Fatigue Crack Growth in Bodies with Thermally Sprayed Coatings

Many applications of thermal spray coatings call for increased fatigue resistance of coated parts. Despite the intensive research, the mechanism of influence of the coating on fatigue properties is still not completely understood. In this paper, the spatio-temporal localization of crack initiation and the dynamics of cracks propagation are studied. The resonance bending fatigue test is employed to test flat specimens with both sides coated. Inconel and low-carbon steel substrates coated with conventional TBC YSZ/NiCoCrAlY composites, and single layer Alumina coatings were tested. The strain distribution on the coating surface is evaluated by the digital image correlation through the whole duration of the fatigue test. Localization of crack initiation sites and the mode of crack propagation in the coated specimen are related to the observed resonance frequency and damping of the sample. The individual phases of specimen degradation, i.e. the material stiffness changes, crack initiation and crack propagation are identified.