A novel technique to characterise and evaluate the interfacial fracture toughness of coatings

Coatings are often used as a covering to enhance the quality and protect the material to which they are applied to. The interface between the coating and substrate is the weakest part of the bimaterial system. Through accurate prediction of the interfacial fracture toughness, coatings can be produced that are more reliable as unanticipated failures are less likely to occur. Currently the tests that are carried out in this field have limitations and are only able to test certain coating–material combinations or are only suitable for certain loading conditions. A simple method using circumferentially notched tensile (CNT) specimen was developed for quantitatively evaluating the adhesion of coatings on substrates. The method is based on common tensile test of materials and is competent for estimating the mixed mode fracture toughness over a range of phase angles. In the first stage the 0, 15, 30, 45 and 60 degree notch angled mild steel cylindrical substrates with electroplated nickel were studied in detail using finite element models. In the second stage the nickel coated steel CNT specimens were tensile tested with a well defined precrack at the interface. In-situ acoustic signals acquisitions and scanning electron microscope were carried out to obtain the crack initiation (critical load) and propagation. From the critical load, critical interface energy release rate as a function of mode mixity was evaluated using J- integrals. The results showed an increase in the interfacial fracture toughness values with the increase in phase angle. In the third stage, the studies were also conducted to evaluate the fracture properties of TiN and TiAlN coatings using CNT specimens. The combined results of computational and experimental analysis showed that any defect or stress concentration at the interface can significantly weaken the adhesion of coating. It has been concluded that this new proposed method is an easier testing method to evaluate the adhesive properties of both ductile and brittle coatings on metallic substrates compared to traditional methods.