Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA12 / P24
Hard Coating of Nanodiamond Composite Films Deposited on Silicon Substrate byCoaxial Arc Plasma Deposition
Ali M. Ali1,2, Mohamed Egiza1,3, Koki Murasawa4, Yasuo Fukui4, Hidenobu Gonda4, Masatoshi Sakurai4, and Tsuyoshi
Yoshitake1
1
Dept. of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka, 8168580, Japan,
2
Dept. of Physics, Al-Azhar University, Cairo 11884, Egypt,3
Dept. of Mechanical Engineering, Kafrelsheikh University, Kafrelsheikh 33516, Egypt
4OSG Corp. 2-17 Shirakumo-cho, Toyokawa, Aichi 442-0018, Japan
Email: abdelgawad_ali@kyudai.jp
Abstract: Nanodiamond Composite (NDC) films deposited on Si substrates with smooth surfaces by coaxial arc plasma
deposition are easily peeled off with increasing film thickness and enhancing hardness of the films. To enlarge the film
thickness at which the films are not peeled off, we suggested to employ NDC buffer layers deposited at a substratetemperature of 550 °C. It was demonstrated that NDC films with thicknesses of more than 1.5 µm, whose hardness andYoung’s modules are 55 and 438 GPa, can be deposited at room temperature. This thickness is as least 4 time larger thanthe maximum thickness of comparably hard hydrogen-free diamond-like carbon films deposited at low temperatures byarc ion plating and is comparable with that of ultrananocrystaline diamond films deposited at substrate temperatures ofhigher than 500 °C by CVD. The NDC buffer layers might play roles in enhancing the adhesion strength due to atomic
interdiffusion at interfaces with the Si substrates and in releasing the internal stress of the upper hard NDC films.
Keywords: Nanodiamond; hard coating; coaxial arc plasma; buffer layer.
1. INTRODUCTION
Nanodiamond Composite (NDC) films [ 1 , 2 ] havereceived much attention since they possess physicalproperties similar to those of diamond and relatively
smooth surfaces [3]. They are promising candidates for avariety of applications such as electrical devices,
detectors, hard coating, microelectromechanical systems(MEMS), and nanoelectromechanical system (NEMS)[4,5]. MEMS and NEMS devices are currently fabricatedon Si substrates, and they are used in applications
involving bending and flexural motion, such as cantileveraccelerometers and vibration sensors [6]. Although Si
substrates are the core of the devices, they have poor
mechanical and tribological properties. To improve theirproperties, hard coating with diamond and related hardmaterials are beneficial to Si substrates in theapplications. In our previous studies, the formation of
NDC films by coaxial arc plasma deposition (CAPD) onSi substrates at a substrate temperature of 550 °C with athickness of approximately 0.4 µm was carried out andtheir hardness and Young’s modulus were at most 23 and
184 GPa, respectively [7].
To increase the thickness at which films can bedeposited without peeling off from substrates, anenhancement in the adhesion strength between films andsubstrates and the release of the internal stress generated
in films are the main key factors. Generally, the internal
stress in films increases with increasing hardness.
In this study, NDC buffer layers deposited
at a substrate temperature of 550 °C were employedfor hard NDC films deposited at different substratetemperatures, and the effects of the buffer layer insertion
on the film thickness at which the NDC films can bedeposited without peeling off were investigated.
2. EXPERIMENTAL PROCEDURES
470 nm-thickness NDC buffer layers weredeposited on Si substrates at the substrate temperature of550 °C and a base pressure of less than 10-4 Pa by CAPD

with graphite targets. A voltage of 100 V was applied tothe arc plasma gun equipped with a 720 µF capacitor. Thedeposition was carried out at a repetition rate of 1 Hz.
Posterior to the deposition of the buffer layers, they werecooled down to room temperature at the base pressure.
After that, NDC films were deposited on the buffer layers
at different substrate temperatures ranging from room
temperature to 550 °C.
The films thickness was measured by a surface
roughnessmeter at edge-steps between the film andsubstrate, which were formed by using a mask during thefilm deposition. In addition, the thickness was also
evaluated from cross-section SEM images. The top-viewimages were taken by scanning electron microscopy(SEM). The crystalline structures of the buffer layer wereexamined by powder X-ray diffraction (XRD) using 12
keV X-rays from synchrotron radiation at beamline 15 ofKyushu Synchrotron Light Research Center/Saga LightSource. For its measurement, the film was peeled offfrom the substrate and powdered. The powder was filled
in a borosilicate glass capillary with an inside diameterof 0.3 mm. The hardness and young’s modulus of theNDC films were estimated by nanoindentation methodwith applied indentation load 0.5 mN.
3. RESULTS AND DISCUSSION
The hardness of the buffer layers was estimated tobe approximately 23 GPa. Figure 1 show the hardness ofthe films deposited at different substrate temperatures.
The hardness increases with decreasing substratetemperature. At the substrate temperature of 550 °C, thehardness become the same value as the buffer layer. The
degradation of the hardness and Young’s modulus withincreasing substrate temperature might be because the sp2
content in an a-C matrix in the NDC films increases withincreasing substrate temperature, in other words,
graphitization is induced thermally.

- 31 -be deposited without peeling off. As reasons for theimprovement, the following two are considered. One isthat the hardness of the buffer layer is not so large. Thebuffer layer is expected to release the internal stress inthe upper films effectively. The other is that atomic
interdiffusion at interfaces between the Si substrates andbuffer layers are actively enhanced since the buffer layers
are deposited at 550 °C, which results in an enhancementin the adhesion strength. In the case of the deposition
directly on the Si substrates at room temperature, theinterdiffusion hardly occurs and the adhesion might
predominantly be due to van der van derWaals adsorption.
By employing the buffer layers, the hardness andthickness are comparably enlarged as compared withthose of films prepared by CVD. In the case of deposition
at elevated substrate temperatures, an internal stress isinduced after coolog down to room temperature due to adifference in the thermal expansion coefficient between
the substrate and film materials, which facilitates thepeeling-off of the films. On the other hand, atomic
interdiffusion at interfaces between substrates and bufferlayers are enhanced at elevated temperatures. Boththermal expansion coefficients of Si and diamond aresmall, thus the effects of the activated atomic
interdiffusion at elevated temperatures might
preferentially appears for the buffer layers in this study.

Fig. 1. Dependence of hardness and Young’s modulus
on substrate temperature.

4. CONCLUSION
Both high hardness and large thickness of NDC films
weresimultaneouslyrealizedbydeposition
employing NDC buffer layers deposited at 550°C. The
NDC buffer layers might play important roles inenhancing the adhesion strength due to atomic
interdiffusion at interfaces with the Si substrates and inreleasing the internal stress of the upper hard NDC films.

Fig. 2. XRD pattern of NDC film deposited at roomtemperature.
To confirm the existence of diamond grains in thefilms, XRD measurements with synchrotron radiation
were employed. An XRD pattern shows peaks due todiamond-111 and 220. They are extremely broad, which
is owing to a small grain size. The size was evaluated tobe approximately 2.4 nm by using Scherrer equation.
There are no peaks due to graphite, which indicated thatthe carbon source is almost completely decomposed into
atomic species by arc plasma.
Since the internal stress in films increases with thehardness, the maximum hardness films deposited at room
temperature should naturally easily be peeled off fromthe substrates. However, the film could be deposited at athickness of 1.5 µm at least. This is the drastic
improvement of the film thickness at which the film can5. ACKNOWLEDGEMENT
This research was partially supported by Osawa
Scientific Studies Grants Foundation, JSPS KAKENHI
Grant-in-Aid for JSPS Fellows (Grant No. 15J00654),
Grant-in-Aid for Young Scientists (B) (Grant No.
26790019), and JST A-STEP (Grant No. AS2511118 M).
The experiment using synchrotron radiation wasperformed at SAGA-LS with the approval of the Kyushu
Synchrotron Light Research Center (Proposal Nos.
1505044S/BL15 and 1410105S/BL12).
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