Friction and Wear Properties of AgCuNi Alloy/Au-Electroplated Layer Sliding Electrical Contact Material

Abstract

Understanding the tribological properties of alloy-based sliding electrical contacts is crucial for both fundamental research and practical applications. Here, to explore the friction, wear, and contact resistance of a AgCuNi alloy/Au-electroplated layer during sliding, a ball-on-disk tribometer was coupled with a source meter. The experiments were conducted under various conditions including a current ranging from 0 to 1.0 A, a normal load ranging from 0.5 to 3.0 N, and a sliding speed of 40 mm/s. The results indicate that the wear of the friction pair is aggravated by both the current and the increase in the normal load. When the current was 0.5 A, the wear loss reached its lowest point. However, as the current increased from 0.5 A to 1.0 A, there was an intensification in Ag transfer from the alloy ball to the Au-electroplated layer, resulting in an increase in wear loss. Both the normal load and current have significant effects on both friction coefficient and contact resistance. The variation in contact resistance over time follows a similar pattern to that of the friction coefficient over time. The formation of a transfer film plays a crucial role in determining contact resistance, wear resistance, and friction coefficient. The experiment demonstrates that optimizing the normal load and current can adjust both the contact resistance and friction coefficient, thereby prolonging service life and ensuring the stability of contacts.

1. Introduction

With the rapid development of aerospace technology and industrial technology, the demand for conductive slip ring electrical contact materials is constantly increasing [1,2]. Sliding electrical contact usually refers to a sliding pair under the action of current [3,4,5], and the friction and wear of electrical contact materials under current (i.e., the friction behavior of a sliding pair under the influence of current) is the main problem that restricts the service performance of electrical contact materials [4,5,6,7]. The study of sliding electrical contact is directly related to the service life and stability of the corresponding friction pair components, being of great significance for improving the service life and reducing the failure of components.

In recent years, many scholars have carried out extensive research on the sliding electrical contact friction behavior in the abovementioned fields. Some scholars have taken the pantograph catenary of railway locomotives as the research object and studied the current-carrying friction properties of copper, carbon, and other related materials [8,9]. It has been found that the wear of materials is affected by current intensity, normal load, sliding speed, and other parameters, providing a scientific basis for reducing wear and improving the service life of components [10,11,12,13]. In the early stages of research, the brush was made up of bundles of copper wire [14,15,16,17]. In the later stages, brushes composed of graphite and other materials appeared [18,19,20]. With the development of science and technology, to improve the performance of the slip ring, metal was added to the brush material [21,22,23]. Therefore, some scholars have conducted research on the sliding electrical contact behavior of some materials, especially graphite, carbon, and copper. At the same time, some scholars have also carried out corresponding research from the perspective of design and processing [24,25,26,27].

Shobert’s research shows that slip ring materials are usually made of Au, Ag, or noble metal alloys [28,29,30,31]. Some scholars have studied the sliding electrical contact friction of slip rings composed of precious metals or precious metal alloys, such as AuAgCu, brass, and Au plating [32,33,34]. AgCuNi alloy is an excellent brush material with good wear resistance and conductivity, and it is widely used in spacecraft [35,36,37,38]. Studying the electrical friction behavior and mechanisms of workpieces with AgCuNi alloy is crucial for component design and application. However, there are few studies on the sliding electrical contact friction of AgCuNi alloy.

In this study, a self-made AgCuNi25-1 alloy was examined to reveal its friction and wear mechanisms, along with those of a Au-electroplated layer, providing a reference to improve the service life and reliability of conductive slip ring friction pairs. The AgCuNi25-1 alloy and Au-electroplated layer were used to carry out ball-on-disk reciprocating sliding friction tests in an atmospheric environment. By studying the effects of current and normal load on the contact resistance, friction, and wear behavior of the friction pair, the tribological behavior of the AgCuNi25-1 alloy and Au electroplated layer in the process of sliding electrical contact was revealed.

2. Experiments

2.1. Materials

A ball-on-disk reciprocating sliding tribometer (Bruker, Penang, Malaysia, UMT3) was used to measure the tribological performance of the alloy ball and Au-electroplated layer, and the system resistance was measured with a source meter (Keithley 2401,Shanghai, China) (Figure 1).

The material of the ball was a AgCuNi alloy (74% Ag, 25% Cu, and 1% Ni), with a diameter of 12.7 mm. The substrate of the disk was brass (62% Cu, 38% Zn), which was plated with Au using the electroplating method. The type of electroplated gold was hard gold (containing traces of cobalt), and nickel was used as an underplate for the gold layer. The dimensions of the disk were ϕ20 × 3 mm, the thickness of the Au-electroplated layer was 3 μm, and the surface roughness of the disk was 0.5 μm. The Vickers hardness of the alloy ball was 200 HV and that of the Au-electroplated disk was 180 HV.

The process used for plate friction pairs is Au plating on the surface of brass. Due to the bonding problem between brass and Au, the surface roughness value cannot be too low. This is caused by the processing technology, and the thickness of the Au coating is relatively large, which can to some extent reduce the impact of roughness on the test results.

2.2. Tests Set-Up and Procedure

Before the test, all tribo-pairs were fully rinsed with ethanol ultrasonically. The friction and electrical contact tests were conducted in a reciprocating mode, with the disk reciprocating with a stroke length of 5 mm. All the tests were conducted in an atmospheric environment at 25 °C (±1 °C) with a relative humidity of 38 ± 2%. The tribology and electrical contact tests were investigated under normal load values of 0.5, 1.0, 2.0, and 3.0 N and current values of 0, 0.5, and 1.0 A, and the sliding speed was 40 mm/s (the reciprocating frequency is 4 Hz). During the experiment, the change in the friction coefficient in the whole process of the friction pair was recorded by a tribometer, and the total resistance of the system was recorded by a source meter. Among them, the source meter provided voltage under a constant current mode (voltage is allowed to change with the change in contact resistance). The contact resistance value was calculated according to Ohm’s law through the measured current and voltage drop through the friction pair [34]. The two wires were, respectively, connected to the ball holder for fixing the sample alloy ball and the lower sample disk, as shown in Figure 1b.

During the experiment, the source meter outputted a constant current (direct current) to the system, and the total resistance of the system was obtained through the I–V curve. In the experiment process, the total resistance of the system was composed of wire resistance, ball holder resistance, and the contact resistance of the friction pair. Except for the contact resistance, the resistance of other parts was fixed. Therefore, in this study, the change in system resistance reflected the change in contact resistance. After the experiment, the electrical contact friction behavior of the friction pair was analyzed by means of white light interference profilometry. In addition, to ensure the repeatability of the measurements, all tests were repeated at least three times.

3. Results

3.1. Friction Coefficient

The corresponding friction coefficient of the friction pair under different normal loads and currents is shown in Figure 2. When the normal load was 0.5 N, 1.0 N, or 2.0 N, with the increase in time, the friction coefficient first increased, then decreased, and finally became stable. For the friction coefficient in the latter two cases, however, the sudden drop was less obvious and it fluctuated greatly after rising. When the normal load was 3.0 N, the characteristics of a sudden drop after the increase in the friction coefficient were not very obvious. The friction coefficient fluctuated greatly initially. With the increase in time, the fluctuation of the friction coefficient gradually decreased and became stable. At the same time, the maximum value of the friction coefficient initially gradually decreased with the increase in normal load. Specifically, for a normal load of 0.5 N, 1.0 N, 2.0 N, and 3.0 N, the maximum values of the friction coefficient were 1.0, 0.9, 0.8, and 0.75, respectively.

When the normal load was 0.5 N, Figure 2a shows the friction coefficients corresponding to different currents. In all cases, the friction coefficient presented the characteristics of a typical running-in curve—that is, the friction coefficient first increased, then suddenly decreased, and then became stable. However, when there was no current, the friction coefficient after stabilization was about 0.51, which was smaller than that with a current. In the cases with a current, although the friction coefficient showed a stable trend after a sudden drop, with the gradual increase in sliding duration, the friction coefficient tended to rise slowly at the final moment. When the normal load was 1.0 N, the corresponding friction coefficient under different currents is shown in Figure 2b, which is similar to the characteristic when the load is 0.5 N.

However, differing from a normal load of 0.5 N, with or without a current, with the gradual increase in time, the friction coefficient showed a slow upward trend right after the drop. When the normal load was 2.0 N, differing from the first two groups, the friction coefficient after stabilization was similar, about 0.65, whether there was a current or not. When the normal load was 3.0 N, the corresponding friction coefficient under different currents is shown in Figure 2d. Differing from the previous three groups, the variation law of this group of friction coefficients is not very obvious. With or without a current, the friction coefficient increased slightly at the initial time point, then decreased, and fluctuated around 0.70. With the increase in time, the fluctuation of the friction coefficient decreased gradually.

3.2. Electrical Contact Performance

The corresponding system resistance under different normal loads and currents is shown in Figure 3. It can be seen from the figure that no matter how large the normal load was, when the current was 0.5 A, the system resistance was greater than that when the current was 1.0 A, and the fluctuation range of the system resistance when the current was 0.5 A was larger than that when the current was 1.0 A.

For system with a small current (0.5 A), the variation in system resistance is not obvious. When the normal load was 0.5 N and 3.0 N, the system resistance increased slowly. When the normal load was 1.0 N and 2.0 N, the system resistance decreased slowly and finally became stable. When the current was 0.5 A, the fluctuation range of system resistance decreased gradually with the increase in normal load.

For system with a large current (1.0 A), the system resistance under different loads gradually decreases and flattens with the friction time, and the resistance of the system resistance increases with the increase in load. When the load is 0.5 N, 1.0 N and 2.0 N, the system resistance finally stabilizes at about 1.4 Ω with the increase in friction time. However, when the load is 3.0 N, the system resistance after stabilization is about 1.47 Ω.

Compared with the variation in friction coefficient, it is found that the change in system resistance with sliding duration is similar to that of friction coefficient, that is, it fluctuates greatly at the initial time. With the increase in sliding duration, the system resistance tends to be stable—that is, the contact resistance of the friction pair remains unchanged in the later period.

4. Discussion

4.1. Friction Coefficient and Transfer Film

To understand the variation law of the friction coefficient as depicted in Figure 2 and Figure 3, we examined the micromorphology of wear scars and wear tracks under corresponding conditions, as shown in Figure 4. When the normal load was 0.5 N, as shown in Figure 4a, a transfer film appeared on the alloy ball—that is, Au on the Au-electroplated layer was transferred to the alloy ball. At the same time, silver spots appeared on the transfer film, and narrow silver banded parts also appeared on the wear tracks—that is, during the reciprocating sliding process of the alloy ball and the Au-electroplated layer, wear debris dominated by Ag was generated on the Au-electroplated layer. With the accumulation of wear debris, this part of the wear debris gradually adhered to the Au transfer film of the alloy ball. When there was no current, the transfer film on the alloy ball was uniform and dense. When there was current, however, the transfer film on the alloy ball had several pits. Therefore, the friction coefficient without a current was lower than that with a current, and this result is similar to that of Grandin [14].

When the normal load was 1.0 N, the micro morphology characteristics of wear scars and wear tracks corresponding to different currents were similar to those when the normal load was 0.5 N, as shown in Figure 4b. Similarly, as shown in Figure 4e, from the profiles of the transfer film, for the transfer film formed by Au adhering to the alloy ball, it can be seen that the end part of the transfer film was smoother when there was no current than when there was a current. Therefore, when there was no current, the transfer film formed on the alloy ball was more uniform; this made the friction coefficient in the case of no current lower than that in the case of a current. However, the amount of Ag adhering to the transfer film was significantly greater than that when the normal load was 0.5 N. Therefore, when the normal load was 1.0 N, the friction coefficient after stabilization with or without a current was greater than that when the load was 0.5 N. At the same time, because of the accumulation of Ag on the transfer film and the widening of the silver band on the wear track compared with the normal load of 0.5 N, the friction coefficient after stabilization tended to rise slowly in the end when there was no current.

When the normal load was 2.0 N, it could be found that compared with a load of 0.5 N and 1.0 N, the width of the wear track and diameter of the wear scar increased with the increase in normal load, as shown in Figure 4c. In the case of no current, it can be observed that Au still existed in the transfer film formed on the alloy ball. In the case of a current being present, the transfer film was covered with a large amount of Ag, and the wear was more severe. Therefore, for the friction coefficient without a current, the initial time presents typical running-in characteristics. After the friction coefficient was stable, with the continuous increase in time, the friction coefficient increased sharply at the final time point, which was close to the stable friction coefficient with a current.

When the normal load was 3.0 N, the corresponding micromorphology of wear scars and wear tracks is shown in Figure 4d. It can be observed that there was no obvious difference between wear scars and wear tracks with or without a current, and there was less distribution of Au on the transfer film. Compared with the smaller normal load, the surface of the transfer film was uneven under a normal load of 3.0 N, as shown in Figure 4f.

From the above discussion, it can be seen that when the normal loads were 0.5 N, 1.0 N and 2.0 N, and there was no current, the friction coefficient after stabilization was lower than that with a current. The introduction of a current and the increase in normal load aggravate the wear of the friction pair. At the same time, this is consistent with the research conclusions of Grandin [13]—that is, the appearance and composition of the transfer film will be affected by the load and current, which in turn will also affect the friction coefficient.

Via the preliminary experiments involving a current-carrying friction test, it was found that both the friction test without a current and the current-carrying friction processes are accompanied by the generation of a transfer film. According to the results of the optical microscope shown in Figure 4a–d, it can be seen that there is a layer of golden transfer film on the surface of the alloy ball, and as the normal load increases, a larger area of silver transfer film gradually appears on the golden transfer film. The above pattern can be inferred from macroscopic experimental results showing that when the normal load is constant, with the increase in current, more silver is transferred to the alloy ball, and the transfer film gradually fails. When the current is constant, the larger the load, the more silver there is in the transfer film, and the friction film gradually fails. Based on this experimental pattern, taking the experimental conditions of a load of 0.5 N, a current of 0.5 A, a sliding speed of 40 mm/s, and a running-in time of 60 min as an example, the transfer film and wear marks were characterized. The results are shown in Figure 5a–d.

Based on these macroscopic observation results and EDS analysis, it can be concluded that the main component of the gold transfer film on the wear spot (balls) is Au, while the main element in the silver transfer film on the surface of the gold transfer film is Ag. The components of the experimental materials used in this study are as follows: ball friction pair (74% Ag, 25% Cu, 1% Ni), Au-plated plate, base brass (62% Cu, 38% Zn), surface Au-plated plate. There is no Ag on the surface of the Au-plated plate. At the beginning of the experiment, the Ag on the alloy ball is transferred to the Au-plated plate to form the wear debris, and the Au on the Au-plated plate is transferred to the surface of the alloy ball to form the Au transfer film. As the wear intensifies, the Ag on the Au-plated plate transfers to the Au transfer film of the alloy ball.

In this experiment, both the current and the load have an effect on the friction transfer film formation, and from the experimental results (Figure 2 and Figure 4), the load has a dominant role. When the load remains the same and the current increases (Figure 2a, when the current is 0.5 A), the friction pair will experience a longer running-in time to form a relatively stable transfer film, and the friction coefficient will decrease. When the current is large enough, the formation of transfer film is accelerated, and the friction pair quickly displays a stable friction coefficient. When the current is constant and the load increases, the failure of the transfer film will be aggravated. When the load is sufficiently large, such as 3 N, increasing the current does not cause significant differences in the friction performance.

It can be concluded that the failure of the transfer film will cause a change in the friction coefficient. Therefore, the analysis of the friction coefficient can reflect the change rule of the transfer film from the beginning to the stable formation to the failure of the transfer film to a certain extent, and the fluctuation degree of the friction coefficient can reflect the evolution process of the transfer film.

4.2. Wear Loss

The friction coefficient after stabilization and the wear loss of the plate corresponding to different loads and currents are shown in Figure 6. Under the condition of no current, the friction coefficient after stabilization gradually increases with the increase in load. This is because under a load of 0.5 N and no current, the transfer film on the alloy ball is the most dense and uniform, while the Ag element adhering to the transfer film is the least, resulting in the lowest friction coefficient after stabilization under this condition. According to the analysis in Figure 4, under the conditions of 0.5 N normal load and no current, the transfer film on the alloy ball is the most dense and uniform, while the Ag element adhered to the transfer film is the least, meaning that the friction coefficient after stabilization is the lowest under this condition. When the normal load is 0.5 N and 1.0 N and a current is applied, the friction coefficient after stabilization is greater than that without a current. When the normal load is 2.0 N and 3.0 N, due to the failure of the transfer film on the alloy ball, there are no significant differences in the stable friction coefficient with or without a current.

As the normal load increases, the wear loss increases with or without a current. From Figure 6a–c, it can be seen that under a normal load of 0.5 N, 1.0 N and 2.0 N, when the current is 0.5 A, the transfer film does not disappear, and the corresponding wear loss is the lowest, which indicates that the anti-wear effect of the material is better when the current is 0.5 A. During the process of loading from 0.5 N to 3.0 N, as the current increases from 0 A to 0.5 A, a decrease in wear loss is observed. This is due to the application of an electric current. In the process of friction shear, a large amount of abrasive particles are accumulated, which causes the shear resistance to increase, so the friction coefficient is high. However, the accumulated abrasive particles rapidly form a transfer film under continuous current and shear, and a solid friction transfer film is formed between the two contact surfaces. At this time, the transfer film is stably formed, and during the subsequent experiments, wear occurs on the transfer film, which protects the friction pair to a certain extent, so the wear loss of the contact surface is reduced.

As the current increases from 0.5 A to 1.0 A, the wear loss increases. This is because with the increase in current, the possible Joule heat also increases [13]. Under the joint action of current and machinery, the wear loss increases. From Figure 4a–d, it can be seen that when the current is 0.5 A, the content of Ag on the wear track is less than that when the current is 1.0 A, which indicates that the increase in current intensifies the transfer of Ag on the alloy ball to the Au-electroplated layer. The increase in Ag content on the Au-electroplated layer leads to the increase in Ag content adhering to the transfer film, which intensifies the wear loss. When the normal load is 3.0 N, due to the intensification of wear, more grooves appear on the transfer film, and the Au-electroplated layer on the plate basically fails. Therefore, under this load condition, the change in wear with current has no obvious law.

4.3. Contact Resistance

Contact resistance is one of the common indicators used to reflect the electrical contact performance [39], which includes constriction resistance and a film resistance [35]. The detection of contact resistance can reflect the wear of friction pairs to a certain extent [36]. In this test system, the current is generated by the source meter and passes through the wire, ball holder, alloy ball, and Au-plated plate. Therefore, in this testing system, the resistance values of the wire and ball holder remain constant during testing. The sliding friction between the alloy ball and the Au-plated plate causes changes in the contact point, which in turn causes changes in the resistance of the ball–plate contact pair, resulting in changes in the resistance value of the testing system. Therefore, the change in the resistance value of the testing system can reflect the change in resistance of the ball–plate contact pair—that is, the change in contact resistance.

As shown in Figure 3, under lower loads, the transfer film can be densely formed with a larger contact area and lower contact resistance. As the load increases to 3 N, the Au-plated plate in the friction pair has already experienced the failure of the Au plating layer. At this time, friction occurs on the substrate and steel ball, and the transfer film changes from a uniformly dense Au-based to a Ag-based transfer film. Therefore, it leads to significant wear on the contact surface, and the contact elements are the brass substrate and Ag, resulting in very high contact resistance under 3 N.

It can be seen from Figure 2 and Figure 3 (when the current is 1.0 A) that the friction coefficient and resistance fluctuate greatly at the initial time point, and then gradually become stable. So, the variation law of the friction coefficient with time is similar to that of contact resistance with time. Therefore, there is a certain correlation between the contact resistance and friction coefficient between the AgCuNi alloy and the Au-electroplated layer, which is similar to the test results of Beake [37]. To further explore the relationship between friction coefficient and contact resistance, their variation in relation to time with current being 1.0 A is shown in Figure 7.

From Figure 7a, it can be seen that when the normal load is 0.5 N, the system resistance rapidly decreases to the lowest value at 700 s, about 1.38 Ω, and then remains stable, so the contact resistance decreases rapidly from the maximum value to the minimum value at about 700 s with the increase in time, and then remains stable. At the same time, the variation law of the friction coefficient with time is similar. At the initial moment, the friction coefficient increases from low to high. At about 700 s, the friction coefficient decreases to its minimum with the formation of a dense and uniform transfer film. At about 2800 s, with the accumulation of wear debris gradually adhering to the transfer film of the alloy ball, the friction coefficient begins to rise slowly, and the contact resistance fluctuates greatly. At the initial time point, due to the contact between the alloy ball and the plate, the contact area is small. With the reciprocating sliding of the friction pair and the running-in of the friction pair, the transfer film on the alloy ball gradually forms, which makes the contact area between the ball and the plate larger, so the conductive area increases [34], while the contact resistance decreases to the minimum value and remains constant. Due to the accumulation and adhesion of wear debris on the transfer film in the later stage, the contact area changes and the contact resistance fluctuates.

When the normal load is 1.0 N and 2.0 N, as shown in Figure 7b,c, the change law of friction coefficient and contact resistance is basically the same. Similarly, the contact resistance changes due to the change in the transfer film.

When the normal load is 3.0 N, as shown in Figure 7d, the friction coefficient does not rise at first and then decreases at the initial time point, but then directly decreases at about 100 s and then rises slowly. This is due to the rapid formation of a transfer film at 3.0 N. However, because of the high contact stress, serious wear, and more wear debris being produced, the transfer film breaks rapidly due to the adhesion of the wear debris after the formation of the transfer film, meaning that the friction coefficient increases slowly after decreasing. At about 500 s, the friction coefficient remains basically stable and fluctuates slightly. Similarly, the contact resistance also tends to rise slowly after 100 s, rather than continuing to decline. This is due to the rupturing of the transfer film, the plastic deformation of the transfer film, and the formation of pits and grooves on its surface, which reduces the contact area and increases the contact resistance. With the accumulation of wear debris, the contact area increases, the conductive area increases, and the contact resistance decreases, after which it finally fluctuates around 1.47 Ω.

As shown in Figure 7, at the beginning of the friction process, the contact resistance suddenly decreases from a larger value and then tends to stabilize. This indicates that at the beginning of the friction process, the two friction pairs are in the running-in process, and the transfer film has not yet formed. The contact surface between the two friction pairs is a rough peak contact with a small contact area, resulting in a higher contact resistance value. As the running-in process progresses, a transfer film gradually forms, and the contact area between the two friction pairs changes from rough peak contact to a uniform and dense transfer film. Therefore, the contact area increases, while the contact resistance decreases and then stabilizes.

It can be seen from the above discussion that, consistent with the research conclusions of Grandin [39] and Jacobson [38], the normal load and current have significant effects on the friction coefficient and contact resistance. The contact resistance is closely related to the contact surface area of the friction pair. At the same time, the friction coefficient is related to the contact area between the two friction pairs. Therefore, the changing trend of the contact resistance is related to the changing law of the friction coefficient. So, the change in contact resistance can directly reflect the formation of a transfer film, while the friction coefficient is the experimental result of the whole system, which cannot directly reflect the formation process of the transfer film. The transfer film determines not only the contact resistance but also the wear resistance and friction coefficient.

5. Conclusions

In this paper, the friction and electrical contact characteristics of a AgCuNi alloy and a Au-electroplated layer under normal load and current were studied. By comparing the friction, wear, and contact resistance under different loads and currents, the following conclusions are obtained:

(1)

When the normal load is less than 3 N and there is no current, the friction coefficient after stabilization is lower than that with a current, and the introduction of a current and the increase in the normal load aggravate the wear of the friction pair.

(2)

Both current and load have an effect on the friction transfer film formation, and the load has a dominant role. When the load is sufficiently large, increasing the current does not cause significant differences in the friction performance.

(3)

When the current is about 1.0 A, there is a certain correlation between the contact resistance and friction coefficient between the AgCuNi alloy and the Au-electroplated layer. The variation law of contact resistance with time is similar to that of the friction coefficient with time.