More recently several researchers have started investigating this wear mechanism and different methods  to evaluate it. Some work has been done by Galary to study scuffing on an SRV [5] as well as to simulate  the FZG test [6]. In the more recent work by Galary, he investigates a method to simulate the FZG scuffing  test on an SRV using custom specimens and fixtures. This new method used the same type of gear steel  and contact pressures as the FZG test, was much less expensive, showed great repeatability, and had a  strong correlation to the FZG test. It also had expanded load stages beyond the standard FZG test, but its  main limitation was that it remained a pure sliding test.  Wedeven has been one of the pioneers in using a rolling/sliding tribometer for scuffing analysis and he has  promoted the value of separating the sliding speed from the entrainment velocity to better understand the  tribological contact [7]. This is accomplished through counter-rotation and is critical in understanding the  wear regime and what is happening in the contact. The approach of Wedeven is different than in this paper  as he approaches the problem by variation of the contact angle between mating bodies as opposed to the  counter-rotation method used in this work.  Experiments  Two different tribological test methods were used in this research. One of the methods is run in pure sliding  under both atmospheric and vacuum conditions (5 x 10-5 Torr min) while the second testing is a mixed  rolling/sliding contact in atmospheric conditions only. The friction/traction properties along with the wear  rates were measured for two multiply alkylated cyclopentane (MAC) hydrocarbon oils formulated with 3%  and 5% lead naphthenate (2001-3PB and 2001-5PB) along with unformulated MAC oil (2001A). The lead  naphthenate used in these samples was vacuum treated prior to formulation to improve vacuum outgassing  characteristics and make it suitable for a space mechanism lubricant.   For the sliding testing, an SRV tribometer was used in a ball on disc configuration. The SRV has a fixed  lower specimen allowing the upper ball to be oscillated at various frequencies while the frictional force is  measured. The contact area is coated with 5 ml of lubricant for the testing that will be run at temperatures  of 20°C and 75°C (controlled during test). For this research, AISI 52100 steel balls and discs were used  that had a Young’s Modulus of 210 GPa and a Poisson ration of 0.30. The balls had a 10-mm diameter with  a roughness (Ra) of 25 nm and a Rockwell hardness of 62 HRC. The 24-mm discs are vacuum arc remelted and had a hardness of 58 HRC with a lapped surface that has a roughness (Rz) of 500 nm. The  440C steel balls and discs that were used had a Young’s Modulus of 210 GPa and a Poisson ration of 0.30.  The balls had a 10-mm diameter with a roughness ( Ra) of 25 nm and a Rockwell hardness of 62 HRC. The  24-mm discs had a hardness of 58 HRC with a lapped surface that has a roughness ( Rz) of 500 nm. All the tests were run in duplicate and with a maximum Hertzian contact stress of 2.12 GPa and a sliding speed of 300 mm/s. The experimental testing methodology is as follows. Test specimens are ultrasonically cleaned in heptane  followed by acetone. The testing apparatus is then assembled, and oil is applied for the experiment. The  lower specimen is then heated to the testing temperature with no force applied to the contact and no  oscillation. Once the testing temperature is reached, the force is applied to the specimens and oscillatory  motion starts. The test continues for two hours or until the coefficient of friction exceeds the test limits or  seizure occurs.   The mixed rolling/sliding testing was performed using a mini-traction machine (MTM) with a ball on-disc  configuration which is shown in Figure 1. In the MTM, the ball and disc are driven independently which  allows any combination of rolling and sliding. The measurement of frictional force is done through a load  cell that is attached to the bearing housing of the ball motor shaft. The lower testing specimen (disc) and  the contact area is completely immersed in oil for testing which is kept at a controlled temperature of 50°C  and 150°C for this testing.  NASA/CP—2018-219887 71