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: lucile.joly-pottuz@insa-lyon.fr

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

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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

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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

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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

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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

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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.

References

1. Ugarte, D.: Curling and closure of graphitic networks under

electron-beam irradiation. Nature 359, 707–709 (1992)

2. Matsumoto, N., Joly-Pottuz, L., Kinoshita, H., Ohmae, N.:

Application of onion-like carbon to micro and nanotribology.

Diamond Relat. Mater. 16, 1227–1230 (2007)

3. Hiraki, J., Mori, H., Taguchi, E., Yasuda, H., Kinoshita, H.,

Ohmae, N.: Transformation of diamond nanoparticles into onion-

like carbon by electron irradiation studied directly inside an ultra-

high vacuum transmission electron microscope. Appl. Phys. Lett.

86, 223101 (2005)

4. Tomita, S., Burian, A., Dore, J.C., LeBolloch, D., Fujii, M.,

Hayashi, S.: Diamond nanoparticles to carbon onions transfor-

mation: X-ray diffraction studies. Carbon 40, 1469–1474 (2002)

5. Mykhaylyk, O.O., Solonin, Y.M., Batchelder, D.N., Brydson, R.:

Transformation of nanodiamond into carbon onions: a compara-

tive study by high-resolution transmission electron microscopy,

electron energy-loss spectroscopy, X-ray diffraction, small-angle

X-ray scattering, and ultraviolet Raman spectroscopy. J. Appl.

Phys. 97, 074302 (2005)

6. Kuznetsov, V.L., Chuvilin, A.L., Butenko, Y.V., Mal’kov, I.Y.,

Titov, V.M.: Onion-like carbon from ultra-disperse diamond.

Chem. Phys. Lett. 222, 343–348 (1994)

7. Greiner, N.R., Phillips, D.S., Johnson, J.D., Volk, F.: Diamonds

in detonation soot. Nature 333, 440–442 (1988)

8. Rapoport, L., Feldman, Y., Homyonfer, M., Cohen, H., Sloan, J.,

Hutchison, J.L., Tenne, R.: Inorganic fullerene-like material as

additives to lubricants: structure-function relationship. Wear

225–229, 975–982 (1999)

9. Rapoport, L., Leschchinsky, V., Lapsker, I., Volovik, Y., Ne-

pomnyashchy, O., Lvovsky, M., Popovitz-Biro, R., Tenne, R.:

Tribological properties of WS2 nanoparticles under mixed

lubrication. Wear 255, 785–793 (2003)

10. Cizaire, L., Vacher, B., Le-Mogne, T., Martin, J.M., Rapoport, L.,

Margolin, A., Tenne, R.: Mechanisms of ultra-low friction by

hollow inorganic fullerene-like MoS2 nanoparticles. Surf. Coat.

Technol. 160, 282–287 (2002)

11. Joly-Pottuz, L., Dassenoy, F., Belin, M., Vacher, B., Martin, J.M.,

Fleischer, N.: Ultra-low friction and wear properties of IF-WS2

under boundary lubrication. Tribol. Lett. 18, 477–485 (2005)

12. Joly-Pottuz, L., Matsumoto, N., Kinoshita, H., Vacher, B., Belin,

M., Montagnac, G., Martin, J.M., Ohmae, N.: Diamond-derived

carbon onions as lubricant additives. Tribol. Int. 41, 69–78 (2008)

13. Joly-Pottuz, L., Ohmae, N.: Carbon-based Nanolubricants. In:

Martin, J.M., Ohmae, N. (eds.) Nanolubricants, pp. 93–147.

Wiley (2008)

14. Moshkovith, A., Perfiliev, V., Verdyan, A., Lapsker, I., Popovitz-

Biro, R., Tenne, R., Rapoport, L.: Sedimentation of IF-WS2

aggregates and a reproducibility of the tribological data. Tribol.

Int. 40, 117–124 (2007)

15. Martin, J.M.: Boundary lubrication by colloıdal systems. General

principles. In: Martin, J.M., Ohmae, N. (eds.) Nanolubricants, pp.

1–13. Wiley (2008)

16. Abe, M., Kataura, H., Kira, H., Kodama, T., Suzuki, S., Achiba,

Y., Kato, K., Takata, M., Fujiwara, A., Matsuda, K., Maniwa, Y.:

Structural transformation from single-wall to double-wall carbon

nanotube bundles. Phys. Rev. B 68, 041405 (2003)

17. Stephan, O., Ajayan, P.M., Colliex, C., Cyrot-Lackmann, F.,

Sandre, E.: Curvature-induced bonding changes in carbon nano-

tubes investigated by electron energy-loss spectrometry. Phys.

Rev. B 53, 13824–13829 (1996)

18. Cabioc’h, T., Girard, J.C., Jaouen, M., Denanot, M.F., Hug, G.:

Carbon onions thin film formation and characterization. Euro-

phys. Lett. 38, 471–475 (1997)

19. Cabioc’h, T., Thune, E., Riviere, J.P., Camelio, S., Girard, J.C.,

Guerin, P., Jaouen, M., Henrard, L., Lambin, P.: Structure and

properties of carbon onion layers onto various substrates. J. Appl.

Phys. 91, 1560–1567 (2002)

20. Terauchi, M.: Electronic structure analyses of BN network

materials using high energy-resolution spectrscopy methods

based on transmission electron microscopy. Microsc. Res. Tech.

69, 531–537 (2006)

21. Tomita, S., Fujii, M., Hayashi, S., Yamamoto, K.: Electron

energy-loss spectroscopy of carbon onions. Chem. Phys. Lett.

305, 225–229 (1999)

22. Xu, S., Tay, B.K., Tan, H.S., Zhong, L., Tu, Y.Q.: Properties of

carbon ion deposited tetrahedral amorphous carbon films as a

function of ion energy. J. Appl. Phys. 79, 7234–7240 (1996)

23. Howe, J.M., Oleshko, V.P.: Application of valence electron

energy-loss spectroscopy and plasmon energy mapping for

determining material properties at the nanoscale. J. Electron

Microsc. 53, 339–351 (2004)

24. Lee, E.H., Hembree, D.M., Rao, G.R., Mansur, L.K.: Raman

scattering from ion-implanted diamond, graphite, and polymers.

Phys. Rev. B 48, 15540–15551 (1993)

25. Richter, A., Ries, R., Smith, R., Henkel, M., Wolf, B.: Nano-

indetation of diamond, graphite and fullerene films. Diamond

Relat. Mater. 9, 170–184 (2000)

26. Colliex, C., Manoubi, T., Ortiz, C.: Electron-energy-loss-spec-

troscopy near-edge fine structures in the iron-oxygen system.

Phys. Rev. B 44, 11402–11411 (1991)

27. Pecharroman, C., Gonzalez-Carreno, T., Iglesias, J.E.: The

infrared dielectric properties of maghemite, c-Fe2O3, from

reflectance measurement on pressed powders. Phys. Chem.

Miner. 22, 21–29 (1995)

28. de Faria, D.L.A., Venancio-Silva, S., de-Oliveira, M.T.: Raman

microspectroscopy of some iron oxides and oxyhydroxydes. J.

Raman Spectrosc. 28, 873–878 (1997)

29. Bosak, A., Serrano, J., Krisch, M., Watanabe, K., Taniguchi, T.,

Kanda, H., Mohr, M., Maultzsch, J., Thomsen, C.: Elasticity of

hexagonal BN and graphite, ESRF Highlights 2006 14–15 (2006)

30. Multani, M.S., Ayyub, P.: Size and pressure driven phase tran-

sitions. Condens. Matter News 1, 25 (1991)

31. Randrianantoandro, N., Mercier, A.M., Hervieu, M., Greneche,

J.M.: Direct phase transformation from hematite to maghemite

during high energy ball milling. Mater. Lett. 47, 150–158 (2001)

Tribol Lett (2008) 30:69–80 79

123

32. Scott, E.R.D., Taylor, G.J., Rubin, A.E., Okada, A., Keil, K.:

Graphite-magnetite agregates in ordinary chondritic meteorites.

Nature 291, 544–546 (1981)

33. Yuansheng, J., Shenghua L.: Superlubricity of in situ generated

protective layer on worn metal surfaces in presence of

Mg6Si4O10(OH)8. In: Erdemir, A., Martin, J.M. (eds.) Superlu-

bricity, vol. 447. Elsevier (2007)

34. Kano, M., Yasuda, Y., Okamoto, Y., Mabuchi, Y., Hamada, T.,

Ueno, T., Ye, J., Konishi, S., Takeshima, S., Martin, J.M., De

Barros Bouchet, M.I., Le Mogne, T.: Ultralow friction of DLC in

presence of glycerol mono-oleate (GMO). Tribol. Lett. 18, 245–

251 (2005)

80 Tribol Lett (2008) 30:69–80

123