- Email: [email protected]
Superlubricity of Tungsten Disulfide Coatings in Ultra High Vacuum
– 14 – Superlubricity of Tungsten Disulfide Coatings in Ultra High Vacuum Lucile Joly-Pottuz1 and Masanori Iwaki2 1 Ecole Centrale de Lyon, 36 avenue Guy de Collongue, Ecully 69134, France 2 Japan Aerospace Exploration Agency (JAXA), 2-1-1 Sengen, Tsukuba 305-8505, Japan
14.1
INTRODUCTION
Tungsten disulfide (WS2 ) is a solid lubricant with a lamellar structure similar to that of MoS2 . However, it has not been so widely studied according to the research literature. Since it possesses a crystal structure similar to that of MoS2 , it is expected to exhibit similar tribological properties including a state of near zero friction or superlubricity. Jamison compared the tribological properties of different metal dichalcogenides and concluded that metal dichalcogenides having the same structure as 2H–MoS2 possess good tribological properties [1]. He also correlated the tribological properties to the axial ratio c0 /na0 , which must exceed 1.87 in order to have a low friction coefficient. This ratio is equal to 1.95 for 2H–MoS2 and 1.96 for 2H–WS2 . In Figure 14.1, the metal elements surrounded by dashed lines form lamellar structures when bonded with sulfur or selenium, but only sulfides and selenides of molybdenum and tungsten have favorable tribological properties (elements surrounded by solid lines in Figure 14.1). By studying the electron distribution in these crystals, Jamison gave another explanation to the good tribological properties of MoS2 and WS2 [2]. In their structure, six nonbonding electrons fill a band and are confined in the structure. These electrons create a net positive charge on the surface layer, promoting easy shear through electrostatic repulsion. WS2 is thermally more stable and resistant to oxidation (about 50 to 100 ◦ C) than MoS2 [3]. The slow rate of oxidation of WS2 can be explained by the formation of tungsten trioxide (WO3 ), which is also known to provide a lower friction coefficient than molybdenum trioxide (MoO3 ). In a dry nitrogen environment, the steady-state friction coefficient of WS2 films grown by pulsed laser deposited on stainless steel against a steel counterface is about 0.04 [4]. A transfer film made of very thin WS2 sheets is observed on the pin side [5]. Analyzed by SEM, the sheets are thin enough (60 nm) to be transparent to the electron beam. Superlubricity Edited by A. Erdemir and J.-M. Martin © 2007 Published by Elsevier B.V.
228
L. Joly-Pottuz and M. Iwaki
Figure 14.1 Portion of the periodic table of elements showing metals that form lamellar structures (dashed line). Mo, W and Nb form lamellar structures with good tribological properties (solid line) [1].
In this chapter, we investigated the tribological properties of WS2 coatings or IF-WS2 coatings in an ultrahigh vacuum and at different temperatures (−130 to 200 ◦ C). Friction experiments were performed in an analytical ultra high vacuum tribometer [6]. This tribometer consists of a linear reciprocating pin-on-flat configuration installed directly inside an ultra high vacuum chamber. The system is equipped with traditional surface analysis techniques, X-ray Photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The pins were made of AISI 52100 steel with a radius of curvature of 4 mm. We used a normal load of 3 N on the pin leading to a maximum Hertzian contact pressure of 470 MPa. A very low friction coefficient was obtained with both types of coatings, indicating the very interesting tribological properties of WS2 coatings.
14.2
WS2 COATINGS
The samples prepared for the experiments were deposited on Si (100) substrates by an RF sputter deposition process with a thickness of 500 nm. Pin-on-flat experiments were conducted under an ultra high vacuum environment (1 × 10−7 Pa) with various temperatures ranging from −130 to 200 ◦ C. Figure 14.2 illustrates the friction profiles of WS2 coatings under these conditions. At room temperature (30 ◦ C), the friction coefficient was about 0.02. The friction coefficient then decreased with decreasing temperature, reaching 0.005 at −130 ◦ C, which is in the superlow friction regime. In contrast, the friction coefficient increased with increasing temperature at temperatures greater than 30 ◦ C. Thus, a linear relationship between the friction coefficient and the temperature can be seen (Figure 14.3). The relationship between the friction coefficient and the temperature was discussed in the 1940s by Eyring [7] then later summarized by Overney et al. [8]. Eyring’s thermalactivation model of lubricated friction predicts a linear relationship of friction and temperature, and Briscoe has experimentally verified this model for solid lubrication [9]. Our results confirm this relationship. Auger analysis (Figure 14.4) demonstrates that less carbon and oxygen can be seen on the pin tested at −130 ◦ C compared to that at room temperature (30 ◦ C), when the amount of sulfur is comparable. This means that a relatively “pure” transfer film of WSx is formed on the counterpart pin at −130 ◦ C. This is attributed to the fact that the chemical reaction is inhibited at lower temperatures.
Superlubricity of Tungsten Disulfide Coatings in Ultra High Vacuum
229
Figure 14.2 Friction profile of WS2 coating at various temperatures.
Figure 14.3 Steady-state friction coefficient of the WS2 coatings as a function of temperature. Line fitting was made by excluding the irregular data at 100 ◦ C and by plotting the line so that the friction coefficient is 0 at 0 K.
A pure and stoichiometric MoS2 coating that contains below 1% oxygen can exhibit an extraordinarily low friction coefficient [10]. However, the WS2 coating prepared for our experiment was considered to have at least several percent oxygen. Therefore, this result indicates that even a disulphide film without special preparation can achieve a superlow friction at cryogenic temperatures.
230
L. Joly-Pottuz and M. Iwaki
Figure 14.4 Auger spectra of tribofilm on the pin after friction test at (a) 30 ◦ C and (b) −130 ◦ C.
Figure 14.5 TEM image of inorganic fullerenes showing their hollow nested structure (courtesy of Dr. Fleischer, Nanomaterials, Ltd.).
14.3
IF -WS2 COATINGS
In an analogy to a carbon onion consisting of concentric multilayered graphitic sheets, fullerene-like nanoparticles of MoS2 and WS2 , called IF-MS2 (M = Mo, W), were synthesized [11–13]. Subsequently, a large-scale synthesis has been reported [14], and new kinds of IF-MS2 were synthesized [15,16]. These nanoparticles are of particular interest for tribological applications. Indeed, their spherical shape, without dangling bonds, confers them a chemical inertness (Figure 14.5), and their hollow structure gives them a high elasticity [17]. Inorganic fullerenes exhibited better tribological properties than the lamellar structure when they were used as additives to lubricating base oils under boundary lubrication [18,19] or mixed lubrication [20]. Another application of IF was obtained by impregnating them into powder materials [21]. The lubrication performance was found to be based on an
Superlubricity of Tungsten Disulfide Coatings in Ultra High Vacuum
231
Figure 14.6 Friction coefficient of IF-WS2 and IF-MoS2 dispersed at 1 wt% in a PAO base oil (v = 2.5 mm/s, P = 1.12 GPa, T = 20 ◦ C). Friction coefficients did not decrease below 0.04.
Figure 14.7 Friction coefficients of MoS2 sputtered films in 45% humidity, MoS2 sputtered films in dry nitrogen and IF-MoS2 films in 45% humidity. A very low friction coefficient was obtained with IF-MoS2 films [22].
exfoliation mechanism of the IF into MS2 single sheets. The sheets generated could then be in incommensurate conditions [19]. However, the friction coefficient never went below 0.01 under these conditions (Figure 14.6). Chhowalla et al. [22] studied thin films of IF-MoS2 deposited by high-pressure arc discharge and observed superlow friction (friction coefficient below 0.01) in a nitrogen atmosphere and under 45% humidity (Figure 14.7). These films had better tribological properties than MoS2 sputtered films tested in the same conditions or in dry nitrogen. They attributed this behavior to the presence of curved MoS2 sheets that prevented oxidation of the sheets.
232
L. Joly-Pottuz and M. Iwaki
Figure 14.8 Friction coefficients obtained with IF-WS2 coatings at 25 ◦ C and −130 ◦ C. A very low friction coefficient (0.006) was obtained.
IF-WS2 coatings were deposited on AISI steel flats by a burnishing process to obtain a thickness of 200 nm. They were tested at 25 ◦ C and −130 ◦ C (Figure 14.8). A superlow friction coefficient of 0.006 was readily obtained after 45 cycles. At 25 ◦ C, the friction coefficient decreased to 0.006 at the beginning of the test and increased regularly until it reached a value of 0.02 after 500 cycles. At −130 ◦ C, the friction coefficient was not stable. High average values measured for friction coefficients are due to high values at the extremities of the wear track (Figure 14.9(a)). Figures 14.9(a) and 14.9(b) depict the direct recording of friction coefficients during one cycle (number 140) at −130 ◦ C and 25 ◦ C. Their comparison indicates that friction coefficients measured at these two temperatures are actually similar except at the end of the scars. The origin of these high values at the end of the scar is not easy to explain but could be attributed to difficulties of working in the same track while the temperature changes. An important point to be noticed is that no scar is observed optically on the flat after friction for these two tribological tests. This means that the increase in friction is not due to the wear of the coating. A transfer film could be observed on the hemispherical pin after the friction tests and was subjected to Raman analysis. This technique is a particularly powerful way to analyze WS2 and to distinguish the lamellar and IF structures (Figure 14.10). 2H–WS2 is crystallized in the D4 6h space group and has 18 modes of lattice vibrations, four of which are Raman active: E2 2g (27 cm−1 ), E1g (323.5 cm−1 ), E1 2g (356.5 cm−1 ) and A1g (420.9 cm−1 ) [23]. Several differences can be observed in the peak positions and intensities of the modes between the two spectra. Even if these differences are difficult to explain, the spectra can be considered as fingerprints for these two structures. A comparison of the spectrum obtained on the transfer film with the ones of pure 2H– WS2 and IF-WS2 reveals that it is composed of a mixture of 60% of 2H–WS2 and 40% of residual IF-WS2 (Figure 14.11). This is in good agreement with the lubrication mechanism of IF used as lubricant additives. IF-WS2 is exfoliated into small single sheets to form a tribofilm on the counterface. This exfoliation is caused by uniaxial pressure exerted over
Superlubricity of Tungsten Disulfide Coatings in Ultra High Vacuum
233
Figure 14.9 Friction coefficient measured at one cycle at −130 ◦ C (a) and 25 ◦ C (b). Friction coefficients measured were similar except at the end of the scar.
Figure 14.10 depicted.
Raman spectra of 2H–WS2 and IF-WS2 . Three characteristic Raman modes of 2H–WS2 are
the nanoparticles inside the contact and occurs even without shear [24]. Tribofilms formed during friction with IF-WS2 were studied using a surface force apparatus and atomic force microscopy (AFM) [25,26]. These analyses reveal that the surface is covered with very thin islands of WS2 well distributed on the surface. Only a few IF at the top of the coatings were exfoliated since we observed no wear scar at the end of the test. The thin tribofilm observed on the pin was sufficient to lead to a very low friction coefficient. Auger analyses confirmed the presence of a tribofilm made of WS2 on the pin after the friction test (Figure 14.12). A very small quantity of oxygen was detected, confirming the advantage of the nested structure that prevented the presence of
234
L. Joly-Pottuz and M. Iwaki
Figure 14.11 Raman analysis of the tribofilm observed after friction test. This spectrum is compared to those of 2H–WS2 and IF-WS2 . A simulation demonstrated that the tribofilm is a mixture of 60% of 2H–WS2 and 40% IF-WS2 .
Figure 14.12
Auger analysis of tribofilm after friction test at 25 ◦ C (a) and −130 ◦ C (b).
oxygen. In the case of the friction test at −130 ◦ C, the presence of carbon can be explained by the condensation of residual hydrocarbons on the cold surface. Transmission electron microscopy of the wear particles collected on the pin after the friction test revealed the presence of layers under incommensurate conditions (Figure 14.13). This could explain the very low friction coefficient measured. The same kind of particles had been observed by Martin et al.1 in the case of the friction of the MoS2 coatings [10] and with IF-WS2 dispersed in the base oil [19]. 1 See Chapter 13.
Superlubricity of Tungsten Disulfide Coatings in Ultra High Vacuum
235
Figure 14.13 HRTEM image of wear particle showing two superimposed layers. The corresponding calculated diffractogram indicates an angle of 30 ◦ between the layers.
14.4
CONCLUSIONS
The WS2 coatings presented very interesting properties in an ultra high vacuum especially at lower temperatures. Thus, they can be envisaged as lubricants for space applications. These favorable results were due to the formation of WS2 sheets on the counterface. The deposition method and the purity of the coatings influenced their properties. The use of IF-WS2 was advantageous in having pure WS2 coatings due to the curvature of the sheets in fullerenes, which preserve from the presence of oxygen. Temperature also influences the tribological properties of the WS2 coatings, and the lower the temperature, the lower is the friction coefficient. The results for IF-WS2 obtained at −130 ◦ C can be considered equal to the results at 25 ◦ C since the friction was high only at the end of the scar and because there was no wear influence. Further studies are necessary to really understand the lubrication mechanism of these coatings, which present very good potential for space applications.
ACKNOWLEDGEMENTS The authors would like to thank Thierry Le Mogne (Ecole Centrale de Lyon) for his support in the surface analyses (XPS, AES) and helpful discussions, and Dr. Niles Fleischer (Nanomaterials, Ltd.) for having supplied the IF-WS2 coatings.
REFERENCES [1] Jamison, W.E., Cosgrove, S.L. ASLE Trans. 14 (1971), 62. [2] Jamison, W.E. ASLE Trans. 15 (1972), 296.
236
L. Joly-Pottuz and M. Iwaki
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Bhushan, B., Gupta, B.K. Handbook of Tribology. McGraw-Hill, New York, 1991. Prasad, S.V., Zabinski, J.S., Dyhouse, V.J. J. Mat. Sci. Lett. 11 (1992), 1282. Prasad, S.V., Zabinski, J.S. J. Mat. Sci. Lett. 12 (1993) 1413. Le-Mogne, T., Martin, J.M., Grossiord, C. In: Symposium Leeds-Lyon. Lyon, 1998. Glasstone, S., Laidler, K.J., Eyring, H. Theory of Rate Processes. McGraw-Hill, New York, 1941. Overney, R.M., Tyndall, G., Frommer, J. Nanotechnology Handbook, 2004, Chapter 29. Briscoe, B.J., Evans, D.C.B. Proc. Roy. Soc. A 380 (1982), 389. Martin, J.M., Donnet, C., Le-Mogne, T., Epicier, T. Phys. Rev. B 48 (1993), 10583. Tenne, R., Margulis, L., Genut, M., Hodes, G. Nature 360 (1995), 444. Feldman, Y., Frey, G.L., Homyonfer, M., Lyakhovitskaya, V., Margulis, L., Cohen, H., Hodes, G., Hutchison, J.L., Tenne, R. J. Am. Chem. Soc. 118 (1996), 5362. Tenne, R., Homyonfer, M., Feldman, Y. Chemistry of Materials 10 (1998), 3225. Feldman, Y., Zak, A., Popovitz-Biro, R., Tenne, R. Solid State Sci. 2 (2000), 663. Schuffenhauer, C., Popovitz-Biro, R., Tenne, R. J. Mat. Chem. 12 (2002), 1587. Schuffenhauer, C., Parkinson, B.A., Jin-Phillip, N.Y., Joly-Pottuz, L., Martin, J.M., Popovitz-Biro, R., Tenne, R. Small 1 (2005), 1100. Rapoport, L., Feldman, Y., Homyonfer, M., Cohen, H., Sloan, J., Hutchison, J.L., Tenne, R. Wear 225–229 (1999), 975. Cizaire, L., Vacher, B., Le-Mogne, T., Martin, J.M., Rapoport, L., Margolin, A., Tenne, R. Surf. Coat. Technol. 160 (2002), 282. Joly-Pottuz, L., Dassenoy, F., Belin, M., Vacher, B., Martin, J.M., Fleischer, N. Tribol. Lett. 18 (2005), 477. Rapoport, L., Fleischer, N., Tenne, R. Adv. Mat. 15 (2003), 651. Rapoport, L., Leshchinsky, V., Lvovsky, M., Lapsker, I., Volovik, Y., Feldman, Y., Popovitz-Biro, R., Tenne, R. Wear 255 (2003), 794. Chhowalla, M., Amaratunga, G.A.J. Nature 407 (2000), 164. Sourisseau, C., Cruege, F., Fouassier, M., Alba, M. Chem. Phys. 150 (1991), 281. Joly-Pottuz, L., Martin, J.M., Dassenoy, F., Belin, M., Montagnac, G., Reynard, B., Fleischer, N. J. Appl. Phys. 99 (2006), 023524. Golan, Y., Drummond, C., Israelachvili, J., Tenne, R. Wear 245 (2000), 190. Drummond, C., Alcantar, N., Israelachvili, J., Tenne, R., Golan, Y. Adv. Funct. Mat. 11 (2001), 348.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
14.1
INTRODUCTION
Tungsten disulfide (WS2 ) is a solid lubricant with a lamellar structure similar to that of MoS2 . However, it has not been so widely studied according to the research literature. Since it possesses a crystal structure similar to that of MoS2 , it is expected to exhibit similar tribological properties including a state of near zero friction or superlubricity. Jamison compared the tribological properties of different metal dichalcogenides and concluded that metal dichalcogenides having the same structure as 2H–MoS2 possess good tribological properties [1]. He also correlated the tribological properties to the axial ratio c0 /na0 , which must exceed 1.87 in order to have a low friction coefficient. This ratio is equal to 1.95 for 2H–MoS2 and 1.96 for 2H–WS2 . In Figure 14.1, the metal elements surrounded by dashed lines form lamellar structures when bonded with sulfur or selenium, but only sulfides and selenides of molybdenum and tungsten have favorable tribological properties (elements surrounded by solid lines in Figure 14.1). By studying the electron distribution in these crystals, Jamison gave another explanation to the good tribological properties of MoS2 and WS2 [2]. In their structure, six nonbonding electrons fill a band and are confined in the structure. These electrons create a net positive charge on the surface layer, promoting easy shear through electrostatic repulsion. WS2 is thermally more stable and resistant to oxidation (about 50 to 100 ◦ C) than MoS2 [3]. The slow rate of oxidation of WS2 can be explained by the formation of tungsten trioxide (WO3 ), which is also known to provide a lower friction coefficient than molybdenum trioxide (MoO3 ). In a dry nitrogen environment, the steady-state friction coefficient of WS2 films grown by pulsed laser deposited on stainless steel against a steel counterface is about 0.04 [4]. A transfer film made of very thin WS2 sheets is observed on the pin side [5]. Analyzed by SEM, the sheets are thin enough (60 nm) to be transparent to the electron beam. Superlubricity Edited by A. Erdemir and J.-M. Martin © 2007 Published by Elsevier B.V.
228
L. Joly-Pottuz and M. Iwaki
Figure 14.1 Portion of the periodic table of elements showing metals that form lamellar structures (dashed line). Mo, W and Nb form lamellar structures with good tribological properties (solid line) [1].
In this chapter, we investigated the tribological properties of WS2 coatings or IF-WS2 coatings in an ultrahigh vacuum and at different temperatures (−130 to 200 ◦ C). Friction experiments were performed in an analytical ultra high vacuum tribometer [6]. This tribometer consists of a linear reciprocating pin-on-flat configuration installed directly inside an ultra high vacuum chamber. The system is equipped with traditional surface analysis techniques, X-ray Photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The pins were made of AISI 52100 steel with a radius of curvature of 4 mm. We used a normal load of 3 N on the pin leading to a maximum Hertzian contact pressure of 470 MPa. A very low friction coefficient was obtained with both types of coatings, indicating the very interesting tribological properties of WS2 coatings.
14.2
WS2 COATINGS
The samples prepared for the experiments were deposited on Si (100) substrates by an RF sputter deposition process with a thickness of 500 nm. Pin-on-flat experiments were conducted under an ultra high vacuum environment (1 × 10−7 Pa) with various temperatures ranging from −130 to 200 ◦ C. Figure 14.2 illustrates the friction profiles of WS2 coatings under these conditions. At room temperature (30 ◦ C), the friction coefficient was about 0.02. The friction coefficient then decreased with decreasing temperature, reaching 0.005 at −130 ◦ C, which is in the superlow friction regime. In contrast, the friction coefficient increased with increasing temperature at temperatures greater than 30 ◦ C. Thus, a linear relationship between the friction coefficient and the temperature can be seen (Figure 14.3). The relationship between the friction coefficient and the temperature was discussed in the 1940s by Eyring [7] then later summarized by Overney et al. [8]. Eyring’s thermalactivation model of lubricated friction predicts a linear relationship of friction and temperature, and Briscoe has experimentally verified this model for solid lubrication [9]. Our results confirm this relationship. Auger analysis (Figure 14.4) demonstrates that less carbon and oxygen can be seen on the pin tested at −130 ◦ C compared to that at room temperature (30 ◦ C), when the amount of sulfur is comparable. This means that a relatively “pure” transfer film of WSx is formed on the counterpart pin at −130 ◦ C. This is attributed to the fact that the chemical reaction is inhibited at lower temperatures.
Superlubricity of Tungsten Disulfide Coatings in Ultra High Vacuum
229
Figure 14.2 Friction profile of WS2 coating at various temperatures.
Figure 14.3 Steady-state friction coefficient of the WS2 coatings as a function of temperature. Line fitting was made by excluding the irregular data at 100 ◦ C and by plotting the line so that the friction coefficient is 0 at 0 K.
A pure and stoichiometric MoS2 coating that contains below 1% oxygen can exhibit an extraordinarily low friction coefficient [10]. However, the WS2 coating prepared for our experiment was considered to have at least several percent oxygen. Therefore, this result indicates that even a disulphide film without special preparation can achieve a superlow friction at cryogenic temperatures.
230
L. Joly-Pottuz and M. Iwaki
Figure 14.4 Auger spectra of tribofilm on the pin after friction test at (a) 30 ◦ C and (b) −130 ◦ C.
Figure 14.5 TEM image of inorganic fullerenes showing their hollow nested structure (courtesy of Dr. Fleischer, Nanomaterials, Ltd.).
14.3
IF -WS2 COATINGS
In an analogy to a carbon onion consisting of concentric multilayered graphitic sheets, fullerene-like nanoparticles of MoS2 and WS2 , called IF-MS2 (M = Mo, W), were synthesized [11–13]. Subsequently, a large-scale synthesis has been reported [14], and new kinds of IF-MS2 were synthesized [15,16]. These nanoparticles are of particular interest for tribological applications. Indeed, their spherical shape, without dangling bonds, confers them a chemical inertness (Figure 14.5), and their hollow structure gives them a high elasticity [17]. Inorganic fullerenes exhibited better tribological properties than the lamellar structure when they were used as additives to lubricating base oils under boundary lubrication [18,19] or mixed lubrication [20]. Another application of IF was obtained by impregnating them into powder materials [21]. The lubrication performance was found to be based on an
Superlubricity of Tungsten Disulfide Coatings in Ultra High Vacuum
231
Figure 14.6 Friction coefficient of IF-WS2 and IF-MoS2 dispersed at 1 wt% in a PAO base oil (v = 2.5 mm/s, P = 1.12 GPa, T = 20 ◦ C). Friction coefficients did not decrease below 0.04.
Figure 14.7 Friction coefficients of MoS2 sputtered films in 45% humidity, MoS2 sputtered films in dry nitrogen and IF-MoS2 films in 45% humidity. A very low friction coefficient was obtained with IF-MoS2 films [22].
exfoliation mechanism of the IF into MS2 single sheets. The sheets generated could then be in incommensurate conditions [19]. However, the friction coefficient never went below 0.01 under these conditions (Figure 14.6). Chhowalla et al. [22] studied thin films of IF-MoS2 deposited by high-pressure arc discharge and observed superlow friction (friction coefficient below 0.01) in a nitrogen atmosphere and under 45% humidity (Figure 14.7). These films had better tribological properties than MoS2 sputtered films tested in the same conditions or in dry nitrogen. They attributed this behavior to the presence of curved MoS2 sheets that prevented oxidation of the sheets.
232
L. Joly-Pottuz and M. Iwaki
Figure 14.8 Friction coefficients obtained with IF-WS2 coatings at 25 ◦ C and −130 ◦ C. A very low friction coefficient (0.006) was obtained.
IF-WS2 coatings were deposited on AISI steel flats by a burnishing process to obtain a thickness of 200 nm. They were tested at 25 ◦ C and −130 ◦ C (Figure 14.8). A superlow friction coefficient of 0.006 was readily obtained after 45 cycles. At 25 ◦ C, the friction coefficient decreased to 0.006 at the beginning of the test and increased regularly until it reached a value of 0.02 after 500 cycles. At −130 ◦ C, the friction coefficient was not stable. High average values measured for friction coefficients are due to high values at the extremities of the wear track (Figure 14.9(a)). Figures 14.9(a) and 14.9(b) depict the direct recording of friction coefficients during one cycle (number 140) at −130 ◦ C and 25 ◦ C. Their comparison indicates that friction coefficients measured at these two temperatures are actually similar except at the end of the scars. The origin of these high values at the end of the scar is not easy to explain but could be attributed to difficulties of working in the same track while the temperature changes. An important point to be noticed is that no scar is observed optically on the flat after friction for these two tribological tests. This means that the increase in friction is not due to the wear of the coating. A transfer film could be observed on the hemispherical pin after the friction tests and was subjected to Raman analysis. This technique is a particularly powerful way to analyze WS2 and to distinguish the lamellar and IF structures (Figure 14.10). 2H–WS2 is crystallized in the D4 6h space group and has 18 modes of lattice vibrations, four of which are Raman active: E2 2g (27 cm−1 ), E1g (323.5 cm−1 ), E1 2g (356.5 cm−1 ) and A1g (420.9 cm−1 ) [23]. Several differences can be observed in the peak positions and intensities of the modes between the two spectra. Even if these differences are difficult to explain, the spectra can be considered as fingerprints for these two structures. A comparison of the spectrum obtained on the transfer film with the ones of pure 2H– WS2 and IF-WS2 reveals that it is composed of a mixture of 60% of 2H–WS2 and 40% of residual IF-WS2 (Figure 14.11). This is in good agreement with the lubrication mechanism of IF used as lubricant additives. IF-WS2 is exfoliated into small single sheets to form a tribofilm on the counterface. This exfoliation is caused by uniaxial pressure exerted over
Superlubricity of Tungsten Disulfide Coatings in Ultra High Vacuum
233
Figure 14.9 Friction coefficient measured at one cycle at −130 ◦ C (a) and 25 ◦ C (b). Friction coefficients measured were similar except at the end of the scar.
Figure 14.10 depicted.
Raman spectra of 2H–WS2 and IF-WS2 . Three characteristic Raman modes of 2H–WS2 are
the nanoparticles inside the contact and occurs even without shear [24]. Tribofilms formed during friction with IF-WS2 were studied using a surface force apparatus and atomic force microscopy (AFM) [25,26]. These analyses reveal that the surface is covered with very thin islands of WS2 well distributed on the surface. Only a few IF at the top of the coatings were exfoliated since we observed no wear scar at the end of the test. The thin tribofilm observed on the pin was sufficient to lead to a very low friction coefficient. Auger analyses confirmed the presence of a tribofilm made of WS2 on the pin after the friction test (Figure 14.12). A very small quantity of oxygen was detected, confirming the advantage of the nested structure that prevented the presence of
234
L. Joly-Pottuz and M. Iwaki
Figure 14.11 Raman analysis of the tribofilm observed after friction test. This spectrum is compared to those of 2H–WS2 and IF-WS2 . A simulation demonstrated that the tribofilm is a mixture of 60% of 2H–WS2 and 40% IF-WS2 .
Figure 14.12
Auger analysis of tribofilm after friction test at 25 ◦ C (a) and −130 ◦ C (b).
oxygen. In the case of the friction test at −130 ◦ C, the presence of carbon can be explained by the condensation of residual hydrocarbons on the cold surface. Transmission electron microscopy of the wear particles collected on the pin after the friction test revealed the presence of layers under incommensurate conditions (Figure 14.13). This could explain the very low friction coefficient measured. The same kind of particles had been observed by Martin et al.1 in the case of the friction of the MoS2 coatings [10] and with IF-WS2 dispersed in the base oil [19]. 1 See Chapter 13.
Superlubricity of Tungsten Disulfide Coatings in Ultra High Vacuum
235
Figure 14.13 HRTEM image of wear particle showing two superimposed layers. The corresponding calculated diffractogram indicates an angle of 30 ◦ between the layers.
14.4
CONCLUSIONS
The WS2 coatings presented very interesting properties in an ultra high vacuum especially at lower temperatures. Thus, they can be envisaged as lubricants for space applications. These favorable results were due to the formation of WS2 sheets on the counterface. The deposition method and the purity of the coatings influenced their properties. The use of IF-WS2 was advantageous in having pure WS2 coatings due to the curvature of the sheets in fullerenes, which preserve from the presence of oxygen. Temperature also influences the tribological properties of the WS2 coatings, and the lower the temperature, the lower is the friction coefficient. The results for IF-WS2 obtained at −130 ◦ C can be considered equal to the results at 25 ◦ C since the friction was high only at the end of the scar and because there was no wear influence. Further studies are necessary to really understand the lubrication mechanism of these coatings, which present very good potential for space applications.
ACKNOWLEDGEMENTS The authors would like to thank Thierry Le Mogne (Ecole Centrale de Lyon) for his support in the surface analyses (XPS, AES) and helpful discussions, and Dr. Niles Fleischer (Nanomaterials, Ltd.) for having supplied the IF-WS2 coatings.
REFERENCES [1] Jamison, W.E., Cosgrove, S.L. ASLE Trans. 14 (1971), 62. [2] Jamison, W.E. ASLE Trans. 15 (1972), 296.
236
L. Joly-Pottuz and M. Iwaki
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Bhushan, B., Gupta, B.K. Handbook of Tribology. McGraw-Hill, New York, 1991. Prasad, S.V., Zabinski, J.S., Dyhouse, V.J. J. Mat. Sci. Lett. 11 (1992), 1282. Prasad, S.V., Zabinski, J.S. J. Mat. Sci. Lett. 12 (1993) 1413. Le-Mogne, T., Martin, J.M., Grossiord, C. In: Symposium Leeds-Lyon. Lyon, 1998. Glasstone, S., Laidler, K.J., Eyring, H. Theory of Rate Processes. McGraw-Hill, New York, 1941. Overney, R.M., Tyndall, G., Frommer, J. Nanotechnology Handbook, 2004, Chapter 29. Briscoe, B.J., Evans, D.C.B. Proc. Roy. Soc. A 380 (1982), 389. Martin, J.M., Donnet, C., Le-Mogne, T., Epicier, T. Phys. Rev. B 48 (1993), 10583. Tenne, R., Margulis, L., Genut, M., Hodes, G. Nature 360 (1995), 444. Feldman, Y., Frey, G.L., Homyonfer, M., Lyakhovitskaya, V., Margulis, L., Cohen, H., Hodes, G., Hutchison, J.L., Tenne, R. J. Am. Chem. Soc. 118 (1996), 5362. Tenne, R., Homyonfer, M., Feldman, Y. Chemistry of Materials 10 (1998), 3225. Feldman, Y., Zak, A., Popovitz-Biro, R., Tenne, R. Solid State Sci. 2 (2000), 663. Schuffenhauer, C., Popovitz-Biro, R., Tenne, R. J. Mat. Chem. 12 (2002), 1587. Schuffenhauer, C., Parkinson, B.A., Jin-Phillip, N.Y., Joly-Pottuz, L., Martin, J.M., Popovitz-Biro, R., Tenne, R. Small 1 (2005), 1100. Rapoport, L., Feldman, Y., Homyonfer, M., Cohen, H., Sloan, J., Hutchison, J.L., Tenne, R. Wear 225–229 (1999), 975. Cizaire, L., Vacher, B., Le-Mogne, T., Martin, J.M., Rapoport, L., Margolin, A., Tenne, R. Surf. Coat. Technol. 160 (2002), 282. Joly-Pottuz, L., Dassenoy, F., Belin, M., Vacher, B., Martin, J.M., Fleischer, N. Tribol. Lett. 18 (2005), 477. Rapoport, L., Fleischer, N., Tenne, R. Adv. Mat. 15 (2003), 651. Rapoport, L., Leshchinsky, V., Lvovsky, M., Lapsker, I., Volovik, Y., Feldman, Y., Popovitz-Biro, R., Tenne, R. Wear 255 (2003), 794. Chhowalla, M., Amaratunga, G.A.J. Nature 407 (2000), 164. Sourisseau, C., Cruege, F., Fouassier, M., Alba, M. Chem. Phys. 150 (1991), 281. Joly-Pottuz, L., Martin, J.M., Dassenoy, F., Belin, M., Montagnac, G., Reynard, B., Fleischer, N. J. Appl. Phys. 99 (2006), 023524. Golan, Y., Drummond, C., Israelachvili, J., Tenne, R. Wear 245 (2000), 190. Drummond, C., Alcantar, N., Israelachvili, J., Tenne, R., Golan, Y. Adv. Funct. Mat. 11 (2001), 348.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]