Measurements ofFriction Coefficients between Oxidized andContaminated Surfaces inVacuum andinAir PeterFrantz"andStephen V.Didziulis° Abstract Thesuccessful operation ofmanymovingmechanical assemblies isdependent uponcontrolling the coefficient offriction(COF)between slidingsurfaces. However, theactualCOFmaydeviatefrom expected valuesduetochanges insurfacecomposition duringstorageandtesting.Theseeffectsare veryimportant forlowcycledevicesthatneverestablish asteadystateoperating condition. Wehave employed anultra-high vacuumtribometer tomeasure thefrictionofcommon devicematerials in environmental conditions specifictospaceapplications. Theinstrument provides forspectroscopic inspection ofsurfacecomposition tocorrelate frictionwithsurfacechemistry. Extended testswere performed todetermine theamountofcyclesrequired toachievesteadystateintheCOFasoxidesand contaminants werewornfromthecontacting surfaces. Introduction Thesuccessful designofmanyspacemechanisms depends upontheappropriate choiceofmaterials to controlthecoefficient offriction(COF)between slidingcomponent surfaces. Inmanycases,adesigner mustrelyuponvaluesfoundintheliterature fortheCOFofcommon material pairs.However, whena mechanism istestedorstoredforextended periods, thecomposition oftheinterface maydeviatefrom thecomposition ofthematerials thatwereusedtodetermine theCOF.Asaresult,theCOFatthe interface ofthemechanism components maybegreaterorlessthanliterature values,depending upon thetypeofmaterials. Asthemechanism iscycledmanytimes,or"run-in",theCOFmaygradually reach asteadystatevaluethatisequaltothosefoundinhandbooks. However, formechanisms thatare expected tooperateforonlyafewcyclesafterstorage, suchasdeployment devices, deviation ofthe COFduringoperation maybelargeenoughtoleadtoanomalous performance characteristics. Furthermore, tribological properties measured inairareoftendifferent fromthosemeasured invacuum andmaybeunreliable forspaceapplications. Deviations oftheCOFfromexpected valuesarisebecause ofdifferences between thesurfaceandbulk composition. Thenatureandoriginofdifferences dependuponthehistoryofthedevice.Onemust consider themanufacturing process, cleaning procedures, testinghistory,andthestoragetimeand environment. Themostcommon differences between thesurfaceandbulkcomposition arethepresence ofanativeoxide,andtheunavoidable layerofadsorbed contaminants. Anotherconsideration isthe transferoflubricants orweardebrisduringtesting. Consider agenericdeployment latch(Figure1)asanexample ofamechanism thatcouldfaildueto surfaceproperties thatdifferfromthoseassumed inthedesignprocess. Duringdeployment, thelatch strikerpassesbetween twocamsbyslidingonthecamsurfaces whileitrotatesthecamsaside(figure 1B).Asitreachesthedeployed configuration (figure1C),thecamsrotatebacktowardstheiroriginal positiontoholdthestrikerinplace.Adevicesuchasthisisoftenexpected tobecycledonlyafewtimes duringtesting,andonceduringdeployment. Duringoperation, thereisaslidingcontactthatisrequired to havelowfriction(soasnottoopposethemotionofthestrikerthroughthecams).Atthisinterface, one maywishtodepositasolidlubricant, suchasabondedfilmofMoS2. Whenthedeployment configuration isreached, thecontactbetween thestrikerandthecamsisrequired tohavehighfriction,sothat separation forcesdonotloosenthelatchbyrotatingthecams.Toachievehighfriction,onemayarrange forthecontacting surfaces tobecleanmetals. "TheAerospace Corporation, ElSegundo, CA Proceedings ofthe34thAerospace Mechanisms Symposium, GoddardSpaceFlightCenter,May10-12,2000 317