Thermal Barrier Coatings are widely used on hot-section components in gas turbine engines to reduce the temperature in the superalloy. Because these components are cooled internally, the thermal gradients through the coating (about 150 mm thick) allow a temperature drop of 100 - 150^oC. This temperature change enables the underlying superalloy to operate at lower temperatures, resulting in considerably enhanced durability, without compromising the gas temperature, while retaining the efficiency. The coatings are multilayered, consisting of a bondcoat deposited on the

superalloy, and a ceramic top coat, typically ZrO₂ as the top layer. The bondcoat protects the substrate from oxidation and the ceramic topcoat provides the thermal protection. As the system is subjected to elevated temperatures, the bond coat oxidizes and a thin layer of aluminum oxide forms between the bondcoat and the topcoat.

The limitation of the present technology is that the coatings spall from the substrate in a stochastic manner. This failure mode prevents the coatings from being used in a "prime-reliant" manner. Should new research enable a minimum durability to be realized, an elevation in the gas temperature could be assured, without exposing the superalloy, whereupon DT could be incorporated into the turbine design. When this become feasible, more efficient turbines will be designed.

Realizing this goal by developing a basic understanding of the mechanisms governing durability provides a focus for the research effort at University of Delaware. Primarily this is done through developing model combining mechanics models with materials science. The research is conducted with a range of collaborators, including industrial, governmental and university partners.

Ratcheting

Ratcheting, or morphological instabilities, are characterized by local imperfections in the TGO that grow on a cyclic basis, eventually causing crack propagation in, and spallation of, the top coat (figure 1). It is characterized by developing during thermal cycling and not isothermal conditions. This failure mechanism is driven by a combination of three non-linear constitutive behaviors in the coating: (1) high temperature inelasticity in the TGO, (2) growth strain in the TGO, and (3) cyclic yielding in the bond coat. The growth strain is induced due to the oxidation process when the new alumina is formed.  Most of the TGO growth occurs as thickening, but a small part is distributed in the grains of the TGO, leading to a lengthening component.  The high temperature inelastic strength of the TGO is often referred to as “growth stress.”  The growth strain is limited by the growth stress, and once the TGO stress reached the level of the growth stress, the lengthening strain is reallocated into thickening strain.  In figure 2 the cases of cyclic versus single cycle scenario are compared, as a function of the cumulative growth strain, showing (a) amplitude increase, (b) tangential stress in the TGO.  The positions designated Y refer to the onset of TGO inelasticity, e.g., growth stress is reached.  In the isothermal scenario, the changes in the displacement essentially stop, while under cyclic conditions, displacements and stresses continue to change once the point of TGO inelasticity is reached.  This is so, since the stresses in the TGO are relaxed during each cycle, allowing for additional accumulation of lengthening strain in the TGO when high temperature is reached.  Ratcheting is observed for systems that have low bond coat yield strength at elevated temperatures, such as Pt-modified alumina, and is subjected to frequent and short cycles, since this allow for relaxation of the TGO during cycling.