Two-dimensional materials having a layered structure comprise a monolayer or multilayers of atomic thickness and ultra-low shear strength. Their high specific surface area, in-plane strength, weak layer-layer interaction, and surface chemical stability result in remarkably low friction and wear-resisting properties. Thus, 2D materials have attracted considerable attention. In recent years, great advances have been made in the scientific research and industrial applications of anti-friction, anti-wear, and lubrication of 2D materials. In this article, the basic nanoscale friction mechanisms of 2D materials including interfacial friction and surface friction mechanisms are summarized. This paper also includes a review of reports on lubrication mechanisms based on the film-formation, self-healing, and ball bearing mechanisms and applications based on lubricant additives, nanoscale lubricating films, and space lubrication materials of 2D materials in detail. Finally, the challenges and potential applications of 2D materials in the field of lubrication were also presented.
Abstract:The remarkable ability of geckos to climb and run rapidly on walls and ceilings has recently received considerable interest from many researchers. Significant progress has been made in understanding the attachment and detachment mechanisms and the fabrication of articulated gecko-inspired adhesives and structured surfaces. This article reviews the direct experiments that have investigated the properties of gecko hierarchical structures, i.e., the feet, toes, setae, and spatulae, and the corresponding models to ascertain the mechanical principles involved. Included in this review are reports on gecko-inspired surfaces and structures with strong adhesion forces, high ratios of adhesion and friction forces, anisotropic hierarchical structures that give rise to directional adhesion and friction, and "intelligent" attachment and detachment motions.
Gecko-inspired surfaces are smart dry adhesive surfaces that have attracted much attention because of their wide range of potential applications. However, strong frictional force, rather than adhesive force, is frequently targeted in most of research in this area. In this study, the interfacial adhesive and frictional properties of a gecko-inspired mushroom-shaped polyurethane pillar array surface have been systematically characterized to design and control the interfacial adhesion of the surface by considering the nanoscale interfacial adhesion, the microscale structural compliance and deformation, and the macro-scale actuation. Matching the movement of the leg springs and the interfacial adhesive characteristics between the pillar array surfaces and substrates, a three-legged clamp prototype has been designed and fabricated to successfully pick up and release light and fragile objects with a smooth upper surface, such as a silicon wafer. These results provide a new insight into not only the theoretical understanding of the integrating adhesion mechanisms, but also the practical applications of utilizing and controlling the adhesive and frictional forces of gecko-inspired surfaces.
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