Abstract
Aluminide coatings were produced by halide-activated pack cementation (HAPC) and halide-activated slurry cementation (HASC) on Ni-base superalloys (UNS N07208, UNS N06230, and UNS N07718). Both HAPC and HASC are in situ chemical vapor deposition processes activated by halide salts. The chemical reactions deposit the desired element (aluminum in this case) on the surface of metallic alloys. The coating element subsequently diffuses into the substrate to modify the surface of the underlying alloy. Under oxidizing conditions, the surface modified alloy forms a passive oxide layer that protects it from high temperature corrosion. In this study, the characteristics of coatings applied by the pack and slurry cementation processes, i.e., thickness, microhardness, microstructure and elemental distribution, were studied using X-ray diffraction (XRD), macrophotography, optical microscopy and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). The HAPC and HASC processes resulted in different coating characteristics; the HAPC process produced more consistent coating microstructures and thicknesses, while the coatings applied by the HASC process were more variable. The effectiveness of HAPC and HASC coatings in high temperature environments was studied using isothermal oxidation testing.
