Tribochemical investigation of DLC coating tested against steel in water using a stable isotopic tracer
Introduction
Diamond-like carbon (DLC) coatings have high hardness and high chemical stability, and exhibit excellent tribological properties, such as low friction, high wear resistance, and so on, in a water environment. For these reasons, DLC coatings are attracting attention as solid lubricants applied widely in water hydraulic systems [1]. A number of tribological investigations in various environments suggest that the transfer layer on the counter parts and the graphitized tribolayer are responsible for the tribological behaviors, and the deposition conditions also affect tribological properties of DLC coatings [2], [3], [4]. Recently, it is reported that a thin lubrication film has been observed on the worn counter-ball surface rubbed against DLC coating in a water environment by TEM (transmission electron microscope) and EDS (energy dispersive X-ray spectroscopy) [5]. It is concluded that the formed lubrication film plays an important role in the tribological properties. However, it is lacking in understanding of the lubrication films on the surfaces of DLC coatings and the counter parts, which is very important in revealing the tribological properties, designing deposition process of DLC, etc. As a part of the fundamental studies on the formation mechanism of lubrication films at the interfaces of DLC coatings and the counter parts, the aim of this work is to investigate the tribochemical reaction of DLC coating in water.
On the other hand, it is well known that many kinds of organic contaminations and water vapor exist in ambient environment, and these compounds are easy to adsorb onto surface of materials. It may be difficult to distinguish the tribochemical products on the surfaces rubbed in water from these adsorbates, because the products may also be composed of carbon, hydrogen and oxygen. In this case, stable-isotopic-tracer method is effective in identifying the source of the components applied in the test by ToF–SIMS (time of flight secondary ion mass spectroscopy) [6]. In order to identify the products formed by rubbing in water more clearly, heavy water (D2O) was used as an isotopic tracer to carry out the friction test of DLC coating and 440C ball pair, and the worn surfaces were analyzed by ToF–SIMS. For comparison, a test was also performed in ion exchanged water (H2O).
Section snippets
Experimental details
DLC coatings were deposited on silicon wafer substrates by thermal electron excited plasma CVD method. A pulsed-bias was applied to the substrates during depositing. The bias voltage, frequency and duty ratio were − 3.0 kV, 1 kHz and 10%, respectively. The deposition time was 180 min. Toluene (C7H8) was used as the carbon source gas. The thickness was 1.4 μm. The analysis result of HFS (hydrogen forward scattering spectrometry) showed that the hydrogen content of the coatings was 24 at.%.
Tribological behaviors
Fig. 1 shows the variation of friction coefficient with sliding time tested in D2O and H2O. The friction behavior of DLC coating and 440C ball pair in D2O is similar to that in H2O: the friction coefficient decreased at the beginning of the sliding, and then came to a relatively stable and low value, around 0.06. The specific wear rates of DLC coating and the mated ball in D2O were 1.9 × 10− 8 and 2.8 × 10− 8 mm3/Nm, respectively, and were in the same level as those in H2O, 1.5 × 10− 8 and 1.9 × 10− 8 mm3
Conclusion
The tribochemical reactions of DLC coatings in water environment have been investigated by means of stable isotopic tracers using deuterium labeled water and ion exchanged water. It has been found that DLC coating tribochemical reacts with water to form CH and OH groups on the surface. It is suggested that OH groups mainly combine with the secondary or tertiary carbons. When C–CH bond was cleaved by mechanical stimulation, it seems that the carbon atom combined with H tends to attack H atom in
Acknowledgments
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.
References (8)
- et al.
Diamond Relat. Mater.
(2004) - et al.
Surf. Coat. Technol.
(1997) - et al.
Diamond Relat. Mater.
(2003) - et al.
Tribol. Int.
(2004)
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