Multifarious Defects Co-Determined Thermal Conductivity in Air Plasma Sprayed Thermal Barrier Coatings

Yttria-stabilized zirconia (YSZ) is the most widely applied thermal barrier coatings (TBCs) materials in gas turbines and aviation engines to prevent components from overheating and to increase the combustion temperature and turbine efficiency. Plasma sprayed coatings usually contain about 15~25% pores, mainly including globular pores, cracks and amorphous phase that to be classified as multifarious and multi-scale defects. They all significantly lower the thermal conductivity, but in drastically different ways depending on their morphologies and orientations. Establishing an accurate correlation between the total porosity and the thermal conductivity requires not only a precise separation and estimation of different kinds of defects but also a reasonable mathematic model to describe their effect on the thermal conductivity. In the present research, cross-section ion polishing and image analysis were chosen as a reliable assembly for visualizing and characterizing multifarious defects of thermal sprayed coatings, through which tiny pores and cracks can be distinctly expressed without any mechanical damage of the microstructure. The effect of different micro-scale defects such as pores and cracks on the thermal conductivity was respectively and quantitatively calculated according to a Bruggeman-based iterative model. Additionally, the nanoscale defects, especially the amorphous phase in the matrix was also taken into consideration, which is proven to be of great importance. The predicted thermal conductivity based on this amorphous-phase-concerned iterative model has achieved a successful evaluation compared to the measured results of coating specimens originated from various spraying processes. This research confirms the applicability of image-analysis-based modeling as a simple, reliable and versatile method for thermal conductivity prediction of porous coating systems.