Diffusion Aluminide Coatings Using Spherical Micro-Sized Aluminium Particles
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.
A. Agüero, J.M. Albella, M.P. Hierro, J. Phillibert and F.J. Pérez Trujillo
M. Juez-Lorenzo et al., "Diffusion Aluminide Coatings Using Spherical Micro-Sized Aluminium Particles", Defect and Diffusion Forum, Vols. 289-292, pp. 261-267, 2009