Stephen V. Pepper

Glenn Research Center, Cleveland, Ohio

Effect of Test Environment on Lifetime of Two

Vacuum Lubricants Determined by Spiral Orbit

Tribometry

NASA/TM—2011-217002

March 2011
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Stephen V. Pepper

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Effect of Test Environment on Lifetime of Two

Vacuum Lubricants Determined by Spiral Orbit

Tribometry

NASA/TM—2011-217002

March 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

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cosponsored by the NASA Langley Research Center and Lockheed Martin Space Systems Company

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I thank Dr. Kenneth W. Street for requesting tests whose results motivated this study.

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NASA/TM—2011-217002 1

Effect of Test Environment on Lifetime of Two Vacuum Lubricants 

Determined by Spiral Orbit Tribometry 

Stephen V. Pepper 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The destruction rates of a perfluoropolyether (PFPE) lubricant, Krytox 143AC, subjected to rolling 

contact with 440C steel in a spiral orbit tribometer at room temperature have been evaluated as a function 

of test environment. The rates in ultrahigh vacuum, 0.213 kPa (1.6 torr) oxygen and one atmosphere of 

dry nitrogen were about the same. Water vapor in the test environment—a few ppm in one atmosphere of 

nitrogen—reduced the destruction rate by up to an order of magnitude. A similar effect of water vapor 

was found for the destruction rate of Pennzane 2001A, an unformulated multiply alkylated cyclopentane 

(MAC) hydrocarbon oil. 

Introduction 

The destruction of liquid lubricant molecules by tribochemical attack (tribochemistry) is wellrecognized, especially for perfluoropolyethers (PFPEs). The destruction or degradation can be manifested 

by the finite lifetime of a mechanism with a limited supply of lubricant. It is also indicated by the 

observation of chemical reaction films on the bearing surface, friction polymer and molecular fragments 

emitted into the environment. Destruction of PFPEs, in particular, has been observed in ball bearings for 

vacuum service in spacecraft and in eccentric bearing tests that operate in the starved or boundary 

lubrication regime, depleting the lubricant supply and causing bearing failure (Refs. 1 and 2). Destruction 

of PFPEs is also a matter of concern in hard disk magnetic storage media (Ref. 3). 

The destruction rate of the lubricant molecules is a function of both the lubricant’s chemical structure 

(Refs. 4 and 5) and bearing substrate chemistry (Refs. 6 and 7). It can also depend on the test environment 

(Refs. 8 to 10). Understanding environmental effects on lubricant’s destruction rate is vital to ensure that 

earth-based life tests of vacuum hardware provide meaningful results. Some tests are performed in “inert” 

gas, such as dry nitrogen, because of significant cost and convenience advantages versus vacuum testing. 

However, the test chamber may still contain trace impurities that compromise the inertness of the test 

environment, even though the chamber may have been backfilled or purged with an inert gas such as 

nitrogen. Impurities such as oxygen or water vapor may be tribochemically active and change the 

lubricant’s lifetime compared to what would be exhibited in vacuum. 

In this paper, the destruction rates of Krytox 143AC, a popular PFPE vacuum lubricant, in rolling 

contact with 440C stainless steel in a spiral orbit tribometer are studied for test environments of ultrahigh 

vacuum (UHV), dry oxygen, dry nitrogen, nitrogen containing water vapor and pure water vapor. The 

destruction rate is expressed as the lifetime of a finite charge of lubricant in a test and the results give a 

ranking of the lifetimes as a function of test environments. Some results are also presented for the effect 

of water vapor on the destruction rate of Pennzane 2001A, an unformulated multiply alkylated 

cyclopentane (MAC) hydrocarbon oil. 

Experimental 

The test instrument is a spiral orbit tribometer (SOT) depicted in Figure 1. This rolling contact 

tribometer is a retainerless thrust bearing with one ball and flat races whose elements and kinematics have 

been described (Ref. 11). The SOT was used to observe the tribochemical destruction of lubricants in 

vacuum and to give a ranking of the lifetimes, or degradation rate, of two different PFPEs (Krytox 143AC 

and Fomblin Z-25) and the unformulated Pennzane 2001A on 52100 steel (Ref. 4). 
NASA/TM—2011-217002 2

Figure 1.—Schematic of the spiral orbit tribometer. 

The 12.7 mm (0.5 in.) diameter ball and the plate specimens were 440C steel. All tests were run at 

room temperature at a mean Hertz pressure of 1.5 GPa and a ball orbit rate of 30 rpm (rolling velocity 

0.071 m/s). The stainless steel, metal-gasketed test chamber was evacuated to 0.13310–8 kPa 

(110–8 torr) with a turbomolecular vacuum pump. A gate valve between the test chamber and the 

turbomolecular pump could be closed to permit the evacuated chamber to be backfilled with a particular 

gas. The nitrogen and oxygen supply bottles were specified to have a water vapor content of 2 ppm. The 

gasses were admitted into the test chamber through an evacuated and baked stainless steel transfer line 

and a variable leak valve. Pure water vapor was admitted into the test chamber through a variable leak 

valve from a water supply that had been thoroughly degassed by the freeze-pump-thaw method. Water 

vapor concentration in the nitrogen atmosphere test environment was determined with a thin film 

hygrometer (Kahn Cermet II) inserted directly into the test chamber. The hygrometer’s readout in 

dewpoint temperature, Td, could be converted to either partial pressure of water vapor or concentration of 

water vapor in ppm through the Magnus formula and then to relative humidity. Test chamber total 

pressure P was determined by a cold cathode ionization gauge for P1.06410–3 kPa (810–3 torr), a 

Pirani gauge for 1.0610–3 kPa (810–3 torr)P0.266 kPa (2 torr) and a diaphragm gauge for 

P0.266 kPa (2 torr). 

The lowest reading of the hygrometer is Td = –99.9 C which corresponds to a water concentration of 

0.014 ppm. This reading is achieved in vacuum and initially upon backfilling the chamber to an 

atmosphere of nitrogen. Eventually, however, water vapor accumulates in the valved-off chamber as 

shown in Figure 2. The accumulation is detectable after about 2104

 orbits (~11 hr) and continues to 

increase thereafter. This water vapor is due to desorption from the chamber’s unbaked interior surfaces. 

The lowest rate of accumulation is observed after the chamber had been evacuated for many days and 

then exposed to room air for a minimal time—approximately 10 min—during insertion of test specimens. 

A lubricant charge of ~25 g for the PFPE and 20 g for the MAC was deposited only on the ball 

from a gas-tight syringe containing a dilute lubricant solution in a volatile solvent. This lubricant amount 

was consumed in a reasonable time—from a few hours to a few days. The speed and temperature of the 

test are similar to values that would be seen in spacecraft bearings, with test acceleration achieved 

through limited lubricant. Friction is recorded as a function of ball orbit and results in a “friction trace”, 

shown in Figure 3.The trace exhibits constant coefficient of friction (0.134) until an abrupt increase at 

~1230 orbits. A friction coefficient exceeding 0.2 is defined as test failure, the point when the lubricant is 

totally consumed. Normalized lifetime is obtained by dividing the orbits to failure by the initial lubricant 

charge. In this test, failure occurred at 1287 orbits and the initial lubricant charge was 25 g, resulting in a 

normalized lifetime of 52 orbits/g. Normalized lifetime is inversely proportional to the lubricant 

destruction rate. 
NASA/TM—2011-217002 3

0

0.1

0.2

0.3

0.4

0.5

0 40000 80000 120000 160000 200000

Orbit Number

H O concentration (ppm) 2

100 hours

Figure 2.—Accumulation of water vapor in an atmosphere of nitrogen. 

0

0.05

0.1

0.15

0.2

0.25

0.3

0 250 500 750 1000 1250 1500

Orbit Number

CoF

25 g Krytox 143ACTM, run in 1.6 Torr O2

1287 orbits

1287 orbits/ 25 g = 52 orbits/g

Figure 3.—Typical friction trace in SOT tests. 
NASA/TM—2011-217002 4

Results 

Krytox 143AC 

The results of four tests of Krytox 143AC run in each of the five specified environments are presented 

in Figure 4. The shortest lifetimes are exhibited by tests run in ultrahigh vacuum (UHV). The longest 

lifetimes are observed by running in the presence of water vapor, with increasing lifetime corresponding 

to increasing water vapor concentration in the test environment. Only 16 ppm water vapor in an 

atmosphere of nitrogen, corresponding to a relative humidity of 0.07 percent, is sufficient to extend the 

lifetime by an order of magnitude relative to testing in vacuum. Pure water vapor itself, without its being 

present in nitrogen gas, also leads to lifetimes much longer than testing in vacuum. Testing in one 

atmosphere of “dry” nitrogen, in which the hygrometer indicated no additional water vapor above its 

lowest reading of Td = –99.9 C (0.014 ppm), gives lifetimes greater than the tests in vacuum, but only 

marginally so. Testing in 0.213 kPa (1.6 torr) dry oxygen, corresponding to ~0.2 percent trace 

contamination of oxygen in a nitrogen atmosphere, also has little effect on lifetime relative to that in 

vacuum. The results presented here thus indicate that water vapor in the test environment exercises a 

“protective” effect on the PFPE lubricant, permitting a longer life before failure. Finally, the coefficient 

of friction of 0.134 was independent of the test environment and thus of the lifetime. 

0

100

200

300

400

500

600

700

Normalized Lifetime, Orbits/g

UHV 1.6 Torr O2 1 atm "dry" N2, 1 atm N2 w/H20

(.03)

(1.7)

(11)

(ppm H20 in N2) (16)

Pure H2O

(Total pressure

 H2O, milliTorr)

(43)

(14)

( 5)

(.3)

Figure 4.—Normalized lifetimes of Krytox 143AC when testing in different environments. 
NASA/TM—2011-217002 5

Pennzane 2001A 

Friction traces for Pennzane 2001A are plotted in Figure 5 for testing in three environments—

ultrahigh vacuum, an atmosphere of nitrogen with a minimum amount of water vapor and an atmosphere 

of nitrogen with a greater concentration of water vapor. The normalized lifetimes for the tests are 

indicated above the individual traces. The minimum concentration of water vapor in an atmosphere of 

nitrogen was achieved simply by closing the gate valve between the chamber and the turbomolecular 

pump and allowing the water vapor to accumulate in a nitrogen atmosphere as shown in Figure 2. The test 

at 75 ppm water vapor was achieved by deliberately admitting water vapor to the isolated test chamber 

from the degassed water vapor source and then backfilling with dry nitrogen. The test in ultrahigh 

vacuum lasted much longer than that of the test of the PFPE in ultrahigh vacuum as demonstrated in 

Reference 4. The tribodegradation rate of the MAC is much lower than that of the PFPE. The lifetime for 

the test in the minimum concentration of water vapor in the nitrogen atmosphere—whose evolution of 

concentration is shown in Figure 2—is greater than the lifetime for the test in ultrahigh vacuum. The 

lifetime for the test in 75 ppm water vapor is greater still. Thus water vapor in the test environment 

extends the lifetime of the MAC as it does for the PFPE. As with the PFPE, the initial coefficient of 

friction of 0.08 is the same for all test environments, regardless of lifetime. 

0 100000 200000 300000 400000

Orbit Number

0.0

0.1

0.2

0.3

CoF

Vacuum Run in 1 atmosphere nitrogen

0<H2O<1 ppm H2O=75 ppm

2925 7902 18,180

Normalized Lifetimes, orbits/g

Figure 5.—Friction traces of Pennzane P2001A in different environments. 
NASA/TM—2011-217002 6

Discussion 

Contact Conditions 

The low constant friction coefficient exhibited in the tests prior to failure is an indication that the contact 

is lubricated. However, the state of the lubricant in the contact is not known. Cann and coworkers (Ref. 12) 

have studied similar systems under a state of reduced availability of liquid lubricant using optical EHL. 

Although the lubricant charge used in the SOT is three orders of magnitude smaller than used in their 

investigations, the SOT system still appears to be in a state of lubrication. Cann et al. presented an expression 

for the film thickness in the contact in the fully starved regime. An evaluation of their expression for the 

present conditions yields a film thickness that is unphysically small (1 nm), confirming that the contact 

regime here is fully starved or parched (Ref. 13). The specific concentration of lubricant molecules in the 

contact is not known, except for the fact that there are enough of them to lubricate the contact with a friction 

coefficient close to that exhibited by a fully flooded contact (Ref. 14). However, there are probably few 

enough so that an appreciable fraction of them are in direct contact with the substrate, where they are 

eventually consumed by tribochemical attack, indicating a failure by elevated friction (Fig. 3). 

In this study, the friction coefficient is independent of the degradation rate. Evidently, the process 

responsible for the establishment of the friction force (possibly of rheological origin) and the process 

responsible for the degradation (tribochemical origin) proceed independently of each other. The tribochemical 

events may be considered second order effects—events that occur relatively seldom compared with the 

rheological friction that is continuous in the contact. If the tribochemical rate of attack was higher, then the 

coefficient of friction might well be different for different test environments that determine different lifetimes. 

Krytox 143AC 

The lifetime increase is evident for even very low concentrations of water vapor in the test environment. 

A concentration of 0.03 ppm in an atmosphere of nitrogen corresponds to a partial pressure of water vapor 

of 0.02 millitorr and lower concentrations may be difficult to achieve. Indeed, the somewhat longer lives for 

the tests in atmospheres of “dry” nitrogen (atmospheres in which the hygrometer maintained its base reading 

of Td = –99.9 C) could be due to water vapor concentrations too low to be registered. The extrapolation of 

the data in Figure 2 to small orbit numbers indicates that there is some lifetime-extending water vapor 

present even for the relatively short lifetime exhibited by this PFPE. This would indicate that the lifetime 

tests conducted in this manner are more sensitive to water vapor than the hygrometer itself. 

The tests conducted with Krytox 143AC in pure water vapor support the assertion that water vapor 

really is the cause for extended lifetimes in nitrogen atmospheres and not some other tribochemically active 

molecule that was introduced into the chamber along with the nitrogen. These results also indicate that life 

testing in “soft” vacuums of about 10–5 torr or less (for which water vapor constitutes a majority of the 

residual gas) can be as valid as testing in a better vacuum. 

The null result of testing in 0.213 kPa (1.6 torr) oxygen establishes that such trace amounts of oxygen in 

nitrogen cannot protect the PFPE lubricant against tribochemical attack. This result relates to a current idea 

that tribological stress exposes clean metal by removing the native oxide, allowing the clean metal to initiate 

tribochemical attack on the lubricant. However, such clean metal would be oxidized by exposure to 

0.213 kPa (1.6 torr) oxygen thus quenching any tribochemical attack by clean metal. If the oxidation is fast 

enough to oxidize clean metal in the contact, then these results imply that the “exposure” mechanism is not 

operative here and that the ball is probably rolling on the native oxide of the steel during the test. 

Pennzane 2001A 

The friction traces in Figure 5 indicate that water vapor in the test environment also extends the lifetime 

of the MAC. It would be desirable to determine if there was any effect on the lifetime by testing in an 

atmosphere of a truly inert gas or oxygen. However, the MAC’s long intrinsic lifetime allows the 

accumulation of water vapor in the test environment—as presently constructed—that would mask the 

possible effects of these species. A study of the effects of oxygen or an inert gas must be conducted in the 

absence of water vapor and remains to be done. 
NASA/TM—2011-217002 7

Mechanisms 

A consideration of the mechanism by which water vapor retards the tribochemical degradation of the 

lubricant molecules begins with the observation that the effect is present for both the PFPE and the MAC. 

The mechanism is not specific to the particular lubricant chemistry and attention is thus directed to the 

substrate. Substrate sites (probably on the steel’s native oxide) are evidently passivated by the water 

vapor. Water molecules might adsorb on the sites physically, that is without chemical reaction, and 

simply sterically block the interaction of the site with the lubricant molecule. Although this physical 

adsorption is well known, appreciable coverage at room temperature occurs at much higher partial 

pressures of water vapor (Ref. 15), rendering this physisorption approach unlikely. An approach that 

considers the passivating chemical reaction of water with the active surface sites may be more fruitful. 

Lewis acid sites have received much attention in the context of PFPE degradation (Ref. 16). This 

approach may also work out for the sites that can attack both the PFPE and the MAC. The purely 

tribological results presented here offer limited chemical insight. 

Other studies (Refs. 8 and 9) of lubricant life extension under a nitrogen cover gas have emphasized 

the role of an oxygen impurity component in the cover gas interacting with both an additive in the 

lubricant and with clean iron exposed by tribological action. The results presented here on oils without 

additives indicate that neither oxygen nor clean iron is a necessary aspect of life extension under an 

impure cover gas. Neither is the presence of an additive an essential aspect of the life extension process. 

Rather, the passivation of active sites on the surface of the bearing steel by water vapor is the governing 

mechanism that reduces the attack on the lubricant molecule itself. 

Application to Testing 

The first implication of these results for testing of space mechanisms is that life-testing devices such 

as bearings in a nitrogen environment to simulate vacuum is permissible with sufficient absence of water 

vapor. However, the severe restrictions required on the water vapor levels may eliminate any cost savings 

versus vacuum testing. The second implication is that the device housing may contribute water molecules 

to the device’s environment, since water always desorbs from an unbaked surface. This water flux 

captured in a housing with limited access to vacuum can provide a degree of protection to the lubricant 

that is not available in a more open geometry such as in the present test arrangement. In this sense the 

present test arrangement provides a very severe test. 

Conclusions 

Both the PFPE and the MAC lubricants are tribochemically degraded in vacuum as evidenced by the 

finite lifetimes they both exhibit in these tests. Tests in controlled environments indicate the lifetime of 

both lubricants is extended if water vapor is present in the environment. The effect of water vapor is 

evident at quite low levels (1 ppm) in the present tests. These results imply that sites must exist on the 

bearing steel’s surface that can attack the lubricant molecules. These sites are chemically passivated by 

water vapor, thus reducing the attack rate and allowing a longer lubricant life. An independent 

understanding and chemical characterization of these sites is not available at present. These results imply 

that testing of those mechanisms whose life is limited by lubricant consumption in an environment with 

water vapor, even at very low concentrations in a nitrogen atmosphere, can lead to mechanism lifetimes 

that are much longer than will be realized in vacuum service. Thus in terms of a “lessons learned”, it is 

advisable to provide water and oxygen sensors for the mechanism test environment. 
NASA/TM—2011-217002 8

References 

1. Carré, D.J. “Perfluoropolyalkylether Oil Degradation: Inference of FeF3 Formation on Steel 

Surfaces Under Boundary Conditions,” ASLE Transactions, 29 (1986), 121–125. 

2. Carré, D.J., “The Performance of Perfluoropolyether Oils Under Boundary Lubrication 

Conditions,” Tribology Transactions 31 (1988), 437–441. 

3. Novotny, V.J., Karis, T.E. and Johhnson, N.W., “Lubricant Removal, Degradation, and Recovery 

on Particulate Magnetic Recording Media,” Journal of Tribology, 114 (1992), 61–67. 

4. Pepper, S.V. and Kingsbury, E.P., “Spiral Orbit Tribometry – Part II: Evaluation of Three Liquid 

Lubricants in Vacuum,” Tribology Transactions, 46 (2003), 65–69. 

5. Bazinet, D.G., Espinosa, M.A., Loewenthal, S.H., Gschwender, L., Jones, W.R., Jr., Predmore, 

R.E., “Life of Scanner Bearings With Four Space Liquid Lubricants,” Proceedings of the 37th 

Aerospace Mechanisms Symposium, Johnson Space Center, May 19–21, 2004. 

6. Carré, D.J., “The Use of Solid Ceramic and Ceramic Hard-Coated Components to Prolong the 

Performance of Perfluoropolyalkylether Lubricants,” Surface and Coatings Technology, 43/44 

(1990), 609–617. 

7. Jones, W.R., Jr., Jansen, M.J., Chen, G-S., Lam, J., Balzer, M., Lo, J., Anderson, M. and Schepis, 

J.P., “The Effect of 17-4PH Stainless Steel on the Lifetime of a Pennzane® Lubricated Microwave 

Limb Sounder Antenna Actuator Assembly Ball Screw for the Aura Spacecraft,” 11th European 

Space Mechanisms and Tribology Symposium (ESMATS 2005), Lucerne, Switzerland, September 

21–23, 2005. 

8. Carré, D.J., “Lead Naphthenate Tribochemistry Under Vacuum and Gaseous Nitrogen Test 

Conditions,” Tribology Letters, 16 (2004), 207–214. 

9. Carré, D.J., “Effect of Test Atmosphere on Moving Mechanical Assembly Test Performance,” 

Proceedings of the 37th Aerospace Mechanisms Symposium, Johnson Space Center, May 19–21, 

2004. 

10. John, P.J., Cutler, J.N. and Sanders. J.H., “Tribological Behavior of a Multialkylated Cyclopentane 

Oil Under Ultrahigh Vacuum Conditions,” Tribology Letters, 9 (2000), 167–173. 

11. Pepper, S.V. and Kingsbury, E.P., “Spiral Orbit Tribometry – Part I: Description of the 

Tribometer,” Tribology Transactions, 46 (2003), 57–64. 

12. Cann, P.M.E., Damiens, B. and Lubrecht, A.A., “The Transition between Fully Flooded and 

Starved Regimes in EHL,” Tribology International, 37 (2004), 859–864. 

13. Kingsbury, E.P., “Parched Elastohydrodynamic Lubrication,” Transactions ASME, J. Tribology, 

107 (1985), 229–233. 

14. Spikes, H.A., “Comparison of Krytox 143ABTM and 143ACTM in Prolonged Mixed 

Sliding/Rolling,” Report TS031/99 of Tribology Section, Department of Mechanical Engineering, 

Imperial College, London. 

15. Israelachvili, J.N., Intermolecular and Surface Forces, 2nd Ed., Academic Press, New York, 1992 

and Gregg, S.J. and Sing, K.S.W., Adsorption, Surface Area and Porosity, 2nd Ed., Academic 

Press, New York, 1982, Chapter 5.5. 

16. Kasai, P.H., “Perfluoropolyethers: Intramolecular Disproportionation,” Macromolecules, 25 (1992), 

6791–6799. 
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Effect of Test Environment on Lifetime of Two Vacuum Lubricants Determined by Spiral 

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14. ABSTRACT

The destruction rates of a perfluoropolyether (PFPE) lubricant, Krytox 143AC, subjected to rolling contact with 440C steel in a spiral orbit 

tribometer at room temperature have been evaluated as a function of test environment. The rates in ultrahigh vacuum, 0.213 kPa (1.6 torr) 

oxygen and one atmosphere of dry nitrogen were about the same. Water vapor in the test environment-a few ppm in one atmosphere of 

nitrogen-reduced the destruction rate by up to an order of magnitude. A similar effect of water vapor was found for the destruction rate of 

Pennzane 2001A, an unformulated multiply alkylated cyclopentane (MAC) hydrocarbon oil.

15. SUBJECT TERMS

Lubricants; Lubricant tests; Water vapor; Ultrahigh vacuum; Oils; Tribometers; Destruction; Friction; Tribology 

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