Thermal transport properties of thermally sprayed coatings: An integrated study of materials, processing and microstructural effects

The complex microstructures of thermally sprayed coatings are very sensitive to processing conditions and have a significant influence on the properties. The thermal transport property is a very important design parameter for thermally sprayed coatings. Despite considerable progress in this area, there is continued need to clarify the interrelationships among processing, microstructure and thermal transport properties. This has been enabled through continued advancements in processing science and control, enhancements in microstructural characterization and new methods of property characterization.;The purpose of this research is to seek a successive pathway to prior efforts in understanding the effect of microstructural defects on the thermal transport property of thermally sprayed coatings. Relationship between microstructure and thermal conductivity is investigated for three sets of plasma sprayed yttria stabilized zirconia (YSZ) coating systems made using different morphology powders, different particle size distribution and controlled modification of particle states via plasma torch parameters. By integrating the results, maps of the thermal conductivity-porosity relationship have been established. Such maps highlight the role of splat thickness and interfaces in thermal conductivity. Furthermore, a new microstructural parameter termed "effective porosity" is proposed which considers the dominating role of interlamellar pores on through thickness thermal transport in thermally sprayed coatings. This effective porosity is rationalized based on the heat transport mechanism and enables better understanding of microstructure-thermal transport property correlation. An inverse linear model and a percolation model are established which can serve as predictive tools for understanding microstructure-thermal conductivity relationships. In addition, a systematic assessment of thermal conductivity anisotropy has been carried out for YSZ, Al2O 3 and several metallic coatings. These results are analyzed from the point of view of modified percolation theory which considers the effect of anisotropic microstructural defects of sprayed coatings on the thermal transport property.;In the case of the ceramic coatings (YSZ, Al2O3), the temperature dependent thermal conductivity is also examined for various starting microstructures in collaboration with the Oak Ridge National Laboratory (ORNL). The decisive role of starting microstructure on temperature dependent thermal conductivity is presented. In addition, sintering effects resulting from thermal cycling and isothermal exposure on both room temperature and temperature dependent thermal conductivity have been carefully examined in an effort to assess the relationship to effective starting microstructure and provide quantitative information for life prediction.;This dissertation also extends to an investigation of thermal conductivity of metal and alloy thermal spray coatings. A range of metallic materials have been considered and the variation of thermal conductivity is interpreted from the point of view of intrinsic attributes (atomic structure, electronic structure and phase structure) as well as extrinsic effects (as a consequence of oxidation and defected microstructure). Finally, in order to achieve precise and reliable measurement of thermal transport property, the applicability and repeatability of both the laser and xenon flash techniques have been examined through the measurements on these coating systems: ceramics, semiconductors, metals, alloys and composites.