Thermal Barrier Coatings Research Articles

Failure behavior of RE2M2O7/YSZ TBCs prepared by EB-PVD in gas thermal shock-CMAS corrosion environment

Three types of thermal barrier coatings (TBCs), YSZ, LZC/YSZ and GZO/YSZ, were deposited by EB-PVD. These coating systems were then subjected to thermal shock-CMAS corrosion testing using a high-temperature gas spray gun. The results demonstrate that the double-layer thermal barrier coatings exhibit excellent resistance to thermal shock-CMAS corrosion. Microstructural analysis revealed that the LZC/YSZ coating mitigated thermal damage owing to the formation of a sealing layer at the CMAS/LZC interface through high-temperature chemical interactions with CMAS. Compared to LZC/YSZ, the GZO/YSZ coatings showed greater resistance to CMAS-induced damage, which was associated with lower theoretical optical basicity values. This research provides valuable insights into the coupled thermal shock-CMAS corrosion failure mechanisms of TBCs on turbine blades and paves the way for new system-safe applications of TBCs.

Fluorescence and temperature sensitive properties of Tb and Eu co-doped 8YSZ thermosensitive ceramic

Incorporating a small amount of rare-earth luminescent elements into thermal barrier coatings is currently the most promising method for determining the temperature distribution of engine blade coatings. In this study, Tb and Eu co-doped 8YSZ ceramic powder (8YSZ:Tb3+&Eu3+) was synthesized using chemical co-precipitation method, and the optimal powder was sintered into ceramic sheet through dry pressing. The phase, morphology, fluorescence properties and temperature sensitivity of 8YSZ:Tb3+&Eu3+ ceramic were studied. Results indicated that 8YSZ:Tb3+&Eu3+ was tetragonal with average grain size ranging from 22 to 28 nm. The luminescence intensity and lifetime of 8YSZ:Tb3+&Eu3+ both reach their maximum when the doping amount of Tb3+ is 1.5 mol%, with the existence of energy transfer from Tb3+ to Eu3+. And the forming processes has little influence on fluorescence performance of 8YSZ:Tb3+&Eu3+ materials. Meanwhile, 8YSZ:1.5Tb&1Eu ceramic reveals excellent temperature sensitivity. At 500 K, the relative sensitivities are 0.519% K-1 and 0.369% K-1 respectively for 8YSZ:1.5Tb&1Eu powder and sheet, while absolute sensitivities are 0.731% K-1 and 0.751% K-1. Consequently, 8YSZ:Tb3+&Eu3+ ceramic is a potential temperature detection material for practical applications.

Crack behaviour and failure mechanisms of air plasma sprayed (Gd, Yb) doped YSZ thermal barrier coatings under oxy-acetylene flame thermal shock

This paper examines the performance and failure mechanisms of atmospheric plasma sprayed (Gd, Yb)-doped YSZ (RYSZ) thermal barrier coatings (TBCs) under oxy-acetylene flame thermal shock conditions. The study involved cyclic thermal shock testing at temperatures of 1400 °C, 1500 °C, and 1600 °C for 10-min intervals until failure occurred, with failure counts recorded as 6, 2, and 2, respectively. During the thermal shock tests, tensile stress in the top coat (TC) caused vertical cracks. The growth of the thermally grown oxide (TGO) layer concentrated stress at the top coat/bond coat interface, leading to transverse interfacial cracks. As the shock temperature increased, the density of vertical cracks and the length of transverse interfacial cracks also increased. Additionally, transverse cracks developed in the TC due to sintering of the surface region at higher shock temperatures. The interaction between vertical and transverse cracks was critical to the spalling of the TC. Variations in crack densities and lengths resulted in different failure modes of the TBCs, depending on the thermal shock temperatures (1400 °C, 1500 °C, and 1600 °C).

Clarifying the effects of composition and phase stabilization on the oxidation resistance in AlCoCrFeNiTi series high-entropy alloys at 1100 °C

The application of high entropy alloys in thermal barrier coating (TBC) systems offers a viable solution for mitigating interfacial instability arising from the elevated-temperature interdiffusion within the bonding layer/matrix. Nevertheless, the elucidation of phase stability under elevated temperatures and the intricate interplay among thermal-mechanical-chemical factors in HEAs remain lacks clarity. Herein, a set of AlxCo10.5Cr10Fe25Ni(50-x)Ti4.5 HEAs featuring desirable high-temperature phase stability, oxidation resistance, and spalling resistance at 1100 °C were meticulously designed. And the effects of phase composition and thermodynamics on the evolution of surface oxides in these HEAs were investigated. Specifically, the formation of the unfavorable σ phase between 860-1200 °C is avoided through the composition adjustment. The oxidation resistance is governed by the presence of large-size columnar grains and the spalling resistance of the Al2O3 oxide film. The spalling resistance is mainly linked to the configuration sequence (Spinel→FeAlTiO5→TiO2) of the outer oxide layer and the phase transition of the matrix either to FCC or BCC. Meanwhile, the formation of FeAlTiO5 assumes a crucial role in the optimization of spalling (>770h) and oxidation resistance (2.6×10-12g·cm-2·s-1) within an Al20Co10.5Cr10Fe25Ni30Ti4.5 alloy. These findings provide guidance for the development of TBC systems endowed with exceptional properties tailored for engineering applications.

Enhancing anti corrosion properties of plasma-sprayed ZrO2-25 wt%CeO2-2.5 wt%Y2O3 thermal barrier coatings via nanostructure engineering

The current research aimed to enhance the corrosion resistance of (i) the CaO-MgO-Al2O3-SiO2 (CMAS) and (ii) the hot corrosion of the Na2SO4-55 wt%V2O5 environments. Conventional thermal barrier coatings (TBCs) with a composition of ZrO2-25 wt%CeO2-2.5 wt%Y2O3 (micro-CYSZ) were replaced by nanostructured coatings (nano-CYSZ) with a similar composition. This replacement was achieved by applying the desired coatings using the atmospheric plasma spraying (APS) process on IN738LC/CoNiCrAlY. Corrosion and hot corrosion tests were conducted in the CMAS and Na2SO4-55 wt%V2O5 environments at 1100 °C and 950 °C with 4-h cycles. In terms of resistance to CMAS corrosion and hot corrosion in the Na2SO4-55 wt%V2O5 environment, the nano-CYSZ coating exhibited better performance compared to the micro-CYSZ with a similar composition. The use of nanostructuring, including the formation of dual structures consisting of unmelted nano-sized particles and melted particles in the top layer of the TBC, resulted in delaying the penetration of molten salts and consequently improving the resistance to the CMAS and Na2SO4-55 wt%V2O5 environments.

Photothermal measurement of material properties for translucent thermal barrier coatings

In this study, a photothermal nondestructive method was proposed to measure the material parameters of semi-transparent or translucent thermal barrier coatings (TBCs). We derived a theoretical model for the photothermal signal from a two-layer semi-infinite material system with a translucent first layer after a pulse laser excitation. Its solution was verified by numerical solution. A data regression algorithm based on a least-squares fitting was used for the determination of the material parameters in the translucent first layer material. To verify this new method, an experimental system was set up with a pulse laser for thermal excitation and an infrared camera for image acquisition of the thermal emission transient from several translucent EBPVD TBC samples. The predicted coating thickness is consistent with the measured value by an optical microscope. The predicted thermal conductivity and optical attenuation coefficients in the absorption and emission band are found to be in good agreement with reference values.

Study on the Thermal Properties According to Deposition Method of Thermal Barrier Coating

Study on the Thermal Properties According to Deposition Method of Thermal Barrier Coating

Study on robot trajectory planning and coating thickness prediction for plasma spraying on complex surface

Plasma spraying techniques are commonly employed for the deposition of thermal barrier coatings (TBCs) due to their efficiency and cost-effectiveness. However, ensuring uniform coating thickness and quality on complex free-form surfaces poses significant challenges. This paper investigates the influence of spraying trajectory and related parameters (spraying distance, angle, velocity) on coating thickness distribution, addressing the need for simplified analysis among numerous variables affecting coating quality. Different from the predominantly existing research focusing on flat or rotationally symmetric substrates, this study delves into the planning of spray trajectories for free-form surfaces, which is crucial for industries dealing with complex components, such as turbine blades. Innovative optimization approaches are employed to refine spray trajectories and improve coating consistency. Through theoretical modeling, simulation and experimental validation, the impact of spray parameters on coating thickness was demonstrated. Both the mean and disperssion coefficient errors of the coating thickness, obtained by the theoretical prediction model and the spray experiments, are lower than 10 %. The normal spray trajectory makes the coatings more evenly distributed, and the coating uniformity is at least 50 % higher than that of the codirectional spraying. This research contributes to the optimization of plasma spraying processes, particularly on irregular surfaces, thereby facilitating the development of high-performance TBCs for industrial applications.

Theoretical investigation of structural, mechanical, electronic, optical, and thermal properties of ternary compounds of heusler alloy ANiSn (A= TI, TH, U) using first principles calculations

In this study, we investigated the ANiSn (A = Ti, Th, U) half-Heusler materials for various properties, including structural, electronic, mechanical, elastic anisotropic, optical, and thermal properties, using Density Functional Theory (DFT) with the Cambridge Serial Total Energy Package (CASTEP) code. The elastic constants satisfied Born's criteria, confirming the thermodynamic and mechanical stability of the ANiSn compounds. Mechanical stability was further assessed through bulk modulus, shear modulus, and Poisson's ratio. Our analysis revealed that TiNiSn and ThNiSn exhibit ductile behaviour, whereas UNiSn is brittle. The calculated elastic modulus indicated that the compounds we studied are elastically anisotropic. The electronic and optical properties confirmed the semiconducting nature of these materials, with significant absorption and conductivity observed in the ultraviolet region. ANiSn is suitable for manufacturing various optoelectronic devices, such as laser diodes (LDs), photodetectors, LEDs, and UV sensors, due to its high absorption coefficient in the IR to UV regions. Additionally, measured Debye and melting temperatures confirmed that TiNiSn is more thermally conductive and can be used in high-temperature structural substances. The low minimum thermal conductivity suggests that UNiSn may be a more efficient material for thermal barrier coatings (TBC) compared to TiNiSn and ThNiSn.

Design of eddy current and capacitance dual-mode sensor for thickness detection of thermal barrier coatings

Abstract Thermal barrier coatings (TBCs) can markedly enhance the service temperature of high-temperature alloys, thereby enhancing the engine thrust-to-weight ratio. However, TBCs must be capable of withstanding a multitude of harsh environments, including high temperatures, high pressures, and high-speed particle impacts, which can potentially lead to damage and failure. Consequently, nondestructive testing of TBCs is of particular importance for the assessment of structural health. As eddy current detection is applicable to conductive materials and capacitive detection is applicable to non-conductive materials, this paper proposes a novel, eddy current/capacitance detection, dual-mode sensor, which combines the advantages of the two detection modes. This study examines the structural configuration and excitation characteristics of the eddy current and capacitance dual-mode sensor. Furthermore, the interaction between the capacitive electrode and the eddy current coil is examined, and the viability of the dual-mode sensor is demonstrated through experimentation. The experimental results demonstrate that the dual-mode sensor exhibits a sensitivity of 2.67 mΩ mm−1 for bond coating thickness detection in eddy current mode with an excitation frequency of 5 MHz, and a maximum sensitivity of 80.31 pF mm−1 for top coating thickness detection in capacitive mode.

Finite element analysis of thermal barrier properties under relevance vector machine in coated pistons and blades

Internal Combustion (IC) engines require Thermal Barrier Coatings (TBCs) to preserve their effectiveness and performance. By coating engine parts with TBCs, thermal fatigue and environmental heat loss may be minimised. To reduce heat loss in IC engines during the combustion stage, piston thermal barrier coating is extremely beneficial. Additionally, it is well known that power plants with higher energy efficiency tend to operate their gas turbines at higher operating temperatures. To produce TBCs for the thermal design of gas turbines and diesel engines with higher performance, an in-depth study on the thermophysical properties of TBCs is required. This article presents an experimental investigation into the combined effect of gas turbine blades and diesel engine pistons coated with a heat barrier, utilising two distinct Thermal Barrier Coatings. Initially, numerical calculations are used to assess the wall heat transfer model’s ability to control temperature and density. The piston and blade top surfaces received plasma-sprayed, Yttria-Stabilised Zirconia (YSZ) with different material combinations on the piston and blades are evaluated. The Finite Element Method is developed for the coating of the ideal thickness to analyse the failure mechanism of the TBC sample. To evaluate the fatigue residual lifespan of TBC pistons, as well as blades in high-power engines, the authors also created a relevant vector Machine-based lifetime prediction model. The CO, HC and NOx emissions, as well as the fuel depletion and braking thermal efficiency, are used to calculate the performance analysis of the coated piston and blades. The results demonstrated improved thermal process endurance and decreased thermal conductivity by 28% when compared to YSZ using the RVM method respectively.

Numerical modelling of CMAS infiltration and dimensionless numbers of anti-infiltration designs of thermal barrier coatings

Resistance to calcium–magnesium–alumina–silicate (CMAS) infiltration and corrosion is a crucial problem for safely applying thermal barrier coatings (TBCs) in advanced aero engines. This work proposes a CMAS infiltration model based on the phase-field method to simulate the CMAS infiltration process in TBCs and analyse the influence factor. Further, a key dimensionless number called relative driving force K=rσcos(θc)tsμϛ2hTC2 was found to determine the CMAS infiltration depth using dimensional analysis. A criterion of K<2 was derived and was considered to be the evaluation standard of TBCs with excellent CMAS infiltration resistance. Based on K minimization, novelty TBCs with microstructure of S-shaped intercolumnar gaps and spherical pores are expected to significantly improve the resistance to CMAS infiltration and corrosion.

Towards enhanced corrosion resistance of PS-PVD TBCs in marine environments by structural design

Thermal barrier coatings (TBCs) prepared by plasma spray-physical vapor deposition (PS-PVD) applied in marine environments encounter a critical challenge of corrosion-induced failure. In this paper, yttria-stabilized zirconia (YSZ) TBCs with different microstructures were prepared via PS-PVD by varying powder feeding rates to investigate the influence of microstructural modifications on corrosion resistance. The effects of structural gradient, initial deposition mechanism, and pore distribution on corrosion resistance were explored. Additionally, the inhibition mechanisms of capillary effect and chemical reaction on molten salt penetration were confirmed. Finally, a microstructure gradient strategy for PS-PVD TBCs was proposed. The structurally graded TBCs tailored using this strategy exhibited excellent corrosion resistance, with the molten salt corrosion resistance surpassing that of conventionally processed PS-PVD TBC by over 50 %. These outcomes highlight the promising potential of the microstructure gradient strategy for enhancing the corrosion resistance of PS-PVD TBCs.

Performance-tunable thermal barrier coating for carbon fiber-reinforced plastic composites via flame spraying

Thermal barrier coatings (TBCs) are essential for improving the heat resistance of materials operating in high-temperature environments. This paper proposes a new method for manufacturing double-layered TBC with graded porosity for carbon fiber-reinforced plastic (CFRP) composites. The TBC was created by a flame spraying process, consisting of relatively dense and porous layers: (1) a dense layer was produced by spraying yttria-stabilized zirconia (YSZ) particles directly onto neat carbon fabric substrate and (2) a porous layer was prepared by co-spraying YSZ particles with sacrificial polyetheretherketone (PEEK) particles. The porosity of the porous layer was controlled by varying a PEEK injection distance (D) and a PEEK feed rate (R). The correlation between porosity and thermal conductivity of the TBC layer was investigated to assess its thermal barrier performance. The TBC fabricated with the D = 5 cm and the R = 1.0 g/min offered the optimal porosity and conductivity. The 640μm-thick TBC with 34% porosity and 0.27 W/m·K thermal conductivity protected the CFRP substrate remarkably under the subjected torch at 500°C, as the TBC layer reduced the surface temperature of CFRP to 230°C. Thermomechanical analysis, following thermal shock tests, revealed that the double-layered TBC/CFRP composite retained 87% and 75% of its pristine flexural strength and modulus, respectively, while the neat CFRP composite was completely burnt out. This study explored the application of flame spray technology to develop highly effective double-layered TBCs with tunable porosity to maximize their thermal barrier performances. All results from the current study provide new insights into the design and development of TBC and CFRP composites, which will benefit a wide range of lightweight high-temperature applications.

Effect of Yb2O3 doping on the thermal and mechanical properties GdAlO3-Gd2Zr2O7 composite with eutectic composition

Rare earth zirconates exhibit potential as thermal barrier coatings, but their limited toughness impedes their widespread use. To address this issue, rare earth aluminates are regarded as effective toughening agents for zirconates. However, the thermal conductivity of composites containing rare earth aluminates and zirconates tends to rise. To mitigate this issue, we have developed a series of Yb2O3-doped GAO-GZO composites. The introduction of Yb2O3 results in the formation of a fluorite phase, with Yb3+ ions occupying Gd sites in both GAO and GZO components. Although the fracture toughness remains relatively unchanged, the hardness of the composite improves. The thermal conductivity decreases with increasing Yb2O3 content due to enhanced phonon-defect scattering. Additionally, the coefficient of thermal expansion is reduced due to the stronger Yb-O bonding compared to Gd-O. This study presents a viable strategy for regulating the mechanical and thermal properties of rare earth zirconates reinforced with aluminates through defect engineering.

Tailoring mechanical, microstructural and toughening characteristics of plasma-sprayed graphene-reinforced samarium niobate coatings for extreme environments

Thermal barrier coatings (TBCs) are advanced ceramic layers applied to metal components to provide insulation and protection against high temperatures in extreme operating environments. This study investigated the effects of graphene nanoplatelet (GNP) reinforcement on samarium niobate (SN: SmNbO4) TBCs for extreme environments. Four ceramic top coat compositions were plasma-sprayed onto Inconel 718 substrates: Yttria-stabilized zirconia (YSZ), SmNbO4 (SN), and SN reinforced with 1 and 1.5 wt% GNPs (SN-1GNP, SN-1.5GNP). The research examined microstructural characteristics, phase evolution, mechanical properties and toughening mechanisms. GNP reinforcement significantly improved coating density, with SN-1.5GNP reaching 97.4 ± 1.64% compared to 91.3 ± 1.69% for SN and 86.6 ± 1.47% for YSZ. Hardness and elastic modulus were enhanced by 86.38% and 57.91% for SN-1GNP, and 101.09% and 65.23% for SN-1.5GNP respectively. Moreover, fracture toughness experienced a significant increase from 1.86 ± 0.4 to 5.48 ± 0.7 MPa·m1/2, facilitated by toughening mechanisms, like splat bridging, GNP pull-out, crack arrest and ferroelastic domain switching. Additionally, the SN-1.5GNP coating exhibited a higher adhesion strength of 36.84 MPa, thereby leading to improved layer distribution and lesser chance of delamination. Compared to YSZ, these findings suggest that GNP-reinforced SN coatings offer enhanced performance for extreme environment applications.

Deciphering Pauling's Third Rule: Uncovering Strong Anharmonicity and Exceptionally Low Thermal Conductivity in TlAgSe for Thermoelectrics

AbstractThe elucidation of chemical bonding, coupled with an exploration of the correlated dynamics of constituent atoms, is essential for unravelling the underlying mechanism responsible for low lattice thermal conductivity (κL) exhibited by a crystalline solid, which is essential for thermoelectrics and thermal barrier coatings. In this regard, Pauling's third empirical rule, which deals with the cationic repulsion due to proximity in the face or edge shared polyhedra in a crystal structure, can bring about the lattice instability required to suppress the κL. Here, we demonstrate the presence of such instability in a ternary selenide, TlAgSe, leading to a ultra‐low κL of 0.17 W/m.K at 573 K. Our study reveals the instability arising from Ag−Ag repulsion within edge‐shared AgSe4 tetrahedra through investigation of the local structure using synchrotron X‐ray pair distribution function (PDF) analysis and supported by first‐principles density functional theory calculations. We observe correlation between weakening in the Ag and the Tl‐sublattice, providing direct experimental evidence of Pauling's third empirical rule. The correlated rattling of Ag and Tl induces a highly anharmonic lattice and low energy optical phonons, resulting in suppressed sound velocity and ultralow κL in TlAgSe. The electronic origin of soft and anharmonic lattice is the presence of filled antibonding states in the valence band near the Fermi level constructed by Ag(4d)−Se(4p) and Tl(6s)−Se(4p) interactions. This work demonstrates that the evidence of dynamic distortion in a crystal lattice is governed by the third empirical rule given by Pauling, which can act as a potential new strategy for diminishing κL in crystalline solids.

PS-PVD thermal barrier coatings microstructure modulation and high-temperature erosion resistance study

Plasma spray-physical vapor deposition (PS-PVD) enables the modulation of both the microstructure and the performance of the coating by modulating the ratio of the three phases (gas, liquid, and solid) present in the jet. Considering the observed deficiency in the erosion resistance of PS-PVD coatings, the study explores improving high-temperature erosion resistance in PS-PVD thermal barrier coatings by adjusting Ar/He flow ratios. At a plasma gas flow rate of 90 SLPM, increasing the Ar/He ratio enhances nanohardness and elastic modulus. An Ar/He ratio of 2:1 provides erosion resistance comparable to standard APS coatings. Raising the plasma flow to 120 SLPM with the same Ar/He ratio forms a dense coating with low erosion rates. The study identifies a strong correlation between the hardness of the coating and erosion resistance. Lateral cracking from high-temperature and high-speed particle erosion is identified as the primary cause of failure.

Mechanical properties, thermal shock resistance and stress evolution of plasma-sprayed 56 wt% Y2O3-stabilized ZrO2 thick thermal barrier coatings

The mechanical properties and thermal shock resistance of plasma-sprayed 56 wt% Y2O3-stabilized ZrO2 thick thermal barrier coatings (TTBCs) deposited with different critical plasma spraying parameter (CPSP) values and its stress evolution were investigated. With the CPSP value increased from 0.83 kW·SLPM−1 to 1.17 kW·SLPM−1, the porosity was decreased from 14.62 ± 0.84 % to 7.00 ± 0.38 % while the bonding strength was increased from 7.82 ± 0.64 MPa to 9.95 ± 0.30 MPa. As a result, the hardness of the coating obtained from the cross-section was increased from 2.16 ± 0.61 GPa to 2.67 ± 0.82 GPa, and the elastic modulus was also increased from 60.47 ± 5.22 GPa to 71.93 ± 6.91 GPa. During thermal cycling test at 1000 °C, thermal cycling lifespan presented a decrease from (83 ± 3) cycles to (44 ± 3) cycles, and coating failure was mainly attributed to the occurrence of partial or whole spallation of the top coat. According to the stress simulation result, residual stress in the coating was accumulated while thermal stress was released with the increase of thermal cycles, and the increasing residual stress was the main failure driver for the coating.

Computer Simulation of Sintering in Porous Structures of Thermal Barrier Coating

Computer Simulation of Sintering in Porous Structures of Thermal Barrier Coating