Effect of Time and Temperature on Thermal Barrier Coating Failure Mode Under Oxidizing Environment

Thermal barrier coatings (TBCs) have been recently introduced to hot section components, such as transition pieces and the first two stages of turbine blades and vanes of advanced F, G, and H class land-based turbine engines. The TBC is typically applied on metallic-coated components. The metallic bond coat provides oxidation and/or corrosion protection. It is generally believed that the primary failure mode of TBCs is delamination and fracture of the top ceramic coating parallel to the bond coat in the proximity of the thermally grown oxide (TGO) between coatings. One of the concerns associated with the use of a TBC as a prime reliant coating is its long-term stability. The effect of long-term operation at typical land based turbine operating temperatures of below $1010°C$ (1850°F) of the failure mode of TBCs is unknown. Long-term isothermal tests were conducted on the thermal barrier-coated specimens at three temperatures, $1010°C$ (1850°F), $1038°C$ (1900°F), and $1066°C$ (1950°F), to determine the effects of long term exposure on the TBC failure location (mode). Following the isothermal testing, the samples were destructively examined to characterize the degradation of the TBC and determine the extent of TGO cracking, TGO growth, bond coat oxidation, and TBC failure location after long term exposure for up to $18,000h$⁠. Optical microscopy and a scanning electron microscope (SEM) attached with an energy dispersive spectroscopy (EDS) system were used to study the degradation of the TBC and bond coatings. The results showed that long term isothermal exposure leads to a change in the TBC failure mode from the delamination of the TBC at the $TGO∕TBC$ interface to the internal oxidation of the bond coat and bond coat delamination. In this paper, the effect of long-term exposure on the delamination of TBC and the bond coat failure mode is discussed.