degradation rate of the PFPE [11]. It is also known that, when not experiencing shearing, spontaneous  degradation of Fomblin Z25 occurs between 190 – 250°C [11]. Together these indicate that the lifetime of  the oil should be severely reduced when operating at 200°C (assuming the lifetime is dictated by the  chemical degradation of the fluid).  It is therefore suggested that the increased lifetime of the hybrid Z25/MoS 2 at elevated temperature is  again a result of temperature-related viscosity changes within the fluid lubricant, allowing ‘reflow’ of fluid  into the running track to occur more easily. This is highly surprising given the low volumes of lubricant  employed during these low fluid volume PoD tests, the fact that the low coefficient of friction suggests that  the lubrication between the contacts is still predominantly provided by the MoS 2 film, and the extent of the  predicted MoS 2 lifetime reduction. By whatever mechanism hybrid lubrication occurs (see below), it is  clearly influenced by operating temperature.  Spur Gear Experimental Campaign  In parallel to the SOT and PoD test programs, a small series of spur gear tests was performed to  determine if the hybrid lubrication effect could be reproduced at component level. These gear tests form  part of an ongoing campaign to evaluate and characterize the performance of solid, fluid, and hybrid  lubricated gears under vacuum and are reported in detail elsewhere [18].  Selected gears were BS4582 class B (DIN867 Q7) (hobbed) precision, manufactured in 17-4PH steel  (Condition A) without further surface treatment. Gears were 0.5 modulus, the pinion having 40 teeth (face  width 5 mm) and the wheel 120 teeth (face width 2.5 mm). It should be noted that the gears do not have a  “hunting tooth” ratio, such that the same teeth contact repeatedly in each revolution.  Whilst the 17-4PH material was selected for similarity with known applications, its relatively low hardness  was chosen to permit both a comparison with earlier work on non-hardened steels and a subsequent  evaluation of the beneficial impact of more typically hard surface treatments (Condition A results in a  minimum hardness of 35 HRC (~333 Hv)). The relatively low gear precision class and surface finish  (hobbed rather than ground gears) were selected for reasons of similarity with earlier test campaigns, and  to permit these production factors to become a variable in the wider context of the larger test campaign. A  unique gear set was employed for each test.  Spur gear tests were performed under vacuum at 23°C, with a preload of 7.5 Nm, equivalent to a peak  stress of 1000 MPa. Sputtered MoS 2, Braycote 601EF grease, and hybrid Braycote 601EF/MoS 2 provided  the lubrication, with the recommended 10-mg/cm2 volume of grease applied directly to the gear teeth.  Rotation speed was 100 RPM for the fluid and hybrid lubricated gears, 50 RPM for the MoS 2 lubricated  gears. Failure of each test was defined as an increase of the long-term torque measurement to 1.25x the  steady-state value. The steady-state torque was determined by averaging the torque after an initial  running-in period.   Given these selected conditions and ratio we might expect these results to provide a demanding (even  perhaps a kind of “worst”) test case for the lubricants used.  NASA/CP—2018-219887 159