Thermal Barrier Coating System Research Articles

Structure and durability evaluation of blast furnace slag coatings and thermal barrier coatings (TBCs) under high temperature conditions

A high temperature aggressive environment is a great challenge for the long lifetime of thermal barrier coatings (TBCs). In this study, the hot corrosion and isothermal oxidation behavior of blast furnace slag (BFS) and yttria stabilized zirconia (YSZ) coated TBC systems were systematically investigated under high temperature conditions. This study aims to analyze the evaluation of microstructural properties of thermally sprayed CoNiCrAlY/BFS and CoNiCrAlY/YSZ coatings before and after high temperature oxidation and hot corrosion tests. For this purpose, the coatings were isothermally oxidized at 900 °C for 8, 24, 50, and 75 h. Hot corrosion behavior of CoNiCrAlY/BFS and CoNiCrAlY/YSZ coatings deposited on Inconel 718 superalloy substrates were studied at 900 °C in a Na2SO4 and V2O5 molten salt environment. CoNiCrAlY/BFS and CoNiCrAlY/YSZ TBC systems exposed to Na2SO4+ V2O5 molten salt at 900 °C investigate the hot corrosion behavior. The microstructure of coatings, formation of thermally grown oxide (TGO) layer, and degradation of coatings during isothermal oxidation and hot corrosion conditions were investigated. The oxidation and hot corrosion mechanisms were discussed. BFS, could be a potential alternative for the YSZ ceramic top coating in TBCs. The TBC system with BFS can be preferred for oxidation damage due to its low cost. YSZ and BFS TBCs are highly resistant to Na2SO4 and V2O5 molten salt environment.

Numerical Simulation of Initial Residual Stress in Thermal Barrier Coatings: Planar geometry model

Thermal barrier coatings (TBCs) are one of the key materials of turbines for propulsion and power generation. A typical TBCs system generally contains four layers, i.e. top ceramic coat (TC), thermally grown oxide (TGO), bond coat (BC) and substrate. The material parameters （such as Young’s modulus, Poisson’s ratio and thermal expansion coefficient） of each layer will result in the different cooling velocity of thermal barrier coatings (TBCs) system during the deposition process. Initial residual stresses in TBCs occur after cooling to the ambient, which play an important role for evaluating the failure and reliability of TBCs. In this paper, the initial residual stresses of a planar TBCs model were obtained by finite element method, the effects of cooling kinds and the thickness of TC and TGO on the fields of initial residual stresses were also considered. The results may be useful for the optimization of TBCs system design and preparation.

Vertical crack distribution effect on the TBC delamination induced by crack growth from the ceramic surface and near the interface

The propagation of vertical crack on the surface of thermal barrier coatings (TBCs) may affect the interface cracking and local spallation. This research aims to establish a TBC model incorporating multiple cracks to comprehensively understand the effects of vertical crack distribution on the coating failure. The continuous TGO growth and ceramic sintering are together introduced in this model. The influence of the vertical crack spacing and non-uniform distribution on the stress state, crack driving force, and dynamic propagation is examined. Moreover, the influence of coating thickness on the crack growth driving is also explored. The results show that large spacing will lead to early crack propagation. The uniform distribution of vertical cracks can delay the spallation. When the spacing is less than 4 times ceramic coat thickness, the cracking driving force will come in a steady-state stage with the increase of vertical crack length. Prefabrication of vertical cracks with spacing less than 0.72 mm on the coating surface can greatly decrease the strain energy. The results in this study will contribute to the construction of an advanced TBC system with long lifetime.

Crack Driving Forces of Atmospheric Plasma-Sprayed Thermal Barrier Coatings

High-temperature operation service conditions can be used to evaluate the durability of Atmospheric Plasma-Sprayed Thermal Barrier Coating systems (APS-TBCs). To evaluate the durability of TBCs within their life span, two different thermal cycling testing results, i.e., isothermal furnace cycling and burner rig cycling tests, are utilized to numerically investigate possible crack driving forces that might lead to the failure of TBCs. Although there are many studies on failure and life prediction, there is still a lack of quantitative evaluation and comparison on the crack driving forces under these two different thermal cycling schemes. In this paper, by using modified analytical models, strain energy release rates (ERRs) are estimated and compared between these two testing approaches using experimental data. A new residual stress model was developed to study the position where the maximum residual stress occurs due to coefficient of thermal expansion (CTE) mismatch at different thermally grown oxide (TGO) thicknesses. The main crack driving forces are identified for two types of thermal cycling. A possible cracking route is found based on the calculated equivalent ERRs with respect to distance from the interface between the topcoat (TC)/TGO layers. The relationship between crack driving force of isothermal furnace and burner cycling tests is also elaborated.

Understanding the thermal barrier performance of plasma-sprayed rare-earth zirconate coating against volcanic ash ingestion

Plasma sprayed thermal barrier coatings (TBC) are widely used to protect gas turbine engine components typically exposed to different types of failures. The state-of-art yttria-stabilized-zirconia (YSZ) TBC fails above 1150 °C under calcium‑magnesium-alumino-silicate (CMAS) or volcanic ash (VA) infiltration and associated thermal cyclic degradation. Rare earth zirconate (REZ) is considered a potential candidate TBCs to perform at much higher temperatures and exhibit better CMAS infiltration resistance. Advanced REZs such as gadolinium zirconate (Gd2Zr2O7, GZ) and yttrium zirconate (Y2Zr2O7,YZ) were chosen for the present study. Free-standing coatings of REZ were deposited by atmospheric plasma spray (APS) and assessed for their VA interaction behavior at elevated temperatures (1150 °C/1250 °C). Single-layered (SL) and double-layered (DL) TBC systems were deposited on superalloy substrates to assess the thermal cycling performance. From the studies, Plasma-sprayed REZ TBCs demonstrate enhanced resistance toward CMAS infiltration and thermal cycling degradation at simulated test conditions, thereby qualifies for futuristic high-performance TBC applications.

Thermal Barrier Coatings (TBCs) Produced by Air Plasma Spray: Repair by Grinding and Waterjet (WJ) Processes

TBCs (Thermal Barrier Coatings) are multilayer structures usually consisting of a ceramic layer, known as top coat, arranged on a connecting layer, known as bond coat, which, in turn, is arranged above the substrate to protect. These structures are applied in hot section parts of gas turbine engines for providing thermal protection. During the component life, some oxidation end/or wear phenomena can occur, therefore the TBC is generally removed and then reapplied. To simplify the manufacturing process making it more cost effectiveness, the aim of this work is to study the possibility to repair a TBC system by removing the only damaged layer by using non-conventional processes and analyzing in detail the morphological roughness to ensure a good adhesion between the layers.

Investigation of HVOF sprayed novel Al1.4Co2.1Cr0.7Ni2.45Si0.2Ti0.14 HEA coating as bond coat material in TBC system

The present study aims to investigate non-stoichiometric Al1.4Co2.1Cr0.7Ni2.45Si0.2Ti0.14 high entropy alloy (HEA) as a bond coat material for the TBC (Thermal Barrier Coating) system. The mechanical activated synthesized HEA was sprayed on a Ni-based superalloy substrate by High-velocity oxy-fuel (HVOF) spraying, and 8 mol% Yttria Stabilized Zirconia (YSZ) was deposited on HEA by Air Plasma Spray (APS). X-Ray Diffraction (XRD) analysis and Scanning Electron Microscopy (SEM) were used to investigate the phases and microstructure of the as-synthesized HEA powder, Ni superalloy/HEA-bond coat, and Ni superalloy/HEA-bond coat/YSZ topcoat. The mechanical properties of the coating like microhardness, Young’s modulus and residual stress between bond coat and YSZ top coat was evaluated using the Nano-Hardness Tester (NHT). The TBC system was investigated for cyclic oxidation at 1050 ℃ for 100 cycles, and its cross-sections were analyzed for TGO (Thermally grown oxides) layer composition, thickness and interdiffusion of elements. The properties of the TBC system containing HEA as a bond coat were compared with those of the conventional TBC system comprising of MCrAlY (AMDRY 365–4) as a bond coat. It was observed that HEA containing TBC displayed exceptional high temperature properties and were comparable to MCrAlY.

Pulsed thermography with laser beam homogenizing for thickness prediction of thin semi-transparent thermal barrier coatings

A pulsed thermography system with laser beam homogenizing is developed to measure the thickness of thermal barrier coatings (TBCs) by numerical fitting. A simplified model is proposed to avoid the complexity of deriving an analytical solution of thermal diffusion in TBC systems that include a thin semi-transparent top coating. A characteristic parameter is identified that captures the system response to fast heating by a short laser pulse and to optothermal effects due to volume heating, bulk emission and multiple reflections in the thin coating. It is demonstrated that small thickness changes can be measured when the unpainted ultra-thin TBC is thermally excited using short laser pulses with flat-top spatial profile. Experimental results show that the proposed laser thermography reconstruction method is fast and reliable when using laser beam shaping with a homogenizer.

Evaluation of Residual Stress Distribution in Thermal Barrier Coating System by Extended Thin Layer-Removal Method for Three-Layered Model

Evaluation of Residual Stress Distribution in Thermal Barrier Coating System by Extended Thin Layer-Removal Method for Three-Layered Model

Interfacial peeling loads in the TBC with an air hole: Analytical solutions and viscoplasticity modelling

This work evaluates the temperature field, stresses, and peeling loads in thermal barrier coatings (TBCs) with air holes subjected to cyclic thermo-mechanical loads during a complete flight. Analytical formulae for the steady-state temperature field and interfacial peeling loads are developed considering thermal gradients in the thickness and longitudinal directions. In addition, viscoplasticity finite element modelling is conducted for the mechanical behaviors of the TBC system. A unified Chaboche-Lemaitre constitutive model that combines a power flow rule with non-linear anisothermal evolution of isotropic and kinematic hardening is integrated into the numerical framework. The results show tensile peeling stress and shear stress are generated in the vicinity of the hole, which motivates the initiation and propagation of hole-edge cracks. The maximum peeling and shear stresses locate close to the thermally grown oxide/bond coat interface and they increase with increasing hole radius. The interfacial peeling moment and shear force could characterize edge cracking in TBCs. As the hole radius increases, the interfacial peeling moment and shear force increase quickly when the hole radius is less than 1 mm but remain stable when the hole radius is beyond 2 mm. This work not only helps to explain the failure mechanisms of hole-edge delamination and spallation in TBCs but also guides the design of thermal barrier coated hot-section components in gas turbines.

Discrete element method to simulate interface delamination and fracture of plasma-sprayed thermal barrier coatings

This contribution deals with a discrete element method (DEM) framework to simulate and investigate the mechanisms leading to the failure of plasma-sprayed thermal barrier coating (TBC) systems. A hybrid lattice-particle approach is proposed to determine residual stress fields induced by the coefficient of thermal expansion mismatch during a cooling-down phase. Besides, this is combined with a mixed-mode cohesive zone model to simulate interface delamination, and the removed discrete element failure criterion to model crack initiation and propagation in TBC system. The context of a unit cell model with a perfectly sinusoidal interface profile is first investigated to highlight the suitability of the proposed DEM-based approach in terms of stress fields and failure process. The case of a real microstructure reproduced by the image processing is then discussed. This underlines the effect of porosity and surface asperities on the failure mechanisms. Results exhibit the potential of the proposed DEM approach to model complex cracks phenomena occurring in TBC systems under thermal loading.

Additive Manufacturing of Columnar Thermal Barrier Coatings by Laser Cladding of Ceramic Feedstock

AbstractThis study presents a new laser‐cladding‐based additive manufacturing technique named Clad2Z. Using a robot‐mounted confocal powder nozzle with axial infrared laser beam, ceramic columns with a diameter of 450 µm and an adjustable height are developed. Influence of laser parameters and robot movements on shape and microstructure is analyzed. As an example application, the common material yttria‐stabilized zirconia (YSZ) is used to deposit columnar‐structured thermal barrier coatings (TBCs). The excellent thermal cycling performance of the Clad2Z samples is demonstrated by burner rig tests and comparing lifetime and failure mechanism with conventional TBC systems.

CMAS corrosion and thermal cycling fatigue resistance of alternative thermal barrier coating materials and architectures: A comparative evaluation

The corrosion of ceramic thermal barrier coatings (TBCs) by molten silicate deposits, usually known as “CMAS” from their main constituents (CaO-MgO-Al2O3-SiO2), is an issue of increasing concern in modern gas turbines as the turbine inlet temperatures are increased to enhance thermodynamic efficiency. Because conventional ZrO2-7wt%Y2O3 (7YSZ) dissolves quite readily in a CMAS melt, many alternative materials have been proposed, but there are not many comparative studies among them. Multi-layer architectures featuring a tougher 7YSZ bottom layer and a more brittle, but more CMAS corrosion-resistant top layer have also been proposed; therefore, a comparison among these architectures is also in order. In this paper we studied comparatively the resistance to CMAS corrosion and to thermal cycling fatigue (an essential pre-requisite for any TBC system) of Gd2Zr2O7, ZrO2–55wt%Y2O3 and Gd/Yb/Y co-doped ZrO2, both in the form of single, dense-vertically cracked (DVC) layers deposited by plasma spraying onto an MCrAlY bond coat, and as top layers with a bottom layer of either porous or DVC 7YSZ. It was found that Gd2Zr2O7 resists CMAS corrosion, without any grain-boundary dissolution, slightly better than does ZrO2–55wt%Y2O3. They both develop a solid Gd- or Y-apatite layer (respectively) at the interface with the CMAS melt, driven by the rather large difference in optical basicity between these compounds and CMAS itself, but the Y-apatite layer is less continuous and, therefore, a bit less protective. Gd/Yb/Y co-doped ZrO2, instead, suffers as much grain-boundary dissolution in contact with molten CMAS as does 7YSZ. A Gd2Zr2O7/porous 7YSZ system would therefore exhibit simultaneously high resistance to CMAS dissolution and to thermal cycling fatigue, although there is a risk that the CMAS melt might infiltrate the segmentation macro-cracks and the microcracks of the Gd2Zr2O7 layer and undermine the porous 7YSZ bottom layer.

Influences of the near-spherical 3D pore on failure mechanism of atmospheric plasma spraying TBCs using a macro-micro integrated model

The near-spherical 3D pores in the thermal barrier coatings (TBCs) by atmospheric plasma spraying (APS) affect the thermal insulation and anti-spalling performance of the coating. To explore the effects of near-spherical 3D pores on the coating failure, a macro-micro integrated model is developed. The randomly distributed pores are implanted into the ceramic layer inside the micro region by the secondary development of Python program. The dynamic propagation, coalescence, or branching of cracks in the ceramic layer are achieved by embedding cohesive elements between adjacent solid elements. The effects of pore characteristic parameters: porosity, aspect ratio, and orientation angle on the crack evolution are investigated. In addition, the effect of TGO on the ceramic cracking is also examined in the porous model. The results show that the stress concentration near the near-spherical pores can induce early crack initiation. Regular spherical pores with a porosity of about 10% are more conducive to the improvement of coating life. When a large number of near-spherical 3D pores appear, TGO thickening still induces premature spallation of the coating. These results can provide important theoretical guidance for the advanced TBC system design with long lifetime.

Evaluation of mechanical properties of YSZ TBCs doped by different ratios of Eu3+ ions after isothermal oxidation

Thermal barrier coatings (TBCs) are essential to improve the thermal insulation performance of high-temperature components. Rare earth element (Eu3+) doped yttrium stabilized zirconia (YSZ) TBCs have been proved to be an ideal solution for non-destructive testing of internal damages. Based on this theory, two types of coatings deposited by air plasma spray (APS) on Hastelloy-X were investigated: (1) Eu3+ doped YSZ (dopant ratios 1 mol%, 2 mol%, 4 mol%, respectively), (2) traditional undoped 8YSZ. Isothermal oxidation treatment at 1100 °C, in increments of 10h until the failure of the coatings are conducted to evaluate the mechanical properties of different coatings. The microscopic morphology and phase of the coatings were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD) patterns, respectively. The indentation testing methods were used to study the apparent interfacial fracture toughness and the hardness of the ceramic top coat. Results show that the Vickers hardness of the top coat increases with the decrease of porosity in the early stage and then decreases with the heat treatment time increasing in the long-term stage. Simultaneously, compared with the undoped 8YSZ coating, the fracture toughness increased with the dopant of Eu3+ ions increasing, from 1 mol% to 2 mol%, nevertheless, that of 4 mol% Eu3+ doped YSZ decreased compared with in the undoped 8 YSZ. For all types of specimens, the interfacial fracture toughness decreases with the increase of isothermal oxidation time. Results also indicate that the content of Eu3+ doping does not affect the microstructure and interfacial morphology of the YSZ coating as well as the growth law of thermally grown oxides (TGO). Furthermore, EDS detection found that the Eu3+ ions almost do not diffuse inside the TBCs system after isothermal oxidation treatment.

Factors affecting cyclic lifetime of EB-PVD thermal barrier coatings with various bond coats

Abstract Standard EB-PVD (electron beam physical vapor deposition) thermal barrier coatings (TBCs) of partially yttria-stabilized zirconia were applied on Ni-base substrate alloys and thermally cycled with T max = 1100 °C. Two different bond coats were investigated: NiCoCrAlY and NiPtAl. The longest lifetimes have been achieved on the Hf-containing Rene142 substrate. A rough interface between TGO (thermally grown oxide) and bond coat with hafnia pegs is characteristic for these alloys. A bond coat heat treatment in Ar–H significantly improved the lifetime of the TBC system with IN100 + PtAl bond coats, whereas reduced lifetimes were found on IN100 + NiCoCrAlY. Spallation of the TBCs was correlated to TGO thickness and microstructure. The TGO growth is faster for NiCoCrAlY than for NiPtAl bond coats, while TBC lifetime is longer for the NiCoCrAlY systems. Tests with reduced cycle length yielded an increased number of cycles to failure for IN100+NiCoCrAlY, but left the time to failure at temperature unchanged. TBC thickness had only a limited influence on cyclic lifetime.

Contemporary Materials Issues for Advanced EB-PVD Thermal Barrier Coating Systems

AbstractThe key issues for thermal barrier coating (TBC) development are high temperature capability and durability under thermal cyclic conditions as experienced in the hot section of gas turbines. Due to the complexity of the system and the interaction of the constituents, performance improvements require a systems approach. However, there are issues closely related to the ceramic top coating and the bond coat, respectively. Reduced thermal conductivity, sintering, and stresses within the ceramic coating are addressed in the paper as well as factors affecting failure of the TBC by spallation. The latter is primarily governed by the formation and growth of the thermally grown oxide scale and therefore related to the bond coat. A strategy for lifetime assessment of TBCs is discussed.

Effect of TiAlCrNb buffer layer on thermal cycling behavior of YSZ/TiAlCrY coatings on γ-TiAl alloys

TiAlCrY has been used as a bonding layer for the thermal barrier coating (TBC) system on TiAl alloys, due to its excellent high temperature oxidation resistance . However, it was found that vertical cracks formed in the TiAlCrY layer during the thermal cycling tests , which would fasten the failure of TBC system. In this work, a TiAlCrNb buffer layer was introduced between TiAlCrY layer and the substrate to alleviate the nucleation and propagation of cracks in the TiAlCrY layer from the perspective of alleviating the stress concentration, aiming to improve the thermal cycling lifetime of the YSZ/TiAlCrY system. The results show that the thermal cycling lifetime of the newly designed YSZ/TiAlCrY/TiAlCrNb system was over 245 cycles by means of water-quenching, which was almost twice of the system without the buffer layer and 3–4 times of the traditional YSZ/MCrAlY system on superalloys . The influence of the TiAlCrNb buffer layer was analyzed based on the combined analysis of microstructure, residual stresses and mechanical properties . This work implies that a proper buffer layer could effectively improve the thermal cycling lifetime of TBCs on TiAl alloys. • A TiAlCrNb buffer layer was firstly introduced to improve the thermal cycling lifetime of the YSZ/TiAlCrY system. • The thermal cycling lifetime of the newly designed YSZ/TiAlCrY/TiAlCrNb system was over 245 cycles by means of water-quenching at 1100 °C, which was almost 3-4 times of the traditional YSZ/MCrAlY system on superalloys. • This work indicates that a suitable buffer layer with excellent mechanical properties can effectively improve the thermal cycle lifetime of the TBCs.

Hot corrosion behavior of dense CYSZ/YSZ bilayer coatings deposited by atmospheric plasma spray in Na2SO4 + V2O5 molten salts

In this work, the hot corrosion (HC) resistance of bilayer thermal barrier coatings (TBC) architectures composed of dense Ceria-Yttria Stabilized Zirconia (CYSZ) and Yttria-Stabilized Zirconia (YSZ) deposited using the Atmospheric Plasma Spray (APS) technique was evaluated. Two kinds of HC tests were performed using different salt concentrations and thermal cycling (adding fresh salt between cycles) with a constant salt mixture (Na 2 SO 4 -V 2 O 5 ) and constant testing temperature (900 °C). The results showed that the HC mechanism changes with the testing procedure, and the outer dense CYSZ layer acts as a barrier to protect the inner layers against HC degradation. However, the formation of vertical cracks during the coating deposition process affected the corrosion resistance of the TBC systems. • Dense CYSZ layer overlay increases the hot corrosion resistance of TBC system. • Mechanisms of coating's degradation were identified. • Vertical cracks reduce the hot corrosion resistance of the TBC system.

Development of Thermal Barrier Coating Systems from Al Microparticles—Part II: Characterisation of Mechanical and Thermal Transport Properties

In this study, the mechanical resistance and the thermal insulation potential of novel thermal barrier coatings (TBCs) made of a foam of hollow alumina particles are assessed through scratch testing, micro-indentation and thermal diffusivity measurements using laser-flash. The thermal diffusivity of the foam coatings ranges between 0.6 × 10−7 and 5 × 10−7 m2·s−1 and is thus comparable with the thermal insulation potential of the standard plasma-sprayed (PS) and electron beam–physical vapour-deposited (EB-PVD) TBCs made of yttria-stabilised zirconia (YSZ). The coatings annealed in more oxidative atmospheres exhibit greater mechanical resistance due to the thickening of the alumina shells and the increased sintering of the foam. However, when the oxidation is poorly tailored, the adhesion of the foam to the substrate decreases due to the presence of unwanted oxide that grows at the substrate/coating interface.