Diffusion Aluminide Coatings Using Spherical Micro-Sized Aluminium Particles

Abstract

Previous work on the oxidation of nano- and micro-sized Al particles revealed a particle size window, where no meta-stable alumina phases were observed. Depositing such particles on an austenitic substrate, diffusion layers with reduced Al contents were obtained. These findings opened new perspectives for investigating the potential impact of the Al particle size and shape on the formation of diffusion aluminide coatings. Spherical Al particles sized in the range of 2 to 5 µm were deposited with a binder by brushing on the austenitic steel X6 CrNi 18-10 (Alloy 304H). For the curing process, the samples were annealed in air at 400°C for 1h. The diffusion effect of Al into the base material was studied in isothermal experiments at 700°C and 900°C with exposure times up to 2000 h in air. The sample surfaces and the diffusion aluminide coatings in cross-section were analysed by field emission scanning electron microscopy (FE-SEM). The results show in the initial state the formation of a diffusion layer consisting of a less aluminium-rich Fe(Cr)-Al phase containing a Fe(Cr)-Al phase with higher content of Al in the region beneath the surface. On further exposure a double-layered structure is found with Kirkendall-pores between the two layers, which may lead to a complete separation of the outer layer. A thin adherent alumina scale is observed on the remaining diffusion layer after 1000 h and 2000 h at both temperatures, however overgrown by Cr2O3 at 900°C. The structure of the diffusion zone beneath agglomerates of Al particles reveals the influence of the particle size on the Al supply for the formation of the aluminide diffusion zone.

Initial Aluminizing Steps of Pure Nickel from Al Micro-Particles

“…Recently, a new concept has been developed within the European project "PARTICOAT" [6] to build in one step protective systems comprising an Al reservoir, an Al 2 O 3 bond coat and a thermal barrier. This is realised by depositing on the alloy surface a slurry [7,8] made of Al µ-particles dispersed in a binder then applying a two steps thermal treatment in order to diffuse Al into the substrate. The first thermal treatment step at about 700°C leads to the formation of a brittle Ni 2 Al 3 layer, whereas the second step at a higher temperature allows to get β-NiAl, which is the expected phase.…”

On the Influence of a Heat Treat for an Aluminizing Progress Based on Al Microparticles Slurry for Model Ni and Ni20Cr. Experimental and Theoretical Approaches.

“…The group of A. Agüero already focused on the use of slurry aluminide coatings on ferritic and martensitic substrates to increase their oxidation resistance against air and water vapor atmospheres for steam turbine components [3][4], while other authors [5][6][7] developed some powder liquid coatings using aluminum powders on ferritic and/or nickel based substrates to be employed in energetic and extreme environments. Such environmentally friendly "slurry-related" alternatives are now under investigation within the framework of the European project "PARTICOAT" [8] to elaborate the aluminide layer combined with a ceramic thermally insulating top-coat onto Ni-based superalloys in a similar manner to the one described by Kolarik et al for austenitic stainless steels [9][10]. During the first step of the heat treatment, a Ni 2 Al 3 diffusion coating is obtained, which is typical of a high activity/low temperature process [11] and during the second step at higher temperature, a β-NiAl diffusion layer, a thin α-Al 2 O 3 bond coat and, on the top of it, a ceramic foam composed of hollow α-alumina sphere are realized.…”