Thermal Conductivity Prediction In Air Plasma Sprayed Thermal Barrier Coatings Containing Multifarious Defects

Air plasma sprayed yttria-stabilized zirconia thermal barrier coatings are widely applied in gas turbines and aviation engines, which usually contain multifarious and multiscale defects, such as pores, cracks, and amorphous layers. They all significantly lower the thermal conductivity of the coating but in drastically different ways depending on their morphologies and orientations. Establishing an accurate correlation between the microstructure 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 thermal conductivity. In this research, cross-section ion polishing and image analysis were chosen as a reliable assembly for characterizing multifarious defects of porous coatings, which was almost undamaged compared with the traditionally mechanical polishing. The effect of different microscale defects on the thermal conductivity was respectively and quantitatively studied to build a mathematical model. A thermal resistance induced by amorphous layers was introduced into the model, which was found to have a linear relationship with the amorphous layer concentration. It was also found a linear relationship between the amorphous layer concentration and the spraying times. The predicted thermal conductivity of porous coatings by multifarious-defect-concerned model fits the data measured using the steady heat flow method very well. 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.