microstructure and tribological properties of magnetron sputtered nc-tic/a-c nanocomposite
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www.elsevier.com/locate/surfcoat
Surface & Coatings Technolo
Microstructure and tribological properties of magnetron sputtered
nc-TiC/a-C nanocomposite
Sam Zhanga,*, Xuan Lam Buia, Jiaren Jiangb, Xiaomin Lic
aSchool of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, SingaporebIntegrated Manufacturing Technologies Institute, National Research Council Canada, 800 Collip Circle, London, Ontario, Canada N6G 4X8
cState Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences,
1295 Ding Xi Road, Shanghai 200050, People’s Republic of China
Abstract
In this study, nanocomposite nc-TiC/a-C coatings were deposited on polished stainless steel substrates and silicon wafers by co-sputtering
of graphite and titanium targets at a low temperature of 150 8C. Atomic force microscopy (AFM), X-ray diffraction (XRD), and X-ray
photoelectron spectroscopy (XPS) were used to investigate the morphology and structure of the coatings. The hardness and tribological
properties were assessed using Nanoindentation and Tribometer. The coatings consisted of two phases: nanocrystalline of TiC embedded in
amorphous matrix of a-C. With Ti content of less than 8 at.%, the coating appeared X-ray amorphous. As Ti increased to 16 at.%,
nanocrystalline phase (nc-TiC) was detected. The crystallite size was in the range of 5 to 16 nm as Ti increased from 16 to 48 at.%. The
maximum hardness of 31 GPa was obtained at 36 at.%Ti. In dry tribotests against stainless steel ball, the coefficient of friction was less than
0.24 as Ti was less than 36 at.%, but abruptly increased to 0.39 as Ti reached 42 at.%. Extremely low coefficient of friction of 0.046 was
obtained with oil lubrication.
D 2004 Elsevier B.V. All rights reserved.
Keywords: a-C; Nanocomposite; Hardness; Friction
1. Introduction
Mechanical and tribological properties of hydrogen-free
amorphous carbon (a-C) coatings have been intensively
studied for about three decades. Compared to hydrogenated
amorphous carbon (a-C:H), a-C shows big advantages, such
as higher hardness and elastic modulus, better wear
resistance, higher thermal stability, lower friction in humid
environments, etc. [1]. a-C has been deposited by various
physical vapor deposition (PVD) techniques, such as
magnetron sputtering [2], pulsed laser deposition [3], filtered
cathodic vacuum arc [4], etc. However, drawbacks, such as
low adhesion, high residual stress, and low toughness, limit
the applications of this material. Various methods, e.g., bias
graded deposition [5], graded interface layering [6], doping
of the growing coating with N, F, Si, or metallic elements
0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2004.10.041
* Corresponding author. Tel.: +65 6790 4400; fax: +65 6791 1859.
E-mail address: msyzhang@ntu.edu.sg (S. Zhang).
[7–9], and subsequent deposition and annealing [10,11],
have been developed to overcome these drawbacks.
Carbon-based nanocomposites, where nanocrystalline
phase is embedded into a-C matrix, are considered a new
class of protective material and found wide range of
engineering applications, such as cutting and machining
tools, bearing, pumps, machine and engine parts [12,13].
Carbides [14,15] or carbon-nitrides [16,17] have been used
as the nanocrystalline phase. These coatings exhibited high
hardness, toughness, good wear resistance and very low
friction in ambient air due to the lubricious a-C matrix
[18,19]. In this work, TiC nanocrystalline phase was
embedded in a-C matrix to form nc-TiC/a-C by co-
sputtering of Ti and graphite targets at low temperature
(150 8C). The coatings were characterized and the correla-
tion between constitution, microstructure, and mechanical
properties were analyzed. The tribological properties of
coatings were investigated in both dry and oil lubrication
conditions.
gy 198 (2005) 206–211
Fig. 1. XPS spectra of C 1s at different Ti (at.%).
S. Zhang et al. / Surface & Coatings Technology 198 (2005) 206–211 207
2. Experimental
2.1. Deposition of coatings
The substrates used in this study were Si (100) wafers
(diameter of 100 mm, thickness of 450 Am, surface
roughness of 2 nm Ra) and 440C steel discs (diameter of
55 mm, thickness of 5.5 mm, polished to surface roughness
of 60 nm Ra). The deposition was done at E303A magnetron
sputtering system (Penta Vacuum—Singapore) [20]. The
substrates were ultrasonically cleaned for 20 min in acetone
followed by 10 min ultrasonic cleaning in ethanol prior to
introduction into the deposition chamber, which was then
pumped to a base pressure of 1.33�10�5 Pa. The substrates
were heated to and maintained at 150 8C for 20 min using a
radiation heater to allow outgassing before plasma cleaning
for 30 min at induced RF bias voltage of �300 V to remove
oxides and contaminants on the surface. Graphite (99.999%)
and Ti (99.995%) targets, located about 100 mm above the
substrate, were co-sputtered at the process chamber pressure
of 0.4 Pa with argon flow rate of 50 sccm. The power
density of graphite target was maintained at 10.5 W/cm2 and
that of Ti target changed for desirable compositions. The
substrate was negatively biased at �150 Vand maintained at
temperature of 150 8C during deposition process. The
coating thickness was controlled through deposition time.
2.2. Characterization
The coating thickness was measured using a profilometer
(Dektak 3SJ) through a sharp step created by masking part
of the substrate. The surface morphology of the coatings
was characterized by atomic force microscopy (AFM) used
in the contact mode (Shimadzu SPM-9500J2). The scanning
area was 2�2 Am2. Structure of the coating was investigated
with a Renishaw Raman spectroscope at 633 nm line excited
with a He–Ne laser and X-ray diffraction (XRD, Philips PW
1830) with CuKa X-ray source at a wavelength of 0.15406
nm. Coating chemistry was analyzed with X-ray photo-
electron spectroscopy (XPS) using a Kratos–Axis spectrom-
eter with monochromatic AlKa (1486.6 eV) X-ray radiation
(15 kV and 10 mA) and hemispherical electron energy
analyzer. The base vacuum of the chamber was 2.66�10�7
Pa. The survey spectra in the range of 0–1100 eV were
Table 1
Specification of Shell Helix 15W-50 engine oil
Kinematic viscosity (cSt) Viscosity
index
Density
(15 8C)(kg/m3)
Flash
pointa
(8C)
Pour
pointb
(8C)40 8C 100 8C
141 19.3 156 886 209 �27
a Flash point: the lowest temperature at which the oil gives off enough
flammable vapor to ignite and produce a flame when an ignition source is
present.b Pour point: the lowest temperature at which the oil is observed to flow.
recorded in 1 eV step for each sample followed by high-
resolution spectra over different elemental peaks in 0.1 eV
step, from which the composition was calculated. Curve
fitting was performed after a Shirley background subtraction
by nonlinear least square fitting using mixed Gauss–Lorentz
function. To remove surface contamination layer, Ar ion
bombardment was carried out for 600 s using a differential
pumping ion gun (Kratos MacroBeam) with an accelerating
voltage of 4 keV and a gas pressure of 1.3�10�5 Pa. The
bombardment was performed at an angle of incidence of 458with respect to the surface normal. Hardness measurement
was conducted at a Nanoindenter (XP) with a Berkovich
diamond indenter. The hardness was determined by con-
tinuous stiffness measurement [21]. The indentation depths
were set not to exceed 10% of the coating thickness to avoid
possible substrate effect.
2.3. Tribotest
Tribological tests were carried out using CSEM tribom-
eter with ball on disc configuration in dry (ambient
environment: 75% humidity, 22 8C) and oil lubrication
conditions. The dry tests were conducted at a sliding speed
of 20 cm/s under 10 N while in oil 5 cm/s under 10 N.
Stainless steel (100Cr6) balls of diameter 6 mm were used
as counterpart. Table 1 lists the specification of the
lubrication oil (Shell Helix 15W-50).
3. Results and discussion
3.1. Chemistry, structure, and morphology of coatings
Fig. 1 plots the XPS spectra of C 1s for nc-TiC/a-C
coatings of different Ti content. Fig. 2 shows their XRD
spectra. Ti and C compositions for all the samples (D1
through D9) were calculated from C 1s and Ti 2p XPS
profiles and tabulated in Table 2. The percentage of C
bonding with Ti to form TiC (the balance is C–C bond in a-
C) was calculated from relative area under the Ti–C bonding
Fig. 2. XRD spectra of coatings at different Ti (at.%).
S. Zhang et al. / Surface & Coatings Technology 198 (2005) 206–211208
peak in Fig. 1. That was done by locking the C 1s peaks at
281.8 eV (for TiC) and 284.6 eV (for a-C), fitting the Ti–C
and C–C peaks, taking the ratio of the Ti–C area over the
total area (Ti–C and C–C).
From Fig. 1, at 0 at.%Ti (i.e., 100% a-C), only C–C bond
was observed at 284.6 eV. As the Ti content in coatings
increased, the carbide (TiC) peak at 281.8 eV appeared and
grew while the C–C peak decreased. At low Ti concen-
tration (8 at.%), the formation of nc-TiC was very limited:
only 2% of carbon bonded with Ti. In addition, as indicated
in XRD spectra (Fig. 2), there were no TiC peaks. Thus, as
far as the coating’s structure is concerned, we consider this
coating as Ti-doped a-C and denoted it as a-C(Ti) in Table 2.
At 16 at.%Ti through 42 at.%Ti, the XRD spectra (cf. Fig.
2) clearly indicated formation of TiC. In these coatings, nc-
TiC imbedded in a-C, thus the coating structure was nc-TiC/
a-C. At 48 at.%Ti and higher, almost no C–C bond was
detected (Fig. 1). Therefore, the structure of the coating was
virtually 100% nc-TiC (the bnanoQ nature will be discussed
in the next section).
The crystal orientations of the nc-TiC could be also seen
in Fig. 2. Peaks at 35.9, 41.7, and 60.48 2h were attributed
to (111), (200), and (220) diffraction mode of TiC. As Ti
increased from 16 to 48 at.%, TiC (111), TiC(200), and
TiC(220) all increased. No dominant texturing was
Table 2
XPS results showing the chemical composition and ratio of carbon bonded
in TiC and a-C of coatings
Coatings Ti
(at.%)
C
(at.%)
Percentage of
C bonded to Ti
Coating
structure
D1 0 100 0 a-C
D2 8 92 2 a-C(Ti)
D3 16 84 5 nc-TiC/a-C
D4 25 75 23 nc-TiC/a-C
D5 30 70 42 nc-TiC/a-C
D6 36 64 56 nc-TiC/a-C
D7 42 58 81 nc-TiC/a-C
D8 48 52 100 TiC
D9 53 47 100 TiC
observed. At higher Ti concentration (30 at.% onwards), a
small shift of TiC peaks to the smaller Bragg’s angle was
seen. That was believed to come from the reduction of
amorphous phase in the coating (thus, stress generated in
the deposition process was relaxed less). Although existing,
the stress effect on microstraining (which affects the XRD
peak width) would be small. Ignoring this effect, the
crystallite size of TiC can be estimated using Debye–
Scherrer formula [22].
D ¼ Kkbcos hð Þ ð1Þ
where K is a constant (K=0.91), D is the mean crystalline
dimension normal to diffracting planes, k is the X-ray
wavelength (k=0.15406 nm), b in radian is the peak width
at half-maximum height, and h is the Bragg’s angle.
The calculated crystallite size of TiC is plotted in Fig. 3.
The nc-TiC size increased with the increase of Ti. On
average, TiC (111) crystallites was larger compared to that
of TiC (200). For TiC (111), crystallite size varied from 5 to
16 nm as the Ti increased from 16 to 48 at.%, comparable to
the TiC crystallite size of nanocomposite coatings deposited
by hybrid technique of pulsed laser and sputtering [18].
With increasing Ti, there will be more and more Ti4+ readily
available for the growth of TiC crystallite. At the same time,
as Ti increases, the relative amount of amorphous carbon is
reduced (Fig. 1 and Table 2); thus, the constraints otherwise
exert on the growth of the crystallites are alleviated. It is
also easily understandable that growing on an existing
crystallite becomes thermodynamically easier than to start a
completely new crystal. All these combined to result in the
increase of the nc-TiC size with increasing Ti.
Fig. 4 shows Raman spectra of coatings with different Ti
contents. The Raman peak of a-C can be observed for
coatings of up to 36 at.%Ti. In coatings of 42 at.%Ti, the
amount of a-C was only 11 at.% of the whole sample [58
at.%�(100 at.%�81 at.%), cf. Table 2], not sufficient to
produce a Raman profile; thus, virtually a straight line
Fig. 3. Size of TiC crystallites determined from XRD vs. Ti (at.%).
Fig. 4. Raman spectra of the nc-TiC/a-C nanocomposite coating of different
Ti (at.%).
Fig. 6. Nanoindentation hardness of the nanocomposite coating of different
Ti (at.%).
S. Zhang et al. / Surface & Coatings Technology 198 (2005) 206–211 209
resulted. The broad peak of a-C was deconvoluted into two
peaks termed G-peak (graphite) at 1530/cm and D-peak
(disorder) at 1350/cm. These peaks are characteristic for
amorphous carbon and their intensity ratio ID/IG is inversely
proportional to the sp3 fraction in the coating [23]. The ID/IGratio of coatings containing Ti of 0, 16, 30, and 36 at.% was
1.1, 1.9, 2.3, and 2.6, respectively. The increase of ID/IGindicates more sp2 fraction in the a-C matrix. Tay et al. [8]
also reported the increase of ID/IG, thus the increase of sp2
fraction, as metals were doped into a-C.
Fig. 5 illustrates the surface roughness of coatings as a
function of Ti. With increasing Ti, surface roughness of the
nc-TiC/a-C coating increased and the roughness accelerated:
at 8 at.%Ti, Ra increased only half a nanometer from that of
a-C (from 3.4 to 3.9 nm); at 25 at.%Ti, Ra increased
considerably to 6.1 nm and reached 15.8 nm at 48 at.%Ti.
From XPS, XRD, and Raman results, it is clear that more Ti
resulted in more crystalline phase with larger grain size,
which contributed to the increase of the surface roughness.
Fig. 5. Surface roughness of the nc-TiC/a-C nanocomposite deposited on Si
wafer.
3.2. Hardness and tribological properties
Hardness of the coatings is summarized in Fig. 6. Pure a-
C coating had the highest hardness of 32 GPa. As Ti
increased, the coating hardness decreased and then
increased owing to different mechanisms: as discussed
afore, at low Ti content, the addition of Ti in the coating
only serves as bdopingQ (virtually no TiC formation).
However, this doping resulted in increase of sp2 fraction
in the a-C matrix (cf. Fig. 4), which resulted in decrease in
hardness. Starting from 16 at.%Ti, nc-TiC formed and the
amount increased with Ti (cf. Table 2 and Fig. 2), resulting
in the recovery of hardness to 31 GPa at 36 at.%Ti after
offsetting the effect of the high sp2 fraction in the matrix. As
Ti further increased from above 36 at.%, the coating
hardness decreased as a result of grain coarsening. The
range of Ti from 30 to 42 at.% allows the production of nc-
TiC/a-C coatings with hardness from 27 to 31 GPa, which is
adequate for most tribological applications.
Fig. 7 shows the coefficient of friction of coatings sliding
against steel ball in dry condition. The coefficient of friction
Fig. 7. Coefficient of friction in dry tribotests of the nc-TiC/a-C
nanocomposite coating of different Ti (at.%).
Fig. 8. Coefficient of friction of uncoated steel substrate, a-C, and nc-TiC/
a-C (30 at.%Ti) in oil lubrication condition.
S. Zhang et al. / Surface & Coatings Technology 198 (2005) 206–211210
increased as more Ti was doped. The coefficient of friction
increased from 0.15 to 0.24 as the Ti increased to 36 at.%.
The a-C matrix contributes much to the decrease in friction
through formation of graphite-rich layer to act as solid
lubricant in humid air between the coating and the counter-
part [19,20,24]. As more Ti was doped in the coating, more
TiC crystallites formed; thus, less a-C matrix was in direct
contact with the wearing ball. Understandably, higher
coefficient of friction resulted. Another reason causing the
increase in friction was the increase of surface roughness
due to the increase of the amount and the size of TiC
crystallites. A drastic increase in coefficient of friction (from
0.24 to 0.39) was experienced as Ti content increased from
36 to 42 at.%. As discussed before, at 42 at.%Ti, the coating
contained only 11 at.% a-C (the balance being nc-TiC); thus,
the coating behaved more like TiC instead of a-C. There-
fore, after the hike, the increase in coefficient of friction did
not experience further quantum increase.
Fig. 8 shows the coefficient of friction of nc-TiC/a-C (30
at.%Ti), a-C, and uncoated steel substrate when sliding
against steel ball in oil lubrication. Different from the
nonlubricated contact, in this case, the oil prevented the
formation of graphite-rich layer; therefore, the friction
mechanism is changed. The friction strongly depends on
the surface roughness, the thickness, and stability of the oil
film between the two sliding surfaces. The stable value of
coefficient of friction obtained from this test were 0.046,
0.053, and 0.107 for nc-TiC/a-C (30 at.%Ti), a-C, and
uncoated steel, respectively. Compared to uncoated sub-
strate, of which the surface roughness was as high as 60 nm,
substrates coated with a-C or nc-TiC/a-C exhibited more
than two times less friction in oil lubrication condition.
Between a-C and nc-TiC/a-C, due to the rougher surface of
nc-TiC/a-C (7.3 nm Ra compared to 3.4 nm Ra of a-C; cf.
Fig. 5), the coefficient of friction of the nanocomposite was
higher than that of a-C at the beginning (below 0.5 Km). As
the running-in phase is over, the nanocomposite coating
became less frictional than a-C because of its better affinity
to oil (owing to the metal in the coating) which resulted
more stable oil film in between the coating and the
counterpart.
4. Conclusion
Nanocrystalline TiC was embedded in a matrix of a-C to
form nanocomposite nc-TiC/a-C coating through co-sputter-
ing of graphite and Ti targets at low deposition temperature
(150 8C). Formation of the nc-TiC was directly related to the
Ti content. At low Ti content, the coatings were X-ray
amorphous, and no appreciable amount of nc-TiC was
formed. The effect was a doping of Ti in a-C [i.e., to form a-
C(Ti)]. That resulted in hardness drop from 32 to 19 GPa
because of the increase in sp2 fraction. From 16 to 36 at.%Ti,
nanocrystalline TiC formed with increasing crystallite size
and amount, resulting in hardness recovery back to 31 GPa
after offsetting the effect of increase in crystallite size and sp2
fraction. As Ti further increased from above 36 at.%, the
coating hardness decreased as a result of grain coarsening.
In dry tribotest, an abrupt increase in coefficient of friction
occurred at 42 at.%Ti because the amount of a-C was no
longer sufficient in providing an effective graphite-rich layer
at the tribosurface. With too much Ti, the coating behaved no
longer like nanocomposite but nanosized polycrystalline
TiC. Extremely low coefficient of friction of 0.046 was
obtained with nc-TiC/a-C in oil lubrication condition.
nc-TiC/a-C coatings show big potential for engineering
applications as it exhibited high hardness, thus, wear
resistance and low friction in both lubricated and non-
lubricated conditions. The properties of the coatings can be
easily modified through changing the microstructure as Ti
content is varied.
Acknowledgement
This work was supported by Nanyang Technical Uni-
versity’s research grant RG12/02.
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