that in a test where the contact stress increases at every stage, the contact patch will also increase at every  stage. This leads to fresh nascent metal being exposed at every stage. This new area of contact does not  have a tribofilm on it and so it is more likely to have aggressive wear. This is why tests run in a progressive  load methodology will typically have failures at the step increase and have lower repeatability.  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 19.05-mm diameter with a roughness ( Rq) of 10 nm and a Rockwell  hardness of 62.5. The discs had a hardness of 60.5 and a roughness ( Rq) of 11 nm. All the tests were run  in duplicate and with a maximum Hertzian contact stress of 1.25 GPa.  The experimental testing methodology is as follows. Test specimens are ultrasonically cleaned in heptane  followed by acetone. The testing apparatus is then assembled and filled with oil for the experiment. The  temperature of the oil is then heated to 150°C while the ball and disc are rotated at a slow speed while not  in contact. This continues for 30 minutes to allow for any chemical absorption of the additives on the  surfaces. After this, there is a 10-minute run-in period with a Hertzian contact stress of 1.25 GPa and an  SRR of 1. Once the run-in has completed, a progressive speed test starts with running stages for 1 minute  and rest stages for 30 seconds. The SRR is varied at each stage in order to maintain the entrainment speed  for each stage but increase the sliding speed at the contact. The test continues until either all stages (51)  are completed (maximum speed for MTM) or scuffing wear and seizure occurs.  Results and Discussion  SRV (Sliding in Atmospheric and Vacuum Conditions)  The results in Tables 1-3 and Figure 2 are from the experiment on the MAC with 0%, 3%, and 5% lead  naphthenate when tested in a mixed film regime, at 20°C on the SRV under both atmospheric and vacuum  conditions. These plots illustrate the wear rate of each material which is determined by the total wear volume  (µm3) per millimeter traveled in the test. The wear volumes were measured using an Ametek 3D Optical  Profilometer.   Table 1. Wear Performance for MAC fluid with 0% Lead at 20°C  Table 2. Wear Performance for MAC fluid with 3% Lead at 20°C  Table 3. Wear Performance for MAC fluid with 5% Lead at 20°C Material Temp ( °C)Environment Specimen Avg Wear Scar (mm2)Disc Wear Volume (μm3)Wear Rate (μm3/mm) 0% Lead 20 Atmosphere 440C 1.939 599,972 0.28 0% Lead 20 Vacuum 440C 1.924 518,400 0.24 0% Lead 20 Atmosphere 52100 2.260 481,018 0.22 0% Lead 20 Vacuum 52100 2.265 410,400 0.19 Material Temp ( °C) Environment Specimen Avg Wear Scar (mm2)Disc Wear Volume (μm3)Wear Rate (μm3/mm) 3% Lead 20 Atmosphere 440C 1.966 410,400 0.19 3% Lead 20 Vacuum 440C 1.945 216,000 0.10 3% Lead 20 Atmosphere 52100 2.278 342,356 0.16 3% Lead 20 Vacuum 52100 2.251 259,200 0.12 Material Temp ( °C)Environment Specimen Avg Wear Scar (mm2)Disc Wear Volume (μm3)Wear Rate (μm3/mm) 5% Lead 20 Atmosphere 440C 2.129 291,600 0.14 5% Lead 20 Vacuum 440C 1.879 172,800 0.08 5% Lead 20 Atmosphere 52100 2.742 263,520 0.12 5% Lead 20 Vacuum 52100 2.242 216,000 0.10 NASA/CP—2018-219887 73