Failure of a Thermal Barrier System Due to a Cyclic Displacement Instability in the Thermally grown Oxide
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FAILURE OF A THERMAL BARRIER SYSTEM DUE TO A CYCLIC DISPLACEMENT INSTABILITY IN THE THERMALLY GROWN OXIDE Daniel R. Mumm and Anthony G. Evans Princeton Materials Institute, Princeton University Princeton, NJ 08540-5211, U.S.A. [email protected] ABSTRACT The mechanism controlling the cyclic failure of a commercial thermal barrier system has been investigated. The system comprises an electron-beam physical vapor deposited (EB-PVD) yttria-stabilized zirconia thermal barrier coating (TBC), deposited on a ( Ni, Pt ) Al bond coating. The thermally grown oxide (TGO) layer that forms between the TBC and bond coat at high temperature is unstable with respect to out of plane displacement, provided initial perturbations are present. With cyclic thermal exposure, the TGO displaces into the bond coat at periodic interfacial sites. The out-of-plane displacements induce strains above the TGO, normal to the interface, that cause cracking. The cracks nucleate either within the TBC layer or at the TBC/TGO interface, and extend laterally until they coalesce with cracks from other sites and coating failure occurs by large scale buckling. The TGO displacements are accommodated by visco-plastic deformation of the underlying bond coat, and are driven by a lateral component of the growth strain in the TGO. The susceptibility of the TGO to out-of-plane displacement depends critically upon the initial morphology of the metal/oxide interface. The observed material responses are compared with predictions of a ‘ratcheting’ model. INTRODUCTION Thermal barrier coatings are widely used in gas turbines for power generation and aeropropulsion [1-6]. They comprise a ceramic layer having minimal thermal conductivity, and sufficient thickness and durability, such that the layer sustains an appreciable temperature gradient in the presence of adequate cooling of the underlying metal. Lowering the temperature of the metallic component prolongs life, regardless of the primary life limiting mechanism: environmental attack (including oxidation), creep rupture, or fatigue. Although the thermally insulating ceramic layer is the key component, the thermal protection system comprises four primary constituents – each of which is dynamic and all of which interact to control the performance and durability of the coating: (i) the thermal barrier coating itself, (ii) the load bearing super-alloy component, treated here as a substrate, (iii) a metallic bond coat (BC) located between the substrate and TBC, and (iv) a thermally grown oxide that forms between the TBC and bond coat layer in service. The ceramic thermal barrier coating is very effective at lowering the exposure temperature of the underlying metal, but is quite permeable to oxygen. Furthermore, the metal temperature is sufficiently high that oxidation will occur if exposed to an oxidizing atmosphere. One of the primary roles of the bond coat then is to promote the growth of a particularly stable and protective TGO layer ( α − Al2O3 ). Thermal barrier coating systems exhibit several modes of failure [6-9].
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