Atomic-scale analysis on a rubber-brass adhesive interface using scanning transmission electron microscopy

At rubber-brass (Cu–Zn alloy) adhesive interfaces, CuxS (x = 0.5–2) layers form during vulcanization, indicating that the mechanical and chemical interactions between rubber and brass strongly depend on the morphologies and chemical compositions of CuxS layers. In this study, an ultrathin cross-sectional sample of a rubber-brass adhesive interface was successfully prepared and observed by scanning transmission electron microscopy (STEM) with atomic resolution. It was revealed that the CuxS layer mainly consisted of two phases: hexagonal Cu2S (h-Cu2S) and orthorhombic CuS (o-CuS). While the h-Cu2S grains had spherical shapes on the brass side of the CuxS layer, o-CuS grains with elongated shapes were on the rubber side of the layer. Moreover, the crystal faces of the CuxS grains and the areal densities of the sulfur atoms in the crystal faces, which would lead to the chemical conditions of the adhesive interface, were identified. This study provides indispensable information to clarify the adhesive mechanisms between rubber and brass.