Many applications of thermally sprayed coatings call for increased fatigue resistance of coated parts. Despite the intensive research in this area, the influence of coating on fatigue is still not completely understood. In this paper, the spatiotemporal localization of crack initiation and the dynamics of crack propagation are studied. The resonance bending fatigue test is employed to test flat specimens with both sides coated. Hastelloy-X substrates coated with classical TBC YSZ/NiCoCrAlY composites were tested. The strain distribution on the coating surface is evaluated by the digital image correlation method (DIC) 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. The individual phases of specimen degradation, i.e. the changes of material properties, crack initiation, and crack propagation are identified. The tested coatings strongly influenced the first two phases, the influence on the crack propagation was less significant.

This paper compares the results of two approaches of instrumented indentation for characterization of mechanical properties of HVOF coatings. Three types of industrially used HVOF sprayed coatings (Cr 3 C 2 -NiCr, WC-Co, (Ti, Mo)(C,N)-NiCo) were selected. The indentation methods applied were: isolated nanoindentation in metallic matrix and carbides with 2 mN peak load and grid indentation with 2 mN peak loads, comprising 400 indentations. The results of the isolated indentation revealed hardness and elastic modulus of the individual phases with surprisingly low standard deviation and in good agreement with the corresponding bulk equivalent. The grid indentation method, based on statistical evaluation of a large number of indentations, was influenced by the carbide-matrix interface, which gave rise to a strong third peak apart from the two peaks corresponding to the hard carbides and softer metallic matrix. This makes the statistical analysis much more complex than using simple bimodal Gaussian fit for separation of matrix and carbide properties. Nevertheless, the results of both grid indentation and isolated nanoindentation compared with microindentation values obtained at higher loads gave important information about the cohesion of the coatings.