High temperature stress and its influence on surface rumpling in NiCoCrAlY bond coat

As a critical technology; thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades; however; the most commonly used MCrAlY bond coats which control air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. This motivates a reconsideration of development of MCrAlY at a fundamental level to understand why the huge efforts in the past three decades has so little impact on industrial application of MCrAlY alloys. Detailed examination of crack trajectories of thermally cycled samples and statistic image analyses of fracture surface of APS TBCs confirmed that APS TBCs predominately fails in top coat. Cracks initiate and propagate along splat boundaries next to interface area. TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in top coat or both. By focusing on the reduction of tensile stress in top coats; three new bond coat alloys have been designed and developed; and the significant progress in TBC lifetime have been achieved by using new alloys. Extremely high thermal cycle lifetime is attributed to the unique properties of new alloys; such as remarkably lower coefficient of thermal expansion (CTE) and weight fraction of β phase; absence of mixed / spinel oxides; and TGO self repair ability; which cannot be achieved by the existed MCrAlY alloys.

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“…Rumpling was also reported in MCrAlY coating [36][37][38]. The increased strength of bond coat can reduce the incidence of rumpling.…”

Section: Strengthening Bond Coats By Hard Phasesmentioning

confidence: 86%

Principle and Practice to Achieve Improvements in TBC Thermal Cycle Lifetime

Sharobem

Zhou

et al. 2021

Thermal Spray 2021: Proceedings From the International Thermal Spray Conference

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“…It has been widely used in the testing and characterization of stress among TBCs. XRD has been used to study the stress evolution in the ceramic top coat and bond coat after operating at different conditions [33][34][35]. Meanwhile, Xiao et al studied the stress evolution of the bond coat during oxidation at 1150°C for different time based on XRD.…”

Section: X-ray Diffraction (Xrd)mentioning

confidence: 99%

“…It was found that the stress in the bond coat mainly generated during the process of cooling from 1150 to 600°C. This may be due to that the β phase deposition during cooling changes the volume of the bond coat [34,35].…”

Section: X-ray Diffraction (Xrd)mentioning

confidence: 99%

Stress among the APS-Prepared TBCs: Testing and Analysis

Qu¹,

He²,

Fang³

2020

Methods for Film Synthesis and Coating Procedures

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Stress state in thermal barrier coatings (TBCs) has an influence on the service property and safety of coatings. The stress in TBCs should be characterized and measured. This is helpful for the guidance of design and preparation, the study of failure mechanism and the promotion of application of TBCs. This chapter reviewed the research progress on the stress measurement for air plasma spray (APS)-prepared TBCs. The origin and category of stress during preparation process and service were discussed. Then, the measurement technology and characterization method for stress in APS-prepared TBCs were focused. The common stress measurement techniques such as X-ray diffraction, neutron diffraction, Raman spectroscopy, photoluminescence piezospectroscopy, curvature measurement, material removal and indentation method were detailed. Furthermore, some suggestions were presented for the future work by summarizing the shortcomings of the exiting research. Methods for Film Synthesis and Coating Procedures2 'partially stabilized' with about 6-8 wt.% yttria (7YSZ), which has the good heatinsulating property and long thermal cycle life. TGO, whose composition is α-Al 2 O 3 , is formed by the reaction between aluminum diffusing from the bond coat and exterior oxygen. TGO can provide good bonding of TBC to bond coat. The bond coat contains the source of elements to create TGO in oxidizing environment and provides oxidation protection, primarily of NiCoCrAlY-or NiAlPt-based compositions. The superalloy substrate, which has high strength at high temperatures, can experience complicated mechanical loads during service [1,2,4].Recently, many methods have been developed to prepare the ceramic top coat, such as air plasma spray (APS) and electron beam physical vapor deposition (EB-PVD) process [5,6]. In APS process, the ceramic feedstock are injected into the high temperature plasma plume, heated to the molten or high-plasticity condition, impinge onto the surface of specimen with a certain momentum, and rapidly solidify. Then the lamellar microstructures, which consist of a large number of overlapped splats, are formed, as shown in Figure 2(a). In EB-PVD process, ingots of a ceramic composition are vaporized in a vacuum chamber using a focused electron beam. The ceramic vapor gradually deposit on the specimen and form columnar microstructure, which are perpendicular to the surface of specimen, as shown in Figure 2(b).TBCs prepared by APS process, which has lower cost, has been widely used in the larger stationary components. However, the failure behavior such as premature spallation of TBCs during preparation and operation process is still an overriding concern. The premature spallation of TBCs may expose the superalloy substrate to hot gases, resulting in the oxidization of superalloy. This may influence the service performance, lifetime and safety of aircraft, even lead to catastrophic damage [7]. The premature spallation of TBCs mainly results from intralayer and interlayer fractures [8]. The fracture behavior of materials is closely...

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“…Although, the application of MCrAlY coatings usually requires a much thicker thickness in industrial application, where the rumpling is less significant, to understand stress evolution in coating still helps to explain the TGO spallation. Yang et al [112] developed a new approach to investigate the coating high temperature stress, results showed that the volume shrinkage of β-γ phase transformation is responsible for the stress build-up in coating, and the grain sliding in response to the high temperature stress in coating is the dominant factor for the surface rumpling.…”

Section: Residual Stress In Coatingmentioning

confidence: 99%

Performance of MCrAlX coatings : Oxidation, Hot corrosion and Interdiffusion

Zhang

2019

Linköping Studies in Science and Technology. Dissertations

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MCrAlY coatings (M=Ni and/or Co) are widely used for the protection of superalloy components against oxidation and hot corrosion in the hot sections of gas turbines. The drive for coating systems to bestow adequate oxidation and corrosion resistance upon the components becomes urgent as an inevitable result of the necessary improvement in engine combustion efficiency and service lifetime. Through careful design of the composition, MCrAlY coating performance can be optimized to meet the needs under different service conditions and component materials, therefore, "MCrAlX", with "X" standing for the minor alloying elements, is used to highlight the effect. In the present thesis, the performance of new MCrAlX coatings is investigated with respect to oxidation, hot corrosion and interactions between coating-superalloy substrates.Coating performance can be affected by many factors, where their impacts may vary at different oxidation stages, therefore experiments are designed by targeting each stage. Investigation on the initial stage oxidation behavior of MCrAlY coatings with post-deposition surface treatments reveals the different growth mechanisms of alumina scales. Results showed that surface treatments significantly reduce the alumina growth rate by suppressing transient alumina development and aiding the early formation of α-Al 2 O 3 , which improves the long-term oxidation performance of the coating. Similarly, the modification of minor alloy elements in MCrAlX coatings also yields a similar effect. Subsequently, study on the oxidation behavior of new MCrAlX coatings is carried out at the steady oxidation stage, followed by the microstructure observation, and thermodynamic and kinetic simulations. As an alternative reactive element to Y, Ce shows a negative effect on the formation of columnar alumina scales of high strain tolerance. In comparison, Fe or Ru addition shows no influence on alumina growth, rather than strengthening the phase stability in the coating and reducing the interdiffusion between coating-substrate through different mechanisms. As the oxidation proceeds to the close-to-end stage, a reliable criterion to estimate the capability of coating to form α-Al 2 O 3 is of great importance to accurately evaluate coating lifetime. A iii iv temperature-dependent critical Al-activity criterion is proposed to better predict the formation of a continuous α-Al 2 O 3 scale based on correction of the elemental activity using commercial thermodynamic databases to replace the empirical Al-concentration based criterion.Severe interdiffusion occurs between coating-substrate during high temperature oxidation, accelerating the degradation of the system. Interdiffusion behavior of diffusion couples of superalloys-MCrAlX coatings are examined. It is highlighted that the the recrystallization of superficial layer of the substrate contributes to the secondary reaction zone formation and element interdiffusion controls subsequent zone thickening.Study on Type I hot corrosion behavior of new MCrAlX coatings shows that the ...

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High temperature stress and its influence on surface rumpling in NiCoCrAlY bond coat

Cited by 61 publications

References 28 publications

Principle and Practice to Achieve Improvements in TBC Thermal Cycle Lifetime

Principle and Practice to Achieve Improvements in TBC Thermal Cycle Lifetime

Stress among the APS-Prepared TBCs: Testing and Analysis

Performance of MCrAlX coatings : Oxidation, Hot corrosion and Interdiffusion

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