Investigation of the adhesion toughness of lacquer to zinc-phosphated steel

Abstract

A shear test under uniform plane stress loading conditions at the interface has been used to evaluate the adhesion toughness of the bond of a fast-cure thermosetting powder coating to zinc-phosphated cold-rolled steel surfaces of two different origins. The shear test is designed to measure the nominal and the net ultimate shear stress (USS) values, as well as the critical stress intensity factor of interfaces. The experimentally measured critical load at which the adhesion failed, together with the ultimate shear stress values calculated and fractographic analysis of shear-failed specimens on mating surfaces, were used to quantify the bond toughness along the interfaces of zinc-phosphated cold-rolled steel-lacquer specimens, and to provide an insight into fracture phenomena and adhesion mechanisms. It was shown that differences between these two layers manifested by change in the USS net value are due to variation in the morphology of the two zinc phosphate conversion layers studied.