Efficacy of Lead Naphthenate for Wear Protection in Mixed Lubrication Regime  Jason T. Galary*,† This research investigated the efficacy of lead naphthenate as a wear additive in multi alkylated  cyclopentane (MAC) fluid. The use of lead naphthenate in MAC lubricants has a history of more than thirty  years with spaceflight heritage. However, despite its history of use in a variety of rolling and sliding  applications, there is still a lack of understanding about this additive's wear protection mechanism and its  performance. This paper will illustrate the efficacy of lead naphthenate to protect a lubricated contact  against wear in a variety of conditions including atmospheric, vacuum, and mixed film lubrication. The  effects of varying levels of lead naphthenate, different compositions of steel (52100 and 440C), and varying  temperatures was investigated.        This research investigates the performance of this additive in simulated contact testing using an SRV as  well as a Mini Traction Machine (MTM). The SRV testing of the lead naphthenate will be in boundary and  mixed film contact under vacuum and atmospheric conditions to better understand how the additive  functions on a lubricated 52100 and 440C contact. The MTM will be utilized for scuffing/galling wear tests  which will be performed in a mixed film and boundary lubrication conditions in a counter-rotation  rolling/sliding contact. Through use of counter-rotation, the entrainment speed can be decoupled so that  the scuffing/galling properties of the lubricant can be determined in boundary and mixed film conditions  over a range of sliding speeds. This also allows for the rolling friction and sliding friction to be decoupled  and measured separately.  Introduction and Background  Lead naphthenate has been used heavily as an anti-wear and extreme pressure additive in multi alkylated  cyclopentane lubricants for high-vacuum space mechanisms. The additive consists of a centralized lead  ion that is bonded with the oxygen atoms of a two carboxylate groups each attached to naphthenate  aromatic rings. The naphthenate aromatic hydrocarbon rings provide the solubility of the lead naphthenate  in different hydrocarbon oils but the way the additive reacts with steel and protects against wear in various  lubrication regimes is still not fully understood.   The historical use of lead naphthenate originates from industrial gears and bearings where it was used an  extreme pressure additive over fifty years ago. Over the last thirty years, it has gained a lot of spaceflight  history although there are still many questions regarding its efficacy, primarily how does it function in various  lubrication regimes, the effects of different metallurgy, and how does the environmental pressure effect its  performance.   With the increasing number of space mechanisms being developed and launched as well as the increasing  length of mission time required, it is critical to have a robust design with high reliability. To improve the  reliability, it is necessary to have long lives for all of the components in the design. This will require that the  lubricants used for space mechanism environments must also improve. The use of lead naphthenate  additive in multi alkylated cyclopentane fluids is a proven additive package with spaceflight history but in  order to develop and advance the lubrication technology for high vacuum space mechanisms, additional  understanding of its tribological performance is needed.   *Applied Science and Tribology, Nye Lubricants, Inc, Fairhaven, MA † Department of Mechanical Engineering, University of Massachusetts Dartmouth Proceedings of the 44th Aerospace Mechanisms Symposium, NASA Glenn Research Center, May 16-18, 2018  NASA/CP—2018-219887 69