Effect of Morphology on Thermal Conductivity of EB-PVD PYSZ TBCs

Kurzfassung

Partially yttria stabilized zirconia (PYSZ) based thermal barrier coatings (TBC) manufactured by electron beam – physical vapour deposition (EB-PVD) process is a crucial part of a system which protects the blades working under severe service conditions at the high pressure sector of aero engines and stationary turbines. These materials show a high strain tolerance relying on their unique coating morphology which is comprised by weakly bonded columns. Growth of these columns occurs in a preferred crystallographic orientation producing inter-columnar gaps in between them. Furthermore, the open porosity is enhanced by the presence of voids between feather-like sub-columns which are surrounding the primary column surfaces inclined toward their growing axis. Finally, rotation of the specimens during the vapour deposition process produces additional intra-columnar closed pores formed inside the primary columns. The results obtained in this work demonstrate that the variation of the parameters during the EB-PVD process alters the columnar morphology of the coatings. Consequently, these morphological changes affect primarily the thermal conductivity which is one of the key physical properties of this material group. This study investigates three different morphologies of PYSZ EB-PVD TBCs in terms of the spatial and geometrical characteristics of their porosities and the corresponding thermal conductivity in as-coated state and after heat treatments at 1100°C for 1h and 100h. Changes of the surface area, shape and size of the open and closed pores after heat treatments are characterized by small-angle neutron scattering (SANS), Brunauer-Emmett-Teller Method (BET) and scanning electron microscope (SEM). The results demonstrate that the thermal conductivity of EB-PVD TBCs is also altered as their microstructure on modification of the EB-PVD process parameters. Correlation of the changes in the thermal conductivity with that of the analysed microstructures, regarding the changes in shape and surface area of the pores in as-coated and heat-treated states, revealed that the thermal conductivity of these coatings is principally influenced by the shape of the pores and secondarily by the pore surface area available at the cross section perpendicular to the heat flux