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The functional group in the molecular chain of lubricants is a key factor for a boundary lubricating film, which can further enhance the friction performance by interfacial tribochemistry. In this paper, the tribological behavior of epoxy/graphene (EP/Gra) composites was investigated through varying lubrication conditions, including dry friction, or different lubricants (such as water, hexadecane, oleylamine, and oleic acid). The porosity and Shore hardness (SHD) of the EP composites were significantly improved by the addition of graphene. Results showed that the porosity of the EP/0.75 wt.% Gra composites was reduced by 75.6% and their SHD increased by 5.4 compared with the EP/0.05 wt.% Gra composites. The average coefficient of friction (COF) of the composites was reduced by at least 27.3% under dry friction conditions. For water as lubricant, the EP/0.5 wt.% Gra composite exhibited the lowest COF (0.0455). The EP/0.75 wt.% Gra composite displayed the lowest COF (0.0466) when hexadecane acted as lubricant. By contrast, EP/0.05 wt.% Gra showed the lowest COF (0.0389) with oleylamine as lubricant, and the EP/0.25 wt.% Gra composites gave the lowest COF (0.0433) under oleic acid. The amine group of oleylamine reacted with the EP group to form a hydroxyl group, resulting in a polymer with low friction adhered to the friction interface. Simultaneously, the EP group reacted with the carboxyl group, forming a network of hydrogen bonds with graphene, which reduced the shear stress on the friction interface. Therefore, the functional groups of lubricants have a vital influence on the tribological properties of frictional composites, which may provide a new approach to solving the problem of wear. [ABSTRACT FROM AUTHOR]

ABSTRACT In order to investigate the impact of micro‐textured surfaces with varying surface density on coating properties, the bionic leaf vein micro‐texture with different surface densities were prepared on the substrate surface by laser processing. Bi2S3 soft coatings were deposited on the textured surfaces by electrohydrodynamic atomization. The influence of textured surface density on the adhesion and tribological properties of the coatings was analysed and discussed by scratch tests and friction wear tests. The results showed a significant increase in the friction coefficient as the surface density increased. However, after reaching a certain point, the friction coefficient tended to decrease. The coatings deposited on the lower surface density (13.9%, 14.5%) have better tribological performance compared with the higher surface density (35.6%, 36.2%). Meanwhile, the adhesion of coatings on the textured substrate enhanced compared with coatings deposited on the polished substrate. A reasonable textured surface density can effectively improve the adhesion and tribological properties of the coating. [ABSTRACT FROM AUTHOR]

This article investigates a class of Duffing nonlinear dynamic systems with fractional‐order dry friction and conducts in‐depth research on the stability, chaotic characteristics, and erosion of the safety basin of this system; the results are verified through numerical simulation. First, the average method is used to approximate the amplitude–frequency relationship of the system, and the accuracy of the analytical results is verified through numerical experiments. Second, the Melnikov method is used to obtain the conditions for the system to enter chaos in the Smale horseshoe sense, and the Melnikov curve is drawn for further verification. Then, bifurcation diagrams are drawn for the changes in various parameters in the system, with a focus on analyzing the influence of friction factors on chaotic bifurcation. By applying the definition and calculation principle of the maximum Lyapunov exponent, and drawing and utilizing the maximum Lyapunov exponent graph, the chaotic state that the system enters under different parameters is more clearly defined. Finally, the evolution law of the safety basin under various parameter changes, especially dry friction changes, is analyzed, and the erosion and bifurcation mechanism of the safety basin is studied. Comparing with the bifurcation diagram, it reveals that chaos primarily contributes to the erosion of the safety basin. [ABSTRACT FROM AUTHOR]

We aim here to investigate a new mathematical model that describes the contact between a viscoelastic body, accounting for long memory and wear effects, and an obstacle referred to as the foundation. The contact model is governed by a normal compliance condition, coupled with Coulomb's law of dry friction for sliding, and wear effects. We derive the variational formulation of the model, which involves coupling of a quasi‐variational inequality with a nonlinear equation. By pursuing the abstract history‐dependent quasi‐variational inequalities and leveraging the fixed point theorem, we establish results concerning both existence and uniqueness. [ABSTRACT FROM AUTHOR]

Polyacrylate materials are widely used in textile adhesives, functional finishing agents, architectural coatings, and other fields. In order to improve their hydrophobicity and flexibility, a novel fluorinated polysiloxane was prepared by acid catalyzed bulk ring opening reaction with octamethylcyclotetrasiloxane (D4), tetramethyltetravinylcyclotetrasiloxane (D4v), and methyltrifluoropropylcyclotrisiloxane (D3F), and its structure was characterized. Then, as a modified monomer, the fluorinated polysiloxane modified polyacrylates was prepared by miniemulsion polymerization. The effects of fluorinated polysiloxane monomers with different vinyl content and molecular weight on the properties of emulsion, film structure and film hydrophobicity were studied. The results showed that when using fluorinated polysiloxane monomer with Mn = 6235, vinyl content of 0.33 mmol/g, and fluorine content of 7.40 mmol/g for modification, the water contact angle on the surface of the adhesive film reached 100.4°, and the water absorption rate was 8.1%, resulting in the best effect. Used for textile printing, the color fastness to dry and wet friction after fabric printing could reach level 4, and the hand feel was soft. [ABSTRACT FROM AUTHOR]

Wear behavior of a fine‐grained dual‐phase high‐entropy carbide/boride ceramics was investigated using ball‐on‐flat dry sliding methods in air, applying rotational and linear reciprocation motion with SiC counterpart. The investigated system showed very high nanohardness of the carbide and boride grains with mean values of 37.4 ± 2.3 and 43.0 ± 2.9 GPa, respectively, with the microhardness of dual system HV1 29.4 ± 2.0 GPa. The stabilized friction coefficient values during the circular tests changing from.62 to.77. During the reciprocal test, the frictional coefficient values are very similar with an average value of.53. The specific wear rates during the circular motion were similar in the range from 4.65 × 10−7 to 1.68 × 10−7 mm3/N m. During the reciprocal test, the wear rates at 5 and 25 N were similar as in the case of circular motion, but at 50 N load, the wear rate increased significantly to the value of 9.11 × 10−6 mm3/N m. The dominant wear mechanisms in all cases were oxidation driven tribochemical reaction and tribolayer formation in boride grains and mechanical wear in carbide grains. During the linear reciprocation test, the loading mode created conditions resulted in relatively low coefficient of friction and very high specific wear rate. [ABSTRACT FROM AUTHOR]

Determining the shear‐velocity dependence of dry granular friction can provide insight into the controlling variables in a dry granular friction law. Some laboratories believe that the quality of this study is at the forefront of the discipline for the following reasons. Results suggest that granular friction is greatly affected by shear‐velocity (v), but shear experiments over the large range of naturally occurring shear‐velocities are lacking. Herein we examined the shear velocity dependence of dry friction for three granular materials, quartz sand, glass beads and fluorspar, across nine orders of magnitude of shear velocity (10−8–2 m/s). Within this range, granular friction exhibited four regimes, following a broad approximate "m" shape including two velocity‐strengthening and two velocity‐weakening regimes. We discuss the possible physical mechanisms of each regime. This shear velocity dependence appeared to be universal for all particle types, shapes, sizes, and for all normal stresses over the tested range. We also found that ultra‐high frequency vibration as grain surfaces were scoured by micro‐chips were formed by spalling at high shear velocities, creating ∼20 μm diameter impact pits on particle surfaces. This study provides laboratory laws of a friction‐velocity (μ‐v) model for granular materials. Plain Language Summary: Numerous phenomena of earth science can be simplified to shearing granular systems, such as landslide gouge and rock avalanches. It is believed that friction varies with shear velocity, and this is no exception for dry granular materials. Studying the law of friction variation over as large a range of velocity changes as possible aids in establishing a full life‐cycle hypothesis for landslides from initiation, through movement to cessation. We sheared granular materials across nine orders of magnitude, from 10−8 to 2 m/s and obtained an "M" shape law for the variation of friction with velocity. By analyzing the acoustic frequency of shear experiments at different velocities, we discovered that the acoustic frequency increased with shear velocity. By comparing the microscopic morphology of the particle surface before and after high‐velocity shearing, we hypothesized that the impact pits may be left by particle collisions during high‐velocity shearing. We proposed that the role of ultra‐high frequency vibration on high‐velocity shear weakening should be considered, which provides a basis for accurately revealing the physical mechanism of granular friction variation with velocity. Key Points: This paper examines the shear velocity dependence of friction for dry granular materials, across nine orders of magnitude in shear velocity (10−8–2 m/s)An "M‐shaped" law for the variation of friction with velocity is obtained including two velocity‐strengthening and two velocity‐weakening lawsThere is microscopic evidence that ultra‐high frequency vibration may cause high velocity shear weakening [ABSTRACT FROM AUTHOR]

Polyimide (PI) has good mechanical properties and thermal stability, while high coefficient (COF) and wear rate limit its wide application. In order to endow PI with better friction and wear resistance, Mo2Ti2C3 MXene was used as solid lubricant to prepare Mo2Ti2C3/PI nanocomposites with different content of Mo2Ti2C3. The effect of the amount of Mo2Ti2C3 on the tribological behavior of PI was explored and the tribological mechanism was also analyzed Tribological tests showed that at 1.2 wt% of Mo2Ti2C3 addition, the composite material demonstrated superior friction and wear resistance, exhibiting a 19.9% decrease in the COF and a 79.7% decrease in the wear rate compared to single PI due to the good self‐lubricating properties of Mo2Ti2C3 MXene. Moreover, Mo2Ti2C3 can also effectively assist PI to form sleek transfer films without interruption or gaps on the steel balls, resulting in a reduced friction COF and wear rate of the material. This work demonstrates that Mo2Ti2C3 MXene is an aussichtsreich additive for polymers to improve the tribological properties and broadens the applications of Mo2Ti2C3/PI composites in friction material area. Highlights: Mo2Ti2C3 significantly improves the tribological performance of polyimide.The thermal properties of Mo2Ti2C3/polyimide are enhanced by the introduction of Mo2Ti2C3.The minimum coefficient of Mo2Ti2C3/PI composites was only 0.246 at the 1.2 wt% content of Mo2Ti2C3.The wear rate of Mo2Ti2C3/PI composites was decreased by 79.7% at the 1.2 wt% addition of Mo2Ti2C3.Mo2Ti2C3 promotes the establishment of tribo‐film to protect the Mo2Ti2C3/PI composites from friction. [ABSTRACT FROM AUTHOR]

The paper discusses an axisymmetric contact problem of indentation of a circular cylindrical punch with a flat base into a homogeneous elastic half-space with friction. It is assumed that the shear contact stresses are related to the normal contact pressure by the law of dry friction, in which the coefficient of friction depends on the radial coordinates of the points of the surfaces in contact and is directly proportional to them. With the help of rotation operators, the solution of the stated problem is reduced to the solution of a singular integral equation of the second kind with variable coefficients, and its closed solution in quadratures is constructed. The numerical calculations display the patterns of change in contact stresses and rigid displacement of the punch depending on the maximum value of the friction coefficient. [ABSTRACT FROM AUTHOR]

Epoxy composites with simultaneously enhanced tribological and mechanical properties filled with basalt belt/reduced graphene oxide/paraffin wax (EP/BF/RGO/PW) have been successfully prepared. Experimental results indicated that the mechanical property of the composites was reinforced by basalt belt, and the tribological property was greatly improved by reduced graphene oxide/paraffin wax at the same time. Thanks to the synergy of PW and RGO sheets, the optimized coefficient of friction and wear rate of the composite declined by 77.5% and 14.4% than pure EP under dry friction conditions of 60 N and 0.42 m s-1. In comparison to pure EP, the tensile strength and modulus of the optimized composite increased by 75.5% and 100% on account of the robust basalt fibers. Thus, the combination of basalt fiber and reduced graphene oxide/paraffin wax can readily realize the introduction of enhancer and oil-impregnation in EP, offering a promising approach for simultaneously improving the mechanical and tribological properties of epoxy. Highlights • A novel BF/RGO/PW 3D skeleton was developed via a facile hydrothermal method. • The BF/RGO/PW can enhance the mechanical and tribological properties of EP. • The basalt fibers account for the improvement in mechanical property. • A reduction of 77.5% in COF and a decrease of 14.4% in wear rate were realized. [ABSTRACT FROM AUTHOR]

Drilling a series of sinuous wellbore trajectories has emerged as a promising operation, offering significant advantages in enhancing reservoir contact. Based on various development cases (Zili and colleagues), it has been observed that horizontal wells with wavy wellbore trajectories tend to exhibit higher production rates. However, extending the length of these wavy trajectories requires careful consideration in terms of safe drilling practices, as the pressure management associated with these intricate trajectories has been seldom explored. In this study, we establish a modified model for annular pressure to predict and assess the open‐hole extend‐reach limit of horizontal wells with wavy wellbore trajectories. The introduced model incorporates dry friction force and annular geometry influenced by cuttings bed, making it more accurate than traditional models based on experimental observations. The wavy wellbore trajectory comprises three types: the up‐dip wellbore, the down‐dip wellbore, and the complex wellbore. Overall, the horizontal interval limits for the down‐dip wellbore and up‐dip wellbore are smaller than those of the corresponding smooth wellbores under varying factors. Notably, the complex wellbore trajectory offers advantages in navigating formations with narrow pressure windows compared to conventionally horizontal wellbores (smooth wellbores), thanks to its longer extended‐reach limit. [ABSTRACT FROM AUTHOR]

Re‐establishing the natural connectivity of rivers using fishways may mitigate the unfavourable effects of dam construction on riverine biodiversity and freshwater fish populations. Knowledge of the swimming performance of target species in specific regions is critical for designing fishways with a high passage efficiency. Substrate roughening with river stones of fishways is considered to improve fish swimming capacity by benefiting from reduced‐velocity zones with lower energetic costs. However, the effectiveness of rough substrates in energy metabolism is rarely tested. We investigated the effect of substrate roughening on the swimming capacity, oxygen consumption and behaviour of Schizothorax wangchiachii from the Heishui River in a flume‐type swimming respirometer. The results showed that substrate roughening improved critical and burst swimming speed by ~12.9% and ~15.0%, respectively, compared to the smooth substrate. Our results demonstrate that increased reduced‐velocity zones, lowered metabolic rate and tail‐beat frequency support our hypothesis that lower energetic costs improve fish swimming performance in rough substrate compared to smooth treatment. The traversable flow velocity model predicted that maximum traversable flow velocity and maximum ascent distance were higher over rough compared to smooth substrate fishways. Fishway substrate roughening may be a practical approach to improve fish swimming upstream for demersal riverine fish. [ABSTRACT FROM AUTHOR]

Rubber compounds with high friction coefficients are widely used in safety shoes, functional shoes, etc. However, in the presence of a liquid lubrication layer, friction coefficients between rubber compounds and the opposing surface are generally very low. In this work, friction coefficients of rubber compounds are improved by a two‐pronged method. First, micron silicon carbides (SiC) with irregular shape and knife‐edge are selected as fillers to improve the plowing friction in dry state, and to improve the adhesive friction in wet state via piercing the liquid lubrication layer. Second, tackifying phenolic resin is added to increase the damping performance of rubber compounds, so as to improve the hysteresis friction in both dry and wet states. The results show that the comprehensive static and dynamic friction properties of rubber compounds are the best in both dry and wet states (0.5 wt% sodium dodecyl sulfate solution or 90 wt% glycerol solution), when SiC size is 7–14 μm and SiC content is 75–90 phr. Phenolic resin can increase the loss factor (tanδ) and loss modulus (E") of rubber compounds, and improve the static and dynamic friction coefficients in wet state (90 wt% glycerol solution). At the same time, it can improve the hardness of rubber compounds and reduce the relative content of SiC, thus reducing the static and dynamic friction coefficients of rubber compounds in dry and wet states (0.5 wt% sodium lauryl sulfate aqueous solution). In general, the optimal content of phenolic resin is 10–15 phr. Highlights: Rubber friction coefficients improved via both adhesion and hysteresis friction.Irregular silicon carbides pierce liquid layer to improve adhesive friction of rubber.Phenolic resin increases hysteresis friction of rubber via damping property. [ABSTRACT FROM AUTHOR]

In this work, the thermal contact problem of a rigid flat punch sliding over functionally graded material (FGM) coating with a surface crack is investigated. The surface of a homogeneous isotropic substrate is ideally coated by FGM. The coefficient of friction on the contact surface is assumed to be constant, and dry Coulomb friction law is applied. The major purpose of this study is to compute the stress intensity factors at the tip of a surface crack under thermomechanical loading. In this perspective, the thermoelastic contact and the surface crack problems are modeled using finite element method. An iterative solution procedure based on finite elements is developed to solve the contact/crack problem until generated frictional heat reach the steady‐state condition. Obtained results are compared to those available in the open literature, and a good agreement is observed. Presented results involve stress intensity factors computed under various thermoelastic sliding conditions. Highlights: A surface crack problem in an FGM coating subjected to sliding contact is examined.Frictional heat due to punch has a remarkable influence on stress intensity factors.The iterative computational method is developed to determine stress intensity factors.Frictional heat and material properties significantly affect stress intensity factors. [ABSTRACT FROM AUTHOR]

Electrophoretic deposition was used to deposit carbon nanotube/polyether ether ketone (CNTs/PEEK) composite coatings onto porous titanium‐tantalum (Ti‐30Ta) substrates at different PEEK concentrations (4.5, 6.0, and 7.5 mg/mL). Coatings were analyzed for thickness, porosity, surface roughness, microhardness and bonding strength, with higher PEEK concentrations producing thicker and more uniform coatings. However, optimal coating thickness showed highest bonding strength; lower and higher thickness led to decreased bonding strength. The tribological properties of the CNTs/PEEK coated Ti‐30Ta samples of different thicknesses (50, 70, and 100 μm) were evaluated using ball‐on‐flat linear reciprocating sliding tests under dry and wet conditions using simulated body fluid (SBF) as a lubricant. The CNTs/PEEK coatings provided excellent tribological protection under dry friction, with thicker coatings having lower friction and negligible wear. However, under wet sliding, the coating's wear rate increased significantly due to softening of the rubbing surface caused by SBF lubrication that increase transfer film onto the counter body surface. Coating with optimal thickness of 74 μm demonstrated the lowest friction and wear under SBF lubrication due to its highest hardness and bonding strength. This study highlights the importance of controlling coating thickness in determining the performance of the CNTs/PEEK coatings for orthopedic implants. [ABSTRACT FROM AUTHOR]

Ultrasonic surface rolling processing (USRP) is used to fabricate textures with different shapes and processing times on the surface of GCr15 substrates. The polytetrafluoroethylene‐polyphenylene sulfide/silicon dioxide (PTFE‐PPS/SiO2) lubricant coating is subsequently deposited on texture surfaces by electrohydrodynamic atomization. USRP texturing not only forms the texture shapes but also realizes the strengthening effects of increased hardness and residual stress. Synergistic effects of the PTFE‐PPS/SiO2 coating and different USRP textures on dry friction and wear properties are investigated. The results show that the synergistic effect of the linear texture with two processing times and the PTFE‐PPS/SiO2 coating achieves better dry friction and wear performance than the individual effect. The texture shape and the processing times both affect the synergistic effect. The increase in processing times has a positive effect on synergistic improvement. The linear texture plays a better synergistic effect compared with that of the wavy texture with the same processing times. The USRP textures affect the tribological properties of texture‐coating samples by mainly influencing the coating‐substrate adhesion. The texture with an appropriate shape and height can improve the adhesion with the coating, and thus synergistically improve the dry friction and wear properties of the substrate. [ABSTRACT FROM AUTHOR]

The Fe2.5Ni2.5CrAl multiprincipal element alloy (MPEA) is a promising material for engineering applications because of its high strength and plasticity values. To evaluate the friction and wear performance, reciprocating dry sliding tests and molecular dynamics (MD) simulations are performed to determine the wear mechanism over a range of length scales. This alloy exhibits a lower average friction coefficient and wear volume loss during dry sliding compared with the well‐known Fe2Ni2CrAl alloy. The worn surface morphology reveals abrasive scratches, grooves, and delamination. The fine wear debris possesses high oxygen content, leading to higher wear resistance. The wear mechanism involves abrasive, adhesive, and oxidative wear. The wear, atomic stress and shear strain, dislocations, and lattice structure are analyzed by MD simulations. Point defects, atomic clusters, and stacking faults are identified in the nanowear process. The behavior of the 1/6<112> $ &amp;amp;amp;amp;amp;amp;amp;gt;$ (Shockley)‐type dislocations is identified as the main dislocation mechanism during sliding. Stacking faults produced during the stress release process are present in the indentation. This work provides a deep insight into the friction and wear behavior of Fe2.5Ni2.5CrAl MPEAs. [ABSTRACT FROM AUTHOR]

A global, bi‐homogeneous, observer for a class of 1‐DOF mechanical systems with Coriolis and centrifugal forces, dry and viscous frictions and non‐vanishing bounded uncertainties/perturbations is proposed, ensuring predefined upper bound of convergence time. The options for convergence acceleration are discussed in a simulation example. [ABSTRACT FROM AUTHOR]

A nonlinear model of an electrically conductive micropolar medium interacting with an external magnetic field is proposed. The deformable state of such a medium is described by two asymmetric tensors: tensor of deformations and bending‐torsion tensor. Both tensors consider both linear and nonlinear terms in rotation and displacement gradients (geometric nonlinearity). The components of the bending‐torsion tensor which have the same indices, describe torsional deformations, while the rest describe bending deformations. The stress state of the medium is described by two asymmetric tensors: stresses tensor and moment stresses tensor. It is assumed, as is customary in magnetoelasticity, that the action of an electromagnetic field on a strain field occurs through Lorentz forces. From the system of Maxwell's equations follow the equations for electric and magnetic induction, which together with electromagnetic equations of state, should be added to the group of equations which describe the dynamics of a micropolar medium. In the frames of the proposed model, a one‐dimensional nonlinear magnetoelastic shear‐rotation wave is considered. In the equations of dynamics, a nonlinear term is considered. This term makes the most significant contribution to wave processes. It is shown that two factors will influence the wave propagation: dispersion and nonlinearity. Nonlinearity leads to the emergence of new harmonics in the wave, which contributes to the appearance of sharp drops in the moving wave profile. Dispersion, on the contrary, smoothed out differences due to differences in phase velocities of harmonic component waves. The combined action of these factors can lead to the formation of stationary waves that propagate at a constant speed without changing shape. Only those cases where a constant component is absent in the deformation wave are physically realizable. Stationary waves can be either periodic or aperiodic. The latter are spatially localized waves—solitons. It is shown that the behavior of "subsonic" and "supersonic" solitons will be qualitatively different. [ABSTRACT FROM AUTHOR]

We propose a novel monolithic pure SPH formulation to simulate fluids strongly coupled with rigid bodies. This includes fluid incompressibility, fluid–rigid interface handling and rigid–rigid contact handling with a viable implicit particle‐based dry friction formulation. The resulting global system is solved using a new accelerated solver implementation that outperforms existing fluid and coupled rigid–fluid simulation approaches. We compare results of our simulation method to analytical solutions, show performance evaluations of our solver and present a variety of new and challenging simulation scenarios. [ABSTRACT FROM AUTHOR]

Real oral processing is the squeezing and shearing between two soft surfaces. The importance of soft palate surface cannot be ignored while focusing on tongue substitutes. Thus the effects of viscoelasticity, roughness of upper jaw substitutes, and fluid rheological properties on lubrication properties were explored by in vitro oral tribology experiments. Different palate substitutes significantly changed the friction curves of pure water, milk, and yogurt. The boundary friction coefficients of pure water and milk are higher under softer or smooth palate substitutes due to stronger viscoelastic responses of friction pairs. Their boundary friction coefficients are lowest at rigid upper jaw substitutes owing to smaller contact angles and deformation. However, the boundary friction coefficient of yogurt is lower owing to its high viscosity, low loss factor, and large particle size under soft friction pairs. In addition, it is highest at rigid palate friction pair because a smaller contact area reduces the entrainment of yogurt, resulting in poor lubricating performance. [ABSTRACT FROM AUTHOR]

In this paper, a new composite coating was prepared on the surface of pure aluminum (Al) by combining the micro‐arc oxidation (MAO) technology with the polyethylene glycol (PEG 400) carbonization technology. The composite coating and the single MAO coating were observed by a scanning electron microscope, and an energy‐dispersive spectrometer, finding that the single MAO coating surface with volcano‐like pores and microcracks, was covered by the carbonized layer of the composite coating where the overall coating thickness was around 19.5 µm, including 17.5 µm of inner MAO coating. The material properties of the composite coating were characterized by X‐ray diffraction and X‐ray photoelectron spectroscopy. The wear resistance of the composite coating was tested under dry friction conditions, finding that the wear width on the composite coating surface was 909.6 µm only, which was around 55.7%, 50.4%, and 58.2% of those for pure Al substrate, single carbonized coating, and single MAO coating, respectively. Then the comprehensive wear resistance of the composite coating was explored under different sliding speeds and lubrication mediums. Finally, the wear‐resisting mechanism of the composite coating was discussed, concluding that the composite coating could effectively reduce adhesive wear and abrasive wear of the Al substrate. [ABSTRACT FROM AUTHOR]

Magnitude of atmospheric turbulence, a key driver of several processes that contribute to aerosol (i.e., particle) deposition, is underrepresented in current models. Various formulations have been developed to model particle dry deposition; all these formulations typically rely on friction velocity and some use additional ad hoc factors to represent enhanced impacts of turbulence. However, none were formally linked with the three‐dimensional (3‐D) turbulence. Here, we propose a set of 3‐D turbulence‐dependent resistance formulations for particle dry deposition simulation and intercompare the performance of new resistance formulations with that obtained from using the existing formulations and measured dry deposition velocity. Turbulence parameters such as turbulence velocity scale, turbulence factor, intensity of turbulence, effective sedimentation velocity, and effective Stokes number are newly introduced into two different particle deposition schemes to improve turbulence strength representation. For an assumed particle size distribution, the newly proposed schemes predict stronger diurnal variation of particle dry deposition velocity and are comparable to corresponding measurements while existing formulations indicate large underpredictions. We also find that the incorporation of new turbulence parameters either introduced or added stronger diurnal variability to sedimentation velocity and collection efficiencies values, making the new schemes predict higher deposition values during daytime and nighttime when compared to existing schemes. The findings from this research may help improve the capability of dry deposition schemes in regional and global models. Plain Language Summary: Aerosols, also known as particulate matter, in the atmosphere can affect ecosystem health through a process called dry deposition, which is a helpful process that can reduce human exposure to air pollutants. There are several processes involved in particle dry deposition and one of the most important processes is the chaotic motions of the atmosphere, which is known as turbulence. However, turbulence strength is underrepresented in mathematical modeling of particle dry deposition. In this study, we introduced several turbulence parameters to improve the representation of turbulence effects on deposition and introduced new formulations. These new formulations are tested in a simple mathematical model and then field measurements are used to evaluate the performance of new formulations as well as existing formulations. Results indicate that the new formulations largely improved results, which are closer to measurements, while existing formulations showed large underestimations. This research offers improved capability of models in estimating particle deposition, and, in turn, hopefully leads to better estimation of particle pollution and related exposures. Key Points: New turbulence parameters are introduced to improve particle deposition estimationNew dry deposition formulations largely reduce biases in deposition velocity estimations [ABSTRACT FROM AUTHOR]

The constitutive behavior of faults is central to many interconnected aspects of earthquake science, from fault dynamics to induced seismicity, to seismic hazards characterization. Yet, a friction law applicable to the range of temperatures found in the brittle crust and upper mantle is still missing. In particular, rocks often exhibit a transition from steady‐state velocity‐strengthening at room temperature to velocity‐weakening in warmer conditions that is poorly understood. Here, we investigate the effect of competing healing mechanisms on the evolution of frictional resistance in a physical model of rate‐, state‐, and temperature‐dependent friction. The yield strength for fault slip depends on the real area of contact, which is modulated by the competition between the growth and erosion of interfacial micro‐asperities. Incorporating multiple healing mechanisms and rock‐forming minerals with different thermodynamic properties allows a transition of the velocity‐ and temperature‐dependence of friction at steady‐state with varying temperatures. We explain the mechanical data for granite, pyroxene, amphibole, shale, and natural fault gouges with activation energies and stress power exponent for weakening of 10–50 kJ/mol and 55–150, respectively, compatible with subcritical crack growth and inter‐granular flow in the active slip zone. Activation energies for the time‐dependent healing process in the range 90–130 kJ/mol in dry conditions and 20–65 kJ/mol in wet conditions indicate the prominence of viscoelastic collapse of microasperities in the absence of water and of pressure‐solution creep, crack healing, and cementation when assisted by pore fluids. Plain Language Summary: Predictions of seismicity on faults depend on our understanding of how rocks break under stress and heal over time. Although this is well documented in laboratory experiments, a physical description of these observations from first principle is still incomplete, preventing extrapolation outside the experimental conditions. Here, we describe the frictional resistance of rocks based on the size of the small asperities that form the real area of contact in a fault zone. While the erosion of asperities during sliding weakens the frictional resistance, any mechanism at the micro‐scale that flattens these contacts strengthens the fault. The temperature dependence of friction and the microstructure of fault gouge representative of different tectonic contexts indicate that rapid sliding occurs by the growth of micro‐cracks and by granular flow in ultrafine‐grained shear zones. Time‐dependent strength recovery occurs by different mechanisms depending on the ambient temperature and water content. In dry conditions, healing occurs by internal viscous deformation of the micro‐asperities. The presence of water enables more efficient healing mechanisms at low temperature, including dissolution and precipitation of minerals and filling of pre‐existing cracks. The different thermodynamic properties of these mechanisms make faults unstable in a specific range of temperature, allowing the initiation of earthquakes. Key Points: A rate‐, state‐, and temperature‐dependent friction law based on the real area of contact explains fault gouge mechanical dataCompeting healing mechanisms enable a transition between velocity‐strengthening and velocity‐weakening friction at steady‐stateTime‐dependent healing is accommodated by low‐temperature plasticity, for example, viscoelastic flow, pressure‐solution creep, or crack healing [ABSTRACT FROM AUTHOR]

An UHMWPE‐based multiscale composite containing graphene oxide, nanodiamonds, and short carbon fibres has shown excellent performance under distilled water lubrication. However, it is crucial to evaluate its tribological performance under conditions which more accurately represent the final application. In this study, the tribological performance of the developed UHMWPE‐based multiscale composite is evaluated and compared with neat UHMWPE under different lubricating conditions: no lubricant (dry), in seawater (SW) and in an environmentally acceptable lubricant (EAL). While neat UHMWPE displays a lower friction and wear in dry conditions, the multiscale composite performs better under SW and EAL lubrication. A maximum reduction in friction coefficient of 77% and specific wear rate of 88% are obtained in SW. Under EAL lubricated conditions, the multiscale composite has a maximum reduction in specific wear rate of up to 75%. [ABSTRACT FROM AUTHOR]

It remains as a challenge to develop binderless WC ceramics that integrate high mechanical properties and low friction wear. Here, we report the preparation of strong and tough WC ceramics with low wear rate by adding graphene nanoplatelets (GNPs) and using oscillatory pressure sintering (OPS) process. The introduced GNPs lead to the formation of nearly fully dense composites with the aid of an oscillatory pressure. The OPS‐prepared WC–0.3‐wt% GNPs composites reached a high flexural strength, hardness, and fracture toughness, being up to 1420 MPa, 24.9 GPa, and 6.89 MPa m1/2, respectively. Moreover, a low friction wear rate of 3.17 × 10−7 mm3 N−1 m−1 is achieved for such composites, which can be ascribed to the formation of a friction lubrication film during dry sliding friction process and their higher mechanical properties. [ABSTRACT FROM AUTHOR]

A novel hydrogenated nitrile butadiene rubber (HNBR) composite filled with organically modified Al2O3 nanoparticle was fabricated. The effects of nanoparticle Al2O3 (nano‐Al2O3) with different particle sizes on the curing, mechanical, and tribological properties of the HNBR composites were studied. Nano‐Al2O3 with different particle sizes (i.e. 10, 50, 100, and 200 nm) was filled into the HNBR in this work. Attenuated total reflection Fourier transform infrared (ATR‐FTIR) spectroscopy was used to investigate the degree of grafting between modified nano‐Al2O3 and rubber matrix. Universal material testing machine was used to study the mechanical properties of the HNBR composites. The tribological properties of the HNBR composites were tested by a ballon‐on‐disk tribometer. The wear morphology of the HNBR composite surface was examined by scanning electron microscopy (SEM). The test results reveal that the nano‐Al2O3 as filler of HNBR composite promoted the vulcanization process, improved tensile and compressive strength, and increased hysteresis energy density and the damping coefficient. And the mechanical properties of the HNBR composite improved gradually with the decrease of nano‐Al2O3 particle size. The friction and wear test results show that the addition of nano‐Al2O3 greatly reduced the dry friction coefficient and specific wear rate of HNBR composites, and the friction coefficient and wear rate decreased gradually with the decrease of nano‐Al2O3 size. The HNBR composite filled with 10 nm nano‐Al2O3 had the better mechanical properties and anti‐wear performance due to that the 10 nm nano‐Al2O3 could form denser cross‐linked network. [ABSTRACT FROM AUTHOR]

The thermal characteristics of four types of dry friction clutch materials (LUK, G95, HCC, and Tiger) are investigated experimentally and numerically in the present work under different working conditions; such as initial sliding angular velocity (ωro), torque (T), and sliding time (ts). The temperature distributions over a cross‐section of friction clutch elements (pressure plate and flywheel) are investigated and optimized during the sliding period (heating phase), and full engagement period (cooling phase). The effect of alternative frictional materials lining of a clutch disc on the thermal behavior of the sliding system under different operating conditions (different angular velocities, torques, and sliding periods) is investigated experimentally. The results showed that the maximum effect on the temperature values occurred when applying maximum torque (4.5 kg·m), maximum initial rotational speed (1200 rpm), slipping period (30 s). However, the temperature values at interface contact decrease when decreasing all the above input conditions values to (2.5 kg·m, 690 rpm, and slipping period to 8 s). The results showed that the temperature reduced (53%) from (180.4°C) for applied torque 4.5 kg·m with initial rotational speed (1200 rpm) and slip period (30 s) to (83.3°C) when applied torque 2.5 kg·m, initial rotational speed (680 rpm) and slip period (8 s) for clutch disc (LUK). It was obtained the same behavior for the other three discs (G95, HCC, and Tiger), but with different values of temperatures. The results show that the temperatures of the pressure plate interface (Tmax = 159.1°C) are higher than those at the flywheel interface (Tmax = 152.7°C), due to the low thermal capacity of pressure plate compared to the flywheel when using G95 frictional material. The experimental optimization results showed that the highest temperatures were obtained when using friction clutch disc (LUK), and minimum temperature when using (HCC) disc, around (20%) reduction when replaced (LUK) material with (HCC) under the same working conditions (T = 4.5 kg·m, ωro = 1200 rpm, and ts = 30 s). [ABSTRACT FROM AUTHOR]

The theory of rate and state friction unifies field, laboratory, and theoretical analysis of the evolution of slip on natural faults. While the observational study of earthquakes and aseismic fault slip is hampered by its strong multi‐scale character in space and time, numerical simulations are well‐positioned to link the laboratory study of grain‐scale processes to the scale at which rock masses move. However, challenges remain in accurately representing the complex and permanently evolving sub‐surface fault networks that exist in nature. Additionally, the common representation of faults as interfaces may miss important physical aspects governing volumetric fault system behavior. In response, we propose a transient viscous rheology that produces shear bands that closely mimic the rate‐ and state‐dependent sliding behavior of equivalent fault interfaces. Critically, we show that the expected tendency of the continuum rheology for runaway localization and mesh dependence can be halted by including an artificial diffusion‐type regularization of anelastic strain rate in the softening law. We demonstrate analytically and numerically using a simplified fault transect that important aspects of the frictional behavior are not significantly affected by the introduced regularization. Any discrepancies with respect to the interfacial description of fault behavior are critically evaluated using one dimensional numerical velocity stepping and spring‐slider experiments. Since no new physical parameters are introduced, our model may be straightforwardly used to extend the existing modeling techniques. The model predicts the emergence of complex patterns of shear localization and delocalization that may inform the interpretation of complex damage distributions observed around faults in nature. Plain Language Summary: How, where, and when earthquakes nucleate is one of the great questions in science and society that, despite steady progress, has hardly been answered to any practical degree. Based on field observations, laboratory experiments, and theoretical work it is believed that a cocktail of escalating mechanical, chemical, and thermal grain‐scale processes cause the sudden and rapid onset of earthquakes. The net effect of these processes are characterized by an immediate strengthening and a gradual weakening response to deformation and are unified in simplified form in the theory of "rate and state friction." This theory is commonly used in computer simulations of earthquake sequences. We point out that rate and state friction, unlike some physical theories of earthquake rupture, does not incorporate a diffusion process such as for example heat conduction. We show the introduction of an artificial diffusion process can prevent the mathematical reduction of a fault zone to a two‐dimensional interface while retaining the properties of the original friction law. This in turn enables simulation techniques that rely on an interface‐free description of the earth and promise to provide new insights into the spontaneous organization of seismic and aseismic phenomena in developing fault zones. Key Points: We reformulate the empirical rate and state friction law as a bulk viscous flow law in terms of anelastic shear strain rateWe show how mesh independence is achieved by including a gradient‐like nonlocal anelastic shear strain rate equivalentWe show analytically and numerically that the proposed continuum model closely reproduces existing results of rate and state friction [ABSTRACT FROM AUTHOR]

In the low speed and heavy load of large machinery rotation or sliding parts, the application of copper base ink plate is very extensive. In this study, the graphene oxide‐polyethylene glycol (GO‐PEG) thin films were prepared on the surface of steel plates by gradient heating, and we simulated the friction and wear state of the solid material when it was brought to the metal surface. And the Fourier transform infrared spectroscopy, X‐ray diffraction, X‐ray energy spectrum analysis, Raman spectroscopy (Raman), and thermogravimetric analysis were used to characterize the structure and properties. The tribological results show that the GO‐PEG composite film prepared by gradient temperature rise method has good lubrication effect. The addition of GO greatly enhances the mechanical properties of the composite films. Compared with the single GO film, the friction coefficient is reduced by 50% and the wear rate is reduced by 85%. The unique characteristics of PEG solved the accumulation phenomenon caused by the strong van der Waals force between GO, then the PEG adhered on GO surfaces caused good lubrication effect. At the same time, the composite film has a broad application prospect in the working environment with little oil and no oil. [ABSTRACT FROM AUTHOR]

Polytetrafluoroethylene (PTFE) composites are commonly used engineering materials owing to their good tribological and mechanical properties. In this study, a friction pair was formed by a glass‐fiber‐reinforced PTFE composite and a chromium‐plated nickel‐based stainless steel plate (18Cr2Ni4WA). Under dry‐friction and oil‐lubrication conditions, friction and wear tests were carried out on the metal plate surface in two roughness ranges and three surface‐texture directions. The friction coefficient, wear rate, width and depth of the wear trace, wear morphology, and surface composition were analyzed. The results showed that the 90° texture direction of the steel plate with respect to the moving direction under oil lubrication and the 0° texture direction under dry friction were the texture directions with the best tribological performance. The friction and wear properties of the PTFE composite under dry friction were better than those under oil lubrication. The 0.3–0.5 μm roughness range of the steel plate was the best roughness range for the tribological properties of the matching pairs. [ABSTRACT FROM AUTHOR]

The lamellar structure and weak interlayer shear force of graphene endow itself potential application value in lubrication fields, while its lubrication performance has been vastly investigated in various fields. However, there are few studies on the composite tribological performance of graphene and ceramic films. In this study, a solid–liquid lubricating system which TiN film slides in graphene aqueous dispersion is developed, aiming to achieve low friction of the TiN ceramic film in macroscale contact. Two kinds of TiN films were obtained by multi‐arc ion plating and magnetron sputtering respectively. The tribological test show that the friction coefficient of the TiN film drops when adding graphene aqueous dispersion and varies with the concentrations of graphene. The results indicate that the graphene might act in two ways to achieve low friction of TiN film, which are liquid lubricating effect of solvents and the formation of smooth sliding interface promoted by graphene. [ABSTRACT FROM AUTHOR]

Ultra‐high molecular weight polyethylene (UHMWPE)/illite‐smectite interstratified clay minerals (ISIC) composites were irradiated by γ‐ray in air and N2 at 75 kGy. The abrasion‐resistance of composites were studied via a ring‐on‐ring friction tester under dry friction conditions. Compared with virgin UHMWPE, the friction coefficient and wear rate of UHMWPE filled with 0.5 wt% ISIC powders and irradiated by γ‐ray in N2 could be further reduced by near 3.1% and 88.0%, respectively. The worn surfaces of the UHMWPE/ISIC composites irradiated in N2 were relatively smooth after dry friction. The composites exhibited relatively high wear resistance. ISIC powders can play the role of heterogeneous nucleation and induce the crystallization of UHMWPE, thereby improving the abrasion‐resistance. The linear molecular chain of UHMWPE was transformed into a three‐dimensional crosslinked structure after irradiation, which can effectively enhance the plastic deformation resistance and then improve the tribological properties. The results show that low filling amount of ISIC powders and γ‐ray irradiation in N2 are effective strategies to improve abrasion resistance of UHMWPE. [ABSTRACT FROM AUTHOR]

The formation of transfer layer especially in polymer‐based composites is unavoidable and essential to investigate the effectiveness of transfer layer under working conditions as it decides the material removal between mating parts. The main aim of the present work is to analyze the performance of transfer layer on specific wear rate (SWR) and coefficient of friction (COF) during dry sliding of hexagonal boron nitride (h‐BN)/prosopis juliflora (PJ) reinforced polymer matrix composite. The mechanical test results exhibited that 136.4%, 145.7%, and 106.2% improvement in hardness, tensile and flexural properties of the hybrid composites compared to pure epoxy. Pin on disc wear test results showed that 15 wt% of h‐BN and 5 wt% of PJ reinforced hybrid composite produced better wear performance of minimum COF as 0.0912 and SWR as 0.0000151 mm3/Nm. ANOVA analysis results showed that load is major contributing parameter on COF and SWR and their percentage of contribution as 25.40% and 79.38% respectively. Worn surface analysis revealed that the reason behind transfer layer formation and its effectiveness under sliding conditions. The major wear mechanisms are identified as micro cutting and micro plowing followed by matrix crack, fiber debonding, and fiber pull out. [ABSTRACT FROM AUTHOR]

The friction clutch is considered a very important element of machines, as it plays a major role in transferring power from the driving part to the driven part. A common application of the friction clutch is in vehicles to connect between the gearbox and the engine. Fast wear occurs as a result of frictional heating that is generated when the clutch is starting to engage. This wear, in addition to the high thermal stresses, will lead to premature failure in the contacting surfaces. The present review highlights the most important studies of the thermal and thermoelastic problems of friction clutches during the last 10 years to show the challenges that were overcome and also the other challenges that needed to find solutions. The present paper will discuss in detail the influence of the frictional heat generated between contact surfaces during slipping and the main factors affecting the thermal behavior of dry friction clutches, such as sliding speed, friction materials, applied pressure, and so on. Furthermore, significant conclusions and remarks based on the available solutions to the thermal problem of the clutch are presented. [ABSTRACT FROM AUTHOR]

Torsional structural behavior of hollow concrete cross‐sections in closed and gaping segment joints Part 2 – The redistribution of torsional shear stresses During the construction of segmental bridges or towers of wind turbines, precast concrete elements are often met dry and the joints get prestressed. By dry friction, the mostly smooth concrete surfaces have to transfer shear stresses from lateral force and torsion. In the first part of this essay [1], two existing verification models for the torsional load‐bearing behavior of dry concrete joints were presented using the example of the tower of a wind turbine and a separate own approach based on the theory of warping torsion was also shown. Based on this approach, a new consistent verification concept is presented below, which shows very good agreement with test data. [ABSTRACT FROM AUTHOR]

A space‐time framework is applied to simulate dense granular flow. Two different numerical experiments are performed: a column collapse and a dam break on an inclined plane. The experiments are modeled as two‐phase flows. The dense granular material is represented by a constitutive model, the μ(I)‐rheology, that is based on the Coulomb's friction law, such that the normal stress applied by the pressure is related to the tangential stress. The model represents a complex viscoplastic material behavior. The interface between the dense granular material and the surrounding light fluid is captured with a level set function. Due to discontinuities close to the the interface, the mesh requires a sufficient resolution. The space‐time approach allows unstructured meshes in time and, therefore a well‐refined mesh in the temporal direction around the interface. In this study, results and performance of a flat and a simplex space time discretization are verified and analyzed. [ABSTRACT FROM AUTHOR]

The water adhesion of a superhydrophobic surface depends on its surface microstructure and chemical composition, which in turn affects its tribological properties. In this paper, a simple and environmentally friendly method was employed to fabricate superhydrophobic aluminium surfaces with different water adhesion. The results confirmed that the superhydrophobic aluminium surfaces with different water adhesion had different tribological properties under dry friction and water lubrication conditions. The sticky superhydrophobic sample with high adhesion had better wear resistance and lower friction coefficient than the slippery superhydrophobic one with very low adhesion. In addition, the related tribological properties were analyzed. This study is helpful to understand the tribological properties of superhydrophobic surfaces with different water adhesion more clearly, which is of great significance for their practical application. [ABSTRACT FROM AUTHOR]

The high temperature solid/liquid lubrication performances of DLC (diamond like carbon) films were investigated by a ball‐on‐disc tribometer under perfluoropolyether (PFPE) oil. The friction and wear behaviours of DLC films sliding against a Si3N4 ball at different friction pairs, sliding speeds and temperatures were evaluated systematically under dry friction. Moreover, tribological properties of DLC films were investigated under PFPE oils lubrication at elevated temperature. The experimental results show that the solid–liquid synergy lubrication is beneficial to improve the tribological properties of the steel‐related friction system and the tribological properties of the friction pair are significantly influenced by lubrication modes under PFPE oil lubrication, especially at high temperature. Tribotests show that the friction system exhibited low friction behaviours under PFPE oil due to the excellent thermal stability of PFPE oil. Coefficient of friction (CoF) of the friction system is as super low as 0.029 under PFPE oil. [ABSTRACT FROM AUTHOR]

This paper presents two sequential friction‐spring damper configurations and develops numerical and analytical models, which describe the systems' behavior. The dampers rely on the non‐smooth characteristics of dry friction to quench unwanted oscillations and only dissipate energy once a preset breakaway force of the friction element is exceeded. Since no additional control is required and the dissipation is only present within a given frequency range, this passive amplitude adaptive dissipation contributes to a higher energy efficiency. The dampers' equations of motion are derived and implemented in a numerical model to gain the first insights into the dampers' behavior. These equations of motion are analyzed via averaging methods in combination with a modal decoupling for nonlinear systems, which lead to the dampers' analytical models. The results from both models are compared and show reasonable agreement within the validity range of asymptotic methods. This work proposes two friction based amplitude adaptive dampers and offers a starting point for future experimental validation. [ABSTRACT FROM AUTHOR]

Summary: A tracking problem is studied in this article where a time‐varying unilateral constraint and disturbances are present. An extension to the method of nonsmooth state transformation is developed. The proposed framework enables synthesis and analysis of control of a rigid body colliding with a time‐varying unilateral constraint. Both the translational and rotational dynamics in three dimensions are covered. The control problem encompasses continuous and discontinuous controllers in the presence of impacts and discontinuous disturbances. This leads to new results in the area of control of systems with impacts and dry friction, a well‐known challenging direction in nonsmooth mechanics. Major contribution of the article is in achieving finite‐time tracking for a class of unilaterally constrained systems where the system dynamics with impacts are converted into one without impacts. Stability results are obtained via the homogeneity of differential inclusions. Numerical simulations are shown to demonstrate specific examples of the theoretical development. [ABSTRACT FROM AUTHOR]

A rectilinear motion of a capsule system along a horizontal rough plane is considered. The system consists of a housing and an internal mass moving periodically along an inclined straight guide rigidly connected to the housing. The friction between the housing and the underlying plane is assumed to be Coulomb's one. The motion of the housing with a periodically changing velocity is investigated. Such a motion may be reversible, in which case the housing alternately moves forward and backward, or irreversible, in which case the housing moves forward or stays at rest and never moves backward. The problem under consideration is the maximization of the distance traveled by the housing for a period with respect to the parameters of the system, for both reversible and irreversible modes of the motion. [ABSTRACT FROM AUTHOR]

Copper‐graphite composites with different copper content are prepared by spark plasma sintering (SPS) at 900 °C using copper‐coated graphite powder and ordinary graphite powder as raw materials. Two groups of specimens are used for friction testing, including the original specimens and K2Cr2O7‐H2SO4‐etched specimens. The density, microstructure, Shore hardness, resistivity and friction and wear properties of the samples were investigated using the Archimedes drainage method, scanning electron microscopy, a hardness tester, a resistance tester, and a friction tester. The results showed that the relative density of the samples with different compositions reached about 90 %, and with an increase of copper content, the density and the relative density of the copper/graphite composites increased. As the copper content increased from 10 wt % to 50 wt %, the Shore hardness of the composites increased from 37 HSD to 59 HSD. Under dry‐friction conditions, with an increase of Cu wt %, the friction coefficients and wear rates of the original specimens showed a downward trend, while those of the chemically etched specimens first increased and then decreased. Under water‐friction conditions, both the original samples and chemically etched samples show very low friction coefficients, about 0.15 and 0.12, respectively, which are far lower than the friction coefficients (∼0.20) obtained under dry‐friction conditions. The wear rates of the original specimens decreased with an increase of copper content, while those of the chemically etched specimens increased first and then decrease. The electrical conductivities of the copper/graphite composites were slightly reduced by chemical etching, but the resistivity of the copper/graphite composites remained at a low level of 10−7 Ω⋅m for a copper content of 40–50 wt %. [ABSTRACT FROM AUTHOR]

In situ filling raises a possibility to restrain the agglomeration of nanomaterials in macromolecule matrices, which usually is encountered in the nanocomposites prepared by a mechanical mixing method. In this work, the nanocomposites of ultra‐high molecular weight polyethylene (UHMWPE) filled with nanosized hydroxyapatite (HAP) were fabricated by an in situ hydrothermal method. The fabricated HAP/UHMWPE nanocomposites exhibited a high dispersion degree of HAP nanoparticles (NPs) and a marked improvement in stiffness, strength, toughness, glass‐transition temperature, and hydrophilicity compared with the matrix and the reference composites prepared by mechanical mixing. Furthermore, pronouncedly decreased coefficients of friction and volume wear rates were observed on the in situ fabricated HAP/UHMWPE nanocomposites under dry friction, the lubrications of water, or cell culture fluid against a steel ring. The in situ fabricating strategy suggests a way to prepare highly dispersed nanocomposites, and the resulting HAP/UHMWPE nanocomposites might indicate a significant clinical prospect. [ABSTRACT FROM AUTHOR]

Dry friction is well known to lead to undesired, self‐excited vibrations which negatively influence a process or cause discomfort. One attempt to quench these oscillations are superposed high‐frequency vibrations. A system consisting of two visco‐elastically coupled masses on a moving belt subject to high‐frequency excitation is analyzed. A multiple scales method is used to find an analytical approach of the system dynamics, which leads to the equation for the slow motion of the system, which is of interest. Linearizing about the quasi‐equilibrium and analyzing the eigenvalues, statements about the stability of the new equilibrium point can be given. These semi‐analytical results are compared to the numerical solution of the full system and the influence of certain parameters is outlined. [ABSTRACT FROM AUTHOR]

Superior wet attachment and friction performance without the need of special external or preloaded normal force, similar to the tree frog's toe pad, is highly essential for biomedical engineering, wearable flexible electronics, etc. Although various pillar surfaces are proposed to enhance wet adhesion or friction, their mechanisms remain on micropillar arrays to extrude interfacial liquid via an external force. Here, two‐level micropillar arrays with nanocavities on top are discovered on the toe pads of a tree frog, and they exhibit strong boundary friction ≈20 times higher than dry and wet friction without the need of a special external or preloaded normal force. Microscale in situ observations show that the specific micro–nano hierarchical pillars in turn trigger three‐level liquid adjusting phenomena, including two‐level liquid self‐splitting and liquid self‐sucking effects. Under these effects, uniform nanometer‐thick liquid bridges form spontaneously on all pillars to generate strong boundary friction, which can be ≈2 times higher than for single‐level pillar surfaces and ≈3.5 times higher than for smooth surfaces. Finally, theoretical models of boundary friction in terms of self‐splitting and self‐sucking are built to reveal the importance of liquid behavior induced by micro–nano hierarchical structure. [ABSTRACT FROM AUTHOR]

In this paper we study boundary optimal control of an evolutionary system governed by a history‐dependent variational‐hemivariational inequality. The inequality is a weak formulation of a dynamic frictional contact problem for a viscoelastic body with a multivalued normal damped response condition and a simplified version of the Coulomb law of dry friction. A continuous dependence result for the solution map is proved and the existence of optimal solutions to the control problem is established. [ABSTRACT FROM AUTHOR]

In order to fully predict the performance of variable displacement vane oil pump (VDVP) during concept design phase， one-dimensional simulation software AMESim is used to model the centralised parameters of VDVP. Several important performance indexes of VDVP are predicted, including the flow, pressure, torque and initial variable point, it is proved by the bench test that the simulation results have a good correlation with the test results. In addition, the effect of leakage on volumetric efficiency is studied, external leakage is smaller than internal leakage, and the higher the temperature, the larger the leakage. The force characteristics of VDVP are studied. Under extreme conditions, the maximum pressure of the internal chamber is about 15.9 bar, and the maximum forces of the stator and rotor are 1920 and 1653 N, respectively. The effect of temperature on the frictional power is studied. Under low-temperature conditions, the viscous frictional power is much greater than the dry frictional power, while the high temperature is the opposite. [ABSTRACT FROM AUTHOR]

Metal rubber (MR) is an elastic porous material with a complex spiral network structure, which is widely used in aerospace, aviation, navigation, medical biomaterials, and other fields. The traditional preparation processes depend on multiple tests or artificial experience, which leads to time‐consuming and laborious. Herein, a virtual manufacturing method is proposed to realize the spatial multipoint dynamic contact analysis of a complex spiral network structure, that is, to study the mechanical properties of MR from the micro‐tribological point of view. Through real‐time capturing the spatial distribution characteristics of each contact point and the number of contact pairs and contact mode, it is found that the number of dry friction contact points between the wires is nonlinear positive correlation with the damping energy dissipation characteristics of MR. The preparation parameters, such as the wire diameter, the spiral wire pitch, and the blank winding angle, are selected to be investigated, and the controlling variable method is used to analyze the single factor influence in the virtual preparation process. The results show that the damping and energy consumption characteristics of MR can be improved effectively by reducing the wire diameter, by reducing the spiral wire pitch, or by increasing the winding angle of the blank. However, too small or too large preparation parameters cause stress concentration and affect the mechanical properties of the actual materials. The quasi‐static compression test is carried out through the preparation of the products, and the error between simulation and experiment results is <8%. Herein, the mechanism of contact friction between wire turns in MR is expounded. Also, this method can effectively guide the production of MR material, reduce the time of material design, and promote the popularization and application of the material. [ABSTRACT FROM AUTHOR]

Titanium‐doped diamond‐like carbon (Ti‐DLC) coatings with Ti concentration of 4 at.% (Ti4at.%‐DLC) and 27 at.% (Ti27at.%‐DLC) were prepared by a hybrid ion beam deposition system for comparison. The tribological behaviors of Ti‐DLC coatings under dry friction and boundary lubrication conditions were systematically investigated. Results showed that, under dry friction, the Ti4at.%‐DLC coating displayed lower friction coefficient (0.07) and wear rate due to the continuous transfer film formed in the sliding interface, while Ti27at.%‐DLC coating was worn out at initial stage due to severe abrasive wear. And under boundary lubrication, both the Ti4at.%‐DLC and Ti27at.%‐DLC coatings showed excellent tribological properties attributing to the formation of oil film between sliding interface. In particular, Ti27at.%‐DLC performed the lowest wear rate of 1.12 × 10−16 m3 N−1 m−1 in this friction case. In conclusion, compared with Ti27at.%‐DLC coating, Ti4at.%‐DLC coating exhibited better tribological performances both under dry friction and boundary lubrication. The present result provides guidance for the selection of DLC coatings according to the realistic environment of starved‐oil and rich‐oil conditions. [ABSTRACT FROM AUTHOR]