Lubricant Reservoirs Research Articles

Friction characteristics of submicrometre-structured surfaces fabricated by particle-assisted near-field enhancement with femtosecond laser

We present friction characteristics of sliding textured silicon surfaces at the submicrometre scale. A two-dimensional submicrometre dimple array on the Si surface is fabricated by femtosecond laser processing. Direct femtosecond laser nano-structuring of the Si (1 0 0) substrate by polystyrene particle-assisted near-field enhancement is used. In the investigated hole diameter domain from 229 to 548 nm, an increase in the friction coefficient with the decrease in the hole size is found experimentally. The fabricated submicrometre dimples act evidently as lubricant reservoirs to supply lubricants and traps to capture wear debris. The fluctuation of the friction coefficient is also increased by reducing the dimple size. The lowest friction coefficient of 1.41 × 10−2 is achieved with the dimple array having a diameter of about 550 nm. This value is 2.6 times lower than that of non-structured substrates.

Sliding Friction and Wear Characteristics of Novel Graphitic Foam Materials

While the use of ORNL developed high-thermal-conductivity graphitic foam materials has been focused on thermal management applications, recent experiments have revealed their potential as bearing surfaces as well. The three primary tribological advantages are: (1) they can efficiently remove frictional heat, (2) their natural porosity can trap wear debris, and (3) their porosity can serve as a lubricant reservoir for the contact surface. A series of pin-on-disk experiments were conducted at both room temperature and 400 °C to compare the sliding friction and wear characteristics of the densified graphitic foam (mated against M-50 tool steel or against alumina) to those of conventional bearing materials like graphite, bearing bronze, poly- tetrafluoroethylene, bearing steel, and a Co-based superalloy. At room temperature and under low contact pressure, the tribological behavior of the densified graphitic foam material was comparable to that of graphite and better than that of other bearing materials. At 400 °C, traditional graphite exhibited a “dusting” wear regime accompanied by a high friction coefficient. In contrast, the graphitic foam demonstrated an ability to maintain low friction and wear at that temperature.

Texturização por Laser ND: YAG e comportamento tribológico de um recobrimento de cromo

Com o intuito de melhorar a condição de lubrificação na superfície de recobrimentos de cromo eletrodepositado em aços-carbono, elaborou-se uma rede regular de furos utilizando um Laser Nd:YAG pulsado. Essa rede de furos foi projetada de modo que cada furo individual funcionasse como reservatório adicional de óleo em condições limítrofes de lubrificação. Técnicas de análise metalográfica e dimensional foram utilizadas para a caracterização das superfícies estudadas. Complementarmente, com o auxílio de uma interpretação analítica da condição limítrofe de lubrificação, foi possível descrever o comportamento tribológico dos recobrimentos texturizado e convencional, quando estes foram ensaiados no teste de pino-sobre-disco. A texturização do recobrimento de cromo levou-o a um significativo aumento da resistência ao "scuffing", sendo até três vezes maior que o recobrimento de cromo convencional, e a um melhor comportamento em relação ao desgaste (perda de massa).

Novel Composite Coatings With 3D Coating Architectures for Tribological Applications Fabricated Using Semiconductor Patterning Processes

ABSTRACTComposite coatings consisting of hard compounds and soft solid lubricant phases have been actively investigated as tribological coatings for reducing friction while improving wear resistance. In most cases structural modifications in these coatings have been achieved by either natural phase separation or sequential layer deposition. In this study, we explore the concept of creating a three-dimensional structure in a tribological composite coating by using well-established semiconductor patterning methods. Using photolithography, we have patterned the substrate surface with a regular array of micron-size islands. The substrate was then coated with various metal and carbide coatings, including Cr and TiC, using sputter deposition. The substrates were sonicated in a solvent to remove the photoresist islands leaving an array of holes. MoS2 was then deposited onto the substrates. A pin-on-disk friction and wear test was conducted on a TiC/MoS2 sample to examine the effects of these islands on wear morphology. The results showed that the MoS2-filled holes can act as lubricant reservoirs, and the areas of the sample coated with TiC/MoS2 provided better wear protection than areas coated with only MoS2.

Effects of ion implantation on microstructure, endurance and wear behavior of IBAD MoS 2

Thin MoS 2 coatings were prepared on steel and Si substrates via ion-beam assisted deposition using conditions known to produce dense, basal oriented microstructures. After deposition, the coated substrates were irradiated with 180 keV Ar 2+ ions at doses of 1×10 15, 1×10 16, and 5×10 16 ions/cm 2. Microstructures of as-deposited and ion irradiated MoS 2 coatings were examined using X-ray diffraction, high resolution transmission electron microscopy, and micro-Raman spectroscopy. Friction, wear and endurance were examined under both unidirectional and reciprocating sliding conditions in dry air. The lowest ion irradiation dose altered the initial microstructure, producing basal oriented, nearly defect-free crystalline domains of MoS 2 in an amorphous matrix; at the highest dose the coatings were nearly amorphous, but the remaining MoS 2 showed mixed orientation. Ion irradiation of the MoS 2 coatings did not significantly modify the friction behavior, and only at the highest dose was the endurance altered, decreasing by more than 75%. Two changes in wear behavior were observed in the highest dosed coatings: (1) accelerated wear of the coating and (2) elimination of solid lubricant reservoirs. Destruction of the solid lubricant replenishment process resulted in premature failure of the highest dosed coating.

Investigation into three-dimensional laser processing of tribological coatings

Tribological coating design has evolved from single layer/single phase to multilayer and composite architectures. The early single-layer coatings were not efficient for arresting cracks, distributing loads, relaxing stress, and preventing adhesive failures. Second-generation coatings with multilayer, gradient, and composite architectures added another dimension to the coating design and allowed much better accommodation of stresses and crack arresting. This paper considers the further evolution of coating design to three dimensions, where the lateral property variation was added to the cross-thickness property variation. The three-dimensional design considerably improved the tribological characteristics of hard coatings, by permitting solid lubricant replenishment inside the friction contacts. A functionally gradient Ti–TiC–TiC/diamond-like carbon coating with an upper layer of a tough nanocrystalline/amorphous composite was used for load support, crack prevention, and stress equalization. This coating was processed with laser irradiation to form grooved tracks along wear paths, which were then filled with MoS 2. This provided a solid lubricant reservoir in the lateral dimension of the coating. The three-dimensional coating was tested in long-duration sliding tests at fixed and cycling humidity. The coating exhibited environmental adaptation with friction coefficients of 0.15 in humid air and 0.02 in dry nitrogen. The wear life was increased by at least one order of magnitude in comparison to that for a hard gradient coating with a top layer of MoS 2 without any three-dimensional laser processing. Discussions of three-dimensional coating tribological properties, friction and wear mechanisms are provided. The environmental adaptation achieved with three-dimensional coating processing can be beneficial for aerospace applications.

Effects of Gas Entrainment on Squeeze Film Damper Performance

Squeeze film dampers are frequently used for stabilization and/or vibration control of rotating machinery. Theoretical analyses to date generally assume an incompressible lubricant. In practice, however, depending on the capacity of the lubricant reservoir, the lubricant at damper inlet contains varying amounts of dissolved gas, which come out of solution to form a “spongy” gas-liquid mixture during damper operation. This paper examines theoretically and experimentally the effects such entrained gases have on damper performance, particularly on damper load capacity and the likelihood of multistable operation. It is shown that under certain operating conditions, a significant delay in the onset of bistable operation is predicted, depending on the fluid film model employed. Preliminary tests indicate that at low bearing parameter values (B ≐ 0.02), the homogeneous compressible film model using the Hayward rather than the Isbin viscosity relationship for gas-liquid mixtures provides the best prediction of damper performance. Of the incompressible film models, the zero pressure truncation predictions are generally quite satisfactory and superior to the commonly used π-film predictions.

The behavior of thin oil films in the presence of porous lubricant reservoirs

Oil filled porous reservoirs are usually incorporated in the bearing cavities of spacecraft to replenish the lubricant that has been lost through evaporation. The replenishment is expected to occur through both vapor and surface transport. It is shown that only for the roughest surfaces (sand-blasted) does oil flow by surface transport from the reservoirs. On smooth surfaces the presence of the reservoir creates an instability in any existing oil film such that de-wetting occurs. In this process no oil leaves the reservoir.

Lubricant Supply Characteristics of a Microporous Polymer

A moldable, polymeric microporous material for use as a lubricant reservoir for rolling element bearings has been studied to determine the parameters which control the rate of lubricant flow. A simplified theory is developed to predict flow rates resulting from body forces or from internal pressure changes. Experimental data and theory are in good agreement. Changes in temperature produce transient changes in flow rate due to an internal pressure change resulting from differential thermal expansion between the lubricant and the polymer matrix. The transience is followed by a steady flow rate due to body forces. Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Boston, Massachusetts, October 5–7, 1976

Lubricant reservoir systems - Thermal considerations

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

Ball Bearing Suspended Flywheels for Attitude Control

The paper identifies and discusses some problem areas encountered during the development and qualification of high reliability ball bearing suspended flywheels. The specifications were met by a design employing damping means on the rotor, leading to ball bearings of acceptable size and moderate torques and torque changes. A special lubricant reservoir assures proper lubrication throughout life. Besides, the qualification and SYMPHONIE flight results obtained until now are presented and assessed.

Lubrication for vacuum applications

The selection and use of lubricants for vacuum applications is a critical part of the mechanisms design phase which is often overlooked. Both wet and dry lubricants can be used for mechanical and electrical moving interfaces in vacuum. Deciding on what type of lubrication system to use and the incorporation of the appropriate hardware design considerations are critical to the performance of the mechanisms. The use of design features such as lubricant reservoirs, labyrinth seals, and barrier films can enable wet lubricated systems to function for long periods in vacuum. Dry lubricated hardware has a finite life due to the sacrificial mode of dry lubricants. The use of suitable dry lubricant materials and designs can allow millions of cycles of operation in vacuum. Dry lubricants have a potential particulate contamination problem while wet lubricants can contaminate by both outgassing and migration by creep.

The performance of steel-backed acetal copolymer bearings

Abstract By bonding to a steel backing, lubricated plastic bearings can compete in cost and performance over a wide range of conditions on equal terms with conventional metal plain bearings, while retaining most of the desirable properties of plastics. Reference is made to instances of the use of plastic or plastic-lined bearings as internal combustion engine crankshaft bearings. The paper then illustrates the performance of lubricated steel-backed thermoplastic bearings by detailing the results of a large scale evaluation programme of DX, a proprietary material based on acetal copolymer, carried out on bush and thrust washer test rigs under a range of practical operating conditions. The reasons behind the adoption of an overall indent pattern on the bore to act as a lubricant reservoir are presented and the effects on DX bearing performance of variables such as rubbing speed, load, lubricant type and direction of loading, temperature, clearance, mating surface finish and material, are discussed and many of the test results presented. Performance comparison is made with oil-impregnated porous bronze and iron and conventional bearing metals. Finally, applications for DX and similar bearing materials are discussed.

The performance of steel-backed acetal copolymer bearings

By bonding to a steel backing, lubricated plastic bearings can compete in cost and performance over a wide range of conditions on equal terms with conventional metal plain bearings, while retaining most of the desirable properties of plastics. Reference is made to instances of the use of plastic or plastic-lined bearings as internal combustion engine crankshaft bearings. The paper then illustrates the performance of lubricated steel-backed thermoplastic bearings by detailing the results of a large scale evaluation programme of DX, a proprietary material based on acetal copolymer, carried out on bush and thrust washer test rigs under a range of practical operating conditions. The reasons behind the adoption of an overall indent pattern on the bore to act as a lubricant reservoir are presented and the effects on DX bearing performance of variables such as rubbing speed, load, lubricant type and direction of loading, temperature, clearance, mating surface finish and material, are discussed and many of the test results presented. Performance comparison is made with oil-impregnated porous bronze and iron and conventional bearing metals. Finally, applications for DX and similar bearing materials are discussed.

Paper 25S: Sliding Noble Metal Electric Contacts

Sliding electric contacts have the unique requirement of low and stable resistance without high friction and wear. Electrical breakdown of surface films, which can establish metallic contact between touching members, occurs only with appreciable circuit voltages. Consequently, noble metals alone are suitable for contacts that serve the low energy circuits of modern computer, communications and aerospace equipment. Approaches used by the engineer to reduce the friction and wear of noble contact metals include: design with low normal loads, careful selection of contact materials and lubrication. Gold coatings, generally as thin electrodeposits, have become the most widely used contact material for separable connector contacts, instrument slip rings and similar devices.The ‘prow formation’ wear mechanism dominates the dry sliding of most noble contact metals, particularly during the early stages of sliding where break-in phenomena occur. Recent work shows that there is a critical load-dependent roughness for prow formation. The wear mechanisms of gold electrodeposits may differ from those of the wrought metal because of their characteristic topographic and crystallographic orientations.Lubricants are often used for sliding noble metal contacts. Solid lubricants are effective only on rough surfaces. When contacts become burnished through use the lubricant reservoirs disappear and high wear commences. Thin films of lubricants having a fluid component, however, do not have this limitation, for they demonstrate hydrodynamic lift and self-healing qualities. Above a certain velocity, characteristic of the system, wear may be negligible with fluid-containing lubricants. Their ability to function on smooth surfaces is important, for the as-plated porosity of electrodeposited metals (which leads to deleterious contact corrosion) is directly related to roughness. Finally, coatings for thin gold-plated contacts have been developed which combine corrosion inhibitors and lubricants.

Paper 4: Opening, Carding and Spinning Preparation Machines

There are no serious bearing problems in opening and carding machines. Except for the high-speed beaters and cylinders, occasionally-oiled plain journals were long regarded as adequate despite the fact that many of them operated under boundary lubrication conditions. Stable hydrodynamic film conditions were maintained in beater and cylinder bearings and some simple form of lubricant reservoir was usually provided. In recent years the introduction of shift working and high machine utilization factors have led to a demand for machines requiring a minimum of attention. This demand is being met by the use of prepacked, sealed ball or roller bearings at all points. Perhaps the most serious and intractable bearing problem in the whole field of textile machinery manufacture is encountered in drawframes and speedframes. For uniformity of the drawing action by which the fibres are straightened and finally attenuated to form a roving, it is necessary for all the drafting rollers to rotate perfectly smoothly. Unfortunately the loading conditions and the rotational speeds of some of the rollers are such that there is often a high liability to the occurrence of spontaneous stick–slip torsional vibrations. In other cases there is an incipient liability to stick–slip vibration and quite small gearing faults, of the appropriate frequency, are sufficient to precipitate high amplitude oscillations. At the frequencies commonly observed the non-uniformity of the drawing action introduces into the sliver or roving a periodic disturbance of a wavelength so short that it is not readily apparent in the delivery from the affected machine. Subsequent drafting, however, increases the wavelength and the ultimate yarn exhibits appreciable variations in mass per unit length and in twist distribution. As these variations are strictly periodic they form objectionable interference patterns when used as weft in either woven or knitted cloths. New machine designs and the use of new types of bearings have largely solved these problems and at the same time greatly increased operating speeds. The comber, too, has been intensively developed during the past ten years and a number of lubrication and wear problems have required attention.

Vapor Deposited Gold Thin Films as Lubricants in Vacuum (10−11 mm Hg)

Gold thin films of 1800 Å to be used as lubricants were vapor deposited on Ni, Ni-Cr, and Ni-Re substrates. Strong bonding (adhesion) and durability between the film and substrate were found to be essential when thin films are used as a lubricant. Factors that were investigated included the selection of the film and substrate material. Strong durability of the thin film is directly related to the type and structure of the interfacial region. Two methods of substrate preparation prior to vapor deposition were investigated: (1) mechanically polished surface and (2) electron bombarded surface. Gold was vapor deposited on the mechanically polished surface at room temperature and on the thermally etched surface at an elevated temperature approximately (800 °F). Strength and durability of the films were investigated in sliding friction experiments with a hemispherical niobium rider sliding on the films at a velocity of 5 ft/min. Results obtained in these friction experiments indicated that the film endurance life was considerably better on the thermally etched surface. This increased film durability with the thermally etched surface is believed to be owing to the formation of a diffusion type interface between the film and the substrate. Because of the disregistry at grain boundaries, a higher rate of diffusion and preferential trapping in and around the grain boundaries occurs, with these regions acting as lubricant reservoirs during the friction experiments.

Size and Speed Effects From Lubricant Breakdown in Plastic Compression

A size dependence of the average pressure for reducing thin, geometrically similar, lubricated disks of aluminum has been associated with the development of lubricant-film breakdown at the periphery. In general, the greater amount of interface sliding (a maximum at the periphery) of larger disks led to earlier breakdown and higher stresses. However, with certain liquid lubricants the size-effect reversed with increasing reduction, the larger disks requiring the smaller stress. This reversal has been related to the presence of a lubricant reservoir, entrapped across the interface, that increases with sample size, supplying the peripheral region and eventually controlling the progress of breakdown. The effects of speed on pressure requirements were attributed to such factors as strain-rate dependence of lubricant shear strength, junction growth across the interface, and changes in the size of lubricant reservoirs.