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℃ with different exposure times.The results reveal that the main phases in the coating are Ni,FeNi3,and hexagonal BN,with α-Al2O3peaks appearing after 2-h oxidation.As the exposure time increases,the diffraction peak intensity of α-Al2O3begins to decrease and Cr2O3peaks appear after 200 h.After 1000 h,Cr2O3becomes dominant with only small amounts of α-Al2O3remaining.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 of7.9×10-4and 8.37×10-5mg2·cm-4·s-1,respectively.Initially,α-Al2O3forms 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.