Oxidation behaviors of Ir–Hf and Ir–Zr coatings under different air pressures at 1800 °C

In order to improve the oxidation resistance of Ir, multilayer Ir–Hf and Ir–Zr coatings with gradually increasing content of alloying elements from inside out were formed in situ on Ir by pack cementation. The oxidation behaviors and microstructural evolution of the Ir–Hf and Ir–Zr coatings oxidized under different air pressures at 1800 °C for 1 h were explored and the oxidation mechanisms were revealed. The results showed that the oxidation resistances of the Ir–Hf and Ir–Zr coatings at 1800 °C were much better than that of pure Ir and increased with the decrease of pressure. At 101 kPa, the mass loss rates of the Ir–Hf and Ir–Zr coatings were 0.52 × 10⁻³ and 0.48 × 10⁻³ mg·cm⁻²·s⁻¹, respectively. The as-oxidized Ir–Hf and Ir–Zr coatings mostly exhibited morphologies of internal oxidation with a quasi-gradient transition structure and both coatings were prone to external oxidation with decreasing pressure. The critical pressure for the internal to external oxidation transition of the Ir–Zr coatings was ~1 kPa and higher than that of the Ir–Hf coatings. © 2023 Elsevier B.V.