•As a general guide the lifetime of grease lubrication under vacuum increases with increasing temperature. This is seemingly counter-intuitive and not in line with our expectations given the known dependence of PFPE lubricant degradation and tribological lifetime upon temperature [11]. It is clear that the grease lifetimes in these PoD tests are dictated by some physical limitations of the  lubricant, rather than tribo-chemical degradation. This can be conclusively demonstrated using a RGA  system, where no evidence of residual gas by-products of the PFPE degradation were observed in any of  the above tests, including those with hybrid Braycote 601EF/MoS 2 lubrication. Failure of the grease is  therefore likely caused by the physical displacement of the fluid lubricant away from the contact zone, in  contrast to the SOT tests, which may also contribute the poor hybrid performance during these PoD tests.   The improved performance of the grease at 200°C also leads to the suggestion that lower viscosity of the  fluid selected may be helpful for the hybrid effect, due to an improved rate of lubricant flow into the  contact zones. This can be demonstrated through repeating of a small selection of tests detailed in Table  3, utilizing the PFPE oil Fomblin Z25 as a replacement to Braycote 601EF. All other test conditions  including the application method were held identical.  Results demonstrated that the Fomblin Z25 oil performed significantly better at PoD level than Braycote  601EF, with RGA data showing shear-induced tribo-chemical degradation of the lubricant under test.  Hybrid Z25/MoS 2 tests were also improved in comparison to Braycote 601EF/MoS 2 (Table 4) but showed  elevated test variability.   Table 4. Performance of hybrid Z25/MoS 2 under vacuum on PoD.  Tests performed at RT, 1500 MPa  Normalized lifetime  Fomblin Z25 MoS 2 Hybrid Z25/MoS 2 2.723 1.000 0.365  Whilst Fomblin Z25 performs better than Braycote 601EF as a hybrid lubricant, it is clear that the highly  encouraging performance observed at SOT is not replicated in the sliding environment of the PoD. This is  potentially due to the significantly lower fluid volumes, and/or fluid film thicknesses, of the SOT testing  campaign in comparison to the PoD tests (for context, the volume of fluid in a typical SOT test is three  orders of magnitude less than the PoD). To explore this possibility, a second phase of PoD testing was  performed utilizing a reduced lubricant volume, applied following the solvent-plating technique employed  on the SOT onto MoS 2-lubricated discs.  PoD Phase Two – Low volume fluid lubrication  Three levels of Fomblin Z25 lubrication were achieved for low volume fluid lubrication tests.  •Discs lubricated with sputtered MoS 2 exposed to a PFPE solvent bath – To confirm that the solvent used for the oil plating technique (PF5060) does not adversely influence the tribological performance of the solid lubricant. •Discs lubricated with sputtered MoS 2 and low mass of Fomblin Z25 (~40 µg). •Discs lubricated with sputtered MoS 2 and high mass of Fomblin Z25 (~400 µg). It should be made clear that this ‘higher’ mass is still two orders of magnitude less than the recommended amount for a fluid lubricated component. All tests were performed under vacuum, 0.6 ms-1 sliding speed, 900-MPa peak contact stress, on  standard 52100 steel test components. Multiple repeat tests were performed under each condition.  Failure of the lubricant was defined as an increase in friction coefficient to ≥ 0.3. Mean friction and  normalized lifetime results are presented in Table 5.  NASA/CP—2018-219887 157