Enhanced cerium migration in ceria-stabilised zirconia

The metal components inside jet engines which are exposed to hot combustion gases often must be thermally insulated with thermal barrier coatings. Most thermal barrier coatings consist of a porous zirconia layer deposited over a NiCoCrAlY layer. However, the zirconia Layer can spall during thermal cycling unless it is stabilised to prevent the zirconia changing between its tetragonal and monoclinic phases. We have studied thermal barrier coatings in which the zirconia was stabilised by alloying with 2.5 wt.% yttria and 25 wt.% ceria. The spatial distributions of the cerium in the zirconia layer before and after heat treatment were studied using electron microscopy (backscattered electron imaging and X-ray analysis). The different phases present were identified by X-ray diffraction. We found that the cerium distribution in the newly-formed zirconia layer was essentially uniform. However, hearing at 1200 degrees C could cause the formation of cerium-rich zones provided the coating was not separated from the NiCoCrAlY layer and the substrate before heating. This implied an interaction between the zirconia and the underlying metal. Experiments in the presence of oxygen getters and experiments in vacuum both suggest that cerium migration in ceria-stabilised zirconia is enhanced at high temperatures (1100 degrees C) under reducing conditions. (C) 1997 Elsevier Science S.A.