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Tribological Performance of Mos2 Coatings in Various Environments

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Tribological Performance of Mos2 Coatings in Various Environments lubricants 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
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