Calcium-Magnesium-Aluminosilicate (CMAS) corrosion resistance of high entropy rare-earth phosphate (Lu0.2Yb0.2Er0.2Y0.2Gd0.2)PO4: A novel environmental barrier coating candidate

Single phase (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ was synthesized, and its thermal properties and CMAS resistance were investigated to explore its potential as an environmental barrier coating (EBC) candidate. The high entropy phosphate (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ displays a lower thermal conductivity (2.86 W m⁻¹ K⁻¹ at 1250 K) than all the single component xenotime phase rare-earth phosphates. Interaction of (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ pellets with CMAS at 1300 °C led to the formation of a dense and uniformed Ca₈MgRE(PO₄)₇ reaction layer, which halted the CMAS penetration into the bulk pellet. At 1400 and 1500 °C the (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄-CMAS corrosion showed CMAS penetrating beyond the reaction layer into the bulk pellet via the grain boundaries, and SiO₂ precipitates remaining at the pellet surface. The effects of duration, temperature, and compositions on the resistance against CMAS corrosion are discussed within the context of optimizing materials design and performance of high entropy rare-earth phosphates as candidates for advanced EBC applications.     

Water vapour corrosion of rare earth monosilicates for environmental barrier coating application

Water vapour corrosion resistance of five rare earth monosilicates Y₂SiO₅, Gd₂SiO₅, Er₂SiO₅, Yb₂SiO₅, and Lu₂SiO₅ was investigated during testing at 1350 °C for up to 166 h in static air with 90% water vapour. Four of the RE-silicates showed little weight gain (0.859 mg cm⁻²) after 166 h of exposure. Prior to testing the microstructure consists of equiaxed grains of 4- 7±0.4 µm. XRD analysis showed that after 50 h exposure to water vapour corrosion Y, Er, Yb and Lu-silicates had both mono and disilicates present on their surfaces as a result of the reaction between monosilicate and water vapour to form disilicate, while Gd-silicate has converted completely to G_4.67Si₃O₁₃ making it less stable for environmental barrier coating application. The microstructures of corroded Y, Er, Yb and Lu-silicates contain ridges and cracks, while that of Gd-silicate contains rounded grains suggesting melting along with striped contract grains.     

Theoretical investigation of phonon contributions to thermal expansion coefficients for rare earth monosilicates RE2SiO5 (RE = Dy,Ho, Er,Tm, Yb and Lu)

Owing to the superior chemical and thermal compatibility with silicon-based ceramics, rare earth (RE) monosilicates RE₂SiO₅ are promising as environmental barrier coating materials for the applications in high-temperatures aero-engines. Herein, thermal expansion and phonon characteristics of RE₂SiO₅ (RE = Dy, Ho, Er, Tm, Yb and Lu) were investigated by first-principles calculations. The thermal expansion coefficients were predicted as a function of temperature and ranged from 8.87 to 7.72 × 10⁻⁶ /K at 1600 K for the studied RE₂SiO₅. The isolated RE and O5 atoms determine the thermal expansion behaviours of RE₂SiO₅ especially at low phonon frequencies; while the [SiO₄] tetrahedrons contribute positively, but less significantly to the thermal expansion. The balance between negative contribution from distortion of RE polyhedrons and positive contribution from stretching of REO bonds is identified as the key to tune the thermal expansion coefficient of RE silicates.     

Thermo-mechanical properties and CMAS resistance of (Ho0.4Yb0.3Lu0.3)2SiO5 solid solution for environmental barrier coating applications

Rare earth monosilicate (RE₂SiO₅) is one of the most promising candidates as an environmental barrier coating (EBC) for SiC_f/SiC ceramic matrix composites. But single-component RE₂SiO₅ is hard to meet the multiple and harsh performance requirements of EBC which brings a significant challenge to their applications. Based on our previous research on single-component RE₂SiO₅ ceramics, (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ solid solution was designed and successfully fabricated in this work. Doping of multiple RE elements endows (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ with excellent thermal insulation properties and matched thermal expansion coefficient with SiC_f/SiC substrates. In addition, it exhibits lower elastic modulus and comparable hardness than that of single-component RE₂SiO₅. (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ also presents good resistance to calcium-magnesium alumino-silicates (CMAS) corrosion. Rational composition design allows (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ to retain the merits of single-component RE₂SiO₅ while taking advantage of the solid solution effect. The results of this work suggest (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ as a promising EBC candidate.     

Theoretical exploration of the abnormal trend in lattice thermal conductivity for monosilicates RE2SiO5 (RE = Dy,Ho, Er,Tm, Yb and Lu)

Rare earth monosilicates RE₂SiO₅ have been considered as promising environmental barrier coating materials for silicon-based ceramics due to their low thermal conductivity and good high-temperature stability. We herein performed a systematic study of the lattice dynamics for RE₂SiO₅ (RE?=?Dy, Ho, Er, Tm, Yb and Lu) using first-principles calculations. The loosely bound rare earth atoms provide large Grüneisen parameters and low phonon group velocities, both of which determine the low thermal conductivity. Theoretical exploration predicts an anomalous increase of lattice thermal conductivity with increment of RE atomic number and the mechanism is explained by the stronger atomic bonding and weaker phonon anharmonicity. Although incorporating heavier atoms has long been considered as an effective way to reduce lattice thermal conductivity, this work addresses the importance of bonding heterogeneity and anharmonicity rather than atomic mass variation. This theoretical study suggests an alternative approach towards the design of new thermal insulating materials.     

High-entropy rare-earth disilicate (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7: A potential environmental barrier coating material

A new high-entropy ceramic (Lu_0.2Yb_0.2Er_0.2Tm_0.2Sc_0.2)₂Si₂O₇ ((5RE_0.2)₂Si₂O₇) was proposed as a potential environmental barrier coating (EBC) material for ceramics matrix composites in this work. Experimental results showed that the (5RE_0.2)₂Si₂O₇ synthesized by solid-phase sintering was a monoclinic solid solution and had good phase stability proved by no obvious absorption/exothermic peak in the DSC curve from room temperature to 1400 °C. It performed a lower coefficient of thermal expansion (2.08 ×10^?6-4.03 ×10^?6 °C^?1) and thermal conductivity (1.76–2.99 W?m^?1?°C^?1) compared with the five single principal RE₂Si₂O₇. In water vapor corrosion tests, (5RE_0.2)₂Si₂O₇ also exhibited better water vapor corrosion resistance attributed to the multiple doping effects. The weight loss was only 3.1831 × 10^?5 g?cm^?2 after 200 h corrosion at 1500 °C, which was lower than that of each single principal RE₂Si₂O₇. Therefore, (5RE_0.2)₂Si₂O₇ could be regarded as a remarkable candidate for EBCs.     

Thin single-phase yttrium-based environmental barrier coating systems for SiC/SiC CMCs

Two thin yttrium silicate-based EBC systems for SiC/SiC CMCs that comprised of three layers were produced via (reactive) magnetron sputtering with pure yttrium disilicate (YDS) and yttrium mono silicate (YMS) layers. The systems survived 1000 1-hour furnace cycle tests at 1250 °C in air interrupted by active cooling for 10 min. One system showed partial delamination of the YMS. The YDS intermediate layer stopped deleterious reactions among YMS and silicon and the silica formation did not cause spallation of the silicates. Some vertical cracks in YMS left possible attack paths for water vapor. Rapid water vapor testing was performed at 1250 °C in 100 vol% water vapor at 10 m/s for 2 h. This test proved that a thin but hermetic and phase pure YMS top layer can protect the system from volatilization: single phase X2-YMS was not attacked by steam whereas a mixed β-YDS/X2-YMS coating showed volatilization of SiO₂.     

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.     

Thermochemical stability and microstructural evolution of Yb2Si2O7 in high-velocity high-temperature water vapor

Thermochemical stability and microstructural evolution of Yb₂Si₂O₇ was studied in high-temperature high-velocity water vapor at temperatures between 1200–1400 °C. Two reactions were shown to occur in the steam environment: Yb₂Si₂O₇ reaction to form Yb₂SiO₅, and further Yb₂SiO₅ reaction to form Yb₂O₃. Parabolic rates of both reactions were observed, and similar reaction enthalpies were determined for each reaction; 207 kJ/mol and 205 kJ/mol, respectively. Densification of the product phase Yb₂SiO₅ shut off pore connectivity for gas transport to the reaction interface at gas velocities exceeding 115?125 m/s and for temperatures of 1300 °C and 1400 °C, resulting in reduced reaction rates at higher velocities. Outward gas diffusion by a silicon hydroxide species is predicted to govern ytterbium silicate reactions with high temperature water vapor. Microstructure changes at high temperatures and velocities were shown to greatly impact the long-term stability of Yb₂Si₂O₇.     

Synthesis,sintering, and grain growth kinetics of Hf6Ta2O17

A systematic study of the solid-state synthesis, pressureless sintering, and grain growth kinetics of Hf₆Ta₂O₁₇ is presented. The ideal conditions for solids-state synthesis of Hf₆Ta₂O₁₇ powder with minimal particle necking was 1250 °C for 2 h in air. The resultant powder has an average particle size of 210 ± 70 nm. The combined synthesis and ball-milling procedure produces highly sinterable Hf₆Ta₂O₁₇ powder, achieving > 97 % of theoretical density after pressureless sintering at 1600 °C for 2 h in air. The grain growth mechanism was sensitive to processing conditions, appearing to be primarily driven by surface diffusion below 1600 °C and grain boundary diffusion above 1650 °C. The respective activation energies for grain growth were found to be Q_S = 659 ± 79 kJ mol⁻¹ and Q_GB = 478 ± 63 kJ mol⁻¹.     

A review on recent applications and future prospects of rare earth oxides in corrosion and thermal barrier coatings,catalysts, tribological,and environmental sectors

Rare earth oxides (REOs) and metals as an important class of materials have generated global interest in modern technology. Here, eight REOs (R₂O₃, R = Yb, Er, Sm, Eu, Y, Gd, Dy, and Ce) are identified those are mostly used in diverse chemical and industrial sectors. In this concise review the applications of these REOs in corrosion protection, thermal barrier coating, hydrophobic coating, catalytic reactions, refrigeration, photoactivity, environmental, and tribological sector are briefly summarized, which are sparsely documented in the existing literature review. In what follows, recently published relevant literature is systematically evaluated and key insights are addressed. The use of REOs as nonstoichiometric compounds or as doping agents to enhance the system performance of specific applications is commendable. Related lab-scale studies, therefore, will invoke the potential exploitation of individual or blended REOs for large-scale commercialization. However, phase transformation and high energy consumption in the thermal coating, multi-step fabrication of hydrophobic coating, the building of aromatic pollutants in catalysis, magnetic entropy shift in refrigeration, and photo corrosion are some of the inherent hindrances of these oxides. These issues warrant prompt actions from the scientific community to deliver cost-effective, efficient, and environmentally sustainable applications. Regardless, this review is expected to provide critical insights into REOs for contrasting industrial applications and encourage interested researchers to dig deeper in finding the best strategic solutions to counter prevalent challenges.     

Composition effects on elastic,thermal and corrosion properties of multiple-RE silicate (Ho1/4Er1/4Yb1/4Lu1/4)2SiO5 as a promising thermal and environmental barrier coating material

(Ho_1/4Er_1/4Yb_1/4Lu_1/4)₂SiO₅ is synthesized and characterized for the application of a promising multifunctional thermal and environmental barrier coating (TEBC) material. X-ray diffraction and scanning electron microscopy analysis indicate that a X2-type multiple-RE silicate (4RE_1/4)₂SiO₅ is formed with homogeneous distribution of the four rare earth species. Dense bulk sample exhibits excellent phase stability up to 1400 °C. Key properties including Young’s modulus, thermal conductivity and thermal expansion coefficient show interesting composition effects. Specially, (Ho_1/4Er_1/4Yb_1/4Lu_1/4)₂SiO₅ demonstrates higher elastic stiffness, lower thermal conductivity, lower thermal expansion coefficient and good resistances to molten CMAS and water vapor corrosions. These results confirm the strategy of multiple-RE engineering that may provide optimal property of advanced TEBCs.     

Robust hydrophobicity and evaporation inertness of rare-earth monosilicates in hot steam at very high temperature

Rare-earth oxides are intrinsically hydrophobic, and this characteristic opens new horizons for their design and diverse applications as robust hydrophobic surfaces. Here, we discover some rare-earth (RE) monosilicates (RE₂SiO₅) that are hydrophobic under ambient conditions. Their hydrophobicity is positively correlated with their hot-steam corrosion at 1400°C. RE³⁺ in RE₂SiO₅ is further found to be a hydration prohibitor, and its species can regulate hydration and volatilization at very high temperatures. These results may provide basis for innovative designs of RE silicates in harsh hot-steam environments for robust environmental barrier coatings (EBC) or as ceramic components.     

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.     

High temperature behavior of HfSiO4 subjected to calcium-magnesium-alumina-silicate (CMAS) and high-velocity water vapor

HfSiO₄ is considered as a candidate for environmental barrier coating (EBC), but there is a lack of comprehensive evaluation of its resistance against corrosive medium. We herein study the behavior of HfSiO₄ against CMAS melt and high-velocity water vapor. HfSiO₄ shows poor resistance to CMAS attack. Si diffusion occurs during CMAS attack, which leads to the formation of HfO₂ and CaSi₂O₅. HfSiO₄ decomposes to form SiO₂ and HfO₂ under the scouring of water vapor, in which SiO₂ forms volatile hydroxide and is taken away by high-velocity steam. HfSiO₄ is not the preferred system for surface layer of EBC system and is expected to be used as intermediate transition layer.     

Microstructures,thermophysical properties and thermal cyclic behaviors of Nd2O3 and Sc2O3 co-doped LaMgAl11O19 thermal barrier coating deposited by plasma spraying

In this study, La_1-xNd_xMgAl_11-xSc_xO₁₉ (x = 0.1, 0.2, 0.3; abbreviated as LNMAS-1, 2, 3) coatings which are supposed to possess better properties than LaMgAl₁₁O₁₉ (LMA) were plasma-sprayed and their high-temperature performance were comparatively investigated. Results show that addition of Nd³⁺ and Sc³⁺ as dopants to LMA endows corresponding coatings with reduced thermal conductivity and enhanced thermal expansion coefficient, while maintaining advantageous phase stability, although still being subjected to amorphization in plasma flame and following crystallization upon high-temperature service. Furthermore, the doping could cause adherence increasing between topcoat/bondcoat, benefiting from improved melting condition, especially in LNMAS-2 and LNMAS-3 coatings, which is related to the specific powder morphology and lowered melting point. During exposure to 1350°C, mechanical performance and structure integrity of doped free-standing LNMAS coatings can be well preserved even after 400 h aging. In thermal cyclic fatigue test, LNMAS-2 and LNMAS-3 coatings undertake thermal cycling lifetime of ～181 and 191 cycles at 1100°C, respectively, 40% durable than that of LMA coating. These preliminary results suggest that LNMAS-2, 3 might be promising candidates for advanced thermal barrier coating applications.     

Microstructure and water corrosion behavior of (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7 high-entropy rare-earth disilicate coating for SiC coated C/C composites

In order to make carbon/carbon composites suitable for application in gas turbine engine, it is necessary to develop environmental barrier coatings (EBCs) to protect them from reacting with water vapor. In our previous work, a novel high-entropy rare-earth disilicate (Lu_0.2Yb_0.2Er_0.2Tm_0.2Sc_0.2)₂Si₂O₇ ((5RE_0.2)₂Si₂O₇) has been developed and verified as a promising candidate for EBCs. In this work, the (5RE_0.2)₂Si₂O₇ coating was synthesized on the surface of SiC coated C/C composites by supersonic atmospheric plasma spraying method. The protective performance and mechanism of this coating under high temperature water vapor environment was explored in detail. Results showed that the weight change of the sample coated with (5RE_0.2)₂Si₂O₇ was only 0.2% after corrosion for 100 h at 1500 ºC, which proved that (5RE_0.2)₂Si₂O₇ coating could significantly improve the resistance of C/C composites against water vapor corrosion. This work may provide theoretical basis for the design and application of high-entropy rare-earth silicates as EBCs.     

Single-phase rare-earth high-entropy zirconates with superior thermal and mechanical properties

Emerging of high-entropy ceramics has brought new opportunities for designing and optimizing materials with desired properties. In the present work, high-entropy rare-earth zirconates (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ and (Yb_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ are designed and synthesized. Both high-entropy ceramics exhibit a single pyrochlore structure with excellent phase stability at 1600 °C. In addition, the Yb-containing system possesses a high coefficient of thermal expansion (10.52 × 10^?6 K^-1, RT～1500 °C) and low thermal conductivity (1.003 W·m^-1 K^-1, 1500 °C), as well as excellent sintering resistance. Particularly, the Yb-containing system has significantly improved fracture toughness (1.80 MPa·mm^1/2) when compared to that of lanthanum zirconate (1.38 MPa·mm^1/2), making it a promising material for thermal barrier coatings (TBCs) applications. The present work indicates that the high-entropy design can be applied for further optimization of the comprehensive properties of the TBCs materials.     

High-temperature water-vapor reaction mechanism of barium strontium aluminosilicate (BSAS)

Environmental barrier coatings (EBCs) are used in commercial turbine engine applications as protection for ceramic matrix composites, yet the high-temperature water vapor reaction mechanism for EBC materials is not fully understood. Here, the water vapor reaction mechanism for barium strontium alumino-silicate (BSAS), an early generation EBC candidate, was determined from the time and temperature dependences of material loss. BSAS water vapor exposures were performed at 1200 °C, 1300 °C, and 1400 °C for 24, 48, and 72 h, at maximum gas velocities of ~ 240 m/s. FactSage thermodynamic calculations were shown to support the experimental findings, where the steam reaction mechanism consisted of volatilization of all BSAS oxide constituents as gaseous metal hydroxide species, i.e. Ba(OH)₂, Sr(OH)₂, Al(OH)₃, and Si(OH)₄ (g).     

Corrosion of RE2Si2O7 (RE=Y,Yb, and Lu) environmental barrier coating materials by molten calcium-magnesium-alumino-silicate glass at high temperatures

With the increased demand for high operating temperature of gas turbine engines, corrosion by molten calcium-magnesium-alumino-silicate (CMAS) exhibits a significant challenge to the development of durable environmental barrier coatings (EBCs). EBC candidates, γ-Y₂Si₂O₇, β-Yb₂Si₂O₇, and β-Lu₂Si₂O₇ were explored on their corrosion resistance to CMAS melts at 1300 °C and 1500 °C for 50 h. Interaction and degradation mechanisms were investigated and the corrosion behaviors showed different trends at high temperatures. At 1300 °C, RE₂Si₂O₇ dissolves into CMAS melts and apatite phases reprecipitate forming a thick recession layer. However, when the temperature increases to 1500 °C, CMAS melts vigorously penetrate through the grain boundary of RE₂Si₂O₇ and ‘blister’ cracks form throughout the samples. The reduced grain boundary stability at 1500 °C promotes the penetration of CMAS melts in RE₂Si₂O₇. Grain boundary engineering is critically demanded to optimize CMAS corrosion at high temperatures.