THERMAL-DIFFUSIVITY OF PLASMA-SPRAYED COATINGS OF ZRO2 WITH 8 WT-PERCENT Y2O3 AND ZRO2 WITH 25 WT-PERCENT CEO2

The thermal diffusivity of coatings several tenths of a millimetre thick is affected by their surface properties. Both types of coatings under study, namely ZrO2 with 8 wt.% Y2O3 and ZrO2 with 25 wt.% CeO2, were found to be semitransparent solids. In such coatings the heat transfer takes place via photon and phonon mechanisms. The thermal diffusivity of the coatings is not a material constant, and with a given wavelength of incident radiation it depends on the coating thickness. This value cannot be exactly determined by the flash method. The flash method can only give an exact determination of the value of so-called ''true'' thermal diffusivity, which refers to the phonon mechanism of heat transfer. This value, however, is not characteristic of the actual mechanism of heat transfer through the coating. Semitransparency was found to be greater for coatings of ZrO2 with 25 wt.% CeO2 than for coatings of ZrO2 with 8 wt.% Y2O3. The thermal diffusivity of both types of coatings decreased when their surfaces were ground. The decrease was greater for thinner coatings. The character of the thermal dependence of thermal diffusivity in the temperature range from 200 to 950-degrees-C was the same for the two types of coatings.