continuity is increased and the acn value is decreased.

Fig. 10. The roundness of CuO phases of simulation and experiment results before arc erosion for AgeCuOI (a and b) and AgeCuOI (c and d) contacts.

Z. Wang et al. / Journal of Alloys and Compounds 828 (2020) 154412

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Fig. 11. The acn values of AgeCuOI (a) and AgeCuOI (b) contacts with increasing make-and-break operation. The acn values of AgeCuOI (c) and AgeCuOI (d) contacts with increasing depth in the vertical section of molten pool.

However, the newly formed CuO skeleton with high anisotropy that can restrict Ag evaporation and flow, thus the skeleton reconstructed rate of CuO is gradually decreased. Therefore, in the later stage of arc erosion, the acn value is decreased slowly with increasing make-and-break operation. Subsequently, the acn value of experiment in AgeCuOS contact is calculated as seen in the red star of Fig. 11b. Similarly, the acn value of simulation is also slightly lower than that of experiment before arc erosion. The surface tension has a little effect on the skeleton reconstruction of CuO in AgeCuOS contact, thus the acn value of experiment is closer to that of simulation compared with AgeCuOI contact. For further assessment of CuO continuity in the vertical section of molten pool, the acn value of CuO after 10
103 make-and-break operation is calculated. Where the molten pool of simulation is divided into five zones, i.e., Z1 to Z5, and three zones of experiments are selected form the five zones, as shown in Fig. 11c and d. Fig. 11c shows the acn curve of AgeCuOI contact with decreasing depth in the vertical section of molten pool. It can be seen that the surface of molten pool exhibits a smaller acn value (about 0.22) and a better continuity of CuO phases, because of the uneven distribution and agglomeration of CuO in molten pool surface. Subsequently, the acn value is quickly increased with decreasing depth of molten pool. In the bottom of molten pool, the acn reaches a maximum value owing to the fewer effect of repetitive remelting and thermal impact, indicating that the CuO phase shows a worse continuity. Moreover, the acn curve of experiment shows a similar trend compared with that of simulation, as seen in the red star of Fig. 11c. The acn value of AgeCuOS contact in the vertical section of molten pool that is also identified and calculated, the acn value of

AgeCuOS contact (about 0.11) is smaller than that of AgeCuOI contact in the surface of molten pool. It can be concluded that the repetitive remelting and thermal impact, which can effectively influence the skeleton reconstruction of CuO. For the acn curve of simulation in AgeCuOS contact, the acn curve is slowly increased with decreasing depth of molten pool, due to the fact that the arc erosion can cause the reconstruction of skeleton in multiple directions, which increases the acn value slowly. Furthermore, the curve trend of experiment is also in agreement with that of simulation as seen in the red star of Fig. 11d. Based on the evaluation and calculation of continuity in both AgeCuOS and AgeCuOI contacts, the skeletal CuO of AgeCuOS contact not only shows a smaller acn value and a better continuity before arc erosion, but also promotes the skeleton reconstruction with multiple directions after arc erosion. In the process of arc erosion, the skeletal CuO can effectively decrease the acn value, and can further improve the arc erosion resistance of AgeCuO contact. From the results and discussions, as presented above, one can observe that the dynamic micromorphology evolution of erosive surface is strongly influenced by the microstructure of CuO in AgeCuO contacts. The skeleton-restricted CuO with continuous microstructure, which can effectively improve the arc erosion resistance for AgeCuO contact. In addition, the present study is based on the simulation and experiment results for the AgeCuO contact materials, but the implications of this work may not limit to this single bulk material. This arc erosion behavior is also seen in other systems, such as the electro-explosive coatings of Ag-based systems [42e44]. If their microstructures of contacts are similar to these AgeCuO contacts, the above methods and routes, in

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principle, will continue to be valid. Therefore, this work may be expected to provide new perspectives on the microstructure design of contact materials with high arc erosion resistance. 5. Conclusion In this paper, we have designed, prepared and constructed the AgeCuO contacts and their 3D scale models, then the arc erosion dynamic of AgeCuO contact materials are systematically studied to reveal the microstructure evolution modes, namely island- and skeleton-restricted CuO evolution. Based on the experimental and simulated characterizations, we focus mainly on the dynamic evolution of microstructural reconstruction under arc erosion in molten pool, the key findings of the present work are summarized below. (1) The arc erosion dynamic of CuO microstructures can be divided into two different evolution modes in the AgeCuOS and AgeCuOI contacts, which exhibit the skeleton-restricted and the island-restricted microstructure after arc erosion. Moreover, the simulated and experimental results both indicated that AgeCuOI contact and AgeCuOS contact show strong and mild microstructure evolution, respectively, in the surface of molten pool. (2) As a result of repetitive thermal impact of make-and-break operations, the crater morphology can be formed in the surface of AgeCuOI contact, while the AgeCuOS contact still shows a relatively smooth surface after arc erosion. It can be attributed to the strong interactions of flowing Ag and evaporating Ag in the surface of molten pool, which can frequently occur in AgeCuOI contact during the make-andbreak operations. (3) The microstructural evolution of CuO plays a dominant role in the formation of surface morphology in molten pool, which can strongly influence the arc erosion resistance of AgeCuO contacts. Under the action of skeletal CuO reconstruction during the arc erosion process, the AgeCuOS contact shows smoother surface of molten pool and better resistance of arc erosion than AgeCuOI contact due to its anisotropic CuO microstructure. (4) The continuity coefficient (acn ) can be used to quantify and evaluate the microstructural continuity of CuO, the acn value of AgeCuOS and AgeCuOI contacts are decreased after arc erosion, and the acn value of CuO for AgeCuOS contact decreases more rapidly than that for AgeCuOI contact with advancing arc erosion. Thus, the CuO of AgeCuOS contact shows better continuity than that of AgeCuOI contact during the arc erosion process. The present work opens a new path into the microstructure effects on arc erosion dynamic, which helps providing guidance for the microstructural design of Ag-based contacts.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Zhe Wang: Writing - original draft. Xueshuo Zhang: Data curation. Song Jiang: Formal analysis. Yinhu Qu: Data curation. Daquan Ou: Software. Jun Wang: Writing - original draft.

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