Wear, 114 (1987) 263 - 274 263
PREPARATION AND PROPERTIES OF DIFFERENT TYPES OF
SPUTTERED MoSz FILMS
V. BUCK*
Lubricants and Tribology Branch, Institute for Technical Physics, Deutsche Forschungs-
und Versuchsanstalt fiir Luft- und Raumfahrt, 7000 Stuttgart 80 (F.R.G.)
(Received March 21,1986; accepted May 2,1986)
Summary
MoS, is widely used as a dry lubricant. For special applications, for
example bearings for space mechanisms, it is applied in the form of sputter-
deposited films. The lamellar structure of this material, the orientation of
the lamellae and the stoichiometry of the coating are important in the sliding
process and thus for the tribological performance.
It is shown that these properties can be varied to a large extent by the
variation of a single parameter in the deposition process. This parameter,
which is usually not controlled quantitatively, is the amount of Hz0 present
in the plasma. Furthermore, it is shown for the first time that all three dif-
ferent types of sputtered MoS, films, namely type I, type II and amorphous,
can be produced by a variation in this parameter solely.
1. Introduction
Since the pioneering work at the National Aeronautics and Space
Administration [l - lo] sputter-deposited films of MoS, have been used as
lubricants for space applications.
The usually obtained films, called type I, are microcrystalline (MoS,-
2H structure) with the c plane perpendicular to the substrate [ 11 - 131.
Recently, the existence of films called type II with the c plane parallel to the
substrate was reported [14, 151 but the only way to obtain this structure
was to use a different target (by a different vendor) [ 14,161. An amorphous
type with poor lubrication properties had already been prepared by Spalvins
using a cooled substrate [ 5, 61.
The morphological and stoichiometric properties of these films are
essential for their tribological performance. However, despite much work in
this field [ 17, 181 no model exists for the influence of the usual sputtering
parameters on the film properties. The purpose of this paper is to emphasize
the importance of contamination of the films by H,O which is present in the
*Present address: FB7 Physics Department, Universitiit GH Essen, Postfach 103764,
4300 Essen 1, F.R.G.
0043-1648/87/$3.50 0 Elsevier Sequoia/Printed in The Netherlands
264
plasma and to show how this indirect parameter of the deposition process
[19] governs the properties of sputtered MO& films. Indeed, all three
reported types can only be obtained by the variation of this parameter.
2. Experimental details
All the films are prepared by conventional r.f. sputtering; special atten-
tion was payed to the amount of Hz0 impurities in the plasma, which can be
controlled over several orders of magnitude, details are given in ref. 19.
The purity of the films depends both on the partial pressure of Hz0
pn,o during deposition and the sputter rate of MO!& Since a comparison of
these quantities is not very expressive, a contamination parameter C is intro-
duced. For this purpose the values of pHzOare formally converted into evap-
oration rates vn,o = COnStaId pH,o Using kIIIgmUi?S eqUatiOn [20] and then
C is defined by the ratio of these rates
c= vMoS,
V H,O + VMoS 2
= V MO%
V %O
if
The quantity C is similar to an impurity concentration and gives an
intuitive feeling for the degree of contamination.
Quantitative relations between this “hidden” parameter and the usual
settings of the sputtering equipment are reported elsewhere [ 191.
3. Results and discussion
3.1. Morphological properties
3.1 .l. Influence of Hz0 contamination
A systematic variation of C leads to pronounced morphological effects.
The variation in reflectivity is striking to the naked eye (Fig. 1). Under
R %1
50 -
_._
40 -
I-
30 -
20 - amorphous
typen type 1
I-
10
1 ------
. ._I
Oj"
10-L 10-j 10-Z to-' 1 c
Fig. 1. Reflectivity R of the films as a function of C.
(4
(4
(e) (f)
Fig. 2. Scanning electron micrographs of films with different contamination parameters
(scale bar represents approximately 100 nm): (a) C = 6 x 10-l; (b) C = 5 x 10-l; (c)
C is; 7 x 10d2; (d) C = 6 x 10e3; (e) C = 5 x 10e4; (f) C = 3 x 10A4.
normal conditions black sooty films are found; however, for the following
two extremes, very impure as well as extremely pure preparation conditions,
films looking rather specular and thus similar by visible inspection are
obtained (a point that possibly can lead to some confusion).
An investigation using a scanning electron microscope gives the follow-
ing results (Figs. 2(a) - 2(f)). The three reported types are clearly distinguished
in the following way: Fig. 2(a), amorphous; Figs. 2(b) - 2(e), type I, vertical
266
lamellae; Fig. 2(f), type II, lamellae parallel to substrate. Furthermore, it
appears that for type I films the tendency for dendritic growth is stronger
for the purer films.
The sizes of the lamellae are also affected. It has been shown earlier
[21] that these morphological properties can be well described by the num-
ber of lamellae per unit surface area II’ but since this quantity turned out to
be proportional to the reciprocal of the film thickness p* the expression
n’P*, which is independent of film thickness, is preferred as a means of char-
acterizing the structure. Its dependence on C is given in Fig. 3.
Besides these morphological effects it must be mentioned that the
deposition rate decreases with increasing Hz0 contamination (Fig. 4). This
might be due to the production of hydrogen in the plasma which is known
to reduce the deposition rate considerably [ 221.
0
10-L 10-j 10-Z 10-l 1 c lo~L 10~3 10-Z 10-l 1 c
Fig. 3. Grain size of films (expressed as n’ p*) as a function of C.
Fig, 4. Deposition rate as a function of C.
3.1.2. Influence of substrate temperature
Heating of the substrate could be a possible way of improving the
purity of the sputtered films, e.g. since the sticking coefficient of the gases is
reduced at higher temperatures. However, under common sputtering condi-
tions, i.e. after some minutes of presputtering, the purity of MO& films
decreases with rising substrate temperature as can be seen from Table 1. This
effect is easily explained by the fact that the heating of the substrate leads to
TABLE 1
Influence of substrate temperature
Substrate pH,O (X lo@ mbar) p* (mg cmP2) N’ (x106 mmP2)
temperature (“C)
100 1.9 0.78 8.3
165 1.8 0.56 10.2
195 2.6 0.51 13.5
240 4.0 0.33 20.7
335 9.8 0.21 30.3
267
G L
400 500 600 T K
Fig. 5. Dependence of C on the substrate temperature T.
a heating of the target (via the plasma) which results in enhanced Hz0 desorp-
tion [ 191. Thus C rises with increasing temperature (Fig. 5) if there is not a
thorough outgassing of the target prior to the deposition and the water
desorption from the target will no longer be affected by the substrate tem-
perature. It could be possible that the observed reduction in wear life of the
films deposited at enhanced temperatures [23] is connected with this
stronger contamination.
3.2. Chemical properties
3.2.1. Influence of Hz0 contamination
In the series of molybdenum compounds molybdenum can occur in dif-
ferent oxidation states (II - VI), where molybdenum(V1) compounds, e.g.
Moos, are the most stable and even mixed oxides are possible, e.g. MOO,-
Moos etc. [ 241. It is well known that sputtered MO& films are usually not
stoichiometric but show a slight deficit in sulphur [13, 25 - 271 (at least
after the target has reached equilibrium with respect to the sputtering rates
of its components [ 281). Furthermore, traces of oxygen can usually be found
within the films (Figs. 6(a) and 6(b)). Both the oxygen and the sulphur con-
tent depend strongly on C, as can be seen from Figs. 7(a) and 7(b). It appears
that the existence of type I films seems to be restricted to the stoichiometry
range 1 < Ns/N,, < 2 and that amorphous films are formed if Ns/N,, > 1.
The fact that type II films appear if Ns/NM, H > 2 and that oxidation processes
L
0 200 400 600 E eV
(b)
0 I
0 5 10 15 t mm
Fig. 6. (a) Auger electron spectrum and (b) depth profile of a typical film: 0, sulphur;A,
molybdenum; 0, oxygen.
268
L
1
(a) O W” 10-L 1om3 1o-2 10-l 1c
Fig. 7. (a) Sulphur and (b) oxygen content of films as a function of C: 0, calculated from
ref. 25; -, see eqn. (3).
by means of H,O in the plasma can be neglected is remarkable. This lack of
oxidation may be the reason why type II films cannot be obtained by reactive
sputtering in an AI-H2S plasma even if Ns/NMO > 2 [ 131.
These effects of C on the stoichiometry are obviously due to oxidation
of MO& by reaction with Hz0 in the plasma. There are two possible reactions
[291
MoS, + H,O - MOOS* + H2
(1)
MOO& + 2H,O - Moos + 2H,S
MoS, + 2H,O - MoOz + 2H,S
(2)
2Mo02 + O2 - 2MoOs
(H2S can be smelt after opening the chamber if Pn,o during sputtering has
been very high.) In both cases MoS,O, films result but the relation between
x and y is different. A simple calculation gives for MO& and MOO, x + y = 2,
for MO,!& and MoS02 x + y = 4 - 3cand for MoSz and Moos 3c + y = 3 - 0.5x.
Therefore if 3c+ y is plotted as a function of x, a possible choice can be
made between these alternatives; from Fig. 8 it can be concluded that MoOz
is the first oxidation product and that reaction II occurs. This reaction leads
to the relations
NS/NMO = 2(1 - C) (3a)
No/NM~ = 2C f background (3b)
that are plotted as full lines in Fig. 7.
As a result of this reaction molybdenum is still present in the moly-
bdenum(V1) oxidation state within the freshly sputtered film and further
oxidation to Moos, which is the final oxidation product [15], occurs after
sputtering under atmospheric conditions.
3.2.2. Final oxidation
The second step, the oxidation of molybdenum(IV) to molybdenum(W)
within the sputtered films, can be investigated, e.g. by X-ray photoelectron
spectroscopy (XPS) or Auger electron spectroscopic analysis. Figure 9(c)
shows the XPS spectrum of a fully oxidized surface that is identical to the
269
A
l/lmax Mo(lV1
0.5..
10
Fig. 8. Sulphur plus oxygen content as a function of sulphur content; included are straight
lines corresponding to the different reaction products.
Fig. 9. XPS spectra of MO& films at different oxidation states: (a) within a freshly depos-
ited film; (b) at the surface after some days of storage (a, Moos); (c) at the surface of a
fully oxidized film.
spectrum of Moos. In contrast, within a freshly deposited film the spectrum
given in Fig. 9(a) can be obtained, showing the characteristic peaks of moly-
bdenum(IV) only. However, after a short exposure to air at the surface a
mixture of molybdenum(IV) and molybdenum(V1) is present, Fig. 9(b) (in
this case the amount of molybdenum(V1) from the intensities of the lines,
i.e. Moos, can be calculated to be 25 at.%). From these spectra the two-step
nature of the oxidation process can clearly be seen.
To determine the total amount of oxygen and the rate of oxidation, the
S-MO ratio can be used. It can be obtained by means of Auger electron spec-
troscopy which has the additional advantage that the use of two rather dif-
ferent electron energies per element allows the simultaneous probing of two
different depths, since the high energy electrons determine the values inside
the substance and the low energy electrons probe the outermost layer.
This method was first applied to type I films by Stewart and Fleischauer
[30] who derived the expression Y = 0.17X + 0.03 where Y is the S-MO
peak-to-peak height for the high energy peaks and X is the low energy peak
ratio. This means that in the case of type I films surface oxidation does not
occur without simultaneous bulk oxidation. However, taking into account
the scatter of the data, a relation Y = cX seems equally justified and more
plausible since Y and X should vanish simultaneously for a zero sulphur
270
TABLE 2
Oxidation behaviour
Sample Surface oxidation Bulk to surface
coefficient X oxidation coef-
ficient Y/X
Type I film, 1.2 0.144
usual surface
after sputtering
Qpe I film as 7.6 0.137
above, wiped
surface
Type I film, 2.5 0.145
usual surface
after 6 months
storage
Type I film, 3.8 0.100
wiped surface
after 6 months
storage
concentration. Thus Y/X (after 2 min sputtering) is taken as a measure of
the ratio of the bulk oxidation to surface oxidation.
The degree of oxidation depends both on the stoichiometry, especially
the oxygen content of the film, and the surface area, i.e. the morphology.
This can be proved by the reduced oxidation of a film which is morphologi-
cally changed by wiping (this pushes the lamellae together and produces a
closed surface [21]). Table 2 shows the results for two parts of the same
sample immediately after deposition and after six months of storage in
humid (50% relative humidity) air; the reduced bulk oxidation of the wiped
sample can be clearly recognized.
3.2.3. influence of substrate bias
To enhance the purity of sputtered films, bias sputtering is a common
practice [31]. In a recent investigation Dimigen et al. [13] determined that a
zero bias gave the best film properties, However, since this investigation was
only carried out for one value of C (or within a very small margin of C), cor-
responding to Ns/NM, * 1.65, this must not necessarily hold for extremely
small values of C. Therefore the influence of a small bias may be an open
question to a certain extent for these rather pure films or type II films. High
bias voltages can certainly be neglected owing to the appearance of different
stoichiometries at the sharp corners (Fig. 10).
3.3. ~ibo~ogic~l properties
The tribologic~ behaviour was investigated using an LFW-1 test machine
(a common device for this purpose [32]) with an oscillatory motion, under
dry nitrogen (2% - 3% relative humidity) at ambient temperature using a con-
271
Ol
0 100 200 300 U&r v
Fig. 10. Stoichiometry vs. substrate bias.
tact pressure of 600 N mme2. The wear mechanism for dry lubricated layers
consists of an abrasive and a fatigue contribution. The relationship between
these two contributions depends strongly on the tribological conditions.
Thus general conclusions will not be given except for the following results
(Fig. 11(a)): the performance of type II films is significantly better than that
of type I films [ 141; amorphous films are not suitable as lubricants [ 61; the
lifetime of type I films decreases strongly with increasing contamination. In
rolling element bearings the fatigue mechanism plays an important role [13,
331, considerations relating the wear life in this case with disorder and con-
tamination are discussed elsewhere [ 341;
The friction coefficient p usually decreases after the start of the test
owing to the tendency of the lamellae to orientate themselves parallel to the
sliding direction. After this running-in process a constant friction coefficient
p,,,_in is achieved until coating failure occurs and the friction torque rises
steeply. /..ialso depends on the film thickness t and surface roughness h of the
samples but for values of the reduced film thickness well above t/h = 1 satur-
ation occurs [ 351. Only this condition is considered further.
The behaviour of P as a function of C is shown in Fig. 11(b). The high
value obtained for amorphous films corresponds to that already reported in
the literature [9]. For type I films we observed an increase in p,,,_in with
decreasing contamination and a constant value of pStart except for high
values of C. This can be explained firstly by the morphological properties
A
.zI
amorphous
,03~;ty~II~ , typ. I /1,_
(a) lo-‘ lo-’ w2 lo-’ i c (b) lo-’ JO-~ lo-’ lo-’ 1 c
Fig. 11. (a) Wear life and (b) friction coefficient as a function of C: 0, pstart; 0, /J,,ine
272
and secondly by an increase in the brittleness of the lamellae with increasing
con~ination, Therefore, for the purest films, ,ustart and p,,,-in are both
high owing to the fact that the planes of easy sliding are orientated perpendic-
ular to the substrate, i.e. the direction of motion. With increasing contamin-
ation the lamellae become more brittle and begin to break, become more
aligned with their planes of easy sliding and more parallel to the substrate
and thus show a decreasing value of F~,,_;~. For the highly con~minat~
films the lamellae are already broken by the application of the load and thus
P,,,_~~ and pstart are both low. These effects may also explain the known
influence of r.f. power on the wear life [36] and the differences reported
concerning the dependence of the wear life on film thickness [13, 37, 381.
Pronounced changes in the friction coefficient with variation in C and thus
with the stoichiometry, as reported for bias-sputtered films [ 13,391, cannot
be detected. Therefore these effects cannot be attributed to changes in
N,/N,, alone but seem to be induced by the bias process. As expected, after
formation of the lamellae for a short time, /Jrun_in is again small for type II
films in which a parallel orientation of the lamellae with respect to the sub-
strate is already established by the deposition process.
4. Conclusions and outlook
Hz0 impurities in the plasma during sputtering, qu~ti~tively expressed
by C, govern all the properties of the deposited films. Since this influence
has been practically neglected up to now, investigations concerning variations
in the usual deposition parameters going beyond only changes in C have to
be carefully re-examined.
Furthermore, the tribological performance of sputtered MO& layers
should be determined q~~titatively with regard to C. On this basis an engi-
neering model for the coating lifetime seems to be possible.
Acknowledgments
Thanks are due to Dr. H. Schmiedel, VG Inst~men~, Wiesbaden, and
to Dr. A. Eicke, Institut fiir Physikalische Elektronik, Universitat Stuttgart,
for the XPS measurements. Furthermore, I am indebted to Dr. M. v. Bradke,
Deutsche Forschungs- und Versuchsanstalt fiir Luft- und Raumfahrt, Stutt-
gart, for his contribution with the scanning electron microscope.
References
1 T. Spalvins, ASLE Trans., 12 (1969) 36.
2 T. Spalvins, Lubr. Eng., 25 (1969) 436.
3 T. Spa&ins, ASLE Trans., 14 (1971) 267.
4 T. Spalvins, NASA Tech. Note TN-D-7169, 1973 (National Aeronautics and Space
Administration, Cleveland, OH).
5 T. Spalvins, ASLE Trans., 17 (1974) 1.
6 T. Spalvins, Thin Solid Films, 53 (1978) 285.
273
7 T. Spalvins, NASA Tech. Memo. TM 81465, 1980 (National Aeronautics and Space
Administration, Cleveland, OH).
8 T. Spaivina, NASA Tech. Memo. TM 82809, 1982 (National Aeronautics and Space
Administration, Cleveland, OH).
9 T. SpaIvins, Thin Solid Films, 96 (1982) 17.
10 T. Spaivins, Proc. 3rd Znt. Conf. on Solid Lubrication, Denver, CO, 1984, in ASLE
Spec. Publ. 14, 1984, p. 201 (American Society of Lubrication Engineers, Park Ridge,
IL).
11 M. T. Lavik and M. E. Campbell, ASLE Trans., 15 (1972) 233.
12 G. G. Shaw and M. T. Lavik, in C. J. Arceneaux (ed.), F’roc. 30th Annu. Meet. of the
Electron Microscopy Society of America, 1972, CIaitor’s Publishing Division, Baton
Rouge, LA, 1972, p. 626.
13 H. Dimigen, H. Hiibsch, P. WiIIich and K. Reichelt, Thin Solid Films, 129 (1985) 79.
14 P. D. Fleischauer, ASLE Trans., 27 (1984) 82.
15 P. D. Fleischauer and L. U. Tolentino, Proc. 3rd Znt. Conf. on Solid Lubrication, Den-
ver, CO, 1984, in ASLE Spec. Publ. 14, 1984, p. 223 (American Society of Lubrica-
tion Engineers, Park Ridge, IL).
16 R. I. Christy, private communication, 1985.
17 Documentation TriboZogy, Vols. 1 - 18, Bundesanstah fur Materialpriifung, Berlin,
1963 - 1984.
18 A. R. Lansdown, Molybdenum disulphide lubrication, Rep. ESRO CR-402, 1974
(European Space Research Organisation).
A. R. Lansdown, Molybdenum disulphide lubrication, a continuation survey
1973 - 1974, Rep. ESA CR-764, 1976 (European Space Agency).
A. R. Lansdown, Molybdenum disulphide lubrication, a continuation survey
1975 - 1976, Rep. 77/363, 1977 (Swansea Tribology Centre) (European Space
Agency Contract, Rep. Ser.).
A. R. Lansdown, Molybdenum disulphide lubrication, a continuation survey
1977 - 1978. In W. R. Burke (ed.), ESA-TRZB-4, 1981 (European Space Agency trib-
series, Paris).
A. R. Lansdown, Molybdenum disulphide lubrication, a continuation survey
1979 - 1980, ESA-TRZB-7,1983 (European Space Agency tribseries, Paris).
A. R. Lansdown, Molybdenum disulphide lubrication, a continuation survey
1981 - 1983, Rep. ESA CR (P)-1973, undated (European Space Agency).
19 V. Buck, Thin Solid Films, 139 (1986) 157.
20 I. Langmuir, Phys. Rev., 2 (1913) 329.
21 V. Buck, Wear, 91 (1983) 281.
22 E. Stern and H. L. &swell, J. Vat. Sci. Technol., 4 (1967) 128.
23 M. Nishimura, M. Nosaka, M. Suzuki and Y. Miyakawa, Proc. 2nd ASLE Conf., 1978,
American Society of Lubrication Engineers, New York, 1978, p. 128.
24 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley, New York,
2nd edn., 1966, Section 30C.
25 T. Ito and K. Nakajima, Philos. Mag., B 37 (1978) 773.
26 H. Dimigen, H. Hiibsch, P. WilIich and K. Reicheh, Thin Solid Films, 64 (1979) 221.
27 G. Rochat, J. Lammer, H. E. Hintermann, P. Niederhiiuser and M. MaiIIat, Proc. 1 Oe
Congr. Znt. de Chronometrie, Gendve, 1979, Societe Suisse de Chronometrie, Buren,
1979, Vol. 3, p. 413.
28 B. C. Stupp, Proc. 3rd Znt. Conf on Solid Lubrication, Denver, CO, 1984, in ASLE
Spec. Publ. 14, 1984, p. 217 (American Society of Lubrication Engineers, Park Ridge,
IL).
29 A. R. Lansdown, Molybdenum disulphide lubrication, Rep. ESRO CR-402, 1974
(European Space Research Organization) p. 28.
30 T. B. Stewart and P. D. Fleischauer, Znorg. Chem., 21 (1982) 2426.
31 L. Maissel, Application of sputtering to the deposition of films. In L. I. Maissel and R.
Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970, pp.
4 - 22.
274
32 M. N. Gardos, Lubr. Eng., 32 (1976)463.
33 M. J. Todd, Tribal. Znt., 15 (1982) 331.
34 V. Buck, Proc. 2nd European Space Mechanisms and Tribology Symp., Meersburg,
October 9 - 11, 1985, in ESA Spec. Publ. SP-231, 1985, p. 155 (European Space
Agency, Nordwijk).
35 T. Shimazaki and 0. Winer, Proc. JSLE Int. Tribology Conf., Tokyo, July 8 - 10,
1985, Elsevier, Amsterdam, 1986, p. 425.
36 R. I. Christy, Thin Solid Films, 73 (1980) 299.
37 R. I. Christy and H. R. Ludwig, Proc. of the 6th Znt. Conf. on Metallurgical Coatings,
San Diego, CA, April 23 - 27, 1979, Vol. 2, Elsevier, Lausanne, 1979, p. 223.
38 C. Menoud, H. Kocher and H. E. Hintermann, in M. Oechsner (ed.), 5th Znt. Conf. Zon
and Plasma Assisted Techniques IPAT’85, Munich, May 13 - 15, 1985, CEP Consul-
tants, Edinburgh, 1985, p. 277.
39 D. R. Wheeler, NASA Tech. Memo. TM-78896, 1978 (National Aeronautics and
Space Administration, Cleveland, OH).
