Due to complete spallation in conventional YSZ after 30 h hot
corrosion test, the cross sectional observation via SEM was not helpful to see any cracks of TGO. Therefore the authors decided to observe the samples before complete failure to identify if any cracks or TGO have been formed during hot corrosion test. Thus the samples after 18 h hot corrosion test were selected to observe via SEM.
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 cor- rosion products and the phase transformation on delamination is discussed at following sections.
Fig. 8(a and b) illustrate the SEM micrographs of the top surface
of the conventional YSZ and the laser clad coating, respectively, after the hot corrosion test. As seen, after 30 h of hot corrosion test, rod-type crystals have formed on the conventional YSZ coating. The EDS analysis (Fig. 8-c) proves that this elongated phase is rich in yttrium and therefore is formed as a result of the reaction of yttria with the molten salt at elevated temperature, showing poor resistance of the YSZ coating. In contrast, these elongated features are scarce on the surface of the laser clad coating (Fig. 8-b), proving significant improvement of the hot corrosion resistance of the TBCs after laser cladding of alumina on the surface of YSZ as a result of the formation a dense imperme- able layer on the top surface of the YSZ coating. The elimination of the imperfections such as voids and cracks after laser cladding, reduces the penetration paths of molten salts into the YSZ layer. The smooth layer of the TBCs after laser cladding (having lower roughness) also decreases the effective contact surface between the YSZ and the molten corrosive salts. The fact that no