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Effects of water environment on tribological properties of DLC rubbed against stainless steel
Wear 263 (2007) 1335–1340
Effects of water environment on tribological properties of DLC rubbed against stainless steel M. Uchidate a,∗ , H. Liu a , A. Iwabuchi a , K. Yamamoto b a
Department of Mechanical Engineering, Faculty of Engineering, Iwate University, 3-5, Ueda 4-chome, Morioka-shi, Iwate 020-8551, Japan b Materials Research Laboratory, Kobe Steel Ltd., 5-5, Takatsukadai 1-chome, Nishi-ku, Kobe 651-2271, Japan Received 13 August 2006; received in revised form 4 October 2006; accepted 12 October 2006 Available online 23 May 2007
Abstract The effects of water environment on tribological properties of DLC against stainless steel were studied for development of the hydraulic pumps, valves and cylinders based on metal. A ball-on-disk tribotester was developed for detailed examination of water environment, such as temperature (20–80 ◦ C), dissolved oxygen (0.01–8 ppm), water pressure (0.1–20 MPa) and dissolved ions. DLC was deposited on the disk by the unbalanced magnetron spattering system. Pure water and quasi-tap water were used to study the effects of dissolved ions. The results showed that temperature and dissolved ions have a major impact on friction and wear of DLC. EPMA, XPS and AES indicated that tribofilm on the metal surface, which consists mainly of C, O and Fe, plays important role for the phenomena. © 2007 Elsevier B.V. All rights reserved. Keywords: Water lubrication; DLC coatings; Friction; Wear; Elevated temperature
1. Introduction Development of water hydraulic systems using metal based materials has been carried on as a national research project in Japan. In this development, combinations of DLC/stainless steel and DLC/brass are expected to achieve low friction and wear under water lubricated conditions because of the superior tribological properties in water [1–3]. Water used in the systems is to be tap water with no additives for ease of use and drainage. Hence, the properties of hydraulic fluid in the system vary depending on the region [4]. Also, the systems will be employed in various water environments in terms of temperature, dissolved oxygen and water pressure depending on use. Therefore, it is necessary to know the effects of factors mentioned above on tribological performance of DLC and counter materials. DLC are thought to be relatively inert and stable in corrosive environment. For example, Yamaguchi et al. investigated friction and wear of DLC against alumina in pure water, 3 mass% NaCl solution and acidic solutions, such as HCl, HNO3 and H2 SO4
∗
Corresponding author. Tel.: +81 196 621 6417; fax: +81 196 621 6417. E-mail address: [email protected] (M. Uchidate).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.10.024
and showed that there is no significant change in pure water and these solutions [5]. On the other hand, Ronkainen et al. showed that water can be aggressive media for DLC rubbed by alumina [6]. This study focuses on the effects of water environment on tribological performance of DLC rubbed against stainless steel. 2. Experimental details A ball-on-disk type tribotester, shown in Fig. 1, which can be operated under controlled water environment in terms of temperature (20–80 ◦ C), water pressure (0.1–20 MPa) and dissolved oxygen (0.01–8 ppm) was developed for this study. About 1 L of water is filled in the autoclave and circulated at the rate of 20 mL/min by a high pressure pump throughout the tests. The autoclave is covered with the temperature chamber with a heating–cooling bath. The amount of dissolved oxygen in water is controlled by aeration of N2 and O2 gas in a reserve tank. Frictional force was measured by the torque meter attached to the driving shaft. Electrodes connected to the potentio-stat were also placed in the chamber for electrochemical measurement as shown in Fig. 1. Turning radius of the ball specimens is 13 mm. Both the ball and disk specimens were made of AISI 630 stainless steel (Fe–17%Cr–4%Ni–4%Cu–1%Nb) hardened in
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M. Uchidate et al. / Wear 263 (2007) 1335–1340
Fig. 1. Schematic diagram of the tribotester for high temperature, high pressure environment in controlled water.
the H900 condition. The diameter of the ball specimen is 9.5 mm. Three ball specimens were used for each single test. Disk specimen is 10 mm in thickness and 50 mm in diameter. DLC and interlayer coatings were deposited using the unbalanced magnetron sputtering system. The substrate surface was sputtercleaned with Ar ion prior to deposition. Cr interlayer with about 50 nm in thickness was deposited on the stainless steel surface to improve adhesion. Then, Cr–C compositional gradient layer with approximately 200 nm in thickness was deposited which was followed by deposition of DLC layer with 1 m in thickness. Mixed gas of 10% CH4 and 90% Ar was introduced in the deposition chamber to add hydrogen in the DLC coating and its content was determined as 30 at.% using the elastic recoil detection analysis (ERDA). The total pressure during the deposition was 0.6 Pa. Bias voltage of −100 V was supplied on the substrate. The deposition temperature was less than 200 ◦ C. The surface hardness of the ball was HV 500 (4.9 GPa). The nanoindentation hardness of the DLC coating was 16 GPa. The surface roughness of both the disk and ball specimens was 0.03 m in Ra.
Fig. 2. Extracted effects of environmental factors and load on wear and friction.
M. Uchidate et al. / Wear 263 (2007) 1335–1340 Table 1 Specifications of quasi-tap water and comparison with tap water sampled in Tokyo Tap water sampled in Tokyo
Quasi-tap water
Dissolved ions (mg/L) Cl− NO3 − SO4 2− Na+ K+ Mg2+ Ca2+
30 12 36 22 2.8 5.0 22
31 13 37 24 2.5 5.3 23
Electrical conductivity (mS/m) pH
28.9 7.2–7.8
29.4 7.4–7.9
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Table 4 Experimental condition for detailed examination of temperature and dissolved ions Turning speed Load Number of revolutions Water pressure Dissolved oxygen Water temperature Water
0.4 m/s 11, 31, 58 N 36,000 (Approx.) 0.1 MPa 7–8 ppm (20, 50, 80) ± 2 ◦ C Pure water, quasi-tap water
Table 2 L9 orthogonal table for examination of environmental factors and load Exp. No.
Load
Temperature
Dissolved oxygen
Water pressure
1 2 3 4 5 6 7 8 9
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
Fig. 3. Dependence on temperature of specific wear rate of DLC.
Numbers of each factor corresponds to the level number shown in Table 3.
Quasi-tap water, which imitates tap water sampled in Tokyo (Table 1), was used to achieve stable quality of water. Pure water (deionized water) was also used for comparison. Experiment was done in two steps; brief examination of environmental factors and detailed examination of temperature. In order to examine the effects of temperature, dissolved oxygen and water pressure with a limited number of experiments, L9 orthogonal array was employed. Layout of factors and conditions are listed in Tables 2 and 3. Load was also included in factors as shown in the tables. Turning speed was 0.4 m/s and number of revolutions was about 36,000 in the L9 experiment. The L9 experiment was carried out in pure water and quasi-tap water. Effects of each factor are extracted by averaging each condition under the assumption that there is no interaction among the factors. The values of mean contact pressure at initial states vary from 0.5 GPa at 10 N to 0.9 GPa at 57 N. And then the effects of temperature and dissolved ions were studied in detail since, as shown later, it was found
Fig. 4. Frictional behavior in different environment.
Table 3 Value of the factors listed in Table 2 Level No.
Load (N)
Temperature (◦ C)
Dissolved oxygen (ppm)
Water pressure (MPa)
1 2 3
10 30 57
20 50 80
0.01 0.1 7
0.1 1 19
Fig. 5. Comparison of average coefficient friction in respective environment.
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Fig. 7. Wear scar profiles in quasi-tap water.
the wear scar with an optical microscope. EPMA, AES and XPS were employed for surface characterization. Fig. 6. SEM image of a wear track on DLC (in quasi-tap water, 57 N, 80 ◦ C).
3. Results and discussion that they have major impacts on tribo-performance by the L9 experiments. Experimental condition for detailed examination of temperature is listed in Table 4. The settings of dissolved oxygen and water pressure are the normal, atmospheric condition. Surface topography of wear track was measured by a stylus profilometer and wear volume was calculated as the volume loss of the wear scar from the mean line of the original surface. Wear volume of the ball specimen was estimated from the width of
3.1. Brief examination of environmental factors Extracted results from L9 experiments are shown in Fig. 2. It is clear from Fig. 2(a) that specific wear rate of DLC increases drastically with an increase in temperature especially in quasitap water. Specific wear rate of steel balls shown in Fig. 2(b) indicates that wear of steel balls is insensitive to the environment. Coefficient of friction shown in Fig. 2(c) shows that drastic increase of coefficient of friction at 80 ◦ C. Overall, dissolved
Fig. 8. SEM images of the wear scars on the steel balls (in quasi-tap water, 57 N).
M. Uchidate et al. / Wear 263 (2007) 1335–1340
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oxygen and water pressure did not show notable effect on friction and wear in these experiments. 3.2. Detailed examination of effects temperature and dissolved ions Fig. 3 shows effects of temperature on wear of DLC at the constant dissolved oxygen and water pressure. It is obvious that high temperature in quasi-tap water increases wear by an order of magnitude from 10−8 mm3 /N m at 20 ◦ C. This tendency is consistent with the results of the L9 experiment. As concerns wear of the steel balls, specific wear rate ranged from 5 × 10−9 to 6 × 10−8 mm3 /N m and no significant effect of temperature was observed. Fig. 4 compares the frictional behavior. The coefficient of friction decreases in an initial run in. Then it increases and reaches a certain value depending on environment. Friction in quasi-tap water is rather fluctuated than that in pure water. Average coefficient of friction after the run in is shown in Fig. 5. It is clear that the coefficient of friction increases with an increase in temperature especially in quasi-tap water. This also agrees to the tendency shown by the L9 experiments. Fig. 6 shows a SEM image of a wear track on DLC. Some scars of rupture together with some abrasive scars were observed as shown in the figure. However, the size and amount of rupture were not large enough to affect the coefficient of friction. Surface profiles of DLC wear scar shown in Fig. 7 exhibit a sign of mild wear even in high temperature conditions. Wear scars of the steel balls are shown in Fig. 8. As can be seen in Fig. 8(b), a kind of tribofilm with mud crack structure is clearly observed at 20 ◦ C. It is reasonable to think that the tribofilm was a hydrated, gel-like product in water and transformed into mud crack structure when it was desiccated for observation. This film appears to become unclear with increasing temperature as shown in Fig. 8(c and d). It was found that the tribofilm shown in Fig. 8(b) consists mainly of O and C by EPMA area mapping. The depth profiles of the wear scar by AES shown in Fig. 9 reveal that the thickness of C-layer decreases with increasing temperature from 500 nm at 20 ◦ C to a few nrs at 80 ◦ C. This fact and the negative impact of high temperature in quasi-tap water, as shown in Figs. 3 and 5, demonstrates that beneficial tribofilm formation on the steel ball was inhibited because of high temperature and dissolved ions. Fig. 10 shows XPS depth profile of the steel ball at 20 ◦ C in quasi-tap water. Small amounts of Ca and Mg were detected in the tribofilm. These elements were not detected on the steel rubbed at 50 and 80 ◦ C. With regard to DLC surface, authors have not succeeded in finding any tribofilms. Vanhulsel et al. showed that degradation of DLC thin toplayer can occur when it is subjected to rubbing even at relatively lower temperatures, such as 100 ◦ C in ambient air [7]. The accelerated degradation may be possible in water with dissolved ions. The tribofilm, mainly consists of Fe, O and C, is formed on the counter steel surface and prevent from direct contact between the steel and DLC surface [6,8]. Oxygen in the tribofilm is probably the result of hydrolysis induced by tribo-chemical reaction because dissolved oxygen did not affect the tribological property as shown in Fig. 2. At higher temperature, the film formation is inhibited (Fig. 8) and causes the direct contact which results in
Fig. 9. AES depth profiles of the wear scars on the steel balls (in quasi-tap water, 57 N).
Fig. 10. XPS depth profiles of the wear scar on the steel ball (in quasi-tap water, 57 N).
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high friction [8]. The film will crack and delaminate partially as the film grows, and loose debris will be produced [9]. Some of them may remain at the interface or be embedded on the steel surface again and increase the friction by preventing smooth contact. This may be related with the increased and fluctuated coefficient of friction after 50–100 m in Fig. 4. and higher friction at higher load in Figs. 2 and 5. Also, the degraded DLC top-layer can be damaged if the debris is hard enough to abrade. 4. Conclusion A series of tribo-test was carried out to examine the effects of water environment on tribo-performance of DLC rubbed against stainless steel. The results reveal that high temperature in tap water environment has the negative impact on friction and wear. Specific wear rate of DLC increased by an order of magnitude when water temperature was rose from 20 to 80 ◦ C in quasi-tap water. Effects of dissolved oxygen and water pressure were also examined briefly but no significant effect was observed. Surface characterization demonstrated that formation of the beneficial tribofilm, which consists mainly of C, O and Fe, on the steel surface. The small amounts of Ca and Mg were also detected in the film rubbed in tap water at 20 ◦ C. The larger wear and higher friction at high temperature in quasi-tap water environment can be explained by degradation of the DLC top-layer and behavior of tribofilm on the steel counter surface. Acknowledgements This work was performed in the national research project on Smart Materials for Tribo-Systems in Drive Unit and financially
supported by New Energy and Industrial Technology Development Organization (NEDO). References [1] J. Andersson, R.A. Erck, A. Erdemir, Friction of diamond-like carbon films in different atmospheres, Wear 254 (2003) 1070–1075. [2] H. Ronkainen, S. Varjus, K. Holmberg, Tribological performance of different DLC coatings in water-lubricated conditions, Wear 249 (2001) 267– 271. [3] T. Ohana, T. Nakamura, M. Suzuki, A. Tanaka, Y. Koga, Tribological properties and characterization of DLC films deposited by pulsed bias CVD, Diamond Relat. Mater. 13 (2004) 1500–1504. [4] A. Iwabuch, M. Uchidate, H. Liu, T. Miura, T. Shimizu, The effect of dissolved ions on tribological properties in water, Life Cycle Tribol. (2005) 431–440. [5] N. Yamaguchi, A. Okamoto, H. Tukahara, K. Demizu, N. Ueda, T. Sone, Y. Hirose, Friction and wear of DLC films on 304 austenitic stainless steel in corrosive solutions, Surf. Coat. Technol. 174–175 (2003) 465– 469. [6] H. Ronkainen, S. Varjus, K. Holmberg, Friction and wear properties in dry, water- and oil-lubricated DLC against alumina and DLC against steel contacts, Wear 222 (1998) 120–128. [7] A. Vanhulsel, B. Blanpain, J.-P. Celis, J. Roos, E. Dekempeneer, J. Smeets, Study of the wear behaviour of diamond-like coatings at elevated temperatures, Surf. Coat. Technol. 98 (1998) 1047–1052. [8] H. Li, T. Xu, C. Wang, J. Chen, H. Zhou, H. Liu, Friction-induced physical and chemical interactions among diamond-like carbon film, steel ball and water and/or oxygen molecules, Diamond Relat. Mater. 15 (2006) 1228–1234. [9] E. Konca, Y.-T. Cheng, A.M. Weiner, J.M. Dasch, A.T. Alpas, Elevated temperature tribological behavior of non-hydrogenated diamond-like carbon coatings against 319 aluminum alloy, Surf. Coat. Technol. 200 (2006) 3996–4005.
Effects of water environment on tribological properties of DLC rubbed against stainless steel M. Uchidate a,∗ , H. Liu a , A. Iwabuchi a , K. Yamamoto b a
Department of Mechanical Engineering, Faculty of Engineering, Iwate University, 3-5, Ueda 4-chome, Morioka-shi, Iwate 020-8551, Japan b Materials Research Laboratory, Kobe Steel Ltd., 5-5, Takatsukadai 1-chome, Nishi-ku, Kobe 651-2271, Japan Received 13 August 2006; received in revised form 4 October 2006; accepted 12 October 2006 Available online 23 May 2007
Abstract The effects of water environment on tribological properties of DLC against stainless steel were studied for development of the hydraulic pumps, valves and cylinders based on metal. A ball-on-disk tribotester was developed for detailed examination of water environment, such as temperature (20–80 ◦ C), dissolved oxygen (0.01–8 ppm), water pressure (0.1–20 MPa) and dissolved ions. DLC was deposited on the disk by the unbalanced magnetron spattering system. Pure water and quasi-tap water were used to study the effects of dissolved ions. The results showed that temperature and dissolved ions have a major impact on friction and wear of DLC. EPMA, XPS and AES indicated that tribofilm on the metal surface, which consists mainly of C, O and Fe, plays important role for the phenomena. © 2007 Elsevier B.V. All rights reserved. Keywords: Water lubrication; DLC coatings; Friction; Wear; Elevated temperature
1. Introduction Development of water hydraulic systems using metal based materials has been carried on as a national research project in Japan. In this development, combinations of DLC/stainless steel and DLC/brass are expected to achieve low friction and wear under water lubricated conditions because of the superior tribological properties in water [1–3]. Water used in the systems is to be tap water with no additives for ease of use and drainage. Hence, the properties of hydraulic fluid in the system vary depending on the region [4]. Also, the systems will be employed in various water environments in terms of temperature, dissolved oxygen and water pressure depending on use. Therefore, it is necessary to know the effects of factors mentioned above on tribological performance of DLC and counter materials. DLC are thought to be relatively inert and stable in corrosive environment. For example, Yamaguchi et al. investigated friction and wear of DLC against alumina in pure water, 3 mass% NaCl solution and acidic solutions, such as HCl, HNO3 and H2 SO4
∗
Corresponding author. Tel.: +81 196 621 6417; fax: +81 196 621 6417. E-mail address: [email protected] (M. Uchidate).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.10.024
and showed that there is no significant change in pure water and these solutions [5]. On the other hand, Ronkainen et al. showed that water can be aggressive media for DLC rubbed by alumina [6]. This study focuses on the effects of water environment on tribological performance of DLC rubbed against stainless steel. 2. Experimental details A ball-on-disk type tribotester, shown in Fig. 1, which can be operated under controlled water environment in terms of temperature (20–80 ◦ C), water pressure (0.1–20 MPa) and dissolved oxygen (0.01–8 ppm) was developed for this study. About 1 L of water is filled in the autoclave and circulated at the rate of 20 mL/min by a high pressure pump throughout the tests. The autoclave is covered with the temperature chamber with a heating–cooling bath. The amount of dissolved oxygen in water is controlled by aeration of N2 and O2 gas in a reserve tank. Frictional force was measured by the torque meter attached to the driving shaft. Electrodes connected to the potentio-stat were also placed in the chamber for electrochemical measurement as shown in Fig. 1. Turning radius of the ball specimens is 13 mm. Both the ball and disk specimens were made of AISI 630 stainless steel (Fe–17%Cr–4%Ni–4%Cu–1%Nb) hardened in
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M. Uchidate et al. / Wear 263 (2007) 1335–1340
Fig. 1. Schematic diagram of the tribotester for high temperature, high pressure environment in controlled water.
the H900 condition. The diameter of the ball specimen is 9.5 mm. Three ball specimens were used for each single test. Disk specimen is 10 mm in thickness and 50 mm in diameter. DLC and interlayer coatings were deposited using the unbalanced magnetron sputtering system. The substrate surface was sputtercleaned with Ar ion prior to deposition. Cr interlayer with about 50 nm in thickness was deposited on the stainless steel surface to improve adhesion. Then, Cr–C compositional gradient layer with approximately 200 nm in thickness was deposited which was followed by deposition of DLC layer with 1 m in thickness. Mixed gas of 10% CH4 and 90% Ar was introduced in the deposition chamber to add hydrogen in the DLC coating and its content was determined as 30 at.% using the elastic recoil detection analysis (ERDA). The total pressure during the deposition was 0.6 Pa. Bias voltage of −100 V was supplied on the substrate. The deposition temperature was less than 200 ◦ C. The surface hardness of the ball was HV 500 (4.9 GPa). The nanoindentation hardness of the DLC coating was 16 GPa. The surface roughness of both the disk and ball specimens was 0.03 m in Ra.
Fig. 2. Extracted effects of environmental factors and load on wear and friction.
M. Uchidate et al. / Wear 263 (2007) 1335–1340 Table 1 Specifications of quasi-tap water and comparison with tap water sampled in Tokyo Tap water sampled in Tokyo
Quasi-tap water
Dissolved ions (mg/L) Cl− NO3 − SO4 2− Na+ K+ Mg2+ Ca2+
30 12 36 22 2.8 5.0 22
31 13 37 24 2.5 5.3 23
Electrical conductivity (mS/m) pH
28.9 7.2–7.8
29.4 7.4–7.9
1337
Table 4 Experimental condition for detailed examination of temperature and dissolved ions Turning speed Load Number of revolutions Water pressure Dissolved oxygen Water temperature Water
0.4 m/s 11, 31, 58 N 36,000 (Approx.) 0.1 MPa 7–8 ppm (20, 50, 80) ± 2 ◦ C Pure water, quasi-tap water
Table 2 L9 orthogonal table for examination of environmental factors and load Exp. No.
Load
Temperature
Dissolved oxygen
Water pressure
1 2 3 4 5 6 7 8 9
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
Fig. 3. Dependence on temperature of specific wear rate of DLC.
Numbers of each factor corresponds to the level number shown in Table 3.
Quasi-tap water, which imitates tap water sampled in Tokyo (Table 1), was used to achieve stable quality of water. Pure water (deionized water) was also used for comparison. Experiment was done in two steps; brief examination of environmental factors and detailed examination of temperature. In order to examine the effects of temperature, dissolved oxygen and water pressure with a limited number of experiments, L9 orthogonal array was employed. Layout of factors and conditions are listed in Tables 2 and 3. Load was also included in factors as shown in the tables. Turning speed was 0.4 m/s and number of revolutions was about 36,000 in the L9 experiment. The L9 experiment was carried out in pure water and quasi-tap water. Effects of each factor are extracted by averaging each condition under the assumption that there is no interaction among the factors. The values of mean contact pressure at initial states vary from 0.5 GPa at 10 N to 0.9 GPa at 57 N. And then the effects of temperature and dissolved ions were studied in detail since, as shown later, it was found
Fig. 4. Frictional behavior in different environment.
Table 3 Value of the factors listed in Table 2 Level No.
Load (N)
Temperature (◦ C)
Dissolved oxygen (ppm)
Water pressure (MPa)
1 2 3
10 30 57
20 50 80
0.01 0.1 7
0.1 1 19
Fig. 5. Comparison of average coefficient friction in respective environment.
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M. Uchidate et al. / Wear 263 (2007) 1335–1340
Fig. 7. Wear scar profiles in quasi-tap water.
the wear scar with an optical microscope. EPMA, AES and XPS were employed for surface characterization. Fig. 6. SEM image of a wear track on DLC (in quasi-tap water, 57 N, 80 ◦ C).
3. Results and discussion that they have major impacts on tribo-performance by the L9 experiments. Experimental condition for detailed examination of temperature is listed in Table 4. The settings of dissolved oxygen and water pressure are the normal, atmospheric condition. Surface topography of wear track was measured by a stylus profilometer and wear volume was calculated as the volume loss of the wear scar from the mean line of the original surface. Wear volume of the ball specimen was estimated from the width of
3.1. Brief examination of environmental factors Extracted results from L9 experiments are shown in Fig. 2. It is clear from Fig. 2(a) that specific wear rate of DLC increases drastically with an increase in temperature especially in quasitap water. Specific wear rate of steel balls shown in Fig. 2(b) indicates that wear of steel balls is insensitive to the environment. Coefficient of friction shown in Fig. 2(c) shows that drastic increase of coefficient of friction at 80 ◦ C. Overall, dissolved
Fig. 8. SEM images of the wear scars on the steel balls (in quasi-tap water, 57 N).
M. Uchidate et al. / Wear 263 (2007) 1335–1340
1339
oxygen and water pressure did not show notable effect on friction and wear in these experiments. 3.2. Detailed examination of effects temperature and dissolved ions Fig. 3 shows effects of temperature on wear of DLC at the constant dissolved oxygen and water pressure. It is obvious that high temperature in quasi-tap water increases wear by an order of magnitude from 10−8 mm3 /N m at 20 ◦ C. This tendency is consistent with the results of the L9 experiment. As concerns wear of the steel balls, specific wear rate ranged from 5 × 10−9 to 6 × 10−8 mm3 /N m and no significant effect of temperature was observed. Fig. 4 compares the frictional behavior. The coefficient of friction decreases in an initial run in. Then it increases and reaches a certain value depending on environment. Friction in quasi-tap water is rather fluctuated than that in pure water. Average coefficient of friction after the run in is shown in Fig. 5. It is clear that the coefficient of friction increases with an increase in temperature especially in quasi-tap water. This also agrees to the tendency shown by the L9 experiments. Fig. 6 shows a SEM image of a wear track on DLC. Some scars of rupture together with some abrasive scars were observed as shown in the figure. However, the size and amount of rupture were not large enough to affect the coefficient of friction. Surface profiles of DLC wear scar shown in Fig. 7 exhibit a sign of mild wear even in high temperature conditions. Wear scars of the steel balls are shown in Fig. 8. As can be seen in Fig. 8(b), a kind of tribofilm with mud crack structure is clearly observed at 20 ◦ C. It is reasonable to think that the tribofilm was a hydrated, gel-like product in water and transformed into mud crack structure when it was desiccated for observation. This film appears to become unclear with increasing temperature as shown in Fig. 8(c and d). It was found that the tribofilm shown in Fig. 8(b) consists mainly of O and C by EPMA area mapping. The depth profiles of the wear scar by AES shown in Fig. 9 reveal that the thickness of C-layer decreases with increasing temperature from 500 nm at 20 ◦ C to a few nrs at 80 ◦ C. This fact and the negative impact of high temperature in quasi-tap water, as shown in Figs. 3 and 5, demonstrates that beneficial tribofilm formation on the steel ball was inhibited because of high temperature and dissolved ions. Fig. 10 shows XPS depth profile of the steel ball at 20 ◦ C in quasi-tap water. Small amounts of Ca and Mg were detected in the tribofilm. These elements were not detected on the steel rubbed at 50 and 80 ◦ C. With regard to DLC surface, authors have not succeeded in finding any tribofilms. Vanhulsel et al. showed that degradation of DLC thin toplayer can occur when it is subjected to rubbing even at relatively lower temperatures, such as 100 ◦ C in ambient air [7]. The accelerated degradation may be possible in water with dissolved ions. The tribofilm, mainly consists of Fe, O and C, is formed on the counter steel surface and prevent from direct contact between the steel and DLC surface [6,8]. Oxygen in the tribofilm is probably the result of hydrolysis induced by tribo-chemical reaction because dissolved oxygen did not affect the tribological property as shown in Fig. 2. At higher temperature, the film formation is inhibited (Fig. 8) and causes the direct contact which results in
Fig. 9. AES depth profiles of the wear scars on the steel balls (in quasi-tap water, 57 N).
Fig. 10. XPS depth profiles of the wear scar on the steel ball (in quasi-tap water, 57 N).
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M. Uchidate et al. / Wear 263 (2007) 1335–1340
high friction [8]. The film will crack and delaminate partially as the film grows, and loose debris will be produced [9]. Some of them may remain at the interface or be embedded on the steel surface again and increase the friction by preventing smooth contact. This may be related with the increased and fluctuated coefficient of friction after 50–100 m in Fig. 4. and higher friction at higher load in Figs. 2 and 5. Also, the degraded DLC top-layer can be damaged if the debris is hard enough to abrade. 4. Conclusion A series of tribo-test was carried out to examine the effects of water environment on tribo-performance of DLC rubbed against stainless steel. The results reveal that high temperature in tap water environment has the negative impact on friction and wear. Specific wear rate of DLC increased by an order of magnitude when water temperature was rose from 20 to 80 ◦ C in quasi-tap water. Effects of dissolved oxygen and water pressure were also examined briefly but no significant effect was observed. Surface characterization demonstrated that formation of the beneficial tribofilm, which consists mainly of C, O and Fe, on the steel surface. The small amounts of Ca and Mg were also detected in the film rubbed in tap water at 20 ◦ C. The larger wear and higher friction at high temperature in quasi-tap water environment can be explained by degradation of the DLC top-layer and behavior of tribofilm on the steel counter surface. Acknowledgements This work was performed in the national research project on Smart Materials for Tribo-Systems in Drive Unit and financially
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