Microstructure characterization of thermal barrier coating systems after controlled exposure to a high temperature

Air plasma spray-deposited thermal barrier coatings were thoroughly characterized after exposure to a high temperature. Microstructural changes in the thermal barrier coatings, as a function of temperature and soaking time, are reported. Pores in the ceramic top coat were redistributed and rounded due to sintering, which increases density and thermal conductivity. Formation of the undesirable monoclinic phase was detected after 600 h at 1100 °C; the amount of this phase increased with exposure time, reaching 18 wt% after 1000 h. The thermally grown oxide layer reached a critical thickness of 5 µm after 600 h at 1100 °C even though no delamination was observed after 1700 h of exposure when the thickness was around 6.7 µm. Ni- and Cr-enriched oxides were formed, which, in theory, should favor crack generation and propagation. After 1000 h, a large amount of precipitates in the Ni-base substrate were observed and characterized.