Electrochemical characterization of plasma sprayed WC–Co coatings by impedance techniques

The porosity of plasma sprayed coatings (which were either conductive when Co was incorporated in a WC matrix or insulating with Al2O3) was tested by impedance techniques in a NaCl aqueous medium. Models of active pores (conductive coating) based on transmission lines or inactive pores (insulating coating) based on lumped parameter circuits were proposed. A good agreement was found between the impedance predicted by the models and the impedances measured in a wide frequency range on both conductive and insulating coatings. In particular, the coatings can be compared on the basis of their porosity. For WC–Co coatings, it was found that the porosity decreases when the Co content increases.