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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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