Performance Of Thermal Barrier Coatings Research Articles

Influence of crack and pore structure characteristics on the thermal protective performance of thermal barrier coatings based on LBM

Influence of crack and pore structure characteristics on the thermal protective performance of thermal barrier coatings based on LBM

Terahertz Nondestructive Measurement of Heat Radiation Performance of Thermal Barrier Coatings Based on Hybrid Artificial Neural Network

Effective control of the micro- and nanostructure of thermal barrier coatings is essential to enhance the thermal radiation performance of the coating, which helps to determine the remaining service life of the coating. This paper proposed a method to measure the radiation properties of thermal barrier coatings by terahertz nondestructive testing technique, using APS-prepared thermal barrier coatings as the object of study. Radiative properties were a comprehensive set of properties characterized by the diffuse reflectance, transmittance, and absorptance of the thermal barrier coating. The coating data in actual service were obtained by scanning electron microscopy and metallographic experiments, and the data were used as the simulation model critical value. The terahertz time-domain simulation data of coatings with different microstructural features were obtained using the finite-different time-domain (FDTD) method. In simulating the real test signals, white noise with a signal-to-noise ratio of 20 dB was added, and fast Fourier transform (FFT), short-time Fourier transform (STFT), and wavelet transform (WT) were used to reduce the noise and compare their noise reduction effects. Different machine learning methods were used to build the model, including support vector machine algorithm (SVM) and k-nearest neighbor algorithm (KNN). The principal component algorithm (PCA) was used to reduce the dimensionality of terahertz time-domain data, and the SVM algorithm and KNN algorithm were optimized using the particle swarm optimization algorithm (PSO) and the ant colony optimization algorithm (ACO), respectively, to improve the robustness of the system. The K-fold cross-validation method was used to construct the model to improve the adaptability of the model. It could be clearly seen that the novel hybrid PCA-ACO-SVM model had superior prediction performance. Finally, this work proposed a novel, convenient, nondestructive, online, safe and highly accurate method for measuring the radiation performance of thermal barrier coatings, which could be used for the judgment of the service life of thermal barrier coatings.

Performance of Atmospheric Plasma-Sprayed Thermal Barrier Coatings on Additively Manufactured Super Alloy Substrates

This work represents a preliminary study of atmospheric plasma-sprayed (APS) Yttria-Stabilized Zirconia (YSZ)-based thermal barrier coatings (TBCs) deposited on forged and additive manufactured (AM) HAYNES®282® (H282) superalloy substrates. The effect of different feedstock morphologies and spray gun designs with radial and axial injection on APS-deposited YSZ layer characteristics such as microstructure, porosity content, roughness, etc., has been investigated. The performance of TBCs in terms of thermal cycling fatigue (TCF) lifetime and erosion behaviour were also comprehensively investigated. In view of the high surface roughness of as-built AM surfaces compared to forged substrates, two different types of NiCoCrAlY bond coats were examined: one involved high-velocity air fuel (HVAF) spraying of a finer powder, and the other involved APS deposition of a coarser feedstock. Despite the process and feedstock differences, the above two routes yielded comparable bond coat surface roughness on both types of substrates. Variation in porosity level in the APS topcoat was observed when deposited using different YSZ feedstock powders employing axial or radial injection. However, the resultant TBCs on AM-derived substrates were observed to possess similar microstructures and functional properties as TBCs deposited on reference (forged) substrates for any given YSZ deposition process and feedstock.

Simulation analysis of thermal insulation performance of diesel engine piston based on PEO and La2Zr2O7 thermal barrier coating

In this research, we explored the application of ‘thermal swing’ thermal barrier coatings (TBCs) in diesel engines, aiming to surpass the performance of conventional TBCs. Traditional TBCs often suffer from limitations like consistently high surface temperatures and a delayed response to sudden variations in cylinder temperatures, which adversely impact engine efficiency. To overcome these challenges, plasma electrolytic oxidation (PEO) technology was employed to engineer TBCs with the sought-after ‘thermal swing’ feature. Comparative simulations were conducted to assess the thermal characteristics of PEO coatings on pistons, in contrast to standard La2Zr2O7 coatings. The steady-state thermal analysis revealed that PEO coatings maintained a lower peak temperature compared to La2Zr2O7 coatings under similar thermal insulation conditions, indicating a superior ability of the PEO coatings in heat dissipation and in sustaining lower surface temperatures.In the transient thermal analysis, focusing on temporal temperature variations, the PEO coatings exhibited a markedly quicker and more robust response to cylinder temperature fluctuations. This indicates that PEO coatings are highly effective in adapting swiftly to temperature changes, thereby contributing to enhanced engine performance. Additionally, the PEO coating demonstrated a notable reduction in the end temperature (TE), which is crucial for mitigating the heating effect on intake gas, in comparison to La2Zr2O7 coatings with equivalent thermal insulation properties. However, it was observed that this advantage significantly decreased when the PEO coating thickness surpassed 0.3 mm. Drawing from these observations, it is concluded that PEO coatings hold several thermal advantages over La2Zr2O7 coatings, particularly in terms of thermal load management and responsiveness to in-cylinder temperature shifts. These benefits are most pronounced when the PEO coating maintains a thickness below 0.3 mm.

Research Progress in the Failure Behavior on the Interface of Thermal Barrier Coatings

This paper mainly introduces the research progress on interface failure behavior in hightemperature alloy surface thermal barrier coating systems. The degradation failure and structural evolution behavior during high-temperature service were analyzed for the matrix/bonding layer interface, bonding layer/TGO interface, and TGO/ceramic layer interface in thermal barrier coatings. The research focus and direction that affect the interface performance of thermal barrier coatings were proposed.

Toughening mechanism in superstructure Hf6Ta2O17 thin film

Fracture toughness is a key indicator to estimate the anti-peeling performance of thermal barrier coatings (TBCs). A unique superstructure endows Hf6Ta2O17 with an outstanding phase stability and mechanical property, which also poses great challenges for studying its intrinsic toughening mechanism. Here, the superstructure Hf6Ta2O17 single crystal film was epitaxially grown on YSZ(011) substrate by pulsed laser deposition. An uncommon ripple pattern near the Vickers indentation crack tips was first reported. The pop-in signals in the load-displacement curve indicated it may be relevant to ferroelastic domain transformation, which was further confirmed by spontaneous strain distribution in HRTEM. The transformation path, predicted by DFT, was conducted in b→c ferroelastic domain through in-plane rotation of Hf–O/Ta–O polyhedrons. The corresponding critical stress for domain transformation was up to 5.83 GPa. In addition, an intrinsic ductility was confirmed with a high Pugh ratio B/G of 2.49, much larger than the critical value (1.75) for brittle-ductile transition. These two mechanisms make contribution to the superior fracture toughness of Hf6Ta2O17. This work provides a deep insight into the toughening mechanism of ceramics and paves the way to design of next-generation TBCs.

Understanding the effect of bondcoat surface treatment on enhanced lifetime of suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are extensively used in gas turbine engines in power generation and aerospace applications. Improvements in TBC performance and lifetime are continuously pursued by the gas turbine industry. Suspension plasma spraying (SPS) has shown promising results in recent years as it could produce a porous columnar TBC microstructure exhibiting low thermal conductivity and high durability. Further improvements in lifetime and fundamental understanding of failure mechanisms would lead to their wider application.Lifetime of a TBC system is dependent not only on the topcoat microstructure and chemistry, but also on topcoat-bondcoat interface and bondcoat characteristics as they influence the growth rate of thermally grown oxide (TGO) layer as well as the mismatch stresses generated in the topcoat during operation. Moreover, in SPS TBCs, the column density and porosity in the topcoat are also affected by the bondcoat surface roughness which in turn affects TBC lifetime. In previous work, it was shown that bondcoat surface treatment by shot peening followed by light grit blasting can significantly enhance the cyclic lifetime of SPS TBCs.The objective of this work was to gain further insight into the lifetime improvements due to surface treatment of bondcoat. Commercial NiCoCrAlY powder was deposited by high velocity air fuel (HVAF) and high velocity oxy fuel (HVOF) spraying while SPS was used to deposit yttria stabilised zirconia topcoat. Before spraying the topcoat, the bondcoat samples were subjected to vacuum heat treatment, shot peening, and/or grit blasting. Residual stress measurements were performed by X-ray diffraction on samples with different variants of bondcoat treatments and compared with Almen strip tests. The lifetime was examined by thermal cyclic fatigue testing. The relationships between changes in bondcoat surface roughness, residual stress state and microstructure due to surface treatments, and resultant topcoat microstructure and lifetime were studied and discussed.

Preparation and high-temperature performance of Sc2O3–Y2O3 co-stabilized ZrO2 thermal barrier coatings

As a promising material for the advanced thermal barrier coatings (TBCs), scandium oxide (Sc2O3) and yttrium oxide (Y2O3) co-stabilized zirconia (ScYSZ) has been given increasing attention during the past decade. In this study, three types of TBCs including YSZ, ScYSZ and ScYSZ-YSZ double ceramic layer (DCL) coatings were deposited by a wide-velocity range high-energy plasma spraying technology. The thermal physical property, phase stability and thermal cycling performance of TBCs were comparatively studied. The results suggested that the thermal conductivity from 200 °C to 1000 °C of ScYSZ coating was reduced by approximately 30 % compared to the YSZ coating. After exposure at 1400 °C for 1000 h, the ScYSZ coating still kept the initial tetragonal phase structure, while the tetragonal phase was completely disappeared in the YSZ coating after 300 h at 1400 °C. Notably, the ScYSZ-YSZ DCL coating demonstrated the highest thermal cycling life within the temperature range of 1320∼1370 °C, exhibiting a remarkable 43 % increase over the single ScYSZ coating and doubling that of the single YSZ coating. The outcomes of this study hold significant promise for practical applications in the realm of high-performance TBCs.

Deposition characteristics, sintering and CMAS corrosion resistance, and mechanical properties of thermal barrier coatings by atmospheric plasma spraying

Atmospheric plasma spraying (APS) was used to fabricate 4.5 mol% Y2O3 stabilized ZrO2 (YSZ) and 4.0 mol% Yb2O3 and 0.5 mol% Y2O3 co-stabilized ZrO2 (YbYSZ) series coatings with different spraying parameters. This study presents the effects of particle size of feed powder and substrate temperature on the microstructure and performance of thermal barrier coatings. Single-pass and multi-pass experiments were conducted to explore the relationship between the splat and the coatings. The experimental results indicate that adjusting particle size of feed powders and deposition temperature can alter the splat behaviour and microstructure, improve anti-corrosion and anti-sintering abilities, and enhance the hardness and elastic modulus. Changes in parameters can cause changes in the microstructure of as-sprayed coatings. Lower substrate temperatures can lead to vertical cracks, finer powders can introduce more microdefects, and conventional sized powders can form larger pores. A coating deposited using a conventional feed powder at a substrate temperature of 800 °C shows excellent sintering resistance. A coating deposited using a fine feed powder on 800 °C substrate exhibits the best calcium‑magnesium-alumina-silicate (CMAS) corrosion resistance.

Analysis of the thermal insulation performance of turbine blade thermal barrier coatings considering inlet non-uniformity

The gases discharged from the new generation of lean-burn combustors usually have hot streaks (HS) and swirls. The thermal performance of rotor thermal barrier coatings (TBCs) exhibits significant non-constant behavior due to inlet non-uniformity and blade row interaction. To investigate the non-constant thermal insulation performance of the blade TBCs, non-constant numerical simulations were carried out for the first stage vane and blade of the High-pressure turbine of the GE-E3 engine. The normalized temperature results for the HS and the combination of HS and swirl cases were compared with the uniform inlet case to investigate the effect of inlet non-uniformity on the surface temperature and thermal insulation effect of the blade TBCs. The results show that a credible investigation of rotor TBCs should consider the unsteady blade row interaction. HS causes high-temperature hot spots on the surface of the TBCs and redistributes them with the change in the direction of the swirl. HS and swirl significantly reduce the turbine blade cooling effectiveness and thermal insulation effectiveness of TBCs. The lowest overall cooling performance of the blades was observed during the 1/4 T stator period. The added swirl reduces the insulation effectiveness's fluctuation value by reducing positive swirl (PSW) by 52.6 % and negative swirl (NSW) by 57.8 %. Therefore, applications and designs of TBCs should consider the effects of HS and swirl.

Evolution of microstructure, thermophysical and mechanical properties and wetting behavior of plasma-sprayed Yb2O3 and Y2O3 co-stabilized ZrO2 coatings during high-temperature exposure and CMAS corrosion

Doping rare-earth elements has been regarded as one of the potential ways to enhance the comprehensive performance of thermal barrier coatings. In the study, the microstructure, thermophysical and mechanical properties, and wetting behavior of Yb2O3-Y2O3 co-stabilized ZrO2 (YbYSZ) were evaluated in detail as compared with Y2O3 stabilized ZrO2 (YSZ) counterpart. During high-temperature exposure and CMAS attack at 1300 °C × 32 h, the samples did not undergo phase transformation, while samples underwent phase transformation under CMAS corrosion. In the cross-section of CMAS penetration, YbYSZ penetration depth decreases by more than 20 % compared to YSZ, and the maximum degradation depth of YSZ and YbYSZ is 27.37 μm and 11.28 μm. Additionally, the thermal conductivity of YbYSZ is lower than that of YSZ. During high-temperature exposure, the hardness and elastic modulus of YSZ show a more significant upward trend than that of YbYSZ. During CMAS corrosion, the hardness and elastic modulus offer a downward trend, with YbYSZ value higher than that of YSZ. The samples were held at 1300 °C × 180 s, the contact angle difference between YSZ (59.2°) and YbYSZ (99.9°) is the largest. The YbYSZ coating exhibits better performance on resistibility to CMAS corrosion and high-temperature exposure.

Internal Diameter Atmospheric-Plasma-Sprayed High-Performance YSZ-Based Thermal Barrier Coatings

Thermal barrier coatings (TBCs) play a crucial role in enhancing the operating temperature of key equipment. However, when TBCs are sprayed on the internal surface of these components, there are issues related to the imperfect internal diameter atmospheric plasma spray (ID-APS) process and the unsatisfactory performance of TBCs in internal holes. Yttria-stabilized zirconia (YSZ), YSZ + Gadolinium zirconate (GZ) and YSZ + tantalate (RETaO4) TBCs were prepared using the SM F210 ID-APS gun. The thermal shock and particle erosion resistance of the coatings were compared, and the relationship between the structure and properties was investigated. The results show that the hardness, fracture toughness and erosion resistance of the three coatings are the largest in the YSZ coating and the smallest in the GZ coating. YSZ coating also has the best thermal shock resistance. These research findings have important guiding significance for the improvement of the ID-APS process.

High precision thickness evaluation of thermal barrier coating with high frequency eddy current testing method

Consisting of a top ceramic coating (TC) and a bond coating (BC) layer, thermal barrier coating (TBC) is a key component of the heavy duty gas turbine blade to protect the substrate material from high working temperature. Thinning of the top coating and the thermally grown oxide (TGO) layer formed between TC and BC cause TBC thickness reduction, and may significantly affect the in-service performance of TBC. High frequency eddy current testing (HF ECT) is considered as an efficient technique for nondestructive evaluation (NDE) of the TBC thickness, but its measurement accuracy is not satisfied up to now. Aiming to realize a high precision TBC thickness evaluation with HF ECT signals, a numerical method for simulating HF ECT of TBC was developed and implemented with consideration of the dielectric properties of TC and TGO material. Through numerical simulations with the developed code, an ECT probe of optimized configuration and robust against lift-off variation was designed and fabricated for the TBC thickness evaluation. A HF ECT measurement system is then setup to investigate the validity of the prototype ECT probe and sizing method experimentally. It was found that the thickness of the TC layer can be evaluated within 10 μm accuracy for the plate TBC specimens, which reveals the HF ECT method and new ECT probe are efficient for NDE of the TBC thickness.

Comparative Study on Different Methods for Prediction of Thermal Insulation Performance of Thermal Barrier Coating Used on Turbine Blades

Comparative Study on Different Methods for Prediction of Thermal Insulation Performance of Thermal Barrier Coating Used on Turbine Blades

CMAS Corrosion Behavior of Nanostructured YSZ and Gd-Yb-Y-Stabilized Zirconia Coatings

With the development of industry, the operating temperature of aero engines and gas turbines continues to increase; developing thermal barrier coatings (TBCs) with superior resistance to CaO-MgO-Al2O3-SiO2 (CMAS) corrosion has become a prominent research focus. In this study, atmospheric plasma spraying (APS) was used to prepare yttria-stabilized zirconia (YSZ), nanostructured yttria-stabilized zirconia (n-YSZ), and Gd-Yb-Y-stabilized zirconia (GYYSZ) coatings. The effects of CMAS exposure on the microstructure, chemical composition, phase transition, and microhardness of the coatings were investigated. Comparative analysis revealed that both phase transition and exfoliation occurred in corroded YSZ and n-YSZ coatings, with n-YSZ exhibiting more pronounced changes. In contrast, GYYSZ coatings remained stable without phase transition and exhibited a smaller increase in microhardness (270 HV0.3). Consequently, doping Gd/Yb/Y elements into ZrO2 can improve the performance of TBCs.

Preparation of Highly Durable Columnar Suspension Plasma Spray (SPS) Coatings by Pre-Oxidation of the CoNiCrAlY Bondcoat

Columnar structured thermal barrier coatings (TBCs) have been intensively investigated due to their potential to enhance the durability and reliability of gas turbine engine components. These coatings consist of vertically aligned columns that provide excellent resistance to thermal cycling. In this study, the lifetime of columnar suspension-plasma-sprayed (SPS) TBCs was evaluated using burner rig tests. The tests were carried out under high-temperature conditions. Significantly, the pre-oxidation of the bondcoat during diffusion bonding treatment was found to have a substantial impact on the performance of the SPS TBCs. The optimized treatment resulted in columnar SPS TBCs demonstrating excellent thermal stability and resistance under the test conditions. The lifetime of the coatings was significantly extended compared to conventional TBCs by pre-oxidation of the CoNiCrAlY bondcoat in argon, which suggests that columnar SPS TBCs have great potential for use in gas turbine engines.

Understanding the Sintering Behavior and its Effect on the Thermal Conductivity of YSZ Coatings for Gas Turbine Applications

Thermal barrier coatings are essential coatings to protect the combustion chamber liner material from harsh environments in modern gas turbines that are used in aerospace and land-based power generation facilities. As there are several different materials to produce thermal barrier coatings, the conventional thermal barrier coating is yttria stabilized zirconia (YSZ). The most common method to manufacture YSZ coating is plasma spraying method due to its flexibility and rapid production capacity. Using plasma spraying, often requires understanding the process parameters effect on the coating structure. As there are many parameters to control coating process the main outcome of all parameters is the particle temperature and velocity during the spraying process hence the coating properties such as hardness, porosity ratio and deposition rates. Furthermore, not only produced microstructure but also during the service conditions sintering behaviour also be considered. Sintering behaviour of thermal barrier coatings results declining of their thermal insulation properties. Therefore, in this study we have evaluated sintering effect of on the thermal conductivity of the plasma sprayed yttria stabilized zirconia coatings. To achieve this objective, we produced free-standing coatings and subjected them to heat treatment, followed by measurements of their thermal conductivities. The results of this study will contribute to a better understanding of the sintering behaviour and its impact on the thermal performance of thermal barrier coatings.

The relationship between the film hole distribution and thermal cycle performance of TBCs by APS and HVOF

Thermal barrier coatings (TBCs) play an important role in protecting hot-end components of aero-engines from high-temperature oxidation and corrosion. The peeling of the TBCs around film holes has emerged as a major source of failure in thermal cycle oxidation. This study examines the failure behavior of TBCs on floating tiles in the combustion chamber of an aero-engine. Ninety thermal cycles were performed on two types of floating tiles with different film hole distribution. Results showed that the surface TBCs of sample No.1 peeled off by approximately 40 % of the total area, while 90 % of the surface TBCs of sample No.2 floating tile remained intact. A thermal cycling experiment of the TBCs on the surface of the floating tiles was performed to determine the stress distribution of the film holes by finite element (FE) simulation. The results indicated that there was a significant stress gradient on the TBCs surface around the film holes after thermal cycles, with 86.09 MPa and 62.84 MPa higher stress gradient around the film holes of sample No.1 floating tile compared to the film hole of sample No.2 floating tile on the surface and back side, respectively. This difference in the stress gradient and the film hole location on the TBCs led to spalling of the TBCs on the surface of both floating tiles. This study elucidates the failure mechanism of TBCs around film holes under thermal cycling through experiments and FE simulations, offering insights to improve the lifetime of TBCs.

A comprehensive Review on the Hot Corrosion and Erosion Performance of thermal Barrier Coatings

A comprehensive Review on the Hot Corrosion and Erosion Performance of thermal Barrier Coatings

Stress evaluation of TBCs with inclined film-cooling hole considering CMAS penetration-induced change of thermo-mechanical properties

CaO-MgO-Al2O3-SiO2 (CMAS) penetration is an essential factor leading to the failure of thermal barrier coatings (TBCs), especially around the film-cooling hole structure of high-temperature gas turbine components. The complex temperature distribution and the free-edge effect around the cooling holes aggravate the influence of CMAS penetration on the failure of the TBCs. In this study, a three-dimensional (3D) model of a TBC-film cooling system considering CMAS attack is presented based on the observation of a retired turbine blade. CMAS penetration-induced change of thermo-mechanical properties of the ceramic top coat (TC) was considered. The thermal gradient around a cooling hole and the uneven growth of the thermally grown oxide (TGO) on the stress distribution in TBCs were investigated. The results showed that the thermal insulation performance of TBCs declined considerably under CMAS penetration, and the induced maximum increase in the temperature at the TC/bond coat (BC)interface reached 60 K. CMAS penetration also led to a considerable stress increase in the TBC-film system. We therefore conclude that CMAS penetration accelerates the crack initiation at the TC/TGO and TGO/BC interfaces.