Influence of Thermal Cycle Frequency on the TGO Growth and Cracking Behaviors of an APS-TBC
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JTTEE5 21:1294–1299 DOI: 10.1007/s11666-012-9824-4 1059-9630/$19.00 Ó ASM International
Influence of Thermal Cycle Frequency on the TGO Growth and Cracking Behaviors of an APS-TBC W.R. Chen, X. Wu, and D. Dudzinski (Submitted May 1, 2012; in revised form June 29, 2012) The durability of thermal barrier coatings (TBCs) is controlled by fracture near the interface between the ceramic topcoat and the metallic bond coat, where a layer of thermally grown oxide (TGO) forms during service exposure. In the present work, the influence of thermal cycle frequency on the oxidation performance, in terms of TGO growth and cracking behavior, of an air-plasma-sprayed (APS) Co-32Ni-21Cr-8Al0.5Y (wt.%) bond coat was studied. The results show that while TGO growth exhibited an initial parabolic growth behavior followed by an accelerated growth stage, higher cycle frequency resulted in a faster TGO growth and a higher crack propagation rate. It is found that a power-law relationship exists between the maximum crack length and the TGO thickness, which is independent of the cycle frequency. This relationship may warrant a TBC life prediction methodology based on the maximum crack length criterion.
Keywords
cracking, holding time, TBC, TGO growth, thermal cycling
1. Introduction The metallic bond coat (BC) is an important constituent of thermal-sprayed thermal barrier coating (TBC) systems. It enhances the adhesion of the ceramic thermal barrier layer (the topcoat or TC) and also provides oxidation and corrosion protection to the substrate metal. At elevated temperatures, however, oxidation of the BC results in the formation of a thermally grown oxide (TGO) layer at the ceramic/BC interface, which protects the BC and substrate from further oxidation but may also initiate cracks if TGO growth is excessive under service conditions. Eventually, oxidation of the BC will cause separation of the ceramic TC layer from the substrate (Ref 1-6). In previous studies, BC oxidation kinetics were usually quantified by specific weight change during thermal exposure at elevated temperatures (Ref 7-11). The results generally showed a parabolic weight-gain behavior followed by weight loss. The weight loss was due to scaling off of the oxide scale formed on the surface of the coating, and therefore a significant part of the oxidation life information was lost. Thus, the weight-gain method may not be suitable to characterize the entire oxidation process, particularly from a durability point of view. The parabolic growth behavior was also observed in terms of the oxide scale thickness as function of the exposure time
(Ref 12-17); however, the effect of cycle frequency has not been studied to a greater extent. It was noted that, in general, high-frequency thermal cycling could significantly degrade the performance of hightemperature alloys and coatings (Ref 18-21), and one explanation was that more frequent cycling might disrupt the oxide scale by introducing a fine network of small cracks or spalled segments and so alleviate the massive strain energy bu
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