Ming Ma

Conventional iron‐based materials suffer from severe wear and oxidation over a wide temperature range, compromising their service reliability. In this work, iron‐based self‐lubricating composites containing graphite and PbO were fabricated via spark plasma sintering, and their tribological behavior was systematically investigated from room temperature to 400 °C. The results show that the composites exhibit high densification, while the combined incorporation of graphite and PbO markedly affects their microhardness and tribological behavior. At room temperature, the composite containing 5 wt% lubricant exhibits the best friction‐reducing performance under loads of 5, 10, and 15 N, owing to the formation of an effective tribofilm on the wear track. At 400 °C, the composite containing 4 wt% lubricant achieves the lowest coefficient of friction and wear rate. A continuous and compact graphite, Pb‐containing phases, and oxides tribofilm forms at high temperature, reducing material loss and promoting a transition from abrasive and adhesive wear to mild oxidative and abrasive wear. This study shows that composite tribofilm formation can effectively suppress wear of iron‐based materials over a broad temperature range.

This study presents a numerical framework for analyzing mixed elastohydrodynamic lubrication (EHL) of rough contact surfaces under grease lubrication, motivated by the need for reliable lubrication design in tribological components such as mechanical face seals and rolling bearings operating in space environments. Considering surface roughness effects, the proposed method integrates an EHL model derived from the Ostwald constitutive equation with the Kogut–Etsion (KE) elastic–plastic asperity contact model. The methodology is demonstrated through a case study of a vacuum grease-lubricated double-row angular contact ball bearing employed in a spacecraft antenna rotation mechanism under low-speed and heavy-load conditions. The governing equations were non-dimensionalized and solved numerically to obtain the lubricant film thickness and pressure distributions under various rotational speeds and axial preloads. The friction torque generated by viscous shear of the lubricant and asperity contact, and the asperity load ratio, were also determined. The novelty of this work lies in two aspects: (i) the integration of the Ostwald grease rheology model and the KE elastic–plastic asperity contact model into a unified mixed EHL framework; and (ii) a systematic investigation of grease-lubricated bearing behavior at low rotational speeds (11.5–55.2 rpm) with explicit consideration of surface roughness. The results indicate that rotational speed and axial preload exert limited influence on film thickness. The film pressure along the rolling direction increases with speed, whereas the asperity contact pressure decreases with speed and increases with preload; the asperity load ratio follows the same trends. The fluid pressure exhibits a single peak on the inlet side of the contact, and no outlet film constriction is observed in the thickness profile. The friction torque decreases with increasing speed, and the asperity load ratio follows the same trend. These findings demonstrate that appropriate adjustment of preload and an increase in rotational speed can reduce both friction torque and the asperity load ratio, thereby improving lubrication conditions and extending bearing service life.

While ion correlation is known to enhance molecular-scale solvation forces, its capacity to improve macroscale boundary lubrication on engineering surfaces remains unverified. This study demonstrates that multivalent electrolyte-induced ion correlation significantly reduces macroscopic boundary friction, achieving up to a 67% reduction on alumina surfaces. This macroscopic enhancement is driven by interfacial electrochemical properties, where highly charged polar oxide interfaces trigger strong electrostatic correlation to restructure confined solvents into a rigid, load-bearing barrier. Ultimately, this work proves that interfacial ion correlation directly dictates and enhances macroscale boundary lubrication, bridging molecular-level force control with practical tribological applications.

Interfacial contaminants remain a major technical challenge for van der Waals (vdW) junctions, severely degrading device performance. Given the inherent difficulty of fabricating contaminant-free vdW interfaces, developing effective postfabrication cleaning technologies is crucial for restoring interfacial quality. Existing technologies include annealing, which results in incomplete removal, and AFM-tip squeezing, which is time-consuming and costly. Here, we invent a complete, rapid, and cost-effective electric-field-assisted cleaning method that simultaneously delivers high spatial and temporal efficiency. For a microscale graphite interface, this method removes over 95% of the contaminant coverage within just 20 ms, restoring the interfacial conductivity to its clean-state level without introducing defects. Compared with previous methods, our method achieves a comparable or higher level of contaminant removal while reducing the required time by at least 4 orders of magnitude. We further demonstrate its practical applicability using a microscale superlubric Schottky generator, which transitions from an insulative to a generating state after cleaning. Importantly, the interfacial cleanliness level achieved by our method satisfies U.S. cleanliness standards for aerospace, semiconductor manufacturing, and medical devices, suggesting its broad applicability across multiple industries.

Sliding electrical contact (SEC) is an important type of electromechanical coupling device where achieving ultralow current-carrying friction and wear under high current density and maintaining excellent conductivity simultaneously has great engineering importance. However, interface electronic coupling and arc erosion under high current density often increase friction and wear. In this work, ultralow wear rate of 1.2×10 ⁻⁸ mm ³ /N·m under current density of 2.5×10 ⁻⁷ A/m ² is achieved during 850 km sliding with velocity of 0.68 m/s, accomplished by single crystalline graphite (SCG) and Au tribo-pair. With in-situ observation and extensive characterization, formation of transfer film from SCG onto counter-pair is revealed to be the primary mechanism for ultralow current-carrying wear rate. Superlubricity (coefficient of friction about 0.007) and low contact resistivity (5.5×10 ⁴ Ω·μm ² ) are also achieved. Lastly, critical current density and normal pressure for maintaining superlubricity in our transfer film system are obtained. Our work provides a promising path to realize device-level ultralow wear under high current density.

The interfacial distance, d, in van der Waals (vdW) junctions reflects the nature of interfacial interaction and plays a crucial role in determining the performance of vdW devices. However, spatially resolved mapping of d remains limited, as existing techniques provide cross-sectional or line-profile data, risk tip-induced damage, or lack quantitative resolution. Here, we introduce an optical interferometry-based method to spatially map d, enabling in situ, real-time, and nondestructive characterization with subnanometer resolution. The method is demonstrated by tracking variations of d during the approaching of a NbSe₂/graphite heterojunction. Also, when applied to a sliding graphene/graphite junction, this method directly visualizes the confined motion of nanoscale third bodies, providing microscopic evidence of anisotropic diffusion during sliding and offering unprecedented experimental insight into the diffusion process. In addition, the interfacial contact states of the sliding junction are resolved, enabling the predictive evaluation of friction, wear, and stress distribution, which are key issues in mechanics and tribology.

When contacting solids slide against each other, friction typically increases as material stiffness decreases due to the resulting increase in real contact area. However, varying trends have been observed, influenced by wear, surface geometry, tribochemistry, and environmental factors. To disentangle these effects, we construct a microscale structural superlubric system composed of graphite slider and graphene substrate, featuring wearless, fully contacted, atomically smooth, and chemically stable interfaces. The normal stiffness is tuned using few-layer graphene with different thicknesses. Experiments show that, for the system with a clean interface, friction remains constant across substrates with varying stiffness. By contrast, a positive stiffness dependence of friction is observed in third body-rich interfaces. A comprehensive framework is provided, clarifying fundamentally how friction and its stiffness dependence originate from the configuration transition of third body molecules and symmetry of the stiffness for the two contacting surfaces, corroborated by molecular dynamics simulations.

Carbon nanomaterials exhibit unique electrokinetic phenomena due to rapid ion transport within the Debye layer, which have been exploited for energy conversion, membrane technology, and liquid lubrication. The electronic properties of solids have been found to influence water permeation and proton transport; however, their effect on ion transport has not been observed. Here, we present an experimental investigation of ion transport in individual double-walled carbon nanotubes (CNTs) of both semiconducting and metallic nature. Systematic measurements show that conductance, streaming current, and osmotic current are larger in semiconducting tubes than in metallic ones. Together with a complete theoretical framework, we found that such behavior is caused by the smaller liquid-solid friction with the same surface charge density for the semiconducting system. As fast ion transport is the key element for efficient energy conversion, in CNTs, the thermoelectric conversion efficiency with ions is two orders of magnitude larger than with electrons, showing the supremacy of ions to recover the waste heat.

Gold thin films play a vital role in a wide range of applications due to their outstanding electrical conductivity, chemical stability, and unique optical properties. They are indispensable in electronic and optoelectronic devices where achieving low contact resistance and high durability is critical. In this study, a technique for fabricating transferable ultrasmooth gold thin films is introduced, in which graphite is used as a mask. Ultrasmooth areas up to 400 μm² are created—two orders of magnitude larger than those achieved by previous techniques. A surface roughness of R_a 0.05 nm and a maximum peak height of 0.20 nm are achieved, representing the lowest values reported to date. Consequently, when forming contact with a graphite flake of 6 μm in size, the actual contact area and conductance are increased by 5 times. By sliding against a SiO₂ microsphere covered with diamond-like carbon (DLC), the friction force is decreased by 50%, and the maximum wear scar depth is reduced by an order of magnitude under a contact stress of 310 MPa. This method is straightforward to execute, cost-effective, and versatile, making it suitable for the fabrication of a wide range of metal films in electrical, tribological, and optical applications.

Structural superlubricity (SSL), a state of ultralow friction and no wear between two solid surfaces in contact, offers a fundamental solution for reducing friction and wear. Recent studies find that the edge pinning of SSL contact dominates the friction. However, its nature remains mysterious due to the lack of direct characterizations on atomic scale. Here, for microscale graphite mesa, we unambiguously reveal the atomic structure and chemical composition of the disordered edge. The friction stress for each contact condition, namely, edge/edge, edge/surface, and surface/surface contacts are quantified, with the ratio being 10⁴:10³:1. The mechanism is revealed by all-atom molecular dynamic simulations, which reproduce the measured friction qualitatively. Inspired by such understanding, through fabricating Si_xN_y caps with tensile stress, we further eliminate the friction caused by the edges through disengaging the edges from the substrate. As a result, an SSL contact with ultralow friction stress of 0.1 kPa or lower is achieved directly.