Relationship of Endurance to Microstructure of IBAD MoS2 Coatings

The endurance of ion-beam-assist-deposited (IBAD) MoS₂ is correlated with coating structure and orientation. Structure and orientation are determined by X-ray diffraction, while endurance is measured with a ball-on-disk tribometer operating in dry air. The IBAD MoS₂ coatings contain both crystalline and non-crystalline components. Only two orientations of the crystalline component are observed: the (001) planes parallel (basal orientation) and perpendicular (edge orientation) to the surface. Endurance of the coating is not related to coating thickness or substrate chemistry, but is related to the relative amount of the two MoS₂ crystal orientations. Coating endurance decreases with increasing edge (100) intensity. Furthermore, MoS₂ coatings with poor crystallinity exhibit good endurance. These results are discussed in terms of a possible oxidative wear mechanism and stress-induced basal re-orientation of the non-crystalline IBAD MoS₂.     

Tribological behaviour and wear mechanism of MoS2–Cr coatings sliding against various counterbody

MoS₂–Cr coatings with different Cr contents have been deposited on high speed steel substrates by closed field unbalanced magnetron (CFUBM) sputtering. The tribological properties of the coatings have been tested against different counterbodies under dry conditions using an oscillating friction and wear tester. The coating microstructures, mechanical properties and wear resistance vary according to the Cr metal-content. MoS₂ tribological properties are improved with a Cr metal dopant in the MoS₂ matrix. The optimum Cr content varies with different counterbodies. Showing especially good tribological properties were MoS₂–Cr8% coating sliding against either AISI 1045 steel or AA 6061 aluminum alloy, and MoS₂–Cr5% coating sliding against bronze. Enhanced tribological behavior included low wear depth on coating, low wear width on counterbody, low friction coefficients and long durability.     

Tribochemistry of MoS3 Nanoparticle Coatings

The tribology of nanoparticles based on transition metal dichalcogenides has been studied extensively. However, evaluation of metal chalcogenides with other stoichiometries has been lacking. We have studied the friction, endurance, and tribochemistry of bonded molybdenum trisulfide (MoS₃) nanoparticle-based coatings for the first time. A facile aqueous chemistry method was used to fabricate the MoS₃ nanoparticles. Pin-on-disk tribometry of an MoS₃ coating using phenolic resin as the binder was conducted in a dry N₂ atmosphere (0.06 % RH, using normal loads of 5 N and 10 N). The results were compared with two types of commercial bonded coatings based on the solid lubricant molybdenum disulfide (MoS₂), as well as a bonded coating we formulated with commercial MoS₂ nanoparticles. Surprisingly, the MoS₃ coating showed similar lubricating ability to the MoS₂-based coatings, exhibiting average μ _k < 0.05 and endurance greater than a million cycles. To evaluate the tribochemistry occurring in the contact region, tribotesting of an MoS₃ coating was halted when steady-state low friction was achieved (i.e., prefailure). Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction on the surface of this wear track showed that the MoS₃ had undergone a tribochemical reaction to form the solid lubricant MoS₂, which explains the excellent lubricity of the coating. This result opens up the possibility of developing MoS₃ nanoparticle-based solid lubricant coatings and MoS₃ nanoparticle additives for oils and greases that are synthetically easier and lower cost than formulations based on MoS₂ nanoparticles.     

Microstructural and wear properties of sputtered carbides and silicides

Sputtered Cr₃C₂, Cr₃Si₂ and MoSi₂ wear-resistant films (0.05–3.5 μm thick) were deposited on metal and glass surfaces. Electron transmission, electron diffraction and scanning electron microscopy were used to determine the microstructural appearance. Strong adherence was obtained with these sputtered films. Internal stresses and defect crystallographic growth structures of various configurations within the film have progressively more undesirable effects for film thicknesses greater than 1.5 μm. Sliding contact and rolling element bearing tests were performed with these sputtered films. Bearings sputtered with a duplex coating (a 0.1 μm thick undercoating of Cr₃Si₂ and subsequently a 0.6 μm coating of MoS₂) produced marked improvement (more than 10.5 × 10⁷ cycles) over straight MoS₂ films.     

Modern solid lubrication: Recent developments and applications of MoS2

An important and growing field of lubrication lies in the use of solid films, although they are in general more expensive than oils or greases, and require specialist attention both in mechanical design and in coating application techniques. In this paper, the general classification of solid lubricant types is reviewed, along with the reasons for choosing, and methods of depositing, solid lubricants, in particular MoS₂. The best‐performing and most flexible technique for making MoS₂ films is by physical vapour deposition (PVD), and the variants of that technology are considered. The intrinsically lubricating, lamellar structure of pure MoS₂ is described, along with a brief summary of its wear and failure modes. Present applications for lubrication by MoS₂ in spacecraft and dry machining are outlined, as are anti‐adhesive uses in extruding and moulding. The current state of the art of modification of MoS₂ films consists in the addition of dopants (co‐sputtering), in multilayering as a series of films, each fulfilling a specific task, or in stacking repeating nano‐metre‐scale films. Composite films of MoS₂ islands in a hard film matrix are also being developed.     

Environmental Effects on the Composition of Surface Films Produced by an Organo-Molybdenum Compound©

The influence, of environments on the properties of the surface films formed, with molybdenum dithiocarbamate (MoDTC) was examined under a reciprocating sliding condition. MoS₂ was formed on the rubbing surfaces in air and oxygen, and molybdenum compounds with an oxidation state lower than MoS₂ were produced in nitrogen and argon. The surface film composed of MoS₂ was effective in reducing the friction and wear, while the molybdenum compound formed in nitrogen or argon had no ability to prevent direct contact between the rubbing surfaces and to reduce the friction. It was a necessary condition for forming the surface film composed of MoS₂ that the environment to be rubbed contained oxygen at a concentration above a certain level.     

Microtribological Performance of Au–MoS2 and Ti–MoS2 Coatings with Varying Contact Pressure

Solid lubricant coatings with co-sputtered metal and MoS₂ have shown favorable macrotribological properties at a wide range of contact stresses and humidity levels. These materials are also candidates for use in microcontacts and micro-electromechanical systems (MEMS), but their performance at this scale is poorly understood. For this study, microtribological properties of Au–MoS₂ and Ti–MoS₂ coatings, with varying metal additives of less than 15 at%, were examined using a nanoindentation instrument. Titanium and gold were chosen for this study as metal additives due to their different influence on the mechanical properties of the coating. The hardness and reduced modulus of the coatings increased with the addition of metal, when compared to pure MoS₂. Reciprocating microscratch tests were performed with two spherical diamond tips (50 and 10 μm radii) in dry air. A range of normal loads were used between 0.2 and 5.0 mN. Friction and wear measurements were analyzed with respect to the variation in the contact pressure and compared to literature studies performed at the macroscale. Correlations were found between the coating mechanical properties, tip-coating adhesion, interfacial shear strength, and the formation of transfer films and tribofilms.     

Friction,Wear, and Evaporation Rates of Various Materials in Vacuum to 10−7 mm Hg

Evaporation data on soft metals, lubricating inorganic compounds, and various reference materials are reported for temperatures from 75 to 1000 F in vacuum as low as 10^?7 mm Hg. Observations on modes of vacuum degradation (e.g., evaporation or dissociation) and methods of experimentation are related. Friction and wear data are presented for several unlubricated metals (e.g., type 440-C steel) and metals coated with inorganic (e.g., MoS₂, CaF₂) as well as with soft metal films in vacuum at ambient pressures between 10^?6 and 10^?7 mm Hg.     

Effect of deposition pressure on microstructure and tribological behavior of MoS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/MoS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-Mo nanoscale multi-layer films

MoS _x /MoS _x -Mo multi-layer films consisted of several bilayers and a surface layer on steel substrate were deposited by d.c. magnetron sputtering at different deposition pressures. Each bilayer contained a MoS _x layer with 80 nm in thickness and a MoS _x -Mo composite layer with 20 nm in thickness. With the increase of deposition pressure, the perpendicular orientation of the basal plane prevailed while the parallel orientation decreased. The tribological properties of the multi-layer films were investigated by using a ball-on-disk tribometer both in vacuum and in humid air. The multi-layer film deposited at 0.24 Pa had a compact, consistent layered structure with high intensity of (002) plane and low S content compared to the others deposited at 0.32 and 0.40 Pa, and showed the lowest friction coefficient and wear rate in humid air.     

Tribology of Co-sputtered Nanocomposite Au/MoS2 Solid Lubricant Films over a Wide Contact Stress Range

Co-sputtered nanocomposite metal/MoS₂ solid lubricant films are traditionally used in high contact stress applications (typically around 1 GPa) because they are hard and conform to the Hertzian contact model, i.e., the coefficient of friction () decreases with increasing contact stress. We are investigating whether appropriate modifications can be made in these films that could make them also work in low contact stress applications, especially sliding electrical contacts (e.g., slip rings) that would benefit from the higher conductivity and environmental robustness of these films. To this end, and also more generally to increase our understanding of how film composition affects performance, we studied the friction and endurance of co-sputtered Au/MoS₂ films in sliding contact in N₂ gas at two vastly different contact stresses, 730 and 0.1 MPa. Seven different film compositions were studied, with Au contents in the range 42-100 at.%, as well as pure MoS₂. The results showed that co-sputtered Au/MoS₂ films outperformed both pure sputtered MoS₂ films and pure sputtered Au films. Optimum films at high contact stress (i.e., those that exhibited the lowest and highest endurance) were films with lower Au contents (i.e., 42 and 59 at.% Au). In contrast, at low contact stress, films with moderately high Au contents (i.e., 75 and 89 at.% Au) performed the best. For films that did not fail by the end of the 2000 m test, Auger Nanoprobe analysis revealed that lubrication was provided by a thin film (1 nm thick) of relatively pure MoS₂, regardless of the contact stress. Based on these results, we hypothesize that at high contact stresses, the low Au content provides optimum amounts of MoS₂ in the contact region, while at low contact stresses, the higher Au contents limit the amount/size of MoS₂ particles that are transferred to the opposing surface, providing a thinner, more uniform transfer film. The results indicate that with appropriate optimization of the metal:MoS₂ ratio, co-sputtered nanocomposite metal/MoS₂ films can be applied to a much wider range of contact stresses than previously studied.     

Effects of Surface Pretreatment of Ti-6AI-4V on the Adhesion and Wear Lifetimes of Sputtered MoS2 Coatings

Improving the adhesion and wear endurance lifetimes of the solid lubricant molybdenum disulfide (MoS₂) on titanium (Ti) alloys was studied in this experimental investigation. Ti-6Al-4V alloy specimens were implanted with gas ions or coated with ceramic layers prior to coating with sputtered MoS₂ to investigate the adhesion and wear lifetimes of the MoS₂ coatings. The greatest improvement in scratch adhesion (2.4 times Ti-6Al-4V coated directly with MoS₂) was observed for an MoS₂/diamond-like carbon/Si multilayer coating. Sliding wear tests revealed the greatest lifetime improvement (3.2 ×) was for an MoS₂/TiC dual-layer coating. Increased MoS₂ adhesion was observed for pretreated surfaces with a Vickers microhardness greater than 800 kgf/mm². Increased adhesion of MoS₂ for bond layers with lower elastic moduli (estimated) is suggested. Therefore the ratio hardness/elastic modulus may be a potential figure of merit for surface pretreatment selection.     

The Role of Cerium in the Resistance of an MoS2-Containing Composite Brush Plating Layer to Humid Atmosphere

MoS₂ is an excellent solid lubricant widely used for reducing friction. However, moisture is very harmful to its solid lubrication property because MoS₂ is easily oxidized to form Mo⁶⁺ and S⁶⁺ in a humid atmosphere. In order to improve its oxidation resistance, a study on the role of a rare earth element Ce in the resistance of a Ni-Cu-P/Mo₂ brush plating layer to the humid atmosphere was carried out. It was found that cerium can effectively stabilize the solid lubrication property of MoS₂ due to its preferential adsorption on the surface of MoS₂ particles, the adsorbed layer serving as a barrier to oxidation. This study shows that the rare earth element Ce can be deposited from a water plating solution.     

Advances in solid lubrication with MoS2 multilayered coatings

The general classification of solid lubricant types is reviewed, along with the reasons for choosing and methods of depositing solid lubricants, in particular MoS₂. The best‐performing and most flexible technique for making MoS₂ films is by physical vapour deposition (PVD), and the variants of that technology are considered. The intrinsically‐lubricating, lamellar structure of pure MoS₂ is described, along with a brief summary of the wear and failure modes. Present applications for lubrication by MoS₂ in spacecraft and dry machining are described. Anti‐adhesion uses in extruding and moulding are also mentioned. The current modification of MoS₂ films is by addition of dopants (co‐sputtering), by multilayering as a series of films each fulfilling a specific task, or by stacking repeating nanometre‐scale films. Composite films of MoS₂ islands in a hard film matrix are also being developed.     

Microstructure, chemical and tribological investigations of Mo x W1−x S y co-sputtered composite films

Mo _x W_1−x S _y composite films were co-sputtered by the combination of MoS₂ and WS₂ targets, which were shown to have much superior tribological performance with lower and more stable friction coefficient, longer durability and higher bearing resistance than pure MoS₂ films in room temperature air with a relative humidity of 45–50%. Especially for the Mo_0.6W_0.4S_1.6 (40 at.% WS₂) composite film, an increase in durability of more than a one order of magnitude was reached. X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy were used to investigate the relationship between the microstructure and the tribological performance of the films. The composite films are shown to have a densified structure and accordingly improved oxidation resistance and lubrication properties. Moreover, the composite films have a lattice expansion in the c direction, along with a reduced the friction within the films.     

Multi-environmental lubrication performance and lubrication mechanism of MoS2/Sb2O3/C composite films

MoS₂–Sb₂O₃–C composite films exhibit adaptive behavior, where surface chemistry changes with environment to maintain the good friction and wear characteristics. In previous work on nanocomposite coatings grown by PVD, this type of material was called a “chameleon” coating. Coatings used in this report were applied by burnishing mixed powders of MoS₂, Sb₂O₃ and graphite. The solid lubricant MoS₂ and graphite were selected to lubricate over a wide and complementary range including vacuum, dry air and humid air. Sb₂O₃ was used as a dopant because it acts synergistically with MoS₂, improving friction and wear properties. The MoS₂–Sb₂O₃–C composite films showed lower friction and longer wear life than either single component MoS₂ or C film in humid air. Very or even super low friction and long wear-life were observed in dry nitrogen and vacuum. The excellent tribological performance was verified and repeated in cycles between humid air and dry nitrogen. The formation of tribo-films at rubbing contacts was studied to identify the lubricating chemistry and microstructure, which varied with environmental conditions. Micro-Raman spectroscopy and Auger electron spectroscopy (AES) were used to determine surface chemistry, while scanning electron microscopy and transmission electron microscopy were used for microstructural analysis. The tribological improvement and lubrication mechanism of MoS₂–Sb₂O₃–C composite films were caused by enrichment of the active lubricant at the contact surface, alignment of the crystal orientation of the lubricant grains, and enrichment of the non lubricant materials below the surface. Sb₂O₃, which is not lubricious, was covered by the active lubricants (MoS₂ – dry, C – humid air). Clearly, the dynamics of friction during environmental cycling cleaned some Sb₂O₃ particles of one lubricant and coated it with the active lubricant for the specific environment. Mechanisms of lubrication and the role of the different materials will be discussed.     

Effect of Atmosphere and Temperature on the Tribological Behavior of the Ti Containing MoS<Subscript>2</Subscript> Coatings Against Aluminum

MoS₂ coatings exhibit low coefficient of friction (COF) when sliding against aluminum; however, the magnitudes of their COF show high sensitivity to environmental conditions. Ti could reduce the sensitivity of the frictional behavior of MoS₂ coatings to moisture. This study examines the tribological properties of Ti containing MoS₂ coating (Ti–MoS₂) tested against an aluminum alloy (Al-6.5% Si) in ambient air (58% relative humidity, RH), dry oxygen, dry air and dry N₂ (< 4% RH) atmospheres. The Ti–MoS₂ coating exhibited similar COF values under an ambient (0.14), dry oxygen (0.15) and dry air (0.16) atmospheres. It was found that oxidation of MoS₂ to MoO₃ was responsible for high COF under these testing conditions as revealed by Energy-dispersive X-ray Spectroscopy (EDS) and micro-Raman spectroscopy. However, a low and stable COF of 0.07 was observed under a dry N₂ condition. This work further showed that the tests performed at elevated temperatures, up to 400 °C in a dry N₂ atmosphere sustained the low and stable COF of the Ti–MoS₂ coatings. The sliding tests performed under a dry N₂ atmosphere prevented the formation of MoO₃ and as a result, the Ti–MoS₂ coatings maintained low COF values. Low COF values were also attributed to the formation of MoS₂ transfer layers.     

Ion bombardment of MoS2 nanoplateletcoatings deposited by electrospraying

The present paper reports on a novel process for deposition ofMoS₂ lubricating coatings by electrospraying aball-milled platelet suspension of stoichiometric MoS₂particles. Ultralow friction coefficients in the range of0.010-0.020, in a dry-nitrogen atmosphere, were obtained with areciprocating ball-on-disk tribometer for a stainless-steel ballsliding on a MoS₂-coated silicon substrate. Thesliding endurance for MoS₂, as deposited, was measuredfor thicknesses ranging from 0.28 to 1.00 m tobe between 0.6 and 44 m. The effect of a low-dose, high-energy,argon-ion bombardment was studied. Thus an ion bombardment with400 keV argon ions to a dose of 18 x 10¹⁵ions/cm² increased the sliding endurance by a factorof about thirty, for a coating thickness of 0.3m.     

Evaluation of Ion-Sputtered Molybdenum Disulfide Bearings for Spacecraft Gimbals

High-density, sputtered molybdenum disulfide films (MoS₂) were investigated as lubricants for the next generation of spacecraft gimbal bearings where low torque signatures and long life are required. Low friction in a vacuum environment, virtually no out-gassing, insensitivity to low temperature, and radiation resistance of these lubricant films are valued in such applications. One hundred and twenty five thousand hours of accumulated bearing lest time were obtained on 24 pairs of flight-quality bearings ion-sputtered with three types of advanced MoS₂ films. Life tests were conducted in a vacuum over a simulated duty cycle for a space pay bad gimbal. Optimum retainer and ball material composition were investigated. Comparisons were made with test bearings lubricated with liquid space lubricants.

Self-lubricating PTFE retainers were required for long life, i.e., > 40 million gimbal cycles. Bearings with polyimide retainers, silicon nitride ceramic balls, or steel balls sputtered with MoS₂ film suffered early torque failure, irrespective of the type of race-sputtered MoS₂ film. Failure generally resulted from excess film or retainer debris deposited in the ball track which tended to jam the bearing. Both grease lubricated and the better MoS₂ film lubricated bearings produced long lives, although the torque with liquid lubricants was lower and less irregular.     

Fabrication and evaluation of D-gun sprayed WC–Co coating with self-lubricating property

A WC–Co coating with self-lubricating property was deposited by detonation gun (D-gun) process, using a commercial WC–Co powder doped with a MoS₂–Ni powder, under a proper spray condition. It is proved that the MoS₂ composition in the feed powder was kept, which is attributed to the protection of Ni around it, and its content is a little higher in the resulting coating. Evaluation on sliding wear property indicates that the MoS₂ composition plays an important role in lowering both coefficient of friction and wear rate for the resulting coating, which is confirmed by observations on wear track, as well as X-ray photoelectron spectroscope (XPS) results on worn surface. It suggests that the deposition of WC–Co coating with self-lubricating property by D-gun spray is feasible by controlling lubricant powder and spray conditions, which can exhibit higher sliding wear resistance.