# Aerosol assisted plasma for the deposition of multifunctional DLC: From the process to the mechanical properties

A. Aussonne, C. Cazot, T. Maerten, A. Riquet, M. Cavarroc-Weimer, Myrtil

L Kahn, R. Clergereaux

► To cite this version:

A. Aussonne, C. Cazot, T. Maerten, A. Riquet, M. Cavarroc-Weimer, et al., Aerosol assisted plasma for the deposition of multifunctional DLC: From the process to the mechanical properties. Surface and Coatings Technology, 2025, 517, pp.132727. 10.1016/j.surfcoat.2025.132727. hal-05290425

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# Aerosol assisted plasma for the deposition of multifunctional DLC: From the process to the mechanical properties☆

A. Aussonne<sup>a,b</sup>, C. Cazot<sup>a</sup>, T. Maerten<sup>c</sup>, A. Riquet<sup>d</sup>, M. Cavarroc-Weimer<sup>d</sup>, M.L. Kahn<sup>a</sup>, R. Clergereaux<sup>b,*</sup>

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## ABSTRACT

Diamond like carbon (DLC) coatings were successfully deposited by aerosol-assisted plasma-enhanced chemical vapor deposition. This approach enables the use of non-volatile, reactive and/or solutions of precursors, offering a flexible route for complex film deposition. Here, liquid droplets of pentane within argon were injected in a RF capacitively coupled plasma. The influences of the injected power and of the aerosol injection rate on the deposition rate, structural characteristics, and mechanical properties were investigated. With a relatively high deposition rate, up to 110 nm·min⁻¹, the balance of matter of this process is very favourable with up to 450 nm deposited per cm³ of pentane injected. Raman and FTIR spectroscopy revealed a progressive increase in sp³ content and carbon network crosslinking, associated with a transition toward denser and more disordered structures. Mechanical testing confirmed a strong dependence on process parameters: at low self-bias, films were soft, brittle, and poorly adhered; at higher self-bias voltage ( > 360 V), coatings exhibited excellent adhesion (HF1–2), no delamination under scratch testing (Lc₃ > 50 N), and hardness >20 GPa. These results demonstrate that aerosol assisted PECVD is a promising method for producing high-performance DLC coatings with tunable properties and high deposition efficiency.

## 1. Introduction

Diamond-like carbon (DLC) thin films are coatings containing both sp² and sp³ hybridized carbon with hydrogen [1]. They are well-known for their excellent chemical inertness, high hardness (up to 50 GPa) [2–4], high wear resistance and low friction coefficient (lower than 0.1) [3] making them excellent protective coatings.

DLC films can be deposited by various physical vapor deposition (PVD) techniques, including magnetron sputtering [3,5,6] and high-power impulse magnetron sputtering (HiPIMS) [2,7] Alternatively, plasma-enhanced chemical vapor deposition (PECVD) was developed. It is particularly attractive because it takes place at low temperatures, making it compatible with temperature-sensitive substrates with a relatively high degree of control.

PECVD involves gases or vapors of hydrocarbon precursors such as methane [8–13], acetylene [11,14–16], ethylene [17], benzene [14] or

toluene [18]. However, such coatings contain a high concentration of hydrogen altering the mechanical properties compared to PVD-grown DLC. Hardness up to ~20 GPa can be obtained by diluting the hydrocarbon precursor in an inert gas to promote sp² bonds saturation, or the application of high self-bias voltages. [19,20] Indeed, it is well-known in PECVD processes that the deposition of DLC coatings is primarily governed by the self-bias voltage [9]. The latter is fixing the ion energy during surface bombardment. It is related to the potential drop across the plasma sheath that accelerates positive ions toward the electrode, thereby affecting both the deposition mechanisms and the structure of the growing film. Finally, the incorporation of metallic atoms or nanoparticles into the carbon matrix is not obvious. Hybrid approaches, such as multi-target sputtering [21] or combined PECVD-PVD processes [22] have been developed. However, such hybrid approached add significant complexity to the process. A more straightforward approach deals with aerosol-assisted PECVD. Through the direct injection of liquid droplets

<footnote><sup>a</sup> Laboratoire de Chimie de Coordination UPR CNRS8241, Université de Toulouse, 205 Route de Narbonne, F-31077, Toulouse cedex 4, France</footnote>

<footnote><sup>b</sup> Laplace, CNRS ULMR5213, UPS, 118 Route de Narbonne, 31062, Toulouse, France</footnote>

<footnote><sup>c</sup> Oerlikon Balzers France, Parc E.S.T.E.R Technopole, 5 All. Skylab, F-87280, Limoges, France</footnote>

<footnote><sup>d</sup> Safran Tech, Rue des jeunes bois, 78772, Magny les Hameaux, France</footnote>

<footnote>* This article is part of a Special issue entitled: 'ICMCTF 2025 (SCT)' published in Surface & Coatings Technology.</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: clergereaux@laplace.univ-tlse.fr (R. Clergereaux).</footnote>
in the plasma reactor, it enables injecting non-volatile, reactive and/or liquid solutions (e.g. containing solutes or nanoparticles) [23–25] independently of their physico-chemical characteristics [26].

This study aims demonstrating the aerosol assisted-PECVD deposition of DLC thin films with mechanical properties. As classically discussed in classical PECVD, aerosol assisted processes are highly controlled by the self-bias voltage. This method also yields high deposition rates of up to 110 nm·min⁻¹ at optimal flow rates and significantly improved material use, with up to 450 nm of film deposited per cm³ of injected pentane. The resulting films were characterized as hydrogenated amorphous carbon with low hydrogen content and high sp³ fraction. Finally, mechanical testing revealed promising properties, with hardness values up to 20 GPa and Young's modulus approaching 180 GPa, demonstrating the potential of aerosol-assisted PECVD for efficient, high-performance DLC deposition.

## 2. Material and methods

### 2.1. Plasma reactor

The deposition has been carried using a RF capacitively coupled plasma from Plasmionique™, Canada. The chamber was evacuated by a primary pump (Pfieffer DUO) to reach a residual pressure of 5.10⁻² mbar.

The bottom electrode was connected to a RF power supply. The injected power was varying from 20 to 100 W, the matching box being set to the automatic mode. As the reactor walls were grounded, the samples placed on this electrode (of 20 cm in diameter) are submitted to ion bombardment related to the high DC self-bias. The latter is recorded all along the deposition process.

A Direct Liquid Injector (DLI) was connected to the reactor through the top shower head. The latter was situated 6 cm above the bottom electrode. Pentane (VWR Chemicals BHD) was pulsed injected as droplets within argon (Air Liquid). The injections were operated at a frequency from 0.1 to 3 Hz, the opening time of the injectors being fixed

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Scheme of the aerosol assisted PECVD setup. The red part represents the DLI system. It is connected to a pentane flask and fed by an argon flow (yellow) to produce the aerosol (orange). The latter is injected in the RF plasma chamber through a shower head (green). The biased electrode (blue) is used as the substrate holder.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Evolutions of a) the working pressure and b) the self-bias voltage over 5 injections at 0.1 Hz. It enables defined values in the steady-state and at the pulse.</figcaption></figure>

at 5 and 10 ms, respectively [26]. The working pressure is measured during the pulsed injection to control the range of pressure. The whole installation is presented in Fig. 1.

### 2.2. Structural and functional characterization

The structural properties of the coatings were investigated on films deposited on silicon wafers without pre-cleaning. The film thickness was measured using a KLA D-600 profilometer. The surface roughness was obtained by a SmartSPM atomic force microscope (AFM). The composition of the coatings was investigated by Raman and Fourier Transformed Infra-Red (FTIR) spectroscopies. The first one is using a Renishaw spectrometer with a 532 nm, 0.01 mW laser for 30 s over 3 accumulations. FTIR was performed over 4 accumulations on a Perkin Elmer spectrometer in transmission mode in dry atmosphere.

Mechanical properties were evaluated on steel samples with a commercial DLC used as underlayer and coated in the same batch by Oerlikon™, i.e. the same deposition time without pre-cleaning. Nanoindentation was performed on a MPS nanoindenter in a Continuous Stiffness Measurement (CSM) mode, where a small oscillatory force is superimposed on the primary load. It allows determining the hardness and the Young's modulus as a function of penetration depth. This enabled detailed profiling of mechanical properties for the entire film thickness. Sixteen indents were performed per sample to ensure reliability. The average Young's modulus and hardness were calculated based on the values obtained for depth between 200 and 400 nm. The
<figure><img src="image_4.png" /></figure>

a)

<figure><img src="image_5.png" /></figure>

b)

<caption>**Fig. 3.** Evolutions of the working pressure and the self-bias voltage in the steady state regime found between two pulses with a) the injection frequency and b) the injected power.</caption>

adherence of the coatings was assessed by Rockwell indent using the HF rating (100 kg) and by microscratch with a load progressively increased from 10 to 50 N (100 N.mm⁻¹) and a scratch speed set to 10 mm.min⁻¹. The critical loads, namely Lc₂ corresponding to the first film perforation and Lc₃ to the full film delamination, were determined by optical microscopy.

# 3. Results

## 3.1. Influence of the process parameters

As previously reported [26], the pulsed aerosol injection is inducing a time evolution of the working pressure and of the self-bias voltage (Fig. 2). Each injection (here at a frequency set to 0.1 Hz) induces a rise of the working pressure (on Fig. 2.a from 5 to 17 Pa) attributed to the pulsed injection of argon as well as liquid pentane vaporization [27]. After the pulse, the pressure returns to steady state. We will define this value as p₀.

These variations directly affect the self-bias voltage (Fig. 2.b). It was previously attributed to the coupling of the RF power that is altered by the pressure variation as well as the presence of liquid droplets in the plasma volume [26]. It induces a transient reduction of the self-bias voltage. Considering the ion bombardment during the steady state between two pulses, the highest value of the self-bias voltage defined as Vbias,₀ is defined.

p₀ and Vbias,₀ are defined for different RF powers and injection frequencies. Crucially, Fig. 3 shows that both are directly affected by the

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

a)

<figure><img src="image_7.png" /></figure>

b)

<caption>**Fig. 4.** Evolutions of the deposition rate with a) the injection frequency and b) the injected power.</caption>

injected power and the aerosol. For example, Fig. 3.a reports the effects of the injection frequency. Increasing the injection frequency from 0.1 to 10 Hz at a given injected power (100 W) raised the working pressure from 5 to 65 Pa. This can be attributed to the accumulation of gases in the plasma reactor as reported in other conditions [27]. With the pressure, the self-bias voltage reduces from -670 to -160 V. In addition, while the injected power from 20 to 100 W does not affect the working pressure (for an injection frequency set to 1 Hz), it directly shifts the self-bias voltage from -160 to -365 V (Fig. 3.b).

These modulations of the working pressure and of the self-bias voltage have a direct effect on the deposition rate (Fig. 4). At a fixed injected power (100 W), increasing the injection frequency led to a higher deposition rate (Fig. 4.a).

Notably, the maximum deposition rate reached 110 nm.min⁻¹ at 1 Hz, which is a really high deposition rate compared to PECVD using gaseous precursors [11,13]. For injection frequency higher than 1 Hz, the deposition rate tends to decrease. It corresponds to the injection frequency when the working pressure significantly increases due to the accumulation of species in the plasma reactor. In addition, with the injected power (Fig. 4.b), the deposition rate increases as classically reported in classical PECVD processes.

Indeed, comparing Figs. 3 and 4 would suggest that the deposition mechanisms are still directly controlled by the self-bias voltage: as frequently reported, DLC deposition is controlled by a complex competition of deposition, etching and sputtering, the latter being controlled by Vbias.

Hence, the film growth rate increases with the self-bias voltage up to
<figure><img src="image_8.png" /><figcaption>Fig. 5. Evolution of the balance of matter with the self-bias voltage for the different conditions reported in Figs. 3 and 4.</figcaption></figure>

a)

<figure><img src="image_9.png" /></figure>

b)

<figure><img src="image_10.png" /><figcaption>Fig. 6. a) Typical Raman spectra of the DLC coating obtained at –365 V. It highlights the classical D and G bands reported for DLC coatings. The positions of these two bands and the band ratio are reported in b) as a function of the self-bias voltage.</figcaption></figure>

a)

<figure><img src="image_11.png" /></figure>

b)

<figure><img src="image_12.png" /><figcaption>Fig. 7. a) Typical FTIR spectra in the 2700–3200 cm⁻¹ range of the DLC coating obtained at 365 V. It highlights an abundant sp³ CHₓ content. The relative evolutions of sp³ CH₃, sp³ CH₂ and sp³ CH are reported in b) as a function of the self-bias voltage.</figcaption></figure>

a threshold, above –360 V, when ion bombardment starts to sputter the films more efficiently than the recombination of species on the surface. It would be the case if the quantity of pentane injected was constant. However, as the volume injected per pulse is constant [26], an increase of the injection frequency corresponds to an increase of the volume of liquid injected per time unit. Furthermore, the deposition rate directly depends on the flow rate. Hence, the deposition rate was divided by the volume of pentane injected per pulse corresponding to a rough estimate of the balance of matter (Fig. 5).

Surprisingly, this parameter increases linearly with the self-bias voltage. Indeed, Fig. 5 shows that the material balance is ranging from ~30 to ~450 nm of deposited film per·cm³ of pentane injected. For comparison, in methane plasma, with a continuous flow rate of 100 cm³·min⁻¹, it means that the deposition rate would reach few hundreds of nm·min⁻¹, a really high value for classical PECVD (generally in the 10 to 100 nm·min⁻¹) [11,13–17].

It means that both the precursor flow rate and self-bias voltage jointly control the thin film deposition. The role of self-bias voltage is consistent with classical PECVD where growth is ion-energy driven [11]. However, it also suggests that ions with few hundreds of eV does not sputter the film during growth, i.e. that they do not significantly affect the plasma deposition processes. Hence, both ion energy and precursor
<figure><img src="image_13.png" /><figcaption>Fig. 8. Rockwel indents on coatings deposited in three conditions.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 9. Microscratch trace on coatings deposited in three conditions.</figcaption></figure>

<table><caption>**Table 1**

mechanical properties of the DLC coatings. The first line corresponds to the commercial substrate properties.</caption><thead><tr><th rowspan="2">Power (W)</th><th rowspan="2">Injection frequency (Hz)</th><th rowspan="2">-V<sub>bias,0</sub> (V)</th><th rowspan="2">Rockwell indentation</th><th colspan="2">Microscratch</th><th rowspan="2">Hardness (GPa)</th><th rowspan="2">Young's modulus (GPa)</th></tr><tr><th>Lc<sub>2</sub> (N)</th><th>Lc<sub>3</sub> (N)</th></tr></thead><tbody><tr><td>20</td><td>1</td><td>155</td><td>HF 4</td><td>13</td><td>22</td><td>3.5 ± 0.5</td><td>37 ± 10</td></tr><tr><td>50</td><td>1</td><td>250</td><td>HF 5</td><td>18</td><td>22</td><td>7.8 ± 0.6</td><td>64 ± 10</td></tr><tr><td>100</td><td>1</td><td>365</td><td>HF1-2</td><td>36</td><td>> 50</td><td>10.0 ± 2.0</td><td>84 ± 19</td></tr><tr><td>100</td><td>0.5</td><td>470</td><td>HF1-2</td><td>34</td><td>> 50</td><td>16.0 ± 4.0</td><td>143 ± 30</td></tr><tr><td>100</td><td>0.2</td><td>640</td><td>HF1-2</td><td>29</td><td>> 50</td><td>19.5 ± 3.0</td><td>175 ± 30</td></tr><tr><td>100</td><td>0.1</td><td>650</td><td>HF 3</td><td>24</td><td>> 50</td><td>16.5 ± 3.0</td><td>180 ± 35</td></tr></tbody></table>

supply become rate-limiting factors. This highlights the remarkable efficiency of aerosol-assisted PECVD using liquid hydrocarbons.

## 3.2. Structural properties of the coatings

The chemical structure of the deposited films was analysed using Raman and FTIR spectroscopy. A typical Raman spectrum of the coating obtained in the condition – 365 V is reported on Fig. 6.a. It shows the typical signature of DLC with the two broad bands namely of disorder (D) band between 1310 and 1358 cm⁻¹ and of graphite (G) band between 1518 and 1542 cm⁻¹ and a fluorescence background related to the hydrogen content [28].

The relative intensity and the position of the bands evolve with the self-bias voltage. As reported in Fig. 6.b, both the D and G bands are red shifted: for the D one, the shift from 1310 to 1358 cm⁻¹ with the self-bias voltage is generally attributed to a reduction in sp² content and a possible increase in hydrogen content, consistent with the DLC composition under high-energy ion bombardment [29]. However, the G band shift from 1518 and 1542 is really high. It would correspond to tetrahedral amorphous carbon. Finally, the band ratio decreases significantly with the self-bias voltage indicating a higher degree of disorder. The

latter could be due to the faster deposition rate observed at high self-bias reducing carbon mobility and inhibiting ordered carbon growth. Hence, the aerosol-assisted plasma deposition process seems to efficiently form various DLC coatings.

FTIR spectra with the typical C—H stretching bands in the 2900–3050 cm⁻¹ (Fig. 7.a) further highlights the presence of hydrogen in the coating [30]. However, the very weak intensity of the bands seems to indicate a low concentration of hydrogen. The latter is then used to further investigate the films composition.

Deconvolution of the C—H stretching reported in Fig. 7.a reveals multiple overlapping bands and a predominance of sp³ carbon atoms. The spectra collected for the different conditions are deconvoluted using the same method. In every case, the sp³ content is ranging from 89 to 97 %. This trend is consistent with the D band red shift observed in Raman spectra. However, this behavior deviates from usual PECVD DLC coatings as sp² content generally increases with the self-bias voltage [29].

The relative areas of those bands are reported in Fig. 7.b as a function of the self-bias voltage. It indicates a decrease in CH₃ content and a corresponding increase in CH₂ while CH contribution keeps constant. This shift reflects a reduction in terminal methyl groups and suggests increased crosslinking in the carbon network. It also supports the
hypothesis of denser, more interconnected carbon structures formed at elevated ion energies.

Aerosol-assisted plasma deposition of DLC coatings show really specific structural properties. Especially, they do not fully correspond to the coatings obtained in classical PECVD processes.

### 3.3. Mechanical properties

To fully assess the coatings, their mechanical properties were measured. For these measurements, 1 µm thick coatings were deposited on steel samples coated with a DLC underlayer to avoid substrate effects. Adhesion was assessed by Rockwell indentation (Fig. 8) and progressive load micro scratch (Fig. 9).

While the coatings deposited at injection frequencies >1 Hz were too soft and poorly adhered to allow for meaningful testing, mechanical properties and adhesion were found to strongly depend on the self-bias voltage (Table 1). All the films deposited with a self-bias over 350 V (corresponding to a RF power of 100 W) displayed a good adhesion with Rockwell indentation ratings of HF1 to HF2, indicating minimal cracking or film delamination. Moreover, no complete delamination of the films was reached by microscratch even with critical load $L_{c3}$ exceeding 50 N. $L_{c2}$ values ranged from 36 to 24 N slightly decreasing with $V_{bias}$ from the optimum obtained for –360 V, which may represent an optimal compromise between densification and internal stress.

However, coatings obtained with a lower self-bias were much more brittle and poorly adhered. They exhibited indentation behavior from HF 4 to HF 5 with significant cracking and delamination. Complete delamination was observed in microscratch at low load ($L_{c3} = 13$ N) and $L_{c2}$ dropped to 8 and 3 N. Nanoindentation revealed also drastic increase of both hardness and Young's modulus in agreement with the increased sp³ content and film densification. Hence, the coating reached impressive mechanical properties with hardness and Young's modulus up to 20 GPa and 180 GPa, respectively.

Finally, the film properties do not appear to be controlled solely by $V_{bias}$. Indeed, for the same value as recorded for 0.1 and 0.2 Hz, the mechanical properties are crucially degraded at lower frequency. It means that the aerosol assisted plasma deposition is more complex. These differences can be attributed to the quantity of matter injected per pulse and/or by the coating post-treatment by a pure argon plasma in between two pulses. For example, compared to 0.2 and 1 Hz, the coating treatment by a pure argon plasma is around 2 or 10 times longer at 0.1 Hz, respectively. These plasma surface interactions can affect the coating surface as well as the final film properties.

### 4. Conclusions

In this study, diamond like carbon thin films were successfully deposited from liquid pentane using an **aerosol-assisted PECVD process**. The deposition rate was shown to depend on both the **injection frequency** and the injected power, both affecting the **self-bias voltage**. Indeed, with the really high efficiency of the process, spectroscopic characterization (Raman, FTIR) revealed an **increase in sp³ bonding** and **film densification** with increasing self-bias, alongside a reduction in terminal $CH_3$ groups and increased crosslinking. These structural changes translated into **significant improvements in mechanical performances**. At low self-bias voltage, the coatings were brittle, weakly adhered to the substrate, and mechanically soft. At higher self-bias voltage, the films exhibited **excellent adhesion** (HF1–2) and **hardness up to 20 GPa**, highlighting the potential of aerosol-assisted PECVD for high-performance DLC coatings.

### CRediT authorship contribution statement

**A. Aussonne:** Writing – original draft, Investigation, Formal

analysis, Data curation. **C. Cazot:** Investigation. **T. Maerten:** Validation, Methodology. **A. Riquet:** Validation, Methodology. **M. Cavarroc-Weimer:** Validation, Supervision. **M.L. Kahn:** Validation, Supervision. **R. Clergereaux:** Writing – review & editing, Validation, Supervision.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

This work was carried out with the financial support of the AID (Agence de l'Innovation de Défense) and with the technical support of Safran, Oerlikon and the ENSIL.

### Data availability

Data will be made available on request.

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