Effect of Time and Temperature on TBC Failure Mode Under Oxidizing Environment

Thermal barrier coatings (TBC) have been recently introduced on hot section components; such as transition pieces and first two stages of turbine blades and vanes of advanced F, G, and H class land-based turbine engines. The TBC coating 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 the 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 below 1010°C (1850°F) on the failure mode of TBCs is unknown. Long-term isothermal tests were conducted on the TBC 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 isothermal testing, the samples were destructively examined to characterize the degradation of TBC and determine the extent of TGO cracking, TGO growth, bond coat oxidation, and TBC failure location after long term exposure for up to 18000 hours. Optical microscopy and 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 delamination of TBC at the TGO/TBC interface to internal oxidation of the bond coat and the bond coat delamination. In this paper, the effect of long-term exposure on delamination of TBC and bond coat failure mode is discussed.