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Microstructures and tribological properties on the composite MoS2 films prepared by a novel two-step method
Materials Chemistry and Physics 91 (2005) 494–499
Microstructures and tribological properties on the composite MoS2 films prepared by a novel two-step method Hai-dou Wanga,∗ , Bin-shi Xua , Jia-jun Liub , Da-ming Zhuangb a
National Key Laboratory for Remanufacturing, Academy of Armored Forces Engineering, No. 21 Dujiakan, Changxindian, Beijing 100072, PR China b Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China Received 23 June 2004; received in revised form 12 December 2004; accepted 20 December 2004
Abstract Solid lubrication composite MoS2 films were prepared on the surface of AISI 1045 steel by a two-step method of multi-arc ion plating Mo and low temperature ion sulfuration. The tribological properties of the composite MoS2 films were investigated on a ball-on-disc wear tester under oil lubrication. AFM and SEM were adopted to analyze their morphologies and compositions of surface, cross-section and wear traces. XRD was utilized to analyze the phase structure and SAM was employed to detect the element variation of film with depth. In the composite MoS2 films, the MoS2 was dominant on the surface and the molybdenum was dominant in the inside. The wear results showed that this film possesses excellent tribological properties. In extreme condition, the failure mechanism of the composite MoS2 film was flaking off. © 2005 Elsevier B.V. All rights reserved. Keywords: Low temperature ion sulfuration; Multi-arc ion plating; Composite MoS2 film; Tribological properties
1. Introduction MoS2 film (or coating) is a kind of excellent solid lubrication film [1,2]. Up to now, the methods, such as catalysis synthesis [3], refined bonding [4], magnetron sputter [5,6], were utilized to prepare the solid lubrication MoS2 films, so as to reduce effectively the friction, wear and scuffing between the frictional-pairs. But because of some weakness, for instance, uneven thickness [3], rough surface [4], or low deposition rate [5,6] and inaccurate atomic ratio between sulfur and molybdenum [7], the methods above were not suitable to the precision frictional-pairs. The authors have been preparing successfully the solid lubrication FeS film on AISI 1045 steel by low temperature ion sulfuration [8]. In the paper, a two-step method, namely multi-arc ion plating Mo and low temperature ion sulfuration, was employed to prepare the solid lubrication com∗
Corresponding author. Tel.: +86 10 66718541; fax: +86 10 66717144. E-mail address: [email protected] (H.-d. Wang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.12.039
posite MoS2 films, and then the microstructures and tribological properties of the composite MoS2 films were studied.
2. Experimental methods The substrate was AISI 1045 steel, heat-treated by quenching and low temperature tempering. The hardness was HRC52 and the surface roughness Ra was 0.13 m. The metal Mo film was first prepared on the surface of 1045 steel in the multi-arc ion plating equipment of model MIP-6-800, and then was treated by low temperature ion sulfuration in the sulfuration equipment of model DW-1. Fig. 1 shows the schematic diagram of MIP-6-800. Three molybdenum targets of Ø150 mm × 30 mm were placed circular symmetrically. The specimens were hung in the rotary trestle near the Mo targets with the uniform motion. When the background vacuum was up to the scheduled value, the inert gas Ar was filled into and the negative bias was
H.-d. Wang et al. / Materials Chemistry and Physics 91 (2005) 494–499
Fig. 1. The schematic diagram of MIP-6-800.
set up. Then the degree of vacuum was adjusted and the arc was generated to create the ion beam flow of molybdenum. Under the negative bias, the ion beam flow of molybdenum was accelerated to rush onto the surface of substrate and deposited a Mo film. The deposition time was 2 h. Fig. 2 shows the schematic diagram of DW-1. The principle of the low temperature ion sulfuration was similar to that of ion nitrification with glow discharge. The workpieces (1045 steel coated with the Mo film) were put on the cathode tray and the container wall was linked to the anode. When the vacuum was up to a certain value, the ammonia gas was filled into the chamber. The high-tension direct current was applied between the cathode and anode to ionize the ammonia gas. Under the effect of cathode voltage drop, the ammonia ions were accelerated to bomb the cathode (workpieces), elevating their temperature. Till the temperature was up to a scheduled value of 190 ◦ C, bombardment stopped. In this temperature the solid sulfur in the chamber became gasification, and the sulfur gas atoms permeated into the Mo film with the crystals and defects, namely sulfurizing, to produce the composite MoS2 films. The friction and wear tests were carried out on a ballon-disc testing machine of model QP-100. The upper sam-
495
ples were AISI SAE 52100 steel ball with diameter 12.7 mm, hardness 770 HV, and the lower samples were the 1045 steel discs with composite MoS2 films, with dimension of Ø60 mm × 5 mm. As comparison, the FeS films with 6 m thick (sulfurized 1045 steel) [9] were also carried out under the same condition. In operation, the upper samples were fixed and the lower samples rotated. All tests were carried out at room temperature, atmospheric environment and lubricated with engine lubrication oil (without any additives). The kinematic viscosity was 37–43 mm2 s−1 at 50 ◦ C and the velocity of oildripping was 2 ml min−1 . The frictional force was recorded directly by X–Y recorder and then converted into the friction coefficient. The wear loss was expressed by the width of the wear scar. Scuffing load was expressed by the weight of load when the film initially scuffed. The scuffing experiment is a destructive experiment. The higher was the scuffing load, the better the anti-scuffing property of the film was. During a fixed load of 70 N and a velocity of 1.60 m s−1 the variation of friction coefficient with time was investigated, the duration time was 60 min. During a fixed time of 37.5 min and a velocity of 1.49 m s−1 the variation of wear loss with load 24, 82, 140, 198 N was investigated. To test the variation of scuffing load with velocity, the velocity variables were employed: 1.12, 1.49, 1.87, 2.25 and 2.70 m s−1 . At each velocity, the sample ran in for 1 min in the beginning at loads of 12 and 70 N, respectively. Then the load was added stepwise. The increment was 58 N and the duration time was 2 min. When the frictional force suddenly increased rapidly, accompanied by evident oscillation and noise, it indicated that scuffing had occurred. At that time, the sum of all weights was the scuffing load. Atomic force microscope (AFM) and scanning electron microscope (SEM) equipped with EDS were employed to analyze the morphologies of surface, cross-section and wear scars of the composite MoS2 films. X-ray diffraction (XRD) was utilized to analyze the phase structure. Scanning Auger microprobe (SAM) was utilized to detect the element variation with depth.
Fig. 2. The schematic diagram of DW-1.
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Fig. 3. Morphologies of surface and cross-section of the MoS2 film.
3. Experimental results 3.1. Microstructures of the composite MoS2 film Fig. 3(a) shows the surface morphologies of the composite MoS2 film prepared by two-step method by AFM, showing the porous characteristics. The dimension of grains on the film surface was under 200 nm. Fig. 3(b) shows the cross-section morphology by SEM, showing that the thickness was only 3 m, and no transition in the interface. Fig. 4 shows the X-ray diffraction pattern of the composite MoS2 film. Besides MoS2 , the FeS was created, showing that the sulfur ion not only permeated through the Mo film, but also penetrated into the 1045 steel substrate and then reacted with some Mo and Fe atoms to form the MoS2 and FeS. The highest simple substance Mo peak showed that majority of Mo was not transformed to MoS2 . It can be considered that the composite MoS2 film prepared by two-step method was a metal-base composite film that solid lubricant MoS2 was contained in the metal Mo. Fig. 5 shows the SAM analysis to the composite MoS2 film, showing that the sulfur content on the film surface was higher, and then decreased with the increase of thickness. To the contrary, the molybdenum content on
Fig. 4. Phase structure of the MoS2 film by XRD.
the film surface was lower, whereas increased with thickness. It could be concluded that the content of solid lubricant MoS2 on the film surface was much more, and the thick, the less. The metal Mo was dominant in the thickness. This conformed to the permeation rule of sulfur element. 3.2. Tribological properties of the composite MoS2 film Fig. 6 shows the changing curves of the tribological properties among the composite MoS2 films, FeS films (sulfurized 1045 steel) and original 1045 steel surfaces. From Fig. 6(a) it can be seen that owing to the good oil lubrication effect, the friction coefficient of all films and surface was stable and descendent slightly. Due to the double lubrication with oil and solid lubricant MoS2 or FeS, the friction coefficient of the composite MoS2 film and FeS film was obviously lower than that of the original 1045 steel surface. Furthermore the friction coefficient of the composite MoS2 film was lower one time than that of the FeS film, showing that the former possessed much excellent solid lubrication.
Fig. 5. Elements variation of the MoS2 film with depth by AES Ar+ sputtering velocity—30 nm min−1 : (a) friction coefficient with time; (b) wear widths with load; (c) scuffing load with velocity.
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Fig. 6. Changing curves of tribological properties of the MoS2 film: (a) morphology of worn surface of the MoS2 film; (b) distribution map of iron; (c) distribution map of molybdenum; (d) distribution map of sulfur.
Fig. 6(b) shows the graph on variation of wear-scar widths with load. The wear-scar widths all increased with the increase of load, but the width values of the composite MoS2 film were less than that of the FeS film and the original 1045 steel surface. Fig. 6(c) shows the variation of scuffing load with velocity. It can be seen that the anti-scuffing property the composite MoS2 film was best and the scuffing load of the composite MoS2 film was the highest at low velocity. The scuffing load of the composite MoS2 film was three or four times than that of the FeS film and original 1045 steel, respectively, when the velocity was at 1.12 m s−1 . But along with the increase of velocity, the scuffing load descended rapidly. This is perhaps because that under the high velocity, the frictional heat destroyed easily the solid lubrication film. Fig. 7(a) shows the scuffing-scar morphology after the composite MoS2 film was scuffed (rotating velocity V = 1.12 m s−1 , scuffing load P = 1056 N, and the duration time T = 36 min). Fig. 7(b–d) shows the surface scanning of Fe, Mo, S elements in the same field of Fig. 7(a). It can be concluded that the failure mechanism of the composite MoS2 film was flaking off.
4. Discussions Why the composite MoS2 film can be prepared by the twostep method? When the multi-arc plating equipment worked, the single Mo atoms or atom groups with energy vaporized from the cathode Mo target rushed onto the substrate under the influence of electric field to pile forming the Mo film. And then, when sulfurizing, the bombardment of ammonia ions to Mo film increased the defects of Mo film. Moreover the atomic diameter of molybdenum is 0.201 nm, being almost one time bigger than that of sulfur of 0.107 nm. So it become possible that during sulfurizing the sulfur atoms permeated into the Mo film along with the gaps and defects to react with Mo to form the compound of MoS2 . According to the interaction rule among molybdenum and other elements in periodic table [10], sulfur does not dissolve in molybdenum on the whole, only reacts with molybdenum and exists in the formation of sul-molyb compound. Based on Fig. 4, the compound was MoS2 . Whereas MoS2 possesses a close-packed hexagonal structure, its lattice constant is a = 0.316 nm, c = 1.229nm. Fig. 8 shows the schematic map of crystal lattice of MoS2 [11]. It can be
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Fig. 7. The morphology of worn surface and distribution map of elements a = 0.316 nm, c = 1.229 nm.
seen that the crystal lattice is the close-packed thin groups formed by three plane layers of S, Mo, S. The thickness of group is 0.625 nm, and slippage happens easily along the close-packed plane [12]. Therefore the friction coefficient
Fig. 8. Schematic diagram of crystal lattice of MoS2 [10].
could be very low and the solid lubrication is very excellent. According to the tribological theory, an ideal friction surface should be soft on the surface, possessing excellent lubricating property, and hard in the sub-surface, giving a sufficient support to the lubrication film and keep a longer service life. Molybdenum is a kind of hard metal, yet MoS2 is quite low, only about 15 HV0.1 [12]. From Fig. 5, in the composite MoS2 film, MoS2 was more on the surface and less in the deep layer and the hard molybdenum is mainly in the deep layer. Thus the composite MoS2 film is actually an ideal friction surface. So the composite MoS2 film possessed the remarkable tribological properties, especially antiscuffing property. But the composite MoS2 film was thin. The bonding between film and substrate was mechanical bonding but not metallurgical one. Under the extreme condition of high velocity and heavy load the film was ultimately scuffed and flake off, as well as lost its solid lubrication effect (see Fig. 7(a)). The success in preparing the MoS2 film through novel two-step method provides a new way to obtain the solid lubrication film. This MoS2 film has a thin thickness, smooth surface, firm bonding with substrate and excellent tribological properties, and is much suitable for the precision frictionalpairs.
H.-d. Wang et al. / Materials Chemistry and Physics 91 (2005) 494–499
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5. Conclusions
References
(a) A novel two-step method, namely multi-arc plating Mo plus low temperature ion sulfuration, was successfully utilized to prepare the composite MoS2 solid lubrication films. (b) To the composite MoS2 films, MoS2 is dominant in the surface and Mo is dominant in the deep layer. They are an ideal friction film and possess excellent tribological properties. (c) Under the extreme condition, the failure mechanism of composite MoS2 films was flaking off.
[1] L.V. Santos, V.J. Trava-Airoldi, K. Iha, Diam. Relat. Mater. 10 (2001) 1049. [2] E. Blando, R. H¨ubler, Surf. Coat. Technol. 158–159 (2002) 685. [3] N. Barreau, J.C. Bern`ede, Thin Solid Films 403–404 (2002) 505. [4] J. Xu, M.H. Zhu, Z.R. Zhou, P. Kapsa, L. Vincent, Wear 255 (2003) 253. [5] G.Z. Xu, Z.R. Zhou, J.J. Liu, Wear 224 (1999) 211. [6] V. Rigato, G. Maggioni, D. Boscarino, L. Sangaletti, L. Depero, V.C. Fox, D. Teer, C. Santini, Surf. Coat. Technol. 116–119 (1999) 176. [7] D.M. Zhuang, J.J. Liu, B.L. Zhu, W.Z. Li, Wear 210 (1997) 45. [8] H.D. Wang, D.M. Zhuang, K.L. Wang, J.J. Liu, Mater. Lett. 57 (2003) 2225. [9] H.D. Wang, B.S. Xu, J.J. Liu, D.M. Zhuang, Vacuum 75 (2004) 353. [10] H.H. Molgonova, Molybdenum Alloys, Metallurgical Industry Press, Beijing, 1984. [11] Z.G. Yan, Lubrication Materials and Lubrication Technology, Chinese Petrochemical Industry Press, Beijing, 2000 (in Chinese). [12] F. Ou, The Handbook of Correct Lubrication Technology, Petroleum Industry Press, Beijing, 1993 (in Chinese).
Acknowledgements This research was granted by National Natural Science Foundation of China (no. 50235030), “863” Project (2003AA331130) and “973” Project (G1999065009).
Microstructures and tribological properties on the composite MoS2 films prepared by a novel two-step method Hai-dou Wanga,∗ , Bin-shi Xua , Jia-jun Liub , Da-ming Zhuangb a
National Key Laboratory for Remanufacturing, Academy of Armored Forces Engineering, No. 21 Dujiakan, Changxindian, Beijing 100072, PR China b Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China Received 23 June 2004; received in revised form 12 December 2004; accepted 20 December 2004
Abstract Solid lubrication composite MoS2 films were prepared on the surface of AISI 1045 steel by a two-step method of multi-arc ion plating Mo and low temperature ion sulfuration. The tribological properties of the composite MoS2 films were investigated on a ball-on-disc wear tester under oil lubrication. AFM and SEM were adopted to analyze their morphologies and compositions of surface, cross-section and wear traces. XRD was utilized to analyze the phase structure and SAM was employed to detect the element variation of film with depth. In the composite MoS2 films, the MoS2 was dominant on the surface and the molybdenum was dominant in the inside. The wear results showed that this film possesses excellent tribological properties. In extreme condition, the failure mechanism of the composite MoS2 film was flaking off. © 2005 Elsevier B.V. All rights reserved. Keywords: Low temperature ion sulfuration; Multi-arc ion plating; Composite MoS2 film; Tribological properties
1. Introduction MoS2 film (or coating) is a kind of excellent solid lubrication film [1,2]. Up to now, the methods, such as catalysis synthesis [3], refined bonding [4], magnetron sputter [5,6], were utilized to prepare the solid lubrication MoS2 films, so as to reduce effectively the friction, wear and scuffing between the frictional-pairs. But because of some weakness, for instance, uneven thickness [3], rough surface [4], or low deposition rate [5,6] and inaccurate atomic ratio between sulfur and molybdenum [7], the methods above were not suitable to the precision frictional-pairs. The authors have been preparing successfully the solid lubrication FeS film on AISI 1045 steel by low temperature ion sulfuration [8]. In the paper, a two-step method, namely multi-arc ion plating Mo and low temperature ion sulfuration, was employed to prepare the solid lubrication com∗
Corresponding author. Tel.: +86 10 66718541; fax: +86 10 66717144. E-mail address: [email protected] (H.-d. Wang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.12.039
posite MoS2 films, and then the microstructures and tribological properties of the composite MoS2 films were studied.
2. Experimental methods The substrate was AISI 1045 steel, heat-treated by quenching and low temperature tempering. The hardness was HRC52 and the surface roughness Ra was 0.13 m. The metal Mo film was first prepared on the surface of 1045 steel in the multi-arc ion plating equipment of model MIP-6-800, and then was treated by low temperature ion sulfuration in the sulfuration equipment of model DW-1. Fig. 1 shows the schematic diagram of MIP-6-800. Three molybdenum targets of Ø150 mm × 30 mm were placed circular symmetrically. The specimens were hung in the rotary trestle near the Mo targets with the uniform motion. When the background vacuum was up to the scheduled value, the inert gas Ar was filled into and the negative bias was
H.-d. Wang et al. / Materials Chemistry and Physics 91 (2005) 494–499
Fig. 1. The schematic diagram of MIP-6-800.
set up. Then the degree of vacuum was adjusted and the arc was generated to create the ion beam flow of molybdenum. Under the negative bias, the ion beam flow of molybdenum was accelerated to rush onto the surface of substrate and deposited a Mo film. The deposition time was 2 h. Fig. 2 shows the schematic diagram of DW-1. The principle of the low temperature ion sulfuration was similar to that of ion nitrification with glow discharge. The workpieces (1045 steel coated with the Mo film) were put on the cathode tray and the container wall was linked to the anode. When the vacuum was up to a certain value, the ammonia gas was filled into the chamber. The high-tension direct current was applied between the cathode and anode to ionize the ammonia gas. Under the effect of cathode voltage drop, the ammonia ions were accelerated to bomb the cathode (workpieces), elevating their temperature. Till the temperature was up to a scheduled value of 190 ◦ C, bombardment stopped. In this temperature the solid sulfur in the chamber became gasification, and the sulfur gas atoms permeated into the Mo film with the crystals and defects, namely sulfurizing, to produce the composite MoS2 films. The friction and wear tests were carried out on a ballon-disc testing machine of model QP-100. The upper sam-
495
ples were AISI SAE 52100 steel ball with diameter 12.7 mm, hardness 770 HV, and the lower samples were the 1045 steel discs with composite MoS2 films, with dimension of Ø60 mm × 5 mm. As comparison, the FeS films with 6 m thick (sulfurized 1045 steel) [9] were also carried out under the same condition. In operation, the upper samples were fixed and the lower samples rotated. All tests were carried out at room temperature, atmospheric environment and lubricated with engine lubrication oil (without any additives). The kinematic viscosity was 37–43 mm2 s−1 at 50 ◦ C and the velocity of oildripping was 2 ml min−1 . The frictional force was recorded directly by X–Y recorder and then converted into the friction coefficient. The wear loss was expressed by the width of the wear scar. Scuffing load was expressed by the weight of load when the film initially scuffed. The scuffing experiment is a destructive experiment. The higher was the scuffing load, the better the anti-scuffing property of the film was. During a fixed load of 70 N and a velocity of 1.60 m s−1 the variation of friction coefficient with time was investigated, the duration time was 60 min. During a fixed time of 37.5 min and a velocity of 1.49 m s−1 the variation of wear loss with load 24, 82, 140, 198 N was investigated. To test the variation of scuffing load with velocity, the velocity variables were employed: 1.12, 1.49, 1.87, 2.25 and 2.70 m s−1 . At each velocity, the sample ran in for 1 min in the beginning at loads of 12 and 70 N, respectively. Then the load was added stepwise. The increment was 58 N and the duration time was 2 min. When the frictional force suddenly increased rapidly, accompanied by evident oscillation and noise, it indicated that scuffing had occurred. At that time, the sum of all weights was the scuffing load. Atomic force microscope (AFM) and scanning electron microscope (SEM) equipped with EDS were employed to analyze the morphologies of surface, cross-section and wear scars of the composite MoS2 films. X-ray diffraction (XRD) was utilized to analyze the phase structure. Scanning Auger microprobe (SAM) was utilized to detect the element variation with depth.
Fig. 2. The schematic diagram of DW-1.
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H.-d. Wang et al. / Materials Chemistry and Physics 91 (2005) 494–499
Fig. 3. Morphologies of surface and cross-section of the MoS2 film.
3. Experimental results 3.1. Microstructures of the composite MoS2 film Fig. 3(a) shows the surface morphologies of the composite MoS2 film prepared by two-step method by AFM, showing the porous characteristics. The dimension of grains on the film surface was under 200 nm. Fig. 3(b) shows the cross-section morphology by SEM, showing that the thickness was only 3 m, and no transition in the interface. Fig. 4 shows the X-ray diffraction pattern of the composite MoS2 film. Besides MoS2 , the FeS was created, showing that the sulfur ion not only permeated through the Mo film, but also penetrated into the 1045 steel substrate and then reacted with some Mo and Fe atoms to form the MoS2 and FeS. The highest simple substance Mo peak showed that majority of Mo was not transformed to MoS2 . It can be considered that the composite MoS2 film prepared by two-step method was a metal-base composite film that solid lubricant MoS2 was contained in the metal Mo. Fig. 5 shows the SAM analysis to the composite MoS2 film, showing that the sulfur content on the film surface was higher, and then decreased with the increase of thickness. To the contrary, the molybdenum content on
Fig. 4. Phase structure of the MoS2 film by XRD.
the film surface was lower, whereas increased with thickness. It could be concluded that the content of solid lubricant MoS2 on the film surface was much more, and the thick, the less. The metal Mo was dominant in the thickness. This conformed to the permeation rule of sulfur element. 3.2. Tribological properties of the composite MoS2 film Fig. 6 shows the changing curves of the tribological properties among the composite MoS2 films, FeS films (sulfurized 1045 steel) and original 1045 steel surfaces. From Fig. 6(a) it can be seen that owing to the good oil lubrication effect, the friction coefficient of all films and surface was stable and descendent slightly. Due to the double lubrication with oil and solid lubricant MoS2 or FeS, the friction coefficient of the composite MoS2 film and FeS film was obviously lower than that of the original 1045 steel surface. Furthermore the friction coefficient of the composite MoS2 film was lower one time than that of the FeS film, showing that the former possessed much excellent solid lubrication.
Fig. 5. Elements variation of the MoS2 film with depth by AES Ar+ sputtering velocity—30 nm min−1 : (a) friction coefficient with time; (b) wear widths with load; (c) scuffing load with velocity.
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Fig. 6. Changing curves of tribological properties of the MoS2 film: (a) morphology of worn surface of the MoS2 film; (b) distribution map of iron; (c) distribution map of molybdenum; (d) distribution map of sulfur.
Fig. 6(b) shows the graph on variation of wear-scar widths with load. The wear-scar widths all increased with the increase of load, but the width values of the composite MoS2 film were less than that of the FeS film and the original 1045 steel surface. Fig. 6(c) shows the variation of scuffing load with velocity. It can be seen that the anti-scuffing property the composite MoS2 film was best and the scuffing load of the composite MoS2 film was the highest at low velocity. The scuffing load of the composite MoS2 film was three or four times than that of the FeS film and original 1045 steel, respectively, when the velocity was at 1.12 m s−1 . But along with the increase of velocity, the scuffing load descended rapidly. This is perhaps because that under the high velocity, the frictional heat destroyed easily the solid lubrication film. Fig. 7(a) shows the scuffing-scar morphology after the composite MoS2 film was scuffed (rotating velocity V = 1.12 m s−1 , scuffing load P = 1056 N, and the duration time T = 36 min). Fig. 7(b–d) shows the surface scanning of Fe, Mo, S elements in the same field of Fig. 7(a). It can be concluded that the failure mechanism of the composite MoS2 film was flaking off.
4. Discussions Why the composite MoS2 film can be prepared by the twostep method? When the multi-arc plating equipment worked, the single Mo atoms or atom groups with energy vaporized from the cathode Mo target rushed onto the substrate under the influence of electric field to pile forming the Mo film. And then, when sulfurizing, the bombardment of ammonia ions to Mo film increased the defects of Mo film. Moreover the atomic diameter of molybdenum is 0.201 nm, being almost one time bigger than that of sulfur of 0.107 nm. So it become possible that during sulfurizing the sulfur atoms permeated into the Mo film along with the gaps and defects to react with Mo to form the compound of MoS2 . According to the interaction rule among molybdenum and other elements in periodic table [10], sulfur does not dissolve in molybdenum on the whole, only reacts with molybdenum and exists in the formation of sul-molyb compound. Based on Fig. 4, the compound was MoS2 . Whereas MoS2 possesses a close-packed hexagonal structure, its lattice constant is a = 0.316 nm, c = 1.229nm. Fig. 8 shows the schematic map of crystal lattice of MoS2 [11]. It can be
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Fig. 7. The morphology of worn surface and distribution map of elements a = 0.316 nm, c = 1.229 nm.
seen that the crystal lattice is the close-packed thin groups formed by three plane layers of S, Mo, S. The thickness of group is 0.625 nm, and slippage happens easily along the close-packed plane [12]. Therefore the friction coefficient
Fig. 8. Schematic diagram of crystal lattice of MoS2 [10].
could be very low and the solid lubrication is very excellent. According to the tribological theory, an ideal friction surface should be soft on the surface, possessing excellent lubricating property, and hard in the sub-surface, giving a sufficient support to the lubrication film and keep a longer service life. Molybdenum is a kind of hard metal, yet MoS2 is quite low, only about 15 HV0.1 [12]. From Fig. 5, in the composite MoS2 film, MoS2 was more on the surface and less in the deep layer and the hard molybdenum is mainly in the deep layer. Thus the composite MoS2 film is actually an ideal friction surface. So the composite MoS2 film possessed the remarkable tribological properties, especially antiscuffing property. But the composite MoS2 film was thin. The bonding between film and substrate was mechanical bonding but not metallurgical one. Under the extreme condition of high velocity and heavy load the film was ultimately scuffed and flake off, as well as lost its solid lubrication effect (see Fig. 7(a)). The success in preparing the MoS2 film through novel two-step method provides a new way to obtain the solid lubrication film. This MoS2 film has a thin thickness, smooth surface, firm bonding with substrate and excellent tribological properties, and is much suitable for the precision frictionalpairs.
H.-d. Wang et al. / Materials Chemistry and Physics 91 (2005) 494–499
499
5. Conclusions
References
(a) A novel two-step method, namely multi-arc plating Mo plus low temperature ion sulfuration, was successfully utilized to prepare the composite MoS2 solid lubrication films. (b) To the composite MoS2 films, MoS2 is dominant in the surface and Mo is dominant in the deep layer. They are an ideal friction film and possess excellent tribological properties. (c) Under the extreme condition, the failure mechanism of composite MoS2 films was flaking off.
[1] L.V. Santos, V.J. Trava-Airoldi, K. Iha, Diam. Relat. Mater. 10 (2001) 1049. [2] E. Blando, R. H¨ubler, Surf. Coat. Technol. 158–159 (2002) 685. [3] N. Barreau, J.C. Bern`ede, Thin Solid Films 403–404 (2002) 505. [4] J. Xu, M.H. Zhu, Z.R. Zhou, P. Kapsa, L. Vincent, Wear 255 (2003) 253. [5] G.Z. Xu, Z.R. Zhou, J.J. Liu, Wear 224 (1999) 211. [6] V. Rigato, G. Maggioni, D. Boscarino, L. Sangaletti, L. Depero, V.C. Fox, D. Teer, C. Santini, Surf. Coat. Technol. 116–119 (1999) 176. [7] D.M. Zhuang, J.J. Liu, B.L. Zhu, W.Z. Li, Wear 210 (1997) 45. [8] H.D. Wang, D.M. Zhuang, K.L. Wang, J.J. Liu, Mater. Lett. 57 (2003) 2225. [9] H.D. Wang, B.S. Xu, J.J. Liu, D.M. Zhuang, Vacuum 75 (2004) 353. [10] H.H. Molgonova, Molybdenum Alloys, Metallurgical Industry Press, Beijing, 1984. [11] Z.G. Yan, Lubrication Materials and Lubrication Technology, Chinese Petrochemical Industry Press, Beijing, 2000 (in Chinese). [12] F. Ou, The Handbook of Correct Lubrication Technology, Petroleum Industry Press, Beijing, 1993 (in Chinese).
Acknowledgements This research was granted by National Natural Science Foundation of China (no. 50235030), “863” Project (2003AA331130) and “973” Project (G1999065009).