Thermal Barrier Coatings Research Articles

First-principles characterization of thermal conductivity in LaPO4 -based alloys

Alloys based on lanthanum phosphate (LaPO4) are often employed as thermal barrier coatings, due to their low thermal conductivity and structural stability over a wide temperature range. To enhance the thermal-insulation performance of these alloys, it is essential to comprehensively understand the fundamental physics governing their heat conduction. Here, we employ the Wigner formulation of thermal transport in conjunction with first-principles calculations to elucidate how the interplay between anharmonicity and compositional disorder determines the thermal properties of La1−xGdxPO4 alloys, and discuss the fundamental physics underlying the emergence and coexistence of particlelike and wavelike heat-transport mechanisms. We also show how the Wigner transport equation correctly describes the thermodynamic limit of a compositionally disordered crystal, while the Boltzmann transport equation does not. Our predictions for microscopic vibrational properties (temperature-dependent Raman spectrum) and for macroscopic thermal conductivity are validated against experiments. Finally, we leverage these findings to devise strategies to optimize the performance of thermal barrier coatings. Published by the American Physical Society 2024

Effect of Bond Coating Surface Morphology on the TBC System Lifetime

A study on the effect of bond coating (BC) surface modifications prior to ceramic deposition is presented. Grit blasting and polishing were used to modify the BC surface finish. Thermal barrier coating (TBC) systems were made of a commercial yttria-stabilized zirconia (YSZ) deposited by electron beam physical vapor deposition on a β-(Ni,Pt)Al-coated Ni-based single crystal superalloy. Surface roughness measurements and scanning electron microscopy observations, before and after thermal cycling at 1100 °C, were performed to investigate the influence of the initial surface treatment on the YSZ/BC interface morphology and top coat spallation resistance. Surface states enhancing TBC spallation resistance have been found. In particular, it is shown that rumpling can be avoided even in the presence of phase transformations in the BC, by grinding samples with P600 SiC paper or by applying an “heavy” grit blasting leading to a thinner BC.

Formation mechanism of an ultra-low thermal conductivity thermal barrier coating deposited using biomimetic hierarchical porous microspheres

The hierarchical porous microsphere is a promising feedstock for thermal barrier coatings (TBCs), due to its excellent flowability, high compression strength, and high temperature structure stability. This study aims to determine the formation mechanism of the coating deposited using hierarchical porous microspheres. For the (Sm0.2Gd0.2Dy0.2Er0.2Yb0.2)2Zr2O7 hierarchical microsphere sintered at 1000 ºC to 1550 ºC, the thermal conductivity increased from 0.21 to 0.64 W/m·K, the compression strength increased from 13 MPa to 80 MPa. The hierarchical porous structure can inhibit the heat flow from the surface to interior, resulting in the low melting degree in the plasma jet. Additionally, the coatings exhibit the characteristics of nanostructured coatings and hierarchical porous zones inherited from the microspheres, as evidenced by a comparison of compression strength and impact strength during deposition. The as-sprayed nanostructured coating shows a significantly lower initial thermal conductivity of 0.33 W/m·K at 1000 ºC compared to the conventional coating.

Influence of Bond Coat Roughness on Adhesion of Thermal Barrier Coatings Deposited by the Electron Beam–Physical Vapour Deposition Process

Thermal barrier coatings (TBCs) are effective protective and insulative coatings on hot section components of turbine engines. The quality and subsequent performance of the TBCs are strongly dependent on the adhesion between the coating and the metal substrate. The adhesion strength of TBCs varies depending on the substrate materials and coating, the coating technique used, the coating application parameters, the substrate surface treatments, and environmental conditions. Therefore, the roughness of the substrate surface has a significant effect on the performance of the TBC system. In this work, the roughness and microstructure of the 7YSZ (7 wt.% yttria-stabilised zirconia) top coat under different bond coat roughness treatments were studied. The purpose of this paper was to investigate the influence of the roughness of the bond coat on the adhesion of 7YSZ TBCs prepared by the electron beam–physical vapour deposition (EB-PVD) process. The VPA (vapour phase aluminium) bond coat was deposited on Inconel 718 nickel superalloy substrate using the above-the-pack technique. The ceramic top coat was applied to the bond coat using the EB-PVD process. The dependence between the TBC coating roughness and the bond coat roughness was determined. Adhesion strength measurements were performed according to the ASTM C 633 standard test method. The highest adhesion value observed in the tensile adhesion tests was 105 MPa. However, it was not determined whether the surface roughness of the bond coat affects the adhesion of the 7YSZ top coat.

Local high temperature phenomena near the complex interfaces of thermal barrier coatings based on non-Fourier model

Thermal barrier coatings (TBCs) consist of a metal substrate (Sub), a bond coat (BC), and a ceramic top coat (TC). The ceramic coating is a porous dielectric layer with a significant relaxation time that cannot be ignored. In high-temperature environments, an irregularly shaped thermally grown oxide layer (TGO) forms between the BC and the TC. Therefore, using non-Fourier model to describe the thermal contact process of TBCs with complex interface can get more accurate results. However, it is difficult to obtain the analytical solution of this model. This paper introduces a finite volume method based on unstructured meshes to address the non-Fourier heat transfer process in the complex interfaces of multilayer TBCs. The gradient reconstruction technique is employed to calculate the cell derivatives on the surface. The method's reliability is confirmed through a comparison with a two-dimensional analytical solution. The results indicate that the thermal wave travels at a limited speed within the TC layer. After passing through the interface, the thermal wave travels very fast within the BC and Sub layers, due to the influence of thermal wave reflection and superposition, obvious local high temperature will be formed near the interface. In addition, the effects of thermal conductivity, relaxation time and interface shapes on the non-Fourier heat conduction process of TBCs are also discussed. The results of this paper are useful for the structural design and failure mechanism research in TBCs.

Synthesis of high-entropy ceramics (Y0.2Yb0.2Lu0.2Eu0.2Er0.2)3Al5O12 by electron beam heating

High-entropy ceramic materials are promising for creating functional ceramics for various purposes. The unique properties of such materials make it possible to create new thermal barrier coatings based on them. Reducing costs, time and simplifying the technology of synthesis of high-entropy ceramics is the subject of numerous studies. In this work, the synthesis of high-entropy ceramics (Y0,2Yb0,2Lu0,2Eu0,2Er0,2)3Al5O12 in a powerful beam of fast electrons was carried out for the first time. A powder mixture of initial oxides, placed in the volume of a massive copper cell, was exposed in air to a short-term exposure to a beam of powerful fast electrons with an energy of 2 MeV and a beam current of 12 mA. The speed of movement of the cuvette with the powder mixture under the beam was 1 cm/s. The total irradiation time was 10 s. During the irradiation process, at least 96 % of the mass of the powder mixture melted, resulting in the formation of highly porous ceramic formations in the form of droplets. X-ray phase analysis showed that the melt droplets are high-entropy ceramics (Y0,2Yb0,2Lu0,2Eu0,2Er0,2)3Al5O12. The powder product remaining in the cuvette and not participating in the formation of droplets contains phases of the original oxides and intermediate phases of synthesized high-entropy ceramics. Electron microscopy with EDS showed a uniform distribution of elements on the surface and in the volume of the formed ceramic droplets.

Degradation of LaPO4-8YSZ composite thick thermal barrier coatings by molten calcium-magnesium-aluminosilicate

The effect of LaPO4 content on the calcium-magnesium-aluminosilicates (CMAS) corrosion resistance of the plasma-sprayed LaPO4-8YSZ composite coatings at 1250 °C was investigated. The microstructure and corrosion products of the LaPO4-8YSZ composite coatings after CMAS attack was determined by using SEM/TEM/SAED. The results indicated that LaPO4 was segregated at the grain boundaries of the composite coatings and the addition of LaPO4 into 8YSZ coating increased their CMAS reactivity, which led to the formation of planar reaction zone while the CMAS infiltration depth decreased from ∼658 μm to ∼119 μm. The CMAS infiltration depth of the composite coatings firstly increased with LaPO4 addition, reaching its maximum for the composite coating with 5 wt% LaPO4, and then showed a downtrend. For the composite coating with 20 wt% LaPO4, a dense apatite layer formed in the corrosion front, which inhibited CMAS penetration and resulted in the shallowest CMAS infiltration depth among the test samples.

(Ba0.5Sr0.5)(Ti0.25Zr0.25Hf0.25Sn0.25)O3: A novel thermal barrier material with low thermal conductivity and excellent resistance to CMAS corrosion

High entropy ceramics are considered as promising materials for thermal barrier coatings due to phonon scattering caused by lattice distortion. In this study, perovskite high-entropy ceramics (Ba0.5Sr0.5)(Ti0.25Zr0.25Hf0.25Sn0.25)O3 (BSTZHS) were successfully prepared by dramatically increasing the conformational entropy of ceramics with complex crystal structures. The results showed that the sample exhibits a single-phase perovskite crystal structure with a dense surface, each element of the sample was uniformly distributed. Moreover, the thermal conductivity of (Ba0.5Sr0.5)(Ti0.25Zr0.25Hf0.25Sn0.25)O3 decreased significantly compared to SrZrO3 and BaZrO3 (the minimum thermal conductivity is 1.37 W/(m·K) at 273–1273 K). This study also identified descriptors for predicting the decrease in thermal conductivity of perovskite high-entropy ceramics. As well, the samples not only exhibit outstanding mechanical properties (Young’s modulus E = 255.7 ± 8.6 GPa、flexural strength Fb = 142.1 ± 4.8 MPa), but also good resistance to CMAS corrosion. Various results support that (Ba0.5Sr0.5)(Ti0.25Zr0.25Hf0.25Sn0.25)O3 is potential to be used for thermal barrier coatings.

Molten CMAS resistance strategy for PS-PVD TBCs based on laser textured and Al-modified bionic structure

Plasma spray-physical vapor deposition (PS-PVD) is a promising third-generation thermal barrier coatings (TBCs) technique. Feather-like columnar TBCs with excellent strain tolerance and low thermal conductivity can be achieved using PS-PVD. However, molten CMAS (CaO–MgO–Al2O3–SiO2) can penetrate coatings and accelerate PS-PVD TBCs failure due to the feather-like columnar structure. Hence, a strategy is proposed to alleviate molten CMAS corrosion. The super-hydrophobicity structure is fabricated via laser texturing on the surface of PS-PVD TBCs to repel molten CMAS wetting and spreading. Then, a thin layer of the Al-film is deposited on the laser-textured surface. Next, the Al-modified layer is in situ synthesized after vacuum heat treatment, preventing the infiltration of molten CMAS into the TBCs and reducing the coating damage. The results show that the contact angle of laser textured and Al-modified PS-PVD TBCs (LT-Al) at room temperature increased from 12.3° to 168.8°. The wetting and spreading behavior of molten CMAS of as-sprayed (AS), laser textured (LT), and LT-Al coatings is observed in situ at 1230 °C for 1800 s. The LT-Al coatings exhibited excellent CMAS corrosion resistance, attributed to the laser-textured micro-nano structures and Al-modified layer protection. The findings may be an effective approach for solving the disadvantage of PS-PVD feather-like columnar structure TBCs.

Ab initio calculations of Nb-based MAX phases as bond coats for thermal barrier coatings

Thermal barrier coatings (TBCs) are essential to the reliable high-temperature operation of gas turbines and engines. They comprise a ceramic top coat (TC), a metallic bond coat (BC), and a superalloy substrate. Metallic BCs are common, but they require a minimal difference in the coefficient of thermal expansion (CTE) between the TC and substrate. Among ceramic materials, MAX phases have high CTE. This study reports our use of ab initio calculations to assess the suitability of MAX phases as TBCs. We model Nb-based MAX phases with 211 and 312 structures that have Al or Si at the A sites and C or N at the X sites. We use the quasi-harmonic approximation to calculate the Young's moduli and CTEs of the materials with respect to temperature. The Nb2SiN MAX phase appears as the most suitable for use as a BC between various ceramic TCs and an Inconel-718 substrate. It is predicted to be effective in relieving thermal stresses due to its high CTE of 10.882 × 10−⁶ K−1 at 1273 K. The results indicate that carbide MAX phases with high Young's modulus and low CTE should be used with caution. They may not accommodate thermal stresses as effectively, potentially leading to material failure or reduced performance. Overall, our study indicates the potential of Nb-based MAX structures for use as BCs.

CMAS resistance characteristics of Gd2(Hf0.7Ce0.3)2O7 thermal barrier material at 1300 °C and 1400 °C

The corrosion resistance to calcium‑magnesium‑aluminum-silicate (CMAS) is a key property affecting the long-term durability of thermal barrier coatings (TBCs). Meanwhile, rare-earth hafnates are considered promising TBC candidates due to their low thermal conductivity and suitable thermal expansion coefficients. Thus, this study explores the CMAS corrosion resistance of Gd2(Hf0.7Ce0.3)2O7 (GH7C3) ceramic material under different conditions and compares it with the binary system Gd2Hf2O7 (GH) ceramic material. The findings indicate that GH7C3 can react chemically with CMAS and rapidly form a dense apatite crystalline layer at the interface between GH7C3 ceramic and CMAS, providing excellent CMAS resistance. Furthermore, the composition of the remaining CMAS melt Furthermore, the composition of the remaining CMAS melt undergoes compositional changes upon interaction, ultimately forming spinel in Mg and Al-enriched regions. In the meantime, CeO2 can promote the rapid formation of apatite crystals, significantly enhancing the CMAS resistance of GH7C3 to CMAS at high temperatures compared to GH.

Synthesis of Lanthanum Pyrochlore–Lanthanum Phosphate Composite Powders for Thermal Barrier Coating Applications

Synthesis of Lanthanum Pyrochlore–Lanthanum Phosphate Composite Powders for Thermal Barrier Coating Applications

Investigation of microstructure and thermal expansion behaviour of a functionally graded YSZ/IN718 composite produced by laser-powder bed fusion

Yttria-stabilized zirconia (YSZ) has been extensively used as a thermal barrier coating (TBC) for high-temperature alloys. In this research, the microstructural attributes and thermal expansion behavior of an additively manufactured functionally graded composite (YSZ/IN718 FGC) were investigated. First, a powder mixture of IN718 and YSZ (ranging from 1 wt% to 4 wt%) was achieved via low-energy ball-milling at 100 rpm. Differential scanning calorimetry (DSC) analysis depicted the melting behaviour of the powder mixtures of different YSZ content. The endothermic peaks in the DSC curve, at ∼1360 ℃, suggested that the energy needed to melt the powder mixture increases with increasing YSZ content. These varying powder compositions were used to fabricate FGC samples using L-PBF. The study investigated the effects of laser power variation along the YSZ gradient, successfully mitigating the fusion defects that had been initially observed in FGC samples. Microscopic analyses, comprising scanning electron microscopy (SEM) and transmission electron microscopy (TEM), depicted the elemental segregation of Nb and Mo at the solidification subgrain boundaries (SSBs). Furthermore, the electron backscattered diffraction (EBSD) results demonstrated the refinement of grains and a reduction in textural intensity, particularly along the YSZ gradient. Energy dispersive spectroscopy (EDS) and electron probe micro analysis (EPMA) were used to validate the uniform dispersion of YSZ particles throughout the composite matrix, with the EPMA line scan affirming a predominantly linear YSZ content variation. Lastly, the coefficients of thermal expansion (CTEs) were evaluated, both parallel and perpendicular to the build direction. A decrease of approximately ∼6 % in CTE was noted along the building or gradation direction. Notably, perpendicular to the building direction, the CTE values for specimens containing 4 wt% and 2 wt% YSZ witnessed significant reductions of ∼32 % and ∼12 %, respectively. This research significantly contributes to the understanding of the microstructural intricacies and thermal behavior of YSZ/IN718 FGC produced by L-PBF process.

Influence of pressure on the different physical features of lead-free double perovskite materials K2SnX6 (X = Cl, and Br): DFT replication

The influence of pressure (0–12 GPa) on several physical features of perovskite materials K2SnX6 (X = Cl, and Br) has been executed through DFT replication coded with CASTEP. Very close relation is noticed concerning the studied and synthesized lattice parameters. The studied compounds are mechanically stable under pressure according to Born's stability criteria. The Pugh's and Poisson's ratios indicate the brittle nature K2SnCl6 at 0 GPa and ductile nature at above 2 GPa. On the other hand, the phase K2SnBr6 exhibits ductile in nature at 0 GPa to high pressure. The increasing behaviors of machinable index with increasing pressure making these materials effectively useable in industrial applications. The determined Vickers hardness (Hv) values at several pressures of both the phases did not exceed 8 GPa which enables them to be more machinable and damage-tolerant. The decrease of band gap with increasing pressure ensures the probable application of these materials in optoelectronic devices. The bond length decreases with increasing pressure and consequently materials become harder. As the static dielectric constant of K2SnBr6 is higher than K2SnCl6, hence K2SnBr6 is more suitable for optoelectronic device applications. In the ultraviolet region, both the materials show their intense peak of absorption and conductivity. Both the studied compounds processes very low thermal conductivity in the entire pressure ranges comparing to the well-known thermal barrier coating (TBC) materials ABO3 which confirming their better uses in industry as TBC materials. Having very high melting temperature at high pressure these compounds are suitable for high-temperature application purposes.

Reaction of a Molten Cr-Si-Base Alloy with Ceramics and a High Entropy Oxide

Due to their higher thermal and chemical stability than other high-temperature materials, chromium-silicon-base (Cr-Si-base) alloys are promising materials for future gas turbines and other high-temperature applications operating under harsh conditions. To enable near-net-shape casting of Cr-Si-base alloys, a compatibility of the alloy melt with the ceramic crucibles and molds is necessary. Additionally, a metal-ceramic contact exists at the interface between thermal barrier coating (TBC) and alloy, where metallic may melts play a role in the case of coating failure and overheating. In this study, molten Cr92Si8 (in at. %) alloy is brought into contact with powders of ceramics commonly used for casting molds or crucibles (e.g. ZrSiO4, Al2O3, 3YSZ), to investigate liquid metal corrosion, interdiffusion, and stabilities. Additionally, the high entropy oxide (Sm0.2Gd0.2Dy0.2Er0.2Yb0.2)2Zr2O7 (HEO), a potential future TBC material, is investigated. Before melting using an electric arc furnace, the powders of the investigated ceramics were mixed with pulverized Cr92Si8 and pressed into alloy-ceramic pairs, to maximize the contact area between molten metal and ceramic. For microstructural investigations and phase analysis, the materials were assessed using scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The widely used mold material ZrSiO4 and the coating BN were found to decompose, while reaction products of SiO2 and CoAl2O4 with the melt were detected. Al2O3, 3YSZ, and the HEO did not show decomposition or corrosion by the melt. Al2O3, 3YSZ, and the HEO are therefore considered as promising crucible, mold, and TBC materials for Cr-Si-base alloys.

Thick Columnar-Structured Thermal Barrier Coatings Using the Suspension Plasma Spray Process

Higher operating temperatures for gas turbine engines require highly durable thermal barrier coatings (TBCs) with improved insulation properties. A suspension plasma spray process (SPS) had been developed for the deposition of columnar-structured TBCs. SPS columnar TBCs are normally achieved at a short standoff distance (50.0 mm–75.0 mm), which is not practical when coating complex-shaped engine hardware since the plasma torch may collide with the components being sprayed. Therefore, it is critical to develop SPS columnar TBCs at longer standoff distances. In this work, a commercially available pressure-based suspension delivery system was used to deliver the suspension to the plasma jet, and a high-enthalpy TriplexPro-210 plasma torch was used for the SPS coating deposition. Suspension injection pressure was optimized to maximize the number of droplets injected into the hot plasma core and achieving the best particle-melting states and deposition efficiency. The highest deposition efficiency of 51% was achieved at 0.34 MPa injection pressure with a suspension flow rate of 31.0 g/min. With the optimized process parameters, 1000 μm thick columnar-structured SPS 8 wt% Y2O3-stabilized ZrO2 (8YSZ) TBCs were successfully developed at a standoff distance of 100.0 mm. The SPS TBCs have a columnar width between 100 μm and 300 μm with a porosity of ~22%. Furnace cycling tests at 1125 °C showed the SPS columnar TBCs had an average life of 1012 cycles, which is ~2.5 times that of reference air-plasma-sprayed dense vertically cracked TBCs with the same coating thickness. The superior durability of the SPS columnar TBCs can be attributed to the high-strain-tolerant microstructure. SEM cross-section characterization indicated the failure of the SPS TBCs occurred at the ceramic top coat and thermally grown oxide (TGO) interface.

A 3D CFD-FEA co-simulation study of low thermal effusivity TBCs applied to the piston and valves of an SI engine

When applied on combustion chamber walls, thermal barrier coatings (TBCs) with low thermal effusivity provide a pathway for reducing heat transfer and improving SI engine efficiency. A 3D CFD-1D FEA co-simulation routine was employed to study the effects of a proprietary TBC on SI engine performance under different permutations of coating the piston, exhaust valves, and intake valves. Marginal reductions (<0.1% points) in total heat transfer and improvements to efficiency were observed when all the three components were coated with the proprietary TBC. Two hypothetical TBC materials with ideally low thermal effusivities were formed by modifying the current material properties and their effect on engine performance was similarly studied at three engine loads at the same engine speed. It was found that coating all the three components with the lowest thermal effusivity TBC offers the largest improvements (∼0.5% points) in net fuel conversion efficiency accompanied by largest reduction (∼1.1% points) in total heat transfer, thus establishing expectations from future TBC materials.

Computational study of the structural, mechanical, electronic, optical and thermal properties of BaLiX (X =P, As, Sb) perovskites

This study explores the structural, optical, mechanical, electronic, and thermal characteristics of ionic semiconductor compounds BaLiX (X = P, As, Sb) using Density Functional Theory (DFT). A comprehensive analysis of BaLiX (X = P, As, Sb) is conducted, proving that the estimated and observed lattice parameters coincide well and confirming mechanical stability through elastic stiffness constants. Electronic band structures reveal direct bandgap semiconductor properties with values of 0.70 eV, 0.30 eV, and 0.95 eV for BaLiP, BaLiAs, and BaLiSb, respectively, suggesting specific applications such as mid-infrared photodetectors, terahertz devices, and near-infrared sensors. Optical property analyses, including energy loss function, reflectivity, refractive index, and absorption coefficient, highlight the potential of these materials for optoelectronic and photovoltaic applications. Despite elastic anisotropy, optical anisotropy remains minimal. The materials exhibit potential for use as thermal barrier coatings (TBC) due to their comparatively lower Debye temperature (D), minimum thermal conductivity (Kmin) and lattice thermal conductivity (kph). Moreover, heat capacity calculations and thermal coefficient of expansion are also calculated.

Finite element simulations of thermal stress distribution in thermal barrier coatings with different mullite whisker arrangements

In recent years, thermal barrier coatings (TBCs) have been extensively employed to safeguard turbine engine blades against high temperatures and harsh operating conditions. Mullite whiskers are frequently utilized to improve the mechanical properties of ceramic matrix composites because of their exceptional characteristics. This study investigates thermal stress in coating systems by analyzing the impact of varying content, aspect ratio, and deflection angle of mullite whiskers using finite element simulations under static conditions. Results demonstrated that incorporating mullite whiskers can mitigate the sudden stress variation between the ceramic coating and the substrate. The addition of mullite whiskers reduced the maximum stress value in the coating system, thereby enhancing its thermal stability and service life. The arrangement of mullite whiskers was controlled during the preparation procedure to ensure their proper orientation. The coating system exhibited higher mechanical properties at a certain position. This study provided a theoretical basis for the design of mullite whisker–reinforced TBCs.

CMAS corrosion resistance of YSZ thermal barrier coatings with Al2O3 and YSZ-Al2O3 protective layers

Molten calcium-magnesium-alumina-silicate (CMAS) corrosion has become a major constraint for the application of YSZ thermal barrier coatings (TBCs). In this paper, Al2O3 and YSZ-Al2O3 protective layers with a thickness of 50 μm were prepared on the surface of 8YSZ TBCs using air plasma spraying technology, and the progressive damage modes of the different TBCs under CMAS exposure were investigated, and the effects of the composition and microstructure of the protective layers on the infiltration and wetting behaviour of CMAS were compared. When the exposure time was shorter, the dense YSZ-Al2O3 had a better impermeability barrier effect. As the interaction time between CMAS and the coating was prolonged, the structure and phase stability of YSZ-Al2O3 were damaged, the thickness was thinned, and Al2O3 had better stability and showed better protective effect. CMAS wettability was mainly related to the roughness of the coating surface, and the wettability was poorer on rough surfaces.