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Characteristics and tribological properties in water of Si-DLC coatings
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Diamond & Related Materials 17 (2008) 7 – 12 www.elsevier.com/locate/diamond
Characteristics and tribological properties in water of Si-DLC coatings Xingyang Wu ⁎, Masahiro Suzuki 1 , Tsuguyori Ohana, Akihiro Tanaka National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 30 July 2006; received in revised form 6 July 2007; accepted 12 September 2007 Available online 19 September 2007
Abstract Si-containing diamond-like carbon (Si-DLC) coatings with a Si content ranging between 0 (DLC) and 10 at.% were deposited by thermal electron excited plasma CVD method, and their characteristics and the tribological properties in water environment were investigated. The results showed that doped Si had little effect on the hardness and Young's modulus of the coatings. Increasing Si content reduced the friction and the wear of the mated ball, although the wear of the coatings increased. The wear of the counter ball occurred mainly in the early stage of rubbing. The Raman and XPS analysis revealed that the tribochemical reaction of Si-DLC coating occurred in water, and SiOx(OH)y gel was formed on the mated ball surface. It is considered that the tribochemical reaction is also responsible for the tribological properties of the Si-DLC coating and the counter ball, and the reaction may be accelerated by increasing the Si content. Failure-resistant capability is strongly governed by the characteristics of the coating, and can be improved by doping Si. There is an optimum Si content for increasing the failure-resistant capability and it was 6.6 at.% in this work. © 2007 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Characteristic; Tribology; Tribochemical reaction
1. Introduction Due to the excellent tribological properties, such as low friction, high wear resistance, in a water environment, diamondlike carbon (DLC) coating is attracting attention as a solid lubricant applied widely to water hydraulic systems [1,2]. Unfortunately, DLC coating easily fails during rubbing in water [1,3], limiting its applications. It is thought that failure of DLC coatings is not only related to the adhesion to substrate, but also determined by their characteristics, such as internal stress, hardness, and so on. Based on these considerations, a number of studies have been carried out on using interlayer [4], doping metallic element [5], ion implantation [6], to improve the adhesion and/or characteristics of DLC coating for increasing the failure-resistant capability. Recently, it has been found that the generation of surface micro-cracks during rubbing is one of the important factors in ⁎ Corresponding author. Tel.: +81 29 861 5080x55274; fax: +81 29 861 4573. E-mail address: [email protected] (X. Wu). 1 Present affiliation: Research & Development Centers, JTEKT C., 24-1 Kokubu Higanjo-cho, Kashiwara, Osaka 582-8588, Japan. 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.09.007
the failure of DLC coating in a water environment, and its failure-resistant property can be improved by adding a top layer of Si-containing DLC (Si-DLC) [7]. It is thought that a Si-DLC layer can alleviate the internal stress of DLC coating to suppress the formation of the micro-cracks. However, the characteristics and tribological behaviors of Si-DLC coating remain unclear. In this work, Si-DLC coatings containing different amounts of Si were deposited on stainless steel substrates by thermal electron excited plasma CVD method, and the characteristics and the tribological behaviors in water were investigated. For comparison, a DLC coating without Si was also deposited. 2. Experimental details Si-DLC coatings were deposited on mirror-finished AISI 630 stainless steel substrates by thermal electron excited plasma CVD equipment (DASH-400DS2P, NANOTEC). Before deposition, the substrate was firstly sputtered by Ar plasma for 15 min at a DC bias of −2.0 kV, and then coated with a Si-DLC interlayer of about 0.05 μm thick at a DC bias of −1.5 kV. The aim of this treatment was to obtain the same adhesion to the substrate for all the coatings. Toluene (T) and hexamethyldisiloxane (HMDS) were
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X. Wu et al. / Diamond & Related Materials 17 (2008) 7–12
Table 1 The amount of Si and various properties of the coatings Coating
S-0
S-1
S-2
S-3
S-4
Amount of Si (except H), at.% Thickness, μm Hardness, GPa Young's modulus, GPa Internal stress, GPa
0 1.0 23 200 − 1.9
2.3 1.0 23 200 − 2.2
3.3 0.9 25 210 −2.2
6.6 1.2 25 220 − 2.0
10 1.0 23 200 − 1.6
surface is flat. After the tests, the worn surfaces were analyzed by Raman spectroscopy. A pin-on-disk test of Si-DLC coating (Table 1, S-3) against an AISI 440C pin with a radius of curvature of 20 mm was also conducted at a load of 10 N, and the worn surfaces were analyzed by XPS to investigate the tribochemical reaction of Si-DLC coating in water. 3. Results and discussion 3.1. Characterization
used as carbon source gases. The amount of Si was controlled by varying the flow rate of the two source gases. A DC bias of −1.5 kV was applied to the substrate during depositing. The coatings deposited on Si wafer specimens simultaneously with the AISI 630 steel substrates were used in the characterizations: the thickness was measured by a profilometer; the hardness and Young's modulus were measured by nanoindentation method, and the values at an indentation depth of 100 nm were used; the internal stress was calculated from the curvature of Si substrate measured by an optical interferometry; the Si content was determined by X-ray photoelectron spectroscopy (XPS). The coatings were also analyzed by Raman spectroscopy. The tribological properties of the coatings were investigated by a ball-on-disk type reciprocating tribometer in a water bath at room temperature. The frequency of reciprocation was 1 Hz (60 cycles/min); the amplitude and the sliding time at a load of 4.8 N were 8 mm and 60 min, and those at loads of 10–60 N were 10 mm and 120 min, respectively. To investigate the transfer behavior of the coatings at the early stage of rubbing, 5, 50 and 100 cycles of rubbing tests were also carried out at a load of 4.8 N. AISI 440C bearing steel balls with a diameter of 4.76 mm were used as counter parts. The main composition of the ball was Cr 16–18%, C 1%, Si b 1%, Mn b 1%, and the balance was Fe. Deionized water was used to carry out the tests, and the contact surfaces were kept underwater during rubbing. The friction coefficient was measured continuously by a load cell. The specific wear rate of the coatings was measured by an optical interferometer, and that of the balls was calculated from the diameter of the scar on the ball, supposing that the worn ball
Fig. 1. Raman spectra of the coatings.
Table 1 shows the Si content and various characteristics of the Si-DLC coatings. The values at an indentation depth of 100 nm were given as the hardness and Young's modulus of the coatings. The hardness, modulus and internal stress of the coatings were only slightly related to the Si content. S-0, S-1 and S-4 coatings showed the lowest hardness and modulus. The hardness of S-2 was the same as S-3. The modulus of S-3 was the highest. The internal stress of S-3 was close to that of S-0, higher than that of S-4, and lower than those of S-1 and S-2. It is clear that the effects of doped Si on the hardness and Young's modulus of the coatings obtained in this work were limited. This result is different from that reported in reference [8]: the hardness and modulus of coatings were decreased markedly by doping Si even in a very small amount. This difference may be caused by a large error in the measurement due to the lack of thickness of those coatings in the reference, or by the material and treatment of substrates, source gases, deposition conditions, etc, since the characteristics of DLC coatings are also related to these parameters. In addition, the roughness of these coatings was about 2 nm. Raman spectra of the coatings are given in Fig. 1. The G peaks of S-0, S-1, S-2, S-3 and S-4 coatings were observed at 1535, 1532, 1528, 1520 and 1511 cm− 1, respectively. The G peak shifted to a lower wave number with increasing Si content. This shift may be due to the increase in the ratio of sp3 to sp2 compositions caused by the formation of O–Si–C [8]. The formation of O–Si–C was confirmed by the XPS analysis.
Fig. 2. Variation of friction coefficients of S-0, S-2 and S-3 coatings as a function of sliding time (load 4.8 N).
X. Wu et al. / Diamond & Related Materials 17 (2008) 7–12
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beginning of the rubbing, and then decreased slowly to a steady and low value around 0.07 after 50 min. The friction curves of S1 and S-4 were similar to those of S-2 and Si-3, respectively. The test was performed two times for each coating, and an average friction coefficient was estimated from the values after 50 min to compare the friction behaviors at the steady state. The results are summarized in Fig. 3, and the error bar showed a range of the friction coefficient. The friction coefficients of S-1 and S-2 coatings were close to that of S-0. On the other hand, S3 showed the lowest and a steadier friction compared with the others. The average friction coefficient of S-4 was close to that of S-4. These results indicate that doping a small amount of Si in the coating has little effect on the friction at the steady state and increasing Si content can reduce the friction. Fig. 3. Average friction coefficient of various coatings (load 4.8 N).
3.2. Friction behavior Fig. 2 gives the variation of friction coefficients of S-0, S-2 and S-3 coatings as a function of sliding time at a load of 4.8 N. After a quick decrease, the friction coefficient of S-0 coating changed gradually and reached to a relatively steady value, around 0.09, after 20 min. For the S-2 coating, the friction coefficient came to a stable state after 6 min. S-3 coating showed a different friction curve from those of S-0 and S-2: the friction coefficient decreased rapidly to a low value below 0.1 at the
3.3. Wear behaviors Fig. 4 shows optical micrographs of the wear scars on the S0, S-3 and S-4 coatings and the mated balls, and profiles of the wear tracks on the coatings. The ball tested against the S-0 coating showed a lager wear scar than those mated with the S-3 and S-4 coatings. Transferred coating material was present on the worn surface of all the mated balls and concentrated at the central part about 0.08 mm wide. This width is close to that of the Hertzian contact area, 0.086 mm. DLC or Si-DLC transfer seemed to occur more easily in the early contact area where the contact pressure was expected to be high.
Fig. 4. Optical micrographs of the wear scars on S-0, S-3 and S-4 coatings and the mated balls, and profiles of the wear tracks on the coatings.
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Fig. 5. Specific wear rates of various coatings and the mated balls (load 4.8 N).
The micrograph of S-0 coating indicates that the coating partially failed due to rubbing. The profile of S-0 was measured at a position without failure. S-4 coating showed the largest wear depth. From the micrographs and profiles of these coatings, it can be seen that the wear track of S-4 was smoother than that of S-3, and that of S-0 was the roughest. These results imply that tribochemical reaction might occur for Si-DLC in water. In addition, the width of the wear tracks on the coatings was also close to that of the Hertzian contact area, and smaller than those of real contact areas observed from the wear scars on the balls. Fig. 5 summarizes the specific wear rates of the coatings and the mated balls. The specific wear rates of both the coating and the ball were clearly related to the Si content of the coatings. The wear of S-0 was visibly lower than the other coatings. The wear rate of the coatings increased with Si content, except that the wear of S-1 was close to that of S-2. It is noteworthy that the wear of S-4 was almost one order higher than that of S-0, although the hardness and Young's modulus were the same. In contrast, the wear rates of the balls mated with S-2, S-3 and S-4 were at the same level, and much lower than that mated with S0. The ball mated with S-1 showed an intermediate wear rate. The wear of the mated balls was effectively reduced by doping Si to coating.
50 N. Under this condition, the maximum contact pressure estimated by the Hertz contact theory is 2.8 GPa. This Si-DLC coating may be a candidate for tribological application in a water environment. The failure-resistant capability of DLC coatings is affected by the adhesion to the substrate as well as other characteristics, such as hardness, modulus, and tensile strength. Since the same treatment, sputter cleaning and interlayer deposition, was performed for all the substrates before the deposition, the adhesions of DLC and Si-DLC coating to the substrates must be the same and so it is clear that the failure-resistant capabilities are strongly governed by their characteristics. The order of the capabilities of S-1, S-2 and S-3 is in agreement with their modulus as given in Table 1. However, S-1 and S-4 showed higher failure-resistant capabilities than S-0, although the hardness and modulus of these coatings were the same. It is thought that the failure-resistant capabilities are also affected by other characteristics, e.g. tensile stress, elasticity, etc. Unfortunately, these properties were not investigated in this work. 3.5. Surface analysis Transferred DLC and Si-DLC on the worn ball surfaces were analyzed by Raman spectroscopy (Fig. 7). The spectrum of the ball mated with S-0 showed a similar feature to that of the coating. However, the spectra of the transferred Si-DLC on the balls mated with Si doped coatings were different from those of the coatings: the G peaks broadened, and the intensities of G band decreased markedly compared with those of the D band, especially for those mated with S-3 and S-4 coatings. It is clear that the chemical composition of the transferred Si-DLC changed due to rubbing in water. This result implies that tribochemical reaction occurs more easily at the interfaces of Si-DLC and the mated ball than the DLC pair. Reportedly, DLC can tribochemically react with water to form hydroxyl groups [9,10]. The doped Si in the coatings may accelerate this reaction. To investigate the tribochemical reaction, a test of pin-on-disk was carried out in water by using S-3 coating and an AISI 440C pin, and the worn surfaces were analyzed by XPS. The Si2p
3.4. Failure-resistant capabilities Friction tests were also carried out at higher loads, and the highest load without causing failure of coating was given as the failure-resistant capability of the coating. The results are shown in Fig. 6. Although partial failure was observed on both S-0 and S-4 coatings at a load of 10 N, S-4 showed a higher failureresistant capability than S-0 in the tests at a load of 5 N (Fig. 4). The failure-resistant capabilities of S-1, S-2 and S-3 were higher than those of S-0 and S-4, and the capability increased markedly with Si content. These results indicate that the failure-resistant capability of DLC coating can be increased by doping Si, and an optimum Si content exists. In this study, S-3 coating containing 6.6 at.% of Si showed the highest failure-resistant capability,
Fig. 6. Failure-resistant capability of various coatings (sliding times: 120 min, amplitudes: 10 mm; ⁎: partial failure was observed for S-0 and S-4).
X. Wu et al. / Diamond & Related Materials 17 (2008) 7–12
Fig. 7. Raman spectra of transferred DLC and Si-DLC on the worn ball surfaces rubbed with various coatings (load 4.8 N).
spectra of the worn surfaces are shown in Fig. 8. For comparison, the spectrum of the original coating surface is also given. The spectrum of the original surface indicates that the Si element is mainly in two chemical states, Si–C (100.7 eV) and C–Si–O (101.7 eV), and O–Si–O bond (103.0 eV) exists in a small amount [11]. The main composition of the worn coating surface is Si–C. However, the spectrum of the pin shows that Si on the worn pin surface mainly consists of gel-type amorphous SiOx (OH)y (103.4 eV) [12] and C–Si–O. Little Si–C was confirmed on the surface. The formation of a SiOH group on the worn ball surface and a decrease in the amount of Si at the worn Si-DLC surface were also confirmed by ToF-SIMS analysis after the test carried out in a heavy water environment [13]. This result suggests that tribochemical reaction of the Si-DLC coating occurred in water resulting in the formation of SiOx(OH)y gel, which can reduce the friction in a water environment [14]. It is well known that the tribological behavior under boundary lubrication conditions is controlled by contact conditions and surface films, including adsorbed and tribochemically formed layers. A number of investigations on tribological properties of
Fig. 8. Si2p XPS spectra of the worn surfaces of S-3 (coating) and the mated pin tested in water at a load of 10 N, and the original surface of the coating.
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Fig. 9. Friction coefficients of S-0 and S-3 coatings rubbed for 5, 50 and 100 cycles at a load of 4.8 N.
DLC coatings in various environments suggest that the transfer layer and the graphitized tribolayer on the counter parts are responsible for the tribological behaviors of DLC coatings [15,16]. Our results show that Si-DLC coatings facilitated reaching a lower friction than DLC. Since the contact conditions, surface roughness, hardness, modulus, etc, are almost the same for Si-DLC and DLC coatings, it is considered that the difference in frictions was caused by surface layers, such as transfer layer, and tribochemical products. In order to reveal the relation between the friction and transfer behaviors, rubbing tests of S-0 and S-3 coatings were carried out in water for 5, 50 and 100 cycles, and the wear scars on the balls were observed and analyzed by Raman spectroscopy. The decrease in the frictions at the beginning of rubbing was observed in all the tests as shown in Fig. 9, and the friction of the S-3 coating was lower than that of S-0 at the same stage, especially at the end of the rubbing. These results are in agreement with those shown in Fig. 2.
Fig. 10. Optical micrographs of the wear scars on the balls after (a) 5 and (b) 100 cycles of rubbing against S-0, and (c) 5 and (d) 100 cycles of rubbing against S-3 (load 4.8 N).
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X. Wu et al. / Diamond & Related Materials 17 (2008) 7–12
4. Conclusion
Fig. 11. Raman spectra of transfer layers on the worn ball surfaces after 100 cycles of tests rubbed with S-0 and S-3 coatings (load 4.8 N).
From the micrographs of wear scars on the balls shown in Fig. 10, it can be seen that transferred DLC of S-0 almost covered the entire wear scars (Fig. 10, (a) and (b)). In contrast, transferred Si-DLC was not observed in the wear scar after 5 cycles of rubbing (Fig. 10, (c)), and only a small amount of transferred Si-DLC existed locally on the scar after 100 cycles of rubbing (Fig. 10, (d)). The wear scars after 50 cycles showed intermediate features between those after 5 and 100 cycles. It is clear that S-0 coating without Si is easier to transfer to the counter ball than S-3. Additionally, the average diameters of the ball wear scars after 100 cycles of rubbing against S-0 and S-3 were 0.14 and 0.10 mm, respectively, and close to those after 60 min, 0.15 and 0.10 mm, as shown in Fig. 4. This fact suggests that the wear of the mated ball mainly occurred in the early stage of rubbing. Raman spectra of transferred DLC and Si-DLC after 100 cycles of rubbing are shown in Fig. 11. A slight increase in the intensity of D band was observed in the spectrum of transferred DLC, and the intensity of G band of transferred SiDLC decreased to the same level as that of the D band. This suggests that rubbing changed the chemical compositions of transferred DLC and Si-DLC, especially for the transferred SiDLC. It is thought that the tribochemical reaction occurs more easily for Si-DLC in water, resulting in changes of chemical composition at the contact surfaces and tribological behaviors. The low friction of the coatings in water resulted not only from the transfer, but also from the surface layer composed of some tribochemical products, such as the SiOx(OH)y gel on the mating ball. Reportedly, a thin lubrication film, which is different from the transferred Si-DLC, has been observed on the ball wear scar rubbed with DLC coating with a top layer of SiDLC in water by using TEM (transmission electron microscope) and EDS (energy dispersive X-ray spectroscopy) [17]. From the above-mentioned results, it is considered that the tribochemical reaction of Si-DLC in a water environment can be accelerated by the doped Si in the coating, and the formed surface layer is one of the important factors in reducing the friction and the wear of the mated ball. On other hand, a large amount of Si may accelerate the chemical wear of the coating, causing high wear as obtained from S-4 coating.
Si-DLC coatings with Si contents of 0 (DLC), 2.3, 3.3, 6.6 and 10 at.% were deposited by thermal electron excited plasma CVD method, and their characteristics and tribological properties in water were investigated. The result showed that the effects of the doped Si on the hardness and modulus of the coatings were limited. Increasing Si content reduced the friction and the wear of the mated ball, although the wear of the coating increased with the Si content. The results of Raman and XPS analysis indicated that tribochemical reaction of Si-DLC coating occurred in water and SiOx(OH)y gel was formed on the mated ball surface. It is considered that the surface layer resulted from tribochemical reaction also plays an important role in reducing the friction and the wear of the counter part, and doped Si may accelerate the reaction. On other hand, a large amount of Si increases the wear of the coating by causing severe chemical wear. The wear of the mated balls was caused mainly in the early stage of the rubbing. Failure-resistant capability was strongly governed by the characteristics of the coating and the capability was improved by doping Si. There is an optimum Si content for increasing the capability and it was 6.6 at.% in this study. 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 [1] Tsuguyori Ohana, Masahiro Suzuki, Takako Nakamura, Akihiro Tanaka, Yoshinori Koga, Diamond Relat. Mater. 13 (2004) 2211. [2] M. Suzuki, T. Ohana, A. Tanaka, Diamond Relat. Mater. 13 (2004) 2216. [3] J. Stallard, D. Mercs, M. Jarratt, D.G. Teer, P.H. Shipway, Surf. Coat. Technol. 177–178 (2004) 545. [4] Chun-Chin Chen, Franklin Chau-Nan Hong, Appl. Surf. Sci. 243 (2005) 296. [5] Jianguo Deng, Manuel Braun, Diamond Relat. Mater. 4 (1995) 936. [6] K.C. Walter, M. Nastasi, C. Munson, Surf. Coat. Technol. 93 (1997) 187. [7] T. Ohana, T. Nakamura, M. Suzuki, A. Tanaka, Y. Koga, Diamond Relat. Mater. 13 (2004) 1500. [8] P. Papakonstantinou, J.F. Zhao, P. Lemoine, E.T. McAdams, J.A. McLaughlin, Diamond Relat. Mater. 11 (2002) 1074. [9] J.E. Olsen, T.E. Fischer, B. Gallois, Wear 200 (1996) 233. [10] X. Wu, T. Ohana, A. Tanaka, T. Kubo, H. Nanao, I. Minami, S. Mori, Diamond Relat. Mater., 16 (2007) 1760. [11] M. Veres, M. Koos, S. Toth, M. Fule, I. Pocsik, A. Toth, M. Mohai, I. Bertoti, Diamond Relat. Mater. 14 (2005) 1051. [12] Hongxuan Li, Tao Xu, Chengbing Wang, Jianmin Chen, Huidi Zhou, Huiwen Liu, Diamond Relat. Mater. 15 (2006) 1585. [13] X. Wu, T. Ohana, A. Tanaka, T. Kubo, H. Nanao, I. Minami, S. Mori, Diamond Relat. Mater., submitted for publication. [14] T. Sugita, K. Ueda, Y. Kanemura, Wear 97 (1984) 1. [15] R. Hauert, U. Muller, Diamond Relat. Mater. 12 (2003) 171. [16] Y. Liu, A. Erdemir, E.I. Meletis, Surf. Coat. Technol. 94–95 (1997) 463. [17] T. Ohana, M. Suzuki, T. Nakamura, A. Tanaka, Y. Koga, Diamond Relat. Mater. 15 (2006) 962.
Diamond & Related Materials 17 (2008) 7 – 12 www.elsevier.com/locate/diamond
Characteristics and tribological properties in water of Si-DLC coatings Xingyang Wu ⁎, Masahiro Suzuki 1 , Tsuguyori Ohana, Akihiro Tanaka National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 30 July 2006; received in revised form 6 July 2007; accepted 12 September 2007 Available online 19 September 2007
Abstract Si-containing diamond-like carbon (Si-DLC) coatings with a Si content ranging between 0 (DLC) and 10 at.% were deposited by thermal electron excited plasma CVD method, and their characteristics and the tribological properties in water environment were investigated. The results showed that doped Si had little effect on the hardness and Young's modulus of the coatings. Increasing Si content reduced the friction and the wear of the mated ball, although the wear of the coatings increased. The wear of the counter ball occurred mainly in the early stage of rubbing. The Raman and XPS analysis revealed that the tribochemical reaction of Si-DLC coating occurred in water, and SiOx(OH)y gel was formed on the mated ball surface. It is considered that the tribochemical reaction is also responsible for the tribological properties of the Si-DLC coating and the counter ball, and the reaction may be accelerated by increasing the Si content. Failure-resistant capability is strongly governed by the characteristics of the coating, and can be improved by doping Si. There is an optimum Si content for increasing the failure-resistant capability and it was 6.6 at.% in this work. © 2007 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Characteristic; Tribology; Tribochemical reaction
1. Introduction Due to the excellent tribological properties, such as low friction, high wear resistance, in a water environment, diamondlike carbon (DLC) coating is attracting attention as a solid lubricant applied widely to water hydraulic systems [1,2]. Unfortunately, DLC coating easily fails during rubbing in water [1,3], limiting its applications. It is thought that failure of DLC coatings is not only related to the adhesion to substrate, but also determined by their characteristics, such as internal stress, hardness, and so on. Based on these considerations, a number of studies have been carried out on using interlayer [4], doping metallic element [5], ion implantation [6], to improve the adhesion and/or characteristics of DLC coating for increasing the failure-resistant capability. Recently, it has been found that the generation of surface micro-cracks during rubbing is one of the important factors in ⁎ Corresponding author. Tel.: +81 29 861 5080x55274; fax: +81 29 861 4573. E-mail address: [email protected] (X. Wu). 1 Present affiliation: Research & Development Centers, JTEKT C., 24-1 Kokubu Higanjo-cho, Kashiwara, Osaka 582-8588, Japan. 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.09.007
the failure of DLC coating in a water environment, and its failure-resistant property can be improved by adding a top layer of Si-containing DLC (Si-DLC) [7]. It is thought that a Si-DLC layer can alleviate the internal stress of DLC coating to suppress the formation of the micro-cracks. However, the characteristics and tribological behaviors of Si-DLC coating remain unclear. In this work, Si-DLC coatings containing different amounts of Si were deposited on stainless steel substrates by thermal electron excited plasma CVD method, and the characteristics and the tribological behaviors in water were investigated. For comparison, a DLC coating without Si was also deposited. 2. Experimental details Si-DLC coatings were deposited on mirror-finished AISI 630 stainless steel substrates by thermal electron excited plasma CVD equipment (DASH-400DS2P, NANOTEC). Before deposition, the substrate was firstly sputtered by Ar plasma for 15 min at a DC bias of −2.0 kV, and then coated with a Si-DLC interlayer of about 0.05 μm thick at a DC bias of −1.5 kV. The aim of this treatment was to obtain the same adhesion to the substrate for all the coatings. Toluene (T) and hexamethyldisiloxane (HMDS) were
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X. Wu et al. / Diamond & Related Materials 17 (2008) 7–12
Table 1 The amount of Si and various properties of the coatings Coating
S-0
S-1
S-2
S-3
S-4
Amount of Si (except H), at.% Thickness, μm Hardness, GPa Young's modulus, GPa Internal stress, GPa
0 1.0 23 200 − 1.9
2.3 1.0 23 200 − 2.2
3.3 0.9 25 210 −2.2
6.6 1.2 25 220 − 2.0
10 1.0 23 200 − 1.6
surface is flat. After the tests, the worn surfaces were analyzed by Raman spectroscopy. A pin-on-disk test of Si-DLC coating (Table 1, S-3) against an AISI 440C pin with a radius of curvature of 20 mm was also conducted at a load of 10 N, and the worn surfaces were analyzed by XPS to investigate the tribochemical reaction of Si-DLC coating in water. 3. Results and discussion 3.1. Characterization
used as carbon source gases. The amount of Si was controlled by varying the flow rate of the two source gases. A DC bias of −1.5 kV was applied to the substrate during depositing. The coatings deposited on Si wafer specimens simultaneously with the AISI 630 steel substrates were used in the characterizations: the thickness was measured by a profilometer; the hardness and Young's modulus were measured by nanoindentation method, and the values at an indentation depth of 100 nm were used; the internal stress was calculated from the curvature of Si substrate measured by an optical interferometry; the Si content was determined by X-ray photoelectron spectroscopy (XPS). The coatings were also analyzed by Raman spectroscopy. The tribological properties of the coatings were investigated by a ball-on-disk type reciprocating tribometer in a water bath at room temperature. The frequency of reciprocation was 1 Hz (60 cycles/min); the amplitude and the sliding time at a load of 4.8 N were 8 mm and 60 min, and those at loads of 10–60 N were 10 mm and 120 min, respectively. To investigate the transfer behavior of the coatings at the early stage of rubbing, 5, 50 and 100 cycles of rubbing tests were also carried out at a load of 4.8 N. AISI 440C bearing steel balls with a diameter of 4.76 mm were used as counter parts. The main composition of the ball was Cr 16–18%, C 1%, Si b 1%, Mn b 1%, and the balance was Fe. Deionized water was used to carry out the tests, and the contact surfaces were kept underwater during rubbing. The friction coefficient was measured continuously by a load cell. The specific wear rate of the coatings was measured by an optical interferometer, and that of the balls was calculated from the diameter of the scar on the ball, supposing that the worn ball
Fig. 1. Raman spectra of the coatings.
Table 1 shows the Si content and various characteristics of the Si-DLC coatings. The values at an indentation depth of 100 nm were given as the hardness and Young's modulus of the coatings. The hardness, modulus and internal stress of the coatings were only slightly related to the Si content. S-0, S-1 and S-4 coatings showed the lowest hardness and modulus. The hardness of S-2 was the same as S-3. The modulus of S-3 was the highest. The internal stress of S-3 was close to that of S-0, higher than that of S-4, and lower than those of S-1 and S-2. It is clear that the effects of doped Si on the hardness and Young's modulus of the coatings obtained in this work were limited. This result is different from that reported in reference [8]: the hardness and modulus of coatings were decreased markedly by doping Si even in a very small amount. This difference may be caused by a large error in the measurement due to the lack of thickness of those coatings in the reference, or by the material and treatment of substrates, source gases, deposition conditions, etc, since the characteristics of DLC coatings are also related to these parameters. In addition, the roughness of these coatings was about 2 nm. Raman spectra of the coatings are given in Fig. 1. The G peaks of S-0, S-1, S-2, S-3 and S-4 coatings were observed at 1535, 1532, 1528, 1520 and 1511 cm− 1, respectively. The G peak shifted to a lower wave number with increasing Si content. This shift may be due to the increase in the ratio of sp3 to sp2 compositions caused by the formation of O–Si–C [8]. The formation of O–Si–C was confirmed by the XPS analysis.
Fig. 2. Variation of friction coefficients of S-0, S-2 and S-3 coatings as a function of sliding time (load 4.8 N).
X. Wu et al. / Diamond & Related Materials 17 (2008) 7–12
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beginning of the rubbing, and then decreased slowly to a steady and low value around 0.07 after 50 min. The friction curves of S1 and S-4 were similar to those of S-2 and Si-3, respectively. The test was performed two times for each coating, and an average friction coefficient was estimated from the values after 50 min to compare the friction behaviors at the steady state. The results are summarized in Fig. 3, and the error bar showed a range of the friction coefficient. The friction coefficients of S-1 and S-2 coatings were close to that of S-0. On the other hand, S3 showed the lowest and a steadier friction compared with the others. The average friction coefficient of S-4 was close to that of S-4. These results indicate that doping a small amount of Si in the coating has little effect on the friction at the steady state and increasing Si content can reduce the friction. Fig. 3. Average friction coefficient of various coatings (load 4.8 N).
3.2. Friction behavior Fig. 2 gives the variation of friction coefficients of S-0, S-2 and S-3 coatings as a function of sliding time at a load of 4.8 N. After a quick decrease, the friction coefficient of S-0 coating changed gradually and reached to a relatively steady value, around 0.09, after 20 min. For the S-2 coating, the friction coefficient came to a stable state after 6 min. S-3 coating showed a different friction curve from those of S-0 and S-2: the friction coefficient decreased rapidly to a low value below 0.1 at the
3.3. Wear behaviors Fig. 4 shows optical micrographs of the wear scars on the S0, S-3 and S-4 coatings and the mated balls, and profiles of the wear tracks on the coatings. The ball tested against the S-0 coating showed a lager wear scar than those mated with the S-3 and S-4 coatings. Transferred coating material was present on the worn surface of all the mated balls and concentrated at the central part about 0.08 mm wide. This width is close to that of the Hertzian contact area, 0.086 mm. DLC or Si-DLC transfer seemed to occur more easily in the early contact area where the contact pressure was expected to be high.
Fig. 4. Optical micrographs of the wear scars on S-0, S-3 and S-4 coatings and the mated balls, and profiles of the wear tracks on the coatings.
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Fig. 5. Specific wear rates of various coatings and the mated balls (load 4.8 N).
The micrograph of S-0 coating indicates that the coating partially failed due to rubbing. The profile of S-0 was measured at a position without failure. S-4 coating showed the largest wear depth. From the micrographs and profiles of these coatings, it can be seen that the wear track of S-4 was smoother than that of S-3, and that of S-0 was the roughest. These results imply that tribochemical reaction might occur for Si-DLC in water. In addition, the width of the wear tracks on the coatings was also close to that of the Hertzian contact area, and smaller than those of real contact areas observed from the wear scars on the balls. Fig. 5 summarizes the specific wear rates of the coatings and the mated balls. The specific wear rates of both the coating and the ball were clearly related to the Si content of the coatings. The wear of S-0 was visibly lower than the other coatings. The wear rate of the coatings increased with Si content, except that the wear of S-1 was close to that of S-2. It is noteworthy that the wear of S-4 was almost one order higher than that of S-0, although the hardness and Young's modulus were the same. In contrast, the wear rates of the balls mated with S-2, S-3 and S-4 were at the same level, and much lower than that mated with S0. The ball mated with S-1 showed an intermediate wear rate. The wear of the mated balls was effectively reduced by doping Si to coating.
50 N. Under this condition, the maximum contact pressure estimated by the Hertz contact theory is 2.8 GPa. This Si-DLC coating may be a candidate for tribological application in a water environment. The failure-resistant capability of DLC coatings is affected by the adhesion to the substrate as well as other characteristics, such as hardness, modulus, and tensile strength. Since the same treatment, sputter cleaning and interlayer deposition, was performed for all the substrates before the deposition, the adhesions of DLC and Si-DLC coating to the substrates must be the same and so it is clear that the failure-resistant capabilities are strongly governed by their characteristics. The order of the capabilities of S-1, S-2 and S-3 is in agreement with their modulus as given in Table 1. However, S-1 and S-4 showed higher failure-resistant capabilities than S-0, although the hardness and modulus of these coatings were the same. It is thought that the failure-resistant capabilities are also affected by other characteristics, e.g. tensile stress, elasticity, etc. Unfortunately, these properties were not investigated in this work. 3.5. Surface analysis Transferred DLC and Si-DLC on the worn ball surfaces were analyzed by Raman spectroscopy (Fig. 7). The spectrum of the ball mated with S-0 showed a similar feature to that of the coating. However, the spectra of the transferred Si-DLC on the balls mated with Si doped coatings were different from those of the coatings: the G peaks broadened, and the intensities of G band decreased markedly compared with those of the D band, especially for those mated with S-3 and S-4 coatings. It is clear that the chemical composition of the transferred Si-DLC changed due to rubbing in water. This result implies that tribochemical reaction occurs more easily at the interfaces of Si-DLC and the mated ball than the DLC pair. Reportedly, DLC can tribochemically react with water to form hydroxyl groups [9,10]. The doped Si in the coatings may accelerate this reaction. To investigate the tribochemical reaction, a test of pin-on-disk was carried out in water by using S-3 coating and an AISI 440C pin, and the worn surfaces were analyzed by XPS. The Si2p
3.4. Failure-resistant capabilities Friction tests were also carried out at higher loads, and the highest load without causing failure of coating was given as the failure-resistant capability of the coating. The results are shown in Fig. 6. Although partial failure was observed on both S-0 and S-4 coatings at a load of 10 N, S-4 showed a higher failureresistant capability than S-0 in the tests at a load of 5 N (Fig. 4). The failure-resistant capabilities of S-1, S-2 and S-3 were higher than those of S-0 and S-4, and the capability increased markedly with Si content. These results indicate that the failure-resistant capability of DLC coating can be increased by doping Si, and an optimum Si content exists. In this study, S-3 coating containing 6.6 at.% of Si showed the highest failure-resistant capability,
Fig. 6. Failure-resistant capability of various coatings (sliding times: 120 min, amplitudes: 10 mm; ⁎: partial failure was observed for S-0 and S-4).
X. Wu et al. / Diamond & Related Materials 17 (2008) 7–12
Fig. 7. Raman spectra of transferred DLC and Si-DLC on the worn ball surfaces rubbed with various coatings (load 4.8 N).
spectra of the worn surfaces are shown in Fig. 8. For comparison, the spectrum of the original coating surface is also given. The spectrum of the original surface indicates that the Si element is mainly in two chemical states, Si–C (100.7 eV) and C–Si–O (101.7 eV), and O–Si–O bond (103.0 eV) exists in a small amount [11]. The main composition of the worn coating surface is Si–C. However, the spectrum of the pin shows that Si on the worn pin surface mainly consists of gel-type amorphous SiOx (OH)y (103.4 eV) [12] and C–Si–O. Little Si–C was confirmed on the surface. The formation of a SiOH group on the worn ball surface and a decrease in the amount of Si at the worn Si-DLC surface were also confirmed by ToF-SIMS analysis after the test carried out in a heavy water environment [13]. This result suggests that tribochemical reaction of the Si-DLC coating occurred in water resulting in the formation of SiOx(OH)y gel, which can reduce the friction in a water environment [14]. It is well known that the tribological behavior under boundary lubrication conditions is controlled by contact conditions and surface films, including adsorbed and tribochemically formed layers. A number of investigations on tribological properties of
Fig. 8. Si2p XPS spectra of the worn surfaces of S-3 (coating) and the mated pin tested in water at a load of 10 N, and the original surface of the coating.
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Fig. 9. Friction coefficients of S-0 and S-3 coatings rubbed for 5, 50 and 100 cycles at a load of 4.8 N.
DLC coatings in various environments suggest that the transfer layer and the graphitized tribolayer on the counter parts are responsible for the tribological behaviors of DLC coatings [15,16]. Our results show that Si-DLC coatings facilitated reaching a lower friction than DLC. Since the contact conditions, surface roughness, hardness, modulus, etc, are almost the same for Si-DLC and DLC coatings, it is considered that the difference in frictions was caused by surface layers, such as transfer layer, and tribochemical products. In order to reveal the relation between the friction and transfer behaviors, rubbing tests of S-0 and S-3 coatings were carried out in water for 5, 50 and 100 cycles, and the wear scars on the balls were observed and analyzed by Raman spectroscopy. The decrease in the frictions at the beginning of rubbing was observed in all the tests as shown in Fig. 9, and the friction of the S-3 coating was lower than that of S-0 at the same stage, especially at the end of the rubbing. These results are in agreement with those shown in Fig. 2.
Fig. 10. Optical micrographs of the wear scars on the balls after (a) 5 and (b) 100 cycles of rubbing against S-0, and (c) 5 and (d) 100 cycles of rubbing against S-3 (load 4.8 N).
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4. Conclusion
Fig. 11. Raman spectra of transfer layers on the worn ball surfaces after 100 cycles of tests rubbed with S-0 and S-3 coatings (load 4.8 N).
From the micrographs of wear scars on the balls shown in Fig. 10, it can be seen that transferred DLC of S-0 almost covered the entire wear scars (Fig. 10, (a) and (b)). In contrast, transferred Si-DLC was not observed in the wear scar after 5 cycles of rubbing (Fig. 10, (c)), and only a small amount of transferred Si-DLC existed locally on the scar after 100 cycles of rubbing (Fig. 10, (d)). The wear scars after 50 cycles showed intermediate features between those after 5 and 100 cycles. It is clear that S-0 coating without Si is easier to transfer to the counter ball than S-3. Additionally, the average diameters of the ball wear scars after 100 cycles of rubbing against S-0 and S-3 were 0.14 and 0.10 mm, respectively, and close to those after 60 min, 0.15 and 0.10 mm, as shown in Fig. 4. This fact suggests that the wear of the mated ball mainly occurred in the early stage of rubbing. Raman spectra of transferred DLC and Si-DLC after 100 cycles of rubbing are shown in Fig. 11. A slight increase in the intensity of D band was observed in the spectrum of transferred DLC, and the intensity of G band of transferred SiDLC decreased to the same level as that of the D band. This suggests that rubbing changed the chemical compositions of transferred DLC and Si-DLC, especially for the transferred SiDLC. It is thought that the tribochemical reaction occurs more easily for Si-DLC in water, resulting in changes of chemical composition at the contact surfaces and tribological behaviors. The low friction of the coatings in water resulted not only from the transfer, but also from the surface layer composed of some tribochemical products, such as the SiOx(OH)y gel on the mating ball. Reportedly, a thin lubrication film, which is different from the transferred Si-DLC, has been observed on the ball wear scar rubbed with DLC coating with a top layer of SiDLC in water by using TEM (transmission electron microscope) and EDS (energy dispersive X-ray spectroscopy) [17]. From the above-mentioned results, it is considered that the tribochemical reaction of Si-DLC in a water environment can be accelerated by the doped Si in the coating, and the formed surface layer is one of the important factors in reducing the friction and the wear of the mated ball. On other hand, a large amount of Si may accelerate the chemical wear of the coating, causing high wear as obtained from S-4 coating.
Si-DLC coatings with Si contents of 0 (DLC), 2.3, 3.3, 6.6 and 10 at.% were deposited by thermal electron excited plasma CVD method, and their characteristics and tribological properties in water were investigated. The result showed that the effects of the doped Si on the hardness and modulus of the coatings were limited. Increasing Si content reduced the friction and the wear of the mated ball, although the wear of the coating increased with the Si content. The results of Raman and XPS analysis indicated that tribochemical reaction of Si-DLC coating occurred in water and SiOx(OH)y gel was formed on the mated ball surface. It is considered that the surface layer resulted from tribochemical reaction also plays an important role in reducing the friction and the wear of the counter part, and doped Si may accelerate the reaction. On other hand, a large amount of Si increases the wear of the coating by causing severe chemical wear. The wear of the mated balls was caused mainly in the early stage of the rubbing. Failure-resistant capability was strongly governed by the characteristics of the coating and the capability was improved by doping Si. There is an optimum Si content for increasing the capability and it was 6.6 at.% in this study. 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 [1] Tsuguyori Ohana, Masahiro Suzuki, Takako Nakamura, Akihiro Tanaka, Yoshinori Koga, Diamond Relat. Mater. 13 (2004) 2211. [2] M. Suzuki, T. Ohana, A. Tanaka, Diamond Relat. Mater. 13 (2004) 2216. [3] J. Stallard, D. Mercs, M. Jarratt, D.G. Teer, P.H. Shipway, Surf. Coat. Technol. 177–178 (2004) 545. [4] Chun-Chin Chen, Franklin Chau-Nan Hong, Appl. Surf. Sci. 243 (2005) 296. [5] Jianguo Deng, Manuel Braun, Diamond Relat. Mater. 4 (1995) 936. [6] K.C. Walter, M. Nastasi, C. Munson, Surf. Coat. Technol. 93 (1997) 187. [7] T. Ohana, T. Nakamura, M. Suzuki, A. Tanaka, Y. Koga, Diamond Relat. Mater. 13 (2004) 1500. [8] P. Papakonstantinou, J.F. Zhao, P. Lemoine, E.T. McAdams, J.A. McLaughlin, Diamond Relat. Mater. 11 (2002) 1074. [9] J.E. Olsen, T.E. Fischer, B. Gallois, Wear 200 (1996) 233. [10] X. Wu, T. Ohana, A. Tanaka, T. Kubo, H. Nanao, I. Minami, S. Mori, Diamond Relat. Mater., 16 (2007) 1760. [11] M. Veres, M. Koos, S. Toth, M. Fule, I. Pocsik, A. Toth, M. Mohai, I. Bertoti, Diamond Relat. Mater. 14 (2005) 1051. [12] Hongxuan Li, Tao Xu, Chengbing Wang, Jianmin Chen, Huidi Zhou, Huiwen Liu, Diamond Relat. Mater. 15 (2006) 1585. [13] X. Wu, T. Ohana, A. Tanaka, T. Kubo, H. Nanao, I. Minami, S. Mori, Diamond Relat. Mater., submitted for publication. [14] T. Sugita, K. Ueda, Y. Kanemura, Wear 97 (1984) 1. [15] R. Hauert, U. Muller, Diamond Relat. Mater. 12 (2003) 171. [16] Y. Liu, A. Erdemir, E.I. Meletis, Surf. Coat. Technol. 94–95 (1997) 463. [17] T. Ohana, M. Suzuki, T. Nakamura, A. Tanaka, Y. Koga, Diamond Relat. Mater. 15 (2006) 962.