2006 SCT 200 DLCandPolaritySaturationOfOILS Kalin

7/31/2019 2006 SCT 200 DLCandPolaritySaturationOfOILS Kalin

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The lubrication of DLC coatings with mineral and biodegradable oils

having different polar and saturation characteristics

M. Kalina,*, J. Viintina, K. Vercammenb, J. Barrigac, A. Arnxekd

aUniversity of Ljubljana, Center for Tribology and Technical Diagnotics, Ljubljana, SloveniabVito, Mol, Belgium

cTekniker, Eibar, SpaindPetrol, Ljubljana, Slovenia

Received 19 January 2005; accepted in revised form 15 March 2005

Available online 10 May 2005

Abstract

Due to improved performance over the last decade, diamond-like carbon (DLC) coatings are more frequently used in highly loaded

mechanical components that sometimes need to operate under boundary- or mixed-lubrication conditions. However, DLC coatings are

considered as inert coatings with a low surface energy and their lubrication ability according to conventional metal-lubrication mechanisms

is therefore questionable. In order to investigate whether the base oil polarity and saturation characteristics play a role in these processes, a

tribological investigation of the a-C:H coating lubricated with natural (sunflower) and synthetic (saturated and un-saturated) biodegradable

oils that posses different amount of polar components and double bonds was performed. For a comparison we also used mineral base oil with

low polar component. The DLC/DLC and steel/steel contacts were tested with base oils without additives and in combination with AW and

EP additives. Despite the higher wear compared to steel/steel contacts, the results suggest that the wear of DLC/DLC contacts can be

importantly improved by using oils with more polar groups and non-saturated molecules. These findings, together with well-known and even

proverbial low-friction properties, suggest a great potential for the use of DLC coatings in combination with biodegradable oils,particularly when additives are present.

D 2005 Elsevier B.V. All rights reserved.

Keywords: DLC coating; Oil; Biodegradable; Lubrication; Polarity; Saturation; Wear; Friction

1. Introduction

Various hard coatings, including DLC, are nowadays

used in many industrial areas. Over the past decade DLC

coatings have shown a great potential for a broader range of

uses due to their excellent properties under a variety ofconditions [16]. This improved performance suggests that

DLC coatings could be suitable for heavily loaded

mechanical components that operate under boundary- or

mixed-lubrication conditions.

Literature data show [7] that more than 10% of all the

lubricants used in Europe are exposed to natural surround-

ings, which increases the already high level of pollution in

these areas. This can occur through the leakage, main-

tenance, filtration of the systems or through deliberate

spillage. Biodegradable oils are one of the possibilities to

partially solve this problem in mechanical systems that are

used in environmentally sensitive places, e.g., machinery inforests, agriculture, mining, construction. The common

biodegradable oils are natural, i.e., rapeseed oil or sunflower

oil, and synthetic esters. Natural biodegradable oils possess

good anti-wear properties and low friction. However, their

oxidation and thermal stability is poor, and this is their

major drawback [8]. On the other hand, although synthetic

esters are more resistant to oxidation and thermal degrada-

tion, their tribological properties are not as good as those of

natural esters [9]. This is especially true for saturated

synthetic esters, while the properties of unsaturated syn-

0257-8972/$ - see front matterD 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2005.03.016

* Corresponding author. Tel.: +386 1 4771 462; fax: +386 1 4771 469.

E-mail address: [email protected] (M. Kalin).

Surface & Coatings Technology 200 (2006) 4515 4522

www.elsevier.com/locate/surfcoat


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thetic esters are in between the two extreme situations. It

could be said that the better the oxidation stability, the

poorer the tribological properties. The properties that affect

the oxidation stability most importantly are the number of

reactive groups and the level of saturation of fatty acids in

these oils, Fig. 1a. More double (or triple) bonds suggest

easier oxidation of the oil. On the other hand, oils with non-saturated molecules and with more polar groups (like

-COOH) posses also more sites for reactions and/or

adsorption with metal surfaces that can provide boundary

lubrication effects [10,11]. Namely, some of the conven-

tional (metal) boundary lubrication mechanisms are due to

the adsorption of oil/additive polar groups at the oxidized

metal surface, Fig. 1b. This property thus influences the

improved tribological behaviour of biodegradable oils as

explained earlier. Accordingly, it is expected that higher

number of polar groups improves the tribological perform-

ance of conventional metal tribo-systems. While the clean

mineral base oils contain no (or little) such polar groups, the

natural biodegradable oils contain them in very high

amounts. The relative ranking of the amount of polar

groups in base oils used in our study (see Experimental) and

expected properties in conventional tribological metal

systems are presented in Fig. 2.Most metal-lubrication mechanisms are based on the

physical or chemical adsorption of polar groups from the oil

and/or additives onto the oxidised metal surface or via a

chemical reaction between the additives and the reactive,

clean metal surfaces under high-temperature and high-stress

conditions [10,11]. So, here lies the major problem with the

lubrication of DLC coatings, i.e., DLC coatings are

considered as inert coatings with a low surface energy

and their lubrication ability is therefore questionable. Some

previous studies suggested that the tribological performance

of DLC cannot be improved by lubrication and additives.

Many efforts have been made in recent years to understandthe lubrication of DLC coatings as part of the European

union research funding scheme known as the XGrowth?

Programme [12]. The effects of various base oils, additives,

coating modifications and operational conditions have been

investigated using various testing geometries and scales

[1316].

In this work we have investigated the effects of polar

characteristics and saturation of molecules of different base

oils on the tribological behaviour of a-C:H coating under

boundary lubrication conditions. Three types of biodegrad-

able oils, i.e., natural (sunflower) and synthetic (saturated

and un-saturated) biodegradable oils that posses different

polar and saturation characteristics were used. For acomparison we also used a mineral base oil with low polar

component. All the oils were tested without additives and in

combination with AW and EP additives. To observe the

pure lubrication effect due only to the properties of the

selected DLC coating, the oils and the additives, and thus to

avoid the expected interactions of the oils and additives with

the steel in steel/DLC contacts [17], only self-mated DLC/

DLC contacts are presented and discussed. As a reference, a

conventional tribological system consisting of steel/steel

contacts was also used.

O

C

OH2C

CO

O

HC

CO

OH2C

Linoleic acid

Oleic acid

Stearic acid

C

OO

C

C

C

Me - O

H H

HH

H H

Polar group

Metal

Non-Polar tail

a)

b)

Fig. 1. (a) Typical triglyceride composition (vegetable oil) with three

different types of fatty acids having different level of saturation (number of

double bonds). (b) A schematic of boundary lubrication mechanism on

metal surface via adsorption of the polar group from oil or additive.

Fig. 2. Schematic presentation of the effect of polar groups and unsaturation of molecules on tribological properties in metal contacts and oxidation stability of

selected oils (see Experimental).

M. Kalin et al. / Surface & Coatings Technology 200 (2006) 4515 45224516


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2. Experimental

A single-layer pure amorphous a-C:H coating (without

any doping elements) prepared using the RF PACVD (13.56

MHz) process with a thickness of 1.78T0.09 Am was used

in all the tests. A Si-based interlayer was employed to

improve the adhesion properties of the coating. Theadhesion of the coatings was investigated with a scratch

tester (Revetest, CSM Instruments SA, Switzerland) and the

average Lc1, Lc2 and Lc3 values [18] were determined as

9.4 N, 11.5 N and 15.6 N. The hardness and the Youngs

modulus of the coating were measured using the depth-

sensing indentation (DSI) technique (NanoTest 600 instru-

ment with Berkovich indenter, Micro Materials Limited,

UK). The hardness of the coating was 21.9 Gpa, and the

Youngs modulus was 157 GPa. The surface roughness Raof the samples, i.e., the balls and discs, was measured after

the coating deposition using the stylus-tip profilometer

(T8000, Hommelwerke GmbH, Schwenningen, Germany)and the average value was 0.06 Am for the discs and 0.02

Am for the balls. The coatings were deposited on the

substrates from DIN 100Cr6 steel. All the steel balls and

discs had a hardness of 850 HV (Leitz Miniload, Wild Leitz

GmbH, D-6330 Wetzlar, Germany), and their roughness

was negligibly lower than that described above. The steel

balls were commercially available, standard ball bearings

with a diameter of 10 mm. The steel flat samples were f24

mm7.9 mm discs. Some of the flat and ball samples wereused as reference steel specimens in the tribological tests,

while the rest of the discs and balls were further coated

using the deposition technique described above. Table 1

summarizes some of the most important material properties.In order to avoid effects of different counter-bodies

(steel) or doping/alloying elements [13,17], only self-mated

contacts were investigated. In the case of coated samples

they are denoted as DLC/DLC. For comparison, steel/steel

contacts were also investigated.

Three different biodegradable base oils were used for this

investigation, i.e., saturated synthetic ester, unsaturated

synthetic ester and high oleic sunflower base oil, and as a

reference, a paraffinic mineral base oil. They all had the

same viscosity, corresponding to grade ISO VG 46. Two

different additives that are used for conventional steel

surfaces were selected. One of the additives was a multi-functional anti-wear/extreme-pressure (AW/EP) additive, a

mixture of amine phosphates, having about 4.8% and 2.7%

of P and N, respectively. The second additive used was a

strong extreme-pressure (EP) additive, dialkyl dithiophos-

phate ester, containing 9.3% of P and 19.8% of S. The

characteristics for the oils and the additives are presented inTable 2.

The wear experiments were performed in a reciprocating

sliding machine. The lower, flat samples were fixed in the

base, while the upper specimens, i.e., the balls, were fixed in

the oscillating holder. In all the experiments, 10 N of normal

load was applied through the loading system, which resulted

in an initial average Hertzian contact stress of about 700

MPa (1 GPa max). A stroke of 1 mm and an oscillating

frequency of 50 Hz were used, resulting in a relative contact

velocity of 0.1 m/s. In each test, the total sliding distance

was 100 m. To ensure high severity of the contact the

temperature was pre-set to 80 -C. Accordingly, the lambda

(k) value (the Tallian parameter [19]) in all the experiments

was less than 0.06, confirming the severe boundary-

lubrication conditions: k


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ern). In addition, a limited number of samples were analysed

with a FT-IR spectrometer equipped with a microscope and

an ATR (Germanium crystal) objective (Spectrum One,

Perkin Elmer Instruments). Also, the XPS (Mg Ka)

measurements were performed in an ultrahigh-vacuum

chamber with a base pressure of 11010 mbar, using a

Perkin-Elmer ESCA/Auger spectrometer with a double-passcylindrical mirror analyzer.

3. Results

As a reference for the contact conditions, steel steel was

used in combination with all the oils. The highest wear

among the biodegradable base oils was found for the

saturated synthetic ester, while the unsaturated ester and the

sunflower base oil performed almost the same, and only

negligibly better, see Fig. 3a. A substantial more than

50%decrease in wear was observed for all the biodegrad-able oils when the additives were used. Also, the relative

ranking between the different biodegradable oils was almost

the same as for the base oils used without additives. The use

of the AW/EP and EP additives resulted in about the same

performance; however, just a little bit lower wear was

always found with the AW/EP additive. The wear loss in the

case of the mineral base oil was slightly lower when

compared to the biodegradable base oils, while with the use

of additives, the wear was similar to that of the biodegrad-

able oils. However, the differences between the various

types of oil are small.

The results of the wear experiment for the self-mated

DLC coatings are presented in Fig. 3b. The results are

similar to those with the steel surfaces; however, thedifferences between the base oils are much more pro-

nounced. Again, the highest wear among the biodegradable

oils was found with the saturated ester, which was also

higher compared to the wear in the steel steel contacts. The

DLC contacts lubricated with the sunflower base oil

experienced about 50% and 40% lower wear levels than

the saturated and unsaturated synthetic esters, respectively.

This wear was even 15% lower than in the case of the steel

contacts. Additives reduced the wear of the DLC coatings

significantly when used with the synthetic esters (more than

30%), but their effect was reduced with sunflower oil, which

showed low wear even without the additives. The wear ofthe DLC contacts with biodegradable oils with additives

was always higher than in the case of the steel contacts. The

mineral oil, which was used as a reference, provided the

poorest wear protection for the coated surfaces when used

without additives and with the AW/EP additives (Fig. 3b).

However, when the EP additives were used, its performance

was comparable to the other results with the DLC-coated

surfaces.

Fig. 4a presents the coefficient of friction obtained in the

steel steel contacts. The highest coefficient of friction

among the biodegradable oils was observed for the saturated

synthetic ester, and the lowest was observed for the

vegetable sunflower oil. Additives did not decrease, buttypically even increased the coefficient of friction. In

general, mineral oil showed the highest values of the

coefficient of friction with or without the additives. It is

also clear that the use of AW/EP additives resulted in higher

values of friction compared to the EP additives, irrespective

of the type of oil used.

The results of the coefficient-of-friction measurements

for self-mated DLC contacts are presented in Fig. 4b. The

values are remarkably lower (20 40%) than the steel

contacts for all the conditions investigated. The highest

friction was measured for the saturated oil, and the lowest

for the sunflower base oil; however, the differences betweenthe unsaturated ester and the sunflower oil are almost

negligible. The use of additives slightly increased the

friction, but in contrast to the steel, the oils with the EP

additives, rather than the AW/EP, resulted in higher levels of

friction. Mineral base oil showed the lowest coefficient of

friction, while the use of the EP additive provided the

highest friction among all the experiments with coated

surfaces.

Fig. 5 shows the SEM images of the steel/steel contacts.

The surfaces that were tested with saturated ester (Fig. 5a)

and mineral oil (Fig. 5c) without additives show clear

evidence of adhesive wear, with a layer formed by smeared

a)

b)

0,0E+00

1,0E-05

2,0E-05

3,0E-05

4,0E-05

5,0E-05

6,0E-05

7,0E-05

8,0E-05

Saturated ester Unsaturated

ester

Sunflower oil Mineral oil

Wearloss(mm

3)

Base oil

Base oil+AW/EP

Base oil+EP

0,0E+00

1,0E-05

2,0E-05

3,0E-05

4,0E-05

5,0E-05

6,0E-05

7,0E-05

8,0E-05


ester


Wearloss(mm

3)

Base oil

Base oil+AW/EP

Base oil+EP

Fig. 3. Wear loss of the ball in the (a) steel/steel and (b) DLC/DLC contacts

as a function of the type of oil. Error bars representTone standard deviation

of the measurements.



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wear debris that subsequently delaminated and caused rather

high wear under these conditions, see Fig 3a. A substantial

amount of deformation, ploughing and delamination can be

seen, in particular with the saturated ester, Fig. 5a. On the

other hand, sunflower base oil (Fig. 5b) results in much less

surface damage, there is no evidence of adhesion and the

original sample preparation scratches can still be clearlyobserved. Despite this, the wear was comparable to the

saturated and mineral base oils, see Fig. 3a. When additives

were added to the oil, the adhesive wear was eliminated in

all contacts (Fig. 5df), in accordance with the much lower

wear loss in these contacts, see Fig. 3a. The surfaces tested

with biodegradable oils with additives are similar and many

original scratches are still visible. In accordance with the

EDS analyses, which revealed traces of phosphorous on the

surfaces (see arrows in Fig. 5df), small islands of such

additive-reacted surfaces can be observed. However, when

mineral oils with additives were used, the surfaces were

much smoother and covered with a pronounced softtribochemical layera result of the reactions of additives

with the steel surfaces and subsequent deformation of the

layer. Obviously, quite different wear mechanisms result

from the use of mineral or present biodegradable oils, even

when employing the same additives. This was also observed

for other coatings and material combinations [17].

Fig. 6 shows SEM images of the DLC worn surfaces

tested with different base oils and with the same oils

combined with the EP additive. The two surfaces tested with

biodegradable base oils again have a similar appearance, see

Fig. 6a and b. Despite clearly measurable wear, these two

surfaces present apparently no-wear conditions with the

Fig. 5. SEM images of the steel surfaces tested with: (a) saturated ester, (b) sunflower oil, and (c) mineral oil. Images (d), (e), and (f) are surfaces tested with

corresponding oils using the AW/EP additive. Arrows indicate locations of the EDS analyses.

a)

b)

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50


ester


Coefficient

offriction

Base oil

Base oil+AW/EP

Base oil+EP

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50


ester


Coefficientoffriction

Base oil

Base oil+AW/EP

Base oil+EP

Fig. 4. Coefficient of friction in the (a) steel/steel and (b) DLC/DLC

contacts as a function of the type of oil. Error bars representTone standard

deviation of the measurements.

M. Kalin et al. / Surface & Coatings Technology 200 (2006) 4515 4522 4519


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original scratches from the sample preparation still visible,

which suggests rather low wear, as seen in Fig. 3b. On the

other hand, mineral base oil resulted in much smoother

surfaces, indicating more intensive run-in and smoothening

of the surfaces, see Fig. 6c. The SEM images are therefore

in agreement with the wear data, showing noticeably higher

wear for the mineral base oil, see Fig. 3b. In the tests when

using the additives, careful inspection of the worn surfaces

showed a quite different appearance and morphology from

the original DLC surface, or from the worn surfaces whenusing only base oils. The worn surfaces with biodegradable

oils (Fig. 6d and e) appear rougher than with the base oils,

and are covered with a very thin and soft layer with many

scratches. Some of the scratches are from the sample

preparation, in agreement with the small amount of wear

(Fig. 3b), but some are due to the sliding action and show

signs of ploughing, indicating the soft nature of the surface.

On the other hand, a quite different appearance of the

surface can be found with the mineral oil with EP additive,

see Fig. 6f. The surface is covered with a clear tribochemical

layer that is very smooth and has an amorphous-like

appearance. This is a common observation with softtribochemical layers [20,21].

4. Discussion

According to conventional lubrication mechanisms with

steel surfaces based on physical and chemical adsorption,

where anti-friction additives and fatty acids with their polar

groups (Fig. 1) play a key role in interactions with the metal

surfaces [10,11], the best tribological performance is

expected for vegetable sunflower base oil (Fig. 2), which

consists of a considerable amount of fatty acids with

unsaturated bonds. Unsaturated synthetic esters have similar

tribological characteristics, but are less favourable due to

their modification for better oxidation stability. In contrast,

saturated synthetic esters (and mineral oils) are much less

prone to these interactions. Our wear data from the tests

with steel surfaces (Fig. 3a) indicate this trend; however, the

differences between the base oils are rather low. This might

suggest that the severity of the contact conditions was

already at the limits of effective lubrication protection when

using only base oils, resulting in a low differentiation of thewear results. In addition, the evidence for adhesion and the

formation of transfer films on some surfaces (Fig. 5a and c)

confirm the above consideration and the poor wear

protection under these conditions. Moreover, when using

oils with additives the wear was significantly lower and the

adhesion was eliminated (Fig. 5df). This was true for all

types of oil, which clearly indicates that additives were

predominantly responsible for the wear protection. The

same behaviour was also observed for reference mineral oil.

In all cases, mild AW/EP additives were slightly more

effective than strong EP additives, and reactions with

phosphorus were always detected by EDS, even with EPadditives.

The coefficient-of-friction results are more indicative and

support the better lubricity of the unsaturated synthetic ester,

and especially that of the sunflower base oil compared to the

saturated synthetic esters or (even more) compared to the

mineral oil, which resulted in a noticeably higher friction in

the steel/steel contacts, see Fig. 4a. On the other hand, for all

the oils used, the friction was higher when additives were

used, which explains the higher wear protection due to the

stronger surface interactions between the additives and the

steel with the higher shear strength of these bonds

[10,22,23]. Indeed, the higher coefficient of friction with

Fig. 6. SEM images of the DLC surfaces tested with base oils: (a) saturated ester, (b) sunflower oil, and (c) mineral oil. Images (d), (e), and (f) are surfaces

tested with corresponding oils using the EP additive.



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AW/EP additives coincides well with the lowest wear for all

the oils used, see Fig. 3a. Despite this, the quite different

appearance of the surfaces tested with biodegradable and

mineral oil (Fig. 5df) suggests that different lubricating

and wear mechanisms are acting in the contact, as was also

observed by [22].

In DLC/DLC contacts tested with sunflower base oil, thewear was clearly the lowest, i.e., about 30% compared to

the saturated and unsaturated synthetic esters without

additives and more than 50% compared to the mineral

base oil, see Fig. 3b. The difference between the base oils

that have very different chemical compositions was there-

fore much more pronounced with the DLC than the steel

surfaces. This suggests that the substantially large amount

of polar and unsaturated molecules that are readily

available in the oil and could interact with the surfaces

(like for high oleic sunflower oil) is even more important

for the tribological behaviour for the non-reactive

surfaces like DLC coatings than for reactive steelsurfaces. The present results suggest a quite simple linear

proportion for this relationship. Nevertheless, the wear of

the DLC surfaces was higher than the steel/steel contacts,

indicating that the shear strength of the adsorbed oil

molecules on the DLC surfaces was lower than on the

steel, resulting in poorer wear protection. As described

previously, the low coefficient of friction is in agreement

with this suggestion due to low interface shear strength, see

Fig. 4b. In agreement with the adsorption mechanism and

shear effect in DLC contacts is also the fact that mineral

base oil showed the lowest coefficient of friction (the least

polar base oil), while the use of the EP additive provided

much higher friction, i.e., the highest among base oils used,due to additive effect [13]. Since the adhesion or any other

catastrophic type of wear of DLC coatings is not

observed (Fig. 6a c), the friction was rather low, in

agreement with the well-known, and even proverbial,

low-friction properties of DLC coatings.

By using the additives, the wear of the DLC coatings was

reduced, and, like with steel, it was very similar for all three

types of biodegradable oils, see Fig. 3b. This again confirms

the critical importance of the additives for the wear

protection, and also with the DLC coatings [13]. A specific

layer has formed in all cases on the DLC surfaces in the

presence of EP additives; see Fig. 6df. Their appearancesuggests that it is most likely that the layers are softer than

the original DLC surfaces and, obviously, they have formed

in the contact as a result of an influence of the additives

used. However, by employing the micro-FTIR and XPS

analyses, no evidence that would indicate a clear chemical

change at the surfaces can be revealed. Thus, based on these

analyses it is presently not possible to explain the

mechanisms of the observed changes and tribological

performance. Primarily, it is not clear whether the chemical

changes as a result of interaction between the additives and

DLC coatings are occurring at the interface, but are smaller

of detection limit of the techniques used, or, for example,

the cleaning process before the analyses was not adequate

and affected the results. Other physical-based phenomena,

like the change in local viscosity and/or the local hot-spot

temperatures [24] that could cause structural changes (i.e.,

graphitisation), should also be explored more in detail.

Because of the clear, empirically determined effect of the

additives, further studies that would reveal the mechanismsare in progress; however, because of the complexity and

large extent they are not included within the scope of this

work.

As with the steel/steel contacts, also with DLC/DLC

contacts different mechanisms can be suggested for the

biodegradable oils and the mineral oil [17]. Namely, in the

case of the biodegradable oils, the surfaces appear less

changed and covered only with a soft and very thin layer,

which is just slightly rougher than the original surface (Fig.

6d and e), while in the case of the mineral oil, a well-

developed amorphous-like layer, which is very different

from the original surfaces is formed, see Fig. 6f. The highwear, in particular with the mineral oil, can thus be

explained by the formation of tribo-layers, which are

typically soft and wear-protective [20,21,25], but in this

case resulted in a higher removal rate than those on steel.

Nevertheless, this wear was still low, in the range of 108

mm3/Nm, corresponding to mild wear.

A general observation in this work was that the wear of

the steel/steel contacts was lower than the wear of the DLC/

DLC contacts (Fig. 3a and b), which indicates that steel

surfaces are indeed better suited for lubrication with the oils

and additives that were used in this investigation than are

the DLC coatings. However, with some other coatings the

results could be more favourable for DLC coatings than inthis study [13]. Moreover, a 3050% reduction of wear due

to the use of additives was found here for the pure DLC/

DLC contacts for all types of oils (Fig. 3b), which is in

agreement with our other findings, showing that the

performance of self-mated coated contacts became compa-

rable to that of steel/steel, if additives are used [13,17]. On

the other hand, the coefficient of friction for all types of base

oil, with and without additives, was up to 30% lower with

the DLC contacts than with the steel surfaces, Fig. 4. The

use of additives slightly increased the coefficient of friction;

however, bearing in mind the substantial reduction in wear,

this seems an acceptable compromise for the DLCcoatings.

The results confirm good tribological performance of

the biodegradable oils in combination with the DLC

surfaces, especially in terms of the low coefficient of

friction. This effect is proportional to the amount of polar

groups and double bonds in their fatty acids, related to

increased adsorption and shear strength at the interface

[10,22,23]. On the contrary, the non-polar mineral oil

performed the worse and caused different types of surface

damage compared to ester-type biodegradable base oils. At

the same time, the wear was in the range of 108 mm3/

Nm, which is reasonably low wear, still corresponding to

M. Kalin et al. / Surface & Coatings Technology 200 (2006) 4515 4522 4521


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mild wear. This suggests a good potential for a selected

combination of oils and DLC coatings to perform well in

conditions of industrial applications under rather severe

boundary-lubrication conditions. Further analyses and field

tests with different industrial systems are in progress; the

first results [26] are very promising and support the above

conclusions.

5. Conclusions

The wear of the steel/steel contacts was lower than that of

the DLC/DLC contacts, which indicates that steel

surfaces are indeed better suited for lubrication with

conventional oils and additives than are the DLC

coatings.

On the contrary, the coefficient of friction for all types of

base oil, with and without additives, was up to 30%

lower with the DLC contacts than with the steel surfaces,which is related to their lower adsorption compared to

that on steel surfaces and to low friction properties of

DLC coatings.

Large amounts of polar groups and unsaturated fatty

acids, which are readily available in the lubricant, like in

high oleic sunflower oil, substantially improve the

efficiency of the base-oil lubrication of inert DLC

surfaces. The results suggest a simple linear propor-

tion for this relationship. The non-polar mineral base oil

showed the worse tribological performance and caused

different DLC coating damage compared to ester-type

biodegradable base oils.

The 30 50% reduction in wear of the DLC/DLCcontacts due to the use of additives with all biodegrad-

able oils suggests the predominant effect of additives on

wear behaviourin agreement with our previous find-

ings based on mineral oil. However, different wear

mechanisms were observed for the biodegradable and the

mineral oils. However, the actual function of additives in

these contacts remains unclear.

The results suggest a good potential for the use of DLC

coatings in combination with biodegradable oils under

boundary-lubrication conditions, in particular when using

oils with additives (AW- and EP-formulated).

Acknowledgements

This work was in part funded by the European

Commission, within the Growth programme of the 5th EU

Framework Project LUBRICOAT (G5RD-CT-2000-00410).

The authors wish to thank all co-workers within this project

for help with various aspects of this work, especially B.

Stenbom and P. Nilsson (Volvo Technology AB, Sweden),

E. Roman (CSIC, Spain), D. Neerinck (Bekaert Dymonics,

Belgium) and F. Kopae (Center for Tribology and Technical

Diagnostics, Slovenia).

References

[1] A. Erdemir, G.R. Fenske, J. Terry, P. Wilbur, Surf. Coat. Technol.

94-94 (1997) 525.

[2] A. Grill, Diamond Relat. Mater. 8 (1999) 428.

[3] A.A. Voevodin, M.S. Donley, J.S. Zabinski, Surf. Coat. Technol. 92

(1997) 42.

[4] Y. Liu, A. Erdemir, E.I. Meletis, Surf. Coat. Technol. 82 (1996) 48.

[5] K. Holmberg, A. Matthews, H. Ronkainen, A. Leyland, Surf. Coat.

Technol. 100101 (1998) 1.

[6] J. Fontaine, C. Donnet, A. Grill, T. LeMogne, Surf. Coat. Technol.

146147 (2001) 286.

[7] W.J. Bartz, Tribol. Int. 31 (1 3) (1998) 35.

[8] A. Igartua, J. Aranzabe, J. Barriga, B. Rodriguez, Cost 516

Tribology Symposium Proceedings, VTT, Espoo, ISBN: 951-38-

4573-7, 1998, p. 135.

[9] B. Kran, J. Viintin, in: J. Viintin, M. Kalin, K. Dohda, S. Jahanmir

(Eds.), Tribology of Mechanical Systems: A Guide to Present and

Future Technologies, ASME press, New York, 2004, p. 107.

[10] R.M. Mortier, S.T. Orszulik, Chemistry and Technology of Lubricants,

2nd edR, Blackie Academic and Professional, Glasgow, 1993.

[11] G.W. Stachowiak, A.W. Batchelor, Engineering Tribology, Tribology

series 24, Elsevier, Amsterdam, 1993.

[12] 5th FP growth project, Lubricoat, Environmentally friendly lubricants

and low friction coatings, A route towards sustainable products and

production processes, 2001 2004 (G5RD-CT-2000-00410).

[13] M. Kalin, J. Viintin, J. Barriga, K. Vercammen, K. Van Acker,

A. Arnxek, Tribol. Lett. 17 (4) (2004) 679.

[14] J. Barriga, M. Kalin, K. Van Acker, K. Vercammen, A. Ortega, M. San

Martn, in: Proceedings of the 11th Nordic Symposium on Tribology,

Tromso, Harstad, Hurtigruten, Norway, June 2004, NORDTRIB 2004,

p. 508.

[15] K. Vercammen, K. Van Acker, A. Vanhulsel, J. Barriga, A. Arnxek,

M. Kalin, J. Meneve, Tribol. Int. 37 (2004) 983.

[16] M. Kalin, J. Vizintin, Wear 259 (112 July) (2005).

[17] M. Kalin, J. Viintin, in: Proceedings of the 11th Nordic Symposium

on Tribology, Tromso, Harstad, Hurtigruten, Norway, June 2004,

NORDTRIB 2004, p. 549.

[18] European Standard EN1071-3:2000:E, Advanced technical ceramics

methods of test for ceramic coatingsPart 3, CEN Management

Centre, Stassartstraat 36, Brussels, Belgium, 2000.

[19] T.E. Tallian, ASLE Trans. 10 (1967) 418.

[20] J. Takadoum, Wear 170 (1993) 285.

[21] M.G. Gee, N.M. Jennett, Wear 193 (1996) 133.

[22] P. Waara, J. Hannu, T. Norrby, A. Byheden, Tribol. Int. 34 (2001) 547.

[23] W. Zhan, Y. Song, T. Ren, W. Liu, Wear 256 (2004) 268.

[24] M. Kalin, Mater. Sci. Eng., A 374 (2004) 390.

[25] M. Kalin, S. Jahanmir, G. Drazic, J. Am. Ceram. Soc. 88 (2)

(2005) 346.

[26] J. Pezdirnik, M. Kalin, J. Viintin, in: J. Viintin, J. Bedenk, M. Kalin

(Eds.), SLOTRIB 04: Proc. of the Conference on Fuels, Tribology

and Ecology, Radenci, Slovenia, November 11 12, Slovenian Society

for Tribology, Ljubljana, 2004, p. 213.


2006 SCT 200 DLCandPolaritySaturationOfOILS Kalin

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