A study of crack initiation and propagation in Ni-Cr thermally sprayed coatings using acoustic emission techniques

Many metallurgical coatings applied by thermal spraying tend to exfoliate or spall through the growth of cracks, presumably initiated at the metal-coating interface or at the substrate. The present study was undertaken to determine whether such cracks are at the substrate-coating interface or within the coating or the substrate themselves. An AISI 1045 steel overlayed with a nickel-chrome alloy was used and the samples were subjected to a positive stress and fatigued using sinusoidal oscillations. In order to obtain a continuous record of crack initiation and propagation, acoustic emission equipment was used. The variation of the events and count rates and energy liberated per event made it possible to determine crack initiation and to follow crack growth. Scanning electron microscopy was used to examine sections of the specimens at different stages of the real-time graphs of acoustic emission. It was found that the cracks initiated in the coating near the interface and not at the substrate or interface as suggested elsewhere. The conditions promoting crack initiation were also determined.