The influence of the composition of atmosphere on the mechanisms of degradation of graphite in planetary ball millers

Article abstract of DOI:10.1016/j.jallcom.2009.10.193

This study focuses on the degradation process of graphite in a planetary ball miller. It is shown that this degradation is only related to the milling mode. The sliding mode is very gentle toward the graphite structure whereas the shock milling mode leads to the quick degradation of the graphite structure producing poorly organized nanoparticles. Nevertheless, the milling mode is not determined by the ball miller rotation speed but by the ability of graphite to form a lubricating layer. The development of this lubricating layer in only possible in presence of oxygen which annihilates the reactive sites created during milling by forming oxygen groups. In this case, the sliding mode predominates. When the oxygen disappears or under inert atmosphere, the reactive sites stick to the ball surface, destroying the graphite lubricating layer and resulting in shock milling mode. This explains the link between the milling atmosphere and the graphite structure evolution in planetary ball millers. This also explains why the graphite structure evolution is far less sensitive toward environment in a vibratory ball miller in which the sliding milling mode is not possible.

Full text of DOI:10.1016/j.jallcom.2009.10.193

Journal of Alloys and Compounds 491 (2010) L15–L19
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
The influence of the composition of atmosphere on the mechanisms of
degradation of graphite in planetary ball millers
Jean-Christophe Rietsch, Roger Gadiou∗, Cathie Vix-Guterl, Joseph Dentzer
Institut de Science des Matériaux de Mulhouse CNRS LRC 7228, 15, rue Jean Starcky, 68057 Mulhouse cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
This study focuses on the degradation process of graphite in a planetary ball miller. It is shown that this
degradation is only related to the milling mode. The sliding mode is very gentle toward the graphite struc-
ture whereas the shock milling mode leads to the quick degradation of the graphite structure producing
poorly organized nanoparticles. Nevertheless, the milling mode is not determined by the ball miller rota-
tion speed but by the ability of graphite to form a lubricating layer. The development of this lubricating
layer in only possible in presence of oxygen which annihilates the reactive sites created during milling
by forming oxygen groups. In this case, the sliding mode predominates. When the oxygen disappears or
under inert atmosphere, the reactive sites stick to the ball surface, destroying the graphite lubricating
layer and resulting in shock milling mode. This explains the link between the milling atmosphere and
the graphite structure evolution in planetary ball millers. This also explains why the graphite structure
evolution is far less sensitive toward environment in a vibratory ball miller in which the sliding milling
mode is not possible.
Received 18 September 2009
Received in revised form 16 October 2009
Accepted 22 October 2009
Available online 31 October 2009
Keywords:
Ball milling
Graphite
Surface area
© 2009 Elsevier B.V. All rights reserved.
In the case of carbon materials, mechanical grinding can be used
to produce a material with specific properties or it can be used as
a model experiment for processes taking place during their use in
industrial applications. For this latter case, we can mentioned the
example of carbon-based materials which are used as friction mate-
rials for industrial applications going from brushes in fuel pump
motors to brakes in aircrafts and racing cars. The rubbing behavior
of these carbon materials depends on the friction conditions (load,
sliding speed, temperature) [1,2] but also on the environment. First
demonstrated by Savage and Campbell on graphite brushes [3,4],
the influence of water, oxygen and all lubricating vapor on wear,
friction and friction film morphology were widely studied, espe-
cially on pin-on-disc and disc-on-disc dynamometers [5–7]. Beside
the film morphology study, several papers focused also on the anal-
ysis of the chemical, physical and textural properties of the debris
generated during the rubbing of the carbon materials [8–10]. The
similarity between the characteristics of the debris and of the car-
bon materials resulting from milling underlines that there is an
analogy between the two processes. Therefore, studying the milling
mechanism can be also useful to obtain a better knowledge of
the evolution of carbon materials submitted to highly energetic
mechanical stresses similar to what encountered during friction.
Mechanical grinding has been widely used to modify the struc-
ture or the surface chemistry of carbon materials [11–14]. Milling of
graphite enables to obtain high surface area graphite (HSAG) with
unusual nanostructures like nanoonions, graphite nanoplatelets
with a low number of stacked graphene layers, etc. [15–17].
Graphite milling in inert or hydrogen atmosphere is also a promis-
ing technique to prepare anodic materials for lithium-ion batteries
[18,19] or hydrogen storage systems [21,20], respectively. The
strong influence of the gas atmosphere on carbon structure evolu-
tion under planetary ball milling was reported by several authors
[22]. Under hydrogen, the structure of a graphitic material is quickly
transformed into poorly organized nanoparticles that chemisorb
large amounts of hydrogen chemically bonded to the carbon. The
presence of C–H bonds shows that the highly reactive sites created
during milling which are able to decompose gaseous hydrogen.
Under inert atmospheres, the graphite structure is also strongly
affected and the reactive sites created during the milling react when
exposed to air, leading to the formation of surface oxygen groups
by oxygen chemisorption. Moreover, the milling also strongly mod-
ify the specific surface area of the graphite and values as high as
400 m2/g can be reached.
On the contrary, the graphite structure is preserved and a
low surface area is maintained under oxygen milling in the same
apparent mechanical conditions as shown by Ong and Yang by
XRD analysis [22]. This phenomenon is attributed to the reaction
between the reactive sites and the oxygen during milling. Ong et
al. proposed that oxygen suppresses the fracture rate by forming
∗
Corresponding author. Tel.: +33 389 608 719; fax: +33 389 608 799.
E-mail address: roger.gadiou@uha.fr (R. Gadiou).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.10.193

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J.-C. Rietsch et al. / Journal of Alloys and Compounds 491 (2010) L15–L19
Fig. 2. Evolution of the mechanism of milling inside the ball miller in correlation
with the oxygen concentration inside the jar.
Fig. 1. Sound intensity emitted by the miller recorded during 3 min at 300 rpm (a)
without graphite under oxygen, (b) with graphite under argon, and (c) with graphite
under oxygen.
that the milling mode is again the shock mode (Fig. 1b). This shock
milling mode persists during the whole experience of the graphite
milling. After 2 h of milling at 300 rpm under argon atmosphere the
specific surface area reaches 400 m2/g (see Fig. 3b). This increase
of the surface area is also related to a strong degradation of the
structure of the material. The Fig. 3a shows that the mean crystallite
thickness Lc decreases from 210 Å down to 60 Å during these first
2 h. On the contrary, when the milling atmosphere is oxygen, the
milling behavior changes (Fig. 1c). During the first 15 s of milling,
the intensity emitted by the experimental setup is high, roughly
similar to the one observed during milling under argon. This shows
that the beginning of the milling occurs in the shock mode. After this
transition phase, the sound intensity quickly decreases to a level
which is typical of a sliding mode of milling. As this transition occurs
oxides on the active centres. However the influence of the gas atmo-
sphere is far less sensitive in vibratory milling devices. Indeed, in
the case of vibratory milling the milling mode is mainly a very ener-
getic shock type interaction whereas in a planetary ball mill, two
different milling modes may occur. The first one is a gentle slid-
ing of the balls along the material while the second one is shock
type interaction similar to the one observed in vibratory milling.
In the latter mode, the energy transferred to the material is far
higher than in the former one, leading to an important degrada-
tion of the graphite material. The two modes can therefore lead to
different materials [23]. The milling mode is generally thought to be
controlled by the ball miller rotation speed. A low rotation speed
should lead to a sliding mode whereas high planetary ball miller
speed should lead to an impact mode.
The milling mode in which the planetary ball miller is running
can be easily detected by the analysis of the sound emitted by the
experimental setup: the sound intensity during the shock mode
is significantly higher than in sliding mode. Therefore, the objec-
tive of this study is to show how the milling mode is controlled by
the gas atmosphere during a planetary ball milling of graphite and
how this milling mode can lead to different degradation states of
graphite. The understanding of the influence of the milling mode on
the carbon degradation is a necessary prerequisite before studying
the influence of the gas environment during milling.
To reach this objective, a graphite (SLX-50 from Timcal SA -
Bodio - Switzerland) was milled in a planetary ball miller with
five balls and a jar in stainless steel (Fritsch Pulverisette 6). The
rotation speed of the planetary ball miller was chosen at 300 rpm.
The jar was equipped with two taps enabling to fulfill the jar with
the desired gas but also to follow up the gas phase composition
and pressure. The gas phase composition was measured by cali-
brated mass spectrometry (Inficon Transpector 2) and the pressure
by a pressure gauge (1000 Torr MKS Baratron). The specific surface
area of the milled carbon materials was obtain by applying the BET
model on the nitrogen adsorption isotherm at 77 K in the relative
pressure range [0.05–0.3]. The XRD spectra were measured for the
milled materials (spectrometer Philips X’pert 2000, monochroma-
tised wavelength Cu k˛ at 1.5405 Å, range 10–60◦, count time 5 s
per 0.05◦ step). The mean thickness of the graphite crystallites Lc
was obtained by applying Scherrer’s equation on the d0 0 2 peak of
the carbon material.
When the miller operates without any carbon material in an
oxygen atmosphere, the shock mode is observed as shown by the
high mean sound intensity recorded (Fig. 1a). If graphite is added in
the jar with an argon atmosphere, the sound intensity recorded is
exactly the same as the one measured with an empty jar, showing
Fig. 3. Influence of the atmosphere in the jar on the evolution of the structural and
textural parameters of the milled material. (a) Average crystallite thickness Lc and
(b) BET surface area SBET
.

J.-C. Rietsch et al. / Journal of Alloys and Compounds 491 (2010) L15–L19
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Fig. 4. MEB images of the surface of the balls after a milling of graphite during 3 min. (a and b) Under argon and (c and d) under oxygen.
after only 15 s, it cannot be related to some structural modification
of the graphite material.
sliding mode is observed during the first 2 h of milling, the rate of
degradation of the graphite material is low and the increase of the
TSA value is not significant. During this phase, the level of structural
order is not modified and the mean Lc value is close to the pristine
one.
At 300 rpm, the sound recording presented in Fig. 2 shows that
this sliding mode phase persists during 1 h 30 min. Then, an inter-
mediate step which have a duration of around 1 h is observed in
which the miller operates alternatively in sliding and shock modes.
This is shown on the sound record by the fast switch between high
and low sound intensity (see Fig. 2). This change of the milling mode
can be related to the evolution of the gas phase composition in
the milling jar. In fact, the oxygen content in the jar atmosphere
decreases during the milling. After 1 h 30 min, about 70% of the
initial amount of oxygen is consumed by chemisorption on the
graphite reactive sites. A small part of the oxygen chemisorbed
groups are then released as CO2. The duration of this intermedi-
ate step in which the miller operates alternatively in sliding and
shock modes is 1 h with our experimental conditions. When the
oxygen and the CO2 are almost consumed, that is after 2 h 30 min,
the milling continue under vacuum conditions and the mechanism
changes to the shock mode. As it was mentioned in the introduction,
the modification of the milling mechanism has a great influence on
the evolution of the properties of the carbon material. Fig. 3a and b
shows the evolution with the milling duration of the specific sur-
face area and of the average thickness of the crystallites Lc of the
carbon material, respectively. Under argon, the graphite is quickly
degraded and its surface area increases up to 500 m2/g. At the same
time, the Lc value decreases down to 50 Å. This shows that the
milling quickly leads to the formation of carbon nanoparticles with
a low structural order compared to the pristine graphite. Under
oxygen, the degradation of the structure of the carbon material is
significantly lower than under an inert atmosphere, this behavior
was already observed by Ong and Yang [22]. In our experiments, the
Then, between 1 h 30 and 2 h 30 min of milling, a period alter-
nating sliding and shock modes takes place. This step is also
characterized by a decrease of the oxygen concentration in the jar.
The modification of the milling mode leads to a significant increase
of the TSA of the graphite which is the indication of an increase of
the rate ofdegradation ofthe carbon material. As aconsequence, the
rate of creation of active sites on the carbon surface increases. This
phenomenon induces an increased consumption of the oxygen and
enhance the transition to the shock mode. Therefore, a feed-back
loop is created which leads to a quick change from sliding mode to
shock mode. The XRD analysis of the milled materials shows that
during this transition phase, the global structural ordering of the
material is mainly kept. Therefore, this change in milling mode is
not directly related to a strong modification of the bulk material.
After 2 h 30 min, the milling mode transits in a pure shock type
mode as shown by the sound intensity (Fig. 2). The oxygen disap-
pears from the gas atmosphere after 3 h of milling. Although less
reactive toward the surface of the carbon material than O2, the CO2
resulting from the carbon–oxygen reaction, is also consumed by the
active sites after 4 h. The jar atmosphere can then be assimilated to
vacuum. In such conditions, the shock mode is the predominant
one leading to a quick destruction of the graphite structure and
to an increase of the surface area as it is observed under argon
atmosphere. This is confirmed by the strong decrease of the value
of the Lc parameter from 210 Å to 90 Å in less than 1 h of milling.
Therefore, our results clearly point out that the degradation pro-

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J.-C. Rietsch et al. / Journal of Alloys and Compounds 491 (2010) L15–L19
Fig. 5. MEB images of the surface of the balls after a milling of graphite during 6 min, the balls have been cleaned by compressed air. (a) Under argon and (b) under oxygen.
cess in the planetary ball miller are strongly linked to the change of
the milling mode which is itself controlled by the gas atmosphere
present during the milling of graphite.
To explain this behavior, a SEM analysis of the surface of the
balls was performed after 3 min of milling under argon and oxygen.
The Fig. 4a and b shows the surface of the ball after 3 min of milling
under argon. The graphite particles are randomly distributed on the
ball surface. On the contrary, under oxygen the graphite particles
lie parallel to the ball surface forming a lubricating graphite layer
(see Fig. 4c and d).
the graphite structure characterized by a strong evolution of its
texture and of its structure. As a consequence nanoparticles with a
low structural organization are produced. Nevertheless, the milling
mode is not determined by the ball miller rotation speed but by the
ability of the graphite to form a lubricating layer or not. This layer
prevents the existence of the shock mode. The development of a
graphite lubricating layer in only possible in presence of oxygen
(or some other reactive molecule) which annihilates the reactive
sites created during milling by forming functional groups. In this
case, the sliding mode predominates. When the oxygen is con-
sumed or under inert atmosphere, the reactive sites can not react
anymore with the jar atmosphere; the milled graphite sticks to
the ball surface and the graphite lubricating layer disappear, pro-
moting a shock milling mode. Hence, the link between the milling
atmosphere and the graphite structure evolution in planetary ball
milling can be explained. This also explains why the graphite
structure evolution is far less sensitive toward environment in a
vibratory ball miller in which the sliding milling mode is not pos-
sible [22].
The surface of the balls was then cleaned by compressed air
(Fig. 5). For the ball used during the milling under argon, a car-
bon layer remains at the surface indicating that an adhesive carbon
layer was formed during the milling (Fig. 5a). On the contrary, in
the case of milling under oxygen, the graphite layer is completely
removed from the ball surface after cleaning and the stainless steel
surface is apparent (Fig. 5b). Therefore the oxygen seems to inhibit
the adhesion of graphite on the steel ball. Under inert atmosphere
(argon, vacuum), the impacts of the ball creates highly reactive sites
on graphite which cannot be stabilized by reactions with the jar
atmosphere since O2 and CO2 are not present. As a consequence,
the existence of these sites promotes the strong stick of the graphite
on the steal ball surface. On the contrary, in the case of milling
under oxygen, the oxygen and the carbon dioxide from the atmo-
sphere react with the reactive sites and hence prevent the adhesion
of graphite on the ball surface. The particles are adsorbed on the
surface through low energy interactions along basal planes. The ori-
entation of the graphite particles is parallel to the sliding direction
and a lubricating layer is formed. This lubricating layer prevents
the ball to be carried by the jar motion. Thus, the planetary ball
milling of graphite under oxygen operates under a sliding gentle
milling mode. Under argon, the lubricating layer is not formed. The
graphite particles are randomly distributed creating roughness. As
the ball is rough, it is carried by the jar motion and centrifugal forces
take the ball down the jar wall. Then, the ball is projected on the
opposite jar wall leading to a shock mode milling.
Our work underlines that the study of the influence of the
milling atmosphere on the evolution of the graphite characteristics
requires to carefully determine the type of milling mode existing
during the milling. In other words, it is not possible to establish a
correlation between the carbon degradation during milling and the
gas atmosphere without taking into account the different milling
modes which can occur in planetary ball millers.
References
[1] N. Murdie, C. Ju, J. Don, F. Fortunato, Carbon 29 (1991) 335–342.
[2] B.K. Yen, T. Ishihara, Wear 196 (1996) 254–262.
[3] W.E. Campbell, R. Kozak, ASME Trans. 70 (1948) 491–498.
[4] R.H. Savage, D.L. Schaefer, J. Appl. Phys. 27 (1956) 136–138.
[5] J.K. Lancaster, J.R. Pritchard, J. Phys. D: Appl. Phys 14 (1981) 747–762.
[6] J. Lancaster, Tribol. Int. 23 (1990) 371–389.
[7] B. Yen, T. Ishihara, I. Yamamoto, J. Mater. Sci. 32 (1997) 681–686.
[8] M. Franc¸ ois, J. Joly, P. Kapsa, P. Jacquemard, Carbon 45 (2007) 124–131.
[9] C. Pevida, P. Jacquemard, J.P. Joly, Carbon 46 (2008) 994–1002.
[10] J.C. Rietsch, J. Dentzer, A. Dufour, F. Schnell, L. Vidal, P. Jacquemard, et al., Carbon
47 (2009) 85–93.
[11] Y.B. Li, B.Q. Wei, J. Liang, Q. Yu, D.H. Wu, Carbon 37 (1999) 493–497.
[12] Y. Chen, J.D. Fitz Gerald, L.T. Chadderton, L. Chaffron, Appl. Phys. Lett. 74 (1999)
2782–2784.
[13] N.J. Welham, V. Berbenni, P.G. Chapman, J. Alloys Compd. 349 (2003)
255–263.
[14] M. Francke, H. Hermann, R. Wenzel, G. Seifert, K. Wetzig, Carbon 43 (2005)
1204–1212.
[15] F. Salver-Disma, J.M. Tarascon, C. Clinard, J.N. Rouzaud, Carbon 37 (1999)
1941–1959.
[16] J.Y. Huang, H. Yasuda, H. Mori, Chem. Phys. Lett. 303 (1999) 130–134.
[17] N. Pierard, A. Fonseca, Z. Konya, I. Willems, G. Van Tendeloo, J.B. Nagy,
Chem.Phys. Lett. 335 (2001) 1–8.
In summary, the influence of the gas atmosphere on the evo-
lution of the carbon properties in a planetary ball miller must be
studied by taking into account the milling mode (sliding or shock
mode). Our work clearly highlights that this milling mode is not
directly controlled by the operating conditions, and that it can
change during the milling depending on the evolution of the com-
position of the atmosphere inside the jar. This change in milling
mode has then a great influence on the evolution of the properties
and on the nature of the milled material.
The sliding mode is very gentle toward the graphite structure
whereas the shock milling mode leads to a quick degradation of

J.-C. Rietsch et al. / Journal of Alloys and Compounds 491 (2010) L15–L19
L19
[18] F. Disma, L. Aymard, L. Dupont, J.M. Tarascon, J. Electrochem. Soc. 143 (1996)
3959–3972.
[21] S. Orimo, G. Majer, T. Fukunaga, A. Züttel, L. Schlapbach, H. Fujii, Appl. Phys.
Lett. 75 (1999) 3093–3095.
[19] B. Gao, C. Bower, J.D. Lorentzen, L. Fleming, A. Kleinhammes, X.P. Tang, et al.,
Chem. Phys. Lett. 327 (2000) 69–75.
[20] T. Ichikawa, D.M. Chen, S. Isobe, E. Gomibuchi, H. Fujii, Mater. Sci. Eng. B 108
(2004) 138–142, EMRS 2003, Symposium C, Nanoscale Materials for Energy
Storage.
[22] T.S. Ong, H. Yang, Carbon 38 (2000) 2077–2085.
[23] A. Calka, D. Wexler, T. Fenwick, Appl. Crystallogr. XX 130 (2007)
219–224.