An Improved Solid Lubricant for Bearings Operating in Space  and Terrestrial Environments  Arindam Paul*, Harpal Singh*†, Kalyan C. Mutyala*‡ and G.L. Doll* Abstract  The lubricity and durability of molybdenum disulfide (MoS 2) is controlled by the interfilm sliding and intrafilm  flow. The primary reason for its ability to reduce friction is attributed to its crystal structure, which allows  easy shearing of MoS 2 layers. Effective lubrication has been achieved under vacuum and dry conditions by  employing MoS 2 as a solid lubricant. However, under humid conditions, the tribological performance of  MoS 2 deteriorates. The deterioration can be offset through the incorporation of certain elements in MoS 2,  which can also improve its tribological and mechanical properties. Demands on robustness and reliability  provide motivations for improvements to broaden the application range of MoS 2 coatings in mechanical  systems. Although Ti-MoS 2 coatings have been shown to perform extremely well in rolling contact, it is  instructive to compare the tribological performance of this coating to MoS 2 and MoS 2 doped with Sb 2O3 and  Au, in sliding contact applications. Results of this investigation indicate that the Ti-MoS 2 coating outperforms  MoS 2 and MoS 2 doped with Sb 2O3 and Au in reciprocating sliding contact experiments performed in  laboratory air at 30°C and 100°C.  Introduction  Solid lubricants are materials that can reduce friction between two surfaces sliding against each other  without the use of a liquid media. Solid lubricants such as MoS 2 have been successfully utilized to reduce  friction and wear in vacuum applications where liquid lubricants cannot be employed. MoS 2-based coatings  are used as solid lubricants in various applications on earth and space such as cutting tools, gears,  bearings, actuators and slip rings, among many others [1-3]. The tendency of MoS 2 to re-orient, from an  initially random orientation, to a state where the {002} basal planes orient parallel to the surface is believed  to be the major reason for its success as a solid lubricant [4, 5]. MoS 2 has a hexagonal crystal structure  (D4h6 – P6 3/mmc) with a low friction coefficient due to the ease of its basal plane shearing. Basal planes  have strong covalent bonds between S-Mo-S and weak van der Waals bonds between the planes.  However, MoS 2 is susceptible to humid environments due to reactive edge sites that inhibit basal plane  shearing and thereby increase its friction coefficient in the presence of oxygen and moisture [6]. Various  elements (such as Ti, Cr, Zr, Au, Pb and Ni) and oxides (SbO X, PbO) have been incorporated into MoS 2 coatings to improve their tribological performance [7–12]. The addition of metals or oxides greatly affects  the tribological, mechanical and structural properties of MoS 2 composite coatings [13,14].  Ti and Sb 2O3/Au doped MoS 2 coatings have found their way into several aerospace applications. The  hardness and Young’s modulus of doped MoS 2 is much higher than sputter-deposited MoS 2. The  improvement in mechanical properties, load bearing capacity, and wear resistance over pure MoS 2 has  also been attributed to the dopant content [1]. Recently, a Ti-MoS 2 coating was developed for rolling contact  applications [4]. Although this particular coating was found to perform excellently as a solid lubricant  material for rolling element bearings operating in laboratory air and vacuum testing, it was also found to  greatly enhance fatigue life of rolling contact applications that operate in lubricated conditions [12]. Although  the friction and wear of these coatings have been extensively studied using unidirectional and reciprocating  sliding, it is beneficial to compare the tribological performances of several varieties of MoS 2 coatings that  are deposited similarly. In this study, Ti-containing, Sb 2O3/Au-containing, and pure MoS 2 coatings were  *Timken Engineered Surfaces Laboratory, University of Akron, Akron, OH †Currently at Sentient Science, West Lafayette, IN ‡Currently at Argonne National Laboratory, Lemont, IL Proceedings of the 44th Aerospace Mechanisms Symposium, NASA Glenn Research Center, May 16-18, 2018 NASA/CP—2018-219887 141