Measurements of the fracture energy of the coating/substrate interfacial region through radial-notched cylindrical specimens

Measurements of the adhesive strength are normally conducted using the direct pull test. However, in the direct pull test failure often occurs at the location of the polymeric adhesive used to prepare the direct pull test specimen when the adhesive strength of the coating/substrate interface is stronger than the strength of the polymeric adhesive. Even when failure does occur at the coating/substrate interface, the direct pull test provides ill-defined properties since the measured failure strength is strongly affected by the strength-controlling defects present at the interface before the test. In this paper we have developed a new test specimen geometry that ensures the propagation of the crack along the coating/substrate interface or the interfacial region, depending on the properties of the interface and the interfacial region. Furthermore, the new specimen provides well-defined properties, i.e. the fracture energy of the coating/substrate interface or the interfacial region. The proposed specimen is validated using oxide coatings on steel substrates prepared via the thermal spray technique.