Thin Film Lubrication Research Articles

Predicting resistance of bearing steels to surface initiated fatigue: Application to hybrid contact

The constant demand for improved energy efficiency in lubricated rolling/ sliding contacts, such as in rolling bearings, is generally addressed through increased power density and the use of low-viscosity (i.e., thin-film) lubricants. In these conditions, mixed lubrication regime can dominate with intensive asperity interactions between two contacting surfaces, thus increasing the risk of surface-initiated fatigue (e.g., in the form of micropitting). In the present study, the influence of steel mechanical properties on the micropitting accumulation in a hybrid contact (steel versus Si3N4 ceramics of different roughness) has been experimentally and theoretically investigated. It was found that higher roughness of Si3N4 surface could increase the risk of micropitting on the steel component. However, despite formation of some mild micropitting, the bearing steels tested here show good performance. For better understanding of the relationship between the mechanical properties of bearing steels and micropitting resistance, a new theoretical framework is introduced. The new micropitting model, based on the finding that the nucleation of a micropit is driven by (i) local accumulation of the plastic strain in mixed rough contact, and (ii) reduction of the total energy by debonding of the plastically deformed zone is introduced. The new micropitting model was shown to match with experiments across the range of test conditions. Micropitting performance maps based on the mechanical properties of steels (yield strength, work hardening and fracture toughness) have been obtained. The new theory has implications not only for predicting micropitting but also for predicting adhesive wear resistance of different steels.

Skiing-inspired robust lubricating composite coatings from thermally sprayed ceramic template

Inspired by the mechanism of skiing, herein, a novel composite coating with excellent lubrication properties was developed by incorporating solid-liquid reversible lubricating phases into the inherent microcracks and pores of thermally sprayed ceramic coatings. The results indicate that the crystalline-amorphous heterostructure formed within the micro-nano cracks, enhancing their cohesive strength and toughness. Compared to traditional ceramic coatings, the friction coefficient of the composite coating has been reduced by 80 %, while the wear rate decreased from 2.89 × 10−4 to 6.44 × 10−7 mm3∙N−1∙m−1. Under the applied load and friction-induced heat, a thin liquid lubrication film is formed. Importantly, the lubricating phase continuously overflows from defects under the induction of frictional heat to replenish the lubricating film, ensuring long-term lubrication of the coating.

Tribological Properties of Hard TiB2 Thin Films Prepared at Low Temperatures Using HiPIMS

Magnetron-sputtered WS2 composite thin films are solid lubricants with excellent performances. However, the low hardness of the WS2 thin films necessitates the further improvement of their wear resistance. For this purpose, an effective strategy is to alternately deposit or code posit WS2 and a hard phase, such as TiB2, to form hard lubricant thin films. Herein, a TiB2 thin film was prepared under the same conditions as those used for depositing the WS2 thin film with a dense structure and excellent tribological properties. Because of the high deposition energy of high-power impulse magnetron sputtering (HiPIMS), the TiB2 thin film possesses a dense structure and leather-like flat surface (hardness = 24.17 GPa). The friction coefficient of the film under different loads ranges between 0.6 and 0.8. The wear rate of the thin film increases with load, mainly because of fatigue wear and abrasive wear. Under high loads, obvious furrow-like wear marks are observed. At different sliding frequencies, except 8 Hz, the friction coefficient of the film ranges from 0.6 to 0.8. The main wear mode is fatigue wear, particularly at increasing sliding frequencies. Although the film possesses a relatively high friction coefficient, its wear resistance is excellent (minimum wear rate = 1.96 × 10−6 mm3/(N·m)).

Wear Reduction on the Roller–Shoe Mechanism at High Operation Loads

The roller–shoe mechanism is a classic mechanical assembly with an essential role in motion transmission. Common rail high-pressure pumps are an example of a complex assembly that uses such a mechanism to transform the rotation motion into a translation one. The rolling element of the mechanism is represented by a cylindrical roller. Although it can carry heavy loads due to its design, a proper surface profile could significantly increase the life of the entire mechanism. A better solution can be achieved using a logarithmic profile. The shoe is the second base element of the mechanism. It is a part with an inner cylindrical surface and it is separated from the roller by a thin lubricant film. Considering this, increasing the hardness of the roller–shoe contact surface can be obtained using a suitable coating. The positive results of this coating are highlighted using endurance tests to which high-pressure pumps are subjected. Therefore, the roller profile and the shoe coating represent two directions for improving the contact between the mechanism transmission elements, in terms of wear reduction. The purpose of this paper is to identify a suitable roller profile and to highlight its impact on the shoe coating.

Modified Reynolds Equation for Thin Film Lubrication with Arbitrarily Enhanced Viscosity Surface Layers and Lubrication Analysis of Micro-Tapered Pad Bearing

Modified Reynolds Equation for Thin Film Lubrication with Arbitrarily Enhanced Viscosity Surface Layers and Lubrication Analysis of Micro-Tapered Pad Bearing

Review of directional liquid transport on surfaces with different structures

Directional liquid transport has many cutting-edge applications, such as fog collection, agricultural drip irrigation, biochemical microreactors, water harvesting, non-powered micro-drug delivery, thin-film lubrication etc. There are many surfaces or linear structures in the natural systems can occur directional transport of water. In this paper, two bionic structures inspired by natural structures and two artificially fabricated surface structures are presented and their flow laws and mechanical mechanisms are described. Thereby, it is analysed that surface-driven external forces, such as the gradient of surface energy and the gradient of Laplace pressure, and surface pinning in other directions are the key points to drive the directional flow of liquids.

Predicting Wear under Boundary Lubrication: A Decisive Statistical Study

The forthcoming revolution in mobility and the use of lubricants to ensure ecological friendliness intensifies the pressure on tribology for predictors in new life cycles, mainly addressing wear. The current paper aims to obtain such predictors by studying how the wear processes that occur in a standard FE8 bearing test rig under thin film lubrication are conducted by the properties of the lubricant rather than simple viscosity parameters. Assuming that the activity of a lubricant with respect to the temperature, surface, and chemicals is a matter of its chemical potential, the results show that the nature of the base oil is a key parameter, apart from the chemical structure of the additives. Moreover, it becomes clear that chemical predictors are changing by altering the conditions they are exposed to. As an important result, the lubricant is effective in the prevention of wear if it has the capacity to uptake and transmit electrical charges due to its polarisability during a wear process.

Fluid–structure interaction of a dynamic seal with a 3D‐printed surface

AbstractAdditive manufacturing (AM) methods are finding more and more applications in the manufacturing of machine parts. One consequence of this method is that generated surface qualities are typically lower than in other processes. This becomes especially relevant when the surface quality is critical for the mechanical function: This is the case in dynamic sealing technologies, for example, in seals between a moving rod and a hydraulic cylinder. Between the seal lip and the polished rod, a thin lubrication film is established with the goal of minimal leakage. How well this goal is reached depends on the surface quality of the seal. For spare part production of seals, we investigate AM methods, which have a surface defined by layers from fused filament fabrication (FFF). We apply a coupled model for fluid–structure interaction of the elastic seal and the film flow. This model provides information on flow field and pressure distribution. Results change when the ‘ideal’ seal lip is replaced by the seal lip geometry from FFF. The continuous loss of solid body contact is replaced by a stepwise loss of contact for the FFF surface.

A numerical study on the impact of lubricant rheology and surface topography on heavily loaded non-conformal contacts

The increasing requirement of high-power density (power throughput/ weight) in modern day machines lead to thin film lubrication condition in various machine components (rolling element bearings, gears, cams, etc,) due to severe loading conditions. Surface roughness features and lubricant rheology plays a vital role in thin film lubrication, and significantly affects the lubrication performance and lifetime of machine components. The present work demonstrates surface topography and lubricant rheology effects on the traction coefficient for heavily loaded non-conformal contacts. The load-sharing concept considering elastic-plastic deformation of asperities, and Carreau shear-thinning rheological model is employed to describe the dry rough contacts and non-Newtonian behavior of lubricant. An influence of surface topography parameters such as roughness, skewness, kurtosis, and pattern ratio on the traction coefficient is discussed. From results, it is found that among different surface topographies, negatively skewed surfaces having isotropic surface pattern exhibit minimum traction coefficient. The load share function and the critical rolling speed are determined for various surface topographies which provides further insights into the surface topography effect on traction coefficient. The findings of present study are noteworthy as they provide a theoretical basis for an assessment of the lubrication performance of heavily loaded non-conformal contacts.

Numerical Study on Thin Film Lubrication Performance with Imperfect Coating under the Effect of the Electrical Double Layer in Point Contact

Revealing the interface effect on lubrication is great significance for designing high performance water-based lubricating friction pairs. In this study, a new thin film lubrication numerical model considering the electrical double layer (EDL) effect with an imperfect coating in point contact is proposed, and the validity and effectiveness of the model is verified. The numerical results show that increasing the zeta potential increases the film thickness, which indicates that the EDL effect can improve the firm lubricated film formation. However, the effect of the zeta potential on the von Mises stress is very small because the liquid film pressure remains constant at different surface potentials. On the other hand, with an increase in coating thickness, the effects of the coating on the film thickness and pressure gradually become evident, but the von Mises stress is affected by the imperfectly bonded interface and coating thickness. Finally, the effect of surface roughness on thin film lubrication was analyzed. The liquid film thickness slightly decreased and the occurrence of stress concentration on the surface coating was evident. The proposed model was expected to provide a calculation basis for solving the thin film lubrication problem of coatings affected by the EDL.

Hysteresis in three-dimensional multi-layer molecularly thin-film lubrication

Abstract For three-dimensional multi-layer molecularly thin-film lubrication system with elastic substrates, roles of hysteresis on tribological properties are investigated by using the multiscale simulation method. It is found that multiple stick-slip transitions with/without hysteresis loops appear in a sliding process and form a quasi-periodic progress with lattice distance. For the few-/multi-layer thin-film lubrication system, as the load increases, the hysteresis length monotonously increases/tends to keep constant. The hysteresis is mainly caused by the relaxation of metastable states of solid atoms in the elastic substrates, which delays the system back to its equilibrium states. In the quasi-periodic shearing progress, the effective elastic coefficients and the hysteresis lengths approximately remain unchanged, which reveals that although the hysteresis loops with the same lengths appear in the sliding process, the total systematic energy is still conserved. These findings not only provide a profound understanding of roles of hysteresis in the thin-film lubrication system but also show the effects of film layers and loads on the systematic tribological properties, which are of great significance for practical applications.

Molecular dynamics simulation of the lubricant conformation changes and energy transfer of the confined thin lubricant film

The lubrication system plays an important role in the reliability of moving links for spacecrafts while lubrication failure has emerged as one of the challenges in developing advanced spacecrafts with long life and high reliability. However, the process and mechanism of lubrication in space are still unclear due to a complex environment. In this study, the lubrication behavior under various pressure loads was studied by analyzing the lubricant conformation changes and energy transfer through molecular dynamics simulations. Results indicated that a “one-step” or “two-step” adjustment in the polymer conformations occurred, and the film exhibited different motion modes and temperature fields under different pressure loads. This work studied polymer thin film lubricant via energy transfer and molecular motion at a molecular level and gave a fundamental understanding on the lubrication behavior and lubrication failure of polymer thin film in space, providing theoretical guidance in developing advanced space lubrication technology.

Chemically-induced active micro-nano bubbles assisting chemical mechanical polishing: Modeling and experiments

The material loss caused by bubble collapse during the micro-nano bubbles auxiliary chemical mechanical polishing (CMP) process cannot be ignored. In this study, the material removal mechanism of cavitation in the polishing process was investigated in detail. Based on the mixed lubrication or thin film lubrication, bubble-wafer plastic deformation, spherical indentation theory, Johnson-Cook (J-C) constitutive model, and the assumption of periodic distribution of pad asperities, a new model suitable for micro-nano bubble auxiliary material removal in CMP was developed. The model integrates many parameters, including the reactant concentration, wafer hardness, polishing pad roughness, strain hardening, strain rate, micro-jet radius, and bubble radius. The model reflects the influence of active bubbles on material removal. A new and simple chemical reaction method was used to form a controllable number of micro-nano bubbles during the polishing process to assist in polishing silicon oxide wafers. The experimental results show that micro-nano bubbles can greatly increase the material removal rate (MRR) by about 400% and result in a lower surface roughness of 0.17 nm. The experimental results are consistent with the established model. In the process of verifying the model, a better understanding of the material removal mechanism involved in micro-nano bubbles in CMP was obtained.

Base lubricants for green stamping: The effects of their structure and viscosity on tribological performance

The requirements for green and sustainable manufacturing mean that stamping lubricants must be continuously re-evaluated and re-designed. In this investigation, the tribological performance of four base oils with different chemical structures (paraffinic and naphthenic) and viscosities (2 and 20 cSt), as well as water, was evaluated for the stamping of steel sheets and compared with a non-lubricated contact. Most lubricants reduce the coefficient of friction and maintain a similar wear coefficient for steel sheets as in dry contacts. Low-viscosity (LV) naphthenic oil performs very like both high-viscosity (HV) oils. A surprising exception is the LV paraffinic oil, with several-times-higher friction and wear compared to dry contact. This is due to the excellent wetting-spreading and very low cohesion forces that enable oil to escape from extremely thin-film contacts because the viscosity is so low, leading to lubricant starvation. In contrast, HV oils provide a sufficiently thick lubricating film, while strong cohesive forces help in the film’s strength, lessening wear, and reducing friction. In thin-film lubrication with LV oils, such as when stamping, it is thus extremely important that the lubricant’s wetting behaviour and viscosity are sufficient to provide enough film in the contact and prevent starvation, thus ensuring lower friction, less wear, and a longer lifetime of the contact.

Analysis of TiO2 Nanolubricant Influence in Micro Deep Drawing of Stainless Steel SUS301

To improve the quality of products produced from microforming, various nanolubricants have been applied in the field of micromanufacturing in recent years. In this paper, the effects of glycerol-based lubricant containing TiO2 NPs (NPs) on micro deep drawing (MDD) of austenitic stainless steel (ASS) SUS301 were studied, and the lubrication mechanism involved was discussed. The MDD experiments were conducted with the SUS301 foils under dry, 1, 2, and 4 wt% TiO2 NP lubrication conditions. The results show that the use of the TiO2 nanolubricants can significantly improve the quality of the drawn cups in terms of decreased wrinkling and surface roughness. Besides, the concentration of TiO2 NPs influences lubricity by reducing friction during the MDD process. The peak drawing force is the lowest when 2 wt% nanolubricant is applied, which drops to 72.54 N from 77.38 N under dry conditions. The micro cup drawn under 2 wt% TiO2 nanolubricant has the best quality among those obtained under all the lubrication conditions. The lubrication mechanisms are derived from the mending effects of TiO2 NPs and the formation of thin lubricant films associated with the open lubricant pockets (OLPs) and close lubricant pocket (CLPs) theory in the MDD. The CLPs function as reservoirs that retain lubricants to counteract the load pressure, whereas the OLPs lead to lubricant leakage due to the higher flow resistance. It was found that the lubricant film and NPs are insufficient at a low concentration (1 wt%), while the lubrication performance can be enhanced with increased NP concentration. However, there exist apparent agglomerations on the surface of the produced micro cup when using 4 wt% nanolubricant, which greatly deteriorates the lubricant performance in the MDD process. It is concluded that the lubricant containing 2 wt% TiO2 NPs demonstrates the best lubrication performance during the MDD of ASS SUS301.

Influences of lubrication on asymmetric peristaltic movement and its bifurcation

Peristaltic motions are used in a variety of industrial and real-world problems such as pumping mixture with such a high solid content from the mining sector, biogases, sewers facilities, food flow through the gastrointestinal system, urinary tract, and fallopian tube of human females where extremely abrasive, gritty, and viscous fluids are present. The goal of this theoretical study is to examine the impact of the lubricated walls on the peristaltic transport of a viscous fluid in an asymmetric medium. The interfacial condition is derived under the assumptions of thin film lubrication and long wavelength approximation. The theory of a dynamical system is utilized to examine the lubrication effects on the position and bifurcation of stagnation points in the flow. For this, three flow distributions, backward flow, trapping, and augmented flow, are discussed. The obtained system of equations is solved numerically by the shooting method based on Newton–Raphson root-finding algorithm in Mathematica. The prime findings for the velocity profile, pressure increase, trapping, and reflux criteria are illustrated through graphs. Bifurcation occurs earlier by increasing the influence of lubrication. The trapping area diminishes and the augmented flow section expands with the lubrication parameter. This will be useful for medical engineering, industrial and physiological systems. The comparison for the particular case of no-slip condition is presented and found to be in excellent agreement.

Realization of ultra-low friction coefficient in thin-film solid lubricant coatings containing nano-sized layers of molybdenum and tantalum disulfides

Solid lubricating coatings consisting of alternating layers of molybdenum and tungsten disulfide were obtained by the method of reactive pulsed laser deposition. The layers were deposited at temperatures of 400 and 500 °C, which caused the formation of a layered packing of atoms characteristic of the hexagonal 2H-phase. The layer thickness was about 10 nm, and the basal planes of the layered atomic packing were oriented predominantly parallel to the substrate surface over the entire thickness of the layers. The study of tribological properties was carried out by the method of sliding a ball over a disk in a nitrogen atmosphere with varying the load on the counter body from 5 to 9 N. The value of the friction coefficient depended on the load on the counter body. The lowest value of the coefficient of friction was 0.01, and it was found under the highest load for coatings obtained at 500 °C. A decrease in the coating deposition temperature had a negative effect on the antifriction properties of the coating.

Effects of Oxygen on Smear Formation in Heat Assisted Magnetic Recording System

Heat-assisted magnetic recording (HAMR) is expected to be a realistic next-generation technology for increasing the recording density of hard disks. However, the magnetic layer is heated above the Curie temperature, and, as a result, the heated lubricant is desorbed from the disk by decomposition and evaporation, which causes a problem as it adheres to the air-bearing surface (ABS) as a smear. In this study, pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) analysis was performed in helium and air environments to investigate the decomposition mechanism of perfluoropolyether (PFPE) lubricant D-4OH by heating and in the presence of oxygen. In the helium environment, thermal decomposition of the end groups was confirmed at 350°C and above with a possibility of main chain decomposition at 450°C. In the air environment, decomposition of the end group was confirmed at 250°C and above, and decomposition of the main chain was confirmed at 450°C. Experiments using a pin-on-disk tester were conducted to confirm what happens to the area of smear when a thin film of D-4OH lubricant coated on an actual disk is laser heated. As a result, it was confirmed that the area of smear decreased even at an oxygen concentration of 5%.

A new method for determining lubrication regimes of piston ring-cylinder liner tribosystem

The time-varying lubrication regime is a unique feature of the piston ring-cylinder liner tribosystem. This study proposes a new method for comparing oil film thickness with critical values, consisting of characteristic topographic parameters (Rpk, Rk, and Rvk, etc.), to determine the lubrication regime. The lubrication regime could be determined as hydrodynamic lubrication, micro-plasto-elastohydrodynamic lubrication, micro-plastohydrodynamic lubrication, thin film lubrication, boundary lubrication, or mixed lubrication. The influence of thermal effects and oil-feed conditions on the lubrication regime is discussed. It is found that the friction coefficient alone is not entirely suitable for evaluating the lubrication regime of the piston ring-cylinder liner tribosystem. Nevertheless, this new method is capable of determining the lubrication regime under different operation conditions.

Semicrystalline Polymer Micro/Nanostructures Formed by Droplet Evaporation of Aqueous Poly(ethylene oxide) Solutions: Effect of Solution Concentration.

Deposits formed after evaporation of sessile droplets, containing aqueous solutions of poly(ethylene oxide), on hydrophilic glass substrates were studied experimentally and mathematically as a function of the initial solution concentration. The macrostructure and micro/nanostructures of deposits were studied using stereo microscopy and atomic force microscopy. A model, based on thin-film lubrication theory, was developed to evaluate the deposit macrostructure by estimating the droplet final height. Moreover, the model was extended to evaluate the micro/nanostructure of deposits by estimating the rate of supersaturation development in connection with the driving force of crystallization. Previous studies had only described the macrostructure of poly(ethylene oxide) deposits formed after droplet evaporation, whereas the focus of our study was the deposit micro/nanostructures. Our atomic force microscopy study showed that regions close to the deposit periphery were composed of predominantly semicrystalline micro/nanostructures in the form of out-of-plane lamellae, which require a high driving force of crystallization. However, deposit central areas presented semicrystalline micro/nanostructures in the form of in-plane terraces and spirals, which require a lower driving force of crystallization. Increasing the initial concentration of solutions led to an increase in the lengths and thicknesses of the out-of-plane lamellae at the deposits' periphery and enhanced the tendency to form spirals in the central areas. Our numerical study suggested that the rate of supersaturation development and thus the driving force of crystallization increased from the center toward the periphery of droplets, and the supersaturation rate was lower for solutions with higher initial concentrations at each radius. Therefore, periphery areas of droplets with lower initial concentrations were suitable for the formation of micro/nanostructures which require higher driving forces, whereas central areas of droplets with higher initial concentration were desirable for the formation of micro/nanostructures which require lower driving forces. These numerical results were in good qualitative agreement with the experimental findings.