# Mechanism of superlubricity of a DLC/Si3N4 contact in the presence of castor oil and other green lubricants

Y. Long, Y. Wang, V. Weihnacht, S. Makowski, M. Kubo, Jean-Michel

Martin, Maria-Isabel De Barros Bouchet

► To cite this version:

Y. Long, Y. Wang, V. Weihnacht, S. Makowski, M. Kubo, et al., Mechanism of superlubricity of a DLC/Si3N4 contact in the presence of castor oil and other green lubricants. Friction, 2022, 10 (10), pp.1693-1706. 10.1007/s40544-022-0601-1. hal-04081506

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Submitted on 13 Mar 2025

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# Mechanism of superlubricity of a DLC/Si₃N₄ contact in the presence of castor oil and other green lubricants

Yun LONG<fnref n="1" />, Yang WANG<fnref n="2" />,<fnref n="3" />, Volker WEIHNACHT<fnref n="4" />, Stefan MAKOWSKI<fnref n="4" />, Momoji KUBO<fnref n="2" />, Jean Michel MARTIN<fnref n="1" />, Maria-Isabel DE BARROS BOUCHET<fnref n="1" />,∗

Received: 03 September 2021 / Revised: 29 November 2021 / Accepted: 24 January 2022

© The author(s) 2022.

**Abstract:** To meet the surging needs in energy efficiency and eco-friendly lubricants, a novel superlubricious technology using a vegetable oil and ceramic materials is proposed. By coupling different hydrogen-free amorphous carbon coatings with varying fraction of sp<fnref n="2" /> and sp<fnref n="3" /> hybridized carbon in presence of a commercially available silicon nitride bulk ceramic, castor oil provides superlubricity although the liquid vegetable oil film in the contact is only a few nanometres thick at most. Besides a partial liquid film possibly separating surfaces in contact, local tribochemical reactions between asperities are essential to maintain superlubricity at low speeds. High local pressure activates chemical degradation of castor oil generating graphitic/graphenic-like species on top of asperities, thus helping both the chemical polishing of surface and its chemical passivation by H and OH species. Particularly, the formation of the formation of –(CH₂–CH₂)ₙ– noligomers have been evidenced to have a major role in the friction reduction. Computer simulation unveils that formation of chemical degradation products of castor oil on friction surfaces are favoured by the quantity of sp<fnref n="2" />-hybridized carbon atoms in the amorphous carbon structure. Hence, tuning sp<fnref n="2" />-carbon content in hydrogen-free amorphous carbon, in particular, on the top layers of the coating, provides an alternative way to control superlubricity achieved with castor oil and other selected green lubricants.

**Keywords:** superlubricity; diamond-like carbon (DLC); sp<fnref n="2" />-hybridized carbon; hydrogen passivation; castor oil

## 1 Introduction

Improving the properties of lubricants and their interaction with surface material contact is an effective answer to energy crisis, resource scarcity, and global warming since good lubrication performances improve energy efficiency and prolong machinery lifetime by inhibiting friction and wear. However, for traditional mineral based oil, extra effort is required to dispose their waste and prohibit leakage of them because used oil attacks male reproductive system, causes tumours, suppresses seedlings growth, etc. [1–3].

Hence, developing more eco-friendly lubricants has been highlighted for the past 30 years.

One option in this context is to substitute traditional mineral based oils with renewable green vegetable oils. For instance, sunflower oil and soybean oil outperform mineral oil and synthetic oil by reducing 50% of friction. This is attributed to polar groups inside vegetable oils, reacting with steel surfaces and facilitating oil film growth [4], however, also contributing to poor oxidation stability [5, 6]. Other vegetable oils like coconut oil, palm oil tend to leave a larger wear scar on metallic steel ball than mineral

<footnote n="1">¹ Ecole Centrale de Lyon, Laboratory of Tribology and System Dynamics, University of Lyon, Ecully 69134, France</footnote>

<footnote n="2">² Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan</footnote>

<footnote n="3">³ Department of Mechanical Systems Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan</footnote>

<footnote n="4">⁴ Fraunhofer Institute for Material and Beam Technology (IWS), Dresden 01277, Germany</footnote>

<footnote>* Corresponding author: Maria-Isabel DE BARROS BOUCHET, E-mail: maria-isabel.de-barros@ec-lyon.fr</footnote>
oil without significantly decreasing coefficient of friction (CoF) [7, 8]. Jatropha oil also causes an increase of CoF compared to mineral oil [9]. In recent years, castor oil (CO) has been reported to show very good lubricity. Quinchia et al. [10] concluded that the hydroxyl functional group makes castor oil viscous and polar, which is the main reason why castor oil overrides lubricant properties of high oleic sunflower and soybean oils. Intriguingly, under boundary lubrication, castor oil enables superlubricity of Nitinol 60 alloy/steel tribopairs, thanks to the formation of metal oxide/hydroxides layers on both surfaces. This metal oxide/hydroxide layer formation is driven by tribochemical reactions between metallic surfaces and castor oil degradation products [11]. However, superlubricity of this system vanishes when sliding speed exceeds 8 m/min [12].

More generally, some esters of glycerol could be efficient to realise superlubricity for ta-C containing tribo-pairs. For instance, with 1% of glycerol mono-oleate (GMO) in polyalphaolefin (PAO4), superlubricity is established for steel/ta-C pairs when sliding speeds are superior to 0.1 m/s [13]. Regarding glycerol (a component of vegetable oils), superlubricity is observed for self-mated ta-C pairs at 80 °C because glycerol enriches ta-C surfaces with easy sheared OH groups [14]. This superlubricity mechanism is also applicable for steel/ta-C pairs [15]. Meanwhile, the tribo-chemical interaction between Si₃N₄ and glycerol can provide superlubricity at 150 °C by forming graphenic carbon nitride nanolayers on Si₃N₄ substrate [16]. However, for steel/steel pairs, glycerol requires the help of inositol [17] or water [18] to achieve superlubricity. Being another component of vegetable oils, unsaturated fatty acids like oleic acid has already demonstrated its ability to graphitize ta-C top surfaces (less than 1 nm) and build up a pathway to superlubricity [19]. Furthermore, reactive sites of oleic acid such as C=C and COOH groups can cross-link on ta-C surfaces. Under shearing, this cross-linked oleic acid breaks, leading to defragmentation of it and surface passivation of ta-C surface [20].

Previous research of castor oil was mainly focused on metal/metal pairs. This work aims to broaden superlubricity range of castor oil by involving hydrogen-free DLC coatings and ceramic materials.

Meanwhile, it provides a comparison between castor oil and conventional lubricants to improve lubricant choice for industrial application. The effect of carbon hybridization was studied by employing both sp²-rich amorphous carbon (a-C) and sp³-rich tetrahedral amorphous carbon (ta-C) coatings. The novel superlubricity mechanism of a-C/Si₃N₄ and ta-C/Si₃N₄ tribopairs lubricated by castor oil is investigated by comparing its performance under low sliding speed (3 mm/s) and identifying the surface chemical changes occurring after friction by X-ray photoelectron spectroscopy (XPS)/X-ray-induced Auger electron spectroscopy (XAES) surface analysis. Furthermore, comparing the coatings with different sp²-hybridized carbon contents, it is proved that tribo-induced aromaticity is beneficial to reach the superlubricity threshold. Complementary numerical simulation results indicate that degradation products from castor oil preferentially occupy sp²-carbon sites compared to sp³ ones.

## 2 Experimental

### 2.1 Materials

Castor oil, glycerol (purity ≥99.5%), ricinoleic acid (purity ≥99%), oleic acid (purity ≥99%) abbreviated as CO, GL, RA, and OA are purchased from Sigma-Aldrich (St. Louis, USA). Poly-alpha-olefin 4 (PAO4) also named as Durasyn® 164 is bought from INEOS oligomers (London, UK). Glycerol monooleate (GMO) provided by Total Energies (Paris, France) is also used in this study. Concerning tribopairs selection, coated AISI 52100 steel pins with 100 mm radius spherical head provided by Rocholl GmbH (Eschelbronn, Germany) are used to slide against Si₃N₄ flats manufactured by hot pressure sintering and provided by Lianyungang High Born Technology Co (Lianyungang, China). The hardness of steel pin is around 8 GPa. Two hydrogen-free amorphous carbon coatings with different contents of sp² and sp³ hybridized carbon atoms are deposited on steel pin surfaces by Fraunhofer IWS (Dresden, Germany). The coatings were applied by pulsed laser-arc technique using a commercial physical vapour deposition (PVD) coating system, consisting of a deposition chamber
with an attached Laser-Arc™ module including a plasma-filtering unit [21]. The samples were mounted in a twofold rotation arrangement corresponding to a situation of an industrial coating of tribological components. Prior to carbon coating, Ar⁺ ion etching was performed, using a hollow-cathode plasma source. After etching, a chromium interlayer with thickness of about 100 nm was deposited by magnetron sputtering. The ratio of sp²/sp³ carbon atoms was adjusted using different temperatures for the deposition process.

After deposition, coating thickness was measured by the ball crater method. Mechanical properties were derived from nanoindentation tests, using a ZwickRoell ZHN nanomechanical test system (Ulm, Germany). Measurements were taken with a Berkovich tip loaded up to 100 mN with quasi continuous stiffness method (QCSM). Young's modulus and hardness of the coating are acquired as described in EN ISO 14577-4:2016, however, for extrapolation of coating indentation modulus, a sigmoid fit model was used instead of the proposed linear fit. Coating Young's modulus was verified independently with surface acoustic wave spectroscopy [22], resulting in well agreeing values. Since hardness, Young's modulus, and density directly correlate with sp³ content for hydrogen-free amorphous carbon, the ratio of sp³ hybridized carbon *F* was approximated from Young's modulus *E* by *F* = *E* / 800 GPa [23].

The carbon coatings were lapped with diamond slurry to mirror finish, removing both growth defects and the native sp² rich top layer formed during deposition. The method to obtain surface roughness (Sa) is detailed in surface analyses section. The different properties of the virgin samples are listed in Table 1.

## 2.2 Friction and wear experiments

A home-made linear reciprocating tribometer with

pin-on-flat configuration was used to perform sliding experiments with a movement following a sine function to vary sliding speed. The maximum speed is reached in the middle of the wear track (noted as mid-point). The tangential force is detected by a piezoelectric sensor. Before running tests, all samples were cleaned by ultra-sonication in n-heptane for 30 min then in acetone for 5 min. Before installing the pin on the flat, 50 µL of lubricants (about one droplet) was added on Si₃N₄ flats with a syringe. All tests were conducted at a temperature of 100 °C and lasted 1.1 h under a constant normal load of 50 N corresponding to an initial maximum Hertzian contact pressure of 262 MPa. Due to the large radius of curvature of DLC pins, the theoretical Hertzian contact diameter is about 600 µm. To ensure a sufficient kinematic length of sliding, the stroke length was fixed at 3 mm. Maximum sliding speeds at mid-point of 3 mm/s were used. The tangential force (*F*ᵢ) and motor position are recorded every 0.001 s. The CoF of one cycle is calculated from half value of *F*ᵢ difference in a round trip at mid-point. It is worthy to mention that the detection limit of CoF is 0.005 in our tribometer with a detection accuracy of ±0.001. Therefore, CoF measured below 0.005 is not representative because it is difficult to extract the signal from the noise. Furthermore, all experiments were conducted 3 times and the CoF differences among those 3 tests are within 5% of each other.

## 2.3 Surface and lubricant analyses

All virgin lubricants were analysed by Fourier-transform Infrared Spectroscopy (FTIR, Perkin Elmer, USA) where diamond/ZnSe crystal is used and scanning range is set up as 4,000–650 cm⁻¹.

Lubricant residues on specimens were removed by ultra-sonicating samples in n-heptane for 5 min.

<table><caption>**Table 1** Chemical and mechanical properties of tribopair materials. The sp³-hybridized carbon content of a-C(30) and ta-C(55) is deduced from Young's modulus. Young's modulus and hardness of the coating are measured by nanoindentation.</caption><thead><tr><th></th><th>Production method</th><th>Coating thickness (μm)</th><th>Sa (nm)</th><th>Indentation modulus (GPa)</th><th>sp³/(sp²+sp³) (%)</th><th>sp²/(sp²+sp³) (%)</th><th>Indentation hardness (GPa)</th></tr></thead><tbody><tr><td>Si₃N₄</td><td>Hot pressing sintering</td><td>—</td><td>6.7</td><td>310</td><td>—</td><td>—</td><td>15.7</td></tr><tr><td>a-C(30)</td><td>Filtered laser-arc PVD</td><td>3.2</td><td>8.5</td><td>237</td><td>30</td><td>70</td><td>30</td></tr><tr><td>ta-C(55)</td><td>Filtered laser-arc PVD</td><td>4.0</td><td>8.1</td><td>521</td><td>65</td><td>35</td><td>55</td></tr></tbody></table>

In the following, the two different carbon coatings are named a-C(30) and ta-C(55) corresponding to their respective hardness in brackets.
Afterwards, samples were optically imaged by digital microscopy (VHX-6000, Keyence, Japan). three dimensional (3D) topographical images were recorded by interferometer (ContourGT-X, Bruker, USA). A 50× objective is used to acquire Sa. This corresponds to an area of 126 µm × 95 µm with lateral resolution of 0.2 µm. An objective with a 10 times larger measured area is also used, where curvature fitting is applied only on wear scar in order to get radius of wear scar curvature. Same fitting process is also used to acquire Sa but in an area of 126 µm × 95 µm. Surface chemistry change on samples after friction were analysed by XAES/XPS analyses with a ULVAC-PHI Versa Probe II spectrometer using Al Kα X-ray source (1,486.6 eV), a take-off angle of 45° and a probed area of 100 µm × 100 µm. In order to avoid the charge effect, a neutralization system was used and C-C/C=C contribution of C1s was indexed at a binding energy (BE) of 284.8 eV. Additionally, spectra of C1s and Auger C KLL regions were obtained with pass energy of 23.5 eV. All peaks were fitted with multipack software using Shirley background. Quantification was carried out using the transmission function of the apparatus and an angular distribution correction for 45° angle. Sensitivity factors were extracted from the Ref. [24], where cross section and escape depth correction were integrated.

The structure difference between a-C(30) and ta-C(55)
materials was investigated by Raman spectroscopy
using a 633 nm laser beam. Spectra were acquired

on three spots, each randomly selected inside and
outside of the wear scar on the coated pin. Laser
power was kept approximately at 1 mW at the sample
surface. By testing different laser powers, it was
ensured that excitation energy was below damage
threshold. After acquisition, carbon peaks are fitted
within a spectral range of 900–1,900 cm⁻¹ to a combined
fit model consisting of a Lorentzian peak (D peak),
BWF peak (G peak), and a linear base line. Fit quality
was good and resulted in R² > 0.993. As parameters
describing the hydrogen-free carbon structures, D/G
peak ratio and G peak position are discussed as
proposed by Ferrari and Robertson [25].

3 Experimental results

3.1 Influence of DLC type

First, sliding tests are conducted with a-C(30)/Si₃N₄
and ta-C(55)/Si₃N₄ friction pairs in order to explore
how DLC structure influences the lubrication ability of
castor oil. All the tests display a very short running-in
period of duration below 15 min followed by a steady
CoF below the value of 0.01 (Fig. 1(a)). This is defined
as the so-called liquid superlubricity regime.

Results are very reproducible as shown for 3
consecutive tests carried out in Fig. 1(a). Interestingly,
steady CoFs depend significantly on the DLC type.
Here, steady CoF is accurately calculated in 2 steps:
first calculating an arithmetic average value of CoFs

<figure><img src="image_2.png" /><figcaption>**Fig. 1** Friction experiments of a-C(30)/Si₃N₄ and ta-C(55)/Si₃N₄ tribpairs lubricated by castor oil in boundary lubrication regime. (a) Friction curves at 3 mm/s; (b, c) optical images of a-C(30) pin and ta-C(55) pin wear scars; 3D profiles of a-C(30) pins (f) before and (d) after friction, ta-C(55) pins (g) before and (e) after friction. 3D profiles have been fitted with a curvature filter to direct demonstrate height distribution of surfaces.</figcaption></figure>

<figure><img src="image_3.png" /></figure>
from the last 10 min of each test; secondly taking previous values and calculating an arithmetic average of 3 repeated tests. As a result, replacing ta-C(55) by a-C(30) leads to about 30% decrease of steady CoF. The steady CoF of ta-C(55)/Si₃N₄ is 0.008 while the one of a-C(30)/Si₃N₄ is 0.0055. Value errors are below ± 0.001.

No wear track can be detected on Si₃N₄ flats by optical microscopy (Fig. S1 in the Electronic Supplementary Material (ESM)), this is rather unexpected because of the lower hardness of Si₃N₄ compared to DLC coatings. On the a-C(30) pin, a homogenous bright and circular wear scar is formed after friction test with a diameter of 650 µm (Fig. 1(b)), a value slightly larger than the Hertzian contact diameter (calculated as 600 µm). This shows a small reduction of the apparent contact pressure from 176 to 150 MPa. DLCs' local height distributions of asperities in 126 µm × 95 µm areas are shown in Figs. 1(d)–1(g). For virgin a-C(30) (Fig. 1(f)), surface is dominated by asperities with a height in the range of 10–20 nm, corresponding to orange colour. After friction, light blue colour becomes more pronounced on a-C(30) wear scar (Fig. 1(d)), indicating that asperities heights are now below 10 nm. As a consequence, the a-C(30) surface is polished reaching a Sa equal to 5.4 nm compared to original Sa of 8.5 nm for a-C(30) (Table 2). Similar results were obtained with the hardest ta-C(55) coating. No visible wear scar is observed on Si₃N₄ side in all tests and no

<table><caption>**Table 2** Comparison of DLC wear status and their lubrication regime in castor oil. The calculations of liquid film thickness and theoretical CoF of Couette flow are detailed in the ESM.</caption><thead><tr><th>DLC(x)/Si<sub>3</sub>N<sub>4</sub></th><th>a-C(30)</th><th>ta-C(55)</th></tr></thead><tbody><tr><td>Steady-state CoF</td><td>0.005</td><td>0.008</td></tr><tr><td>Theoretical CoF considering Couette flow</td><td>0.0005</td><td>0.0005</td></tr><tr><td>Wear scar diameter (mm)</td><td>0.62</td><td>0.63</td></tr><tr><td>Radius of curvature inside the wear scar (mm)</td><td>125</td><td>105</td></tr><tr><td>Apparent contact pressure (MPa)</td><td>148</td><td>170</td></tr><tr><td>Roughness Sa before test (nm)</td><td>8.5</td><td>8.1</td></tr><tr><td>Roughness Sa after test (nm)</td><td>5.4</td><td>5.4</td></tr><tr><td>Oil film thickness before test (nm)</td><td>3.5</td><td>3.5</td></tr><tr><td>Lambda before test</td><td>0.30</td><td>0.31</td></tr><tr><td>Film thickness after test (nm)</td><td>3.9</td><td>3.5</td></tr><tr><td>Lambda after test</td><td>0.40</td><td>0.36</td></tr></tbody></table>

morphological change has been recorded, either. This denies the possibility of large-scale materials transfer from DLCs side to Si₃N₄. However, this similarity doesn't explain why the CoF of a-C(30) stabilizes at 0.0055 while the steady CoF of ta-C(55) is significantly higher at 0.008. It is interesting to notice that the wear scar on the pin is not completely truncated but that the radius of curvature inside the scar is a little bit larger than 100 µm inducing a very low wear volume < 4,000 µm³. And the wear depth of a-C is around 25 nm while the wear depth of ta-C is 10 nm. This is included in the calculation of film thickness.

The question is which lubrication regime is at work in our conditions. We performed elastohydrodynamic lubrication (EHL) calculations to determine the theoretical minimum film thickness for both cases. Because of the low roughness inside the wear scar, a fluid film could support the load and could give low friction. Also, we calculated the theoretical CoF considering Couette flow and infinitely smooth surfaces (detailed in the ESM: Theoretical CoF arisen from liquid) in the contact area. As shown in Table 2, lambda value is inferior to 1 both before and after shearing. This indicates friction tests are conducted in boundary lubrication.

First, we assume that the wear and polishing mechanism on the pin take place during the running-in period where friction is higher. The calculated film thickness is about 5 nm in both cases corresponding to a lambda value below unity (typical of boundary lubrication) and Couette friction associated is much lower than 0.01. In the literature it is generally accepted that EHL is not at work when the film thickness is below 5 nm. Moreover, in our case, castor oil is a relatively large molecule with a non-planar 3-D configuration providing a pressure-viscosity coefficient above 10 GPa⁻¹. The spherical average size of the molecule in the liquid can be estimated between 3 and 4 nm (at least twice the length of the ricinoleic acid). In these conditions the gap of 5 nm in the contact does not permit the molecules to slide easily on each like in a liquid and the definition of viscosity in this kind of interface is more than questionable so that we can confirm that the EHL standard calculation cannot apply in our case. It is more probable that the molecule is quickly decomposed
under shear stress as we will see in the atomistic
calculations. Then the superlubricity regime is more
dominated by surface chemistry than by oil viscosity
parameters.

To reveal underlying chemical mechanism, XPS/
XAES analysis is used to investigate surface chemistry
changes inside and outside of wear scars for DLC
pins. As we already mentioned, Si₃N₄ flats could not
be analysed because the wear scar is not detectable
by optical microscopy, so only DLC pin counterparts
are analysed after friction.

Due to lack of characteristic peaks other that of carbon
and oxygen, spectra are calibrated for all samples by
fixing the C1s photopeak at the same BE of 284.8 eV
corresponding to C–C/C–H chemical bonds. Obviously
the full width at half maximum (FWHM) of C–C/C=C
peak inside wear scar is significantly larger than
outside (Fig. 2, Table 3) regardless of the DLC type.

<figure><img src="image_4.png" /><figcaption>**Fig. 2** C 1s and O 1s XPS spectra of DLC pins after testing where (a, c) recorded inside of wear scar for a-C(30) and ta-C(55) while (b, d) are recorded outside of wear scar for a-C(30) and ta-C(55).</figcaption></figure>

This strongly suggests that a significant part of sp²
carbon is formed inside the wear scar and has been
produced by the tribochemical reactions.

Moreover, by comparing outside and inside of
wear scars, stronger contributions of C=O, C=O, and
O=C=O (peaking at 286.3, 287.5, and 289.0 eV,
respectively) as well as C–OH on the O 1s photopeak
are found inside of wear scar. In particular, the increase
of C–OH content inside wear scar is compared with
outside: a-C(30) shows an increase of 11% of C–O
while ta-C(55) exhibits 14% increase. This oxygen
content increases inside DLC wear scars could
originate from the chemical reaction of castor oil (or
its degradation products) with DLC material.

Since the change in sp² carbon content before and
after friction cannot be deduced accurately from the
fitting of the XPS C1s peak, we investigated the Auger
C KLL first-derivative signal in the XPS spectra
(so-called XAES) to identify more accurately the
hybridization state and chemical bonding of carbon
on DLC surface.

In Fig. 3, we measured the distance between the
highest and the lowest kinetic energies that is known
as the *D* parameter [26] A linear relationship is often
accepted between *D* parameter and sp²/sp³ ratio on
a carbon surface [27]. Our results show that a larger
*D* parameter is always observed inside wear scar
compared to outside, and this for both type of DLC.
The *D* parameters of a-C(30), ta-C(55) wear scar are
18.4, 16.4 eV, respectively. Knowing that *D* parameter
of graphite is 20 eV and *D* parameter of diamond is
14.2 eV [28], a 2 eV difference between a-C(30) and
ta-C(55) wear scars means 24% difference for sp²
content. The sp² content of a-C(30) wear scar reaches
71%. This enrichment in sp² carbon is located at the
extreme surface of the wear scar since the Auger C
KLL signal comes from a depth of only 1.5 nm compared
with the C 1s signal originating from 5 nm depth
(in the ESM: XPS information depth calculation).

Interestingly, an additional peak at 261.5 eV clearly
appears between the highest and the lowest energies
inside a-C(30) wear scar showing the lowest friction
coefficient. Such extra peak shows up while analysing
pure polyethylene (PE) but not in polystyrene (PS)
and polyacetylene (PA) (Fig. 3) [29]. This peak also
exists in DLC coatings which are fabricated by adopting

<figure><img src="image_5.png" /></figure>
<table><caption>**Table 3** Fitting details of Fig. 2.</caption><thead><tr><th></th><th colspan="3">C 1s</th><th colspan="3">O 1s</th></tr><tr><th></th><th>C–C/C=C</th><th>C–O</th><th>C=O</th><th>O–C=O</th><th>C–OH</th><th>C–O</th><th>C=O</th></tr></thead><tbody><tr><th>BE (eV)</th><td>284.8</td><td>286.3</td><td>287.5</td><td>290.0</td><td>533.7</td><td>532.4</td><td>531.5</td></tr><tr><th colspan="8">a-C(30) inside wear scar</th></tr><tr><th>FWHM (eV)</th><td>1.6</td><td>1.4</td><td>1.4</td><td>1.2</td><td>1.6</td><td>1.5</td><td>1.5</td></tr><tr><th>Content (%)</th><td>84.3</td><td>12.0</td><td>2.6</td><td>1.1</td><td>36.7</td><td>51.2</td><td>12.1</td></tr><tr><th colspan="8">a-C(30) outside wear scar</th></tr><tr><th>FWHM (eV)</th><td>1.2</td><td>1.4</td><td>1.4</td><td>1.2</td><td>1.6</td><td>1.5</td><td>1.5</td></tr><tr><th>Content (%)</th><td>90.0</td><td>9.4</td><td>—</td><td>0.6</td><td>25.2</td><td>55.5</td><td>19.3</td></tr><tr><th colspan="8">ta-C(55) inside wear scar</th></tr><tr><th>FWHM (eV)</th><td>1.4</td><td>1.4</td><td>1.4</td><td>1.4</td><td>1.6</td><td>1.5</td><td>1.5</td></tr><tr><th>Content (%)</th><td>79.2</td><td>16.7</td><td>3.1</td><td>1.0</td><td>34.9</td><td>51.8</td><td>13.3</td></tr><tr><th colspan="8">ta-C(55) outside wear scar</th></tr><tr><th>FWHM (eV)</th><td>1.2</td><td>1.4</td><td>1.4</td><td>1.4</td><td>1.6</td><td>1.5</td><td>1.5</td></tr><tr><th>Content (%)</th><td>83.0</td><td>13.8</td><td>2.1</td><td>1.1</td><td>20.2</td><td>49.0</td><td>30.8</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 3** Comparison of the first derivation of XAES CKLL spectra performed inside (red) and outside (black) wear track of a-C(30), inside (green) and outside (blue) wear track of ta-C(55), graphite (violet), polyethylene-PE (light brown), polyacetylene-PA (cyan), and mixture of PE and PA (dark brown). The spectra for PA–PE, PE, and PA are reproduced. Reproduced with permission from Ref. [29], © American Chemical Society, 2018.</figcaption></figure>

CH=CH as precursor but not in those coatings produced by using CH₄ and C₆H₆ [30]. And it has been recently remarked after friction test of hydrogenated DLC (a-C:H) in vacuum and attributed to –(CH₂–CH₂)ₙ–

oligomers in agreement with atomistic calculations [31]. Hence, XAES data indicates that –(CH₂–CH₂)ₙ– oligomers are present on a-C(30) wear scar. It is worthy to highlight that PE+PA mixture also owns this peak. So the existence of alkene groups on a-C(30) surface can’t be excluded. Here, both a-C(30) and ta-C(55) are hydrogen-free DLC and therefore, the source of –(CH₂–CH₂)ₙ– oligomers formation on DLC is certainly castor oil degradation. This is consistent with the fact that no clear peak at 261.5 eV is detected outside of wear scar.

Summarizing our analytical data, the top surface of a-C(30) is found to be polished after shearing, enriched in sp² carbon and partially terminated by some hydroxyl groups and mostly alkene and alkane groups. This situation is typically favourable for giving a very low shear strength offering an efficient pathway to limit the energy dissipation at the top of asperities [32].

## 3.2 Results with other green lubricants

Lubrication of DLC/Si₃N₄ pair is investigated with some other interesting green lubricants: glycerol monooleate (GMO), Glycerol, oleic acid, ricinoleic acid, and PAO4 synthetic base oil. When analysed by infrared spectrometer, GMO, glycerol, ricinoleic acid, and castor oil show wide OH stretching vibration in
between 3,000–3,650 cm⁻¹ while oleic acid and ricinoleic
acid show a broad band in the region 2,500–3,300 cm⁻¹
which features OH stretching of carboxylic acid
functions [33]. As expected, no OH stretch has been
observed for PAO4 (Fig. 4(d)). Interestingly, by sliding
a-C(30)/Si₃N₄ under the identical experimental
conditions, superlubricity is only observed for GMO,
glycerol, ricinoleic acid, and castor oil containing OH
groups bonded with CH or CH₂ (Fig. 4(a)). As for
oleic acid, its OH function only exists in carboxylic
group and final CoF is recorded as 0.012 at 1.1 h.
Summarising, a higher CoF shows up when no
OH group is present in lubricants like PAO4. This
observation is consistent with the fact that OH
groups benefits friction reduction as already reported
elsewhere [12]. It is worthy to note that under the
same conditions, a-C(30) shows a lower friction
coefficient than ta-C(55) while sliding in ricinoleic
acid (Fig. 4(b)). A higher *D* parameter in the Auger peak
derivative is detected on a-C(30) wear scar comparing
with the one of ta-C(55) (Fig. S2 in the ESM).
Accompanying the enlargement of *D* parameter, a

significant peak at 261.5 eV is also observed for a-C(30)
wear scars under lubrication by GMO and CO where
superlubricity is reached (CoF << 0.01). At the opposite,
the peak is weaker for RA (CoF about 0.01) and outside
wear scar in all cases. This shows that CHₓ-termination
plays a key role in the superlow friction achievement.

Raman spectra of a-C(30) and ta-C(55) coatings
inside and outside the wear track were analysed
regarding G peak position and D/G peak height ratio,
and results are shown in Fig. 4(c). No noteworthy D
peak is found for ta-C(55) which is typical of super
hard coatings, indicating a fully amorphous sp³-rich
carbon network [25]. After sliding, the structure is
unchanged and no D peak appeared, still indicating a
fully amorphous structure. Only a very small shift of
the G peak to lower wavenumbers is observed, which
could be attributed to intrinsic stress relaxation [34]
or minor sp³ to sp² rehybridization.

In contrast, a-C(30) shows a distinct D shoulder in
the initial state, commonly found for a-C coatings
deposited with high deposition rates or elevated
deposition temperature. The fraction of the rigid

<figure><img src="image_7.png" /><figcaption>**Fig. 4** (a) Final CoFs for a-C(30)/Si₃N₄ lubricated by different lubricants at 100 °C, 3 mm/s; (b) friction curves of both a-C(30)/Si₃N₄ and ta-C(55)/Si₃N₄ lubricated by RA under the same conditions. The inset image represents the molecular structure of RA with carbon in grey, oxygen in red, and hydrogen in white spheres. (c) D and G peaks information acquired from Raman spectrums, where arrows indicate change from initial surface (outside) to wear scar (inside); and (d) FTIR spectra of different lubricants.</figcaption></figure>

<figure><img src="image_8.png" /></figure>
network of sp³ carbon is much smaller, leaving sp²
carbon atoms enough space to rearrange and increasing
order during or after deposition. As expected, the
initial a-C(30) structure comprises some amorphous
sp³ carbon network, some amorphous sp² network and
some nano-clustered sp² sites, which are responsible
for the shoulder at the D peak position. After sliding,
the D peak has increased and the G peak has shifted
to higher wave numbers, resulting from an increase
in structured sp² sites. It can be seen that such increase
of sp² nanocluster took place with all lubricants with
a-C(30), regardless of the friction level. This is in
contrast with ta-C(55), where no initial sp² nanoclusters
were present and also did not develop during sliding.
Since Raman spectroscopy is probing to a depth of
1 µm, much deeper from the surface than XAES, the
sp² ordering process resulting from sliding is likely a
mechanical, shear-induced process [20].

In brief, both Raman and XAES confirm that
a-C(30) with initial nano-clustered sp² sites are easier
to realise sp² ordering under shearing than ta-C(55).
And since low friction behaviours of a-C(30)/Si₃N₄ is
also influenced by lubricants. The way how castor oil
participate tribo-chemical reaction is further elucidated
by simulation.

4 Atomistic simulation

To understand what happens at the real contact
asperities and clarify the mechanisms of superlubricity,
atomistic simulations are performed based on the
molecular dynamics (MD) simulation approach with
a reactive force field (ReaxFF) [35]. The ReaxFF
parameter set for hydrocarbon system was reported
elsewhere [36]. It was developed based on the parameter
set of ReaxFFc2013 [37] and has been proved to be able
to describe the mechanical properties of solid-state
carbon materials very well by comparing with first-
principle calculations and experiments [38, 39]. As
shown in Fig. 5, a Si3N4 upper slab slides against
a bottom ta-C (50% sp3) slab in the presence of
RA molecules. For simulations, castor oil can be
simplified to be the triglyceride species formed from
ricinoleic acid and glycerol. Hence the reactions of
ricinoleic acid and castor oil with the surfaces should
be very similar. Moreover, as shown before, both

glycerol and ricinoleic acid provide superlow friction
in tribological tests. The molecular structure of
ricinoleic acid is shown below the simulation model
in Fig. 5. For both Si₃N₄ and ta-C, the surfaces are fully
terminated by hydrogen terminations and have the
same sinusoidal shape with a height of 1 nm. During
the friction simulation, atoms in the top layer of
upper Si₃N₄ substrate are kept rigid and forcibly slid
along x-direction with a speed of 100 m/s. Meanwhile,
a normal pressure of 10 GPa is applied to the rigid
layer. Atoms in the bottom layer of lower ta-C substrate
are also kept rigid and stationary during the whole
friction simulation. The layers neighbouring to the
rigid layers are thermostat layers, which means,
temperature of the thermostat layers is always controlled
at 300 K during the simulation. Velocity–Verlet
algorithm is used to calculate the motion of each atom
in the simulation system with a time step of 0.25 fs/step.
The friction simulation is performed for totally 150 ps.
In these simulations, we did not measure the friction
force.

Figure 6 shows the snapshots of friction simulation of Si₃N₄ against ta-C. From the snapshots, even though we observe a slight elastic deformation of Si₃N₄ during the sliding (Figs. 6(b) and 6(c)), there is no visible damage of both Si₃N₄ and ta-C surfaces after 150 ps of friction. Then, to obtain clear insights into the reactions between ricinoleic acid molecules and surfaces and then compare with experimental observations, the upper Si₃N₄ substrate is uplifted after the friction (Fig. 6(d)). We clearly confirm the tribochemical

<figure><img src="image_9.png" /><figcaption>**Fig. 5** Simulation model of the Si₃N₄/ta-C friction pair. Ricinoleic acid molecules are between two substrates.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 6** Snapshots of friction simulation of Si₃N₄ against ta-C lubricated by ricinoleic acid molecules. (a) Initial state; (b) at 40 ps; (c) at 150 ps; and (d) uplifted after 150 ps. (e) and (f) are magnifications of the Si₃N₄ and ta-C surfaces respectively in (d), where in (e, f), hydrogen atoms as well as the small gas-phase molecules that do not bond to either Si₃N₄ or ta-C substrates have been hidden for clarity, while the colour of sp² and sp³ type carbon in oil are assigned to dark blue and rose respectively to visualize the distributions of carbon atoms on surfaces. Alkanes and alkenes groups have been highlighted by red cycles.</figcaption></figure>

reaction of ricinoleic acid molecules on both Si₃N₄ and ta-C surfaces, agreeing with the experimental observations and chemical analyses (Fig. 2). On the Si₃N₄ side, the oil adsorption occurs accompanied with the surface amorphization. Alkane, alkene, and some OH terminations are visible (Figs. 6(e) and 6(f)). Then we investigate the distribution of sp² and sp³ type carbon atoms that move from the oil to Si₃N₄ and ta-C surface, as shown in Figs. 6(e) and 6(f) respectively where hydrogen atoms and the small gas-phase molecules that do not bond to either Si₃N₄ or ta-C substrates are invisible for clarity. In Figs. 6(e) and 6(f), although each RA molecule has only one alkene group (C=C bond), both Si3N4 and ta-C surfaces contain a plenty of sp² type carbon atoms perhaps owing to the decomposition and dehydration of RA molecules. On the Si₃N₄ surface, it is interesting to observe that sp² type carbon atoms are much closer to the surface than sp³ type ones, indicating the higher reactivity of sp² type carbon. While on the ta-C surface, the amounts of sp³ and sp² type carbon atoms are almost identical. We further focus on the unabsorbed

molecules after friction. Figure 7 shows some typical unabsorbed molecules remaining in the simulation cell, which mainly includes alkanes, aldehydes, alcohols, and other complex small molecules. All of them were originally a part of ricinoleic acids. These results indicate a decomposition of ricinoleic acid molecule during the friction, highlighting the critical role of the contact pressure and shear between the asperities on the initiation of tribochemical reactions.

Based on above simulation results, the tribochemical reactions regarding ricinoleic acid at the friction interface of Si₃N₄/ta-C can be concluded as following: the ricinoleic acid molecules trapped between asperities are firstly decomposed into various small active molecule fragments under shearing, and then a part of these small molecules is attached to the ta-C surfaces forming a chemisorbed layer. Considering the above experiments in which solid contact between surface asperities exist and make a large contribution to the overall friction, this adsorbed layer on the asperity surface can effectively prevent this solid contact of asperities and hence decrease the friction

<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 7** Typical decomposition products of ricinoleic acid after friction. Here white, grey, and red indicate hydrogen, carbon, and oxygen, respectively.</figcaption></figure>

force of the solid contact region, acting as the origin of superlubricity in experiments.

Next, to understand the difference in frictional behaviour observed in the presence of castor oil between the materials a-C(30) and ta-C(55) in the experiments, we investigate which kind of hybridized carbon atoms (sp, sp², or sp³) on ta-C surface is preferred to react with ricinoleic acid. Figure 8(a) shows the percentage of each hybridization states that are preferentially bonded to ricinoleic acid fragments. For the reacted carbon, sp, sp², and sp³-hybridized carbon atoms are 3.29%, 55.26%, and 41.45%, respectively, indicating that ricinoleic acid clearly

preferentially reacts with sp²-hybridized carbon atoms compared to sp³-ones. Moreover, the evolution of hybridization ratio after friction for a ta-C material with an initial sp² ratio around 50% is displayed by the comparison between Figs. 8(b) and 8(c). The amounts of sp and sp²-hybridized carbon (0.37% and 51.07%, respectively) are higher after friction than the original amounts in ta-C (0.19% and 49.16%, respectively). On the opposite, the sp³-hybridized carbon atoms decrease from 50.64% to only 48.56% after friction (only a few tens ps). Above results indicate that ricinoleic acid fragments are more likely to react with sp and sp²-hybridized carbons. In addition, it was observed that the sp and sp² amounts in ta-C increased slightly after friction while the sp³ ratio decreased (comparison of Figs. 8(b) and 8(c)). This tendency to re-hybridize in the sp and sp² states helps to reduce friction by providing more active adsorption sites. The result of the simulation is consistent with our experiments (Figs. 2 and 3). For the a-C(30) sample, even though there is no significant change in the amount of surface asperities in real contact compared with ta-C(55) due to the similar surface roughness, the higher sp²-carbon amount in ta-C(30) sample and its ability to form more structured sp² sites make the surface more likely to react with the castor oil molecules, improving the formation of the adsorbed oil layer and preventing direct solid contact between ta-C and Si₃N₄.

## 5 Discussions

In both CO and RA, a-C/Si₃N₄ tribo-pair is preferred than ta-C/Si₃N₄ in terms of friction reduction. Moreover, replacing ta-C/Si₃N₄ by a-C/Si₃N₄ further decrease CoF is observed reaching the superlubricity regime. Such reduction of CoF originates from high initial sp²

<figure><img src="image_13.png" /><figcaption>**Fig. 8** Percentage of each kind of hybridized carbon atoms in simulations. (a) Carbon atoms that are bonded with ricinoleic acid fragments; (b) and (c) carbon atoms in ta-C before and after friction, respectively.</figcaption></figure>
carbon content of a-C comparing with ta-C. It not only facilitates the sp² ordering on DLC surfaces but also influences oil molecules chemisorption on DLC surfaces because sp² carbon is usually more reactive than sp³ carbon (Fig. 8).

However, oil molecules don't stay intact in the contact, undergoing a cascade of chemical decomposition into small fragments (Fig. 7) covering both DLC and Si₃N₄. Among all these fragments, mixed chains of alkane and alkene play a major role and they are also found by simulation. The existence of alkane and alkene groups on a-C after friction test is demonstrated by XAES analysis (Fig. 3). Interestingly, RA molecule has only one alkene group. So, the formation of alkene chains is most likely due to dehydration of alkane chains under shearing. These alkane and alkene chains passivate both surfaces and support partially the load and avoid direct contact between asperities [40].

On the other hand, by comparing tribological performance of a-C/Si₃N₄ in different lubricants, those with OH functions are preferred to minimize CoF. Their advantage derives from two aspects: firstly, OH function serves as anchoring spot to bridge oil molecule and surfaces [20]; secondly, the OH function helps for surface hydroxylation. DLC surfaces passivated by OH functions have been always associated with low friction in previous works [41, 12]. However, in our case, CH₃-termination (by both alkanes and alkenes) is the dominant mechanism because the characteristic CH₂ feature at 261.5 eV in the Auger CKLL peak is more intense when friction is low and this independently of the oxygen content of the lubricant (see the case of GMO for example in Fig. S2 in the ESM).

## 6 Conclusions

This study enlarges the potential application range of castor oil as a superlubricant under boundary lubrication. Previously evidenced for nickel alloys, it is here enlarged to Si₃N₄ ceramics and H-free DLC (a-C or ta-C), a highly wear-resistant tribopair, and this even under severe boundary lubrication conditions (low speed and lambda ratio <1). This green superlubricity mechanism is enabled by mitigating

the shear strength at the level of asperities by the tribo-induced formation of graphitic/graphenic nanolayers, surface hydroxylation, and –(CH₂–CH₂)ₘ– oligomers on surface. Such mechanism preferentially occurs on sp²-rich amorphous carbon coatings, initially containing some nano-clustered graphite sites such as a-C(30). The importance of carbon re-hybridization process from sp³ to sp² on the chemical adsorption and reaction of the castor oil molecular fragments is highlighted using atomistic calculation and additional action on the flattening top surface of asperities at a nano-scale could be expected. Thus, through tailoring initial sp²-hybridized carbon content at the ta-C surface, friction can be governed.

## Acknowledgements

This research is supported by TOTAL, Solaize Research Center and Federal Ministry of Economic Affairs and Energy Germany (BMWi) within project CHEOPS3 (Funding number 03ET1286B). The authors wish to express their acknowledgement to the Agence Nationale de la Recherche (ANR) for partial funding of ULVAC-PHI Versa Probe II XPS spectrometer.

**Electronic Supplementary Material:** Supplementary material is available in the online version of this article at https://doi.org/10.1007/s40544-022-0601-1.

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<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>
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**Yun LONG.** He received his bachelor degree in material physics at 2011 from Northeastern University, Shenyang, China. Then, he pursued master degree of nano engineering at Institut National

des Sciences Appliquées, Lyon, France. And he was granted with doctor degree by Ecole Centrale de Lyon, France in 2020. His research interests include fundamental study of superlubricity mechanism and commercial additives development.

<figure><img src="image_17.png" /></figure>

**Maria Isabel De Barros BOUCHET.** She received her doctoral degree on the “Development and characterisation of diamond coatings for tribological applications” from Institut de Combustion, Aérother-

mique, Réactivité et Environnement (ICARE, CNRS)

mique, Réactivité et Environnement (ICARE, CNRS) in France. She joined the Laboratory of Tribology and System Dynamics at Ecole Centrale de Lyon from 2001. She is a lecturer in the Department of Materials and Surface Sciences and deputy director of research of école centrale de Lyon (ECL). Her research activities cover surface science, tribochemistry, and superlubricity mechanisms.