ARTICLE IN PRESS
Tribology International 42 (2009) 584–590
Contents lists available at ScienceDirect
Tribology International
journal homepage: www.elsevier.com/locate/triboint
Application of diamond-like carbon coatings to elastomers frictional surfaces
L. Martı́nez a,, R. Nevshupa a,d, L. Álvarez a, Y. Huttel a, J. Méndez a, E. Román a, E. Mozas b, J.R. Valdés b,
M.A. Jimenez b, Y. Gachon c, C. Heau c, F. Faverjon c
a
Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Cantoblanco, 28049 Madrid, Spain
Instituto Tecnológico de Aragón, Maria de Luna 8, 50018 Zaragoza, Spain
HEF R&D, ZI Sud Rue Benoit Fourneyron, F42166 Andrezieux Boutheon Cedex, France
d
Bauman Moscow State Technical University, 2-Baumanskaia 5, 105005 Moscow, The Russian Federation
b
c
a r t i c l e in f o
a b s t r a c t
Article history:
Received 27 September 2007
Received in revised form
26 June 2008
Accepted 30 June 2008
Available online 23 August 2008
Nitrile-butyl rubber-like materials were coated with amorphous hydrogenated diamond-like carbon
(DLC) coatings in order to modify their surface and tribological properties. Measurements of water
contact angle were performed by the sessile drop method and showed that the coated samples are more
hydrophobic with water contact angles up to 1161. The surface free energy of the elastomers was
calculated by the acid–base regression method considering polar and dispersive contributions and the
results were correlated with changes in the surface chemistry measured by X-ray photoelectron
spectroscopy. It has been found that the lower presence of oxygen functional groups on the elastomer
surfaces led to lower surface free energies, even though the polar contribution was not predominant.
We also found that the DLC coatings led to a significant decrease of the surface free energy (up to 16%)
and that there is a good correlation between the surface free energy values and the corresponding water
contact angle values. The coefficient of friction was also measured and presented a significant decrease
after coating with DLC.
& 2008 Elsevier Ltd. All rights reserved.
Keywords:
Elastomer
Diamond-like coating
Surface energy
X-ray photoelectron spectroscopy
Friction coefficient
1. Introduction
Acrylonitrile–butadiene rubbers (NBR) and hydrogenated
acrylonitrile–butadiene rubbers (HNBR) are widely used in the
automotive industry, due to their moderate cost, excellent
resistance to oils, fuels and greases, processability and very good
resistance to swelling by aliphatic hydrocarbons [1]. One effective
way to increase the life in service of machine elements is to apply
a thin coating of amorphous diamond-like carbon (DLC). Such
coatings presented excellent tribological properties as they
increase the frictional and wear performance of machine elements
[2]. These DLC coatings have been extensively/predominantly
used over metals, ceramics and other inorganic materials.
Recently, Nakahigashi et al. [3] reported lower friction and wear
when coating elastomers with elastic DLC. The idea of applying
a hard DLC film on elastomer materials was considered at first
with reluctance because it does not sound logical to apply a thin
hard film on a deformable substrate without the occurrence of
interfacial delamination. Once it was proved to be possible, there
exists a wide field of application for coated elastomer components
in mechanics and especially automotive industry. In this work,
DLC coatings were applied over a series of elastomers of the same
Corresponding author. Tel.: +34 913349000; fax: +34 913720623.
E-mail address: lidia.martinez@icmm.csic.es (L. Martı́nez).
0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.triboint.2008.06.016
family (NBR) in order to study the variation of these frictional
surface properties and consider further optimization of the films.
Water contact angle (CA) measurements proportioned information about the hydrophobicity of the surface that is crucial for
applications in various branches of industry [4]. A surface is
considered to be hydrophobic when the water CA is over 901 [5].
CA is typically the property measured for the non-stick coatings to
estimate their surface energy. In the present work, the obtained
results on the water CA were compared with a detailed analysis
of the surface free energy calculated by the acid–base regression
method. In addition, the influence of the surface chemistry
and morphology was studied by means of X-ray photoelectron
spectroscopy (XPS) and atomic force microscopy (AFM). Finally,
the effect of DLC coatings deposited on the elastomer and/or the
counterbody on the tribological performance of the elastomers
was studied by measuring the coefficient of friction (COF) and by
evaluating the friction-related noise and wear.
2. Experimental details
2.1. Materials and treatments
A comparison of the performance of NBR 7201, NBR 9003,
NBR 8003 and HNBR 8001 elastomers was carried out with and
without DLC coatings deposited on their surface.
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Deposition of DLC films was performed in a vacuum deposition
unit equipped with radio frequency (RF) biasing. The chamber was
pumped down to 3 106 mbar residual pressure. The Ar+ plasma
etching of the surface lasted for 1 h; after that an organosilicon
precursor was introduced in the plasma for 7 min. Afterwards,
acetylene was introduced in the plasma for 3 h. During coating
deposition the bias voltage was reduced. As elastomer materials
are really sensitive to temperature, parameters were optimized
to keep the temperature of each step of the process below
the maximal admissible temperature (120 1C). Depending on the
bias, the maximal temperature ranged between 75 and 100 1C.
Preliminary experiments were performed, where the process was
interrupted after heating and etching. The samples were observed
afterwards, and neither noticeable surface modification nor
plasticizer exhaust was observed.
Optical observation of a film deposited on elastomer (Fig. 1)
shows a regular crack network along the valleys of the surface. It
is now admitted that the presence of these crack networks is a
positive thing as it allows flexibility of the coating when the
substrate is bent without interfacial delamination [6]. The
thickness of the DLC coating on the elastomer was measured by
a Mahr stylus profilometer and the resulting thickness was 3 mm.
The counterbody used for tribological tests was a brass alloy
(EN12164 CW164N). The PECVD process was used for the
deposition of DLC on the counterparts. After degreasing and
cleaning in ultrasonic bath, samples were introduced in a vacuum
chamber with a base pressure of 2 104 mbar and heated at
150 1C for 2 h for outgassing. Subsequently, Ar was introduced
(60 sccm) in the chamber for plasma etching of the surface during
45 min. A pulsed bias voltage was applied to ignite the plasma.
Then an organosilicon precursor was introduced in the plasma for
1 h. Finally, acetylene was introduced in the plasma for 1 h.
2.2. CA measurement and surface free energy calculation
Static CA measurements were carried out at room temperature
by the sessile drop method using water droplets of 4 ml. The
surface energy evaluation system (SEE) [4] was used for the
acquisition of the drop images and further analysis of the drop
shape. The CA measurements were repeated several times on each
sample and the obtained results were averaged in order to
minimize the measurement uncertainty. The recorded pictures
were analyzed manually by the user, providing full control of
CA determination.
The surface free energy was calculated by the acid–base
regression method [4] using the same SEE system. This method
585
enables to determine the electron-donor and electron-acceptor
components of the surface energy. The total free surface energy
(g) is the sum of non-polar Lifshitz–Van der Waals (gLW) and polar
acid–base (gAB) components. This last gAB component includes
electron-acceptor (g+) and electron-donor (g) components,
which are not additive [7]:
pffiffiffiffiffiffiffiffiffiffiffi
gAB ¼ 2 g gþ
(1)
Five liquids were used for this analysis: water, glycerol, ethylene
glycol, formamide and diiodomethane, whose properties are
detailed in Table 1.
2.3. XPS measurements
All the samples were characterized by XPS. The measurements
were carried out in an ultra-high vacuum (UHV) chamber with a
base pressure of 1 1010 mbar, using a Phoibos 100 ESCA/Auger
spectrometer, with non-monochromatized MgKa (1253.6 eV)
X-rays. Wide energy range scan spectra and narrow energy scan
spectra were recorded. The wide scans provided complete
information about all the elements present in the samples while
the narrow scans provided more accurate chemical information
on specific elements and chemical bonds. The narrow scan spectra
were recorded with an analyser pass energy of 15 eV. The
contribution of the MgKa satellite lines was subtracted and
the spectra were subjected to a Shirley background subtraction
formalism [8]. The binding energy scale was calibrated with
respect to the C 1s peak at 285 eV [9,10].
2.4. AFM measurements
The surface morphology was examined using AFM in the
dynamic mode in order to avoid stick–slip problems during
the measurements. The tip used was a silicon polygon-based
pyramid with a height of 10–15 mm and a typical tip radius of
o7 nm. The analysis of the images was carried out with WSxM
software [11].
2.5. Tribological tests
Tribological tests were conducted on a Plint TE77 tribometer
with reciprocating configuration [12], which includes a control
unit incorporating COMPEND acquisition software. The tests were
carried out at a temperature of 2372 1C with a relative humidity
between 55% and 70%. The system provides sequence control of
load, frequency and temperature in addition to the data acquisition of measured parameters. Fig. 2 shows an image of the
reciprocating test system of the TE 77 tribometer.
The rubber specimens were square plates of side 10 mm and
thickness 2 mm, glued to the steel tooling using cyanoacrylate
adhesive. The coated surfaces of the rubber were cleaned with a
clean laboratory tissue and the uncoated ones were cleaned with
acetone.
The reciprocating friction tests consisted of eight steps that
combined different values of normal load and friction frequency,
Table 1
Components of surface free energy (in mJ/m2) of reference liquids
Fig. 1. Optical microphotograph of a DLC-coated elastomer surface.
Liquid
gtot
gLW
gAB
g
g+
Water
Glycerol
Ethylene glycol
Diiodomethane
Formamide
72.80
64.00
48.00
50.80
58.00
21.80
34.00
29.00
50.80
39.00
51.00
30.00
19.00
0.00
19.00
25.50
57.40
30.10
0.00
39.60
25.50
3.92
3.00
0.00
2.28
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Normal
Force
Reciprocating
motion
Steel tool
Brass
counterpart
Rubber
specimen
Fig. 2. General scheme of the reciprocating motion Plint TE77 tribometer.
Table 2
Normal load, oscillating frequency and duration of the steps of reciprocating friction tests
Test description
Step
No.
ID
1
2
3
4
5
6
Running-in
Test #1
Test #2
Test #3
Test #4
Test #5
Pause
Test #6
Final test
7
8
Normal force (N)
Pressure (MPa)
Oscillating frequency (Hz)
Max. linear velocity (mm/s)
Time (s)
Distance (mm)
10
10
10
10
10
40
0
10
10
0.1
0.1
0.1
0.1
0.1
0.4
0.1
0.1
0.1
2.15
0.125
0.215
2.15
0.215
0.215
0
0.215
2.15
101.3
5.89
10.1
101.3
10.1
10.1
0
10.1
101.3
300
120
120
120
120
120
60
120
120
9675
225
387
3870
387
387
0
387
3870
as specified in Table 2. Various combinations of coated and
uncoated elastomers and brass counterbodies were studied. All
the tests were performed at room temperature with a linear
stroke of the reciprocating motion of 15 mm.
3. Results
3.1. CA measurements and surface free energy calculations
Fig. 3 represents the comparison of the water CA values
measured on each elastomer before and after coating with DLC. In
all cases, an increase in the water CA was registered after DLC
deposition. The NBR-coated elastomers had CA values above 1001,
which represented an increase of at least a 10% of CA after the DLC
coating. On the contrary, the HNBR 8001 is the only elastomer
tested whose CA remained below 1001 in particular and 841 after
coating with DLC. For this elastomer only a small variation of the
CA close to the experimental uncertainty was registered.
The crack network observed in the DLC surfaces (Fig. 1) can
influence the final value of the CA measured. A higher surface
roughness usually leads to lower CA. However, in our experiments
the coatings with similar crack structure induced different
modifications on the CA measured, depending on the substrate.
The importance of the substrate on the CA measured on the DLCcoated samples is especially evident from different behaviour
observed on the hydrogenated HNBR in comparison to the NBRs.
Furthermore, in the case of the HNBR, the variation in the CA after
coating with DLC is within the experimental uncertainty and,
Fig. 3. Water CA measurements before and after coating with DLC.
therefore, this suggests that surface roughness is not a determinant factor in the final value measured.
Total surface free energy and its components were calculated
using acid–base regression method and are shown on Table 3. For
the three NBR elastomers, the total surface free energy decreased
after the DLC coating, whereas the HNBR showed the opposite
tendency. The dispersive component (gLW) was the main contribution to the surface free energy. In the case of the NBR
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elastomers, both the polar and dispersive components of the
surface free energy presented a tendency to decrease with the DLC
coating that involved a decrease in the total surface free energy.
Only in the case of NBR 8002, a small increase in gLW was
measured. This tendency was also observed on the HNBR 8001,
Table 3
Surface free energy of the studied samples determined by the acid–base regression
method
Samplea
Surface free energy (mJ/m2)
g
gLW
gAB
g+
g
NBR 7201
U
C
51.0
42.8
49.7
42.4
1.3
0.4
1.4
0.2
0.3
0.2
NBR 9003
U
C
36.5
34.1
36.2
33.6
0.3
0.5
0.01
0.38
3.6
0.1
NBR 8002
U
C
36.7
33.3
31.2
32.6
5.5
0.7
1.2
0.7
6.3
0.1
HNBR 8001
U
C
38.0
41.2
35.8
39.5
2.2
1.7
0.3
0.3
4.5
2.4
a
U—uncoated, C—coated with DLC.
587
where gLW increased and gAB remained above 1 mJ/m2, mainly due
to a high contribution of the electron-donor component (g).
3.2. Surface analysis
In Table 4 the chemical composition (%at) of the elastomer
surfaces obtained from the wide energy range scan XPS spectra is
compared for uncoated elastomers and after DLC coating. The
main elements present on the samples before and after coating
with DLC were carbon, oxygen and nitrogen. Small amounts of
other elements have been found. Among these elements sulphur
and zinc are typically used in curing processes of rubber [14];
silicon is also typically present on the elastomer, while the origin
of tin, argon and chlorine is unclear. In the present analysis these
elements were not taken into account as it was supposed that
they have no real influence on the surface material properties [1].
An increase in the oxygen content was the only modification
observed in the surface composition after adding the DLC
coatings.
In order to investigate changes in the main bonding of the
elastomers, the curve-fitting procedure of XPS spectra of the C 1s
core level peaks was performed. The results of the curve fittings
are illustrated in Fig. 4 and are summarized in Table 5. The four
components used for the fitting are derived from the expected
chemistry of the samples, taking into account the natural
oxidation process of the elastomers. The main contribution to
the carbon peak was fixed at 285 eV, which corresponds either to
Table 4
Chemical composition of the samples tested (%at)
Table 5
Percentage of the C 1s peak area of XPS core level spectra of the NBR samples
Samplea
Samplea
Composition (%at)
% C 1s core levels
C
O
N
Ar
Cl
Sn
Zn
Si
NBR 7201
U
C
88.4
87.7
6.8
7.9
0.5
2.7
2.7
–
0.4
0.1
–
–
0.2
Traces
1
1.6
NBR 7201
U
C
98
95
NBR 9003
U
C
90.7
88.6
6.9
7.9
–
1.3
1.1
–
0.6
0.4
0.1
0.5
0.1
–
0.5
1.3
NBR 9003
U
C
NBR 8002
U
C
98.2
98.3
1
1.3
–
–
–
–
–
–
–
–
–
–
0.8
0.3
HNBR 8001
U
C
91.7
89.1
4
6.5
3.6
3.5
–
–
–
–
–
–
–
–
0.7
0.9
CQO
O–C ¼ O
0
3
0
1
2
1
90
88
6
9
1
1
3
2
NBR 8002
U
C
97
93
3
7
0
0
0
0
HNBR 8001
U
C
91
86
5
11
4
2
0
1
a
U—uncoated; C—coated with DLC.
C=O
C-C/C-H
O-C = O
C-O
C=O
C-O
HNBR 8001
292
290
288
286
284
C–O
U—uncoated, C—coated with DLC.
C-C/C-H
Intensity (a.u.)
a
C–C/C–H
282 280 292 290
Binding energy (eV)
HNBR 8001 +DLC
288
286
284
Fig. 4. C 1s core levels of the uncoated (a) and coated with DLC (b) HNBR.
282
280
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Fig. 5. AFM images of uncoated (left) and coated with DLC (right) NBR 7201 elastomer.
the C–C or the C–H bond [10]. The following components at higher
binding energies correspond to oxidized components: C–O bonds
at 286.3 eV, CQO at 288.1 eV and O–CQO at 289 eV [15].
The CQC bond was included in the C–C component as the gap
between these binding energies is only 0.3 eV [7]. The CRN bond
from the NBR structure was not considered due to the small
contribution of nitrogen to the final composition. After these
considerations, the variation of each contribution after the DLC
coating was analysed keeping the binding energy and the fullwidth at half-maximum (FWHM) of each untreated elastomers
fixed.
The results evidenced a decrease in the C–C contribution
corresponding to the backbone structure of the elastomers after
coating with DLC, as a consequence of the formation of oxygen
functional groups, mainly in the form of C–O, and also O–CQO. In
the case of the CQO contribution, the changes observed were
smaller than on the other oxygen-containing components. Among
the tested samples, the HNBR elastomer presented the highest
increase in O functional groups.
Variation of the surface morphology of the samples at
nanoscale was studied by AFM. Fig. 5 presents as an example
the surface of the NBR 7201 elastomer before (left) and after
(right) DLC coating. It can be observed that the surface appearance
of the elastomers is modified with the DLC coating. Among the
several parameters that can be used to characterize the surface
roughness, three different roughness parameters were measured
in this work: the root mean square (RMS), the maximum height
difference (Rmax) and the average roughness (Ra). All of them
revealed an increase in the surface roughness of the elastomers by
a factor of four after the DLC coating. This was surprising since
after DLC deposition a lower surface roughness was expected, as it
usually occurs on metals [13]. However, over these elastomers,
DLC coatings produced an increase in the surface roughness on
nanoscale.
3.3. Tribological tests
The COF of the elastomers was measured along the eight steps
specified in Table 2. Additionally, the loss of weight of the tested
surfaces was measured after friction tests. Four different combinations of coated and uncoated specimens and counterbodies
were studied for each elastomer. Fig. 6 illustrates the results of the
friction tests, whereas Table 6 displays the obtained values.
It can be observed that for most elastomers DLC coating
produced a significant decrease in the COF, in tests with both
coated and uncoated counterparts. The NBR 8002 is the only
elastomer that presented a significant decrease in the COF when
the counterpart was not coated. When the friction tests were
Fig. 6. Example of one of the friction tests performed on NBR 8002.
performed with uncoated elastomer and coated counterpart, there
also appears a significant reduction in the COF, especially for NBR
8002 and HNBR 8001, but not as large as the reduction achieved
by coating the elastomers.
When the elastomer is coated, there is a tendency to reduce
friction noise. Nevertheless, this conclusion must be handled with
care, since only the qualitative appearance of noise has been
registered, and a detailed analysis of the magnitude and frequency
of the noise has not been performed.
Finally, wear analysis failed to provide any conclusive evidence,
since the weight losses during the friction test (25 min) were
negligible with respect to the weight of the samples (around 11 g),
and wear tracks were not observed.
4. Discussion
In this work, the CA of water measured by the sessile drop
method was used as a first approximation to estimate the surface
free energy of DLC coatings on elastomers. The results obtained
presented a good correlation between the CA measured and the
surface free energy calculated by the acid–base regression
method.
For NBR elastomers coated with DLC, the CA of water increased
from 10% to 22%. These results clearly indicate the increase in the
ARTICLE IN PRESS
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Table 6
Coefficient of friction of the elastomers
Step
p (MPa)
Coefficient of function
Combination elastomer/counterbodya
NBR 7201
Running-in
Test #1
Test #2
Test #3
Test #4
Test #5
Test #6
Final test
Friction noise
Lost weight (mg)
NBR 9003
Running-in
Test #1
Test #2
Test #3
Test #4
Test #5
Test #6
Final test
Friction noise
Lost weight (mg)
NBR 8002
Running-in
Test #1
Test #2
Test #3
Test #4
Test #5
Test #6
Final test
Friction Noise
Lost weight (mg)
HNBR 8001
Running-in
Test #1
Test #2
Test #3
Test #4
Test #5
Test #6
Final test
Friction noise
Lost weight (mg)
a
C/C
U/C
C/U
U/U
0.1
0.1
0.1
0.1
0.1
0.4
0.1
0.1
1.07
0.65
0.84
1.03
0.87
0.38
0.73
0.94
0.1
4.34
3.10
3.83
4.50
4.32
1.76
4.30
4.60
+
0.6
0.99
0.48
0.69
0.98
0.67
0.36
0.72
1.05
0
6.0
3.5
4.9
5.0
4.7
2.0
5.0
4.6
+
0.7
0.1
0.1
0.1
0.1
0.1
0.4
0.1
0.1
1.03
0.72
0.96
1.37
0.92
0.38
0.87
1.25
0
3.61
3.29
4.09
3.57
3.88
1.59
3.68
2.62
+
0.3
0.96
0.61
0.88
1.01
0.84
0.38
0.82
1.01
0.2
5.27
3.54
4.28
4.69
4.17
1.74
4.24
4.43
+
0.1
0.1
0.1
0.1
0.1
0.1
0.4
0.1
0.1
1.31
1.39
1.52
1.20
1.23
0.65
1.15
0.99
+
0.1
2.86
2.27
2.63
2.23
2.12
1.10
2.23
2.23
+
0.3
0.56
0.44
0.52
0.53
0.50
0.27
0.52
0.49
+
0.2
4.41
2.75
3.08
4.52
3.13
1.15
3.14
4.63
0.2
0.1
0.1
0.1
0.1
0.1
0.4
0.1
0.1
1.00
0.45
0.57
0.88
0.61
0.30
0.57
0.87
0.2
3.77
3.05
3.56
3.36
3.52
1.40
3.23
3.11
+
0.5
0.76
0.38
0.55
0.78
0.55
0.23
0.50
0.78
0
5.89
3.50
4.80
5.88
4.55
1.83
5.05
6.20
+
0.1
C—coated, U—uncoated.
hydrophobic character of the elastomer surfaces. This finding is
consistent with previous works, where an increase in the
hydrophobic properties of the DLC-coated elastomers was
explained by the presence of are a mixture of sp2 and sp3
hybridized carbon bonds in the DLC coating. This response is
usually related to a lower reactivity of the coated surface. Indeed,
in our experiments we measured a decrease in the surface free
energy of the NBR elastomers coated with DLC. Characterization of
hydrophobicity of the elastomer surfaces by the measurement of
the CA of water is a good method to determine the modifications
on the polarity of the elastomer surfaces and the corresponding
chemical activity after DLC coating [16].
For the hydrogenated HNBR elastomer, a different behaviour
was observed. The variations in the CA measured were statistically insignificant. In addition, this is the only material tested
whose surface free energy increased after coating with DLC. It is
suggested that hydrogenation of unsaturated bonds to form HNBR
589
results in different reactivity of the elastomer towards the DLC
coatings. Despite the fact that the same type of DLC coating was
deposited on all the elastomers, the extent of the modifications
was different depending on the substrate. This is clearly evident
from the comparison of the CA and the surface free energy for
coated NBR and HNBR elastomers.
In addition to the surface chemistry, the CA can be modified by
changing the surface roughness. More specifically the increase in
the roughness of hydrophobic surfaces may result in a drastic
decrease of water CA [17]. However, in the calculation of the surface
free energy we could not take into account this factor. Analysis of
the surface morphology shows different patterns on micro and
nanoscales. As shown in Fig. 1, on microscale the surface pattern is
characterized by a net of cracks and peeled-off zones. This net of
cracks with distance between them from 5 to 50 mm can be formed
as a consequence of the large difference in mechanical properties,
i.e. elastic modulus and hardness, of the coating and the elastomer
substrates. This distance is much larger than the size of the AFM
images, but much smaller than the size of the water droplet.
Although all coated surfaces have equivalent surface roughness, the
effect of the roughness on the CA should be the same for all
elastomers. Thus, the difference in CA measured for coated NBR and
HNBR elastomers must be explained by different factors including
surface and bulk chemistry of substrates.
The chemical surface analysis evidenced an increase in the
polar oxygen functional groups on the coated surfaces. The
presence of these groups leads to stronger attraction of polar
liquids like water to the surface [18]. Therefore, lower CAs and
higher surface free energies are expected for surfaces containing
more polar oxygen functional groups. On the other hand, these
groups are mainly responsible for the increase in the acid
component (g) of gAB, while this component is not predominant
in the surface free energy.
The small changes in the surface chemistry and surface free
energy may lead to small changes in the surface shear stress, which,
in turn, not only leads to direct changes on the friction coefficient,
but also affects the tangential deformation of the underlying
elastomer substrate. Indeed, this deformation controls the energy
dissipation during the friction of elastomers by elastic hysteresis.
The increase in the surface shear stress results in an increase of the
dissipated energy in the volume and vice versa. Therefore, the effect
of the initial small surface changes on the friction coefficient is
significantly enhanced. In our case, the small decrease observed in
the surface free energy may result in a decrease in the adhesive
component of the friction coefficient. Also, the CA of water
decreased, thus decreasing the capillary component of friction.
From the NBR family, the NBR 8002 presents some peculiarities. The untreated elastomer presents the highest gAB values to
the surface free energy due to an important g contribution. This
material presented the highest increase in the water CA after
coating with DLC. The difference in this sample could be explained
on the light of its surface composition. It presented the largest
amount of carbon before and after DLC coating, which represented
a low contribution of polar groups and, consequently, this sample
exhibited the highest hydrophobic properties before and after
coating deposition. In particular, this elastomer also presented the
most significant decrease in the COF, which can be due to a
reduction of the adhesive component of friction.
5. Conclusions
It was demonstrated that coating NBR and HNBR elastomers
with DLC is a good method to improve the performance of these
materials. It produces an increase in hydrophobic properties,
surface roughness at nanometer and micrometer scales and
ARTICLE IN PRESS
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L. Martı́nez et al. / Tribology International 42 (2009) 584–590
decrease in the COF. However, a strong influence of the substrate
characteristics was found on the final properties of DLC coating.
Despite the fact that a similar surface roughness at micro and
nanoscale was found in all the samples, the final surface
properties change depending on the elastomer substrate tested.
The measurement of water CA can be used as an efficient way
of estimating the evolution of the surface free energy. A
comparison between the CA and the surface free energy measured
by the acid–base regression method reflects a good correlation
between both parameters.
The hydrogenated HNBR elastomer presented a different
behaviour from the non-hydrogenated elastomers (NBRs). A larger
amount of oxygen functional groups on the surface of HNBR after
DLC coating induced the lowest increase in hydrophobic properties. As a result, HNBR is the only material that showed an
increased surface free energy upon DLC coating.
DLC coatings produce a significant decrease in the COF of the
elastomers and reduce their friction noise.
Acknowledgements
The authors acknowledge the financial support of the EU from
the Sixth framework programme in the KRISTAL Project no.
515837-2. L. Martı́nez and Y. Huttel acknowledge the Spanish
‘‘Ministerio de Educación y Ciencia’’ for the ‘‘Juan de la Cierva’’ and
‘‘Ramón y Cajal’’ programmes, respectively. R. Nevshupa acknowledges the ‘‘Marie Curie’’ programme (MIF1-CT-2006-22067).
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