ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
7. Singh, H., et al. "An investigation of material and tribological properties of Sb 2O3/Au-doped MoS 2 solid lubricant films under sliding and rolling contact in different environments." Surface and Coatings Technology 284  (2015): 281-289. 8. Stupp, Bernard C. "Synergistic effects of metals co-sputtered with MoS 2." Thin solid films  84.3 (1981): 257-266. 9. Singh, H., et al. "Tribological performance and coating characteristics of sputter-Deposited Ti-Doped MoS 2 in rolling and sliding contact." Tribology Transactions  58.5 (2015): 767-777. 10. Singh, Harpal, et al. "An atom probe tomography investigation of Ti–MoS 2 and MoS 2–Sb 2 O3–Au films." Journal of Materials Research  32.9 (2017): 1710-1717. 11. Pope, L. E., and J. K. G. Panitz. "The effects of hertzian stress and test atmosphere on the friction coefficients of MoS 2 coatings." Surface and Coatings Technology  36.1-2 (1988): 341-350. 12. Mutyala, Kalyan C., et al. "Deposition, characterization, and performance of tribological coatings on spherical rolling elements." Surface and Coatings Technology  284 (2015): 302-309. 13. Stoyanov, Pantcho, et al. "Microtribological performance of Au–MoS 2 and Ti–MoS 2 coatings with varying contact pressure." Tribology letters  40.1 (2010): 199-211. 14. Windom, Bret C., W. G. Sawyer, and David W. Hahn. "A Raman spectroscopic study of MoS 2 and MoO 3: applications to tribological systems." Tribology Letters  42.3 (2011): 301-310. 15. Berthier, Y., M. Godet, and M. Brendle. "Velocity accommodation in friction." Tribology Transactions 32.4 (1989): 490-496. 16. Godet, Maurice. "Third-bodies in tribology." Wear  136.1 (1990): 29-45. NASA/CP—2018-219887 149
subsequent vacuum running has been shown to be related to the in-air dwell period and the extent of  moist air running (Figure 2) [6], and is more severe if the sputtered MoS 2 is exposed to in-air heating [7].  Figure 1. Low-torque life of MoS 2 lubricated ED20  bearings in vacuum as a function of in-air operation  prior to vacuum testing [4] Figure 2. PoD revs required to achieve low friction  performance of sputtered MoS 2 under vacuum  following moist air dwell period [6]  This deleterious effect on subsequent lifetime under vacuum is of concern for space mechanisms  applications, where demonstration of a successful deployment on ground is often demanded as part of  the qualification program. Whilst the use of a protective dry nitrogen environment can theoretically be  employed to protect the sputtered MoS 2 lubricated components during ‘on ground’ operation, this may  have practical limitations especially at spacecraft level.  Numerous attempts have been made to improve the lubricating performance of sputtered MoS 2 films in  moist air, including doped variants. However, this paper concentrates only on improving the performance  of the existing film.  Fluid Lubrication – PFPE oils  Perfluoropolyether (PFPE) type fluids are well suited for applications in space due to their low vapor  pressures, low pour points, resistance to radiation and atomic oxygen, good tribological properties, and  being highly chemically inert [8]. Z-type PFPEs (such as Fomblin Z25, Brayco 815Z, and greases based  upon these oils such as Braycote 601EF) are constructed from linear polymer chains and have been  employed extensively as lubricants in spacecraft mechanisms for many decades [9]. However, under  boundary conditions such lubricants are susceptible to chemical degradation, resulting in increased  friction coefficients, material wear, and eventually component failure [10, 11]. This tribo-chemical  degradation occurs primarily through the reaction between the polymer chains and chemically active sites  in the substrate steel [12]. Nevertheless Z-type PFPE fluids are commonly used in spacecraft  mechanisms.  PFPE fluid lubricants also offer the advantage that their tribological performance in vacuum is not  compromised by prior operation in moist air, as displayed by sputtered MoS 2. In fact, lifetimes of PFPE  fluids (and multiply alkylated cyclopentanes) are shown to be extended in moist air in comparison to  vacuum [13, 14].  Hybrid Lubrication  The suggestion is occasionally made within the space mechanisms community that a form of hybrid  lubrication may circumvent the restriction on in-air operation of MoS 2, through the application of a  controlled quantity of PFPE fluid lubricant to a component lubricated in the conventional way with  sputtered MoS 2, thus ‘protecting’ the MoS 2 from the moist environment. The suggestion states that the  fluid lubricant will provide low friction during operation in moist air and will subsequently be lost (either  through evaporation or tribo-chemical degradation depending upon the fluid) under vacuum, allowing the  NASA/CP—2018-219887 152
(hopefully) uncompromised MoS 2 film to provide low friction and long life for the remainder of operation  under vacuum.  The potential advantage of synergistic behavior between the fluid and solid lubricating constituents also  exists, where performance of the whole lubrication solution is greater than the sum of its parts. Essentially  this occurs where the presence of one lubricant constituent prolongs the operational performance of the  other, and vice-versa. This behavior may occur on a physical and/or chemical level.  This paper details the recent testing campaigns at ESTL to understand and characterize the potential of  hybrid lubrication of PFPE fluids and sputtered MoS 2 with respect to the advantages stated above,  performed at both tribometer and component level. These testing campaigns shall be discussed  individually.  Spiral Orbit Tribometer Experimental Campaign  SOT Phase One – Feasibility Study  A feasibility demonstration of hybrid lubrication was performed using a Spiral Orbit Tribometer (SOT). The  SOT is essentially a rolling tribometer, where a solid or low volume of fluid lubricant can be assessed  under representative conditions of an angular contact bearing operating within the boundary regime.  Details of the SOT are described elsewhere [15].  SOT test balls of 52100 steel were first lubricated with sputtered MoS 2. These same test balls were then  lubricated with ~50 µg of PFPE oil Fomblin Z25 via solvent plating, providing hybrid Z25/MoS 2 lubrication.  Hybrid lubricated balls were inserted into the SOT test chamber and rotated in moist air for a defined  duration and subsequently under high vacuum to failure. Test conditions were 2.25-GPa peak contact  stress, 100-RPM rotation speed, and 23°C. Failure of the hybrid lubricant was defined as an increase in  friction coefficient to ≥ 0.28.  Initial feasibility results demonstrated encouraging behavior, with indications that a small volume of fluid  lubricant acts to protect the sputtered MoS 2 from elevated wear rates in moist air, and some degree of  extension in life is observed under vacuum. In addition, the friction coefficient of the hybrid lubrication was  found as 0.02 (Figure 3), identical to the value for MoS 2 alone in vacuum [16]. Given that Z25 provides a  friction coefficient of 0.1 under vacuum on the SOT, we can say with some confidence that the MoS 2 was  providing lubrication at this time. A more detailed account of this initial feasibility study is provided  elsewhere [17].  Figure 3. Hybrid Z25/MoS 2 rolling in air (blue) and vacuum (red). Elevated frictional noise during vacuum  running is an artifact introduced by the analysis software and is not real   NASA/CP—2018-219887 153
Given the encouraging performance of the hybrid Z25/MoS 2 lubrication, additional SOT tests were  performed to further investigate this behavior.  SOT Phase Two – Detailed SOT Study  Phase Two of SOT testing was performed in an equivalent manner to Phase One, with the following  alterations.  •Tests performed at three contact stresses (3.00 GPa, 2.25 GPa & 1.50 GPa peak). •Tests performed with varying durations of in-air running prior to vacuum. •In-air running durations defined as percentages of in-vacuum MoS 2 life. Prior to hybrid lubrication testing, the lifetimes of MoS 2 under vacuum at the above contact stress was  assessed. From these lifetimes the in-air durations required for the Phase Two hybrid testing can be  calculated. The required in-air running durations are given in Table 1, with L x being the in-vacuum MoS 2 life at a given contact stress S X.  Table 1. Required in-air running for SOT Phase One  Peak stress S 1 (3.00 GPa) S 2 (2.25 GPa) S 3 (3.00 GPa)  Required in-air  operation 0.0005 L 1 0.0005 L 2 0.0005 L 3 0.005 L 1 0.005 L 2 0.005 L 3 0.05 L 1 0.05 L 2 0.05 L 3 0.5 L 1 0.5 L 2 0.5 L 3 Results of Phase Two SOT testing on hybrid lubrication are presented in Table 2.  Except for S1-D, all tests passed the in-air duration of running without displaying evidence of lubricant  failure, with S1-D displaying failure after operating for 98% of the required orbits. These results allow us  to state that the application of ~50 μg of Fomblin Z25 oil can allow balls lubricated with sputtered MoS 2 to  run in-air for 50% of their in-vacuum lifetime without displaying evidence of failure.  Following in-air testing each sample was run until failure under vacuum conditions. Vacuum lifetimes  show that in almost all cases vacuum lifetimes were extended in comparison to sputtered MoS 2 alone.   Table 2. Moist-air and subsequent normalized vacuum lifetimes of hybrid Z25/MoS 2 lubricated SOT tests  ID Peak Stress  (GPa) Required inair operation In-air  Pass/Fail Subsequent vacuum life  / MoS 2-only life Total life / predicted  vacuum life  S1-A 3.00 0.0005 L 1 Pass 2.199 L 1 2.176  S1-B 3.00 0.005 L 1 Pass 1.501 L 1 1.484  S1-C 3.00 0.05 L 1 Pass 2.106 L 1 2.130  S1-D 3.00 0.5 L 1 Fail (98%) 0.000 L 1 0.486  S2-A 2.25 0.0005 L 2 Pass 1.521 L 2 1.463  S2-B 2.25 0.005 L 2 Pass 3.644 L 2 3.550  S2-C 2.25 0.05 L 2 Pass 2.456 L 2 2.399  S2-D 2.25 0.5 L 2 Pass 0.002 L 2 0.482  S3-A 1.50 0.0005 L 3 Pass 1.210 L 3 1.103  S3-B 1.50 0.005 L 3 Pass 4.686 L 3 4.291  S3-C 1.50 0.05 L 3 Pass 2.976 L 3 2.833  S3-D 1.50 0.5 L 3 Pass 2.392 L 3 2.648  NASA/CP—2018-219887 154
Figure 4. Measured lifetimes of hybrid Z25/MoS 2 lubrication as factor of predicted life  Using the predictions of fluid lifetimes taken from [16], we can calculate the individual contributions from  the fluid and solid components of these tests under vacuum (assuming no prior in-air running). Such  calculations demonstrate that for all Phase Two SOT tests (except for S1-D and S2-D), the total hybrid  lubrication lifetime is longer than that of the individual lubricant constituents (Figure 4). That is to say  there is a synergistic lubrication effect.  Life of PFPE/MoS 2 lubrication > Life of PFPE lubrication + Life of MoS 2 lubrication  In addition, the steady state friction coefficient in during the vacuum stage of testing was significantly  below 0.1 in all cases. This suggests that the MoS 2 was providing the lubrication during the in-vacuo  stage of all hybrid tests throughout the extended life.  Post-test inspections of the test pieces showed mixed regions of MoS 2 debris captured within the  degraded PFPE oil, displaced to the edges of the ball tracks. No dusting of loose MoS 2 debris was  observed, in contrast to the typical post-test observations of MoS 2 alone on the SOT.  Pin-on-Disc Tribometer Experimental Campaign  PoD Phase One – High volume fluid lubrication  To assess the potential for hybrid lubrication in a pure sliding environment a series of Pin-on-Disc (PoD)  tests were performed under the following test conditions.  •Discs lubricated with Braycote 601EF grease only. •Discs lubricated with sputtered MoS 2 only. •Disc lubricated with both sputtered MoS 2 and Braycote 601EF. Where possible the same test disc was used as for the MoS 2-only test. Grease lubricant was applied following standard ESTL procedure recommending 10 mg/cm2 to a disc of  surface area 4.4 cm2, amounting to 44 mg onto the surface of each test disc. The grease was applied by  syringe and then distributed using a ISO class-5 wipe to the necessary volume of grease, measured  using a microbalance.  All tests were performed under vacuum, 0.6 ms-1 sliding speed, on standard 52100 steel PoD test pieces.  Contact stress and temperature was varied as detailed below. Failure of the lubricant was defined as an  increase in friction coefficient to ≥ 0.3. Friction and lifetime results are provided in Table 3.  NASA/CP—2018-219887 155
Table 3. Tribological behaviors of PoD test campaign of hybrid Braycote 601EF/MoS 2 lubrication.  Lifetimes are normalized to behavior of MoS 2-only, RT, 1500 MPa  Test ID Lubricant Peak stress  (MPa) Temp Normalized  Lifetime Friction  coefficient PoD.A1 Braycote 601EF 850 23 0.001 0.20  PoD.B1 MoS 2 850 23 8.591 0.03  PoD.C1 Braycote 601EF/MoS 2 850 23 0.253 0.06  PoD.A2 Braycote 601EF 1500 23 0.005 0.15  PoD.B2 MoS 2 1500 23 1.000 0.02  PoD.C2 Braycote 601EF/MoS 2 1500 23 0.272 0.04  PoD.A3 Braycote 601EF 2000 23 0.000 0.17  PoD.B3 MoS 2 2000 23 0.060 0.06  PoD.C3 Braycote 601EF/MoS 2 2000 23 0.070 0.04  PoD.A4 Braycote 601EF 850 120 0.004 0.26  PoD.B4 MoS 2 850 120 1.867 0.10  PoD.C4 Braycote 601EF/MoS 2 850 120 0.413 0.15  PoD.A5 Braycote 601EF 1500 120 0.012 0.26  PoD.B5 MoS 2 1500 120 0.194 0.06  PoD.C5 Braycote 601EF/MoS 2 1500 120 0.008 0.16  PoD.A6 Braycote 601EF 2000 120 0.014 0.26  PoD.B6 MoS 2 2000 120 0.007 0.06  PoD.C6 Braycote 601EF/MoS 2 2000 120 0.039 0.04  Considering hybrid lubrication, the Braycote 601EF/MoS 2 tests are disappointing in comparison to the  SOT testing campaign, with the following conclusions drawn (demonstrated in Figure 5).  •The addition of MoS 2 to a grease lubricated surfaces will increase the lifetime significantly. •The addition of grease to MoS 2 lubricated surfaces will not increase the lifetime significantly. . Figure 5. Hybrid Braycote 601EF/MoS 2 lifetimes as a factor of Braycote 601EF-only (left) and sputtered  MoS 2 only (right) lifetimes  In addition, the friction coefficient of the Braycote 601EF/MoS 2 tests is, in most cases, elevated in  comparison to MoS 2. Other observations are made from the test data above.  •The test results demonstrate that under vacuum conditions the lifetime of MoS 2 decreases with increasing temperature and contact stress. This is in line with previous data and expectations. NASA/CP—2018-219887 156
Table 5. Performance of varying levels of hybrid Z25/MoS 2 lubrication under vacuum on PoD. Tests  performed at 900 MPa, 23°C. Lifetimes normalized to MoS 2-only life at room temperature  Lubricant Life / MoS 2 life (RT) Friction coefficient  MoS 2 only 1.000 0.03  MoS 2 with solvent bath 0.998 0.03  MoS 2 with Z25 (~40 µ g) 1.271 0.03  MoS 2 with Z25 (~400 µ g) 1.848 0.03  Results demonstrate no tribological influence from the solvent bath, demonstrating that a PF5060 solvent  immersion does not influence the lifetime of the sputtered MoS 2 film in a measurable manner.  Hybrid lubricated tests show an increase in sliding lifetime in comparison to MoS 2, with the higher mass of  the applied fluid lubricant increasing the success of the hybrid lubrication (Figure 6). In addition, the  friction coefficient was not compromised by the addition of the oil onto the sputtered MoS 2.   Figure 6. Normalized performance of Z25/MoS 2 lubrication under vacuum  The above tests were then repeated under vacuum at elevated temperature (200°C), displaying greater  sliding lifetimes than those achieved at room temperature, again with no increase in friction coefficient  (see Table 6). This is a surprising observation given that the degradation lifetimes of both the sputtered  MoS 2 films and Fomblin Z25 oil are known to dependent upon operating temperature.  Table 6. Comparison of hybrid Z25/MoS 2 lubrication at room temperature and 200°C under vacuum  Lubricant Temp (°C) Life / MoS 2-only life (RT) Friction coefficient  MoS 2 with Z25 (~400 µg) 23 1.848 0.03  MoS 2 with Z25 (~400 µg) 200 2.261 0.03  Using data from the Phase One PoD testing (Table 3) we observe that the sliding lifetime of sputtered  MoS 2 films under vacuum at 120°C is reduced to ~10 – 20% of the lifetime achieved at room temperature.  Although the nature of this reduction in life is not fully known (i.e. to what level the increased reaction  between oxygen/moisture with the MoS 2 film, and the softening of the substrate steel factor into this  reduction), and the dataset is not sufficient enough to produce a confident life vs. temperature  relationship, it is clear that the sliding life of MoS 2 at 200°C is predicted to be <10 – 20% of the room  temperature lifetime for a given contact stress.  A similar relationship exists for Fomblin Z25, where the rolling lifetime of the fluid lubricant at 100°C is  reduced to ~30 – 50% of the lifetime achieved at room temperature on the SOT, due to an increase in the  NASA/CP—2018-219887 158
Figure 7. Conventional gear test setup for ESTL’s miniature gear/pinion testing rig  Tests were performed using ESTL’s miniature gear/pinion testing rig, utilizing the 4-square principle with  compliant gear preloading (Figure 7). The test gears are supported on both sides by rolling element  bearings to avoid any misalignment/stiffness issues. The compliant preload system maintains axial  alignment between the gear shafts by supporting the shafts with plain bushings and locating the two  shafts with a recess and boss. The torque was assessed via a transducer.  Figure 8. Mean torque behavior of spur gear tests (parasitic torques removed)  Results from the gear testing was unfortunately inconclusive. Whilst true that the hybrid lubricated test  lasted longer than the sum of the lifetimes for the solid and fluid lubricated gears individually, this lifetime  extension is extremely minimal and likely falls within the margin of error. The elevated torque value is also  disappointing, suggesting that for the hybrid lubricated gears the lubrication was predominantly being  provided by the fluid, and not the MoS 2.  Table 7. Summary of spur gear test behavior  Lubricant Lifetime (revs) Steady-state  torque (mNm) End-of-life  torque (mNm)  Braycote 601EF 545,545 33.4 41.8  MoS 2 12,525 15.2 19.0  Braycote 601EF / MoS 2 567,490 42.3 52.9  NASA/CP—2018-219887 160
a 0.5-µm MoS 2 film, assuming fluid distribution over the ball only. These test cases demonstrate an  extension in life provided by the hybrid lubrication, where the fluid film thickness is 5-50% of the originally  deposited MoS 2 film.  In instances where a significantly larger mass of fluid lubricant is applied to a sputtered MoS 2 film, typical  of the mass applied to a grease-only lubricated component (e.g. Phase One PoD tests and spur gear  tests), the fluid film thickness is closer to 50 µm, and the positive effect of hybrid lubrication is essentially  lost. This demonstrates that a threshold may exist above which the application of additional fluid lubricant  onto the surface of the sputtered MoS 2 film is detrimental to the performance, rather than an  improvement.   Figures 9 and 10 suggests a mechanism by which this process may occurs, whereby a thicker fluid film is  proposed to dislodge the lightly adhered MoS 2 transfer film from the contact via viscous drag effects  (assuming Model 3). This threshold for fluid lubricant volume would appear to exist at a significantly lower  volume than the amount prescribed by the standard grease lubrication procedure.  Figure 9. MoS 2 transfer film formation (3rd body) of high fluid volume hybrid lubrication  Figure 10. MoS 2 transfer film formation (3rd body) of low fluid volume hybrid lubrication  The improved performance of hybrid lubrication at elevated temperatures suggests that the viscosity of  fluid also is a factor in determining the success of hybrid lubrication, with the reduced viscosity of the  Fomblin Z25 at 200°C potentially allowing for greater re-flow of the lubricant within the contact zones,  producing longer life. It is known that the physical lifetime of Fomblin Z25 is longer than the more viscous  grease Braycote 601EF when assessed on a PoD tribometer, despite their tribo-chemical degradation  lives behaviors identical [16], due to reflow effects. Given the above it is likely that a relationship exists  whereby the success of hybrid lubrication is governed by the lubricant film thickness, and the physical  properties (i.e. viscosity) of the fluid itself (as well, potentially, as the surface roughness of the substrate).   The true mechanism of hybrid lubrication of PFPE/MoS 2 is likely to be a combination of these above  models.  NASA/CP—2018-219887 162
Non-PFPE fluids in Hybrid Lubrication  The above hypothesis states that the success of hybrid lubrication depends primarily upon the ability of  the fluid constituent to retain the 3rd-body MoS 2 transfer film within the contact zone, a phenomenon  related to related to film thickness (or ratio of film thickness to MoS 2 thickness) and viscosity of the fluid. If  this is the case, the tribological properties of the fluid itself may be second order effects compared to the  physical properties of the fluid (vapor pressure, viscosity), potentially allowing for the selection of more  favorable fluids to achieve hybrid lubrication.  For instance, the lower viscosity offered by the MAC fluid Nye 2001a may prove advantageous in a hybrid  regime. However, this fluid is characterized by a relatively higher vapor pressure, and so potentially is not  so attractive for elevated temperature operations. In such cases the ultra-low vapor pressure fluid  Fomblin Z60 may be a viable candidate, despite its poorer tribo-chemical lifetime [16].   Table 8. Vapor pressures and viscosities for potential candidate fluids for hybrid lubrication  Fomblin Z25 Nye 2001a Fomblin Z60  Vapor pressure (mbar)  20°C  38°C  100°C 2.13 x 10-13 4.13 x 10-12 3.73 x 10-091.53 x 10-11 1.34 x 10-10 4.74 x 10-081.47 x 10-16 7.33 x 10-15 5.47 x 10-11 Kinematic viscosity (cSt)  20°C  40°C  100°C 263  157  49 297  108  14.6 600  355  98  Viscosity index 358 137 360  An SOT testing campaign to explore the potential of other fluids in a hybrid lubrication regime is planned  to take place at ESTL in 2018.  Conclusions  The work presented here (and elsewhere by ESTL) demonstrates the potential for hybrid lubrication, but  also the limitations. Under the right conditions the lifetime of a sputtered MoS 2 film can be significantly  extended under vacuum and protected from a reasonable degree of moist-air operation, with no  consequential increase in friction coefficient. In other cases, however, attempts at hybrid lubrication have  resulted only in the loss of the good tribological properties of the sputtered MoS 2, resulting in shorter  operational lifetimes.  These results would suggest that the physical properties of the applied fluid lubricant are potentially more  important to the success of hybrid lubrication than their tribo-chemical properties (which more dictate their  performance alone). For successful hybrid lubrication a layer of fluid lubricant of sufficient volume is  required to protect the MoS 2, but this layer must be sufficiently thin, mobile and/or fluid to ensure it does  not disrupt the formation of the MoS 2 transfer film or provide a fully fluid lubrication regime over the MoS 2 (i.e. the friction/torque retains the characteristic of solid lubrication).   Component and tribometer level testing activities are continuing at ESTL to verify this hypothesis, and to  demonstrate the potential of hybrid lubrication at angular contact bearing level.  NASA/CP—2018-219887 163