NASA / TMm1998-208654

Evaluation of Several Space Lubricants Using a

Vacuum Four-Ball Tribometer

William R. Jones, Jr.

Lewis Research Center, Cleveland, Ohio

Agnieszka K. Poslowski

John Carroll University, University Heights, Ohio

Bradley A. Shogrin

Case Western Reserve University, Cleveland, Ohio

Pilar Herrera-Fierro

Ohio Aerospace Institute, Brook Park, Ohio

Mark J. Jansen

AYT Corporation, Brook Park, Ohio

October 1998

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:1 1i ]
NASA/TM--1998-208654

Evaluation of Several Space Lubricants Using a

Vacuum Four-Ball Tribometer

William R. Jones, Jr.

Lewis Research Center, Cleveland, Ohio

Agnieszka K. Poslowski

John Carroll University, University Heights, Ohio

Bradley A. Shogrin

Case Western Reserve University, Cleveland, Ohio

Pilar Herrera-Fierro

Ohio Aerospace Institute, Brook Park, Ohio

Mark J. Jansen

AYT Corporation, Brook Park, Ohio

Prepared for the International Tribology Conference and Exposition

sponsored by the American Society of Mechanical Engineers

and the Society of Tribologists and Lubrication Engineers

Toronto, Canada, October 25-28, 1998

National Aeronautics and

Space Administration

Lewis Research Center

October 1998
NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A03

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Price Code: A03

!:1 | i
USING A VACUUM FOUR-BALL TRIBOMETER

William R. Jones, Jr.

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 4413 5

Agnieszka K. Poslowski

John Carroll University

University Heights, Ohio 44118

Bradley A. Shogrin

Case Western Reserve University

Cleveland, Ohio 44106

Pilar Herrera-Fierro

Ohio Aerospace Institute

Brookpark, Ohio 44142

Mark J. Jansen

AYT Corp.

Brookpark, Ohio 44142

SUMMARY

The friction and wear behavior of seven space lubricants was investigated under boundary lubrication conditions using a vacuum four-ball tribometer. Three of the lubricants were perfluoropolyethers (143AC, S-200,

and Z-25). Three were synthetic hydrocarbons (a multiply alkylated cyclopentane, 2001a), and a formulated

version with an antiwear and an antioxidant additive (2001). The third hydrocarbon was an unformulated polyalphaolefin (PAO-100). An unformulated silahydrocarbon (SiHC) was also evaluated. Test conditions

included: a pressure <6.7x10 4 Pa, a 200N load, a sliding velocity of 28.8 mm/sec (100 rpm), and room temperature (~23 °C). The wear rate for each lubricant was determined from the slope of wear volume as a function of sliding distance. The lowest wear rate (0.033xl0 -9 mm3/mm) was obtained with the silahydrocarbon.

The formulated synthetic hydrocarbon had a wear rate of 0.037x10 .9 mm3/mm, which was a 36 percent reduction compared to the unformulated fluid. The polyalphaolefin had the highest wear rate of the non-PFPE fluids.

Of the perfluoropolyethers (PFPEs), wear rates decreased by about 50 percent from Z-25 (I.7xi0 -L_mm3/mm) to

S-200 (0.70x10" mm3/mm) to 143AC (0.21x10" mm_/mm).

INTRODUCTION

In the past, space components such as electronics, batteries, and thermal and optical systems were the

main causes of spacecraft failure (Ref. 1). However, with the many advances taking place in improving these

components, tribology is being exposed as the major roadblock to achieving critical mission requirements such

as increased mission lifc and greater reliability. Spacecraft designers are constantly in need of tribological

data (e.g., torque, wear characteristics, lubricant degradation, and outgassing characteristics) for various

material/lubricant combinations (Ref. 2).

Liquid lubricants are frequently used in space mechanisms because they are associated with low

mechanical noise, no wear in the elastohydrodynamic 0EHL) regime, ease of replenishment, ability to remove

wear debris, and an insensitivity to environmental factors (Ref. 2). Esters, mineral oils, polyalphaolefins, perfluoropolyethers (PFPE), synthetic multiply-alkylated cyclopentanes (Ref. 3), and silahydrocarbons (Ref. 4)

are the different chemical base stocks with space heritage or are potential candidates for space applications.

The class of synthetic hydrocarbon fluids called multiply-alkylated cyclopentanes (MACs) have received increasing attention as possible space lubricants. These lubricants, in which two to five alkyl groups are attached

to one cyclopentane ring, are the result of reacting dicyclopentadiene with commercially available alcohols

(Ref. 3). They can be synthesized with a wide range of physical properties depending upon thc number and

NAS A/TM-- 1998-208654 1
natureof thealkylsubstituent groups(Ref.3).Hence,thewideavailabilityof anumberof commercialcoholsallowsforthepreparationof alkylcyclopentanes withvastcombinationsof properties. Also,sinceMACs

arehydrocarbons, manyof thecommerciallyavailableantiwear,extremepressure, andantioxidantadditives

aresolubleinthemandcanbeusedtofurtherenhancetheirperformance. However,onlyoneMACiscommerciallyavailablewhichisamixtureof thedi-andtri- substituted(2-octyldodecyl) cyclopentane.

Unlikethehydrocarbons, perfluoropolyethers areextensivelyusedneat(pure)asspacelubricantsbecause

of theirinsolubilitywiththeabovementionedadditives.Thesefluidshavehighintrinsicthermal(Ref.5)and

chemical stabilities(Ref.6)andareparticularlyeffectiveintheelastohydrodynamic regime(Ref.7).However,PFPEsdecomposewhentheyareincontactwithLewisacids(refs.8 to 10),variousmetalsandtheir

oxides(refs.11to 13),andundercertaintribologicalconditions(refs.14to 16).Also,whenusedasboundary

lubricants, PFPEsexhibitmorevariableandlesspredictablebehaviorthanconventional lubricants,suchas

hydrocarbons, esters,andpolyalphaolefins (Ref.14).

A newclassof syntheticlubricants, knownasthesilahydrocarbons, arebeingdevelopedforuseinspace

(refs.4, 17and18).Thesematerialscontainonlysilicon,carbon,andhydrogen. Theyareunimolecular compoundswithextremelyowvolatility.Bychangingthelengthof thecarbonchain,aseriesofcompoundswith

arangeof viscositiescanbeproduced. In addition,thesecompoundswill readilyacceptconventional lubricant

additivesdevelopedforhydrocarbons.

Therefore, theobjectiveof thisworkwastocomparethesteadystatewearratesandcoefficientsof frictionof severalPFPEs,hydrocarbons, andasilahydrocarbon. A vacuumfour-ballapparatuswasemployedto

evaluatethelubricants. First,theresultsof thePFPEswereevaluatedagainsteachotherandthencomparedto

thehydrocarbons andsilahydrocarbon. Theeffectsof theadditivesinthehydrocarbononthereductionof wear

wasalsoshown.Surfaceanalysis(FTIRandRamanspectroscopies) wasperformedonsomepost-testwear

specimens.

EXPERIMENTAL

Lubricants

Frictionandwearcharacteristics ofsevenlubricantsandlubricantformulationswereevaluatedin vacuum.

Thestructuresof theselubricantsappearinTableI, andsomeof theirphysicalproperties(frommanufacturer's

data)appearinTableI1.Ofthese,threewerecommerciallyavailablePFPEs:S-200,Z-25,and143AC.These

productsareformedbypolymerizationof perfluorinatedmonomers. 143ACissynthesizedbyCsFcatalyzed

polymerizationf hexafluoropropene oxide,andit iscomposedof aseriesof branchedpolymers(Ref.19).The

linearpolymer,Z-25,isproducedbyUVcatalyzedphoto-oxidationftetrafluoroethylene (Ref.20).Thethird

polymer,S-200,is synthesizedby polymerizationof tetrafluorooxetane followedbydirectfluorination(Ref.

21).Thesynthetichydrocarbons testedincludedanunformulatedMAC(2001a)andanadditiveversion(2001).

TheMACbasefluidisamixtureof di-andtri(2-octyldodecyl)cyclopentane, andtheformulatedversioncontainsthebasefluid,amixtureof phosphateesters(anantiwearadditive),andahinderedphenol(anantioxidant).Thcothcrhydrocarbonisahighmolecularweightunformulatedpolyalphaolefin(PAO-100). Thelast

fluidis anunformulatedpentasilahydrocarbon, Si[C3H_,Si(C_2H25)314.

Tribometer

A four-balltribometer(Fig.1),operatingin theboundarylubricationmode,wasusedtomeasuresteady

statewearratesandfrictioncoefficientsfor eachlubricant.Specimenconfiguration(Fig.2)isthesameasthe

ordinaryfour-ballapparatus, except9.5mm(3/8in.)diameter440Cstainlesssteclprecisionbearingballs

(gradc10)areused.A completedescriptionof thisdeviceappearsinreference22.

TestProcedure

Cleaning and Preparation.--The test balls and the lubricant cup were ultrasonically cleaned sequentially

in baths of hexane, acetone, and ethanol for 10 rain. The balls and lubricant cup were dried with nitrogen after

the final ultrasonic cleaning. Just prior to rig assembly, a final cleaning of the balls occurred with a 15 min

UV/ozone treatment, rotating the balls every 5 min. Within 5 rain of this treatment, one ball was placed into

the nitrogen-purged tribometer and used as the rotating ball. The other three balls were secured into the lubricant cup and covered with the test fluid (-5 ml). The cup was then placed into a bell jar for 30 rain which was

NAS A/TM-- 1998-208654

-1 !]
evacuated with a mechanical pump to remove dissolved gases. The cup was placed on the stage inside the

tribometcr and the chamber was evacuated.

_.--The experiment was started after reaching a chamber pressure of <6.7xl0 _ Pa. The stage was

pneumatically loaded against the upper ball and rotation initiated. All tests were performed at room temperature, a load of 200N (an initial Hertzian stress of 3.5 GPa), and a sliding speed of 28.8 mm/sec (rotating bail

speed of 100 rpm). The frictional torque was monitored and recorded throughout the experiment. A Hall-effect

sensor provided an indirect measurement of torque by determining the relative angular displacement of the

lubricant cup which was attached to a flex pivot. The system is calibrated statically by using a dead weight

loading fixture. Between 4 and 6 tests were performed on each lubricant.

Wear was determined by measuring the wear scar diameters on the three stationary balls using an optical

microscope and calculating an average. The sample stage on the optical microscope was designed so that the

wear scars could be measured without removing the balls from the cup. The experiment was then continued

using the same set of balls. A complete test run was four hours in length with interruptions every hour for wear

scar measurements. At test conclusion, a wear rate (mm_/mm) was calculated from the slope of the straight

line which was obtained from a plot of wear volume as a function of sliding distance. Correlation coefficients

were typically >0.97. An example of data from a typical test (PFPE Z-25) is shown in figure 3. Each of the four

data points represents the average of the three balls for one wear scar measurement, which typically vary

+5 percent.

Surface Analysis

One stationary ball from one test using Z-25, 143AC, S-200 (rinsed in trichlorotrifluoroethane) and MAC

2001a (rinsed in hexane) was analyzed with a Fourier transform infrared microscope (_-FTIR) in the reflectance mode using a Grazing Angle Objective (GAO) and a I00 !am diameter spot size. The spcctral resolution

and acquisition time were 8 cm -_ and 400 sec, respectively. A gold coated glass slide was used to acquire a

background spectrum. Also, Raman spectra of the steel surfaces were taken on the same balls. Balls from tests

with MAC 2001, PAO-100 and the silahydrocarbon were examined visually, but not analyzed.

Raman analysis was performed using a 15mW Ar laser (514.5 nm wavelength), with an analyzing spot

diameter of 2 lain, and a 50X objective. The acquisition time was 50 sec and the spectral range was 400 to

3200 cm -I. The instrument was aligned to both the type 2A diamond line at 1331.2 cm _and Si (100) at

520.8 cm _. Care was taken when exposing the films to the laser beam since damage can occur that mimics

degradation produced by a tribological process. This was done by limiting the acquisition time and checking

standards periodically to assure that no artifacts were being introduced_

RESULTS

Wear

Wear results, including standard deviations, appear in Table III for all lubricants. The number of tests

performed with each lubricant appears within the parentheses in the respective lubricant column. The

PFPEs possessed higher wear rates than the hydrocarbons or silahydrocarbon. The PFPE wear rates decreased

by approximately 50 percent from Z-25 (1.7x10 -'J mm3/mm) to S-200 (0.70x10 c, mm3/mm) to 143AC

(0.21x10,7 mm._/mm). MAC 2001 (formulated) had a 36 percent reduction in wear rate as compared to the unformulated fluid (2001a). However, both had a lower wear rate than any of the PFPEs. The unformulated poly-

'alphaolefin (PAO-100) yielded a higher wear rate (0.13x10 q mm3/mm) than the MACs. The lowest wear rate

was obtained with the unformulated silahydrocarbon (0.033x10 -Q mm3/mm). Wear results are shown graphically

in Figure 4, where the error bars represent one standard deviation.

Friction

The friction coefficient results are also shown in Table III. The mean friction coefficient values are indicated, while the ranges attained throughout the runs for each lubricant are within the parentheses. Thc mean

friction results are shown graphically in Fgure 5, where the error bars represent the minimum and maximum

friction coefficient observed for each lubricant. All lubricants had a mean friction coefficient between 0.08 to

0.12.

NASA/TM--1998-208654 3
SurfaceAnalysis

Thewearexperimentsweredoneintwostages.Therefore, all lubricantpost-testspecimenswerenotanalyzed.Areasonandoffof thewearscarswereinspcctedfortribologicallyinducedproductsleftonthemetal

surfaces. Organicandinorganiccompoundswereidentifiedonthesurfaceof thepost-wcarballsusingthe

PFPElubricants, butonlyorganicspecieswerefoundforthesynthetichydrocarbon(MAC2001a).Brown

depositswereobservedonthcoff-scarandon-scarrcgionsof theexaminedthen143AC.Ontheotherhand,the

p.-FTIRspectrafortheMACidentifiedonlyoneweaksignalbetween2850balls.Z-25andS-200showedthe

mostdebrisintheoff scaregions,with143ACformingconsiderablyless,andthe2001aproducingfairlyclean

surfaces. Theon-scarregionscontainedbothcleanmetalandareasofresiduefor alllubricants.

FTIR.--TheM-FTIRon-scarspectrarevealedthepresenceofhydroxidc(3000to3600cm-_),organic

fluorocarbons(1000to 1350cm-_),andcarboxylate(~1650cmJ)forthePFPElubricatedballsandonlyC-H

signals(2850to 3000cm-l) forthchydrocarbon(2001a).OfthePFPEs, theZ-25lubricatedsteelregisteredthe

strongestignalsfor hydroxide, organicfluorocarbons, andacarboxylateattheoff-scarareasfollowedby

S-200and143AC.AreprescntativespectrumforZ-25isshowninFigure6.Thisspectrumissimilarto the

spectrumobtainedfor 143ACinball-on-diskexperiments(Ref.23).On-scarareasyicidedthesamespeciesfor

thethreePFPElubricantsbutinreducedamounts.

Qualitatively,the_-FTIRanalysisshowedthatthequantityofreactionproductsonthesteelsurfaces

usingthePFPElubricantswasgreatestforZ-25,followedbyS-200,andthen143AC.Ontheotherhand,the

la-FTIRspectrafortheMACidentifiedonlyoneweaksignalbetween2850to3000cm_forC-H.Thissuggests

thatthesynthetichydrocarbon(2001a)producesnegligibleresidueunderthesetribologicalconditions.

Raman.--The Raman spectral analysis idcntificd a carbonaceous cross-linked residue with graphitic characteristics as the major deposit on and off of the scarred areas for all of the lubricants analyzed (the formulated MAC and thc silahydrocarbon specimens were not analyzed since they were included from an earlier

study). Also, 143AC showed a distinct signal for an inorganic oxide residue, with the other lubricants revealing

much smaller quantities (Fig. 7). This assignment was verified with iron oxide standards. Overall, during the

tribological process, the lubricants undergo decomposition which eventually degrades their long organic chains

into a final carbonaceous form. This has been observed in other experiments performed in a nitrogen atmosphcre using a ball-on-disk apparatus and a branched PFPE lubricant (143AC) (Ref. 23). The degradation of

fluorinated polymers in lubricated contacts in vacuum has been well documented. In a series of papers Sugimoto and Miyake (refs. 24 to 26) studied the progressive degradation of sputtered films of polychlorotrifluoroethylene and polytetrafluoroethylene sliding against 440C steel. Using a variety of surface analytical

techniques, they clearly demonstrated the progressive conversion of these materials into a fluorine deficient

material and finally, into an amorphous carbon network.

DISCUSSION

PFPE base lubricants normally operate in the corrosive wear regime under boundary lubrication conditions

(Ref. 7). For a detailed discussion of the boundary lubrication regime as well as corrosive wear effects, referenccs 27 and 28 are suggested. In boundary contacts, PFPE fluids react with the bearing surfaces producing a

series of corrosive products. These products then react with existing surface oxides resulting in the production

of metal fluorides (rcfs. 13 and 15). Fluorides are effective solid lubricants which provide protection for the

contact surfaces, thus, reducing wear and friction, compared to the unlubricated case. Nonetheless, the fluorides also attack and decompose the PFPEs which causes the production of even more reactive species. The

surface fluorides are constantly removed from thc sliding contact region which is the reason for a high substrate wear rate (i.e. corrosive wear) (refs. 15 and 29). Overall, the very reaction that enables unformulatcd

PFPEs to protect surfaces, eventually leads to their destruction. Normally, the amount of degradation products

around the wear scar correlates with the wear rate (Ref. 23). This is also reflected in the strength of the features in the infrared spectra (Rcf. 29). In thc present study, the amount of degradation products shown by FTIR

also correlated with wear rate for the three PFPEs.

Mori et al., (Ref. 30) and Masuko et al., (Ref. 31) have reported friction and wear propertics on essentially

the same PFPE lubricants in vacuum. Mori et al., reported the same order for traction coefficients as shown in

the present work: 143AC > S-200 > Z-25. Masuko et al., reportcd wear rates on the same fluids to be: Z-25 >

S-200 > 143AC. This also agrees with the results presented herein. Both authors concluded that the high wear

rate and low friction coefficient observed with the Z-25 type of PFPE is due to its high reactivity with steel

surfaces thus generating metal fluorides which promoted chemical (i.e., corrosive wear).

Mori ctal., (Ref. 15) also observed the formation of high molecular weight polymers which were polymerized or cross-linked during the sliding experiments. More recently, Pepper and Kingsbury (Ref. 32) observed

NASA/TM-- 1998-208654 4
frictionpolymerformationinball-on-platetestsinvacuumwiththeZ-25lubricant.Gasevolutionaccompanied

theformationof solid-likematerialsinthetribological contactsinbothstudies.

In contrasttothePFPEs,unformulatedhydrocarbonlubricantsarelessreactiveandoperateintheadhesivewearregimcunderboundaryconditions(Ref.28).Someprotectivefrictionpolymer(Ref.33)isformed,

buttoenhanceperformance, hydrocarbons areformulatedwithreactiveantiwearorextremepressureadditives.

Inthiscase,theMACwastestedwithandwithoutheantiwearandantioxidant compounds. Resultshoweda

36percentdecreaseinwearratefortheformulatedMAC(2001),ascomparedto theunformulatedversion

(2001a).However,anevenlowerwearratewasobservedwiththeunformulatedsilahydrocarbon. Themechanismof wearreductionisnotclearwiththisparticularlubricant.

Throughsurfaceanalysis,organicandinorganiccompoundshavebeenidentifiedonthesurfaceof the

post-wear ballsusingthePFPElubricantsandanorganicompoundusingtheMAC.Thepost-wearPFPElubricatedballscontainedhydroxide,inorganicoxide,organicfluorocarbons andcarbonylcompounds, andthe

MAClubricatedballscontainedalkanes.ForboththeMACandthePFPEs,graphiticarbonwasidentifiedon

thesurface,oxidewasalsoidentifiedforthePFPElubricatedballs.Theinorganiccomponent of thePFPE

lubricatedsurfacelayerconsistedof theoriginaloxideandhydroxide.Theorganicomponents(carbonyl

groups,organicfluorocarbons, andgraphiticarbon)canbeattributedto thenondegradedlubricantanddegradationproducts.TheMACsurfaceresidueonlyconsistedofalkanesandgraphiticarbon,resultinginafairly

cleansurfacc.

SUMMARYOFRESULTS

1.Thelowestwearratewasobtainedwithanunformulatedsilahydrocarbon (0.033×109mm3/mm).

2.Thewearratesof theperfluoropolyethers werehigherthanthehydrocarbons, decreasingby

approximately50percentfromZ-25(1.7×109mm3/mm) toS-200(0.70×10_mm_/mm)to 143AC

(0.21×I09mm._/mm).

3.TheformulatedMAC(2001)(0.037×10.9mm3/mm) hada36percentwearratereductionascomparedto

theunformulatedversion(2001a)(0.058×10-9mm3/mm). Ahigherwearratewasobtainedwiththeother

hydrocarbon, anunformulatedpolyalphaolefin(PAO-100)(0.13×109mm3/mm).

4.Infraredspectroscopy andRamanspectroscopy revealeddegradationproductsonthePFPElubricated,

andtoalesserextent,ontheMAC2001alubricatedsurfaces. Theamountof degradationproductsobservedintheinfraredcorrelatedwiththeamountof wear.

5.Asaresultof thetribologicalprocesstowhichtheselubricantsaresubjected, acommondegradation

productforall lubricants(exceptthesilahydrocarbon) isahighlycross-linkedcarbonaceous product

withgraphiticcharacteristics.

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NASA/TM--1998-208654 5
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21. Ohsaka, Y.: "Recent Advances in Synthetic Lubricating Oils. (8). Perfluorinated Polyethers," Petrotech

(Tokyo), 8, 9, pp. 840-843, (1985).

22. Masuko, M., Jones, W.R., Jr., Jansen, R., Ebibara, B., Pepper, S.V., and Helmick, L.: "A Vacuum FourBall Tribometer to Evaluate Liquid Lubricants for Space Applications," Lub. Eng., 5, 11, pp. 871-875,

(1994).

23. Hcrrera-Fierro, P., Shogrin, B. and Jones, William R., Jr.: "Spectroscopic Analysis of Perfluoropolyethcr

Lubricant Degradation During Boundary Lubrication," NASA TM-107299, Lewis Research Center, Sept.

(1996).

24. Sugimoto, I., and Miyake, S.: "Surface Microanalytical Study on the Tribological Interface Between the

Sputtered Fluoropolymer Film and a Sliding Ball," J. Appl. Phys., 65, 2, pp. 767-774, (1989).

25. Sugimoto, I., and Miyake, S.: "High Lubrication Performance of Tribologically Oriented Fluoropolymer

Molecules Analyzed by Polarizcd Infrared Microspectroscopy," J. Appl. Phys., 67, 9, pp. 4083-4089,

(1990).

26. Sugimoto, I., and Miyake, S.: "Tribological Behavior of C-F Bonds in Plasma-Synthesized Fluoropolymers

Analyzed by Polarized Infrared Microspectroscopy," J. Appl. Phys., 70, 5, pp. 2618-2622, (1991).

27. Jones, W.R., Jr.: "Boundary Lubrication-Revisited," NASA TM-82858, Lewis Research Center, Cleveland,

OH (1982).

28. Rowe, C.N.: "Wear-Corrosion and Erosion," in Interdisciplinary Approach to Liquid Lubricant Technology,

NASA SP-318, ed. by P.M. Ku, pp. 527-568, (1973).

29. Shogrin, B., Jones, W.R., Jr., Herrera-Fierro, P., Lin, T-Y and Kawa, H.: "Evaluation of BoundaryEnhancement Additivcs for Perfluoropolyethers," NASA TM-107393, Lewis Research Center, Cleveland,

OH (1996).

30. Mori, S., ltoh, M., and Moriuchi, T.: "Relationship between the Tribological Properties of Perfluoroalkyl

Polyether Oils and their Molecular Structure (Part 1): Lubricating Properties," Japanese J. of Trib., 40,

I I, pp. 1-12, (1995).

31. Masuko, M., Fujinami, I., and Okabe, H.: "Lubrication Performance of Perfluoropolyalkylethers under

High Vacuum," Wear, 159, pp. 249-256, (1992).

32. Pepper, S. and Kingsbury, E.P.: "Destruction of Fomblin Z-25 by Different Bearing Metals," Proceedings,

Int. Rolling Element Bearing Symp., Orlando, FL, April 28-30, (1997).

33. Lauer, J.L. and Jones, W.R.: "Friction Polymers," ASLE Special Publication SP-21, Tribology and

Mechanics of Magnetic Storage Systems, Vol. III, pp. 14-23, October, (1986).

NASA/TM--1998-208654
TABLE I.--PRIMARY CHEMICAL STRUCTURES OF SPACE LUBRICANTS TESTED

Name Structure

PFPE S-200 C _F 70(CF2 CF 2CF 2 O),_C 2F.s

PFPE 143 AC C_F70[CF(CF 3)CF20]_C2F 5

PF PE Z-25 CF 30(CF 2CF: O) _(CF _O) _CFs

MAC 2001a Mixture of di- and tri(2-octyldodecyl)-c;,,clopentane

MAC 2001 (Formulated) Mixture of di- and tri(2-octyldodecyl)-cyclopentane + a Mixture

of Phosphate Eslers (an antiwear additive) and a Hindered Phenol

(an anlioxidant)

PAO- 100 Mixture of Branched Hydrocarbons

Pentasilaydrocarbon SiHC Si[C3H_Si(C_2H__0_I_

TABLE II.--PHYSICAL PROPERTIES OF TEST LUBRICANTS

(.Manufacturer's Data)

Lubricant Average Viscosity at Viscosity Pour pont, Vapor pressure.

molecular 20 °C, cS index °C (ton') at 20 °C

weight

PFPE Z-25 9500 255 355 _ 29 x 10-_-

PFTE 143 AC 6250 800 134 -35 2x 10-x

PFPE S-200 8400 500 210 -53 Ix 10a_

MAC 2001a 868 _ 107 137 -57 h3x 10-u

MAC 2001 868 _ 107 137 -55 1,Ix 10 _

PAO- I (90 2000 _'1400 [ 168 -20 NM

SiHC 2336 _'143 [ 169 NM NM

aAt 40 °C

_At 25 °C

NM--Not measured.

TABLE III.--WEAR RATES AND FRICTION COEFFICIENTS OF THE TEST LUBRICANTS

Lubricant, number runs Mean wear rule, Wear rate standard Mean and range of

x 10" mm_/mm deviation, friction coefficient

x I0-" mm_/mm

PFPE 143 AC (6) 0.21 0.030 0.12 (0.08-0.20)

PFPE S-200 (6) 0.70 0.31 0.11 (0.09-0.13)

PFPE Z-25 (4) 1.7 0.16 0.08 (0.05-0.10)

MAC 2001a (4) 0.058 0.007 0.11 (0.08-0.15)

MAC 2001 (Fornmlated) (5) 0.037 0.014 0.09 (0.08-0.13)

Silahydrocarbon SiHC (4) 0.033 0.007 0.08 (0.07-0.09)

Polyalphaolefin PAO- 100 (5) 0.13 0. I I 0. I 0 (0.09-0.14)

NASA/TM--1998-208654 7
r;

..._ Ferrofluklic

, feedthrough

To

pumping

system

Lold

¢el4

Figure 1.--Vacuum tribometer.

NASA/TM-- 1998-208654
I

StaUonaty balls

/

/

Figure 2.mSpecimen Configuration.

NASA/TM--1998-208654 9
E

E

E

_=

8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.0

0.0

Four-Ball Apparatus

200N Load, Vacuum (<6.7x10 -4 Pa), 28.8 mm/sec Sliding Speed, -23 °C

E

E

E

E

&

e,-

I 1 I l

1.0 2.0 3.0 4.0

Sliding Distance (x105 mm)

Figure 3.--Wear volume as a function of sliding distance for PFPE Z-25.

Four-Ball Apparatus

200N Load, Vacuum (<6.7x10 -4 Pa), 28.8 mm/sec Sliding Speed, -23 °C

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.0

I

,,J-

----,t---.--

I I"' l_ I I I

_ o _ 0

Lubncant

Figure 4.--Mean wear rates of the lubricants tested. Error bars represent one

standard deviation.

I

5.0

NASA/TM--1998-208654 10

i'l 1i _
L_

t-

"¢: It.

C

(9

0.10

0.05

0.00

Four-Ball Apparatus

200N Load, Vacuum (<6.7x10 -4 Pa), 28.8 mm/sec Sliding Speed, -23 °C

0"30 I

0.15

0

<

,¢

I

I I

]

O

O

O4

I I I I I

o cq 0

Lubricant

Figure 5.--Mean friction coefficients of the lubricants tested. Error bars represent

one standard deviation.

0

r"

£

o

03

<_

0.200

0.1 O0

0.000

4000 3000 2000 1000

Wavenumber (cm -1)

Figure 6.--Infrared spectrum of wear debris off wear scar for

PFPE Z-25 lubricant.

NASA/TM--1998-208654 II
.-2

¢-

I O,,d_ / 1

1000 2000 3()00

Wavenumber (cm-1)

Figure 7.--Raman spectra of wear debris off wear scar for three PFPE

lubricants (Z-25, 143AC, S-200).

NASA/TM--1998-208654 12 i
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVE'i_ED

October 1998 Technical Memorandum

4. TITLE AND SUBTITLE

Evaluation of Several Space Lubricants Using a Vacuum Four-Ball Tribometer

6. AUTHOR(S)

William R. Jones, Jr., Agnieszka K. Poslowski, Bradley A. Shogrin, Pilar HerreraFierro, and Mark J. Jansen

7. PERFORMING ORGANIZATION NAME(B) AND ADDRESS(ES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

5. FUNDING NUMBERS

WU-616-31-71-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-I1374

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASATM--1998-208654

11. SUPPLEMENTARY NOTES

Prepared for the International Tribology Conference and Exposition, sponsored by the American Society of Mechanical Engineers, and

the Society of Tribologists and Lubrication Engineers, Toronto, Canada, October 25-28, ]998. William R. Jones, Jr., Lewis Research

Center, Cleveland, Ohio; Agnieszka K. Poslowski, John Carroll University, University Heights, Ohio 44118; Bradley A. Shogrin, Case

Western Reserve University, Cleveland, Ohio 44106; Pilaf Herrera-Fierro; Ohio Aerospace Institute, Brook Park, Ohio 44142; Mark J.

Jansen, AYT Corporation, Brook Park, Ohio 44142. Responsible person, William R. Jones, Jr., organization code 5140, (216) 433-6051.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 27 Distribution: Nonstandard

This publication is available from the NASA Center for AeroSpace Information, (301) 621-0390

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The friction and wear behavior of seven space lubricants was investigated under boundary lubrication conditions using a

vacuum four-ball tribomcter. Three of the lubricants were perfluoropolyethers (143AC, S-200, and Z-25). Three were

synthetic hydrocarbons (a multiply alkylated cyclopentane, 2001a), and a formulated version with an antiwear and an

antioxidant additive (2001). The third hydrocarbon was an unformulated polyalphaolefin (PAO-100). An unformulated

silahydrocarbon (SiHC) was also evaluated. Test conditions included: a pressure <6.7 x 10 4 Pa, a 200N load, a sliding

velocity of 28.8 mm/sec (100 RPM), and room temperature (~23°C). The wear ratc for each lubricant was determined frorr

the slope of wear volume as a function of sliding distance. The lowest wear rate (0.033 x 10 9 mm_/mm) was obtained with

the silahydrocarbon. The formulated synthetic hydrocarbon had a wear rate of 0.037 x 10 `7mmVmm, which was a 36c_

reduction compared to the unformulated fluid. The polyalphaolefin had the highest wear rate of the non-PFPE fluids. Of

the perfluoropolyethers (PFPEs), wear rates decreased by about 50% from Z-25 (I.7 x IO ° mmVmm) to S-200 (0.70 x 10 -°

mm_/mm) to 143AC (0.21 x 10 -'_mm_/mm).

14. SUBJECT TERMS

Spacecraft; Lubrication

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18

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