Superlubricity achieved by polyvinylpyrrolidone at Si₃N₄/sapphire interfaces

Polymers have complex molecular structures that often lead to inter-chain friction and hinder movement, making it difficult to achieve superlubricity. However, in the field of hydration lubrication, the electronegative interface of ceramics readily adsorbs water molecules, creating a protective water film that covers the frictional interface and effectively reduces friction. To achieve hydration lubrication, it is essential to create a continuous lubricating film by selectively enriching specific functional groups of adsorbed water molecules from the polymer solution onto the ceramic surface. By adsorbing a hydrated layer composed of polyvinylpyrrolidone with pyrrolidone groups onto a negatively charged Si₃N₄/sapphire interface, we have enabled the formation of a continuous lubricating film. Research has found that the interaction between the polymer chain structure of Polyvinylpyrrolidone molecules (such as PVP10000) in solution and water molecules could exhibit excellent superlubricity. Under contact pressure exceeding 198 MPa, the coefficients of friction could be reduced to 0.004-0.007. Through detailed surface analyses and sophisticated simulations, we uncovered the underlying mechanism. The pyrrolidone moieties of PVP formed hydrogen bonds with the Si₃N₄ surface, transforming the initially difficult frictional interface into a PVP/sapphire interface with significantly reduced sliding energy barriers. These findings highlight the vital role of PVP in superlubricity and hydration lubrication, and provide a theoretical and experimental basis for the design of materials and lubricants with exceptional lubricating properties.