Performance of MoS 2 Coated Gears Exposed to Humid Air During Storage  Timothy Krantz*, Claef Hakun**, Zachary Cameron*, Iqbal Shareef+ and Michael Dube++ Abstract  The purpose of this work was to study the effect of exposure to humid air on the durability of a  molybdenum disulfide (MoS 2) dry film lubricant on spur gears operated in vacuum. This study was  motivated by the James Webb Space Telescope (JWST) Mission. Some mechanisms of the JWST using  MoS 2 dry film lubricants have been exposed to humid air during storage as a subassembly and after  integration into a higher-level assembly. In this study MoS 2 dry film lubricant was applied to steel spur test  gears and subsequently tested in vacuum environment. One-half of the gears had essentially zero time  exposure to humid air prior to testing, and the other half were exposed to humid air of 57 percent relative  humidity up to 77 days prior to testing. All tests were completed at constant torque and speed. On  average the film durability was shorter for gears exposed to humid air compared to those with zero  exposure. For the unexposed gears, the durability ranged from 53,300 to 190,300 pinion revolutions with  an average value of 100,200 and a median value of 83,500 revolutions. For the exposed gears, the  durability ranged from 21,000 to 84,700 pinion revolutions with an average value of 64,900 and a median  value of 68,800 revolutions. Using the unexposed gears as a baseline, the exposure reduced the average  durability by 35 percent and the median value of durability by 18 percent. Red-brown coloration was  noted on some of the gear teeth that had been exposed to humid air. The colored regions appeared as  soon as 17 days after exposure to humid air. SEM inspections showed that at least some of these colored  areas included material raised above the surrounding MoS 2 film.  Introduction  The purpose of this work was to study the effect of exposure to humid air on the durability of a  molybdenum disulfide (MoS 2) dry film lubricant on spur gears operated in vacuum.  This study was motivated by the James Webb Space Telescope (JWST) Mission, and the study is one  part of a NASA Engineering Safety Center effort to evaluate potential risks and performance effects to  JWST instrument mechanisms and components lubricated with sputtered MoS 2 coatings. The coatings  are exposed to humid air environments during integration, ground operations, and storage prior to launch.  The mechanism configurations included in the scope of the study are the fine guidance sensor (FGS)  coarse focus mechanism, the FGS fine focus mechanism, the FGS dual-wheel mechanism, and the nearinfrared camera focus adjust mechanism. Some details about the design, life testing, and engineering  enhancements of these mechanisms have been reported [1-3]. The mentioned mechanisms have  sputtered MoS 2 dry film lubricants on gears and other components. The practical effects of exposure to  humidity on the performance of MoS 2 are not fully understood, and to our knowledge a dedicated study of  such effects has not been conducted for MoS 2 films lubricating gear teeth. The focus of this study was the  durability of a MoS 2 dry film lubricant on spur test gears that have been exposed to humid air up to 77  days.  *NASA Glenn Research Center, Cleveland, OH **  NASA Goddard Space Flight Center, Greenbelt MD + Bradley University, Peoria, IL ++ NASA Langley Research Center (NESC), Hampton, VA Proceedings of the 44th Aerospace Mechanisms Symposium, NASA Glenn Research Center, May 16-18, 2018 NASA/CP—2018-219887 127