ORIGINAL PAPER
Anti-wear and Friction Reducing Mechanisms of CarbonNano-onions as Lubricant Additives
L. Joly-Pottuz Æ B. Vacher Æ N. Ohmae ÆJ. M. Martin Æ T. Epicier
Received: 3 February 2008 / Accepted: 6 March 2008 / Published online: 18 March 2008
� Springer Science+Business Media, LLC 2008
Abstract Carbon nano-onions have better tribological
properties than graphite powder when used as additives
dispersed in a poly-alpha-olefin base oil. Carbon nano-
onions give a better dispersion in the liquid base oil due to
their nanometre-scale size. In particular, the anti-wear
efficiency of carbon onions under boundary lubrication and
mild wear regime is much better than that of graphite
powder. This effect can be attributed to the different
structure of the carbon layers in the two species. High-
resolution transmission electron microscopy and electron
energy-loss spectroscopy were used to characterize the
carbon samples, and significant differences in the carbon
layer spacing and the density were shown. Wear debris
were also observed by the same techniques. In the tribofilm
material we depicted new chemical and crystal nano-
structures species similar to some microstructures observed
in chondrite meteorites, the most interesting one being
maghemite iron oxide. The origin of the low friction and
wear is still largely unknown.
Keywords Carbon nano-onions � HRTEM �Electron energy-loss spectroscopy � Tribology �Anti-wear additive
1 Introduction
The slippery feel of graphite has been known for centuries
and the industrial production of pyrolytic graphite began in
the 1950s. Graphite powder has been used in engine oils
and metalworking applications for a long time. Carbon
nano-onions, discovered by Ugarte in 1992 [1], also called
giant fullerenes in the literature, present a structure similar
to inorganic MS2 fullerenes, except that they are exclu-
sively made of carbon and they are not hollow in the
centre. Thus, we may expect that this carbon species pre-
sents also interesting tribological properties as already
suggested in the literature [2].
An easy way to synthesize carbon onions is by trans-
forming nanodiamond particles into carbon onions, either
by irradiation by electron beam or by thermal annealing.
Hiraki et al. [3] synthesized carbon onions by irradiation of
nanodiamond inside a high vacuum transmission electron
microscope (HV-TEM). Another easier method to obtain the
onions is the annealing of nanodiamond inside a vacuum
chamber. Depending on the annealing temperature, carbon
onions obtained have different structures [4]. Recently,
Mikhaylik et al. [5] studied the transformation of nanodia-
mond particles into carbon onions via annealing at 1,000 and
1,500 �C by coupling several analyses: high-resolution
transmission electron microscopy (HRTEM), electron
energy-loss spectroscopy (EELS), SAXS and UV Raman
spectroscopy. A mechanism of transformation of nanodia-
mond particles into carbon onions has been proposed by
Kuznetsov et al. [6]. It has demonstrated that a single graphite
layer is first formed at the periphery of the particle by the
transformation of (111) planes of diamond into (001) planes
of graphite. Since it is possible to generate nanodiamond
particles with the same size by detonation method [7], carbon
onions with same size can be also obtained afterwards.
L. Joly-Pottuz (&) � T. Epicier
MATEIS, Institut National des Sciences Appliquees, 7 avenue
Jean Capelle, Villeurbanne Cedex 69621, France
e-mail: [email protected]
B. Vacher � J. M. Martin
LTDS, Ecole Centrale de Lyon, BP 163, Ecully 69134, France
N. Ohmae
Department of Mechanical Engineering, Faculty of Engineering,
Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
123
Tribol Lett (2008) 30:69–80
DOI 10.1007/s11249-008-9316-3
Lubricating nanoparticles (so-called nanolubricants) are
currently envisaged as potentially available solid lubricant
additives in many tribological systems. Inorganic fuller-
enes of MS2 have already been extensively studied as
additives incorporated into oil or grease [8–11]. They
present excellent tribological properties, especially in mild-
wear conditions. One key point of the mechanism is that
such nanolubricants present both friction reducing and anti-
wear properties at typically low temperatures.
Previous results showed that the structure of the carbon
onions has an influence on their tribological properties [12,
13]. Indeed the presence of diamond cores was found to
give a detrimental effect on the anti-wear properties of
carbon nano-onions. One may assume that the lubrication
mechanism of onions is based on a progressive exfoliation
of carbon onions to liberate small pseudo-graphitic stacks
or even graphene sheets, similar to the ones previously
observed for inorganic fullerenes. But the observation of
graphite single (graphene) sheets is very difficult by TEM
and they were not observed by STM on the surface after
friction tests [2].
In this paper we carefully compare carbon nano-onions
to graphite powder as additives in synthetic base oil. We
used a specific technique which allows a full character-
ization of carbon material. Electron energy-loss
spectroscopy in the TEM (TEM-EELS) and particularly the
study of the low-loss plasmon peak was used to charac-
terize nano-onions and wear debris and to compare their
physical properties to those of graphite.
2 Experimental Section
Carbon nano-onions were synthesized by annealing dia-
mond nanoparticles at 1,700 �C for 10 min. Commercial
diamond nanoparticles (CBN diamond type UDD, Global
diamond CO, Ltd) were deposited in a tantalum crucible
that was electrically heated and temperature was controlled
by using a Re3%W-Re25%W thermocouple. Graphite, also
tested as a lubricant additive for comparison, was a pyro-
lytic graphite powder for analysis with an average grain
size of 60 lm and a purity of 99%. Graphite has already
been used as lubricant, and its lubrication mechanism is
based on the shear between its large graphitic lamellae. But
the large size of the lamellae could avoid them to enter
easily the contact area. Furthermore, they have to be
aligned inside the contact area. Tiny carbon onions may
enter easily the contact area and their round shape could be
an advantage.
Carbon nano-onions and graphite powder were dis-
persed at 0.1 wt.% in a poly-alpha-olefin (PAO) synthetic
base oil, using an ultrasonic bath. Dispersions of nano-
particles in oil are not always stable. Moshkovitz et al. [14]
studied the effect of the mixing time by ultrasonic bath on
the tribological properties of WS2 inorganic fullerenes
dispersed in paraffin oil. The increase in the mixing time
leads to a decrease in the size of nanoparticles aggregates
and to a better reproducibility of friction experiments.
After 5 h of mixing, the fraction of aggregates with an
important size drastically decreases. We also performed
experiments on the sedimentation time of inorganic ful-
lerenes of several diameter [15]. We observed that the
sedimentation time decreases when the mean diameter of
the nanoparticles increases. This result is not surprising
since Van der Waals forces, responsible of nanoparticles
aggregation, are more important for nanoparticles with a
large diameter. Since our nanoparticles have a mean
diameter of 10 nm, we assumed that carbon onions are
easier to disperse than inorganic fullerenes with a mean
diameter of 120 nm. Thus, we used a mixing time of 5 h to
disperse nanoparticles in oil.
A pin-on-flat tribometer with both surfaces made of
AISI 52100 steel was used for the tribological tests. Two
droplets of the lubricant were deposited on the flat before
the experiment. Measurements were performed in humid
ambient air (30–35 RH) and at room temperature (25 �C)
with a sliding velocity of 2.5 mm/s. Different contact
pressures were used to test the lubricant properties: 0.83,
1.12, 1.42 GPa (corresponding to normal loads of 2, 5 and
10 N, respectively). Thus, experiments were performed
under boundary lubrication in the mild-wear regime. Two
kinds of friction experiments were performed: 500 cycles
friction tests corresponding to a total sliding distance of
2.5 m and 10,000 cycles durability friction tests (sliding
distance of 50 m). Each test was performed at least five
times to insure the reproducibility of the results.
In order to observe carbon nano-onions by TEM-EELS,
a drop of highly diluted dispersion in ethanol is deposited
onto a 30 nm thick lacey carbon film mounted on a copper
grid. Graphite powder was preliminary ground to obtain
smaller and curved sheets. TEM-EELS analyses of some
stacks with c-axis normal to the electron beam can be thus
performed. After friction, wear particles are collected
under the optical microscope using a micromanipulator, by
gently sliding a TEM grid directly on the unwashed wear
scar on the pin, in order to collect only material originating
from the contact area. Afterwards, the grid is immersed in
pure heptane for 30 min in order to eliminate any residual
oil. Analytical TEM was performed on a JEOL 2010 FEG
microscope operating at 200 kV accelerating voltage
equipped a Gatan 666 PEELS (energy resolution 1.2 eV).
A LEO 912 microscope operating at 120 kV equipped with
an omega filter was also used to perform zero-loss dark
field (DF) images and selected-area electron diffractions
(SAEDs). Indeed, the use of the filter allows the elimina-
tion of inelastic electrons, which greatly improves the
70 Tribol Lett (2008) 30:69–80
123
spatial resolution in the images and the sharpness in dif-
fraction patterns.
3 Results
3.1 TEM-EELS Characterization of Carbon
Nano-onions
Carbon nano-onions have a spheroidal shape with a typical
diameter \10 nm, as shown by the HRTEM image in
Fig. 1. They are composed of a ten of nested graphitic
carbon shells in a giant fullerene-like structure and most of
particles do not contain any visible residual diamond core
but possibly a C60 molecule. Zero-loss SAED was per-
formed on the sample containing a large quantity of
nanoparticles. Figure 1 also shows the Debye-Scherrer-
type diffraction pattern obtained, which essentially exhibits
diffuse rings. Figure 2 shows the intensity profiles recorded
on zero-loss electron diffraction patterns obtained for nano-
onions and diamond nanoparticles, together with the dif-
fraction lines found in the literature for crystalline diamond
and graphite (JCPDS data 23-0064 and 06-0675). First, we
observe a diffuse (002) line corresponding to the basal
plane distance between graphite layers at about 0.37 nm.
This is about 10% more than the 0.336 nm value found for
bulk graphite and this can be attributed to the small size
effect and to the curvature of the graphitic layers in the
nested structure. A thermal expansion anisotropy of
graphite-like structures has already been reported by Abe
et al. [16]. By using a thermal expansion coefficient of
2.7 � 10-5 K-1 and a graphite interlayer distance of
0.344 nm, a distance of 0.355 nm is found for a synthesis
temperature of 1,700 K, which is smaller than the one
observed in our case. Second, we observe diffuse reflexions
peaking near 4.8 and 8 nm-1 which could correspond to
the (101) and (110) atomic plane distances in graphite.
These reflexions could also been attributed to some resid-
ual nanodiamond material after the synthesis of nano-
onions. However, Raman spectroscopy and HRTEM
unambiguously confirmed the absence of diamond in nano-
onions [12]. The carbon onions studied here have a small
diameter with a high curvature of the planes. This curva-
ture does not allow stacking sequences to appear as already
observed by Tomita et al. [4].
Figure 3 compares the low-loss selected area EELS
spectra obtained for carbon nano-onions and a ‘standard’ of
graphite with the electron beam parallel and normal to the
c-axis, respectively. The low-loss plasmon peak essentially
corresponds to excitation of (p + r) valence electrons and
its maximum is located at 24.3 ± 0.1 eV for carbon nano-
onions. It is well known that graphite is not isotropic and
that the maximum energy of the plasmon loss peak depends
on the crystal orientation under the electron beam. Similar
changes have already been observed on the K-edge fine
structure at 285 eV energy loss [17]. From our data, a
maximum at 27 ± 0.1 eV is observed for graphite with the
electron beam parallel to the c-axis. This maximum shifts
down to 26 eV for graphite with the electron beam normal
to the c-axis. A ‘shoulder’ can also be observed on this
spectrum near 19 eV energy loss and could correspond to a
surface plasmons excitation since the analysis was typi-
cally performed on the edge of curved graphite particle.
The p/p* excitation is centred at 6.1 eV for carbon nano-Fig. 1 TEM picture of carbon onions showing the structure of one
onion. In frame, the SAED pattern of the whole sample is shown
Fig. 2 Intensity profiles of SAED pattern of carbon onions compared
to the one of nanodiamond used as synthesis precursor. Positions of
reflections of graphite (black lines) and diamond (dotted lines) are
indicated
Tribol Lett (2008) 30:69–80 71
123
onions compared to 7 eV for graphite sheets. These data
are in good agreement with values observed by Cabioc’h
et al [18, 19] on carbon onions with a p/p* peak at 5.7 eV
and a plasmon loss peak maximum at 24 eV for a
momentum transfer of 0.15 A-1 (corresponding nearly to
our operating conditions). These significant differences are
basically attributed to the curvature of sheets, the coupling
of electrons on the spherical shells being well different
from the coupling in the planar case. The same trend has
been observed for boron nitride nanotubes and cones [20].
In our conditions, mainly r bonds are excited in the
spectrum and this represents the stronger carbon bond
contribution in nano-onions. The K-edge EELS spectra in
Fig. 3b are in good agreement with the plasmon spectra
and clearly indicated the graphitic structure of carbon in
onions, as observed by Tomita et al. [21].
3.2 Mechanical Properties of Carbon Nano-onions
Xu et al. [22] have used EELS to characterize the hydro-
gen-free tetrahedral amorphous carbon films. They studied
several carbon films material deposited with different bias
voltage. From their results, it is possible to find a good
correlation between the density q of the carbon films tested
and the maximum of the low-loss plasmon peak (Ep). Thus,
Xu et al. [22] obtained an empirical relation as follows:
log q ¼ 1.0519 log Ep þ 1.9563:
Using this relation, the density of graphite and nano-
onions was found equal to 2,897 and 2,593 kg/m3,
respectively. Although these values are higher than the data
found in the literature, the density calculated by EELS for
nano-onions indicates a 10% decrease compared to
graphite. On the other hand, SAED data revealed a 10%
expansion of the basal planes distance between carbon
sheets in the carbon onions. These results are in quite good
agreement with the decrease in the density of nano-onions
compared to graphite. Tentatively, Howe and Oleshko [23]
made an estimation of the ‘‘physical’’ or ‘‘electronic’’
hardness of a carbon material from the energy of the
maximum of its plasmon loss peak in the EELS spectrum.
For isotropic or amorphous carbon compounds, an empir-
ical relation was obtained as follows:
log Hm ¼ �7.44þ 6.1 log Ep:
Using this relation, we found a hardness of about
25 GPa for graphite and 10 GPa for carbon nano-onions. It
is to be noticed that in these conditions, the hardness given
by Oleshko’s relation for graphite is maximum and corre-
sponds to the contribution of ‘‘in plane’’ covalent carbon
bonds. There is a huge discrepancy between our values
obtained by EELS and those determined from classical
nanohardness measurements in the literature. Graphite is
usually said to be one of the softest solid material known to
man and the measured hardness is found to lie between 0.1
[24] and 2.4 GPa [25]. This kind of disagreement was also
observed on carbon black and soot material. However, it is
well known that soots can abrade steel like a cutting tool.
So, we suspect that some graphite particles can scratch
steel as well.
To conclude, the nested carbon structure in nano-onions
is less dense, less ordered and less hard ‘‘electronically’’
than in graphite crystal, mainly because of the nearly
spherical curvature of the carbon shells. On the other hand,
there is no apparent edge in the structure. It is then
Fig. 3 EELS spectra of carbon onions compared to the one of
graphite: (a) plasmon energy range, (b) C K-edge
72 Tribol Lett (2008) 30:69–80
123
expected large differences in the tribological response of
these materials in stressed mechanical contacts.
3.3 Tribological Properties of Carbon Onions
Used as lubricant additives, the tribological properties of
carbon nano-onions and graphite powder were compared
under boundary lubrication conditions and at ambient
temperature. Quite similar friction reducing properties
were observed for the two carbon structures (Fig. 4). The
addition of each solid additive powder leads to a significant
decrease in friction below 0.1 compared to pure PAO base
oil ([0.15). Of course, this is not very surprising because
graphite powder was already known to provide low friction
under boundary lubrication.
In order to test the durability of the tribological prop-
erties of carbon onions, we performed longer friction tests
of 10,000 cycles (representing a total sliding distance of
50 m) at 1.12 GPa. Figure 5 shows the results obtained for
two successive tests that have been performed. A steady-
state friction coefficient of 0.08 was obtained throught the
test. Friction tests were also performed with graphite and
PAO (data not shown here) and the same value of friction
coefficient was observed than the one measured during the
friction tests of 500 cycles. These results confirm the
excellent tribological properties of carbon onions on a long
test duration.
Table 1 presents the different wear scar diameters
measured at the end of friction tests of Fig. 4 and friction
tests of 10,000 cycles. Although graphite effectively
reduces wear compared to PAO base oil alone, carbon
nano-onions added to PAO base oil reveal to be much more
efficient than graphite. Indeed, wear observed with carbon
onions is 20–30% lower than the one observed with
graphite powder, this behaviour being the same for several
contact pressures. Figure 6 presents the wear scar observed
on the flat after friction test with carbon onions and
graphite at 0.83 GPa. With graphite, a scratch due to the
initial polishing of the flat is not visible inside the wear
scar. This proves that the wear is important in the case of
graphite. With carbon onions, the width of the wear scar is
lower, confirming the tendency observed on the pins. Wear
observed on the pin after the 10,000 cycles friction tests
(represented in italic in the table) confirm the results
obtained after the short friction tests. Thus, carbon nano-
onions present far better anti-wear properties than graphite
in our conditions.
3.4 TEM-EELS Analyses of Wear Debris
In order to understand the lubrication mechanism of carbon
onions, wear particles were collected on the wear scar of
the steel ball after the friction test with carbon onions at
1.72 GPa and they were investigated by TEM-EELS. They
are flat and elongated particles with a typical length of 1–
2 lm and some wear debris are deposited on the holey
carbon film of the grid. Analyses were preferably per-
formed on the part of the particle which is inside a hole of
the carbon film to avoid the contribution of amorphous
carbon in the TEM images and the EELS spectra. Figure 7
presents a zero-loss bright field TEM image of typical wear
debris and more precisely a detail of this particle in a hole
Fig. 4 Friction coefficients obtained at several contact pressures
(0.83–1.72 GPa) with the dispersion of 0.1 wt.% in PAO of carbon
onion or graphite
Fig. 5 Friction coefficients obtained with the dispersion of carbon
onions at 1.12 GPa for long friction experiments (10,000 cycles
corresponding to a total sliding distance of 50 m)
Tribol Lett (2008) 30:69–80 73
123
in the carbon film. The Debye-Scherrer electron diffraction
pattern obtained on this particle presents several continu-
ous rings centred at 3.41, 3.95, 4.76 and 6.16 nm-1
(Fig. 8a). A comparison of the intensity profiles recorded
on a line-scan on the pattern and the one of pristine carbon
onions indicates the existence of new reflexions (Fig. 8b)
and a weak but clearly visible contribution of the (002)
basal plane distances of graphite. Inter-reticular distances
calculated for these new rings (0.293, 0.253, 0.210 and
0.162 nm, respectively) fit well with distances in maghe-
mite structure (cubic iron oxide c-Fe2O3, JCPDS data 39-
1346 with a = 0.835 nm): 0.295 nm for {220}, 0.252 nm
for {311}, 0.209 nm for {400} and 0.161 nm for {511}.
The magnetite iron oxide Fe3O4 (JCPDS data 19-0629 with
a = 0.839 nm) is also consistent with these measurements
because of its similar distances: (e.g. 0.297 nm for {220}
instead of 0.295 nm in maghemite, 0.253 nm for {311}
instead of 0.252 nm). Indeed the maghemite structure is
actually an iron-deficient magnetite, and both structures
cannot be easily distinguished by electron diffraction.
Moreover, no other iron oxide can fit the present experi-
mental data, and neither hematite a-Fe2O3, with a
rhombohedral crystal structure very different from the
cubic oxides, nor goethite (FeOOH) which has its strongest
diffraction line at 0.418 nm. Figure 9a presents an HRTEM
image of an isolated nanoparticle which can be identified as
maghemite or magnetite, observed under a [1–10] azimuth.
Table 1 Wear scar diameters on pins measured after friction tests of 500 cycles at several contact pressures (0.83–1.72 GPa) for the dispersion
of 0.1 wt.% in PAO of carbon onion or graphite
Contact pressure (Gpa) PAO + 0.1 wt.% carbon onions PAO + 0.1 wt.% graphite PAO Hertz diameter
0.83 90 130 170 68
1.12 115 145 175 92
175 225 320
1.42 135 155 180 116
1.72 115 140 150 106
Wear scar observed after friction tests of 10,000 cycles at 1.12 GPa are indicated in italics
Fig. 6 Wear scars observed on the flat after friction tests at 0.83 GPa
with: (a) carbon onions, (b) graphite
Fig. 7 TEM image of a wear particle. In frame, the part of the wear
particle analysed in detail is presented
74 Tribol Lett (2008) 30:69–80
123
In order to clarify if these nanoparticles have a ma-
ghemite or a magnetite structure, we have undertaken an
EELS study. Previous work [26] on various oxides (FeO, aand c Fe2O3, Fe3O4) shows that those compositions can be
confidently measured by the conventional method for
chemical analysis in EELS, which consists in integrating
the spectra intensities in a certain energy window below the
O–K and Fe–L2,3 edges, respectively, after usual correc-
tions. Figure 9b is the EELS spectrum of the particle
shown in Fig. 9a acquired with a small probe of about
5 nm.
From these data one can determine an atomic ratio O/Fe
equal to 1.54 ± 0.08, which is much more consistent with
the maghemite composition (O/Fe = 1.5) than with the
magnetite composition (O/Fe = 1.333). Several EELS
measurements have been made on other particles and
similar results were found, the average over 14 particles
being O/Fe = 1.58. During this investigation, an interest-
ing structural evolution of theses particles was observed
under electron beam irradiation in the TEM. Figure 10
presents HRTEM images of an iron oxide nanoparticle
before and after irradiation. Before irradiation (Fig. 10a),
the diffractogram is consistent with a [1–10] projection of
Fig. 8 (a) SAED pattern of the whole particle. Positions of the
objective aperture used to obtain DF images are represented. (b)
Intensity profiles of SAED pattern of wear particles and carbon onions
are compared (positions of reflexions of maghemite iron oxide are
indicated)Fig. 9 TEM identification of a maghemite nanoparticle observed in
the wear particle; (a) HRTEM image with the corresponding
numerical diffractogram showing spatial frequencies associated with
interplanar distances in perfect agreement with the {111} lattice
planes as expected in the [1,-1,0] azimuth; (b): corresponding EELS
spectrum acquired with an unfocussed probe of about 5 nm on the
particle leading to an atomic ratio O/Fe = (integration of the O–K
and Fe–L2,3 edge areas was done over an energy window D = 50 eV
after subtraction of the background extrapolated as shown)
Tribol Lett (2008) 30:69–80 75
123
maghemite (or magnetite), and the EELS analysis deter-
mines a ratio O/Fe equal to 1.47 ± 0.06, which again
confirms the maghemite structure. After only a few seconds
of normal exposure under the electron beam (Fig. 10b), a
doubling of one interplanar distance is clearly observed in
the image and also evidenced on the diffractogram. When
focussing the incident beam as a more intense nanoprobe in
order to perform local EELS measurements, one can
evidence a spectacular chemical evolution, since the oxy-
gen content drops drastically from O/Fe = 1.67 ± 0.09 at
the beginning of the experiment down to O/Fe equal to
about 0.2 within less than 1 min. Although these results
would require further investigation to be understood
properly (which is however beyond the scope of the present
work), some reasonable hypotheses can be drawn out from
crystallographic considerations on the maghemite struc-
ture. Indeed, maghemite has an inverse spinel structure and
contains iron vacancies. One can thus assume that vacancy
ordering, promoted by electron irradiation, takes place,
which may lead to the period doubling observed in
Fig. 10b. In addition, the presence of such vacancies may
also facilitate the departure of oxygen atoms under high
electron flux irradiation. Ordering in maghemite is not a
surprising feature since it has already been evidenced [27]
under the form of a tetragonal form (space group: P 43 21
2), consisting in a c-axis three times larger than the cubic
parameter. This stable structure cannot however explain the
present observations which were repeated several times on
different oxide particles.
To better understand the nanostructure of the wear
particle, zero-loss DF images on the same particle were
performed by EF-TEM on a selection of the different rings
observed. The position of the objective aperture used is
indicated on the diffraction pattern in Fig. 8. First DF
image (Fig. 11a) was performed on a part of the diffuse
ring which corresponds to the (002) reflection of graphite
basal planes. A homogeneous distribution of very small
white dots is observed in the image, which may correspond
to carbon onions contributions. Same experiments (data not
shown here) were performed on a sample of pure carbon
nano-onions preparation and similar images were obtained.
Since this image does not ascertain unambiguously the
presence of intact carbon nano-onions in the wear debris,
an EELS spectrum at the carbon K-edge was performed on
the same area, and the comparison of this spectrum with
that of pristine nano-onions gave strong evidence for the
presence of some intact nano-onions in the wear debris
material. This is furthermore consistent with the evidence
of remaining carbon onions such as can be observed at the
bottom of micrographs as shown in Fig. 10. Dark-field
images performed on the other rings corresponding to iron
oxide (Fig. 11b–d) clearly show the presence of iron oxide
nanoparticles well distributed in the carbon onion network.
No preferential order is observed in the structure of the
wear debris.
3.5 Surface Analysis of the Wear Track
by Raman Spectroscopy
After washing the surface, Raman analysis was performed
on the wear track using a 514.5 nm excitation wavelength.
Fig. 10 HRTEM image of an iron oxide nanoparticle (a) before
irradiation and (b) after irradiation with the measured evolution of the
atomic ratio O/Fe as a function of time. In both cases, the numerical
diffractograms correspond to the circled area. Note the planar distance
doubling in (b) associated with the additional spatial frequency
arrowed in the diffractogram
76 Tribol Lett (2008) 30:69–80
123
The spectrum obtained in Fig. 12 shows, despite of low
signal/ratio, several peaks easily attributed to maghemite
when compared with Raman spectra of maghemite and
magnetite found in the literature which are completely
different [28]. Indeed, three broad structures at 350, 500
and 700 cm-1 can be observed, which are consistent with
the maghemite structure. These peaks are not observed in
other oxides or oxyhydroxides.
4 Discussion: Anti-wear Mechanisms of Carbon
Nano-onions: The Origin of Maghemite
The first explanation for better anti-wear properties of
nano-onions could be related to the fact that graphite
crystal nanoparticles are intrinsically much harder
(25 GPa) than bearing steel (typically 10 GPa for bearing
steel) and can induce some scratches when they pass inside
the contact, this depending of course on the angle of the
crystal compared to the steel surface. For example, the
elastic moduli of pyrolitic hexagonal graphite has recently
been measured by inelastic X-ray scattering [29] and it is
shown that the C11 contribution is equal to 1.1 TPa giving a
value for ‘‘in-basal plane’’ Young’s modulus of individual
graphene sheets. When sheets are superimposed in a crystal
stack and in the absence of water molecules then the
graphite nanoparticle may scratch the steel surface, the
edge acting as a cutting tool. The situation is different in
the case of carbon nano-onions. Carbon nano-onions have
typically no edge and they can easily slide and even roll on
the surface. Moreover, should the nano-onion structure
exfoliate in individual nano-sheets, these 2-D curved
structures will certainly not damage the surface because
the layers are flexible and will certainly prefer to adhere on
the steel surface or roll in a cylinder instead of abrading the
steel. This could explain the lower wear observed with
carbon nano-onions.
Another interesting feature is the existence of some
typical iron oxide materials in the wear debris, preferably
maghemite as suggested by the TEM study. Our Raman
study definitely confirms that the oxide detected in the wear
scar is maghemite. Despite the fact that hematite (a-Fe2O3)
is the more stable oxide species, it has never been detected
in the present study. In fact, as attested by HRTEM and
Fig. 11 DF images obtained for
the several positions of the
objective aperture (a–d
corresponding to positions 1–4
in Fig. 7)
Tribol Lett (2008) 30:69–80 77
123
EFTEM observations, oxide particles lying in the wear
debris have a very small grain size of typically 5–10 nm
(see Figs. 9 and 10), and it is well known that the structure
of stable iron oxides is size-dependent. When the crystallite
size is above 30 nm, it is well established that the more
stable phase of iron oxide is hematite, a corundum-type
structure which is particularly abrasive. However, within
the size range 5–30 nm, maghemite (c-Fe2O3), the cubic
spinel observed here, is preferred. Below typically 5 nm,
the formation of amorphous particles is favoured. This
phenomenon, called ‘‘size-driven phase transition’’ [30],
has been observed during a milling procedure using steel
balls, which produces a decrease in hematite grain size
with the emergence of nanocrystalline grains of maghemite
[31]. Furthermore, because maghemite and graphite have
similar atomic plane distances lying in the 0.2 nm values
(see Fig. 8b), it can be supposed that the oxide could be
stabilized by the graphitic structure of the carbon sheets.
Indeed, HRTEM images show that the iron oxide particles
have a nanometre size and are practically embedded and
homogeneously distributed in the carbon structure. It is
interesting to notice that this kind of graphite–magnetite–
maghemite composite microstructure has already been
observed in ordinary chondritic meteorites and interstellar
grains [32], a material which is submitted to severe
solicitations when entering the atmosphere and hitting the
earth.
Eventually, we propose the following model for the
onion-induced tribofilm structure. It is composed of a
mosaic of graphitic sheets and intact onions bound by
lubricious iron oxides nanoparticles. Low friction has
already been observed with Fe3O4 and FeOOH nanoparti-
cles dispersed on Fe–C matrix [33]. In the model proposed
by Yuansheng et al. [33], Fe3O4 act as lubricious oxides
and FeOOH supply hydrogen on the counter surface. A
similar mechanism might be involved in the case of carbon
onions with the formation of a carbon film containing
lubricious iron oxides and with the presence of OH groups
on their surface, which would however remain almost
impossible to detect in TEM. Hydroxylation of diamond-
like carbon coatings was also found to be at the origin of
superlow friction coefficient during friction between sur-
faces coated by DLC coatings and lubricated by glycerol
mono-oleate [34]. Maghemite is a ferrimagnetic material
(like magnetite) and has a high electrical resistivity (at the
opposite of magnetite). Each nanoparticles behaves like a
ferromagnet in the tribofilm material. The possibility of
repulsive forces between the two counterparts cannot be
excluded. More work would be necessary to validate this
mechanism.
5 Conclusion
Carbon nano-onions present better tribological properties
than graphite powder when used as lubricant additives in
PAO base oil for steel surfaces (specifically anti-wear
efficiency). The lubrication mechanism of nano-onions is
not yet fully understood, but HRTEM, EELS, EFTEM and
Raman analyses give important clues for a better
comprehension.
High-resolution TEM coupled with EELS was used to
fully characterize carbon onions before friction and to
clearly distinguish their properties from those of graphite.
From plasmon-loss EELS results, carbon onions can be
considered less ‘‘electronically hard’’ than graphite. As a
consequence, they are far less abrasive than graphite
micron size particles. This softer behaviour may also be
explained from their spherical nested structure (without
edges).
Structural changes of carbon onions during friction tests
were studied by electron diffraction and HRTEM coupled
with DF imaging. After friction, results give evidence for
the presence of residual intact carbon onions and iron
oxides nanoparticles inside the tribofilm material. Iron
oxide was never found in the hematite form (the stable
structure which is known to be highly abrasive) but in the
unstable form of maghemite, which is most probably sta-
bilized by a size effect. The presence of magnetite, which
remains difficult to infirm on a simple basis of diffraction,
Fig. 12 (a) Raman analysis of the wear scar with carbon onions after
500 cycles (k = 514.5 nm) compared to spectra of maghemite (b)
and hematite (c) from [28]
78 Tribol Lett (2008) 30:69–80
123
is rejected according to the EELS analysis. Moreover,
Raman analyses performed inside the wear scar confirm
that maghemite is preferentially formed. In this case, we
speculate a stabilization of maghemite by both the ‘‘size-
driven phase transition’’ and the proximity of graphitic
structures, as observed in interstellar grains and chondrite
meteorites. Thus, the tribofilm formed by carbon onions
could be able to trap large abrasive wear particles and
convert them into ultrafine lubricious iron oxides ones, thus
preventing the tribological surfaces from further abrasive
wear process. The origin of friction reduction is tentatively
attributed to the presence of OH termination at the top of
the oxides. Further works are necessary to confirm this
point.
Acknowledgements The CLYM (Centre Lyonnais de Microscopie)
is gratefully acknowledged for the access to the transmission electron
microscopes.
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