Role of residual stresses on interface toughness of thermally sprayed coatings

In order to create and propagate a crack in the interface plane, Vickers indents were applied to polished cross-sections at the interface of Cr3C2/NiCr coatings obtained by the high velocity oxy-fuel spraying process. Using this procedure, it is possible to calculate an apparent interface toughness. For as-sprayed specimen, it was shown first that the interface toughness varies like a function of one over the square root of the coating thickness. The value extrapolated to an infinite thickness is assumed to represent the adhesive properties of the coating. Same tests were performed on annealed specimen for which it was expected a severe modification of the residual stresses state. In these conditions, an apparent interface toughness was found independent on the coating thickness. Moreover, this value is equal to the value extrapolated for the as-sprayed specimen. This very important and new result allows to modify the model for the interface toughness in order to take into account a stress parameter.