1 Introduction

Silicon-incorporated diamond-like carbon (Si-DLC) films have excellent properties such as low friction, high durability, and stability against humidity and temperature (Oguri and Arai 1991). Moreover, Si addition improves surface roughness and adhesion strength. These superior properties of the Si-DLC films make them as candidates for a protective coating in micro and nano-system such as microelectromechanical systems (MEMS) and magnetic disk drives. In the case of magnetic disk drives, since the thickness of protective DLC coatings decreases to several nanometers to achieve much higher recording density, DLC coatings which have better tribological and anti-corrosion properties are required (Bhushan 1999).

Ultrathin organic films such as perfluoropolyethers are employed in MEMS (Lee et al. 2000) and magnetic disk drives (Homola 1996) to protect the system from stiction and corrosion. The strong bonding between solid surface and organic molecules is required to reduce these problems. The lubricant bonding toward the solid surfaces is related to the structure and chemistry of the solid surface, since the end groups of organic molecules are adsorbed to adsorption sites on the solid surfaces (Yanagisawa 1994). To obtain high molecular bonding, it is necessary to increase the number of adsorption sites on the solid surface, we consider, which can be achieved by modifying the solid surface. In the case of protective DLC coatings, some elements such as silicon can be added in the carbon network for this. In this article, we review various properties of the Si-DLC films and explain the low friction and high molecular bonding mechanism and good corrosion performance of the Si-DLC films.

2 Deposition of Si-DLC films

The Si-DLC films were deposited using a bipolar-type plasma based ion implantation and deposition (PBII&D) (Miyagawa 2002). The schematic diagram of the PBII is shown in Fig. 1. The glow discharge plasma is formed around the target surface by applying a positive pulse voltage directly to the target, and the plasma is omnidirectionally implanted into (or deposited on) the target surface by a subsequent negative high pulse voltage. The PBII processing with bipolar pulses provides improved adhesion strength between the DLC film and the target. A gas mixture of tetramethylsilane [TMS, Si(CH3)4] and toluene was used for the deposition of the Si-DLC films. The ratio of TMS and toluene in the gas mixture were change to obtain the Si-DLC film with various Si contents, whereas the deposition pressure was kept constant. The deposition conditions of the Si-DLC films are shown in Table 1. The composition and microstructure of the deposited Si-DLC films were measured and analyzed using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The film thickness was measured with a microstylus profilometer.

Fig. 1
figure 1

Schematic diagram of PBII&D used for the deposition of Si-DLC films

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Table 1 Deposition conditions for the Si-DLC films
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3 Microstructure of Si-DLC Films

Figure 2 shows (1) Raman spectra of the Si-DLC films, and (2) I D/I G ratio and G-peak position determined from the Raman spectra. The G- and D-peak positions of the Si-free DLC film and the change of the G-peak positions for the Si-DLC films are indicated using arrows. The Si-free DLC film shows a typical diamond-like structure with peaks centered at 1,539 (G band) and 1,373 cm−1 (D band). With the increasing Si content, the G-peak becomes more pronounced relative to the D-peak, and the peak for 29 at. % Si-DLC film can be fitted by a single Gaussian. The G-peak position is shifted to a low wavenumber from 1,539 to 1,461 cm−1, and the intensity ratio, I D/I G, decreases from 0.81 to 0.15 with the increasing Si content up to 29 at.%. Incorporation of more Si atoms into the carbon network reduces the phonon vibration frequency because the Si atom is heavier than the C atom, causing a G-peak shift to low wavenumbers (Abbas et al. 2005a, b). The D-peak is due to six-folder (sp2) rings (Robertson 2002). The Si-incorporation causes an opening up of the sp2 rings and decreases of the sp2 cluster size because the Si atoms can not form the π bonds (Abbas et al. 2005, b), in a result, I D/I G decreases with the increasing Si content. These behaviors indicate that the sp3 bonds increase with the increasing Si content in the Si-DLC film.

Fig. 2
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a Raman spectra of the Si-DLC films, b I D/I G ratio and G-peak position determined from the Raman spectra

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Figure 3 shows the depth profiles of oxygen contents in the Si-free and Si-DLC film surfaces. The oxygen contents were measured using XPS before and after Ar+ ion sputtering. The sputter rate was about 0.05 nm/s. It is observed that the oxygen content in the DLC film surface increases due to Si addition. Furthermore, the oxygen content on the Si-DLC film surface increases with the increasing Si content. The oxygen content in the Si-free DLC film surface becomes negligible after Ar+ ion sputtering for 20 s, which indicates that the oxide thickness on the Si-free DLC surface is about 1 nm. On the other hand, the Si-DLC film has a thicker oxide layer compared to that of the Si-free DLC film.

Fig. 3
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The depth profiles of oxygen contents in the Si-free and Si-DLC film surfaces measured with XPS

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Recently, the bonding characteristics of perfluoropolyether (PFPE, Fomblin ZDOL4000) molecules, having a hydroxyl end group, regarding to the Si-DLC films were investigated using conventional dip coating (Choi et al. 2006). It was found that Si incorporation enhances the molecular bonding of PFPE lubricant on the DLC film surfaces, and the bonding increases with the increasing Si content in the DLC film. These results are attributed to structural and chemical modifications of the DLC surfaces due to Si incorporation into the carbon network. The molecular bonding of lubricant is related to the kinds of functional end groups of lubricants, the kinds of solid surfaces (i.e., the number of adsorption sites) and the contamination of solid surfaces as illustrated in Fig. 4. Yanagisawa reported that oxygen-containing functional groups and dangling bonds are effective adsorption sites for the PFPE molecules having hydroxyl end groups (Yanagisawa 1994). Kasai described the sputtered carbon films as stacks of spheroidal granules of several nanometers in diameter. Each individual granule would have either the sp2 (graphitic) bonding scheme or sp3 (diamond-like) bonding scheme, and the large number of dangling bonds are generated and trapped only within the tight three dimensional networks of diamond-like granules (Kasai 2000). The Si addition to the DLC surface increases the sp3 bonds (i.e., the number of dangling bonds) and the oxygen groups in the carbon surface as exhibited from Raman (Fig. 2) and XPS (Fig. 3) results, which explains the good bonding properties of PFPE molecules toward the Si-DLC film.

Fig. 4
figure 4

Schematic diagram of lubricant bonding on the DLC film surface

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4 Friction properties of Si-DLC films

Figure 5 shows the internal stress and friction coefficient of the Si-DLC films deposited on Si substrates. The internal stress? was calculated using Stoney equation by measuring the curvature radii of the DLC-coated silicon substrates with a microstylus profilometer. The internal stress of the DLC films deposited by PBII exhibits a very low value compared to that of DLC film deposited by conventional PECVD process (Wu and Hon 1997). The internal stress further decreases below 0.2 GPa due to the Si addition. Wu and Hon reported that the low internal stress of the Si-DLC films is due to the polymer-like structure of the films, i.e., high hydrogen content and low hardness (Wu and Hon 1997).

Fig. 5
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The internal stress and friction coefficient of the Si-DLC films

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The friction coefficients of the DLC films deposited on steel substrates (SKD61) were measured by a ball-on-flat type reciprocal friction tester in ambient air. The applied load, the reciprocal sliding distance, the sliding speed, and diameter of the steel balls (SUJ2) were 0.98 N, 8 mm, 0.5 Hz, and 3 mm, respectively. The film thickness of the Si-free DLC and Si-DLC films varied from 1 to 2 μm regarding to the deposition ratio. The deposition ratio decreased with increasing the TMS volume fraction in the gas mixture. The friction coefficient of the DLC film decreases to a very low value of 0.04 due to Si-incorporation. Figure 6 shows (1) SEM, (2) Si mapping and (3) O mapping images of steel ball surface slid against 29 at.% Si-DLC film. The Si and O mapping images were obtained by electron probe microanalysis (EPMA). The bright regions of Fig. 6b and c are Si-rich and O-rich regions, respectively, and the dark regions of Fig. 6b and c are Fe-rich regions originating from the steel ball. As can be seen in Fig. 6, the distribution of Si on worn ball surface corresponds to that of O, which indicates that the silicon oxide is transferred on the steel ball surface after sliding. The low friction of the Si-DLC films is related to the formation of silicon-rich oxide debris and transferred silicon oxide layers on the steel ball surfaces. The silicon oxide layers on the Si-DLC film (see Fig. 3) and steel ball prevent from the metal/DLC contact between the Si-DLC film and steel ball when sliding, which results in a low friction. In the case of the Si-free DLC film, the steel ball surface is seriously damaged as shown in Fig. 7, which results in a high friction coefficient. Another possible explanation on the low friction properties of the Si-DLC films is the effect of water lubrication. The adsorbed water molecules on SiOH groups existing on the Si-DLC film surfaces reduce shear strength between the Si-DLC film and steel ball (Patton et al. 2000).

Fig. 6
figure 6

a SEM, b Si mapping, c O mapping images of steel ball surface after sliding against 29 at.% Si-DLC film. The bright regions of (b) and (c) are Si-rich and O-rich regions, respectively. The dark regions of (b) and (c) are Fe-rich regions originating from the steel ball

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Fig. 7
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SEM image of the steel ball surface after sliding against Si-free DLC film

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5 Anti-corrosion properties of Si-DLC films

Potentiodynamic polarization experiments were conducted to evaluate the corrosion performance of the Si-DLC films in aqueous 0.05 M NaCl solution. The Si-DLC films were deposited on glass plates coated with magnesium alloy films. The film thickness of the DLC films were adjusted to 800 nm from the deposition rates. The magnesium films were deposited using an ion beam sputter with a magnesium alloy target (AZ31, Mg−3%Al-1%Zn). A Pt sheet and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. For comparison purpose, a result of substrate (i.e., glass plate coated with magnesium alloy film) is also included.

Figure 8a is the potentiodynamic curves which are usually obtained from the potentiodynamic polarization experiments. The curve is the total current, that is, the sum of the anodic and cathodic currents. Figure 8b is obtained by converting the current density (the vertical axis of Fig. 8a) as the logarithm of absolute current density. The sharp point in the curve is the point where the current changes signs as the reaction changes from cathodic to anodic (or from anodic to cathodic). The corrosion currents and corrosion potentials are obtained from Fig. 8b by Tafel analysis (Stippich et al. 1998), and the values are shown in Table 2. The corrosion potential of the Si-free DLC coated sample was shifted 0.6 V to positive direction compared to the substrate. Moreover, the corrosion potential of the Si-DLC coated sample was shifted to more positive value compared to that of the Si-free DLC coated sample. It indicates that the corrosion protection performance of the Si-DLC film is superior to that of the Si-free DLC film. The corrosion current density of the Si-DLC coated sample is three orders of magnitude lower than that of the Si-free DLC coated sample, whereas the corrosion current of the Si-free DLC coated sample is comparable to that of uncoated substrate. The lower this value, the better the corrosion protection property, because the corrosion current is directly related to the corrosion rate. XPS measurements exhibited that silicon oxide layer exists on the Si-DLC surface. The insulating silicon oxide layer results in good anti-corrosion properties of the Si-DLC films (Fig. 3). Another reason is the very low internal stress of the Si-DLC films as shown in Fig. 5. The high internal stress causes the surface cracking and, consequently, poor anti-corrosion properties.

Fig. 8
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Polarization curves for the Si-free DLC and Si-DLC films deposited on magnesium alloy films

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Table 2 Corrosion potentials and corrosion currents
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6 Conclusion

The various properties of the Si-DLC films—microstructural, tribological, anti-corrosion and PFPE bonding properties - were investigated. The results are as follows:

  1. 1.

    The analysis of Raman spectroscopy indicated that the sp3 bonds in the DLC film increase with the increasing Si content. Also, the Si-DLC films have high oxygen contents on the surfaces. These explain the high molecular bonding properties of PFPE lubricant toward the Si-DLC film surface due to increased adsorption sites.

  2. 2.

    The silicon oxide layer on the Si-DLC film and the transferred silicon oxide layer on the steel ball prevent from the metal/DLC contact between the Si-DLC film and steel ball when sliding, which results in a low friction.

  3. 3.

    The Si-DLC film shows a low internal stress, and Si incorporation in the DLC film creates a thick insulating oxide layer on the film. These lead to significant improvements in the corrosion properties of the DLC film.