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Thin Solid Films 475

Amorphous carbon and carbon nitride multilayered films prepared by

shielded arc ion plating

K.H. Leea,*, R. Ohtaa, H. Sugimuraa, Y. Inoueb, O. Takaia,c, H. Sugimurad

aDepartment of Materials Processing Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, JapanbResearch Center for Nuclear Materials Recycle, Nagoya University, Nagoya 464-8603, Japan

cCenter for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8603, JapandDepartment of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan

Available online 11 September 2004

Abstract

Multilayered films consisting of amorphous carbon (a-C) and carbon nitride (a-CN) have been prepared by shielded arc ion plating

(SAIP). Hardness and wear resistance of the multilayered films were measured with a nanoindenter interfaced with an atomic force

microscope (AFM). Friction coefficients of the multilayered films were determined as well against a SUJ2 (SAE 52100) bearing ball

using a ball-on-disc tribo-tester. The a-CN films deposited on a hard a-C film prepared at Vb=0 or �300 V were harder than the single

a-CN films directly deposited on the silicon substrate. The hardness of these layered samples was 4 GPa greater than that of the single

layers. The hardness of a-C (60 nm)/a-CN (60 nm, Vb=�300 V)/a-C (120 nm) triple layer film was 5 GPa higher than that of the

single layered a-C. The wear resistance of the all layers was better than sapphire, although sapphire is harder than the single and

double layers. The friction coefficients of the triple layers were low and stable. The triple layer showed the lowest friction coefficient

of about 0.1.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Amorphous carbon (a-C); Amorphous carbon nitride (a-CN); Multilayer; Shielded arc ion plating (SAIP); Nanoindentation

1. Introduction

Amorphous carbon (a-C) and carbon nitride (a-CN) films

have become widely employed as protective hard coatings

in the last few decades due to their excellent mechanical

properties. These films are practically applicable to many

tribological and mechanical applications including magnetic

hard disks, microelectromechanical systems, biomedical

implants, cutting tools, molds and bearings [1–9]. Although

these films have the outstanding mechanical properties, they

have some disadvantages in utilization as hard protective

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.07.016

* Corresponding author. Current address: Department of Materials

Science and Engineering, Graduate School of Engineering, Kyoto

University, Sakyo-ku, Kyoto 606-8501, Japan. Tel.: +1 81 75 753 9130

fax: +81 75 753 9130.

E-mail address:

kyung-hwang.lee@materials.mbox.media.kyoto-u.ac.jp (K.H. Lee).

;

coating materials because of their exfoliation [10]. In the

case of hard a-C with a high sp3 fraction, thick films were

difficult to be prepared owing to their high compressive

internal stress, which caused readily delamination of the

films. Although the internal stress decreases when nitrogen

is incorporated into the a-C film, its hardness decreases

simultaneously. According to Goiti et al. [11], an a-C film

consisting of layers with low and high internal stresses

deposited using alternative (positive/negative) bias voltages

by a sputtering method had a totally reduced internal stress

while remaining other excellent mechanical properties.

Moreover, mechanical properties of an a-CN film were

improved by forming a multiplelayer as well [12]. Here we

report on the multilayered films prepared through an

alternative deposition of low and high internal stress films

by shielded arc ion plating (SAIP), in order to obtain a thick

film with lower total internal stress and satisfactory

mechanical properties. Hardness and wear resistance of

(2005) 308–312

Fig. 1. Schematic diagram of the shielded arc ion plating (SAIP) apparatus.

K.H. Lee et al. / Thin Solid Films 475 (2005) 308–312 309

the multilayered films were measured by a nanoindentation

system. Friction coefficients of the films were determined

using a ball-on-disc tribo-tester.

2. Experimental details

Multilayered films composed of amorphous carbon (a-C)

and carbon nitride (a-CN) were prepared by shielded arc ion

plating (SAIP) system (Nissin electric, MAV-15.2N) using a

graphite target (Toyo Tanso IG510, ash; 10 ppm, a 64�32

mm). An n-type silicon (100) substrate of 400 Am thick was

ultrasonically cleaned in acetone and methanol in that order

before loading into the vacuum chamber. The basic pressure

of the chamber was evacuated below a pressure of

2.3�10�3 Pa. Prior to deposition, ion sputter cleaning was

applied for 10 min at argon or nitrogen gas pressure of 10 Pa

with a substrate bias (Vb) of �700 V. These gases with a

purity of 99.999% were introduced into the chamber

through a mass flow controller. The dc arc current was set

at 60 A. Gas pressures of argon and nitrogen were fixed at 1

Pa for depositing a-C and a-CN films, respectively.

Fig. 2. Schematic of the layer structures for

A residual stress in each layer was calculated from the

curvature radii of a bare Si substrate and that after deposited

with the film. The thickness and curvature radii were

determined by a stylus profilometer (Mitutoyo SV-600).

Hardness and wear depth of the layers were measured by a

nanoindention (Hysitron, TriboScope) interfaced with an

atomic force microscope (AFM, JEOL, JSPM–4210) using a

diamond tip (Berkovich type: 65.38 of half angle). The

hardness was determined a load–unload curve with from a

peak load force of 500 AN. The wear resistance was

evaluated on the basis of wear depth of the top layer. The

tip scanning was repeated 30 cycles in a 1-Am2 area at a

scanning rate of 2.8 Am/s and a load force of 30 AN. Ball-on-disc tribo-tests were conducted under a load of 5 N and a

sliding speed of 100 mm/s using a SUJ2 (SAE 52100) ball as

a partner material. A diameter of the ball was 6 mm and the

total sliding distance was 300 m with a 2-mm rotation radius.

3. Results and discussion

3.1. Film deposition

Carbon macroparticles (0.1–100 mm) were emitted from

the target due to arc discharge. These macroparticles are not

normally ionized and do not react fully with nitrogen

species. In order to interrupt the deposition of macro-

particles in the film, a shielding plate made of stainless steel

was set between the target and the substrate as shown in Fig.

1. The distance between the target and the shielding plate

was 120 mm and that between the plate and the substrate

was 40 mm. The substrate temperature was not maintained

constant in this experiment. Single layers were deposited on

the Si substrate. Fig. 2 shows the schematic of the layer

structures for the a-C and a-CN multilayered films. One was

the a-C film prepared under Vb=�100 V (Layer 3) and two

were the a-CN films prepared under Vb=0 (Layer 1) and

�300 V (Layer 2). Double layers were fabricated by

depositing a top layer on ground (Layer 4) or Vb=�300 V

(Layer 5) under nitrogen arc plasma on Layer 3. Triple

layers were fabricated by depositing an a-C film at Vb of

the a-C and a-CN multilayered films.

Table 1

The layer structure, bias voltage (Vb), thickness, compressive internal stress, and of a-C and a-CN multilayered films

Sample

number

Surface

film type

Layer structure Vb (-V) Thickness

(nm)

Internal stress

(-GPa)

Sample

name

No. 1 a-CN a-CN/Si Grounded 140/Si 1.4 Layer 1

No. 2 a-CN a-CN/Si 300 130/Si 2.4 Layer 2

No. 3 a-C a-C/Si 100 120/Si 10.3 Layer 3

No. 4 a-CN a-CN/a-C/Si Grounded/100 80/120/Si 6.9 Layer 4

No. 5 a-CN a-CN/a-C/Si 300/100 60/120/Si 6.5 Layer 5

No. 6 a-C a-C/a-CN/a-C/Si 100/Grounded/100 60/80/120/Si 7.6 Layer 6

No. 7 a-C a-C/a-CN/a-C/Si 100/300/100 60/60/120/Si 13.8 Layer 7

K.H. Lee et al. / Thin Solid Films 475 (2005) 308–312310

�100 V on Layers 4 or 5. These triple layers are expressed

as Layers 6 and 7, respectively. Layer 3 was the hardest film

and also had the greatest internal compressive stress [13].

Layers 1 and 2, that is, a-CN films directly deposited on the

Si substrates, were relatively soft and hard among the a-CN

films in our study [14]. Internal stresses of Layers 1 and 2

were about four and seven times lower in comparison with

that of Layer 3. In order to obtain a thick film with a lower

internal stress, the multilayered films were prepared through

an alternative deposition of low and high internal stress

films. Marechal et al. [15] measured the stress of silver films

on Si substrates using the following equation:

r ¼ Es

6 1� msð Þt2stf

1

Rfs

� 1

Rs

��

where Rs and Rfs are the radii of curvatures for the substrate

and the film, respectively. Here ts is the substrate thickness

Fig. 3. Hardness (a) and a relative wear depth to sapphire (b) of the layers.

and tf is the film thickness. Young’s modulus and Poisson’s

ratio of substrate are represented as the Es and ms. The layerstructure, bias voltage, thickness, internal stress, and hard-

ness of the a-C and a-CN multilayered films are summarized

in Table 1.

Fig. 4. Wear patterns on layers and sapphire.

Fig. 5. Friction coefficients of the layers.

K.H. Lee et al. / Thin Solid Films 475 (2005) 308–312 311

3.2. Mechanical properties

The film exfoliation was frequently observed when the

film having a high internal stress became to some extent in

our previous study [16]. However, that was not found in this

study. The internal stress of Layer 4 is 5.5 GPa greater than

that of Layer 1. The stress of Layer 5 is 4.1 GPa greater than

that of Layer 2. However, the total compressive internal

stress of the triple layer; that is, Layer 6 is lower than that of

the single, i.e., Layer 3, although the thickness of the triple

layer is two times. Fig. 3(a) shows hardness of the layers.

The hardness of the layers is ranging from 10 to 46 GPa. In

the a-CN films, those are directly deposited on silicon

substrate, Layer 2 applied a proper Vb=�300 V became

harder than Layer 1. Layers 4 and 5 of double layers, those

are deposited the a-CN films on the top layer, are 3.4 and

3.8 GPa harder than Layers 1 and 2, respectively, owing to

the insertion of the superhard a-C film beneath each of the a-

CN films. The single a-C film, i.e., Layer 3, shows a

hardness of 42 GPa. The triple layers, i.e., Layers 6 and 7,

are harder than Layer 3, although the intermediate a-CN

films are softer than Layer 3. This means that mechanical

properties of all layers are obtained without influences of Si

substrate and intermediate a-CN films.

It is very difficult to estimate an absolute wear depth

value because the diamond probe also gradually wears down

when used repeatedly. Thus, we evaluate wear resistance by

comparing wear depths of a sample and sapphire. Fig. 3(b)

shows relative wear depths of the samples to sapphire. Fig. 4

shows wear patterns on the samples and sapphire produced

by the diamond tip pressed into the surfaces at a load force

of 30 AN. The tip-scanning was conducted 30 cycles at a

scanning rate of 2.8 Am/s. The wear resistance of the

samples was quantified in terms of the depth of the wear

pattern determined from the AFM images. The wear

resistance of all the samples is better than sapphire and

fused quartz. These sapphire and fused quartz have high

hardness about 35F5 and 9.5F0.5 Gpa, respectively. Layers

2 and 5, of which top layers are the a-CN films prepared by

supplying proper �Vb, showed improved wear resistances

in comparison with Layers 1 and 4, i.e., the a-CN films

prepared while grounded. However, the wear depths of the

double layers, i.e., Layers 4 and 5, are deeper than the single

layer samples, i.e., Layers 1 and 2, although the hardness of

Layers 4 and 5 is harder than Layers 1 and 2, respectively.

Moreover, the wear resistance of sapphire is poorest,

although sapphire has harder than the single and double

layers. In addition, the wear depths of Layers 6 and 7 are

deeper than Layer 3. Effects of hardness on wear resistance

are not straightforward.

The results of friction characteristics show somewhat

different compared with the wear resistance. Fig. 5 shows

the results of friction coefficient of the samples. The friction

coefficient of a bare silicon substrate hastily increases about

0.8 at the start of the test. The friction coefficients of Layers

1 and 2 abruptly increase at a sliding distance about 180 m.

On the contrary, the friction coefficients of Layers 4 and 5

are stable through the test. The friction coefficients of Layer

3 are unstable and slightly high at a start of test. The friction

coefficients of Layers 6 and 7 are clearly low and stable as

average friction coefficient of 0.1. Layer 6 shows the lowest

friction coefficient. This sample showed a lower compres-

sive internal stress than Layer 7.

4. Conclusion

In order to obtain a thick film with a lower internal stress

and sufficient mechanical properties, we fabricated multi-

layered films by means of the alternative deposition

technique of low and high internal stress layers. The

hardness of the a-CN films improved when that composed

the double layer compared with the film directly deposited

on the substrate. The hardness of the a-C film in the triple

layer was also harder than the single a-C film. The wear

resistance of all the samples was better than sapphire, in

spite of the fact that sapphire was harder than the single and

double layers. We understood that the hardness was not

directly related to wear resistance, although hardness greatly

governed the improvement of wear resistance. The friction

coefficients of double layered a-CN films were better than

those of the single layers due to their thickness. The triple

layer showed the lowest friction coefficient. The multilayer

systems fabricated by SAIP were found to show the better,

mechanical characteristics than the single layers directly

deposited on the substrates. The results shown in this paper

K.H. Lee et al. / Thin Solid Films 475 (2005) 308–312312

will give crucial information in order to design and fabricate

practically applicable tribological thin films.

Acknowledgements

This work was supported by a Grant-in-aid for Scientific

Researches of the Ministry of Education, Science, Sports

and Culture of Japan. The authors would like to thank Dr.

Kazuki Kawata (Oriental Engineering) for conducting the

ball-on-disc tests.

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