Effect of Co addition on oxidation behavior of IrAl

Although iridium is promising to be as an effective oxygen diffusion barrier (ODB) for ultrahigh-temperature structural materials, iridium itself is not applied to be a practical oxidation resistant coating due to its poor oxidation resistance caused by gaseous IrO3 formation. Recently the authors proposed B2 IrAl-base smart coatings which can form an iridium layer as ODB and an aluminum layer as a protective coating on the iridium layer by oxidation. However, the oxidation resistance of binary IrAl alloys is still insufficient. In this study, the effect of Co addition to IrAl on oxidation behavior was studied where Co addition was expected to enhance B2 phase stability. Binary IrAl and IrAl containing 10 mol% Co were fabricated by reactive hot pressing method and oxidation behavior was evaluated using simultaneous differential scanning calorimetry (DSC) and thermogravimetry (TG) in O2 up to 1873 K. X-ray difrractometry (XRD) and Scanning electron microscopy (SEM) were also carried out for alloy characterization. It was revealed for heating oxidation up to 1863 K in O2 that the oxidized products confirmed by XRD were Ir, IrO2 and Al2O3 for both the alloys and that Co2AlO4 was recognized only for Co-added IrAl. It was also found for isothermal oxidation up to 1673 K that thin and continuous Al2O3 layer was formed at surface only for Co-added IrAl. It is concluded that oxidation resistance of IrAl alloys was dramatically improved by Co addition.