Tribological properties and characterization of DLC films deposited by pulsed bias CVD

doi:10.1016/j.diamond.2003.11.019

Diamond and Related Materials

Volume 13, Issues 4–8, April–August 2004, Pages 1500-1504

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Abstract

The tribological properties of diamond-like carbon (DLC) films in ambient air and aqueous environment were investigated. The films were deposited on the stainless steel substrate by pulsed-bias CVD. The films were severely damaged with the friction in aqueous condition, and micro-cracks were observed on the wear track of DLC film. The multi-layer structure having hard DLC film covered with Si-doping DLC layer induced improvement of tribological properties of films in aqueous condition. The Si-doping DLC layer induced alleviation of the internal stress of film and prevented generation of the micro-cracks. The multi-layer DLC film covered with Si-DLC layer (Si concentration was 7.3 at.%) proofed against friction test at load of 11.3 N with few partial ruptures, and the friction coefficient was 0.07. Furthermore, it was found that the wear of counter materials was extremely low. The wear rate of a stainless-steel ball was 3.2×10⁻⁹ mm³/Nm at load of 9.4 N.

Introduction

Diamond-like carbon (DLC) films have received considerable interest for more than a decade because of their excellent tribological properties such as low-friction coefficient, high wear resistance and high hardness. Recently, the development of DLC films, with good tribological properties in water, has been expected in connection with environmental problems. In order to apply the DLC film to a solid lubricant in an aqueous environment, the development of films with not only low-friction but also high wear resistance is important. Baba and Hatada reported good adhesion of amorphous carbon deposited by pulsed-bias deposition [1]. Pulse-biased technique in the plasma process was known as a PBII (plasma based ion implantation). The PBII technique has been applied to coating of DLC films for the complex shapes [2], [3]. Wazumi et al. applied the plasma CVD pulse-biased process to the deposition of DLC films [4]. These DLC films showed excellent tribological properties because of their low internal stress. The internal stress in the film decreases in the pulse-biased technique. In this paper we investigated the deposition of DLC films on steel substrates by pulsed-bias deposition and resulting tribological properties in ambient air and water environments. The DLC films were deposited at various voltages. Furthermore, we investigated the improvement of Si-doped DLC (Si-DLC) by over-coating on the DLC films. It was known that doping of silicon in DLC films was useful for the improvement of tribological properties in water. Ronkainen et al. showed improved tribological performance of multilayer structure consisting of a-C:H and a-Si_1−x–C_x:H layers [5]. However, Dress et al. reported that de-adhesion of coating occurs at an early stage with subsequent destruction of the coating [6]. Therefore, it is considered that the restrain of the damage at the early stage is important for wear resistance. An over coating of Si-DLC will be expected to lead the improvement of wear properties of DLC film in water.

Section snippets

Experimental

DLC films were deposited on steel disks using electron-excited plasma CVD implementing an ion source gun with a hot filament and an anode (DASH-400DS2P, NANOTEC). Details of the experimental setup are described elsewhere [4]. The substrate material was nitriding AISI 440C stainless steel (Hv=900). The substrates were polished to a surface roughness of Ra 0.01 μm. Si(100) substrates were used for the analyses of XPS and mechanical properties such as internal stress and hardness. Before

Results and discussion

Friction coefficients of DLC films prepared by pulsed bias were approximately in the range from 0.15 to 0.25 under the air environment. The specific wear rates of these films at load of 9.4 N were 1.9×10⁻⁷ (DLC-1), 1.2×10⁻⁷ (DLC-2) and 5.7×10⁻⁸ mm³/Nm (DLC-3). In the friction test in aqueous condition, DLC films showed low friction coefficient of less than 0.1 (Fig. 1). However, the DLC film prepared by the pulsed bias at −1.0 kV was damaged severely at load of 1.9 N. Furthermore, the DLC films

Conclusion

The tribological properties of carbon films on the stainless steel substrate were investigated in ambient air and aqueous environment. The hard carbon film prepared by pulsed-bias CVD method showed low-friction and good adhesion in air condition. However, the friction in aqueous condition induced severe damage. Micro-cracks were observed on the wear track of DLC films. A mechanism of the dramatic rupture was proposed. The contact stress leads to micro-cracks on the wear surface, and then the

Acknowledgments

The authors express their gratitude to Mr M. Kanomata for his assistance in experiments. This study was supported by the New Energy and Industrial Science and Technology Development Organization of Japan, as a part of a project of the Ministry of Economy, Trade and Industry of Japan, entitled ‘Surface-modification technologies for friction reduction in various machinery’.

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