Damage Accumulation Mechanisms in Thermal Barrier Coatings

Abstract

Spallation is a major failure condition experienced by thermal barrier coatings (TBCs) subjected to thermal and mechanical loads. Although evidence of spallation is substantiated and mechanistic models to describe the failure condition is prevalent in literature, the progressive nature of damage evolution leading to spallation has not been addressed adequately. In this paper, we investigated the damage evolution in partially stabilized zirconia TBC on Nickel-based single crystal superalloy, Rene N5. Thermal cycles were imposed on button specimens with Electron Beam - Plasma Vapor Deposition (EB-PVD) TBC coating. The bond coat was PtAl. The temperature range used was 200–1177C. Progressive damage evolution was tracked using microscopy on samples subjected to a series of thermal cycles. Fick’s law can describe the thermally grown oxide (TGO) buildup for early cycles. However, at higher number of thermal cycles, damage in the form of microcracks and their coalescence results in the loss of integrity of the TGO. Thus, both oxidation kinetics and damage appears to have significant roles to play as it relates to spallation. As these microcracks coalesce to form major delamination cracks or interlayer separation, the susceptibility for coating buckling is increased. The delamination cracks finally consume the TGO layer. The loss of TBC integrity from the bond coat and the substrate facilitates its buckling during cool down from elevated temperature. Our estimations show that a delamination crack length of about sixteen times the TBC thickness is needed for the current material system to initiate buckling. Progressive microcrack linking is a possible mechanism to develop such critical delamination crack lengths. Physical evidence of buckling was found in specimens prior to complete spallation.