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 …

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.

In order to further improve the hot corrosion resistance of yttria-stabilized zirconia (YSZ), an Al{sub 2}O{sub 3} overlay of 58 {micro}m thick was deposited on the surface of YSZ by electron-beam physical vapor deposition. Hot corrosion tests were performed on the YSZ coatings with {gamma}-Al{sub 2}O{sub 3} overlay and {alpha}-Al{sub 2}O{sub 3} overlay in molten salt mixture (Na2SO4 + 5wt%V2O5) at 950 C. The {alpha}-Al{sub 2}O{sub 3} overlay was obtained by the post-annealing of g-Al{sub 2}O{sub 3} overlay at 1200 C for 1h. The results showed that compared with the hot corrosion resistance of YSZ coating with 25 {micro}m thick {gamma}-Al{sub 2}O{sub 3} overlay, either thickening {gamma}-Al{sub 2}O{sub 3} overlay or employing {alpha}-Al{sub 2}O{sub 3} overlay could impair the hot corrosion resistance of YSZ coating, because the tensile stresses developed in the alumina overlay in both cases due to the mismatch in thermal expansion coefficient (TEC) between alumina and zirconia resulted in cracking of Al{sub 2}O{sub 3} overlay. The formation of cracks increased contact area between molten salt and Al{sub 2}O{sub 3} overlay, and also the penetration rate of molten salt into Al{sub 2}O{sub 3} overlay and YSZ coating, leading a faster and greater degradation of YSZ coating upon exposure. …