Improvement of service life of advanced thermal barrier coatings by nanostructuring and architecture modification

Thermal barrier coatings (TBCs) are heat resistant and protective layers used in hot section components of gas turbine engines. A conventional thermal barrier coating system consists of a thermal spayed Ni/Co-CrAlY bond coat, providing the oxidation and hot corrosion resistance to the underlying superalloy, and a porous yttria stabilized zirconia (YSZ) top coat (either plasma sprayed or deposited by EB-PVD) acting as an insulating layer. On-service durability of TBCs mainly depends on porosity, compliance, toughness and thermal properties of the ceramic top coat and on the ability of the bond coat to produce an efficient oxygen barrier through the formation of a consistent alumina layer (thermally grown oxide, TGO) at the interface with the top coat. Different approaches to the optimization of the mentioned properties are proposed: i) the use of nanostructured YSZ top coats (for increased high temperature mechanical properties and improved sintering resistance); ii) the deposition of a thin Al2O3-PVD sputtered overlay on the bond coat prior to top coating with YSZ (for decreased aluminum depletion of the Ni/Co-CrAlY coating and reduced thermal oxide growth stresses development). The results of experimental investigations were analyzed considering both the microstructural features (microstructure evolution, oxide growth rates) and the engineering properties (high temperature mechanical behavior, thermal shock resistance); moreover a thermo-mechanical finite element model was developed implementing the oxidation kinetics in order to estimate the effects of the PVD film on the mechanical stress field of the TBC system.