302 and advance the lubrication technology  for high vacuum space mechanisms, additional understanding of  its tribological performance is needed  to facilitate  innovations .  

When considering how to research the efficacy of lead naphthenate and how it functions as a tribofilm, it is  important to look at various methodologies to characterize its performance. In the beginning stages, this is  easiest performed by using simulated contact testing. In this respect, an SRV makes the best choice as it  is very flexible regarding the contact mechanics and envi ronmental conditions. The availability of SOT  testing is somewhat limited but a Mini Traction Machine (MTM) can be used in a manner to simulate the  same type of application conditions with a ball running in an orbital pattern on a disc with a mixture of  rolling/sliding. While the MTM cannot currently run in a vacuum condition, it can be used to understand the  fundamental complexities of lead naphthenate operating in a rolling/sliding mixed application. When this is  combined with the vacuum/atmospheric testi ng on the SRV, the performance of the lead naphthenate can  be made clearer.   In more recent research  done on lead naphthenate using the SRV and other sliding tribometers, results  have typically been inconclusive regarding the efficacy of this additive to protect a lubricated contact. The  standard SRV testing includes conditions that could be inappropriat e for evaluating many anti -wear  additives including lead naphthenate. This includes the sliding speed which would create a thicker film  preventing the additive function from being studied. The contact stress is also much higher in standard  ASTM tests which  will influence how the tribofilms are created and in the case of lead naphthenate, the  higher contact stresses will create additional wear which will react and consume available lead naphthenate  making it unavailable to create a protective tribofilm.  

From the authors previous  resear ch [7], the following was determined:  In oscillatory testing, Lead  Naphthenate had better anti -wear performance at lower temperatures (~20°C) and increasing the amount  of lead directly reduced the wear rate. Under high vacuum conditions, the samples with lead naphthenate  offered twice the wear protection with 440C performing the best. At higher temperatures (75°C) the wear  rate was almost double for the samples with a higher content of lead across all experiments. The 75°C  testing in pure oscillatory sliding on 52100 steel, showed that samples with lead naphthenate in an  atmospheric environment performed worse as th e concentration of lead increased. However,  when in a  vacuum environment, as the concentration increased the wear rate decreased.  In 440C testing, samples  with lead offered up to twice the wear protection over the neat samples in a vacuum environment.  In mixed  rolling/sliding experiments, Lead Naphthenate had better anti -wear performance at high temperatures and  increasing the amount of lead directly reduced the wear rate. At lower temperatures (50°C) in the counter - rotation wear test, lead naphthenate offered three times the wear protection when in a 3% concentration  and eleven times the protection for 5% lead compared to the neat MAC fluid. At higher temperatures   (150°C) in the counter -rotation wear test, lead naphthenate offered five times the wear protec tion when in  a 3% concentration and twenty times the protection for 5% lead compared to the neat MAC fluid. There  appears to be is a transition point between 3% and 5% lead naphthenate where the available lead can react  with both the surface and worn metal  to create a strong lead anti -wear tribofilm. This tribofilm that is created  is between 2- 4µm thick and while it will cause an overall increase in friction at the surface, the wear of the  contact is greatly reduced.  

Previous Lessons Learned from AMS 2018:   - The additive function of lead naphthenate is a combination of physical absorption through rubbing  or pressure and chemisorption.   - Depending on the mechanics in the contact (sliding versus rolling), the effect of temperature had a  significant influence . This appears to indicate that higher concentrations of lead would be required  for more severe applications involving pure sliding and/or high temperatures as the lead is  consumed faster through reaction with surface layer steel oxides and sublayers.   - In rolling and mixed contacts, lead naphthenate creates a strong tribofilm on the steel surface that  aids in protecting from wear but at the same time will increase the friction in the contact.   - The formation and durability of lead naphthenate tribofilm is depende nt on the environment with  higher performance coming under vacuum conditions. It is also believed the lack of oxygen