Applied Surface Science 242 (2005) 261–269
www.elsevier.com/locate/apsusc
Structure and properties of diamond-like carbon nanocomposite
films containing copper nanoparticles
Chun-Chin Chen, Franklin Chau-Nan Hong*
Department of Chemical Engineering and Center for Micro-Nano Technology,
National Cheng Kung University, Tainan 701, Taiwan
Received 1 February 2004; received in revised form 14 July 2004; accepted 20 August 2004
Available online 13 October 2004
Abstract
Diamond-like carbon (DLC) nanocomposite films, containing copper (Cu) nanocrystallites, were synthesized and studied.
Cu bonds very weakly with carbon, and does not form a carbide phase. Therefore, Cu nanoparticles can be easily formed in a
DLC matrix by depositing Cu and carbon together. The mechanical properties of DLC films that contain Cu nanoparticles are
interesting since the film toughness may be increased by grain–matrix interface sliding. Hard, tough and stress-free DLC/Cu
films were prepared by a sputtering Cu target in an argon/acetylene atmosphere while biasing the substrate with a radio
frequency power supply. The residual stress of the film, calculated by Stoney’s equation, was as low as 0.7 GPa. The reduced
stress and the increased film toughness increased the critical load from 66 N for a conventional DLC film to 80 N for the DLC/Cu
film, as measured in a scratch test. However, the DLC/Cu films were slightly less hard than the DLC films.
# 2004 Elsevier B.V. All rights reserved.
PACS: 61.43.Er; 61.46.+w; 62.20. x; 62.20.Qp; 62.40.+i
Keywords: Diamond-like carbon (DLC); Nanocomposite film; Copper; Nanoparticles; Mechanical properties
1. Introduction
Diamond-like carbon (DLC) films have been
extensively investigated over the last three decades.
DLC films exhibit superb mechanical properties,
including high hardness and low friction coefficients.
These films are very useful in precision machining and
* Corresponding author. Tel.: +886 6 2757575x62662;
fax: +886 6 2385423.
E-mail address: hong@mail.ncku.edu.tw (F.-N. Hong).
manufacturing. The importance of hard and abrasionresistant coatings for machining application is
evidenced by the rapidly growing market for cutting
tools with wear-resistant coatings. Besides hardness,
many film properties including toughness, high
adhesion, and low surface energy, are yet to meet
the requirements of various applications. DLC films
have been synthesized by sputtering, laser ablation,
cathodic arc and chemical vapor deposition (CVD)
[1–5]. The bombardment of energetic carbon species
during deposition is critical for the growth of DLC
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2004.08.036
262
C.-C. Chen, F.C.-N. Hong / Applied Surface Science 242 (2005) 261–269
films. The ion energy is the most important parameter
for determining the characteristics of DLC films.
However, the ion bombardment tends to result in the
highly-dense packing of carbon atoms in the film,
yielding a very high compressive stress therein. The
internal compressive stress depends strongly on the
distortion of the bond length and the angle of sp3bonded carbon. A very high compressive stress tends
to detach the film from the substrate, when the film
thickness increases above a critical value. The internal
stress can be reduced by different mechanisms.
Adding a new element to the DLC matrix, which
consists mostly of carbon and hydrogen, is a
commonly used method. The third incorporated
element is normally Si, Ti or W, all of which exhibit
bond strongly with the carbon matrix. In this work,
soft and ductile metallic copper nanocrystallites were
embedded in the hard amorphous DLC matrix to
increase the toughness and stress in the film. Copper
was also used to prevent the formation of bonds
between the nanocrystallite and the carbon matrix,
facilitating grain–matrix interface sliding, which
increases the film’s ductility [6,7]. No Cu metal
carbide phase has even been formed.
Fig. 1. A schematic diagram of the experimental set-up employed in this study.
C.-C. Chen, F.C.-N. Hong / Applied Surface Science 242 (2005) 261–269
2. Experimental
Fig. 1 schematically depicts the experimental setup. A plasma CVD system equipped with a Cu
sputtering gun was used to deposit DLC/Cu
nanocomposite films on the biased substrate using
acetylene/argon mixtures. The substrate holder was
biased with a radio frequency (RF) power supply of
13.56 MHz (Huttinger). Cu was deposited by
sputtering from an unbalanced magnetron gun using
a negative pulse power supply (100 kHz, ENI). The
DLC films were deposited by accelerating the
acetylene ions to bombard the substrate using RF
bias, and Cu nanoparticles were deposited by the
sputtering of copper target with Ar ions. The Cu
target (99.9% in purity) was cleaned using argon
sputtering for 10 min before deposition. Stainless
steel (SKD11) substrates (30 mm 7 mm) and Si
substrates (20 mm 20 mm 0.38 mm) were used.
Before deposition, SKD11 substrates were polished
to a mirror finish with 0.3 mm Al2O3 particles;
ultrasonically cleaned for 20 min in acetone, and
then blown dry in nitrogen. A thin buffer layer about
0.1 mm was deposited using Hexamethldisiloxane
(HMDSO, 99%) reactant on the SKD11 substrate
before depositing DLC or DLC/Cu films. The buffer
layer was grown at 8 10 3 Torr, with a substrate
bias of 120 V, using an RF power of 110 W and for
20 min. Silicon substrates were cleaned first in
acetone and then in HF solution. After the substrate
had been loaded, the base pressure of the deposition
chamber was pumped down to under 10 5 Torr.
The substrates were then cleaned by Ar ion bombardment for 20 min. The pressure was maintained at
1 10 3 Torr during deposition. Table 1 lists the
deposition parameters.
The self-bias normally depends on the geometry
of the reactor. The deposition chamber used in this
work was 450 mm in internal diameter and 500 mm
high. The substrate holder was 300 mm in diameter.
The copper sputtering target was 150 mm in
diameter. The distance between the target and the
substrate was 290 mm. The self-bias varied as the
inverse the diameter of the substrate holder. The selfbias of the substrate at an RF power of 400 W was
440 V when the sputtering gun was turned off.
Turning up the power of the sputtering gun from
280 to 320 W only slightly increased the substrate
263
Table 1
Experimental conditions for incorporating Cu nanocrystals in DLC
films
Substrate power (RF)
RF self bias (substrate bias)
Sputter power (pulse dc)
Base pressure
Pre-clean gas
Working gas
Working pressure
Sputtering target
Depositing time
400 W
450 V (with sputtering)
440 V (without sputtering)
280–320 W
<10 5 Torr
Ar
Ar:C2H2 = 1:1
1 10 3 Torr
Cu
30 min
bias to 450 V at the same RF power. The pressure
and Ar/C2H2 ratio in the gas were maintained at
1 10 3 Torr and 1, respectively, during deposition.
The self-bias was not influenced by the Ar/C2H2 gas
ratio, and was only slightly sensitive to the gas
pressure. The self-bias was reduced from 450 to
430 V by increasing the pressure from 1 10 3 to
8 10 3 Torr.
The thickness and chemical composition of the
films were analyzed by scanning electron microscopy
(SEM) and energy dispersion spectroscopy (EDS),
respectively. The structure of the deposited film was
characterized using a Raman spectrometer (Dilor) and
an Ar ion laser that emits 514.5 nm radiation as the
excitation source. The films were investigated using
optical microscopy (OM) and high-resolution transmission electron microscopy (HRTEM). Stoney’s
equation [8] was used to determine the stress in the
film by measuring the its curvature using an a-step
profilometer. The strength of the adhesion between the
film and the substrate was measured using a scratch
tester. The scratch tip was a diamond stylus with a
radius of 300 mm. The rate of increase of load was
1 N/s and the speed of the stage was 0.1 mm/s. The
hardness of the film was determined by nanoindentation (Digital). A Berkovich diamond indenter with a
tip radius of about 50 nm was used to make the
measurements. The total included angle on this tip was
142.38. The sample was indented with maximum loads
in the range of 500–1500 mN to keep the indentation
depth less than 10% of the film thickness, to minimize
the substrate effect. Rockwell indentation was also
used to evaluate the adhesion and toughness of the
film.
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Table 2
The compositions and residual stresses of DLC nanocomposite films containing Cu nanoparticles
Sample
Sputtering power (W)
Composition of Cu (at.%)
Residual stresses (GPa)
Film thickness (nm)
B1
C1
C2
C3
0
280
300
320
0
11
16
23
2.9
0.7
1.4
1.2
240
230
200
250
Table 3
The copper deposition rate without C2H2
Sputtering power (pulse power) (W)
Deposition rate (nm/h)
280
300
320
50
85
125
3. Results and discussion
3.1. Film composition and residual stresses
Table 2 presents thickness and the residual stress of
the film in relation to the Cu content. The Cu content
was altered by varying the pulse power of the
sputtering gun. Table 3 presents the deposition rate
of copper without C2H2, in relation to the pulse power.
The analysis by energy dispersive X-ray spectroscopy
(EDS) revealed that the Cu content increased
monotonically with the pulse power of the sputtering
gun. In Fig. 2, the glow discharge spectroscopy (GDS)
analysis shows that Cu is uniformly distributed
through the thickness of the film. The stress analysis
based on Stoney’s equation demonstrated that
incorporating Cu nanoparticles into the DLC film
clearly reduced the residual stress (comparing B1 with
C1, C2 and C3). However, the DLC film with the
lowest Cu content (C1), rather than that with the
highest Cu content, had the lowest residual stress. All
depositions had been repeated three times and the
results were highly reproducible. As discussed below,
characterizations of the film structure by Raman
spectroscopy explain results.
3.2. Structure of the DLC films that contain
Cu nanoparticles
The C1 and C3 films were characterized by
HRTEM; Fig. 3 depicts the bright-field images.
Evidently, the DLC film was a nanocomposite film
that contained Cu nanocrystals of sizes 15–30 nm.
Table 4 summarizes the d-spacings of the C1 and C3
films determined by selective area diffraction.
Comparing the spacings of copper and graphite in
JCPDS files clearly revealed that only Cu crystallites
were formed. The density of the nanoparticles, but not
their size, increased with the power of the sputtering
gun was increased from 280 to 320 W.
3.3. Raman analysis of the DLC films that contain
Cu nanoparticles
Fig. 2. The GDS analysis of the C1 film. Cu percentage in the film is
plotted vs. the depth of the film from the surface.
Fig. 4 presents Raman spectra of the samples, B1,
C1 and C3. The crystal structure of copper is facecentered cubic (FCC), and so is centrosymmetric, so
Raman spectra are obtained from the DLC matrix
only. Table 5 shows the results of fitting the Raman
spectra with Gaussian peaks. Comparing C1 with B1
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265
Fig. 3. TEM bright field images and diffraction patterns for the (a) C1 and (b) C3 films. The inset shows the high resolution image of the Cu
particle in the C3 specimen.
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Table 4
The diffraction indices of C1 and C3 films compared with Cu (JCPDS#01-1242) and graphite (JCPDS#75-2078) indices
Cu (JCPDS#01-1242)
Index
Our C1 sample
Our C3 sample
d-value
111
2.08
200
1.80
2.48
2.07
2.08
Graphite (JCPDS#75-2078)
Index
d-value
111
3.34
010
110
2.08
1.95
222
112
1.67
1.62
221
1.46
110
232
120
333
1.23
1.19
1.15
1.12
1.80
1.50
220
311
222
400
331
420
422
1.27
1.08
1.04
0.90
0.83
0.81
0.74
1.27
1.30
1.07
1.07
0.94
0.85
0.81
0.74
0.92
0.84
0.81
0.74
indicates demonstrates that adding 11% Cu to the film
shifted the G band and the D band to a higher
wavenumber; increased the ratio of intensity of the D
band to that of the G band, ID/IG, and reduced the
FWHM of the G band. Accordingly, Raman spectra
demonstrated that adding 11% Cu in the DLC film
Fig. 4. Raman spectra of the DLC films.
increased the content of sp2 carbon. Then, increasing
the sputtering power from 280 W (C1 sample) to
320 W (C3 sample) further increased the Cu content
from 11 to 23%, shifting the G and the D bands to
lower wavenumbers, reducing the intensity ratio, ID/
IG, and increasing the FWHM of the G band. Hence,
Raman results revealed that increasing Cu content
from 11 to 23% reduced the sp2 carbon content.
Increasing the sputtering power from zero (B1 sample)
to 320 W (C3 sample) shifted the G band and D band
to slightly lower wavenumbers; reduced the ID/IG
ratio, and increased the FWHM of the G band,
indicating a fall in the sp2 carbon percentage in the
film. This conclusion is drawn from the statement of
Ferrari and Robertson [9]. Table 2 indicates that all the
films—C1, C2 and C3—have similar thickness. The
same signal to noise ratios in the Raman spectra also
support the claim that all the films have similar
thicknesses. The Raman intensity associated with C3
appears to be reduced by the rise of the background.
The decline of the luminescence background in the
Raman spectra is related to impurities in the films.
The extent of the decline in the luminescence background in the Raman spectra indirectly measures the
C.-C. Chen, F.C.-N. Hong / Applied Surface Science 242 (2005) 261–269
267
Table 5
Raman analysis of the DLC nanocomposite films containing Cu nanoparticles
Sample
G center (cm 1)
D center (cm 1)
FWHM of G (cm 1)
ID/IG
Stress (GPa)
B1
C1
C3
1539
1566
1538
1324
1424
1321
178
126
185
0.31
0.74
0.27
2.9
0.7
1.2
hydrogen content in the films. In Fig. 4, the decline of
the background of the C3 sample is less than those for
the C1 and B1 samples. Hence, the C3 film contains
less hydrogen than the C1 or B1 film.
The film with 11% Cu (the C1 film) had the lowest
stress, probably because it had the most sp2 carbon in
the DLC matrix. The fall in the sp2 carbon content in
the film as the Cu content increased from 11% (C1) to
23% (C3) may have been caused by the increase in the
sputtering power from 280 to 320 W. The plasma
density and the degree of gas ionization increased with
the sputtering power. The fluxes of highly energetic
ions that bombarded the growing films thus increased
with respect to those of low-energy neutral species,
such as hydrocarbon radicals. Accordingly, the mean
kinetic energy of all species that struck the film
increased, favoring the formation of sp3 carbon and
reducing both the sp2 carbon content and the hydrogen
content in the C3 film. The increase of the ion energy
during the film deposition favored the formation of sp3
carbon in this work. A moderate energetic ion flux that
sufficed to penetrate the growing film surface was
required to generate a higher fraction of sp3-carbon in
the diamond-like films. Further energetic ion flux may
generate graphite-like films with a larger fraction of
sp2 carbon because of the energy relaxation of a
thermal spike. Therefore, the high fraction of sp3
carbon in the C3 film due to the increase of ion flux
reveals that the ion flux was moderate under the
deposition conditions in this work. In summary, the
lower stresses of the C1 and C3 films than of the B1
film were not related to the increase in the fraction of
sp2 carbon in the film, but to the effect of incorporating
Cu nanoparticles in the carbon matrix.
3.4. Mechanical properties of the DLC films that
contain Cu nanoparticles
Table 6 lists the stresses, hardness and critical loads
of the DLC films, including their sp3 carbon contents
and their Cu contents. Incorporating Cu nanoparticles
in the DLC matrix clearly reduced the film stress.
However, the stress in the C3 film (23% Cu) exceeded
that in the C1 film (11% Cu), probably because of the
higher sp3 carbon content. Nanoindentation measurements revealed that the film hardness fell monotonically as the Cu content in the films increased. The
hardness of the pure DLC film was 22 GPa. However,
as the Cu content in the film was increased to 23%, the
hardness of the DLC films fell to 15 GPa. These results
are in contrast with those in other reports on superhard
nanocomposite films such as TiN/Si3N4 and ZrN/Cu,
which revealed a maximum hardness for some
compositions of hard nanocrystallites embedded in
amorphous regions [6,7,10]. The difference, however,
is explained by the absence of strong chemical bonds
between Cu nanocrystallites and the DLC matrix [11],
and by the ductility of Cu nanocrystals within the
amorphous DLC matrix. These two factors enable
easy sliding of the grain–matrix interface in the DLC/
Cu composites [6], increasing the ductility, and
thereby improving toughness [12]. Pure Cu is very
soft, with a hardness of under 1 GPa. Notably, copper
and graphite will never form an alloy, because the
Table 6
Hardness and critical load of DLC nanocomposite films
Sample
sp3 carbon content
Cu composition (%)
Stress (GPa)
Hardness (GPa)
Critical load (N)
B1
C1
C3
Medium
Low
High
0
11
23
2.9
0.7
1.2
22
16
15
66
80
74
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C.-C. Chen, F.C.-N. Hong / Applied Surface Science 242 (2005) 261–269
bonding between carbon and copper is very weak. The
HRTEM results in Fig. 3 verified the formation of only
Cu nanocrystallites (15–30 nm) in the amorphous
carbon matrix.
DLC films were further deposited on SKD11 steel
substrates under B1, C1 and C3 conditions, and the
critical load was in each case measured in a scratch
test. Critical loads depend strongly on the coating
adhesive, the cohesive strength, and the frictional
force between the diamond stylus and the coating
surface. The adhesion should increase with the critical
load. Table 6 reveals that the critical load of pure DLC
films was 66 N the lowest value. The critical loads of
the C1 and C3 films were higher at 80 and 74 N,
Fig. 5. The OM pictures of the DLC nanocomposite films: (a) B1 and (b) C1, after Rockwell indentation tests.
C.-C. Chen, F.C.-N. Hong / Applied Surface Science 242 (2005) 261–269
respectively. Adding Cu nanoparticles to the DLC
films reduced the film stress and so increased its
toughness, and therefore, the critical load. The effect
of the adhesion of Cu nanocrystallites on the steel
substrate is unclear.
Film toughness was measured by conducting
Rockwell indentation tests at a load high enough
for significant deformation of both the film and the
substrate [7]. Fig. 5(a) is an OM picture of the pure
DLC film, B1, following indentation. Cracks were
clearly observed on the edge. Pure DLC film was
brittle in Rockwell indentation under a load of 150 g.
Stress analysis revealed that the pure DLC film
exhibited large residual compression stress, which
may have accounted for the failure of adhesion.
Fig. 5(b) displays an OM picture of the DLC film that
contains Cu nanocrystallites, C1, following indentation. No crack in the DLC nanocomposite film is
observed. The hardness of the film in Fig. 5(b) was
16 GPa, which is similar to that of hydrogenated
diamond-like carbon. The ductility of the DLC
nanocomposite film that contained Cu nanoparticles
is consistent with the concept of the sliding of the
grain–matrix interface.
4. Conclusion
Copper nanoparticles were embedded in the DLC
matrix to increase the film ductility. TEM images
demonstrated that the DLC nanocomposite films
contained a high concentration of 15–30 nm Cu
nanoparticles. Increasing the Cu content in the DLC
matrix slightly reduced the film hardness from 22 GPa
for pure DLC to 16 GPa for the DLC nanocomposite
film that contained 11% Cu. Embedding copper
nanoparticles in the DLC film increased the critical
load in the adhesion test from 66 N for pure DLC to
80 N for DLC nanocomposite (11% Cu). Incorporating Cu nanoparticles in the DLC films reduced the
film stress (0.7 GPa) and increased the film toughness,
increasing of the critical load in the adhesion
269
measurements. However, the film hardness was
slightly reduced. Our results demonstrated that the
DLC film hardness was about 10 GPa when the film
stress was reduced to under 1 GPa. The weak bonding
between copper and carbon allows the tough and hard
Cu-DLC nanocomposite film to be grown, because of
the sliding of the grain–matrix interface.
Acknowledgement
Financial support for this work from the Ministry of
Economic Affairs under Contract No. 91-EC-17-A07-S1-0018 and from Center for Micro-Nano Technology, National Cheng Kung University is gratefully
acknowledged.
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