Injection molded direct joining (IMDJ) is a promising metal–plastic direct joining method, which first treats the metal surface and then injects melted plastic onto the surface via injection insert molding. The mechanical interlocking is widely accepted as one of the joining mechanisms. On the other hand, the existence of the chemical interaction, which can form between polar groups/molecules on the metal surface and functional groups in the plastic, is still unclear. Thus, the objective of this study is to verify whether the chemical interaction takes effect on the joining between metal and plastic. IR spectrums are usually used to characterize the chemical interaction. However, the general IR system is limited because it does not reach nanoscale spatial resolution. Here, we solved this limitation by applying AFM-IR. Aluminum plates with different surface chemical conditions were prepared and then joined with polyamide 6 (PA6) via IMDJ. We then characterized the joint interface with AFM-IR. The IR spectrum change showed that the hydrogen bonding formed between -OH on the aluminum surface and -CONH in PA6. The results showed that the hydrogen bonding existed, and AFM-IR was able to characterize IR spectrums at the metal–plastic interface in a nanoscale.
