Progress in life time modeling of APS-TBC Part I: residual, thermal and growth stresses including the role of thermal fatigue

Abstract

For about the last two decades there has been an effort to produce a reasonable life time model for the thermal barrier coating systems (TBC) that are used in the gas turbine industry. However, recent advances in testing technology, namely acoustic emission (AE) analysis and Raman spectroscopy, have provided many new insights into the life of TBCs. This new technology used in conjunction with more traditional testing, such as the four point bend mechanical test, has provided much needed data for the development of a life time model.

Part I of this paper is devoted to the stress situation that exists in isothermally and cyclically oxidized TBC systems. The focus of the current TBC research is on the air plasma sprayed (APS) systems. However, some results for the electron beam physical vapor deposited systems (EB–PVD) are presented for the purpose of providing insight into the role of interface roughness and the stresses in the thermally grown oxide (TGO). AE analysis of isothermally and cyclically oxidized samples is used to address the role of “desk top failure” and of thermal fatigue. The possible role of intrinsic growth stress is addressed by presenting the measurement data from the oxidation of freestanding NiCoCrAlY foil. Findings are supported by thermal stress calculations and micrographs.

Part II of this paper is devoted to the development of a life time model. Here TBC top coat failure and life time is considered as a two-step process, where step 1 is time to delamination macro-cracking and step 2 is time to through macro-cracking. This two-step failure mechanism comes directly from critical strain measurement data and traditional coating failure theory. Included in the model is damage accumulation due to bond coat oxidation and due to thermal fatigue. The damage terms in the model have their origins in AE data from cyclic oxidation samples. The life time model is then used in concert with measured micro-crack lengths. Finally, a full model is presented for isothermal and cyclic oxidation of APS–TBC.

Keywords:

• life time modeling
• residual stress
• thermal stress
• growth stress
• thermal fatigue
• acoustic emission