Hertzian stress analysis and characterization of thermal barrier coatings containing unidirectional vertical cracks

Abstract Finite element method (FEM) analysis and experimental studies are conducted for designing thermal barrier coatings using yttria-added gadolinium zirconia (Y-GZO) and yttria-stabilized zirconia (YSZ) under indentation contact stresses. The designed thermal barrier coating contains unidirectional vertical cracks (UDCs) to alleviate stress when subjected to thermal cycling or thermal shock. The optimal bilayer thermal barrier coating comprises porous YSZ as the bottom layer and dense Y-GZO material containing UDCs as the top layer. To investigate the advantages of bilayer thermal barrier coatings containing UDCs, the thermal and mechanical durability of the bilayer coatings are compared with those of a single-layer-coated YSZ without UDCs. Modeling and FEM analysis show that the thermal barrier coating containing UDCs shows stress alleviation contours under the same indentation load. Additionally, the coatings containing UDCs with different sequences of dense and porous layers exhibit excellent advantages from the viewpoint of stress relaxation. Results show that the bilayer thermal barrier coatings containing UDCs exhibit better durability against interface delamination during thermal cycling and thermal shock tests in comparison with single-layer coating without UDCs. The bilayer thermal barrier coating containing UDCs exhibit good mechanical behavior since the modulus of elasticity and hardness remain unchanged during thermal shock cycling and exhibit excellent abrasion resistance. These results show that a durable coating layer can be designed as a thermal barrier coating for gas turbines.