Post-test inspection of gears showed clear evidence of lubricant failure, with varying degrees of scuffing  and pitting of the teeth leading to metallic wear.  It is recognized that despite the similar fluid lubricant volumes employed, significant differences exist  between the gear and tribometer testing which may influence the lubricant behavior, and hence the  success of hybrid lubrication. The PoD tribometer is an assessment of the lubricant behavior in a pure  sliding regime only, whilst the motion on the SOT is predominantly rolling. In contrast, the spur gears  experience a combination of rolling motion during gear meshing at the pitch point, combined with sliding  as the mesh moves from the pitch point. This meshing action of the gears can act to redistribute the fluid  lubricant in the contact, bringing about an extension in life for the fluid lubricated gears, and also  potentially altering the behavior in the hybrid lubricated case in a way not comparable to the PoD (or  SOT).  Discussion of Results and Theory of Hybrid Lubrication  From these experimental test results the commonality can be drawn that the key factor in the success of  hybrid PFPE/MoS 2 lubrication appears to the mass and/or viscosity of the fluid layer applied to the  sputtered MoS 2. This observation shall now be discussed in the context of three proposed models for the  hybrid lubricating behavior.  It has been shown that hybrid lubrication has the potential for two attractive tribological behaviors.  •Hybrid lubrication can act to protect the sputtered MoS 2 film from elevated degradation/wear/oxidation when operating in moist air. •Hybrid lubrication provides a vacuum lifetime greater than the vacuum lifetime of its constituent parts (i.e. it is synergistic), with no subsequent increase in friction/torque. The mechanism by which these behaviors arise is not clear, but three general models are proposed.  •Model 1) Protection of PFPE fluid from degradation  – It is known that the lifetime of a PFPE lubricant can be extended through physical protection of the fluid from reaction with the substrate steel, in this instance provided by an MoS 2 film. However, if this model were to dominate we would expect to observe a higher friction coefficient, more akin to fluid lubrication than MoS 2, when observing an extension in life. This is not the case during SOT and PoD testing. •Model 2) Protection of sputtered MoS 2 film from degradation  – An alternative model has been proposed whereby the layer of PFPE fluid upon the surface of the MoS 2 film acts to protect the solid lubricant from reacting with the environment, prolonging the wear life of the MoS 2. This model is attractive to explain the protection seen by the MoS 2 film when rolling in moist air but is less applicable to explain the extension of life under vacuum, where the presence of moisture/oxygen is severely reduced. •Model 3) MoS 2 transfer film establishment  – It is understood that the production of a 3rd-body transfer film is vital for successful lubrication of MoS 2 [19]. Given that this 3rd-body transfer film is produced from what is essentially ‘wear debris’ of the MoS 2, it is proposed that the physical presence of a viscous fluid within and around the contact zone helps to retain this MoS 2 debris, and is advantageous to the formation/protection of a 3rd-body layer. It is clear that the success of hybrid lubrication is sensitive to the film thickness of the applied oil (or ratio  of fluid film thickness to MoS 2 film thickness). If we assume an equal distribution of oil, the fluid film  thickness achieved during the Phase Two PoD tests ranged from 0.05 to 0.55 µm, applied onto a ~1-µm  sputtered MoS 2 film. SOT tests described above employed a similar fluid film thickness of ~0.17 µm over  NASA/CP—2018-219887 161