Tribological Behaviour of Diamond-Like Carbon Films used in Automotive Application: A Comparison

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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

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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



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.



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.



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



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

Tribological Behaviour of Diamond-Like Carbon Films used in Automotive Application: A Comparison

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