- Email: [email protected]
Frictional behavior of DLC films in a water environment
Diamond and Related Materials 13 (2004) 1464–1468
Frictional behavior of DLC films in a water environment M. Suzukia,*, A. Tanakaa, T. Ohanaa, W. Zhangb a
Research Center for Advanced Carbon Materials, AIST Tsukuba Central 5, AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b National Key Laboratory of Remanufacture, No.21, Dujiakan, Changxindian, Beijing 100072, PR China
Abstract Three types of Ar incorporated DLC films were deposited using a thermal electron excited plasma CVD apparatus at bias voltages of y0.5 to y3 kV. Their friction and wear properties were evaluated under water and air environments using a ball-onplate type reciprocating friction tester. The anti-wear properties of Ar-DLC films in water were superior to that in air; the smallest wear rate in water was approximately 8=10y9 mm3 yNm. The reduction in the friction coefficient in water was not clear, compared to that in air. The lowest friction coefficient in water was approximately 0.1. The transferred materials were observed on the wear scar of the mating ball in every experiment, but the transferred materials in water were greater in quantity than those in air. Furthermore, there seemed to be a qualitative difference in transferred materials between water and air environments, based on micro-Raman analysis. The difference in such transfer phenomena seemed to affect both the friction coefficient and wear rates. These friction and wear behaviors suggest that our Ar-DLC films are good candidates for use in rubbing machine parts when in use with water hydraulic systems. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Friction; Wear; Coatings
1. Introduction Numerous machines and instruments have used oil hydraulic systems as driving systems. However, leakage of oil from such hydraulic systems tends to pollute factories and natural environments, such as soil, rivers, lakes and oceans. To eliminate this pollution source, water hydraulic systems are replacing oil systems. These systems are also useful in disaster prevention and improving safety and sanitation. However, systems using water have some technical problems, including controllability, tribology, corrosion, and reliability. This is especially true with tribological problems such as high friction, large wear, and seizures. To overcome such problems, it is necessary to develop the tribomaterials, which display excellent tribological properties in a water environment. Some recent research efforts have demonstrated that diamond-like carbon (DLC) films can exhibit excellent tribological properties in both water environments and ambient air w1–5x. These results suggest that DLC films *Corresponding author. Tel.: q81-29-861-3442; fax: q81-29-8614636. E-mail address: [email protected] (M. Suzuki).
are very promising candidates for the coating of sliding parts used in water hydraulic systems. However, detailed tribological properties of different film types and the mechanisms of low friction and high wear resistance have not yet been obtained. Recently, Zhang et al. reported that an Ar incorporated DLC (Ar-DLC) film shows excellent low friction and wear properties in dry air, N2, and O2 environments w6x. There is the possibility that this Ar-DLC film may have good tribological properties, even when in a water environment. In this study, Ar-DLC films were deposited by a plasma-enhanced CVD technique, changing substrate bias voltage, and their friction and wear properties were fully determined in both water and air environments. 2. Experiments 2.1. Film deposition DLC films were deposited on silicon wafers using thermal electron excited plasma CVD apparatus and C6H6q Ar gas. Three types of Ar incorporated DLC films were prepared at different bias voltage of y0.5 kV (Ar-DLC1), y2.0 kV (Ar-DLC2) and y3.0 kV
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.068
M. Suzuki et al. / Diamond and Related Materials 13 (2004) 1464–1468
Fig. 1. Raman spectra of Ar-DLC films deposited at different bias voltages.
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had a typical DLC Raman spectrum exhibiting a shouldered peak (D-band at 1350 cmy1) and a broad peak (G-band at 1570 cmy1). However, the intensity ratio of the D-band to G-band, Id yIg, of Ar-DLC1 was smaller than those of other films. The various properties of three films are summarized in Table 1. The hardness was measured by a nanoindenter and the hydrogen content was analyzed with an elastic recoil detection analysis (ERDA). The hardness decreased from 39 to 27 GPa by increasing the negative substrate bias voltage. The hydrogen content of Ar-DLC1 was 29 at.%, and the hydrogen content of Ar-DLC2 and Ar-DLC3 were approximately 20 at.%. The increase in film hardness is generally accompanied by a decrease in hydrogen content. However, the hardness of our Ar-DLC film increased with an increase in the hydrogen content, although the reason for this is unknown. The film surface roughness, Ra, ranged from 1.5 to 2 nm, irrespective of film type. 3.2. Friction and wear
(Ar-DLC3), respectively. Details of deposition were reported elsewhere w6x. 2.2. Friction and wear testing All tests were conducted by a ball-on-plate reciprocating friction tester. The friction part was soaked in an ion-exchange water bath, except for the test in ambient air. The plate specimen was an Ar-DLC film and the mating ball specimen (diameter of 4.76 mm) was made of hardened martensite stainless steel (AISI 440C, Hvs 830). The reciprocating friction stroke was 8 mm and the normal load was given by the dead weight. Prior to testing, specimens were cleaned with petroleum benzene and acetone in an ultrasonic cleaner, and then dried in desiccators. Tests were conducted at normal loads of 5.7, 9.4, and 11.3 N (maximum Hertzian stress, 1.2, 1.5 and 1.6 GPa). The average sliding speed was 16 mmys (60 cycleymin) and the friction cycle was 7200 cycles. The wear volume of these Ar-DLC films was calculated by measuring wear scars with an atomic force microscope (AFM) and the mating ball wear was obtained by measurement of the wear scar diameter on the ball. The wear scar of the Ar-DLC films and mating balls was observed using optical microscopy. Transferred materials on the wear scar of the mating ball and the wear tracks of DLC films were determined using microlaser Raman spectroscopy and X-ray photoelectron spectroscopy.
The friction behavior of three Ar-DLC films is shown in Fig. 2. In water, the friction coefficient generally decreased at an extremely early stage and then increased gradually, approaching a steady-state value. However, the friction in air started at a significantly high value and then converged to a steady state value after a gradual decrease. The minimum friction coefficient in water was clearly smaller than that in air with no exceptions. However, the difference in the value of the steady friction coefficient between water and air environments was unclear, regardless of the type of film. The friction fluctuation in water was smaller than in air for all films tested. This clear difference in steady friction was not found among the three types of films when in both water and air environments. The friction coefficient of Ar-DLC2 and Ar-DLC3 tended to decrease when increasing the load, although the Ar-DLC1 film was independent of the load. Fig. 3 is the specific wear rate of Ar-DLC films and mating balls. The specific wear rate of every Ar-DLC film when in water was clearly smaller than that in air, without exception. There was no clear difference in wear rates among these three films. In a water environment, Ar-DLC1 and Ar-DLC3 films showed a very small value of approximately 8=10y9 mm3 yNm at 9.4 N. The wear Table 1 Various properties of three Ar-DLC films
3. Results and discussion 3.1. Characterization of Ar-DLC films The Raman spectra of Ar-DLC films deposited using different bias voltages are shown in Fig. 1. Every film
Ar-DLC1 Ar-DLC2 Ar-DLC3
Hardness (GPa)
Hydrogen content (at.%)
Roughness Ra (nm)
38 32 27
28.6 21.4 20.3
2.0 2.0 1.5
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M. Suzuki et al. / Diamond and Related Materials 13 (2004) 1464–1468
Fig. 3. Specific wear rates of three Ar-DLC films in water and air. Fig. 2. Friction behaviors of three types of Ar-DLC films in water and air.
rate of every film tended to decrease when increasing the normal load in water, although this tendency was not seen with Ar-DLC3 in air. The mating ball wear with Ar-DLC2 and Ar-DLC3 was larger in water than that in air, while with Ar-DLC1, the dependency of the wear rate on the environment was reversed. In general, the ball wear was smaller than the film wear, irrespective of the film type. There are no clear differences in ball wear between Ar-DLC2 and Ar-DLC3, but the ball wear when slid against Ar-DLC1 was somewhat smaller than that with other films. The dependency of ball wear rate on the normal load was different with different film types.
to that in air. The wear scar features of Ar-DLC3 were similar to that of Ar-DLC2. Fig. 5 are the wear scars on mating balls observed by optical microscopy. The wear scar on the ball was
3.3. Wear scars The wear scar profiles of Ar-DLC films, measured by an AFM, are shown in Fig. 4. The wear depth of ArDLC2 in water was clearly shallower than that in air, while the wear scar width in water seemed to be slightly larger than that in air. However, the wear depth and the wear scar width of Ar-DLC1 in water were comparable
Fig. 4. Wear scar profiles on Ar-DLC2 films at 9.4 N.
M. Suzuki et al. / Diamond and Related Materials 13 (2004) 1464–1468
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Fig. 5. Wear scars of mating balls sliding against Ar-DLC2 at 9.4 N.
clearly covered by transferred materials, irrespective of the environment. The amount of transferred materials in water, however, was greater than that in air. The same tendency was observed on the mating ball when slid against other films. The wear scar on the mating ball was analyzed by micro-laser Raman spectroscopy and X-ray photoelectron spectroscopy. The Raman spectrum showed that there was a clear qualitative difference in the transferred materials between water and air environments (Fig. 6). In air, some graphitization occurred in transferred materials, while no clear change from the virgin Ar-DLC film was recognized in water. The analytical results by XPS are summarized in Table 2. This result suggests that the wear scars, covered with transferred materials, are largely composed of carbon and oxygen. The carbon content of these wear scars is a little greater when under a water environment than that in air. This detailed analysis, unfortunately, could not separate the qualitative
differences between two friction surfaces under both water and air, although it showed the existence of compounds on both wear scares; including Fe2O3, Fe3O4, FeOOH, Cr(OH)3, Cr2O3, etc. 3.4. Discussion Three types of Ar incorporated DLC films, deposited by changing the bias voltage from y0.5 to y3 kV, showed a moderate difference in characteristics such as H content, hardness, and the ratio of D over G peak area in the Raman spectra. No clear difference in friction coefficient, however, appeared among three films. Film wear also did not appear to be clearly dependant on the film type. These results suggest that the friction and wear in both water and air were not sensitive to film structure and composition, at least in this study. The friction behavior in water was different from that in air, although the differences in the steady state friction were not as clear. The wear rate of every film in water was always lower than that in air, irrespective of the normal load used. However, the mating ball surfaces in water were covered with a greater amount of transferred materials than those in air. The transferred material produced under a water environment differed qualitatively from that produced in air, based on Raman analysis. In any case, the difference in the transfer phenomenon between the mating balls must correlate with the difference in friction and wear behavior between water and air environments, although the details remain unknown. Furthermore, the features identified in Table 2 Results of XPS analysis of wear scars on mating balls sliding against Ar-DLC2 film at 9.4 N
Fig. 6. Raman spectra of wear scars on mating ball sliding against Ar-DLC2 at 11.3 N.
Ar-DLC2 in air Ar-DLC2 in water Ar-DLC2* *
C
O
Ar
N
Si
Cr
Fe
Co
39 48 91
41 30 8.6
– – -0.5
1 -1 –
6 -0.5 –
5 5 –
5 5 –
-1 -1 –
The atomic ratio of Ar-DLC film before tribological tests.
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M. Suzuki et al. / Diamond and Related Materials 13 (2004) 1464–1468
the wear scars on Ar-DLC films in water differed from that in air, although the differences with Ar-DLC1 were unclear. These differences in the wear scar features seem to be correlated with differences in the wear rate of films between water and air environments. In water, all types of Ar-DLC films had a low friction coefficient of approximately 0.1 and a low wear rate of less than 2=10y8 mm3 yNm. Moreover, the mating ball wear exhibited a low level of 10y9 mm3 yNm. This friction and wear behaviors suggest that our Ar-DLC films are good candidates for use in rubbing machine parts with water hydraulic systems. 4. Conclusions Three types of Ar incorporated DLC films were deposited using a thermal electron excited plasma CVD method. Their friction and wear properties were evaluated under water and air environments using a ball-onplate type friction tester. The main results are as follows: 1. The anti-wear property of Ar-DLC films in water was superior to that in air; the smallest wear rate in water was approximately 8=10y9 mm3 yNm. 2. The reduction of friction coefficient in water was not clear, compared to that in air. The lowest friction coefficient in water was approximately 0.1. 3. The transferred materials were observed on the wear scar of the mating ball in every experiment, but the transferred materials in water were richer than those
in air. Furthermore, there seemed to be the qualitative difference in transferred materials between water and air environments. The difference in such transfer phenomena seemed to affect both friction coefficient and wear rate. Acknowledgments The authors express their gratitude to Mr Tomoki Nishikawa for his assistance in this experiment. 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, entitled ‘Surface-modification technologies for friction reduction in various machinery’. References w1x D. Neerinck, P. Persoone, M. Sercu, A. Goel, D. Kester, D. Bray, Diamond Relat. Mater. 7 (1998) 468. w2x H. Ronkainen, S. Varjus, K. Holmberg, Wear 222 (1998) 120. w3x H. Ronkainen, S. Varjus, K. Holmberg, Wear 249 (2001) 267–271. w4x D. Sheejs, B.K. Tay, S.P. Lau, L.N. Numg, Surf. Coat. Technol. 146–147 (2001) 410–416. w5x J. Jiang, S. Zhang, R.D. Arnell, Surf. Coat. Technol. 167 (2003) 221–225. w6x W. Zhang, A. Tanaka, K. Wazumi, Y. Koga, Thin Solid Films 416 (2002) 145–152.
Frictional behavior of DLC films in a water environment M. Suzukia,*, A. Tanakaa, T. Ohanaa, W. Zhangb a
Research Center for Advanced Carbon Materials, AIST Tsukuba Central 5, AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b National Key Laboratory of Remanufacture, No.21, Dujiakan, Changxindian, Beijing 100072, PR China
Abstract Three types of Ar incorporated DLC films were deposited using a thermal electron excited plasma CVD apparatus at bias voltages of y0.5 to y3 kV. Their friction and wear properties were evaluated under water and air environments using a ball-onplate type reciprocating friction tester. The anti-wear properties of Ar-DLC films in water were superior to that in air; the smallest wear rate in water was approximately 8=10y9 mm3 yNm. The reduction in the friction coefficient in water was not clear, compared to that in air. The lowest friction coefficient in water was approximately 0.1. The transferred materials were observed on the wear scar of the mating ball in every experiment, but the transferred materials in water were greater in quantity than those in air. Furthermore, there seemed to be a qualitative difference in transferred materials between water and air environments, based on micro-Raman analysis. The difference in such transfer phenomena seemed to affect both the friction coefficient and wear rates. These friction and wear behaviors suggest that our Ar-DLC films are good candidates for use in rubbing machine parts when in use with water hydraulic systems. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Friction; Wear; Coatings
1. Introduction Numerous machines and instruments have used oil hydraulic systems as driving systems. However, leakage of oil from such hydraulic systems tends to pollute factories and natural environments, such as soil, rivers, lakes and oceans. To eliminate this pollution source, water hydraulic systems are replacing oil systems. These systems are also useful in disaster prevention and improving safety and sanitation. However, systems using water have some technical problems, including controllability, tribology, corrosion, and reliability. This is especially true with tribological problems such as high friction, large wear, and seizures. To overcome such problems, it is necessary to develop the tribomaterials, which display excellent tribological properties in a water environment. Some recent research efforts have demonstrated that diamond-like carbon (DLC) films can exhibit excellent tribological properties in both water environments and ambient air w1–5x. These results suggest that DLC films *Corresponding author. Tel.: q81-29-861-3442; fax: q81-29-8614636. E-mail address: [email protected] (M. Suzuki).
are very promising candidates for the coating of sliding parts used in water hydraulic systems. However, detailed tribological properties of different film types and the mechanisms of low friction and high wear resistance have not yet been obtained. Recently, Zhang et al. reported that an Ar incorporated DLC (Ar-DLC) film shows excellent low friction and wear properties in dry air, N2, and O2 environments w6x. There is the possibility that this Ar-DLC film may have good tribological properties, even when in a water environment. In this study, Ar-DLC films were deposited by a plasma-enhanced CVD technique, changing substrate bias voltage, and their friction and wear properties were fully determined in both water and air environments. 2. Experiments 2.1. Film deposition DLC films were deposited on silicon wafers using thermal electron excited plasma CVD apparatus and C6H6q Ar gas. Three types of Ar incorporated DLC films were prepared at different bias voltage of y0.5 kV (Ar-DLC1), y2.0 kV (Ar-DLC2) and y3.0 kV
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.068
M. Suzuki et al. / Diamond and Related Materials 13 (2004) 1464–1468
Fig. 1. Raman spectra of Ar-DLC films deposited at different bias voltages.
1465
had a typical DLC Raman spectrum exhibiting a shouldered peak (D-band at 1350 cmy1) and a broad peak (G-band at 1570 cmy1). However, the intensity ratio of the D-band to G-band, Id yIg, of Ar-DLC1 was smaller than those of other films. The various properties of three films are summarized in Table 1. The hardness was measured by a nanoindenter and the hydrogen content was analyzed with an elastic recoil detection analysis (ERDA). The hardness decreased from 39 to 27 GPa by increasing the negative substrate bias voltage. The hydrogen content of Ar-DLC1 was 29 at.%, and the hydrogen content of Ar-DLC2 and Ar-DLC3 were approximately 20 at.%. The increase in film hardness is generally accompanied by a decrease in hydrogen content. However, the hardness of our Ar-DLC film increased with an increase in the hydrogen content, although the reason for this is unknown. The film surface roughness, Ra, ranged from 1.5 to 2 nm, irrespective of film type. 3.2. Friction and wear
(Ar-DLC3), respectively. Details of deposition were reported elsewhere w6x. 2.2. Friction and wear testing All tests were conducted by a ball-on-plate reciprocating friction tester. The friction part was soaked in an ion-exchange water bath, except for the test in ambient air. The plate specimen was an Ar-DLC film and the mating ball specimen (diameter of 4.76 mm) was made of hardened martensite stainless steel (AISI 440C, Hvs 830). The reciprocating friction stroke was 8 mm and the normal load was given by the dead weight. Prior to testing, specimens were cleaned with petroleum benzene and acetone in an ultrasonic cleaner, and then dried in desiccators. Tests were conducted at normal loads of 5.7, 9.4, and 11.3 N (maximum Hertzian stress, 1.2, 1.5 and 1.6 GPa). The average sliding speed was 16 mmys (60 cycleymin) and the friction cycle was 7200 cycles. The wear volume of these Ar-DLC films was calculated by measuring wear scars with an atomic force microscope (AFM) and the mating ball wear was obtained by measurement of the wear scar diameter on the ball. The wear scar of the Ar-DLC films and mating balls was observed using optical microscopy. Transferred materials on the wear scar of the mating ball and the wear tracks of DLC films were determined using microlaser Raman spectroscopy and X-ray photoelectron spectroscopy.
The friction behavior of three Ar-DLC films is shown in Fig. 2. In water, the friction coefficient generally decreased at an extremely early stage and then increased gradually, approaching a steady-state value. However, the friction in air started at a significantly high value and then converged to a steady state value after a gradual decrease. The minimum friction coefficient in water was clearly smaller than that in air with no exceptions. However, the difference in the value of the steady friction coefficient between water and air environments was unclear, regardless of the type of film. The friction fluctuation in water was smaller than in air for all films tested. This clear difference in steady friction was not found among the three types of films when in both water and air environments. The friction coefficient of Ar-DLC2 and Ar-DLC3 tended to decrease when increasing the load, although the Ar-DLC1 film was independent of the load. Fig. 3 is the specific wear rate of Ar-DLC films and mating balls. The specific wear rate of every Ar-DLC film when in water was clearly smaller than that in air, without exception. There was no clear difference in wear rates among these three films. In a water environment, Ar-DLC1 and Ar-DLC3 films showed a very small value of approximately 8=10y9 mm3 yNm at 9.4 N. The wear Table 1 Various properties of three Ar-DLC films
3. Results and discussion 3.1. Characterization of Ar-DLC films The Raman spectra of Ar-DLC films deposited using different bias voltages are shown in Fig. 1. Every film
Ar-DLC1 Ar-DLC2 Ar-DLC3
Hardness (GPa)
Hydrogen content (at.%)
Roughness Ra (nm)
38 32 27
28.6 21.4 20.3
2.0 2.0 1.5
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M. Suzuki et al. / Diamond and Related Materials 13 (2004) 1464–1468
Fig. 3. Specific wear rates of three Ar-DLC films in water and air. Fig. 2. Friction behaviors of three types of Ar-DLC films in water and air.
rate of every film tended to decrease when increasing the normal load in water, although this tendency was not seen with Ar-DLC3 in air. The mating ball wear with Ar-DLC2 and Ar-DLC3 was larger in water than that in air, while with Ar-DLC1, the dependency of the wear rate on the environment was reversed. In general, the ball wear was smaller than the film wear, irrespective of the film type. There are no clear differences in ball wear between Ar-DLC2 and Ar-DLC3, but the ball wear when slid against Ar-DLC1 was somewhat smaller than that with other films. The dependency of ball wear rate on the normal load was different with different film types.
to that in air. The wear scar features of Ar-DLC3 were similar to that of Ar-DLC2. Fig. 5 are the wear scars on mating balls observed by optical microscopy. The wear scar on the ball was
3.3. Wear scars The wear scar profiles of Ar-DLC films, measured by an AFM, are shown in Fig. 4. The wear depth of ArDLC2 in water was clearly shallower than that in air, while the wear scar width in water seemed to be slightly larger than that in air. However, the wear depth and the wear scar width of Ar-DLC1 in water were comparable
Fig. 4. Wear scar profiles on Ar-DLC2 films at 9.4 N.
M. Suzuki et al. / Diamond and Related Materials 13 (2004) 1464–1468
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Fig. 5. Wear scars of mating balls sliding against Ar-DLC2 at 9.4 N.
clearly covered by transferred materials, irrespective of the environment. The amount of transferred materials in water, however, was greater than that in air. The same tendency was observed on the mating ball when slid against other films. The wear scar on the mating ball was analyzed by micro-laser Raman spectroscopy and X-ray photoelectron spectroscopy. The Raman spectrum showed that there was a clear qualitative difference in the transferred materials between water and air environments (Fig. 6). In air, some graphitization occurred in transferred materials, while no clear change from the virgin Ar-DLC film was recognized in water. The analytical results by XPS are summarized in Table 2. This result suggests that the wear scars, covered with transferred materials, are largely composed of carbon and oxygen. The carbon content of these wear scars is a little greater when under a water environment than that in air. This detailed analysis, unfortunately, could not separate the qualitative
differences between two friction surfaces under both water and air, although it showed the existence of compounds on both wear scares; including Fe2O3, Fe3O4, FeOOH, Cr(OH)3, Cr2O3, etc. 3.4. Discussion Three types of Ar incorporated DLC films, deposited by changing the bias voltage from y0.5 to y3 kV, showed a moderate difference in characteristics such as H content, hardness, and the ratio of D over G peak area in the Raman spectra. No clear difference in friction coefficient, however, appeared among three films. Film wear also did not appear to be clearly dependant on the film type. These results suggest that the friction and wear in both water and air were not sensitive to film structure and composition, at least in this study. The friction behavior in water was different from that in air, although the differences in the steady state friction were not as clear. The wear rate of every film in water was always lower than that in air, irrespective of the normal load used. However, the mating ball surfaces in water were covered with a greater amount of transferred materials than those in air. The transferred material produced under a water environment differed qualitatively from that produced in air, based on Raman analysis. In any case, the difference in the transfer phenomenon between the mating balls must correlate with the difference in friction and wear behavior between water and air environments, although the details remain unknown. Furthermore, the features identified in Table 2 Results of XPS analysis of wear scars on mating balls sliding against Ar-DLC2 film at 9.4 N
Fig. 6. Raman spectra of wear scars on mating ball sliding against Ar-DLC2 at 11.3 N.
Ar-DLC2 in air Ar-DLC2 in water Ar-DLC2* *
C
O
Ar
N
Si
Cr
Fe
Co
39 48 91
41 30 8.6
– – -0.5
1 -1 –
6 -0.5 –
5 5 –
5 5 –
-1 -1 –
The atomic ratio of Ar-DLC film before tribological tests.
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M. Suzuki et al. / Diamond and Related Materials 13 (2004) 1464–1468
the wear scars on Ar-DLC films in water differed from that in air, although the differences with Ar-DLC1 were unclear. These differences in the wear scar features seem to be correlated with differences in the wear rate of films between water and air environments. In water, all types of Ar-DLC films had a low friction coefficient of approximately 0.1 and a low wear rate of less than 2=10y8 mm3 yNm. Moreover, the mating ball wear exhibited a low level of 10y9 mm3 yNm. This friction and wear behaviors suggest that our Ar-DLC films are good candidates for use in rubbing machine parts with water hydraulic systems. 4. Conclusions Three types of Ar incorporated DLC films were deposited using a thermal electron excited plasma CVD method. Their friction and wear properties were evaluated under water and air environments using a ball-onplate type friction tester. The main results are as follows: 1. The anti-wear property of Ar-DLC films in water was superior to that in air; the smallest wear rate in water was approximately 8=10y9 mm3 yNm. 2. The reduction of friction coefficient in water was not clear, compared to that in air. The lowest friction coefficient in water was approximately 0.1. 3. The transferred materials were observed on the wear scar of the mating ball in every experiment, but the transferred materials in water were richer than those
in air. Furthermore, there seemed to be the qualitative difference in transferred materials between water and air environments. The difference in such transfer phenomena seemed to affect both friction coefficient and wear rate. Acknowledgments The authors express their gratitude to Mr Tomoki Nishikawa for his assistance in this experiment. 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, entitled ‘Surface-modification technologies for friction reduction in various machinery’. References w1x D. Neerinck, P. Persoone, M. Sercu, A. Goel, D. Kester, D. Bray, Diamond Relat. Mater. 7 (1998) 468. w2x H. Ronkainen, S. Varjus, K. Holmberg, Wear 222 (1998) 120. w3x H. Ronkainen, S. Varjus, K. Holmberg, Wear 249 (2001) 267–271. w4x D. Sheejs, B.K. Tay, S.P. Lau, L.N. Numg, Surf. Coat. Technol. 146–147 (2001) 410–416. w5x J. Jiang, S. Zhang, R.D. Arnell, Surf. Coat. Technol. 167 (2003) 221–225. w6x W. Zhang, A. Tanaka, K. Wazumi, Y. Koga, Thin Solid Films 416 (2002) 145–152.