While there are a wide variety of atmospheric tribology tests, there are far fewer that operate under high  vacuum conditions. The three most important of these vacuum tribology tests are the 4-ball, SRV, and the  SOT, all of which have their particular tradeoffs.   The use of a 4-ball wear test under vacuum provides good comparison to a large amount of published data  as this test has been used for decades in various lubricants. However, this test has a high Hertzian contact  stress that operates in a pure sliding mode typically within the mixed lubrication regime. Because of the  high speeds this test is typically operated at, the sliding motion, and operating in a single rotational direction,  strong films are created which prevent this test from being able to properly evaluate anti-wear or extreme  pressure additives. The SRV, while similar to the 4-ball wear, offers more flexibility and operates in an  oscillating motion which allows for better evaluation of lubricant additives. The SRV also provides a variety  of contact geometries, loads, and speeds to help simulate a tribological contact. The SOT is in a class by  itself as it was purposely designed to simulate angular contact bearings in a vacuum environment and has  a rolling/pivoting motion.   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 in regards to the contact mechanics and environmental 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  complexities of lead naphthenate operating in a rolling/sliding mixed application. When this is combined  with the vacuum/atmospheric testing on the SRV, the performance of the lead naphthenate can be made  clearer.   To understand the functionality of lead naphthenate as an anti-wear additive, it is important to understand  the genesis of wear in order to be able to reduce or control it. There are many forms of wear including  adhesive, abrasive, and fretting , however they all have a starting point where the first damage to the surface  occurs and an ending point which is either a seizure or contact fatigue. Recent work had been done to  illustrate how scuffing wear is believed to be the point where this wear starts [1] [13].  Scuffing wear has been described by many ways over the years. It can be used in cases synonymously  with galling or scoring, all of which to some degree are different stages of the same wear phenomenon.  When a contact is supported by a lubricating film whether it is in Boundary, Mixed, or an EHD lubrication  regime, both the additives and fluid are performing different roles to prevent wear in the contact. The  scuffing point or genesis of wear, which I will define as instantaneous breakdown of the fluid film supporting  the contact, marks the time when wear will begin to occur, and the severity is then related to the strength  of the tribofilm. The breakdown of the fluid film can be caused by several methods including, increased  contact stress, entrainment speed, or degradation.   Over the last 100 years there has been considerable work done to develop the knowledge surrounding  scuffing wear and contact fatigue. In the 30’s and 40’s, Blok did a lot of research to study the scuffing  properties of lubricants with respect to their viscosity [2]. He was also the first to research and publish work  on how counter-rotation works to decouple the sliding and entrainment speeds. This will eliminate torque  effects and separate the different frictional elements.  Over the years, the research into scuffing has led to the creation of many different test methods or  equipment. Some of these include the 4-Ball Wear test which has been used for scuffing and contact fatigue  testing. The FZG gear test is one of the more well-known scuffing tests but has limitations due to its cost,  reproducibility, and being able to investigate the scuffing before catastrophic damage to the gear occurs [3]  [4].  NASA/CP—2018-219887 70