or0404—effect of a tribochemical reacted film on friction and wear properties of dlc coatings

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S96

Effect of a Tribochemical Reacted Film onFriction and Wear Properties of DLC Coatings

Kentaro Yoshida,* Takahiro Horiuchi, Makoto Kano, Masao Kumagai

Diamond-like carbon (DLC) coatings provide low friction properties without lubricants and,with lubricants, should provide super low friction. In this study, the friction and wearproperties of DLC coatings with lubrication in the sliding contact area were evaluated tofind an environmentally friendly material combination that can provide super low friction(i.e., a coefficient of friction lower than 0.01). The friction andwear properties of a steel pin on aDLC coated disk with lubrication were evaluated by using environmentally friendly fluids(organic acid or alcohol) as lubricants. In a sliding test of tetrahedral amorphous carbon (ta-C)lubricated with DL (a mixture of dextrorotatory and levorotatory molecules)-lactic acid, a superlow coefficient of friction (0.01) was obtained at the end of the test and was much lower thanthat of an uncoated disk and an amorphous hydrogenated carbon (a-C:H) disk. The coefficientof friction obtained with DL-lactic acid was lower than that seen for acetic acid or glycerol. Theta-C disk lubricated with DL-lactic acid showed a small wear scar width. The main reason forthe reduced friction and wear is probably attributable to the formation of a tribochemicalreacted film in a tribochemical reaction with the acid. An oxidation film (white colored layer)of FeO also formed on the mating pin under DL-lactic acid lubrication in the sliding contactarea. These results showed not only the difference in the adsorbed lubricants due to thechemical structure of DLC, but also that the oxidized condition of the steel pin influenced thecoefficient of friction.

Introduction

In recent years, surface treatmentswith hard coatings have

been used to reduce wear and the coefficient of friction.

Diamond-like carbon (DLC) coatings in particular have

attracted a lot of attention because of their high hardness

and lower friction properties compared with other hard

coatings such as titanium nitride (TiN). DLC has been

applied invariousfields, including slidingautomotiveparts

and cutting tools. It is also reported that a tribochemical

reacted film formed through the interaction between the

K. Yoshida, T. Horiuchi, M. Kano, M. KumagaiDepartment of Materials Engineering, Kanagawa IndustrialTechnology Center, 705-1 Shimo-imaizumi, Ebina, Kanagawa 243-0435, JapanFax: (þ81) 46 236 1525; E-mail: [email protected]

Plasma Process. Polym. 2009, 6, S96–S101

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

coating and additives provides a low coefficient of friction

when a DLC coating is used with lubrication.[1,2] It is

necessary to choose environmentally friendly lubricants

when the combination of a DLC coating and a lubricant

is selected, because the European chemical regulations,

such as the ‘‘Regulation on the Registration, Evaluation and

the Authorisation of Chemicals’’, are expected to become

increasinglymore stringent in the future. Environmentally

friendlymaterial in this paper is defined as one having low

toxicity and high biodegradability.[3–5] Such materials do

not contain any elements like nitrogen, phosphorus, sulfur

and chlorine. Research has been done on the tribological

behavior resulting fromacombination of aDLC coating and

a biodegradable lubricant,[6] but there are no reports in the

literature about the low friction provided by a combination

of aDLC coating andan environmentally friendly lubricant.

This study examined the effect of the large friction

DOI: 10.1002/ppap.200930405

Effect of a Tribochemical Reacted Film on Friction . . .

Figure 1. Pin-on-disk test rig.

Table 1. Test conditions.

Dimensions and material

of fixed pin (mm)

11.8 dia.� 9.5 long

Dimensions and material

of disk (mm)

33.0 dia.� 3.0 thick

Volume of lubricant (mL) 200

Sliding speed (mm � s�1) 50

Load (N) 5

Hertzian pressure (MPa) 360

Sliding time (s) 1 800

Room temperature (K) 296

Relative humidity (%) 62–75

reduction obtained by the combination of DLC and various

lubricants. The results of this study are expected to be

applied to lubricant design in various cases involving the

use of DLC.

The background behind this study includes the super

lubrication performance (i.e., a coefficient of friction lower

than0.01) reported for tetrahedral amorphous carbon (ta-C)

lubricated with glycerol at 353K.[7–9] It is well known that

the carboxyl group in higher fatty acids is adsorbed at the

frictional surface of the contact between two metals and

reduces the coefficient of friction.[10] With a DLC coating,

even a simple low molecular substance such as glycerol

seems tobeable to reduce the coefficientof friction, because

the hydroxyl group in glycerol is adsorbed on the DLC

surface.[9] This implies that the coefficient of friction can be

reduced evenmore by using lowmolecular lubricants with

an active carboxyl group rather than a hydroxyl group. For

that reason, DL (a mixture of dextrorotatory and levorota-

tory molecules)-lactic acid and acetic acid having the

carboxyl group were chosen as environmentally friendly

lubricants in this study. Glycerol was evaluated as a

lubricant.

The objectives of this study were to find a DLC coating,

lubricated with an environmentally friendly fluid, capable

of reducing friction markedly and to investigate the

condition of the sliding surface between the DLC coating

and steel. Although there have been several studies about

super low friction,[7–13] most of them were conducted in

special environments, such as in an inert gas,[11] or in a

vacuum. Finding a material combination that achieves

super low friction at room temperature in air can be a

shortcut to a successful practical application.

Friction tests of DLC coatings combined with various

lubricants were conducted at room temperature in air. It

was reported in previous research that the hydroxyl group

in the lubricant was adsorbed on the DLC coating surface

and a tribochemical reacted film (tribofilm) was formed in

the interaction with the DLC coating.[7–9] In this study, the

friction andwear properties of twoDLC coatings, a-C:H and

ta-C, were evaluated and the relationship with friction and

wear was made clear in detail.

Experimental Part

Experimental Procedure

A sliding testwas conducted by pressing a pin on a rotating disk, as

shown in Figure 1. The pin anddiskweremade of bearing steel AISI

52100. The test conditions are shown in Table 1. A fixed crown-

shapedpinwitha radiusof curvatureof 75.3mmwasused toapply

a load of 5N. The hertzian pressure was 360MPa. The shape of the

contact between the pin and the disk was a uniform oval. The

lubricant was supplied by wetting the disk before the test.



DLC Coatings

Disks were separately coated with two kinds of DLC: amorphous

hydrogenated carbon (a-C:H), which was deposited by radio

frequency plasma enhanced chemical vapor deposition (RF-

PECVD),[14] and ta-C, whichwas depositedwith a T-shaped filtered

arc deposition system (T-FAD) that positively collected the

droplets.[15] Figure 2 shows the configuration of the T-FAD system.

Table 2 gives the deposition conditions and properties of the DLC

coatings.

Lubricants

DL-Lactic acid and acetic acid were used as lubricants and were

wetted on the disk before the sliding test. Glycerol was used for

comparison with the acids. The reason why the strong acids with

high purity and concentration were used was to evaluate the

influence of the functional group. Table 3 shows the properties of

the test lubricants.

Surface Analysis

Thewear scarwidthwasmeasuredwithanopticalmicroscope, and

the wear scars were observed in detail using scanning electron

www.plasma-polymers.org S97

K. Yoshida, T. Horiuchi, M. Kano, M. Kumagai

Figure 2. T-shape filtered deposition (T-FAD) system.

Table 2. Deposition conditions and properties of DLCcoatings.[14,1,5]

a-C:H ta-C

Coating method Plasma CVD PVD (T-FAD)

Coating time (min) 180 64

Atmosphere temperature (K) — 333

Substrate temperature (K) 473 <423

Material source (CH3)4Si,C2H2 Graphite

Other gas Ar —

Thickness (mm) 1.1 0.3

Hardness (GPa) 20 61

Roughness (Ra) (nm) 2.3 2.4

Table 3. Properties of the test lubricants.

DL-lactic acid Acetic acid Glycerol

Rational

formula

C2H4(OH)(COOH) CH3(COOH) C3H5(OH)3

pH <1.0 <1.0 7.0

Purity (%) 90.5 >99 >99

Viscosity

(mPa � s)57 1 1 003

S98

microscopy (SEM). The hardness of the DLC coated layer was

measuredwith a nanoindentor (themaximum indentation load is

2mN). The conditions of the iron and oxygen were also analyzed

after the test byX-rayphotoelectron spectroscopy (XPS) tomeasure

oxidation on the pin.

Results and Discussion

Reduction of Coefficient of Friction by DL-Lactic Acid

Figure 3(a) shows the coefficient of friction when DL-lactic

acidwas used as the lubricant. The change in the coefficient

of friction of the uncoated disk over timewas scattered and

wavy, and the coefficient of friction at the end of the test

was 0.07. In contrast, the fluctuation of the coefficient of

friction of the a-C:H and ta-C coated disks over time was



smooth and small throughout the sliding test. In addition,

the coefficient of frictionof the ta-C coateddisk at the endof

the sliding test was 0.01, which is much lower than that

of the a-C:H coated disk. The functional group from DL-lactic

acid might have been adsorbed more easily on the ta-C

coating, because there were more dangling bonds for the

ta-C coated disks than for the a-C:H coated ones.[7]

Figure 3(b) shows thewear scarwidth of the pin and disk

after the test as measured from observation images

obtained with the optical microscope. The wear scar width

of the pins was generally larger than that of the disks

because the same part of the pin was in contact with the

rotating disk. Although the wear scar width of both the a-

C:H and ta-C coated disks was very short that on the pins

testedwith the ta-C coated diskwas noticeably longer. This

was probably due to the hardness of ta-C (61GPa), which

was much higher than that of a-C:H (20GPa).

Figure 3(c) shows SEM images of wear scars on the pin

and disk after the test. The right side of the pin image is the

contact area and the left side is the non-contact area. A

white colored layer due to oxidation of the steel was

generated in the non-contact area, but was not observed in

the contact area. The formation and destruction of the

oxidationfilmwaspresumably repeated in the contact area

due to severe pressure. In contrast, wear scars were not

observed on either the ta-C or a-C:H coated disks, whereas a

wear scar was clearly observed on the uncoated disk.

Influence on Friction and Wear of the ta-C CoatingWith Several Lubricants

The friction properties of the ta-C coating lubricated with

aceticacidandglycerolwereevaluated for comparisonwith

the results for DL-lactic acid. The coefficients of friction

obtained with acetic acid and glycerol were higher than

that seen for DL-lactic acid. Figure 4(a) shows the coefficient

of friction for the acetic acid, glycerol, and DL-lactic acid

lubricants.

When acetic acid was used, a brown colored layer was

observedandweardebrisweredetectedon thematingsteel

pin. A wear scar was also observed on the disk. The results

showed that the average of the coefficient of friction was

high. When glycerol was used, no colored layer was

observed on the pin. Under these test conditions, DL-lactic

acid effectively reduced friction though the molecular

structure. The pH of acetic acid was similar to that of DL-

lactic acid. A marked effect on reducing the coefficient of

friction was not observed for glycerol at 296K in

comparison with glycerol at 353K.[7–9]

After the test, the wear scar widths of the disk and pin

weremeasured using an opticalmicroscope and the results

are shown in Figure 4(b). Wear scars were clearly observed

on the disk lubricated with acetic acid. These wear scars

DOI: 10.1002/ppap.200930405


Figure 3. (a) Coefficient of friction during sliding with DL-lactic acid lubricant. (b) Wear scar width of pin and disk with DL-lactic acid lubricant.(c) SEM images of wear scar of pin and disk with DL-lactic acid lubricant.

were probably formed in a tribochemical reaction with the

acid. Thewear scars on the pinswere largerwith both acids

thanwithglycerol. Theamountofwearontheta-Cdiskmay

have been related to the oxidation condition of the mating

pin.

Figure 4(c) shows SEM images of wear scars on the pin

and disk after the test. Oxidation of the pin increased in the

order of glycerol, DL-lactic acid, and acetic acid. In the case of

acetic acid in particular, fine cracks caused by oxidation

damage were observed on the pin. As for the disks,

delamination of the ta-C coating on the disk was observed

with acetic acid and the damage was severe. It is

unknown whether wear or corrosion of the pin influenced

delamination of the ta-C disk, but the degree of oxidation

of the steel pin seemed to influence the friction of the ta-C

disk.



The condition of the iron and oxygen was analyzed by

XPS in order to understand the difference in oxidation on

the surface of the steel pins for which DL-lactic acid and

acetic acid were used. Figure 5 shows the spectra of the

iron–oxygen bond as measured by XPS. The DL-lactic acid

spectrum shows a lower level of Fe2O3 than the acetic acid

spectrum, and FeOwas only detected in the spectrum from

DL-lactic acid. Therefore, the white colored layer was

presumably iron oxide, consisting of FeO and Fe2O3, and

was rigid and tight. The difference in oxidation on the

mating steel pin influenced friction.

It is inferred that three factorswere involved in reducing

the coefficient of friction by the combination of the ta-C

coating and DL-lactic acid.

� A
tight, rigid oxidation film was formed on the mating
steel pin. An oxidation film of FeO seemed to work to


K. Yoshida, T. Horiuchi, M. Kano, M. Kumagai

Figure 4. (a) Coefficient of friction during sliding test with ta-C DLC coating. (b) Wear scar width of disk and pin with ta-C DLC coating.(c) SEM images of wear scar of disk and pin with ta-C DLC coating.

Figure 5. Spectra of iron–oxygen bonds as measured by XPS withorganic acid lubricants.

S100Pl

�

reduce friction. This implies that moderate oxidation of

the mating steel was an important factor.

� T
he carboxyl and hydroxyl groups in DL-lactic acid
seemed to bond strongly with the dangling bonds on the

surface of the ta-C coating.[8,9]

� D
L-lactic acid seemed to be conducive to the formation of
a very low-shear tribochemical reacted film through

asma Process. Polym. 2009, 6, S96–S101

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

interaction with the oxidation film on the steel pin and

the functional group in DL-lactic acid. The resultant

tribofilm reduced friction.[8,9]

Conclusion

In this study, the friction and wear properties of two DLC

coatings, a-C:H and ta-C, were evaluated when lubricated

with DL-lactic acid, acetic acid, and glycerol. The results

obtained are summarized below:

(1) I
n sliding tests with disks lubricated with DL-lactic acid
at room temperature, the coefficient of friction of the

ta-C coated disk was 0.01 at the end of the test, which

wasmuch lower than that of the uncoated disk and the

a-C:H coated disk.

(2) T
he reduction of the coefficient of friction was
presumably due to the strong connections formed by

both the hydroxyl group and the carboxyl group in DL-

lactic acid with the dangling bonds on the surface of

the ta-C coating.

(3) T
here was no clearly observable wear scar on the ta-C
coated disk with the DL-lactic acid lubricant, but there

were clear wear scars on the disk with the acetic

DOI: 10.1002/ppap.200930405


Plas

� 20

acid lubricant. These wear scars were probably formed

by an accelerated tribochemical reaction with the acid.

(4) A
difference in oxidation of the mating steel pin was
observed between the DL-lactic acid and acetic acid

lubricants. An oxidation film of FeO formed on the pin

in the case of DL-lactic acid. This showed that the

condition of the steel pin also influenced the coefficient

of friction. It is inferred that the oxidation film of FeO

worked to reduce friction.

(5) F
or effectively reducing friction, a suitable balance of
conditions was important, including the properties of

the DLC coating, the functional group in the lubricants,

the oxidation of the mating steel pin, and the test

conditions for load, speed and contact shape. The

optimum combination of conditions offers the possi-

bility of reducing the coefficient of friction further.

Acknowledgements: This study was supported by the Environ-ment-friendly Functional Surface Project of the Ministry ofEducation, Culture, Sports, Science, and Technology of Japan andby Kanagawa Prefecture.

Received: September 5, 2008; Accepted: February 19, 2009; DOI:10.1002/ppap.200930405

Keywords: diamond-like carbon (DLC); organic substances;oxidation; tribology; wear

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or0404—effect of a tribochemical reacted film on friction and wear properties of dlc coatings

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