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Autonomous underwater vehicles (AUVs) have various applications in both military and civilian fields. A wider operation area and more complex tasks require better overall range performance of AUVs. However, until recently, there have been few unified criteria for evaluating the range performance of AUVs. In the present work, a unified range index, i.e., L*, considering the cruising speed, the sailing distance, and the volume of an AUV, is proposed for the first time, which can overcome the shortcomings of previous criteria using merely one single parameter, and provide a uniform criterion for the overall range performance of various AUVs. After constructing the expression of the L* index, the relevant data of 49 AUVs from 12 countries worldwide have been collected, and the characteristics of the L* range index in different countries and different categories were compared and discussed. Furthermore, by analyzing the complex factors affecting the range index, methods to enhance the L* range index value, such as efficiency enhancement and drag reduction, have been introduced and discussed. Under this condition, the work proposes a unified and scientific criterion for evaluating the range performance of AUVs for the first time, provides valuable theoretical insight for the development of AUVs with higher performance, and then arouses more attention to the application of the cutting-edge superlubricity technology to the field of underwater vehicles, which might greatly help to accelerate the coming of the era of the superlubricitive engineering.

Counterion adsorption at the solid–liquid interface affects numerous applications. However, the counterion adsorption density in the Stern layer has remained poorly evaluated. Here we report the direct determination of surface charge density at the shear plane between the Stern layer and the diffuse layer. By the Grahame equation extension and streaming current measurements for different solid surfaces in different aqueous electrolytes, we are able to obtain the counterion adsorption density in the Stern layer, which is mainly related to the surface charge density but is less affected by the bulk ion concentration. The charge inversion concentration is further found to be sensitive to the ion type and ion valence rather than to the charged surface, which is attributed to the ionic competitive adsorption and ion–ion correlations. Our findings offer a framework for understanding ion distribution in many physical and chemical processes where the Stern layer is ubiquitous.

Purpose: The purpose of this paper is to study the aerodynamic characteristics of Ahmed body in longitudinal and lateral platoons under crosswind by computational fluid dynamics simulation. It helps to improve the aerodynamic characteristics of vehicles by providing theoretical basis and engineering direction for the development and progress of intelligent transportation. Design/methodology/approach: A two-car platoon model is used to compare with the experiment to prove the accuracy of the simulation method. The simplified Ahmed body model and the Reynolds Averaged N-S equation method are used to study the aerodynamic characteristics of vehicles at different distances under cross-winds. Findings: When the longitudinal distance x/L = 0.25, the drag coefficients of the middle and trailing cars at β = 30° are improved by about 272% and 160% compared with β = 10°. The side force coefficients of the middle and trailing cars are increased by 50% and 62%. When the lateral distance y/W = 0.25, the side force coefficients of left and middle cars at β = 30° are reduced by 38% and 37.5% compared with β = 10°. However, the side force coefficient of the right car are increased by about 84.3%. Originality/value: Most of the researches focus on the overtaking process, and there are few researches on the neat lateral platoon. The innovation of this paper is that in addition to studying the aerodynamic characteristics of longitudinal driving, the aerodynamic characteristics of neat lateral driving are also studied, and crosswind conditions are added. The authors hope to contribute to the development of intelligent transportation. [ABSTRACT FROM AUTHOR]

Superlubricity control is of great interest in both industry and scientific research, and several methods have been proposed to achieve this goal. In this work, ultraviolet (UV) light was introduced into titanium dioxide (TiO2) and silicon nitride (Si3N4) tribosystems to accomplish photoinduced superlubricity. The friction coefficients (COFs) between Si3N4balls and TiO2plates in the mixtures of sulfuric acid (H2SO4) solution and glycerol solution were obviously reduced, and the system entered the superlubricity region (COF < 0.01) after UV illumination at a speed of 56 mm/s. However, the COF was much larger without UV treatment than that with UV treatment. The formation of silica (SiO2) layers on the surfaces of Si3N4balls and the elastohydrodynamic effects were determined to be fundamental to the low friction in this experiment, and the enhancement of the combination between the TiO2surface and the hydroxy group of glycerol by UV illumination was the key to the photoinduced superlubricity in this system. These findings showed one method for achieving superlubricity by introducing a light field that could be further applied to special working conditions.

Moiré superlattices have emerged as an unprecedented manipulation tool for engineering correlated quantum phenomena in van der Waals heterostructures. With moiré potentials as a naturally configurable solid-state that sustains high exciton density, interlayer excitons in transition metal dichalcogenide heterostructures are expected to achieve high-temperature exciton condensation. However, the exciton degeneracy state is usually optically inactive due to the finite momentum of interlayer excitons. Experimental observation of dark interlayer excitons in moiré potentials remains challenging. Here we directly visualize the dark interlayer exciton transport in WS2/h-BN/WSe2heterostructures using femtosecond transient absorption microscopy. We observe a transition from classical free exciton gas to quantum degeneracy by imaging temperature-dependent exciton transport. Below a critical degeneracy temperature, exciton diffusion rates exhibit an accelerating downward trend, which can be explained well by a nonlinear quantum diffusion model. These results open the door to quantum information processing and high-precision metrology in moiré superlattices.

Purpose: The purpose of this paper is to investigate the optimal average drag coefficient of the Ahmed body for mixed platoon driving under crosswind and no crosswind conditions using the response surface optimization method. This study has extraordinary implications for the planning of future intelligent transportation. Design/methodology/approach: First, the single vehicle and vehicle platoon models are validated. Second, the configuration with the lowest average drag coefficient under the two conditions is obtained by response surface optimization. At the same time, the aerodynamic characteristics of the mixed platoon driving under different conditions are also analyzed. Findings: The configuration with the lowest average drag coefficient under no crosswind conditions is 0.3 L for longitudinal spacing and 0.8 W for lateral spacing, with an average drag coefficient of 0.1931. The configuration with the lowest average drag coefficient under crosswind conditions is 10° for yaw angle, 0.25 L for longitudinal spacing, and 0.8 W for lateral spacing, with an average drag coefficient of 0.2251. Compared to the single vehicle, the average drag coefficients for the two conditions are reduced by 25.1% and 41.3%, respectively. Originality/value: This paper investigates the lowest average drag coefficient for mixed platoon driving under no crosswind and crosswind conditions using a response surface optimization method. The computational fluid dynamics (CFD) results of single vehicle and vehicle platoon are compared and verified with the experimental results to ensure the reliability of this study. The research results provide theoretical reference and guidance for the planning of intelligent transportation. [ABSTRACT FROM AUTHOR]

In this study, the tribological behaviors of graphene as a lubricant additive for steel/copper and steel/steel friction pairs were compared. For the steel/copper friction pair, the graphene sheets remarkably decreased the coefficient of friction and wear scar depth under low loads, but these slightly increased under high loads. The steel/steel friction pair showed excellent tribological properties even under high loads. Severe plastic deformation on the copper surface reduced the stability of the graphene tribofilm because of a rough copper transfer film on the steel during the running-in period. The results provide a better understanding of the mechanism of graphene as a lubricant additive.

Graphene is a promising material as a lubricant additive for reducing friction and wear. Here, a dispersing method which combines chemical modification of graphene by octadecylamine and dicyclohexylcarbodiimide with a kind of effective dispersant has been successfully developed to achieve the remarkable dispersion stability of graphene in base oil. The stable dispersion time of modified graphene (0.5 wt%) with dispersant (1 wt%) in PAO-6 could be up to about 120 days, which was the longest time reported so far. At the same time, the lubricant exhibits a significant improvement of tribological performance for a steel ball to plate tribo-system with a normal load of 2 N. The coefficient of friction between sliding surfaces was ~0.10 and the depth of wear track on plate was ~21 nm, which decreased by about 44% and 90% when compared to pure PAO-6, respectively. Furthermore, the analysis of the lubricating mechanisms in regard to the sliding-induced formation of nanostructured tribo-film has been contacted by using Raman spectra and TEM.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have layered structures with excellent tribological properties. Since the energy difference between hexagonal-molybdenum ditelluride (2H-MoTe2) and distorted octahedral-molybdenum ditelluride (1T’-MoTe2) is very small among the transition metal dichalcogenides (TMDCs), MoTe2becomes one of the most promising candidates for phase engineering. In our experiment, we found that the friction force and friction coefficient (COF) of 2H-MoTe2were an order of magnitude smaller than those of 1T’-MoTe2by the atomic force microscope (AFM) experiments. The friction difference between 1T’-MoTe2and 2H-MoTe2was further verified in molecular dynamics (MD) simulations. The density functional theory (DFT) calculations suggest that the friction contrast is related to the difference in sliding energy barrier of the potential energy surface (PES) for a tip sliding across the surface. The PES obtained from the DFT calculation indicates that the maximum energy barrier and the minimum energy path (MEP) energy barrier of 2H-MoTe2are both smaller than those of 1T’-MoTe2, which means that less energy needs to be dissipated during the sliding process. The difference in energy barrier of the PES could be ascribed to its larger interlayer spacing and weaker Mo–Te interatomic interactions within the layers of 2H-MoTe2than those of 1T’-MoTe2. The obvious friction difference between 1T’-MoTe2and 2H-MoTe2not only provides a new non-destructive means to detect the phase transition by the AFM, but also provides a possibility to tune friction by controlling the phase transition, which has the potential to be applied in extreme environments such as space lubrication.

Ultralow friction polymer composites were prepared by adding oil-loaded microcapsules into epoxy (EP) resin. Mono-dispersed polystyrene (PS)/poly alpha olefin (PAO) microcapsules with a diameter of ~2 μm and a shell thickness of ~ 30 nm were prepared by solvent evaporation method in an oil-in-water emulsion. The lubrication behaviors of the EP resin composites with oil-loaded microcapsules have been investigated under different loads and sliding speeds. As compared with the pure EP resin, the friction coefficient of the composite could be reduced to 4% (from 0.71 to 0.028) and the wear rate could be decreased up to two orders of magnitude. It was demonstrated that the released PAO oil from the microcapsules during the friction process produced a boundary lubricating film, which could prevent the direct contact of two rubbing surfaces, and thus leading to an extremely low friction coefficient and wear rate. Moreover, the composites with microcapsules could achieve comparable lubrication properties to the case under the external lubrication condition, while the former case could effectively minimize the lubricant leakage and improve the lubrication efficiency.

The presence of defects such as vacancies has a significant impact on the frictional properties of 2D materials that are excellent solid lubricants. In this study, we demonstrate that the nonmonotonic effect of Te vacancy defects on the friction of MoTe2is related to the change in the maximum sliding energy barrier due to the variation in tip position. The experimental results of atomic force microscopy suggest that the friction shows an overall increasing trend with the increase in Te vacancy density, but this variation is nonmonotonic. Molecular dynamics simulations show that the increase in friction force with defect density can be attributed to the large and more sliding energy barriers that the tip has to overcome. Furthermore, the nonmonotonic variation of friction with defect density is dominated by the change of the maximum sliding potential barrier caused by the variation of tip position perpendicular to the sliding direction during the sliding process. Additionally, the uneven charge distribution due to charge transfer occurring at the defect also contributes to the increase in friction. This work shows the mechanism of the effect of Te vacancy defects on the friction of MoTe2, which provides guidance for the modulation of the frictional properties of solid lubricants.

Cations can achieve excellent hydration lubrication at smooth interfaces under both microscale and macroscale conditions due to the boundary layer composed of hydration shells surrounding charges, but what about anions? Commonly used friction pairs are negatively charged at the solid/solution interface. Achieving anionic adsorption through constructing positively charged surfaces is a prerequisite for studying the hydration lubrication of anions. Here we report the hydration layer composed of anions adsorbed on the positively charged polymer/sapphire interface at acidic electrolyte solutions with pH below the isoelectric point, which contributes to the hydration lubrication of anions. Strongly hydrated anions (for the case of SO42–) exhibit stable superlubricity comparable to cations, with strikingly low boundary friction coefficient of 0.003–0.007 under contact pressures above 15 MPa without a running-in period. The hydration lubrication performance of anions is determined by both the ionic hydration strength and ion adsorption density based on the surface potential and tribological experiments. The results shed light on the role of anions in superlubricity and hydration lubrication, which may be relevant for understanding the lubrication mechanism and improving lubrication performance in acidic environments, for example, in acid pumps, sealing rings of compressors for handling acidic media, and processing devices of nuclear waste.

The issues regarding energy dissipation and component damage caused by the interface friction between a friction pair attract enormous attention to friction reduction. The key-enabling technique to realize friction reduction is the use of lubricants. The lubricants smooth the contact interfaces, achieving an ultralow friction contact, which is called superslippery or superlubricity. At present, superslippery and superlubricity are two isolated research topics. There is a lack of unified definition on superslippery and superlubricity from the viewpoint of tribology. Herein, this review aims at exploring the differences and relations between superslippery and superlubricity from their origin and application scenarios. Meanwhile, the challenges for developing superslippery surface and superlubricity surface are discussed. In addition, perspectives on the interactive development of these two surfaces are presented. We hope that our discussion can provide guidance for designing superslippery or superlubricity surfaces by using varies drag-reduction technologies.

Engineering nanocontainers with encapsulated inhibitors onto graphene has been an emerging technology for developing self-healing anticorrosion coatings. However, the loading contents of inhibitors are commonly limited by inhomogeneous nanostructures of graphene platforms. Here, we propose an activation-induced ultrathin graphene platform (UG-BP) with the homogeneous growth of polydopamine (PDA) nanocontainers encapsulated with benzotriazole (BTA). Ultrathin graphene prepared by catalytic exfoliation and etching activation provides an ideal platform with an ultrahigh specific surface area (1646.8 m2/g) and homogeneous active sites for the growth of PDA nanocontainers, which achieves a high loading content of inhibitors (40 wt %). The obtained UG-BP platform exhibits pH-sensitive corrosion inhibition effects due to its charged groups. The epoxy/UG-BP coating possesses integrated properties of enhanced mechanical properties (>94%), efficient pH-sensitive self-healing behaviors (98.5% healing efficiency over 7 days), and excellent anticorrosion performance (4.21 × 109Ω·cm2over 60 days), which stands out from previous related works. Moreover, the interfacial anticorrosion mechanism of UG-BP is revealed in detail, which can inhibit the oxidation of Fe2+and promote the passivation of corrosion products by a dehydration process. This work provides a universal activation-induced strategy for developing loading-enhanced and tailor-made graphene platforms in extended smart systems and demonstrates a promising smart self-healing coating for advanced anticorrosion applications.

An amorphous carbon (a-C) film shows substantial potential for friction and wear reduction. In this work, the robust superlubricity state with a coefficient of friction of 0.002 at the maximal pressure of 1.15 GPa was realized when lithium citrate (LC) was applied as the lubricating additive in ethylene glycol (EG) to lubricate the Si3N4/a-C friction pair based on the ball-on-plate friction test. The wear rate of the a-C film was 4.5 × 10–10mm3/N·m, which was reduced by 98.3% compared to that of the film lubricated with EG. Friction promoted the chemisorption of the LC molecules via the tribochemical reaction between the carboxylate radicals and the a-C film. The exposed lithium ions could adsorb water molecules to form a hydration layer, providing extremely low shear strength. Furthermore, the colloidal silica layer formed on the Si3N4ball via the tribochemical reaction could reduce friction. It was difficult to destroy the formed tribochemical films under high contact pressure because they were robust, preventing the direct contact of the friction pair and resulting in the near-zero wear of the a-C film.

An extremely low friction state was observed on the gold surface induced by applying a specific negative potential in cationic surfactant solution. The friction force showed a remarkable reduction from 8.3 to 3.5 × 10−2nN (reduced by 99.6%) with increasing the period of negative applied potential, and the final friction coefficient could reduce down to 3 × 10−4. The extremely low friction state was robust, and it also exhibited an excellent load bearing capacity, which cannot be damaged by a high load. Moreover, the extremely low friction state achieved under negative applied potential could keep stable even after the removal of potential, but failed in a short time, once a specific positive potential was applied. It was demonstrated that there was a stable electro-adsorption of surfactant molecules on the gold surface induced by applying a negative potential, leading to the formation of a bilayer structure on the gold surface. The hydration layers of the bilayer on the gold surface and micelles on the silica probe provided a shear plane with an extremely low shear strength, leading to the extremely low friction state on the gold surface. This study provides a method to achieve extremely low friction state by applied potential.

In this study, a robust macroscale liquid superlubricity with a coefficient of friction of 0.004 was achieved by introducing molybdenum carbide (Mo2CTx) MXene nanoparticles as lubricating additives in a lithium hexafluorophosphate-based ionic liquid at Si3N4—sapphire interfaces. The maximal contact pressure in the superlubricity state could reach 1.42 GPa, which far exceeds the limit of the superlubricity regime in previous studies. The results indicate that a composite tribofilm (mainly containing molybdenum oxide and phosphorus oxide) that formed at the interface by a tribochemical reaction contributed to the excellent antiwear performance. Furthermore, the extremely low shear strength of the tribofilm and the interlayers of Mo2CTxMXene contributed to the superlubricity. This work demonstrates the promising potential of Mo2CTxMXene in improving superlubricity properties, which could accelerate the application of superlubricity in mechanical systems.

Extensive efforts have been made to pursue a low-friction state with promising applications in many fields, such as mechanical and biomedical engineering. Among which, the load capacity of the low-friction state has been considered to be crucial for industrial applications. Here, we report a low friction under ultrahigh contact pressure by building a novel self-assembled fluorinated azobenzene layer on an atomically smooth highly-oriented pyrolytic graphite (HOPG) surface. Sliding friction coefficients could be as low as 0.0005 or even lower under a contact pressure of up to 4 GPa. It demonstrates that the low friction under ultrahigh contact pressure is attributed to molecular fluorination. The fluorination leads to effective and robust lubrication between the tip and the self-assembled layer and enhances tighter rigidity which can reduce the stress concentration in the substrate, which was verified by density functional theory (DFT) and molecular dynamics (MD) simulation. This work provides a new approach to avoid the failure of ultralow friction coefficient under relatively high contact pressure, which has promising potential application value in the future.

Macroscale superlubricity has attracted increasing attention owing to its high significance in engineering and economics. We report the superlubricity of engineering materials by the addition of partially oxidized black phosphorus (oBP) in an oleic acid (OA) oil environment. The phosphorus oxides produced by active oxidation exhibit lower friction and quick deposition performance compared to BP particles. The H-bond (-COOH⋯O-P, or -COOH⋯O=P) formed between P-O bond (or P=O) and OA molecule could benefit the lubricating state and decrease the possibility of direct contact between rough peaks. The analysis of the worn surface indicates that a three-layer tribofilm consisting of amorphous carbon, BP crystal, and phosphorus oxide forms during the friction, which replaces the shear interface from the steel/steel to carbon—oBP/carbon—oBP layer and enables macroscale superlubricity.

Diamond-like carbon (DLC) films have significant potential to provide solutions for the friction reduction and the lubricity problem of mechanical moving friction pairs. However, the realization of excellent lubrication or even superlubricity and long lifetime under heavy loading conditions is still a great challenge, which is crucial for the applications of DLC in harsh environments. Here, we construct a group of property-strengthening Si-DLC/PLC multilayer films that could withstand ultrahigh contact stresses and achieve robust superlubricity. Under a peak Hertz contact stress of up to 2.37 GPa, the setup of a bilayer thickness of 324 nm enables the multilayered film (an overall film thickness of 1.53 μm) to achieve a superlow coefficient of friction toward 0.001 and an ultralow wear rate of 3.13 × 10–9mm3/Nm. An alternating load reciprocating friction test emphasizes that this strengthening nanostructured Si-DLC/PLC multilayer possesses a kind of load self-adaptation because of its in situ nanoclustering transformation and local ordering of sp2-C phases at the sliding interface. The genesis of self-adaptation to the applied load is evaluated comprehensively to reveal its strengthening and toughening structural characteristics and robustness of the near-zero friction and wear features. The findings provide a significant design criterion for carbon-based solid lubricants applicable to harsh loading environments.

Triboelectric nanogenerator (TENG) has been seen as one of the most promising energy harvesting technologies. However, there are an irreconcilable contradiction between low friction and high electrical output performance of the TENG. Here, we have devised a macroscale liquid superlubric triboelectric nanogenerator based on solid-liquid-solid structure, leveraging the concept of liquid superlubricity technology, resulting in a remarkable increase in both the open circuit voltage and short-circuit current by 53.0 % and 58.4 %, respectively. This huge increase in electrical output performance is accompanied by a 99.1 % reduction in friction coefficient (≈ 0.0025) and 99.993 % reduction in wear rate (≈ 0.76×10−7mm3/N·m), when compared to those of a lubricant-free triboelectric nanogenerator. This work delves into the lubrication and charge transfer mechanisms. The liquid superlubricity technology achieves friction reduction through a hybrid mechanism combining boundary and hydrodynamic lubrication. And the high outputs arise from the charges transfer at solid-liquid interface. The triboelectric charges generated by the friction pair are transferred from solid to the lubricating liquid. Meanwhile, the lubricating liquid promotes electron transfer and contributes additional electrons. Our work provides profound insights into the lubrication and charge transfer mechanisms at solid-liquid interfaces, and addresses the paradox of high output and low friction in TENGs.

Dynamic friction occurs not only between two contact objects sliding against each other, but also between two relative sliding surfaces several nanometres apart. Many emerging micro- and nano-mechanical systems that promise new applications in sensors or information technology may suffer or benefit from noncontact friction. Herein we demonstrate the distance-dependent friction energy dissipation between the tip and the heterogeneous polymers by the bimodal atomic force microscopy (AFM) method driving the second order flexural and the first order torsional vibration simultaneously. The pull-in problem caused by the attractive force is avoided, and the friction dissipation can be imaged near the surface. The friction dissipation coefficient concept is proposed and three different contact states are determined from phase and energy dissipation curves. Image contrast is enhanced in the intermediate setpoint region. The work offers an effective method for directly detecting the friction dissipation and high resolution images, which overcomes the disadvantages of existing methods such as contact mode AFM or other contact friction and wear measuring instruments.

Electrons and phonons are regarded as the microscopic carriers of friction energy dissipation and their coupling is a typical dissipation mode. However, due to the lack of ultrafast detection technic, the friction mechanism about electron-phonon coupling remains unexplained. Here, using high resolution non-contact atomic force microscopy and ultrafast pump-probe spectroscopy, we find that interlayer electron-phonon coupling dissipation channel in WS2/graphene heterostructures can be enhanced by defects and the electron-phonon scattering time is accelerated from 0.62psto 0.27ps. The enhanced electron-phonon coupling leads to significant energy dissipation. We further quantitatively model the friction with dissipation rate to control the friction energy dissipation by ultrafast interlayer electron-phonon coupling. This work provides a new way to understand the mechanism of electron-phonon coupling in friction.

The design and development of self-lubricating materials aid in enhancing the tribological performance and prolonging the service life of aero self-lubricating spherical plain bearings. Herein, monodisperse hollow mesoporous carbon nanospheres (HMCNs) with thin shell thickness and uniform particle size were synthesized. Afterward, oil-containing nanocapsules (Oil@HMCNs) were obtained by the impregnation method, which was an innovative approach to prepare nanoscale capsules. The Oil@HMCNs were monodispersed, and the oil content rate could reach 45 wt %. The nanocapsules were applied as additives to achieve the self-lubricating properties of epoxy resin (EP). It was revealed that the coefficient of friction (COF) and the wear rate of EP in dry friction can be decreased from 0.56 to 0.084 and from 50.15 × 10–6to 0.27 × 10–6mm3/N/m, respectively. The tribological properties of EP composites in dry friction are better than those of EP with external oil. According to the analysis of friction pairs, the antifriction performance was contributed by the released automatic transmission fluid 6HP (ATF6) from nanocapsules. Meanwhile, the presence of ATF6 effectively could inhibit fatigue wear, and the decrease of compressive strength and surface hardness led to plastic deformation and mechanical polishing. Both resulted in a lower wear rate of EP. In conclusion, this work explored the application possibility of hollow mesoporous materials in lubrication. The prepared EP composites exhibited excellent tribological performances and self-lubricating properties, which were expected to be applied to self-lubricating spherical plain bearings and other mechanical parts.

Mysteries about the origin of friction have remained for centuries. Especially, how friction is tuned by an electric field is still unclear. Present tuning mechanisms mainly focus on the atomic configurations and electrostatic force, yet the role of interfacial electronic properties is not fully understood. Here, we investigate a unique friction tuning effect induced by an electric current in a conductive atomic force microscopy experiment and uncover two main tuning mechanisms of friction by the fluctuation of electronic properties during sliding: (1) electric-field-induced electron density redistribution and (2) current-induced electron transfer. We put forward an electronic level friction model unraveling the relationship between the friction tuning and the electronic property fluctuation (EPF) under electric field/current, which is applicable to tribosystems ranging from conductors to semiconductors and insulators, including two-dimensional material interfaces. This model provides theoretical guidance for tribosystem design and friction control, proposing a new perspective in understanding the origin of friction.

The appearance of multidrug-resistant bacteria and their biofilms presents a serious threat to modern medical systems. Herein, we fabricated a novel gold-nanorod-based chemo-photothermal-integrated antimicrobial platform with surface-charge-switchable and near-infrared (NIR)-induced size-transformable activities that show an enhanced killing efficiency against methicillin-resistant Staphylococcus aureus (MRSA) in both planktonic and biofilm phenotypes. The nanocomposites are prepared by in situ copolymerization using N-isopropyl acrylamide (NIPAM), acrylic acid (AA), and N-allylmethylamine (MAA) as monomers on the surfaces of gold nanorods (GNRs). Ciprofloxacin (CIP) is loaded onto polymer shells of nanocomposites with a loading content of 9.8%. The negatively charged nanocomposites switch to positive upon passive accumulation at the infectious sites, which promotes deep biofilm penetration and bacterial adhesion of the nanoparticles. Subsequently, NIR irradiation triggers the nanocomposites to rapidly shrink in volume, further increasing the depth of biofilm penetration. The NIR-triggered, ultrafast volume shrinkage causes an instant release of CIP on the bacterial surface, realizing the synergistic benefits of chemo-photothermal therapy. Both in vitroand in vivoevidence demonstrate that drug-loaded nanocomposites could eradicate clinical MRSAbiofilms. Taken together, the multifunctional chemo-photothermal-integrated antimicrobial platform, as designed, is a promising antimicrobial agent against MRSAinfections.

The temperature distribution along the surface of evaporating droplets can affect significantly the flow field inside the liquid and consequently the deposition pattern on the substrate. Although a “phase diagram” for the temperature distribution along the droplet surface was revealed by the numerical simulations, its experimental verification has still not been reported. In this paper, the surface temperature of evaporating droplets has been observed by using an infrared (IR) camera. The experimental observations show that three different patterns of temperature distribution along the droplet surface occur in succession with the change of the contact angle during the evaporation process, which is in good agreement with the theoretical predictions by the “phase diagram” of the surface temperature distribution. Furthermore, the effects of evaporative cooling on the “phase diagram” of sessile droplets have been explored. The numerical results indicate that the evaporative cooling effect can alter the size of the phase regions in the “phase diagram”. These results may provide a better understanding of the evaporation process of drying sessile droplets.

Although grease can effectively lubricate machines, lubrication failure may occur under high speed and heavy load conditions. In this study, Mn3O4/graphene nanocomposites (Mn3O4#G) were synthetized using a hydrothermal method as lubricant additives. The lubrication properties of compound grease with Mn3O4#G nanocomposite additive under heavy contact loads of 600–900 N (3.95–4.59 GPa) were investigated. First, the nanocomposites were dispersed into L-XBCEA 0 lithium grease via successive electromagnetic stirring, ultrasound vibration, and three-roll milling. Compound grease with additives of commercial graphene (Com#G) was also investigated for comparison. Tribological test results revealed that the trace amounts of Mn3O4#G (as low as 0.02 wt%) could reduce the coefficient of friction (COF) of grease significantly. When the concentration of Mn3O4#G was 0.1 wt%, the COF and wear depth were 43.5% and 86.1%, lower than those of pure graphene, respectively. In addition, under the effect of friction, the microstructure of graphene in Mn3O4#G nanocomposites tends to be ordered and normalized. Furthermore, most of the Mn3O4transformed into Mn2O3owing to the high temperature generated from friction. Using the Ar gas cluster ion beam sputtering method, the thickness of the tribofilm was estimated to be 25–34 nm. Finally, the improvement of the lubrication properties was attributed to the synergistic effect of the adsorbed tribofilm, i.e., the graphene island effect and the filling effect of Mn3O4#G.

Precise therapy has become prevalent in clinical practice owing to its accurate and efficient targeting treatment of diseases. Such treatments involving polymersomes as carriers have great potential to lesion sites without damage to normal tissues. However, due to the inherent thick hydrophobic layer of polymersomes, an instantaneous release response to external stimuli remains a challenge. To tackle this challenge, here, we report on the synthesis and applications of azobenzene-containing photochromic vesicles as delivery vectors. These vesicles are assembled from small-molecule amphiphiles that have been developed to provide a fast response and promote instantaneous release due to molecular size reduction compared with macromolecular polymersomes. After cross-linking, the stability of vesicles under a physiological environment is notably enhanced. By varying UV and visible light irradiation, the “gate” of vesicles can be opened and closed reversibly for the controlled release of capsuled cargoes. In vitro experiments display that the vesicles can be applied to load cysteamine for eliminating excess reactive oxygen. The synthesized vesicles here show high performance in controlled and instantaneous release in cells both in time and space. By our approach, oxidative damage to cellular biomolecules can be substantially reduced.

Hydroxyapatites (HAPs) are usually coated on the surface of an implant to improve the osseointegration with defect bone tissue. Besides, conducting polymers have the advantages of good conductivity, reasonable biocompatibility, and easy of modification, which endow them applicable to electrical stimulation therapy. However, it still remains a great challenge to fabricate hybrid coating combing HAP with conducting polymer on implant surface efficiently. In this work, phytic acid-doped polyaniline (PANI) were successfully synthesized on medical titanium (Ti) sheets. By virtue of the abundant anodic phosphoric groups of phytic acid, HAP nanocrystals were biomineralized on PANI. The PANI-HAP hybrid layer exhibits good cell compatibility with MC3T3 cells. More importantly, HAP nanocrystals and PANI operate synergistically on cell proliferation and osteogenesis through electrical stimulation. Alkaline phosphatase activity and extracellular calcium contents of cells on PANI-HAP display 3-fold and 2.6-fold increases, compared with bare Ti sheets, respectively. The valid integration of mineralization and electrical stimulation in this work renders an efficient strategy for implant coating, which might have potential applications in bone-related defects.

Applied potential can control the friction on electrical conductor surfaces in both air and liquid environments. Using a three-electrode electrochemical cell filled with aqueous ionic solution under atomic force microscopy, a potential-dependent friction phenomenon was observed on a graphitic surface. With an applied positive potential on the graphitic surface, the friction force increased over that observed under neutral applied potential and increased with increasing positive potential. With the applied negative potential on the graphitic surface, lubricity was observed compared to the case with the neutral potential on the graphitic surface. The applied potential also changed the interaction force between the probe and graphitic surface, where the positive potential resulted in a stronger attractive force, while the negative potential induced a stronger repulsive force. The mechanism of the friction and interaction force modulation under the applied potential was ascribed to the rearrangement of hydrated cations and anions on the graphitic surface under different potentials, resulting in electrostatic forces and hydrogen bonds varying as a function of potential. This study demonstrates that the friction on a graphitic surface can be modulated by the surface potential in aqueous ionic solutions, which may provide a novel method for reducing friction in aqueous environments.

Graphite has been conventionally believed to exhibit an inferior lubricating performance with significantly larger friction coefficient and wear rate in a vacuum environment than in ambient air. Dangling bonds at the edge planes of graphite, accounting for the high friction in inert atmosphere are saturated by chemisorbed vapor molecules in air, which contributes to low surface adhesion and low friction. However, there is still a lack of direct experimental evidence whether basal planes of graphite excluding the negative effects of edges or dangling bonds shows intrinsic lubricity when sliding under ultrahigh vacuum (UHV) conditions. By the interlayer friction measurement enabled by graphite flake-wrapped atomic force microscope tips in UHV, we show a record-low friction coefficient of 4 × 10–5(slope of friction vs normal force curve) when sliding between graphite layers, which is much lower than that in ambient air. This discrepancy manifests the intrinsic sliding frictional behavior between the graphite basal planes when the tribo-materials and experimental conditions are well-designed and strictly controlled. In addition, the temperature dependence of the kinetic friction between the graphite layers has been investigated under UHV conditions over the temperature range of 125–448 K, which is consistent with the thermally activated process.

Reversible friction regulation is of long-standing great interest in the fields of both industry and scientific research, so some materials and theories have been developed aiming to solve this problem. Light-sensitive materials are promising because of the easy controllable switching of the properties and structures. Here, a reversible light-controlled macrolubrication was realized by regulating the performance of nanoscale light-sensitive molecules adsorbed on contact surfaces. In this work, symmetric diarylethene and asymmetric diarylethene had been designed and synthesized as functional materials. The friction forces were found to be obviously increased upon exposure to ultraviolet light and decayed to the initial value under visible light. In addition, the friction coefficient changed alternately with ultraviolet and visible illumination. According to the results of experiments and simulation of material properties, the behavior was suggested to be attributed to the difference in shear stiffness of the nanoscale diarylethene molecule adsorption layer triggered by two wavelength lights. This work not only provides a new lubrication regulation technology but also develops intelligent engineering materials.

Introduction of graphene-family nanoflakes in liquid results in a reduction in friction and enhanced wear resistance. However, the high demand for dispersity and stability of the nanoflakes in liquid largely restricted the choice of graphene-family nanoflakes thus far. This study proposed a new strategy to overcome this limitation, involving the formation of a graphene coating with deposited graphene-family nanoflakes, followed by the lubrication of the coating with glycerol solution. Pristine graphene (PG), fluorinated graphene (FG), and graphene oxide (GO) nanoflakes were chosen to be deposited on the respective SiO2substrates to form graphene coatings, and then an aqueous solution of glycerol was used as lubricant. The coefficient of friction (COF) and wear rate were reduced for all deposited coatings. However, the PG coating exhibited better lubrication and antiwear performance than FG and GO coatings. A robust superlubricity with COF of approximately 0.004 can be achieved by combining glycerol with the PG coating. The superlubricity mechanism was attributed to the formation of a tribofilm, mainly composed of graphene nanoflakes in the contact zone. The extremely low friction achieved on the hydrophobic graphene coating with liquid can aid in the development of a high-performing new lubrication system for industrial applications.

We present a detailed first-principle study of few-layer arsenic and antimony electronic structures. The band structures of 2D arsenic and antimony are calculated by a hybrid functional with the spin–orbital coupling. The results show that the band gaps of arsenene (monolayer arsenic) and antimonene (monolayer antimony) are 1.93 and 1.52 eV, respectively. It is observed that the band gaps narrow in trilayer arsenic and bilayer antimony. The band alignment with HfO2and other 2D materials is calculated to show that HfO2is a good candidate as a gate oxide in field effect transistors. It is found that point defects such as a single vacancy or adatom will introduce several defect states in arsenene in the middle of the band gap. Meanwhile, the defect formation energy becomes negative when the Fermi level is close to the band edges. By comparison, the defect formation energy in antimonene is always positive so that the Fermi level pinning should be suppressed in contact with the reactive metal.

The interfaces between two-dimensional (2D) materials and the silicon dioxide (SiO2)/silicon (Si) substrate, generally considered as a solid–solid mechanical contact, have been especially emphasized for the structure design and the property optimization in microsystems and nanoengineering. The basic understanding of the interfacial structure and dynamics for 2D material-based systems still remains one of the inevitable challenges ahead. Here, an interfacial mobile water layer is indicated to insert into the interface of the degraded black phosphorus (BP) flake and the SiO2/Si substrate owing to the induced hydroxyl groups during the ambient degradation. A super-slippery degraded BP/SiO2interface was observed with the interfacial shear stress (ISS) experimentally evaluated as low as 0.029 ± 0.004 MPa, being comparable to the ISS values of incommensurate rigid crystalline contacts. In-depth investigation of the interfacial structure through nuclear magnetic resonance spectroscopy and in situ X-ray photoelectron spectroscopy depth profiling revealed that the interfacial liquid water was responsible for the super-slippery BP/SiO2interface with extremely low shear stress. This finding clarifies the strong interactions between degraded BP and water molecules, which supports the potential wider applications of the few-layer BP nanomaterial in biological lubrication.

Triboluminescence (TL) refers to the luminescence phenomenon at the material surface under the action of pressure or shear. This fascinating phenomenon can directly convert mechanical energy into light emission without the need for other auxiliary components; therefore, it attracts more and more researchers to conduct research in different wavelength ranges, such as X-ray, ultraviolet, visible light, and terahertz. However, there have been few reports on the study of the near-infrared (NIR) range, which is very important in the integrity of the triboluminescence research. In this research, we found that NIR light with a wavelength ranging from 800 to 1000 nm was generated by friction between solid metals and a quartz crystal. Analysis of the cross section of the quartz disk after friction revealed that the TL phenomenon had a strong relationship with the doping of metal grains into the silica. Density functional theory (DFT) and X-ray photoelectron spectroscopy were also conducted to further identify the results. We infer that such light emission arises from the implantation of metal grains into the surface of the quartz, which forms a metal–insulator junction with amorphous silica. Moreover, electron transition between the metal and the insulator, followed by a transition at the center of the defects, causes near-infrared light emission. Our research reveals the infrared luminescence behavior from a different perspective, the transfer of materials, and perhaps deepens the understanding of the near-infrared emission mechanism.

Aqueous lubrication in nature is attracting increasing attention in the tribological fields for reducing friction energy consumption and improving anti-wear durability. Generally, adding nanolubricant additives is one of the most important strategies to effectively enhance the interface performance under boundary lubrication via the formation of a protective tribofilm on rubbing surfaces. However, the adsorbed tribofilms are unstable and are prone to failure during friction, and the interaction mechanism between the tribofilms and frictional interfaces is partly disclosed. In this study, inspired by mussels, an in situ-assembled polydopamine (PDA) tribofilm is achieved with PDA nanoparticles as aqueous lubricant additives, which shows excellent lubrication properties. The coefficient of friction is interface-independent and is reduced by as much as 83%. The results show that the PDA tribofilm can not only form chemical bonding with metal interfaces but also present a synergistic lubrication effect with the upper ceramic surface. Especially, a self-repairing effect of the PAD tribofilm is observed, by which the ultrastable lubricating properties can be achieved during friction, and thus, the friction and wear can be effectively controlled. This work provides an effective method for improving the interface stability of friction pairs under aqueous lubrication and also shows great meaning for industrial applications.

In modern electric equipment, especially electric vehicles, inverter control systems can lead to complex shaft voltages and bearing currents. Within an electric motor, many parts have electrical failure problems, and among which bearings are the most sensitive and vulnerable components. In recent years, electrical failures in bearing have been frequently reported in electric vehicles, and the electrical failure of bearings has become a key issue that restricts the lifetime of all-electric motor-based power systems in a broader sense. The purpose of this review is to provide a comprehensive overview of the bearing premature failure in the mechanical systems exposed in an electrical environment represented by electric vehicles. The electrical environments in which bearing works including the different components and the origins of the shaft voltages and bearing currents, as well as the typical modes of electrical bearing failure including various topographical damages and lubrication failures, have been discussed. The fundamental influence mechanisms of voltage/current on the friction/lubrication properties have been summarized and analyzed, and corresponding countermeasures have been proposed. Finally, a brief introduction to the key technical flaws in the current researches will be made and the future outlook of frontier directions will be discussed.

It is well known that groove texture with a careful design can be used to enhance the load-carrying capacity of oil film under the conditions of hydrodynamic lubrication. In this study, a general parametric model was developed, and agenetic algorithm-sequential quadratic programming hybrid method was adopted to obtain the global-optimum profile of the groove texture. The optimized profiles at different rotating speeds are all chevrons. The numerical analysis results verified the effect of the optimization. In addition to the numerical optimization, experiments were conducted to validate the superiority of the optimized results.The experimental results show that the optimized groove texture can efficiently reduce the coefficient of friction (COF) and the temperature rise of the specimen. In particular, the optimized groove textures can achieve stable ultra-low COF values (COF < 0.01) under certain conditions.

Robust superlubricity of 2D materials could be obtained by transferring graphene on the tip surface for the formation of interlayer friction of heterojunction, owing to the availability of stable interfacial incommensurate contact. Nevertheless, the material transfer mechanisms assisting superlubricity via atomic force microscopy (AFM) probe are still hardly comprehended. In this work, we reported a superlow friction coefficient (0.003) of the WS2/graphene heterojunction governed by graphene flake-transferred AFM tips and achieved a superlubricity state of velocity independence. Both low adhesion of the heterojunction and excellent wear-resistance for tip were also observed, which were attributed to the extremely low interface interaction during the incommensurate contact. The in-depth investigation on the frictional contact zones of probes was performed through high-resolution transmission electron microscopy. The observations emphasize the prevailing mechanisms of tribo-induced interfacial material transfer when AFM probes scan on the surface of 2D materials. The evolution of the superlubricity state principally depends on the establishment of interfacial nanostructures in the self-adaptive running-in period, by different contact mechanics and tribo-reconstructing pathways. These results stimulate a technical route to develop superlubricious tribopairs of 2D materials and guide a promising perspective in the engineering system.

In thin-film lubrication (TFL), generally, the viscosity of the lubricant and its coefficient of friction (CoF) increase. Finding a method to reduce the CoF in TFL is a significant challenge for tribologists. In the present work, we report a robust superlubricity attained by using polyalkylene glycols (PAGs, polar molecules) and poly-α-olefins (PAOs, nonpolar molecules) as lubricants on steel/steel friction pairs that have been pre-treated by wearing-in with polyethylene glycol aqueous solution (PEG(aq)). A steady superlubricity state with a CoF of 0.0045 for PAG100 and 0.006 for PAO6 could be maintained for at least 1 h. Various affecting factors, including the sliding velocity, normal load, and viscosity of the lubricants, were investigated. Element analysis proved that composite tribochemical layers were deposited on the worn region after the treatment with PEG(aq). These layers were formed by the tribochemical reactions between PEG and steel and composed of various substances including oxides, iron oxides, FeOOH, and Fe(OH)3, which contributed to the superlubricity. In addition to the tribochemical layers, ordered layers and a fluid layer were formed by the PAGs and PAOs during the superlubricity periods. All the three types of layers contributed to the superlubricity, indicating that it was attained in the TFL regime. Accordingly, a mechanism was proposed for the superlubricity of the PAGs and PAOs in the TFL regime in this work. This study will increase the scientific understanding of the superlubricity in the TFL regime and reveal, in the future, the potential for designing superlubricity systems on steel surfaces for industrial applications.

Newly emerging two-dimensional (2D) materials of MXenes possess lots of merits, which provide potential solutions for the lubrication issues in harsh conditions. Here, preliminary efforts were devoted to developing an MXene-based 2D composite coating and its antiwear interfacial performance in ambient environments. Macroscale and atomic-scale characterizations were utilized to explore the lubrication behaviors of the composite coating to clarify the influence of the coating composition and tribo-test parameters in the establishment of ultra-wear-resistant sliding interfaces. The results highlighted a unique lubrication mechanism for the 2D MXene composite coating. They suggested that the MXene/nanodiamond coating exhibited almost no wear when rubbed against a polytetrafluoroethylene (PTFE) ball. A nanostructured tribofilm with unprecedented bonding features was in situ formed along the sliding interface. The ultra-wear resistance highly depended on the combined effects of shielding and self-lubrication of PTFE, layer shearing of MXenes, and self-rolling of nanodiamond. These discoveries clearly enrich the 2D material-based lubrication theories and offer technical guidance for designing and exploiting high-performance ultra-wear-resistant materials.

In the present work, we show that the superlubricity can be achieved when the polyhydroxy alcohol solutions are mixed with acid solutions. The lowest friction coefficients between 0.003 and 0.006 are obtained on a traditional tribometer with a high pressure under the lubrication of these mixtures. Experimental results indicate that the superlubricity mechanism is in accordance with that under the lubrication of the mixture of glycerol and acid solutions in the study by Li et al. (Li, J. J.; Zhang, C. H.; Ma, L. R.; Liu, Y. H.; Luo, J. B.Superlubricity achieved with mixtures of acids and glycerol. Langmuir2013, 29, 271−275). It is also found that the superlubricity is closely dependent upon the concentration of polyhydroxy alcohol and the number of hydroxyl groups in the molecular structure of polyhydroxy alcohol. However, the number of carbon atoms and the arrangement of hydroxyl groups in the molecular structure almost have no effect on superlubricity.

Platooning can reduce air drag when driving at high speed and improve aerodynamic characteristics. It can not only improve fuel economy, driving comfort, and safety of drivers but also reduce emissions and alleviate traffic congestion. At the same time, the energy recovery of vehicle driving is also in line with the general trend of international environmental governance. It has played an active role in reducing emissions and improving the ecological environment. In this paper, the effects of vehicle longitudinal spacing, number of vehicles and crosswind on the aerodynamic performance of longitudinal and side-by-side platoons are reviewed, as well as energy recovery. The study found that the three-vehicle platoon reduces drag by 30% compared to the two-vehicle platoon. Comparing the aerodynamic drag of the three-vehicle platoon with that of the single vehicle, the drag coefficient of the first and last vehicles can be reduced by 20%, and the drag coefficient of the middle vehicle can be reduced by about 50%. As the crosswind speed and crosswind angle increase, the drag coefficient and pressure of each member in the platoon increase. Recycling wind energy through wind turbines and alternators can increase the driving range of electric vehicles by 23%. The study of the aerodynamic characteristics and energy recovery of platooning vehicles can provide direction and theoretical basics for the development and sustainable development of intelligent transportation.

Surface texture shape significantly influences the tribological and sealing properties of fluid mechanical face seals. Considering that it is a contradiction to simultaneously improve load capacity and reduce leakage, a numerical multi-objective optimization model is proposed herein to improve the load-carrying capacity while reducing the volume leakage rate via texture shape optimization. The mass-conserving cavitating boundary condition and the non-dominated sorting genetic algorithm II are employed for the optimization. The numerical results indicate that the optimal texture shape is spiral-like under the lowest volume leakage rate, which becomes an asymmetric flat-front chevron with increasing load-carrying capacity and volume leakage rate. The optimal shape outperforms four regular shapes (all obtained by multi-objective optimization) at different rotation speeds and sealing pressures. The cavitation within the optimal shape ensures zero leakage in fluid face seals. The comprehensive performance of the texture shape obtained by multi-objective optimization is superior to that obtained by single-objective optimization.

Graphene-related materials are promising solid lubricants owing to their easy shear between lattice layers. However, the coefficient of friction (COF) of graphene is not sufficiently low at the macroscale, and the lubrication performance is largely restricted by the external environment. In this study, we fabricated a fluorinated graphene (FG) coating on a stainless-steel substrate by a simple electrophoretic deposition in ethanol. The FG coating exhibited an excellent lubrication performance, which reduced the COF by 54.0 and 66.2% compared to those of pristine graphene and graphene oxide coatings, respectively. The lubrication enhancement of FG coating is attributed to its extremely low surface energy and interlaminar shear strength. The formation of ionic metal–fluorine chemical bonds provided a robust solid tribofilm and transfer layer on the friction pairs, which further increased the lubrication performance of the FG coating. The limited influence of the humidity on the lubrication performance of the FG coating is attributed to the hydrophobicity of the FG nanoflakes, which could prevent the influence of water molecules on the sliding interface. The excellent lubrication performance and better environmental adaptability of the FG make it a promising solid lubricant for mechanical engineering applications.