Evaluation of thickness, porosity and pore shape of plasma sprayed TBC by electrochemical impedance spectroscopy

An equivalent circuit of electrochemical impedance spectroscopy (EIS) was suggested to model the morphological structure of nonexposure thermal barrier coating (TBC). The morphological properties of as-coated air plasma sprayed (APS) TBC were correlated with the EIS parameters. A linear relationship between TBC topcoat thickness, Dc, and ceramic resistance, Rc, has been found particularly when the TBC have the same porosity. The TBC porosity, P, shows to be a function of ceramic thickness, Dc, and porosity, P; in particular, it shows a proportional relationship with ceramic capacitance, Cc, when ceramic thickness, Dc, is kept unchanged. The pore size and shape in the TBC topcoat can be assessed by the pore resistance, Rp, as measured by EIS. The results indicate that electrochemical impedance spectroscopy can be used for quality assurance of TBC and, in particular, to evaluate TBC ceramic topcoat thickness, porosity and kinds of defects.