Optimization of chemical vapor deposition process parameters for aluminizing

Ni-based super alloys such as Inconel 738LC, MAR-M-200, CMSX-4 are used in the hot sections of industrial gas turbines due to their excellent mechanical strength and creep resistance at high service temperatures. To protect turbine blades from thermo-mechanical failure, oxidation/corrosion resistant coatings are necessary to extend the life time. For this purpose, diffusion coatings including chromized, siliconized, aluminide coatings have been studied by different groups. Among them, aluminide coatings are widely preferred in turbine applications due to its ability to form stable oxides that prevent oxygen diffusion to substrate materials. Aluminide coatings are generally applied by pack cementation, above-pack-process, slurry and chemical vapor deposition (CVD) methods. Although pack cementation is simpler and cheaper coating method, CVD has advantages over other coating processes because of its high ability to control process variables such as process gas flows and their ratios, chamber temperature, chlorinator temperature and system pressure; hence coating microstructure and homogeneity in composition. In this research, effect of CVD process variables on gas phase aluminizing are investigated to obtain highly pure and chemically uniform β phase NiAl compound with high Al content (35 - 55 %at.) aluminide coating on IN738LC substrates. Before CVD process, turbine blade surfaces are analyzed by linear profilometer to observe influence of surface roughness on aluminide coating microstructure and aluminum uniformity through substrate. Also substrates are weighed by precision scales. After aluminizing process, surface roughness and weight measurements are conducted again to observe any change due to CVD process. Coating microstructures, phases and chemical compositions are characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy (WDS). Cross section analysis are performed by SEM to measure aluminide coating thickness consist of β-NiAl and interdiffusion zone (IDZ). Effect of CVD parameters on β-NiAl to IDZ ratio are investigated to obtain higher ratio such as 5:1. For identification of coating phases, X-ray crystallography (XRD) technique is used to observe formation of undesirable Ni – Al coating phases such as Ni2Al3, Ni3Al