Variation Of Friction Force Research Articles

Behaviors of carbon atoms induced by friction in mechanical polishing of diamond

Molecular dynamics simulation is employed to represent the mechanical polishing of diamond crystal, in which two diamond substrates with free surface of {100} and {110} faces are established. Firstly, the polishing direction related to variations of friction force on the both substrates are analyzed, and the non-diamond carbon atoms produced due to the phase transformation, mainly sp2 and amorphous sp3 hybridizations, in the surface layer are further extracted. Then, the distribution planes of ordered sp2 hybridizations are fitted, on which the length and angles of C–C bonds are also calculated. Finally, dependence of the length of dislocation lines and the distribution area of localized von Mises shear strain in the polishing direction are discussed. The results suggest that friction force decreases gradually when the polishing direction transiting from the ‘soft’ direction to the ‘hard’ direction, and the same variation tendency is also observed on the phase transformation, length of dislocation lines as well as distribution area of localized shear strain. More interestingly, the appearance of ordered sp2 structures, which can be considered as graphitization of diamond, has an obvious directionality. It always generates on the {100} crystal planes, regardless of polishing on the {100} and {110} faces.

Flow-Spurt Transition under Shear Deformation of Concentrated Suspensions

The rheological properties of concentrated suspensions containing 55 and 60 vol % aluminum particles are studied. This concentration range corresponds to the transition from the viscoelastic to the elastoplastic behavior of the suspensions. Moreover, this range is most favorable for powder injection molding of metals and ceramics. The measurements are performed in a rheometer at different shear stresses. Elastoplastic deformations take place at low stresses, while separation, i.e., the rupture of the adhesion contact between the bulk suspension and the boundary of a solid wall of the rheometer, is observed with an increase in the stress. The rupture of the adhesion contact occurs with time, and this process is characterized by a dependence of the lifetime (durability) of the contact on stress. This dependence is of an exponential character. The replacement of smooth working surfaces of the rheometer by rough ones leads to some increase in the stresses. This reflects the replacement of the adhesion separation by the formation of discontinuities in the bulk suspension at the asperities of the rough surface. The observed behavior of the concentrated suspensions in the aforementioned range of compositions is represented by a mechanical model, into which a new element has been introduced. It is represented by a friction pair with variable length (and, accordingly, variable friction force). The spurt arises when a threshold force that induces displacement over the friction surface is reached.

Theoretical analysis and experimental investigation of hysteretic performance of self-centering variable friction damper braces

Traditional buildings, even with normal self-centering energy dissipation (SCED) braces, might suffer from large deformations and high mode effects under an extremely strong earthquake, leading to a concentration of inter-story drift in upper floors. To satisfy the requirements of resilience, larger post-yield stiffness and higher energy dissipation, a novel brace with pretensioned basalt fiber-reinforced polymer (BFRP) tendons and variable friction dampers (VFDs) was developed, and this brace was termed the self-centering variable friction damper (SC-VFD) brace. The variable stiffness and sliding force of the VFD system were theoretically analyzed. This was followed by quasi-static experiments on two VFD and two SC-VFD braces with different parameters. The theoretical analysis and experimental results revealed the same tendencies, demonstrating the reliability and feasibility of SC-VFD braces. The hysteretic curve of the SC-VFD braces exhibited stable and normal flag-shape behavior before the second activation, while the hysteresis curve indicated a variable flag-shape behavior with second activation, variable friction force, and larger post-yield stiffness when the brace slid at slope section. Compared with the VFD brace, the SC-VFD brace had the same energy dissipation ability but less residual displacement and lower equivalent viscous damping ratio. More combinations of disc springs in series resulted in smaller axial forces and lowers post-yielding stiffness, thereby decreasing the energy dissipation capability.

Numerical verification of variable friction cladding connection for multihazard mitigation

The motion of cladding systems can be leveraged to mitigate natural and man-made hazards. The literature counts various examples of connections enhanced with passive energy dissipation capabilities at connections. However, because such devices are passive, their mitigation performance is typically limited to specific excitations. The authors have recently proposed a novel variable friction cladding connection capable of mitigating hazards semi-actively. The variable friction cladding connection is engineered to transfer lateral forces from the cladding element to the structural system. Its variation in friction force is generated by a toggle-actuated variable normal force applied onto sliding friction plates. In this study, a multiobjective motion-based design methodology integrating results from the previous work is proposed to leverage the variable friction cladding connection for nonsimultaneous wind, seismic, and blast hazard mitigation. The procedure starts with the quantification of each hazard and performance objectives. It is followed by the selection of dynamic parameters enabling prescribed performance under wind and seismic loads, after which an impact rubber bumper is designed to satisfy motion requirements under blast. Last, the peak building responses are computed and iterations conducted on the design parameters on the satisfaction of the motion objectives. The motion-based design procedure is verified through numerical simulations on two example buildings subjected to the three nonsimultaneous hazards. The performance of the variable friction cladding connection is also assessed and compared against different control cases. Results show that the motion-based design procedure yields a conservative design approach in meeting all of the motion requirements and that the variable friction cladding connection performs significantly well at mitigating vibrations.

Variation of Static Frictional Forces in the Fixed Orthodontic System

Currently, the aesthetic appearance of fixed orthodontic appliances is an important factor for patients seeking orthodontic treatment. The aim of this study was to evaluate static frictional forces generated by different types of round 0.016 NiTi archwires, with and without aesthetic coating, when coupled with monocrystalline alumina brackets. The static frictional force was determined using a testing machine for measuring compression and tension forces. The results showed a variation of static frictional forces depending on the type of archwire used. Uncoated archwires have produced the lowest static frictional forces. Partially aesthetic coated archwires have generated lower static frictional forces than fully aesthetic coated ones. Fully aesthetic coated archwires have produced the highest static frictional forces, but no significant differences were noted between archwires with polymer and epoxy resin coating.

Variation of Static Frictional Forces in the Fixed Orthodontic System

Currently, the aesthetic appearance of fixed orthodontic appliances is an important factor for patients seeking orthodontic treatment. The aim of this study was to evaluate static frictional forces generated by different types of round 0.016` NiTi archwires, with and without aesthetic coating, when coupled with monocrystalline alumina brackets. The static frictional force was determined using a testing machine for measuring compression and tension forces. The results showed a variation of static frictional forces depending on the type of archwire used. Uncoated archwires have produced the lowest static frictional forces. Partially aesthetic coated archwires have generated lower static frictional forces than fully aesthetic coated ones. Fully aesthetic coated archwires have produced the highest static frictional forces, but no significant differences were noted between archwires with polymer and epoxy resin coating.

Transition from Self-Organized Criticality into Self-Organization during Sliding Si3N4 Balls against Nanocrystalline Diamond Films

The paper investigates the variation of friction force (Fx) during reciprocating sliding tests on nanocrystalline diamond (NCD) films. The analysis of the friction behavior during the run-in period is the focus of the study. The NCD films were grown using microwave plasma-enhanced chemical vapor deposition (MW-PECVD) on single-crystalline diamond SCD(110) substrates. Reciprocating sliding tests were conducted under 500 and 2000 g of normal load using Si3N4 balls as a counter body. The friction force permanently varies during the test, namely Fx value can locally increase or decrease in each cycle of sliding. The distribution of friction force drops (dFx) was extracted from the experimental data using a specially developed program. The analysis revealed a power-law distribution f−µ of dFx for the early stage of the run-in with the exponent value (µ) in the range from 0.6 to 2.9. In addition, the frequency power spectrum of Fx time series follows power-law distribution f−α with α value in the range of 1.0–2.0, with the highest values (1.6–2.0) for the initial stage of the run-in. No power-law distribution of dFx was found for the later stage of the run-in and the steady-state periods of sliding with the exception for periods where a relatively extended decrease of coefficient of friction (COF) was observed. The asperity interlocking leads to the stick-slip like sliding at the early stage of the run-in. This tribological behavior can be related to the self-organized criticality (SOC). The emergence of dissipative structures at the later stages of the run-in, namely the formation of ripples, carbonaceous tribolayer, etc., can be associated with the self-organization (SO).

DETERMINATION OF RANDOM FRICTION FORCES ON THE BIOLOGICAL SURFACES OF A HUMAN HIP JOINT WITH A PHOSPHOLIPID BILAYER

The paper presented concerns a new mathematical form of the stochastic theory of hydrodynamic friction forces occurring on real human hip joint surfaces with a phospholipids bilayer. This paper particularly presents a new review of stochastic analytical considerations realized by the authors for friction forces estimation during hydrodynamic lubrication of biological surfaces performed on the basis of the gap height measurements in the human hip joint. After numerous experimental measurements, it directly follows that the random unsymmetrical increments and decrements of the gap height of human joints have an important influence on the load carrying capacities and finally on the friction forces and wear of cooperating cartilage surfaces. The main topic demonstrates the impact of the variations of expectancy values and the standard deviation of the human joint gap height on the friction forces occurring in the human joint. Moreover, an evident connection is observed between the apparent dynamic viscosity and the features of the cartilage surface coated by the phospholipid cells. Hence, after the abovementioned remarks, follows the corollary that the influence of the gap height stochastic variations and random surfaces coated by the PL cells tend indirectly from the apparent viscosity into the friction force variations. The synthetic, complex elaborations of the results obtained indicate the influence of the random roughness and stochastic growth of living biological cartilage surfaces on the friction forces distribution.

Variable friction cladding connection for seismic mitigation

Cladding systems are conventionally designed to serve an architectural purpose and provide environmental protection for building occupants. Recent research has been conducted to enhance structural resiliency by leveraging cladding systems against man-made and natural hazards. The vast majority of the work includes the use of sacrificial cladding panels and energy dissipating connectors. These passive protection systems, though effective, have typically targeted a single hazard one-at-a-time because of their limited frequency bandwidths. A novel semi-active friction connection has been previously proposed by the authors to leverage the cladding motion for mitigating blast and wind hazards. This semi-active friction device, termed variable friction cladding connection (VFCC), is designed to laterally connect cladding elements to the structural system and dissipate energy via friction. Its variable friction force is generated onto the sliding friction plates upon which a variable normal force is applied via actuated toggles. Because of its semi-active capabilities, the VFCC could be used over wide-band excitation frequencies and is thereby, an ideal candidate for multiple hazard mitigation. The VFCC in its passive in situ mode has been previously designed to mitigate air-blast effects towards the structure and its semi-active scheme has been applied to wind hazard mitigation. In this paper, a motion-based design (MBD) procedure is developed to apply the VFCC to seismic hazard mitigation, completing its application against multiple hazards. The MBD procedure begins with the quantification of seismic load and performance objectives, and afterwards, dynamic parameters of the cladding connection are selected based on non-dimensional analytical solutions. Simulations are conducted on two example buildings to verify and demonstrate the motion-based design methodology. Results show the semi-actively controlled VFCC is capable of mitigating the seismic vibrations of structures, demonstrating the promise of the semi-active cladding system for field applications.

Research on the simulation modelling of high‐voltage switchgear ring‐shaped spring contact force

In order to explore the simulation method of the contact force for the ring-shaped spring and provide the support for the design of high-voltage switchgear-rated current and mechanical operating power, the parametric equation was derived, based on NX software the three-dimensional model was built, considering the influence of contact, friction, and large deformation, the non-linear dynamic finite element simulation model for the process of assembling and operating was established, the stress, resistance force, and friction force at any moment were obtained by simulation, and the variation of resistance and friction force during contact insertion was analysed. The results show that during the mechanical operating process, the resistance force rapidly increased to the extreme value and then gradually decreased to a stable value of 238N, and the friction force quickly increased and then slowly rose to the steady value of 238N, which is consistent with the experience and proved the rationality of the simulation model. Finally, according to the Coulomb friction law, the contact force of the spring contact was calculated as 1190N.

BACKGROUND TO THE ARRANGEMENT OF AUTOMATING AND VOLVOUS PROCESSES IN THE FORM-FORMING MACHINES WHEN TREATING THE PREPARATIONS AND DETAILS BY THE PLASTIC DEFORMATION METHOD

Purpose. Analysis of surface-hardening technology for cold processing of metals, taking into account the deformation-wave processes occurring in the area in front of the shaping tool. Methodology. To establish the reasons for obtaining finished products with increased roughness, insufficient contact strength, fracturing and peeling of the surface layers, as well as the occurrence of significant dynamic loads in the mechanisms of machines for machining parts using plastic deformation, a two-mass design model is used in which the working tool is connected by friction with the moving processed workpiece. Findings. The results of determining the conditions for the occurrence of self-oscillations in the system “workpiece - tool” are presented. Based on the analysis of the friction characteristics as a function of the mutual slip rate between the workpiece and the shaping tool, the range of slip speeds is set, at which the self-oscillating processes for the friction pair can be excited: "tool - workpiece". Originality consists in the fact that the influence of the variable friction force on the occurrence of sustained self-oscillations between the workpiece being processed and the tool in the machines for “cold” plastic deformation of the metal has been established. The conditions for the occurrence and absence of self-oscillations in the zone of contact between the tool and the workpiece are found. Practical value lies in the fact that the modes of operation of machines for processing products by plastic deformation have been revealed under which unregulated roughness, fracturing and “peeling” of the parts being machined can occur, as well as a high dynamic background during the operation of the shaping tool of driving and power mechanisms. Practical recommendations are given to improve the performance of finished products and reduce vibroactivity in the drive. Keywords: running in, dragging, knurling, plastic deformation, elastic deformation wave, roller, tool, blank.

Effects of surface chemical groups and environmental media on tribochemical running-in behaviors of silicon surface

Abstract In order to offer insight into the removal of outmost silicon atoms, the various tribochemical running-in processes at the Si/SiO2 interfaces were investigated. The running-in of Si–SiOx surface in humid air exhibited a transition from protrusion to groove. By contrast, the initial wear of Si–H surface began with a nearly wearless state in air and revealed apparent material removal after only one sliding cycle in water. The variations in wear rate and friction force suggested the changes of interfacial dangling bonds and energy dissipation. Not only the removal of native oxide layer but also the condensation of interfacial water molecules and the change of surface chemical groups were proposed to interpret the tribochemical running-in mechanisms of silicon surface.

On the metal/ZnO contacts in a sliding-bending piezoelectric nanogenerator

We carry out molecular dynamics (MD) simulations and analyze the piezoelectric and mechanical properties of a ZnO nanowire (NW) under mechanical bending in a sliding-bending piezoelectric nanogenerator. The computational model contains a vertically aligned ZnO NW along the [0001] direction and a Pt (111) metal tip. MD simulations are performed to mimic the interfacial sliding dynamics of an atomic force microscope (AFM) metal tip sliding over a ZnO NW. Ionic charges for Zn and O in the ZnO NW are used to calculate the piezoelectric potential distributions in a grid-point matrix. Simulation results reveal the very detailed distributions of the piezoelectric potential within the ZnO NW, which are different from the continuum field theory predictions. The very similar variations of the lateral friction and contact forces versus the sliding distance of the AFM metal tip over the ZnO NW during repeated forward-backward sliding suggest that the metal/ZnO contact is robust. Moreover, the substantial contact area at the metal/ZnO interface suggests that a Schottky contact should be established at the interface, which needs further investigations.

DNA translocation through polyelectrolyte-modified nanopores: An analytical approximation.

An analytical model for the electrophoretic speed of DNA translocating through nanopore functionalized with polyelectrolyte (PE) brush is developed for the first time. The electrophoretic speed depends on DNA surface potential, applied electric field, viscosity, and permittivity of solution along with velocity and electrostatic potential at liquid-polyelectrolyte layer (PEL) interface where the interface seemed to behave similar to that of a solid-state nanopore wall. Under the limit of Debye-Hückel linearization, the electrostatic potential at liquid-PEL interface and at DNA surface have been calculated. Velocity at liquid-PEL interface has been estimated by assuming a linear variation of hydrodynamic frictional force within the PEL. It is observed that velocity and electrostatic potential at liquid-PEL interface strongly depend on PE charge density and softness parameter. Present analytical results show excellent agreement with exact numerical results (i.e., without any approximation) at a higher salt concentration where Debye-Hückel linearization is applicable.

Physical mechanism of ice/structure interaction

ABSTRACTTo obtain the effect of velocity and structural natural frequency (structural stiffness) on ice failure, an extended dynamic Van der Pol-based single degree-of-freedom ice/structure interaction model is developed. Three basic modes of response were reproduced: intermittent crushing, frequency lock-in and continuous crushing. Further analysis on the physical mechanism of ice/structure interaction is presented on the basis of feedback mechanism and energy mechanism, respectively. Internal effect and external effect from ice and structure were both explained in the feedback branch. Based on reproduced results, energy exchanges at different configurations are computed from the energy conservation using the first law of thermodynamics. A general conclusion on the predominant type of vibration when the ice velocity increases during the interaction process is forced, self-excited and forced in each of the three modes of responses. Ice force variations also show that there is more impulse energy during the lock-in range. Moreover, ice-induced vibration demonstrates an analogy of friction-induced self-excited vibration. Finally, the similarity between strain-stress curve and Stribeck curve shows that static and kinetic friction force variations are attributed to ice force characteristic, and can be used to explain the lower effective pressure magnitude during continuous crushing than the peak pressure during intermittent crushing.

Effect of contact pressure and stress ratio on the fretting fatigue behaviour of Ti-6Al-4V

Fretting fatigue behaviour of Ti-6Al-4V was studied at two different contact pressures (150MPa and 300MPa) and two different stress ratios (0.1 and 0.7). The variation of frictional force and tangential force coefficient (TFC) with the number of fretting cycles was studied to understand the fretting fatigue behaviour of the alloy. As evidenced by the variation of frictional force with time as well as frictional force versus cyclic load hysteresis-type plots, gross sliding was present at lower contact pressures and lower stress ratios, leading to higher TFC values and surface roughness. Oxide particles were detected in the slip region and in the initiated fretting cracks indicating fretting debris. While the effect of increasing contact pressure in increasing the fretting fatigue life was clearly seen at higher stress ratio, it was not observed at lower stress ratio. At both contact pressures, a significant increase in life with an increase in stress ratio was noticed. The contact problem was analysed using the existing numerical tools to obtain the contact stresses. From these stresses, fretting fatigue lives were estimated as a sum of initiation lives estimated from multi-axial fatigue parameters and propagation lives estimated from fracture mechanics approach. A good agreement while using Findley parameter indicates that the adopted procedure can effectively account for the effects of contact pressure as well as stress ratio.

Effect of counterface roughness on the friction of bionic wall-shaped microstructures for gecko-like attachments

Hairy adhesive systems involved in gecko locomotion have drawn the interest of many researchers regarding the development of bionic solutions for fast and reversible adhesive technologies. To date, despite extensive efforts to design gecko-inspired adhesive surfaces, adhesion and friction capacities are often evaluated using smooth and rigid counterfaces, in general glass, whereas most natural and artificial surfaces inevitably have a certain level of roughness. For that reason, in this study experiments tested the effects of the substrate roughness on the friction of bionic wale-shaped microstructures for gecko-like attachments. To this end, 12 substrates with different isotropic roughness were prepared using the same Epoxy material. Friction force was measured under various normal loads. It was concluded that classical roughness parameters, considered separately, are not appropriate to explain roughness-related variations in friction force. This has led us to develop a new integrative roughness parameter that combines characteristics of the surface. The parameter is capable of classifying the obtained experimental results in a readable way. An analytical model based on the experimental results has been developed to predict the variation of the friction force as a function of counterface roughness and applied normal load.

Alteration of friction force of a harmonically excited elastomeric pillar

The friction force of a single elastomeric pillar on a smooth substrate is investigated under harmonic base excitation. Friction force variation is acquired for various excitation frequencies with a constant base acceleration where the pillar stays in stick regime. The peeling behavior of the pillar for different harmonic base displacements is discussed. The apparent work of adhesion is demonstrated via energy equilibrium at the contact interface. It was observed that a harmonic force may alter the interface adhesive nature, and the adhesion and friction coupling can be tuned via the excitation frequency.

Friction characteristics of hybrid magnesium alloy composites reinforced with carbon nanotube and cerium

PurposeThe paper aims to clarify the hybrid Mg alloy composites reinforced with multi-walled carbon nanotube (MWCNT) and Cerium (Ce) rare earth element tribological properties were investigated by using pin-on-disk test configuration under dry and lubricated sliding conditions.Design/methodology/approachThe paper opted for an experimental study with composite samples under lubricated conditions. Their tribological properties were investigated by using pin-on-disk test configuration under dry and lubricated sliding conditions. Wear tests were carried out at 20°C temperature. Wear behavior of Mg alloy composites was evaluated as maximum coefficient of friction and the variation of the maximum frictional forces of the samples.FindingsThe reinforcements such as Ce and MWCNT have a decreasing effect between 100 and 200 rpm speed tests for friction coefficient and friction force of Mg alloy. The microstructure has an important effect on the wear mechanism. There can be both adhesive and abrasive wear mechanism for the same composite at different sliding speeds. It is determined that there is no systematic relationship between reinforcement percent and sliding speed related to wear behavior of AZ 41 matrix composites reinforced with CNT and Ce. The results indicate that different wear mechanisms occur at different sliding speeds. The sliding speed has effect on both an increment and decrement for wear.Research limitations/implicationsBecause of the chosen research approach, the research results may lack generalizability. Therefore, researchers are encouraged to test the different reinforced composite samples further.Practical implicationsThe paper includes implications for the development of a hybrid composite reinforced with rare earth elements and MWCNT.Originality/valueThis paper fulfils an identified need to study a hybrid composite reinforced with rare earth elements and MWCNT.

Experimental research on the tactile perception from fingertip skin friction

Skin friction is a main perception channel for humankind to recognize object surface properties. However, the relationship between tactile perception and skin friction is a typical bio-tribology interdisciplinary problem, which is rarely studied both in biology and tribology fields. In order to study the brain stimulation response to the fingertips sliding friction, cognitive related fingertip friction testing was carried out, and the tactile perception evoked by the friction and vibration of various surface textures was investigated in this paper, human subjective tactile experiments was carried out through twenty volunteers testing for the tactile perception of surface properties, focused on three fabric samples (cotton, linen and silk) and three paper samples (photo paper, kraft paper and normal paper). The friction coefficient and frictional vibration signals were measured, and the brain response signals stimulated by the fingertip friction were simultaneously recorded and analyzed. The research works in this paper are beneficial for the deep understanding of tactile perception mechanism by fingertips, and building up a biomimetic model for smart robotic fingers.The research results indicate that the cerebral cortex responds sensitively to the variations in fingertips frictional force and vibration amplitudes. The human tactile perception is very sensitive to the fabric texture properties, which are described by the dimensions such as rough-smooth, coarse-fine and complex-uniform. The higher values of the fabric texture dimensions, the lower values in friction coefficient, and the more sensitive of tactile perception for the surface textures. Furthermore, it is noticed that the amplitude and distribution of brain signals response, i.e. ERPs (event related potentials) evoked by fingertip friction, is affected by the surface texture or friction coefficient. It is found that the response sensibility shown on the beginning peaks of ERP waveform has strong relation to the frictional stimulus on fingertip friction. The parameter of P3 latency (i.e. the third positive peak of ERP) is the distinct identifying information for the tactile perception of surface textures.