Investigation of defects in thermal sprayed coatings using impedance spectroscopy

Thermal spray (TS) coatings and materials including thermal barrier, tribological and anti-corrosive coatings have established application across a number of engineering fields. TS is attractive for these systems due to its low cost, ability to coat large areas and flexibility in material feedstock. These attributes, along with improvements in process diagnostics have spurred the exploration of TS for more functional applications including fuel cells, conformal electronic sensors and biomedical implants. Successful implementation of TS coatings in these systems will require more robust characterization of their mechanical behavior; to date this has been limited and in practice most measurements are carried out in a pass/fail manner. Little is known about the intrinsic or progressive behavior of the coatings under repeated loading. This is important as the microstructure of TS coatings comprises layers of micron-thick flattened particles ('splats') separated by interfaces, the bonding between which is not well understood. These interfaces represent potential short crack growth sites throughout the material. A lot of works has demonstrated that microstructurally short cracks propagate at substantially higher growth rates than long cracks at equivalent driving forces under both quasi-static and cycling loading conditions in ceramics and their composites and metals. Short cracks in a naturally broken material like TS will have a different mechanism.;Mechanical properties and fracture behavior have been examined in TS, but via conventional methods. That is to say, a large notch is introduced and allowed to propagate. This method completely neglects the existing microstructure of a TS material, which is lamellar and contains a number of near-horizontal cracks. In this study, a new approach to damage monitoring in TS coatings, using through thickness impedance spectroscopy to detect changes in dielectric properties is introduced. The goal of this research is to understand the fatigue behavior and microstructure changes of thermal spray coatings under low strain cycling. Results showed a significant increase in relative dielectric constant of thermal sprayed coatings under tension (R=0) which is due to the growth of micro-scale pre-existing cracks. Yttria-Stabilized Zirconia (YSZ) were deposited on tapered cantilever beam substrates to provide constant in-plane cyclic strains. Relative dielectric constant of coatings was measured via impedance spectroscopy. A Finite Element (FE) model of a randomly distributed network of short crack pairs under a Paris law type stable crack growth was used to model the growth of micro-scale preexisting cracks in coatings. Experiments confirm growth of micro-cracks in thermal spray coatings which are in good comparison with Finite Element (FE) models.