Corrosion behaviour of NiCrFeAl-hBN seal coatings in oxidation environments at a high temperature

The purpose of this investigation was to evaluate the microstructure, high-temperature oxidation behaviour, and hardness of a seal coating under controlled exposures at 750 °C with different exposure times. The results reveal that the main phases in the coating are Ni, FeNi₃, and hexagonal BN, with α-Al₂O₃ peaks appearing after 2-h oxidation. As the exposure time increases, the diffraction peak intensity of α-Al₂O₃ begins to decrease and Cr₂O₃ peaks appear after 200 h. After 1000 h, Cr₂O₃ becomes dominant with only small amounts of α-Al₂O₃ remaining. The high-temperature oxidation process of the coating includes three stages: the fast stage, transition stage, and slow stage. The oxidation rate is constant in each stage, and in the fast stage and slow stages with values of 7.59 × 10⁻⁴ and 8.37 × 10⁻⁵ mg²·cm⁻⁴·s⁻¹, respectively. Initially, α-Al₂O₃ forms near the pores in the coating, followed by the formation of Cr₂O₃. As the exposure time increases, oxidation penetrates into the interior of the coating along phase boundaries to form island-shaped inner oxide layers with high concentrations of Cr, Ni, and Fe. Additionally, hardness increases with increasing oxide percentage.