224 As discussed above, the total magnitude of oxidation of the samples to MoO 3 at 53% RH is ~3%. However,  this percentage represents the entire sample, and does not t ake into account the likelihood that a higher  amount of oxidation occurs near the surfaces of the piles of powders. In fact, a  whitish color indicative of  MoO 3 was visible on the surfaces of MoS 2 piles samples stored at 53% RH for 1½ and 3 years , which was  not apparent in the region belo w the surface of the piles.  

Figure 2.  Relative amount (in percent) of powder MoS 2 samples that were oxidized to MoO 3 by  exposure to air. Results are shown over 3 years storage for samples stored at three humidities.  

MoS 2 solid lubricant coatings used on spacecraft include burnished films, sputter -deposited coatings, and  pressure- sprayed coatings  that are considerably thinner than the macroscopic samples evaluated in this  earlier study (i.e., 1 –10 μm thick). As such, the amount of oxidation relative to the lubricant thick ness  for  thin coatings is potentially much greater compared to the more macroscopic samples in the MoS 2 powder  study. In fact, for very thin or porous coatings, a  previous study showed that oxidation could be as high as  40–100% throughout the depth of the coating.18 In less porous coatings, the amount of oxidation might be  lower. However, the amount of oxidation would be significant in either case because the oxidation is  enhanced at the surface of the coatings, where tribological interactions in lubricated devices occur.   Whether lubricant degradation due to MoS 2 oxidation adversely affects device p erformance depends on  the application, the type of coating, and the tribological requirements, such as fri ction/torque and number  of duty cycles. Applications involving low numbers of duty cycles and wide friction margins are more robust with respect to such degradation. For example, latches and other release mechanisms will only operate once on orbit (including a few cycles during ground test), and the dominant concern is to ensure that a maximum coefficient of friction (COF) is not reached and to prevent metal -to-metal contact that would cause  galling or cold- welding. In such cases, the increase in friction due to partial conversion of the MoS 2 to MoO 3  may not be enough to significantly affect deployment. In contrast, scanner and gimbal bearings involve higher contact stresses and must oper ate for millions of cycles. Also, excessive MoO 3 production could  prevent the bearings from meeting requirements for low torque noise. For these applications, even a small amount of degradation could be significant. For bearings containing self -lubricating retainers, a film of MoO 3  on the bearing race surfaces could affect the rate of formation and uniformity of transfer films. Poor transfer film formation could result in the appearance of torque noise and lowered lifetime.    The oxidation of MoS 2 is believed to be many times higher at the more chemically reactive edge plane  surfaces of a MoS 2 crystallite, rather than the relatively unreactive basal plane surfaces.14 In resin- bonded  coatings  (containing powdered MoS 2 as the lubricant) , these reactive edge sites are protected signifi cantly  from moisture by the sur rounding binder matrix. As such, bonded coatings are generally less sensitive to  humid air exposure. However, even for these coatings, a thin film of pure MoS 2 forms on the surface of a  burnished/run- in coating, which is then susceptible to oxidation. The result is that there w ill be initially  increased fric tion until this oxidized surface layer is worn off.