Lubricant Droplets Research Articles

Through-tool minimum quantity lubrication and effect on machinability

This paper simulated through-tool minimum quantity lubrication (MQL) to characterize micromist droplet size and distribution. The effect of nozzle surface roughness and air pressure was experimentally performed to study the lubricant droplet and its effectiveness in micromilling of Inconel alloy. The external MQL simulated internal flow in commercially available drill with internal cooling channels. Droplets were collected on a glass plate from which the average airborne diameters and standard deviation were calculated. The droplet diameter and distribution were most sensitive when using nozzle with rough internal surface. MQL at different conditions was used in micromilling of Inconel 718 blocks that were additively printed by selective laser melting technique. Micromist generated from a rough nozzle at 550 kPa effectively improved tool life and produced micromilled slots with surface finish Sa of 1.5 μm.

Film building in circular EHL contact lubricated by oil droplet groups

AbstractNumerical calculations have been conducted to study elastohydrodynamic lubrication (EHL) with lubricant in the form of droplet groups, which was aimed to simulate air‐oil lubrication. Two types of lubricant droplet distributions, ie, 1 row of droplets along the centreline of the contact or 2 rows of droplets on each side of the centreline, were adopted. The results showed that continuous EHL film could be obtained in spite that discontinuous lubricant droplets were supplied, which is attributed to the squeeze spread and rejoining of the droplets at the inlet. The total effective lubrication time for full film lubrication has a linear correlation with the number of the droplets, and there exists an optimum droplet spacing distance for continuous full film lubrication. For a constant volume of lubricant, calculations indicated that for most efficient lubrication, there is a trade‐off between the droplet size and the number of droplets divided.

Effects of Various Lubricants and Fuels on Pre-Ignition in a Turbocharged Direct-Injection Spark-Ignition Engine

Recently, downsized turbocharged direct-injection engines with high efficiency and power have drawn attention worldwide. However, abnormal combustion can occur when these engines are driven at low speeds with a high load, causing potentially fatal damage to these engines. This phenomenon pre-ignition is known to be caused by lubricant droplets, and the calcium, zinc, and molybdenum components of the lubricant affect the pre-ignition. We performed several experiments to determine the effects of these components, various fuels, and different coolant temperatures on the occurrence of pre-ignition. We also carried out a separate analysis to investigate and compare how each lubricant component affects the frequency and intensity of pre-ignition with each fuel.

Behaviors of a micro oil droplet in an EHL contact

Oil–air lubrication supplies lubricants in the form of droplets to elastohydrodynamic lubrication (EHL) contacts, such as those in high-speed spindle bearings. However, there is a paucity of information related to understanding the lubrication behaviors of oil droplets within EHL contacts. In this study, behaviors of lubricant droplets, in terms of spreading around a static contact as well as passing through a rolling contact, were studied with an optical ball-on-disk EHL test rig. Influences of oil droplet size, viscosity, and surface tension on droplet spreading were examined. Lubricating film formation was also investigated when droplets traveled through the EHL contact region. The results indicated that droplet size and running speed significantly influenced film profiles. With increasing entrainment speeds, a small droplet passed through the contact without spreading and generated films with a significant depression in the central contact region.

Tribological evaluation of contact-charged electrostatic spray lubrication as a new near-dry machining technique

To minimize lubricant usage and friction at the chip–tool interface, a new near-dry machining technique called “contact-charged electrostatic spray lubrication” (CCESL) is proposed. In this study, the chargeability, penetrability, and wettability of charged lubricant droplets under CCESL conditions were investigated. The atomization and tribological properties of CCESL were compared with those of conventional minimal quantity lubrication (MQL) techniques under various testing-conditions. The experimental results suggest that CCESL improves tribological performance considerably and reduces lubricant consumption compared with conventional MQL. In addition, XPS analysis was used to investigate worn surfaces. The enhanced tribological performance of the new technique is attributed to the formation of a lubricating layer comprising an adsorption film and an oxide layer, which improves the surface quality.

Tribological Performance and Lubrication Mechanism of Contact-Charged Electrostatic Spray Lubrication Technique

To minimize friction at the chip–tool interface and the amount of lubricant usage, a new near-dry machining technique called “contact-charged electrostatic spray lubrication (CCESL) technique” was proposed. The chargeability, penetrability, and wettability of lubricant droplets under CCESL condition were analyzed. The atomization and tribological performance of the CCESL technique were compared with those of the existing minimal quantity lubrication (MQL) technique under different testing conditions. The experimental results suggest that the CCESL technique considerably improves the anti-wear and anti-friction properties compared with the existing MQL technique. In addition, to understand its lubrication mechanism, the morphology and main elements of the worn surface were characterized using an optical microscope and X-ray photoelectron spectroscopy, respectively. In the CCESL technique, an ordered molecule layer which provides effective lubrication is formed on the rubbing surface as polar functional groups in the lubricant molecule are oriented. The enhanced tribological performance is attributed to the fact that the technique can provide more abundant lubricant and oxygen for the interface of frictional pairs to promote the formation of an abundant lubricating layer comprising adsorption and oxide films, which improve the worn surface quality.

Pre-Ignition of Gasoline-Air Mixture Triggered by a Lubricant Oil Droplet

Pre-Ignition of Gasoline-Air Mixture Triggered by a Lubricant Oil Droplet

Spreading Kinetics of Lubricant Droplets on Magnetic Recording Disks

The spreading of droplets of perfluoropolyether lubricants is studied on unlubricated, carbon-overcoated disk surfaces, the type used in magnetic recording disk drives. As the number of hydroxyl end groups on the Fomblin-Z backbone is increased from zero to two to four (Z → Zdol → Ztetraol), the spreading kinetics slows down dramatically for each addition of two hydroxyl end groups, and with eight hydroxyl groups, the lubricant ZTMD no longer spreads. Faster spreading kinetics is observed initially when a central droplet is still available to feed lubricants into the molecularly thin lubricant film. Much slower spreading kinetics is observed once this central droplet has dissipated to form a pancake-shaped film less than a monolayer thickness. During the initial spreading stage with the central droplet, the radius R of the area covered by the spreading film grows as R ~ t ν , with ν ~ 1/3, while during the final spreading stage, ν ~ 0.1. The faster spreading mechanism during the initial spreading stage is attributed to the en masse flow of lubricant molecules in the lubricant film driven by the backing pressure of the lubricant remaining in the droplet. The slower spreading kinetics once the central droplet has dissipated is well described by a thickness-dependent diffusion coefficient D(h) obtained by modeling lubricant migration as Poiseuille flow driven by the disjoining pressure gradient.

Effect of Slider Surface Energy on Head Disk Interface Performance

Study of lubricant-coated sliders showed that low slider surface energy increases in stable slider flying and reduces lubricant droplet formation on the disk surface at ultra-narrow clearance. Both component- and drive-level evaluation showed the lubricant pick-up and the flying height it causes were reduced significantly. Furthermore, there was no magnetic spacing increase at the drive-level for the lubricant-coated slider when using contact detection to set the clearance methods.

Experimental Evidence of Lubricant Droplet Transfer From Slider to Disk

To enable the smaller slider-disk spacing that is required for the continual increase in data storage density, the interaction between the slider and the lubricant layer must be understood. In this paper, an interesting experimental phenomenon of sudden lubricant droplet transfer from the slider to the disk is reported and investigated. This phenomenon is different from the lubricant moguls and ripples previous researchers have identified. Is it expected that this sudden lubricant drop-off will degrade head-disk spacing and magnetic signal. Therefore, it is imperative to understand the mechanism and the driving conditions so that it can be prevented.

Simulation on contact between the droplet and the slider at head-disk interface based on water-hammer pressure model

To understand the cause of read/write error due to lube accumulation, a model to simulate the slider’s response to the contact impact, which can occur between a lubricant droplet on the disk and a slider, was developed. The contact impact model is based on the water-hammer pressure model with an additional damping force, where the wave-shock pressure is assumed to function as the contact pressure, and the damping force defines the damping characteristics of the impact which are due to the lubricant’s high viscosity and squeeze between the droplet and slider contact area along the slider local velocity direction. The transient contact impact is dependent on lube droplet density, disk velocity, pitch angle of the slider, and contact area between the droplet and the slider. The measured read/write signal jump due to lube pickup can be explained by the simulation results. This modeling and simulation are helpful to us in understanding the read/write signal loss due to a lube droplet at head disk interface.

Spreading of droplets on lubricant-patterned substrates

Droplet spreading behaviors on lubricant-patterned substrates are investigated by using molecular dynamics simulations to explore application potentials in magnetic storage drive systems. Microscopic spreading processes are studied by both potential fields of lubricant-patterned substrates and single molecule movements in lubricant droplets. The potential fields indicate that the wall molecules patterned on the substrates attract the mobile ones in the lubricant droplets. Due to the attraction force, the mobile molecules experience difficulties in diffusing freely along the substrates. The single molecule movements in lubricant droplets demonstrate that during the diffusion process, the mobile molecules encounter, adsorb, encompass, and disengage the wall ones. The spreading behaviors are significantly impacted by the bonded ratio. The potential fields indicate that as the bonded ratio increases, the attractive regions of wall molecules merge to overlap, which indicate combined interactions formed by the adjacent wall molecules.

A Water Evaporation Based Model for Lubricant Dryoff on Die Surfaces Heated Beyond the Leidenfrost Point

Lubricants in hot forging are applied to the heated die surface by spraying dilute water based graphite suspensions through high-pressure nozzles. To avoid undesirable steam formation, these lubricant droplets need to completely dry before deformation begins. This paper presents an analytical model to estimate evaporation time for the impacting drops at surface temperatures above Leidenfrost, 300–450°C. It is based on the suspension of deformed droplet on a vapor cushion sustained by a continuous supply of water from the drying droplet. Model predictions are validated by single droplet experiments with different surface temperatures, dilution ratios, and impacting speeds.

Modeling and Simulation of the Interaction Between Lubricant Droplets on the Slider Surface and Air Flow Within the Head/Disk Interface of Disk Drives

Modeling and Simulation of the Interaction Between Lubricant Droplets on the Slider Surface and Air Flow Within the Head/Disk Interface of Disk Drives

A two–dimensional model for the interaction between lubricant droplet on the slider surface and air flow within the head/disk interface of disk drives

We numerically study the interaction phenomenon between a lubricant droplet located on the slider’s air bearing surface and the air flow within the head/disk interface of disk drives. A very strong interaction between the lubricant droplet motion and air flow is discovered for ultra-low-flying slider design. The lubricant droplet is driven to migrate towards the slider’s trailing edge by a net effect of air shearing, air bearing pressure gradient, and disjoining pressure gradient. The droplet simultaneously undergoes significant shape deformation. The moving and deforming droplet in turn modifies the effective air bearing surface profile and induces a corresponding air flow adjustment. For a simplified two-dimensional (2D) slider with a fixed flying attitude, our results show that the moving and deforming lubricant droplet induces significant air bearing force and pitch moment change. The interaction becomes stronger when the size of the droplet and the disk’s linear velocity are increased, and when the flying height and pitch angle of the slider are reduced. Our results indicate that a lubricant droplet contamination entering the air bearing surface may impose a non-negligible disturbance on the slider’s flying dynamics.

Spreading Behavior of Water Based Graphite Lubricants on Hot Die Surfaces

In hot forging, water-based-graphite lubricants are sprayed on the die surface to provide a thin adherent graphite film for the desired metal flow. The film formation depends on the spread of the lubricant droplets and their dry-off. With dryoff-times being very small, the spreading is essentially isothermal. Therefore, the fundamentals of spreading droplets are investigated through droplet mechanics and the similarity principles. Relations are derived between the spray parameters and the non-dimensional spread coefficient using single droplet experiments. These relations together with measured rheological properties are used to predict spread ratio for the various lubricant dilution ratios using validated numerical models.

An effect of dewetting of lubricated surfaces on friction and wear properties

To meet the demand of increased storage density for magnetic data storage, stronger and thinner lubricant films for head disk interface in hard disk drives is desired. It is reported that perfluoropolyether lubricant with polar end groups is dewetted and brings about the flying instability of a magnetic head. In this report the effects of dewetting phenomenon on friction and wear properties are studied. Observations of lubricant film by optical microscopy and friction tests were carried out. The observation showed the lubricated surface became dewetted with time and lubricant droplets were formed. The dewetted area increased with time as shown in the figure and, at the same time, the number of droplets decreased. These results suggest the growth of droplets. From the results of friction tests, it was found that the friction coefficient increased and the lifetime reduced with the growth of droplets. The reason for this will be discussed in this research.

The interactions between Z-Tetraol perfluoropolyether lubricant and amorphous nitrogenated- and hydrogenated-carbon surfaces and silicon nitride

The interactions that occur between the hydroxyl-terminated perfluoropolyether Z-Tetraol and the ion beam-deposited amorphous hydrogenated CH x, sputtered amorphous nitrogenated carbon CN x, and sputtered amorphous silicon nitride SiN x surfaces, are investigated via surface energy measurements, kinetic measurements, and ab initio calculations. The film thickness dependence of the Z-Tetraol dispersive surface energy can be fit using a repulsive van der Waals potential, where the repulsion between the Z-Tetraol main chain and the underlying surface increases in the order: CH x (IBC) < CN x < SiN x, due to the increased average separation between the lubricant main chain and the underlying surface. Ab initio calculations on model dimers quantify the separation distances and binding energy of the lubricant/surface interactions. The Z-Tetraol thickness dependence of the polar surface energy indicates that strong polar interactions occur between the applied lubricant and the underlying surface. The decrease in the polar surface energy is correlated to the increased level of bonding between the hydroxyl end groups of Z-Tetraol and the polar sites of the underlying surface. Further analysis of the surface energy data identifies the critical film thickness at which film instability occurs to be near 17.5±1.0 Å for the 2200 molecular weight of Z-Tetraol on the various surfaces. Images of the disk surfaces showing the evolution of lubricant droplets due to dewetting corroborate the surface energy data analysis.

Development of life prediction model and improvement of a die steel for hot forging die : Miyahara, M., Nomoto, K., Umino, M., Sera, T., Kurokawa, Y. and Okada, Y. Sumitomo Metals (Apr. 1996) 48 (2), 76–84

In hot forging, water-based-graphite lubricants are sprayed on the die surface to provide a thin adherent graphite film for the desired metal flow. The film formation depends on the spread of the lubricant droplets and their dry-off. With dryoff-times being very small, the spreading is essentially isothermal. Therefore, the fundamentals of spreading droplets are investigated through droplet mechanics and the similarity principles. Relations are derived between the spray parameters and the non-dimensional spread coefficient using single droplet experiments. These relations together with measured rheological properties are used to predict spread ratio for the various lubricant dilution ratios using validated numerical models.

Method and apparatus for coating recorded disks with a lubricant

The surfaces of a recorded disc are coated with a thin layer of lubricant by forming in an atomizer chamber droplets of the lubricant supported in air. The droplets are formed by passing a flow of air through a supply of the lubricant in the atomizer chamber at a rate and pressure such that the air causes the lubricant to be blown into the space above the supply as fine droplets. The air laden with the lubricant droplets flows out of the atomizer housing to a pair of nozzles in a coating chamber. The nozzles direct the air laden with the lubricant droplets onto the surface of the recorded disc which passes between the nozzles. Excess lubricant which does not coat the disc is collected in the coating chamber and carried back to the atomizer housing.