Tsinghua Science and Technology, 10(6): pp. 699-703. Retrieved March 13, 2006 from http://nano.materials.drexel.edu/Papers/CDC_Tibology_TSINGHUA.pdf. DOI: http://dx.doi.org/10.1016/S1007-0214(05)70138-3Carbide-derived carbon (CDC) is a form of carbon produced by reacting metal carbides, such as SiC or TiC, with halogens at temperatures high enough to produce fast kinetics, but too low to permit the re-arrangement of the carbon atoms into an equilibrium graphitic structure. The structure of CDC is derivative of the original carbide structure and contains nanoscale porosity and both sp2 and sp3 bonded carbon in a variety of nanoscale structures. CDC can be produced as a thin film on hard carbides to improve their tri-bological performance. CDC coatings are distinguished by their low friction coefficients and high wear resis-tance in many important industrial environments and by their resistance to spallation and delamination. The tribology of CDC coatings on SiC surfaces is described in detail

Erdemir, Ali

Ersoy, Daniel

Gogotsi, Yury G.

Kovalchenko, Andriy

Lee, Allen

McNallan, Michael

Welz, Sascha

White, Christopher

Zhu, Ranyi

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TSINGHUA SCIENCE AND TECHNOLOGY 
ISSN 1007-0214 05/11 pp699-703    
Volume 10, Number 6, December 2005 
Nano-Structured Carbide-Derived Carbon Films 
and Their Tribology 
Michael McNallan1,**, Daniel Ersoy1, Ranyi Zhu1, Allen Lee1, Christopher White1,  
Sascha Welz1, Yury Gogotsi2, Ali Erdemir3, Andriy Kovalchenko3 
 
1. CME Department M/C 246, University of Illinois at Chicago, 842 W. Taylor St., Chicago, IL 60607, USA; 
2. Department of Materials Science and Engineering and A. J. Drexel Nanotechnology Institute,  
Drexel University, Philadelphia, PA 19104, USA; 
3. Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA 
 
Abstract: Carbide-derived carbon (CDC) is a form of carbon produced by reacting metal carbides, such as 
SiC or TiC, with halogens at temperatures high enough to produce fast kinetics, but too low to permit the re-
arrangement of the carbon atoms into an equilibrium graphitic structure. The structure of CDC is derivative 
of the original carbide structure and contains nanoscale porosity and both sp2 and sp3 bonded carbon in a 
variety of nanoscale structures. CDC can be produced as a thin film on hard carbides to improve their tri-
bological performance. CDC coatings are distinguished by their low friction coefficients and high wear resis-
tance in many important industrial environments and by their resistance to spallation and delamination. The 
tribology of CDC coatings on SiC surfaces is described in detail. 
Key words: carbon; coatings; tribology; friction; chlorination 
 
Introduction 
Carbide-derived carbon (CDC) refers to a unique 
structure of carbon produced when metal carbides, 
such as SiC, are exposed to halogens at moderately 
elevated temperatures. The halogens react with the 
metal component of the carbide to form volatile 
species, such as SiCl4, but because CCl4 is not 
thermodynamically stable at high temperatures, the 
carbon is left behind[1-3]. When the process is 
performed at temperatures that are low in comparison 
to the graphitization temperature of carbon, the 
structure of the CDC will be a metastable combination 
of nanoporous amorphous carbon, nanocrystalline 
diamond, and nested fullerene structures[4-7]. 
 Received: 2005-03-16 
﹡﹡ To whom correspondence should be addressed.  
E-mail: McNallan@uic.edu 
 
CDC materials have many potential  applications  
deriving from their unique physical and chemical 
properties. The applications that may be most easily 
exploited are in tribology, where CDC functions as a 
solid lubricant film. CDC layers in the thickness range 
from 1 to 100 µm can be grown quickly on hard car-
bides, such as SiC or WC, by exposure to flowing 
chlorine gas at temperatures in the range of 800℃ to 
1000℃ . These carbides are widely used in wear-
sensitive applications, such as bearings or pump seal 
rings, because of their high hardness and wear resis-
tance. CDC coatings can be applied to the surfaces of 
SiC components with minimal dimensional change, 
and produce a substantial reduction in the friction coef-
ficient during sliding[8-10]. The CDC coating is much 
more wear resistant than graphitic carbon. The me-
chanical hardness of CDC is affected by structural 
variations, and changes gradually from a high value 
near  the  CDC/SiC  interface to a lower value at the 
external surface. This graded hardness matches the 
  Tsinghua Science and Technology, December 2005, 10(6): 699-703 
 
700 
hardness of the underlying SiC and reduces the suscep-
tibility of CDC to spallation and delamination in com-
parison to vapor-deposited  diamond  or  diamond-like  
carbon. 
The tribological properties of CDC can be related to 
the properties of the carbide substrate and the process-
ing conditions under which the CDC conversion layer 
was produced. CDC layers have been formed on com-
mercial SiC materials widely used in applications such 
as pump seal rings by exposure to flowing Ar-3.5% 
(vol) Cl2 gas mixtures in fused silica furnace tubes at 
temperatures on the order of 1000 . The resulting ℃
CDC coatings have been characterized by optical and 
electron microscopy, Raman and X-ray emission spec-
troscopy, mechanical testing by nano-indentation, and 
in a pin-on-disk tribology tester. The results provide 
some insights into the performance of these coatings 
and the mechanism by which they reduce tribological 
losses. 
1 Experimental Details 
The apparatus in which the CDC layers were grown 
has been described elsewhere[1,2]. It consists of a fused 
silica reaction tube in a resistance heated electric fur-
nace, fitted to a gas flow and purification system set up 
for handling mixtures of argon, chlorine, and hydrogen. 
In a typical experiment, a SiC specimen is placed 
within the furnace, and the furnace is purged with the 
chlorine-containing gas mixtures before heating to the 
intended reaction temperature. The furnace is held at 
the test temperature for a fixed period of time (often on 
the order of one hour), and then the furnace is cooled 
to near room temperature in the flowing gas mixture so 
that the test specimen can be removed for characteriza-
tion. The gas flow system is designed such that the 
chlorine flows only through glass or polytetrafluoro-
ethylene (PTFE) tubing between leaving the supply 
cylinder and being exhausted to a fume hood to avoid 
corrosion losses of the gas. Halide deposits must be 
removed periodically from the reaction tube to avoid 
flow interruptions due to plugging.  
CDC coatings on SiC and other carbides have been 
produced in varying thickness and have been charac-
terized by optical and electron microscopy and by X-
ray and Raman spectroscopy. The tribological perform-
ance of the layers has been evaluated in pin-on-disk tri-
bology tests as well as more specialized seal tests. 
Figure 1 is an optical micrograph of a CDC coating 
produced on sintered alpha SiC after one hour in flow-
ing Ar-3.5% Cl2 at 1000 . The thickness of the layer ℃
ranges from 10 to 20 µm. The convolutions in the 
SiC/CDC interface result from local variations in the 
rate of conversion of SiC to CDC due to the differ-
ences in orientation of the SiC grains. These convolu-
tions interlock the CDC coating with the SiC substrate 
and prevent delamination during tribological testing. 
 
Fig. 1 Cross-sectional optical micrograph of CDC 
produced on sintered alpha SiC after one-hour treat-
ment in flowing Ar-3.5% Cl2 at 1000 . The upper ℃
dark-grey layer is epoxy, CDC coating is in the middle, 
and the bottom part of the micrograph shows SiC. 
The interface between the SiC and the CDC is fur-
ther strengthened by a gradient in the mechanical 
properties of the CDC as it approaches the SiC/CDC 
interface.   Figure  2  shows  the  results   of  nano- 
 
Fig. 2 Cross-sectional micrograph and results of 
nano-indentation tests on CDC near SiC/CDC inter-
face on CDC produced in flowing Ar-3.5% Cl2 at 
1000℃. 
 
Michael McNallan et al：Nano-Structured Carbide-Derived Carbon ……  701
indentation tests performed as a function of position 
every 3 µm across the CDC/SiC interface. In this ex-
periment, a thick CDC layer was grown by exposing 
sintered alpha SiC to flowing Ar-3.5% Cl2 at 1000  ℃
for 68 h. Throughout most of the CDC layer, the mate-
rial is relatively soft, with a hardness of 1 to 2 GPa. 
However, within a few micrometers of the SiC/CDC 
interface, the hardness increases to the range of 20-30 
GPa. The hardness of the CDC near the interface 
matches well with that of the SiC, so that there is no 
sharp change in mechanical properties at the interface 
that could concentrate stresses there, leading to de-
lamination and spallation of the layer. No spallation 
failures of CDC have been observed in any of the tri-
bological tests performed on CDC-coated SiC. 
2 Tribological Performance 
Figure 3 shows the friction coefficient during a pin-on-
disk tribology test performed on sintered alpha SiC and 
CDC produced by treatment in flowing Ar-2.77% Cl2 -
1% H2 at 1000 . In these ℃ tests, the pin was a 0.9525-
cm-diameter silicon nitride ball sliding against a flat 
surface of SiC at a velocity of 1.25 cm/s with a load of 
5 N. Untreated SiC shows a friction coefficient on the 
order of 0.6-0.7, typical of values for this ceramic. After 
treatment to produce a CDC layer, the friction 
coefficient drops to the range of 0.1 to 0.2, more typical 
of friction coefficients for graphitic lubricants. This 
lubrication effect was maintained throughout a long-
term test continuing for over 6 h without showing any 
sign of lubrication failure or significant wear of the 
coating. 
 
Fig. 3 Friction coefficient versus sliding distance 
from pin-on-disk tribology test performed on un-
treated SiC and CDC-coated SiC produced in Ar-2% 
Cl2-0.75% H2 at 1000℃.  
Testing performed under conditions similar to that in 
Fig. 3 showed that the performance of CDC was insen-
sitive to changes in the test conditions, including varia-
tions in applied load from 1 to 16 N, sliding velocity 
from 50 to 550 r/min, and changes in humidity level 
from dry nitrogen with 0% relative humidity to sur-
faces wet with liquid water. Friction coefficients were 
consistently in the range of 0.1 to 0.3 for all of these 
tests. The results of pin-on-disk tests performed in dry 
nitrogen are most illustrative of the advantages of CDC 
over graphitic carbon solid lubricants. Figure 4 shows 
the results of pin-on-disk tests performed in dry nitro-
gen on CDC and other forms of low friction carbon.  
 
Fig. 4 Friction coefficient versus time for pin-on-disk 
tribology tests performed in dry nitrogen on graphite, 
glassy carbon, and CDC grown on SiC (Relative hu-
midity is 0%). 
Graphite and other sp2 bonded forms of carbon have 
low friction coefficients in sliding wear because of the 
weak chemical bonding between the 2-D graphene lay-
ers in the structure. Water molecules terminate active 
bonding sites along graphene sheet edges, permitting 
them to slide past each other easily. In vacuum or in a 
dry, inert environment, this lubricant water is absent, 
and the friction coefficient is much higher.  
As shown in Fig. 4, when a graphite sample is tested 
in dry nitrogen, its initial friction coefficient is compa-
rable to that of CDC, but after approximately 5 min of 
testing, when the layer of graphite that had been hy-
drated during exposure to laboratory air before the test 
had worn through, the friction coefficient rose by about 
50%. This rise is accompanied by drastically acceler-
ated wear of the graphite, leading to rapid failure. 
Similar behavior is observed for glassy carbon in the 
same environment, with the accelerated friction and 
wear beginning after only about one minute of testing. 
CDC, in contrast, maintains a low friction coefficient 
throughout the test. Wear rates were determined by 
 
  Tsinghua Science and Technology, December 2005, 10(6): 699-703 
 
702 
profilometry at the conclusion of the tests. They were 
approximately 1.1×10−5 mm3/Nm for graphite, ap-
proximately 1.9×10−6 mm3/Nm for glassy carbon, and 
was below the detection limit for the instrument, esti-
mated as 3×10−8 mm3/Nm, for CDC. Both the friction 
and wear performance of CDC are far superior to those 
of other sp2 bonded carbon under dry conditions. 
The excellent performance of CDC in dry environ-
ments indicates that the mechanism for low friction in 
this material may be different from that in other sp2 
bonded carbons. This finding suggests that CDC may 
have applications in vacuum systems and other envi-
ronments where conventional lubrication is difficult. A 
possible explanation for this behavior is the presence 
of nested fullerenic structures, described as carbon on-
ions, in the CDC coating[4,7]. Figure 5 is a transmission 
electron micrograph of these structures at the surface 
of the CDC. The physical transformations of the car-
bon leading to the formation of such structures have 
been discussed elsewhere[6,7]. Because of their unique 
morphology, the friction-reducing characteristics of 
these structures may be less dependent on the presence 
of adsorbed water molecules than for other sp2 bonded 
carbons. 
 
Fig. 5 Transmission electron micrograph showing 
nested fullerenic structures or carbon onions on the 
CDC surface.  
3 Hydrogen Treatment of CDC 
The friction coefficients of CDC sliding against silicon 
nitride are substantially reduced when the CDC is 
treated in hydrogen gas at 800  for several hours after ℃
synthesis, as shown in Fig. 6. The friction coefficient is 
reduced by approximately a factor of two after the hy-
drogen treatment, and the friction coefficients in this 
test fall well below 0.1, demonstrating an improvement 
over graphite, even in humid conditions. 
 
Fig. 6 Friction coefficients versus time for pin-on-
disk tests performed at 60 r/min under 2 N load in dry 
nitrogen using CDC as-synthesized and after an 8-h 
treatment in Ar-5%H2 at 800℃. 
The effect of the hydrogen treatment may be to 
change the adsorbed gas species on the carbon surfaces. 
Because of their nano-porous nature, CDCs have a 
substantial capacity for adsorption of gases, such as 
hydrogen[11,12]. The presence of adsorbed gases can 
have a substantial effect on tribological performance of 
carbon films[13]. In the as-synthesized state, CDC 
contains a substantial amount of residual chlorine. This 
chlorine may contribute to the low friction 
performance of the CDC, but it may not be the 
optimum adsorbed gas species for producing low 
friction coefficients. Although this chlorine is not 
removed by high temperature treatments in vacuum or 
inert gas, hydrogen treatment at 800℃ substantially 
reduces the chlorine content of the CDC. Hydrogen 
can react with adsorbed Cl to produce HCl (g), which 
is a more stable gas molecule than Cl2 (g), the form in 
which it must vaporize when hydrogen is not present. 
Excess hydrogen in the environment is likely to 
replace chlorine on the available adsorption sites on 
the carbon surfaces. With hydrogen termination of the 
chemical bonds on the carbon surfaces, the interactions 
between the contact surfaces are minimized, and the 
friction coefficient for the CDC is reduced compared 
to that in the as-synthesized material. 
4 Summary 
Carbide-derived carbon can be synthesized as coatings 
on SiC and other hard carbide ceramics by exposure to 
flowing chlorine at temperatures on the order of 
1000 . CDC℃ -coated SiC shows reduced friction 
coefficients in sliding compared with untreated SiC. 
 
Michael McNallan et al：Nano-Structured Carbide-Derived Carbon ……  703
The CDC coatings are resistant to wear and to 
spallation and delamination in comparison to other 
low-friction carbon coatings. The tribological 
performance of CDC may be related to the nanoscale 
structure of the carbon produced by the halogenation 
reaction, which contains nanoscale diamond, carbon 
onions, and nanoporous amorphous carbon. Further 
treatment of CDC in hydrogen at high temperature 
reduces the content of residual chlorine in the structure 
and further reduces the friction coefficient during 
sliding. 
References 
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silicon carbide by reaction with chlorine containing gases. 
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A I, Danishevskii A M, Popov  V  V,  Gordeev  S  K,  
Grechinskaya A V. A study of nanoporous carbon ob-
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