Adhesion of coatings to sheet metal under plastic deformation

This study presents a new technique to evaluate adhesion of coatings to sheet metal under plastic deformation. Based on the concept of the forming limit diagram (FLD), the present work evaluates coating performance with the notch-coating adhesion test, the cross-hatch tape test (ASTM D3359), accelerated conditioning, the uniaxial tensile test, and the rectangular stretch bend test. Experiments were conducted on two types of polyvinylidene fluoride (PVDF)-coated sheet metals: one with polyester primer and other with polyurethane primer. The areas with coating failures were identified, and results of the evaluation were plotted according to major and minor strains depicting coating durability. The constructed durability limit diagram can be used in conjunction with the FLD to determine the feasibility of a complex forming operation. Optical microscopy and energy-dispersive X-ray spectroscopy (EDS) were employed to understand the nature and composition of debonded surfaces after conditioning and application of strain. Comparative results were presented in terms of number of damaged grids in each sample for different applied strains and different durations of conditioning. This procedure combines the key attributes of various experimental techniques and shows that the methodology is successful in characterizing the strength and weakness of two different types of coated materials.