Sliding between two objects under very high load generally involves direct solid–solid contact at molecular/atomic level, the mechanism of which is far from clearly disclosed yet. Those microscopic solid–solid contacts could easily lead to local melting of rough surfaces. At extreme conditions, this local melting could propagate to the seizure and welding of the entire interface. Traditionally, the microscopic solid–solid contact is alleviated by various lubricants and additives based on their improved mechanical properties. In this work, we realized the state-of-the-art of extreme pressure lubrication by utilizing the high thermal diffusivity of liquid metal, 2 orders of magnitude higher than general organic lubricants. The extreme pressure lubrication property of gallium based liquid metal (GBLM) was compared with gear oil and poly-α-olefin in a four-ball test. The liquid metal lubricates very well at an extremely high load (10 kN, the maximum capability of a four-ball tester) at a rotation speed of 1800 rpm for a duration of several minutes, much better than traditional organic lubricants which typically break down within seconds at a load of a few kN. Our comparative experiments and analysis showed that this superextreme pressure lubrication capability of GBLM was attributed to the synergetic effect of the ultrafast heat dissipation of GBLM and the low friction coefficient of FeGa3 tribo-film. The present work demonstrated a novel way of improving lubrication capability by enhancing the lubricant thermal properties, which might lead to mechanical systems with much higher reliability.Keywords: frictional heat; liquid metal; lubricant; thermal conductivity; thermal diffusivity;

•Fluctuation of friction force change affects the generation of friction noise.•FRC has a smaller stick-slip phenomenon because of better viscoelasticity.•Synthetic rubber has obvious stick-slip phenomenon and high frictional noise.This study investigated the different stick-slip behaviours of ultra high molecular weight polyethylene (UHMWPE), synthetic rubber (SR) and fiber resin composite (FRC) polymer materials with ZCuSn10Zn2 plates under water-lubricated conditions on an UMT-3 tribo-tester. The stick-slip phenomena of the polymers at the starting processes were significant and led to different frictional noise emissions. With better hydrophilicity and viscoelasticity, fiber resin composite resulted in the smallest frictional noise. A better hydrophobicity and worse self-lubricating property resulted in the largest stick-slip phenomenon and frictional noise of synthetic rubber. Thus the results disclosed the relationship between the lubrication property of the polymers and their frictional noise performance, and could be of help for the selection of high performance water lubrication materials.

Gecko adhesion has inspired the fabrication of various dry adhesive surfaces, most of which are developed to be used under atmospheric conditions. However, applications of gecko-inspired surfaces can be expanded to vacuum and even space environment due to the characteristics of van der Waals interactions, which are always present between materials regardless of the surrounding environment. In this paper, a controllable, anisotropic dry adhesion in vacuum is demonstrated with gecko-inspired wedged dry adhesive surfaces fabricated using an ultraprecision diamond cutting mold. The adhesion and friction properties of the wedge-structured surfaces are systematically characterized in loading–pulling mode and loading–dragging–pulling mode. The surfaces show significant anisotropic adhesion (Pad ≈ 10.5 kPa vs Pad ≈ 0.7 kPa) and friction (Pf ≈ 50 kPa vs Pf ≈ 30 kPa) when actuated in gripping and releasing direction, respectively. The wedge-structured surfaces in vacuum show comparable properties as exposed in atmosphere. A three-legged gripper is designed to pick up, hold, and release a patterned silicon wafer in vacuum. The study demonstrates a green, high-yield, and low-cost method to fabricate a reliable and durable mold for gecko inspired anisotropic dry adhesive surfaces and the potential application of dry adhesive surface in vacuum.

Bioinspired mushroom-shaped micropillar recently has attracted considerable interest from researchers on adhesion-functionalized artificial surface due to its prominent dry adhesive property. Understanding the interface behavior and further exploring the physical mechanism are of significance for properly designing the structure dimension with enhanced performance. However, the friction contribution to such type of adhesive structures is mostly overlooked in previous investigations. In this paper, by revisiting the detachment behavior associated with the calculation of the critical dynamic crack size making the contact interface destabilized, it is demonstrated that the friction force does work in the detachment process of mushroom-shaped micropillars. The calculated maximum pull-off forces are in good agreement with experimental results and the friction force contribution can reach up to about 41% of the total adhesive force when the tip diameter is 2.6 times the diameter of the supporting pillar. The present model provides a deeper insight into the mushroom-shaped dry adhesives and may be helpful in future bioinspired dry adhesives designs.

•Effect of hydrodynamic on ultra-low friction coefficient between a self- mated Si3N4 pair in water has been investigated.•Surface wear has been measured after a long-duration test under ultra-low friction conditions.•The mechanism of ultralow friction is determined, attributing to a pure hydrodynamic effect.•A two-step running-in method is presented to rapidly achieve ultralow friction.The aim of this work was to determine the mechanism of ultra-low friction between a self-mated Si3N4 pair lubricated with water after running-in. The key to solving this problem was to determine the lubrication regime of the interface under ultra-low friction conditions. According to the Stribeck curve, the variations of friction coefficient with load and velocity were measured to evaluate the hydrodynamic effect; and the variation under different lubricating oils was examined to investigate the influence of viscosity and eliminate the influence of any tribochemical reaction between Si3N4 and water. Surface wear after a long-duration friction test under ultra-low friction conditions was also evaluated. It was found that the ultra-low friction mechanism was attributed to a pure hydrodynamic effect.

Modern composite materials and lignum vitae are widely studied and used in water-lubricated bearings. Noise, wear and tear, environmental pollution, and limited resources restrict the application of these materials. Thus, alternatives to lignum vitae for water-lubricated bearings should be identified and developed. In this work, the chemical composition and mechanical and tribological properties of water-, vegetable oil-, and gear oil-immersed and wax- and water-extracted Excentrodendron hsienmu (E. hsienmu) samples were studied. Results showed that the friction coefficients of the water- and gear oil-immersed and waxand water-extracted E. hsienmu samples are significantly smaller than those of the wax- and water-extracted lignum vitae samples. The grinding crack widths of the water-extracted gear oil-immersed E. hsienmu samples are smaller than those of the water-immersed lignum vitae samples. The preliminary results show that it is possible to provide an environmentally friendly method that could replace lignum vitae and other composite materials for water-lubricated bearings.

Dispersion in water of two-dimensional (2D) nanosheets is conducive to their practical applications in fundamental science communities due to their abundance, low cost, and ecofriendliness. However, it is difficult to achieve stable aqueous 2D material suspensions because of the intrinsic hydrophobic properties of the layered materials. Here, we report an effective and economic way of producing various 2D nanosheets (h-BN, MoS2, MoSe2, WS2, and graphene) as aqueous dispersions using carbon quantum dots (CQDs) as exfoliation agents and stabilizers. The dispersion was prepared through a liquid phase exfoliation. The as-synthesized stable 2D nanosheets based dispersions were characterized by UV–vis, HRTEM, AFM, Raman, XPS, and XRD. The solutions based on CQD decorated 2D nanosheets were utilized as aqueous lubricants, which realized a friction coefficient as low as 0.02 and even achieved a superlubricity under certain working conditions. The excellent lubricating properties were attributed to the synergetic effects of the 2D nanosheets and CQDs, such as good dispersion stability and easy-sliding interlayer structure. This work thus proposes a novel strategy for the design and preparation of high-performance water based green lubricants.Keywords: 2D nanosheets; carbon quantum dots; dispersion stability; friction coefficient; synergistic effect;

•Effect of running-in on the friction behavior in water has been investigated.•A three-step friction test method is presented to achieve ultralow friction.•Surface microtopography and macrowaviness have been measured after running-in.•The mechanism of ultralow friction is attributed to hydrodynamic effect.Effect of running-in process on friction behavior of graphite sliding against cemented carbide in deionized water was investigated using a standard tribometer Plint TE92 in a ring-on-ring contact configuration. A stable and ultralow friction coefficient of about 0.003 between graphite and cemented carbide rings was obtained after a three-step friction test method: Step 1 was rapid pre-sliding at standard operating conditions and then the test apparatus was stopped; Step 2 was the accelerated running-in process under higher load and velocity, and the test apparatus was stopped again; The wear particles on the specimen surface were cleaned, and used water was replaced by new water from step 3. According to the measurements of suface microtopography and macrowaviness, surface geometric features in the interface after three-step friction test reached the optimal harmonious state. Variations of friction coefficient with speed and load were also investigated. The ultralow friction mechanism was attributed to the hydrodynamic effect.

We investigated the biphasic resistive pulses during particle translocation through cylindrical nanopores at low salt concentration by simulation, and the effects of electrolyte concentration, surface charge, electric potential, and pore geometry were systematically discussed. The formation of positive peaks in the pulses is ascribed to the surface charge on the particle and the pore. The peak current enhancement/decline ratio increases linearly with the particle surface charge density but decreases with the salt concentration increase. We find that there is an optimum electric potential for the peak current enhancement ratio to reach the maximum value. When a negatively charged particle is at the orifice of the pore on the low/high potential side, the ion concentration inside and around the pore is significantly depleted/enriched, while inverse electric potential or inverse surface charge has an opposite effect. The extent of such ion modulation is larger with a longer pore. The peak current enhancement/decline ratio is quantitatively linked to the percent of ion concentration enrichment/depletion inside and around the pore, by considering particle occupied volume and concentration change.

A multi-physical signal correlation analysis method is proposed to identify the different tribological properties of materials. The acoustic emission (AE), contact resistance, and frictional force behaviors during dry sliding between four metals, 45# carbon steel, YG12 carbide, 2A12 aluminum alloy, and H62 brass, have been studied. Both positive and negative correlations between the root mean square of the amplified AE (AE RMS) signal and the frictional coefficient have been found in the experiments. In addition, the AE RMS signal and the contact resistance changed with changing sliding speed and normal load in different ways. The different correlation behaviors have been attributed to diverse tribological states under different experimental conditions due to different material characteristics. The correlation analysis provides a new method of quantitatively identifying the tribological states and the AE sources during frictional interaction. The observed anomalous correlations between the AE signal and frictional coefficient should be properly considered according to the different material properties during industrial friction condition monitoring using AE technology.

•Higher pH value leads to higher critical shear stress for the onset of shear thickening.•Higher critical shear stress corresponds to higher low-shear-rate viscosity at high pH value.•Correlation between low-shear-rate viscosity and critical shear stress could change.The effects of pH on the initial shear thinning and subsequent thickening behaviors of fumed silica suspensions with increased shear rate were investigated. The critical shear stress observed at the onset of the shear thickening of suspensions can be tuned through the pH of added aqueous solutions. The increase in pH of the addition shifts the onset of shear thickening to higher shear stresses. This finding is consistent with the trend of prediction models that involve the hydrocluster mechanism for shear thickening. A high critical shear stress at a relatively high pH corresponds to a high value of low-shear-rate viscosity, coincident with a high friction coefficient between two silica surfaces that are lubricated with high pH solutions. The correlation between low-shear-rate viscosity and critical shear stress of suspensions depends on whether the added dilute solution is HCl or NaOH.

•We AE technique for analyzing the tribological properties of the BMG.•We compare the tribological properties of three types of materials with BMG.•We compare the previous researches on BMG tribological properties with ours.The tribological properties of zirconium-based bulk metallic glass (BMG) sliding against polymers, steels, and ceramics at different loads and speeds were investigated. Acoustic emission (AE) technology was used to analyze wear states. The frictional coefficients of the BMG slid against the steel and ceramic balls were high but decreased with increased applied normal load and sliding speed. As the steel balls were more ductile than the ceramic ones, the steel–BMG sliding pairs generated weaker AE signals and exhibited larger wear rates. The BMG tested against the polymer balls had much lower and more stable frictional coefficients than the ones against the steel or ceramic balls because of transferred polymer layers on the BMG surfaces. The BMG against the polymer balls also exhibited the highest AE signals among the three types of counter materials used, indicating that abrasive wear dominated in the polymer-BMG sliding pairs. These results demonstrated the potential application of the BMG as a new tribomaterial that could be an alternative to the traditional crystalline metals for various counter materials.

Gecko-inspired surfaces are smart dry adhesive surfaces that have attracted much attention because of their wide range of potential applications. However, strong frictional force, rather than adhesive force, is frequently targeted in most of research in this area. In this study, the interfacial adhesive and frictional properties of a gecko-inspired mushroom-shaped polyurethane pillar array surface have been systematically characterized to design and control the interfacial adhesion of the surface by considering the nanoscale interfacial adhesion, the microscale structural compliance and deformation, and the macro-scale actuation. Matching the movement of the leg springs and the interfacial adhesive characteristics between the pillar array surfaces and substrates, a three-legged clamp prototype has been designed and fabricated to successfully pick up and release light and fragile objects with a smooth upper surface, such as a silicon wafer. These results provide a new insight into not only the theoretical understanding of the integrating adhesion mechanisms, but also the practical applications of utilizing and controlling the adhesive and frictional forces of gecko-inspired surfaces.Keywords: adhesion; clamp; gecko inspired; interface; peel; pillar surface;

The remarkable ability of geckos to climb and run rapidly on walls and ceilings has recently received considerable interest from many researchers. Significant progress has been made in understanding the attachment and detachment mechanisms and the fabrication of articulated gecko-inspired adhesives and structured surfaces. This article reviews the direct experiments that have investigated the properties of gecko hierarchical structures, i.e., the feet, toes, setae, and spatulae, and the corresponding models to ascertain the mechanical principles involved. Included in this review are reports on gecko-inspired surfaces and structures with strong adhesion forces, high ratios of adhesion and friction forces, anisotropic hierarchical structures that give rise to directional adhesion and friction, and “intelligent” attachment and detachment motions.

A structure parameter, Sn = ηcγ̇/τE, is proposed to represent the increase of effective viscosity due to the introduction of particles into a viscous liquid and to analyze the shear behavior of electrorheological (ER) fluids. Sn can divide the shear curves of ER fluids, τ/E2 versus Sn, into three regimes, with two critical values Snc of about 10–4 and 10–2, respectively. The two critical Snc are applicable to ER fluids with different particle volume fractions φ in a wide range of shear rate γ̇ and electric field E. When Sn < 10–4, the shear behavior of ER fluids is mainly dominated by E and by shear rate when Sn > 10–2. The electric current of ER fluids under E varied with shear stress in the same or the opposite trend in different shear rate ranges. Snc also separates the conductivity variation of ER fluids into three regimes, corresponding to different structure evolutions. The change of Sn with particle volume fraction and E has also been discussed. The shear thickening in ER fluids can be characterized by Snc(L) and Snc(H) with a critical value about 10–6. As an analogy to friction, the correspondence between τ/E2 and friction coefficient, Sn and bearing numbers, as well as the similarity between the shear curve of ER fluids and the Stribeck curve of friction, indicate a possible friction origin in ER effect.

Effect of magnetic field on the friction between two crossed steel cylinders in reciprocating motion in ambient air was experimentally studied in a speed range of 0.012–0.24 m/s and a load range of 0.5–6 N. Upon the applying of an external magnetic field, the friction force and the acoustic emission (AE) signal increased while the wear rate decreased. The increase of friction force is mainly ascribed to the magnetic field induced attractive force between cylinders acting as an extra normal load, which can be substituted by a mechanically exerted equivalent normal load to obtain the similar lateral force and AE signal. While the friction coefficient decreased with the increase of the normal load, the friction coefficient upon applying magnetic field decreased due to a higher equivalent normal load. The relationships among friction coefficient, sliding velocity, normal load and AE signal have been extensively discussed. Analysis of the energy transition from friction to AE signal showed that the external magnetic field could suppress the elastic wave releasing during friction. Also the wear was alleviated by the lubrication of debris concentrated around the contact region in the presence of a magnetic field.Graphical abstractHighlights► Friction coefficient will decrease due to the high equivalent normal load. ► Net normal load induced by magnetic field equals to the force exerted mechanically. ► Acoustic emission releasing rate is slightly restrained under the magnetic field. ► The wear is levitated due to the concentration of the debris around contact point.

Geckos can climb steadily and quickly on different surfaces under various environmental conditions. Extensive experiments have been conducted using gecko toe, setal array, single seta and single spatula to explore the physical principles of gecko adhesion and friction. This work focuses on the effect of environment (i.e. substrate materials and relative humidity [RH]) and experimental conditions (i.e. preload, sliding velocity, and sliding direction) on gecko adhesion and friction through an isolated gecko setal array. Experimental results show that when sliding a setal array in gripping direction, adhesion and friction can be enhanced by more than 200% by increasing surface energy, 60% by increasing relative humidity, and only slightly by increasing sliding velocity; an appropriate preload increase can also cause an enhancement. When RH >40%, adhesion and friction become saturated, and higher sliding velocity and preload can realize the saturation at a lower RH. When a preload is oversized in gripping direction or when sliding a setal array against substrates in releasing direction, the setal array deforms severely and always exhibits a preload-dominated repulsion. A strong anisotropic property is revealed when sliding a setal array in gripping and releasing direction. These results help understand the remarkable abilities of gecko swiftly controlling strong attachment and easy detachment, which inspire the design and fabrication of gecko-inspired reversible dry adhesives.