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Frictional properties of DLC films in low-pressure hydrogen conditions
Author’s Accepted Manuscript Frictional properties of DLC films in low-pressure Hydrogen conditions Hikaru Okubo, Ryo Tsuboi, Shinya Sasaki
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PII: DOI: Reference:
S0043-1648(15)00190-8 http://dx.doi.org/10.1016/j.wear.2015.03.018 WEA101386
To appear in: Wear Received date: 23 September 2014 Revised date: 22 January 2015 Accepted date: 21 March 2015 Cite this article as: Hikaru Okubo, Ryo Tsuboi and Shinya Sasaki, Frictional properties of DLC films in low-pressure Hydrogen conditions, Wear, http://dx.doi.org/10.1016/j.wear.2015.03.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Frictional properties of DLC films in low-pressure hydrogen conditions Hikaru Okubo1, Ryo Tsuboi2, Shinya Sasaki3* 1
Graduate School, Tokyo University of Science 6-3-1 Niijuku, Katsushika-ku, Tokyo, 125-8585, Japan. 2 3
Daido University, Japan.
Tokyo University of Science, Japan.
*Corresponding author: Shinya Sasaki Tel.: +81 3 5876 1334, E-mail address: [email protected], [email protected]
Abstract The effects of hydrogen gas on the frictional behavior of diamond-like carbon (DLC) films were investigated by varying the hydrogen pressure during a friction test. Hydrogenated DLC (a-C:H) and non-hydrogenated DLC (ta-C) coatings have been tested. The ta-C/ta-C tribopair exhibited a linearly relationship between the hydrogen pressure and the average friction coefficients. On the other hand, the a-C:H/a-C:H tribopair exhibited an extremely low friction coefficient of less than 0.01 when the hydrogen pressure reached 5000 Pa. In addition, the friction coefficients from the tests at increased and decreased hydrogen pressures exhibited loop-like behavior. It is considered that the hydrogen pressure and the hydrogen content of the DLC film has an effect on the tribofilms and that the tribofilms played an important role in the loop-like frictional behavior. The evaluation of the hydrogen content of the DLC films was carried out by the G-peak method. The value of logarithm of (N/S) can be used as a measure of the hydrogen content, where the intensity of the fitted G-peak; S, the maximum intensity of the G-peak; and N, the intensity of the photoluminescence background. The behavior of logarithm of (N/S) from the Raman spectra for various hydrogen pressures indicated an interesting relationship between the average friction coefficient and the hydrogen content of tribofilm on a-C:H ball. The relationship showed the average friction coefficient decreases as a linear relationship with the hydrogen content increases. These results show that the hydrogen pressure and tribofilms also play an important role in the superlow friction of the DLC films. Keywords: DLC, Raman, Hydrogen, superlow friction, a-C:H, ta-C.
1. INTRODUCTION A diamond-like carbon (DLC) film is a thin film of amorphous carbon that has sp2 and sp3 hybridized orbitals. Further, DLC films are expected to be utilized in various applications owing to their excellent tribological properties [1]. It is possible to control the mechanical properties of DLC films with deposition techniques, and it is well-known that these mechanical properties depend on the hydrogen content of the DLC films [2]. Their structures and compositions greatly affect the friction and wear properties [3] [4]. In particular, hydrogenated amorphous carbon (a-C:H) films are of special interest because some of these films exhibit a low friction coefficient lower than 0.01 and down to 0.001 in high-vacuum or inert environments owing to the build-up of tribofilms on the counter faces [4] [5]. This phenomenon is known as the “superlow friction” of a-C:H films. However, not all DLC films exhibit superlow friction, and the DLC films with low hydrogen content could not exhibit extremely low friction [6]. Therefore, the hydrogen content is the key factor for the superlow friction of DLC films. Further, the effect of the hydrogenation condition also plays an important role in the superlow friction of the DLC films. At high hydrogen pressures of 500 Pa and 1000 Pa, superlow friction is preserved owing to healing of the transfer film on the counter faces [7]. Many researchers have published papers regarding the superlow friction of DLC films and proposed models of the mechanism [7] [8]. One of the models assumed that the superlow friction originated from the weak Van der Waals interaction of the hydrogen atoms between the sliding surfaces [8]. However, the tribological mechanisms of superlow friction have not yet been fully comprehended. In our previous work [9], it was reported that super-low friction appeared for a hydrogenated DLC tribopair sliding in a hydrogenated gas. In addition, the time-of-flight mass spectrometry (TOF-SIMS) results showed that a deuterium ion was observed on the surface of the specimens marked by deuterated gas. This result indicated that the hydrogenated gas affects the superlow friction of DLC films on sliding surfaces. However, the influence of the hydrogenated gas on the superlow friction was not clarified. In this study, the effects of hydrogen gas on the frictional behavior of DLC films were investigated by friction tests with varying hydrogen pressures. 2. EXPERIMENTAL DETAILS Steel disks (24 mm × t7.9 mm, ISO100Cr6) and balls (4 mm, ISO100Cr6) coated with DLC films were used as test specimens. The Vickers hardness of steel disks and balls is in
the range 800 - 900 HV. The hardness, coating thickness, surface roughness, and hydrogen content of the DLC films were measured using a Tribo-indenter (Ti750, Hysitrion, US), Calotest, surface profile meter (1500SD3, Tokyo Seimeitsu Ltd., Japan) and Elastic recoil detection analysis (ERDA), respectively. Two types of coated DLC films were used, and their properties are listed in Table 1. The two types of coatings were deposited by different techniques; the hydrogenated DLC (a-C:H, hydrogen content: 30 atomic %) and non-hydrogenated DLC (ta-C, hydrogen content: free) coatings were deposited by plasma-enhanced chemical vapor deposition (PECVD) and arc ion plating (AIP) deposition, respectively. The surface roughness of Ra was 0.0060 mm for a-C:H film and 0.0103 mm for ta-C film. The friction tests were carried out using the ball-on-disk-type tribo-tester in Fig. 1, in which the test environment could be changed. The test conditions were a load of 2.5 N, a rotation radius of 5 mm, and a rotation sp m eed of 6 rpm (The Hertz'ian pressure: 1.16 GPa,
Radius of contact area :40 μm, Sliding velocity: 3.14 mm/s). All tests were conducted for hydrogenated conditions. In this study, the hydrogen pressure was varied from 500 Pa to 5000 Pa in increments of 500 Pa every three minutes during the friction tests, as shown in Fig. 2. After the hydrogen pressure reached 5000 Pa, it decreased from 5000 Pa to 500 Pa in increments of 500 Pa every three minutes. The average friction coefficient was evaluated at each hydrogen pressure. After the friction tests, the wear tracks were then examined by confocal laser scanning microscopy (LEXT OLS3500, Olympus, JP) and Raman spectroscopy (NRS-3200, JASCO, JP). 3. RESULTS AND DISCUSSION 3.1. Friction tests Friction tests were conducted on the ta-C/ta-C and a-C:H/a-C:H tribopairs for hydrogenated conditions. The frictional behaviors of the ta-C and a-C:H films in hydrogen conditions are shown in Fig. 3 as a function of the sliding time. As shown in Fig. 3, the frictional behaviors of the DLC films depended on hydrogen pressures. The relationship between the hydrogen pressure and the average friction coefficient for the ta-C/ta-C tribopair is shown in Fig. 4. When the hydrogen pressure was gradually increased, the average friction coefficient decreased linearly. On the other hand, the friction coefficients linearly increased when the hydrogen pressure subsequently decreased. Fig. 5 shows the relationship between the hydrogen pressure and the average friction coefficient for the a-C:H/a-C:H tribopair. In contrast to the ta-C/ta-C tribopair, the observed
relationship was nonlinear. Instead, the friction coefficient gradually decreased between 500 Pa and 3500 Pa as the hydrogen pressure increased. At a hydrogen pressure of 5000 Pa, the friction coefficient exhibited superlow friction. Moreover, the friction behaviors were completely different for the increase and decrease in hydrogen pressure. The friction coefficients of the test for decreasing pressure exhibited lower values at each point of hydrogen pressure compared with those of the corresponding points of the test for increasing pressure. In Fig. 4 and Fig. 5, it is clear that the a-C:H film exhibits a lower friction coefficient than the ta-C film, indicating that the hydrogen content of the DLC films plays a key role in superlow friction. On the other hand, both tribopairs of DLC films exhibited interesting relationships between the hydrogen pressure and the average friction coefficient. The ta-C/ta-C tribopair exhibited a linearly relationship between the hydrogen pressure and the average friction coefficient. For the a-C:H/a-C:H tribopair, the results from the tests for increasing and decreasing hydrogen pressures exhibit loop-like behavior. It is considered that the hydrogen pressure and hydrogen content have an effect on the tribofilms and that the tribofilms significantly influenced the loop-like behavior of the friction coefficient. However, it is not enough to discuss this behavior from the only the results of the friction tests. The description of this behavior will be discussed with the Raman spectra of the wear track. 3.2 Wear track images The three types of specimens were examined by confocal laser scanning microscopy: the disk and ball before and after the entire friction test (increase in H2 followed by decrese in H2) as well as at a hydrogen pressure of 5000 Pa. Fig. 6 shows the images of the ta-C specimens that indicate that tribofilms were observed on the ball after the friction test and at a hydrogen pressure of 5000 Pa. For the a-C:H/a-C:H tribopair, the tribofilm on the a-C:H ball was also observed at a hydrogen pressure of 5000 Pa and after the friction test (Fig. 7). It appears that the tribofilm on a-C:H ball existed as polymer-like film. It is estimated that the polymer-like film was key factor for superlow friction of a-C:H film. Fig. 8 (a, b) shows the typical cross-sectional profiles of these tribofilms for the a-C:H and ta-C balls at a hydrogen pressure of 5000 Pa. The tribofilm of the ta-C film has a thickness of 1–3 μm. However, the thickness was not able to be determined for the tribofilm of the a-C:H film. These surface profiles indicate that the types of tribofilms are different for the ta-C and a-C:H films. It is considered that their difference is related to the frictional behavior of the DLC films under hydrogenated conditions. 3.3 Raman analysis
Raman spectroscopic analyses were carried out for the three types of specimens: the disk and ball before the friction test, after the friction test, and at a hydrogen pressure of 5000 Pa. The Raman spectra were obtained using a 514 nm diode laser with a maximum output power of 0.5 mW in conjunction with an objective lens having a magnification of 20x. The analyzed points are marked in the images as red points in Fig. 6 and Fig. 7. In the cases of the analyses of the ball specimens, the Raman analyses were conducted on the tribofilm observed in Fig. 6 and Fig. 7. In this study, the degree of graphitization is estimated by calculating the ratio of D to G intensities [10]. In addition, the evolution of the hydrogen content of the a-C:H films was estimated by another calculation method. From the previous works [11,12], a method using the G-peak intensity could be divided as shown in Fig. 9 for two intensities: the fluorescent intensity called the N peak and the scattering intensity called the S peak. Further, the logarithm of (N/S) has a linear relationship with the hydrogen content of the DLC film. However, the method cannot be used below the H2 contents 20 at.%. This is clearly the case for the ta-C film. Therefore, this method was used to estimate the hydrogen content of the a-C:H/a-C:H tribopair. In Fig. 10 (a, b), Raman spectra of the as-deposited ta-C and a-C:H films are compared with the spectra obtained at a hydrogen pressure of 5000 Pa and after the friction test. A significant structural transformation was not observed for either the ta-C or a-C:H disks. However, a structural transformation existed for the tribofilms of both the ta-C and a-C:H balls at a hydrogen pressure of 5000 Pa and after the friction test. A comparison of the ratio of D to G intensities for the ta-C and a-C:H balls is shown in Fig. 11. A drastic increase in the intensity ratio was observed for the ta-C ball at a hydrogen pressure of 5000 Pa and after the friction test. These results show that only the ta-C ball exhibits the effects of graphitization [10]. It is widely known that the graphitization of DLC films is closely related to their frictional properties. We believed that it is necessary for achieving low friction of the ta-C film to form the graphite layer (as shown in Fig.6). However, there are no differences the ratio of D to G intensities (the degree of graphitization) between at a hydrogen pressure of 5000 Pa and after the friction test even if the friction coefficient causes a dramatically changes (Fig. 4). In other words, the friction coefficient of the ta-C film
changed depending on hydrogen pressure (Fig.4). Therefore, the hydrogen pressure in the test atmosphere dominates the frictional behavior for the ta-C/ta-C tribopair. For the a-C:H/a-C:H tribopair, the a-C:H ball at a hydrogen pressure of 5000 Pa exhibited a low intensity ratio compared with the ratio before and after the friction test. From these results, the hydrogen pressure affected the composition of the tribofilm of the a-C:H film. A comparison of the values of log (N/S) for the a-C:H disk and ball are shown in Fig. 12. For the a-C:H disk, the values did not appear to change during the friction tests. On the other
hand, Fig. 12 shows a remarkable change in the values for the a-C:H ball. At a hydrogen pressure of 5000 Pa, the value of log (N/S) were higher than the value before the friction test. The result show that the tribofilm on the a-C:H ball obtained a large amount of hydrogen during the friction test with a higher hydrogen pressure. Therefore, it is considered that the hydrogen content of tribofilm is the key factor for the superlow friction of a-C:H films. 3.4. Effect of hydrogen pressure on frictional properties of a-C:H film The effects of hydrogen gas on the frictional behavior of the DLC films were investigated. In particular, a-C:H/a-C:H tribopair exhibited interesting frictional behavior (Fig. 5). The hydrogen pressure had no effect on the frictional properties when the hydrogen pressure was increased from 500 Pa to 3500 Pa. However, the a-C:H film exhibited superlow friction at the high hydrogen pressure of 5000 Pa because the formation of the tribofilm included a large hydrogen content. From the tests for decreasing hydrogen pressure, the a-C:H film also exhibited a low friction coefficient at higher hydrogen pressures. However, the average friction coefficient gradually increased as the hydrogen pressure decreased. The reason for these phenomena was the absence of tribofilms on the sliding surface at lower hydrogen pressures. Therefore, these test results for increasing and decreasing hydrogen pressures show the loop-like behavior in Fig. 5. Moreover, Fig. 13 shows images of the tribofilm on a a-C:H ball and the Raman spectra obtained from the tribofilm at hydrogen pressures of 4500 Pa when the pressure was increased and 1500 Pa when the hydrogen pressure decreased. At a hydrogen pressure of 4500 Pa, a tribofilm was observed, similar to the film at a hydrogen pressure of 5000 Pa (Fig. 7). In contrast, it appears that the tribofilm gradually disappeared owing to sliding at a hydrogen pressure of 1500 Pa when the pressure was decreased. After the friction test, the tribofilm including a large amount of hydrogen was hardly observed, as shown in Fig. 7. Therefore, these results indicate that the hydrogen pressure affects the frictional properties as well as the formation of the tribofilm. On the other hand, the behavior of log (N/S) from the Raman spectra for various hydrogen pressures exhibited a linearly relationship between the average friction coefficient and log (N/S) for the a-C:H ball as shown in Fig. 14. The average friction coefficient decreases as log (N/S) increases, demonstrating that the tribofilm with high hydrogen content exhibited a low friction coefficient. The difference observed under hydrogen conditions between high and low frictions could be explained by the surface interactions between the hydrogen-rich tribofilm and a-C:H surface. In tribofilm with a higher hydrogen content, hydrogen atoms are considered to cover the a-C:H surface, leading to weak Van der Waals interactions between the sliding
surfaces; whereas in tribofilm with a lower hydrogen content there are not enough hydrogen atoms to shield the strong interactions between the π-orbitals of sp2 carbon double bonds [13]. Moreover, the difference observed under hydrogen conditions between high and low frictions could also be explained by hydrogen pressure and formation of tribofilm. For high hydrogen pressures, it is estimated that the amount of supplied hydrogen is more than that of desorbed hydrogen as shown in Fig 15 (a). The condition enables hydrogen-rich tribofilm to form on a-C:H ball. For low hydrogen pressures, it is estimated that the amount of desorbed hydrogen is more than that of supplied hydrogen as shown in Fig. 15(b). This shows that it is difficult to form hydrogen-rich tribofilm on a-C:H ball at low hydrogen pressures. Therefore, the hydrogen pressure strongly affects the frictional properties of the a-C:H films through the formation of tribofilms, and that the tribofilm with high hydrogen content play an important role in the superlow friction of the a-C:H film. 4. CONCLUSIONS The effects of hydrogen gas on the frictional behavior of DLC films were investigated by varying the hydrogen pressure during friction tests. The main conclusions are as follows: •
The ta-C/ta-C tribopair exhibited a proportional relationship between the hydrogen
pressure and the average friction coefficient. In addition, the results of the Raman spectroscopic analysis showed that the ta-C balls exhibited the effects of graphitization. However, the hydrogen pressure in the test atmosphere dominates the frictional behavior of the ta-C/ta-C tribopair because there was no difference in the degree of graphitization for the specimens at a hydrogen pressure of 5000 Pa and after the friction test. •
At a higher hydrogen pressure of 5000 Pa, the a-C:H film exhibited superlow friction
owing to the formation of tribofilms including a large amount of hydrogen. In addition, the results from the tests for increasing and decreasing hydrogen pressures exhibited hysteretic frictional behavior. It is considered that the hydrogen pressure and tribofilms play the key roles in that relationship. •
For a-C:H/a-C:H tribopair, the average friction coefficient decreased as the hydrogen
content of tribofilm on the a-C:H ball increased for various hydrogen pressures. From this relationship, the hydrogen content of the tribofilm is the key factor for superlow friction. 5. REFERENCES
1.
J. Robertson, Diamond-like amorphous carbon. Materials Science and Engineering: R
37 (2002) 129–281. 2.
A. Grill, Diamond-like carbon: state of the art. Diamond Relat. Mater. 8 (1999)
428–434 3.
A. Grill, Tribology of diamond like carbon and related materials: an updated review.
Surface and Coatings Technology 94–95 (1997) 507–513. 4.
C. Donnet, M. Belin, JC. Auge, JM. Martin, A. Grill, V. Patel, Tribochemistry of
diamond-like carbon coatings in various environments. Surface and Coatings Technology 68–69 (1994) 626–631. 5.
A. Erdemir, OL. Eryilmaz, G. Fenske, Synthesis of diamond like carbon films with
superlow friction and wear properties. Journal of Vacuum Science & Technology A 18 (2000)1987–1992. 6.
S. Miyake, S. Takahashi, I. Watanabe, H. Yoshihara, Friction and wear behavior of
hard carbon films. A S L E Transactions (1987) 30 121–127. 7.
J. Fontaine, M. Belin, T. Le Mogne, A. Grill, How to restore superlow friction of
DLC: the healing effect of hydrogen gas. Tribology International 37 (2004) 869–877. 8.
A. Erdemir, Genesis of superlow friction and wear in diamond like carbon films.
Tribology International 37 (2004) 1005–1012. 9.
K. Oshima, Y. Tokuta, R. Tsuboi S. Sasaki M. Kawaguchi, Effect of surrounding
hydrogen gas on friction and wear characteristics of DLC films. Proceedings of the 15th Int. Conference on Experimental Mechanics, POR, 2012, pp. 905–906. 10.
JC. Sanchez-Lopez, A. Erdemir, C. Donnet, TC. Rojas, Friction-induced structural
transformations of diamond-like carbon coatings under various atmospheres. Surface and Coatings Technology 163–164 (2003) 444–450. 11.
K. Miura, M. Nakamura, Analysis of hydrogen concentration in DLC films by Raman
Spectroscopy. Journal of the Surface Finishing Society of Japan 59 (2008) 203–205.
12.
Junho Choi, Keisuke Ishii, Takahisa Kato, Masahiro Kawaguchi, Wonsik Lee,
Structural and mechanical properties of DLC films prepared by bipolar PBII&D. Diamond & Related Materials 20 (2011) 845–848 13.
J. Fontaine, T. Le Mogne, J.L. Loubet, M. Belin, Achieving superlow friction with
hydrogenated amorphous carbon: some key requirements. Thin Solid Films 482 (2005) 99–108.
List of Table Table 1 Properties
of the DLC coatings Table1 Properties of the DLC coatings.
Table 1 a-C:H
ta-C
Ra
mm
0.0060
0.0103
Hardness
GPa
20.5
72.6
Hydrogen contents
atomic %
30
Free
Coating thickness
μm
1-2
1-2
List of Figures Fig. 1 Ball-on-disk tribo-tester.
Fig. 2 Hydrogen pressure variation versus sliding time. Fig. 3 Frictional behavior of the DLC films in low-pressure hydrogen conditions. Fig. 4 Hydrogen pressure effect on the friction coefficients of the ta-C/ta-C tribopair. Fig. 5 Hydrogen pressure effect on the friction coefficient of the a-C:H/a-C:H tribopair. Fig. 6 Optical micrographs of the initial surface and wear scars for the ta-C/ta-C tribopair. Fig. 7 Optical micrographs of the initial surface and wear scars for the a-C:H/a-C:H tribopair. Fig. 8 Cross-sectional profiles of the typical worn surfaces for (a) a-C:H and (b) ta-C balls at a hydrogen pressure of 5000 Pa.. Fig. 9 Typical Raman spectrum of the DLC films. Fig. 10 Results of the Raman spectroscopic analyses for (a) ta-C/ta-C and a-C:H/a-C:H tribopairs. Fig. 11 Values of ID/IG from the Raman spectra of both DLC films. Fig. 12 The values of log(N/S) from the Raman spectra of both DLC films. Fig. 13 Optical micrographs of the wear scars and the Raman spectra of the a-C:H balls at several hydrogen pressures. Fig.14. Relationship between the value of log (N/S) of the a-C:H ball and the average friction coefficient.
Fig. 15 Schematic of the formation of tribofilm on a-C:H ball in (a) high hydrogen pressure condition and (b) low hydrogen pressure condition.
Hydrogen gas
Load
Ball Disk
Strain gauge & Weight
Pump
Fig. 1 Ball-on-disk tribo-tester
Hydrogen pressure [ Pa ]
5000
2500
500 0
1000
2000
3000
Sliding time [ s ]
Fig. 2 Hydrogen pressure variation versus sliding time.
Hydrogen Pressure ta−C/ta−C tribopair a−C:H/a−C:H tribopair
5000
0.2
0 0
0
1000
2000
Hydrogen pressure [Pa]
Friction coefficient
0.4
3000
Sliding time [s]
Fig. 3 Frictional behavior of the DLC films in low-pressure hydrogen conditions
Average friction coefficient
0.15
ta−C H2 increase ta−C H2 decrease H2 increase
0.1
H2 decrease 0.05
0 500
2500
5000
Hydrogen pressure [ Pa ]
Fig. 4 Hydrogen pressure effect on the friction coefficients of the ta-C/ta-C tribopair.
Average friction coefficient
0.15
a−C:H H2 increase a−C:H H2 decrease
0.1
H2 increase
0.05
H2 decrease
0 500
2500
5000
Hydrogen pressure [ Pa ]
Fig. 5 Hydrogen pressure effect on the friction coefficient of the a-C:H/a-C:H tribopair.
S. D.
S. D.
S. D.
S. D.
Fig. 6 Optical micrographs of the initial surface and wear scars for the ta-C/ta-C tribopair.
S. D.
S. D.
S. D.
S. D.
Fig. 7 Optical micrographs of the initial surface and wear scars for the a-C:H/a-C:H tribopair.
(a)
30
20
40
Tribofilm Initial surface m] Depth [ [μm] Height
m] height [ [μm] Height
40
Position [ [mm] m] Positon [μm]
(b)
Tribofilm Initial surface
30
20
Position [[mm] m] Positon [μm]
Fig. 8 Cross-sectional profiles of the typical worn surfaces for (a) a-C:H and (b) ta-C balls at a hydrogen pressure of 5000 Pa.
G peak
D peak S
Baseline N Raman shift [cm-1]
Fig. 9 Typical Raman spectrum of the DLC films.
(a)
Before test At 5000 Pa
1200
1500
1800
900
−1
Before test At 5000 Pa
1200
At 5000 Pa
1500
1800 −1
1800
a−C:H BALL
After test
Intensity [ arb unit ]
Intensity [ arb unit ]
Before test
a−C:H DISK
Raman shift [ cm ]
1500
Raman shift [ cm ]
After test
900
1200
−1
Raman shift [ cm ]
(b)
ta−C BALL
After test
Intensity [ arb unit ]
Intensity [ arb unit ]
After test
900
Before test At 5000 Pa
ta−C DISK
900
1200
1500
1800 −1
Raman shift [ cm ]
Fig. 10 Results of the Raman spectroscopic analyses for (a) ta-C/ta-C and a-C:H/a-C:H tribopairs.
ID / I G
1.5
ta−C BALL
a−C:H BALL
1
0.5
0 0
Before 5000Pa After
Before 5000Pa After
Fig. 11 Values of ID/IG from the Raman spectra of both DLC films
a−C:H DISK
Log( N / S )
1.5
a−C:H BALL
1
0.5
0 0
Before
5000Pa
After
Before
5000Pa
After
Fig. 12 The values of log(N/S) from the Raman spectra of both DLC films.
S. D.
S. D.
Fig. 13 Optical micrographs of the wear scars and the Raman spectra of the a-C:H balls at several hydrogen pressures.
Fig.14. Relationship between the value of log (N/S) of the a-C:H ball and the average friction coefficient.
Fig.15. Schematic of the formation of tribofilm on a-C:H ball in (a) high hydrogen pressure condition and (b) low hydrogen pressure condition.
HIGHLIGHTS The friction behavior of ta-C and a-C:H varied depending on hydrogen pressures. The a-C:H showed superlow friction when the hydrogen pressure reached 5000 Pa. The friction coefficient decreased with increment of hydrogen within tribofilm.
www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(15)00190-8 http://dx.doi.org/10.1016/j.wear.2015.03.018 WEA101386
To appear in: Wear Received date: 23 September 2014 Revised date: 22 January 2015 Accepted date: 21 March 2015 Cite this article as: Hikaru Okubo, Ryo Tsuboi and Shinya Sasaki, Frictional properties of DLC films in low-pressure Hydrogen conditions, Wear, http://dx.doi.org/10.1016/j.wear.2015.03.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Frictional properties of DLC films in low-pressure hydrogen conditions Hikaru Okubo1, Ryo Tsuboi2, Shinya Sasaki3* 1
Graduate School, Tokyo University of Science 6-3-1 Niijuku, Katsushika-ku, Tokyo, 125-8585, Japan. 2 3
Daido University, Japan.
Tokyo University of Science, Japan.
*Corresponding author: Shinya Sasaki Tel.: +81 3 5876 1334, E-mail address: [email protected], [email protected]
Abstract The effects of hydrogen gas on the frictional behavior of diamond-like carbon (DLC) films were investigated by varying the hydrogen pressure during a friction test. Hydrogenated DLC (a-C:H) and non-hydrogenated DLC (ta-C) coatings have been tested. The ta-C/ta-C tribopair exhibited a linearly relationship between the hydrogen pressure and the average friction coefficients. On the other hand, the a-C:H/a-C:H tribopair exhibited an extremely low friction coefficient of less than 0.01 when the hydrogen pressure reached 5000 Pa. In addition, the friction coefficients from the tests at increased and decreased hydrogen pressures exhibited loop-like behavior. It is considered that the hydrogen pressure and the hydrogen content of the DLC film has an effect on the tribofilms and that the tribofilms played an important role in the loop-like frictional behavior. The evaluation of the hydrogen content of the DLC films was carried out by the G-peak method. The value of logarithm of (N/S) can be used as a measure of the hydrogen content, where the intensity of the fitted G-peak; S, the maximum intensity of the G-peak; and N, the intensity of the photoluminescence background. The behavior of logarithm of (N/S) from the Raman spectra for various hydrogen pressures indicated an interesting relationship between the average friction coefficient and the hydrogen content of tribofilm on a-C:H ball. The relationship showed the average friction coefficient decreases as a linear relationship with the hydrogen content increases. These results show that the hydrogen pressure and tribofilms also play an important role in the superlow friction of the DLC films. Keywords: DLC, Raman, Hydrogen, superlow friction, a-C:H, ta-C.
1. INTRODUCTION A diamond-like carbon (DLC) film is a thin film of amorphous carbon that has sp2 and sp3 hybridized orbitals. Further, DLC films are expected to be utilized in various applications owing to their excellent tribological properties [1]. It is possible to control the mechanical properties of DLC films with deposition techniques, and it is well-known that these mechanical properties depend on the hydrogen content of the DLC films [2]. Their structures and compositions greatly affect the friction and wear properties [3] [4]. In particular, hydrogenated amorphous carbon (a-C:H) films are of special interest because some of these films exhibit a low friction coefficient lower than 0.01 and down to 0.001 in high-vacuum or inert environments owing to the build-up of tribofilms on the counter faces [4] [5]. This phenomenon is known as the “superlow friction” of a-C:H films. However, not all DLC films exhibit superlow friction, and the DLC films with low hydrogen content could not exhibit extremely low friction [6]. Therefore, the hydrogen content is the key factor for the superlow friction of DLC films. Further, the effect of the hydrogenation condition also plays an important role in the superlow friction of the DLC films. At high hydrogen pressures of 500 Pa and 1000 Pa, superlow friction is preserved owing to healing of the transfer film on the counter faces [7]. Many researchers have published papers regarding the superlow friction of DLC films and proposed models of the mechanism [7] [8]. One of the models assumed that the superlow friction originated from the weak Van der Waals interaction of the hydrogen atoms between the sliding surfaces [8]. However, the tribological mechanisms of superlow friction have not yet been fully comprehended. In our previous work [9], it was reported that super-low friction appeared for a hydrogenated DLC tribopair sliding in a hydrogenated gas. In addition, the time-of-flight mass spectrometry (TOF-SIMS) results showed that a deuterium ion was observed on the surface of the specimens marked by deuterated gas. This result indicated that the hydrogenated gas affects the superlow friction of DLC films on sliding surfaces. However, the influence of the hydrogenated gas on the superlow friction was not clarified. In this study, the effects of hydrogen gas on the frictional behavior of DLC films were investigated by friction tests with varying hydrogen pressures. 2. EXPERIMENTAL DETAILS Steel disks (24 mm × t7.9 mm, ISO100Cr6) and balls (4 mm, ISO100Cr6) coated with DLC films were used as test specimens. The Vickers hardness of steel disks and balls is in
the range 800 - 900 HV. The hardness, coating thickness, surface roughness, and hydrogen content of the DLC films were measured using a Tribo-indenter (Ti750, Hysitrion, US), Calotest, surface profile meter (1500SD3, Tokyo Seimeitsu Ltd., Japan) and Elastic recoil detection analysis (ERDA), respectively. Two types of coated DLC films were used, and their properties are listed in Table 1. The two types of coatings were deposited by different techniques; the hydrogenated DLC (a-C:H, hydrogen content: 30 atomic %) and non-hydrogenated DLC (ta-C, hydrogen content: free) coatings were deposited by plasma-enhanced chemical vapor deposition (PECVD) and arc ion plating (AIP) deposition, respectively. The surface roughness of Ra was 0.0060 mm for a-C:H film and 0.0103 mm for ta-C film. The friction tests were carried out using the ball-on-disk-type tribo-tester in Fig. 1, in which the test environment could be changed. The test conditions were a load of 2.5 N, a rotation radius of 5 mm, and a rotation sp m eed of 6 rpm (The Hertz'ian pressure: 1.16 GPa,
Radius of contact area :40 μm, Sliding velocity: 3.14 mm/s). All tests were conducted for hydrogenated conditions. In this study, the hydrogen pressure was varied from 500 Pa to 5000 Pa in increments of 500 Pa every three minutes during the friction tests, as shown in Fig. 2. After the hydrogen pressure reached 5000 Pa, it decreased from 5000 Pa to 500 Pa in increments of 500 Pa every three minutes. The average friction coefficient was evaluated at each hydrogen pressure. After the friction tests, the wear tracks were then examined by confocal laser scanning microscopy (LEXT OLS3500, Olympus, JP) and Raman spectroscopy (NRS-3200, JASCO, JP). 3. RESULTS AND DISCUSSION 3.1. Friction tests Friction tests were conducted on the ta-C/ta-C and a-C:H/a-C:H tribopairs for hydrogenated conditions. The frictional behaviors of the ta-C and a-C:H films in hydrogen conditions are shown in Fig. 3 as a function of the sliding time. As shown in Fig. 3, the frictional behaviors of the DLC films depended on hydrogen pressures. The relationship between the hydrogen pressure and the average friction coefficient for the ta-C/ta-C tribopair is shown in Fig. 4. When the hydrogen pressure was gradually increased, the average friction coefficient decreased linearly. On the other hand, the friction coefficients linearly increased when the hydrogen pressure subsequently decreased. Fig. 5 shows the relationship between the hydrogen pressure and the average friction coefficient for the a-C:H/a-C:H tribopair. In contrast to the ta-C/ta-C tribopair, the observed
relationship was nonlinear. Instead, the friction coefficient gradually decreased between 500 Pa and 3500 Pa as the hydrogen pressure increased. At a hydrogen pressure of 5000 Pa, the friction coefficient exhibited superlow friction. Moreover, the friction behaviors were completely different for the increase and decrease in hydrogen pressure. The friction coefficients of the test for decreasing pressure exhibited lower values at each point of hydrogen pressure compared with those of the corresponding points of the test for increasing pressure. In Fig. 4 and Fig. 5, it is clear that the a-C:H film exhibits a lower friction coefficient than the ta-C film, indicating that the hydrogen content of the DLC films plays a key role in superlow friction. On the other hand, both tribopairs of DLC films exhibited interesting relationships between the hydrogen pressure and the average friction coefficient. The ta-C/ta-C tribopair exhibited a linearly relationship between the hydrogen pressure and the average friction coefficient. For the a-C:H/a-C:H tribopair, the results from the tests for increasing and decreasing hydrogen pressures exhibit loop-like behavior. It is considered that the hydrogen pressure and hydrogen content have an effect on the tribofilms and that the tribofilms significantly influenced the loop-like behavior of the friction coefficient. However, it is not enough to discuss this behavior from the only the results of the friction tests. The description of this behavior will be discussed with the Raman spectra of the wear track. 3.2 Wear track images The three types of specimens were examined by confocal laser scanning microscopy: the disk and ball before and after the entire friction test (increase in H2 followed by decrese in H2) as well as at a hydrogen pressure of 5000 Pa. Fig. 6 shows the images of the ta-C specimens that indicate that tribofilms were observed on the ball after the friction test and at a hydrogen pressure of 5000 Pa. For the a-C:H/a-C:H tribopair, the tribofilm on the a-C:H ball was also observed at a hydrogen pressure of 5000 Pa and after the friction test (Fig. 7). It appears that the tribofilm on a-C:H ball existed as polymer-like film. It is estimated that the polymer-like film was key factor for superlow friction of a-C:H film. Fig. 8 (a, b) shows the typical cross-sectional profiles of these tribofilms for the a-C:H and ta-C balls at a hydrogen pressure of 5000 Pa. The tribofilm of the ta-C film has a thickness of 1–3 μm. However, the thickness was not able to be determined for the tribofilm of the a-C:H film. These surface profiles indicate that the types of tribofilms are different for the ta-C and a-C:H films. It is considered that their difference is related to the frictional behavior of the DLC films under hydrogenated conditions. 3.3 Raman analysis
Raman spectroscopic analyses were carried out for the three types of specimens: the disk and ball before the friction test, after the friction test, and at a hydrogen pressure of 5000 Pa. The Raman spectra were obtained using a 514 nm diode laser with a maximum output power of 0.5 mW in conjunction with an objective lens having a magnification of 20x. The analyzed points are marked in the images as red points in Fig. 6 and Fig. 7. In the cases of the analyses of the ball specimens, the Raman analyses were conducted on the tribofilm observed in Fig. 6 and Fig. 7. In this study, the degree of graphitization is estimated by calculating the ratio of D to G intensities [10]. In addition, the evolution of the hydrogen content of the a-C:H films was estimated by another calculation method. From the previous works [11,12], a method using the G-peak intensity could be divided as shown in Fig. 9 for two intensities: the fluorescent intensity called the N peak and the scattering intensity called the S peak. Further, the logarithm of (N/S) has a linear relationship with the hydrogen content of the DLC film. However, the method cannot be used below the H2 contents 20 at.%. This is clearly the case for the ta-C film. Therefore, this method was used to estimate the hydrogen content of the a-C:H/a-C:H tribopair. In Fig. 10 (a, b), Raman spectra of the as-deposited ta-C and a-C:H films are compared with the spectra obtained at a hydrogen pressure of 5000 Pa and after the friction test. A significant structural transformation was not observed for either the ta-C or a-C:H disks. However, a structural transformation existed for the tribofilms of both the ta-C and a-C:H balls at a hydrogen pressure of 5000 Pa and after the friction test. A comparison of the ratio of D to G intensities for the ta-C and a-C:H balls is shown in Fig. 11. A drastic increase in the intensity ratio was observed for the ta-C ball at a hydrogen pressure of 5000 Pa and after the friction test. These results show that only the ta-C ball exhibits the effects of graphitization [10]. It is widely known that the graphitization of DLC films is closely related to their frictional properties. We believed that it is necessary for achieving low friction of the ta-C film to form the graphite layer (as shown in Fig.6). However, there are no differences the ratio of D to G intensities (the degree of graphitization) between at a hydrogen pressure of 5000 Pa and after the friction test even if the friction coefficient causes a dramatically changes (Fig. 4). In other words, the friction coefficient of the ta-C film
changed depending on hydrogen pressure (Fig.4). Therefore, the hydrogen pressure in the test atmosphere dominates the frictional behavior for the ta-C/ta-C tribopair. For the a-C:H/a-C:H tribopair, the a-C:H ball at a hydrogen pressure of 5000 Pa exhibited a low intensity ratio compared with the ratio before and after the friction test. From these results, the hydrogen pressure affected the composition of the tribofilm of the a-C:H film. A comparison of the values of log (N/S) for the a-C:H disk and ball are shown in Fig. 12. For the a-C:H disk, the values did not appear to change during the friction tests. On the other
hand, Fig. 12 shows a remarkable change in the values for the a-C:H ball. At a hydrogen pressure of 5000 Pa, the value of log (N/S) were higher than the value before the friction test. The result show that the tribofilm on the a-C:H ball obtained a large amount of hydrogen during the friction test with a higher hydrogen pressure. Therefore, it is considered that the hydrogen content of tribofilm is the key factor for the superlow friction of a-C:H films. 3.4. Effect of hydrogen pressure on frictional properties of a-C:H film The effects of hydrogen gas on the frictional behavior of the DLC films were investigated. In particular, a-C:H/a-C:H tribopair exhibited interesting frictional behavior (Fig. 5). The hydrogen pressure had no effect on the frictional properties when the hydrogen pressure was increased from 500 Pa to 3500 Pa. However, the a-C:H film exhibited superlow friction at the high hydrogen pressure of 5000 Pa because the formation of the tribofilm included a large hydrogen content. From the tests for decreasing hydrogen pressure, the a-C:H film also exhibited a low friction coefficient at higher hydrogen pressures. However, the average friction coefficient gradually increased as the hydrogen pressure decreased. The reason for these phenomena was the absence of tribofilms on the sliding surface at lower hydrogen pressures. Therefore, these test results for increasing and decreasing hydrogen pressures show the loop-like behavior in Fig. 5. Moreover, Fig. 13 shows images of the tribofilm on a a-C:H ball and the Raman spectra obtained from the tribofilm at hydrogen pressures of 4500 Pa when the pressure was increased and 1500 Pa when the hydrogen pressure decreased. At a hydrogen pressure of 4500 Pa, a tribofilm was observed, similar to the film at a hydrogen pressure of 5000 Pa (Fig. 7). In contrast, it appears that the tribofilm gradually disappeared owing to sliding at a hydrogen pressure of 1500 Pa when the pressure was decreased. After the friction test, the tribofilm including a large amount of hydrogen was hardly observed, as shown in Fig. 7. Therefore, these results indicate that the hydrogen pressure affects the frictional properties as well as the formation of the tribofilm. On the other hand, the behavior of log (N/S) from the Raman spectra for various hydrogen pressures exhibited a linearly relationship between the average friction coefficient and log (N/S) for the a-C:H ball as shown in Fig. 14. The average friction coefficient decreases as log (N/S) increases, demonstrating that the tribofilm with high hydrogen content exhibited a low friction coefficient. The difference observed under hydrogen conditions between high and low frictions could be explained by the surface interactions between the hydrogen-rich tribofilm and a-C:H surface. In tribofilm with a higher hydrogen content, hydrogen atoms are considered to cover the a-C:H surface, leading to weak Van der Waals interactions between the sliding
surfaces; whereas in tribofilm with a lower hydrogen content there are not enough hydrogen atoms to shield the strong interactions between the π-orbitals of sp2 carbon double bonds [13]. Moreover, the difference observed under hydrogen conditions between high and low frictions could also be explained by hydrogen pressure and formation of tribofilm. For high hydrogen pressures, it is estimated that the amount of supplied hydrogen is more than that of desorbed hydrogen as shown in Fig 15 (a). The condition enables hydrogen-rich tribofilm to form on a-C:H ball. For low hydrogen pressures, it is estimated that the amount of desorbed hydrogen is more than that of supplied hydrogen as shown in Fig. 15(b). This shows that it is difficult to form hydrogen-rich tribofilm on a-C:H ball at low hydrogen pressures. Therefore, the hydrogen pressure strongly affects the frictional properties of the a-C:H films through the formation of tribofilms, and that the tribofilm with high hydrogen content play an important role in the superlow friction of the a-C:H film. 4. CONCLUSIONS The effects of hydrogen gas on the frictional behavior of DLC films were investigated by varying the hydrogen pressure during friction tests. The main conclusions are as follows: •
The ta-C/ta-C tribopair exhibited a proportional relationship between the hydrogen
pressure and the average friction coefficient. In addition, the results of the Raman spectroscopic analysis showed that the ta-C balls exhibited the effects of graphitization. However, the hydrogen pressure in the test atmosphere dominates the frictional behavior of the ta-C/ta-C tribopair because there was no difference in the degree of graphitization for the specimens at a hydrogen pressure of 5000 Pa and after the friction test. •
At a higher hydrogen pressure of 5000 Pa, the a-C:H film exhibited superlow friction
owing to the formation of tribofilms including a large amount of hydrogen. In addition, the results from the tests for increasing and decreasing hydrogen pressures exhibited hysteretic frictional behavior. It is considered that the hydrogen pressure and tribofilms play the key roles in that relationship. •
For a-C:H/a-C:H tribopair, the average friction coefficient decreased as the hydrogen
content of tribofilm on the a-C:H ball increased for various hydrogen pressures. From this relationship, the hydrogen content of the tribofilm is the key factor for superlow friction. 5. REFERENCES
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List of Table Table 1 Properties
of the DLC coatings Table1 Properties of the DLC coatings.
Table 1 a-C:H
ta-C
Ra
mm
0.0060
0.0103
Hardness
GPa
20.5
72.6
Hydrogen contents
atomic %
30
Free
Coating thickness
μm
1-2
1-2
List of Figures Fig. 1 Ball-on-disk tribo-tester.
Fig. 2 Hydrogen pressure variation versus sliding time. Fig. 3 Frictional behavior of the DLC films in low-pressure hydrogen conditions. Fig. 4 Hydrogen pressure effect on the friction coefficients of the ta-C/ta-C tribopair. Fig. 5 Hydrogen pressure effect on the friction coefficient of the a-C:H/a-C:H tribopair. Fig. 6 Optical micrographs of the initial surface and wear scars for the ta-C/ta-C tribopair. Fig. 7 Optical micrographs of the initial surface and wear scars for the a-C:H/a-C:H tribopair. Fig. 8 Cross-sectional profiles of the typical worn surfaces for (a) a-C:H and (b) ta-C balls at a hydrogen pressure of 5000 Pa.. Fig. 9 Typical Raman spectrum of the DLC films. Fig. 10 Results of the Raman spectroscopic analyses for (a) ta-C/ta-C and a-C:H/a-C:H tribopairs. Fig. 11 Values of ID/IG from the Raman spectra of both DLC films. Fig. 12 The values of log(N/S) from the Raman spectra of both DLC films. Fig. 13 Optical micrographs of the wear scars and the Raman spectra of the a-C:H balls at several hydrogen pressures. Fig.14. Relationship between the value of log (N/S) of the a-C:H ball and the average friction coefficient.
Fig. 15 Schematic of the formation of tribofilm on a-C:H ball in (a) high hydrogen pressure condition and (b) low hydrogen pressure condition.
Hydrogen gas
Load
Ball Disk
Strain gauge & Weight
Pump
Fig. 1 Ball-on-disk tribo-tester
Hydrogen pressure [ Pa ]
5000
2500
500 0
1000
2000
3000
Sliding time [ s ]
Fig. 2 Hydrogen pressure variation versus sliding time.
Hydrogen Pressure ta−C/ta−C tribopair a−C:H/a−C:H tribopair
5000
0.2
0 0
0
1000
2000
Hydrogen pressure [Pa]
Friction coefficient
0.4
3000
Sliding time [s]
Fig. 3 Frictional behavior of the DLC films in low-pressure hydrogen conditions
Average friction coefficient
0.15
ta−C H2 increase ta−C H2 decrease H2 increase
0.1
H2 decrease 0.05
0 500
2500
5000
Hydrogen pressure [ Pa ]
Fig. 4 Hydrogen pressure effect on the friction coefficients of the ta-C/ta-C tribopair.
Average friction coefficient
0.15
a−C:H H2 increase a−C:H H2 decrease
0.1
H2 increase
0.05
H2 decrease
0 500
2500
5000
Hydrogen pressure [ Pa ]
Fig. 5 Hydrogen pressure effect on the friction coefficient of the a-C:H/a-C:H tribopair.
S. D.
S. D.
S. D.
S. D.
Fig. 6 Optical micrographs of the initial surface and wear scars for the ta-C/ta-C tribopair.
S. D.
S. D.
S. D.
S. D.
Fig. 7 Optical micrographs of the initial surface and wear scars for the a-C:H/a-C:H tribopair.
(a)
30
20
40
Tribofilm Initial surface m] Depth [ [μm] Height
m] height [ [μm] Height
40
Position [ [mm] m] Positon [μm]
(b)
Tribofilm Initial surface
30
20
Position [[mm] m] Positon [μm]
Fig. 8 Cross-sectional profiles of the typical worn surfaces for (a) a-C:H and (b) ta-C balls at a hydrogen pressure of 5000 Pa.
G peak
D peak S
Baseline N Raman shift [cm-1]
Fig. 9 Typical Raman spectrum of the DLC films.
(a)
Before test At 5000 Pa
1200
1500
1800
900
−1
Before test At 5000 Pa
1200
At 5000 Pa
1500
1800 −1
1800
a−C:H BALL
After test
Intensity [ arb unit ]
Intensity [ arb unit ]
Before test
a−C:H DISK
Raman shift [ cm ]
1500
Raman shift [ cm ]
After test
900
1200
−1
Raman shift [ cm ]
(b)
ta−C BALL
After test
Intensity [ arb unit ]
Intensity [ arb unit ]
After test
900
Before test At 5000 Pa
ta−C DISK
900
1200
1500
1800 −1
Raman shift [ cm ]
Fig. 10 Results of the Raman spectroscopic analyses for (a) ta-C/ta-C and a-C:H/a-C:H tribopairs.
ID / I G
1.5
ta−C BALL
a−C:H BALL
1
0.5
0 0
Before 5000Pa After
Before 5000Pa After
Fig. 11 Values of ID/IG from the Raman spectra of both DLC films
a−C:H DISK
Log( N / S )
1.5
a−C:H BALL
1
0.5
0 0
Before
5000Pa
After
Before
5000Pa
After
Fig. 12 The values of log(N/S) from the Raman spectra of both DLC films.
S. D.
S. D.
Fig. 13 Optical micrographs of the wear scars and the Raman spectra of the a-C:H balls at several hydrogen pressures.
Fig.14. Relationship between the value of log (N/S) of the a-C:H ball and the average friction coefficient.
Fig.15. Schematic of the formation of tribofilm on a-C:H ball in (a) high hydrogen pressure condition and (b) low hydrogen pressure condition.
HIGHLIGHTS The friction behavior of ta-C and a-C:H varied depending on hydrogen pressures. The a-C:H showed superlow friction when the hydrogen pressure reached 5000 Pa. The friction coefficient decreased with increment of hydrogen within tribofilm.