Advanced Thermal Barrier Coatings Research Articles

The influence of the elemental valence on the thermophysical properties of high entropy (La0.2Sm0.2Eu0.2Yb0.2Y0.2)2(Ce0.5Zr0.5)2O7 coatings for thermal barrier application

Commercial yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are no longer sufficient for the stringent demands in the high operating temperature and excellent thermophysical properties of the high-performance gas turbines and aviation engines. High entropy ceramics, distinguished by their superior thermophysical properties, have risen as a potential substitute for TBCs. However, the realm of atmospheric plasma spraying (APS) in the creation of these advanced TBCs remains relatively unexplored. In this work, three distinct high-entropy (La0.2Sm0.2Eu0.2Yb0.2Y0.2)2(Ce0.5Zr0.5)2O7 coatings were developed through the meticulous alteration of spraying power. The effects of spraying power on the microstructure, valence states of constituent elements and thermophysical properties of the coatings were deeply investigated. The results indicated that all the as-sprayed coatings exhibit a defect fluorite structure. Spraying power does have a significant impact on the thermophysical properties of the coatings. With the decrease in spraying power, the thermal conductivity of the coatings shows a progressive reduction, whereas the coefficient of thermal expansion (CTE) exhibits a gradual increase. This trend is closely related to valence states of constituent elements in the coatings. The resulting coating exhibits a low thermal conductivity of 0.54 W⋅m-1⋅K-1, a high CTE of 11.22×10-6·K-1 and a superior phase stability at elevated temperatures.

Tailoring the stoichiometry in NdNbO4 for advanced thermal barrier coating applications

AbstractRENbO4 ceramics have the potential to serve as a viable alternative to YSZ for advanced thermal barrier coatings. However, the fabrication of stoichiometric plasma spray RENbO4 coatings remains unexplored. Herein, we proposed a strategy to deposit a stoichiometric NdNbO4 coating using the atmospheric plasma spray technique. This was accomplished by the addition of excess Nb2O5 in feedstock powder to compensate for the evaporated Nb2O5 during the plasma spray process. The influence of powder composition and critical plasma spray parameters (CPSP) on the microstructure and phase formation of NdNbO4 coatings were characterized. Further, quantitative assessment of Nb2O5 evaporation with varying plasma parameters was also characterized using energy‐dispersive X‐ray fluorescence spectroscopy. Moreover, the support vector machines (SVM) model was used to construct a relation between the initial powder composition, CPSP, and stoichiometry of the coating. This strategy can be used for the fabrication of stoichiometric RENbO4 coatings for next‐generation thermal barrier coatings.

Computational design of high-entropy rare earth aluminum garnets for advanced thermal and environmental barrier coatings

Computational design of high-entropy rare earth aluminum garnets for advanced thermal and environmental barrier coatings

Heterogeneous multilayered delamination of thermally grown oxide accelerating spallation of thermal barrier coatings

Long lifetime of thermal barrier coatings (TBCs) is limited by the localized thermally grown oxide (TGO) accumulation. Herein, the heterogeneous multilayered delamination mechanism of TGO is revealed. TGO cracking preferentially occurs at the peak of bond coat due to mismatch and extends toward the flank region under TGO growth. The thinner sublayer after first delamination is mainly attributed to loss of ceramic constraint. The delamination accumulation increases the YSZ crack driving force and provides a fast propagation channel for YSZ crack to pass through TGO. These results provide a guidance for the development of advanced TBC with higher oxidation resistance.

Comprehensive Understanding of TGO Morphology Effect on the Thermal Barrier Coatings Failure Under Free Edges

Abstract The growth stresses induced by the thermally grown oxide (TGO) will be amplified at the free-edge site, making the free-edge site a weak part of the thermal barrier coatings (TBCs). In this study, the TBCs failure behavior is investigated based on different TGO morphologies under free edges. The thermomechanical model is established by creating straight lines and simplified sinusoidal curves, respectively. Dynamic TGO growth is realized by the secondary development of the subroutine. The cohesive element is inserted at the TC/TGO interface to simulate the delamination. The stress evolution near different TGO morphologies under the influence of the free edge are examined. In addition, the interfacial cracking behavior near the free edge is also explored. The results show that the appearance of the free edge will deteriorate the stress condition in the nearby area, change the preferred cracking area, and induce the earlier failure behavior. The straight line morphology has the most “friendly” stress distribution. The sinusoidal curves have peaks and valleys, and different areas of the TGO shape are different under the influence of the free edge, but all of them have the effect of stress “convergence”. These results can provide significant guidance to develop the next-generation advanced TBCs.

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.

Interfacial Stability between High-Entropy (La0.2Yb0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 and Yttria-Stabilized Zirconia for Advanced Thermal Barrier Coating Applications

(La0.2Yb0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 (HEZ) has shown considerable promise as a novel thermal barrier coating material for temperatures exceeding 1300 °C. This study systematically investigates the interfacial stability of (La0.2Yb0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 with yttria-stabilized zirconia (YSZ), which is of paramount importance for its application in double-layer thermal barrier coatings. Our findings highlight that rare earth elements with a smaller radius diffuse more easily into the YSZ lattice, resulting in a broader diffusion zone. Simultaneously, the incorporation of rare earth elements into the YSZ lattice inhibits tetragonal-to-monoclinic phase transformation. Compared to La2Zr2O7/YSZ, HEZ/YSZ demonstrates superior high-temperature stability, which could be attributed to the higher fracture toughness and lower thermal expansion coefficient of HEZ, the absence of t-m transformation and the formation of a continuous gradient diffusion layer that minimizes interface stress. This study offers a practical strategy for designing materials for durable double-layer thermal barrier coating systems.

Effect of feedstock powder size on the microstructure and thermal conductivity of quasi-eutectic LaYbZr2O7 TBCs

The effect of average feedstock powder size on the evolution of microstructure and thermal conductivity are comprehensively analyzed. It was found that the porosity and thermal conductivity of the LaYbZr2O7 coatings were positively correlated with the average powder size. The conventional notion that higher porosity results in lower thermal conductivity is challenged by the above results. The abnormal porosity and thermal conductivity relationship can be ascribed to the density of horizontal cracks which dominates the thermal conductivity reduction. Reducing the average powder size leads to a greater concentration of horizontal cracks and hence reduced thermal conductivity. Moreover, the LaYbZr2O7 coatings not only show significantly lower initial thermal conductivity of under 0.70 W m−1 K−1, but also exhibit stronger sintering resistance in terms of porosity stability and thermal insulation ability compared to the conventional YSZ coatings. The present work affords a new insight in developing advanced thermal barrier coatings with ultralow thermal conductivity.

Comprehensive Understanding of the Effect of TGO Growth Modes on Thermal Barrier Coating Failure Based on a Simulation.

The growth stress induced by thermally grown oxide (TGO) is one of the main reasons for the failure of thermal barrier coatings (TBCs). In this study, the failure behavior of TBCs was examined based on different growth modes of TGO. A TBC thermo-mechanical model with a simplified sinusoidal interface morphology was established by the secondary development of a numerical simulation. The plasticity and creep behavior of materials were considered. Based on the subroutine development, the non-uniform growth of the TGO layer was realized. Cohesive elements were also applied to the TC/TGO interface. The stress distribution and evolution at the TC/TGO interface were investigated. Then, the cracking behavior near the interface was studied. The results show that lateral growth causes the off-valley site to replace the previous off-peak site as a vulnerable site. The non-uniform growth accelerates damage in the off-valley site, which leads to a change in the failure behavior. These results will provide significant guidance for understanding the TBC failure and the development of advanced TBCs.

Layer structure formation induced by in-situ deposit surface temperature under different scanning methods during thermal spraying

Plasma spraying-physical vapor deposition (PS-PVD) and atmospheric plasma spraying (APS), as new thermal spraying process, could prepare advanced thermal barrier coatings by successive scanning deposition layer by layer. To investigate the layer structure of the coating, the transient in-situ deposit surface temperature was investigated by varying the scanning methods. Results show that, compared to unidirectional scanning method, alternating scanning method results in doubling heating duration, higher in-situ deposit temperature rising and thereby thicker well-bonded layer structure during each spray cycle for the coating deposited at the edge position of the scanning path. The erosion resistance of the coatings deposited by unidirectional scanning method is independent on deposition positions including edge position and center position. However, the erosion resistance of the coatings deposited at edge position by alternating scanning method is one time higher than those deposited by unidirectional scanning method. Finally, the erosion mechanism of layer spallation is revealed to explain the difference between different scanning methods.

Temperature-dependent interatomic force constants and phonon coherent resonance contribution in quaternary non-centrosymmetric chalcogenides BaAg2SnSe4

Phonon heat transfer by thermoelectric modules undoubtedly plays a pivotal role for the future waste heat recovery and utilization. BaAg2SnSe4, a prototypical glasslike compound among quaternary non-centrosymmetric chalcogenides, demonstrates remarkable thermoelectric performance due to its exceptionally low lattice thermal conductivity (κL). Even with a comprehensive grasp of the underlying cause behind ultra-low κL and the subsequent heightened thermoelectric conversion efficiency, the temperature-dependent lattice dynamics and glass-like attributes governing κL continue to pose unresolved questions. In this study, we investigate the phonon thermal transport properties of quaternary BaAg2SnSe4 by incorporating the influence of phonon coherent resonance, thereby departing from the conventional Peierls's framework. Based on the temperature dependence of the interatomic force constants and coherences’ contribution to total lattice thermal conductivity (κL(C)), we intriguingly observe that the particle-like driven lattice thermal conductivity (κL(P)) and κL(C) have the quite significant and dominant role for describing ultralow phonon transport behaviors in the middle-high-temperature regions. Specially, enhanced phonon anharmonicity coupled with excited coherent phonons due to increasing temperature yields two close values, with 700 K values of 0.16 and 0.13 W/mK predicted for BaAg2SnSe4, respectively, accelerating its application in thermoelectric field. Furthermore, we also find that the off-diagonal terms of heat flux operators in κL exist distinction for different phonons, with zero contribution modes exhibiting temperature-independent phonon coherence effect. Temperature response of the force constants tensors and coherent resonance proposed in our work can be employed as a powerful tool, which would substantially be expected to inspire a broad revisiting phonon transport study in various device applications, including advanced thermal barrier coatings and superior thermoelectrics.

Phase evolution, thermophysical and mechanical properties of high-entropy (Ce0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 ceramic for advanced thermal barrier coatings

Single-phase high-entropy rare-earth zirconate (Ce0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 with large thermal expansion coefficient (11.69 ×10−6 K−1, 25–1500 °C), low thermal conductivity (1.07 W m−1 K−1, 1500 °C), and high fracture toughness (1.84 ± 0.03 MPa m1/2, 25 °C) was successfully synthesized using reverse coprecipitation and high-temperature calcination. The underlying mechanism governing the observed results was analyzed by considering the ionic bond strength, crystal lattice energy, and bond length. X-ray diffraction and Raman spectroscopy results demonstrated that the pyrochlore structured sample has excellent high-temperature phase stability. Moreover, after being heated at 1600 °C for 10–30 h, the average grain size of this sample only increases from 0.95 µm to 1.47 µm, indicating a sluggish grain growth rate. The outstanding combination of thermophysical and mechanical properties of the (Ce0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 emphasizes its enormous potential for next-generation thermal barrier coating applications.

Development of Gd2O3 doped yttria stabilized zirconia based thermal barrier coating for improved high temperature oxidation and erosion resistance

Rear-earth doped advanced thermal barrier coatings (TBCs) have been developed by in-situ doping of Gd into YSZ matrix by atmospheric plasma spray coating of YSZ and Gd2O3 (6 to 15 mol %) premixed powders. These modified TBCs are deposited over CoNiCrAlY bond coated Inconel 718 substrate. Diffusion of Gd or Gd3+ into YSZ matrix resulted in further stabilization of its tetragonal phase which is confirmed by detailed X-Ray diffraction analysis. Microstructure and chemical compositions of doped TBCs are examined by scanning electron microscopy and energy dispersive spectroscopy, respectively. Incorporation of Gd2O3 resulted in relatively denser and smoother coating compared to pure YSZ coating for identical process parameters due to finer particle size and complete melting of Gd2O3 powders. However, in-situ doped coatings are sinter resistant at elevated temperature unlike pure YSZ coating which aids in retention of porosities during high temperature exposure and provide better thermal performances. Modified TBCs showed higher hardness and Young's modulus, as revealed by micro and nano hardness measurements. High temperature isothermal oxidation studies carried out between 1173 K and 1273 K allowed evaluation of the efficacy of modified TBCs for oxidation resistance and related kinetics. The results show that modified TBCs provide better oxidation resistance compared to pure YSZ coating. Erosion tests of TBCs both at ambient and elevated temperature (773 K) showed that modified coatings are more resistant to erosion compared to YSZ coating.

The microstructure and property evolution of LaYbZr2O7 thermal barrier coatings

The search for advanced thermal barrier coatings with high sintering resistance and low thermal conductivity has become increasingly urgent in the development of advanced gas turbines. In this study, we successfully fabricated a LaYbZr2O7 coating with exceptional thermal and mechanical properties. The microstructure and property evolution of the coating were comprehensively investigated after annealing at high temperatures. The as-sprayed LaYbZr2O7 coating exhibited a single pure fluorite phase, which subsequently transformed into a fluorite + pyrochlore phase after sintering at 1300 °C for 20 h. The phase transition resulted in a 0.4 % volume change, which occurred at high temperatures without causing any abrupt volume changes during heating and cooling processes. In comparison to YSZ coatings, the LaYbZr2O7 coating displayed an extremely low thermal conductivity both before and after heat treatment. Even after being exposed to 1200 °C for 200 h, its thermal conductivity remained at approximately 0.9 W·m−1·K−1, indicating excellent sintering resistance. The coating also exhibited sluggish grain growth, with an average grain size of <400 nm after exposure to high temperatures. The presence of superfine grains in the LaYbZr2O7 coating contributed to its high fracture toughness and hardness. These findings indicate that the LaYbZr2O7 coating is a promising advanced thermal barrier material for high-performance gas turbines.

An in-situ oxide-dispersion-strengthened AlCoCrFeNiY high-entropy alloy composite coating prepared by AC-HVAF with superior oxidation and spallation resistance

We report an in-situ oxide-dispersion-strengthened AlCoCrFeNiY high-entropy alloy composite coating (ODS-HEA composite coating) with exceptional oxidation and spallation resistance at 1100 °C and 1150 °C. The coating is manufactured by activated combustion-high velocity air fuel (AC-HVAF) and has an ultrafine-grained microstructure that comprises γ, β and nanoscale oxide particles (Al2O3, Cr2O3 and Y2O3) formed in-situ in coating processing. The thermally grown oxides (TGOs) on the coating at 1100 °C and 1150 °C are exclusively α-Al2O3 with a columnar grain structure. The parabolic rate constant of the coating is about half of that of the conventional NiCoCrAlY coating (CNA coating) at both 1100 °C and 1150 °C, respectively. The lower TGO growth rate is primarily attributed to its coarser TGO grains that reduce the efficient oxygen diffusivity. Compared to the CNA coating, a higher TGO/coating interfacial toughness is achieved via inhibiting the formation of interfacial pores for ODS-HEA composite coating. The low TGO growth rate decreases the driving force for TGO spallation and the high interfacial toughness increases the TGO adhesion, thus contributing to the strong spallation resistance for the ODS-HEA composite coating. The superior oxidation and spallation resistance of the ODS-HEA composite coating at 1100 °C and 1150 °C make it a candidate bond coat for advanced thermal barrier coatings.

Highly anti-sintering and toughened pyrochlore (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 high-entropy ceramic for advanced thermal barrier coatings

Single-phase high-entropy rare-earth zirconate (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 with excellent high-temperature phase stability at 1600 °C was designed and synthesized. After being heated at 1600 °C for 1 ∼ 50 h, the average grain size of the sample increases only from 0.74 μm to 2.22 μm, indicating a high anti-sintering ability. The fracture toughness is 2.07 ± 0.03 MPa m1/2 (25 °C), which is 50% higher than lanthanum zirconate. The local lattice distortions induced by the atomic interactions and atomic-size mismatch are considered to be responsible for the strong anti-sintering ability and improved fracture toughness. In addition, the thermal expansion coefficient and thermal conductivity of (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 are 10.59 × 10-6 K-1 (25 ∼ 1500 °C) and 1.14 ± 0.09 W·m-1·K-1 (1500 °C), respectively. The outstanding combination of anti-sintering ability, mechanical behavior, and thermophysical properties of (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 highlights its great potential for next-generation thermal barrier coating applications.

High Temperature Resistant Coatings for Strategic Aero space Applications

The aerospace components operating in hot sections of aero-engines and combustors experience extreme environments. Typically, the components are subjected to high service temperatures exceeding 1100°C and oxidizing conditions. Protective coatings are essential for preventing oxidation-induced dimensional degradation of the components and enhancing their high temperature capability as well as durability. Defence Metallurgical Research Laboratory (DMRL) has developed a variety of metallic and ceramic thermal barrier coating (TBC) systems for Ni-base superalloys, and refractory Nb-alloys for strategic aerospace applications involving ultra-high temperatures and high flow velocities. These coatings have demonstrated significant effectiveness against thermal degradation at temperatures as high as 2000 °C during oxidation in static air as well as in dynamic conditions involving high flow velocities (Mach &gt; 2). The present article provides an overview of the advanced oxidation resistant and thermal barrier coatings developed in DMRL. The effectiveness of the TBCs in preventing dimensional degradation of the metallic and composite substrate materials has been evaluated at the laboratory scale. The developed TBCs have the potential for use in aero-engines and propulsion systems of hypervelocity vehicles.

Multi-Scale Structural Design and Advanced Materials for Thermal Barrier Coatings with High Thermal Insulation: A Review

Thermal barrier coatings (TBCs) are a fundamental technology used in high-temperature applications to protect superalloy substrate components. However, extreme high-temperature environments present many challenges for TBCs, such as the degradation of their thermal and mechanical properties. Hence, highly insulating, long-life TBCs must be developed to meet higher industrial efficiency. This paper reviews the main factors influencing the thermal insulation performance of TBCs, such as material, coating thickness, and structure. The heat transfer mechanism of the coating is summarized, and the degradation mechanism of the thermal insulation is analyzed from the perspective of the coating structure. Finally, the recent advances in improving the thermal insulation and lifetime of coatings are reviewed in terms of advanced materials and structural design, which will benefit advanced TBCs in future engineering applications and provide guidance for the next generation of high thermal insulating TBCs.

Exploring the increasing behavior of thermal conductivity for high-entropy zirconates at high temperatures

Up to present, the discussion on the increase of thermal conductivity at high temperatures for thermal barrier coatings is not comprehensive enough. The present study systematically explored the increased behavior of high temperature thermal conductivity for (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xTix)2O7 high-entropy ceramics. Above 600 ℃, the thermal conductivities of x = 0.1–0.5 components increase with composition and temperature. The significantly reduced emissivity and improved transmittance imply that the reason for the increase of high temperature thermal conductivity is the remarkable improvement of photon thermal conductivity. Importantly, this study provides a route to explore the different behavior of high temperature thermal conductivity by investigating phonon, photon and electronic heat conduction behaviors, which is also conducive to design and develop advanced thermal barrier coatings with low thermal conductivity.

Rare-earth high-entropy aluminate-toughened-zirconate dual-phase composite ceramics for advanced thermal barrier coatings

Superb toughening is achieved by incorporating a secondary ferroelastic phase in high-entropy rare-earth zirconate 5RE2Zr2O7 (HZ). Here, we report an enhancement of 64% in fracture toughness through the addition of 30mol% high-entropy rare-earth aluminate 5REAlO3 (HA) to the HZ matrix (30HA). The aforementioned rare-earth elements RE are La, Sm, Eu, Gd, and Yb. The present dual-phase composite ceramic 30HA has a large fracture toughness of 2.77 ± 0.14 MPa m1/2, along with excellent high-temperature phase stability, resulting in good usage for potential thermal barrier coating applications. Particularly, the fracture toughness of the dual-phase composite ceramics at first increases to a maximum and then drops suddenly, as the mole fraction of HA increases from 0 to 50%. A clear definition of fitting parameters and their physical significance is provided for a better interpretation of the experimental data. The present toughening mechanism sheds light on microstructure engineering in high-entropy ceramics for excellent mechanical properties.