Arc Erosion Properties of the Ag-Cr₂AlC Contact Material

Abstract

This study investigates the arc performance of Ag-Cr₂AlC composite materials. Spark plasma sintering method was employed to prepare the Ag-Cr₂AlC composite material. A self-made arc erosion device was utilized to erode the material with different times of arc. The surface of the material was categorized into three distinct areas: the eroded center area, the eroded edge area, and the heat-affected area. After one time of arc erosion, the material exhibits a relatively flat surface with a small erosion area. However, after one hundred arc erosions, the eroded area has significantly increased, accompanied by numerous splashes, protrusions, and pores. The action of the arc leads to the decomposition and oxidation of the Ag-Cr₂AlC composite material, resulting in the formation of Ag₂O, Al₂O₃, and Cr₂O₃ on the surface. During the process of 100 arc erosions, the breakdown current value remains relatively stable, ranging from 20 to 35 A. From the first to the 70th arc erosion, the breakdown strength consistently varies between 3 × 10⁶ V/m and 6 × 10⁶ V/m. Subsequently, there is an observed enhancement in breakdown strength, leading to the appearance of ageing. These findings establish a theoretical foundation for the application of silver-based electrical contact materials.

1. Introduction

Electric contact materials are crucial components in various electrical devices, particularly in switches, relays, and connectors, where they facilitate the flow of electrical current. The high temperature generated by the arc can melt the contact, damage the insulation material, and potentially ignite the entire electrical equipment [1,2]. The presence of the arc prolongs the time required to open the switch of electrical appliances and exacerbates the consequences of short-circuit failures within the power system [3]. The generation and maintenance of arcs involves a series of complex processes that span multiple disciplines, including physics, chemistry, electrical engineering, thermodynamics, and mechanics. Therefore, it is essential to evaluate the damage to the electric contact material caused by the arc in advance [4].

Common electric contact materials primarily consist of silver-based compounds. Silver is recognized as the most conductive metal, possessing low contact resistance and excellent antioxidant properties [5]. Consequently, silver is extensively utilized in electrical contact components that require high precision and reliability, such as high-precision relays and small switches [6]. Although silver-based electrical contact materials are widely utilized in electrical engineering, several limitations persist. Firstly, there is insufficient resistance to arc erosion; under conditions of high current or frequent switching operations, these materials are vulnerable to arc erosion, which results in surface roughness, increased contact resistance, and ultimately compromises the reliability and lifespan of the equipment. Secondly, the anti-wear performance is limited; over prolonged use, electrical contact materials experience mechanical wear, leading to a reduction in contact area and an increase in contact resistance, thereby adversely affecting equipment performance. Lastly, poor high-temperature stability is a concern; in elevated temperature environments, silver-based materials are susceptible to oxidation and softening, which result in a decline in material performance. To mitigate the disadvantages associated with sterling silver, small amounts of other metallic elements are often incorporated into the silver, resulting in various alloys such as AgCu alloy, AgNi alloy, AgW alloy, and Ag-based composite materials [7,8,9,10,11]. MAX phases are a class of nanolaminated ternary ceramics characterized by the general chemical formula M_n+1AX_n, where M represents an early transition metal (e.g., Ti, V, Cr), A denotes an A-group element (e.g., Al, Si), X signifies carbon or nitrogen, and n typically takes on values of 1, 2, or 3. These materials exhibit a distinctive combination of metallic and ceramic properties, including high hardness, good electrical and thermal conductivity, excellent thermal shock resistance, and machinability. Such characteristics render MAX phases promising candidates for applications in high-temperature structural materials, electrode materials, and components for nuclear reactors [12,13]. Therefore, Ag-MAX composite materials are expected to be used in the field of brush and electric contact [14,15,16,17].

Considering the limitations of silver-based electrical contact materials and the advantages of MAX-phase materials, conducting an in-depth study of Ag-Cr₂AlC electrical contact materials holds significant theoretical and practical importance. The Ag-Cr₂AlC composite material was prepared by spark plasma sintering (SPS). The material was eroded with one and 100 times of arc erosions to explore the impact of arc number on the performance of the Ag-Cr₂AlC composite material, including its erosion area, breakdown strength, morphologies and compositions. The findings of this research aim to expand the applications of Ag-based contact materials.

2. Experimental Procedures

2.1. Sample Preparation

Ag powder (purity > 99.9%, 200 mesh, Beijing Nonferrous Metals Research Institute, Beijing, China) and Cr₂AlC powder (purity > 98%, 200 mesh, Laizhou Kai Kai Ceramic Materials Company Ltd., Laizhou, China) were mechanically mixed for two hours using a mixer, with a Cr₂AlC volume fraction of 20%. The mixed powder was then placed into a spark plasma sintering furnace (SPS-3T-3-MIN, Shanghai Chenhua Science Technology Corp., Ltd., Shanghai, China), where it was heated to 700 °C at an elevated rate of 100 °C/min and held at that temperature for 30 min. Following this, the samples were cooled in the furnace before being removed for grinding and polishing. The Archimedes’ method was employed to determine the material densities. A microhardness tester (YZHV-IZP, Shanghai Aolong Xingdi Testing Instrument Co., Ltd., Shanghai, China) was employed to assess the hardness of Ag-Cr₂AlC composites. The microhardness was evaluated by applying a positive quadrangular conical diamond indenter, with an angle of 136° between the top two opposite faces, onto the surface of the material under test. A specific test force was applied, maintained for a designated duration, and subsequently released to measure the diagonal length of the resulting indentation. The load utilized for this assessment was 100 g. The microhardness of the Ag-Cr₂AlC composites was calculated in accordance with Equation (1) following the release of the load after a 30 s hold.

$$\mathrm{HV}={\displaystyle \frac{0.1891F}{d^2}}$$

(1)

where HV is the microhardness, F is the test force (N), and d is the mean value of the diagonal length of the indentation (mm). Compositional analyses of the sintered bulk materials were carried out using X-ray diffractometer (XRD, SmartLabSE, Tokyo, Japan). The distribution of Ag and Cr₂AlC materials was assessed using optical microscopy (LIOO M110T, Beijing Jinghao Yongcheng Trading Co., Beijing, China). Additionally, a thermal field emission scanning electron microscope (TFE-SEM, ZEISS Gemini 500, Carl Zeiss AG, Oberkochen, Germany), equipped with energy dispersive X-ray spectroscopy (EDS, Oxford Aztec UltimMax100, Oxford Instruments, London, UK), was employed to analyze and document the morphology and composition of the Ag-Cr₂AlC composites.

2.2. Arc Erosion Test

Using a self-made arc erosion device, the loading voltage of the two electrodes was set to 7 kV. Here, Ag-Cr₂AlC composite was used as the cathode and W-rod with tip as the anode, and the voltage of the two electrodes was set to be 7 kV, and subsequently the cathode and anode were allowed to move slowly relative to each other. Under the influence of a strongly nonuniform field, electron avalanche and positive streamer were generated, causing the originally insulated air to conduct and ultimately leading to an electric arc. This arc resulted in erosion damage to the cathode. Arc erosion experiments were carried out 1 and 100 times to compare the effect of number of discharges on the arc erosion properties of Ag-Cr₂AlC composites. The composition and morphology of the surface after arc erosion were analyzed. Additionally, a digital oscilloscope (GA1102CAL) was utilized to record the breakdown current and arc life for 1 and 100 cycles, after which the breakdown strength was calculated.

The surface appearance analysis following arc erosion was conducted using a thermal field emission scanning electron microscope (TFE-SEM, ZEISS Gemini 500, Carl Zeiss AG, Oberkochen, Germany), which is equipped with energy dispersive X-ray spectroscopy (EDS, Oxford Aztec Ul-timMax100, Oxford Instruments, London, UK) and three-dimensional laser scanning morphology (3D LSCM, Keyence VK-X250, Osaca, Japan). Composition analysis employed a combination of EDS and Raman spectra (Lab RAM-HR, HORIBA, Kyoto, Japan), which was used to detect the type of bonding over the wavenumber range of 100 to 1200 cm⁻¹. The Raman spectra were excited with a Nd: YAG laser, with an incident power of 4 mW.

3. Results and Discussion

3.1. Physical Properties

The composition of the sintered sample was examined, as illustrated in Figure 1. Peaks corresponding to Ag (PDF#04-0783) and Cr₂AlC (PDF#29-0017) materials were identified in Figure 1a. Figure 1b presents a microscopic image of the Ag-Cr₂AlC composite material, revealing the presence of two distinct phases. The dark grey phase is uniformly distributed across the surface of the light grey phase. A point-scanning analysis was conducted on the dark grey phase at position 1 (green dot in Figure 1b), with the results displayed in Figure 1c. From Figure 1c, it is evident that this phase corresponds to the Cr₂AlC material.

The densities of Ag-20 vol.% Cr₂AlC composites were determined to be 93.3% using the Archimedes’ method. Upon applying a load of 0.98 N to the surface of Ag-Cr₂AlC with a microhardness tester, the resulting formation is illustrated in Figure 2. The diagonal length of the indentation measured by the microhardness tester is presented in

Table 1. Microhardness measurement values corresponding to Figure 2.

Test Point	d1 (μm)	d2 (μm)	d (μm)	Hardness (HV)
(a)	55.346	55.994	55.67	59.8
(b)	55.188	55.404	55.296	60.6
(c)	54.972	54.109	54.5405	62.3

, and the average microhardness, calculated from three measurements, was found to be 60.9 HV. In Table 1, d1 and d2 represent the diagonal lengths of the indentations, and d represents the average value of the diagonal lengths. The indentation images suggest non-uniform deformation around the particles. This non-uniformity is indeed related to the heterogeneous microstructure of the Ag-Cr₂AlC composite material. The composite consists of a soft Ag matrix and hard Cr₂AlC particles, which exhibit significantly different mechanical properties. During indentation the soft Ag matrix undergoes plastic deformation more readily, while the hard Cr₂AlC particles resist deformation, resulting in localized stress concentrations and non-uniform deformation patterns around the particles. This behavior aligns with previous studies on metal-ceramic composites, where the hardness mismatch between the matrix and reinforcement phases leads to similar deformation characteristics.

3.2. Arc Erosion Performance

The Ag-Cr₂AlC composites were subjected to one and 100 times of arc erosion at a load voltage of 7 kV, with the eroded morphology illustrated in Figure 3a and Figure 3b, respectively. As observed in Figure 3a, the eroded shape exhibits a rounded shape. Based on the varying colors following erosion, the eroded area can be categorized into three distinct regions: the eroded center area, the eroded edge area, and the heat-affected area, as depicted in Figure 3a. The eroded center area corresponds to the region within circle 1, while the eroded edge area is defined as the space enclosed between circle 1 and circle 2. The heat-affected area is located between circle 2 and circle 3. In Figure 3b, the same divisions are applied; however, due to the limited magnification of the device, the heat-affected zone is not depicted within the image. The software equipped with the 3D LSCM was utilized to measure the arc erosion area. The erosion area of Ag-Cr₂AlC composites after one arc erosion is approximately 1.32 × 10⁶ μm², while the erosion area after 100 arc erosions is approximately 3.84 × 10⁶ μm². A comparison of Figure 3a,b reveals that, after 100 times of arc erosion, the eroded center area is significantly larger than that observed after a single erosion. This trend is also evident in both the eroded edge area and the heat-affected area. The morphology of the eroded center area is further enlarged in Figure 3c and Figure 3d, respectively. The morphology appears relatively flat, with only the edge of the eroded center exhibiting a morphology of protrusion, as shown by arrow in Figure 3c. Conversely, Figure 3d displays a topography characterized by numerous protrusions, as indicated by the arrows.

The micrographs of the surface of Ag-Cr₂AlC composites after one and 100 times of arc erosion are shown in Figure 4a,b and Figure 4c,d, respectively. In Figure 4a, the eroded morphology is relatively flat, and basically the distribution of the matrix and reinforcement phases can be recognized. The eroded edges are narrower, as indicated by the arrows in Figure 4b. The materials in rectangles 1 and 2 were analyzed through EDS, as shown in Figure 5a,b. From Figure 5a, it is evident that rectangle 1 primarily contains the elements Ag and O, along with a minor presence of Al. Meanwhile, rectangle 2 predominantly consists of the elements Cr, Al, and C, with a small amount of Ag. This suggests that after one arc erosion, only a limited quantity of Ag oxidized on the surface, and only a small fraction of the Cr₂AlC phase decomposed. Small amounts of elemental Si and Na were also detected in both EDS results, which may be impurities left over from polishing and cleaning. After 100 times of arc erosion, the eroded center area depicted in the local magnification of Figure 4c, material melting and solidification can be observed. Focusing on the eroded edge area, it is observed that the width of the edge is greater than that observed after one time of arc erosion, measuring over 100 µm. There are many spatter particles of different sizes outside the eroded edge area, as the area shown by the red circle in Figure 4d, where the spatter particles are larger. From the eroded center towards the edge, the particle size gradually becomes smaller, as indicated by the blue arrow in Figure 4d. The eroded edge exhibits distinct morphological characteristics of material melting and re-solidifying, while numerous holes are formed, as illustrated by the blue circle in Figure 4d. Point-scanning of the material at rectangles 3 and 4 were conducted, and the results are presented in Figure 5c,d. Elemental analysis revealed that Ag, O, and C were predominantly detected at rectangle 3, along with trace amounts of Al and W. In comparison to the results from one arc erosion, these findings indicate that a significant quantity of molten Ag was sputtered onto the surface of the material, while W elements from the anode were deposited onto the surface of the cathode following 100 cycles of arc erosions. At rectangle 4, the elements Ag, O, and C were primarily examined. The morphology observed at this location is a result of the melting and re-solidification of metallic silver. Spectroscopic analysis reveals that the Ag-Cr₂AlC composites exhibit oxides on their surface, and Cr₂AlC decomposed following both 1 and 100 arc erosions. After 100 cycles of erosion, the melted silver was subjected to arc forces, resulting in the sputtering of particles of varying sizes. The splashed/sputtered Ag particles observed after 100 arc erosions could indeed contribute to the formation of conductive debris, which may have significant implications for the electrical performance of the contact material. Conductive debris could potentially lead to short circuits, increased contact resistance, or even arc reignition under certain conditions. We suggest that future studies could explore surface modifications or protective coatings to minimize the splashing of Ag particles and reduce debris formation. In Figure 4, traces of material melting and re-solidification are evident. Additionally, Figure 3 shows that the eroded area resulting from 100 arc erosions is significantly larger than that from a single arc burn. Moreover, after one arc erosion, as depicted in Figure 4, the amount of material ejected from the melt pool is minimal; conversely, after 100 arc erosions, a substantial number of particles are observed to be splashing.

The 3D laser scan images following 1 and 100 arc erosions are presented in Figure 6. The color coordinates in this figure indicates height, with red representing a bump and blue indicating a depression. In Figure 6a, there is an absence of blue areas at the eroded center, suggesting that a single arc erosion does not produce a depression. Conversely, Figure 6b clearly shows that the eroded center is blue, while the edges are raised in red. This observation indicates that after multiple arc erosions, a concentrated eroded crater morphology has formed at the center, while the edges exhibit a raised morphology due to the arc force [18,19,20].

Raman spectroscopy was employed to analyze the composition of the eroded center area following 1 and 100 arc erosions. The laser light from the Raman spectrometer is directed at the green cross in the inset of Figure 7. Figure 7a presents the Raman spectrum of the Ag-Cr₂AlC composite following one arc erosion, revealing the presence of Cr₂O₃ (R060892), Al₂O₃ (X050045), and Ag₂O on the surface [21,22,23]. Figure 7b displays the Raman spectrum of the Ag-Cr₂AlC composite after 100 arc erosions, where the same substances—Cr₂O₃, Al₂O₃, and Ag₂O—are also detected on the surface. Combined with the energy spectrum results in Figure 1, these findings indicate that the Ag-Cr₂AlC composite material experiences oxidation after both one and 100 arc erosions.

Under the action of the electric field, the original insulation of the air breakdown occurs, the current instantaneously reaches its maximum value, this maximum value is the breakdown current. Figure 8 shows the curve of current-time after one discharge. This time the breakdown current is 28 A. The breakdown currents for 100 discharges are recorded in Figure 9a. It is evident that when the loading voltage is set at 7 kV, the breakdown current values remain relatively stable, fluctuating between 20 A and 35 A.

Breakdown strength is employed to evaluate the susceptibility to arc formation; a higher breakdown strength indicates a lower likelihood of arc occurrence. The breakdown strength can be calculated using Equation (2) [24].

$$\mathrm{E}={\displaystyle \frac{U}{d}}$$

(2)

where E is the breakdown strength (V/m), U is the loading voltage (V), and d(m) is the distance between the two electrodes. The breakdown strength of 100 arc erosions is calculated and presented in Figure 9b. It can be observed that when the number of discharges ranges from the 1st to the 70th, the breakdown strength remains relatively stable between 3 × 10⁶ V/m and 6 × 10⁶ V/m. Subsequently, as the arc erosion reaches the 90th discharge, the breakdown strength gradually increases to 11.38 × 10⁶ V/m, indicating that the arc of the Ag-Cr₂AlC material becomes more resistant to occurrence after numerous discharges. This phenomenon is known as ageing, which has also been reported in other literature [25,26,27,28]. Starting from the 91st discharge, the breakdown strength again stabilizes between 3 × 10⁶ V/m and 6 × 10⁶ V/m. According to the Fowler-Nordheim equation, the field enhancement factor (β) is intrinsically linked to the surface geometry [29,30]. As illustrated in Figure 3c,d, an increase in the number of discharges results in the formation of a pronounced protrusion morphology on the surface. The tips of these protrusions exhibit a smaller radius of curvature, which contributes to a higher β-value. This enhancement significantly amplifies the local electric field, thereby accelerating electron emission prior to breakdown and ultimately reducing the breakdown strength. In our previous discussion, we analyzed the breakdown strength of T₃SiC₂ material at a voltage of 7 kV [18]. The breakdown strength of Ti₃SiC₂ after 100 cycles of arc erosions remains stable, ranging from 1.2 × 10⁶ V/m to 1.3 × 10⁶ V/m. Notably, the breakdown strength of Ag-Cr₂AlC composites exceeds that of pure Ti₃SiC₂ material, indicating that Ag-Cr₂AlC composite material exhibits superior resistance to arc erosion. This observation reinforces the conclusion that Ag-Cr₂AlC composites possess enhanced resistance to arc erosion compared to pure Ti₃SiC₂ material.

4. Conclusions

In this study, the arc erosion properties of Ag-Cr₂AlC composites were systematically investigated from both theoretical and practical perspectives using a combination of the finite element method and experiments.

(1) Ag-20 vol.% Cr₂AlC composites were prepared using SPS, resulting in a hardness of 60.9 HV and a densification of 93.3%. The Cr₂AlC particles were uniformly distributed on the surface of the Ag matrix.

(2) The Ag-Cr₂AlC composite material underwent both single and multiple arc erosions using a self-made arc erosion device. The eroded area was categorized into three distinct regions: the eroded center area, the eroded edge area, and the heat-affected area. Following single arc erosion, the surface of the material appeared relatively flat. In contrast, after 100 arc erosions, the surface exhibited numerous pores, scattered particles, and protrusions morphology. Additionally, Ag₂O, Al₂O₃, and Cr₂O₃ were formed on the surface of the material after both a single and 100 arc erosion cycles.

(3) During 100 arc erosions, the breakdown current value remained relatively stable, ranging from 20 to 35 A. From the first to the seventieth discharge, the breakdown strength was consistently stable, fluctuating between 3 × 10⁶ V/m and 6 × 10⁶ V/m. However, after the 70th discharge, the breakdown strength increased and the phenomenon of ageing appeared. At the 90th arc erosion, the highest breakdown strength is 11.38 × 10⁶ V/m. As the number of discharges increases, the breakdown strength diminishes due to the formation of numerous protrusions features on eroded surface, which in turn amplify the local electric field.