Plastic-elastic Model for Water-based Lubrication Considering Surface Force

Water-based lubrication is an effective method to achieve superlubricity,which implies a friction coefficient in the order of 10−3 or lower.Recent numerical,analytical,and experimental studies confirm that the surface force effect is crucial for realizing water-based superlubricity.To enhance the contribution of the surface force,soft and plastic materials can be utilized as friction pair materials because of their effect in increasing the contact area.A new numerical model of water-based lubrication that considers the surface force between plastic and elastic materials is developed in this study to investigate the effect of plastic flow in water-based lubrication.Considering the complexity of residual stress accumulation in lubrication problems,a simplified plastic model is proposed,which merely calculates the result of the dry contact solution and avoids repeated calculations of the plastic flow.The results of the two models show good agreement.Plastic deformation reduces the local contact pressure and enhances the function of the surface force,thus resulting in a lower friction coefficient.

Characteristics of surface and subsurface of formed thread parts by axial-infeed thread rolling process

The thread rolling process has been widely applied to manufacture high-performance thread parts.In this process,the evolutions of surface and subsurface are frequently introduced,which affect the working performance of manufactured parts.In this study,an axial-infeed thread rolling process(ATRP)is employed,and the macro-meso surface characteristics under different lubrications and operating conditions are investigated.Moreover,the distributions of microstructure and hardness on the subsurface of formed tooth are analyzed in detail,along with the study of stress state and yield strength change.It is found that the MoS_(2)grease is more effective in reducing the surface roughness and defects than the lubrication oil and water-base graphite during the ATRP process.Increasing rolling speed improves the quality of surface morphology and can reduce the surface roughness.On the subsurface of bottom and flank,intensive shear stress occurs in a narrow region,resulting in the elongation and refinement of the grains and increasing the low angle grain boundary fraction.Based on the grain size and plastic strain,the yield strength is predicted.The maximum yield strength and hardness on the bottom of formed tooth are improved by 41.2% and 39.4%,respectively.

Unlocking the secrets behind liquid superlubricity:A state-ofthe-art review on phenomena and mechanisms

Superlubricity,the state of ultralow friction between two sliding surfaces,has become a frontier subject in tribology.Here,a state-of-the-art review of the phenomena and mechanisms of liquid superlubricity are presented based on our ten-year research,to unlock the secrets behind liquid superlubricity,a major approach to achieve superlubricity.An overview of the discovery of liquid superlubricity materials is presented from five different categories,including water and acid-based solutions,hydrated materials,ionic liquids(ILs),two-dimensional(2D)materials as lubricant additives,and oil-based lubricants,to show the hydrodynamic and hydration contributions to liquid superlubricity.The review also discusses four methods to further expand superlubricity by solving the challenge of lubricants that have a high load-carrying capacity with a low shear resistance,including enhancing the hydration contribution by strengthening the hydration strength of lubricants,designing friction surfaces with higher negative surface charge densities,simultaneously combining hydration and hydrodynamic contribution,and using 2D materials(e.g.,graphene and black phosphorus)to separate the contact of asperities.Furthermore,uniform mechanisms of liquid superlubricity have been summarized for different liquid lubricants at the boundary,mixed,and hydrodynamic lubrication regimes.To the best of our knowledge,almost all the immense progresses of the exciting topic,superlubricity,since the first theoretical prediction in the early 1990s,focus on uniform superlubricity mechanisms.This review aims to guide the research direction of liquid superlubricity in the future and to further expand liquid superlubricity,whether in a theoretical research or engineering applications,ultimately enabling a sustainable state of ultra-low friction and ultra-low wear as well as transformative improvements in the efficiency of mechanical systems and human bodies.

Trajectory Optimization for Cellular-Connected UAV Under Outage Duration Constraint

Enabling cellular access for unmanned aerial vehicles(UAVs)is a practically appealing solution to realize their high-quality communications with the ground for ensuring safe and efficient operations.In this paper,we study the trajectory design for a cellular-connected UAV that needs to fly from given initial to final locations,while communicating with the ground base stations(GBSs)subject to a minimum signal-to-noise ratio(SNR)requirement along its flight.However,due to various practical considerations such as GBSs’locations and coverage range as well as UAV’s trajectory and mobility constraints,the SNR target may not be met at certain time periods during the flight,each termed as an outage duration.In this paper,we first propose a general outage cost function in terms of outage durations in the flight,which includes the two commonly used metrics,namely total outage duration and maximum outage duration as special cases.Based on it,we formulate a UAV trajectory optimization problem to minimize its mission completion time,subject to a constraint on the maximum tolerable outage cost.To tackle this challenging(non-convex)optimization problem,we first transform it into a tractable form and thereby reveal some useful properties of the optimal trajectory solution.Based on these properties,we further simplify the problem and propose efficient algorithms to check its feasibility and obtain optimal as well as low-complexity suboptimal solutions for it by leveraging graph theory and convex optimization techniques.Numerical results show that our proposed trajectory designs outperform that by the conventional method of dynamic programming,in terms of both performance and complexity.