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The friction–reduction model of the iron sulfide film prepared by plasma source ion sulfuration
Surface & Coatings Technology 201 (2007) 5236 – 5239 www.elsevier.com/locate/surfcoat
The friction–reduction model of the iron sulfide film prepared by plasma source ion sulfuration Hai-dou Wang a,⁎,1 , Bin-shi Xu a , Jia-jun Liu b , Da-ming Zhuang b a
National Key Lab for remanufacturing, Academy of Armored Forces Engineering, Beijing, 100072, China b Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China Available online 23 October 2006
Abstract A new plasma source method, low temperature ion sulfuration, was utilized to prepare the FeS film on the AISI 1045 steel surface. The antifriction property was tested on the ball-on-plate tester and the results showed the film holds the excellent performance. AFM and SEM were adopted to analyze the morphologies of surfaces and wear scars of the FeS film. SAM was employed to detect the surface element distribution with depth. The physical friction–reduction model of the FeS film was formed and was verified by SAM analysis. Results showed that in the course of friction, FeS film formed a dynamic balance between decomposing and recomposing. Therefore the thin FeS film could last a long solid lubrication time. © 2006 Published by Elsevier B.V. PACS: 81.15.Ef; 81.15.-z; 81.40.Pq Keywords: Iron sulfide film; Plasma source; Ion sulfuration; Friction–reduction; Model
1. Introduction To the precise frictional-pairs, in order to improve the tribological performance, the methods, such as perfecting the lubrication oil [1,2], or increasing the surface hardness [3,4], are commonly utilized. Yet it's not effectual at all events. The increase of hardness, could cause the decrease of materials toughness, not in favor of anti-fatigue, as well as increase the severe wear for opposite components. Under some extreme condition, such as high temperature, high speed, vacuum, the lubrication oil does not work well. Thus it's a good choice to select solid lubrication film [5,6]. Since the last 90s, the solid lubrication iron sulfide (FeS) and its film, which is prepared by the plasma source low temperature ion sulfuration on the ferrous metals with several micrometers thick, became the important solid lubrication material and film [7–9]. The FeS film can obstruct efficiently the direct contact and decrease obviously the metals wear. In the paper, the anti-friction mechanism of the FeS
⁎ Corresponding author. Tel.: +86 10 66718541; fax: +86 10 66717144. E-mail address: [email protected] (H. Wang). 1 Supported by the NSFC(50575225). 0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.07.226
film would be studied, anti-friction model would be established, and the correctness of the model would be proved. 2. Experimental methods The AISI 1045 steel, with hardness HRC55, was selected as substrate. The sulfuration course could be narrated as following [7]: the workpieces (steel) were put on the cathode tray with DC voltage applied between the components and the container wall. The solid sulfur within the furnace became gasification when the temperature was up to 180 °C, and the sulfur gas atoms permeated into the steel with the crystals and defects, namely sulfurizing, to form the compound FeS. The friction tests were carried out on a ball-on-disc testing machine. The upper sample was 52100 steel ball with diameter 12.7 mm, hardness 770 HV, and the lower samples were the AISI 1045 steel discs with FeS films, with dimension of 60 mm (in diameter) × 5 mm (in thickness). During friction, the upper sample was fixed, the lower disc rotated. The test was in a lubrication condition. The Scanning Electron Microscopy (SEM) was utilized to observe and analyze the morphologies of cross-section and worn scar of FeS film. The Atomic Force Microscopy (AFM)
H. Wang et al. / Surface & Coatings Technology 201 (2007) 5236–5239
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efficient with load, due to the periodic equidifferent increasing of load, the friction condition deteriorated continuously, so as the friction coefficient increased continuously. But no matter how variation the friction coefficient with sliding time or load, owing to the double lubrication of “fluid plus solid lubrication”, the friction coefficient of FeS film was lower than that of the uncoated plain steel. The break of the friction coefficient of uncoated steel in Fig. 2(b) was due to the experience stop, resulting from the scuffing owing to the overload. 3.2. Model and verification of friction–reduction mechanism It could be found from the Fe–S binary phase diagram [11] that the solubility of S in Fe is very low. In the temperature of 930 °C, the solubility of sulfur in ferrite is only 0.02%, and almost zero at 700 °C, so that in the FeS film, sulfur does not solid dissolve within iron, but exists as the sulfur–iron compound (mainly FeS). FeS possesses a close-packed hexagonal structure, with the easy sliding along the close-packed plane, to improve the lubrication condition between the frictional-pairs. When the flash temperature from frictional heat was over the molten point of FeS, the FeS film would be decomposed, and then its solid lubrication lost. But the frictional heat has dual influence. Under high frictional heat, on the one hand, FeS was decomposed to Fe and S, on the other hand, the activated S atoms retained on the worn surface could react with iron atoms
Fig. 1. Surface and cross-section morphologies of the FeS film. a) Surface, (b) cross-section.
was used to observe the surface morphologies. The Scanning Auger Microprobe (SAM) was utilized to analyze the element variation of boundary lubrication film with depth. 3. Results and discussion 3.1. Characterization and tribological performance Fig. 1(a) and (b) are the surface and cross-section morphologies of FeS film by AFM and SEM respectively. Most of the surface grains on the FeS film were less than 100 nm. In cross-section, the film was an uninterrupted white belt with about 5 μm thick. The XRD patterns of FeS film could be found in Ref. [10], Both FeS and FeS2 were produced, but only FeS is the solid lubricant because FeS2 doesn't possess the closepacked hexagonal structure. Fig. 2(a) and (b) are the variation curves of friction coefficient with time, under fixed velocity 1.60 m/s and fixed load 50 N, and with load, under fixed velocity 1.49 m/s and increasing load with equidifferent 58 N respectively. It can be seen that the variation between friction coefficient and sliding time, the coefficient values were stable, almost a line, showing that in the test, due to the appropriate velocity and moderate load, the frictional-pairs operated steadily. Whereas the variation between friction co-
Fig. 2. Variation of friction coefficient under oil lubrication. (a) With sliding time, (b) with load.
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Fig. 3. The physical friction–reduction model of FeS film.
to reproduce FeS [12]. Thus FeS keeps the dynamic balance between decomposing and recomposing on the worn surface and can reduce the friction coefficient continuously. Furthermore, though the FeS film on the frontal place contacted with opposite was destroyed, the FeS film in the marginal place still retained, and therefore the anti-friction performance lasted all the while. Fig. 3 is the friction–reduction physical model. In (a), no contact, no wear. In (b), wear just happens. At this time, wear, namely shearing, appears within the FeS film, substrate is free from wear. In (c), wear continues, the FeS film in the right side will be worn off, but in the marginal place still retained and worked continuously. Under the high load and frictional heat, FeS was decomposed to S and Fe atoms; on the other hand, due to the crystal lattice in the substrate materials distorted and defects increased, the decomposed activated S atoms diffused into the substrate along with the defects and crystal boundary, to form a sulfur diffusing area. In (d), the film has been worn off, and the substrate was worn. Owing to the frictional heat, solid lubricant FeS was reproduced between activated S atoms and Fe atoms. Therefore although the integrated FeS film in the right side did not exist, it retained all the time that the reproduced FeS in the sulfur diffusing area and the integrated FeS film in the marginal place. That's the reason why the FeS film with only 5 μm thick could take always the solid lubrication effect under the severe condition. The correctness of the model can be proved by the SAM results. Fig. 4(a) is the worn scar after scuffing, with the serious wear. Fig. 4(b) is the SAM analysis to (a). It can be found that within the depth below surface, there were not only the iron from substrate and oxygen absorbed from air, but also the diffusing
Fig. 4. Worn scar morphology of the FeS film and distribution of elements with depth. (a)Worn scar, (b)Results of SAM. (Ar+ sputtering velocity: 30 nm/min).
H. Wang et al. / Surface & Coatings Technology 201 (2007) 5236–5239
sulfur. The sulfur content decreased gradually from surface to inner. The existence and change trend of sulfur demonstrated that under the load and frictional heat, when FeS film was worn off, there surely existed a sulfur diffusing area in the depth of substrate. 4. Conclusions The solid lubrication FeS film was prepared by the plasma source low temperature ion sulfuration on the ferrous metals. Most of the surface grains on the FeS film were less than 100 nm. In cross-section, the film was an uninterrupted black belt with about 5 μm thick. The friction coefficient of FeS film is obviously lower than that of AISI 1045 steel, showing its remarkable tribological performance. The thin solid lubrication FeS film has a long service time. There existed a sulfur diffusing area in the depth of substrate, being suitable for the re-forming of solid lubricant FeS. Acknowledgements This paper was financially supported by the Natural Science Foundation of China (50575225).
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Y.S. Ma, J.J. Liu, L.Q. Zheng, Tribol. Int. 28 (1995) 329. Y.P. Cao, L.G. Yu, W.M. Liu, Wear 244 (2000) 126. H. Kenichi, Tribol. Int. 28 (1995) 279. A. Roy, I. Manna, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 297 (2001) 85. C. Donnet, A. Erdemir, Surf. Coat. Technol. 180–181 (2004) 76. S. Hiroyuki, M. Yasuo, J. Mater. Process. Technol. 140 (2003) 25. D.M. Zhuang, Y.R. Liu, J.J. Liu, X.D. Fang, M.X. Guan, Y. Cui, Wear 225–229 (1999) 799. P. Zhang, J.J. Zhao, W.Z. Han, Surf. Coat. Technol. 131 (2000) 386. N. Zhang, D.M. Zhuang, J.J. Liu, X.D. Fang, M.X. Guan, Surf. Coat. Technol. 132 (2000) 1. H.D. Wang, D.M. Zhuang, K.L. Wang, J.J. Liu, Mater. Lett. 57 (2003) 2225. B.D. Hong, Z.K. Yao, Chemical Heat Treatment, Heilongjian People Press, Harbin, 1981 (in Chinese). N. Zhang, D.M. Zhuang, J.J. Liu, X.D. Fang, M.X. Guan, Appl. Surf. Sci. 161 (2000) 170.
The friction–reduction model of the iron sulfide film prepared by plasma source ion sulfuration Hai-dou Wang a,⁎,1 , Bin-shi Xu a , Jia-jun Liu b , Da-ming Zhuang b a
National Key Lab for remanufacturing, Academy of Armored Forces Engineering, Beijing, 100072, China b Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China Available online 23 October 2006
Abstract A new plasma source method, low temperature ion sulfuration, was utilized to prepare the FeS film on the AISI 1045 steel surface. The antifriction property was tested on the ball-on-plate tester and the results showed the film holds the excellent performance. AFM and SEM were adopted to analyze the morphologies of surfaces and wear scars of the FeS film. SAM was employed to detect the surface element distribution with depth. The physical friction–reduction model of the FeS film was formed and was verified by SAM analysis. Results showed that in the course of friction, FeS film formed a dynamic balance between decomposing and recomposing. Therefore the thin FeS film could last a long solid lubrication time. © 2006 Published by Elsevier B.V. PACS: 81.15.Ef; 81.15.-z; 81.40.Pq Keywords: Iron sulfide film; Plasma source; Ion sulfuration; Friction–reduction; Model
1. Introduction To the precise frictional-pairs, in order to improve the tribological performance, the methods, such as perfecting the lubrication oil [1,2], or increasing the surface hardness [3,4], are commonly utilized. Yet it's not effectual at all events. The increase of hardness, could cause the decrease of materials toughness, not in favor of anti-fatigue, as well as increase the severe wear for opposite components. Under some extreme condition, such as high temperature, high speed, vacuum, the lubrication oil does not work well. Thus it's a good choice to select solid lubrication film [5,6]. Since the last 90s, the solid lubrication iron sulfide (FeS) and its film, which is prepared by the plasma source low temperature ion sulfuration on the ferrous metals with several micrometers thick, became the important solid lubrication material and film [7–9]. The FeS film can obstruct efficiently the direct contact and decrease obviously the metals wear. In the paper, the anti-friction mechanism of the FeS
⁎ Corresponding author. Tel.: +86 10 66718541; fax: +86 10 66717144. E-mail address: [email protected] (H. Wang). 1 Supported by the NSFC(50575225). 0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.07.226
film would be studied, anti-friction model would be established, and the correctness of the model would be proved. 2. Experimental methods The AISI 1045 steel, with hardness HRC55, was selected as substrate. The sulfuration course could be narrated as following [7]: the workpieces (steel) were put on the cathode tray with DC voltage applied between the components and the container wall. The solid sulfur within the furnace became gasification when the temperature was up to 180 °C, and the sulfur gas atoms permeated into the steel with the crystals and defects, namely sulfurizing, to form the compound FeS. The friction tests were carried out on a ball-on-disc testing machine. The upper sample was 52100 steel ball with diameter 12.7 mm, hardness 770 HV, and the lower samples were the AISI 1045 steel discs with FeS films, with dimension of 60 mm (in diameter) × 5 mm (in thickness). During friction, the upper sample was fixed, the lower disc rotated. The test was in a lubrication condition. The Scanning Electron Microscopy (SEM) was utilized to observe and analyze the morphologies of cross-section and worn scar of FeS film. The Atomic Force Microscopy (AFM)
H. Wang et al. / Surface & Coatings Technology 201 (2007) 5236–5239
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efficient with load, due to the periodic equidifferent increasing of load, the friction condition deteriorated continuously, so as the friction coefficient increased continuously. But no matter how variation the friction coefficient with sliding time or load, owing to the double lubrication of “fluid plus solid lubrication”, the friction coefficient of FeS film was lower than that of the uncoated plain steel. The break of the friction coefficient of uncoated steel in Fig. 2(b) was due to the experience stop, resulting from the scuffing owing to the overload. 3.2. Model and verification of friction–reduction mechanism It could be found from the Fe–S binary phase diagram [11] that the solubility of S in Fe is very low. In the temperature of 930 °C, the solubility of sulfur in ferrite is only 0.02%, and almost zero at 700 °C, so that in the FeS film, sulfur does not solid dissolve within iron, but exists as the sulfur–iron compound (mainly FeS). FeS possesses a close-packed hexagonal structure, with the easy sliding along the close-packed plane, to improve the lubrication condition between the frictional-pairs. When the flash temperature from frictional heat was over the molten point of FeS, the FeS film would be decomposed, and then its solid lubrication lost. But the frictional heat has dual influence. Under high frictional heat, on the one hand, FeS was decomposed to Fe and S, on the other hand, the activated S atoms retained on the worn surface could react with iron atoms
Fig. 1. Surface and cross-section morphologies of the FeS film. a) Surface, (b) cross-section.
was used to observe the surface morphologies. The Scanning Auger Microprobe (SAM) was utilized to analyze the element variation of boundary lubrication film with depth. 3. Results and discussion 3.1. Characterization and tribological performance Fig. 1(a) and (b) are the surface and cross-section morphologies of FeS film by AFM and SEM respectively. Most of the surface grains on the FeS film were less than 100 nm. In cross-section, the film was an uninterrupted white belt with about 5 μm thick. The XRD patterns of FeS film could be found in Ref. [10], Both FeS and FeS2 were produced, but only FeS is the solid lubricant because FeS2 doesn't possess the closepacked hexagonal structure. Fig. 2(a) and (b) are the variation curves of friction coefficient with time, under fixed velocity 1.60 m/s and fixed load 50 N, and with load, under fixed velocity 1.49 m/s and increasing load with equidifferent 58 N respectively. It can be seen that the variation between friction coefficient and sliding time, the coefficient values were stable, almost a line, showing that in the test, due to the appropriate velocity and moderate load, the frictional-pairs operated steadily. Whereas the variation between friction co-
Fig. 2. Variation of friction coefficient under oil lubrication. (a) With sliding time, (b) with load.
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Fig. 3. The physical friction–reduction model of FeS film.
to reproduce FeS [12]. Thus FeS keeps the dynamic balance between decomposing and recomposing on the worn surface and can reduce the friction coefficient continuously. Furthermore, though the FeS film on the frontal place contacted with opposite was destroyed, the FeS film in the marginal place still retained, and therefore the anti-friction performance lasted all the while. Fig. 3 is the friction–reduction physical model. In (a), no contact, no wear. In (b), wear just happens. At this time, wear, namely shearing, appears within the FeS film, substrate is free from wear. In (c), wear continues, the FeS film in the right side will be worn off, but in the marginal place still retained and worked continuously. Under the high load and frictional heat, FeS was decomposed to S and Fe atoms; on the other hand, due to the crystal lattice in the substrate materials distorted and defects increased, the decomposed activated S atoms diffused into the substrate along with the defects and crystal boundary, to form a sulfur diffusing area. In (d), the film has been worn off, and the substrate was worn. Owing to the frictional heat, solid lubricant FeS was reproduced between activated S atoms and Fe atoms. Therefore although the integrated FeS film in the right side did not exist, it retained all the time that the reproduced FeS in the sulfur diffusing area and the integrated FeS film in the marginal place. That's the reason why the FeS film with only 5 μm thick could take always the solid lubrication effect under the severe condition. The correctness of the model can be proved by the SAM results. Fig. 4(a) is the worn scar after scuffing, with the serious wear. Fig. 4(b) is the SAM analysis to (a). It can be found that within the depth below surface, there were not only the iron from substrate and oxygen absorbed from air, but also the diffusing
Fig. 4. Worn scar morphology of the FeS film and distribution of elements with depth. (a)Worn scar, (b)Results of SAM. (Ar+ sputtering velocity: 30 nm/min).
H. Wang et al. / Surface & Coatings Technology 201 (2007) 5236–5239
sulfur. The sulfur content decreased gradually from surface to inner. The existence and change trend of sulfur demonstrated that under the load and frictional heat, when FeS film was worn off, there surely existed a sulfur diffusing area in the depth of substrate. 4. Conclusions The solid lubrication FeS film was prepared by the plasma source low temperature ion sulfuration on the ferrous metals. Most of the surface grains on the FeS film were less than 100 nm. In cross-section, the film was an uninterrupted black belt with about 5 μm thick. The friction coefficient of FeS film is obviously lower than that of AISI 1045 steel, showing its remarkable tribological performance. The thin solid lubrication FeS film has a long service time. There existed a sulfur diffusing area in the depth of substrate, being suitable for the re-forming of solid lubricant FeS. Acknowledgements This paper was financially supported by the Natural Science Foundation of China (50575225).
5239
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Y.S. Ma, J.J. Liu, L.Q. Zheng, Tribol. Int. 28 (1995) 329. Y.P. Cao, L.G. Yu, W.M. Liu, Wear 244 (2000) 126. H. Kenichi, Tribol. Int. 28 (1995) 279. A. Roy, I. Manna, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 297 (2001) 85. C. Donnet, A. Erdemir, Surf. Coat. Technol. 180–181 (2004) 76. S. Hiroyuki, M. Yasuo, J. Mater. Process. Technol. 140 (2003) 25. D.M. Zhuang, Y.R. Liu, J.J. Liu, X.D. Fang, M.X. Guan, Y. Cui, Wear 225–229 (1999) 799. P. Zhang, J.J. Zhao, W.Z. Han, Surf. Coat. Technol. 131 (2000) 386. N. Zhang, D.M. Zhuang, J.J. Liu, X.D. Fang, M.X. Guan, Surf. Coat. Technol. 132 (2000) 1. H.D. Wang, D.M. Zhuang, K.L. Wang, J.J. Liu, Mater. Lett. 57 (2003) 2225. B.D. Hong, Z.K. Yao, Chemical Heat Treatment, Heilongjian People Press, Harbin, 1981 (in Chinese). N. Zhang, D.M. Zhuang, J.J. Liu, X.D. Fang, M.X. Guan, Appl. Surf. Sci. 161 (2000) 170.