Friction最新文献

Water friction in nanoconfinement is of great importance in water lubrication and membrane-based applications, yet remains fraught with doubts despite great efforts. Our molecular dynamics simulations demonstrate that the first water layer adjacent to the surface plays an important role in interfacial friction. Applying a uniform strain to the surface (changing the lattice constant) can induce a significant change in friction and is quite different for the hydrophilic and hydrophobic cases. Specifically, in the hydrophilic case, there is maximum friction when the lattice constant approaches the preferential oxygen‒oxygen distance of the first water layer (a constant value), and the further it deviates, the smaller the friction. The maximum friction corresponds to the most ordered first water layer. While in the hydrophobic case, the friction increases monotonically with increasing lattice constant, which hardly changes the first water layer structure but only increases the difficulty of water molecular jump (meaning jump from one equilibrium position to another). Starting from the molecular jump in the first water layer, theoretical dependence of friction on the molecular activation barrier and shear velocity is established, which provides a reasonable explanation for the friction behavior. Moreover, the water transport behavior in nanochannels supports the finding of the friction dependence on the lattice constant, suggesting great potential for improving and controlling water transport. Our results not only provide a novel understanding of nanoconfined water friction but are also instructive for friction control and water transport.

In actual applications, the pocket surface of the porous polyimide (PPI) cage often suffers blackening wear, which may lead to lubrication failure. However, the mechanism of blackening wear is still controversy. In this study, the impact of oxygen concentration on the blackening wear was investigated using a sealed double-contact friction tester by controlling the ventilation of argon gas. Then four types of oils, as well as steel balls and Si₃N₄ balls, were used to study the effect of oil decomposition on blackening wear. Finally, the effect of PPI decomposition was studied under dry friction condition for obtaining high friction temperature. The results indicate that iron oxides are the primary factor of blackening on the worn surface of PPI, while oil and PPI decomposition are the minor factor. At a concentration of 0.1% oxygen or when Si₃N₄ balls are used as rubbing-pair materials, there is only slight blackening wear on the PPI worn surface. Therefore, the influence of iron oxides on the blackening of PPI worn surfaces is crucial. Under low oxygen concentration (0.1%), higher unsaturation of oil may lead to slight blackening wear of PPI due to oil decomposition, but the impact is much lower. This study provides necessary insights into the mechanism of blackening wear of PPI cages.

In this work, few-layer graphene oxide (GO) sheets are investigated as aqueous lubricant additives between ceramic surfaces. Three batches of GO sheets with various lateral sizes were selected, and the lateral sizes were mainly within the ranges of 0.5±0.2, 5.9±2.1, and 59.1±17.1 μm. The weight concentration of the GO sheets in the aqueous lubricant ranged from 0.0005 to 0.8 wt%. The lubrication regime for the friction tests was kept at boundary lubrication. The GO sheets can enhance lubricity by entering the contact area and preventing the sliding surfaces from contacting each other directly, and lubricity is determined by the coverage of the contact area. For each batch of GO sheets, as the concentration increases, the coverage rate of the contact area increases; thus, the coefficient of friction (COF) and wear volume decrease. However, when the GO sheet concentration is very high, the COF approaches a stable value since the contact area is already fully covered by GO sheets, but the wear volume increases slightly due to the high acidity. Moreover, GO sheets with larger lateral sizes can lead to a smoother contact interface. Therefore, at the same concentration, GO sheets with larger lateral sizes can lead to lower COFs and wear volumes. These findings provide a general strategy for improving the performance of lubricants with two-dimensional (2D) material additives in a broad range of mechanical applications.

During friction at extremely low surface pressures, oleyl acid phosphate (OLAP) has an interesting phenomenon: The friction coefficient has a positive gradient in the velocity range of boundary lubrication to mixed lubrication transitions, and the friction coefficient decreases as the running-in time increases. This phenomenon is presumed to be due to the action of the boundary layer; therefore, we analyzed friction surfaces with friction test oil still present via a near edge X-ray absorption fine structure. The results were then combined with first-principles calculations to investigate the chemical state of the boundary layer in a state close to that of sliding. As a result, the iron salt of a phosphate ester and the coordination structure of an iron-centered phosphate ester were generated by OLAP-added oil and aggregated near the interface with the base material during friction. Furthermore, a boundary friction model that considers non-Newtonian characteristics was applied to an experimentally obtained friction diagram to verify the effect of the boundary layer on the friction characteristics. The maximum effective viscosity calculated from a function obtained by fitting the friction diagram was approximately 3,000 Pa·s, which was equivalent to that of common grease. These results indicated that the characteristic frictional properties of the OLAP are due to the action of its grease-like organic boundary layer.

The achievement of a superlubric state with vanishing friction and negligible wear has important applications in minimizing energy dissipation and prolonging the service life of moving mechanical systems. However, the search for a superlubricious oil system applicable to industrial fields remains a major challenge. In this work, we demonstrate for the first time that precisely employing polyether modification for silicone oil molecules could induce direct superlubricity and superlow wear for engineering steel tribopairs. Superlubricity originates from the fact that polyether-modified silicone oil (PESO) can effectively employ polyether functional groups to interact with friction surfaces, during which a complex tribochemical reaction process can be induced under the catalytic role of friction, where an organic lubricious film composed mainly of carbon, silicon and oxygen can be induced in situ, which can not only effectively passivate friction surfaces but also enable superlubric sliding by virtue of its easy-to-shear nature. Furthermore, iron oxides and chromium oxides could also be confirmed to be distributed within the tribofilm, which is desirable for increasing the load-bearing capability of the tribofilm and toughness. Thus, a remarkable superlubricity of 0.01 without running-in combined with superlow wear was realized at the same time. The results of this work show high promise in promoting the industrial use of oil superlubricity and revolutionizing the development of mechanical systems.

Cobalt-based alloys are widely used in aerospace and machinery due to their excellent mechanical properties, where extraordinary wear performance is also desirable to ensure stable operation. However, there is still scarce information on the tribological mechanism of the building block, the cobalt metal, especially under different humidity. The insight into the wear mechanism of Co under different humidity is crucial for the study of the tribological performance of Co-based alloys as well as exploring their potential applications under various working conditions. Here, we report the investigation of the humidity effect on the wear behavior of Co. The results showed that the Co exhibited an ultralow wear characteristic under the humid air environment (RH 70%) with the wear rate of 2.15 × 10^-7 mm³/Nm and dramatically increased by three orders of magnitude to 1.47 × 10^-4 mm³/Nm for dry ambient (~5% RH). Surface analysis revealed that the tribochemistry dominated the whole wearing process, with the worn surface almost fully covered by cobalt oxide, Co₃O₄, when subjected to the humid environment, whilst a small amount of oxide layers was only observed within the wear grooves under RH 5% testing condition. The stripe test results unraveled the evolution of this protective oxide generation, and the FIB/SEM of the cross-sections at different sliding stages bore out the role of tribochemistry for triggering such self-protection behavior. Our work provides a fundamental understanding of the wear mechanisms of Co metal, and we anticipate that this finding can offer valuable guidance for further improving the wear performance of cobalt-based alloys in the future.

In response to the issues of differences significantly in service stages, performance, and materials used in current-carrying tribo-pairs across different fields. This paper utilizes a self-made micro-sliding current-carrying friction test machine to study the evolution of surface damage during the current-carrying friction process. The results indicate that during all the testing processes, the contact resistance experiences four stages: the smooth surface stage, the transition stage, the optimal friction surface stage, and the failure stage. Variations in load can affect the duration of these stages. The smooth surface stage and optimal friction surface stages exhibit good electrical conductivity properties. The number of cycles during the smooth surface stage decreases with increasing load, reaching a maximum of 518 cycles at a wire diameter of 0.4 mm and a load of 0.025 N. The number of cycles during the optimal friction surface stage increases with increasing load, reaching 9955 cycles at a wire diameter of 1.0 mm and a load of 3.2 N. From the perspective of damage, intense electric arc erosion significantly impacts the current-carrying friction process, and it should be avoided throughout the entire service life of the friction pair. From an engineering perspective, the service stages of "short" lifespan friction pairs such as electrical connectors should correspond to the smooth surface stage. For "long" lifespan friction pairs such as pantograph strips and brushes, the service stages should correspond to the optimal friction surface stage, note that the friction pairs should be run-in.

In this study, Cerium oxide (CeO₂) nanoflowers were uniformly grown on the surface of chromium aluminum carbide (Cr₂AlC) particles via a simple and efficient co-precipitation approach, which resulted in the preparation of the hybrids referred to as Cr₂AlC@CeO₂. The CeO₂ nanoparticles exhibited a capacity to alternate between the oxidation states of Ce³⁺ and Ce⁴⁺ under stress, forming a protective layer to repair damaged surfaces and reduce friction and wear at the nanoscale. The Cr₂AlC@CeO₂ hybrids were utilized to enhance the tribological performance of carbon fiber (CF) and polytetrafluoroethylene fiber (PTFE) blended fabrics (CF/PTFE fabric) phenolic composites, and the friction test indicated that when the filler content reached 4.0 wt%, the wear rate of the fabric composites was 2.79×10^-14 m³ (N·m)^-1, which was 59% lower than that of the pure composites, and the coefficient of friction was decreased by 39%. This enhancement was attributed to the formation of an adaptive tribofilm with a thickness ranging from 85 nm to 113 nm on the corresponding surface. The analysis of the worn surface and the tribofilm revealed a synergistic enhancement effect of Cr₂AlC and CeO₂. The Cr₂AlC@CeO₂ reinforced fabric composites (Cr₂AlC@CeO₂/fabric composites) exhibited the best wear resistance due to the superior load-bearing capacity of Cr₂AlC and the outstanding lubricating properties of CeO₂.

The underwater surface haptics based on friction modulation provide new concepts for immersive extended reality haptic technology. The previous study proposed a modulation method by using voltage-controlled technique in saline solutions, demonstrating that finger friction was actively adjusted by external voltages. This work further investigated the voltage-controlled finger friction on 304 stainless steel plates with two surface roughness, in sodium dodecyl sulfate (SDS) solutions with three concentrations. By changing the roughness, the effects of both increasing friction and reducing friction were observed. This can be attributed to the competitive mechanisms between the adsorption/desorption of surfactant and the formation/dissolution of metal oxide film. This study improves the understanding of the mechanisms of finger friction based on voltage-controlled technique and verifies the feasibility of underwater friction modulation.

Hexagonal boron nitride (h-BN) nanosheets are widely used as key additives to enhance the performance of lubricants under harsh working conditions due to their excellent high-temperature resistance, good chemical stability, and layered structure similar to graphene. Recently, it has been found that diurea modification can endow h-BN nanosheets with unique ultrasonic-induced thickening properties, enabling their use in the formulation of ultrasonic-responsive nanogel lubricants. However, the mechanism of diureas in regulating the thickening properties and lubrication performance of h-BN nanosheets is still not well elucidated. Therefore, in this paper, h-BN nanosheets were modified with three diureas with straight-chain structures, and the effects of chain-length on the ultrasonic-induced thickening capabilities and lubricating properties in polyalphaolefin 8 (PAO8) were investigated. It was found that the thickening capabilities of modified h-BN nanosheets mainly depended on the interaction between the diureas and PAO8. Tribological tests showed that the nanogel formed by the modified h-BN nanosheets under sonication exhibited better lubrication performance than PAO8 under startup, varying speeds, and heavy load conditions. The high fluidity and high h-BN content were found to be essential for attaining optimized lubrication performance of h-BN nanogels.