Interface Characterization of Fusion Bonded Composites Made from Laser-Pretreated Steel and Glass Fiber-Reinforced Thermoplastics Using Electrochemical Impedance Spectroscopy
Authors: Gundlach Christian , Mengedoht Miriam , Kandula Maja , Hartwig Sven 
Published: December 2024

Material-hybrid designs made of metal and plastic are becoming increasingly important in the automotive and aerospace industries. They place new demands on joining technology in particular. One promising process is the fusion bonding of metals and fiber-reinforced thermoplastics. In this process, the thermoplastic matrix material of the fiber-reinforced composite component is melted and a direct adhesive bond is created with the metallic joining partner. This joining technology has already been extensively studied in the past. In particular, the interface between the joining partners has been identified as a critical factor for the long-term quality of the joint. The interface can be altered by influences during production, such as e-coat process or the effects of harsh environmental conditions during operation like corrosion and thermomechanical loads. Determining the extent of these influences on the interface condition and thus the quality of the joint is difficult, but it is essential to support the use of the joining method in safety-critical areas. Previous investigations of this joining method have mostly been carried out destructively using tensile shear tests and non-destructively using fiber optic and piezo ceramic sensors, but all approaches have only shown limited significance with regard to the metal-plastic interface in detail. Electrochemical impedance spectroscopy (EIS) as a very sensitive measurement method could represent an almost non-destructive and at the same time very sensitive alternative which is currently not reported for this use case in the literature. In previous internal investigations, we found that EIS can be used to measure layer thicknesses of up to 3.5 mm, contrary to the usual use of this test method for characterizing the properties of thin coatings, corrosion behavior and battery performance. In fusion bonding applications, typical material thicknesses of thermoplastics above the interface are around 1.5 mm and are thus well above the usual layer thicknesses of approx. 20 µm to 100 µm for typical EIS applications.
This study aims to examine how EIS can be used advantageously for the interface characterization of fusion bonded specimens and up to which extent the EIS can provide a statement about the metal-plastic interface. In this study, a joint of a laser-pretreated steel and a glass fiber-reinforced PA6 is investigated. We measured specimens directly after production, after a treatment similar to an automotive e-coat process and after certain intervals of a salt spray test. Using the Kramers-Kronig transformation, the EIS data was validated and qualitatively analyzed with Nyquist and Bode diagrams, the impedance at 0.1 Hz, the maximum impedance and equivalent circuits. Tensile shear and edge shear test serve as destructive reference tests to determine strength parameters of the joints and evaluate the respective fracture patterns. This opens up the possibility to correlate the results of EIS and destructive measurements. In summary, EIS is suitable for characterizing the metal-plastic interface, even with polymer thicknesses of 1.5 mm, which are comparatively high for EIS. Due to the very low phase angle and the resulting low dielectricity of the glass fiber-reinforced thermoplastic, changes in the impedance data are directly related to the interface condition. The measured impedance spectra could be differentiated with Nyquist and Bode diagrams, the impedance at 0.1 Hz and the maximum impedance.