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Lubricants, Volume 6, Issue 2 (June 2018)

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Cover Story (view full-size image) The high-load tribological contacts present in automotive braking systems give rise to the [...] Read more.
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Open AccessArticle Synergistic Effect of Nanodiamond and Phosphate Ester Anti-Wear Additive Blends
Received: 20 May 2018 / Revised: 5 June 2018 / Accepted: 13 June 2018 / Published: 18 June 2018
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Abstract
Nanodiamonds are known to improve tribological performance when added to lubricants, but their impact on additives that may already be present in the lubricant is poorly documented. Here, we report on a study of their effects on thermal reaction films formed from tricresyl
[...] Read more.
Nanodiamonds are known to improve tribological performance when added to lubricants, but their impact on additives that may already be present in the lubricant is poorly documented. Here, we report on a study of their effects on thermal reaction films formed from tricresyl phosphate (TCP) on Fe substrates immersed in a dibasic ester basestock when blended with TCP. Thermal reaction film formation temperatures were recorded in-situ by monitoring the reaction film formation on both Fe and air baked Fe surfaces using a quartz crystal microbalance (QCM). The nanodiamonds were found to raise the thermal reaction film formation temperature by 18 °C, possibly by raising the activation energy for the reaction, but they were not observed to affect the thickness or rate of formation of the films. The nanodiamonds, moreover, were observed to trigger thermal reaction film formation on air baked Fe surfaces that otherwise were highly resistance to reaction film formation. The surface morphology, roughness, and thickness of the thermal reaction films, as measured by atomic force microscopy (AFM), are reported as well as their chemical compositions, as studied with Electron Dispersive X-ray Spectroscopy (EDS). The coefficients of friction measured on the thermal reaction films during dry solid–solid contact are also reported. Full article
(This article belongs to the Special Issue Wear Resistant Materials)
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Open AccessArticle Dispersion Stability and Lubrication Performance Correlation of Vegetable Oil-In-Water Emulsions with Nanoparticle-Shielded Oil Droplets
Received: 3 May 2018 / Revised: 31 May 2018 / Accepted: 6 June 2018 / Published: 9 June 2018
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Abstract
Vegetable oil-in-water (VO/W) emulsions are bio-based metal working lubricants. The emulsions’ lubrication performance depends on the stability of oil droplets. In this paper, the oil droplets’ dispersion stability and lubrication of emulsions containing TiO2/SiO2 nanoparticles (NPs) as dispersant and lubrication
[...] Read more.
Vegetable oil-in-water (VO/W) emulsions are bio-based metal working lubricants. The emulsions’ lubrication performance depends on the stability of oil droplets. In this paper, the oil droplets’ dispersion stability and lubrication of emulsions containing TiO2/SiO2 nanoparticles (NPs) as dispersant and lubrication agents have been investigated. Enhanced dispersion of NP-shielded oil droplets was found. Increasing the NPs’ mass fraction initially lowers the average size of NP-shielded droplets up to the saturation of the droplets’ surface with NPs at 0.5 wt % mass fraction. NPs also form NP agglomerates in emulsions, more so after the droplets’ surfaces have been saturated with NPs. There is an apparent minimum quantity of NPs (~0.5 wt %) required to ensure sustained dispersions of the droplets which is thought to be related to the oil concentration and the droplets’ total surface-area-to-volume ratio. Below the required quantity of NPs, partially shielded and fully shielded droplets coexist. The partially shielded droplets initially attract other droplets and undergo limited coalescence but retain their long-term stability. A small quantity of NPs improves the antiwear property of the lubricants. However, emulsions with NPs have slightly higher friction than the NP-free emulsion due to the reduced strength of the tribofilm. Despite the increased friction, the tribofilm formed in presence of NPs can easily be removed from the surface with water, indicating cleaner surfaces after the lubrication (i.e., less oil residue on the surfaces), which, for the sake of cleanliness, is favourable in many applications. Full article
(This article belongs to the Special Issue Biolubrication and Biomimetic Lubrication)
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Open AccessArticle The Running-in of Lubricated Metal-Metal Contacts—A Review on Ultra-Low Wear Systems
Received: 11 April 2018 / Revised: 11 May 2018 / Accepted: 15 May 2018 / Published: 8 June 2018
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Abstract
The running-in of lubricated metal–metal contacts leading to ultra-low wear is inseparably connected with the formation of the third body and vice versa. Adequate tribological stressing provides the system with a power density that leads to complex changes of topography, near-surface morphology and
[...] Read more.
The running-in of lubricated metal–metal contacts leading to ultra-low wear is inseparably connected with the formation of the third body and vice versa. Adequate tribological stressing provides the system with a power density that leads to complex changes of topography, near-surface morphology and chemical composition. During the running-in these changes proceed until the system shows small friction and ultra-low wear rates and performs stable with low sensitivity to external perturbations. By means of high-resolution wear measurement as well as physical and chemical analysis the capability of a tribological system to develop the third body can be determined. Moreover, the running-in can be controlled by sample finishing, oil additivation and the sequence of initial stressing steps. This contribution summarizes 20 years of own research on ultra-low wear systems and its applications. Full article
(This article belongs to the Special Issue Applied Nanotribology)
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Open AccessArticle Vibration Technologies for Friction Reduction to Overcome Weight Transfer Challenge in Horizontal Wells Using a Multiscale Friction Model
Received: 27 March 2018 / Revised: 14 May 2018 / Accepted: 17 May 2018 / Published: 29 May 2018
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Abstract
Drag reduction technologies mainly include the mechanical method and the chemical method. Mechanical drag reduction technologies are widespread in the drilling field due to their environmental friendliness and ease of use. Vibration technologies are among some of the most-used mechanical drag reduction technologies.
[...] Read more.
Drag reduction technologies mainly include the mechanical method and the chemical method. Mechanical drag reduction technologies are widespread in the drilling field due to their environmental friendliness and ease of use. Vibration technologies are among some of the most-used mechanical drag reduction technologies. However, various types of vibration tools have the negative effect of obstructing the promotion and application of mechanical drag reduction technologies. This paper widely investigated the types and applications of vibration tools. A drilling agitator system and slider drilling technology were included. The structure and mechanism of the vibration tool were studied. A multiscale friction model was proposed based on the Dahl model according to the drilling environment. The model was verified using experimental data. The multiscale friction model was used to analyze the drag reduction mechanism and the effect of different kinds of vibration technologies. Simulations demonstrated that vibration technologies can effectively reduce the axial friction of the drill string. Longitudinal vibration can reduce the axial friction such that the dependence of the reduced coefficient of friction on the reduced velocity does not change significantly after the reduced oscillation amplitude exceeds the critical value of one. Axial friction decreased with the increasing amplitude of the radial vibration. However, the reduction effect has no relationship with the rate of penetration (ROP). Torsional vibration and rocking motion can reduce the axial friction force. When the oscillation amplitude increases, the axial friction decreases. The reduction effect of slider drilling technology is better than torsional vibration and two other technologies in terms of friction reduction. Full article
(This article belongs to the Special Issue Oil and Gas Tribology)
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Open AccessArticle Low Friction, Lubricity, and Durability of Polymer Brush Coatings, Characterized Using the Relaxation Tribometer Technique
Received: 16 March 2018 / Revised: 29 April 2018 / Accepted: 1 May 2018 / Published: 20 May 2018
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Abstract
Among the possible solutions for achieving low friction, polymer brushes that are grafted onto surfaces are good candidates. The tribological characterization of such layers becomes more difficult when the friction is lower: the signal-to-noise ratio of the friction force that is measured with
[...] Read more.
Among the possible solutions for achieving low friction, polymer brushes that are grafted onto surfaces are good candidates. The tribological characterization of such layers becomes more difficult when the friction is lower: the signal-to-noise ratio of the friction force that is measured with conventional tribometers impedes the precise quantification. Therefore, we have applied a new technique that has been developed at the Laboratory of Tribology and System Dynamics (LTDS), called the ‘oscillating relaxation tribometer’. The advantage of this original technique is that it characterizes low friction with unequalled sensitivity. The lower the friction, the better the precision, and it permits obtaining the ‘friction law’ directly from robust and rapid experimental tests. In this study, the samples that have been used are the ionic liquid-type polymer brushes (ILPBs) with different thicknesses, which have been grafted onto silicon wafers and steel coupons. The counter-face is a mirror-polished steel ball. We show that (i) a thick ILPB layer on silicon is very resistant to high contact pressure, up to 555 MPa; (ii) the friction behavior that is obtained is close to that of a Newtonian viscous one, even under maximum normal loads; (iii) poorer results are obtained for the thinner sample; and (iv) the repetition, up to 5000 oscillations on the same surface, does not affect the friction damping of the contact, which demonstrates that this film provides a favorable resistance to friction under severe contact conditions. In addition, the feasibility of grafting onto steel surfaces is demonstrated. The results are then discussed, with respect to friction and dissipation. Full article
(This article belongs to the Special Issue Selected Papers in the Sixth World Tribology Congress (WTC 2017))
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Open AccessArticle The Fundamental Regularities of the Evolution of Elastic Vortices Generated in the Surface Layers of Solids under Tangential Contact Loading
Received: 12 April 2018 / Revised: 12 May 2018 / Accepted: 16 May 2018 / Published: 18 May 2018
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Abstract
Conventionally discussed dynamic mechanisms of elastic strain energy redistribution in near-contact surface regions include P and S elastic wave pulses radiating from the contact surface. At the same time, the elastic strain energy can be transferred by localized vortex-like elastic waves (Rayleigh, Love,
[...] Read more.
Conventionally discussed dynamic mechanisms of elastic strain energy redistribution in near-contact surface regions include P and S elastic wave pulses radiating from the contact surface. At the same time, the elastic strain energy can be transferred by localized vortex-like elastic waves (Rayleigh, Love, Stoneley wave, and so on). In the paper, we numerically studied the main features of the formation and propagation of localized vortex-like waves in the surface layers under the contact zone. The study was done using the numerical method of movable cellular automata. We showed that the initial phase of dynamic contact interaction with a nonzero tangential component of contact velocity is accompanied by the formation of a so-called elastic vortex. The elastic vortex is a fully dynamic object, which is characterized by shear stress concentration and propagates at the shear wave speed. We first revealed the ability of the elastic vortex to propagate toward the bulk of the material and transfer elastic strain energy deep into the surface layer in a localized manner. We analyzed the dependence of the direction of vortex propagation on the tangential contact velocity, contact pressure and Young’s modulus of the material. The results of the study are important for better understanding the dynamic mechanisms contributing to inelastic strain accumulation or gradual degradation of surface layers. Full article
(This article belongs to the Special Issue Computer Simulation in Tribology and Friction)
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Open AccessArticle Thermal–Hydrodynamic Behaviour of Coated Pivoted Pad Thrust Bearings: Comparison between Babbitt, PTFE and DLC
Received: 25 December 2017 / Revised: 4 May 2018 / Accepted: 8 May 2018 / Published: 17 May 2018
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Abstract
The hydrodynamic lubrication and thermal analysis of tilting pad thrust bearings has been a major subject for many studies in the field of tribology. There is only a limited number of studies regarding thrust bearings with coated surfaces. The purpose of this study
[...] Read more.
The hydrodynamic lubrication and thermal analysis of tilting pad thrust bearings has been a major subject for many studies in the field of tribology. There is only a limited number of studies regarding thrust bearings with coated surfaces. The purpose of this study is to build a parametric, iterative algorithm in order to perform a complete thermal and hydrodynamic lubrication analysis for pivoted pad thrust bearings with coatings. The analytical model is mainly based on the energy, continuity and Navier–Stokes equations, which are solved numerically with the Semi-Implicit Method for Pressure Linked Equations Consistent (SIMPLEC) method. The analysis focuses on a single pivoted pad of the thrust bearing. The thermal properties of the coating material are taken into account and the resulting thermal and flow fields are solved. The basic hydrodynamic and tribological characteristics are calculated for an uncoated, a Babbitt coated, a PTFE coated and a diamond like carbon (DLC) coated pivoted pad thrust bearing. The pressure and the film thickness distribution, as well as the load capacity and the frictional forces, are determined for several pad positions and velocities of the rotor. A mineral oil lubricant is used to estimate the shear thinning or thickening effects on the pad tribological performance. The results indicate that pads coated with PTFE and DLC show lower friction forces compared to the common steel and Babbitt applications. At the same time, the DLC coating seems to affect the bearing’s flow and thermal fields less than the PTFE, making it more suitable for thrust bearings applications. Full article
(This article belongs to the Special Issue Friction and Lubrication of Sliding Bearings)
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Open AccessArticle Effect of Surface Texturing Parameters on the Lubrication Characteristics of an Axial Piston Pump Valve Plate
Received: 30 March 2018 / Revised: 10 May 2018 / Accepted: 14 May 2018 / Published: 16 May 2018
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Abstract
In this article, a geometrical model of different microtextures is established for an axial piston pump valve plate. A finite differential method was used to solve the Reynolds equation for the oil film thickness and pressure, which were simulated under different microtextures. The
[...] Read more.
In this article, a geometrical model of different microtextures is established for an axial piston pump valve plate. A finite differential method was used to solve the Reynolds equation for the oil film thickness and pressure, which were simulated under different microtextures. The influence of microtexture shape and structure on performance was studied and optimal parameters sought. Different convergence gaps are formed by different microtexture radii, and they produce different hydrodynamic effects. The lubrication characteristics of the valve plate are better when a microtexture is used and are influenced by the type of microtexture. We reached the following conclusions: (1) The lubrication characteristics of the valve plate are influenced by different microtexture parameters and can be improved by optimizing the microtexture parameters; (2) There is an optimal parameter combination when adding microtexture with three shapes (spherical, cylindrical and square) and the optimal dimensionless oil film pressure lubrication characteristics can be obtained; (3) The degree of improvement in the dimensionless oil film pressure lubrication characteristics was (listed from highest to lowest): micro-hemispherical texture > micro-cylindrical texture > micro-square texture. Full article
(This article belongs to the Special Issue Tribological Performance of Textured Surfaces)
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Open AccessArticle A Comparison Study on Wear Behaviors of Mo and Al2O3-Mo Coatings from RT to 300 °C
Received: 11 April 2018 / Revised: 8 May 2018 / Accepted: 8 May 2018 / Published: 11 May 2018
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Abstract
Mo and Al2O3-Mo coatings are fabricated on a low-carbon steel substrate using atmospheric plasma spraying. The microstructure and mechanical properties of two as-sprayed coatings, with a particular focus on the tribological behaviors from room temperature to 300 °C, are
[...] Read more.
Mo and Al2O3-Mo coatings are fabricated on a low-carbon steel substrate using atmospheric plasma spraying. The microstructure and mechanical properties of two as-sprayed coatings, with a particular focus on the tribological behaviors from room temperature to 300 °C, are comparatively investigated in this study. Microstructural analysis of the coatings shows that the porosity of the Al2O3-Mo coating is higher than that of Mo coating. The addition of Al2O3 particles reduces the coating–substrate adhesion strength. The Al2O3-Mo coating, in comparison to the Mo coating, shows improved mechanical properties, such as hardness and wear resistance. The friction coefficients of both coatings increase with further increases in test temperatures. The friction coefficient of the Al2O3-Mo coating, tested above 100 °C, is lower than that of the Mo coating. The wear failure mechanisms of the two coatings are delamination, brittle fracture, oxidation and adhesion wear. In addition, local plastic deformation was also found in the Mo coating. Full article
(This article belongs to the Special Issue Tribocorrosion of Surface Engineered Materials)
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Open AccessArticle Detailed Investigations on the Oil Flow in Dip-Lubricated Gearboxes by the Finite Volume CFD Method
Received: 9 February 2018 / Revised: 10 April 2018 / Accepted: 2 May 2018 / Published: 9 May 2018
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Abstract
Three main concerns are in the focus of the development of geared transmissions nowadays: load carrying capacity, noise–vibration–harshness (NVH) behavior, and efficiency. Increasing the efficiency of modern gearboxes contributes significantly to the reduction of energy consumption and the saving of resources. Gearboxes are
[...] Read more.
Three main concerns are in the focus of the development of geared transmissions nowadays: load carrying capacity, noise–vibration–harshness (NVH) behavior, and efficiency. Increasing the efficiency of modern gearboxes contributes significantly to the reduction of energy consumption and the saving of resources. Gearboxes are frequently designed conservatively with an oversupply of oil to guarantee operational reliability. An oversupply of oil results in an unnecessarily high amount of oil kept in motion and to high no-load losses. Detailed information on the oil distribution in the early design stages of gearboxes would help to optimize the lubrication and to increase the efficiency. Thereby, CFD (computational fluid dynamics) methods offer a very flexible way to visualize the oil flow inside gearboxes with much fewer restrictions compared to measurements with transparent gearbox designs. In this study, a verified CFD model based on the finite volume method is used to investigate the oil flow in a single-stage gearbox. Different oil viscosities and circumferential speeds are considered. The investigations focus on the oil flow. The gear churning loss, as part of the no-load loss, is additionally considered. Experimental validation is obtained by high-speed camera recordings and measurements at the FZG no-load power loss test rig. The results show very strong agreement between simulation and measurement. The results show that CFD simulations can visualize the oil flow behavior with a very high degree of detail. Full article
(This article belongs to the Special Issue Selected Papers in the Sixth World Tribology Congress (WTC 2017))
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Open AccessArticle Three-Dimensional DEM Modelling of Ball Bearing with Lubrication Regime Prediction
Received: 28 February 2018 / Revised: 2 May 2018 / Accepted: 3 May 2018 / Published: 8 May 2018
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Abstract
This paper deals with an efficient 3D modelling of a radial ball bearing to predict the operating lubrication regime under mechanical loading and mounting conditions by using the Discrete Element Method (DEM). Due to the relevance of such an approach, especially for multicontact
[...] Read more.
This paper deals with an efficient 3D modelling of a radial ball bearing to predict the operating lubrication regime under mechanical loading and mounting conditions by using the Discrete Element Method (DEM). Due to the relevance of such an approach, especially for multicontact systems, the lubrication regime associated with specific operating conditions can be predicted accurately. By means of an elastohydrodynamic lubrication formulation depending on parameters related to the size of contact area, mechanical properties of materials, roughness and fluid viscosity, the lubricant film thickness is predicted and used to take into consideration the fluid film damping effect and friction coefficient variation. The lubrication regime can be identified according to Stribeck curve with the assumption of a piezo-viscous-elastic behaviour of the lubricant. The numerical simulations performed with MULTICOR-3D software on an operating ball bearing shown that the lubrication regime at the rolling element-raceway contact can be easily monitored and quantitatively identified. To assess the efficiency of the discrete modelling, a parametric study is carried out in order to exhibit how the operating conditions affect the lubrication regimes and the fluid film spread in the loaded zone. The adequacy between the choice of lubricant and the bearing tribofinition is sought to optimize the component lifetime. Full article
(This article belongs to the Special Issue Computer Simulation in Tribology and Friction)
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Open AccessArticle Tribological and Rheological Characterization of New Completely Biogenic Lubricating Greases: A Comparative Experimental Investigation
Received: 22 March 2018 / Revised: 30 April 2018 / Accepted: 2 May 2018 / Published: 5 May 2018
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Abstract
Against the background of raw material shortage and the ever-expanding environmental consciousness, the use of biodegradable greases becomes more and more important. The aim of this experimental work is to investigate the tribological response of completely biodegradable greases. Complete biodegradable lubricating greases were
[...] Read more.
Against the background of raw material shortage and the ever-expanding environmental consciousness, the use of biodegradable greases becomes more and more important. The aim of this experimental work is to investigate the tribological response of completely biodegradable greases. Complete biodegradable lubricating greases were formulated with high-oleic sunflower oil (HOSO) and/or castor oil, and different biodegradable thickener agents such as natural cellulose fibers of different lengths and some derivatives, as well as glyceryl and sorbitan stearates. To investigate the friction process, the model greases were tribologically examined with a nanotribometer at a normal force of 200 mN using a material combination of a steel ball on a steel disc. All frictional results, along with the volumes of wear tracks and micrographs of the main contacting area on the steel plate, are presented and discussed. In addition to this, rotational transient flow measurements were carried out on a rheometer at different temperatures to monitor the evolution of the shear stress with time at a constant shear rate, and to characterize the internal friction behavior by quantifying the energy density. All results were also analyzed from an energetic point of view. Full article
(This article belongs to the Special Issue Lubricating Greases 2017)
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Open AccessArticle Modeling the Friction Boundary Layer of an Entire Brake Pad with an Abstract Cellular Automaton
Received: 13 April 2018 / Revised: 26 April 2018 / Accepted: 2 May 2018 / Published: 4 May 2018
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Abstract
The principle energy exchange of a brake system occurs in the tribological boundary layer between the pad and the disc. The associated phenomena are primarily responsible for the dynamics of brake systems. The wear debris forms flat contact structures, or “patches,” which carry
[...] Read more.
The principle energy exchange of a brake system occurs in the tribological boundary layer between the pad and the disc. The associated phenomena are primarily responsible for the dynamics of brake systems. The wear debris forms flat contact structures, or “patches,” which carry the majority of the normal load in the system and are highly influential on the friction behavior of the system. A new simulation tool is presented, which is capable of rapidly performing simulations of the contact between an entire brake pad and disc. The “Abstract Cellular Automaton” simulations accurately model the patch coverage state of a brake pad surface based on the system’s load history. This can be used to simulate the complex dissipation phenomena within the tribological contact of the entire pad, including time-dependent local friction coefficients, wear and wear debris transport, and vibrational effects on highly differing scales. Full article
(This article belongs to the Special Issue Computer Simulation in Tribology and Friction)
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Open AccessArticle Molecular Dynamics Modeling of the Sliding Performance of an Amorphous Silica Nano-Layer—The Impact of Chosen Interatomic Potentials
Received: 21 March 2018 / Revised: 27 April 2018 / Accepted: 1 May 2018 / Published: 3 May 2018
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Abstract
The sliding behavior of an amorphous silica sample between two rigid surfaces is in the focus of the present paper. Molecular Dynamics using a classical Tersoff’s potential and a recently developed ReaxFF potential was applied for simulating sliding within a thin film corresponding
[...] Read more.
The sliding behavior of an amorphous silica sample between two rigid surfaces is in the focus of the present paper. Molecular Dynamics using a classical Tersoff’s potential and a recently developed ReaxFF potential was applied for simulating sliding within a thin film corresponding to a tribofilm formed from silica nanoparticles. The simulations were performed at different temperatures corresponding to moderate and severe tribological stressing conditions. Simulations with both potentials revealed the need of considering different temperatures in order to obtain a sound interpretation of experimental findings. The results show the striking differences between the two potentials not only in terms of magnitude of the resistance stress (about one order of magnitude) but also in terms of friction mechanisms. The expected smooth sliding regime under high temperature conditions was predicted by both simulations, although with Tersoff’s potential smooth sliding was obtained only at the highest temperature. On the other hand, at room temperature Tersoff-style calculations demonstrate stick-slip behavior, which corresponds qualitatively with our experimental findings. Nevertheless, comparison with a macroscopic coefficient of friction is not possible because simulated resistance stresses do not depend on the applied normal pressure. Full article
(This article belongs to the Special Issue Computer Simulation in Tribology and Friction)
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Open AccessArticle Development of a Constitutive Model for Friction in Bulk Metal Forming
Received: 13 March 2018 / Revised: 25 April 2018 / Accepted: 25 April 2018 / Published: 28 April 2018
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Abstract
This paper presents a systematic procedure for the development of a constitutive model of friction with focus on the application in bulk metal forming simulations. The empirically based friction model describes friction as a function of sliding distance and the most relevant friction
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This paper presents a systematic procedure for the development of a constitutive model of friction with focus on the application in bulk metal forming simulations. The empirically based friction model describes friction as a function of sliding distance and the most relevant friction influencing parameters. The latter were determined by means of designed experiments. An optimal friction model is obtained as a trade-off between model accuracy and complexity by using stepwise nonlinear regression and a modified version of the Akaike information criterion. Within this study, the procedure is applied to determine a friction model for tube drawing. However, the same approach can also be used for modeling friction of any other bulk metal forming process. Full article
(This article belongs to the Special Issue Tribology in Metal Forming)
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Open AccessArticle Influence of Test Stand and Contact Size Sensitivity on the Friction Coefficient in Sheet Metal Forming
Received: 6 April 2018 / Revised: 23 April 2018 / Accepted: 24 April 2018 / Published: 27 April 2018
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Abstract
The precise knowledge of frictional behavior is highly relevant for accurate modelling in sheet metal forming simulations. This allows e.g., the precise prediction of restraining forces which, in turn, determines an optimal draw bead strategy and blank-texture-development for automotive components. As a result,
[...] Read more.
The precise knowledge of frictional behavior is highly relevant for accurate modelling in sheet metal forming simulations. This allows e.g., the precise prediction of restraining forces which, in turn, determines an optimal draw bead strategy and blank-texture-development for automotive components. As a result, tryout loops can be avoided and thus production costs can be reduced. Nevertheless, the benefit of this detailed friction description is often ignored by the use of a constant friction coefficient. Finding a practical solution has motivated numerous research projects in recent decades. In this context, many efforts have been made to develop test stands to gain a better understanding of friction and to determine load-dependent friction coefficients for simulations. However, different test stands for friction investigation show a big quantitative difference in friction value which makes the direct use of the values in finite element simulation questionable. Therefore, the focus of this paper is to compare two different common strip drawing tests and detect the sources of deviation. In particular, the influence of the contact area between tool and blank is investigated. The results indicate that while the effect of the different test stands is negligible, a high dependency of the friction coefficient on the contact area was shown. This phenomenon is caused by macroscopic lubricant distribution over the contact area, which varies according to the size of the tools. The results show a potential field of research in categorizing different friction test stands and resolving the issue of quantitative non-comparable coefficients of friction. Full article
(This article belongs to the Special Issue Tribology in Metal Forming)
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Open AccessArticle Dynamical Simulations of a Flexible Rotor in Cylindrical Uncavitated and Cavitated Lubricated Journal Bearings
Received: 5 February 2018 / Revised: 16 April 2018 / Accepted: 20 April 2018 / Published: 25 April 2018
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Abstract
Due to requirements of their operating conditions, such as high speed, high flexibility and high efficiency, rotating machines are designed to obtain larger operating ranges. These operating conditions can increase the risk of fluid-induced instability. In fact, the presence of non-linear fluid forces
[...] Read more.
Due to requirements of their operating conditions, such as high speed, high flexibility and high efficiency, rotating machines are designed to obtain larger operating ranges. These operating conditions can increase the risk of fluid-induced instability. In fact, the presence of non-linear fluid forces when the threshold speed is overcome by the rotational speed, can generate rotor lateral self-excited vibrations known as “oil whirl” or “oil whip”. These instabilities derive from the interaction between the rotor and the sliding bearing and they are typically sub-synchronous and they contribute to eventual rubbing between rotor and stator with consequent damage to the rotating machines. For these reasons, the aim of this paper is to numerically investigate the differences in the dynamic behaviour of a flexible rotor supported by cylindrical lubricated journal bearings. The study considers two different cases, uncavitated and cavitated lubricated films, in order to develop an original Matlab-Simulink algorithm for the numerical solution of the differential non-linear equations of motion of the unbalanced flexible rotor supported on hydrodynamic journal bearings. The bearings were modelled as uncavitated and cavitated (π-Film) short bearings derived from classical Reynolds’ theory. Dynamic simulation allowed prediction of the shape and size of the orbit performed by the system and evaluation of the vibrating phenomena exerted by the rotor during the motion. The results show that cavitation completely modifies the behaviour of the system in every aspect. The analysis of the diagrams obtained showed that the proposed algorithm provides consistent results and represents a valuable instrument for dynamic analysis of rotating systems. Full article
(This article belongs to the Special Issue Tribology of Machine Elements--Smart Lubricants)
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Open AccessArticle Material Characterization and Influence of Sliding Speed and Pressure on Friction and Wear Behavior of Self-Lubricating Bearing Materials for Hydropower Applications
Received: 16 March 2018 / Revised: 16 April 2018 / Accepted: 20 April 2018 / Published: 24 April 2018
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Abstract
Nowadays, hydropower plants are forced to have more frequent power control and the self-lubricated bearings used in the applications are one of the most critical components affected by the continuously changing operating conditions. In this study, microstructure and composition of two commercially available
[...] Read more.
Nowadays, hydropower plants are forced to have more frequent power control and the self-lubricated bearings used in the applications are one of the most critical components affected by the continuously changing operating conditions. In this study, microstructure and composition of two commercially available bearing materials (Orkot TXM Marine and Thordon ThorPlas) used in hydropower turbines were studied. In addition, the influence of sliding speed and applied pressure on the friction and wear behavior of the materials was investigated systematically for dry sliding conditions. The bearing materials were characterized using X-ray microtomography, Nuclear Magnetic Resonance (NMR) spectroscopy and Inductively Coupled Plasma–Sector Field Mass Spectrometry (ICP-SFMS) techniques. Friction and wear tests were carried out with a polymer pin sliding against a stainless steel (SS2333) plate with a linear reciprocating motion. Test conditions were: room temperature, 9–28 MPa pressure and 10–40 mm/s sliding speed ranges. Surface analysis of the polymer pins and the wear tracks were performed by optical profilometry, Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) techniques. Test results show that, for both materials, the coefficient of friction (COF) is decreasing at higher pressures. Surface analysis reveals higher concentrations of solid lubricants in the transfer layers formed at higher pressures, explaining the decrease in COF. Furthermore, the specific wear rate coefficients are increasing at higher sliding speeds, especially at lower pressures. Results of this study demonstrate that, under dry sliding conditions, changes in sliding speed and pressure have a significant influence on the tribological behavior of these bearing materials. Full article
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Open AccessArticle Simple Prediction Method for Rubber Adhesive Friction by the Combining Friction Test and FE Analysis
Received: 7 March 2018 / Revised: 16 April 2018 / Accepted: 17 April 2018 / Published: 18 April 2018
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Abstract
In the design and development of rubber products, it is important to evaluate the contact load dependency of the friction coefficient. In particular, since the pressure distribution varies depending on the dimensions of sliding bodies and the pattern of the contact surface, a
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In the design and development of rubber products, it is important to evaluate the contact load dependency of the friction coefficient. In particular, since the pressure distribution varies depending on the dimensions of sliding bodies and the pattern of the contact surface, a simplified and accurate evaluation method that can take these influences into account is desired. In this study, we proposed a prediction method for the adhesive friction between rubber specimens of arbitrary shapes with arbitrary roughness and a smooth hard surface, by combining the: (1) friction theory considering the influence of roughness; (2) basic friction test; and (3) finite element analysis. Further, we verified the effectiveness of the proposed method by comparing the predicted results with the measurement results of friction between a hemispherical PDMS specimen and a PMMA flat plate and between a PDMS block specimen with a grooved surface and a flat prism. Results show that the prediction accuracy of the contact load dependency of the friction coefficient is reasonably good. Full article
(This article belongs to the Special Issue Computer Simulation in Tribology and Friction)
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Open AccessArticle Tribocorrosion of Fe-Based Amorphous Coating in Simulated Body Fluids
Received: 19 March 2018 / Revised: 9 April 2018 / Accepted: 12 April 2018 / Published: 18 April 2018
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Abstract
An arc-sprayed Fe-based amorphous coating with high hardness and low porosity was prepared. A tribo-electrochemical approach was used to study the tribocorrosion behaviour of the amorphous coating. The volume wear losses of the amorphous coating with different sliding paths in dry, 0.9% NaCl,
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An arc-sprayed Fe-based amorphous coating with high hardness and low porosity was prepared. A tribo-electrochemical approach was used to study the tribocorrosion behaviour of the amorphous coating. The volume wear losses of the amorphous coating with different sliding paths in dry, 0.9% NaCl, and PBS solutions were measured, as well as the friction coefficient and the polarization curves in static and dynamic situations. The volume wear loss with the linear sliding path is higher than those with circular and triangle paths. Since the ions in the solution accelerate the wear, the volume loss of the amorphous coating in 0.9% NaCl solution is higher than dry and in PBS solution. The wear loss of 316L stainless steel (SS) is about 1.7 times more than the amorphous coating in PBS solution under a load of 10 N. Although 316L SS possesses better corrosion resistance than the amorphous coating in the static situation, the corrosion resistance of the amorphous coating is much better than that of 316L SS during tribocorrosion. The wear mechanism of the amorphous coating includes abrasive wear accompanying with corrosive wear. For the intrinsic superior corrosion resistance, amorphous coating shows the prospective tribology application in the corrosion environment. Full article
(This article belongs to the Special Issue Tribocorrosion of Surface Engineered Materials)
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Open AccessArticle Laser Surface Texturing of TiAl Multilayer Films—Effects of Microstructure and Topography on Friction and Wear
Received: 16 March 2018 / Accepted: 15 April 2018 / Published: 16 April 2018
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Abstract
Laser surface texturing is an efficient way to control the friction and wear properties of materials. Although described in many papers, most previous work relates to a pure topographic view of laser-textured surfaces. As lasers are heat sources, their thermal impact during treatment
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Laser surface texturing is an efficient way to control the friction and wear properties of materials. Although described in many papers, most previous work relates to a pure topographic view of laser-textured surfaces. As lasers are heat sources, their thermal impact during treatment can be high enough to modify the material’s microstructure or surface chemistry and affect tribological properties as well. This research took a closer look at the microstructure of laser-textured TiAl multilayers, besides topographic aspects. Direct laser interference patterning was used to create well-defined line-like surface textures in TiAl multilayers with differing lateral feature sizes in the micron range. High-resolution techniques such as TEM and XRD highlighted the effect of this method on microstructure, and in particular, the phase situation of the TiAl multilayer. Thermal simulations demonstrated that the maximum achievable temperatures were around 2000 K, thus being high enough to melt Ti and Al. Cooling rates on the order of 109 K/s depended on the lateral feature size, potentially leading to metastable microstructures. Finally, ball-on-disk tests on as-textured TiAl specimens showed a reduction in wear under dry conditions depending on the periodicity of the line-like textures used. Full article
(This article belongs to the Special Issue Improvement of Friction and Wear by Laser Surface Texturing)
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Open AccessArticle Importance of Emulsification in Calibrating Infrared Spectroscopes for Analyzing Water Contamination in Used or In-Service Engine Oil
Received: 9 March 2018 / Revised: 4 April 2018 / Accepted: 9 April 2018 / Published: 12 April 2018
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Abstract
Using Fourier transform infrared (FT-IR) spectroscopy we investigated the water content of SAE 15W–40 diesel engine lubricating oil at various levels of contamination to establish instrument calibration standards for measuring water contamination in used or in-service engine oil by the standards of ASTM
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Using Fourier transform infrared (FT-IR) spectroscopy we investigated the water content of SAE 15W–40 diesel engine lubricating oil at various levels of contamination to establish instrument calibration standards for measuring water contamination in used or in-service engine oil by the standards of ASTM International. Since some known additives in consumer grade engine oil possess slightly hydrophilic properties, this experiment avoided changing the sample matrix with supplemental additives, such as adding surfactants, to achieve homogeneity of the original sample. The impact of sampling time after contamination on the spectral absorption signature was examined in an attempt to improve the accuracy of water contamination quantification and determine if water-soluble potassium bromide (KBr) windows were suitable for analyzing water in oil emulsions. Analysis of variance (ANOVA) modeling and limit of detection calculations were used to predict the ability to discriminate contamination levels over time. Our results revealed that the amount of water concentration in engine oil could be misinterpreted depending on the timing of the FT-IR measurement of the calibration standard after initial water contamination. Also, KBr windows are not sufficiently etched due to the limited window interaction with water molecules within micelles of emulsions to alter FT-IR spectral signatures. Full article
(This article belongs to the Special Issue Oil and Gas Tribology)
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Open AccessArticle Pathways of Dissipation of Frictional Energy under Boundary Lubricated Sliding Wear of Martensitic Materials
Received: 6 March 2018 / Revised: 26 March 2018 / Accepted: 3 April 2018 / Published: 11 April 2018
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Abstract
The challenges of technical systems subjected to friction and wear become more demanding with steadily increasing stresses. Besides safety matters, failure of tribologically loaded systems can cause tremendous maintenance costs. Because of the lack of a general wear prediction model, tribometer tests must
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The challenges of technical systems subjected to friction and wear become more demanding with steadily increasing stresses. Besides safety matters, failure of tribologically loaded systems can cause tremendous maintenance costs. Because of the lack of a general wear prediction model, tribometer tests must be used in order to investigate wear behaviour of materials in certain tribological systems. Any well-aimed optimizations of tribological contacts requires a comprehensive understanding of friction and wear mechanisms. Otherwise the transferability into technical applications is questionable because of the wide range of applied loads, lubrication conditions, and materials microstructures. In this study, specimens with different topographies and subsurface microstructures were investigated prior to and after tribological testing. The analyses of surface and subsurface characteristics were performed by means of complementary high-resolution electron-microscopy techniques. The study attempted to link the findings to the wear behavior in order to gain information about the pathways of dissipation and transformation of frictional energy into wear. It was found that the dissipation pathways of base body and counter body were different, resulting in diverse tribological behaviour. Nonetheless, the presence of a near-surface grain-refined layers (tribomaterial) supported by a sub-surface strain gradient appears to provide a beneficial influence. Despite the fact that any direct or even conclusive relation to the topographies or subsurface microstructures cannot be given, the discussion provides some hints on how to analyse such systems for their characteristic mechanisms. In addition to the capability of such approach as one step of understanding, its limitations are shown and briefly discussed as well. Full article
(This article belongs to the Special Issue Applied Nanotribology)
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Open AccessArticle Extension of One-Dimensional Models for Hyperelastic String Structures under Coulomb Friction with Adhesion
Received: 28 February 2018 / Revised: 2 April 2018 / Accepted: 6 April 2018 / Published: 10 April 2018
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Abstract
A stretching behavior of knitted and woven textiles is modeled. In our work, the yarns are modeled as one-dimensional hyperelastic strings with frictional contact. Capstan law known for Coulomb’s friction of yarns is extended to an additional adhesion due to gluing of filaments
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A stretching behavior of knitted and woven textiles is modeled. In our work, the yarns are modeled as one-dimensional hyperelastic strings with frictional contact. Capstan law known for Coulomb’s friction of yarns is extended to an additional adhesion due to gluing of filaments on the yarn surface or some chemical reaction. Two-step Newton’s method is applied for the solution of the large stretching with sliding evolution in the contact nodes. The approach is illustrated on a hysteresis of knitted textile and on the force-strain curve for a woven pattern and both compared with experimental effective curves. Full article
(This article belongs to the Special Issue Computer Simulation in Tribology and Friction)
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Open AccessArticle Effect of Base Oil Type in Grease Composition on the Lubricating Film Formation in EHD Contacts
Received: 29 January 2018 / Revised: 15 March 2018 / Accepted: 23 March 2018 / Published: 9 April 2018
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Abstract
The service life of rolling bearings is significantly affected by the film formation in elastohydrodynamic (EHD) contacts, which depends on the operating conditions, like rotational speed or temperature. In grease lubricated EHD contacts, the film formation is determined by the grease consistency and
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The service life of rolling bearings is significantly affected by the film formation in elastohydrodynamic (EHD) contacts, which depends on the operating conditions, like rotational speed or temperature. In grease lubricated EHD contacts, the film formation is determined by the grease consistency and composition, i.e., thickener and base oil type as well as properties of the bleed oil, which is released from the grease during operation. Thus, the film formation of grease lubricated contacts as compared to base oil lubricated contacts can be different. With increasing rolling speed, the film thickness of oil lubricated contacts usually grows. However, in case of grease lubricated contacts, which are not fully flooded, the film thickness remains constant or even decreases with further increasing rotational speed. This effect is referred to as starvation. Since the onset of starvation depends on the grease composition, the film formation of two different grease compositions is investigated in this study. The film thickness measurements are performed on a ball-on-disc tribometer for each grease, as well as the corresponding bleed and pure base oils. Thereby, the characteristic rotational speed leading to the onset of starvation has been identified in dependence of the grease composition and the differences in the lubricating film formation of base oil, bleed oil, and grease lubricated EHD contacts have been discussed. The investigations should help to establish an advanced understanding of the physical mechanisms leading to the onset of starvation to encourage future work with focus on a method to predict the film formation in grease lubricated EHD contacts. Full article
(This article belongs to the Special Issue Lubricating Greases 2017)
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Open AccessArticle Ab Initio Molecular Dynamics Simulation of Tribochemical Reactions Involving Phosphorus Additives at Sliding Iron Interfaces
Received: 22 March 2018 / Revised: 30 March 2018 / Accepted: 4 April 2018 / Published: 5 April 2018
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Abstract
We performed, for the first time to our knowledge, fully ab initio molecular dynamics simulations of additive tribochemistry in boundary lubrication conditions. We consider an organophosphourus additive that has been experimentally shown to reduce friction in steel-on-steel sliding contacts thanks to the tribologically-induced
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We performed, for the first time to our knowledge, fully ab initio molecular dynamics simulations of additive tribochemistry in boundary lubrication conditions. We consider an organophosphourus additive that has been experimentally shown to reduce friction in steel-on-steel sliding contacts thanks to the tribologically-induced formation of an iron phosphide tribofilm. The simulations allow us to observe in real time the molecular dissociation at the sliding iron interface under pressure and to understand the mechanism of iron phosphide formation. We discuss the role played by the mechanical stress by comparing the activation times for molecular dissociation observed in the tribological simulations at different applied loads with that expected on the basis of the dissociation barrier. Full article
(This article belongs to the Special Issue Selected Papers in the Sixth World Tribology Congress (WTC 2017))
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Open AccessReview Advances in Tribology of Lubricin and Lubricin-Like Synthetic Polymer Nanostructures
Received: 2 January 2018 / Revised: 6 March 2018 / Accepted: 15 March 2018 / Published: 4 April 2018
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Abstract
Articular cartilage surrounds the ends of diarthrodial joints (most common movable joints) and during motion, it experiences a wide range of loading conditions while remaining under exceedingly low-friction and wear-free conditions. This remarkable tribological performance stems from complex interactions between the synovial fluid
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Articular cartilage surrounds the ends of diarthrodial joints (most common movable joints) and during motion, it experiences a wide range of loading conditions while remaining under exceedingly low-friction and wear-free conditions. This remarkable tribological performance stems from complex interactions between the synovial fluid and articular cartilage. In fact, lubricin and hyaluronic acid (HA) that are part of the synovial fluid are now known to be the key contributors to effective joint lubrication and wear protection. Studies involving animal models and artificial systems suggest that lubricin and HA molecules may work in tandem to produce a highly synergistic effect for lubrication. However, latest observations suggest that lubricin has significant potential for protecting the articular joints, probably more than HA. Recently, lurbicin-related friction regulation in soft eye tissues, where much lower forces are involved compared to knee joints for instance, has been shown to be related to dry eye disease and contact lens discomfort. As such, lubricin’s role in natural friction regulation is very complex. Moreover, partially unresolved water-lubricin interactions are essential for lubrication and load carrying function in the joints. The chemical structure of lubricin has inspired several chemists to synthesize new copolymers and polymer brushes that function just like lubricin in order to design new synthetic or bio-based lubricants with ultra-low-friction coefficients. Hence, lubricin has emerged as a key natural molecule for bioinspired tribology. The aim of this review is to present the latest advances in understanding of lubricin’s function in joint lubrication and in soft tissue friction (i.e., human eye) and document what has been achieved so far in transforming this biomedical knowledge into new polymer design for advanced engineering tribology. It is hoped that this review will catalyze research and development efforts in obtaining very stable and high load-bearing polymer-based ultra-low-friction surfaces via biomimicry. Full article
(This article belongs to the Special Issue Biolubrication and Biomimetic Lubrication)
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