Research Article
Methodology for predicting the life of plasma-sprayed thermal barrier coating system considering oxidation-induced damage

https://doi.org/10.1016/j.jmst.2021.07.018Get rights and content

Highlights

  • l

    A novel life prediction methodology that is not bound by the temperature history and type of TBC was established.

  • l

    A virtual S-N diagram method for calculating and evaluating the damage to TBCs is introduced for the first time.

  • l

    The damage can be evaluated by deriving the intrinsic β value according to the TBC.

  • l

    The higher the stress conversion rate, the higher the probability of crack formation in the valley.

Abstract

The lifespan of plasma-sprayed thermal barrier coating (TBC) systems is difficult to predict owing to the variety of microstructures and deterioration histories. In this study, we developed a novel TBC damage model to reflect deterioration histories; thus, it can be applied to various TBCs. Damage to TBCs is classified into oxidation and mechanical damage; therefore, a detailed deterioration history can be reflected. In addition, by introducing a virtual SN diagram, a life prediction model that can be applied to TBCs with various microstructures was established. We used the proposed damage and life prediction models in isothermal aging and thermal cycle tests with different aging cycles. The predicted lifespan of TBCs by using the proposed models was within 95% of the results obtained by performing actual tests in the temperature range of 1150–1350 °C.

Introduction

Thermal barrier coatings (TBCs) are films applied to the surface of superalloy components. They consist of a ceramic material that acts as insulation and metal material for bonding and oxidation resistance. TBCs play a key role in raising the operating temperature of superalloy parts above the temperature capability of the original material [1], [2], [3]. However, spallation occurs owing to the thermal stress caused by the difference in the thermal expansion coefficient (TEC) of the materials. Therefore, solving the spallation problem is essential for the development of high-performance materials. Many studies have been conducted to identify the factors involved and to explore the damage mechanism [4], [5], [6].

The spallation life of TBCs may vary depending on the constituent material, manufacturing method, process parameters, and surface conditions of the TBCs. According to Wang et al. [7], the process parameters determine the microstructure of the top coating (TC). They also found that the thermal and mechanical properties, such as elastic modulus and thermal conductivity, vary depending on the microstructure [8]. TC properties help determine the thermal stress inside a TBC, which affects the spallation life of TBCs [9], [10], [11]. Kromer et al.[12] found that the stress gradient of TBCs varies depending on the surface shape and affects the growth behavior of thermally grown oxide (TGO), thus affecting the spallation life of TBCs [13], [14], [15].

As a result, the lifespan of TBCs varies depending on various factors and is closely associated with thermal stress. Researchers have selected thermal stress as a spallation criterion and suggested a model in which spallation occurs when the TGO reaches a critical thickness [16], a crack propagation damage model [17], and a damage model considering various factors [18]. However, these models cannot consider the influence of growing TGO—more specifically, the fluctuating thermal stress and deterioration history, which increase at high temperatures. Moreover, different lifespans might be achieved with a similar coating depending on the test conditions [19,20]. As the operation of gas turbines is changing because of recent environmental regulations, a damage model is needed to consider these changes [21], [22].

In this study, we established a damage model that can reflect various deterioration histories by simultaneously considering the oxidation and mechanical damage of TBCs. In addition, because it is simpler than the existing models, we aimed to realize a life prediction model that can be generally applied to TBCs. We manufactured specimens and coated them via atmospheric plasma spraying (APS). Subsequently, isothermal aging and thermal cycle tests were conducted with different thermal cycles in the temperature range of 1150–1300 °C to determine the spallation life. Finite element (FE) analysis was performed to analyzed the stress at the interface of the TC in terms of TGO thickness. Based on the fluctuating cumulative stress due to oxidation and thermal fatigue and fractography, we identified the mechanism of damage to TBCs. A life prediction model was developed and verified by using the thermal cycle test results. Consequently, we present a methodology for predicting the life of TBCs, which can reflect the history of deterioration, for the first time.

Section snippets

Specimen preparation

Because the oxidation behavior and spallation life of TBCs are sensitive to changes in the microstructure of the TC, specifying the manufacturing process and the condition of the specimen is desirable. Fig. 1 shows the microstructure of the TBC specimen manufactured in this study, and Table 1 lists the process parameters. The same process was used as that used in our previous study on the oxidation model [23]. That process is also applied to commercial gas turbine blades. BC (MCrAlY) was

Results

Fig. 3 shows the test interval and spallation life results of the isothermal tests for each temperature. When the temperature was 1200 °C, spallation occurred over an average of six cycles. In addition, when the test interval was 25 h, the lifetime was in the range of 125–150 h. In particular, because the temperature was cooled to room temperature six times from 1200 °C, the accumulated damage resulting from the TGO growth in each of these six cycles caused spallation of the TBCs. Comparably,

FE analysis

We performed FE analysis to analyze the mechanical damage caused by the oxidation of the TGOs. According to Eriksson et al. [27], when the surface of the BC and the shape of the TGO were expressed in a form with periodicity, sinusoidal function simulations demonstrated the highest similarity with the real surface. This method has been widely used [28], [29], [30]. Furthermore, by applying the above method, the amplitude (A) and wavelength (λ) of the TGO can be derived from Eqs. (3) and (4), in

Conclusions

In this study, we analyzed the mechanisms of oxidation and spallation of TBCs. Subsequently, we developed a life prediction model that can simultaneously reflect oxidation and mechanical damage. The key results of this study are as follows:

  • (1)

    TGO grows as the oxidation of TBC proceeds and causes a stress-conversion phenomenon that changes the location of the maximum stress region from peak to valley. The stress conversion phenomenon proceeds rapidly as the temperature increases; thus, the number

Acknowledgments

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant, funded by the Korean government (MOTIE) (No. 20193310100030), and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2018R1A2A1A05077886).

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