Lince, Loewenthal, and Clark provide a recent and thorough discussion of previous studies regarding  MoS 2 aging and oxidation. They comment that long-term storage life test data for sputter-deposited MoS 2 coatings are uncommon [4]. They also completed and reported a study, motivated by the JWST  observatory, on the degradation of three nanocomposite MoS 2 coatings during storage in air. The tests  used to evaluate endurance were pin-on-disk tests, and storage times were up to 2.3 years. They found  that exposure to air could degrade the endurance of the coatings by up to 55 percent. The severity of  degradation depended on both the exposure duration and coating composition.  Experimental Equipment  Test Gears  Readily available stock gears with appropriate center distance were selected and customized for use in  this study. The customizations of the stock gear design were the bore diameters, sized for a commercial  off-the-shelf keyless shaft-locking device, and custom face widths. The pinions and gears were 3-mm  module spur gears with standard tooth proportions. The pinions had 26 teeth and a 13-mm face width.  The gears had 48 teeth and 10-mm face width. There were six pairs of pinions and gears. The material  was S45C steel (equivalent to AISI 1045). The teeth were induction hardened to surface hardness of HRc  50-60 and ground. The test gears were coated with MoS 2 by sputtering. While certain mechanisms of the JWST use  nanocomposite MoS 2 coatings [4], the mechanisms of interest for this work have a pure MoS 2 coating,  and so the test gears were provided with such a composition. The chamber capacity required two coating  runs. Witness coupons were in the chamber during sputtering. The thickness of the coating on the  witness coupons were reported by the vendor to be 37,000 angstroms for one run and 30,000 angstroms  for the second run. After coating the test gears were sealed in bags using a dry inert cover gas by the  coating vendor. Gears to be tested as unexposed remained in the sealed bags until the start of the  installation procedure. The time from the opening of the bag until the gears were in a vacuum condition in  the gear test rig was minimized to all practical extent.  The tooth surface roughness was measured along the involute profile direction of a randomly selected  pinion tooth using a stylus profilometer prior to sputtering. Another tooth was inspected after sputtering  and subsequent exposure to humid air prior to testing. The data were filtered using an ISO standard  Gaussian filter with 0.8-mm cutoff and 300:1 bandwidth. The resulting calculated roughness average  value, 0.42 micrometer Ra, was the same prior to and after sputtering. The peak-to-valley range of the  roughness-filtered data was approximately the same as the measured thickness of the coating on witness  samples that were in the sputtering chamber with the gears. The roughness topography features were the  same prior to and after sputtering (Fig. 1).   One half of the available tooth surfaces were exposed to humid air prior to testing. The exposure was  done in a closed chamber with the gears placed on a perforated plate. Beneath the plate was a saturated  solution of water and sodium bromide. The saturated salt solution provided a relative humidity of  approximately 57 percent, this value being near the upper limit 60 percent relative humidity of certain  storage conditions for the mechanisms of interest.  NASA/CP—2018-219887 128