Thermal barrier coatings (TBC) reduce the thermal load of highly loaded gas turbine components. State-of-the-art TBCs consist of yttria stabilized zirconia and are deposited by thermal spray techniques (e. g. atmospheric plasma spraying) or electron beam physical vapor deposition (EB-PVD). The resulting microstructure is dependent on the process and strongly influences the coating´s properties and time to failure. Columnar structures exhibit a higher lifetime and are thus favored.
This talk investigates an alternative deposition technique (gas flow sputtering - GFS), which also gives rise to columnar microstructures. These microstructures can be altered by changing the process parameters and the surface condition of the substrate.
The substrate temperature has been identified to be the crucial parameter. Between 500 and 800°C, four different groups of microstructures are observed and described utilizing SEM, FIB and XRD. The principal shape of the columns is explained using a growth model.
To evaluate GFS coatings for the use as TBCs, thermal cycling experiments were conducted. Delaminations were linked to a thermally grown mixed-oxide zone, and two failure mechanisms were found: Due to low stiffness, highly porous microstructures cannot resist buckling of the thermally grown oxide (TGO), resulting in local delaminations. For denser coatings, segmentation cracks develop due to pronounced intercolumnar sintering, growing together with delamination cracks at the interface. In conclusion, the GFS process allows to manufacture various different microstructures, which seem to be promising for future use as TBCs.