The effect of grain size, porosity and yttria content on the thermal conductivity of nanocrystalline zirconia

In order to accommodate the ever increasing inlet temperatures of gas turbines, air plasma sprayed (APS) or electron beam physically vapor deposited (EB-PVD) yttria stabilized zirconia thermal barrier coatings (TBC`s) are used to insulate the metallic surfaces. Because of its historic use as a TBC, the thermal diffusivity and conductivity of single crystal and polycrystalline stabilized zirconia have been the subject of numerous experimental investigations. However, to the knowledge of the authors, the thermal conductivity of nanocrystalline (gain size < 100 nm) zirconia has not yet been determined. To ascertain whether or when grain boundary effects begin to dominate thermal conductivity, k, values for a variety of nanocrystalline zirconias of different densities (60--100%), grain sizes (30--400 nm), and purities (0--15wt.% yttria) are compared in this work. Finally the measured values are compared with the thermal conductivities of commercially available air plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) coatings.