Failure characteristics during cyclic oxidation of yttria stabilized zirconia thermal barrier coatings deposited via electron beam physical vapor deposition on platinum aluminide and on NiCoCrAlY bond coats with processing modifications for improved performances

Abstract

In this study, the cyclic oxidation lives of the current state-of-the-art thermal barrier coating (TBC) systems (heavy grit-blasted Pt aluminide and NiCoCrAlY bond coats with EBPVD TBCs) were investigated first, followed by TBC systems that were modified based on the results obtained on the failure of the state-of-the-art TBC systems. The specimens were subjected to cyclic oxidation testing, mostly at 1100°C, in a bottom-loading furnace in laboratory air. Optical and scanning electron microscopy techniques were used to characterize the as-processed and failed specimens. The state-of-the-art TBC systems with NiCoCrAlY bond coats failed as the result of defects that were identified as TBC defects, transient oxides, surface defects, and reactive element-rich oxide protrusions. On the other hand, the failures of the state-of-the-art TBC systems with Pt aluminide bond coats were due to deformation of the bond coat by a mechanism known as ratcheting. The stored strain energy in the thermally grown oxide (TGO) was also a factor that contributed to the failure of both systems. Most of the modifications performed on the state-of-the-art TBC systems improved their lives to some extent. In the case of NiCoCrAlY systems, elimination or at least minimization of the identified defects was responsible for the improvements, whereas the prevention of the ratcheting type of failure was the main reason for the improvement in lives in the case of Pt aluminide systems. On the other hand, other issues, such as slower growth of the TGO as well as improved TGO/bond coat interfacial toughnesses with some of the modifications, were observed to be contributing factors in the improved lives. Based on the observations on the failure of both the state-of-the-art as well as the modified TBC systems, the surface condition of the bond coats and the morphology of the TBCs close to the TGO were found to have a first-order effect on the failure of TBC systems. The characteristics of the TGO, such as composition, growth rate, and adherence both to the bond coat and the TBC, as well as the characteristics of the bond coats were also observed to have an effect on the failures. Recommendations for future work that should be pursued to better define the conditions necessary for optimized TBC performances are given.

Comparison of the Failures during Cyclic Oxidation of Yttria-Stabilized (7 to 8 Weight Percent) Zirconia Thermal Barrier Coatings Fabricated via Electron Beam Physical Vapor Deposition and Air Plasma Spray

“…The number of cycles to failure decreases markedly with increasing exposure temperature (approximately a factor of 10 for each 100 K (100°C) increase). An Arrhenius plot of these data [18] yielded an apparent activation energy of 356 kJ/mole, which is quite close to that for the parabolic rate constant for the growth of a-alumina.…”

“…The MCrAlY bond coats were deposited on the superalloy substrate using an argon-shrouded plasma spray process. In order to attempt to minimize the defects in EBPVD TBCs on NiCoCrAlY bond coats, [18] the bond coats were polished to Ra~0.3 lm compared to the Rã 3 lm for the state of the art bond coat surfaces that were initially grit blasted. Also, some bond coats were plated with~5 lm of platinum and annealed prior to TBC deposition.…”

“…The details of the failure mechanisms for these two systems were discussed in an earlier publication. [18] Some work also has been done on no bond coat systems for EBPVD TBCs on Rene`N5 substrates. A summary of cyclic lives of the various specimens is presented in Table I along with the average lives of the improved systems that will be described next in this article.…”

Program Evaluation for U.S. Army Lifelong Learning Centers (LLCs): Extension to Military Operational Speciality (MOS)-Based LLCs

“…As a result, the interface toughness between the TGO and the bond coat with both γ and β phases contacting the TGO at the early stage of oxidation is higher than the interface between the TGO and the top γ layer in the later oxidation stages. Therefore, the delamination occurred along the TBC/TGO interface during the early stage of oxidation when a continuous γ top layer has not been formed, as observed in the indentation tests [24] and the wedge impression tests [22] of the TBCs. Low interface toughness between Al 2 O 3 and Ni has been reported in the literature, especially when segregation of contaminants were present in combination with ambient moisture [25,26].…”

“…The surface morphology of the initially spalled oxide scale with small fraction of exposed bond coat around the hottest spot region of the bar specimen (Figure 6a) appear similar to the bond coat surface exposed by wedge impression of a TBC coating with NiCoCrAlY BC after 10 h exposure at 1100 °C [14]. Depending on the amount of the stored strain energy in the TGO as well as the concentration and frequency of defects (transient oxides, TBC defects, reactive element-rich oxide protrusions, porosities, and surface defects), failure of the NiCoCrAlY systems can occur along either the TGO/TBC or the TGO/bond coat interfaces [24]. A weaker TBC/TGO interface fracture resistance compared to that of the TGO/BC interface was thus proposed by Mumm and Evans [22] at the early stage of oxidation when neither the strain energy nor the defects are significant.…”