High temperature thermal fatigue causes the failure of Thermal Barrier Coating (TBC) systems. This paper addresses the development of thick TBCs, focusing attention on the microstructure and the porosity of the Yttria Partially Stabilized Zirconia (YPSZ) coating, in relation to its resistance to thermal cycling fatigue. Thick TBCs, with different grade of porosity, were produced by means of a CoNiCrAlY bond coat and Yttria Partially Stabilised Zirconia top coat, both sprayed by Air Plasma Spray. The thermal fatigue resistance of new TBC systems and the evolution of the coatings before and after thermal cycling were evaluated. The limit of thermal fatigue resistance increases with amount of porosity in the top coat. Raman analysis shows that the compressive in-plane stress increases in the TBC systems after thermal cycling, nevertheless the increasing rate has a trend contrary to the porosity level of top coat.

Yttria Partially Stabilized Zirconia (YPSZ) coatings are used as Thermal Barrier Coatings (TBCs) because of their capability to improve gas turbine performance by allowing higher turbine inlet temperature and reduced cooling air flow. Usually, YPSZ layers are applied by plasma spray on Ni-based superalloys pre-coated with a bond coat layer of MCrAlY, where M is Co, Ni, or both. In this work we used Raman spectroscopy to study the performance of TBCs during thermal cycling at two different temperature (1273K and 1373K). Raman scattering is capable of detecting monoclinic zirconia and to analyze residual stresses of the coatings with an higher sensitivity than traditional X-ray diffraction measurements. Raman spectra were analyzed by deconvolution methods in order to study the evolution of the relative intensities and position of different bands. We found that the position of the 635 cm-1 band shifts to higher values of wave number during thermal cycling and by comparing this data with those reported in literature, it is possible to retrieve the value of residual stresses in zirconia coating. We also observed a relation between the intensities of the bands at 602 cm-1 and 635 cm-1 band and the number of thermal cycles performed.