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Diamond & Related Material

Thermal stability of diamond-like carbon films deposited by plasma based

ion implantation technique with bipolar pulses

Junho Choi *, Soji Miyagawa, Setsuo Nakao, Masami Ikeyama, Yoshiko Miyagawa

Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora,

Shimoshidami, Moriyama-ku, Nagoya, Aichi 463-8560, Japan

Available online 27 December 2005

Abstract

Silicon incorporated diamond-like carbon (DLC) films were deposited using a bipolar-type plasma based ion implantation technique, and the

effect of the positive pulse voltage on the thermal stability of the DLC films was investigated. The positive pulse voltage was varied from 2.0 to

6.3 kV while the negative pulse voltage was maintained at �5.0 kV. The deposited DLC films were annealed in air for 1 h at constant

temperatures of 500 and 600 -C. As a result, an optimum positive pulse voltage exists for the high deposition rate and thermal stability of the DLC

films, which correlates with the surface temperature during the film deposition. The DLC film deposited at a positive pulse of 4.0 kV, whose

surface temperature during deposition was 300 -C, showed a typical DLC Raman spectrum even though the annealing temperature increased to

600 -C, and exhibited good friction properties. This high thermal stability is attributed to the effect of pre-annealing during the deposition and the

formation of a stable and thick silicon oxide layer on the DLC surface when annealed in air. On the other hand, the thermal stability of the DLC

film deposited at 2.0 kV, whose surface temperature during deposition was 150 -C, quickly deteriorated when the film was annealed at 600 -C.D 2005 Elsevier B.V. All rights reserved.

Keywords: Diamond-like carbon; Thermal stability; Friction; Bipolar-type PBII

1. Introduction

Diamond-like carbon (DLC) films have attracted much

interest in the past three decades due to their high hardness,

chemical inertness, extremely low friction and high wear

resistance. In spite of this, thermal degradation of the DLC

films, i.e., graphitization and hydrogen effusion limits their

high temperature application. It has been reported that DLC

films maintain stable properties up to about 400 -C while the

graphitization of the films starts above this temperature [1]. It

is required that DLC films should endure up to 600 -C under

air conditions to achieve high temperature applications such

as mold coating for magnesium or aluminum extrusion.

Though numerous studies have been conducted to investigate

the thermal stability of DLC films, most of them treated the

thermal properties of DLC films in a vacuum environment

[2–4].

In the present study, DLC films were deposited by a bipolar-

type plasma based ion implantation (PBII) technique, and the

0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.diamond.2005.11.013

* Corresponding author.

E-mail address: junho.choi@aist.go.jp (J. Choi).

effect of the positive pulse voltages on the thermal stability of

the DLC films in air was investigated.

2. Experiments

A bipolar-type PBII system [5] was used for deposition of

the DLC films on steel and Si substrates. The steel substrates

were used for the friction measurements. The deposition

process of the DLC films is as follows: (1) the substrate

surfaces are sputter-cleaned with an Ar plasma to remove any

organic contaminants and oxide layer, (2) the substrates are

irradiated with nitrogen ions using N2 plasma to harden the

surfaces, (3) carbon ions using CH4 plasma are implanted into

the substrates using a high negative pulse voltage, (4) a SiC

layer is deposited using tetramethylsilane plasma to improve

the adhesion between the substrate and DLC film, and then (5)

the DLC films are deposited under the following conditions—

precursor gas: a mixture of toluene/tetramethylsilane; deposi-

tion pressure: 0.1 Pa; positive pulse voltages: 2.0, 4.0, 4.5 and

6.3 kV; negative pulse voltage: �5 kV; pulse frequency: 4 kHz.The deposited DLC films were annealed in air for 1 h at

constant temperatures of 500 and 600 -C.

s 15 (2006) 948 – 951

www.els

800 1000 1200 1400 1600 1800 2000

Raman Shift (cm-1)

(c) annealed at 600 °C

4.0 kV

(b) annealed at 500 °C

Co

un

ts (

arb

. un

its)

2.0 kV

4.5 kV

6.3 kV

2.0 kV

4.0 kV

4.5 kV6.3 kV

(a) as-deposited4.0 kV2.0 kV

4.5 kV

6.3 kV

Fig. 1. Raman spectra of DLC films as deposited and annealed at 500 and

600 -C.

J. Choi et al. / Diamond & Related Materials 15 (2006) 948–951 949

The composition and microstructure of the DLC films were

investigated using X-ray photoelectron spectroscopy (XPS),

elastic recoil detection analysis (ERDA) and Raman spectros-

copy. The hardness of the films was measured by a

nanoindentor. The friction coefficients were measured by a

ball-on-flat type reciprocal friction tester. The applied load, the

reciprocal sliding distance, the sliding speed, and diameter of

the steel balls were 0.98 N, 8 mm, 0.5 Hz, and 3 mm,

respectively. We corrected the last sentence in the Experiments

section as follows: The film thickness and substrate curvatures

were measured with a microstylus profilometer. From the

measured curvatures, the internal stress was determined using

Stoney equation. The temperature of the steel substrates during

the DLC film deposition was monitored using an infrared

thermometer.

3. Results and discussion

3.1. Deposition of the DLC films

The properties of the DLC films deposited at various

positive pulse voltages are shown in Table 1. The surface

temperature during the film deposition linearly increases with

the increasing positive pulse voltage due to enhanced electron

bombardment. The hardness and internal stress are gradually

reduced with the increasing positive pulse voltage due to

graphitization of the DLC films, that is, the G band shifts to

higher wavenumbers and the broad shoulder D band is

enhanced as the positive pulse voltage increases as shown in

the Raman spectra (Fig. 1(a)). Though the DLC films exhibit

an enhanced graphitization with the increasing positive pulse,

the deposited DLC films show reasonably a high hardness and

very low internal stress. The atomic concentration of Si

decreases as the positive pulse increases, whereas the atomic

concentration of C increases. This may be due to the difference

in the dissociation energy of the tetramethylsilane and toluene,

as described by Bhusari and Kshirsagar [6].

3.2. Properties of the DLC films after thermal annealing

Raman spectra of the DLC films annealed at 500 and 600 -Cin air are also shown in Fig. 1(b) and (c). It is apparent that the

graphitization of the DLC films is further enhanced due to the

annealing. On the other hand, the DLC film deposited at 4.0 kV

still exhibits a typical DLC spectrum even though the annealing

temperature was increased to 600 -C. It should be noted that

the Raman spectrum of the DLC film deposited at 2.0 kV

shows an asymmetric single peak when the film is annealed at

Table 1

Properties of as-deposited DLC films

Positive pulse (kV) Surface temp. (-C) Hardness (GPa)

2.0 150 24.59

4.0 300 23.47

4.5 450 19.92

6.3 550 22.18

500 -C, but spectrum measured after annealing at 600 -C is

drastically changed.

Fig. 2(a) shows the thickness change of the DLC film before

and after annealing at 500 -C. The film thickness is the

averaged values of every 4-measurement. The experimental

error was less than T60 nm. The film thickness (i.e., the

deposition rate) increases with the increasing positive pulses up

to 4 kV, and decreases as the positive pulse further increases to

6.3 kV. The deposition rate is dependent on the plasma density

and the atomic mobility on the substrate. The increased

positive pulse increases the plasma density near the substrate

surfaces, resulting in a high deposition rate, whereas the high

atomic mobility due to too high a temperature can lead to

reflection of the atoms from the substrate and the deposition

rate eventually decreases [7]. After annealing at 500 -C, thefilm thickness slightly decreases over the positive pulse range.

Fig. 2(b) shows the change of the H concentration in the

films before and after annealing. As the positive pulse voltage

increases to 6.3 kV, the H is drastically effused due to a high

surface temperature of 550 -C during the deposition.

3.3. Friction and wear properties

Fig. 3 shows the friction coefficients of the DLC films

before and after annealing. The as-deposited DLC films show

Internal stress (GPa) Composition (at.%)

C Si H

0.434 62.4 17.3 20.3

0.184 62.2 14.2 23.6

0.093 66.1 11.2 22.6

0.105 75.8 12.4 11.8

0 100 200 300 400 500

0

20

40

60

80

Etching Time (sec)

Ato

mic

% o

f C

0

20

40

60

Ato

mic

% o

f O

0

20

40

60

(c)

(b)

2.0 kV

4.0 kV

6.3 kV

Ato

mic

% o

f S

i

(a)

600

Fig. 4. Depth profiles of atomic concentration in the 500 -C-annealed-DLCfilms measured by XPS. The etching was conducted using Ar+ ions.

2 3 4 5 6 710

15

20

25

Ato

mic

% o

f H

Positive Pulse (kV)

0

1

2

3

(b)

as-deposited

annealed at 500 °C

as-deposited

annealed at 500 °C

Film

Th

ickn

ess

(µm

)

(a)

Fig. 2. Changes in (a) film thickness and (b) H concentration before and after

annealing at 500 -C.

J. Choi et al. / Diamond & Related Materials 15 (2006) 948–951950

low friction coefficients of less than 0.1. For the DLC films

annealed at 500 -C, the friction coefficients maintain a low

value of less than 0.1 up to a positive pulse of 4.5 kV, where the

friction coefficient increases to 0.14 as the positive pulse

increases to 6.3 kV. These results are related to both the

enhanced graphitization and formation of a silicon oxide (SiOx)

layer on the DLC surfaces by annealing. Fig. 4 shows the depth

profiles of the atomic concentration in the 500 -C-annealed-DLC films measured by XPS. It is observed that the C

concentration on the annealed DLC surfaces is very low and

the surfaces are covered with SiOx. The thickness of the SiOx

layer on the DLC film deposited at 4.0 kV is much thicker than

2 3 4 5 6 70.0

0.2

0.4

0.6

Fri

ctio

n C

oef

fici

ent

Positive Pulse (kV)

as-deposited

annealed at 500 °C

annealed at 600 °C

Fig. 3. Friction coefficients of DLC films before and after annealing.

that on the DLC film deposited at 6.3 kV as shown in Fig. 4.

Since the thickness of the SiOx layer would be directly related

to the amount of originally incorporated Si, this result is

understandable (refer Table 1). On the other hand, the SiOx

thickness of the DLC film deposited at 2.0 kV is slightly thin

compared to that deposited at 4.0 kV (i.e. as shown in Fig. 4,

the oxygen concentration decreases to zero after etching for

about 450 s in the case of 4.0 kV, whereas the oxygen

concentration in the film deposited at 2.0 kV decreases to zero

after etching for about 350 s.) despite the fact that the Si

concentration of the former is greater than that of the latter as

shown in Table 1.

In general, DLC film incorporating much hydrogen results

in a weaker matrix against wear, causing a higher friction

coefficient, whereas the incorporation of the small amount of

hydrogen results in a denser structure, thus improving the

friction and wear properties [8]. However, our friction results

show that the DLC films deposited at 4.0 kV has a stable and

low friction in spite of the higher H concentration compared to

other films. This result is attributed to a thick and stable SiOx

layer formed on the DLC films as mentioned above. The SiOx

debris created by a rubbing motion is transferred to the steel

ball surface without rupture of the thick SiOx layer on the DLC

surfaces. The sliding between the silicon oxide surfaces results

in a low friction coefficient [9]. The transferred layer of SiOx

on the steel ball was confirmed by Electron Probe Micro-

J. Choi et al. / Diamond & Related Materials 15 (2006) 948–951 951

Analysis (EPMA) measurements. However, we do not know

why the DLC film deposited at 4.0 kV represents a thicker SiOx

layer compared to the DLC film deposited at 2.0 kV. It seems

that this is related to the H concentration, but further study is

necessary to clarify this.

As the annealing temperature increases to 600 -C, the frictioncoefficients of the DLC films deposited at 2.0, 4.5 and 6.3 kV

abruptly increase due to rupture of the DLC films, whereas the

DLC film deposited at 4.0 kV still shows a low friction. This

high thermal stability is attributed to the effect of pre-annealing

at 300 -C during deposition process and the formation of a stable

and thick silicon oxide layer on the DLC surface mentioned

above. Also, the thick and stable SiOx layer possibly inhibits H

from effusing. On the other hand, it is not understood why the

friction coefficient of the DLC film deposited at 2.0 kV is

drastically changed. One possible reason is degradation of the

films due to the explosive effusion of H and then graphitization

as indicated by the drastic change of the Raman spectrum (Fig.

1). For the DLC film deposited at 2.0 kV, the contribution of the

pre-annealing to the friction properties is not expected due to low

deposition temperature during the deposition process.

4. Conclusions

The DLC films were deposited using a bipolar-type PBII,

and the effect of the positive pulse voltage on the thermal

stability of the DLC films was investigated. An optimum

positive pulse voltage exists for the high deposition rate and

thermal stability of the DLC films, which correlates with the

surface temperature during the film deposition. The DLC film

deposited at a positive pulse of 4.0 kV (its surface temperature

during deposition is 300 -C) shows a typical DLC Raman

spectrum and a low friction even though the annealing

temperature increases to 600 -C. This high thermal stability

is attributed to the formation of a stable and thick SiOx layer on

the DLC surface and pre-annealing during the deposition

process. On the other hand, the thermal properties of the DLC

film deposited at 2.0 kV abruptly deteriorated when the film is

annealed at 600 -C, which is due to the explosive effusion of H

and then graphitization. For the DLC film deposited at 2.0 kV,

the contribution of the pre-annealing to the friction properties is

not expected due to the low deposition temperature during the

deposition process.

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