State Friction Research Articles

Effect of initial fabric from sample preparation on the mechanical behaviour of a carbonate sand from the South China Sea

Mega reclamation projects have been undertaken on the coral reefs. However, research on the geotechnical properties of the hydraulically filled carbonate sand is limited, and the impact of the initial fabrics from different densification methods on the mechanical behaviour of carbonate sand remains unclear. In this work, a systematic experimental investigation on the effect of initial fabric induced by sample preparation method on the mechanical behaviour of carbonate sand has been performed. Four sample preparation methods, including moist tamping, vibration, dry pluviation and water pluviation were adopted to reconstitute samples with distinct fabrics. It is found that, for a single preparation method, the particle breakage as well as the peak friction angle of carbonate sand is affected by the drainage condition significantly, and both the critical state friction angle (φ'cs) and the compressibility (λ) of carbonate sand are strongly stress dependent. For samples prepared by different methods, an obvious divergence of critical state points can be seen in the specific volume and mean effective stress plane at low stress levels. The dry pluviation samples show the lowest φ'cs and highest λ, due to the highest anisotropy. In contrast, moist tamping samples give the highest φ'cs and lowest λ, due to the most isotropic fabric.

Dynamic Characteristics Analysis of the First-Stage Fixed-Axis Gear Pair with Wear Fault for a Wind Turbine Gearbox Considering Different Friction States

Dynamic Characteristics Analysis of the First-Stage Fixed-Axis Gear Pair with Wear Fault for a Wind Turbine Gearbox Considering Different Friction States

Analysis of Flow and Fluctuation Characteristics in Coated Slag Using a 2D Model in the Meniscus Region of Mold

Steel is mainly produced through continuous casting; molten steel flows into the mold from the tundish, where it cools and then enters the secondary cooling zone, ultimately solidifying into a billet. During the continuous casting production process, the quality of the casting billet is mainly related to the lubrication state of the coated slag. In the upper part of the mold, the consumption of liquid protective slag directly affects the friction state of the initial solidified billet shell. Therefore, the flow and fluctuation characteristics of coated slag in the meniscus area are very important. There is limited research on the flow and fluctuation characteristics of coated slag in the meniscus area, and little consideration has been given to the shape of the meniscus. In this work, a two-dimensional numerical model for the flow and fluctuation of coated slag in the meniscus region was established, and the transient flow velocity of protective slag and molten steel at each moment of the vibration cycle was obtained, as well as the fluctuation of the slag/steel interface in the meniscus region. The results show that when the surface mold vibrated upwards, the protective slag in the meniscus area flowed clockwise. When the mold moved downwards, the protective slag in the slag pool generated a counterclockwise flow vortex. When the mold was in a positive slip state, the negative pressure formed by the upward flow of the protective slag on the meniscus and the inertia force of steel liquid pushed the meniscus toward the inner wall of the mold. During negative slip, the flow of coated slag generated positive pressure on the slag/steel interface, pushing the meniscus toward the steel liquid, and at the initial moment of negative slip, the steel liquid overflowed into the slag channel. This model could provide a theoretical basis for the flow control of protective slag.

Constitutive Behavior of Rocks During the Seismic Cycle

AbstractEstablishing a constitutive law for fault friction is a crucial objective of earthquake science. However, the complex frictional behavior of natural and synthetic gouges in laboratory experiments eludes explanations. Here, we present a constitutive framework that elucidates the rate, state, and temperature dependence of fault friction under the relevant sliding velocities and temperatures of the brittle lithosphere during seismic cycles. The competition between healing mechanisms, such as viscoelastic collapse, pressure‐solution creep, and crack sealing, explains the low‐temperature stability transition from steady‐state velocity‐strengthening to velocity‐weakening as a function of slip‐rate and temperature. In addition, capturing the transition from cataclastic flow to semi‐brittle creep accounts for the stabilization of fault slip at elevated temperatures. We calibrate the model using extensive laboratory data on synthetic albite and granite gouge, and on natural samples from the Alpine Fault and the Mugi Mélange in the Shimanto accretionary complex in Japan. The constitutive model consistently explains the evolving frictional response of fault gouge from room temperature to 600°C for sliding velocities ranging from nanometers to millimeters per second. The frictional response of faults can be uniquely determined by the in situ lithology and the prevailing hydrothermal conditions.

Study on the ice friction characteristics

Ice friction plays a vital role in different fields, including glaciology, polar runways, and winter sports, the mechanisms of which are complex and affected by many parameters, such as temperature, velocity, and load. It is commonly accepted that the frictional characteristics of ice can be attributed to a lubricating layer. Sphere-on-ice friction experiments were conducted to study the influence of temperature, pressure, and velocity on ice friction. The lubrication state of ice friction under different conditions was analyzed, and it was found that the contact angle and roughness were crucial factors. Characterizations of the worn surfaces done using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and three-dimensional white-light interference indicated that abrasive and oxidation wear were the main wear mechanisms arising from ice friction. The underlying mechanism was explored using molecular dynamics (MD) simulations. The results suggested that the effects of velocity, temperature, and load on the presence of hydrogen bonding alter the fluidity of the lubricating layer existing between the ice and sphere, which in turn affects the ice friction characteristics. In the lubricating layer, the concentrations of water molecules near the sliders and ice were higher than those in the inner water. Moreover, the adsorption energy verified that water molecules tended to be adsorbed on the ice and sliders. In addition, the calculation results indicated that some water molecules were re-oriented in the ice friction because spikes were observed in the radial distribution function (RDF) graphs.

Stress–dilatancy behaviour of remoulded Fujinomori clay

Abstract The effect of the degree of consolidation and the stress path on the behaviour of remoulded Fujinomori clay for drained triaxial compression and extension was analysed using the Frictional State Concept. It is shown that the stress–dilatancy behaviour can be approximated by a linear general dilatancy equation given by the critical frictional state angle and two soil parameters. The newly formulated dilatant failure state is represented on the stress ratio plastic dilatancy plane by points lying on the friction state line defined by the friction state angle and the Friction State Concept parameters α=0 and β=1. It has been shown that the stress ratio–plastic dilatancy relationship, which is very rarely used in the interpretation of test results, is important for a complete description of the behaviour of soils during shearing.

Evaluating Sand Particle Surface Smoothness Using a New Computer-Based Approach to Improve the Characterization of Macroscale Parameters

The analysis of sands, and the foundation systems with which they interact, are largely dependent on macroscale behavioral parameters that represent the aggregated response of several microscale characteristics. This research paper examines the influence of surface texture, or smoothness, on the behavior of sands. The challenge of estimating or measuring smoothness, due to its microscale feature domain, is addressed through an examination of six artificially graded sand specimens. These specimens are evaluated both visually and numerically to characterize their surface smoothness. The first approach described is a simple visual method that uses a smoothness scale consistent with those of roundness and sphericity. This method, which can be performed with a tool as simple as a hand lens, evaluates a group of representative particles collectively. The second approach is also a visual evaluation, but it utilizes images obtained via scanning electronic microscopy, traditional optical microscopy, and newer low-cost digital microscopes that can be rapidly connected to a smartphone or laptop. To validate these visual estimates, a novel third approach is introduced. This approach is a more objective numerical analysis measurement technique that enables rapid and economic quantification of smoothness. This technique may assist both practitioners and academics in their understanding of the macroscale response of coarse-grained soils. In addition to the visual methods, this research also conducted several laboratory index tests to observe the mechanical behavior of the specimens, considering their particle shape and surface smoothness properties. The results indicate that angular sands have greater minimum and maximum void ratios, a larger difference between the minimum and maximum void ratios, greater critical state friction angles, and greater flow rates through an orifice of fixed size. When adjusted for surface smoothness using the proposed approach, the behavior of the sands—particularly the limit void ratio results—appears to be more predictable in some cases. These results provide additional evidence of particle smoothness contributing to the strength behavior of sand, which may be particularly useful in the domains of slope stability, land reclamation, soil–structure interaction, and soil dynamics.

Controllable Friction of an Epoxy Composite via Thermal Treatment

Smart surfaces with controllable friction have generated considerable attention lately. However, most composites prepared with traditional fillers cannot achieve “real-time” friction conversion. Herein, a new smart surface was designed to achieve different friction coefficients (0.65 and 0.12). Different coefficients of friction were reversibly and precisely controlled via heating. Via friction and heating, 1H,1H,2H,2H-perfluorohexyl hexadecane (PHHD), a kind of phase-change material—paraffin wax—was released from the microcapsules, and a stable and complete film was formed. It changed the interface from “solid-solid” to “solid-liquid” in a dry friction state. The composite contains microcapsules that prevent phase separation between PHHD and matrix, which enables the composite to have a long service time and switchable friction performance. In addition, this composite can maintain its extraordinary ability even in harsh environments like UV irradiation. By demonstrating switchable friction based on changes in the interactions between contact interfaces, this work provides a new principle for designing smart tribological composites.

The cohesive zone crack analogue for incomplete contacts under mild wear conditions

Fretting fatigue may appear in mechanical components that are in contact under vibrational loading, inducing oscillatory frictional stresses, often combined with alternating bulk stresses. Fretting fatigue cracking is important, with its initiation being difficult to estimate. This work extends a well-known fretting fatigue model, the crack analogue model that examines fretting fatigue induced by incomplete contacts of metallic surfaces. This is done by inserting a mode II type cohesive zone at the contact edges. In our previous work on fretting fatigue of complete contacts, we proposed mixed mode I and II cohesive zones because of the nature of the stress fields around the contact edges. The proposed cohesive zones address several issues related to the mechanics of friction and wear, by combining them in the case of mild wear where asperity plastic shakedown is the dominant inelastic mechanism. Thus, the micromechanical behaviour of the surface topology and its evolution are naturally introduced into the analysis. The surface roughness becomes an internal variable that evolves with the cyclic loading, establishing a steady state friction coefficient. Alternatively, friction evolution can be established pointwise by monitoring the accumulated micro-slip. The work of friction dissipates the mechanical work and at the same time becomes the precursor of large scale wear. A predictive methodology for fretting crack nucleation is proposed through the fatigue threshold stress intensity factor used in ordinary fatigue.

In Situ Formation of MoS2 on the Surface of CF to Improve the Tribological Properties of PUE.

The roller is an important part of the belt conveyor used in coal transportation. Due to the harsh environment of coal mines, the rollers are in a state of high load and high friction for a long time, which causes wear failure and has a serious impact on the reliability and safety of the equipment. In order to prepare roller material with excellent bearing performance and friction performance, CF/PUE composites were prepared by pouring method with polyurethane as the matrix and carbon fiber as reinforcement. Due to the low surface activity of unmodified carbon fibers and poor bonding performance with the matrix, MoS2 was generated on the surface of carbon fiber by the in situ generation method in this paper. It was found that the mechanical properties of MoS2/CF/PUE composites were better when the CF content was 0.3 wt%. The Shore hardness reached 92.2 HA, which is 10% higher than pure polyurethane. The tensile strength was 38.44 MPa, which is 53% higher than pure polyurethane. The elongation at break was 850%, which is 16% higher than pure polyurethane. The maximum compressive stress was 2.32 MPa, which is 42% higher than pure polyurethane. The friction coefficient was much lower than that of pure PUE composites, the friction coefficient was 0.284, which is 59% lower than pure polyurethane.

Atomic-scale interfacial instability of hydrogenated amorphous carbon film at elevated temperature

Tribochemical reactions can modify the surface structure and chemical properties of amorphous carbon films during sliding, thereby altering their tribological behavior. Understanding the effects of temperature on the reaction process is important for the widespread application of amorphous carbon films, particularly at elevated temperatures. Upon sliding at temperatures higher than 373 K, the a-C:H film with 40 at.% H demonstrated atomic interfacial instability, disrupting the interfacial ordered structures with increased friction and wear. Reactive molecular dynamics (MD) simulations further revealed that the interfacial material transfer of hydrocarbon clusters in the a-C:H film with 40 at.% H at elevated temperatures was triggered by mechanochemical exfoliation of graphitic rings, leading to significant wear of the interfacial structures. When the hydrogen content decreased to 10 at.% H, more carbon chains were mechanochemically fragmented between the sliding surfaces to trap the interfacial atoms, resulting in stabilized, ordered structures and a low friction state.

Tribological properties of high-speed steel surface with texture and vertical fibers

Inadequate lubrication of the two touching surfaces during friction can lead to severe wear, especially in metal cutting. Therefore, a surface with synergistic anti-friction effect of texture and solid lubricant was proposed to improve lubrication. A mesh texture with excellent wettability was prepared on the high-speed steel (HSS) surface by laser, and then nylon fibers were vertically implanted into the grooves of the texture using the electrostatic flocking technology. The friction and wear state of different surfaces (smooth, textured, flocking) under dry/oil-lubricated were studied by a linear reciprocating wear tester. The coefficient of friction (COF) under different working conditions was used to analyze the anti-friction properties, and the wear rate was used to evaluate the wear resistance of the surface. The results showed that the tribological properties of flocking surfaces were better than those of the other two surfaces. This is because the addition of nylon fibers eases shear at the edges of the texture. The broken fibers form a solid lubricating film on the specimen surface, which prevents the surface from being scratched by debris. In addition, it is found that COF decreases with increasing load. Finally, the rapid wettability of the oil droplets on the flocking surface shows the great potential of the surface for lubrication and anti-friction.

Physical state of water controls friction of gabbro-built faults

Earthquakes often occur along faults in the presence of hot, pressurized water. Here we exploit a new experimental device to study friction in gabbro faults with water in vapor, liquid and supercritical states (water temperature and pressure up to 400 °C and 30 MPa, respectively). The experimental faults are sheared over slip velocities from 1 μm/s to 100 mm/s and slip distances up to 3 m (seismic deformation conditions). Here, we show with water in the vapor state, fault friction decreases with increasing slip distance and velocity. However, when water is in the liquid or supercritical state, friction decreases with slip distance, regardless of slip velocity. We propose that the formation of weak minerals, the chemical bonding properties of water and (elasto)hydrodynamic lubrication may explain the weakening behavior of the experimental faults. In nature, the transition of water from liquid or supercritical to vapor state can cause an abrupt increase in fault friction that can stop or delay the nucleation phase of an earthquake.

Challenges pertaining to particulate matter emission of toxic formulations and prospects on using green ingredients for sustainable eco-friendly automotive brake composites

This review provides current insights on designing brake friction composites and states the extent of deviation since its historical evolution, the nuances of braking-induced contact dynamics, the adverse effect of brake wear debris followed by the contemporary bio-fiber/ naturally derived composition alternatives based on life cycle analysis implications and formulation design strategy assessment in view of the toxicity consequences of metallic constituents in brake friction composites. To reduce this adverse environmental impact, bio-based resinous binders, plant fibers, and naturally derived additives have been highlighted with their roles and contribution to tribological performance attributes particularly elucidated in terms of brake composites. Moreover, the role of bio-based natural friction additives and degradable solid waste as fillers in controlling braking and sliding-induced contact dynamics has been elaborated. Thus the paper comprehensively analyses the compositional influence pertaining to bio-based/naturally occurring/degradable solid waste as alternative materials to sustainably design new friction compositions while addressing the objective of minimizing braking-induced particulate matter emission and microplastic generation. The review further explores the life cycle analysis of brake friction composition of the ideologically appropriate one to that of the conventional composites in the broader realms of SWOT analysis and strategy assessment to explore the incorporation of green ingredients as a class of functionally integral components into brake composites.

Mechanical behavior and particle crushing of marine carbonate gravel in Xisha Islands, South China Sea

Carbonate gravelly soils are widely distributed in sub-tropical marine areas. During construction and engineering operations, carbonate gravel undergoes considerable particle crushing, which has remarkable effects on their engineering properties. A series of large-diameter oedometer tests, large-diameter drained and undrained triaxial shear tests were conducted on carbonate gravel specimens taken from the South China Sea. The carbonate gravel specimens exhibit significant particle crushing at common engineering pressure levels, and the particle crushing highly depends on the initial density, loading history and stress path. Three fragmentation modes, fracture, attrition, and abrasion, are notable in carbonate gravel specimens, which induces significant variations of PSD curves and greatly effects the mechanical properties. Particle crushing in carbonate gravel specimens increases the compressibility resulting in the compression index increases with the increase of surcharge pressure; decreases the pressure-hardening resulting in the initial Young’s and secant moduli slightly increase with the increase of initial confining pressure; and decreases the dilatancy resulting in the peak and critical state friction angles significantly deceases with the increase of initial confining pressure. Particle crushing also shows great effect on the CSL of the carbonate gravel specimens in both p ′ - q space and e - p ′ space.

Motion Simulation Analysis and Experimental Verification of Drill String Rotation Controller

Abstract Conventional directional drilling mainly relies on the sliding of screw motor and drill string to realize the change of wellbore trajectory, so the friction force is large. The use of friction reduction tools that generate axial force to change the friction state can only achieve partial friction reduction, and drill string is still in a sliding state during directional drilling. These problems are solved by using rotary steerable drilling system, which can achieve directional drilling when drill string rotates. But its use cost is high. Considering the principle of high efficiency and economy, a new friction reduction tool called drill string rotation controller is proposed, which is also used to reduce friction during directional drilling through drill string rotation. By adjusting the pump pressure, the meshing state of spline module of drill string rotation controller is changed to realize the conversion of drilling mode. In rotary drilling mode, upper drill string, drill string rotation controller, and bottom hole assembly rotate together. In directional drilling mode, upper drill string rotates and drill string rotation controller slides with bottom hole assembly. The function of the tool is verified by field experiment, and motion simulation of the tool is carried out. The results show that when the driving torque is set to 25,000 N · m, more reverse torque can be overcome in the directional drilling mode, and the drilling fluid pressure is set to 25 MPa, which can be converted to the rotary drilling mode faster.

Superlubricity of Materials: Progress, Potential, and Challenges.

This review paper provides a comprehensive overview of the phenomenon of superlubricity, its associated material characteristics, and its potential applications. Superlubricity, the state of near-zero friction between two surfaces, presents significant potential for enhancing the efficiency of mechanical systems, thus attracting significant attention in both academic and industrial realms. We explore the atomic/molecular structures that enable this characteristic and discuss notable superlubric materials, including graphite, diamond-like carbon, and advanced engineering composites. The review further elaborates on the methods of achieving superlubricity at both nanoscale and macroscale levels, highlighting the influence of environmental conditions. We also discuss superlubricity's applications, ranging from mechanical systems to energy conservation and biomedical applications. Despite the promising potential, the realization of superlubricity is laden with challenges. We address these technical difficulties, specifically those related to achieving and maintaining superlubricity, and the issues encountered in scaling up for industrial applications. The paper also underscores the sustainability concerns associated with superlubricity and proposes potential solutions. We conclude with a discussion of the possible future research directions and the impact of technological innovations in this field. This review thus provides a valuable resource for researchers and industry professionals engaged in the development and application of superlubric materials.

Bioinspired ultra-lubrication hydrogels with controllably switchable lubrication

Switchable lubrication materials have great potential in the preparation of intelligent grabbing devices, tissue materials, and intelligent soft robots. In nature, numerous living organisms can dynamically change their frictional behaviors. Inspired by the lubrication to the astringency of the oral cavity, mucin was incorporated with poly(vinyl alcohol) (PVA) to emulate this behavior from an ultra-low friction state to a high friction state. The results show that mucin allows for the reduction in coefficient of friction by 95.8 % relative to mucin-free hydrogels. Furthermore, large-scale switchable lubrication (μ∼0.009 to μ∼0.47) is achieved through the interaction between mucin and tannic acid (TA). The approach may offer a method for preparing intelligent grabbing devices, tissue materials, and intelligent soft robots.

High-output soft-contact fiber-structure triboelectric nanogenerator and its sterilization application

Infectious diseases are spreading rapidly with the flow of the world’s population, and the prevention of epidemic diseases is particularly important for public and personal health. Therefore, there is an urgent need to develop a simple, efficient and non-toxic method to control the spread of bacteria and viruses. The newly developed triboelectric nanogenerator (TENG) can generate a high voltage, which inhibits bacterial reproduction. However, the output performance is the main factor limiting real-world applications of TENGs. Herein, we report a soft-contact fiber-structure TENG to avoid insufficient friction states and to improve the output, especially at a high rotation speed. Rabbit hair, carbon nanotubes, polyvinylidene difluoride film and paper all contain fiber structures that are used to guarantee soft contact between the friction layers and improve the contact state and abrasion problem. Compared with a direct-contact triboelectric nanogenerator, the outputs of this soft-contact fiber-structure TENG are improved by about 350%. Meanwhile, the open-circuit voltage can be enhanced to 3440 V, which solves the matching problems when driving high-voltage devices. A TENG-driven ultraviolet sterilization system is then developed. The bactericidal rate of this sterilization system can reach 91%, which significantly reduces the risk of disease spread. This work improves a forward-looking strategy to improve the output and service life of the TENG. It also expands the applications of self-powered TENG sterilization systems.

A review on tribology, characterization and lubricants for water-based drilling fluids

The challenges associated with drilling large displacement horizontal wells are numerous, and their adverse effects can lead to severe damage or even shut down of the drilling operation. Frictional torque of the tubular column is one of the most common problems encountered when applying water-based drilling fluids to the drilling of extended reach wells. This is because during the drilling process, there is continuous friction between the drilling tool and the rock. When water-based drilling fluid cannot effectively reduce friction, it will cause difficulty in lifting or lowering the drilling tool, resulting in sticking, and thus affecting the drilling operation. Most petroleum engineers use different lubricants to reduce friction and wear during drilling.The purpose of this study is to present the friction and lubrication state during drilling, the mechanism of action of boundary lubrication, lubrication evaluation and analysis methods, and friction reduction and wear resistance related to water-based drilling fluid lubricants reported in literature for reducing friction.Based on the review, we recommend a broad evaluation of lubricants using different lubricity performance evaluation methods to determine the true lubricity performance in the coexistence of multiple frictions during drilling. In addition, we recommend that future researchers enhance the application of numerical simulations in the analysis of lubricant lubrication mechanisms to address the difficulty of direct experiments in boundary lubrication laboratories. Any experimental program requires at least five characterization techniques, including friction coefficient, wear resistance testing, scanning electron microscopy, energy dispersive X-ray spectroscopy, and electron spectroscopy, to provide a detailed understanding of the friction reduction mechanism of the lubricant used. Lubricants based on bionic technology with good water solubility, low biotoxicity, and easy biodegradability are highly encouraged because of their availability and good lubricating properties.