Surface Lubrication Research Articles

Tribological behavior prediction of composite MoS2-TiO2 coating using Taguchi coupled artificial neural network approach

Solid lubricants have been popularly used in tribological applications by virtue of their excellent lubricating properties in extreme environmental conditions. The tribological performance of such lubricated surfaces can be significantly enhanced by optimization of the coating material properties. In the present study, the composite MoS2-TiO2 coating is fabricated and studied the different parameters affecting the tribological behavior of the developed coating using design of experiment Techniques such as Taguchi and artificial neural network (ANN) approach. The optimization study was performed by considering the various input parameters such as addition of wt.% of TiO2 into the MoS2 base matrix, contact pressure, sliding speed, and pin surface temperature which affects the coating’s material tribological properties. The Taguchi approach reveals that pin surface temperature has the highest influence, followed by contact pressure, and wt.% addition of TiO2. The responses predicted using both Taguchi and ANN were compared and it has been observed that ANN helps to predict the responses more accurately compared to the Taguchi approach.

Tribological properties of a helical gear pair considering interfacial slip between lubricant and tooth surface

AbstractIn this study, an interfacial slip model including the limiting shear stress is proposed and applied to the thermal elastohydrodynamic lubrication (EHL) analysis of a helical gear pair. The main difference between the proposed model and the classical EHL model is that, the term of entrainment velocity in Reynolds equation is modified. The influences of interfacial slip, thermal effect, initial limiting shear stress and operating conditions on the tribological properties are evaluated. Due to the interfacial slip, the pressure distribution moves towards the inlet region, and the fluctuation distributions of entrainment velocity and film thickness are similar to the trigonometric function. The influence of thermal effect on interfacial slip cannot be ignored, especially in the case of high speed and heavy load. As the input torque and input rotational speed increase, the interfacial slip gradually extends to the whole meshing process.

Ice-Shedding Polymer Coatings with High Hardness but Low Ice Adhesion

Ice readily sheds from weak oil-swollen polymer gels but tends to adhere to mechanically robust coatings. This paper reports bilayer coatings that simultaneously possess high bulk hardness but low ice adhesion. These coatings are prepared by cocuring a triisocyanate, P#'-g-PDMS [a methacrylate polyol bearing poly(dimethylsiloxane) (PDMS) side chains with # being 1, 2, or 3 and g denoting graft], and optionally a methacrylate polyol P#. The self-assembly of the system during coating formation yields a PDMS brush layer on the surface of the cross-linked polyurethane matrix. After the surface PDMS layer is lubricated with a silicone oil, this coating exhibits an ice adhesion τ that is 10 000-fold lower than that of a triisocyanate/P# coating. Ice slides under its own weight on such a coating at a tilt angle of 3°. Yet, the coating matrix is harder than poly(ethylene terephthalate), a widely used plastic. Additionally, such a coating maintains its low τ values for more than 10 consecutive icing/deicing cycles. Subsequent increases in τ are reversed by allowing time for the replenishment of the depleted surface lubricant with that released from the coating matrix. This design opens the door for effective yet hard ice-shedding polymer coatings.

How Polytetrafluoroethylene Lubricates Iron: An Atomistic View by Reactive Molecular Dynamics.

The tribochemistry and transfer film formation at the metal/polymer interface plays an essential role in surface protection, wear reduction, and lubrication. Although the topic has been studied for decades, challenges persist in clarifying the nanoscale mechanism and dynamic evolution of tribochemical reactions. To investigate the tribochemistry between iron and polytetrafluoroethylene (PTFE) in ambient and cryogenic environments, we have trained and expanded a ReaxFF reactive force field to describe iron-oxygen-water-PTFE systems (C/H/O/F/Fe). Using ReaxFF molecular dynamics simulations, we find that mechanical shearing of single asperity induced the degradation of PTFE molecules and radicals, showing subsequent oxidation and hydroxylation reactions of the radicals initiated by C-C bond cleavage, in agreement with previous experimental observations. Furthermore, we studied mechanisms of interfacial tribochemical reactions and formation of transfer films. We found that tribochemical wear and Fe-C and Fe-F bonding networks are important mechanisms for anchoring molecular chains to form a transfer film on the iron countersurface. Hydroxyl groups can dehydrogenate to form short and strong chelation bonds with the Fe2O3 countersurface. A friction-induced oriented molecular layer plays a key role in reducing friction, which is responsible for the excellent lubrication property. By varying temperatures in the range of 10-300 K, we found a nonmonotonic change in friction with a maxima at 100 K. At cryogenic temperatures, the molecular mobility was obviously suppressed, while the chain rigidity was enhanced, resulting in the less oriented interface and brittle-like shear interface, which is responsible for nonmonotonic friction. This work elucidates mechanisms of tribochemical reactions and transfer film formation between iron and PTFE at the atomistic level, facilitating design and development of self-lubricating materials, especially under harsh conditions.

Super-lubricating hybrid elastomer with rapid photothermal sterilization and strong anti-cell adhesion

Silicone elastomer has been widely used as raw materials for manufacturing medical soft catheters. The surface properties of silicone elastomer, such as bactericidal performance, anti-cell adhesion and surface lubrication, are essential features to determine whether the catheters can fully play the role in transporting liquid or gas to organisms. To realize the integration of interfacial functions, we develop a super-lubricating hybrid elastomer with rapid photothermal sterilization and strong anti-cell adhesion capacities. Specifically, a hybrid elastomer is prepared by grafting one kind of lubricating polymers with anti-cell adhesion onto near-infrared (NIR) photothermal Fe3O4 nanoparticles-loaded polydimethylsiloxane substrate. Through the hydration lubrication, a super-low friction is realized on the surface of this hybrid elastomer. More importantly, this elastomer achieves a rapid photothermal bactericidal performance by local heating triggered by NIR irradiation. In addition, this hybrid elastomer shows a strong anti-adhesion effect against HepG2 cells. Based on these merits, this hybrid elastomer shows great potential for producing biomedical catheters that may realize anti-tissue adhesion, rapid sterilization, and reduction of interfacial resistance between catheters and biological tissues.

Improvement of the lubrication performance of an ester base oil with coated ferrite nanoadditives for different material pairs

In the present work, lubrication properties (friction and wear) of a synthetic ester oil, tris(2-ethylhexyl) trimellitate (TOTM) containing ferrite nanoparticles coated with oleic acid (F3O4-OA) were investigated for two different material pairs: steel ball-steel disc and silicon nitride ball-steel disc. Thus, four TOTM nanolubricants were formulated: TOTM + 0.010 wt% Fe3O4-OA, TOTM + 0.015 wt% Fe3O4-OA, TOTM + 0.020 wt% Fe3O4-OA and TOTM + 0.025 wt% Fe3O4-OA showing all of them a moderate time stability due to the oleic acid coating. Wettability behaviour of the ferrite-based nanolubricants on steel surface was analysed, revealing that the addition of Fe3O4-OA nanoparticles in TOTM decreases the contact angle between the steel surface and TOTM lubricant surface. Friction sliding tests were performed with the neat TOTM and with the formulated nanolubricants under a 20 N of load. All nanolubricants showed lower coefficients of friction than those reached with TOTM base oil for both material pairs. Worn area was significantly reduced for all Fe3O4-OA concentrations in the steel-steel contact and for the highest concentrations in the silicon nitride-steel contact. Specifically, the largest achieved reductions were for the TOTM + 0.010 wt% F3O4-OA nanolubricant: 43% reduction in friction (silicon nitride-steel) and reductions of 17% in wear track width, 42% in wear track deep and 36% in area (steel-steel). In addition, roughness analysis and Raman microscopy of the tested discs showed that tribofilm formation and surface repairing mechanisms occur.

Effects of Laser Surface Texturing and Lubrication on the Vibrational and Tribological Performance of Sliding Contact

Various textures are fabricated by a picosecond laser machine on the surfaces of circular stainless steel specimens. Vibrational and tribological effects of laser surface textures are investigated by means of a tribometer and a data acquisition and signal processing (DASP) system. Experimental results show that surface textures can reduce the coefficients of friction (COFs), enhance the wear resistance, and improve the dynamical performance of frictional surfaces. In this study, the surface with micro circular dimples in diameter of 150 μm or textured area density of 25% has the best tribological and dynamical performance. Compared with the non-textured surface, the surface with circular dimples in diameter of 150 μm and 15% textured area density has 27% reduction of COFs, 95% reduction of frictional vibrations, and 66% reduction of frictional noise. The frictional vibrations and noise in the sliding contacts can be effectively reduced by adding graphene to the lubrication oil, and the surface textures enhance the frictional noise reduction performance of lubrication.

Initial losing behavior of pre-tightening force for threaded fastener during repeated tightening

The clamping force of the bolted joint is a key factor in maintaining its service performance. However, when the torque wrench is removed after tightening, there will always be a certain degree of loss in the pre-tightening force. Herein, the stress state of the bolt and the nut during tightening and service was analyzed. The repeatability experiment of the threaded fasteners during repeated tightening was carried out to investigate the pre-tightening force loss after the removal of torque wrench (initial loss) and the pre-tightening force loss in a period of service (service loss). Different forms of lubrication was considered, namely without lubrication, thread lubrication and nut surface lubrication. The influence of the pre-tightening force, repeated tightening and lubrication on the initial loss and the service loss was analyzed according to the experimental results. It showed that the pre-tightening force and its loss have a large dispersion. At the first tightening, the pre-tightening force may increase after the initial loss stage. When the lubricant is used, the service loss has a significant positive correlation with its pre-tightening force. The use of lubricant at the threads is easier to lead the service loss of pre-tightening force.

Effect of Textured Surfaces in Hydrodynamic Lubrication

Effect of Textured Surfaces in Hydrodynamic Lubrication

Comparison of Graphene as an Oil Additive with Conventional Automotive Additives for the Lubrication of Steel and Dlc-Coated Surfaces

Comparison of Graphene as an Oil Additive with Conventional Automotive Additives for the Lubrication of Steel and Dlc-Coated Surfaces

Probing protein aggregation at buried interfaces: distinguishing between adsorbed protein monomers, dimers, and a monomer-dimer mixture in situ.

Protein adsorption on surfaces greatly impacts many applications such as biomedical materials, anti-biofouling coatings, bio-separation membranes, biosensors, antibody protein drugs etc. For example, protein drug adsorption on the widely used lubricant silicone oil surface may induce protein aggregation and thus affect the protein drug efficacy. It is therefore important to investigate the molecular behavior of proteins at the silicone oil/solution interface. Such an interfacial study is challenging because the targeted interface is buried. By using sum frequency generation vibrational spectroscopy (SFG) with Hamiltonian local mode approximation method analysis, we studied protein adsorption at the silicone oil/protein solution interface in situ in real time, using bovine serum albumin (BSA) as a model. The results showed that the interface was mainly covered by BSA dimers. The deduced BSA dimer orientation on the silicone oil surface from the SFG study can be explained by the surface distribution of certain amino acids. To confirm the BSA dimer adsorption, we treated adsorbed BSA dimer molecules with dithiothreitol (DTT) to dissociate these dimers. SFG studies on adsorbed BSA after the DTT treatment indicated that the silicone oil surface is covered by BSA dimers and BSA monomers in an approximate 6 : 4 ratio. That is to say, about 25% of the adsorbed BSA dimers were converted to monomers after the DTT treatment. Extensive research has been reported in the literature to determine adsorbed protein dimer formation using ex situ experiments, e.g., by washing off the adsorbed proteins from the surface then analyzing the washed-off proteins, which may induce substantial errors in the washing process. Dimerization is a crucial initial step for protein aggregation. This research developed a new methodology to investigate protein aggregation at a solid/liquid (or liquid/liquid) interface in situ in real time using BSA dimer as an example, which will greatly impact many research fields and applications involving interfacial biological molecules.

A Review of Modeling and Mitigation Techniques for Bearing Currents in Electrical Machines With Variable-Frequency Drives

The converter switching in variable-frequency drives can generate high frequency common mode voltage between the machine winding and the converter ground, leading to high frequency parasitic currents which can flow through the machine bearings unless precautions are taken in design and installation. These parasitic and unintended currents in the bearings cause deterioration of the lubrication film and surface damage to the rolling elements of the bearings. These problems will be exacerbated as wide-bandgap semiconductors with faster switching rise times start becoming more widespread in variable-frequency drives. This paper reviews the modelling and mitigation techniques of high frequency bearing currents in inverter fed AC drives. It aims to provide a solid base for the research community to further understanding the bearing currents phenomenon and helping to find novel improved technique for their mitigation and measurement.

Errata:Modified Reynolds Equations for Thin Film Lubrication with Saturated High-Viscosity Surface Layer and Lubrication Analysis of Tapered Pad Bearing [tribologist Vol.66 No.6 470-481]，The Effect of Wheel Rolling Direction Reversal on Rolling Contact Fatigue Crack and Wear [tribologist Vol.67 No.8 567-580

Errata:Modified Reynolds Equations for Thin Film Lubrication with Saturated High-Viscosity Surface Layer and Lubrication Analysis of Tapered Pad Bearing [tribologist Vol.66 No.6 470-481]，The Effect of Wheel Rolling Direction Reversal on Rolling Contact Fatigue Crack and Wear [tribologist Vol.67 No.8 567-580

Modified Reynolds Equations for Thin Film Lubrication with Saturated High-Viscosity Surface Layer and Lubrication Analysis of Tapered Pad Bearing

Modified Reynolds Equations for Thin Film Lubrication with Saturated High-Viscosity Surface Layer and Lubrication Analysis of Tapered Pad Bearing

Microstructure of continuous shear thickening colloidal suspensions determined by rheo-VSANS and rheo-USANS.

Research on shear thickening colloidal suspensions demonstrates that measurements of the microstructure can elucidate the source of the rheological material properties in the shear thickened state as well as critically test simulations and theory based on a variety of mechanisms such as enhanced lubrication hydrodynamics, elastohydrodynamics, and contact friction. Prior work on continuous shear thickening dispersions with a well-defined shear thickened state identified the formation of hydroclusters as characteristic of this state, determined the anisotropy in the nearest neighbor distribution, and used this information to test prevailing theories and simulations. However, important questions remain about the mesoscale (i.e., particle cluster scale) microstructure of the shear thickened state. Here we employ neutron scattering methods applied to shearing colloidal dispersions of spherical particles with two extremes of friction and lubrication surface properties to resolve the longer-length scale microstructure in the shear thickened state. Hydroclusters are shown to be highly localized, in agreement with prior neutron scattering and direct optical measurements, but in disagreement with the most recent simulations that predict a longer-range structure formation. These results combined with prior measurements provide experimental evidence about the length scale of microstructure formation in continuous shear thickening suspensions necessary to improve our understanding of the phenomenon as well as guide theoretical investigations that quantitatively link nanoscale forces to macroscopic properties in the shear thickened state.

Experimental study of durable low-friction concrete contacts for precast segmental columns with resettable sliding joints

Planar sliding joints have been introduced to precast segmental bridge columns to alleviate the detrimental effects of earthquakes. However, residual sliding movements may result from strong shaking. Resettable sliding joints comprising V-shaped or W-shaped interfaces of gentle inclinations have been proposed to achieve better resilience against seismic actions in regions of moderate to high seismicity. However, possible deterioration of the frictional performance at the concrete contact surfaces may affect the resetting performance of these joints. In this regard, the tribological features, performance and durability at the interface of resettable sliding joints in precast segmental bridge columns were investigated experimentally utilizing 3D printed moulds and computed tomography scanning technology. Various surface treatment and lubrication methods were evaluated monthly up to 360 days after casting based on a modified cyclic direct shear apparatus at high loading pressure. Results show that high-performance PTFE grease lubricant is useful in achieving and maintaining favourable coefficients of friction as low as 0.05.

Нeat and mass transfer models at boundary lubrication to determine the transition temperatures

At present, kinetic and thermodynamic methods for assessing the lubricating effect of oils are being increasingly developed. At the limit friction, the reduction of friction and wear of surfaces is due to the ability of the lubricant to form layers of adsorption or chemical origin on the surface. Analytical models of transition temperatures and wear in the limit lubrication mode must be used to mathematically describe the processes in the subsystems and the transition between them. The Fourier equation of thermal conductivity is accepted as the basic calculated dependence. It is assumed that the process of heat propagation under the conditions of formation of lubricating films is not Markovian, i.e. the magnitude of the heat flux is determined by the entire "history" of heat transfer in a certain elementary volume. The equation of motion of a lubricating film over the surface of a body that is being lubricated is obtained from the equation of motion for a Newtonian continuous medium. As a result, nonlinear heat and mass transfer models are obtained to determine the transition temperatures in the formation of boundary lubricating films in the concept of structural-thermodynamic approaches to describe the processes of boundary lubrication of surfaces

Comparative Analysis of Machine Learning Methods for Predicting Robotized Incremental Metal Sheet Forming Force.

This paper proposes a method for extracting information from the parameters of a single point incremental forming (SPIF) process. The measurement of the forming force using this technology helps to avoid failures, identify optimal processes, and to implement routine control. Since forming forces are also dependent on the friction between the tool and the sheet metal, an innovative solution has been proposed to actively control the friction forces by modulating the vibrations that replace the environmentally unfriendly lubrication of contact surfaces. This study focuses on the influence of mechanical properties, process parameters and sheet thickness on the maximum forming force. Artificial Neural Network (ANN) and different machine learning (ML) algorithms have been applied to develop an efficient force prediction model. The predicted forces agreed reasonably well with the experimental results. Assuming that the variability of each input function is characterized by a normal distribution, sampling data were generated. The applicability of the models in an industrial environment is due to their relatively high performance and the ability to balance model bias and variance. The results indicate that ANN and Gaussian process regression (GPR) have been identified as the most efficient methods for developing forming force prediction models.

Investigation on anisotropic tribological properties of superhydrophobic/superlipophilic lead bronze surface textured by femtosecond laser

Superhydrophobic and superlipophilic surfaces have been widely used in rail transit, petroleum equipment, aerospace, and other fields in recent years. Most of these surfaces have the disadvantage of poor wear resistance. However, the surface of the suitable microstructure has antifriction and wear-resistant properties. The femtosecond laser is easy to control, has good repeatability, and has great advantages in preparing special wetted surfaces with microstructures. In this work, an ultrafast femtosecond laser is used to prepare a superhydrophobic/superlipophilic lead bronze surface. It is found that the sample immediately after laser processing has a high content of surface polar bonds and high free energy, which is a super-hydrophilic surface. After being placed in the air for two months, the adsorption of hydrocarbons on the surface increases the C-C/C-H non-polar bond content, and the surface free energy decreases, showing superhydrophobic/superlipophilic characteristics. The friction behavior of superhydrophobic/superlipophilic surfaces in oil lubrication and dry friction is different, and the anisotropy of the surface structure will also lead to the anisotropy of the tribological properties. There mainly occurred fatigue wear. This work provides a method for the preparation of anisotropic antifriction and wear-resistant superhydrophobic/superlipophilic surfaces, and variable surface microstructures could be prepared for various friction conditions.

Green Superlubricity Enabled by Only One Water Droplet on Plant Oil-Infused Surfaces

The increase in energy loss due to friction and the use of large amounts of lubricants to improve it are major challenges we face from a global environmental perspective. Pitcher-plant-inspired liquid-infused surfaces (LISs) are emerging super-repellent surfaces against liquids. However, their coefficient of friction (CoF) against solids is higher than that of conventional lubricant surfaces. Herein, we demonstrate superlubricity with a single water droplet placed on a LIS holding oleic acid, a component of plant oil. When a water droplet is placed on the fluid layer, the CoF under reciprocating and rotating friction is 0.012 and 0.0098, respectively. A force in the direction opposite to the loading due to the Laplace pressure on the droplet and an autonomous positional movement of the water accompanied by the optimization of surface energy maintain the fluid-lubrication state and prevent direct contact between the surface and the friction material, resulting in a decrease of the dependence of the CoF on the friction velocity. The key technology here will not only present a novel strategy for preparing LISs against solids but also serve as a step toward a sustainable green strategy for friction reduction and lubrication, which would greatly reduce energy loss and environmental degradation.