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 particles on bonded lubricant films of MoS2 and PTFE in vacuum [6, 7]. Results show that the particles easily Friction tests were carried out to investigate effects wore away the films. Ishibashi et al. conducted of dust particles covering the moon surface on wear tribological tests for several ceramics [8, 9], showing properties of materials for space components. Especially, wear rates for hardness of the ceramics. In this study, influences of particle sizes on wear were evaluated friction tests were carried out in vacuum to investigate using a lunar soil simulant. The simulant was sieved the wears of some metal materials and a few types of into particle sizes of less than 32 µm and 100-300 µm. coatings without binder materials, and changes in their The particles were put on contact areas of specimens frictional performance by lunar soil simulant. Those before the friction tests. The friction tests were data will be useful for design and material selection for conducted for several kinds of materials in vacuum. component moving on the moon. Tested materials were a few metals: stainless steel, titanium and aluminum alloys. A few coatings, PTFE films and MoS film that have been used for space, were 2 2. EXPERIMENTAL DETAILS also evaluated. Results show that wear of most of the materials increased by existence of simulant of lunar 2.1 Lunar soil simulant dust particles. Differences in friction were not significant but differences in wear were found between The lunar soil simulant [10, 11] was made from basaltic lava; it simulates the hardness and distribution particle sizes. The effects of dust particles on the wear of particle size to real lunar dust particles, regolith, of tested materials are reported. 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. 1. INTRODUCTION The simulant was sieved into two particle sizes to Activities on the moon are the next targets of space evaluate the particle size effects: less than 32 µm, and development around the world. Also in Japan, "Kaguya", 100-300 µm. Fig. 1 shows photographs of both sizes of which is a full-scale lunar exploration project since dust particles by optical microscope. 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 250 µm 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. Particle sizes of crawlers, wheels of rovers) [5]. However, effects of the less than 32 µm 100 - 300 µm moon dust particles on parts in vacuum have not been (<32 µm) clarified. Acquisition of data about legolith effects is necessary. Fig. 1 Photographs of lunar soil simulant classified into particle sizes. Some studies of tribological effects of regolith have been started. Matsumoto et al. evaluated effects of dust

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 Fig. 3 Friction behaviors of the 440C roller used for space applications were deposited inside the specimen against arch-shaped specimens arch-shaped specimens. The coatings tested in this study made of (a) 440C, (b) Ti alloy and (c) Al alloy with and without dust particles. 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 15 mm/s. The test duration was 500 rotations of the deposited on 440C stainless steel (PTFE film B). The roller specimen. 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 3. RESULTS AND DISCUSSIONS arch-shaped specimens are presented in Table 1. 3.1 Results for bare metals Friction coefficients of the 440C roller specimen 2.3 Friction tests against three arch-shaped metal specimens are presented respectively in Figs. 3(a), 3(b) and 3(c). High friction The lunar soil simulant of 5 mg, with a certain coefficients of 0.8 to 1.0 were observed irrespective of particle size, was put on the contact area of the the existence of dust particles for the 440C-440C arch-shaped specimen. Then a friction test was started combination. For the combination of 440C against Ti in vacuum by putting a rotating roller specimen on an alloy, relatively low friction coefficients of 0.6 were arch-shaped specimen with an applied load. The obtained with dust particles in spite of the high value of vacuum pressure was less than 1 × 10-4 Pa. Friction tests 0.8 in the case of no particles. Dust particles acted as a were conducted at a load of 10 N and a sliding speed of 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. 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. (a) 440C against 440C 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 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 (c) 440C against Al alloy 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 Fig. 4 morphologies and surface profiles of wear arch-shaped specimen. Long scratched scars were tracks of both roller specimen and apparent on the roller specimen with particle of 100-300 arch-shaped specimens after the friction µm; it was more obvious than with <32 µm particles. tests with and without dust 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 Fig. 6 Friction behaviors and wear tracks of PTFE film A with and without dust PTFE film B with and without dust particles. particles.

Friction behavior of PTFE film B on 440C and 3.3 Discussions results of observation after the tests are presented in Fig. The wear mechanism is expected to be abrasive wear 6. The friction coefficient showed a high value of 1.2 when dust particles existed at the contact surface without particles, whereas significant wear was not although the scratch track sizes differed between found. Compared to the non-particle test, lower friction particle sizes. However, in some cases, e.g. 440C-440C of 0.6 was observed with particles; it did not depend on and 440C-Ti alloy combinations, morphology of the the particle size. Some wear debris was found on the wear tracks by particle size of 100-300 µm was roller specimen but the wear depths of the coating were observed as identical to those obtained from small and similar to the case of no particles. This non-particle tests, apparently because of moving out of coating showed excellent wear resistance for the lunar the particles. Wear by larger particles seemed to differ regolith except for the high friction in the case of no with the movement and action of the particles: the particles. particle stayed at the contact surface, it left there soon or The low friction coefficient of 0.1 was obtained for it was crashed to a smaller size. Action of the dust MoS2 film on 440C, as depicted in Fig. 7. However, particles probably depended on test conditions, surface because of the dust particles’ existence, high friction activation, static electricity of surfaces, and so on. and much wear were observed. The film was worn away Results obtained in this study also show that by particles early in the tests because the friction materials without high hardness, such as aluminum coefficient and wear morphology were almost identical alloy, are difficult to use in a moon dust environment. to results of the 440C against bare 440C tests. Low However, wear resistance against dust particles was hardness of the film and thin film thickness seemed to improved by PTFE coatings. To select materials and cause these results. 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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12. Matsumoto, K. and Suzuki, M., “Evaluation of 4. SUMMARY Various PTFE Films for Space use”, Proc. of Some metal materials and coatings used for space International Tribology Conf. Kobe, (2005). 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.