Friction Mode Research Articles

Influence of temperature on the dynamics of formation of granic sleeps and connected elevation dynamics in sliding conditions

The running-in process is accompanied by a change in microgeometry, as a result of which some constant roughness is established, which is characteristic of given friction conditions, and the physical and mechanical properties of the surface layers also change, since plastic deformations usually predominate in the contact.
 The thickness of the surface layers that have undergone changes during external friction depends on the stress state in the zones of their actual contact and heating during friction. The stress state in the zone of actual contact of the bodies is characterized by indentation or crushing of surface microroughnesses, as well as by elastic or plastic states of the latter. Surface heating during friction depends on the thermophysical properties of the contacting bodies and the friction mode. An increase in the temperature of the surface layers causes not only their softening, but also greatly increases the rate of physical and chemical processes in them. This leads to saturation of the surface layers with environmental gas molecules, oxidizing slicks, and also to an increase in the concentration of defects in these layers

Wear resistance of structural steels nitroded in cyclic-commuted discharge at limit modes of friction

The article discusses the method of conducting tribological studies at the limit modes of friction of nitrided and non-nitrided steels 20 and 45 in order to achieve a comparison of the results of laboratory tests with operational data. The relationship of structural phases in time is significantly influenced by the initial state of the surface and its physical and mechanical characteristics, pressure on the contact surface, sliding speed, and all these parameters for the limit mode of friction are closely related. Carrying out tests on the wear resistance of samples made of different materials and with significantly different characteristics of the surface layer at the same parameters of the test regime is impossible in most cases, since the obtained results are problematic to compare.

NEZHEBOVSKY V.V., BEREZHNYI R.A., PERMYAKOV O.A., KLOCHKO O.O., RYABCHENKO S.V., USTINENKO O.V. SYSTEMS OF THE PARAMETERS OF THE CONDITION OF THE WORKING SURFACES AND DIMENSIONS ACCURACY OF THE CYLINDRICAL HARDENED GEAR WHEELS OF THE CLEANING COMBINED UKD

The issues of efficiency improvement in hydrodynamic (liquid) or limit (semi-liquid) modes of friction of gears with an asymmetric profile and the effect of the liquid on tooth damage, power loss and temperature in contact, as well as the role of an elastic medium that dampens shocks and changes contact lines, are considered. In many gears, the tooth load per tooth is much higher and applied for longer periods of time. The asymmetrical shape of the tooth reflects this functional difference. In the case of asymmetric gears, the standard symmetrical toothed tool rail is modified by changing the pressure angle of one of the side surfaces. However, this simplistic approach to the design of an asymmetric transmission significantly limits the ability to maximize the performance of a wide range of possible applications of these transmissions. As the frequency of impact interactions increases, the lubricating layer of high-speed gears with an asymmetric profile ceases to respond to pressure changes, and the load value, which determines the smallest layer thickness over time, approaches the minimum stationary component of the spectrum of dynamic loads. At the same time, noise and vibration levels are slightly reduced. Therefore, in the calculations of the strength, contact endurance and seizure of high-speed gears with an asymmetric profile, the complex influence of the criterion parameters of the lubricant must be taken into account not only when determining the allowable stresses, but also when determining the amount of dynamic load amplification due to the impact interaction of the teeth.

Shear thickening in dense suspensions: an experimental study

<h2>Abstract</h2> Shear thickening is an intriguing rheological behaviour which consists in a brutal increase in the viscosity above a critical shear rate. It is famously encountered in suspensions of corn starch in water. Despite having been discovered in the early 1930's, its underlying mechanisms remained a mystery for a long time. In 2013–14, numerical and theoretical works [1–3] put forward a frictional transition scenario to explain this phenomenon. In this talk, I will present experimental work investigating this frictional transition scenario. In order to test the ideas of this model, one has to go further than standard rheological techniques, since they do not provide access to the frictional state of the measured suspension. I will thus focus on the techniques that we developed in order to evidence the frictional transition and link it to the presence of a shear-thickening behaviour.

Force Transmission Analysis and Optimization of Bowden Cable on Body in a Flexible Exoskeleton.

The Bowden cable is a significant force transmission equipment for a flexible exoskeleton. However, the previous researches of Bowden cable had emphasized on the data from experimenting test board, instead of on human body, which produced the inaccurate assisting analysis of the flexible exoskeleton. In this paper, a flexible exoskeleton for assisting knee extension was proposed, which provided an on-body condition. Then, the friction force and its influencing factors between the wire rope and sheath of the Bowden cable from the motor to the anchor of knee have been analyzed. The segment models of force transmission with the concern of three kinds of friction modes were established, and the relationship between various lengths and bending angles of Bowden cable was fitted to the equations of curve. Furthermore, the association rule between the force transmission and the lengths of Bowden cable was obtained, based on which, the optimal force transmission efficiency was 78.68% when the length value of the Bowden cable was 475 mm. A flexible exoskeleton prototype was assembled; then, the experiments with force transmission and metabolic cost have been developed. The results showed that the force transmission efficiency had strong association with the lengths of Bowden cable, instead of the transmission velocities. Furthermore, this knee assistance exoskeleton reduced the net metabolic cost of the testees during walking. These experiments results corroborated the force transmission modeling and simulation of the Bowden cable on body we proposed in this paper.

Variation of Structural and Dynamical Flexibility of Myelin Basic Protein in Response to Guanidinium Chloride.

Myelin basic protein (MBP) is intrinsically disordered in solution and is considered as a conformationally flexible biomacromolecule. Here, we present a study on perturbation of MBP structure and dynamics by the denaturant guanidinium chloride (GndCl) using small-angle scattering and neutron spin–echo spectroscopy (NSE). A concentration of 0.2 M GndCl causes charge screening in MBP resulting in a compact, but still disordered protein conformation, while GndCl concentrations above 1 M lead to structural expansion and swelling of MBP. NSE data of MBP were analyzed using the Zimm model with internal friction (ZIF) and normal mode (NM) analysis. A significant contribution of internal friction was found in compact states of MBP that approaches a non-vanishing internal friction relaxation time of approximately 40 ns at high GndCl concentrations. NM analysis demonstrates that the relaxation rates of internal modes of MBP remain unaffected by GndCl, while structural expansion due to GndCl results in increased amplitudes of internal motions. Within the model of the Brownian oscillator our observations can be rationalized by a loss of friction within the protein due to structural expansion. Our study highlights the intimate coupling of structural and dynamical plasticity of MBP, and its fundamental difference to the behavior of ideal polymers in solution.

Анализ работы опорных подшипников турбомашин на экологически чистых жидкостях

The subject of study in this article is the cost-effectiveness of the rotors of powerful turbines and their fire safety. Improvement of these indicators is possible by replacing oil lubrication systems with environmentally adaptive lubricants (EAL), in particular water or aqueous solutions. The main disadvantage of using water and other low-viscosity aqueous solutions for operating plain bearings is the lack of a hydrodynamic effect at low speeds of the rotor during start and stop of the turbine. The current study experimentally studies the possibility of replacing oil lubrication systems with EAL, in particular water or aqueous solutions. The objectives of the study: 1) to ensure that in the operating range of speeds and loads, the bearing operates in a stable hydrodynamic mode; 2) show that the use of a hydrostatic lifting system provides a liquid mode of friction in the range under study of loads and sliding speeds; 3) to determine the level of pressures at which the rotor rises. When solving this problem, the following methods were used. On the experimental stand, a full-scale radial bearing with babbitt filling of the working surface of the liner was tested. During the experiment, the following parameters were varied and measured: rotor speed, bearing load, lubricant consumption, working fluid temperature in front of and out of the bearing, water pressure (condensate) in the hydraulic system, pressure in the lubricating layer and chambers of the bearing. The main parameter by which the efficiency of the bearing was evaluated was the thickness of the lubricating layer of the working fluid. The following results were obtained. In the first stage, when the speed of the rotor is zero, the value of the pressure of the fluid supply, which ensures the emergence of the shaft. In the second stage, the dependence of the relative eccentricity on the shaft speed and the speed at which it is possible to turn off the rotor lifting system. Conclusions. In the investigated range of speed and loads, the bearing operated in a stable hydrodynamic mode. The use of a hydrostatic lifting system provided a liquid mode of friction in the range under study of sliding speeds (including starting and stopping the rotor). The temperature of the working surface of the bearing in a stable hydrodynamic mode depends little on the external load and sliding speed.

Tribological mechanism of micro/meso/macroscopic textured surfaces under different normal forces, relative velocities, and sliding directions

Friction behavior under different operating conditions, such as normal forces, relative velocities, and length scales of the textured surfaces, is a crucial topic in tribology. In this study, different tribology mechanisms of micro/meso/macroscopic textured surfaces were investigated, considering the effects of asperities and operating conditions. A prediction model of the friction coefficient was proposed and validated for polished and microscopic, mesoscopic, and macroscopic textured surfaces. Subsequently, reciprocating sliding friction experiments (sphere-on-plate) were carried out to estimate the friction coefficients of the specimens in contact with an aluminum sphere (Al 99.9). The results indicate that a larger normal force, relative velocity, and length scale produce more wear debris, which transforms the friction mode from adhesive friction to abrasive friction.

Dissipative structure suppression as a way to increase the sustainable improvement of the frame support bearing capacity

Introduction. A yield frame support is a basic means of providing the underground roadways’ stability in the deep mines operating in the hard ground control condition when the ratio of the ground pressure to the unconfined strength of surrounding rock mass exceeds 0.33. The operators adjust the nominal bearing capacity of the frames at 0.55 of their maximum or peak resistance Fmax because the frames operate in a dry friction mode spontaneously generating oscillation, which causes variation of the resistance in the range from 0.1 up to 0.9 of Fmax . Aim/tasks. We intended to reduce the dynamic oscillation of the support resistance that allows for increasing their bearing capacity. Methodology. We used a computer simulation by FLA3D model, indoor testing of the frames, and actual measurements in an underground coal mine. Results. We revealed a specific behavior of the frame support that reduced the limit of the bearing capacity by 45%. During yielding, the frames generate dissipative structures (DS) that become apparent due to the dynamic nature of stick-slip friction following the yielding process. We demonstrated that DS control and especially its suppression provides a sustainable increase in the frame bearing capacity. Conclusions. Improvement of the yield frames design and especially their clamps are the prospective way to control DS and provide sustainable enhancement of the frame bearing capacity.

Friction-Dominated Carrier Excitation and Transport Mechanism for GaN-Based Direct-Current Triboelectric Nanogenerators.

The semiconductor triboelectric nanogenerator (TENG) based on the tribovoltaic effect has the characteristics of direct current and high current density, but the energy transfer and conversion mechanism is not completely clear. Here, a series of gallium nitride (GaN)-based semiconductor direct-current TENGs (SDC-TENGs) are investigated for clarifying the carrier excitation and transport mechanism. During the friction process, the external output current always flows from GaN to silicon or aluminum, regardless of the direction of the built-in electric field, because of the semiconductor types. These results reveal that the carrier transport direction is dominated by the interfacial electric field formed by triboelectrification, which is also verified under different bias voltages. Moreover, the characteristics dependent on the frictional force have been systematically investigated under different normal forces and frictional modes. The open-circuit voltage and short-circuit current of SDC-TENG are both increased with a larger frictional force, which shows that the more severe friction results in both a larger interface electric field and more excited carriers. The maximum voltage can reach 25 V for lighting up a series of LEDs, which is enhanced by four times compared to the cutting-edge reported SDC-TENGs. This work has clarified the friction-dominated carrier excitation and transport mechanism for the tribovoltaic effect, which demonstrates the great potential of semiconductor materials for frictional energy recovery and utilization.

On friction measurements and certification of slip‐preventing materials for road freight usages

AbstractSlip‐preventing materials are often used as tertiary packaging to improve cargo securement and simplify truck loading. There are many national and international standards on their safe usage based on the extra friction they can provide. However, it is challenging to quantify the coefficient of friction (CoF) of slip‐preventing materials. This paper presents CoFs of three slip‐preventing materials and one baseline condition measured on different surfaces and contact pressures. These measurements show interesting phenomena that affect the result variability. For instance, 60% of the pairs of surfaces tested were affected by the contact pressure. Hence, the load of the freight seems to affect performance of the slip‐preventing materials. This interaction happened in different ways such as changes in the friction mode (constant slip and stick slip at difference frequencies and magnitudes). The typical CoF values available in standards as well as standardized measurement methods do not consider very well these effects nor how to deal with high variability of CoF measurements. This can lead to either over or under CoF estimations. To overcome these limitations, this paper presents a statistical method to quantify slip‐preventing material CoFs based on variability and confidence intervals of the measurements.

Polyacrylamide hydrogel lubricates cartilage after biochemical degradation and mechanical injury.

Intra-articular injections of hyaluronic acid have been a mainstay of osteoarthritis treatment for decades. However, controversy surrounds the mechanism of action and efficacy of this therapy. As such, there has been recent interest in developing synthetic lubricants that lubricate cartilage. Recently, a synthetic 4 wt% polyacrylamide (pAAm) hydrogel was shown to effectively decrease lameness in horses. However, its mechanism of action and ability to lubricate cartilage is unknown. The goal of this study was to characterize the lubricating ability of this hydrogel and determine its efficacy for healthy and degraded cartilage. The study utilized previously established IL-1β-induced biochemical degradation and mechanical impact injury models to degrade cartilage. The lubricating ability of the hydrogel was then characterized using a custom-built tribometer using a glass counterface and friction was evaluated using the Stribeck framework for articular cartilage. pAAm hydrogel was shown to significantly lower the friction coefficient of cartilage explants from both degradation models (30%-40% reduction in friction relative to controls). A striking finding from this study was the aggregation of the pAAm hydrogel at the articulating surface. The surface aggregation was observed in the histological sections of explants from all treatment groups after tribological evaluation. Using the Stribeck framework, the hydrogel was mapped to higher Sommerfeld numbers and was characterized as a viscous lubricant predominantly in the minimum friction mode. In summary, this study revealed that pAAm hydrogel lubricates native and degraded cartilage explants effectively and may have an affinity for the articulating surface of the cartilage.

Precise Controlling of Friction and Adhesion on Reprogrammable Shape Memory Micropillars.

Microstructured surfaces with stimuli-responsive performances have aroused great attention in recent years, but it still remains a significant challenge to endow surfaces with precisely controlled morphological changes in microstructures, so as to get the precise control of regional properties (e.g., friction, adhesion). Herein, a kind of carbonyl iron particle-doped shape memory polyurethane micropillar with precisely controllable morphological changes is realized, upon remote near-infrared light (NIR) irradiation. Owing to the reversible transition of micropillars between bent and upright states, the micro-structured surface exhibits precisely controllable low-to-high friction transitions, together with the changes of friction coefficient ranging from ∼0.8 to ∼1.2. Hence, the changes of the surface friction even within an extremely small area can be precisely targeted, under local NIR laser irradiation. Moreover, the water droplet adhesion force of the surface can be reversibly switched between ∼160 and ∼760 μN, demonstrating the application potential in precisely controllable wettability. These features indicate that the smart stimuli-responsive micropillar arrays would be amenable to a variety of applications that require remote, selective, and on-demand responses, such as a refreshable Braille display system, micro-particle motion control, lab-on-a-chip, and microfluidics.

Исследование механизмов трения и оценка трибологических характеристик твердосмазочных покрытий, нанесенных разными методами

The article analyzes the wear dynamics of solid lubricant coatings (SLC) of suspension, magnetron-high-frequency and diffusion methods of application. Calculated dependences are presented to assess the tribotechnical characteristics of the SLC. It was found that the service life of SLC based on MoS2 HF application in the considered ranges of surface temperature variations differs slightly from the that of SLC suspension application VNII NP 212. The wear of diffusion SLC M804 (Dimolit-4) in the steady-state friction mode is 34 µm. The wear rate of diffusion SLC М804 under vacuum conditions at a sliding speed of V = 0.2 m/s with the increase in contact pressure from 1 to 8 MPa increases by a factor of 2 and is on average 4.5 times higher than that of friction pairs with SLC VNII NP 212. Anti-friction characteristics of diffusion SLC in steady state friction modes at temperatures up to 600 °C were slightly higher than the similar characteristics for SLC with a binder. With an increase in heating temperatures to the limiting value of 800 °C, the friction coefficient of the diffusion SLC M801 and M810 (based on NbS2) decreases to the values of ffr = 0.03–0.04.

Gyroscope-Structured Triboelectric Nanogenerator for Harvesting Multidirectional Ocean Wave Energy.

Wave motion in the ocean can generate plentiful energy, but it is difficult to harvest wave energy for practical use because of the low frequency and random directional characteristics of wave motion. In this paper, a gyroscope-structured triboelectric nanogenerator (GS-TENG) is proposed for harvesting multidirectional ocean wave energy. Its inner and outer generation units can operate independently in different directions, and they all adopt the friction mode of surface contact. While realizing noninterference multidirectional energy harvesting, the power generation area is increased. In the experiments, under acceleration of 6 m/s2 with variations in excitation angle, the GS-TENG can output direct currents of 0.8-3.2 μA, and the open-circuit voltages of the inner and outer generation units can reach 730 and 160 V, respectively. When the devices are networked and placed in the water, the electrical energy generated by the GS-TENGs can enable commercial thermometers to operate normally. The attenuation of direct-current output by the GS-TENG in the experiment of 30 days in water is about 8%, which verifies the good durability of the device in the water environment. Therefore, the GS-TENG has excellent application prospects in the wave energy harvesting field.

TRIBOLOGICAL CONTACTIN MATERIALS SYSTEMS"CuCrNiZrTi-M500", "CuCrNiZrTi-5140H", "CuCrNiZrTi-3310H"

The limited information on the manifestation of the parameters of the molecular component of friction -the shear strength of the adhesive bond τ0and the piezoelectric coefficient βof the molecular component for each specific study predetermines the use of purposeful determination of parameters by modeling shear on small-sized samples in order to increase the objectivity and accuracy of the result assessment. The article poses the task of conducting definitive tribotechnical tests of the high-entropy alloy to assess its adaptability to loading and lubrication conditions during contact with steel, cast iron and to establish the nature of the manifestation of the parameters of adhesion properties by modeling shear on small samples. It was found that under conditions of drop lubrication, the surfaces of the samples of the studied materials appear to be compatible and are satisfactorily run-in. In this case, the parameters of high-speed and force loading determine the manifestation of boundary lubrication. The time reaching the steady-state friction mode is practically the same for all systems, and is observed after 7.5 minutes. The obtained graphic patterns and parameters of their mathematical approximation made it possible to determine the nature of the change in the adhesive properties of a high-entropy alloy with a change in shear rates.

Evaluation of the antiseptic activity of 5% alcoholic povidone-iodine solution using four different modes of application: a randomized open-label study

Before some invasive procedures, such as injections, surgical incision or intravascular catheter insertions, alcoholic antiseptics (e.g., alcoholic povidone-iodine (PVP-I)) are widely used to prevent infection. This randomized, open-label study investigated the impact of mode of application (which includes both application technique and volume) on the antiseptic activity of 5% alcoholic PVP-I solution. Alcoholic PVP-I was administered to the backs of healthy adults using four modes of application: (A) concentric circle method, 3mL; (B) concentric circle method, 10mL; (C) back-and-forth friction method, 3mL; (D) back-and-forth friction method, 10mL. antiseptic activity of alcoholic PVP-I, assessed via change from baseline in log10/cm2 colony-forming units (cfu) count for total aerobic and facultative anaerobic bacteria. Safety was monitored. A total of 113 healthy participants were screened; 32 were randomized. Alcoholic PVP-I showed significant antiseptic activity with all modes of application (P<0.001 for each), providing an overall mean decrease from baseline in cfu count of >3 log10/cm2 (P<0.001). Significantly greater efficacy was seen with back-and-forth friction (modes C and D) versus concentric circles (modes A and B): covariate adjusted change in log10/cm2 cfu count 0.22; 90% confidence intervals: 0.07, 0.37 (P=0.017). No safety issues were observed. Alcoholic PVP-I demonstrated high antiseptic activity for all modes of application. Greater efficacy was achieved with back-and-forth friction versus concentric circles, showing that application technique may influence antiseptic activity; these findings suggest that when comparing the efficacy of antiseptic substances (e.g., alcoholic PVP-I and alcoholic chlorhexidine), comparable application techniques should be used.

Design and Control of Multi-Plate MR Clutch Featuring Friction and Magnetic Field Control Modes.

A magnetorheological (MR) multi-plate clutch was proposed with both mechanical friction mode and magnetic field control modes. The magnetic field control mode was based on an MR fluid coupler that changed its viscous properties according to the density of an applied magnetic field. This mode was used in the early stage of clutch operation to reduce the impact of friction between the disc and plate, and eliminate to the extent possible the difference in their relative speeds when contacting each other in later stages. Once the rotational speed difference between the disc and plate was reduced, the clutch was operated in mechanical friction mode by compressing the friction surfaces together. A torque modeling equation was then derived for each mode based on the Bingham model of the MR fluid, and the transmission torque of the proposed multi-plate clutch was derived using these equations as well as magnetic field analysis results obtained using ANSYS Maxwell. A multi-plate MR clutch was then fabricated, and its torque transmission characteristics were evaluated in the magnetic field control and mechanical friction modes. The results confirmed that the model-based torque calculations were consistent with the observed transmission torque. Finally, control algorithms for mechanical friction only and mixed mechanical friction/magnetic field control torque tracking of the proposed MR multi-plate clutch were designed, and their performances were evaluated when applying unit step command, half-sine-wave command, and rotational speed changes. The results indicated that the torque tracking control was performed smoothly, demonstrating the advantages of the proposed clutch.

Titanium Drawing

To improve the quality of human life, sometimes, surgical interventions are required to replace or retain damaged tissue during reparative regeneration. Titanium and titanium alloys are well-proven biocompatible materials. The methods of modeling the phase composition of the titanium alloy can be used to predict the chemical and physical-mechanical properties of implants and suture material. In this research, the features of the titanium drawing process are investigated, recommendations are given for thermo-mechanical processing and the choice of a lubricant. Modeling the structure and phase composition of alloys allows predicting their properties and choosing the optimal technological parameters for all drawing transitions. Drawing must be performed in a friction mode close to the hydrodynamic regime with intermediate annealing to control the phase composition of titanium, restore the plasticity resource, and heal crystal structure defects. The composition of atmospheric gases during annealing makes it possible to control the phase composition and properties of the alloy. The use of exfoliated graphite is proposed as a lubricant applied to the anodized titanium surface.

Synchronous nanogenerator with intermittent sliding friction self-excitation for water wave energy harvesting

The triboelectric nanogenerator (TENG) is a promising technology for harvesting low-frequency ocean energy, while a low surface charge density sets back its industrialization. Normally, owing to insufficient friction the traditional contact-separation mode has difficulty achieving a high surface charge density as that of sliding friction mode, which also can hardly maintain long-term sustenance due to abrasion. Herein, we designed a synchronous contact-separation triboelectric nanogenerator (SC-TENG) with intermittent sliding friction self-excitation that can achieve the dual functions of long-term sustenance of a high surface charge density and low friction-induced loss. A movable pendulum structure rotates intermittently to replenish the charge for its own, thus sustaining a high surface charge density for continuous contact-separation working mode. Several comparative experiments are conducted, showing short-time sliding friction made it easier to improve the surface charge density and output performance than long-term contact-separation friction. The single TENG with short-term friction increases by seven-fold in the transferred charges, 3.2 times in output current, compared to the TENG without friction. The SC-TENG could generate a high voltage of 1284 V and maximum peak power of 8.3 mW at 1 Hz. With those capacities, the TENG can easily capture the water wave energy to power a commercial thermohygrometer and portable wearable device. SC-TENG proposes an innovative design and a new strategy for preparing the TENG with high output and low loss.