Fis

Fe 2p

80

O1s

60

_Cr 2p

1s

40 4

Mo 8d
S2p

20

|

1200 4000 00 606 400 200 °

binding energy (eV)
(a) grease MAPLUB® SH051

nit)

8.00 9.00 10.00

Figure 5: XPS and EDAX spectra of the scrub zone indicating the presence of a
fluorocarbon product and the absence of MoS,

NASA/TM—2002-211875 10


Relative Lifetimes of MAPLUB" Greases for Space Applications

Mario Marchetti’ and William R. Jones, Jr.
National Aeronautics and Space Administration
Glenn Research Center
Cleveland, Ohio 44135

Jacques Sicre
Centre National D’Etudes Spatiales
Toulouse, France

Abstract
A Spiral Orbit Tribometer was employed to evaluate the tribological behavior and
relative lifetimes of several commercially available greases under ultrahigh vacuum. These
greases are either based on a multiply alkylated cyclopentane oil, or a perfluoropolyalkylether
oil, and a thickener made of polytetrafluoroethylene (PTFE) telomer. The multiply alkylated
cyclopentane (MAC) greases yielded long lifetimes, while perfluoropolyalkylether (PFPE)
greases yielded short lifetimes.

I- Introduction

Extended mission lifetimes and improvements to other spacecraft components, such
as electronics, batteries, and computers have placed increased burdens on space lubrication
systems [1]. Liquid or grease lubrication is commonly used to extend lifetimes and minimize
wear, torque, and noise [2]. Thus, the reliability of spacecraft moving mechanical assemblies
(MMAs) clearly depends on the lubricant employed to assure that mission objectives will be
attained. Accelerated testing has become mandatory and critical.

Full scale life testing [3] or actual component testing [4, 5, 6] is desirable, but both
are costly and time consuming. Various accelerated tests are available to evaluate the relative
lifetime, torque, wear rate, friction coefficient or degradation rate of the lubricant. These
include the eccentric bearing test apparatus [7, 8], the vacuum four-ball tribometer [9, 10],
and the spiral orbit rolling contact tribometer [11].

The Spiral Orbit Tribometer (SOT) [11] reproduces the kinematics of an angular
contact bearing. The lubricated lifetime, friction coefficient, contact resistance, and
degradation products can be determined, monitored, and analyzed. The relative lifetimes of
lubricants measured with the SOT have correlated well with actual bearing life tests [4]. The
lifetime is, indeed, inversely proportional to the degradation rate of the lubricant. The SOT’s
ability to study oils and solid lubricants has been extended to greases [12, 13, 14], which
represent many of the lubricants used in MMAs.

The objective of the work reported here was to compare the tribological behavior of
several commercially available greases, which are used for the lubrication of space
mechanisms.

II- Materials and testing
1- The Spiral Orbit Tribometer

The Spiral Orbit Tribometer (SOT) simulates an angular contact bearing (Figure 1).
A 12.7 mm (1/2 inch) diameter ball rolls between a fixed plate and a rotary plate, running at

“National Research Council—NASA Resident Research Associate at Glenn Research Center.

NASA/TM—2002-211875 1



MAC-greases

MAPLUB’ SH050

MAPLUB’ SH051

RheolubeTM 2000

(60 strikes, 25°C)

appearance white black light brown
base oil MAC MAC MAC
additi . , a phosphate, an amine
ae) none Rue and a hindered phenol
thickener PTFE PTFE soap of sodium
n-octadecylterephthalamate
dropping point (°C) N/A N/A > 260
worked penetration 348 365 276

oil separation
(mass %)

4.2 (100°C, 30 h)

3.8 (100°C, 30 h)

3.3 (100°C, 24 h)

temperature range
min/max (°C)

—40 / 200

—40 / 200

4!

a

/ 125

PFPE-gtreases

NLGI grade 0-1 0 2}

MAPLUB* PF100

MAPLUB” PF101

Krytox” 240AC

strikes (25°C)

appearance white black white
base oil linear PFPE linear PFPE branched PFPE
additive(s) none MoS, none
thickener PARE PERE PIFE:
dropping point (°C) N/A N/A N/A
worked penetration, 60 269 281 265-295

oil separation
(mass %)

2.7 (100°C, 30 h)

3.3 (100°C, 30 h)

3.0 (99°C, 30 h)

temperature range
min/max (°C)

—60 / 130

—60 / 130

—35 / 285

NLGI grade

5

2

2

NASA/TM—2002-211875

Table 1: Grease compositions and properties



The way the friction coefficient increased during the tests changes from one grease
to the other. In the case of PFPE-greases, the friction coefficient was nearly steady, and then
increased sharply at failure. The MAC-greases have shown a more Progtessiy e and
continuous increase, and no abrupt failure

(Figure 3). Arrows in Figures 3 and 4 indicate
changes in the friction traces of the different lubricants, putting in ev idence several stages to
be discussed below.

IV- Discussion

The lifetimes obtained with these greases are consistent with the ones previously
obtained with oils and greases based on similar materials. The results have confirmed the
stability of MAC oil in the boundary lubrication regime, while the PFPE based lubricants
yielded very low lifetimes due to the autocatalytic degradation mechanism. The PFPE-
greases can only be safely used with materials reducing degradation, such as TiC [17], ion-
implantation of nitrogen [18] or SiN, or TiN
various space mechanisms.

The presence of MoS, improved the lifetime of the MAC-grease by about 50% but
MoS, presence did not affect the PFPE-based grease lifetime. An improvement due to the
presence of MoS, in grease was already shown in the past [20], and can be explained by the
small amount of additive within the lubricant. MoS, generally ranged around 1% in volume,

[19], some of which are now being used in

enough to provide the grease a black color. Since tests were run with approximately 50 Ug of
grease deposited on the surface of the ball, there are only traces of MoS,. No traces of this
material were detected either on balls or tracks after the tests, using XPS and EDAX analysis
(Figure 5). In the case of MAC-grease, the degradation process is slow, so the system can
rely on the small amount of MoS, present to improve its lifetime. In the case of the PFPE-
grease, it would mean that the lifetime could not be attributed to the traces of additives, due
to the quick decomposition of both the oil and the thickener. Another possibility is a
chemical reaction between the MoS, and the free radicals usually generated as the lubricant
degrades. In all cases, both tracks and ball were covered by a layer of fluorinated friction
polymer, a result of the degradation of PTFE and/or the lubricant, shown by XPS analysis.
The degradation of the lubricant is faster with the PFPE-grease, allowing the MoS, particles
to become quickly embedded in the friction polymer where they are inactive. Moreover, the
improvement could be so small that it would be much less than the standard deviation. The
more gradual decomposition of the MAC-grease allows the MoS, to reach the tribological
surfaces and lubricate the contact.

Some differences have also appeared in the friction traces of the various greases.
This aspect was already discussed in a previous paper [13]. Some examples of the friction
traces are given in Figures 3 and 4. There is a clear distinction between the traces according
to the type of base oil involved.

The friction traces of the MAC-greases show a more progressive failure. It took
several thousands orbits to reach the friction limit. On the other hand, the behavior of the
PFPE-greases could be divided in three different stages: a short and rapid increase in the
friction coefficient, a steady increase region, and then the sudden failure.

The common point between the MAPLUB* greases is the type of thickener. All are
based on PTFE. As a matter of fact, MAPLUB" greases based on MAC have not shown the
same behavior as the MAC based Rheolube® 2000 (thickener made of an ester soap), which
have a precursor of failure described in a former study [13]. The Rheolube® 2000 has
additives, and a different thickener, which could explain the long lifetime obtained compared

NASA/TM—2002-211875 3


210 rpm. The load, providing a mean hertzian stress of 1.5 GPa, was applied through the
fixed plate. The combination of high load, moderate speed, and of the small amount of
lubricant (approximately 50 Ug) allowed the system to operate in the boundary lubrication
regime. The ball was rolling and pivoting in a spiral and maintained in the orbit by a guide
plate at a mean orbit radius of approximately 21 mm. The force exerted by the ball on the
guide plate, and normal to the guide plate, was used to determine the friction coefficient,
since the ball was sliding between the disks when also in contact with the guide plate (scrub
area). The resistance of the contacts between the ball and the plates was calculated from the
voltage drop across the plates. Evaluation of the greases was conducted at room temperature
(= 23°C), and under ultrahigh vacuum (1.3.10° Pa). As the lubricant was tribologically
stressed, it was degraded and eventually consumed. Test conclusion was defined when a
friction coefficient of 0.28 was attained. Normalized lubricant lifetime (or inversely, its
degradation rate) was then defined as the number of orbits to failure divided by the initial
amount of lubricant in micrograms.

2- Materials preparation

Greases considered in this study are either based on a MAC or a PFPE oil and a
tetrafluoroethylene telomer as a thickener. They were developed at INSA Lyon (France), in
collaboration with CNES (French Space Agency) [15]. Their characteristics and properties
are summarized in Table 1. For comparison, data for Rheolube” 2000 and Krytox” 240AC,
both greases for space applications, are also given.

For the SOT tests, the greases were applied only to the ball by rolling it several times
between two elastic membranes made of polyethylene. The small amount of grease
deposited on the ball (30 to 60 lg) was determined using a balance with an accuracy of + 2
lig. The edges of the wear tracks on the SOT plates, where some of the lubricant was
transferred during the test, were analyzed with an infrared micro-spectrometer. It confirmed
that both oil and thickener were present on the ball surface.

All specimens were made of AISI 440C stainless steel. For tribological purposes, ball
and plate surfaces were polished to a roughness Ra of 0.05 Um. The cleaning process,
described earlier [16], is based on an alumina slurry, deionized water, drying with filtered
nitrogen, and exposure to UV/ozone.

III- Tribological response of the greases
1- Normalized Lifetime

All greases were tested three times with the SOT. The data, reported in Figure 2,
clearly show the lifetimes of the MAC-greases to be much greater than PFPE-greases by
three orders of magnitude. The results obtained were consistent with the ones from
Rheolube” 2000 [13] and Krytox” 240AC [14], two others greases used for space. The
presence of MoS, in the PFPE-grease did not improve its lifetime, as was the case with the
MAC-grease.

2- Friction Coefficient

The friction traces (Figures 3 and 4) of the MAPLUB" greases changed with the base
oil used. In the case of PFPE-greases, the friction coefficient was higher (0.12 to 0.13) than
with the MAC-greases (0.09 to 0.10). The MAC-greases have shown a long lifetime, and had
a greater ability to lubricate the contact than the PFPE had.

NASA/TM—2002-211875 2



G),

10.

12"

3°

14,

53
16.

18.

9),

20.

Carré D., Kalogeras C.G., Didziulis S.V., Fleischauer P.D., Bauer R., “Recent
Experience with Synthetic Hydrocarbon Lubricants for Spacecraft Applications,” The
Aerospace Corporation, E] Segundo, CA. Report n° TR-95(5935)-3, 1995.

Masuko M., Jones W. R., Jr., Jansen R., Ebihara B., Pepper S.V., Helmick L.R., “A
Vacuum Four-Ball Tribometer to Evaluate Liquid Lubricants for Space
Applications,” Lubrication Engineering, volume 50 (1995) 871.

Jones W.R., Jr., Poslowski A.K., Shogrin B.A., Herrera-Fierro P., Jansen MJ.,
“Eyaluation of Several Space Lubricants Using a Vacuum Four-Ball Tribometer.”
NASA TM-208654, 1998.

Jones W.R., Jr., Pepper S.V., Jansen M.J., Nguyen Q.N., Kingsbury E.P., Loewenthal
S., Predmore R., “A New Apparatus to Evaluate Lubricants for Space Applications —
The Spiral Orbit Tribometer (SOT),” International Spring Fuels & Lubricants
Meeting & Exposition, Paris (France), June 19-22, 2000.

Marchetti M., Jones W.R., Jr., Street K.W., Pepper S.V., Jansen MJ., “Preliminary
Evaluation of Greases for Space Mechanisms Using a Vacuum Spiral Orbit
Tribometer,’” NLGI 68" Conference, West Palm Beach (Florida), October 27-31
2001.

Marchetti M., Jones W.R., Jr., Street K.W., Wheeler D., Dixon D., Jansen MJ.,
Kimura H., “Tribological Performance of Some Pennzane® Based Greases for
Vacuum Applications.” Tribology Letters (7 press).

Marchetti M., Jones W.R., Jr., “Degradation of PFPE Based Greases under Ultrahigh
Vacuum” (to be published).

Prat P., Ph.D. Thesis, INSA Lyon, France (97 ISAL 018), 14" February 1996.

Jansen M,J., Jones W.R. Jr., Wheeler D.R., “Evaluation of Non-Ozone-Depleting-
Chemical Cleaning Methods for Space Mechanisms Using a Vacuum Spiral Orbit
Rolling Contact Tribometer,” Lubrication Engineering, vol. 57, n° 10, October 2001,
pp. 22-26.

Jones W.R. Jr., Pepper S.V., Wheeler D.R., Jansen M,J., Nguyen Q., Schréer A., “The
Effect of Stress and TiC Coated Balls on Lifetime of a Perfluoropolyalkylether Using
a Vacuum Rolling Contact Tribometer”. Tribology Transactions, volume 43 (2000), 4,
pp. 685-688.

Shogrin B., Jones W.R. Jr., Wilbur PJ., Herrera-Fierro P., Williamson D.L., “The
Effects of lon Implantation on the Tribology of Perfluoropolyether-Lubricated 440C
Stainless Steel Couples,” NASA TM—106965, August 1995.

Carré DJJ., “The Use of Ceramic and Ceramic Hard-Coated Components to Prolong
the Performance of Perfluoropolyalkylether Lubricants,” Aerospace Corporation
Report SSD-TR-90-32, 30 September 1990.

Vukasovich M.S., Kelly W.D., “The Effect of MoS, on the Performance of Some
Model Greases,” Lubrication Engineering, vol. 28, April 1972, pp. 125-134.

NASA/TM—2002-211875 5)



NASA/TM—2002-211875 Ig p:

Relative Lifetimes of MAPLUB® Greases
for Space Applications

Mario Marchetti and William R. Jones, Jr.
Glenn Research Center, Cleveland, Ohio

Jacques Sicre
Centre National D’Etudes Spatiales, Toulouse, France

September 2002


Coefficient of Friction

a number of orbits
0 100000 200000 300000 400000

(a) grease MAPLUB" SH050

Coefficient of Friction

® number of orbits

° 100000 200000 300000 400000

(b) grease MAPLUB® SH051

Coefficient of Friction

¢ number of orbits

o 100000 200000, 300000 400000, 500000, 600000

(c) grease RheolubeTM 2000 [13]
Figure 3: Examples of friction coefficient traces for MAC-greases

NASA/TM—2002-211875 8


Rotating plate

(tilted up for clarity) — f =n
\ ASS

Ball orbit

Guide plate —
\

Force
transducer —

L \ Ball
Spiral orbit —/
Stationary plate

Figure 1: The Spiral Orbit Tribometer

100000
18965
ee -
S 7790
8 10000 = PAE
4
=
S 1000
§
S
5p 125
5 100
= 40
5 i -
3)
=| 10
2
be
3
1 : = SORE Z
OSH 050 OSH 051  Rheolube 2000
OOPF 100 OPF 101 OKrytox 240AC

Figure 2: Normalized lifetimes of the test greases (with standard deviation)

NASA/TM—2002-211875 a


to the corresponding MAPLUB®. The combination of a fluorinated thickener and a multiply
alkylated cyclopentane base oil has led to a behavior which is a combination of the
tribological characteristics of the components. The presence of the MAC as a base oil has
lead to a greater lifetime, similar to the Rheolube® 2000, while the PTFE thickener caused
the friction coefficient to increase progressively, as observed with the PFPE-greases
(Figure 4). This aspect would suggest that both PTFE and PFPE are subject to degradation.

‘V-Summarty of the results

The MAPLUB® greases based on Pennzane” oil have shown a much greater lifetime
than PFPE-based greases. Their lifetimes are of the same order of magnitude as Rheolube®
2000, also based on a Pennzane” oil. The presence of MoS, has increased the lifetime of the
MAC-grease. The presence of PTFE compound within the grease caused a progressive and
continuous increase in the friction coefficient of the lubricant tested with the SOT.

VI-Conclusion

The Spiral Orbit Tribometer is clearly able to make a distinction between the
capabilities of different fluid lubricants. According to the composition of the grease (nature
of the base oil, thickener, additive), a clear distinction can be made between their tribological
response and their friction coefficient trace.

VII- References

il, Fleischauer P.D., Hilton M.R., Assessment of the Tribological Requirements of
Advanced Spacecraft Mechanisms,” The Aerospace Corporation, El Segundo, CA,
Report n° TOF-0090 (5064)-1, 1991

De Jones W.R., Jr., Jansen M,J., “Modern tribology Handbook — volume 2,” Section 4,

pp. 1159-1186, CRC Press: Boca Raton, London, New York, Washington D.C.

ISBNO-8493-8403-6.

Brown P.L., Miller J.B., Jones W.R., Jr., Rasmussen K., Wheeler D.R., Rana M., Peri

F., “The Clouds and Earth's Radiant Energy System Elevation Bearing Assembly Life

Test,” Proceedings of the 33“ Space Mechanisms Symposium, May 1999, pp. 197—

AWE

4. Loewenthal S.H., Jones W.R., Jr., Predmore R., “Life of Pennzane and 815Z-
Lubricated Instrument Bearings Cleaned with Non-CFC Solvents,’ NASA TM—
209392, 1999.

Is, VanDyk S., Dietz B., Street K., Jones W., Jansen M., Dube M., Sharma R., Predmore
R., “The Role of Bearing and Scan Mechanism Life Testing in Flight Qualification of
The MODIS Instrument,” Proceedings of the 35° Aerospace Mechanisms
Symposium, May 2001, pp. 1-14.

6. Gill S., Rowntree R.A., “Interim Results from ESTL Studies on Static Adhesion and
the Performance of Pennzane SHF X-2000 in Ball Bearings,” Proceedings of the 6
European Space Mechanisms and Tribology Symposium, Technopark, Ziirich,
Switzerland, 4-6 October 1995, pp. 279-291.

Ts Kalogeras C., Hilton M., Carré D., Didziulis S., Fleischauer P., “The
Use of Screening Tests in Spacecraft Lubricant Evaluation,’ The Aerospace
Corporation, E] Segundo, CA. Report n° TR-93(3935)-6, 1993.

cS)

NASA/TM—2002-211875 4


This work was performed while the first author held a National Research Council Research Associateship Award in

NASA Center for Aerospace Information

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Acknowledgments

the Tribology and Surface Science Branch, NASA Glenn Research Center.

Trade names or manufacturers’ names are used in this report for
identification only. This usage does not constitute an official
endorsement, either expressed or implied, by the National
Aeronautics and Space Administration.

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Relative Lifetimes of MAPLUB® Greases for Space Applications

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Mario Marchetti, William R. Jones, Jr., and Jacques Sicre

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Mario Marchetti, National Research Council—National Research Associate at Glenn Research Center; William R. Jones,
Jr., NASA Glenn Research Center; and Jacques Sicre, Centre National D’ Etudes Spatiales, Département Mécanismes,
18 avenue Edouard Belin, BPi 1416, 31055 Toulouse cedex, France. Responsible person, Mario Marchetti, organization

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A Spiral Orbit Tribometer was employed to evaluate the tribological behavior and relative lifetimes of several commer-
cially available greases under ultrahigh vacuum. These greases are either based on a multiply alkylated cyclopentane oil,
or a perfluoropolyalkylether oil, and a thickener made of polytetrafluoroethylene (PTFE) telomer. The multiply alkylated
cyclopentane (MAC) greases yielded long lifetimes, while perfluoropolyalkylether (PFPE) greases yielded short life-

14. SUBJECT TERMS

Spiral orbiter tribometer; Space mechanisms

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NASA/TM—2002-211875

Relative Lifetimes of MAPLUB® Greases
for Space Applications

Mario Marchetti and William R. Jones, Jr.
Glenn Research Center, Cleveland, Ohio

Jacques Sicre
Centre National D’Etudes Spatiales, Toulouse, France

National Aeronautics and
Space Administration

Glenn Research Center

September 2002


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oa f

Coefficient of Friction

° number of orbits

° 1000 2000 3000 4000

(a) grease MAPLUB" PF100

03

oa

Coefficient of Friction

° number of orbits
0 1000 2000 3000

(b) grease MAPLUB® PF101

03

Coefficient of Friction

i number of orbits
0 1000 2000 3000 4000 3000 6000 7000

(c) grease Krytox" 240AC
Figure 4: Examples of friction coefficient traces for PFPE-greases

NASA/TM—2002-211875 9)