High-temperature corrosion of sintered RE_(2)Si_(2)O_(7)(RE=Yb and Ho)environmental barrier coating materials by volcanic ash

Rare-earth silicates are promising environmental barrier coatings(EBCs)that can protect SiC_(f)/Si C_(m)substrates in next-genera tion gas turbine blades.Notably,RE_(2)Si_(2)O_(7)(RE=Yb and Ho)shows potential as an EBC due to its coefficient of thermal expansion(CTE)compatible with substrates and high resistance to water vapor corrosion.The target operating temperature for next-generation tur bine blades is 1400°C.Corrosion is inevitable during adhesion to molten volcanic ash,and thus,understanding the corrosion behavior o the material is crucial to its reliability.This study investigates the high-temperature corrosion behavior of sintered RE_(2)Si_(2)O_(7)(RE=Yb and Ho).Samples were prepared using a solid-state reaction and hot-press method.They were then exposed to volcanic ash at 1400°C for 224,and 48 h.After 48 h of exposure,volcanic ash did not react with Yb_(2)Si_(2)O_(7)but penetrated its interior,causing damage.Meanwhile Ho_(2)Si_(2)O_(7)was partially dissolved in the molten volcanic ash,forming a reaction zone that prevented volcanic ash melts from penetrating the interior.With increasing heat treatment time,the reaction zone expanded,and the thickness of the acicular apatite grains increased The Ca:Si ratios in the residual volcanic ash were mostly unchanged for Yb_(2)Si_(2)O_(7)but decreased considerably over time for Ho_(2)Si_(2)O_(7).Th Ca in volcanic ash was consumed and formed apatite,indicating that RE^(3+)ions with large ionic radii(Ho>Yb)easily precipitated apatit from the volcanic ash.

Oxidation Performance of Ytterbium Disilicate/Silicon Environmental Barrier Coating via Optimized Air Plasma Spraying

Environmental barrier coatings (EBCs) play a critical role in mitigating the degradation of SiCf/SiC ceramic matrix composites (CMCs) in complex combustion environment, and improve the service life of thermal engine components. In this paper, by adjusting the parameters of atmospheric plasma spraying (APS), the spraying process of ytterbium disilicate (Yb2Si2O7) under a lower power has been optimized. A two-layer EBC system consisting of ytterbium disilicate and silicon is prepared on the SiCf/SiC composite substrate by using optimized technological parameters. The thermal resistance and water oxygen corrosion resistance of such two-layer EBC system are investigated. The results indicate that the current ytterbium disilicate/silicon EBC system exhibits good phase stability, excellent water vapor and oxygen corrosion resistance. However, the exposed silicon bonding layer tends to generate an excessive thermal growth oxide (TGO) layer known as SiO2, leading to an early spallation of the coating.

Xenotime-type high-entropy(Dy_(1/7)Ho_(1/7)Er_(1/7)Tm_(1/7)Yb_(1/7)Lu_(1/7)Y_(1/7))PO_(4):A promising thermal/environmental barrier coating material for SiCf/SiC ceramic matrix composites

Rare-earth phosphates(REPO4)are regarded as one of the promising thermal/environmental barrier coating(T/EBC)materials for SiCf/SiC ceramic matrix composites(SiC-CMCs)owing to their excellent resistance to water vapor and CaO–MgO–Al_(2)O_(3)–SiO_(2)(CMAS).Nevertheless,a relatively high thermal conductivity(κ)of the REPO_(4) becomes the bottleneck for their practical applications.In this work,novel xenotime-type high-entropy(Dy_(1/7)Ho_(1/7)Er_(1/7)Tm_(1/7)Yb_(1/7)Lu_(1/7)Y_(1/7))PO4(HE(7RE_(1/7))PO_(4))has been designed and synthesized for the first time to solve this issue.HE(7RE_(1/7))PO_(4) with a homogeneous rare-earth element distribution exhibits high thermal stability up to 1750℃and good chemical compatibility with SiO_(2) up to 1400℃.In addition,the thermal expansion coefficient(TEC)of HE(7RE_(1/7))PO_(4)(5.96×10^(−6)℃^(−1) from room temperature(RT)to 900℃)is close to that of the SiC-CMCs.What is more,the thermal conductivities of HE(7RE_(1/7))PO_(4)(from 4.38 W·m^(−1)·K^(−1) at 100℃to 2.25 W·m^(−1)·K^(−1) at 1300℃)are significantly decreased compared to those of single-component REPO4 with the minimum value ranging from 9.90 to 4.76 W·m^(−1)·K^(−1).These results suggest that HE(7RE_(1/7))PO_(4) has the potential to be applied as the T/EBC materials for the SiC-CMCs in the future.

Investigation on the preparation of Si/mullite/Yb_2Si_2O_7 environmental barrier coatings onto silicon carbide

With the development of aero-engine,gas import temperatures of hot section structural materials are increasingly higher.Metal alloy materials due to the rapidly decreased mechanical properties at relative high temperature are gradually replaced with silicon-based non-oxide silicon carbide ceramics.However,silicon carbide ceramic materials tend to spall and deform in engine combustion environment,need environmental barrier coatings for the protection of the matrix.The preparation of Si/mullite/Yb2Si2O7 envir...

Recent progress in thermal/environmental barrier coatings and their corrosion resistance

Thermal/environmental barrier coatings(T/EBCs)play important roles in jet and/or gas turbine engines to protect the Ni-based superalloys and/or ceramic matrix composite substrates from the high-temperature airflow damage.Great efforts have been contributed to searching for enhanced T/EBC materials to improve the efficiency of the engines,which is the key of improving thrust-to-weight ratio and energy saving.The practical candidates,rare earth-contained materials,are widely used for T/EBCs in gas turbines due to their excellent properties such as low thermal conductivity,high melting point,hightemperature strength and durability as exhibited in yttriastabilized zirconia,pyrochlore oxides and rare earth silicates.In addition to the intrinsic properties,the microstructures obtained by different synthesis processes and the service performances,as well as the underlying failure mechanism,are also significant to this specific application.However,the main challenges for T/EBCs developments are T/EBC materials selection with balanced properties and their anti-corrosion performances at higher operating temperature.In this review,we summarized the progress in their fabrication techniques and mechanical/thermal properties of typically rare earth-contained T/EBCs,together with their anti-corrosion performance under the condition of molten salts or oxides(such as Na2SO4,V2O5and NaVO3),calcium–magnesium–alumina–silicate(CMAS)and high-temperature water vapor.

Effect of microstructure evolution and crystal structure on thermal properties for plasma-sprayed RE_(2)SiO_(5)(RE=Gd,Y,Er)environmental barrier coatings

In this work,the microstructure evolution,thermal expansion,thermal conductivity,and thermal shock resistance properties of the plasma-sprayed Xl-Gd_(2)SiO_(5),X2-Y_(2)SiO_(5),and X2-Er_(2)SiO_(5)coatings were evaluated and compared by experimental measurement and theoretical exploration.Results showed that significant microstructure evolution such as crystallization of amorphous phase,grain growth,and defects reduction was observed in the RE_(2)SiO_(5)coatings after thermal aging at 1400℃.The Xl-Gd_(2)SiO_(5)coating exhibited higher CTE values than the X2-Y_(2)SiO_(5)and X2-Er_(2)SiO_(5)coatings,which was related to their crystal structure.The thermal conductivity of thermal-aged RE_(2)SiO_(5)coating was much higher than that of the as-sprayed RE_(2)SiO_(5)coating,and thermal conductivity was determined not only by crystal structure but also mainly by the microstructure of the coatings.The X2-Y_(2)SiO_(5)and X2-Er_(2)SiO_(5)coatings with lower thermal mismatch stre s ses presented much better thermal shock resistance than the X1-Gd_(2)SiO_(5)coating.

Pressure infiltration of molten aluminum for densification of environmental barrier coatings

Environmental barrier coatings(EBCs)effectively protect the ceramic matrix composites(CMCs)from harsh engine environments,especially steam and molten salts.However,open pores inevitably formed during the deposition process provide the transport channels for oxidants and corrosives,and lead to premature failure of EBCs.This research work proposed a method of pressure infiltration densification which blocked these open pores in the coatings.These results showed that it was difficult for aluminum to infiltrate spontaneously,but with the increase of external gas pressure and internal vacuum simultaneously,the molten aluminum obviously moved forward,and finally stopped infiltrating at a depth of a specific geometry.Based on the wrinkled zigzag pore model,a mathematical relationship between the critical pressure with the infiltration depth and the pore intrinsic geometry was established.The infiltration results confirmed this relationship,indicating that for a given coating,a dense thick film can be obtained by adjusting the internal and external gas pressures to drive a melt infiltration.

Mechanism of vertical crack formation in Yb_(2)SiO_(5) coatings deposited via plasma spray-physical vapor deposition

Plasma spray-physical vapor deposition(i.e.,PS-PVD)is a promising method for obtaining advanced environmental barrier coatings(EBCs).The EBCs must meet some requirements in the application,in which the thermal cycle performance affects the service lifetime.The preparation of artificial vertical cracks in Yb_(2)SiO_(5) coatings is an effective approach for meeting the requirements above because vertical cracks provide a strain tolerance.To clarify the formation mechanism of vertical cracks during the PSPVD,the effects of coating thickness and substrate temperature on the formation of vertical cracks were investigated.In addition,the interactions of spray powder and plasma flame during coating deposition were also characterized by optical spectroscopy.It is indicated that vertical cracks are formed due to a thermal expansion mismatch between Yb_(2)SiO_(5) and mullite coating,transient cooling after deposition and the nucleation of evaporated Yb_(2)SiO_(5) as well.

Fabrication of barium-strontium aluminosilicate coatings on C/SiC composites via laser cladding

Barium-strontium aluminosilicate （BSAS） and Si/BSAS coatings were fabricated on the surface of C/SiC composites via a two-step laser cladding process. The microstructure, mechanical properties, and the water vapor corrosion behavior of the samples were investigated. The BSAS coating was found to be tightly bonded to the substrate and only a few pores and microcracks were observed. The introduction of a silicon middle layer was revealed to reduce thermal stress and promote the healing of defects formed during the laser cladding process. To evaluate the corrosion resistance, the BSAS and Si/BSAS-coated C/SiC com- posites were exposed to an atmosphere of 50% H2O and 50% O2 at 1250 ℃. The resulting weight change and flexural strength were measured as a function of the corrosion time. The addition of the silicon middle layer below the BSAS top layer resulted in a better resistance to water vapor corrosion. Furthermore, the Si/BSAS-coated samples showed a lower weight loss and a smaller reduction in flexural strength than the BSAS-coated and the uncoated samples during water vapor corrosion. Thus, laser cladding is dem- onstrated to be an effective and feasible method to fabricate high-quality ceramic coatings on C/SiC composites. The introduction of a silicon middle layer can inhibit defect formation during the laser clad- ding process and protect the composite from water vapor corrosion.

Simultaneous manipulations of thermal expansion and conductivity in symbiotic ScTaO_(4)/SmTaO_(4) composites via multiscale effects

Effective manipulations of thermal expansion and conductivity are significant for improving operational performances of protective coatings,thermoelectric,and radiators.This work uncovers determinant mechanisms of the thermal expansion and conductivity of symbiotic ScTaO_(4)/SmTaO_(4) composites as thermal/environmental barrier coatings(T/EBCs),and we consider the effects of interface stress and thermal resistance.The weak bonding and interface stress among composite grains manipulate coefficient of thermal expansion(CTE)stretching from 6.4×10^(−6) to 10.7×10^(−6) K^(−1) at 1300℃,which gets close to that of substrates in T/EBC systems.The multiscale effects,including phonon scattering at the interface,mitigation of the phonon speed(vp),and lattice point defects,synergistically depress phonon thermal transports,and we estimate the proportions of different parts.The interface thermal resistance(R)reduces the thermal conductivity(k)by depressing phonon speed and scattering phonons because of different acoustic properties and weak bonding between symbiotic ScTaO_(4) and SmTaO_(4) ceramics in the composites.This study proves that CTE of tantalates can be artificially regulated to match those of different substrates to expand their applications,and the uncovered multiscale effects can be used to manipulate thermal transports of various materials.

Influence of average radii of RE3+ ions on phase structures and thermal expansion coefficients of high-entropy pyrosilicates

High-entropy pyrosilicate element selection is relatively blind, and the thermal expansion coefficient (CTE) of traditional β-type pyrosilicate is not adjustable, making it difficult to meet the requirements of various types of ceramic matrix composites (CMCs). The following study aimed to develop a universal rule for high-entropy pyrosilicate element selection and to achieve directional control of the thermal expansion coefficient of high-entropy pyrosilicate. The current study investigates a high-entropy design method for obtaining pyrosilicates with stable β-phase and γ-phase by introducing various rare-earth (RE) cations. The solid-phase method was used to create 12 different types of high-entropy pyrosilicates with 4–6 components. The high-entropy pyrosilicates gradually transformed from β-phase to γ-phase with an increase in the average radius of RE^(3+) ions ( r¯(RE^(3+))). The nine pyrosilicates with a small r¯(RE^(3+)) preserve β-phase or γ-phase stability at room temperature to the maximum of 1400 ℃. The intrinsic relationship between the thermal expansion coefficient, phase structure, and RE–O bond length has also been found. This study provides the theoretical background for designing high-entropy pyrosilicates from the perspective of r¯(RE^(3+)). The theoretical guidance makes it easier to synthesize high-entropy pyrosilicates with stable β-phase or γ-phase for the use in environmental barrier coatings (EBCs). The thermal expansion coefficient of γ-type high-entropy pyrosilicate can be altered through component design to match various types of CMCs.

In-situ observation and mechanism of calcium–magnesium–alumina–silicate(CMAS)melts-induced degradation of RE_(2)SiO_(5)(RE=Tb,Dy,Ho,Y,Er,Tm,and Yb)ceramics at 1500℃

Rare earth(RE)silicate is one of the most promising environmental barrier coatings for silicon-based ceramics in gas turbine engines.However,calcium-magnesium-alumina-silicate(CMAS)corrosion becomes much more serious and is the critical challenge for RE silicate with the increasing operating temperature.Therefore,it is quite urgent to clarify the mechanism of high-temperature CMAS-induced degradation of RE silicate at relatively high temperatures.Herein,the interaction between RE_(2)SiO_(5) and CMAS up to 1500℃was investigated by a novel high temperature in-situ observation method.High temperature promotes the growth of the main reaction product(Ca_(2)RE_(8)(SiO_(4))6O_(2))fast along the[001]direction,and the precipitation of short and horizontally distributed Ca_(2)RE_(8)(SiO_(4))6O_(2) grains was accelerated during the cooling process.The increased temperature increases the solubility of RE elements,decreases the viscosity of CMAS,and thus elevates the corrosion reaction rate,making RE_(2)SiO_(5) fast interaction with CMAS and less affected by RE element species.

(La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4: A high-entropy rare-earth phosphate monazite ceramic with low thermal conductivity and good compatibility with Al2O3

Low thermal conductivity, matched thermal expansion coefficient and good compatibility are general requirements for the environmental/thermal barrier coatings(EBCs/TBCs) and interphases for Al2O3 f/Al2O3 composites. In this work, a novel high-entropy(HE) rare-earth phosphate monazite ceramic (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4 is designed and successfully synthesized. This new type of HE rare-earth phosphate monazite exhibits good chemical compatibility with Al2O3, without reaction with Al2O3 as high as 1600℃ in air. Moreover, the thermal expansion coefficient(TEC) of HE (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4(8.9 × 10^-6/℃ at 300–1000℃) is close to that of Al2O3. The thermal conductivity of HE (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4 at room temperature is as low as 2.08 W·m^-1·K^-1, which is about 42% lower than that of La PO4. Good chemical compatibility, close TEC to that of Al2O3, and low thermal conductivity indicate that HE (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4 is suitable as a candidate EBC/TBC material and an interphase for Al2O3 f/Al2O3 composites.

High-entropy(Nd_(0.2)Sm_(0.2)Eu_(0.2)Y_(0.2)Yb_(0.2))_(4)Al_(2)O_(9) with good high temperature stability,low thermal conductivity,and anisotropic thermal expansivity

The critical requirements for the environmental barrier coating(EBC)materials of silicon-based ceramic matrix composites(CMCs)include good tolerance to harsh environments,thermal expansion matches with the interlayer mullite,good high-temperature phase stability,and low thermal conductivity.Cuspidine-structured rare-earth aluminates RE_(4)Al_(2)O_(9) have been considered as candidates of EBCs for their superior mechanical and thermal properties,but the phase transition at high temperatures is a notable drawback of these materials.To suppress the phase transition and improve the phase stability,a novel cuspidine-structured rare-earth aluminate solid solution(Nd_(0.2)Sm_(0.2)Eu_(0.2)Y_(0.2)Yb_(0.2))_(4)Al_(2)O_(9) was designed and successfully synthesized inspired by entropy stabilization effect of high-entropy ceramics(HECs).The as-synthesized HE(Nd_(0.2)Sm_(0.2)Eu_(0.2)Y_(0.2)Yb_(0.2))_(4)Al_(2)O_(9) exhibits a close thermal expansion coefficient(6.96×10^(-6) K^(-1) at 300-1473 K)to that of mullite,good phase stability from 300 to 1473 K,and low thermal conductivity(1.50 W·m^(-1)·K^(-1) at room temperature).In addition,strong anisotropic thermal expansion has been observed compared to Y_(4)Al_(2)O_(9) and Yb_(4)Al_(2)O_(9).The mechanism for low thermal conductivity is attributed to the lattice distortion and mass difference of the constituent atoms,and the anisotropic thermal expansion is due to the anisotropic chemical bonding enhanced by the large size rare-earth cations.

Mechanical and thermal properties of RETaO4(RE=Yb,Lu,Sc)ceramics with monoclinic-prime phase

As candidate thermal/environmental barrier coatings(T/EBCs),the structure characteristics and comprehensive properties of monoclinic-prime(m')RETaO4(RE=Yb,Lu,Sc)with excellent Al2O3/SiO2 chemical compatibility are studied.Excellent thermal insulation protection will be provided by m'RETaO4 due to their low thermal conductivity(~1.6 Wm^-1 K^-1,900℃)and prominent thermal radiation resistance,which is much better than those of YSZ(~2.5 Wm^-1 K^-1,1000℃)and La-12 Zr2O7(~2.0 Wm^-1 K,900℃).The thermal expansion coefficients(TECs)are 3.0–8.0×10^-6 K^-1(200-1200℃),which is suitable for T/EBCs applications.Furthermore,absence of phase transition and extraordinary chemical compatibility with Al2O3/SiO2 up to 1500℃indicate the potential application prospect.The documented governing mechanisms of m'RETaO4 properties will enable researchers to promote their application in the future investigation.

Interaction of multicomponent disilicate(Yb_(0.2)Y_(0.2)Lu_(0.2)Sc_(0.2)Gd_(0.2))_(2)Si_(2)O_(7)with molten calcia-magnesia-aluminosilicate

Environmental barrier coating(EBC)materials that are resistant against molten calcia-magnesia-aluminosilicate(CMAS)corrosion are urgently required.Elerein,multicomponent rare-earth(RE)disilicate((Yb_(0.2)Y_(0.2)Lu_(0.2)Sc_(0.2)Gd_(0.2))_(2)Si_(2)O_(7),(5RE)_(2)Si_(2)O_(7))was investigated with regard to its CMAS interaction behavior at 1400°C.Compared with the individual RE disilicates,the(5RE)2Si2C>7 material exhibited improved resistance against CMAS attack.The dominant process involved in the interaction of(5RE)_(2)Si_(2)O_(7)with CMAS was reaction-recrystallization.A dense and continuous reaction layer protected the substrate from rapid corrosion at high temperatures.The results demonstrated that multicomponent strategy of RE species in disilicate can provide a new perspective in the development of promising EBC materials with improved corrosion resistance.

(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5) high-entropy ceramic with low thermal conductivity,tunable thermal expansion coefficient,and excellent resistance to CMAS corrosion

Low thermal conductivity,compatible thermal expansion coefficient,and good calcium–magnesium–aluminosilicate(CMAS)corrosion resistance are critical requirements of environmental barrier coatings for silicon-based ceramics.Rare earth silicates have been recognized as one of the most promising environmental barrier coating candidates for good water vapor corrosion resistance.However,the relatively high thermal conductivity and high thermal expansion coefficient limit the practical application.Inspired by the high entropy effect,a novel rare earth monosilicate solid solution(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)was designed to improve the overall performance.The as-synthesized(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)shows very low thermal conductivity(1.07 W·m-1·K-1 at 600℃).Point defects including mass mismatch and oxygen vacancies mainly contribute to the good thermal insulation properties.The thermal expansion coefficient of(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)can be decreased to(4.0–5.9)×10^(-6)K^(-1)due to severe lattice distortion and chemical bonding variation,which matches well with that of SiC((4.5–5.5)×10^(-6)K^(-1)).In addition,(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)presents good resistance to CMAS corrosion.The improved performance of(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)highlights it as a promising environmental barrier coating candidate.

Water vapor corrosion behaviors of high-entropy pyrosilicates

In this study,the water vapor corrosion resistance of two types of high-entropy pyrosilicates((Yb_(0.2)Y_(0.2)Lu_(0.2)Ho_(0.2)Er_(0.2))_(2)Si_(2)O_(7)((5RE1/5)_(2)Si_(2)O_(7))and(Yb_(0.2)5Lu_(0.2)5Ho_(0.2)5Er_(0.2)5)_(2)Si_(2)O_(7)((4RE_(1/4))_(2)Si_(2)O_(7)))and two single-component pyrosilicates(Yb_(2)Si_(2)O_(7) and Lu_(2)Si_(2)O_(7))were evaluated at 1350℃ for 50e100 h,and the initial corrosion behaviors of these pyrosilicates were studied.The results showed that the final corrosion products of the four types of pyrosilicates were all X2-type monosilicates,exhibiting similar corrosion phenomena.However,(4RE_(1/4))_(2)Si_(2)O_(7) generated many nanoscale monosilicate grains during corrosion.The corrosion resistance of Lu_(2)Si_(2)O_(7) was clearly better than those of the others,and(4RE_(1/4))_(2)Si_(2)O_(7) exhibited the worst corrosion resistance.The corrosion mechanism of the pyrosilicate blocks was analyzed from the perspectives of grain size,bulk hydrophobicity,and binding energy.This study potentially provides a theoretical basis for the preparation of high-entropy pyrosilicates with different atomic ratios according to the different properties of the various rare earth elements.

Durable dual-state duplex Si-HfO_(2)with excellent oxidation and cracking resistance

The lifetime of Si bond coatings in environmental barrier coatings is constrained by phase-transition-induced cracking of the SiO_(2)scale.In this study,Si-HfO_(2)dual-state duplex composite materials are proposed to address this issue by partially forming HfSiO_(4)and minimizing the SiO_(2)content.The as-prepared composite exhibited a structure comprising discrete HfO_(2)“bricks”embedded in a continuous Si“mortar”,while the oxidized state transformed into discrete HfSiO_(4)“bricks”within continuous thin SiO_(2)“mortars”.The results indicate that continuous thin SiO_(2)contributes to reducing the oxidation rate to a level comparable to that of pure Si,and discrete HfSiO_(4)particles aid in relieving phase transition-induced stress and inhibiting crack propagation,thereby enhancing oxidation and cracking resistance simultaneously.Consequently,the composite with 20 mol%HfO_(2)and a mean particle size of~500 nm at 1370℃exhibited a service lifetime 10 times greater than that of pure Si.This research provides valuable insights for designing Si-based bond coatings with improved service lifetime.