Damage Induced by Thermal Cycling of Thermal Barrier Coatings

Abstract

Different forms of damage have been identified during the course of a systematic study of the effects of thermal cycling on the life and spallation behavior of electron beam deposited thermal barrier coatings (TBC) on a Ni-base superalloy with a ( Ni,Pt)Al bond coat. The growth rate of the α-Al2O3 oxide scale on the bond coat surface appears to be the major factor determining the upper limit of TBC lifetime. Besides oxidation, there are a number of other degradation processes which occur prior to TBC failure and may significantly reduce durability of the TBC. These range from interface separation between the TBC and the bond coat, associated with “rumpling” of the bond coat surface, to the development of sub-surface cavitation in the bond coat, to “wrinkling” of the TBC itself. The incidence of the individual types of damage depends on the thermal cycling profile used, the bond coat surface pre-treatment and, probably, also its chemical composition. This unexpected variety of degradation modes further demonstrates that, although final failure is generally by buckling and subsequent spallation of the TBC, the underlying mechanisms can be far more complex.