Thin Lubrication Research Articles

Efficient computational modelling of low loaded bearings of turbocharger rotors

The overall efficiency of turbochargers is a strictly monitored variable in all applications. Increasing the overall efficiency is closely related to the mechanical efficiency and hence the rotor bearing system. This paper presents a new computational model describing the behaviour of the thrust bearings of the turbocharger rotor under different operating conditions. The model assumes steady state turbulent flow of lubricant through a thin lubrication gap in the form of a two-phase fluid. Fluid flow is affected by centrifugal forces due to high rotor speeds and by the change in lubricant properties due to pressure, temperature and shear rate. The model is designed to allow a very fast and efficient steady state solution for many input parameter variations, while maintaining a sustainable physical depth of description of the reality. The new computational model is confronted with three-dimensional higher-level models and verified by technical experiments on a real combustion engine turbocharger.

Effect of airborne contaminants on the macroscopic anti-wear performance of chemical vapor deposition graphene

With ultra-high mechanical strength and inherent lubrication property, graphene exhibits great potential as an atomically thin solid lubricant to address wear problems that affect mechanical function and longevity. Despite the excellent wear resistance of graphene at the nanoscale, wear of graphene is difficult to avoid at micro- to macroscale friction tests in ambient conditions and sensitive to the environmental atmosphere and humidity. As an atomically thin material, contaminants from the environment have a non-ignorable effect on graphene surface properties. To study the influence of airborne contaminants on graphene macroscale wear behavior, we performed sliding tests on new (as-prepared) and aged monolayer chemical vapor deposition grown graphene using a 5 mm-radius lens under load down to 0.1mN. The atomic force microscope characterization demonstrated the existence of airborne contaminants on aged graphene. The tribological experiments showed the aged graphene exhibited better wear resistance than the new one due to that the airborne contaminants separated the counterpart from the graphene wrinkles, from where graphene wear was initiated.

Green Tribology: Orientation Properties of Diamond-Like Carbon Coatings of Friction Units in Lubricating Media

Multidisciplinary conceptual approach to studying the friction in friction couples differing in the friction surfaces, their structure, structural organization in thin layers, lubricants, and loads in the contact zones using different methods for recording the sliding and rolling friction is presented. The possibility of controlling the boundary friction using deposited coatings and the orientation effect of these coatings on lubricating materials in friction couples was demonstrated. Application of diamond-like carbon coatings onto working surfaces of parts of heavy-duty friction units (engines, tooth gears, contacts between a metalworking tool and a part being machined, etc.) allows the content of additives in lubricating oils to be significantly reduced and thus the consumption of lubricating materials to be decreased, up to complete abandonment of the use of oils containing expensive or environmentally unfriendly additives.

Solvothermal preparation and tribological performance of g‐C 3 N 4 /TiO 2 hybrids as oil‐based lubricant additives

TiO2 nanospheres wrapped by graphene-like g-C3N4 nanosheets were fabricated through the one-pot solvothermal process and applied as lubricant additives in paraffin base oil to characterise the tribology performances. Moreover, g-C3N4/TiO2 composites were investigated by different characterisation methods such as X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy, indicating the formation of g-C3N4/TiO2 composites. Subsequently, the friction experiment of the resulting samples using g-C3N4/TiO2 hybrids as additives in base oil were tested by a multispecimen friction and wear tester (UMT-2), which indicate g-C3N4/TiO2 composites possessed excellent tribological property including significantly reduced frictional coefficient and wear rate as an oil additive, owing to the presence of a thin physical lubricating film on the surface of the matrix.

Friction Modifier Additives, Synergies and Antagonisms

There is growing interest in reducing friction in lubricated machine components to thereby increase the energy efficiency of machines. One important way to minimise friction is to employ friction modifier additives to reduce friction in thin film boundary lubrication conditions. There are currently three main types of friction modifier additive, organic friction modifiers, oil soluble organomolybdenum friction modifiers and functionalised polymers. In common practice, a single such additive is generally employed in a formulated lubricant, but it is of interest to explore whether combinations of two friction modifier additives may prove beneficial. In this study, the performance of eight commercial friction modifier additives spanning all three main types was first measured in three quite different friction tests. The aim was to identify the contact conditions under which each additive was most effective. Additive solutions in both a base oil and a formulated engine oil were investigated. In general, functionalised polymers were most beneficial in sliding–rolling contacts, while oil soluble organomolybdenum friction modifiers worked best in severe, reciprocating sliding conditions. However, all friction modifier additive response was strongly affected by the other additives present in formulated engine oils. The friction performance of combinations of friction modifier additives was then explored. When two different friction modifiers additives were combined in solution, several possible outcomes were observed. The most common was for one of the additives to predominate, to give friction that was characteristic of that additive alone, while in some cases friction lay between the values produced by either additive on its own. In a few cases the additives behaved antagonistically so that the combination gave higher friction than either additive by itself. In a few cases true synergy was observed, where a combination of two additives produced lower friction in a given test that either individual component at the same overall concentration. Another, and possibly more important synergy could also occur, however, when a pair of FMs worked more effectively than either individual additive over the range of test conditions present in different friction tests. This study suggests that optimal combinations of FMs may provide a means of reducing boundary friction and thus increasing the efficiency of machines, especially if the latter contain a range of lubricated machine components that operate with different types of tribological contacts.

Diprotic Ammonium Succinate Ionic Liquid in Thin Film Aqueous Lubrication and in Graphene Nanolubricant

An ammonium succinate protic ionic liquid (PIL) has been used as additive in water and as base lubricant for graphene dispersions in sapphire-stainless steel lubrication. The tribological performance of a water-based lubricant containing the PIL additive di[bis(2-hydroxyethyl)ammonium] succinate (DSu) in 1 wt% proportion (Water + DSu) has been studied. Both neat DSu and Water + DSu lubricants present a high friction coefficient during the running-in period. Elimination of the running-in high friction period has been achieved with a DSu layer generated on the steel surface by evaporation of water, before the sliding begins. This DSu surface layer reduces the wear rate in two orders of magnitude with respect to full-fluid Water + DSu and to neat DSu lubricants. The high friction running-in period can also be eliminated by the addition of 0.05 wt% few-layers graphene (G) to DSu. The new (DSu + 0.05G) nanolubricant also prevents wear and surface damage on stainless steel. Surface interactions are discussed from contact angles, SEM and TEM microscopies, XPS surface analysis, and Raman microscopy results.

Methodology of Turbulence Parameter Correction in Water-Lubricated Thrust Bearings

Oil-lubricated bearings are widely used in high-speed rotating machines such as those found in automotive industries and aerospace. However, environmental issues and risk-averse operations are resulting in the removal of oil and the replacement of all sealed oil bearings with reliable water-lubricated bearings. The low viscosity of water increases Reynolds numbers drastically and therefore makes water-lubricated bearings prone to turbulence effects. This requires finer meshes for finite element modeling when compared to oil-lubricated bearings as the low-viscosity fluid produces a very thin lubricant film. Analyzing water-lubricated bearings can also produce convergence and accuracy issues in traditional oil-based analysis codes. Fitting the velocity profile with experiments having a nondimensional wall distance y+ in a certain range results in Ng-optimized Reichardt's constants k and δ+. The definition of y+ can be used to approximate the first layer thickness calculated for a uniform mesh. On the condition that the y+ is fixed to that of a standard oil bearing for which an oil-bearing code was validated, the number of elements across the film thickness and coefficients used in the eddy-viscosity equation can be adjusted to allow for convergence with other fluids other than that which the traditional oil-bearing code was designed for. This study proposed a new methodology to preserve the y+ value to make water-lubricated thrust bearing models valid. A method for determining the required number of cross-film elements in water-lubricated bearings was found. The results of this study could aid in improving future designs and models of water-lubricated bearings.

Microdroplet self-propulsion during dropwise condensation on lubricant-infused surfaces.

Water vapor condensation is common in nature and widely used in industrial applications, including water harvesting, power generation, and desalination. As compared to traditional filmwise condensation, dropwise condensation on lubricant-infused surfaces (LIS) can lead to an order-of-magnitude increase in heat transfer rates. Small droplets (D ≤ 100 μm) account for nearly 85% of the total heat transfer and droplet sweeping plays a crucial role in clearing nucleation sites, allowing for frequent re-nucleation. Here, we focus on the dynamic interplay of microdroplets with the thin lubricant film during water vapor condensation on LIS. Coupling high-speed imaging, optical microscopy, and interferometry, we show that the initially uniform lubricant film re-distributes during condensation. Governed by lubricant height gradients, microdroplets as small as 2 μm in diameter undergo rigorous and gravity-independent self-propulsion, travelling distances multiples of their diameters at velocities up to 1100 μm s-1. Although macroscopically the movement appears to be random, we show that on a microscopic level capillary attraction due to asymmetrical lubricant menisci causes this gravity-independent droplet motion. Based on a lateral force balance analysis, we quantitatively find that the sliding velocity initially increases during movement, but decreases sharply at shorter inter-droplet spacing. The maximum sliding velocity is inversely proportional to the oil viscosity and is strongly dependent of the droplet size, which is in excellent agreement with the experimental observations. This novel and non-traditional droplet movement is expected to significantly enhance the sweeping efficiency during dropwise condensation, leading to higher nucleation and heat transfer rates.

Sink dynamics of aqueous drops on lubricating fluid coated hydrophilic surfaces

Slippery behavior of aqueous drops on lubricating oil coated solid surfaces depends strongly on the wettability of the solid substrates. Stable slippery behavior is observed, with floating aqueous drops, if underneath solid substrates are hydrophobic. For hydrophilic surfaces, lubricating oil film underneath an aqueous drop becomes unstable and ruptures due to attractive van der Waals interaction, leading to sinking of the aqueous drop. Here, we report the sinking behavior of aqueous drops on thin lubricating oil coated hydrophobic and hydrophilic surfaces. The sinking behavior of aqueous drops is analyzed in terms of change in the apparent contact angle with time. The sink time of drops depends on both the wettability of the substrate as well as viscosity of the lubricating layer, while the final apparent contact angle depends only on wettability.

Interfacial molecular deformation mechanism for low friction of MoS2 determined using ReaxFF-MD simulation

Two-dimensional layered molybdenum disulfide (MoS2) has been recognized as a promising material for an atomic thin solid lubricant. In this study, we investigated the microscopic friction behavior of MoS2 using a reactive force-field (ReaxFF) molecular dynamics simulation. The effects of the interlayer distance and temperature on the frictional force during the sliding process were studied. The results showed that the friction was mainly affected by the interaction between S atoms at the interface. As the distance between layers decreased, the gradually increasing interlayer repulsive interaction deformed the Mo-S bond of the MoS2 and changed the positions of S atoms at the interface. The migration of S atoms at the interface contributed to the low friction behavior of the MoS2.

Atomic force microscopic measurement of a used cylinder liner for prediction of boundary friction

Accurate simulation performs a crucial role in the design and development of new modern internal combustion engines. In the case of piston rings, simulations are used to effectively predict generated friction and power loss of proposed designs. These are consequences of viscous shear of a thin lubricant film, likewise boundary friction caused by direct interaction of piston rings with the cylinder liner/bore surface. The most commonly used model for determining boundary friction is that of Greenwood and Tripp. The model requires the pressure coefficient of boundary shear strength of asperities from the softer of the contacting surfaces as an input. This parameter needs to be measured. The paper describes the process of measurement using an Atomic Force Microscope (AFM), both for a dry surface and that wetted by the presence of a lubricant layer. For realistic results, the investigated specimen is a used, tested engine cylinder liner where boundary active lubricant additives are bonded to its surface as well as combustion products. This approach is as opposed to the previously reported works using new flat surfaces with base oil or partially formulated lubricants and has not been previously reported in the literature. The results show that for used cylinder liners, the measured boundary shear strength of asperities varies according to location along the stroke. Results are reported for the top dead centre, mid-stroke and bottom dead centre locations. The measurements are subsequently used with two-dimensional Reynolds solution for a top compression ring-liner contact, where it is found that accurate localised predictions of generated friction and power loss can be made instead of the usual average value approach reported in the literature.

Micro-wear and friction of carbon film with molecularly thin PFPE lubricants

When a surface protective diamond-like-carbon (DLC) film experiences high speed contact, it can be degraded and damaged by mechanical stress and frictional temperature rise. In this study, the thermomechanical contact behavior of thin DLC film on a magnetic recording disk was systematically investigated with respect to the contact speed (1.2 m/s ~ 36.1 m/s) using a newly-developed experiment technique. For comparison, experiments were performed on the DLC film samples with and without molecularly thin perfluoropolyether (PFPE) lubricants on the surface. Using the post experiment samples, atomic force microscopy (AFM) and micro Raman measurements were carried out to examine the mechanisms of surface wear and material degradation. It was observed that the higher contact speed (12.0 m/s & 36.1 m/s) caused earlier failure and wider wear track on the surface than the lower contact speed (i.e., 1.2 m/s). When a monolayer thick PFPE lubricant is applied onto the surface, it significantly improved the friction and wear performance of the DLC film. Based on the Raman analysis, it could be found that the DLC on the contact area showed a clear evidence of thermal degradation, i.e., graphitization process, by frictional heat generation.

Contact angle hysteresis and oil film lubrication in electrowetting with two immiscible liquids

Electrowetting (EW) of water drops in ambient oil has found a wide range of applications including lab-on-a-chip devices, display screens, and variable focus lenses. The efficacy of all these applications is dependent on the contact angle hysteresis (CAH), which is generally reduced in the presence of ambient oil due to thin lubrication layers. While it is well-known that AC voltage reduces the effective contact angle hysteresis (CAH) for EW in ambient air, we demonstrate here that CAH for EW in ambient oil increases with increasing AC and DC voltage. Taking into account the disjoining pressure of the fluoropolymer-oil-water system, short range chemical interactions, viscous oil entrainment, and electrostatic stresses, we find that this observation can be explained by progressive thinning of the oil layer underneath the drop with increasing voltage. This exposes the droplet to the roughness of the underlying solid and thereby increases hysteresis.

Low friction states for thin solid lubricant film of MoS2

PurposeDuring the operation of Wendelstein 7-X (W7-X), any mechanical disturbance such as stick-slip may cause quenching of superconducting (SC) coils. The friction behavior of MoS2 lubrication (thin film) for narrow support elements between the SC coils in W7-X is rather important, as there is a design requirement for a coefficient of friction (COF) 0.05 between the sliding surfaces to control the stress contribution (from friction).Design/methodology/approachThe author has carried out intensive calibrations or verifications using verified models considering previous friction tests on various samples which measured the COF in 4.2 K, 77 K and room temperature conditions (at high vacuum) to simulate the actual working condition.FindingsThe author has given useful explanations and diagnosis for previous anomalous scattered data. To improve the performance of MoS2, the author has predicted its better COF (0.002 via tuning of the activation volume), which could be a superlubricating state for MoS2 thin films considering the long-term operation requirement W7-X.Originality/valueIn this paper, the author has adopted Eyring’s approach to predict the low COF (0.002 via tuning of the activation volume), which could be a superlubricating state for MoS2 thin films considering the long-term operation requirement W7-X. Finally, some recent progresses about the possible few-layer MoS2 role in the electromagnetic loads have been provided.

Mass transfer in Taylor flow: Transfer rate modelling from measurements at the slug and film scale

Mass transfer in non-reactive gas–liquid Taylor flow has been studied at the unit cell scale with high resolution non-invasive experimental techniques for a large variety of hydrodynamic regimes at high inertia (30 ≤ Reb ≤ 1430). The planar laser induced fluorescence with inhibition (PLIF-I) technique has been used to measure the local oxygen concentration fields in different liquid phases (tap water, water and Breox solutions at different concentrations) in order to vary the Schmidt number Sc. The concentration field can be separated into a film region, corresponding to the thin lubrication film extended all along the channel wall, and the remainder of the liquid which makes up the slug region. It has been found that even though the global mass transfer is mainly driven by the rate of transfer in the slug, the film plays a significant role as a source of oxygen, in addition to the bubble caps, to feed the slug.In the investigated circular capillary, fed by means of a T-mixer, two contrasted configurations have been observed in the liquid phase (slugs and films), depending on a critical bubble Reynolds number of ∼ 300, where the time-averaged concentration fields are found to differ considerably. For large Reynolds number, particle image velocimetry (PIV) measurements have revealed low temporal fluctuations at the rear of the bubble, possibly due to the presence of adsorbed contaminants, that tends to increase mixing in the slug. Despite this difference, the mass transfer dynamics were found to be controlled in all cases by the intensity of the recirculating motion in the slug, which is directly related to the bubble velocity for these cases of thin films.A new scaling law has been proposed for the overall Sherwood number, based on Reb and Sc, which satisfactorily describes the overall mass transfer of the experimental results for Reb > 120 to an accuracy of ±11%.

Thin-Film and Marginal Lubrication of Polyetherketone–Steel Sliding Contacts at High Temperature and High Speed

ABSTRACTPolyetherketone (PEK) is a suitable material for tribological systems that specifically require the properties of high chemical resistance, low component weight, seizure resistance under starved lubrication conditions, and operation at higher temperatures than many other engineering polymers can survive. PEK is used with a liquid lubricant at high temperatures and velocities to reduce friction and also to control unstable friction and wear, particularly in the region of the material's glass transition temperature, Tg.Intermittent and marginal lubrication using representative high-temperature synthetic lubricants was applied to high-speed, high-temperature PEK–steel sliding contacts to determine the effectiveness of lubrication under these conditions. Variations in the stability of the thin lubricant films were observed, particularly under different load conditions. Under low load conditions, the lubricant polarity and the related ability to form a film in the contact were important. Under high load conditions, the thermal stability of the lubricant became more important in retaining stability in the friction and wear mechanisms. Though not ideal practice, marginal lubrication of PEK–steel sliding contacts can be achieved by selection of an appropriate lubricant, even in the glass transition region of PEK.

Approaches for Achieving Superlubricity in Two-Dimensional Materials.

Controlling friction and reducing wear of moving mechanical systems is important in many applications, from nanoscale electromechanical systems to large-scale car engines and wind turbines. Accordingly, multiple efforts are dedicated to design materials and surfaces for efficient friction and wear manipulation. Recent advances in two-dimensional (2D) materials, such as graphene, hexagonal boron nitride, molybdenum disulfide, and other 2D materials opened an era for conformal, atomically thin solid lubricants. However, the process of effectively incorporating 2D films requires a fundamental understanding of the atomistic origins of friction. In this review, we outline basic mechanisms for frictional energy dissipation during sliding of two surfaces against each other, and the procedures for manipulating friction and wear by introducing 2D materials at the tribological interface. Finally, we highlight recent progress in implementing 2D materials for friction reduction to near-zero values-superlubricity-across scales from nano- up to macroscale contacts.

Real-Time Observation of Molecularly Thin Lubricant Films on Head Sliders Using Rotating-Compensator-Based Ellipsometric Microscopy

Lubricant transfer or pickup from a disk to a head slider is a crucial issue in designing the head–disk interfaces of hard disk drives (HDDs). A method based on ellipsometric microscopy is presented for observing thin lubricant film on a head slider in real time. The use of rotating-compensator ellipsometry (RCE) results in one order of magnitude higher temporal resolution (0.6 frames/s) and several times higher thickness resolution (0.2 nm) compared to null-ellipsometry-based microscopy, which was recently proposed. RCE-based ellipsometric microscopy will be useful in clarifying the mechanism of lubricant pickup for higher density HDDs.

Is the trend of Stribeck curves followed by nano-lubrication with molecularly thin liquid lubricant films?

Abstract Using a lateral force microscope with our original dual-axis probe, we measured the friction of molecularly thin films of perfluoropolyether lubricants at low sliding speeds. Merging these results with those measured at high sliding speeds using a home-built pin-on-disk type friction tester, we were able to explore nano-lubrication properties over a wide sliding-speed range from 1 μm/s to 0.2 m/s. The perfluoropolyether films displayed similar trends of an initial decrease followed by an increase in the friction coefficient as the sliding speed increases. The sliding speed at the inflection point decreased with increasing viscosity of the bulk lubricant. We conclude that the trend of Stribeck curves characterizing lubrication with bulk liquids is followed by nano-lubrication with molecularly thin liquid films.

Experimental investigation on the rheological properties of castor oil at different temperatures

Castor oil is one of the most enormous potential environment-friendly lubricants, and used as the base oil and additives in industrial applications. In this paper, the rheological property of castor oil was investigated using the optical interferometric test rig to analyze its flow properties under the micro gap. These results indicated that the central film thickness of castor oil in the point contact zone underwent the initial increased and then stable variation with the entrainment speed at different temperatures. The temperature and entrainment speed played an important role on the lubrication state and oil shape variation along the transverse and entrainment direction. At low temperature, the contact zone was almost in the thin lubrication regime at low entrainment speed, but was in the elastohydrodynamic lubrication regime at high entrainment speed. The side leakage went through the initial increased then decreased trend with the temperature and entrainment speed. These research results would further develop the engineering application of castor oil.