Review Long-Term Storage Considerations for Lubricants

Michael Buttery *, Simon Lewis, Anthony Kent, Rachel Bingley and Matthew Cropper European Space Laboratory (ESTL), ESR Technology, 202 Cavendish Place, Birchwood Park, Warrington WA3 6WU, UK; [email protected] (S.L.); [email protected] (A.K.); [email protected] (R.B.); [email protected] (M.C.) * Correspondence: [email protected]; Tel.: +44-(0)-1925-84-3437

 Received: 28 February 2020; Accepted: 13 March 2020; Published: 13 March 2020 

Abstract: Spacecraft mechanisms commonly undergo extended periods of storage, either on-ground, or in-flight and there are an increasing number of missions for which some element of long-term storage may be required. Despite the obvious potential for degradation of lubricants during storage which might impact mechanism functionality or life and so even become mission-threatening, today’s understanding of storage phenomena is rather incomplete. This paper provides consolidation and review of recent experimental studies in this area and considers the range of storage conditions and associated degradation phenomena which could impact different lubricants. Whilst some storage best practice guidelines exist, experimental verification of the impact of storage phenomena has rarely been carried out and test data is rather scarce and incomplete. Given the absence of comprehensive data to support design, selection or the development of storage protocols, it is shown that for all lubricant types careful control of storage and test environments combined with monitoring of the evolving tribological performance during periodic mechanism exercising are presently the most effective storage risk mitigations.

Keywords: ; spacecraft; ; tribology; long-term storage

1. Introduction Spacecraft mechanism components are commonly stored before or after spacecraft integration for a considerable time. Such storage can occur prior to launch (on-ground storage), or in-flight where a mechanism remains dormant but exposed to the flight environment potentially for some years prior to operation. Whilst the “ideally benign” on-ground storage conditions for the most sensitive lubricants (e.g., sputtered MoS2) are typically considered as dry nitrogen (DN2) of 1–3% ppm moisture, for practical reasons some relaxation of this criteria is commonly followed. Extended on-ground storage at spacecraft level is often under a continuous long-term purge of semi-dry nitrogen (~10–20% relative humidity (RH)), with periodic “exercising” of the components under DN2. For in-flight storage, mechanisms are exposed to the thermal vacuum conditions of space, but usually without any possibility for “exercise”. Storage periods can continue for months and occasionally for many years both before and after launch. The general consideration is that this storage period under “benign” conditions will have a negligible impact on the tribological behaviour of the employed lubricants (assuming they are correctly applied). However recent activities (at ESTL and elsewhere) have cast some doubt onto this understanding, for both storage and operational environment conditions. There are growing concerns over the impact of on-ground or in-flight storage on subsequent performance, especially where such storage is for longer than the originally specified design requirement for the mechanism and/or under sub-optimal conditions.

Lubricants 2020, 8, 32; doi:10.3390/lubricants8030032 www.mdpi.com/journal/lubricants Lubricants 2020, 8, 32 2 of 15

The areas of concern regarding the change in the tribological performance of lubricants can be grouped into the following three categories.

Operational Environment—the environment in which the lubricant is sheared during operation • (e.g., DN2 for ground testing, vacuum for flight). On-ground Storage Environment—non-operational exposure environment prior to shearing. • Lubricants may be stored either fully applied to the respective components (such as for angular contact bearings), or prior to application (such as fluid lubricants stored within containers). Dwell and In-Flight Storage Effects—non-operational periods following some degree of shearing, • typically without separating the contacting surfaces and often, but not always, in a vacuum (e.g., during in-flight dormant periods).

This review presents recent experimental data generated by ESTL into the potential effects of the above on the tribological performance of solid and fluid lubricants, together with insights gleaned from over 40 years of consultancy and testing support to the European space mechanisms industry. As a general statement, little explicit data exists into the effect of long-term storage on spacecraft lubricants, and as a result, there are no industry-established software tools or models that can be employed. Data generated during long-term storage investigations produce, by its very nature, a sample size that is unlikely to be adequate for Weibull analysis (maximum likelihood estimators and fitting of degradation models). Likewise, studying thermo-kinetic models of any observed degradation chemical processes is hampered by the small sample size and the absence of an extensive array of measured properties. Therefore, one must draw from available datasets to gain confidence in the impact of these factors. These phenomena must be adequately understood as lubricant degradation during spacecraft storage or in-flight could be mission-compromising.

2. Materials and Methodologies

2.1. Environmental Control Much of the motivation for the work described herein stems from concerns surrounding the creation and maintenance of suitable test (and storage) environments, and the subsequent impact of these environments on the tribological components within a spacecraft mechanism. It is frequently reported that the level of humidity control and precision of monitoring practically achieved, both in general and specifically when considering mechanism storage post-integration at spacecraft level is relatively poor, and this may contribute to the variation in behaviours often observed. The limitations of the most relevant methods of environmental control are outlined below. A general comment is made that, in the opinion of ESTL, test/storage environments are often not monitored with enough accuracy, nor sufficiently local to the points of tribological contact.

Evacuation of the test chamber—the quality and level of the vacuum achieved within a testing • chamber depend not only on the dimensions of the chamber itself, its contents and the pump system but also the prior history of the test chamber hardware both in terms of maintenance and chemical exposure. Furthermore, it is not normally possible to define an absolute operating pressure for the full duration of any tests, rather tests are defined by a threshold start pressure with the chamber pressure allowed to (usually) decrease at some uncontrolled rate after the test starts. Gas purging—not only does the quality of purge gases vary but the concentration of trace elements • is often impossible to determine or control. Also, particularly for larger test setups, leaks of moist air into the test chamber are possible and not always appropriately mitigated, monitored or minimised. The progression of purge gasses into the tribological contacts of a mechanism, particularly where convoluted pathways or labyrinth seals are employed, must also be considered. Without integrated and direct purge lines implemented in the mechanism design from the start, Lubricants 2020, 8, 32 3 of 15

transport of purge gas into the cavities within mechanisms may be relatively slow/ineffective meaning the actual local environment of the tribological contacts is not fully known. Laboratory/cleanroom air—specifications can allow for significant variation in environmental • factors such as temperature and RH. This variation is inevitable as maintaining a tight tolerance on the environment of a cleanroom would be prohibitively expensive.

2.2. Tribometers and Test Facilities Tribological test activities described within this review were performed using either a Pin-on-Disc (PoD) or Spiral Orbit Tribometer (SOT). The differences and relative merits of each form of tribometer have been discussed in various literature [1,2], but the simplest consideration is to state that the PoD operates under predominantly sliding contact, whilst the SOT operates under predominantly rolling. Dependent upon the exact nature of the end application and the selected lubricant, either a tribometer may be more appropriate for a given study. Angular contact bearing tests were also performed to support certain testing activities.

3. Operational Environment

3.1. MoS2 Operation with Environment It is well known that the tribological properties of certain lubricants are highly dependent upon the operational environment. Sputtered MoS2, for example, is known to display significantly higher friction coefficient and increased wear rates when operating in moist air due to rapid degradation of the thin film at the exposed boundaries of the S-Mo-S layers, caused by the presence of moisture and oxygen acting to degrade the low shear strength properties of the film. Whilst there are many reactions involving MoS2, oxygen and water vapour, two seem energetically most favoured.

2MoS2 + 4H2O + O2 = 2MoO3 + 4H2S (1)

2MoS2 + 4H2O + 9O2 = 2MoO3 + 4H2SO4 (2) In ESTL’s experience, reaction (2) takes place in condensing environments such as those of high humidity (not normal for most storage) in which the temperature fluctuates significantly. In these situations, the corrosive acidic product can damage the lubricant coating and for this reason, such environments can be particularly damaging for MoS2-lubricated components. Where the relative humidity and temperature are controlled, as in a cleanroom or in other kinds of on-ground storage, reaction (2) above does not appear to occur and instead, an oxide will form by the route given in equation (1) above. Of the above reactions, it is clear that (1), which occurs at a low rate under cleanroom conditions is the main concern for most credible mechanism/spacecraft level storage conditions. However, the favourable properties of the lubricant under vacuum are known to be reversible, with the cycling of the test environment between air and vacuum allowing the performance of the lubricant to recover [3]. It is also known that some duration of in-air running, even if short, can have a profoundly detrimental effect on the subsequent life of the MoS2 coating under vacuum due to the increased wear rate of the lubricant film in the presence of moisture (Figure1a) [4]. The tribological properties of sputtered MoS2 under DN2 are also not consistent with those achieved in a vacuum, with the lubricant displaying an extension in sliding lifetime under DN2 [5]. Recent work performed by ESTL to support the Euclid mission [6] and by others [7] observe the same behaviour, with the latter describing this increase in durability as a consequence of beneficial rd contamination from the DN2 environments acting to improve the efficiency of the MoS2 3 body lubrication. It is clear therefore that DN2 is not an accurate representation of vacuum for the operation of sputtered MoS2. One question commonly asked is, “How moist is too moist for operation or storage?” European industry guidelines state that for solid lubricants (i.e., MoS2); Lubricants 2020, 8, 32 4 of 15

Prior to use, solid lubricated components should be stored in sealed bags purged with a dry inert Lubricants• 2020, 8, 32 4 of 15 gas (i.e., DN2). Lubricants 2020, 8, 32 4 of 15 • Any operational cycles should be performed under a DN2 atmosphere (<15 ppm moisture) as this • Any operational cycles should be performed under a DN2 atmosphere (<15 ppm moisture) as • is known to be “safe”. thisAny is known operational to be cycles“safe”. should be performed under a DN2 atmosphere (<15 ppm moisture) as • Operationthis is known in environments to be “safe”. with humidity levelslevels aboveabove 5%5% RHRH is known to cause MoS22 coatings • • to degradeOperation i.e., in environments the lifetime ofof with thethe lubricantlubricanthumidity coatingcoatinglevels above decreasesdecreases 5% RH is known to cause MoS2 coatings to degrade i.e., the lifetime of the lubricant coating decreases

(a()a ) (b(b) )

Figure 1. (a) Low-torque life of MoS2 2lubricated bearings in a vacuum as a function of the amount of FigureFigure 1. (1.a )(a Low-torque) Low-torque life life of of MoS MoS2 lubricated lubricated bearingsbearings in in a a vacuum vacuum as as a afunction function of of the the amount amount of of in-airin-air operation operation prior prior to to vacuum vacuum testing testing [4]; [4]; ( b(b)) regions regions ofof safe and deleteriousdeleterious impact impact on on MoS MoS2 lifetime2 lifetime in-air operation prior to vacuum testing [4]; (b) regions of safe and deleterious impact on MoS2 lifetime showingshowing assessment assessment humidities humidities [[8].8 [8].]. Nu Numbers Numbersmbers indicate indicate environments by by [8]. [[8].8].

However,However, muchmuch much of of theof the experimentalthe experimental experimental evidence evidenceevidence for these forfor these guidelines guidelinesguidelines is limited isis limited limited and based and and onbased based historical on on historicaldatahistorical which data maydata which nowhich longer may may no be no accurate.longer longer be be Inaccurate. accurate. addition, InIn the addition,addition, region thethe of moisture regionregion of of betweenmoisture moisture thatbetween between defined that that as “safe”defineddefined and as as “deleterious”“safe” “safe” and and “deleterious” “deleterious” is not well documentedis is not not well well docudocu (Figurementedmented1b) and,(Figure although 1b)1b) and, and, this although although region this of this uncertainty region region of of uncertaintyshoulduncertainty be avoided should should whenever be be avoided avoided possible, whenever whenever due to possible,possible, the issues due of to achieving the issuesissues dry ofof conditions achievingachieving (describeddry dry conditions conditions above) (describedthese(described environments above) above) these maythese environments representenvironments many may may practically represen represent achievablet manymany practically test conditions. achievable achievable test test conditions. conditions. A PoDA PoD test test campaign campaign was was performed performed toto investigateinvestiginvestigate the tribological tribological performance performance of of sputtered sputtered MoS2 within this hitherto poorly documented humidity region [8]. These tests suggest that in an MoS2 withinwithin this hitherto poorly documented humidity region [[8].8]. These These tests tests suggest suggest that that in an environment of up to 2% RH/~500 ppm moisture (achieved in N2), there is no evidence of a environment ofof up up to to 2% 2% RH /RH/~500~500 ppm ppm moisture moisture (achieved (achieved in N2), in there N2 is), nothere evidence is no ofevidence a detrimental of a detrimental effect of water vapour on the tribological lifetime of sputtered MoS2 (Figure 2a). Above detrimentaleffect of water effect vapour of water on the vapour tribological on the tribological lifetime of sputtered lifetime of MoS sputtered(Figure MoS2a).2 (Figure Above this2a). Above value, this value, the anticipated drop-off in lifetime is observed. This suggests2 that the deleterious effect of thisthe anticipatedvalue, the anticipated drop-off in drop-off lifetime in is observed.lifetime is ob Thisserved. suggests This thatsuggests the deleterious that the deleterious effect of moisture effect of moisture does not occur until a moderately high RH, and the statements for operation given above moisturedoesare not somewhat occurdoes untilnot conservative occur a moderately until (Figure a moderately high 2b). RH, and high the RH, statements and the for statements operation for given operation above are given somewhat above areconservative somewhat (Figure conservative2b). (Figure 2b).

(a) (b)

Figure 2. (a) Normalised(a) lifetime of sputtered MoS2 vs. humidity (in N2). Performed(b) at 1.5 GPa Peak

Hertzian contact stress [8]; (b) New proposed regions of safe and deleterious impact on MoS2 lifetime Figure 2. (a) Normalised lifetime of sputteredsputtered MoSMoS2 vs. humidity (in N2).). Performed Performed at at 1.5 1.5 GPa GPa Peak following new data. 2 2 Hertzian contact stress [[8];8]; (b) New proposed regions ofof safesafe and deleterious impact on MoS22 lifetime 3.2.following Some Period new of data. Running in Moist Environments

3.2. SomeThe Period impact of Running of short in duration Moist Environments (i.e., not fully to failure) of operation under these “safe” and “deleterious” environments on the subsequent vacuum lifetime of sputtered MoS2 has also been The impact of short duration (i.e., not fully to failure) of operation under these “safe” and “deleterious” environments on the subsequent vacuum lifetime of sputtered MoS2 has also been

Lubricants 2020, 8, 32 5 of 15

3.2. Some Period of Running in Moist Environments The impact of short duration (i.e., not fully to failure) of operation under these “safe” and “deleterious” environments on the subsequent vacuum lifetime of sputtered MoS2 has also been Lubricants 2020, 8, 32 5 of 15 investigated. MoS2 coatings were sheared under N2 at a controlled moisture level for some predefined duration,investigated. before MoS evacuating2 coatings the were chamber sheared andunder allowing N2 at a controlled the test to moisture continue level to for failure someunder predefined vacuum. duration,Results showedbefore evacuating that, for thethe “safe”chamber region and allowing ( 2% RH the DN test ),to the continue cumulative to failure lifetime underobtained vacuum. of the ≤ 2 sputteredResults MoS2 showedfilm (N2 that,revolutions for the “safe” performed region plus(≤2% subsequent RH DN2), the vacuum cumulative life) waslifetime consistent obtained (Figure of 3). Thisthe suggests sputtered that, MoS when2 film su (Nffi2ciently revolutions dry, performed operation plus in nitrogen subsequent does vacuum not significantly life) was consistent influence the (Figure 3). This suggests that, when sufficiently dry, operation in nitrogen does not significantly sputtered MoS2 beyond cumulative wear. influence the sputtered MoS2 beyond cumulative wear.

FigureFigure 3. Normalised 3. Normalised total total sliding sliding life life ofof sputtered MoS MoS2 following2 following varying varying periods periods of N2 ofoperation. N2 operation. Yellow bar indicates typical variance.Yellow bar indicates typical variance.

MoreMore interesting interesting is is the the observationobservation that, that, with with prior prior shearing shearing in moist in moist(>2% RH) (>2% environment, RH) environment, the the cumulativecumulative lifetime lifetime of of sputtered sputtered MoS MoS2 is2 seenis seen in inincrease increase in some in some instances. instances. Observations Observations of this of this extension in MoS2 life when exposed to moist environments have been made before [6,9,10] but were extension in MoS2 life when exposed to moist environments have been made before [6,9,10] but were commonlycommonly attributed attributed to testto test variability variability (amongst (amongst other other things).things). However, given given that that this this increased increased life life appears to occur only at higher RH values, and an improved understanding of the nature of MoS2 appears to occur only at higher RH values, and an improved understanding of the nature of MoS2 variability on the PoD [10], this assumption should be re-evaluated. It is argued that the adage of variability on the PoD [10], this assumption should be re-evaluated. It is argued that the adage of exposure and/or shearing under a moist environment being universally detrimental to sputtered exposure and/or shearing under a moist environment being universally detrimental to sputtered MoS MoS2 is potentially too simplistic, and under some (as yet undefined) conditions, exposure to 2 is potentiallymoisture may too simplistic,improve the and subsequent under some in-vacuum (as yet lifetime undefined) of the conditions, lubricant. However, exposure tountil moisture better may improveunderstood, the subsequent it is still in-vacuumessential that lifetime the guidelines of the lubricant. above be However,followed for until the better purposes understood, of risk it is stillmitigation. essential that the guidelines above be followed for the purposes of risk mitigation.

3.3. Hybrid3.3. Hybrid Lubrication Lubrication to to Mitigate Mitigate Environmental Environmental Behaviours Behaviours TheThe suggestion suggestion is is occasionally occasionally mademade within th thee space space mechanisms mechanisms community community that a that form a formof of hybrid lubrication may circumvent the restriction on in-air operation of MoS2, through the hybrid lubrication may circumvent the restriction on in-air operation of MoS2, through the application of aapplication controlled quantityof a controlled of PFPE quantity fluid lubricantof PFPE fl touid a componentlubricant to lubricateda component in thelubricated conventional in the way conventional way with sputtered MoS2, thus protecting the MoS2 from the moist environment. The with sputtered MoS , thus protecting the MoS from the moist environment. The suggestion states that suggestion states2 that the fluid lubricant will 2provide low friction during operation in moist air and the fluidwill subsequently lubricant will be providelost (through low frictioneither evapor duringation operation or tribo-chemical in moist degradation) air and will subsequentlyunder vacuum, be lost (throughallowing either the evaporation(hopefully) uncompromised or tribo-chemical MoS degradation)2 film to provide under low vacuum, friction and allowing long life the for (hopefully) the uncompromisedremainder of operation MoS2 film under to vacuum. provide low friction and long life for the remainder of operation under vacuum.Feasibility trials have been performed using the SOT where 52,100 steel test balls were first lubricatedFeasibility with trials sputtered have beenMoS2. performedThese same test using balls the were SOT then where lubricated 52,100 with steel ~50test μg of balls PFPE were oil first applied via solvent plating. Hybrid lubricated balls were inserted into the SOT test chamber and lubricated with sputtered MoS2. These same test balls were then lubricated with ~50 µg of PFPE oil appliedrotated via in solvent moist air plating. for a defined Hybrid duration lubricated and subs ballsequently were under inserted a high into vacuum the SOT to failure. test chamber Results and demonstrated encouraging behaviour, with indications that a small volume of fluid lubricant acts to rotated in moist air for a defined duration and subsequently under a high vacuum to failure. Results protect the sputtered MoS2 from elevated wear rates in moist air. In addition, the friction coefficient of the hybrid lubrication was found to be ~0.02 (Figure 4), identical to the value for MoS2 alone in

Lubricants 2020, 8, 32 6 of 15 demonstrated encouraging behaviour, with indications that a small volume of fluid lubricant acts to protect the sputtered MoS2 from elevated wear rates in moist air. In addition, the friction coefficient of the hybrid lubrication was found to be ~0.02 (Figure4), identical to the value for MoS 2 alone in vacuumLubricants [2]. Given 2020, 8, 32 that Z25 provides a friction coefficient of ~0.1 under vacuum on the6 of 15 SOT [2], it can be saidvacuum with some[2]. Given confidence that Z25 provides that the a friction MoS coefficient2 was providing of ~0.1 under lubrication vacuum on the at SOT this [2], time it can and had been protectedbe said from with a reasonablesome confidence degree that ofthe moist MoS2 was air-operation. providing lubrication at this time and had been protected from a reasonable degree of moist air-operation.

Figure 4. Hybrid PFPE/MoS2 rolling in air (blue) and vacuum (red). Elevated frictional noise during Figure 4. Hybrid PFPE/MoS2 rolling in air (blue) and vacuum (red). Elevated frictional noise during vacuum running is an artefact introduced by the analysis software and is not real. vacuum running is an artefact introduced by the analysis software and is not real. Furthermore, under ideal conditions (believed to be dependent upon the physical properties of Furthermore,the fluid, such underas viscosity ideal and conditions film thickness (believed [11]), the tolifetime be dependent of a sputtered upon MoS2 the film physical can be properties of the fluid,significantly such extended as viscosity under andvacuum, film with thickness no conseque [11ntial]), theincrease lifetime in friction of a coefficient, sputtered through MoS 2 film can be significantlythe application extended of a underfluid film. vacuum, Component- with and no trib consequentialometer-level research increase activities in friction are continuing coeffi atcient, through ESTL to verify this hypothesis. the application of a fluid film. Component- and tribometer-level research activities are continuing at ESTL to3.4. verify Lead Operation this hypothesis. with Environment Little experimental evidence exists as to the impact of the operational environment on thin 3.4. Lead Operation with Environment lubricating films of lead. [12] states that an oxide layer of lead does not adversely influence the friction Littlecoefficient experimental of the bulk material, evidence butexists SOT activities as to theperformed impact by ESTL of the show operational clear differences environment in the on thin tribological properties of the lubricant when rolling under vacuum and laboratory air. The friction lubricatingcoefficient films of of lead lead. under [12 air] states on the thatSOT is an measured oxide layer as ≥0.13, of leadin comparison does not to adversely≤0.05 under influencevacuum, the friction coefficientwith ofa lifetime the bulk of ~10 material,6 orbits in the but air SOT (typically activities 2 × 107 performedachieved under by vacuum). ESTLshow clear differences in the tribologicalIn properties addition, exposure of the to lubricant in-air heating when (up to rolling 8-h at 200°C) under has vacuum also been and shown laboratory to increase air.the The friction coefficientstart-up of leadrolling under friction, air reduce on the the SOT rolling is measuredlifetime, and as increase0.13, the in comparisonrun-in duration to required0.05 underto vacuum, achieve low friction on the SOT [13]. If we assume that the ≥reaction at the coating surface≤ is purely with a lifetime of ~106 orbits in the air (typically 2 107 achieved under vacuum). oxidation, and the moisture content of the environment× remains the same (a simplification), then a Inbrief addition, period of exposure in-air heating to in-aircan be heatingequated to (up a longer to 8-h period at 200of in-air◦C) exposure has also at beenambient shown periods. to increase the start-upWith rolling this logic, friction, the above reduce observations the rolling for lifetime,lead are similar and increaseto those for the MoS run-in2, where duration periods of required in-air to achieve low frictionexposure on are the known SOT to [13 temporarily]. If we assume increase the that friction the reaction coefficient at of thethe lubricant coating under surface vacuum is purely [3] oxidation, and the3.5. moisture Fluid Lubricant content Performance of the environment with Environment remains the same (a simplification), then a brief period of in-air heating can be equated to a longer period of in-air exposure at ambient periods. With this logic, It is often assumed that the tribological behaviour of fluid lubricated components will be the aboveinsensitive observations to the operational for lead environment, are similar but to thosein many for instances, MoS2, wherethis may periods be a simplification. of in-air exposure are knownExperimental to temporarily investigations increase conducted the friction using coe theffi SOcientT show of that the the lubricant tribo-consumption under vacuum rate of PFPE [3]. oil is greatly affected by the presence of small quantities of water vapour [14]. The degradation rate 3.5. Fluidof PFPE Lubricant [15] and Performance MAC lubricants with [16] Environment are also known to be influenced by surface cleanliness, leading to the suggestion that the presence of water vapour (or other contaminants such as an oxide layer) on It isthe often substrate assumed surface that acts theto retard tribological the degradation behaviour rate of of vacuum fluid lubricatedfluid lubricants. components Under shear, will thebe insensitive to the operationalnative oxide layer environment, present on the surface but in of many the subs instances,trate steel is thisremoved, may revealing be a simplification. the nascent metal. Experimental investigations conducted using the SOT show that the tribo-consumption rate of PFPE oil is greatly affected by the presence of small quantities of water vapour [14]. The degradation rate of PFPE [15] and MAC lubricants [16] are also known to be influenced by surface cleanliness, leading to the suggestion that the presence of water vapour (or other contaminants such as an oxide layer) on the substrate Lubricants 2020, 8, 32 7 of 15

This nascent steel is much more reactive than the original oxide layer, resulting in accelerated

Lubricantschemical2020 degradation, 8, 32 of the fluid. 7 of 15 In an ambient air environment, and if oxidation occurs fast enough, this oxide layer can be replenished, retarding the degradation rate of the fluid. This has been experimentally demonstrated surfacein PoD (sliding) acts to retard and SOT the degradation(rolling) investigations, rate of vacuum where fluid the lubricants.lubricating Underlifetimes shear, of a thePFPE native and MAC- oxide layerbased present grease on(Braycote the surface 601EF of the and substrate MAPLUB steel SH100-b) is removed, were revealing found to the be nascent significantly metal. higher This nascent in an steelambient is much air environment more reactive (Figure than the 5a), original with marginally oxide layer, lower resulting friction in acceleratedcoefficients chemical [17]. PFPE degradation lubricants ofhave the also fluid. been shown to provide extended lifetimes when operating in a modest vacuum (10-2 bar) [8]. AssumingIn an ambient that airthe environment, residual pressure and ifwith oxidationin a vacuum occurs fastchamber enough, is predominantly this oxide layer moisture can be replenished,released from retarding the internal the degradationsurfaces, this rate correlate of the fluid.s well Thiswith has the beenconsideratio experimentallyn that the demonstrated presence of inmoisture PoD (sliding) results in and increased SOT (rolling) lubricant investigations, lifetimes. where the lubricating lifetimes of a PFPE and MAC-basedCuriously grease the (Braycotelifetimes obtained 601EF and via MAPLUB operation SH100-b) in DN2 wereare also found increased to be significantly (Figure 5a). higher Whilst in it an is ambientclear that air the environment humidity content (Figure 5withina), with a marginallynitrogen environment lower friction impacts coe fficients upon [the17]. lifetime PFPE lubricants of PFPE 2 havelubricants also been (Figure shown 5b), to it provideis also clear extended that the lifetimes moisture when alone operating cannot in be a modestthe sole vacuumcause of(10 this− increasebar) [8]. Assumingin life. This that can the be residualfurther demonstrated pressure within via a vacuumthe observation chamber of is increased predominantly rolling moisturelifetimes releasedof PFPE fromfluids the in internalmoist nitrogen, surfaces, when this compared correlates wellwith withambi theent considerationair of equal RH that [8]. the The presence influence of of moisture oxygen resultshas been in suggested increased lubricantas a potential lifetimes. explanation for this behaviour.

(a) (b)

FigureFigure 5.5. ((a) NormalisedNormalised lifetimelifetime (orbits(orbits//µμg) ofof BraycoteBraycote 601EF601EF andand MaplubMaplub SH100bSH100b greasegrease underunder variousvarious environmentsenvironments asas assessedassessed onon thethe SpiralSpiral OrbitOrbit TribometerTribometer (SOT)(SOT) [[17];17]; ((bb)) NormalisedNormalised lifetimelifetime

(orbits(orbits/ug)/ug) ofof PFPEPFPE oiloil (Fomblin(Fomblin Z25)Z25) withwith varyingvarying humidityhumidityin inN N22 (with(with linearlinear extrapolation)extrapolation) [[8].8].

CuriouslyThese observations the lifetimes lead obtained to the viainterpretation operation in that DN 2asare long also as increased the expected (Figure friction/torque5a). Whilst it isdifferences clear that theare humidityconsidered, content on-ground within operations a nitrogen of environment fluid lubricated impacts components upon the lifetimein non-vacuum of PFPE lubricantsenvironments (Figure can5 b),be itconsidered is also clear “safe”; that the i.e., moisture they will alone not cannot result bein thecompromised sole cause of behaviour this increase of the in life.lubricated This can component be further during demonstrated subsequent via the vacuum observation operation. of increased However, rolling the ab lifetimesove data of suggests PFPE fluids also inthat, moist IF the nitrogen, target whenis to test compared on the ground with ambient with minimal air of equal cumulative RH [8]. The tribo-chemical influence of degradation, oxygen has been one suggestedwould be asbetter a potential performing explanation the test forin a this moist behaviour. N2 (say 45% RH) environment, rather than in moist air. These observations lead to the interpretation that as long as the expected friction/torque differences are considered, on-ground operations of fluid lubricated components in non-vacuum environments can be4. On-Ground considered “safe”; Storage i.e., Behaviours they will not result in compromised behaviour of the lubricated component during subsequent vacuum operation. However, the above data suggests also that, IF the target In addition to the operational environment, attention must be given to pre-operational storage is to test on the ground with minimal cumulative tribo-chemical degradation, one would be better conditions for lubricated components. Storage conditions are ideally selected on the assumption that performing the test in a moist N (say 45% RH) environment, rather than in moist air. the environment is benign, but often2 this has not been explicitly demonstrated. 4. On-Ground Storage Behaviours 4.1. Maturation of Sputtered MoS2 In addition to the operational environment, attention must be given to pre-operational storage Recent experimental investigations suggest that sputtered MoS2 may “mature” upon storage, conditions for lubricated components. Storage conditions are ideally selected on the assumption that such that the tribological properties of freshly deposited MoS2 differ following some period of the environment is benign, but often this has not been explicitly demonstrated. storage. This has been investigated in detail by ESTL where a series of PoD tests were performed over

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4.1. Maturation of Sputtered MoS2

Recent experimental investigations suggest that sputtered MoS2 may “mature” upon storage, such that the tribological properties of freshly deposited MoS2 differ following some period of storage. This has been investigated in detail by ESTL where a series of PoD tests were performed over a period of 60 days on a single batch of sputtered MoS2 lubricated 52100-steel discs. Test coupons were stored in DN2 prior to the assessment. Results demonstrated a marked increase in lubricant lifetime (but also some increased lifetime variance) as storage duration increases, suggesting that exposure to the commonly considered benign DN2 acts to alter the lubricant performance. A parallel test campaign performed on the SOT displayed the same behaviour, demonstrating that this maturation effect is not unique to the sliding motion of the PoD tribometer. Steady-state friction coefficient did not show a strong correlation with the storage environment, and no relationship between steady-state friction and lifetime was observed during these tests. Little evidence was seen of a correlation between coating thickness and tribological performance, in agreement with previous observations [15]. A follow-up campaign of PoD tests was performed by ESTL under identical conditions, with sputtered MoS2 lubricated samples stored for up to 12-months (in DN2 and laboratory air). Results from this experiment showed evidence of increasing lubricant lifetime over the first 60 days, followed by a plateau in which the tribological behaviour (including lifetime) was unchanged. A final phase of the experimental campaign was performed utilising “opportunistic” lubricant samples from ESTL’s own MoS2 long-past production activities. MoS2 test coupons from production dating back some 16-years and exposed in the intervening time to uncontrolled (i.e., room air) storage were assessed via PoD, and their tribological performance compared against their original behaviour at the time of production. These lubricated coupons showed no statistically significant change in coating lifetime as a result of such storage. Taken together these experimental investigations provide evidence of a “maturation” effect of MoS2. The initial phase of this maturation appears to occur over a few months following the deposition of the coating, before plateauing and maintaining consistent tribological behaviour beyond this point, implying some level of passivation-type behaviour. The exact details of how this maturation occurs are not known but may be related to the observation from [7] that the presence of contaminants (including H2O, CO2, O, N2 and H2) within an operational environment is favourable for the MoS2 lubrication.

4.2. Long Term Storage of Fluids Fluid lubricants are particularly susceptible to effects of on-ground storage which may influence their tribological properties. These include oxidation effects which have the potential to change the viscosity of lubricants over time (particularly MAC-based oils), absorption of moisture, loss of additives, and chemical degradation as well as physical effects such as base oil separation (for greases) and lubricant creep. As a result, fluid lubricant manufacturers often provide guidelines for lubricant storage (often at low temperatures) and shelf lives of typically ~5yrs. However recent SOT testing performed by ESTL on historical PFPE oils, some stored under uncontrolled conditions (i.e., room air) for 40-years, have shown similar tribological behaviour to new as-formulated PFPE oils, with no reduction in lubricating lifetime. This indicates that there may be no significant effect of uncontrolled air storage on the tribological performance of PFPE oils.

4.3. Phenolic Cages In many applications of fluid lubrication, bearings are fitted with cotton fabric reinforced phenolic cages (retainers). It is common practice to vacuum-impregnate such cages with the base-oil of the fluid lubricant used in the bearings to saturate the cotton fabric with the lubricant oil prior to use (and inhibit the uptake of the free oil within the bearing). There is however evidence [18] that even “fully impregnated” cages when continuously wetted by further exposure to oil may absorb a significant Lubricants 2020, 8, 32 9 of 15

additional oil mass as the oil is gradually taken up by the phenolic matrix itself. It can be shown that the typical capacity of the cage for this additional oil uptake, represents a disturbingly high percentage of available free oil. For example, the infiltration of oil into the phenolic matrix is a slow process, taking months, but as shown in Figure6 the total mass absorbed can be relatively high. Assuming this oil is Lubricantsabsorbed 2020 entirely, 8, 32 from the free oil quantity typically added to a bearing, the proportion of oil taken-up9 of 15 can quickly reach 100%. This phenomenon is potentially an overlooked lubricant loss route which may proportion of oil taken-up can quickly reach 100%. This phenomenon is potentially an overlooked cause the onset of starvation/dry running within a bearing. lubricant loss route which may cause the onset of starvation/dry running within a bearing.

Figure 6. Oil mass uptake by weight (left axis) and potential impact on the proportion of free oil (Basis: Figure28 µL 6. free Oil oilmass available, uptake by storage weight period (left axis) up to and 9 months) potential (right impact axis) on [ 18the]. proportion of free oil (Basis: 28μl free oil available, storage period up to 9 months) (right axis) [18]. 4.4. Creep Barriers 4.4. Creep Barriers Creep barriers are commonly used to mitigate and control the unwanted creep of lubricant within a fluidCreep lubricated barriers application.are commonly Shelf used lives to for mitigate creep barriers and control are often the quotedunwanted as 1 creep year, andof lubricant Minimum withinOrder a Quantitiesfluid lubricated (MOQ) application. can be, relative Shelf to lives the quantity for creep needed, barriers prohibitively are often quoted large (e.g., as 1 MOQ year, ~5and kg). MinimumProduct lead-timesOrder Quantities may also (MOQ) be long, can and be, asrelative a result, to itthe is commonquantity needed, that a creep prohibitively barrier intended large (e.g., for an MOQapplication ~5kg). Product may be formallylead-times borderline may also “life-expired” be long, and as or evena result, fully it beyondis common its quoted that a creep shelf lifebarrier when intendedrequired for for an use. application In order may to verify be formally whether borderlin the performancee “life-expired” of such or formally even fully expired beyond creep its quoted barriers shelfremains life when acceptable required for for a flightuse. In application, order to verify a re-lifing whether process the performance must be completed, of such formally with a expired series of creeptests barriers performed remains and theacceptable results comparedfor a flight against application, the desired a re-lifing/known process behaviour must be of completed, the in-life product. with a seriesThese of tests tests should performed include and an the optical results inspection compar ifed the against creep the barrier desired/known fluid, a check behaviour of the UV of marker the in- (if lifethe product. creep barrier These containstests should one), include surface an energy optical assessment inspection (i.e., if the via creep contact barrier angle), fluid, crosshatch a check adhesion of the UVtest, marker and FTIR (if the analysis. creep barrier contains one), surface energy assessment (i.e., via contact angle), crosshatch adhesion test, and FTIR analysis. 4.5. Corrosion 4.5. CorrosionSubstrate corrosion can occur in the presence of a condensing environment. This may be caused bySubstrate residual contamination corrosion can occur (e.g., chlorides)in the presence or by of the a acidiccondensing by-product environment. of the reaction This may between be caused MoS 2 byand residual moisture contamination in a condensing (e.g., chlorides) environment or by (Reaction the acidic (2) by-product above). This of the can reaction initiate between corrosion MoS of the2 andsubstrate moisture and in is a relevant condensing to components environment lubricated (Reactio withn (2) PVD above). MoS This2 and can self-lubricating initiate corrosion polymers of the that substratecontain and MoS is2 relevantwhich may to components be exposed lubricated to a condensing with PVD environment. MoS2 and self-lubricating The speed of this polymers reaction that is a containfunction MoS of2 temperature. which may be In aexposed non-condensing to a condensing environment, environment. the oxidation The ofspeed MoS of2 (Reaction this reaction (1), above) is a functionis limited of totemperature. a very thin outerIn a non-conden surface layersing (~100 environment, Angstroms) the and oxidation the reaction of MoS product2 (Reaction is non-acidic, (1), above)meaning is limited that in to the a very absence thin ofouter other surface reacting layer species, (~100 Angstroms) corrosion is and unlikely the reaction to be an product issue. Substrate is non- acidic,corrosion meaning is typically that in identified the absence by detailedof other visualreacting inspection. species, corrosion is unlikely to be an issue. SubstrateIn addition,corrosion thereis typically is the riskidenti that,fied in by the detailed presence visual of a inspection. moist environment, corrosion is initiated betweenIn addition, metals there with is highly the risk diff that,erent in electrochemical the presence of potentials.a moist environment, For example, corrosion mechanisms is initiated having between metals with highly different electrochemical potentials. For example, mechanisms having several different metallic interfaces (e.g., from industry a threshold of >0.25V electro-chemical potential difference is taken as a cause for concern) in local contact (bearings, housings, end-caps and clamp rings for example) could be at risk from galvanic corrosion. In this case, a representative “equivalent assembly” or witness sample subassembly may be more useful for assessing the impact of storage.

Lubricants 2020, 8, 32 10 of 15 several different metallic interfaces (e.g., from industry a threshold of >0.25V electro-chemical potential difference is taken as a cause for concern) in local contact (bearings, housings, end-caps and clamp rings for example) could be at risk from galvanic corrosion. In this case, a representative “equivalent assembly” or witness sample subassembly may be more useful for assessing the impact of storage.

4.6. Seals Under Loading Creep under load of polymeric seals (O-rings) has been reported as a concern by the recent European spacecraft programmes. This may be quantified at the component level by a load vs. deflection measurement made as a sealed interface is offloaded.

5. Dwell Effects and In-Flight Storage In-flight operation and storage (i.e., prolonged in-vacuum stasis prior to operation—as for example in robotic devices for planetary missions or in devices required for de-orbiting spacecraft at end-of-mission) will bring its own considerations, challenges and mitigations. Where possible for fluid-lubricated mechanisms, the orientation of the spacecraft or mechanism should be selected to minimise its average temperature during the storage period. This is due to lubricant creep, degradation (including due to potential autocatalysis of PFPEs), and the evaporation rate of oils all displaying strong dependencies upon temperature. This measure will inhibit most degradation phenomena and maintain the fluid lubricant within the mechanism to the maximum extent – but may not always be feasible for spacecraft operational reasons. For in-flight storage, the environment can be highly variable dependent on the mission profile. Whilst typically high/ultra-high vacuum and ranging from, say 40 to +80 C, the environment − ◦ for in-flight storage can also encompass more extreme thermal excursions for exposed items. For mechanisms on different types of science missions a full range from ~20K to ~600K and environments including CO2,H2, traces of CH4 or, for LEO, atomic oxygen could be relevant depending on mechanism location and mission type. 13 The vacuum level in GEO is typically of order 10− mbar, however, near a satellite, the pressure 10 is higher, about 10− mbar, as a result of outgassing from the satellite body. Within the spacecraft itself the pressure is yet higher. Calculations have shown, for a Solar Array Drive Mechanism (SADM) 5 for example, that the internal pressure reaches ~10− mbar soon after orbital height is attained and 7 decreases during the first year of operation to about 10− mbar (comprising ~80% water vapour and ~20% carbon monoxide) depending on the balance between the pump-out speed of venting orifices and declining outgassing rates [19]. Furthermore, in the case of reaction wheels there is the possibility of in-flight storage as well as operation under a partial vacuum (believed to be typically around 3 10− mbar).

5.1. Evaporation Effects Over a long duration storage period, the loss of fluid lubricant through evaporation must be considered. This is particularly true in instances where storage temperature is elevated for some or all of the storage period (of greater concern during in-flight storage than on-ground). Such evaporation is generally controlled through labyrinth sealing. Losses due to evaporation can be estimated using the Langmuir equation approach. However, in general, a lubricant may have components of different molecular weights and a range of volatilities. This can render analysis using the Langmuir equation and published vapour pressure data too simplistic, the true situation is that the rate of mass loss, even for nominally single component fluids without additives is dependent on the thermal history of the lubricant. Indeed, there is evidence [18] that the Langmuir equation may predict higher loss rates than found in practice by approximately an order of magnitude at typical operating temperatures. Lubricants 2020, 8, 32 11 of 15

5.2. Fluid Separation and Creep Concern exists regarding the separation of the oils (also known as “oil bleed”) from the grease, allowing migration to other surfaces. This can cause changes in the consistency of the grease and in so doing may alter its tribological properties. In the extreme, it may create lubrication-starved conditions during initial operations. Oil separation usually occurs on static components and is thus likely to occur during prolonged storage. It is considered that when using oils and greases in long-term applications or when mechanisms are subjected to long periods of storage, it is advisable to run the unit in question periodically to redistribute the lubricant if there has been pooling of oil due to oilLubricants separation 2020, 8 or, 32 slumping due to storage conditions and orientation – implicitly this suggests11 of 15 such periodicthe unit “exercising” in question periodically can locally to re-mix redistribute oil back the into lubricant the grease. if there In has this been respect, pooling the of use oil due of creep to oil barriers isseparation also important or slumping as it restricts due to thestorage surface conditions migration and orientation of oil from – theimplicitly lubricated this suggests area. Without such such barriers,periodic oil “exercising” could be lost can irretrievably locally re-mix during oil back prolonged into the grease. storage In from this respect, lubricated the areas.use of creep barriersWhere is also a small important quantity as it of restricts oil is used, the surface 1-g migration migration is of minimised oil from the as lubricated the lubricant area. tendsWithout to remain withinsuch thebarriers, contact oil could and on be thelost surfacesirretrievably of the during component, prolonged due storage to surface from lubricated energy e ffareas.ects. This is true for relativelyWhere small a small quantities quantity of of oil, oil ofis used, order 1-g ~1% migration to <~5% is minimised free-volume as the fill lubricant (i.e., this tends amounts to remain typically to ~1withinµL/mm the bearing contact PCD).and on However, the surfaces for of larger the compon volumeent, fills, due as to might surface be energy used ifeffects. viscous This losses is true are less a concernfor relatively or total small evaporative quantities losses of oil, overof order the ~1% mission to <~5% duration free-volume expected fill (i.e., to bethis high amounts (due typically to, for example, to ~1μl/mm bearing PCD). However, for larger volume fills, as might be used if viscous losses are high-temperature operation and/or long lifetime), migration in 1-g is a much more likely phenomenon less a concern or total evaporative losses over the mission duration expected to be high (due to, for asexample, the mass high-temperature of the fluid body operation is high and/or compared long tolifetime), the retaining migration viscous in 1-g forcesis a much and more substrate likely oil-film surfacephenomenon energetics. as the Such mass 1-g of migrationthe fluid body may is impact high compared the subsequent to the retaining flight lifetime viscous of forces the mechanism and if thesubstrate oil is able oil-film to exit surface the bearing energetics. envelope. Such 1-g migration may impact the subsequent flight lifetime of theDuring mechanism on-ground if the oil storage is able to creep exit the isdriven bearing by envelope. surface energy differentials and also by gravity, and can beDuring minimised on-ground partly storage by control creep of is surfacedriven by features surface (lessenergy likely differentials on smooth and surfaces,also by gravity, or to migrate aroundand can sharp be minimised edges) and partly (primarily) by control by theof surface appropriate features application (less likely of on anti-creep smooth surfaces, coatings or which to have a lowermigrate surface around energy sharp edges) than the and lubricating (primarily) fluid.by the appropriate application of anti-creep coatings which have a lower surface energy than the lubricating fluid. 5.3. Dwell Behaviours of Solid Lubricants 5.3. Dwell Behaviours of Solid Lubricants The influence of in-situ (vacuum and air) dwell periods on the subsequent in-vacuum tribological The influence of in-situ (vacuum and air) dwell periods on the subsequent in-vacuum properties of PVD MoS2 and lead has been investigated at PoD and angular contact bearing level [20]. tribological properties of PVD MoS2 and lead has been investigated at PoD and angular contact bearingWhen level assessed [20]. in pure sliding, exposure to an in-air dwell period acted to increase the start-up frictionWhen coeffi assessedcient of in sputtered pure sliding, MoS exposure2. The rateto an of in-air increase dwell in period friction acted with to increase exposure the time start-up appears to 5 levelfriction off at coefficient exposure ofperiods sputtered of MoS 10 2s,. The demonstrating rate of increase a saturationin friction with effect. exposure Steady-state time appears friction to values arelevel una offffected at exposure by in-air periods dwell of periods,105 s, demonstrating but the recovery a saturation period effect. (i.e., Steady-state the number friction of disc values revolutions requiredare unaffected to re-establish by in-air lowdwell friction periods, in but vacuum) the reco increasesvery period logarithmically (i.e., the number (Figure of disc7). revolutions required to re-establish low friction in vacuum) increases logarithmically (Figure 7).

FigureFigure 7. 7.Start-up Start-up friction friction andand revsrevs toto recoveryrecovery underunder vacuum of of sputtered MoS MoS2 2followingfollowing in-air in-air dwell (datadwell shows (data 1.50shows GPa 1.50GPa Peak Peak Hertzian Hertzian contact contact pressure pressure [20 [20].].

MoS2 lubricated bearings (fitted with PGM-HT cages) displayed higher start-up torque in vacuum as a result of periods of dwell in both laboratory air and vacuum environments [20]. The most significant increase in torque was a factor of ~2.75x higher than the run-in torque (i.e., less than the equivalent effect observed under a pure sliding motion but potentially significant in relation to mechanism torque margins), occurring after an exposure period of 106 s in air. The number of

Lubricants 2020, 8, 32 12 of 15

MoS2 lubricated bearings (fitted with PGM-HT cages) displayed higher start-up torque in vacuum as a result of periods of dwell in both laboratory air and vacuum environments [20]. The most significant increase in torque was a factor of ~2.75 higher than the run-in torque (i.e., less than the equivalent × effect observed under a pure sliding motion but potentially significant in relation to mechanism torque margins), occurring after an exposure period of 106 s in air. The number of revolutions required to recover low torque also increased with increasing dwell time, in a similar manner to that seen at the PoD level. After being correctly run-in in vacuum, PVD lead-lubricated bearings display no measured effects of dwell, either in vacuum or laboratory air, up to 106 s.

5.4. Auto-Catalytic Degradation Perfluorinated oil-based fluid lubricants such as the grease Braycote 601EF or Fomblin Z25 oil are highly susceptible to chemical degradation, whereby a reaction occurs between the fluorine released from the oil by the action of shearing and iron within the substrate steel [21]. This reaction produces the compound FeF3 which further increases the rate of fluid degradation. This reaction acts to destroy the long-chain polymers within the fluid, removing the attractive low shear properties of the fluid, causing a rapid increase in friction coefficient and ultimately restricting the lifetime of these lubricants. Although usually understood to occur during operation (i.e., shearing), it has been postulated that once this reaction has been initiated through the shearing of the fluid (for example during mechanism run-in, acceptance test or initial in-flight commissioning), degradation could potentially continue when the fluid is not being sheared due to the existing presence of FeF3 within the contacts [22]. Such behaviour is referred to as the Autocatalytic Effect (ACE) and could potentially cause continued degradation of PFPE lubricants, following initial shearing, when stationary (e.g., degradation potentially continuing during both ground storage and during periods of in-flight inactivity, such as for missions having a long cruise phase). The concern is that such behaviour could impact the overall lubricant lifetime or produce significantly higher torques upon restart of a mechanism following an extended dwell period either on the ground or in flight. Despite its theoretical feasibility, to date there is limited experimental evidence of auto-catalytic degradation of a PFPE lubricant from tribometer testing. Numerous experimental PoD and SOT campaigns performed by ESTL have been inconclusive in observing this effect in all instances beside those where test conditions were selected to be deliberately harsh and designed to induce autocatalytic degradation [22]. Nevertheless, ACE remains a concern, albeit theoretical, for fluid lubricated mechanisms.

5.5. Periodic Actuation/Exercising At the mechanism level, general practice is to always maintain mechanisms intended for on-ground storage in a dry nitrogen purge and to occasionally “exercise” the mechanism. The term exercise is used to describe the periodic operation on the ground for a low number of cycles in order to recover pre-storage levels of performance, or to detect incipient storage issues at a time when preventative action or rectification measures may be implemented. Whilst the requirement to exercise a mechanism during storage is usually taken as being most applicable to grease or oil lubricated devices for which the exercise operations may re-distribute or replenish the local supply of oil or grease within the tribo-contacts (discussed above), it should also be noted that limited exercising may also be appropriate for solid lubricated mechanisms in order to avoid or minimise the impact of localised lubricant film indentations occasionally reported due to lubricant creep within preloaded contacts. In addition to these effects local to preloaded, lubricated tribo-contacts, consideration must also be given to the avoidance or potential impact of preload spring (or diaphragm) stress-relaxation during storage (an early life effect which can be designed-out). Whilst it is conventional to calculate the required qualification lifetime test duration based upon foreseen on-ground and in-orbit operational cycles, it should be noted that any additional operational Lubricants 2020, 8, 32 13 of 15 cycles required as a result of the need to carry out the periodic exercising of a device during long-term storage on-ground must preferably also be included (and appropriately factored) in the calculation of qualification lifetime. However, this may be practically difficult in some circumstances as the need for long-term storage cannot always be anticipated, it being potentially required due to prior launch failures or other unforeseen circumstances occurring after qualification and delivery of flight hardware. The frequency and duration of exercise operations during ground tests need to be considered carefully. The following considerations are applicable for different applications:

For some applications, a few revolutions or cycles may represent a significant proportion of the • in-flight lifetime. In these cases, the duration should be considerably reduced and may ultimately be replaced by small oscillations through an angle large enough to provide overlap of ball positions on bearing raceways (so preventing local lubricant build-up). In other applications, operation in dry nitrogen or in air for a prolonged duration may generate • excessive particulate debris or lubricant degradation which could compromise subsequent 5 performance (e.g., for lead lubrication operated cumulatively beyond 10 cycles in air, or for MoS2 where even a few revs in the air can greatly impact subsequent in-vacuum lifetime [4]). Therefore, the exercise duration (and environment) should be carefully chosen to minimise such impacts. In some applications, exercising the device will have some unique and potentially high risks • (e.g., requiring de-integration from a spacecraft) which need to be weighed against the benefits obtained from the exercise regime.

Therefore, the main consideration for exercising a device is to perform the minimum cycles to mitigate any storage phenomena and confirm beyond doubt that the unit is in-family with its pre-storage performance. Tribological devices exhibit a range of behaviours depending on the environment, previous test history and many other factors, however, the aim of the mechanism exercise regime is to achieve, upon completion of each exercise event, in-family, in-specification and stable behaviour considering the test environment. This should be close to the behaviour at the commencement of storage assuming the same setup and test conditions.

6. Conclusions The increased interest in interplanetary, lunar and other robotic missions, combined with the need to employ mechanical systems for spacecraft de-orbit or in-orbit servicing seems likely to drive the need for a better understanding of in-orbit storage phenomena. Furthermore, some recent programmes in which the economies associated with batch-builds of sequences of satellites, as well as the occasional un-planned launch delays dictate that a better understanding is required of on-ground storage effects and the potential mitigations. Although some storage best practice guidelines exist (including those discussed above) to date little experimental verification has been performed. Therefore, many concerns exist which cannot easily be allayed by experimental data, indeed where experimental data does exist, this is often “opportunistic” and may provide quite limited statistical confidence. All space lubricants are tribologically susceptible to environmental effects during operation and storage, but with widely differing degrees of severity. As a general statement, the above experimental investigations show that it is important, whenever possible, to operate both solid and fluid lubricated mechanism components under environments representative of their intended application. In addition, the monitoring and understanding of storage and operational environments is extremely important to gain confidence in the tribological performance of spacecraft mechanism components to be used in future ambitious missions.

Author Contributions: M.B. is the main author of the paper with inputs, contributions, and reviews by S.L., A.K. and R.B. Experimental activities, analysis, inspections and data interpretation was performed by all authors. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded in part by ESA-ESTEC. Lubricants 2020, 8, 32 14 of 15

Acknowledgments: This review paper discusses research activities and programs performed by many staff at the European Space Tribology Laboratory, past and present. Additionally, inputs and guidance towards the research was provided by the ESA-ESTEC TEC-MSM team. The authors gratefully acknowledge this support. Conflicts of Interest: The authors declare no conflict of interest.

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