Effects of Prehafnizing on Morphological Development of a Chemical Vapor Deposition Aluminide Coating Formed on Single-Crystal Ni-Based Superalloy

We examined a sequential Hf doping procedure, which consisted of (1) "prehafnizing" the surface of a single-crystal Ni-based superalloy (RENÉ N5) with HfCl₄ and H₂, and (2) aluminizing with AlCl₃ and H₂, as a means of incorporating Hf as a dopant in the aluminide coating matrix. The prehafnized layer on RENÉ N5 substrate significantly altered the growth behavior and therefore the morphology of the resulting aluminide coating. With the prehafnizing step, the coating layer became much thinner with a significant amount of Hf incorporated as Hf-rich phases (Hf₂Ni₇, Hf₃Ni₇, and/or Hf₈Ni₂₁). However, the Hf-rich phases segregated to the coating surface and retarded the inward Al diffusion required to form the β-NiAl coating matrix. The sequential Hf doping procedure provided a mechanism to incorporate a significant amount of Hf in the coating, but did not produce a uniform distribution of Hf as a dopant. The results were compared to those observed for a continuous doping procedure that was previously studied, and were discussed in the context of understanding the limitations of these procedures.