Oxidation Resistance of Thermal Barrier Coatings Based on Hollow Alumina Particles

Abstract

Thermal barrier coatings were synthesized in a single step process from a slurry containing Al microspheres onto different Ni-based superalloys. Upon growth of the coating a top coat of hollow alumina spheres linked to an aluminium diffused coating through an alumina TGO formed. The isothermal and cyclic oxidation tests at different temperatures (900 till 1100°C) up to 1000 h or 1500 cycles revealed progressive growth of different thermal oxides depending on the substrate composition. Faster degradation of the coatings occurred in the titanium-rich substrates (e.g. IN-738LC and PWA1483) compared to the titanium-poor ones (CM-247LC and René N5). By comparing with conventional low activity aluminide coatings, it appeared that the incorporation of alloying elements (notably Ti and Ta) to the diffused layers upon the high activity slurry coating process is responsible for such fastest degradation.

Development of thermal barrier coating systems from Al microparticles. Part I: Influence of processing conditions on the mechanisms of formation

“…Therefore, they cannot be applied to other sections that now require thermal insulation due to the increase of the turbine inlet temperatures. Among the alternative coating techniques, slurries containing Al microspheres appear particularly attractive as they can form a top foam of hollow alumina microspheres in addition to an aluminide coating [1][2][3][4][5]. It has been demonstrated that just a 40 µm -thick top foam indeed confers thermal insulation equivalent to those of 400 µm thick conventional plasma-sprayed coatings made of yttria-stabilized zirconia between 400°C and 900°C [6].…”

Electron Microscopy Characterization of the High Temperature Degradation of the Aluminide Layer on Turbine Blades Made of a Nickel Superalloy

“…The diffusional processes taking place in the coatings under exposure to the temperatures applied in the present study can be explained based on the results of the work by Pedraza et al [ 31 ], who investigated both the isothermal and cyclic oxidation of aluminum-rich coatings. The current paper is complementary to [ 31 ] because it focuses on the very top near outer layer of the coating.…”

“…The diffusional processes taking place in the coatings under exposure to the temperatures applied in the present study can be explained based on the results of the work by Pedraza et al [ 31 ], who investigated both the isothermal and cyclic oxidation of aluminum-rich coatings. The current paper is complementary to [ 31 ] because it focuses on the very top near outer layer of the coating. The current paper demonstrates that with the exception of the formation of the alumina layer, all other occurring processes such as coarsening of the microstructure of the coatings, formation of pores, outward diffusion of Fe and Cr to the surface, degrade the insulation and protective properties of the heat-resistant coatings.…”