Effects of Porosity and Thermal Ageing on In-Plane Cracking Behavior of Thermal Barrier Coatings
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Effects of Porosity and Thermal Ageing on In-Plane Cracking Behavior of Thermal Barrier Coatings Z. Zhang1, J. Kameda1,2, A. H. Swanson1, S. Sakurai3 and M. Sato4 1 Ames Laboratory, Iowa State University, Ames IA 5011 USA 2 Office of Naval Research International Field Office, Tokyo 106 Japan 3 Hitachi Works, Hitachi Ltd., Hitachi 317 Japan 4 Tohoku Electric Power Co., Sendai 980 Japan ABSTRACT This paper describes the effects of porosity and thermal ageing at 950oC for 4000 hr. in air on in-plane cracking behavior of plasma-sprayed thermal barrier coating (TBC) made up of 8 % yittria-stabilized zirconia. The in-plane TBC cracking was analyzed by a protruded TBC bend testing technique together with finite element stress analysis. As-deposited and aged TBC protruded specimens showed a large variation of porosity depending on the location of specimen extraction. The critical local tensile stress (σ*) necessary for the initiation of in-plane cracks for each specimen with different porosity was determined using elastic moduli (E) estimated from the porosity dependence of E. The σ* for in-plane cracking of the as-deposited TBC initially increased with increasing porosity and showed a peak when the porosity reached 0.23. It was shown that in-plane cracking at the interface of TBC and thermally grown oxides required much higher σ* than that at the interface of TBC and bond coatings. The thermal ageing led to a slight increase in σ* for away-from-interfacial TBC cracking. The dependence of in-plane TBC cracking behavior on the porosity is discussed in terms of effective critical stress via the Griffith criterion for porous materials. INTRODUCTION Advanced multilayer coating systems are being applied to maintain the integrity of elevated temperature components. Thermal barrier coatings (TBC), made up of yttria stabilized zirconia (YSZ) with high toughness, and MCrAlY bond coatings (BC) are deposited over a Ni based alloy substrate by a plasma spraying method to shield the thermal conduction and to impede elevated temperature oxidizing attack of the substrate. The dispersion of pores further promotes the thermal shielding effect of TBC. In-service degradation of TBC occurs due to microstructural evolution and thermal cycling. The formation of thermally grown oxide (TGO) near the TBC/BC interface results from high permeability of oxygen through the YSZ. Moreover, tensile/compressive residual stresses are built-up during in-service thermal cycling due to the thermal mismatch between the ceramic TBC, TGO and metallic BC/substrate. Thus, TBC delamination arises from in-plane cracking near the TBC/TGO interface, which is controlled in a complex manner by the TGO, residual stress and porosity [1-3]. For the purpose of examining the integrity of advanced high temperature components and developing better TBC systems, several mechanical testing methods [4-7] are applied to characterize TBC delamination induced while in-service. Nevertheless, the mechanisms of TBC delamination have not been fully understood due to the difficulty in cont
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