Thermal barrier coatings (TBCs) are widely used in gas turbines to reduce thermal exposure of structural components and increase turbine efficiency. They typically consist of a MCrAlY bond coat and a YSZ topcoat. At high temperatures, a thermally grown oxide (TGO) layer forms between the bond coat and topcoat. If this layer is a continuous scale of alumina, it will act as a diffusion barrier to suppress the formation of other detrimental oxides, thus helping to protect the substrate from further oxidation. It has been reported, however, that other oxides, such as chromia, spinel, and NiO, may form along with the TGO layer, ultimately leading to TBC failure. To investigate such claims, coatings of comparable thickness were deposited by various spraying methods onto a superalloy substrate using a powder of the same composition. Samples were isothermally oxidized at 1273 K for different periods up to 3000 hours. The samples were examined before and after furnace tests and the results are presented and discussed.

High temperature thermal fatigue causes the failure of Thermal Barrier Coating (TBC) systems. Due to the difference in thickness and microstructure between thick TBCs and traditional thin TBCs, they cannot be assumed a-priori to possess the same failure mechanisms. Thick TBCs, consisting of a CoNiCrAlY bond coat and Yttria Partially Stabilised Zirconia top coat with different values of porosity, were produced by Air Plasma Spray. Thermal fatigue resistance limit of TBCs was tested by Furnace Cycling Tests (FCT) according to the specifications of an Original Equipment Manufacturer (OEM). TBC systems were analyzed before and after FCT. The morphological and chemical evolution of CoNiCrAlY/TGO microstructure was studied. Sintering effect, residual stress, phase transformation and fracture toughness were evaluated in the ceramic Top Coat. All the tested samples passed FCT according to the specification of an important OEM. Thermal fatigue resistance increases with the amount of porosity in the top coat. The compressive in-plane stresses increase in the TBC systems after thermal cycling, nevertheless the increasing rate has a trend contrary to the porosity level of top coat. The data suggest that the spallation happens at the TGO/Top Coat interface. The failure mechanism of thick TBCs subjected to thermal fatigue was eventually found to be similar to the failure mechanism of thin TBC systems made by APS.

In order to improve gas turbine performance it is possible to decrease back flow gases in the high temperature combustion region of the turbo machine reducing shroud/rotor gap. Thick and porous TBC systems and composite CoNiCrAlY/Al 2 O 3 coatings made by Air Plasma Spray (APS) and composite NiCrAlY/graphite coatings made by Laser Cladding were studied as possible high temperature abradable seal on shroud. Oxidation and thermal fatigue resistance of the coatings were assessed by means of isothermal and cyclic oxidation tests. Tested CoNiCrAlY/Al 2 O 3 and NiCrAlY/graphite coatings after 1000 hours at 1100°C do not show noticeable microstructural modification. The oxidation resistance of new composite coatings satisfied Original Equipment Manufacturer (OEM) specification. Thick and porous TBC systems passed the thermal fatigue test according to the considered OEM procedures. According to the OEM specification for abradable coatings the hardness evaluation suggests that these kinds of coatings must be used with abrasive tipped blades. Thick and porous TBC coating has shown good abradability using tipped blades.

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.

This paper compares two methods for determining the composition of Ti/TiN coatings deposited by reactive plasma spraying. The coatings were obtained by spraying titanium powder in a low-pressure N2/Ar atmosphere. The resulting film has a variable nitrogen content in the form of titanium nitrides, depending on gas partial pressure, total pressure, sample-source distance, and other parameters. The composition of the film was determined using X-ray diffraction and X-ray photoelectron spectroscopy. The two techniques provide similar results and either can be used for the compositional characterization of these coatings.