Effect of bond coat nature and thickness on mechanical characteristic and contact damage of zirconia-based thermal barrier coatings

Mechanical characterization and contact damage of zirconia-based thermal barrier coatings (TBCs) have been investigated using nanoindentation and Hertzian indentation tests. Two types of TBC systems with different bond coat thicknesses of 60 ± 20 and 280 ± 20 μm were prepared using two different processes: air plasma spraying (APS) and high-velocity oxygen fuel (HVOF). Top coats were coated onto each bond coat using the APS process. The TBC system with the bond coat formed using the HVOF process shows a step-like decrease in mechanical properties on passing from the substrate to the top coat, and the bond coat formed using the HVOF process indicates higher values of E and H than that prepared using the APS process. The mechanical properties directly affect indentation stress–strain curves of the TBC systems, and the stress–strain curves of the TBC system with the APS bond coat are significantly lower than those with the HVOF process. The bond coat thickness plays an important role in limiting the effect of damage, indicating that the thin bond coat enhances contact damage and transmits the damage to the substrate, whereas the thick bond coat limits the damage to within the bond coat at a relatively low load of P = 500 N. As the indentation load and the number of cycles increase, the contact damage is enhanced and becomes more severe in the TBC system with the APS bond coat. The fracture modes in the top coat and between the top coat and the bond coat of the TBC system are dependent on the bond coat nature, including the accumulation of damage and its development at high loads of 1000 and 1500 N. The TBC system with the APS bond coat shows a bigger and more intensive damage region.