# How transfer film formation in bronze/silver-graphite sliding contact drives its electrical performance

Timothée Doutriaux, Siegfried Fouvry, Sébastien Larousse, Manon Isard, Olivier Graton, Maria Isabel de Barros Bouchet

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Timothée Doutriaux, Siegfried Fouvry, Sébastien Larousse, Manon Isard, Olivier Graton, et al. How transfer film formation in bronze/silver-graphite sliding contact drives its electrical performance. Wear, 2025, 570, pp.205976. 10.1016/j.wear.2025.205976. hal-05378051

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Submitted on 22 Nov 2025

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Distributed under a Creative Commons Attribution 4.0 International License
How transfer film formation in bronze/silver-graphite
sliding contact drives its electrical performance

Timothée Doutriaux<fnref n="1" />,<fnref n="2" />*, Siegfried Fouvry<fnref n="1" />,<fnref n="3" />, Sébastien Larousse<fnref n="2" />, Manon Isard<fnref n="4" />, Olivier Graton<fnref n="1" />, Maria Isabel de Barros Bouchet<fnref n="1" />

E-mail: [doutriaux.timothee@gmail.com](mailto:doutriaux.timothee@gmail.com)

**Keywords:** Electrical contact, Transfer film and third bodies, Solid lubrication, Wear.

**Abstract**

Recent interest in improving lifespan of electrical slip rings for application in wind turbines has renewed the need for deeper understanding of sliding electrical contact operation. In contact slip rings using graphite-based brush technology, the formation of transfer film, or "patina", on the surface of the metal ring is a well-known phenomenon [1], [2]. In this scope, the study of its formation in a bronze/silver-graphite contact has been conducted. The test bench is based on a real industrial slip ring specific for wind turbine application. The evolution of the transfer film was investigated by means of different surface characterization methods. EDX analyses permitted to establish a direct relation between transfer chemical composition, transfer thickness (h) and electrical resistance of the contact (Rc), highlighting the role of the transfer film in slip ring electrical performance. In particular, the presence of contaminating Cu2O oxides is emerging as a key factor in controlling both h and Rc. Optical microscopy and profilometry, allowed to propose a scenario of build-up following by destruction of the transfer film, explaining at the same time the origin of these Cu2O oxides. Focused Ion Beam (FIB) nanomachining, followed by

<footnote n="1">¹Ecole Centrale de Lyon, CNRS, ENTPE, LTDS, UMR 5513, 69130 Ecully, France</footnote>

<footnote n="2">²Everaxis Industries, 18 rue de Montréal, BP 439, Ville-la-Grand, Annemasse, 74108, France</footnote>

<footnote n="3">³MinesParis, PSL University, Centre for Material Sciences (MAT), UMR 7633 CNRS, 91003 Evry, France</footnote>

<footnote n="4">⁴Everaxis Aerospace and Defence, 5 avenue Denis Papin, BP 36, Le Plessis-Robinson, 92350, France</footnote>

<footnote>* Corresponding author</footnote>
Transmission Electron Microscopy (TEM) enrich this scenario and correlates the surface
observations, revealing a strong evolution of the composition and morphology of the transfer pads
from very carbonaceous, poorly graphitized transfers to a much more metallic and graphitized
composition. In this last state, a lamellar structure is observed, with successive lamellae of silver
separated by thin graphite layers. Raman spectroscopy also permits to study the crystalline
structure of the graphite transferred on the ring surface and shows a progressive re-organization
of the crystallites. While this reorganisation should be beneficial in a tribological perspective, it is
important to note that this mechanism is entirely independent of both the growth-destruction of
the transfer film and the evolution of Rc.

# 1. Introduction

The reliable operation and longevity of wind turbines are critical for the sustainable production of renewable energy. Within these turbines, electrical slip rings play an essential role, enabling the transmission of electrical signals and power from the stationary systems to the rotating components, usually embedded within the blades. These components can be used for monitoring, such as strain gauges, or for icing prevention, like electrical resistances for example. Ensuring the durability and the performance of these slip rings is therefore crucial, as any failure could lead to costly maintenance and downtime.

Graphite-based brush technology is commonly used in electrical slip rings and requires regular
maintenance operations. Managing these sliding electrical contacts is thus one of the key
challenges in improving the durability of the electrical slip rings. During operation, the sliding of
the brushes induces the formation of a transfer film, commonly referred to as "patina", on the
surface of the metal rings on which they slide [1], [2]. This transfer film is a complex layer formed
as a result of material transfer from the brush to the ring [3], [4]. The transferred material can
affect the electrical and tribological performance of the contact and therefore plays a critical role
in the overall efficiency and reliability of the slip ring. Previous studies have highlighted the
importance of the transfer film in modifying the contact resistance and protecting the underlying
metal surface [2], [4], [5], [6]. The existence of two "modes" of operation of the metal-graphite
brushes has also been demonstrated in the 80's [7], [8]. The two modes depend directly on the
chemical composition and structure of the transfer film. In mode I, the film has a lamellar structure
of parallel silver and graphite layers. In this mode, the sliding electrical contact operates in a high
resistance/low friction state. Conversely, in mode II, the transfer film is composed of silver
fragments welded together with some graphite particles embedded. This mode is associated with
low resistance/high friction operation. According to the authors, the transition from mode I to
mode II would occur when the surface temperature reaches about 160°C, temperature where the
water vapour is desorpted from the surface and can no longer help the graphite to shear and bond
with silver. All these studies demonstrate the extensive knowledge of sliding metal-graphite
contacts. However, in most of the work cited, experiments were carried out under severe
conditions compared to those encountered in wind turbine applications. In addition, the
experiments were usually short, with test durations usually limited to a few hours. Furthermore,
most of these studies were carried out between 1970 and 1995. The massive advances in surface
characterisation techniques since then could allow to provide a deeper understanding of the
contact behaviour.

This study aims to fill this knowledge gap by investigating the formation and evolution of the
transfer film in a bronze/silver-graphite contact, commonly used in wind turbine slip rings. Using
a test rig designed to replicate industrial conditions, the electrical resistance of the sliding contact
was monitored. A comprehensive suite of surface characterisation techniques, including routine
analyses such as optical microscopy, optical profilometry, Raman spectroscopy, and more detailed
analyses such as Scanning Electron Microscopy (SEM) or Energy Dispersive X-ray spectroscopy
(EDX) were used to analyse the transfer film's morphology, thickness, and crystalline structure.
Focused Ion Beam (FIB) processing was also carried out on the transfer pads to produce cross
sections observable by Transmission Electron Microscopy (TEM).

These analyses revealed a correlation between transfer chemical composition, transfer thickness
and the electrical contact resistance, providing new insights into the role of the transfer film in
slip ring performance. In addition, the study reveals a significant evolution in the composition and
structure of the transfer film over time. These findings contribute to a deeper understanding of
the mechanisms of transfer film formation and its influence on the electrical properties of slip
rings.

**2. Materials and methods**

**2.1. Device under test**

An industrial slip ring (see Figure 1.a and .b) is composed of twelve equally spaced 34 mm
diameter bronze rings mounted on a rotating shaft. Polymer discs placed in between every two
bronze rings ensure electrical isolation. Two metal-graphite brushes slide on the same track of
each bronze ring (Figure 1.c). The brushes are obtained by a sintering process of graphite and
silver powders, resulting in a heterogeneous 80 wt% silver and 20 wt% graphite composition, as
illustrated in Figure 1.d. Each brush is welded to the edge of a metal strip, which is fixed at the
other end to the stator part of the bench. The elastic bending of the strip applies the normal force
of the brush onto the ring. The brushes have a parallelepiped shape with dimensions of
3 × 8 × 4 mm³, except for one face, which is concave. This concave face matches the ring's radius of curvature, creating a curved plane-to-plane contact (see Figure 1.e).

<figure><img src="image_2.png" /><figcaption>Figure 1. (a) 3D CAD view, (b) photography of an industrial slip ring, (c) schematic of the electrical path in a single ring-brush assembly, (d) SEM image of a sintered silver-graphite brush surface, (e) photography of the curved plane-plane contact.</figcaption></figure>

A power supply provides a 5 A DC electrical current, which flows through all the sliding electrical contacts. For each ring, the current enters by one of the two brushes (called “A”), flows through the ring, and leaves by the second brush (called “B”) (see Figure 1.c). On the stator side, a soldered wire provides the electrical connection from brush B of the first ring to brush A of the second ring. This structure is repeated for all rings so that all sliding contacts are connected in series. The last brush, brush B of ring 12, is wired back to the power supply to close the circuit loop. This electrical structure, specific to this study, differs from an industrial slip ring and allows the same electrical load to be applied to all the contacts.

## 2.2. Test rig

The study of the brush wear and electrical behaviour of the different contacts in the slip ring was conducted on the test bench presented in Figure 2.a. This setup enables the evaluation of commercially available industrial slip ring as described in Section 2.1, under controlled laboratory environment. The engine block consists of an asynchronous motor, coupled with a speed reducer
that allows a precise tuning of the rotating speed. It drives the shaft's rotational motion. The entire structure is installed on dampers to minimise vibration transmission and mechanically isolate the bench from its environment. The maximal speed allowed by the speed reducer is approximately 81 rpm, which corresponds to a linear sliding velocity of 0.144 m/s at the circumference of the rings. An incremental encoder is mounted on the drive shaft to follow the rotating velocity and count the number of revolutions throughout the test. It also provides the angular position of the shaft with a step of 1.8 degrees (200 pulses per revolution).

<figure><img src="image_3.png" /><figcaption>Figure 2 (a) Test bench used for the study, (b) photography of slip ring's axis with the special conducting disc between rings 5 and 6, (c) schematic of the voltage drop measurement method, (d) equivalent electrical structure at contact 6A.</figcaption></figure>

The electrical contact resistance is calculated from a voltage drop measurement, by means of a Keysight 3458A Digital MultiMeter (DMM). Due to material limitations, the voltage drop could only be measured on one of the 24 sliding contacts, the brush A of ring 6. A pseudo four-point probe configuration is used to measure the voltage drop (see Figure 2.b, c and d). All the other contacts, although not electrically instrumented, were used to provide interrupted sliding scars in
order to characterise the evolution of the wear track with the test duration. As previously described, the current flows through the brush 6A, ring 6 and brush 6B. One of the multimeter probes is connected to the brush 6A. Exceptionally, a conducting disk instead of a polymer disc separates the ring 6 from the adjacent ring 5 (see Figure 2.b and c). Brush 5A is then used as the second probe for the multimeter. Obviously, no current flowed through ring 5 and its brushes to make the measurement possible. In this configuration, the measured potential drop results from three contributions: the contact resistance between the brush and the ring, the brush body resistance and metallic strip body resistance (Figure 2.d). The last two contributions are considered constant throughout the tests, and their combined value is approximately 620 µΩ, which is considered negligible.

A software developed with LabVIEW 2022 controls and monitors the test. An acquisition device provides the interface between the PC and the measurement devices. The voltage measurement is triggered by the signal from the encoder so that each measurement is made at specific and constant position around the ring. The multimeter used has a sample rate of 7 500 readings per second. At a pre-selected cumulated number of cycles, the average value of the contact resistance is calculated, giving a contact resistance value for that cycle. The standard deviation is also calculated from the measurements during this specific cycle, giving what is referred to the electrical noise. It can be interpreted as an indicator of the stability of the resistance along the wear track. These two values are used to evaluate the electrical performance.

## 2.3. Test conditions and protocol

The results presented here actually come from 3 distinct tests. All three tests were run in the exact same conditions, presented in Table 1. For practical reasons, there was no electrical measurement during the first two tests, but they permitted to better understand the tribological and wear behaviour of the contact. As explained in Section 2.1 the bench allows, through the rotation of a slip ring, to test 12 microsystems "Brush A – Ring – Brush B" at the same time. To study the evolution of the contact over time, the motor was stopped at specific number of rotations to collect two of the rings each time. The test rig was then immediately restarted. Each test carrying 12 rings, this procedure is repeated 6 times for each test, until all the rings are collected on the axle. In parallel, the samples collected undergo a characterization routine, composed by different surface analyses techniques. Collecting systematically two rings introduces a redundancy and provides insight into the repeatability of the behaviour. A representative subset of these revolutions was selected for this study.
<table><caption>Table 1. Test conditions.</caption><thead><tr><td>Parameter</td><td>Value</td></tr></thead><tbody><tr><td>Slip ring material</td><td>Bronze</td></tr><tr><td>Slip ring diameter (mm)</td><td>34</td></tr><tr><td>Slip ring roughness Ra (μm) ; Rz (μm) ; Rsk<br/>Transversal direction</td><td>0.4 ; 1.8 ; 0.5</td></tr><tr><td>Brush material</td><td>Silver-Graphite (80wt% Ag – 20wt%C)</td></tr><tr><td>Brush face area (mm²)</td><td>24</td></tr><tr><td>Brush roughness Ra (μm) ; Rz (μm) ; Rsk<br/>Transversal direction<br/>Longitudinal direction</td><td>0.8 ; 7.5 ; -1.2<br/>0.8 ; 5.4 ; -2.0</td></tr><tr><td>Brush normal force (N)</td><td>0.5</td></tr><tr><td>Sliding speed (m/s)</td><td>0.144</td></tr><tr><td>Electrical current (A)</td><td>5</td></tr><tr><td>Gas environment</td><td>Ambient air, ~23°C, ~40 %RH</td></tr></tbody></table>

## 2.4. Characterization techniques

Different surface characterization techniques were employed to a comprehensive understanding of the surface modifications and the formation of the tribofilm, which are crucial for assessing the wear and electrical performance of the contacts. Optical micrographs and rapid 3D profiles were obtained using a Keyence VHX-7000 numerical microscope. A detailed dimensional information of the transfers formed on the ring surface was provided by a Wyko NT9300 optical profilometer. All 3D profiles and thickness measurements presented in this paper were conducted in Vertical Scanning Interferometry (VSI) mode, with a 20x objective. A microscope Tescan Mira3 FEG, associated to an Oxford Instruments XMAXN 80 EDX detector, provided all the presented SEM and EDX analyses. The electron beam was set at an accelerating voltage of 10 or 15 keV.

Raman spectra were recorded on the rings using a Renishaw InVia Raman microscope. A 633 nm wavelength laser was employed for excitation, and focused on the samples using a 50x objective. Spectra were collected in Extended Mode, covering a range from -100 cm⁻¹ to 3200cm⁻¹. Each measurement was conducted with a 10-second excitation time, using a laser power limited to 5%, corresponding to around 0.5 mW on the sample. Two accumulations were performed to enhance the signal-to-noise ratio.
A Focused Ion Beam (FIB) was employed to realise cross sections in the body of the transfer film. The nanomachining is preceded by a step of deposition. This is done in two steps: a first Pt/C deposition assisted by electron beam, to grow the layer without damaging the sample underneath, followed by a second deposition on top, this time assisted by an ion beam. The cross sections are then machined in the surface, and thinned down to less than 100 nm thick so it can be observable by Transmission Electron Microscopy (TEM). The TEM 2100F from JEOL has provided images of the thin blades. It is a 200 kV field emission analytical microscope with a resolution of 0.2 nm.

# 3. Results

## 3.1. Evolution of electrical contact resistance

Figure 3 displays the mean electrical resistance and noise during the first 1M cycles (corresponding to 107 km of brush sliding distance). Three main periods can be distinguished on this graph. At the beginning of the test, a fast and large increase in contact resistance is measured. This increase lasts until 100 000 cycles, where the resistance reaches a maximum value of 40 mΩ. Then, we observe a little decrease from 40 mΩ to 25 mΩ. The value scatters around 25 mΩ between 200 000 and 500 000 cycles. All the way, the electrical noise stays at a value around 5 mΩ. After around 520 000 cycles, a very clear decrease of both the electrical resistance and electrical noise is observed, reaching respectively near 10 mΩ and 2.5 mΩ. The change in the aspect of the curves is also striking, as the values are much more stable. This suggests that a change in the tribological system is to be expected.

<figure><img src="image_4.png" /><figcaption>Figure 3. Evolution of electrical resistance and noise as a function of the number of cycles and related sliding distance.</figcaption></figure>
## 3.2. Structural evolution of the transfer film

Number of different surface characterizations have been carried out, on both the rings' and the brushes' surfaces. Figure 4 shows optical micrographs of the two surfaces after test at different number of cycles. Typical transformations are visible, with material transferred on the ring surface, which suffer almost no wear, whereas brush surface shows abrasion marks, wear, and probably some transfers from the ring too.

<figure><img src="image_5.png" /><figcaption>Figure 4. Optical micrographs of rings (left) and brushes (rights) after 100 and 5M cycles.</figcaption></figure>

However, specific attention was drawn to the ring due to the material transferred onto its surface, which can alter the tribological and electrical behaviour of the contact. Optical microscopy imaging, associated with Energy Dispersive X-ray Spectroscopy (EDX) mapping, give visual, structural and compositional information about the tribofilm. Additionally, optical profilometry gives information on the thickness of the transferred material. Figure 5 shows these three analyses, performed on thick transfers formed on the surfaces of the rings after different number of cycles. After just a hundred cycles, brush material has already transferred onto the ring (Figure 5.a). They are however very few and localised at the edges of the sliding zone, as shown in Figure 4.a. These first transfers are few hundred microns long. They are naturally composed of carbon and silver, and these two distinct phases are clearly observed at this state (see Figure 5.a, optical images and EDX mappings). Between 100 and 1 000 cycles, more and more similar transfer pads appear on the ring surface, showing also larger dimensions (Figure 5.b). At the prow of several transfers, finely grinded silver/graphite wear debris accumulate (Figure 5.b). After 10 000 cycles, the heterogeneous structure of the first transfer pads is no longer visible (Figure 5.c). They now seem to be entirely covered with a dense, compacted amalgam of silver and graphite. Horizontal
marks, parallel to the sliding direction, are visible on the transfer pad. The 3D profiles presented in Figure 5 show all along a growth in the thickness of the transfers.

<figure><img src="image_6.png" /><figcaption>Figure 5. Transfer pads and films on the ring surface after different number of cycles. From left to right, optical microscopy image, 3D profile, and EDX mappings of silver and carbon. For all 3D profiles, the window is 0.44x0.59 mm² and the z-scale goes from -2 µm to 7 µm.</figcaption></figure>
Figure 5.d shows a transfer pad at 100 000 cycles in a similar homogeneous structure than that after 10 000 cycles. Nonetheless, it is much thicker, with a mean height of around 4 µm and culminating at near 10 µm in some points. The transfer pads also expand on the ring surface, mostly in the sliding direction but also crosswise, to a lesser extent. We were indeed able to find some transfers several millimetres long. We also notice a more chaotic aspect, with the presence of powdery debris, pieces of transfer pads detached shown with optical microscopy, and height irregularities with profilometry measurement. After 500 000 cycles (i.e. 54 km of brush sliding distance), most of the ring surface is now covered with thin transfers, which appear brighter than the thick pads previously observed. This change in appearance could indicate an increasing silver content (Figure 5.e). We are now more inclined to speak of transfer *film* instead of *pads*. Afterwards, and until 5 000 000 cycles, the evolution of the ring surface is more continuous. On the surface observations (Figure 5.f and Figure 5.g), the film appears progressively brighter and smoother. Further detailed inspection has revealed the presence of blisters within the transfer pads. Figure 6 presents optical micrographs of these blisters, combined with 3D profiles obtained with the optical microscope. The blisters observed are few microns high above the surface, and can even reach 10 µm (see Figure 6.a). It is noteworthy that these blisters were observed as early as 10⁴ cycles (see Figure 6.a) with some persisting even after 5 million cycles as illustrated in Figure 6.c. This blister phenomenon promotes the detachment of large platelets of transfer layers.

<figure><img src="image_7.png" /><figcaption>Figure 6. Example of blisters on different transfer films, (a) after 10,000 cycles, (b) after 3.5 million cycles, (c) after 5 million cycles. The red rectangle in the images on first row indicates the position of the image in second row. Third row is a 3D profile of the blister obtained with the optical microscope. It gives an idea of the height of the blisters.</figcaption></figure>
### 3.3. Correlation between transfer thickness and electrical performance

Figure 7.a compares the evolution of electrical contact resistance and the transfer thickness as a function of the siding cycles using a log scale to better illustrate the fast initial transient period. Transfer thickness values are calculated from typical 3D profiles as displayed in Figure 5, using the uncovered bronze substrate as a reference. For each transfer pad, the mean height value is computed. Several values (from 6 to 12) are obtained from different pads for each number of cycles. The graph displays the mean value at each number of cycles, whereas the error bars show the associated standard deviation. The results indicate an increase in thickness from 100 to 100,000 cycles, followed by a gradual decrease until 1 million cycles, consistent with the trend observed in the 3D profiles from Figure 5. From Figure 7.a, a correlation between mean transfer thickness and electrical resistance is evident. When plotting these two values on a scatter plot (Figure 7.b), a linear relationship seems to emerge.

<figure><img src="image_8.png" /><figcaption>Figure 7. (a) Evolution of transfer thickness and electrical resistance as a function of the number of cycles. (b) Electrical resistance as a function of mean transfer thickness. The linear regression has been calculated without taking into account the out-of-range point (*).</figcaption></figure>

It is important to note that the values of electrical resistance were obtained from a single test, while the transfer thickness values were derived from several other samples. The only point where both electrical resistance and transfer thickness values were obtained from the same sample is displayed in green. Its position fits well with the linear regression extracted from the interrupted tests. However, one data point deviates from this linear trend, exhibiting a high electrical resistance (30 mΩ) despite the smallest transfer thickness (0.8 µm). This outlier corresponds to the last point on Figure 7.a, collected just before the sharp drop in electrical resistance. Although the mechanisms appear robust, there are inherent uncertainties in the
kinetics of transfer formation due to experimental biases, which can cause variations of few thousand cycles between samples. For clarity, the linear trend line shown in Figure 7.b has been calculated without including this outlier. The electrical contact resistance can therefore be approximated through the following linear relationship:

$$R_c = K_h \times h \quad (1)$$

with $K_h = 12.64 \text{ m}\Omega/\mu\text{m}$.

### 3.4. Correlation between transfer composition, transfer thickness and electrical performance

The former investigation underlines a link between electrical resistance and transfer thickness. However, it seems important to explain this evolution and, in particular, to compare these two values with the chemical composition of the third body. EDX surface analysis conducted on different transfers confirms the correlation between the electrical resistance and the atomic composition of these latter (Figure 8). The minimum electrical resistances (≈ 10 mΩ) observed before 10⁴ cycles and then after 10⁶ cycles correspond to significant silver contents (> 35at%). On the other hand, the region associated with the highest electrical resistances (> 20 mΩ) between 10⁴ and 10⁶ cycles is associated with a significant increase in the copper and oxygen contents of around 15at% and 20at% respectively. It is therefore possible to explain the variation in electrical resistance not only as a function of the thickness of the transfers, but also as a function of their composition.
<figure><img src="image_9.png" /><figcaption>Figure 8. Evolution of the transfer composition [at%] from EDX analysis (balance: carbon), transfer thickness and electrical resistance as a function of the number of cycles.</figcaption></figure>

This chemical analysis suggests that the increase in electrical resistance would be directly related to a significant rising in the copper content of the transfer. Copper, a non-noble element, reacts with the oxygen of ambient air to form oxides, causing an increase in the oxygen content. Figure 9 suggests a quasi-linear evolution of the oxygen concentration as a function of the copper.

<figure><img src="image_10.png" /><figcaption>Figure 9. Evolution of thickness and oxygen concentration of the transfers as a function of copper concentration measured in the transfers.</figcaption></figure>

Oxygen reacting with copper promotes the formation of copper oxides, which are known to significantly decay the conductivity of the debris beds. To clarify the nature of these copper oxides, micro-Raman analyses have been performed outside the sliding track and within transfer
pad after 10⁵ cycles (Figure 10). Both suggest the presence of Cu₂O type oxide on the native surface but also within the thick transfer pad. From this analysis, it can be concluded a link between the electrical resistance and copper oxide concentration.

<figure><img src="image_11.png" /><figcaption>Figure 10. Raman spectrum on the bronze ring outside the track (left) and within a transfer pad (right).</figcaption></figure>

Having established a link between the chemical composition of the transfers (i.e. copper contamination) and the evolution of the electrical resistance, it seems interesting to refine the analysis of the structure of the transfers, in particular to explain the non-monotonous evolution of the transfer thickness and the final transition towards thin, silver rich and electrically conductive transfers.

### 3.5. TEM observation of the transfers

Focused Ion Beam (FIB) cross sections have been carried out on three transfer pads at different number of cycles. A thin blade is extracted from each transfer, transversal to the sliding direction, and then observed with Transmission Electron Microscopy (TEM). These observations, presented in Figure 11, give very useful information on the real composition and structure of the transfers, and their evolution. On these TEM images, silver appears black since it is the heavier element of the sample, whereas bronze is dark grey and carbon is light grey. On each low magnification image (Figure 11.1) the red dashed lines delimit the transfer from the substrate and the platine protective layer, while the white squares indicate the location where high-magnification images in Figure 13 were taken). From 100 to 5 million cycles, we observe a significant enrichment in silver across the entire body of the transfer, with a notable concentration near the surface (see Figure 11). We also notice different thicknesses (0,7 µm, 2,5 µm and 1,2 µm respectively for Figure 11.A, .B and .C), confirming the evolution shown with optical profilometry (see Figure 7.a). However, the exact values can be different for the two techniques because optical profilometry provides a global mean height value whereas FIB cross sections give localised height values within a single transfer.
<figure><img src="image_12.png" /><figcaption>**Figure 11.** TEM observations (bright field) of thin blades nanomachined in different transfer pads showed in the optical images on the left. The white squares indicate the location where high-magnification images (Figure 12) were taken.</figcaption></figure>

At 100 cycles, contrary to what the optical images might have suggested, the transfer is mainly composed of carbon with metallic particles incorporated. Large, elongated silver particles can be observed close to the surface (Figure 11.A2). Some can be more than 1 µm long and about 20 nm thick. The sample also shows much smaller, round metallic particles, which appear less contrasted with the carbonaceous matrix (Figure 11.A2 and .A3). They are about 5 nm in diameter or less. At 10 000 cycles (Figure 11.B), the thick silver particles appear significantly more compacted but are still embedded within the carbon. Additionally, the round metallic nanoparticles, 5 nm in diameter, are still visible within the carbon matrix (Figure 11.B3). At 5 million cycles, a thin gap, parallel to the bronze surface, separates the transfer into two distinct phases (Figure 11.C1). Above the gap, the transfer consists almost entirely of silver and appears very dense (Figure 11.C2). It is arranged in a lamellar structure, with very thin carbon layers (around 10 nm thick) separating the metal lamellae (Figure 12.C). Below the gap, the structure is more similar to the transfer observed at 10 000 cycles, with more ovoid silver particles embodied in a carbon matrix (Figure 11.C3). They are less than 100 nm in diameter.
Figure 12 displays TEM images of the thin transfer sections at a higher magnification, allowing visualisation of the crystalline arrangement of the different phases. White squares in Figure 11.1 give an indication of the position of each high-magnification image presented in Figure 12. The diffraction patterns of several zones in each image are also presented. The transmitted electrons are diffused in a predictive way if the structure is periodic (crystalline). The resulting pattern then shows bright spots, corresponding to gathered electrons. On the contrary, for amorphous structures, the electrons are randomly diffused, resulting in a uniform blurred halo. Diffraction patterns A1, B1 and C1 (Figure 12) confirm silver as the predominant metallic particles in the carbon matrix. At 100 cycles, carbon is mostly amorphous (Figure 12.A3) but few organized parallel planes are visible, typical of the graphitic structure (Figure 12.A2). This crystallised structure becomes significantly more prominent after 10 000 cycles (Figure 12.B2), and 5 million cycles where it is the dominant form of carbon (Figure 12.C2).

<figure><img src="image_13.png" /><figcaption>Figure 12. High-magnification TEM images (bright field) of the three transfer pads. Bottom: diffraction patterns of subregions, showing metallic particles (A.1, B.1, C.1) and graphite crystals (A.2, B.2, C.2). The presence of amorphous carbon is also evidenced in A.3.</figcaption></figure>

The very near interface between the transfer and the bronze is also a valuable source of information. On all samples, we can observe what we identified as a thin layer of copper Cu₂O oxides (Figure 11.A3, B3 and C3). This layer is around 3 nm thick, which is consistent with the
literature [9, p. 151]. It seems that the interface is made by the contact of this oxide layer with the
carbon matrix, but in reality both amorphous carbon and embodied silver nanoparticles make
contact with it (see Figure 13).

<figure><img src="image_14.png" /><figcaption>Figure 13. High magnification TEM image (bright field) of the bronze-transfer interface after 100 cycles.</figcaption></figure>

**3.6. Investigation of graphite crystallinity of the transfer film by Raman spectroscopy**

We performed several Raman measurements within the wear track of the ring to investigate the crystalline structure of the graphite in the transfer film. The analysed depth in Raman is commonly estimated to be a few hundred angstroms for graphitic materials [10], [11], [12]. On each ring, 8 measurements were made perpendicular to the direction of the track as shown in Figure 14.a. The average spectrum is then computed and is used to assess the crystalline graphite state within the transfer film. Figure 14.b shows the evolution of the Raman spectrum collected on the wear track as a function of the number of cycles. The spectrum of Highly Oriented Pyrolitic Graphite (HOPG), a very pure form of graphite with very few defects, is also displayed as a reference. For visible excitation, the first order Raman spectrum of a perfect crystalline graphite has a single feature at 1580 cm⁻¹, called the G band. It comes from the bond stretching of all the sp² carbon atoms in the rings that compose a graphite layer. In graphite samples with a certain amount of defects, a second band appears at around 1350 cm⁻¹, usually called the D band. This band is commonly assigned to the amount of defects in the crystal. Tuinstra and Koenig [13] revealed a linear relationship between the ratio of the D peak intensity, I_D, to that of the G peak, I_G, with the crystallite size, L_a. Later on, a general equation has been established for all excitation wavelengths λ_I in the visible range [14]:
$$ \frac{I_D}{I_G} = (2.4 * 10^{-10}) \lambda_I^4 * \frac{1}{L_a} \quad (3) $$

By recording Raman spectra and calculating the $I_D/I_G$ ratio for different rings, we were able to compare the graphite crystallinity during the test. In all the spectra collected on the wear track (see Figure 14.b), the D band is very intense – always more than the G band – indicating a high amount of defects. The shift of the G band from 1580 cm⁻¹ in HOPG (indicated by the dotted line) to 1605 cm⁻¹ on the wear track is also evident and is characteristic of nanocrystalline graphite [15]. From 100 cycles to 1 million cycles, the increasing intensity of the G band compared to the D band is also noticeable.

<figure><img src="image_15.png" /><figcaption>Figure 14. (a) Schematic of the position of the 8 Raman measurements for each ring. Each red cross represents a measurement. (b) Raman spectrum of the ring's wear track for different number of cycles. (c) Evolution of the $I_D/I_G$ ratio recorded on the rings wear tracks.</figcaption></figure>

Figure 14.c displays the evolution of the $I_D/I_G$ ratio on the rings wear tracks as a function of the number of cycles. The results show a decreasing ratio during the first 30 000 revolutions, from 2.5 to 1.4. The measurement being performed at 633 nm, by using the equation (3), it traduces an evolution of the crystallite size from 15 nm to 28 nm. The ratio then reaches a plateau at 1.4,
indicating a steady-state. The ratio recorded on the graphite grain of a new brush shows the good quality of the initial graphite.

## 4. Discussion

### 4.1. Influence of transfer film chemical composition and thickness on electrical contact resistance

The former investigations underline a relation between the electrical contact resistance and the chemical composition of the transfers (Figure 8), which support the load carrying and therefore the current conduction. A key parameter to formalize this dependency could be the oxygen concentration. Indeed, it has been shown in previous fretting wear investigations of silver and gold plated electrical contacts that electrical failure can be associated with an increase in oxygen concentration [16], [17], [18], [19]. Figure 15 displays the electrical resistance as a function of oxygen concentration measured within the transfer. It supports this tendency since a fast rising of the electrical resistance as a function of the oxygen concentration is observed. A linear approximation gives a negative offset, which is not physically consistent. Hence, by extrapolating a positive electrical resistance offset around $R_{c0[0\%at]}$ = 3 mΩ, a simple power law formulation was approximated:

$$R_c = K_{[O at\%]} \times [O at\%]^{n_{[O at\%]}} + R_{c0[0\%at]} \quad (2)$$

with $K_{[O at\%]}$= 0.3 and $n_{[O at\%]}$=1.5.

<figure><img src="image_16.png" /><figcaption>Figure 15: Evolution of the electrical resistance as a function of the oxygen concentration measured in the transfers.</figcaption></figure>

According to Raman analyses (Figure 10) and taking into account the similar evolution of [Cu at%] and [O at%] (Figure 8), the oxygen concentration present in the transfer is mainly coming from
Cu₂O oxides embedded within the transfer. A key question is thus the explanation of the origin this Cu₂O oxides in the transfers, since they originate from the Ag-C brushes. To investigate this aspect, optical and SEM-EDX analyses of ring and brush surfaces were conducted at different number of rotation cycles (Figure 16). Values of [Cu at%] where obtained from the EDX mappings presented here, which were realised in the exact same conditions.

<figure><img src="image_17.png" /><figcaption>Figure 16. Optical and SEM-EDX comparison of brush and ring surfaces for different rotation cycles.</figcaption></figure>

The optical micrographs of the ring surfaces confirm the monotonous increase in surface coverage resulting from the transfer process (dark grey aspect). In the meantime, the worn surface of the brush displays a significant quantity of copper. However, this contamination is non-monotonous since it first increases from 100 to 10⁵ cycles, then decreases. These observations permit to construct the following scenario.

At the beginning of the test, the first transfer pads from brush to ring are rich in silver and carbon, with low concentration of copper (i.e. Cu₂O) (Figure 8). Indeed, the surface of the brush is not yet contaminated with copper transfer from the interaction with the ring (Figure 16) with a [Cu at%] of about 1.4%. With the continuous rotation, more and more Cu₂O oxides and potentially metallic bronze from the ring are transferred to the brush surface as displayed in Figure 16. After 10⁵ cycles, the copper concentration measured reaches nearly 5at%. Since the transfer process from brush to ring is still running, it can explain the rising of Cu₂O oxide concentration within the
transfer pads (Figure 8). However, after 10⁵ cycles, it was found that an increasing amount of the ring surface was covered by transfer (see Figure 16), reducing the proportion of direct contacts between the bronze surface (mainly covered by Cu₂O oxides, see Figure 10) and the Ag-C brush surface. Therefore, the Cu₂O oxide contamination of the brush surface tends to decrease, as confirmed in Figure 16 where the copper concentration decreases to 2.5 after 5.10⁶ cycles. As the brush surface becomes less contaminated by Cu₂O oxides, it can be understood the decrease of both [Cu at%] and [O at%] within the transfer pads. After this hypothesis, the presence of Cu₂O observed within the transfer layer covering the ring is in fact induced by a round trip, where Cu₂O are first transferred from ring to brush, and then back from brush to ring. This hypothesis also permits to explain the non-monotonous evolution of the electrical resistance, mainly affected by the presence of copper oxides.

However, electrical resistance also depends on the transfer thickness, h (see Figure 7.b). In fact, transfer thickness and copper concentration are also correlated, as suggested by Figure 9. Two hypotheses, probably combined, can be considered to explain this trend. First, it can be assumed that a certain amount of copper remains in metallic state in the debris bed. Acting as a binder, it would favor the cohesion of the debris bed and therefore promote a thicker third body layer. Secondly, the copper oxides, formed by highly polar covalent bonds, can promote cohesion within the debris bed by reacting with water molecules. This mechanism has already been observed for titanium oxides [20], [21]. This second hypothesis has the particular advantage of explaining the higher oxygen content compared to that of copper. Indeed, by complexing copper oxides Cu₂O with water molecules, the proportion of oxygen measured is increased. Micro Raman analyses did not reveal the presence of copper hydroxides. However, their effective presence is not to be excluded, since these structures are very unstable and can degrade under the effect of laser radiation.

In conclusion, this chemical analysis suggests that the simultaneous increase in the concentration of copper and oxygen associated with the formation of copper oxides or even hydroxides would induce an increase in the resistivity but also in the thickness of the transfers. These two combined effects cause a significant rising in electrical resistance as illustrated in Figure 7.b and Figure 15. However, looking ahead to very long life cycles, since the bronze surface will be no more in direct contact with the brush, the Cu₂O supply in the tribological process will be no more effective. Therefore, it can be concluded that this Cu₂O contamination process is only a transient phenomenon.

A next step in this discussion will now concern the mechanisms of transfer formation subsequent delamination, which removes the thick and low conductive transfers and finally promotes the formation of thin, silver-enriched pads providing low and stable electrical resistance.
## 4.2. Formation, destruction mechanisms and structural evolution of the transfer layer

By integrating (i) the general observation of the contacting surfaces, (ii) the analysis of the chemical evolution of the transfers and (iii) the evolution of the microstructure of the latter in cross section, it is possible to propose the following scenario of formation and destruction of the transfers (Figure 17).

<figure><img src="image_18.png" /><figcaption>Figure 17. Schematic of the transfer film formation and destruction, reaching a layered Ag-rich structure.</figcaption></figure>

At the beginning of the rotation, the first transfers are heterogeneous and their structure is similar to the structure of the brushes from which they are coming (Figure 17.a). In particular, they are localised at the edges of the sliding track (i.e. at the edges of the contact), probably because it is where the theoretical local pressure profile is at its maximum [22], [23]. It is challenging to affirm whether the transfers are the result of adhesive or abrasive mechanisms. The ring surface has not been polished or treated and presents a significant roughness ($R_a = 0.4$ µm, $R_z = 1.8$ µm, $R_{sk} = 0.5$) which makes abrasive mechanisms probable. Regarding adhesive mechanisms, material compatibility studies show that copper is highly compatible with silver, and has a good, albeit lesser, affinity with carbon. Therefore, adhesive mechanisms are also likely. However, TEM images of the transfer sections (Figure 13) reveal the presence of a continuous oxide layer on top of the bronze substrate. Raman spectroscopy conducted on pristine bronze ring surfaces confirms its
presence even prior to the first brush contact and identify it as Cu₂O (see Figure 10). This native
oxide layer should reduce the likelihood of adhesive transfers to occur. Previous studies [3], [24],
[25] explained that the transfer films were formed by weld formation between silver from the
brush and copper from the ring, subsequent to a disruption of this protective layer. Our findings
and TEM observations show that this disruption is not achieved in our experiments since the oxide
layer is still present under the transfer pads. Despite the slightly different tribo-electrical and
environmental conditions imposed here, this result contrasts with what could have been stated in
the literature.

Between 10² and 10⁵ cycles (Figure 17.b), the transfer pads grow on the ring surface and display a refined, more homogeneous microstructure, due to the cyclic shear straining imposed by the successive rotations. It also corresponds to an increase in the layer thickness, with the contributive action of copper, copper oxides and possibly copper hydroxides, as discussed in the previous section. The compaction of wear debris is associated to the decrease in I<sub>D</sub>/I<sub>G</sub> ratio, indicating graphite crystalline reordering. One interesting information deduced from the observation of the pads (see Figure 5.c) is the presence of parallel grooves collinear to the sliding direction with confirm that the pads are more and more supporting the contact (i.e. decreasing the direct contact between brush and bronze ring surface) ; this type of transfer is also softer than the original brush materials.

After 100 000 cycles (Figure 17.c), the pads reach a critical thickness associated with a maximum
concentration of copper oxides, corresponding to the maximum of electrical resistance (Figure 8).
The pads are becoming too thick, and more affected by the interfacial shear strain. This load is
similar to a fatigue load, where low stresses are applied periodically. Cracks appear within the
transfer pads and they undergo a bulking delamination process. The hypothesis of a delamination
process by in-body cracking is supported by the observation of remaining transfer material on the
ring's surface after the destruction process. The observation of blisters on the transfer pads also
corroborates delamination (see Figure 6). This phenomenon has already been observed for layer-
lattice materials, such as graphite or MoS<sub>2</sub>, but also in DLC coatings [26], [27], [28], [29]. It
provides information on the elastic nature of the transfers, which would otherwise have cracked
and flaked. The presence of blisters also indicates severe local tribological loading imposed to the
transfer pads, and that the adhesive bond between the pads and the ring surface is stronger than
the cohesive strength of the pads themselves. It is important to note that the delamination process
does not affect the quality of the graphite on the ring surface since the I<sub>D</sub>/I<sub>G</sub> ratio remains constant.
The reason must be a scale effect, since the transfer pads are delaminated in the micrometre range
(the fragments can be more than 100 µm long), whereas the graphite crystallite size is around
30 nm according to Raman measurements and the Tuinstra and Koenig's equation (1). Therefore,
the delamination process does not affect the graphite crystal structure and vice versa. It should be positive from a tribological point of view, since a well-aligned graphite lubricates the contact and reduces mechanical energy loss, thus preserving the lifetime of the sliding electrical contact. Alternatively, it can be noted that no correlation was found between the graphite crystallinity and the electrical performance of the transfer layer.

After 10⁶ cycles (Figure 17.d) and the complete removal of the thick, enriched copper oxides transfer pads and the decrease of the copper oxide concentration within the remaining transfers, the system reaches an optimal state of low electrical resistance. Indeed, TEM observations confirm a layered structure with a quasi-pure Ag top layer on top of thin transfers, favouring the electrical conduction. Subsequently, transfer thickness, electrical resistance and electrical noise do not change much, indicating the establishment of a steady-state. However, the presence of a gap within the transfer at 5 million cycles and parallel to the bronze surface evidenced by TEM observations (Figure 11, C-1), suggests that the transfer could have been delaminated again if left under rotation. However, as mentioned previously, this transfer would keep a low Cu₂O concentration, since the supply of copper from the bronze substrate is no more effective due to the protection of the ring surface by the transfer film itself. Figure 6 also shows blisters within the body of aged transfer pads (3.5 and 5 million cycles), which supports this hypothesis. It is likely that the gap observed in Figure 11, C-1 would have given birth to a similar blister if left under the tribological loading. Two hypotheses can explain the presence of this gap. First, it might be the result of crack nucleation and growth. Dense lamellar structure is observed above the gap, while the underlying structure appears unchanged compared to the transfers at 10 000 cycles. In this scenario, the marked contrast in the structural composition of the pad can be attributed to structural changes driven by the mechanical load imposed by the sliding, the crack protecting the underlying material from shear stress, while the material on top undergo transformation while suffering the load. In the other scenario, the gap is the result of a low adhesion between two different phases within the transfer, the unloading of the contact inducing stress relaxation within the transfer and permitting its opening. The bottom phase, which structure is similar to the transfer at 10 000 cycles, may have been transferred at the same period and have resisted to the delamination process. The Ag-rich lamellar phase may have been transferred on top afterwards, but with loose adhesion between the phases due to structural differences.

**5. Conclusion**

Experiments were conducted on test rig using an industrial slip ring. The electrical and tribological behaviour of silver-graphite brushes against bronze rings was investigated in conditions close to the real application. From the comprehensive physico-chemical surface
characterization of the rings and the monitoring of electrical resistance, a scenario of formation
of the transfer film has been established, with a direct correlation with the evolution of electrical
performance. The formation occurs in two distinct phases, a construction of the transfer pads by
nucleation then growth, followed by a second phase of partial destruction of the transferred
materials. A lamellar structure of graphite and silver layers of few and few tenths of nanometres,
respectively, has been evidenced. It is the result of transformations induced by mechanical stress.
The high silver density in the transfer pads explains the excellent electrical performance after 10⁶
cycles. Raman measurements on the ring surface revealed that the graphite wear products
undergo a crystalline reorganization during sliding, quickly reaching a steady state. This optimum
crystalline state persists despite transfer destruction and structural changes, probably due to a
scale effect. This is promising from a tribological point of view, as high graphite crystallinity
ensures efficient lubrication without compromising the electrical performance of the transfer
layer.

Electrical resistance is intimately correlated with transfer chemical composition. EDX analysis
shows copper contamination within the transfer layer. This contamination increases resistivity
through copper oxide formation and may also be related to transfer thickness through enhanced
cohesive properties of the debris. Based on these interactions, a power law formulation is
proposed to estimate electrical contact resistance as a function of oxygen (oxide) concentration
in the transfer layer.
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