# Effect of current intensity and polarity on the electrical performances of slip ring contacts

Corentin Ferreira, Manon Isard, Sophie Noël, Aurore Brézard-Oudot, Frédéric Houzé, Philippe Teste

► To cite this version:

Corentin Ferreira, Manon Isard, Sophie Noël, Aurore Brézard-Oudot, Frédéric Houzé, et al. Effect of current intensity and polarity on the electrical performances of slip ring contacts. Journal of Tribology, 2026, 148 (3), pp.034602. 10.1115/1.4070001. hal-05279533

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Submitted on 20 Oct 2025

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**Corentin Ferreira**

Université Paris-Saclay,
CentraleSupélec, CNRS,
Sorbonne Université,
Laboratoire de Génie Électrique
et Électronique de Paris,
Gif-sur-Yvette 91192, France;
Everaxis Aerospace and Defence,
Le Plessis-Robinson 92350, France
e-mail: corentin17@live.fr

**Manon Isard**

Everaxis Aerospace & Defence,
Le Plessis-Robinson 92350, France
e-mail: manon.isard@everaxis.com

**Sophie Noël<fnref n="1" />**

Université Paris-Saclay,
CentraleSupélec, CNRS,
Sorbonne Université,
Laboratoire de Génie Électrique
et Électronique de Paris,
Gif-sur-Yvette 91192, France
e-mail: sophie.noel@centralesupelec.fr

**Aurore Brézard Oudot**

Université Paris-Saclay,
CentraleSupélec, CNRS,
Sorbonne Université,
Laboratoire de Génie Électrique
et Électronique de Paris,
Gif-sur-Yvette 91192, France
e-mail: aurore.brezardoudot@geeps.
centralesupelec.fr

**Frédéric Houzé**

Université Paris-Saclay,
CentraleSupélec, CNRS,
Sorbonne Université,
Laboratoire de Génie Électrique
et Électronique de Paris,
Gif-sur-Yvette 91192, France
e-mail: frederic.houze@centralesupelec.fr

**Philippe Testé**

Université Paris-Saclay,
CentraleSupélec, CNRS,
Sorbonne Université,
Laboratoire de Génie Électrique
et Électronique de Paris,
Gif-sur-Yvette 91192, France
e-mail: philippe.teste@centralesupelec.fr

# Effect of Current Intensity and Polarity on the Electrical Performances of Slip Ring Contacts

In this investigation, the effect of current has been studied on sliding electrical contacts between bulk gold-alloy wires and gold-alloy-plated rings. Two parameters have been examined: current intensity, and direction of current. For this purpose, a model slip ring has been used, which includes four electrically independent groups of four rings. For each group, the electrical diagram requires the current to flow alternately from the wires to the ring (cathode ring) and from the ring to the wires (anode ring). The contact voltage has been measured with an equivalent four-probe method, and the morphology of the worn surfaces has been observed by means of an optical microscope and a Scanning Electron Microscope (SEM); its composition has been characterized by Energy Dispersive X-ray spectroscopy (EDX). First, the results show that the current allows to reduce the contact voltage and the wear of surfaces. Secondly, the results show an influence of the polarity of the current: the anode rings present much higher contact voltage, and a greater loss of the gold-plating compared to the cathode rings. In this paper, the results obtained are presented and a hypothesis is proposed to explain the polarity effect on the contacts. [DOI: 10.1115/1.4070001]

Keywords: slip ring, gold contact, electrical sliding contact, current intensity, polarity effect, coatings, electrical contacts, fluid film lubrication, third bodies, wear

## 1 Introduction

A slip ring is a device that allows transmission of an electric current from a static structure to a rotating one, and vice versa. Such a system can be found in many industrial applications, like

<footnote n="1">¹Corresponding author.</footnote>

<footnote>Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received June 17, 2025; final manuscript received August 25, 2025; published online October 16, 2025. Assoc. Editor: Robert L. Jackson.</footnote>
the deicing of rotating helicopter blades or the orientation of a satellite's solar panels. For some applications, the quality of the transmitted electrical current is very important. Indeed, in order to transmit a signal, it is necessary to have minimal electrical noise, i.e., a minimal variation around the contact voltage. During sliding, several phenomena occur at the contact interface, including friction, wear, and material transfer, which all have an impact on the tribological behavior of the contact but also on its electrical behavior. An electric current in the contact adds another layer of complexity to the study of such a system. Depending on the voltage, intensity, or polarity, electrical phenomena such as arcing [1,2] or high temperatures [3] can cause premature contact failure. For this study, it was decided to focus on the effect of current intensity and the direction of current.

Many papers highlight the influence of numerous parameters on the contacts of a slip ring. For a contact similar to the one under study, Fu et al. [4] highlighted the benefits of incorporating palladium (3.5%) into gold to reduce wear on contact elements, Xie et al. [5] studied the influence of normal force on the friction coefficient and contact resistance; Xie et al. [5], Sawa et al. [6] and Ueno [7] observed the influence of speed on contact resistance; Ueno et al. [7], Takada et al. [8] and Jia et al. [9] observed the influence of the current intensity. Jia et al. [9] showed that an increase in the current intensity involves an increase in wear of the surfaces. A similar observation was recently made by Wu et al. [10]. Studying polarity effects can be of practical importance because phenomena related to the orientation of the electric field can occur at the interface and modify the surfaces. Argibay et al. [11] and Sawa et al. [6] observed that the polarity of the current could have an influence on the tribological and electrical performance of the contacts. Indeed, on a model slip ring where the AgPdCu wires slide against gold alloy-coated rings, Sawa et al. [6] observed that the electrical resistance of the anode rings was superior to that of the cathode rings. This polarity phenomenon has also been observed on graphite contacts [12–14]. The explanation given for this type of contact is linked to the transfer of material depending on the polarity of the current, which leads to the formation of a graphite deposit (called a patina) on the ring. The formation of such a deposit is favored when the ring is the anode, i.e., when the current flows from the ring to the brush. This has an impact on the coefficient of friction, contact resistance, and wear.

In this article, we report on experimental work carried out on an endurance test bench using contact elements similar to those employed in the targeted industrial applications, which is an original approach compared with the previous work. We will successfully (i) describe the test device with the method for electrical measurement, (ii) present and discuss the results obtained on the effect of current intensity, and (iii) present and discuss the results obtained on the effect of current polarity.

## 2 Experiment

### 2.1 Tests.
The design of the slip ring studied in this paper is represented in Fig. 1. It includes the geometries and materials

<figure><img src="image_4.png" /><figcaption>Fig. 1 Illustration of the wire/ring contacts</figcaption></figure>

used in the manufactured products. The electrical current flows between structures through a sliding contact composed of a 0.4 mm diameter bulk gold-alloy wire confined in a V-groove geometry of a 40 mm diameter brass ring plated with hard gold alloy covered by a gold-flash finish, forming the slip ring system. The wire is in double contact with the V-groove of the ring and ensures both the contact resistance as well as the contact force. To improve the durability and reliability of the contact over time, contact zones are multiplied by adding a second wire in the same V-groove facing the first one and are lubricated with an ester synthetic oil. Details of the composition and properties of gold alloys, as well as oil formulation, cannot be given for reasons of confidentiality.

The design of the endurance bench allows a wide variety of test conditions, such as normal force, speed, environment, lubrication, and materials in contact. This bench is made up of four groups, each composed of four rings in contact with wires, that are independent except for the rotation speed, which is the same for all groups. The rings are made up of 5 V-grooves. In each V-groove, two wires facing each other slide against the ring. A channel is defined by a V-groove in contact with the two associated wires, representing four elementary contacts. Each group, therefore, consists of 20 channels.

For this study, adaptations have been made for laboratory investigation. In particular, the electrical performance of the contacts was measured during the tests using a 4-probe equivalent measurement. The independence of groups means that they can be powered by different current sources, and different cabling paths can be imposed depending on requirements. For the purposes of this study, the electrical diagram is identical for each group, with the imposed current intensity possibly being different in each group. A simplified view of this diagram is shown in Fig. 2, where only a single wire per V-groove is represented. In each group, the current flows through the rings, alternatively from the wires to the ring and from the ring to the wires. When the current flows from the wires to the ring, the ring is referred to as the cathode ring, and when the current flows from the ring to the wires, the ring is referred to as the anode ring. In order to study the influence of the current, it was decided to impose different power supply conditions for the four groups: test conditions are summarized in Table 1. Groups 1 and 2 were powered with the same polarity and two different intensities, while Group 4 was powered in the opposite direction, so the even rings become the cathode rings and the odd rings become the anode rings. Note that the current intensity per channel given in the table is a nominal value corresponding to one-fifth of that chosen for the group considered; the actual current per channel may be slightly different depending on the contact resistances specific to each channel. Group 3 was not powered.

### 2.2 Electrical Measurements on the Contact.
During the test, the contact voltage on each channel is measured at regular intervals. At the start of the rotation, measurements are taken every 30 min, then every hour, twice a day, progressing to once a week. In this way, it is possible to monitor changes in the electrical resistance of the contacts over time.

The electrical measurement is done as shown in the diagram of Fig. 3. It shows two rings with 5 V-grooves connected. The two wires sliding in each V-groove are represented by a single wire. The wire corresponding to channel $n^{\circ}1$ is connected to the anode terminal of the current source, and channels $n^{\circ}6, 7, 8, 9$, and $10$ are connected to the cathode terminal of the current source. In this way, the current flows through channel $n^{\circ}1$ via the wire and then the ring, and exits through channels $n^{\circ}6, 7, 8, 9$, and $10$ thanks to the connection between the rings. Channels $n^{\circ}2, 3, 4$, and $5$ are not connected to the current source, so no current flows through them. Ideally, to measure the contact voltage of channel $n^{\circ}1$, the measurement should be done between point A and point B, but due to the rotation of the rings, point B is not directly accessible. Here, the non-powered channels, i.e., channels $n^{\circ}2, 3, 4$, and $5$
<figure><img src="image_5.png" /><figcaption>Fig. 2 Electrical diagram common to all four groups of four rings. Each ring has 5 V-grooves with two wires per groove. Here, for ease of understanding, only one wire per groove is shown. The direction and intensity of the imposed current vary from one group to another.</figcaption></figure>

are used: because of the contacts between the ring and the non-powered wires, the potential in the ring, i.e., at the output of the contact on channel n°1, is identical to that of the non-powered wires. So measuring the voltage between point A and point C is the same as measuring the voltage between point A and point B. The electrical measurements are therefore taken between the wires of the channel to be measured and the non-powered wires.

For the purposes of this study, a current source of 100 mA is used, and the channel voltage is measured using an oscilloscope, Teledyne LeCroy HDO6000B, Courtaboeuf, France, after amplification to a gain of 100. The variation of the contact voltage is an indicator of the quality of the transmission of an electrical signal. To measure the contact voltage variation, the peak-to-peak voltage is selected. This voltage corresponds to the maximum variation of the contact voltage, and it reflects the performance of a channel. A channel is considered to have good (respectively poor) electrical performance when its electrical resistance is lower (respectively higher) than 40 mΩ (this is a common maximum value for slip ring resistive noise under 100 mA).

**2.3 Analysis.** The results include contact voltage measurements for each channel during the test and observations using an optical microscope and a Scanning Electron Microscope (SEM)–Energy Dispersive X-ray spectroscopy (EDX) after the test.

With a fixed current of 100 mA during the electrical measurements, it is possible to calculate the resistance for each channel with Ohm's law. Then, the electrical results can be grouped by ring or by test condition under a mean value or a median value. These data can be plotted to show the median resistance for selected test conditions as a function of the number of revolutions, or to compare the resistance value for each ring in a bar graph. In both cases, this grouping of data makes it possible to directly compare the electrical resistance of each ring (cathode or anode) according to the power supply conditions.

After the test, the surfaces of every track (wires and rings) were observed under an optical microscope (Olympus DSX1000, Rungis, France). These observations allowed the selection of wires and rings for which additional observations should be performed. The selected rings were cut, mounted, and polished in order to measure the loss of thickness of the gold plating. Also,

the selected samples have been observed under a SEM-EDX (Thermo Scientific Phenom XL G2 desktop, Les Ulis, France) in order to perform chemical analysis of the wear tracks. The various observations aim to identify the effects of the different test conditions.

**3 Influence of the Current Intensity**

Because of the difference in electrical results depending on the polarity reported in the literature, which is to the detriment of the anode parts [5], this section focuses on channels involving cathode rings, i.e., when the current flows from the wires to the rings. Consequently, eight channels are selected for each current intensity.

**3.1 Electrical Results.** Figure 4 compares the electrical performance for each current intensity by plotting the median resistance of the eight channels selected (cathode rings) as a function of the number of revolutions. The interquartile range is also shown for the two non-zero intensities (vertical bars on the dark curves in solid red lines and dotted blue lines). It can be observed that the dispersion of the resistance values measured is of the same order of magnitude overall, although slightly lower for the 2 A current than for 1 A. For the case with no current, the wide dispersion of measurements has not been represented to avoid stretching the ordinate scale and preserve legibility.

On this graph, it is possible to distinguish the three amperages in relation to the electrical results. For no power supply, the median resistance is higher, as confirmed by Table 2 in which the associated interquartile ranges have also been mentioned. For the other two amperages, the electrical performances are close, with an advantage for the higher intensity, which shows a lower median channel resistance compared with the 1 A per V-groove intensity. Otherwise, it should be noted that a period of non-powering between two measurements (between 45 and 50 million revolutions) caused an increase in channel resistance toward values close to those of the

<table><caption>Table 1 Test conditions</caption><thead><tr><th>Designation</th><th>Group 1</th><th>Group 2</th><th>Group 3</th><th>Group 4</th></tr></thead><tbody><tr><td>Rotating speed (RPM)</td><td></td><td></td><td>489</td><td></td></tr><tr><td>Linear speed (m/s)</td><td></td><td></td><td>1</td><td></td></tr><tr><td>Current intensity<br>(A per V-groove, nominal)</td><td>2</td><td>1</td><td>0</td><td>-1</td></tr><tr><td>Voltage limit (V)</td><td></td><td></td><td>5.5</td><td></td></tr><tr><td>Number of revolutions</td><td></td><td></td><td>60,000,000</td><td></td></tr><tr><td>Lubricant</td><td></td><td></td><td>Synthetic ester oil</td><td></td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Fig. 3 Simplified electrical diagram showing two rings connected together to illustrate the electrical measurement of channel n°1</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4 Evolution of the median resistance per measurement as a function of the number of revolutions for the three selected amperages (0, 1, and 2 A per V-groove). For 1-A and 2-A vertical bars representing the interquartile range are also shown.</figcaption></figure>

<table><caption>Table 2 Median resistance and interquartile range on the entire test and on the last measurement for each current intensity</caption><thead><tr><th rowspan="2">Ampérage (A per V-groove)</th><th colspan="2">On the entire test</th><th colspan="2">On the last measurement</th></tr><tr><th>Median resistance (mΩ)</th><th>Interquartile range (mΩ)</th><th>Median resistance (mΩ)</th><th>Interquartile range (mΩ)</th></tr></thead><tbody><tr><td>0</td><td>21</td><td>20</td><td>33</td><td>18</td></tr><tr><td>1</td><td>11</td><td>4</td><td>16</td><td>7</td></tr><tr><td>2</td><td>8</td><td>4</td><td>10</td><td>2</td></tr></tbody></table>

non-powered group. The discussion in Sec. 3.3 suggests possible explanations for the trends observed.

**3.2 Surface Observations.** In all three cases, optical microscope and SEM-EDX observations gave similar results. To illustrate these results, the SEM-EDX observations of one channel at 1 A per V-groove were selected. Beforehand, the surfaces are cleaned in an ultrasonic bath with propanol and then acetone. On the wires (Fig. 5(a) and Table 3), the presence of a layer in both contact areas was noted. The associated EDX measurements showed that this layer is very rich in gold with a mass concentration of 95%w, the remainder being copper. On the rings side (Fig. 5(b) and Table 3), the chemical composition of the wear track is similar, with the presence of two zones: a majority zone in dark gray (red target identified 2) where the mass concentration is close to that of the gold plating showing the elimination of the top coat, and a minority zone in light gray (green target identified 3) where the mass concentration is very rich in gold, close to that of the layer formed on the wires. Moreover, the cross sections show a difference in thickness loss between the powered groups and the non-powered group. In the case of no power supply, the loss of thickness is 3.5 µm with a wear track width of 125 µm. For a current intensity of 1 A per V-groove, the loss is 1 µm with a wear track width of 141 µm, whereas it is 1.4 µm with a wear track width of 142 µm for a current intensity of 2 A per V-groove.

**3.3 Discussion.** In their work, Sawa and coauthors [6], Takada et al. [8], and Jia et al. [9] observed a more or less marked difference in the wear of samples as a function of current intensity. This result depends on the materials in contact and on the current intensity. Ueno and Takada have almost identical test conditions, except for the materials in contact and the speed. Ueno uses a silver-graphite pin rubbing against a gold-coated ring, while Takada uses a copper-graphite pin rubbing against a steel ring. Ueno's results show that the difference in wear of the samples is very slight, whereas

Takada's results show a difference in wear of the samples. For his part, Jia uses a CuAgCr wire rubbing on a copper alloy disc with high current intensities. He found that the higher the current intensity, the greater the wear. As we have not carried out tests with intermediate intensities, we do not know whether wear could be further reduced below 1 µm.

In this study, the use of an electric current was observed to reduce the wear of the gold alloy plating of the ring. Without current, the loss of thickness is around 3.5 µm, whereas it is less than 1.5 µm with current. Between the current intensity of 1 and 2 A per V-groove, there is a slight increase in loss of thickness with the increase in the current intensity. Except that, the observations of the samples give similar results.

Electrically, the intensity of the current has an effect on the electrical performance of the channels. A higher current results in lower channel resistance. This may be due to an increase in temperature at the contact due to the Joule effect. The Joule effect depends on the current intensity; in this case, the higher the current intensity, the greater the power dissipated by the Joule effect. In this way, a higher local temperature is generated, leading to greater softening of the surfaces, which in turn increases the contact area between the surfaces and consequently reduces contact resistance. Also, the softening of the surfaces has an impact on their mechanical properties, such as the shear modulus. With an increase in temperature, the shear modulus decreases, and the surfaces may deform more easily, limiting the formation of debris. This could explain why the loss of thickness of the plating is higher without an electrical current.

# 4 Influence of Current Polarity

**4.1 Electrical Results.** As mentioned before, a difference in electrical performance between the cathode and anode rings has been observed in previous works. Figure 6 shows the electrical resistance for each ring of the 4 groups after 30 million revolutions. For groups 1 and 2, the cathode rings are odd-numbered, and the
<figure><img src="image_8.png" /><figcaption>Fig. 5 SEM-EDX observations—Back Scattered electrons Detector (BSD)—(a) of the wear track on a wire (after 6.10<sup>7</sup> revolutions, with cleaning)—x750; (b) of the wear track on one face of a V-groove—x1000 (1 A/V-groove; cathode ring). The markers identified 1 indicate measurement points outside the wear tracks, while the markers identified 2 and 3 indicate those taken within the tracks. The corresponding chemical compositions are given in Table 3, where it is interesting to note the evolution of the proportion of Au.</figcaption></figure>

<table><caption>Table 3 EDX results linked to Fig. 5</caption><thead><tr><th rowspan="3">Zone</th><th colspan="4">A (wire)</th><th colspan="4">B (ring)</th></tr><tr><th colspan="4">Average weight concentration (%wt)</th><th colspan="4">Average weight concentration (%wt)</th></tr><tr><th>Au</th><th>Ag</th><th>Cu</th><th>X</th><th>Au</th><th>Ag</th><th>Cu</th><th>X</th></tr></thead><tbody><tr><td>1 Blue</td><td>54</td><td>33</td><td>13</td><td>0</td><td>83</td><td>0</td><td>7</td><td>10</td></tr><tr><td>2 Red</td><td>97</td><td>0</td><td>3</td><td>0</td><td>75</td><td>0</td><td>17</td><td>8</td></tr><tr><td>3 Green</td><td>—</td><td>—</td><td>—</td><td>—</td><td>90</td><td>0</td><td>8</td><td>2</td></tr></tbody></table>

anode rings are even-numbered. For Group 4, it is the opposite. Current intensity was 1 A per V-groove. Group 3 was non-powered. For the cathode rings, the electrical results show a lower and less dispersed channel resistance compared with the anode rings. The case of Group 4 shows that the current polarity is responsible for the difference in electrical performance between the rings and not their positions within the group. It should be noted that the contact resistance measurement protocol makes it possible to exclude the hypothesis of a thermoelectric effect (for more details see the discussion in Sec. 4.3). Furthermore, Group 3 shows that the electrical performance of the four rings is similar, with quite low contact resistance values in the absence of an electric current.

During the tests on the different axes, several events disrupted the rotation or the power supply of the different groups. For example, in Fig. 4, a non-powering period caused an increase in the track resistance to values close to those of the non-powered unit. It is possible to consider four scenarios, detailed in Table 4, in order to observe whether the difference in electrical performance between the cathode and anode rings is maintained. It can be seen that the absence of current does not maintain the difference in performance between the rings.

These results raise the question of the reversibility of the phenomenon, since the absence of current seems to cause the channel resistance to tend toward values similar to non-powered channels from the start of the rotation. For this reason, in a later stage of the test, it was decided to reverse the polarity of the current for Group 4, i.e., to impose on it the direction of current common to groups 1 and 2. As Fig. 7 shows, after 59.5 million revolutions, the change in polarity reversed the electrical results for Group 4. This shows that the phenomenon is reversible and that the change in behavior is quite rapid.

Another interesting point is the beginning of the phenomenon. It has been observed that the difference in electrical performance between the cathode and anode rings is not immediate. In fact, from the various tests carried out (with different rotation speeds, for example, not shown here), it is possible to determine wide intervals of appearance of this phenomenon as a function of linear speed:

- Between 10 and 20 million revolutions at 0.5 m/s.

- Between 6 and 12 million revolutions at 1 m/s (speed condition of the measurements presented above).

- Between 0.5 and 6 million revolutions at 2 m/s.

The beginning of the phenomenon seems to show a certain dependence on speed: the higher the speed, the faster the phenomenon appears.

## 4.2 Surface Observations.
For both polarities, observations on the wires (Figs. 5(a) and 8(a)) highlight the presence of a layer in the contact areas. EDX measurements (Tables 3 and 5) show that this layer is very rich in gold with a mass concentration of over 95%, the remainder being copper. On the rings side, the EDX measurements show a similar chemical composition of the trace (Tables 3 and 5) with the presence of two zones: a majority zone in dark gray (red target identified 2) where the mass concentration is close to that of the gold plating showing the elimination of the top coat, and a minority zone in light gray (green target identified 3) where the mass concentration is very rich in gold, close to that of the layer formed on the wires. The cross sections show different thickness losses depending on the polarity. At 1 A per V-groove, for the cathode ring (Fig. 5(b)), the thickness loss of the gold plating is approximately 1 µm with an average wear track width of 141 µm. For the anode ring (Fig. 8(b)), the loss of gold plating thickness is more than 2 µm with an average wear track width of 132 µm. Despite similarities in many aspects, it seems that the polarity of the current causes additional wear to the gold-plating when the ring is the anode, i.e., when the current flows from the ring to the wires.

## 4.3 Discussion.
The assembly of a group on the test device requires some rings to be cathodes and other anodes, depending on the direction of the current. The obtained results show that polarity has an influence on the electrical performances of the channel.
<figure><img src="image_9.png" /><figcaption>Fig. 6 Average contact resistance with median resistance and standard deviation for each ring after 30 million revolutions. We observe for Groups 1 and 2 lower and less dispersed channel resistance for cathode rings compared to anode rings. The polarity of the current for Group 4 is opposite to that of Groups 1 and 2, and we do indeed observe opposite behavior according to the ring's position.</figcaption></figure>

<table><caption>Table 4 Existence of the difference in electrical performance between the cathode and anode rings according to four different scenarios</caption><thead><tr><th>Scenario</th><th>Rotation Current</th><th colspan="2"><sup>a</sup> Current</th></tr></thead><tbody><tr><td>Difference cathode/anode</td><td>YES</td><td>NO</td><td>YES NO</td></tr></tbody></table>

Indeed, after several million revolutions, a significant difference between the electrical performance of the cathode and anode rings was observed. The cathode rings have a low contact resistance, while the anode rings have a much higher one with a wide dispersion. The tests showed that this phenomenon appears in the presence of a current (no effect observed for non-powered contacts), and whatever the position of the ring in the assembly. In addition, several scenarios were tested in order to observe the behavior of this phenomenon. After a period of non-powering, the channel resistance of the cathode and anode rings tends toward values close to the non-powered channels since the start of the rotation.

Also, the presence of a current during rotation or in static enables the difference in electrical performance between the two polarities to be maintained. This shows that the phenomenon is present but not constantly. In fact, reversing the direction of the current in a group has shown that the phenomenon is reversible.

The surface observations show additional wear on the anode rings with a higher loss in thickness of the gold plating. Before looking for an explanation of a particular physical phenomenon in the contact, first the experimental set-up and, more specifically, the electrical measurement were checked. The measured channel is supplied with a current of 100 mA, and the reading voltage is taken between the wire of the channel to be measured and the non-powered wires of other channels. As a result, the measurement is always taken in the same current direction, which avoids the formation of a thermocouple. It was still necessary to check whether the electrical measurement gave the same result in both directions of current. To do this, two channels were measured independently (one cathode channel and one anode channel) and then measured in series in both current directions. In the first direction, the sum of the two resistances is equal to the resistance of the two channels in series. In the other direction, there is a difference of 7 mΩ. This difference is small and does not explain the large one in electrical

<figure><img src="image_10.png" /><figcaption>Fig. 7 Average contact resistance with median resistance and standard deviation for each ring after 59.5 million revolutions. From 30 million revolutions, the polarity of the current was reversed for Group 4, returning it to the situation of Groups 1 and 2, and we can now clearly observe a behavior similar to that of Groups 1 and 2, proving the reversibility of the phenomenon</figcaption></figure>

<footnote><sup>a</sup>Measurement carried out after restarting rotation.</footnote>
performances between the cathode and anode rings. These results eliminate electrical measurement as the cause of the difference between the cathode and anode rings.

Physical phenomena may be responsible for the difference in electrical results depending on polarity:

- The formation of electrostatic charges between two neutral bodies during friction. In the case of an open circuit, the charges do not evacuate but accumulate. However, it is difficult to develop it in the contact despite the presence of an insulating lubricant, which can cause electrostatic charges to appear. In this case, the lubricant does not completely separate the two conductive surfaces, which limits the accumulation of electrostatic charges in the contact.

- The difference in potential between the gold and copper, and the electrical continuity between the two elements, can result in galvanic corrosion. Despite the presence of green spots in the lubricant showing the oxidation of the copper, there were no notable differences in the wear of samples between the anode and cathode paths (except for the thickness loss of the gold plating), and the oxidation is present regardless of the direction of the current. Furthermore, it does not explain the disappearance of the differences in electrical performance between the rings after the test has been stopped and without power.

- Electromigration, which is defined as the displacement of atoms in a conductor induced by a flow of electrons, occurs mainly at high current densities, between 10 and $10^3$ A/mm$^2$. In the case of the study, according to Hertz's theory, contact between a wire and one face of a V-groove on a ring involves a contact area of $1.84 \times 10^{-4}$ mm$^2$. For a current of 0.25 A per contact, this results in a current density of $0.14 \times 10^4$ A/mm$^2$, which is theoretically sufficient to produce electromigration effects. In order to understand how this phenomenon could induce a difference in electrical performance between the cathode and anode rings, it is interesting to look at the consequences of electromigration in both current directions:

* Case 1: The current flows from the wire to the ring, a situation referred to as "cathode ring" (odd rings of Groups 1 and 2; even rings of Group 4 up to $3 \times 10^7$ revolutions).

* Gaps are created in the ring.

* Accumulation of metal ions on the wire.

* Case 2: Current flows from the ring to the wire, situation referred to as "anode ring" (even rings of Groups 1 and 2; odd rings of Group 4 between $3 \times 10^7$ and $5.9 \times 10^7$ revolutions).

* Gaps are created in the wire.

### Accumulation of metal ions on the ring.

The results showed that case 2 gave a poorer electrical performance than case 1. It is possible to distinguish two origins for the poor electrical results: the wire or the ring. The first requires the contact resistance to be high but fairly stable around a certain value. However, this does not seem to be the case, as shown in Fig. 9. In the second case, the electrical performance of the channel becomes poor when the wire encounters an accumulation of metal ions on the ring. This accumulation causes periodic disturbances as seen in Fig. 9. This may be one explanation for the appearance of the phenomenon after a certain number of revolutions. However, despite the coherent arguments, there were no notable differences in the wear of samples between the cathode and anode paths (except for the loss in thickness of the gold plating). This highlights the fact that electromigration is a minority phenomenon in the contact.

The three phenomena proposed above do not fully explain the differences in electrical performance between the cathode and anode rings. Looking at the data in Fig. 9, the periodic increases in contact voltage lead us to believe that there was a defect point on the rings. However, at the end of the tests, surface observations do not reveal any difference in wear between the rings or any noticeable defects. Consequently, this defect point may be due to wear of the samples in the contact between the ring and the wire. On the other hand, debris from a ring or a wire is normally conductive and cannot cause such significant variations in contact voltage. However, an oxide can cause this type of variation, as it will significantly affect the electrical contact area between the two surfaces. On the wires, green spots in the lubricant, identified as oxidized copper particles, were observed. It is thought that the copper from the gold-plating plays a role since the differences in electrical performance between the rings are also observed with other gold platings containing copper, but are absent with pure gold plating. Whichever the gold-plating, gold-rich clusters (>95%wt) form on the wire, triggering the formation of a galvanic couple between the copper from the wear of the gold-plating and the gold present on the wires. Thus, the time of appearance for the difference in electrical performance between the rings is explained by the formation of a gold-rich layer on the wires, the wear of the gold plating causing the presence of copper-containing debris in the contact, and the oxidation of the copper. However, this does not explain why one direction of current is better than the other.

The hypothesis developed here to explain the observations made is linked to the mobility of oxidized debris ($Cu_xO_y$) in the contact. This oxidized debris can be, for example, a copper (II) oxide, CuO, which is known to deteriorate contact properties. Among the insightful works on this issue, Tamai studied in detail the effect

<figure><img src="image_11.png" /><figcaption>Fig. 8 SEM-EDX observations—BSD—(a) of the wear track on a wire (after $6.10^7$ revolutions, with cleaning)—$x750$; and (b) of the wear track on one face of a V-groove—$x1000$ (1 A per V-groove; anode ring)</figcaption></figure>
<table><caption>Table 5 EDX Results linked to Fig. 8</caption><thead><tr><th rowspan="3">Zone</th><th colspan="4">A (wire)</th><th colspan="4">B (ring)</th></tr><tr><th colspan="4">Average weight concentration (%wt)</th><th colspan="4">Average weight concentration (%wt)</th></tr><tr><th>Au</th><th>Ag</th><th>Cu</th><th>X</th><th>Au</th><th>Ag</th><th>Cu</th><th>X</th></tr></thead><tbody><tr><td>1 Blue</td><td>54</td><td>33</td><td>13</td><td>0</td><td>83</td><td>0</td><td>7</td><td>10</td></tr><tr><td>2 Red</td><td>97</td><td>0</td><td>3</td><td>0</td><td>75</td><td>0</td><td>17</td><td>7</td></tr><tr><td>3 Green</td><td>—</td><td>—</td><td>—</td><td>—</td><td>91</td><td>0</td><td>9</td><td>1</td></tr></tbody></table>

of oxide growth on contact resistance in copper contact elements [15]. He showed that cuprous oxide Cu₂O is the first to form in a relatively thin layer and has almost no electrical effect, whereas cupric oxide CuO, which forms subsequently, causes a significant increase in contact resistance, associated with a change in the transport mechanism linked to its greater thickness. In CuO, the copper atom has a double bond with the oxygen atom. The difference in electronegativity between the copper atom (1.9) and the oxygen atom (3.4) makes their bond polar, with the oxygen atom acting as the cathode and the copper atom as the anode:

- In the case where the current flows from the wires toward the ring cathode ring, (Fig. 10(a)), the oxygen atom is oriented toward the wire and the copper atom toward the ring. On the one hand, the oxygen atom has little affinity with gold, and on the other hand, the bond energy between the copper atom and the oxygen atom (343 kJ/mol) is higher than the bond energies between the copper atom and the gold atom (232 kJ/mol) or another copper atom (202 kJ/mol). Whether on the wire or on the ring, copper oxide does not form preferential bonds with samples.

- In the case where the current flows from the ring toward the wires, anode ring (Fig. 10(b)), the oxygen atom is oriented toward the ring and the copper atom toward the wire. This orientation facilitates the bonds between the oxygen atom of the copper oxide and the copper atoms of the gold plating.

Consequently, the orientation of the copper according to the current polarity affects its mobility. In the first case, the copper oxide does not form preferential bonds with the samples, while in the second case, the copper oxide forms preferential bonds with the ring, which has an impact on its mobility in the contact (this can have an impact on the wear of the gold plating; hence, the additional wear when the ring is anode). This makes it possible to explain the effects of the current direction despite a similar aspect of wear, the stopping of the differences during periods of non-power supply, and the reversibility of the phenomenon (in a non-powered situation, friction can break previously established bonds fairly quickly).

<figure><img src="image_12.png" /><figcaption>Fig. 9 Screenshot of the contact voltage as a function of time obtained on the oscilloscope for the anode channel</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 10 Orientation of the copper oxide depending on the direction of the current—(a) when the current flows from the wire to the ring; and (b) when the current flows from the ring to the wire</figcaption></figure>

## 5 Conclusion

In this study, the influence of current was investigated on sliding electrical contacts between gold alloy wires and gold alloy-plated rings. The design of the test device allows four groups to be run under different test conditions. Here, the only parameters that changed were the current intensity and polarity. During the test, the electrical resistance of each channel was recorded, and at the end of the test, the surfaces were observed using an optical microscope and an SEM-EDX.

The influence of current intensity was measured using three amperages: 0, 1, and 2 A per V-groove. The electrical results show that the higher the current intensity, the lower the channel resistance. This reduction in channel resistance with current has been attributed to Joule effect heating. The surface observations gave similar results except regarding the thickness loss of the gold alloy plating of the ring, which was significantly greater in the absence of current.

The influence of the current direction has been observed on the different groups. Current flow in a group of 4 rings requires an alternation between cathode and anode rings. The electrical results have shown a much lower channel resistance when the rings were cathodes. This difference in electrical results between the cathode and anode rings appeared after several million revolutions and was reversible. In addition, cross-sectional observations have shown additional wear on the anode rings. The speculative assumption put forward to explain this phenomenon is linked to the mobility of copper oxide debris in the contact: when the current flows from the wires to the ring, the copper oxide does not form preferential bonds with the samples, whereas in the opposite case, the copper oxide forms preferential bonds with the ring having an impact on its mobility in the contact. The mobility reduction of the oxides involves a variation of the contact area between the surfaces, and therefore of the contact resistance.

To go further, additional analysis would be required, consisting of recovering wear debris present in the lubricant or on surfaces at different stages in the life of the contact, and carrying out chemical analysis to try to understand the nature of this debris. Moreover, it is possible to imagine other test conditions to observe if the phenomenon still occurs. For example, a renewal of lubricant would drain debris away from the contact, and the difference in electrical results between the cathode and anode rings may disappear.

## Acknowledgment

This work was carried out under a “CIFRE” agreement managed by the ANRT (a cross-sector public-private network for French research) between the GeePs laboratory and Everaxis Aerospace and Defence company, with funding from Everaxis.

## Conflict of Interest

There are no conflicts of interest.
# Data Availability Statement

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

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