To improve the hot corrosion resistance of YSZ thermal barrier coatings, a 25 {micro}m and a 2 {micro}m thick Al{sub 2}O{sub 3} overlay were deposited by HVOF thermal spray onto to the surface of YSZ coating. Oxidation at high temperature and hot corrosion tests showed that Al{sub 2}O{sub 3} overlay deposited on the YSZ TBCs surface can not only reduce the hot corrosion rate, but also significantly prevents the bond coat from oxidation.

To improve the hot corrosion resistance of YSZ thermal barrier coatings, a 25 {micro}m thick Al{sub 2}O{sub 3} overlay were deposited by HVOF thermal spray, respectively, onto to the surface of YSZ coating. In the next reporting period, we will measure or calculate the residue stress within Al{sub 2}O{sub 3} overlay and YSZ coating to study the mechanism of effect of Al{sub 2}O{sub 3} overlay on spalling of YSZ coating. However, due to the thermal expansion mismatch between YSZ coating and Al{sub 2}O{sub 3} overlay, such surface modification using Al{sub 2}O{sub 3} overlay might deteriorate strain tolerance of the TBC. In the present work, in order to investigate the effect of Al{sub 2}O{sub 3} overlay on residual stress developed in the samples during cooling after hot corrosion at high temperature, Finite Element method (FEM) was employed to determine the detailed stress states in the test specimens after cooling. The results showed that there is no high stress concentration at the interface between the YSZ and the bond coat for TBCs system without Al{sub 2}O{sub 3} overlay. On the other hand, the maximum compressive stress with a value of approximately, -330 MPa occurred within the Al{sub 2}O{sub 3} overlay. The maximum …

In order to improve the hot corrosion resistance of conventional YSZ TBC system, the overlay of Al{sub 2}O{sub 3} coating was deposited on the TBC by EB-PVD techniques. Hot corrosion tests were carried out on the TBC with and without Al{sub 2}O{sub 3} coating in molten salts mixtures (Na{sub 2}SO{sub 4} + 5%V{sub 2}O{sub 5}) at 950 C for 10h. The microstructures of TBC and overlay before and after exposure were examined by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDX) and X-ray diffraction (XRD). It has been found that TBC will react with V{sub 2}O{sub 5} to form YVO{sub 4}. A substantial amount of M-phase was formed due to the leaching of Y{sub 2}O{sub 3} from YSZ. Al{sub 2}O{sub 3} overlay coating deposited by EB-PVD was dense, continues and adherent to the TBC. As a result, overlay Al{sub 2}O{sub 3} coating can prevent the YSZ from the attack by molten salts containing vanadium and arrest the penetration of salts into the YSZ along porous and cracks in the YSZ TBC, although there were some cracks in overlay Al{sub 2}O{sub 3} coating and at the interface between alumina and zirconia formed during hot corrosion tests due to the …