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Defects, Stresses and Cracks in Thermal Barrier Coatings
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Defects, Stresses and Cracks in Thermal Barrier Coatings

A special issue of Coatings (ISSN 2079-6412). This special issue belongs to the section "Ceramic Coatings and Engineering Technology".

Deadline for manuscript submissions: 30 October 2024 | Viewed by 17737

Special Issue Editors

Dr. Yongle Sun

E-Mail Website
Guest Editor
Welding Engineering and Laser Processing Centre, School of Aerospace, Manufacturing and Transport, Cranfield University, Cranfield MK43 0AL, UK
Interests: additive manufacturing; welding; coatings; cellular materials; applied mechanics; metals processing technology; multi-physics modelling
Special Issues, Collections and Topics in MDPI journals
Dr. Peng Jiang

E-Mail Website
Guest Editor
State Key Laboratory for Strength and Vibration of Mechanical Structures, Department of Engineering Mechanics, Xi’an Jiaotong University, Xi'an, China
Interests: thermal barrier coatings; high-temperature experimental mechanics; nondestructive measurement; spray forming technology

Special Issue Information

Dear Colleagues,

Gas turbines are the energy source at the heart of aircraft, ships, power plants, and many other energy-demanding industrial systems. Thermal barrier coatings (TBCs) are a key technology for gas turbines. To improve turbine performance, the inlet gas temperature of advanced heavy-duty gas turbine has reached 1600 °C, which poses a severe challenge to the integrity of turbine blades. Compared with superalloy technology (work temperature increased by 1–2 °C per year) and cooling gas film technology (causing lower thermal efficiency and more processing difficulty), TBCs are recognised as the most effective and promising technical route to continuously enhance the high temperature performance and service life of gas turbines.

In TBCs, the microstructure of each constituent layer and the interface between different layers are complex, and they are commonly susceptible to intricate or unpredictable defects. During service, the TBCs are exposed to extremely harsh environment and subjected to mechanical and thermal loads, high-temperature oxidation, erosion, and corrosion, among others. Consequently, high stresses can be generated and concentrated around structural/geometrical defects. They act as the internal force to drive the nucleation and propagation of cracks, which can lead to TBC fracture and delamination. The ultimate failure of TBCs, which is manifested as spallation, not only means the loss of thermal protection for the metal substrate but also produces peeled fragments which can collide with the turbine blades rotating at high speeds, posing a catastrophic threat to the blades and even the whole turbine system. Therefore, it is crucial to characterise, determine, understand, and mitigate the defects, stresses, and cracks which are closely associated with the failure mechanisms of TBCs.

In recent years, the research on TBC technology has led to developments of new materials, processes, structures, and characterisation and simulation methods. Examples of these new technologies include rare earth-doped ceramic coatings that are more resistant to sintering, PS-PVD, and SPS processes, double ceramic layer coating structures, TBCs with fabricated vertical cracks, etc. The research on defects, stresses, and cracks in the new TBCs will underpin the understanding and improvement of these promising technologies.

This Special Issue will serve as a forum for communication and dissemination of recent research on defects, stresses, and cracks in TBCs. It will cover but be not limited to the following topics:

  • Detection, characterisation, and mitigation of TBC defects;
  • Measurement and prediction of TBC stresses;
  • Crack initiation and growth in TBCs;
  • Experiments and simulations on TBC stresses/crack-related oxidation, sintering, erosion, CMAS corrosion, etc.;
  • Failure mechanisms of TBCs;
  • TBC life prediction models.

Dr. Yongle Sun
Dr. Peng Jiang
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Coatings is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • thermal barrier coating system
  • microstructural defects
  • stress analysis
  • fracture mechanics
  • failure mechanisms

Published Papers (7 papers)

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Research

11 pages, 7746 KiB  
Article
In Situ High-Temperature Tensile Fracture Mechanism of PS-PVD EBCs
by Dongling Yang, Junling Liu, Jungui Zhang, Xinghua Liang and Xiaofeng Zhang
Coatings 2022, 12(5), 655; https://doi.org/10.3390/coatings12050655 - 11 May 2022
Cited by 2 | Viewed by 1526
Abstract
Environmental barrier coatings (EBCs) are increasingly being used in the high-temperature sections of gas turbines because of their protective effects on SiC fiber-reinforced SiC ceramic matrix composites (SiCf/SiC CMCs) when subjected to high-temperature water oxygen corrosion. The objective of this study [...] Read more.
Environmental barrier coatings (EBCs) are increasingly being used in the high-temperature sections of gas turbines because of their protective effects on SiC fiber-reinforced SiC ceramic matrix composites (SiCf/SiC CMCs) when subjected to high-temperature water oxygen corrosion. The objective of this study was to investigate the failure behavior of EBCs prepared on SiCf/SiC CMC matrix materials under coupled high-temperature and load conditions. A plasma spray-physical vapor deposition (PS-PVD) method was used to prepare Si/3Al2O3·2SiO2/Yb2SiO5 EBC composite coatings on the surface of SiCf/SiC ceramic matrix composites. In situ scanning electron microscopy was used to study the evolutionary behavior of the coating surface cracks at different temperatures and the failure and fracture mechanism of the coating/substrate when held at 766 °C and subjected to different loading conditions. The results show that no significant crack extension occurred on the coating surface as the temperature of the coated specimen increased from room temperature to 766 °C in the absence of an applied tensile load, indicating that the effect of a single temperature factor on the failure of the specimen was negligible. However, under coupled high-temperature and load conditions, the specimens fractured at a load of 340 N when subjected to 766 °C, indicating that the coated sample is more likely to fail when subjected to high-temperature and tensile loading. The step-like fracture exhibits features consistent with the coating fracture and spalling caused by surface cracks extending from the coating surface to the interior. The spalling, large crack formation and step-like shape of the fracture in the coating and the substrate indicate that cracks were generated between the coating and the substrate under the coupled high-temperature and load conditions. The generation and extension of cracks in both parts eventually led to full specimen rupture. Full article
(This article belongs to the Special Issue Defects, Stresses and Cracks in Thermal Barrier Coatings)
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13 pages, 6685 KiB  
Article
Experimental and Simulation Analysis of the Evolution of Residual Stress Due to Expansion via CMAS Infiltration in Thermal Barrier Coatings
by Shaochen Tseng, Chingkong Chao, Dongxu Li and Xueling Fan
Coatings 2021, 11(10), 1148; https://doi.org/10.3390/coatings11101148 - 23 Sep 2021
Cited by 7 | Viewed by 2227
Abstract
The failure behavior of thermal barrier coatings (TBCs) involves multilayered systems infiltrated with calcium–magnesium–alumino-silicates (CMAS). The metastable tetragonal phase is mainly composed of 7YSZ (7 mol.% Y2O3-stabilized ZrO2), and it destabilizes into the Y-lean tetragonal phase, which [...] Read more.
The failure behavior of thermal barrier coatings (TBCs) involves multilayered systems infiltrated with calcium–magnesium–alumino-silicates (CMAS). The metastable tetragonal phase is mainly composed of 7YSZ (7 mol.% Y2O3-stabilized ZrO2), and it destabilizes into the Y-lean tetragonal phase, which may be induced by CMAS infiltration, and transforms into a monoclinic phase during cooling. The phase transformation leads to volume expansion around the CMAS-rich layer. Furthermore, it is shown that the spalling of the coating system emerges when the surface of the coating system is subjected to significant residual stress. In this study, a double-cantilever beam model is established to describe the macroscopic phenomenon of thermal buckling induced via CMAS. The result of the buckle height is used to demonstrate the consistency of the experiment and finite element simulation. The experimental parameters are imported into a multilayer cantilever beam model to analyze the interfacial stresses due to CMAS infiltration. The finite element results indicate that the phase transformation leads to damage in the coating system wherein the interfacial stresses due to phase transformation are 27% higher than those in the model without phase transformation. Full article
(This article belongs to the Special Issue Defects, Stresses and Cracks in Thermal Barrier Coatings)
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12 pages, 9364 KiB  
Article
Rear Earth Oxide Multilayer Deposited by Plasma Spray-Physical Vapor Deposition for Envisaged Application as Thermal/Environmental Barrier Coating
by Jie Zhong, Dongling Yang, Shuangquan Guo, Xiaofeng Zhang, Xinghua Liang and Xi Wu
Coatings 2021, 11(8), 889; https://doi.org/10.3390/coatings11080889 - 26 Jul 2021
Cited by 9 | Viewed by 2076
Abstract
SiC fiber-reinforced SiC ceramic matrix composites (SiCf/SiC CMCs) are being increasingly used in the hot sections of gas turbines because of their light weight and mechanical properties at high temperatures. The objective of this investigation was the development of a thermal/environmental [...] Read more.
SiC fiber-reinforced SiC ceramic matrix composites (SiCf/SiC CMCs) are being increasingly used in the hot sections of gas turbines because of their light weight and mechanical properties at high temperatures. The objective of this investigation was the development of a thermal/environmental barrier coating (T/EBC) composite coating system consisting of an environmental barrier coating (EBC) to protect the ceramic matrix composites from chemical attack and a thermal barrier coating (TBC) that insulates and reduces the ceramic matrix composites substrate temperature for increased lifetime. In this paper, a plasma spray-physical vapor deposition (PS-PVD) method was used to prepare multilayer Si–HfO2/Yb2Si2O7/Yb2SiO5/Gd2Zr2O7 composite coatings on the surface of SiCf/SiC ceramic matrix composites. The purpose of this study is to develop a coating with resistance to high temperatures and chemical attack. Different process parameters are adopted, and their influence on the microstructure characteristics of the coating is discussed. The water quenching thermal cycle of the coating at high temperatures was tested. The results show that the structure of the thermal/environmental barrier composite coating changes after water quenching because point defects and dislocations appear in the Gd2Zr2O7 and Yb2SiO5 coatings. A phase transition was found to occur in the Yb2SiO5 and Yb2Si2O7 coatings. The failure mechanism of the T/EBC composite coating is mainly spalling when the top layer penetrates cracks and cracking occurs in the interface of the Si–HfO2/Yb2Si2O7 coating. Full article
(This article belongs to the Special Issue Defects, Stresses and Cracks in Thermal Barrier Coatings)
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14 pages, 3202 KiB  
Article
Optimal Data Processing Method for the Application of Eu3+ Photoluminescence Piezospectroscopy in Thermal Barrier Coatings
by Yanheng Zhang, Ning Lu and Wei Qiu
Coatings 2021, 11(6), 678; https://doi.org/10.3390/coatings11060678 - 4 Jun 2021
Cited by 8 | Viewed by 1983
Abstract
Thermal barrier coatings (TBCs) are widely used to protect gas turbine blades but internal stress near the interface in TBCs is one of the main causes of thermal barrier failure under thermal cycling. A non-destructive inspection technique based on Eu3+ photoluminescence piezospectroscopy [...] Read more.
Thermal barrier coatings (TBCs) are widely used to protect gas turbine blades but internal stress near the interface in TBCs is one of the main causes of thermal barrier failure under thermal cycling. A non-destructive inspection technique based on Eu3+ photoluminescence piezospectroscopy has been successfully used to analyze the residual stress in TBCs, but systematic and quantitative evaluation of data processing is still needed, especially with respect to the identification of peak positions. In this work, processing methods for Eu3+ photoluminescence spectroscopy data were studied to characterize TBC internal stress. Both physical and numerical experiments were carried out where Eu3+ luminescence spectra were obtained from a sample of europium-doped yttria-stabilized zirconia (YSZ:Eu3+) under step-by-step uniaxial loading, and the simulated spectra were numerically deduced from the measured spectra. The peak shifts were then obtained by processing the spectral data in different ways (Gaussian, Lorentzian, pseudo-Voigt fitting, and the barycenter method), and comparing the results. We found that the Gaussian function, rather than the commonly used Lorentzian function, is the most appropriate method for the application of Eu3+ photoluminescence piezospectroscopy in TBCs because it provides sufficient sensitivity, stability and confidence for quantitative stress analysis. Full article
(This article belongs to the Special Issue Defects, Stresses and Cracks in Thermal Barrier Coatings)
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13 pages, 5987 KiB  
Article
Mitigation of Platinum Depletion in Platinum Diffused Single Phase Bond Coat on CMSX-4 Superalloy
by Mingwen Bai, Ying Chen, Yongle Sun and Ping Xiao
Coatings 2021, 11(6), 669; https://doi.org/10.3390/coatings11060669 - 31 May 2021
Cited by 1 | Viewed by 2520
Abstract
Pt-diffused bond coat with a mixture of γ/γ’ phase has just been developed in the recent decades as a cheaper alternative to the Pt-enriched β-phase Aluminide bond coat that contains a higher content of Al. However, concerns are raised on the inevitable depletion [...] Read more.
Pt-diffused bond coat with a mixture of γ/γ’ phase has just been developed in the recent decades as a cheaper alternative to the Pt-enriched β-phase Aluminide bond coat that contains a higher content of Al. However, concerns are raised on the inevitable depletion of Pt near the coating interface that may endanger the component after long-term service. In this study, modified Pt-diffused bond coats with a single phase (γ or γ’) were made by applying selective etching on CMSX-4 single crystal superalloys prior to the electroplating of Pt. The single-phase bond coats show distinctive diffusion behaviour in comparison with the conventional γ/γ’ bond coat. Surprisingly, Pt remains more stable in the γ’-phase bond coat with significantly less depletion after diffusion, which implies a potential in saving a considerable amount of Pt. On the other hand, however, the depletion of Pt is more severe in the γ-phase bond coat. The mechanism that governs the diffusion behavior of Pt in the γ and γ’-phase was also discussed that mainly concerns with thermodynamic and kinetic factors. Full article
(This article belongs to the Special Issue Defects, Stresses and Cracks in Thermal Barrier Coatings)
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8 pages, 2176 KiB  
Article
Effects of Roughness on Stresses in an Oxide Scale Formed on a Superalloy Substrate
by Yang Zhao, Fan Sun, Peng Jiang and Yongle Sun
Coatings 2021, 11(4), 479; https://doi.org/10.3390/coatings11040479 - 19 Apr 2021
Cited by 2 | Viewed by 1997
Abstract
The effects of surface roughness on the stresses in an alumina scale formed on a Fecralloy substrate are investigated. Spherical indenters were used to create indents with different radii and depths to represent surface roughness and then the roughness effect was studied comprehensively. [...] Read more.
The effects of surface roughness on the stresses in an alumina scale formed on a Fecralloy substrate are investigated. Spherical indenters were used to create indents with different radii and depths to represent surface roughness and then the roughness effect was studied comprehensively. It was found that the residual stresses in the alumina scale formed around the rough surface are almost constant and they are dominated by the curvature rather than the depth of the roughness. Oxidation changes the surface roughness. The edge of the indent was sharpened after oxidation and the residual stress there was released presumably due to cracking. The residual stresses in the alumina scale decrease with increase in oxidation time, while the substrate thickness has little effect, given that the substrate is thicker than the alumina scale. Furthermore, the effect of roughness on the oxide growth stress is analysed. This work indicates that the surface roughness should be considered for evaluation of stresses in coatings. Full article
(This article belongs to the Special Issue Defects, Stresses and Cracks in Thermal Barrier Coatings)
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14 pages, 12934 KiB  
Article
Investigations on the Diffusion of Platinum between CMSX-4 Superalloy and Platinum-Enriched Bond Coat
by Mingwen Bai, Ying Chen and Ping Xiao
Coatings 2021, 11(4), 441; https://doi.org/10.3390/coatings11040441 - 11 Apr 2021
Cited by 6 | Viewed by 2777
Abstract
The depletion of Pt in Pt-enriched bond coats due to inter-diffusion with superalloys has been a critical concern for the long-term oxidation resistance of thermal barrier coatings. This study investigated the diffusion behaviour of Pt between CMSX-4 superalloys and two commercial Pt-enriched bond [...] Read more.
The depletion of Pt in Pt-enriched bond coats due to inter-diffusion with superalloys has been a critical concern for the long-term oxidation resistance of thermal barrier coatings. This study investigated the diffusion behaviour of Pt between CMSX-4 superalloys and two commercial Pt-enriched bond coats comprising intermetallic γ′/γ-phase or β-phase, with the aim to understand the mechanism that leads to the depletion of Pt at high temperatures. The results demonstrated that the diffusion of Pt in superalloy disrupts its phase equilibrium, causes a significant lattice parameter misfit between the γ-phase and γ′-phase, and results in the formation of large γ′-grains with irregular shapes and random orientations. In addition, by using the Thermo-Calc software, Pt was found to have negative chemical interactions with both Al and Ta that stabilise Pt by decreasing its chemical activity. The depletion of Al due to the growth of Al2O3 scale during oxidation increases the activity of Pt and therefore accelerates the inwards depletion of Pt towards superalloys. Full article
(This article belongs to the Special Issue Defects, Stresses and Cracks in Thermal Barrier Coatings)
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