Thermal Barrier Coating Failure

Dr. Daniel Balint
Engineer
Exponent, Inc.

Abstract

Multilayer thermal barrier coatings (TBCs) deposited on superalloy blades in turbine engines provide protection from operating temperatures that can exceed 1500°C. Internal cooling establishes a thermal gradient across the coating that prolongs the lifetime of the underlying blade. TBCs are complex, dynamic systems that evolve with thermal cycling. One of the dominant failure modes comprises cracking from undulation growth, or rumpling, of the compressed oxide layer that grows between the ceramic topcoat and the intermetallic bond coat. Interaction between the stress in the bond coat induced by the constraining effect of the thick substrate blade and normal tractions applied at the surface of the bond coat by the compressed oxide layer allows undulations to grow incrementally over many engine cycles. Rumples crack the topcoat locally, eventually leading to large spalls as the cracks coalesce. Oxide rumpling is highly nonlinear and characterized by more than twenty material and geometric parameters, necessitating a robust yet computationally efficient model. A technological perspective on thermal barrier coatings in turbine engines and a survey of failure modes will be presented, followed by a summary of a recent analytical model of oxide rumpling. A selection of results obtained using the model will be shown to elucidate the mechanics and replicate trends observed in a series of seminal experiments. Viable ways of designing more tolerant TBCs based on the predictions of the model will be discussed.

Thursday, January 12, 2006
337 Towne Bldg.
2:00 – 3:00 p.m.