Failure mechanism of an out-of-pack nickel aluminide coating cyclically oxidised at 1100 °C with water accelerated cooling

The failure mechanism of a nickel aluminide coating cyclically oxidised for 1 h-cycles at 1100 °C and cooled by liquid water is described using a cyclic oxidation rig allowing very rapid cooling (15.9 s) between 1100 °C and ambient temperature. Repeated cycling first provoked the development of cracks into the coating as well as martensitic transformations (β/L10) within the β-NiAl phase. The coating subsequently oxidised internally through the cracks during the oxidation stage. With increased exposure, these zones grew laterally, thus consuming the entire Al available in the intermetallic phase. A degradation mechanism based on crack formation and internal oxidation is proposed.