We study the frictional behavior of both elastic and viscoelastic thin coatings bonded to a seemingly rigid substrate and sliding against a rough profile in the presence of Coulomb friction at the ...interface. The aim is to explore the effect of the coupling between the normal and tangential displacement fields arising from the finiteness of the material thickness and to quantify the contribution this can have on energy losses.
We found that, due to normal–tangential coupling, asymmetric contacts and consequently additional friction are observed even for purely elastic layers, indeed associated with zero bulk energy dissipation. Furthermore, enhanced viscoelastic friction is reported in the case of viscoelastic coatings due to coupling, this time also entailing larger bulk energy dissipation.
Geometric coupling also introduces additional interactions involving the larger scales normal displacements, which leads to a significant increase of the contact area, under given normal load, compared to the uncoupled contacts.
These results show that, in the case of contact interfaces involving thin deformable coating bonded to significantly stiffer substrate, the effect of interfacial shear stresses on the frictional and contact behavior cannot be neglected.
Nonequilibrium molecular dynamics (NEMD) simulations have provided unique insights into the nanoscale behaviour of lubricants under shear. This review discusses the early history of NEMD and its ...progression from a tool to corroborate theories of the liquid state, to an instrument that can directly evaluate important fluid properties, towards a potential design tool in tribology. The key methodological advances which have allowed this evolution are also highlighted. This is followed by a summary of bulk and confined NEMD simulations of liquid lubricants and lubricant additives, as they have progressed from simple atomic fluids to ever more complex, realistic molecules. The future outlook of NEMD in tribology, including the inclusion of chemical reactivity for additives, and coupling to continuum methods for large systems, is also briefly discussed.
The slip and friction behavior of n-hexadecane, confined between organic friction modifier surfactant films adsorbed on hematite surfaces, has been studied using nonequilibrium molecular dynamics ...simulations. The influence of the surfactant type and coverage, as well as the applied shear rate and pressure, has been investigated. A measurable slip length is only observed for surfactant films with a high surface coverage, which provide smooth interfaces between well-defined surfactant and hexadecane layers. Slip commences above a critical shear rate, beyond which the slip length first increases with increasing shear rate and then asymptotes toward a constant value. The maximum slip length increases significantly with increasing pressure. Systems and conditions which show a larger slip length typically give a lower friction coefficient. Generally, the friction coefficient increases linearly with logarithmic shear rate; however, it shows a much stronger shear rate dependency at low pressure than at high pressure. Relating slip and friction, slip only occurs above a critical shear stress, after which the slip length first increases linearly with increasing shear stress and then asymptotes. This behavior is well-described using previously proposed slip models. This study provides a more detailed understanding of the slip of alkanes on surfactant monolayers. It also suggests that high coverage surfactant films can significantly reduce friction by promoting slip, even when the surfaces are well-separated by a lubricant.
This paper describes a new computational fluid dynamics methodology for modelling elastohydrodynamic contacts. A finite-volume technique is implemented in the ‘OpenFOAM’ package to solve the ...Navier-Stokes equations and resolve all gradients in a lubricated rolling-sliding contact. The method fully accounts for fluid-solid interactions and is stable over a wide range of contact conditions, including pressures representative of practical rolling bearing and gear applications. The elastic deformation of the solid, fluid cavitation and compressibility, as well as thermal effects are accounted for. Results are presented for rolling-sliding line contacts of an elastic cylinder on a rigid flat to validate the model predictions, illustrate its capabilities, and identify some example conditions under which the traditional Reynolds-based predictions deviate from the full CFD solution.
•A new, CFD based numerical method for modelling EHD lubrication is presented.•The model utilises finite volume technique for both fluid and solid domains to produce a two-way coupled FSI model.•The model is implemented in open source package OpenFOAM and validated against published data.•Stability over a wide-range of conditions including those found in practical bearing applications is illustrated.•An example case where CFD predictions deviate from Reynolds based approaches is shown.
The influence of surface roughness and contact adhesion on the rolling behaviour of dry particles has been investigated. Rough particle surfaces are approximated using an array of spheres, the ...properties of which are informed by random processes. An analytical model has been derived by considering the torques that a particle experiences. Two mechanisms of rolling resistance are explored – a stationary particle experiencing a tangential force, and a dynamically rolling particle. The analytical model is found to agree well with simulations of the equivalent system using the discrete element model. Adhesive forces are found to increase rolling resistance in all cases. The complex consequences of varying the height variance and length scale of the surface roughness are reproduced accurately by the analytical model.
Display omitted
•A model of rolling resistance for rough, adhesive particles is developed.•Roughness is described using spherical asperities, their properties informed by the statistics of random fields.•Analytical results are found to fit well to numerical calculations of the same properties using DEM.
Articular cartilage lesions are a particular challenge for regenerative medicine strategies as cartilage function stems from a complex depth-dependent organization. Tissue engineering scaffolds that ...vary in morphology and function offer a template for zone-specific cartilage extracellular matrix (ECM) production and mechanical properties. We fabricated multi-zone cartilage scaffolds by the electrostatic deposition of polymer microfibres onto particulate-templated scaffolds produced with 0.03 or 1.0mm3 porogens. The scaffolds allowed ample space for chondrocyte ECM production within the bulk while also mimicking the structural organization and functional interface of cartilage’s superficial zone. Addition of aligned fibre membranes enhanced the mechanical and surface properties of particulate-templated scaffolds. Zonal analysis of scaffolds demonstrated region-specific variations in chondrocyte number, sulfated GAG-rich ECM, and chondrocytic gene expression. Specifically, smaller porogens (0.03mm3) yielded significantly higher sGAG accumulation and aggrecan gene expression. Our results demonstrate that bilayered scaffolds mimic some key structural characteristics of native cartilage, support in vitro cartilage formation, and have superior features to homogeneous particulate-templated scaffolds. We propose that these scaffolds offer promise for regenerative medicine strategies to repair articular cartilage lesions.
PDF
