Anisotropic mechanical properties and contact damage in air-plasma-sprayed thermal barrier coatings with bond coating nature and thermal exposure condition

The anisotropic mechanical properties and contact damage of air-plasma-sprayed (APS) zirconia-based thermal barrier coatings (TBCs) have been investigated using Vickers and Hertzian indentation tests as functions of the nature of the bond coating and the degree of thermal exposure. The hardness values of the TBC systems are dependent on the applied load at relatively low loads, and became saturated at a load of 30 N, independent of the nature of the bond coating or the degree of exposure. The values of the top coating obtained on the top surface from the Vickers indentation tests were higher than those on the sectional plane, indicating that there is an anisotropic strain behavior due to the microstructure. The regions near to the interface of the top coating and the thermally grown oxide (TGO) layer show higher values after thermal exposure, whereas the values of the APS bond coating increased and the indentation values of the high-velocity oxygen fuel (HVOF) sprayed bond coating slightly decreased after thermal exposure, owing to resintering and element deficiency during thermal exposure, respectively. In contact damage tests, the TBC system with the HVOF bond coating showed less damage than the TBC system with the APS bond coating. The shape of the damage was different between the two systems. After thermal exposure, the damage was reduced in both TBC systems, and the cracking or delamination formed at the regions near to the interface of the top coating and the TGO layer in both TBC systems.