Materials Transactions, Vol. 45, No. 5 (2004) pp. 1601 to 1606
Special Issue on Recent Research and Developments in Titanium and Its Alloys
#2004 The Japan Institute of Metals
Formation of Diamond-Like Carbon Based Double-Layer Film
on Ti-6Al-4V Substrate by Ionization Deposition
Tsuyoshi Mano1 , Osamu Sugiyama1 , Yoshio Shibuya1 , Hiroshi Nakayama1 and Osamu Takai2
1
2
Fuji Industrial Research Institute of Shizuoka Prefecture, Fuji 417-8550, Japan
Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8603, Japan
A double-layer film, in which the top layer was a diamond-like carbon (DLC) film and the bottom layer was a compositionally graded film
of silicon and carbon compounds with decreasing C/Si atomic ratio to a substrate, was successfully formed on a Ti-6Al-4V substrate by an
ionization deposition method. In the film deposition process, a benzene vapor was used for the DLC deposition as a source gas, and
hexamethyldisiloxane and benzene vapors were used as source gasses for the compositionally graded film of silicon and carbon compounds. The
results of ball-on-disk and scratching tests showed that the double-layer film with graded composition provided a low friction coefficient, high
wear resistance and good adhesion with the Ti-6Al-4V substrate compared to a single-layer DLC film. The DLC-based double-layer film
developed in this study is much effective in wide applications of the Ti-6Al-4V alloy for which the use to antifriction components has been
difficult.
(Received November 19, 2003; Accepted January 29, 2004)
Keywords: titanium alloy, diamond-like carbon (DLC), double-layer film, graded composition, ionization deposition, X-ray photoelectron
spectroscopy, low friction, wear resistance, adhesion
1.
Introduction
Ti-6Al-4V alloy is one of the most promising materials for
mechanical and biomedical uses because it has excellent
properties such as high strength, high ductility and high
corrosion resistance. However, the wear resistance of Ti-6Al4V is often weak, which results in the difficulty to use for
sliding parts without any surface treatment.1–3) In order to
overcome this obstacle, some researches on implantation of
nitrogen or carbon into the Ti-6Al-4V alloy or formation of
TiN coating on it by a PVD process were carried out.4–7)
However, the modified layer by the implantation process has
insufficient thickness to be durable for a long period.
Moreover, TiN coating has a high friction coefficient under
dry condition, and its corrosion resistance is not enough,
either.8–10) Consequently, development of novel coating
material which provides high wear and corrosion resistance
to Ti-6Al-4V has been intensely desired.
Diamond-like carbon (DLC) coatings have good properties, such as high hardness, low friction coefficient and high
corrosion resistance.11–14) However, the adhesion of hard
DLC films with substrates is often weak, due to their high
internal stress.15,16) Therefore the substrates which can offer
good adhesion to DLC are limited, such as WC-Co alloy,
silicon, etc. Hence there are very few reports17) of the
deposition of DLC on Ti-6Al-4V.
In this research, formation of a DLC based double-layer
film on the Ti-6Al-4V substrate by an ionization deposition
method18,19) was carried out to improve the wear resistance
and adhesion for the Ti-6Al-4V substrate. The feature of this
DLC based double-layer film is that the top layer is thick
DLC of high hardness and the bottom layer is silicon-carbon
compositionally graded compounds with decreasing C/Si
atomic ratio to the substrate. Furthermore, this double-layer
deposition process can be performed successively without the
extinction of plasma. The wear resistance and adhesion of the
DLC based double-layer film were examined by a ball-on-
disk test and a scratch test. Further, the chemical bonding and
composition of the compositionally graded film were
evaluated with an X-ray photoelectron spectrometer (XPS).
2.
Experimental Procedure
An ionization deposition apparatus (Nanotec, DASH450DS) was used for the preparation of the DLC based
double-layer films. Source gasses were hexamethyldisiloxane
(abbreviated as HMDSO; [(CH3 )3 Si]2 O, >99:0% in purity)
and benzene (C6 H6 , >99:9% in purity). Distance between a
substrate and an anode was set to be 180 mm. Substrates were
mirror-polished Ti-6Al-4V plates (Ra <0:05 mm, annealed;
34 mm 34 mm 3.2 mm). Ar ion bombardment for 20 min
was performed prior to the deposition in order to clean up the
surface of the substrate.
The deposition was carried out under the conditions, gas
pressure 0.15 Pa, filament current 30 A, anodic current
0.35 A, and negative bias voltage of the substrate 1500 V.20)
These conditions were fixed throughout the deposition
process for a set of all the layers.
Three types of layer-structures with DLC films were
formed in the present study as follows: (1) ‘‘single-layer
DLC’’ (abbreviated as DLC), (2) ‘‘DLC/single composition
layer’’ (abbreviated as DLC/SCL), and (3) ‘‘DLC/graded
composition layer’’ (abbreviated as DLC/GCL). In the
‘‘DLC’’ structure, the DLC film was deposited directly onto
the substrate. In the ‘‘DLC/SCL’’ or ‘‘DLC/GCL’’ structure,
the SCL or GCL was deposited onto the substrate first and
then the DLC film was done onto it. The DLC films were
prepared by using a benzene vapor as a source gas. The SCL
was prepared from the source gas of HMDSO alone, and the
GCL was prepared from the source gasses of both HMDSO
and benzene. In the preparation of the GCL, an adding
volume ratio of HMDSO to benzene, [HMDSO]/[benzene]
was decreased in a stepped mode as follows; 1.0 at first 5 min,
0.75 from 5 to 10 min, 0.50 from 10 to 15 min and 0.25 from
T. Mano, O. Sugiyama, Y. Shibuya, H. Nakayama and O. Takai
3.1 Wear resistance and adhesion
The nanoindentation hardness of three types of DLC-based
films was measured in advance of the friction and wear
experiments. The hardness of DLC, DLC/SCL and DLC/
GCL surfaces, on which each maximum indentation depth
was about 110 nm, was 32 GPa, 29 GPa, and 30 GPa,
respectively. That is, a hard DLC film was formed on the
surface and the influence of a difference among the layerstructures on hardness was not observed.
Figure 1 shows the variation of coefficients of friction with
increasing number of cycles for three types of DLC-based
films and the Ti-6Al-4V substrate obtained by the ball-ondisk test. The Ti-6Al-4V substrate has high coefficient of
friction, about 0.40, from the early cycles. The DLC shows
low coefficient of friction of 0.10 or less up to about 700
cycles, however the coefficient of friction is increasing
rapidly after that cycle. The reason of this result is that the
DLC film removed and the direct contact of the substrate with
a ball occurred. It became clear by the observation of the
DLC
DLC / SCL
0.60
DLC / GCL
Substrate (Ti-6Al-4V)
0.40
DLC
0.20
DLC/SCL
DLC/GCL
0.0
0
200
400
600
800
1000
Number of cycles, N
Fig. 1 Variations of the coefficients of friction with increasing number of
cycles in the ball-on-disk test for three types of DLC-based films and the
Ti-6Al-4V substrate.
optical microscope. The DLC/SCL and the DLC/GCL are
maintaining with the low coefficient of friction. Moreover,
the lifetime of the DLC/SCL and the DLC/GCL was 70700
cycles and more than 100000 cycles, respectively. The DLC/
GCL maintained the low coefficient of friction over the long
time.
Figure 2 shows the wear volumes of the samples tested at
1000 cycles and at 60000 cycles. The Ti-6Al-4V and the
DLC had severe wear volumes more than 0.3 mm3 . The
measurement of wear volume was unable at 60000 cycles due
to their over-wearing apparent. The DLC/SCL and the DLC/
GCL have better wear resistance when compared to these two
samples. For the DLC/SCL, the wear volume at 60000 cycles
(0.0035 m3 ) has about 1.5 times larger than that the wear
volume at 1000 cycles (0.0025 m3 ). The wear resistance of
the DLC/SCL is falling with increasing number of cycles.
For the DLC/GCL, both the wear volumes at 1000 cycles and
60000 cycles are around 0.002 m3 . That is, the DLC/GCL
exhibits better wear resistance than the DLC/SCL.
0.350
At 1000 cycles
At 60000 cycles
0.300
0.006
0.004
0.002
No data
Results and Discussion
Substrate (Ti-6Al-4V)
No data
N
3.
0.80
Coefficient of friction
15 to 20 min.
Total thicknesses of DLC, DLC/SCL and DLC/GCL were
fixed at about 1.5 mm, which were measured with a contacting
profilometer, by controlling deposition time. The thicknesses
of the SCL and the GCL were about 0.25 mm.
Hardness of the films was measured with a nanoindentation hardness tester (CSM, NHT) using a Vickers type
indenter under the conditions where maximum load was
5.0 mN and load-unloading velocity was 10 mN/min.
Friction and wear experiments were carried out under dry
air at room temperature using a ball-on-disk type wear tester
(CSM, Tribometer). An alumina ball (radius; 3.2 mm,
Vickers hardness; 14 GPa) was used as a counterpart
material. A normal load of 20 N was applied on top of the
ball. A sliding velocity was 0.2 m/s, number of cycles were
1000 and 60000 (wear distance; 31.4 m and 1884 m, respectively). The coefficient of friction was measured during the
tests. The reproducibility of the measurements was good and
the uncertainty was negligible. After the tests, to calculate
wear volume, measurements of the cross sectional areas of
the wear tracks were conducted at four different places and
the measured values were then averaged. The wear tracks
were characterized with an optical microscope (Keyence,
VHX-100) and an electron probe microanalyzer (EPMA:
Shimadzu, EPMA-1600).
A scratch tester (CSM, Revetest) equipped with a radius of
200 mm diamond indenter was used at the load velocity of
100 N/min to estimate the adhesion of DLC-based films. A
critical load was defined as the load at which the frictional
force began to increase rapidly.21)
Composition and chemical bonding state of the GCL were
analyzed with an X-ray photoelectron spectrometer (XPS:
Shimadzu, ESCA-K1) using MgK radiation of 12 kV20 mA. Ar ion etching for 10 to 250 min at a 10 min interval
was performed for the depth profiling. The analyzing surface
area was approximately 5.0 mm 5.0 mm. The absolute
values for the energy scale were calibrated by referencing
them to the O1s binding energy of O-Si as 531.5 eV.22)
Wear volume, V / mm3
1602
0
DLC
DLC/SCL
DLC/GCL
Substrate
(Ti-6Al-4V)
Fig. 2 Wear volumes of three types of DLC-based films and the Ti-6Al-4V
substrate obtained at 1000 cycles and 60000 cycles.
Formation of Diamond-Like Carbon Based Double-Layer Film on Ti-6Al-4V Substrate by Ionization Deposition
1603
film. Figure 3(b) shows the tracks for the DLC/SCL. Long
and slender scars are observed in some places on the wear
tracks at 60000 cycles. The depth from the surface to the
scars was about 1.3 mm, and the presence of silicon and
carbon was confirmed by the EPMA measurement. Therefore
the ‘‘SCL’’ was exposed in these scars. Thin and shallow
wear tracks were observed for the DLC/GCL at 60000
cycles, and no particle and scar were observed along the wear
tracks, as shown in Fig. 3(c). Since there was no significant
difference of the bonding states in Raman spectra as well as
the surface hardness among the three types of the DLC-based
films, the enhancement of wear resistance for DLC/GCL is
not owing to an improvement of the top DLC layer.
The scratch test was performed in order to investigate the
adhesion strength between each DLC-based film and the Ti6Al-4V substrate. Figure 4 shows the critical loads obtained
by the scratch test, in which average values of three repetitive
measurements and their standard deviations are drawn as
vertical columns and error-bars, respectively. The critical
loads of the DLC/SCL and DLC/GCL were 28 N and 34 N,
which were approximately 2.5 and 3.0 times larger than the
critical load (11 N) of the DLC. As a result, the insertion of
the intermediate layers, especially of this GCL film, was
approved to be fairly effective on the enhancement of
adhesion strength between the top DLC film and the
substrate.
A residual stress value was roughly estimated from the
displacements in the bending curvature of silicon wafer
substrates (34 mm 34 mm 0.5 mm) with the DLC, the
DLC/SCL, and the DLC/GCL film. As a result, the residual
stress was about 5.4 GPa, 2.0 GPa, and 1.4 GPa as compressive, respectively, and the DLC/GCL film was the lowest.
The reduction of this residual stress of DLC/GCL presumably enhanced the adhesion with the substrate. Furthermore,
it is speculated that the reason why the DLC layer of the DLC
film was peeled off in the ball-on-disk wear test is possibly
because the adhesion strength of the DLC layer with the
substrate was insufficient. That is, the excellent adhesion
between the DLC film and the substrate is considered to have
contributed also to the improvement of the wear resistance by
the ball-on disk tests of normal load of 20 N.
Fig. 3 Optical micrographs of the wear track surfaces of three types of
DLC-based films: (a) the DLC at 1000 cycles, (b) the DLC/SCL at 60000
cycles and (c) the DLC/GCL at 60000 cycles.
Figure 3 shows the wear track surfaces of three types of
DLC-based films observed with an optical microscope. In the
case of the DLC shown in Fig. 3(a), wide and deep wear
tracks are formed at 1000 cycles, and the metal surface of Ti6Al-4V is exposed. Many particles are seen in the wear
tracks. The main element of these particles is carbon from the
elemental analysis by EPMA, and they are debris of the DLC
Critical load, L / N
50
40
30
20
10
0
DLC
Fig. 4
DLC/SCL
DLC/GCL
Critical loads of three types of DLC-based films.
T. Mano, O. Sugiyama, Y. Shibuya, H. Nakayama and O. Takai
Ti
Si
O
0
50
100
150
200
250
Etching time, t / min
Intensity (arb.unit)
Ti
Fig. 5 XPS compositional change along the depth direction from surface to
substrate for the GCL film obtained with Ar etching (etching time: 0 to
250 min).
Si-C
C
Si-O
Si
O
C4
Intensity (arb.unit)
C
Ti
silicon content increases simultaneously with increasing
etching time. These contents change in a stepped mode,
corresponding to the intermittent changes in the gas volume
ratio by the 5 min intervals in the deposition process. This
compositional change contributes to relaxation of the
residual stress between the DLC layer and the substrate.
Figure 6 shows the variation of O1s , Ti2p , C1s and Si2p
photoelectron spectra of the GCL with the etching time. At
the etching time of 10 min (near the GCL surface), C1s peak
intensity is highest. After that, it decreases with increasing
etching time, corresponding to the compositional change
shown in Fig. 5. Moreover, the peak shift from ‘‘C3’’ to ‘‘C4’’
was observed with increasing etching time. It shifted from
284.5 to 283.7 eV between the etching times of 10 min and
200 min. The reason of this peak shift will be discussed later.
The intensity of Si2p peak becomes high till the etching time
of about 180 min, corresponding to inversely decrease in that
of C1s . At the same etching time, Ti2p peak appears which
belongs to the substrate. The significant peak shifts of Si2p do
not occur. The binding energy of Si2p peak is close to
100.4 eV of Si-C, a reference value.22) Here, Si was not
bonded to a methyl group since this value much differs from
105.9 eV22) of Si(CH3 )4 . Small O1s peak appears and
disappears being synchronized with Si2p peak. This peak is,
therefore, possibly assigned to O-Si. The deconvolution of
Si2p showed the relative area for Si-O bond (102.2 eV) as
about 1% in the total Si2p area. It is thought that a little
amount of Si-O species also exists in a large amount of Si-C
species. This Si-O probably originated from siloxane bonds,
Si-O-Si, in HMDSO as a source gas.
Figure 7 shows the enlarged C1s spectra at the etching time
of 20 min, 150 min and 200 min paying attention to C1s of
Fig. 6. These peaks were fitted to a deconvolution of a
Gaussian profile.23,24) At the etching time of 20 min, as shown
in Fig. 7(a), the binding energy before the deconvolution of
C1s spectrum is 284.5 eV, and it is very close to 284.4 eV22)
C5
3.2 Analysis of the GCL film
The DLC/GCL on the Ti-6Al-4V substrate was approved
to be fairly effective on the enhancement of wear resistance
and adhesion. In order to clarify the reason of these
improvements, the composition and chemical bonding state
of the GCL were analyzed. Although X-ray diffraction
measurement of this GCL was performed, any peaks except
for the Ti-6Al-4V substrate were not observed. Thus, the
GCL was considered to be amorphous or microcrystalline
structure. From XPS analysis, existence of carbon, silicon
and oxygen was observed in the GCL film. Figure 5 shows
the compositional change along the depth direction from the
film surface to the substrate obtained by changing the etching
time from 0 to 250 min. The carbon content decreases but the
C3
1604
Binding energy, E/ eV
B
Fig. 6
Variation of O1s , Ti2p , C1s and Si2p photoelectron spectra of the GCL with the etching time.
Formation of Diamond-Like Carbon Based Double-Layer Film on Ti-6Al-4V Substrate by Ionization Deposition
these facts into account, the reduction in the residual stress of
the DLC/GCL is due to the changes in these compositions
and chemical bonding states, which results in the improvement of the adhesion with the substrate.
C3
(a)
C1 C2
C4 C5
4.
Intensity (arb.unit)
1605
Conclusion
Formation of the DLC based double-layer film on the Ti6Al-4V substrate by an ionization deposition method was
carried out in order to improve the wear resistance and
adhesion of the film. The DLC/GCL on the substrate was
approved to be fairly effective on the enhancement of wear
resistance and adhesion, comparing with the DLC and the
DLC/SCL. The GCL was characterized by the change in
composition and chemical bonding state. The results led us
that the improvement in adhesion and wear resistance of the
top DLC layer was based on this change in the composition
and chemical bonding state in the GCL. The DLC/GCL
double-layer film is much effective in wide applications of
Ti-6Al-4V for which the use for antifriction components has
been difficult.
(b)
Acknowledgements
(c)
The collaboration of Mr. M. Yoshioka, Shizuoka Industrial
Research Institute of Shizuoka Prefecture, with the XPS
measurements is gratefully acknowledged. The authors
would also like to thank Mr. A. Nishiguchi and Dr. T.
Sumiya, Nanotec Co., for their helpful discussion on this
study.
290
286
282
278
Binding energy, E/ eV
Fig. 7 C1s XPS spectra of the GCL at various etching time: (a) 20 min, (b)
150 min and (c) 200 min.
(C3) of graphite. It is thought that the graphite-rich layer
contributes to the binding strength between the top DLC layer
and the GCL. Moreover, 286.7 eV22) (C1) of C-O bond,
285.5 eV22) (C2) of C-H, and 283.3 eV22) (C4) of C-Si also
exist slightly. It could be separated into two high peaks of C3
at graphite and C4 at C-Si and one low peak of C1 at C-O, as
shown in Fig. 7(b). That is, the composition in the GCL is
probably a mixture of graphite and silicon carbide. In Fig.
7(c), the 281.8 eV22) (C5) peak of the C-Ti bond is newly
indicated in addition to C3, C4 and C1. Accordingly,
existence of titanium carbide implies that the chemically
bonding reaction has occurred between the deposited GCL
and the Ti-6Al-4V substrate.
From the results of the compositional depth profile by the
XPS analysis, the GCL was characterized as the compositional change of silicon from carbon in a stepped mode.
Furthermore, based on the analysis of the chemical bonding
state, it was confirmed that variation of components occurred
from ‘‘graphite’’ to ‘‘silicon carbide + graphite’’ and ‘‘silicon
carbide + graphite + titanium carbide’’, which corresponded
to the positions from the top to bottom of the GCL. Taking
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