Study of aluminium bonded joints by analytical electron microscopy: Microstructural changes with aging

This study concerns the influence of aging on the microstructural characteristics of each component in bonded joint systems consisting of an aluminium alloy substrate (2024) clad with an aluminium sheet, an oxide layer obtained by surface treatment ('optimized' chromic-sulphuric acid etching, chromic acid anodizing, or phosphoric acid anodizing), an epoxy-novolac-based composite primer, and an epoxy-based adhesive film. The samples studied were obtained by ultrathin cross-section microtomy of the bonded joints (before and after aging). The microcharacterization was carried out using a transmission electron microscope (TEM) equipped with an X-ray energy-dispersive spectrometer (EDS), an electron energy-loss spectrometer (EELS), and an energy-filtered imaging system. Detailed analyses revealed that microstructural changes occur during aging in the primer in contact with the oxidized substrate and that the thickness of the modified zone depends on the underlying oxide. In addition, it is shown that a 'true' interphase about 200 nm thick is formed between the primer and adhesive by a diffusional process.