Thermal Barrier Coating Modeling for Stress Analysis

Abstract

Thermal barrier coatings (TBCs) have been used widely in aerospace and land-based gas turbines. The TBC system consists of a top coat layer, a thermally grown oxide (TGO), a bond coat layer and a substrate. The growth kinetics of the TGO significantly affects the durability of TBCs. At a critical TGO thickness, the growth stresses exceed the ceramic-bond coat interface strength, resulting in TBC system failure. Regardless of the deposition method used, it is vitally important to accurately predict the TBC lifetime by investigating the determinants of the failure. The main objective of this study was to investigate the effect of oxidation stress induced by TGO layer in high temperature cycling environment through a series of reliable numerical simulations. Indeed, this oxidation stress is a known factor of interface degradation, and may result in failure of the ceramic-metal interface. A 2-D finite element model of the TBC was built via ANSYS APDL software, to conduct parametric studies of increasing complexity. The model accounted for elasticity first, before creep was integrated. Then, the model included swelling induced by phase transformation associated with oxidation, incorporating the effect of volumetric expansion of the newly grown TGO. This coupled oxidation constitutive approach was implemented for a typical air plasma spray deposited TBC coating. The interfacial radial stresses induced by the gradual oxidation were investigated. Different morphologies of the TBC interface were also considered to analyze the roughness effect on interface stresses. A complete model including swelling, creep, aging effects on the TBC layers at a given roughness was finally investigated.