Microstructure Analysis of the Pack Cementation Aluminide Coatings Modified by CeO₂ Addition

Minor additions of reactive elements (Y, Ce, Hf, etc.) can improve the oxidation behavior of high temperature alloys and coatings such as aluminide coatings. In the current investigation cerium-modified aluminide coatings were developed via additions of CeO₂ to the powder mixture of high activity pack cementation aluminizing. Nickel-based substrates were aluminized at 750 °C and then diffusion annealed at 1080 °C. Microstructure of the coatings was studied using scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The results were compared with the characterization results of specimens achieved by aluminizing an electrodeposited Ni–CeO₂ interlayer at 1030 °C. The coatings modified by CeO₂ addition to the pack showed a dense Al-rich NiAl surface layer with cerium distributed in the coating, however, the coatings modified by Ni–CeO₂ interlayer revealed a porous Ni-rich aluminide layer containing CeO₂ particles. Increasing the CeO₂ content of the pack decreased the thickness of the final coating, but the presence of CeO₂ particles in the interlayer increased the thickness of the final aluminide coating. The observed microstructural features were explained using thermodynamic analysis of equilibrium concentration of the involved gaseous species.