Hyperthermal atomic oxygen interaction with MoS2 lubricants and
relevance to space environmental effects in low earth orbit – effects on
friction coefficient and wear-life
M. Tagawaa,*, M. Muromotoa, S. Hachiuea, K. Yokotaa, N. Ohmaea, K. Matsumotob and M. Suzukib
aDepartment of Mechanical Engineering, Kobe University, 1-1 Rokko-dai, Nada, Kobe, 657-8501, Japan
bJapan Aerospace Exploration Agency, Jindaiji-Higashi, 7-44-1, Chofu, Tokyo, 182-8522, Japan
Received 7 December 2003; accepted 14 December 2004
Wear-life of the MoS2 film was evaluated by in situ tribological testing under 5 eV atomic oxygen exposures which simulated
atomic oxygen environment in low Earth orbit. A combination of a laser-detonation atomic oxygen source and a conventional pin-
on-disk friction tester was used to perform tribological tests. It was confirmed that the friction coefficient was not affected by atomic
oxygen exposure when atomic oxygen fluence was low; however, the friction coefficient increased with increasing atomic oxygen
fluence and it reached as high as 0.05 at the atomic oxygen fluence of 3.4 · 1016 atoms/cm2/cycle (nine times larger than the normal
value). Effect of atomic oxygen on the wear-life of the film has much more drastic. With atomic oxygen fluence of 1.7 · 1016 atoms/
cm2/cycle, wear-life of the film was reduced less than one-tenth of that during ex situ testing result of the same film. It was also
observed that the wear-life of the film was inversely proportional to the atomic oxygen fluence between sliding passes.
KEY WORDS: atomic oxygen, MoS2, space environmental effect, tribology, low earth orbit, lubricant
1. Introduction
The energetic atomic oxygen reaction with materials
used in the exterior of a spacecraft operated in low Earth
orbit (LEO) is one of the key issues in future long-life
spacecraft [1,2]. Much attention has been paid to poly-
mer degradation due to atomic oxygen attack in the last
decade. Beside polymeric materials used in thermal
control systems, lubricants are also one of the key
materials to be tested. A lubrication malfunction in a
space system can result in serious damage to a satellite.
In many cases, there exists no back-up system for such
tribological failure. Therefore, it should be stressed that
the influence of atomic oxygen on lubricants must be
evaluated before the mission. Both liquid and solid
lubricants are used in space systems, however, to avoid
vaporization at high temperature and low pressure
(which may cause contamination problems to other
optical surfaces), or radiation-induced decomposition,
solid lubricants have often been selected for external
applications. Among many types of solid lubricants,
MoS2 lubrication film is most successfully applied in
LEO satellites. External lubrication for the Exposure
Facility of Japanese Experimental Module of the
International Space Station, however, requires high
reliability and much longer life of MoS2 lubricants, even
after a long exposure to the space environment.
In order to evaluate the effect of high-energy (5 eV,
which comes from orbital velocity of spacecraft) impact
of atomic oxygen to MoS2-based lubricants, some re-
search groups have studied the tribological properties of
the atomic oxygen-exposed MoS2 lubricants. Results of
the previous studies are summarized in table 1. As listed
in table 1, such research results were often contradic-
tory. Arita evaluated the tribological properties of hy-
perthermal atomic oxygen-exposed sputter-deposited
and binder-type MoS2 films [3]. He found that the
atomic oxygen-exposed samples showed initial high
friction, but recovered normal values after wear. He also
reported no effect on wear-life of the film. Dugger also
reported similar results [4]. Arita and Dugger both used
a laser detonation atomic oxygen source to form 5 eV
atomic oxygen beam. Martin et al. used CO2
laser-heated molecular beam source to produce atomic
oxygen source which can provide 1.5 eV atomic oxygen
beam [5,6]. They reported similar results but they ob-
served isotropic oxidation of MoS2 single crystal that is
contradictory with Arita’s data. They also reported that
high temperature sputtered film has oxidation resistance
against atomic oxygen attack. Dursch and Pippin tested
tribological properties of the binder-type MoS2 film.
They used radio frequency (RF)-discharge atomic oxy-
gen source. With 0.5 N load, slight increase in friction
coefficient was obvious after atomic oxygen exposure [7].
In contrast, Yamaguchi et al. used low-energy high-flux
atomic oxygen source (DC arc-jet source) to study tri-
bological properties of sputtered MoS2 films [8]. They
*To whom correspondence should be addressed.
E-mail: tagawa@mech.kobe-u.ac.jp
1023-8883/05/0400–0437/0
2005 Springer Science+Business Media, Inc.
Tribology Letters, Vol. 18, No. 4, April 2005 (
2005) 437
DOI: 10.1007/s11249-004-3594-1

reported that the friction coefficient became lower and
wear-life became longer after atomic oxygen exposure.
Also Matsumoto, who used a laser-detonation atomic
oxygen source, reported the initial low friction and
elongation of wear-life for high temperature sputtered
film [9]. These results are contradictory to the others
shown above. These discrepancies may be due to the
difference in the experimental conditions in each exper-
iment such as beam energy, fluence, sample preparation
and so on. Therefore, it is important to clarify the
experimental conditions, including beam characteristics,
in order to access these tribological properties of MoS2
lubricant in an LEO space environment.
Until today, most of the tribological tests regarding
atomic oxygen effects have been carried out after atomic
oxygen exposure was completed; i.e., post-exposure
tests. It is always doubted whether or not ex situ and in
situ tests give the same results. In situ tribological testing
results were reported by Wei et al. [10]. They reported
stable friction and wear properties under atomic oxygen
environment. However, they used a thermal O-atom
source where the translational energy of 5 eV is ne-
glected. Recent studies suggested the importance of the
role of impinging energy on the surface reaction of
atomic oxygen with polymers [11]. Thus, the role of
impinging energy of atomic oxygen should also be
considered in atomic oxygen effects on lubricants. Such
a study was recently reported by Tagawa [12,13]. He
reported in situ tribological testing results under 5 eV
hyperthermal atomic oxygen beam exposure. The flu-
ence dependence in the initial high friction was clearly
observed. Unlike Arita’s report, he found no recovery
effect in the case of in situ tribological testing. However,
none of these ground-based studies reported the wear-
life of the film under simulated atomic oxygen envi-
ronment in LEO.
In this study, we report ground-based simulation
results of the atomic oxygen effects on the wear-life as
well as friction coefficient of MoS2 sputter-deposited
films, which simulates both the impinging energy and
flux of atomic oxygen in LEO. Wear-life of the MoS2
sputtered film under various atomic oxygen testing
conditions, including conventional ex situ and more
realistic in situ testing conditions, were evaluated and
compared in this study. The tribological testing results
of the same sputtered MoS2 film flown on the STS-85
flight were also referred.
2. Experimental details
The MoS2 specimens used in this study were sputter-
deposited MoS2 films. The sputtered films were prepared
by the RF-sputtering technique at the Japan Aerospace
Exploration Agency. Sputtering conditions of the RF
sputtered specimens are reported elsewhere [5]. The
samples prepared by the same procedure have been
aboard STS-85 and correlated with the ground-based
experiments reported here.
The atomic oxygen source used in this study was
based upon the laser-induced detonation phenomenon
which was originally developed by the Physical Sciences
Incorporation [14]. The atomic oxygen source was at-
tached to the testing facility developed in our laboratory
[15]. The atomic oxygen beam was monitored by a time-
of-flight measurement system consisting of a quadrupole
mass spectrometer and a multi-channel scalar. The mean
energy of the hyperthermal atomic oxygen was calcu-
lated to be 4.5 eV. The atomic oxygen fraction in the
beam was approximately 45% and balance was molec-
ular oxygen. The atomic oxygen flux of the beam was
measured by an Ag-coated quartz crystal microbalance
(QCM) with an accommodation coefficient of 0.62 [16].
A typical atomic oxygen flux at the sample position
(47 cm from the nozzle) was estimated to be
2.4 · 1014 atoms/cm2/s. Note that atomic oxygen flux is
inversely proportional to square of the nozzle-sample
distance.
The Auger electron spectroscopy (AES) and X-ray
photoelectron spectroscopy (XPS) used in this study
Table 1.
Some tribological testing results of MoS2 in a simulated LEO space environment reported previously.
Source MoS2 Sample and
Friction Conditions
AO Exposure Energy
and Fluence
Test Results
Arita [3] RF sputtered on SUS440C,
Ti–6Al–4V pin, 10 N
Hyperthermal (PSI)
5 eV, 1020 AO/cm2
Ex situ Initial high friction (20–30%)
Oxide: 6 nm, Crystallite dependence
Matsumoto [5] RF sputtered on SUS440C,
SUS440C ball, 10 N
Hyperthermal (PSI) &
LEO 5 eV, 1020 AO/cm2
Ex situ Initial low friction, elongation of wear-life
for 160 C deposited sample by AO-exposure
Martin [6,7] RF sputtered on Sapphire,
SUS440C pin, 0.3 N
Hyperthermal (LANL)
1.5 eV, 1024 AO/cm2
Ex situ Oxide: MoO2, MoO3, SO2, Crystalline
independence, initial high friction on 60 C,
but no effect on 200 C deposited samples
Wei [10] Ion beam assisted sputter
film, Al2O3 ball
Thermal (RF Asher) 0.02 eV,
flux unknown
In situ Stable friction and wear properties
Tagawa [12,13] RF sputtered on SUS440C,
Ti–6Al–4V pin, 2 N
Hyperthermal (Kobe University)
5 eV, 1018 AO/cm2
In situ Oxide: MoO3, SO, initial high friction with
AO exposure as low as 1017 AO/cm2
438 M. Tagawa et al./Hyperthermal AO interaction with MoS2