Synthesis of hafnium-doped beta-nickel-aluminum coatings using a hot-wall CVD reactor by sequential and continuous doping procedures

The dynamic versatility of chemical vapor deposition (CVD) offers the possibility of controlling, proactively as well as precisely, the concentration and distribution of Hafnium as a beneficial dopant while the surface of a single crystal Nickel superalloy is aluminized to form an aluminide (β-NiAl) coating. Because of metallurgical complexity and processing reproducibility problems, the Hf doping approach has not been yet properly demonstrated for use in aircraft engines and power generation gas turbines.; In this study, a laboratory-scale, hot-wall CVD reactor was specifically designed and constructed to perform critical Hf doping experiments, with emphasis on understanding the effects of “sequential” and “continuous” doping procedures on the incorporation behavior of Hf and the development of coating microstructure. The sequential doping procedure, which consisted of “hafnizing” the Ni superalloy surface with HfCl₄ and H₂ followed by aluminizing with AlCl₃ and H ₂, resulted in significant Hf incorporation through the formation of Hf-rich precipitates. However, these precipitates acted as diffusion barriers to retard the subsequent growth of the β-NiAl coating matrix.; In contrast, the continuous doping procedure, in which HfCl₄ and AlCl₃ were simultaneously introduced with H₂, required rather high HfCl₄ concentrations to dope the aluminide coating near the apparent solubility limit of Hf in the β-NiAl matrix. The segregation of Hf and the formation of a thin γ′-Ni₃Al layer (∼0.5 μm) at the coating surface were consistently observed for the continuous doping experiments. These observations suggested that: (1) the coating growth occurred at the interface between the γ′ -Ni₃Al layer and the β-NiAl coating matrix and (2) the incorporation of Hf into the growing β-NiAl matrix was most likely dictated by the difference in Hf solubility between the γ′-Ni ₃Al and β-NiAl phases.; From a practical point of view, the results from this study showed that: (1) the simple doping procedures are ineffective in terms of proactively controlling the concentration and distribution of Hf in the coating matrix and (2) time-resolved delivery of the dopant precursor, HfCl₄, may be used as a key variable for process optimization.