Doctor of Philosophy (PhD)
Thermal barrier coatings (TBCs) play an important role in protecting the superalloy components from the hot gas stream in gas-turbine engines. Driven by the higher inlet temperature of modern gas turbine engines for higher efficiency and less harmful gas emission, exploration of TBC new materials and thermal radiation effects of TBCs have attracted more attentions recently. In this study, thermochemical compatibility of Gd2Zr2O7 (GZ) and yttria-stabilized-zirconia (YSZ) at 1300 ºC was investigated. Single, double and three ceramic-layered GZ based TBCs were prepared and their thermal cycling properties were performed under different thermal conditions. Thermochemical compatibility investigation showed that YSZ tended to transform into a solid solution with GZ structure but higher ZrO2 concentration compared to stoichiometric GZ due to the ion diffusion in the process of heat treatment, which would cause the volume change and thermal stress. Thermal cycling results showed that delamination usually occurred from the sample edge in YSZ layer close to the YSZ/thermally grown oxide (TGO) interface through a sequence of crack nucleation, propagation and coalescence events. The underlying motivation was the compression in the TGO around interface imperfections caused by TGO growth and thermal expansion misfit between the ceramic layer and the bond coat upon cooling. Sintering of GZ shrinked the ceramic layer, introduced the in-plane tensile stress and caused the vertical cracks. Failure of TBCs was accelerated by increasing the temperature and cycle frequency. The reflectance and transmittance spectra of YSZ and GZ coatings were measured and four-flux, two-flux and Kubelka-Munk models were deduced to determine the absorption and scattering coefficients. Results showed that scattering coefficients of GZ are higher than that of YSZ in low wavelength range indicating that GZ can reflect more thermal radiation energy to the hot gas stream and permit less radiation to penetrate the coating, which efficiently decreases the bond coat temperature and prolongs the life span of TBCs. To determine TBC absorption and scattering coefficients, Kubelka-Munk model is over simplified by neglecting the interface reflectivity, while the two-flux model can be used as a simplification of the four-flux model when the coating thickness is larger than ~200µm.
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Wang, Li, "Thermal cycling and thermal radiation performances of novel thermal barrier coatings" (2013). LSU Doctoral Dissertations. 3701.