Ultrathin Lubricant Film Research Articles

Online monitoring of oil film thickness of journal bearing in aviation fuel gear pump

As the key support of aviation fuel pumps, journal bearings are operated under extreme conditions of high temperature, high pressure and low viscosity media, therefore, the broken of the ultra-thin lubrication film is the main cause of lubrication failure and abnormal wear. However, the dynamic hydraulic lubrication (DHL) film is still blind due to the lack of non-destructive online monitoring techniques. This paper presents the study of ultrasonic-based measurement for the dynamic oil film thickness of fuel pump. The circumferential oil film thicknesses in a journal bearing are measured with the complex model and the resonance model, meanwhile, the minimum oil film thickness is extracted by fitting the measured points with the initial circle. Under varying operating conditions, the measured results are verified with classical DHL simulations, and the maximum relative error is shown to be lower than 15%.

Prediction on Flow and Thermal Characteristics of Ultrathin Lubricant Film of Hydrodynamic Journal Bearing.

This paper focuses on the flow and thermal characteristics of the lubricant film in the micro clearance of a hydrodynamic journal bearing (HJB) at high rotating speed. A thermohydrodynamic (THD) method consists of the Reynolds equation coupled with energy and viscosity-temperature equation with considering the cavitation is put forward. The 3D surface diagrams of the lubricant film thickness, pressure, temperature, liquid mass fraction, flow rate and heat dissipation distributions under different geometric, operating, slip and no-slip boundary conditions are systemically exhibited and analyzed. The results show that with the rise of eccentricity or length diameter ratio, the maximum peaks of pressure, temperature and heat dissipation are rapidly increased, the cavitation is aggravated, and the flow rate is accelerated in different extent. As the bearing speed accelerating, the maximum peak of temperature is strongly increased, whereas, the distinction between peaks of flow rate and heat dissipation is magnified and reduced, respectively. It provides a fruitful inside view of the inner flow and thermal characterizations of HJB for further understanding its flow-thermal interaction mechanisms and offers theoretical support for improving its working performance.

Investigations on start-up performances of novel hybrid metal rubber-bump foil bearings

In the frequent start-up process of aerodynamic bearings, contact easily occurs between journal and bearing bush. In this research, novel hybrid metal rubber-bump foil bearings (MR-BFBs) were proposed. Based on rough surface contact and ultra-thin film lubrication theory, we studied the start-up performances of MR-BFBs. The results showed that the changes in both RSm and Ra caused great changes in the parameters pmax and A0. The increased start-up times led to a decrease in the load ratio Wc/W0 and the total friction coefficient, and an increase in both the linear velocity, the gas film pressure and the minimum gas film thickness. With the increase in the composite roughness, both the critical rotation speed and start-up time exhibited an upward trend.

Laser-induced depletion of ultrathin PFPE lubricants using a quantitative coarse-grained model.

Knowledge of laser-pulsed processing of ultrathin liquids on solid surfaces is of great importance in advancing the understanding of synthesis, characterization and applications of functional nanofilms. In this work, we performed molecular dynamics simulations coupled with a novel quantitative coarse-grained model to study laser-induced local heating and depletion behaviors of Fomblin Z2000 lubricants. It was found that the Fomblin lubricant films experience severe evaporation and thermodiffusion-related degradation under laser heating. The ultrathin lubricants inevitably lose their frictional resistance under extremely high temperatures and thermal gradients (i.e., up to 990K and 65.4K/nm) that are required for FePt media in HAMR systems. The factors that influence the local thermal depletion, such as laser power, spot size, and scanning velocity, were investigated as well. The results clearly show that the laser power has the greatest influence on the relative maximum temperature change and subsequently the severe lubricant film depletion during rapid laser heating, while the spot size and scanning velocity play a relatively weaker role in the laser-induced local thermal instability. The findings in this work are thus believed to provide molecular scale insights into laser-induced local heating and depletion of ultrathin lubricant films from a precise quantitative perspective.

Theoretical Investigation of Transient Ultra-Thin Lubricant Film During Rapid Deceleration

Theoretical Investigation of Transient Ultra-Thin Lubricant Film During Rapid Deceleration

Friction Characteristics of Ultrathin Perfluoropolyether Boundary Lubricant Films Subjected to Laser Irradiation Heating in Heat-Assisted Magnetic Recording

We investigated the laser-heating-temperature dependence of the friction characteristics of magnetic disks coated with boundary lubricant films by using a pin-on-disk tester and discussed the fundamental characteristics observed. In the contact area of the glass pin and disk, we observed the area that showed a buildup and depletion of the lubricant film thickness. The amount of buildup and depletion increased with increasing heating temperature. It was found that the friction coefficient of each lubricant film initially decreased as the temperature increased. Thereafter, the minimum value of the friction coefficient of each test lubricant occurred at a specific temperature, resulting in an increase in the friction coefficient inversely as the temperature further increased. It was suggested that the specific temperature corresponds to the transition temperature at which the test lubricant film evaporates. In addition, it was also suggested that the thermal durability of the lubricant film improved because of ultraviolet treatment.

Stick-slip boundary friction mode as a second-order phase transition with an inhomogeneous distribution of elastic stress in the contact area.

This article presents an investigation of the dynamical contact between two atomically flat surfaces separated by an ultrathin lubricant film. Using a thermodynamic approach we describe the second-order phase transition between two structural states of the lubricant which leads to the stick–slip mode of boundary friction. An analytical description and numerical simulation with radial distributions of the order parameter, stress and strain were performed to investigate the spatial inhomogeneity. It is shown that in the case when the driving device is connected to the upper part of the friction block through an elastic spring, the frequency of the melting/solidification phase transitions increases with time.

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.

Molecular dynamics study of ultrathin lubricant films with functional end groups: Thermal-induced desorption and decomposition

Molecular dynamics simulation is employed to study the thermal durability and stability of molecularly thin lubricant films with functional end groups. A coarse-grained polymer model that describes intrinsic bond breakage is presented. Using this novel model, we investigated the bead number density profile to quantify the lubricant weight loss that occurs during heat treatment process. The density profile, which was partially computed by a hyperbolic tangent function, reveals that both the liquid–vapor interfacial thickness and the solid–vapor separation increase with temperature. Nonetheless, the liquid phase diminishes at high temperatures and the solid–vapor separation begins to decay as the temperature increases further. We also investigated the lubricant weight and bond loss profiles to provide an insight on the thermal instability of nonfunctional and functional lubricant films. Our simulation results show that the functionality of the lubricant film can greatly influence lubricant desorption albeit a slight effect on lubricant decomposition. For both nonfunctional and functional thin lubricant films, lubricant desorption is the domination depletion process and is attributed as the root for lubricant degradation and failure.

Lubricant depletion due to moving laser heating: A molecular dynamics simulation study

Heat-assisted magnetic recording has been perceived as an enabling technology of next generation hard disk drives. In this paper, molecular dynamics simulations of ultrathin polymeric lubricant film under moving laser heating are carried out. The effect of the laser spot size is examined and the surface morphologies show that the lubricant undergoes severe depletion through evaporation and thermodiffusion. Raised ridges due to non-equilibrium thermocapillary stress are formed around the depletion track towards the laser moving direction when ultra-small laser spot size is used. Lubricant depletion at different laser spot sizes results in thermodiffusion-related degradation. More intriguingly, a critical laser spot size is observed, at which the film exhibits maximum thermodiffusion.

Effect of Surface Forces on Ultrathin Film Lubrication

Lubricated contact with nanoscale oil film is modeled for friction. Effect of the van der Waals pressure and the solvation pressure forces on such ultrathin lubricating oil film is considered while finding the friction and other parameters. Hydrodynamic action is represented using transient thermoelastohydrodynamics. Net pressure due to hydrodynamic, solvation, and van der Waals’ action is integrated over the contact area to find contact load. Conjunctional friction due to thermal activation of such ultrathin film is derived using the Eyring model. Effect of molecular dimension on friction is studied.

A-1-1 両末端・片末端水酸基を有するPFPE超薄膜複合潤滑膜の摩擦・凝着力特性

A-1-1 両末端・片末端水酸基を有するPFPE超薄膜複合潤滑膜の摩擦・凝着力特性

Hysteresis Behavior in the Stick–Slip Mode at the Boundary Friction

A tribological system is considered that consists of two atomically smooth solid surfaces separated by an ultrathin lubricant film. A thermodynamic model based on the Landau theory of phase transitions is built that describes the behavior of this system in the boundary friction mode. The free energy density for an ultrathin lubricant film is given in the form of expansion in series in terms of the powers of order parameter that is reduced to the shear modulus of the lubricant. The kinetics of the system is studied on the basis of a model describing first-order phase transitions between kinetic modes of friction. It is shown that in the presence of spring between the external drive and block the width of temperature hysteresis increases versus fixed coupling.

A Molecular Dynamics Study of the Transition from Ultra-Thin Film Lubrication Toward Local Film Breakdown

The transition from ultra-thin lubrication to dry friction under high pressure and shear is studied using molecular dynamics: the quantity of lubricant in the confined film is progressively reduced toward solid-body contact. A quantized layer structure is observed for n-alkanes confined between smooth, wettable walls, featuring an alternation of well-layered, low friction configurations, and disordered ones, characterized by high friction, and heat generation. The molecular structure influences the ordering of the fluid and the resulting shear stress. In fact, Lennard-Jones fluids are characterized by low friction due to the absence of interlayer bridges, opposed to the always entangled states and high shear stresses for branched molecules. Surface geometry and wettability also affect the behavior of the confined lubricant. The presence of nanometer-scale roughness frustrates the ordering of the fluid molecules, leading to high friction states. Furthermore, local film breakdown can be observed when the asperities come into contact, with strong wall–wall interactions causing the maximum in shear stress. Finally, friction is limited to a small, constant value by the presence of smooth, non-wettable surfaces in the system due to the occurrence of wall slip.

Pereryvchastyy rezhym mezhovoho tertya yak fazovyy perekhid pershoho rodu

A tribological system consisting of two contacting blocks has been considered. One of them is arranged between two springs, the other is driven periodically. The kinetics of the system has been studied in the boundary friction mode, when an ultrathin lubricant film is contained between the atomically smooth surfaces. In order to describe the film state, the expression for the free energy density is used in the form of an expansion in a power series in the order parameter, the latter being reduced to the shear modulus of a lubricant. The stick-slip mode is shown to be realized in a wide range of parameters, being a result of the periodic first-order phase transitions between kinetic friction regimes. The behavior of the system governed by internal and external parameters has been predicted.

6PM2-C-3 熱アシスト磁気記録におけるレーザ照射による潤滑膜減耗特性(6PM2-C OS6 マイクロナノトライボロジー(3))

To achieve magnetic recording densities of more than 10 Tb/in^2, the head-disk interface (HDI) spacing should be less than 2-3 nm. Thus far, thermally assisted magnetic recording (TAMR) technology has been studied and developed to achieve such ultra-high magnetic recording densities. However, the use of ultra-thin liquid lubricant films as well as DLC thin films on the disk surface in TAMR leads to critical issues with regard to the HDI. In addition, HDD technology with head-disk assembly mechanism in which He gas is enclosed has been developed. In this study, the effect of lubricant bonding mechanisms on lubricant depletion has been studied. Furthermore, the mechanisms of lubricant depletion due to laser heating have bean investigated and discussed.

Melting of the ultrathin lubricant film between atomically-flat solid surfaces as second-order phase transition

Melting of the ultrathin lubricant film between atomically-flat solid surfaces as second-order phase transition

Adhesion and Friction Behaviors of Probes Approaching Magnetic Disk Surfaces

In this study, we focused on the adhesion and friction behaviors of probes approaching magnetic disk surfaces. In order to observe these behaviors, a micro-tribotester was newly developed to evaluate the friction and adhesion forces of the ultrathin lubricant film on magnetic disks. The contact area between the spherical glass surface and the disk surface was measured using light intensities and Newton's rings and approximated by the JKR model. An adhesion force corresponding to a pull-off force acted on the interface between the glass surface and the disk surface. The friction coefficients on disks with different lubricant thicknesses were measured and the coefficient was found to decrease as the lubricant thickness increased. The pull-off force gradually increased as lubricant thickness increased.

Depletion of monolayer liquid lubricant films induced by high-frequency pulsed-laser heating in thermally assisted magnetic recording

In this study, lubricant depletion due to high-frequency pulsed-laser heating was investigated for lubricant films with thicknesses of both more than and less than one monolayer. A conventional lubricant, Zdol2000, was used. It was found that the critical temperature at which the lubricant begins to deplete owing to laser heating was strongly dependent on the lubricant film thickness. In the case in which the thickness of the lubricant film was less than one monolayer, this temperature was approximately 170 °C higher than it was when the thickness was more than one monolayer. To analyze the lubricant depletion mechanism, we examined the tested lubricant film using temperature programmed desorption (TPD) spectroscopy. It was found that the lubricant depletion characteristics due to laser heating could be explained using the experimental TPD results for the tested lubricant film, and that the depletion mechanism involves the desorption or decomposition of the lubricant molecules, which interact with the diamond-like carbon thin films when the lubricant film thickness is less than one monolayer. Further, the results of TPD and of a thermogravimetric analysis (TGA) of the lubricant were compared. The thermal robustness of the ultra-thin liquid lubricant films was found to be greater than that of the bulk lubricant materials.

Molecular Dynamics Simulation for Lubricant Shear Properties During Heating

This paper discusses the shear properties of ultra-thin lubricant film on hard disk media during heating. A molecular dynamics method is used on four different types of lubricant to better understand the phenomena on the atomic scale. We investigate the shear mobility of lubricant for each temperature between 30°C and 300°C, assuming there is an air shear-induced migration on a carbon overcoat. The simulation results show that a thick lubricant film has a mobile and restricted layer, while a thin film which thickness is less than 1 nm , increases the slippage at lubricant/overcoat interface. At a high temperature, a lubricant with a flexible main chain tends to have a higher level of mobility. The difference in lubricant shear property is closely related to the flexibility of main chain and the amount of functional groups of lubricant which can affect the shear viscosity and adhesion to the carbon overcoat.