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), CuxS, and ZnO/ZnS, between the steel cords and rub-ber. During degradation, CuxS 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 ch...