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), CuS, and ZnO/ZnS between the steel cords and rubber. During degradation, CuS 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 CuS leads to stronger adhesion than a valence of x = 1.