Tribological Behaviour of Diamond-Like Carbon Films used in Automotive Application: A Comparison
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S478
Tribological Behaviour of Diamond-LikeCarbon Films used in Automotive Application:A Comparison
Olivier Jarry,* Cedric Jaoul, Pascal Tristant, Therese Merle-Mejean,Maggy Colas, Christelle Dublanche-Tixier, Helene Ageorges, Claude Lory,Jean-Marie Jacquet
Excellent tribological behaviour such as high wear resistance and very low friction coefficientmake diamond-like carbon (DLC) very interesting for automotive applications in order toreduce friction losses in engines. In this work, mechanical properties, structure and tribolo-gical behaviour of two DLC films (DLC-A and B) designed for automotive applications arecompared. Better hardness and Young modulus of one of the coating are investigated byRaman analysis. Two laboratory tribometers with and without lubrication are used toevaluate the tribological behaviour of both coatings. In each test, a running-in period isclearly identified. Most of the wear takes place during this period which also presents a higherfriction coefficient. Even if the structure of both DLC films remains stable in depth during thetest, the DLC-B has the best wear resistance for both lubricated and unlubricated conditions.Later on, this best wear resistance of DLC-B is validated by measurement on test rig.
Introduction
Environmental impact and energy consumption of engines
are becoming an important issue for automotive industry.
One of the main ways to decrease CO2 emissions is to reduce
friction losses in automotive engines. Lubrication plays an
important role in the reduction of these friction losses but
efficiency of lubricant and therefore friction and wear
O. Jarry, C. Lory, J.-M. JacquetSorevi, Parc Ester Technopole, 5 Allee Skylab, BP 6810, 87068Limoges, FranceFax: (þ33) (0)5 55 38 13’35; E-mail: [email protected]. Jaoul, P. Tristant, C. Dublanche-TixierUniversity of Limoges, CNRS, Sciences des Procedes Ceramiques etde Traitements de Surface FranceENSIL, Parc Ester Technopole, 16 rue d’Atlantis, BP 6804, 87068Limoges Cedex, FranceT. Merle-Mejean, M. Colas, H. AgeorgesFaculty of Sciences and Technics, 123 avenue Albert Thomas,87060 Limoges Cedex, France
Plasma Process. Polym. 2009, 6, S478–S482
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mechanisms are different depending on engine parts
concerned. Stribeck curve allows describing different
lubrication regimes.[1–3] Some components as valve train
parts operate under boundary lubrication where oil film is
not enough thick to totally separate the surfaces. Cam/
follower pair represents the most critical interface of valve
train that is the reason why this contact will be studied
in this work. In boundary lubricated regime, surface
properties are key parameters governing the tribological
behaviour of the system. Diamond-like carbon (DLC)
coatings can be used in automotive applications due to
excellent tribological behaviour such as a high wear
resistance and very low friction coefficient.[1,2,4,5] But the
mechanical properties of these films depend mostly on the
proportion of sp2 to sp3 carbon bonds and hydrogen
content.[6]
The aim of this work is to obtain an overall characteriza-
tion of properties and structure of two DLC coatings.
Afterwards, tests on tribometers in both lubricated and
unlubricated conditions are performed in the laboratory in
DOI: 10.1002/ppap.200931007
Tribological Behaviour of Diamond-Like Carbon Films . . .
order to compare their tribological behaviours. Finally,
measurements on test rig should allow corroborating
results previously obtained on tribometers.
Experimental Part
Two series of DLC commercial films were provided by Sorevi
Bekaert. These coatings are labelled DLC-A and B. They were
deposited onto AISI M2 polished flat samples by PE-CVD technique
using an industrial scale R&D reactor. The average roughness (Ra) of
both coated and uncoated samples was around 0.02mm while DLC
layer thickness was 1.5mm. DLC film adhesion was improved by
interlayer system between the steel sample and the DLC film
consisting of transition layers obtained by physical vapour
deposition (PVD). Thus an adhesion value over 30 N for both films
was obtained by scratch test under constant load. This high level of
adhesion allows applying coatings on mechanical parts like finger
followers or camshafts without any risks of delamination.
Chemical composition of DLC layers was determined by elastic
recoil detection analysis (ERDA) and Rutherford backscattering
spectrometry (RBS) using an ion beam of Heþ operating at 2.3 MeV.
Density of the DLC films was calculated combining ERDA-RBS
results with coating thickness measurements obtained by calotest.
Residual stresses in the films were calculated from the curvature of
silicon sample strips after DLC deposition, using the Stoney
equation.[7] Contact angle with water was measured by means of a
goniometer. Hardness (H) and Young Modulus (E) were estimated
by nanoindentation by the means of MTS Nanoindenter1 XP
working with a continuous stiffness modulus (CSM) which allows
obtaining H and E as a function of depth penetration. The Oliver and
Pharr method has been used.[8] Raman spectrometry was used to
investigate the structure of as-deposited DLC films, wear tracks and
transfer films. Raman spectra were performed on a Jobin Yvon
spectrometer (model 64000) in backscattering mode, using the
514.5 nm line of an argon ion laser. Laser power on the sample is
about 4 mW with a 50� magnification objective. Spectra acquired
were fitted by using two Gaussian peaks labelled G and D which are
characteristic of the sp2 hybridized carbon. Three values are
extracted from the curve fitting: Full-width at half maximum of
the G peak (FWHMG), ratio between intensities of the two peaks ID/
IG and the position of the G peak. FWHMG increases as the disorder
increases, i.e., clustering decreases, and clear correlation between
FWHMG and mechanical properties has already been shown.[9,10]
ID/IG is used as a measure of the sp2 phase organized in clusters.[11]
The slope of the raw spectra was observed since it is characteristic of
photoluminescence (PL) background which increases with hydro-
gen content.[9,10]
Wear measurements were performed in ambient air with a
rotating ball-on-disc tribometer at room temperature without
lubrication. The sliding speed was 0.1 m � s�1 with a 6 mm radius
wear track. The uncoated AISI 52000 steel balls with a diameter of
6 mm were static whereas the coated samples were in rotation. The
Hertz contact pressure is around 1.5 GPa. Wear volume of steel balls
was determined by optical microscopy measuring the diameter of
the circular wear scar. Wear volume of the DLC coated samples was
calculated after measuring the cross-section of the wear track by
profilometry. Two tests at 30 000 sliding cycles and one test at
Plasma Process. Polym. 2009, 6, S478–S482
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4 000 cycles were performed for both coatings. The last test with
reduced sliding distance was carried out to check if the wear
resistance is constant in function of the sliding distance.
Friction and wear measurements in lubricated conditions were
realized using a reciprocating pin-on-disk tribometer in order to
roughly approach engine conditions. DLC coated M2 samples and
uncoated AISI 52000 steel pin with a radius of 3 mm were used.
Automotive commercial oil heated at a 363 K temperature was
chosen as lubricant. The tests were conducted under 50 N normal
load. Sliding distance for each test was 1 456 m (22 h) with a 4.6 mm
stroke length. Finally, wear and friction measurements were
performed on a test rig. Finger followers were DLC coated and slid
against uncoated polished camshaft. Eight cycles of one hour were
performed. Each cycle was divided into six steps corresponding to
different engine speeds. During the test, torque measurement and
horse power were continuously registered in order to evaluate
friction behaviour of each coating. Results for the third cycle were
not registered due to technical failure. At the end of the run,
remaining DLC thickness was measured on several followers by
calotest to estimate the wear resistance of the films.
Results and Discussion
Overall Characterization
First part of the work is dedicated to the characterization
and the comparison of DLC films (see Table 1). DLC-B
presents better mechanical properties than DLC-A since its
hardness and its Young modulus are higher. DLC-B also has
the highest density: this is in accordance with its better
mechanical properties because a higher density is typical of
a structure containing more sp3 hybridized carbon and less
hydrogen.[9,10] ERDA-RBS measurements confirm the lower
hydrogen content of DLC-B. According to Raman character-
istics also reported in Table 1, DLC-B presents the highest
FWHMG and also the lowest ID/IG ratio which indicates that
the phase organized in rings is reduced in the case of DLC-B
film. Hence, these values reveal that sp3 carbon hybridiza-
tion is promoted in the DLC-B structure in comparison with
DLC-A structure. Slopes of Raman spectra corroborate
results obtained by ERDA-RBS: level of PL is higher in the
case of DLC-A traducing a more hydrogenated structure.
Both reduction of the hydrogen content and a more sp3
hybridized structure explain and confirm the improvement
in mechanical properties noticed for DLC-B films.
DLC films obtained by PE-CVD are classified in the hard
amorphous carbon hydrogenated (a-C : H) films in the
ternary phase diagram proposed by Robertson.[4] Mechan-
ical properties detailed by Robertson for this kind of films
are generally lower (H� 20 GPa). Moreover results obtained
for DLC-B films are close to properties of hydrogenated
tetrahedral amorphous carbon films (ta-C : H, H� 50 GPa).
The increase in hardness does not induce higher residual
stress as both films present similar values. Actually both
DLC coatings possess high level of compressive stresses.[12]
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O. Jarry et al.
Table 1. Main characteristics of the DLC films compared in this work.
Characteristics DLC-A DLC-B Method
Hardness (GPa) 32.7� 1.8 37.2� 1.6 Nanoindentation
Young Modulus (GPa) 246� 14 286� 10 Nanoindentation
H content (%) 32.3� 1.0 29.0� 1.0 ERDA-RBS
Density (g � cm�3) 1.6 1.9 ERDA-RBSþ calotest
FWHMG 176.0 188.8 Deconvolution of Raman spectra
ID/IG 0.47 0.61 Deconvolution of Raman spectra
Residual stress (GPa) �3.7� 0.3 �3.4� 0.1 Si sample curvature
Water contact angle (8) 69.4 66.3 Goniometer
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Nevertheless, for films studied here, these elevated residual
stresses are not responsible for coating delamination since
during scratch test, coating failure mostly occurs at
substrate-interlayer interface.
Water contact angle is slightly lower for DLC-B, but
deviation appears insufficient to lead to different tribolo-
gical behaviours in lubricated conditions.
Unlubricated Ball-On-Disc Tribometer
Tribological tests in unlubricated conditions with fresh
coated sample are divided in three sessions: 30 000, 30 000
again and 4 000 revolutions. For all samples, friction
coefficients vary between 0.1 and 0.2, whatever the tests.
Curves of friction are very much disturbed probably due to
the formation of debris. Nevertheless it is impossible to link
the instabilities of the friction coefficient to wear results.
Bar-graphs of the Figure 1 show that wear volumes of DLC
coated samples are in the same range after 4 000 and
30 000 cycles. This is also valid for the wear of steel balls.
Thus the wear occurs essentially at the beginning of the test
during the running-in period. After this period, both the
Figure 1. Wear volume of DLC films sliding against the steel ballfor different sliding cycle numbers in ambient atmosphere with-out lubrication.
Plasma Process. Polym. 2009, 6, S478–S482
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surfaces are accommodated. As roughness is reduced and
the surfaces become less abrasive, the wear rate decreases.
Moreover the contact area is larger so that the contact
pressure is reduced since the load applied onto the ball
remains constant during the whole test.[13] However,
surface accommodation cannot explain the similarity of
results after 4 000 and 30 000 cycles. The other reason must
be the formation of the transfer film during the running-in
period.[14] Indeed, for each test, formation of a very thin
transfer film is observed on the contact area of the steel ball.
Formation of this tribofilm is well known to be responsible
for the friction coefficient reduction. Raman spectrum of
this tribofilm shown in Figure 2 presents a very graphitic
character. As graphite is very easy to shear, the tribofilm
promotes the reduction of the friction coefficient. Reducing
the friction at constant load leads to decrease drastically
shear stresses in both DLC films and steel surface. These two
combined phenomena can explain inconstant wear rate
during the ball-on-disc test.
Wear volumes obtained for DLC-B are very low and
reproducible (Figure 1). On the other hand, DLC-A presents
higher wear volumes with very bad reproducibility. Indeed,
in the first session of tests, wear of DLC-A is slightly higher
Figure 2. Raman spectra of as-deposited DLC films, of DLC filmwear tracks and transfer layer observed on the steel ball.
DOI: 10.1002/ppap.200931007
Tribological Behaviour of Diamond-Like Carbon Films . . .
than the one registered for DLC-B, whereas DLC-A wear rate
is drastically increased in the second and third sessions.
Wear track of the DLC-A film tested in the first session
appears very smooth as also seen for all tracks of DLC-B. This
kind of wear is typical of abrasive wear. On the other hand
DLC-A film is worn out and sublayers are visible for tracks of
second and third sessions. Figure 2 presents the Raman
spectrum of DLC-A film before wear test and the spectrum of
the remaining DLC in the wear track of the second session.
Structure of the DLC coatings in the wear tracks remains
similar to as-deposited structures for both DLC films. No
graphitization in depth is visible which could explain the
fast wear of DLC-A. Wear volumes measured on steel ball are
similar for both DLC films and all sessions.
Before the formation of tribofilm, the 1.5 GPa contact
pressure associated with high friction generate high shear
stresses and could explain the higher wear rate during the
running-in period. Moreover, the formation of this tribofilm
and therefore friction coefficient are influenced by atmo-
sphere condition and RH of the air.[15,16] This phenomenon
induces bad reproducibility for the wear results. If the
running-in period is too long, DLC film is worn out. Particles
detached during this period are very abrasive and increase
the abrasive wear phenomena. It can be concluded that
the load used in this test leads to friction conditions very
close to the limit resistance of the DLC-A film. The higher
mechanical properties of DLC-B allows it to resist to these
test conditions, leading to a very reproducible low wear.
Higher hardness of this film does not generate more wear of
the steel counterpart due to the formation of the tribofilm.
Pin-On-Disk in Lubricated Conditions
Figure 3 shows evolution of friction coefficient for both DLC
films in lubricated conditions. Running-in period is again
observed with a decrease of the friction coefficient to 0.08.
As seen in unlubricated tests, this period corresponds with
the formation of the transfer layer and the accommodation
Figure 3. Coefficient of friction of DLC films sliding against steel inlubricated conditions.
Plasma Process. Polym. 2009, 6, S478–S482
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of the two surfaces. The visible tribofilm on the steel pin and
the increase in contact area can also explain the reduction of
friction during this period. Friction behaviour of both DLC
films is similar. In contrast to the unlubricated test, friction
curves do not present any instability leading to more stable
measurements. It is confirmed by the morphology of wear
tracks which seem to be very smooth. Film wear volume is
slightly higher in the case of DLC-A whereas steel pin wear
remains similar.
As predicted, the low differences in water contact angle
and hydrogen content do not influence the frictional
behaviour of the DLC films in lubricated conditions.
Chemical composition and surface morphology of both
DLC are too close to influence the lubricant attachment at
the DLC surface and therefore friction coefficient remains
the same. Actually, the friction behaviour is governed by the
properties (e.g. viscosity) of the lubricant. Nevertheless, the
presence of a thin transfer film on the steel pins indicates
that the lubricant film is not thick enough to avoid contact
between the two surfaces. Most of the wear probably occurs
during the running-in period until tribofilm formation.
Better wear resistance observed for DLC-B is again
attributed to its hardness. Indeed as abrasive wear is
directly linked to hardness, it can be concluded that DLC-B
has a better wear resistance than DLC-A.
Test Rig Measurements
In order to validate previous results, test rig measurements
reproducing real conditions are carried out. It is admitted
that cam/follower system operates in boundary lubrica-
tion.[1] In this regime, the lubricant film is not thick enough
to totally separate the two surfaces in contact. Hence, this
system is governed by both unlubricated and lubricated
contact rules. As described by the Stribeck curve, lubrication
regime depends mostly on the lubricant viscosity and the
sliding speed.[1–3] Due to this reason tests are performed at
different engine speeds. Figure 4 shows the friction
measurement for each cycle and for each DLC coating.
Horse power needed to reach all sliding speeds is clearly
higher during the first cycle of one hour. This result is in
accordance with tests already performed on tribometer in
dry and lubricated conditions. Indeed, a running-in period is
also necessary to reduce friction losses. It is difficult to
distinguish the presence of a transfer film on the steel cam
after the tests. Hence, the main mechanism responsible for
the running-in period should be the accommodation of each
finger follower/cam pair leading to smoother surfaces and
not the formation of the transfer layer. In the first cycle,
friction behaviour is similar for both DLC coated parts
except at the highest engine speed. After the running-in
period, friction behaviours of DLC films remain close with a
slight advantage for DLC-B that disappears at the maximum
speed engine. At this regime, running-in period is a little bit
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O. Jarry et al.
Figure 4. Measurements of the power needed to reach differentengine speeds on a test rig during eight cycles of one hour and inthe frame: remaining DLC thickness on finger followers after thewhole test.
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longer for DLC-B. Figure 4 also shows results of wear
measurements in terms of the remaining DLC thickness. All
finger followers have the same initial DLC thickness around
1.3mm. Each follower is marked by a letter and a number.
Letters correspond to DLC film: A for DLC-A film and B for
DLC-B film. Numbers are attributed as a function of the
finger follower position in the cylinder head. Indeed, oil
pressure is different depending on the distance with oil
injection. Finger followers A4, A5, B4 and B5 are less
lubricated than the other ones. This lower lubrication does
not induce wear increase for followers coated with DLC-B,
whereas a more pronounced wear is clearly visible for A4
and A5 parts. Almost no wear was observed for well-
lubricated finger followers. The reduction of oil pressure at
some location is responsible for wear of DLC-A film. Highest
hardness of DLC-B allows followers to endure more severe
conditions. Better wear behaviour of DLC-B can also explain
the slight difference observed in terms of friction. Indeed,
detachment of debris could be responsible for energy losses
and deterioration of the wear behaviour of DLC-A coated
parts.
Conclusion
In this work, mechanical properties, structure and tribolo-
gical behaviour of two DLC films designed for automotive
applications are compared. Higher hardness and Young
modulus of DLC-B coating are explained by Raman analysis.
More sp3 hybridized structure of DLC-B with lower
hydrogen content account for increase in hardness and
density. This gain does not generate significant modifica-
tions of water contact angle and residual stresses. Two
laboratory tribometers with and without lubrication are
used to evaluate the tribological behaviour of the two
coatings. In each test, a running-in period is clearly
Plasma Process. Polym. 2009, 6, S478–S482
� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
identified. Friction coefficient is higher and most of the
wear takes place during this period. Smoothening of the
contact surfaces associated with formation of a graphitic
transfer film leads to decrease in friction coefficient and
wear. Structure of both DLC films remains stable in depth
during the test. DLC-B clearly has the best wear resistance
for both lubricated and unlubricated conditions. On the
other hand, friction remains similar for both DLC coatings.
Hence, in lubricated conditions, friction coefficient seems to
be governed by lubricant characteristics. Later on, the best
wear resistance of DLC-B is validated by measurement on
test rig. Wear resistance improvement in both dry and
lubricated conditions has been successfully achieved
without an impact on the frictional behaviour. In the
future, engine components could be coated with both DLC
films to be tested in real engines. Next evolution to decrease
energy losses in engine motors should be the development
of lubricant formulation associated with high wear
resistant DLC coating.
Acknowledgements: Authors are thankful to B. Baccaud(SOFRANCE) for her help and permission to carry out watercontact angle measurements. ANRT and the Limousin Region aregratefully acknowledged for financial support.
Received: September 17, 2008; Accepted: March 5, 2009; DOI:10.1002/ppap.200931007
Keywords: coatings; diamond-like carbon (DLC); films; plasma-enhanced chemical vapour deposition (PE-CVD); tribology; wear
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DOI: 10.1002/ppap.200931007