Determination of interfacial fracture toughness of coatings using circumferentially notched cylindrical substrate

The evaluation of interfacial strength is a challenging task due to the complexity of loading, the thickness of coating and the material conditions of interface cracking and coating spallation. In this study, a new method using circumferentially notched tensile (CNT) specimens is proposed for evaluating the interfacial fracture toughness of coated materials. The 0°, 15°, 30° and 45° notch angled mild steel cylindrical substrates with electroplated nickel were tensile tested. A well defined precrack was introduced at the interface for quantitative evaluation of adhesion. In situ acoustic signals were acquired to obtain the crack initiation or the critical load. The crack initiation and propagation were analysed using scanning electron microscope. Finite element analyses were used to evaluate the critical interface energy release rate as a function of mode mixity from the obtained critical load. The results showed an increase in the interfacial fracture toughness values with an increase in the phase angle. Based on the findings, it has been concluded that this proposed method is an easier testing method to evaluate the adhesive properties of coatings on metallic substrates compared to traditional methods.