[Significance] Rapid expansion of global rail transit requires higher operating speeds for high-speed trains, posing considerable challenges to the safety and reliability of braking systems, particularly under demanding conditions such as long, continuous, steep slopes. In such scenarios, stable speed regulation requires prolonged mechanical friction braking to complement electrical braking. This extended braking action causes a rapid temperature increase at the brake disc/pad interface, and this temperature often reaches extreme levels. Such thermal stress leads to significant degradation in braking performance and reduced reliability of brake components, with a greater risk of brake failure. Despite the critical nature of these issues, research on the tribological behavior and dynamic responses of braking systems under prolonged slope conditions remains insufficient. [Progress] This review synthesizes experimental studies and numerical simulations to elucidate the mechanisms underlying braking system performance degradation on long steep slopes. Experimental research reveals wear patterns and damage behaviors of friction pairs during prolonged braking, highlighting the roles of heat accumulation and friction reduction at the brake disc/pad interface. Fully coupled thermal-mechanical-wear finite element models have been employed to explore the interrelated effects of temperature, stress, and wear throughout the braking process. Lumped parameter models offer a detailed characterization of contact behaviors in friction pairs and their impact on dynamic system responses, incorporating principles from fractal theory and Hertz contact theory to develop mathematical models for contact stiffness and damping. Additionally, two-degree-of-freedom models have been utilized to analyze braking system stability under realistic operational conditions. Furthermore, dynamic models incorporating wheel/rail adhesion have been developed to examine the coupled torsional interactions between the brake disc/pad subsystem and the wheel/rail subsystem, as well as the impacts of the interactions on system vibration behavior. These models also assess the influence of diverse service conditions, brake disc/pad friction properties, and wheel/rail adhesion characteristics on system stability and vibration dynamics, thereby revealing the interaction mechanisms among various components of the braking system. [Conclusions and Prospects] Future research should account for complex environmental factors encountered at high altitudes and on steep slopes and elucidate the mechanisms of braking degradation under multi-factor coupling. Particular attention should be given to the effects of low temperatures, snow, and low pressure on the performance of friction pairs. Moreover, the integration of intelligent monitoring and predictive technologies will be crucial for developing efficient real-time monitoring systems capable of dynamically assessing braking performance and identifying potential failure risks at an early stage. These advancements will enhance the safe operation of trains under challenging conditions and provide a robust theoretical and technical foundation for improving the braking performance and safety of high-speed trains while contributing to the sustainable growth of the rail industry.

[Significance] This review highlights the progress and challenges in chip atomic layer polishing. Chips are fundamental to the modern information society. According to Moore's Law, the chip feature size is shrinking and approaching the physical limit. At the same time, advanced packaging technologies such as hybrid bonding continuously evolve. These create pressing needs to develop atomic layer polishing, a technique that enables extremely precise material removal at the atomic layer level, to achieve surfaces with atomic-level precision for demanding processes such as photolithography and bonding. Currently, chemical mechanical polishing (CMP) is the only key technology in chip manufacturing capable of simultaneously achieving local and global planarization of the wafer surface, with the potential to realize atomic layer polishing. This review provides a systematic summary of the mechanisms and processes of CMP for chip substrate surfaces and interconnect heterogeneous surfaces. [Progress] Significant progress has been made in the controlled removal with a single atomic layer precision at the microscopic level and in the CMP with surface roughness close to the theoretical limit at the macroscopic level for the monocrystalline silicon substrate. These advances highlight the extreme precision processing capability of CMP for the wafer surface. Furthermore, ongoing developments in multi-field assisted CMP and energy particle beam polishing hold promise for enabling atomic layer polishing for new substrates like GaN, SiC and diamond. In the case of interconnect heterogeneous surfaces, two material removal modes in CMP are summarized from a tribological perspective based on the interactions between the abrasive and material surfaces: mechanical plowing and chemical bonding. Copper, cobalt, and nickel are mainly removed through the mechanical plowing mode, while tantalum, ruthenium, and titanium are mainly removed through chemical bonding. According to this foundation, control principles and methods for achieving equivalent removal of heterogeneous surfaces are proposed based on different material removal modes. In the mechanical plowing material removal mode, corrosion and its impact on the mechanical strength of the material surface can be adjusted through the modulation of the effects of oxidation, complexation, and corrosion inhibition, as well as their synergistic effects. This approach allows for controlling the material removal rate (MRR). In the chemical bonding material removal mode, the number of reactive sites and their influence on interfacial chemical bonds can be regulated through the adjustment of pH, oxidation, and ionic strength, along with their synergistic effects, thus controlling the MRR. According to these control principles, a systematic summary of the planarization processes for interconnect heterogeneous surfaces, such as copper/tantalum, copper/cobalt, and copper/ruthenium, is provided. Finally, based on existing research progress, it is proposed to leverage the synergistic effects of mechanical, chemical, and electrical/optical/plasma/energy beams to confine chemical reactions and mechanical-chemical reactions to the surface atomic layer, to achieve atomic layer polishing. Specifically, for the mechanical plowing material removal mode, the focus is on designing the molecular structures of chemical additives to precisely modulate the effects of oxidation, complexation, and corrosion inhibition. This approach confines the corrosion behavior to the outermost atomic layer while controlling the mechanical action of the abrasive to enable precise, controllable atomic layer removal. For the chemical bonding material removal mode, the investigation focuses on confining chemical bonding reactions to the outermost atomic layer, weakening back bonds, and simultaneously controlling the mechanical action of the abrasive to disrupt chemical bonds between the outermost and sub-surface atoms, thus achieving controlled atomic layer removal. [Conclusions and Prospects] This review highlights the growing demand for atomic-level manufacturing of high-end chips and the development of atomic layer polishing. It provides a systematic summary of the mechanisms, progress, and challenges in atomic layer polishing, with the aim of offering critical theoretical and technical support for the atomic-level manufacturing of advanced chips. The findings from this research have potential applications in key areas such as high-end optical components and superlubricity devices.

[Significance] The tribological properties of ceramic materials are crucial for the long-term reliability of ceramic components. Understanding the friction and wear mechanisms of ceramics is essential for designing, optimizing, and improving the operating performance. Numerical simulation methods, because of their low cost and high efficiency, are valuable for analyzing tribological behavior. They allow for real-time analysis of stress, temperature, cracks, and molecular motion during friction and wear. These capabilities make numerical simulations a widely discussed approach in tribology research. However, most studies on the tribological behavior of ceramics using simulations remain fragmented and lack systematic induction and summary. [Progress] This paper categorizes numerical simulations of ceramic tribological behavior into three main methods: finite element method (FEM), molecular dynamics (MD), and discrete element method (DEM). The applicable scenarios, research status, and limitations of each method are reviewed. FEM uses mathematical approximations to solve differential equations, simulating real-world physical systems. Initially, it was applied to study elastic stress distribution on ceramic surfaces during friction, serving primarily as an experimental support tool. Over time, FEM has advanced to incorporate surface fracture analysis, thermomechanical coupling, and wear modeling. Recent developments allow FEM to investigate subsurface crack initiation, crack propagation, and temperature distribution at friction interfaces under high-stress conditions, such as those in ceramic cutting tools and machining. Furthermore, FEM-based wear models can quantitatively estimate the wear volume of ceramic surfaces; however, they are highly dependent on experimental data, limiting their general applicability. MD simulations, based on Newton’s laws of motion, track the trajectories of atoms and molecules during ceramic friction and wear processes by modeling interatomic interactions. This method provides a detailed view of the microfriction and wear mechanisms in ceramics. However, current research is primarily focused on SiC ceramics, with limited research on other ceramics. DEM simulations model ceramics as a collection of discrete elements and predict their tribological behavior based on interactions between these elements. This approach overcomes the continuous medium assumption and provides insights into microcrack initiation and propagation during ceramic friction and wear. However, its application is limited, primarily focusing on ceramic cutting tools and grinding wheels. [Conclusions and Prospects] Numerical simulations are crucial for understanding the tribological behavior and mechanisms of ceramic materials and components. While its use is increasingly widespread, existing studies often focus on specific scales and boundary conditions, hindering a comprehensive understanding of the tribological mechanisms of ceramics. Moreover, a single numerical simulation method cannot completely account for the complex physical and chemical boundary conditions involved. Therefore, the development of multiscale, multifield simulation methods is essential. Additionally, tribological information methods based on machine learning and artificial intelligence can enhance data correlations, improve empirical parameter exploration, and accelerate numerical simulations with approximate calculations. Integrating these advanced techniques with traditional numerical methods can create more efficient and innovative computational tools for ceramic tribology.

[Significance] The capture, transport, and collection of underwater methane and other fuel gases are essential for addressing global environmental and energy challenges. Methane, a potent greenhouse gas, has a global warming potential that is 25 times greater than CO₂, making underwater methane leaks a severe threat to climate stability and global health, and a challenge to China's dual carbon targets. In addition, as the US, Europe, and Japan advance their strategic goals for ocean exploration robots, China urgently needs to develop its underwater robots. Current equipment, reliant on cables and/or batteries limits endurance, Nonetheless, capturing underwater fuel gases offers opportunities for energy self-sufficiency and extended operational capabilities. The capture and utilization of underwater methane and other gases are vital for reducing greenhouse gas emissions, promoting environmental health, addressing energy shortages, and enhancing the endurance of underwater equipment. [Progress] Recent advances in bubble capture, transport, and collection stem from interdisciplinary research merging micronanotechnology, material science, and fluid mechanics. Researchers have employed noncontact techniques, including electric fields, magnetic fields, and sound waves, to improve bubble stability and optimize their movement. Studying bubble physicochemical properties has helped overcome challenges such as rupture, coalescence, and trajectory oscillations caused by external disturbances, including fluid flow and temperature changes. Micronanotechnology has enabled precise manipulation over bubble interfacial behavior by leveraging surface structures and interfacial energy. Techniques such as using hydrophobic surfaces and capillary forces have improved bubble capture, whereas microstructured surfaces and optimized fluid channels allow precise, efficient transport. Advanced materials, including responsive polymers, further improve dynamic control of bubble flow paths, increasing overall efficiency. Notable progress has been made in gas collection. Porous materials and functionalized membranes now enable efficient gas separation and aggregation. Biomimetic structures inspired by natural systems, along with superhydrophobic surfaces, have improved bubble capture and stability, presenting promising solutions for integrated gas recovery systems. [Conclusions and Prospects] Despite these advancements, considerable challenges remain. Bubbles in underwater environments are highly vulnerable to external disturbances, making their stable capture and efficient transport difficult. Furthermore, interactions between bubbles of varying sizes during transport can reduce separation efficiency and directional control, whereas inconsistent aggregation during collection further limits overall efficiency. Future research should address these challenges by integrating nanomaterials and advancing interfacial modification techniques for improved selectivity and precision of bubble capture in complex environments. Analyzinging the relationship between bubble properties and environmental factors through simulations and experiments can refine strategies for trajectory control, size classification, and stability. Moreover, the development of novel materials, including superhydrophobic and multifunctional surfaces, combined with innovations in external field applications (electric, magnetic, and optical), offers tremendous potential to revolutionize underwater gas recovery systems. These approaches, combined with advancements in theoretical models and experimental techniques, hold the promise of groundbreaking improvements in the efficiency and controllability of gas capture, transport, and collection processes. These efforts will support sustainable energy utilization and contribute to mitigating climate impacts and advancing ocean exploration technologies.

[Objective] As integrated circuit (IC) technology progresses to 7-nm nodes and beyond, cobalt (Co) has emerged as a promising substitute for copper (Cu) in interconnects. This shift is driven by Co's shorter electron mean free path, excellent electromigration resistance, and superior deposition characteristics. This study addresses the challenges associated with the selective removal of Co and titanium (Ti) barrier layers during the chemical mechanical polishing (CMP) process while also achieving global wafer planarization. By optimizing the functional groups in the slurry, the process enhances the selectivity of the removal process. [Methods] This research integrates CMP experiments with electrochemical tests, static etch experiments, and nanoscratch tests to analyze the removal behaviors of Co and Ti within Co interconnect heterostructures. Electrochemical tests are used to assess the impact of chemical reactions on material removal, while nanoscratch tests evaluate the mechanical strength of wafer surfaces after chemical exposure. The study focuses on the role of complexing agents containing amino (—NH₂) and carboxyl (—COOH) functional groups in improving removal efficiency. By correlating electrochemical data with removal rates, the action mechanisms of these functional groups in the slurry are explored. Additionally, the study analyzes how adjusting the abrasive concentration in the slurry affects Ti removal efficiency. [Results] Experimental results demonstrate significant differences in the removal mechanisms of Co and Ti. Co removal is predominantly driven by chemical corrosion, significantly accelerated by amino functional groups. Mechanical action also plays a role, contributing to rapid Co removal. This behavior is attributed to strong complexation reactions between Co ions and amino groups, facilitating Co dissolution and enhancing the chemical corrosion process. Conversely, carboxyl functional groups have a relatively minor impact on Co removal. Ti removal is primarily led by mechanical action, with chemical corrosion playing a minor role. Increasing the abrasive concentration in the slurry significantly enhances the Ti removal rate. The study confirms that by optimizing the types and concentrations of functional groups in the slurry, selective removal of Co and Ti can be effectively controlled. A comprehensive database has been developed documenting the specific effects of different amino and carboxyl groups under various conditions. Furthermore, the study validates a proposed strategy for controlling the removal rates of heterogeneous Co/Ti structures through patterned wafer experiments. These experiments explored time thresholds for the bulk and barrier CMP processes. The process parameters for the two-step polishing of Co interconnect wafers were optimized, achieving a defect-free Co interconnect structure. During the bulk CMP step, —NH₂ group-based agents were employed, with polishing times controlled between 1.5 and 2.5 min to prevent excessive dishing. Simultaneously, reducing the abrasive concentration lowered the Ti removal rate, further optimizing selectivity between different materials and ensuring superior surface quality. For the barrier CMP step, complexing agents containing an appropriate amount of —COOH groups were used, with polishing times around one minute. An increase in abrasive concentration enhanced the mechanical action of Ti removal. [Conclusions] This optimization strategy not only reduces the Co removal rate but also increases the Ti removal rate, effectively minimizing height differences between material interfaces and achieving the desired planarization effect during the polishing process. These findings provide valuable insights into the removal mechanisms of Co and Ti in the CMP process. They also establish effective strategies for enhancing selectivity and overall process efficiency, offering a theoretical framework for tailoring CMP slurry formulations to meet the specific requirements of advanced IC manufacturing.

[Objective] Conductive slip rings, essential components in rotary electrical systems, often experience contact instability due to surface ablation because of electrical arc discharge. This study examines how the position of ablation pits influences contact stability within slip rings, highlighting their detrimental impact on electrical performance. By integrating multiscale characterization, it explores the structure, composition, and properties of ablation pits formed under operational conditions. The findings aim to deepen the understanding of their effects and identify strategies for mitigating their impact. [Methods] To investigate the positional dependence of ablation pits and their impact on electrical contact stability, this study employs a comprehensive multitechnique approach. The surface topography and morphology of the ablation pits are characterized using surface profile measurements. This technique provides high-resolution data on pit depth, width, and overall surface texture. Nanoindentation tests evaluated hardness and elastic modulus variations across different pit regions within the ablation pits, identifying localized changes caused by arc discharge. Raman spectroscopy and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analyses examined the chemical and structural alterations within the pits. Raman spectroscopy detected molecular-level alterations, such as the presence of graphitic or disordered carbon, whereas SEM-EDS offered data on elemental compositions. Conductive atomic force microscopy (C-AFM) measured electrical conductivity variations across different pit regions, linking material changes to the slip ring performance. By combining these techniques, the study provides a thorough examination of the effects of ablation on the mechanical, chemical, and electrical properties of the slip ring material. [Results] The results demonstrate a clear positional dependence of the ablation pit characteristics, with significant variations in morphology and material properties across different regions. Surface profiling showed that pits in the central area were deeper and more defined than the outer regions that appeared shallower. Nanoindentation results indicated high hardness and elastic modulus in the pit center and inner ring regions, suggesting localized transformation of the material owing to high-temperature arc discharge. Conversely, the outer regions exhibited low hardness, indicative of extensive material degradation. Raman spectroscopy results highlighted the presence of disordered and graphitic carbon deposits in the inner and central regions of the pits, further contributing to high local conductivity. These findings were supported by C-AFM measurements, which confirmed considerably increased conductivity in the central and inner regions owing to carbonaceous deposits formed during the discharge process. Finally, SEM-EDS analysis reveals compositional gradients within the pits, with high concentrations of carbon and oxygen near the center and copper depletion toward the edges, suggesting complex interactions between copper, carbon, and oxygen during ablation. [Conclusions] Ablation pits introduce mechanical and electrical heterogeneities, remarkably influencing contact stability. The positional differences in pit properties are directly linked to the arc discharge process, driving localized surface modifications and material transformations. The study highlights the complex interplay among mechanical properties, electrical conductivity, and material composition within the ablation pits, offering valuable insights into the mechanisms underlying contact instability in conductive slip rings. The results improve the understanding of surface ablation and inform material design and operational strategies for mitigating its adverse effects. Addressing the challenges posed by ablation pits plays a key role in advancing the performance and reliability of conductive slip rings in demanding, high-performance applications.

[Objective] Droplet microarrays are widely used in electronic device manufacturing, high-throughput cell screening, and microsensors. Unlike traditional micropipetting techniques, the confined adsorption method—using a hydrophilic/hydrophobic patterned substrate to generate droplet microarrays—offers higher efficiency through parallel processing. In this method, specific regions of a substrate are modified to be hydrophilic using methods like plasma treatment or vacuum ultraviolet irradiation, while other regions are made hydrophobic via self-assembled monolayer films. The substrate is then immersed in a liquid and withdrawn at a constant speed, causing the liquid to selectively adsorb onto the hydrophilic regions, resulting in droplet microarrays. For droplets in equilibrium with a specific hydrophilic surface, the maximum height is sufficient to fully characterize the droplet's shape and volume. Thus, controlling the maximum height allows for the regulation of droplet shape and volume. However, accurately quantifying the maximum height of confined liquid adsorption droplets is challenging, both theoretically and experimentally, because of the multiscale dynamics involved in the adsorption process. These challenges include issues such as three-phase contact line pinning and sliding, along with the extremely small contact angle at the droplet's edge, which leads to substantial fast evaporation losses. Although precise theoretical models and experimental data are available for idealized cases—such as infinitely large hydrophilic areas or infinitely long hydrophilic lines—these do not apply to more general scenarios. Specifically, when the lengths and widths of hydrophilic regions are comparable and both are smaller than the capillary length, no sufficiently accurate theoretical model exists. The lack of a precise model limits the systematic control of the maximum height of droplets in microarrays formed on hydrophilic regions. To address this gap, we developed a theoretical model and conducted numerical analysis to explore the maximum height of adsorbed droplets within hydrophilic regions smaller than the capillary length. [Methods] The confined adsorption process on hydrophilic/hydrophobic substrates is numerically modeled using phase-field dynamics and lubrication approximation methods. Experimentally, the maximum height of the confined adsorption droplets is determined by measuring the residual surface morphology of the solid after drying the solution. High-speed imaging captures the liquid adsorption dynamics on hydrophilic patterns. Finally, the theoretical and experimental results are compared qualitatively and quantitatively. [Results] The comparative results show that both the lubrication approximation and phase-field methods effectively simulate the two stages of liquid adsorption in hydrophilic regions. Compared with the phase-field method, the lubrication approximation method more accurately characterizes liquid bridge breakup and satellite droplet formation. Conversely, the phase-field method clarifies the relationship between the maximum droplet height and capillary number, revealing the weakening effect of viscous forces on droplet formation during confined adsorption. [Conclusions] These findings offer valuable insights into the precise regulation of the maximum droplet height in confined adsorption-generated droplet microarrays.

[Objective] Molybdenum disulfide (MoS₂) is a multifunctional material primarily used in lubrication, electronics, and catalysis. MoS₂ films are widely utilized in the aerospace industry due to their excellent lubrication properties. These films are applied in aircraft landing gear, engine components, and moving parts of spacecraft to ensure efficient operation and minimize frictional wear. However, under high-temperature conditions, MoS₂ films are susceptible to oxidation into molybdenum trioxide, significantly degrading their lubricating performance and restricting their applicability in high-temperature environments. [Methods] Herein, MoS₂ films were enhanced by doping them with amorphous carbon to improve their mechanical properties and high-temperature tribological performance. Using direct-current magnetron sputtering, medium-frequency magnetron sputtering, and high-power pulsed composite sputtering techniques, MoS₂-C composite films were fabricated. The effects of doping amorphous carbon and its concentration on the microstructure, mechanical properties, and tribological performance of MoS₂ films were thoroughly investigated. [Results] The results revealed that the MoS₂-C composite films exhibited a preferential orientation of the (002) crystal plane. Amorphous carbon incorporation into the MoS₂ matrix resulted in a dense and uniform structure while reducing surface roughness. This structural modification enhanced the mechanical and tribological properties of the films. Doping MoS₂-C composite films with an optimal amount of amorphous carbon significantly improved their mechanical properties. Their nanohardness and elastic modulus reached 5.50 and 82.53 GPa, respectively, while substrate adhesion strength increased to 8.30 N, approximately 3.6 times higher than that of pure MoS₂ films. These improvements suggest that amorphous carbon addition enhances the mechanical strength and durability of the films. At room temperature, both MoS₂ and MoS₂-C composite films exhibited poor tribological performance, primarily due to the infiltration of moisture molecules from air into the MoS₂ interlayers. This results in MoS₂ oxidation, compromising the lubrication properties of the films. Meanwhile, the tribological performance of MoS₂-C composite films substantially improved in a vacuum environment, attributed to the isolation from oxygen, preventing oxidation and allowing the films to maintain their lubricating properties. Under high-temperature conditions (100 ℃-300 ℃), MoS₂-C films outperformed pure MoS₂ films by maintaining a lower friction coefficient. MoS₂-C films with 37.41% atomic percentage of carbon exhibited the lowest wear rate of 9.75×10^-8 mm/(N·m) while showing a friction coefficient of 0.008 at 200 ℃, which is the lowest value among all samples. Notably, at 300 ℃, pure MoS₂ films quickly failed due to oxidation, whereas MoS₂-C composite films retained a lower friction coefficient and longer wear life. This improvement is primarily attributed to the incorporation of carbon, which effectively inhibits MoS₂ oxidation in high-temperature environments. [Conclusions] MoS₂-C composite films exhibit enhanced wear resistance and load-bearing capacity at elevated temperatures. These findings suggest that doping amorphous carbon into MoS₂ films significantly improves their tribological and mechanical properties, especially under high-temperature conditions. MoS₂-C composite films demonstrate excellent wear resistance and prolonged service life, making them promising for high-temperature lubrication applications. By optimizing the carbon content, it is possible to further enhance the high-temperature lubrication performance of MoS₂ films while maintaining their excellent mechanical properties. This provides new possibilities for developing advanced tribological coatings that effectively perform under harsh operating conditions.

[Objective] The water-lubricated tail bearing is a critical component of a ship's propulsion system. Its stability and reliability significantly affect the safety of ship operations. Under low-speed, heavy-load conditions, forming a stable hydrodynamic lubrication water film becomes challenging, often resulting in poor lubrication. This can cause micro-defects on the composite material's surface. To address this, microcapsules containing diisocyanate are incorporated into the composite material, enabling it to autonomously repair such micro-defects. This study explores how the mass fraction of self-healing microcapsules affects the self-repairing ability, mechanical properties, and tribological performance while also analyzing the underlying mechanisms. [Methods] Self-healing microcapsules containing active IPDI self-healing agents were prepared using a combination of Pickering emulsion and in situ polymerization methods. Composite materials infused with these microcapsules were then fabricated using hot pressing. The mechanical properties of the composites were analyzed using differential scanning calorimetry, dynamic mechanical analysis, and mechanical performance tests. Scratch tests were employed to assess the self-repairing capabilities of the composites, while an Rtec tribometer was used to evaluate their tribological properties. The worn surfaces were examined using a scanning electron microscope and laser confocal microscopy. [Results] The addition of self-healing microcapsules negatively impacted the mechanical properties of the composite materials as the microcapsule mass fraction increased. Specifically, the crystallinity of the composites containing 5%, 10%, and 15% microcapsules decreased to 9.87%, 12.37%, and 14.50%, respectively, compared to UHMWPE-1. The storage modulus decreased by 28.33%, 31.8%, and 38.61% while bending strength decreased by 13.56%, 18.29%, and 26.58%. When the microcapsule mass fraction exceeded 10%, the decline in mechanical properties accelerated. This was attributed to poor microcapsule dispersion of microcapsules within the matrix material content, which reduced rigidity and elasticity. Regarding self-repairing performance, the self-healing efficiencies of UHMWPE-I5, UHMWPE-I10, and UHMWPE-I15 composites reached 16%, 33%, and 78%, respectively. However, the tribological properties degraded under low-speed, heavy-load working conditions (Condition 2). Compared to UHMWPE-1, the average friction coefficients of UHMWPE-I5, UHMWPE-I10, and UHMWPE-I15 increased by 22.68%, 49.03%, and 101.72%, while wear volumes grew by 66.88%, 67.57%, and 73.42%. Additionally, higher microcapsule content led to more pronounced adhesive wear on the composite surface. Similarly to the mechanical properties, the decline in tribological properties intensified when the microcapsule mass fraction exceeded 10%. [Conclusions] This study analyzed the impact of self-healing microcapsules on composite material performance, focusing on mechanical properties, tribological behavior, and self-repairing ability. As the microcapsule mass fraction increased, the self-repairing performance improved significantly but at the expense of reduced mechanical and tribological properties. The optimal microcapsule mass fraction was identified as 10%, striking a balance between maintaining mechanical and tribological integrity and achieving effective self-repairing capabilities. These findings lay a solid experimental foundation for optimizing self-healing water-lubricated composite materials.

[Objective] Water-based lubrication has gained significant attention in tribology due to its availability, eco-friendliness, non-flammability, high thermal conductivity, and excellent cleaning properties. Replacing oil-based lubricants, which pose environmental risks, is an effective way toward achieving green tribology. However, water-based lubricants typically face challenges such as low viscosity, susceptibility to corrosion, and inferior lubrication performance. Water-soluble poly(ionic liquid)s, which combine the benefits of polymers and ionic liquids, offer potential as multifunctional water-based lubricant additives to enhance the physicochemical and tribological properties of water-based lubricants. [Methods] Through rational molecular structure design, we developed protic poly(ionic liquids) (PPILs) water-based lubricating additives, PPD-N, by combining a polymer chain visco-enhancing structure with a proton-type ionic liquids lubricating structure. PPD-N demonstrates excellent viscosity enhancement, corrosion resistance, and lubricating properties. Kinematic and dynamic viscosities of different water-based lubricating fluids were investigated at 25 ℃ and 40 ℃ using a Pinkevitch Viscometer and a rotational rheometer, with the commercial viscosity builder, Koreox W55000, serving as the control. Following the national standard GB/T 6144—2010, we evaluated corrosion inhibition performance on first-grade gray cast iron using immersion corrosion tests, comparing deionized water, 6% PPD-N, and Koreox W55000 aqueous solution. The friction reduction performance of PPD-N additives was assessed using the SRV-V tester, while its anti-wear properties were characterized using a fully automated real-color confocal microscope. Elastohydrodynamic lubrication properties of PPD-N were investigated by optical interferometry. The surface micromorphology of wear patches was observed using scanning electron microscopy. We also investigated the lubrication mechanisms of the additives using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). [Results] The results of ¹H NMR, FT-IR, GPC and TGA tests confirmed the successful synthesis of PPD-N, which demonstrated excellent thermal stability with 5% and 10% thermal decomposition temperatures of 249.7 ℃ and 268.9 ℃, respectively. Adding PPD-N can significantly improve the viscosity of water-based lubricants, with viscosity increasing proportionally to the PPIL additive content. PPD-N also effectively reduced the corrosion of cast iron sheets caused by water-based lubricants, outperforming commercial water-based viscosity builders at equivalent concentrations. At a 6% concentration, the PPD-N aqueous solution achieved a coefficient of friction and wear volume of 0.106 and 22.89×10^-5 mm³, respectively, a reduction of about 66% and 85% compared to water’s coefficient of friction (0.314) and wear volume (148.20×10^-5 m³). Elastohydrodynamic lubrication tests revealed that the PPD-N-containing aqueous solution increased the film thickness at the friction interface with rising velocity. Both the central film thickness and minimum film thickness at the lubricant outlet were significantly higher than those of deionized water. Based on XPS and ToF-SIMS analyses, the lubrication mechanism of PPD-N can be attributed to the formation of tribochemical reaction films and adsorption films at the friction interface. These films effectively prevent direct contact between friction surfaces, endowing the water-based lubricant with superior tribological performance. [Conclusions] Compared to the commercial viscosity builder, Koreox W55000, PPD-N additives demonstrate superior lubricity and anti-wear properties. They significantly enhance the viscosity of water-based lubricants and effectively inhibit cast iron corrosion in water. Free of phosphorus, sulfur, and halogens, PPD-N is simple to synthesize, environmentally friendly, and has great potential as a viscosity-building lubricant additive for non-flammable hydraulic fluids and fully synthetic water-based metalworking fluids.

[Objective] The control moment gyroscope (CMG) is a crucial actuator for spacecraft, such as space stations and high-resolution satellites, enabling attitude regulation and pointing control. The CMG offers high efficiency and generates significant output torque. It mainly consists of a high-speed rotor, a low-speed gimbal, and bearings. Microturbulence within the CMG, which limits the precision of spacecraft attitude control, arises from the flexible characteristics of the high-speed rotor and bearing wear failure. High-speed bearings, commonly used in spaceflight, operate under challenging conditions of time-varying load and speed, making them prone to wear and lubrication failure due to anomalous ball sliding. This is a major contributing factor to CMG failure and micro-vibration. Therefore, this study develops a CMG dynamics model that accounts for rotor flexibility, the coupling effect between the gimbal, bearing, and rotor, and bearing contact behavior. The model can be used to analyze the motion state of the balls and investigate the ball sliding mechanism. [Methods] The flexible shaft in the CMG is modeled as a Timoshenko beam unit, and a finite element dynamics model of the rotor is constructed, considering the rotor's flexible deformation, gyroscopic effect, and unbalanced forces and moments. According to the cylindrical coordinate system of the bearing, a set of dynamic equations is developed for the balls, raceways, and cage. The mass and gyro matrices are derived through the calculation of the kinetic energy of the CMG system using the Lagrange method. The nonlinear dynamics equations of the system are then obtained by coupling the bearing dynamics equations with the flexible rotor dynamics equations. In this model, the time-varying high-speed bearing support stiffness allows for real-time coupling between bearing analysis and system dynamics. An experimental platform is established for in-situ measurement of high-speed ball motion in the CMG, and binocular vision technology is employed to track the spatial motion of the balls. The model's accuracy is validated through experimental results. [Results] The model is further used to investigate the effect of rotor flexibility on the balls' behavior. Both low-frequency and high-frequency fluctuations define the dynamic response of a high-speed bearing's rolling element in a CMG. The variation period is half of the gimbal rotation period, and the low-frequency variations are mostly influenced by the gyroscopic torque generated by the gimbal rotation. High-frequency fluctuations are mainly influenced by rotor speed and bearing parameters, manifesting as the cage characteristic frequency. When rotor flexibility is considered, the gyroscopic moment is converted into a greater radial force. This change alters the dynamic behavior of the balls in both the “load-bearing” and “non-load-bearing” zones of the bearing, leading to more significant variations in the balls' contact force with the raceway and the contact angle. When rotor flexibility is considered, the spin-to-roll ratio of the inner raceway increases, and the pitch angles of the balls are smaller than those with a rigid rotor. [Conclusions] This study develops a CMG dynamics model that incorporates rotor flexibility, gimbal-bearing-rotor coupling effect, and balls' contact behavior to reveal the spatial motion characteristics of the high-speed bearing balls in the CMG. An experimental platform for in-situ measurement of high-speed ball motion in the CMG is constructed, and the ball's motion is measured using binocular vision technology. The accuracy of the theoretical model is confirmed. To further understand the spatial motion dynamics of the rolling body and explore bearing wear inhibition techniques, the impact of rotor flexibility on rolling body motion is also examined.

[Objective] Chemical mechanical polishing (CMP) is an ultra-precision machining technology for hard and brittle materials. The technology has garnered significant attention from researchers worldwide owing to its low cost of processing equipment and relatively simple operational process. In the CMP process, the polishing pad plays a critical role. It not only serves as the medium for storing the polishing slurry and abrasive particles but also transfers the processing load. This paper presents a fluid-structure coupling analysis framework to elucidate the fiber structure of the polishing pad and the interaction mechanism between the polishing slurry and the workpiece during the polishing process. [Methods] An innovative fluid-structure coupling analysis framework is proposed to investigate the interaction mechanism between the fiber structure of the polishing pad, the slurry, and the workpiece during the polishing process. Through comprehensive experimental verification and numerical modeling, the polishing efficiency differences between the ordered plain weave polishing pad and the disordered nonwoven polishing pad were systematically evaluated. To meet the ultra-precision processing requirements of fused silica materials, a green and environmentally friendly polishing slurry was specially developed. The slurry consisted of cerium oxide abrasive, hydrogen peroxide, guar gum, and deionized water. The composition of the polishing slurry and the polishing process were optimized through a single-factor test. Subsequent experiments were conducted on polishing pads with different textures (ordered plain weave fabric and disordered nonwovens) using the optimized slurry and process. The experimental procedure was further optimized through a single-factor test. [Results] The experimental results showed that the surface roughness (Sa) of the nonwoven polishing pad was as low as 0.181 nm, which was significantly better than that of the plain weave pad (0.486 nm). In addition, the thickness of the subsurface damage layer caused by the nonwoven pad was reduced to 4.14 nm, approximately one-third of that of the plain weave pad (about 12.12 nm). A comparison of the surface elements of fused silica before and after polishing revealed that the polished sample had no impurity residue on the surface after cleaning. The difference in fiber structure between the two polishing pads only affected mechanical removal during the polishing process but did not influence the chemical reaction. Furthermore, the fluid-structure coupling model analysis revealed that the nonwoven polishing pad exhibited a more uniform fiber stress distribution during polishing, with the maximum stress value being only about 0.5 MPa, considerably lower than the 6 MPa observed for the plain weave pad. In addition, the stress distribution of the slurry in the system was more random and uniform, which optimized the stress transfer and diffusion efficiency throughout the overall polishing system and promoted uniform material removal. [Conclusions] In conclusion, this paper highlights the advantages of the disordered nonwoven polishing system in the precision processing of fused silica from both experimental and numerical perspectives. It also provides valuable insights for the analysis, design, and manufacturing practices of complex polishing systems involving fiber aggregate polishing pads, slurry, and workpieces.

[Objective] Rolling bearings are a key component in the operation of numerical control machine tool, directly affecting the reliability and safety of the entire equipment. Under large load impacts, defects such as pits, scratches, and spalling can form on the bearing raceway and rolling element surfaces. Compared with a single defect, a composite defect results in a larger impact amplitude in the time-domain waveform and more complex main frequency components in the spectrum. Therefore, it is essential to study the compound failure characteristics of rolling bearings to further clarify their failure mechanisms and dynamic behaviors. [Methods] The present study uses ANSYS Workbench to establish a dynamic simulation model of an angular contact ball bearing with composite faults. The simulation results of the bearing composite fault characteristics were compared with the theoretical calculations. The comparison shows an error of less than 3%, thereby verifying the accuracy of the model. The dynamic response behavior of the composite fault bearing at different rotational speeds was then analyzed. Additionally, a composite bearing fault feature identification method based on optimized variational modal decomposition was proposed. The proposed method can be employed to effectively eliminate noise interference in the extraction of bearing fault features. The original search method of the northern eagle optimization (NGO) algorithm exhibits a linear decreasing trend, which hinders the effective balance between global search and local development capabilities. To address this issue, the positive cosine strategy and position optimization search algorithm are employed to replace the position update method in the NGO algorithm during the search phase, thereby improving the original search method. The improved NGO algorithm is then applied to the adaptive parameter search for VMD. Once the optimal parameters are determined, the simulated vibration signals undergo variational mode decomposition, followed by signal reconstruction of the eigenmode function based on the maximum kurtosis principle, enabling spectral analysis. Furthermore, the rolling bearing accelerated life test dataset from Xi'an Jiaotong University was used for bearing fault feature extraction to verify the effectiveness of the SPNGO-VMD composite fault feature extraction method proposed in this study. Finally, a quantitative comparison is presented to assess the differences in feature extraction performance between SPNGO-VMD and the other methods. The analysis is based on search efficiency, global convergence, and fault feature extraction accuracy, and different methods are evaluated using quantitative indexes. [Results] The research findings indicate that before and after the defective rolling element passes through the outer ring, the equivalent stress is mainly concentrated at the edge of the outer ring defect. The passage of a rolling element through a defect causes localized stress concentration and release within the bearing, decreasing stress. The VMD optimized by the improved NGO algorithm effectively extracts the characteristic frequency components of the composite faults in the bearings, with the frequency peaks corresponding to the theoretically calculated fault frequencies. The improved algorithm demonstrated better search efficiency, global convergence, and fault feature extraction accuracy than other feature extraction models. The algorithm can consistently find the global optimal solution in fewer iterations, avoiding local optima and extracting fault frequency features with greater accuracy. [Conclusions] The simulation model provides a foundation for the experimental design of defective composite bearings. The study results not only aid in identifying and diagnosing potential faults but also offer a theoretical basis for designing more reliable bearing systems.

[Objective] The quartz pendulum is a critical sensitive component in quartz flexible accelerometers, and the stress distribution of the gold coating on both sides directly affects the deformation, zero position, and measurement accuracy of the pendulum. Enhancing the precision of the accelerometer requires an understanding of the influence of stress distribution on both sides of the pendulum on its zero position. This paper proposes a non-contact strain measurement method based on capacitance, introducing a high-precision, in-situ strain measurement technique. This method provides effective technical support for detecting small deformations of the quartz pendulum in complex environments. [Methods] This study begins by calculating the fundamental modes of the quartz pendulum using a finite element simulation model, which serves as the basis for constructing and calibrating the measurement device. Thermal loads are then applied to both sides of the quartz pendulum's coating in the simulation to induce stress, allowing the resulting deformation of the pendulum to be calculated. The relationship between the stress difference on both sides of the pendulum and its deformation, as well as the corresponding capacitance change, is subsequently derived. This process establishes the stress-strain-displacement-capacitance variation relationship, providing theoretical guidance for the development of the quartz pendulum stress-strain measurement device. Afterward, a non-contact strain measurement method based on capacitance is proposed, and a multi-channel quartz pendulum stress-strain measurement system is developed utilizing the AD7747 chip. Through the adjustment of the displacement platform to modify the pendulum position, a series of corresponding capacitance values are recorded. A linear relationship is then fitted to calibrate the device. Finally, the system is validated, and its ability to accurately measure micro-deformations as small as 20 nm is demonstrated. [Results] Using finite element simulation, this study successfully establishes the relationship between the stress difference on both sides of the quartz pendulum and the resulting deformation and capacitance changes, providing a crucial theoretical foundation for quartz pendulum strain measurements. Traditionally, measuring the deformation of a quartz pendulum requires disassembly and the use of a white light interferometer—a cumbersome process that cannot reliably capture the pendulum's true deformation. In this paper, we present an innovative in-situ, online, non-contact strain measurement method based on capacitance, enabling continuous monitoring of the quartz pendulum's deformation state. Furthermore, a stress-strain measurement system for the quartz pendulum is developed and calibrated. The system can operate accurately under various environmental conditions. [Conclusions] This paper presents a high-precision, non-contact, in-situ measurement method for detecting small deformations of the quartz pendulum. Through finite element model simulation and analysis, the relationship between the stress difference on both sides of the pendulum and the resulting capacitance changes is clarified, and the relationship between stress, strain, displacement, and capacitance is established. An in-situ measurement system based on capacitance is also designed and constructed. Following calibration and verification, the system demonstrates the capability to measure micro-deformations as small as 20 nm. This work provides an effective and precise method for detecting small deformations of the quartz pendulum and for conducting in-situ measurements in complex environmental conditions.

[Objective] The rapid evolution of the high-end equipment manufacturing industry, from traditional manual methods to advanced digital intelligent manufacturing, has significantly improved production precision and contributed to technological advancements across various sectors. As the demand for the reliability and durability of critical components in aerospace, semiconductor manufacturing, and other industries continues to rise, there is an increasing focus on understanding microscale defects caused by friction. These friction-induced microdefects, including microcracks and phase transitions, present significant challenges. Although these defects are often subtle and not easily observed under standard conditions, their cumulative effects can significantly impact material performance. Currently, systematic investigations into the influence of friction-induced defects on the optical properties of semiconducting materials remain limited. [Methods] This research addresses this gap by focusing on monolayer molybdenum disulfide (MoS₂), a widely studied two-dimensional (2D) material known for its exceptional electronic and optical properties. Monolayer MoS₂ samples, free of defects, were prepared through the mechanical exfoliation method. Microscale friction-induced defects were introduced via atomic force microscopy under controlled loading conditions to ensure reproducibility and minimize significant structural damage. After defect introduction, the optical properties were analyzed via photoluminescence (PL) spectroscopy and Raman microscopy. [Results] The experimental results showed that friction-induced defects generated new defect energy levels within the MoS₂ structure. These defect states served as non-radiative recombination centers, leading to a substantial reduction in PL intensity and exciton lifetime. Steady-state PL measurements showed a significant decrease in fluorescence intensity in defect-affected regions compared with pristine regions. Time-resolved PL spectroscopy further quantified the reduction in exciton lifetime, from 0.198 ns in pristine regions to 0.128 ns in defected regions, confirming that defects introduced non-radiative dissipation channels for excitons. Raman spectroscopic analysis confirmed the structural integrity of the samples after friction, with observable shifts in vibrational modes indicating the presence of localized stress fields and defect-induced modifications. Temperature-dependent PL studies highlighted the pronounced impact of friction defects at low temperatures. As the temperature decreased, defect-induced PL peaks became dominant in the spectral profile, particularly below 60 K; this behavior is attributed to the enhanced capture of excitons by defect states owing to reduced thermal activation at lower temperatures. Moreover, the defect-induced spectral features became more prominent with higher excitation powers, highlighting the influence of the features on carrier dynamics and non-radiative recombination pathways. These results emphasize the critical role of defect management in optimizing the performance of MoS₂-based optoelectronic devices. [Conclusions] The presence of friction-induced defects not only reduces PL efficiency but also shortens exciton lifetimes, posing challenges for applications that require high quantum yield and stable emission properties. Therefore, strategies to mitigate defect formation during manufacturing and operational processes are crucial for improving device reliability and efficiency. This study elucidates the physical mechanisms driving defect-induced alterations in the optical properties of 2D materials, offering a foundation for improved material design and device engineering. Future research will focus on extending these findings to other 2D materials, such as WS₂ and BN, and incorporating advanced characterization techniques, such as electron microscopy and first-principles calculations, to investigate defect formation at the atomic scale. By addressing these challenges, this research plays a key role in advancing the development of next-generation, high-performance optoelectronic devices suited for demanding industrial applications.

[Objective] Polymer materials exhibit promising prospects in various applications, such as ship bearings and artificial joints, due to their low density as well as high toughness, corrosion resistance, and foreign-matter tolerance. However, in engineering applications, there is often a need to reduce friction. Modifying polymers to enhance their surface binding capacity with hydrated ions and utilizing the hydration effect to reduce friction, represents an important strategy in the tribological design of polymer friction pairs. Polymer materials typically possess a low elastic modulus, and the hydration effect introduces nonlinearity into iterative algorithms. Consequently, a robust numerical model for simulating the contact behavior of such low-elastic-modulus materials under the hydration effect remains elusive. [Methods] This paper presents a numerical model for simulating the contact behavior of low-elastic-modulus polymer materials under the hydration effect. Taking ultra-high-molecular-weight polyethylene (UHMWPE) and sapphire as an example, a rough surface with a Gaussian-distributed topography is constructed to analyze the influence of surface forces and material elastic modulus on material contact behavior. [Results] The findings reveal that low-elastic-modulus materials exhibit low contact pressures, with pressure distribution across the contact area potentially fully supported by the hydration layer, resulting in the formation of nanoscale gaps. Calculations indicate that when the maximum pressure in the contact area is less than the maximum repulsive pressure provided by surface forces, the two surfaces can be completely separated. Conversely, when a part of the pressure in the contact area exceeds the maximum repulsive force, partial separation occurs, with regions where the contact pressure is less than the maximum surface force remaining fully separated. A low elastic modulus promotes the reduction of pressure in the contact area, facilitating surface separation and friction reduction. This study shows that surface topography has a minor effect on contact calculations considering the surface forces of low-elastic-modulus materials but has a significant effect in the case of high-elastic-modulus materials. Furthermore, the elastic modulus significantly affects model convergence: model convergence becomes challenging at low elastic moduli. [Conclusions] First, under static contact conditions, the surfaces of low-elastic-modulus materials can be completely separated by surface forces. Calculations demonstrate that contact pressures in low-elastic-modulus materials are sufficiently low and smaller than the maximum inter-surface force, resulting in complete separation of the surfaces and the entire load being borne by the hydration layer, forming a continuous nanoscale gap between the surfaces. In addition, due to a large contact area and low elastic modulus, polymer materials are less prone to plastic deformation. Second, the surface topography of low-elastic-modulus polymer materials has a limited influence on the effect of surface forces and the distribution of contact pressure. Contrasting results are obtained for high-elastic-modulus materials. Therefore, for friction pairs based on the hydration effect used in hard materials, attention must be paid to surface finishing; this requirement is less critical for soft materials. For low-elastic-modulus materials, rough secondary surfaces can enhance the load-bearing capacity of the hydration layer. Finally, low elastic moduli significantly increased the difficulty of convergence in iterative algorithms. This study found that secondary details on the surface of low-elastic-modulus polymer materials do not significantly affect the contact calculation results, enabling the use of low computational mesh densities. However, material elastic modulus significantly affects the convergence of iterative algorithms used for calculating surface forces, with low elastic moduli leading to great convergence challenges and necessitating strict parameter constraints to achieve convergence. This study lays the foundation for subsequent numerical studies on mixed lubrication in polymers under the action of surface forces.

[Objective] Superlubricity is a state of motion characterized by near-zero friction and negligible wear on tribo-affected materials. It represents a groundbreaking technological approach to mitigating friction-induced material degradation and mechanical equipment failure. From a surface engineering perspective, achieving superlubricity relies heavily on the design and development of both bulk and surface tribo-materials. Solid superlubricity can be achieved under specific conditions, such as ultra-high vacuum or dry inert gaseous environments, and offers distinct advantages, including the ability to sustain high normal loads and extreme temperatures. Diamond-like carbon and layered materials such as molybdenum disulfide can achieve superlubricity through their inherent surface characteristics. However, in the practical operating conditions of mechanical components, the complex and often harsh contact environments present significant challenges for a simple, homogeneous lubricant to sustain exceptional lubricity. Heterogeneous systems composed of at least two types of lubricants offer a promising solution. [Methods] This research investigates a heterogeneous tribo-interface composed of hard hydrogenated carbon films and nanocrystalline-doped transition metal disulfides. The hard hydrogenated carbon films were synthesized by an ion beam deposition system using hydrocarbon gaseous sources as processed precursor. Specific molecular structure such as aromatic-ring species like methylbenzene (C₇H₈) was chosen for tuning the superior property and surface passivation capacity in the film. The correlation between the sp²/sp³ ratio and hydrogen content in the carbon matrix can be controlled by the pulse-biased ion energy. The silver-doped MoS₂ or WS₂ films were prepared by the ion beam assisted magnetron sputtering method. Multilayered structures were established by alternatively depositing each individual layers using different modes. Afterwards, the study focuses on characterizing the mechanical properties, nanostructures, and tribological behaviors of the system. [Results] The macroscale superlubricity performance and its influencing parameters, particularly the contact pressures ranging from 0 to 3.2 GPa, are analyzed for the tribo-systems GLCH/WS₂-Ag and SUJ2/WS₂-Ag. A superlow friction coefficient (COF<0.01) was achieved for a wide range of contact pressure, generally with the decreasing evolution trend as the gradual increasement in the applied normal load regardless of the counterface materials. The heterogeneous sliding interfaces are even capable of bearing a maximum Hertz contact pressure of 7.78 GPa, corresponding to an average value of 3.2 GPa. The duration test further verifies the robustness of the system with a prolonged sliding life-span in the term of 330 000 reciprocating cycles (1 353 m) along with a very low material wear rate. The in-depth analysis of the morphologies and nanostructures of tribofilms at the contact interface reveals the stress-induced evolution of graphene-like carbon transfer layers and WS₂-derived shear bands occurred along the sliding interface. [Conclusions] The above results emphasize that the in-situ formed composite structure provides a synergetic lubrication effect for the maintenance of a superlubricity state in harsh contract conditions. These findings clarify the mechanisms underlying the in-situ formation and ordering of multiple lubricating phases, enabling superlubricity under ultra-high contact pressures.