Monitoring laser cleaning for enhanced adhesion of FIPG silicone adhesive in automotive industry

Adhesive bonding and sealing of safety-relevant bonding joints in the automotive industry requires automatable monitoring of cleaning processes to reproducibly ensure clean adherend surfaces and finally sufficient adhesion. The industry lacks combined experimental data that include the cleaning of precisely contaminated surfaces, the monitoring thereof and the subsequent adhesive bonding to design these production processes holistically. Therefore, this study investigates laser-induced fluorescence (LIF) and laser-induced breakdown spectroscopy (LIBS) as inline-capable monitoring methods for laser cleaning of aluminium specimens contaminated with cooling lubricant by varying the contamination grammage and the laser cleaning power density. The results of the methods are validated with measurements using a Fourier transform infrared spectroscopy with an attenuated total reflectance unit (FTIR-ATR). Additionally, the adhesion properties of a model room temperature vulcanised (RTV) silicone adhesive are evaluated using lap shear tests. This adhesive is characteristically used as a formed-in-place gasket (FIPG) for sealing applications. The results show a complete and consistent picture of the interaction of all applied analytical methods. LIF, LIBS and FTIR-ATR are able to detect contaminations between 0.15 and 2.0 g m- 2. Subsequently, they determine a laser power density onset threshold for laser cleaning of a 2.0 g m-2 contaminated specimen, which was verified in lap shear analysis. The fracture patterns show decreasing cohesive failure with increasing contamination grammage, which increases after laser cleaning with a laser power density above the determined threshold.