Graceful behavior during CMAS corrosion of a high-entropy rare-earth zirconate for thermal barrier coating material

The corrosion resistance to calcium-magnesium-alumino-silicates (CMAS) is critically important for the thermal barrier coatings (TBCs). High-entropy zirconate (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ (HEZ) ceramics with low thermal conductivity, high coefficient of thermal expansion and good durability to thermal shock is expected to be a good candidate for the next-generation TBCs. In this work, the CMAS corrosion of HEZ at 1300°C was firstly investigated and compared with the well-studied La₂Zr₂O₇ (LZ). It is found that the HEZ ceramics showed a graceful behavior to CMAS corrosion, obviously much better than the LZ ceramics. The HEZ suffered from CMAS corrosion only through dissolution and re-precipitation, while additional grain boundary corrosion existed in the LZ system. The precipitated high-entropy apatite showed fine-grained structure, resulting in a reaction layer without cracks. This study reveals that HEZ is a promising candidate for TBCs with extreme resistance to CMAS corrosion.     

Microstructural characterization of GZ/CYSZ thermal barrier coatings after thermal shock and CMAS+hot corrosion test

In this study, first, Gd₂Zr₂O₇/ceria–yttria stabilized zirconia (GZ/CYSZ) TBCs having multilayered and functionally graded designs were subjected to thermal shock (TS) test. The GZ/CYSZ functionally graded coatings displayed better thermal shock resistance than multilayered and single layered Gd₂Zr₂O₇ coatings. Second, single layered YSZ and functionally graded eight layered GZ/CYSZ coating (FG8) having superior TS life time were selected for CMAS + hot corrosion test. CMAS + hot corrosion tests were carried out in the same experiment at once. Furthermore, to generate a thermal gradient, specimens were cooled from the back surface of the substrate while heating from the top surface of the TBC by a CO₂ laser beam. Microstructural characterizations showed that the reaction products were penetrated locally inside of the YSZ. On the other hand, a reaction layer having ∼6 μm thickness between CMAS and Gd₂Zr₂O₇ was seen. This reaction layer inhibited to further penetration of the reaction products inside of the FG8.     

A new class of high-entropy fluorite oxides with tunable expansion coefficients,low thermal conductivity and exceptional sintering resistance

High-temperature thermal barrier coating (TBC) materials are desired for the development of high-efficient gas turbines and diesel engines. Herein, to meet up with this requirement, a new class of high-entropy fluorite-type oxides (HEFOs) has been synthesized via a solid-state reaction method. Comparing to La₂Ce₂O₇, a promising TBC material, the HEFOs exhibit similar high thermal expansion coefficients (TECs) of 11.92×10⁻⁶∼12.11×10⁻⁶ K^-1 at temperatures above 673 K but a better TEC matching performance at the temperature range of 473–673 K. It is also found that through tuning the average A-site cation radius, the TEC of the HEFOs could be tailored efficiently. The HEFOs also possess low thermal conductivities of 1.52-1.55 W∙m^-1∙K^-1 at room temperature, which is much lower than that of La₂Ce₂O₇ and comparable to pyrochlores as Gd₂Zr₂O₇. Moreover, the HEFOs display good sintering resistance and phase stability even at temperatures as high as 1873 K. The combination of these fascinating properties makes the HEFOs good candidates for thermal barrier coating and thermal insulating materials.     

Characterization of stress in a thermal barrier coating during CMAS corrosion using Ce3+ photoluminescence spectroscopy

The stress caused by calcium–magnesium–alumino–silicate (CMAS) corrosion is a critical factor in thermal barrier failure of thermal barrier coatings (TBCs). For the service safety of TBCs, it is important to characterize the stress inside TBCs during CMAS corrosion using a nondestructive and accurate method. In this study, photoluminescence spectroscopy technology was applied to characterize the stress in TBCs during CMAS corrosion. First, TBC specimens containing yttrium–aluminum–garnet doped with trace Ce³⁺ ions (YAG:Ce³⁺)/yttrium oxide partially stabilized zirconia double-ceramic-layer were prepared by atmospheric plasma spraying. Then, CMAS corrosion experiments were performed using the TBC specimens, and a mechanical model was derived based on Ce³⁺ photoluminescence spectroscopy to investigate the stress in the TBCs. Finally, the microstructure, extent of CMAS corrosion and stress field in TBC specimens, was characterized. The results reveal that the penetration of CMAS leads to local stress concentration and a nonlinear stress distribution from the outside surface to the inside of the YAG:Ce³⁺ layer. In addition, an increase in corrosion time, temperature, and CMAS concentration can significantly influence the evolution of the stress field in TBCs.     

Tuning stoichiometry of high-entropy oxides for tailorable thermal expansion coefficients and low thermal conductivity

Thermal barrier coating materials with proper thermal expansion coefficient (TEC), low thermal conductivity, and good high-temperature stability are of great significance for their applications in next-generation turbine engines. Herein, we report a new class of high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x with different Ce⁴⁺ contents synthesized by a solid-state reaction method. They exhibit different crystal structures at different Ce⁴⁺ content, including a bixbyite single phase without Ce⁴⁺ doping (x = 0), bixbyite-fluorite dual-phase in the RE₂O₃-rich region (0 < x < 2), and fluorite single phase in the stoichiometric (x = 2) and CeO₂-rich region (x > 2). The high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x exhibit tailorable TECs at a large range of 9.04 × 10^–6–13.12 × 10^–6 °C^–1 and engineered low thermal conductivity of 1.79–2.63 W·m^–1·K^–1. They also possess good sintering resistance and high-temperature phase stability. These results reveal that the high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x are promising candidates for thermal barrier coating materials as well as thermally insulating materials and refractories.     

CMAS corrosion resistance,thermal shock resistance and numerical simulation of novel surface micromesh thermal barrier coatings

Thermal barrier coatings (TBCs) are widely used as insulating layers to protect the underlying metallic structure of gas turbine blades. However, the thermal cycling performance of TBCs is affected by their complex working environments, which may shorten their service life. Previous studies have shown that preparing a mesh structure in the bonding layer can relieve thermal stress and improve the bonding strength, thereby prolonging the service life of TBCs. In this paper, a micromesh structure was prepared on the surface of the bonding layer via wet etching. The microstructure and failure mechanism of the micromesh TBCs after CMAS (CaO-MgO-Al₂O₃-SiO₂) thermal erosion were investigated. Numerical simulation was combined with thermal shock experiments to study the stress distribution of the micromesh-structured TBCs. The results showed that the circular convex structure can effectively improve the CMAS corrosion resistance and thermal shock resistance of TBCs.     

Mechanism of enhanced corrosion resistance against molten CMAS for pyrosilicates by high-entropy design

Meeting service requirements at temperatures above 1400°C is challenging for the CMAS corrosion resistance of single-component pyrosilicates. This research presents a high-entropy design approach for pyrosilicates using ionic radius modulation. This method enhances pyrosilicates’ resistance to CMAS corrosion by regulating the apatite's quantity formed to obstruct CMAS melt infiltration while avoiding excessive reactions. We investigated the corrosion behavior of two types of single-component pyrosilicates (Lu₂Si₂O₇ and Yb₂Si₂O₇) with a small ionic radius of rare-earth elements (REEs), three types of β-type pyrosilicates ((Ho_1/4Er_1/4Yb_1/4Lu_1/4)₂Si₂O₇, (Y_1/5Ho_1/5Er_1/5Yb_1/5Lu_1/5)₂Si₂O₇ and (Y_1/6Ho_1/6Er_1/6Tm_1/6Yb_1/6Lu_1/6)₂Si₂O₇), and one γ-type pyrosilicate ((Gd_1/4Dy_1/4Yb_1/4Lu_1/4)₂Si₂O₇) with a larger average ionic radius of REEs at 1450–1550°C. The analysis of the residual CMAS and apatite compositions showed the differences in the behavior of different REEs in the reaction with CMAS and the conditions required for the reaction to proceed.     

复合氧化物材料的负热膨胀机理

介绍了相转变、桥氧原子的横向热振动、刚性多面体的旋转耦合、固体内压转变、相界面弯曲、阳离子迁移等六种模式的负热膨胀机理。并对其应用前景和发展趋势进行了预测     

New class of high-entropy rare-earth niobates with high thermal expansion and oxygen insulation

Tailoring the structure and properties of materials using the high-entropy (HE) effect is of significant interest in the fields of environmental and thermal barrier coatings (TBCs). In this work, a new class of dense HE rare-earth niobates was successfully prepared by a solid-phase reaction method, including (Sm_1/5Dy_1/5Ho_1/5Er_1/5Yb_1/5)NbO₄ (5HERN), (Sm_1/6Dy_1/6Ho_1/6Er_1/6Yb_1/6Lu_1/6)NbO₄ (6HERN), (Sm_1/7Dy_1/7Ho_1/7Er_1/7Yb_1/7Lu_1/7Gd_1/7)NbO₄ (7HERN), and (Sm_1/8Dy_1/8Ho_1/8Er_1/8Yb_1/8Lu_1/8Gd_1/8Tm_1/8)NbO₄ (8HERN), along with eight single rare-earth niobates (RENbO₄, RE = Sm, Dy, Ho, Er, Yb, Lu, Gd, and Tm). X-ray diffraction analysis showed that 5–8HERN are single-phase solid solutions with a monoclinic structure (space group C12/c1). The thermal expansion coefficients of 7HERN and 8HERN exceed 11 × 10⁻⁶ K⁻¹ at 1200°C and are much higher than those of the RENbO₄ compositions (10.13–10.74 × 10⁻⁶ K⁻¹) and other some HE rare-earth oxides (10.27–10.87 × 10⁻⁶ K⁻¹). Importantly, 5–8HERN have lower oxygen-ion conductivity and higher activation energy than yttrium-stabilized zirconia (YSZ) and the RENbO₄ compositions. The oxygen-ion conductivity of 5HERN (7.52 × 10⁻⁷ S cm⁻¹, 900°C) was 10⁵ times lower than that of YSZ (0.01 S cm⁻¹, 750°C). The hardness of 5–8HERN is ∼7.81–8.46 GPa and these compositions have low intrinsic lattice thermal conductivity at high temperature (1.28–1.69 W m⁻¹ K⁻¹ at 900°C). The mechanism by which the HE effect improved the material properties was elucidated. Young's modulus, hardness, thermal expansion coefficient, and intrinsic lattice thermal conductivity are linearly related to the mass, size, and distortion degree of samples. In contrast, the oxygen-ion conductivity depends on both the degrees of disorder and distortion and the oxygen-ion vacancy concentration. Based on their overall performance, especially their high thermal expansion coefficients and excellent oxygen-barrier performance, HE rare-earth niobates show potential for further development as TBC materials.     

Transient thermal stress due to the penetration of calcium-magnesium-alumino-silicate in EB-PVD thermal barrier coating system

Electron beam-physical vapor deposited (EB-PVD) thermal barrier coating system (TBCs) are vulnerable to the degradations induced by the penetration of calcium- magnesium-alumino-silicate (CMAS). In this work, we conduct a numerical study to investigate the effect of CMAS penetration on the development of transient thermal stress in EB-PVD TBCs with the columnar microstructure. A two-dimensional periodical model is developed, taking into account the columnar microstructure of EB-PVD TBCs and the CMAS penetration. We found that the CMAS penetration would induce a field of high in-plane tensile stress in TC upon the rapid cooling, promoting the initiation of the vertical cracks from top surface toward to the bottom of TC. Meanwhile, the accumulation of out-of-plane tensile stress tends to occur at the side edges of EB-PVD columns near three main regions: closely beneath the top surface of TC, at the interface between CMAS penetrated and non-penetrated zone, and close to the TC/BC interface. Therefore, the horizontal cracks are likely to initiate from the side edges of EB-PVD columns at these three regions, which agrees well with experiments.     

CMAS corrosion of YSZ thermal barrier coatings obtained by different thermal spray processes

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al₂O₃-SiO₂ (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.     

Phase stability and tensorial thermal expansion properties of single to high-entropy rare-earth disilicates

Temperature limitations in nickel-base superalloys have resulted in the emergence of SiC-based ceramic matrix composites as a viable replacement for gas turbine components in aviation applications. Higher operating temperatures allow for reduced fuel consumption but present a materials design challenge related to environmental degradation. Rare-earth disilicates (RE₂Si₂O₇) have been identified as coatings that can function as environmental barriers and minimize hot component degradation. In this work, single- and multiple-component rare-earth disilicate powders were synthesized via a sol-gel method with compositions selected to exist in the monoclinic C 2/m phase (β phase). Phase stability in multiple cation compositions was shown to follow a rule of mixtures and the C 2/m phase could be realized for compositions that contained up to 25% dysprosium, which typically only exists in a triclinic, P $\overline{1}$, phase. All compositions exhibited phase stability from room temperature to 1200°C as assessed by X-ray diffraction. The thermal expansion tensors for each composition were determined from high-temperature synchrotron X-ray diffraction and accompanying Rietveld refinements. It was observed that ytterbium-containing compositions had larger changes in the α₃₁ shear component with increasing temperature that led to a rotation of the principal axes. Principal axes rotation of up to 47° were observed for ytterbium disilicate. The results suggest that microstructure design and crystallographic texture may be essential future avenues of investigation to ensure thermo-mechanical robustness of rare-earth disilicate environmental barrier coatings.     

The influence of laser treatment on hot corrosion behavior of plasma-sprayed nanostructured yttria stabilized zirconia thermal barrier coatings

In this study, the effect of laser glazing on the hot corrosion behavior of nanostructured thermal barrier coatings (TBCs) was investigated. To this end, the hot corrosion test of plasma-sprayed and laser-glazed thermal barrier coatings conducted against 45 wt.% Na₂SO₄ + 55 wt.% V₂O₅ molten salt at 910 °C for 30 h in open air atmosphere. The results obtained from hot corrosion test showed that the reaction between Y₂O₃ and the corrosive salt produced YVO₄, leached Y₂O₃ from YSZ and led to the progressive destabilization transformation of YSZ from tetragonal to the monoclinic phase. The lifetimes of the plasma-sprayed TBCs were enhanced approximately twofold by laser glazing. Reducing the reactive specific surface area of the dense glazed layer with the molten salts and improving the stress accommodation through network cracks produced by laser glazing were the main enhancement mechanisms accounting for TBC life extension.     

Compositional pathways and anisotropic thermal expansion of high-entropy transition metal diborides

The recent discovery of high entropy transition metal diborides (HEBs) has sparked renewed interest in ultra-high temperature ceramics (UHTCs). Presently, transition metal (Me) oxides based boro-carbo/thermal reduction (BCTR) syntheses show great promise as relatively cheap production methods, but also may present limits to attain single phase pure HEBs. Herein, by selectively tuning the concentration of boron and carbon, the reducing agents of Me oxide mixture (Me = Ti, Ta, Nb, Zr and Hf), and exploiting high-resolution synchrotron X-ray powder diffraction, we first identified and quantified the formation of intermediate phases during the BCTR synthesis, with the ultimate intent to achieve a full dense (Ti,Ta,Nb,Zr,Hf)B₂ solid solution (SS). Additional insight was obtained by temperature dependent diffraction, which highlighted, for the first time in this class of materials, anisotropic thermal expansion, most likely at the origin of the SS micro-cracking, as was also observed by electron microscopy.     

LaMeAl11O19(Me=Cu,Zn)陶瓷体材料的抗CMAS性能研究

随着燃气涡轮机的应用温度不断提升,陶瓷材料的抗CaO-MgO-Al₂O₃-SiO₂(CMAS)性能越来越重要。通过X射线衍射(XRD)、扫描电镜(SEM)等测试方法,研究了LaMeAl₁₁O₁₉(Me=Cu,Zn)陶瓷体材料在不同温度和时间条件下的抗CMAS腐蚀行为。结果表明,LaZnAl₁₁O₁₉(LZA)和LaCuAl₁₁O₁₉(LCA)体材料的腐蚀产物都包括透辉石(Ca(Mg,Al)(Si,Al)O₇)和钙长石(CaAl₂Si₂O₈)。随着腐蚀温度的提高和时间的延长,腐蚀深度增加,Ca(Mg,Al)(Si,Al)O₇逐渐转变为CaAl₂Si₂O₈。LZA和LCA体材料的CMAS腐蚀可以用“溶解-析出”机制解释。体材料逐渐溶解到CMAS中,形成Ca(Mg,Al)(Si,Al)O₇,进而逐渐转变为CaAl₂Si₂O₈,使难以结晶的透辉石相转变为易结晶的钙长石相。La原子为析晶的晶核,CMAS玻璃相与体材料之间存在界面能,这些因素共同促进了CaAl₂Si₂O₈在CMAS内部以及两者的界面处析出厚板状晶体。     

Effects of Yb and Sc co-doping on the thermal properties and CMAS corrosion of garnet-type (Y3-xYbx)(Al5-xScx)O12 ceramics

Thermal barrier coatings with excellent thermal performance and corrosion resistance are essential for improving the performance of aero-engines. In this paper, (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ (x = 0, 0.1, 0.2, 0.3) thermal barrier coating materials were synthesized by a combination of sol-gel method and ball milling refinement method. The thermal properties of the (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ ceramics were significantly improved by increasing Yb and Sc doping content. Among designed ceramics, (Y_2.8Yb_0.2)(Al_4.8Sc_0.2)O₁₂ (YS-YAG) showed the lowest thermal conductivity (1.58 Wm^?1K^?1, at 800 °C) and the highest thermal expansion coefficient (10.7 × 10^?6 K^?1, at 1000 °C). In addition, calcium-magnesium- aluminum -silicate (CMAS) corrosion resistance of YS-YAG was further investigated. It was observed that YS-YAG ceramic effectively prevented CMAS corrosion due to its chemical inertness to CMAS as well as its unique and complex structure. Due to the excellent thermal properties and CMAS corrosion resistance, YS-YAG is considered to be prospective material for thermal barrier coatings.     

Synthesis and characterization of ytterbium oxide: A novel CMAS-resistant environmental barrier coating material

Demand for more powerful aircraft promotes development of ceramic matrix composites and environmental barrier coating (EBC). A promising EBC material, ytterbium oxide (Yb₂O₃), was fabricated by hot pressing, and its properties were systemically investigated. The evaluation of thermal properties provides a baseline for the application of Yb₂O₃ on SiC_f/SiC or Al₂O_3f/Al₂O₃ composites. The performance in water vapor and molten calcium–magnesium–aluminosilicate (CMAS) environments indicates its excellent durability in harsh environment. Compared with rare-earth silicates, the thermochemical interactions between ytterbium oxide and CMAS changed greatly with the absence of silicon oxide. Reactions of ytterbium oxide with CMAS form several reaction products, including apatite, garnet, and silicocarnotite. The crystallization of garnet and silicocarnotite could effectively consume and solidify the CMAS melt, which prevents the melt infiltration and mitigates the further corrosion.     

Structure,thermal properties and hot corrosion behaviors of Gd2Hf2O7 as a potential thermal barrier coating material

In this work, Gd₂Hf₂O₇ ceramics were synthesized and investigated as a potential thermal barrier coating (TBC) material. The phase composition, microstructure and associated thermal properties of Gd₂Hf₂O₇ ceramics were characterized systematically. Results show that the thermal conductivity of Gd₂Hf₂O₇ ceramics is 1.40 Wm⁻¹K⁻¹ at 1200 °C, ~25% lower than that of 8 wt% yttria partially stabilized zirconia (8YSZ). Gd₂Hf₂O₇ ceramics also present large thermal expansion coefficients, which decrease from 12.0 × 10⁻⁶ K⁻¹ to 11.3 × 10⁻⁶ K⁻¹ (300–1200 °C). Besides, the hot corrosion behaviors of Gd₂Hf₂O₇ ceramics exposed to V₂O₅ and Na₂SO₄ + V₂O₅ salts at temperatures of 900–1200 °C were discussed in great detail. We pay much attention on the corrosion process, corrosion mechanism and corrosion damage of Gd₂Hf₂O₇ ceramics subjected to molten V₂O₅ and Na₂SO₄ + V₂O₅ salts at different temperatures.     

Evolution of porosity,crack density,and CMAS penetration in thermal barrier coatings subjected to burner rig testing

Degradation of thermal barrier coatings (TBCs) in gas-turbine engines due to calcium–magnesium–aluminosilicate (CMAS) glassy deposits from various sources has been a persistent issue since many years. In this study, state of the art electron microscopy was correlated with X-ray refraction techniques to elucidate the intrusion of CMAS into the porous structure of atmospheric plasma sprayed (APS) TBCs and the formation and growth of cracks under thermal cycling in a burner rig. Results indicate that the sparse nature of the infiltration as well as kinetics in the burner rig are majorly influenced by the wetting behavior of the CMAS. Despite the obvious attack of CMAS on grain boundaries, the interaction of yttria-stabilized zirconia (YSZ) with intruded CMAS has no immediate impact on structure and density of internal surfaces. At a later stage the formation of horizontal cracks is observed in a wider zone of the TBC layer.