Relation Between Shear Relaxation and Molecular Structure of Lubricating Oils

The viscoelastic property of various lubricating oils was measured with the oscillating crystal technique. The relation between the shear relaxation behavior and the molecular structure of the lubricating oils is discussed. Eyring's theory for viscous flow is used to explain the relaxation behavior from a molecular point of view. Some insight into a procedure for estimating the relaxation time from the molecular structure is presented.     

Analytical Solution of the General Linear Viscoelastic Radial Load Equation for Rotary Shaft Seals

The viscoelastic radial load equation for a cylindrical shell representing the seal was derived from the static solution by the correspondence principle and from first principles. The equation decoupled in space and time by a normal mode analysis was solved both by conventional methods with an impact initial condition and rather more efficiently by the Laplace transform method, for a general linear Maxwell body in plane stress. The solution was in the form of a recurrence relation, allowing representation of material properties by any number of Maxwell elements. When subjected to the standard creep test, step radial load, the seal responded with an impact displacement, followed by transient and finally steady creep. The equation was also solved by the finite element method with the semidiscrete approximation. The solution for stress relaxation and oscillatory strain are given in the Appendix.     

Tribological Properties of Hard Carbon Films on Zirconia Ceramics

In this study, the authors investigated the tribological properties of hard diamondlike carbon (DLC) films on magnesia-partially stabilized zirconia (MgO-PSZ) substrates over a wide range of bads, speeds, temperatures, and counterface materials. The films were 2 μm thick and produced by ion-beam deposition at room temperature. Tribological tests were conducted on a ball-on-disk machine with MgO-PSZ balls, in open air of 30 to 50% relative humidity under contact loads of 1 to 50 N, at sliding velocities of 0.1 to 6 m/s, and at temperatures of 400°C. Al₂O₃ and Si₃N₄ balls were also rubbed against the DLC-coaled MgO-PSZ disks, primarily to assess their friction and wear performance and to compare it with that of MgO-PSZ balls. A series of long-duration lifetime tests was run at speeds of 1, 2, and 6 m/s under a 5 N load to assess the durability of these DLC films. Results showed that the friction coefficients of MgO-PSZ balls sliding against MgO-PSZ disks were 0.5-0.8, and the average specific wear rates of MgO-PSZ balls ranged from 1 × 10^?5 to 5 × 10^?4 mm³/N·m, depending on sliding velocity, contact load, and ambient temperature. The friction coefficients of MgO-PSZ balls sliding against the DLC-coaled MgO-PSZ disks ranged from 0.03 to 0.1. The average specific wear rates of MgO-PSZ, balls were reduced by three to four orders of magnitude when rubbed against the DLC-coaled disks. These DLC films could last 1.5 to 4 million cycles, depending on sliding velocity. Scanning electron microscopy and micro-laser Raman spectroscopy were used to elucidate the microstructural and chemical nature of the DLC films and worn surfaces.     

The Effects of Dopants on the Chemistry and Tribology of Sputter-Deposited MoS2 Films

The fact. that dopants improve the friction and wear properties of sputtered MoS₂ films is well known. However, the role of dopants in the mechanisms governing friction and wear are not well understood. The purpose of this work is to gain a fundamental understanding of their role by co-depositing a number of materials, i.e., Ni, Fe, Au, and. Sb₂O₃, with MoS₂ and evaluating their effects on film chemistry, crystallinity, microstructure, and tribology. Friction and wear measurements were collected, using ball-on-flat and dual-rub shoe tribom-eters. Other physical and chemical properties were obtained using SEM, XPS, XRD) and Raman spectroscopy. Crystalline MoS₂ was seen in all of the films. In Sb₂O₃-doped films, an amorphous phase was also observed. The presence of dopants caused film densification and affected crystallite size. They had little effect on the overall crystallite orientation. In addition, dopants caused a reduction in the mean and. variance of the friction coefficient and an increase in wear life. The correlation between dopants, film properties, and tribology is discussed in detail.     

Microstructure,Physical Properties,and Tribological Characteristics of Composite Solid Lubricant Coatings with Gas Atomized BaF2-CaF2

PS304 is a NASA-developed composite solid lubricant for friction and wear reduction at high temperatures. The microstructure, physical properties, and tribological performance of PS304 using standard comminuted BaF₂-CaF₂ and novel gas atomized BaF₂-CaF₂ were compared. Two atomized BaF₂-CaF₂ particle size distributions were used. Overall, feedstock powder with atomized BaF₂-CaF₂ flowed more freely than standard PS304 feedstock. The cohesion strength of coatings with atomized BaF₂-CaF₂ was lower than that of standard PS304, while the hardness of all of the studied coatings was essentially the same (approximately 58 HRA). Pin wear was lower against the new coatings and the coefficients of friction were essentially the same for all coatings. These results indicate that atomized BaF₂-CaF₂ can be substituted for comminuted BaF₂-CaF₂, providing a high-volume BaF₂-CaF₂ fabrication technique to reduce the cost of PS304 and improve its commercial availability.     

Starting and Steady-State Friction Torque of Grease-Lubricated Rolling Element Bearings at Low Temperatures—Part II: Correlation with Less-Complex Test Methods

Today, torque measurements are conducted in full-scale test rigs, either standardized or specially designed. This is a time-consuming approach, especially at low temperatures, and it is therefore of high priority to find some less complex test method(s) correlating with bearing friction at different temperatures. The paper deals with five possible candidate test methods to predict low temperature starting and running friction: apparent viscosity, base oil viscosity, sliding friction, base oil traction properties and yield stress. The pros and cons of these methods are discussed and empirical expressions are presented for the methods where good correlation was found.

It is stated that base oil viscosity at the temperature of interest is the best method to predict bearing friction torque at low temperature starting.     

Lubricating Performance of Organic Sulfides Under Repeated Rubbing Conditions

Repeated four-ball tests of squalane with or without organic sulfides have been carried out to investigate the relationship between the properties of surface film formed on the rubbing surfaces and the lubricating performance of several organic sulfides.

With dibenzyl disulfide and dibenzyl monosulfide, friction was remarkably reduced and load-carrying capacity raised in the second run with the additive after the first run without the additive, presumably in which only an oxide film had been formed. In contrast, for diphenyl disulfide, the repetition of rubbing tests, each with additive, was effective.

EPMA, XPS(ESCA) and electron diffraction techniques have been applied to examine surface films formed during rubbing process. It is confirmed that the formation of surface films with the optimum composition of oxide and sulfide has brought about the excellent lubricating performance of these organic sulfides.     

Thrust-Washer Evaluation of Self-Lubricating PS304 Composite Coatings in High Temperature Sliding Contact

PS304 self-lubricating composite coatings were successfully deposited on steel substrates at various plasma spray facilities using mixtures blended from commercially obtained constituent particles. Coatings were evaluated in thrust-washer tests against Inconel X-750 at low contact pressures to 40kPa, sliding speed of 5Amis, and either ambient temperature or 500 °C chosen to simulate conditions in airfoil bearings during startup and shutdown contact. Wear factors for all PS304 coatings tested, regardless of contact pressure and temperature, ranged from 1–3*10^?4 mm³/Nm while coefficients of friction of approximately μ =0.5 were measured in all cases. While wear and friction behavior of PS304 in air foil bearings appear to have been simulated, surface roughening was observed in these thrust-washer tests which used continuous sliding contact, as opposed to the evolution of smoother surfaces observed in high-temperature foil bearings experiencing cyclic startup/shutdown. Wear-induced surface smoothening of PS304 was additionally simulated in thrust-washer tests with sliding contact instead imposed intermittently.     

High Velocity Oxyfuel Deposition for Low Surface Roughness PS304 Self-Lubricating Composite Coatings

Efforts were made to achieve lower (R _q < 0.1 μm) initial RMS roughnesses of PS304 coatings so that they may be considered for foil bearings operating under increasingly severe conditions that result in smaller air film thicknesses. Attainable roughness of conventional plasma-sprayed PS304 coatings has been typically R _q > 0.25 μm, as limited by porosity in the deposited coating and surface irregularities correspondingly formed upon finishing. Initial attempts at achieving dense coatings by instead using a high-velocity oxyfuel (HVOF) flame-spraying process failed due to insufficient heating and softening of the NiCr and Cr ₂ O ₃ constituents of the PS304 feed powder, which rebounded from the steel target substrate and resulted in low deposition yield. Efficient HVOF deposition by a hydrogen-fueled system was achieved using NiCr and Cr ₂ O ₃ constituent particles of reduced size that were more effectively heated. The resultant dense coatings provided roughnesses as low as R _q = 0.05 μm upon polishing. Tribological performance of these HVOF coatings was evaluated against Inconel X-750 in thrust-washer tests at a sliding speed of 5.4 m/s, with contact pressures of 20 and 40 kPa, and ambient or 500°C temperatures. The wear and friction performances in direct sliding contact, as would exist upon loss of separating air-bearing film, of the PS304 coatings produced by HVOF are found to be similar to those deposited by the plasma spray process.     

Tribological Characteristics of NiAl Matrix Composites with 1.5 wt% Graphene at Elevated Temperatures: An Experimental and Theoretical Study

In this study, the tribological properties of NiAl matrix composites with 1.5 wt% graphene nanoplatelets (NAG) at elevated temperatures were simulated and tested. NAG exhibits excellent characteristics at 100 and 200°C due to the formation of metal oxides and graphene nanoplatelets lubrication film. Furthermore, the removed layer thickness, the stress distributions, and the high stresses of the affected layer have been estimated theoretically. At 400°C, the friction coefficient increased due to the absence of the lubrication layer. In addition, the wear rate increased due to the excessive stresses, the increased layer thickness removal, and the propagation of subsurface cracks.     

Tribological Properties of Carbon Coatings Produced by High Temperature Chlorination of Silicon Carbide

The tribological properties of highly disordered graphitic carbon layers formed on silicon carbide (SiC) substrates by reaction with chlorine and chlorine-hydrogen gas mixtures at 1000 °C were studied. Si was selectively removed from the near surface of SiC by chlorine gas, leaving behind a layer of carbon having high structural density and strong bonding characteristics. Tribological tests showed that the carbon films were highly adherent and able to reduce friction coefficients of the base SiC by factors of up to seven. There was little or no change in the factional behavior of carbon layers when sliding velocity and load were increased. Low friction coefficients (～0.1) could be obtained under wet, dry, polished, and rough conditions. The initially rough carbon surface underwent plastic flow producing a smooth, self-adjusting carbon layer. Structural morphology and the amount of disorder in the carbon layers were correlated with the friction and wear performance of the resultant films.     

Carbon-Coated Sliders and Their Effect on Carbon Oxidation Wear

Alumina-titanium carbide composite sliders used in magnetic recording were coated with diamondlike carbon (DLC) to permit exploration of their effect on tribochemical wear. For comparison, testing was performed on both coated and uncoated sliders. Gases sampled directly from the sliding interface between a carbon-coated thin-film magnetic recording disk and the slider contained carbon dioxide in both dry nitrogen and dry oxygen environments. In the oxygen environment, uncoated sliders produce carbon dioxide at a rate 10 times greater than coated sliders. This suggests that catalysts in the slider composite material are necessary for carbon oxidation wear.     

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 Pulsed Laser Annealing on Wear and Corrosion Properties of Electroless Ni-P Plating

Electroless nickel-phosphorous alloy was plated onto quenched and hardened stainless steel. Laser surface treatments by YAG laser beams were carried out with the objective of improving -both the wear and the corrosion resistance of the platings. The friction and wear properties of the laser-treated platings were compared to unplated, untreated plated, and 400°C heat-treated plated substrates in pin-on-disc tests under unlubricated conditions. It was shown that laser treatments gave about a ten percent reduction in the friction coefficient compared with unplated substrates, and that controlled laser-treated plating improved the wear resistance of untreated plating to a level equivalent to 400°C heat-treated plating. It was found that the salt-spray corrosion resistance of the laser-treated platings was much greater than the unplated material and the 400°C heat-treated plating. The laser treatment is capable of improving both wear and corrosion resistance; and the wear resistance is externally governed by the hardness of the plating, and the corrosion resistance is governed by the crystal structure, especially the amount of amorphous nickel.     

Wear and Friction of TiAl Matrix Self-Lubricating Composites against Si3N4 in Air at Room and Elevated Temperatures

More durable, low-friction bearing materials over a wide temperature range are needed for turbine components and other high-temperature bearing applications. The current study reported the tribological properties of TiAl matrix self-lubricating composites (TMC) containing MoS₂ (a low-temperature lubricant, below 500°C), hBN (a medium-temperature lubricant, below 600°C), and Ti₃SiC₂ (a high-temperature lubricant, above 600°C) designated as MhT against an Si₃N₄ counterface at temperatures ranging from 25 to 800°C in air. The load was 10 N and the sliding speed was 0.2 m/s for all tests. Tribological studies indicated that TMC containing MhT showed a lower friction coefficient and wear rate in comparison to TiAl-based alloy at all test temperatures, which was attributed to the excellent synergetic lubricating effect of MoS₂, hBN, and Ti₃SiC₂. TMC containing 5 wt% MhT exhibited the best tribological properties over a wide temperature range.     

Tribological Properties of TiAl Matrix Self-Lubricating Composites Containing Multilayer Graphene and Ti3SiC2 at High Temperatures

An investigation is conducted on the unexplored synergistic effects of multilayer graphene (MLG) and Ti₃SiC₂ in self-lubricating composites for use in high-temperature friction and wear applications. The tribological properties of TiAl matrix self-lubricating composites with different solid lubricant additions (Ti₃SiC₂-MLG, MLG) are investigated from room temperature to 800°C using a rotating ball-on-disk configuration. Tribological results suggest the evolution of lubrication properties of MLG and the excellent synergistic lubricating effect of MLG and Ti₃SiC₂ as the testing temperature changes. It can be deduced that MLG has great potential applications as a promising high-temperature solid lubricant within 400°C, and a combination of MLG and Ti₃SiC₂ is an effective way to achieve and maintain desired tribological properties over a wide temperature range.     

Tribological Properties of PTFE and PTFE Composites at Different Temperatures

The friction and wear properties of polytetrafluoroethylene (PTFE) and its composites with fillers such as bronze, glass fiber, carbon fiber, carbon, graphite, and polymer were studied at ambient temperature and high temperature. The wear resistance and hardness were enhanced by the fillers. Results showed that the wear resistance of all composites was much higher than that of pure PTFE. Pure PTFE has the lowest friction coefficient at ambient temperature (temperature: 23 ± 2°C, humidity: 50 ± 10%) but highest friction coefficient at high temperature (above 100°C). The PTFE composite filled with bronze showed the best wear resistance at ambient temperature but the poorest wear resistance at high temperature. The carbon-graphite- or polymer-filled PTFE composite showed a lower friction coefficient and moderate wear resistance at both ambient and high temperature.     

Experimental Analysis of the Wear,Life and Behavior of PTFE Coated Thrust Washer Bearings Under Non-Axisymmetric Loading

In this work the behavior and life of a PTFE coating on a flat thrust washer bearing is investigated. The thrust washer bearing is located between a helical gear and its carrier, and is subjected to non-axisymmetric loading and wear. The volume of worn material is approximated by measuring the difference in height between the worn and unworn surfaces. It was also found that the surface roughness of tested washers increases with the severity of wear, in most cases. After a finite number of cycles the effective coefficient of friction between the surfaces increases, suggesting that the coating is wearing off and losing effectiveness. The rate at which the coating wears off also varies with load and speed, hence, there is a region of operation that minimizes the wear and friction.     

Wear Characteristics of Diamond Tools in Machining Ceramics

Wear characteristics of natural diamonds used to machine ceramic materials at a depth of cut of 2 μm and a speed of 1600 m/min were investigated. The diamond tools used for the machining tests were inspected using the Laue back reflection technique and scanning electron microscopy (SEM). Wear characteristics of the diamond tools appeared to be influenced by the material properties of the ceramics being machined, the build-up on the tips of the diamond tools, and the crystallographic orientations of the diamond crystals. Three wear patterns were identified: single-flat wear, double-flat wear, and micro-chipping wear. The single-flat and double-flat wear patterns were primarily observed in machining silicon nitride and alumina; the microchipping wear pattern was observed in machining silicon carbide. The wear rate for the microchipping pattern was found to be one to two orders of magnitude higher than those for other wear patterns. Silicon nitride wore the diamond tools faster than alumina did; however, it often formed built-up lips which reduced the wear of the diamond tools.