Effect of feed gas composition effects on the nanotribological properties of diamond-like carbon films
Highlights
► Tribological diamond-like carbon (DLC) coatings were prepared. ► RF-plasma-enhanced chemical vapor deposition with H2 and CH4 gas mixture ► Integrated intensity ratios (ID/IG) have been correlated with sp2/sp3 ratios. ► Tribological properties of DLC films studied by nanoindentation
Introduction
For decades, diamond-like carbon (DLC) films have attracted a desirable interest from both industry and the scientific community. DLC is a unique coating material for hardening the surface of moving mechanical parts, especially for tribological applications. DLC coatings are attractive for tribological applications because of their favorable properties including low friction coefficient, high mechanical hardness and excellent chemical inertness. DLC coatings have been the subject of growing attention within tribological applications as a result of their high hardness, high wear resistance, low friction coefficient, excellent chemical inertness, and their potential for use in a variety of energy-saving applications. Previous studies show that preparation methods can result in different compositions and rations, hence affect the film properties (composition, carbon bonding, surface roughness, mechanical strength, tribological behavior and electrical resistivity) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. In general, the sp3- and sp2-bonded carbons have been revealed to coexist in amorphous or crystalline carbon microstructures and nanostructures [11], [12], [13], [14], [15], [16], [17].
Fontaine et al. [17] studied the effect of the hydrogen contents on the friction coefficient of DLC films. Their results showed that the friction coefficient for lower hydrogenated DLC films rises up to 0.6. By contrast, in the case of DLC films with the higher hydrogen content, the friction decreases down to lower values (less than 0.01). Therefore, the hydrogen content of the DLC films and the nature of the carbon–hydrogen bonds are now widely recognized to play a paramount role on the friction coefficient observed. A similar performance has also been reported for the tribological behaviors of hydrogenated and hydrogen-free DLC coatings by Ronkainen et al. [18]. Recently, many research works have been published on tribological phenomena of DLC films using a pin-on-disk tester or ball-on-disk tribometer [19], [20], [21], [22]. However, such conventional tests based on continuum theory might break down when the physical dimension falls in the nanometer regime. Recent development of nanotechnology provides a unique opportunity to microscopically manipulate various nano-mechanisms to achieve desirable tribological performances and good interfacial behaviors. In this study, we have employed nanoindentation to characterize tribological properties of DLC thin films for which a depth-sensing technique also was utilized to determine mechanical properties through the analysis of the load-depth curve. These DLC films have been deposited on a silicon substrate using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) method with pure methane gas (CH4) or a mixture of H2 and CH4 gases as the source gas. The nanotribological and nanowear properties for the DLC films were investigated by the nanoscratch tests. The topography of the wear tracks of samples prepared using various H2 contents in the fed gas was examined by scanning probe microscopy (SPM). Moreover, the role of H2 content in the deposition process on the properties of DLC films was examined using Raman spectroscopy, which has proven to be a powerful method to characterize DLC films.
Section snippets
Experiments
In this present study, the DLC films were deposited on a silicon substrate using the capacitively-coupled radio frequency (RF) plasma enhanced chemical vapor deposition (RF-PECVD) method [23]. The substrates were mounted on the RF-powered electrode with a negative dc self-bias of 220–250 V. The electrode was cooled in flowing water to keep the substrate near room temperature during the deposition of DLC films, and the substrate temperature was measured using a K-type thermocouple. The substrate
Result and discussion
The Raman analyses are performed for various DLC films derived from pure methane and various hydrogenated methane gas discharge plasmas. The results, shown in Fig. 1, indicate that the film structure contains both G-band and D-band, in which the two peaks in the curves correspond to peaks for G-band and D-band, the former being centered at 1560 cm− 1, the latter at 1350 cm− 1. In general, the shift of G band to a lower wavenumber usually reflects the increase of sp3 bonds, in which the ID/IG ratio
Conclusions
In this present study, DLC films were deposited by RF-PECVD using a mixture of hydrogen and methane gases. Raman spectroscopy results show that the integrated intensity ratios (ID/IG) of each profile correlate with sp2/sp3 ratios. The present result also indicates that the sp2/sp3 ratio and hydrogen-to-carbon ratio in DLC films strongly depended on hydrogen content in the source gas. In addition, a series of experimental nanoindentation, nanowear and nanoscratch tests has been employed to
Acknowledgment
The authors gratefully acknowledge the support provided to this research by the National Science Council, Taiwan, NSC 100-2120-M-194-002 and NSC 100-2120-M-194-004. The support of AFOSR under Contract No. FA4869-06-1-0056 AOARD 114105 is also acknowledged.
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