Organic-Inorganic Hybrid Solid Lubricants Based on 2D Silver Thiolate Coordination Polymers Featuring Precise...

Layered solid lubricants, such as molybdenum disulfide, are widely used to reduce friction and wear at moving contact surfaces. However, their limited structural variety makes it challenging to tailor their lubrication properties for specific applications. This study demonstrated the potential of two-dimensional silver thiolate (AgSR) coordination polymers, which feature distinctive organic-inorganic lamellar crystal structures, as tunable solid lubricants based on the precise structural design. X-ray diffraction measurements showed that various AgSR compounds with linear alkylthiolato and para-substituted benzenethiolato ligands formed layered structures with high crystallinity. Ball-on-plate friction tests were performed on powder-supported specimens to assess the solid lubricating properties of AgSR. The AgSR-supported specimens with alkylthiolato ligands demonstrated a coefficient of friction (COF) of approximately 0.1, considerably lower than that of the well-known layered materials, MoS_2, and alkylammonium-intercalated mica. This exceptional lubricity of AgSR is likely attributed to their unique interlayer structures, which exhibit minimal interdigitation of alkyl chains. Scanning electron microscopy and energy-dispersive X-ray spectroscopy analyses of the worn surfaces of the AgSR-supported specimens revealed that AgSR compounds create lubricating films on the sliding substrates. Furthermore, for a series of AgSR compounds featuring para-substituted benzenethiolato ligands, the COF values were highly affected by the functional groups at the para position, which affected their interlayer interactions. Strong interlayer interactions, such as hydrogen bonds, resulted in considerable increases in the COF to approximately 0.7, suggesting that the lubrication properties of AgSR are derived from the ease of cleavage in their layered crystal structures. These findings imply that AgSR offers considerable advantages over traditional inorganic layered materials for the tailored development of custom solid lubricants.