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On the influence of a heat treatment for an aluminizing process based on Al microparticles slurry for model Ni and Ni20Cr. Experimental and theoretical approaches

: Rannou, B.; Mollard, M.; Bouchaud, B.; Balmain, J.; Bonnet, G.; Kolarik, V.; Pedraza, F.


Bezverkhyy, I.:
Diffusion in materials. International Conference on Diffusion in Materials, DIMAT 2011. Selected, peer reviewed papers : July 3 - 8, 2011, Dijon, France
Durnten-Zurich: TTP, 2012 (Defect and Diffusion Forum 323/325)
ISBN: 978-3-03-785397-9
International Conference on Diffusion in Materials (DIMAT) <8, 2011, Dijon>
Fraunhofer ICT ()

The use of thermal barrier coating systems allows superalloys to withstand higher operating temperatures in aeroengine turbines. Aiming at providing oxidation protection to such substrates, an aluminum-rich layer is deposited to form the alpha-Al2O3 scale over which a ceramic layer (i.e. YSZ layer) is applied to provide thermal insulation. A new approach is now being investigated within the FP7 European project « PARTICOAT », in which a single step process is employed by applying micro-sized aluminum particles. The particles are mixed in a binder and deposited by brushing or spraying on the substrate surface. During a heat treatment, the particles sinter and oxidize to form a top coat composed of hollow con-joint alumina spheres and simultaneously, an Al-rich diffusion zone is formed in the substrate. For a better understanding of the diffusion / growth processes, preliminary tests were carried out on pure nickel and Ni20Cr model alloys prior to further application on commercial superalloys. The effect of the heat treatment on the coating characteristics (number of layers, thickness, composition, homogeneity, etc.) was particularly investigated to emphasize the mechanisms of diffusion governing the growth of the coatings. The establishment of the diffused layers occurred very readily even at intermediate temperatures (650 and 700°C). However, the layers formed did not match perfectly with the thermodynamic modeling because of the quick incorporation of Ni into molten Al at intermediate temperatures (650°C). In contrast, at higher temperatures (700 and 1100°C) the phases predicted by Thermocalc are in good agreement with the observed thickness of the diffused layers. The incorporation of Cr as an alloying element restrained Al ingress by segregation of Cr even at very low temperatures aluminizing temperatures (625°C).