Hybrid Lubrication of PFPE Fluids and Sputtered MoS 2 Michael Buttery*, Anthony Kent*, Dave Forster* and Achilleas Vortselas*  Abstract  We present an overview of the recent activities performed by the European Space Tribology Laboratory  (ESTL) into the potential of hybrid lubrication of PFPE fluids (Fomblin Z25 & Braycote 601EF) and  sputtered MoS 2. Test campaigns were performed using a spiral orbit tribometer (SOT), pin-on-disc  tribometer (PoD), and at spur-gear level.  Results demonstrated mixed behavior of hybrid lubrication. In the best case the lifetime is extended  beyond that predicted by the individual constituent lubricants, with no elevation in friction coefficient. In  the worst case the application of a grease to the sputtered MoS 2 appears to inhibit the favorable  tribological behavior of the solid lubricant film, reducing the lifetime and elevating the friction/torque.   The degree of success of hybrid lubrication appears to be related to the physical properties of the applied  fluid lubricant (film thickness, viscosity), rather than the tribo-chemical lifetime. A model is proposed by  which this behavior occurs.  We gratefully acknowledge that this work was funded by the European Space Agency.  Introduction  For a mechanism engineer lubricant selection often comes down to a trade-off between solid and fluid  lubrication, with merits and disadvantages to both solutions. Solid lubricants are typically chosen in  situations where temperature restraints preclude the use of fluid lubricants (due to evaporative losses or  viscous torque increases), as well as applications for which contamination is a major consideration (e.g.  involving optics). Fluid lubrication is typically selected for applications operating at high speeds over  medium-to-long periods (high duty). Fluid lubricants also typically display lower torque noise and higher  thermal conductance.  Solid Lubrication – Sputtered molybdenum disulphide  Molybdenum disulphide (MoS 2), deposited as a thin film via physical vapor deposition (PVD), is  commonly used as a solid lubricant within high vacuum and spacecraft mechanism applications. Such  films yield very low friction and relatively long lives when operated under high vacuum conditions [1].  These tribological properties are maintained over a wide range of temperatures [2, 3]. As such, MoS 2 coatings are used routinely to lubricate spacecraft mechanisms. However, when operated in moist air the  coatings adsorb water molecules and this affects their shear properties which in turn causes the friction to  increase (by up to an order of magnitude) [1].  Furthermore, the coating oxidizes and, as a result, wears at a much more rapid pace than would be the  case in vacuum. Thus, operation in moist air severely reduces the subsequent in-vacuum life of the  coatings [4, 5]. This reduction in life is shown to be dependent upon running duration in-air, with even  short running periods producing dramatic reductions in subsequent in-vacuum life (Figure 1).  Following operation in air, friction coefficient values can return to their low vacuum-running values, but  often only after a period of high friction. The magnitude and duration of this increased friction upon  *ESTL (European Space Tribology Laboratory), ESR Technology Ltd., Warrington, U.K. Proceedings of the 44th Aerospace Mechanisms Symposium, NASA Glenn Research Center, May 16-18, 2018 NASA/CP—2018-219887 151