Thermophysical properties and thermal cycling behavior of plasma sprayed thick thermal barrier coatings

Thick yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) of 1.5 mm thickness were produced using atmospheric plasma spraying (APS). The microstructures, thermophysical properties and failure behaviour of the coatings were studied. The coating sprayed at high substrate temperature contained a fine segmentation crack network consisting of circular segments with a diameter of 0.7 mm. This coating had a thermal conductivity of up to 1.75 W/mK at 1200 °C. The thermal expansion appeared to be insensitive to the coating microstructure, and a thermal expansion coefficient of 11.2 K−1 was determined. The segmentation crack network was stable during thermal shock testing, and high segmentation crack density gave rise to improve the thermal shock resistance. The coating with a segmentation crack density of 2.7 mm−1 had a lifetime of more than 1770 cycles at a surface temperature of 1238 °C and approximately 300 cycles at 1335 °C. Failure of the TBCs occurred by spalling and delamination cracking, while oxidation of the bond coat did not play a significant role as failure mechanism. Additionally, phase transformation of the YSZ coating was not observed.