Concluding Remarks  It is necessary to understand the detailed characteristics of a lubricant for use in space since maintenance  can almost never be performed after launch. Therefore, the mission-unique environments must be  considered when selecting a lubricant. WS 2 was considered as an option for lubrication in high-temperature  space environments. The test results presented confirm the possibility of WS 2 as a solid lubricant for  elevated temperature space applications.   Three types of WS 2 lubricant films were prepared: bonded film with inorganic binder, sputtered film with  titanium interlayer and shot peening film. The results of this study show that all WS 2 coatings tested  produced very low measured friction coefficients.   This study had two purposes. The first was to evaluate WS 2 solid lubricants for elevated temperature in  vacuum using current technology. One approach was to compare the tribological properties of the WS 2 coatings to MoS 2 coatings since MoS 2 and WS 2 share similar crystal structures. Furthermore, MoS 2 has  many previous studies showing it to be an excellent solid lubricant in a vacuum environment. The other  approach was to compare the performance of pure WS 2 coatings produced with different methods of  application. The second purpose was to verify the low friction mechanism of WS 2 Sputtering films which  can be applied without binder and with high purity. Shot peening WS 2 is able to apply the coating without  binder or interlayer. These test methods are better to investigate the friction properties of WS 2 than only  from analytical results.  The friction coefficients of WS 2 shot peening films are extremely low at room temperature and also at  elevated temperatures. This lubricant film showed increasing friction coefficient with an increase in test  temperature, however friction coefficient at 400°C is still under 0.1. The low friction mechanism has been  confirmed based by the result of XRD analysis after the friction tests. All the WS 2 shot peening specimens  show a preferred orientation of (0002) basal plane aligning parallel on the disc surface. It is therefore  concluded that the friction coefficient values of WS 2 strongly depend on the (0002) crystal structure and  lattice defects in the crystal. These results suggest that behaviors of the lattice defect play an important role  on the low friction mechanism of WS 2 that relies on sliding of the (0002) plane.  Endurance life of WS 2 bonded films were longer than the other coatings. However, friction coefficient  increased over life. It is proposed from results of shot peening XRD analysis that the orientation (0002)  plane aligning parallel on top surface is one of reason for increasing of friction coefficient.   The testing performed showed WS 2 to have good performance and endurance life at elevated temperatures.  Based on a comparison of the test results between WS 2 and MoS 2 bonded film, it can be concluded that  WS 2 is a promising solid lubricant candidate at elevated temperature.  Acknowledgments  The authors wish to thank Drs. M. Suzuki, K. Matsumoto and H. Shiomi of Japan Aerospace Exploration  Agency (JAXA) for their technical assistances and helpful discussion. A measurement of friction coefficient  was carried out at JAXA. Stainless steel base material was fabricated by Uchiyama and Uetake of Teikyo  University. The bonded films of WS 2 and MoS 2 were supplied from Kawamura Corporation laboratory. WS 2 sputtering film produced with Prof. Koike, Associate Prof. Sutou and Mr. Toyoda of Tohoku University in  Japan. The shot peening WS 2 substrate were made by Japan Lubricant LTD. They cooperated for this  research. I would like to say sincere thanks to all of the people who supported this research.  NASA/CP—2018-219887 125