Microstructure and oxidation resistance of Si-modified aluminide coatings prepared by slurry sintering on K447A superalloy

Si-modified aluminide coatings and pure aluminide coatings were prepared on K447A superalloy via slurry sintering method using Ni₂Al₃ alloy powders instead of Al powders. A double-layer coating mainly composed of β-NiAl phase was formed after sintering at 1100 °C for 1 h. Si addition promoted the growth of coating during the sintering process. A series of silicides including CrSi₂, Cr₅Si₃, and Ni₁₆Cr₆Si₇ were formed in Si-modified coatings and distributed along the NiAl grain boundaries. After oxidation at 1150 °C for 150 h, Al₂O₃ scale was formed on the surface of aluminide coatings and phase transformation of β-NiAl to γ’-Ni₃Al occurred inside the coatings. Si addition in the coating had a significant impact on reducing oxidation mass gain and retarded the transformation of β-NiAl to γ’-Ni₃Al phase during high temperature oxidation, thus improving high-temperature oxidation resistance. However, the addition of Si powders exceeding 7 wt% weakened the modification effect, and many brittle phases formed in the interdiffusion zone, thereby leading to decrease in oxidation resistance.