Effect of thermal fatigue on mechanical characteristics and contact damage of zirconia-based thermal barrier coatings with HVOF-sprayed bond coat

Mechanical characteristics, such as hardness, elastic modulus and indentation stress–strain curves, and contact damage of a thermal barrier coating (TBC) system with a top coat prepared using an air-plasma spraying (APS) process and a bond coat using a high-velocity oxygen flow (HVOF) process have been investigated using the nanoindentation and Hertzian indentation tests, as a function of the thermal fatigue condition. The bond coat and the top coat deposited on the substrate make the TBC system soft, showing lower stress–strain curves than that of the substrate. Thermal fatigue does not affect the stress–strain curves, except for thermal fatigue for 500 h. However, the thermally grown oxide (TGO) layer thickness is dependent on the exposure time under thermal fatigue, showing a nominal thickness of approximately 4 μm after thermal fatigue for 500 h, independent of the number of thermal fatigue cycles. The values of hardness, H, in each component are not greatly affected by thermal fatigue, except for thermal fatigue for 500 h, whereas the value of elastic modulus, E, in the bond coat is dominantly affected by thermal fatigue with a smaller increase for the other components—top coat and substrate. The H/E ratio for the top coat is higher than those for the bond coat and the substrate, indicating that resintering of the top coat occurs during thermal fatigue. The top coat acts as a protection layer for contacts, resulting in reduced damage to the substrate. As the exposure time is increased in the thermal fatigue experiments, the damage to the top coat is inhibited with less crack coalescence. The higher stiffness in the bond coat induces a cracking or delamination at the interface between the bond coat and the substrate, whereas thermal fatigue increases the mechanical properties, especially E, of the bond coat and enhances the damage tolerance of the TBC system.