Sintering resistance of suspension plasma sprayed 7YSZ TBC under isothermal and cyclic oxidation

doi:10.1016/j.jeurceramsoc.2019.12.046

Journal of the European Ceramic Society

Volume 40, Issue 5, May 2020, Pages 2030-2041

https://doi.org/10.1016/j.jeurceramsoc.2019.12.046 Get rights and content

Abstract

This study examines sintering resistance of a thermal barrier coating (TBC), composed of a 7YSZ suspension plasma sprayed (SPS) top coat (TC), an air plasma sprayed (APS) NiCoCrAl bond coat (BC), and an INCONEL 625 substrate, under isothermal and cyclic conditions with a peak temperature of 1080 °C for 400, 800, and 1300 h/cycles. Microstructure, phase composition and microstrain were examined using SEM and XRD. Mechanical properties of fracture toughness, hardness and elastic modulus were obtained using nano-indentation. Samples under cyclic conditions presented faster sintering rate than under isothermal condition due to larger compressive strain and frequent heating and cooling cycles. Faster degradation of mechanical properties due to sintering leads to shorter lifetime of SPS coating under cyclic conditions. Moreover, vertical cracks within SPS coatings reduces compressive stress leading to a greater lifetime as compared to APS coatings exposed to similar conditions.

Introduction

Thermal barrier coatings (TBCs) are applied onto the metal components of hot section parts in gas turbine engines because of their outstanding thermal insulation ability [1]. There are usually four layers in a TBC system: i) superalloy substrate, ii) oxidation resistance metallic bond coat (BC), iii) thermally grown oxide (TGO) and iv) a thermally insulating ceramic top coat (TC) [2]. Zirconia (ZrO₂) has been a widely used TC material due to its higher thermal expansion coefficient, low thermal conductivity, and good erosion resistance. However, pure zirconia is rarely used because it is susceptible to cracking during cooling as a result of phase transformation. As a result, yttrium oxide (Y₂O₃) or MgO, with its cation valence less than that of the Zr atom, is added to stabilize the charge balance, rendering zirconia more resistant to phase transformation cracking. Nowadays yttria stabilized zirconia (YSZ) is the most common TC for aerospace applications [3].

The two widely used deposition methods for YSZ TCs are air plasma spray (APS) and electron beam physical vapor deposition (EB-PVD). APS deposited TCs have high porosity, so they possess added thermal insulation and can be produced with lower deposition cost [4]. Unlike APS, EB-PVD deposited TCs feature a columnar microstructure. The columns give TCs improved strain and thermo-shock tolerance, but the deposition cost is higher than APS and the deposition rate is low (around 1−2 μm/s) [[5], [6], [7]]. Suspension plasma spray (SPS) is a relatively new deposition technology, which has the potential to combine the advantages of the two previously mentioned deposition technologies [8]. SPS can deposit nano-structured, vertically cracked (VC), as well as EB-PVD liked columnar structures by modifying the substrate temperature, plasma torch temperature, droplet size, and stand-off distances (SOD) [9]. VC-structured SPS TCs have been proven to have improved mechanical properties and higher strain tolerance [10], but more research is necessary, especially in assessing their long-term durability.

One of the concerns for long-term durability of YSZ-TCs is how the mechanical properties degrade over time under high temperature thermal exposure. Sintering of YSZ occurs with exposure to high temperatures leading to densification, cracking, and phase transformation [11,12]. Subsequently, elastic modulus, hardness and fracture toughness change with time, contributing to accelerated failure of coatings [13]. The sintering performances of APS and EB-PBD deposited TCs have been well studied, but there is limited information on how VC-structured TCs sinter under high temperature environments. In this study, the effects of thermal exposure on sintering, mechanical properties, phase transformations, and microstructures of VC-structured SPS TCs are investigated. Both isothermal and cyclic thermal conditions are employed to identify the different sintering performances and the effects of thermal stress.

Section snippets

Material and coating processes

The TBC system investigated in this study incorporates INCONEL 625 as the substrate, a Praxair Co-210-24 NiCoCrAl bond coating (BC) with a thickness of 150 μm, and a suspension plasma sprayed (SPS) 7YSZ (7 mol.% Y₂O₃ = 14 mol.% YO_1.5, Northwest Mettech Corp) top coating (TC) with a thickness of 300 μm. All coatings were sprayed using the Axial III high power plasma torch (Northwest Mettech Corp, Vancouver, Canada) and Nanofeed 350 suspension feed system. With a 3-anode/3-cathode DC plasma

Microstructure evolution of TCs

Surface microstructures of SPS 7YSZ after isothermal and cyclic oxidation tests after different exposure durations/cycles are summarized in Fig. 3. As these increases, so does the quantity of “mud-cracks” due to sintering, which are undiscernible on the as-received sample (). Comparing the samples after isothermal and cyclic tests, the cyclic tested samples feature a smaller cluster size with a greater number of subcracks in each cluster. After 1300 cycles, some of the micro-cracks within the

SPS TC sintering behaviour under isothermal and cyclic conditions

The SPS 7YSZ TCs have been observed to form more VCs in the form of mud-cracks at surface, and with wider crack gaps after thermal tests, as shown in the Fig. 3. When a 7YSZ TC is exposed to a high temperature environment, the sintering leads to lateral shrinkage. The shrinkage induces extra in-plane tensile stress in TC while being constrained by the substrate [30,31]. When the stress reaches a critical sintering stress value σ_s, shrinkage slows down and the stress generates a new VC

Conclusion

This study investigate sintering resistance of 7YSZ thermal barrier coatings (TBCs) deposited by suspension plasma spraying (SPS) onto an INCONEL 625 substrate under isothermal and cyclic thermal conditions with a peak temperature of 1080 °C. The specimens were examined after intervals of 400, 800 and 1300 h for isothermal and 400, 800, 1300 cycles for cyclic condition. Microstructural and compositional analyses were carried out using SEM while the occurrence of phase transformation and changes

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank the NRC for providing access to the testing facilities for the experiments. Continued financial support from Natural Sciences and Engineering Research Council of Canada (NSERC, Grant ID: 05862) discovery grant (awarded to Dr. Huang) is greatly appreciated.

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Citation Excerpt :
The {1 0 1}t peak near 30.2° was chosen to calculate the lattice deformation because it was the peak with the highest intensity [17]. The effect of background noise from factors such as the amorphous phase and background radiation on the peak height and width are limited owing to the strong intensity [17,52]. After isothermal oxidation from 0 to 75 h, the diffraction peak gradually shifted to a smaller angle (Table 2).
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Original ArticleSintering resistance of suspension plasma sprayed 7YSZ TBC under isothermal and cyclic oxidation

Abstract

Introduction

Section snippets

Material and coating processes

Microstructure evolution of TCs

SPS TC sintering behaviour under isothermal and cyclic conditions

Conclusion

Declaration of Competing Interest

Acknowledgements

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Original Article
Sintering resistance of suspension plasma sprayed 7YSZ TBC under isothermal and cyclic oxidation