# Plasma electrolytic oxidation of aluminium in electrolytes containing various concentrations of carbon black nanoparticles

L. Magniez, C. da Silva Tousch, Sébastien Fontana, S. Cahen, J. Martin, C.

Hérold, G. Henrion

► To cite this version:

L. Magniez, C. da Silva Tousch, Sébastien Fontana, S. Cahen, J. Martin, et al., Plasma electrolytic oxidation of aluminium in electrolytes containing various concentrations of carbon black nanoparticles. Surface and Coatings Technology, 2023, 473, pp.129990. 10.1016/j.surfcoat.2023.129990. hal-04205574

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Submitted on 13 Sep 2023

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
Plasma electrolytic oxidation of aluminium in electrolytes containing
various concentrations of carbon black nanoparticles

3

L. Magniez¹, C. Da Silva Tousch¹, S. Fontana¹, S. Cahen¹, J. Martin¹,², C. Hérold¹,

G. Henrion¹, ², *

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¹ Université de Lorraine, CNRS, Institut Jean Lamour, F-54000 Nancy, France

² Université de Lorraine, Laboratoire d'Excellence Design of Alloy Metals for low-mAss

Structures ('LabEx DAMAS'), F-57045 Metz, France

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*Corresponding author: [gerard.henrion@univ-lorraine.fr](mailto:gerard.henrion@univ-lorraine.fr) (G. Henrion) ; (+33) 372 742 489

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1 Abstract

2 Incorporation of carbon-based nanoparticles into ceramic coatings during plasma electrolytic
3 oxidation (PEO) is promising for the synthesis of new composite layers on lightweight metals.

4 Specifically, the present study focuses on the incorporation of carbon black (CB)

5 nanoparticles into PEO alumina layers. For this purpose, PEO of aluminium is performed in
6 silicate-based electrolytes containing various concentrations of dispersed carbon black

7 nanoparticles (from 0 to 6 g·L⁻¹). The influence of this concentration on the microstructure of
8 the achieved PEO coatings is investigated by combining complementary characterization
9 techniques (scanning electron microscopy, X-ray diffraction and Raman spectroscopy).

10 Results show that using concentrations up to 6 g·L⁻¹ tend to limit the morphological

11 inhomogeneity between the edges and the centre of the treated samples. Moreover, the

12 addition of carbon black nanoparticles results in a sponge-like outermost sublayer covering

13 larger areas of the surface with abilities to host a higher amount of these nanoparticles. It is
14 also evidenced that CB nanoparticles do not suffer any further structural degradation during

15 their incorporation. In addition, cross-checked results show that the presence of dispersed CB
16 nanoparticles slightly affect the coating average growth rate. As for potential future

17 applications, the electrical volume conductivity of grown carbon-alumina composite coatings
18 is also measured.

19

20 **Keywords:** Plasma Electrolytic Oxidation (PEO); Aluminium; Alumina coating; Carbon
21 black; Nanoparticle; Electrical volume conductivity

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23
# 1. Introduction

Over the last 25 years, plasma electrolytic oxidation (PEO) also known as micro-arc oxidation (MAO) has been widely studied due to its high potential to advantageously replace anodizing that often uses hazardous chemical compounds and requires several steps of cleaning, oxidation and sealing. Moreover, PEO makes it possible to grow thick and protective oxide coatings on valve metals (e.g. Al, Ti; Mg, Zr) in a single step and using a non-hazardous alkaline electrolyte [1-6]. More recently, PEO has gained a new interest by considering the possibility to elaborate ceramic-based composite coatings by incorporating elements dissolved or dispersed in the electrolyte into the inherent porosity of the PEO layers.

Indeed, it seems fairly obvious that the network of pores represents privileged sites in which particles added to the electrolyte can be trapped [7-13].

The mechanisms of incorporation of particles into the PEO coating are still unclear. Some authors, however, suggest that the insertion of particles into the layers takes place according to two mechanisms that can occur either separately or simultaneously:

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(i) the transport of the particles from the electrolyte to the electrode and through the extreme surface porosity under the effect of the electric field (electrophoresis). This transport is predominant during the anodic phase, especially for particles with a negative zeta potential [8, 13, 14], and

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(ii) the deposition and trapping of particles on the inner walls of pores mainly driven by the electrolyte infiltration [7, 15].

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According to these mechanisms, it appears that several factors can influence the insertion of particles in the PEO layers:
1

- their electrokinetic potential. In most cases, particles exhibit a negative zeta potential in

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an alkaline medium, and this potential increases in absolute value with the increase in

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electrolyte pH [16];

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- the relative size and shape of particles and pores [7, 17];

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- the composition of the electrolyte, keeping in mind that the addition of particles in the

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electrolyte can modify its conductivity [18-20];

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- the electrical parameters of the process (applied voltage and current waveforms), which

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define the electric field during the anodic and/or cathodic phases as well as the micro-

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discharge characteristics and behaviour;

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- the nature and concentration of particles dispersed in the electrolyte.

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Finally, the incorporation of particles or elements from the electrolytic bath can be either inert

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– the particles are simply trapped in the available pores, like physisorption – or reactive. In

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the latter case, the incorporated elements react with the host sites and their environment,

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which can result in modifications of the microstructure and/or the composition of the layers

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[21, 22]. Depending on the desired properties of the PEO coatings, there is a wide range of

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particles that can be dispersed into the electrolyte and incorporated in the PEO layers. These

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particles include metals, ceramics, nitrides, sulfides, polymers [9, 10, 21, 23-26]. The review

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paper by Lu *et al.* [8] gives an extensive overview of the most used particles to improve the

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PEO coating properties.

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Despite the large number of papers reporting on the PEO with particle incorporation, those

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dealing with the incorporation of carbon-based nanoparticles only account for less than 8%

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[27]. Moreover, most of these papers focus on the final properties of the composite coatings

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and only few relate to the process itself and the mechanisms of incorporation of particles

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during PEO.
1. Carbon black (CB) nanoparticles have been widely used for many decades for various applications [28], essentially as a pigment and reinforcing phase in the tire industry. Due to their electrical properties, carbon blacks are also used in various kinds of batteries [29-31] and can be incorporated in polymer matrix to elaborate conductive carbon-polymer composites [32, 33]. Hence, the present work focuses on the incorporation of carbon black nanoparticles in the PEO coatings by dispersing CB with various concentrations in the electrolyte, and on their influence on the characteristics of the resulting coatings, including the electrical volume conductivity.

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## 2. Material and methods

### 2.1 Experimental set up

12. The PEO treatments were carried out in a 2 L beaker filled with the electrolyte containing a carbon black suspension (Fig. 1). The choice of working with a small volume tank was motivated by the presence of carbon nanoparticles. Indeed, a small volume reduces the required quantity of particles, thus reducing their consumption as well as waste by-products.
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14
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16 Moreover, thermal regulation of the electrolyte usually involves circulating it through a heat exchanger with a pump, which is not suitable when solid particles are dispersed in the electrolyte. Indeed the particles could accumulate in the recirculation pipes and heat exchanger with the risk of clogging it. Particles could also be lost at the internal walls of the pipes and heat exchanger resulting in a decreasing particle concentration in the electrolyte during the process. Instead, cold water at a temperature of 18 °C flowed in a cooling copper coil that surrounded the electrodes, thus keeping the electrolyte temperature around 40 °C.
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23 Rectangular samples (32 × 50 × 5 mm³ in size) made of Al-1050 (Table 1) were used as working electrodes. The processed aluminium sample and two titanium counter electrodes

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1 (120 × 50 × 1 mm³) were immersed in the electrolyte. The distance between the working electrode and each counter electrode was set at 20 mm. During the PEO treatment, a magnetic stirrer was used to prevent the CB nanoparticles sedimentation, and to ensure a homogeneous electrolyte temperature.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Experimental setup for PEO treatments with dispersed carbon black nanoparticles in the electrolyte.</figcaption></figure>

8

Table 1

Elemental composition of the 1050 aluminium substrate.

<table><thead><tr><th>Element</th><th>Fe, Ti, Zn</th><th>Si</th><th>Cu</th><th>Mg</th><th>Mn</th><th>Al</th></tr></thead><tbody><tr><td>wt.%</td><td>&lt; 0.5</td><td>&lt; 0.25</td><td>0.05</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>Balance</td></tr></tbody></table>

11

The aluminium sample and the counter electrodes were connected to a pulsed bipolar current supply (Ceratronic® CER004 [34]). The anodic and cathodic current waveforms could be adjusted in amplitude and duration to set the positive and negative charge quantity at the desired values. In the present work, the current parameters were set according to Table 2; a schematic drawing of the current waveform is depicted in Fig. 2. A full description of the current waveform is given in [35]. Within this set of parameters, the negative to positive

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1 charge quantity ratio was slightly lower than 1 (~ 0.88), which favoured the occurrence of the

2 "soft" sparking regime [35-37].

3

<figure><img src="image_3.png" /></figure>

4 **Fig. 2.** Schematic drawing of one period of the current waveform indicating the time at each

5 change in the current rising/falling rate according to Table 2.

6 **Table 2**

7 Parameters of the current waveform.

<table><tbody><tr><td colspan="2">Current pulse frequency (Hz)</td><td>100</td></tr><tr><td rowspan="5">Anodic current pulse</td><td>Rise time (ms)</td><td>0.6</td></tr><tr><td>Pulse duration (ms)</td><td>2.6</td></tr><tr><td>Fall time (ms)</td><td>0.3</td></tr><tr><td>Amplitude (A)</td><td>15</td></tr><tr><td>Charge per pulse (C)</td><td>0.046</td></tr><tr><td colspan="2">Delay between anodic and cathodic pulses (ms)</td><td>0.3</td></tr><tr><td rowspan="5">Cathodic current pulse</td><td>Rise time (ms)</td><td>0.3</td></tr><tr><td>Pulse duration (ms)</td><td>5.5</td></tr><tr><td>Fall time (ms)</td><td>0.3</td></tr><tr><td>Amplitude (A)</td><td>-9</td></tr><tr><td>Charge per pulse (C)</td><td>0.052</td></tr></tbody></table>
<table><tr><td>Delay between cathodic and anodic pulses (ms)</td><td>0.1</td></tr></table>

1

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**Table 3**

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Concentration of dispersed carbon black nanoparticles used in different electrolytes, and corresponding sample identification.

<table><tr><td>CB (g·L<sup>-1</sup>)</td><td>0</td><td>1</td><td>2</td><td>4</td><td>6</td></tr><tr><td>Sample name</td><td>CB0</td><td>CB1</td><td>CB2</td><td>CB4</td><td>CB6</td></tr></table>

18

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Carbon black nanoparticles (Vulcan XC-72) were provided by Cabot Corp.

Thermogravimetric analysis (TGA Setsys Evolution 1750 Setaram) of carbon black
nanoparticles was performed prior to their dispersion in the electrolyte. The CB powder was
heated in a platinum crucible under dry air from ambient temperature to 1000 °C with
3 °C min⁻¹ heating rate. TGA revealed that CB nanoparticles were of high purity (less than
0.7 wt% impurities). Scanning electron microscopy (SEM FEI Quanta 650 FEG),
transmission electron microscopy (TEM Phillips CM200), and Raman spectroscopy with an
exciting radiation wavelength of 532 nm in the range 800-2000 cm⁻¹ (Horiba Jobin-Yvon
LabRam HR800) were used to characterize the produced PEO coatings as well as the as-
received CB nanoparticles. SEM and TEM micrographs and the Raman spectrum of the
pristine CB nanoparticles are shown in Fig. 3 and Fig. 4, respectively.

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** TEM (A) and SEM (B) micrographs of the pristine carbon black nanoparticles used for dispersion in the PEO electrolyte.</figcaption></figure>
<figure><img src="image_5.png" /></figure>

1

2 **Fig. 4.** Raman spectrum (λ = 532 nm) of the pristine carbon black nanoparticles used for dispersion in the PEO electrolyte.

3

4

5 SEM and TEM micrographs reveal that CB nanoparticles are agglomerated in pristine state.

6 Their spherical shape is observed by TEM experiments, and one can note the presence of two
7 main sizes of particles, about 20 and 50 nm in diameter. The Raman spectrum recorded in the
8 800-2000 cm⁻¹ region shows the presence of the typical D and G bands for carbon materials
9 [39].

10 After the PEO treatment, the processed samples were rinsed with ethanol, cleaned in an
11 ultrasonic bath in ethanol in order to remove all non-incorporated CB nanoparticles, dried,
12 and stored in a dry environment before *ex situ* characterizations.

13

14 2.2 *Ex situ* characterization of the PEO coatings

15 Cross-sections of the processed samples were examined by SEM (FEI – Quanta 650 FEG)
16 working in backscattered electron mode at 20 kV accelerating voltage. Prior to SEM
1 observations, samples were cut, mounted in resin and finely polished with 1 µm diamond

2 paste. Top surface of the processed samples were observed by SEM in secondary electron

3 mode at 20 kV accelerating voltage. To avoid surface charging issues from improper electron

4 evacuation during SEM observations, the samples were coated with a 10 nm gold thin film.

5 Observations were systematically done at both the centre and the edge of each sample in order

6 to account for possible coating inhomogeneity due to edge effect. The centre and edge areas

7 of the sample are defined as depicted in Fig. 5.

8 For each zone (edge and centre), the coating thickness was determined as the average value of

9 40 measurements (20 on each side, front and back, of the sample) taken randomly on the

10 considered zone by an Eddy current gauge (Fischer Isoscope FMP10-FTA3.3H). Moreover, to

11 get a better overview of the variation of thickness over the sample surface, thickness

12 measurements were also done using ImageJ software on SEM cross-section views at rather

13 low magnification. A hundred measurements regularly spaced of 30 µm were taken and

14 averaged in the two zones. The phase composition of the coatings was determined by X-ray

15 diffraction (XRD) measurements in Bragg-Brentano geometry using the Cu-Kα₁ radiation at

16 λ = 0.1542 nm (Bruker D8 ADVANCE diffractometer). Finally, CB nanoparticles

17 incorporated through the PEO coatings were also characterized by Raman spectroscopy

18 measurements performed with an exciting radiation wavelength of 532 nm in the range 800-

19 2000 cm⁻¹.

20 Because of the high value of the initial estimates of the sample resistance, electrometric

21 methods must be used to precisely determine the volume conductivity of the composite

22 coatings. Conductivity measurements were performed at room temperature using a Keithley

23 610BR electrometer in ohmmeter mode. It was connected to the test set-up by a short length

24 coaxial transmission cable and PTFE insulated connectors. The measuring device consisted of
1 two brass clamps on a PTFE block, shielded by a 2 mm thick copper box. The volume

2 resistance of the coating was measured between a 2.5 cm² area covered with silver paint on

3 the surface coating (Fig. 5) and the underlying aluminium substrate.

<figure><img src="image_6.png" /></figure>

5 **Fig. 5.** Schematic drawing showing the SEM observation areas at the centre and at the edge of the PEO processed samples, and the delimited silvered area for conductivity measurements.

7

8 **3. Results and discussion**

9 3.1 Visual aspects the PEO treated samples.

10 Fig. 6 shows macroscopic views of the sample surface after PEO treatment for various
11 concentrations of CB nanoparticles dispersed in the electrolyte. Depending on this
12 concentration, the samples exhibit a more or less dark grey colour as compared with the
13 nearly white colour coating of the sample processed without carbon black nanoparticles
14 (CB0). This indicates that carbon nanoparticles are indeed incorporated, at least on the top-
15 most sublayer of the PEO coatings. An inhomogeneity in the colouring can also be observed
16 between the centre and the edges of the samples, the edges being darker in case of a treatment
17 with CB nanoparticles. Such an inhomogeneity is likely due to a stronger electric field at the
edges as already pointed out in references [40, 41], which results in a higher growth rate and
likely a greater particle incorporation. Moreover, as the concentration in nanoparticles
increases, the visual aspect of the coatings becomes more homogeneous with a larger
expansion of the dark zones from the edges to the centre, up to a concentration of 6 g·L⁻¹
(CB6).

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Macroscopic views of the sample surface after PEO treatment using different concentrations of carbon black nanoparticles dispersed in the electrolyte.</figcaption></figure>

## 3.2 Morphology of the PEO coatings.

Top-surface SEM micrographs of the processed samples shown in Fig. 7 evidence different morphologies of the PEO layers according to whether they were achieved with or without CB nanoparticles. Sample CB0 exhibits in the centre a mix of a pancake-like morphology as usually reported for PEO processed aluminium samples in silicate-containing electrolyte [2] and a morphology that looks like a pancake-like structure that was damaged, likely because of discharges igniting there. The edge of the sample is only covered with such a damaged pancake-like morphology. Adding CB nanoparticles in the base electrolyte results in a slightly different morphology of the layers that exhibits a sponge-like morphology with few remaining pancake-like structures, with the sponge-like structure covering larger area as the CB concentration in the electrolyte increases. Furthermore, this sponge-like morphology is a well-known feature associated with the "soft" sparking regime as encountered when supplying the

<figure><img src="image_8.png" /></figure>
electrodes with a pulse bipolar current waveform having an anodic to cathodic charge quantity ratio slightly lower than 1 [35, 42]. Therefore, this change in surface morphology is likely related to an earlier transition to the "soft" sparking regime when the concentration in carbonaceous nanoparticles in the electrolyte increases. Indeed, we recently reported that the time needed for the PEO process to switch from arc to "soft" sparking regime linearly decreases with the increase in the content of carbon nanoparticles, whether they are carbon nanotubes [43] or carbon black nanoparticles [44]. It is also worth noting that, specifically for the sample CB6 treated in presence of the highest concentration in CB nanoparticles, the surface morphology of the oxide coating is less subject to the edge effect and remains quite similar both at the centre and the edges of the sample (Fig. 7A & 7B), which confirms visual observations (see section 3.1).

At higher magnification (Fig. 7D), SEM observation reveals the presence of nanoparticles in the porosity of the coatings for all samples processed with CB nanoparticles dispersed in the electrolyte. Analyses of the surface of all samples by Raman spectroscopy confirm that these nanoparticles imbedded in the coating surface porosity consist indeed of carbon black nanoparticles (see section 3.5).

The changes in coating morphology are also noticeable on SEM cross-section views (Fig. 8). Sample CB0 exhibits a rather dense inner sublayer and a thin outer sublayer mainly composed of pancake-like structures. In contrast, the sample processed with a low concentration of nanoparticles (CB1) presents a combined arrangement of pancake-like structures and sponge-like ones, while with a higher CB concentration in the electrolyte (CB6), the sponge-like structure dominates and covers the whole sample surface.
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Top view SEM micrographs of the samples processed with different carbon black concentrations. Low magnification SEM images taken at the centre (A) and at the edge (B) of the samples. Medium (C) and high (D) magnification SEM images showing nanoparticles in the porosity of samples processed with carbon black nanoparticles dispersed in the electrolyte. The dash lines in A delimit zones with different morphology. S-L, P-L, and DP-L stand for sponge-like, pancake-like, and damaged pancake-like structures, respectively.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Cross-section SEM micrographs of the samples processed with different carbon black concentrations. S-L, P-L, and DP-L stand for sponge-like, pancake-like, and damaged pancake-like structures, respectively.</figcaption></figure>
1. 3.3 Growth rate of the PEO layers.

2. As observed on SEM cross-section views (Fig. 8), the effect of introducing CB nanoparticles in the electrolyte on the coating thickness is not that obvious. Therefore, thickness measurements were performed according to the measurement procedures described in section 2.2. Thickness values obtained by using an Eddy current probe (Fig. 9A) show that the average thickness is rather the same whatever the CB concentration dispersed in the electrolyte. However, Eddy current probe has a measurement area of the order of a few square millimetres. To get more localized measurements, thickness values were also determined from cross-section SEM pictures (Fig. 9B). Several observations arise from these measurements.

10. First, the coating inhomogeneity observed on macroscopic views (Fig. 6) is linked to an uneven thickness of the coating over the sample, with a much thicker layer at the edges than at the centre. Second, it appears that the inhomogeneity is less pronounced as the CB concentration in the electrolyte increases, as evidenced by a lower dispersion in the data points (Fig. 9 and Table 4). Third, it is noticeable that the dispersion of thickness values decreases as the concentration in CB increases. Finally, it is worth noting that the sample processed without CB (CB0) exhibits a slightly lower average growth rate than those treated with CB, irrespective of the CB concentration in the electrolyte.

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Coating thickness measured (A) with Eddy current probe and (B) from SEM pictures. Average thickness values in the centre (white bars) and at the edges (grey bars) of the sample,</figcaption></figure>
1 together with all data points (black dots) that were used to determine the average thickness

2 values. See section 2.2 for the measurement procedure.

3

4 **Table 4**

5 Standard deviation of coating thickness values measured with Eddy current probe and from

6 SEM pictures.

<table><thead><tr><th>Sample</th><th></th><th></th><th>CB0</th><th>CB1</th><th>CB2</th><th>CB4</th><th>CB6</th></tr></thead><tbody><tr><td rowspan="4">Standard deviation (μm)</td><td rowspan="2">Eddy current</td><td>Centre</td><td>4.4</td><td>4.7</td><td>4.7</td><td>2.6</td><td>2.7</td></tr><tr><td>Edge</td><td>15.7</td><td>12.1</td><td>8.3</td><td>9.3</td><td>8.0</td></tr><tr><td rowspan="2">SEM</td><td>Centre</td><td>14.0</td><td>9.7</td><td>14.8</td><td>13.2</td><td>9.6</td></tr><tr><td>Edge</td><td>10.7</td><td>10.8</td><td>10.5</td><td>8.0</td><td>8.6</td></tr></tbody></table>

7

8 3.4 Crystallographic phase composition of the PEO coatings

9 X-ray diffraction (XRD) measurements were carried out in order to identify the different
10 polymorphs of aluminium oxide formed during PEO of aluminium in presence or not of
dispersed CB nanoparticles, and to estimate their relative phase proportion. The
11 corresponding X-ray diffraction patterns are shown in Fig. 10. Whatever the concentration in
12 CB, diffraction peaks are fine and well defined, indicating a good crystallinity of the coatings.
13 The aluminium peaks are due to the metallic substrate. All other peaks are due to the PEO
14 coatings in which two main crystalline polymorphic phases of aluminium oxide are detected:
15 the rhombohedral α-Al₂O₃ that corresponds to the stable alumina phase formed at high
16 temperatures (> 1100 °C) and the cubic γ-Al₂O₃ that corresponds to a low temperature
17 metastable phase. Some small diffraction peaks corresponding to monoclinic θ-Al₂O₃ are also
18 detected. As proposed by some authors [45, 46], the relative proportion of α- and γ-Al₂O₃ was
19
evaluated as a function of the concentration in CB particle from the relative area of the (113) reflexion peak of α-Al₂O₃ at 43.6° (Iα) to that of the (400) one of γ-Al₂O₃ at 45.9° (Iγ). The value of this Iα/Iγ ratio, given in Table 5, is close to 0.5 for all samples suggesting that the presence of CB nanoparticles does not modify the crystallographic phase proportion of PEO oxide coatings achieved on aluminium.

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** XRD patterns (Cu-Kα₁, λ = 0.1542 nm) recorded on aluminium samples PEO processed with different concentrations in carbon black nanoparticles dispersed in the electrolyte. Al is for aluminium (PDF 04-0787); α for α-Al₂O₃ (PDF 46-1212); γ for γ-Al₂O₃ (PDF 74-2206) and θ for θ-Al₂O₃ (PDF 23-1009).</figcaption></figure>

<caption>**Table 5**</caption>

Relative integrated intensity of α-Al₂O₃ (113) and γ-Al₂O₃ (400) diffraction peaks at 43.6° and 45.9°, respectively, for different concentrations of CB nanoparticles.

<table><thead><tr><th>Sample</th><th>CB0</th><th>CB1</th><th>CB2</th><th>CB4</th><th>CB6</th></tr></thead><tbody><tr><td>Iα/Iγ</td><td>0.49</td><td>0.43</td><td>0.48</td><td>0.49</td><td>0.51</td></tr></tbody></table>

Distributed under a Creative Commons Attribution | 4.0 International licence
https://creativecommons.org/licenses/by/4.0/

<figure><img src="image_13.png" /></figure>
1

2 3.5 Characterization of PEO coatings by Raman spectroscopy

3 Fig. 11 shows the Raman spectra recorded on the surface (at the edge area) of CB0, CB1, and

4 CB6 samples. As a reminder, data from Fig. 4 (multiplied by 4 for an easier comparison) are

5 also plotted in Fig. 11. As expected, no vibrational band of CB is detected for sample CB0

6 processed without nanoparticles. In contrast, for samples CB1 and CB6, the spectra reveal
7 two bands attributed to carbon nanoparticles: the D band around 1350 cm⁻¹ associated with

8 the structural defects of the carbon black nanoparticles, and the G band around 1600 cm⁻¹ due

9 to the in-plane vibrations of sp² carbon atoms. It is also worth noting that the spectra of the
10 CB1 and CB6 samples are superimposable, which means that increasing the concentration of

11 CB in the electrolyte does not modify the structural properties of the incorporated CB

12 nanoparticles. Moreover, the comparison of these spectra with that of the pristine CB

13 nanoparticles does not evidence any significant difference, thus indicating that incorporated

14 CB nanoparticles are not degraded by the PEO process. These results clearly show that CB

15 nanoparticles are efficiently incorporated in the PEO coating, without being damaged.

16

17

18
<figure><img src="image_14.png" /><figcaption>Fig. 11. Raman spectra (λ = 532 nm) of the surface of PEO coatings obtained with different concentrations in carbon black nanoparticles dispersed into the electrolyte (0, 1 and 6 g·L⁻¹). Spectra were recorded at the edge area of the samples.</figcaption></figure>

## 3.6 Electrical properties of the PEO coatings

The volume conductivity σV (S·cm⁻¹) was calculated from the volume resistance value RV (Ω) according to the following equation:

$$ \sigma_V = \frac{l}{R_V} \times \frac{e}{A} \qquad (1) $$

where e (cm) is the average thickness of the coating measured on the edge of the sample by Eddy current probe, and A (cm²) is the area of the measuring zone.

The values given in Table 6 show a volume conductivity of the order of 10⁻¹² S·cm⁻¹, which corresponds to values reported for usual porous alumina layers like those deposited by thermal spray [47-49], with some defects and/or impurities.

<figure><img src="image_15.png" /></figure>
1 Values vary slightly, but remain within this range, despite the incorporation of carbon black
2 nanoparticles in the layers.

3

4 **Table 6**

5 Volume resistance $R_V$ and volume electrical conductivity $\sigma_V$ of the PEO coatings obtained
6 with different CB concentrations dispersed into the electrolyte.

<table><thead><tr><th>Sample</th><th>CB0</th><th>CB1</th><th>CB2</th><th>CB4</th><th>CB6</th></tr></thead><tbody><tr><td>$R_V$ (10<sup>9</sup> Ω)</td><td>1.65</td><td>1.45</td><td>1.50</td><td>1.05</td><td>2.25</td></tr><tr><td>$\sigma_V$ (10<sup>-12</sup> S·cm<sup>-1</sup>)</td><td>1.65</td><td>2.07</td><td>2.05</td><td>2.90</td><td>1.28</td></tr></tbody></table>

7

8 **3.7 Discussion**

9 From the results and observations presented above, it can be suggested that the CB
10 nanoparticles do not incorporate deeply in the layers and are likely located in the outer porous
11 sublayer. To sustain this hypothesis, Raman spectra were recorded along the coating thickness
12 of sample CB6. For this, the coating was detached from the aluminium substrate by immersing
13 the sample in a 1M NaOH solution for about 2 hours. After dissolution of the substrate, the
14 resulting self-supported layer was broken to reveal the transverse coating cross section on which
15 Raman spectroscopy was performed. This cross section was also observed using a ZEISS
16 Gemini SEM 500 in SE mode at 2 kV accelerating voltage, which allowed us to work without
17 depositing a conductive gold layer prior to the observations. Fig. 12 shows the Raman spectra
18 recorded at various depths of the coating, together with a SEM micrograph on which the
19 locations of the measurement points are reported.

20

21 These spectra clearly evidence that carbon particles are only detected in the outer porous
22 sublayer. This is in agreement with the incorporation mechanism driven by the electrolyte
23 filling the pores left after the extinction of micro-discharges ignited at the surface, as proposed
24 by O'Hara *et al.* [13] and by Da Silva Tousch *et al.* [50]. Nevertheless, the absence of CB
25 fingerprint in the dense sublayer does not necessarily mean that no particles are trapped in that
26 sublayer. It could be also that CB nanoparticles be embedded in that sublayer with an amount

<figure><img src="image_16.png" /></figure>
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below the Raman detection limit, which would also be consistent with the results of electrical conductivity measurements.

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8

The preferential incorporation in the outer sublayer is also consistent with the macroscopic views of the samples (Fig. 6) and the related SEM observations (Fig. 7 & 8). Indeed, as the CB concentration in the electrolyte increases, the sponge-like structure expands on larger area on the sample. Such a structure has a higher porosity and, consequently, more sites in which CB particles can be trapped. This results in a more homogeneous black colour of the samples processed with a high CB concentration (Fig. 6).

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As for the electrical conduction properties of the carbon-alumina composite, two issues must be addressed. First, the CB nanoparticles must be evenly distributed across the layer thickness. One possibility could be to favour the porous layer and inhibit the growth of the dense inner layer by working out of soft sparking conditions [35, 36]. It is also well known that supplying the electrodes with DC current or voltage favours the growth of porous PEO coatings on aluminium [51]. Such a DC mode could therefore be of interest to incorporate more particles into the layers. Incorporating CB nanoparticles into the dense sublayer could also be a solution to grow a carbon-alumina conductive composite, similarly to carbon-polymer conductive composites [32, 33]. This however is still a concern since the dense layer can be formed either by diffusion mechanisms of aluminium and oxygen species from the substrate and the electrolyte respectively, or by densification of the porous layer under soft sparking conditions. In the former case, CB nanoparticles could unlikely penetrate the dense layer. In the latter case, one would expect CB nanoparticles present in the porous layer to be trapped in the densified layer, which would be an asset towards conductive PEO layers. Second, incorporated CB nanoparticles must percolate in order to constitute a continuous current pathway. This implies that most of the inner pores are open in order to form a network of cavities that could be filled with CB nanoparticles.

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27

Increasing the porosity would however likely be detrimental to the corrosion and wear resistance of the coatings, which might be an issue depending on the applications.
<figure><img src="image_17.png" /></figure>

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<caption>Fig. 12. A) SEM micrograph of a cross section of a self-supported PEO layer (CB6) with indication of the points where Raman spectra were recorded. B) Raman spectra (λ = 532 nm) recorded at several depths across the layer as indicated in the SEM image in A).</caption>

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**4. Conclusions**

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The present study related to the incorporation of carbon black nanoparticles (CB) into alumina layers formed by plasma electrolytic oxidation of aluminium with carbon black nanoparticles dispersed at various concentrations in the processing electrolyte. In particular, the main objective was to take advantage of complementary characterization techniques to study how the presence of CB influences the produced PEO alumina coating and its characteristics including the electrical volume conductivity. The information presented allows the following conclusions to be drawn:

14

* CB preferentially incorporate throughout the sponge-like structure of the coating.

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Moreover, the increase in the CB concentration promotes a larger covering of the sponge-like structure, and a higher incorporation of CB nanoparticles into the overall processed surface.
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* The presence of CB nanoparticles results in a slight increase in the average growth rate of the PEO coatings (as compared with processing without particles). Whatever the concentration in CB nanoparticles, the average growth rate of the PEO coatings remains close to 1.2 µm·min⁻¹.

* CB do not affect the crystallographic phase composition and coatings mainly consist of α and γ-alumina polymorphs in more or less identical proportion.

* CB nanoparticles do not seem to undergo any structural degradation from the surrounding plasma of the micro-discharges.

* In the range of 0 to 6 g·L⁻¹ CB concentration in the electrolyte, the electrical volume conductivity remains constant at about 10⁻¹² S·cm⁻¹. It is therefore suspected that because incorporation mainly takes place in the outermost porous sublayer, the percolation threshold through the dense inner sublayer is not being reached.
# Acknowledgments

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* This work was funded by the French Agence Nationale de la Recherche (ANR) under project CAMFRE - ANR21-CE08-0029

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* The authors acknowledge the Conseil Régional de la Région Grand Est for granting C. Da Silva Tousch's PhD work under decision 19_GE_049.

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* The contribution of the characterization platforms of Institut Jean Lamour is highly acknowledged.

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* The authors greatly acknowledge Mrs Anna Nomine for her contribution in TEM characterization of carbon black nanoparticles, and the MPA team at Institut Jean Lamour for the realization of some parts or the experimental device by additive manufacturing.

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# Data availability

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The data that support the findings of this study are available from the corresponding author, upon reasonable request.

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# Declaration of competing interest

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The authors declare no competing financial interests.

22
1 CRediT author statement

2 **L. Magniez**: Methodology, investigation, formal analysis, methodology, visualization, writing

3 - original draft, writing - review & editing.

4 **C. Da Silva Tousch**: Methodology, investigation, formal analysis, writing - review & editing.

5 **S. Fontana**: Formal analysis, supervision, writing - review & editing.

6 **S. Cahen**: Formal analysis, supervision, writing - review & editing.

7 **J. Martin**: Conceptualisation, formal analysis, supervision, methodology, writing - review & editing.

9 **C. Hérold**: Supervision, formal analysis, writing – review & editing.

10 **G. Henrion**: Conceptualization, supervision, project administration, funding acquisition, writing - review & editing.

12

13

14 Authorship statement

15 The submission of the manuscript has been approved by all co-authors.

16
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20

21
1 List of Tables

2

3 Table 1

4 Elemental composition of the 1050 aluminium substrate.

<table><thead><tr><th>Element</th><th>Fe, Ti, Zn</th><th>Si</th><th>Cu</th><th>Mg</th><th>Mn</th><th>Al</th></tr></thead><tbody><tr><td>wt.%</td><td>&lt; 0.5</td><td>&lt; 0.25</td><td>0.05</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>Balance</td></tr></tbody></table>

5

6
1

2 Table 2

3 Parameters of the current waveform.

<table><tbody><tr><td colspan="2">Current pulse frequency (Hz)</td><td>100</td></tr><tr><td rowspan="5">Anodic current pulse</td><td>Rise time (ms)</td><td>0.6</td></tr><tr><td>Pulse duration (ms)</td><td>2.6</td></tr><tr><td>Fall time (ms)</td><td>0.3</td></tr><tr><td>Amplitude (A)</td><td>15</td></tr><tr><td>Charge per pulse (C)</td><td>0.046</td></tr><tr><td colspan="2">Delay between anodic and cathodic pulses (ms)</td><td>0.3</td></tr><tr><td rowspan="5">Cathodic current pulse</td><td>Rise time (ms)</td><td>0.3</td></tr><tr><td>Pulse duration (ms)</td><td>5.5</td></tr><tr><td>Fall time (ms)</td><td>0.3</td></tr><tr><td>Amplitude (A)</td><td>-9</td></tr><tr><td>Charge per pulse (C)</td><td>0.052</td></tr><tr><td colspan="2">Delay between cathodic and anodic pulses (ms)</td><td>0.1</td></tr></tbody></table>

4

5

6
1

2 Table 3

3 Concentration of dispersed carbon black nanoparticles used in different electrolytes, and

4 corresponding sample identification.

<table><tr><td>CB (g·L<sup>-1</sup>)</td><td>0</td><td>1</td><td>2</td><td>4</td><td>6</td></tr><tr><td>Sample name</td><td>CB0</td><td>CB1</td><td>CB2</td><td>CB4</td><td>CB6</td></tr></table>

5

6

7
1

2 Table 4

3 Standard deviation of coating thickness values measured with Eddy current probe and from

4 SEM pictures.

<table><thead><tr><th>Sample</th><th></th><th>CB0</th><th>CB1</th><th>CB2</th><th>CB4</th><th>CB6</th></tr></thead><tbody><tr><td rowspan="6">Standard deviation (µm)</td><td rowspan="2">Eddy current</td><td>Centre</td><td>4.4</td><td>4.7</td><td>4.7</td><td>2.6</td><td>2.7</td></tr><tr><td>Edge</td><td>15.7</td><td>12.1</td><td>8.3</td><td>9.3</td><td>8.0</td></tr><tr><td rowspan="2">SEM</td><td>Centre</td><td>14.0</td><td>9.7</td><td>14.8</td><td>13.2</td><td>9.6</td></tr><tr><td>Edge</td><td>10.7</td><td>10.8</td><td>10.5</td><td>8.0</td><td>8.6</td></tr></tbody></table>

5

6

7
1

Table 5

Relative integrated intensity of α-Al₂O₃ (113) and γ-Al₂O₃ (400) diffraction peaks at 43.6° and 45.9°, respectively, for different concentrations of CB nanoparticles.

4

<table><thead><tr><th>Sample</th><th>CB0</th><th>CB1</th><th>CB2</th><th>CB4</th><th>CB6</th></tr></thead><tbody><tr><td>I<sub>α</sub>/I<sub>γ</sub></td><td>0.49</td><td>0.43</td><td>0.48</td><td>0.49</td><td>0.51</td></tr></tbody></table>

5

6

7
1

Table 6

3 Volume resistance $R_V$ and volume electrical conductivity $\sigma_V$ of the PEO coatings obtained
4 with different CB concentrations dispersed into the electrolyte.

<table><thead><tr><th>Sample</th><th>CB0</th><th>CB1</th><th>CB2</th><th>CB4</th><th>CB6</th></tr></thead><tbody><tr><td>R<sub>V</sub> (10<sup>9</sup> Ω)</td><td>1.65</td><td>1.45</td><td>1.50</td><td>1.05</td><td>2.25</td></tr><tr><td>σ<sub>V</sub> (10<sup>-12</sup> S·cm<sup>-1</sup>)</td><td>1.65</td><td>2.07</td><td>2.05</td><td>2.90</td><td>1.28</td></tr></tbody></table>

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1 **Figure captions.**

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**Fig. 1.** Experimental setup for PEO treatments with dispersed carbon black nanoparticles in the electrolyte.

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**Fig. 2.** Schematic drawing of one period of the current waveform indicating the time at each change in the current rising/falling rate according to Table 2.

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**Fig. 3.** TEM (A) and SEM (B) micrographs of the pristine carbon black nanoparticles used for dispersion in the PEO electrolyte.

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**Fig. 4.** Raman spectrum (λ = 532 nm) of the pristine carbon black nanoparticles used for dispersion in the PEO electrolyte.

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**Fig. 5.** Schematic drawing showing the SEM observation areas at the centre and at the edge of the PEO processed samples, and the delimited silvered area for conductivity measurements.

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**Fig. 6.** Macroscopic views of the sample surface after PEO treatment using different concentrations of carbon black nanoparticles dispersed in the electrolyte.

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**Fig. 7.** Top view SEM micrographs of the samples processed with different carbon black concentrations. Low magnification SEM images taken at the centre (A) and at the edge (B) of the samples. Medium (C) and high (D) magnification SEM images showing nanoparticles in
1 the porosity of samples processed with CB nanoparticles dispersed in the electrolyte. The
2 dash lines in A delimit zones with different morphology. S-L, P-L, and DP-L stand for
3 sponge-like, pancake-like, and damaged pancake-like structures, respectively.
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5 **Fig. 8.** Cross-section SEM micrographs of the samples processed with different carbon black
6 concentrations. S-L, P-L, and DP-L stand for sponge-like, pancake-like, and damaged
7 pancake-like structures, respectively

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9 **Fig. 9.** Coating thickness measured (A) with Eddy current probe and (B) from SEM pictures.
10 Average thickness values in the centre (white bars) and at the edges (grey bars) of the sample,
11 together with all data points (black dots) that were used to determine the average thickness
12 values. See section 2.2 for the measurement procedure.

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14 **Fig. 10.** XRD patterns (Cu-Kα₁, λ = 0.1542 nm) recorded on aluminium samples PEO
15 processed with different concentrations in carbon black nanoparticles dispersed in the
16 electrolyte. Al is for aluminium (PDF 04-0787); α for α-Al₂O₃ (PDF 46-1212); γ for γ-Al₂O₃
17 (PDF 74-2206) and θ for θ-Al₂O₃ (PDF 23-1009).

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19 **Fig. 11.** Raman spectra (λ = 532 nm) of the surface of PEO coatings obtained with different
20 concentrations in carbon black nanoparticles dispersed into the electrolyte (0, 1 and 6 g·L⁻¹).
21 Spectra were recorded at the edge area of the samples.

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23 **Fig. 12.** A) SEM micrograph of a cross section of a self-supported PEO layer (CB6) with
24 indication of the points where Raman spectra were recorded. B) Raman spectra (λ = 532 nm)
25 recorded at several depths across the layer as indicated in the SEM image in A).

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1 **List of Figures**

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<figure><img src="image_18.png" /></figure>

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<caption>17 **Fig. 1.**</caption>

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<figure><img src="image_19.png" /></figure>

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<caption>Fig. 2.</caption>

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<figure><img src="image_20.png" /></figure>
<figure><img src="image_21.png" /><figcaption>Fig. 3.</figcaption></figure>

<figure><img src="image_22.png" /></figure>

<footnote>Distributed under a Creative Commons Attribution | 4.0 International licence
https://creativecommons.org/licenses/by/4.0/</footnote>
<figure><img src="image_23.png" /><figcaption>Fig. 4.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>Fig. 5.</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>Fig. 6.</figcaption></figure>
A

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<figure><img src="image_26.png" /><figcaption>Fig. 7.</figcaption></figure>

<figure><img src="image_27.png" /></figure>
<figure><img src="image_28.png" /><figcaption>Fig. 8.</figcaption></figure>

<figure><img src="image_29.png" /></figure>
<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /><figcaption>Fig. 9.</figcaption></figure>

<figure><img src="image_34.png" /></figure>
<figure><img src="image_35.png" /><figcaption>Fig. 10</figcaption></figure>

<figure><img src="image_36.png" /></figure>
<figure><img src="image_37.png" /><figcaption>Fig. 11.</figcaption></figure>
<figure><img src="image_38.png" /><figcaption>Fig. 12.</figcaption></figure>

<figure><img src="image_39.png" /></figure>