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Improvement of tribological characteristics under water lubrication of DLC-coatings by surface polishing
Wear 267 (2009) 2167–2172
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Improvement of tribological characteristics under water lubrication of DLC-coatings by surface polishing Maiko Tokoro a,∗ , Yusuke Aiyama a , Masabumi Masuko a , Akihito Suzuki a , Hirotaka Ito b , Kenji Yamamoto b a b
Tokyo Institute of Technology, Department of Chemical Engineering, Graduate School of Science and Engineering, 12-1 O-okayama 2-chome, Meguro-ku, Tokyo 152-8552, Japan Materials Research Laboratory, Kobe Steel Ltd., 5-5 Takatsuka-dai Nishi-ku 1-chome, Kobe, Hyogo 651-2271, Japan
a r t i c l e
i n f o
Article history: Received 8 October 2008 Received in revised form 2 March 2009 Accepted 10 April 2009 Available online 23 May 2009 Keywords: Water lubrication DLC-coatings Surface polishing Wettability
a b s t r a c t To achieve a hydraulic power system, it is important to control tribology because water has a lack of lubricity. Therefore, coated surface is necessary under water lubrication. Diamond-like-carbon (DLC)coating is known as a useful material because of its high hardness and low friction, therefore it can be used as a coating durable for the water lubrication. Deposition methods of DLC-coating are developed in various ways. Particles called “droplets” are observed on the surface of DLC-coating depends on deposition methods and it can affect friction and wear properties. In this study, DLC-coating was prepared using a multi-cathode unbalanced magnetron sputtering (UBMS) system. The surface was polished with diamond slurry solution and aero lap to remove droplets. DLC-coatings were evaluated by tribo-tests before and after polishing. It is considered that some surface modification occurred. Moreover, the results of tribotests show that friction coefficients became lower and more stable than before polishing. Although partial delamination was observed after tribo-tests without polishing, no appreciable wear was observed after polishing. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In order to minimize the consumption of fossil fuels and mineral oils for preventing environmental destruction, use of water as a working fluid of hydraulic system instead of mineral oil is one of the solutions. Although the use of water has many advantages, the low lubricity of water has to be solved to develop the reliable water hydraulic systems. Therefore, tribology is the key technology to overcome the problem of water. Some inorganic materials or coatings on metal surfaces are known to have a good lubricity under water lubrication [1,2]. For example, silicon nitride (Si3 N4 ) and silicon carbide (SiC) show low friction in water environments [3–7]. In spite of having good lubricity under water lubrication, silicon nitride or silicon carbide cannot be applied to the commercial power systems due to the cost or their low machinability. Diamond-like-carbon (DLC) is known as a low-cost coating durable not only for under dry condition but also for under water lubrication [8,9]. The authors have already reported that the DLC-coating showed low friction coefficient and sliding speed dependency of friction coefficient is very small under water lubri-
∗ Corresponding author. E-mail address: [email protected] (M. Tokoro). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.04.009
cation [10]. There have been many reports on DLC-coating because it has unique properties, such as chemical inertness, mechanical, tribological and optical properties [11–17]. It has been known that small particles called “droplets” are occasionally observed on the surface of DLC-coatings. The degree of droplets formation varies depending on the deposition method. Especially, it is unavoidable in case of sputtering. Some experiments show that generation of droplets can be controlled using a filter [18]. However, degree of droplets formation is not always same even under the same condition and same deposition method, and in addition, it is unpredictable. From the tribological point of view, effect of the droplets should be clarified to maintain good lubricity. In this study, the droplets were removed from DLC surface using two types of polishing method, and their effect on tribology under dry and water lubrication conditions was compared. 2. Experimental 2.1. Tribometer Friction and wear properties were evaluated using a ball-ondisk tribometer. A flat disk was installed on a rotating shaft with a disk holder inside a cup. A ball was set in a ball holder. Heat-treated stainless steel (SUS630) ball having a diameter of 9.53 mm (3/8 in.) was used. In this study, the friction experiments were carried out
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Fig. 1. SEM images of the surface of DLC-coatings after diamond slurry polishing DLC (d-s), after aero lap polishing DLC (a-l), and before polishing DLC (non).
under a constant axial load of 4.7 N and a sliding speed of 0.1 m/s. Total sliding distance in each experiment was fixed to 500 m except when DLC-coating was delaminated. All the friction experiments were carried out at room temperature and under dry or water lubrication condition. For water lubrication, the cup was filled with water and a disk in the cup was immersed in distilled water. Friction force was monitored during friction experiments with data acquisition rate of 5 Hz. All the results of the friction experiments shown in this paper are the averaged friction coefficient value obtained by a moving average method. Friction experiments were conducted several times in each experimental condition, and the typical experimental results that represented the result in each condition, are displayed in this paper.
on the surface, followed by washing with ethanol by ultrasonic cleaning. Another method for removing droplets was a dry polishing called aero lap polishing. The aero lap polishing (Yamashita Works Co., Ltd.) is a lapping technique using shot particles with fine diamond powder on the surface, which are resilient and tacky containing a little water therein, blasted by air blow against the surface of target. In this paper, DLC-coatings with diamond slurry polishing, aero lap polishing and without polishing are tentatively defined as—DLC (d-s), DLC (a-l) and DLC (non), respectively.
2.2. Coated disk specimens The DLC-coatings were deposited using a multi-cathode UBMS system. Details of this deposition instrument are described elsewhere [19]. Substrate material used in this study was a mirror polished precipitation hardened stainless steel disk. After the substrates were introduced in a vacuum chamber, the system was evacuated down to <1 mPa, followed by an Ar ion plasma etching process. The Cr metal layer was deposited on the substrate as a bonding layer and the Cr–C gradient layer was formed on the Cr layer by increasing the carbon content toward the surface. Finally, an approximately 500–600-nm thick DLC layer was deposited on top of the bonding and gradient layer under an Ar–CH4 atmosphere (CH4 , 10 vol%). The substrate temperature and the bias voltage were approximately 200 ◦ C and 100 V, respectively, during the deposition. Hardness was approximately 17 GPa. Hydrogen content was approximately 30 at%. 2.3. Surface polishing of DLC-coating disks In order to remove droplets on the surface of coatings, two types of polishing method were used. One method was a typical wet polishing using diamond slurry aqueous liquid (particle size: 0.125 m). The coating was polished for 400 s to remove droplets
Fig. 2. Effect of washing treatment of DLC (a-l) on friction trace.
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3. Results and discussion 3.1. Change of the surface morphology and wettability by polishing Fig. 1 shows the image of the surfaces by scanning electron microscope before and after polishing. On the surface of DLCcoating without polishing, a large number of droplets (small
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particles) were observed, while after polishing either with the diamond slurry method or with the aero lap method, droplets were almost removed. However, it was found by the SEM observation that the surface of DLC-coating after the aero lap polishing was damaged and many streaky small scratches were generated on the surface. Moreover, DLC surface became hydrophilic after both polishing. Wettability of the surface was evaluated by contact angle measurement with water. The contact angle of water on DLC before
Fig. 3. Optical microscope images of the surface of DLC (a-l) and balls after friction experiments with and without washing treatment.
Fig. 4. Friction traces of each coating under dry and water lubrication conditions.
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polishing was 72.4◦ , which shows hydrophobic. The contact angle was changed to 48.5◦ on DLC (d-s), and to 66.1◦ on DLC (a-l), respectively. It is considered that polishing with the diamond slurry changed the surface of DLC-coating more hydrophilic, since the lapping with diamond slurry was carried out with the enough amount of water. Moreover, it was observed that the surface of DLC-coating changed into hydrophilic after sliding under water lubrication in our previous study. It is speculated that the hydrophilic nature of DLC surface might be brought by the formation of chemically bonded –OH group on the surface or some sort of hydrate formation. Contact angle measurements verified that the surface changed toward hydrophilic after polishing though the degree was different. It is suggested by the wettability that the surface conditions of DLC was different after two different polishing methods. 3.2. Tribological results 3.2.1. Effect of surface washing After polishing with diamond slurry, DLC-coatings were washed by ultrasonic cleaning with ethanol in order to remove diamond slurry remained on the surface. In order to compare the friction behavior under the same conditions, the same treatments were applied to all the coatings. Then friction examinations were conducted under dry condition and water lubrication in order to see if the effect of the difference of polishing methods existed. Although no significant difference of friction behavior between with and without washing was observed both on DLC (d-s) and
DLC (non), the apparent difference in the friction behavior of DLC (a-l) was observed after washing. The friction of DLC (a-l) under dry condition became higher than that of before washing as shown in Fig. 2. It is considered that the aero lap particles containing a slight amount of water on the surface were removed by washing, and the water with those particles affected the friction behaviors. However, the friction behaviors were not very different under water lubrication. Fig. 3 shows the images of the surface of DLC-coatings and balls after the experiments under dry condition. As shown in this figure, the wear of both DLC-coatings and balls increased after washing. In addition, amount of DLC transfer onto the ball surface became more after washing than that of without washing. This corresponds to the high friction. This is discussed in the next section. 3.2.2. Effect of water lubrication Friction behaviors observed under dry condition and water lubrication conditions are compared for three types of DLCs, i.e. DLC (non), DLC (d-s) and DLC (a-l), in Fig. 4. In all three DLCs, friction coefficient was smaller under water lubrication than under dry condition, in the initial region of the experiment. In addition, change in friction coefficient was more stable under water lubrication than under dry condition for all coatings. This might be explained by the introduction of the hydrodynamic lubrication effect with water. Particularly for DLC (d-s), due to the hydrophilic change of surface property by polishing, most stable friction trace was observed, which might be due to the ease of entry of water into the narrow friction surface. However, for DLC (non) and DLC
Fig. 5. Optical microscope images of counter ball surface. (a) DLC (non) dry, (b) DLC (non) water, (c) DLC (d-s) dry, (d) DLC (d-s) water, (e) DLC (a-l) dry and (f) DLC (a-l) water.
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Fig. 6. Optical microscope images of wear track of coatings with surface profile measured with stylus-type profilometer. (a) DLC (non) dry, (b) DLC (non) water, (c) DLC (d-s) dry, (d) DLC (d-s) water, (e) DLC (a-l) dry and (f) DLC (a-l) water.
(d-s), friction coefficients became similar around 0.18 for DLC (non) and 0.1 for DLC (d-s) before the sliding distance of 500 m. Fig. 5 shows the images of ball surfaces. It was observed that the amount of DLC transfer was larger under dry condition than under water lubrication. Initial high friction enhanced transfer onto the counter surface and then large amount of transfer was produced. After that, it was expected to make the friction coefficient lower in the successive period. Water inhibited adhesion between DLC and the steel surface resulted in small transfer, and then brought low friction by some tribo-film on DLC surface generated by friction. Fig. 6 shows the images of wear tracks of coatings. Delamination like local damage was observed on the surface of DLC (non) and DLC (d-s) under dry condition. The width of wear track was smaller under dry condition than that under water lubrication, related to the wear scar size of the ball. Droplets on the surface of DLC (non) might cause the local delamination under dry condition. In contrast, the local damage of surface of DLC (non) caused by droplets was reduced due to hydrodynamic lubrication effect and/or some hydrophilic tribo-film formation. It brought the surface wear across the entire friction track without local delamination under water lubrication. For DLC (d-s), local delamination under dry condition, which was shallower than for DLC (non), was observed even droplets on the surface were removed by polishing. It can be explained due to the lack of enough lubricity, though the hydrophilicity of the surface contributed to less wear compared to DLC (non). The wear of the surface of DLC (d-s) under water lubrication can be also explained
by the hydrodynamic lubrication effect among coatings and some tribo-film formation. For DLC (a-l), larger amount of transfer was observed under dry condition as in DLC (non) under dry, but the friction coefficient kept showing high value. Although the reason has not been clarified yet, it is considered that the aero lap polishing might generate micro-cracks or pits in the DLC-coating layer, and they made the coating weaker against friction. As a result, the transfer on the ball surface from the DLC-coating became larger, but friction coefficient kept high due to the cracks. Of course, as this is highly speculated explanation, more experimental proof is necessary. 3.2.3. Comparison of friction trace under different sliding conditions Friction traces of three types of DLC-coatings were put together under water lubrication and dry condition, separately as shown in Fig. 7. Under dry condition, DLC polished with the diamond slurry, DLC (d-s), showed the lowest friction coefficient and the smallest amount of transfer. This might be caused that the surface of coating became smoother by removing droplets, and became hydrophilic by polishing as explained in Section 3.1. This surface layer might bring good lubricity. Even though the DLC (a-l) was polished coating, it showed the highest friction coefficient and the largest amount of transfer. Moreover, with regard to the surface morphology of coatings, as shown in Fig. 6, local delamination was observed on both DLC (d-s) and DLC (non) under dry condition, while no delamination was observed
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Fig. 7. Comparison of friction traces of all coatings under each dry and water lubrication conditions.
on DLC (a-l). This may be considered because of the difference in condition of the surfaces. Micro-cracks were expected to exist on the surface of the DLC (a-l) as explained in Section 3.1 due to the aero lap polishing, therefore DLC (a-l) showed relatively high friction coefficient and the coating could be easily removed and the wear spread more widely resulted in no local delamination. Under water lubrication, DLC (a-l) showed the lowest and very stable friction coefficient, contrary to dry condition, though it showed worse result for both wear track and the wear scar size of the ball. It is considered that the small cracks on the surface could hold some water during experiments and caused low and stable friction. However, by the same reason, DLC (a-l) has tendency to be destroyed due to the presence of the small cracks, then it showed large wear over the entire region of the contact surface. Among the different DLC surfaces, DLC (d-s) showed the best friction and wear properties. 4. Conclusion DLC-coatings on SUS630 substrate were evaluated under water lubrication and dry condition. As droplets that were generated during the coating process were observed on the surface of coatings, it was tried to remove those droplets by polishing. Friction experiments were conducted with the diamond slurry polished surface, DLC (d-s), the aero lap polished surface, DLC (a-l), and without polishing surface, DLC (non), under both dry and water lubrication condition. Under dry condition, the transfer on the ball surface from DLC-coating was expected to make friction coefficient lower. For DLC (a-l), it showed highest friction coefficient under dry condition, while friction coefficient was very stable and lowest under water lubrication. It is considered that the aero lap particles generated micro-cracks on the surface of coating, and therefore those cracks could hold water but caused high friction and large wear. Therefore, it is considered that both hydrodynamic lubrication effect due to the generation of cracks which could hold some water and the change of surface property to hydrophilic by polishing or tribo-film formation could bring lower friction. With all the results considered, DLC (d-s) had best friction and wear properties under both water lubrication and dry condition.
References [1] H. Heshmat, S. Jahanmir, Tribological behavior of ceramics at high sliding speeds in steam, Tribol. Lett. 17 (2004) 359–366. [2] A. Eldemir, C. Bindal, G.R. Fenske, Tribological properties of hard carbon films on zirconia ceramics, Tribol. Trans. 39 (1996) 735–744. [3] S. Jahanmir, Y. Ozmen, L.K. Ives, Water lubrication of silicon nitride in sliding, Tribol. Lett. 17 (2004) 409–417. [4] L. Jordi, C. Iliev, T.E. Fischer, Lubrication of silicon nitride and silicon carbide by water: running in, wear and operation of sliding bearings, Tribol. Lett. 17 (2004) 367–376. [5] Q. Wang, Q. Xue, W. Liu, J. Chen, Tribological characteristics of nanometer Si3 N4 filled poly (ether ether ketone) under distilled water lubrication, J. Appl. Polym. Sci. 79 (2001) 1394–1400. [6] J. X.U, K. Kato, T. Hirayama, The transition of wear mode during the running-in process of silicon nitride sliding in water, Wear 205 (1997) 55–63. [7] R.S. Gates, S.M. Hsu, Tribochemistry between water and Si3 N4 and SiC: induction time analysis, Tribol. Lett. 17 (2004) 399–407. [8] A. Eldemir, M. Halter, G.R. Fenske, C. Zuiker, R. Csencsits, A.R. Krauss, D.M. Gruen, Friction and wear mechanisms of smooth diamond films during sliding in air and dry nitrogen, Tribol. Trans. 40 (1997) 667–675. [9] A. Eldemir, Proc. Inst. Mech. Eng. Part J: J. Eng. Tribol. 216 (2002) 387. [10] M. Masuko, A. Suzuki, Y. Sagae, M. Tokoro, K. Yamamoto, Friction characteristics of inorganic or organic thin coatings on solid surfaces under water lubrication, Tribol. Int. 39 (2006) 1601–1608. [11] H. Ronkainen, S. Varjus, K. Holmberg, Friction and wear properties in dry, waterand oil-lubricated DLC against alumina and DLC against steel contacts, Wear 222 (1998) 120–128. [12] H.I. Kim, J.R. Lince, O.L. Eryilmaz, A. Erdemir, Environmental effects on the friction of hydrogenated DLC films, Tribol. Lett. 21 (2006) 53–58. [13] J. Xu, H. Fan, H. Kousaka, N. Umehara, D. Diao, W. Liu, Growth and properties of hydrogen-free DLC films deposited by surface-wave-sustained plasma, Diamond Relat. Mater. 16 (2007) 161–166. [14] R. Paul, S. Dalui, S.N. Das, R. Bhar, A.K. Pal, Hydrophobicity in DLC films prepared by electrodeposition technique, Appl. Surf. Sci. 255 (2008) 1705–1711. [15] W. Tillmann, E. Vogli, F. Hoffmann, Low-friction diamond-like carbon (DLC)-layers for humid environment, Thin Solid Films 516 (2007) 262–266. [16] P. Lindholm, S. Bjorklund, F. Svahn, Method and surface roughness aspects for the design of DLC coatings, Wear 261 (2006) 107–111. [17] V.P. Poliakov, C.J.M. Siqueira, W. Veiga, I.A. Hummelgen, C.M. Lepienski, G.G. Kirpilenko, S.T. Dechandt, Physical and tribological properties of hard amorphous DLC films deposited on different substrates, Diamond Relat. Mater. 13 (2004) 1511–1515. [18] I.I. Aksenov, V.E. Strelnitskij, V.V. Vasilyev, D.Y. Zaleskij, Efficiency of magnetic plasma filters, Surf. Coat. Technol. 163–164 (2003) 118–127. [19] K. Yamamoto, K. Matsukado, Effect of hydrogenated DLC-coating hardness on the tribological properties under water lubrication, Tribol. Int. 39 (2006) 1609–1614.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Improvement of tribological characteristics under water lubrication of DLC-coatings by surface polishing Maiko Tokoro a,∗ , Yusuke Aiyama a , Masabumi Masuko a , Akihito Suzuki a , Hirotaka Ito b , Kenji Yamamoto b a b
Tokyo Institute of Technology, Department of Chemical Engineering, Graduate School of Science and Engineering, 12-1 O-okayama 2-chome, Meguro-ku, Tokyo 152-8552, Japan Materials Research Laboratory, Kobe Steel Ltd., 5-5 Takatsuka-dai Nishi-ku 1-chome, Kobe, Hyogo 651-2271, Japan
a r t i c l e
i n f o
Article history: Received 8 October 2008 Received in revised form 2 March 2009 Accepted 10 April 2009 Available online 23 May 2009 Keywords: Water lubrication DLC-coatings Surface polishing Wettability
a b s t r a c t To achieve a hydraulic power system, it is important to control tribology because water has a lack of lubricity. Therefore, coated surface is necessary under water lubrication. Diamond-like-carbon (DLC)coating is known as a useful material because of its high hardness and low friction, therefore it can be used as a coating durable for the water lubrication. Deposition methods of DLC-coating are developed in various ways. Particles called “droplets” are observed on the surface of DLC-coating depends on deposition methods and it can affect friction and wear properties. In this study, DLC-coating was prepared using a multi-cathode unbalanced magnetron sputtering (UBMS) system. The surface was polished with diamond slurry solution and aero lap to remove droplets. DLC-coatings were evaluated by tribo-tests before and after polishing. It is considered that some surface modification occurred. Moreover, the results of tribotests show that friction coefficients became lower and more stable than before polishing. Although partial delamination was observed after tribo-tests without polishing, no appreciable wear was observed after polishing. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In order to minimize the consumption of fossil fuels and mineral oils for preventing environmental destruction, use of water as a working fluid of hydraulic system instead of mineral oil is one of the solutions. Although the use of water has many advantages, the low lubricity of water has to be solved to develop the reliable water hydraulic systems. Therefore, tribology is the key technology to overcome the problem of water. Some inorganic materials or coatings on metal surfaces are known to have a good lubricity under water lubrication [1,2]. For example, silicon nitride (Si3 N4 ) and silicon carbide (SiC) show low friction in water environments [3–7]. In spite of having good lubricity under water lubrication, silicon nitride or silicon carbide cannot be applied to the commercial power systems due to the cost or their low machinability. Diamond-like-carbon (DLC) is known as a low-cost coating durable not only for under dry condition but also for under water lubrication [8,9]. The authors have already reported that the DLC-coating showed low friction coefficient and sliding speed dependency of friction coefficient is very small under water lubri-
∗ Corresponding author. E-mail address: [email protected] (M. Tokoro). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.04.009
cation [10]. There have been many reports on DLC-coating because it has unique properties, such as chemical inertness, mechanical, tribological and optical properties [11–17]. It has been known that small particles called “droplets” are occasionally observed on the surface of DLC-coatings. The degree of droplets formation varies depending on the deposition method. Especially, it is unavoidable in case of sputtering. Some experiments show that generation of droplets can be controlled using a filter [18]. However, degree of droplets formation is not always same even under the same condition and same deposition method, and in addition, it is unpredictable. From the tribological point of view, effect of the droplets should be clarified to maintain good lubricity. In this study, the droplets were removed from DLC surface using two types of polishing method, and their effect on tribology under dry and water lubrication conditions was compared. 2. Experimental 2.1. Tribometer Friction and wear properties were evaluated using a ball-ondisk tribometer. A flat disk was installed on a rotating shaft with a disk holder inside a cup. A ball was set in a ball holder. Heat-treated stainless steel (SUS630) ball having a diameter of 9.53 mm (3/8 in.) was used. In this study, the friction experiments were carried out
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Fig. 1. SEM images of the surface of DLC-coatings after diamond slurry polishing DLC (d-s), after aero lap polishing DLC (a-l), and before polishing DLC (non).
under a constant axial load of 4.7 N and a sliding speed of 0.1 m/s. Total sliding distance in each experiment was fixed to 500 m except when DLC-coating was delaminated. All the friction experiments were carried out at room temperature and under dry or water lubrication condition. For water lubrication, the cup was filled with water and a disk in the cup was immersed in distilled water. Friction force was monitored during friction experiments with data acquisition rate of 5 Hz. All the results of the friction experiments shown in this paper are the averaged friction coefficient value obtained by a moving average method. Friction experiments were conducted several times in each experimental condition, and the typical experimental results that represented the result in each condition, are displayed in this paper.
on the surface, followed by washing with ethanol by ultrasonic cleaning. Another method for removing droplets was a dry polishing called aero lap polishing. The aero lap polishing (Yamashita Works Co., Ltd.) is a lapping technique using shot particles with fine diamond powder on the surface, which are resilient and tacky containing a little water therein, blasted by air blow against the surface of target. In this paper, DLC-coatings with diamond slurry polishing, aero lap polishing and without polishing are tentatively defined as—DLC (d-s), DLC (a-l) and DLC (non), respectively.
2.2. Coated disk specimens The DLC-coatings were deposited using a multi-cathode UBMS system. Details of this deposition instrument are described elsewhere [19]. Substrate material used in this study was a mirror polished precipitation hardened stainless steel disk. After the substrates were introduced in a vacuum chamber, the system was evacuated down to <1 mPa, followed by an Ar ion plasma etching process. The Cr metal layer was deposited on the substrate as a bonding layer and the Cr–C gradient layer was formed on the Cr layer by increasing the carbon content toward the surface. Finally, an approximately 500–600-nm thick DLC layer was deposited on top of the bonding and gradient layer under an Ar–CH4 atmosphere (CH4 , 10 vol%). The substrate temperature and the bias voltage were approximately 200 ◦ C and 100 V, respectively, during the deposition. Hardness was approximately 17 GPa. Hydrogen content was approximately 30 at%. 2.3. Surface polishing of DLC-coating disks In order to remove droplets on the surface of coatings, two types of polishing method were used. One method was a typical wet polishing using diamond slurry aqueous liquid (particle size: 0.125 m). The coating was polished for 400 s to remove droplets
Fig. 2. Effect of washing treatment of DLC (a-l) on friction trace.
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3. Results and discussion 3.1. Change of the surface morphology and wettability by polishing Fig. 1 shows the image of the surfaces by scanning electron microscope before and after polishing. On the surface of DLCcoating without polishing, a large number of droplets (small
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particles) were observed, while after polishing either with the diamond slurry method or with the aero lap method, droplets were almost removed. However, it was found by the SEM observation that the surface of DLC-coating after the aero lap polishing was damaged and many streaky small scratches were generated on the surface. Moreover, DLC surface became hydrophilic after both polishing. Wettability of the surface was evaluated by contact angle measurement with water. The contact angle of water on DLC before
Fig. 3. Optical microscope images of the surface of DLC (a-l) and balls after friction experiments with and without washing treatment.
Fig. 4. Friction traces of each coating under dry and water lubrication conditions.
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polishing was 72.4◦ , which shows hydrophobic. The contact angle was changed to 48.5◦ on DLC (d-s), and to 66.1◦ on DLC (a-l), respectively. It is considered that polishing with the diamond slurry changed the surface of DLC-coating more hydrophilic, since the lapping with diamond slurry was carried out with the enough amount of water. Moreover, it was observed that the surface of DLC-coating changed into hydrophilic after sliding under water lubrication in our previous study. It is speculated that the hydrophilic nature of DLC surface might be brought by the formation of chemically bonded –OH group on the surface or some sort of hydrate formation. Contact angle measurements verified that the surface changed toward hydrophilic after polishing though the degree was different. It is suggested by the wettability that the surface conditions of DLC was different after two different polishing methods. 3.2. Tribological results 3.2.1. Effect of surface washing After polishing with diamond slurry, DLC-coatings were washed by ultrasonic cleaning with ethanol in order to remove diamond slurry remained on the surface. In order to compare the friction behavior under the same conditions, the same treatments were applied to all the coatings. Then friction examinations were conducted under dry condition and water lubrication in order to see if the effect of the difference of polishing methods existed. Although no significant difference of friction behavior between with and without washing was observed both on DLC (d-s) and
DLC (non), the apparent difference in the friction behavior of DLC (a-l) was observed after washing. The friction of DLC (a-l) under dry condition became higher than that of before washing as shown in Fig. 2. It is considered that the aero lap particles containing a slight amount of water on the surface were removed by washing, and the water with those particles affected the friction behaviors. However, the friction behaviors were not very different under water lubrication. Fig. 3 shows the images of the surface of DLC-coatings and balls after the experiments under dry condition. As shown in this figure, the wear of both DLC-coatings and balls increased after washing. In addition, amount of DLC transfer onto the ball surface became more after washing than that of without washing. This corresponds to the high friction. This is discussed in the next section. 3.2.2. Effect of water lubrication Friction behaviors observed under dry condition and water lubrication conditions are compared for three types of DLCs, i.e. DLC (non), DLC (d-s) and DLC (a-l), in Fig. 4. In all three DLCs, friction coefficient was smaller under water lubrication than under dry condition, in the initial region of the experiment. In addition, change in friction coefficient was more stable under water lubrication than under dry condition for all coatings. This might be explained by the introduction of the hydrodynamic lubrication effect with water. Particularly for DLC (d-s), due to the hydrophilic change of surface property by polishing, most stable friction trace was observed, which might be due to the ease of entry of water into the narrow friction surface. However, for DLC (non) and DLC
Fig. 5. Optical microscope images of counter ball surface. (a) DLC (non) dry, (b) DLC (non) water, (c) DLC (d-s) dry, (d) DLC (d-s) water, (e) DLC (a-l) dry and (f) DLC (a-l) water.
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Fig. 6. Optical microscope images of wear track of coatings with surface profile measured with stylus-type profilometer. (a) DLC (non) dry, (b) DLC (non) water, (c) DLC (d-s) dry, (d) DLC (d-s) water, (e) DLC (a-l) dry and (f) DLC (a-l) water.
(d-s), friction coefficients became similar around 0.18 for DLC (non) and 0.1 for DLC (d-s) before the sliding distance of 500 m. Fig. 5 shows the images of ball surfaces. It was observed that the amount of DLC transfer was larger under dry condition than under water lubrication. Initial high friction enhanced transfer onto the counter surface and then large amount of transfer was produced. After that, it was expected to make the friction coefficient lower in the successive period. Water inhibited adhesion between DLC and the steel surface resulted in small transfer, and then brought low friction by some tribo-film on DLC surface generated by friction. Fig. 6 shows the images of wear tracks of coatings. Delamination like local damage was observed on the surface of DLC (non) and DLC (d-s) under dry condition. The width of wear track was smaller under dry condition than that under water lubrication, related to the wear scar size of the ball. Droplets on the surface of DLC (non) might cause the local delamination under dry condition. In contrast, the local damage of surface of DLC (non) caused by droplets was reduced due to hydrodynamic lubrication effect and/or some hydrophilic tribo-film formation. It brought the surface wear across the entire friction track without local delamination under water lubrication. For DLC (d-s), local delamination under dry condition, which was shallower than for DLC (non), was observed even droplets on the surface were removed by polishing. It can be explained due to the lack of enough lubricity, though the hydrophilicity of the surface contributed to less wear compared to DLC (non). The wear of the surface of DLC (d-s) under water lubrication can be also explained
by the hydrodynamic lubrication effect among coatings and some tribo-film formation. For DLC (a-l), larger amount of transfer was observed under dry condition as in DLC (non) under dry, but the friction coefficient kept showing high value. Although the reason has not been clarified yet, it is considered that the aero lap polishing might generate micro-cracks or pits in the DLC-coating layer, and they made the coating weaker against friction. As a result, the transfer on the ball surface from the DLC-coating became larger, but friction coefficient kept high due to the cracks. Of course, as this is highly speculated explanation, more experimental proof is necessary. 3.2.3. Comparison of friction trace under different sliding conditions Friction traces of three types of DLC-coatings were put together under water lubrication and dry condition, separately as shown in Fig. 7. Under dry condition, DLC polished with the diamond slurry, DLC (d-s), showed the lowest friction coefficient and the smallest amount of transfer. This might be caused that the surface of coating became smoother by removing droplets, and became hydrophilic by polishing as explained in Section 3.1. This surface layer might bring good lubricity. Even though the DLC (a-l) was polished coating, it showed the highest friction coefficient and the largest amount of transfer. Moreover, with regard to the surface morphology of coatings, as shown in Fig. 6, local delamination was observed on both DLC (d-s) and DLC (non) under dry condition, while no delamination was observed
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Fig. 7. Comparison of friction traces of all coatings under each dry and water lubrication conditions.
on DLC (a-l). This may be considered because of the difference in condition of the surfaces. Micro-cracks were expected to exist on the surface of the DLC (a-l) as explained in Section 3.1 due to the aero lap polishing, therefore DLC (a-l) showed relatively high friction coefficient and the coating could be easily removed and the wear spread more widely resulted in no local delamination. Under water lubrication, DLC (a-l) showed the lowest and very stable friction coefficient, contrary to dry condition, though it showed worse result for both wear track and the wear scar size of the ball. It is considered that the small cracks on the surface could hold some water during experiments and caused low and stable friction. However, by the same reason, DLC (a-l) has tendency to be destroyed due to the presence of the small cracks, then it showed large wear over the entire region of the contact surface. Among the different DLC surfaces, DLC (d-s) showed the best friction and wear properties. 4. Conclusion DLC-coatings on SUS630 substrate were evaluated under water lubrication and dry condition. As droplets that were generated during the coating process were observed on the surface of coatings, it was tried to remove those droplets by polishing. Friction experiments were conducted with the diamond slurry polished surface, DLC (d-s), the aero lap polished surface, DLC (a-l), and without polishing surface, DLC (non), under both dry and water lubrication condition. Under dry condition, the transfer on the ball surface from DLC-coating was expected to make friction coefficient lower. For DLC (a-l), it showed highest friction coefficient under dry condition, while friction coefficient was very stable and lowest under water lubrication. It is considered that the aero lap particles generated micro-cracks on the surface of coating, and therefore those cracks could hold water but caused high friction and large wear. Therefore, it is considered that both hydrodynamic lubrication effect due to the generation of cracks which could hold some water and the change of surface property to hydrophilic by polishing or tribo-film formation could bring lower friction. With all the results considered, DLC (d-s) had best friction and wear properties under both water lubrication and dry condition.
References [1] H. Heshmat, S. Jahanmir, Tribological behavior of ceramics at high sliding speeds in steam, Tribol. Lett. 17 (2004) 359–366. [2] A. Eldemir, C. Bindal, G.R. Fenske, Tribological properties of hard carbon films on zirconia ceramics, Tribol. Trans. 39 (1996) 735–744. [3] S. Jahanmir, Y. Ozmen, L.K. Ives, Water lubrication of silicon nitride in sliding, Tribol. Lett. 17 (2004) 409–417. [4] L. Jordi, C. Iliev, T.E. Fischer, Lubrication of silicon nitride and silicon carbide by water: running in, wear and operation of sliding bearings, Tribol. Lett. 17 (2004) 367–376. [5] Q. Wang, Q. Xue, W. Liu, J. Chen, Tribological characteristics of nanometer Si3 N4 filled poly (ether ether ketone) under distilled water lubrication, J. Appl. Polym. Sci. 79 (2001) 1394–1400. [6] J. X.U, K. Kato, T. Hirayama, The transition of wear mode during the running-in process of silicon nitride sliding in water, Wear 205 (1997) 55–63. [7] R.S. Gates, S.M. Hsu, Tribochemistry between water and Si3 N4 and SiC: induction time analysis, Tribol. Lett. 17 (2004) 399–407. [8] A. Eldemir, M. Halter, G.R. Fenske, C. Zuiker, R. Csencsits, A.R. Krauss, D.M. Gruen, Friction and wear mechanisms of smooth diamond films during sliding in air and dry nitrogen, Tribol. Trans. 40 (1997) 667–675. [9] A. Eldemir, Proc. Inst. Mech. Eng. Part J: J. Eng. Tribol. 216 (2002) 387. [10] M. Masuko, A. Suzuki, Y. Sagae, M. Tokoro, K. Yamamoto, Friction characteristics of inorganic or organic thin coatings on solid surfaces under water lubrication, Tribol. Int. 39 (2006) 1601–1608. [11] H. Ronkainen, S. Varjus, K. Holmberg, Friction and wear properties in dry, waterand oil-lubricated DLC against alumina and DLC against steel contacts, Wear 222 (1998) 120–128. [12] H.I. Kim, J.R. Lince, O.L. Eryilmaz, A. Erdemir, Environmental effects on the friction of hydrogenated DLC films, Tribol. Lett. 21 (2006) 53–58. [13] J. Xu, H. Fan, H. Kousaka, N. Umehara, D. Diao, W. Liu, Growth and properties of hydrogen-free DLC films deposited by surface-wave-sustained plasma, Diamond Relat. Mater. 16 (2007) 161–166. [14] R. Paul, S. Dalui, S.N. Das, R. Bhar, A.K. Pal, Hydrophobicity in DLC films prepared by electrodeposition technique, Appl. Surf. Sci. 255 (2008) 1705–1711. [15] W. Tillmann, E. Vogli, F. Hoffmann, Low-friction diamond-like carbon (DLC)-layers for humid environment, Thin Solid Films 516 (2007) 262–266. [16] P. Lindholm, S. Bjorklund, F. Svahn, Method and surface roughness aspects for the design of DLC coatings, Wear 261 (2006) 107–111. [17] V.P. Poliakov, C.J.M. Siqueira, W. Veiga, I.A. Hummelgen, C.M. Lepienski, G.G. Kirpilenko, S.T. Dechandt, Physical and tribological properties of hard amorphous DLC films deposited on different substrates, Diamond Relat. Mater. 13 (2004) 1511–1515. [18] I.I. Aksenov, V.E. Strelnitskij, V.V. Vasilyev, D.Y. Zaleskij, Efficiency of magnetic plasma filters, Surf. Coat. Technol. 163–164 (2003) 118–127. [19] K. Yamamoto, K. Matsukado, Effect of hydrogenated DLC-coating hardness on the tribological properties under water lubrication, Tribol. Int. 39 (2006) 1609–1614.