This research aims to investigate the effects of employing cryo-milled and milled MCrAlY feedstock powders on the oxidation behaviour of low-pressure plasma sprayed (LPPS) and HVOF-sprayed coatings deposited onto a Ni-based superalloy substrate. Commercially-available powders with three different chemical compositions were selected and sprayed both in standard condition and after milling and cryo-milling processes. The LPPS and HVOF coatings, deposited onto an Inconel substrate, were diffusion-treated at 1080 °C (according to the industrial standard) and subjected to isothermal and cyclic oxidation tests. The outcomes of these tests show that transient oxidation is suppressed in the coatings obtained from milled MCrAlY systems, whose overall resistance to cyclic oxidation (number of cycles to failure) is approximately two times greater than that of standard coatings. This difference is not related to the nanostructural features induced on the powder particles by the milling process, because, after the diffusion treatment, all coatings exhibit identical γ-β two-phase microstructure, with no trace of the original nanostructure. The improvement is ascribed to the fine dispersion of nanometric Al 2 O 3 grains within the milled powder particles: in the sprayed coatings, these nanometric oxides act as nuclei and favour the direct formation of an Al 2 O 3 oxide scale.

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.

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.