Microstructural evolution of a NiCoCrAlY coating on an IN100 substrate

The microstructure of a NiCoCrAlY bond coat (BC) of a thermal barrier coating (TBC) system is investigated in dependence on the heat treatment temperature between 770 and 980 degrees C by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Besides the major gamma and phases, additionally gamma precipitates within the beta phase, alpha-Cr precipitates within the phase, grains of the Cr-rich intermetallic sigma phase and M23X6 phase occur. The gamma' precipitates are observed within the bond coat up to a certain distance from the substrate depending on the temperature. This range decreases with temperature between 920 and 800 degrees C linearly. At higher temperatures, almost no gamma' precipitates are detected. The a-Cr precipitates within the grains are found within all [ grains for heat treatment temperatures below 770 degrees C. For higher temperatures up to 860 degrees C, they exist only within p grains near the substrate and are not proven for temperatures higher than 890 degrees C. The sigma grains are abundant after heat treatment at 770 degrees C, decreasing in number with increasing temperature until disappearing after heat treatment at 980 degrees C. For the M23X6, no temperature dependence was observed. The microstructural evolution of the NiCoCrAlY coating is suggested to be used as post mortem temperature indicator. (c) 2004 Elsevier B.V. All rights reserved.