Fig. 7-a shows the high magnification of the TBC in the inter- face of the bond coat and the YSZ coating after 18 h hot corrosion test. This micrograph clearly shows the delamination mechanism of the YSZ. An interfacial crack has formed in the top coat near the interface of the bond coat and the YSZ, where the EDS result (Fig. 7-b), confirmed the formation of TGO. The effect of the corrosion products and the phase transformation on delamination is discussed at following sections.