TBCs have been used in the last 50 years in gas turbines to reduce the metal temperature of cooled hot components and provide protection against oxidation. TBCs are complex systems formed by 3 layers:
Bond layer: made of diffused aluminide, platinum or MCrAlY that provides bonding between the base metal and the top ceramic coating, as well as protection of the base metal against oxidation.
Thermally Grown Oxide (TGO): an intermediate layer of oxide which is formed though exposure to high temperatures during the manufacturing process and operation.
Outer ceramic coating: usually stabilized yttria-zirconia which provides the thermal insulation. Due to its low thermal conductivity (~1.5-2W/mK), this coat can create a temperature gradient of approximately 150°C in 200-500µm, which induces strong thermal stresses.
This article contains excerpts from the paper, “Lifetime of gas turbines hot section parts in an oil and gas environment” by Bernard Quoix, Amélie Pesquet and Pablo Bellocq of Total E&P at the 2018 Turbomachinery and Pump Symposia.
Yttria-zirconia is commonly used because its thermal expansion coefficient (9 x 10-6 m/K) is not far
from that of nickel superalloys (14 to 16 x 10-6 m/K) and this minimizes the thermally induced stress between the coating and base material. The thickness of this layer is designated to provide a compromise between thermal insulation and thermally induced stress which has a direct impact on durability of the coating.
TBCs typically fail by delamination/spallation and expose the base materials to an environment they were not designed to withstand. Various complex damage mechanisms can interact and produce delamination/spallation of TBCs:
Thermal gradients and transients produce thermal stress in the coating due to the difference in thermal expansion which result in fatigue damage.
During operation, TGO thickness grows (due to oxidation) and induces “growth” stresses. Additionally, when the TBC is cooled (unit shut down), relatively high compressive residual stresses (up to 4-6 GPa) is developed in the TGO due to the difference in thermal growth of the various layers. This can lead to undulation of the TGO.
Creep of bond coat: Changes in TGO and in service temp gradients lead to relatively high inter layer stress which at high temperatures produce considerable creep deformation on the bond layer.
Excessive oxidation of the bond layer: As the TGO grows, the amount of Al in the bond coat is reduced and at some point, oxides, which are more brittle than Al2O3, start developing. These brittle oxides reduce the fatigue life of the coating.
Sintering of the ceramic coat: This happens at high temperatures and has three main effects: a) increase in thermal conductivity degrading the thermal insulation function of the coat; b) local increase of the elastic modulus (E) and consequently increase in stress (strain is fixed by thermal expansions); c) local shrinkage at the top of the coat initiating cracks.
Erosion damage and Foreign body impact. Erosion of minute particles degrades the TBC. Additionally, high gas speeds produces shear in the outer layer which can be of alternating nature (blades passing vanes) and eventually leads to fatigue crack nucleation in the ceramic layer.
Modification of the structure due to molten deposits: Calcium, magnesium, aluminum or silicates present in the air can melt in the combustion chamber and diffuse through the TBC. When they reach the solidification temperature, they form crystals inside the TBC distorting its structure and making it fragile.
As it can be appreciated, the damage mechanisms of TBCs are quite complex and they all interact with each other. The existing TBC lifing models in the open literature try to capture some of the described mechanisms (or a combination of 2 or even 3) and also include some simplified failure criteria. Such models can be tuned to a specific TBC system (composition, manufacturing process, dimensions, etc.) and in a particular component and operational envelope.