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.