The coating application for enhanced adhesion between GFRP composites and cataphoresis-coated (KTL) steel – part 1: mechanical properties

Purpose This research aims to investigate how the adhesion performance of GFRP composite components, commonly used in railway vehicles, is affected when bonded to cataphoresis coated steel substrate surfaces. Design/methodology/approach In this context, the aim was to determine the optimal adhesion parameters for bonding GFRP samples with natural and primed surfaces to steel samples with cataphoresis coatings. Then, single-lap joint samples with different bond thicknesses of 1 mm, 2 mm and 3 mm were prepared. Finally, tensile tests were performed on the samples. Findings The results showed that GFRP specimens with natural surfaces, characterised by the highest surface roughness, exhibited the lowest bond strength. But, the highest bonding performance was achieved in specimens where primed GFRP was bonded to cataphoresis coated steel, especially with a bond thickness of 1 mm, and achieving a yield strength of 20 MPa. This situation explains the characteristic difference between surface roughness and chemical coating, which are two essential pre-treatments in adhesive bonding. While surface roughness provides mechanical interlocking, excessive roughness can hinder the adhesive’s wetting ability, causing it to remain suspended on the surface as described in the Cassie–Baxter theorem. In contrast, it has been observed that, despite low surface roughness, chemical coatings enhance the bonding between primer paint and adhesive molecules, and – as stated in the Wenzel theorem – improve the surface wettability. Originality/value As a preliminary preparation in the adhesive method, primer paint is applied to steel surfaces and GFRP material surfaces in classical industrial applications. In this research, the application of the catapheresis process to the steel substrate instead of primer paint and the bonding of primer-painted GFRP materials to this surface make a unique contribution to the research.