tribology online, vol. 11, no. 2 (2016) pp. 203-208

Tribology Online, 11, 2 (2016) 203-208. ISSN 1881-2198

DOI 10.2474/trol.11.203

Copyright © 2016 Japanese Society of Tribologists 203

Article

Effects of Surface Texture for Improving Friction Properties of Hydrogenated

Amorphous Carbon Films

Yuuki Tokuta1)*, Masahiro Kawaguchi2) and Shinya Sasaki3)

1)Tokyo Metropolitan Industrial Technology Research Institute Joto Branch 7-2-5 Aoto, Katsushika-ku, Tokyo 125-0062, Japan

2)Tokyo Metropolitan Industrial Technology Research Institute 2-4-10 Aomi, Koto-ku, Tokyo 135-0064, Japan

3)Tokyo University of Science 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan

*Corresponding author: [email protected]

( Manuscript received 30 July 2015; accepted 14 January 2016; published 30 April 2016 ) ( Presented at the International Tribology Conference Tokyo 2015, 16-20 September, 2015 )

Hydrogenated amorphous carbon (a-C:H) films exhibit excellent friction properties such as high mechanical hardness, high wear resistance, and low friction. a-C:H films have amorphous structures generally composed of sp2- and sp3-hybridized carbon, which bring about extraordinary friction properties. Some reports have focused on the low-friction mechanism of a-C:H films, concluding that it is induced by the existence of graphitized wear particles at the sliding interface. It is possible to consider the existence of graphitized wear particles as the most important factor in achieving the lower friction of a-C:H films. We focus on the effects of surface texture in trapping the graphitized particles at the sliding interface and discuss the role of surface texture with regard to the friction properties of a-C:H films. Micro slurry-jet erosion (MSE) surface machining was employed to manipulate the surface texture on a high-carbon chromium-bearing steel substrate, upon which a-C:H films were deposited. The friction properties of a-C:H films deposited on a mirror-like polished substrate (a-C:H/mirror-like) and on an MSE-produced substrate (a-C:H/MSE-produced) were compared using a reciprocating-type ball-on-disk sliding tester. From the results of friction testing, it is confirmed that a-C:H/MSE-produced films indicated lower friction coefficients compared with the a-C:H/mirror-like case. Scanning electron microscopy (SEM) and Raman spectroscopy were performed to study the friction improvement mechanism of the a-C:H/MSE-produced films. SEM revealed the existence of wear particles in the wear track of a-C:H/MSE-produced films. It is confirmed by Raman spectroscopic analysis that these wear particles’ structure was changed, adopting a graphite-like structure. From these results, it is possible to consider that the existence of graphitized wear particles induced lower shearing resistance at the sliding interface, enabling friction improvement. Keywords: hard coating, hydrogenated amorphous carbon, surface texture, friction

1. Introduction

In recent years, the use of the hydrogenated amorphous carbon (a-C:H) films have attracted attention as a surface improvement technology. a-C:H films exhibit excellent friction properties such as high mechanical hardness, high wear resistance, and low friction [1-4]. These types of films have an amorphous structure generally composed of sp2- and sp3-hybridized carbon and hydrogen, leading to good friction properties. a-C:H films adopt complicated bonding in spite of their simple elemental composition. Because of their extraordinary structure, a-C:H films attract academic and

industrial interest. They are used in many industrial applications such as cutting tools, steel molds, and automobile sliding parts, and their potential fields of application are expanding [5-8].

On the other hand, possible friction reduction mechanisms for an a-C:H films have been reported, such as reduced shearing resistance of the sliding interface. In particular, as a mechanism that explains why a-C:H films exhibit excellent friction properties in a non-lubricated condition, Liu et al. have reported on wear particles generated by friction effects which improve friction properties [9,10]. In these reports, the structure of these particles (whose size is less than 5 nm) was found to have

Yuuki Tokuta, Masahiro Kawaguchi and Shinya Sasaki

Japanese Society of Tribologists (http://www.tribology.jp/) Tribology Online, Vol. 11, No. 2 (2016) / 204

changed to a graphitized structure (graphitization) during sliding, and reduction of the friction coefficient was induced by the existence of wear particles at the sliding interface. From these suggestions, it is possible to consider that graphitization of wear particles is the most important mechanism in promoting the friction properties of a-C:H films. Further improvement of a-C:H film friction properties is achieved by devising an effective method of utilizing graphitized wear particles.

In this research, we focused on surface texturing as a new method of utilizing the wear particles in a-C:H films. By changing the film surface topography to enable efficient generation of graphitized wear particles and trapping them into concavities, surface texturing increases the number of wear particles at the sliding interface between the films and the opposing surface. The wear particles in a-C:H films are expected to be minute, of nanometer order. It is considered that nanometer-order surface machining is suitable for trapping wear particles into surface valleys. From the point of view of trapping nanometer-size wear particles, we applied micro slurry-jet erosion (MSE) surface machining to produce the desired surface texturing. MSE surface machining enables us to attain surface irregularity of nanometer order. We investigated the effect of the surface texturing produced by MSE surface machining in the reduction of the friction coefficient of a-C:H films. Furthermore, we tried to reveal the friction and wear mechanism of surface-textured a-C:H films from an academic perspective.

2. Experimental methods

2.1. Producing of surface texturing on a-C:H films The a-C:H films used in our study were deposited on

a high-carbon chromium-bearing steel (ISO-100Cr6) disk via the unbalanced magnetron sputtering (UBMS) method. The disk diameter was 24 mm. The disk substrate was polished to a mirror-like surface of Ra 0.006 μm. MSE surface machining was applied against a polished disk substrate, and surface texturing was produced at the substrate surface. A schematic illustration of MSE surface machining (MSE-N401, Palmeso Co., Ltd.) is shown in Fig. 1. The surface texturing was produced by projection of alumina particles. Figure 2 shows the result of SEM observation of alumina particles used in this study. It was confirmed that the average alumina particle size was nearly 1 μm. After MSE surface machining, a-C:H film was deposited

Fig. 1 Schematic illustration of MSE

Fig. 2 SEM observation of alumina particle

Fig. 3 3D surface profiles measured by white light interferometer (a) The a-C:H/mirror-like films (b) The a-C:H/MSE-produced films

Effects of Surface Texture for Improving Friction Properties of Hydrogenated Amorphous Carbon Films


on the textured substrate surface to produce different surface forms of the films. Figure 3(a) shows the three-dimensional surface profile of the a-C:H films deposited on the mirror-like polished substrate (a-C:H/mirror-like), measured using a white light interferometer. Figure 3(b) shows the three-dimensional surface profile of a-C:H films deposited on the MSE-produced substrate (a-C:H/MSE-produced). Two- dimensional surface profiles of each film were shown in Fig. 4(a,b) respectively. The surface profile parameters Pa and Pz (defined in ISO 4287-1997) of a-C:H films and alumina ball were shown in Table. 1. Each parameter was calculated from two-dimensional surface profile data. Friction properties of these samples were compared to literature regarding the relationship between surface texturing and friction properties of a-C:H films.

2.2. Friction testing of a-C:H films Friction properties were investigated using a

reciprocating-type ball-on-disk sliding tester. An Al2O3 ball 8 mm in diameter, was used as the ball specimen. The schematic illustration of the sliding test is shown in Fig. 5. The applied load, sliding speed, sliding distance, and total sliding cycles were 5 N, 5 mm/s, 5 mm, and 100 cycles, respectively. The friction test was performed in a non-lubricated condition.

2.3. Elucidation of wear and friction mechanism by structural analysis

We employed optical and scanning electron microscopes (SEM) to the wear track of the a-C:H films to elucidate the friction and wear mechanism. SEM observation was performed at a magnification of 5000

and the vacuum degree in the chamber was less than 1.5 × 10-3 Pa. Micro-laser Raman spectroscopy was applied to analyze structural changes in the wear track of the a-C:H films. Raman spectroscopy is a nondestructive technique popularly used for probing the structure of a-C:H films to investigate their different bond types and sp2-configuration. In particular, Raman-active benzene rings and disordered structure can provide some information on the structure of a-C:H films [11-13]. Raman spectroscopy was performed on a Raman apparatus using a YAG laser with a wavelength of 532 nm, power of 0.25 mW, and laser spot size of 5.0 μm, and spectra were recorded in the range of 800-2,000 cm-1. The Raman spectra were fitted based on two Gaussian curves using curve-fitting software. We compared the spectra at the wear track of the a-C:H/mirror-like films and the a-C:H/MSE-produced films.

3. Results and discussion

3.1. Friction testing of a-C:H films Figure 6 shows friction behavior of the a-C:H/mirror-

like films and the a-C:H/MSE-produced films. From this figure, a break-in period was observed in the case of the a-C:H/mirror-like films over 0-25 cycles. The a-C:H/mirror-like films after 25 cycles indicated stable behavior, and the friction coefficient was approximately 0.17 at the end of the test. The a-C:H/MSE-produced

Fig. 4 2D surface profiles measured by white light interferometer (b) The a-C:H/MSE-produced films (a) The a-C:H/mirror-like films

Fig. 5 Schematic illustration of friction test

Table 1 Surface profile parameters Pa and Pz



films had a lower friction coefficient compared with a-C:H/mirror-like films, and a break-in period could not be observed. At the end of the test, the friction coefficient of a-C:H/MSE-produced films was 0.11.

The results of the friction tests showed that the friction coefficient decreased with surface texturing on the a-C:H film surfaces. It is possible to consider that surface texturing had the effect of reducing the friction coefficient and preventing the occurrence of a break-in period for the a-C:H films.

3.2. Improving the mechanism of friction properties in a-C:H/MSE-produced films

Figure 7 shows optical images observed using an optical microscope at the wear track of the ball and disk after the sliding test. In the case of the a-C:H/mirror-like films, adhesion of wear particles was observed around the wear track on both sides of the ball and disk. This result indicates that wear particles were pushed out from the sliding interface by friction with the opposing surface. On the other hand, in the case of a-C:H/MSE-produced films, existence of wear particles could not be observed around the wear track of the ball and disk. It is possible to consider that wear particles were trapped at the wear track of the a-C:H/MSE-produced film surfaces and were not pushed out from the sliding interface. This means that a-C:H/mirror-like films and a-C:H/MSE-produced films have different friction and wear mechanisms. In the case of a-C:H/MSE-produced films, it is suggested that improvement of friction properties was induced by the existence of wear particles at the sliding interface.

As a result of observing the film surface from the diagonal direction by SEM, observation images outside of the wear track on the a-C:H/MSE-produced films are shown in Fig. 8(a,b) shows an observation image at the

Fig. 6 Result of friction test

(a) Outside of the wear track (b) Wear track

Fig. 8 SEM observation images of the a-C:H/MSE-produced films

Fig. 7 Optical images of wear track both side of ball and disk

Effects of Surface Texture for Improving Friction Properties of Hydrogenated Amorphous Carbon Films


wear track. From Fig. 8(a), the existence of fine irregularities, which were produced by MSE surface machining, was confirmed clearly outside of the wear track. From Fig. 8(b), fine irregularities seen outside of the wear track could not be observed at the wear track, and a smooth surface was formed at the wear track of the a-C:H/MSE-produced films. From these results, it is possible to consider that the smoothed surface of the wear track was induced by clogging of wear particles into the valleys of surface texture. It is suggested that wear particles were trapped at the wear track in the case of a-C:H/MSE-produced films.

In order to investigate the existence of trapped wear particles, surface observation using SEM was employed after ultrasonic cleaning of the a-C:H/MSE-produced films. The a-C:H/MSE-produced films were cleaned with petroleum benzene in an ultrasonic bath, repeated for 10 minutes and 6 times. Figure 9(a,b) show observation SEM images of the wear track before cleaning and after cleaning, respectively. In the images before cleaning, discoloration areas could be observed at the wear track. It is possible to consider that the discoloration areas were caused by the existence of wear particles. In the case of films after cleaning, observation results at the wear track and outside of the wear track showed similar colors. This indicates that wear particles trapped in the surface texture were removed by ultrasonic cleaning. From these surface observation results, it appears that wear particles generated by sliding wear were trapped in the concavities of the surface texture. Because wear particles are constantly present at the sliding interface, a lower friction coefficient was achieved.

3.3. Lubrication mechanism due to wear particles in a-C:H films

To investigate the effects of wear particles in inducing excellent friction properties, film structure analysis was accomplished by Raman spectroscopy. Figure 10 shows the comparison of the intensity ratio ID/IG measured at and outside of the wear track. Raman spectroscopy was performed before and after cleaning. In the case of the a-C:H/mirror-like films, the results at and outside of the wear track indicate similar ID/IG ratios.

This means that there was no structural change at the sliding interface of the a-C:H/mirror-like films. In the case of the a-C:H/MSE-produced films, the ID/IG ratio at the wear track before cleaning indicated a higher value compared with outside of the wear track. In general, increase of the ID/IG ratio value indicates graphitization of the a-C:H films structure. This result suggests that graphitized wear particles were trapped at the wear track before cleaning. On the other hand, in the results at the wear track after cleaning, reduction of ID/IG ratio was observed compared with the wear track before cleaning. This means that graphitized wear particles at the wear track were removed by cleaning in an ultrasonic bath. From these results, it is suggested that reduction of the friction coefficient was induced by the presence of graphitized wear particles at the sliding interface in the case of a-C:H/MSE-produced films. On the other hand, the Raman spectrum attributed to the a-C:H film structure could not be confirmed at the wear track of the ball. Low friction of the films is not attributed to transferred a-C:H films, and it is suggested that improvement of friction properties is caused by the existence of wear particles at the sliding interface.

From these results, it is considered that a-C:H/MSE-produced possess two steps about wear and friction process as a one of the hypothesis. The schematic illustration of hypothesis about wear and friction mechanism was shown in Fig. 11.

(a) Before cleaning (b) After cleaning

Fig. 9 Comparison of SEM images of before cleaning and after cleaning

Fig. 10 Result of Raman spectroscopy



STEP 1: The peaks of surface texture are worn, and the sliding interface forms a break-in shape immediately. As a result, the initial break-in period shortens.

STEP 2: Wear particles are trapped into concavities of the surface texture, which prevent removal of wear particles from the wear track. Lower sharing resistance of graphitized wear particles induces a low friction coefficient.

From the above, it is possible to consider wear particle graphitization and their presence at the sliding interface as two dominant factors in improving the friction properties of a-C:H films in a non-lubricated condition. We plan to reveal more detail mechanism about low-friction of a-C:H/MSE-produced.

4. Conclusions

In this study, a-C:H films were deposited onto high-carbon chromium-bearing steel (ISO-100Cr6) via the UBMS method and surface texture on the a-C:H film was produced by MSE surface machining. A comparison of friction properties was made using a reciprocating-type ball-on-disk sliding tester between a-C:H films deposited onto a mirror-like polished substrate surface and those deposited onto an MSE-produced substrate surface. Surface observations using SEM and optical microscopy and structural analysis using Raman spectroscopy were performed to reveal the friction and wear mechanism of a-C:H films. Our findings are summarized follows: 1) It is confirmed that MSE-produced a-C:H films

show lower friction coefficients compared with a-C:H/mirror-like films.

2) In the case of a-C:H/MSE-produced films, wear particles are trapped in concavities of the surface texture. Trapped wear particles at the sliding interface are graphitized by friction and contribute to the low friction of a-C:H/MSE-produced films.

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Fig. 11 Schematic illustration of wear and friction mechanism of MSE-produced DLC

tribology online, vol. 11, no. 2 (2016) pp. 203-208

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