Effect of Al2O3 scales from pre-oxidation on the microstructural evolution and phase transition of NiAlHf coatings at 1200 °C

The pre-oxidation treatment was conducted for NiAlHf coatings in a sealed glass tube with low oxygen atmosphere at 900 °C. The microstructure evolution and oxidation resistance of the NiAlHf coatings with and without pre-oxidation treatment were systematically investigated via isothermal oxidation at 1200 °C for up to 200 h in laboratory air. The results revealed that a dense Al2O3 scale was formed on the NiAlHf coating surface after pre-oxidation treatment, which could effectively reduce oxygen pressure and Al consumption on the NiAlHf coating surface. Furthermore, the NiAlHf coating microstructure evolution was effectively delayed from β-NiAl to the γ′-Ni3Al phase due to Al2O3 scale formation by pre-oxidation treatment. Oxidation resistance of the NiAlHf coating with pre-oxidation treatment was improved after 200 h of oxidation, of which the oxidation rate constant was enhanced by 40% compared to the NiAlHf coating without pre-oxidation treatment. The mechanism of oxygen diffusion inhibition by pre-oxidation was also discussed. This research was beneficial to understand the effect of pre-treatment on the NiAHf coating microstructure evolution and could further guide the improvement of the oxidation resistance of NiAlHf coatings for long-term oxidation. •A dense Al2O3 layer is prepared by pre-oxidation treatment.•Al2O3 layer can reduce the pressure of oxygen on the surface of NiAlHf coatings, effectively delaying the consumption of Al and transformation form β-NiAl to γ'-Ni3Al phase.•Through pre-oxidation treatment, the weight gain is reduced and oxidation rate constant is enhanced by 40%.