- Email: [email protected]
Solid lubricating films produced by ion bombardment of sputter deposited MoSx films
Sm~fiu'e aJ~d('outi~tgs Techmd~ey, 51 (1992) I Ig 123
118
Solid lubricating films produced by ion bombardment of sputter
deposited MoSx films Niels Jorgem M i k k e l s e n * a n d G. S o r e n s e n Institute of Physi¢',~, University ofAarhus, DK-~VOtX)Aarhus (" !Denmark .~
Abstcact Several attempts to enhance the wear resistance (the "sliding life") of sputter deposited MoS, lilms have previously been made. However. sputter deposition of MoS,. films is often difficult to control owing to changes in substrate temperature and prc~nce of water vapour in the plasma during deposition, In the present study MoS,. films were deposited under extremely well controlled conditions with respect to both the deposition temperature and the water vapour pressure, 4~X)keV A r bombardment, performed after film deposition at doses varying from 3 × I(P 3 Ar ~ c m ~ to I x I(P ~ Ar" cm -' is shown to have a minor influence on film composition, and a marked influem.~eon film structure, The ion b~nnbarded films were subjected to tribological investigations and the sliding life of the various ion bombarded MoS~ layers was related to film structure. Using this approach it was po,~sible to gain valuable information on the relation between MoS, lilm structure and tribological bchaviour. Thus, it was shown that the characteristic columnar plate*like MoS,~ structure is only ~d'~tained when H.,O is pre~nt in the plasma, and high flucnce (around I × 10t~ Ar- cm- -') ion beam enhanced sliding life is only found when bombarding these plate*like lilms, Furthermore, it is shown that the presence of substantial amounts of oxygen in the films (about 25 at,%l is not detrimental to the tribological behaviour, unless the oxygen i n d ~ inferior plate*like structures,
I. latroLaetioa Sputter deposited MoS~ films have been subjected to a large number of experimental studies where several attempts to enhance the wear resistance (the "sliding life") of the system have been made. Thus, Chevallier et al, [1], Kobs et al. [2, 3] andd Mikkelsen andd coworkers [4-6] have shown the feasibility of increasing the sliding life of the films by 100 k e y ion bombardment. The ion bombardment has been shown to have a minor influence on film composition and a marked influence on film structure [4, 5]. Thus, 100 keV ion bombardment may be a powerful tool to study the relation between film structure (mass density, morphology andd crystallinity) and tribological behaviour. It is well known that the structure of MoS~ films plays an important role in the sliding life [7-11], andd it has been shown by guck [12] that the presence of water vapour in the plasma during deposition has a major influence on both film structure, film composition andd oxygen contamination, and sliding life, It is, however, not possible from these studies to conclude whether or not the changes in sliding lives are related to the structural changes and/or altered film composition or comamination, The films studied by Buck were produced at 100 "C, and Spalvins [13] has shown that the crystal*Present address: J, Mikkeisen, Danish Technological Institute, Teknologiparken, DK-8000 Aarhus C, Denmark,
0257-8972/92:$5.00
linity of sputtered MoS~ films increases dramatically when the deposition temperature exceeds approximately 20 "C, Thus, film depositions carried out at ambient temperatures can be difficult to control, since the substrate temperature can increase to 70 "C or more, owing to process-induced heating. in the present study MoS~ films were deposited under extremely well controlled conditions with respect to both deposition temperature and water vapour pressure. Using 400 keY Ar + bombardment, performed after film deposition, the film structures were changed without altering the film composition and contamination. This approach makes it possible to gain valuable information on the relation between film structure andd sliding life.
2, Exlterimemal
~oeedwre
2.1. Film deposition
Solid lubricating MoS~ films were deposited in an r.f. diode sputtering chamber onto AISI 52100 bearing steel discs at two substrate temperatures. In all cams the deposited films had a thickness of 96 lag cm 2 (corresponding to 200 nm for MoS2 bulk density). Prior to film deposition the discs were mechanically polished with diamond paste to a surface roughness of R, ~ 2 nm, followed by ultrasonic cleaning in ethanol and demineralized water. Films were deposited both as "AT" films,
,q" 1992
ElsevierSequoia, All rights r¢~rved
N, J, Mikkelsen, G, Soren.wa / Ion bombardment c4"MoS.~ films
at ambient temperature (26+ 5 °C), and as "HT" films at 200+5 "C. A liquid nitrogen cooling trap in the deposition crambe,r, adjacent to both the argon inlet and the plasma region, minimized the residual water vapour pressure, Furthermore, it was possible to increase the water vapour pressure in the plasma during the process by introducing water vapour into the chamber through a needle valve. Thus, MoS.~ films can be deposited both under extremely dry plasma conditions, using the cooling trap, and under humid plasma conditions by deliberately introducing H20 vapour into the chamber, In the "dry" mode (with the cooling trap), the background pressure before sputtering was I × 10-s Pa, and the ¢ondRioned system had an estimated residual-gas pressure of about 5 x 10 -4 Pa during film deposition. In the "H20" depositions, the partial water-vapour pressure was 1 × 10- 2 Pa. Thus, four different deposition conditions were applied: HT-H20, HT-dry, AT-H20, and AT-dry. In all cases, the depositions were carried out under the following general conditions: target-substrate distance 3 cm; power density 5 W can-2; deposition rate 8.01~gcm-2min-l; target presputtering 60rain at 5 W cm-2; argon pressure 2.5 Pa; substrate sputter-etch cleaning 9 rain, 1.7 keV 20 mA d.c. at 7.5 Pa argon pressure (sputtering off about 50 nm of the substrate surface), 2.2, Ion bombardment Ion bombardment of the MoSx layers was performed after film deposition, with a 400 keV Ar + mass-separated beam, Ion fluences were 3×1013, I×10 !4, 3x1014, 1 × 10is, 3 x 10is, and I × 10 !6 Ar + c m - 2 and the sample temperature never exceeded 60 °C, The background pressure during implantation was less than 1,2 × 10 -4 Pa, and the beam current density on the sample was kept at 3 ~.A cm-2 The ion energies and fluences chosen ensured that most of the ions penetrated the film and, furthermore, the induced radiation damage in the film (estimated from TRIM calculations [14])was almost homogeneous from the surface to the film-substrate interface, ranging from about 0.06 at the lowest dose, to 20 displacements per film atom at the highest dose. The ion bombardmerR was thus expected to cause a varying degree of atomic displacement, from almost no damage to a total mixing of the film layer atoms. 2.3. Film properties and tribological performance The stoichlometry and film thickness (micrograms per square centimetre) were measured by Rutherford backscattering spectrometry (RBS). The oxygen content which, as shown below, is an important parameter, was measured for films deposited onto carbon substrates.
119
The geometrical thicknesses and surface profiles were measured by a Dektak 3030 stylus profilometer, and by combining the RBS thickness measurements with the profifometer data, the film density could be evaluated. The film morphology and cxystallinity were studied by scanning electron microscopy (SEM) and transmission ele~ron microscopy (TEM). The TEM measurements were made on films deposited o n t o aluminium and steel. After removal of the metal substrates by mechanical polishing, followed by ion-milling (for steel) or by chemical etching for aluminium, the films were ready for TEM. The friction and wear measurements were taken in a dry nitrogen atmosphere, using a reciprocating "ball-ondisc" tribometer under conditions described in ref. 5. The sliding life is used as a measure of the wear resistance of the system and is defined as the time from the beginning of the test until the coeffw,ient of friction /~ rises dramatically from typically #~,0.03 to values exceeding 0,2,
3. E x l ~ ~ l
res~
3,1, Film characterization After daily use of the sputtering chamber for about 2 months, the system was conditioned, giving stable and reproducible deposition conditions, The MoS~ films had an x value of about 1,7 and a controllable oxygen content, as shown in Table 1. The oxygen content in the films from the dry runs is similar to the lowest values observed by Buck [12], and for the HaO runs the oxygen c,ontent 08%-27%) is similar to the values measured when the HzO vapour pressure is not controlled [12, 15-17]. Figure ! shows the morphology of as-deposited HT films for both the HT-H20 and HT-dry deposition conditions, it is observed that the morphology changes
l~tm Fig, I, Scanning electron microlffaphs showing the morphology of Mo$~ films made at 200 ~C: left, film deposited under humid plasma conditions (HT-H~O film}; right, film deposited under extremely dry plasma conditions (HT-dry film),
N, J, Mild
120
TABLE I, The stoiehiometry x and oxygen content of Mo$~ layers made under controlled conditions Delx~,xition temperature
Plasma contamination
Film cormposition After maximum Ar" radiation
As-del~sited
AT (26 C + 5 CI HT
Dry H~O Dry
1,68 ± 0,02 1,77+0,01 1,63 +0,01
(2iX) ('+5 'C)
H~O
1.62+03)I
from the very characteristic columnar plate-like "MoS~ structure" [9, 18-23] to a more dense and featureless morphology when the H 2 0 content in the plasma is decreased. X-ray diffraction analysis (not shown here) reveals that the grain size in AT films is below the resolution limit of the measurement, i.e. the diffraction spectra are similar to spectra from amorphous films. As expected, the HT films are more crystalline than the AT films, and X-ray measurements indicate that the HT-H20 films are more crystalline than the HT-dry films, in contrast to the findings of Buck [ 16], it is observed that the presence of H 2 0 in the plasma has no influence on film orientation. Both the HT-H20 and HT-dry films have about 90% of the basal planes perpendicular and 10% parallel to the substrate, i.e. they are type I films [9]. The TEM images shown in Fig, 2 of the HT-dry, HTH20, AT-dry, and AT-H20 films after ion bombardment at various doses exhibit the following characteristics. For the lowest doses, the film morphology exhibits no significant changes compared with as-deposited films and it is observed that the HT-dry films to some extent resemble the AT films, For the as-deposited AT films the structure is deaase, fibrous and glassy, and the influence of H 2 0 vapour content in the plasma is limited. With increasing bombardment doses, the HT-H20 film morphology is dramatically changed. The lamellar columns seem to be smeared out, and at I × l0 j6 Ar + cm 2 the film structure has changed completely to a fully dense, structureless morphology. For the AT films, only minor changes are observed, but the as-deposited dense structure changes to a more featureless morphology similar to that of the I ×10 ~ Ar ÷ cm 2 bombarded HT films.
3.2. Tribological characterization Figures 3 and 4 show for the HT and AT film depositions respectively the relation between film density and crystallinity (lower parts), and the sliding lives (upper parts). The tribological tests were performed on 12 samples: three similar samples produced under each of
Oxygen content (% atoms)
x
Oxygen content I% atoms)
6,9 + 0,7 25.3 ___+0.3 6,3 +_0,5 18,0 ± 0.2
1,66 __.+0,02 1,77 ± 0,02 1.60 + 0,01 1.60 4- 0,02
7.0 +___0.¢-, 25.9 + 0A 6,3 ± 0,7 14,4 ± O,7
the four different deposition conditions. On each sample parts of the layers were bombarded by Ar* at the various doses. Seven tests were performed on each sample, each representing the various Ar + doses, including a test on a non-treated area. Each .sliding life result is thus the mean value of three replicated tests. It is observed that for all MoS.~ layers, the films are amorphous at doses exceeding the "transition-region'" hetwcen I x l 0 ~ and I ×10 !s Ar + cm -a, For the ion beam amorphized films the TEM diffraction patterns exhibit only diffuse haloes with no trace of diffraction rings due to polycrystalline structures. Furthermore, with high resolution TEM lattice imaging it was not possible to observe any periodic fringes from the (002) basal planes in the ion beam amorphized films. For all films tested, the coefficient of friction was between 0,03 and 0.06, confirming previous results showing low friction for ion-beam amorphized MoS~ films [4, 5]. For the HT-H20 plate-like films the benefits from ion implantation are cfearly illustrated. The bombardment increases the sliding life by a factor of about five for I×10 !~Ar + cm -2 and a factor of 2,6 for I × 10Is Ar + cm ', thereby confirming the results from ref. 5. However, it is observed from the results shown in Figs. 3 and 4 that high-dose {(3-10)×10~SAr + cm--') ion-beam enhanced wear resistance seems to be obtained only when columnar plate-like films (the HT-H20 films) are bombarded. Films deposited at 200 :C under extremely dry conditions (the HT-dry films) have, in the as-deposited state, a sliding life which is four times as long as that of the H 2 0 films, and at high doms (amorphous films), no beneficial effect from ion bombardment is observed. For the AT films, no significant differences in the sliding life between the "dry" and the "H20" films are observed. But the endurance life of the as-deposited AT films is about seven times that of the HToH,O films. and at high doses the implantation reduces the wear resistance of the former. These observations seem to imply that marked benefits from high-dose (around I × l0 .l~ Ar ÷¢m a ) 100 keV ion
N,J, Mikkeisen, G, Sorensen / Ion bombardment of MoS~Jilms
HT-H20
{
121
w'*
/
HT-
AT-H20
II
AT-dry
3xl~ 4
Ixl~
3xl~
IxlO~
Des, JAr÷/earl 0A Fig, 2, TEM bright field images of HT-H~O, HT-dry, AT-HzO, and AT-dry MoS~ films bombarded with 400 keV Ar + at various doses,
bombardment are obtained only when the as-deposited layers are porous lamellar films. The MoS~ films made by Kobs et al. E2, 3] (although deposited at 60 °C) may all have been the lamellar plate-like films owing to the l~esence of H20 in the plasma. The ion-beam-reduced film thicknesses reported by K obs et al. in ref. 3 could confirm this. In the case of iow-dose ion bombardment (at (0.31) × lot 4 Ar + cm -2, where radiation damage is still limited) the sliding life is enhanced by a factor of 1.3-1.9 for the four different films. This cannot he explained by enhancement of the film densities, which in this dose range is marginal. It is evident from the results in Figs 3 at~d 4 that for more glassy films (i.e. the AT and the HT-dry films), which are much denser in the as-deposited state than the columnar HT-H20 film, the wear resistance decreases with it~cxeasing amorphization beyond the transition region. This may imply that, when the films
are tested under the present conditions, the amorl~hous state of the MoS~ films is not optimum with respect to tribological behaviour. The maximum sliding life for the HT-HzO films obtained at 1 xlot 6 Ar + can -= cannot he attributed to the amorphous structure, because HT-HzO films bombarded with 3 × l0 ts Ar + can -2 are completely amorphous but have a sliding life of only half the value found for the highest dose. This may he explained by the fact that the film demsity at the lower dose ( 3 x l o t S A r + c m -2) is still not as high as for the ! ×lot6Ar + can -2 dose. Furthermore, when studying TEM images in Fig. 2, it is observed that the HT-H20 films only become fully dense and smooth at the highest dose. When comparing Figs 3 and 4, it is observed that the sliding life is not related to film density alone. Thus, the density of HT-H20 films bombarded with 3×lots Ar + can -2 is about 4.1 g c m -3, very similar to the 4.4 g c m - 3 of the as-deposited AT-H20 film, but the latter exhibits a sliding life three times longer.
N, J. Mikkelsea, G, Soren,sen : Ion bombardment of MoS.jilm,~
122 j
~-T ¸¸~" ~
.
lxl0S i__1_,,_~_ HT-dry
I "'-- I
T
'l /P~"
I
HT-H~O
I
I
" --T~
I
-------
T " ¢ ~ " r - " -t--- ~ ' r - - "
~ --'1- ............ "1
AT-H20
I AT- dry
o
R
0
N 5x10¢
CO
3±
_e
}
lx10s
>.,
% I-0 Z
, I , I
C3
51
5x10¢
--I
, __. I J
Bulk
E
HT-HzO
HT-dry
Bulk
.=
~.
_
t
~?
J t..-
~
~ :/--~
gso Ig .
3
g
AT-dry
g-
5 I.L
l
I
I
I
AT-H20
g-1
E
0
I ;: ,
as
dep,
I
,
10~' 10 ~
1~16~
I .. ,
as
I
,
I
~
I
10~' 10~s 10~
Bulk
7
>-
dep, Ar* DOSE (Ar'/cm 2)
Fig, 3, Above: sliding life measurements of HT-dry and HT-H=O films bombarded with 400 keV At* at various doses; test conditions, rteipa"ocaring ball-on-disc, dry nitrogen atmosphear¢, e 5 mm AISI 52100 steel ball, 7 Hz and 2 mm amplitude, Below: the corrtsponding film mass dr.nsities and structurt~, Film grain sizes arc added,
Z LIJ CZ _i LL
3
I
2
~E
When tested under the present conditions, improved wear remistanc¢ by 100 keY argon bombardment (doses in the range I x 10!s-I x 10 ~ Ar + cm -2) is only obtained for columnar plate-like films, Results in refs, 4-6 and the findings of Kobs et al, [2, 3] (from tests under simihr conditions), showing enhanced wear resistance of sputter deposited MoS~ layers by 100 keY argon bombardment in the same dose range, arc all based on "typical" MoS:, films with a plate-like morphology, Spalvins [13] has described the wear and failure mode of sputtered MoS~ films as premature brcaking-off of the plate-like columns due to lack of intercolumnar cohesion combined with film brRtfencss, This is in agreement with the findings of Buck [ 12], where plates in the film break off during wear. The rcsuRs from the present study show that the typical as-deposited columnar plate-like MoS:, films exhibit inferior wear resistance compared with more dense and featureless films; this is probably explained by breaking-off of the columns, as shown in a previous
zo~ ?
~ ~-~
li
1 O'
I ..,
QS
d~p
4, Diseassm
~
~
L
I
10it. 10is 1016
as
101~ 10~s 10is
dep
Ar" DOSE (Ar'Icm~) Fig, 4, Above: sliding life measurtments of AT-dry and AT-H20 films bombarded with 400 kcV A t - at various doses: test conditions, reciprocating ball-oh-disc, dry nitrogen atmosphere, e 5 mm AIS! 52100 steel ball, 7 Hz and 2 mm amplitude. Below: the ¢orrcspondiog film mass densities and structurts, Film grain sizes art add~l,
publication [5], Ion bombardment at high doses is thus acting like a repair tool for plate-like films, where the columns are smeared out and the voids filled out, finally at the highest doses, to be transformed to non-columnar fully dense films. For low-dose ion bombardment in the range 3 × l 0 t3 3 × 1014 Ar + cm -2, the sliding lives for both featureless and columnar film types are increased by a factor of about !.3--1.9. In this dose range, the structural changes arc marginal, but at the film-subslrate interface chemical bonds may be modified which may resuR in an enhanced film-substrate adhesion [24], even though no significant film-substrate intermixing occurs, Oxygen contamination of sputtered MoS~ films due to H 2 0 vapour in the plasma (Table I) only causes the
N. J. Mikkelsen, G. Soren.wn / Ion bombardment of MoS~films
tribofogical behaviour of the film to deteriorate if the
123
itdereaees
presence of H20 has induced a plate-like morphology. Thus oxygen is not an important intrinsic parameter, but acts more as a catalyst for columnar film growth, which may then have a detrimental effect on the tribological properties, 5. C o c k , sims
The prese~ study has shown the feasibility of ion bombardment for studying basic phenomena in MoS~ solid lub6cation, and surface modificationby ion beams has been shown to be a useful tool for understanding the complex nature of MoS~, sputter deposited films. The tribological behaviour of sputtered MoS~ seems to be governed exclusively by structural properties. When tested under the present conditions, the optimum wear resistant film structure should thus exhibit a nanocrystalline, dense and featureless morphology; whereas an increase in the grain size (induced by a combination of elevated deposition temperatures and oxygen film contamination) reduces the wear resistance, probably owing to a reduced film mass density resulting from a plate-like structure. This indicates that an "~leal" grain size for MoSx films may exist [18]. The reason for this, however, may he that the '~feaF' grain size reflects an optimum balance between the degree of crystallinity and high film density.
The work was supported by the Danish Development P r o g r a m m e for Materials Technology, Centre for Surface T e c h n o l o g y - Dry Coating Processes.
I J, Chevallier, S, Ok,sen, G, ~re.e~en and k Gul~a, ApI~, Phys, Len,, 48 (1986) 876, 2 K, Kob~ H. D/migen, H, H / ~ h , H.J, Tolle, R, Leut©neckcr and H. Ryssel,Appl. Phys. Lett., 49 (1986) 496. 3 K. Kob~ H. Dimigen, H. Hiibsch, H. J. Tolle, R. Leutenecker and H. Ryssel, Mater. Sci. Eng., 90 (1987) 281. 4 I'4.J. Mikkelsen,J. Chevallier,G, Smensen and C, A. Straede, Appl. Phys. Lett.. 52 (1988) 1130. 5 1,4.J. Mikk©lsen and G. Sm'ensen, Mater. Res. Soc. Syrup. Proc.. :40 (1989) 265, 6 N.J. Mikkehen and G. Sorensen, Miner. Sci. Eng.. 115 (1989) 343. 7 R. 1. Christy and H. R. Ludwig, Thin Solid Films. 64 (1979) 223. 8 R, I. Christy, Thin Solid Films, 73 (1980) 299, 9 P.D. Fleischauer, ASLE Trans.. 27 (1984) 82. 10 E.W. Roberts, 20th American Mechanisms Symposium, NASA Conf. PubL 2423, 1986 (NASA, Washinston, DC), p. 103. 11 M.R. Hilton, R. Bauer and P.D. Fleischauer. Thin Solid Films. 188 (1990) 219. 12 V. Buck, Wear. 114 (1987) 263. 13 T. Spalvins, NASA TM-8.LT63, 1984 (NASA, Washington, DC).
14 J. F. Ziegler, J.P. Biersack and U. Littmark, in J. F. Ziegler (ed.) The StOpl~nf of Ions in Solids, Vol. 1, Pergamon, New York, 1985, p, 109. 15 J. R. Lince, J. Mater. Res.. 5(I) (1989) 1990. 16 V. l~ck~ Vacuura-Teknik. 4 (1988) !11. 17 S. Fayelle, P. D. Ehni and I. L. Singer, in D. Dowson, C. M. Taylor and M. Godet (eds.), Mechanics of Coatings. Tribology Series. 17,
Elsevier, Amsterdam, 1990, p, 129, 18 P, D, Fle/schauex and R. Bauer, ASLE Trmls,, 30 (1987) 160, 19 P. D, Fleischauer, M, R. Hilton and R, Bauer, in D. Dowson, C, M, Taylor and M. Godet (eds.), Mechanics of Cominfs. Tribolo~y Series. 17, Elsevier, Amsterdam, 1990, p. 121. 20 M.R. Hilton and P. D. Fleischauer, Miner. Res. Soc. Syrup. Proc.
21 22 23 24
140 (1989) 277. M.R. Hilton and P.D. Fleischauer, J. Miner. Res.. 5 (1990) 406. T. Spalvins, Thin Solid Films. 73 (1980) 291. J.R. IAnoeand P. D. Flei~ohauer, J. Miner. Res.. 2(6) (1987) 827. J. E. E. Baglin, G. J. Clark and J. Bottiger, Miner. Res. Soc. Syrup. Proc.. 25 (1984) 329.
118
Solid lubricating films produced by ion bombardment of sputter
deposited MoSx films Niels Jorgem M i k k e l s e n * a n d G. S o r e n s e n Institute of Physi¢',~, University ofAarhus, DK-~VOtX)Aarhus (" !Denmark .~
Abstcact Several attempts to enhance the wear resistance (the "sliding life") of sputter deposited MoS, lilms have previously been made. However. sputter deposition of MoS,. films is often difficult to control owing to changes in substrate temperature and prc~nce of water vapour in the plasma during deposition, In the present study MoS,. films were deposited under extremely well controlled conditions with respect to both the deposition temperature and the water vapour pressure, 4~X)keV A r bombardment, performed after film deposition at doses varying from 3 × I(P 3 Ar ~ c m ~ to I x I(P ~ Ar" cm -' is shown to have a minor influence on film composition, and a marked influem.~eon film structure, The ion b~nnbarded films were subjected to tribological investigations and the sliding life of the various ion bombarded MoS~ layers was related to film structure. Using this approach it was po,~sible to gain valuable information on the relation between MoS, lilm structure and tribological bchaviour. Thus, it was shown that the characteristic columnar plate*like MoS,~ structure is only ~d'~tained when H.,O is pre~nt in the plasma, and high flucnce (around I × 10t~ Ar- cm- -') ion beam enhanced sliding life is only found when bombarding these plate*like lilms, Furthermore, it is shown that the presence of substantial amounts of oxygen in the films (about 25 at,%l is not detrimental to the tribological behaviour, unless the oxygen i n d ~ inferior plate*like structures,
I. latroLaetioa Sputter deposited MoS~ films have been subjected to a large number of experimental studies where several attempts to enhance the wear resistance (the "sliding life") of the system have been made. Thus, Chevallier et al, [1], Kobs et al. [2, 3] andd Mikkelsen andd coworkers [4-6] have shown the feasibility of increasing the sliding life of the films by 100 k e y ion bombardment. The ion bombardment has been shown to have a minor influence on film composition and a marked influence on film structure [4, 5]. Thus, 100 keV ion bombardment may be a powerful tool to study the relation between film structure (mass density, morphology andd crystallinity) and tribological behaviour. It is well known that the structure of MoS~ films plays an important role in the sliding life [7-11], andd it has been shown by guck [12] that the presence of water vapour in the plasma during deposition has a major influence on both film structure, film composition andd oxygen contamination, and sliding life, It is, however, not possible from these studies to conclude whether or not the changes in sliding lives are related to the structural changes and/or altered film composition or comamination, The films studied by Buck were produced at 100 "C, and Spalvins [13] has shown that the crystal*Present address: J, Mikkeisen, Danish Technological Institute, Teknologiparken, DK-8000 Aarhus C, Denmark,
0257-8972/92:$5.00
linity of sputtered MoS~ films increases dramatically when the deposition temperature exceeds approximately 20 "C, Thus, film depositions carried out at ambient temperatures can be difficult to control, since the substrate temperature can increase to 70 "C or more, owing to process-induced heating. in the present study MoS~ films were deposited under extremely well controlled conditions with respect to both deposition temperature and water vapour pressure. Using 400 keY Ar + bombardment, performed after film deposition, the film structures were changed without altering the film composition and contamination. This approach makes it possible to gain valuable information on the relation between film structure andd sliding life.
2, Exlterimemal
~oeedwre
2.1. Film deposition
Solid lubricating MoS~ films were deposited in an r.f. diode sputtering chamber onto AISI 52100 bearing steel discs at two substrate temperatures. In all cams the deposited films had a thickness of 96 lag cm 2 (corresponding to 200 nm for MoS2 bulk density). Prior to film deposition the discs were mechanically polished with diamond paste to a surface roughness of R, ~ 2 nm, followed by ultrasonic cleaning in ethanol and demineralized water. Films were deposited both as "AT" films,
,q" 1992
ElsevierSequoia, All rights r¢~rved
N, J, Mikkelsen, G, Soren.wa / Ion bombardment c4"MoS.~ films
at ambient temperature (26+ 5 °C), and as "HT" films at 200+5 "C. A liquid nitrogen cooling trap in the deposition crambe,r, adjacent to both the argon inlet and the plasma region, minimized the residual water vapour pressure, Furthermore, it was possible to increase the water vapour pressure in the plasma during the process by introducing water vapour into the chamber through a needle valve. Thus, MoS.~ films can be deposited both under extremely dry plasma conditions, using the cooling trap, and under humid plasma conditions by deliberately introducing H20 vapour into the chamber, In the "dry" mode (with the cooling trap), the background pressure before sputtering was I × 10-s Pa, and the ¢ondRioned system had an estimated residual-gas pressure of about 5 x 10 -4 Pa during film deposition. In the "H20" depositions, the partial water-vapour pressure was 1 × 10- 2 Pa. Thus, four different deposition conditions were applied: HT-H20, HT-dry, AT-H20, and AT-dry. In all cases, the depositions were carried out under the following general conditions: target-substrate distance 3 cm; power density 5 W can-2; deposition rate 8.01~gcm-2min-l; target presputtering 60rain at 5 W cm-2; argon pressure 2.5 Pa; substrate sputter-etch cleaning 9 rain, 1.7 keV 20 mA d.c. at 7.5 Pa argon pressure (sputtering off about 50 nm of the substrate surface), 2.2, Ion bombardment Ion bombardment of the MoSx layers was performed after film deposition, with a 400 keV Ar + mass-separated beam, Ion fluences were 3×1013, I×10 !4, 3x1014, 1 × 10is, 3 x 10is, and I × 10 !6 Ar + c m - 2 and the sample temperature never exceeded 60 °C, The background pressure during implantation was less than 1,2 × 10 -4 Pa, and the beam current density on the sample was kept at 3 ~.A cm-2 The ion energies and fluences chosen ensured that most of the ions penetrated the film and, furthermore, the induced radiation damage in the film (estimated from TRIM calculations [14])was almost homogeneous from the surface to the film-substrate interface, ranging from about 0.06 at the lowest dose, to 20 displacements per film atom at the highest dose. The ion bombardmerR was thus expected to cause a varying degree of atomic displacement, from almost no damage to a total mixing of the film layer atoms. 2.3. Film properties and tribological performance The stoichlometry and film thickness (micrograms per square centimetre) were measured by Rutherford backscattering spectrometry (RBS). The oxygen content which, as shown below, is an important parameter, was measured for films deposited onto carbon substrates.
119
The geometrical thicknesses and surface profiles were measured by a Dektak 3030 stylus profilometer, and by combining the RBS thickness measurements with the profifometer data, the film density could be evaluated. The film morphology and cxystallinity were studied by scanning electron microscopy (SEM) and transmission ele~ron microscopy (TEM). The TEM measurements were made on films deposited o n t o aluminium and steel. After removal of the metal substrates by mechanical polishing, followed by ion-milling (for steel) or by chemical etching for aluminium, the films were ready for TEM. The friction and wear measurements were taken in a dry nitrogen atmosphere, using a reciprocating "ball-ondisc" tribometer under conditions described in ref. 5. The sliding life is used as a measure of the wear resistance of the system and is defined as the time from the beginning of the test until the coeffw,ient of friction /~ rises dramatically from typically #~,0.03 to values exceeding 0,2,
3. E x l ~ ~ l
res~
3,1, Film characterization After daily use of the sputtering chamber for about 2 months, the system was conditioned, giving stable and reproducible deposition conditions, The MoS~ films had an x value of about 1,7 and a controllable oxygen content, as shown in Table 1. The oxygen content in the films from the dry runs is similar to the lowest values observed by Buck [12], and for the HaO runs the oxygen c,ontent 08%-27%) is similar to the values measured when the HzO vapour pressure is not controlled [12, 15-17]. Figure ! shows the morphology of as-deposited HT films for both the HT-H20 and HT-dry deposition conditions, it is observed that the morphology changes
l~tm Fig, I, Scanning electron microlffaphs showing the morphology of Mo$~ films made at 200 ~C: left, film deposited under humid plasma conditions (HT-H~O film}; right, film deposited under extremely dry plasma conditions (HT-dry film),
N, J, Mild
120
TABLE I, The stoiehiometry x and oxygen content of Mo$~ layers made under controlled conditions Delx~,xition temperature
Plasma contamination
Film cormposition After maximum Ar" radiation
As-del~sited
AT (26 C + 5 CI HT
Dry H~O Dry
1,68 ± 0,02 1,77+0,01 1,63 +0,01
(2iX) ('+5 'C)
H~O
1.62+03)I
from the very characteristic columnar plate-like "MoS~ structure" [9, 18-23] to a more dense and featureless morphology when the H 2 0 content in the plasma is decreased. X-ray diffraction analysis (not shown here) reveals that the grain size in AT films is below the resolution limit of the measurement, i.e. the diffraction spectra are similar to spectra from amorphous films. As expected, the HT films are more crystalline than the AT films, and X-ray measurements indicate that the HT-H20 films are more crystalline than the HT-dry films, in contrast to the findings of Buck [ 16], it is observed that the presence of H 2 0 in the plasma has no influence on film orientation. Both the HT-H20 and HT-dry films have about 90% of the basal planes perpendicular and 10% parallel to the substrate, i.e. they are type I films [9]. The TEM images shown in Fig, 2 of the HT-dry, HTH20, AT-dry, and AT-H20 films after ion bombardment at various doses exhibit the following characteristics. For the lowest doses, the film morphology exhibits no significant changes compared with as-deposited films and it is observed that the HT-dry films to some extent resemble the AT films, For the as-deposited AT films the structure is deaase, fibrous and glassy, and the influence of H 2 0 vapour content in the plasma is limited. With increasing bombardment doses, the HT-H20 film morphology is dramatically changed. The lamellar columns seem to be smeared out, and at I × l0 j6 Ar + cm 2 the film structure has changed completely to a fully dense, structureless morphology. For the AT films, only minor changes are observed, but the as-deposited dense structure changes to a more featureless morphology similar to that of the I ×10 ~ Ar ÷ cm 2 bombarded HT films.
3.2. Tribological characterization Figures 3 and 4 show for the HT and AT film depositions respectively the relation between film density and crystallinity (lower parts), and the sliding lives (upper parts). The tribological tests were performed on 12 samples: three similar samples produced under each of
Oxygen content (% atoms)
x
Oxygen content I% atoms)
6,9 + 0,7 25.3 ___+0.3 6,3 +_0,5 18,0 ± 0.2
1,66 __.+0,02 1,77 ± 0,02 1.60 + 0,01 1.60 4- 0,02
7.0 +___0.¢-, 25.9 + 0A 6,3 ± 0,7 14,4 ± O,7
the four different deposition conditions. On each sample parts of the layers were bombarded by Ar* at the various doses. Seven tests were performed on each sample, each representing the various Ar + doses, including a test on a non-treated area. Each .sliding life result is thus the mean value of three replicated tests. It is observed that for all MoS.~ layers, the films are amorphous at doses exceeding the "transition-region'" hetwcen I x l 0 ~ and I ×10 !s Ar + cm -a, For the ion beam amorphized films the TEM diffraction patterns exhibit only diffuse haloes with no trace of diffraction rings due to polycrystalline structures. Furthermore, with high resolution TEM lattice imaging it was not possible to observe any periodic fringes from the (002) basal planes in the ion beam amorphized films. For all films tested, the coefficient of friction was between 0,03 and 0.06, confirming previous results showing low friction for ion-beam amorphized MoS~ films [4, 5]. For the HT-H20 plate-like films the benefits from ion implantation are cfearly illustrated. The bombardment increases the sliding life by a factor of about five for I×10 !~Ar + cm -2 and a factor of 2,6 for I × 10Is Ar + cm ', thereby confirming the results from ref. 5. However, it is observed from the results shown in Figs. 3 and 4 that high-dose {(3-10)×10~SAr + cm--') ion-beam enhanced wear resistance seems to be obtained only when columnar plate-like films (the HT-H20 films) are bombarded. Films deposited at 200 :C under extremely dry conditions (the HT-dry films) have, in the as-deposited state, a sliding life which is four times as long as that of the H 2 0 films, and at high doms (amorphous films), no beneficial effect from ion bombardment is observed. For the AT films, no significant differences in the sliding life between the "dry" and the "H20" films are observed. But the endurance life of the as-deposited AT films is about seven times that of the HToH,O films. and at high doses the implantation reduces the wear resistance of the former. These observations seem to imply that marked benefits from high-dose (around I × l0 .l~ Ar ÷¢m a ) 100 keV ion
N,J, Mikkeisen, G, Sorensen / Ion bombardment of MoS~Jilms
HT-H20
{
121
w'*
/
HT-
AT-H20
II
AT-dry
3xl~ 4
Ixl~
3xl~
IxlO~
Des, JAr÷/earl 0A Fig, 2, TEM bright field images of HT-H~O, HT-dry, AT-HzO, and AT-dry MoS~ films bombarded with 400 keV Ar + at various doses,
bombardment are obtained only when the as-deposited layers are porous lamellar films. The MoS~ films made by Kobs et al. E2, 3] (although deposited at 60 °C) may all have been the lamellar plate-like films owing to the l~esence of H20 in the plasma. The ion-beam-reduced film thicknesses reported by K obs et al. in ref. 3 could confirm this. In the case of iow-dose ion bombardment (at (0.31) × lot 4 Ar + cm -2, where radiation damage is still limited) the sliding life is enhanced by a factor of 1.3-1.9 for the four different films. This cannot he explained by enhancement of the film densities, which in this dose range is marginal. It is evident from the results in Figs 3 at~d 4 that for more glassy films (i.e. the AT and the HT-dry films), which are much denser in the as-deposited state than the columnar HT-H20 film, the wear resistance decreases with it~cxeasing amorphization beyond the transition region. This may imply that, when the films
are tested under the present conditions, the amorl~hous state of the MoS~ films is not optimum with respect to tribological behaviour. The maximum sliding life for the HT-HzO films obtained at 1 xlot 6 Ar + can -= cannot he attributed to the amorphous structure, because HT-HzO films bombarded with 3 × l0 ts Ar + can -2 are completely amorphous but have a sliding life of only half the value found for the highest dose. This may he explained by the fact that the film demsity at the lower dose ( 3 x l o t S A r + c m -2) is still not as high as for the ! ×lot6Ar + can -2 dose. Furthermore, when studying TEM images in Fig. 2, it is observed that the HT-H20 films only become fully dense and smooth at the highest dose. When comparing Figs 3 and 4, it is observed that the sliding life is not related to film density alone. Thus, the density of HT-H20 films bombarded with 3×lots Ar + can -2 is about 4.1 g c m -3, very similar to the 4.4 g c m - 3 of the as-deposited AT-H20 film, but the latter exhibits a sliding life three times longer.
N, J. Mikkelsea, G, Soren,sen : Ion bombardment of MoS.jilm,~
122 j
~-T ¸¸~" ~
.
lxl0S i__1_,,_~_ HT-dry
I "'-- I
T
'l /P~"
I
HT-H~O
I
I
" --T~
I
-------
T " ¢ ~ " r - " -t--- ~ ' r - - "
~ --'1- ............ "1
AT-H20
I AT- dry
o
R
0
N 5x10¢
CO
3±
_e
}
lx10s
>.,
% I-0 Z
, I , I
C3
51
5x10¢
--I
, __. I J
Bulk
E
HT-HzO
HT-dry
Bulk
.=
~.
_
t
~?
J t..-
~
~ :/--~
gso Ig .
3
g
AT-dry
g-
5 I.L
l
I
I
I
AT-H20
g-1
E
0
I ;: ,
as
dep,
I
,
10~' 10 ~
1~16~
I .. ,
as
I
,
I
~
I
10~' 10~s 10~
Bulk
7
>-
dep, Ar* DOSE (Ar'/cm 2)
Fig, 3, Above: sliding life measurements of HT-dry and HT-H=O films bombarded with 400 keV At* at various doses; test conditions, rteipa"ocaring ball-on-disc, dry nitrogen atmosphear¢, e 5 mm AISI 52100 steel ball, 7 Hz and 2 mm amplitude, Below: the corrtsponding film mass dr.nsities and structurt~, Film grain sizes arc added,
Z LIJ CZ _i LL
3
I
2
~E
When tested under the present conditions, improved wear remistanc¢ by 100 keY argon bombardment (doses in the range I x 10!s-I x 10 ~ Ar + cm -2) is only obtained for columnar plate-like films, Results in refs, 4-6 and the findings of Kobs et al, [2, 3] (from tests under simihr conditions), showing enhanced wear resistance of sputter deposited MoS~ layers by 100 keY argon bombardment in the same dose range, arc all based on "typical" MoS:, films with a plate-like morphology, Spalvins [13] has described the wear and failure mode of sputtered MoS~ films as premature brcaking-off of the plate-like columns due to lack of intercolumnar cohesion combined with film brRtfencss, This is in agreement with the findings of Buck [ 12], where plates in the film break off during wear. The rcsuRs from the present study show that the typical as-deposited columnar plate-like MoS:, films exhibit inferior wear resistance compared with more dense and featureless films; this is probably explained by breaking-off of the columns, as shown in a previous
zo~ ?
~ ~-~
li
1 O'
I ..,
QS
d~p
4, Diseassm
~
~
L
I
10it. 10is 1016
as
101~ 10~s 10is
dep
Ar" DOSE (Ar'Icm~) Fig, 4, Above: sliding life measurtments of AT-dry and AT-H20 films bombarded with 400 kcV A t - at various doses: test conditions, reciprocating ball-oh-disc, dry nitrogen atmosphere, e 5 mm AIS! 52100 steel ball, 7 Hz and 2 mm amplitude. Below: the ¢orrcspondiog film mass densities and structurts, Film grain sizes art add~l,
publication [5], Ion bombardment at high doses is thus acting like a repair tool for plate-like films, where the columns are smeared out and the voids filled out, finally at the highest doses, to be transformed to non-columnar fully dense films. For low-dose ion bombardment in the range 3 × l 0 t3 3 × 1014 Ar + cm -2, the sliding lives for both featureless and columnar film types are increased by a factor of about !.3--1.9. In this dose range, the structural changes arc marginal, but at the film-subslrate interface chemical bonds may be modified which may resuR in an enhanced film-substrate adhesion [24], even though no significant film-substrate intermixing occurs, Oxygen contamination of sputtered MoS~ films due to H 2 0 vapour in the plasma (Table I) only causes the
N. J. Mikkelsen, G. Soren.wn / Ion bombardment of MoS~films
tribofogical behaviour of the film to deteriorate if the
123
itdereaees
presence of H20 has induced a plate-like morphology. Thus oxygen is not an important intrinsic parameter, but acts more as a catalyst for columnar film growth, which may then have a detrimental effect on the tribological properties, 5. C o c k , sims
The prese~ study has shown the feasibility of ion bombardment for studying basic phenomena in MoS~ solid lub6cation, and surface modificationby ion beams has been shown to be a useful tool for understanding the complex nature of MoS~, sputter deposited films. The tribological behaviour of sputtered MoS~ seems to be governed exclusively by structural properties. When tested under the present conditions, the optimum wear resistant film structure should thus exhibit a nanocrystalline, dense and featureless morphology; whereas an increase in the grain size (induced by a combination of elevated deposition temperatures and oxygen film contamination) reduces the wear resistance, probably owing to a reduced film mass density resulting from a plate-like structure. This indicates that an "~leal" grain size for MoSx films may exist [18]. The reason for this, however, may he that the '~feaF' grain size reflects an optimum balance between the degree of crystallinity and high film density.
The work was supported by the Danish Development P r o g r a m m e for Materials Technology, Centre for Surface T e c h n o l o g y - Dry Coating Processes.
I J, Chevallier, S, Ok,sen, G, ~re.e~en and k Gul~a, ApI~, Phys, Len,, 48 (1986) 876, 2 K, Kob~ H. D/migen, H, H / ~ h , H.J, Tolle, R, Leut©neckcr and H. Ryssel,Appl. Phys. Lett., 49 (1986) 496. 3 K. Kob~ H. Dimigen, H. Hiibsch, H. J. Tolle, R. Leutenecker and H. Ryssel, Mater. Sci. Eng., 90 (1987) 281. 4 I'4.J. Mikkelsen,J. Chevallier,G, Smensen and C, A. Straede, Appl. Phys. Lett.. 52 (1988) 1130. 5 1,4.J. Mikk©lsen and G. Sm'ensen, Mater. Res. Soc. Syrup. Proc.. :40 (1989) 265, 6 N.J. Mikkehen and G. Sorensen, Miner. Sci. Eng.. 115 (1989) 343. 7 R. 1. Christy and H. R. Ludwig, Thin Solid Films. 64 (1979) 223. 8 R, I. Christy, Thin Solid Films, 73 (1980) 299, 9 P.D. Fleischauer, ASLE Trans.. 27 (1984) 82. 10 E.W. Roberts, 20th American Mechanisms Symposium, NASA Conf. PubL 2423, 1986 (NASA, Washinston, DC), p. 103. 11 M.R. Hilton, R. Bauer and P.D. Fleischauer. Thin Solid Films. 188 (1990) 219. 12 V. Buck, Wear. 114 (1987) 263. 13 T. Spalvins, NASA TM-8.LT63, 1984 (NASA, Washington, DC).
14 J. F. Ziegler, J.P. Biersack and U. Littmark, in J. F. Ziegler (ed.) The StOpl~nf of Ions in Solids, Vol. 1, Pergamon, New York, 1985, p, 109. 15 J. R. Lince, J. Mater. Res.. 5(I) (1989) 1990. 16 V. l~ck~ Vacuura-Teknik. 4 (1988) !11. 17 S. Fayelle, P. D. Ehni and I. L. Singer, in D. Dowson, C. M. Taylor and M. Godet (eds.), Mechanics of Coatings. Tribology Series. 17,
Elsevier, Amsterdam, 1990, p, 129, 18 P, D, Fle/schauex and R. Bauer, ASLE Trmls,, 30 (1987) 160, 19 P. D, Fleischauer, M, R. Hilton and R, Bauer, in D. Dowson, C, M, Taylor and M. Godet (eds.), Mechanics of Cominfs. Tribolo~y Series. 17, Elsevier, Amsterdam, 1990, p. 121. 20 M.R. Hilton and P. D. Fleischauer, Miner. Res. Soc. Syrup. Proc.
21 22 23 24
140 (1989) 277. M.R. Hilton and P.D. Fleischauer, J. Miner. Res.. 5 (1990) 406. T. Spalvins, Thin Solid Films. 73 (1980) 291. J.R. IAnoeand P. D. Flei~ohauer, J. Miner. Res.. 2(6) (1987) 827. J. E. E. Baglin, G. J. Clark and J. Bottiger, Miner. Res. Soc. Syrup. Proc.. 25 (1984) 329.