Lubricant Thickness Research Articles

Computational fluid dynamics analysis of top compression ring in mixed lubrication

The top compression ring design of an internal combustion engine has an impact on ring in-plane motion and its lubrication conditions at the ring-cylinder liner contact. In this paper, the geometrical dimensions of the top compression piston ring-cylinder system were obtained from an actual four-stroke motorbike engine. The top ring tribological behaviour was characterized by a Computational Fluid Dynamics (CFD) simulation including the effects of asperity contact. Based on the numerical solution of the Navier–Stokes equations and taking into account realistic engine running conditions, the effect of the in-plane top ring motion in quasi-static equilibrium was determined. The simulation model was validated by the numerical and experimental results of similar investigations of other researchers. Good predictions were achieved by solving the Navier-Stokes equations because the pressure gradient into the lubricant film was accounted for. The effects of ring curvature at the ends of the stroke were studied. The results show that a flatter ring profile has a sufficient minimum lubricant thickness at reversal points, showing reasonably lower boundary friction than that of the higher curvature. Higher heights of the curvature profile promote significantly mixed lubrication, in which the power losses and the burning of excess lubricating oil are increased. The proposed simulation model can be expanded to any set of compression rings where a minimum simulation time is required.

Optimising the design of textured surfaces for reducing lubricated friction coefficient

AbstractUnder operating conditions which are unfavourable for lubrication, such as high load and low velocity, the use of textured surfaces significantly promotes the formation of a thick lubricant film and an improvement of the friction coefficient. This paper relates to the manufacture of textures using a photolithography and chemical etching process. Different surface geometries, texturing densities and depths were designed to analyse the influence of these parameters. The friction coefficient was measured in a ball‐on‐disc tribometer under different lubrication regimes, and the results have been used to develop an artificial neural network with texturing optimisation potential. © 2016 The Authors Lubrication Science published by John Wiley & Sons Ltd.

Tribological design of a multistep journal bearing

Journal bearings are machine elements with widespread application. The extent of their use, from the automobile engines to gas turbines, gives a significant incentive for improvement of their performance. Various approaches focusing on the journal bearing design have been used in the past in order to reduce power losses and increase load carrying capacity of journal bearings, including artificial texturing, axial and circumferential grooves and bushing specific shapes (e.g. three-lobe bearings). In this paper, the multistep journal bearing is examined. Its performance benefits are quantified. A maximum friction coefficient improvement of 38% is obtained with simultaneous 9.7% improvement of load capacity in comparison to the plain bearing. The specific operating conditions under which these benefits occur are established. The physical mechanism responsible for these improvements is discussed by correlating pressure, shear stress distribution, lubricant thickness and bushing geometry. The ultimate objective of this work is to suggest certain design principles that may adjust the improvement of the desired operating characteristics of the multistep journal bearing. The surface configuration of the bushing is relatively simple and is thus easy to be manufactured.

Measurement of Thickness Distribution of Molecularly Thin Lubricant Films on Head Sliders Using Ellipsometric Microscopy

Lubricant transfer or pickup from a disk to head slider is a crucial issue in designing the head–disk interfaces with a reduced head–disk gap. Observation of lubricant on the slider is difficult with the conventional methods due to their limited lateral resolution. In this paper, nanometer-thick lubricant on a slider was observed by using vertical-objective-based ellipsometric microscopy (VEM). The lubricant thickness was evaluated using a null-ellipsometric method. The thickness measured by VEM agreed fairly well with that by atomic force microscopy. The temporal change in the thickness of a nanometer-thick lubricant film distribution around a read/write element in the slider was successfully observed with a lateral resolution of the order of $1 ~\mu \text{m}$ .

Lubricant Transfer Model at the Head-Disk Interface in Magnetic Storage Considering Lubricant–Lubricant Interaction

Interactions among molecularly thin lubricant–solid and lubricant–vapor interfaces become increasingly strong when the slider approaches closer to the disk (or media) surface, which is desirable for higher storage areal densities. Lubricant transfers from the disk to the slider as the head-media spacing decreases below a critical value, leading to loss of protection for the solid thin films. The present work proposes a new criterion for lubricant pickup to occur by improving the lubricant transfer model via considering interactions between the lubricant films, in addition to lubricant–solid interaction. The effect of lubricant roughness is also included in the proposed model, which is then used to predict critical solid distances as a function of lubricant roughness and average lubricant thickness. It is found that the interaction force between the slider solid and the disk lubricant plays a weaker role than that between the lubricant on the slider and the lubricant on the disk in determining the critical solid distance. The physics for the dependence of the critical clearance on the bonding ratio is explained, and a linear model is developed.

Investigations on transitions of lubrication states for water lubricated bearing. Part II: further insight into the film thickness ratio lambda

Purpose As the companion paper of Part I, this paper aims to get more insight into the essence of lambda and to reveal its nature and role in the transition of lubrication states. Mixed lubrication (ML) model with micro-asperities contacts has been discussed in details in Part I. Design/methodology/approach Mimetic algorithm is used to get numerical solutions. Relationships between film thickness ratios and lubrication states transition with different external loads, rotating speeds, radial clearances, elastic modulus, surface hardness and roughness parameters are obtained. Findings The characteristic parameters of transitions from boundary lubrication (BL) to ML and ML to hydrodynamic lubrication (HL) are studied to determine how these parameters change with above factors. Finally, the essence and major influencing factors of lambda are summarized for such bearings. Originality/value In Part II, the authors believe that the paper presents for the first time: further insight into the essence of the lambda ratio, and its role in the lubrication states transition are given; the determinations of the characteristic parameters of transition from BL to ML and ML to HL are investigated for the first time; the characteristic parameters of transitions from BL to ML and ML to HL are also studied to determine how parameters (external load, rotating speed, radial clearance, elastic modulus, surface hardness and roughness parameter) change with above factors; a summary of the essence and major influencing factors of lambda for such bearings is given.

The effects of lubricant and tendon mass variances on the coefficient of friction in unbonded post-tensioning tendons

In unbonded post-tensioning tendons, the coefficient of friction varies from one design standard to another. This variation is caused by the large number of complex factors that must be considered in design. These factors include the thickness of lubricant or grease present in the system, clearance between the strand and the plastic sheath, and surface characteristics of the strand and plastic sheath. In order to determine the effect of these factors a series of friction tests were performed on two different diameters of strand, namely 12.7 mm and 15.24 mm diameters. Through a regression analysis, it was found that the frictional force decreases with an increase in the thickness of the grease, and that friction increases with an increase in the mass of the strand. The degree of friction was found to be dependent on the surface characteristics of the strand and plastic sheath, clearance between the plastic sheath and the strand, and the extrusion process of the plastic sheath.

Effects of head wear performance with respect to the lubricant thickness on hard disk media for ultra-thin lubricants

Magnetic head wear performance was studied using various thicknesses of D-4OH lubricant that covers hard disk media. The characteristic head wear behavior to over-push the dynamic flying height power differed for each lubricant thickness. The difference in the behavior was highly dependent on the media friction coefficient. The surface free energy also varied as a function of the lubricant thickness. According to the lubricant coverage analysis, we found that the head wear behavior and the friction were dependent on the lubricant coverage change as a function of the lubricant thickness. Furthermore, the molecular dynamics method was applied to investigate the lubricant molecular distribution; the coverage changed with the lubricant thickness similar to that in the experiment. Finally, a head---disk interface mechanism under an over-pushed state was discussed, and meniscus bridge formation was suggested to have occurred at touchdown power for each lubricant thickness.

Terraced spreading of nanometer-thin lubricant using molecular dynamics

Ultra-thin lubricant films are essential in the design of nanoscale systems and devices as surface effects become increasingly important on the nanoscale. We have used molecular dynamics simulations to quantify terraced spreading of perfluoropolyether lubricant on a flat substrate as a function of polymer chain length, lubricant thickness, and functional end groups of the lubricant and the substrate. In addition, we have investigated the physical mechanisms that drive terraced lubricant spreading on a flat substrate. The results show that terraced lubricant spreading follows a process of diffusion and instability, where functional lubricant end groups are attracted to other functional end groups to form clusters that organize into layers. These distinct layers of functional end groups cause the lubricant thickness profile to take on a terraced shape, where layers correspond to the locations at which terraced formations occur. The presence of functional end groups determines the locations of both layer and terrace formations, and greatly affects lubricant spreading.

Analysis of Porous Layered Journal Bearing Lubricated with Ferrofluid

This paper presents an analysis of porous layered long journal bearing lubricated with ferrofluid using displaced infinitely long wire magnetic field model. The ferrofluid flow in the porous region is analyzed using modified Brinkman model. Nondimensional pressure and shear stress expressions are derived using Reynolds boundary conditions. Nondimensional load capacity and coefficient of friction are evaluated under the influence of permeability of porous media, porous layer thickness, lubricant layer thickness, magnetic field intensity and distance ratio parameter. A porous layered journal bearing lubricated with ferrofluid increases the load carrying capacity and reduces the coefficient of friction.

イオン結合末端を有する極薄膜PFPE潤滑剤のDLC表面への吸着特性

Affinity to DLC surface of two types of PFPE lubricant films were compared. One lubricant material (SNH2) was Deminaum main-chain with single ionic end-group and other lubricant material (Z-tetraol) was Fomblin main-chain with double hydroxyl end-groups. SNH2 showed much thicker lubricant thickness than Z-tetraol at the same lubricant concentration. However, SNH2 showed larger polar surface energy than Z-tetraol. The reason was estimated that the ionic function had stronger polarity and that polarity increased the polar surface energy.

Analysis of accumulated lubricant for air–helium gas mixture in HDDs

For higher areal density, full helium as well as air---helium gas mixtures are currently being investigated as candidates for use in advanced drives. Moreover, the head mechanical spacing and the amount of accumulated lubricant on the bottom surface of the slider should be reduced for higher areal density. To analyze the characteristics of accumulated lubricant under these conditions, gas property equations for binary gas mixtures as functions of helium fraction ratio and temperature were derived. Accumulated lubricant equations were derived by considering the attractive forces, such as the dispersive and polar effects. An increase in the helium fraction ratio reduces the small shear stress on the bottom surface of the slider, which leads to a decrease in lubricant transfer capability. As a result, the amount of total lubricant transfer and accumulation are increased markedly. Also, with a decrease in lubricant thickness, the amount of accumulated lubricant increases greatly with a high helium fraction ratio. In conclusion, the HDI problems induced by lubricant transfer should be considered in full helium-filled or air---helium gas mixture drives.

Laser-Induced Thermo-Desorption of Perfluoropolyether Lubricant from the Surface of a Heat-Assisted Magnetic Recording Disk: Lubricant Evaporation and Diffusion

Heat-assisted magnetic recording (HAMR) storage technology will thermally stress the lubricant film typically applied to the storage disk surface. Different lubricant loss and morphology change mechanisms have been hypothesized to occur during information writing. A loss to of the lubricant film will dramatically affect its overall mechanical and chemical performance of the head–disk interface, decreasing its reliability and its durability. Thus an all optical pump–probe method was used to study the effect of fast thermal transients (106 K/s) on a lubricant film on HAMR media. Thermal transients (Bhushan and Cheng in J Appl Phys 81:5390, 1997) with peak temperatures above the HAMR media Curie temperature (T c) were found to remove by evaporation the lubricant within the heated region creating a lubricant depression in the otherwise continuous film. No accumulation of lubricant volume was observed to take place in the cooler regions of the thermal spot, indicating that thermocapillary shear stress is not an important mechanism of lubricant thickness change with the optical spot size used (65 μm). The onset of lubricant loss was observed to begin at approximately 610 K and was totally removed at 823 K. The change in depth of the lubricant depression with time showed that no structural terms contributed to the disjoining pressure for the lubricant thickness range studied. From this change, the diffusion coefficient of the lubricant on the carbon overcoat surface was determined to be 1 × 10−13 m2/s by fitting Fick’s second law to the normalized lubricant thickness. The importance of these observations on the operating HAMR head–disk interface is discussed.

Tribological Properties and Nanomechanics of Cu-BTA Composite Nanooils

This project investigated the tribological properties and nanomechanics of Cu-benzotriazole (BTA) composite nanooils. Cu-BTA nanoparticles were synthesized by a thermal decomposition process. Cu-BTA nanoparticles were added into paraffin oil to form the nanooils. Cu-BTA explores the nanomechanics of sphere geometry functions as a rolling medium for friction lower. BTA nanoparticles functions as a protector from oxidation of the Cu nanoparticles in various test circumstances. Tribological experiments were conducted using a pin-on-disk (ASTM G99) test for the wear scar diameter, friction coefficient, and morphology of worn surfaces. The experiment results revealed the dispersion capability of the benzotriazole-capped Cu nanoparticles and indicated the dispersing stability in liquid paraffin oil for the BTA-capped surface of Cu nanoparticles. The testing results show that the Cu-BTA nanoparticle used as an additive in paraffin oil at an appropriate concentration exhibits better tribological properties than those of pure paraffin oil. Cu-BTA functioning as an additives have different anti-wear abilities due to its small size effect and a high absolute viscosity given high Herser number, corresponds to relatively thick lubricant film and an larged elastohydrodynamic lubrication area. A thin film or powder consisting of spherical Cu-BTA nanoparticles on pin-on-disk (ASTM G99) test iron surface protests against damage from relative rolling movement, which reduces friction and wear.

A parametric investigation on the microelastohydrodynamic lubrication of power law fluid lubricated line contact

A parametric study of microelastohydrodynamic lubrication contact between rough cylinder and plane lubricated with non-Newtonian power law fluid is carried out. A Gaussian rough surface is considered and modeled using stochastic approach. A theoretical solution of pressure distribution, fluid film thickness, and friction for given speeds, material properties, slide-to-roll ratio, and power law index, is obtained by solution of Reynolds equation using finite element method techniques. An isotropic roughness pattern is considered and results are obtained for wide range of operating parameters. The results indicate that the shear-thickening fluids provide thicker lubricant films as compared to shear-thinning fluids. The minimum fluid film thickness is observed to decrease with an increase of load and it increases with an increase of speed and material parameter. Slide-to-roll ratio has a marginal influence of the minimum film thickness. Under certain operating conditions, softer materials with shear-thickening lubricants yield thicker lubricant films than the harder materials lubricated with shear-thinning fluids. The coefficient of friction too is influenced by the rheology of lubricants with shear-thinning fluids providing larger values of coefficient of friction than the shear-thickening fluids.

Artificial Neural Network Model for Monitoring Oil Film Regime in Spur Gear Based on Acoustic Emission Data

The thickness of an oil film lubricant can contribute to less gear tooth wear and surface failure. The purpose of this research is to use artificial neural network (ANN) computational modelling to correlate spur gear data from acoustic emissions, lubricant temperature, and specific film thickness (λ). The approach is using an algorithm to monitor the oil film thickness and to detect which lubrication regime the gearbox is running either hydrodynamic, elastohydrodynamic, or boundary. This monitoring can aid identification of fault development. Feed-forward and recurrent Elman neural network algorithms were used to develop ANN models, which are subjected to training, testing, and validation process. The Levenberg-Marquardt back-propagation algorithm was applied to reduce errors. Log-sigmoid and Purelin were identified as suitable transfer functions for hidden and output nodes. The methods used in this paper shows accurate predictions from ANN and the feed-forward network performance is superior to the Elman neural network.

S1110402 CNx膜の潤滑油中における低摩擦発現に及ぼす油の粘度と油膜厚さの影響

The carbonaceous hard coating such as Diamond-Like Carbon is one of the materials to achieve low friction. It was reported that Carbon Nitride (CNx) coating showed lower friction coefficient than any other DLC coating when it slid in lubricated environment of PAO. But the mechanism of CNx's lower friction coefficient is not clarified. In this study, we clarified the effect of viscosity of lubricant and lubricant film thickness between contact surfaces to low friction of CNx coatings. We conducted pendulum-type test in PAO2, PAO4 and PAO8 lubricated conditions. From the results, friction coefficient of CNx coatings decreased with the increase of viscosity of lubricant, 0.0876 in PAO2, 0.0784 in PAO4 and 0.0609 in PAO8, while friction condition represented by film parameter(Λ, the ratio of lubricant thickness and surface roughness) showed boundary lubrication. And we also conducted sapphire-pressing test under reflectance spectrometry to clarify the existence of lubricant film between contact surfaces. From these results, reflectance of CNx coating after oil dripped surface decreased at the rate of 65.3 % before dripping more than that of ta-C coating (17.6 %).

External Electric Field Assisted Ultraviolet Irradiation for Bonding of Lubricant Film on Magnetic Disks

In this paper, we sought to determine if, during ultraviolet (UV) exposure, the external electric field could control the bonded lubricant thickness. During UV irradiation, the effect of the external electric field on both the bonded lubricant thickness and surface free energy was studied. The lubricant films after UV irradiation were analyzed using time-of-flight secondary ion mass spectroscopy (TOF-SIMS). Because of the negative electric field, the polar component of the surface free energy was reduced, while the bonded lubricant thickness increased. Based on the results of the TOF-SIMS analysis, we concluded that these findings resulted from the negative electric field accelerating the photoelectron emission, and because of these photoelectrons, the bonding between the end groups and diamond-like carbon surface increased.

Hydrodynamic Performance Produced by Nanotexturing in Submicrometer Clearance With Surface Roughness

Surface texturing is a promising way to expand the hydrodynamic lubrication regime and thereby modify the tribological properties of sliding surfaces. Spiral-groove textures in particular have attracted much attention over the past several decades because they produce a thicker lubrication film in the gap. However, no research has been reported on the effect of periodic texturing with a several 100 nm depth on hydrodynamic performance in submicrometer clearance with surface roughness. The purpose of the study reported here was to investigate the effect of such nanotexturing on hydrodynamic performance. This was done by conducting ring-on-disk friction tests, focusing on the existence of surface roughness in the narrow clearance. The samples were rings with various degrees of surface roughness and disks with spiral-groove textures produced by femtosecond laser processing. The friction coefficients experimentally obtained were plotted as a Stribeck curve and compared with a theoretical one calculated using a Reynolds equation formulated from two physical models, the Patir–Cheng average flow model and a sinusoidal wave model. The results showed that surface roughness did not affect the friction coefficient in the hydrodynamic lubrication regime. However, the hydrodynamic lubrication regime gradually shrank with an increase in surface roughness, and mild transitions to the mixed lubrication regime were observed at higher rotational speeds. The minimum clearances reached at the transition speed were almost the same, about 200–300 nm, for all experiments regardless of surface roughness.

Simulation of the Performance of Various PFPE Lubricants Under Heat Assisted Magnetic Recording Conditions

Heat assisted magnetic recording (HAMR) is proposed for the next generation of hard disk drives. In HAMR systems, a laser beam heats the disk magnetic layer to the Curie temperature. This may cause the thin film lubricant coating the disk to deplete due to evaporation and surface tension gradient. In this study, we perform simulations for the Z-tetraol family of lubricants with four hydroxyl end-groups, including Z-tetraol 1200 as a low molecular weight member of the family and Z-tetraol 2200 as a high molecular weight of the family, and also for ZTMD (2,200 Da) with eight hydroxyl groups as a multi-dentate lubricant, which is manufactured based on the Z-tetraol family. All studies are performed for four cases of lubricant thicknesses including 5, 7, 12, and 14A. These numbers are chosen in order to provide a fair comparison with a previous study for Z-dol. We also investigate the relative effects of evaporation with respect to the thermocapillary shear stress. It is found that after a 2 ns illumination of the laser, a trough and two side ridges across the down-track direction can be seen in the lubricant. The performances of the lubricants can be ranked mainly based on the trough depth and also evaporation such that better lubricants show less deformation and trough depth under equal conditions of thermal spot size and peak temperature. We also found that all of the lubricants deplete rapidly and their depletion speed decreases gradually.