Diamond & Related Materials 15 (2006) 1223 – 1227
www.elsevier.com/locate/diamond
A study of hard diamond-like carbon films in mid-frequency
dual-magnetron sputtering
Yu Xiang a,*, Wang Cheng-biao a, Liu Yang b, Yu De-yang b
a
School of Engineering and Technology, China University of Geosciences, Beijing 100083, China
b
Beijing Powertech Co,. Ltd., Beijing 100072, China
Received 14 December 2004; received in revised form 7 August 2005; accepted 22 September 2005
Available online 22 November 2005
Abstract
A series of hydrogen-free diamond-like carbon (DLC) films were deposited by a mid-frequency dual-magnetron sputtering under basic
conditions of Cr and C target power density between 6 and 18 W/cm2, bias voltage in a range of 100 V to 200 V, and a pure argon atmosphere.
Microstructure, microhardness, adhesion, friction and wear properties were investigated for the DLC films to be used as protective films on cutting
tools and forming dies, etc. The DLC films exhibited some combined superior properties: high hardness of 30 – 46 GPa, good adhesion of critical
load of 50 – 65 N, and friction coefficient about 0.1 in air condition. Properties of the magnetron-sputtered carbon films showed a strong
dependence on flux and energy of ion bombardment during growth of the films.
D 2005 Published by Elsevier B.V.
Keywords: Cr-doped DLC film; Mid-frequency dual-magnetron sputtering; Multilayered and graded structure; Properties
1. Introduction
Due to having some prominent tribological properties
such as high hardness, low friction coefficient and high wear
resistance, diamond-like carbon (DLC) films are of great
interest for usages of wear-resistant films on cutting tool,
forming die, etc. Accordingly, numerous studies on quality
improvement of such film have been made in various
techniques, e.g. arc evaporation, HF-plasma or microwaveplasma CVD and unbalance magnetron sputtering [1].
Hydrogenated DLC film has limited microhardness due to
inclusion of H atoms. A main limitation in hydrogen-free
DLC film applications is its poor adhesion to steel substrates
due to high residual stress in the DLC films and/or the large
discrepancy of mechanical properties between the DLC films
and steel substrates [2]. Some studies have attempted to
improve the adhesion between DLC film and its substrates
by applying adherent layer such as B and Ti to the film
interface [3]. Methods enhancing the mechanical properties
were also reported, such as doping TiN, TiCN and TiC
* Corresponding author. Tel.: +86 10 82320998; fax: +86 10 82321883.
E-mail address: yuyuxiang7546@sina.com (Y. Xiang).
0925-9635/$ - see front matter D 2005 Published by Elsevier B.V.
doi:10.1016/j.diamond.2005.09.040
layers in DLC matrix [4]. A graded composition and
microstructure may be used to improve adhesion and
microhardness of the graded film. In this study, several
multilayered and graded DLC films with a single-phase
homogeneous carbon layer were synthesized by selectively
using C and Cr targets in a mid-frequency dual-magnetron
sputtering.
The pulsed plasma technique was applied to both magnetron
drives and substrate bias in this experiment. Resent studies
demonstrated that mid-frequency (20 – 100 kHz) magnetron
sputtering offered some advantages over traditional DC
magnetron sputtering for deposition of metastable films like
DLC and CNx [5]. Pulsing the discharge can significantly
modify the characteristics of magnetron plasma, pulsing the
substrate bias voltage can increase the ion current drawn at the
substrate, which have some important effects on properties of
the me tastable films [6].
This paper focuses on improving properties of the
hydrogen-free DLC film by controlling process parameters
like target current, substrate bias voltage, etc., during the midfrequency dual-magnetron sputtering. Structure, deposition
process and resulting properties of the DLC film are
investigated.
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Y. Xiang et al. / Diamond & Related Materials 15 (2006) 1223 – 1227
2. Experimental details
2.1. Coating equipment
Cr-doped hydrogen-free DLC films were deposited using
unbalanced dual-magnetron sputtering in a coating rig SP8050
(Beijing Powertech Co., Ltd.), whose cross-sectional view is
schematically shown in Fig. 1. As can be seen, four pairs of
dual-magnetron targets including two chromium (Cr) and six
graphite (C) ones were set in the cylindrical coating rig. Two
planar magnetrons of each pair of dual-magnetron targets were
placed in vacuum chamber in face-to-face configuration
approximately 70 cm apart, respectively. The substrate holder
was placed in center area. A unipolar-pulse DC power supply
on the substrates and four pulsed AC power supplies of 40
kHz, on each pair of dual-magnetrons, were used to control the
substrate bias voltage, substrate bias current and target power.
The duty ratio of the pulsed substrate bias voltage was 80% in
Ar ion cleaning and was 60% in the following deposition
processes. The mid-frequency power supplies used a currentconstant mode to control the magnetron targets at constant
values of 2, 4 and 6 A, respectively, corresponding to target
power increased from 2 to 8 kW and target voltage 3 from 200
to 800 V. The C and Cr targets had a uniform size of
420 80 8 mm3 and a purity of 99.9%. Argon gas was
introduced through each target to enhance sputtering of the
targets. Process parameters in each procedure could be
automatically controlled by PLC (Programmable Logical
Controller) through presetting.
2.2. Sample preparation and Ar ion cleaning
Sheets of M2 high-speed steel (HSS) in a uniform size of
20 20 3 mm3 were used as coupons for mechanical and
tribological tests, and some coupons of Si(100) wafers in a
uniform size of 8 8 0.2 mm3 were used for cross-sectional
SEM and Raman tests. Surfaces of the steel sheets were mirror
polished to surface roughness of R a 0.05 Am. All coupons were
ultrasonically cleaned in acetone bath for 20 min, and were
then blown dry with nitrogen.
After pumping down by a turbo-molecular pump to a base
pressure of 2.0 10 3 Pa and before depositing, surfaces of the
coupons were further cleaned by Ar ion bombardment under
the bias voltage of 800 V at pressure of 3.0 Pa for 15 min, in
order to remove some adhering impurities on substrates.
2.3. Deposition procedures
After the substrate was heated to 100 -C, deposition process
was started in condition that the depositing pressure maintained
at 0.3 Pa in argon atmosphere and the bias voltage was in a
rang of 100 V to 200 V. The deposition procedures
included three optional steps: firstly, Cr interlayer about 0.2 Am
on the substrate was sputtered from two Cr targets, where Cr
target current kept at 4 A; secondly, transitional layer of Crx Cy
about 1 Am on Cr interlayer was co-sputtered from two Cr and
six C targets, where Cr target current decreased from 10 A to 1
A and C target current increased from 2 A to 6 A, continuously;
thirdly, top DLC layer about 1 Am was sputtered from six C
targets, where C target currents were at constant values of 2, 4
and 6 A, respectively.
2.4. Surface analysis methods
Scanning electron microscope (SEM) combined with
energy-dispersive X-ray (EDX) analyzer, Quant 200, was used
to investigate the morphologies and elemental compositions.
Raman spectroscope, PHI-6100/SAM, was used to investigate
the microstructure. Nanoindenter (MML1 Nanotester 600) and
scratch tester (LICP-CAS SW-97) were used to investigate the
microhardness and adhesion, respectively. Ball-on-disk tester
(LICP-CAS DD-92) was used to investigate the friction and
wear property under conditions: DLC film was deposited on
the disc, and counterpart ball used was a A6 mm Al2O3 ball;
loads of 1 N, 2 N and 5 N were applied in air; rotated speed
was constant at 1000 r/min, equivalent to 0.6 m/s of linear
speed; tested time was about 15 min. Surface profilometer was
used to measure the cross-sectional area of the wear track so as
to calculate the wear volume of the DLC film. Optical
microscope attached with a camera was used for analysis of
wear traces.
3. Results and discussions
Properties of the graded DLC film in unbalanced magnetron
sputtering show a strong dependence on flux and energy of ion
bombardment during growth of the film [7]. According to our
experimental results of mid-frequency dual-magnetron sputtering, the substrate bias current, an index of ion flux, had some
close correlations with process parameters, such as: target
Fig. 1. Schematic diagram of cross-sectional coating rig SP8050.
1
Micro Materials Limited. LICP-CAS: Lanzhou Institute of Chemical
Physics, Chinese Academy of Science.
Y. Xiang et al. / Diamond & Related Materials 15 (2006) 1223 – 1227
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power, substrate bias voltage, substrate temperature and gas
pressure. For example, in condition that target power was 6 kW
or substrate bias voltage was 250 V at substrate temperature
less than 150 -C and argon pressure of 0.3 Pa, the substrate bias
current had a high value of 5 A for the carbon deposition. This
value could be even higher with increasing argon pressure
from 0.3 Pa to 1 Pa and substrate temperature from 150 -C to
450 -C. The energy of ion bombardment was increased with
the increasing bias voltage. For a convenient operation, the
properties of DLC films were controlled by adjusting the target
current and substrate bias voltage under condition of the
constant substrate temperature and sputtering pressure.
3.1. Microstructure and deposition character
Surface and cross-section SEM images of DLC films at
100 V of the pulsed bias voltage are shown in Fig. 2. As
shown in Fig. 2a, the film showed a very smooth morphology
with no visible loose particles from the targets, whose average
surface roughness, R a, is about 4 nm. During unbalance
magnetron sputtering on precise forming die, substrate arcing
is somewhat inevitable [8]. If the localized surface arcing
persists, some droplets and holes from the running arcs may
make the die unusable. The mid-frequency magnetron sputtering may be an effective method to reduce the defects induced
by substrate arcing. In Fig. 2b, the multilayered and graded
structure, containing layers of Cr, Cx Cry and DLC, can be
observed. Apart from the imperfect smoothness of crosssection surface due to sampling difficulty, R a å 0.2 Am in the
wide areas; interfaces between each layer of the graded DLC
film were well bonded without any visible flaw, of which
Cx Cry layer were embedded in DLC matrix. Such graded
structure may be used to cope with the residual stress in DLC
Fig. 3. A typical Raman spectrum of DLC film deposited at
bias voltage.
100 V of pulsed
film and the large discrepancy of physical properties between
DLC film and the substrate. The integral structure of graded
DLC film is dense and compact, which is vital in achieving
high hardness for wear resistance. A typical Raman spectrum
of the DLC film is shown in Fig. 3. The spectrum can be fitted
to two Gaussian peaks, one at approximately 1565 cm 1 (‘‘G’’
peak), and another at approximately 1338 cm 1 (‘‘D’’ peak).
The D-to-G bond ratio of I D/I G is 1.74. Similar to report by
Yang and Teer [9], the metal Cr and Cr containing layers
bolstered the stress management in hard a-C films. In this way,
the stress state of me tal-doped DLC film could be stepwise
adjusted in graded structure from outer boundary through
multilayer to inner boundary and substrate by specified process
control.
Deposition rate of the DLC film was high, more than 0.8
Am/h, which was about twice of reported one using traditional
sputtering [10]. The deposition rate was closely related with
target power in this current-constant mode and thus could be
controlled. When the biasing voltages was at 100, 150, and
200 V, meanwhile carbon target current kept at 2 A, the
deposition rates were measured at 0.96, 0.94 and 0.87 Am/h,
respectively. When the C target currents was at 2, 4 and 6 A,
meanwhile the bias voltage kept at 100 V, the deposition
rates were measured at 0.96, 1.06, 1.14 Am/h, respectively.
Above results were obtained in such condition: compared with
chromium, carbon deposition rate was relatively low owing to
the low sputtering yield of graphite. However, if the biasing
voltage exceeded 200 V or the target current exceeded 6 A in
above condition, the deposition rate was not increased and may
be decreased little due to too intense ion bombardment, similar
results can be seen in literature [11].
3.2. Mechanical properties
Fig. 2. Surface and cross-section SEM images showing structure of Cr-doped
DLC film on Si wafer.
3.2.1. Microhardness
To accurately measure microhardness of DLC film in
micron-scale thickness, a two-step penetration method in
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Y. Xiang et al. / Diamond & Related Materials 15 (2006) 1223 – 1227
improvement in the adhesion, and the best one was 65 N of L c
from the graded DLC film at substrate bias voltage of 160 V.
3.3. Friction and wear property
Friction and wear property of DLC films of hard one with
34 GPa and superhard one with 45 GPa was investigated in the
ball-on-disc tester, whose frictional results are shown in Fig. 6.
At a 7 load of 5 N, the average friction coefficient of the superhard one is about 0.08, and that of the hard one is about 0.1. At
the low loads of 1 N, 2 N and 5 N, the DLC films show some
low and constant friction coefficients between 0.15 and 0.08,
Fig. 4. Influence of target current and substrate bias voltages on mirohardness
of DLC films.
nanoindenter tests was used, i.e. after obtaining the test results
of large-load, a penetration load range could be determined to
ensure that range of deformation under the indenter was within
DLC layer and did not extend into the transitional layer or
substrate. As shown in Fig. 4, microhardness of the hydrogenfree DLC films is between 34 GPa and 46 GPa. The
microhardness value is higher than that of hydrogen carbon
films (a-C:H) and a-C films, as reported in literature [12]. The
compactness of DLC film may contribute to such improved
microhardness. The microhardness value increases with the
increasing bias voltages, a typical result of increased residual
stress by ion bombardment [13,14]. Target current also shows a
clear effect on microhardness of the film.
3.2.2. Adhesion
In case of using the graded structure, influences of interlayer
design and substrate bias on film adhesion were evaluated by
scratch test, whose results are shown in Fig. 5. Being as index
of adhesion force between film and substrate, the critical load
L c, was determined by onset of abrupt variations in friction
forces, as a diamond stylus scratched with continuous loading
on DLC films. Three types of DLC films were tested for
evaluating their adhesions. In Fig. 5A, a pure DLC film
showed a poor adhesion, peeled off the substrate surface at 16
N of L c, due to the poor interface compatibility. In Fig. 5B, a
DLC film with Cr interlayer was used to improve the adhesion.
For a convenient analysis, L c is divided into L c1 (critical load
for cohesion) and L c2 (critical load for adhesion), determined
by the first and secondary onsets of abrupt variations of the
friction forces. The DLC film was peeled off the substrate
surface at 21 N of L c1, but L c2 was lasted to 60 N. Cr interlayer
seemed to play an important role on adhesion improvement. In
Fig. 5C,D, the graded DLC films with Cr interlayer and Cx Cry
transitional layer at substrate bias voltages of 100 V and
160 V were tested. Scratch results demonstrated a significant
Fig. 5. Influence of transitional layer and substrate bias voltages on adhesion of
DLC films (target current 6 A).
Y. Xiang et al. / Diamond & Related Materials 15 (2006) 1223 – 1227
Fig. 6. Two comparative curves of friction coefficients of DLC films with
different hardness of 34 GPa and 45 GPa. Optical micrographs of wear tracks
were from the discs at the end of 500 m of sliding distance.
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The DLC film possessed a graded and multilayered
structure. The Cr interlayer played a role of adhesion
improvement. The structural integrity was beneficial for both
the high microhardness and good adhesion of the DLC films.
The effect of target current and bias voltage on the properties of
the DLC film was also examined. It was found that the DLC
film possessed some combined advisable properties: a very
smooth morphology, microhardness in range of 32– 46 GPa,
adhesion of L c up to 65 N and a reliable friction coefficient
about 0.1. The super-hard DLC film had the behavior of both
good adhesion and lower friction coefficient, and thus it is
predicted that it may be used the one as a high-quality wearresistant film in the industrial applications.
Acknowledgements
and these fiction coefficients do not show an obvious change
with the different loads. As shown from the optical micrographs in Fig. 6, in the test range of 500 m, no obvious failure
of DLC films was observed. The sliding wear rates of the DLC
films are in a range of 2.833 10 8 –8.653 10 8 mm3/Nm at
a load of 2 N and at a sliding distance of 500 m. The wear rate
of the super-hard DLC film is 2.833 10 8 mm3/Nm, and that
of the hard one is 8.646 10 8 mm3/Nm. After collecting the
wear debris from the ball and disc and filtering the debris in
acetone bath, it was found in the EDX analysis that the main
element content of the debris was carbon, so abrasion was
supposed as the wear mechanism. Under the same test
condition, the superhard DLC film, microhardness more than
40 GPa, showed the lower volumetric wear rate and friction
coefficient than that of the hard one, microhardness between 30
GPa and 40 GPa. In this case, the wear resistance of DLC film
might be correlated with the hardness of DLC film [15,16].
4. Conclusion
A series of hydrogen-free DLC films were successfully
synthesized using the mid-frequency dual-magnetron sputtering. The process parameters, structure and properties of Crdoped graded DLC film have been investigated for the films to
be used as the protective films. The properties of these DLC
films showed a clear dependence on the flux and energy of ion
bombardment during growth of the film.
This project has been supported by the National Natural
Science Foundation of China with Grant No. 50475057 and by
The Tribology Science Fund of National Tribology Laboratory
of Tsinghua University in Beijing.
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