WEAR OF MATERIALS IN LUNAR DUST ENVIRONMENT
Koji MATSUMOTO (1), Mineo SUZUKI (1), Shin-ichiro NISHIDA (2), and Sachiko WAKABAYASHI (2)
(1)
Japan Aerospace Exploration Agency (JAXA), Aerospace Research and Development Directorate,
7-44-1 Jindaiji-higashimachi, Chofu, Tokyo 182-8522, Japan
Phone: +81-422-40-3181, E-mail: [email protected]
(2)
Japan Aerospace Exploration Agency (JAXA), JAXA Space Exploration Center,
3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
ABSTRACT
Friction tests were carried out to investigate effects
of dust particles covering the moon surface on wear
properties of materials for space components. Especially,
influences of particle sizes on wear were evaluated
using a lunar soil simulant. The simulant was sieved
into particle sizes of less than 32 µm and 100-300 µm.
The particles were put on contact areas of specimens
before the friction tests. The friction tests were
conducted for several kinds of materials in vacuum.
Tested materials were a few metals: stainless steel,
titanium and aluminum alloys. A few coatings, PTFE
films and MoS2 film that have been used for space, were
also evaluated. Results show that wear of most of the
materials increased by existence of simulant of lunar
dust particles. Differences in friction were not
significant but differences in wear were found between
particle sizes. The effects of dust particles on the wear
of tested materials are reported.
1. INTRODUCTION
Activities on the moon are the next targets of space
development around the world. Also in Japan, "Kaguya",
which is a full-scale lunar exploration project since
NASA's Apollo program, obtained important data using
several kinds of observation instruments. The Japan
Aerospace Exploration Agency (JAXA) will also
progress to research and development on a subsequent
explorer following Kaguya and observation and
experimentation systems [1, 2]. The next steps for lunar
exploration are landing and moving on the lunar surface.
The moon is covered by large amounts of dust particles,
called regolith, which should make serious problems for
tribological component on the moon [3, 4]. Moreover,
for activities on the moon, not only tribological parts
sealed from dust particles but also some parts exposed
directly to the dust environment will be necessary (e.g.
crawlers, wheels of rovers) [5]. However, effects of the
moon dust particles on parts in vacuum have not been
clarified. Acquisition of data about legolith effects is
necessary.
Some studies of tribological effects of regolith have
been started. Matsumoto et al. evaluated effects of dust
particles on bonded lubricant films of MoS2 and PTFE
in vacuum [6, 7]. Results show that the particles easily
wore away the films. Ishibashi et al. conducted
tribological tests for several ceramics [8, 9], showing
wear rates for hardness of the ceramics. In this study,
friction tests were carried out in vacuum to investigate
the wears of some metal materials and a few types of
coatings without binder materials, and changes in their
frictional performance by lunar soil simulant. Those
data will be useful for design and material selection for
component moving on the moon.
2. EXPERIMENTAL DETAILS
2.1
Lunar soil simulant
The lunar soil simulant [10, 11] was made from
basaltic lava; it simulates the hardness and distribution
of particle size to real lunar dust particles, regolith,
which were extracted and retrieved during the Apollo
program. The particles’ medium size was 70 µm. As
with real regolith, its major constituent is silicon oxide.
The simulant was sieved into two particle sizes to
evaluate the particle size effects: less than 32 µm, and
100-300 µm. Fig. 1 shows photographs of both sizes of
dust particles by optical microscope.
250 µm
Particle sizes of
less than 32 µm
100 - 300 µm
(<32 µm)
Fig. 1 Photographs of lunar soil simulant
classified into particle sizes.
Roller specimen
(Coatings)
Load
Dust particles
Arch-shaped specimen
Fig. 2 Configuration of test specimens.
Table 1 Combinations of test specimen materials
Arch-shaped specimen
Roller specimen
Coating
Substrate
Stainless steel (440C)
Titanium alloy (Ti-6Al-4V)
440C
Aluminum alloy (A6061)
stainless steel
PTFE A
A6061
PTFE B
440C
MoS2
440C
2.2
Test specimens
The configuration of the friction test is presented in
Fig. 2. A roller specimen rotates against an arch-shaped
specimen [6, 7]. This configuration was designed to
restrain dust particles from leaving the contact area
easily. The roller specimen had a curvature in the axial
direction to apply a load uniformly; it was made of
440C stainless steel. Arch-shaped specimens were made
of various metals: stainless steel (440C), titanium alloy
(Ti-6Al-4V) and aluminum alloy (A6061). Coatings
used for space applications were deposited inside the
arch-shaped specimens. The coatings tested in this study
were PTFE films and MoS2 film. The PTFE films were
made by impregnating PTFE into many minute holes on
hard anodized aluminum on the aluminum alloy surface
(PTFE film A) [12] and into a non-electrolytic nickel
layer, which was containing numerous micropores and
deposited on 440C stainless steel (PTFE film B). The
thicknesses of the PTFE films were 20-50 µm. The
MoS2 film was sputter-deposited using an
RF-magnetron. The film thickness was approximately
2.0 µm. Combinations of roller specimen and
arch-shaped specimens are presented in Table 1.
2.3
Friction tests
The lunar soil simulant of 5 mg, with a certain
particle size, was put on the contact area of the
arch-shaped specimen. Then a friction test was started
in vacuum by putting a rotating roller specimen on an
arch-shaped specimen with an applied load. The
vacuum pressure was less than 1 × 10-4 Pa. Friction tests
were conducted at a load of 10 N and a sliding speed of
Fig. 3 Friction behaviors of the 440C roller
specimen against arch-shaped specimens
made of (a) 440C, (b) Ti alloy and (c) Al
alloy with and without dust particles.
15 mm/s. The test duration was 500 rotations of the
roller specimen.
3. RESULTS AND DISCUSSIONS
3.1
Results for bare metals
Friction coefficients of the 440C roller specimen
against three arch-shaped metal specimens are presented
respectively in Figs. 3(a), 3(b) and 3(c). High friction
coefficients of 0.8 to 1.0 were observed irrespective of
the existence of dust particles for the 440C-440C
combination. For the combination of 440C against Ti
alloy, relatively low friction coefficients of 0.6 were
obtained with dust particles in spite of the high value of
0.8 in the case of no particles. Dust particles acted as a
lubricant. Their rotation on contact surface seemed to
decrease friction. Especially, smaller particles showed a
lower value at the beginning of the test. Unstable
behavior shown later might have occurred by reduction
of particles at the contact area. For the Al alloy
arch-shaped specimen, no difference in friction behavior
was found between tests except that the non-particle test
showed a slightly lower value than the tests with
particles at the start of the tests.
(a) 440C against 440C
Photographs of wear tracks of both roller and
arch-shaped specimens by microscopic observation and
surface profiles are depicted in Fig. 4. Much adhesive
wear of both specimens occurred for the combination of
440C-440C. Smaller particles operated to restrain
adhesive wear by reducing contacts between specimens.
However, adhesive wear by 100-300 µm particles was
greater than that occurring without particles. Removal
of surface contaminants or oxides by the particles and
subsequent removal of the particles themselves might
have increased the adhesive wear.
Severe adhesive wear was not observed in the case of
440C against different metals without particles.
However, the amount of wear increased by dust
particles. Much wear was observed on both 440C roller
specimen and Ti alloy arch-shaped specimen with
particle size of less than 32 µm. With larger particles, Ti
alloy specimens showed little wear, as in the case with
no particles. The aluminum alloy arch-shaped
specimens showed the most wear with each particle size
although the counterpart of 440C roller specimens
showed no noticeable wear.
(b) 440C against Ti alloy
3.2
(c) 440C against Al alloy
Fig. 4 morphologies and surface profiles of wear
tracks of both roller specimen and
arch-shaped specimens after the friction
tests with and without dust particles.
Results for coatings
Figure 5 presents friction coefficients of PTFE film
A on Al alloy and wear tracks of the coatings and 440C
roller specimens after the tests. The test without dust
particles started with a friction coefficient of about 0.8
and became stable at a value of approximately 0.5. The
friction coefficient of the coating with particle size of
<32 µm increased gradually to 0.9 from the test start
and became stable at 0.8. The friction behavior of the
test with particle size of 100-300 µm was similar to that
found in the non-particle test. Wear in the test without
dust particles was greater than that of non-coated Al
alloy. However, the wear of arch-shaped specimens by
particles decreased with the coating. Wear resistance
was improved by the coating in case of existence of dust
particles. On other hand, the wear of the roller specimen
increased compared to the case of the bare Al alloy
arch-shaped specimen. Long scratched scars were
apparent on the roller specimen with particle of 100-300
µm; it was more obvious than with <32 µm particles.
The larger particles seemed to be removed from the
contact area early in the test, so friction behavior was
similar to that of the test with no particles.
Fig. 5 Friction behaviors and wear tracks of
PTFE film A with and without dust
particles.
Friction behavior of PTFE film B on 440C and
results of observation after the tests are presented in Fig.
6. The friction coefficient showed a high value of 1.2
without particles, whereas significant wear was not
found. Compared to the non-particle test, lower friction
of 0.6 was observed with particles; it did not depend on
the particle size. Some wear debris was found on the
roller specimen but the wear depths of the coating were
small and similar to the case of no particles. This
coating showed excellent wear resistance for the lunar
regolith except for the high friction in the case of no
particles.
The low friction coefficient of 0.1 was obtained for
MoS2 film on 440C, as depicted in Fig. 7. However,
because of the dust particles’ existence, high friction
and much wear were observed. The film was worn away
by particles early in the tests because the friction
coefficient and wear morphology were almost identical
to results of the 440C against bare 440C tests. Low
hardness of the film and thin film thickness seemed to
cause these results.
Fig. 6 Friction behaviors and wear tracks of
PTFE film B with and without dust
particles.
3.3
Discussions
The wear mechanism is expected to be abrasive wear
when dust particles existed at the contact surface
although the scratch track sizes differed between
particle sizes. However, in some cases, e.g. 440C-440C
and 440C-Ti alloy combinations, morphology of the
wear tracks by particle size of 100-300 µm was
observed as identical to those obtained from
non-particle tests, apparently because of moving out of
the particles. Wear by larger particles seemed to differ
with the movement and action of the particles: the
particle stayed at the contact surface, it left there soon or
it was crashed to a smaller size. Action of the dust
particles probably depended on test conditions, surface
activation, static electricity of surfaces, and so on.
Results obtained in this study also show that
materials without high hardness, such as aluminum
alloy, are difficult to use in a moon dust environment.
However, wear resistance against dust particles was
improved by PTFE coatings. To select materials and
coatings for activities on the moon, hardness is an
important consideration; however, its relationship to
counterpart materials, toughness, particles adhesion and
so on should be also considered. With consideration for
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Fig. 7 Friction behaviors and wear tracks of
MoS2 sputtered film with and without dust
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them, acquisition of further data related to other
materials, coatings and treatments, along with different
combinations are necessary to enhance wear resistance
more.
4. SUMMARY
Some metal materials and coatings used for space
were evaluated on wear and friction properties in
vacuum and lunar dust environment.
Wear of most of the metal materials increased by
existence of simulant of lunar dust particles.
Differences in friction were not great, but differences in
wear were observed between particle sizes.
Regarding the coatings, most coatings tested in this
study also had much wear or wore away easily by dust
particles. However, a PTFE impregnated film having
high wear resistance against dust particles was
identified.
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Lunar Regolith Simulant (Section 2) – Wear
Characteristics in Rolling Contacts –”, Proc. of JAST
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of Slope Mobility Testbed using Simulated Lunar
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Various Types of Lunar Soil Simulants by Hydrogen
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