Environmental Barrier Coatings Research Articles

Silicon‐based bond coatings for environmental barrier coatings: Present status and prospective

AbstractEnvironmental barrier coatings (EBCs) are indispensable for the service of SiC‐based turbine engines. The Si‐bond coating is a critical layer that prevents oxidants from penetrating SiC substrates and determines the service lifetimes of EBCs. In this study, the oxidation behaviors and failure mechanisms of Si‐based bond coatings were reviewed. The large growth rate and phase transformation of thermally grown oxides (TGOs, SiO2) seriously deteriorate the service of Si‐bond coatings. The low melting point of Si further limits its application in next‐generation engines above 1 427°C. The results show that an isolated particle healing (IPH) treatment decreased the oxidation rate of the Si‐bond coating by ∼24% at 1 300°C. Moreover, the Si–HfO2 and Si‐stabilizer (Si–Al2O3 or Si‐mullite) composite/duplex bond coatings can eliminate SiO2 phase transitions, thus improving the service lifetime. In addition, rare earth silicide (RESi), SiC and SiO2–HfO2 composite show potential for use in next‐generation EBCs above 1 427°C. This review provides guidance for designing Si‐based bond coatings with improved service lifetime.

Process parameters for enhanced microstructure and composition of as-sprayed Yb2Si2O7 environmental barrier coatings via atmospheric plasma spray

Achieving high crystallinity levels, dense, crack-free microstructures with near-stoichiometric phase composition and controlled secondary phase formation are crucial for enhancing the performance of environmental barrier coatings (EBCs). To investigate the impact of APS process parameters on the as-sprayed crystallinity, microstructure, and phase composition of Yb2Si2O7 EBCs, various spraying conditions were explored by adjusting torch power, nozzle size, powder feedrate, stand-off distance, and adding a shroud attachment to the torch. The velocity of particles controlled the density of the EBCs by an interplay between deposition efficiency and splat flattening ratios. Increasing nozzle size from 5/16″ to 1/2″ reduced particle velocity by up to 40 %, resulting in improved deposition efficiencies, porosity levels and coating thickness. However, larger nozzles led to higher silicon evaporation and up to 8 wt% more stable Yb2SiO5 (I2/a) secondary phase was formed. Increasing the stand-off distance from 50 mm had minimal effect on microstructure, but metastable Yb2SiO5 (P21/c) was detected (up to 9 wt%) at longer stand-off distances of 100 and 150 mm. Raising the powder feedrate from 15 gr/min to 60 gr/min improved deposition efficiency (up to 9 %) and density (up to 15 %) of EBCs, yet resulted in higher silicon-depleted phase formation (up to 12 wt%). Adding a shroud to the torch nozzle exit led to severe silicon evaporation and formation of up to 42 wt% and 6 wt% of Yb2SiO5 (I2/a) and Yb2O3, respectively. Moreover, due to the shroud's funnelling effect on smaller particles as well as particle overheating, porous microstructures with up to 28 % porosity levels were formed. High levels of crystallinity (∼75–91 %) were achieved in the as-sprayed condition for all the coatings, indicating real-time crystallization during spraying in all the spraying conditions.

Mechanical Properties and Thermal Conductivity of Y-Si and Gd-Si Silicides: First-Principles Calculations

The traditional Si bonding layer in environmental barrier coatings has a low melting point (1414 °C), which is a significant challenge in meeting the requirements of the next generation higher thrust-to-weight ratio aero-engines. To seek new bonding layer materials with higher melting points, the mechanical properties of Y-Si and Gd-Si silicides were calculated by the first-principles method. Subsequently, empirical formulae were employed to compute the sound velocities, Debye temperatures, and the minimum coefficients of thermal conductivity for the YSi, Y5Si4, Y5Si3, GdSi, and Gd5Si4. The results showed that Y5Si4 has the best plasticity and ductility among all these materials. In addition, Gd5Si4 has the minimum Debye temperature (267 K) and thermal conductivity (0.43 W m−1 K−1) compared with others. The theoretical calculation results indicate that some silicides in the Y-Si and Gd-Si systems possess potential application value in high-temperature bonding layers for thermal and/or environmental barrier coating.

High-throughput composition screening of high-entropy rare-earth monosilicates for superior CMAS corrosion resistance up to 1873 K

Superior calcium-magnesium-aluminum-silicate (CMAS) corrosion resistance is the key to the applications of high-entropy rare-earth monosilicates (HEREMs) as environmental barrier coatings (EBCs) for next-generation gas turbine engines. Here, we report a high-throughput composition screening strategy to achieve superior CMAS corrosion resistance in HEREMs for the first time. Specifically, we first fabricate ten kinds of (Ho0.25Yb0.25Lu0.25X0.25)2SiO5 (X = Sc, Tm, Er, Y, Dy, Gd, Eu, Sm, Nd, and La) HEREM samples using a high-throughput pressure-less sintering technique, and subsequently screen their CMAS corrosion resistance at 1673 K via a high-throughput testing apparatus. Among all the as-fabricated samples, the HEREM-Er samples are found to have the best CMAS corrosion resistance, even up to 1873 K, which outperforms the EBC materials that have been reported. Such superior CMAS corrosion resistance of the HEREM-Er samples is primarily attributed to the formation of a dense apatite barrier layer, which results from its decreased melting point, as confirmed by first-principles calculations and molecular dynamic simulations. Our work provides a guide for designing exceptional anti-CMAS corrosion high-entropy rare-earth silicates.

Plasma sprayed duplex ytterbium disilicate/monosilicate EBCs and the transformation from ytterbia to ytterbium monosilicate during burner rig testing

Environmental Barrier Coating systems (EBCs) were thermally cycled in a burner rig test facility. Yb2Si2O7 (YbDS) layer was deposited by air plasma spraying while two suspension plasma sprayed Yb2SiO5 (YbMS) microstructures were evaluated in duplex (YbDS/YbMS) systems: columnar and segmentation cracked. EBCs underwent 2000 cycles at a surface temperature of 1300 °C without signs of delamination failure. A porous YbMS layer formed at the base of intercolumnar gaps and segmentation cracks in the duplex systems, presumably due to reactions with entrapped water vapor. Furthermore, Yb2O3 depletion zones were evident at both the surface and YbDS/YbMS interface of the duplex EBCs.

Microstructural, thermal characterization and CMAS corrosion resistance of novel quaternary (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 high entropy disilicate material

A novel (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 high entropy disilicate quaternary composition was synthesized from commercial oxide powders using ball milling and sintering processes as a candidate material for environmental barrier coatings (EBC). As-synthesized high entropy disilicate powders were sintered at different durations (12, 18, and 24 h) at 1600 °C in a muffle furnace before characterization. The XRD and SEM analyses revealed the single-phase monoclinic structure (β-type) with homogeneous elemental distribution for the sintered samples. The (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 samples exhibited low thermal diffusivity coefficient, low thermal conductivity, a close coefficient of thermal expansion (CTE) to SiC and a high temperature stability. The (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 samples were subjected to CMAS corrosion tests at 1300 °C with different durations (2, 12, and 24 h) to evaluate CMAS corrosion resistance. Additionally, Yb2Si2O7 samples were prepared and subjected to molten CMAS in the same way for comparison. Based on the results, the CMAS corrosion resistance was improved with (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 composition.

Corrosion resistance and microstructural evolution of Yb-Al-Si-O glass-ceramics under molten Ca-Mg-Al-Si environment at 1350 °C

The intrinsic characteristics of Yb2O3-Al2O3-SiO2 (YbAS) glass-ceramics and their resistance to corrosion by molten calcium magnesium aluminosilicate (CMAS) at 1350 °C were systematically investigated. YbAS glass-ceramics, characterized by diverse compositions, maintaining phase stability over extended durations at elevated temperatures. After CMAS corrosion for 50 h, the main reaction phases were CaAl2Si2O8 and Ca2Yb8(SiO4)6O2, accompanied by multiple ion solid solution garnet in certain components. The glass component positioned along the eutectic line of Yb2Si2O7 and mullite, with an OB value marginally exceeding that of CMAS, exhibits superior corrosion resistance to CMAS. This finding offers valuable insights for the subsequent design of environmental barrier coatings (EBCs).

Compositional design and phase formation capability of high-entropy rare-earth disilicates from machine learning and decision fusion

A key strategy for designing environmental barrier coatings is to incorporate multiple rare-earth (RE) components into β- and γ-RE2Si2O7 to achieve multifunctional performance optimization. However, the polymorphic phase presents significant challenges for the design of multicomponent RE disilicates. Here, employing decision fusion, a machine learning (ML) method is crafted to identify multicomponent RE disilicates, showcasing notable accuracy in prediction. The well-trained ML models evaluated the phase formation capability of 117 (RE10.25RE20.25Yb0.25Lu0.25)2Si2O7 and (RE11/6RE21/6RE31/6Gd1/6Yb1/6Lu1/6)2Si2O7, which are unreported in experiments and validated by first-principles calculations. Utilizing model visualization, essential factors governing the formation of (RE10.25RE20.25Yb0.25Lu0.25)2Si2O7 are pinpointed, including the average radius of RE3+ and variations in different RE3+ combinations. On the other hand, (RE11/6RE21/6RE31/6Gd1/6Yb1/6Lu1/6)2Si2O7 must take into account the average mass and the electronegativity deviation of RE3+. This work combines material-oriented ML methods with formation mechanisms of multicomponent RE disilicates, enabling the efficient design of superior materials with exceptional properties for the application of environmental barrier coatings.

Composition-driven superior CMAS corrosion resistance of high-entropy rare-earth disilicates

Achieving superior calcium-magnesium-aluminosilicate (CMAS) corrosion resistance in high-entropy rare-earth disilicates (HEREDs) is crucial to their applications as environmental barrier coatings in aerospace. Stimulated by the vast compositional field of HEREDs, we achieve exceptional CMAS corrosion resistance in HEREDs at elevated temperatures via a composition-driven strategy for the first time. Specifically, the equimolar 3- to 8-cation high-entropy rare-earth disilicate (3–8HERED) samples are first successfully designed and fabricated using a high-throughput pressure-less sintering technique and then their CMAS corrosion resistance up to 1773 K is evaluated. Among all the as-fabricated samples, the as-fabricated 7HERED samples are found to exhibit the best CMAS corrosion resistance. The outstanding CMAS corrosion resistance of the as-fabricated 7HERED samples is primarily attributed to pronounced sluggish diffusion induced by configurational entropy, with the synergistic effect of their chemical activity, which facilitates the formation of dense apatite protective layers. Our work provides valuable ways to design remarkable anti-CMAS corrosion high-entropy ceramics.

Bimodal nanomechanical performance of nanostructured Lu2Si2O7 environmental barrier coating

Lutetium disilicate (Lu2Si2O7) is considered a promising material for environmental barrier coatings because of its remarkable high-temperature stability and low thermal conductivity. In this study, nanostructured Lu2Si2O7 coating was prepared successfully using as-prepared feedstocks via atmospheric plasma spraying. The nanomechanical performance of the coating was studied. The results showed that the coating exhibited a bimodal nanomechanical performance attributed to the bimodal distribution of its microstructure; i.e., in the coating, the melted lamella exhibited higher elastic modulus and nanohardness, whereas the unmelted and semi-melted parts processed low elastic modulus and nanohardness. The elastic modulus and nanohardness of the coating were 143.438 ± 25.449 GPa and 9.239 ± 3.514 GPa, respectively. Furthermore, the elastic and plastic works were 9.018–14.752 nJ and 12.792–25.793 nJ, respectively. The average value of the microhardness dissipation parameter of the coating was 0.615, demonstrated that the remarkable wear resistance of the coating.

Nanostructured Yb2SiO5/mullite-SiC/Si environmental barrier coating with long-term stability at 1250 °C

In this work, nanostructured Yb2SiO5/mullite-SiC/Si environmental barrier coatings (EBCs) were prepared to investigate the effect of heat treatment on the microstructure and phase transition of nanostructured Yb2SiO5/mullite-SiC/Si coatings. The Yb2SiO5 content and microstructure evolution in the coatings were thoroughly analyzed before and after heat treatment. The results demonstrate a slight decrease in the porosities of nanostructured Yb2SiO5 and mullite-SiC coatings, accompanied by a gradual growth in grain sizes with increasing heat treatment time, indicating the nanostructured EBCs show good resistance to sintering. Moreover, the thickness of thermal growth oxidation layer is only 200 nm at 1250 °C for 500 h of heat treatment. In particular, the presence of the Yb2O3 phase has been observed to exert a favorable influence on the thermal stability of nanostructured EBCs. This work provides an innovative strategy for designing advanced EBCs with excellent high-temperature thermal stability.

Mechanism of stress evolution in environmental barrier coatings considering the presence of crack channels

The roles of crack channel transitions and thermally grown oxides (TGO) in the failure of environmental barrier coatings (EBCs) should be clarified. Hence, in this study, the laws of penetration and delamination crack evolution were investigated using a virtual crack extension method for multilayer materials to gain insights into the transformation conditions with a framework for a coupled thermal/mechanical/chemical numerical model of TGO containing channel cracks developed based on this law. The framework integrates diffusion, TGO growth, and phase transformation of the oxides. A theoretical model for high-temperature gas corrosion of EBCs coupled with deformation, mass diffusion, and chemical reactions was proposed, with the results indicating that the cracks transitioned from vertical to exfoliation cracks in the ytterbium dibasic silicate (YbDS) layer. The coupling effect of cracks and stresses led to non-uniform growth of the TGO by affecting the oxygen diffusion and inflow rates. The y-direction stresses at the YbDS/TGO interface near the crack channel changed from compressive to tensile stresses after cooling, making the first location to undergo delamination failure. The proposed model can provide better guidance for designing EBCs systems and supporting the life prediction of EBCs in service environments.

An investigation into the erosion and wear mechanisms observed in abradable ytterbium disilicate environmental barrier coatings

Abradable environmental barrier coatings (EBCs) are used to reduce clearances between blades and casings in the hottest parts of aero gas turbines, increasing the overall efficiency of the turbine, however, research into the mechanical performance of such coatings is limited. The aim of this work was to better understand the relationship between microstructure and erosion and wear performance in abradable EBCs, since publicly available literature regarding such coatings is scarce and the potential efficiency gains in gas turbines are significant. In this study, ytterbium disilicate abradable EBCs containing three different porosity levels (determined by fugitive polyester phase addition) were deposited using atmospheric plasma spraying with porosity levels of 8, 15 and 21.5% by area. These coatings were then characterised in numerous ways; the porosity was quantified, the thermal conductivity was measured, the superficial hardness and erosion resistance were measured, and finally, the coatings were subjected to a rig test designed to simulate in-service cutting mechanisms against a tipped turbine blade. The results show that increasing the level of porosity via increasing the amount of pore forming phase in the feedstock, led to reduced erosion resistance and improved cutting by a turbine blade.

Effect of calcium–magnesium–aluminum silicate composition on reaction behavior for X1‐Gd2SiO5 and X2‐Er2SiO5 environmental barrier coatings

AbstractRare‐earth (RE) silicates environmental barrier coatings (EBCs) are prone to accelerate failure resulted from molten calcium–magnesium–aluminum silicate (CMAS), which presents significant challenges to the design and development of durable EBCs. In this study, X1‐Gd2SiO5 and X2‐Er2SiO5 coatings were explored on their corrosion resistance to CMAS melts with different Ca/Si ratios (1.1, 0.75, 0.64, and 0.42) at 1350°C for 25 and 50 h, respectively. Results showed that with the Ca/Si ratio increasing, CMAS viscosity decreased and a more intense corrosion reaction with coatings occurred. Garnet phase was only observed in X2‐Er2SiO5 coating corroded by CMAS with high Ca/Si ratios, which could be beneficial for improving corrosion resistance. The X2‐Er2SiO5 phase, which is composed of [SO4] and [REO6]/[REO7] polyhedrons, exhibits better corrosion resistance due to its more stable crystal structure, lower reactivity with CMAS, and the radius difference between the RE ions and Ca2+.

(Gd0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 and (Sc0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 high-entropy rare-earth disilicates as promising materials for environmental barrier coatings

The novel potential environmental barrier coating materials, two high-entropy rare earth disilicates (HEDs) (Gd0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 and (Sc0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 were prepared by the solid-phase method. Both the HEDs showed remarkable high-temperature phase stability and an appropriate thermal expansion coefficient (CTE) that matched that of SiCf/SiC. In addition, both the HEDs possessed above 20% lower thermal conductivity at 1000 °C (1.37 and 1.42 W·m−1·K−1, respectively) compared to traditional single-component rare earth disilicate material YbDS, and they also exhibited higher mechanical properties and good resistance to water vapour corrosion at 1300 °C. Moreover, the corresponding free-standing coatings obtained after deposition by atmospheric plasma spraying (APS) showed excellent resistance to CMAS corrosion, primarily because of the formation of an apatite layer.

Thermal shock resistance of a novel (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 environmental barrier coating

AbstractMulticomponent rare earth (RE) disilicates have been recognized as promising candidates for environmental barrier coatings (EBCs). In this work, a novel EBC system with (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 as top coat and Si as bond coat was deposited on SiC‐based substrates by atmospheric plasma spraying technology and subjected to 200 thermal shock cycles at 1350°C. The composition and microstructure evolution in the coating during thermal cycling were investigated using x‐ray diffraction analysis and scanning electron microscopy. The results show that the as‐prepared EBC remains dense and integrated, and no obvious spalling or penetrating cracks could be observed in the coating after 200 thermal shock cycles. Furthermore, the average thickness of thermal growth oxide layer between (6RE1/6)2Si2O7 top coat and Si bond coat is 0.71 ± 0.17 µm and 0.98 ± 0.23 µm before and after the thermal cycling, respectively, indicating the excellent thermal shock resistance of (6RE1/6)2Si2O7 EBC. The present study presents a novel EBC with excellent thermal shock ability and is expected to contribute to the development of EBC for advanced SiCf/SiC ceramic matrix composites.

Phase and properties evolution of plasma sprayed rare earth silicate coatings

Rare-earth silicates have been pursued for many years as candidates for Environmental Barrier Coatings for the protection of silicon carbide (SiC) based components in the new generation of aero and power production turbine engines. The properties and behavior of these materials are usually reported based on theoretical studies or measurements done on bulk specimens, but their prospective use is in the form of coatings. One of the most extended ways of depositing these coatings is plasma spray which produces partially or totally amorphous deposit and changes towards SiO2 lean chemical composition. In this work is comparatively discussed the effect of plasma spray deposition on Yb2Si2O7, Y2Si2O7 and the mixed cation silicate (Y0.5Yb0.5)2Si2O7 coatings. The phase evolution with temperature and its impact on the coefficient of thermal expansion, microstructure and thermal conductivity are described. The stability of crystalline phases, properties and absence of cracks in its microstructure makes (Y0.5Yb0.5)2Si2O7 a more optimum candidate among the studied rare-earth silicates for environmental barrier applications.

Mechanical and Thermal Properties of the Hf–Si System: First-Principles Calculations

The relatively low melting point of a traditional Si bonding layer limits the upper servicing temperature of environmental barrier coatings (EBC). To explore suitable high temperature bonding layers and expedite the development of EBC, first-principles calculation was used to evaluate the mechanical properties and thermal conductivity of HfSi2, HfSi, Hf5Si4, Hf3Si2, and Hf2Si with much higher melting points than that of Si. Among them, HfSi2 has the lowest modulus capable of good modulus matching with SiC substrate. In addition, these Hf-Si compounds have much lower high temperature thermal conductivity with Hf2Si being the lowest of 0.63 W m−1 K−1, which is only half of Si, capable of improved heat insulation.

Isothermal Oxidation and Thermal Shock Resistance of Thick and Porous LaMgAl11O19 Abradable Topcoat

An exploration of the plasma-sprayed abradable sealing coatings (ASCs) of a thick and porous LaMgAl11O19 topcoat onto SiC/SiC ceramic matrix composites (CMCs) is detailed in this study. Interlayers comprising Si/Si + Yb2Si2O7/Yb2SiO5 environmental barrier coatings (EBCs) were strategically employed, considering their function in protecting the SiC/SiC CMCs from recession and mitigating thermal expansivity misfit. An isothermal oxidation test was conducted at 1300 °C and resulted in the formation of bubble and glassy melt on the side surface of the coated sample, while a significant reaction layer emerged at the Yb2SiO5/LaMgAl11O19 interface near the edge. The localized temperature rise caused by the exothermic oxidation of the SiC/SiC substrate was determined to be the underlying factor for bubble generation. The temperature-dependent viscosity of the melt contributed to various bubble characteristics, and due to the enrichment of Al ions, the glassy melt exacerbated the degradation of the Yb2SiO5 layer. After a thermal shock test at 1300 °C, the substrate on the uncoated backside of the sample experienced fracture, while the front coating remained intact. However, due to the presence of a through-coating crack, an internal crack network also developed within the substrate.

Thermal properties and calcium-magnesium-alumino-silicate (CMAS) interaction of novel γ-phase ytterbium-doped yttrium disilicate (γ-Y1.5Yb0.5Si2O7) environmental barrier coating material

Rare-earth disilicates are promising candidates for thermal and environmental barrier coatings (TEBC) in gas turbines that safeguard SiCf/SiC ceramic matrix composites (CMCs) from thermal degradation and environmental attacks. Here, we report a systematic investigation on novel TEBC material, γ-Y1.5Yb0.5Si2O7. The γ-phase quarter molar ytterbium–doped yttrium disilicate exhibited low thermal conductivity (1.72 W·m−1·K−1 at 1200 °C) and reduced intrinsic thermal expansion (3.17 ± 0.22 × 10−6 K−1 up to 1000 °C), ensuring promisingly effective thermal insulation and minimized thermal stress with CMC substrates. Using density functional theory (DFT), the heat capacity of γ-Y1.5Yb0.5Si2O7 was predicted higher than that of undoped γ-Y2Si2O7. Comparing these predictions to results calculated using the Neumann–Kopp (NK) rule revealed only minor variations. A metastable CMAS interaction byproduct, cyclosilicate phase Ca3RE2(Si3O9)2, was identified based on energy dispersive X-ray spectrometer (EDS) and electron backscatter diffraction (EBSD) techniques, appearing at 1300 °C but disappearing at 1400 °C. The γ-Y1.5Yb0.5Si2O7 exhibited good CMAS resistance on both dense pellets and sprayed coatings, forming a protective apatite (Ca2RE8(SiO4)6O2) interlayer that effectively hindered CMAS infiltration at evaluated temperatures. The relatively higher Y:Yb atomic ratio (> 3) in the apatite grains indicate differential reactivity with molten CMAS and provides crucial insights into the CMAS corrosion mechanism. These findings highlight the potential of γ-Y1.5Yb0.5Si2O7 as a CMC coating material, emphasizing the need for tailored microstructural optimization as a thermal sprayed coating to enhance long-term performance in extreme gas turbine environments.