Microstructural investigation of low-activity and high-activity aluminide coatings fabricated by vapor phase...

To improve the oxidation and hot corrosion resistance of components, parts for gas turbine engines made of nickel-based superalloys are often coated with aluminide coatings. Various methods apply these coatings on superalloys. This research examined the vapor phase aluminizing method. The samples were aluminized through a single-step vapor phase at 1050 °C for a duration of 4 h. This method separates the vapor phase formation chamber from the coating chamber, produces Al_xCl_y vapor in the coating powder input, and transports it to the coating chamber via argon gas to form the aluminide coating. The powder composition was 18 wt% Al, 3 wt% NH₄Cl, and 79 wt% Al₂O₃, and the coating powder inputs considered were 20, 30, 35, and 100 g. After coating, the microstructure and formation of the coating were evaluated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Energy-Dispersive X-ray Spectroscopy (EDS) mapping. The results showed that the thickness of coated samples increased with using more coating powder input from 21.3 μm for 20 g to 68.7 μm for 100 g. The results revealed that powder weights of 20, 30, and 35 g formed low-activity high-temperature (LAHT) aluminide coatings with a double-layer microstructure, with the upper layer free of precipitates, indicating outward diffusion of Ni. The formation of a triple-layer coating for a powder weight of 100 g revealed the presence of precipitates in the upper layer, signifying the inward diffusion of Al. In all coating layers, the matrix phase was β-NiAl, where the outer layer was Al-rich and the interdiffusion zone (IDZ) was Ni-rich.