ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
292   Figure 7. TFF measurements of oil mobility after wear in the high vacuum tribometer  

During the thinning experiments shown in F igure 7, the film thickness was monitored interferometrically and  plotted in Figure 8. Here, we compare the unworn base oil (green diamonds) with the low speed (red  triangles and blue circles)  and high speed ( purple squares ) test results . The results after the high- speed  tests are within the experimental uncertainty of the unworn base oil tests. There are no detectable e ffects  of running under EHD conditions for the times and pressures that were used. However, the impeded flow  after the boundary tests are clearly seen in the lower slope (higher viscosity). Another distinguishing feature  is that the slope continued to decrease as the film thickness reduced further in the later portions of the test.  This implies that viscosity continued to increase with reduced thickness. Another observation is that the  onset of interfacial flow increased from 100 nm with the unworn film to over 180 nm after low -speed testing  and is due to the presence of much larger molecular weight species  formed during degradationpolymerization.  

Figure 9 provides a helpful visualization of the distribution of worn lubricant products in the vicinity of the  contact. The image on the left shows the contact spot on the ball after testing, and the image on the right  shows the sliding contact band on the coupon disk. The Hertzian contact spot, the zone of intimate contact  between the ball and flat is at the center of the ring on the left -hand image. The “butterfly wing” lobes are  caused by residue in the path of oil flow due to compression of the oil film on the surfaces and distortion of  the oil meniscus caused by the dynamics of  the sliding contact. The inlet zone of the contact is to the right,  and the outlet is on the left. The reflow of oil in this pattern with each ball pass on the disk causes wear  products to be swept from the Hertzian contact zone and deposited throughout t he area covered by the  base of the meniscus on the flat and the cap of the meniscus on the ball (details below). This results in  adlayers of polymerized lubricant in the areas where oil flow is necessary for  resupply of oil to the contact.  Deposits of polymeric residue may impede flow of oil to the contact, reducing the volume of oil in the inlet  zone, resulting in reduced EHD film thickness.
293   Figure 8. Results from measurements of oil mobility after wear in the high vacuum tribometer  

Figure 9. Optical micrographs of the ball and coupon surfaces after wear in the tribometer, showing  patterns of lubricant wear product deposition  

Film Stability and Wetting Tests  

Resupply of oil to the active tribological contacts also depends on the stability and wettability of the fluid on  the substrate. These, too, can depend on composition and film thickness . Thin film stability tests  have been  done to quantitatively evaluate wetting  versus dewetting behavior by monitoring static evolution of a thi n  fluid film (h ~3  µm) deposited on a surface (e.g. 440C, 52100, SiOx). This is highly instructive for multi - component systems, such as formulated Pennzane oils, where additives can have an interfacial preference  and modify the interfacial landscape (γsl, the surface tension at the solid/liquid interface) . 

Each wetting test was conducted on two  oxygen plasma cleaned 440C steel coupons [10]. Each test fluid  was drop cast onto a coupon at room temperature. The fluids were cast from 100 µl of heptane solution   that was diluted to 2.5% by volume. The drop cast technique preserves the test fluid stoichiometry and  enhances diffusion for a uniform film that is close to its thermodynamically equilibrium state. Once applied,
294 the coupon is covered with a petri dish to allow time for stabilization. After 15 minutes, the dish is removed  to allow the solvent  to evaporate. Two photos are then acquired every 15 minutes with the coupon at room  temperature. The coupon is then placed on a hot plate at 40° C for 15 minutes and t hen photographed  again.   

Figure 10 shows time dependent results for three different lubricant component samples. The top row  shows thin film stability of a phosphate additive commonly used in aerospace mechanisms. After  15 minutes, there are numerous loc alized regions of dewetting across the surface. This fluid does not  maintain a stable fluid on clean 440C steel. The second row shows the time sequence of a thin film of  unformulated MAC oil. Dewetting is initiated at the edges of the coupon at room temper ature, and the film  rapidly retracts at 40 °C. A thin film of MAC oil,  formulated with the phosphate additive, is shown in the third  row. Here, we find that the film is stable at all temperatures that were tested. It is evident that the additives  are essent ial to maintain a stable film of oil in the laboratory environment.  

Figure 10. Photographs of steel coupons during oil wettability tests  

Oil Meniscus Evolution  

To help prolong the life of moving mechanisms, the l essons learned regarding mobility of lubricants must  be ultimately applied to models of the supply rate of oil to the ball/race interface. The ultimate objective is  to devise and test strategies for sustaining oil and promoting oil resupply at the critical ball /race interfaces.  These efforts are guided by a theoretical model of oil uptake from the surrounding surfaces.  As oil is drawn  into the meniscus via surface tension and capillary forces, it fills the volume defined by a cylindrically  symmetric annulus, with  measurable base (b), waist, and cap ( a) radii. A time interval optical microscopic  imaging system was constructed to record images of a physical  contact  while the oil is flooding the  meniscus .[8, 12]  This system enables dynamic volume measurements over long periods of time. A  screenshot  of the meniscus analysis system is shown in F igure 11. Here, the meniscus profile, meniscus
295 volume and other elements describing the meniscus geometry are extracted and measured from optical  images.    

Figure 11. Screenshot showing the  imaging window in a home- made application to measure the uptake of  oil in the meniscus between a ball and flat.  

An example of the evolution of these parameters during oil uptake in the meniscus is shown in F igure 12.  In this case, the formulated MAC oil was cast as a thin film of 2.85 µm thickness on a clean 440C steel  substrate at 25 °C. A 440C ball was then brought  into contact with the film at time t = 0, and the cap radius  (a), meniscus height (h), and meniscus volume were monitored with time using the automatic imaging  system. In this case , the meniscus growth was monitored for the first 180 hr and was only parti ally filled at  that point. Each parameter was normalized to its value at 17 0 hr, so that they can be plotted together. We  find continuous growth in volume that starts immediately upon contact, and is taken up at a rate that is  controlled by thin film flow on the substrate and the changing capillary pressure as the ball/race interface  floods.  

Figure 13 shows the meniscus volume as a function of time, using a more viscous gyroscope oil. This series  of tests was run until the volume saturated after approximately one day of uptake from a thin film. The data  are compared against the results of a theoretical model which will be described elsewhere.
296   Figure 12. Three different geometric parameters of an MAC oil meniscus are shown as the capillary  between a ball and flat is flooded with oil from a thin film on the flat  

Figure 13. Time evolution of a ball/flat oil meniscus flooded by a thin film on the flat substrate  

Effects of Composition on Oil Film Thickness  

Changes in oil composition and mobility can ultimately affect the thickness  and starvation of a protective oil  film in the rolling contacts of high- speed bearings. The results above show the effective increase in viscosity  of oil in the vicinity of a tribological contact due to polymeri zation. These results show the effects of changes  in mobility due to deposition of lubricant degradation products, and illustrates that  these products remain in  the active tribological contacts. This would alter the molecular weight distribution as the mechanism ages,  skewing it to heavier molecules and higher effective viscosity. To study the potential effects of aging on  lubricant film thickness in spacecraft mechanisms, we have conducted EHD film thickness measurements  on operating angular contact bearings using lubricants of similar chemical constituents and various  molecular weights.
298 mobility of the more viscous oil, impeding the reflow of oil from the periphery of the contact back into the  inlet zone. These results show the degree to which film thickness depends on viscosity.   

As lubricant deteriorates in an aging satellite mechanism, we may expect similar increases in film thickness.  However, the experiments shown here were performed after only 500 hours of operation. While this is  enough time for run- in of the lubricant and stabilization of the film thickness, it is not sufficiently long to  reach the more severely starved conditions of a late- life satellite mechanism. For that reason, we suspect  these film thicknesses are not representative of such conditions. Future experiments will explor e the effects  of lubricant depletion.  

Figure 15. Dependence of EHD film thickness on speed, for two lubricants with similar chemistry and  molecular architecture, but different molecular weight and viscosity. Circles are experimental  measurements;  solid lines are fits to Hamrock -Dowson model  

Conclusions  

Lubrication is an essential component of any moving mechanical assembly. In most space mechanisms,  where the lubricant cannot be easily replenished, degradation and loss of the oil ultimately leads to changes  in performance and an end to component  life. Our goals have been to develop a better understanding of  how to manage those changes in performance and to delay the end of life. The experimental techniques  and the selected results shown here demonstrate our approach to achieving these goals.   

Among the lessons learned from this study are that: 1) Spacecraft component life often depends on  management of thin films in an evolving environment, 2) Oil mobility decreases with film thickness, 3) Oil  mobili ty decreases as it degrades in a tribological contact, 3) new techniques can detect subtle changes in  lubricant composition, 4) these changes can affect wettability, mobility, and resupply of oil to critical  contacts, and 5) impeded resupply can reduce the thickness and stability of a protective film between two  surfaces.   0510152025303540 0 500 1000 1500 2000 2500 3000EHD Film Thickness ( µin) Rotational Speed (rpm)Comparison of PAO10 with PAO40 Oil PAO40 Experiment PAO10 Experiment PAO40 Theory PAO10 Theory
299 References  

1. Palmgren, A. and B. Snare, Influence of Load and Motion on the Lubrication and Wear of Rolling  Elements.  Inst. Of Mech. Eng., 1957. 79 : p. 454- 458.  2. Helt, J.M., Density, Dynamic and Kinematic Viscosity of Bearing Lubricating Oils and Additives .  2019, The Aerospace Corporation: El Segundo, CA.   3. Messaâdi, A., et al., A New Equation Relating the Viscosity Arrhenius Temperature and the  Activation Energy for Some Newtonian Classical Solvents.  Journal of Chemistry, 2015. 2015(3):  p. 12 pages.   4. Deryagin, B.V. and V.V. Karasev, The Study of the Boundary Viscosity of Organic Liquids by the  Blow -off Method.  Russian Chemical Reviews, 1988. 57(7): p. 634- 647.  5. Scarpulla, M.A., C.M. Mate, and M.D. Carter, Air shear driven flow of thin perfluoropolyether  polymer films.  The Journal of Chemical Physics, 2003. 118(7): p. 3368- 3375.   6. Berendsen, C.W.J., et al., Rupture of Thin Liquid Films Induced  by Impinging Air -Jets.  Langmuir,  2012. 28(26): p. 9977- 9985.   7. Helt, J.M., High -Vacuum Tribometry Tests of Fluid Lubricants on Bearing Steels . 2013, The  Aerospace Corporation: El Segundo, CA.   8. Helt, J.M. and P.F. Frantz, Investigating the Reflow of Gyr oscope Lubricant SRS160 on Bearing  Surfaces: Analysis Methods and Techniques . 2013, The Aerospace Corporation: El Segundo,  CA.  9. Helt, J.M., High vacuum tribometry tests of fluid lubricants on bearing steels , in ACS National  Meeting . 2014: San Francisco C A.  10. Helt, J.M., Procedure for Separating Pennzane Oil from Formulated Oils and Rheolube Based  Greases . 2016, The Aerospace Corporation: El Segundo, CA.   11. Helt, J.M. and J.J. Kirsch, High Vacuum Tribometer for Fluid Lubricated Bearing Material Testing:   Part 1 –  Instrument Design. 2010, The Aerospace Corporation: El Segundo, CA.   12. Didziulis, S., J. Helt, and P. Frantz, Laboratory Studies of Spacecraft Fluid Lubricants , in ACS  National Meeting . 2019: San Diego CA.   13. Ward, P.C., A.R. Leveille, and P.P. Frantz, Measuring the EHD Film Thickness in a Rotating Ball  Bearing, in Proceedings of the 39th Aerospace Mechanisms Symposium. 2008: NASA Marshall  Space Flight Center.   14. Helt, J.M., EHD Bearing Test Unit: Final Design . 2016, The Aerospace Corporation: El Segundo,  CA.  15. Helt, J.M., EHD Bearing Test Unit: Setup & Operation Overview . 2017, The Aerospace  Corporation: El Segundo, CA.   16. Helt, J.M., EHD Bearing Test Unit: Automated Test and Analysis Framework . 2018, The  Aerospace Corporation: El Segundo, C A.  17. Dowson, D. and G.R. Higginson, Elasto -hydrodynamic lubrication the fundamentals of roller and  gear lubrication.  1966.   18. Hamrock, B.J. and D. Dowson, Isothermal Elastohydrodynamic Lubrication of Point Contacts:  Part III —Fully Flooded Results.  Journ al of Tribology, 1977. 99(2): p. 264- 275.  19. Coy, J.J. and E.V. Zaretsky, Some Limitations in Applying Classical EHD Film Thickness  Formulas to a High- Speed Bearing.  Journal of Tribology, 1981. 103(2): p. 295- 301.
303 promotes this life due to the lack of oxide formation on the steel and degradation of the lead  naphthenate.   - In general, an increase in lead content will decrease the wear rate.  

Figure 1: Mixed Film Oscillatory Wear Results from AMS 2018  

Scope of Work  

The following work was covered in this research.   

• Fluid Lubricant Evaluation: The performance of five multi- alkylated cyclopentane (MAC) fluid   lubricants were performed included the heritage material Synthetic Oil 2001A, two lead  naphthenate versions (2001- 3PB and 2001- 5PB),  and comparable versions that utilize triphenyl  phosphate (2001B) and tricresyl phosphate (2001T) .  • Grease Lubricant Evaluation: The grease counterparts to the materials in the fluid evaluation  (Rheolube 2000F , 2004, 2000- 5PB, 2000B, and 2000T ) were tested under a vacuum bearing  configur ation.  

Testing Apparatus  

Three 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 other two methods   utilize  a mixed rolling/sliding contact in atmospheric conditions for both mixed film and boundary lubrication.  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.  

Spiral Orbit Tribometer   The Spiral Orbit Tribometer (SOT) is a single contact simulation that replicates Angular Contact Ball  Bearings in a semi -starved lubrication mode while under vacuum. It is simply a ball held between two  parallel  plates which essentially makes it a thrust bearing. The SOT utilizes two flat, concentric discs  between which a ½” lubricated ball rolls and pivots during test operation.  A very small amount of lubricant,  approximately 50μg, is applied to the ball to ensure the system operates in the boundary lubrication regime,  providing a fast and efficient screening method of lubricant performance.  The upper disc applies a load  which can be varied depending on the desired Hertzian Contact Stress (material, load, and bal l diameter  dependent).        During test operation, the lack of a ball retainer allows the ball to orbit in an opening spiral pattern whose  orbit radius continuously increases throughout the course of one revolution of the bottom sample disc.  Since
304 there is no retainer, if this orbit was left unchecked, it would result in the ball orbiting out of the two parallel  sample discs, causing the two plates to contact each other.  Fortunately, as the ball reaches the completion  of each revolution, it is pushed back into its original orbit radius by contacting what is known as the guide  plate.  The test is initiated with the sample ball touching the guide plate, which sets the initial orbit radius,  and is adjustable.  The guide plate is oriented in a position such that it  contacts the center of the ball on  each revolution. Attached to the guide plate is a charge amplifier force transducer.  This force transducer  measures the reactionary force required for the ball to be scrubbed back into its initial orbit diameter. This  location throughout the orbit is identified as the scrub region. As the lubricant becomes consumed or  degrades, this force will naturally increase.     

Figure 2. Spiral Orbit Tribometer mechanisms, Image courtesy of Spiralab  

Vacuum Bearing Testing :  The apparatus used for these experiments tests a single angular contact ball bearing. The system operates  at a vacuum level better than 5.0 x 10- 7 Pa, from 1 to 500 R PM, up to 200°C, and loads to 450 N. The  system uses dead weight loading and either a heat lamp or band heater. The system also measures cross  bearing electrical resistance, which is used to monitor the operating regime. Bearing torque, load, chamber  press ure, and cross bearing resistance are recorded using a data acquisition system. The system uses a  single SKF 7204 1219 ( 52100 steel) angular contact bearing. The bearing has an outside diameter of 47  mm, a bore of 20 mm, eleven 12.7 mm balls and a steel  retainer. The bearing is mounted in a fixture that  holds the outer race and rotates the inner race. Temperature information is gathered from a thermocouple  mounted just below the inner race.  

Wear Testing:   Three different wear rates were calculated including a mixed film in pure sliding, mixed film (rolling/sliding),  and boundary (rolling/sliding). A normalized wear rate for all these tests were calculated to compare  materials tested. The wear rate will indicate the volume of wear (µm3) over a distance traveled (in  millimeters) which will normalize the data in the case of premature failures. For the pure sliding test, the  ASTM  D-5707 SRV Coefficient of Friction test was used. For both the mixed film and boundary testing in a  rolling/sliding contact, a custom experimental method using a Mini Traction Machine (MTM)  (as show in  Figure 3) was used [1][2][4][7][ 13]. In the MTM, the ball and disc are driven independently which allows any  combination of rolling and sliding. The measurement of friction force is done through a load cell that is  attached to the bearing housing of the ball motor shaft.
306 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.  

Procedures  

Sample Preparation   All test specimens are ultrasonically cleaned in heptane followed by acetone. For the SOT testing, the fluid  lubricants were plated via the preparation of a solution of lubricant diluted into an appropriate solvent. This  solution was then applied directly to a rotating ball and the solvent allowed to evaporate from the surface.  This left the fluid lubricant on the ball’s surface. The lubricant plating method allows for the application of  very smal l amounts of lubricant. The application of grease was done by rubbing the ball between cleanroom  grade polyethelye sheets until and even coating of lubricant was applied. The target amount of applied  lubricant for both fluids and greases were 50 μg. The contact area for the SRV was coated with 5 ml of  lubricant  while the MTM specimens were fully flooded.  

Results and Discussion  

SRV (Sliding in Atmospheric and Vacuum Conditions)   The results in  Tables 1- 3 and Figure 4 are from the experiment on  the MAC  with 0%, 3%, and 5% lead  naphthenate when tested in a boundary lubricating  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 1.302 599,972 0.28 0% Lead 20 Vacuum 440C 1.294 518,400 0.24 0% Lead 20 Atmosphere 52100 1.499 481,018 0.22 0% Lead 20 Vacuum 52100 1.420 410,400 0.19 Material Temp ( °C) Environment Specimen Avg Wear Scar (mm2)Disc Wear Volume (μm3)Wear Rate (μm3/mm) 3% Lead 20 Atmosphere 440C 2.409 410,400 0.19 3% Lead 20 Vacuum 440C 1.942 216,000 0.10 3% Lead 20 Atmosphere 52100 1.950 342,356 0.16 3% Lead 20 Vacuum 52100 1.820 259,200 0.12 Material Temp ( °C)Environment Specimen Avg Wear Scar (mm2)Disc Wear Volume (μm3)Wear Rate (μm3/mm) 5% Lead 20 Atmosphere 440C 1.468 291,600 0.14 5% Lead 20 Vacuum 440C 1.259 172,800 0.08 5% Lead 20 Atmosphere 52100 1.389 259,200 0.12 5% Lead 20 Vacuum 52100 1.242 216,000 0.10
307   Figure 4: Boundary  Lubrication Wear Rate at 20°C  

In atmospheric conditions, the wear rates for 52100 steel was consistently lower than the 440C. Under  vacuum conditions , all of the samples had lower wear rates than the atmospheric tests, the samples with  lead had almost half the wear than neat oils, and the amount of lead made a small difference in the wear.   When  looking at the average wear scar there is no correlation to the disc wear volume of the wear rate.  Until recently, most published papers used the average wear scar  to compare efficacy of wear additives  and performance in testing. Using this measurement simply gives you a dimension of the worn area with  no indication of how much material was removed. The  results from this study as well as those presented  by St. Pierre  [9] have illustrated that  two-dimensional  wear measurements cannot be relied on to  understand what is going on in a mechanism or trib ological contact. By using 3D profilometry, a deeper  understanding of what is going on can be attained. The results in Tables 4- 6 and Figure 5  are for the multiply  alkylated cyclopentane with 0%, 3%, and 5% lead naphthenate when tested in a boundary  lubric ating  regime, at 75°C on the SRV under both atmospheric and vacuum conditions.  

Table 4: Wear Performance for MAC fluid with 0% Lead at 75°C  

Table 5: Wear Performance for MAC fluid with 3% Lead at 75°C  

Table 6: Wear Performance for MAC fluid with 5% Lead at 75°C  

Material Temp ( °C)Environment Specimen Avg Wear Scar (mm2) Disc Wear Volume (μm3)Wear Rate (μm3/mm) 0% Lead 75 Atmosphere 440C 1.437 1,080,000 0.50 0% Lead 75 Vacuum 440C 1.786 972,000 0.45 0% Lead 75 Atmosphere 52100 1.513 263,796 0.12 0% Lead 75 Vacuum 52100 1.826 276,480 0.13 Material Temp ( °C)Environment Specimen Avg Wear Scar (mm2) Disc Wear Volume (μm3)Wear Rate (μm3/mm) 3% Lead 75 Atmosphere 440C 1.449 972,000 0.45 3% Lead 75 Vacuum 440C 1.824 691,200 0.32 3% Lead 75 Atmosphere 52100 1.346 622,811 0.29 3% Lead 75 Vacuum 52100 1.236 259,200 0.12 Material Temp ( °C)Environment Specimen Avg Wear Scar (mm2) Disc Wear Volume (μm3) Wear Rate (μm3/mm) 5% Lead 75 Atmosphere 440C 3.003 756000 0.35 5% Lead 75 Vacuum 440C 1.874 388800 0.18 5% Lead 75 Atmosphere 52100 3.335 612415 0.28 5% Lead 75 Vacuum 52100 1.842 162000 0.08