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Abstract Inspired by the dynamic wet adhesive systems in nature, various artificial adhesive surfaces have been developed but still face different challenges. Crucially, the theoretical mechanics of wet adhesives has never been sufficiently revealed. Here, we develop a novel adhesive mechanism for governing wet adhesion and investigate the biological models of honeybee arolium for reproducing the natural wet adhesive systems. Micro-nano structures of honeybee arolium and arolium-prints were observed by Cryogenic scanning electron microscopy (Cryo-SEM), and the air pockets were found in the contact interface notably. Subsequently, the adhesive models with a three-phase composite interface (including air pockets, liquid secretion, and hexagonal frames of arolium), were formed to analyze the wet adhesion of honeybee arolium. The results of theoretical calculations and experiments indicated an enhanced adhesive mechanism of the honeybee by liquid self-sucking effects and air-embolism effects. Under these effects, normal and shear adhesion can be adjusted by controlling the proportion of liquid secretion and air pockets in the contact zone. Notably, the air-embolism effects contribute to the optimal coupling of smaller normal adhesion with greater shear adhesion, which is beneficial for the high stride frequency of honeybees. These works can provide a fresh perspective on the development of bio-inspired wet adhesive surfaces.

Abstract Three-dimensional (3D) frictional contact model of functionally graded magneto-electro-elastic (FGMEE) material with a conducting spherical punch under electromagnetic fields is presented. Two types of imperfect bonding interface of layers, dislocation-like interface and force-like interface, are considered. Frequency response functions (FRFs) for multilayered MEE material with imperfect interface subjected to unit mechanical, electric, and magnetic loads are derived. The FRFs are used with the semi-analytical method (SAM) to solve present multiphysical contact problem. The present model is verified by comparing with literatures and the finite element method (FEM) and used to study the contact problem of FGMEE film imperfectly bonded on homogenous MEE half-space under electromagnetic fields. Parametric studies are conducted to reveal the effects of imperfect interfaces and also film properties including gradient index and thickness.

Abstract Polytetrafluoroethylene (PTFE) has been widely used as a lubrication additive for reducing friction and wear; however, the hydrophobic nature of PTFE restricts its application in eco-friendly water-based lubrication systems. In this study, for the first time, we designed novel PTFE@silica Janus nanoparticles (JNs) to meet the requirement for additives in water-based lubricants, which have excellent dispersion stability in water attributed to the unique amphiphilic structure. By introducing the lubrication of the aqueous dispersion of the JNs with a concentration of 0.5 wt%, the coefficient of friction (COF) and wear volume were reduced by 63.8% and 94.2%, respectively, comparing to those with the lubrication of pure water. Meanwhile, the JNs suspension also exhibits better lubrication and wear-resistance performances comparing to commercial silica and PTFE suspensions. The excellent tribological behaviors of PTFE@silica JNs as nano-additives could be attributed to the synergetic effect of the two components, where the PTFE provided lubrication through the formed tribofilms on the friction pairs, and the rigid silica further enhanced the wear-resistance performance. Most importantly, the unique structure of JNs makes it possible to use PTFE as an additive in water-lubrication systems. Our study shed light on the design and application of novel JNs nanomaterials as additives to meet the requirements of future industrial applications.

Abstract Superlubricity has drawn substantial attention worldwide while the energy crisis is challenging human beings. Hence, numerous endeavors are bestowed to design materials for superlubricity achievement at multiple scales. Developments in graphene-family materials, such as graphene, graphene oxide, and graphene quantum dots, initiated an epoch for atomically thin solid lubricants. Nevertheless, superlubricity achieved with graphene-family materials still needs fundamental understanding for being applied in engineering in the future. In this review, the fundamental mechanisms for superlubricity that are achieved with graphene-family materials are outlined in detail, and the problems concerning graphene superlubricity and future progress in superlubricity are proposed. This review concludes the fundamental mechanisms for graphene superlubricity and offers guidance for utilizing graphene-family materials in superlubricity systems.

Abstract An oil-impregnated porous polyimide (PI) retainer is used in space rolling bearings to improve the lubrication performance, which depends on the release of lubricant from the pores, and therefore is closely related to the pore size. To study the effect of pore size, in this work, PI materials with different pore sizes were prepared by preheating the retainer tube billet during the limit pressing process, and then the friction tests were conducted with the ball-on-ring mode. The results show that the applied load deforms the pores, allowing the lubricant to be squeezed out from the pore; the centrifugal effect induced by rotation also makes the lubricant migrate out of the pore. Therefore, for the same pore sizes, the friction coefficients decrease with the increasing loads and rotation speeds. In addition, it was found that there exists an optimal pore size for the best lubrication properties of porous PI material. Furthermore, the optimal pore size should be larger for lubricants with high viscosity. The microscopic mechanism for lubricant outflow from pores is clarified by molecular dynamic simulations. The insights gained in this study can guide the preparation of oil-impregnated porous retainers under different working conditions.

Abstract Around 1,000 peer-reviewed papers were selected from 3,450 articles published during 2020–2021, and reviewed as the representative advances in tribology research worldwide. The survey highlights the development in lubrication, wear and surface engineering, biotribology, high temperature tribology, and computational tribology, providing a show window of the achievements of recent fundamental and application researches in the field of tribology.

Abstract There are few experimental results available on film thickness at speeds above 5 m/s and they are almost all based on the optical ball-on-disc test rig. In contrast to the contacts in a rolling bearing, in which the lubricant in the oil reservoir distributes symmetrically, ball-on-disc contact shows asymmetry of lubricant distribution due to centrifugal effects. In order to closely imitate the contact occurring between the ball and the outer ring of a ball bearing, this study proposes an experimental model based on ball-on-glass ring contact. An optical matrix method is used to analyze the optical system, which is composed of a steel ball-lubricant-chromium-coated glass ring. Based on the optical analysis, the measurement system is improved in order to obtain a high quality interference image, which makes it possible to measure the film thickness at high-speeds conditions.

Abstract Oil–air lubrication supplies lubricants in the form of droplets to elastohydrodynamic lubrication (EHL) contacts, such as those in high-speed spindle bearings. However, there is a paucity of information related to understanding the lubrication behaviors of oil droplets within EHL contacts. In this study, behaviors of lubricant droplets, in terms of spreading around a static contact as well as passing through a rolling contact, were studied with an optical ball-on-disk EHL test rig. Influences of oil droplet size, viscosity, and surface tension on droplet spreading were examined. Lubricating film formation was also investigated when droplets traveled through the EHL contact region. The results indicated that droplet size and running speed significantly influenced film profiles. With increasing entrainment speeds, a small droplet passed through the contact without spreading and generated films with a significant depression in the central contact region.