I. Introduction
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 (1). 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 (2,3,4). However, to the knowledge of the authors, the thermal conductivity of nanocrystalline (grain size ฯฝ 100 nm) zirconia has not yet been determined.
Recently it has become possible to synthesize (5,6) 60% to 99 ฯฉ % dense yttria stabilized zirconia bodies having grain sizes below 100 nm and an equiaxed grain structure with potential for TBC applications. Theoretical calculations (7) suggest that the thermal conductivity of such fine grained yttria stabilized zirconias will be lower than that of their coarse grained counterparts. The low thermal conductivity of traditional stabilized zirconia is due largely to phonon scattering by vacancies in the oxygen lattice with a mean spacing less than the intrinsic mean free path. It is expected that in nanocrystalline zirconia, a grain size of nanocrystalline dimensions would additionally limit the mean free path of some of the normal phonon modes and thus contribute to a further decrease in thermal conductivity (7,8,9,10). If such an effect, as predicted by theory, could be exploited for nanocrystalline zirconia it would increase its potential value as a TBC material. 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.
II. Experimental
Nanocrystalline powders containing 0, 5.8, 8 and 15wt% yttria, corresponding to 0, 3.3, 4.5 and 8.8mol% yttria were synthesized by the co-precipitation method (11). Commercially available 5.3wt.%