ORIGINAL PAPER

Tribological Behavior of Carbide-derived Carbon Coating on SiCPolycrystal against SAE52100 Steel in Moderately Humid Air

Fei Gao Æ Jinjun Lu Æ Weimin Liu

Received: 6 March 2007 / Accepted: 4 June 2007 / Published online: 23 June 2007

� Springer Science+Business Media, LLC 2007

Abstract In this article, carbide-derived carbon coating

(CDC) on a substrate of silicon carbide was produced by

chlorination at 1000 �C. The influence of dechlorination on

the friction and wear of CDC coating sliding against

SAE52100 steel was investigated. It was found that

dechlorination is crucial and necessary for obtaining good

tribological performance of CDC coating against steel. The

CDC coatings exhibit excellent tribological performance in

air at loads lower than 30 N and provide good protection

from wear damage of steel as well. The tribological per-

formance of CDC coating against steel is superior to that of

commercially available graphite.

Keywords Carbide derived carbon � Friction and wear �Steel

Introduction

Tribology of carbon is always an attractive and challenging

field to tribologists. Tribology of carbon (graphite, dia-

mond, diamond-like carbon, and fullerene) nowadays

covers a wide range of topics: from additive for oil and

grease, solid lubricant for composites, and, in the past

decades to low- and super-low-friction carbon coatings

[1–5]. Recently, carbide-derived carbon (CDC) has

received attention due to its excellent tribological perfor-

mance [6–12]. According to the definition, CDC is a pure

carbon material produced by selective extraction of metal

atoms from a carbide crystal lattice by halogens, super-

critical water, oxygen at a low-partial pressure, other et-

chants, or in vacuum [6]. The advantages of CDC coating

over carbon coating by conventional vapor deposition

techniques are in many aspects: high growth rate, good

coating adherence, unlimited thickness, no delamination of

coatings, and no risk of high residual stress [7]. Meanwhile,

CDC coating by high-temperature chlorination of SiC

without hydrogen is believed to have an excellent tribo-

logical performance in both humid and dry air [7]. CDC

coating on top of carbides is found to be versatile as a solid

lubricant in various environments, from dry air to high

humid air [7, 12] and is expected to find many related

applications. It should be pointed out that the counterbody

used in previous studies was silicon nitride [7–12], which is

hard, chemically inert, and easily surface finished. Up to

now, no attempt has been made to investigate the friction

and wear of CDC coating sliding against steel which is

widely used and is subject to chemical attack. Since CDC

has a nanoporous structure and a surface of high-specific

surface area [6], it allows adsorption of different kinds of

gases, such as chlorine. However, there is no information

on what kind of influence the adsorbed gases could have,

especially when the counterbody is made of steel. There-

fore, a systematic investigation of the friction and wear

behavior (wear of the coating is included in this paper) of

CDC coating sliding against steel is necessary. In this

article, a CDC coating on a-SiC was prepared by chlori-

nation at 1000 �C in a flowing gas Ar-5%Cl2. The influence

of dechlorination of the coating on the friction and wear in

moderately humid air is undoubtedly a very important part

of this work. Commercially available graphite was used as

F. Gao � J. Lu (&) � W. Liu

State Key Laboratory of Solid Lubrication, Lanzhou Institute of

Chemical Physics, Chinese Academy of Sciences, Lanzhou

730000, P.R. China

e-mail: jjlu@lzb.ac.cn

F. Gao

Graduate School of the Chinese Academy of Sciences, Beijing

100039, P.R. China

123

Tribol Lett (2007) 27:339–345

DOI 10.1007/s11249-007-9240-y

the baseline for comparison of the tribological test results.

The friction and wear of the CDC coating against steel in

moderately humid air are investigated and discussed.

Experimental Details

Materials

The a-SiC block (98 wt.% purity) is commercially avail-

able from Shanghai Institute of Ceramics, Chinese Acad-

emy of Sciences. It is fabricated via pressureless sintering.

The density is 3.12 g/cm3. Before chlorination, the SiC

samples were sectioned into 20 · 10 · 2 mm cubes, fol-

lowed by grinding (diamond wheel) and polishing (dia-

mond paste) to a surface roughness Ra of 0.02 lm.

Silicon nitride balls and SAE52100 balls (HRC60-65)

used in this study are commercially available from

Shanghai Institute of Materials, China. All the balls have a

diameter of 3 mm. The surface roughness (Ra) of Si3N4

ball and SAE52100 ball are 0.02 and 0.08 lm, respec-

tively. As the baseline for tribological tests, commercial

graphite available from Shanghai Sxcarbon Technology

Co., Ltd was chosen. This material is considered as a 3H-

carbon material and can be used in many friction-reducing

applications. The 3H means high purity in graphite, highly

dense, and high strength. The density is 1.80 g/cm3 and

compressive strength is 72.6 MPa.

Preparation of CDC Coating

The apparatus used for preparation of CDC coating is de-

signed and manufactured according to previous literature

[6]. In order to perform the chlorination, the sectioned SiC

samples were exposed to flowing gas Ar + (4–5)% Cl2 for

20 h. Every step is strictly controlled to make sure the

reaction is correctly performed. For example, it is neces-

sary to pre-purge Ar gas through the reaction tube for at

least 30 min before the experiment. In this study, a heating

rate of 50 �C per min was used. Dechlorination was per-

formed to remove residual Cl2 in the coating [6]. In order

to study the effect of the adsorbed Cl2 on the friction and

wear of the CDC coating, some samples without dechlo-

rination were deliberately used for tribological tests. X-ray

photoelectron spectroscopy (XPS, PHI-5702, Physical

Electronic, USA) is used to evaluate the surface before and

after dechlorination since it is a surface sensitive tool. A

thin gold coating (less than 2 nm in thickness) is sputtered

on the CDC coating before XPS. The peak of Au4f is used

as the internal standard for calibration. Meanwhile, it is

necessary to ultrasonically clean the CDC coating in an

acetone bath to remove the loosely attached, powdery top

carbon layer, as described in the literature [7]. The average

surface roughness of the CDC coating is 2.26 lm, which

indicates a very rough surface after the chlorination. For

tribological tests, very careful polishing on paper (manual

polishing, A4 paper) was used to make the average surface

roughness of the CDC coating to be 0.85 lm. The pol-

ishing was done dry and without polishing compound.

Meanwhile, due to the ability to self-adjust to the count-

erbody of the CDC coating produced in pure chlorine [7],

CDC coatings with high-surface roughness were used.

Friction and Wear Tests

Friction and wear tests were performed on a UMT-2MT

tribo-meter (CETR, USA) with a ball-on-disk configuration

at 20–21 �C in air (relative humidity is 49–51%). The ball

which is made of either SAE52100 or Si3N4 with a diam-

eter of 3 mm, makes oscillating movement (5 mm in

amplitude) on the top of CDC coating or commercially

available graphite. The tribological tests were conducted

with varied normal loads (2–40 N) and frequencies

(2–20 Hz, corresponding from 0.02 to 0.2 m/s in sliding

speed). All the specimens were ultrasonically cleaned in an

acetone bath prior to the tribological tests. A MicroX-

AM 3D profiler was used to measure the worn volumes of

the disks after friction and wear testing. The diameters of

the wear scars on the test balls were measured on an optical

microscope and were converted to wear volumes. All wear

rates were calculated based on the worn volumes and the

sliding distance. The worn surfaces were investigated by

using scanning electron microscopy (SEM, JSM-5600LV

JEOL).

Results and Discussion

Effect of Dechlorination on the Tribological

Performance

Figure 1 shows the XPS spectra of the CDC coating before

and after dechlorination. It is clear that the adsorbed Cl2 in

the coating could be totally removed by means of dechlo-

rination. Another result of the dechlorination might be the

partial removal of the adsorbed oxygen species, which is

based on the fact that there was pronounced reduction of

O1s peak and O auger line after dechlorination. As a matter

of fact, the outermost surface of the CDC coating was loose

in structure and had a high-surface area, which enabled

adsorption of different gases. The adsorbed species, such as

O2, H2O, etc., may have some influence on the tribological

behavior of the CDC coating. However, more experiments

and very detailed discussion are necessary to make it clear.

340 Tribol Lett (2007) 27:339–345

123

Figure 2 shows the typical friction coefficient as a

function of sliding time of CDC coating against SAE52100

steel before and after dechlorination. It is not surprising to

see that dechlorination is very crucial for improved fric-

tional behavior of CDC coating sliding against steel. The

friction coefficient was higher than 0.25 and unstable

(fluctuated after 4,000 s) for samples with no dechlorina-

tion. After dechlorination, the friction coefficient was as

low as 0.15 and remains very stable even after 5 h. The

chemical attack of Cl2 on the steel caused severe corrosion,

and therefore, severe wear (the diameter of wear scar of

steel ball was 1.31 mm) as steel slides against CDC coating

without dechlorination.

Unlike 52100 steel, Si3N4 showed similar tribological

behavior sliding against CDC coatings with and without

dechlorination. The friction coefficients of the CDC coat-

ings without dechlorination treatment sliding against Si3N4

balls at 5 N, 10 N, and 30 N (at a fixed speed of 0.02 m/s)

were 0.14, 0.13 and 0.11, respectively. For the CDC

coatings sliding against Si3N4 balls under the same con-

ditions, friction coefficients were 0.13, 0.11, and 0.10,

respectively. In all cases, the wear of Si3N4 balls was too

low to measure. It is clear that dechlorination of CDC

coatings was not necessary for the CDC/Si3N4 tribo-cou-

ples. It is, due to the chemical inertness and resistance to

corrosion of Si3N4 balls. This result is in good agreement

with previous results [8].

Friction and Wear after Dechlorination

Effect of Load

Figure 3a shows friction coefficients of the commercially

available graphite and CDC against SAE52100 under dif-

ferent loads. The friction coefficients of the commercially

available graphite against SAE52100 were in the range

from 0.23 to 0.27 and seemed to be independent of normal

load. The friction coefficients of the CDC coating sliding

against SAE52100 were in the range from 0.12 to 0.14

under the given loads. It is clear that the typical friction

coefficients of SAE52100 against the CDC coating were

only half of the values of the same steel against commer-

cially available graphite.

Figure 3b shows the wear rates of the commercially

available graphite and the CDC coating against SAE52100

under different loads. The commercially available graphite

showed poor wear resistance under the given loads. In

addition, the wear rates of the commercially available

graphite increased very rapidly with increasing normal

loads. For example, the wear rate of the commercial

graphite at 5 N is on the order of 10–4 mm3m–1 while the

wear rate at 10 N was on the order of 10–3 mm3m–1. In

contrast, the CDC coating exhibited very good wear

resistance, especially at 5 N and 10 N. Although the wear

rates of the CDC coating increased considerably at loads of

30 N and 40 N, the values were still very low compared to

that of the commercially available graphite.

Figure 3c shows the wear rate of SAE52100 sliding

against commercially available graphite and the CDC

coating. It is important to measure the wear rate of the steel

in order to make a comparison on the lubricity of the

commercially available graphite and the CDC coating. As

seen in Fig. 3c, the CDC coating provided good protection

from wear damage of the steel ball, even at 30 N and 40 N.

In this sense, the tribological performance of the CDC

coating was superior to that of the commercially available

graphite.

Fig. 1 XPS spectra of the surface of the CDC coating before and

after dechlorination

Fig. 2 Typical friction coefficient as a function of sliding time of the

CDC coating sliding against SAE52100 steel before and after

dechlorination

Tribol Lett (2007) 27:339–345 341

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Effect of Speed

Figure 4 shows the influence of sliding speed on the fric-

tion and wear of the commercially available graphite and

the CDC coating sliding against SAE52100. In both cases,

the friction coefficients tend to decrease as speed increases

from 0.02 to 0.2 m/s (Fig. 4a). For the commercially

available graphite, friction coefficients at 0.02 m/s and

0.20 m/s are 0.24 and 0.13, respectively. For CDC coating,

the friction coefficients at speeds higher and equal to

0.10 m/s were lower than 0.10, which makes the CDC

coating very attractive for many applications.

As seen in Fig. 4b, the wear rates of the commercially

available graphite tended to decrease as the speed

Fig. 3 Tribological properties of the commercially available graphite and the CDC against steel under different loads at 0.02 m/s (a) friction

coefficient, (b) wear rates of the commercial graphite and the CDC coating, (c) wear rate of steel

Fig. 4 The influence of speed on the friction and wear of the commercially available graphite and the CDC coating sliding against steel a)

friction coefficient, b) wear rate

342 Tribol Lett (2007) 27:339–345

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increases, e.g. from 1.90 · 10–3 mm3m–1 at 0.02 m/s to

2.43 · 10–4 mm3m-1 at 0.20 m/s. The wear rates of the

CDC coating were substantially lower compared with those

of the commercially available graphite. The wear rates of

the CDC tended to decrease as the speed increases; how-

ever, the wear rate of the CDC at 0.20 m/s was consider-

ably higher than at other lower speeds, as shown in Fig. 4b.

It would be reasonable to make a tribological test at speeds

higher than 0.20 m/s to see what happens. However, 20 Hz

(0.20 m/s) was the highest frequency that could be em-

ployed for the tribometer.

For a tribological test with oscillating movement, higher

speed means a shorter time for exposure to ambient gases.

In other words, the time for the readsorption of gases on

worn surface is inversely proportional to the sliding speed.

The fact of low-friction coefficient at high speed might be

related to the adsorption and desorption of gases. The

higher-contact temperatures that occur at higher-sliding

speeds may have an influence on friction. Those higher

surface temperatures could affect the tribochemical

behavior (adsorption and desorption of gases as well as

oxidation) and the mechanical behavior of the contacting

materials.

Worn Surface

Figures 5a and 5b show the worn surfaces of the com-

mercially available graphite and the CDC coating at 30 N

and 0.02 m/s. The worn surface of the commercially

available graphite in Fig. 5a was typical and was charac-

terized by many crevices. Plastic deformation could also be

found in Fig. 5a. However, it should be noted that the

crevices, which were the consequence of the cracks initi-

ated from the sub-surface and plastic flow, gave birth to a

large number of wear particles, and thereby, yielded high-

wear rate. For the CDC coating, the worn surface at 30 N

and 0.02 m/s was free of cracks and was characterized by

plastic deformation, see Fig. 5b. It was smooth with only

several plastically deformed grooves along the sliding

direction (the grooves correspond to the abrasion of hard

particles or asperities on the surface of SAE52100 ball, see

Fig. 5b). The worn surfaces of the CDC coating at low

loads (e.g. 5 N and 10 N) were very smooth in the contact

areas (Fig. 5c).

Figure 6 shows the worn surfaces of steel balls after

sliding against CDC coating with and without dechlorina-

tion, as well as commercial graphite. The diameter of the

wear scar on steel ball against CDC coating before,

dechlorination was much bigger than that on steel ball

against CDC coating after dechlorination, Figs. 6a and 6b.

Meanwhile, it can be seen that the worn surface of steel

ball, in Fig. 6a was covered by, tribochemical products

(Fig. 6a). At 5 N and 0.02 m/s, the worn surface of steel

ball was smooth (Fig. 6b). However, several grooves par-

allel to the sliding direction can be found on the worn

surface of steel ball under high load (30 N in Fig. 6c). As

seen in Figs. 6b, 6c and 6d, wear particles on steel ball in

sliding against commercial graphite were much more than

on steel ball against CDC coating.

Fig. 5 SEM micrographs of the worn surfaces of a) the commercially

available graphite at 30 N and 0.02 m/s, b) the CDC coating at 30 N

and 0.02 m/s, and c) the CDC coating at 40 N, 30 N and 10 N (wear

tracks from left to right)

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Comparison

In this work, the tribological behavior of the CDC coating

sliding against SAE52100 in air at moderately relative

humidity has been investigated. Commercially available

graphite which is used as the baseline for the tribological

tests, exhibited considerable friction-reducing ability at low

loads and high speeds. However, the wear resistance of the

graphite could hardly meet the requirements of many tri-

bological applications. The cracks initiated from the sub-

surface and their propagation were responsible for the high

wear of the commercially available graphite. The CDC

coating, however, which is composed of nested fullerenic

structures [8], described as carbon onions [9–10], had low

friction coefficient. Plastic deformation, rather than crack

initiation and propagation, were extremely useful for

maintaining low wear of the coating (Figs. 5b and 5c). The

fact that the wear rates of CDC coating increased rapidly at

loads higher than 30 N might be attributed to the structure

of the coating. For a CDC coating with a thickness of

90 lm (after removal of the loosely attached top layer), a

black carbon layer, which was not as dense as the gray

carbon layer underneath, is less than 10 lm in thickness.

The cross-sectional image of CDC coating shows black

carbon layer and gray carbon layer by using a field emis-

sion scanning electron microscopy (FESEM), as shown in

Fig. 7. The load-bearing capacity of CDC coating might be

determined by the black carbon layer but is not the topic of

this article.

Conclusions

Carbide derived carbon coatings with 90 lm thickness

were synthesized on SiC by chlorination at 1000 �C.

Dechlorination was found to be crucial and necessary for

Fig. 6 SEM micrographs of the worn surfaces of SAE52100 sliding against a) the CDC coating before dechlorination at 5 N and 0.02 m/s, b)

the CDC coating after dechlorination at 5 N and 0.02 m/s, c) the CDC coating after dechlorination at 40 N and 0.02 m/s, and d) the

commercially available graphite at 10 N and 0.10 m/s

Fig. 7 FESEM micrographs of the cross-sectional view of the CDC

coating

344 Tribol Lett (2007) 27:339–345

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the tribological performance of CDC coating in sliding

against steel. CDC coatings exhibited excellent tribologi-

cal performance in air at loads lower than 30 N and

provided good protection from wear damage of steel as

well. The tribological performance of CDC coating

against steel is superior to that of commercially available

graphite.

Acknowledgment The present work is financially supported by,

National Natural Science Foundation of China (No. 50675216) and

Xibuzhiguang of Chinese Academy of Sciences 2003.

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