Tribological Performance of Mos2 Coatings in Various Environments

Article Tribological Performance of MoS2 Coatings in Various Environments

Thomas Gradt 1,* and Thomas Schneider 2

1 Federal Institute for Materials Research and Testing (BAM), 12205 Berlin, Germany 2 AiF Projekt GmbH, 13156 Berlin, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-30-8104-3531

Academic Editor: Ga Zhang Received: 29 February 2016; Accepted: 29 August 2016; Published: 7 September 2016

Abstract: Molybdenum disulﬁde (MoS2) is a well-known solid lubricant for tribosystems running in vacuum or dry gases. Problems arise due to its sensitivity to humidity, which is a drawback for its application under ambient conditions. However, by using a physical vapor deposition (PVD) process, deposition parameters can be optimized not only to gain a coatings structure with favorable frictional properties but also to minimize the sensitivity to attack by water molecules. Therefore, an improved tribological behavior even under moist conditions can be achieved. MoS2 coatings are also candidates for being applied at cryogenic temperatures. They already have proven their suitability, e.g., for sliding support elements between superconducting magnets of the nuclear fusion-experiment Wendelstein 7-X. However, these coatings were exclusively produced for this particular application and the utilization for more common tribosystems may be precluded due to cost considerations. In view of a wider range of applications, pure and Cr containing PVD-MoS2 coatings with an optimized structure were tested under varying environments including hydrogen gas and cryogenic temperatures. Results of the most promising variant are presented in this paper.

Keywords: solid lubrication; MoS2; extreme environments; hydrogen; cryogenic environment

1. Introduction—Lubricating Properties of MoS2

For vacuum and inert gaseous environment, molybdenum disulfide (MoS2) is a well suited and widely applied solid lubricant [1]. In vacuum, solid lubricants of this type show minimum friction coefficients as low as 0.03, and, under certain conditions, even lower. However, a major drawback for the application of MoS2 is its sensitivity to the presence of water vapor. In normal humid air, friction coefficients between 0.15 and 0.30, accompanied by high wear, are observed. For low temperature environment some authors report a minimum temperature of 88 K for special liquid lubricants [2]. However, commercially available oils and greases are only applicable at temperatures above 173 K. Furthermore, the cryogenic liquids themselves are not able to build up hydrodynamic lubrication [3]. Therefore, for components such as bearings, liners, moving seals, and valves, dry sliding, dry rolling or non-contacting systems must be employed. There are some special applications that require smooth sliding surfaces in vacuum environment at cryogenic temperatures. One example is the support structure of large superconducting magnets for nuclear fusion devices [4]. These magnets are cooled by liquid helium, which has a boiling temperature of 4.2 K. During cool down and ramping up of the magnetic field, the superconducting coils experience large forces due to thermal shrinkage of the materials and an increasing electrical current in the magnetic field. This results in a very slow tangential movement of several mm at the supports of the individual coils. Because frictional heat and mechanical shocks are able to initiate a normal conducting zone, a lubricant must be employed that guarantees low friction sliding without stick-slip at these

Lubricants 2016, 4, 32; doi:10.3390/lubricants4030032 www.mdpi.com/journal/lubricants Lubricants 2016, 4, 32 2 of 13 Lubricants 2016, 4, 32 2 of 13 stick-slipsupports. at Tests these have supports. shown Te thatsts have only shown MoS2 thatcoatings only withoutMoS2 coatings any additions without any are ableadditions to meet are suchable torequirements meet such requirements [5]. [5]. SimilarSimilar toto graphite,graphite, MoS MoS2 crystallizes2 crystallizes in in a lamellar a lamellar structure. structure. The MoTheatoms Mo atoms form hexagonalform hexagonal planes planeswith covalently with covalently bonded bonded S atoms S at atoms both sides.at both The sides. spacing The betweenspacing between the Mo and the SatomsMo and is Satoms 0.154 nm, is 0.154between nm, the between Satoms the to Satoms the nearest to the MoS nearest2 plane MoS 0.3082plane nm. 0.308 Between nm. theBetween planes the only planes weak only Van-der-Waals weak Van- der-Waalsforces are acting,forces resultingare acting, in resulting easy shearing in easy tangential shearing to tangential the planes, to whichthe planes, is responsible which is forresponsible the good forlubricating the good properties. lubricating Molecular properties. dynamic Molecular simulations dynamic show simulations that during show sliding that a tribofilm during issliding formed a tribofilmwith basal is orientationformed with of basal the lattice orientation planes of [6 ].the These lattice model planes calculations [6]. These model also show calculations that, with also perfectly show that,aligned with layers, perfectly a minimum aligned friction layers, coefficient a minimum of 0.006 friction is possible, coefficient due of to 0.006 the repulsive is possible, Coulomb due to forces the repulsivebetween theCoulomb sulfur atoms.forces between the sulfur atoms. MoSMoS22 isis applied applied by by burnishing, burnishing, ph physicalysical or or chemical chemical vapor vapor depo depositionsition (PVD, (PVD, CVD), CVD), or or with bindersbinders as anti-friction (AF) coatings. It It can can also also serve serve as as friction reducing component in polymer compositescomposites [1]. [1]. Anti-frictionAnti-friction (AF) coatings coatings consist consist of one one or or more more solid lubricants and, in most most cases, an an organic organic binder.binder. Some Some variants, variants, in inpart particularicular those those containing containing MoS MoS2 or2 polytetrafluoroethyleneor polytetrafluoroethylene (PTFE), (PTFE), are suitableare suitable for low for lowtemperature temperature operation. operation. ManyMany AF AF coatings need a certain running in for achieving the desired properties. One extreme casecase isis shownshown in in Figure Figure1. It1. is It a sprayedis a sprayed MoS 2MoScoating2 coating with polycarbamidewith polycarbamide binder, binder, which was which specially was speciallydesigned designed for space for applications space applications [7]. Figure [7].1 aFigu showsre 1a a shows scanning a scanning electron electron microscopy microscopy (SEM) image(SEM) imageof the virginof the coating,virgin coating, which consistswhich consists of only of loosely only loosely adhered adhered particles. particles. During During loading loading and sliding and slidingthe particles the particles are compacted are compacted and get and a preferredget a preferred order, order, resulting resulting in a veryin a very smooth smooth film film (Figure (Figure1b). 1b).In Figure In Figure2, the 2, friction the friction coefficients coefficients of this coatingof this againstcoating 100Cr6against steel 100Cr6 balls steel (AISI balls 52100) (AISI are 52100) shown are for shownseveral for cryogenic several environmentscryogenic environments [8]. It can [8]. be seenIt can that be itseen provides that it effectiveprovides lubricationeffective lubrication in all of the in allinvestigated of the investigated media. In media. cryogenic In cryogenic environment, environment, friction is considerable friction is considerable lower than atlower room than temperature. at room temperature.The lowest friction The lowest coefficient friction is 0.02 coefficient at 77 K is in 0.02 vacuum. at 77 K In in most vacuum. cases, In higher most loads cases, and higher velocities loads giveand velocitieslower friction, give lower which friction, is a common which feature is a common of MoS 2featurecoatings. of MoS2 coatings.

(a) (b)

FigureFigure 1. AFAF coating coating with with polycarbamide binder binder [7]: [7]: ( (aa)) virgin virgin coating; coating; and and ( (bb)) after after sliding sliding in in liquid liquid heliumhelium (LHe).

By using a PVD process, deposition parameters can be optimized to gain coatings with a more By using a PVD process, deposition parameters can be optimized to gain coatings with a more textured structure orienting the (002) basal planes parallel to the substrate surface [9]. A similar textured structure orienting the (002) basal planes parallel to the substrate surface [9]. A similar concept concept has been used by Koch et al. [5] for coatings developed for support elements in the has been used by Koch et al. [5] for coatings developed for support elements in the superconducting superconducting magnet system of the WENDELSTEIN W7-X fusion experiment. They are able to magnet system of the WENDELSTEIN W7-X fusion experiment. They are able to withstand extreme withstand extreme operational conditions such as temperatures as low as 5 K in combination with operational conditions such as temperatures as low as 5 K in combination with high vacuum. Other high vacuum. Other authors report friction coefficients between 0.015 and 0.06 in liquid nitrogen authors report friction coefﬁcients between 0.015 and 0.06 in liquid nitrogen (LN2) and minimum wear (LN2) and minimum wear on both sliding surfaces for Ti containing PVD-MoS2 coatings [10,11]. on both sliding surfaces for Ti containing PVD-MoS2 coatings [10,11].

Lubricants 2016, 4, 32 3 of 13

Lubricants 2016, 4, 32 3 of 13 Lubricants 2016, 4, 32 3 of 13

Figure 2. Coefficient of friction of a MoS2-AF coating with polycarbamide binder in air and cryogenic Figure 2. Coefﬁcient of friction of a MoS -AF coating with polycarbamide binder in air and Figure 2. Coefficient of friction of a MoS2-AF 2coating with polycarbamide binder in air and cryogenic media [8]. cryogenicmedia media [8]. [8].

A general feature of MoS2 coatings is that the friction coefficient decreases with increasing load, A general feature of MoS2 coatings is that the friction coefficient decreases with increasing load, A general feature of MoS2 coatings is that the friction coefficient decreases with increasing load, whichwhich makes makes them them suitable suitable for for high-loaded high-loaded friction friction contacts.contacts. The The friction friction curve curve of aof PVD a PVD coating coating for for which makes them suitable for high-loaded friction contacts. The friction curve of a PVD coating for such ansuch application an application in cryogenics in cryogenics is isshown shown in in Figure Figure 3. This is is the the type type of of coating coating which which is used is used for for such an application in cryogenics is shown in Figure3. This is the type of coating which is used for the narrowthe narrow support support elements elements in inthe the WENDELSTEIN WENDELSTEIN 7-X magnetmagnet system. system. It wasIt was chosen chosen because because it it providesthe narrowprovides low support lowfriction friction elements without without instic stic thek-slipk-slip WENDELSTEIN effects effects andand needs 7-X nearly magnetnearly no nosystem. running-in running-in It was[12]. [12]. chosenOnly Only the because firstthe first it cycleprovides cycledeviates lowdeviates frictionnoticeably noticeably without from from the stick-slip the following. following. effects During During and needs the nextnext nearly few few cycles nocycles running-in a stablea stable frictional frictional [12]. Onlybehavior behavior the first iscycle establishedis deviates established with noticeably with friction friction from coefficients coefficients the following. in in the the During ra rangenge thebetween next few 0.030.03 cyclesand and 0.06. 0.06. a stable Only Only frictionalvery very small small behavior static static is frictionestablishedfriction peaks withpeaks occur friction occur and and coefficientsno nostick-slip stick-slip in effects theeffects range are are visible. betweenvisible. StaticStatic 0.03 and andand dynamic 0.06.dynamic Only friction friction very have small have almost static almost the friction the same values. No coating failure occurred within the test duration, which was several hundred cycles samepeaks values. occur andNo coating no stick-slip failure effects occurred are within visible. the Static test duration, and dynamic which friction was several have almost hundred the cycles same for this particular application. A further result of this test was that the roughness of the substrate has forvalues. this particular No coating application. failure occurred A further within result the testof this duration, test was which that the was roughness several hundred of the substrate cycles for has this particularonly negligible application. influence A further on the result steady of state this friction test was coefficient. that the Other roughness authors of report the substrate an optimum has only only negligiblesubstrate roughness influence of on R athe = 0.1 steady to 0.15 state µm friction [10]. The coefficient. sample with Other a roughness authors reportof Rz = an2 µmoptimum is negligible influence on the steady state friction coefficient. Other authors report an optimum substrate substrateapproximately roughness within of R thata = range.0.1 to 0.15 µm [10]. The sample with a roughness of Rz = 2 µm is µ µ approximatelyroughness of R withina = 0.1 that to 0.15 range.m [10]. The sample with a roughness of Rz = 2 m is approximately within that range. 0,25 Rz = 2 mic 0,250,2 RzRz = = 2 6-8 mic mic 0,20,15 0,1 Rz = 6-8 mic 0,15 0,05 0,1 0 0,05 -0,05 0-0,1

-0,05friction coefficient ? -0,15 -0,1-0,2

friction coefficient ? -0,15-0,25

-0,2 Figure 3. First friction-0,25 cycles of a sputtered MoS2 coating against Al-bronze in LHe environment; two samples with different substrate roughness; load: 250 N; average sliding velocity: 0.2 mm·s −1 (negative Figurevalues 3. First indicate friction the reciprocating cycles of a motion) sputtered [12]. MoS 2 coating against Al-bronze in LHe environment; Figure 3. First friction cycles of a sputtered MoS2 coating against Al-bronze in LHe environment; two two samples with different substrate roughness; load: 250 N; average sliding velocity: 0.2 mm·s−1 −1 samplesBecause with differentthe application substrate of roughness;MoS2 coatings load: in 250 normal N; average humid sliding air is still velocity: a problem 0.2 mm·s a test (negative program (negative values indicate the reciprocating motion) [12]. valuesfor further indicate optimization the reciprocating by a variation motion) of [12]. deposition parameters for pure MoS2 and doping by Cr atoms was conducted. The results of the best variants are shown in the following. Because the application of MoS coatings in normal humid air is still a problem a test program for Because the application of MoS22 coatings in normal humid air is still a problem a test program further optimization by a variation of deposition parameters for pure MoS and doping by Cr atoms for further optimization by a variation of deposition parameters for pure2 MoS2 and doping by Cr atomswas conducted. was conducted. The results The results of the bestof the variants best variants are shown are shown in the following.in the following.

Lubricants 2016, 4, 32 4 of 13 Lubricants 2016, 4, 32 4 of 13

2. Materials Materials and and Methods Methods

2.1. Test Rig for Tribological MeasurementsMeasurements atat TemperaturesTemperatures below below 120 120 K K Tribological investigations at cryogenic temperatures require special devices (cryotribometers) in respect of of cooling cooling system, system, thermal thermal insulation, insulation, an andd instrumentation. instrumentation. Figure Figure 4 4shows shows an an example example of ofan anapparatus apparatus for forpin-on-disc pin-on-disc tests tests in liquid in liquid helium helium (LHe) (LHe) [13]. [ 13The]. Theliquid liquid coolant coolant is filled is ﬁlled directly directly into intothe sample the sample chamber chamber and the and friction the friction couple couple is immersed is immersed completely completely into the into liquid the liquid cryogen cryogen (bath (bathcryostat cryostat operation). operation). Therefore, Therefore, the environmental the environmental temperature temperature is eq isual equal to the to theboiling boiling temperature temperature Tb Tofb theof the coolant coolant (LN (LN2: T2b:T = b77.3= 77.3 K; LH K;2 LH: Tb2 :T= 20.4b = 20.4K; LHe: K; LHe: Tb = T4.2b = K). 4.2 The K). Theadvantage advantage of this of thismethod method is a isvery a very effective effective cooling cooling of the of sample. the sample. The Thefrictional frictional heat heatis not is only not only removed removed by heat by heatconduction conduction and andconvection, convection, but also but by also evaporation by evaporation of the ofliquid. the liquid. The cryostat The cryostat shown shownin Figure in 4 Figure also allows4 also cooling allows coolingindependently independently of the boiling of the boilingtemperature temperature of the coolan of thet. coolant. In this case, In this the case, coolant the coolant flows through ﬂows through a heat aexchanger heat exchanger and the and sample the sample is surrounded is surrounded by vacuum by vacuum or contact or contact gas. With gas. Withadditional additional heating, heating, it is itpossible is possible to adjust to adjust the thetemperature temperature between between 4.2 4.2 K K(with (with LHe-cooling) LHe-cooling) and and room room temperature independently from from the the pressure pressure in in the the sample sample ch chamber.amber. The The sample sample chamber chamber can can be be evacuated evacuated to to a aresidual residual pressure pressure in in the the order order of of 10 10−−5 5mbarmbar and and therefore, therefore, the the cryotr cryotribometeribometer is is also also suitable for friction tests in high vacuum.

Figure 4. Cryotribometer CT3, suitable for cryogenic, vacuum and H22 environment.

2.2. Materials and Sample Preparation The disc-shaped specimens were coated by an unbalanced magnetron sputter process on a PVD coating plant H-O-T TT 300300 atat thethe ChairChair ofof EngineeringEngineering Design,Design, Friedrich-Alexander-UniversitätFriedrich-Alexander-Universität Erlangen-Nürnberg [14]. [14]. Samp Samplesles were were held held in front in front of the of thetarget target without without additional additional rotation, rotation, thus thusenabling enabling an adjustment an adjustment of a of well a well defined deﬁned target/substrate target/substrate distance. distance. Coating Coating thickness thickness was was kept constant at 1.8 µµmm ±± 0.30.3 µm,µm, adapting deposition time time accordingly. To avoid unintended annealing effects during deposition, the substratesubstrate material consisted of secondary hardened high speed steel 1.3343 (S(S 6-5-2),6-5-2), which which is is tempering-resistant tempering-resistant up up to 450to 450◦C; °C; hardness hardness remained remained at approximately at approximately 62 HRC. 62 HRC. Five different coatings were tested, three variants of pure MoS2 and two Cr containing types Five different coatings were tested, three variants of pure MoS2 and two Cr containing types (MoS2:Cr). One coating denoted as “standard” (STD) was identiﬁed by a preceding extensive variation (MoS2:Cr). One coating denoted as “standard” (STD) was identified by a preceding extensive

Lubricants 2016, 4, 32 5 of 13 of deposition parameters, resulting in different mechanical and tribological properties. The methods andLubricants results of2016 these, 4, 32 tests are reported in detail in Reference [14]. 5 of 13 Before the deposition, the substrates were polished and ultrasonically cleaned in acetone and variation of deposition parameters, resulting in different mechanical and tribological properties. The isopropyl alcohol. Additionally, they were prepared by an argon-ion etching process at 2.3 Pa, methods and results of these tests are reported in detail in Reference [14]. applyingBefore a 600 Vthe pulsed deposition, DC bias the tosubstrates the substrates were poli forshed half and an hour. ultrasonically Argon also cleaned served in acetone as working and gas for sputtering.isopropyl alcohol. Additionally, they were prepared by an argon-ion etching process at 2.3 Pa, applyingThe deposition a 600 V pulsed parameters DC bias for to the the standard substrates coating for half were an hour. a cathode Argon voltagealso served of 800 as working V, a deposition gas ◦ temperaturefor sputtering. of 150 C, an argon pressure of 0.51 Pa and flow rate of 80 sccm, a distance of 90 mm betweenThe the deposition sputter cathode parameters and thefor the sample standard and coating a resulting were cathodea cathode current voltage ofof 2.09800 V, A. a Thisdeposition type was furthertemperature developed of in150 order °C, an to argon produce pressure coatings of 0.51 with Pa higher and flow intrinsic rate of stress. 80 sccm, The a results distance of theof 90 two mm most promisingbetween variants the sputter are showncathode in and the the following. sample and Both a resu types,lting denoted cathode ascurrent HS and of 2.09 LS, hadA. This higher type intrinsicwas stressesfurther than developed the standard in order coating to produce (200 MPa), coatings but with to a higher different intrinsic degree stress. (HS: The 370 results MPa, LS:of the 300 two MPa). For bothmost variantspromising the variants cathode are voltage shown was in the 700 followin V and theg. Both temperature types, denoted 50 ◦C. as For HS the and LS LS, type, had the higher cathode intrinsic stresses than the standard coating (200 MPa), but to a different degree (HS: 370 MPa, LS: 300 distance was reduced to 65 mm and the Ar flow rate increased to 115 sccm. MPa). For both variants the cathode voltage was 700 V and the temperature 50 °C. For the LS type, Additionally two coatings were doped with chromium at a high (120 A) and low (70 A) Cr cathode the cathode distance was reduced to 65 mm and the Ar flow rate increased to 115 sccm. current, respectively, resulting in chromium contents of 10 and 5 at%. In the figures, these variants are Additionally two coatings were doped with chromium at a high (120 A) and low (70 A) Cr referredcathode to as current, CH and respectively, CL. resulting in chromium contents of 10 and 5 at%. In the figures, these variantsThe tribological are referred tests to as were CH carriedand CL. out in ball-on-disc configuration. Hardened 100Cr6 steel balls (AISI 52100)The withtribological a diameter tests were of 4 mmcarried served out in as ball-on-disc counter bodies. configuration. If not otherwise Hardened indicated 100Cr6 steel in the balls figure captions,(AISI the52100) applied with a load diameter was 10of 4 N mm (maximum served as Hertziancounter bodies. contact If not pressure: otherwise 1860.6 indicated MPa) in and the the figure sliding velocitycaptions, was 0.1the msapplied−1. load was 10 N (maximum Hertzian contact pressure: 1860.6 MPa) and the −1 3 slidingThe tests velocity were was carried 0.1 ms out in. high vacuum, in vacuum with H2O vapor at a pressure of 1.4 × 10 Pa, in gaseousThe hydrogen tests were at carried normal out and in hi lowgh vacuum, pressure in (1 vacuum× 104 Pa) with as H well2O vapor as in at liquid a pressure hydrogen. of 1.4 × 103 4 Pa,The in weargaseous coefficients hydrogen were at normal determined and low bypressure stylus (1 profilometry, × 10 Pa) as well measuring as in liquid the hydrogen. wear track at four differentThe positions wear coefficients (shifted by were 90◦). determined The coefficient by stylus of friction profilometry, (COF) wasmeasuring averaged the forwear a slidingtrack at distancefour different positions (shifted by 90°). The coefficient of friction (COF) was averaged for a sliding of approximately 1000 m, excluding the running-in phase. In order to test the wear life, the tests were distance of approximately 1000 m, excluding the running-in phase. In order to test the wear life, the run until coating failure was detected (failure criterion: COF > 0.25). tests were run until coating failure was detected (failure criterion: COF > 0.25). 3. Results 3. Results 3.1. Vacuum Environment 3.1. Vacuum Environment UnderUnder vacuum vacuum conditions conditions all variants all variants showed showed friction friction coefficients coefficients below below 0.1 and 0.1 wear and coefficients wear −7 3 −1 −1 in thecoefficients range of in 1 the to range 3 × 10 of 1 tomm 3 × 10·N−7 mm·m3·N−.1·m As−1. As an an example, example, Figure Figure5 5 showsshows a a friction friction plot plot for for a PVD-MoSa PVD-MoS2 coating2 coating in in high high vacuum. vacuum. TheThe coatingcoating is is similar similar to to the the one one used used in inthe the Wendelstein Wendelstein 7-X 7-X fusionfusion experiment experiment [5]. [5]. The The peaks peaks to to negative negative directiondirection are are caused caused by by test test stops stops and and restarts. restarts. It can It canbe be seenseen that that the the friction friction coefficient coefficient is is around around 0.06 0.06 and and thethe wear life life more more than than 350,000 350,000 cycles. cycles.

Figure 5. Coefficient of friction of a PVD-MoS2 coating [5] under high vacuum conditions (residual Figure 5. Coefﬁcient of friction of a PVD-MoS2 coating [5] under high vacuum conditions (residual −5 −1 pressure:pressure: 2 × 210 × −105 mbar); mbar); counterbody:counterbody: 10-mm 10-mm 100Cr6 100Cr6 ball ball,, load: load: 40 N; 40 sliding N; sliding velocity: velocity: 0.5 ms 0.5. ms−1.

Lubricants 2016, 4, 32 6 of 13 Lubricants 2016, 4, 32 6 of 13

3.2.3.2. AirAir EnvironmentEnvironment withwith 50%50% HumidityHumidity AsAs expected,expected, inin normalnormal air with a wa waterter vapor partial partial pressure pressure of of 1.4 1.4 ×× 10103 Pa,3 Pa, corresponding corresponding to Lubricants 2016, 4, 32 6 of 13 to50% 50% relative relative humidity, humidity, the the COF COF increased increased to tovalues values between between 0.1 0.1 and and 0.25 0.25 and and the the wear wear coefficient coefﬁcient up −5 −5 3 3−1 −−1 −1 upto to103.2. 10 mmAirmm Environment·N ·N·m ,· mwhich with, which50% is Humiditymore is more than than one one order order of of magnitude magnitude higher higher than than underunder vacuumvacuum conditions.conditions. AA typical typical example example of theof the friction friction development development is shown is shown in Figure in 6Figure. However, 6. However, best variants best As expected, in normal air with a water vapor partial pressure of 1.4 × 103 Pa, corresponding to variants showed values in the order−6 of 103·−6 mm−1· 3·N−1−1·m−1, which is comparable to the wear of diamond- showed50% values relative in humidity, the order the of COF 10 increasedmm N to valuesm , whichbetween is 0.1 comparable and 0.25 and to the wear wear coefficient of diamond-like up carbonlike carbonto (DLC)10−5 (DLC)mm coatings,3·N coatings,−1·m−1 developed, which developed is formore operation for than operation one under order under high of magnitude high humidity humidity [higher15]. As[15]. than expected, As under expected, coatingsvacuum coatings that werethat were appropriateconditions. appropriate A for typical ambient for example ambient air conditions of air the conditions friction had a development high had degree a high of isdegree (002)shown basalof in (002) Figure plane basal orientation.6. However, plane orientation. Thisbest was detectedThis variantswas by detected X-ray showed diffraction by values X-ray in the (XRD),diffraction order which of 10 (XRD),−6 wasmm3 ·N carriedwhic−1·mh−1 , wasoutwhich forcarried is the comparable coating out for withto the the thecoatingwear best of diamond-with performance the best inperformance humidlike carbon environment. in (DLC)humid coatings, environment. The result developed is The shown for result operation in is Figure show under7n. in Therefore, high Figure humidity 7. byTherefore, means [15]. As of byexpected, a means suitable coatings of adapteda suitable that were appropriate for ambient air conditions had a high degree of (002) basal plane orientation. PVD-processadapted PVD-process MoS2 coatings MoS2 withoutcoatings further without dopants further can dopants be developed can be thatdeveloped are able that to sufﬁcientlyare able to withstandsufficientlyThis was adverse withstand detected conditions byadverse X-ray such diffractionconditions as high (XRD), such humidity. whicas highh Thewas humidity. resultscarried ofout The the for correspondingresultsthe coating of the with corresponding test the programbest performance in humid environment. The result is shown in Figure 7. Therefore, by means of a suitable andtest theprogram effect and of the the deposition effect of the parameters deposition are para reportedmeters in are detail reported in Reference in detail [14 in]. Reference [14]. adapted PVD-process MoS2 coatings without further dopants can be developed that are able to sufficiently withstand adverse conditions such as high humidity. The results of the corresponding test program and the effect of the deposition parameters are reported in detail in Reference [14].

Figure 6. Example of the friction increase of a MoS2 coating in air with 50% relative humidity. Figure 6. Example of the friction increase of a MoS coating in air with 50% relative humidity. Figure 6. Example of the friction increase of a MoS2 coating in air with 50% relative humidity.

FigureFigure 7. X-ray 7. X-ray diffraction diffraction (XRD) (XRD) patterns: patterns: Dominating Dominating (002) orientation orientation of ofthe the coating coating with with highest highest durabilityFiguredurability 7. X-ray under under diffraction moist moist conditions. conditions. (XRD) patterns: Pattern Patternof Dominatingof the the uncoateduncoated (002) sample sample orientation for for comparison comparison of the [14].coating [14 Reproduced]. Reproducedwith highest withdurability permissionwith permission under from moist from “Surface conditions. “Surfa andce and CoatingsPattern Coatings of Technology”; Technology”;the uncoated published sample for by by Elsevier,comparison Elsevier, 2013. 2013. [14]. Reproduced with permission from “Surface and Coatings Technology”; published by Elsevier, 2013. Figure 8a shows a high resolution transmission electron microscopy (HR-TEM) image of a Figurecoating8 awith shows higher a high internal resolution stress. It transmission still has a highly electron ordered microscopy layered structure (HR-TEM) which image is visible of a coating in Figure 8a shows a high resolution transmission electron microscopy (HR-TEM) image of a with higher internal stress. It still has a highly ordered layered structure which is visible in the coating with higher internal stress. It still has a highly ordered layered structure which is visible in

Lubricants 2016, 4, 32 7 of 13 Lubricants 2016, 4, 32 7 of 13 Lubricants 2016, 4, 32 7 of 13 upperthe upper region. region. The selectedThe selected area area diffraction diffraction (SAED) (SAE imageD) image of Figureof Figure8b 8b clearly clearly shows shows reﬂexes reflexes of of the (002)thethe planes. (002)upper planes. region. The selected area diffraction (SAED) image of Figure 8b clearly shows reflexes of the (002) planes.

(a) (b) (a) (b) Figure 8. (a) High resolution transmission electron microscopy (HR-TEM) and (b) selected area FigureFigure 8. 8.(a ()a High) High resolution resolution transmissiontransmission electronelectron microscopy microscopy (HR-TEM) (HR-TEM) and and (b ()b selected) selected area area diffraction (SAED) images of a MoS2 coating with higher internal stress. diffraction (SAED) images of a MoS coating with higher internal stress. diffraction (SAED) images of a MoS2 2 coating with higher internal stress. 3.3. Low Pressure Water Vapor Environment 3.3.3.3. Low Low Pressure Pressure Water Water Vapor Vapor Environment Environment In order to test the influence of water vapor without the presence of oxygen, only H2O gas was filledIn In orderinto order the to vacuumto test test the the chamber influence influence unt of ofil water watera pressure vaporvapor corresponding withoutwithout the presence to 100% ofrH of oxygen, oxygen,was reached. only only H In H2O 2Figure Ogas gas was 9, was filledfilled into into the the vacuum vacuum chamber chamber until untila a pressurepressure corresponding to to 100% 100% rH rH was was reached. reached. In InFigure Figure 9, 9, it can be seen that, for a MoS2:Cr variant with 5 at% Cr, the friction coefficient stabilized at about 0.17, it canwhichit can be beis seen seenconsiderably that, that, for for a a MoShigher MoS2:Cr2:Cr than variantvariant in inert withwith enviro 5 at% at%nment Cr, Cr, the the but friction friction still lowcoefficient coefficient for solid stabilized stabilized lubricants. at atabout This about 0.17,test 0.17, which is considerably higher than in inert environment but still low for solid lubricants. This test whichclearly is shows considerably that oxidation higher to than MoO in3 is inert not environmentthe only acting but wear still mechanism. low for solid lubricants. This test clearly shows that oxidation to MoO3 is not the only acting wear mechanism. clearly shows that oxidation to MoO3 is not the only acting wear mechanism.

Figure 9. Friction of a MoS2:Cr coating (5 at% Cr) in low pressure water vapor at room temperature. FigureFigure 9. 9.Friction Friction of of a a MoS MoS2:Cr2:Cr coating coating (5(5 at%at% Cr) in in low low pressure pressure water water vapor vapor at at room room temperature. temperature. Figure 10 shows the friction and wear data for all five variants in water vapor. The friction coefficientsFigureFigure 10 of 10 showsthe shows pure the LSthe frictionand friction HS coatings andand wearwear are datadata abou forfort 0.13 allall and fivefive 0.17, variants respectively, in in water water for vapor. vapor.the Cr The containing The friction friction coefficientsvariants.coefficients For of of theall the optimized pure pure LS LS and andcoatings, HS HS coatings coatings the wear areare coefficients aboutabout 0.13 areand and in 0.17, 0.17,the respectively,order respectively, of 10−6 mmfor for the3·N the −Cr1·m Cr containing−1. containing For the −6 3 −1 −1 variants.standardvariants. For coatingFor all all optimizedoptimized both, friction coatings, coatings, and wear the the wear are wear cosiderably coefficients coefficients higher. are arein the This in order the proves order of 10 the of mm lower 10−·N6 mmsensitivity·m3·.N For−1 the ·tom − 1. Forhumiditystandard the standard ofcoating these coating both,variants. friction both, and friction wear andare cosiderably wear are cosiderably higher. This higher.proves the This lower proves sensitivity the lower to humidity of these variants. sensitivity to humidity of these variants.

Lubricants 2016, 4, 32 8 of 13 Lubricants 2016, 4, 32 8 of 13

Lubricants 2016, 4, 32 8 of 13

Figure 10. Friction and wear coefﬁcients of optimized MoS2 coatings vs. 100Cr6 in low pressure H2O Figure 10. Friction and wear coefficients of optimized MoS2 coatings vs. 100Cr6 in low pressure H2O vapor; load: 10 N, sliding velocity: 0.1 ms−1. vapor;Figure load: 1010. N, Friction sliding and velocity: wear coefficients 0.1 ms−1 .of optimized MoS2 coatings vs. 100Cr6 in low pressure H2O vapor; load: 10 N, sliding velocity: 0.1 ms−1. 3.4.3.4. Hydrogen Hydrogen Environment Environment 3.4. Hydrogen Environment Tests in gaseous hydrogen were performed at 100 mbar and 1 bar environmental pressure. Tests Testsin gaseous in gaseous hydrogen hydrogen were were performed performed at at 100 100 mbar mbar and 11 barbar environmental environmental pressure. pressure. All All All variants showed a low COF, comparable to or even lower than the vacuum values. Figure 11 variantsvariants showed showed a low a COF,low COF, comparable comparable to toor or even even lower lower thanthan the vacuumvacuum values. values. Figure Figure 11 showes11 showes showes a friction plot for the reference coationg in H gas at 1 bar. Friction was very stable with a COF a frictiona friction plot for plot the for reference the reference coationg coationg in inH 2H gas2 gas at at 1 21 bar. bar. Friction Friction waswas very very stable stable with with a COF a COF of 0.03 of 0.03 andof 0.03 theand andwear the the wearlife wear with life lifewith 340,000 with 340,000 cycles 340,000 cycles was cycles was satisfactory. satisfactory. was satisfactory. ResultsResultsResults were were were similar similar similar for for forthe the theCr Cr Cralloyed alloyed alloyed types typestypes (Figure (Figure 12),12), againagain again showing showing showing friction friction friction and and andwear wear wear comparablecomparablecomparable to to vacuum vacuum to vacuum conditions. conditions. conditions. The The The wear wear life life life was was was longest longest longest in H in22 gasHgas2 atgas at 1 1bar. at bar. 1 After bar. After about After about 100 about 100 h, or 100h, or h, 430,000or 430,000430,000 cycles, cycles, cycles, the thetest the test wastest waswas stopped stopped without without without showing showing showing coating coating coating failure. failure.

FigureFigure 11. Friction 11. Friction of aof MoS a MoS2 coating2 coating (STD) (STD) vs. 100Cr6 100Cr6 in in H H2 gas2 gas at room at room temperature. temperature.

Figure 11. Friction of a MoS2 coating (STD) vs. 100Cr6 in H2 gas at room temperature.

Lubricants 2016,, 44,, 3232 9 of 13

Lubricants 2016, 4, 32 9 of 13

Figure 12. Friction of MoS2:Cr (type CL) vs. 100Cr6 in H2 gas at room temperature. Figure 12. Friction of MoS2:Cr (type CL) vs. 100Cr6 in H2 gas at room temperature. Figure 12. Friction of MoS2:Cr (type CL) vs. 100Cr6 in H2 gas at room temperature. Figure 13 shows a comparison of the friction and wear behavior between high and low pressure Figure 13 shows a comparison of the friction and wear behavior between high and low pressure hydrogen andand vacuum vacuum environment environment for thefor fourthe variants.four variants. Between Between vacuum vacuum and low pressureand lowhydrogen, pressure hydrogen and vacuum environment for the four variants. Between vacuum and low pressure thehydrogen, differences the differences are small. are Only small. the Only pure the coatings pure coatings with optimized with optimized structure structure showed showed higher higher wear. hydrogen, the differences are small. Only the pure coatings with optimized structure showed higher Inwear. hydrogen In hydrogen gas at normalgas at normal pressure pressure all variants all varian showedts showed higher wear higher with wear the with exception the exception of the standard of the wear. In hydrogen gas at normal pressure all variants showed higher wear with the exception of the coating,standard which coating, showed which a similarshowed wear a similar rate and wear distinctively rate and distinctively lower friction lower than friction in vacuum. than in In vacuum. addition, standard coating, which showed a similar wear rate and distinctively lower friction than in vacuum. the friction of the pure variant with lower intrinsic stress was as low as for the standard variant. InIn addition, addition, the the friction friction ofof thethe purepure variant withwith lower lower intrinsic intrinsic stress stress was was as aslow low as asfor for the the standard standard However,variant.variant. However, However, these results these these are resultsresults based are on based only on twoon onlyonly tests two two for tests tests each for for condition. each each condition. condition. Therefore, Therefore, Therefore, only theonly only general the the tendencygeneralgeneral tendency totendency low friction toto lowlow and frictionfriction wear inand hydrogen wearwear inin environment hydrogenhydrogen environment environment is veriﬁed. Theis isverified. pressureverified. The dependenceThe pressure pressure is lessdependencedependence clear and is is needs less less clear clear to be andand proved needsneeds by to further be provedproved tests. by by further further tests. tests.

Figure 13. Friction and wear coefficients of optimized MoS2 coatings vs. 100Cr6 in vacuum and H2 Figure 13. Friction and wear coefﬁcients of optimized MoS2 coatings vs. 100Cr6 in vacuum and H2 Figureenvironment; 13. Friction load: and 10 N, wear sliding coeffi velocity:cients 0.1 of msoptimized−1; STD:−1 reference MoS2 coatings coating; vs. MoS 100Cr62:Cr: CL: in 5vacuum at% Cr; CH:and H2 environment; load: 10 N, sliding velocity: 0.1 ms ; STD: reference coating; MoS2:Cr: CL: 5 at% Cr; −1 CH:environment;10 at% 10 at% Cr; Cr;pure load: pure coatings 10 coatings N, slidingwith with higher velocity: higher internal 0.1 internal msstress:; stress:STD: LS: 300 reference LS: MPa; 300 HS: MPa; coating; 370 HS: MPa. MoS 370 MPa.2:Cr: CL: 5 at% Cr; CH: 10 at% Cr; pure coatings with higher internal stress: LS: 300 MPa; HS: 370 MPa.

Lubricants 2016, 4, 32 10 of 13 LubricantsLubricants 20162016,, 44,, 3232 10 of 13 3.5. Cryogenic Environment 3.5. Cryogenic Environment 3.5. CryogenicA relatively Environment long endurance was measured for MoS2:Cr (5 at% Cr) at 77 K in liquid nitrogen (FigureA relatively14). In contrast, long endurance in liquid helium,was measured which is for also MoS an2 :Crinert (5 environment,at% Cr) at 77 the K inCOF liquid increased nitrogen to A relatively long endurance was measured for MoS :Cr (5 at% Cr) at 77 K in liquid nitrogen 0.2(Figure after 14).only In 3000 contrast, cycles in (Figure liquid 15).helium, In addition, which isthe also large an2 fluctuationinert environment, indicated the coating COF increasedfailure. to (Figure 14). In contrast, in liquid helium, which is also an inert environment, the COF increased to 0.2 0.2 afterAdditional only 3000 experiments cycles (Figure were 15). conducted In addition, in liquid the large hydrogen fluctuation (LH2 )indicated at 20 K. In coating this environment, failure. after only 3000 cycles (Figure 15). In addition, the large ﬂuctuation indicated coating failure. for mostAdditional samples, experiments the COF was were initially conducted as low in as liquid approximately hydrogen 0.05,(LH2 )while at 20 theK. InCr-containing this environment, types Additional experiments were conducted in liquid hydrogen (LH ) at 20 K. In this environment, yieldedfor most 0.06. samples, However, the COF as can was be initially seen in asthe low friction as approximately curve of Figure 0.05, 16,2 while wear thelife Cr-containingwas short. The types COF for most samples, the COF was initially as low as approximately 0.05, while the Cr-containing types beganyielded to 0.06. rise However, steeply beyond as can be0.1 seen after in only the friction 1000 and curve 3000 of Figurecycles for16, wearall variants. life was A short. maximum The COF in yielded 0.06. However, as can be seen in the friction curve of Figure 16, wear life was short. The COF durabilitybegan to rise was steeply achieved beyond at 4000 0.1 cycles after before only 1000the test and was 3000 aborted cycles at for COF all >variants. 0.25. Again, A maximum the variants in began to rise steeply beyond 0.1 after only 1000 and 3000 cycles for all variants. A maximum in withdurability optimized was achieved structure at showed 4000 cycles no improvemen before the testt in wascomparison aborted toat theCOF standard > 0.25. Again, coating. the variants durability was achieved at 4000 cycles before the test was aborted at COF > 0.25. Again, the variants with optimized structure showed no improvement in comparison to the standard coating. with optimized structure showed no improvement in comparison to the standard coating.

−1 Figure 14. Friction of MoS2:Cr (5 at% Cr) vs. 100Cr6 in LN2, load: 10 N, sliding velocity: 0.1 ms−1. Figure 14. Friction of MoS2:Cr (5 at% Cr) vs. 100Cr6 in LN2, load: 10 N, sliding velocity: 0.1 ms . Figure 14. Friction of MoS2:Cr (5 at% Cr) vs. 100Cr6 in LN2, load: 10 N, sliding velocity: 0.1 ms−1.

−1 Figure 15. Friction of MoS2:Cr (5 at% Cr) vs. 100Cr6 in LHe, load: 10 N, sliding velocity: 0.1 ms . Figure 15. Friction of MoS2:Cr (5 at% Cr) vs. 100Cr6 in LHe, load: 10 N, sliding velocity: 0.1 ms−1.

Figure 15. Friction of MoS2:Cr (5 at% Cr) vs. 100Cr6 in LHe, load: 10 N, sliding velocity: 0.1 ms−1.

Lubricants 2016, 4, 32 11 of 13 Lubricants 2016, 4, 32 11 of 13

FigureFigure 16. FrictionFriction of of MoS MoS2:Cr2:Cr (5 (5at% at% Cr) Cr) vs. vs.100Cr6 100Cr6 in liquid in liquid hydrogen hydrogen (LH2), (LH load:2), 10 load: N, sliding 10 N, slidingvelocity: velocity: 0.1 ms−1 0.1. ms−1.

4.4. Discussion Discussion

The results for the optimized variants show that already pure MoS2 coatings with increased The results for the optimized variants show that already pure MoS2 coatings with increased internalinternal stressstress have have a highera higher durability durability in humidin humid environment. environment. Doping Doping with with Cr does Cr notdoes result not result in lower in frictionlower friction and the and wear the is wear not influenced is not influenced significantly. significantly. The reason The for reason the lower for the sensitivity lower sensitivity to humidity to humidity of the coatings with optimized structure may be the lower number of MoS2 crystallite edges of the coatings with optimized structure may be the lower number of MoS2 crystallite edges at the surface,at the surface, which which are postulated are postulated to be favoriteto be favorite areas areas for water for water adsorption adsorption [1]. Water[1]. Water molecules molecules at the at the crystallite edges may be responsible for blocking crystallite alignment and shearing of the MoS2- crystallite edges may be responsible for blocking crystallite alignment and shearing of the MoS2-planes. Thisplanes. mechanism This mechanism is active is onlyactive at only the surfaceat the surface and crystallite and crystallite edges edges and mayand may be an be explanation an explanation for thefor the fact fact that that Cr-atoms Cr-atoms in thein the bulk bulk materials materials do do not not have have a positivea positive influence. influence. Because, Because, even even forfor thethe bestbest variants,variants, numerousnumerous activeactive sitessites areare presentpresent atat thethe surface,surface, aa lowlow frictionfriction coefficientcoefficient similarsimilar toto thethe vacuumvacuum valuevalue cannotcannot bebe achieved.achieved. In contrast to H2O, low pressure H2 environment at room temperature enhances the friction In contrast to H2O, low pressure H2 environment at room temperature enhances the friction properties of MoS2. For all variants, friction is considerably lower than in humid environment. properties of MoS2. For all variants, friction is considerably lower than in humid environment. DependingDepending onon thethe coatingcoating type,type, wearwear isis unchangedunchanged oror upup toto oneone orderorder ofof magnitudemagnitude lower.lower. InIn normalnormal pressurepressure hydrogen,hydrogen, allall newnew variantsvariants showshow aa slightlyslightly higherhigher frictionfriction andand aa wearwear raterate similarsimilar toto humidhumid environment.environment. However,However, thethe wearwear lifelife isis alsoalso longlong inin hydrogen gasgas at 1 bar. Because, for such coatings, mostmost ofof thethe filmfilm thicknessthickness isis removedremoved duringduring runningrunning in,in, wearwear raterate isis notnot directlydirectly relatedrelated toto thethe wearwear life.life. Contrary Contrary to to humid humid environmen environment,t, in in hydrogen, hydrogen, a a remaining remaining thin thin layer layer seems seems to to be be stable stable for for a along long time. time. TheThe differencesdifferences inin thethe tribologicaltribological behaviorbehavior ofof allall testedtested variantsvariants betweenbetween thethe differentdifferent environmentsenvironments show that that oxidation oxidation is is not not the the only only mechanism mechanism responsible responsible for forincreasing increasing friction friction and wear of MoS2. H2O molecules seem to impede the alignment and shearing of the MoS2 layers resulting and wear of MoS2.H2O molecules seem to impede the alignment and shearing of the MoS2 layers in higher friction. In contrast, H2 may act as termination of reactive edges of the MoS2 planes, which resulting in higher friction. In contrast, H2 may act as termination of reactive edges of the MoS2 planes, is a mechanism similar to hydrogen in carbon coatings [15]. As a second effect, H2 may trap residual which is a mechanism similar to hydrogen in carbon coatings [15]. As a second effect, H2 may trap residualoxygen in oxygen the environment. in the environment. ThereThere isis nono straightstraight forwardforward explanationexplanation forfor thethe veryvery shortshort wearwear lifelife ofof allall testedtested coatingscoatings inin LHeLHe and LH2; in particular, because at 77 K in liquid nitrogen, more than 100,000 friction cycles with a and LH2; in particular, because at 77 K in liquid nitrogen, more than 100,000 friction cycles with a COF COF not higher than 0.03 are still possible. However, in earlier tests with other MoS2 variants, not higher than 0.03 are still possible. However, in earlier tests with other MoS2 variants, deviations in thedeviations tribological in the behavior tribological between behavior 77 and between 4.2 K were 77 and also 4.2 detected K were [16 also]. One detected reason [16]. for One early reason failure for in early failure in LHe and LH2 may be thermal expansion mismatch between the coating and the LHe and LH2 may be thermal expansion mismatch between the coating and the substrate. The stress causedsubstrate. by thisThe effectstress increasescaused by with this decreasingeffect increase temperature.s with decreasing For a verification temperature. of this For effect, a verification tests with of coatingsthis effect, with tests different with coatings internal with stresses different are planned. internal stresses are planned.

Lubricants 2016, 4, 32 12 of 13

5. Conclusions

Under humid environmental conditions, MoS2 usually exhibits high friction and short wear life. Improvements can be achieved by optimizing the deposition process in order to an orientation of the (002) basal plane parallel to the substrate surface or by doping with metal atoms, e.g., Cr. Gaseous H environment at room temperature enhances the tribological properties of MoS2 coatings. Friction and wear are low and comparable to the vacuum values under different hydrogen gas pressures at room temperature. Early coating failure is observed in liquid hydrogen with a boiling temperature of 20 K. However, in liquid nitrogen at 77 K, friction is low and wear life is satisfactory. In liquid hydrogen as well as in liquid helium (4.2 K), friction is only low at the very beginning of sliding. After a few friction cycles, all variants showed early failure. Thus, in this extreme temperature range, thermal mismatch seems to dominate the tribological behavior and should be accounted for in future. The initially low COF could motivate more elaborate testing in the future on this topic as well as on wear reduction. Operating components such as pumps and valves reliably in (liquid) hydrogen could become more important as hydrogen gains momentum as medium for energy storage and conversion.

Acknowledgments: This work is funded by the German Research Foundation (DFG) as a part of the project Tribological Optimization of Molybdenum Disulfide PVD-Coatings for Varying Environmental Conditions (GR 1002/8-1, ME 1029/17-1, SZ 258/1-1). The coatings were provided Bernd Vierneusel, researcher at the Chair of Engineering Design, Friedrich-Alexander-Universität Erlangen-Nürnberg. The HR-TEM and SAED images were contributed by Werner Österle of BAM, Division 5.1, Materialography, Fractography and Ageing of Engineering Materials. Author Contributions: Thomas Schneider designed and carried out the experiments in vacuum, inert, and hydrogen environment. Thomas Gradt wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript: COF Coefﬁcient of friction LN2 Liquid nitrogen LH2 Liquid hydrogen LHe Liquid helium PVD Physical vapor deposition CVD Chemical vapor deposition HR-TEM High resolution transmission electron microscope SAED Selected area electron diffraction RT Room temperature STD reference or standard coating HS coating variant with an internal stress of 370 MPa LS coating variant with an internal stress of 300 MPa CH MoS2:Cr coating with 10 at% Cr CL MoS2:Cr coating with 5 at% Cr

References

1. Landsdown, A.R. Tribology Series 35: Molybdenium Disulphide Lubrication; Dowson, D., Ed.; Elsevier: Amsterdam, The Netherlands, 1999. 2. Kragelsky, I.V.; Alisin, V.V. Chapter 14 “Friction at low temperatures”. In Tribology Handbook, Friction Wear Lubrication; Pergamon Press: Oxford, UK, 1981; pp. 75–92. 3. Bozet, J.-L. Modelling of friction and wear for designing cryogenic valves. Tribol. Int. 2001, 34, 207–215. [CrossRef] 4. Gasparotto, M.; Elio, F.; Heinemann, B.; Jaksic, N.; Mendelevitch, B.; Simon-Weidner, J.; Streibl, B. The WENDELSTEIN 7-X mechanical structure support elements: Design and tests. Fusion Eng. Des. 2005, 74, 161–165. [CrossRef]

5. Koch, F.; Nocentini, R.; Heinemann, B.; Linding, S.; Junghans, P.; Bolt, H. MoS2 coatings for the narrow support elements of the W-7X nonplanar coils. Fusion Eng. Des. 2007, 82, 1614–1620. [CrossRef] Lubricants 2016, 4, 32 13 of 13

6. Morita, Y.; Onodera, T.; Suzuki, A.; Sahnoun, R.; Koyama, M.; Tsuboi, H.; Hatakeyama, N.; Endou, A.; Takaba, H.; Kubo, M.; et al. Development of a new molecular dynamics method for tribochemical reaction

and its application to formation dynamics of MoS2 triboﬁlm. Appl. Surf. Sci. 2008, 254, 7618–7621. [CrossRef] 7. Yukhno, T.P.; Vvedenskij, Y.V.; Sentyurikhina, L.N. Low temperature investigations on frictional behaviour and wear resistance of solid lubricant coatings. Tribol. Intern. 2001, 34, 293–298. [CrossRef] 8. Gradt, T.; Hübner, W.; Ostrovskaya, Y. Tribologisches Verhalten von Gleitlacken für kryogene Anwendungen. Tribol. Schmier. 2004, 51, 37–40. 9. Zhang, X.; Prakash, B.; Lauwerens, W.; Zhu, X.; He, J.; Celis, J.-P. Low friction MoSx coatings resistant to wear in ambient air of low and high humidity. Tribol. Lett. 2003, 14, 131–135. [CrossRef] 10. Roberts, E.W. Thin solid lubricant ﬁlms in space. Tribol. Int. 1990, 23, 95–104. [CrossRef] 11. Subramonian, B.; Kato, K.; Adachi, K.; Basu, B. Experimental evaluation of friction and wear properties of solid lubricant coatings on SUS440C steel in liquid nitrogen. Tribol. Lett. 2005, 20, 263–272. [CrossRef] 12. Gradt, T.; Aßmus, K. Tribological behaviour of solid lubricants at low Temperatures. In Proceedings of the 21st International Cryogenic Engineering Conference (ICEC 21), Prague, Czech Republic, 17–21 July 2006; pp. 173–176. 13. Gradt, T.; Börner, H.; Schneider, T. Low temperature tribometers and the behaviour of ADLC-Coatings in cryogenic environment. Tribol. Int. 2001, 34, 225–230. [CrossRef]

14. Vierneusel, B.; Tremmel, S.; Wartzack, S. Humidity resistant MoS2 coatings deposited by unbalanced magnetron sputtering. Surf. Coat. Technol. 2013, 235, 97–107. [CrossRef] 15. Ronkainen, H.; Holmberg, K. Tribology of Diamond-Like Carbon Films; Donnet, C., Erdemir, A., Eds.; Springer: New York, NY, USA, 2008; pp. 155–200. 16. Gradt, T.; Schneider, T.; Lingertat, J.; Junghanns, P.; Aßmus, K.; Schauer, F. Cryogenic vacuum tests of scaled down narrow support elements for the W7-X coil system. Fusion Eng. Des. 2009, 84, 840–884. [CrossRef]

© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).