Degradation of a Metal–Polymer Interface Observed by Element-Specific Focused Ion Beam-Scanning Electron Microscopy

The degradation of a metal–polymer interface was studied in three dimensions using focused ion beam-scanning electron microscopy (FIB-SEM) with energy-dispersive X-ray spectroscopy. A brass-rubber interface, which is important for tires, was examined as an example of a metal-polymer interface. Brass-plated steel cords were embedded in rubber, which was then vulcanized. The brass-rubber interface was treated at 70 °C under 96% humidity for up to 14 days (a wet-heat aging treatment). FIB-SEM provided clear three-dimensional images of the adhesive layer consisting of brass (CuZn), Cu x S, and ZnO/ZnS between the steel cords and rubber. During degradation, Cu x S at the interfaces diffused into the rubber, resulting in the direct contact of bare steel with rubber. The lack of a substantial adhesive layer explained the degradation of mechanical properties after the wet–heat treatment. In addition, electron diffraction and electron energy loss spectroscopy revealed that the Cu2S crystals in the adhesive layer changed to crystal-like CuS during the degradation, which also caused a degradation of mechanical properties because a high Cu valence of x ≈ 2 in Cu x S leads to stronger adhesion than a valence of x = 1.