Browsing by Subject "Tribology"

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Open Access
Computational homogenization of soft matter friction: Isogeometric framework and elastic boundary layers
(John Wiley and Sons Ltd, 2014) Temizer, I.
SUMMARY: A computational contact homogenization framework is established for the modeling and simulation of soft matter friction. The main challenges toward the realization of the framework are (1) the establishment of a frictional contact algorithm that displays an optimal combination of accuracy, efficiency, and robustness and plays a central role in (2) the construction of a micromechanical contact test within which samples of arbitrary size may be embedded and which is not restricted to a single deformable body. The former challenge is addressed through the extension of mixed variational formulations of contact mechanics to a mortar-based isogeometric setting where the augmented Lagrangian approach serves as the constraint enforcement method. The latter challenge is addressed through the concept of periodic embedding, with which a periodically replicated C1-continuous interface topography is realized across which not only pending but also ensuing contact among simulation cells will be automatically captured. Two-dimensional and three-dimensional investigations with unilateral/bilateral periodic/random roughness on two elastic micromechanical samples demonstrate the overall framework and the nature of the macroscopic frictional response. © 2014 John Wiley & Sons, Ltd.
Open Access
Effect of triboelectric charges on friction and wear of polymers at macro scale
(2017-12) Sayfidinov, Khaydarali
The interest towards the study of underlying mechanism behind tribology has gained enormous attention recently since almost one-fourth of the total produced global energy is consumed by friction and wear. Dry sliding or rubbing two dielectric polymers on each other results in surface charging showing significant effects on friction coefficients and wear. Determination of the correlation between triboelectricity and tribologic events like friction and wear, the control of friction coefficient, and reducing wear by surface charging constitutes the main idea and research topic of this thesis. However, tribological events are very complicated considering the fact that diverse processes encompassing of physical and chemical changes occur at the counterface. Therefore, the fundamentals of friction is still controversial. Owing to tribological actions that occur due to contact between different phases of the matter, interfaces generate tribocharges due to electron, ion, and material transfer mechanisms. Even though the fundamental mechanism is still vague and under debate, it is believed that static electrification due to tribological actions are utterly because of electron transfer. Current studies unveiled that physical based phenomena are not the only source of surface electrification but also chemical changes such as bond rupturing and following surface oxidation that can take place as a result of mechanical actions on an insulating polymer. Consequently, these two groups of surface events; surface electrification and friction are expected to demonstrate a mutual relation, and detailed study concerning this relation needs to be investigated in order to solve e.g. energy loss and wear problems in tribology. To achieve this goal, it is essential to understand the main mechanisms and processes involved, and reveal the connections between tribological events and establish a relationship between all the intrinsic and extrinsic properties of materials from molecular, nano to meso scale. Thus, in this study, we investigated the contribution of triboelectrification to friction by taking into account some factors - surface area, load, atmosphere - between common polymers and pure cellulose under dry friction conditions.
Open Access
Model description of friction on planar and buckled two dimensional materials
(2018-01) Uzlu, Hasan Burkay
The law of friction has been known since the 18th century but yet, the development on the tribology field was established in the last decades mainly by the invention of frictional force microscope (FFM), which enabled scientist to study friction on atomic levels. To describe the friction phenomena at nanoscale, molecular dynamics (MD) and density functional theory (DFT) models are commonly used, popular models and detailed information about friction can be obtained via those models. On the other hand, reduced-order simplified models such as Prandtl-Tomlinson (PT) model can also provide essential information about friction phenomena and understanding a phenomenon via a simplified model is always motivate. In this thesis, Prandtl-Tomlinson model is generalized into three dimensions and the model is illustrated in both two and three dimensions on various quasi two dimensional crystal structures such as graphene, silicene, germanene and hexagonal boron nitride. By solving the equation of motion of the PT model numerically, friction curves and some parametric dependences of the friction such as anisotropy and friction dependence on external loading force is analyzed. We concluded that the PT model in three dimensions provides good results and can be used to analyze friction phenomena to save from computational cost in MD and DFT models.
Open Access
Nanoscale tribology of graphene grown by chemical vapor deposition and transferred onto silicon oxide substrates
(Cambridge University Press, 2016) Demirbaş, T.; Baykara, M. Z.
We present a comprehensive nanoscale tribological characterization of single-layer graphene grown by chemical vapor deposition (CVD) and transferred onto silicon oxide (SiO2) substrates. Specifically, the nanotribological properties of graphene samples are studied via atomic force microscopy (AFM) under ambient conditions using calibrated probes, by measuring the evolution of friction force with increasing normal load. The effect of using different probes and post-transfer cleaning procedures on frictional behavior is evaluated. A new method of quantifying lubrication performance based on measured friction coefficient ratios of graphene and SiO2 is introduced. A comparison of lubrication properties with mechanically-exfoliated graphene is performed. Results indicate that CVD-grown graphene constitutes a very good solid lubricant on SiO2, reducing friction coefficients by ∼ 90% for all investigated samples. Finally, the effect of wrinkles associated with CVD-grown graphene on measured friction values is quantitatively analyzed, with results revealing a substantial increase in friction on these structural defects.
Open Access
A new approach for modeling viscoelastic thin film lubrication
(Elsevier BV, 2021-03-27) Biancofiore, Luca; Ahmed, Humayun
Lubricants can exhibit significant viscoelastic effects due to the addition of high molecular weight polymers. The overall behavior of the mixture is vastly different from a simpler Newtonian fluid. Therefore, understating the influence of viscoelasticity on the load carrying capacity of the film is essential for lubricated contacts. A new modeling technique based on lubrication theory is proposed to take into account viscoelastic effects. As a result, we obtain a modified equation for the pressure, i.e. the viscoelastic Reynolds (VR) equation. We have first examined a parabolic slider to mimic a roller bearing configuration. An increase of the load carrying capacity is observed when polymers are added to the lubricant. Furthermore, our results are compared with existing models based on the lubrication approximation and direct numerical simulations (DNS). For small Weissenberg number (), i.e. the ratio between the polymer relaxation time and the residence time scale, VR predicts the same pressure of the linearized model, in which is the perturbation parameter ( is the ratio between the vertical length scale and the horizontal length scale). However, the difference grows rapidly as viscoelastic effects become stronger. Excellent quantitative and qualitative agreement is observed between DNS and our model over small to moderate Weissenberg number. While DNS is numerically unstable at high values of the Weissenberg number, VR does not have the same issue allowing to capture the evolution of the stress and pressure also when the viscoelastic effects are strong. It is shown that even in high shear flows, normal stresses have the largest impact on load carrying capacity and thus cannot be neglected. Furthermore, the additional pressure due to viscoelasticity comprises two components, the first one due to the normal stress and the second one due to the shear stress. Afterwards, the methodology used for the parabolic slider is extended to a plane slider where, instead, the load decreases by adding polymers to the fluid. In particular, under the effect of the polymers surface slopes enhance the rate at which pressure gradients increase, whereas curvature opposes this along the contact. Therefore, the increase of the load carrying capacity observed for viscoelastic lubricants is due to its shape close to the inlet, which is steeper than the plane slider.
Open Access
Observer based friction cancellation in mechanical systems
(IEEE, 2014-10) Odabaş, Caner; Morgül, Ömer
An adaptive nonlinear observer based friction compensation for a special time delayed system is presented in this paper. Considering existing delay, an available Coulomb observer is modified and closed loop system is formed by using a Smith predictor based controller as if the process is delay free. Implemented hierarchical feedback system structure provides two-degree of freedom and controls both velocity and position separately. For this purpose, controller parametrization method is used to extend Smith predictor structure to the position control loop for different types of inputs and disturbance attenuation. Simulation results demonstrate that without requiring much information about friction force, the method can significantly improve the performance of a control system in which it is applied. © 2014 Institute of Control, Robotics and Systems (ICROS).
Open Access
Structural superlubricity of platinum on graphite under ambient conditions: the effects of chemistry and geometry
(American Institute of Physics Inc., 2017) Özoǧul, A.; Ipek, S.; Durgun, Engin; Baykara, M. Z.
An investigation of the frictional behavior of platinum nanoparticles laterally manipulated on graphite has been conducted to answer the question of whether the recent observation of structural superlubricity under ambient conditions [E. Cihan, S. Ipek, E. Durgun, and M. Z. Baykara, Nat. Commun. 7, 12055 (2016)] is exclusively limited to the gold-graphite interface. Platinum nanoparticles have been prepared by e-beam evaporation of a thin film of platinum on graphite, followed by post-deposition annealing. Morphological and structural characterization of the nanoparticles has been performed via scanning electron microscopy and transmission electron microscopy, revealing a crystalline structure with no evidence of oxidation under ambient conditions. Lateral manipulation experiments have been performed via atomic force microscopy under ambient conditions, whereby results indicate the occurrence of structural superlubricity at mesoscopic interfaces of 4000-75 000 nm2, with a noticeably higher magnitude of friction forces when compared with gold nanoparticles of similar contact areas situated on graphite. Ab initio simulations of sliding involving platinum and gold slabs on graphite confirm the experimental observations, whereby the higher magnitude of friction forces is attributed to stronger energy barriers encountered by platinum atoms sliding on graphite, when compared with gold. On the other hand, as predicted by theory, the scaling power between friction force and contact size is found to be independent of the chemical identity of the sliding atoms, but to be determined by the geometric qualities of the interface, as characterized by an average "sharpness score" assigned to the nanoparticles.
Open Access
Tactile perception by friction induced vibrations
(2011) Fagiani, R.; Massi, F.; Chatelet, E.; Berthier, Y.; Akay, A.
When a finger moves to scan the surface of an object (haptic sensing), the sliding contact generates vibrations that propagate in the finger skin activating the receptors (mechanoreceptors) located in the skin, allowing the brain to identify objects and perceive information about their properties. The information about the surface of the object is transmitted through vibrations induced by friction between the skin and the object scanned by the fingertip. The mechanoreceptors transduce the stress state into electrical impulses that are conveyed to the brain. A clear understanding of the mechanisms of the tactile sensing is fundamental to numerous applications, like the development of artificial tactile sensors for intelligent prostheses or robotic assistants, and in ergonomics. While the correlation between surface roughness and tactile sensation has already been reported in literature, the vibration spectra induced by the finger-surface scanning and the consequent activation of the mechanoreceptors on the skin have received less attention. In this paper, frequency analysis of signals characterizing surface scanning is carried out to investigate the vibration spectrum measured on the finger and to highlight the changes shown in the vibration spectra as a function of characteristic contact parameters such as scanning speed, roughness and surface texture. An experimental set-up is developed to recover the vibration dynamics by detecting the contact force and the induced vibrations; the bench test has been designed to guarantee reproducibility of measurements at the low amplitude of the vibrations of interest, and to perform measurements without introducing external noise. Two different perception mechanisms, as a function of the roughness wavelength, have been pointed out. The spectrum of vibration obtained by scanning textiles has been investigated. © 2011 Elsevier Ltd. All rights reserved.
Open Access
Tuning macroscopic sliding friction at soft contact interfaces: interaction of bulk and surface heterogeneities
(Elsevier Ltd, 2016) Kılıç, K. İ.; Temizer, İ.
Macroscopic frictional response of soft interfaces is strongly governed by the interaction of surface heterogeneities such as micro-texture features with bulk heterogeneities such as voids or inclusions beneath the highly deformable surface. This microscopic interaction manifests itself on the macroscale as an interface response that is reminiscent of stick-slip. Consequently, the accompanying macroscopic friction signal exhibits strong oscillations around a mean value, which itself significantly differs from its microscopic value due to finite deformations. In this work, a mechanism is proposed which enables the tuning of the macroscopic friction signal of soft interfaces. Specifically, it is demonstrated that optimally positioning subsurface particles in the vicinity of micro-texture features can significantly reduce observed oscillations, thereby allowing control of macroscopic sliding friction. © 2016 Elsevier Ltd
Open Access
Viscoelastic effects in lubricated contacts in the presence of cavitation
(2020-11) Gamaniel, Samuel Shari
A model is proposed to study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Previous studies on viscoelastic lubricants did not consider the presence of cavitation, rather reported negative pressures in regions where cavitation was expected to occur. The proposed model uses the Oldroyd-B constitutive model to describe the presence of cavitation and assumes that the Deborah number (De), the ratio between polymer relaxation time and flow time scale, is small. In doing so, the viscoelastic thin film equations can be linearised in a similar approach to what was pioneered by ”Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model.” The zeroth order solution in De corresponds to the Reynolds equation and has been modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We model several bearing configurations for the flow of viscoelastic lubricants using (i) a cosine/parabolic profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. Introducing viscoelasticity to the cavitating journal bearing decreases the length of the non-active (cavitation) region due to an increasing pressure distribution in the lubricant film. This results in an increase to the load carrying capacity with increasing De corroborating the beneficial influence of the polymers in fluid film bearings. The pocket profile is shown to either increase or decrease the load carrying capacity with increasing viscoelastic effects, depending on the location of surface texturing at the contact. An oscillating squeeze flow problem is modeled for viscoelastic lubricants between two flat plates with motion only at the top surface. A reduction in the load carrying capacity at larger values of De is observed as film reformation is seen to be retarded with increasing viscoelastic effects. As viscoelastic effects become stronger, the nonactive region is grows continuously until reaching a value of De beyond which a full film reformation does not occur upon the inception of cavitation. The study is extended to a direct numerical simulations using the openFoam toolbox. A model that couples a solver for incompressible, isothermal, two phase flow with interaction between the phases and a solver for viscoelastic fluids is proposed. However, DNS are only valid for lower values of De as instabilities occur as a result of the non-linear coupling.