In this study, a disk-shaped thermal barrier coating system for a gas turbine blade was isothermally aged at $1100^{circ}C$ for various times up to 800 h. The microstructures of the bond coat were observed by using an optical microscope for each aging condition. Diffusion of primary elements consisting of the bond coat was characterized by using electron probe microanalyzer mapping. Semi-quantitative analysis by using scanning electron microscopy energy-dispersive X-ray was insufficient to sense variations in the Al content within the bond coat according to exposure time. An area fraction of ${eta}$-NiAl phases within the bond coat was calculated by image analysis, and a reasonable approach to estimate time for complete depletion of the Al-rich ${eta}$-NiAl phase within the bond coat was suggested.