Static Friction Behavior Research Articles

Modeling static friction behavior of elastic–plastic spherical adhesive microcontact in full-stick condition

The static friction behavior of an elastic–plastic spherical adhesive microcontact between a rigid flat and a deformable sphere under combined normal and tangential loading is studied by the finite element method (FEM). The contact between the sphere and the rigid flat is assumed to be full-stick, and the sliding inception is related to a loss of tangential stiffness. The intermolecular force between the rigid flat and the sphere is assessed by the Lennard–Jones (LJ) potential, which is applied to the sphere and the rigid flat by a user subroutine. The evolution of the adhesive force with tangential displacement in the full-stick condition is revealed. The results indicate that the increasing effect of adhesive energy on the static friction coefficient gradually diminishes with an increase in the adhesive energy and the external normal load. Finally, based on an extensive parametric study, an empirical dimensionless expression is obtained to predict the static friction coefficient of the spherical adhesive microcontact considering the intermolecular force.

Static Friction Behavior of Spherical Contact With Ultrathin Soft Coating

Abstract A surprising behavior of ultrathin soft coating is described in the relevant literature showing discontinuity (sharp drop) between the static friction coefficients before and after an ultrathin soft coating is applied on a harder substrate. Existing finite element (FE) models are unable to explain this unusual behavior since they suffer from convergence problem due to excessive distortion of elements in such ultrathin films. To address this problem, an FE model based on the coupled Eulerian–Lagrangian (CEL) method is presented. This method is able to capture the unexpected behavior of ultrathin soft coatings and hence, to provide a reliable scientific explanation to the experimental observation reported in the literature.

A model for breakaway distance and maximum static friction to study the static frictional behavior of the secondary seal in non-contacting mechanical seals

Abstract The secondary seal is a critical element of non-contacting mechanical seals. However, its frictional behavior in non-equilibrium condition is not well known. For studying this, we developed a finite element (FE) model on breakaway distance and maximum static friction by applying a piecewise-loading technology and validated this model by comparing the simulated and experimental results. Using the model, we demonstrated the saltation of the seal at the breakaway distance, which would lead to leakage and collision of mechanical seal. Moreover, we investigated both stages of sliding and the transformation of seal's contacting region.

Insights into the static friction behavior of Ni-based superalloys

The purpose of this study was to provide a better understanding of the static friction behavior of Ni-based superalloys over a wide range of temperatures. The static friction coefficient was investigated using a custom-built high-temperature apparatus in a flat-on-flat contact configuration. Measurements were performed on Inconel 718, using a variety of counterface materials (i.e. Ni-Cr coating, ME16, Ti-6246 and Inconel 718).The static friction coefficient was overall higher for the experiments at elevated temperatures for all counterface materials against Inconel 718. Ex situ analysis by means of FIB/SEM and XPS revealed the formation of metal oxide layers on the surfaces of the different alloys after testing at elevated temperature. In addition, atomistic simulations were performed in order to elucidate the interfacial mechanism leading to the differences in static friction. The observations of the simulations correlated well with the experiments, where the shear strength of the frictional interfaces was evidently higher for the oxide-on-oxide systems compared to the nickel-on-nickel.

An anisotropic friction model in non-bonded flexible risers

The present paper addresses small scale friction tests and an anisotropic friction model to predict friction for the interface between the anti-wear tape and tensile armors in non-bonded flexible risers. Small scale friction tests were carried out to study the static and dynamic friction behavior, measuring the transition distance between static and dynamic friction and the associated friction coefficients under variable contact pressure. In addition, the anti-wear tape shear modulus was measured to validate the analytical solution of shear interaction stick stiffness, K0, applied in previously proposed friction models. As grooves in the anti-wear tape surface were observed in the full scale test pipe, an anisotropic friction model was formulated and implemented into a computer code. Since the slip behavior will be influenced by the friction scaling factors, the K0 and the sliding direction, numerical studies were carried out to investigated their effects on the friction forces and the axial stress ranges at the outermost fiber position. This was followed by a sensitivity study with respect to the effect of applying 1D (loxodromic) and 2D (geodesic) friction models on the axial stress range at the same position. At last, the effect of applying measured transition behavior in the static friction model was investigated.

On the Transition from Static to Dynamic Boundary Friction of Lubricated PEEK for a Spreading Adhesive Contact by Macroscopic Oscillatory Tribometry

The tribology of lubricated poly(ether ether ketone) (PEEK)—steel tribosystems was investigated in the static and boundary friction regime. Pentaerythrite ester and trimellitic acid ester were used as lubricants. The lubricants differed in their molecular structure but showed only minor differences in their rheological and cohesive energetic properties. In order to investigate the effect of the lubricants on static and dynamic friction, macroscopic oscillatory tribometry experiments and gliding experiments were carried out. The surface and interfacial energies of PEEK, lubricant, and steel, which can be used to determine e.g. the spreading tendency of the lubricant, were employed to explain the tribological characteristics of the system. The gliding experiments exhibited a velocity dependence of the coefficient of friction which indicates a “rate-and-state” type of friction. Trimellitic acid ester resulted in the lowest static and dynamic friction values due to its high tendency to spread. A different static friction behavior was found for pentaerythrite ester, which indicates a possible lubricant-induced mobilization of the polymer surface chains. This finding was supported by an increased interaction energy measured by contact angle measurements. The macroscopic oscillatory tribometry was shown to be a unique and precise method to investigate the transition from static to dynamic friction and to quantify macroscopic adhesive friction. Both types of experiments used a polished steel surface as a frictional partner, which favors the adhesive component of friction. However, the results for the lubricated contacts of polished steel surfaces in the boundary lubrication regime can give some insight into effects of the polymer transfer to rough steel surfaces in the mixed lubrication regime.

A Study of the Static/Kinetic Friction Behavior of PTFE-Based Fabric Composites

ABSTRACTThree different polytetrafluoroethylene (PTFE)-based fabric composites were prepared. The static/kinetic friction behaviors of these composites under different loads and speeds were studied. A 3D laser microscope and profile measurement apparatus were used for analysis of the morphology and weave structure of the composites, and the contact temperature of these composites under different loads and speeds was monitored continuously using a high-precision thermal resistor. In addition, a dynamic mechanical analysis (DMA) apparatus was used to explore the thermal and mechanical properties of PTFE-based fabric composites. The results demonstrated that speed/load, weave structure, and fiber form have an important influence on static and kinetic friction behavior of the fabric composites. Generally, the static friction coefficient is greater than the kinetic friction coefficient, except when considering light load conditions. Under light load conditions, the static friction coefficient is equal to the kinetic friction coefficient. In addition, the kinetic friction coefficient first increased and then decreased with increased speed, but the static coefficient increased first and then remained at an almost constant value. At all sliding speeds, the static and kinetic friction coefficients of tape yarn composites are better than those of the multifilament yarn composites. Weave structure has no effect on the static friction coefficient, but it has a significant influence on the kinetic friction coefficient.

Static skin friction behavior of a single micropile in sand

Numerical pile segment analyses were conducted using an advanced soil model to investigate the development of side resistance and the soil behavior around a micropile installed in sand and subjected to static axial loading. A series of static pile segment analyses were performed for micropiles and Conventional Drilled Cast-In-Place (CDCIP) piles considering effects of pile installation, relative density of sand and pile diameter. Results of analyses were used to determine the effects of pile installation, pile diameter, and relative density of sand on static shear behavior of a micropile and changes in effective stresses and volumetric strains in the soil adjacent to the interface. Relatively high side resistance and stiff t-z response were characteristic features of a micropile compared with CDCIP piles. Results of static numerical pile segment analyses for micropiles were compared and found to exhibit good agreement with pile loading tests and design methods.

Dry frictional contact of metal asperities: A dislocation dynamics analysis

Discrete dislocation plasticity simulations are performed to investigate the static frictional behavior of a metal asperity on a large single crystal, in contact with a rigid platen. The focus of this study is on understanding the relative importance of contact slip opposed to plasticity in a single asperity at the micrometer size scale, where plasticity is size dependent.Slip of a contact point is taken to occur when the shear traction exceeds the normal traction at that point times a microscopic friction coefficient. Plasticity initiates through the nucleation of dislocations from Frank-Read sources in the metal and is modeled as the collective motion of edge dislocations.Results show that plasticity can delay or even suppress full slip of the contact. This generally happens when the friction coefficient is large. However, if the flattening depth is sufficiently large to induce nucleation of a large dislocation density, slip is suppressed even when the friction coefficient is very small. This study also shows that when self-similar asperities of different size are flattened to the same depth and subsequently loaded tangentially, their frictional behavior appears size independent. However, when they are submitted to the same contact pressure, smaller asperities slip while larger asperities deform plastically.

Static friction of porous bioceramic β-TCP on intestinal mucus films

The static friction behavior between a porous bioceramic material and an intestinal mucus film was investigated in order to develop a new intestine robotic endoscope. Here, the friction couple is porous β-tricalcium phosphate (β-TCP) and an artificial intestine mucus film. The effect of pore size of the β-TCP material on the friction behavior is investigated. The results illustrated that in this friction system there is a relatively small normal force upon the intestinal mucus film of the intestine wall during locomotion. The maximum static friction force in this friction couple varies with the pore size of the porous β-TCP material.

Static friction coefficient of some plastics against steel and aluminum under different contact conditions

The static friction behavior was investigated in the case of POM-based polymers containing additives, designated DE20076, DE100KM and DE12017 as well as Nylon-66 designated ZY103HSL, when in contact with AISI 1045 steel and 6061 aluminum alloy. A designed apparatus that detects the exact moment corresponding to the transition from rest to slip was used to measure the coefficient of static friction. The impact of normal load, dwell time before slip, apparent area of contact and lubrication by water and paraffin oil on the static friction characteristics was studied. A decrease of the static coefficient of friction with low normal load followed by a levelling off governed by its effect on the real area of contact and stiffness of the counterface was observed. This also yielded a linear decrease of slip acceleration as well as stick time. A polynomial equation, which increases monotonically, was found to express well the effect of stick time on static friction, independently of the materials in contact. Adhesive effects were generated by the presence of both lubricants with the result of increase of the static coefficient of friction.

Static friction in polysilicon surface micromachines

Surface micromachines of polycrystalline silicon were used to investigate the dependence of static friction in microelectromechanical systems on the external load, apparent contact area, and environmental conditions. An analytical model of the micromachine at the inception of sliding was used to determine the normal load consisting of the restoring and levitation forces exerted by the micromachine's comb-drive actuators. The apparent shear strength at the contact interface(s) exhibited a nonlinear dependence on the apparent contact pressure. Relatively higher static coefficient of friction and interfacial shear strength were obtained in room air than vacuum ambient. The static coefficient of friction was found to depend on the normal load, apparent contact area, and ambient conditions (i.e., relative humidity). Electrical contact resistance measurements indicated that sliding in room air promoted thickening of the native oxide film at asperity contacts. The experimental evidence suggests that modification of the surface topography occurred at the asperity level. However, these submicroscopic changes in the surface topography did not affect the overall static friction behavior, for the test cycles simulated in the friction experiments.

잔류응력의 표면파손과 마멸에 대한 영향

Break-in is an intentional treatment to enhance the performance life of machinery parts and to maintain static friction behavior. Most studies on break-in have concerned only about surface conditions such as roughness or film formation. But the exact mechanism of break-in has not been found yet. Friction, scuffing behavior and wear of AISI 1045 were studied in relation to break-in and residual stress. The cylinder-on-disk type tribometer was used with the line-contact geometry. Scuffing tests were carried out using a constant load of 730N. In the break-in procedure the step load was applied from 100N to 200N. In this experiment, it was found that the break-in helps compressive residual stress to be formed well enough to enhance the scuffing life during the scuffing test. Specimens that had high compressive residual stress induced by shot-peening show better wear resistance than those were not shot-peened. Results of scuffing test, break-in procedure and wear amount in relation to residual stress have been discussed.

Dynamic behavior and modelling of the pre-sliding static friction

One of the main difficulties in dealing with positioning problems with extremely high precision is the static friction and the induced nonlinear behavior existing in the mechanism. In this research, the phenomenological study of the static friction behavior before normal slip starts is conducted for direct drive torque motor systems. It is found that the qualitative behavior of static friction is consistent. Furthermore, based on the experimental results published in the literature, the pre-sliding behavior is similar for various mechanical mechanisms including friction between two simple materials and friction of complicated systems that have many friction sources. It shows that the pre-sliding motion is mainly the combination of two kinds of motion: plastic deformation that demonstrates creep and work hardening, and nonlinear spring deformation that demonstrates hysteresis loop with special memory and wipe out effects. These behaviors are very close to those of most plastic materials. A new static friction model that consists of a plastic module and a nonlinear spring module is introduced. This model is of a simple structure and is proved to match all those qualitative behaviors. Parameter estimation and simulation algorithms are also described. Simulation results show that this model does coincide with those experimental results conducted by the authors as well as by other investigators.

Sensor-accuracy precision positioning with the existence of static friction part I - dynamic model of friction during the pre-sliding range

One of the main difficulties in dealing with precision positioning problems is the friction force and the induced nonlinear behavior existing in the mechanism. In this research, the static friction behavior is observed experimentally for a direct drive torque motor system. It is found that the qualitative behavior of static friction can be summarized in several simple rules. The deformation in stiction phase is the result of two motions: plastic deformation that demonstrates creep and work hardening, and nonlinear spring deformation that demonstrates hysteresis with memory and wipe out effects. It is also found that these phenomena are common to lots of mechanical friction. Furthermore, it is very close to the behavior of a standard plastic solid. A new static friction model that consists of a plastic module and a nonlinear spring module is introduced. This model is of a simple structure and is proved to satisfy all those rules relating to qualitative behavior. Simulation results show that this model does match those experimental results conducted by the authors as well as by other investigators.

Measurement of static friction coefficient between flat surfaces

An experimental device, called the centrifugal friction apparatus (CFA), was developed which is capable of measuring the static coefficient of friction, μ, between flat surfaces with very low normal forces. Experiments were performed with the CFA using the polished surface of n-type arsenic doped silicon wafers to investigate static frictional behavior under differing light load conditions. For loads less than 100 mgf, the average μ and the standard deviation were found to increase as the normal load decreased. For loads between 100 mgf and 1.1 gf μ was found to be a constant in the range of 0.30−0.40. Results suggest that despite extreme smoothness the real area of contact is still significantly smaller than the apparent area of contact and μ is still a function of load. Tests with solid particles between the silicon mating surfaces showed that under low load conditions ( <0.9 Pa), the change in μ was negligible for both abrasive particles and for solid lubricants. At the higher loads used ( ≈475–575 Pa) the expected changes in μ were starting to occur for both types of solid particles.

The Role of Relative Humidity, Surface Roughness and Liquid Build-Up on Static Friction Behavior of the Head/Disk Interface

Stiction at the head/disk interface has become one of the major concerns as smoother surfaces are required to achieve lower flying heights of magnetic heads over magnetic disks. In this paper, static friction forces on three types of disk samples with different surface roughness values were measured at various relative humidities. It was found that static friction coefficients were well correlated with total thickness of liquid (lubricant and adsorbed water) at the head/disk interface. The experimental data also agreed fairly well with the calculated values based on a proposed stiction model. It is implied in the stiction model that the bearing ratio or the shape of asperity height distribution, especially the part of high asperities, determines the stiction force. Moreover, long-term stiction was investigated on the unlubricated disk surfaces at 80 percent relative humidity and on the lubricated disks at 5 percent relative humidity to separate the effects of water build-up and lubricant build-up at the head/disk interface. It appears that long-term stiction occurs only when enough mobile lubricant is present and the thickness of liquid at the head/disk interface is close to a critical thickness value which is related to surface roughness values.