Shroudless turbine designs offer the advantages of weight reduction and lower mechanical loads on the one hand but bear challenges as high gap sensitivity and high temperatures of the static parts on the other hand. In the last years, a lot of work was carried out in order to develop a sealing system for a shroudless design consisting of an abrasive blade tip coating and an abradable segment coating addressing all the requirements defined. Aside from being abradable, the segment coatings have to be mechanically stable, withstand high thermo-mechanical loadings and have to work for thicknesses larger than 1 mm.
Due to the limited temperature capability of the currently used segment coating material yttria-stabilised zirconia, which combines advantageously a suitable thermal conductivity with a high thermal expansion coefficient, new ceramic materials for the segment coating had to be developed.
A very promising sealing system combines an abrasive blade tip coating with an yttria-stabilised zirconia / magnesia alumina spinel double-layer abradable coating system with a 3D interface structure between the bond coat and the ceramic coatings. The present work gives an overview of the development and the performance of this sealing system.