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Super-low friction of MoS2 coatings in various environments
Tribology
ELSEVIER SCIENCE:
0301-679X(95)00094-1
International Vol. 29, No. 2. pp. 123-128, 1996 Copyright @ 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0301-679X1961315.00
Super-low friction of MoS, coatings in various environments C. Donnet,
J. M. Martin,
Th. Le Mogne
and M. Belin
The ultra-low friction coefficient (typically in the lo-* range) of MoS,-based coatings is generally associated with the frictioninduced orientation of ‘easy-shear’ planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used an AES/XPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing the deposition of oxygen-free MoS, PVD coatings and the performance of friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS, coatings deposited on AISI 52100 steel was studied in ultrahigh vacuum (UHV: 5 x lop8 Pa), high vacuum (HV: lop3 Pa), dry nitrogen (lo5 Pa) and ambient air (lo5 Pa). ‘Super-low’ friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to a calculated interfacial shear strength in the range of 1 MPa, about ten times lower than for standard coatings. Low friction coefficients of about 0.013-0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger electron spectroscopy shows no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the ‘super-low’ friction behaviour (lop3 range) can be attributed to superlubricity, obtained for a oarticular combination of cvstalloaraphic orientation and the absence of contaminants, leading’to a c&&derable decrease in the inter-facial shear strength. Keywords: gaseous
MO&, vacuum, environment
super-low
friction,
thin film, so/id lubricant,
Introduction Molybdenum disulphide (MO&) is a well-known lamellar solid lubricant with a hexagonal structure. Extensive surveys dealing with the tribological behaviour of
Laboratoire de Tribologie et Dynamique des Systkmes, URA CNRS 8.55, hole Centrale de Lyon, B.P. 163, F-69131 l?culiy Cedex, France. Received 15 December 1993; revised 24 March 1994; accepted 4 May 1995
Tribology
sputter-deposited MoS, coatings exist in the literature1-4. Ultra-low friction of MO& coatings has been identified when running friction tests in the absence of oxygen and/or water vapour. In such conditions, and denending on the normal load in the sliding contact, friction cogfficient values between 0.01 and 0.05 have been measured, which already represent uncommon values in solid film lubrication. The mechanisms of ultra-low friction of MO& (typically observed in dry nitrogen or in high vacuum) can be summarized with the three following conditions*: International
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2 1996
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Super-low
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of MO!?,: C. Donnet
et al.
According to the Hertzian theory, the friction coefficient in the sphere-on-plane configuration below the elastic limit depends on three variableslo as:
Notation AES HV PVD RF TEM UHV XPS
Auger electron spectroscopy high vacuum physical vapour deposition radio frequency transmission electron microscopy ultra-high vacuum X-ray photoelectron spectroscopy
p = So.~.(3.R/4.E)2f3.W-1J3
the built-up of a MoS2 transfer film onto the frictional counterface. This transfer film is formed due to good adhesion MO& to the opposing material; 0 a friction-induced orientation of the (0001) basal planes of the MoSz grains in the interfacing region, parallel to the sliding direction (in the transfer film, the film itself and eventually the third body in general). It is anticipated that friction-induced basal plane orientation occurs very early at the beginning of sliding, by a simultaneous orientation-switching of all the individual nanometre-scale MO& grains in the interface; l the absence of contaminants. Carbon and oxygen are well-known contaminants of MO&, coming from the residual gas during the sputtering process or from the MO& target. Special attention must be paid to the effect of water vapour coming from the ambient air or from a humid environment during storage, for example. It has been suggested that liquid water could be formed by capillary condensation of vapour in the defects of the MoS2 crystal structure and that water could then modify the easy shear between basal plane9. l
Recently, using a dedicated ultra-high vacuum analytical tribometer, a ‘super-low’ friction behaviour has been identified (average friction coefficient in the 10e3 range) when testing in an ultra-high vacuum a sputtered MO& coating free from impurities such as carbon, oxygen and water vapour. In some cases, the recorded tangential force was hardly detectable as if the friction force had completely vanished. Although the assputtered grain size was not modified by the tribological process, friction-induced orientation of the MO& grains in the contact interface was clearly demonstrated by electron diffraction and high resolution TEM studies, carried out on selected wear debris collected at the end of the friction test6r7. Consequently the three conditions listed above are verified and thus are not sufficient alone to explain the origin of the ‘superlow’ friction behaviour. The Bowden and Tabor model of friction provides a good starting point for understanding how a thin solid film can reduce frictions. The friction coefficient is assumed to depend on the normal load W, the real area of contact A and the shear strength S of the inter-facial film as: p = S.AIW
(1) The shear strength S of solids at high pressure has been observed to have a pressure dependence, which can be approximated by9: s = so + ff.P 124
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(3)
where E is the composite elastic modulus of the contacting materials, and R is the radius of the ball. This model assumes that the real contact area corresponds to the Hertzian zone, as calculated in Eq. (3). This assumption is justified by the soft behaviour of MoS2 film and debris in the contact, leading to a good accommodation in the contact geometry. It has been proven to be valid for thin MoS2 coatingslo. Therefore, friction measurements can be used to determine So and (Y values : friction of MoS2 in dry air leads to So = 25 MPa and (Y = O.OO1lo, friction in vacuum leads to So = 7 MPa and (Y = O.OO1ll. From these data, it appears that MoS, coatings have So values which depend on the atmosphere during friction, since the presence of oxygen (in dry air) increases the shear strength of the film. These results suggest that surface chemistry due to atmospheric conditions during friction processes is of great importance in the tribological performance and mechanical characteristics of MoS2 thin films. Taking into account this approach, different questions are raised, in order to go further in the understanding of the friction mechanism of these coatings: l
l
How do different kinds of atmosphere (ultra-high vacuum, poor vacuum, nitrogen and air) alter the friction behaviour of MoS2 for a given structure and film composition? According to Eq. (l), what are the respective contributions of the shear strength S and the real area of contact A in the different levels of friction observed in various atmospheres? In particular, is a ‘super-low’ friction behaviour due to a low value of the shear strength S or to a reduction of the real contact area A?
Wheelerl* recently suggested that a partial pressure of pure oxygen could reduce the friction by influencing the transfer of interfacial material to the pin in such a way as to reduce the area of contact. As an attempt to progress further in this field, we performed tribological tests (pin-on-disc configuration) on pure and stoichiometric sputter-deposited MoS2 coatings in various controlled atmospheres, at a given contact pressure. Experiments were carried out using an ultrahigh vacuum analytical tribometer coupled with a preparation chamber, which has already been described in detail13. With this apparatus, sputter-deposited MoS2 thin films were obtained on steel and analysed by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy ( AES). Chemical analyses were performed by AES inside and outside the wear tracks after each test carried out in the different atmospheres. Optical micrographs gave an estimation of the real areas of contact in the sliding contact, thus allowing the determination of the shear strength S, depending on the atmosphere during friction. In the following experiments, the normal load, the pin radius and thus the contact pressure were not changed, because of technical difficulties in the UHV tribometer. Conse2 1996
Super-low
quently, the computed values of S,, S and fy cannot be determined to a high degree of precision.
Friction measurements atmospheres Elaboration coatings
in various
and characterization
of MoS,
Radio-frequency magnetron sputtering of MO& was performed on a cleaned bearing steel (AISI 52100) surface at room temperature, using a previously degassed MO& target. A 120 nm thick film was obtained by sputtering MO& at 7.4 W/cm2 with a current intensity of 300 mA. An in situ analysis of the coating was performed by XPS’. The Mo3d5/2 and S2p core level peaks were respectively centred at 228.8 -+ 0.2 eV and 162.0 2 0.2 eV, in agreement with already published values corresponding to MoS214. The chemical composition of the film was calculated using the Mo3d3/2 and S2p peak intensities and sensitivity factors published by Briggs et a1.15. The experimental S : MO ratio was 2.04, thus suggesting the film to be nearly stoichiometric. Auger electron spectroscopy analyses did not show any trace of contamination elements such as carbon or oxygen, within the detection limits of the technique. The TEM analysis of the coating has been already published”.
Tribologicai
friction
of MO&:
C. Donnet
et a/.
conditions
The friction measurements were carried out using a reciprocating pin-on-flat tribometer. The calibration procedure and the sensitivity of the force measurement device has already been presented in detai16. The tribological parameters were the following: using an AISI 52100 steel hemispherical pin (4 mm radius of curvature) as a hard material, and several identical MoS,-on-steel coatings, we performed 100 cycle-tests with a normal load of 1.0 N, corresponding to a mean contact pressure of 0.37 GPa, a linear sliding speed of 0.5 mm/s and a 3 mm wear track length. Tribological tests were performed in various kinds of controlled atmosphere: l l l
l
ultra-high vacuum (UHV) 5 x lo-* Pa high vacuum (HV) 10-j Pa dry nitrogen (d-N,) lo5 Pa, relative RH < 1% ambient air (at-Air) lo5 Pa, relative RH = 40%
Effect of atmosphere
on friction
humidity humidity
coefficient
We show the typical evolution of the average friction coefficient, as a function of the number of cycles in UHV, in HV and in d-N2 (Fig 1). These results have a very good repeatability. In both UHV and d-N* conditions, the friction coefficient begins at 0.01 but drastically decreases to 0.001-0.003 a few cycles later.
0.020 /
10
20
30
40
50
60
70
80
90
100
Fig 1 Average friction coejjkient versus number of cycles of a MO,!& coating tested in ultra-high vacuum (CJHV: 5 x lOpa Pa), high vacuum (HV: lop3 Pa) and dry nitrogen (d-N,: 105 Pa). Normal load: 1.0 N, mean contact pressure.. 0.37 GPa. The experimental error for the friction force measurement is 5 mN, inducing a 0.005 error in the friction Tribology
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Super-low
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of MO&:
C. Donnet
et al.
In the case of the UHV test, negative values arise from the calculation indicating that, in this case, the noise can be larger than the signal from the transducer. In UHV, we find a vanishing of the friction force, which has never been observed before. In HV conditions, the friction coefficient remains stable between 0.015 and 0.018 over the hundred cycles. The tests performed in at-Air (RH = 40%) gave an average friction coefficient between 0.15 and 0.20 (not shown), as already published16. AES spectra performed inside the wear tracks are presented in Fig 2(b)-(d). The AES spectrum of the initial film is presented in Fig 2(a). No oxygen or carbon was detected inside the wear track performed in UHV and HV. No nitrogen was detected inside the wear track performed in d-Nz. Oxygen and carbon were detected inside and outside the wear track performed in at-Air. These results show that sliding in UHV, HV and d-N* conditions does not induce any chemical reaction in the sliding contact. Moreover, AES confirms the absence of an Fe signal in all cases, thus indicating that in all the wear tracks, MoS, is
still present after the 100 cycle tests. On the other hand, no iron was transferred from the pin. Estimation of the shear strength MoS, films
values of the
From the Hertzian theory, we can deduce the theoretical diameter of the apparent contact zone DHz, 58 pm in the experimental conditions above. This calculated value can be compared to experimental ones deduced from the optical micrographs, as shown in Fig 3. Whatever the nature of the atmosphere during friction, the diameter of the contact zones Dexp has values between 60 and 70 pm. Although precise measurement of the track width is not easy due to blurred edges as seen with the optical contrast, the experimental values D exp are all of the same order of magnitude as the theoretical one DHz. From these results and according to Eq. (l), one can deduce that the friction coefficient of MoS,-coated steel under elastic conditions is controlled by the shear strength S of the interfacial film, which directly depends on the nature of the atmosphere
/ UHV
Energy (eV) Fig 2 Auger spectra carried out on the MO& surface, outside the wear track (a), inside the wear track performed (b) in UHV (5.10p8 Pa), (c) in HV (lop3 Pa), (d) in ambient air (105 Pa, RH = 40%), (e) in dry nitrogen (1W Pa). Primary electron energy: 5 keV 126
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Super-low
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of MO&:
C. Donnet
et al.
b
a
50 pm Fig 3 Optical micrograph of wear scars on: (a) the steel pin, (6) the MoS, thin coated steel plane. Surrounding the Herztian zone (dotted line), we can observe a small number of wear fragments
during friction. Table 1 gives the shear strength values S of the interfacial film, calculated from Eq. (1) using contact width of 58 pm. From Eq. (2), the pressureindependent So value is less than the S values. However, assuming (Y = 0.001 whatever the atmosphere during friction, as already mentioned in the literature3, S,, is in the same range as S. To conclude, the super-low regime is characterized by an interface shear strength of the order of 1 MPa. Origin of super-low nitrogen
friction
in UHV
and dry
As mentioned in the introduction, the ultra-low friction of MoS, in high vacuum is generally attributed to friction-induced basal plane orientation. Friction coefficients in the 10e2 range correspond to interface film shear strength values in the range of 10 MPa (assuming (Y = 0.001, see Eq. (2)). Super-low friction of pure and stoichometric MoS, reaches the lo-” range and we have shown that this effect can be explained by a decrease of the interface shear strength Table 1 Average friction coefficient of a MoS, coating against a steel pin at cycle N = 100 and corresponding shear strength S of the interfacial film in various atmospheres during friction. See text for the calculation procedure of S Atmosphere during friction
UHV HV d-N2 at-Air
Average friction coefficient at cycle N = 100 0.002 0.013 0.003 0.150
Calculated shear strength S (MPa)
0.7 4.9 1.1 56.0 Tribology
S to approximately 1 MPa, with no significant change of the contact area. The decrease in the shear strength has been correlated to a superlubricating state during friction. Superlubricity is the state in which two contacting surfaces exhibit no resistance to sliding. Shinjo et af.” have shown that superlubricity is related to the atomistic origin of friction and that the phenomenon appears when the sum of the forces acting on each moving atom against the entire system vanishes. A specific case of superlubrication is frictional anisotropyis when incommensurate contacting surfaces are sliding against each other, which can be the case of two contacting crystal lattices at a certain misfit angle. The existence of superlubricity has been recently suggested for a hexagonal symmetry of the crystal: experiments were carried out by measuring friction between two contacting surfaces of cleaved mica, when changing the lattice misfit angle 19. The friction force can be lowered by one order of magnitude. No frictional anisotropy could be seen in ambient atmosphere, showing that the absence of surface contaminants is a determining factor in reaching the superlubricating state. However, no direct experimental evidence for this mechanism has been given by the authors. In our opinion, the present results could be explained in terms of frictional anisotropy in UHV or in d-N2. They are consistent with complementary results already published’, giving strong evidence for superlubricity of MoS2 coatings. Considering these data, the effect of the atmosphere during friction can play a role at two levels: l
a change in the nature of the MoS2 crystal by chemical reaction (oxidation, atomic substitution), modifying both the basal plane orientation and the rotational disorder between grains. It has been suggested that the substitution of sulphur by oxygen atoms could be at the origin of an increase of the basal plane distance and could explain a decrease of the shear strength20. Alternatively, the formation international
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C. Donnet
et al.
of bulk oxides makes friction drastically increase by eliminating ‘easy shear’ basal planes; some contamination or adsorption of the gas species onto the sulphur-rich basal planes, eliminating the frictional anisotropy effect by masking the atomic lattice. This effect has been shown by Shinjo et al., using freshly cleaved mica surfaces”.
l
We observed that nitrogen did not modify the UHV super-low friction behaviour of MoS2 and this is in agreement with previous work in the ultra-low friction regime, indicating that no reaction takes place between the gas and the solid in the working conditions (as confirmed by AES investigations). An introduction of a low partial pressure of humid air has an effect very similar to the presence of impurities in the coating material. The friction mechanism seems to be deeply affected by the presence of oxygen or water vapour. As no chemical reaction between these two impurities and MoS2 occurred, only physical processes have to be taken into account, such as liquid water condensation in crystal defects. More work is necessary to clarify if only the crystal orientation is affected or if only contamination is involved in the processes21.
Conclusion We have studied the tribological properties of RFmagnetron sputtered MoS2 coatings deposited on cleaned bearing steel surfaces, and tested in different environments, using an ultra-high vacuum analytical tribometer with a sphere on plane reciprocating contact. Great care was taken to measure the very low friction coefficients accurately, by optimizing the data processing and the calibration procedure of the friction coefficient. The following conclusions can be drawn.
(1)
(2)
The friction coefficient exhibited by the MoS2 coating in ultra-high vacuum (5 x lo-* Pa) and in dry nitrogen ( lo3 Pa) was extraordinarily low. Its calculated average value on each cycle was below 0.003 and in some cases the tangential force was hardly detectable, as if the friction force had completely vanished. The friction coefficient in high vacuum ( lop3 Pa) remains stable between 0.015 and 0.018 during the friction process, whereas the friction coefficient in ambient air (RH = 40%) is between 0.15 and 0.20.
(3)
128
The width of the contact zones is very close to the theoretical width calculated from the Hertzian theory. Consequently, the experimental values of the average friction coefficients are correlated to values of the shear strength of the interfacial film: 0.7-1.1 MPa in ultra-high vacuum or dry
Tribology
International
nitrogen, 4.9 MPa in high vacuum and 56 MPa in ambient air. In ultra-high vacuum, the vanishing of the friction force is attributed to a superlubricating state associated with frictional anisotropy of basal plane oriented MoS, grains. In dry nitrogen, it seems that the situation is similar and that no tribochemical reaction occurs between MoS, and nitrogen. In a low partial pressure of water vapour, the increase of the shear strength cannot be attributed to a chemical interaction. It seems that physisorption of water has to be involved in the mechanism.
(4)
(5)
References 1. Spalvins T. Vat. Sci. Technol. 1987 AS, (2), 212 Roberts E.W. Tribal. Int. 1990, 23 (2), 95 3. Singer LL. Fundamental in Friction: Macroscopic and Microscopic Processes (Eds I.L. Singer and H.M. Pollock) Kluwer 2.
Academic,
Amsterdam,
1992, 237
4. Fieischauer P.D. and Bauer R. Tribol. Trans. 1988, 31 (2), 239 5. Uemura M., Saito K. and Nakao K. Tribal. Trans.. 1990, 33, (4) 5.51 6. Donnet C., Le Mogne Th. and Martin J.M. Surf Coat. Technol. 1993,
62, 406
7. Martin J.M., Donnet C., Le Mogne Th. and Epicier Th. Phys. Rev. B, 1993, 48, (14), 10583 8. Bowden F.P. and Tabor D. The Friction and Lubrication of Solids, Clarendon Press, Oxford, 1964, Part 1: pp. 110-121, Part 2: pp. 158-185 9. Bridgeman P.W. Proc. Am. Acad. Arts Sci. 1936, 71, 387 10. Singer I.L., Bolster R.N., Wegand J, Fayeuile S. and Stupp R.C. Appl. Phys. Lett. 1990, 57, 995 11. Roberts E.W. Tribology, Friction, Lubrication and Wear, Fifty Years On, Proc. I. Mech. E., London, 1987, 503 12. Wheeler D.R. Thin Solid Films, 1993, 223, 78 13. Martin J.M. and L.e Mogne Th. Surf. Coat. Techn. 1991, 49, 427 14. Stewart T.B. and Fleischauer P.D. Znorg. Chem. 1982,21,2426 15. Briggs D. and Seah M.P. Practical Surface Analysis by Auger and X-Ray Electron Spectroscopies, Wiley Science, New York, 1985
16. 17. 18. 19.
Moser J. and Levy F. J. Mater. Res. 1993, 8, (l), 206 Sbinjo K. and Hirano M. Surf. Sci. 1993, 283, 473 Sokoloff J.B. Phys. Rev. B. 1990, 42, 760 Hirano M., Shinjo K., Kaneko R. and Murato Y. Phys. Rev. Lett. 1991, 67, (19), 2642 20. Lince J.R., Hilton M.R. and Bommannavar A.S. Surf. Coat. Tech.
1990,
43144,
640
21. Uemura M., Saito K. and Nakao K., Tribol. (4), 551
Volume 29 Number 2 1996
Trans.
1990,
33,
ELSEVIER SCIENCE:
0301-679X(95)00094-1
International Vol. 29, No. 2. pp. 123-128, 1996 Copyright @ 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0301-679X1961315.00
Super-low friction of MoS, coatings in various environments C. Donnet,
J. M. Martin,
Th. Le Mogne
and M. Belin
The ultra-low friction coefficient (typically in the lo-* range) of MoS,-based coatings is generally associated with the frictioninduced orientation of ‘easy-shear’ planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used an AES/XPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing the deposition of oxygen-free MoS, PVD coatings and the performance of friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS, coatings deposited on AISI 52100 steel was studied in ultrahigh vacuum (UHV: 5 x lop8 Pa), high vacuum (HV: lop3 Pa), dry nitrogen (lo5 Pa) and ambient air (lo5 Pa). ‘Super-low’ friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to a calculated interfacial shear strength in the range of 1 MPa, about ten times lower than for standard coatings. Low friction coefficients of about 0.013-0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger electron spectroscopy shows no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the ‘super-low’ friction behaviour (lop3 range) can be attributed to superlubricity, obtained for a oarticular combination of cvstalloaraphic orientation and the absence of contaminants, leading’to a c&&derable decrease in the inter-facial shear strength. Keywords: gaseous
MO&, vacuum, environment
super-low
friction,
thin film, so/id lubricant,
Introduction Molybdenum disulphide (MO&) is a well-known lamellar solid lubricant with a hexagonal structure. Extensive surveys dealing with the tribological behaviour of
Laboratoire de Tribologie et Dynamique des Systkmes, URA CNRS 8.55, hole Centrale de Lyon, B.P. 163, F-69131 l?culiy Cedex, France. Received 15 December 1993; revised 24 March 1994; accepted 4 May 1995
Tribology
sputter-deposited MoS, coatings exist in the literature1-4. Ultra-low friction of MO& coatings has been identified when running friction tests in the absence of oxygen and/or water vapour. In such conditions, and denending on the normal load in the sliding contact, friction cogfficient values between 0.01 and 0.05 have been measured, which already represent uncommon values in solid film lubrication. The mechanisms of ultra-low friction of MO& (typically observed in dry nitrogen or in high vacuum) can be summarized with the three following conditions*: International
Volume
29 Number
2 1996
123
Super-low
friction
of MO!?,: C. Donnet
et al.
According to the Hertzian theory, the friction coefficient in the sphere-on-plane configuration below the elastic limit depends on three variableslo as:
Notation AES HV PVD RF TEM UHV XPS
Auger electron spectroscopy high vacuum physical vapour deposition radio frequency transmission electron microscopy ultra-high vacuum X-ray photoelectron spectroscopy
p = So.~.(3.R/4.E)2f3.W-1J3
the built-up of a MoS2 transfer film onto the frictional counterface. This transfer film is formed due to good adhesion MO& to the opposing material; 0 a friction-induced orientation of the (0001) basal planes of the MoSz grains in the interfacing region, parallel to the sliding direction (in the transfer film, the film itself and eventually the third body in general). It is anticipated that friction-induced basal plane orientation occurs very early at the beginning of sliding, by a simultaneous orientation-switching of all the individual nanometre-scale MO& grains in the interface; l the absence of contaminants. Carbon and oxygen are well-known contaminants of MO&, coming from the residual gas during the sputtering process or from the MO& target. Special attention must be paid to the effect of water vapour coming from the ambient air or from a humid environment during storage, for example. It has been suggested that liquid water could be formed by capillary condensation of vapour in the defects of the MoS2 crystal structure and that water could then modify the easy shear between basal plane9. l
Recently, using a dedicated ultra-high vacuum analytical tribometer, a ‘super-low’ friction behaviour has been identified (average friction coefficient in the 10e3 range) when testing in an ultra-high vacuum a sputtered MO& coating free from impurities such as carbon, oxygen and water vapour. In some cases, the recorded tangential force was hardly detectable as if the friction force had completely vanished. Although the assputtered grain size was not modified by the tribological process, friction-induced orientation of the MO& grains in the contact interface was clearly demonstrated by electron diffraction and high resolution TEM studies, carried out on selected wear debris collected at the end of the friction test6r7. Consequently the three conditions listed above are verified and thus are not sufficient alone to explain the origin of the ‘superlow’ friction behaviour. The Bowden and Tabor model of friction provides a good starting point for understanding how a thin solid film can reduce frictions. The friction coefficient is assumed to depend on the normal load W, the real area of contact A and the shear strength S of the inter-facial film as: p = S.AIW
(1) The shear strength S of solids at high pressure has been observed to have a pressure dependence, which can be approximated by9: s = so + ff.P 124
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(2) International
Volume
29 Number
+ a
(3)
where E is the composite elastic modulus of the contacting materials, and R is the radius of the ball. This model assumes that the real contact area corresponds to the Hertzian zone, as calculated in Eq. (3). This assumption is justified by the soft behaviour of MoS2 film and debris in the contact, leading to a good accommodation in the contact geometry. It has been proven to be valid for thin MoS2 coatingslo. Therefore, friction measurements can be used to determine So and (Y values : friction of MoS2 in dry air leads to So = 25 MPa and (Y = O.OO1lo, friction in vacuum leads to So = 7 MPa and (Y = O.OO1ll. From these data, it appears that MoS, coatings have So values which depend on the atmosphere during friction, since the presence of oxygen (in dry air) increases the shear strength of the film. These results suggest that surface chemistry due to atmospheric conditions during friction processes is of great importance in the tribological performance and mechanical characteristics of MoS2 thin films. Taking into account this approach, different questions are raised, in order to go further in the understanding of the friction mechanism of these coatings: l
l
How do different kinds of atmosphere (ultra-high vacuum, poor vacuum, nitrogen and air) alter the friction behaviour of MoS2 for a given structure and film composition? According to Eq. (l), what are the respective contributions of the shear strength S and the real area of contact A in the different levels of friction observed in various atmospheres? In particular, is a ‘super-low’ friction behaviour due to a low value of the shear strength S or to a reduction of the real contact area A?
Wheelerl* recently suggested that a partial pressure of pure oxygen could reduce the friction by influencing the transfer of interfacial material to the pin in such a way as to reduce the area of contact. As an attempt to progress further in this field, we performed tribological tests (pin-on-disc configuration) on pure and stoichiometric sputter-deposited MoS2 coatings in various controlled atmospheres, at a given contact pressure. Experiments were carried out using an ultrahigh vacuum analytical tribometer coupled with a preparation chamber, which has already been described in detail13. With this apparatus, sputter-deposited MoS2 thin films were obtained on steel and analysed by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy ( AES). Chemical analyses were performed by AES inside and outside the wear tracks after each test carried out in the different atmospheres. Optical micrographs gave an estimation of the real areas of contact in the sliding contact, thus allowing the determination of the shear strength S, depending on the atmosphere during friction. In the following experiments, the normal load, the pin radius and thus the contact pressure were not changed, because of technical difficulties in the UHV tribometer. Conse2 1996
Super-low
quently, the computed values of S,, S and fy cannot be determined to a high degree of precision.
Friction measurements atmospheres Elaboration coatings
in various
and characterization
of MoS,
Radio-frequency magnetron sputtering of MO& was performed on a cleaned bearing steel (AISI 52100) surface at room temperature, using a previously degassed MO& target. A 120 nm thick film was obtained by sputtering MO& at 7.4 W/cm2 with a current intensity of 300 mA. An in situ analysis of the coating was performed by XPS’. The Mo3d5/2 and S2p core level peaks were respectively centred at 228.8 -+ 0.2 eV and 162.0 2 0.2 eV, in agreement with already published values corresponding to MoS214. The chemical composition of the film was calculated using the Mo3d3/2 and S2p peak intensities and sensitivity factors published by Briggs et a1.15. The experimental S : MO ratio was 2.04, thus suggesting the film to be nearly stoichiometric. Auger electron spectroscopy analyses did not show any trace of contamination elements such as carbon or oxygen, within the detection limits of the technique. The TEM analysis of the coating has been already published”.
Tribologicai
friction
of MO&:
C. Donnet
et a/.
conditions
The friction measurements were carried out using a reciprocating pin-on-flat tribometer. The calibration procedure and the sensitivity of the force measurement device has already been presented in detai16. The tribological parameters were the following: using an AISI 52100 steel hemispherical pin (4 mm radius of curvature) as a hard material, and several identical MoS,-on-steel coatings, we performed 100 cycle-tests with a normal load of 1.0 N, corresponding to a mean contact pressure of 0.37 GPa, a linear sliding speed of 0.5 mm/s and a 3 mm wear track length. Tribological tests were performed in various kinds of controlled atmosphere: l l l
l
ultra-high vacuum (UHV) 5 x lo-* Pa high vacuum (HV) 10-j Pa dry nitrogen (d-N,) lo5 Pa, relative RH < 1% ambient air (at-Air) lo5 Pa, relative RH = 40%
Effect of atmosphere
on friction
humidity humidity
coefficient
We show the typical evolution of the average friction coefficient, as a function of the number of cycles in UHV, in HV and in d-N2 (Fig 1). These results have a very good repeatability. In both UHV and d-N* conditions, the friction coefficient begins at 0.01 but drastically decreases to 0.001-0.003 a few cycles later.
0.020 /
10
20
30
40
50
60
70
80
90
100
Fig 1 Average friction coejjkient versus number of cycles of a MO,!& coating tested in ultra-high vacuum (CJHV: 5 x lOpa Pa), high vacuum (HV: lop3 Pa) and dry nitrogen (d-N,: 105 Pa). Normal load: 1.0 N, mean contact pressure.. 0.37 GPa. The experimental error for the friction force measurement is 5 mN, inducing a 0.005 error in the friction Tribology
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In the case of the UHV test, negative values arise from the calculation indicating that, in this case, the noise can be larger than the signal from the transducer. In UHV, we find a vanishing of the friction force, which has never been observed before. In HV conditions, the friction coefficient remains stable between 0.015 and 0.018 over the hundred cycles. The tests performed in at-Air (RH = 40%) gave an average friction coefficient between 0.15 and 0.20 (not shown), as already published16. AES spectra performed inside the wear tracks are presented in Fig 2(b)-(d). The AES spectrum of the initial film is presented in Fig 2(a). No oxygen or carbon was detected inside the wear track performed in UHV and HV. No nitrogen was detected inside the wear track performed in d-Nz. Oxygen and carbon were detected inside and outside the wear track performed in at-Air. These results show that sliding in UHV, HV and d-N* conditions does not induce any chemical reaction in the sliding contact. Moreover, AES confirms the absence of an Fe signal in all cases, thus indicating that in all the wear tracks, MoS, is
still present after the 100 cycle tests. On the other hand, no iron was transferred from the pin. Estimation of the shear strength MoS, films
values of the
From the Hertzian theory, we can deduce the theoretical diameter of the apparent contact zone DHz, 58 pm in the experimental conditions above. This calculated value can be compared to experimental ones deduced from the optical micrographs, as shown in Fig 3. Whatever the nature of the atmosphere during friction, the diameter of the contact zones Dexp has values between 60 and 70 pm. Although precise measurement of the track width is not easy due to blurred edges as seen with the optical contrast, the experimental values D exp are all of the same order of magnitude as the theoretical one DHz. From these results and according to Eq. (l), one can deduce that the friction coefficient of MoS,-coated steel under elastic conditions is controlled by the shear strength S of the interfacial film, which directly depends on the nature of the atmosphere
/ UHV
Energy (eV) Fig 2 Auger spectra carried out on the MO& surface, outside the wear track (a), inside the wear track performed (b) in UHV (5.10p8 Pa), (c) in HV (lop3 Pa), (d) in ambient air (105 Pa, RH = 40%), (e) in dry nitrogen (1W Pa). Primary electron energy: 5 keV 126
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b
a
50 pm Fig 3 Optical micrograph of wear scars on: (a) the steel pin, (6) the MoS, thin coated steel plane. Surrounding the Herztian zone (dotted line), we can observe a small number of wear fragments
during friction. Table 1 gives the shear strength values S of the interfacial film, calculated from Eq. (1) using contact width of 58 pm. From Eq. (2), the pressureindependent So value is less than the S values. However, assuming (Y = 0.001 whatever the atmosphere during friction, as already mentioned in the literature3, S,, is in the same range as S. To conclude, the super-low regime is characterized by an interface shear strength of the order of 1 MPa. Origin of super-low nitrogen
friction
in UHV
and dry
As mentioned in the introduction, the ultra-low friction of MoS, in high vacuum is generally attributed to friction-induced basal plane orientation. Friction coefficients in the 10e2 range correspond to interface film shear strength values in the range of 10 MPa (assuming (Y = 0.001, see Eq. (2)). Super-low friction of pure and stoichometric MoS, reaches the lo-” range and we have shown that this effect can be explained by a decrease of the interface shear strength Table 1 Average friction coefficient of a MoS, coating against a steel pin at cycle N = 100 and corresponding shear strength S of the interfacial film in various atmospheres during friction. See text for the calculation procedure of S Atmosphere during friction
UHV HV d-N2 at-Air
Average friction coefficient at cycle N = 100 0.002 0.013 0.003 0.150
Calculated shear strength S (MPa)
0.7 4.9 1.1 56.0 Tribology
S to approximately 1 MPa, with no significant change of the contact area. The decrease in the shear strength has been correlated to a superlubricating state during friction. Superlubricity is the state in which two contacting surfaces exhibit no resistance to sliding. Shinjo et af.” have shown that superlubricity is related to the atomistic origin of friction and that the phenomenon appears when the sum of the forces acting on each moving atom against the entire system vanishes. A specific case of superlubrication is frictional anisotropyis when incommensurate contacting surfaces are sliding against each other, which can be the case of two contacting crystal lattices at a certain misfit angle. The existence of superlubricity has been recently suggested for a hexagonal symmetry of the crystal: experiments were carried out by measuring friction between two contacting surfaces of cleaved mica, when changing the lattice misfit angle 19. The friction force can be lowered by one order of magnitude. No frictional anisotropy could be seen in ambient atmosphere, showing that the absence of surface contaminants is a determining factor in reaching the superlubricating state. However, no direct experimental evidence for this mechanism has been given by the authors. In our opinion, the present results could be explained in terms of frictional anisotropy in UHV or in d-N2. They are consistent with complementary results already published’, giving strong evidence for superlubricity of MoS2 coatings. Considering these data, the effect of the atmosphere during friction can play a role at two levels: l
a change in the nature of the MoS2 crystal by chemical reaction (oxidation, atomic substitution), modifying both the basal plane orientation and the rotational disorder between grains. It has been suggested that the substitution of sulphur by oxygen atoms could be at the origin of an increase of the basal plane distance and could explain a decrease of the shear strength20. Alternatively, the formation international
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of bulk oxides makes friction drastically increase by eliminating ‘easy shear’ basal planes; some contamination or adsorption of the gas species onto the sulphur-rich basal planes, eliminating the frictional anisotropy effect by masking the atomic lattice. This effect has been shown by Shinjo et al., using freshly cleaved mica surfaces”.
l
We observed that nitrogen did not modify the UHV super-low friction behaviour of MoS2 and this is in agreement with previous work in the ultra-low friction regime, indicating that no reaction takes place between the gas and the solid in the working conditions (as confirmed by AES investigations). An introduction of a low partial pressure of humid air has an effect very similar to the presence of impurities in the coating material. The friction mechanism seems to be deeply affected by the presence of oxygen or water vapour. As no chemical reaction between these two impurities and MoS2 occurred, only physical processes have to be taken into account, such as liquid water condensation in crystal defects. More work is necessary to clarify if only the crystal orientation is affected or if only contamination is involved in the processes21.
Conclusion We have studied the tribological properties of RFmagnetron sputtered MoS2 coatings deposited on cleaned bearing steel surfaces, and tested in different environments, using an ultra-high vacuum analytical tribometer with a sphere on plane reciprocating contact. Great care was taken to measure the very low friction coefficients accurately, by optimizing the data processing and the calibration procedure of the friction coefficient. The following conclusions can be drawn.
(1)
(2)
The friction coefficient exhibited by the MoS2 coating in ultra-high vacuum (5 x lo-* Pa) and in dry nitrogen ( lo3 Pa) was extraordinarily low. Its calculated average value on each cycle was below 0.003 and in some cases the tangential force was hardly detectable, as if the friction force had completely vanished. The friction coefficient in high vacuum ( lop3 Pa) remains stable between 0.015 and 0.018 during the friction process, whereas the friction coefficient in ambient air (RH = 40%) is between 0.15 and 0.20.
(3)
128
The width of the contact zones is very close to the theoretical width calculated from the Hertzian theory. Consequently, the experimental values of the average friction coefficients are correlated to values of the shear strength of the interfacial film: 0.7-1.1 MPa in ultra-high vacuum or dry
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nitrogen, 4.9 MPa in high vacuum and 56 MPa in ambient air. In ultra-high vacuum, the vanishing of the friction force is attributed to a superlubricating state associated with frictional anisotropy of basal plane oriented MoS, grains. In dry nitrogen, it seems that the situation is similar and that no tribochemical reaction occurs between MoS, and nitrogen. In a low partial pressure of water vapour, the increase of the shear strength cannot be attributed to a chemical interaction. It seems that physisorption of water has to be involved in the mechanism.
(4)
(5)
References 1. Spalvins T. Vat. Sci. Technol. 1987 AS, (2), 212 Roberts E.W. Tribal. Int. 1990, 23 (2), 95 3. Singer LL. Fundamental in Friction: Macroscopic and Microscopic Processes (Eds I.L. Singer and H.M. Pollock) Kluwer 2.
Academic,
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1992, 237
4. Fieischauer P.D. and Bauer R. Tribol. Trans. 1988, 31 (2), 239 5. Uemura M., Saito K. and Nakao K. Tribal. Trans.. 1990, 33, (4) 5.51 6. Donnet C., Le Mogne Th. and Martin J.M. Surf Coat. Technol. 1993,
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7. Martin J.M., Donnet C., Le Mogne Th. and Epicier Th. Phys. Rev. B, 1993, 48, (14), 10583 8. Bowden F.P. and Tabor D. The Friction and Lubrication of Solids, Clarendon Press, Oxford, 1964, Part 1: pp. 110-121, Part 2: pp. 158-185 9. Bridgeman P.W. Proc. Am. Acad. Arts Sci. 1936, 71, 387 10. Singer I.L., Bolster R.N., Wegand J, Fayeuile S. and Stupp R.C. Appl. Phys. Lett. 1990, 57, 995 11. Roberts E.W. Tribology, Friction, Lubrication and Wear, Fifty Years On, Proc. I. Mech. E., London, 1987, 503 12. Wheeler D.R. Thin Solid Films, 1993, 223, 78 13. Martin J.M. and L.e Mogne Th. Surf. Coat. Techn. 1991, 49, 427 14. Stewart T.B. and Fleischauer P.D. Znorg. Chem. 1982,21,2426 15. Briggs D. and Seah M.P. Practical Surface Analysis by Auger and X-Ray Electron Spectroscopies, Wiley Science, New York, 1985
16. 17. 18. 19.
Moser J. and Levy F. J. Mater. Res. 1993, 8, (l), 206 Sbinjo K. and Hirano M. Surf. Sci. 1993, 283, 473 Sokoloff J.B. Phys. Rev. B. 1990, 42, 760 Hirano M., Shinjo K., Kaneko R. and Murato Y. Phys. Rev. Lett. 1991, 67, (19), 2642 20. Lince J.R., Hilton M.R. and Bommannavar A.S. Surf. Coat. Tech.
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21. Uemura M., Saito K. and Nakao K., Tribol. (4), 551
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Trans.
1990,
33,