ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
228   Figure 6.  Enduran ce of Hohman Plating Sb 2O3-MoS 2 coatings as a function of days  stored in 59%  RH air . (See Figure 4 caption.) The endurance has reduced by about 27% for a storage time   of 1.1 years.  

Solid lubricant coatings typically exhibit increased friction at the beginning of testing due to the ini tial coating  surface chemical state, morphology, and crystallinity. After steady -state conditions have been reached (i.e.,  post run- in), the friction is generally at a minimum.  Early -life friction was meas ured during separate (higher  data rate) scans for the Ni -MoS 2 and Au- Sb2O3-MoS 2 coatings stored in N 2 and for storage in air for 2.3  years  (these were only run for a few hundred cycl es, i.e., not to failure). R esults are shown in Table 1. Also  shown are the stea dy-state values obtained from the original endurance runs.  

The early -life friction for the Au -Sb2O3-MoS 2 coating (without air storage) is slightly lower than that for the  Ni-MoS 2 coating. After storage for 2.3 years , the early -life friction increases slightly for both types of  coatings. For the steady -state values, the friction for the Au- Sb2O3-MoS 2 coating (without air storage) is  lower than that for the Ni -MoS 2 coating. After storage, the steady -state fric tion does not change significantly  for either coating.  

Table 1.  Friction Values Obtained from Sputter -Deposited MoS 2 Coatings Before and After  Storage in 59% RH Air for 2.3 years (tested in dry N 2)  Sample  Maximum COF measured   at Start of Test  Steady State/   Minimum COF   Stored in N 2 Stored in Air  Stored in N 2 Stored in Air   Ni-MoS 2 0.22 ±.01  0.25 ±.005  .008 ±.002  .007 ±.002   Au-Sb2O3-MoS 2 0.18 ±.01  0.22 ±.01  .005 ±.002  .003 ±.001      Surface Chemical Results   In order to help understand the chemical changes occurring in the coatings that gave rise to the changes in friction and endurance, X -ray Photoelectron Spectroscopy  (XPS) was used to study unworn areas of the  coatings. Our main interest was in the oxidation state of Mo since the most significant result of storage in  humid air is to oxidize the lubri cating MoS 2 to nonlubricating MoO 3, as discussed above.  

Figure 7  shows XPS spectra in the Mo 3d region of the Ni -MoS 2 coating that had been stored in dry N 2,  along with one stored for 2.3 years  in air with 59% RH. In the spectra, the Mo 3d doublet for MoS 2 overlaps
229 with that for MoO 3, giving a struc ture that appears to be  three peaks . This necessitated using linear least  squares deconvolution to determine the molecular ratios of the two species . The ratio of the MoO 3 to MoS 2  spectrum indicates the amount of oxidation that has occurred through the detection depth. The two spectra  in Figure 7 indicate that  some of the MoS 2 has been oxidized to MoO 3 at the surface of the coating stored  in dry N 2. However, significantly more of the MoS 2 has been oxidized to MoO 3 for the coating stored in  humid air.  

Figure 7.  XPS spectra in the Mo 3d region for Hohman Plating Ni -MoS 2 coatings stored in dry N 2  and stored in 59% RH air for 2.3 years. In the spectra, the Mo 3d doublet for MoS 2 overlaps with  that for MoO 3. The spectra are normalized so that they show equal contrib utions of MoO 3. The  MoO 3:MoS 2 ratio is significantly higher for the coating stored in humid air.    Figure 8 shows corresponding spectra for the Au- Sb 2O3-MoS 2 coating. The two spectra indicate that  some of the MoS 2 has been oxidized to MoO 3 at the surface of the coating stored in dry N 2, although  somewhat less than for the Ni -MoS 2 coating. In addition, although some of the MoS 2 in the Au- Sb2O3MoS 2 coating has been further oxidized to MoO 3 after storing in humid air, the change is considerably  less pro nounced that for the Ni -MoS 2 coating.
230   Figure 8.  XPS spectra in the Mo 3d region for Hohman Plating Au -Sb2O3-MoS 2 coatings stored in  dry N 2 and stored in 59% RH air for 2.3 years. In the spectra, the Mo 3d doublet for MoS 2 overlaps  with that for MoO 3. The spectra are normaliz ed so that they show equal contributions of MoO 3. The  MoO 3:MoS 2 ratio is only slightly higher for the coating stored in humid air.  

Table 2 gives the calculated ratios of MoO 3 to MoS 2 for the spectra shown in Figures 6 and 7. For the Ni - MoS 2 coating stored in N 2, there is slightly more oxidized material present in the surface region than nonoxidized material, as shown by a MoO 3:MoS 2 ratio of 1.15. H owever, after storing in humid air for 2.3 years ,  the ratio has almost doubled to 2.16, indicating considerable oxidation has occurred during humid air storage.  

Table 2.  MoO 3:MoS 2 Ratios Obtained from XPS Spectra of Sputter -Deposited MoS 2 Coatings   Sample  Storage Condition  MoO 3:MoS 2  Ni-MoS 2 Stored in N 2 1.15  Stored in 59% RH Air for 2.3 years  2.16  Au-Sb2O3-MoS 2 Stored in N 2 1.02  Stored in 59% RH Air for 2.3 years  1.14   For the Au- Sb 2O3-MoS 2 coating stor ed in N 2, Table 2 shows that about the same amount of oxidized  material is present in the surface region as non- oxidized material, as shown by a MoO 3:MoS 2 ratio of 1.02;  this represents slightly less surface oxidation than for th e Ni-MoS 2 coating stored in N 2. For the Au- Sb2O3MoS 2 coating stored in 59% RH air for 2.3 years , the value of 1.14 indicates that only about 10% greater  oxidation has occurred during humid air storage.  Storage in air clearly has less of an effect on the AuSb2O3-MoS 2 coating than the Ni -MoS 2 coating.    To investigate oxidation throughout the bulk of the coatings, Ar + ion sputter depth profiles were obtained.  Figure 9a shows the early stages of the depth pro file of the Ni -MoS 2 coating stored in N 2. At longer  sputtering times, about 25 at% O is seen in the coating, indicating that significant oxygen was pr esent in  the coating as deposited. This is commonly due to residual water vapor in the chamber during the coating  process, which results in the formation of a MoS xO2-x solid solution, rather than conversion of the Mo to a  higher oxidation state.22 However, the surface of the coating shows that significantly more O is present (55
231 at%), which as shown in Figure 7, is d ue substantially to oxidation of the MoS 2 to MoO 3. This probably  occurred during brief exposure to the air when not being stored in N 2. The region of surface ox idation  appears to be about 0.5- nm thick (based on estimates of thickness vs. sputtering time). For the Ni -MoS 2  coating stored in air for 2.3 years , there is more O seen at the surface (65 at%) (see Figure 9b), and i n  addition, the oxidized layer is about three times thicker than that for the coating stored in N 2.  

Figure 9.  XPS sputter depth profiles for Hohman Plating Ni -MoS 2 coatings. Profiles are shown for  (a) a coating stored in dry N 2, and (b) a coating stored in 59% RH air for 2.3 years . (Ni is not shown  because of its low intensity .) The profiles show an enhancement in oxygen near the surface (i.e.,  at low sputtering tim es) due to surface oxidation of the MoS 2. The coating stored in humid air for  2.3 years shows a higher level of oxidation at the surface, and a thicker oxidized layer. (Measured  atomic percentages after extended sputtering differ from actual bulk values du e to preferential  sputtering of one element over another.)  

A corresponding depth profile for the Au- Sb2O3-MoS 2 coating stored in dry N 2 (not shown) also indicates  the presence of oxygen at the surface, but at a much lower level (12 at%) than for the corresponding Ni - MoS 2 coating. The Au- Sb2O3-MoS 2 coating stored in air for 2.3 years  shows increased oxygen at the  surface (18 at%), but the thickness of the oxidized layer s for the two Au-Sb2O3-MoS 2 coatings appear   similar.   

The XPS results correlate wit h the endurance results during pin- on-disk testing. Specifically, both the Ni - MoS 2 and Au- Sb2O3-MoS 2 coatings show increased oxidation at their surfaces after 2.3 years  of storage in  59% RH air, and both show decreased endurance. The increase in oxidation due to air stor age is  significantly greater for the Ni -MoS 2 coating compared to the Au- Sb2O3-MoS 2 coating, and the decrease in  endurance is correspondingly  greater for  the Ni -MoS 2 coating.  

   (a)  (b)
232 Conclusions  

The results of this study demonstrate that the endurance of nanocomposite, sputter -deposited MoS 2  coatings that are typically  used on spacecraft can be significantly degraded after storage for two years  under typical storage conditions (i.e., storage in air at 59% RH) . Specifically, pin-on-disk friction testing   showed that that sputter -deposited Ni-MoS 2 coating s degrade by 55% over 2.3 years of storage, while AuSb2O3-MoS 2 coating s degrade by 20% over the same period. Sb 2O3-MoS 2 coatings degrade by 17% over   1.1 years of storage.  

The reduction in endurance was shown to result from oxidation of lubricating MoS 2 to non- lubricating MoO 3  at the  surface of the coatings.  This was demonstrated by XPS of the surfaces of the coatings, including Ar+  ion sputter depth profiles. All coatings exhibited a small amount of oxidation of their surfaces , even those  stored in dry N 2 due to brief exposure of the coatings to air. After storage for 2.3 years, a Ni -MoS 2 coating   stored in 59% RH air showed significantly greater surface oxidation than a coating stored in dry N 2, while  Au-Sb2O3-MoS 2 coatings showed a relatively small increase in oxidation stored under the same conditions.  These results demonstrate that the degree of oxidation correlates with a decrease in endurance.    Storage effects aside, Au -Sb 2O3-MoS 2 and Sb2O3-MoS 2 coatings were shown to greatly outperform  the Ni - MoS 2 coating , by 18 × and 26 ×, respectively. This may be explained partly by the higher amount of  cosputterant in the Sb2O3-containing coatings relative to the Ni -MoS 2 coating, since a major effect of forming  the nanocomposite is to densify the coatings and reduce their crystallinity. Among other effects, this results in a harder, more fracture tough coating that resists wear . 6,9 With respect to oxidation sensitivity, denser,  more amorphous coatings would be more likely to resist intrusion of water and oxygen into the coat ings,  reducing the likelihood  of oxidation to nonlubricating MoO 3.   The results of this study may be used as a guide to storing hardware lubricated with sputter -deposited  MoS 2-based coatings . For devices that require a limited number of cycles (i.e., deplo yment or release  devices), degradation at typical humidities (i.e., <60% RH) will not have a significant effect on performance. However, the degradation must be taken into account for high cycle mechanisms, as endurance can be lowered by 2 0 to 55% during storage for two years at typical humidities, depending on the type of coating.  Storage in dry N 2 or if necessary in air at lower humidities (i.e., <30% RH) can significantly slow the  endurance degradation rate.3,18   Regarding the FAM storage question, we showed that although the MoS 2 coatings exhibited some  degradation in friction performance after the first year of storage, the reduced cycle life of 1.5 million cycles  for the Au- Sb2O3-MoS 2 coating and 2.4  million cycles for the Sb 2O3-MoS 2 coating was more than adequate  to meet the NIRCam cycle life requirement.   

References
234                                                                                                                                                               19. J.K.G. Panitz, L.E. Pope, J.E. Lyons, and D.J. Staley, “The tribology properties of  MoS 2 coatings  in  vacuum, low relative humidity and high relat ive humidity environments,” J. Vac. Sci. Technol. A, 6(3),  May/Jun (1988)   20. J.R. Lince and P.D. Fleischauer,  “Crystallinity of rf -sputtered MoS 2 films,”  J. Mater. Res. , 2 (1987)  827.   21. M. Iwaki, S. Obara and K. Imagawa, “ The Effect of Storage Conditions on the Tribological Properties  of Solid Lubricants ,” Proc. of 10th ESMATS, ESA SP- 653, (2003 )  22. J. R. Lince, M. R. Hilton, and A. S. Bommannavar, “EXAFS of sputter -deposited MoS 2 films,” Thin  Solid Films  264 (1995) 120.  

© The Aerospace Corporation 2016
237 a system of rolling elements, the amount of frictional wear on any components especially under no load is  greatly reduced. This allows us to study the particle generation solely based on the lubricant and speed of  the test. With the addition of a lubricant t hat provides a protective film  between all surfaces, the main  component of wear on the system is the lubricating grease, which then generates the particles being  examined in this study. This ensures that the particles generated in the dynamic system are purely  generated from the lubricating grease. Please see Figures  1 through 4 for the Dynamic Particle Generation  testing apparatus.    In Figure 1, the testing apparatus  is illustrated showing the clean air supply that moves over the testing  equipment, the ball screw , and tunnel. To supply clean, filtered air to the system, a laminar flow clean air  handler supplies clean ISO 2 Class air to the system . The air handler can deliver filtered air at a range of  velocities, depending on operator input, but for the sake of repeatability and comparison, a velocity of 1 m/s  ±0.25 m/s was used for all testing. This value was decided on after investigating the average volume of  clean air turned over in a clean air environment. This filtered air passes over the test system as the ball screw assembly operates for the length of the test .  

Figure 1.  Dynamic Particle Generating test apparatus    Figures 2 , 3, and 4 shows a more detailed view of the particle generation test unit and components.
239   Figure 4.  Dynamic Particle Generating test apparatus  

 Particles are collected via an inlet tube mounted at the end of the ball screw assembly  (Figure 4.) . The  location of the pick -up tube is key, as it captures only particles generated by the grease on the ball screw,  and none generated by the servo motor, bearings, linear guides, or flex coupling . The pickup tube leads to  a Light -scattering Airborne Particle Counter . The particle counter features simultaneous particle  measurement of sizes from 0.1  µm and above  all the way to 5 µm and above via the use of a transverse  light-scattering system which provides the most accurate and repeatable measurements available.    During the test , a particle count profile is then constructed, plotting the count of each category vs . test  duration. This profile chart is an important asset to have in order to understand the behavior of the grease  being run as two greases may share the same cl eanliness value (ISO, JIS, or FED) , but may have  completely different particle distribution profiles over the duration of the test .    The Dynamic Particle Generation test apparatus has the ability to run at speeds from  200 to 2400 RPM but  in this study, we looked at 200, 1200, and 2400 RPM respectively . This translates to 0 .02, 0.1, and    0.21 m/s of linear velocity, respectively . Comparison between greases should only be conducted at like  rotational speeds, as particle generation typically increases as RPM increases.    Grease is applied to the ball screw assembly at 200  RPM with a sample volume of 2 cc bei ng applied via  syringe and a 10- cycle run -in to evenly distribute the grease. The test begins with the motor stationary while  the particle counter takes a series of background readings . The number of readings taken is user defined.  The average of these background readings are subtracted from the particle count under dynamic conditions  before ISO, JIS, or Federal Classes are calculated. This ensures that the  particles being counted are only  those produced by the grease and not the filtered air passing over the system .   

At the conclusion of the test, the collected data is compared against the particle  classification tables  and  the ISO, Federal, and JIS class ification s are determined. The ISO cleanliness levels are determined by the  following formula and T able 2.
240 Cn= 10N x [ 0.1/D ]2.08  from [2]  

Where C n = represents the maximum permitted concentration (in particle/m3 of air) of airborne particles that are  equal to or larger than the considered particle size; C n is rounded to the nearest whole number   N = ISO class number, which must be a multiple of 0.1 and be 9 or less   D = the particle size in microns  

Table 2.  ISO Particle Classifications in Air  

   Table 3.  Federal Standard Particle Classifications in Air  

Repeatability and Statistical Significance   To validate this new experimental test method, materials were tested with a minimum of three replications .  Afterwards  a statistical  analysis of variance was performed on the sample sets to look for any statistically  significant differences. Different lots of the same material and different sections of a specific batch were  also tested. The results of this statistical analysis can be found in the t echnical paper entitled “Investigation  into the Dynamic Particle Generation of Lubricating Greases”  [5]. 

  CLASS 0.1  micron0.2  micron0.3  micron0.5 micron 1 micron 5 microns ISO1 10 2 ISO2 100 24 10 4 ISO3 1,000 237 102 35 8 ISO4 10,000 2,370 1,020 352 83 ISO5 100,000 23,700 10,200 3,520 832 29 ISO6 1,000,000 237,000 102,000 35,200 8,320 293 ISO7 352,000 83,200 2,930 ISO8 3,520,000 832,000 29,300 ISO9 35,200,000 8,320,000 293,000Number of Particles per Cubic Meter by Micrometer Size 0.1 0.2 0.3 0.5 5 1 35 7.5 3 1 10 350 75 30 10 100 750 300 100 1,000 1,000 7 10,000 10,000 70 100,000 100,000 700CLASSMEASURED PARTICLE SIZE (MICROMETERS)
241 Results and Discussion  

Table 4 and 5 summarizes the ISO and Federal classifications for the five greases that were studi ed in  this experiment at 200, 1200, and 2 400 RPM respectively.  

Table 4.  ISO Classification of Aerospace Greases  

Table 5.  Federal Classification of Aerospace Greases  

It is apparent from Tables 4 and 5 that at the lowest rotational speeds , all of the lubricants performed   equivalently. As the speeds increased as well as friction between the lubricants and the bearings, the  materials started performing differently.    The two PFPE greases received identical ISO and Federal ratings so they appear to be equivalent from the  perspective of how much particle contamination they generate into the environment. Looking at the particle generation graphs in Figures 5 and 6, a different story is told as the curves are dramatically different at  2400 RPM. While both of these greases are PTFE thickened and made from similar viscosity linear  structured PFPE fluids, their tendency to generate particles  less than 1 µ is very different. It is well kno wn  that PTFE and PFPE chemistries have weak bonding strengths (Van Der Waals) between the molecules and surfaces during sliding motion [6].    However , the difference seen here between these two materials is most likely the effect of the difference in  PTFE used in the material. There are many different grades of PTFE as well as manufacturing processes  ranging from emulsion formed polymers to irradiated polymers. These differences in manufacturing processes as well as residuals of surfactants and other chemic al processing aids can possibly have a  significant effect on the ability of the PFPE and PTFE to bind into a strong lubricant system. This combined with differences in the particle size distribution of the PTFE polymers creates factors that generate more environmental contamination through particle generation.    Braycote 601EF NyeTorr 6300 Rheolube 2000 Rheolube 2000F NyeTorr 6200 200 RPM ISO 1 ISO 1 ISO 1 ISO 1 ISO 1 1200 RPM ISO 4 ISO 4 ISO 4 ISO 2 ISO 4 2400 RPM ISO 5 ISO 5 ISO 6 ISO 3 ISO 5 Braycote 601EF NyeTorr 6300 Rheolube 2000 Rheolube 2000F NyeTorr 6200 200 RPM Class 1 Class 1 Class 1 Class 1 Class 1 1200 RPM Class 100 Class 100 Class 100 Class 1 Class 100 2400 RPM Class 1,000 Class 1,000 Class 10,000 Class 100 Class 1,000
242   Figure 5 .  Dynamic Particle Generation of Braycote 601EF  

Figure 6.  Dynamic Particle Generation of NyeTorr 6300  

The results for the MAC -based greases showed some potential performance benefits  compared to the  PFPE lubricants but it did depend greatly on thickener. The Rheolube 2000 and 2000F share the same  base fluid while the NyeTorr 6200 utilizes the Pennzane Ultra X -2000 base fluid which is processed  additionally to reduce its outgassing. The sodium thickener used in the 2000 grease is apparently sensitive  to the shear as the material generates more particles as the rotational speed and sliding friction increases.  In Figure 7, the particle generation trend can be seen to continually increase over time. The majority of  these particles were small (less than 1  µ) and the continual increase in the amount detected per cubic cm  of air is most likely attributed to the processing of the thickener/grease and the bonding strength of the  sodium thickener  resulting in the ultimate shear stability of the thickener .   01000200030004000500060007000 1 54 107 160 213 266 319 372 425 478 531 584 637 690 743 796 849 902 955 1008 1061 1114 1167 1220 1273 1326 1379 1432 1485 1538 1591 1644Braycote 601EF  2400 RPM 1200 RPM 200 RPM 0200040006000800010000120001400016000 1 34 67 100 133 166 199 232 265 298 331 364 397 430 463 496 529 562 595 628 661 694 727 760 793 826 859 892 925 958 991 1024NyeTorr 6300 2400 RPM 1200 RPM 200 RPM
243   Figure 7 .  Dynamic Particle Generation of Rheolube 2000  

As the 2000F and the 2000 grease both share the X2000 base oil and similar  additive packages, we can  see the direct effect of the thickener of the particle generation. It is well known that the MAC s possess  superior film strength and adhesion to metal surfaces when compared to PFPE’ s so we would expect low  particle generation as  the probability of particles being shed to the air would be lower. With the thickener of  the 2000F having strong covalent bonding within the PTFE molecules [ 6] combined with the strong film  strength of the MAC , it showed to be the superior lubricant in th is study for particle generation.  

Figure 8.  Dynamic Particle Generation of Rheolube 2000F    0100020003000400050006000700080009000 1 54 107 160 213 266 319 372 425 478 531 584 637 690 743 796 849 902 955 1008 1061 1114 1167 1220 1273 1326 1379 1432 1485 1538 1591 1644Rheolube 2000 2400 RPM 1200 RPM 200 RPM 0100200300400500600 1 55 109 163 217 271 325 379 433 487 541 595 649 703 757 811 865 919 973 1027 1081 1135 1189 1243 1297 1351 1405 1459 1513 1567 1621 1675Rheolube 2000F 2400 RPM 1200 RPM 200 RPM