215
Tribology Letters, Vol. 22, No. 3, June 2006 (Ó 2006)
DOI: 10.1007/s11249-006-9084-x
Effect of nitrogen atoms desorption on the friction of the CNx coating
against Si3N4 ball in nitrogen gas
T. Tokoroyamaa,*, M. Gotob, N. Umeharaa, T. Nakamurac and F. Hondad
a
Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan
Ube National College of Technology, 2-14-1, Takiwadai, Ube, 755-8555, Japan
c
Nagoya Institute of Technology, Gokiso-cho, showa-ku, Nagoya, Aichi, 466-8555, Japan
d
Toyota Technological Institute, 2-12-1, Hisakata, Tempaku-ku, Nagoya, 468-8550, Japan
b
Phase transition of CNx coatings by sliding against a Si3N4 ball has been studied by Auger electron spectroscopy (AES) and
X-ray photoelectron spectroscopy (XPS) to understand this super-low friction phenomena in N2. A pin-on-disk type tribometer
was constructed to determine the tribological properties of this coating when sliding against a Si3N4 ball in N2. The analytical
results by AES and XPS showed that the nitrogen atoms desorbed from the top layers of the coating, and that the layers changed to
a graphite-like structure without nitrogen during a friction coefficient decrease to lower than 0.01. The structural transition of CNx
is discussed in this paper.
KEY WORDS: CNx, low friction, N2, AES, XPS, graphite
1. Introduction
Carbon hard coating such as Diamond-Like Carbon
(DLC) has been much awaited for use as a protective
coating on sliding interfaces. The advantages of DLC
coatings are their high mechanical strength, chemical
inertness, high wear resistance, and low friction coefficient [1–6]. These coatings can be applied as solid
lubricants for high-speed sliding, high pressure conditions, high temperature environments, and vacuum
situations.
Amorphous Carbon Nitride (CNx) coating is one of
the promising materials among carbon system hard
coatings used to obtain excellent mechanical properties
[7–11]. The mechanical properties and the friction
coefficient of the coatings depend on their synthesis
techniques [12–16]. In particular Umehara et al.
reported that CNx coating showed a friction coefficient
lower than 0.01 when was slid against a Si3N4 ball in N2
[17,18]. The mechanism of the low friction coefficient
was considered to be that the sliding surface changed to
a graphite-like structure by sliding in N2. Since the
friction coefficient of the graphite layer was not as low as
that of CNx, the specific characteristic structure of
graphite on the CNx could govern the low friction
coefficient. It is important to understand the detailed
mechanism of the low friction coefficient of CNx the
optimum design of a solid lubricant. However, the
effects of both nitrogen atoms in the coating and from
*To whom correspondence should be addressed.
E-mail: [email protected]
ambient N2 gas on the transformation of surface layers
of CNx remain unclear.
Information on the chemical state of a sliding
interface is very important to understand the effect of
the extremely low friction coefficient of CNx in N2
gas. Surface analyses such as Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy
(XPS) are the powerful techniques used to analyze
both surface composition and the chemical state with
a depth resolution of a few nanometers. The peak
position in their spectra provides information on the
chemical composition of samples, and the peak
profiles indicate the chemical state of elements simultaneously.
Direct comparison of surfaces before and after sliding
over an extended period has been insufficient to understand structural changes of the surface for a plausible
low friction mechanism of CNx. Thus, the variation of
chemical compositions in the wear track on the CNx
surface with the number of sliding cycles was observed
using AES because the spatial resolution of AES is
significantly higher than that of XPS. The phase transition of the topmost layer of CNx before and after
sliding was also analyzed with AES, and the result was
compared with the XPS data obtained from the same
samples with the special preparation. The surface analyses of both AES and XPS provided important information on surface conditions such as chemical
composition and state because of their sufficient sensitivity with respect to the information from the surface
layers of the wear surface. The purpose of this paper is
to clarify the chemical state of the CNx surface as a
1023-8883/06/0600–0215/0 Ó 2006 Springer Science+Business Media, Inc.
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T. Tokoroyama et al./Effect of nitrogen atoms desorption on the friction of the CNx coating
function of the friction coefficient. AES was employed to
investigate the relationship between the friction coefficient and the nitrogen concentration on a CNx surface.
The phase transition on the CNx surface was also
examined using AES and XPS. The mechanism of the
low friction coefficient of CNx is discussed in terms of
the bond strength between carbon and nitrogen atoms in
the coatings.
2. Experimental
2.1. Apparatus
A schematic illustration of the pin-on-disk type tribometer is shown in figure 1. The substrate covered with
CNx coating was attached to a rotatable table, and the
ball was fixed on top of an arm with a load sensor.
Normal load and friction force were detected with strain
gauges on leaf springs. The sensitivities of the load
sensor for normal load and friction force were 0.047 and
0.0265 mN/mV, respectively. Both normal load and
friction force were continuously recorded during the
friction test with a sampling frequency of 1 Hz. The
sliding speed was set to 0.26 m/s.
The tribometer was positioned in a vacuum chamber
with a basic pressure, 6.7 10)2 Pa which was evacuated by a rotary pump. The 99.8% purity N2 gas was
introduced into the chamber and maintained throughout the tests.
2.2. Specimens
A CNx coating 100 nm thick was deposited on a
Si(100) wafer by the ion beam assisted deposition
(IBAD) method, as shown in figure 2. The vacuum
chamber was evacuated to 1.0 10)4 Pa before introducing nitrogen gas up to 2.2 10)2 Pa. Prior to the
deposition, the Si substrate was sputter-cleaned for
5 min by nitrogen ions (N+) with the energy of
1.0 KeV. The deposition process of CNx coating
consisted of two essentials: one was the sputtering
deposition of amorphous carbon on the substrate, and
the other an ion-mixing process of the coating by N+
Figure 2. Configuration of CNx coating preparation.
ions. Therefore, CNx coating was produced by a combination of both deposition and mixing processes. The
target of the sputtering process was sintered graphite
with 99.999% purity, which was bombarded by argon
ions with the energy of 1 KeV and a 100 lA current.
N+ ions for the ion-mixing process had the energy of
0.5 KeV and an ion current density of 30 lA/cm2. The
deposition rate of CNx coating was 0.9 nm/min, which
was measured by quartz crystal microbalance (QCM).
The QCM was calibrated by thickness results measured
by a profilometer.
The atomic concentration of nitrogen in the CNx
coating was approximately 20 at% according to XPS
analysis. The surface roughness was 0.72 nmRa, which
was measured by atomic force microscope (AFM).
Nano-indentation measurements showed the hardness
of the CNx coating was 25.2 GPa with a maximum
indentation load of 20 lN, and an indentation speed of
0.6 nm/s.
A silicon nitride ball with a diameter of 8.0 mm was
selected as the counter material, and this is widely used
in industrial applications as the ball bearing. The
hardness of the ball was 15 GPa by micro-Vickers
hardness measurements, and the roughness of the surface was 30 nmRy measured by AFM.
2.3. Experimental procedures
Figure 1. Schematic of pin-on-disk type tribometer.
The substrate with the CNx coating and the ball were
ultrasonically cleaned in acetone for 15 min, and then
installed in the tribometer.
Friction tests were carried out to determine the
transition in the friction coefficient as a function of the
sliding cycles. The substrate was settled on the rotatable
table of the tribometer, and the Si3N4 ball was in contact
with the CNx coating. The maximum Hertzian contact
pressure was 400 MPa, and the Hertz contact diameter
was approximately 20 lm under a normal contact load
of 0.1 N. The sliding speed was set to 0.26 m/s, which
was controlled by adjusting the revolution number of
T. Tokoroyama et al./Effect of nitrogen atoms desorption on the friction of the CNx coating
the table. The atmospheric condition was maintained at
1 105 Pa of N2 during the friction tests.
The nitrogen concentration on the worn surface of
the coatings was analyzed by AES with a beam energy
of 5 kV and a probe current of 100 nA as a function of
the sliding cycles. The phase transition of the topmost
layer of CNx before and after sliding was also analyzed
by AES, and the result was compared with the XPS data
obtained from the same samples. The X-ray source for
XPS analysis was selected as monochromized Al Ka
with a beam diameter of 100 lm.
To obtain the exact signals of photoelectrons from
the worn surface by XPS, the size of the worn surface
must be larger than the beam diameter. A large area of
worn surface was prepared by increasing the number of
the wear tracks 20 lm wide to avoid admixing of the
signals from the as-deposited surface. The distance from
the rotating center of the table to the sliding position
was varied from 4.0 mm to 8.0 mm with feed increments
of 1.8 lm. As a result, a 4 4 mm2 area showing a
friction coefficient lower than 0.01 was prepared for the
surface analysis, which was sufficiently large compared
with the beam diameter of the X-ray in XPS.
3. Results and discussion
3.1. Surface chemistry of sliding surface of CNx coating
Figure 3 shows the friction coefficient of CNx coating
sliding against the Si3N4 ball in an N2 atmosphere as a
function of the sliding cycles. The friction coefficient
during the first sliding cycles to approximately 8000
cycles was in the range of 0.3. The friction coefficient
then decreased gradually down to 0.2 with sliding cycles
at 8000 cycles to 10,000 cycles. Then, after a long running-in period, it decreased to approximately 0.01 at the
end of the friction cycles (16,000 cycles). The transition
of the friction coefficient of CNx coating in N2 was
similar to that reported previously [17,18].
Figure 3. Friction behavior of CNx coating sliding against Si3N4 ball
as a function of sliding cycles. AES analyses were carried out at points
(a), (b), (c), and (d).
217
The topographies of the wear tracks on the coating
and the counter surfaces were observed by AFM, as
shown in figures 4 and 5, respectively. In the first 10,000
cycles, the CNx coating had gradually worn approximately to 3 nm depth, and 25 lm width. After the
16,000 sliding cycles, the wear track widened to
approximately 70 lm. The transfer layer on the counter
surface developed gradually as the sliding cycles proceeded, as shown in figure 5. In our previous research,
the transfer layer built-up on the Si3N4 ball surface
before the friction coefficient drastically decreased [19].
The results by AES analysis between 150 eV and
550 eV on the worn surfaces of the CNx coating are
shown in figure 6. C KLL, N KLL and O KLL peaks
presented at around 258, 375 and 507 eV, respectively,
and the O KLL peak showed oxygen molecules
adsorbed on the CNx coating surface [20]. It is clear that
the N KLL peak decreased with the number of sliding
cycles, and the relative intensity ratios of N KLL to C
KLL on the worn surfaces were then plotted as a
function of sliding cycles as shown in figure 7. The
intensity ratio of N KLL/C KLL of the first to 1000
cycles was the same value, indicating that no exodiffusion had taken place at these cycles. Then its ratio
decreased with the number of sliding cycles, and finally
the ratio fell to approximately zero. This suggests that N
atom exodiffusion from the CNx coating had been
simultaneously taking place with the build-up of the
transfer layer at the running-in period. Umehara et al.
attributed the low friction mechanism of CNx coating
sliding against the Si3N4 ball in N2 to a graphitization of
the surface layers of the CNx coating [17,18]. However,
they did not point out the role of N atoms in the CNx
coating. The AES result presented here, however, denied
the possibility of a strong-bond-formation between N
and C atoms in the surface layers.
3.2. Phase transition of sliding interface
As mentioned above, we first demonstrated N atom
exodiffusion from a CNx coating surface by sliding
against a Si3N4 ball in N2. From the viewpoint of N
atom exodiffusion, the AES analysis for C KLL is
shown in figure 8 as a function of the amount of N
atoms included in CNx surfaces and highly-oriented
pyroritic graphite (HOPG). The C KLL peak of 263 eV
suggested carbonaceous contamination which could be
detected by a high electron beam current [21]. However,
all these samples were cleaned the same way before
analysis. Therefore, these surfaces were considered as
having almost the same amount of contamination.
However, a shoulder peak at around 237.2 eV was
observed on the HOPG surface, whereas hardly any
shoulder was observed on the as-deposited surface of
CNx. Further, it is readily noticed that the shoulder
peak which appeared on the HOPG appears on the
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T. Tokoroyama et al./Effect of nitrogen atoms desorption on the friction of the CNx coating
Figure 4. AFM images of the wear tracks on CNx coating (a) 0 cycles, (b) 1000 cycles, (c) 10,000 cycles, and (d) 16,000 cycles.
Figure 5. AFM images of the counter surfaces of Si3N4 ball (a) 0 cycles, (b) 1000 cycles, (c) 10,000 cycles, and (d) 16,000 cycles.
T. Tokoroyama et al./Effect of nitrogen atoms desorption on the friction of the CNx coating
80000
219
Beam energy: 5 kV
70000 Beam current: 100 nA
N = 16000
Intensity
60000
N = 10000
50000
40000
N = 1000
30000
20000
10000
0
550
N=0
O-KLL
C-KLL
N-KLL
500
450
400
350
300
250
200
150
Kinetic energy, eV
Figure 6. Auger electron spectra of CNx coating after sliding number
of cycles N = 0, 1000, 10,000, and 16,000 sliding cycles.
Figure 8. C KLL spectra of as-deposited CNx, worn track of CNx,
and HOPG.
worn surface of the coating, which showed a remarkable
friction coefficient of 0.01.
Figure 9 shows C1s spectra obtained from asdeposited CNx, the wear track of CNx, and HOPG in
XPS analysis. The FWHM of C1s became small by
sliding, and the FWHM value approached that of
HOPG. This indicates that the as-deposited CNx surface
became more graphitic and contained fewer C–N and
C=N bonds but the majority of clusterized sp2 sites.
Furthermore, figure 10 shows the magnified spectra of
C1s peaks of as-deposited CNx, and the worn surfaces
of the CNx, diamond, and HOPG. The peak positions
of the diamond and HOPG were 285.30 and 284.55 eV,
respectively. The peak positions of C1s of the worn
surface of CNx shifted slightly toward the HOPG peak
position. These results indicated that the top layer of
the worn surface on CNx changed to a slightly more
ordered structure without N atoms compared with the
amorphous structure of the original CNx. The ordering
approached that of a graphite structure.
The low friction mechanism of hydrogenated DLC
was discussed on the basis of surface graphitization by
Erdemir from the result of Raman spectroscopy
[4,22,23]. However, other authors propose a low friction
mechanism in which an electrostatic repulsion between
the surfaces of hydrogen-terminated DLC may
counteract the weak van der Waals attraction [24].
However, such repulsive forces of hydrogen cannot explain the super-low friction phenomena of CNx coating,
because it did not include hydrogen in the structure and
was not hydrogen-terminated too. Therefore, the
experimental results of N atom exodiffusion from CNx
surface demonstrated the process of CNx surface
graphitization as proposed by Umehara et al. [17,18].
We thus support the hypothesis of a wear-induced
graphitization to support the stable and low friction
nature of DLC films [22,23].
To increase the graphite phase on a sliding surface, it
is necessary to break the C–C sp3 bonds, C–N sp3 bonds
and/or C=N sp2 bonds involved. In our previous
research, XPS analysis of CNx coating exhibited two
peaks that were 398.5 and 399.5 eV, respectively [25].
These peaks indicate that 398.5 eV shows C–N sp3
hybridization [26], and 399.8 eV shows C=N sp2
hybridization [27]. Since the friction surface received an
energy greater than 610 kJ/mol, given the loss of nitrogen atoms from the CNx coating surface, the C=N
bonds were considered to have definitely dissociated
because of their lower binding energy of 305 kJ/mol
[28].
Figure 7. The relationship between friction coefficient and N KLL/C
KLL intensity for wear tracks on CNx coating surface.
Figure 9. C1s spectra on as-deposited CNx, worn track of CNx, and
HOPG.
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T. Tokoroyama et al./Effect of nitrogen atoms desorption on the friction of the CNx coating
Acknowledgment
The authors thank Professor Koji Kato of Tohoku
University for using IBAD equipment.
References
Figure 10. Magnified spectra of C1s peaks of CNx (before and after
sliding), diamond and HOPG.
In light of these discussions and proposals about low
friction mechanisms, a transfer layer must be formed
during the running-in period and wear-induced graphitization of coating surface is needed for super-low
friction phenomena to take place. Therefore, if a CNx
and/or DLC surface are formed by C–N sp3 bonds, it is
suggested that the super-low friction phenomena may
appear from a short running-in period. To clarify the
detailed mechanism of the low friction coefficient in N2,
a full understanding of the process of this graphitelike-structure formation is necessary, and this is the next
problem to be solved.
4. Conclusion
In this paper, the effect of nitrogen atoms and their
contribution to the low friction coefficient of CNx was
discussed in terms of the phase transition of the topmost
layers of the coatings by friction. AES and XPS results
demonstrated that the topmost layers of the coating
changed to a graphite-like structure without nitrogen
atoms when the friction coefficient decreased to below
0.01, and the graphitization of the topmost layers was
attributed to the low friction coefficient in N2 gas. We
first indicated N atom exodiffusion from the CNx surface during friction. From the viewpoint of binding
energy, it is clear that frictional energy exceeded at least
610 kJ/mol which is C=N sp2 hybridization. The
effectiveness of surface analysis by AES and XPS has
also been demonstrated to clarify the transition in
chemical state of the sliding surface with fine depth and
spatial resolution. The results contribute useful information to help understand the low friction phenomena
of CNx coating in N2.
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