Effect of cooling rate and substrate thickness on spallation of alumina scale on Fecralloy

FeCrAlY sheets of different thicknesses (0.477–7.608 mm) have been oxidized at 1200 °C in air for 25 h to form alumina scales, with cooling to room temperature at rates from 1 °C/min to 100 °C/min. Spallation of the alumina scale occurs with intermediate cooling rates, e.g. 5, 10 and 30 °C/min, whereas no spallation is evident in samples with the highest cooling rate of 100 °C/min, suggesting that the residual stress is not the only factor to drive the spallation. For a fixed cooling rate of 30 °C/min, the spalling is more pronounced on thicker substrates than on a thin FeCrAlY alloy substrate where the thicker substrate leads to higher residual stresses in the alumina scale. Chemical analysis reveals the Cr-carbide segregation at grain boundaries of the Fecralloy and the alumina/Fecralloy interface. The degree of segregation decreased with increase of the cooling rate, and appears to be similar with change of substrate thickness. Meanwhile, the grain size of Fecralloy decreases with increase in cooling rate. It appears that the combination of residual stresses, grain size change and the Cr-carbide segregation controls the spallation of the alumina scale. Based on the spallation phenomena, the alumina/FeCrAlY alloy interfacial toughness has been determined tending to increase with increase of cooling rate. The result suggested that the interface is degraded by the carbide segregation and change in Fecralloy composition.