Spallation Behaviour of Alumina Scale Formed on FeCrAlY Alloy After Isothermal Oxidation

The spallation behaviour of alumina scales grown on FeCrAlY alloy was investigated. Substrates with different thicknesses were oxidized at 1200 °C for 25 h and cooled at various cooling rates. Generally, the scale formed on a thicker substrate or with a faster cooling rate exhibits a larger compressive stress. However, the failure behaviour of alumina scales is more complicated than expected for a compressed film. Specifically, (i) the extent of the spallation is not proportional to the residual stress in the oxide; (ii) the spallation does not occur immediately after cooling, but requires a period of incubation. This indicates that the residual stress is not the sole reason for the failure of scales. It was found that the carbide forms at the oxide–metal interface after cooling, which acts in conjunction with the residual stress to control the spallation of oxides. In addition, the mechanics analysis suggests that the microscopic roughness at the interface is another important factor.