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Friction and wear of DLC films on 304 austenitic stainless steel in corrosive solutions
Surface and Coatings Technology 174 – 175 (2003) 465–469
Friction and wear of DLC films on 304 austenitic stainless steel in corrosive solutions N. Yamauchia,*, A. Okamotoa, H. Tukaharaa, K. Demizua, N. Uedaa, T. Sonea, Y. Hiroseb a Technology Research Institute of Osaka Prefecture, 7-1, Ayumino 2, Izumi, Osaka 594-1157, Japan Department of Material Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
b
Abstract Diamond-like carbon (DLC) films are well known for their high hardness, low friction and excellent wear resistance. In the present work, the friction coefficient and wear resistance of DLC films deposited on 304 austenitic stainless steel were investigated in corrosive solutions. DLC films were deposited on the substrate by the CVD method using radio frequency (13.56 MHz) plasma. A mixture of methane (CH4) and hydrogen (H2) was used as processing gas. The CH4 concentration was varied from 20 to 100%. DLC films were subjected to friction and wear tests using a ball-on-flat surface friction apparatus in solutions of 3 mass% NaCl, 0.05 N HCl, 0.05 N H2 SO4 and 0.05 N HNO3 . The flat surface was oscillated against the alumina ball. In the tests performed within the corrosive solution, the friction coefficients of the DLC films and substrate were approximately 0.1 and 0.5, respectively. The DLC films deposited on the 304 substrate showed a remarkably lower volume of wear than the substrate alone. Accordingly, the DLC coating process was confirmed to be effective in decreasing the friction coefficient and improving the corrosion wear resistance of 304 in 3 mass% NaCl and acidic solutions. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Friction; Wear; DLC; Corrosion; Stainless steel; Plasma CVD
1. Introduction In recent years, diamond-like carbon (DLC) films have been widely studied for their excellent mechanical and tribological properties. Besides their high hardness, smooth surfaces and chemical inertness, DLC films can provide the excellent property of a low friction coefficient at various conditions. Many workers have investigated the wear characteristics of DLC films in dry conditions w1–6x. While these studies have been of great utility, they do not apply to the materials used in various corrosive environments. As DLC is chemically inert, materials coated with DLC are likely to show excellent corrosion resistance. Thus, it is important to investigate the wear characteristics of DLC films in corrosive environment. In this study, DLC films were deposited on 304 austenitic stainless steel by the plasma CVD method using radio frequency (RF). After the film characteri*Corresponding author. Tel.: q81-725512716; fax: q81725512749. E-mail address: [email protected] (N. Yamauchi).
zation of Raman spectrometry and glow discharge spectrometry, the friction and wear properties of the films were investigated in solutions of 3 mass% sodium chloride (NaCl), 0.05 normal hydrochloric acid (HCl), nitric acid (HNO3) and sulfuric acid (H2SO4). 2. Experimental procedure 2.1. Film preparation The substrate material was a commercial grade AISI 304 austenitic stainless steel. Each substrate was shaped as a disk and measured 25=2 mm in diameter and thickness, respectively. The substrate surface was finished with 1200 grit emery paper, then by cleaning them with ethanol and acetone. After depositing a silicon interlayer of 0.3 mm in thickness by the ion beam sputter method, the DLC films were deposited the substrates at a temperature below 473 K by the RF plasma CVD method w6x. The supplied power and frequency were 300 W and 13.56 MHz, respectively. A mixture of methane (CH4) and hydrogen (H2) gas was used as processing gas, and gas
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00406-7
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N. Yamauchi et al. / Surface and Coatings Technology 174 – 175 (2003) 465–469
2.3.2. Friction and wear tests in acidic solutions Films coated at a 100% CH4 concentration were prepared for the friction and wear tests in solutions of 0.05 N HCl, 0.05 N HNO3, 0.05 N H2SO4 and distilled water. The friction and wear test in acidic solutions were carried out as the same conditions of test in solution of a 3 mass% NaCl (see Section 2.3.1). 3. Results and discussion 3.1. Film property and structure Fig. 1. The schematic diagram of friction and wear apparatus in corrosive solutions.
pressure was adjusted to 26.6 Pa. Negative self-bias was from y450 to y500 V. Deposition rate was 2.0 mmyh at 60% CH4 (see Section 3.1.2). 2.2. Film characterization DLC films were deposited at 20, 40, 60, 80 and 100% CH4 concentrations for film characterization. Structures of DLC films were characterized by Raman spectrometry and the hydrogen content of them were evaluated by glow discharge spectrometry (GDS). A Knoop hardness tester at a load of 0.49 N was also used to measure the surface hardness. 2.3. Friction and wear tests in corrosive solutions The thicknesses of DLC films are thought to influence the results of friction and wear tests. However, as later described, the thicknesses of the films in this experiments increased as the CH4 concentrations increased in the gas mixture (see Section 3.1.2). Thus, for the films used for the friction and wear tests, the deposition time was also varied to obtain a constant thickness of approximately 2.2 mm. 2.3.1. Friction and wear tests in 3 mass% NaCl solution The substrates coated at 20, 60 and 100% CH4 concentration were prepared for the friction and wear tests in a 3 mass% NaCl solution. Fig. 1 shows the schematic diagram of a friction and wear tests in corrosive solutions w7x. The tests were carried out using a ball-on-flat type surface friction apparatus in a 3 mass% NaCl solution at room temperature. The flat substrate was oscillated against the ball (alumina counterface, 4.8 mm diameter, 1800 HV). Alumina was chosen as the material for the ball because of its chemically inertness under various corrosive environments. A load of 3.92 N was applied by a dead weight. The sliding speed, sliding length and sliding time were 20 mm sy1, 5 mm and 14.4 ks (4 h), respectively.
3.1.1. Raman spectrometry Fig. 2 shows the Raman spectra of films prepared at various CH4 concentrations. Generally, Raman spectra of DLC films have been evaluated by the G-band at approximately 1540–1580 cmy1 and D-band at approximately 1350 cmy1. The G-band represents graphite and the D-band represents a disordered graphite-like structure (not diamond) w8x. As can be seen in Fig. 2, the films deposited at all CH4 concentrations have typical shapes of DLC spectra. The G-band was broad in shape and located at more or less the same position in all of the films. However, in the film deposited at the 20% CH4 concentration, the G-band shifted toward a higher frequency and the Dband appeared at approximately 1370 cmy1. This suggested that the 20% CH4 film had more of a graphite-like structure than other films. 3.1.2. Film thickness and hardness Fig. 3 shows the relationship between the CH4 concentration in gas mixture and DLC film thickness. The deposition time was 3 h. The thickness of DLC films was measured using a surface roughness tester at the boundary between the deposited and undeposited sec-
Fig. 2. Raman spectra of DLC films.
N. Yamauchi et al. / Surface and Coatings Technology 174 – 175 (2003) 465–469
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Fig. 5. The relationship between CH4 concentration and HyC relative intensity by GDS. Fig. 3. The relationship between CH4 concentration and DLC film thickness (deposition time; 3 h).
tions of the substrate. As can be seen in Fig. 3, the thickness increased as the CH4 concentration increased in the gas mixture. Fig. 4 shows Knoop hardness for films prepared at various CH4 concentrations. The hardness was measured directly from the surface. The maximum hardness, approximately 2600 HK, was observed in the 100% CH4 film, whereas the 20% CH4 film had a hardness of approximately 500 HK. The Raman spectra of DLC deposited at 40, 60, 80 and 100% CH4 were similar in shape, but the surface hardnesses were not the same. The film thickness was thought to influence the Knoop hardness, as the hardness was measured from the surface. However, the same tendency was also observed in the films deposited at a constant thickness. These findings suggested that the difference of hardness was dependent on the hydrogen content in the films (see Section 3.1.3). More detailed studies are required to identify the relationship between DLC film structure and hardness.
Fig. 4. Knoop hardness of DLC films.
3.1.3. GDS analysis The hydrogen content in a DLC film is generally known to influence the film property of hardness w9x. Fig. 5 shows the relationship between CH4 concentration in gas mixture and hydrogenycarbon (HyC) relative intensity of DLC films determined by GDS. Rather than directly reflecting the hydrogen content of a DLC film, the HyC relative intensity shows the change of the hydrogen content, indirectly. The HyC relative intensity of DLC films decreased as CH4 concentration increased in the gas mixture. The HyC relative intensity of the film deposited at 100% CH4 reached 0.37, a somewhat high value, as a result of the H atoms in the CH4 molecule. 3.2. Friction and wear test in corrosive solutions 3.2.1. Friction and wear test in 3 mass% NaCl solution Fig. 6 shows the relationship between the testing time and the friction coefficients of various substrates in 3 mass% NaCl solution. The friction coefficients of DLC films were much lower than that of the 304 substrate (approx. 0.07 vs. approx. 0.5). The behavior of friction coefficient of DLC at the 60% CH4 concentration was almost the same as that of DLC at 100% CH4, in spite
Fig. 6. Friction coefficient of DLC in 3 mass% NaCl solution.
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N. Yamauchi et al. / Surface and Coatings Technology 174 – 175 (2003) 465–469
Fig. 9. Comparison of cross-sectional area of wear tracks in 3 mass% NaCl solution. Fig. 7. Surface profiles of wear tracks in 3 mass% NaCl solution.
of the differences of the surface hardness and hydrogen content. The friction coefficient of the 20% CH4 film was higher than that of the other DLC films at the beginning of the test, and after 5000 s it became unstable and the film began to peel. Fig. 7 shows the surface profiles of wear tracks of DLC films deposited at all CH4 concentrations. Fig. 8 show surface images of the tracks of a DLC film deposited at 20% CH4 viewed through an optical microscope. The wear tracks of the 304 substrate and Si interlayer substrate were deep and broad, while the tracks of the 60 and 100% CH4 films were unclear and narrow. As can be seen in Fig. 8, the 20% CH4 film was partially peeled. The track of the peeled part of the DLC deposited at 20% CH4 was similar to that of the
Fig. 8. Surface images of wear tracks: DLC (CH4 20%).
304 substrate. However, the unpeeled part of track was as thin and narrow as the tracks of the other DLC films. Cross-sectional area of wear was calculated from the surface profile of wear track. Fig. 9 shows a comparison of cross-sectional area of wear of all films. The crosssectional area of the peeled part of the 20% CH4 film was similar to that of the 304 substrate, whereas the cross-sectional area of the unpeeled part was higher than that of the other DLC films. The 60 and 100% CH4 films showed excellent wear resistance in a 3 mass% NaCl solution, whereas the 20% CH4 film had a somewhat higher friction coefficient and cross-sectional area. This suggested that the film structure measured by Raman analysis was an
Fig. 10. Friction coefficients of DLC and 304 in three types of acidic solutions and distilled water.
N. Yamauchi et al. / Surface and Coatings Technology 174 – 175 (2003) 465–469
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sectional area of wear track. These effects were probably caused by the corrosion products on the 304 substrate. The variation of the friction coefficient and crosssectional area of wear track of a DLC film were not similar to those of the 304 substrate. The DLC film was shown to resist these acidic solutions. 4. Conclusions
Fig. 11. Surface profiles of wear tracks in acidic solutions.
important factor in the friction and wear characteristics in the 3 mass% NaCl solution. 3.2.2. Friction and wear tests in acidic solutions As the 100% CH4 DLC film showed excellent wear resistance in the 3 mass% NaCl solution, the friction and wear tests in acidic solutions were carried out only on the 100% CH4 DLC film. Fig. 10 shows the friction coefficients of a 304 substrate and DLC film deposited at 100% CH4 in three types of acidic solutions and distilled water, respectively. The corrosion products in the acidic solutions seemed to decrease the friction coefficients than the 304 substrate in distilled water. On the other hand, the friction coefficient of the DLC film was not higher in distilled water than in the acidic solutions. Fig. 11 shows the surface profiles of wear tracks in acidic solutions. The cross-sectional area of wear track on the 304 substrate was much larger than that on a DLC film. The friction coefficients of the 304 substrate in the acidic solutions increased in the following order: H2SO4-HCl-HNO3-distilled water. The cross-sectional area of wear track of a 304 substrate in acidic solutions increased in the following order: distilled water-H2SO4-HCl-HNO3 . Thus, the acidic solutions were shown to decrease the friction coefficient of the 304 substrate, yet simultaneously increase the cross-
A DLC film was deposited on a 304 substrate with an Si interlayer using the RF plasma CVD method. The CH4 concentration in the processing gas (CH4qH2) was varied from 20 to 100%. All of the films produced in this study showed Raman spectra typical for DLC. The G-band of the Raman spectrum of the film deposited at 20% CH4 was shifted toward a higher frequency. The film hardnesses increased as the hydrogen content in the DLC films decreased. The 60 and 100% CH4 DLC films had low friction coefficients and high wear resistance in the 3 mass% NaCl solution. The DLC film of the 20% CH4 film was partially peeled at this condition. The DLC film deposited at 100% CH4 on a 304 substrate had low friction coefficients and high wear resistance in 0.05 N HCl, 0.05 N H2SO4 and 0.05 N HNO3 solutions. The DLC coating process was effective in the decreasing the friction coefficient and improving the corrosion wear resistance of 304 in 3 mass% NaCl and acidic solutions. Acknowledgments The authors would like to thank Mr Y. Miki and Mr T. Taniguchi of Nara Prefectural Institute of Industrial Technology for supporting this study. References w1x E. Liu, B. Blanpain, X. Shi, J.-P. Celis, H.-S. Tan, B.-K. Tay, L.-K. Cheah, J.R. Roos, Surf. Coat. Technol. 106 (1998) 72–80. w2x A. Erdemir, Surf. Coat. Technol. 146–147 (2001) 292–297. w3x J. Jiang, R.D.A.rnell Jin Ton, Tribol. Int. 30 (1997) 613–625. w4x K. Miyoshi, R.L.C. Wu, A. Garscadden, Surf. Coat. Technol. 54y55 (1992) 428–434. w5x S. Miyake, Surf. Coat. Technol. 54y55 (1992) 563–569. w6x K. Demidzu, T. Sone, K. Natsukawa, Y. Fujishima, J. Soc. Mater. Sci. Jpn. 42 (1993) 997–1003, in Japanese. w7x N. Yamauchi, N. Ueda, K. Demizu, A. Okamoto, T. Sone, K. Oku, T. Konda, K. Ichii, K. Akamastu, in: T. Bell, K. Akamatsu (Eds.), Stainless Steel 2000, Maney Publishing, London, 2001, pp. 247–261. w8x Y. Taki, O. Takai, Thin Solid Films 316 (1998) 45–50. w9x L. Martinu, A. Raveh, A. Domingue, L. Bertrand, J.E. Klemberg-Sapieha, S.C. Gujrathi, M.R. Wertheimer, Thin Solid Films 208 (1992) 42–47.
Friction and wear of DLC films on 304 austenitic stainless steel in corrosive solutions N. Yamauchia,*, A. Okamotoa, H. Tukaharaa, K. Demizua, N. Uedaa, T. Sonea, Y. Hiroseb a Technology Research Institute of Osaka Prefecture, 7-1, Ayumino 2, Izumi, Osaka 594-1157, Japan Department of Material Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
b
Abstract Diamond-like carbon (DLC) films are well known for their high hardness, low friction and excellent wear resistance. In the present work, the friction coefficient and wear resistance of DLC films deposited on 304 austenitic stainless steel were investigated in corrosive solutions. DLC films were deposited on the substrate by the CVD method using radio frequency (13.56 MHz) plasma. A mixture of methane (CH4) and hydrogen (H2) was used as processing gas. The CH4 concentration was varied from 20 to 100%. DLC films were subjected to friction and wear tests using a ball-on-flat surface friction apparatus in solutions of 3 mass% NaCl, 0.05 N HCl, 0.05 N H2 SO4 and 0.05 N HNO3 . The flat surface was oscillated against the alumina ball. In the tests performed within the corrosive solution, the friction coefficients of the DLC films and substrate were approximately 0.1 and 0.5, respectively. The DLC films deposited on the 304 substrate showed a remarkably lower volume of wear than the substrate alone. Accordingly, the DLC coating process was confirmed to be effective in decreasing the friction coefficient and improving the corrosion wear resistance of 304 in 3 mass% NaCl and acidic solutions. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Friction; Wear; DLC; Corrosion; Stainless steel; Plasma CVD
1. Introduction In recent years, diamond-like carbon (DLC) films have been widely studied for their excellent mechanical and tribological properties. Besides their high hardness, smooth surfaces and chemical inertness, DLC films can provide the excellent property of a low friction coefficient at various conditions. Many workers have investigated the wear characteristics of DLC films in dry conditions w1–6x. While these studies have been of great utility, they do not apply to the materials used in various corrosive environments. As DLC is chemically inert, materials coated with DLC are likely to show excellent corrosion resistance. Thus, it is important to investigate the wear characteristics of DLC films in corrosive environment. In this study, DLC films were deposited on 304 austenitic stainless steel by the plasma CVD method using radio frequency (RF). After the film characteri*Corresponding author. Tel.: q81-725512716; fax: q81725512749. E-mail address: [email protected] (N. Yamauchi).
zation of Raman spectrometry and glow discharge spectrometry, the friction and wear properties of the films were investigated in solutions of 3 mass% sodium chloride (NaCl), 0.05 normal hydrochloric acid (HCl), nitric acid (HNO3) and sulfuric acid (H2SO4). 2. Experimental procedure 2.1. Film preparation The substrate material was a commercial grade AISI 304 austenitic stainless steel. Each substrate was shaped as a disk and measured 25=2 mm in diameter and thickness, respectively. The substrate surface was finished with 1200 grit emery paper, then by cleaning them with ethanol and acetone. After depositing a silicon interlayer of 0.3 mm in thickness by the ion beam sputter method, the DLC films were deposited the substrates at a temperature below 473 K by the RF plasma CVD method w6x. The supplied power and frequency were 300 W and 13.56 MHz, respectively. A mixture of methane (CH4) and hydrogen (H2) gas was used as processing gas, and gas
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00406-7
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N. Yamauchi et al. / Surface and Coatings Technology 174 – 175 (2003) 465–469
2.3.2. Friction and wear tests in acidic solutions Films coated at a 100% CH4 concentration were prepared for the friction and wear tests in solutions of 0.05 N HCl, 0.05 N HNO3, 0.05 N H2SO4 and distilled water. The friction and wear test in acidic solutions were carried out as the same conditions of test in solution of a 3 mass% NaCl (see Section 2.3.1). 3. Results and discussion 3.1. Film property and structure Fig. 1. The schematic diagram of friction and wear apparatus in corrosive solutions.
pressure was adjusted to 26.6 Pa. Negative self-bias was from y450 to y500 V. Deposition rate was 2.0 mmyh at 60% CH4 (see Section 3.1.2). 2.2. Film characterization DLC films were deposited at 20, 40, 60, 80 and 100% CH4 concentrations for film characterization. Structures of DLC films were characterized by Raman spectrometry and the hydrogen content of them were evaluated by glow discharge spectrometry (GDS). A Knoop hardness tester at a load of 0.49 N was also used to measure the surface hardness. 2.3. Friction and wear tests in corrosive solutions The thicknesses of DLC films are thought to influence the results of friction and wear tests. However, as later described, the thicknesses of the films in this experiments increased as the CH4 concentrations increased in the gas mixture (see Section 3.1.2). Thus, for the films used for the friction and wear tests, the deposition time was also varied to obtain a constant thickness of approximately 2.2 mm. 2.3.1. Friction and wear tests in 3 mass% NaCl solution The substrates coated at 20, 60 and 100% CH4 concentration were prepared for the friction and wear tests in a 3 mass% NaCl solution. Fig. 1 shows the schematic diagram of a friction and wear tests in corrosive solutions w7x. The tests were carried out using a ball-on-flat type surface friction apparatus in a 3 mass% NaCl solution at room temperature. The flat substrate was oscillated against the ball (alumina counterface, 4.8 mm diameter, 1800 HV). Alumina was chosen as the material for the ball because of its chemically inertness under various corrosive environments. A load of 3.92 N was applied by a dead weight. The sliding speed, sliding length and sliding time were 20 mm sy1, 5 mm and 14.4 ks (4 h), respectively.
3.1.1. Raman spectrometry Fig. 2 shows the Raman spectra of films prepared at various CH4 concentrations. Generally, Raman spectra of DLC films have been evaluated by the G-band at approximately 1540–1580 cmy1 and D-band at approximately 1350 cmy1. The G-band represents graphite and the D-band represents a disordered graphite-like structure (not diamond) w8x. As can be seen in Fig. 2, the films deposited at all CH4 concentrations have typical shapes of DLC spectra. The G-band was broad in shape and located at more or less the same position in all of the films. However, in the film deposited at the 20% CH4 concentration, the G-band shifted toward a higher frequency and the Dband appeared at approximately 1370 cmy1. This suggested that the 20% CH4 film had more of a graphite-like structure than other films. 3.1.2. Film thickness and hardness Fig. 3 shows the relationship between the CH4 concentration in gas mixture and DLC film thickness. The deposition time was 3 h. The thickness of DLC films was measured using a surface roughness tester at the boundary between the deposited and undeposited sec-
Fig. 2. Raman spectra of DLC films.
N. Yamauchi et al. / Surface and Coatings Technology 174 – 175 (2003) 465–469
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Fig. 5. The relationship between CH4 concentration and HyC relative intensity by GDS. Fig. 3. The relationship between CH4 concentration and DLC film thickness (deposition time; 3 h).
tions of the substrate. As can be seen in Fig. 3, the thickness increased as the CH4 concentration increased in the gas mixture. Fig. 4 shows Knoop hardness for films prepared at various CH4 concentrations. The hardness was measured directly from the surface. The maximum hardness, approximately 2600 HK, was observed in the 100% CH4 film, whereas the 20% CH4 film had a hardness of approximately 500 HK. The Raman spectra of DLC deposited at 40, 60, 80 and 100% CH4 were similar in shape, but the surface hardnesses were not the same. The film thickness was thought to influence the Knoop hardness, as the hardness was measured from the surface. However, the same tendency was also observed in the films deposited at a constant thickness. These findings suggested that the difference of hardness was dependent on the hydrogen content in the films (see Section 3.1.3). More detailed studies are required to identify the relationship between DLC film structure and hardness.
Fig. 4. Knoop hardness of DLC films.
3.1.3. GDS analysis The hydrogen content in a DLC film is generally known to influence the film property of hardness w9x. Fig. 5 shows the relationship between CH4 concentration in gas mixture and hydrogenycarbon (HyC) relative intensity of DLC films determined by GDS. Rather than directly reflecting the hydrogen content of a DLC film, the HyC relative intensity shows the change of the hydrogen content, indirectly. The HyC relative intensity of DLC films decreased as CH4 concentration increased in the gas mixture. The HyC relative intensity of the film deposited at 100% CH4 reached 0.37, a somewhat high value, as a result of the H atoms in the CH4 molecule. 3.2. Friction and wear test in corrosive solutions 3.2.1. Friction and wear test in 3 mass% NaCl solution Fig. 6 shows the relationship between the testing time and the friction coefficients of various substrates in 3 mass% NaCl solution. The friction coefficients of DLC films were much lower than that of the 304 substrate (approx. 0.07 vs. approx. 0.5). The behavior of friction coefficient of DLC at the 60% CH4 concentration was almost the same as that of DLC at 100% CH4, in spite
Fig. 6. Friction coefficient of DLC in 3 mass% NaCl solution.
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N. Yamauchi et al. / Surface and Coatings Technology 174 – 175 (2003) 465–469
Fig. 9. Comparison of cross-sectional area of wear tracks in 3 mass% NaCl solution. Fig. 7. Surface profiles of wear tracks in 3 mass% NaCl solution.
of the differences of the surface hardness and hydrogen content. The friction coefficient of the 20% CH4 film was higher than that of the other DLC films at the beginning of the test, and after 5000 s it became unstable and the film began to peel. Fig. 7 shows the surface profiles of wear tracks of DLC films deposited at all CH4 concentrations. Fig. 8 show surface images of the tracks of a DLC film deposited at 20% CH4 viewed through an optical microscope. The wear tracks of the 304 substrate and Si interlayer substrate were deep and broad, while the tracks of the 60 and 100% CH4 films were unclear and narrow. As can be seen in Fig. 8, the 20% CH4 film was partially peeled. The track of the peeled part of the DLC deposited at 20% CH4 was similar to that of the
Fig. 8. Surface images of wear tracks: DLC (CH4 20%).
304 substrate. However, the unpeeled part of track was as thin and narrow as the tracks of the other DLC films. Cross-sectional area of wear was calculated from the surface profile of wear track. Fig. 9 shows a comparison of cross-sectional area of wear of all films. The crosssectional area of the peeled part of the 20% CH4 film was similar to that of the 304 substrate, whereas the cross-sectional area of the unpeeled part was higher than that of the other DLC films. The 60 and 100% CH4 films showed excellent wear resistance in a 3 mass% NaCl solution, whereas the 20% CH4 film had a somewhat higher friction coefficient and cross-sectional area. This suggested that the film structure measured by Raman analysis was an
Fig. 10. Friction coefficients of DLC and 304 in three types of acidic solutions and distilled water.
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sectional area of wear track. These effects were probably caused by the corrosion products on the 304 substrate. The variation of the friction coefficient and crosssectional area of wear track of a DLC film were not similar to those of the 304 substrate. The DLC film was shown to resist these acidic solutions. 4. Conclusions
Fig. 11. Surface profiles of wear tracks in acidic solutions.
important factor in the friction and wear characteristics in the 3 mass% NaCl solution. 3.2.2. Friction and wear tests in acidic solutions As the 100% CH4 DLC film showed excellent wear resistance in the 3 mass% NaCl solution, the friction and wear tests in acidic solutions were carried out only on the 100% CH4 DLC film. Fig. 10 shows the friction coefficients of a 304 substrate and DLC film deposited at 100% CH4 in three types of acidic solutions and distilled water, respectively. The corrosion products in the acidic solutions seemed to decrease the friction coefficients than the 304 substrate in distilled water. On the other hand, the friction coefficient of the DLC film was not higher in distilled water than in the acidic solutions. Fig. 11 shows the surface profiles of wear tracks in acidic solutions. The cross-sectional area of wear track on the 304 substrate was much larger than that on a DLC film. The friction coefficients of the 304 substrate in the acidic solutions increased in the following order: H2SO4-HCl-HNO3-distilled water. The cross-sectional area of wear track of a 304 substrate in acidic solutions increased in the following order: distilled water-H2SO4-HCl-HNO3 . Thus, the acidic solutions were shown to decrease the friction coefficient of the 304 substrate, yet simultaneously increase the cross-
A DLC film was deposited on a 304 substrate with an Si interlayer using the RF plasma CVD method. The CH4 concentration in the processing gas (CH4qH2) was varied from 20 to 100%. All of the films produced in this study showed Raman spectra typical for DLC. The G-band of the Raman spectrum of the film deposited at 20% CH4 was shifted toward a higher frequency. The film hardnesses increased as the hydrogen content in the DLC films decreased. The 60 and 100% CH4 DLC films had low friction coefficients and high wear resistance in the 3 mass% NaCl solution. The DLC film of the 20% CH4 film was partially peeled at this condition. The DLC film deposited at 100% CH4 on a 304 substrate had low friction coefficients and high wear resistance in 0.05 N HCl, 0.05 N H2SO4 and 0.05 N HNO3 solutions. The DLC coating process was effective in the decreasing the friction coefficient and improving the corrosion wear resistance of 304 in 3 mass% NaCl and acidic solutions. Acknowledgments The authors would like to thank Mr Y. Miki and Mr T. Taniguchi of Nara Prefectural Institute of Industrial Technology for supporting this study. References w1x E. Liu, B. Blanpain, X. Shi, J.-P. Celis, H.-S. Tan, B.-K. Tay, L.-K. Cheah, J.R. Roos, Surf. Coat. Technol. 106 (1998) 72–80. w2x A. Erdemir, Surf. Coat. Technol. 146–147 (2001) 292–297. w3x J. Jiang, R.D.A.rnell Jin Ton, Tribol. Int. 30 (1997) 613–625. w4x K. Miyoshi, R.L.C. Wu, A. Garscadden, Surf. Coat. Technol. 54y55 (1992) 428–434. w5x S. Miyake, Surf. Coat. Technol. 54y55 (1992) 563–569. w6x K. Demidzu, T. Sone, K. Natsukawa, Y. Fujishima, J. Soc. Mater. Sci. Jpn. 42 (1993) 997–1003, in Japanese. w7x N. Yamauchi, N. Ueda, K. Demizu, A. Okamoto, T. Sone, K. Oku, T. Konda, K. Ichii, K. Akamastu, in: T. Bell, K. Akamatsu (Eds.), Stainless Steel 2000, Maney Publishing, London, 2001, pp. 247–261. w8x Y. Taki, O. Takai, Thin Solid Films 316 (1998) 45–50. w9x L. Martinu, A. Raveh, A. Domingue, L. Bertrand, J.E. Klemberg-Sapieha, S.C. Gujrathi, M.R. Wertheimer, Thin Solid Films 208 (1992) 42–47.