Analysis of pore morphology and thermal conductivity of alternative EB-PVD deposited TBC-compositions

Kurzfassung

Thermal barrier coatings (TBCs) consist of a ceramic top coat deposited onto a bond-coated superalloy substrate and are used to increase lifetime and efficiency of highly loaded turbine blades and vanes by reducing the average metal temperature and mitigating the detrimental effects of hot spots. Electron-beam physical vapor deposition (EB-PVD) of the standard top coat, partially yttria-stabilized zirconia (PYSZ), produces various anisotropic pores which contribute in the reduction of thermal conductivity. However, increases in combustion temperature for improved turbine efficiency causing greater thermal loading alters the pore morphology resulting in an increase of thermal conductivity. Long-term dignity of the ceramic top coat of TBCs can be provided through the development of new materials with ultra-low thermal conductivity, excellent phase stability and improved sintering-resistance. Fully yttria (14 wt.% Y2O3) stabilised zirconia (FYSZ) yields intrinsically lower thermal conductivity. Further optimization of the TBC properties is possible by tailoring its microstructure, since an important relationship exists between thermal conductivity and process-induced pore structure of EB-PVD-TBCs. Pore morphology and microstructure of EB-PVD pyrochlore differ significantly from those of PYSZ and FYSZ. The influence of the pore morphology on reduction of thermal conductivity and resistance to thermal loading are not well-investigated in the new TBC compositions. This work describes the anisotropic nano-sized pore morphology in FYSZ and Pyrochlore (La2Zr2O7) TBCs obtained by small angle neutron scattering (SANS) analysis method. In order to differentiate between the 3D closed and open pores in 400 µm thick coatings, a contrast matching liquid SANS technique was employed. Samples in several arrangements such as free-standing EB-PVD-layers and TBCs deposited on metallic substrates were analysed by the Laser-Flash technique for their thermal diffusivity in the as-coated condition and after ageing at 1100°C in air. The pore migration and pore morphology changes were investigated by advanced TEM/SEM-analysis on FIB sections. Thermal derived changes in crystal structure as well as pore size and morphology observed by TEM and SANS have been correlated with the thermal conductivity changes.