Effect of thermal treatment on the effective thermal conductivity of YPSZ coatings

Abstract

Thermally sprayed coatings present effective properties strongly different from those of the primary bulk material. In particular, the actual thermal conductivity of Yttria Partially Stabilized Zirconia (YPSZ) coatings is typically twice lower than the thermal conductivity of dense YPSZ. The architecture of the porous network plays a major role on this decrease: thin inter-lamellar cracks act as thermal resistance and contribute to decrease the effective thermal conductivity more efficiently than globular pores. From this situation, an in-house code has been developed since a few years: this code implements a finite difference method to perform calculations directly on micrographs of coating cross-sections obtained by SEM. Each pixel of the intermediate binary picture is interpreted as a cell of integration of the heat conduction equation. A thermal gradient is applied between the top and bottom edges and a system of linear equations is formed and solved, providing the thermal flux flowing through the structure and the corresponding effective thermal conductivity. In the present study, the case of YPSZ coatings before and after thermal treatment was considered. The numerical results are in rather good agreement with experimental data: the thermal treatment tends to close a part of the thinnest pores, thus providing a decrease of the pore level and an increase of the effective thermal conductivity of the produced coatings.