Maximizing β-NiAl/IDZ Ratio of High-Growth CVD Aluminide Coatings on Ni-Based Superalloys for Turbine Engine Applications

Turbine engine components operating in harsh, corrosive environments rely on protective aluminide diffusion coatings to shield against damage. Among all processing techniques, chemical vapor deposition (CVD) offers distinct advantages in tailoring coating architectures due to its precise thermodynamic control over variables such as temperature, total system pressure, precursor activity, and carrier/reactant gas velocities. However, optimizing deposition rates against in situ diffusion kinetics to maximize the outer β-NiAl layer relative to the underlying inter-diffusion zone (IDZ) presents a challenge while satisfying β-NiAl phase homogeneity throughout the diffusion coating grown over nickel-based superalloys. This study demonstrates the formation of aluminide coatings in a low aluminum activity regime to maximize the β-NiAl/IDZ ratio while maintaining high growth rates under varying CVD process conditions such as temperature, pressure, (HCl, hydrogen and argon) gas flows, HCl/hydrogen ratios, chlorinator temperature, process time, and substrate surface roughness and, for two of the commonly used substrate material types (IN738LC and IN939), for advanced propulsion applications. Univariate experiments revealed optimal growth rates and maximum β-NiAl/IDZ thickness ratios are achieved at 1050 °C, under 100 mbar pressure, and with an HCl flow of 125–250 sccm, while prior substrate surface grinding ensures consistent coating thickness and uniformity. The higher initial Cr content in IN939 leads to a greater accumulation of chromium precipitates within the IDZ due to limited solubility in the β-NiAl layer.