THERMAL-CONDUCTIVITY OF THERMAL BARRIER COATINGS

In thermal barrier coatings and other ceramic oxides, heat is conducte d by lattice waves, and also by a radiative component which becomes si gnificant at high temperatures. The theory of heat conduction by latti ce waves is reviewed in the equipartition limit (above room temperatur e). The conductivity is composed of contributions from a spectrum of w aves, determined by the frequency dependent attenuation length. Intera ction between lattice waves (intrinsic processes), scattering by atomi c scale point defects and scattering by extended imperfections such as grain boundaries, each limit the attenuation length in different part s of the spectrum. Intrinsic processes yield a spectral conductivity w hich is independent of frequency. Point defects reduce the contributio n of the high frequency spectrum, grain boundaries and other extended defects that of the low frequencies. These reductions are usually inde pendent of each other. Estimates will be given for zirconia containing 7wt% Y2O3, and for yttrium aluminum garnet. They will be compared to measurements. The effects of grain size, cracks and porosity will be d iscussed both for the lattice and the radiative components. While the lattice component of the thermal conductivity is reduced substantially by decreasing the grain size to nanometers, the radiative component r equires pores or other inclusions of micrometer scale. (C) 1998 Elsevi er Science S.A. All rights reserved.