Solid-lubrication properties of copper benzene-1,4-dicaboxylate, a metal–organic framework with a two-dimensional layered crystal structure†

Abstract

Solid lubricants are widely used to control friction and wear in moving contact areas. In particular, inorganic materials with layered crystal structures, such as graphite, are a well-known category of solid lubricant. However, their structural designability is restricted because of their chemically stable nature, making it difficult to control their tribological characteristics. In this study, the solid lubricity of copper(II) benzene-1,4-dicarboxylate (CuBDC), a two-dimensional metal–organic framework (2D-MOF), was investigated as a new type of solid lubricant with structural diversity. The tribological measurements of powder-supported specimens revealed that CuBDC exhibited good lubrication properties comparable to those of typical solid lubricants, such as graphite and polytetrafluoroethylene. From the scanning electron microscopy observations of the worn surfaces of the CuBDC-supported specimen, the layered crystal structure of CuBDC effectively formed a smooth wear surface at the contact area. In contrast, specimens supporting copper(II) benzoate and copper(II) benzene-1,3,5-tricarboxylate, which have similar chemical natures as CuBDC, exhibited high frictional force, reflecting on the difference in their crystal structures. Furthermore, the transformation of the CuBDC crystal by thermal treatment, which afforded interlayer coordination bonds, increased the friction coefficient. These results suggest that the solid lubricity of CuBDC originates from its layered crystal structure. Thus, the 2D-MOFs with layered crystal structures are potential candidates for solid lubricants with good property tunability.