Wednesday, 13 October 2004 - 2:15 PM

This presentation is part of : Failure of Ceramics and Ceramic Composites

Tracking Interfacial Toughness Loss in Thermal Barrier Coating Systems

Jack L. Beuth1, Qin Ma1, Frederick Pettit2, Gerald Meier2, and Matthew Stiger2. (1) Carnegie Mellon University, Department of Mechanical Engineering, Pittsburgh, PA 15213, (2) University of Pittsburgh, Department of Materials Science and Engineering, Pittsburgh, PA 15260

A principal concern with ceramic thermal barrier coatings (TBCs) used in gas turbines is their loss of adhesion during service, leading to coating spallation. In this presentation, an overview is given of an indentation test used to quantify decreases in interfacial toughness of TBC systems due to isothermal or cyclic high-temperature exposures in dry or wet air. The indentation test involves penetration of the TBC and the oxide layer below it, inducing plastic deformation in the underlying metal bond coat and superalloy substrate. This plastic deformation induces a compressive radial strain away from the indent, which drives an axisymmetric delamination of the TBC and oxide layers.

Test results are presented tracking the "apparent" loss of toughness (that could include changes in the TBC system that contribute to adhesion loss independent of interfacial damage) for EBPVD TBC systems as a function of isothermal exposures from 1100C to 1200C. Analyses of these tests are presented which quantify the relative importance of oxide thickening, TBC sintering and interfacial damage in decreasing apparent TBC system adhesion. The importance of as-processed interfacial toughness in determining the rate of toughness loss with exposure and in determining TBC life is demonstrated. Results are also presented that give insight into the importance of steam exposure and cyclic thermal loading compared to isothermal loadings in dry air. Finally a description is given of extensions of the test to include different indenter shapes and the indentation of cylindrically shaped substrates.

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