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Humidity resistant MoSx films prepared by pulsed magnetron sputtering
Surface and Coatings Technology 131 Ž2000. 216᎐221
Humidity resistant MoS x films prepared by pulsed magnetron sputtering W. Lauwerens a,U , Jihui Wang a , J. Navratil a , E. Wieers ¨ a , J. D’haena , L.M. Stals a , J.P. Celis b, Y. Bruynseraede c a
Institute for Materials Research, Limburg Uni¨ ersity Centrum, B-3590 Diepenbeek, Belgium Department of MTM, Katholieke Uni¨ ersiteit Leu¨ en, de Croylaan 2, B-3001 Leu¨ en, Belgium c Department of Physics, Katholieke Uni¨ ersiteit Leu¨ en, Celestijnenlaan 200D, B-3001 Leu¨ en, Belgium b
Abstract It is well known that MoS2 is a good lubricant, but the lubricity of MoS x thin films is greatly affected by the deposition parameters, especially when used under environmental conditions of high relative humidity. In this work, MoS x films were prepared by magnetron sputtering using a bipolar pulsed power supply. Several deposition parameters such as argon pressure, substrate temperature, substrate bias voltage, cathode power and deposition time were varied. Composition, morphology and structure were investigated by a number of techniques including energy dispersive spectroscopy ŽEDS., Rutherford back scattering ŽRBS., scanning electron microscopy ŽSEM. and X-ray diffraction ŽXRD.. Tribological properties were measured with a ball-on-disk fretting tester. The results show that MoS x films prepared at low argon pressure Žbelow 0.4 Pa. and low substrate temperature Žroom temperature. have a low friction coefficient and long wear life. These films have a remarkable low sulfur content Ž xf 1 and even smaller, in contrast to frequently reported values of xs 1.2; 1.8., a featureless morphology and only a strong basal plane Ž002. diffraction peak. The relative humidity, up to values of 90%, has only a small effect on the friction coefficient and wear life. The structure of the films and the friction and wear mechanism are discussed in view of the low sulfur content. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: MoS x lubricating films; Pulsed sputtering; Friction; Wear
1. Introduction Thin films of molybdenum disulfide are well known for their lubrication properties and are mainly used in space and vacuum applications w1,2x. The films are mostly deposited by sputtering resulting in MoS x layers with 0.9- x- 2.1 w3x. When used under environmental conditions, especially at high relative humidity, the lubricity is, however, greatly affected w4x. In view of a growing interest in using low friction thin films under atmospheric conditions Že.g. dry machining., research on MoS x thin films is concentrating on stabilizing the properties in open air applications.
U
Corresponding author.
The structure of sputtered MoS x films depends on the deposition conditions. Two typical structures are known w1,5x. The first Žtype I films. is characterized by basal planes with perpendicular orientation to the substrate surface and by a porous, columnar structure. The reactive edge sites of the structure are exposed which easily leads to the formation of MoO 3 in humid air and which is detrimental for the friction properties. The second type of film structure Žtype II. has the basal planes parallel to the substrate surface. This basal orientation is better for lubrication, not only because of the right direction of the sliding planes, but also because of a more dense structure. This structure is more suited to withstand oxidation when exposed to air. DC, DC magnetron and mainly RF sputtering have been used to prepare the layers w6᎐8x. The need to
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 7 9 6 - 9
W. Lauwerens et al. r Surface and Coatings Technology 131 (2000) 216᎐221
grow structures with basal orientation has led to higher energy techniques as IBAD and closed field unbalanced sputtering. Also layers are grown with metal dopants, with a metalrMoS x multilayer structure or with a nano-composite structure w9᎐12x. This paper reports on the properties of MoS x films prepared by bipolar pulsed magnetron sputtering. Bipolar pulsed sputtering is normally used to prevent ‘target poisoning’ when sputtering a metal target in a reactive gas that forms a non-conducting film w13x. However, when sputtering a bad conducting target Žlike pressed MoS2 targets., it is expected that the positive pulse removes any excess charge on the target surface and thereby prevents arcs and micro-arcs during sputtering.
2. Experimental Films were deposited by a planar magnetron sputtering system equipped with an unbalancing coil. The magnetron was fed using pulsed power. For this purpose a magpuls QP-1000r6r15 bp power supply was used, which is capable of providing bipolar pulses up to 33 kHz. The length of the negative and positive pulse can be separately adjusted, but their magnitude is equal. In case of pulsed plasma, the negative pulse was set at 40 s and the positive at 10 s. Fig. 1 shows the line form of the pulses. The MoS2 target ŽTarget Materials, Inc.. with diameter of 75 mm had a purity of 99% and the substrate to target distance was 65 mm. As the substrate material, hardened and polished Ž R a s 0.05 m. discs Ž25 mm diameter. of 440C stainless steel were used. The base pressure in the deposition chamber was approximately 1 = 10y3 Pa. Before deposition, the sample was sputter cleaned and the target was pre-sputtered with a shutter in front of the substrate.
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The main deposition parameter changed was the argon pressure. Samples have been deposited at argon pressures Ž PAr . between 0.15 and 2.4 Pa. Also, the influence of the substrate temperature ŽTS - 80⬚C and TS s 300⬚C. and the substrate bias voltage Ž VB s 0 V and VB s y100 V. have been investigated. Most of the samples were prepared at a target power of 8 Wrcm2 , but also a lower Ž4 Wrcm2 . and a higher Ž12 Wrcm2 . level were used. For comparison a few samples were deposited using DC power at 8 Wrcm2 . The typical deposition time was 14 min which resulted in film thicknesses of approximately 1.5 m at a power of 8 Wrcm2 . Structural analysis of the films was done by X-ray diffraction ŽXRD. measurements with a Siemens D5000 diffractometer in coupled ᎐2 geometry using CuK ␣ radiation. The composition of the films, i.e. the ratio of S to Mo expressed by the parameter x, was determined by energy dispersive X-ray spectrometry ŽEDX.. Because of a partial overlap of the molybdenum L and sulfur K lines, the EDX analysis was checked by Rutherford backscattering spectrometry. Here, some MoS x films were deposited on silicon substrates. The results were in agreement within the experimental errors. X-Ray photoelectron spectrometry ŽXPS. was used for depth profiling and for the determination of possible contaminants. Scanning electron microscopy ŽSEM. was used to study the morphology of the layers. To study friction and wear properties, fretting tests were performed in air at 23⬚C and relative humidity ŽRH. levels of - 10%, 50% and 90% under unlubricated contact conditions Žsee w14x for a detailed description of the test device.. Corundum balls with a diameter of 10 mm and surface roughness R a s 0.2 m were used as counterbodies. The vibration frequency was 10 Hz and the linear displacement stroke was 100 m. A normal load of 1 N and a test duration of 30 000 cycles were used. The volumetric wear was measured by scanning the wear scar with a laser profilometer according to a procedure described previously w15x.
3. Results 3.1. Structure and composition
Fig. 1. Line form of the bipolar current pulses used for plasma excitation.
Fig. 2 shows the XRD pattern of some selected samples. Curve Ža. is representative for films deposited at low pressure Ž PAr F 0.4 Pa. and low substrate temperature. The spectrum shows only a pronounced Ž002. reflection peak. This indicates a structure in which the basal planes are oriented parallel with the substrate surface. At higher deposition pressures Ž0.9 Pa., as indicated by the spectrum of curve Žb., also Ž100., Ž101. and Ž110. reflections are observed. It was found that at
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case of low Ar pressure and for films deposited at negative bias voltage. The largest x-values were found in films deposited at high pressure and high substrate temperature. However, surprisingly low values of x, ranging between 0.8 and 1.0, were found in films with only basal plane orientation ŽFig. 2, curve a.. XPS measurements revealed oxygen and carbon contaminants. O was present at a level of 3᎐5 at.% and C at 5 to 8 at.%. Depth profiling indicated that the composition and contamination level was constant throughout the film thickness. No systematic trend in the contamination level could be found. We note that contamination levels of O and C in the range 5᎐15 at.% are frequently reported w17x.
Fig. 2. XRD patterns of some films deposited at 8 Wrcm2 and: Ža. PAr s 0.4 Pa, TS - 80⬚C, VB s 0 V; Žb. PAr s 0.9 Pa, TS - 80⬚C, VB s 0 V; Žc. same as Žb. but VB s y100 V; Žd. same as Ža. but DC power; Že. PAr s 1.6 Pa, TS s 300⬚C, VB s 0 V. The arrows indicate Žhkl. reflections of 2H MoS2 .
even higher Ar pressures Ž1.5 Pa and above., the Ž002. reflection was reduced and the other were increased. In this structure both parallel and perpendicular orientations of the basal planes are present. An additional Ar bombardment Žnegative bias., shown by spectrum Žc., only slightly reduced the Ž101. and Ž100. reflection intensities. Curve Žd. shows the spectrum of a film deposited with DC power, but the other deposition conditions were the same as for the film of spectrum Ža.. The fact that here, contrary to spectrum Ža., also Ž101. and Ž110. reflections are present, is a remarkable result. Spectrum Že. indicates that in films deposited at high temperature perpendicular orientation of the basal planes prevails. The different film morphologies that were observed are shown in Fig. 3. The SEM image of Fig. 3a is typical for MoS x films and indicates a porous columnar structure. This morphology was observed mainly in films deposited at high argon pressure Ž PAr ) 1.0 Pa. and high substrate temperatures. The cauliflower-like structure of Fig. 3b corresponds to films deposited at intermediate pressures Ž0.4 - PAr - 1.0 Pa. and is probably less porous. Fig. 3c shows a dense and featureless morphology. Films with this morphology were grown at pressures below 0.4 Pa and low substrate temperature and were also characterized by the XRD pattern of Fig. 2, curve a, indicating parallel orientation of the basal planes. It was also noticed that these films had a bright metallic appearance, contrary to the others which had a grey look. The bright appearance was also noticed by other researchers Že.g. w16x.. The composition of most of the films was characterized by a value of x ŽSrMo ratio. varying between 1.2 and 1.5. Some trends could be seen: x was smaller in
Fig. 3. Surface SEM images showing three typical morphologies: Ža. needle-like, Žb. cauliflower-like, Žc. featureless.
W. Lauwerens et al. r Surface and Coatings Technology 131 (2000) 216᎐221
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Fig. 4. Averaged coefficient of friction as a function of Ar pressure and bias voltage during deposition.
3.2. Friction and wear properties Fig. 4 gives the averaged friction coefficient as a function of the deposition pressure for zero bias and y100 V bias at two levels of RH. It is noticed that at - 10% RH the friction coefficient remains almost constant. At 50% RH the friction coefficient decreases with decreasing deposition pressure and seems to reach a minimum at 0.4 Pa. The negative bias during deposition gives friction coefficients that are systematically below the zero bias data points. At low pressure Ž PAr s 0.4 Pa. the difference between biased or unbiased deposition vanishes. In Fig. 5 the coefficient of friction as a function of fretting cycles is given for two films. The film in Fig. 5a represents a typical film deposited at PAr s 0.4 Pa, low substrate temperature and zero bias ŽXRD of Fig. 2a curve and morphology of Fig. 3c.. At - 10% and 50% RH the friction coefficient is practically the same and varies between 0.04 and 0.08. At 90% RH the friction coefficient is somewhat higher but remains fairly low at a value of approximately 0.12. Fig. 5b shows a different picture. This film is made using DC power, but the other deposition conditions were equal to those of the film in Fig. 5a. Especially at 50% RH the friction coefficient increases quickly to a level of approximately 0.3. Fig. 6 shows the wear volume of the same films as in Fig. 5 measured after the fretting test. The film made with bipulsed power shows a low wear volume that is almost the same for - 10% and 50% RH and increases by a factor of approximately 3 at 90% RH. The film made with DC power shows a low wear volume at - 10% RH, but a very high at 50% RH. At low Ar pressure and low substrate temperature some other experiments have been done. When changing the deposition power from 8 Wrcm2 to 4 Wrcm2 , it was found that the properties of the films remained
Fig. 5. Coefficient of friction as a function of the fretting cycles at different levels of humidity; Ža. typical film with the best properties, Žb. film prepared with the same conditions as Ža., but using DC power. At 90% RH the lifetime of the DC film was less than 30 000 cycles.
the same. An increase to 12 Wrcm2 , however, made the resulting films less resistant to humidity. This result can probably be ascribed to an increased rate of water vapour evolving from the target at high power. It is known that the presence of water vapour during deposition can alter the properties to a large extent w18x. It was also noted that the film properties were independent on film thickness for thicknesses ranging between 0.15 and 3.0 m.
Fig. 6. Wear volume at different levels of the relative humidity for the two films of Fig. 5.
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4. Discussion Most of the trends that were found as a function of the deposition parameters, have already been reported. At high deposition pressure Ž PAr ) 1 Pa. andror substrate temperature one usually finds a lamellar structure with perpendicular orientation of the basal planes w19x. Lowering the pressure during deposition leads to a gradual increase in the density of the film, as is indicated by Fig. 3b and Fig. 4 Žalso reported in w6x.. The decreasing friction coefficient at 50% RH with decreasing deposition pressure is interpreted as a denser structure and hence less susceptible to the uptake of moist air which is detrimental to the friction properties. The denser structure at low PAr and, as appears from Fig. 4, also as a consequence of Ar bombardment, is in agreement with the principles of sputtering. At low pressure the free path length is larger and hence sputtered species are less thermalized when reaching the substrate. The enhanced energy of the incident particles can also explain the lower S content in these films because of preferential re-sputtering of S. At PAr - 0.4 Pa the structure is very dense with pronounced basal plane orientation and bright appearance. This is in agreement with results obtained by others at a low deposition pressure w5,6,16,20x. However, the value of x obtained in this study Ž0.8᎐1. is lower than previous reported values and it is usually reported that x-values lower than approximately 1 correspond to an amorphous structure, which is clearly not the case here Žsee Fig. 2, curve a.. The XRD pattern of Fig. 2a resembles very well the spectra recorded by Muller et al. w6x who suggest a ¨ turbostratic structure for their thick and compact films deposited at low pressure Ž0.1 Pa.. This structure was also suggested in w21x for IBAD deposited films of high density and with basal orientation. The turbostratic or nearly random layered structure ŽNRLS. is characterized by the arbitrary rotational position of crystallites around an axis perpendicular to the substrate. We suggest that the structure of our very dense and low S films can be described by this structure which probably explains some degree of order in combination with the low S content. The low friction coefficient and the good wear resistance in humid conditions up to 90% RH ŽFig. 5a. of the films made by bipolar pulsed power can probably be ascribed to the very dense and low S containing structure. However, the minimum at 0.4 Pa in Fig. 4 can mean that going to lower pressures and further reducing the S content increases the friction, indicating that there is an optimal x-value. The wear volume of the film made by pulsed power ŽFig. 6. is excellent in our opinion. Because there are not many published data at such high humidity level, we can only compare with the data of Muller et al. w6x. They report an ¨
increase in wear rate of a factor of 17 when the humidity increases from - 5% to 98% RH, which is clearly more than the factor of 3 Žfor 90% RH. in our case. A very remarkable result is the properties of the film deposited by DC power. Friction and wear are much higher than for a film made with pulsed power, although not abnormal in comparison with published values. In w16x is reported a friction coefficient of 0.2 at 60% RH for dense and compact films made by DC magnetron sputtering. At this moment we do not have an explanation yet and further research is needed to point out whether there are fundamental changes in the deposition process when going from DC to pulsed power.
5. Conclusion MoS x thin films prepared by pulsed magnetron sputtering show very good friction and wear properties, also in moist air and hence were called humidity resistant. They show a very dense structure with pronounced basal plane orientation. Films made by DC power under identical deposition conditions show less tribological properties. Understanding the differences between both deposition processes can lead to a further improvement of the films made by pulsed power.
Acknowledgements This research is part of the Eureka project funded by the Flemish government ŽLubrimat 960235., a Belgium-China international project supported by the Flemish government and the PR China government Žproject BIL 96r35., and the IUAP P4r33 project funded by the Belgian government. References w1x T. Spalvins, J.Vac. Sci. Technol. A5 Ž1987. 212. w2x M.R. Hilton, P.D. Fleischauer, Surf. Coatings Technol. 54r55 Ž1992. 435. w3x ? Dimingen, H. Hubsch, P. Willich, K. Reichelt, Thin Solid ¨ Films 12 Ž1985. 79. w4x J. Moser, F. Levy, ´ Thin Solid Films 228 Ž1993. 257. w5x F. Levy, ´ J. Moser, Surf. Coatings Technol. 68r69 Ž1994. 433. w6x C. Muller, C. Menoud, M. Maillat, H.E. Hintermann, Surf. ¨ Coatings Technol. 36 Ž1988. 351. w7x E.W. Roberts, Tribol. Trans. 31 Ž1988. 239. w8x T. Spalvins, J. Vac. Sci. Technol. A5 Ž1987. 212. w9x L.E. Seitzman, R.N. Bolster, I.L. Singer, Surf. Coatings Technol. 52 Ž1992. 93. w10x D.G. Teer, J. Hampshire, V. Fox, V. Bellido-Gonzales, Surf. Coatings Technol. 94r95 Ž1997. 572. w11x M.R. Hilton, G. Jayaram, D.L. Marks, J. Mater. Res. 13 Ž4. Ž1998. 1022.
W. Lauwerens et al. r Surface and Coatings Technology 131 (2000) 216᎐221 w12x R. Gilmore, M.A. Baker, P.N. Gibson et al., Surf. Coatings Technol. 108r109 Ž1998. 345. w13x R.A. Scholl, Surf. Coatings Technol. 98 Ž1998. 823. w14x H. Mohrbacher, J.P. Celis, J.R. Roos, Tribol. Int. 28 Ž5. Ž1995. 269. w15x P. Campbell, J.P. Celis, J.R. Roos, O. Van Der Biest, Wear 174 Ž1994. 47. w16x A. Aubert, J.Ph. Nabot, J. Ernoult, Ph. Renaux, Surf. Coatings Technol. 41 Ž1990. 127.
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w17x G. Jayaram, N. Doraiswamy, M.R. Hilton, Surf. Coatings Technol. 68r69 Ž1994. 439. w18x V. Buck, Wear 114 Ž1987. 263. w19x M.R. Hilton, R. Bauer, S.V. Didziulis, M.T. Dugger, J.M. Keem, J. Scholhamer, Surf. Coatings Technol. 53 Ž1992. 13. w20x R.N. Bolster, I.L. Singer, J.C. Wegand, S. Fayeulle, C.R. Gosset, Surf. Coatings Technol. 46 Ž1991. 207. w21x D.N. Dunn, L.E. Seitzman, I.L. Singer, J. Mater. Res. 13 Ž1998. 3001.
Humidity resistant MoS x films prepared by pulsed magnetron sputtering W. Lauwerens a,U , Jihui Wang a , J. Navratil a , E. Wieers ¨ a , J. D’haena , L.M. Stals a , J.P. Celis b, Y. Bruynseraede c a
Institute for Materials Research, Limburg Uni¨ ersity Centrum, B-3590 Diepenbeek, Belgium Department of MTM, Katholieke Uni¨ ersiteit Leu¨ en, de Croylaan 2, B-3001 Leu¨ en, Belgium c Department of Physics, Katholieke Uni¨ ersiteit Leu¨ en, Celestijnenlaan 200D, B-3001 Leu¨ en, Belgium b
Abstract It is well known that MoS2 is a good lubricant, but the lubricity of MoS x thin films is greatly affected by the deposition parameters, especially when used under environmental conditions of high relative humidity. In this work, MoS x films were prepared by magnetron sputtering using a bipolar pulsed power supply. Several deposition parameters such as argon pressure, substrate temperature, substrate bias voltage, cathode power and deposition time were varied. Composition, morphology and structure were investigated by a number of techniques including energy dispersive spectroscopy ŽEDS., Rutherford back scattering ŽRBS., scanning electron microscopy ŽSEM. and X-ray diffraction ŽXRD.. Tribological properties were measured with a ball-on-disk fretting tester. The results show that MoS x films prepared at low argon pressure Žbelow 0.4 Pa. and low substrate temperature Žroom temperature. have a low friction coefficient and long wear life. These films have a remarkable low sulfur content Ž xf 1 and even smaller, in contrast to frequently reported values of xs 1.2; 1.8., a featureless morphology and only a strong basal plane Ž002. diffraction peak. The relative humidity, up to values of 90%, has only a small effect on the friction coefficient and wear life. The structure of the films and the friction and wear mechanism are discussed in view of the low sulfur content. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: MoS x lubricating films; Pulsed sputtering; Friction; Wear
1. Introduction Thin films of molybdenum disulfide are well known for their lubrication properties and are mainly used in space and vacuum applications w1,2x. The films are mostly deposited by sputtering resulting in MoS x layers with 0.9- x- 2.1 w3x. When used under environmental conditions, especially at high relative humidity, the lubricity is, however, greatly affected w4x. In view of a growing interest in using low friction thin films under atmospheric conditions Že.g. dry machining., research on MoS x thin films is concentrating on stabilizing the properties in open air applications.
U
Corresponding author.
The structure of sputtered MoS x films depends on the deposition conditions. Two typical structures are known w1,5x. The first Žtype I films. is characterized by basal planes with perpendicular orientation to the substrate surface and by a porous, columnar structure. The reactive edge sites of the structure are exposed which easily leads to the formation of MoO 3 in humid air and which is detrimental for the friction properties. The second type of film structure Žtype II. has the basal planes parallel to the substrate surface. This basal orientation is better for lubrication, not only because of the right direction of the sliding planes, but also because of a more dense structure. This structure is more suited to withstand oxidation when exposed to air. DC, DC magnetron and mainly RF sputtering have been used to prepare the layers w6᎐8x. The need to
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 7 9 6 - 9
W. Lauwerens et al. r Surface and Coatings Technology 131 (2000) 216᎐221
grow structures with basal orientation has led to higher energy techniques as IBAD and closed field unbalanced sputtering. Also layers are grown with metal dopants, with a metalrMoS x multilayer structure or with a nano-composite structure w9᎐12x. This paper reports on the properties of MoS x films prepared by bipolar pulsed magnetron sputtering. Bipolar pulsed sputtering is normally used to prevent ‘target poisoning’ when sputtering a metal target in a reactive gas that forms a non-conducting film w13x. However, when sputtering a bad conducting target Žlike pressed MoS2 targets., it is expected that the positive pulse removes any excess charge on the target surface and thereby prevents arcs and micro-arcs during sputtering.
2. Experimental Films were deposited by a planar magnetron sputtering system equipped with an unbalancing coil. The magnetron was fed using pulsed power. For this purpose a magpuls QP-1000r6r15 bp power supply was used, which is capable of providing bipolar pulses up to 33 kHz. The length of the negative and positive pulse can be separately adjusted, but their magnitude is equal. In case of pulsed plasma, the negative pulse was set at 40 s and the positive at 10 s. Fig. 1 shows the line form of the pulses. The MoS2 target ŽTarget Materials, Inc.. with diameter of 75 mm had a purity of 99% and the substrate to target distance was 65 mm. As the substrate material, hardened and polished Ž R a s 0.05 m. discs Ž25 mm diameter. of 440C stainless steel were used. The base pressure in the deposition chamber was approximately 1 = 10y3 Pa. Before deposition, the sample was sputter cleaned and the target was pre-sputtered with a shutter in front of the substrate.
217
The main deposition parameter changed was the argon pressure. Samples have been deposited at argon pressures Ž PAr . between 0.15 and 2.4 Pa. Also, the influence of the substrate temperature ŽTS - 80⬚C and TS s 300⬚C. and the substrate bias voltage Ž VB s 0 V and VB s y100 V. have been investigated. Most of the samples were prepared at a target power of 8 Wrcm2 , but also a lower Ž4 Wrcm2 . and a higher Ž12 Wrcm2 . level were used. For comparison a few samples were deposited using DC power at 8 Wrcm2 . The typical deposition time was 14 min which resulted in film thicknesses of approximately 1.5 m at a power of 8 Wrcm2 . Structural analysis of the films was done by X-ray diffraction ŽXRD. measurements with a Siemens D5000 diffractometer in coupled ᎐2 geometry using CuK ␣ radiation. The composition of the films, i.e. the ratio of S to Mo expressed by the parameter x, was determined by energy dispersive X-ray spectrometry ŽEDX.. Because of a partial overlap of the molybdenum L and sulfur K lines, the EDX analysis was checked by Rutherford backscattering spectrometry. Here, some MoS x films were deposited on silicon substrates. The results were in agreement within the experimental errors. X-Ray photoelectron spectrometry ŽXPS. was used for depth profiling and for the determination of possible contaminants. Scanning electron microscopy ŽSEM. was used to study the morphology of the layers. To study friction and wear properties, fretting tests were performed in air at 23⬚C and relative humidity ŽRH. levels of - 10%, 50% and 90% under unlubricated contact conditions Žsee w14x for a detailed description of the test device.. Corundum balls with a diameter of 10 mm and surface roughness R a s 0.2 m were used as counterbodies. The vibration frequency was 10 Hz and the linear displacement stroke was 100 m. A normal load of 1 N and a test duration of 30 000 cycles were used. The volumetric wear was measured by scanning the wear scar with a laser profilometer according to a procedure described previously w15x.
3. Results 3.1. Structure and composition
Fig. 1. Line form of the bipolar current pulses used for plasma excitation.
Fig. 2 shows the XRD pattern of some selected samples. Curve Ža. is representative for films deposited at low pressure Ž PAr F 0.4 Pa. and low substrate temperature. The spectrum shows only a pronounced Ž002. reflection peak. This indicates a structure in which the basal planes are oriented parallel with the substrate surface. At higher deposition pressures Ž0.9 Pa., as indicated by the spectrum of curve Žb., also Ž100., Ž101. and Ž110. reflections are observed. It was found that at
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case of low Ar pressure and for films deposited at negative bias voltage. The largest x-values were found in films deposited at high pressure and high substrate temperature. However, surprisingly low values of x, ranging between 0.8 and 1.0, were found in films with only basal plane orientation ŽFig. 2, curve a.. XPS measurements revealed oxygen and carbon contaminants. O was present at a level of 3᎐5 at.% and C at 5 to 8 at.%. Depth profiling indicated that the composition and contamination level was constant throughout the film thickness. No systematic trend in the contamination level could be found. We note that contamination levels of O and C in the range 5᎐15 at.% are frequently reported w17x.
Fig. 2. XRD patterns of some films deposited at 8 Wrcm2 and: Ža. PAr s 0.4 Pa, TS - 80⬚C, VB s 0 V; Žb. PAr s 0.9 Pa, TS - 80⬚C, VB s 0 V; Žc. same as Žb. but VB s y100 V; Žd. same as Ža. but DC power; Že. PAr s 1.6 Pa, TS s 300⬚C, VB s 0 V. The arrows indicate Žhkl. reflections of 2H MoS2 .
even higher Ar pressures Ž1.5 Pa and above., the Ž002. reflection was reduced and the other were increased. In this structure both parallel and perpendicular orientations of the basal planes are present. An additional Ar bombardment Žnegative bias., shown by spectrum Žc., only slightly reduced the Ž101. and Ž100. reflection intensities. Curve Žd. shows the spectrum of a film deposited with DC power, but the other deposition conditions were the same as for the film of spectrum Ža.. The fact that here, contrary to spectrum Ža., also Ž101. and Ž110. reflections are present, is a remarkable result. Spectrum Že. indicates that in films deposited at high temperature perpendicular orientation of the basal planes prevails. The different film morphologies that were observed are shown in Fig. 3. The SEM image of Fig. 3a is typical for MoS x films and indicates a porous columnar structure. This morphology was observed mainly in films deposited at high argon pressure Ž PAr ) 1.0 Pa. and high substrate temperatures. The cauliflower-like structure of Fig. 3b corresponds to films deposited at intermediate pressures Ž0.4 - PAr - 1.0 Pa. and is probably less porous. Fig. 3c shows a dense and featureless morphology. Films with this morphology were grown at pressures below 0.4 Pa and low substrate temperature and were also characterized by the XRD pattern of Fig. 2, curve a, indicating parallel orientation of the basal planes. It was also noticed that these films had a bright metallic appearance, contrary to the others which had a grey look. The bright appearance was also noticed by other researchers Že.g. w16x.. The composition of most of the films was characterized by a value of x ŽSrMo ratio. varying between 1.2 and 1.5. Some trends could be seen: x was smaller in
Fig. 3. Surface SEM images showing three typical morphologies: Ža. needle-like, Žb. cauliflower-like, Žc. featureless.
W. Lauwerens et al. r Surface and Coatings Technology 131 (2000) 216᎐221
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Fig. 4. Averaged coefficient of friction as a function of Ar pressure and bias voltage during deposition.
3.2. Friction and wear properties Fig. 4 gives the averaged friction coefficient as a function of the deposition pressure for zero bias and y100 V bias at two levels of RH. It is noticed that at - 10% RH the friction coefficient remains almost constant. At 50% RH the friction coefficient decreases with decreasing deposition pressure and seems to reach a minimum at 0.4 Pa. The negative bias during deposition gives friction coefficients that are systematically below the zero bias data points. At low pressure Ž PAr s 0.4 Pa. the difference between biased or unbiased deposition vanishes. In Fig. 5 the coefficient of friction as a function of fretting cycles is given for two films. The film in Fig. 5a represents a typical film deposited at PAr s 0.4 Pa, low substrate temperature and zero bias ŽXRD of Fig. 2a curve and morphology of Fig. 3c.. At - 10% and 50% RH the friction coefficient is practically the same and varies between 0.04 and 0.08. At 90% RH the friction coefficient is somewhat higher but remains fairly low at a value of approximately 0.12. Fig. 5b shows a different picture. This film is made using DC power, but the other deposition conditions were equal to those of the film in Fig. 5a. Especially at 50% RH the friction coefficient increases quickly to a level of approximately 0.3. Fig. 6 shows the wear volume of the same films as in Fig. 5 measured after the fretting test. The film made with bipulsed power shows a low wear volume that is almost the same for - 10% and 50% RH and increases by a factor of approximately 3 at 90% RH. The film made with DC power shows a low wear volume at - 10% RH, but a very high at 50% RH. At low Ar pressure and low substrate temperature some other experiments have been done. When changing the deposition power from 8 Wrcm2 to 4 Wrcm2 , it was found that the properties of the films remained
Fig. 5. Coefficient of friction as a function of the fretting cycles at different levels of humidity; Ža. typical film with the best properties, Žb. film prepared with the same conditions as Ža., but using DC power. At 90% RH the lifetime of the DC film was less than 30 000 cycles.
the same. An increase to 12 Wrcm2 , however, made the resulting films less resistant to humidity. This result can probably be ascribed to an increased rate of water vapour evolving from the target at high power. It is known that the presence of water vapour during deposition can alter the properties to a large extent w18x. It was also noted that the film properties were independent on film thickness for thicknesses ranging between 0.15 and 3.0 m.
Fig. 6. Wear volume at different levels of the relative humidity for the two films of Fig. 5.
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4. Discussion Most of the trends that were found as a function of the deposition parameters, have already been reported. At high deposition pressure Ž PAr ) 1 Pa. andror substrate temperature one usually finds a lamellar structure with perpendicular orientation of the basal planes w19x. Lowering the pressure during deposition leads to a gradual increase in the density of the film, as is indicated by Fig. 3b and Fig. 4 Žalso reported in w6x.. The decreasing friction coefficient at 50% RH with decreasing deposition pressure is interpreted as a denser structure and hence less susceptible to the uptake of moist air which is detrimental to the friction properties. The denser structure at low PAr and, as appears from Fig. 4, also as a consequence of Ar bombardment, is in agreement with the principles of sputtering. At low pressure the free path length is larger and hence sputtered species are less thermalized when reaching the substrate. The enhanced energy of the incident particles can also explain the lower S content in these films because of preferential re-sputtering of S. At PAr - 0.4 Pa the structure is very dense with pronounced basal plane orientation and bright appearance. This is in agreement with results obtained by others at a low deposition pressure w5,6,16,20x. However, the value of x obtained in this study Ž0.8᎐1. is lower than previous reported values and it is usually reported that x-values lower than approximately 1 correspond to an amorphous structure, which is clearly not the case here Žsee Fig. 2, curve a.. The XRD pattern of Fig. 2a resembles very well the spectra recorded by Muller et al. w6x who suggest a ¨ turbostratic structure for their thick and compact films deposited at low pressure Ž0.1 Pa.. This structure was also suggested in w21x for IBAD deposited films of high density and with basal orientation. The turbostratic or nearly random layered structure ŽNRLS. is characterized by the arbitrary rotational position of crystallites around an axis perpendicular to the substrate. We suggest that the structure of our very dense and low S films can be described by this structure which probably explains some degree of order in combination with the low S content. The low friction coefficient and the good wear resistance in humid conditions up to 90% RH ŽFig. 5a. of the films made by bipolar pulsed power can probably be ascribed to the very dense and low S containing structure. However, the minimum at 0.4 Pa in Fig. 4 can mean that going to lower pressures and further reducing the S content increases the friction, indicating that there is an optimal x-value. The wear volume of the film made by pulsed power ŽFig. 6. is excellent in our opinion. Because there are not many published data at such high humidity level, we can only compare with the data of Muller et al. w6x. They report an ¨
increase in wear rate of a factor of 17 when the humidity increases from - 5% to 98% RH, which is clearly more than the factor of 3 Žfor 90% RH. in our case. A very remarkable result is the properties of the film deposited by DC power. Friction and wear are much higher than for a film made with pulsed power, although not abnormal in comparison with published values. In w16x is reported a friction coefficient of 0.2 at 60% RH for dense and compact films made by DC magnetron sputtering. At this moment we do not have an explanation yet and further research is needed to point out whether there are fundamental changes in the deposition process when going from DC to pulsed power.
5. Conclusion MoS x thin films prepared by pulsed magnetron sputtering show very good friction and wear properties, also in moist air and hence were called humidity resistant. They show a very dense structure with pronounced basal plane orientation. Films made by DC power under identical deposition conditions show less tribological properties. Understanding the differences between both deposition processes can lead to a further improvement of the films made by pulsed power.
Acknowledgements This research is part of the Eureka project funded by the Flemish government ŽLubrimat 960235., a Belgium-China international project supported by the Flemish government and the PR China government Žproject BIL 96r35., and the IUAP P4r33 project funded by the Belgian government. References w1x T. Spalvins, J.Vac. Sci. Technol. A5 Ž1987. 212. w2x M.R. Hilton, P.D. Fleischauer, Surf. Coatings Technol. 54r55 Ž1992. 435. w3x ? Dimingen, H. Hubsch, P. Willich, K. Reichelt, Thin Solid ¨ Films 12 Ž1985. 79. w4x J. Moser, F. Levy, ´ Thin Solid Films 228 Ž1993. 257. w5x F. Levy, ´ J. Moser, Surf. Coatings Technol. 68r69 Ž1994. 433. w6x C. Muller, C. Menoud, M. Maillat, H.E. Hintermann, Surf. ¨ Coatings Technol. 36 Ž1988. 351. w7x E.W. Roberts, Tribol. Trans. 31 Ž1988. 239. w8x T. Spalvins, J. Vac. Sci. Technol. A5 Ž1987. 212. w9x L.E. Seitzman, R.N. Bolster, I.L. Singer, Surf. Coatings Technol. 52 Ž1992. 93. w10x D.G. Teer, J. Hampshire, V. Fox, V. Bellido-Gonzales, Surf. Coatings Technol. 94r95 Ž1997. 572. w11x M.R. Hilton, G. Jayaram, D.L. Marks, J. Mater. Res. 13 Ž4. Ž1998. 1022.
W. Lauwerens et al. r Surface and Coatings Technology 131 (2000) 216᎐221 w12x R. Gilmore, M.A. Baker, P.N. Gibson et al., Surf. Coatings Technol. 108r109 Ž1998. 345. w13x R.A. Scholl, Surf. Coatings Technol. 98 Ž1998. 823. w14x H. Mohrbacher, J.P. Celis, J.R. Roos, Tribol. Int. 28 Ž5. Ž1995. 269. w15x P. Campbell, J.P. Celis, J.R. Roos, O. Van Der Biest, Wear 174 Ž1994. 47. w16x A. Aubert, J.Ph. Nabot, J. Ernoult, Ph. Renaux, Surf. Coatings Technol. 41 Ž1990. 127.
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w17x G. Jayaram, N. Doraiswamy, M.R. Hilton, Surf. Coatings Technol. 68r69 Ž1994. 439. w18x V. Buck, Wear 114 Ž1987. 263. w19x M.R. Hilton, R. Bauer, S.V. Didziulis, M.T. Dugger, J.M. Keem, J. Scholhamer, Surf. Coatings Technol. 53 Ž1992. 13. w20x R.N. Bolster, I.L. Singer, J.C. Wegand, S. Fayeulle, C.R. Gosset, Surf. Coatings Technol. 46 Ž1991. 207. w21x D.N. Dunn, L.E. Seitzman, I.L. Singer, J. Mater. Res. 13 Ž1998. 3001.