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Superlubricity of MoS2: crystal orientation mechanisms
Surface and Coatings Technology, 68/69 (1994) 427—432
427
Superlubricity of MoS2: crystal orientation mechanisms J. M. Martina, H. Pascala, C. Donneta, Th. Le Mognea, J. L. Loubeta, Th. Epicier” aEcole Centrale de Lyon. Laboratoire de Tribologie et Dynamique des Systèmes, URA CNRS 855, Dpt. de Technologie des Surfaces, BP 163, 69131 Ecully, France blnstitut National des Sciences Appliquees de Lyon, Groupe d’Etudes de Metallurgie Physique et de Physique des Matériaux, URA CNRS 341, 69621 Villeurbanne Cedex, France
Abstract We have investigated the origin of the extraordinary low friction coefficient (in the 10-s range or even less) ofpure and stoichiometric sputtered MoS2 coatings, in ultrahigh vacuum. In these conditions, shear strengths of the interface as low as 1 MPa were measured. Importantly, the tribometer was operating in macroscopic contact conditions, typically at a long time-length scale. Friction-induced orientation of (0001) basal planes of MoS, grains parallel to the sliding direction was first verified by means of electron diffraction. Friction-induced rotation of these crystals around the c axis, during intercrystallite slip in the contact, was investigated by high resolution transmission electron microscopy performed on selected wear fragments. Atomic force microscopy at atomic resolution was also carried out on the surface inside and outside the wear scar. The data indicated that the vanishing of the friction force was due to frictional anisotropy in the interface between nanometre-scale domains in rotational disorder (intercrystallite slippage of incommensurate sulphur-rich hexagonal lattices). The term superlubricity was used here because of the zero friction state that could be theoretically predicted in these conditions. Finally, the mechanisms of MoS, superlubricity are thought to depend on the proper combination of the grain size, the two crystal orientation effects and the absence of contaminants.
1. Introduction 1.1. Mechanisms of MoS2 lubrication in terms of crystal structure 1.1.1. Basal plane orientation It has been recognized early that, when sputtered MoS2 coatings are allowed to slide relative to each other, the crystallites reorient with their basal planes (within a few degrees) parallel to the direction of sliding [1,2]. The main technique that has been used to demonstrate this effect is grazing angle X-ray diffraction directly carried out on the worn surface. Another way is to examine cleaved or wear fragments by electron diffraction in the transmission electron microscope [3]. As demonstrated by Auger electron spectroscopy and low energy electron diffraction, the remarkable lubricating effect of MoS2 exists even in the case of films of nanometre-scale thickness [4]. This is an indication of a predominant surface rather than a bulk effect. 1.1.2. Intragranular shear The easy shear in the basal plane orientation is considered to be mainly responsible for the very low friction coefficient of MoS2 and other metal dichalcogenides. Recently, as a consequence of the basal plane orientation, the mechanism of intragranular shear between sulphur-rich atomic planes (S—S glide) has been
0257—8972/94/$7.00 SSDI 0257-8972(94)08019-U
analysed in detail from the point of view of the crystal structure by Takahashi and Okada [5], using high resolution transmission electron microscopy (HRTEM). It has been shown that the hexagonal—rhombohedral stacking transition could be associated with S—S glide in the (0001) plane through (a/2) [0110], just above the slip plane. Considering the characteristic crystal structure of MoS2, this transformation has been shown to occur reversibly during mechanical shearing of rhombohedral MoS2. This may provide an explanation of the easy glide of the lamellar compound in terms of successive inverse transformations, allowing slipping over relatively large distances. Dealing with the theoretical aspects of energy dissipation in sliding crystal surfaces, Sokoloff [6] has shown that, in the absence of dislocation movement, intragranular shear of MoS2 is unlikely to occur. The applied stress necessary is practically equivalent to the rupture strength of the material. 1.1.3. Intercrystallite slip Intercrystallite slip is an alternative to intragranular shear to explain the lubricating effect of MoS2 in the steady state regime, as recently pointed out by Hilton and Fleischauer [7]. Intercrystallite slip needs a transfer film to be formed first on the antagonist surface. Sokoloff [6] studied the slippage at the interface between two incommensurate lattices by solving the problem of the
© 1994
—
Elsevier Science S.A. All rights reserved
428
f.M. Martin et a!.
/
sliding of a crystal lattice whose surface interacts with a periodic potential of period incommensurate with the lattice periodicity. He found that the dissipative strength is a factor io’~ smaller than for a commensurate lattice, indicating a virtual vanishing of the friction force in this case. Shinjo and Hirano [8] recently showed that incommensurate surfaces are easily produced when two similar crystal lattices with a certain misfit angle are allowed to slide up on each other. In the case of hexagonal lattices, for example, it was pointed out that the misfit angle is 30°and the situation was referred to as frictional anisotropy. From the theoretical point of view, it can be shown that, in such a situation, “low friction trajectories” can be found along selected crystal directions and that, quantitatively, the friction force can completely vanish leading to the so-called zero friction or superlubricity [9]. Atomic-level simulations (molecular dynamic (MD) calculations for example) can also be used to study friction at the atomic level. MD simulations of friction between hydrogenated diamond surfaces, for example, lead to the same conclusion that zero friction is possible [10]. Therefore, during MoS2 basal plane friction-induced orientation, the effect of the individual crystal rotation around the c axis is important for both intragranular shear and intercrystallite slip. This aspect, however, has not been highlighted in the literature in order to explain the ultralow friction coefficient of sputtered coatings. In this study, we used two complementary techniques that give the atomic resolution: the crystal orientation ofMoS2grainsinthebulkwasinvestigatedbyHRTEM, performed on electron-transparent wear fragments. Furthermore, the surface of oriented MoS2 grains can be probed by atomic force microscopy (AFM) directly inside the wear track.
Superlubricity of MoS
2
data generation and logging can be found elsewhere [11,12]. The collection of wear fragments for HRTEM studies was made at the end of the test (100 cycles) as follows: first we observed the wear scar of the pin with an optical microscope and, approaching a holey carbon film mounted on a copper grid, wear debris surrounding the wear scar stuck naturally onto carbon. Special attention was paid to avoid rubbing the carbon film onto the wear particles. At low magnification in the transmission electron microscope, among the great amount of debris, some flake-like electron-transparent particles were selected for further investigation at high magnification in a 400 kV accelerating voltage microscope. For AFM imaging, we used an Si3N4 pyramidal tip, with a radius of curvature of 50 nm and a stiffness cantilever of 0.064 N m’. The normal load was set up to 1.0 RN and the effective contact force was measured to be 50.0 nN. The scanning frequency was 20 Hz. The scanning system was calibrated using on a cleaved sample of graphite (highly oriented pyrolytic graphite), where the nearest atomic distance is known to be 0.246 nm.
3. Results and discussion 3.1. Friction coefficient and shear strength of MoS2 Fig. 1 presents in vacuo friction curves as a function of time for MoS2 coatings (MoS2-ECL in the following). 100 cycles were run at a 0.3mm s~sliding speed on a 3 mm track length and corresponding to different normal loads. As a comparison, some friction curves obtained in similar tribological conditions but with commercial sputter-deposited MoS2 coatings originating from other 0.04
o.o~
2. Experimental details R.f. magnetron sputtering of MoS2 was performed on steel surfaces (AISI 52100) at ambient temperature, using a previously degassed MoS2 target. As already reported earlier [11], the MoS2 coating can be described as polycrystalline pure molybdenite (stoichiometric molybdenum disulphide) with a grain size of a few nanometres and a preferential basal plane orientation perpendicular to the surface (edge orientation). Friction tests were performed in ultrahigh vacuum conditions (1 nPa partial pressure) using a specially modified reciprocating pin-on-flat tribometer. The tribometer is capable of measuring very low friction coefficients (in the milhrange) even when macroscopic tribological conditions are applied on in situ elaborated pristine coatings. Details on the calibration procedures,
~ o.o~ ~ ~ 0
50
~
100
I ~C CS
Fig. I. Evolution of the friction coefficient of sputtered MoS2 coatings as a function ofthe number of cycles, in ultrahigh vacuum conditions. Comparison of in situ elaborated MoS2-ECL (~ A, A) with MoS2 coatings originating from other laboratories (~, A, •). For the different normal loads W, the shear strength t has been calculated at the end of the friction test A, W= 1 N, t= 1.8 MPa; •, W= 1 N, t= 21 MPa; A, W=1N, T~1.5MPa;A, W~lN, r=9.4MPa; A, W= 2 N, r=0.9 MPa; •, W=l N, r=6.6 MPa.
f.M. Martin et a!.
/
laboratories are also presented. Results clearly indicate that, in the steady state regime, the average friction coefficients of MoS2-ECL films are the smallest (generally below 5 x 10~) and that in some cases the tangential force was scarcely measurable (with the equipment at hand, forces below 2 mN corresponding to friction coefficients below iO~with a 2 N normal load are not detectable). It is interesting to examine the beginning of the friction curves, corresponding to the evolution between the first and four cycles approximately: the MoS2-ECL coatings give low friction immediately, except for test fl which had been previously in contact with humid air for one week. All the commercial coatings were in contact with air for a long time before the tests. This indicates that the contamination of the film by the environment (water vapour and oxygen) affects only the extreme surface and causes in vacuo friction to increase only over the first cycles. The differentiation between the coatings clearly appears ten cycles afterwards: our films maintain very low friction to reach the millirange (so-called superlow), whereas the other films develop a regular increase to reach the 10-2 range, which corresponds to classical values depicted in the literature [13]. Shear strengths of the interface films at the end of the test were calculated from the values of the friction coefficient and the hertzian contact pressure, using the model developed by Singer [14]. The results are also reported in Fig. 1 and show that there is approximately one order of magnitude between the two kinds of coatings: MoS2-ECL films produce interface shear strengths as low as 1 MPa. We deduce from these data that the extraordinary low friction of the films is not due to specific contact mechanics conditions but to material properties of the MoS2 interface film, as detailed in the following section. 3.2. High resolution transmission electron microscopy study ofwear fragments HRTEM is a powerful technique to determine the crystal structure changes at the atomic scale and to study the faults and the dislocations in the Mo52 structure. Lattice fringe HRTEM images show the atomic scale stacking of layers that are in good agreement with the calculated simulations. Takahashi and Shiojiri [15] have already performed a detailed study of the layer structure and stacking faults in MoS2 and WS2 powders using HRTEM. For this HRTEM study, wear debris were used from the end of the friction test ~/, where the friction coefficient stabilized around an average value of 0.003. Fig. 2 shows electron diffraction patterns in the selected area mode performetl in the transmission electron microscope and the corresponding high resolution images obtained for two different specimens: the original MoS2 coating
Superlubricity of MoS
2
429
(Fig. 2(a)), and the edge of a typical wear particle from the friction test (Fig. 2(b)). It is very clear that the original film is mainly edge oriented, with an azimuthal disorder of the c axis direction in the coating plane. The crystal structure is hexagonal and the grain size is approximately ten times the basal plane distance, i.e. 7 nm. As can be seen in Fig. 2(a), the MoS2 crystals have many defects: curled up atomic planes, dislocations, voids etc. Friction-induced orientation of basal planes in wear debris (corresponding to the absence of the (002) ring in the diffraction pattern) is clearly seen in Fig. 2(b). However, from the diffraction pattern of the same area, no recrystallization of MoS2 is visible in this experiment and the grain size is practically unchanged. The diffraction pattern of the wear debris indicates a disorder of the grain in the (100) and (110) directions. It is not evident at this stage whether the grains are superimposed or simply juxtaposed. If the grain are superimposed, Moire rotation patterns are expected in the HRTEM images. Fig. 3 represents an enlargement of the TEM image of the MoS2 wear particle of Fig. 2(b) and shows atomic resolution in very thin regions. In these working conditions, the correspondence between the electron image and the calculated projected potential has already been discussed earlier [12]. Fig. 3 indicates the presence of characteristic Moire patterns in some areas where the rotation angle can be easily measured by calculating the diffractograms of the digitized areas. It is interesting to observe that the atomic commensurability between two superimposed crystals seems to disappear when the rotation angle is near 30°,as predicted by the theoretical calculations of Hirano et al. [9]. Near 6°,for example, there are still several regions of atomic coincidence between the basal planes. Unfortunately, the rotation angles cannot be easily calculated if there are more than two superimposed stackings. Consequently, the analysis is limited to the edges of the particle where there is a distribution of the misfit angles. We believe that these data demonstrate the existence of frictional anisotropy between oriented MoS2 grains and are at the origin of the extraordinary low friction. As recently pointed out by Isshiki et al. [16] on WS2, the calculated images for [0110] hexagonal MoS2 show only (0002) lattice fringes and it is not possible to discriminate the stacking sequences. In contrast, the [2110] images can give very interesting information on the atomic configuration and on the arrangement of the sMos or SM0S columns. Particularly interesting is the alternate chevron-like image whose spots correspond to exact atom positions. For example, a change from the hexagonal to the rhombohedral structure will appear in the HRTEM image in regions where the same chevrons are repeated: the hexagonal packing appears as rows of right-side up and upside-down triangles aligned on the
430
f.M. Martin et a!.
/
Superlubricity of MoS
2
(002) (110) (100)
-
..~
a
9nm
b
Fig. 2. Friction-induced orientation of the basal planes of MoS2: (a) HRTEM micrograph and corresponding electron diffraction pattern of the pristine coating; (b) HRTEM micrograph and electron diffraction pattern of a flake-like wear particle. Note the absence of the (002) ring in the diffraction pattern of the wear particle and the fact that no preferential orientation effect is observed in the other directions (100) and (110).
Fig. 3. HRTEM micrograph of an MoS2 wear particle, showing atomic resolution. The electron beam is parallel to the c axis. The calculated diffractograms on the micrograph correspond to image frames showing characteristic Moire patterns. Zones where atomic coincidence has disappeared are correlated with a rotation angle of two superimposed crystals equal to about 30°.
f.M. Martin et al.
/
(0001) layer (a zigzag structure), whereas, in the rhombohedral order, the stacking sequence is composed of rows of triangles aligned in the same direction and the interface between the regions directly gives information on the stacking faults. Fig. 4 shows an HRTEM image of the edge of a curled wear particle which is oriented with the electron beam parallel to the basal plane. Atomic resolution can be observed in the S—Mo—S layer and, in some areas with good orientation, the chevronlike structure can be observed. This shows that some rhombohedral (3R) stacking can be detected and that intergranular shear in MoS2 grains might occur during the process. Unfortunately, the edge-oriented pristine coating does not allow this kind of imaging and we do not know whether this 3R stacking is present before the friction process. 3.3. Atomicforce microscopy in the wear track An AFM picture of the pristine coated surface is shown in Fig. 5(a), indicating that the as-grown MoS2 coating is not atomically smooth and that the grain size is in the 5 nm range, in good agreement with the TEM study. Tentatively, high resolution AFM images gave no clear atomic resolution of the termination of the S—Mo—S sandwiches at the surface. Fig. 5(b) shows a high resolution AFM image obtained inside the wear scar and the corresponding numerical diffractogram. The hexagonal symmetry of the atomic arrangement present on the surface is easily seen. The interatomic distance was calculated to be
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.
,
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Superlubricity of Mo5
2
431
0.316 nm + 0.003 nm, in agreement with a previous AFM work on cleaved molybdenite surfaces [17]. The image also suggests that a part of the crystal (in the upper left) is slightly disoriented, indicating the presence of a grain boundary. This strongly supports the idea that the sliding surface could be composed of a mosaic of atomically clean basal plane oriented grains, with azimuthal disorder around the c axis.
4. Conclusion The origin of the superlow friction of pure MoS2 coatings in ultrahigh vacuum conditions was studied from the point of view of crystal structure. The mechanisms of crystal orientation processes were investigated by means of electron diffraction in the transmission electron microscope, HRTEM and AFM. The results are as follows. (i) The pristine MoS2 film is polycrystalline and composed of grains whose sizes lie in the nanometre range. These grains are mostly edge oriented in our case, with azimuthal disorder of the c axis within the coating plane. (ii) Friction-induced orientation of the grains was found both on the rubbed surface and in most of the wear fragments, with the basal plane of the crystal structure parallel to the sliding direction. No grain size increase was observed during this process. (iii) Intragranular shear inside the Mo52 grain may
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mm Fig. 4. HRTFM micrograph of the edge of an MoS, wear particle, with the electron beam normal to the c axis. Details of the atomic arrangement in the S—Mo—S sandwishes can be observed. An area with the [2110] orientation depicting a rhombohedral stacking (chevrons aligned in the same direction) is also shown.
432
f.M. Martin et al.
/
2
particles although it could also originate from the original coating. (iv) Frictional anisotropy during intercrystallite slip was clearly identified in the interface products. The presence of incommensurate atomic basal planes in slippage conditions (rotation of approximately 30° between two superimposed crystals) has been experimentally established by HRTEM. Therefore, we have demonstrated that the conditions of the superlubric state (or zero friction) can be approached in some places of the sliding interface. Therefore, the drastic decrease in the friction force can be attributed to this phenomenon. More work is necessary to find the driving forces of these orientation mechanisms: do they depend on the original film structure (a memory effect), or are they induced by friction to minimize the energy dissipation during sliding?
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a
References
F.F.T.
_______________________________________
~
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Ii
nm
b
Fig. 5. AFM images of the MoS, coating: (a) low magnification image of the pristine MoS2 coating, showing the presence of grains; (b) high resolution image inside the wear scar giving evidence for basal plane orientation and the cleanliness of the exposed surface to friction.
occur via the hexagonal—rhombohedral phase transformation. Actually, some S—Mo—S stacking with the rhombohedral arrangement was observed in the wear
[11] [12] [13] [14]
[15] [16] [17]
E. W. Roberts, Tribol. mt., 23 (2) (1990) 95. P. D. Fleischauer and R. Bauer, Tribo!. Trans., 31(1988) 239. A. Mogami and A. Okitsu, Proc. Eurotrib 85 (1), 1985, p. 45. N. Takahashi and K. Okada, Proc. ASLE mt. Conf. on Solid Lubrication, Denver, CO, 1978, ASLE, New York, 1978, p. 14. N. Takahashi, Wear, 124 (1988) 279. J. B. Sokoloff, Phys. Rev. B, 42 (1) (1990) 760. M. R. Hilton and P. D. Fleischauer, J. Mater. Res., 5 (2) (1990) 406. K. Shinjo and M. Hirano, Surf. Sci., 283 (1993) 473. M. Hirano, K. Shinjo, R. Kaneko and Y. Murato, Phys. Rev. Lett., 67 (1991) 2642. J. A. Harrison, C. T. White, R. J. Colton and D. W. Brenner, Phys. Rev. B,46 (1992) 9700. C. Donnet, Tb. Le Mogne and J. M. Martin, Surf. Coat. Technol., 62 (1993) 406. J. M. Martin, C. Donnet and Tb. Le Mogne, Phys. Rev. B, 48 (14) (1993) 10583. T. Spalvins, J. Vac. Set. Technol. A,5 (1987) 212. I. L. Singer, in I. L. Singer and H. M. Pollock (eds.), Fundamentals in Friction: Macroscopic and Microscopic Processes, Kluwer, Dordrecht, 1992, p. 237. N. Takahashi and M. Shiojiri, Wear, 167 (1993) 163. T. Isshiki K. Nishio, I. Aoyagi, Y. Yabuuchi, N. Takahashi, H. Saijo and H. Shiojiri, Wear, 170 (1993) 55. C. M. Lieber and Y. Kim, Thin Solid Films, 206 (1991) 355.
427
Superlubricity of MoS2: crystal orientation mechanisms J. M. Martina, H. Pascala, C. Donneta, Th. Le Mognea, J. L. Loubeta, Th. Epicier” aEcole Centrale de Lyon. Laboratoire de Tribologie et Dynamique des Systèmes, URA CNRS 855, Dpt. de Technologie des Surfaces, BP 163, 69131 Ecully, France blnstitut National des Sciences Appliquees de Lyon, Groupe d’Etudes de Metallurgie Physique et de Physique des Matériaux, URA CNRS 341, 69621 Villeurbanne Cedex, France
Abstract We have investigated the origin of the extraordinary low friction coefficient (in the 10-s range or even less) ofpure and stoichiometric sputtered MoS2 coatings, in ultrahigh vacuum. In these conditions, shear strengths of the interface as low as 1 MPa were measured. Importantly, the tribometer was operating in macroscopic contact conditions, typically at a long time-length scale. Friction-induced orientation of (0001) basal planes of MoS, grains parallel to the sliding direction was first verified by means of electron diffraction. Friction-induced rotation of these crystals around the c axis, during intercrystallite slip in the contact, was investigated by high resolution transmission electron microscopy performed on selected wear fragments. Atomic force microscopy at atomic resolution was also carried out on the surface inside and outside the wear scar. The data indicated that the vanishing of the friction force was due to frictional anisotropy in the interface between nanometre-scale domains in rotational disorder (intercrystallite slippage of incommensurate sulphur-rich hexagonal lattices). The term superlubricity was used here because of the zero friction state that could be theoretically predicted in these conditions. Finally, the mechanisms of MoS, superlubricity are thought to depend on the proper combination of the grain size, the two crystal orientation effects and the absence of contaminants.
1. Introduction 1.1. Mechanisms of MoS2 lubrication in terms of crystal structure 1.1.1. Basal plane orientation It has been recognized early that, when sputtered MoS2 coatings are allowed to slide relative to each other, the crystallites reorient with their basal planes (within a few degrees) parallel to the direction of sliding [1,2]. The main technique that has been used to demonstrate this effect is grazing angle X-ray diffraction directly carried out on the worn surface. Another way is to examine cleaved or wear fragments by electron diffraction in the transmission electron microscope [3]. As demonstrated by Auger electron spectroscopy and low energy electron diffraction, the remarkable lubricating effect of MoS2 exists even in the case of films of nanometre-scale thickness [4]. This is an indication of a predominant surface rather than a bulk effect. 1.1.2. Intragranular shear The easy shear in the basal plane orientation is considered to be mainly responsible for the very low friction coefficient of MoS2 and other metal dichalcogenides. Recently, as a consequence of the basal plane orientation, the mechanism of intragranular shear between sulphur-rich atomic planes (S—S glide) has been
0257—8972/94/$7.00 SSDI 0257-8972(94)08019-U
analysed in detail from the point of view of the crystal structure by Takahashi and Okada [5], using high resolution transmission electron microscopy (HRTEM). It has been shown that the hexagonal—rhombohedral stacking transition could be associated with S—S glide in the (0001) plane through (a/2) [0110], just above the slip plane. Considering the characteristic crystal structure of MoS2, this transformation has been shown to occur reversibly during mechanical shearing of rhombohedral MoS2. This may provide an explanation of the easy glide of the lamellar compound in terms of successive inverse transformations, allowing slipping over relatively large distances. Dealing with the theoretical aspects of energy dissipation in sliding crystal surfaces, Sokoloff [6] has shown that, in the absence of dislocation movement, intragranular shear of MoS2 is unlikely to occur. The applied stress necessary is practically equivalent to the rupture strength of the material. 1.1.3. Intercrystallite slip Intercrystallite slip is an alternative to intragranular shear to explain the lubricating effect of MoS2 in the steady state regime, as recently pointed out by Hilton and Fleischauer [7]. Intercrystallite slip needs a transfer film to be formed first on the antagonist surface. Sokoloff [6] studied the slippage at the interface between two incommensurate lattices by solving the problem of the
© 1994
—
Elsevier Science S.A. All rights reserved
428
f.M. Martin et a!.
/
sliding of a crystal lattice whose surface interacts with a periodic potential of period incommensurate with the lattice periodicity. He found that the dissipative strength is a factor io’~ smaller than for a commensurate lattice, indicating a virtual vanishing of the friction force in this case. Shinjo and Hirano [8] recently showed that incommensurate surfaces are easily produced when two similar crystal lattices with a certain misfit angle are allowed to slide up on each other. In the case of hexagonal lattices, for example, it was pointed out that the misfit angle is 30°and the situation was referred to as frictional anisotropy. From the theoretical point of view, it can be shown that, in such a situation, “low friction trajectories” can be found along selected crystal directions and that, quantitatively, the friction force can completely vanish leading to the so-called zero friction or superlubricity [9]. Atomic-level simulations (molecular dynamic (MD) calculations for example) can also be used to study friction at the atomic level. MD simulations of friction between hydrogenated diamond surfaces, for example, lead to the same conclusion that zero friction is possible [10]. Therefore, during MoS2 basal plane friction-induced orientation, the effect of the individual crystal rotation around the c axis is important for both intragranular shear and intercrystallite slip. This aspect, however, has not been highlighted in the literature in order to explain the ultralow friction coefficient of sputtered coatings. In this study, we used two complementary techniques that give the atomic resolution: the crystal orientation ofMoS2grainsinthebulkwasinvestigatedbyHRTEM, performed on electron-transparent wear fragments. Furthermore, the surface of oriented MoS2 grains can be probed by atomic force microscopy (AFM) directly inside the wear track.
Superlubricity of MoS
2
data generation and logging can be found elsewhere [11,12]. The collection of wear fragments for HRTEM studies was made at the end of the test (100 cycles) as follows: first we observed the wear scar of the pin with an optical microscope and, approaching a holey carbon film mounted on a copper grid, wear debris surrounding the wear scar stuck naturally onto carbon. Special attention was paid to avoid rubbing the carbon film onto the wear particles. At low magnification in the transmission electron microscope, among the great amount of debris, some flake-like electron-transparent particles were selected for further investigation at high magnification in a 400 kV accelerating voltage microscope. For AFM imaging, we used an Si3N4 pyramidal tip, with a radius of curvature of 50 nm and a stiffness cantilever of 0.064 N m’. The normal load was set up to 1.0 RN and the effective contact force was measured to be 50.0 nN. The scanning frequency was 20 Hz. The scanning system was calibrated using on a cleaved sample of graphite (highly oriented pyrolytic graphite), where the nearest atomic distance is known to be 0.246 nm.
3. Results and discussion 3.1. Friction coefficient and shear strength of MoS2 Fig. 1 presents in vacuo friction curves as a function of time for MoS2 coatings (MoS2-ECL in the following). 100 cycles were run at a 0.3mm s~sliding speed on a 3 mm track length and corresponding to different normal loads. As a comparison, some friction curves obtained in similar tribological conditions but with commercial sputter-deposited MoS2 coatings originating from other 0.04
o.o~
2. Experimental details R.f. magnetron sputtering of MoS2 was performed on steel surfaces (AISI 52100) at ambient temperature, using a previously degassed MoS2 target. As already reported earlier [11], the MoS2 coating can be described as polycrystalline pure molybdenite (stoichiometric molybdenum disulphide) with a grain size of a few nanometres and a preferential basal plane orientation perpendicular to the surface (edge orientation). Friction tests were performed in ultrahigh vacuum conditions (1 nPa partial pressure) using a specially modified reciprocating pin-on-flat tribometer. The tribometer is capable of measuring very low friction coefficients (in the milhrange) even when macroscopic tribological conditions are applied on in situ elaborated pristine coatings. Details on the calibration procedures,
~ o.o~ ~ ~ 0
50
~
100
I ~C CS
Fig. I. Evolution of the friction coefficient of sputtered MoS2 coatings as a function ofthe number of cycles, in ultrahigh vacuum conditions. Comparison of in situ elaborated MoS2-ECL (~ A, A) with MoS2 coatings originating from other laboratories (~, A, •). For the different normal loads W, the shear strength t has been calculated at the end of the friction test A, W= 1 N, t= 1.8 MPa; •, W= 1 N, t= 21 MPa; A, W=1N, T~1.5MPa;A, W~lN, r=9.4MPa; A, W= 2 N, r=0.9 MPa; •, W=l N, r=6.6 MPa.
f.M. Martin et a!.
/
laboratories are also presented. Results clearly indicate that, in the steady state regime, the average friction coefficients of MoS2-ECL films are the smallest (generally below 5 x 10~) and that in some cases the tangential force was scarcely measurable (with the equipment at hand, forces below 2 mN corresponding to friction coefficients below iO~with a 2 N normal load are not detectable). It is interesting to examine the beginning of the friction curves, corresponding to the evolution between the first and four cycles approximately: the MoS2-ECL coatings give low friction immediately, except for test fl which had been previously in contact with humid air for one week. All the commercial coatings were in contact with air for a long time before the tests. This indicates that the contamination of the film by the environment (water vapour and oxygen) affects only the extreme surface and causes in vacuo friction to increase only over the first cycles. The differentiation between the coatings clearly appears ten cycles afterwards: our films maintain very low friction to reach the millirange (so-called superlow), whereas the other films develop a regular increase to reach the 10-2 range, which corresponds to classical values depicted in the literature [13]. Shear strengths of the interface films at the end of the test were calculated from the values of the friction coefficient and the hertzian contact pressure, using the model developed by Singer [14]. The results are also reported in Fig. 1 and show that there is approximately one order of magnitude between the two kinds of coatings: MoS2-ECL films produce interface shear strengths as low as 1 MPa. We deduce from these data that the extraordinary low friction of the films is not due to specific contact mechanics conditions but to material properties of the MoS2 interface film, as detailed in the following section. 3.2. High resolution transmission electron microscopy study ofwear fragments HRTEM is a powerful technique to determine the crystal structure changes at the atomic scale and to study the faults and the dislocations in the Mo52 structure. Lattice fringe HRTEM images show the atomic scale stacking of layers that are in good agreement with the calculated simulations. Takahashi and Shiojiri [15] have already performed a detailed study of the layer structure and stacking faults in MoS2 and WS2 powders using HRTEM. For this HRTEM study, wear debris were used from the end of the friction test ~/, where the friction coefficient stabilized around an average value of 0.003. Fig. 2 shows electron diffraction patterns in the selected area mode performetl in the transmission electron microscope and the corresponding high resolution images obtained for two different specimens: the original MoS2 coating
Superlubricity of MoS
2
429
(Fig. 2(a)), and the edge of a typical wear particle from the friction test (Fig. 2(b)). It is very clear that the original film is mainly edge oriented, with an azimuthal disorder of the c axis direction in the coating plane. The crystal structure is hexagonal and the grain size is approximately ten times the basal plane distance, i.e. 7 nm. As can be seen in Fig. 2(a), the MoS2 crystals have many defects: curled up atomic planes, dislocations, voids etc. Friction-induced orientation of basal planes in wear debris (corresponding to the absence of the (002) ring in the diffraction pattern) is clearly seen in Fig. 2(b). However, from the diffraction pattern of the same area, no recrystallization of MoS2 is visible in this experiment and the grain size is practically unchanged. The diffraction pattern of the wear debris indicates a disorder of the grain in the (100) and (110) directions. It is not evident at this stage whether the grains are superimposed or simply juxtaposed. If the grain are superimposed, Moire rotation patterns are expected in the HRTEM images. Fig. 3 represents an enlargement of the TEM image of the MoS2 wear particle of Fig. 2(b) and shows atomic resolution in very thin regions. In these working conditions, the correspondence between the electron image and the calculated projected potential has already been discussed earlier [12]. Fig. 3 indicates the presence of characteristic Moire patterns in some areas where the rotation angle can be easily measured by calculating the diffractograms of the digitized areas. It is interesting to observe that the atomic commensurability between two superimposed crystals seems to disappear when the rotation angle is near 30°,as predicted by the theoretical calculations of Hirano et al. [9]. Near 6°,for example, there are still several regions of atomic coincidence between the basal planes. Unfortunately, the rotation angles cannot be easily calculated if there are more than two superimposed stackings. Consequently, the analysis is limited to the edges of the particle where there is a distribution of the misfit angles. We believe that these data demonstrate the existence of frictional anisotropy between oriented MoS2 grains and are at the origin of the extraordinary low friction. As recently pointed out by Isshiki et al. [16] on WS2, the calculated images for [0110] hexagonal MoS2 show only (0002) lattice fringes and it is not possible to discriminate the stacking sequences. In contrast, the [2110] images can give very interesting information on the atomic configuration and on the arrangement of the sMos or SM0S columns. Particularly interesting is the alternate chevron-like image whose spots correspond to exact atom positions. For example, a change from the hexagonal to the rhombohedral structure will appear in the HRTEM image in regions where the same chevrons are repeated: the hexagonal packing appears as rows of right-side up and upside-down triangles aligned on the
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Fig. 2. Friction-induced orientation of the basal planes of MoS2: (a) HRTEM micrograph and corresponding electron diffraction pattern of the pristine coating; (b) HRTEM micrograph and electron diffraction pattern of a flake-like wear particle. Note the absence of the (002) ring in the diffraction pattern of the wear particle and the fact that no preferential orientation effect is observed in the other directions (100) and (110).
Fig. 3. HRTEM micrograph of an MoS2 wear particle, showing atomic resolution. The electron beam is parallel to the c axis. The calculated diffractograms on the micrograph correspond to image frames showing characteristic Moire patterns. Zones where atomic coincidence has disappeared are correlated with a rotation angle of two superimposed crystals equal to about 30°.
f.M. Martin et al.
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(0001) layer (a zigzag structure), whereas, in the rhombohedral order, the stacking sequence is composed of rows of triangles aligned in the same direction and the interface between the regions directly gives information on the stacking faults. Fig. 4 shows an HRTEM image of the edge of a curled wear particle which is oriented with the electron beam parallel to the basal plane. Atomic resolution can be observed in the S—Mo—S layer and, in some areas with good orientation, the chevronlike structure can be observed. This shows that some rhombohedral (3R) stacking can be detected and that intergranular shear in MoS2 grains might occur during the process. Unfortunately, the edge-oriented pristine coating does not allow this kind of imaging and we do not know whether this 3R stacking is present before the friction process. 3.3. Atomicforce microscopy in the wear track An AFM picture of the pristine coated surface is shown in Fig. 5(a), indicating that the as-grown MoS2 coating is not atomically smooth and that the grain size is in the 5 nm range, in good agreement with the TEM study. Tentatively, high resolution AFM images gave no clear atomic resolution of the termination of the S—Mo—S sandwiches at the surface. Fig. 5(b) shows a high resolution AFM image obtained inside the wear scar and the corresponding numerical diffractogram. The hexagonal symmetry of the atomic arrangement present on the surface is easily seen. The interatomic distance was calculated to be
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0.316 nm + 0.003 nm, in agreement with a previous AFM work on cleaved molybdenite surfaces [17]. The image also suggests that a part of the crystal (in the upper left) is slightly disoriented, indicating the presence of a grain boundary. This strongly supports the idea that the sliding surface could be composed of a mosaic of atomically clean basal plane oriented grains, with azimuthal disorder around the c axis.
4. Conclusion The origin of the superlow friction of pure MoS2 coatings in ultrahigh vacuum conditions was studied from the point of view of crystal structure. The mechanisms of crystal orientation processes were investigated by means of electron diffraction in the transmission electron microscope, HRTEM and AFM. The results are as follows. (i) The pristine MoS2 film is polycrystalline and composed of grains whose sizes lie in the nanometre range. These grains are mostly edge oriented in our case, with azimuthal disorder of the c axis within the coating plane. (ii) Friction-induced orientation of the grains was found both on the rubbed surface and in most of the wear fragments, with the basal plane of the crystal structure parallel to the sliding direction. No grain size increase was observed during this process. (iii) Intragranular shear inside the Mo52 grain may
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particles although it could also originate from the original coating. (iv) Frictional anisotropy during intercrystallite slip was clearly identified in the interface products. The presence of incommensurate atomic basal planes in slippage conditions (rotation of approximately 30° between two superimposed crystals) has been experimentally established by HRTEM. Therefore, we have demonstrated that the conditions of the superlubric state (or zero friction) can be approached in some places of the sliding interface. Therefore, the drastic decrease in the friction force can be attributed to this phenomenon. More work is necessary to find the driving forces of these orientation mechanisms: do they depend on the original film structure (a memory effect), or are they induced by friction to minimize the energy dissipation during sliding?
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References
F.F.T.
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[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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nm
b
Fig. 5. AFM images of the MoS, coating: (a) low magnification image of the pristine MoS2 coating, showing the presence of grains; (b) high resolution image inside the wear scar giving evidence for basal plane orientation and the cleanliness of the exposed surface to friction.
occur via the hexagonal—rhombohedral phase transformation. Actually, some S—Mo—S stacking with the rhombohedral arrangement was observed in the wear
[11] [12] [13] [14]
[15] [16] [17]
E. W. Roberts, Tribol. mt., 23 (2) (1990) 95. P. D. Fleischauer and R. Bauer, Tribo!. Trans., 31(1988) 239. A. Mogami and A. Okitsu, Proc. Eurotrib 85 (1), 1985, p. 45. N. Takahashi and K. Okada, Proc. ASLE mt. Conf. on Solid Lubrication, Denver, CO, 1978, ASLE, New York, 1978, p. 14. N. Takahashi, Wear, 124 (1988) 279. J. B. Sokoloff, Phys. Rev. B, 42 (1) (1990) 760. M. R. Hilton and P. D. Fleischauer, J. Mater. Res., 5 (2) (1990) 406. K. Shinjo and M. Hirano, Surf. Sci., 283 (1993) 473. M. Hirano, K. Shinjo, R. Kaneko and Y. Murato, Phys. Rev. Lett., 67 (1991) 2642. J. A. Harrison, C. T. White, R. J. Colton and D. W. Brenner, Phys. Rev. B,46 (1992) 9700. C. Donnet, Tb. Le Mogne and J. M. Martin, Surf. Coat. Technol., 62 (1993) 406. J. M. Martin, C. Donnet and Tb. Le Mogne, Phys. Rev. B, 48 (14) (1993) 10583. T. Spalvins, J. Vac. Set. Technol. A,5 (1987) 212. I. L. Singer, in I. L. Singer and H. M. Pollock (eds.), Fundamentals in Friction: Macroscopic and Microscopic Processes, Kluwer, Dordrecht, 1992, p. 237. N. Takahashi and M. Shiojiri, Wear, 167 (1993) 163. T. Isshiki K. Nishio, I. Aoyagi, Y. Yabuuchi, N. Takahashi, H. Saijo and H. Shiojiri, Wear, 170 (1993) 55. C. M. Lieber and Y. Kim, Thin Solid Films, 206 (1991) 355.