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Models for ratcheting

Ratcheting is a failure mechanism in thermal barrier systems motivated by a complex interaction of dynamic processes occurring in the various layers of the system. The mode of failure is typical of systems involving Pt-modified aluminide bond coats. In essence, it involves amplification of initial undulations in the bond coat as a result of stresses arising in the system, both at temperature as well as during thermal cycling, creating separations from the thermal barrier layer which eventually coalesce and cause spallation.

A key feature of the mechanism is the lateral extension of the thermally grown oxide (TGO) as it thickens. This is ascribed to new aluminum oxide forming at the grain boundaries creating a growth strain that must be accommodated elastically or, at high temperature, by flow of the TGO, the bond coat, or both. Interdiffusion between the bond coat and the substrate is a major contributor to the evolution in several important ways, including swelling as a result of asymmetric fluxes, phase transformations with associated volume changes, and displacive (martensitic) transformations that change the flow characteristics of the metal.

While the details of the ratcheting phenomena and associated mechanisms remain under investigation, models have been developed to explore the contributions of different parameters and exposure conditions. These models are contained in a number of publications, and some of them have been transferred to industry for further development.

Informations connexes

Reported by

University of California, Santa Barbara
2361A Engineering II, Materials Department
93106-5050 Santa Barbara, California
United States