Mechanisms of formation of slurry aluminide coatings from Al and Cr microparticles

The mechanisms of formation of nickel aluminide coatings from alternate deposition of Al and Cr microparticles and subsequent heat treatment in Ar were studied on pure nickel. For this purpose, tailored amounts of Al and Cr water-based slurries were successively deposited on pure nickel following two different architectures (Al/Cr and Cr/Al double-layer systems). Regardless of the coating architecture, the addition of Cr microparticles was found to decrease the thermodynamic activity of Al upon aluminizing through the formation of AlxCry phases. This considerably limited the inward diffusion of Al towards the substrate at low temperature (e.g. 650 °C). Whereas the formation of δ-Ni2Al3 was still observed with the Al/Cr double-layer system, its formation was completely suppressed with the Cr/Al one. By adjusting the composition of the deposited layers, i.e. the thickness of both Cr and Al layers, it was possible to directly form the β-NiAl phase with a further annealing at high temperature (e.g. 1000 °C for 3 h). This fostered the outward diffusion of nickel and the dissolution of synthesized AlxCry phases. Undissolved Cr-rich phases were also observed in the diffusion layers, which confirmed the outward growth of the coating typical of low-activity aluminizing. •Double-layer deposits (Al/Cr and Cr/Al) with tailored amounts of Al and Cr successfully elaborated by slurry on pure Ni.•The Ni substrate-Al/Cr configuration results in high-activity aluminide coatings.•The Ni substrate-Cr/Al configuration results in low-activity aluminide coatings.•The mechanisms of formation of such coatings involve the appearance of intermediate AlxCry intermetallics.