Solid Film Lubrication at High Temperature

The behavior of several solid film lubricants has been experimentally established as a function of temperature. These films are formed of a suspension of lubricant particles (graphite, molybdenum disulfide) in a thermosetting resin baked onto a hard surface. The test consists of heating such a layer and continually observing the friction coefficient until failure. Sliding conditions of high unit loading and low rubbing velocity were chosen to provide a comparison between the films. They were evaluated as to the highest temperature for which the friction coefficient remained low and subsequently as to the number of load cycles sustained at temperature before an abrupt increase in friction. Results for all films were similar in that failure was caused by a breakdown of the binder rather than the lubricating particles and that the minimum friction coefficient (～ 0.03) was observed just before the film failure at temperatures as high as 1200° F.     

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.     

Lubrication by Transferred Films of Solid Lubricants

Experiments are described to determine the lubricating ability of various solid films, particularly graphite and molybdenum disulfide, transferred from a compact to a steel surface. The amount of lubricant transferred depends markedly on the surface finish of the steel, and the most effective films are formed on relatively rough surfaces. In these conditions, bonding between the film and its substrate appears to be primarily mechanical. Determinations of the scuffing load and endurance tests show that replenishment of the lubricant film by continuous transfer to compensate for wear is only possible to a limited extent. With molybdenum disulfide, the film ultimately wears away, but with graphite, failure occurs when the mean surface temperature exceeds about 100 C. The load-carrying capacity of transferred films of molybdenum disulfide is appreciably greater than that of graphite films.     

Friction and Wear Properties of MoS<Subscript>2</Subscript>-Overcoated Laser Surface-Textured Silver-Containing Nickel-Based Alloy at Elevated Temperatures

The solid lubricant that is coated on a flat surface is easily removed during friction. Surface texture dimples, which act as reservoirs of solid lubricant, can prolong the wear life of solid lubricant films. We textured silver-containing nickel-based alloys by a pulse laser and filled the micro-dimples with molybdenum disulfide powders. The tribological properties of the alloys were tested by rubbing against alloyed steel on a ring-on-disk tribometer at temperatures ranging from room temperature to 600°C . After laser surface texturing, the friction coefficients of the silver-containing nickel-based alloy smeared with molybdenum disulfide powders were reduced at temperatures ranging from room temperature to 400°C. With increasing dimple density, the wear life of the MoS₂ film increased while the wear rate of the nickel-based alloy decreased. The wear life of the textured surface with a dimple density of 11.2% exceeded 10,000 m at room temperature. We conclude that molybdenum disulfide and its oxides stored in the micro-dimples play a role in lubrication at room temperature and high temperatures, respectively.     

Tribological Enhancement of Aluminum by Porous Anodic Films Containing Solid Lubricants of MoS2 Precursors©

Porous anodic films containing molybdenum disulfide precursors were developed for self-lubricating purposes on aluminum by an initial anodizing and a subsequent re-anodizing process. The self-lubricating films were then examined with respect to the morphology, microstructure, and composition of the anodic film material and the lubricant, using X-ray diffraction, electron microscopy, energy dispersive X-ray analysis and X-ray photo-electron spectroscopy. The dry sliding wear of aluminum supporting such self-lubricating films was significantly reduced, as a result of greatly reduced coefficients of friction. The enhanced lubricity, due to the MoS₂ precursors contained within the porous anodic film, leads to wear mode changes from severe abrasive and adhesive wear for uncoated aluminum, to a mild film fatigue wear, for aluminum supporting the self-lubricating anodic films. The wear mechanism change is suggested by the wear and friction curves, as well as confirmed by wear track morphology.     

The effect of branching on slip and rheological properties of lubricants in molecular dynamics simulation of Couette shear flow

Molecularly thin liquid films of alkane in extreme conditions in a thin film lubrication regime have been investigated. To get an insight into the effects of molecular architecture in the behaviour of these thin lubricant films we have studied six different molecules, mainly isomers of C₃₀. In this work the effect of branching on rheological properties and behaviour of lubricant film is examined. Our study shows viscosity and normal stress effects depend on the degree of branching. Dynamics of the molecules and their orientation are also affected by the degree of branching. A weaker layering near the wall is observed for branched molecules. Slip between the wall and lubricant film also was larger for branched molecules. Branched molecules had less tendency to change their orientation under the flow. The results obtained here could be helpful in designing new lubricants at the molecular level.     

New Sulfide Addition Agents for Lubricant Materials

Two sulfides, AsSbS₄, and AsAsS₄ were prepared by aqueous precipitation methods. Property studies covered lubricant characteristics, and comparisons were made with As₂S₃, As₂S₃, Sb₂S₃ and Sb₂S₃. The various compounds were examined as chemical addition agents for diester lubricating grease and organic resin solid film lubricants. Details are presented for lubricant formulation and preparation, as well as laboratory test methods. The single most significant result is the dual response of AsSbS₄ to steel-steel and steel-molybdenum. Previous studies have shown that sulfur compounds as a class are preferential lubricants for molybdenum; however, a lubricating effect for steel by metallic sulfides has been essentially limited to MoS₂.     

Solid lubrication of ball bearings for spacecraft mechanisms

The advantages of solid lubricants over oils and greases for the lubrication of ball bearings in spacecraft are briefly discussed. The techniques of transfer film, of ion plating and of sputtering are well suited to such bearing lubrication. Three specific lubricants, which account for the majority of applications: ptfe-composite, rf-sputtered MoS₂ and ionplated lead film, are first discussed and then compared in terms of durability and torque characteristics. It is concluded that any of these solid films may be acceptable, depending upon the required bearing duty. The limitations of each lubricant are considered and compared     

Evaluation of Dry Powdered Lubricants at 1000 F in a Modified Four-ball Wear Machine

The friction and wear behavior of selected solid lubricants and wear specimens has been studied in a modified four-ball wear machine. Data were obtained at 1000 F for sliding speeds of 120 and 700 ft per min under initial Hertzian contact stresses of about 250,000 psi. Conventional solid lubricant material ssuch as lead oxide (litharge), molybdenum disulfide, and graphite were evaluated. A new dry solid lubricant, metal-free phthalocyanine, was studied and was usually found to have superior lubricating ability. Wear specimens of AISI Type M-1 tool steel and titanium carbide-nickel-molybdenum cermet showed the most promise for high-temperature sliding systems. Initial coefficients of friction less than 0.1 were obtained with combinations of these materials.     

Study on Lubricant Depletion Induced by Laser Heating in Thermally Assisted Magnetic Recording Systems: Effect of Lubricant Thickness and Bonding Ratio

In this study, we have carried out fundamental research on lubricant depletion due to laser heating in thermally assisted magnetic recording. In particular, we investigated the effects of lubricant film thickness and lubricant bonding ratio on lubricant depletion. Conventional lubricants Zdol2000 and Ztetraol2000 were used. The lubricant depletion characteristics due to laser heating were found to depend largely on the lubricant film thickness and material. That is, for films thicker than one monolayer, the lubricant depletion depth increased with the laser-irradiation duration, whereas the thickness of the lubricant after laser irradiation on the diamond-like carbon (DLC) films tended to remain at a constant film thickness of one monolayer. The lubricant depletion width gradually increased as the laser irradiation duration increased. The increasing trends for the lubricant depletion width were quantitatively very similar and almost independent of the initial lubricant film thickness. However, for lubricant films with thicknesses less than one monolayer, the lubricant depletion depth was very small. The lubricant depletion width increased remarkably to several hundred micrometers as the laser irradiation duration increased. The lubricant depletion depth and width were much smaller for Ztetraol2000 than Zdol2000. In addition, the lubricant-bonding ratio was found to greatly affect the lubricant depletion characteristics due to laser heating. In other words, the lubricant depletion depth and width decreased as the bonding ratio increased. The lubricant depletion mechanism involves the evaporation of mobile lubricant molecules when the maximum attained temperature is less than 100 °C. Another suggested lubricant depletion mechanism involves the thermocapillary stress effect, which is induced by the disk surface temperature gradient resulting from the non-uniformity of the laser spot intensity distribution.     

Effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives

This paper describes the effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives. In the experiments, the contact slider dynamics as well as ultra-thin liquid lubricants behavior are investigated using three types of lubricants, which have different end-groups and molecular weight as a function of lubricant film thickness. The dynamics of a contact slider is mainly monitored using acoustic emission (AE). The disks are also examined with a scanning micro-ellipsometer before and after contact slider experiments. It is found that the lubricant film thickness instability occurs as a result of slider–disk contacts, when the lubricant film thickness is thicker than one monolayer. Their unstable lubricant behavior depends on the chemical structure of functional end-groups and molecular weight. In addition, it is also found that the AE RMS values, which indicate the contact slider dynamics, are almost same, independent of the end-groups and molecular weight for the lubricants, when the lubricant film thickness is approximately one monolayer. The molecular weight, however, affects the contact slider dynamics, when the lubricant film thickness is less than one monolayer. In other words, the AE RMS values increase remarkably as the molecular weight for the lubricant increases. When the lubricant film thickness is more than one monolayer, the AE RMS values decrease because of the effect of mobile lubricant layer, while the lubricant instability affects the contact slider dynamics. Therefore, it may be concluded that the lubricant film thickness should be designed to be approximately one monolayer thickness region in order to achieve contact recording for future head–disk interface.     

Water Adsorption on Lubricated a-CH x in Humid Environments

Humidity influences the tribological performance of the head-disk interface in magnetic data storage devices. In this work a quartz crystal microbalance was used to measure the uptake of water on amorphous hydrogenated carbon (a-CH _x ) films at room temperature and pressures of water corresponding to relative humidities of 25%. These experiments have used a-CH _x films of varying thickness with and without lubricant. The lubricants used included Fomblin Z-03, Z-disoc, and Z-tetraol deposited on the surfaces of a-CH _x films of various thickness. The amount of water adsorbed on the unlubricated a-CH _x films is roughly independent of a-CH _x film thickness. The presence of the lubricant reduces the amount of adsorbed water; however, the amount of water adsorbed in the presence of a lubricant does not depend significantly on the type of lubricant. These observations imply that water is adsorbed on the surfaces of the lubricant or the a-CH _x film rather than being absorbed in their bulk.     

Influence of lubricants on the performance of journal bearings – a review

ABSTRACT

The lubricant properties have a significant influence on the static and dynamic performance characteristics of journal bearing such as load-carrying capability, minimum fluid film thickness, maximum pressure, lubricant flow rate, damping coefficients, stiffness coefficients, etc. The present document reviews the behaviour of various lubricants such as power-law lubricants, couple stress lubricants, micropolar lubricants, ionic liquid lubricants and space lubricants. The influence of these lubricants on the performance of hydrostatic, hydrodynamic and hybrid journal bearings is discussed. An effort is made to develop the understanding to choose the suitable lubricant for journal bearings for different journal bearing configurations. Journal bearings operated with non-Newtonian lubricants have shown better performance compared to Newtonian lubricants. Ionic liquid lubricants have shown high potential in vacuum applications and extreme temperature environment such as in bearings of spacecraft moving mechanical assemblies.     

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.     

Behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives

In this paper, we describe the behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives. In the experiments, the ultrathin liquid lubricant film behavior is investigated using Zdol and cyclotriphosphazene-terminated PFPE lubricant which have different end groups as a function of lubricant film thickness. The disks are examined with a scanning microellipsometer before and after contact slider experiments. It is found that the lubricant film thickness profiles almost do not change, when the lubricant film thickness is less than one monolayer. It can also be observed that lubricant film thickness instability due to dewetting occurs as a result of slider-disk contacts for the tested lubricants and the films undergo spontaneous redistributions, resulting in significantly nonuniform film thickness profiles, when the lubricant film thickness is thicker than one monolayer. In addition, it is found that the observed behavior of ultrathin liquid lubricant films for cyclotriphosphazine-terminated PFPE lubricant contrasts markedly with that for Zdol. The difference between cyclotriphosphazene-terminated PFPE lubricant and Zdol is only the functional end group. Therefore, it may be concluded that their unstable lubricant behavior depends on the chemical structure of functional end groups.     

Detecting Wear and Migration of Solid-Film Lubricants Using Simultaneous Exoelectron Emission

This study was carried out to see if solid-lubricant films on metal surfaces wear monotonically, or if there is occasional migration from a region on the wear track with copious lubricant to one from which the lubricant has been worn away. To facilitate this study, a new pin-on-disk apparatus was constructed with the capability of continuously measuring the friction and the exoelectron emission from tile wear track. Sliding tests were carried out using a circular wear track which, initially, traversed a path partly on an unlubricated surface and partly on a surface covered by a solid-lubricant film. It was found that lubricant films of rubbed graphite and molybdenum disulfide showed no spreading of the lubricant along the wear track, but polytetrafluoroethylene had some capability for migration and, hence, healing of a denuded wear track.     

The Effect of Solvents on the Perfluoropolyether Lubricants Used on Rigid Magnetic Recording Media

The adsorption characteristics and tribological properties of the perfluoropolyether (PFPE) lubricants Zdol and Z-Tetraol on amorphous nitrogenated CN_x carbon are investigated as a function of solvent used to apply the lubricants. The solvents used in these studies include perfluorohexane, CF₃CHFCHFCF₂CF₃ and C₄F₉OCH₃. Deposition studies indicate that the applied thickness of PFPE films is strongly solvent-dependent that can be related to differences in the solubility parameters between the various lubricants and solvents. The results of ab initio computations on the molecular electronic structure of the solvent molecules show that their solvent power is correlated to their polarity and in particular to the acidity of the protons on the CF₃CHFCHFCF₂CF₃ and C₄F₉OCH₃ molecules. Tribological reliability, as measured by contact start-stop testing, slider-disk clearance, lubricant pickup by the slider, lubricant smearing on the disk surface, etc., is independent of solvent and is limited to the physical properties of the adsorbed lubricant film. The kinetics of lubricant mobility are charateristic of confined liquids that are independent of solvent as shown by lubricant flow profiles, bonding kinetics, and contact angle goniometry.     

Effect of Various Lubricants and Base Materials on Friction at Ultrahigh Loads

A series of high load, low-speed sliding friction tests was made on eight greases and eighteen dry lubricants at normal unit loads from 10,000 psi to 150,000 psi. Four different substrate materials were used having a range of hardnesses from Rockwell C 18 to Rockwell C 55. The ultimate load capability of both greases and dry films is a function of substrate hardness with the best ultimate load capability being provided by inorganically bonded molybdenum disulfide films with small amounts of graphite added. The coefficient of friction of the greases appears to be an inverse function of substrate hardness and a direct function of the normal load. The coefficient of friction of the dry lubricants is an inverse function of the normal load, but does not appear to be related to the substrate hardness.     

An investigation of friction and wear performances of bonded molybdenum disulfide solid film lubricants in fretting conditions

The friction and wear performances of bonded MoS₂ solid film lubricants with the counterpart steel ball rubbing were investigated in fretting wear conditions in order to inquire into the load-carrying capacity and wear mechanisms of bonded MoS₂ solid film lubricants under dry friction conditions. Experimental results show that the bonded MoS₂ solid film lubricants have excellent anti-friction and wear-resistance performances within a wide load range between 20 N and 800 N and within a wide oscillatory frequency range between 5 Hz and 30 Hz. It is found through analyses of the transfer films formed in the surface of the counterpart steel ball investigated by SEM, XPS and AES, that the thickness of the transfer film formed is about 38 nm and the oxidation of MoS₂ in the transfer films does not occur during dry friction process. The high load and frequency promote the formation of a compact transfer films. The compact transfer films are believed to be the predominant mechanism giving rise to high load-carrying capacity, and excellent wear-resistance performances of the bonded MoS₂ solid film lubricants.     

Advanced lubricants for heat engines

An advanced liquid lubricant for heat engines has been developed and tested successfully in a prototype engine. The lubricant possesses superior oxidation stability and high temperature stability. With the advent of new engine designs, stability should be measured in terms of both the temperature and the time for which the lubricant is subjected to it. This lubricant is designed to provide friction and wear protection for three to five minutes at 425°C (800°F) at the ring zone and maintains stability at an oil sump temperature of 171°C. The lubricant has been evaluated by the Cummins Engine Company. Out of a field of several dozen lubricants, six lubricants were selected for a prototype 200 h endurance tests. The NIST lubricant was one of the two lubricants that successfully finished the endurance testing. This paper provides an overview of the key lubricant design considerations, including oxidation and thermal stability, volatility, and deposit control, the prototype engine test conditions and the results.